4 perlfunc - Perl builtin functions
8 The functions in this section can serve as terms in an expression.
9 They fall into two major categories: list operators and named unary
10 operators. These differ in their precedence relationship with a
11 following comma. (See the precedence table in L<perlop>.) List
12 operators take more than one argument, while unary operators can never
13 take more than one argument. Thus, a comma terminates the argument of
14 a unary operator, but merely separates the arguments of a list
15 operator. A unary operator generally provides scalar context to its
16 argument, while a list operator may provide either scalar or list
17 contexts for its arguments. If it does both, scalar arguments
18 come first and list argument follow, and there can only ever
19 be one such list argument. For instance,
20 L<C<splice>|/splice ARRAY,OFFSET,LENGTH,LIST> has three scalar arguments
21 followed by a list, whereas L<C<gethostbyname>|/gethostbyname NAME> has
22 four scalar arguments.
24 In the syntax descriptions that follow, list operators that expect a
25 list (and provide list context for elements of the list) are shown
26 with LIST as an argument. Such a list may consist of any combination
27 of scalar arguments or list values; the list values will be included
28 in the list as if each individual element were interpolated at that
29 point in the list, forming a longer single-dimensional list value.
30 Commas should separate literal elements of the LIST.
32 Any function in the list below may be used either with or without
33 parentheses around its arguments. (The syntax descriptions omit the
34 parentheses.) If you use parentheses, the simple but occasionally
35 surprising rule is this: It I<looks> like a function, therefore it I<is> a
36 function, and precedence doesn't matter. Otherwise it's a list
37 operator or unary operator, and precedence does matter. Whitespace
38 between the function and left parenthesis doesn't count, so sometimes
39 you need to be careful:
41 print 1+2+4; # Prints 7.
42 print(1+2) + 4; # Prints 3.
43 print (1+2)+4; # Also prints 3!
44 print +(1+2)+4; # Prints 7.
45 print ((1+2)+4); # Prints 7.
47 If you run Perl with the L<C<use warnings>|warnings> pragma, it can warn
48 you about this. For example, the third line above produces:
50 print (...) interpreted as function at - line 1.
51 Useless use of integer addition in void context at - line 1.
53 A few functions take no arguments at all, and therefore work as neither
54 unary nor list operators. These include such functions as
55 L<C<time>|/time> and L<C<endpwent>|/endpwent>. For example,
56 C<time+86_400> always means C<time() + 86_400>.
58 For functions that can be used in either a scalar or list context,
59 nonabortive failure is generally indicated in scalar context by
60 returning the undefined value, and in list context by returning the
63 Remember the following important rule: There is B<no rule> that relates
64 the behavior of an expression in list context to its behavior in scalar
65 context, or vice versa. It might do two totally different things.
66 Each operator and function decides which sort of value would be most
67 appropriate to return in scalar context. Some operators return the
68 length of the list that would have been returned in list context. Some
69 operators return the first value in the list. Some operators return the
70 last value in the list. Some operators return a count of successful
71 operations. In general, they do what you want, unless you want
75 A named array in scalar context is quite different from what would at
76 first glance appear to be a list in scalar context. You can't get a list
77 like C<(1,2,3)> into being in scalar context, because the compiler knows
78 the context at compile time. It would generate the scalar comma operator
79 there, not the list concatenation version of the comma. That means it
80 was never a list to start with.
82 In general, functions in Perl that serve as wrappers for system calls
83 ("syscalls") of the same name (like L<chown(2)>, L<fork(2)>,
84 L<closedir(2)>, etc.) return true when they succeed and
85 L<C<undef>|/undef EXPR> otherwise, as is usually mentioned in the
86 descriptions below. This is different from the C interfaces, which
87 return C<-1> on failure. Exceptions to this rule include
88 L<C<wait>|/wait>, L<C<waitpid>|/waitpid PID,FLAGS>, and
89 L<C<syscall>|/syscall NUMBER, LIST>. System calls also set the special
90 L<C<$!>|perlvar/$!> variable on failure. Other functions do not, except
93 Extension modules can also hook into the Perl parser to define new
94 kinds of keyword-headed expression. These may look like functions, but
95 may also look completely different. The syntax following the keyword
96 is defined entirely by the extension. If you are an implementor, see
97 L<perlapi/PL_keyword_plugin> for the mechanism. If you are using such
98 a module, see the module's documentation for details of the syntax that
101 =head2 Perl Functions by Category
104 Here are Perl's functions (including things that look like
105 functions, like some keywords and named operators)
106 arranged by category. Some functions appear in more
107 than one place. Any warnings, including those produced by
108 keywords, are described in L<perldiag> and L<warnings>.
112 =item Functions for SCALARs or strings
113 X<scalar> X<string> X<character>
115 =for Pod::Functions =String
117 L<C<chomp>|/chomp VARIABLE>, L<C<chop>|/chop VARIABLE>,
118 L<C<chr>|/chr NUMBER>, L<C<crypt>|/crypt PLAINTEXT,SALT>,
119 L<C<fc>|/fc EXPR>, L<C<hex>|/hex EXPR>,
120 L<C<index>|/index STR,SUBSTR,POSITION>, L<C<lc>|/lc EXPR>,
121 L<C<lcfirst>|/lcfirst EXPR>, L<C<length>|/length EXPR>,
122 L<C<oct>|/oct EXPR>, L<C<ord>|/ord EXPR>,
123 L<C<pack>|/pack TEMPLATE,LIST>,
124 L<C<qE<sol>E<sol>>|/qE<sol>STRINGE<sol>>,
125 L<C<qqE<sol>E<sol>>|/qqE<sol>STRINGE<sol>>, L<C<reverse>|/reverse LIST>,
126 L<C<rindex>|/rindex STR,SUBSTR,POSITION>,
127 L<C<sprintf>|/sprintf FORMAT, LIST>,
128 L<C<substr>|/substr EXPR,OFFSET,LENGTH,REPLACEMENT>,
129 L<C<trE<sol>E<sol>E<sol>>|/trE<sol>E<sol>E<sol>>, L<C<uc>|/uc EXPR>,
130 L<C<ucfirst>|/ucfirst EXPR>,
131 L<C<yE<sol>E<sol>E<sol>>|/yE<sol>E<sol>E<sol>>
133 L<C<fc>|/fc EXPR> is available only if the
134 L<C<"fc"> feature|feature/The 'fc' feature> is enabled or if it is
135 prefixed with C<CORE::>. The
136 L<C<"fc"> feature|feature/The 'fc' feature> is enabled automatically
137 with a C<use v5.16> (or higher) declaration in the current scope.
139 =item Regular expressions and pattern matching
140 X<regular expression> X<regex> X<regexp>
142 =for Pod::Functions =Regexp
144 L<C<mE<sol>E<sol>>|/mE<sol>E<sol>>, L<C<pos>|/pos SCALAR>,
145 L<C<qrE<sol>E<sol>>|/qrE<sol>STRINGE<sol>>,
146 L<C<quotemeta>|/quotemeta EXPR>,
147 L<C<sE<sol>E<sol>E<sol>>|/sE<sol>E<sol>E<sol>>,
148 L<C<split>|/split E<sol>PATTERNE<sol>,EXPR,LIMIT>,
149 L<C<study>|/study SCALAR>
151 =item Numeric functions
152 X<numeric> X<number> X<trigonometric> X<trigonometry>
154 =for Pod::Functions =Math
156 L<C<abs>|/abs VALUE>, L<C<atan2>|/atan2 Y,X>, L<C<cos>|/cos EXPR>,
157 L<C<exp>|/exp EXPR>, L<C<hex>|/hex EXPR>, L<C<int>|/int EXPR>,
158 L<C<log>|/log EXPR>, L<C<oct>|/oct EXPR>, L<C<rand>|/rand EXPR>,
159 L<C<sin>|/sin EXPR>, L<C<sqrt>|/sqrt EXPR>, L<C<srand>|/srand EXPR>
161 =item Functions for real @ARRAYs
164 =for Pod::Functions =ARRAY
166 L<C<each>|/each HASH>, L<C<keys>|/keys HASH>, L<C<pop>|/pop ARRAY>,
167 L<C<push>|/push ARRAY,LIST>, L<C<shift>|/shift ARRAY>,
168 L<C<splice>|/splice ARRAY,OFFSET,LENGTH,LIST>,
169 L<C<unshift>|/unshift ARRAY,LIST>, L<C<values>|/values HASH>
171 =item Functions for list data
174 =for Pod::Functions =LIST
176 L<C<grep>|/grep BLOCK LIST>, L<C<join>|/join EXPR,LIST>,
177 L<C<map>|/map BLOCK LIST>, L<C<qwE<sol>E<sol>>|/qwE<sol>STRINGE<sol>>,
178 L<C<reverse>|/reverse LIST>, L<C<sort>|/sort SUBNAME LIST>,
179 L<C<unpack>|/unpack TEMPLATE,EXPR>
181 =item Functions for real %HASHes
184 =for Pod::Functions =HASH
186 L<C<delete>|/delete EXPR>, L<C<each>|/each HASH>,
187 L<C<exists>|/exists EXPR>, L<C<keys>|/keys HASH>,
188 L<C<values>|/values HASH>
190 =item Input and output functions
191 X<I/O> X<input> X<output> X<dbm>
193 =for Pod::Functions =I/O
195 L<C<binmode>|/binmode FILEHANDLE, LAYER>, L<C<close>|/close FILEHANDLE>,
196 L<C<closedir>|/closedir DIRHANDLE>, L<C<dbmclose>|/dbmclose HASH>,
197 L<C<dbmopen>|/dbmopen HASH,DBNAME,MASK>, L<C<die>|/die LIST>,
198 L<C<eof>|/eof FILEHANDLE>, L<C<fileno>|/fileno FILEHANDLE>,
199 L<C<flock>|/flock FILEHANDLE,OPERATION>, L<C<format>|/format>,
200 L<C<getc>|/getc FILEHANDLE>, L<C<print>|/print FILEHANDLE LIST>,
201 L<C<printf>|/printf FILEHANDLE FORMAT, LIST>,
202 L<C<read>|/read FILEHANDLE,SCALAR,LENGTH,OFFSET>,
203 L<C<readdir>|/readdir DIRHANDLE>, L<C<readline>|/readline EXPR>,
204 L<C<rewinddir>|/rewinddir DIRHANDLE>, L<C<say>|/say FILEHANDLE LIST>,
205 L<C<seek>|/seek FILEHANDLE,POSITION,WHENCE>,
206 L<C<seekdir>|/seekdir DIRHANDLE,POS>,
207 L<C<select>|/select RBITS,WBITS,EBITS,TIMEOUT>,
208 L<C<syscall>|/syscall NUMBER, LIST>,
209 L<C<sysread>|/sysread FILEHANDLE,SCALAR,LENGTH,OFFSET>,
210 L<C<sysseek>|/sysseek FILEHANDLE,POSITION,WHENCE>,
211 L<C<syswrite>|/syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET>,
212 L<C<tell>|/tell FILEHANDLE>, L<C<telldir>|/telldir DIRHANDLE>,
213 L<C<truncate>|/truncate FILEHANDLE,LENGTH>, L<C<warn>|/warn LIST>,
214 L<C<write>|/write FILEHANDLE>
216 L<C<say>|/say FILEHANDLE LIST> is available only if the
217 L<C<"say"> feature|feature/The 'say' feature> is enabled or if it is
218 prefixed with C<CORE::>. The
219 L<C<"say"> feature|feature/The 'say' feature> is enabled automatically
220 with a C<use v5.10> (or higher) declaration in the current scope.
222 =item Functions for fixed-length data or records
224 =for Pod::Functions =Binary
226 L<C<pack>|/pack TEMPLATE,LIST>,
227 L<C<read>|/read FILEHANDLE,SCALAR,LENGTH,OFFSET>,
228 L<C<syscall>|/syscall NUMBER, LIST>,
229 L<C<sysread>|/sysread FILEHANDLE,SCALAR,LENGTH,OFFSET>,
230 L<C<sysseek>|/sysseek FILEHANDLE,POSITION,WHENCE>,
231 L<C<syswrite>|/syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET>,
232 L<C<unpack>|/unpack TEMPLATE,EXPR>, L<C<vec>|/vec EXPR,OFFSET,BITS>
234 =item Functions for filehandles, files, or directories
235 X<file> X<filehandle> X<directory> X<pipe> X<link> X<symlink>
237 =for Pod::Functions =File
239 L<C<-I<X>>|/-X FILEHANDLE>, L<C<chdir>|/chdir EXPR>,
240 L<C<chmod>|/chmod LIST>, L<C<chown>|/chown LIST>,
241 L<C<chroot>|/chroot FILENAME>,
242 L<C<fcntl>|/fcntl FILEHANDLE,FUNCTION,SCALAR>, L<C<glob>|/glob EXPR>,
243 L<C<ioctl>|/ioctl FILEHANDLE,FUNCTION,SCALAR>,
244 L<C<link>|/link OLDFILE,NEWFILE>, L<C<lstat>|/lstat FILEHANDLE>,
245 L<C<mkdir>|/mkdir FILENAME,MODE>, L<C<open>|/open FILEHANDLE,EXPR>,
246 L<C<opendir>|/opendir DIRHANDLE,EXPR>, L<C<readlink>|/readlink EXPR>,
247 L<C<rename>|/rename OLDNAME,NEWNAME>, L<C<rmdir>|/rmdir FILENAME>,
248 L<C<select>|/select FILEHANDLE>, L<C<stat>|/stat FILEHANDLE>,
249 L<C<symlink>|/symlink OLDFILE,NEWFILE>,
250 L<C<sysopen>|/sysopen FILEHANDLE,FILENAME,MODE>,
251 L<C<umask>|/umask EXPR>, L<C<unlink>|/unlink LIST>,
252 L<C<utime>|/utime LIST>
254 =item Keywords related to the control flow of your Perl program
257 =for Pod::Functions =Flow
259 L<C<break>|/break>, L<C<caller>|/caller EXPR>,
260 L<C<continue>|/continue BLOCK>, L<C<die>|/die LIST>, L<C<do>|/do BLOCK>,
261 L<C<dump>|/dump LABEL>, L<C<eval>|/eval EXPR>,
262 L<C<evalbytes>|/evalbytes EXPR>, L<C<exit>|/exit EXPR>,
263 L<C<__FILE__>|/__FILE__>, L<C<goto>|/goto LABEL>,
264 L<C<last>|/last LABEL>, L<C<__LINE__>|/__LINE__>,
265 L<C<next>|/next LABEL>, L<C<__PACKAGE__>|/__PACKAGE__>,
266 L<C<redo>|/redo LABEL>, L<C<return>|/return EXPR>,
267 L<C<sub>|/sub NAME BLOCK>, L<C<__SUB__>|/__SUB__>,
268 L<C<wantarray>|/wantarray>
270 L<C<break>|/break> is available only if you enable the experimental
271 L<C<"switch"> feature|feature/The 'switch' feature> or use the C<CORE::>
272 prefix. The L<C<"switch"> feature|feature/The 'switch' feature> also
273 enables the C<default>, C<given> and C<when> statements, which are
274 documented in L<perlsyn/"Switch Statements">.
275 The L<C<"switch"> feature|feature/The 'switch' feature> is enabled
276 automatically with a C<use v5.10> (or higher) declaration in the current
277 scope. In Perl v5.14 and earlier, L<C<continue>|/continue BLOCK>
278 required the L<C<"switch"> feature|feature/The 'switch' feature>, like
281 L<C<evalbytes>|/evalbytes EXPR> is only available with the
282 L<C<"evalbytes"> feature|feature/The 'unicode_eval' and 'evalbytes' features>
283 (see L<feature>) or if prefixed with C<CORE::>. L<C<__SUB__>|/__SUB__>
284 is only available with the
285 L<C<"current_sub"> feature|feature/The 'current_sub' feature> or if
286 prefixed with C<CORE::>. Both the
287 L<C<"evalbytes">|feature/The 'unicode_eval' and 'evalbytes' features>
288 and L<C<"current_sub">|feature/The 'current_sub' feature> features are
289 enabled automatically with a C<use v5.16> (or higher) declaration in the
292 =item Keywords related to scoping
294 =for Pod::Functions =Namespace
296 L<C<caller>|/caller EXPR>, L<C<import>|/import LIST>,
297 L<C<local>|/local EXPR>, L<C<my>|/my VARLIST>, L<C<our>|/our VARLIST>,
298 L<C<package>|/package NAMESPACE>, L<C<state>|/state VARLIST>,
299 L<C<use>|/use Module VERSION LIST>
301 L<C<state>|/state VARLIST> is available only if the
302 L<C<"state"> feature|feature/The 'state' feature> is enabled or if it is
303 prefixed with C<CORE::>. The
304 L<C<"state"> feature|feature/The 'state' feature> is enabled
305 automatically with a C<use v5.10> (or higher) declaration in the current
308 =item Miscellaneous functions
310 =for Pod::Functions =Misc
312 L<C<defined>|/defined EXPR>, L<C<formline>|/formline PICTURE,LIST>,
313 L<C<lock>|/lock THING>, L<C<prototype>|/prototype FUNCTION>,
314 L<C<reset>|/reset EXPR>, L<C<scalar>|/scalar EXPR>,
315 L<C<undef>|/undef EXPR>
317 =item Functions for processes and process groups
318 X<process> X<pid> X<process id>
320 =for Pod::Functions =Process
322 L<C<alarm>|/alarm SECONDS>, L<C<exec>|/exec LIST>, L<C<fork>|/fork>,
323 L<C<getpgrp>|/getpgrp PID>, L<C<getppid>|/getppid>,
324 L<C<getpriority>|/getpriority WHICH,WHO>, L<C<kill>|/kill SIGNAL, LIST>,
325 L<C<pipe>|/pipe READHANDLE,WRITEHANDLE>,
326 L<C<qxE<sol>E<sol>>|/qxE<sol>STRINGE<sol>>,
327 L<C<readpipe>|/readpipe EXPR>, L<C<setpgrp>|/setpgrp PID,PGRP>,
328 L<C<setpriority>|/setpriority WHICH,WHO,PRIORITY>,
329 L<C<sleep>|/sleep EXPR>, L<C<system>|/system LIST>, L<C<times>|/times>,
330 L<C<wait>|/wait>, L<C<waitpid>|/waitpid PID,FLAGS>
332 =item Keywords related to Perl modules
335 =for Pod::Functions =Modules
337 L<C<do>|/do EXPR>, L<C<import>|/import LIST>,
338 L<C<no>|/no MODULE VERSION LIST>, L<C<package>|/package NAMESPACE>,
339 L<C<require>|/require VERSION>, L<C<use>|/use Module VERSION LIST>
341 =item Keywords related to classes and object-orientation
342 X<object> X<class> X<package>
344 =for Pod::Functions =Objects
346 L<C<bless>|/bless REF,CLASSNAME>, L<C<dbmclose>|/dbmclose HASH>,
347 L<C<dbmopen>|/dbmopen HASH,DBNAME,MASK>,
348 L<C<package>|/package NAMESPACE>, L<C<ref>|/ref EXPR>,
349 L<C<tie>|/tie VARIABLE,CLASSNAME,LIST>, L<C<tied>|/tied VARIABLE>,
350 L<C<untie>|/untie VARIABLE>, L<C<use>|/use Module VERSION LIST>
352 =item Low-level socket functions
355 =for Pod::Functions =Socket
357 L<C<accept>|/accept NEWSOCKET,GENERICSOCKET>,
358 L<C<bind>|/bind SOCKET,NAME>, L<C<connect>|/connect SOCKET,NAME>,
359 L<C<getpeername>|/getpeername SOCKET>,
360 L<C<getsockname>|/getsockname SOCKET>,
361 L<C<getsockopt>|/getsockopt SOCKET,LEVEL,OPTNAME>,
362 L<C<listen>|/listen SOCKET,QUEUESIZE>,
363 L<C<recv>|/recv SOCKET,SCALAR,LENGTH,FLAGS>,
364 L<C<send>|/send SOCKET,MSG,FLAGS,TO>,
365 L<C<setsockopt>|/setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL>,
366 L<C<shutdown>|/shutdown SOCKET,HOW>,
367 L<C<socket>|/socket SOCKET,DOMAIN,TYPE,PROTOCOL>,
368 L<C<socketpair>|/socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL>
370 =item System V interprocess communication functions
371 X<IPC> X<System V> X<semaphore> X<shared memory> X<memory> X<message>
373 =for Pod::Functions =SysV
375 L<C<msgctl>|/msgctl ID,CMD,ARG>, L<C<msgget>|/msgget KEY,FLAGS>,
376 L<C<msgrcv>|/msgrcv ID,VAR,SIZE,TYPE,FLAGS>,
377 L<C<msgsnd>|/msgsnd ID,MSG,FLAGS>,
378 L<C<semctl>|/semctl ID,SEMNUM,CMD,ARG>,
379 L<C<semget>|/semget KEY,NSEMS,FLAGS>, L<C<semop>|/semop KEY,OPSTRING>,
380 L<C<shmctl>|/shmctl ID,CMD,ARG>, L<C<shmget>|/shmget KEY,SIZE,FLAGS>,
381 L<C<shmread>|/shmread ID,VAR,POS,SIZE>,
382 L<C<shmwrite>|/shmwrite ID,STRING,POS,SIZE>
384 =item Fetching user and group info
385 X<user> X<group> X<password> X<uid> X<gid> X<passwd> X</etc/passwd>
387 =for Pod::Functions =User
389 L<C<endgrent>|/endgrent>, L<C<endhostent>|/endhostent>,
390 L<C<endnetent>|/endnetent>, L<C<endpwent>|/endpwent>,
391 L<C<getgrent>|/getgrent>, L<C<getgrgid>|/getgrgid GID>,
392 L<C<getgrnam>|/getgrnam NAME>, L<C<getlogin>|/getlogin>,
393 L<C<getpwent>|/getpwent>, L<C<getpwnam>|/getpwnam NAME>,
394 L<C<getpwuid>|/getpwuid UID>, L<C<setgrent>|/setgrent>,
395 L<C<setpwent>|/setpwent>
397 =item Fetching network info
398 X<network> X<protocol> X<host> X<hostname> X<IP> X<address> X<service>
400 =for Pod::Functions =Network
402 L<C<endprotoent>|/endprotoent>, L<C<endservent>|/endservent>,
403 L<C<gethostbyaddr>|/gethostbyaddr ADDR,ADDRTYPE>,
404 L<C<gethostbyname>|/gethostbyname NAME>, L<C<gethostent>|/gethostent>,
405 L<C<getnetbyaddr>|/getnetbyaddr ADDR,ADDRTYPE>,
406 L<C<getnetbyname>|/getnetbyname NAME>, L<C<getnetent>|/getnetent>,
407 L<C<getprotobyname>|/getprotobyname NAME>,
408 L<C<getprotobynumber>|/getprotobynumber NUMBER>,
409 L<C<getprotoent>|/getprotoent>,
410 L<C<getservbyname>|/getservbyname NAME,PROTO>,
411 L<C<getservbyport>|/getservbyport PORT,PROTO>,
412 L<C<getservent>|/getservent>, L<C<sethostent>|/sethostent STAYOPEN>,
413 L<C<setnetent>|/setnetent STAYOPEN>,
414 L<C<setprotoent>|/setprotoent STAYOPEN>,
415 L<C<setservent>|/setservent STAYOPEN>
417 =item Time-related functions
420 =for Pod::Functions =Time
422 L<C<gmtime>|/gmtime EXPR>, L<C<localtime>|/localtime EXPR>,
423 L<C<time>|/time>, L<C<times>|/times>
425 =item Non-function keywords
427 =for Pod::Functions =!Non-functions
429 C<and>, C<AUTOLOAD>, C<BEGIN>, C<CHECK>, C<cmp>, C<CORE>, C<__DATA__>,
430 C<default>, C<DESTROY>, C<else>, C<elseif>, C<elsif>, C<END>, C<__END__>,
431 C<eq>, C<for>, C<foreach>, C<ge>, C<given>, C<gt>, C<if>, C<INIT>, C<le>,
432 C<lt>, C<ne>, C<not>, C<or>, C<UNITCHECK>, C<unless>, C<until>, C<when>,
433 C<while>, C<x>, C<xor>
438 X<portability> X<Unix> X<portable>
440 Perl was born in Unix and can therefore access all common Unix
441 system calls. In non-Unix environments, the functionality of some
442 Unix system calls may not be available or details of the available
443 functionality may differ slightly. The Perl functions affected
446 L<C<-I<X>>|/-X FILEHANDLE>, L<C<binmode>|/binmode FILEHANDLE, LAYER>,
447 L<C<chmod>|/chmod LIST>, L<C<chown>|/chown LIST>,
448 L<C<chroot>|/chroot FILENAME>, L<C<crypt>|/crypt PLAINTEXT,SALT>,
449 L<C<dbmclose>|/dbmclose HASH>, L<C<dbmopen>|/dbmopen HASH,DBNAME,MASK>,
450 L<C<dump>|/dump LABEL>, L<C<endgrent>|/endgrent>,
451 L<C<endhostent>|/endhostent>, L<C<endnetent>|/endnetent>,
452 L<C<endprotoent>|/endprotoent>, L<C<endpwent>|/endpwent>,
453 L<C<endservent>|/endservent>, L<C<exec>|/exec LIST>,
454 L<C<fcntl>|/fcntl FILEHANDLE,FUNCTION,SCALAR>,
455 L<C<flock>|/flock FILEHANDLE,OPERATION>, L<C<fork>|/fork>,
456 L<C<getgrent>|/getgrent>, L<C<getgrgid>|/getgrgid GID>,
457 L<C<gethostbyname>|/gethostbyname NAME>, L<C<gethostent>|/gethostent>,
458 L<C<getlogin>|/getlogin>,
459 L<C<getnetbyaddr>|/getnetbyaddr ADDR,ADDRTYPE>,
460 L<C<getnetbyname>|/getnetbyname NAME>, L<C<getnetent>|/getnetent>,
461 L<C<getppid>|/getppid>, L<C<getpgrp>|/getpgrp PID>,
462 L<C<getpriority>|/getpriority WHICH,WHO>,
463 L<C<getprotobynumber>|/getprotobynumber NUMBER>,
464 L<C<getprotoent>|/getprotoent>, L<C<getpwent>|/getpwent>,
465 L<C<getpwnam>|/getpwnam NAME>, L<C<getpwuid>|/getpwuid UID>,
466 L<C<getservbyport>|/getservbyport PORT,PROTO>,
467 L<C<getservent>|/getservent>,
468 L<C<getsockopt>|/getsockopt SOCKET,LEVEL,OPTNAME>,
469 L<C<glob>|/glob EXPR>, L<C<ioctl>|/ioctl FILEHANDLE,FUNCTION,SCALAR>,
470 L<C<kill>|/kill SIGNAL, LIST>, L<C<link>|/link OLDFILE,NEWFILE>,
471 L<C<lstat>|/lstat FILEHANDLE>, L<C<msgctl>|/msgctl ID,CMD,ARG>,
472 L<C<msgget>|/msgget KEY,FLAGS>,
473 L<C<msgrcv>|/msgrcv ID,VAR,SIZE,TYPE,FLAGS>,
474 L<C<msgsnd>|/msgsnd ID,MSG,FLAGS>, L<C<open>|/open FILEHANDLE,EXPR>,
475 L<C<pipe>|/pipe READHANDLE,WRITEHANDLE>, L<C<readlink>|/readlink EXPR>,
476 L<C<rename>|/rename OLDNAME,NEWNAME>,
477 L<C<select>|/select RBITS,WBITS,EBITS,TIMEOUT>,
478 L<C<semctl>|/semctl ID,SEMNUM,CMD,ARG>,
479 L<C<semget>|/semget KEY,NSEMS,FLAGS>, L<C<semop>|/semop KEY,OPSTRING>,
480 L<C<setgrent>|/setgrent>, L<C<sethostent>|/sethostent STAYOPEN>,
481 L<C<setnetent>|/setnetent STAYOPEN>, L<C<setpgrp>|/setpgrp PID,PGRP>,
482 L<C<setpriority>|/setpriority WHICH,WHO,PRIORITY>,
483 L<C<setprotoent>|/setprotoent STAYOPEN>, L<C<setpwent>|/setpwent>,
484 L<C<setservent>|/setservent STAYOPEN>,
485 L<C<setsockopt>|/setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL>,
486 L<C<shmctl>|/shmctl ID,CMD,ARG>, L<C<shmget>|/shmget KEY,SIZE,FLAGS>,
487 L<C<shmread>|/shmread ID,VAR,POS,SIZE>,
488 L<C<shmwrite>|/shmwrite ID,STRING,POS,SIZE>,
489 L<C<socket>|/socket SOCKET,DOMAIN,TYPE,PROTOCOL>,
490 L<C<socketpair>|/socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL>,
491 L<C<stat>|/stat FILEHANDLE>, L<C<symlink>|/symlink OLDFILE,NEWFILE>,
492 L<C<syscall>|/syscall NUMBER, LIST>,
493 L<C<sysopen>|/sysopen FILEHANDLE,FILENAME,MODE>,
494 L<C<system>|/system LIST>, L<C<times>|/times>,
495 L<C<truncate>|/truncate FILEHANDLE,LENGTH>, L<C<umask>|/umask EXPR>,
496 L<C<unlink>|/unlink LIST>, L<C<utime>|/utime LIST>, L<C<wait>|/wait>,
497 L<C<waitpid>|/waitpid PID,FLAGS>
499 For more information about the portability of these functions, see
500 L<perlport> and other available platform-specific documentation.
502 =head2 Alphabetical Listing of Perl Functions
507 X<-r>X<-w>X<-x>X<-o>X<-R>X<-W>X<-X>X<-O>X<-e>X<-z>X<-s>X<-f>X<-d>X<-l>X<-p>
508 X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
516 =for Pod::Functions a file test (-r, -x, etc)
518 A file test, where X is one of the letters listed below. This unary
519 operator takes one argument, either a filename, a filehandle, or a dirhandle,
520 and tests the associated file to see if something is true about it. If the
521 argument is omitted, tests L<C<$_>|perlvar/$_>, except for C<-t>, which
522 tests STDIN. Unless otherwise documented, it returns C<1> for true and
523 C<''> for false. If the file doesn't exist or can't be examined, it
524 returns L<C<undef>|/undef EXPR> and sets L<C<$!>|perlvar/$!> (errno).
525 With the exception of the C<-l> test they all follow symbolic links
526 because they use C<stat()> and not C<lstat()> (so dangling symlinks can't
527 be examined and will therefore report failure).
529 Despite the funny names, precedence is the same as any other named unary
530 operator. The operator may be any of:
532 -r File is readable by effective uid/gid.
533 -w File is writable by effective uid/gid.
534 -x File is executable by effective uid/gid.
535 -o File is owned by effective uid.
537 -R File is readable by real uid/gid.
538 -W File is writable by real uid/gid.
539 -X File is executable by real uid/gid.
540 -O File is owned by real uid.
543 -z File has zero size (is empty).
544 -s File has nonzero size (returns size in bytes).
546 -f File is a plain file.
547 -d File is a directory.
548 -l File is a symbolic link (false if symlinks aren't
549 supported by the file system).
550 -p File is a named pipe (FIFO), or Filehandle is a pipe.
552 -b File is a block special file.
553 -c File is a character special file.
554 -t Filehandle is opened to a tty.
556 -u File has setuid bit set.
557 -g File has setgid bit set.
558 -k File has sticky bit set.
560 -T File is an ASCII or UTF-8 text file (heuristic guess).
561 -B File is a "binary" file (opposite of -T).
563 -M Script start time minus file modification time, in days.
564 -A Same for access time.
565 -C Same for inode change time (Unix, may differ for other
572 next unless -f $_; # ignore specials
576 Note that C<-s/a/b/> does not do a negated substitution. Saying
577 C<-exp($foo)> still works as expected, however: only single letters
578 following a minus are interpreted as file tests.
580 These operators are exempt from the "looks like a function rule" described
581 above. That is, an opening parenthesis after the operator does not affect
582 how much of the following code constitutes the argument. Put the opening
583 parentheses before the operator to separate it from code that follows (this
584 applies only to operators with higher precedence than unary operators, of
587 -s($file) + 1024 # probably wrong; same as -s($file + 1024)
588 (-s $file) + 1024 # correct
590 The interpretation of the file permission operators C<-r>, C<-R>,
591 C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
592 of the file and the uids and gids of the user. There may be other
593 reasons you can't actually read, write, or execute the file: for
594 example network filesystem access controls, ACLs (access control lists),
595 read-only filesystems, and unrecognized executable formats. Note
596 that the use of these six specific operators to verify if some operation
597 is possible is usually a mistake, because it may be open to race
600 Also note that, for the superuser on the local filesystems, the C<-r>,
601 C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
602 if any execute bit is set in the mode. Scripts run by the superuser
603 may thus need to do a L<C<stat>|/stat FILEHANDLE> to determine the
604 actual mode of the file, or temporarily set their effective uid to
607 If you are using ACLs, there is a pragma called L<C<filetest>|filetest>
608 that may produce more accurate results than the bare
609 L<C<stat>|/stat FILEHANDLE> mode bits.
610 When under C<use filetest 'access'>, the above-mentioned filetests
611 test whether the permission can(not) be granted using the L<access(2)>
612 family of system calls. Also note that the C<-x> and C<-X> tests may
613 under this pragma return true even if there are no execute permission
614 bits set (nor any extra execute permission ACLs). This strangeness is
615 due to the underlying system calls' definitions. Note also that, due to
616 the implementation of C<use filetest 'access'>, the C<_> special
617 filehandle won't cache the results of the file tests when this pragma is
618 in effect. Read the documentation for the L<C<filetest>|filetest>
619 pragma for more information.
621 The C<-T> and C<-B> tests work as follows. The first block or so of
622 the file is examined to see if it is valid UTF-8 that includes non-ASCII
623 characters. If so, it's a C<-T> file. Otherwise, that same portion of
624 the file is examined for odd characters such as strange control codes or
625 characters with the high bit set. If more than a third of the
626 characters are strange, it's a C<-B> file; otherwise it's a C<-T> file.
627 Also, any file containing a zero byte in the examined portion is
628 considered a binary file. (If executed within the scope of a L<S<use
629 locale>|perllocale> which includes C<LC_CTYPE>, odd characters are
630 anything that isn't a printable nor space in the current locale.) If
631 C<-T> or C<-B> is used on a filehandle, the current IO buffer is
633 rather than the first block. Both C<-T> and C<-B> return true on an empty
634 file, or a file at EOF when testing a filehandle. Because you have to
635 read a file to do the C<-T> test, on most occasions you want to use a C<-f>
636 against the file first, as in C<next unless -f $file && -T $file>.
638 If any of the file tests (or either the L<C<stat>|/stat FILEHANDLE> or
639 L<C<lstat>|/lstat FILEHANDLE> operator) is given the special filehandle
640 consisting of a solitary underline, then the stat structure of the
641 previous file test (or L<C<stat>|/stat FILEHANDLE> operator) is used,
642 saving a system call. (This doesn't work with C<-t>, and you need to
643 remember that L<C<lstat>|/lstat FILEHANDLE> and C<-l> leave values in
644 the stat structure for the symbolic link, not the real file.) (Also, if
645 the stat buffer was filled by an L<C<lstat>|/lstat FILEHANDLE> call,
646 C<-T> and C<-B> will reset it with the results of C<stat _>).
649 print "Can do.\n" if -r $a || -w _ || -x _;
652 print "Readable\n" if -r _;
653 print "Writable\n" if -w _;
654 print "Executable\n" if -x _;
655 print "Setuid\n" if -u _;
656 print "Setgid\n" if -g _;
657 print "Sticky\n" if -k _;
658 print "Text\n" if -T _;
659 print "Binary\n" if -B _;
661 As of Perl 5.10.0, as a form of purely syntactic sugar, you can stack file
662 test operators, in a way that C<-f -w -x $file> is equivalent to
663 C<-x $file && -w _ && -f _>. (This is only fancy syntax: if you use
664 the return value of C<-f $file> as an argument to another filetest
665 operator, no special magic will happen.)
667 Portability issues: L<perlport/-X>.
669 To avoid confusing would-be users of your code with mysterious
670 syntax errors, put something like this at the top of your script:
672 use 5.010; # so filetest ops can stack
679 =for Pod::Functions absolute value function
681 Returns the absolute value of its argument.
682 If VALUE is omitted, uses L<C<$_>|perlvar/$_>.
684 =item accept NEWSOCKET,GENERICSOCKET
687 =for Pod::Functions accept an incoming socket connect
689 Accepts an incoming socket connect, just as L<accept(2)>
690 does. Returns the packed address if it succeeded, false otherwise.
691 See the example in L<perlipc/"Sockets: Client/Server Communication">.
693 On systems that support a close-on-exec flag on files, the flag will
694 be set for the newly opened file descriptor, as determined by the
695 value of L<C<$^F>|perlvar/$^F>. See L<perlvar/$^F>.
704 =for Pod::Functions schedule a SIGALRM
706 Arranges to have a SIGALRM delivered to this process after the
707 specified number of wallclock seconds has elapsed. If SECONDS is not
708 specified, the value stored in L<C<$_>|perlvar/$_> is used. (On some
709 machines, unfortunately, the elapsed time may be up to one second less
710 or more than you specified because of how seconds are counted, and
711 process scheduling may delay the delivery of the signal even further.)
713 Only one timer may be counting at once. Each call disables the
714 previous timer, and an argument of C<0> may be supplied to cancel the
715 previous timer without starting a new one. The returned value is the
716 amount of time remaining on the previous timer.
718 For delays of finer granularity than one second, the L<Time::HiRes> module
719 (from CPAN, and starting from Perl 5.8 part of the standard
720 distribution) provides
721 L<C<ualarm>|Time::HiRes/ualarm ( $useconds [, $interval_useconds ] )>.
722 You may also use Perl's four-argument version of
723 L<C<select>|/select RBITS,WBITS,EBITS,TIMEOUT> leaving the first three
724 arguments undefined, or you might be able to use the
725 L<C<syscall>|/syscall NUMBER, LIST> interface to access L<setitimer(2)>
726 if your system supports it. See L<perlfaq8> for details.
728 It is usually a mistake to intermix L<C<alarm>|/alarm SECONDS> and
729 L<C<sleep>|/sleep EXPR> calls, because L<C<sleep>|/sleep EXPR> may be
730 internally implemented on your system with L<C<alarm>|/alarm SECONDS>.
732 If you want to use L<C<alarm>|/alarm SECONDS> to time out a system call
733 you need to use an L<C<eval>|/eval EXPR>/L<C<die>|/die LIST> pair. You
734 can't rely on the alarm causing the system call to fail with
735 L<C<$!>|perlvar/$!> set to C<EINTR> because Perl sets up signal handlers
736 to restart system calls on some systems. Using
737 L<C<eval>|/eval EXPR>/L<C<die>|/die LIST> always works, modulo the
738 caveats given in L<perlipc/"Signals">.
741 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
743 my $nread = sysread $socket, $buffer, $size;
747 die unless $@ eq "alarm\n"; # propagate unexpected errors
754 For more information see L<perlipc>.
756 Portability issues: L<perlport/alarm>.
759 X<atan2> X<arctangent> X<tan> X<tangent>
761 =for Pod::Functions arctangent of Y/X in the range -PI to PI
763 Returns the arctangent of Y/X in the range -PI to PI.
765 For the tangent operation, you may use the
766 L<C<Math::Trig::tan>|Math::Trig/B<tan>> function, or use the familiar
769 sub tan { sin($_[0]) / cos($_[0]) }
771 The return value for C<atan2(0,0)> is implementation-defined; consult
772 your L<atan2(3)> manpage for more information.
774 Portability issues: L<perlport/atan2>.
776 =item bind SOCKET,NAME
779 =for Pod::Functions binds an address to a socket
781 Binds a network address to a socket, just as L<bind(2)>
782 does. Returns true if it succeeded, false otherwise. NAME should be a
783 packed address of the appropriate type for the socket. See the examples in
784 L<perlipc/"Sockets: Client/Server Communication">.
786 =item binmode FILEHANDLE, LAYER
787 X<binmode> X<binary> X<text> X<DOS> X<Windows>
789 =item binmode FILEHANDLE
791 =for Pod::Functions prepare binary files for I/O
793 Arranges for FILEHANDLE to be read or written in "binary" or "text"
794 mode on systems where the run-time libraries distinguish between
795 binary and text files. If FILEHANDLE is an expression, the value is
796 taken as the name of the filehandle. Returns true on success,
797 otherwise it returns L<C<undef>|/undef EXPR> and sets
798 L<C<$!>|perlvar/$!> (errno).
800 On some systems (in general, DOS- and Windows-based systems)
801 L<C<binmode>|/binmode FILEHANDLE, LAYER> is necessary when you're not
802 working with a text file. For the sake of portability it is a good idea
803 always to use it when appropriate, and never to use it when it isn't
804 appropriate. Also, people can set their I/O to be by default
805 UTF8-encoded Unicode, not bytes.
807 In other words: regardless of platform, use
808 L<C<binmode>|/binmode FILEHANDLE, LAYER> on binary data, like images,
811 If LAYER is present it is a single string, but may contain multiple
812 directives. The directives alter the behaviour of the filehandle.
813 When LAYER is present, using binmode on a text file makes sense.
815 If LAYER is omitted or specified as C<:raw> the filehandle is made
816 suitable for passing binary data. This includes turning off possible CRLF
817 translation and marking it as bytes (as opposed to Unicode characters).
818 Note that, despite what may be implied in I<"Programming Perl"> (the
819 Camel, 3rd edition) or elsewhere, C<:raw> is I<not> simply the inverse of C<:crlf>.
820 Other layers that would affect the binary nature of the stream are
821 I<also> disabled. See L<PerlIO>, L<perlrun>, and the discussion about the
822 PERLIO environment variable.
824 The C<:bytes>, C<:crlf>, C<:utf8>, and any other directives of the
825 form C<:...>, are called I/O I<layers>. The L<open> pragma can be used to
826 establish default I/O layers.
828 I<The LAYER parameter of the L<C<binmode>|/binmode FILEHANDLE, LAYER>
829 function is described as "DISCIPLINE" in "Programming Perl, 3rd
830 Edition". However, since the publishing of this book, by many known as
831 "Camel III", the consensus of the naming of this functionality has moved
832 from "discipline" to "layer". All documentation of this version of Perl
833 therefore refers to "layers" rather than to "disciplines". Now back to
834 the regularly scheduled documentation...>
836 To mark FILEHANDLE as UTF-8, use C<:utf8> or C<:encoding(UTF-8)>.
837 C<:utf8> just marks the data as UTF-8 without further checking,
838 while C<:encoding(UTF-8)> checks the data for actually being valid
839 UTF-8. More details can be found in L<PerlIO::encoding>.
841 In general, L<C<binmode>|/binmode FILEHANDLE, LAYER> should be called
842 after L<C<open>|/open FILEHANDLE,EXPR> but before any I/O is done on the
843 filehandle. Calling L<C<binmode>|/binmode FILEHANDLE, LAYER> normally
844 flushes any pending buffered output data (and perhaps pending input
845 data) on the handle. An exception to this is the C<:encoding> layer
846 that changes the default character encoding of the handle.
847 The C<:encoding> layer sometimes needs to be called in
848 mid-stream, and it doesn't flush the stream. C<:encoding>
849 also implicitly pushes on top of itself the C<:utf8> layer because
850 internally Perl operates on UTF8-encoded Unicode characters.
852 The operating system, device drivers, C libraries, and Perl run-time
853 system all conspire to let the programmer treat a single
854 character (C<\n>) as the line terminator, irrespective of external
855 representation. On many operating systems, the native text file
856 representation matches the internal representation, but on some
857 platforms the external representation of C<\n> is made up of more than
860 All variants of Unix, Mac OS (old and new), and Stream_LF files on VMS use
861 a single character to end each line in the external representation of text
862 (even though that single character is CARRIAGE RETURN on old, pre-Darwin
863 flavors of Mac OS, and is LINE FEED on Unix and most VMS files). In other
864 systems like OS/2, DOS, and the various flavors of MS-Windows, your program
865 sees a C<\n> as a simple C<\cJ>, but what's stored in text files are the
866 two characters C<\cM\cJ>. That means that if you don't use
867 L<C<binmode>|/binmode FILEHANDLE, LAYER> on these systems, C<\cM\cJ>
868 sequences on disk will be converted to C<\n> on input, and any C<\n> in
869 your program will be converted back to C<\cM\cJ> on output. This is
870 what you want for text files, but it can be disastrous for binary files.
872 Another consequence of using L<C<binmode>|/binmode FILEHANDLE, LAYER>
873 (on some systems) is that special end-of-file markers will be seen as
874 part of the data stream. For systems from the Microsoft family this
875 means that, if your binary data contain C<\cZ>, the I/O subsystem will
876 regard it as the end of the file, unless you use
877 L<C<binmode>|/binmode FILEHANDLE, LAYER>.
879 L<C<binmode>|/binmode FILEHANDLE, LAYER> is important not only for
880 L<C<readline>|/readline EXPR> and L<C<print>|/print FILEHANDLE LIST>
881 operations, but also when using
882 L<C<read>|/read FILEHANDLE,SCALAR,LENGTH,OFFSET>,
883 L<C<seek>|/seek FILEHANDLE,POSITION,WHENCE>,
884 L<C<sysread>|/sysread FILEHANDLE,SCALAR,LENGTH,OFFSET>,
885 L<C<syswrite>|/syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET> and
886 L<C<tell>|/tell FILEHANDLE> (see L<perlport> for more details). See the
887 L<C<$E<sol>>|perlvar/$E<sol>> and L<C<$\>|perlvar/$\> variables in
888 L<perlvar> for how to manually set your input and output
889 line-termination sequences.
891 Portability issues: L<perlport/binmode>.
893 =item bless REF,CLASSNAME
898 =for Pod::Functions create an object
900 This function tells the thingy referenced by REF that it is now an object
901 in the CLASSNAME package. If CLASSNAME is an empty string, it is
902 interpreted as referring to the C<main> package.
903 If CLASSNAME is omitted, the current package
904 is used. Because a L<C<bless>|/bless REF,CLASSNAME> is often the last
905 thing in a constructor, it returns the reference for convenience.
906 Always use the two-argument version if a derived class might inherit the
907 method doing the blessing. See L<perlobj> for more about the blessing
908 (and blessings) of objects.
910 Consider always blessing objects in CLASSNAMEs that are mixed case.
911 Namespaces with all lowercase names are considered reserved for
912 Perl pragmas. Builtin types have all uppercase names. To prevent
913 confusion, you may wish to avoid such package names as well.
914 It is advised to avoid the class name C<0>, because much code erroneously
915 uses the result of L<C<ref>|/ref EXPR> as a truth value.
917 See L<perlmod/"Perl Modules">.
921 =for Pod::Functions +switch break out of a C<given> block
923 Break out of a C<given> block.
925 L<C<break>|/break> is available only if the
926 L<C<"switch"> feature|feature/The 'switch' feature> is enabled or if it
927 is prefixed with C<CORE::>. The
928 L<C<"switch"> feature|feature/The 'switch' feature> is enabled
929 automatically with a C<use v5.10> (or higher) declaration in the current
933 X<caller> X<call stack> X<stack> X<stack trace>
937 =for Pod::Functions get context of the current subroutine call
939 Returns the context of the current pure perl subroutine call. In scalar
940 context, returns the caller's package name if there I<is> a caller (that is, if
941 we're in a subroutine or L<C<eval>|/eval EXPR> or
942 L<C<require>|/require VERSION>) and the undefined value otherwise.
943 caller never returns XS subs and they are skipped. The next pure perl
944 sub will appear instead of the XS sub in caller's return values. In
945 list context, caller returns
948 my ($package, $filename, $line) = caller;
950 With EXPR, it returns some extra information that the debugger uses to
951 print a stack trace. The value of EXPR indicates how many call frames
952 to go back before the current one.
955 my ($package, $filename, $line, $subroutine, $hasargs,
958 $wantarray, $evaltext, $is_require, $hints, $bitmask, $hinthash)
961 Here, $subroutine is the function that the caller called (rather than the
962 function containing the caller). Note that $subroutine may be C<(eval)> if
963 the frame is not a subroutine call, but an L<C<eval>|/eval EXPR>. In
964 such a case additional elements $evaltext and C<$is_require> are set:
965 C<$is_require> is true if the frame is created by a
966 L<C<require>|/require VERSION> or L<C<use>|/use Module VERSION LIST>
967 statement, $evaltext contains the text of the C<eval EXPR> statement.
968 In particular, for an C<eval BLOCK> statement, $subroutine is C<(eval)>,
969 but $evaltext is undefined. (Note also that each
970 L<C<use>|/use Module VERSION LIST> statement creates a
971 L<C<require>|/require VERSION> frame inside an C<eval EXPR> frame.)
972 $subroutine may also be C<(unknown)> if this particular subroutine
973 happens to have been deleted from the symbol table. C<$hasargs> is true
974 if a new instance of L<C<@_>|perlvar/@_> was set up for the frame.
975 C<$hints> and C<$bitmask> contain pragmatic hints that the caller was
976 compiled with. C<$hints> corresponds to L<C<$^H>|perlvar/$^H>, and
977 C<$bitmask> corresponds to
978 L<C<${^WARNING_BITS}>|perlvar/${^WARNING_BITS}>. The C<$hints> and
979 C<$bitmask> values are subject to change between versions of Perl, and
980 are not meant for external use.
982 C<$hinthash> is a reference to a hash containing the value of
983 L<C<%^H>|perlvar/%^H> when the caller was compiled, or
984 L<C<undef>|/undef EXPR> if L<C<%^H>|perlvar/%^H> was empty. Do not
985 modify the values of this hash, as they are the actual values stored in
988 Furthermore, when called from within the DB package in
989 list context, and with an argument, caller returns more
990 detailed information: it sets the list variable C<@DB::args> to be the
991 arguments with which the subroutine was invoked.
993 Be aware that the optimizer might have optimized call frames away before
994 L<C<caller>|/caller EXPR> had a chance to get the information. That
995 means that C<caller(N)> might not return information about the call
996 frame you expect it to, for C<< N > 1 >>. In particular, C<@DB::args>
997 might have information from the previous time L<C<caller>|/caller EXPR>
1000 Be aware that setting C<@DB::args> is I<best effort>, intended for
1001 debugging or generating backtraces, and should not be relied upon. In
1002 particular, as L<C<@_>|perlvar/@_> contains aliases to the caller's
1003 arguments, Perl does not take a copy of L<C<@_>|perlvar/@_>, so
1004 C<@DB::args> will contain modifications the subroutine makes to
1005 L<C<@_>|perlvar/@_> or its contents, not the original values at call
1006 time. C<@DB::args>, like L<C<@_>|perlvar/@_>, does not hold explicit
1007 references to its elements, so under certain cases its elements may have
1008 become freed and reallocated for other variables or temporary values.
1009 Finally, a side effect of the current implementation is that the effects
1010 of C<shift @_> can I<normally> be undone (but not C<pop @_> or other
1011 splicing, I<and> not if a reference to L<C<@_>|perlvar/@_> has been
1012 taken, I<and> subject to the caveat about reallocated elements), so
1013 C<@DB::args> is actually a hybrid of the current state and initial state
1014 of L<C<@_>|perlvar/@_>. Buyer beware.
1019 X<directory, change>
1021 =item chdir FILEHANDLE
1023 =item chdir DIRHANDLE
1027 =for Pod::Functions change your current working directory
1029 Changes the working directory to EXPR, if possible. If EXPR is omitted,
1030 changes to the directory specified by C<$ENV{HOME}>, if set; if not,
1031 changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
1032 variable C<$ENV{'SYS$LOGIN'}> is also checked, and used if it is set.) If
1033 neither is set, L<C<chdir>|/chdir EXPR> does nothing and fails. It
1034 returns true on success, false otherwise. See the example under
1035 L<C<die>|/die LIST>.
1037 On systems that support L<fchdir(2)>, you may pass a filehandle or
1038 directory handle as the argument. On systems that don't support L<fchdir(2)>,
1039 passing handles raises an exception.
1042 X<chmod> X<permission> X<mode>
1044 =for Pod::Functions changes the permissions on a list of files
1046 Changes the permissions of a list of files. The first element of the
1047 list must be the numeric mode, which should probably be an octal
1048 number, and which definitely should I<not> be a string of octal digits:
1049 C<0644> is okay, but C<"0644"> is not. Returns the number of files
1050 successfully changed. See also L<C<oct>|/oct EXPR> if all you have is a
1053 my $cnt = chmod 0755, "foo", "bar";
1054 chmod 0755, @executables;
1055 my $mode = "0644"; chmod $mode, "foo"; # !!! sets mode to
1057 my $mode = "0644"; chmod oct($mode), "foo"; # this is better
1058 my $mode = 0644; chmod $mode, "foo"; # this is best
1060 On systems that support L<fchmod(2)>, you may pass filehandles among the
1061 files. On systems that don't support L<fchmod(2)>, passing filehandles raises
1062 an exception. Filehandles must be passed as globs or glob references to be
1063 recognized; barewords are considered filenames.
1065 open(my $fh, "<", "foo");
1066 my $perm = (stat $fh)[2] & 07777;
1067 chmod($perm | 0600, $fh);
1069 You can also import the symbolic C<S_I*> constants from the
1070 L<C<Fcntl>|Fcntl> module:
1072 use Fcntl qw( :mode );
1073 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
1074 # Identical to the chmod 0755 of the example above.
1076 Portability issues: L<perlport/chmod>.
1078 =item chomp VARIABLE
1079 X<chomp> X<INPUT_RECORD_SEPARATOR> X<$/> X<newline> X<eol>
1085 =for Pod::Functions remove a trailing record separator from a string
1087 This safer version of L<C<chop>|/chop VARIABLE> removes any trailing
1088 string that corresponds to the current value of
1089 L<C<$E<sol>>|perlvar/$E<sol>> (also known as C<$INPUT_RECORD_SEPARATOR>
1090 in the L<C<English>|English> module). It returns the total
1091 number of characters removed from all its arguments. It's often used to
1092 remove the newline from the end of an input record when you're worried
1093 that the final record may be missing its newline. When in paragraph
1094 mode (C<$/ = ''>), it removes all trailing newlines from the string.
1095 When in slurp mode (C<$/ = undef>) or fixed-length record mode
1096 (L<C<$E<sol>>|perlvar/$E<sol>> is a reference to an integer or the like;
1097 see L<perlvar>), L<C<chomp>|/chomp VARIABLE> won't remove anything.
1098 If VARIABLE is omitted, it chomps L<C<$_>|perlvar/$_>. Example:
1101 chomp; # avoid \n on last field
1102 my @array = split(/:/);
1106 If VARIABLE is a hash, it chomps the hash's values, but not its keys,
1107 resetting the L<C<each>|/each HASH> iterator in the process.
1109 You can actually chomp anything that's an lvalue, including an assignment:
1111 chomp(my $cwd = `pwd`);
1112 chomp(my $answer = <STDIN>);
1114 If you chomp a list, each element is chomped, and the total number of
1115 characters removed is returned.
1117 Note that parentheses are necessary when you're chomping anything
1118 that is not a simple variable. This is because C<chomp $cwd = `pwd`;>
1119 is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as
1120 C<chomp( $cwd = `pwd` )> which you might expect. Similarly,
1121 C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than
1122 as C<chomp($a, $b)>.
1131 =for Pod::Functions remove the last character from a string
1133 Chops off the last character of a string and returns the character
1134 chopped. It is much more efficient than C<s/.$//s> because it neither
1135 scans nor copies the string. If VARIABLE is omitted, chops
1136 L<C<$_>|perlvar/$_>.
1137 If VARIABLE is a hash, it chops the hash's values, but not its keys,
1138 resetting the L<C<each>|/each HASH> iterator in the process.
1140 You can actually chop anything that's an lvalue, including an assignment.
1142 If you chop a list, each element is chopped. Only the value of the
1143 last L<C<chop>|/chop VARIABLE> is returned.
1145 Note that L<C<chop>|/chop VARIABLE> returns the last character. To
1146 return all but the last character, use C<substr($string, 0, -1)>.
1148 See also L<C<chomp>|/chomp VARIABLE>.
1151 X<chown> X<owner> X<user> X<group>
1153 =for Pod::Functions change the ownership on a list of files
1155 Changes the owner (and group) of a list of files. The first two
1156 elements of the list must be the I<numeric> uid and gid, in that
1157 order. A value of -1 in either position is interpreted by most
1158 systems to leave that value unchanged. Returns the number of files
1159 successfully changed.
1161 my $cnt = chown $uid, $gid, 'foo', 'bar';
1162 chown $uid, $gid, @filenames;
1164 On systems that support L<fchown(2)>, you may pass filehandles among the
1165 files. On systems that don't support L<fchown(2)>, passing filehandles raises
1166 an exception. Filehandles must be passed as globs or glob references to be
1167 recognized; barewords are considered filenames.
1169 Here's an example that looks up nonnumeric uids in the passwd file:
1172 chomp(my $user = <STDIN>);
1174 chomp(my $pattern = <STDIN>);
1176 my ($login,$pass,$uid,$gid) = getpwnam($user)
1177 or die "$user not in passwd file";
1179 my @ary = glob($pattern); # expand filenames
1180 chown $uid, $gid, @ary;
1182 On most systems, you are not allowed to change the ownership of the
1183 file unless you're the superuser, although you should be able to change
1184 the group to any of your secondary groups. On insecure systems, these
1185 restrictions may be relaxed, but this is not a portable assumption.
1186 On POSIX systems, you can detect this condition this way:
1188 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
1189 my $can_chown_giveaway = ! sysconf(_PC_CHOWN_RESTRICTED);
1191 Portability issues: L<perlport/chown>.
1194 X<chr> X<character> X<ASCII> X<Unicode>
1198 =for Pod::Functions get character this number represents
1200 Returns the character represented by that NUMBER in the character set.
1201 For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
1202 chr(0x263a) is a Unicode smiley face.
1204 Negative values give the Unicode replacement character (chr(0xfffd)),
1205 except under the L<bytes> pragma, where the low eight bits of the value
1206 (truncated to an integer) are used.
1208 If NUMBER is omitted, uses L<C<$_>|perlvar/$_>.
1210 For the reverse, use L<C<ord>|/ord EXPR>.
1212 Note that characters from 128 to 255 (inclusive) are by default
1213 internally not encoded as UTF-8 for backward compatibility reasons.
1215 See L<perlunicode> for more about Unicode.
1217 =item chroot FILENAME
1222 =for Pod::Functions make directory new root for path lookups
1224 This function works like the system call by the same name: it makes the
1225 named directory the new root directory for all further pathnames that
1226 begin with a C</> by your process and all its children. (It doesn't
1227 change your current working directory, which is unaffected.) For security
1228 reasons, this call is restricted to the superuser. If FILENAME is
1229 omitted, does a L<C<chroot>|/chroot FILENAME> to L<C<$_>|perlvar/$_>.
1231 B<NOTE:> It is mandatory for security to C<chdir("/")>
1232 (L<C<chdir>|/chdir EXPR> to the root directory) immediately after a
1233 L<C<chroot>|/chroot FILENAME>, otherwise the current working directory
1234 may be outside of the new root.
1236 Portability issues: L<perlport/chroot>.
1238 =item close FILEHANDLE
1243 =for Pod::Functions close file (or pipe or socket) handle
1245 Closes the file or pipe associated with the filehandle, flushes the IO
1246 buffers, and closes the system file descriptor. Returns true if those
1247 operations succeed and if no error was reported by any PerlIO
1248 layer. Closes the currently selected filehandle if the argument is
1251 You don't have to close FILEHANDLE if you are immediately going to do
1252 another L<C<open>|/open FILEHANDLE,EXPR> on it, because
1253 L<C<open>|/open FILEHANDLE,EXPR> closes it for you. (See
1254 L<C<open>|/open FILEHANDLE,EXPR>.) However, an explicit
1255 L<C<close>|/close FILEHANDLE> on an input file resets the line counter
1256 (L<C<$.>|perlvar/$.>), while the implicit close done by
1257 L<C<open>|/open FILEHANDLE,EXPR> does not.
1259 If the filehandle came from a piped open, L<C<close>|/close FILEHANDLE>
1260 returns false if one of the other syscalls involved fails or if its
1261 program exits with non-zero status. If the only problem was that the
1262 program exited non-zero, L<C<$!>|perlvar/$!> will be set to C<0>.
1263 Closing a pipe also waits for the process executing on the pipe to
1264 exit--in case you wish to look at the output of the pipe afterwards--and
1265 implicitly puts the exit status value of that command into
1266 L<C<$?>|perlvar/$?> and
1267 L<C<${^CHILD_ERROR_NATIVE}>|perlvar/${^CHILD_ERROR_NATIVE}>.
1269 If there are multiple threads running, L<C<close>|/close FILEHANDLE> on
1270 a filehandle from a piped open returns true without waiting for the
1271 child process to terminate, if the filehandle is still open in another
1274 Closing the read end of a pipe before the process writing to it at the
1275 other end is done writing results in the writer receiving a SIGPIPE. If
1276 the other end can't handle that, be sure to read all the data before
1281 open(OUTPUT, '|sort >foo') # pipe to sort
1282 or die "Can't start sort: $!";
1283 #... # print stuff to output
1284 close OUTPUT # wait for sort to finish
1285 or warn $! ? "Error closing sort pipe: $!"
1286 : "Exit status $? from sort";
1287 open(INPUT, 'foo') # get sort's results
1288 or die "Can't open 'foo' for input: $!";
1290 FILEHANDLE may be an expression whose value can be used as an indirect
1291 filehandle, usually the real filehandle name or an autovivified handle.
1293 =item closedir DIRHANDLE
1296 =for Pod::Functions close directory handle
1298 Closes a directory opened by L<C<opendir>|/opendir DIRHANDLE,EXPR> and
1299 returns the success of that system call.
1301 =item connect SOCKET,NAME
1304 =for Pod::Functions connect to a remote socket
1306 Attempts to connect to a remote socket, just like L<connect(2)>.
1307 Returns true if it succeeded, false otherwise. NAME should be a
1308 packed address of the appropriate type for the socket. See the examples in
1309 L<perlipc/"Sockets: Client/Server Communication">.
1311 =item continue BLOCK
1316 =for Pod::Functions optional trailing block in a while or foreach
1318 When followed by a BLOCK, L<C<continue>|/continue BLOCK> is actually a
1319 flow control statement rather than a function. If there is a
1320 L<C<continue>|/continue BLOCK> BLOCK attached to a BLOCK (typically in a
1321 C<while> or C<foreach>), it is always executed just before the
1322 conditional is about to be evaluated again, just like the third part of
1323 a C<for> loop in C. Thus it can be used to increment a loop variable,
1324 even when the loop has been continued via the L<C<next>|/next LABEL>
1325 statement (which is similar to the C L<C<continue>|/continue BLOCK>
1328 L<C<last>|/last LABEL>, L<C<next>|/next LABEL>, or
1329 L<C<redo>|/redo LABEL> may appear within a
1330 L<C<continue>|/continue BLOCK> block; L<C<last>|/last LABEL> and
1331 L<C<redo>|/redo LABEL> behave as if they had been executed within the
1332 main block. So will L<C<next>|/next LABEL>, but since it will execute a
1333 L<C<continue>|/continue BLOCK> block, it may be more entertaining.
1336 ### redo always comes here
1339 ### next always comes here
1341 # then back the top to re-check EXPR
1343 ### last always comes here
1345 Omitting the L<C<continue>|/continue BLOCK> section is equivalent to
1346 using an empty one, logically enough, so L<C<next>|/next LABEL> goes
1347 directly back to check the condition at the top of the loop.
1349 When there is no BLOCK, L<C<continue>|/continue BLOCK> is a function
1350 that falls through the current C<when> or C<default> block instead of
1351 iterating a dynamically enclosing C<foreach> or exiting a lexically
1352 enclosing C<given>. In Perl 5.14 and earlier, this form of
1353 L<C<continue>|/continue BLOCK> was only available when the
1354 L<C<"switch"> feature|feature/The 'switch' feature> was enabled. See
1355 L<feature> and L<perlsyn/"Switch Statements"> for more information.
1358 X<cos> X<cosine> X<acos> X<arccosine>
1362 =for Pod::Functions cosine function
1364 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
1365 takes the cosine of L<C<$_>|perlvar/$_>.
1367 For the inverse cosine operation, you may use the
1368 L<C<Math::Trig::acos>|Math::Trig> function, or use this relation:
1370 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
1372 =item crypt PLAINTEXT,SALT
1373 X<crypt> X<digest> X<hash> X<salt> X<plaintext> X<password>
1374 X<decrypt> X<cryptography> X<passwd> X<encrypt>
1376 =for Pod::Functions one-way passwd-style encryption
1378 Creates a digest string exactly like the L<crypt(3)> function in the C
1379 library (assuming that you actually have a version there that has not
1380 been extirpated as a potential munition).
1382 L<C<crypt>|/crypt PLAINTEXT,SALT> is a one-way hash function. The
1383 PLAINTEXT and SALT are turned
1384 into a short string, called a digest, which is returned. The same
1385 PLAINTEXT and SALT will always return the same string, but there is no
1386 (known) way to get the original PLAINTEXT from the hash. Small
1387 changes in the PLAINTEXT or SALT will result in large changes in the
1390 There is no decrypt function. This function isn't all that useful for
1391 cryptography (for that, look for F<Crypt> modules on your nearby CPAN
1392 mirror) and the name "crypt" is a bit of a misnomer. Instead it is
1393 primarily used to check if two pieces of text are the same without
1394 having to transmit or store the text itself. An example is checking
1395 if a correct password is given. The digest of the password is stored,
1396 not the password itself. The user types in a password that is
1397 L<C<crypt>|/crypt PLAINTEXT,SALT>'d with the same salt as the stored
1398 digest. If the two digests match, the password is correct.
1400 When verifying an existing digest string you should use the digest as
1401 the salt (like C<crypt($plain, $digest) eq $digest>). The SALT used
1402 to create the digest is visible as part of the digest. This ensures
1403 L<C<crypt>|/crypt PLAINTEXT,SALT> will hash the new string with the same
1404 salt as the digest. This allows your code to work with the standard
1405 L<C<crypt>|/crypt PLAINTEXT,SALT> and with more exotic implementations.
1406 In other words, assume nothing about the returned string itself nor
1407 about how many bytes of SALT may matter.
1409 Traditionally the result is a string of 13 bytes: two first bytes of
1410 the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
1411 the first eight bytes of PLAINTEXT mattered. But alternative
1412 hashing schemes (like MD5), higher level security schemes (like C2),
1413 and implementations on non-Unix platforms may produce different
1416 When choosing a new salt create a random two character string whose
1417 characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
1418 '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of
1419 characters is just a recommendation; the characters allowed in
1420 the salt depend solely on your system's crypt library, and Perl can't
1421 restrict what salts L<C<crypt>|/crypt PLAINTEXT,SALT> accepts.
1423 Here's an example that makes sure that whoever runs this program knows
1426 my $pwd = (getpwuid($<))[1];
1428 system "stty -echo";
1430 chomp(my $word = <STDIN>);
1434 if (crypt($word, $pwd) ne $pwd) {
1440 Of course, typing in your own password to whoever asks you
1443 The L<C<crypt>|/crypt PLAINTEXT,SALT> function is unsuitable for hashing
1444 large quantities of data, not least of all because you can't get the
1445 information back. Look at the L<Digest> module for more robust
1448 If using L<C<crypt>|/crypt PLAINTEXT,SALT> on a Unicode string (which
1449 I<potentially> has characters with codepoints above 255), Perl tries to
1450 make sense of the situation by trying to downgrade (a copy of) the
1451 string back to an eight-bit byte string before calling
1452 L<C<crypt>|/crypt PLAINTEXT,SALT> (on that copy). If that works, good.
1453 If not, L<C<crypt>|/crypt PLAINTEXT,SALT> dies with
1454 L<C<Wide character in crypt>|perldiag/Wide character in %s>.
1456 Portability issues: L<perlport/crypt>.
1461 =for Pod::Functions breaks binding on a tied dbm file
1463 [This function has been largely superseded by the
1464 L<C<untie>|/untie VARIABLE> function.]
1466 Breaks the binding between a DBM file and a hash.
1468 Portability issues: L<perlport/dbmclose>.
1470 =item dbmopen HASH,DBNAME,MASK
1471 X<dbmopen> X<dbm> X<ndbm> X<sdbm> X<gdbm>
1473 =for Pod::Functions create binding on a tied dbm file
1475 [This function has been largely superseded by the
1476 L<C<tie>|/tie VARIABLE,CLASSNAME,LIST> function.]
1478 This binds a L<dbm(3)>, L<ndbm(3)>, L<sdbm(3)>, L<gdbm(3)>, or Berkeley
1479 DB file to a hash. HASH is the name of the hash. (Unlike normal
1480 L<C<open>|/open FILEHANDLE,EXPR>, the first argument is I<not> a
1481 filehandle, even though it looks like one). DBNAME is the name of the
1482 database (without the F<.dir> or F<.pag> extension if any). If the
1483 database does not exist, it is created with protection specified by MASK
1484 (as modified by the L<C<umask>|/umask EXPR>). To prevent creation of
1485 the database if it doesn't exist, you may specify a MODE of 0, and the
1486 function will return a false value if it can't find an existing
1487 database. If your system supports only the older DBM functions, you may
1488 make only one L<C<dbmopen>|/dbmopen HASH,DBNAME,MASK> call in your
1489 program. In older versions of Perl, if your system had neither DBM nor
1490 ndbm, calling L<C<dbmopen>|/dbmopen HASH,DBNAME,MASK> produced a fatal
1491 error; it now falls back to L<sdbm(3)>.
1493 If you don't have write access to the DBM file, you can only read hash
1494 variables, not set them. If you want to test whether you can write,
1495 either use file tests or try setting a dummy hash entry inside an
1496 L<C<eval>|/eval EXPR> to trap the error.
1498 Note that functions such as L<C<keys>|/keys HASH> and
1499 L<C<values>|/values HASH> may return huge lists when used on large DBM
1500 files. You may prefer to use the L<C<each>|/each HASH> function to
1501 iterate over large DBM files. Example:
1503 # print out history file offsets
1504 dbmopen(%HIST,'/usr/lib/news/history',0666);
1505 while (($key,$val) = each %HIST) {
1506 print $key, ' = ', unpack('L',$val), "\n";
1510 See also L<AnyDBM_File> for a more general description of the pros and
1511 cons of the various dbm approaches, as well as L<DB_File> for a particularly
1512 rich implementation.
1514 You can control which DBM library you use by loading that library
1515 before you call L<C<dbmopen>|/dbmopen HASH,DBNAME,MASK>:
1518 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
1519 or die "Can't open netscape history file: $!";
1521 Portability issues: L<perlport/dbmopen>.
1524 X<defined> X<undef> X<undefined>
1528 =for Pod::Functions test whether a value, variable, or function is defined
1530 Returns a Boolean value telling whether EXPR has a value other than the
1531 undefined value L<C<undef>|/undef EXPR>. If EXPR is not present,
1532 L<C<$_>|perlvar/$_> is checked.
1534 Many operations return L<C<undef>|/undef EXPR> to indicate failure, end
1535 of file, system error, uninitialized variable, and other exceptional
1536 conditions. This function allows you to distinguish
1537 L<C<undef>|/undef EXPR> from other values. (A simple Boolean test will
1538 not distinguish among L<C<undef>|/undef EXPR>, zero, the empty string,
1539 and C<"0">, which are all equally false.) Note that since
1540 L<C<undef>|/undef EXPR> is a valid scalar, its presence doesn't
1541 I<necessarily> indicate an exceptional condition: L<C<pop>|/pop ARRAY>
1542 returns L<C<undef>|/undef EXPR> when its argument is an empty array,
1543 I<or> when the element to return happens to be L<C<undef>|/undef EXPR>.
1545 You may also use C<defined(&func)> to check whether subroutine C<func>
1546 has ever been defined. The return value is unaffected by any forward
1547 declarations of C<func>. A subroutine that is not defined
1548 may still be callable: its package may have an C<AUTOLOAD> method that
1549 makes it spring into existence the first time that it is called; see
1552 Use of L<C<defined>|/defined EXPR> on aggregates (hashes and arrays) is
1553 no longer supported. It used to report whether memory for that
1554 aggregate had ever been allocated. You should instead use a simple
1557 if (@an_array) { print "has array elements\n" }
1558 if (%a_hash) { print "has hash members\n" }
1560 When used on a hash element, it tells you whether the value is defined,
1561 not whether the key exists in the hash. Use L<C<exists>|/exists EXPR>
1562 for the latter purpose.
1566 print if defined $switch{D};
1567 print "$val\n" while defined($val = pop(@ary));
1568 die "Can't readlink $sym: $!"
1569 unless defined($value = readlink $sym);
1570 sub foo { defined &$bar ? $bar->(@_) : die "No bar"; }
1571 $debugging = 0 unless defined $debugging;
1573 Note: Many folks tend to overuse L<C<defined>|/defined EXPR> and are
1574 then surprised to discover that the number C<0> and C<""> (the
1575 zero-length string) are, in fact, defined values. For example, if you
1580 The pattern match succeeds and C<$1> is defined, although it
1581 matched "nothing". It didn't really fail to match anything. Rather, it
1582 matched something that happened to be zero characters long. This is all
1583 very above-board and honest. When a function returns an undefined value,
1584 it's an admission that it couldn't give you an honest answer. So you
1585 should use L<C<defined>|/defined EXPR> only when questioning the
1586 integrity of what you're trying to do. At other times, a simple
1587 comparison to C<0> or C<""> is what you want.
1589 See also L<C<undef>|/undef EXPR>, L<C<exists>|/exists EXPR>,
1590 L<C<ref>|/ref EXPR>.
1595 =for Pod::Functions deletes a value from a hash
1597 Given an expression that specifies an element or slice of a hash,
1598 L<C<delete>|/delete EXPR> deletes the specified elements from that hash
1599 so that L<C<exists>|/exists EXPR> on that element no longer returns
1600 true. Setting a hash element to the undefined value does not remove its
1601 key, but deleting it does; see L<C<exists>|/exists EXPR>.
1603 In list context, usually returns the value or values deleted, or the last such
1604 element in scalar context. The return list's length corresponds to that of
1605 the argument list: deleting non-existent elements returns the undefined value
1606 in their corresponding positions. When a
1607 L<keyE<sol>value hash slice|perldata/KeyE<sol>Value Hash Slices> is passed to
1608 C<delete>, the return value is a list of key/value pairs (two elements for each
1609 item deleted from the hash).
1611 L<C<delete>|/delete EXPR> may also be used on arrays and array slices,
1612 but its behavior is less straightforward. Although
1613 L<C<exists>|/exists EXPR> will return false for deleted entries,
1614 deleting array elements never changes indices of existing values; use
1615 L<C<shift>|/shift ARRAY> or L<C<splice>|/splice
1616 ARRAY,OFFSET,LENGTH,LIST> for that. However, if any deleted elements
1617 fall at the end of an array, the array's size shrinks to the position of
1618 the highest element that still tests true for L<C<exists>|/exists EXPR>,
1619 or to 0 if none do. In other words, an array won't have trailing
1620 nonexistent elements after a delete.
1622 B<WARNING:> Calling L<C<delete>|/delete EXPR> on array values is
1623 strongly discouraged. The
1624 notion of deleting or checking the existence of Perl array elements is not
1625 conceptually coherent, and can lead to surprising behavior.
1627 Deleting from L<C<%ENV>|perlvar/%ENV> modifies the environment.
1628 Deleting from a hash tied to a DBM file deletes the entry from the DBM
1629 file. Deleting from a L<C<tied>|/tied VARIABLE> hash or array may not
1630 necessarily return anything; it depends on the implementation of the
1631 L<C<tied>|/tied VARIABLE> package's DELETE method, which may do whatever
1634 The C<delete local EXPR> construct localizes the deletion to the current
1635 block at run time. Until the block exits, elements locally deleted
1636 temporarily no longer exist. See L<perlsub/"Localized deletion of elements
1637 of composite types">.
1639 my %hash = (foo => 11, bar => 22, baz => 33);
1640 my $scalar = delete $hash{foo}; # $scalar is 11
1641 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
1642 my @array = delete @hash{qw(foo baz)}; # @array is (undef,33)
1644 The following (inefficiently) deletes all the values of %HASH and @ARRAY:
1646 foreach my $key (keys %HASH) {
1650 foreach my $index (0 .. $#ARRAY) {
1651 delete $ARRAY[$index];
1656 delete @HASH{keys %HASH};
1658 delete @ARRAY[0 .. $#ARRAY];
1660 But both are slower than assigning the empty list
1661 or undefining %HASH or @ARRAY, which is the customary
1662 way to empty out an aggregate:
1664 %HASH = (); # completely empty %HASH
1665 undef %HASH; # forget %HASH ever existed
1667 @ARRAY = (); # completely empty @ARRAY
1668 undef @ARRAY; # forget @ARRAY ever existed
1670 The EXPR can be arbitrarily complicated provided its
1671 final operation is an element or slice of an aggregate:
1673 delete $ref->[$x][$y]{$key};
1674 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
1676 delete $ref->[$x][$y][$index];
1677 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1680 X<die> X<throw> X<exception> X<raise> X<$@> X<abort>
1682 =for Pod::Functions raise an exception or bail out
1684 L<C<die>|/die LIST> raises an exception. Inside an L<C<eval>|/eval EXPR>
1685 the exception is stuffed into L<C<$@>|perlvar/$@> and the L<C<eval>|/eval
1686 EXPR> is terminated with the undefined value. If the exception is
1687 outside of all enclosing L<C<eval>|/eval EXPR>s, then the uncaught
1688 exception is printed to C<STDERR> and perl exits with an exit code
1689 indicating failure. If you need to exit the process with a specific
1690 exit code, see L<C<exit>|/exit EXPR>.
1692 Equivalent examples:
1694 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
1695 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
1697 Most of the time, C<die> is called with a string to use as the exception.
1698 You may either give a single non-reference operand to serve as the
1699 exception, or a list of two or more items, which will be stringified
1700 and concatenated to make the exception.
1702 If the string exception does not end in a newline, the current
1703 script line number and input line number (if any) and a newline
1704 are appended to it. Note that the "input line number" (also
1705 known as "chunk") is subject to whatever notion of "line" happens to
1706 be currently in effect, and is also available as the special variable
1707 L<C<$.>|perlvar/$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1709 Hint: sometimes appending C<", stopped"> to your message will cause it
1710 to make better sense when the string C<"at foo line 123"> is appended.
1711 Suppose you are running script "canasta".
1713 die "/etc/games is no good";
1714 die "/etc/games is no good, stopped";
1716 produce, respectively
1718 /etc/games is no good at canasta line 123.
1719 /etc/games is no good, stopped at canasta line 123.
1721 If LIST was empty or made an empty string, and L<C<$@>|perlvar/$@>
1722 already contains an exception value (typically from a previous
1723 L<C<eval>|/eval EXPR>), then that value is reused after
1724 appending C<"\t...propagated">. This is useful for propagating exceptions:
1727 die unless $@ =~ /Expected exception/;
1729 If LIST was empty or made an empty string,
1730 and L<C<$@>|perlvar/$@> contains an object
1731 reference that has a C<PROPAGATE> method, that method will be called
1732 with additional file and line number parameters. The return value
1733 replaces the value in L<C<$@>|perlvar/$@>; i.e., as if
1734 C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >> were called.
1736 If LIST was empty or made an empty string, and L<C<$@>|perlvar/$@>
1737 is also empty, then the string C<"Died"> is used.
1739 You can also call L<C<die>|/die LIST> with a reference argument, and if
1740 this is trapped within an L<C<eval>|/eval EXPR>, L<C<$@>|perlvar/$@>
1741 contains that reference. This permits more elaborate exception handling
1742 using objects that maintain arbitrary state about the exception. Such a
1743 scheme is sometimes preferable to matching particular string values of
1744 L<C<$@>|perlvar/$@> with regular expressions.
1746 Because Perl stringifies uncaught exception messages before display,
1747 you'll probably want to overload stringification operations on
1748 exception objects. See L<overload> for details about that.
1749 The stringified message should be non-empty, and should end in a newline,
1750 in order to fit in with the treatment of string exceptions.
1751 Also, because an exception object reference cannot be stringified
1752 without destroying it, Perl doesn't attempt to append location or other
1753 information to a reference exception. If you want location information
1754 with a complex exception object, you'll have to arrange to put the
1755 location information into the object yourself.
1757 Because L<C<$@>|perlvar/$@> is a global variable, be careful that
1758 analyzing an exception caught by C<eval> doesn't replace the reference
1759 in the global variable. It's
1760 easiest to make a local copy of the reference before any manipulations.
1763 use Scalar::Util "blessed";
1765 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1766 if (my $ev_err = $@) {
1767 if (blessed($ev_err)
1768 && $ev_err->isa("Some::Module::Exception")) {
1769 # handle Some::Module::Exception
1772 # handle all other possible exceptions
1776 If an uncaught exception results in interpreter exit, the exit code is
1777 determined from the values of L<C<$!>|perlvar/$!> and
1778 L<C<$?>|perlvar/$?> with this pseudocode:
1780 exit $! if $!; # errno
1781 exit $? >> 8 if $? >> 8; # child exit status
1782 exit 255; # last resort
1784 As with L<C<exit>|/exit EXPR>, L<C<$?>|perlvar/$?> is set prior to
1785 unwinding the call stack; any C<DESTROY> or C<END> handlers can then
1786 alter this value, and thus Perl's exit code.
1788 The intent is to squeeze as much possible information about the likely cause
1789 into the limited space of the system exit code. However, as
1790 L<C<$!>|perlvar/$!> is the value of C's C<errno>, which can be set by
1791 any system call, this means that the value of the exit code used by
1792 L<C<die>|/die LIST> can be non-predictable, so should not be relied
1793 upon, other than to be non-zero.
1795 You can arrange for a callback to be run just before the
1796 L<C<die>|/die LIST> does its deed, by setting the
1797 L<C<$SIG{__DIE__}>|perlvar/%SIG> hook. The associated handler is called
1798 with the exception as an argument, and can change the exception,
1800 calling L<C<die>|/die LIST> again. See L<perlvar/%SIG> for details on
1801 setting L<C<%SIG>|perlvar/%SIG> entries, and L<C<eval>|/eval EXPR> for some
1802 examples. Although this feature was to be run only right before your
1803 program was to exit, this is not currently so: the
1804 L<C<$SIG{__DIE__}>|perlvar/%SIG> hook is currently called even inside
1805 L<C<eval>|/eval EXPR>ed blocks/strings! If one wants the hook to do
1806 nothing in such situations, put
1810 as the first line of the handler (see L<perlvar/$^S>). Because
1811 this promotes strange action at a distance, this counterintuitive
1812 behavior may be fixed in a future release.
1814 See also L<C<exit>|/exit EXPR>, L<C<warn>|/warn LIST>, and the L<Carp>
1820 =for Pod::Functions turn a BLOCK into a TERM
1822 Not really a function. Returns the value of the last command in the
1823 sequence of commands indicated by BLOCK. When modified by the C<while> or
1824 C<until> loop modifier, executes the BLOCK once before testing the loop
1825 condition. (On other statements the loop modifiers test the conditional
1828 C<do BLOCK> does I<not> count as a loop, so the loop control statements
1829 L<C<next>|/next LABEL>, L<C<last>|/last LABEL>, or
1830 L<C<redo>|/redo LABEL> cannot be used to leave or restart the block.
1831 See L<perlsyn> for alternative strategies.
1836 Uses the value of EXPR as a filename and executes the contents of the
1837 file as a Perl script:
1839 # load the exact specified file (./ and ../ special-cased)
1842 do '../foo/stat.pl';
1844 # search for the named file within @INC
1848 C<do './stat.pl'> is largely like
1852 except that it's more concise, runs no external processes, and keeps
1853 track of the current filename for error messages. It also differs in that
1854 code evaluated with C<do FILE> cannot see lexicals in the enclosing
1855 scope; C<eval STRING> does. It's the same, however, in that it does
1856 reparse the file every time you call it, so you probably don't want
1857 to do this inside a loop.
1859 Using C<do> with a relative path (except for F<./> and F<../>), like
1863 will search the L<C<@INC>|perlvar/@INC> directories, and update
1864 L<C<%INC>|perlvar/%INC> if the file is found. See L<perlvar/@INC>
1865 and L<perlvar/%INC> for these variables. In particular, note that
1866 whilst historically L<C<@INC>|perlvar/@INC> contained '.' (the
1867 current directory) making these two cases equivalent, that is no
1868 longer necessarily the case, as '.' is not included in C<@INC> by default
1869 in perl versions 5.26.0 onwards. Instead, perl will now warn:
1871 do "stat.pl" failed, '.' is no longer in @INC;
1872 did you mean do "./stat.pl"?
1874 If L<C<do>|/do EXPR> can read the file but cannot compile it, it
1875 returns L<C<undef>|/undef EXPR> and sets an error message in
1876 L<C<$@>|perlvar/$@>. If L<C<do>|/do EXPR> cannot read the file, it
1877 returns undef and sets L<C<$!>|perlvar/$!> to the error. Always check
1878 L<C<$@>|perlvar/$@> first, as compilation could fail in a way that also
1879 sets L<C<$!>|perlvar/$!>. If the file is successfully compiled,
1880 L<C<do>|/do EXPR> returns the value of the last expression evaluated.
1882 Inclusion of library modules is better done with the
1883 L<C<use>|/use Module VERSION LIST> and L<C<require>|/require VERSION>
1884 operators, which also do automatic error checking and raise an exception
1885 if there's a problem.
1887 You might like to use L<C<do>|/do EXPR> to read in a program
1888 configuration file. Manual error checking can be done this way:
1890 # Read in config files: system first, then user.
1891 # Beware of using relative pathnames here.
1892 for $file ("/share/prog/defaults.rc",
1893 "$ENV{HOME}/.someprogrc")
1895 unless ($return = do $file) {
1896 warn "couldn't parse $file: $@" if $@;
1897 warn "couldn't do $file: $!" unless defined $return;
1898 warn "couldn't run $file" unless $return;
1903 X<dump> X<core> X<undump>
1909 =for Pod::Functions create an immediate core dump
1911 This function causes an immediate core dump. See also the B<-u>
1912 command-line switch in L<perlrun>, which does the same thing.
1913 Primarily this is so that you can use the B<undump> program (not
1914 supplied) to turn your core dump into an executable binary after
1915 having initialized all your variables at the beginning of the
1916 program. When the new binary is executed it will begin by executing
1917 a C<goto LABEL> (with all the restrictions that L<C<goto>|/goto LABEL>
1919 Think of it as a goto with an intervening core dump and reincarnation.
1920 If C<LABEL> is omitted, restarts the program from the top. The
1921 C<dump EXPR> form, available starting in Perl 5.18.0, allows a name to be
1922 computed at run time, being otherwise identical to C<dump LABEL>.
1924 B<WARNING>: Any files opened at the time of the dump will I<not>
1925 be open any more when the program is reincarnated, with possible
1926 resulting confusion by Perl.
1928 This function is now largely obsolete, mostly because it's very hard to
1929 convert a core file into an executable. As of Perl 5.30, it must be invoked
1932 Unlike most named operators, this has the same precedence as assignment.
1933 It is also exempt from the looks-like-a-function rule, so
1934 C<dump ("foo")."bar"> will cause "bar" to be part of the argument to
1935 L<C<dump>|/dump LABEL>.
1937 Portability issues: L<perlport/dump>.
1940 X<each> X<hash, iterator>
1945 =for Pod::Functions retrieve the next key/value pair from a hash
1947 When called on a hash in list context, returns a 2-element list
1948 consisting of the key and value for the next element of a hash. In Perl
1949 5.12 and later only, it will also return the index and value for the next
1950 element of an array so that you can iterate over it; older Perls consider
1951 this a syntax error. When called in scalar context, returns only the key
1952 (not the value) in a hash, or the index in an array.
1954 Hash entries are returned in an apparently random order. The actual random
1955 order is specific to a given hash; the exact same series of operations
1956 on two hashes may result in a different order for each hash. Any insertion
1957 into the hash may change the order, as will any deletion, with the exception
1958 that the most recent key returned by L<C<each>|/each HASH> or
1959 L<C<keys>|/keys HASH> may be deleted without changing the order. So
1960 long as a given hash is unmodified you may rely on
1961 L<C<keys>|/keys HASH>, L<C<values>|/values HASH> and
1962 L<C<each>|/each HASH> to repeatedly return the same order
1963 as each other. See L<perlsec/"Algorithmic Complexity Attacks"> for
1964 details on why hash order is randomized. Aside from the guarantees
1965 provided here the exact details of Perl's hash algorithm and the hash
1966 traversal order are subject to change in any release of Perl.
1968 After L<C<each>|/each HASH> has returned all entries from the hash or
1969 array, the next call to L<C<each>|/each HASH> returns the empty list in
1970 list context and L<C<undef>|/undef EXPR> in scalar context; the next
1971 call following I<that> one restarts iteration. Each hash or array has
1972 its own internal iterator, accessed by L<C<each>|/each HASH>,
1973 L<C<keys>|/keys HASH>, and L<C<values>|/values HASH>. The iterator is
1974 implicitly reset when L<C<each>|/each HASH> has reached the end as just
1975 described; it can be explicitly reset by calling L<C<keys>|/keys HASH>
1976 or L<C<values>|/values HASH> on the hash or array, or by referencing
1977 the hash (but not array) in list context. If you add or delete
1978 a hash's elements while iterating over it, the effect on the iterator is
1979 unspecified; for example, entries may be skipped or duplicated--so don't
1980 do that. Exception: It is always safe to delete the item most recently
1981 returned by L<C<each>|/each HASH>, so the following code works properly:
1983 while (my ($key, $value) = each %hash) {
1985 delete $hash{$key}; # This is safe
1988 Tied hashes may have a different ordering behaviour to perl's hash
1991 The iterator used by C<each> is attached to the hash or array, and is
1992 shared between all iteration operations applied to the same hash or array.
1993 Thus all uses of C<each> on a single hash or array advance the same
1994 iterator location. All uses of C<each> are also subject to having the
1995 iterator reset by any use of C<keys> or C<values> on the same hash or
1996 array, or by the hash (but not array) being referenced in list context.
1997 This makes C<each>-based loops quite fragile: it is easy to arrive at
1998 such a loop with the iterator already part way through the object, or to
1999 accidentally clobber the iterator state during execution of the loop body.
2000 It's easy enough to explicitly reset the iterator before starting a loop,
2001 but there is no way to insulate the iterator state used by a loop from
2002 the iterator state used by anything else that might execute during the
2003 loop body. To avoid these problems, use a C<foreach> loop rather than
2006 This prints out your environment like the L<printenv(1)> program,
2007 but in a different order:
2009 while (my ($key,$value) = each %ENV) {
2010 print "$key=$value\n";
2013 Starting with Perl 5.14, an experimental feature allowed
2014 L<C<each>|/each HASH> to take a scalar expression. This experiment has
2015 been deemed unsuccessful, and was removed as of Perl 5.24.
2017 As of Perl 5.18 you can use a bare L<C<each>|/each HASH> in a C<while>
2018 loop, which will set L<C<$_>|perlvar/$_> on every iteration.
2019 If either an C<each> expression or an explicit assignment of an C<each>
2020 expression to a scalar is used as a C<while>/C<for> condition, then
2021 the condition actually tests for definedness of the expression's value,
2022 not for its regular truth value.
2025 print "$_=$ENV{$_}\n";
2028 To avoid confusing would-be users of your code who are running earlier
2029 versions of Perl with mysterious syntax errors, put this sort of thing at
2030 the top of your file to signal that your code will work I<only> on Perls of
2033 use 5.012; # so keys/values/each work on arrays
2034 use 5.018; # so each assigns to $_ in a lone while test
2036 See also L<C<keys>|/keys HASH>, L<C<values>|/values HASH>, and
2037 L<C<sort>|/sort SUBNAME LIST>.
2039 =item eof FILEHANDLE
2048 =for Pod::Functions test a filehandle for its end
2050 Returns 1 if the next read on FILEHANDLE will return end of file I<or> if
2051 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
2052 gives the real filehandle. (Note that this function actually
2053 reads a character and then C<ungetc>s it, so isn't useful in an
2054 interactive context.) Do not read from a terminal file (or call
2055 C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
2056 as terminals may lose the end-of-file condition if you do.
2058 An L<C<eof>|/eof FILEHANDLE> without an argument uses the last file
2059 read. Using L<C<eof()>|/eof FILEHANDLE> with empty parentheses is
2060 different. It refers to the pseudo file formed from the files listed on
2061 the command line and accessed via the C<< <> >> operator. Since
2062 C<< <> >> isn't explicitly opened, as a normal filehandle is, an
2063 L<C<eof()>|/eof FILEHANDLE> before C<< <> >> has been used will cause
2064 L<C<@ARGV>|perlvar/@ARGV> to be examined to determine if input is
2065 available. Similarly, an L<C<eof()>|/eof FILEHANDLE> after C<< <> >>
2066 has returned end-of-file will assume you are processing another
2067 L<C<@ARGV>|perlvar/@ARGV> list, and if you haven't set
2068 L<C<@ARGV>|perlvar/@ARGV>, will read input from C<STDIN>; see
2069 L<perlop/"I/O Operators">.
2071 In a C<< while (<>) >> loop, L<C<eof>|/eof FILEHANDLE> or C<eof(ARGV)>
2072 can be used to detect the end of each file, whereas
2073 L<C<eof()>|/eof FILEHANDLE> will detect the end of the very last file
2076 # reset line numbering on each input file
2078 next if /^\s*#/; # skip comments
2081 close ARGV if eof; # Not eof()!
2084 # insert dashes just before last line of last file
2086 if (eof()) { # check for end of last file
2087 print "--------------\n";
2090 last if eof(); # needed if we're reading from a terminal
2093 Practical hint: you almost never need to use L<C<eof>|/eof FILEHANDLE>
2094 in Perl, because the input operators typically return L<C<undef>|/undef
2095 EXPR> when they run out of data or encounter an error.
2098 X<eval> X<try> X<catch> X<evaluate> X<parse> X<execute>
2099 X<error, handling> X<exception, handling>
2105 =for Pod::Functions catch exceptions or compile and run code
2107 C<eval> in all its forms is used to execute a little Perl program,
2108 trapping any errors encountered so they don't crash the calling program.
2110 Plain C<eval> with no argument is just C<eval EXPR>, where the
2111 expression is understood to be contained in L<C<$_>|perlvar/$_>. Thus
2112 there are only two real C<eval> forms; the one with an EXPR is often
2113 called "string eval". In a string eval, the value of the expression
2114 (which is itself determined within scalar context) is first parsed, and
2115 if there were no errors, executed as a block within the lexical context
2116 of the current Perl program. This form is typically used to delay
2117 parsing and subsequent execution of the text of EXPR until run time.
2118 Note that the value is parsed every time the C<eval> executes.
2120 The other form is called "block eval". It is less general than string
2121 eval, but the code within the BLOCK is parsed only once (at the same
2122 time the code surrounding the C<eval> itself was parsed) and executed
2123 within the context of the current Perl program. This form is typically
2124 used to trap exceptions more efficiently than the first, while also
2125 providing the benefit of checking the code within BLOCK at compile time.
2126 BLOCK is parsed and compiled just once. Since errors are trapped, it
2127 often is used to check if a given feature is available.
2129 In both forms, the value returned is the value of the last expression
2130 evaluated inside the mini-program; a return statement may also be used, just
2131 as with subroutines. The expression providing the return value is evaluated
2132 in void, scalar, or list context, depending on the context of the
2133 C<eval> itself. See L<C<wantarray>|/wantarray> for more
2134 on how the evaluation context can be determined.
2136 If there is a syntax error or runtime error, or a L<C<die>|/die LIST>
2137 statement is executed, C<eval> returns
2138 L<C<undef>|/undef EXPR> in scalar context, or an empty list in list
2139 context, and L<C<$@>|perlvar/$@> is set to the error message. (Prior to
2140 5.16, a bug caused L<C<undef>|/undef EXPR> to be returned in list
2141 context for syntax errors, but not for runtime errors.) If there was no
2142 error, L<C<$@>|perlvar/$@> is set to the empty string. A control flow
2143 operator like L<C<last>|/last LABEL> or L<C<goto>|/goto LABEL> can
2144 bypass the setting of L<C<$@>|perlvar/$@>. Beware that using
2145 C<eval> neither silences Perl from printing warnings to
2146 STDERR, nor does it stuff the text of warning messages into
2147 L<C<$@>|perlvar/$@>. To do either of those, you have to use the
2148 L<C<$SIG{__WARN__}>|perlvar/%SIG> facility, or turn off warnings inside
2149 the BLOCK or EXPR using S<C<no warnings 'all'>>. See
2150 L<C<warn>|/warn LIST>, L<perlvar>, and L<warnings>.
2152 Note that, because C<eval> traps otherwise-fatal errors,
2153 it is useful for determining whether a particular feature (such as
2154 L<C<socket>|/socket SOCKET,DOMAIN,TYPE,PROTOCOL> or
2155 L<C<symlink>|/symlink OLDFILE,NEWFILE>) is implemented. It is also
2156 Perl's exception-trapping mechanism, where the L<C<die>|/die LIST>
2157 operator is used to raise exceptions.
2159 Before Perl 5.14, the assignment to L<C<$@>|perlvar/$@> occurred before
2161 of localized variables, which means that for your code to run on older
2162 versions, a temporary is required if you want to mask some, but not all
2165 # alter $@ on nefarious repugnancy only
2169 local $@; # protect existing $@
2170 eval { test_repugnancy() };
2171 # $@ =~ /nefarious/ and die $@; # Perl 5.14 and higher only
2172 $@ =~ /nefarious/ and $e = $@;
2174 die $e if defined $e
2177 There are some different considerations for each form:
2183 Since the return value of EXPR is executed as a block within the lexical
2184 context of the current Perl program, any outer lexical variables are
2185 visible to it, and any package variable settings or subroutine and
2186 format definitions remain afterwards.
2190 =item Under the L<C<"unicode_eval"> feature|feature/The 'unicode_eval' and 'evalbytes' features>
2192 If this feature is enabled (which is the default under a C<use 5.16> or
2193 higher declaration), EXPR is considered to be
2194 in the same encoding as the surrounding program. Thus if
2195 S<L<C<use utf8>|utf8>> is in effect, the string will be treated as being
2196 UTF-8 encoded. Otherwise, the string is considered to be a sequence of
2197 independent bytes. Bytes that correspond to ASCII-range code points
2198 will have their normal meanings for operators in the string. The
2199 treatment of the other bytes depends on if the
2200 L<C<'unicode_strings"> feature|feature/The 'unicode_strings' feature> is
2203 In a plain C<eval> without an EXPR argument, being in S<C<use utf8>> or
2204 not is irrelevant; the UTF-8ness of C<$_> itself determines the
2207 Any S<C<use utf8>> or S<C<no utf8>> declarations within the string have
2208 no effect, and source filters are forbidden. (C<unicode_strings>,
2209 however, can appear within the string.) See also the
2210 L<C<evalbytes>|/evalbytes EXPR> operator, which works properly with
2213 Variables defined outside the C<eval> and used inside it retain their
2214 original UTF-8ness. Everything inside the string follows the normal
2215 rules for a Perl program with the given state of S<C<use utf8>>.
2217 =item Outside the C<"unicode_eval"> feature
2219 In this case, the behavior is problematic and is not so easily
2220 described. Here are two bugs that cannot easily be fixed without
2221 breaking existing programs:
2227 It can lose track of whether something should be encoded as UTF-8 or
2232 Source filters activated within C<eval> leak out into whichever file
2233 scope is currently being compiled. To give an example with the CPAN module
2234 L<Semi::Semicolons>:
2236 BEGIN { eval "use Semi::Semicolons; # not filtered" }
2239 L<C<evalbytes>|/evalbytes EXPR> fixes that to work the way one would
2242 use feature "evalbytes";
2243 BEGIN { evalbytes "use Semi::Semicolons; # filtered" }
2250 Problems can arise if the string expands a scalar containing a floating
2251 point number. That scalar can expand to letters, such as C<"NaN"> or
2252 C<"Infinity">; or, within the scope of a L<C<use locale>|locale>, the
2253 decimal point character may be something other than a dot (such as a
2254 comma). None of these are likely to parse as you are likely expecting.
2256 You should be especially careful to remember what's being looked at
2263 eval { $x }; # CASE 4
2265 eval "\$$x++"; # CASE 5
2268 Cases 1 and 2 above behave identically: they run the code contained in
2269 the variable $x. (Although case 2 has misleading double quotes making
2270 the reader wonder what else might be happening (nothing is).) Cases 3
2271 and 4 likewise behave in the same way: they run the code C<'$x'>, which
2272 does nothing but return the value of $x. (Case 4 is preferred for
2273 purely visual reasons, but it also has the advantage of compiling at
2274 compile-time instead of at run-time.) Case 5 is a place where
2275 normally you I<would> like to use double quotes, except that in this
2276 particular situation, you can just use symbolic references instead, as
2279 An C<eval ''> executed within a subroutine defined
2280 in the C<DB> package doesn't see the usual
2281 surrounding lexical scope, but rather the scope of the first non-DB piece
2282 of code that called it. You don't normally need to worry about this unless
2283 you are writing a Perl debugger.
2285 The final semicolon, if any, may be omitted from the value of EXPR.
2289 If the code to be executed doesn't vary, you may use the eval-BLOCK
2290 form to trap run-time errors without incurring the penalty of
2291 recompiling each time. The error, if any, is still returned in
2292 L<C<$@>|perlvar/$@>.
2295 # make divide-by-zero nonfatal
2296 eval { $answer = $a / $b; }; warn $@ if $@;
2298 # same thing, but less efficient
2299 eval '$answer = $a / $b'; warn $@ if $@;
2301 # a compile-time error
2302 eval { $answer = }; # WRONG
2305 eval '$answer ='; # sets $@
2307 If you want to trap errors when loading an XS module, some problems with
2308 the binary interface (such as Perl version skew) may be fatal even with
2309 C<eval> unless C<$ENV{PERL_DL_NONLAZY}> is set. See
2312 Using the C<eval {}> form as an exception trap in libraries does have some
2313 issues. Due to the current arguably broken state of C<__DIE__> hooks, you
2314 may wish not to trigger any C<__DIE__> hooks that user code may have installed.
2315 You can use the C<local $SIG{__DIE__}> construct for this purpose,
2316 as this example shows:
2318 # a private exception trap for divide-by-zero
2319 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
2322 This is especially significant, given that C<__DIE__> hooks can call
2323 L<C<die>|/die LIST> again, which has the effect of changing their error
2326 # __DIE__ hooks may modify error messages
2328 local $SIG{'__DIE__'} =
2329 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
2330 eval { die "foo lives here" };
2331 print $@ if $@; # prints "bar lives here"
2334 Because this promotes action at a distance, this counterintuitive behavior
2335 may be fixed in a future release.
2337 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
2338 L<C<next>|/next LABEL>, L<C<last>|/last LABEL>, or
2339 L<C<redo>|/redo LABEL> cannot be used to leave or restart the block.
2341 The final semicolon, if any, may be omitted from within the BLOCK.
2345 =item evalbytes EXPR
2350 =for Pod::Functions +evalbytes similar to string eval, but intend to parse a bytestream
2352 This function is similar to a L<string eval|/eval EXPR>, except it
2353 always parses its argument (or L<C<$_>|perlvar/$_> if EXPR is omitted)
2354 as a string of independent bytes.
2356 If called when S<C<use utf8>> is in effect, the string will be assumed
2357 to be encoded in UTF-8, and C<evalbytes> will make a temporary copy to
2358 work from, downgraded to non-UTF-8. If this is not possible
2359 (because one or more characters in it require UTF-8), the C<evalbytes>
2360 will fail with the error stored in C<$@>.
2362 Bytes that correspond to ASCII-range code points will have their normal
2363 meanings for operators in the string. The treatment of the other bytes
2364 depends on if the L<C<'unicode_strings"> feature|feature/The
2365 'unicode_strings' feature> is in effect.
2367 Of course, variables that are UTF-8 and are referred to in the string
2371 evalbytes 'print ord $a, "\n"';
2379 Source filters activated within the evaluated code apply to the code
2382 L<C<evalbytes>|/evalbytes EXPR> is available starting in Perl v5.16. To
2383 access it, you must say C<CORE::evalbytes>, but you can omit the
2385 L<C<"evalbytes"> feature|feature/The 'unicode_eval' and 'evalbytes' features>
2386 is enabled. This is enabled automatically with a C<use v5.16> (or
2387 higher) declaration in the current scope.
2392 =item exec PROGRAM LIST
2394 =for Pod::Functions abandon this program to run another
2396 The L<C<exec>|/exec LIST> function executes a system command I<and never
2397 returns>; use L<C<system>|/system LIST> instead of L<C<exec>|/exec LIST>
2398 if you want it to return. It fails and
2399 returns false only if the command does not exist I<and> it is executed
2400 directly instead of via your system's command shell (see below).
2402 Since it's a common mistake to use L<C<exec>|/exec LIST> instead of
2403 L<C<system>|/system LIST>, Perl warns you if L<C<exec>|/exec LIST> is
2404 called in void context and if there is a following statement that isn't
2405 L<C<die>|/die LIST>, L<C<warn>|/warn LIST>, or L<C<exit>|/exit EXPR> (if
2406 L<warnings> are enabled--but you always do that, right?). If you
2407 I<really> want to follow an L<C<exec>|/exec LIST> with some other
2408 statement, you can use one of these styles to avoid the warning:
2410 exec ('foo') or print STDERR "couldn't exec foo: $!";
2411 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
2413 If there is more than one argument in LIST, this calls L<execvp(3)> with the
2414 arguments in LIST. If there is only one element in LIST, the argument is
2415 checked for shell metacharacters, and if there are any, the entire
2416 argument is passed to the system's command shell for parsing (this is
2417 C</bin/sh -c> on Unix platforms, but varies on other platforms). If
2418 there are no shell metacharacters in the argument, it is split into words
2419 and passed directly to C<execvp>, which is more efficient. Examples:
2421 exec '/bin/echo', 'Your arguments are: ', @ARGV;
2422 exec "sort $outfile | uniq";
2424 If you don't really want to execute the first argument, but want to lie
2425 to the program you are executing about its own name, you can specify
2426 the program you actually want to run as an "indirect object" (without a
2427 comma) in front of the LIST, as in C<exec PROGRAM LIST>. (This always
2428 forces interpretation of the LIST as a multivalued list, even if there
2429 is only a single scalar in the list.) Example:
2431 my $shell = '/bin/csh';
2432 exec $shell '-sh'; # pretend it's a login shell
2436 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
2438 When the arguments get executed via the system shell, results are
2439 subject to its quirks and capabilities. See L<perlop/"`STRING`">
2442 Using an indirect object with L<C<exec>|/exec LIST> or
2443 L<C<system>|/system LIST> is also more secure. This usage (which also
2444 works fine with L<C<system>|/system LIST>) forces
2445 interpretation of the arguments as a multivalued list, even if the
2446 list had just one argument. That way you're safe from the shell
2447 expanding wildcards or splitting up words with whitespace in them.
2449 my @args = ( "echo surprise" );
2451 exec @args; # subject to shell escapes
2453 exec { $args[0] } @args; # safe even with one-arg list
2455 The first version, the one without the indirect object, ran the I<echo>
2456 program, passing it C<"surprise"> an argument. The second version didn't;
2457 it tried to run a program named I<"echo surprise">, didn't find it, and set
2458 L<C<$?>|perlvar/$?> to a non-zero value indicating failure.
2460 On Windows, only the C<exec PROGRAM LIST> indirect object syntax will
2461 reliably avoid using the shell; C<exec LIST>, even with more than one
2462 element, will fall back to the shell if the first spawn fails.
2464 Perl attempts to flush all files opened for output before the exec,
2465 but this may not be supported on some platforms (see L<perlport>).
2466 To be safe, you may need to set L<C<$E<verbar>>|perlvar/$E<verbar>>
2467 (C<$AUTOFLUSH> in L<English>) or call the C<autoflush> method of
2468 L<C<IO::Handle>|IO::Handle/METHODS> on any open handles to avoid lost
2471 Note that L<C<exec>|/exec LIST> will not call your C<END> blocks, nor
2472 will it invoke C<DESTROY> methods on your objects.
2474 Portability issues: L<perlport/exec>.
2477 X<exists> X<autovivification>
2479 =for Pod::Functions test whether a hash key is present
2481 Given an expression that specifies an element of a hash, returns true if the
2482 specified element in the hash has ever been initialized, even if the
2483 corresponding value is undefined.
2485 print "Exists\n" if exists $hash{$key};
2486 print "Defined\n" if defined $hash{$key};
2487 print "True\n" if $hash{$key};
2489 exists may also be called on array elements, but its behavior is much less
2490 obvious and is strongly tied to the use of L<C<delete>|/delete EXPR> on
2493 B<WARNING:> Calling L<C<exists>|/exists EXPR> on array values is
2494 strongly discouraged. The
2495 notion of deleting or checking the existence of Perl array elements is not
2496 conceptually coherent, and can lead to surprising behavior.
2498 print "Exists\n" if exists $array[$index];
2499 print "Defined\n" if defined $array[$index];
2500 print "True\n" if $array[$index];
2502 A hash or array element can be true only if it's defined and defined only if
2503 it exists, but the reverse doesn't necessarily hold true.
2505 Given an expression that specifies the name of a subroutine,
2506 returns true if the specified subroutine has ever been declared, even
2507 if it is undefined. Mentioning a subroutine name for exists or defined
2508 does not count as declaring it. Note that a subroutine that does not
2509 exist may still be callable: its package may have an C<AUTOLOAD>
2510 method that makes it spring into existence the first time that it is
2511 called; see L<perlsub>.
2513 print "Exists\n" if exists &subroutine;
2514 print "Defined\n" if defined &subroutine;
2516 Note that the EXPR can be arbitrarily complicated as long as the final
2517 operation is a hash or array key lookup or subroutine name:
2519 if (exists $ref->{A}->{B}->{$key}) { }
2520 if (exists $hash{A}{B}{$key}) { }
2522 if (exists $ref->{A}->{B}->[$ix]) { }
2523 if (exists $hash{A}{B}[$ix]) { }
2525 if (exists &{$ref->{A}{B}{$key}}) { }
2527 Although the most deeply nested array or hash element will not spring into
2528 existence just because its existence was tested, any intervening ones will.
2529 Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
2530 into existence due to the existence test for the C<$key> element above.
2531 This happens anywhere the arrow operator is used, including even here:
2534 if (exists $ref->{"Some key"}) { }
2535 print $ref; # prints HASH(0x80d3d5c)
2537 Use of a subroutine call, rather than a subroutine name, as an argument
2538 to L<C<exists>|/exists EXPR> is an error.
2541 exists &sub(); # Error
2544 X<exit> X<terminate> X<abort>
2548 =for Pod::Functions terminate this program
2550 Evaluates EXPR and exits immediately with that value. Example:
2553 exit 0 if $ans =~ /^[Xx]/;
2555 See also L<C<die>|/die LIST>. If EXPR is omitted, exits with C<0>
2557 universally recognized values for EXPR are C<0> for success and C<1>
2558 for error; other values are subject to interpretation depending on the
2559 environment in which the Perl program is running. For example, exiting
2560 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
2561 the mailer to return the item undelivered, but that's not true everywhere.
2563 Don't use L<C<exit>|/exit EXPR> to abort a subroutine if there's any
2564 chance that someone might want to trap whatever error happened. Use
2565 L<C<die>|/die LIST> instead, which can be trapped by an
2566 L<C<eval>|/eval EXPR>.
2568 The L<C<exit>|/exit EXPR> function does not always exit immediately. It
2569 calls any defined C<END> routines first, but these C<END> routines may
2570 not themselves abort the exit. Likewise any object destructors that
2571 need to be called are called before the real exit. C<END> routines and
2572 destructors can change the exit status by modifying L<C<$?>|perlvar/$?>.
2573 If this is a problem, you can call
2574 L<C<POSIX::_exit($status)>|POSIX/C<_exit>> to avoid C<END> and destructor
2575 processing. See L<perlmod> for details.
2577 Portability issues: L<perlport/exit>.
2580 X<exp> X<exponential> X<antilog> X<antilogarithm> X<e>
2584 =for Pod::Functions raise I<e> to a power
2586 Returns I<e> (the natural logarithm base) to the power of EXPR.
2587 If EXPR is omitted, gives C<exp($_)>.
2590 X<fc> X<foldcase> X<casefold> X<fold-case> X<case-fold>
2594 =for Pod::Functions +fc return casefolded version of a string
2596 Returns the casefolded version of EXPR. This is the internal function
2597 implementing the C<\F> escape in double-quoted strings.
2599 Casefolding is the process of mapping strings to a form where case
2600 differences are erased; comparing two strings in their casefolded
2601 form is effectively a way of asking if two strings are equal,
2604 Roughly, if you ever found yourself writing this
2606 lc($this) eq lc($that) # Wrong!
2608 uc($this) eq uc($that) # Also wrong!
2610 $this =~ /^\Q$that\E\z/i # Right!
2614 fc($this) eq fc($that)
2616 And get the correct results.
2618 Perl only implements the full form of casefolding, but you can access
2619 the simple folds using L<Unicode::UCD/B<casefold()>> and
2620 L<Unicode::UCD/B<prop_invmap()>>.
2621 For further information on casefolding, refer to
2622 the Unicode Standard, specifically sections 3.13 C<Default Case Operations>,
2623 4.2 C<Case-Normative>, and 5.18 C<Case Mappings>,
2624 available at L<https://www.unicode.org/versions/latest/>, as well as the
2625 Case Charts available at L<https://www.unicode.org/charts/case/>.
2627 If EXPR is omitted, uses L<C<$_>|perlvar/$_>.
2629 This function behaves the same way under various pragmas, such as within
2630 L<S<C<"use feature 'unicode_strings">>|feature/The 'unicode_strings' feature>,
2631 as L<C<lc>|/lc EXPR> does, with the single exception of
2632 L<C<fc>|/fc EXPR> of I<LATIN CAPITAL LETTER SHARP S> (U+1E9E) within the
2633 scope of L<S<C<use locale>>|locale>. The foldcase of this character
2634 would normally be C<"ss">, but as explained in the L<C<lc>|/lc EXPR>
2636 changes that cross the 255/256 boundary are problematic under locales,
2637 and are hence prohibited. Therefore, this function under locale returns
2638 instead the string C<"\x{17F}\x{17F}">, which is the I<LATIN SMALL LETTER
2639 LONG S>. Since that character itself folds to C<"s">, the string of two
2640 of them together should be equivalent to a single U+1E9E when foldcased.
2642 While the Unicode Standard defines two additional forms of casefolding,
2643 one for Turkic languages and one that never maps one character into multiple
2644 characters, these are not provided by the Perl core. However, the CPAN module
2645 L<C<Unicode::Casing>|Unicode::Casing> may be used to provide an implementation.
2647 L<C<fc>|/fc EXPR> is available only if the
2648 L<C<"fc"> feature|feature/The 'fc' feature> is enabled or if it is
2649 prefixed with C<CORE::>. The
2650 L<C<"fc"> feature|feature/The 'fc' feature> is enabled automatically
2651 with a C<use v5.16> (or higher) declaration in the current scope.
2653 =item fcntl FILEHANDLE,FUNCTION,SCALAR
2656 =for Pod::Functions file control system call
2658 Implements the L<fcntl(2)> function. You'll probably have to say
2662 first to get the correct constant definitions. Argument processing and
2663 value returned work just like L<C<ioctl>|/ioctl
2664 FILEHANDLE,FUNCTION,SCALAR> below. For example:
2667 my $flags = fcntl($filehandle, F_GETFL, 0)
2668 or die "Can't fcntl F_GETFL: $!";
2670 You don't have to check for L<C<defined>|/defined EXPR> on the return
2671 from L<C<fcntl>|/fcntl FILEHANDLE,FUNCTION,SCALAR>. Like
2672 L<C<ioctl>|/ioctl FILEHANDLE,FUNCTION,SCALAR>, it maps a C<0> return
2673 from the system call into C<"0 but true"> in Perl. This string is true
2674 in boolean context and C<0> in numeric context. It is also exempt from
2676 L<C<Argument "..." isn't numeric>|perldiag/Argument "%s" isn't numeric%s>
2677 L<warnings> on improper numeric conversions.
2679 Note that L<C<fcntl>|/fcntl FILEHANDLE,FUNCTION,SCALAR> raises an
2680 exception if used on a machine that doesn't implement L<fcntl(2)>. See
2681 the L<Fcntl> module or your L<fcntl(2)> manpage to learn what functions
2682 are available on your system.
2684 Here's an example of setting a filehandle named C<$REMOTE> to be
2685 non-blocking at the system level. You'll have to negotiate
2686 L<C<$E<verbar>>|perlvar/$E<verbar>> on your own, though.
2688 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
2690 my $flags = fcntl($REMOTE, F_GETFL, 0)
2691 or die "Can't get flags for the socket: $!\n";
2693 fcntl($REMOTE, F_SETFL, $flags | O_NONBLOCK)
2694 or die "Can't set flags for the socket: $!\n";
2696 Portability issues: L<perlport/fcntl>.
2701 =for Pod::Functions the name of the current source file
2703 A special token that returns the name of the file in which it occurs.
2705 =item fileno FILEHANDLE
2708 =item fileno DIRHANDLE
2710 =for Pod::Functions return file descriptor from filehandle
2712 Returns the file descriptor for a filehandle or directory handle,
2714 filehandle is not open. If there is no real file descriptor at the OS
2715 level, as can happen with filehandles connected to memory objects via
2716 L<C<open>|/open FILEHANDLE,EXPR> with a reference for the third
2717 argument, -1 is returned.
2719 This is mainly useful for constructing bitmaps for
2720 L<C<select>|/select RBITS,WBITS,EBITS,TIMEOUT> and low-level POSIX
2721 tty-handling operations.
2722 If FILEHANDLE is an expression, the value is taken as an indirect
2723 filehandle, generally its name.
2725 You can use this to find out whether two handles refer to the
2726 same underlying descriptor:
2728 if (fileno($this) != -1 && fileno($this) == fileno($that)) {
2729 print "\$this and \$that are dups\n";
2730 } elsif (fileno($this) != -1 && fileno($that) != -1) {
2731 print "\$this and \$that have different " .
2732 "underlying file descriptors\n";
2734 print "At least one of \$this and \$that does " .
2735 "not have a real file descriptor\n";
2738 The behavior of L<C<fileno>|/fileno FILEHANDLE> on a directory handle
2739 depends on the operating system. On a system with L<dirfd(3)> or
2740 similar, L<C<fileno>|/fileno FILEHANDLE> on a directory
2741 handle returns the underlying file descriptor associated with the
2742 handle; on systems with no such support, it returns the undefined value,
2743 and sets L<C<$!>|perlvar/$!> (errno).
2745 =item flock FILEHANDLE,OPERATION
2746 X<flock> X<lock> X<locking>
2748 =for Pod::Functions lock an entire file with an advisory lock
2750 Calls L<flock(2)>, or an emulation of it, on FILEHANDLE. Returns true
2751 for success, false on failure. Produces a fatal error if used on a
2752 machine that doesn't implement L<flock(2)>, L<fcntl(2)> locking, or
2753 L<lockf(3)>. L<C<flock>|/flock FILEHANDLE,OPERATION> is Perl's portable
2754 file-locking interface, although it locks entire files only, not
2757 Two potentially non-obvious but traditional L<C<flock>|/flock
2758 FILEHANDLE,OPERATION> semantics are
2759 that it waits indefinitely until the lock is granted, and that its locks
2760 are B<merely advisory>. Such discretionary locks are more flexible, but
2761 offer fewer guarantees. This means that programs that do not also use
2762 L<C<flock>|/flock FILEHANDLE,OPERATION> may modify files locked with
2763 L<C<flock>|/flock FILEHANDLE,OPERATION>. See L<perlport>,
2764 your port's specific documentation, and your system-specific local manpages
2765 for details. It's best to assume traditional behavior if you're writing
2766 portable programs. (But if you're not, you should as always feel perfectly
2767 free to write for your own system's idiosyncrasies (sometimes called
2768 "features"). Slavish adherence to portability concerns shouldn't get
2769 in the way of your getting your job done.)
2771 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
2772 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
2773 you can use the symbolic names if you import them from the L<Fcntl> module,
2774 either individually, or as a group using the C<:flock> tag. LOCK_SH
2775 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
2776 releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
2777 LOCK_SH or LOCK_EX, then L<C<flock>|/flock FILEHANDLE,OPERATION> returns
2778 immediately rather than blocking waiting for the lock; check the return
2779 status to see if you got it.
2781 To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
2782 before locking or unlocking it.
2784 Note that the emulation built with L<lockf(3)> doesn't provide shared
2785 locks, and it requires that FILEHANDLE be open with write intent. These
2786 are the semantics that L<lockf(3)> implements. Most if not all systems
2787 implement L<lockf(3)> in terms of L<fcntl(2)> locking, though, so the
2788 differing semantics shouldn't bite too many people.
2790 Note that the L<fcntl(2)> emulation of L<flock(3)> requires that FILEHANDLE
2791 be open with read intent to use LOCK_SH and requires that it be open
2792 with write intent to use LOCK_EX.
2794 Note also that some versions of L<C<flock>|/flock FILEHANDLE,OPERATION>
2795 cannot lock things over the network; you would need to use the more
2796 system-specific L<C<fcntl>|/fcntl FILEHANDLE,FUNCTION,SCALAR> for
2797 that. If you like you can force Perl to ignore your system's L<flock(2)>
2798 function, and so provide its own L<fcntl(2)>-based emulation, by passing
2799 the switch C<-Ud_flock> to the F<Configure> program when you configure
2800 and build a new Perl.
2802 Here's a mailbox appender for BSD systems.
2804 # import LOCK_* and SEEK_END constants
2805 use Fcntl qw(:flock SEEK_END);
2809 flock($fh, LOCK_EX) or die "Cannot lock mailbox - $!\n";
2810 # and, in case we're running on a very old UNIX
2811 # variant without the modern O_APPEND semantics...
2812 seek($fh, 0, SEEK_END) or die "Cannot seek - $!\n";
2817 flock($fh, LOCK_UN) or die "Cannot unlock mailbox - $!\n";
2820 open(my $mbox, ">>", "/usr/spool/mail/$ENV{'USER'}")
2821 or die "Can't open mailbox: $!";
2824 print $mbox $msg,"\n\n";
2827 On systems that support a real L<flock(2)>, locks are inherited across
2828 L<C<fork>|/fork> calls, whereas those that must resort to the more
2829 capricious L<fcntl(2)> function lose their locks, making it seriously
2830 harder to write servers.
2832 See also L<DB_File> for other L<C<flock>|/flock FILEHANDLE,OPERATION>
2835 Portability issues: L<perlport/flock>.
2838 X<fork> X<child> X<parent>
2840 =for Pod::Functions create a new process just like this one
2842 Does a L<fork(2)> system call to create a new process running the
2843 same program at the same point. It returns the child pid to the
2844 parent process, C<0> to the child process, or L<C<undef>|/undef EXPR> if
2846 unsuccessful. File descriptors (and sometimes locks on those descriptors)
2847 are shared, while everything else is copied. On most systems supporting
2848 L<fork(2)>, great care has gone into making it extremely efficient (for
2849 example, using copy-on-write technology on data pages), making it the
2850 dominant paradigm for multitasking over the last few decades.
2852 Perl attempts to flush all files opened for output before forking the
2853 child process, but this may not be supported on some platforms (see
2854 L<perlport>). To be safe, you may need to set
2855 L<C<$E<verbar>>|perlvar/$E<verbar>> (C<$AUTOFLUSH> in L<English>) or
2856 call the C<autoflush> method of L<C<IO::Handle>|IO::Handle/METHODS> on
2857 any open handles to avoid duplicate output.
2859 If you L<C<fork>|/fork> without ever waiting on your children, you will
2860 accumulate zombies. On some systems, you can avoid this by setting
2861 L<C<$SIG{CHLD}>|perlvar/%SIG> to C<"IGNORE">. See also L<perlipc> for
2862 more examples of forking and reaping moribund children.
2864 Note that if your forked child inherits system file descriptors like
2865 STDIN and STDOUT that are actually connected by a pipe or socket, even
2866 if you exit, then the remote server (such as, say, a CGI script or a
2867 backgrounded job launched from a remote shell) won't think you're done.
2868 You should reopen those to F</dev/null> if it's any issue.
2870 On some platforms such as Windows, where the L<fork(2)> system call is
2871 not available, Perl can be built to emulate L<C<fork>|/fork> in the Perl
2872 interpreter. The emulation is designed, at the level of the Perl
2873 program, to be as compatible as possible with the "Unix" L<fork(2)>.
2874 However it has limitations that have to be considered in code intended
2875 to be portable. See L<perlfork> for more details.
2877 Portability issues: L<perlport/fork>.
2882 =for Pod::Functions declare a picture format with use by the write() function
2884 Declare a picture format for use by the L<C<write>|/write FILEHANDLE>
2885 function. For example:
2888 Test: @<<<<<<<< @||||| @>>>>>
2889 $str, $%, '$' . int($num)
2893 $num = $cost/$quantity;
2897 See L<perlform> for many details and examples.
2899 =item formline PICTURE,LIST
2902 =for Pod::Functions internal function used for formats
2904 This is an internal function used by L<C<format>|/format>s, though you
2905 may call it, too. It formats (see L<perlform>) a list of values
2906 according to the contents of PICTURE, placing the output into the format
2907 output accumulator, L<C<$^A>|perlvar/$^A> (or C<$ACCUMULATOR> in
2908 L<English>). Eventually, when a L<C<write>|/write FILEHANDLE> is done,
2909 the contents of L<C<$^A>|perlvar/$^A> are written to some filehandle.
2910 You could also read L<C<$^A>|perlvar/$^A> and then set
2911 L<C<$^A>|perlvar/$^A> back to C<"">. Note that a format typically does
2912 one L<C<formline>|/formline PICTURE,LIST> per line of form, but the
2913 L<C<formline>|/formline PICTURE,LIST> function itself doesn't care how
2914 many newlines are embedded in the PICTURE. This means that the C<~> and
2915 C<~~> tokens treat the entire PICTURE as a single line. You may
2916 therefore need to use multiple formlines to implement a single record
2917 format, just like the L<C<format>|/format> compiler.
2919 Be careful if you put double quotes around the picture, because an C<@>
2920 character may be taken to mean the beginning of an array name.
2921 L<C<formline>|/formline PICTURE,LIST> always returns true. See
2922 L<perlform> for other examples.
2924 If you are trying to use this instead of L<C<write>|/write FILEHANDLE>
2925 to capture the output, you may find it easier to open a filehandle to a
2926 scalar (C<< open my $fh, ">", \$output >>) and write to that instead.
2928 =item getc FILEHANDLE
2929 X<getc> X<getchar> X<character> X<file, read>
2933 =for Pod::Functions get the next character from the filehandle
2935 Returns the next character from the input file attached to FILEHANDLE,
2936 or the undefined value at end of file or if there was an error (in
2937 the latter case L<C<$!>|perlvar/$!> is set). If FILEHANDLE is omitted,
2939 STDIN. This is not particularly efficient. However, it cannot be
2940 used by itself to fetch single characters without waiting for the user
2941 to hit enter. For that, try something more like:
2944 system "stty cbreak </dev/tty >/dev/tty 2>&1";
2947 system "stty", '-icanon', 'eol', "\001";
2950 my $key = getc(STDIN);
2953 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
2956 system 'stty', 'icanon', 'eol', '^@'; # ASCII NUL
2960 Determination of whether C<$BSD_STYLE> should be set is left as an
2961 exercise to the reader.
2963 The L<C<POSIX::getattr>|POSIX/C<getattr>> function can do this more
2964 portably on systems purporting POSIX compliance. See also the
2965 L<C<Term::ReadKey>|Term::ReadKey> module on CPAN.
2968 X<getlogin> X<login>
2970 =for Pod::Functions return who logged in at this tty
2972 This implements the C library function of the same name, which on most
2973 systems returns the current login from F</etc/utmp>, if any. If it
2974 returns the empty string, use L<C<getpwuid>|/getpwuid UID>.
2976 my $login = getlogin || getpwuid($<) || "Kilroy";
2978 Do not consider L<C<getlogin>|/getlogin> for authentication: it is not
2979 as secure as L<C<getpwuid>|/getpwuid UID>.
2981 Portability issues: L<perlport/getlogin>.
2983 =item getpeername SOCKET
2984 X<getpeername> X<peer>
2986 =for Pod::Functions find the other end of a socket connection
2988 Returns the packed sockaddr address of the other end of the SOCKET
2992 my $hersockaddr = getpeername($sock);
2993 my ($port, $iaddr) = sockaddr_in($hersockaddr);
2994 my $herhostname = gethostbyaddr($iaddr, AF_INET);
2995 my $herstraddr = inet_ntoa($iaddr);
3000 =for Pod::Functions get process group
3002 Returns the current process group for the specified PID. Use
3003 a PID of C<0> to get the current process group for the
3004 current process. Will raise an exception if used on a machine that
3005 doesn't implement L<getpgrp(2)>. If PID is omitted, returns the process
3006 group of the current process. Note that the POSIX version of
3007 L<C<getpgrp>|/getpgrp PID> does not accept a PID argument, so only
3008 C<PID==0> is truly portable.
3010 Portability issues: L<perlport/getpgrp>.
3013 X<getppid> X<parent> X<pid>
3015 =for Pod::Functions get parent process ID
3017 Returns the process id of the parent process.
3019 Note for Linux users: Between v5.8.1 and v5.16.0 Perl would work
3020 around non-POSIX thread semantics the minority of Linux systems (and
3021 Debian GNU/kFreeBSD systems) that used LinuxThreads, this emulation
3022 has since been removed. See the documentation for L<$$|perlvar/$$> for
3025 Portability issues: L<perlport/getppid>.
3027 =item getpriority WHICH,WHO
3028 X<getpriority> X<priority> X<nice>
3030 =for Pod::Functions get current nice value
3032 Returns the current priority for a process, a process group, or a user.
3033 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
3034 machine that doesn't implement L<getpriority(2)>.
3036 C<WHICH> can be any of C<PRIO_PROCESS>, C<PRIO_PGRP> or C<PRIO_USER>
3037 imported from L<POSIX/RESOURCE CONSTANTS>.
3039 Portability issues: L<perlport/getpriority>.
3042 X<getpwnam> X<getgrnam> X<gethostbyname> X<getnetbyname> X<getprotobyname>
3043 X<getpwuid> X<getgrgid> X<getservbyname> X<gethostbyaddr> X<getnetbyaddr>
3044 X<getprotobynumber> X<getservbyport> X<getpwent> X<getgrent> X<gethostent>
3045 X<getnetent> X<getprotoent> X<getservent> X<setpwent> X<setgrent> X<sethostent>
3046 X<setnetent> X<setprotoent> X<setservent> X<endpwent> X<endgrent> X<endhostent>
3047 X<endnetent> X<endprotoent> X<endservent>
3049 =for Pod::Functions get passwd record given user login name
3053 =for Pod::Functions get group record given group name
3055 =item gethostbyname NAME
3057 =for Pod::Functions get host record given name
3059 =item getnetbyname NAME
3061 =for Pod::Functions get networks record given name
3063 =item getprotobyname NAME
3065 =for Pod::Functions get protocol record given name
3069 =for Pod::Functions get passwd record given user ID
3073 =for Pod::Functions get group record given group user ID
3075 =item getservbyname NAME,PROTO
3077 =for Pod::Functions get services record given its name
3079 =item gethostbyaddr ADDR,ADDRTYPE
3081 =for Pod::Functions get host record given its address
3083 =item getnetbyaddr ADDR,ADDRTYPE
3085 =for Pod::Functions get network record given its address
3087 =item getprotobynumber NUMBER
3089 =for Pod::Functions get protocol record numeric protocol
3091 =item getservbyport PORT,PROTO
3093 =for Pod::Functions get services record given numeric port
3097 =for Pod::Functions get next passwd record
3101 =for Pod::Functions get next group record
3105 =for Pod::Functions get next hosts record
3109 =for Pod::Functions get next networks record
3113 =for Pod::Functions get next protocols record
3117 =for Pod::Functions get next services record
3121 =for Pod::Functions prepare passwd file for use
3125 =for Pod::Functions prepare group file for use
3127 =item sethostent STAYOPEN
3129 =for Pod::Functions prepare hosts file for use
3131 =item setnetent STAYOPEN
3133 =for Pod::Functions prepare networks file for use
3135 =item setprotoent STAYOPEN
3137 =for Pod::Functions prepare protocols file for use
3139 =item setservent STAYOPEN
3141 =for Pod::Functions prepare services file for use
3145 =for Pod::Functions be done using passwd file
3149 =for Pod::Functions be done using group file
3153 =for Pod::Functions be done using hosts file
3157 =for Pod::Functions be done using networks file
3161 =for Pod::Functions be done using protocols file
3165 =for Pod::Functions be done using services file
3167 These routines are the same as their counterparts in the
3168 system C library. In list context, the return values from the
3169 various get routines are as follows:
3172 my ( $name, $passwd, $gid, $members ) = getgr*
3173 my ( $name, $aliases, $addrtype, $net ) = getnet*
3174 my ( $name, $aliases, $port, $proto ) = getserv*
3175 my ( $name, $aliases, $proto ) = getproto*
3176 my ( $name, $aliases, $addrtype, $length, @addrs ) = gethost*
3177 my ( $name, $passwd, $uid, $gid, $quota,
3178 $comment, $gcos, $dir, $shell, $expire ) = getpw*
3181 (If the entry doesn't exist, the return value is a single meaningless true
3184 The exact meaning of the $gcos field varies but it usually contains
3185 the real name of the user (as opposed to the login name) and other
3186 information pertaining to the user. Beware, however, that in many
3187 system users are able to change this information and therefore it
3188 cannot be trusted and therefore the $gcos is tainted (see
3189 L<perlsec>). The $passwd and $shell, user's encrypted password and
3190 login shell, are also tainted, for the same reason.
3192 In scalar context, you get the name, unless the function was a
3193 lookup by name, in which case you get the other thing, whatever it is.
3194 (If the entry doesn't exist you get the undefined value.) For example:
3196 my $uid = getpwnam($name);
3197 my $name = getpwuid($num);
3198 my $name = getpwent();
3199 my $gid = getgrnam($name);
3200 my $name = getgrgid($num);
3201 my $name = getgrent();
3204 In I<getpw*()> the fields $quota, $comment, and $expire are special
3205 in that they are unsupported on many systems. If the
3206 $quota is unsupported, it is an empty scalar. If it is supported, it
3207 usually encodes the disk quota. If the $comment field is unsupported,
3208 it is an empty scalar. If it is supported it usually encodes some
3209 administrative comment about the user. In some systems the $quota
3210 field may be $change or $age, fields that have to do with password
3211 aging. In some systems the $comment field may be $class. The $expire
3212 field, if present, encodes the expiration period of the account or the
3213 password. For the availability and the exact meaning of these fields
3214 in your system, please consult L<getpwnam(3)> and your system's
3215 F<pwd.h> file. You can also find out from within Perl what your
3216 $quota and $comment fields mean and whether you have the $expire field
3217 by using the L<C<Config>|Config> module and the values C<d_pwquota>, C<d_pwage>,
3218 C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
3219 files are supported only if your vendor has implemented them in the
3220 intuitive fashion that calling the regular C library routines gets the
3221 shadow versions if you're running under privilege or if there exists
3222 the L<shadow(3)> functions as found in System V (this includes Solaris
3223 and Linux). Those systems that implement a proprietary shadow password
3224 facility are unlikely to be supported.
3226 The $members value returned by I<getgr*()> is a space-separated list of
3227 the login names of the members of the group.
3229 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
3230 C, it will be returned to you via L<C<$?>|perlvar/$?> if the function
3232 C<@addrs> value returned by a successful call is a list of raw
3233 addresses returned by the corresponding library call. In the
3234 Internet domain, each address is four bytes long; you can unpack it
3235 by saying something like:
3237 my ($w,$x,$y,$z) = unpack('W4',$addr[0]);
3239 The Socket library makes this slightly easier:
3242 my $iaddr = inet_aton("127.1"); # or whatever address
3243 my $name = gethostbyaddr($iaddr, AF_INET);
3245 # or going the other way
3246 my $straddr = inet_ntoa($iaddr);
3248 In the opposite way, to resolve a hostname to the IP address
3252 my $packed_ip = gethostbyname("www.perl.org");
3254 if (defined $packed_ip) {
3255 $ip_address = inet_ntoa($packed_ip);
3258 Make sure L<C<gethostbyname>|/gethostbyname NAME> is called in SCALAR
3259 context and that its return value is checked for definedness.
3261 The L<C<getprotobynumber>|/getprotobynumber NUMBER> function, even
3262 though it only takes one argument, has the precedence of a list
3263 operator, so beware:
3265 getprotobynumber $number eq 'icmp' # WRONG
3266 getprotobynumber($number eq 'icmp') # actually means this
3267 getprotobynumber($number) eq 'icmp' # better this way
3269 If you get tired of remembering which element of the return list
3270 contains which return value, by-name interfaces are provided in standard
3271 modules: L<C<File::stat>|File::stat>, L<C<Net::hostent>|Net::hostent>,
3272 L<C<Net::netent>|Net::netent>, L<C<Net::protoent>|Net::protoent>,
3273 L<C<Net::servent>|Net::servent>, L<C<Time::gmtime>|Time::gmtime>,
3274 L<C<Time::localtime>|Time::localtime>, and
3275 L<C<User::grent>|User::grent>. These override the normal built-ins,
3276 supplying versions that return objects with the appropriate names for
3277 each field. For example:
3281 my $is_his = (stat($filename)->uid == pwent($whoever)->uid);
3283 Even though it looks as though they're the same method calls (uid),
3284 they aren't, because a C<File::stat> object is different from
3285 a C<User::pwent> object.
3287 Many of these functions are not safe in a multi-threaded environment
3288 where more than one thread can be using them. In particular, functions
3289 like C<getpwent()> iterate per-process and not per-thread, so if two
3290 threads are simultaneously iterating, neither will get all the records.
3292 Some systems have thread-safe versions of some of the functions, such as
3293 C<getpwnam_r()> instead of C<getpwnam()>. There, Perl automatically and
3294 invisibly substitutes the thread-safe version, without notice. This
3295 means that code that safely runs on some systems can fail on others that
3296 lack the thread-safe versions.
3298 Portability issues: L<perlport/getpwnam> to L<perlport/endservent>.
3300 =item getsockname SOCKET
3303 =for Pod::Functions retrieve the sockaddr for a given socket
3305 Returns the packed sockaddr address of this end of the SOCKET connection,
3306 in case you don't know the address because you have several different
3307 IPs that the connection might have come in on.
3310 my $mysockaddr = getsockname($sock);
3311 my ($port, $myaddr) = sockaddr_in($mysockaddr);
3312 printf "Connect to %s [%s]\n",
3313 scalar gethostbyaddr($myaddr, AF_INET),
3316 =item getsockopt SOCKET,LEVEL,OPTNAME
3319 =for Pod::Functions get socket options on a given socket
3321 Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
3322 Options may exist at multiple protocol levels depending on the socket
3323 type, but at least the uppermost socket level SOL_SOCKET (defined in the
3324 L<C<Socket>|Socket> module) will exist. To query options at another
3325 level the protocol number of the appropriate protocol controlling the
3326 option should be supplied. For example, to indicate that an option is
3327 to be interpreted by the TCP protocol, LEVEL should be set to the
3328 protocol number of TCP, which you can get using
3329 L<C<getprotobyname>|/getprotobyname NAME>.
3331 The function returns a packed string representing the requested socket
3332 option, or L<C<undef>|/undef EXPR> on error, with the reason for the
3333 error placed in L<C<$!>|perlvar/$!>. Just what is in the packed string
3334 depends on LEVEL and OPTNAME; consult L<getsockopt(2)> for details. A
3335 common case is that the option is an integer, in which case the result
3336 is a packed integer, which you can decode using
3337 L<C<unpack>|/unpack TEMPLATE,EXPR> with the C<i> (or C<I>) format.
3339 Here's an example to test whether Nagle's algorithm is enabled on a socket:
3341 use Socket qw(:all);
3343 defined(my $tcp = getprotobyname("tcp"))
3344 or die "Could not determine the protocol number for tcp";
3345 # my $tcp = IPPROTO_TCP; # Alternative
3346 my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
3347 or die "getsockopt TCP_NODELAY: $!";
3348 my $nodelay = unpack("I", $packed);
3349 print "Nagle's algorithm is turned ",
3350 $nodelay ? "off\n" : "on\n";
3352 Portability issues: L<perlport/getsockopt>.
3355 X<glob> X<wildcard> X<filename, expansion> X<expand>
3359 =for Pod::Functions expand filenames using wildcards
3361 In list context, returns a (possibly empty) list of filename expansions on
3362 the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
3363 scalar context, glob iterates through such filename expansions, returning
3364 undef when the list is exhausted. This is the internal function
3365 implementing the C<< <*.c> >> operator, but you can use it directly. If
3366 EXPR is omitted, L<C<$_>|perlvar/$_> is used. The C<< <*.c> >> operator
3367 is discussed in more detail in L<perlop/"I/O Operators">.
3369 Note that L<C<glob>|/glob EXPR> splits its arguments on whitespace and
3371 each segment as separate pattern. As such, C<glob("*.c *.h")>
3372 matches all files with a F<.c> or F<.h> extension. The expression
3373 C<glob(".* *")> matches all files in the current working directory.
3374 If you want to glob filenames that might contain whitespace, you'll
3375 have to use extra quotes around the spacey filename to protect it.
3376 For example, to glob filenames that have an C<e> followed by a space
3377 followed by an C<f>, use one of:
3379 my @spacies = <"*e f*">;
3380 my @spacies = glob '"*e f*"';
3381 my @spacies = glob q("*e f*");
3383 If you had to get a variable through, you could do this:
3385 my @spacies = glob "'*${var}e f*'";
3386 my @spacies = glob qq("*${var}e f*");
3388 If non-empty braces are the only wildcard characters used in the
3389 L<C<glob>|/glob EXPR>, no filenames are matched, but potentially many
3390 strings are returned. For example, this produces nine strings, one for
3391 each pairing of fruits and colors:
3393 my @many = glob "{apple,tomato,cherry}={green,yellow,red}";
3395 This operator is implemented using the standard C<File::Glob> extension.
3396 See L<File::Glob> for details, including
3397 L<C<bsd_glob>|File::Glob/C<bsd_glob>>, which does not treat whitespace
3398 as a pattern separator.
3400 If a C<glob> expression is used as the condition of a C<while> or C<for>
3401 loop, then it will be implicitly assigned to C<$_>. If either a C<glob>
3402 expression or an explicit assignment of a C<glob> expression to a scalar
3403 is used as a C<while>/C<for> condition, then the condition actually
3404 tests for definedness of the expression's value, not for its regular
3407 Portability issues: L<perlport/glob>.
3410 X<gmtime> X<UTC> X<Greenwich>
3414 =for Pod::Functions convert UNIX time into record or string using Greenwich time
3416 Works just like L<C<localtime>|/localtime EXPR> but the returned values
3417 are localized for the standard Greenwich time zone.
3419 Note: When called in list context, $isdst, the last value
3420 returned by gmtime, is always C<0>. There is no
3421 Daylight Saving Time in GMT.
3423 Portability issues: L<perlport/gmtime>.
3426 X<goto> X<jump> X<jmp>
3432 =for Pod::Functions create spaghetti code
3434 The C<goto LABEL> form finds the statement labeled with LABEL and
3435 resumes execution there. It can't be used to get out of a block or
3436 subroutine given to L<C<sort>|/sort SUBNAME LIST>. It can be used to go
3437 almost anywhere else within the dynamic scope, including out of
3438 subroutines, but it's usually better to use some other construct such as
3439 L<C<last>|/last LABEL> or L<C<die>|/die LIST>. The author of Perl has
3440 never felt the need to use this form of L<C<goto>|/goto LABEL> (in Perl,
3441 that is; C is another matter). (The difference is that C does not offer
3442 named loops combined with loop control. Perl does, and this replaces
3443 most structured uses of L<C<goto>|/goto LABEL> in other languages.)
3445 The C<goto EXPR> form expects to evaluate C<EXPR> to a code reference or
3446 a label name. If it evaluates to a code reference, it will be handled
3447 like C<goto &NAME>, below. This is especially useful for implementing
3448 tail recursion via C<goto __SUB__>.
3450 If the expression evaluates to a label name, its scope will be resolved
3451 dynamically. This allows for computed L<C<goto>|/goto LABEL>s per
3452 FORTRAN, but isn't necessarily recommended if you're optimizing for
3455 goto ("FOO", "BAR", "GLARCH")[$i];
3457 As shown in this example, C<goto EXPR> is exempt from the "looks like a
3458 function" rule. A pair of parentheses following it does not (necessarily)
3459 delimit its argument. C<goto("NE")."XT"> is equivalent to C<goto NEXT>.
3460 Also, unlike most named operators, this has the same precedence as
3463 Use of C<goto LABEL> or C<goto EXPR> to jump into a construct is
3464 deprecated and will issue a warning. Even then, it may not be used to
3465 go into any construct that requires initialization, such as a
3466 subroutine, a C<foreach> loop, or a C<given>
3467 block. In general, it may not be used to jump into the parameter
3468 of a binary or list operator, but it may be used to jump into the
3469 I<first> parameter of a binary operator. (The C<=>
3470 assignment operator's "first" operand is its right-hand
3471 operand.) It also can't be used to go into a
3472 construct that is optimized away.
3474 The C<goto &NAME> form is quite different from the other forms of
3475 L<C<goto>|/goto LABEL>. In fact, it isn't a goto in the normal sense at
3476 all, and doesn't have the stigma associated with other gotos. Instead,
3477 it exits the current subroutine (losing any changes set by
3478 L<C<local>|/local EXPR>) and immediately calls in its place the named
3479 subroutine using the current value of L<C<@_>|perlvar/@_>. This is used
3480 by C<AUTOLOAD> subroutines that wish to load another subroutine and then
3481 pretend that the other subroutine had been called in the first place
3482 (except that any modifications to L<C<@_>|perlvar/@_> in the current
3483 subroutine are propagated to the other subroutine.) After the
3484 L<C<goto>|/goto LABEL>, not even L<C<caller>|/caller EXPR> will be able
3485 to tell that this routine was called first.
3487 NAME needn't be the name of a subroutine; it can be a scalar variable
3488 containing a code reference or a block that evaluates to a code
3491 =item grep BLOCK LIST
3494 =item grep EXPR,LIST
3496 =for Pod::Functions locate elements in a list test true against a given criterion
3498 This is similar in spirit to, but not the same as, L<grep(1)> and its
3499 relatives. In particular, it is not limited to using regular expressions.
3501 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
3502 L<C<$_>|perlvar/$_> to each element) and returns the list value
3504 elements for which the expression evaluated to true. In scalar
3505 context, returns the number of times the expression was true.
3507 my @foo = grep(!/^#/, @bar); # weed out comments
3511 my @foo = grep {!/^#/} @bar; # weed out comments
3513 Note that L<C<$_>|perlvar/$_> is an alias to the list value, so it can
3515 modify the elements of the LIST. While this is useful and supported,
3516 it can cause bizarre results if the elements of LIST are not variables.
3517 Similarly, grep returns aliases into the original list, much as a for
3518 loop's index variable aliases the list elements. That is, modifying an
3519 element of a list returned by grep (for example, in a C<foreach>,
3520 L<C<map>|/map BLOCK LIST> or another L<C<grep>|/grep BLOCK LIST>)
3521 actually modifies the element in the original list.
3522 This is usually something to be avoided when writing clear code.
3524 See also L<C<map>|/map BLOCK LIST> for a list composed of the results of
3528 X<hex> X<hexadecimal>
3532 =for Pod::Functions convert a hexadecimal string to a number
3534 Interprets EXPR as a hex string and returns the corresponding numeric value.
3535 If EXPR is omitted, uses L<C<$_>|perlvar/$_>.
3537 print hex '0xAf'; # prints '175'
3538 print hex 'aF'; # same
3539 $valid_input =~ /\A(?:0?[xX])?(?:_?[0-9a-fA-F])*\z/
3541 A hex string consists of hex digits and an optional C<0x> or C<x> prefix.
3542 Each hex digit may be preceded by a single underscore, which will be ignored.
3543 Any other character triggers a warning and causes the rest of the string
3544 to be ignored (even leading whitespace, unlike L<C<oct>|/oct EXPR>).
3545 Only integers can be represented, and integer overflow triggers a warning.
3547 To convert strings that might start with any of C<0>, C<0x>, or C<0b>,
3548 see L<C<oct>|/oct EXPR>. To present something as hex, look into
3549 L<C<printf>|/printf FILEHANDLE FORMAT, LIST>,
3550 L<C<sprintf>|/sprintf FORMAT, LIST>, and
3551 L<C<unpack>|/unpack TEMPLATE,EXPR>.
3556 =for Pod::Functions patch a module's namespace into your own
3558 There is no builtin L<C<import>|/import LIST> function. It is just an
3559 ordinary method (subroutine) defined (or inherited) by modules that wish
3560 to export names to another module. The
3561 L<C<use>|/use Module VERSION LIST> function calls the
3562 L<C<import>|/import LIST> method for the package used. See also
3563 L<C<use>|/use Module VERSION LIST>, L<perlmod>, and L<Exporter>.
3565 =item index STR,SUBSTR,POSITION
3566 X<index> X<indexOf> X<InStr>
3568 =item index STR,SUBSTR
3570 =for Pod::Functions find a substring within a string
3572 The index function searches for one string within another, but without
3573 the wildcard-like behavior of a full regular-expression pattern match.
3574 It returns the position of the first occurrence of SUBSTR in STR at
3575 or after POSITION. If POSITION is omitted, starts searching from the
3576 beginning of the string. POSITION before the beginning of the string
3577 or after its end is treated as if it were the beginning or the end,
3578 respectively. POSITION and the return value are based at zero.
3579 If the substring is not found, L<C<index>|/index STR,SUBSTR,POSITION>
3583 X<int> X<integer> X<truncate> X<trunc> X<floor>
3587 =for Pod::Functions get the integer portion of a number
3589 Returns the integer portion of EXPR. If EXPR is omitted, uses
3590 L<C<$_>|perlvar/$_>.
3591 You should not use this function for rounding: one because it truncates
3592 towards C<0>, and two because machine representations of floating-point
3593 numbers can sometimes produce counterintuitive results. For example,
3594 C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
3595 because it's really more like -268.99999999999994315658 instead. Usually,
3596 the L<C<sprintf>|/sprintf FORMAT, LIST>,
3597 L<C<printf>|/printf FILEHANDLE FORMAT, LIST>, or the
3598 L<C<POSIX::floor>|POSIX/C<floor>> and L<C<POSIX::ceil>|POSIX/C<ceil>>
3599 functions will serve you better than will L<C<int>|/int EXPR>.
3601 =item ioctl FILEHANDLE,FUNCTION,SCALAR
3604 =for Pod::Functions system-dependent device control system call
3606 Implements the L<ioctl(2)> function. You'll probably first have to say
3608 require "sys/ioctl.ph"; # probably in
3609 # $Config{archlib}/sys/ioctl.ph
3611 to get the correct function definitions. If F<sys/ioctl.ph> doesn't
3612 exist or doesn't have the correct definitions you'll have to roll your
3613 own, based on your C header files such as F<< <sys/ioctl.h> >>.
3614 (There is a Perl script called B<h2ph> that comes with the Perl kit that
3615 may help you in this, but it's nontrivial.) SCALAR will be read and/or
3616 written depending on the FUNCTION; a C pointer to the string value of SCALAR
3617 will be passed as the third argument of the actual
3618 L<C<ioctl>|/ioctl FILEHANDLE,FUNCTION,SCALAR> call. (If SCALAR
3619 has no string value but does have a numeric value, that value will be
3620 passed rather than a pointer to the string value. To guarantee this to be
3621 true, add a C<0> to the scalar before using it.) The
3622 L<C<pack>|/pack TEMPLATE,LIST> and L<C<unpack>|/unpack TEMPLATE,EXPR>
3623 functions may be needed to manipulate the values of structures used by
3624 L<C<ioctl>|/ioctl FILEHANDLE,FUNCTION,SCALAR>.
3626 The return value of L<C<ioctl>|/ioctl FILEHANDLE,FUNCTION,SCALAR> (and
3627 L<C<fcntl>|/fcntl FILEHANDLE,FUNCTION,SCALAR>) is as follows:
3629 if OS returns: then Perl returns:
3631 0 string "0 but true"
3632 anything else that number
3634 Thus Perl returns true on success and false on failure, yet you can
3635 still easily determine the actual value returned by the operating
3638 my $retval = ioctl(...) || -1;
3639 printf "System returned %d\n", $retval;
3641 The special string C<"0 but true"> is exempt from
3642 L<C<Argument "..." isn't numeric>|perldiag/Argument "%s" isn't numeric%s>
3643 L<warnings> on improper numeric conversions.
3645 Portability issues: L<perlport/ioctl>.
3647 =item join EXPR,LIST
3650 =for Pod::Functions join a list into a string using a separator
3652 Joins the separate strings of LIST into a single string with fields
3653 separated by the value of EXPR, and returns that new string. Example:
3655 my $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
3657 Beware that unlike L<C<split>|/split E<sol>PATTERNE<sol>,EXPR,LIMIT>,
3658 L<C<join>|/join EXPR,LIST> doesn't take a pattern as its first argument.
3659 Compare L<C<split>|/split E<sol>PATTERNE<sol>,EXPR,LIMIT>.
3666 =for Pod::Functions retrieve list of indices from a hash
3668 Called in list context, returns a list consisting of all the keys of the
3669 named hash, or in Perl 5.12 or later only, the indices of an array. Perl
3670 releases prior to 5.12 will produce a syntax error if you try to use an
3671 array argument. In scalar context, returns the number of keys or indices.
3673 Hash entries are returned in an apparently random order. The actual random
3674 order is specific to a given hash; the exact same series of operations
3675 on two hashes may result in a different order for each hash. Any insertion
3676 into the hash may change the order, as will any deletion, with the exception
3677 that the most recent key returned by L<C<each>|/each HASH> or
3678 L<C<keys>|/keys HASH> may be deleted without changing the order. So
3679 long as a given hash is unmodified you may rely on
3680 L<C<keys>|/keys HASH>, L<C<values>|/values HASH> and L<C<each>|/each
3681 HASH> to repeatedly return the same order
3682 as each other. See L<perlsec/"Algorithmic Complexity Attacks"> for
3683 details on why hash order is randomized. Aside from the guarantees
3684 provided here the exact details of Perl's hash algorithm and the hash
3685 traversal order are subject to change in any release of Perl. Tied hashes
3686 may behave differently to Perl's hashes with respect to changes in order on
3687 insertion and deletion of items.
3689 As a side effect, calling L<C<keys>|/keys HASH> resets the internal
3690 iterator of the HASH or ARRAY (see L<C<each>|/each HASH>) before
3691 yielding the keys. In
3692 particular, calling L<C<keys>|/keys HASH> in void context resets the
3693 iterator with no other overhead.
3695 Here is yet another way to print your environment:
3697 my @keys = keys %ENV;
3698 my @values = values %ENV;
3700 print pop(@keys), '=', pop(@values), "\n";
3703 or how about sorted by key:
3705 foreach my $key (sort(keys %ENV)) {
3706 print $key, '=', $ENV{$key}, "\n";
3709 The returned values are copies of the original keys in the hash, so
3710 modifying them will not affect the original hash. Compare
3711 L<C<values>|/values HASH>.
3713 To sort a hash by value, you'll need to use a
3714 L<C<sort>|/sort SUBNAME LIST> function. Here's a descending numeric
3715 sort of a hash by its values:
3717 foreach my $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
3718 printf "%4d %s\n", $hash{$key}, $key;
3721 Used as an lvalue, L<C<keys>|/keys HASH> allows you to increase the
3722 number of hash buckets
3723 allocated for the given hash. This can gain you a measure of efficiency if
3724 you know the hash is going to get big. (This is similar to pre-extending
3725 an array by assigning a larger number to $#array.) If you say
3729 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
3730 in fact, since it rounds up to the next power of two. These
3731 buckets will be retained even if you do C<%hash = ()>, use C<undef
3732 %hash> if you want to free the storage while C<%hash> is still in scope.
3733 You can't shrink the number of buckets allocated for the hash using
3734 L<C<keys>|/keys HASH> in this way (but you needn't worry about doing
3735 this by accident, as trying has no effect). C<keys @array> in an lvalue
3736 context is a syntax error.
3738 Starting with Perl 5.14, an experimental feature allowed
3739 L<C<keys>|/keys HASH> to take a scalar expression. This experiment has
3740 been deemed unsuccessful, and was removed as of Perl 5.24.
3742 To avoid confusing would-be users of your code who are running earlier
3743 versions of Perl with mysterious syntax errors, put this sort of thing at
3744 the top of your file to signal that your code will work I<only> on Perls of
3747 use 5.012; # so keys/values/each work on arrays
3749 See also L<C<each>|/each HASH>, L<C<values>|/values HASH>, and
3750 L<C<sort>|/sort SUBNAME LIST>.
3752 =item kill SIGNAL, LIST
3757 =for Pod::Functions send a signal to a process or process group
3759 Sends a signal to a list of processes. Returns the number of arguments
3760 that were successfully used to signal (which is not necessarily the same
3761 as the number of processes actually killed, e.g. where a process group is
3764 my $cnt = kill 'HUP', $child1, $child2;
3765 kill 'KILL', @goners;
3767 SIGNAL may be either a signal name (a string) or a signal number. A signal
3768 name may start with a C<SIG> prefix, thus C<FOO> and C<SIGFOO> refer to the
3769 same signal. The string form of SIGNAL is recommended for portability because
3770 the same signal may have different numbers in different operating systems.
3772 A list of signal names supported by the current platform can be found in
3773 C<$Config{sig_name}>, which is provided by the L<C<Config>|Config>
3774 module. See L<Config> for more details.
3776 A negative signal name is the same as a negative signal number, killing process
3777 groups instead of processes. For example, C<kill '-KILL', $pgrp> and
3778 C<kill -9, $pgrp> will send C<SIGKILL> to
3779 the entire process group specified. That
3780 means you usually want to use positive not negative signals.
3782 If SIGNAL is either the number 0 or the string C<ZERO> (or C<SIGZERO>),
3783 no signal is sent to the process, but L<C<kill>|/kill SIGNAL, LIST>
3784 checks whether it's I<possible> to send a signal to it
3785 (that means, to be brief, that the process is owned by the same user, or we are
3786 the super-user). This is useful to check that a child process is still
3787 alive (even if only as a zombie) and hasn't changed its UID. See
3788 L<perlport> for notes on the portability of this construct.
3790 The behavior of kill when a I<PROCESS> number is zero or negative depends on
3791 the operating system. For example, on POSIX-conforming systems, zero will
3792 signal the current process group, -1 will signal all processes, and any
3793 other negative PROCESS number will act as a negative signal number and
3794 kill the entire process group specified.
3796 If both the SIGNAL and the PROCESS are negative, the results are undefined.
3797 A warning may be produced in a future version.
3799 See L<perlipc/"Signals"> for more details.
3801 On some platforms such as Windows where the L<fork(2)> system call is not
3802 available, Perl can be built to emulate L<C<fork>|/fork> at the
3804 This emulation has limitations related to kill that have to be considered,
3805 for code running on Windows and in code intended to be portable.
3807 See L<perlfork> for more details.
3809 If there is no I<LIST> of processes, no signal is sent, and the return
3810 value is 0. This form is sometimes used, however, because it causes
3811 tainting checks to be run. But see
3812 L<perlsec/Laundering and Detecting Tainted Data>.
3814 Portability issues: L<perlport/kill>.
3823 =for Pod::Functions exit a block prematurely
3825 The L<C<last>|/last LABEL> command is like the C<break> statement in C
3827 loops); it immediately exits the loop in question. If the LABEL is
3828 omitted, the command refers to the innermost enclosing
3829 loop. The C<last EXPR> form, available starting in Perl
3830 5.18.0, allows a label name to be computed at run time,
3831 and is otherwise identical to C<last LABEL>. The
3832 L<C<continue>|/continue BLOCK> block, if any, is not executed:
3834 LINE: while (<STDIN>) {
3835 last LINE if /^$/; # exit when done with header
3839 L<C<last>|/last LABEL> cannot return a value from a block that typically
3840 returns a value, such as C<eval {}>, C<sub {}>, or C<do {}>. It will perform
3841 its flow control behavior, which precludes any return value. It should not be
3842 used to exit a L<C<grep>|/grep BLOCK LIST> or L<C<map>|/map BLOCK LIST>
3845 Note that a block by itself is semantically identical to a loop
3846 that executes once. Thus L<C<last>|/last LABEL> can be used to effect
3847 an early exit out of such a block.
3849 See also L<C<continue>|/continue BLOCK> for an illustration of how
3850 L<C<last>|/last LABEL>, L<C<next>|/next LABEL>, and
3851 L<C<redo>|/redo LABEL> work.
3853 Unlike most named operators, this has the same precedence as assignment.
3854 It is also exempt from the looks-like-a-function rule, so
3855 C<last ("foo")."bar"> will cause "bar" to be part of the argument to
3856 L<C<last>|/last LABEL>.
3863 =for Pod::Functions return lower-case version of a string
3865 Returns a lowercased version of EXPR. This is the internal function
3866 implementing the C<\L> escape in double-quoted strings.
3868 If EXPR is omitted, uses L<C<$_>|perlvar/$_>.
3870 What gets returned depends on several factors:
3874 =item If C<use bytes> is in effect:
3876 The results follow ASCII rules. Only the characters C<A-Z> change,
3877 to C<a-z> respectively.
3879 =item Otherwise, if C<use locale> for C<LC_CTYPE> is in effect:
3881 Respects current C<LC_CTYPE> locale for code points < 256; and uses Unicode
3882 rules for the remaining code points (this last can only happen if
3883 the UTF8 flag is also set). See L<perllocale>.
3885 Starting in v5.20, Perl uses full Unicode rules if the locale is
3886 UTF-8. Otherwise, there is a deficiency in this scheme, which is that
3887 case changes that cross the 255/256
3888 boundary are not well-defined. For example, the lower case of LATIN CAPITAL
3889 LETTER SHARP S (U+1E9E) in Unicode rules is U+00DF (on ASCII
3890 platforms). But under C<use locale> (prior to v5.20 or not a UTF-8
3891 locale), the lower case of U+1E9E is
3892 itself, because 0xDF may not be LATIN SMALL LETTER SHARP S in the
3893 current locale, and Perl has no way of knowing if that character even
3894 exists in the locale, much less what code point it is. Perl returns
3895 a result that is above 255 (almost always the input character unchanged),
3896 for all instances (and there aren't many) where the 255/256 boundary
3897 would otherwise be crossed; and starting in v5.22, it raises a
3898 L<locale|perldiag/Can't do %s("%s") on non-UTF-8 locale; resolved to "%s".> warning.
3900 =item Otherwise, If EXPR has the UTF8 flag set:
3902 Unicode rules are used for the case change.
3904 =item Otherwise, if C<use feature 'unicode_strings'> or C<use locale ':not_characters'> is in effect:
3906 Unicode rules are used for the case change.
3910 ASCII rules are used for the case change. The lowercase of any character
3911 outside the ASCII range is the character itself.
3916 X<lcfirst> X<lowercase>
3920 =for Pod::Functions return a string with just the next letter in lower case
3922 Returns the value of EXPR with the first character lowercased. This
3923 is the internal function implementing the C<\l> escape in
3924 double-quoted strings.
3926 If EXPR is omitted, uses L<C<$_>|perlvar/$_>.
3928 This function behaves the same way under various pragmas, such as in a locale,
3929 as L<C<lc>|/lc EXPR> does.
3936 =for Pod::Functions return the number of characters in a string
3938 Returns the length in I<characters> of the value of EXPR. If EXPR is
3939 omitted, returns the length of L<C<$_>|perlvar/$_>. If EXPR is
3940 undefined, returns L<C<undef>|/undef EXPR>.
3942 This function cannot be used on an entire array or hash to find out how
3943 many elements these have. For that, use C<scalar @array> and C<scalar keys
3944 %hash>, respectively.
3946 Like all Perl character operations, L<C<length>|/length EXPR> normally
3948 characters, not physical bytes. For how many bytes a string encoded as
3949 UTF-8 would take up, use C<length(Encode::encode('UTF-8', EXPR))>
3950 (you'll have to C<use Encode> first). See L<Encode> and L<perlunicode>.
3955 =for Pod::Functions the current source line number
3957 A special token that compiles to the current line number.
3959 =item link OLDFILE,NEWFILE
3962 =for Pod::Functions create a hard link in the filesystem
3964 Creates a new filename linked to the old filename. Returns true for
3965 success, false otherwise.
3967 Portability issues: L<perlport/link>.
3969 =item listen SOCKET,QUEUESIZE
3972 =for Pod::Functions register your socket as a server
3974 Does the same thing that the L<listen(2)> system call does. Returns true if
3975 it succeeded, false otherwise. See the example in
3976 L<perlipc/"Sockets: Client/Server Communication">.
3981 =for Pod::Functions create a temporary value for a global variable (dynamic scoping)
3983 You really probably want to be using L<C<my>|/my VARLIST> instead,
3984 because L<C<local>|/local EXPR> isn't what most people think of as
3985 "local". See L<perlsub/"Private Variables via my()"> for details.
3987 A local modifies the listed variables to be local to the enclosing
3988 block, file, or eval. If more than one value is listed, the list must
3989 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
3990 for details, including issues with tied arrays and hashes.
3992 The C<delete local EXPR> construct can also be used to localize the deletion
3993 of array/hash elements to the current block.
3994 See L<perlsub/"Localized deletion of elements of composite types">.
3996 =item localtime EXPR
3997 X<localtime> X<ctime>
4001 =for Pod::Functions convert UNIX time into record or string using local time
4003 Converts a time as returned by the time function to a 9-element list
4004 with the time analyzed for the local time zone. Typically used as
4008 my ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
4011 All list elements are numeric and come straight out of the C `struct
4012 tm'. C<$sec>, C<$min>, and C<$hour> are the seconds, minutes, and hours
4013 of the specified time.
4015 C<$mday> is the day of the month and C<$mon> the month in
4016 the range C<0..11>, with 0 indicating January and 11 indicating December.
4017 This makes it easy to get a month name from a list:
4019 my @abbr = qw(Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec);
4020 print "$abbr[$mon] $mday";
4021 # $mon=9, $mday=18 gives "Oct 18"
4023 C<$year> contains the number of years since 1900. To get a 4-digit
4028 To get the last two digits of the year (e.g., "01" in 2001) do:
4030 $year = sprintf("%02d", $year % 100);
4032 C<$wday> is the day of the week, with 0 indicating Sunday and 3 indicating
4033 Wednesday. C<$yday> is the day of the year, in the range C<0..364>
4034 (or C<0..365> in leap years.)
4036 C<$isdst> is true if the specified time occurs during Daylight Saving
4037 Time, false otherwise.
4039 If EXPR is omitted, L<C<localtime>|/localtime EXPR> uses the current
4040 time (as returned by L<C<time>|/time>).
4042 In scalar context, L<C<localtime>|/localtime EXPR> returns the
4045 my $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
4047 The format of this scalar value is B<not> locale-dependent but built
4048 into Perl. For GMT instead of local time use the
4049 L<C<gmtime>|/gmtime EXPR> builtin. See also the
4050 L<C<Time::Local>|Time::Local> module (for converting seconds, minutes,
4051 hours, and such back to the integer value returned by L<C<time>|/time>),
4052 and the L<POSIX> module's L<C<strftime>|POSIX/C<strftime>> and
4053 L<C<mktime>|POSIX/C<mktime>> functions.
4055 To get somewhat similar but locale-dependent date strings, set up your
4056 locale environment variables appropriately (please see L<perllocale>) and
4059 use POSIX qw(strftime);
4060 my $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
4061 # or for GMT formatted appropriately for your locale:
4062 my $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
4064 Note that C<%a> and C<%b>, the short forms of the day of the week
4065 and the month of the year, may not necessarily be three characters wide.
4067 The L<Time::gmtime> and L<Time::localtime> modules provide a convenient,
4068 by-name access mechanism to the L<C<gmtime>|/gmtime EXPR> and
4069 L<C<localtime>|/localtime EXPR> functions, respectively.
4071 For a comprehensive date and time representation look at the
4072 L<DateTime> module on CPAN.
4074 Portability issues: L<perlport/localtime>.
4079 =for Pod::Functions +5.005 get a thread lock on a variable, subroutine, or method
4081 This function places an advisory lock on a shared variable or referenced
4082 object contained in I<THING> until the lock goes out of scope.
4084 The value returned is the scalar itself, if the argument is a scalar, or a
4085 reference, if the argument is a hash, array or subroutine.
4087 L<C<lock>|/lock THING> is a "weak keyword"; this means that if you've
4089 by this name (before any calls to it), that function will be called
4090 instead. If you are not under C<use threads::shared> this does nothing.
4091 See L<threads::shared>.
4094 X<log> X<logarithm> X<e> X<ln> X<base>
4098 =for Pod::Functions retrieve the natural logarithm for a number
4100 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
4101 returns the log of L<C<$_>|perlvar/$_>. To get the
4102 log of another base, use basic algebra:
4103 The base-N log of a number is equal to the natural log of that number
4104 divided by the natural log of N. For example:
4108 return log($n)/log(10);
4111 See also L<C<exp>|/exp EXPR> for the inverse operation.
4113 =item lstat FILEHANDLE
4118 =item lstat DIRHANDLE
4122 =for Pod::Functions stat a symbolic link
4124 Does the same thing as the L<C<stat>|/stat FILEHANDLE> function
4125 (including setting the special C<_> filehandle) but stats a symbolic
4126 link instead of the file the symbolic link points to. If symbolic links
4127 are unimplemented on your system, a normal L<C<stat>|/stat FILEHANDLE>
4128 is done. For much more detailed information, please see the
4129 documentation for L<C<stat>|/stat FILEHANDLE>.
4131 If EXPR is omitted, stats L<C<$_>|perlvar/$_>.
4133 Portability issues: L<perlport/lstat>.
4137 =for Pod::Functions match a string with a regular expression pattern
4139 The match operator. See L<perlop/"Regexp Quote-Like Operators">.
4141 =item map BLOCK LIST
4146 =for Pod::Functions apply a change to a list to get back a new list with the changes
4148 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
4149 L<C<$_>|perlvar/$_> to each element) and composes a list of the results of
4150 each such evaluation. Each element of LIST may produce zero, one, or more
4151 elements in the generated list, so the number of elements in the generated
4152 list may differ from that in LIST. In scalar context, returns the total
4153 number of elements so generated. In list context, returns the generated list.
4155 my @chars = map(chr, @numbers);
4157 translates a list of numbers to the corresponding characters.
4159 my @squares = map { $_ * $_ } @numbers;
4161 translates a list of numbers to their squared values.
4163 my @squares = map { $_ > 5 ? ($_ * $_) : () } @numbers;
4165 shows that number of returned elements can differ from the number of
4166 input elements. To omit an element, return an empty list ().
4167 This could also be achieved by writing
4169 my @squares = map { $_ * $_ } grep { $_ > 5 } @numbers;
4171 which makes the intention more clear.
4173 Map always returns a list, which can be
4174 assigned to a hash such that the elements
4175 become key/value pairs. See L<perldata> for more details.
4177 my %hash = map { get_a_key_for($_) => $_ } @array;
4179 is just a funny way to write
4183 $hash{get_a_key_for($_)} = $_;
4186 Note that L<C<$_>|perlvar/$_> is an alias to the list value, so it can
4187 be used to modify the elements of the LIST. While this is useful and
4188 supported, it can cause bizarre results if the elements of LIST are not
4189 variables. Using a regular C<foreach> loop for this purpose would be
4190 clearer in most cases. See also L<C<grep>|/grep BLOCK LIST> for a
4191 list composed of those items of the original list for which the BLOCK
4192 or EXPR evaluates to true.
4194 C<{> starts both hash references and blocks, so C<map { ...> could be either
4195 the start of map BLOCK LIST or map EXPR, LIST. Because Perl doesn't look
4196 ahead for the closing C<}> it has to take a guess at which it's dealing with
4197 based on what it finds just after the
4198 C<{>. Usually it gets it right, but if it
4199 doesn't it won't realize something is wrong until it gets to the C<}> and
4200 encounters the missing (or unexpected) comma. The syntax error will be
4201 reported close to the C<}>, but you'll need to change something near the C<{>
4202 such as using a unary C<+> or semicolon to give Perl some help:
4204 my %hash = map { "\L$_" => 1 } @array # perl guesses EXPR. wrong
4205 my %hash = map { +"\L$_" => 1 } @array # perl guesses BLOCK. right
4206 my %hash = map {; "\L$_" => 1 } @array # this also works
4207 my %hash = map { ("\L$_" => 1) } @array # as does this
4208 my %hash = map { lc($_) => 1 } @array # and this.
4209 my %hash = map +( lc($_) => 1 ), @array # this is EXPR and works!
4211 my %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
4213 or to force an anon hash constructor use C<+{>:
4215 my @hashes = map +{ lc($_) => 1 }, @array # EXPR, so needs
4218 to get a list of anonymous hashes each with only one entry apiece.
4220 =item mkdir FILENAME,MODE
4221 X<mkdir> X<md> X<directory, create>
4223 =item mkdir FILENAME
4227 =for Pod::Functions create a directory
4229 Creates the directory specified by FILENAME, with permissions
4230 specified by MODE (as modified by L<C<umask>|/umask EXPR>). If it
4231 succeeds it returns true; otherwise it returns false and sets
4232 L<C<$!>|perlvar/$!> (errno).
4233 MODE defaults to 0777 if omitted, and FILENAME defaults
4234 to L<C<$_>|perlvar/$_> if omitted.
4236 In general, it is better to create directories with a permissive MODE
4237 and let the user modify that with their L<C<umask>|/umask EXPR> than it
4239 a restrictive MODE and give the user no way to be more permissive.
4240 The exceptions to this rule are when the file or directory should be
4241 kept private (mail files, for instance). The documentation for
4242 L<C<umask>|/umask EXPR> discusses the choice of MODE in more detail.
4244 Note that according to the POSIX 1003.1-1996 the FILENAME may have any
4245 number of trailing slashes. Some operating and filesystems do not get
4246 this right, so Perl automatically removes all trailing slashes to keep
4249 To recursively create a directory structure, look at
4250 the L<C<make_path>|File::Path/make_path( $dir1, $dir2, .... )> function
4251 of the L<File::Path> module.
4253 =item msgctl ID,CMD,ARG
4256 =for Pod::Functions SysV IPC message control operations
4258 Calls the System V IPC function L<msgctl(2)>. You'll probably have to say
4262 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
4263 then ARG must be a variable that will hold the returned C<msqid_ds>
4264 structure. Returns like L<C<ioctl>|/ioctl FILEHANDLE,FUNCTION,SCALAR>:
4265 the undefined value for error, C<"0 but true"> for zero, or the actual
4266 return value otherwise. See also L<perlipc/"SysV IPC"> and the
4267 documentation for L<C<IPC::SysV>|IPC::SysV> and
4268 L<C<IPC::Semaphore>|IPC::Semaphore>.
4270 Portability issues: L<perlport/msgctl>.
4272 =item msgget KEY,FLAGS
4275 =for Pod::Functions get SysV IPC message queue
4277 Calls the System V IPC function L<msgget(2)>. Returns the message queue
4278 id, or L<C<undef>|/undef EXPR> on error. See also L<perlipc/"SysV IPC">
4279 and the documentation for L<C<IPC::SysV>|IPC::SysV> and
4280 L<C<IPC::Msg>|IPC::Msg>.
4282 Portability issues: L<perlport/msgget>.
4284 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
4287 =for Pod::Functions receive a SysV IPC message from a message queue
4289 Calls the System V IPC function msgrcv to receive a message from
4290 message queue ID into variable VAR with a maximum message size of
4291 SIZE. Note that when a message is received, the message type as a
4292 native long integer will be the first thing in VAR, followed by the
4293 actual message. This packing may be opened with C<unpack("l! a*")>.
4294 Taints the variable. Returns true if successful, false
4295 on error. See also L<perlipc/"SysV IPC"> and the documentation for
4296 L<C<IPC::SysV>|IPC::SysV> and L<C<IPC::Msg>|IPC::Msg>.
4298 Portability issues: L<perlport/msgrcv>.
4300 =item msgsnd ID,MSG,FLAGS
4303 =for Pod::Functions send a SysV IPC message to a message queue
4305 Calls the System V IPC function msgsnd to send the message MSG to the
4306 message queue ID. MSG must begin with the native long integer message
4307 type, be followed by the length of the actual message, and then finally
4308 the message itself. This kind of packing can be achieved with
4309 C<pack("l! a*", $type, $message)>. Returns true if successful,
4310 false on error. See also L<perlipc/"SysV IPC"> and the documentation
4311 for L<C<IPC::SysV>|IPC::SysV> and L<C<IPC::Msg>|IPC::Msg>.
4313 Portability issues: L<perlport/msgsnd>.
4318 =item my TYPE VARLIST
4320 =item my VARLIST : ATTRS
4322 =item my TYPE VARLIST : ATTRS
4324 =for Pod::Functions declare and assign a local variable (lexical scoping)
4326 A L<C<my>|/my VARLIST> declares the listed variables to be local
4327 (lexically) to the enclosing block, file, or L<C<eval>|/eval EXPR>. If
4328 more than one variable is listed, the list must be placed in
4331 The exact semantics and interface of TYPE and ATTRS are still
4332 evolving. TYPE may be a bareword, a constant declared
4333 with L<C<use constant>|constant>, or L<C<__PACKAGE__>|/__PACKAGE__>. It
4335 currently bound to the use of the L<fields> pragma,
4336 and attributes are handled using the L<attributes> pragma, or starting
4337 from Perl 5.8.0 also via the L<Attribute::Handlers> module. See
4338 L<perlsub/"Private Variables via my()"> for details.
4340 Note that with a parenthesised list, L<C<undef>|/undef EXPR> can be used
4341 as a dummy placeholder, for example to skip assignment of initial
4344 my ( undef, $min, $hour ) = localtime;
4353 =for Pod::Functions iterate a block prematurely
4355 The L<C<next>|/next LABEL> command is like the C<continue> statement in
4356 C; it starts the next iteration of the loop:
4358 LINE: while (<STDIN>) {
4359 next LINE if /^#/; # discard comments
4363 Note that if there were a L<C<continue>|/continue BLOCK> block on the
4365 executed even on discarded lines. If LABEL is omitted, the command
4366 refers to the innermost enclosing loop. The C<next EXPR> form, available
4367 as of Perl 5.18.0, allows a label name to be computed at run time, being
4368 otherwise identical to C<next LABEL>.
4370 L<C<next>|/next LABEL> cannot return a value from a block that typically
4371 returns a value, such as C<eval {}>, C<sub {}>, or C<do {}>. It will perform
4372 its flow control behavior, which precludes any return value. It should not be
4373 used to exit a L<C<grep>|/grep BLOCK LIST> or L<C<map>|/map BLOCK LIST>
4376 Note that a block by itself is semantically identical to a loop
4377 that executes once. Thus L<C<next>|/next LABEL> will exit such a block
4380 See also L<C<continue>|/continue BLOCK> for an illustration of how
4381 L<C<last>|/last LABEL>, L<C<next>|/next LABEL>, and
4382 L<C<redo>|/redo LABEL> work.
4384 Unlike most named operators, this has the same precedence as assignment.
4385 It is also exempt from the looks-like-a-function rule, so
4386 C<next ("foo")."bar"> will cause "bar" to be part of the argument to
4387 L<C<next>|/next LABEL>.
4389 =item no MODULE VERSION LIST
4393 =item no MODULE VERSION
4395 =item no MODULE LIST
4401 =for Pod::Functions unimport some module symbols or semantics at compile time
4403 See the L<C<use>|/use Module VERSION LIST> function, of which
4404 L<C<no>|/no MODULE VERSION LIST> is the opposite.
4407 X<oct> X<octal> X<hex> X<hexadecimal> X<binary> X<bin>
4411 =for Pod::Functions convert a string to an octal number
4413 Interprets EXPR as an octal string and returns the corresponding
4414 value. (If EXPR happens to start off with C<0x>, interprets it as a
4415 hex string. If EXPR starts off with C<0b>, it is interpreted as a
4416 binary string. Leading whitespace is ignored in all three cases.)
4417 The following will handle decimal, binary, octal, and hex in standard
4420 $val = oct($val) if $val =~ /^0/;
4422 If EXPR is omitted, uses L<C<$_>|perlvar/$_>. To go the other way
4423 (produce a number in octal), use L<C<sprintf>|/sprintf FORMAT, LIST> or
4424 L<C<printf>|/printf FILEHANDLE FORMAT, LIST>:
4426 my $dec_perms = (stat("filename"))[2] & 07777;
4427 my $oct_perm_str = sprintf "%o", $perms;
4429 The L<C<oct>|/oct EXPR> function is commonly used when a string such as
4431 to be converted into a file mode, for example. Although Perl
4432 automatically converts strings into numbers as needed, this automatic
4433 conversion assumes base 10.
4435 Leading white space is ignored without warning, as too are any trailing
4436 non-digits, such as a decimal point (L<C<oct>|/oct EXPR> only handles
4437 non-negative integers, not negative integers or floating point).
4439 =item open FILEHANDLE,EXPR
4440 X<open> X<pipe> X<file, open> X<fopen>
4442 =item open FILEHANDLE,MODE,EXPR
4444 =item open FILEHANDLE,MODE,EXPR,LIST
4446 =item open FILEHANDLE,MODE,REFERENCE
4448 =item open FILEHANDLE
4450 =for Pod::Functions open a file, pipe, or descriptor
4452 Opens the file whose filename is given by EXPR, and associates it with
4455 Simple examples to open a file for reading:
4457 open(my $fh, "<", "input.txt")
4458 or die "Can't open < input.txt: $!";
4462 open(my $fh, ">", "output.txt")
4463 or die "Can't open > output.txt: $!";
4465 (The following is a comprehensive reference to
4466 L<C<open>|/open FILEHANDLE,EXPR>: for a gentler introduction you may
4467 consider L<perlopentut>.)
4469 If FILEHANDLE is an undefined scalar variable (or array or hash element), a
4470 new filehandle is autovivified, meaning that the variable is assigned a
4471 reference to a newly allocated anonymous filehandle. Otherwise if
4472 FILEHANDLE is an expression, its value is the real filehandle. (This is
4473 considered a symbolic reference, so C<use strict "refs"> should I<not> be
4476 If three (or more) arguments are specified, the open mode (including
4477 optional encoding) in the second argument are distinct from the filename in
4478 the third. If MODE is C<< < >> or nothing, the file is opened for input.
4479 If MODE is C<< > >>, the file is opened for output, with existing files
4480 first being truncated ("clobbered") and nonexisting files newly created.
4481 If MODE is C<<< >> >>>, the file is opened for appending, again being
4482 created if necessary.
4484 You can put a C<+> in front of the C<< > >> or C<< < >> to
4485 indicate that you want both read and write access to the file; thus
4486 C<< +< >> is almost always preferred for read/write updates--the
4487 C<< +> >> mode would clobber the file first. You can't usually use
4488 either read-write mode for updating textfiles, since they have
4489 variable-length records. See the B<-i> switch in L<perlrun> for a
4490 better approach. The file is created with permissions of C<0666>
4491 modified by the process's L<C<umask>|/umask EXPR> value.
4493 These various prefixes correspond to the L<fopen(3)> modes of C<r>,
4494 C<r+>, C<w>, C<w+>, C<a>, and C<a+>.
4496 In the one- and two-argument forms of the call, the mode and filename
4497 should be concatenated (in that order), preferably separated by white
4498 space. You can--but shouldn't--omit the mode in these forms when that mode
4499 is C<< < >>. It is safe to use the two-argument form of
4500 L<C<open>|/open FILEHANDLE,EXPR> if the filename argument is a known literal.
4502 For three or more arguments if MODE is C<|->, the filename is
4503 interpreted as a command to which output is to be piped, and if MODE
4504 is C<-|>, the filename is interpreted as a command that pipes
4505 output to us. In the two-argument (and one-argument) form, one should
4506 replace dash (C<->) with the command.
4507 See L<perlipc/"Using open() for IPC"> for more examples of this.
4508 (You are not allowed to L<C<open>|/open FILEHANDLE,EXPR> to a command
4509 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>, and
4510 L<perlipc/"Bidirectional Communication with Another Process"> for
4513 In the form of pipe opens taking three or more arguments, if LIST is specified
4514 (extra arguments after the command name) then LIST becomes arguments
4515 to the command invoked if the platform supports it. The meaning of
4516 L<C<open>|/open FILEHANDLE,EXPR> with more than three arguments for
4517 non-pipe modes is not yet defined, but experimental "layers" may give
4518 extra LIST arguments meaning.
4520 In the two-argument (and one-argument) form, opening C<< <- >>
4521 or C<-> opens STDIN and opening C<< >- >> opens STDOUT.
4523 You may (and usually should) use the three-argument form of open to specify
4524 I/O layers (sometimes referred to as "disciplines") to apply to the handle
4525 that affect how the input and output are processed (see L<open> and
4526 L<PerlIO> for more details). For example:
4528 open(my $fh, "<:encoding(UTF-8)", $filename)
4529 || die "Can't open UTF-8 encoded $filename: $!";
4531 opens the UTF8-encoded file containing Unicode characters;
4532 see L<perluniintro>. Note that if layers are specified in the
4533 three-argument form, then default layers stored in ${^OPEN} (see L<perlvar>;
4534 usually set by the L<open> pragma or the switch C<-CioD>) are ignored.
4535 Those layers will also be ignored if you specify a colon with no name
4536 following it. In that case the default layer for the operating system
4537 (:raw on Unix, :crlf on Windows) is used.
4539 Open returns nonzero on success, the undefined value otherwise. If
4540 the L<C<open>|/open FILEHANDLE,EXPR> involved a pipe, the return value
4541 happens to be the pid of the subprocess.
4543 On some systems (in general, DOS- and Windows-based systems)
4544 L<C<binmode>|/binmode FILEHANDLE, LAYER> is necessary when you're not
4545 working with a text file. For the sake of portability it is a good idea
4546 always to use it when appropriate, and never to use it when it isn't
4547 appropriate. Also, people can set their I/O to be by default
4548 UTF8-encoded Unicode, not bytes.
4550 When opening a file, it's seldom a good idea to continue
4551 if the request failed, so L<C<open>|/open FILEHANDLE,EXPR> is frequently
4552 used with L<C<die>|/die LIST>. Even if L<C<die>|/die LIST> won't do
4553 what you want (say, in a CGI script,
4554 where you want to format a suitable error message (but there are
4555 modules that can help with that problem)) always check
4556 the return value from opening a file.
4558 The filehandle will be closed when its reference count reaches zero.
4559 If it is a lexically scoped variable declared with L<C<my>|/my VARLIST>,
4561 means the end of the enclosing scope. However, this automatic close
4562 does not check for errors, so it is better to explicitly close
4563 filehandles, especially those used for writing:
4566 || warn "close failed: $!";
4568 An older style is to use a bareword as the filehandle, as
4570 open(FH, "<", "input.txt")
4571 or die "Can't open < input.txt: $!";
4573 Then you can use C<FH> as the filehandle, in C<< close FH >> and C<<
4574 <FH> >> and so on. Note that it's a global variable, so this form is
4575 not recommended in new code.
4577 As a shortcut a one-argument call takes the filename from the global
4578 scalar variable of the same name as the filehandle:
4581 open(ARTICLE) or die "Can't find article $ARTICLE: $!\n";
4583 Here C<$ARTICLE> must be a global (package) scalar variable - not one
4584 declared with L<C<my>|/my VARLIST> or L<C<state>|/state VARLIST>.
4586 As a special case the three-argument form with a read/write mode and the third
4587 argument being L<C<undef>|/undef EXPR>:
4589 open(my $tmp, "+>", undef) or die ...
4591 opens a filehandle to a newly created empty anonymous temporary file.
4592 (This happens under any mode, which makes C<< +> >> the only useful and
4593 sensible mode to use.) You will need to
4594 L<C<seek>|/seek FILEHANDLE,POSITION,WHENCE> to do the reading.
4596 Perl is built using PerlIO by default. Unless you've
4597 changed this (such as building Perl with C<Configure -Uuseperlio>), you can
4598 open filehandles directly to Perl scalars via:
4600 open(my $fh, ">", \$variable) || ..
4602 To (re)open C<STDOUT> or C<STDERR> as an in-memory file, close it first:
4605 open(STDOUT, ">", \$variable)
4606 or die "Can't open STDOUT: $!";
4608 The scalars for in-memory files are treated as octet strings: unless
4609 the file is being opened with truncation the scalar may not contain
4610 any code points over 0xFF.
4612 Opening in-memory files I<can> fail for a variety of reasons. As with
4613 any other C<open>, check the return value for success.
4615 See L<perliol> for detailed info on PerlIO.
4619 open(my $log, ">>", "/usr/spool/news/twitlog");
4620 # if the open fails, output is discarded
4622 open(my $dbase, "+<", "dbase.mine") # open for update
4623 or die "Can't open 'dbase.mine' for update: $!";
4625 open(my $dbase, "+<dbase.mine") # ditto
4626 or die "Can't open 'dbase.mine' for update: $!";
4628 open(my $article_fh, "-|", "caesar <$article") # decrypt
4630 or die "Can't start caesar: $!";
4632 open(my $article_fh, "caesar <$article |") # ditto
4633 or die "Can't start caesar: $!";
4635 open(my $out_fh, "|-", "sort >Tmp$$") # $$ is our process id
4636 or die "Can't start sort: $!";
4639 open(my $memory, ">", \$var)
4640 or die "Can't open memory file: $!";
4641 print $memory "foo!\n"; # output will appear in $var
4643 You may also, in the Bourne shell tradition, specify an EXPR beginning
4644 with C<< >& >>, in which case the rest of the string is interpreted
4645 as the name of a filehandle (or file descriptor, if numeric) to be
4646 duped (as in L<dup(2)>) and opened. You may use C<&> after C<< > >>,
4647 C<<< >> >>>, C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>.
4648 The mode you specify should match the mode of the original filehandle.
4649 (Duping a filehandle does not take into account any existing contents
4650 of IO buffers.) If you use the three-argument
4651 form, then you can pass either a
4652 number, the name of a filehandle, or the normal "reference to a glob".
4654 Here is a script that saves, redirects, and restores C<STDOUT> and
4655 C<STDERR> using various methods:
4658 open(my $oldout, ">&STDOUT") or die "Can't dup STDOUT: $!";
4659 open(OLDERR, ">&", \*STDERR) or die "Can't dup STDERR: $!";
4661 open(STDOUT, '>', "foo.out") or die "Can't redirect STDOUT: $!";
4662 open(STDERR, ">&STDOUT") or die "Can't dup STDOUT: $!";
4664 select STDERR; $| = 1; # make unbuffered
4665 select STDOUT; $| = 1; # make unbuffered
4667 print STDOUT "stdout 1\n"; # this works for
4668 print STDERR "stderr 1\n"; # subprocesses too
4670 open(STDOUT, ">&", $oldout) or die "Can't dup \$oldout: $!";
4671 open(STDERR, ">&OLDERR") or die "Can't dup OLDERR: $!";
4673 print STDOUT "stdout 2\n";
4674 print STDERR "stderr 2\n";
4676 If you specify C<< '<&=X' >>, where C<X> is a file descriptor number
4677 or a filehandle, then Perl will do an equivalent of C's L<fdopen(3)> of
4678 that file descriptor (and not call L<dup(2)>); this is more
4679 parsimonious of file descriptors. For example:
4681 # open for input, reusing the fileno of $fd
4682 open(my $fh, "<&=", $fd)
4686 open(my $fh, "<&=$fd")
4690 # open for append, using the fileno of $oldfh
4691 open(my $fh, ">>&=", $oldfh)
4693 Being parsimonious on filehandles is also useful (besides being
4694 parsimonious) for example when something is dependent on file
4695 descriptors, like for example locking using
4696 L<C<flock>|/flock FILEHANDLE,OPERATION>. If you do just
4697 C<< open(my $A, ">>&", $B) >>, the filehandle C<$A> will not have the
4698 same file descriptor as C<$B>, and therefore C<flock($A)> will not
4699 C<flock($B)> nor vice versa. But with C<< open(my $A, ">>&=", $B) >>,
4700 the filehandles will share the same underlying system file descriptor.
4702 Note that under Perls older than 5.8.0, Perl uses the standard C library's'
4703 L<fdopen(3)> to implement the C<=> functionality. On many Unix systems,
4704 L<fdopen(3)> fails when file descriptors exceed a certain value, typically 255.
4705 For Perls 5.8.0 and later, PerlIO is (most often) the default.
4707 You can see whether your Perl was built with PerlIO by running
4708 C<perl -V:useperlio>. If it says C<'define'>, you have PerlIO;
4709 otherwise you don't.
4711 If you open a pipe on the command C<-> (that is, specify either C<|-> or C<-|>
4712 with the one- or two-argument forms of
4713 L<C<open>|/open FILEHANDLE,EXPR>), an implicit L<C<fork>|/fork> is done,
4714 so L<C<open>|/open FILEHANDLE,EXPR> returns twice: in the parent process
4716 of the child process, and in the child process it returns (a defined) C<0>.
4717 Use C<defined($pid)> or C<//> to determine whether the open was successful.
4719 For example, use either
4721 my $child_pid = open(my $from_kid, "-|") // die "Can't fork: $!";
4725 my $child_pid = open(my $to_kid, "|-") // die "Can't fork: $!";
4731 # either write $to_kid or else read $from_kid
4733 waitpid $child_pid, 0;
4735 # am the child; use STDIN/STDOUT normally
4740 The filehandle behaves normally for the parent, but I/O to that
4741 filehandle is piped from/to the STDOUT/STDIN of the child process.
4742 In the child process, the filehandle isn't opened--I/O happens from/to
4743 the new STDOUT/STDIN. Typically this is used like the normal
4744 piped open when you want to exercise more control over just how the
4745 pipe command gets executed, such as when running setuid and
4746 you don't want to have to scan shell commands for metacharacters.
4748 The following blocks are more or less equivalent:
4750 open(my $fh, "|tr '[a-z]' '[A-Z]'");
4751 open(my $fh, "|-", "tr '[a-z]' '[A-Z]'");
4752 open(my $fh, "|-") || exec 'tr', '[a-z]', '[A-Z]';
4753 open(my $fh, "|-", "tr", '[a-z]', '[A-Z]');
4755 open(my $fh, "cat -n '$file'|");
4756 open(my $fh, "-|", "cat -n '$file'");
4757 open(my $fh, "-|") || exec "cat", "-n", $file;
4758 open(my $fh, "-|", "cat", "-n", $file);
4760 The last two examples in each block show the pipe as "list form", which is
4761 not yet supported on all platforms. A good rule of thumb is that if
4762 your platform has a real L<C<fork>|/fork> (in other words, if your platform is
4763 Unix, including Linux and MacOS X), you can use the list form. You would
4764 want to use the list form of the pipe so you can pass literal arguments
4765 to the command without risk of the shell interpreting any shell metacharacters
4766 in them. However, this also bars you from opening pipes to commands
4767 that intentionally contain shell metacharacters, such as:
4769 open(my $fh, "|cat -n | expand -4 | lpr")
4770 || die "Can't open pipeline to lpr: $!";
4772 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
4774 Perl will attempt to flush all files opened for
4775 output before any operation that may do a fork, but this may not be
4776 supported on some platforms (see L<perlport>). To be safe, you may need
4777 to set L<C<$E<verbar>>|perlvar/$E<verbar>> (C<$AUTOFLUSH> in L<English>)
4778 or call the C<autoflush> method of L<C<IO::Handle>|IO::Handle/METHODS>
4779 on any open handles.
4781 On systems that support a close-on-exec flag on files, the flag will
4782 be set for the newly opened file descriptor as determined by the value
4783 of L<C<$^F>|perlvar/$^F>. See L<perlvar/$^F>.
4785 Closing any piped filehandle causes the parent process to wait for the
4786 child to finish, then returns the status value in L<C<$?>|perlvar/$?> and
4787 L<C<${^CHILD_ERROR_NATIVE}>|perlvar/${^CHILD_ERROR_NATIVE}>.
4789 The filename passed to the one- and two-argument forms of
4790 L<C<open>|/open FILEHANDLE,EXPR> will
4791 have leading and trailing whitespace deleted and normal
4792 redirection characters honored. This property, known as "magic open",
4793 can often be used to good effect. A user could specify a filename of
4794 F<"rsh cat file |">, or you could change certain filenames as needed:
4796 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
4797 open(my $fh, $filename) or die "Can't open $filename: $!";
4799 Use the three-argument form to open a file with arbitrary weird characters in it,
4801 open(my $fh, "<", $file)
4802 || die "Can't open $file: $!";
4804 otherwise it's necessary to protect any leading and trailing whitespace:
4806 $file =~ s#^(\s)#./$1#;
4807 open(my $fh, "< $file\0")
4808 || die "Can't open $file: $!";
4810 (this may not work on some bizarre filesystems). One should
4811 conscientiously choose between the I<magic> and I<three-argument> form
4812 of L<C<open>|/open FILEHANDLE,EXPR>:
4814 open(my $in, $ARGV[0]) || die "Can't open $ARGV[0]: $!";
4816 will allow the user to specify an argument of the form C<"rsh cat file |">,
4817 but will not work on a filename that happens to have a trailing space, while
4819 open(my $in, "<", $ARGV[0])
4820 || die "Can't open $ARGV[0]: $!";
4822 will have exactly the opposite restrictions. (However, some shells
4823 support the syntax C<< perl your_program.pl <( rsh cat file ) >>, which
4824 produces a filename that can be opened normally.)
4826 If you want a "real" C L<open(2)>, then you should use the
4827 L<C<sysopen>|/sysopen FILEHANDLE,FILENAME,MODE> function, which involves
4828 no such magic (but uses different filemodes than Perl
4829 L<C<open>|/open FILEHANDLE,EXPR>, which corresponds to C L<fopen(3)>).
4830 This is another way to protect your filenames from interpretation. For
4834 sysopen(my $fh, $path, O_RDWR|O_CREAT|O_EXCL)
4835 or die "Can't open $path: $!";
4837 print $fh "stuff $$\n";
4839 print "File contains: ", readline($fh);
4841 See L<C<seek>|/seek FILEHANDLE,POSITION,WHENCE> for some details about
4842 mixing reading and writing.
4844 Portability issues: L<perlport/open>.
4846 =item opendir DIRHANDLE,EXPR
4849 =for Pod::Functions open a directory
4851 Opens a directory named EXPR for processing by
4852 L<C<readdir>|/readdir DIRHANDLE>, L<C<telldir>|/telldir DIRHANDLE>,
4853 L<C<seekdir>|/seekdir DIRHANDLE,POS>,
4854 L<C<rewinddir>|/rewinddir DIRHANDLE>, and
4855 L<C<closedir>|/closedir DIRHANDLE>. Returns true if successful.
4856 DIRHANDLE may be an expression whose value can be used as an indirect
4857 dirhandle, usually the real dirhandle name. If DIRHANDLE is an undefined
4858 scalar variable (or array or hash element), the variable is assigned a
4859 reference to a new anonymous dirhandle; that is, it's autovivified.
4860 Dirhandles are the same objects as filehandles; an I/O object can only
4861 be open as one of these handle types at once.
4863 See the example at L<C<readdir>|/readdir DIRHANDLE>.
4870 =for Pod::Functions find a character's numeric representation
4872 Returns the numeric value of the first character of EXPR.
4873 If EXPR is an empty string, returns 0. If EXPR is omitted, uses
4874 L<C<$_>|perlvar/$_>.
4875 (Note I<character>, not byte.)
4877 For the reverse, see L<C<chr>|/chr NUMBER>.
4878 See L<perlunicode> for more about Unicode.
4883 =item our TYPE VARLIST
4885 =item our VARLIST : ATTRS
4887 =item our TYPE VARLIST : ATTRS
4889 =for Pod::Functions +5.6.0 declare and assign a package variable (lexical scoping)
4891 L<C<our>|/our VARLIST> makes a lexical alias to a package (i.e. global)
4892 variable of the same name in the current package for use within the
4893 current lexical scope.
4895 L<C<our>|/our VARLIST> has the same scoping rules as
4896 L<C<my>|/my VARLIST> or L<C<state>|/state VARLIST>, meaning that it is
4897 only valid within a lexical scope. Unlike L<C<my>|/my VARLIST> and
4898 L<C<state>|/state VARLIST>, which both declare new (lexical) variables,
4899 L<C<our>|/our VARLIST> only creates an alias to an existing variable: a
4900 package variable of the same name.
4902 This means that when C<use strict 'vars'> is in effect, L<C<our>|/our
4903 VARLIST> lets you use a package variable without qualifying it with the
4904 package name, but only within the lexical scope of the
4905 L<C<our>|/our VARLIST> declaration. This applies immediately--even
4906 within the same statement.
4914 our $foo; # alias to $Foo::foo
4915 print $foo; # prints 23
4918 print $Foo::foo; # prints 23
4920 print $foo; # ERROR: requires explicit package name
4922 This works even if the package variable has not been used before, as
4923 package variables spring into existence when first used.
4928 our $foo = 23; # just like $Foo::foo = 23
4930 print $Foo::foo; # prints 23
4932 Because the variable becomes legal immediately under C<use strict 'vars'>, so
4933 long as there is no variable with that name is already in scope, you can then
4934 reference the package variable again even within the same statement.
4939 my $foo = $foo; # error, undeclared $foo on right-hand side
4940 our $foo = $foo; # no errors
4942 If more than one variable is listed, the list must be placed
4947 An L<C<our>|/our VARLIST> declaration declares an alias for a package
4948 variable that will be visible
4949 across its entire lexical scope, even across package boundaries. The
4950 package in which the variable is entered is determined at the point
4951 of the declaration, not at the point of use. This means the following
4955 our $bar; # declares $Foo::bar for rest of lexical scope
4959 print $bar; # prints 20, as it refers to $Foo::bar
4961 Multiple L<C<our>|/our VARLIST> declarations with the same name in the
4963 scope are allowed if they are in different packages. If they happen
4964 to be in the same package, Perl will emit warnings if you have asked
4965 for them, just like multiple L<C<my>|/my VARLIST> declarations. Unlike
4966 a second L<C<my>|/my VARLIST> declaration, which will bind the name to a
4967 fresh variable, a second L<C<our>|/our VARLIST> declaration in the same
4968 package, in the same scope, is merely redundant.
4972 our $bar; # declares $Foo::bar for rest of lexical scope
4976 our $bar = 30; # declares $Bar::bar for rest of lexical scope
4977 print $bar; # prints 30
4979 our $bar; # emits warning but has no other effect
4980 print $bar; # still prints 30
4982 An L<C<our>|/our VARLIST> declaration may also have a list of attributes
4985 The exact semantics and interface of TYPE and ATTRS are still
4986 evolving. TYPE is currently bound to the use of the L<fields> pragma,
4987 and attributes are handled using the L<attributes> pragma, or, starting
4988 from Perl 5.8.0, also via the L<Attribute::Handlers> module. See
4989 L<perlsub/"Private Variables via my()"> for details.
4991 Note that with a parenthesised list, L<C<undef>|/undef EXPR> can be used
4992 as a dummy placeholder, for example to skip assignment of initial
4995 our ( undef, $min, $hour ) = localtime;
4997 L<C<our>|/our VARLIST> differs from L<C<use vars>|vars>, which allows
4998 use of an unqualified name I<only> within the affected package, but
5001 =item pack TEMPLATE,LIST
5004 =for Pod::Functions convert a list into a binary representation
5006 Takes a LIST of values and converts it into a string using the rules
5007 given by the TEMPLATE. The resulting string is the concatenation of
5008 the converted values. Typically, each converted value looks
5009 like its machine-level representation. For example, on 32-bit machines
5010 an integer may be represented by a sequence of 4 bytes, which will in
5011 Perl be presented as a string that's 4 characters long.
5013 See L<perlpacktut> for an introduction to this function.
5015 The TEMPLATE is a sequence of characters that give the order and type
5016 of values, as follows:
5018 a A string with arbitrary binary data, will be null padded.
5019 A A text (ASCII) string, will be space padded.
5020 Z A null-terminated (ASCIZ) string, will be null padded.
5022 b A bit string (ascending bit order inside each byte,
5024 B A bit string (descending bit order inside each byte).
5025 h A hex string (low nybble first).
5026 H A hex string (high nybble first).
5028 c A signed char (8-bit) value.
5029 C An unsigned char (octet) value.
5030 W An unsigned char value (can be greater than 255).
5032 s A signed short (16-bit) value.
5033 S An unsigned short value.
5035 l A signed long (32-bit) value.
5036 L An unsigned long value.
5038 q A signed quad (64-bit) value.
5039 Q An unsigned quad value.
5040 (Quads are available only if your system supports 64-bit
5041 integer values _and_ if Perl has been compiled to support
5042 those. Raises an exception otherwise.)
5044 i A signed integer value.
5045 I An unsigned integer value.
5046 (This 'integer' is _at_least_ 32 bits wide. Its exact
5047 size depends on what a local C compiler calls 'int'.)
5049 n An unsigned short (16-bit) in "network" (big-endian) order.
5050 N An unsigned long (32-bit) in "network" (big-endian) order.
5051 v An unsigned short (16-bit) in "VAX" (little-endian) order.
5052 V An unsigned long (32-bit) in "VAX" (little-endian) order.
5054 j A Perl internal signed integer value (IV).
5055 J A Perl internal unsigned integer value (UV).
5057 f A single-precision float in native format.
5058 d A double-precision float in native format.
5060 F A Perl internal floating-point value (NV) in native format
5061 D A float of long-double precision in native format.
5062 (Long doubles are available only if your system supports
5063 long double values _and_ if Perl has been compiled to
5064 support those. Raises an exception otherwise.
5065 Note that there are different long double formats.)
5067 p A pointer to a null-terminated string.
5068 P A pointer to a structure (fixed-length string).
5070 u A uuencoded string.
5071 U A Unicode character number. Encodes to a character in char-
5072 acter mode and UTF-8 (or UTF-EBCDIC in EBCDIC platforms) in
5075 w A BER compressed integer (not an ASN.1 BER, see perlpacktut
5076 for details). Its bytes represent an unsigned integer in
5077 base 128, most significant digit first, with as few digits
5078 as possible. Bit eight (the high bit) is set on each byte
5081 x A null byte (a.k.a ASCII NUL, "\000", chr(0))
5083 @ Null-fill or truncate to absolute position, counted from the
5084 start of the innermost ()-group.
5085 . Null-fill or truncate to absolute position specified by
5087 ( Start of a ()-group.
5089 One or more modifiers below may optionally follow certain letters in the
5090 TEMPLATE (the second column lists letters for which the modifier is valid):
5092 ! sSlLiI Forces native (short, long, int) sizes instead
5093 of fixed (16-/32-bit) sizes.
5095 ! xX Make x and X act as alignment commands.
5097 ! nNvV Treat integers as signed instead of unsigned.
5099 ! @. Specify position as byte offset in the internal
5100 representation of the packed string. Efficient
5103 > sSiIlLqQ Force big-endian byte-order on the type.
5104 jJfFdDpP (The "big end" touches the construct.)
5106 < sSiIlLqQ Force little-endian byte-order on the type.
5107 jJfFdDpP (The "little end" touches the construct.)
5109 The C<< > >> and C<< < >> modifiers can also be used on C<()> groups
5110 to force a particular byte-order on all components in that group,
5111 including all its subgroups.
5115 Larry recalls that the hex and bit string formats (H, h, B, b) were added to
5116 pack for processing data from NASA's Magellan probe. Magellan was in an
5117 elliptical orbit, using the antenna for the radar mapping when close to
5118 Venus and for communicating data back to Earth for the rest of the orbit.
5119 There were two transmission units, but one of these failed, and then the
5120 other developed a fault whereby it would randomly flip the sense of all the
5121 bits. It was easy to automatically detect complete records with the correct
5122 sense, and complete records with all the bits flipped. However, this didn't
5123 recover the records where the sense flipped midway. A colleague of Larry's
5124 was able to pretty much eyeball where the records flipped, so they wrote an
5125 editor named kybble (a pun on the dog food Kibbles 'n Bits) to enable him to
5126 manually correct the records and recover the data. For this purpose pack
5127 gained the hex and bit string format specifiers.
5129 git shows that they were added to perl 3.0 in patch #44 (Jan 1991, commit
5130 27e2fb84680b9cc1), but the patch description makes no mention of their
5131 addition, let alone the story behind them.
5135 The following rules apply:
5141 Each letter may optionally be followed by a number indicating the repeat
5142 count. A numeric repeat count may optionally be enclosed in brackets, as
5143 in C<pack("C[80]", @arr)>. The repeat count gobbles that many values from
5144 the LIST when used with all format types other than C<a>, C<A>, C<Z>, C<b>,
5145 C<B>, C<h>, C<H>, C<@>, C<.>, C<x>, C<X>, and C<P>, where it means
5146 something else, described below. Supplying a C<*> for the repeat count
5147 instead of a number means to use however many items are left, except for:
5153 C<@>, C<x>, and C<X>, where it is equivalent to C<0>.
5157 <.>, where it means relative to the start of the string.
5161 C<u>, where it is equivalent to 1 (or 45, which here is equivalent).
5165 One can replace a numeric repeat count with a template letter enclosed in
5166 brackets to use the packed byte length of the bracketed template for the
5169 For example, the template C<x[L]> skips as many bytes as in a packed long,
5170 and the template C<"$t X[$t] $t"> unpacks twice whatever $t (when
5171 variable-expanded) unpacks. If the template in brackets contains alignment
5172 commands (such as C<x![d]>), its packed length is calculated as if the
5173 start of the template had the maximal possible alignment.
5175 When used with C<Z>, a C<*> as the repeat count is guaranteed to add a
5176 trailing null byte, so the resulting string is always one byte longer than
5177 the byte length of the item itself.
5179 When used with C<@>, the repeat count represents an offset from the start
5180 of the innermost C<()> group.
5182 When used with C<.>, the repeat count determines the starting position to
5183 calculate the value offset as follows:
5189 If the repeat count is C<0>, it's relative to the current position.
5193 If the repeat count is C<*>, the offset is relative to the start of the
5198 And if it's an integer I<n>, the offset is relative to the start of the
5199 I<n>th innermost C<( )> group, or to the start of the string if I<n> is
5200 bigger then the group level.
5204 The repeat count for C<u> is interpreted as the maximal number of bytes
5205 to encode per line of output, with 0, 1 and 2 replaced by 45. The repeat
5206 count should not be more than 65.
5210 The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
5211 string of length count, padding with nulls or spaces as needed. When
5212 unpacking, C<A> strips trailing whitespace and nulls, C<Z> strips everything
5213 after the first null, and C<a> returns data with no stripping at all.
5215 If the value to pack is too long, the result is truncated. If it's too
5216 long and an explicit count is provided, C<Z> packs only C<$count-1> bytes,
5217 followed by a null byte. Thus C<Z> always packs a trailing null, except
5218 when the count is 0.
5222 Likewise, the C<b> and C<B> formats pack a string that's that many bits long.
5223 Each such format generates 1 bit of the result. These are typically followed
5224 by a repeat count like C<B8> or C<B64>.
5226 Each result bit is based on the least-significant bit of the corresponding
5227 input character, i.e., on C<ord($char)%2>. In particular, characters C<"0">
5228 and C<"1"> generate bits 0 and 1, as do characters C<"\000"> and C<"\001">.
5230 Starting from the beginning of the input string, each 8-tuple
5231 of characters is converted to 1 character of output. With format C<b>,
5232 the first character of the 8-tuple determines the least-significant bit of a
5233 character; with format C<B>, it determines the most-significant bit of
5236 If the length of the input string is not evenly divisible by 8, the
5237 remainder is packed as if the input string were padded by null characters
5238 at the end. Similarly during unpacking, "extra" bits are ignored.
5240 If the input string is longer than needed, remaining characters are ignored.
5242 A C<*> for the repeat count uses all characters of the input field.
5243 On unpacking, bits are converted to a string of C<0>s and C<1>s.
5247 The C<h> and C<H> formats pack a string that many nybbles (4-bit groups,
5248 representable as hexadecimal digits, C<"0".."9"> C<"a".."f">) long.
5250 For each such format, L<C<pack>|/pack TEMPLATE,LIST> generates 4 bits of result.
5251 With non-alphabetical characters, the result is based on the 4 least-significant
5252 bits of the input character, i.e., on C<ord($char)%16>. In particular,
5253 characters C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
5254 C<"\000"> and C<"\001">. For characters C<"a".."f"> and C<"A".."F">, the result
5255 is compatible with the usual hexadecimal digits, so that C<"a"> and
5256 C<"A"> both generate the nybble C<0xA==10>. Use only these specific hex
5257 characters with this format.
5259 Starting from the beginning of the template to
5260 L<C<pack>|/pack TEMPLATE,LIST>, each pair
5261 of characters is converted to 1 character of output. With format C<h>, the
5262 first character of the pair determines the least-significant nybble of the
5263 output character; with format C<H>, it determines the most-significant
5266 If the length of the input string is not even, it behaves as if padded by
5267 a null character at the end. Similarly, "extra" nybbles are ignored during
5270 If the input string is longer than needed, extra characters are ignored.
5272 A C<*> for the repeat count uses all characters of the input field. For
5273 L<C<unpack>|/unpack TEMPLATE,EXPR>, nybbles are converted to a string of
5278 The C<p> format packs a pointer to a null-terminated string. You are
5279 responsible for ensuring that the string is not a temporary value, as that
5280 could potentially get deallocated before you got around to using the packed
5281 result. The C<P> format packs a pointer to a structure of the size indicated
5282 by the length. A null pointer is created if the corresponding value for
5283 C<p> or C<P> is L<C<undef>|/undef EXPR>; similarly with
5284 L<C<unpack>|/unpack TEMPLATE,EXPR>, where a null pointer unpacks into
5285 L<C<undef>|/undef EXPR>.
5287 If your system has a strange pointer size--meaning a pointer is neither as
5288 big as an int nor as big as a long--it may not be possible to pack or
5289 unpack pointers in big- or little-endian byte order. Attempting to do
5290 so raises an exception.
5294 The C</> template character allows packing and unpacking of a sequence of
5295 items where the packed structure contains a packed item count followed by
5296 the packed items themselves. This is useful when the structure you're
5297 unpacking has encoded the sizes or repeat counts for some of its fields
5298 within the structure itself as separate fields.
5300 For L<C<pack>|/pack TEMPLATE,LIST>, you write
5301 I<length-item>C</>I<sequence-item>, and the
5302 I<length-item> describes how the length value is packed. Formats likely
5303 to be of most use are integer-packing ones like C<n> for Java strings,
5304 C<w> for ASN.1 or SNMP, and C<N> for Sun XDR.
5306 For L<C<pack>|/pack TEMPLATE,LIST>, I<sequence-item> may have a repeat
5307 count, in which case
5308 the minimum of that and the number of available items is used as the argument
5309 for I<length-item>. If it has no repeat count or uses a '*', the number
5310 of available items is used.
5312 For L<C<unpack>|/unpack TEMPLATE,EXPR>, an internal stack of integer
5313 arguments unpacked so far is
5314 used. You write C</>I<sequence-item> and the repeat count is obtained by
5315 popping off the last element from the stack. The I<sequence-item> must not
5316 have a repeat count.
5318 If I<sequence-item> refers to a string type (C<"A">, C<"a">, or C<"Z">),
5319 the I<length-item> is the string length, not the number of strings. With
5320 an explicit repeat count for pack, the packed string is adjusted to that
5321 length. For example:
5323 This code: gives this result:
5325 unpack("W/a", "\004Gurusamy") ("Guru")
5326 unpack("a3/A A*", "007 Bond J ") (" Bond", "J")
5327 unpack("a3 x2 /A A*", "007: Bond, J.") ("Bond, J", ".")
5329 pack("n/a* w/a","hello,","world") "\000\006hello,\005world"
5330 pack("a/W2", ord("a") .. ord("z")) "2ab"
5332 The I<length-item> is not returned explicitly from
5333 L<C<unpack>|/unpack TEMPLATE,EXPR>.
5335 Supplying a count to the I<length-item> format letter is only useful with
5336 C<A>, C<a>, or C<Z>. Packing with a I<length-item> of C<a> or C<Z> may
5337 introduce C<"\000"> characters, which Perl does not regard as legal in
5342 The integer types C<s>, C<S>, C<l>, and C<L> may be
5343 followed by a C<!> modifier to specify native shorts or
5344 longs. As shown in the example above, a bare C<l> means
5345 exactly 32 bits, although the native C<long> as seen by the local C compiler
5346 may be larger. This is mainly an issue on 64-bit platforms. You can
5347 see whether using C<!> makes any difference this way:
5349 printf "format s is %d, s! is %d\n",
5350 length pack("s"), length pack("s!");
5352 printf "format l is %d, l! is %d\n",
5353 length pack("l"), length pack("l!");
5356 C<i!> and C<I!> are also allowed, but only for completeness' sake:
5357 they are identical to C<i> and C<I>.
5359 The actual sizes (in bytes) of native shorts, ints, longs, and long
5360 longs on the platform where Perl was built are also available from
5363 $ perl -V:{short,int,long{,long}}size
5369 or programmatically via the L<C<Config>|Config> module:
5372 print $Config{shortsize}, "\n";
5373 print $Config{intsize}, "\n";
5374 print $Config{longsize}, "\n";
5375 print $Config{longlongsize}, "\n";
5377 C<$Config{longlongsize}> is undefined on systems without
5382 The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, C<L>, C<j>, and C<J> are
5383 inherently non-portable between processors and operating systems because
5384 they obey native byteorder and endianness. For example, a 4-byte integer
5385 0x12345678 (305419896 decimal) would be ordered natively (arranged in and
5386 handled by the CPU registers) into bytes as
5388 0x12 0x34 0x56 0x78 # big-endian
5389 0x78 0x56 0x34 0x12 # little-endian
5391 Basically, Intel and VAX CPUs are little-endian, while everybody else,
5392 including Motorola m68k/88k, PPC, Sparc, HP PA, Power, and Cray, are
5393 big-endian. Alpha and MIPS can be either: Digital/Compaq uses (well, used)
5394 them in little-endian mode, but SGI/Cray uses them in big-endian mode.
5396 The names I<big-endian> and I<little-endian> are comic references to the
5397 egg-eating habits of the little-endian Lilliputians and the big-endian
5398 Blefuscudians from the classic Jonathan Swift satire, I<Gulliver's Travels>.
5399 This entered computer lingo via the paper "On Holy Wars and a Plea for
5400 Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980.
5402 Some systems may have even weirder byte orders such as
5407 These are called mid-endian, middle-endian, mixed-endian, or just weird.
5409 You can determine your system endianness with this incantation:
5411 printf("%#02x ", $_) for unpack("W*", pack L=>0x12345678);
5413 The byteorder on the platform where Perl was built is also available
5417 print "$Config{byteorder}\n";
5419 or from the command line:
5423 Byteorders C<"1234"> and C<"12345678"> are little-endian; C<"4321">
5424 and C<"87654321"> are big-endian. Systems with multiarchitecture binaries
5425 will have C<"ffff">, signifying that static information doesn't work,
5426 one must use runtime probing.
5428 For portably packed integers, either use the formats C<n>, C<N>, C<v>,
5429 and C<V> or else use the C<< > >> and C<< < >> modifiers described
5430 immediately below. See also L<perlport>.
5434 Also floating point numbers have endianness. Usually (but not always)
5435 this agrees with the integer endianness. Even though most platforms
5436 these days use the IEEE 754 binary format, there are differences,
5437 especially if the long doubles are involved. You can see the
5438 C<Config> variables C<doublekind> and C<longdblkind> (also C<doublesize>,
5439 C<longdblsize>): the "kind" values are enums, unlike C<byteorder>.
5441 Portability-wise the best option is probably to keep to the IEEE 754
5442 64-bit doubles, and of agreed-upon endianness. Another possibility
5443 is the C<"%a">) format of L<C<printf>|/printf FILEHANDLE FORMAT, LIST>.
5447 Starting with Perl 5.10.0, integer and floating-point formats, along with
5448 the C<p> and C<P> formats and C<()> groups, may all be followed by the
5449 C<< > >> or C<< < >> endianness modifiers to respectively enforce big-
5450 or little-endian byte-order. These modifiers are especially useful
5451 given how C<n>, C<N>, C<v>, and C<V> don't cover signed integers,
5452 64-bit integers, or floating-point values.
5454 Here are some concerns to keep in mind when using an endianness modifier:
5460 Exchanging signed integers between different platforms works only
5461 when all platforms store them in the same format. Most platforms store
5462 signed integers in two's-complement notation, so usually this is not an issue.
5466 The C<< > >> or C<< < >> modifiers can only be used on floating-point
5467 formats on big- or little-endian machines. Otherwise, attempting to
5468 use them raises an exception.
5472 Forcing big- or little-endian byte-order on floating-point values for
5473 data exchange can work only if all platforms use the same
5474 binary representation such as IEEE floating-point. Even if all
5475 platforms are using IEEE, there may still be subtle differences. Being able
5476 to use C<< > >> or C<< < >> on floating-point values can be useful,
5477 but also dangerous if you don't know exactly what you're doing.
5478 It is not a general way to portably store floating-point values.
5482 When using C<< > >> or C<< < >> on a C<()> group, this affects
5483 all types inside the group that accept byte-order modifiers,
5484 including all subgroups. It is silently ignored for all other
5485 types. You are not allowed to override the byte-order within a group
5486 that already has a byte-order modifier suffix.
5492 Real numbers (floats and doubles) are in native machine format only.
5493 Due to the multiplicity of floating-point formats and the lack of a
5494 standard "network" representation for them, no facility for interchange has been
5495 made. This means that packed floating-point data written on one machine
5496 may not be readable on another, even if both use IEEE floating-point
5497 arithmetic (because the endianness of the memory representation is not part
5498 of the IEEE spec). See also L<perlport>.
5500 If you know I<exactly> what you're doing, you can use the C<< > >> or C<< < >>
5501 modifiers to force big- or little-endian byte-order on floating-point values.
5503 Because Perl uses doubles (or long doubles, if configured) internally for
5504 all numeric calculation, converting from double into float and thence
5505 to double again loses precision, so C<unpack("f", pack("f", $foo)>)
5506 will not in general equal $foo.
5510 Pack and unpack can operate in two modes: character mode (C<C0> mode) where
5511 the packed string is processed per character, and UTF-8 byte mode (C<U0> mode)
5512 where the packed string is processed in its UTF-8-encoded Unicode form on
5513 a byte-by-byte basis. Character mode is the default
5514 unless the format string starts with C<U>. You
5515 can always switch mode mid-format with an explicit
5516 C<C0> or C<U0> in the format. This mode remains in effect until the next
5517 mode change, or until the end of the C<()> group it (directly) applies to.
5519 Using C<C0> to get Unicode characters while using C<U0> to get I<non>-Unicode
5520 bytes is not necessarily obvious. Probably only the first of these
5523 $ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
5524 perl -CS -ne 'printf "%v04X\n", $_ for unpack("C0A*", $_)'
5526 $ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
5527 perl -CS -ne 'printf "%v02X\n", $_ for unpack("U0A*", $_)'
5529 $ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
5530 perl -C0 -ne 'printf "%v02X\n", $_ for unpack("C0A*", $_)'
5532 $ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
5533 perl -C0 -ne 'printf "%v02X\n", $_ for unpack("U0A*", $_)'
5534 C3.8E.C2.B1.C3.8F.C2.89
5536 Those examples also illustrate that you should not try to use
5537 L<C<pack>|/pack TEMPLATE,LIST>/L<C<unpack>|/unpack TEMPLATE,EXPR> as a
5538 substitute for the L<Encode> module.
5542 You must yourself do any alignment or padding by inserting, for example,
5543 enough C<"x">es while packing. There is no way for
5544 L<C<pack>|/pack TEMPLATE,LIST> and L<C<unpack>|/unpack TEMPLATE,EXPR>
5545 to know where characters are going to or coming from, so they
5546 handle their output and input as flat sequences of characters.
5550 A C<()> group is a sub-TEMPLATE enclosed in parentheses. A group may
5551 take a repeat count either as postfix, or for
5552 L<C<unpack>|/unpack TEMPLATE,EXPR>, also via the C</>
5553 template character. Within each repetition of a group, positioning with
5554 C<@> starts over at 0. Therefore, the result of
5556 pack("@1A((@2A)@3A)", qw[X Y Z])
5558 is the string C<"\0X\0\0YZ">.
5562 C<x> and C<X> accept the C<!> modifier to act as alignment commands: they
5563 jump forward or back to the closest position aligned at a multiple of C<count>
5564 characters. For example, to L<C<pack>|/pack TEMPLATE,LIST> or
5565 L<C<unpack>|/unpack TEMPLATE,EXPR> a C structure like
5568 char c; /* one signed, 8-bit character */
5573 one may need to use the template C<c x![d] d c[2]>. This assumes that
5574 doubles must be aligned to the size of double.
5576 For alignment commands, a C<count> of 0 is equivalent to a C<count> of 1;
5581 C<n>, C<N>, C<v> and C<V> accept the C<!> modifier to
5582 represent signed 16-/32-bit integers in big-/little-endian order.
5583 This is portable only when all platforms sharing packed data use the
5584 same binary representation for signed integers; for example, when all
5585 platforms use two's-complement representation.
5589 Comments can be embedded in a TEMPLATE using C<#> through the end of line.
5590 White space can separate pack codes from each other, but modifiers and
5591 repeat counts must follow immediately. Breaking complex templates into
5592 individual line-by-line components, suitably annotated, can do as much to
5593 improve legibility and maintainability of pack/unpack formats as C</x> can
5594 for complicated pattern matches.
5598 If TEMPLATE requires more arguments than L<C<pack>|/pack TEMPLATE,LIST>
5599 is given, L<C<pack>|/pack TEMPLATE,LIST>
5600 assumes additional C<""> arguments. If TEMPLATE requires fewer arguments
5601 than given, extra arguments are ignored.
5605 Attempting to pack the special floating point values C<Inf> and C<NaN>
5606 (infinity, also in negative, and not-a-number) into packed integer values
5607 (like C<"L">) is a fatal error. The reason for this is that there simply
5608 isn't any sensible mapping for these special values into integers.
5614 $foo = pack("WWWW",65,66,67,68);
5616 $foo = pack("W4",65,66,67,68);
5618 $foo = pack("W4",0x24b6,0x24b7,0x24b8,0x24b9);
5619 # same thing with Unicode circled letters.
5620 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
5621 # same thing with Unicode circled letters. You don't get the
5622 # UTF-8 bytes because the U at the start of the format caused
5623 # a switch to U0-mode, so the UTF-8 bytes get joined into
5625 $foo = pack("C0U4",0x24b6,0x24b7,0x24b8,0x24b9);
5626 # foo eq "\xe2\x92\xb6\xe2\x92\xb7\xe2\x92\xb8\xe2\x92\xb9"
5627 # This is the UTF-8 encoding of the string in the
5630 $foo = pack("ccxxcc",65,66,67,68);
5633 # NOTE: The examples above featuring "W" and "c" are true
5634 # only on ASCII and ASCII-derived systems such as ISO Latin 1
5635 # and UTF-8. On EBCDIC systems, the first example would be
5636 # $foo = pack("WWWW",193,194,195,196);
5638 $foo = pack("s2",1,2);
5639 # "\001\000\002\000" on little-endian
5640 # "\000\001\000\002" on big-endian
5642 $foo = pack("a4","abcd","x","y","z");
5645 $foo = pack("aaaa","abcd","x","y","z");
5648 $foo = pack("a14","abcdefg");
5649 # "abcdefg\0\0\0\0\0\0\0"
5651 $foo = pack("i9pl", gmtime);
5652 # a real struct tm (on my system anyway)
5654 $utmp_template = "Z8 Z8 Z16 L";
5655 $utmp = pack($utmp_template, @utmp1);
5656 # a struct utmp (BSDish)
5658 @utmp2 = unpack($utmp_template, $utmp);
5659 # "@utmp1" eq "@utmp2"
5662 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
5665 $foo = pack('sx2l', 12, 34);
5666 # short 12, two zero bytes padding, long 34
5667 $bar = pack('s@4l', 12, 34);
5668 # short 12, zero fill to position 4, long 34
5670 $baz = pack('s.l', 12, 4, 34);
5671 # short 12, zero fill to position 4, long 34
5673 $foo = pack('nN', 42, 4711);
5674 # pack big-endian 16- and 32-bit unsigned integers
5675 $foo = pack('S>L>', 42, 4711);
5677 $foo = pack('s<l<', -42, 4711);
5678 # pack little-endian 16- and 32-bit signed integers
5679 $foo = pack('(sl)<', -42, 4711);
5682 The same template may generally also be used in
5683 L<C<unpack>|/unpack TEMPLATE,EXPR>.
5685 =item package NAMESPACE
5687 =item package NAMESPACE VERSION
5688 X<package> X<module> X<namespace> X<version>
5690 =item package NAMESPACE BLOCK
5692 =item package NAMESPACE VERSION BLOCK
5693 X<package> X<module> X<namespace> X<version>
5695 =for Pod::Functions declare a separate global namespace
5697 Declares the BLOCK or the rest of the compilation unit as being in the
5698 given namespace. The scope of the package declaration is either the
5699 supplied code BLOCK or, in the absence of a BLOCK, from the declaration
5700 itself through the end of current scope (the enclosing block, file, or
5701 L<C<eval>|/eval EXPR>). That is, the forms without a BLOCK are
5702 operative through the end of the current scope, just like the
5703 L<C<my>|/my VARLIST>, L<C<state>|/state VARLIST>, and
5704 L<C<our>|/our VARLIST> operators. All unqualified dynamic identifiers
5705 in this scope will be in the given namespace, except where overridden by
5706 another L<C<package>|/package NAMESPACE> declaration or
5707 when they're one of the special identifiers that qualify into C<main::>,
5708 like C<STDOUT>, C<ARGV>, C<ENV>, and the punctuation variables.
5710 A package statement affects dynamic variables only, including those
5711 you've used L<C<local>|/local EXPR> on, but I<not> lexically-scoped
5712 variables, which are created with L<C<my>|/my VARLIST>,
5713 L<C<state>|/state VARLIST>, or L<C<our>|/our VARLIST>. Typically it
5714 would be the first declaration in a file included by
5715 L<C<require>|/require VERSION> or L<C<use>|/use Module VERSION LIST>.
5716 You can switch into a
5717 package in more than one place, since this only determines which default
5718 symbol table the compiler uses for the rest of that block. You can refer to
5719 identifiers in other packages than the current one by prefixing the identifier
5720 with the package name and a double colon, as in C<$SomePack::var>
5721 or C<ThatPack::INPUT_HANDLE>. If package name is omitted, the C<main>
5722 package as assumed. That is, C<$::sail> is equivalent to
5723 C<$main::sail> (as well as to C<$main'sail>, still seen in ancient
5724 code, mostly from Perl 4).
5726 If VERSION is provided, L<C<package>|/package NAMESPACE> sets the
5727 C<$VERSION> variable in the given
5728 namespace to a L<version> object with the VERSION provided. VERSION must be a
5729 "strict" style version number as defined by the L<version> module: a positive
5730 decimal number (integer or decimal-fraction) without exponentiation or else a
5731 dotted-decimal v-string with a leading 'v' character and at least three
5732 components. You should set C<$VERSION> only once per package.
5734 See L<perlmod/"Packages"> for more information about packages, modules,
5735 and classes. See L<perlsub> for other scoping issues.
5740 =for Pod::Functions +5.004 the current package
5742 A special token that returns the name of the package in which it occurs.
5744 =item pipe READHANDLE,WRITEHANDLE
5747 =for Pod::Functions open a pair of connected filehandles
5749 Opens a pair of connected pipes like the corresponding system call.
5750 Note that if you set up a loop of piped processes, deadlock can occur
5751 unless you are very careful. In addition, note that Perl's pipes use
5752 IO buffering, so you may need to set L<C<$E<verbar>>|perlvar/$E<verbar>>
5753 to flush your WRITEHANDLE after each command, depending on the
5756 Returns true on success.
5758 See L<IPC::Open2>, L<IPC::Open3>, and
5759 L<perlipc/"Bidirectional Communication with Another Process">
5760 for examples of such things.
5762 On systems that support a close-on-exec flag on files, that flag is set
5763 on all newly opened file descriptors whose
5764 L<C<fileno>|/fileno FILEHANDLE>s are I<higher> than the current value of
5765 L<C<$^F>|perlvar/$^F> (by default 2 for C<STDERR>). See L<perlvar/$^F>.
5772 =for Pod::Functions remove the last element from an array and return it
5774 Pops and returns the last value of the array, shortening the array by
5777 Returns the undefined value if the array is empty, although this may
5778 also happen at other times. If ARRAY is omitted, pops the
5779 L<C<@ARGV>|perlvar/@ARGV> array in the main program, but the
5780 L<C<@_>|perlvar/@_> array in subroutines, just like
5781 L<C<shift>|/shift ARRAY>.
5783 Starting with Perl 5.14, an experimental feature allowed
5784 L<C<pop>|/pop ARRAY> to take a
5785 scalar expression. This experiment has been deemed unsuccessful, and was
5786 removed as of Perl 5.24.
5789 X<pos> X<match, position>
5793 =for Pod::Functions find or set the offset for the last/next m//g search
5795 Returns the offset of where the last C<m//g> search left off for the
5796 variable in question (L<C<$_>|perlvar/$_> is used when the variable is not
5797 specified). This offset is in characters unless the
5798 (no-longer-recommended) L<C<use bytes>|bytes> pragma is in effect, in
5799 which case the offset is in bytes. Note that 0 is a valid match offset.
5800 L<C<undef>|/undef EXPR> indicates
5801 that the search position is reset (usually due to match failure, but
5802 can also be because no match has yet been run on the scalar).
5804 L<C<pos>|/pos SCALAR> directly accesses the location used by the regexp
5805 engine to store the offset, so assigning to L<C<pos>|/pos SCALAR> will
5806 change that offset, and so will also influence the C<\G> zero-width
5807 assertion in regular expressions. Both of these effects take place for
5808 the next match, so you can't affect the position with
5809 L<C<pos>|/pos SCALAR> during the current match, such as in
5810 C<(?{pos() = 5})> or C<s//pos() = 5/e>.
5812 Setting L<C<pos>|/pos SCALAR> also resets the I<matched with
5813 zero-length> flag, described
5814 under L<perlre/"Repeated Patterns Matching a Zero-length Substring">.
5816 Because a failed C<m//gc> match doesn't reset the offset, the return
5817 from L<C<pos>|/pos SCALAR> won't change either in this case. See
5818 L<perlre> and L<perlop>.
5820 =item print FILEHANDLE LIST
5823 =item print FILEHANDLE
5829 =for Pod::Functions output a list to a filehandle
5831 Prints a string or a list of strings. Returns true if successful.
5832 FILEHANDLE may be a scalar variable containing the name of or a reference
5833 to the filehandle, thus introducing one level of indirection. (NOTE: If
5834 FILEHANDLE is a variable and the next token is a term, it may be
5835 misinterpreted as an operator unless you interpose a C<+> or put
5836 parentheses around the arguments.) If FILEHANDLE is omitted, prints to the
5837 last selected (see L<C<select>|/select FILEHANDLE>) output handle. If
5838 LIST is omitted, prints L<C<$_>|perlvar/$_> to the currently selected
5839 output handle. To use FILEHANDLE alone to print the content of
5840 L<C<$_>|perlvar/$_> to it, you must use a bareword filehandle like
5841 C<FH>, not an indirect one like C<$fh>. To set the default output handle
5842 to something other than STDOUT, use the select operation.
5844 The current value of L<C<$,>|perlvar/$,> (if any) is printed between
5845 each LIST item. The current value of L<C<$\>|perlvar/$\> (if any) is
5846 printed after the entire LIST has been printed. Because print takes a
5847 LIST, anything in the LIST is evaluated in list context, including any
5848 subroutines whose return lists you pass to
5849 L<C<print>|/print FILEHANDLE LIST>. Be careful not to follow the print
5851 parenthesis unless you want the corresponding right parenthesis to
5852 terminate the arguments to the print; put parentheses around all arguments
5853 (or interpose a C<+>, but that doesn't look as good).
5855 If you're storing handles in an array or hash, or in general whenever
5856 you're using any expression more complex than a bareword handle or a plain,
5857 unsubscripted scalar variable to retrieve it, you will have to use a block
5858 returning the filehandle value instead, in which case the LIST may not be
5861 print { $files[$i] } "stuff\n";
5862 print { $OK ? *STDOUT : *STDERR } "stuff\n";
5864 Printing to a closed pipe or socket will generate a SIGPIPE signal. See
5865 L<perlipc> for more on signal handling.
5867 =item printf FILEHANDLE FORMAT, LIST
5870 =item printf FILEHANDLE
5872 =item printf FORMAT, LIST
5876 =for Pod::Functions output a formatted list to a filehandle
5878 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that
5879 L<C<$\>|perlvar/$\> (the output record separator) is not appended. The
5880 FORMAT and the LIST are actually parsed as a single list. The first
5881 argument of the list will be interpreted as the
5882 L<C<printf>|/printf FILEHANDLE FORMAT, LIST> format. This means that
5883 C<printf(@_)> will use C<$_[0]> as the format. See
5884 L<sprintf|/sprintf FORMAT, LIST> for an explanation of the format
5885 argument. If C<use locale> (including C<use locale ':not_characters'>)
5886 is in effect and L<C<POSIX::setlocale>|POSIX/C<setlocale>> has been
5887 called, the character used for the decimal separator in formatted
5888 floating-point numbers is affected by the C<LC_NUMERIC> locale setting.
5889 See L<perllocale> and L<POSIX>.
5891 For historical reasons, if you omit the list, L<C<$_>|perlvar/$_> is
5893 to use FILEHANDLE without a list, you must use a bareword filehandle like
5894 C<FH>, not an indirect one like C<$fh>. However, this will rarely do what
5895 you want; if L<C<$_>|perlvar/$_> contains formatting codes, they will be
5896 replaced with the empty string and a warning will be emitted if
5897 L<warnings> are enabled. Just use L<C<print>|/print FILEHANDLE LIST> if
5898 you want to print the contents of L<C<$_>|perlvar/$_>.
5900 Don't fall into the trap of using a
5901 L<C<printf>|/printf FILEHANDLE FORMAT, LIST> when a simple
5902 L<C<print>|/print FILEHANDLE LIST> would do. The
5903 L<C<print>|/print FILEHANDLE LIST> is more efficient and less error
5906 =item prototype FUNCTION
5911 =for Pod::Functions +5.002 get the prototype (if any) of a subroutine
5913 Returns the prototype of a function as a string (or
5914 L<C<undef>|/undef EXPR> if the
5915 function has no prototype). FUNCTION is a reference to, or the name of,
5916 the function whose prototype you want to retrieve. If FUNCTION is omitted,
5917 L<C<$_>|perlvar/$_> is used.
5919 If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
5920 name for a Perl builtin. If the builtin's arguments
5921 cannot be adequately expressed by a prototype
5922 (such as L<C<system>|/system LIST>), L<C<prototype>|/prototype FUNCTION>
5923 returns L<C<undef>|/undef EXPR>, because the builtin
5924 does not really behave like a Perl function. Otherwise, the string
5925 describing the equivalent prototype is returned.
5927 =item push ARRAY,LIST
5930 =for Pod::Functions append one or more elements to an array
5932 Treats ARRAY as a stack by appending the values of LIST to the end of
5933 ARRAY. The length of ARRAY increases by the length of LIST. Has the same
5936 for my $value (LIST) {
5937 $ARRAY[++$#ARRAY] = $value;
5940 but is more efficient. Returns the number of elements in the array following
5941 the completed L<C<push>|/push ARRAY,LIST>.
5943 Starting with Perl 5.14, an experimental feature allowed
5944 L<C<push>|/push ARRAY,LIST> to take a
5945 scalar expression. This experiment has been deemed unsuccessful, and was
5946 removed as of Perl 5.24.
5950 =for Pod::Functions singly quote a string
5954 =for Pod::Functions doubly quote a string
5958 =for Pod::Functions quote a list of words
5962 =for Pod::Functions backquote quote a string
5964 Generalized quotes. See L<perlop/"Quote-Like Operators">.
5968 =for Pod::Functions +5.005 compile pattern
5970 Regexp-like quote. See L<perlop/"Regexp Quote-Like Operators">.
5972 =item quotemeta EXPR
5973 X<quotemeta> X<metacharacter>
5977 =for Pod::Functions quote regular expression magic characters
5979 Returns the value of EXPR with all the ASCII non-"word"
5980 characters backslashed. (That is, all ASCII characters not matching
5981 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
5982 returned string, regardless of any locale settings.)
5983 This is the internal function implementing
5984 the C<\Q> escape in double-quoted strings.
5985 (See below for the behavior on non-ASCII code points.)
5987 If EXPR is omitted, uses L<C<$_>|perlvar/$_>.
5989 quotemeta (and C<\Q> ... C<\E>) are useful when interpolating strings into
5990 regular expressions, because by default an interpolated variable will be
5991 considered a mini-regular expression. For example:
5993 my $sentence = 'The quick brown fox jumped over the lazy dog';
5994 my $substring = 'quick.*?fox';
5995 $sentence =~ s{$substring}{big bad wolf};
5997 Will cause C<$sentence> to become C<'The big bad wolf jumped over...'>.
6001 my $sentence = 'The quick brown fox jumped over the lazy dog';
6002 my $substring = 'quick.*?fox';
6003 $sentence =~ s{\Q$substring\E}{big bad wolf};
6007 my $sentence = 'The quick brown fox jumped over the lazy dog';
6008 my $substring = 'quick.*?fox';
6009 my $quoted_substring = quotemeta($substring);
6010 $sentence =~ s{$quoted_substring}{big bad wolf};
6012 Will both leave the sentence as is.
6013 Normally, when accepting literal string input from the user,
6014 L<C<quotemeta>|/quotemeta EXPR> or C<\Q> must be used.
6016 In Perl v5.14, all non-ASCII characters are quoted in non-UTF-8-encoded
6017 strings, but not quoted in UTF-8 strings.
6019 Starting in Perl v5.16, Perl adopted a Unicode-defined strategy for
6020 quoting non-ASCII characters; the quoting of ASCII characters is
6023 Also unchanged is the quoting of non-UTF-8 strings when outside the
6025 L<C<use feature 'unicode_strings'>|feature/The 'unicode_strings' feature>,
6026 which is to quote all
6027 characters in the upper Latin1 range. This provides complete backwards
6028 compatibility for old programs which do not use Unicode. (Note that
6029 C<unicode_strings> is automatically enabled within the scope of a
6030 S<C<use v5.12>> or greater.)
6032 Within the scope of L<C<use locale>|locale>, all non-ASCII Latin1 code
6034 are quoted whether the string is encoded as UTF-8 or not. As mentioned
6035 above, locale does not affect the quoting of ASCII-range characters.
6036 This protects against those locales where characters such as C<"|"> are
6037 considered to be word characters.
6039 Otherwise, Perl quotes non-ASCII characters using an adaptation from
6040 Unicode (see L<https://www.unicode.org/reports/tr31/>).
6041 The only code points that are quoted are those that have any of the
6042 Unicode properties: Pattern_Syntax, Pattern_White_Space, White_Space,
6043 Default_Ignorable_Code_Point, or General_Category=Control.
6045 Of these properties, the two important ones are Pattern_Syntax and
6046 Pattern_White_Space. They have been set up by Unicode for exactly this
6047 purpose of deciding which characters in a regular expression pattern
6048 should be quoted. No character that can be in an identifier has these
6051 Perl promises, that if we ever add regular expression pattern
6052 metacharacters to the dozen already defined
6053 (C<\ E<verbar> ( ) [ { ^ $ * + ? .>), that we will only use ones that have the
6054 Pattern_Syntax property. Perl also promises, that if we ever add
6055 characters that are considered to be white space in regular expressions
6056 (currently mostly affected by C</x>), they will all have the
6057 Pattern_White_Space property.
6059 Unicode promises that the set of code points that have these two
6060 properties will never change, so something that is not quoted in v5.16
6061 will never need to be quoted in any future Perl release. (Not all the
6062 code points that match Pattern_Syntax have actually had characters
6063 assigned to them; so there is room to grow, but they are quoted
6064 whether assigned or not. Perl, of course, would never use an
6065 unassigned code point as an actual metacharacter.)
6067 Quoting characters that have the other 3 properties is done to enhance
6068 the readability of the regular expression and not because they actually
6069 need to be quoted for regular expression purposes (characters with the
6070 White_Space property are likely to be indistinguishable on the page or
6071 screen from those with the Pattern_White_Space property; and the other
6072 two properties contain non-printing characters).
6079 =for Pod::Functions retrieve the next pseudorandom number
6081 Returns a random fractional number greater than or equal to C<0> and less
6082 than the value of EXPR. (EXPR should be positive.) If EXPR is
6083 omitted, the value C<1> is used. Currently EXPR with the value C<0> is
6084 also special-cased as C<1> (this was undocumented before Perl 5.8.0
6085 and is subject to change in future versions of Perl). Automatically calls
6086 L<C<srand>|/srand EXPR> unless L<C<srand>|/srand EXPR> has already been
6087 called. See also L<C<srand>|/srand EXPR>.
6089 Apply L<C<int>|/int EXPR> to the value returned by L<C<rand>|/rand EXPR>
6090 if you want random integers instead of random fractional numbers. For
6095 returns a random integer between C<0> and C<9>, inclusive.
6097 (Note: If your rand function consistently returns numbers that are too
6098 large or too small, then your version of Perl was probably compiled
6099 with the wrong number of RANDBITS.)
6101 B<L<C<rand>|/rand EXPR> is not cryptographically secure. You should not rely
6102 on it in security-sensitive situations.> As of this writing, a
6103 number of third-party CPAN modules offer random number generators
6104 intended by their authors to be cryptographically secure,
6105 including: L<Data::Entropy>, L<Crypt::Random>, L<Math::Random::Secure>,
6106 and L<Math::TrulyRandom>.
6108 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
6109 X<read> X<file, read>
6111 =item read FILEHANDLE,SCALAR,LENGTH
6113 =for Pod::Functions fixed-length buffered input from a filehandle
6115 Attempts to read LENGTH I<characters> of data into variable SCALAR
6116 from the specified FILEHANDLE. Returns the number of characters
6117 actually read, C<0> at end of file, or undef if there was an error (in
6118 the latter case L<C<$!>|perlvar/$!> is also set). SCALAR will be grown
6120 so that the last character actually read is the last character of the
6121 scalar after the read.
6123 An OFFSET may be specified to place the read data at some place in the
6124 string other than the beginning. A negative OFFSET specifies
6125 placement at that many characters counting backwards from the end of
6126 the string. A positive OFFSET greater than the length of SCALAR
6127 results in the string being padded to the required size with C<"\0">
6128 bytes before the result of the read is appended.
6130 The call is implemented in terms of either Perl's or your system's native
6131 L<fread(3)> library function. To get a true L<read(2)> system call, see
6132 L<sysread|/sysread FILEHANDLE,SCALAR,LENGTH,OFFSET>.
6134 Note the I<characters>: depending on the status of the filehandle,
6135 either (8-bit) bytes or characters are read. By default, all
6136 filehandles operate on bytes, but for example if the filehandle has
6137 been opened with the C<:utf8> I/O layer (see
6138 L<C<open>|/open FILEHANDLE,EXPR>, and the L<open>
6139 pragma), the I/O will operate on UTF8-encoded Unicode
6140 characters, not bytes. Similarly for the C<:encoding> layer:
6141 in that case pretty much any characters can be read.
6143 =item readdir DIRHANDLE
6146 =for Pod::Functions get a directory from a directory handle
6148 Returns the next directory entry for a directory opened by
6149 L<C<opendir>|/opendir DIRHANDLE,EXPR>.
6150 If used in list context, returns all the rest of the entries in the
6151 directory. If there are no more entries, returns the undefined value in
6152 scalar context and the empty list in list context.
6154 If you're planning to filetest the return values out of a
6155 L<C<readdir>|/readdir DIRHANDLE>, you'd better prepend the directory in
6156 question. Otherwise, because we didn't L<C<chdir>|/chdir EXPR> there,
6157 it would have been testing the wrong file.
6159 opendir(my $dh, $some_dir) || die "Can't opendir $some_dir: $!";
6160 my @dots = grep { /^\./ && -f "$some_dir/$_" } readdir($dh);
6163 As of Perl 5.12 you can use a bare L<C<readdir>|/readdir DIRHANDLE> in a
6164 C<while> loop, which will set L<C<$_>|perlvar/$_> on every iteration.
6165 If either a C<readdir> expression or an explicit assignment of a
6166 C<readdir> expression to a scalar is used as a C<while>/C<for> condition,
6167 then the condition actually tests for definedness of the expression's
6168 value, not for its regular truth value.
6170 opendir(my $dh, $some_dir) || die "Can't open $some_dir: $!";
6171 while (readdir $dh) {
6172 print "$some_dir/$_\n";
6176 To avoid confusing would-be users of your code who are running earlier
6177 versions of Perl with mysterious failures, put this sort of thing at the
6178 top of your file to signal that your code will work I<only> on Perls of a
6181 use 5.012; # so readdir assigns to $_ in a lone while test
6186 X<readline> X<gets> X<fgets>
6188 =for Pod::Functions fetch a record from a file
6190 Reads from the filehandle whose typeglob is contained in EXPR (or from
6191 C<*ARGV> if EXPR is not provided). In scalar context, each call reads and
6192 returns the next line until end-of-file is reached, whereupon the
6193 subsequent call returns L<C<undef>|/undef EXPR>. In list context, reads
6194 until end-of-file is reached and returns a list of lines. Note that the
6195 notion of "line" used here is whatever you may have defined with
6196 L<C<$E<sol>>|perlvar/$E<sol>> (or C<$INPUT_RECORD_SEPARATOR> in
6197 L<English>). See L<perlvar/"$/">.
6199 When L<C<$E<sol>>|perlvar/$E<sol>> is set to L<C<undef>|/undef EXPR>,
6200 when L<C<readline>|/readline EXPR> is in scalar context (i.e., file
6201 slurp mode), and when an empty file is read, it returns C<''> the first
6202 time, followed by L<C<undef>|/undef EXPR> subsequently.
6204 This is the internal function implementing the C<< <EXPR> >>
6205 operator, but you can use it directly. The C<< <EXPR> >>
6206 operator is discussed in more detail in L<perlop/"I/O Operators">.
6209 my $line = readline(STDIN); # same thing
6211 If L<C<readline>|/readline EXPR> encounters an operating system error,
6212 L<C<$!>|perlvar/$!> will be set with the corresponding error message.
6213 It can be helpful to check L<C<$!>|perlvar/$!> when you are reading from
6214 filehandles you don't trust, such as a tty or a socket. The following
6215 example uses the operator form of L<C<readline>|/readline EXPR> and dies
6216 if the result is not defined.
6218 while ( ! eof($fh) ) {
6219 defined( $_ = readline $fh ) or die "readline failed: $!";
6223 Note that you have can't handle L<C<readline>|/readline EXPR> errors
6224 that way with the C<ARGV> filehandle. In that case, you have to open
6225 each element of L<C<@ARGV>|perlvar/@ARGV> yourself since
6226 L<C<eof>|/eof FILEHANDLE> handles C<ARGV> differently.
6228 foreach my $arg (@ARGV) {
6229 open(my $fh, $arg) or warn "Can't open $arg: $!";
6231 while ( ! eof($fh) ) {
6232 defined( $_ = readline $fh )
6233 or die "readline failed for $arg: $!";
6238 Like the C<< <EXPR> >> operator, if a C<readline> expression is
6239 used as the condition of a C<while> or C<for> loop, then it will be
6240 implicitly assigned to C<$_>. If either a C<readline> expression or
6241 an explicit assignment of a C<readline> expression to a scalar is used
6242 as a C<while>/C<for> condition, then the condition actually tests for
6243 definedness of the expression's value, not for its regular truth value.
6250 =for Pod::Functions determine where a symbolic link is pointing
6252 Returns the value of a symbolic link, if symbolic links are
6253 implemented. If not, raises an exception. If there is a system
6254 error, returns the undefined value and sets L<C<$!>|perlvar/$!> (errno).
6255 If EXPR is omitted, uses L<C<$_>|perlvar/$_>.
6257 Portability issues: L<perlport/readlink>.
6264 =for Pod::Functions execute a system command and collect standard output
6266 EXPR is executed as a system command.
6267 The collected standard output of the command is returned.
6268 In scalar context, it comes back as a single (potentially
6269 multi-line) string. In list context, returns a list of lines
6270 (however you've defined lines with L<C<$E<sol>>|perlvar/$E<sol>> (or
6271 C<$INPUT_RECORD_SEPARATOR> in L<English>)).
6272 This is the internal function implementing the C<qx/EXPR/>
6273 operator, but you can use it directly. The C<qx/EXPR/>
6274 operator is discussed in more detail in L<perlop/"I/O Operators">.
6275 If EXPR is omitted, uses L<C<$_>|perlvar/$_>.
6277 =item recv SOCKET,SCALAR,LENGTH,FLAGS
6280 =for Pod::Functions receive a message over a Socket
6282 Receives a message on a socket. Attempts to receive LENGTH characters
6283 of data into variable SCALAR from the specified SOCKET filehandle.
6284 SCALAR will be grown or shrunk to the length actually read. Takes the
6285 same flags as the system call of the same name. Returns the address
6286 of the sender if SOCKET's protocol supports this; returns an empty
6287 string otherwise. If there's an error, returns the undefined value.
6288 This call is actually implemented in terms of the L<recvfrom(2)> system call.
6289 See L<perlipc/"UDP: Message Passing"> for examples.
6291 Note that if the socket has been marked as C<:utf8>, C<recv> will
6292 throw an exception. The C<:encoding(...)> layer implicitly introduces
6293 the C<:utf8> layer. See L<C<binmode>|/binmode FILEHANDLE, LAYER>.
6302 =for Pod::Functions start this loop iteration over again
6304 The L<C<redo>|/redo LABEL> command restarts the loop block without
6305 evaluating the conditional again. The L<C<continue>|/continue BLOCK>
6306 block, if any, is not executed. If
6307 the LABEL is omitted, the command refers to the innermost enclosing
6308 loop. The C<redo EXPR> form, available starting in Perl 5.18.0, allows a
6309 label name to be computed at run time, and is otherwise identical to C<redo
6310 LABEL>. Programs that want to lie to themselves about what was just input
6311 normally use this command:
6313 # a simpleminded Pascal comment stripper
6314 # (warning: assumes no { or } in strings)
6315 LINE: while (<STDIN>) {
6316 while (s|({.*}.*){.*}|$1 |) {}
6321 if (/}/) { # end of comment?
6330 L<C<redo>|/redo LABEL> cannot return a value from a block that typically
6331 returns a value, such as C<eval {}>, C<sub {}>, or C<do {}>. It will perform
6332 its flow control behavior, which precludes any return value. It should not be
6333 used to exit a L<C<grep>|/grep BLOCK LIST> or L<C<map>|/map BLOCK LIST>
6336 Note that a block by itself is semantically identical to a loop
6337 that executes once. Thus L<C<redo>|/redo LABEL> inside such a block
6338 will effectively turn it into a looping construct.
6340 See also L<C<continue>|/continue BLOCK> for an illustration of how
6341 L<C<last>|/last LABEL>, L<C<next>|/next LABEL>, and
6342 L<C<redo>|/redo LABEL> work.
6344 Unlike most named operators, this has the same precedence as assignment.
6345 It is also exempt from the looks-like-a-function rule, so
6346 C<redo ("foo")."bar"> will cause "bar" to be part of the argument to
6347 L<C<redo>|/redo LABEL>.
6354 =for Pod::Functions find out the type of thing being referenced
6356 Examines the value of EXPR, expecting it to be a reference, and returns
6357 a string giving information about the reference and the type of referent.
6358 If EXPR is not specified, L<C<$_>|perlvar/$_> will be used.
6360 If the operand is not a reference, then the empty string will be returned.
6361 An empty string will only be returned in this situation. C<ref> is often
6362 useful to just test whether a value is a reference, which can be done
6363 by comparing the result to the empty string. It is a common mistake
6364 to use the result of C<ref> directly as a truth value: this goes wrong
6365 because C<0> (which is false) can be returned for a reference.
6367 If the operand is a reference to a blessed object, then the name of
6368 the class into which the referent is blessed will be returned. C<ref>
6369 doesn't care what the physical type of the referent is; blessing takes
6370 precedence over such concerns. Beware that exact comparison of C<ref>
6371 results against a class name doesn't perform a class membership test:
6372 a class's members also include objects blessed into subclasses, for
6373 which C<ref> will return the name of the subclass. Also beware that
6374 class names can clash with the built-in type names (described below).
6376 If the operand is a reference to an unblessed object, then the return
6377 value indicates the type of object. If the unblessed referent is not
6378 a scalar, then the return value will be one of the strings C<ARRAY>,
6379 C<HASH>, C<CODE>, C<FORMAT>, or C<IO>, indicating only which kind of
6380 object it is. If the unblessed referent is a scalar, then the return
6381 value will be one of the strings C<SCALAR>, C<VSTRING>, C<REF>, C<GLOB>,
6382 C<LVALUE>, or C<REGEXP>, depending on the kind of value the scalar
6383 currently has. But note that C<qr//> scalars are created already
6384 blessed, so C<ref qr/.../> will likely return C<Regexp>. Beware that
6385 these built-in type names can also be used as
6386 class names, so C<ref> returning one of these names doesn't unambiguously
6387 indicate that the referent is of the kind to which the name refers.
6389 The ambiguity between built-in type names and class names significantly
6390 limits the utility of C<ref>. For unambiguous information, use
6391 L<C<Scalar::Util::blessed()>|Scalar::Util/blessed> for information about
6392 blessing, and L<C<Scalar::Util::reftype()>|Scalar::Util/reftype> for
6393 information about physical types. Use L<the C<isa> method|UNIVERSAL/C<<
6394 $obj->isa( TYPE ) >>> for class membership tests, though one must be
6395 sure of blessedness before attempting a method call.
6397 See also L<perlref> and L<perlobj>.
6399 =item rename OLDNAME,NEWNAME
6400 X<rename> X<move> X<mv> X<ren>
6402 =for Pod::Functions change a filename
6404 Changes the name of a file; an existing file NEWNAME will be
6405 clobbered. Returns true for success, false otherwise.
6407 Behavior of this function varies wildly depending on your system
6408 implementation. For example, it will usually not work across file system
6409 boundaries, even though the system I<mv> command sometimes compensates
6410 for this. Other restrictions include whether it works on directories,
6411 open files, or pre-existing files. Check L<perlport> and either the
6412 L<rename(2)> manpage or equivalent system documentation for details.
6414 For a platform independent L<C<move>|File::Copy/move> function look at
6415 the L<File::Copy> module.
6417 Portability issues: L<perlport/rename>.
6419 =item require VERSION
6426 =for Pod::Functions load in external functions from a library at runtime
6428 Demands a version of Perl specified by VERSION, or demands some semantics
6429 specified by EXPR or by L<C<$_>|perlvar/$_> if EXPR is not supplied.
6431 VERSION may be either a literal such as v5.24.1, which will be
6432 compared to L<C<$^V>|perlvar/$^V> (or C<$PERL_VERSION> in L<English>),
6433 or a numeric argument of the form 5.024001, which will be compared to
6434 L<C<$]>|perlvar/$]>. An exception is raised if VERSION is greater than
6435 the version of the current Perl interpreter. Compare with
6436 L<C<use>|/use Module VERSION LIST>, which can do a similar check at
6439 Specifying VERSION as a numeric argument of the form 5.024001 should
6440 generally be avoided as older less readable syntax compared to
6441 v5.24.1. Before perl 5.8.0 (released in 2002), the more verbose numeric
6442 form was the only supported syntax, which is why you might see it in
6445 require v5.24.1; # run time version check
6446 require 5.24.1; # ditto
6447 require 5.024_001; # ditto; older syntax compatible
6450 Otherwise, L<C<require>|/require VERSION> demands that a library file be
6451 included if it hasn't already been included. The file is included via
6452 the do-FILE mechanism, which is essentially just a variety of
6453 L<C<eval>|/eval EXPR> with the
6454 caveat that lexical variables in the invoking script will be invisible
6455 to the included code. If it were implemented in pure Perl, it
6456 would have semantics similar to the following:
6462 my ($filename) = @_;
6463 if ( my $version = eval { version->parse($filename) } ) {
6464 if ( $version > $^V ) {
6465 my $vn = $version->normal;
6466 croak "Perl $vn required--this is only $^V, stopped";
6471 if (exists $INC{$filename}) {
6472 return 1 if $INC{$filename};
6473 croak "Compilation failed in require";
6476 foreach $prefix (@INC) {
6478 #... do other stuff - see text below ....
6480 # (see text below about possible appending of .pmc
6481 # suffix to $filename)
6482 my $realfilename = "$prefix/$filename";
6483 next if ! -e $realfilename || -d _ || -b _;
6484 $INC{$filename} = $realfilename;
6485 my $result = do($realfilename);
6486 # but run in caller's namespace
6488 if (!defined $result) {
6489 $INC{$filename} = undef;
6490 croak $@ ? "$@Compilation failed in require"
6491 : "Can't locate $filename: $!\n";
6494 delete $INC{$filename};
6495 croak "$filename did not return true value";
6500 croak "Can't locate $filename in \@INC ...";
6503 Note that the file will not be included twice under the same specified
6506 The file must return true as the last statement to indicate
6507 successful execution of any initialization code, so it's customary to
6508 end such a file with C<1;> unless you're sure it'll return true
6509 otherwise. But it's better just to put the C<1;>, in case you add more
6512 If EXPR is a bareword, L<C<require>|/require VERSION> assumes a F<.pm>
6513 extension and replaces C<::> with C</> in the filename for you,
6514 to make it easy to load standard modules. This form of loading of
6515 modules does not risk altering your namespace, however it will autovivify
6516 the stash for the required module.
6518 In other words, if you try this:
6520 require Foo::Bar; # a splendid bareword
6522 The require function will actually look for the F<Foo/Bar.pm> file in the
6523 directories specified in the L<C<@INC>|perlvar/@INC> array, and it will
6524 autovivify the C<Foo::Bar::> stash at compile time.
6526 But if you try this:
6528 my $class = 'Foo::Bar';
6529 require $class; # $class is not a bareword
6531 require "Foo::Bar"; # not a bareword because of the ""
6533 The require function will look for the F<Foo::Bar> file in the
6534 L<C<@INC>|perlvar/@INC> array and
6535 will complain about not finding F<Foo::Bar> there. In this case you can do:
6537 eval "require $class";
6541 require "Foo/Bar.pm";
6543 Neither of these forms will autovivify any stashes at compile time and
6544 only have run time effects.
6546 Now that you understand how L<C<require>|/require VERSION> looks for
6547 files with a bareword argument, there is a little extra functionality
6548 going on behind the scenes. Before L<C<require>|/require VERSION> looks
6549 for a F<.pm> extension, it will first look for a similar filename with a
6550 F<.pmc> extension. If this file is found, it will be loaded in place of
6551 any file ending in a F<.pm> extension. This applies to both the explicit
6552 C<require "Foo/Bar.pm";> form and the C<require Foo::Bar;> form.
6554 You can also insert hooks into the import facility by putting Perl code
6555 directly into the L<C<@INC>|perlvar/@INC> array. There are three forms
6556 of hooks: subroutine references, array references, and blessed objects.
6558 Subroutine references are the simplest case. When the inclusion system
6559 walks through L<C<@INC>|perlvar/@INC> and encounters a subroutine, this
6560 subroutine gets called with two parameters, the first a reference to
6561 itself, and the second the name of the file to be included (e.g.,
6562 F<Foo/Bar.pm>). The subroutine should return either nothing or else a
6563 list of up to four values in the following order:
6569 A reference to a scalar, containing any initial source code to prepend to
6570 the file or generator output.
6574 A filehandle, from which the file will be read.
6578 A reference to a subroutine. If there is no filehandle (previous item),
6579 then this subroutine is expected to generate one line of source code per
6580 call, writing the line into L<C<$_>|perlvar/$_> and returning 1, then
6581 finally at end of file returning 0. If there is a filehandle, then the
6582 subroutine will be called to act as a simple source filter, with the
6583 line as read in L<C<$_>|perlvar/$_>.
6584 Again, return 1 for each valid line, and 0 after all lines have been
6586 For historical reasons the subroutine will receive a meaningless argument
6587 (in fact always the numeric value zero) as C<$_[0]>.
6591 Optional state for the subroutine. The state is passed in as C<$_[1]>.
6595 If an empty list, L<C<undef>|/undef EXPR>, or nothing that matches the
6596 first 3 values above is returned, then L<C<require>|/require VERSION>
6597 looks at the remaining elements of L<C<@INC>|perlvar/@INC>.
6598 Note that this filehandle must be a real filehandle (strictly a typeglob
6599 or reference to a typeglob, whether blessed or unblessed); tied filehandles
6600 will be ignored and processing will stop there.
6602 If the hook is an array reference, its first element must be a subroutine
6603 reference. This subroutine is called as above, but the first parameter is
6604 the array reference. This lets you indirectly pass arguments to
6607 In other words, you can write:
6609 push @INC, \&my_sub;
6611 my ($coderef, $filename) = @_; # $coderef is \&my_sub
6617 push @INC, [ \&my_sub, $x, $y, ... ];
6619 my ($arrayref, $filename) = @_;
6620 # Retrieve $x, $y, ...
6621 my (undef, @parameters) = @$arrayref;
6625 If the hook is an object, it must provide an C<INC> method that will be
6626 called as above, the first parameter being the object itself. (Note that
6627 you must fully qualify the sub's name, as unqualified C<INC> is always forced
6628 into package C<main>.) Here is a typical code layout:
6634 my ($self, $filename) = @_;
6638 # In the main program
6639 push @INC, Foo->new(...);
6641 These hooks are also permitted to set the L<C<%INC>|perlvar/%INC> entry
6642 corresponding to the files they have loaded. See L<perlvar/%INC>.
6644 For a yet-more-powerful import facility, see
6645 L<C<use>|/use Module VERSION LIST> and L<perlmod>.
6652 =for Pod::Functions clear all variables of a given name
6654 Generally used in a L<C<continue>|/continue BLOCK> block at the end of a
6655 loop to clear variables and reset C<m?pattern?> searches so that they
6657 expression is interpreted as a list of single characters (hyphens
6658 allowed for ranges). All variables (scalars, arrays, and hashes)
6659 in the current package beginning with one of
6660 those letters are reset to their pristine state. If the expression is
6661 omitted, one-match searches (C<m?pattern?>) are reset to match again.
6662 Only resets variables or searches in the current package. Always returns
6665 reset 'X'; # reset all X variables
6666 reset 'a-z'; # reset lower case variables
6667 reset; # just reset m?one-time? searches
6669 Resetting C<"A-Z"> is not recommended because you'll wipe out your
6670 L<C<@ARGV>|perlvar/@ARGV> and L<C<@INC>|perlvar/@INC> arrays and your
6671 L<C<%ENV>|perlvar/%ENV> hash.
6673 Resets only package variables; lexical variables are unaffected, but
6674 they clean themselves up on scope exit anyway, so you'll probably want
6675 to use them instead. See L<C<my>|/my VARLIST>.
6682 =for Pod::Functions get out of a function early
6684 Returns from a subroutine, L<C<eval>|/eval EXPR>,
6685 L<C<do FILE>|/do EXPR>, L<C<sort>|/sort SUBNAME LIST> block or regex
6686 eval block (but not a L<C<grep>|/grep BLOCK LIST> or
6687 L<C<map>|/map BLOCK LIST> block) with the value
6688 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
6689 context, depending on how the return value will be used, and the context
6690 may vary from one execution to the next (see
6691 L<C<wantarray>|/wantarray>). If no EXPR
6692 is given, returns an empty list in list context, the undefined value in
6693 scalar context, and (of course) nothing at all in void context.
6695 (In the absence of an explicit L<C<return>|/return EXPR>, a subroutine,
6696 L<C<eval>|/eval EXPR>,
6697 or L<C<do FILE>|/do EXPR> automatically returns the value of the last expression
6700 Unlike most named operators, this is also exempt from the
6701 looks-like-a-function rule, so C<return ("foo")."bar"> will
6702 cause C<"bar"> to be part of the argument to L<C<return>|/return EXPR>.
6705 X<reverse> X<rev> X<invert>
6707 =for Pod::Functions flip a string or a list
6709 In list context, returns a list value consisting of the elements
6710 of LIST in the opposite order. In scalar context, concatenates the
6711 elements of LIST and returns a string value with all characters
6712 in the opposite order.
6714 print join(", ", reverse "world", "Hello"); # Hello, world
6716 print scalar reverse "dlrow ,", "olleH"; # Hello, world
6718 Used without arguments in scalar context, L<C<reverse>|/reverse LIST>
6719 reverses L<C<$_>|perlvar/$_>.
6721 $_ = "dlrow ,olleH";
6722 print reverse; # No output, list context
6723 print scalar reverse; # Hello, world
6725 Note that reversing an array to itself (as in C<@a = reverse @a>) will
6726 preserve non-existent elements whenever possible; i.e., for non-magical
6727 arrays or for tied arrays with C<EXISTS> and C<DELETE> methods.
6729 This operator is also handy for inverting a hash, although there are some
6730 caveats. If a value is duplicated in the original hash, only one of those
6731 can be represented as a key in the inverted hash. Also, this has to
6732 unwind one hash and build a whole new one, which may take some time
6733 on a large hash, such as from a DBM file.
6735 my %by_name = reverse %by_address; # Invert the hash
6737 =item rewinddir DIRHANDLE
6740 =for Pod::Functions reset directory handle
6742 Sets the current position to the beginning of the directory for the
6743 L<C<readdir>|/readdir DIRHANDLE> routine on DIRHANDLE.
6745 Portability issues: L<perlport/rewinddir>.
6747 =item rindex STR,SUBSTR,POSITION
6750 =item rindex STR,SUBSTR
6752 =for Pod::Functions right-to-left substring search
6754 Works just like L<C<index>|/index STR,SUBSTR,POSITION> except that it
6755 returns the position of the I<last>
6756 occurrence of SUBSTR in STR. If POSITION is specified, returns the
6757 last occurrence beginning at or before that position.
6759 =item rmdir FILENAME
6760 X<rmdir> X<rd> X<directory, remove>
6764 =for Pod::Functions remove a directory
6766 Deletes the directory specified by FILENAME if that directory is
6767 empty. If it succeeds it returns true; otherwise it returns false and
6768 sets L<C<$!>|perlvar/$!> (errno). If FILENAME is omitted, uses
6769 L<C<$_>|perlvar/$_>.
6771 To remove a directory tree recursively (C<rm -rf> on Unix) look at
6772 the L<C<rmtree>|File::Path/rmtree( $dir )> function of the L<File::Path>
6777 =for Pod::Functions replace a pattern with a string
6779 The substitution operator. See L<perlop/"Regexp Quote-Like Operators">.
6781 =item say FILEHANDLE LIST
6784 =item say FILEHANDLE
6790 =for Pod::Functions +say output a list to a filehandle, appending a newline
6792 Just like L<C<print>|/print FILEHANDLE LIST>, but implicitly appends a
6793 newline at the end of the LIST instead of any value L<C<$\>|perlvar/$\>
6794 might have. To use FILEHANDLE without a LIST to
6795 print the contents of L<C<$_>|perlvar/$_> to it, you must use a bareword
6796 filehandle like C<FH>, not an indirect one like C<$fh>.
6798 L<C<say>|/say FILEHANDLE LIST> is available only if the
6799 L<C<"say"> feature|feature/The 'say' feature> is enabled or if it is
6800 prefixed with C<CORE::>. The
6801 L<C<"say"> feature|feature/The 'say' feature> is enabled automatically
6802 with a C<use v5.10> (or higher) declaration in the current scope.
6805 X<scalar> X<context>
6807 =for Pod::Functions force a scalar context
6809 Forces EXPR to be interpreted in scalar context and returns the value
6812 my @counts = ( scalar @a, scalar @b, scalar @c );
6814 There is no equivalent operator to force an expression to
6815 be interpolated in list context because in practice, this is never
6816 needed. If you really wanted to do so, however, you could use
6817 the construction C<@{[ (some expression) ]}>, but usually a simple
6818 C<(some expression)> suffices.
6820 Because L<C<scalar>|/scalar EXPR> is a unary operator, if you
6822 parenthesized list for the EXPR, this behaves as a scalar comma expression,
6823 evaluating all but the last element in void context and returning the final
6824 element evaluated in scalar context. This is seldom what you want.
6826 The following single statement:
6828 print uc(scalar(foo(), $bar)), $baz;
6830 is the moral equivalent of these two:
6833 print(uc($bar), $baz);
6835 See L<perlop> for more details on unary operators and the comma operator,
6836 and L<perldata> for details on evaluating a hash in scalar contex.
6838 =item seek FILEHANDLE,POSITION,WHENCE
6839 X<seek> X<fseek> X<filehandle, position>
6841 =for Pod::Functions reposition file pointer for random-access I/O
6843 Sets FILEHANDLE's position, just like the L<fseek(3)> call of C C<stdio>.
6844 FILEHANDLE may be an expression whose value gives the name of the
6845 filehandle. The values for WHENCE are C<0> to set the new position
6846 I<in bytes> to POSITION; C<1> to set it to the current position plus
6847 POSITION; and C<2> to set it to EOF plus POSITION, typically
6848 negative. For WHENCE you may use the constants C<SEEK_SET>,
6849 C<SEEK_CUR>, and C<SEEK_END> (start of the file, current position, end
6850 of the file) from the L<Fcntl> module. Returns C<1> on success, false
6853 Note the emphasis on bytes: even if the filehandle has been set to operate
6854 on characters (for example using the C<:encoding(UTF-8)> I/O layer), the
6855 L<C<seek>|/seek FILEHANDLE,POSITION,WHENCE>,
6856 L<C<tell>|/tell FILEHANDLE>, and
6857 L<C<sysseek>|/sysseek FILEHANDLE,POSITION,WHENCE>
6858 family of functions use byte offsets, not character offsets,
6859 because seeking to a character offset would be very slow in a UTF-8 file.
6861 If you want to position the file for
6862 L<C<sysread>|/sysread FILEHANDLE,SCALAR,LENGTH,OFFSET> or
6863 L<C<syswrite>|/syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET>, don't use
6864 L<C<seek>|/seek FILEHANDLE,POSITION,WHENCE>, because buffering makes its
6865 effect on the file's read-write position unpredictable and non-portable.
6866 Use L<C<sysseek>|/sysseek FILEHANDLE,POSITION,WHENCE> instead.
6868 Due to the rules and rigors of ANSI C, on some systems you have to do a
6869 seek whenever you switch between reading and writing. Amongst other
6870 things, this may have the effect of calling stdio's L<clearerr(3)>.
6871 A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position:
6875 This is also useful for applications emulating C<tail -f>. Once you hit
6876 EOF on your read and then sleep for a while, you (probably) have to stick in a
6877 dummy L<C<seek>|/seek FILEHANDLE,POSITION,WHENCE> to reset things. The
6878 L<C<seek>|/seek FILEHANDLE,POSITION,WHENCE> doesn't change the position,
6879 but it I<does> clear the end-of-file condition on the handle, so that the
6880 next C<readline FILE> makes Perl try again to read something. (We hope.)
6882 If that doesn't work (some I/O implementations are particularly
6883 cantankerous), you might need something like this:
6886 for ($curpos = tell($fh); $_ = readline($fh);
6887 $curpos = tell($fh)) {
6888 # search for some stuff and put it into files
6890 sleep($for_a_while);
6891 seek($fh, $curpos, 0);
6894 =item seekdir DIRHANDLE,POS
6897 =for Pod::Functions reposition directory pointer
6899 Sets the current position for the L<C<readdir>|/readdir DIRHANDLE>
6900 routine on DIRHANDLE. POS must be a value returned by
6901 L<C<telldir>|/telldir DIRHANDLE>. L<C<seekdir>|/seekdir DIRHANDLE,POS>
6902 also has the same caveats about possible directory compaction as the
6903 corresponding system library routine.
6905 =item select FILEHANDLE
6906 X<select> X<filehandle, default>
6910 =for Pod::Functions reset default output or do I/O multiplexing
6912 Returns the currently selected filehandle. If FILEHANDLE is supplied,
6913 sets the new current default filehandle for output. This has two
6914 effects: first, a L<C<write>|/write FILEHANDLE> or a L<C<print>|/print
6915 FILEHANDLE LIST> without a filehandle
6916 default to this FILEHANDLE. Second, references to variables related to
6917 output will refer to this output channel.
6919 For example, to set the top-of-form format for more than one
6920 output channel, you might do the following:
6927 FILEHANDLE may be an expression whose value gives the name of the
6928 actual filehandle. Thus:
6930 my $oldfh = select(STDERR); $| = 1; select($oldfh);
6932 Some programmers may prefer to think of filehandles as objects with
6933 methods, preferring to write the last example as:
6935 STDERR->autoflush(1);
6937 (Prior to Perl version 5.14, you have to C<use IO::Handle;> explicitly
6940 Portability issues: L<perlport/select>.
6942 =item select RBITS,WBITS,EBITS,TIMEOUT
6945 This calls the L<select(2)> syscall with the bit masks specified, which
6946 can be constructed using L<C<fileno>|/fileno FILEHANDLE> and
6947 L<C<vec>|/vec EXPR,OFFSET,BITS>, along these lines:
6949 my $rin = my $win = my $ein = '';
6950 vec($rin, fileno(STDIN), 1) = 1;
6951 vec($win, fileno(STDOUT), 1) = 1;
6954 If you want to select on many filehandles, you may wish to write a
6955 subroutine like this:
6960 for my $fh (@fhlist) {
6961 vec($bits, fileno($fh), 1) = 1;
6965 my $rin = fhbits(\*STDIN, $tty, $mysock);
6969 my ($nfound, $timeleft) =
6970 select(my $rout = $rin, my $wout = $win, my $eout = $ein,
6973 or to block until something becomes ready just do this
6976 select(my $rout = $rin, my $wout = $win, my $eout = $ein, undef);
6978 Most systems do not bother to return anything useful in C<$timeleft>, so
6979 calling L<C<select>|/select RBITS,WBITS,EBITS,TIMEOUT> in scalar context
6980 just returns C<$nfound>.
6982 Any of the bit masks can also be L<C<undef>|/undef EXPR>. The timeout,
6984 in seconds, which may be fractional. Note: not all implementations are
6985 capable of returning the C<$timeleft>. If not, they always return
6986 C<$timeleft> equal to the supplied C<$timeout>.
6988 You can effect a sleep of 250 milliseconds this way:
6990 select(undef, undef, undef, 0.25);
6992 Note that whether L<C<select>|/select RBITS,WBITS,EBITS,TIMEOUT> gets
6993 restarted after signals (say, SIGALRM) is implementation-dependent. See
6994 also L<perlport> for notes on the portability of
6995 L<C<select>|/select RBITS,WBITS,EBITS,TIMEOUT>.
6997 On error, L<C<select>|/select RBITS,WBITS,EBITS,TIMEOUT> behaves just
6998 like L<select(2)>: it returns C<-1> and sets L<C<$!>|perlvar/$!>.
7000 On some Unixes, L<select(2)> may report a socket file descriptor as
7001 "ready for reading" even when no data is available, and thus any
7002 subsequent L<C<read>|/read FILEHANDLE,SCALAR,LENGTH,OFFSET> would block.
7003 This can be avoided if you always use C<O_NONBLOCK> on the socket. See
7004 L<select(2)> and L<fcntl(2)> for further details.
7006 The standard L<C<IO::Select>|IO::Select> module provides a
7007 user-friendlier interface to
7008 L<C<select>|/select RBITS,WBITS,EBITS,TIMEOUT>, mostly because it does
7009 all the bit-mask work for you.
7011 B<WARNING>: One should not attempt to mix buffered I/O (like
7012 L<C<read>|/read FILEHANDLE,SCALAR,LENGTH,OFFSET> or
7013 L<C<readline>|/readline EXPR>) with
7014 L<C<select>|/select RBITS,WBITS,EBITS,TIMEOUT>, except as permitted by
7015 POSIX, and even then only on POSIX systems. You have to use
7016 L<C<sysread>|/sysread FILEHANDLE,SCALAR,LENGTH,OFFSET> instead.
7018 Portability issues: L<perlport/select>.
7020 =item semctl ID,SEMNUM,CMD,ARG
7023 =for Pod::Functions SysV semaphore control operations
7025 Calls the System V IPC function L<semctl(2)>. You'll probably have to say
7029 first to get the correct constant definitions. If CMD is IPC_STAT or
7030 GETALL, then ARG must be a variable that will hold the returned
7031 semid_ds structure or semaphore value array. Returns like
7032 L<C<ioctl>|/ioctl FILEHANDLE,FUNCTION,SCALAR>:
7033 the undefined value for error, "C<0 but true>" for zero, or the actual
7034 return value otherwise. The ARG must consist of a vector of native
7035 short integers, which may be created with C<pack("s!",(0)x$nsem)>.
7036 See also L<perlipc/"SysV IPC"> and the documentation for
7037 L<C<IPC::SysV>|IPC::SysV> and L<C<IPC::Semaphore>|IPC::Semaphore>.
7039 Portability issues: L<perlport/semctl>.
7041 =item semget KEY,NSEMS,FLAGS
7044 =for Pod::Functions get set of SysV semaphores
7046 Calls the System V IPC function L<semget(2)>. Returns the semaphore id, or
7047 the undefined value on error. See also
7048 L<perlipc/"SysV IPC"> and the documentation for
7049 L<C<IPC::SysV>|IPC::SysV> and L<C<IPC::Semaphore>|IPC::Semaphore>.
7051 Portability issues: L<perlport/semget>.
7053 =item semop KEY,OPSTRING
7056 =for Pod::Functions SysV semaphore operations
7058 Calls the System V IPC function L<semop(2)> for semaphore operations
7059 such as signalling and waiting. OPSTRING must be a packed array of
7060 semop structures. Each semop structure can be generated with
7061 C<pack("s!3", $semnum, $semop, $semflag)>. The length of OPSTRING
7062 implies the number of semaphore operations. Returns true if
7063 successful, false on error. As an example, the
7064 following code waits on semaphore $semnum of semaphore id $semid:
7066 my $semop = pack("s!3", $semnum, -1, 0);
7067 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
7069 To signal the semaphore, replace C<-1> with C<1>. See also
7070 L<perlipc/"SysV IPC"> and the documentation for
7071 L<C<IPC::SysV>|IPC::SysV> and L<C<IPC::Semaphore>|IPC::Semaphore>.
7073 Portability issues: L<perlport/semop>.
7075 =item send SOCKET,MSG,FLAGS,TO
7078 =item send SOCKET,MSG,FLAGS
7080 =for Pod::Functions send a message over a socket
7082 Sends a message on a socket. Attempts to send the scalar MSG to the SOCKET
7083 filehandle. Takes the same flags as the system call of the same name. On
7084 unconnected sockets, you must specify a destination to I<send to>, in which
7085 case it does a L<sendto(2)> syscall. Returns the number of characters sent,
7086 or the undefined value on error. The L<sendmsg(2)> syscall is currently
7087 unimplemented. See L<perlipc/"UDP: Message Passing"> for examples.
7089 Note that if the socket has been marked as C<:utf8>, C<send> will
7090 throw an exception. The C<:encoding(...)> layer implicitly introduces
7091 the C<:utf8> layer. See L<C<binmode>|/binmode FILEHANDLE, LAYER>.
7093 =item setpgrp PID,PGRP
7096 =for Pod::Functions set the process group of a process
7098 Sets the current process group for the specified PID, C<0> for the current
7099 process. Raises an exception when used on a machine that doesn't
7100 implement POSIX L<setpgid(2)> or BSD L<setpgrp(2)>. If the arguments
7101 are omitted, it defaults to C<0,0>. Note that the BSD 4.2 version of
7102 L<C<setpgrp>|/setpgrp PID,PGRP> does not accept any arguments, so only
7103 C<setpgrp(0,0)> is portable. See also
7104 L<C<POSIX::setsid()>|POSIX/C<setsid>>.
7106 Portability issues: L<perlport/setpgrp>.
7108 =item setpriority WHICH,WHO,PRIORITY
7109 X<setpriority> X<priority> X<nice> X<renice>
7111 =for Pod::Functions set a process's nice value
7113 Sets the current priority for a process, a process group, or a user.
7114 (See L<setpriority(2)>.) Raises an exception when used on a machine
7115 that doesn't implement L<setpriority(2)>.
7117 C<WHICH> can be any of C<PRIO_PROCESS>, C<PRIO_PGRP> or C<PRIO_USER>
7118 imported from L<POSIX/RESOURCE CONSTANTS>.
7120 Portability issues: L<perlport/setpriority>.
7122 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
7125 =for Pod::Functions set some socket options
7127 Sets the socket option requested. Returns L<C<undef>|/undef EXPR> on
7128 error. Use integer constants provided by the L<C<Socket>|Socket> module
7130 LEVEL and OPNAME. Values for LEVEL can also be obtained from
7131 getprotobyname. OPTVAL might either be a packed string or an integer.
7132 An integer OPTVAL is shorthand for pack("i", OPTVAL).
7134 An example disabling Nagle's algorithm on a socket:
7136 use Socket qw(IPPROTO_TCP TCP_NODELAY);
7137 setsockopt($socket, IPPROTO_TCP, TCP_NODELAY, 1);
7139 Portability issues: L<perlport/setsockopt>.
7146 =for Pod::Functions remove the first element of an array, and return it
7148 Shifts the first value of the array off and returns it, shortening the
7149 array by 1 and moving everything down. If there are no elements in the
7150 array, returns the undefined value. If ARRAY is omitted, shifts the
7151 L<C<@_>|perlvar/@_> array within the lexical scope of subroutines and
7152 formats, and the L<C<@ARGV>|perlvar/@ARGV> array outside a subroutine
7153 and also within the lexical scopes
7154 established by the C<eval STRING>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>,
7155 C<UNITCHECK {}>, and C<END {}> constructs.
7157 Starting with Perl 5.14, an experimental feature allowed
7158 L<C<shift>|/shift ARRAY> to take a
7159 scalar expression. This experiment has been deemed unsuccessful, and was
7160 removed as of Perl 5.24.
7162 See also L<C<unshift>|/unshift ARRAY,LIST>, L<C<push>|/push ARRAY,LIST>,
7163 and L<C<pop>|/pop ARRAY>. L<C<shift>|/shift ARRAY> and
7164 L<C<unshift>|/unshift ARRAY,LIST> do the same thing to the left end of
7165 an array that L<C<pop>|/pop ARRAY> and L<C<push>|/push ARRAY,LIST> do to
7168 =item shmctl ID,CMD,ARG
7171 =for Pod::Functions SysV shared memory operations
7173 Calls the System V IPC function shmctl. You'll probably have to say
7177 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
7178 then ARG must be a variable that will hold the returned C<shmid_ds>
7179 structure. Returns like ioctl: L<C<undef>|/undef EXPR> for error; "C<0>
7180 but true" for zero; and the actual return value otherwise.
7181 See also L<perlipc/"SysV IPC"> and the documentation for
7182 L<C<IPC::SysV>|IPC::SysV>.
7184 Portability issues: L<perlport/shmctl>.
7186 =item shmget KEY,SIZE,FLAGS
7189 =for Pod::Functions get SysV shared memory segment identifier
7191 Calls the System V IPC function shmget. Returns the shared memory
7192 segment id, or L<C<undef>|/undef EXPR> on error.
7193 See also L<perlipc/"SysV IPC"> and the documentation for
7194 L<C<IPC::SysV>|IPC::SysV>.
7196 Portability issues: L<perlport/shmget>.
7198 =item shmread ID,VAR,POS,SIZE
7202 =for Pod::Functions read SysV shared memory
7204 =item shmwrite ID,STRING,POS,SIZE
7206 =for Pod::Functions write SysV shared memory
7208 Reads or writes the System V shared memory segment ID starting at
7209 position POS for size SIZE by attaching to it, copying in/out, and
7210 detaching from it. When reading, VAR must be a variable that will
7211 hold the data read. When writing, if STRING is too long, only SIZE
7212 bytes are used; if STRING is too short, nulls are written to fill out
7213 SIZE bytes. Return true if successful, false on error.
7214 L<C<shmread>|/shmread ID,VAR,POS,SIZE> taints the variable. See also
7215 L<perlipc/"SysV IPC"> and the documentation for
7216 L<C<IPC::SysV>|IPC::SysV> and the L<C<IPC::Shareable>|IPC::Shareable>
7219 Portability issues: L<perlport/shmread> and L<perlport/shmwrite>.
7221 =item shutdown SOCKET,HOW
7224 =for Pod::Functions close down just half of a socket connection
7226 Shuts down a socket connection in the manner indicated by HOW, which
7227 has the same interpretation as in the syscall of the same name.
7229 shutdown($socket, 0); # I/we have stopped reading data
7230 shutdown($socket, 1); # I/we have stopped writing data
7231 shutdown($socket, 2); # I/we have stopped using this socket
7233 This is useful with sockets when you want to tell the other
7234 side you're done writing but not done reading, or vice versa.
7235 It's also a more insistent form of close because it also
7236 disables the file descriptor in any forked copies in other
7239 Returns C<1> for success; on error, returns L<C<undef>|/undef EXPR> if
7240 the first argument is not a valid filehandle, or returns C<0> and sets
7241 L<C<$!>|perlvar/$!> for any other failure.
7244 X<sin> X<sine> X<asin> X<arcsine>
7248 =for Pod::Functions return the sine of a number
7250 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
7251 returns sine of L<C<$_>|perlvar/$_>.
7253 For the inverse sine operation, you may use the C<Math::Trig::asin>
7254 function, or use this relation:
7256 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
7263 =for Pod::Functions block for some number of seconds
7265 Causes the script to sleep for (integer) EXPR seconds, or forever if no
7266 argument is given. Returns the integer number of seconds actually slept.
7268 May be interrupted if the process receives a signal such as C<SIGALRM>.
7271 local $SIG{ALRM} = sub { die "Alarm!\n" };
7274 die $@ unless $@ eq "Alarm!\n";
7276 You probably cannot mix L<C<alarm>|/alarm SECONDS> and
7277 L<C<sleep>|/sleep EXPR> calls, because L<C<sleep>|/sleep EXPR> is often
7278 implemented using L<C<alarm>|/alarm SECONDS>.
7280 On some older systems, it may sleep up to a full second less than what
7281 you requested, depending on how it counts seconds. Most modern systems
7282 always sleep the full amount. They may appear to sleep longer than that,
7283 however, because your process might not be scheduled right away in a
7284 busy multitasking system.
7286 For delays of finer granularity than one second, the L<Time::HiRes>
7287 module (from CPAN, and starting from Perl 5.8 part of the standard
7288 distribution) provides L<C<usleep>|Time::HiRes/usleep ( $useconds )>.
7289 You may also use Perl's four-argument
7290 version of L<C<select>|/select RBITS,WBITS,EBITS,TIMEOUT> leaving the
7291 first three arguments undefined, or you might be able to use the
7292 L<C<syscall>|/syscall NUMBER, LIST> interface to access L<setitimer(2)>
7293 if your system supports it. See L<perlfaq8> for details.
7295 See also the L<POSIX> module's L<C<pause>|POSIX/C<pause>> function.
7297 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
7300 =for Pod::Functions create a socket
7302 Opens a socket of the specified kind and attaches it to filehandle
7303 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for
7304 the syscall of the same name. You should C<use Socket> first
7305 to get the proper definitions imported. See the examples in
7306 L<perlipc/"Sockets: Client/Server Communication">.
7308 On systems that support a close-on-exec flag on files, the flag will
7309 be set for the newly opened file descriptor, as determined by the
7310 value of L<C<$^F>|perlvar/$^F>. See L<perlvar/$^F>.
7312 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
7315 =for Pod::Functions create a pair of sockets
7317 Creates an unnamed pair of sockets in the specified domain, of the
7318 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
7319 for the syscall of the same name. If unimplemented, raises an exception.
7320 Returns true if successful.
7322 On systems that support a close-on-exec flag on files, the flag will
7323 be set for the newly opened file descriptors, as determined by the value
7324 of L<C<$^F>|perlvar/$^F>. See L<perlvar/$^F>.
7326 Some systems define L<C<pipe>|/pipe READHANDLE,WRITEHANDLE> in terms of
7327 L<C<socketpair>|/socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL>, in
7328 which a call to C<pipe($rdr, $wtr)> is essentially:
7331 socketpair(my $rdr, my $wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
7332 shutdown($rdr, 1); # no more writing for reader
7333 shutdown($wtr, 0); # no more reading for writer
7335 See L<perlipc> for an example of socketpair use. Perl 5.8 and later will
7336 emulate socketpair using IP sockets to localhost if your system implements
7337 sockets but not socketpair.
7339 Portability issues: L<perlport/socketpair>.
7341 =item sort SUBNAME LIST
7344 =item sort BLOCK LIST
7348 =for Pod::Functions sort a list of values
7350 In list context, this sorts the LIST and returns the sorted list value.
7351 In scalar context, the behaviour of L<C<sort>|/sort SUBNAME LIST> is
7354 If SUBNAME or BLOCK is omitted, L<C<sort>|/sort SUBNAME LIST>s in
7355 standard string comparison
7356 order. If SUBNAME is specified, it gives the name of a subroutine
7357 that returns an integer less than, equal to, or greater than C<0>,
7358 depending on how the elements of the list are to be ordered. (The
7359 C<< <=> >> and C<cmp> operators are extremely useful in such routines.)
7360 SUBNAME may be a scalar variable name (unsubscripted), in which case
7361 the value provides the name of (or a reference to) the actual
7362 subroutine to use. In place of a SUBNAME, you can provide a BLOCK as
7363 an anonymous, in-line sort subroutine.
7365 If the subroutine's prototype is C<($$)>, the elements to be compared are
7366 passed by reference in L<C<@_>|perlvar/@_>, as for a normal subroutine.
7367 This is slower than unprototyped subroutines, where the elements to be
7368 compared are passed into the subroutine as the package global variables
7369 C<$a> and C<$b> (see example below).
7371 If the subroutine is an XSUB, the elements to be compared are pushed on
7372 to the stack, the way arguments are usually passed to XSUBs. C<$a> and
7375 The values to be compared are always passed by reference and should not
7378 You also cannot exit out of the sort block or subroutine using any of the
7379 loop control operators described in L<perlsyn> or with
7380 L<C<goto>|/goto LABEL>.
7382 When L<C<use locale>|locale> (but not C<use locale ':not_characters'>)
7383 is in effect, C<sort LIST> sorts LIST according to the
7384 current collation locale. See L<perllocale>.
7386 L<C<sort>|/sort SUBNAME LIST> returns aliases into the original list,
7387 much as a for loop's index variable aliases the list elements. That is,
7388 modifying an element of a list returned by L<C<sort>|/sort SUBNAME LIST>
7389 (for example, in a C<foreach>, L<C<map>|/map BLOCK LIST> or
7390 L<C<grep>|/grep BLOCK LIST>)
7391 actually modifies the element in the original list. This is usually
7392 something to be avoided when writing clear code.
7394 Historically Perl has varied in whether sorting is stable by default.
7395 If stability matters, it can be controlled explicitly by using the
7401 my @articles = sort @files;
7403 # same thing, but with explicit sort routine
7404 my @articles = sort {$a cmp $b} @files;
7406 # now case-insensitively
7407 my @articles = sort {fc($a) cmp fc($b)} @files;
7409 # same thing in reversed order
7410 my @articles = sort {$b cmp $a} @files;
7412 # sort numerically ascending
7413 my @articles = sort {$a <=> $b} @files;
7415 # sort numerically descending
7416 my @articles = sort {$b <=> $a} @files;
7418 # this sorts the %age hash by value instead of key
7419 # using an in-line function
7420 my @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
7422 # sort using explicit subroutine name
7424 $age{$a} <=> $age{$b}; # presuming numeric
7426 my @sortedclass = sort byage @class;
7428 sub backwards { $b cmp $a }
7429 my @harry = qw(dog cat x Cain Abel);
7430 my @george = qw(gone chased yz Punished Axed);
7432 # prints AbelCaincatdogx
7433 print sort backwards @harry;
7434 # prints xdogcatCainAbel
7435 print sort @george, 'to', @harry;
7436 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
7438 # inefficiently sort by descending numeric compare using
7439 # the first integer after the first = sign, or the
7440 # whole record case-insensitively otherwise
7443 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
7448 # same thing, but much more efficiently;
7449 # we'll build auxiliary indices instead
7453 push @nums, ( /=(\d+)/ ? $1 : undef );
7457 my @new = @old[ sort {
7458 $nums[$b] <=> $nums[$a]
7460 $caps[$a] cmp $caps[$b]
7464 # same thing, but without any temps
7465 my @new = map { $_->[0] }
7466 sort { $b->[1] <=> $a->[1]
7469 } map { [$_, /=(\d+)/, fc($_)] } @old;
7471 # using a prototype allows you to use any comparison subroutine
7472 # as a sort subroutine (including other package's subroutines)
7474 sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are
7477 my @new = sort Other::backwards @old;
7479 # guarantee stability
7481 my @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
7483 Warning: syntactical care is required when sorting the list returned from
7484 a function. If you want to sort the list returned by the function call
7485 C<find_records(@key)>, you can use:
7487 my @contact = sort { $a cmp $b } find_records @key;
7488 my @contact = sort +find_records(@key);
7489 my @contact = sort &find_records(@key);
7490 my @contact = sort(find_records(@key));
7492 If instead you want to sort the array C<@key> with the comparison routine
7493 C<find_records()> then you can use:
7495 my @contact = sort { find_records() } @key;
7496 my @contact = sort find_records(@key);
7497 my @contact = sort(find_records @key);
7498 my @contact = sort(find_records (@key));
7500 C<$a> and C<$b> are set as package globals in the package the sort() is
7501 called from. That means C<$main::a> and C<$main::b> (or C<$::a> and
7502 C<$::b>) in the C<main> package, C<$FooPack::a> and C<$FooPack::b> in the
7503 C<FooPack> package, etc. If the sort block is in scope of a C<my> or
7504 C<state> declaration of C<$a> and/or C<$b>, you I<must> spell out the full
7505 name of the variables in the sort block :
7508 my $a = "C"; # DANGER, Will Robinson, DANGER !!!
7510 print sort { $a cmp $b } qw(A C E G B D F H);
7512 sub badlexi { $a cmp $b }
7513 print sort badlexi qw(A C E G B D F H);
7515 # the above prints BACFEDGH or some other incorrect ordering
7517 print sort { $::a cmp $::b } qw(A C E G B D F H);
7519 print sort { our $a cmp our $b } qw(A C E G B D F H);
7521 print sort { our ($a, $b); $a cmp $b } qw(A C E G B D F H);
7523 sub lexi { our $a cmp our $b }
7524 print sort lexi qw(A C E G B D F H);
7526 # the above print ABCDEFGH
7528 With proper care you may mix package and my (or state) C<$a> and/or C<$b>:
7538 say sort { $a->{our $a} <=> $a->{our $b} }
7539 qw{ huge normal tiny small big};
7541 # prints tinysmallnormalbighuge
7543 C<$a> and C<$b> are implicitly local to the sort() execution and regain their
7544 former values upon completing the sort.
7546 Sort subroutines written using C<$a> and C<$b> are bound to their calling
7547 package. It is possible, but of limited interest, to define them in a
7548 different package, since the subroutine must still refer to the calling
7549 package's C<$a> and C<$b> :
7552 sub lexi { $Bar::a cmp $Bar::b }
7554 ... sort Foo::lexi ...
7556 Use the prototyped versions (see above) for a more generic alternative.
7558 The comparison function is required to behave. If it returns
7559 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
7560 sometimes saying the opposite, for example) the results are not
7563 Because C<< <=> >> returns L<C<undef>|/undef EXPR> when either operand
7564 is C<NaN> (not-a-number), be careful when sorting with a
7565 comparison function like C<< $a <=> $b >> any lists that might contain a
7566 C<NaN>. The following example takes advantage that C<NaN != NaN> to
7567 eliminate any C<NaN>s from the input list.
7569 my @result = sort { $a <=> $b } grep { $_ == $_ } @input;
7571 =item splice ARRAY,OFFSET,LENGTH,LIST
7574 =item splice ARRAY,OFFSET,LENGTH
7576 =item splice ARRAY,OFFSET
7580 =for Pod::Functions add or remove elements anywhere in an array
7582 Removes the elements designated by OFFSET and LENGTH from an array, and
7583 replaces them with the elements of LIST, if any. In list context,
7584 returns the elements removed from the array. In scalar context,
7585 returns the last element removed, or L<C<undef>|/undef EXPR> if no
7587 removed. The array grows or shrinks as necessary.
7588 If OFFSET is negative then it starts that far from the end of the array.
7589 If LENGTH is omitted, removes everything from OFFSET onward.
7590 If LENGTH is negative, removes the elements from OFFSET onward
7591 except for -LENGTH elements at the end of the array.
7592 If both OFFSET and LENGTH are omitted, removes everything. If OFFSET is
7593 past the end of the array and a LENGTH was provided, Perl issues a warning,
7594 and splices at the end of the array.
7596 The following equivalences hold (assuming C<< $#a >= $i >> )
7598 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
7599 pop(@a) splice(@a,-1)
7600 shift(@a) splice(@a,0,1)
7601 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
7602 $a[$i] = $y splice(@a,$i,1,$y)
7604 L<C<splice>|/splice ARRAY,OFFSET,LENGTH,LIST> can be used, for example,
7605 to implement n-ary queue processing:
7609 while (my @next_n = splice @_, 0, $n) {
7610 say join q{ -- }, @next_n;
7614 nary_print(3, qw(a b c d e f g h));
7620 Starting with Perl 5.14, an experimental feature allowed
7621 L<C<splice>|/splice ARRAY,OFFSET,LENGTH,LIST> to take a
7622 scalar expression. This experiment has been deemed unsuccessful, and was
7623 removed as of Perl 5.24.
7625 =item split /PATTERN/,EXPR,LIMIT
7628 =item split /PATTERN/,EXPR
7630 =item split /PATTERN/
7634 =for Pod::Functions split up a string using a regexp delimiter
7636 Splits the string EXPR into a list of strings and returns the
7637 list in list context, or the size of the list in scalar context.
7638 (Prior to Perl 5.11, it also overwrote C<@_> with the list in
7639 void and scalar context. If you target old perls, beware.)
7641 If only PATTERN is given, EXPR defaults to L<C<$_>|perlvar/$_>.
7643 Anything in EXPR that matches PATTERN is taken to be a separator
7644 that separates the EXPR into substrings (called "I<fields>") that
7645 do B<not> include the separator. Note that a separator may be
7646 longer than one character or even have no characters at all (the
7647 empty string, which is a zero-width match).
7649 The PATTERN need not be constant; an expression may be used
7650 to specify a pattern that varies at runtime.
7652 If PATTERN matches the empty string, the EXPR is split at the match
7653 position (between characters). As an example, the following:
7655 print join(':', split(/b/, 'abc')), "\n";
7657 uses the C<b> in C<'abc'> as a separator to produce the output C<a:c>.
7660 print join(':', split(//, 'abc')), "\n";
7662 uses empty string matches as separators to produce the output
7663 C<a:b:c>; thus, the empty string may be used to split EXPR into a
7664 list of its component characters.
7666 As a special case for L<C<split>|/split E<sol>PATTERNE<sol>,EXPR,LIMIT>,
7667 the empty pattern given in
7668 L<match operator|perlop/"m/PATTERN/msixpodualngc"> syntax (C<//>)
7669 specifically matches the empty string, which is contrary to its usual
7670 interpretation as the last successful match.
7672 If PATTERN is C</^/>, then it is treated as if it used the
7673 L<multiline modifier|perlreref/OPERATORS> (C</^/m>), since it
7674 isn't much use otherwise.
7676 C<E<sol>m> and any of the other pattern modifiers valid for C<qr>
7677 (summarized in L<perlop/qrE<sol>STRINGE<sol>msixpodualn>) may be
7678 specified explicitly.
7680 As another special case,
7681 L<C<split>|/split E<sol>PATTERNE<sol>,EXPR,LIMIT> emulates the default
7683 command line tool B<awk> when the PATTERN is either omitted or a
7684 string composed of a single space character (such as S<C<' '>> or
7685 S<C<"\x20">>, but not e.g. S<C</ />>). In this case, any leading
7686 whitespace in EXPR is removed before splitting occurs, and the PATTERN is
7687 instead treated as if it were C</\s+/>; in particular, this means that
7688 I<any> contiguous whitespace (not just a single space character) is used as
7689 a separator. However, this special treatment can be avoided by specifying
7690 the pattern S<C</ />> instead of the string S<C<" ">>, thereby allowing
7691 only a single space character to be a separator. In earlier Perls this
7692 special case was restricted to the use of a plain S<C<" ">> as the
7693 pattern argument to split; in Perl 5.18.0 and later this special case is
7694 triggered by any expression which evaluates to the simple string S<C<" ">>.
7696 As of Perl 5.28, this special-cased whitespace splitting works as expected in
7697 the scope of L<< S<C<"use feature 'unicode_strings">>|feature/The
7698 'unicode_strings' feature >>. In previous versions, and outside the scope of
7699 that feature, it exhibits L<perlunicode/The "Unicode Bug">: characters that are
7700 whitespace according to Unicode rules but not according to ASCII rules can be
7701 treated as part of fields rather than as field separators, depending on the
7702 string's internal encoding.
7704 If omitted, PATTERN defaults to a single space, S<C<" ">>, triggering
7705 the previously described I<awk> emulation.
7707 If LIMIT is specified and positive, it represents the maximum number
7708 of fields into which the EXPR may be split; in other words, LIMIT is
7709 one greater than the maximum number of times EXPR may be split. Thus,
7710 the LIMIT value C<1> means that EXPR may be split a maximum of zero
7711 times, producing a maximum of one field (namely, the entire value of
7712 EXPR). For instance:
7714 print join(':', split(//, 'abc', 1)), "\n";
7716 produces the output C<abc>, and this:
7718 print join(':', split(//, 'abc', 2)), "\n";
7720 produces the output C<a:bc>, and each of these:
7722 print join(':', split(//, 'abc', 3)), "\n";
7723 print join(':', split(//, 'abc', 4)), "\n";
7725 produces the output C<a:b:c>.
7727 If LIMIT is negative, it is treated as if it were instead arbitrarily
7728 large; as many fields as possible are produced.
7730 If LIMIT is omitted (or, equivalently, zero), then it is usually
7731 treated as if it were instead negative but with the exception that
7732 trailing empty fields are stripped (empty leading fields are always
7733 preserved); if all fields are empty, then all fields are considered to
7734 be trailing (and are thus stripped in this case). Thus, the following:
7736 print join(':', split(/,/, 'a,b,c,,,')), "\n";
7738 produces the output C<a:b:c>, but the following:
7740 print join(':', split(/,/, 'a,b,c,,,', -1)), "\n";
7742 produces the output C<a:b:c:::>.
7744 In time-critical applications, it is worthwhile to avoid splitting
7745 into more fields than necessary. Thus, when assigning to a list,
7746 if LIMIT is omitted (or zero), then LIMIT is treated as though it
7747 were one larger than the number of variables in the list; for the
7748 following, LIMIT is implicitly 3:
7750 my ($login, $passwd) = split(/:/);
7752 Note that splitting an EXPR that evaluates to the empty string always
7753 produces zero fields, regardless of the LIMIT specified.
7755 An empty leading field is produced when there is a positive-width
7756 match at the beginning of EXPR. For instance:
7758 print join(':', split(/ /, ' abc')), "\n";
7760 produces the output C<:abc>. However, a zero-width match at the
7761 beginning of EXPR never produces an empty field, so that:
7763 print join(':', split(//, ' abc'));
7765 produces the output S<C< :a:b:c>> (rather than S<C<: :a:b:c>>).
7767 An empty trailing field, on the other hand, is produced when there is a
7768 match at the end of EXPR, regardless of the length of the match
7769 (of course, unless a non-zero LIMIT is given explicitly, such fields are
7770 removed, as in the last example). Thus:
7772 print join(':', split(//, ' abc', -1)), "\n";
7774 produces the output S<C< :a:b:c:>>.
7776 If the PATTERN contains
7777 L<capturing groups|perlretut/Grouping things and hierarchical matching>,
7778 then for each separator, an additional field is produced for each substring
7779 captured by a group (in the order in which the groups are specified,
7780 as per L<backreferences|perlretut/Backreferences>); if any group does not
7781 match, then it captures the L<C<undef>|/undef EXPR> value instead of a
7783 note that any such additional field is produced whenever there is a
7784 separator (that is, whenever a split occurs), and such an additional field
7785 does B<not> count towards the LIMIT. Consider the following expressions
7786 evaluated in list context (each returned list is provided in the associated
7789 split(/-|,/, "1-10,20", 3)
7792 split(/(-|,)/, "1-10,20", 3)
7793 # ('1', '-', '10', ',', '20')
7795 split(/-|(,)/, "1-10,20", 3)
7796 # ('1', undef, '10', ',', '20')
7798 split(/(-)|,/, "1-10,20", 3)
7799 # ('1', '-', '10', undef, '20')
7801 split(/(-)|(,)/, "1-10,20", 3)
7802 # ('1', '-', undef, '10', undef, ',', '20')
7804 =item sprintf FORMAT, LIST
7807 =for Pod::Functions formatted print into a string
7809 Returns a string formatted by the usual
7810 L<C<printf>|/printf FILEHANDLE FORMAT, LIST> conventions of the C
7811 library function L<C<sprintf>|/sprintf FORMAT, LIST>. See below for
7812 more details and see L<sprintf(3)> or L<printf(3)> on your system for an
7813 explanation of the general principles.
7817 # Format number with up to 8 leading zeroes
7818 my $result = sprintf("%08d", $number);
7820 # Round number to 3 digits after decimal point
7821 my $rounded = sprintf("%.3f", $number);
7823 Perl does its own L<C<sprintf>|/sprintf FORMAT, LIST> formatting: it
7825 function L<sprintf(3)>, but doesn't use it except for floating-point
7826 numbers, and even then only standard modifiers are allowed.
7827 Non-standard extensions in your local L<sprintf(3)> are
7828 therefore unavailable from Perl.
7830 Unlike L<C<printf>|/printf FILEHANDLE FORMAT, LIST>,
7831 L<C<sprintf>|/sprintf FORMAT, LIST> does not do what you probably mean
7832 when you pass it an array as your first argument.
7833 The array is given scalar context,
7834 and instead of using the 0th element of the array as the format, Perl will
7835 use the count of elements in the array as the format, which is almost never
7838 Perl's L<C<sprintf>|/sprintf FORMAT, LIST> permits the following
7839 universally-known conversions:
7842 %c a character with the given number
7844 %d a signed integer, in decimal
7845 %u an unsigned integer, in decimal
7846 %o an unsigned integer, in octal
7847 %x an unsigned integer, in hexadecimal
7848 %e a floating-point number, in scientific notation
7849 %f a floating-point number, in fixed decimal notation
7850 %g a floating-point number, in %e or %f notation
7852 In addition, Perl permits the following widely-supported conversions:
7854 %X like %x, but using upper-case letters
7855 %E like %e, but using an upper-case "E"
7856 %G like %g, but with an upper-case "E" (if applicable)
7857 %b an unsigned integer, in binary
7858 %B like %b, but using an upper-case "B" with the # flag
7859 %p a pointer (outputs the Perl value's address in hexadecimal)
7860 %n special: *stores* the number of characters output so far
7861 into the next argument in the parameter list
7862 %a hexadecimal floating point
7863 %A like %a, but using upper-case letters
7865 Finally, for backward (and we do mean "backward") compatibility, Perl
7866 permits these unnecessary but widely-supported conversions:
7869 %D a synonym for %ld
7870 %U a synonym for %lu
7871 %O a synonym for %lo
7874 Note that the number of exponent digits in the scientific notation produced
7875 by C<%e>, C<%E>, C<%g> and C<%G> for numbers with the modulus of the
7876 exponent less than 100 is system-dependent: it may be three or less
7877 (zero-padded as necessary). In other words, 1.23 times ten to the
7878 99th may be either "1.23e99" or "1.23e099". Similarly for C<%a> and C<%A>:
7879 the exponent or the hexadecimal digits may float: especially the
7880 "long doubles" Perl configuration option may cause surprises.
7882 Between the C<%> and the format letter, you may specify several
7883 additional attributes controlling the interpretation of the format.
7884 In order, these are:
7888 =item format parameter index
7890 An explicit format parameter index, such as C<2$>. By default sprintf
7891 will format the next unused argument in the list, but this allows you
7892 to take the arguments out of order:
7894 printf '%2$d %1$d', 12, 34; # prints "34 12"
7895 printf '%3$d %d %1$d', 1, 2, 3; # prints "3 1 1"
7901 space prefix non-negative number with a space
7902 + prefix non-negative number with a plus sign
7903 - left-justify within the field
7904 0 use zeros, not spaces, to right-justify
7905 # ensure the leading "0" for any octal,
7906 prefix non-zero hexadecimal with "0x" or "0X",
7907 prefix non-zero binary with "0b" or "0B"
7911 printf '<% d>', 12; # prints "< 12>"
7912 printf '<% d>', 0; # prints "< 0>"
7913 printf '<% d>', -12; # prints "<-12>"
7914 printf '<%+d>', 12; # prints "<+12>"
7915 printf '<%+d>', 0; # prints "<+0>"
7916 printf '<%+d>', -12; # prints "<-12>"
7917 printf '<%6s>', 12; # prints "< 12>"
7918 printf '<%-6s>', 12; # prints "<12 >"
7919 printf '<%06s>', 12; # prints "<000012>"
7920 printf '<%#o>', 12; # prints "<014>"
7921 printf '<%#x>', 12; # prints "<0xc>"
7922 printf '<%#X>', 12; # prints "<0XC>"
7923 printf '<%#b>', 12; # prints "<0b1100>"
7924 printf '<%#B>', 12; # prints "<0B1100>"
7926 When a space and a plus sign are given as the flags at once,
7927 the space is ignored.
7929 printf '<%+ d>', 12; # prints "<+12>"
7930 printf '<% +d>', 12; # prints "<+12>"
7932 When the # flag and a precision are given in the %o conversion,
7933 the precision is incremented if it's necessary for the leading "0".
7935 printf '<%#.5o>', 012; # prints "<00012>"
7936 printf '<%#.5o>', 012345; # prints "<012345>"
7937 printf '<%#.0o>', 0; # prints "<0>"
7941 This flag tells Perl to interpret the supplied string as a vector of
7942 integers, one for each character in the string. Perl applies the format to
7943 each integer in turn, then joins the resulting strings with a separator (a
7944 dot C<.> by default). This can be useful for displaying ordinal values of
7945 characters in arbitrary strings:
7947 printf "%vd", "AB\x{100}"; # prints "65.66.256"
7948 printf "version is v%vd\n", $^V; # Perl's version
7950 Put an asterisk C<*> before the C<v> to override the string to
7951 use to separate the numbers:
7953 printf "address is %*vX\n", ":", $addr; # IPv6 address
7954 printf "bits are %0*v8b\n", " ", $bits; # random bitstring
7956 You can also explicitly specify the argument number to use for
7957 the join string using something like C<*2$v>; for example:
7959 printf '%*4$vX %*4$vX %*4$vX', # 3 IPv6 addresses
7962 =item (minimum) width
7964 Arguments are usually formatted to be only as wide as required to
7965 display the given value. You can override the width by putting
7966 a number here, or get the width from the next argument (with C<*>)
7967 or from a specified argument (e.g., with C<*2$>):
7969 printf "<%s>", "a"; # prints "<a>"
7970 printf "<%6s>", "a"; # prints "< a>"
7971 printf "<%*s>", 6, "a"; # prints "< a>"
7972 printf '<%*2$s>', "a", 6; # prints "< a>"
7973 printf "<%2s>", "long"; # prints "<long>" (does not truncate)
7975 If a field width obtained through C<*> is negative, it has the same
7976 effect as the C<-> flag: left-justification.
7978 =item precision, or maximum width
7981 You can specify a precision (for numeric conversions) or a maximum
7982 width (for string conversions) by specifying a C<.> followed by a number.
7983 For floating-point formats except C<g> and C<G>, this specifies
7984 how many places right of the decimal point to show (the default being 6).
7987 # these examples are subject to system-specific variation
7988 printf '<%f>', 1; # prints "<1.000000>"
7989 printf '<%.1f>', 1; # prints "<1.0>"
7990 printf '<%.0f>', 1; # prints "<1>"
7991 printf '<%e>', 10; # prints "<1.000000e+01>"
7992 printf '<%.1e>', 10; # prints "<1.0e+01>"
7994 For "g" and "G", this specifies the maximum number of significant digits to
7997 # These examples are subject to system-specific variation.
7998 printf '<%g>', 1; # prints "<1>"
7999 printf '<%.10g>', 1; # prints "<1>"
8000 printf '<%g>', 100; # prints "<100>"
8001 printf '<%.1g>', 100; # prints "<1e+02>"
8002 printf '<%.2g>', 100.01; # prints "<1e+02>"
8003 printf '<%.5g>', 100.01; # prints "<100.01>"
8004 printf '<%.4g>', 100.01; # prints "<100>"
8005 printf '<%.1g>', 0.0111; # prints "<0.01>"
8006 printf '<%.2g>', 0.0111; # prints "<0.011>"
8007 printf '<%.3g>', 0.0111; # prints "<0.0111>"
8009 For integer conversions, specifying a precision implies that the
8010 output of the number itself should be zero-padded to this width,
8011 where the 0 flag is ignored:
8013 printf '<%.6d>', 1; # prints "<000001>"
8014 printf '<%+.6d>', 1; # prints "<+000001>"
8015 printf '<%-10.6d>', 1; # prints "<000001 >"
8016 printf '<%10.6d>', 1; # prints "< 000001>"
8017 printf '<%010.6d>', 1; # prints "< 000001>"
8018 printf '<%+10.6d>', 1; # prints "< +000001>"
8020 printf '<%.6x>', 1; # prints "<000001>"
8021 printf '<%#.6x>', 1; # prints "<0x000001>"
8022 printf '<%-10.6x>', 1; # prints "<000001 >"
8023 printf '<%10.6x>', 1; # prints "< 000001>"
8024 printf '<%010.6x>', 1; # prints "< 000001>"
8025 printf '<%#10.6x>', 1; # prints "< 0x000001>"
8027 For string conversions, specifying a precision truncates the string
8028 to fit the specified width:
8030 printf '<%.5s>', "truncated"; # prints "<trunc>"
8031 printf '<%10.5s>', "truncated"; # prints "< trunc>"
8033 You can also get the precision from the next argument using C<.*>, or from a
8034 specified argument (e.g., with C<.*2$>):
8036 printf '<%.6x>', 1; # prints "<000001>"
8037 printf '<%.*x>', 6, 1; # prints "<000001>"
8039 printf '<%.*2$x>', 1, 6; # prints "<000001>"
8041 printf '<%6.*2$x>', 1, 4; # prints "< 0001>"
8043 If a precision obtained through C<*> is negative, it counts
8044 as having no precision at all.
8046 printf '<%.*s>', 7, "string"; # prints "<string>"
8047 printf '<%.*s>', 3, "string"; # prints "<str>"
8048 printf '<%.*s>', 0, "string"; # prints "<>"
8049 printf '<%.*s>', -1, "string"; # prints "<string>"
8051 printf '<%.*d>', 1, 0; # prints "<0>"
8052 printf '<%.*d>', 0, 0; # prints "<>"
8053 printf '<%.*d>', -1, 0; # prints "<0>"
8057 For numeric conversions, you can specify the size to interpret the
8058 number as using C<l>, C<h>, C<V>, C<q>, C<L>, or C<ll>. For integer
8059 conversions (C<d u o x X b i D U O>), numbers are usually assumed to be
8060 whatever the default integer size is on your platform (usually 32 or 64
8061 bits), but you can override this to use instead one of the standard C types,
8062 as supported by the compiler used to build Perl:
8064 hh interpret integer as C type "char" or "unsigned
8065 char" on Perl 5.14 or later
8066 h interpret integer as C type "short" or
8068 j interpret integer as C type "intmax_t" on Perl
8069 5.14 or later; and prior to Perl 5.30, only with
8070 a C99 compiler (unportable)
8071 l interpret integer as C type "long" or
8073 q, L, or ll interpret integer as C type "long long",
8074 "unsigned long long", or "quad" (typically
8076 t interpret integer as C type "ptrdiff_t" on Perl
8078 z interpret integer as C type "size_t" on Perl 5.14
8081 As of 5.14, none of these raises an exception if they are not supported on
8082 your platform. However, if warnings are enabled, a warning of the
8083 L<C<printf>|warnings> warning class is issued on an unsupported
8084 conversion flag. Should you instead prefer an exception, do this:
8086 use warnings FATAL => "printf";
8088 If you would like to know about a version dependency before you
8089 start running the program, put something like this at its top:
8091 use 5.014; # for hh/j/t/z/ printf modifiers
8093 You can find out whether your Perl supports quads via L<Config>:
8096 if ($Config{use64bitint} eq "define"
8097 || $Config{longsize} >= 8) {
8098 print "Nice quads!\n";
8101 For floating-point conversions (C<e f g E F G>), numbers are usually assumed
8102 to be the default floating-point size on your platform (double or long double),
8103 but you can force "long double" with C<q>, C<L>, or C<ll> if your
8104 platform supports them. You can find out whether your Perl supports long
8105 doubles via L<Config>:
8108 print "long doubles\n" if $Config{d_longdbl} eq "define";
8110 You can find out whether Perl considers "long double" to be the default
8111 floating-point size to use on your platform via L<Config>:
8114 if ($Config{uselongdouble} eq "define") {
8115 print "long doubles by default\n";
8118 It can also be that long doubles and doubles are the same thing:
8121 ($Config{doublesize} == $Config{longdblsize}) &&
8122 print "doubles are long doubles\n";
8124 The size specifier C<V> has no effect for Perl code, but is supported for
8125 compatibility with XS code. It means "use the standard size for a Perl
8126 integer or floating-point number", which is the default.
8128 =item order of arguments
8130 Normally, L<C<sprintf>|/sprintf FORMAT, LIST> takes the next unused
8131 argument as the value to
8132 format for each format specification. If the format specification
8133 uses C<*> to require additional arguments, these are consumed from
8134 the argument list in the order they appear in the format
8135 specification I<before> the value to format. Where an argument is
8136 specified by an explicit index, this does not affect the normal
8137 order for the arguments, even when the explicitly specified index
8138 would have been the next argument.
8142 printf "<%*.*s>", $a, $b, $c;
8144 uses C<$a> for the width, C<$b> for the precision, and C<$c>
8145 as the value to format; while:
8147 printf '<%*1$.*s>', $a, $b;
8149 would use C<$a> for the width and precision, and C<$b> as the
8152 Here are some more examples; be aware that when using an explicit
8153 index, the C<$> may need escaping:
8155 printf "%2\$d %d\n", 12, 34; # will print "34 12\n"
8156 printf "%2\$d %d %d\n", 12, 34; # will print "34 12 34\n"
8157 printf "%3\$d %d %d\n", 12, 34, 56; # will print "56 12 34\n"
8158 printf "%2\$*3\$d %d\n", 12, 34, 3; # will print " 34 12\n"
8159 printf "%*1\$.*f\n", 4, 5, 10; # will print "5.0000\n"
8163 If L<C<use locale>|locale> (including C<use locale ':not_characters'>)
8164 is in effect and L<C<POSIX::setlocale>|POSIX/C<setlocale>> has been
8166 the character used for the decimal separator in formatted floating-point
8167 numbers is affected by the C<LC_NUMERIC> locale. See L<perllocale>
8171 X<sqrt> X<root> X<square root>
8175 =for Pod::Functions square root function
8177 Return the positive square root of EXPR. If EXPR is omitted, uses
8178 L<C<$_>|perlvar/$_>. Works only for non-negative operands unless you've
8179 loaded the L<C<Math::Complex>|Math::Complex> module.
8182 print sqrt(-4); # prints 2i
8185 X<srand> X<seed> X<randseed>
8189 =for Pod::Functions seed the random number generator
8191 Sets and returns the random number seed for the L<C<rand>|/rand EXPR>
8194 The point of the function is to "seed" the L<C<rand>|/rand EXPR>
8195 function so that L<C<rand>|/rand EXPR> can produce a different sequence
8196 each time you run your program. When called with a parameter,
8197 L<C<srand>|/srand EXPR> uses that for the seed; otherwise it
8198 (semi-)randomly chooses a seed. In either case, starting with Perl 5.14,
8199 it returns the seed. To signal that your code will work I<only> on Perls
8200 of a recent vintage:
8202 use 5.014; # so srand returns the seed
8204 If L<C<srand>|/srand EXPR> is not called explicitly, it is called
8205 implicitly without a parameter at the first use of the
8206 L<C<rand>|/rand EXPR> operator. However, there are a few situations
8207 where programs are likely to want to call L<C<srand>|/srand EXPR>. One
8208 is for generating predictable results, generally for testing or
8209 debugging. There, you use C<srand($seed)>, with the same C<$seed> each
8210 time. Another case is that you may want to call L<C<srand>|/srand EXPR>
8211 after a L<C<fork>|/fork> to avoid child processes sharing the same seed
8212 value as the parent (and consequently each other).
8214 Do B<not> call C<srand()> (i.e., without an argument) more than once per
8215 process. The internal state of the random number generator should
8216 contain more entropy than can be provided by any seed, so calling
8217 L<C<srand>|/srand EXPR> again actually I<loses> randomness.
8219 Most implementations of L<C<srand>|/srand EXPR> take an integer and will
8221 truncate decimal numbers. This means C<srand(42)> will usually
8222 produce the same results as C<srand(42.1)>. To be safe, always pass
8223 L<C<srand>|/srand EXPR> an integer.
8225 A typical use of the returned seed is for a test program which has too many
8226 combinations to test comprehensively in the time available to it each run. It
8227 can test a random subset each time, and should there be a failure, log the seed
8228 used for that run so that it can later be used to reproduce the same results.
8230 B<L<C<rand>|/rand EXPR> is not cryptographically secure. You should not rely
8231 on it in security-sensitive situations.> As of this writing, a
8232 number of third-party CPAN modules offer random number generators
8233 intended by their authors to be cryptographically secure,
8234 including: L<Data::Entropy>, L<Crypt::Random>, L<Math::Random::Secure>,
8235 and L<Math::TrulyRandom>.
8237 =item stat FILEHANDLE
8238 X<stat> X<file, status> X<ctime>
8242 =item stat DIRHANDLE
8246 =for Pod::Functions get a file's status information
8248 Returns a 13-element list giving the status info for a file, either
8249 the file opened via FILEHANDLE or DIRHANDLE, or named by EXPR. If EXPR is
8250 omitted, it stats L<C<$_>|perlvar/$_> (not C<_>!). Returns the empty
8251 list if L<C<stat>|/stat FILEHANDLE> fails. Typically
8254 my ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
8255 $atime,$mtime,$ctime,$blksize,$blocks)
8258 Not all fields are supported on all filesystem types. Here are the
8259 meanings of the fields:
8261 0 dev device number of filesystem
8263 2 mode file mode (type and permissions)
8264 3 nlink number of (hard) links to the file
8265 4 uid numeric user ID of file's owner
8266 5 gid numeric group ID of file's owner
8267 6 rdev the device identifier (special files only)
8268 7 size total size of file, in bytes
8269 8 atime last access time in seconds since the epoch
8270 9 mtime last modify time in seconds since the epoch
8271 10 ctime inode change time in seconds since the epoch (*)
8272 11 blksize preferred I/O size in bytes for interacting with the
8273 file (may vary from file to file)
8274 12 blocks actual number of system-specific blocks allocated
8275 on disk (often, but not always, 512 bytes each)
8277 (The epoch was at 00:00 January 1, 1970 GMT.)
8279 (*) Not all fields are supported on all filesystem types. Notably, the
8280 ctime field is non-portable. In particular, you cannot expect it to be a
8281 "creation time"; see L<perlport/"Files and Filesystems"> for details.
8283 If L<C<stat>|/stat FILEHANDLE> is passed the special filehandle
8284 consisting of an underline, no stat is done, but the current contents of
8285 the stat structure from the last L<C<stat>|/stat FILEHANDLE>,
8286 L<C<lstat>|/lstat FILEHANDLE>, or filetest are returned. Example:
8288 if (-x $file && (($d) = stat(_)) && $d < 0) {
8289 print "$file is executable NFS file\n";
8292 (This works on machines only for which the device number is negative
8295 On some platforms inode numbers are of a type larger than perl knows how
8296 to handle as integer numerical values. If necessary, an inode number will
8297 be returned as a decimal string in order to preserve the entire value.
8298 If used in a numeric context, this will be converted to a floating-point
8299 numerical value, with rounding, a fate that is best avoided. Therefore,
8300 you should prefer to compare inode numbers using C<eq> rather than C<==>.
8301 C<eq> will work fine on inode numbers that are represented numerically,
8302 as well as those represented as strings.
8304 Because the mode contains both the file type and its permissions, you
8305 should mask off the file type portion and (s)printf using a C<"%o">
8306 if you want to see the real permissions.
8308 my $mode = (stat($filename))[2];
8309 printf "Permissions are %04o\n", $mode & 07777;
8311 In scalar context, L<C<stat>|/stat FILEHANDLE> returns a boolean value
8313 or failure, and, if successful, sets the information associated with
8314 the special filehandle C<_>.
8316 The L<File::stat> module provides a convenient, by-name access mechanism:
8319 my $sb = stat($filename);
8320 printf "File is %s, size is %s, perm %04o, mtime %s\n",
8321 $filename, $sb->size, $sb->mode & 07777,
8322 scalar localtime $sb->mtime;
8324 You can import symbolic mode constants (C<S_IF*>) and functions
8325 (C<S_IS*>) from the L<Fcntl> module:
8329 my $mode = (stat($filename))[2];
8331 my $user_rwx = ($mode & S_IRWXU) >> 6;
8332 my $group_read = ($mode & S_IRGRP) >> 3;
8333 my $other_execute = $mode & S_IXOTH;
8335 printf "Permissions are %04o\n", S_IMODE($mode), "\n";
8337 my $is_setuid = $mode & S_ISUID;
8338 my $is_directory = S_ISDIR($mode);
8340 You could write the last two using the C<-u> and C<-d> operators.
8341 Commonly available C<S_IF*> constants are:
8343 # Permissions: read, write, execute, for user, group, others.
8345 S_IRWXU S_IRUSR S_IWUSR S_IXUSR
8346 S_IRWXG S_IRGRP S_IWGRP S_IXGRP
8347 S_IRWXO S_IROTH S_IWOTH S_IXOTH
8349 # Setuid/Setgid/Stickiness/SaveText.
8350 # Note that the exact meaning of these is system-dependent.
8352 S_ISUID S_ISGID S_ISVTX S_ISTXT
8354 # File types. Not all are necessarily available on
8357 S_IFREG S_IFDIR S_IFLNK S_IFBLK S_IFCHR
8358 S_IFIFO S_IFSOCK S_IFWHT S_ENFMT
8360 # The following are compatibility aliases for S_IRUSR,
8361 # S_IWUSR, and S_IXUSR.
8363 S_IREAD S_IWRITE S_IEXEC
8365 and the C<S_IF*> functions are
8367 S_IMODE($mode) the part of $mode containing the permission
8368 bits and the setuid/setgid/sticky bits
8370 S_IFMT($mode) the part of $mode containing the file type
8371 which can be bit-anded with (for example)
8372 S_IFREG or with the following functions
8374 # The operators -f, -d, -l, -b, -c, -p, and -S.
8376 S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
8377 S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)
8379 # No direct -X operator counterpart, but for the first one
8380 # the -g operator is often equivalent. The ENFMT stands for
8381 # record flocking enforcement, a platform-dependent feature.
8383 S_ISENFMT($mode) S_ISWHT($mode)
8385 See your native L<chmod(2)> and L<stat(2)> documentation for more details
8386 about the C<S_*> constants. To get status info for a symbolic link
8387 instead of the target file behind the link, use the
8388 L<C<lstat>|/lstat FILEHANDLE> function.
8390 Portability issues: L<perlport/stat>.
8395 =item state TYPE VARLIST
8397 =item state VARLIST : ATTRS
8399 =item state TYPE VARLIST : ATTRS
8401 =for Pod::Functions +state declare and assign a persistent lexical variable
8403 L<C<state>|/state VARLIST> declares a lexically scoped variable, just
8404 like L<C<my>|/my VARLIST>.
8405 However, those variables will never be reinitialized, contrary to
8406 lexical variables that are reinitialized each time their enclosing block
8408 See L<perlsub/"Persistent Private Variables"> for details.
8410 If more than one variable is listed, the list must be placed in
8411 parentheses. With a parenthesised list, L<C<undef>|/undef EXPR> can be
8413 dummy placeholder. However, since initialization of state variables in
8414 such lists is currently not possible this would serve no purpose.
8416 L<C<state>|/state VARLIST> is available only if the
8417 L<C<"state"> feature|feature/The 'state' feature> is enabled or if it is
8418 prefixed with C<CORE::>. The
8419 L<C<"state"> feature|feature/The 'state' feature> is enabled
8420 automatically with a C<use v5.10> (or higher) declaration in the current
8429 =for Pod::Functions no-op, formerly optimized input data for repeated searches
8431 At this time, C<study> does nothing. This may change in the future.
8433 Prior to Perl version 5.16, it would create an inverted index of all characters
8434 that occurred in the given SCALAR (or L<C<$_>|perlvar/$_> if unspecified). When
8435 matching a pattern, the rarest character from the pattern would be looked up in
8436 this index. Rarity was based on some static frequency tables constructed from
8437 some C programs and English text.
8440 =item sub NAME BLOCK
8443 =item sub NAME (PROTO) BLOCK
8445 =item sub NAME : ATTRS BLOCK
8447 =item sub NAME (PROTO) : ATTRS BLOCK
8449 =for Pod::Functions declare a subroutine, possibly anonymously
8451 This is subroutine definition, not a real function I<per se>. Without a
8452 BLOCK it's just a forward declaration. Without a NAME, it's an anonymous
8453 function declaration, so does return a value: the CODE ref of the closure
8456 See L<perlsub> and L<perlref> for details about subroutines and
8457 references; see L<attributes> and L<Attribute::Handlers> for more
8458 information about attributes.
8463 =for Pod::Functions +current_sub the current subroutine, or C<undef> if not in a subroutine
8465 A special token that returns a reference to the current subroutine, or
8466 L<C<undef>|/undef EXPR> outside of a subroutine.
8468 The behaviour of L<C<__SUB__>|/__SUB__> within a regex code block (such
8469 as C</(?{...})/>) is subject to change.
8471 This token is only available under C<use v5.16> or the
8472 L<C<"current_sub"> feature|feature/The 'current_sub' feature>.
8475 =item substr EXPR,OFFSET,LENGTH,REPLACEMENT
8476 X<substr> X<substring> X<mid> X<left> X<right>
8478 =item substr EXPR,OFFSET,LENGTH
8480 =item substr EXPR,OFFSET
8482 =for Pod::Functions get or alter a portion of a string
8484 Extracts a substring out of EXPR and returns it. First character is at
8485 offset zero. If OFFSET is negative, starts
8486 that far back from the end of the string. If LENGTH is omitted, returns
8487 everything through the end of the string. If LENGTH is negative, leaves that
8488 many characters off the end of the string.
8490 my $s = "The black cat climbed the green tree";
8491 my $color = substr $s, 4, 5; # black
8492 my $middle = substr $s, 4, -11; # black cat climbed the
8493 my $end = substr $s, 14; # climbed the green tree
8494 my $tail = substr $s, -4; # tree
8495 my $z = substr $s, -4, 2; # tr
8497 You can use the L<C<substr>|/substr EXPR,OFFSET,LENGTH,REPLACEMENT>
8498 function as an lvalue, in which case EXPR
8499 must itself be an lvalue. If you assign something shorter than LENGTH,
8500 the string will shrink, and if you assign something longer than LENGTH,
8501 the string will grow to accommodate it. To keep the string the same
8502 length, you may need to pad or chop your value using
8503 L<C<sprintf>|/sprintf FORMAT, LIST>.
8505 If OFFSET and LENGTH specify a substring that is partly outside the
8506 string, only the part within the string is returned. If the substring
8507 is beyond either end of the string,
8508 L<C<substr>|/substr EXPR,OFFSET,LENGTH,REPLACEMENT> returns the undefined
8509 value and produces a warning. When used as an lvalue, specifying a
8510 substring that is entirely outside the string raises an exception.
8511 Here's an example showing the behavior for boundary cases:
8514 substr($name, 4) = 'dy'; # $name is now 'freddy'
8515 my $null = substr $name, 6, 2; # returns "" (no warning)
8516 my $oops = substr $name, 7; # returns undef, with warning
8517 substr($name, 7) = 'gap'; # raises an exception
8519 An alternative to using
8520 L<C<substr>|/substr EXPR,OFFSET,LENGTH,REPLACEMENT> as an lvalue is to
8522 replacement string as the 4th argument. This allows you to replace
8523 parts of the EXPR and return what was there before in one operation,
8524 just as you can with
8525 L<C<splice>|/splice ARRAY,OFFSET,LENGTH,LIST>.
8527 my $s = "The black cat climbed the green tree";
8528 my $z = substr $s, 14, 7, "jumped from"; # climbed
8529 # $s is now "The black cat jumped from the green tree"
8531 Note that the lvalue returned by the three-argument version of
8532 L<C<substr>|/substr EXPR,OFFSET,LENGTH,REPLACEMENT> acts as
8533 a 'magic bullet'; each time it is assigned to, it remembers which part
8534 of the original string is being modified; for example:
8537 for (substr($x,1,2)) {
8538 $_ = 'a'; print $x,"\n"; # prints 1a4
8539 $_ = 'xyz'; print $x,"\n"; # prints 1xyz4
8541 $_ = 'pq'; print $x,"\n"; # prints 5pq9
8544 With negative offsets, it remembers its position from the end of the string
8545 when the target string is modified:
8548 for (substr($x, -3, 2)) {
8549 $_ = 'a'; print $x,"\n"; # prints 1a4, as above
8551 print $_,"\n"; # prints f
8554 Prior to Perl version 5.10, the result of using an lvalue multiple times was
8555 unspecified. Prior to 5.16, the result with negative offsets was
8558 =item symlink OLDFILE,NEWFILE
8559 X<symlink> X<link> X<symbolic link> X<link, symbolic>
8561 =for Pod::Functions create a symbolic link to a file
8563 Creates a new filename symbolically linked to the old filename.
8564 Returns C<1> for success, C<0> otherwise. On systems that don't support
8565 symbolic links, raises an exception. To check for that,
8568 my $symlink_exists = eval { symlink("",""); 1 };
8570 Portability issues: L<perlport/symlink>.
8572 =item syscall NUMBER, LIST
8573 X<syscall> X<system call>
8575 =for Pod::Functions execute an arbitrary system call
8577 Calls the system call specified as the first element of the list,
8578 passing the remaining elements as arguments to the system call. If
8579 unimplemented, raises an exception. The arguments are interpreted
8580 as follows: if a given argument is numeric, the argument is passed as
8581 an int. If not, the pointer to the string value is passed. You are
8582 responsible to make sure a string is pre-extended long enough to
8583 receive any result that might be written into a string. You can't use a
8584 string literal (or other read-only string) as an argument to
8585 L<C<syscall>|/syscall NUMBER, LIST> because Perl has to assume that any
8586 string pointer might be written through. If your
8587 integer arguments are not literals and have never been interpreted in a
8588 numeric context, you may need to add C<0> to them to force them to look
8589 like numbers. This emulates the
8590 L<C<syswrite>|/syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET> function (or
8593 require 'syscall.ph'; # may need to run h2ph
8594 my $s = "hi there\n";
8595 syscall(SYS_write(), fileno(STDOUT), $s, length $s);
8597 Note that Perl supports passing of up to only 14 arguments to your syscall,
8598 which in practice should (usually) suffice.
8600 Syscall returns whatever value returned by the system call it calls.
8601 If the system call fails, L<C<syscall>|/syscall NUMBER, LIST> returns
8602 C<-1> and sets L<C<$!>|perlvar/$!> (errno).
8603 Note that some system calls I<can> legitimately return C<-1>. The proper
8604 way to handle such calls is to assign C<$! = 0> before the call, then
8605 check the value of L<C<$!>|perlvar/$!> if
8606 L<C<syscall>|/syscall NUMBER, LIST> returns C<-1>.
8608 There's a problem with C<syscall(SYS_pipe())>: it returns the file
8609 number of the read end of the pipe it creates, but there is no way
8610 to retrieve the file number of the other end. You can avoid this
8611 problem by using L<C<pipe>|/pipe READHANDLE,WRITEHANDLE> instead.
8613 Portability issues: L<perlport/syscall>.
8615 =item sysopen FILEHANDLE,FILENAME,MODE
8618 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
8620 =for Pod::Functions +5.002 open a file, pipe, or descriptor
8622 Opens the file whose filename is given by FILENAME, and associates it with
8623 FILEHANDLE. If FILEHANDLE is an expression, its value is used as the real
8624 filehandle wanted; an undefined scalar will be suitably autovivified. This
8625 function calls the underlying operating system's L<open(2)> function with the
8626 parameters FILENAME, MODE, and PERMS.
8628 Returns true on success and L<C<undef>|/undef EXPR> otherwise.
8630 The possible values and flag bits of the MODE parameter are
8631 system-dependent; they are available via the standard module
8632 L<C<Fcntl>|Fcntl>. See the documentation of your operating system's
8633 L<open(2)> syscall to see
8634 which values and flag bits are available. You may combine several flags
8635 using the C<|>-operator.
8637 Some of the most common values are C<O_RDONLY> for opening the file in
8638 read-only mode, C<O_WRONLY> for opening the file in write-only mode,
8639 and C<O_RDWR> for opening the file in read-write mode.
8640 X<O_RDONLY> X<O_RDWR> X<O_WRONLY>
8642 For historical reasons, some values work on almost every system
8643 supported by Perl: 0 means read-only, 1 means write-only, and 2
8644 means read/write. We know that these values do I<not> work under
8645 OS/390 and on the Macintosh; you probably don't want to
8646 use them in new code.
8648 If the file named by FILENAME does not exist and the
8649 L<C<open>|/open FILEHANDLE,EXPR> call creates
8650 it (typically because MODE includes the C<O_CREAT> flag), then the value of
8651 PERMS specifies the permissions of the newly created file. If you omit
8652 the PERMS argument to L<C<sysopen>|/sysopen FILEHANDLE,FILENAME,MODE>,
8653 Perl uses the octal value C<0666>.
8654 These permission values need to be in octal, and are modified by your
8655 process's current L<C<umask>|/umask EXPR>.
8658 In many systems the C<O_EXCL> flag is available for opening files in
8659 exclusive mode. This is B<not> locking: exclusiveness means here that
8660 if the file already exists,
8661 L<C<sysopen>|/sysopen FILEHANDLE,FILENAME,MODE> fails. C<O_EXCL> may
8663 on network filesystems, and has no effect unless the C<O_CREAT> flag
8664 is set as well. Setting C<O_CREAT|O_EXCL> prevents the file from
8665 being opened if it is a symbolic link. It does not protect against
8666 symbolic links in the file's path.
8669 Sometimes you may want to truncate an already-existing file. This
8670 can be done using the C<O_TRUNC> flag. The behavior of
8671 C<O_TRUNC> with C<O_RDONLY> is undefined.
8674 You should seldom if ever use C<0644> as argument to
8675 L<C<sysopen>|/sysopen FILEHANDLE,FILENAME,MODE>, because
8676 that takes away the user's option to have a more permissive umask.
8677 Better to omit it. See L<C<umask>|/umask EXPR> for more on this.
8679 Note that under Perls older than 5.8.0,
8680 L<C<sysopen>|/sysopen FILEHANDLE,FILENAME,MODE> depends on the
8681 L<fdopen(3)> C library function. On many Unix systems, L<fdopen(3)> is known
8682 to fail when file descriptors exceed a certain value, typically 255. If
8683 you need more file descriptors than that, consider using the
8684 L<C<POSIX::open>|POSIX/C<open>> function. For Perls 5.8.0 and later,
8685 PerlIO is (most often) the default.
8687 See L<perlopentut> for a kinder, gentler explanation of opening files.
8689 Portability issues: L<perlport/sysopen>.
8691 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
8694 =item sysread FILEHANDLE,SCALAR,LENGTH
8696 =for Pod::Functions fixed-length unbuffered input from a filehandle
8698 Attempts to read LENGTH bytes of data into variable SCALAR from the
8699 specified FILEHANDLE, using L<read(2)>. It bypasses
8700 buffered IO, so mixing this with other kinds of reads,
8701 L<C<print>|/print FILEHANDLE LIST>, L<C<write>|/write FILEHANDLE>,
8702 L<C<seek>|/seek FILEHANDLE,POSITION,WHENCE>,
8703 L<C<tell>|/tell FILEHANDLE>, or L<C<eof>|/eof FILEHANDLE> can cause
8704 confusion because the
8705 perlio or stdio layers usually buffer data. Returns the number of
8706 bytes actually read, C<0> at end of file, or undef if there was an
8707 error (in the latter case L<C<$!>|perlvar/$!> is also set). SCALAR will
8709 shrunk so that the last byte actually read is the last byte of the
8710 scalar after the read.
8712 An OFFSET may be specified to place the read data at some place in the
8713 string other than the beginning. A negative OFFSET specifies
8714 placement at that many characters counting backwards from the end of
8715 the string. A positive OFFSET greater than the length of SCALAR
8716 results in the string being padded to the required size with C<"\0">
8717 bytes before the result of the read is appended.
8719 There is no syseof() function, which is ok, since
8720 L<C<eof>|/eof FILEHANDLE> doesn't work well on device files (like ttys)
8721 anyway. Use L<C<sysread>|/sysread FILEHANDLE,SCALAR,LENGTH,OFFSET> and
8722 check for a return value of 0 to decide whether you're done.
8724 Note that if the filehandle has been marked as C<:utf8>, C<sysread> will
8725 throw an exception. The C<:encoding(...)> layer implicitly
8726 introduces the C<:utf8> layer. See
8727 L<C<binmode>|/binmode FILEHANDLE, LAYER>,
8728 L<C<open>|/open FILEHANDLE,EXPR>, and the L<open> pragma.
8730 =item sysseek FILEHANDLE,POSITION,WHENCE
8733 =for Pod::Functions +5.004 position I/O pointer on handle used with sysread and syswrite
8735 Sets FILEHANDLE's system position I<in bytes> using L<lseek(2)>. FILEHANDLE may
8736 be an expression whose value gives the name of the filehandle. The values
8737 for WHENCE are C<0> to set the new position to POSITION; C<1> to set it
8738 to the current position plus POSITION; and C<2> to set it to EOF plus
8739 POSITION, typically negative.
8741 Note the emphasis on bytes: even if the filehandle has been set to operate
8742 on characters (for example using the C<:encoding(UTF-8)> I/O layer), the
8743 L<C<seek>|/seek FILEHANDLE,POSITION,WHENCE>,
8744 L<C<tell>|/tell FILEHANDLE>, and
8745 L<C<sysseek>|/sysseek FILEHANDLE,POSITION,WHENCE>
8746 family of functions use byte offsets, not character offsets,
8747 because seeking to a character offset would be very slow in a UTF-8 file.
8749 L<C<sysseek>|/sysseek FILEHANDLE,POSITION,WHENCE> bypasses normal
8750 buffered IO, so mixing it with reads other than
8751 L<C<sysread>|/sysread FILEHANDLE,SCALAR,LENGTH,OFFSET> (for example
8752 L<C<readline>|/readline EXPR> or
8753 L<C<read>|/read FILEHANDLE,SCALAR,LENGTH,OFFSET>),
8754 L<C<print>|/print FILEHANDLE LIST>, L<C<write>|/write FILEHANDLE>,
8755 L<C<seek>|/seek FILEHANDLE,POSITION,WHENCE>,
8756 L<C<tell>|/tell FILEHANDLE>, or L<C<eof>|/eof FILEHANDLE> may cause
8759 For WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>,
8760 and C<SEEK_END> (start of the file, current position, end of the file)
8761 from the L<Fcntl> module. Use of the constants is also more portable
8762 than relying on 0, 1, and 2. For example to define a "systell" function:
8764 use Fcntl 'SEEK_CUR';
8765 sub systell { sysseek($_[0], 0, SEEK_CUR) }
8767 Returns the new position, or the undefined value on failure. A position
8768 of zero is returned as the string C<"0 but true">; thus
8769 L<C<sysseek>|/sysseek FILEHANDLE,POSITION,WHENCE> returns
8770 true on success and false on failure, yet you can still easily determine
8776 =item system PROGRAM LIST
8778 =for Pod::Functions run a separate program
8780 Does exactly the same thing as L<C<exec>|/exec LIST>, except that a fork is
8781 done first and the parent process waits for the child process to
8782 exit. Note that argument processing varies depending on the
8783 number of arguments. If there is more than one argument in LIST,
8784 or if LIST is an array with more than one value, starts the program
8785 given by the first element of the list with arguments given by the
8786 rest of the list. If there is only one scalar argument, the argument
8787 is checked for shell metacharacters, and if there are any, the
8788 entire argument is passed to the system's command shell for parsing
8789 (this is C</bin/sh -c> on Unix platforms, but varies on other
8790 platforms). If there are no shell metacharacters in the argument,
8791 it is split into words and passed directly to C<execvp>, which is
8792 more efficient. On Windows, only the C<system PROGRAM LIST> syntax will
8793 reliably avoid using the shell; C<system LIST>, even with more than one
8794 element, will fall back to the shell if the first spawn fails.
8796 Perl will attempt to flush all files opened for
8797 output before any operation that may do a fork, but this may not be
8798 supported on some platforms (see L<perlport>). To be safe, you may need
8799 to set L<C<$E<verbar>>|perlvar/$E<verbar>> (C<$AUTOFLUSH> in L<English>)
8800 or call the C<autoflush> method of L<C<IO::Handle>|IO::Handle/METHODS>
8801 on any open handles.
8803 The return value is the exit status of the program as returned by the
8804 L<C<wait>|/wait> call. To get the actual exit value, shift right by
8805 eight (see below). See also L<C<exec>|/exec LIST>. This is I<not> what
8806 you want to use to capture the output from a command; for that you
8807 should use merely backticks or
8808 L<C<qxE<sol>E<sol>>|/qxE<sol>STRINGE<sol>>, as described in
8809 L<perlop/"`STRING`">. Return value of -1 indicates a failure to start
8810 the program or an error of the L<wait(2)> system call (inspect
8811 L<C<$!>|perlvar/$!> for the reason).
8813 If you'd like to make L<C<system>|/system LIST> (and many other bits of
8814 Perl) die on error, have a look at the L<autodie> pragma.
8816 Like L<C<exec>|/exec LIST>, L<C<system>|/system LIST> allows you to lie
8817 to a program about its name if you use the C<system PROGRAM LIST>
8818 syntax. Again, see L<C<exec>|/exec LIST>.
8820 Since C<SIGINT> and C<SIGQUIT> are ignored during the execution of
8821 L<C<system>|/system LIST>, if you expect your program to terminate on
8822 receipt of these signals you will need to arrange to do so yourself
8823 based on the return value.
8825 my @args = ("command", "arg1", "arg2");
8827 or die "system @args failed: $?";
8829 If you'd like to manually inspect L<C<system>|/system LIST>'s failure,
8830 you can check all possible failure modes by inspecting
8831 L<C<$?>|perlvar/$?> like this:
8834 print "failed to execute: $!\n";
8837 printf "child died with signal %d, %s coredump\n",
8838 ($? & 127), ($? & 128) ? 'with' : 'without';
8841 printf "child exited with value %d\n", $? >> 8;
8844 Alternatively, you may inspect the value of
8845 L<C<${^CHILD_ERROR_NATIVE}>|perlvar/${^CHILD_ERROR_NATIVE}> with the
8846 L<C<W*()>|POSIX/C<WIFEXITED>> calls from the L<POSIX> module.
8848 When L<C<system>|/system LIST>'s arguments are executed indirectly by
8849 the shell, results and return codes are subject to its quirks.
8850 See L<perlop/"`STRING`"> and L<C<exec>|/exec LIST> for details.
8852 Since L<C<system>|/system LIST> does a L<C<fork>|/fork> and
8853 L<C<wait>|/wait> it may affect a C<SIGCHLD> handler. See L<perlipc> for
8856 Portability issues: L<perlport/system>.
8858 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
8861 =item syswrite FILEHANDLE,SCALAR,LENGTH
8863 =item syswrite FILEHANDLE,SCALAR
8865 =for Pod::Functions fixed-length unbuffered output to a filehandle
8867 Attempts to write LENGTH bytes of data from variable SCALAR to the
8868 specified FILEHANDLE, using L<write(2)>. If LENGTH is
8869 not specified, writes whole SCALAR. It bypasses buffered IO, so
8870 mixing this with reads (other than C<sysread)>),
8871 L<C<print>|/print FILEHANDLE LIST>, L<C<write>|/write FILEHANDLE>,
8872 L<C<seek>|/seek FILEHANDLE,POSITION,WHENCE>,
8873 L<C<tell>|/tell FILEHANDLE>, or L<C<eof>|/eof FILEHANDLE> may cause
8874 confusion because the perlio and stdio layers usually buffer data.
8875 Returns the number of bytes actually written, or L<C<undef>|/undef EXPR>
8876 if there was an error (in this case the errno variable
8877 L<C<$!>|perlvar/$!> is also set). If the LENGTH is greater than the
8878 data available in the SCALAR after the OFFSET, only as much data as is
8879 available will be written.
8881 An OFFSET may be specified to write the data from some part of the
8882 string other than the beginning. A negative OFFSET specifies writing
8883 that many characters counting backwards from the end of the string.
8884 If SCALAR is of length zero, you can only use an OFFSET of 0.
8886 B<WARNING>: If the filehandle is marked C<:utf8>, C<syswrite> will raise an exception.
8887 The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer.
8888 Alternately, if the handle is not marked with an encoding but you
8889 attempt to write characters with code points over 255, raises an exception.
8890 See L<C<binmode>|/binmode FILEHANDLE, LAYER>,
8891 L<C<open>|/open FILEHANDLE,EXPR>, and the L<open> pragma.
8893 =item tell FILEHANDLE
8898 =for Pod::Functions get current seekpointer on a filehandle
8900 Returns the current position I<in bytes> for FILEHANDLE, or -1 on
8901 error. FILEHANDLE may be an expression whose value gives the name of
8902 the actual filehandle. If FILEHANDLE is omitted, assumes the file
8905 Note the emphasis on bytes: even if the filehandle has been set to operate
8906 on characters (for example using the C<:encoding(UTF-8)> I/O layer), the
8907 L<C<seek>|/seek FILEHANDLE,POSITION,WHENCE>,
8908 L<C<tell>|/tell FILEHANDLE>, and
8909 L<C<sysseek>|/sysseek FILEHANDLE,POSITION,WHENCE>
8910 family of functions use byte offsets, not character offsets,
8911 because seeking to a character offset would be very slow in a UTF-8 file.
8913 The return value of L<C<tell>|/tell FILEHANDLE> for the standard streams
8914 like the STDIN depends on the operating system: it may return -1 or
8915 something else. L<C<tell>|/tell FILEHANDLE> on pipes, fifos, and
8916 sockets usually returns -1.
8918 There is no C<systell> function. Use
8919 L<C<sysseek($fh, 0, 1)>|/sysseek FILEHANDLE,POSITION,WHENCE> for that.
8921 Do not use L<C<tell>|/tell FILEHANDLE> (or other buffered I/O
8922 operations) on a filehandle that has been manipulated by
8923 L<C<sysread>|/sysread FILEHANDLE,SCALAR,LENGTH,OFFSET>,
8924 L<C<syswrite>|/syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET>, or
8925 L<C<sysseek>|/sysseek FILEHANDLE,POSITION,WHENCE>. Those functions
8926 ignore the buffering, while L<C<tell>|/tell FILEHANDLE> does not.
8928 =item telldir DIRHANDLE
8931 =for Pod::Functions get current seekpointer on a directory handle
8933 Returns the current position of the L<C<readdir>|/readdir DIRHANDLE>
8934 routines on DIRHANDLE. Value may be given to
8935 L<C<seekdir>|/seekdir DIRHANDLE,POS> to access a particular location in
8936 a directory. L<C<telldir>|/telldir DIRHANDLE> has the same caveats
8937 about possible directory compaction as the corresponding system library
8940 =item tie VARIABLE,CLASSNAME,LIST
8943 =for Pod::Functions +5.002 bind a variable to an object class
8945 This function binds a variable to a package class that will provide the
8946 implementation for the variable. VARIABLE is the name of the variable
8947 to be enchanted. CLASSNAME is the name of a class implementing objects
8948 of correct type. Any additional arguments are passed to the
8949 appropriate constructor
8950 method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
8951 or C<TIEHASH>). Typically these are arguments such as might be passed
8952 to the L<dbm_open(3)> function of C. The object returned by the
8953 constructor is also returned by the
8954 L<C<tie>|/tie VARIABLE,CLASSNAME,LIST> function, which would be useful
8955 if you want to access other methods in CLASSNAME.
8957 Note that functions such as L<C<keys>|/keys HASH> and
8958 L<C<values>|/values HASH> may return huge lists when used on large
8959 objects, like DBM files. You may prefer to use the L<C<each>|/each
8960 HASH> function to iterate over such. Example:
8962 # print out history file offsets
8964 tie(my %HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
8965 while (my ($key,$val) = each %HIST) {
8966 print $key, ' = ', unpack('L', $val), "\n";
8969 A class implementing a hash should have the following methods:
8971 TIEHASH classname, LIST
8973 STORE this, key, value
8978 NEXTKEY this, lastkey
8983 A class implementing an ordinary array should have the following methods:
8985 TIEARRAY classname, LIST
8987 STORE this, key, value
8989 STORESIZE this, count
8995 SPLICE this, offset, length, LIST
9002 A class implementing a filehandle should have the following methods:
9004 TIEHANDLE classname, LIST
9005 READ this, scalar, length, offset
9008 WRITE this, scalar, length, offset
9010 PRINTF this, format, LIST
9014 SEEK this, position, whence
9016 OPEN this, mode, LIST
9021 A class implementing a scalar should have the following methods:
9023 TIESCALAR classname, LIST
9029 Not all methods indicated above need be implemented. See L<perltie>,
9030 L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>.
9032 Unlike L<C<dbmopen>|/dbmopen HASH,DBNAME,MASK>, the
9033 L<C<tie>|/tie VARIABLE,CLASSNAME,LIST> function will not
9034 L<C<use>|/use Module VERSION LIST> or L<C<require>|/require VERSION> a
9035 module for you; you need to do that explicitly yourself. See L<DB_File>
9036 or the L<Config> module for interesting
9037 L<C<tie>|/tie VARIABLE,CLASSNAME,LIST> implementations.
9039 For further details see L<perltie>, L<C<tied>|/tied VARIABLE>.
9044 =for Pod::Functions get a reference to the object underlying a tied variable
9046 Returns a reference to the object underlying VARIABLE (the same value
9047 that was originally returned by the
9048 L<C<tie>|/tie VARIABLE,CLASSNAME,LIST> call that bound the variable
9049 to a package.) Returns the undefined value if VARIABLE isn't tied to a
9055 =for Pod::Functions return number of seconds since 1970
9057 Returns the number of non-leap seconds since whatever time the system
9058 considers to be the epoch, suitable for feeding to
9059 L<C<gmtime>|/gmtime EXPR> and L<C<localtime>|/localtime EXPR>. On most
9060 systems the epoch is 00:00:00 UTC, January 1, 1970;
9061 a prominent exception being Mac OS Classic which uses 00:00:00, January 1,
9062 1904 in the current local time zone for its epoch.
9064 For measuring time in better granularity than one second, use the
9065 L<Time::HiRes> module from Perl 5.8 onwards (or from CPAN before then), or,
9066 if you have L<gettimeofday(2)>, you may be able to use the
9067 L<C<syscall>|/syscall NUMBER, LIST> interface of Perl. See L<perlfaq8>
9070 For date and time processing look at the many related modules on CPAN.
9071 For a comprehensive date and time representation look at the
9077 =for Pod::Functions return elapsed time for self and child processes
9079 Returns a four-element list giving the user and system times in
9080 seconds for this process and any exited children of this process.
9082 my ($user,$system,$cuser,$csystem) = times;
9084 In scalar context, L<C<times>|/times> returns C<$user>.
9086 Children's times are only included for terminated children.
9088 Portability issues: L<perlport/times>.
9092 =for Pod::Functions transliterate a string
9094 The transliteration operator. Same as
9095 L<C<yE<sol>E<sol>E<sol>>|/yE<sol>E<sol>E<sol>>. See
9096 L<perlop/"Quote-Like Operators">.
9098 =item truncate FILEHANDLE,LENGTH
9101 =item truncate EXPR,LENGTH
9103 =for Pod::Functions shorten a file
9105 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
9106 specified length. Raises an exception if truncate isn't implemented
9107 on your system. Returns true if successful, L<C<undef>|/undef EXPR> on
9110 The behavior is undefined if LENGTH is greater than the length of the
9113 The position in the file of FILEHANDLE is left unchanged. You may want to
9114 call L<seek|/"seek FILEHANDLE,POSITION,WHENCE"> before writing to the
9117 Portability issues: L<perlport/truncate>.
9120 X<uc> X<uppercase> X<toupper>
9124 =for Pod::Functions return upper-case version of a string
9126 Returns an uppercased version of EXPR. This is the internal function
9127 implementing the C<\U> escape in double-quoted strings.
9128 It does not attempt to do titlecase mapping on initial letters. See
9129 L<C<ucfirst>|/ucfirst EXPR> for that.
9131 If EXPR is omitted, uses L<C<$_>|perlvar/$_>.
9133 This function behaves the same way under various pragmas, such as in a locale,
9134 as L<C<lc>|/lc EXPR> does.
9137 X<ucfirst> X<uppercase>
9141 =for Pod::Functions return a string with just the next letter in upper case
9143 Returns the value of EXPR with the first character in uppercase
9144 (titlecase in Unicode). This is the internal function implementing
9145 the C<\u> escape in double-quoted strings.
9147 If EXPR is omitted, uses L<C<$_>|perlvar/$_>.
9149 This function behaves the same way under various pragmas, such as in a locale,
9150 as L<C<lc>|/lc EXPR> does.
9157 =for Pod::Functions set file creation mode mask
9159 Sets the umask for the process to EXPR and returns the previous value.
9160 If EXPR is omitted, merely returns the current umask.
9162 The Unix permission C<rwxr-x---> is represented as three sets of three
9163 bits, or three octal digits: C<0750> (the leading 0 indicates octal
9164 and isn't one of the digits). The L<C<umask>|/umask EXPR> value is such
9165 a number representing disabled permissions bits. The permission (or
9166 "mode") values you pass L<C<mkdir>|/mkdir FILENAME,MODE> or
9167 L<C<sysopen>|/sysopen FILEHANDLE,FILENAME,MODE> are modified by your
9168 umask, so even if you tell
9169 L<C<sysopen>|/sysopen FILEHANDLE,FILENAME,MODE> to create a file with
9170 permissions C<0777>, if your umask is C<0022>, then the file will
9171 actually be created with permissions C<0755>. If your
9172 L<C<umask>|/umask EXPR> were C<0027> (group can't write; others can't
9173 read, write, or execute), then passing
9174 L<C<sysopen>|/sysopen FILEHANDLE,FILENAME,MODE> C<0666> would create a
9175 file with mode C<0640> (because C<0666 &~ 027> is C<0640>).
9177 Here's some advice: supply a creation mode of C<0666> for regular
9178 files (in L<C<sysopen>|/sysopen FILEHANDLE,FILENAME,MODE>) and one of
9179 C<0777> for directories (in L<C<mkdir>|/mkdir FILENAME,MODE>) and
9180 executable files. This gives users the freedom of
9181 choice: if they want protected files, they might choose process umasks
9182 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
9183 Programs should rarely if ever make policy decisions better left to
9184 the user. The exception to this is when writing files that should be
9185 kept private: mail files, web browser cookies, F<.rhosts> files, and
9188 If L<umask(2)> is not implemented on your system and you are trying to
9189 restrict access for I<yourself> (i.e., C<< (EXPR & 0700) > 0 >>),
9190 raises an exception. If L<umask(2)> is not implemented and you are
9191 not trying to restrict access for yourself, returns
9192 L<C<undef>|/undef EXPR>.
9194 Remember that a umask is a number, usually given in octal; it is I<not> a
9195 string of octal digits. See also L<C<oct>|/oct EXPR>, if all you have
9198 Portability issues: L<perlport/umask>.
9201 X<undef> X<undefine>
9205 =for Pod::Functions remove a variable or function definition
9207 Undefines the value of EXPR, which must be an lvalue. Use only on a
9208 scalar value, an array (using C<@>), a hash (using C<%>), a subroutine
9209 (using C<&>), or a typeglob (using C<*>). Saying C<undef $hash{$key}>
9210 will probably not do what you expect on most predefined variables or
9211 DBM list values, so don't do that; see L<C<delete>|/delete EXPR>.
9212 Always returns the undefined value.
9213 You can omit the EXPR, in which case nothing is
9214 undefined, but you still get an undefined value that you could, for
9215 instance, return from a subroutine, assign to a variable, or pass as a
9216 parameter. Examples:
9219 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
9223 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
9224 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
9225 select undef, undef, undef, 0.25;
9226 my ($x, $y, undef, $z) = foo(); # Ignore third value returned
9228 Note that this is a unary operator, not a list operator.
9231 X<unlink> X<delete> X<remove> X<rm> X<del>
9235 =for Pod::Functions remove one link to a file
9237 Deletes a list of files. On success, it returns the number of files
9238 it successfully deleted. On failure, it returns false and sets
9239 L<C<$!>|perlvar/$!> (errno):
9241 my $unlinked = unlink 'a', 'b', 'c';
9243 unlink glob "*.bak";
9245 On error, L<C<unlink>|/unlink LIST> will not tell you which files it
9247 If you want to know which files you could not remove, try them one
9250 foreach my $file ( @goners ) {
9251 unlink $file or warn "Could not unlink $file: $!";
9254 Note: L<C<unlink>|/unlink LIST> will not attempt to delete directories
9256 superuser and the B<-U> flag is supplied to Perl. Even if these
9257 conditions are met, be warned that unlinking a directory can inflict
9258 damage on your filesystem. Finally, using L<C<unlink>|/unlink LIST> on
9259 directories is not supported on many operating systems. Use
9260 L<C<rmdir>|/rmdir FILENAME> instead.
9262 If LIST is omitted, L<C<unlink>|/unlink LIST> uses L<C<$_>|perlvar/$_>.
9264 =item unpack TEMPLATE,EXPR
9267 =item unpack TEMPLATE
9269 =for Pod::Functions convert binary structure into normal perl variables
9271 L<C<unpack>|/unpack TEMPLATE,EXPR> does the reverse of
9272 L<C<pack>|/pack TEMPLATE,LIST>: it takes a string
9273 and expands it out into a list of values.
9274 (In scalar context, it returns merely the first value produced.)
9276 If EXPR is omitted, unpacks the L<C<$_>|perlvar/$_> string.
9277 See L<perlpacktut> for an introduction to this function.
9279 The string is broken into chunks described by the TEMPLATE. Each chunk
9280 is converted separately to a value. Typically, either the string is a result
9281 of L<C<pack>|/pack TEMPLATE,LIST>, or the characters of the string
9282 represent a C structure of some kind.
9284 The TEMPLATE has the same format as in the
9285 L<C<pack>|/pack TEMPLATE,LIST> function.
9286 Here's a subroutine that does substring:
9289 my ($what, $where, $howmuch) = @_;
9290 unpack("x$where a$howmuch", $what);
9295 sub ordinal { unpack("W",$_[0]); } # same as ord()
9297 In addition to fields allowed in L<C<pack>|/pack TEMPLATE,LIST>, you may
9298 prefix a field with a %<number> to indicate that
9299 you want a <number>-bit checksum of the items instead of the items
9300 themselves. Default is a 16-bit checksum. The checksum is calculated by
9301 summing numeric values of expanded values (for string fields the sum of
9302 C<ord($char)> is taken; for bit fields the sum of zeroes and ones).
9304 For example, the following
9305 computes the same number as the System V sum program:
9309 unpack("%32W*", readline) % 65535;
9312 The following efficiently counts the number of set bits in a bit vector:
9314 my $setbits = unpack("%32b*", $selectmask);
9316 The C<p> and C<P> formats should be used with care. Since Perl
9317 has no way of checking whether the value passed to
9318 L<C<unpack>|/unpack TEMPLATE,EXPR>
9319 corresponds to a valid memory location, passing a pointer value that's
9320 not known to be valid is likely to have disastrous consequences.
9322 If there are more pack codes or if the repeat count of a field or a group
9323 is larger than what the remainder of the input string allows, the result
9324 is not well defined: the repeat count may be decreased, or
9325 L<C<unpack>|/unpack TEMPLATE,EXPR> may produce empty strings or zeros,
9326 or it may raise an exception.
9327 If the input string is longer than one described by the TEMPLATE,
9328 the remainder of that input string is ignored.
9330 See L<C<pack>|/pack TEMPLATE,LIST> for more examples and notes.
9332 =item unshift ARRAY,LIST
9335 =for Pod::Functions prepend more elements to the beginning of a list
9337 Does the opposite of a L<C<shift>|/shift ARRAY>. Or the opposite of a
9338 L<C<push>|/push ARRAY,LIST>,
9339 depending on how you look at it. Prepends list to the front of the
9340 array and returns the new number of elements in the array.
9342 unshift(@ARGV, '-e') unless $ARGV[0] =~ /^-/;
9344 Note the LIST is prepended whole, not one element at a time, so the
9345 prepended elements stay in the same order. Use
9346 L<C<reverse>|/reverse LIST> to do the reverse.
9348 Starting with Perl 5.14, an experimental feature allowed
9349 L<C<unshift>|/unshift ARRAY,LIST> to take
9350 a scalar expression. This experiment has been deemed unsuccessful, and was
9351 removed as of Perl 5.24.
9353 =item untie VARIABLE
9356 =for Pod::Functions break a tie binding to a variable
9358 Breaks the binding between a variable and a package.
9359 (See L<tie|/tie VARIABLE,CLASSNAME,LIST>.)
9360 Has no effect if the variable is not tied.
9362 =item use Module VERSION LIST
9363 X<use> X<module> X<import>
9365 =item use Module VERSION
9367 =item use Module LIST
9373 =for Pod::Functions load in a module at compile time and import its namespace
9375 Imports some semantics into the current package from the named module,
9376 generally by aliasing certain subroutine or variable names into your
9377 package. It is exactly equivalent to
9379 BEGIN { require Module; Module->import( LIST ); }
9381 except that Module I<must> be a bareword.
9382 The importation can be made conditional by using the L<if> module.
9384 In the C<use VERSION> form, VERSION may be either a v-string such as
9385 v5.24.1, which will be compared to L<C<$^V>|perlvar/$^V> (aka
9386 $PERL_VERSION), or a numeric argument of the form 5.024001, which will
9387 be compared to L<C<$]>|perlvar/$]>. An exception is raised if VERSION
9388 is greater than the version of the current Perl interpreter; Perl will
9389 not attempt to parse the rest of the file. Compare with
9390 L<C<require>|/require VERSION>, which can do a similar check at run
9391 time. Symmetrically, C<no VERSION> allows you to specify that you
9392 want a version of Perl older than the specified one.
9394 Specifying VERSION as a numeric argument of the form 5.024001 should
9395 generally be avoided as older less readable syntax compared to
9396 v5.24.1. Before perl 5.8.0 released in 2002 the more verbose numeric
9397 form was the only supported syntax, which is why you might see it in
9399 use v5.24.1; # compile time version check
9401 use 5.024_001; # ditto; older syntax compatible with perl 5.6
9403 This is often useful if you need to check the current Perl version before
9404 L<C<use>|/use Module VERSION LIST>ing library modules that won't work
9405 with older versions of Perl.
9406 (We try not to do this more than we have to.)
9408 C<use VERSION> also lexically enables all features available in the requested
9409 version as defined by the L<feature> pragma, disabling any features
9410 not in the requested version's feature bundle. See L<feature>.
9411 Similarly, if the specified Perl version is greater than or equal to
9412 5.12.0, strictures are enabled lexically as
9413 with L<C<use strict>|strict>. Any explicit use of
9414 C<use strict> or C<no strict> overrides C<use VERSION>, even if it comes
9415 before it. Later use of C<use VERSION>
9416 will override all behavior of a previous
9417 C<use VERSION>, possibly removing the C<strict> and C<feature> added by
9418 C<use VERSION>. C<use VERSION> does not
9419 load the F<feature.pm> or F<strict.pm>
9422 The C<BEGIN> forces the L<C<require>|/require VERSION> and
9423 L<C<import>|/import LIST> to happen at compile time. The
9424 L<C<require>|/require VERSION> makes sure the module is loaded into
9425 memory if it hasn't been yet. The L<C<import>|/import LIST> is not a
9426 builtin; it's just an ordinary static method
9427 call into the C<Module> package to tell the module to import the list of
9428 features back into the current package. The module can implement its
9429 L<C<import>|/import LIST> method any way it likes, though most modules
9430 just choose to derive their L<C<import>|/import LIST> method via
9431 inheritance from the C<Exporter> class that is defined in the
9432 L<C<Exporter>|Exporter> module. See L<Exporter>. If no
9433 L<C<import>|/import LIST> method can be found, then the call is skipped,
9434 even if there is an AUTOLOAD method.
9436 If you do not want to call the package's L<C<import>|/import LIST>
9437 method (for instance,
9438 to stop your namespace from being altered), explicitly supply the empty list:
9442 That is exactly equivalent to
9444 BEGIN { require Module }
9446 If the VERSION argument is present between Module and LIST, then the
9447 L<C<use>|/use Module VERSION LIST> will call the C<VERSION> method in
9448 class Module with the given version as an argument:
9454 BEGIN { require Module; Module->VERSION(12.34) }
9456 The L<default C<VERSION> method|UNIVERSAL/C<VERSION ( [ REQUIRE ] )>>,
9457 inherited from the L<C<UNIVERSAL>|UNIVERSAL> class, croaks if the given
9458 version is larger than the value of the variable C<$Module::VERSION>.
9460 The VERSION argument cannot be an arbitrary expression. It only counts
9461 as a VERSION argument if it is a version number literal, starting with
9462 either a digit or C<v> followed by a digit. Anything that doesn't
9463 look like a version literal will be parsed as the start of the LIST.
9464 Nevertheless, many attempts to use an arbitrary expression as a VERSION
9465 argument will appear to work, because L<Exporter>'s C<import> method
9466 handles numeric arguments specially, performing version checks rather
9467 than treating them as things to export.
9469 Again, there is a distinction between omitting LIST (L<C<import>|/import
9470 LIST> called with no arguments) and an explicit empty LIST C<()>
9471 (L<C<import>|/import LIST> not called). Note that there is no comma
9474 Because this is a wide-open interface, pragmas (compiler directives)
9475 are also implemented this way. Some of the currently implemented
9481 use sigtrap qw(SEGV BUS);
9482 use strict qw(subs vars refs);
9483 use subs qw(afunc blurfl);
9484 use warnings qw(all);
9485 use sort qw(stable);
9487 Some of these pseudo-modules import semantics into the current
9488 block scope (like L<C<strict>|strict> or L<C<integer>|integer>, unlike
9489 ordinary modules, which import symbols into the current package (which
9490 are effective through the end of the file).
9492 Because L<C<use>|/use Module VERSION LIST> takes effect at compile time,
9493 it doesn't respect the ordinary flow control of the code being compiled.
9494 In particular, putting a L<C<use>|/use Module VERSION LIST> inside the
9495 false branch of a conditional doesn't prevent it
9496 from being processed. If a module or pragma only needs to be loaded
9497 conditionally, this can be done using the L<if> pragma:
9499 use if $] < 5.008, "utf8";
9500 use if WANT_WARNINGS, warnings => qw(all);
9502 There's a corresponding L<C<no>|/no MODULE VERSION LIST> declaration
9503 that unimports meanings imported by L<C<use>|/use Module VERSION LIST>,
9504 i.e., it calls C<< Module->unimport(LIST) >> instead of
9505 L<C<import>|/import LIST>. It behaves just as L<C<import>|/import LIST>
9506 does with VERSION, an omitted or empty LIST,
9507 or no unimport method being found.
9513 Care should be taken when using the C<no VERSION> form of L<C<no>|/no
9514 MODULE VERSION LIST>. It is
9515 I<only> meant to be used to assert that the running Perl is of a earlier
9516 version than its argument and I<not> to undo the feature-enabling side effects
9519 See L<perlmodlib> for a list of standard modules and pragmas. See
9520 L<perlrun|perlrun/-m[-]module> for the C<-M> and C<-m> command-line
9521 options to Perl that give L<C<use>|/use Module VERSION LIST>
9522 functionality from the command-line.
9527 =for Pod::Functions set a file's last access and modify times
9529 Changes the access and modification times on each file of a list of
9530 files. The first two elements of the list must be the NUMERIC access
9531 and modification times, in that order. Returns the number of files
9532 successfully changed. The inode change time of each file is set
9533 to the current time. For example, this code has the same effect as the
9534 Unix L<touch(1)> command when the files I<already exist> and belong to
9535 the user running the program:
9538 my $atime = my $mtime = time;
9539 utime $atime, $mtime, @ARGV;
9541 Since Perl 5.8.0, if the first two elements of the list are
9542 L<C<undef>|/undef EXPR>,
9543 the L<utime(2)> syscall from your C library is called with a null second
9544 argument. On most systems, this will set the file's access and
9545 modification times to the current time (i.e., equivalent to the example
9546 above) and will work even on files you don't own provided you have write
9549 for my $file (@ARGV) {
9550 utime(undef, undef, $file)
9551 || warn "Couldn't touch $file: $!";
9554 Under NFS this will use the time of the NFS server, not the time of
9555 the local machine. If there is a time synchronization problem, the
9556 NFS server and local machine will have different times. The Unix
9557 L<touch(1)> command will in fact normally use this form instead of the
9558 one shown in the first example.
9560 Passing only one of the first two elements as L<C<undef>|/undef EXPR> is
9561 equivalent to passing a 0 and will not have the effect described when
9562 both are L<C<undef>|/undef EXPR>. This also triggers an
9563 uninitialized warning.
9565 On systems that support L<futimes(2)>, you may pass filehandles among the
9566 files. On systems that don't support L<futimes(2)>, passing filehandles raises
9567 an exception. Filehandles must be passed as globs or glob references to be
9568 recognized; barewords are considered filenames.
9570 Portability issues: L<perlport/utime>.
9577 =for Pod::Functions return a list of the values in a hash
9579 In list context, returns a list consisting of all the values of the named
9580 hash. In Perl 5.12 or later only, will also return a list of the values of
9581 an array; prior to that release, attempting to use an array argument will
9582 produce a syntax error. In scalar context, returns the number of values.
9584 Hash entries are returned in an apparently random order. The actual random
9585 order is specific to a given hash; the exact same series of operations
9586 on two hashes may result in a different order for each hash. Any insertion
9587 into the hash may change the order, as will any deletion, with the exception
9588 that the most recent key returned by L<C<each>|/each HASH> or
9589 L<C<keys>|/keys HASH> may be deleted without changing the order. So
9590 long as a given hash is unmodified you may rely on
9591 L<C<keys>|/keys HASH>, L<C<values>|/values HASH> and
9592 L<C<each>|/each HASH> to repeatedly return the same order
9593 as each other. See L<perlsec/"Algorithmic Complexity Attacks"> for
9594 details on why hash order is randomized. Aside from the guarantees
9595 provided here the exact details of Perl's hash algorithm and the hash
9596 traversal order are subject to change in any release of Perl. Tied hashes
9597 may behave differently to Perl's hashes with respect to changes in order on
9598 insertion and deletion of items.
9600 As a side effect, calling L<C<values>|/values HASH> resets the HASH or
9601 ARRAY's internal iterator (see L<C<each>|/each HASH>) before yielding the
9602 values. In particular,
9603 calling L<C<values>|/values HASH> in void context resets the iterator
9604 with no other overhead.
9606 Apart from resetting the iterator,
9607 C<values @array> in list context is the same as plain C<@array>.
9608 (We recommend that you use void context C<keys @array> for this, but
9609 reasoned that taking C<values @array> out would require more
9610 documentation than leaving it in.)
9612 Note that the values are not copied, which means modifying them will
9613 modify the contents of the hash:
9615 for (values %hash) { s/foo/bar/g } # modifies %hash values
9616 for (@hash{keys %hash}) { s/foo/bar/g } # same
9618 Starting with Perl 5.14, an experimental feature allowed
9619 L<C<values>|/values HASH> to take a
9620 scalar expression. This experiment has been deemed unsuccessful, and was
9621 removed as of Perl 5.24.
9623 To avoid confusing would-be users of your code who are running earlier
9624 versions of Perl with mysterious syntax errors, put this sort of thing at
9625 the top of your file to signal that your code will work I<only> on Perls of
9628 use 5.012; # so keys/values/each work on arrays
9630 See also L<C<keys>|/keys HASH>, L<C<each>|/each HASH>, and
9631 L<C<sort>|/sort SUBNAME LIST>.
9633 =item vec EXPR,OFFSET,BITS
9634 X<vec> X<bit> X<bit vector>
9636 =for Pod::Functions test or set particular bits in a string
9638 Treats the string in EXPR as a bit vector made up of elements of
9639 width BITS and returns the value of the element specified by OFFSET
9640 as an unsigned integer. BITS therefore specifies the number of bits
9641 that are reserved for each element in the bit vector. This must
9642 be a power of two from 1 to 32 (or 64, if your platform supports
9645 If BITS is 8, "elements" coincide with bytes of the input string.
9647 If BITS is 16 or more, bytes of the input string are grouped into chunks
9648 of size BITS/8, and each group is converted to a number as with
9649 L<C<pack>|/pack TEMPLATE,LIST>/L<C<unpack>|/unpack TEMPLATE,EXPR> with
9650 big-endian formats C<n>/C<N> (and analogously for BITS==64). See
9651 L<C<pack>|/pack TEMPLATE,LIST> for details.
9653 If bits is 4 or less, the string is broken into bytes, then the bits
9654 of each byte are broken into 8/BITS groups. Bits of a byte are
9655 numbered in a little-endian-ish way, as in C<0x01>, C<0x02>,
9656 C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example,
9657 breaking the single input byte C<chr(0x36)> into two groups gives a list
9658 C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>.
9660 L<C<vec>|/vec EXPR,OFFSET,BITS> may also be assigned to, in which case
9661 parentheses are needed
9662 to give the expression the correct precedence as in
9664 vec($image, $max_x * $x + $y, 8) = 3;
9666 If the selected element is outside the string, the value 0 is returned.
9667 If an element off the end of the string is written to, Perl will first
9668 extend the string with sufficiently many zero bytes. It is an error
9669 to try to write off the beginning of the string (i.e., negative OFFSET).
9671 If the string happens to be encoded as UTF-8 internally (and thus has
9672 the UTF8 flag set), L<C<vec>|/vec EXPR,OFFSET,BITS> tries to convert it
9673 to use a one-byte-per-character internal representation. However, if the
9674 string contains characters with values of 256 or higher, a fatal error
9677 Strings created with L<C<vec>|/vec EXPR,OFFSET,BITS> can also be
9678 manipulated with the logical
9679 operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit
9680 vector operation is desired when both operands are strings.
9681 See L<perlop/"Bitwise String Operators">.
9683 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
9684 The comments show the string after each step. Note that this code works
9685 in the same way on big-endian or little-endian machines.
9688 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
9690 # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
9691 print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P')
9693 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
9694 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
9695 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
9696 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
9697 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
9698 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
9700 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
9701 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
9702 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
9705 To transform a bit vector into a string or list of 0's and 1's, use these:
9707 my $bits = unpack("b*", $vector);
9708 my @bits = split(//, unpack("b*", $vector));
9710 If you know the exact length in bits, it can be used in place of the C<*>.
9712 Here is an example to illustrate how the bits actually fall in place:
9718 unpack("V",$_) 01234567890123456789012345678901
9719 ------------------------------------------------------------------
9724 for ($shift=0; $shift < $width; ++$shift) {
9725 for ($off=0; $off < 32/$width; ++$off) {
9726 $str = pack("B*", "0"x32);
9727 $bits = (1<<$shift);
9728 vec($str, $off, $width) = $bits;
9729 $res = unpack("b*",$str);
9730 $val = unpack("V", $str);
9737 vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
9738 $off, $width, $bits, $val, $res
9742 Regardless of the machine architecture on which it runs, the
9743 example above should print the following table:
9746 unpack("V",$_) 01234567890123456789012345678901
9747 ------------------------------------------------------------------
9748 vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000
9749 vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000
9750 vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000
9751 vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000
9752 vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000
9753 vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000
9754 vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000
9755 vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000
9756 vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000
9757 vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000
9758 vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000
9759 vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000
9760 vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000
9761 vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000
9762 vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000
9763 vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000
9764 vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000
9765 vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000
9766 vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000
9767 vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000
9768 vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000
9769 vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000
9770 vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000
9771 vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000
9772 vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000
9773 vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000
9774 vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000
9775 vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000
9776 vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000
9777 vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100
9778 vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010
9779 vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001
9780 vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000
9781 vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000
9782 vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000
9783 vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000
9784 vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000
9785 vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000
9786 vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000
9787 vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000
9788 vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000
9789 vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000
9790 vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000
9791 vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000
9792 vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000
9793 vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000
9794 vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000
9795 vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010
9796 vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000
9797 vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000
9798 vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000
9799 vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000
9800 vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000
9801 vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000
9802 vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000
9803 vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000
9804 vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000
9805 vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000
9806 vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000
9807 vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000
9808 vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000
9809 vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000
9810 vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100
9811 vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001
9812 vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000
9813 vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000
9814 vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000
9815 vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000
9816 vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000
9817 vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000
9818 vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000
9819 vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000
9820 vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000
9821 vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000
9822 vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000
9823 vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000
9824 vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000
9825 vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000
9826 vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000
9827 vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100
9828 vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000
9829 vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000
9830 vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000
9831 vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000
9832 vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000
9833 vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000
9834 vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000
9835 vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010
9836 vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000
9837 vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000
9838 vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000
9839 vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000
9840 vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000
9841 vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000
9842 vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000
9843 vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001
9844 vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000
9845 vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000
9846 vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000
9847 vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000
9848 vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000
9849 vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000
9850 vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000
9851 vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000
9852 vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000
9853 vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000
9854 vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000
9855 vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000
9856 vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000
9857 vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000
9858 vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000
9859 vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000
9860 vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000
9861 vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000
9862 vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000
9863 vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000
9864 vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000
9865 vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000
9866 vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000
9867 vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100
9868 vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000
9869 vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000
9870 vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000
9871 vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010
9872 vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000
9873 vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000
9874 vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000
9875 vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
9880 =for Pod::Functions wait for any child process to die
9882 Behaves like L<wait(2)> on your system: it waits for a child
9883 process to terminate and returns the pid of the deceased process, or
9884 C<-1> if there are no child processes. The status is returned in
9885 L<C<$?>|perlvar/$?> and
9886 L<C<${^CHILD_ERROR_NATIVE}>|perlvar/${^CHILD_ERROR_NATIVE}>.
9887 Note that a return value of C<-1> could mean that child processes are
9888 being automatically reaped, as described in L<perlipc>.
9890 If you use L<C<wait>|/wait> in your handler for
9891 L<C<$SIG{CHLD}>|perlvar/%SIG>, it may accidentally wait for the child
9892 created by L<C<qx>|/qxE<sol>STRINGE<sol>> or L<C<system>|/system LIST>.
9893 See L<perlipc> for details.
9895 Portability issues: L<perlport/wait>.
9897 =item waitpid PID,FLAGS
9900 =for Pod::Functions wait for a particular child process to die
9902 Waits for a particular child process to terminate and returns the pid of
9903 the deceased process, or C<-1> if there is no such child process. A
9904 non-blocking wait (with L<WNOHANG|POSIX/C<WNOHANG>> in FLAGS) can return 0 if
9905 there are child processes matching PID but none have terminated yet.
9906 The status is returned in L<C<$?>|perlvar/$?> and
9907 L<C<${^CHILD_ERROR_NATIVE}>|perlvar/${^CHILD_ERROR_NATIVE}>.
9909 A PID of C<0> indicates to wait for any child process whose process group ID is
9910 equal to that of the current process. A PID of less than C<-1> indicates to
9911 wait for any child process whose process group ID is equal to -PID. A PID of
9912 C<-1> indicates to wait for any child process.
9916 use POSIX ":sys_wait_h";
9920 $kid = waitpid(-1, WNOHANG);
9925 1 while waitpid(-1, WNOHANG) > 0;
9927 then you can do a non-blocking wait for all pending zombie processes (see
9929 Non-blocking wait is available on machines supporting either the
9930 L<waitpid(2)> or L<wait4(2)> syscalls. However, waiting for a particular
9931 pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the
9932 system call by remembering the status values of processes that have
9933 exited but have not been harvested by the Perl script yet.)
9935 Note that on some systems, a return value of C<-1> could mean that child
9936 processes are being automatically reaped. See L<perlipc> for details,
9937 and for other examples.
9939 Portability issues: L<perlport/waitpid>.
9942 X<wantarray> X<context>
9944 =for Pod::Functions get void vs scalar vs list context of current subroutine call
9946 Returns true if the context of the currently executing subroutine or
9947 L<C<eval>|/eval EXPR> is looking for a list value. Returns false if the
9949 looking for a scalar. Returns the undefined value if the context is
9950 looking for no value (void context).
9952 return unless defined wantarray; # don't bother doing more
9953 my @a = complex_calculation();
9954 return wantarray ? @a : "@a";
9956 L<C<wantarray>|/wantarray>'s result is unspecified in the top level of a file,
9957 in a C<BEGIN>, C<UNITCHECK>, C<CHECK>, C<INIT> or C<END> block, or
9958 in a C<DESTROY> method.
9960 This function should have been named wantlist() instead.
9963 X<warn> X<warning> X<STDERR>
9965 =for Pod::Functions print debugging info
9967 Emits a warning, usually by printing it to C<STDERR>. C<warn> interprets
9968 its operand LIST in the same way as C<die>, but is slightly different
9969 in what it defaults to when LIST is empty or makes an empty string.
9970 If it is empty and L<C<$@>|perlvar/$@> already contains an exception
9971 value then that value is used after appending C<"\t...caught">. If it
9972 is empty and C<$@> is also empty then the string C<"Warning: Something's
9975 By default, the exception derived from the operand LIST is stringified
9976 and printed to C<STDERR>. This behaviour can be altered by installing
9977 a L<C<$SIG{__WARN__}>|perlvar/%SIG> handler. If there is such a
9978 handler then no message is automatically printed; it is the handler's
9979 responsibility to deal with the exception
9980 as it sees fit (like, for instance, converting it into a
9981 L<C<die>|/die LIST>). Most
9982 handlers must therefore arrange to actually display the
9983 warnings that they are not prepared to deal with, by calling
9984 L<C<warn>|/warn LIST>
9985 again in the handler. Note that this is quite safe and will not
9986 produce an endless loop, since C<__WARN__> hooks are not called from
9989 You will find this behavior is slightly different from that of
9990 L<C<$SIG{__DIE__}>|perlvar/%SIG> handlers (which don't suppress the
9991 error text, but can instead call L<C<die>|/die LIST> again to change
9994 Using a C<__WARN__> handler provides a powerful way to silence all
9995 warnings (even the so-called mandatory ones). An example:
9997 # wipe out *all* compile-time warnings
9998 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
10000 my $foo = 20; # no warning about duplicate my $foo,
10001 # but hey, you asked for it!
10002 # no compile-time or run-time warnings before here
10005 # run-time warnings enabled after here
10006 warn "\$foo is alive and $foo!"; # does show up
10008 See L<perlvar> for details on setting L<C<%SIG>|perlvar/%SIG> entries
10010 examples. See the L<Carp> module for other kinds of warnings using its
10011 C<carp> and C<cluck> functions.
10013 =item write FILEHANDLE
10020 =for Pod::Functions print a picture record
10022 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
10023 using the format associated with that file. By default the format for
10024 a file is the one having the same name as the filehandle, but the
10025 format for the current output channel (see the
10026 L<C<select>|/select FILEHANDLE> function) may be set explicitly by
10027 assigning the name of the format to the L<C<$~>|perlvar/$~> variable.
10029 Top of form processing is handled automatically: if there is insufficient
10030 room on the current page for the formatted record, the page is advanced by
10031 writing a form feed and a special top-of-page
10032 format is used to format the new
10033 page header before the record is written. By default, the top-of-page
10034 format is the name of the filehandle with C<_TOP> appended, or C<top>
10035 in the current package if the former does not exist. This would be a
10036 problem with autovivified filehandles, but it may be dynamically set to the
10037 format of your choice by assigning the name to the L<C<$^>|perlvar/$^>
10038 variable while that filehandle is selected. The number of lines
10039 remaining on the current page is in variable L<C<$->|perlvar/$->, which
10040 can be set to C<0> to force a new page.
10042 If FILEHANDLE is unspecified, output goes to the current default output
10043 channel, which starts out as STDOUT but may be changed by the
10044 L<C<select>|/select FILEHANDLE> operator. If the FILEHANDLE is an EXPR,
10045 then the expression
10046 is evaluated and the resulting string is used to look up the name of
10047 the FILEHANDLE at run time. For more on formats, see L<perlform>.
10049 Note that write is I<not> the opposite of
10050 L<C<read>|/read FILEHANDLE,SCALAR,LENGTH,OFFSET>. Unfortunately.
10054 =for Pod::Functions transliterate a string
10056 The transliteration operator. Same as
10057 L<C<trE<sol>E<sol>E<sol>>|/trE<sol>E<sol>E<sol>>. See
10058 L<perlop/"Quote-Like Operators">.
10062 =head2 Non-function Keywords by Cross-reference
10072 These keywords are documented in L<perldata/"Special Literals">.
10090 These compile phase keywords are documented in L<perlmod/"BEGIN, UNITCHECK, CHECK, INIT and END">.
10100 This method keyword is documented in L<perlobj/"Destructors">.
10132 These operators are documented in L<perlop>.
10142 This keyword is documented in L<perlsub/"Autoloading">.
10166 These flow-control keywords are documented in L<perlsyn/"Compound Statements">.
10170 The "else if" keyword is spelled C<elsif> in Perl. There's no C<elif>
10171 or C<else if> either. It does parse C<elseif>, but only to warn you
10172 about not using it.
10174 See the documentation for flow-control keywords in L<perlsyn/"Compound
10187 These flow-control keywords related to the experimental switch feature are
10188 documented in L<perlsyn/"Switch Statements">.