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/$_>.
1231 B<NOTE:> It is good security practice to do C<chdir("/")>
1232 (L<C<chdir>|/chdir EXPR> to the root directory) immediately after a
1233 L<C<chroot>|/chroot FILENAME>.
1235 Portability issues: L<perlport/chroot>.
1237 =item close FILEHANDLE
1242 =for Pod::Functions close file (or pipe or socket) handle
1244 Closes the file or pipe associated with the filehandle, flushes the IO
1245 buffers, and closes the system file descriptor. Returns true if those
1246 operations succeed and if no error was reported by any PerlIO
1247 layer. Closes the currently selected filehandle if the argument is
1250 You don't have to close FILEHANDLE if you are immediately going to do
1251 another L<C<open>|/open FILEHANDLE,EXPR> on it, because
1252 L<C<open>|/open FILEHANDLE,EXPR> closes it for you. (See
1253 L<C<open>|/open FILEHANDLE,EXPR>.) However, an explicit
1254 L<C<close>|/close FILEHANDLE> on an input file resets the line counter
1255 (L<C<$.>|perlvar/$.>), while the implicit close done by
1256 L<C<open>|/open FILEHANDLE,EXPR> does not.
1258 If the filehandle came from a piped open, L<C<close>|/close FILEHANDLE>
1259 returns false if one of the other syscalls involved fails or if its
1260 program exits with non-zero status. If the only problem was that the
1261 program exited non-zero, L<C<$!>|perlvar/$!> will be set to C<0>.
1262 Closing a pipe also waits for the process executing on the pipe to
1263 exit--in case you wish to look at the output of the pipe afterwards--and
1264 implicitly puts the exit status value of that command into
1265 L<C<$?>|perlvar/$?> and
1266 L<C<${^CHILD_ERROR_NATIVE}>|perlvar/${^CHILD_ERROR_NATIVE}>.
1268 If there are multiple threads running, L<C<close>|/close FILEHANDLE> on
1269 a filehandle from a piped open returns true without waiting for the
1270 child process to terminate, if the filehandle is still open in another
1273 Closing the read end of a pipe before the process writing to it at the
1274 other end is done writing results in the writer receiving a SIGPIPE. If
1275 the other end can't handle that, be sure to read all the data before
1280 open(OUTPUT, '|sort >foo') # pipe to sort
1281 or die "Can't start sort: $!";
1282 #... # print stuff to output
1283 close OUTPUT # wait for sort to finish
1284 or warn $! ? "Error closing sort pipe: $!"
1285 : "Exit status $? from sort";
1286 open(INPUT, 'foo') # get sort's results
1287 or die "Can't open 'foo' for input: $!";
1289 FILEHANDLE may be an expression whose value can be used as an indirect
1290 filehandle, usually the real filehandle name or an autovivified handle.
1292 =item closedir DIRHANDLE
1295 =for Pod::Functions close directory handle
1297 Closes a directory opened by L<C<opendir>|/opendir DIRHANDLE,EXPR> and
1298 returns the success of that system call.
1300 =item connect SOCKET,NAME
1303 =for Pod::Functions connect to a remote socket
1305 Attempts to connect to a remote socket, just like L<connect(2)>.
1306 Returns true if it succeeded, false otherwise. NAME should be a
1307 packed address of the appropriate type for the socket. See the examples in
1308 L<perlipc/"Sockets: Client/Server Communication">.
1310 =item continue BLOCK
1315 =for Pod::Functions optional trailing block in a while or foreach
1317 When followed by a BLOCK, L<C<continue>|/continue BLOCK> is actually a
1318 flow control statement rather than a function. If there is a
1319 L<C<continue>|/continue BLOCK> BLOCK attached to a BLOCK (typically in a
1320 C<while> or C<foreach>), it is always executed just before the
1321 conditional is about to be evaluated again, just like the third part of
1322 a C<for> loop in C. Thus it can be used to increment a loop variable,
1323 even when the loop has been continued via the L<C<next>|/next LABEL>
1324 statement (which is similar to the C L<C<continue>|/continue BLOCK>
1327 L<C<last>|/last LABEL>, L<C<next>|/next LABEL>, or
1328 L<C<redo>|/redo LABEL> may appear within a
1329 L<C<continue>|/continue BLOCK> block; L<C<last>|/last LABEL> and
1330 L<C<redo>|/redo LABEL> behave as if they had been executed within the
1331 main block. So will L<C<next>|/next LABEL>, but since it will execute a
1332 L<C<continue>|/continue BLOCK> block, it may be more entertaining.
1335 ### redo always comes here
1338 ### next always comes here
1340 # then back the top to re-check EXPR
1342 ### last always comes here
1344 Omitting the L<C<continue>|/continue BLOCK> section is equivalent to
1345 using an empty one, logically enough, so L<C<next>|/next LABEL> goes
1346 directly back to check the condition at the top of the loop.
1348 When there is no BLOCK, L<C<continue>|/continue BLOCK> is a function
1349 that falls through the current C<when> or C<default> block instead of
1350 iterating a dynamically enclosing C<foreach> or exiting a lexically
1351 enclosing C<given>. In Perl 5.14 and earlier, this form of
1352 L<C<continue>|/continue BLOCK> was only available when the
1353 L<C<"switch"> feature|feature/The 'switch' feature> was enabled. See
1354 L<feature> and L<perlsyn/"Switch Statements"> for more information.
1357 X<cos> X<cosine> X<acos> X<arccosine>
1361 =for Pod::Functions cosine function
1363 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
1364 takes the cosine of L<C<$_>|perlvar/$_>.
1366 For the inverse cosine operation, you may use the
1367 L<C<Math::Trig::acos>|Math::Trig> function, or use this relation:
1369 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
1371 =item crypt PLAINTEXT,SALT
1372 X<crypt> X<digest> X<hash> X<salt> X<plaintext> X<password>
1373 X<decrypt> X<cryptography> X<passwd> X<encrypt>
1375 =for Pod::Functions one-way passwd-style encryption
1377 Creates a digest string exactly like the L<crypt(3)> function in the C
1378 library (assuming that you actually have a version there that has not
1379 been extirpated as a potential munition).
1381 L<C<crypt>|/crypt PLAINTEXT,SALT> is a one-way hash function. The
1382 PLAINTEXT and SALT are turned
1383 into a short string, called a digest, which is returned. The same
1384 PLAINTEXT and SALT will always return the same string, but there is no
1385 (known) way to get the original PLAINTEXT from the hash. Small
1386 changes in the PLAINTEXT or SALT will result in large changes in the
1389 There is no decrypt function. This function isn't all that useful for
1390 cryptography (for that, look for F<Crypt> modules on your nearby CPAN
1391 mirror) and the name "crypt" is a bit of a misnomer. Instead it is
1392 primarily used to check if two pieces of text are the same without
1393 having to transmit or store the text itself. An example is checking
1394 if a correct password is given. The digest of the password is stored,
1395 not the password itself. The user types in a password that is
1396 L<C<crypt>|/crypt PLAINTEXT,SALT>'d with the same salt as the stored
1397 digest. If the two digests match, the password is correct.
1399 When verifying an existing digest string you should use the digest as
1400 the salt (like C<crypt($plain, $digest) eq $digest>). The SALT used
1401 to create the digest is visible as part of the digest. This ensures
1402 L<C<crypt>|/crypt PLAINTEXT,SALT> will hash the new string with the same
1403 salt as the digest. This allows your code to work with the standard
1404 L<C<crypt>|/crypt PLAINTEXT,SALT> and with more exotic implementations.
1405 In other words, assume nothing about the returned string itself nor
1406 about how many bytes of SALT may matter.
1408 Traditionally the result is a string of 13 bytes: two first bytes of
1409 the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
1410 the first eight bytes of PLAINTEXT mattered. But alternative
1411 hashing schemes (like MD5), higher level security schemes (like C2),
1412 and implementations on non-Unix platforms may produce different
1415 When choosing a new salt create a random two character string whose
1416 characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
1417 '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of
1418 characters is just a recommendation; the characters allowed in
1419 the salt depend solely on your system's crypt library, and Perl can't
1420 restrict what salts L<C<crypt>|/crypt PLAINTEXT,SALT> accepts.
1422 Here's an example that makes sure that whoever runs this program knows
1425 my $pwd = (getpwuid($<))[1];
1427 system "stty -echo";
1429 chomp(my $word = <STDIN>);
1433 if (crypt($word, $pwd) ne $pwd) {
1439 Of course, typing in your own password to whoever asks you
1442 The L<C<crypt>|/crypt PLAINTEXT,SALT> function is unsuitable for hashing
1443 large quantities of data, not least of all because you can't get the
1444 information back. Look at the L<Digest> module for more robust
1447 If using L<C<crypt>|/crypt PLAINTEXT,SALT> on a Unicode string (which
1448 I<potentially> has characters with codepoints above 255), Perl tries to
1449 make sense of the situation by trying to downgrade (a copy of) the
1450 string back to an eight-bit byte string before calling
1451 L<C<crypt>|/crypt PLAINTEXT,SALT> (on that copy). If that works, good.
1452 If not, L<C<crypt>|/crypt PLAINTEXT,SALT> dies with
1453 L<C<Wide character in crypt>|perldiag/Wide character in %s>.
1455 Portability issues: L<perlport/crypt>.
1460 =for Pod::Functions breaks binding on a tied dbm file
1462 [This function has been largely superseded by the
1463 L<C<untie>|/untie VARIABLE> function.]
1465 Breaks the binding between a DBM file and a hash.
1467 Portability issues: L<perlport/dbmclose>.
1469 =item dbmopen HASH,DBNAME,MASK
1470 X<dbmopen> X<dbm> X<ndbm> X<sdbm> X<gdbm>
1472 =for Pod::Functions create binding on a tied dbm file
1474 [This function has been largely superseded by the
1475 L<C<tie>|/tie VARIABLE,CLASSNAME,LIST> function.]
1477 This binds a L<dbm(3)>, L<ndbm(3)>, L<sdbm(3)>, L<gdbm(3)>, or Berkeley
1478 DB file to a hash. HASH is the name of the hash. (Unlike normal
1479 L<C<open>|/open FILEHANDLE,EXPR>, the first argument is I<not> a
1480 filehandle, even though it looks like one). DBNAME is the name of the
1481 database (without the F<.dir> or F<.pag> extension if any). If the
1482 database does not exist, it is created with protection specified by MASK
1483 (as modified by the L<C<umask>|/umask EXPR>). To prevent creation of
1484 the database if it doesn't exist, you may specify a MODE of 0, and the
1485 function will return a false value if it can't find an existing
1486 database. If your system supports only the older DBM functions, you may
1487 make only one L<C<dbmopen>|/dbmopen HASH,DBNAME,MASK> call in your
1488 program. In older versions of Perl, if your system had neither DBM nor
1489 ndbm, calling L<C<dbmopen>|/dbmopen HASH,DBNAME,MASK> produced a fatal
1490 error; it now falls back to L<sdbm(3)>.
1492 If you don't have write access to the DBM file, you can only read hash
1493 variables, not set them. If you want to test whether you can write,
1494 either use file tests or try setting a dummy hash entry inside an
1495 L<C<eval>|/eval EXPR> to trap the error.
1497 Note that functions such as L<C<keys>|/keys HASH> and
1498 L<C<values>|/values HASH> may return huge lists when used on large DBM
1499 files. You may prefer to use the L<C<each>|/each HASH> function to
1500 iterate over large DBM files. Example:
1502 # print out history file offsets
1503 dbmopen(%HIST,'/usr/lib/news/history',0666);
1504 while (($key,$val) = each %HIST) {
1505 print $key, ' = ', unpack('L',$val), "\n";
1509 See also L<AnyDBM_File> for a more general description of the pros and
1510 cons of the various dbm approaches, as well as L<DB_File> for a particularly
1511 rich implementation.
1513 You can control which DBM library you use by loading that library
1514 before you call L<C<dbmopen>|/dbmopen HASH,DBNAME,MASK>:
1517 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
1518 or die "Can't open netscape history file: $!";
1520 Portability issues: L<perlport/dbmopen>.
1523 X<defined> X<undef> X<undefined>
1527 =for Pod::Functions test whether a value, variable, or function is defined
1529 Returns a Boolean value telling whether EXPR has a value other than the
1530 undefined value L<C<undef>|/undef EXPR>. If EXPR is not present,
1531 L<C<$_>|perlvar/$_> is checked.
1533 Many operations return L<C<undef>|/undef EXPR> to indicate failure, end
1534 of file, system error, uninitialized variable, and other exceptional
1535 conditions. This function allows you to distinguish
1536 L<C<undef>|/undef EXPR> from other values. (A simple Boolean test will
1537 not distinguish among L<C<undef>|/undef EXPR>, zero, the empty string,
1538 and C<"0">, which are all equally false.) Note that since
1539 L<C<undef>|/undef EXPR> is a valid scalar, its presence doesn't
1540 I<necessarily> indicate an exceptional condition: L<C<pop>|/pop ARRAY>
1541 returns L<C<undef>|/undef EXPR> when its argument is an empty array,
1542 I<or> when the element to return happens to be L<C<undef>|/undef EXPR>.
1544 You may also use C<defined(&func)> to check whether subroutine C<func>
1545 has ever been defined. The return value is unaffected by any forward
1546 declarations of C<func>. A subroutine that is not defined
1547 may still be callable: its package may have an C<AUTOLOAD> method that
1548 makes it spring into existence the first time that it is called; see
1551 Use of L<C<defined>|/defined EXPR> on aggregates (hashes and arrays) is
1552 no longer supported. It used to report whether memory for that
1553 aggregate had ever been allocated. You should instead use a simple
1556 if (@an_array) { print "has array elements\n" }
1557 if (%a_hash) { print "has hash members\n" }
1559 When used on a hash element, it tells you whether the value is defined,
1560 not whether the key exists in the hash. Use L<C<exists>|/exists EXPR>
1561 for the latter purpose.
1565 print if defined $switch{D};
1566 print "$val\n" while defined($val = pop(@ary));
1567 die "Can't readlink $sym: $!"
1568 unless defined($value = readlink $sym);
1569 sub foo { defined &$bar ? $bar->(@_) : die "No bar"; }
1570 $debugging = 0 unless defined $debugging;
1572 Note: Many folks tend to overuse L<C<defined>|/defined EXPR> and are
1573 then surprised to discover that the number C<0> and C<""> (the
1574 zero-length string) are, in fact, defined values. For example, if you
1579 The pattern match succeeds and C<$1> is defined, although it
1580 matched "nothing". It didn't really fail to match anything. Rather, it
1581 matched something that happened to be zero characters long. This is all
1582 very above-board and honest. When a function returns an undefined value,
1583 it's an admission that it couldn't give you an honest answer. So you
1584 should use L<C<defined>|/defined EXPR> only when questioning the
1585 integrity of what you're trying to do. At other times, a simple
1586 comparison to C<0> or C<""> is what you want.
1588 See also L<C<undef>|/undef EXPR>, L<C<exists>|/exists EXPR>,
1589 L<C<ref>|/ref EXPR>.
1594 =for Pod::Functions deletes a value from a hash
1596 Given an expression that specifies an element or slice of a hash,
1597 L<C<delete>|/delete EXPR> deletes the specified elements from that hash
1598 so that L<C<exists>|/exists EXPR> on that element no longer returns
1599 true. Setting a hash element to the undefined value does not remove its
1600 key, but deleting it does; see L<C<exists>|/exists EXPR>.
1602 In list context, usually returns the value or values deleted, or the last such
1603 element in scalar context. The return list's length corresponds to that of
1604 the argument list: deleting non-existent elements returns the undefined value
1605 in their corresponding positions. When a
1606 L<keyE<sol>value hash slice|perldata/KeyE<sol>Value Hash Slices> is passed to
1607 C<delete>, the return value is a list of key/value pairs (two elements for each
1608 item deleted from the hash).
1610 L<C<delete>|/delete EXPR> may also be used on arrays and array slices,
1611 but its behavior is less straightforward. Although
1612 L<C<exists>|/exists EXPR> will return false for deleted entries,
1613 deleting array elements never changes indices of existing values; use
1614 L<C<shift>|/shift ARRAY> or L<C<splice>|/splice
1615 ARRAY,OFFSET,LENGTH,LIST> for that. However, if any deleted elements
1616 fall at the end of an array, the array's size shrinks to the position of
1617 the highest element that still tests true for L<C<exists>|/exists EXPR>,
1618 or to 0 if none do. In other words, an array won't have trailing
1619 nonexistent elements after a delete.
1621 B<WARNING:> Calling L<C<delete>|/delete EXPR> on array values is
1622 strongly discouraged. The
1623 notion of deleting or checking the existence of Perl array elements is not
1624 conceptually coherent, and can lead to surprising behavior.
1626 Deleting from L<C<%ENV>|perlvar/%ENV> modifies the environment.
1627 Deleting from a hash tied to a DBM file deletes the entry from the DBM
1628 file. Deleting from a L<C<tied>|/tied VARIABLE> hash or array may not
1629 necessarily return anything; it depends on the implementation of the
1630 L<C<tied>|/tied VARIABLE> package's DELETE method, which may do whatever
1633 The C<delete local EXPR> construct localizes the deletion to the current
1634 block at run time. Until the block exits, elements locally deleted
1635 temporarily no longer exist. See L<perlsub/"Localized deletion of elements
1636 of composite types">.
1638 my %hash = (foo => 11, bar => 22, baz => 33);
1639 my $scalar = delete $hash{foo}; # $scalar is 11
1640 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
1641 my @array = delete @hash{qw(foo baz)}; # @array is (undef,33)
1643 The following (inefficiently) deletes all the values of %HASH and @ARRAY:
1645 foreach my $key (keys %HASH) {
1649 foreach my $index (0 .. $#ARRAY) {
1650 delete $ARRAY[$index];
1655 delete @HASH{keys %HASH};
1657 delete @ARRAY[0 .. $#ARRAY];
1659 But both are slower than assigning the empty list
1660 or undefining %HASH or @ARRAY, which is the customary
1661 way to empty out an aggregate:
1663 %HASH = (); # completely empty %HASH
1664 undef %HASH; # forget %HASH ever existed
1666 @ARRAY = (); # completely empty @ARRAY
1667 undef @ARRAY; # forget @ARRAY ever existed
1669 The EXPR can be arbitrarily complicated provided its
1670 final operation is an element or slice of an aggregate:
1672 delete $ref->[$x][$y]{$key};
1673 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
1675 delete $ref->[$x][$y][$index];
1676 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1679 X<die> X<throw> X<exception> X<raise> X<$@> X<abort>
1681 =for Pod::Functions raise an exception or bail out
1683 L<C<die>|/die LIST> raises an exception. Inside an L<C<eval>|/eval EXPR>
1684 the exception is stuffed into L<C<$@>|perlvar/$@> and the L<C<eval>|/eval
1685 EXPR> is terminated with the undefined value. If the exception is
1686 outside of all enclosing L<C<eval>|/eval EXPR>s, then the uncaught
1687 exception is printed to C<STDERR> and perl exits with an exit code
1688 indicating failure. If you need to exit the process with a specific
1689 exit code, see L<C<exit>|/exit EXPR>.
1691 Equivalent examples:
1693 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
1694 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
1696 Most of the time, C<die> is called with a string to use as the exception.
1697 You may either give a single non-reference operand to serve as the
1698 exception, or a list of two or more items, which will be stringified
1699 and concatenated to make the exception.
1701 If the string exception does not end in a newline, the current
1702 script line number and input line number (if any) and a newline
1703 are appended to it. Note that the "input line number" (also
1704 known as "chunk") is subject to whatever notion of "line" happens to
1705 be currently in effect, and is also available as the special variable
1706 L<C<$.>|perlvar/$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1708 Hint: sometimes appending C<", stopped"> to your message will cause it
1709 to make better sense when the string C<"at foo line 123"> is appended.
1710 Suppose you are running script "canasta".
1712 die "/etc/games is no good";
1713 die "/etc/games is no good, stopped";
1715 produce, respectively
1717 /etc/games is no good at canasta line 123.
1718 /etc/games is no good, stopped at canasta line 123.
1720 If LIST was empty or made an empty string, and L<C<$@>|perlvar/$@>
1721 already contains an exception value (typically from a previous
1722 L<C<eval>|/eval EXPR>), then that value is reused after
1723 appending C<"\t...propagated">. This is useful for propagating exceptions:
1726 die unless $@ =~ /Expected exception/;
1728 If LIST was empty or made an empty string,
1729 and L<C<$@>|perlvar/$@> contains an object
1730 reference that has a C<PROPAGATE> method, that method will be called
1731 with additional file and line number parameters. The return value
1732 replaces the value in L<C<$@>|perlvar/$@>; i.e., as if
1733 C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >> were called.
1735 If LIST was empty or made an empty string, and L<C<$@>|perlvar/$@>
1736 is also empty, then the string C<"Died"> is used.
1738 You can also call L<C<die>|/die LIST> with a reference argument, and if
1739 this is trapped within an L<C<eval>|/eval EXPR>, L<C<$@>|perlvar/$@>
1740 contains that reference. This permits more elaborate exception handling
1741 using objects that maintain arbitrary state about the exception. Such a
1742 scheme is sometimes preferable to matching particular string values of
1743 L<C<$@>|perlvar/$@> with regular expressions.
1745 Because Perl stringifies uncaught exception messages before display,
1746 you'll probably want to overload stringification operations on
1747 exception objects. See L<overload> for details about that.
1748 The stringified message should be non-empty, and should end in a newline,
1749 in order to fit in with the treatment of string exceptions.
1750 Also, because an exception object reference cannot be stringified
1751 without destroying it, Perl doesn't attempt to append location or other
1752 information to a reference exception. If you want location information
1753 with a complex exception object, you'll have to arrange to put the
1754 location information into the object yourself.
1756 Because L<C<$@>|perlvar/$@> is a global variable, be careful that
1757 analyzing an exception caught by C<eval> doesn't replace the reference
1758 in the global variable. It's
1759 easiest to make a local copy of the reference before any manipulations.
1762 use Scalar::Util "blessed";
1764 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1765 if (my $ev_err = $@) {
1766 if (blessed($ev_err)
1767 && $ev_err->isa("Some::Module::Exception")) {
1768 # handle Some::Module::Exception
1771 # handle all other possible exceptions
1775 If an uncaught exception results in interpreter exit, the exit code is
1776 determined from the values of L<C<$!>|perlvar/$!> and
1777 L<C<$?>|perlvar/$?> with this pseudocode:
1779 exit $! if $!; # errno
1780 exit $? >> 8 if $? >> 8; # child exit status
1781 exit 255; # last resort
1783 As with L<C<exit>|/exit EXPR>, L<C<$?>|perlvar/$?> is set prior to
1784 unwinding the call stack; any C<DESTROY> or C<END> handlers can then
1785 alter this value, and thus Perl's exit code.
1787 The intent is to squeeze as much possible information about the likely cause
1788 into the limited space of the system exit code. However, as
1789 L<C<$!>|perlvar/$!> is the value of C's C<errno>, which can be set by
1790 any system call, this means that the value of the exit code used by
1791 L<C<die>|/die LIST> can be non-predictable, so should not be relied
1792 upon, other than to be non-zero.
1794 You can arrange for a callback to be run just before the
1795 L<C<die>|/die LIST> does its deed, by setting the
1796 L<C<$SIG{__DIE__}>|perlvar/%SIG> hook. The associated handler is called
1797 with the exception as an argument, and can change the exception,
1799 calling L<C<die>|/die LIST> again. See L<perlvar/%SIG> for details on
1800 setting L<C<%SIG>|perlvar/%SIG> entries, and L<C<eval>|/eval EXPR> for some
1801 examples. Although this feature was to be run only right before your
1802 program was to exit, this is not currently so: the
1803 L<C<$SIG{__DIE__}>|perlvar/%SIG> hook is currently called even inside
1804 L<C<eval>|/eval EXPR>ed blocks/strings! If one wants the hook to do
1805 nothing in such situations, put
1809 as the first line of the handler (see L<perlvar/$^S>). Because
1810 this promotes strange action at a distance, this counterintuitive
1811 behavior may be fixed in a future release.
1813 See also L<C<exit>|/exit EXPR>, L<C<warn>|/warn LIST>, and the L<Carp>
1819 =for Pod::Functions turn a BLOCK into a TERM
1821 Not really a function. Returns the value of the last command in the
1822 sequence of commands indicated by BLOCK. When modified by the C<while> or
1823 C<until> loop modifier, executes the BLOCK once before testing the loop
1824 condition. (On other statements the loop modifiers test the conditional
1827 C<do BLOCK> does I<not> count as a loop, so the loop control statements
1828 L<C<next>|/next LABEL>, L<C<last>|/last LABEL>, or
1829 L<C<redo>|/redo LABEL> cannot be used to leave or restart the block.
1830 See L<perlsyn> for alternative strategies.
1835 Uses the value of EXPR as a filename and executes the contents of the
1836 file as a Perl script:
1838 # load the exact specified file (./ and ../ special-cased)
1841 do '../foo/stat.pl';
1843 # search for the named file within @INC
1847 C<do './stat.pl'> is largely like
1851 except that it's more concise, runs no external processes, and keeps
1852 track of the current filename for error messages. It also differs in that
1853 code evaluated with C<do FILE> cannot see lexicals in the enclosing
1854 scope; C<eval STRING> does. It's the same, however, in that it does
1855 reparse the file every time you call it, so you probably don't want
1856 to do this inside a loop.
1858 Using C<do> with a relative path (except for F<./> and F<../>), like
1862 will search the L<C<@INC>|perlvar/@INC> directories, and update
1863 L<C<%INC>|perlvar/%INC> if the file is found. See L<perlvar/@INC>
1864 and L<perlvar/%INC> for these variables. In particular, note that
1865 whilst historically L<C<@INC>|perlvar/@INC> contained '.' (the
1866 current directory) making these two cases equivalent, that is no
1867 longer necessarily the case, as '.' is not included in C<@INC> by default
1868 in perl versions 5.26.0 onwards. Instead, perl will now warn:
1870 do "stat.pl" failed, '.' is no longer in @INC;
1871 did you mean do "./stat.pl"?
1873 If L<C<do>|/do EXPR> can read the file but cannot compile it, it
1874 returns L<C<undef>|/undef EXPR> and sets an error message in
1875 L<C<$@>|perlvar/$@>. If L<C<do>|/do EXPR> cannot read the file, it
1876 returns undef and sets L<C<$!>|perlvar/$!> to the error. Always check
1877 L<C<$@>|perlvar/$@> first, as compilation could fail in a way that also
1878 sets L<C<$!>|perlvar/$!>. If the file is successfully compiled,
1879 L<C<do>|/do EXPR> returns the value of the last expression evaluated.
1881 Inclusion of library modules is better done with the
1882 L<C<use>|/use Module VERSION LIST> and L<C<require>|/require VERSION>
1883 operators, which also do automatic error checking and raise an exception
1884 if there's a problem.
1886 You might like to use L<C<do>|/do EXPR> to read in a program
1887 configuration file. Manual error checking can be done this way:
1889 # Read in config files: system first, then user.
1890 # Beware of using relative pathnames here.
1891 for $file ("/share/prog/defaults.rc",
1892 "$ENV{HOME}/.someprogrc")
1894 unless ($return = do $file) {
1895 warn "couldn't parse $file: $@" if $@;
1896 warn "couldn't do $file: $!" unless defined $return;
1897 warn "couldn't run $file" unless $return;
1902 X<dump> X<core> X<undump>
1908 =for Pod::Functions create an immediate core dump
1910 This function causes an immediate core dump. See also the B<-u>
1911 command-line switch in L<perlrun>, which does the same thing.
1912 Primarily this is so that you can use the B<undump> program (not
1913 supplied) to turn your core dump into an executable binary after
1914 having initialized all your variables at the beginning of the
1915 program. When the new binary is executed it will begin by executing
1916 a C<goto LABEL> (with all the restrictions that L<C<goto>|/goto LABEL>
1918 Think of it as a goto with an intervening core dump and reincarnation.
1919 If C<LABEL> is omitted, restarts the program from the top. The
1920 C<dump EXPR> form, available starting in Perl 5.18.0, allows a name to be
1921 computed at run time, being otherwise identical to C<dump LABEL>.
1923 B<WARNING>: Any files opened at the time of the dump will I<not>
1924 be open any more when the program is reincarnated, with possible
1925 resulting confusion by Perl.
1927 This function is now largely obsolete, mostly because it's very hard to
1928 convert a core file into an executable. As of Perl 5.30, it must be invoked
1931 Unlike most named operators, this has the same precedence as assignment.
1932 It is also exempt from the looks-like-a-function rule, so
1933 C<dump ("foo")."bar"> will cause "bar" to be part of the argument to
1934 L<C<dump>|/dump LABEL>.
1936 Portability issues: L<perlport/dump>.
1939 X<each> X<hash, iterator>
1944 =for Pod::Functions retrieve the next key/value pair from a hash
1946 When called on a hash in list context, returns a 2-element list
1947 consisting of the key and value for the next element of a hash. In Perl
1948 5.12 and later only, it will also return the index and value for the next
1949 element of an array so that you can iterate over it; older Perls consider
1950 this a syntax error. When called in scalar context, returns only the key
1951 (not the value) in a hash, or the index in an array.
1953 Hash entries are returned in an apparently random order. The actual random
1954 order is specific to a given hash; the exact same series of operations
1955 on two hashes may result in a different order for each hash. Any insertion
1956 into the hash may change the order, as will any deletion, with the exception
1957 that the most recent key returned by L<C<each>|/each HASH> or
1958 L<C<keys>|/keys HASH> may be deleted without changing the order. So
1959 long as a given hash is unmodified you may rely on
1960 L<C<keys>|/keys HASH>, L<C<values>|/values HASH> and
1961 L<C<each>|/each HASH> to repeatedly return the same order
1962 as each other. See L<perlsec/"Algorithmic Complexity Attacks"> for
1963 details on why hash order is randomized. Aside from the guarantees
1964 provided here the exact details of Perl's hash algorithm and the hash
1965 traversal order are subject to change in any release of Perl.
1967 After L<C<each>|/each HASH> has returned all entries from the hash or
1968 array, the next call to L<C<each>|/each HASH> returns the empty list in
1969 list context and L<C<undef>|/undef EXPR> in scalar context; the next
1970 call following I<that> one restarts iteration. Each hash or array has
1971 its own internal iterator, accessed by L<C<each>|/each HASH>,
1972 L<C<keys>|/keys HASH>, and L<C<values>|/values HASH>. The iterator is
1973 implicitly reset when L<C<each>|/each HASH> has reached the end as just
1974 described; it can be explicitly reset by calling L<C<keys>|/keys HASH>
1975 or L<C<values>|/values HASH> on the hash or array, or by referencing
1976 the hash (but not array) in list context. If you add or delete
1977 a hash's elements while iterating over it, the effect on the iterator is
1978 unspecified; for example, entries may be skipped or duplicated--so don't
1979 do that. Exception: It is always safe to delete the item most recently
1980 returned by L<C<each>|/each HASH>, so the following code works properly:
1982 while (my ($key, $value) = each %hash) {
1984 delete $hash{$key}; # This is safe
1987 Tied hashes may have a different ordering behaviour to perl's hash
1990 The iterator used by C<each> is attached to the hash or array, and is
1991 shared between all iteration operations applied to the same hash or array.
1992 Thus all uses of C<each> on a single hash or array advance the same
1993 iterator location. All uses of C<each> are also subject to having the
1994 iterator reset by any use of C<keys> or C<values> on the same hash or
1995 array, or by the hash (but not array) being referenced in list context.
1996 This makes C<each>-based loops quite fragile: it is easy to arrive at
1997 such a loop with the iterator already part way through the object, or to
1998 accidentally clobber the iterator state during execution of the loop body.
1999 It's easy enough to explicitly reset the iterator before starting a loop,
2000 but there is no way to insulate the iterator state used by a loop from
2001 the iterator state used by anything else that might execute during the
2002 loop body. To avoid these problems, use a C<foreach> loop rather than
2005 This prints out your environment like the L<printenv(1)> program,
2006 but in a different order:
2008 while (my ($key,$value) = each %ENV) {
2009 print "$key=$value\n";
2012 Starting with Perl 5.14, an experimental feature allowed
2013 L<C<each>|/each HASH> to take a scalar expression. This experiment has
2014 been deemed unsuccessful, and was removed as of Perl 5.24.
2016 As of Perl 5.18 you can use a bare L<C<each>|/each HASH> in a C<while>
2017 loop, which will set L<C<$_>|perlvar/$_> on every iteration.
2018 If either an C<each> expression or an explicit assignment of an C<each>
2019 expression to a scalar is used as a C<while>/C<for> condition, then
2020 the condition actually tests for definedness of the expression's value,
2021 not for its regular truth value.
2024 print "$_=$ENV{$_}\n";
2027 To avoid confusing would-be users of your code who are running earlier
2028 versions of Perl with mysterious syntax errors, put this sort of thing at
2029 the top of your file to signal that your code will work I<only> on Perls of
2032 use 5.012; # so keys/values/each work on arrays
2033 use 5.018; # so each assigns to $_ in a lone while test
2035 See also L<C<keys>|/keys HASH>, L<C<values>|/values HASH>, and
2036 L<C<sort>|/sort SUBNAME LIST>.
2038 =item eof FILEHANDLE
2047 =for Pod::Functions test a filehandle for its end
2049 Returns 1 if the next read on FILEHANDLE will return end of file I<or> if
2050 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
2051 gives the real filehandle. (Note that this function actually
2052 reads a character and then C<ungetc>s it, so isn't useful in an
2053 interactive context.) Do not read from a terminal file (or call
2054 C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
2055 as terminals may lose the end-of-file condition if you do.
2057 An L<C<eof>|/eof FILEHANDLE> without an argument uses the last file
2058 read. Using L<C<eof()>|/eof FILEHANDLE> with empty parentheses is
2059 different. It refers to the pseudo file formed from the files listed on
2060 the command line and accessed via the C<< <> >> operator. Since
2061 C<< <> >> isn't explicitly opened, as a normal filehandle is, an
2062 L<C<eof()>|/eof FILEHANDLE> before C<< <> >> has been used will cause
2063 L<C<@ARGV>|perlvar/@ARGV> to be examined to determine if input is
2064 available. Similarly, an L<C<eof()>|/eof FILEHANDLE> after C<< <> >>
2065 has returned end-of-file will assume you are processing another
2066 L<C<@ARGV>|perlvar/@ARGV> list, and if you haven't set
2067 L<C<@ARGV>|perlvar/@ARGV>, will read input from C<STDIN>; see
2068 L<perlop/"I/O Operators">.
2070 In a C<< while (<>) >> loop, L<C<eof>|/eof FILEHANDLE> or C<eof(ARGV)>
2071 can be used to detect the end of each file, whereas
2072 L<C<eof()>|/eof FILEHANDLE> will detect the end of the very last file
2075 # reset line numbering on each input file
2077 next if /^\s*#/; # skip comments
2080 close ARGV if eof; # Not eof()!
2083 # insert dashes just before last line of last file
2085 if (eof()) { # check for end of last file
2086 print "--------------\n";
2089 last if eof(); # needed if we're reading from a terminal
2092 Practical hint: you almost never need to use L<C<eof>|/eof FILEHANDLE>
2093 in Perl, because the input operators typically return L<C<undef>|/undef
2094 EXPR> when they run out of data or encounter an error.
2097 X<eval> X<try> X<catch> X<evaluate> X<parse> X<execute>
2098 X<error, handling> X<exception, handling>
2104 =for Pod::Functions catch exceptions or compile and run code
2106 C<eval> in all its forms is used to execute a little Perl program,
2107 trapping any errors encountered so they don't crash the calling program.
2109 Plain C<eval> with no argument is just C<eval EXPR>, where the
2110 expression is understood to be contained in L<C<$_>|perlvar/$_>. Thus
2111 there are only two real C<eval> forms; the one with an EXPR is often
2112 called "string eval". In a string eval, the value of the expression
2113 (which is itself determined within scalar context) is first parsed, and
2114 if there were no errors, executed as a block within the lexical context
2115 of the current Perl program. This form is typically used to delay
2116 parsing and subsequent execution of the text of EXPR until run time.
2117 Note that the value is parsed every time the C<eval> executes.
2119 The other form is called "block eval". It is less general than string
2120 eval, but the code within the BLOCK is parsed only once (at the same
2121 time the code surrounding the C<eval> itself was parsed) and executed
2122 within the context of the current Perl program. This form is typically
2123 used to trap exceptions more efficiently than the first, while also
2124 providing the benefit of checking the code within BLOCK at compile time.
2125 BLOCK is parsed and compiled just once. Since errors are trapped, it
2126 often is used to check if a given feature is available.
2128 In both forms, the value returned is the value of the last expression
2129 evaluated inside the mini-program; a return statement may also be used, just
2130 as with subroutines. The expression providing the return value is evaluated
2131 in void, scalar, or list context, depending on the context of the
2132 C<eval> itself. See L<C<wantarray>|/wantarray> for more
2133 on how the evaluation context can be determined.
2135 If there is a syntax error or runtime error, or a L<C<die>|/die LIST>
2136 statement is executed, C<eval> returns
2137 L<C<undef>|/undef EXPR> in scalar context, or an empty list in list
2138 context, and L<C<$@>|perlvar/$@> is set to the error message. (Prior to
2139 5.16, a bug caused L<C<undef>|/undef EXPR> to be returned in list
2140 context for syntax errors, but not for runtime errors.) If there was no
2141 error, L<C<$@>|perlvar/$@> is set to the empty string. A control flow
2142 operator like L<C<last>|/last LABEL> or L<C<goto>|/goto LABEL> can
2143 bypass the setting of L<C<$@>|perlvar/$@>. Beware that using
2144 C<eval> neither silences Perl from printing warnings to
2145 STDERR, nor does it stuff the text of warning messages into
2146 L<C<$@>|perlvar/$@>. To do either of those, you have to use the
2147 L<C<$SIG{__WARN__}>|perlvar/%SIG> facility, or turn off warnings inside
2148 the BLOCK or EXPR using S<C<no warnings 'all'>>. See
2149 L<C<warn>|/warn LIST>, L<perlvar>, and L<warnings>.
2151 Note that, because C<eval> traps otherwise-fatal errors,
2152 it is useful for determining whether a particular feature (such as
2153 L<C<socket>|/socket SOCKET,DOMAIN,TYPE,PROTOCOL> or
2154 L<C<symlink>|/symlink OLDFILE,NEWFILE>) is implemented. It is also
2155 Perl's exception-trapping mechanism, where the L<C<die>|/die LIST>
2156 operator is used to raise exceptions.
2158 Before Perl 5.14, the assignment to L<C<$@>|perlvar/$@> occurred before
2160 of localized variables, which means that for your code to run on older
2161 versions, a temporary is required if you want to mask some, but not all
2164 # alter $@ on nefarious repugnancy only
2168 local $@; # protect existing $@
2169 eval { test_repugnancy() };
2170 # $@ =~ /nefarious/ and die $@; # Perl 5.14 and higher only
2171 $@ =~ /nefarious/ and $e = $@;
2173 die $e if defined $e
2176 There are some different considerations for each form:
2182 Since the return value of EXPR is executed as a block within the lexical
2183 context of the current Perl program, any outer lexical variables are
2184 visible to it, and any package variable settings or subroutine and
2185 format definitions remain afterwards.
2189 =item Under the L<C<"unicode_eval"> feature|feature/The 'unicode_eval' and 'evalbytes' features>
2191 If this feature is enabled (which is the default under a C<use 5.16> or
2192 higher declaration), EXPR is considered to be
2193 in the same encoding as the surrounding program. Thus if
2194 S<L<C<use utf8>|utf8>> is in effect, the string will be treated as being
2195 UTF-8 encoded. Otherwise, the string is considered to be a sequence of
2196 independent bytes. Bytes that correspond to ASCII-range code points
2197 will have their normal meanings for operators in the string. The
2198 treatment of the other bytes depends on if the
2199 L<C<'unicode_strings"> feature|feature/The 'unicode_strings' feature> is
2202 In a plain C<eval> without an EXPR argument, being in S<C<use utf8>> or
2203 not is irrelevant; the UTF-8ness of C<$_> itself determines the
2206 Any S<C<use utf8>> or S<C<no utf8>> declarations within the string have
2207 no effect, and source filters are forbidden. (C<unicode_strings>,
2208 however, can appear within the string.) See also the
2209 L<C<evalbytes>|/evalbytes EXPR> operator, which works properly with
2212 Variables defined outside the C<eval> and used inside it retain their
2213 original UTF-8ness. Everything inside the string follows the normal
2214 rules for a Perl program with the given state of S<C<use utf8>>.
2216 =item Outside the C<"unicode_eval"> feature
2218 In this case, the behavior is problematic and is not so easily
2219 described. Here are two bugs that cannot easily be fixed without
2220 breaking existing programs:
2226 It can lose track of whether something should be encoded as UTF-8 or
2231 Source filters activated within C<eval> leak out into whichever file
2232 scope is currently being compiled. To give an example with the CPAN module
2233 L<Semi::Semicolons>:
2235 BEGIN { eval "use Semi::Semicolons; # not filtered" }
2238 L<C<evalbytes>|/evalbytes EXPR> fixes that to work the way one would
2241 use feature "evalbytes";
2242 BEGIN { evalbytes "use Semi::Semicolons; # filtered" }
2249 Problems can arise if the string expands a scalar containing a floating
2250 point number. That scalar can expand to letters, such as C<"NaN"> or
2251 C<"Infinity">; or, within the scope of a L<C<use locale>|locale>, the
2252 decimal point character may be something other than a dot (such as a
2253 comma). None of these are likely to parse as you are likely expecting.
2255 You should be especially careful to remember what's being looked at
2262 eval { $x }; # CASE 4
2264 eval "\$$x++"; # CASE 5
2267 Cases 1 and 2 above behave identically: they run the code contained in
2268 the variable $x. (Although case 2 has misleading double quotes making
2269 the reader wonder what else might be happening (nothing is).) Cases 3
2270 and 4 likewise behave in the same way: they run the code C<'$x'>, which
2271 does nothing but return the value of $x. (Case 4 is preferred for
2272 purely visual reasons, but it also has the advantage of compiling at
2273 compile-time instead of at run-time.) Case 5 is a place where
2274 normally you I<would> like to use double quotes, except that in this
2275 particular situation, you can just use symbolic references instead, as
2278 An C<eval ''> executed within a subroutine defined
2279 in the C<DB> package doesn't see the usual
2280 surrounding lexical scope, but rather the scope of the first non-DB piece
2281 of code that called it. You don't normally need to worry about this unless
2282 you are writing a Perl debugger.
2284 The final semicolon, if any, may be omitted from the value of EXPR.
2288 If the code to be executed doesn't vary, you may use the eval-BLOCK
2289 form to trap run-time errors without incurring the penalty of
2290 recompiling each time. The error, if any, is still returned in
2291 L<C<$@>|perlvar/$@>.
2294 # make divide-by-zero nonfatal
2295 eval { $answer = $a / $b; }; warn $@ if $@;
2297 # same thing, but less efficient
2298 eval '$answer = $a / $b'; warn $@ if $@;
2300 # a compile-time error
2301 eval { $answer = }; # WRONG
2304 eval '$answer ='; # sets $@
2306 If you want to trap errors when loading an XS module, some problems with
2307 the binary interface (such as Perl version skew) may be fatal even with
2308 C<eval> unless C<$ENV{PERL_DL_NONLAZY}> is set. See
2311 Using the C<eval {}> form as an exception trap in libraries does have some
2312 issues. Due to the current arguably broken state of C<__DIE__> hooks, you
2313 may wish not to trigger any C<__DIE__> hooks that user code may have installed.
2314 You can use the C<local $SIG{__DIE__}> construct for this purpose,
2315 as this example shows:
2317 # a private exception trap for divide-by-zero
2318 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
2321 This is especially significant, given that C<__DIE__> hooks can call
2322 L<C<die>|/die LIST> again, which has the effect of changing their error
2325 # __DIE__ hooks may modify error messages
2327 local $SIG{'__DIE__'} =
2328 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
2329 eval { die "foo lives here" };
2330 print $@ if $@; # prints "bar lives here"
2333 Because this promotes action at a distance, this counterintuitive behavior
2334 may be fixed in a future release.
2336 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
2337 L<C<next>|/next LABEL>, L<C<last>|/last LABEL>, or
2338 L<C<redo>|/redo LABEL> cannot be used to leave or restart the block.
2340 The final semicolon, if any, may be omitted from within the BLOCK.
2344 =item evalbytes EXPR
2349 =for Pod::Functions +evalbytes similar to string eval, but intend to parse a bytestream
2351 This function is similar to a L<string eval|/eval EXPR>, except it
2352 always parses its argument (or L<C<$_>|perlvar/$_> if EXPR is omitted)
2353 as a string of independent bytes.
2355 If called when S<C<use utf8>> is in effect, the string will be assumed
2356 to be encoded in UTF-8, and C<evalbytes> will make a temporary copy to
2357 work from, downgraded to non-UTF-8. If this is not possible
2358 (because one or more characters in it require UTF-8), the C<evalbytes>
2359 will fail with the error stored in C<$@>.
2361 Bytes that correspond to ASCII-range code points will have their normal
2362 meanings for operators in the string. The treatment of the other bytes
2363 depends on if the L<C<'unicode_strings"> feature|feature/The
2364 'unicode_strings' feature> is in effect.
2366 Of course, variables that are UTF-8 and are referred to in the string
2370 evalbytes 'print ord $a, "\n"';
2378 Source filters activated within the evaluated code apply to the code
2381 L<C<evalbytes>|/evalbytes EXPR> is available starting in Perl v5.16. To
2382 access it, you must say C<CORE::evalbytes>, but you can omit the
2384 L<C<"evalbytes"> feature|feature/The 'unicode_eval' and 'evalbytes' features>
2385 is enabled. This is enabled automatically with a C<use v5.16> (or
2386 higher) declaration in the current scope.
2391 =item exec PROGRAM LIST
2393 =for Pod::Functions abandon this program to run another
2395 The L<C<exec>|/exec LIST> function executes a system command I<and never
2396 returns>; use L<C<system>|/system LIST> instead of L<C<exec>|/exec LIST>
2397 if you want it to return. It fails and
2398 returns false only if the command does not exist I<and> it is executed
2399 directly instead of via your system's command shell (see below).
2401 Since it's a common mistake to use L<C<exec>|/exec LIST> instead of
2402 L<C<system>|/system LIST>, Perl warns you if L<C<exec>|/exec LIST> is
2403 called in void context and if there is a following statement that isn't
2404 L<C<die>|/die LIST>, L<C<warn>|/warn LIST>, or L<C<exit>|/exit EXPR> (if
2405 L<warnings> are enabled--but you always do that, right?). If you
2406 I<really> want to follow an L<C<exec>|/exec LIST> with some other
2407 statement, you can use one of these styles to avoid the warning:
2409 exec ('foo') or print STDERR "couldn't exec foo: $!";
2410 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
2412 If there is more than one argument in LIST, this calls L<execvp(3)> with the
2413 arguments in LIST. If there is only one element in LIST, the argument is
2414 checked for shell metacharacters, and if there are any, the entire
2415 argument is passed to the system's command shell for parsing (this is
2416 C</bin/sh -c> on Unix platforms, but varies on other platforms). If
2417 there are no shell metacharacters in the argument, it is split into words
2418 and passed directly to C<execvp>, which is more efficient. Examples:
2420 exec '/bin/echo', 'Your arguments are: ', @ARGV;
2421 exec "sort $outfile | uniq";
2423 If you don't really want to execute the first argument, but want to lie
2424 to the program you are executing about its own name, you can specify
2425 the program you actually want to run as an "indirect object" (without a
2426 comma) in front of the LIST, as in C<exec PROGRAM LIST>. (This always
2427 forces interpretation of the LIST as a multivalued list, even if there
2428 is only a single scalar in the list.) Example:
2430 my $shell = '/bin/csh';
2431 exec $shell '-sh'; # pretend it's a login shell
2435 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
2437 When the arguments get executed via the system shell, results are
2438 subject to its quirks and capabilities. See L<perlop/"`STRING`">
2441 Using an indirect object with L<C<exec>|/exec LIST> or
2442 L<C<system>|/system LIST> is also more secure. This usage (which also
2443 works fine with L<C<system>|/system LIST>) forces
2444 interpretation of the arguments as a multivalued list, even if the
2445 list had just one argument. That way you're safe from the shell
2446 expanding wildcards or splitting up words with whitespace in them.
2448 my @args = ( "echo surprise" );
2450 exec @args; # subject to shell escapes
2452 exec { $args[0] } @args; # safe even with one-arg list
2454 The first version, the one without the indirect object, ran the I<echo>
2455 program, passing it C<"surprise"> an argument. The second version didn't;
2456 it tried to run a program named I<"echo surprise">, didn't find it, and set
2457 L<C<$?>|perlvar/$?> to a non-zero value indicating failure.
2459 On Windows, only the C<exec PROGRAM LIST> indirect object syntax will
2460 reliably avoid using the shell; C<exec LIST>, even with more than one
2461 element, will fall back to the shell if the first spawn fails.
2463 Perl attempts to flush all files opened for output before the exec,
2464 but this may not be supported on some platforms (see L<perlport>).
2465 To be safe, you may need to set L<C<$E<verbar>>|perlvar/$E<verbar>>
2466 (C<$AUTOFLUSH> in L<English>) or call the C<autoflush> method of
2467 L<C<IO::Handle>|IO::Handle/METHODS> on any open handles to avoid lost
2470 Note that L<C<exec>|/exec LIST> will not call your C<END> blocks, nor
2471 will it invoke C<DESTROY> methods on your objects.
2473 Portability issues: L<perlport/exec>.
2476 X<exists> X<autovivification>
2478 =for Pod::Functions test whether a hash key is present
2480 Given an expression that specifies an element of a hash, returns true if the
2481 specified element in the hash has ever been initialized, even if the
2482 corresponding value is undefined.
2484 print "Exists\n" if exists $hash{$key};
2485 print "Defined\n" if defined $hash{$key};
2486 print "True\n" if $hash{$key};
2488 exists may also be called on array elements, but its behavior is much less
2489 obvious and is strongly tied to the use of L<C<delete>|/delete EXPR> on
2492 B<WARNING:> Calling L<C<exists>|/exists EXPR> on array values is
2493 strongly discouraged. The
2494 notion of deleting or checking the existence of Perl array elements is not
2495 conceptually coherent, and can lead to surprising behavior.
2497 print "Exists\n" if exists $array[$index];
2498 print "Defined\n" if defined $array[$index];
2499 print "True\n" if $array[$index];
2501 A hash or array element can be true only if it's defined and defined only if
2502 it exists, but the reverse doesn't necessarily hold true.
2504 Given an expression that specifies the name of a subroutine,
2505 returns true if the specified subroutine has ever been declared, even
2506 if it is undefined. Mentioning a subroutine name for exists or defined
2507 does not count as declaring it. Note that a subroutine that does not
2508 exist may still be callable: its package may have an C<AUTOLOAD>
2509 method that makes it spring into existence the first time that it is
2510 called; see L<perlsub>.
2512 print "Exists\n" if exists &subroutine;
2513 print "Defined\n" if defined &subroutine;
2515 Note that the EXPR can be arbitrarily complicated as long as the final
2516 operation is a hash or array key lookup or subroutine name:
2518 if (exists $ref->{A}->{B}->{$key}) { }
2519 if (exists $hash{A}{B}{$key}) { }
2521 if (exists $ref->{A}->{B}->[$ix]) { }
2522 if (exists $hash{A}{B}[$ix]) { }
2524 if (exists &{$ref->{A}{B}{$key}}) { }
2526 Although the most deeply nested array or hash element will not spring into
2527 existence just because its existence was tested, any intervening ones will.
2528 Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
2529 into existence due to the existence test for the C<$key> element above.
2530 This happens anywhere the arrow operator is used, including even here:
2533 if (exists $ref->{"Some key"}) { }
2534 print $ref; # prints HASH(0x80d3d5c)
2536 Use of a subroutine call, rather than a subroutine name, as an argument
2537 to L<C<exists>|/exists EXPR> is an error.
2540 exists &sub(); # Error
2543 X<exit> X<terminate> X<abort>
2547 =for Pod::Functions terminate this program
2549 Evaluates EXPR and exits immediately with that value. Example:
2552 exit 0 if $ans =~ /^[Xx]/;
2554 See also L<C<die>|/die LIST>. If EXPR is omitted, exits with C<0>
2556 universally recognized values for EXPR are C<0> for success and C<1>
2557 for error; other values are subject to interpretation depending on the
2558 environment in which the Perl program is running. For example, exiting
2559 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
2560 the mailer to return the item undelivered, but that's not true everywhere.
2562 Don't use L<C<exit>|/exit EXPR> to abort a subroutine if there's any
2563 chance that someone might want to trap whatever error happened. Use
2564 L<C<die>|/die LIST> instead, which can be trapped by an
2565 L<C<eval>|/eval EXPR>.
2567 The L<C<exit>|/exit EXPR> function does not always exit immediately. It
2568 calls any defined C<END> routines first, but these C<END> routines may
2569 not themselves abort the exit. Likewise any object destructors that
2570 need to be called are called before the real exit. C<END> routines and
2571 destructors can change the exit status by modifying L<C<$?>|perlvar/$?>.
2572 If this is a problem, you can call
2573 L<C<POSIX::_exit($status)>|POSIX/C<_exit>> to avoid C<END> and destructor
2574 processing. See L<perlmod> for details.
2576 Portability issues: L<perlport/exit>.
2579 X<exp> X<exponential> X<antilog> X<antilogarithm> X<e>
2583 =for Pod::Functions raise I<e> to a power
2585 Returns I<e> (the natural logarithm base) to the power of EXPR.
2586 If EXPR is omitted, gives C<exp($_)>.
2589 X<fc> X<foldcase> X<casefold> X<fold-case> X<case-fold>
2593 =for Pod::Functions +fc return casefolded version of a string
2595 Returns the casefolded version of EXPR. This is the internal function
2596 implementing the C<\F> escape in double-quoted strings.
2598 Casefolding is the process of mapping strings to a form where case
2599 differences are erased; comparing two strings in their casefolded
2600 form is effectively a way of asking if two strings are equal,
2603 Roughly, if you ever found yourself writing this
2605 lc($this) eq lc($that) # Wrong!
2607 uc($this) eq uc($that) # Also wrong!
2609 $this =~ /^\Q$that\E\z/i # Right!
2613 fc($this) eq fc($that)
2615 And get the correct results.
2617 Perl only implements the full form of casefolding, but you can access
2618 the simple folds using L<Unicode::UCD/B<casefold()>> and
2619 L<Unicode::UCD/B<prop_invmap()>>.
2620 For further information on casefolding, refer to
2621 the Unicode Standard, specifically sections 3.13 C<Default Case Operations>,
2622 4.2 C<Case-Normative>, and 5.18 C<Case Mappings>,
2623 available at L<http://www.unicode.org/versions/latest/>, as well as the
2624 Case Charts available at L<http://www.unicode.org/charts/case/>.
2626 If EXPR is omitted, uses L<C<$_>|perlvar/$_>.
2628 This function behaves the same way under various pragmas, such as within
2629 L<S<C<"use feature 'unicode_strings">>|feature/The 'unicode_strings' feature>,
2630 as L<C<lc>|/lc EXPR> does, with the single exception of
2631 L<C<fc>|/fc EXPR> of I<LATIN CAPITAL LETTER SHARP S> (U+1E9E) within the
2632 scope of L<S<C<use locale>>|locale>. The foldcase of this character
2633 would normally be C<"ss">, but as explained in the L<C<lc>|/lc EXPR>
2635 changes that cross the 255/256 boundary are problematic under locales,
2636 and are hence prohibited. Therefore, this function under locale returns
2637 instead the string C<"\x{17F}\x{17F}">, which is the I<LATIN SMALL LETTER
2638 LONG S>. Since that character itself folds to C<"s">, the string of two
2639 of them together should be equivalent to a single U+1E9E when foldcased.
2641 While the Unicode Standard defines two additional forms of casefolding,
2642 one for Turkic languages and one that never maps one character into multiple
2643 characters, these are not provided by the Perl core. However, the CPAN module
2644 L<C<Unicode::Casing>|Unicode::Casing> may be used to provide an implementation.
2646 L<C<fc>|/fc EXPR> is available only if the
2647 L<C<"fc"> feature|feature/The 'fc' feature> is enabled or if it is
2648 prefixed with C<CORE::>. The
2649 L<C<"fc"> feature|feature/The 'fc' feature> is enabled automatically
2650 with a C<use v5.16> (or higher) declaration in the current scope.
2652 =item fcntl FILEHANDLE,FUNCTION,SCALAR
2655 =for Pod::Functions file control system call
2657 Implements the L<fcntl(2)> function. You'll probably have to say
2661 first to get the correct constant definitions. Argument processing and
2662 value returned work just like L<C<ioctl>|/ioctl
2663 FILEHANDLE,FUNCTION,SCALAR> below. For example:
2666 my $flags = fcntl($filehandle, F_GETFL, 0)
2667 or die "Can't fcntl F_GETFL: $!";
2669 You don't have to check for L<C<defined>|/defined EXPR> on the return
2670 from L<C<fcntl>|/fcntl FILEHANDLE,FUNCTION,SCALAR>. Like
2671 L<C<ioctl>|/ioctl FILEHANDLE,FUNCTION,SCALAR>, it maps a C<0> return
2672 from the system call into C<"0 but true"> in Perl. This string is true
2673 in boolean context and C<0> in numeric context. It is also exempt from
2675 L<C<Argument "..." isn't numeric>|perldiag/Argument "%s" isn't numeric%s>
2676 L<warnings> on improper numeric conversions.
2678 Note that L<C<fcntl>|/fcntl FILEHANDLE,FUNCTION,SCALAR> raises an
2679 exception if used on a machine that doesn't implement L<fcntl(2)>. See
2680 the L<Fcntl> module or your L<fcntl(2)> manpage to learn what functions
2681 are available on your system.
2683 Here's an example of setting a filehandle named C<$REMOTE> to be
2684 non-blocking at the system level. You'll have to negotiate
2685 L<C<$E<verbar>>|perlvar/$E<verbar>> on your own, though.
2687 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
2689 my $flags = fcntl($REMOTE, F_GETFL, 0)
2690 or die "Can't get flags for the socket: $!\n";
2692 fcntl($REMOTE, F_SETFL, $flags | O_NONBLOCK)
2693 or die "Can't set flags for the socket: $!\n";
2695 Portability issues: L<perlport/fcntl>.
2700 =for Pod::Functions the name of the current source file
2702 A special token that returns the name of the file in which it occurs.
2704 =item fileno FILEHANDLE
2707 =item fileno DIRHANDLE
2709 =for Pod::Functions return file descriptor from filehandle
2711 Returns the file descriptor for a filehandle or directory handle,
2713 filehandle is not open. If there is no real file descriptor at the OS
2714 level, as can happen with filehandles connected to memory objects via
2715 L<C<open>|/open FILEHANDLE,EXPR> with a reference for the third
2716 argument, -1 is returned.
2718 This is mainly useful for constructing bitmaps for
2719 L<C<select>|/select RBITS,WBITS,EBITS,TIMEOUT> and low-level POSIX
2720 tty-handling operations.
2721 If FILEHANDLE is an expression, the value is taken as an indirect
2722 filehandle, generally its name.
2724 You can use this to find out whether two handles refer to the
2725 same underlying descriptor:
2727 if (fileno($this) != -1 && fileno($this) == fileno($that)) {
2728 print "\$this and \$that are dups\n";
2729 } elsif (fileno($this) != -1 && fileno($that) != -1) {
2730 print "\$this and \$that have different " .
2731 "underlying file descriptors\n";
2733 print "At least one of \$this and \$that does " .
2734 "not have a real file descriptor\n";
2737 The behavior of L<C<fileno>|/fileno FILEHANDLE> on a directory handle
2738 depends on the operating system. On a system with L<dirfd(3)> or
2739 similar, L<C<fileno>|/fileno FILEHANDLE> on a directory
2740 handle returns the underlying file descriptor associated with the
2741 handle; on systems with no such support, it returns the undefined value,
2742 and sets L<C<$!>|perlvar/$!> (errno).
2744 =item flock FILEHANDLE,OPERATION
2745 X<flock> X<lock> X<locking>
2747 =for Pod::Functions lock an entire file with an advisory lock
2749 Calls L<flock(2)>, or an emulation of it, on FILEHANDLE. Returns true
2750 for success, false on failure. Produces a fatal error if used on a
2751 machine that doesn't implement L<flock(2)>, L<fcntl(2)> locking, or
2752 L<lockf(3)>. L<C<flock>|/flock FILEHANDLE,OPERATION> is Perl's portable
2753 file-locking interface, although it locks entire files only, not
2756 Two potentially non-obvious but traditional L<C<flock>|/flock
2757 FILEHANDLE,OPERATION> semantics are
2758 that it waits indefinitely until the lock is granted, and that its locks
2759 are B<merely advisory>. Such discretionary locks are more flexible, but
2760 offer fewer guarantees. This means that programs that do not also use
2761 L<C<flock>|/flock FILEHANDLE,OPERATION> may modify files locked with
2762 L<C<flock>|/flock FILEHANDLE,OPERATION>. See L<perlport>,
2763 your port's specific documentation, and your system-specific local manpages
2764 for details. It's best to assume traditional behavior if you're writing
2765 portable programs. (But if you're not, you should as always feel perfectly
2766 free to write for your own system's idiosyncrasies (sometimes called
2767 "features"). Slavish adherence to portability concerns shouldn't get
2768 in the way of your getting your job done.)
2770 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
2771 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
2772 you can use the symbolic names if you import them from the L<Fcntl> module,
2773 either individually, or as a group using the C<:flock> tag. LOCK_SH
2774 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
2775 releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
2776 LOCK_SH or LOCK_EX, then L<C<flock>|/flock FILEHANDLE,OPERATION> returns
2777 immediately rather than blocking waiting for the lock; check the return
2778 status to see if you got it.
2780 To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
2781 before locking or unlocking it.
2783 Note that the emulation built with L<lockf(3)> doesn't provide shared
2784 locks, and it requires that FILEHANDLE be open with write intent. These
2785 are the semantics that L<lockf(3)> implements. Most if not all systems
2786 implement L<lockf(3)> in terms of L<fcntl(2)> locking, though, so the
2787 differing semantics shouldn't bite too many people.
2789 Note that the L<fcntl(2)> emulation of L<flock(3)> requires that FILEHANDLE
2790 be open with read intent to use LOCK_SH and requires that it be open
2791 with write intent to use LOCK_EX.
2793 Note also that some versions of L<C<flock>|/flock FILEHANDLE,OPERATION>
2794 cannot lock things over the network; you would need to use the more
2795 system-specific L<C<fcntl>|/fcntl FILEHANDLE,FUNCTION,SCALAR> for
2796 that. If you like you can force Perl to ignore your system's L<flock(2)>
2797 function, and so provide its own L<fcntl(2)>-based emulation, by passing
2798 the switch C<-Ud_flock> to the F<Configure> program when you configure
2799 and build a new Perl.
2801 Here's a mailbox appender for BSD systems.
2803 # import LOCK_* and SEEK_END constants
2804 use Fcntl qw(:flock SEEK_END);
2808 flock($fh, LOCK_EX) or die "Cannot lock mailbox - $!\n";
2809 # and, in case we're running on a very old UNIX
2810 # variant without the modern O_APPEND semantics...
2811 seek($fh, 0, SEEK_END) or die "Cannot seek - $!\n";
2816 flock($fh, LOCK_UN) or die "Cannot unlock mailbox - $!\n";
2819 open(my $mbox, ">>", "/usr/spool/mail/$ENV{'USER'}")
2820 or die "Can't open mailbox: $!";
2823 print $mbox $msg,"\n\n";
2826 On systems that support a real L<flock(2)>, locks are inherited across
2827 L<C<fork>|/fork> calls, whereas those that must resort to the more
2828 capricious L<fcntl(2)> function lose their locks, making it seriously
2829 harder to write servers.
2831 See also L<DB_File> for other L<C<flock>|/flock FILEHANDLE,OPERATION>
2834 Portability issues: L<perlport/flock>.
2837 X<fork> X<child> X<parent>
2839 =for Pod::Functions create a new process just like this one
2841 Does a L<fork(2)> system call to create a new process running the
2842 same program at the same point. It returns the child pid to the
2843 parent process, C<0> to the child process, or L<C<undef>|/undef EXPR> if
2845 unsuccessful. File descriptors (and sometimes locks on those descriptors)
2846 are shared, while everything else is copied. On most systems supporting
2847 L<fork(2)>, great care has gone into making it extremely efficient (for
2848 example, using copy-on-write technology on data pages), making it the
2849 dominant paradigm for multitasking over the last few decades.
2851 Perl attempts to flush all files opened for output before forking the
2852 child process, but this may not be supported on some platforms (see
2853 L<perlport>). To be safe, you may need to set
2854 L<C<$E<verbar>>|perlvar/$E<verbar>> (C<$AUTOFLUSH> in L<English>) or
2855 call the C<autoflush> method of L<C<IO::Handle>|IO::Handle/METHODS> on
2856 any open handles to avoid duplicate output.
2858 If you L<C<fork>|/fork> without ever waiting on your children, you will
2859 accumulate zombies. On some systems, you can avoid this by setting
2860 L<C<$SIG{CHLD}>|perlvar/%SIG> to C<"IGNORE">. See also L<perlipc> for
2861 more examples of forking and reaping moribund children.
2863 Note that if your forked child inherits system file descriptors like
2864 STDIN and STDOUT that are actually connected by a pipe or socket, even
2865 if you exit, then the remote server (such as, say, a CGI script or a
2866 backgrounded job launched from a remote shell) won't think you're done.
2867 You should reopen those to F</dev/null> if it's any issue.
2869 On some platforms such as Windows, where the L<fork(2)> system call is
2870 not available, Perl can be built to emulate L<C<fork>|/fork> in the Perl
2871 interpreter. The emulation is designed, at the level of the Perl
2872 program, to be as compatible as possible with the "Unix" L<fork(2)>.
2873 However it has limitations that have to be considered in code intended
2874 to be portable. See L<perlfork> for more details.
2876 Portability issues: L<perlport/fork>.
2881 =for Pod::Functions declare a picture format with use by the write() function
2883 Declare a picture format for use by the L<C<write>|/write FILEHANDLE>
2884 function. For example:
2887 Test: @<<<<<<<< @||||| @>>>>>
2888 $str, $%, '$' . int($num)
2892 $num = $cost/$quantity;
2896 See L<perlform> for many details and examples.
2898 =item formline PICTURE,LIST
2901 =for Pod::Functions internal function used for formats
2903 This is an internal function used by L<C<format>|/format>s, though you
2904 may call it, too. It formats (see L<perlform>) a list of values
2905 according to the contents of PICTURE, placing the output into the format
2906 output accumulator, L<C<$^A>|perlvar/$^A> (or C<$ACCUMULATOR> in
2907 L<English>). Eventually, when a L<C<write>|/write FILEHANDLE> is done,
2908 the contents of L<C<$^A>|perlvar/$^A> are written to some filehandle.
2909 You could also read L<C<$^A>|perlvar/$^A> and then set
2910 L<C<$^A>|perlvar/$^A> back to C<"">. Note that a format typically does
2911 one L<C<formline>|/formline PICTURE,LIST> per line of form, but the
2912 L<C<formline>|/formline PICTURE,LIST> function itself doesn't care how
2913 many newlines are embedded in the PICTURE. This means that the C<~> and
2914 C<~~> tokens treat the entire PICTURE as a single line. You may
2915 therefore need to use multiple formlines to implement a single record
2916 format, just like the L<C<format>|/format> compiler.
2918 Be careful if you put double quotes around the picture, because an C<@>
2919 character may be taken to mean the beginning of an array name.
2920 L<C<formline>|/formline PICTURE,LIST> always returns true. See
2921 L<perlform> for other examples.
2923 If you are trying to use this instead of L<C<write>|/write FILEHANDLE>
2924 to capture the output, you may find it easier to open a filehandle to a
2925 scalar (C<< open my $fh, ">", \$output >>) and write to that instead.
2927 =item getc FILEHANDLE
2928 X<getc> X<getchar> X<character> X<file, read>
2932 =for Pod::Functions get the next character from the filehandle
2934 Returns the next character from the input file attached to FILEHANDLE,
2935 or the undefined value at end of file or if there was an error (in
2936 the latter case L<C<$!>|perlvar/$!> is set). If FILEHANDLE is omitted,
2938 STDIN. This is not particularly efficient. However, it cannot be
2939 used by itself to fetch single characters without waiting for the user
2940 to hit enter. For that, try something more like:
2943 system "stty cbreak </dev/tty >/dev/tty 2>&1";
2946 system "stty", '-icanon', 'eol', "\001";
2949 my $key = getc(STDIN);
2952 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
2955 system 'stty', 'icanon', 'eol', '^@'; # ASCII NUL
2959 Determination of whether C<$BSD_STYLE> should be set is left as an
2960 exercise to the reader.
2962 The L<C<POSIX::getattr>|POSIX/C<getattr>> function can do this more
2963 portably on systems purporting POSIX compliance. See also the
2964 L<C<Term::ReadKey>|Term::ReadKey> module on CPAN.
2967 X<getlogin> X<login>
2969 =for Pod::Functions return who logged in at this tty
2971 This implements the C library function of the same name, which on most
2972 systems returns the current login from F</etc/utmp>, if any. If it
2973 returns the empty string, use L<C<getpwuid>|/getpwuid UID>.
2975 my $login = getlogin || getpwuid($<) || "Kilroy";
2977 Do not consider L<C<getlogin>|/getlogin> for authentication: it is not
2978 as secure as L<C<getpwuid>|/getpwuid UID>.
2980 Portability issues: L<perlport/getlogin>.
2982 =item getpeername SOCKET
2983 X<getpeername> X<peer>
2985 =for Pod::Functions find the other end of a socket connection
2987 Returns the packed sockaddr address of the other end of the SOCKET
2991 my $hersockaddr = getpeername($sock);
2992 my ($port, $iaddr) = sockaddr_in($hersockaddr);
2993 my $herhostname = gethostbyaddr($iaddr, AF_INET);
2994 my $herstraddr = inet_ntoa($iaddr);
2999 =for Pod::Functions get process group
3001 Returns the current process group for the specified PID. Use
3002 a PID of C<0> to get the current process group for the
3003 current process. Will raise an exception if used on a machine that
3004 doesn't implement L<getpgrp(2)>. If PID is omitted, returns the process
3005 group of the current process. Note that the POSIX version of
3006 L<C<getpgrp>|/getpgrp PID> does not accept a PID argument, so only
3007 C<PID==0> is truly portable.
3009 Portability issues: L<perlport/getpgrp>.
3012 X<getppid> X<parent> X<pid>
3014 =for Pod::Functions get parent process ID
3016 Returns the process id of the parent process.
3018 Note for Linux users: Between v5.8.1 and v5.16.0 Perl would work
3019 around non-POSIX thread semantics the minority of Linux systems (and
3020 Debian GNU/kFreeBSD systems) that used LinuxThreads, this emulation
3021 has since been removed. See the documentation for L<$$|perlvar/$$> for
3024 Portability issues: L<perlport/getppid>.
3026 =item getpriority WHICH,WHO
3027 X<getpriority> X<priority> X<nice>
3029 =for Pod::Functions get current nice value
3031 Returns the current priority for a process, a process group, or a user.
3032 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
3033 machine that doesn't implement L<getpriority(2)>.
3035 C<WHICH> can be any of C<PRIO_PROCESS>, C<PRIO_PGRP> or C<PRIO_USER>
3036 imported from L<POSIX/RESOURCE CONSTANTS>.
3038 Portability issues: L<perlport/getpriority>.
3041 X<getpwnam> X<getgrnam> X<gethostbyname> X<getnetbyname> X<getprotobyname>
3042 X<getpwuid> X<getgrgid> X<getservbyname> X<gethostbyaddr> X<getnetbyaddr>
3043 X<getprotobynumber> X<getservbyport> X<getpwent> X<getgrent> X<gethostent>
3044 X<getnetent> X<getprotoent> X<getservent> X<setpwent> X<setgrent> X<sethostent>
3045 X<setnetent> X<setprotoent> X<setservent> X<endpwent> X<endgrent> X<endhostent>
3046 X<endnetent> X<endprotoent> X<endservent>
3048 =for Pod::Functions get passwd record given user login name
3052 =for Pod::Functions get group record given group name
3054 =item gethostbyname NAME
3056 =for Pod::Functions get host record given name
3058 =item getnetbyname NAME
3060 =for Pod::Functions get networks record given name
3062 =item getprotobyname NAME
3064 =for Pod::Functions get protocol record given name
3068 =for Pod::Functions get passwd record given user ID
3072 =for Pod::Functions get group record given group user ID
3074 =item getservbyname NAME,PROTO
3076 =for Pod::Functions get services record given its name
3078 =item gethostbyaddr ADDR,ADDRTYPE
3080 =for Pod::Functions get host record given its address
3082 =item getnetbyaddr ADDR,ADDRTYPE
3084 =for Pod::Functions get network record given its address
3086 =item getprotobynumber NUMBER
3088 =for Pod::Functions get protocol record numeric protocol
3090 =item getservbyport PORT,PROTO
3092 =for Pod::Functions get services record given numeric port
3096 =for Pod::Functions get next passwd record
3100 =for Pod::Functions get next group record
3104 =for Pod::Functions get next hosts record
3108 =for Pod::Functions get next networks record
3112 =for Pod::Functions get next protocols record
3116 =for Pod::Functions get next services record
3120 =for Pod::Functions prepare passwd file for use
3124 =for Pod::Functions prepare group file for use
3126 =item sethostent STAYOPEN
3128 =for Pod::Functions prepare hosts file for use
3130 =item setnetent STAYOPEN
3132 =for Pod::Functions prepare networks file for use
3134 =item setprotoent STAYOPEN
3136 =for Pod::Functions prepare protocols file for use
3138 =item setservent STAYOPEN
3140 =for Pod::Functions prepare services file for use
3144 =for Pod::Functions be done using passwd file
3148 =for Pod::Functions be done using group file
3152 =for Pod::Functions be done using hosts file
3156 =for Pod::Functions be done using networks file
3160 =for Pod::Functions be done using protocols file
3164 =for Pod::Functions be done using services file
3166 These routines are the same as their counterparts in the
3167 system C library. In list context, the return values from the
3168 various get routines are as follows:
3171 my ( $name, $passwd, $gid, $members ) = getgr*
3172 my ( $name, $aliases, $addrtype, $net ) = getnet*
3173 my ( $name, $aliases, $port, $proto ) = getserv*
3174 my ( $name, $aliases, $proto ) = getproto*
3175 my ( $name, $aliases, $addrtype, $length, @addrs ) = gethost*
3176 my ( $name, $passwd, $uid, $gid, $quota,
3177 $comment, $gcos, $dir, $shell, $expire ) = getpw*
3180 (If the entry doesn't exist, the return value is a single meaningless true
3183 The exact meaning of the $gcos field varies but it usually contains
3184 the real name of the user (as opposed to the login name) and other
3185 information pertaining to the user. Beware, however, that in many
3186 system users are able to change this information and therefore it
3187 cannot be trusted and therefore the $gcos is tainted (see
3188 L<perlsec>). The $passwd and $shell, user's encrypted password and
3189 login shell, are also tainted, for the same reason.
3191 In scalar context, you get the name, unless the function was a
3192 lookup by name, in which case you get the other thing, whatever it is.
3193 (If the entry doesn't exist you get the undefined value.) For example:
3195 my $uid = getpwnam($name);
3196 my $name = getpwuid($num);
3197 my $name = getpwent();
3198 my $gid = getgrnam($name);
3199 my $name = getgrgid($num);
3200 my $name = getgrent();
3203 In I<getpw*()> the fields $quota, $comment, and $expire are special
3204 in that they are unsupported on many systems. If the
3205 $quota is unsupported, it is an empty scalar. If it is supported, it
3206 usually encodes the disk quota. If the $comment field is unsupported,
3207 it is an empty scalar. If it is supported it usually encodes some
3208 administrative comment about the user. In some systems the $quota
3209 field may be $change or $age, fields that have to do with password
3210 aging. In some systems the $comment field may be $class. The $expire
3211 field, if present, encodes the expiration period of the account or the
3212 password. For the availability and the exact meaning of these fields
3213 in your system, please consult L<getpwnam(3)> and your system's
3214 F<pwd.h> file. You can also find out from within Perl what your
3215 $quota and $comment fields mean and whether you have the $expire field
3216 by using the L<C<Config>|Config> module and the values C<d_pwquota>, C<d_pwage>,
3217 C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
3218 files are supported only if your vendor has implemented them in the
3219 intuitive fashion that calling the regular C library routines gets the
3220 shadow versions if you're running under privilege or if there exists
3221 the L<shadow(3)> functions as found in System V (this includes Solaris
3222 and Linux). Those systems that implement a proprietary shadow password
3223 facility are unlikely to be supported.
3225 The $members value returned by I<getgr*()> is a space-separated list of
3226 the login names of the members of the group.
3228 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
3229 C, it will be returned to you via L<C<$?>|perlvar/$?> if the function
3231 C<@addrs> value returned by a successful call is a list of raw
3232 addresses returned by the corresponding library call. In the
3233 Internet domain, each address is four bytes long; you can unpack it
3234 by saying something like:
3236 my ($w,$x,$y,$z) = unpack('W4',$addr[0]);
3238 The Socket library makes this slightly easier:
3241 my $iaddr = inet_aton("127.1"); # or whatever address
3242 my $name = gethostbyaddr($iaddr, AF_INET);
3244 # or going the other way
3245 my $straddr = inet_ntoa($iaddr);
3247 In the opposite way, to resolve a hostname to the IP address
3251 my $packed_ip = gethostbyname("www.perl.org");
3253 if (defined $packed_ip) {
3254 $ip_address = inet_ntoa($packed_ip);
3257 Make sure L<C<gethostbyname>|/gethostbyname NAME> is called in SCALAR
3258 context and that its return value is checked for definedness.
3260 The L<C<getprotobynumber>|/getprotobynumber NUMBER> function, even
3261 though it only takes one argument, has the precedence of a list
3262 operator, so beware:
3264 getprotobynumber $number eq 'icmp' # WRONG
3265 getprotobynumber($number eq 'icmp') # actually means this
3266 getprotobynumber($number) eq 'icmp' # better this way
3268 If you get tired of remembering which element of the return list
3269 contains which return value, by-name interfaces are provided in standard
3270 modules: L<C<File::stat>|File::stat>, L<C<Net::hostent>|Net::hostent>,
3271 L<C<Net::netent>|Net::netent>, L<C<Net::protoent>|Net::protoent>,
3272 L<C<Net::servent>|Net::servent>, L<C<Time::gmtime>|Time::gmtime>,
3273 L<C<Time::localtime>|Time::localtime>, and
3274 L<C<User::grent>|User::grent>. These override the normal built-ins,
3275 supplying versions that return objects with the appropriate names for
3276 each field. For example:
3280 my $is_his = (stat($filename)->uid == pwent($whoever)->uid);
3282 Even though it looks as though they're the same method calls (uid),
3283 they aren't, because a C<File::stat> object is different from
3284 a C<User::pwent> object.
3286 Many of these functions are not safe in a multi-threaded environment
3287 where more than one thread can be using them. In particular, functions
3288 like C<getpwent()> iterate per-process and not per-thread, so if two
3289 threads are simultaneously iterating, neither will get all the records.
3291 Some systems have thread-safe versions of some of the functions, such as
3292 C<getpwnam_r()> instead of C<getpwnam()>. There, Perl automatically and
3293 invisibly substitutes the thread-safe version, without notice. This
3294 means that code that safely runs on some systems can fail on others that
3295 lack the thread-safe versions.
3297 Portability issues: L<perlport/getpwnam> to L<perlport/endservent>.
3299 =item getsockname SOCKET
3302 =for Pod::Functions retrieve the sockaddr for a given socket
3304 Returns the packed sockaddr address of this end of the SOCKET connection,
3305 in case you don't know the address because you have several different
3306 IPs that the connection might have come in on.
3309 my $mysockaddr = getsockname($sock);
3310 my ($port, $myaddr) = sockaddr_in($mysockaddr);
3311 printf "Connect to %s [%s]\n",
3312 scalar gethostbyaddr($myaddr, AF_INET),
3315 =item getsockopt SOCKET,LEVEL,OPTNAME
3318 =for Pod::Functions get socket options on a given socket
3320 Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
3321 Options may exist at multiple protocol levels depending on the socket
3322 type, but at least the uppermost socket level SOL_SOCKET (defined in the
3323 L<C<Socket>|Socket> module) will exist. To query options at another
3324 level the protocol number of the appropriate protocol controlling the
3325 option should be supplied. For example, to indicate that an option is
3326 to be interpreted by the TCP protocol, LEVEL should be set to the
3327 protocol number of TCP, which you can get using
3328 L<C<getprotobyname>|/getprotobyname NAME>.
3330 The function returns a packed string representing the requested socket
3331 option, or L<C<undef>|/undef EXPR> on error, with the reason for the
3332 error placed in L<C<$!>|perlvar/$!>. Just what is in the packed string
3333 depends on LEVEL and OPTNAME; consult L<getsockopt(2)> for details. A
3334 common case is that the option is an integer, in which case the result
3335 is a packed integer, which you can decode using
3336 L<C<unpack>|/unpack TEMPLATE,EXPR> with the C<i> (or C<I>) format.
3338 Here's an example to test whether Nagle's algorithm is enabled on a socket:
3340 use Socket qw(:all);
3342 defined(my $tcp = getprotobyname("tcp"))
3343 or die "Could not determine the protocol number for tcp";
3344 # my $tcp = IPPROTO_TCP; # Alternative
3345 my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
3346 or die "getsockopt TCP_NODELAY: $!";
3347 my $nodelay = unpack("I", $packed);
3348 print "Nagle's algorithm is turned ",
3349 $nodelay ? "off\n" : "on\n";
3351 Portability issues: L<perlport/getsockopt>.
3354 X<glob> X<wildcard> X<filename, expansion> X<expand>
3358 =for Pod::Functions expand filenames using wildcards
3360 In list context, returns a (possibly empty) list of filename expansions on
3361 the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
3362 scalar context, glob iterates through such filename expansions, returning
3363 undef when the list is exhausted. This is the internal function
3364 implementing the C<< <*.c> >> operator, but you can use it directly. If
3365 EXPR is omitted, L<C<$_>|perlvar/$_> is used. The C<< <*.c> >> operator
3366 is discussed in more detail in L<perlop/"I/O Operators">.
3368 Note that L<C<glob>|/glob EXPR> splits its arguments on whitespace and
3370 each segment as separate pattern. As such, C<glob("*.c *.h")>
3371 matches all files with a F<.c> or F<.h> extension. The expression
3372 C<glob(".* *")> matches all files in the current working directory.
3373 If you want to glob filenames that might contain whitespace, you'll
3374 have to use extra quotes around the spacey filename to protect it.
3375 For example, to glob filenames that have an C<e> followed by a space
3376 followed by an C<f>, use one of:
3378 my @spacies = <"*e f*">;
3379 my @spacies = glob '"*e f*"';
3380 my @spacies = glob q("*e f*");
3382 If you had to get a variable through, you could do this:
3384 my @spacies = glob "'*${var}e f*'";
3385 my @spacies = glob qq("*${var}e f*");
3387 If non-empty braces are the only wildcard characters used in the
3388 L<C<glob>|/glob EXPR>, no filenames are matched, but potentially many
3389 strings are returned. For example, this produces nine strings, one for
3390 each pairing of fruits and colors:
3392 my @many = glob "{apple,tomato,cherry}={green,yellow,red}";
3394 This operator is implemented using the standard C<File::Glob> extension.
3395 See L<File::Glob> for details, including
3396 L<C<bsd_glob>|File::Glob/C<bsd_glob>>, which does not treat whitespace
3397 as a pattern separator.
3399 If a C<glob> expression is used as the condition of a C<while> or C<for>
3400 loop, then it will be implicitly assigned to C<$_>. If either a C<glob>
3401 expression or an explicit assignment of a C<glob> expression to a scalar
3402 is used as a C<while>/C<for> condition, then the condition actually
3403 tests for definedness of the expression's value, not for its regular
3406 Portability issues: L<perlport/glob>.
3409 X<gmtime> X<UTC> X<Greenwich>
3413 =for Pod::Functions convert UNIX time into record or string using Greenwich time
3415 Works just like L<C<localtime>|/localtime EXPR> but the returned values
3416 are localized for the standard Greenwich time zone.
3418 Note: When called in list context, $isdst, the last value
3419 returned by gmtime, is always C<0>. There is no
3420 Daylight Saving Time in GMT.
3422 Portability issues: L<perlport/gmtime>.
3425 X<goto> X<jump> X<jmp>
3431 =for Pod::Functions create spaghetti code
3433 The C<goto LABEL> form finds the statement labeled with LABEL and
3434 resumes execution there. It can't be used to get out of a block or
3435 subroutine given to L<C<sort>|/sort SUBNAME LIST>. It can be used to go
3436 almost anywhere else within the dynamic scope, including out of
3437 subroutines, but it's usually better to use some other construct such as
3438 L<C<last>|/last LABEL> or L<C<die>|/die LIST>. The author of Perl has
3439 never felt the need to use this form of L<C<goto>|/goto LABEL> (in Perl,
3440 that is; C is another matter). (The difference is that C does not offer
3441 named loops combined with loop control. Perl does, and this replaces
3442 most structured uses of L<C<goto>|/goto LABEL> in other languages.)
3444 The C<goto EXPR> form expects to evaluate C<EXPR> to a code reference or
3445 a label name. If it evaluates to a code reference, it will be handled
3446 like C<goto &NAME>, below. This is especially useful for implementing
3447 tail recursion via C<goto __SUB__>.
3449 If the expression evaluates to a label name, its scope will be resolved
3450 dynamically. This allows for computed L<C<goto>|/goto LABEL>s per
3451 FORTRAN, but isn't necessarily recommended if you're optimizing for
3454 goto ("FOO", "BAR", "GLARCH")[$i];
3456 As shown in this example, C<goto EXPR> is exempt from the "looks like a
3457 function" rule. A pair of parentheses following it does not (necessarily)
3458 delimit its argument. C<goto("NE")."XT"> is equivalent to C<goto NEXT>.
3459 Also, unlike most named operators, this has the same precedence as
3462 Use of C<goto LABEL> or C<goto EXPR> to jump into a construct is
3463 deprecated and will issue a warning. Even then, it may not be used to
3464 go into any construct that requires initialization, such as a
3465 subroutine, a C<foreach> loop, or a C<given>
3466 block. In general, it may not be used to jump into the parameter
3467 of a binary or list operator, but it may be used to jump into the
3468 I<first> parameter of a binary operator. (The C<=>
3469 assignment operator's "first" operand is its right-hand
3470 operand.) It also can't be used to go into a
3471 construct that is optimized away.
3473 The C<goto &NAME> form is quite different from the other forms of
3474 L<C<goto>|/goto LABEL>. In fact, it isn't a goto in the normal sense at
3475 all, and doesn't have the stigma associated with other gotos. Instead,
3476 it exits the current subroutine (losing any changes set by
3477 L<C<local>|/local EXPR>) and immediately calls in its place the named
3478 subroutine using the current value of L<C<@_>|perlvar/@_>. This is used
3479 by C<AUTOLOAD> subroutines that wish to load another subroutine and then
3480 pretend that the other subroutine had been called in the first place
3481 (except that any modifications to L<C<@_>|perlvar/@_> in the current
3482 subroutine are propagated to the other subroutine.) After the
3483 L<C<goto>|/goto LABEL>, not even L<C<caller>|/caller EXPR> will be able
3484 to tell that this routine was called first.
3486 NAME needn't be the name of a subroutine; it can be a scalar variable
3487 containing a code reference or a block that evaluates to a code
3490 =item grep BLOCK LIST
3493 =item grep EXPR,LIST
3495 =for Pod::Functions locate elements in a list test true against a given criterion
3497 This is similar in spirit to, but not the same as, L<grep(1)> and its
3498 relatives. In particular, it is not limited to using regular expressions.
3500 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
3501 L<C<$_>|perlvar/$_> to each element) and returns the list value
3503 elements for which the expression evaluated to true. In scalar
3504 context, returns the number of times the expression was true.
3506 my @foo = grep(!/^#/, @bar); # weed out comments
3510 my @foo = grep {!/^#/} @bar; # weed out comments
3512 Note that L<C<$_>|perlvar/$_> is an alias to the list value, so it can
3514 modify the elements of the LIST. While this is useful and supported,
3515 it can cause bizarre results if the elements of LIST are not variables.
3516 Similarly, grep returns aliases into the original list, much as a for
3517 loop's index variable aliases the list elements. That is, modifying an
3518 element of a list returned by grep (for example, in a C<foreach>,
3519 L<C<map>|/map BLOCK LIST> or another L<C<grep>|/grep BLOCK LIST>)
3520 actually modifies the element in the original list.
3521 This is usually something to be avoided when writing clear code.
3523 See also L<C<map>|/map BLOCK LIST> for a list composed of the results of
3527 X<hex> X<hexadecimal>
3531 =for Pod::Functions convert a hexadecimal string to a number
3533 Interprets EXPR as a hex string and returns the corresponding numeric value.
3534 If EXPR is omitted, uses L<C<$_>|perlvar/$_>.
3536 print hex '0xAf'; # prints '175'
3537 print hex 'aF'; # same
3538 $valid_input =~ /\A(?:0?[xX])?(?:_?[0-9a-fA-F])*\z/
3540 A hex string consists of hex digits and an optional C<0x> or C<x> prefix.
3541 Each hex digit may be preceded by a single underscore, which will be ignored.
3542 Any other character triggers a warning and causes the rest of the string
3543 to be ignored (even leading whitespace, unlike L<C<oct>|/oct EXPR>).
3544 Only integers can be represented, and integer overflow triggers a warning.
3546 To convert strings that might start with any of C<0>, C<0x>, or C<0b>,
3547 see L<C<oct>|/oct EXPR>. To present something as hex, look into
3548 L<C<printf>|/printf FILEHANDLE FORMAT, LIST>,
3549 L<C<sprintf>|/sprintf FORMAT, LIST>, and
3550 L<C<unpack>|/unpack TEMPLATE,EXPR>.
3555 =for Pod::Functions patch a module's namespace into your own
3557 There is no builtin L<C<import>|/import LIST> function. It is just an
3558 ordinary method (subroutine) defined (or inherited) by modules that wish
3559 to export names to another module. The
3560 L<C<use>|/use Module VERSION LIST> function calls the
3561 L<C<import>|/import LIST> method for the package used. See also
3562 L<C<use>|/use Module VERSION LIST>, L<perlmod>, and L<Exporter>.
3564 =item index STR,SUBSTR,POSITION
3565 X<index> X<indexOf> X<InStr>
3567 =item index STR,SUBSTR
3569 =for Pod::Functions find a substring within a string
3571 The index function searches for one string within another, but without
3572 the wildcard-like behavior of a full regular-expression pattern match.
3573 It returns the position of the first occurrence of SUBSTR in STR at
3574 or after POSITION. If POSITION is omitted, starts searching from the
3575 beginning of the string. POSITION before the beginning of the string
3576 or after its end is treated as if it were the beginning or the end,
3577 respectively. POSITION and the return value are based at zero.
3578 If the substring is not found, L<C<index>|/index STR,SUBSTR,POSITION>
3582 X<int> X<integer> X<truncate> X<trunc> X<floor>
3586 =for Pod::Functions get the integer portion of a number
3588 Returns the integer portion of EXPR. If EXPR is omitted, uses
3589 L<C<$_>|perlvar/$_>.
3590 You should not use this function for rounding: one because it truncates
3591 towards C<0>, and two because machine representations of floating-point
3592 numbers can sometimes produce counterintuitive results. For example,
3593 C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
3594 because it's really more like -268.99999999999994315658 instead. Usually,
3595 the L<C<sprintf>|/sprintf FORMAT, LIST>,
3596 L<C<printf>|/printf FILEHANDLE FORMAT, LIST>, or the
3597 L<C<POSIX::floor>|POSIX/C<floor>> and L<C<POSIX::ceil>|POSIX/C<ceil>>
3598 functions will serve you better than will L<C<int>|/int EXPR>.
3600 =item ioctl FILEHANDLE,FUNCTION,SCALAR
3603 =for Pod::Functions system-dependent device control system call
3605 Implements the L<ioctl(2)> function. You'll probably first have to say
3607 require "sys/ioctl.ph"; # probably in
3608 # $Config{archlib}/sys/ioctl.ph
3610 to get the correct function definitions. If F<sys/ioctl.ph> doesn't
3611 exist or doesn't have the correct definitions you'll have to roll your
3612 own, based on your C header files such as F<< <sys/ioctl.h> >>.
3613 (There is a Perl script called B<h2ph> that comes with the Perl kit that
3614 may help you in this, but it's nontrivial.) SCALAR will be read and/or
3615 written depending on the FUNCTION; a C pointer to the string value of SCALAR
3616 will be passed as the third argument of the actual
3617 L<C<ioctl>|/ioctl FILEHANDLE,FUNCTION,SCALAR> call. (If SCALAR
3618 has no string value but does have a numeric value, that value will be
3619 passed rather than a pointer to the string value. To guarantee this to be
3620 true, add a C<0> to the scalar before using it.) The
3621 L<C<pack>|/pack TEMPLATE,LIST> and L<C<unpack>|/unpack TEMPLATE,EXPR>
3622 functions may be needed to manipulate the values of structures used by
3623 L<C<ioctl>|/ioctl FILEHANDLE,FUNCTION,SCALAR>.
3625 The return value of L<C<ioctl>|/ioctl FILEHANDLE,FUNCTION,SCALAR> (and
3626 L<C<fcntl>|/fcntl FILEHANDLE,FUNCTION,SCALAR>) is as follows:
3628 if OS returns: then Perl returns:
3630 0 string "0 but true"
3631 anything else that number
3633 Thus Perl returns true on success and false on failure, yet you can
3634 still easily determine the actual value returned by the operating
3637 my $retval = ioctl(...) || -1;
3638 printf "System returned %d\n", $retval;
3640 The special string C<"0 but true"> is exempt from
3641 L<C<Argument "..." isn't numeric>|perldiag/Argument "%s" isn't numeric%s>
3642 L<warnings> on improper numeric conversions.
3644 Portability issues: L<perlport/ioctl>.
3646 =item join EXPR,LIST
3649 =for Pod::Functions join a list into a string using a separator
3651 Joins the separate strings of LIST into a single string with fields
3652 separated by the value of EXPR, and returns that new string. Example:
3654 my $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
3656 Beware that unlike L<C<split>|/split E<sol>PATTERNE<sol>,EXPR,LIMIT>,
3657 L<C<join>|/join EXPR,LIST> doesn't take a pattern as its first argument.
3658 Compare L<C<split>|/split E<sol>PATTERNE<sol>,EXPR,LIMIT>.
3665 =for Pod::Functions retrieve list of indices from a hash
3667 Called in list context, returns a list consisting of all the keys of the
3668 named hash, or in Perl 5.12 or later only, the indices of an array. Perl
3669 releases prior to 5.12 will produce a syntax error if you try to use an
3670 array argument. In scalar context, returns the number of keys or indices.
3672 Hash entries are returned in an apparently random order. The actual random
3673 order is specific to a given hash; the exact same series of operations
3674 on two hashes may result in a different order for each hash. Any insertion
3675 into the hash may change the order, as will any deletion, with the exception
3676 that the most recent key returned by L<C<each>|/each HASH> or
3677 L<C<keys>|/keys HASH> may be deleted without changing the order. So
3678 long as a given hash is unmodified you may rely on
3679 L<C<keys>|/keys HASH>, L<C<values>|/values HASH> and L<C<each>|/each
3680 HASH> to repeatedly return the same order
3681 as each other. See L<perlsec/"Algorithmic Complexity Attacks"> for
3682 details on why hash order is randomized. Aside from the guarantees
3683 provided here the exact details of Perl's hash algorithm and the hash
3684 traversal order are subject to change in any release of Perl. Tied hashes
3685 may behave differently to Perl's hashes with respect to changes in order on
3686 insertion and deletion of items.
3688 As a side effect, calling L<C<keys>|/keys HASH> resets the internal
3689 iterator of the HASH or ARRAY (see L<C<each>|/each HASH>) before
3690 yielding the keys. In
3691 particular, calling L<C<keys>|/keys HASH> in void context resets the
3692 iterator with no other overhead.
3694 Here is yet another way to print your environment:
3696 my @keys = keys %ENV;
3697 my @values = values %ENV;
3699 print pop(@keys), '=', pop(@values), "\n";
3702 or how about sorted by key:
3704 foreach my $key (sort(keys %ENV)) {
3705 print $key, '=', $ENV{$key}, "\n";
3708 The returned values are copies of the original keys in the hash, so
3709 modifying them will not affect the original hash. Compare
3710 L<C<values>|/values HASH>.
3712 To sort a hash by value, you'll need to use a
3713 L<C<sort>|/sort SUBNAME LIST> function. Here's a descending numeric
3714 sort of a hash by its values:
3716 foreach my $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
3717 printf "%4d %s\n", $hash{$key}, $key;
3720 Used as an lvalue, L<C<keys>|/keys HASH> allows you to increase the
3721 number of hash buckets
3722 allocated for the given hash. This can gain you a measure of efficiency if
3723 you know the hash is going to get big. (This is similar to pre-extending
3724 an array by assigning a larger number to $#array.) If you say
3728 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
3729 in fact, since it rounds up to the next power of two. These
3730 buckets will be retained even if you do C<%hash = ()>, use C<undef
3731 %hash> if you want to free the storage while C<%hash> is still in scope.
3732 You can't shrink the number of buckets allocated for the hash using
3733 L<C<keys>|/keys HASH> in this way (but you needn't worry about doing
3734 this by accident, as trying has no effect). C<keys @array> in an lvalue
3735 context is a syntax error.
3737 Starting with Perl 5.14, an experimental feature allowed
3738 L<C<keys>|/keys HASH> to take a scalar expression. This experiment has
3739 been deemed unsuccessful, and was removed as of Perl 5.24.
3741 To avoid confusing would-be users of your code who are running earlier
3742 versions of Perl with mysterious syntax errors, put this sort of thing at
3743 the top of your file to signal that your code will work I<only> on Perls of
3746 use 5.012; # so keys/values/each work on arrays
3748 See also L<C<each>|/each HASH>, L<C<values>|/values HASH>, and
3749 L<C<sort>|/sort SUBNAME LIST>.
3751 =item kill SIGNAL, LIST
3756 =for Pod::Functions send a signal to a process or process group
3758 Sends a signal to a list of processes. Returns the number of arguments
3759 that were successfully used to signal (which is not necessarily the same
3760 as the number of processes actually killed, e.g. where a process group is
3763 my $cnt = kill 'HUP', $child1, $child2;
3764 kill 'KILL', @goners;
3766 SIGNAL may be either a signal name (a string) or a signal number. A signal
3767 name may start with a C<SIG> prefix, thus C<FOO> and C<SIGFOO> refer to the
3768 same signal. The string form of SIGNAL is recommended for portability because
3769 the same signal may have different numbers in different operating systems.
3771 A list of signal names supported by the current platform can be found in
3772 C<$Config{sig_name}>, which is provided by the L<C<Config>|Config>
3773 module. See L<Config> for more details.
3775 A negative signal name is the same as a negative signal number, killing process
3776 groups instead of processes. For example, C<kill '-KILL', $pgrp> and
3777 C<kill -9, $pgrp> will send C<SIGKILL> to
3778 the entire process group specified. That
3779 means you usually want to use positive not negative signals.
3781 If SIGNAL is either the number 0 or the string C<ZERO> (or C<SIGZERO>),
3782 no signal is sent to the process, but L<C<kill>|/kill SIGNAL, LIST>
3783 checks whether it's I<possible> to send a signal to it
3784 (that means, to be brief, that the process is owned by the same user, or we are
3785 the super-user). This is useful to check that a child process is still
3786 alive (even if only as a zombie) and hasn't changed its UID. See
3787 L<perlport> for notes on the portability of this construct.
3789 The behavior of kill when a I<PROCESS> number is zero or negative depends on
3790 the operating system. For example, on POSIX-conforming systems, zero will
3791 signal the current process group, -1 will signal all processes, and any
3792 other negative PROCESS number will act as a negative signal number and
3793 kill the entire process group specified.
3795 If both the SIGNAL and the PROCESS are negative, the results are undefined.
3796 A warning may be produced in a future version.
3798 See L<perlipc/"Signals"> for more details.
3800 On some platforms such as Windows where the L<fork(2)> system call is not
3801 available, Perl can be built to emulate L<C<fork>|/fork> at the
3803 This emulation has limitations related to kill that have to be considered,
3804 for code running on Windows and in code intended to be portable.
3806 See L<perlfork> for more details.
3808 If there is no I<LIST> of processes, no signal is sent, and the return
3809 value is 0. This form is sometimes used, however, because it causes
3810 tainting checks to be run. But see
3811 L<perlsec/Laundering and Detecting Tainted Data>.
3813 Portability issues: L<perlport/kill>.
3822 =for Pod::Functions exit a block prematurely
3824 The L<C<last>|/last LABEL> command is like the C<break> statement in C
3826 loops); it immediately exits the loop in question. If the LABEL is
3827 omitted, the command refers to the innermost enclosing
3828 loop. The C<last EXPR> form, available starting in Perl
3829 5.18.0, allows a label name to be computed at run time,
3830 and is otherwise identical to C<last LABEL>. The
3831 L<C<continue>|/continue BLOCK> block, if any, is not executed:
3833 LINE: while (<STDIN>) {
3834 last LINE if /^$/; # exit when done with header
3838 L<C<last>|/last LABEL> cannot return a value from a block that typically
3839 returns a value, such as C<eval {}>, C<sub {}>, or C<do {}>. It will perform
3840 its flow control behavior, which precludes any return value. It should not be
3841 used to exit a L<C<grep>|/grep BLOCK LIST> or L<C<map>|/map BLOCK LIST>
3844 Note that a block by itself is semantically identical to a loop
3845 that executes once. Thus L<C<last>|/last LABEL> can be used to effect
3846 an early exit out of such a block.
3848 See also L<C<continue>|/continue BLOCK> for an illustration of how
3849 L<C<last>|/last LABEL>, L<C<next>|/next LABEL>, and
3850 L<C<redo>|/redo LABEL> work.
3852 Unlike most named operators, this has the same precedence as assignment.
3853 It is also exempt from the looks-like-a-function rule, so
3854 C<last ("foo")."bar"> will cause "bar" to be part of the argument to
3855 L<C<last>|/last LABEL>.
3862 =for Pod::Functions return lower-case version of a string
3864 Returns a lowercased version of EXPR. This is the internal function
3865 implementing the C<\L> escape in double-quoted strings.
3867 If EXPR is omitted, uses L<C<$_>|perlvar/$_>.
3869 What gets returned depends on several factors:
3873 =item If C<use bytes> is in effect:
3875 The results follow ASCII rules. Only the characters C<A-Z> change,
3876 to C<a-z> respectively.
3878 =item Otherwise, if C<use locale> for C<LC_CTYPE> is in effect:
3880 Respects current C<LC_CTYPE> locale for code points < 256; and uses Unicode
3881 rules for the remaining code points (this last can only happen if
3882 the UTF8 flag is also set). See L<perllocale>.
3884 Starting in v5.20, Perl uses full Unicode rules if the locale is
3885 UTF-8. Otherwise, there is a deficiency in this scheme, which is that
3886 case changes that cross the 255/256
3887 boundary are not well-defined. For example, the lower case of LATIN CAPITAL
3888 LETTER SHARP S (U+1E9E) in Unicode rules is U+00DF (on ASCII
3889 platforms). But under C<use locale> (prior to v5.20 or not a UTF-8
3890 locale), the lower case of U+1E9E is
3891 itself, because 0xDF may not be LATIN SMALL LETTER SHARP S in the
3892 current locale, and Perl has no way of knowing if that character even
3893 exists in the locale, much less what code point it is. Perl returns
3894 a result that is above 255 (almost always the input character unchanged),
3895 for all instances (and there aren't many) where the 255/256 boundary
3896 would otherwise be crossed; and starting in v5.22, it raises a
3897 L<locale|perldiag/Can't do %s("%s") on non-UTF-8 locale; resolved to "%s".> warning.
3899 =item Otherwise, If EXPR has the UTF8 flag set:
3901 Unicode rules are used for the case change.
3903 =item Otherwise, if C<use feature 'unicode_strings'> or C<use locale ':not_characters'> is in effect:
3905 Unicode rules are used for the case change.
3909 ASCII rules are used for the case change. The lowercase of any character
3910 outside the ASCII range is the character itself.
3915 X<lcfirst> X<lowercase>
3919 =for Pod::Functions return a string with just the next letter in lower case
3921 Returns the value of EXPR with the first character lowercased. This
3922 is the internal function implementing the C<\l> escape in
3923 double-quoted strings.
3925 If EXPR is omitted, uses L<C<$_>|perlvar/$_>.
3927 This function behaves the same way under various pragmas, such as in a locale,
3928 as L<C<lc>|/lc EXPR> does.
3935 =for Pod::Functions return the number of characters in a string
3937 Returns the length in I<characters> of the value of EXPR. If EXPR is
3938 omitted, returns the length of L<C<$_>|perlvar/$_>. If EXPR is
3939 undefined, returns L<C<undef>|/undef EXPR>.
3941 This function cannot be used on an entire array or hash to find out how
3942 many elements these have. For that, use C<scalar @array> and C<scalar keys
3943 %hash>, respectively.
3945 Like all Perl character operations, L<C<length>|/length EXPR> normally
3947 characters, not physical bytes. For how many bytes a string encoded as
3948 UTF-8 would take up, use C<length(Encode::encode('UTF-8', EXPR))>
3949 (you'll have to C<use Encode> first). See L<Encode> and L<perlunicode>.
3954 =for Pod::Functions the current source line number
3956 A special token that compiles to the current line number.
3958 =item link OLDFILE,NEWFILE
3961 =for Pod::Functions create a hard link in the filesystem
3963 Creates a new filename linked to the old filename. Returns true for
3964 success, false otherwise.
3966 Portability issues: L<perlport/link>.
3968 =item listen SOCKET,QUEUESIZE
3971 =for Pod::Functions register your socket as a server
3973 Does the same thing that the L<listen(2)> system call does. Returns true if
3974 it succeeded, false otherwise. See the example in
3975 L<perlipc/"Sockets: Client/Server Communication">.
3980 =for Pod::Functions create a temporary value for a global variable (dynamic scoping)
3982 You really probably want to be using L<C<my>|/my VARLIST> instead,
3983 because L<C<local>|/local EXPR> isn't what most people think of as
3984 "local". See L<perlsub/"Private Variables via my()"> for details.
3986 A local modifies the listed variables to be local to the enclosing
3987 block, file, or eval. If more than one value is listed, the list must
3988 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
3989 for details, including issues with tied arrays and hashes.
3991 The C<delete local EXPR> construct can also be used to localize the deletion
3992 of array/hash elements to the current block.
3993 See L<perlsub/"Localized deletion of elements of composite types">.
3995 =item localtime EXPR
3996 X<localtime> X<ctime>
4000 =for Pod::Functions convert UNIX time into record or string using local time
4002 Converts a time as returned by the time function to a 9-element list
4003 with the time analyzed for the local time zone. Typically used as
4007 my ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
4010 All list elements are numeric and come straight out of the C `struct
4011 tm'. C<$sec>, C<$min>, and C<$hour> are the seconds, minutes, and hours
4012 of the specified time.
4014 C<$mday> is the day of the month and C<$mon> the month in
4015 the range C<0..11>, with 0 indicating January and 11 indicating December.
4016 This makes it easy to get a month name from a list:
4018 my @abbr = qw(Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec);
4019 print "$abbr[$mon] $mday";
4020 # $mon=9, $mday=18 gives "Oct 18"
4022 C<$year> contains the number of years since 1900. To get a 4-digit
4027 To get the last two digits of the year (e.g., "01" in 2001) do:
4029 $year = sprintf("%02d", $year % 100);
4031 C<$wday> is the day of the week, with 0 indicating Sunday and 3 indicating
4032 Wednesday. C<$yday> is the day of the year, in the range C<0..364>
4033 (or C<0..365> in leap years.)
4035 C<$isdst> is true if the specified time occurs during Daylight Saving
4036 Time, false otherwise.
4038 If EXPR is omitted, L<C<localtime>|/localtime EXPR> uses the current
4039 time (as returned by L<C<time>|/time>).
4041 In scalar context, L<C<localtime>|/localtime EXPR> returns the
4044 my $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
4046 The format of this scalar value is B<not> locale-dependent but built
4047 into Perl. For GMT instead of local time use the
4048 L<C<gmtime>|/gmtime EXPR> builtin. See also the
4049 L<C<Time::Local>|Time::Local> module (for converting seconds, minutes,
4050 hours, and such back to the integer value returned by L<C<time>|/time>),
4051 and the L<POSIX> module's L<C<strftime>|POSIX/C<strftime>> and
4052 L<C<mktime>|POSIX/C<mktime>> functions.
4054 To get somewhat similar but locale-dependent date strings, set up your
4055 locale environment variables appropriately (please see L<perllocale>) and
4058 use POSIX qw(strftime);
4059 my $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
4060 # or for GMT formatted appropriately for your locale:
4061 my $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
4063 Note that C<%a> and C<%b>, the short forms of the day of the week
4064 and the month of the year, may not necessarily be three characters wide.
4066 The L<Time::gmtime> and L<Time::localtime> modules provide a convenient,
4067 by-name access mechanism to the L<C<gmtime>|/gmtime EXPR> and
4068 L<C<localtime>|/localtime EXPR> functions, respectively.
4070 For a comprehensive date and time representation look at the
4071 L<DateTime> module on CPAN.
4073 Portability issues: L<perlport/localtime>.
4078 =for Pod::Functions +5.005 get a thread lock on a variable, subroutine, or method
4080 This function places an advisory lock on a shared variable or referenced
4081 object contained in I<THING> until the lock goes out of scope.
4083 The value returned is the scalar itself, if the argument is a scalar, or a
4084 reference, if the argument is a hash, array or subroutine.
4086 L<C<lock>|/lock THING> is a "weak keyword"; this means that if you've
4088 by this name (before any calls to it), that function will be called
4089 instead. If you are not under C<use threads::shared> this does nothing.
4090 See L<threads::shared>.
4093 X<log> X<logarithm> X<e> X<ln> X<base>
4097 =for Pod::Functions retrieve the natural logarithm for a number
4099 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
4100 returns the log of L<C<$_>|perlvar/$_>. To get the
4101 log of another base, use basic algebra:
4102 The base-N log of a number is equal to the natural log of that number
4103 divided by the natural log of N. For example:
4107 return log($n)/log(10);
4110 See also L<C<exp>|/exp EXPR> for the inverse operation.
4112 =item lstat FILEHANDLE
4117 =item lstat DIRHANDLE
4121 =for Pod::Functions stat a symbolic link
4123 Does the same thing as the L<C<stat>|/stat FILEHANDLE> function
4124 (including setting the special C<_> filehandle) but stats a symbolic
4125 link instead of the file the symbolic link points to. If symbolic links
4126 are unimplemented on your system, a normal L<C<stat>|/stat FILEHANDLE>
4127 is done. For much more detailed information, please see the
4128 documentation for L<C<stat>|/stat FILEHANDLE>.
4130 If EXPR is omitted, stats L<C<$_>|perlvar/$_>.
4132 Portability issues: L<perlport/lstat>.
4136 =for Pod::Functions match a string with a regular expression pattern
4138 The match operator. See L<perlop/"Regexp Quote-Like Operators">.
4140 =item map BLOCK LIST
4145 =for Pod::Functions apply a change to a list to get back a new list with the changes
4147 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
4148 L<C<$_>|perlvar/$_> to each element) and composes a list of the results of
4149 each such evaluation. Each element of LIST may produce zero, one, or more
4150 elements in the generated list, so the number of elements in the generated
4151 list may differ from that in LIST. In scalar context, returns the total
4152 number of elements so generated. In list context, returns the generated list.
4154 my @chars = map(chr, @numbers);
4156 translates a list of numbers to the corresponding characters.
4158 my @squares = map { $_ * $_ } @numbers;
4160 translates a list of numbers to their squared values.
4162 my @squares = map { $_ > 5 ? ($_ * $_) : () } @numbers;
4164 shows that number of returned elements can differ from the number of
4165 input elements. To omit an element, return an empty list ().
4166 This could also be achieved by writing
4168 my @squares = map { $_ * $_ } grep { $_ > 5 } @numbers;
4170 which makes the intention more clear.
4172 Map always returns a list, which can be
4173 assigned to a hash such that the elements
4174 become key/value pairs. See L<perldata> for more details.
4176 my %hash = map { get_a_key_for($_) => $_ } @array;
4178 is just a funny way to write
4182 $hash{get_a_key_for($_)} = $_;
4185 Note that L<C<$_>|perlvar/$_> is an alias to the list value, so it can
4186 be used to modify the elements of the LIST. While this is useful and
4187 supported, it can cause bizarre results if the elements of LIST are not
4188 variables. Using a regular C<foreach> loop for this purpose would be
4189 clearer in most cases. See also L<C<grep>|/grep BLOCK LIST> for a
4190 list composed of those items of the original list for which the BLOCK
4191 or EXPR evaluates to true.
4193 C<{> starts both hash references and blocks, so C<map { ...> could be either
4194 the start of map BLOCK LIST or map EXPR, LIST. Because Perl doesn't look
4195 ahead for the closing C<}> it has to take a guess at which it's dealing with
4196 based on what it finds just after the
4197 C<{>. Usually it gets it right, but if it
4198 doesn't it won't realize something is wrong until it gets to the C<}> and
4199 encounters the missing (or unexpected) comma. The syntax error will be
4200 reported close to the C<}>, but you'll need to change something near the C<{>
4201 such as using a unary C<+> or semicolon to give Perl some help:
4203 my %hash = map { "\L$_" => 1 } @array # perl guesses EXPR. wrong
4204 my %hash = map { +"\L$_" => 1 } @array # perl guesses BLOCK. right
4205 my %hash = map {; "\L$_" => 1 } @array # this also works
4206 my %hash = map { ("\L$_" => 1) } @array # as does this
4207 my %hash = map { lc($_) => 1 } @array # and this.
4208 my %hash = map +( lc($_) => 1 ), @array # this is EXPR and works!
4210 my %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
4212 or to force an anon hash constructor use C<+{>:
4214 my @hashes = map +{ lc($_) => 1 }, @array # EXPR, so needs
4217 to get a list of anonymous hashes each with only one entry apiece.
4219 =item mkdir FILENAME,MODE
4220 X<mkdir> X<md> X<directory, create>
4222 =item mkdir FILENAME
4226 =for Pod::Functions create a directory
4228 Creates the directory specified by FILENAME, with permissions
4229 specified by MODE (as modified by L<C<umask>|/umask EXPR>). If it
4230 succeeds it returns true; otherwise it returns false and sets
4231 L<C<$!>|perlvar/$!> (errno).
4232 MODE defaults to 0777 if omitted, and FILENAME defaults
4233 to L<C<$_>|perlvar/$_> if omitted.
4235 In general, it is better to create directories with a permissive MODE
4236 and let the user modify that with their L<C<umask>|/umask EXPR> than it
4238 a restrictive MODE and give the user no way to be more permissive.
4239 The exceptions to this rule are when the file or directory should be
4240 kept private (mail files, for instance). The documentation for
4241 L<C<umask>|/umask EXPR> discusses the choice of MODE in more detail.
4243 Note that according to the POSIX 1003.1-1996 the FILENAME may have any
4244 number of trailing slashes. Some operating and filesystems do not get
4245 this right, so Perl automatically removes all trailing slashes to keep
4248 To recursively create a directory structure, look at
4249 the L<C<make_path>|File::Path/make_path( $dir1, $dir2, .... )> function
4250 of the L<File::Path> module.
4252 =item msgctl ID,CMD,ARG
4255 =for Pod::Functions SysV IPC message control operations
4257 Calls the System V IPC function L<msgctl(2)>. You'll probably have to say
4261 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
4262 then ARG must be a variable that will hold the returned C<msqid_ds>
4263 structure. Returns like L<C<ioctl>|/ioctl FILEHANDLE,FUNCTION,SCALAR>:
4264 the undefined value for error, C<"0 but true"> for zero, or the actual
4265 return value otherwise. See also L<perlipc/"SysV IPC"> and the
4266 documentation for L<C<IPC::SysV>|IPC::SysV> and
4267 L<C<IPC::Semaphore>|IPC::Semaphore>.
4269 Portability issues: L<perlport/msgctl>.
4271 =item msgget KEY,FLAGS
4274 =for Pod::Functions get SysV IPC message queue
4276 Calls the System V IPC function L<msgget(2)>. Returns the message queue
4277 id, or L<C<undef>|/undef EXPR> on error. See also L<perlipc/"SysV IPC">
4278 and the documentation for L<C<IPC::SysV>|IPC::SysV> and
4279 L<C<IPC::Msg>|IPC::Msg>.
4281 Portability issues: L<perlport/msgget>.
4283 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
4286 =for Pod::Functions receive a SysV IPC message from a message queue
4288 Calls the System V IPC function msgrcv to receive a message from
4289 message queue ID into variable VAR with a maximum message size of
4290 SIZE. Note that when a message is received, the message type as a
4291 native long integer will be the first thing in VAR, followed by the
4292 actual message. This packing may be opened with C<unpack("l! a*")>.
4293 Taints the variable. Returns true if successful, false
4294 on error. See also L<perlipc/"SysV IPC"> and the documentation for
4295 L<C<IPC::SysV>|IPC::SysV> and L<C<IPC::Msg>|IPC::Msg>.
4297 Portability issues: L<perlport/msgrcv>.
4299 =item msgsnd ID,MSG,FLAGS
4302 =for Pod::Functions send a SysV IPC message to a message queue
4304 Calls the System V IPC function msgsnd to send the message MSG to the
4305 message queue ID. MSG must begin with the native long integer message
4306 type, be followed by the length of the actual message, and then finally
4307 the message itself. This kind of packing can be achieved with
4308 C<pack("l! a*", $type, $message)>. Returns true if successful,
4309 false on error. See also L<perlipc/"SysV IPC"> and the documentation
4310 for L<C<IPC::SysV>|IPC::SysV> and L<C<IPC::Msg>|IPC::Msg>.
4312 Portability issues: L<perlport/msgsnd>.
4317 =item my TYPE VARLIST
4319 =item my VARLIST : ATTRS
4321 =item my TYPE VARLIST : ATTRS
4323 =for Pod::Functions declare and assign a local variable (lexical scoping)
4325 A L<C<my>|/my VARLIST> declares the listed variables to be local
4326 (lexically) to the enclosing block, file, or L<C<eval>|/eval EXPR>. If
4327 more than one variable is listed, the list must be placed in
4330 The exact semantics and interface of TYPE and ATTRS are still
4331 evolving. TYPE may be a bareword, a constant declared
4332 with L<C<use constant>|constant>, or L<C<__PACKAGE__>|/__PACKAGE__>. It
4334 currently bound to the use of the L<fields> pragma,
4335 and attributes are handled using the L<attributes> pragma, or starting
4336 from Perl 5.8.0 also via the L<Attribute::Handlers> module. See
4337 L<perlsub/"Private Variables via my()"> for details.
4339 Note that with a parenthesised list, L<C<undef>|/undef EXPR> can be used
4340 as a dummy placeholder, for example to skip assignment of initial
4343 my ( undef, $min, $hour ) = localtime;
4352 =for Pod::Functions iterate a block prematurely
4354 The L<C<next>|/next LABEL> command is like the C<continue> statement in
4355 C; it starts the next iteration of the loop:
4357 LINE: while (<STDIN>) {
4358 next LINE if /^#/; # discard comments
4362 Note that if there were a L<C<continue>|/continue BLOCK> block on the
4364 executed even on discarded lines. If LABEL is omitted, the command
4365 refers to the innermost enclosing loop. The C<next EXPR> form, available
4366 as of Perl 5.18.0, allows a label name to be computed at run time, being
4367 otherwise identical to C<next LABEL>.
4369 L<C<next>|/next LABEL> cannot return a value from a block that typically
4370 returns a value, such as C<eval {}>, C<sub {}>, or C<do {}>. It will perform
4371 its flow control behavior, which precludes any return value. It should not be
4372 used to exit a L<C<grep>|/grep BLOCK LIST> or L<C<map>|/map BLOCK LIST>
4375 Note that a block by itself is semantically identical to a loop
4376 that executes once. Thus L<C<next>|/next LABEL> will exit such a block
4379 See also L<C<continue>|/continue BLOCK> for an illustration of how
4380 L<C<last>|/last LABEL>, L<C<next>|/next LABEL>, and
4381 L<C<redo>|/redo LABEL> work.
4383 Unlike most named operators, this has the same precedence as assignment.
4384 It is also exempt from the looks-like-a-function rule, so
4385 C<next ("foo")."bar"> will cause "bar" to be part of the argument to
4386 L<C<next>|/next LABEL>.
4388 =item no MODULE VERSION LIST
4392 =item no MODULE VERSION
4394 =item no MODULE LIST
4400 =for Pod::Functions unimport some module symbols or semantics at compile time
4402 See the L<C<use>|/use Module VERSION LIST> function, of which
4403 L<C<no>|/no MODULE VERSION LIST> is the opposite.
4406 X<oct> X<octal> X<hex> X<hexadecimal> X<binary> X<bin>
4410 =for Pod::Functions convert a string to an octal number
4412 Interprets EXPR as an octal string and returns the corresponding
4413 value. (If EXPR happens to start off with C<0x>, interprets it as a
4414 hex string. If EXPR starts off with C<0b>, it is interpreted as a
4415 binary string. Leading whitespace is ignored in all three cases.)
4416 The following will handle decimal, binary, octal, and hex in standard
4419 $val = oct($val) if $val =~ /^0/;
4421 If EXPR is omitted, uses L<C<$_>|perlvar/$_>. To go the other way
4422 (produce a number in octal), use L<C<sprintf>|/sprintf FORMAT, LIST> or
4423 L<C<printf>|/printf FILEHANDLE FORMAT, LIST>:
4425 my $dec_perms = (stat("filename"))[2] & 07777;
4426 my $oct_perm_str = sprintf "%o", $perms;
4428 The L<C<oct>|/oct EXPR> function is commonly used when a string such as
4430 to be converted into a file mode, for example. Although Perl
4431 automatically converts strings into numbers as needed, this automatic
4432 conversion assumes base 10.
4434 Leading white space is ignored without warning, as too are any trailing
4435 non-digits, such as a decimal point (L<C<oct>|/oct EXPR> only handles
4436 non-negative integers, not negative integers or floating point).
4438 =item open FILEHANDLE,EXPR
4439 X<open> X<pipe> X<file, open> X<fopen>
4441 =item open FILEHANDLE,MODE,EXPR
4443 =item open FILEHANDLE,MODE,EXPR,LIST
4445 =item open FILEHANDLE,MODE,REFERENCE
4447 =item open FILEHANDLE
4449 =for Pod::Functions open a file, pipe, or descriptor
4451 Opens the file whose filename is given by EXPR, and associates it with
4454 Simple examples to open a file for reading:
4456 open(my $fh, "<", "input.txt")
4457 or die "Can't open < input.txt: $!";
4461 open(my $fh, ">", "output.txt")
4462 or die "Can't open > output.txt: $!";
4464 (The following is a comprehensive reference to
4465 L<C<open>|/open FILEHANDLE,EXPR>: for a gentler introduction you may
4466 consider L<perlopentut>.)
4468 If FILEHANDLE is an undefined scalar variable (or array or hash element), a
4469 new filehandle is autovivified, meaning that the variable is assigned a
4470 reference to a newly allocated anonymous filehandle. Otherwise if
4471 FILEHANDLE is an expression, its value is the real filehandle. (This is
4472 considered a symbolic reference, so C<use strict "refs"> should I<not> be
4475 If three (or more) arguments are specified, the open mode (including
4476 optional encoding) in the second argument are distinct from the filename in
4477 the third. If MODE is C<< < >> or nothing, the file is opened for input.
4478 If MODE is C<< > >>, the file is opened for output, with existing files
4479 first being truncated ("clobbered") and nonexisting files newly created.
4480 If MODE is C<<< >> >>>, the file is opened for appending, again being
4481 created if necessary.
4483 You can put a C<+> in front of the C<< > >> or C<< < >> to
4484 indicate that you want both read and write access to the file; thus
4485 C<< +< >> is almost always preferred for read/write updates--the
4486 C<< +> >> mode would clobber the file first. You can't usually use
4487 either read-write mode for updating textfiles, since they have
4488 variable-length records. See the B<-i> switch in L<perlrun> for a
4489 better approach. The file is created with permissions of C<0666>
4490 modified by the process's L<C<umask>|/umask EXPR> value.
4492 These various prefixes correspond to the L<fopen(3)> modes of C<r>,
4493 C<r+>, C<w>, C<w+>, C<a>, and C<a+>.
4495 In the one- and two-argument forms of the call, the mode and filename
4496 should be concatenated (in that order), preferably separated by white
4497 space. You can--but shouldn't--omit the mode in these forms when that mode
4498 is C<< < >>. It is safe to use the two-argument form of
4499 L<C<open>|/open FILEHANDLE,EXPR> if the filename argument is a known literal.
4501 For three or more arguments if MODE is C<|->, the filename is
4502 interpreted as a command to which output is to be piped, and if MODE
4503 is C<-|>, the filename is interpreted as a command that pipes
4504 output to us. In the two-argument (and one-argument) form, one should
4505 replace dash (C<->) with the command.
4506 See L<perlipc/"Using open() for IPC"> for more examples of this.
4507 (You are not allowed to L<C<open>|/open FILEHANDLE,EXPR> to a command
4508 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>, and
4509 L<perlipc/"Bidirectional Communication with Another Process"> for
4512 In the form of pipe opens taking three or more arguments, if LIST is specified
4513 (extra arguments after the command name) then LIST becomes arguments
4514 to the command invoked if the platform supports it. The meaning of
4515 L<C<open>|/open FILEHANDLE,EXPR> with more than three arguments for
4516 non-pipe modes is not yet defined, but experimental "layers" may give
4517 extra LIST arguments meaning.
4519 In the two-argument (and one-argument) form, opening C<< <- >>
4520 or C<-> opens STDIN and opening C<< >- >> opens STDOUT.
4522 You may (and usually should) use the three-argument form of open to specify
4523 I/O layers (sometimes referred to as "disciplines") to apply to the handle
4524 that affect how the input and output are processed (see L<open> and
4525 L<PerlIO> for more details). For example:
4527 open(my $fh, "<:encoding(UTF-8)", $filename)
4528 || die "Can't open UTF-8 encoded $filename: $!";
4530 opens the UTF8-encoded file containing Unicode characters;
4531 see L<perluniintro>. Note that if layers are specified in the
4532 three-argument form, then default layers stored in ${^OPEN} (see L<perlvar>;
4533 usually set by the L<open> pragma or the switch C<-CioD>) are ignored.
4534 Those layers will also be ignored if you specify a colon with no name
4535 following it. In that case the default layer for the operating system
4536 (:raw on Unix, :crlf on Windows) is used.
4538 Open returns nonzero on success, the undefined value otherwise. If
4539 the L<C<open>|/open FILEHANDLE,EXPR> involved a pipe, the return value
4540 happens to be the pid of the subprocess.
4542 On some systems (in general, DOS- and Windows-based systems)
4543 L<C<binmode>|/binmode FILEHANDLE, LAYER> is necessary when you're not
4544 working with a text file. For the sake of portability it is a good idea
4545 always to use it when appropriate, and never to use it when it isn't
4546 appropriate. Also, people can set their I/O to be by default
4547 UTF8-encoded Unicode, not bytes.
4549 When opening a file, it's seldom a good idea to continue
4550 if the request failed, so L<C<open>|/open FILEHANDLE,EXPR> is frequently
4551 used with L<C<die>|/die LIST>. Even if L<C<die>|/die LIST> won't do
4552 what you want (say, in a CGI script,
4553 where you want to format a suitable error message (but there are
4554 modules that can help with that problem)) always check
4555 the return value from opening a file.
4557 The filehandle will be closed when its reference count reaches zero.
4558 If it is a lexically scoped variable declared with L<C<my>|/my VARLIST>,
4560 means the end of the enclosing scope. However, this automatic close
4561 does not check for errors, so it is better to explicitly close
4562 filehandles, especially those used for writing:
4565 || warn "close failed: $!";
4567 An older style is to use a bareword as the filehandle, as
4569 open(FH, "<", "input.txt")
4570 or die "Can't open < input.txt: $!";
4572 Then you can use C<FH> as the filehandle, in C<< close FH >> and C<<
4573 <FH> >> and so on. Note that it's a global variable, so this form is
4574 not recommended in new code.
4576 As a shortcut a one-argument call takes the filename from the global
4577 scalar variable of the same name as the filehandle:
4580 open(ARTICLE) or die "Can't find article $ARTICLE: $!\n";
4582 Here C<$ARTICLE> must be a global (package) scalar variable - not one
4583 declared with L<C<my>|/my VARLIST> or L<C<state>|/state VARLIST>.
4585 As a special case the three-argument form with a read/write mode and the third
4586 argument being L<C<undef>|/undef EXPR>:
4588 open(my $tmp, "+>", undef) or die ...
4590 opens a filehandle to a newly created empty anonymous temporary file.
4591 (This happens under any mode, which makes C<< +> >> the only useful and
4592 sensible mode to use.) You will need to
4593 L<C<seek>|/seek FILEHANDLE,POSITION,WHENCE> to do the reading.
4595 Perl is built using PerlIO by default. Unless you've
4596 changed this (such as building Perl with C<Configure -Uuseperlio>), you can
4597 open filehandles directly to Perl scalars via:
4599 open(my $fh, ">", \$variable) || ..
4601 To (re)open C<STDOUT> or C<STDERR> as an in-memory file, close it first:
4604 open(STDOUT, ">", \$variable)
4605 or die "Can't open STDOUT: $!";
4607 The scalars for in-memory files are treated as octet strings: unless
4608 the file is being opened with truncation the scalar may not contain
4609 any code points over 0xFF.
4611 Opening in-memory files I<can> fail for a variety of reasons. As with
4612 any other C<open>, check the return value for success.
4614 See L<perliol> for detailed info on PerlIO.
4618 open(my $log, ">>", "/usr/spool/news/twitlog");
4619 # if the open fails, output is discarded
4621 open(my $dbase, "+<", "dbase.mine") # open for update
4622 or die "Can't open 'dbase.mine' for update: $!";
4624 open(my $dbase, "+<dbase.mine") # ditto
4625 or die "Can't open 'dbase.mine' for update: $!";
4627 open(my $article_fh, "-|", "caesar <$article") # decrypt
4629 or die "Can't start caesar: $!";
4631 open(my $article_fh, "caesar <$article |") # ditto
4632 or die "Can't start caesar: $!";
4634 open(my $out_fh, "|-", "sort >Tmp$$") # $$ is our process id
4635 or die "Can't start sort: $!";
4638 open(my $memory, ">", \$var)
4639 or die "Can't open memory file: $!";
4640 print $memory "foo!\n"; # output will appear in $var
4642 You may also, in the Bourne shell tradition, specify an EXPR beginning
4643 with C<< >& >>, in which case the rest of the string is interpreted
4644 as the name of a filehandle (or file descriptor, if numeric) to be
4645 duped (as in L<dup(2)>) and opened. You may use C<&> after C<< > >>,
4646 C<<< >> >>>, C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>.
4647 The mode you specify should match the mode of the original filehandle.
4648 (Duping a filehandle does not take into account any existing contents
4649 of IO buffers.) If you use the three-argument
4650 form, then you can pass either a
4651 number, the name of a filehandle, or the normal "reference to a glob".
4653 Here is a script that saves, redirects, and restores C<STDOUT> and
4654 C<STDERR> using various methods:
4657 open(my $oldout, ">&STDOUT") or die "Can't dup STDOUT: $!";
4658 open(OLDERR, ">&", \*STDERR) or die "Can't dup STDERR: $!";
4660 open(STDOUT, '>', "foo.out") or die "Can't redirect STDOUT: $!";
4661 open(STDERR, ">&STDOUT") or die "Can't dup STDOUT: $!";
4663 select STDERR; $| = 1; # make unbuffered
4664 select STDOUT; $| = 1; # make unbuffered
4666 print STDOUT "stdout 1\n"; # this works for
4667 print STDERR "stderr 1\n"; # subprocesses too
4669 open(STDOUT, ">&", $oldout) or die "Can't dup \$oldout: $!";
4670 open(STDERR, ">&OLDERR") or die "Can't dup OLDERR: $!";
4672 print STDOUT "stdout 2\n";
4673 print STDERR "stderr 2\n";
4675 If you specify C<< '<&=X' >>, where C<X> is a file descriptor number
4676 or a filehandle, then Perl will do an equivalent of C's L<fdopen(3)> of
4677 that file descriptor (and not call L<dup(2)>); this is more
4678 parsimonious of file descriptors. For example:
4680 # open for input, reusing the fileno of $fd
4681 open(my $fh, "<&=", $fd)
4685 open(my $fh, "<&=$fd")
4689 # open for append, using the fileno of $oldfh
4690 open(my $fh, ">>&=", $oldfh)
4692 Being parsimonious on filehandles is also useful (besides being
4693 parsimonious) for example when something is dependent on file
4694 descriptors, like for example locking using
4695 L<C<flock>|/flock FILEHANDLE,OPERATION>. If you do just
4696 C<< open(my $A, ">>&", $B) >>, the filehandle C<$A> will not have the
4697 same file descriptor as C<$B>, and therefore C<flock($A)> will not
4698 C<flock($B)> nor vice versa. But with C<< open(my $A, ">>&=", $B) >>,
4699 the filehandles will share the same underlying system file descriptor.
4701 Note that under Perls older than 5.8.0, Perl uses the standard C library's'
4702 L<fdopen(3)> to implement the C<=> functionality. On many Unix systems,
4703 L<fdopen(3)> fails when file descriptors exceed a certain value, typically 255.
4704 For Perls 5.8.0 and later, PerlIO is (most often) the default.
4706 You can see whether your Perl was built with PerlIO by running
4707 C<perl -V:useperlio>. If it says C<'define'>, you have PerlIO;
4708 otherwise you don't.
4710 If you open a pipe on the command C<-> (that is, specify either C<|-> or C<-|>
4711 with the one- or two-argument forms of
4712 L<C<open>|/open FILEHANDLE,EXPR>), an implicit L<C<fork>|/fork> is done,
4713 so L<C<open>|/open FILEHANDLE,EXPR> returns twice: in the parent process
4715 of the child process, and in the child process it returns (a defined) C<0>.
4716 Use C<defined($pid)> or C<//> to determine whether the open was successful.
4718 For example, use either
4720 my $child_pid = open(my $from_kid, "-|") // die "Can't fork: $!";
4724 my $child_pid = open(my $to_kid, "|-") // die "Can't fork: $!";
4730 # either write $to_kid or else read $from_kid
4732 waitpid $child_pid, 0;
4734 # am the child; use STDIN/STDOUT normally
4739 The filehandle behaves normally for the parent, but I/O to that
4740 filehandle is piped from/to the STDOUT/STDIN of the child process.
4741 In the child process, the filehandle isn't opened--I/O happens from/to
4742 the new STDOUT/STDIN. Typically this is used like the normal
4743 piped open when you want to exercise more control over just how the
4744 pipe command gets executed, such as when running setuid and
4745 you don't want to have to scan shell commands for metacharacters.
4747 The following blocks are more or less equivalent:
4749 open(my $fh, "|tr '[a-z]' '[A-Z]'");
4750 open(my $fh, "|-", "tr '[a-z]' '[A-Z]'");
4751 open(my $fh, "|-") || exec 'tr', '[a-z]', '[A-Z]';
4752 open(my $fh, "|-", "tr", '[a-z]', '[A-Z]');
4754 open(my $fh, "cat -n '$file'|");
4755 open(my $fh, "-|", "cat -n '$file'");
4756 open(my $fh, "-|") || exec "cat", "-n", $file;
4757 open(my $fh, "-|", "cat", "-n", $file);
4759 The last two examples in each block show the pipe as "list form", which is
4760 not yet supported on all platforms. A good rule of thumb is that if
4761 your platform has a real L<C<fork>|/fork> (in other words, if your platform is
4762 Unix, including Linux and MacOS X), you can use the list form. You would
4763 want to use the list form of the pipe so you can pass literal arguments
4764 to the command without risk of the shell interpreting any shell metacharacters
4765 in them. However, this also bars you from opening pipes to commands
4766 that intentionally contain shell metacharacters, such as:
4768 open(my $fh, "|cat -n | expand -4 | lpr")
4769 || die "Can't open pipeline to lpr: $!";
4771 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
4773 Perl will attempt to flush all files opened for
4774 output before any operation that may do a fork, but this may not be
4775 supported on some platforms (see L<perlport>). To be safe, you may need
4776 to set L<C<$E<verbar>>|perlvar/$E<verbar>> (C<$AUTOFLUSH> in L<English>)
4777 or call the C<autoflush> method of L<C<IO::Handle>|IO::Handle/METHODS>
4778 on any open handles.
4780 On systems that support a close-on-exec flag on files, the flag will
4781 be set for the newly opened file descriptor as determined by the value
4782 of L<C<$^F>|perlvar/$^F>. See L<perlvar/$^F>.
4784 Closing any piped filehandle causes the parent process to wait for the
4785 child to finish, then returns the status value in L<C<$?>|perlvar/$?> and
4786 L<C<${^CHILD_ERROR_NATIVE}>|perlvar/${^CHILD_ERROR_NATIVE}>.
4788 The filename passed to the one- and two-argument forms of
4789 L<C<open>|/open FILEHANDLE,EXPR> will
4790 have leading and trailing whitespace deleted and normal
4791 redirection characters honored. This property, known as "magic open",
4792 can often be used to good effect. A user could specify a filename of
4793 F<"rsh cat file |">, or you could change certain filenames as needed:
4795 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
4796 open(my $fh, $filename) or die "Can't open $filename: $!";
4798 Use the three-argument form to open a file with arbitrary weird characters in it,
4800 open(my $fh, "<", $file)
4801 || die "Can't open $file: $!";
4803 otherwise it's necessary to protect any leading and trailing whitespace:
4805 $file =~ s#^(\s)#./$1#;
4806 open(my $fh, "< $file\0")
4807 || die "Can't open $file: $!";
4809 (this may not work on some bizarre filesystems). One should
4810 conscientiously choose between the I<magic> and I<three-argument> form
4811 of L<C<open>|/open FILEHANDLE,EXPR>:
4813 open(my $in, $ARGV[0]) || die "Can't open $ARGV[0]: $!";
4815 will allow the user to specify an argument of the form C<"rsh cat file |">,
4816 but will not work on a filename that happens to have a trailing space, while
4818 open(my $in, "<", $ARGV[0])
4819 || die "Can't open $ARGV[0]: $!";
4821 will have exactly the opposite restrictions. (However, some shells
4822 support the syntax C<< perl your_program.pl <( rsh cat file ) >>, which
4823 produces a filename that can be opened normally.)
4825 If you want a "real" C L<open(2)>, then you should use the
4826 L<C<sysopen>|/sysopen FILEHANDLE,FILENAME,MODE> function, which involves
4827 no such magic (but uses different filemodes than Perl
4828 L<C<open>|/open FILEHANDLE,EXPR>, which corresponds to C L<fopen(3)>).
4829 This is another way to protect your filenames from interpretation. For
4833 sysopen(my $fh, $path, O_RDWR|O_CREAT|O_EXCL)
4834 or die "Can't open $path: $!";
4836 print $fh "stuff $$\n";
4838 print "File contains: ", readline($fh);
4840 See L<C<seek>|/seek FILEHANDLE,POSITION,WHENCE> for some details about
4841 mixing reading and writing.
4843 Portability issues: L<perlport/open>.
4845 =item opendir DIRHANDLE,EXPR
4848 =for Pod::Functions open a directory
4850 Opens a directory named EXPR for processing by
4851 L<C<readdir>|/readdir DIRHANDLE>, L<C<telldir>|/telldir DIRHANDLE>,
4852 L<C<seekdir>|/seekdir DIRHANDLE,POS>,
4853 L<C<rewinddir>|/rewinddir DIRHANDLE>, and
4854 L<C<closedir>|/closedir DIRHANDLE>. Returns true if successful.
4855 DIRHANDLE may be an expression whose value can be used as an indirect
4856 dirhandle, usually the real dirhandle name. If DIRHANDLE is an undefined
4857 scalar variable (or array or hash element), the variable is assigned a
4858 reference to a new anonymous dirhandle; that is, it's autovivified.
4859 Dirhandles are the same objects as filehandles; an I/O object can only
4860 be open as one of these handle types at once.
4862 See the example at L<C<readdir>|/readdir DIRHANDLE>.
4869 =for Pod::Functions find a character's numeric representation
4871 Returns the numeric value of the first character of EXPR.
4872 If EXPR is an empty string, returns 0. If EXPR is omitted, uses
4873 L<C<$_>|perlvar/$_>.
4874 (Note I<character>, not byte.)
4876 For the reverse, see L<C<chr>|/chr NUMBER>.
4877 See L<perlunicode> for more about Unicode.
4882 =item our TYPE VARLIST
4884 =item our VARLIST : ATTRS
4886 =item our TYPE VARLIST : ATTRS
4888 =for Pod::Functions +5.6.0 declare and assign a package variable (lexical scoping)
4890 L<C<our>|/our VARLIST> makes a lexical alias to a package (i.e. global)
4891 variable of the same name in the current package for use within the
4892 current lexical scope.
4894 L<C<our>|/our VARLIST> has the same scoping rules as
4895 L<C<my>|/my VARLIST> or L<C<state>|/state VARLIST>, meaning that it is
4896 only valid within a lexical scope. Unlike L<C<my>|/my VARLIST> and
4897 L<C<state>|/state VARLIST>, which both declare new (lexical) variables,
4898 L<C<our>|/our VARLIST> only creates an alias to an existing variable: a
4899 package variable of the same name.
4901 This means that when C<use strict 'vars'> is in effect, L<C<our>|/our
4902 VARLIST> lets you use a package variable without qualifying it with the
4903 package name, but only within the lexical scope of the
4904 L<C<our>|/our VARLIST> declaration. This applies immediately--even
4905 within the same statement.
4913 our $foo; # alias to $Foo::foo
4914 print $foo; # prints 23
4917 print $Foo::foo; # prints 23
4919 print $foo; # ERROR: requires explicit package name
4921 This works even if the package variable has not been used before, as
4922 package variables spring into existence when first used.
4927 our $foo = 23; # just like $Foo::foo = 23
4929 print $Foo::foo; # prints 23
4931 Because the variable becomes legal immediately under C<use strict 'vars'>, so
4932 long as there is no variable with that name is already in scope, you can then
4933 reference the package variable again even within the same statement.
4938 my $foo = $foo; # error, undeclared $foo on right-hand side
4939 our $foo = $foo; # no errors
4941 If more than one variable is listed, the list must be placed
4946 An L<C<our>|/our VARLIST> declaration declares an alias for a package
4947 variable that will be visible
4948 across its entire lexical scope, even across package boundaries. The
4949 package in which the variable is entered is determined at the point
4950 of the declaration, not at the point of use. This means the following
4954 our $bar; # declares $Foo::bar for rest of lexical scope
4958 print $bar; # prints 20, as it refers to $Foo::bar
4960 Multiple L<C<our>|/our VARLIST> declarations with the same name in the
4962 scope are allowed if they are in different packages. If they happen
4963 to be in the same package, Perl will emit warnings if you have asked
4964 for them, just like multiple L<C<my>|/my VARLIST> declarations. Unlike
4965 a second L<C<my>|/my VARLIST> declaration, which will bind the name to a
4966 fresh variable, a second L<C<our>|/our VARLIST> declaration in the same
4967 package, in the same scope, is merely redundant.
4971 our $bar; # declares $Foo::bar for rest of lexical scope
4975 our $bar = 30; # declares $Bar::bar for rest of lexical scope
4976 print $bar; # prints 30
4978 our $bar; # emits warning but has no other effect
4979 print $bar; # still prints 30
4981 An L<C<our>|/our VARLIST> declaration may also have a list of attributes
4984 The exact semantics and interface of TYPE and ATTRS are still
4985 evolving. TYPE is currently bound to the use of the L<fields> pragma,
4986 and attributes are handled using the L<attributes> pragma, or, starting
4987 from Perl 5.8.0, also via the L<Attribute::Handlers> module. See
4988 L<perlsub/"Private Variables via my()"> for details.
4990 Note that with a parenthesised list, L<C<undef>|/undef EXPR> can be used
4991 as a dummy placeholder, for example to skip assignment of initial
4994 our ( undef, $min, $hour ) = localtime;
4996 L<C<our>|/our VARLIST> differs from L<C<use vars>|vars>, which allows
4997 use of an unqualified name I<only> within the affected package, but
5000 =item pack TEMPLATE,LIST
5003 =for Pod::Functions convert a list into a binary representation
5005 Takes a LIST of values and converts it into a string using the rules
5006 given by the TEMPLATE. The resulting string is the concatenation of
5007 the converted values. Typically, each converted value looks
5008 like its machine-level representation. For example, on 32-bit machines
5009 an integer may be represented by a sequence of 4 bytes, which will in
5010 Perl be presented as a string that's 4 characters long.
5012 See L<perlpacktut> for an introduction to this function.
5014 The TEMPLATE is a sequence of characters that give the order and type
5015 of values, as follows:
5017 a A string with arbitrary binary data, will be null padded.
5018 A A text (ASCII) string, will be space padded.
5019 Z A null-terminated (ASCIZ) string, will be null padded.
5021 b A bit string (ascending bit order inside each byte,
5023 B A bit string (descending bit order inside each byte).
5024 h A hex string (low nybble first).
5025 H A hex string (high nybble first).
5027 c A signed char (8-bit) value.
5028 C An unsigned char (octet) value.
5029 W An unsigned char value (can be greater than 255).
5031 s A signed short (16-bit) value.
5032 S An unsigned short value.
5034 l A signed long (32-bit) value.
5035 L An unsigned long value.
5037 q A signed quad (64-bit) value.
5038 Q An unsigned quad value.
5039 (Quads are available only if your system supports 64-bit
5040 integer values _and_ if Perl has been compiled to support
5041 those. Raises an exception otherwise.)
5043 i A signed integer value.
5044 I An unsigned integer value.
5045 (This 'integer' is _at_least_ 32 bits wide. Its exact
5046 size depends on what a local C compiler calls 'int'.)
5048 n An unsigned short (16-bit) in "network" (big-endian) order.
5049 N An unsigned long (32-bit) in "network" (big-endian) order.
5050 v An unsigned short (16-bit) in "VAX" (little-endian) order.
5051 V An unsigned long (32-bit) in "VAX" (little-endian) order.
5053 j A Perl internal signed integer value (IV).
5054 J A Perl internal unsigned integer value (UV).
5056 f A single-precision float in native format.
5057 d A double-precision float in native format.
5059 F A Perl internal floating-point value (NV) in native format
5060 D A float of long-double precision in native format.
5061 (Long doubles are available only if your system supports
5062 long double values _and_ if Perl has been compiled to
5063 support those. Raises an exception otherwise.
5064 Note that there are different long double formats.)
5066 p A pointer to a null-terminated string.
5067 P A pointer to a structure (fixed-length string).
5069 u A uuencoded string.
5070 U A Unicode character number. Encodes to a character in char-
5071 acter mode and UTF-8 (or UTF-EBCDIC in EBCDIC platforms) in
5074 w A BER compressed integer (not an ASN.1 BER, see perlpacktut
5075 for details). Its bytes represent an unsigned integer in
5076 base 128, most significant digit first, with as few digits
5077 as possible. Bit eight (the high bit) is set on each byte
5080 x A null byte (a.k.a ASCII NUL, "\000", chr(0))
5082 @ Null-fill or truncate to absolute position, counted from the
5083 start of the innermost ()-group.
5084 . Null-fill or truncate to absolute position specified by
5086 ( Start of a ()-group.
5088 One or more modifiers below may optionally follow certain letters in the
5089 TEMPLATE (the second column lists letters for which the modifier is valid):
5091 ! sSlLiI Forces native (short, long, int) sizes instead
5092 of fixed (16-/32-bit) sizes.
5094 ! xX Make x and X act as alignment commands.
5096 ! nNvV Treat integers as signed instead of unsigned.
5098 ! @. Specify position as byte offset in the internal
5099 representation of the packed string. Efficient
5102 > sSiIlLqQ Force big-endian byte-order on the type.
5103 jJfFdDpP (The "big end" touches the construct.)
5105 < sSiIlLqQ Force little-endian byte-order on the type.
5106 jJfFdDpP (The "little end" touches the construct.)
5108 The C<< > >> and C<< < >> modifiers can also be used on C<()> groups
5109 to force a particular byte-order on all components in that group,
5110 including all its subgroups.
5114 Larry recalls that the hex and bit string formats (H, h, B, b) were added to
5115 pack for processing data from NASA's Magellan probe. Magellan was in an
5116 elliptical orbit, using the antenna for the radar mapping when close to
5117 Venus and for communicating data back to Earth for the rest of the orbit.
5118 There were two transmission units, but one of these failed, and then the
5119 other developed a fault whereby it would randomly flip the sense of all the
5120 bits. It was easy to automatically detect complete records with the correct
5121 sense, and complete records with all the bits flipped. However, this didn't
5122 recover the records where the sense flipped midway. A colleague of Larry's
5123 was able to pretty much eyeball where the records flipped, so they wrote an
5124 editor named kybble (a pun on the dog food Kibbles 'n Bits) to enable him to
5125 manually correct the records and recover the data. For this purpose pack
5126 gained the hex and bit string format specifiers.
5128 git shows that they were added to perl 3.0 in patch #44 (Jan 1991, commit
5129 27e2fb84680b9cc1), but the patch description makes no mention of their
5130 addition, let alone the story behind them.
5134 The following rules apply:
5140 Each letter may optionally be followed by a number indicating the repeat
5141 count. A numeric repeat count may optionally be enclosed in brackets, as
5142 in C<pack("C[80]", @arr)>. The repeat count gobbles that many values from
5143 the LIST when used with all format types other than C<a>, C<A>, C<Z>, C<b>,
5144 C<B>, C<h>, C<H>, C<@>, C<.>, C<x>, C<X>, and C<P>, where it means
5145 something else, described below. Supplying a C<*> for the repeat count
5146 instead of a number means to use however many items are left, except for:
5152 C<@>, C<x>, and C<X>, where it is equivalent to C<0>.
5156 <.>, where it means relative to the start of the string.
5160 C<u>, where it is equivalent to 1 (or 45, which here is equivalent).
5164 One can replace a numeric repeat count with a template letter enclosed in
5165 brackets to use the packed byte length of the bracketed template for the
5168 For example, the template C<x[L]> skips as many bytes as in a packed long,
5169 and the template C<"$t X[$t] $t"> unpacks twice whatever $t (when
5170 variable-expanded) unpacks. If the template in brackets contains alignment
5171 commands (such as C<x![d]>), its packed length is calculated as if the
5172 start of the template had the maximal possible alignment.
5174 When used with C<Z>, a C<*> as the repeat count is guaranteed to add a
5175 trailing null byte, so the resulting string is always one byte longer than
5176 the byte length of the item itself.
5178 When used with C<@>, the repeat count represents an offset from the start
5179 of the innermost C<()> group.
5181 When used with C<.>, the repeat count determines the starting position to
5182 calculate the value offset as follows:
5188 If the repeat count is C<0>, it's relative to the current position.
5192 If the repeat count is C<*>, the offset is relative to the start of the
5197 And if it's an integer I<n>, the offset is relative to the start of the
5198 I<n>th innermost C<( )> group, or to the start of the string if I<n> is
5199 bigger then the group level.
5203 The repeat count for C<u> is interpreted as the maximal number of bytes
5204 to encode per line of output, with 0, 1 and 2 replaced by 45. The repeat
5205 count should not be more than 65.
5209 The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
5210 string of length count, padding with nulls or spaces as needed. When
5211 unpacking, C<A> strips trailing whitespace and nulls, C<Z> strips everything
5212 after the first null, and C<a> returns data with no stripping at all.
5214 If the value to pack is too long, the result is truncated. If it's too
5215 long and an explicit count is provided, C<Z> packs only C<$count-1> bytes,
5216 followed by a null byte. Thus C<Z> always packs a trailing null, except
5217 when the count is 0.
5221 Likewise, the C<b> and C<B> formats pack a string that's that many bits long.
5222 Each such format generates 1 bit of the result. These are typically followed
5223 by a repeat count like C<B8> or C<B64>.
5225 Each result bit is based on the least-significant bit of the corresponding
5226 input character, i.e., on C<ord($char)%2>. In particular, characters C<"0">
5227 and C<"1"> generate bits 0 and 1, as do characters C<"\000"> and C<"\001">.
5229 Starting from the beginning of the input string, each 8-tuple
5230 of characters is converted to 1 character of output. With format C<b>,
5231 the first character of the 8-tuple determines the least-significant bit of a
5232 character; with format C<B>, it determines the most-significant bit of
5235 If the length of the input string is not evenly divisible by 8, the
5236 remainder is packed as if the input string were padded by null characters
5237 at the end. Similarly during unpacking, "extra" bits are ignored.
5239 If the input string is longer than needed, remaining characters are ignored.
5241 A C<*> for the repeat count uses all characters of the input field.
5242 On unpacking, bits are converted to a string of C<0>s and C<1>s.
5246 The C<h> and C<H> formats pack a string that many nybbles (4-bit groups,
5247 representable as hexadecimal digits, C<"0".."9"> C<"a".."f">) long.
5249 For each such format, L<C<pack>|/pack TEMPLATE,LIST> generates 4 bits of result.
5250 With non-alphabetical characters, the result is based on the 4 least-significant
5251 bits of the input character, i.e., on C<ord($char)%16>. In particular,
5252 characters C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
5253 C<"\000"> and C<"\001">. For characters C<"a".."f"> and C<"A".."F">, the result
5254 is compatible with the usual hexadecimal digits, so that C<"a"> and
5255 C<"A"> both generate the nybble C<0xA==10>. Use only these specific hex
5256 characters with this format.
5258 Starting from the beginning of the template to
5259 L<C<pack>|/pack TEMPLATE,LIST>, each pair
5260 of characters is converted to 1 character of output. With format C<h>, the
5261 first character of the pair determines the least-significant nybble of the
5262 output character; with format C<H>, it determines the most-significant
5265 If the length of the input string is not even, it behaves as if padded by
5266 a null character at the end. Similarly, "extra" nybbles are ignored during
5269 If the input string is longer than needed, extra characters are ignored.
5271 A C<*> for the repeat count uses all characters of the input field. For
5272 L<C<unpack>|/unpack TEMPLATE,EXPR>, nybbles are converted to a string of
5277 The C<p> format packs a pointer to a null-terminated string. You are
5278 responsible for ensuring that the string is not a temporary value, as that
5279 could potentially get deallocated before you got around to using the packed
5280 result. The C<P> format packs a pointer to a structure of the size indicated
5281 by the length. A null pointer is created if the corresponding value for
5282 C<p> or C<P> is L<C<undef>|/undef EXPR>; similarly with
5283 L<C<unpack>|/unpack TEMPLATE,EXPR>, where a null pointer unpacks into
5284 L<C<undef>|/undef EXPR>.
5286 If your system has a strange pointer size--meaning a pointer is neither as
5287 big as an int nor as big as a long--it may not be possible to pack or
5288 unpack pointers in big- or little-endian byte order. Attempting to do
5289 so raises an exception.
5293 The C</> template character allows packing and unpacking of a sequence of
5294 items where the packed structure contains a packed item count followed by
5295 the packed items themselves. This is useful when the structure you're
5296 unpacking has encoded the sizes or repeat counts for some of its fields
5297 within the structure itself as separate fields.
5299 For L<C<pack>|/pack TEMPLATE,LIST>, you write
5300 I<length-item>C</>I<sequence-item>, and the
5301 I<length-item> describes how the length value is packed. Formats likely
5302 to be of most use are integer-packing ones like C<n> for Java strings,
5303 C<w> for ASN.1 or SNMP, and C<N> for Sun XDR.
5305 For L<C<pack>|/pack TEMPLATE,LIST>, I<sequence-item> may have a repeat
5306 count, in which case
5307 the minimum of that and the number of available items is used as the argument
5308 for I<length-item>. If it has no repeat count or uses a '*', the number
5309 of available items is used.
5311 For L<C<unpack>|/unpack TEMPLATE,EXPR>, an internal stack of integer
5312 arguments unpacked so far is
5313 used. You write C</>I<sequence-item> and the repeat count is obtained by
5314 popping off the last element from the stack. The I<sequence-item> must not
5315 have a repeat count.
5317 If I<sequence-item> refers to a string type (C<"A">, C<"a">, or C<"Z">),
5318 the I<length-item> is the string length, not the number of strings. With
5319 an explicit repeat count for pack, the packed string is adjusted to that
5320 length. For example:
5322 This code: gives this result:
5324 unpack("W/a", "\004Gurusamy") ("Guru")
5325 unpack("a3/A A*", "007 Bond J ") (" Bond", "J")
5326 unpack("a3 x2 /A A*", "007: Bond, J.") ("Bond, J", ".")
5328 pack("n/a* w/a","hello,","world") "\000\006hello,\005world"
5329 pack("a/W2", ord("a") .. ord("z")) "2ab"
5331 The I<length-item> is not returned explicitly from
5332 L<C<unpack>|/unpack TEMPLATE,EXPR>.
5334 Supplying a count to the I<length-item> format letter is only useful with
5335 C<A>, C<a>, or C<Z>. Packing with a I<length-item> of C<a> or C<Z> may
5336 introduce C<"\000"> characters, which Perl does not regard as legal in
5341 The integer types C<s>, C<S>, C<l>, and C<L> may be
5342 followed by a C<!> modifier to specify native shorts or
5343 longs. As shown in the example above, a bare C<l> means
5344 exactly 32 bits, although the native C<long> as seen by the local C compiler
5345 may be larger. This is mainly an issue on 64-bit platforms. You can
5346 see whether using C<!> makes any difference this way:
5348 printf "format s is %d, s! is %d\n",
5349 length pack("s"), length pack("s!");
5351 printf "format l is %d, l! is %d\n",
5352 length pack("l"), length pack("l!");
5355 C<i!> and C<I!> are also allowed, but only for completeness' sake:
5356 they are identical to C<i> and C<I>.
5358 The actual sizes (in bytes) of native shorts, ints, longs, and long
5359 longs on the platform where Perl was built are also available from
5362 $ perl -V:{short,int,long{,long}}size
5368 or programmatically via the L<C<Config>|Config> module:
5371 print $Config{shortsize}, "\n";
5372 print $Config{intsize}, "\n";
5373 print $Config{longsize}, "\n";
5374 print $Config{longlongsize}, "\n";
5376 C<$Config{longlongsize}> is undefined on systems without
5381 The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, C<L>, C<j>, and C<J> are
5382 inherently non-portable between processors and operating systems because
5383 they obey native byteorder and endianness. For example, a 4-byte integer
5384 0x12345678 (305419896 decimal) would be ordered natively (arranged in and
5385 handled by the CPU registers) into bytes as
5387 0x12 0x34 0x56 0x78 # big-endian
5388 0x78 0x56 0x34 0x12 # little-endian
5390 Basically, Intel and VAX CPUs are little-endian, while everybody else,
5391 including Motorola m68k/88k, PPC, Sparc, HP PA, Power, and Cray, are
5392 big-endian. Alpha and MIPS can be either: Digital/Compaq uses (well, used)
5393 them in little-endian mode, but SGI/Cray uses them in big-endian mode.
5395 The names I<big-endian> and I<little-endian> are comic references to the
5396 egg-eating habits of the little-endian Lilliputians and the big-endian
5397 Blefuscudians from the classic Jonathan Swift satire, I<Gulliver's Travels>.
5398 This entered computer lingo via the paper "On Holy Wars and a Plea for
5399 Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980.
5401 Some systems may have even weirder byte orders such as
5406 These are called mid-endian, middle-endian, mixed-endian, or just weird.
5408 You can determine your system endianness with this incantation:
5410 printf("%#02x ", $_) for unpack("W*", pack L=>0x12345678);
5412 The byteorder on the platform where Perl was built is also available
5416 print "$Config{byteorder}\n";
5418 or from the command line:
5422 Byteorders C<"1234"> and C<"12345678"> are little-endian; C<"4321">
5423 and C<"87654321"> are big-endian. Systems with multiarchitecture binaries
5424 will have C<"ffff">, signifying that static information doesn't work,
5425 one must use runtime probing.
5427 For portably packed integers, either use the formats C<n>, C<N>, C<v>,
5428 and C<V> or else use the C<< > >> and C<< < >> modifiers described
5429 immediately below. See also L<perlport>.
5433 Also floating point numbers have endianness. Usually (but not always)
5434 this agrees with the integer endianness. Even though most platforms
5435 these days use the IEEE 754 binary format, there are differences,
5436 especially if the long doubles are involved. You can see the
5437 C<Config> variables C<doublekind> and C<longdblkind> (also C<doublesize>,
5438 C<longdblsize>): the "kind" values are enums, unlike C<byteorder>.
5440 Portability-wise the best option is probably to keep to the IEEE 754
5441 64-bit doubles, and of agreed-upon endianness. Another possibility
5442 is the C<"%a">) format of L<C<printf>|/printf FILEHANDLE FORMAT, LIST>.
5446 Starting with Perl 5.10.0, integer and floating-point formats, along with
5447 the C<p> and C<P> formats and C<()> groups, may all be followed by the
5448 C<< > >> or C<< < >> endianness modifiers to respectively enforce big-
5449 or little-endian byte-order. These modifiers are especially useful
5450 given how C<n>, C<N>, C<v>, and C<V> don't cover signed integers,
5451 64-bit integers, or floating-point values.
5453 Here are some concerns to keep in mind when using an endianness modifier:
5459 Exchanging signed integers between different platforms works only
5460 when all platforms store them in the same format. Most platforms store
5461 signed integers in two's-complement notation, so usually this is not an issue.
5465 The C<< > >> or C<< < >> modifiers can only be used on floating-point
5466 formats on big- or little-endian machines. Otherwise, attempting to
5467 use them raises an exception.
5471 Forcing big- or little-endian byte-order on floating-point values for
5472 data exchange can work only if all platforms use the same
5473 binary representation such as IEEE floating-point. Even if all
5474 platforms are using IEEE, there may still be subtle differences. Being able
5475 to use C<< > >> or C<< < >> on floating-point values can be useful,
5476 but also dangerous if you don't know exactly what you're doing.
5477 It is not a general way to portably store floating-point values.
5481 When using C<< > >> or C<< < >> on a C<()> group, this affects
5482 all types inside the group that accept byte-order modifiers,
5483 including all subgroups. It is silently ignored for all other
5484 types. You are not allowed to override the byte-order within a group
5485 that already has a byte-order modifier suffix.
5491 Real numbers (floats and doubles) are in native machine format only.
5492 Due to the multiplicity of floating-point formats and the lack of a
5493 standard "network" representation for them, no facility for interchange has been
5494 made. This means that packed floating-point data written on one machine
5495 may not be readable on another, even if both use IEEE floating-point
5496 arithmetic (because the endianness of the memory representation is not part
5497 of the IEEE spec). See also L<perlport>.
5499 If you know I<exactly> what you're doing, you can use the C<< > >> or C<< < >>
5500 modifiers to force big- or little-endian byte-order on floating-point values.
5502 Because Perl uses doubles (or long doubles, if configured) internally for
5503 all numeric calculation, converting from double into float and thence
5504 to double again loses precision, so C<unpack("f", pack("f", $foo)>)
5505 will not in general equal $foo.
5509 Pack and unpack can operate in two modes: character mode (C<C0> mode) where
5510 the packed string is processed per character, and UTF-8 byte mode (C<U0> mode)
5511 where the packed string is processed in its UTF-8-encoded Unicode form on
5512 a byte-by-byte basis. Character mode is the default
5513 unless the format string starts with C<U>. You
5514 can always switch mode mid-format with an explicit
5515 C<C0> or C<U0> in the format. This mode remains in effect until the next
5516 mode change, or until the end of the C<()> group it (directly) applies to.
5518 Using C<C0> to get Unicode characters while using C<U0> to get I<non>-Unicode
5519 bytes is not necessarily obvious. Probably only the first of these
5522 $ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
5523 perl -CS -ne 'printf "%v04X\n", $_ for unpack("C0A*", $_)'
5525 $ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
5526 perl -CS -ne 'printf "%v02X\n", $_ for unpack("U0A*", $_)'
5528 $ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
5529 perl -C0 -ne 'printf "%v02X\n", $_ for unpack("C0A*", $_)'
5531 $ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
5532 perl -C0 -ne 'printf "%v02X\n", $_ for unpack("U0A*", $_)'
5533 C3.8E.C2.B1.C3.8F.C2.89
5535 Those examples also illustrate that you should not try to use
5536 L<C<pack>|/pack TEMPLATE,LIST>/L<C<unpack>|/unpack TEMPLATE,EXPR> as a
5537 substitute for the L<Encode> module.
5541 You must yourself do any alignment or padding by inserting, for example,
5542 enough C<"x">es while packing. There is no way for
5543 L<C<pack>|/pack TEMPLATE,LIST> and L<C<unpack>|/unpack TEMPLATE,EXPR>
5544 to know where characters are going to or coming from, so they
5545 handle their output and input as flat sequences of characters.
5549 A C<()> group is a sub-TEMPLATE enclosed in parentheses. A group may
5550 take a repeat count either as postfix, or for
5551 L<C<unpack>|/unpack TEMPLATE,EXPR>, also via the C</>
5552 template character. Within each repetition of a group, positioning with
5553 C<@> starts over at 0. Therefore, the result of
5555 pack("@1A((@2A)@3A)", qw[X Y Z])
5557 is the string C<"\0X\0\0YZ">.
5561 C<x> and C<X> accept the C<!> modifier to act as alignment commands: they
5562 jump forward or back to the closest position aligned at a multiple of C<count>
5563 characters. For example, to L<C<pack>|/pack TEMPLATE,LIST> or
5564 L<C<unpack>|/unpack TEMPLATE,EXPR> a C structure like
5567 char c; /* one signed, 8-bit character */
5572 one may need to use the template C<c x![d] d c[2]>. This assumes that
5573 doubles must be aligned to the size of double.
5575 For alignment commands, a C<count> of 0 is equivalent to a C<count> of 1;
5580 C<n>, C<N>, C<v> and C<V> accept the C<!> modifier to
5581 represent signed 16-/32-bit integers in big-/little-endian order.
5582 This is portable only when all platforms sharing packed data use the
5583 same binary representation for signed integers; for example, when all
5584 platforms use two's-complement representation.
5588 Comments can be embedded in a TEMPLATE using C<#> through the end of line.
5589 White space can separate pack codes from each other, but modifiers and
5590 repeat counts must follow immediately. Breaking complex templates into
5591 individual line-by-line components, suitably annotated, can do as much to
5592 improve legibility and maintainability of pack/unpack formats as C</x> can
5593 for complicated pattern matches.
5597 If TEMPLATE requires more arguments than L<C<pack>|/pack TEMPLATE,LIST>
5598 is given, L<C<pack>|/pack TEMPLATE,LIST>
5599 assumes additional C<""> arguments. If TEMPLATE requires fewer arguments
5600 than given, extra arguments are ignored.
5604 Attempting to pack the special floating point values C<Inf> and C<NaN>
5605 (infinity, also in negative, and not-a-number) into packed integer values
5606 (like C<"L">) is a fatal error. The reason for this is that there simply
5607 isn't any sensible mapping for these special values into integers.
5613 $foo = pack("WWWW",65,66,67,68);
5615 $foo = pack("W4",65,66,67,68);
5617 $foo = pack("W4",0x24b6,0x24b7,0x24b8,0x24b9);
5618 # same thing with Unicode circled letters.
5619 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
5620 # same thing with Unicode circled letters. You don't get the
5621 # UTF-8 bytes because the U at the start of the format caused
5622 # a switch to U0-mode, so the UTF-8 bytes get joined into
5624 $foo = pack("C0U4",0x24b6,0x24b7,0x24b8,0x24b9);
5625 # foo eq "\xe2\x92\xb6\xe2\x92\xb7\xe2\x92\xb8\xe2\x92\xb9"
5626 # This is the UTF-8 encoding of the string in the
5629 $foo = pack("ccxxcc",65,66,67,68);
5632 # NOTE: The examples above featuring "W" and "c" are true
5633 # only on ASCII and ASCII-derived systems such as ISO Latin 1
5634 # and UTF-8. On EBCDIC systems, the first example would be
5635 # $foo = pack("WWWW",193,194,195,196);
5637 $foo = pack("s2",1,2);
5638 # "\001\000\002\000" on little-endian
5639 # "\000\001\000\002" on big-endian
5641 $foo = pack("a4","abcd","x","y","z");
5644 $foo = pack("aaaa","abcd","x","y","z");
5647 $foo = pack("a14","abcdefg");
5648 # "abcdefg\0\0\0\0\0\0\0"
5650 $foo = pack("i9pl", gmtime);
5651 # a real struct tm (on my system anyway)
5653 $utmp_template = "Z8 Z8 Z16 L";
5654 $utmp = pack($utmp_template, @utmp1);
5655 # a struct utmp (BSDish)
5657 @utmp2 = unpack($utmp_template, $utmp);
5658 # "@utmp1" eq "@utmp2"
5661 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
5664 $foo = pack('sx2l', 12, 34);
5665 # short 12, two zero bytes padding, long 34
5666 $bar = pack('s@4l', 12, 34);
5667 # short 12, zero fill to position 4, long 34
5669 $baz = pack('s.l', 12, 4, 34);
5670 # short 12, zero fill to position 4, long 34
5672 $foo = pack('nN', 42, 4711);
5673 # pack big-endian 16- and 32-bit unsigned integers
5674 $foo = pack('S>L>', 42, 4711);
5676 $foo = pack('s<l<', -42, 4711);
5677 # pack little-endian 16- and 32-bit signed integers
5678 $foo = pack('(sl)<', -42, 4711);
5681 The same template may generally also be used in
5682 L<C<unpack>|/unpack TEMPLATE,EXPR>.
5684 =item package NAMESPACE
5686 =item package NAMESPACE VERSION
5687 X<package> X<module> X<namespace> X<version>
5689 =item package NAMESPACE BLOCK
5691 =item package NAMESPACE VERSION BLOCK
5692 X<package> X<module> X<namespace> X<version>
5694 =for Pod::Functions declare a separate global namespace
5696 Declares the BLOCK or the rest of the compilation unit as being in the
5697 given namespace. The scope of the package declaration is either the
5698 supplied code BLOCK or, in the absence of a BLOCK, from the declaration
5699 itself through the end of current scope (the enclosing block, file, or
5700 L<C<eval>|/eval EXPR>). That is, the forms without a BLOCK are
5701 operative through the end of the current scope, just like the
5702 L<C<my>|/my VARLIST>, L<C<state>|/state VARLIST>, and
5703 L<C<our>|/our VARLIST> operators. All unqualified dynamic identifiers
5704 in this scope will be in the given namespace, except where overridden by
5705 another L<C<package>|/package NAMESPACE> declaration or
5706 when they're one of the special identifiers that qualify into C<main::>,
5707 like C<STDOUT>, C<ARGV>, C<ENV>, and the punctuation variables.
5709 A package statement affects dynamic variables only, including those
5710 you've used L<C<local>|/local EXPR> on, but I<not> lexically-scoped
5711 variables, which are created with L<C<my>|/my VARLIST>,
5712 L<C<state>|/state VARLIST>, or L<C<our>|/our VARLIST>. Typically it
5713 would be the first declaration in a file included by
5714 L<C<require>|/require VERSION> or L<C<use>|/use Module VERSION LIST>.
5715 You can switch into a
5716 package in more than one place, since this only determines which default
5717 symbol table the compiler uses for the rest of that block. You can refer to
5718 identifiers in other packages than the current one by prefixing the identifier
5719 with the package name and a double colon, as in C<$SomePack::var>
5720 or C<ThatPack::INPUT_HANDLE>. If package name is omitted, the C<main>
5721 package as assumed. That is, C<$::sail> is equivalent to
5722 C<$main::sail> (as well as to C<$main'sail>, still seen in ancient
5723 code, mostly from Perl 4).
5725 If VERSION is provided, L<C<package>|/package NAMESPACE> sets the
5726 C<$VERSION> variable in the given
5727 namespace to a L<version> object with the VERSION provided. VERSION must be a
5728 "strict" style version number as defined by the L<version> module: a positive
5729 decimal number (integer or decimal-fraction) without exponentiation or else a
5730 dotted-decimal v-string with a leading 'v' character and at least three
5731 components. You should set C<$VERSION> only once per package.
5733 See L<perlmod/"Packages"> for more information about packages, modules,
5734 and classes. See L<perlsub> for other scoping issues.
5739 =for Pod::Functions +5.004 the current package
5741 A special token that returns the name of the package in which it occurs.
5743 =item pipe READHANDLE,WRITEHANDLE
5746 =for Pod::Functions open a pair of connected filehandles
5748 Opens a pair of connected pipes like the corresponding system call.
5749 Note that if you set up a loop of piped processes, deadlock can occur
5750 unless you are very careful. In addition, note that Perl's pipes use
5751 IO buffering, so you may need to set L<C<$E<verbar>>|perlvar/$E<verbar>>
5752 to flush your WRITEHANDLE after each command, depending on the
5755 Returns true on success.
5757 See L<IPC::Open2>, L<IPC::Open3>, and
5758 L<perlipc/"Bidirectional Communication with Another Process">
5759 for examples of such things.
5761 On systems that support a close-on-exec flag on files, that flag is set
5762 on all newly opened file descriptors whose
5763 L<C<fileno>|/fileno FILEHANDLE>s are I<higher> than the current value of
5764 L<C<$^F>|perlvar/$^F> (by default 2 for C<STDERR>). See L<perlvar/$^F>.
5771 =for Pod::Functions remove the last element from an array and return it
5773 Pops and returns the last value of the array, shortening the array by
5776 Returns the undefined value if the array is empty, although this may
5777 also happen at other times. If ARRAY is omitted, pops the
5778 L<C<@ARGV>|perlvar/@ARGV> array in the main program, but the
5779 L<C<@_>|perlvar/@_> array in subroutines, just like
5780 L<C<shift>|/shift ARRAY>.
5782 Starting with Perl 5.14, an experimental feature allowed
5783 L<C<pop>|/pop ARRAY> to take a
5784 scalar expression. This experiment has been deemed unsuccessful, and was
5785 removed as of Perl 5.24.
5788 X<pos> X<match, position>
5792 =for Pod::Functions find or set the offset for the last/next m//g search
5794 Returns the offset of where the last C<m//g> search left off for the
5795 variable in question (L<C<$_>|perlvar/$_> is used when the variable is not
5796 specified). This offset is in characters unless the
5797 (no-longer-recommended) L<C<use bytes>|bytes> pragma is in effect, in
5798 which case the offset is in bytes. Note that 0 is a valid match offset.
5799 L<C<undef>|/undef EXPR> indicates
5800 that the search position is reset (usually due to match failure, but
5801 can also be because no match has yet been run on the scalar).
5803 L<C<pos>|/pos SCALAR> directly accesses the location used by the regexp
5804 engine to store the offset, so assigning to L<C<pos>|/pos SCALAR> will
5805 change that offset, and so will also influence the C<\G> zero-width
5806 assertion in regular expressions. Both of these effects take place for
5807 the next match, so you can't affect the position with
5808 L<C<pos>|/pos SCALAR> during the current match, such as in
5809 C<(?{pos() = 5})> or C<s//pos() = 5/e>.
5811 Setting L<C<pos>|/pos SCALAR> also resets the I<matched with
5812 zero-length> flag, described
5813 under L<perlre/"Repeated Patterns Matching a Zero-length Substring">.
5815 Because a failed C<m//gc> match doesn't reset the offset, the return
5816 from L<C<pos>|/pos SCALAR> won't change either in this case. See
5817 L<perlre> and L<perlop>.
5819 =item print FILEHANDLE LIST
5822 =item print FILEHANDLE
5828 =for Pod::Functions output a list to a filehandle
5830 Prints a string or a list of strings. Returns true if successful.
5831 FILEHANDLE may be a scalar variable containing the name of or a reference
5832 to the filehandle, thus introducing one level of indirection. (NOTE: If
5833 FILEHANDLE is a variable and the next token is a term, it may be
5834 misinterpreted as an operator unless you interpose a C<+> or put
5835 parentheses around the arguments.) If FILEHANDLE is omitted, prints to the
5836 last selected (see L<C<select>|/select FILEHANDLE>) output handle. If
5837 LIST is omitted, prints L<C<$_>|perlvar/$_> to the currently selected
5838 output handle. To use FILEHANDLE alone to print the content of
5839 L<C<$_>|perlvar/$_> to it, you must use a bareword filehandle like
5840 C<FH>, not an indirect one like C<$fh>. To set the default output handle
5841 to something other than STDOUT, use the select operation.
5843 The current value of L<C<$,>|perlvar/$,> (if any) is printed between
5844 each LIST item. The current value of L<C<$\>|perlvar/$\> (if any) is
5845 printed after the entire LIST has been printed. Because print takes a
5846 LIST, anything in the LIST is evaluated in list context, including any
5847 subroutines whose return lists you pass to
5848 L<C<print>|/print FILEHANDLE LIST>. Be careful not to follow the print
5850 parenthesis unless you want the corresponding right parenthesis to
5851 terminate the arguments to the print; put parentheses around all arguments
5852 (or interpose a C<+>, but that doesn't look as good).
5854 If you're storing handles in an array or hash, or in general whenever
5855 you're using any expression more complex than a bareword handle or a plain,
5856 unsubscripted scalar variable to retrieve it, you will have to use a block
5857 returning the filehandle value instead, in which case the LIST may not be
5860 print { $files[$i] } "stuff\n";
5861 print { $OK ? *STDOUT : *STDERR } "stuff\n";
5863 Printing to a closed pipe or socket will generate a SIGPIPE signal. See
5864 L<perlipc> for more on signal handling.
5866 =item printf FILEHANDLE FORMAT, LIST
5869 =item printf FILEHANDLE
5871 =item printf FORMAT, LIST
5875 =for Pod::Functions output a formatted list to a filehandle
5877 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that
5878 L<C<$\>|perlvar/$\> (the output record separator) is not appended. The
5879 FORMAT and the LIST are actually parsed as a single list. The first
5880 argument of the list will be interpreted as the
5881 L<C<printf>|/printf FILEHANDLE FORMAT, LIST> format. This means that
5882 C<printf(@_)> will use C<$_[0]> as the format. See
5883 L<sprintf|/sprintf FORMAT, LIST> for an explanation of the format
5884 argument. If C<use locale> (including C<use locale ':not_characters'>)
5885 is in effect and L<C<POSIX::setlocale>|POSIX/C<setlocale>> has been
5886 called, the character used for the decimal separator in formatted
5887 floating-point numbers is affected by the C<LC_NUMERIC> locale setting.
5888 See L<perllocale> and L<POSIX>.
5890 For historical reasons, if you omit the list, L<C<$_>|perlvar/$_> is
5892 to use FILEHANDLE without a list, you must use a bareword filehandle like
5893 C<FH>, not an indirect one like C<$fh>. However, this will rarely do what
5894 you want; if L<C<$_>|perlvar/$_> contains formatting codes, they will be
5895 replaced with the empty string and a warning will be emitted if
5896 L<warnings> are enabled. Just use L<C<print>|/print FILEHANDLE LIST> if
5897 you want to print the contents of L<C<$_>|perlvar/$_>.
5899 Don't fall into the trap of using a
5900 L<C<printf>|/printf FILEHANDLE FORMAT, LIST> when a simple
5901 L<C<print>|/print FILEHANDLE LIST> would do. The
5902 L<C<print>|/print FILEHANDLE LIST> is more efficient and less error
5905 =item prototype FUNCTION
5910 =for Pod::Functions +5.002 get the prototype (if any) of a subroutine
5912 Returns the prototype of a function as a string (or
5913 L<C<undef>|/undef EXPR> if the
5914 function has no prototype). FUNCTION is a reference to, or the name of,
5915 the function whose prototype you want to retrieve. If FUNCTION is omitted,
5916 L<C<$_>|perlvar/$_> is used.
5918 If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
5919 name for a Perl builtin. If the builtin's arguments
5920 cannot be adequately expressed by a prototype
5921 (such as L<C<system>|/system LIST>), L<C<prototype>|/prototype FUNCTION>
5922 returns L<C<undef>|/undef EXPR>, because the builtin
5923 does not really behave like a Perl function. Otherwise, the string
5924 describing the equivalent prototype is returned.
5926 =item push ARRAY,LIST
5929 =for Pod::Functions append one or more elements to an array
5931 Treats ARRAY as a stack by appending the values of LIST to the end of
5932 ARRAY. The length of ARRAY increases by the length of LIST. Has the same
5935 for my $value (LIST) {
5936 $ARRAY[++$#ARRAY] = $value;
5939 but is more efficient. Returns the number of elements in the array following
5940 the completed L<C<push>|/push ARRAY,LIST>.
5942 Starting with Perl 5.14, an experimental feature allowed
5943 L<C<push>|/push ARRAY,LIST> to take a
5944 scalar expression. This experiment has been deemed unsuccessful, and was
5945 removed as of Perl 5.24.
5949 =for Pod::Functions singly quote a string
5953 =for Pod::Functions doubly quote a string
5957 =for Pod::Functions quote a list of words
5961 =for Pod::Functions backquote quote a string
5963 Generalized quotes. See L<perlop/"Quote-Like Operators">.
5967 =for Pod::Functions +5.005 compile pattern
5969 Regexp-like quote. See L<perlop/"Regexp Quote-Like Operators">.
5971 =item quotemeta EXPR
5972 X<quotemeta> X<metacharacter>
5976 =for Pod::Functions quote regular expression magic characters
5978 Returns the value of EXPR with all the ASCII non-"word"
5979 characters backslashed. (That is, all ASCII characters not matching
5980 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
5981 returned string, regardless of any locale settings.)
5982 This is the internal function implementing
5983 the C<\Q> escape in double-quoted strings.
5984 (See below for the behavior on non-ASCII code points.)
5986 If EXPR is omitted, uses L<C<$_>|perlvar/$_>.
5988 quotemeta (and C<\Q> ... C<\E>) are useful when interpolating strings into
5989 regular expressions, because by default an interpolated variable will be
5990 considered a mini-regular expression. For example:
5992 my $sentence = 'The quick brown fox jumped over the lazy dog';
5993 my $substring = 'quick.*?fox';
5994 $sentence =~ s{$substring}{big bad wolf};
5996 Will cause C<$sentence> to become C<'The big bad wolf jumped over...'>.
6000 my $sentence = 'The quick brown fox jumped over the lazy dog';
6001 my $substring = 'quick.*?fox';
6002 $sentence =~ s{\Q$substring\E}{big bad wolf};
6006 my $sentence = 'The quick brown fox jumped over the lazy dog';
6007 my $substring = 'quick.*?fox';
6008 my $quoted_substring = quotemeta($substring);
6009 $sentence =~ s{$quoted_substring}{big bad wolf};
6011 Will both leave the sentence as is.
6012 Normally, when accepting literal string input from the user,
6013 L<C<quotemeta>|/quotemeta EXPR> or C<\Q> must be used.
6015 In Perl v5.14, all non-ASCII characters are quoted in non-UTF-8-encoded
6016 strings, but not quoted in UTF-8 strings.
6018 Starting in Perl v5.16, Perl adopted a Unicode-defined strategy for
6019 quoting non-ASCII characters; the quoting of ASCII characters is
6022 Also unchanged is the quoting of non-UTF-8 strings when outside the
6024 L<C<use feature 'unicode_strings'>|feature/The 'unicode_strings' feature>,
6025 which is to quote all
6026 characters in the upper Latin1 range. This provides complete backwards
6027 compatibility for old programs which do not use Unicode. (Note that
6028 C<unicode_strings> is automatically enabled within the scope of a
6029 S<C<use v5.12>> or greater.)
6031 Within the scope of L<C<use locale>|locale>, all non-ASCII Latin1 code
6033 are quoted whether the string is encoded as UTF-8 or not. As mentioned
6034 above, locale does not affect the quoting of ASCII-range characters.
6035 This protects against those locales where characters such as C<"|"> are
6036 considered to be word characters.
6038 Otherwise, Perl quotes non-ASCII characters using an adaptation from
6039 Unicode (see L<http://www.unicode.org/reports/tr31/>).
6040 The only code points that are quoted are those that have any of the
6041 Unicode properties: Pattern_Syntax, Pattern_White_Space, White_Space,
6042 Default_Ignorable_Code_Point, or General_Category=Control.
6044 Of these properties, the two important ones are Pattern_Syntax and
6045 Pattern_White_Space. They have been set up by Unicode for exactly this
6046 purpose of deciding which characters in a regular expression pattern
6047 should be quoted. No character that can be in an identifier has these
6050 Perl promises, that if we ever add regular expression pattern
6051 metacharacters to the dozen already defined
6052 (C<\ E<verbar> ( ) [ { ^ $ * + ? .>), that we will only use ones that have the
6053 Pattern_Syntax property. Perl also promises, that if we ever add
6054 characters that are considered to be white space in regular expressions
6055 (currently mostly affected by C</x>), they will all have the
6056 Pattern_White_Space property.
6058 Unicode promises that the set of code points that have these two
6059 properties will never change, so something that is not quoted in v5.16
6060 will never need to be quoted in any future Perl release. (Not all the
6061 code points that match Pattern_Syntax have actually had characters
6062 assigned to them; so there is room to grow, but they are quoted
6063 whether assigned or not. Perl, of course, would never use an
6064 unassigned code point as an actual metacharacter.)
6066 Quoting characters that have the other 3 properties is done to enhance
6067 the readability of the regular expression and not because they actually
6068 need to be quoted for regular expression purposes (characters with the
6069 White_Space property are likely to be indistinguishable on the page or
6070 screen from those with the Pattern_White_Space property; and the other
6071 two properties contain non-printing characters).
6078 =for Pod::Functions retrieve the next pseudorandom number
6080 Returns a random fractional number greater than or equal to C<0> and less
6081 than the value of EXPR. (EXPR should be positive.) If EXPR is
6082 omitted, the value C<1> is used. Currently EXPR with the value C<0> is
6083 also special-cased as C<1> (this was undocumented before Perl 5.8.0
6084 and is subject to change in future versions of Perl). Automatically calls
6085 L<C<srand>|/srand EXPR> unless L<C<srand>|/srand EXPR> has already been
6086 called. See also L<C<srand>|/srand EXPR>.
6088 Apply L<C<int>|/int EXPR> to the value returned by L<C<rand>|/rand EXPR>
6089 if you want random integers instead of random fractional numbers. For
6094 returns a random integer between C<0> and C<9>, inclusive.
6096 (Note: If your rand function consistently returns numbers that are too
6097 large or too small, then your version of Perl was probably compiled
6098 with the wrong number of RANDBITS.)
6100 B<L<C<rand>|/rand EXPR> is not cryptographically secure. You should not rely
6101 on it in security-sensitive situations.> As of this writing, a
6102 number of third-party CPAN modules offer random number generators
6103 intended by their authors to be cryptographically secure,
6104 including: L<Data::Entropy>, L<Crypt::Random>, L<Math::Random::Secure>,
6105 and L<Math::TrulyRandom>.
6107 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
6108 X<read> X<file, read>
6110 =item read FILEHANDLE,SCALAR,LENGTH
6112 =for Pod::Functions fixed-length buffered input from a filehandle
6114 Attempts to read LENGTH I<characters> of data into variable SCALAR
6115 from the specified FILEHANDLE. Returns the number of characters
6116 actually read, C<0> at end of file, or undef if there was an error (in
6117 the latter case L<C<$!>|perlvar/$!> is also set). SCALAR will be grown
6119 so that the last character actually read is the last character of the
6120 scalar after the read.
6122 An OFFSET may be specified to place the read data at some place in the
6123 string other than the beginning. A negative OFFSET specifies
6124 placement at that many characters counting backwards from the end of
6125 the string. A positive OFFSET greater than the length of SCALAR
6126 results in the string being padded to the required size with C<"\0">
6127 bytes before the result of the read is appended.
6129 The call is implemented in terms of either Perl's or your system's native
6130 L<fread(3)> library function. To get a true L<read(2)> system call, see
6131 L<sysread|/sysread FILEHANDLE,SCALAR,LENGTH,OFFSET>.
6133 Note the I<characters>: depending on the status of the filehandle,
6134 either (8-bit) bytes or characters are read. By default, all
6135 filehandles operate on bytes, but for example if the filehandle has
6136 been opened with the C<:utf8> I/O layer (see
6137 L<C<open>|/open FILEHANDLE,EXPR>, and the L<open>
6138 pragma), the I/O will operate on UTF8-encoded Unicode
6139 characters, not bytes. Similarly for the C<:encoding> layer:
6140 in that case pretty much any characters can be read.
6142 =item readdir DIRHANDLE
6145 =for Pod::Functions get a directory from a directory handle
6147 Returns the next directory entry for a directory opened by
6148 L<C<opendir>|/opendir DIRHANDLE,EXPR>.
6149 If used in list context, returns all the rest of the entries in the
6150 directory. If there are no more entries, returns the undefined value in
6151 scalar context and the empty list in list context.
6153 If you're planning to filetest the return values out of a
6154 L<C<readdir>|/readdir DIRHANDLE>, you'd better prepend the directory in
6155 question. Otherwise, because we didn't L<C<chdir>|/chdir EXPR> there,
6156 it would have been testing the wrong file.
6158 opendir(my $dh, $some_dir) || die "Can't opendir $some_dir: $!";
6159 my @dots = grep { /^\./ && -f "$some_dir/$_" } readdir($dh);
6162 As of Perl 5.12 you can use a bare L<C<readdir>|/readdir DIRHANDLE> in a
6163 C<while> loop, which will set L<C<$_>|perlvar/$_> on every iteration.
6164 If either a C<readdir> expression or an explicit assignment of a
6165 C<readdir> expression to a scalar is used as a C<while>/C<for> condition,
6166 then the condition actually tests for definedness of the expression's
6167 value, not for its regular truth value.
6169 opendir(my $dh, $some_dir) || die "Can't open $some_dir: $!";
6170 while (readdir $dh) {
6171 print "$some_dir/$_\n";
6175 To avoid confusing would-be users of your code who are running earlier
6176 versions of Perl with mysterious failures, put this sort of thing at the
6177 top of your file to signal that your code will work I<only> on Perls of a
6180 use 5.012; # so readdir assigns to $_ in a lone while test
6185 X<readline> X<gets> X<fgets>
6187 =for Pod::Functions fetch a record from a file
6189 Reads from the filehandle whose typeglob is contained in EXPR (or from
6190 C<*ARGV> if EXPR is not provided). In scalar context, each call reads and
6191 returns the next line until end-of-file is reached, whereupon the
6192 subsequent call returns L<C<undef>|/undef EXPR>. In list context, reads
6193 until end-of-file is reached and returns a list of lines. Note that the
6194 notion of "line" used here is whatever you may have defined with
6195 L<C<$E<sol>>|perlvar/$E<sol>> (or C<$INPUT_RECORD_SEPARATOR> in
6196 L<English>). See L<perlvar/"$/">.
6198 When L<C<$E<sol>>|perlvar/$E<sol>> is set to L<C<undef>|/undef EXPR>,
6199 when L<C<readline>|/readline EXPR> is in scalar context (i.e., file
6200 slurp mode), and when an empty file is read, it returns C<''> the first
6201 time, followed by L<C<undef>|/undef EXPR> subsequently.
6203 This is the internal function implementing the C<< <EXPR> >>
6204 operator, but you can use it directly. The C<< <EXPR> >>
6205 operator is discussed in more detail in L<perlop/"I/O Operators">.
6208 my $line = readline(STDIN); # same thing
6210 If L<C<readline>|/readline EXPR> encounters an operating system error,
6211 L<C<$!>|perlvar/$!> will be set with the corresponding error message.
6212 It can be helpful to check L<C<$!>|perlvar/$!> when you are reading from
6213 filehandles you don't trust, such as a tty or a socket. The following
6214 example uses the operator form of L<C<readline>|/readline EXPR> and dies
6215 if the result is not defined.
6217 while ( ! eof($fh) ) {
6218 defined( $_ = readline $fh ) or die "readline failed: $!";
6222 Note that you have can't handle L<C<readline>|/readline EXPR> errors
6223 that way with the C<ARGV> filehandle. In that case, you have to open
6224 each element of L<C<@ARGV>|perlvar/@ARGV> yourself since
6225 L<C<eof>|/eof FILEHANDLE> handles C<ARGV> differently.
6227 foreach my $arg (@ARGV) {
6228 open(my $fh, $arg) or warn "Can't open $arg: $!";
6230 while ( ! eof($fh) ) {
6231 defined( $_ = readline $fh )
6232 or die "readline failed for $arg: $!";
6237 Like the C<< <EXPR> >> operator, if a C<readline> expression is
6238 used as the condition of a C<while> or C<for> loop, then it will be
6239 implicitly assigned to C<$_>. If either a C<readline> expression or
6240 an explicit assignment of a C<readline> expression to a scalar is used
6241 as a C<while>/C<for> condition, then the condition actually tests for
6242 definedness of the expression's value, not for its regular truth value.
6249 =for Pod::Functions determine where a symbolic link is pointing
6251 Returns the value of a symbolic link, if symbolic links are
6252 implemented. If not, raises an exception. If there is a system
6253 error, returns the undefined value and sets L<C<$!>|perlvar/$!> (errno).
6254 If EXPR is omitted, uses L<C<$_>|perlvar/$_>.
6256 Portability issues: L<perlport/readlink>.
6263 =for Pod::Functions execute a system command and collect standard output
6265 EXPR is executed as a system command.
6266 The collected standard output of the command is returned.
6267 In scalar context, it comes back as a single (potentially
6268 multi-line) string. In list context, returns a list of lines
6269 (however you've defined lines with L<C<$E<sol>>|perlvar/$E<sol>> (or
6270 C<$INPUT_RECORD_SEPARATOR> in L<English>)).
6271 This is the internal function implementing the C<qx/EXPR/>
6272 operator, but you can use it directly. The C<qx/EXPR/>
6273 operator is discussed in more detail in L<perlop/"I/O Operators">.
6274 If EXPR is omitted, uses L<C<$_>|perlvar/$_>.
6276 =item recv SOCKET,SCALAR,LENGTH,FLAGS
6279 =for Pod::Functions receive a message over a Socket
6281 Receives a message on a socket. Attempts to receive LENGTH characters
6282 of data into variable SCALAR from the specified SOCKET filehandle.
6283 SCALAR will be grown or shrunk to the length actually read. Takes the
6284 same flags as the system call of the same name. Returns the address
6285 of the sender if SOCKET's protocol supports this; returns an empty
6286 string otherwise. If there's an error, returns the undefined value.
6287 This call is actually implemented in terms of the L<recvfrom(2)> system call.
6288 See L<perlipc/"UDP: Message Passing"> for examples.
6290 Note that if the socket has been marked as C<:utf8>, C<recv> will
6291 throw an exception. The C<:encoding(...)> layer implicitly introduces
6292 the C<:utf8> layer. See L<C<binmode>|/binmode FILEHANDLE, LAYER>.
6301 =for Pod::Functions start this loop iteration over again
6303 The L<C<redo>|/redo LABEL> command restarts the loop block without
6304 evaluating the conditional again. The L<C<continue>|/continue BLOCK>
6305 block, if any, is not executed. If
6306 the LABEL is omitted, the command refers to the innermost enclosing
6307 loop. The C<redo EXPR> form, available starting in Perl 5.18.0, allows a
6308 label name to be computed at run time, and is otherwise identical to C<redo
6309 LABEL>. Programs that want to lie to themselves about what was just input
6310 normally use this command:
6312 # a simpleminded Pascal comment stripper
6313 # (warning: assumes no { or } in strings)
6314 LINE: while (<STDIN>) {
6315 while (s|({.*}.*){.*}|$1 |) {}
6320 if (/}/) { # end of comment?
6329 L<C<redo>|/redo LABEL> cannot return a value from a block that typically
6330 returns a value, such as C<eval {}>, C<sub {}>, or C<do {}>. It will perform
6331 its flow control behavior, which precludes any return value. It should not be
6332 used to exit a L<C<grep>|/grep BLOCK LIST> or L<C<map>|/map BLOCK LIST>
6335 Note that a block by itself is semantically identical to a loop
6336 that executes once. Thus L<C<redo>|/redo LABEL> inside such a block
6337 will effectively turn it into a looping construct.
6339 See also L<C<continue>|/continue BLOCK> for an illustration of how
6340 L<C<last>|/last LABEL>, L<C<next>|/next LABEL>, and
6341 L<C<redo>|/redo LABEL> work.
6343 Unlike most named operators, this has the same precedence as assignment.
6344 It is also exempt from the looks-like-a-function rule, so
6345 C<redo ("foo")."bar"> will cause "bar" to be part of the argument to
6346 L<C<redo>|/redo LABEL>.
6353 =for Pod::Functions find out the type of thing being referenced
6355 Examines the value of EXPR, expecting it to be a reference, and returns
6356 a string giving information about the reference and the type of referent.
6357 If EXPR is not specified, L<C<$_>|perlvar/$_> will be used.
6359 If the operand is not a reference, then the empty string will be returned.
6360 An empty string will only be returned in this situation. C<ref> is often
6361 useful to just test whether a value is a reference, which can be done
6362 by comparing the result to the empty string. It is a common mistake
6363 to use the result of C<ref> directly as a truth value: this goes wrong
6364 because C<0> (which is false) can be returned for a reference.
6366 If the operand is a reference to a blessed object, then the name of
6367 the class into which the referent is blessed will be returned. C<ref>
6368 doesn't care what the physical type of the referent is; blessing takes
6369 precedence over such concerns. Beware that exact comparison of C<ref>
6370 results against a class name doesn't perform a class membership test:
6371 a class's members also include objects blessed into subclasses, for
6372 which C<ref> will return the name of the subclass. Also beware that
6373 class names can clash with the built-in type names (described below).
6375 If the operand is a reference to an unblessed object, then the return
6376 value indicates the type of object. If the unblessed referent is not
6377 a scalar, then the return value will be one of the strings C<ARRAY>,
6378 C<HASH>, C<CODE>, C<FORMAT>, or C<IO>, indicating only which kind of
6379 object it is. If the unblessed referent is a scalar, then the return
6380 value will be one of the strings C<SCALAR>, C<VSTRING>, C<REF>, C<GLOB>,
6381 C<LVALUE>, or C<REGEXP>, depending on the kind of value the scalar
6382 currently has. But note that C<qr//> scalars are created already
6383 blessed, so C<ref qr/.../> will likely return C<Regexp>. Beware that
6384 these built-in type names can also be used as
6385 class names, so C<ref> returning one of these names doesn't unambiguously
6386 indicate that the referent is of the kind to which the name refers.
6388 The ambiguity between built-in type names and class names significantly
6389 limits the utility of C<ref>. For unambiguous information, use
6390 L<C<Scalar::Util::blessed()>|Scalar::Util/blessed> for information about
6391 blessing, and L<C<Scalar::Util::reftype()>|Scalar::Util/reftype> for
6392 information about physical types. Use L<the C<isa> method|UNIVERSAL/C<<
6393 $obj->isa( TYPE ) >>> for class membership tests, though one must be
6394 sure of blessedness before attempting a method call.
6396 See also L<perlref> and L<perlobj>.
6398 =item rename OLDNAME,NEWNAME
6399 X<rename> X<move> X<mv> X<ren>
6401 =for Pod::Functions change a filename
6403 Changes the name of a file; an existing file NEWNAME will be
6404 clobbered. Returns true for success, false otherwise.
6406 Behavior of this function varies wildly depending on your system
6407 implementation. For example, it will usually not work across file system
6408 boundaries, even though the system I<mv> command sometimes compensates
6409 for this. Other restrictions include whether it works on directories,
6410 open files, or pre-existing files. Check L<perlport> and either the
6411 L<rename(2)> manpage or equivalent system documentation for details.
6413 For a platform independent L<C<move>|File::Copy/move> function look at
6414 the L<File::Copy> module.
6416 Portability issues: L<perlport/rename>.
6418 =item require VERSION
6425 =for Pod::Functions load in external functions from a library at runtime
6427 Demands a version of Perl specified by VERSION, or demands some semantics
6428 specified by EXPR or by L<C<$_>|perlvar/$_> if EXPR is not supplied.
6430 VERSION may be either a literal such as v5.24.1, which will be
6431 compared to L<C<$^V>|perlvar/$^V> (or C<$PERL_VERSION> in L<English>),
6432 or a numeric argument of the form 5.024001, which will be compared to
6433 L<C<$]>|perlvar/$]>. An exception is raised if VERSION is greater than
6434 the version of the current Perl interpreter. Compare with
6435 L<C<use>|/use Module VERSION LIST>, which can do a similar check at
6438 Specifying VERSION as a numeric argument of the form 5.024001 should
6439 generally be avoided as older less readable syntax compared to
6440 v5.24.1. Before perl 5.8.0 (released in 2002), the more verbose numeric
6441 form was the only supported syntax, which is why you might see it in
6444 require v5.24.1; # run time version check
6445 require 5.24.1; # ditto
6446 require 5.024_001; # ditto; older syntax compatible
6449 Otherwise, L<C<require>|/require VERSION> demands that a library file be
6450 included if it hasn't already been included. The file is included via
6451 the do-FILE mechanism, which is essentially just a variety of
6452 L<C<eval>|/eval EXPR> with the
6453 caveat that lexical variables in the invoking script will be invisible
6454 to the included code. If it were implemented in pure Perl, it
6455 would have semantics similar to the following:
6461 my ($filename) = @_;
6462 if ( my $version = eval { version->parse($filename) } ) {
6463 if ( $version > $^V ) {
6464 my $vn = $version->normal;
6465 croak "Perl $vn required--this is only $^V, stopped";
6470 if (exists $INC{$filename}) {
6471 return 1 if $INC{$filename};
6472 croak "Compilation failed in require";
6475 foreach $prefix (@INC) {
6477 #... do other stuff - see text below ....
6479 # (see text below about possible appending of .pmc
6480 # suffix to $filename)
6481 my $realfilename = "$prefix/$filename";
6482 next if ! -e $realfilename || -d _ || -b _;
6483 $INC{$filename} = $realfilename;
6484 my $result = do($realfilename);
6485 # but run in caller's namespace
6487 if (!defined $result) {
6488 $INC{$filename} = undef;
6489 croak $@ ? "$@Compilation failed in require"
6490 : "Can't locate $filename: $!\n";
6493 delete $INC{$filename};
6494 croak "$filename did not return true value";
6499 croak "Can't locate $filename in \@INC ...";
6502 Note that the file will not be included twice under the same specified
6505 The file must return true as the last statement to indicate
6506 successful execution of any initialization code, so it's customary to
6507 end such a file with C<1;> unless you're sure it'll return true
6508 otherwise. But it's better just to put the C<1;>, in case you add more
6511 If EXPR is a bareword, L<C<require>|/require VERSION> assumes a F<.pm>
6512 extension and replaces C<::> with C</> in the filename for you,
6513 to make it easy to load standard modules. This form of loading of
6514 modules does not risk altering your namespace, however it will autovivify
6515 the stash for the required module.
6517 In other words, if you try this:
6519 require Foo::Bar; # a splendid bareword
6521 The require function will actually look for the F<Foo/Bar.pm> file in the
6522 directories specified in the L<C<@INC>|perlvar/@INC> array, and it will
6523 autovivify the C<Foo::Bar::> stash at compile time.
6525 But if you try this:
6527 my $class = 'Foo::Bar';
6528 require $class; # $class is not a bareword
6530 require "Foo::Bar"; # not a bareword because of the ""
6532 The require function will look for the F<Foo::Bar> file in the
6533 L<C<@INC>|perlvar/@INC> array and
6534 will complain about not finding F<Foo::Bar> there. In this case you can do:
6536 eval "require $class";
6540 require "Foo/Bar.pm";
6542 Neither of these forms will autovivify any stashes at compile time and
6543 only have run time effects.
6545 Now that you understand how L<C<require>|/require VERSION> looks for
6546 files with a bareword argument, there is a little extra functionality
6547 going on behind the scenes. Before L<C<require>|/require VERSION> looks
6548 for a F<.pm> extension, it will first look for a similar filename with a
6549 F<.pmc> extension. If this file is found, it will be loaded in place of
6550 any file ending in a F<.pm> extension. This applies to both the explicit
6551 C<require "Foo/Bar.pm";> form and the C<require Foo::Bar;> form.
6553 You can also insert hooks into the import facility by putting Perl code
6554 directly into the L<C<@INC>|perlvar/@INC> array. There are three forms
6555 of hooks: subroutine references, array references, and blessed objects.
6557 Subroutine references are the simplest case. When the inclusion system
6558 walks through L<C<@INC>|perlvar/@INC> and encounters a subroutine, this
6559 subroutine gets called with two parameters, the first a reference to
6560 itself, and the second the name of the file to be included (e.g.,
6561 F<Foo/Bar.pm>). The subroutine should return either nothing or else a
6562 list of up to four values in the following order:
6568 A reference to a scalar, containing any initial source code to prepend to
6569 the file or generator output.
6573 A filehandle, from which the file will be read.
6577 A reference to a subroutine. If there is no filehandle (previous item),
6578 then this subroutine is expected to generate one line of source code per
6579 call, writing the line into L<C<$_>|perlvar/$_> and returning 1, then
6580 finally at end of file returning 0. If there is a filehandle, then the
6581 subroutine will be called to act as a simple source filter, with the
6582 line as read in L<C<$_>|perlvar/$_>.
6583 Again, return 1 for each valid line, and 0 after all lines have been
6585 For historical reasons the subroutine will receive a meaningless argument
6586 (in fact always the numeric value zero) as C<$_[0]>.
6590 Optional state for the subroutine. The state is passed in as C<$_[1]>.
6594 If an empty list, L<C<undef>|/undef EXPR>, or nothing that matches the
6595 first 3 values above is returned, then L<C<require>|/require VERSION>
6596 looks at the remaining elements of L<C<@INC>|perlvar/@INC>.
6597 Note that this filehandle must be a real filehandle (strictly a typeglob
6598 or reference to a typeglob, whether blessed or unblessed); tied filehandles
6599 will be ignored and processing will stop there.
6601 If the hook is an array reference, its first element must be a subroutine
6602 reference. This subroutine is called as above, but the first parameter is
6603 the array reference. This lets you indirectly pass arguments to
6606 In other words, you can write:
6608 push @INC, \&my_sub;
6610 my ($coderef, $filename) = @_; # $coderef is \&my_sub
6616 push @INC, [ \&my_sub, $x, $y, ... ];
6618 my ($arrayref, $filename) = @_;
6619 # Retrieve $x, $y, ...
6620 my (undef, @parameters) = @$arrayref;
6624 If the hook is an object, it must provide an C<INC> method that will be
6625 called as above, the first parameter being the object itself. (Note that
6626 you must fully qualify the sub's name, as unqualified C<INC> is always forced
6627 into package C<main>.) Here is a typical code layout:
6633 my ($self, $filename) = @_;
6637 # In the main program
6638 push @INC, Foo->new(...);
6640 These hooks are also permitted to set the L<C<%INC>|perlvar/%INC> entry
6641 corresponding to the files they have loaded. See L<perlvar/%INC>.
6643 For a yet-more-powerful import facility, see
6644 L<C<use>|/use Module VERSION LIST> and L<perlmod>.
6651 =for Pod::Functions clear all variables of a given name
6653 Generally used in a L<C<continue>|/continue BLOCK> block at the end of a
6654 loop to clear variables and reset C<m?pattern?> searches so that they
6656 expression is interpreted as a list of single characters (hyphens
6657 allowed for ranges). All variables (scalars, arrays, and hashes)
6658 in the current package beginning with one of
6659 those letters are reset to their pristine state. If the expression is
6660 omitted, one-match searches (C<m?pattern?>) are reset to match again.
6661 Only resets variables or searches in the current package. Always returns
6664 reset 'X'; # reset all X variables
6665 reset 'a-z'; # reset lower case variables
6666 reset; # just reset m?one-time? searches
6668 Resetting C<"A-Z"> is not recommended because you'll wipe out your
6669 L<C<@ARGV>|perlvar/@ARGV> and L<C<@INC>|perlvar/@INC> arrays and your
6670 L<C<%ENV>|perlvar/%ENV> hash.
6672 Resets only package variables; lexical variables are unaffected, but
6673 they clean themselves up on scope exit anyway, so you'll probably want
6674 to use them instead. See L<C<my>|/my VARLIST>.
6681 =for Pod::Functions get out of a function early
6683 Returns from a subroutine, L<C<eval>|/eval EXPR>,
6684 L<C<do FILE>|/do EXPR>, L<C<sort>|/sort SUBNAME LIST> block or regex
6685 eval block (but not a L<C<grep>|/grep BLOCK LIST> or
6686 L<C<map>|/map BLOCK LIST> block) with the value
6687 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
6688 context, depending on how the return value will be used, and the context
6689 may vary from one execution to the next (see
6690 L<C<wantarray>|/wantarray>). If no EXPR
6691 is given, returns an empty list in list context, the undefined value in
6692 scalar context, and (of course) nothing at all in void context.
6694 (In the absence of an explicit L<C<return>|/return EXPR>, a subroutine,
6695 L<C<eval>|/eval EXPR>,
6696 or L<C<do FILE>|/do EXPR> automatically returns the value of the last expression
6699 Unlike most named operators, this is also exempt from the
6700 looks-like-a-function rule, so C<return ("foo")."bar"> will
6701 cause C<"bar"> to be part of the argument to L<C<return>|/return EXPR>.
6704 X<reverse> X<rev> X<invert>
6706 =for Pod::Functions flip a string or a list
6708 In list context, returns a list value consisting of the elements
6709 of LIST in the opposite order. In scalar context, concatenates the
6710 elements of LIST and returns a string value with all characters
6711 in the opposite order.
6713 print join(", ", reverse "world", "Hello"); # Hello, world
6715 print scalar reverse "dlrow ,", "olleH"; # Hello, world
6717 Used without arguments in scalar context, L<C<reverse>|/reverse LIST>
6718 reverses L<C<$_>|perlvar/$_>.
6720 $_ = "dlrow ,olleH";
6721 print reverse; # No output, list context
6722 print scalar reverse; # Hello, world
6724 Note that reversing an array to itself (as in C<@a = reverse @a>) will
6725 preserve non-existent elements whenever possible; i.e., for non-magical
6726 arrays or for tied arrays with C<EXISTS> and C<DELETE> methods.
6728 This operator is also handy for inverting a hash, although there are some
6729 caveats. If a value is duplicated in the original hash, only one of those
6730 can be represented as a key in the inverted hash. Also, this has to
6731 unwind one hash and build a whole new one, which may take some time
6732 on a large hash, such as from a DBM file.
6734 my %by_name = reverse %by_address; # Invert the hash
6736 =item rewinddir DIRHANDLE
6739 =for Pod::Functions reset directory handle
6741 Sets the current position to the beginning of the directory for the
6742 L<C<readdir>|/readdir DIRHANDLE> routine on DIRHANDLE.
6744 Portability issues: L<perlport/rewinddir>.
6746 =item rindex STR,SUBSTR,POSITION
6749 =item rindex STR,SUBSTR
6751 =for Pod::Functions right-to-left substring search
6753 Works just like L<C<index>|/index STR,SUBSTR,POSITION> except that it
6754 returns the position of the I<last>
6755 occurrence of SUBSTR in STR. If POSITION is specified, returns the
6756 last occurrence beginning at or before that position.
6758 =item rmdir FILENAME
6759 X<rmdir> X<rd> X<directory, remove>
6763 =for Pod::Functions remove a directory
6765 Deletes the directory specified by FILENAME if that directory is
6766 empty. If it succeeds it returns true; otherwise it returns false and
6767 sets L<C<$!>|perlvar/$!> (errno). If FILENAME is omitted, uses
6768 L<C<$_>|perlvar/$_>.
6770 To remove a directory tree recursively (C<rm -rf> on Unix) look at
6771 the L<C<rmtree>|File::Path/rmtree( $dir )> function of the L<File::Path>
6776 =for Pod::Functions replace a pattern with a string
6778 The substitution operator. See L<perlop/"Regexp Quote-Like Operators">.
6780 =item say FILEHANDLE LIST
6783 =item say FILEHANDLE
6789 =for Pod::Functions +say output a list to a filehandle, appending a newline
6791 Just like L<C<print>|/print FILEHANDLE LIST>, but implicitly appends a
6792 newline. C<say LIST> is simply an abbreviation for
6793 C<{ local $\ = "\n"; print LIST }>. To use FILEHANDLE without a LIST to
6794 print the contents of L<C<$_>|perlvar/$_> to it, you must use a bareword
6795 filehandle like C<FH>, not an indirect one like C<$fh>.
6797 L<C<say>|/say FILEHANDLE LIST> is available only if the
6798 L<C<"say"> feature|feature/The 'say' feature> is enabled or if it is
6799 prefixed with C<CORE::>. The
6800 L<C<"say"> feature|feature/The 'say' feature> is enabled automatically
6801 with a C<use v5.10> (or higher) declaration in the current scope.
6804 X<scalar> X<context>
6806 =for Pod::Functions force a scalar context
6808 Forces EXPR to be interpreted in scalar context and returns the value
6811 my @counts = ( scalar @a, scalar @b, scalar @c );
6813 There is no equivalent operator to force an expression to
6814 be interpolated in list context because in practice, this is never
6815 needed. If you really wanted to do so, however, you could use
6816 the construction C<@{[ (some expression) ]}>, but usually a simple
6817 C<(some expression)> suffices.
6819 Because L<C<scalar>|/scalar EXPR> is a unary operator, if you
6821 parenthesized list for the EXPR, this behaves as a scalar comma expression,
6822 evaluating all but the last element in void context and returning the final
6823 element evaluated in scalar context. This is seldom what you want.
6825 The following single statement:
6827 print uc(scalar(foo(), $bar)), $baz;
6829 is the moral equivalent of these two:
6832 print(uc($bar), $baz);
6834 See L<perlop> for more details on unary operators and the comma operator,
6835 and L<perldata> for details on evaluating a hash in scalar contex.
6837 =item seek FILEHANDLE,POSITION,WHENCE
6838 X<seek> X<fseek> X<filehandle, position>
6840 =for Pod::Functions reposition file pointer for random-access I/O
6842 Sets FILEHANDLE's position, just like the L<fseek(3)> call of C C<stdio>.
6843 FILEHANDLE may be an expression whose value gives the name of the
6844 filehandle. The values for WHENCE are C<0> to set the new position
6845 I<in bytes> to POSITION; C<1> to set it to the current position plus
6846 POSITION; and C<2> to set it to EOF plus POSITION, typically
6847 negative. For WHENCE you may use the constants C<SEEK_SET>,
6848 C<SEEK_CUR>, and C<SEEK_END> (start of the file, current position, end
6849 of the file) from the L<Fcntl> module. Returns C<1> on success, false
6852 Note the emphasis on bytes: even if the filehandle has been set to operate
6853 on characters (for example using the C<:encoding(UTF-8)> I/O layer), the
6854 L<C<seek>|/seek FILEHANDLE,POSITION,WHENCE>,
6855 L<C<tell>|/tell FILEHANDLE>, and
6856 L<C<sysseek>|/sysseek FILEHANDLE,POSITION,WHENCE>
6857 family of functions use byte offsets, not character offsets,
6858 because seeking to a character offset would be very slow in a UTF-8 file.
6860 If you want to position the file for
6861 L<C<sysread>|/sysread FILEHANDLE,SCALAR,LENGTH,OFFSET> or
6862 L<C<syswrite>|/syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET>, don't use
6863 L<C<seek>|/seek FILEHANDLE,POSITION,WHENCE>, because buffering makes its
6864 effect on the file's read-write position unpredictable and non-portable.
6865 Use L<C<sysseek>|/sysseek FILEHANDLE,POSITION,WHENCE> instead.
6867 Due to the rules and rigors of ANSI C, on some systems you have to do a
6868 seek whenever you switch between reading and writing. Amongst other
6869 things, this may have the effect of calling stdio's L<clearerr(3)>.
6870 A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position:
6874 This is also useful for applications emulating C<tail -f>. Once you hit
6875 EOF on your read and then sleep for a while, you (probably) have to stick in a
6876 dummy L<C<seek>|/seek FILEHANDLE,POSITION,WHENCE> to reset things. The
6877 L<C<seek>|/seek FILEHANDLE,POSITION,WHENCE> doesn't change the position,
6878 but it I<does> clear the end-of-file condition on the handle, so that the
6879 next C<readline FILE> makes Perl try again to read something. (We hope.)
6881 If that doesn't work (some I/O implementations are particularly
6882 cantankerous), you might need something like this:
6885 for ($curpos = tell($fh); $_ = readline($fh);
6886 $curpos = tell($fh)) {
6887 # search for some stuff and put it into files
6889 sleep($for_a_while);
6890 seek($fh, $curpos, 0);
6893 =item seekdir DIRHANDLE,POS
6896 =for Pod::Functions reposition directory pointer
6898 Sets the current position for the L<C<readdir>|/readdir DIRHANDLE>
6899 routine on DIRHANDLE. POS must be a value returned by
6900 L<C<telldir>|/telldir DIRHANDLE>. L<C<seekdir>|/seekdir DIRHANDLE,POS>
6901 also has the same caveats about possible directory compaction as the
6902 corresponding system library routine.
6904 =item select FILEHANDLE
6905 X<select> X<filehandle, default>
6909 =for Pod::Functions reset default output or do I/O multiplexing
6911 Returns the currently selected filehandle. If FILEHANDLE is supplied,
6912 sets the new current default filehandle for output. This has two
6913 effects: first, a L<C<write>|/write FILEHANDLE> or a L<C<print>|/print
6914 FILEHANDLE LIST> without a filehandle
6915 default to this FILEHANDLE. Second, references to variables related to
6916 output will refer to this output channel.
6918 For example, to set the top-of-form format for more than one
6919 output channel, you might do the following:
6926 FILEHANDLE may be an expression whose value gives the name of the
6927 actual filehandle. Thus:
6929 my $oldfh = select(STDERR); $| = 1; select($oldfh);
6931 Some programmers may prefer to think of filehandles as objects with
6932 methods, preferring to write the last example as:
6934 STDERR->autoflush(1);
6936 (Prior to Perl version 5.14, you have to C<use IO::Handle;> explicitly
6939 Portability issues: L<perlport/select>.
6941 =item select RBITS,WBITS,EBITS,TIMEOUT
6944 This calls the L<select(2)> syscall with the bit masks specified, which
6945 can be constructed using L<C<fileno>|/fileno FILEHANDLE> and
6946 L<C<vec>|/vec EXPR,OFFSET,BITS>, along these lines:
6948 my $rin = my $win = my $ein = '';
6949 vec($rin, fileno(STDIN), 1) = 1;
6950 vec($win, fileno(STDOUT), 1) = 1;
6953 If you want to select on many filehandles, you may wish to write a
6954 subroutine like this:
6959 for my $fh (@fhlist) {
6960 vec($bits, fileno($fh), 1) = 1;
6964 my $rin = fhbits(\*STDIN, $tty, $mysock);
6968 my ($nfound, $timeleft) =
6969 select(my $rout = $rin, my $wout = $win, my $eout = $ein,
6972 or to block until something becomes ready just do this
6975 select(my $rout = $rin, my $wout = $win, my $eout = $ein, undef);
6977 Most systems do not bother to return anything useful in C<$timeleft>, so
6978 calling L<C<select>|/select RBITS,WBITS,EBITS,TIMEOUT> in scalar context
6979 just returns C<$nfound>.
6981 Any of the bit masks can also be L<C<undef>|/undef EXPR>. The timeout,
6983 in seconds, which may be fractional. Note: not all implementations are
6984 capable of returning the C<$timeleft>. If not, they always return
6985 C<$timeleft> equal to the supplied C<$timeout>.
6987 You can effect a sleep of 250 milliseconds this way:
6989 select(undef, undef, undef, 0.25);
6991 Note that whether L<C<select>|/select RBITS,WBITS,EBITS,TIMEOUT> gets
6992 restarted after signals (say, SIGALRM) is implementation-dependent. See
6993 also L<perlport> for notes on the portability of
6994 L<C<select>|/select RBITS,WBITS,EBITS,TIMEOUT>.
6996 On error, L<C<select>|/select RBITS,WBITS,EBITS,TIMEOUT> behaves just
6997 like L<select(2)>: it returns C<-1> and sets L<C<$!>|perlvar/$!>.
6999 On some Unixes, L<select(2)> may report a socket file descriptor as
7000 "ready for reading" even when no data is available, and thus any
7001 subsequent L<C<read>|/read FILEHANDLE,SCALAR,LENGTH,OFFSET> would block.
7002 This can be avoided if you always use C<O_NONBLOCK> on the socket. See
7003 L<select(2)> and L<fcntl(2)> for further details.
7005 The standard L<C<IO::Select>|IO::Select> module provides a
7006 user-friendlier interface to
7007 L<C<select>|/select RBITS,WBITS,EBITS,TIMEOUT>, mostly because it does
7008 all the bit-mask work for you.
7010 B<WARNING>: One should not attempt to mix buffered I/O (like
7011 L<C<read>|/read FILEHANDLE,SCALAR,LENGTH,OFFSET> or
7012 L<C<readline>|/readline EXPR>) with
7013 L<C<select>|/select RBITS,WBITS,EBITS,TIMEOUT>, except as permitted by
7014 POSIX, and even then only on POSIX systems. You have to use
7015 L<C<sysread>|/sysread FILEHANDLE,SCALAR,LENGTH,OFFSET> instead.
7017 Portability issues: L<perlport/select>.
7019 =item semctl ID,SEMNUM,CMD,ARG
7022 =for Pod::Functions SysV semaphore control operations
7024 Calls the System V IPC function L<semctl(2)>. You'll probably have to say
7028 first to get the correct constant definitions. If CMD is IPC_STAT or
7029 GETALL, then ARG must be a variable that will hold the returned
7030 semid_ds structure or semaphore value array. Returns like
7031 L<C<ioctl>|/ioctl FILEHANDLE,FUNCTION,SCALAR>:
7032 the undefined value for error, "C<0 but true>" for zero, or the actual
7033 return value otherwise. The ARG must consist of a vector of native
7034 short integers, which may be created with C<pack("s!",(0)x$nsem)>.
7035 See also L<perlipc/"SysV IPC"> and the documentation for
7036 L<C<IPC::SysV>|IPC::SysV> and L<C<IPC::Semaphore>|IPC::Semaphore>.
7038 Portability issues: L<perlport/semctl>.
7040 =item semget KEY,NSEMS,FLAGS
7043 =for Pod::Functions get set of SysV semaphores
7045 Calls the System V IPC function L<semget(2)>. Returns the semaphore id, or
7046 the undefined value on error. See also
7047 L<perlipc/"SysV IPC"> and the documentation for
7048 L<C<IPC::SysV>|IPC::SysV> and L<C<IPC::Semaphore>|IPC::Semaphore>.
7050 Portability issues: L<perlport/semget>.
7052 =item semop KEY,OPSTRING
7055 =for Pod::Functions SysV semaphore operations
7057 Calls the System V IPC function L<semop(2)> for semaphore operations
7058 such as signalling and waiting. OPSTRING must be a packed array of
7059 semop structures. Each semop structure can be generated with
7060 C<pack("s!3", $semnum, $semop, $semflag)>. The length of OPSTRING
7061 implies the number of semaphore operations. Returns true if
7062 successful, false on error. As an example, the
7063 following code waits on semaphore $semnum of semaphore id $semid:
7065 my $semop = pack("s!3", $semnum, -1, 0);
7066 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
7068 To signal the semaphore, replace C<-1> with C<1>. See also
7069 L<perlipc/"SysV IPC"> and the documentation for
7070 L<C<IPC::SysV>|IPC::SysV> and L<C<IPC::Semaphore>|IPC::Semaphore>.
7072 Portability issues: L<perlport/semop>.
7074 =item send SOCKET,MSG,FLAGS,TO
7077 =item send SOCKET,MSG,FLAGS
7079 =for Pod::Functions send a message over a socket
7081 Sends a message on a socket. Attempts to send the scalar MSG to the SOCKET
7082 filehandle. Takes the same flags as the system call of the same name. On
7083 unconnected sockets, you must specify a destination to I<send to>, in which
7084 case it does a L<sendto(2)> syscall. Returns the number of characters sent,
7085 or the undefined value on error. The L<sendmsg(2)> syscall is currently
7086 unimplemented. See L<perlipc/"UDP: Message Passing"> for examples.
7088 Note that if the socket has been marked as C<:utf8>, C<send> will
7089 throw an exception. The C<:encoding(...)> layer implicitly introduces
7090 the C<:utf8> layer. See L<C<binmode>|/binmode FILEHANDLE, LAYER>.
7092 =item setpgrp PID,PGRP
7095 =for Pod::Functions set the process group of a process
7097 Sets the current process group for the specified PID, C<0> for the current
7098 process. Raises an exception when used on a machine that doesn't
7099 implement POSIX L<setpgid(2)> or BSD L<setpgrp(2)>. If the arguments
7100 are omitted, it defaults to C<0,0>. Note that the BSD 4.2 version of
7101 L<C<setpgrp>|/setpgrp PID,PGRP> does not accept any arguments, so only
7102 C<setpgrp(0,0)> is portable. See also
7103 L<C<POSIX::setsid()>|POSIX/C<setsid>>.
7105 Portability issues: L<perlport/setpgrp>.
7107 =item setpriority WHICH,WHO,PRIORITY
7108 X<setpriority> X<priority> X<nice> X<renice>
7110 =for Pod::Functions set a process's nice value
7112 Sets the current priority for a process, a process group, or a user.
7113 (See L<setpriority(2)>.) Raises an exception when used on a machine
7114 that doesn't implement L<setpriority(2)>.
7116 C<WHICH> can be any of C<PRIO_PROCESS>, C<PRIO_PGRP> or C<PRIO_USER>
7117 imported from L<POSIX/RESOURCE CONSTANTS>.
7119 Portability issues: L<perlport/setpriority>.
7121 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
7124 =for Pod::Functions set some socket options
7126 Sets the socket option requested. Returns L<C<undef>|/undef EXPR> on
7127 error. Use integer constants provided by the L<C<Socket>|Socket> module
7129 LEVEL and OPNAME. Values for LEVEL can also be obtained from
7130 getprotobyname. OPTVAL might either be a packed string or an integer.
7131 An integer OPTVAL is shorthand for pack("i", OPTVAL).
7133 An example disabling Nagle's algorithm on a socket:
7135 use Socket qw(IPPROTO_TCP TCP_NODELAY);
7136 setsockopt($socket, IPPROTO_TCP, TCP_NODELAY, 1);
7138 Portability issues: L<perlport/setsockopt>.
7145 =for Pod::Functions remove the first element of an array, and return it
7147 Shifts the first value of the array off and returns it, shortening the
7148 array by 1 and moving everything down. If there are no elements in the
7149 array, returns the undefined value. If ARRAY is omitted, shifts the
7150 L<C<@_>|perlvar/@_> array within the lexical scope of subroutines and
7151 formats, and the L<C<@ARGV>|perlvar/@ARGV> array outside a subroutine
7152 and also within the lexical scopes
7153 established by the C<eval STRING>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>,
7154 C<UNITCHECK {}>, and C<END {}> constructs.
7156 Starting with Perl 5.14, an experimental feature allowed
7157 L<C<shift>|/shift ARRAY> to take a
7158 scalar expression. This experiment has been deemed unsuccessful, and was
7159 removed as of Perl 5.24.
7161 See also L<C<unshift>|/unshift ARRAY,LIST>, L<C<push>|/push ARRAY,LIST>,
7162 and L<C<pop>|/pop ARRAY>. L<C<shift>|/shift ARRAY> and
7163 L<C<unshift>|/unshift ARRAY,LIST> do the same thing to the left end of
7164 an array that L<C<pop>|/pop ARRAY> and L<C<push>|/push ARRAY,LIST> do to
7167 =item shmctl ID,CMD,ARG
7170 =for Pod::Functions SysV shared memory operations
7172 Calls the System V IPC function shmctl. You'll probably have to say
7176 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
7177 then ARG must be a variable that will hold the returned C<shmid_ds>
7178 structure. Returns like ioctl: L<C<undef>|/undef EXPR> for error; "C<0>
7179 but true" for zero; and the actual return value otherwise.
7180 See also L<perlipc/"SysV IPC"> and the documentation for
7181 L<C<IPC::SysV>|IPC::SysV>.
7183 Portability issues: L<perlport/shmctl>.
7185 =item shmget KEY,SIZE,FLAGS
7188 =for Pod::Functions get SysV shared memory segment identifier
7190 Calls the System V IPC function shmget. Returns the shared memory
7191 segment id, or L<C<undef>|/undef EXPR> on error.
7192 See also L<perlipc/"SysV IPC"> and the documentation for
7193 L<C<IPC::SysV>|IPC::SysV>.
7195 Portability issues: L<perlport/shmget>.
7197 =item shmread ID,VAR,POS,SIZE
7201 =for Pod::Functions read SysV shared memory
7203 =item shmwrite ID,STRING,POS,SIZE
7205 =for Pod::Functions write SysV shared memory
7207 Reads or writes the System V shared memory segment ID starting at
7208 position POS for size SIZE by attaching to it, copying in/out, and
7209 detaching from it. When reading, VAR must be a variable that will
7210 hold the data read. When writing, if STRING is too long, only SIZE
7211 bytes are used; if STRING is too short, nulls are written to fill out
7212 SIZE bytes. Return true if successful, false on error.
7213 L<C<shmread>|/shmread ID,VAR,POS,SIZE> taints the variable. See also
7214 L<perlipc/"SysV IPC"> and the documentation for
7215 L<C<IPC::SysV>|IPC::SysV> and the L<C<IPC::Shareable>|IPC::Shareable>
7218 Portability issues: L<perlport/shmread> and L<perlport/shmwrite>.
7220 =item shutdown SOCKET,HOW
7223 =for Pod::Functions close down just half of a socket connection
7225 Shuts down a socket connection in the manner indicated by HOW, which
7226 has the same interpretation as in the syscall of the same name.
7228 shutdown($socket, 0); # I/we have stopped reading data
7229 shutdown($socket, 1); # I/we have stopped writing data
7230 shutdown($socket, 2); # I/we have stopped using this socket
7232 This is useful with sockets when you want to tell the other
7233 side you're done writing but not done reading, or vice versa.
7234 It's also a more insistent form of close because it also
7235 disables the file descriptor in any forked copies in other
7238 Returns C<1> for success; on error, returns L<C<undef>|/undef EXPR> if
7239 the first argument is not a valid filehandle, or returns C<0> and sets
7240 L<C<$!>|perlvar/$!> for any other failure.
7243 X<sin> X<sine> X<asin> X<arcsine>
7247 =for Pod::Functions return the sine of a number
7249 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
7250 returns sine of L<C<$_>|perlvar/$_>.
7252 For the inverse sine operation, you may use the C<Math::Trig::asin>
7253 function, or use this relation:
7255 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
7262 =for Pod::Functions block for some number of seconds
7264 Causes the script to sleep for (integer) EXPR seconds, or forever if no
7265 argument is given. Returns the integer number of seconds actually slept.
7267 May be interrupted if the process receives a signal such as C<SIGALRM>.
7270 local $SIG{ALRM} = sub { die "Alarm!\n" };
7273 die $@ unless $@ eq "Alarm!\n";
7275 You probably cannot mix L<C<alarm>|/alarm SECONDS> and
7276 L<C<sleep>|/sleep EXPR> calls, because L<C<sleep>|/sleep EXPR> is often
7277 implemented using L<C<alarm>|/alarm SECONDS>.
7279 On some older systems, it may sleep up to a full second less than what
7280 you requested, depending on how it counts seconds. Most modern systems
7281 always sleep the full amount. They may appear to sleep longer than that,
7282 however, because your process might not be scheduled right away in a
7283 busy multitasking system.
7285 For delays of finer granularity than one second, the L<Time::HiRes>
7286 module (from CPAN, and starting from Perl 5.8 part of the standard
7287 distribution) provides L<C<usleep>|Time::HiRes/usleep ( $useconds )>.
7288 You may also use Perl's four-argument
7289 version of L<C<select>|/select RBITS,WBITS,EBITS,TIMEOUT> leaving the
7290 first three arguments undefined, or you might be able to use the
7291 L<C<syscall>|/syscall NUMBER, LIST> interface to access L<setitimer(2)>
7292 if your system supports it. See L<perlfaq8> for details.
7294 See also the L<POSIX> module's L<C<pause>|POSIX/C<pause>> function.
7296 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
7299 =for Pod::Functions create a socket
7301 Opens a socket of the specified kind and attaches it to filehandle
7302 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for
7303 the syscall of the same name. You should C<use Socket> first
7304 to get the proper definitions imported. See the examples in
7305 L<perlipc/"Sockets: Client/Server Communication">.
7307 On systems that support a close-on-exec flag on files, the flag will
7308 be set for the newly opened file descriptor, as determined by the
7309 value of L<C<$^F>|perlvar/$^F>. See L<perlvar/$^F>.
7311 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
7314 =for Pod::Functions create a pair of sockets
7316 Creates an unnamed pair of sockets in the specified domain, of the
7317 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
7318 for the syscall of the same name. If unimplemented, raises an exception.
7319 Returns true if successful.
7321 On systems that support a close-on-exec flag on files, the flag will
7322 be set for the newly opened file descriptors, as determined by the value
7323 of L<C<$^F>|perlvar/$^F>. See L<perlvar/$^F>.
7325 Some systems define L<C<pipe>|/pipe READHANDLE,WRITEHANDLE> in terms of
7326 L<C<socketpair>|/socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL>, in
7327 which a call to C<pipe($rdr, $wtr)> is essentially:
7330 socketpair(my $rdr, my $wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
7331 shutdown($rdr, 1); # no more writing for reader
7332 shutdown($wtr, 0); # no more reading for writer
7334 See L<perlipc> for an example of socketpair use. Perl 5.8 and later will
7335 emulate socketpair using IP sockets to localhost if your system implements
7336 sockets but not socketpair.
7338 Portability issues: L<perlport/socketpair>.
7340 =item sort SUBNAME LIST
7343 =item sort BLOCK LIST
7347 =for Pod::Functions sort a list of values
7349 In list context, this sorts the LIST and returns the sorted list value.
7350 In scalar context, the behaviour of L<C<sort>|/sort SUBNAME LIST> is
7353 If SUBNAME or BLOCK is omitted, L<C<sort>|/sort SUBNAME LIST>s in
7354 standard string comparison
7355 order. If SUBNAME is specified, it gives the name of a subroutine
7356 that returns an integer less than, equal to, or greater than C<0>,
7357 depending on how the elements of the list are to be ordered. (The
7358 C<< <=> >> and C<cmp> operators are extremely useful in such routines.)
7359 SUBNAME may be a scalar variable name (unsubscripted), in which case
7360 the value provides the name of (or a reference to) the actual
7361 subroutine to use. In place of a SUBNAME, you can provide a BLOCK as
7362 an anonymous, in-line sort subroutine.
7364 If the subroutine's prototype is C<($$)>, the elements to be compared are
7365 passed by reference in L<C<@_>|perlvar/@_>, as for a normal subroutine.
7366 This is slower than unprototyped subroutines, where the elements to be
7367 compared are passed into the subroutine as the package global variables
7368 C<$a> and C<$b> (see example below).
7370 If the subroutine is an XSUB, the elements to be compared are pushed on
7371 to the stack, the way arguments are usually passed to XSUBs. C<$a> and
7374 The values to be compared are always passed by reference and should not
7377 You also cannot exit out of the sort block or subroutine using any of the
7378 loop control operators described in L<perlsyn> or with
7379 L<C<goto>|/goto LABEL>.
7381 When L<C<use locale>|locale> (but not C<use locale ':not_characters'>)
7382 is in effect, C<sort LIST> sorts LIST according to the
7383 current collation locale. See L<perllocale>.
7385 L<C<sort>|/sort SUBNAME LIST> returns aliases into the original list,
7386 much as a for loop's index variable aliases the list elements. That is,
7387 modifying an element of a list returned by L<C<sort>|/sort SUBNAME LIST>
7388 (for example, in a C<foreach>, L<C<map>|/map BLOCK LIST> or
7389 L<C<grep>|/grep BLOCK LIST>)
7390 actually modifies the element in the original list. This is usually
7391 something to be avoided when writing clear code.
7393 Historically Perl has varied in whether sorting is stable by default.
7394 If stability matters, it can be controlled explicitly by using the
7400 my @articles = sort @files;
7402 # same thing, but with explicit sort routine
7403 my @articles = sort {$a cmp $b} @files;
7405 # now case-insensitively
7406 my @articles = sort {fc($a) cmp fc($b)} @files;
7408 # same thing in reversed order
7409 my @articles = sort {$b cmp $a} @files;
7411 # sort numerically ascending
7412 my @articles = sort {$a <=> $b} @files;
7414 # sort numerically descending
7415 my @articles = sort {$b <=> $a} @files;
7417 # this sorts the %age hash by value instead of key
7418 # using an in-line function
7419 my @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
7421 # sort using explicit subroutine name
7423 $age{$a} <=> $age{$b}; # presuming numeric
7425 my @sortedclass = sort byage @class;
7427 sub backwards { $b cmp $a }
7428 my @harry = qw(dog cat x Cain Abel);
7429 my @george = qw(gone chased yz Punished Axed);
7431 # prints AbelCaincatdogx
7432 print sort backwards @harry;
7433 # prints xdogcatCainAbel
7434 print sort @george, 'to', @harry;
7435 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
7437 # inefficiently sort by descending numeric compare using
7438 # the first integer after the first = sign, or the
7439 # whole record case-insensitively otherwise
7442 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
7447 # same thing, but much more efficiently;
7448 # we'll build auxiliary indices instead
7452 push @nums, ( /=(\d+)/ ? $1 : undef );
7456 my @new = @old[ sort {
7457 $nums[$b] <=> $nums[$a]
7459 $caps[$a] cmp $caps[$b]
7463 # same thing, but without any temps
7464 my @new = map { $_->[0] }
7465 sort { $b->[1] <=> $a->[1]
7468 } map { [$_, /=(\d+)/, fc($_)] } @old;
7470 # using a prototype allows you to use any comparison subroutine
7471 # as a sort subroutine (including other package's subroutines)
7473 sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are
7476 my @new = sort Other::backwards @old;
7478 # guarantee stability
7480 my @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
7482 Warning: syntactical care is required when sorting the list returned from
7483 a function. If you want to sort the list returned by the function call
7484 C<find_records(@key)>, you can use:
7486 my @contact = sort { $a cmp $b } find_records @key;
7487 my @contact = sort +find_records(@key);
7488 my @contact = sort &find_records(@key);
7489 my @contact = sort(find_records(@key));
7491 If instead you want to sort the array C<@key> with the comparison routine
7492 C<find_records()> then you can use:
7494 my @contact = sort { find_records() } @key;
7495 my @contact = sort find_records(@key);
7496 my @contact = sort(find_records @key);
7497 my @contact = sort(find_records (@key));
7499 C<$a> and C<$b> are set as package globals in the package the sort() is
7500 called from. That means C<$main::a> and C<$main::b> (or C<$::a> and
7501 C<$::b>) in the C<main> package, C<$FooPack::a> and C<$FooPack::b> in the
7502 C<FooPack> package, etc. If the sort block is in scope of a C<my> or
7503 C<state> declaration of C<$a> and/or C<$b>, you I<must> spell out the full
7504 name of the variables in the sort block :
7507 my $a = "C"; # DANGER, Will Robinson, DANGER !!!
7509 print sort { $a cmp $b } qw(A C E G B D F H);
7511 sub badlexi { $a cmp $b }
7512 print sort badlexi qw(A C E G B D F H);
7514 # the above prints BACFEDGH or some other incorrect ordering
7516 print sort { $::a cmp $::b } qw(A C E G B D F H);
7518 print sort { our $a cmp our $b } qw(A C E G B D F H);
7520 print sort { our ($a, $b); $a cmp $b } qw(A C E G B D F H);
7522 sub lexi { our $a cmp our $b }
7523 print sort lexi qw(A C E G B D F H);
7525 # the above print ABCDEFGH
7527 With proper care you may mix package and my (or state) C<$a> and/or C<$b>:
7537 say sort { $a->{our $a} <=> $a->{our $b} }
7538 qw{ huge normal tiny small big};
7540 # prints tinysmallnormalbighuge
7542 C<$a> and C<$b> are implicitly local to the sort() execution and regain their
7543 former values upon completing the sort.
7545 Sort subroutines written using C<$a> and C<$b> are bound to their calling
7546 package. It is possible, but of limited interest, to define them in a
7547 different package, since the subroutine must still refer to the calling
7548 package's C<$a> and C<$b> :
7551 sub lexi { $Bar::a cmp $Bar::b }
7553 ... sort Foo::lexi ...
7555 Use the prototyped versions (see above) for a more generic alternative.
7557 The comparison function is required to behave. If it returns
7558 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
7559 sometimes saying the opposite, for example) the results are not
7562 Because C<< <=> >> returns L<C<undef>|/undef EXPR> when either operand
7563 is C<NaN> (not-a-number), be careful when sorting with a
7564 comparison function like C<< $a <=> $b >> any lists that might contain a
7565 C<NaN>. The following example takes advantage that C<NaN != NaN> to
7566 eliminate any C<NaN>s from the input list.
7568 my @result = sort { $a <=> $b } grep { $_ == $_ } @input;
7570 =item splice ARRAY,OFFSET,LENGTH,LIST
7573 =item splice ARRAY,OFFSET,LENGTH
7575 =item splice ARRAY,OFFSET
7579 =for Pod::Functions add or remove elements anywhere in an array
7581 Removes the elements designated by OFFSET and LENGTH from an array, and
7582 replaces them with the elements of LIST, if any. In list context,
7583 returns the elements removed from the array. In scalar context,
7584 returns the last element removed, or L<C<undef>|/undef EXPR> if no
7586 removed. The array grows or shrinks as necessary.
7587 If OFFSET is negative then it starts that far from the end of the array.
7588 If LENGTH is omitted, removes everything from OFFSET onward.
7589 If LENGTH is negative, removes the elements from OFFSET onward
7590 except for -LENGTH elements at the end of the array.
7591 If both OFFSET and LENGTH are omitted, removes everything. If OFFSET is
7592 past the end of the array and a LENGTH was provided, Perl issues a warning,
7593 and splices at the end of the array.
7595 The following equivalences hold (assuming C<< $#a >= $i >> )
7597 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
7598 pop(@a) splice(@a,-1)
7599 shift(@a) splice(@a,0,1)
7600 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
7601 $a[$i] = $y splice(@a,$i,1,$y)
7603 L<C<splice>|/splice ARRAY,OFFSET,LENGTH,LIST> can be used, for example,
7604 to implement n-ary queue processing:
7608 while (my @next_n = splice @_, 0, $n) {
7609 say join q{ -- }, @next_n;
7613 nary_print(3, qw(a b c d e f g h));
7619 Starting with Perl 5.14, an experimental feature allowed
7620 L<C<splice>|/splice ARRAY,OFFSET,LENGTH,LIST> to take a
7621 scalar expression. This experiment has been deemed unsuccessful, and was
7622 removed as of Perl 5.24.
7624 =item split /PATTERN/,EXPR,LIMIT
7627 =item split /PATTERN/,EXPR
7629 =item split /PATTERN/
7633 =for Pod::Functions split up a string using a regexp delimiter
7635 Splits the string EXPR into a list of strings and returns the
7636 list in list context, or the size of the list in scalar context.
7637 (Prior to Perl 5.11, it also overwrote C<@_> with the list in
7638 void and scalar context. If you target old perls, beware.)
7640 If only PATTERN is given, EXPR defaults to L<C<$_>|perlvar/$_>.
7642 Anything in EXPR that matches PATTERN is taken to be a separator
7643 that separates the EXPR into substrings (called "I<fields>") that
7644 do B<not> include the separator. Note that a separator may be
7645 longer than one character or even have no characters at all (the
7646 empty string, which is a zero-width match).
7648 The PATTERN need not be constant; an expression may be used
7649 to specify a pattern that varies at runtime.
7651 If PATTERN matches the empty string, the EXPR is split at the match
7652 position (between characters). As an example, the following:
7654 print join(':', split(/b/, 'abc')), "\n";
7656 uses the C<b> in C<'abc'> as a separator to produce the output C<a:c>.
7659 print join(':', split(//, 'abc')), "\n";
7661 uses empty string matches as separators to produce the output
7662 C<a:b:c>; thus, the empty string may be used to split EXPR into a
7663 list of its component characters.
7665 As a special case for L<C<split>|/split E<sol>PATTERNE<sol>,EXPR,LIMIT>,
7666 the empty pattern given in
7667 L<match operator|perlop/"m/PATTERN/msixpodualngc"> syntax (C<//>)
7668 specifically matches the empty string, which is contrary to its usual
7669 interpretation as the last successful match.
7671 If PATTERN is C</^/>, then it is treated as if it used the
7672 L<multiline modifier|perlreref/OPERATORS> (C</^/m>), since it
7673 isn't much use otherwise.
7675 C<E<sol>m> and any of the other pattern modifiers valid for C<qr>
7676 (summarized in L<perlop/qrE<sol>STRINGE<sol>msixpodualn>) may be
7677 specified explicitly.
7679 As another special case,
7680 L<C<split>|/split E<sol>PATTERNE<sol>,EXPR,LIMIT> emulates the default
7682 command line tool B<awk> when the PATTERN is either omitted or a
7683 string composed of a single space character (such as S<C<' '>> or
7684 S<C<"\x20">>, but not e.g. S<C</ />>). In this case, any leading
7685 whitespace in EXPR is removed before splitting occurs, and the PATTERN is
7686 instead treated as if it were C</\s+/>; in particular, this means that
7687 I<any> contiguous whitespace (not just a single space character) is used as
7688 a separator. However, this special treatment can be avoided by specifying
7689 the pattern S<C</ />> instead of the string S<C<" ">>, thereby allowing
7690 only a single space character to be a separator. In earlier Perls this
7691 special case was restricted to the use of a plain S<C<" ">> as the
7692 pattern argument to split; in Perl 5.18.0 and later this special case is
7693 triggered by any expression which evaluates to the simple string S<C<" ">>.
7695 As of Perl 5.28, this special-cased whitespace splitting works as expected in
7696 the scope of L<< S<C<"use feature 'unicode_strings">>|feature/The
7697 'unicode_strings' feature >>. In previous versions, and outside the scope of
7698 that feature, it exhibits L<perlunicode/The "Unicode Bug">: characters that are
7699 whitespace according to Unicode rules but not according to ASCII rules can be
7700 treated as part of fields rather than as field separators, depending on the
7701 string's internal encoding.
7703 If omitted, PATTERN defaults to a single space, S<C<" ">>, triggering
7704 the previously described I<awk> emulation.
7706 If LIMIT is specified and positive, it represents the maximum number
7707 of fields into which the EXPR may be split; in other words, LIMIT is
7708 one greater than the maximum number of times EXPR may be split. Thus,
7709 the LIMIT value C<1> means that EXPR may be split a maximum of zero
7710 times, producing a maximum of one field (namely, the entire value of
7711 EXPR). For instance:
7713 print join(':', split(//, 'abc', 1)), "\n";
7715 produces the output C<abc>, and this:
7717 print join(':', split(//, 'abc', 2)), "\n";
7719 produces the output C<a:bc>, and each of these:
7721 print join(':', split(//, 'abc', 3)), "\n";
7722 print join(':', split(//, 'abc', 4)), "\n";
7724 produces the output C<a:b:c>.
7726 If LIMIT is negative, it is treated as if it were instead arbitrarily
7727 large; as many fields as possible are produced.
7729 If LIMIT is omitted (or, equivalently, zero), then it is usually
7730 treated as if it were instead negative but with the exception that
7731 trailing empty fields are stripped (empty leading fields are always
7732 preserved); if all fields are empty, then all fields are considered to
7733 be trailing (and are thus stripped in this case). Thus, the following:
7735 print join(':', split(/,/, 'a,b,c,,,')), "\n";
7737 produces the output C<a:b:c>, but the following:
7739 print join(':', split(/,/, 'a,b,c,,,', -1)), "\n";
7741 produces the output C<a:b:c:::>.
7743 In time-critical applications, it is worthwhile to avoid splitting
7744 into more fields than necessary. Thus, when assigning to a list,
7745 if LIMIT is omitted (or zero), then LIMIT is treated as though it
7746 were one larger than the number of variables in the list; for the
7747 following, LIMIT is implicitly 3:
7749 my ($login, $passwd) = split(/:/);
7751 Note that splitting an EXPR that evaluates to the empty string always
7752 produces zero fields, regardless of the LIMIT specified.
7754 An empty leading field is produced when there is a positive-width
7755 match at the beginning of EXPR. For instance:
7757 print join(':', split(/ /, ' abc')), "\n";
7759 produces the output C<:abc>. However, a zero-width match at the
7760 beginning of EXPR never produces an empty field, so that:
7762 print join(':', split(//, ' abc'));
7764 produces the output S<C< :a:b:c>> (rather than S<C<: :a:b:c>>).
7766 An empty trailing field, on the other hand, is produced when there is a
7767 match at the end of EXPR, regardless of the length of the match
7768 (of course, unless a non-zero LIMIT is given explicitly, such fields are
7769 removed, as in the last example). Thus:
7771 print join(':', split(//, ' abc', -1)), "\n";
7773 produces the output S<C< :a:b:c:>>.
7775 If the PATTERN contains
7776 L<capturing groups|perlretut/Grouping things and hierarchical matching>,
7777 then for each separator, an additional field is produced for each substring
7778 captured by a group (in the order in which the groups are specified,
7779 as per L<backreferences|perlretut/Backreferences>); if any group does not
7780 match, then it captures the L<C<undef>|/undef EXPR> value instead of a
7782 note that any such additional field is produced whenever there is a
7783 separator (that is, whenever a split occurs), and such an additional field
7784 does B<not> count towards the LIMIT. Consider the following expressions
7785 evaluated in list context (each returned list is provided in the associated
7788 split(/-|,/, "1-10,20", 3)
7791 split(/(-|,)/, "1-10,20", 3)
7792 # ('1', '-', '10', ',', '20')
7794 split(/-|(,)/, "1-10,20", 3)
7795 # ('1', undef, '10', ',', '20')
7797 split(/(-)|,/, "1-10,20", 3)
7798 # ('1', '-', '10', undef, '20')
7800 split(/(-)|(,)/, "1-10,20", 3)
7801 # ('1', '-', undef, '10', undef, ',', '20')
7803 =item sprintf FORMAT, LIST
7806 =for Pod::Functions formatted print into a string
7808 Returns a string formatted by the usual
7809 L<C<printf>|/printf FILEHANDLE FORMAT, LIST> conventions of the C
7810 library function L<C<sprintf>|/sprintf FORMAT, LIST>. See below for
7811 more details and see L<sprintf(3)> or L<printf(3)> on your system for an
7812 explanation of the general principles.
7816 # Format number with up to 8 leading zeroes
7817 my $result = sprintf("%08d", $number);
7819 # Round number to 3 digits after decimal point
7820 my $rounded = sprintf("%.3f", $number);
7822 Perl does its own L<C<sprintf>|/sprintf FORMAT, LIST> formatting: it
7824 function L<sprintf(3)>, but doesn't use it except for floating-point
7825 numbers, and even then only standard modifiers are allowed.
7826 Non-standard extensions in your local L<sprintf(3)> are
7827 therefore unavailable from Perl.
7829 Unlike L<C<printf>|/printf FILEHANDLE FORMAT, LIST>,
7830 L<C<sprintf>|/sprintf FORMAT, LIST> does not do what you probably mean
7831 when you pass it an array as your first argument.
7832 The array is given scalar context,
7833 and instead of using the 0th element of the array as the format, Perl will
7834 use the count of elements in the array as the format, which is almost never
7837 Perl's L<C<sprintf>|/sprintf FORMAT, LIST> permits the following
7838 universally-known conversions:
7841 %c a character with the given number
7843 %d a signed integer, in decimal
7844 %u an unsigned integer, in decimal
7845 %o an unsigned integer, in octal
7846 %x an unsigned integer, in hexadecimal
7847 %e a floating-point number, in scientific notation
7848 %f a floating-point number, in fixed decimal notation
7849 %g a floating-point number, in %e or %f notation
7851 In addition, Perl permits the following widely-supported conversions:
7853 %X like %x, but using upper-case letters
7854 %E like %e, but using an upper-case "E"
7855 %G like %g, but with an upper-case "E" (if applicable)
7856 %b an unsigned integer, in binary
7857 %B like %b, but using an upper-case "B" with the # flag
7858 %p a pointer (outputs the Perl value's address in hexadecimal)
7859 %n special: *stores* the number of characters output so far
7860 into the next argument in the parameter list
7861 %a hexadecimal floating point
7862 %A like %a, but using upper-case letters
7864 Finally, for backward (and we do mean "backward") compatibility, Perl
7865 permits these unnecessary but widely-supported conversions:
7868 %D a synonym for %ld
7869 %U a synonym for %lu
7870 %O a synonym for %lo
7873 Note that the number of exponent digits in the scientific notation produced
7874 by C<%e>, C<%E>, C<%g> and C<%G> for numbers with the modulus of the
7875 exponent less than 100 is system-dependent: it may be three or less
7876 (zero-padded as necessary). In other words, 1.23 times ten to the
7877 99th may be either "1.23e99" or "1.23e099". Similarly for C<%a> and C<%A>:
7878 the exponent or the hexadecimal digits may float: especially the
7879 "long doubles" Perl configuration option may cause surprises.
7881 Between the C<%> and the format letter, you may specify several
7882 additional attributes controlling the interpretation of the format.
7883 In order, these are:
7887 =item format parameter index
7889 An explicit format parameter index, such as C<2$>. By default sprintf
7890 will format the next unused argument in the list, but this allows you
7891 to take the arguments out of order:
7893 printf '%2$d %1$d', 12, 34; # prints "34 12"
7894 printf '%3$d %d %1$d', 1, 2, 3; # prints "3 1 1"
7900 space prefix non-negative number with a space
7901 + prefix non-negative number with a plus sign
7902 - left-justify within the field
7903 0 use zeros, not spaces, to right-justify
7904 # ensure the leading "0" for any octal,
7905 prefix non-zero hexadecimal with "0x" or "0X",
7906 prefix non-zero binary with "0b" or "0B"
7910 printf '<% d>', 12; # prints "< 12>"
7911 printf '<% d>', 0; # prints "< 0>"
7912 printf '<% d>', -12; # prints "<-12>"
7913 printf '<%+d>', 12; # prints "<+12>"
7914 printf '<%+d>', 0; # prints "<+0>"
7915 printf '<%+d>', -12; # prints "<-12>"
7916 printf '<%6s>', 12; # prints "< 12>"
7917 printf '<%-6s>', 12; # prints "<12 >"
7918 printf '<%06s>', 12; # prints "<000012>"
7919 printf '<%#o>', 12; # prints "<014>"
7920 printf '<%#x>', 12; # prints "<0xc>"
7921 printf '<%#X>', 12; # prints "<0XC>"
7922 printf '<%#b>', 12; # prints "<0b1100>"
7923 printf '<%#B>', 12; # prints "<0B1100>"
7925 When a space and a plus sign are given as the flags at once,
7926 the space is ignored.
7928 printf '<%+ d>', 12; # prints "<+12>"
7929 printf '<% +d>', 12; # prints "<+12>"
7931 When the # flag and a precision are given in the %o conversion,
7932 the precision is incremented if it's necessary for the leading "0".
7934 printf '<%#.5o>', 012; # prints "<00012>"
7935 printf '<%#.5o>', 012345; # prints "<012345>"
7936 printf '<%#.0o>', 0; # prints "<0>"
7940 This flag tells Perl to interpret the supplied string as a vector of
7941 integers, one for each character in the string. Perl applies the format to
7942 each integer in turn, then joins the resulting strings with a separator (a
7943 dot C<.> by default). This can be useful for displaying ordinal values of
7944 characters in arbitrary strings:
7946 printf "%vd", "AB\x{100}"; # prints "65.66.256"
7947 printf "version is v%vd\n", $^V; # Perl's version
7949 Put an asterisk C<*> before the C<v> to override the string to
7950 use to separate the numbers:
7952 printf "address is %*vX\n", ":", $addr; # IPv6 address
7953 printf "bits are %0*v8b\n", " ", $bits; # random bitstring
7955 You can also explicitly specify the argument number to use for
7956 the join string using something like C<*2$v>; for example:
7958 printf '%*4$vX %*4$vX %*4$vX', # 3 IPv6 addresses
7961 =item (minimum) width
7963 Arguments are usually formatted to be only as wide as required to
7964 display the given value. You can override the width by putting
7965 a number here, or get the width from the next argument (with C<*>)
7966 or from a specified argument (e.g., with C<*2$>):
7968 printf "<%s>", "a"; # prints "<a>"
7969 printf "<%6s>", "a"; # prints "< a>"
7970 printf "<%*s>", 6, "a"; # prints "< a>"
7971 printf '<%*2$s>', "a", 6; # prints "< a>"
7972 printf "<%2s>", "long"; # prints "<long>" (does not truncate)
7974 If a field width obtained through C<*> is negative, it has the same
7975 effect as the C<-> flag: left-justification.
7977 =item precision, or maximum width
7980 You can specify a precision (for numeric conversions) or a maximum
7981 width (for string conversions) by specifying a C<.> followed by a number.
7982 For floating-point formats except C<g> and C<G>, this specifies
7983 how many places right of the decimal point to show (the default being 6).
7986 # these examples are subject to system-specific variation
7987 printf '<%f>', 1; # prints "<1.000000>"
7988 printf '<%.1f>', 1; # prints "<1.0>"
7989 printf '<%.0f>', 1; # prints "<1>"
7990 printf '<%e>', 10; # prints "<1.000000e+01>"
7991 printf '<%.1e>', 10; # prints "<1.0e+01>"
7993 For "g" and "G", this specifies the maximum number of significant digits to
7996 # These examples are subject to system-specific variation.
7997 printf '<%g>', 1; # prints "<1>"
7998 printf '<%.10g>', 1; # prints "<1>"
7999 printf '<%g>', 100; # prints "<100>"
8000 printf '<%.1g>', 100; # prints "<1e+02>"
8001 printf '<%.2g>', 100.01; # prints "<1e+02>"
8002 printf '<%.5g>', 100.01; # prints "<100.01>"
8003 printf '<%.4g>', 100.01; # prints "<100>"
8004 printf '<%.1g>', 0.0111; # prints "<0.01>"
8005 printf '<%.2g>', 0.0111; # prints "<0.011>"
8006 printf '<%.3g>', 0.0111; # prints "<0.0111>"
8008 For integer conversions, specifying a precision implies that the
8009 output of the number itself should be zero-padded to this width,
8010 where the 0 flag is ignored:
8012 printf '<%.6d>', 1; # prints "<000001>"
8013 printf '<%+.6d>', 1; # prints "<+000001>"
8014 printf '<%-10.6d>', 1; # prints "<000001 >"
8015 printf '<%10.6d>', 1; # prints "< 000001>"
8016 printf '<%010.6d>', 1; # prints "< 000001>"
8017 printf '<%+10.6d>', 1; # prints "< +000001>"
8019 printf '<%.6x>', 1; # prints "<000001>"
8020 printf '<%#.6x>', 1; # prints "<0x000001>"
8021 printf '<%-10.6x>', 1; # prints "<000001 >"
8022 printf '<%10.6x>', 1; # prints "< 000001>"
8023 printf '<%010.6x>', 1; # prints "< 000001>"
8024 printf '<%#10.6x>', 1; # prints "< 0x000001>"
8026 For string conversions, specifying a precision truncates the string
8027 to fit the specified width:
8029 printf '<%.5s>', "truncated"; # prints "<trunc>"
8030 printf '<%10.5s>', "truncated"; # prints "< trunc>"
8032 You can also get the precision from the next argument using C<.*>, or from a
8033 specified argument (e.g., with C<.*2$>):
8035 printf '<%.6x>', 1; # prints "<000001>"
8036 printf '<%.*x>', 6, 1; # prints "<000001>"
8038 printf '<%.*2$x>', 1, 6; # prints "<000001>"
8040 printf '<%6.*2$x>', 1, 4; # prints "< 0001>"
8042 If a precision obtained through C<*> is negative, it counts
8043 as having no precision at all.
8045 printf '<%.*s>', 7, "string"; # prints "<string>"
8046 printf '<%.*s>', 3, "string"; # prints "<str>"
8047 printf '<%.*s>', 0, "string"; # prints "<>"
8048 printf '<%.*s>', -1, "string"; # prints "<string>"
8050 printf '<%.*d>', 1, 0; # prints "<0>"
8051 printf '<%.*d>', 0, 0; # prints "<>"
8052 printf '<%.*d>', -1, 0; # prints "<0>"
8056 For numeric conversions, you can specify the size to interpret the
8057 number as using C<l>, C<h>, C<V>, C<q>, C<L>, or C<ll>. For integer
8058 conversions (C<d u o x X b i D U O>), numbers are usually assumed to be
8059 whatever the default integer size is on your platform (usually 32 or 64
8060 bits), but you can override this to use instead one of the standard C types,
8061 as supported by the compiler used to build Perl:
8063 hh interpret integer as C type "char" or "unsigned
8064 char" on Perl 5.14 or later
8065 h interpret integer as C type "short" or
8067 j interpret integer as C type "intmax_t" on Perl
8068 5.14 or later; and prior to Perl 5.30, only with
8069 a C99 compiler (unportable)
8070 l interpret integer as C type "long" or
8072 q, L, or ll interpret integer as C type "long long",
8073 "unsigned long long", or "quad" (typically
8075 t interpret integer as C type "ptrdiff_t" on Perl
8077 z interpret integer as C type "size_t" on Perl 5.14
8080 As of 5.14, none of these raises an exception if they are not supported on
8081 your platform. However, if warnings are enabled, a warning of the
8082 L<C<printf>|warnings> warning class is issued on an unsupported
8083 conversion flag. Should you instead prefer an exception, do this:
8085 use warnings FATAL => "printf";
8087 If you would like to know about a version dependency before you
8088 start running the program, put something like this at its top:
8090 use 5.014; # for hh/j/t/z/ printf modifiers
8092 You can find out whether your Perl supports quads via L<Config>:
8095 if ($Config{use64bitint} eq "define"
8096 || $Config{longsize} >= 8) {
8097 print "Nice quads!\n";
8100 For floating-point conversions (C<e f g E F G>), numbers are usually assumed
8101 to be the default floating-point size on your platform (double or long double),
8102 but you can force "long double" with C<q>, C<L>, or C<ll> if your
8103 platform supports them. You can find out whether your Perl supports long
8104 doubles via L<Config>:
8107 print "long doubles\n" if $Config{d_longdbl} eq "define";
8109 You can find out whether Perl considers "long double" to be the default
8110 floating-point size to use on your platform via L<Config>:
8113 if ($Config{uselongdouble} eq "define") {
8114 print "long doubles by default\n";
8117 It can also be that long doubles and doubles are the same thing:
8120 ($Config{doublesize} == $Config{longdblsize}) &&
8121 print "doubles are long doubles\n";
8123 The size specifier C<V> has no effect for Perl code, but is supported for
8124 compatibility with XS code. It means "use the standard size for a Perl
8125 integer or floating-point number", which is the default.
8127 =item order of arguments
8129 Normally, L<C<sprintf>|/sprintf FORMAT, LIST> takes the next unused
8130 argument as the value to
8131 format for each format specification. If the format specification
8132 uses C<*> to require additional arguments, these are consumed from
8133 the argument list in the order they appear in the format
8134 specification I<before> the value to format. Where an argument is
8135 specified by an explicit index, this does not affect the normal
8136 order for the arguments, even when the explicitly specified index
8137 would have been the next argument.
8141 printf "<%*.*s>", $a, $b, $c;
8143 uses C<$a> for the width, C<$b> for the precision, and C<$c>
8144 as the value to format; while:
8146 printf '<%*1$.*s>', $a, $b;
8148 would use C<$a> for the width and precision, and C<$b> as the
8151 Here are some more examples; be aware that when using an explicit
8152 index, the C<$> may need escaping:
8154 printf "%2\$d %d\n", 12, 34; # will print "34 12\n"
8155 printf "%2\$d %d %d\n", 12, 34; # will print "34 12 34\n"
8156 printf "%3\$d %d %d\n", 12, 34, 56; # will print "56 12 34\n"
8157 printf "%2\$*3\$d %d\n", 12, 34, 3; # will print " 34 12\n"
8158 printf "%*1\$.*f\n", 4, 5, 10; # will print "5.0000\n"
8162 If L<C<use locale>|locale> (including C<use locale ':not_characters'>)
8163 is in effect and L<C<POSIX::setlocale>|POSIX/C<setlocale>> has been
8165 the character used for the decimal separator in formatted floating-point
8166 numbers is affected by the C<LC_NUMERIC> locale. See L<perllocale>
8170 X<sqrt> X<root> X<square root>
8174 =for Pod::Functions square root function
8176 Return the positive square root of EXPR. If EXPR is omitted, uses
8177 L<C<$_>|perlvar/$_>. Works only for non-negative operands unless you've
8178 loaded the L<C<Math::Complex>|Math::Complex> module.
8181 print sqrt(-4); # prints 2i
8184 X<srand> X<seed> X<randseed>
8188 =for Pod::Functions seed the random number generator
8190 Sets and returns the random number seed for the L<C<rand>|/rand EXPR>
8193 The point of the function is to "seed" the L<C<rand>|/rand EXPR>
8194 function so that L<C<rand>|/rand EXPR> can produce a different sequence
8195 each time you run your program. When called with a parameter,
8196 L<C<srand>|/srand EXPR> uses that for the seed; otherwise it
8197 (semi-)randomly chooses a seed. In either case, starting with Perl 5.14,
8198 it returns the seed. To signal that your code will work I<only> on Perls
8199 of a recent vintage:
8201 use 5.014; # so srand returns the seed
8203 If L<C<srand>|/srand EXPR> is not called explicitly, it is called
8204 implicitly without a parameter at the first use of the
8205 L<C<rand>|/rand EXPR> operator. However, there are a few situations
8206 where programs are likely to want to call L<C<srand>|/srand EXPR>. One
8207 is for generating predictable results, generally for testing or
8208 debugging. There, you use C<srand($seed)>, with the same C<$seed> each
8209 time. Another case is that you may want to call L<C<srand>|/srand EXPR>
8210 after a L<C<fork>|/fork> to avoid child processes sharing the same seed
8211 value as the parent (and consequently each other).
8213 Do B<not> call C<srand()> (i.e., without an argument) more than once per
8214 process. The internal state of the random number generator should
8215 contain more entropy than can be provided by any seed, so calling
8216 L<C<srand>|/srand EXPR> again actually I<loses> randomness.
8218 Most implementations of L<C<srand>|/srand EXPR> take an integer and will
8220 truncate decimal numbers. This means C<srand(42)> will usually
8221 produce the same results as C<srand(42.1)>. To be safe, always pass
8222 L<C<srand>|/srand EXPR> an integer.
8224 A typical use of the returned seed is for a test program which has too many
8225 combinations to test comprehensively in the time available to it each run. It
8226 can test a random subset each time, and should there be a failure, log the seed
8227 used for that run so that it can later be used to reproduce the same results.
8229 B<L<C<rand>|/rand EXPR> is not cryptographically secure. You should not rely
8230 on it in security-sensitive situations.> As of this writing, a
8231 number of third-party CPAN modules offer random number generators
8232 intended by their authors to be cryptographically secure,
8233 including: L<Data::Entropy>, L<Crypt::Random>, L<Math::Random::Secure>,
8234 and L<Math::TrulyRandom>.
8236 =item stat FILEHANDLE
8237 X<stat> X<file, status> X<ctime>
8241 =item stat DIRHANDLE
8245 =for Pod::Functions get a file's status information
8247 Returns a 13-element list giving the status info for a file, either
8248 the file opened via FILEHANDLE or DIRHANDLE, or named by EXPR. If EXPR is
8249 omitted, it stats L<C<$_>|perlvar/$_> (not C<_>!). Returns the empty
8250 list if L<C<stat>|/stat FILEHANDLE> fails. Typically
8253 my ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
8254 $atime,$mtime,$ctime,$blksize,$blocks)
8257 Not all fields are supported on all filesystem types. Here are the
8258 meanings of the fields:
8260 0 dev device number of filesystem
8262 2 mode file mode (type and permissions)
8263 3 nlink number of (hard) links to the file
8264 4 uid numeric user ID of file's owner
8265 5 gid numeric group ID of file's owner
8266 6 rdev the device identifier (special files only)
8267 7 size total size of file, in bytes
8268 8 atime last access time in seconds since the epoch
8269 9 mtime last modify time in seconds since the epoch
8270 10 ctime inode change time in seconds since the epoch (*)
8271 11 blksize preferred I/O size in bytes for interacting with the
8272 file (may vary from file to file)
8273 12 blocks actual number of system-specific blocks allocated
8274 on disk (often, but not always, 512 bytes each)
8276 (The epoch was at 00:00 January 1, 1970 GMT.)
8278 (*) Not all fields are supported on all filesystem types. Notably, the
8279 ctime field is non-portable. In particular, you cannot expect it to be a
8280 "creation time"; see L<perlport/"Files and Filesystems"> for details.
8282 If L<C<stat>|/stat FILEHANDLE> is passed the special filehandle
8283 consisting of an underline, no stat is done, but the current contents of
8284 the stat structure from the last L<C<stat>|/stat FILEHANDLE>,
8285 L<C<lstat>|/lstat FILEHANDLE>, or filetest are returned. Example:
8287 if (-x $file && (($d) = stat(_)) && $d < 0) {
8288 print "$file is executable NFS file\n";
8291 (This works on machines only for which the device number is negative
8294 On some platforms inode numbers are of a type larger than perl knows how
8295 to handle as integer numerical values. If necessary, an inode number will
8296 be returned as a decimal string in order to preserve the entire value.
8297 If used in a numeric context, this will be converted to a floating-point
8298 numerical value, with rounding, a fate that is best avoided. Therefore,
8299 you should prefer to compare inode numbers using C<eq> rather than C<==>.
8300 C<eq> will work fine on inode numbers that are represented numerically,
8301 as well as those represented as strings.
8303 Because the mode contains both the file type and its permissions, you
8304 should mask off the file type portion and (s)printf using a C<"%o">
8305 if you want to see the real permissions.
8307 my $mode = (stat($filename))[2];
8308 printf "Permissions are %04o\n", $mode & 07777;
8310 In scalar context, L<C<stat>|/stat FILEHANDLE> returns a boolean value
8312 or failure, and, if successful, sets the information associated with
8313 the special filehandle C<_>.
8315 The L<File::stat> module provides a convenient, by-name access mechanism:
8318 my $sb = stat($filename);
8319 printf "File is %s, size is %s, perm %04o, mtime %s\n",
8320 $filename, $sb->size, $sb->mode & 07777,
8321 scalar localtime $sb->mtime;
8323 You can import symbolic mode constants (C<S_IF*>) and functions
8324 (C<S_IS*>) from the L<Fcntl> module:
8328 my $mode = (stat($filename))[2];
8330 my $user_rwx = ($mode & S_IRWXU) >> 6;
8331 my $group_read = ($mode & S_IRGRP) >> 3;
8332 my $other_execute = $mode & S_IXOTH;
8334 printf "Permissions are %04o\n", S_IMODE($mode), "\n";
8336 my $is_setuid = $mode & S_ISUID;
8337 my $is_directory = S_ISDIR($mode);
8339 You could write the last two using the C<-u> and C<-d> operators.
8340 Commonly available C<S_IF*> constants are:
8342 # Permissions: read, write, execute, for user, group, others.
8344 S_IRWXU S_IRUSR S_IWUSR S_IXUSR
8345 S_IRWXG S_IRGRP S_IWGRP S_IXGRP
8346 S_IRWXO S_IROTH S_IWOTH S_IXOTH
8348 # Setuid/Setgid/Stickiness/SaveText.
8349 # Note that the exact meaning of these is system-dependent.
8351 S_ISUID S_ISGID S_ISVTX S_ISTXT
8353 # File types. Not all are necessarily available on
8356 S_IFREG S_IFDIR S_IFLNK S_IFBLK S_IFCHR
8357 S_IFIFO S_IFSOCK S_IFWHT S_ENFMT
8359 # The following are compatibility aliases for S_IRUSR,
8360 # S_IWUSR, and S_IXUSR.
8362 S_IREAD S_IWRITE S_IEXEC
8364 and the C<S_IF*> functions are
8366 S_IMODE($mode) the part of $mode containing the permission
8367 bits and the setuid/setgid/sticky bits
8369 S_IFMT($mode) the part of $mode containing the file type
8370 which can be bit-anded with (for example)
8371 S_IFREG or with the following functions
8373 # The operators -f, -d, -l, -b, -c, -p, and -S.
8375 S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
8376 S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)
8378 # No direct -X operator counterpart, but for the first one
8379 # the -g operator is often equivalent. The ENFMT stands for
8380 # record flocking enforcement, a platform-dependent feature.
8382 S_ISENFMT($mode) S_ISWHT($mode)
8384 See your native L<chmod(2)> and L<stat(2)> documentation for more details
8385 about the C<S_*> constants. To get status info for a symbolic link
8386 instead of the target file behind the link, use the
8387 L<C<lstat>|/lstat FILEHANDLE> function.
8389 Portability issues: L<perlport/stat>.
8394 =item state TYPE VARLIST
8396 =item state VARLIST : ATTRS
8398 =item state TYPE VARLIST : ATTRS
8400 =for Pod::Functions +state declare and assign a persistent lexical variable
8402 L<C<state>|/state VARLIST> declares a lexically scoped variable, just
8403 like L<C<my>|/my VARLIST>.
8404 However, those variables will never be reinitialized, contrary to
8405 lexical variables that are reinitialized each time their enclosing block
8407 See L<perlsub/"Persistent Private Variables"> for details.
8409 If more than one variable is listed, the list must be placed in
8410 parentheses. With a parenthesised list, L<C<undef>|/undef EXPR> can be
8412 dummy placeholder. However, since initialization of state variables in
8413 such lists is currently not possible this would serve no purpose.
8415 L<C<state>|/state VARLIST> is available only if the
8416 L<C<"state"> feature|feature/The 'state' feature> is enabled or if it is
8417 prefixed with C<CORE::>. The
8418 L<C<"state"> feature|feature/The 'state' feature> is enabled
8419 automatically with a C<use v5.10> (or higher) declaration in the current
8428 =for Pod::Functions no-op, formerly optimized input data for repeated searches
8430 At this time, C<study> does nothing. This may change in the future.
8432 Prior to Perl version 5.16, it would create an inverted index of all characters
8433 that occurred in the given SCALAR (or L<C<$_>|perlvar/$_> if unspecified). When
8434 matching a pattern, the rarest character from the pattern would be looked up in
8435 this index. Rarity was based on some static frequency tables constructed from
8436 some C programs and English text.
8439 =item sub NAME BLOCK
8442 =item sub NAME (PROTO) BLOCK
8444 =item sub NAME : ATTRS BLOCK
8446 =item sub NAME (PROTO) : ATTRS BLOCK
8448 =for Pod::Functions declare a subroutine, possibly anonymously
8450 This is subroutine definition, not a real function I<per se>. Without a
8451 BLOCK it's just a forward declaration. Without a NAME, it's an anonymous
8452 function declaration, so does return a value: the CODE ref of the closure
8455 See L<perlsub> and L<perlref> for details about subroutines and
8456 references; see L<attributes> and L<Attribute::Handlers> for more
8457 information about attributes.
8462 =for Pod::Functions +current_sub the current subroutine, or C<undef> if not in a subroutine
8464 A special token that returns a reference to the current subroutine, or
8465 L<C<undef>|/undef EXPR> outside of a subroutine.
8467 The behaviour of L<C<__SUB__>|/__SUB__> within a regex code block (such
8468 as C</(?{...})/>) is subject to change.
8470 This token is only available under C<use v5.16> or the
8471 L<C<"current_sub"> feature|feature/The 'current_sub' feature>.
8474 =item substr EXPR,OFFSET,LENGTH,REPLACEMENT
8475 X<substr> X<substring> X<mid> X<left> X<right>
8477 =item substr EXPR,OFFSET,LENGTH
8479 =item substr EXPR,OFFSET
8481 =for Pod::Functions get or alter a portion of a string
8483 Extracts a substring out of EXPR and returns it. First character is at
8484 offset zero. If OFFSET is negative, starts
8485 that far back from the end of the string. If LENGTH is omitted, returns
8486 everything through the end of the string. If LENGTH is negative, leaves that
8487 many characters off the end of the string.
8489 my $s = "The black cat climbed the green tree";
8490 my $color = substr $s, 4, 5; # black
8491 my $middle = substr $s, 4, -11; # black cat climbed the
8492 my $end = substr $s, 14; # climbed the green tree
8493 my $tail = substr $s, -4; # tree
8494 my $z = substr $s, -4, 2; # tr
8496 You can use the L<C<substr>|/substr EXPR,OFFSET,LENGTH,REPLACEMENT>
8497 function as an lvalue, in which case EXPR
8498 must itself be an lvalue. If you assign something shorter than LENGTH,
8499 the string will shrink, and if you assign something longer than LENGTH,
8500 the string will grow to accommodate it. To keep the string the same
8501 length, you may need to pad or chop your value using
8502 L<C<sprintf>|/sprintf FORMAT, LIST>.
8504 If OFFSET and LENGTH specify a substring that is partly outside the
8505 string, only the part within the string is returned. If the substring
8506 is beyond either end of the string,
8507 L<C<substr>|/substr EXPR,OFFSET,LENGTH,REPLACEMENT> returns the undefined
8508 value and produces a warning. When used as an lvalue, specifying a
8509 substring that is entirely outside the string raises an exception.
8510 Here's an example showing the behavior for boundary cases:
8513 substr($name, 4) = 'dy'; # $name is now 'freddy'
8514 my $null = substr $name, 6, 2; # returns "" (no warning)
8515 my $oops = substr $name, 7; # returns undef, with warning
8516 substr($name, 7) = 'gap'; # raises an exception
8518 An alternative to using
8519 L<C<substr>|/substr EXPR,OFFSET,LENGTH,REPLACEMENT> as an lvalue is to
8521 replacement string as the 4th argument. This allows you to replace
8522 parts of the EXPR and return what was there before in one operation,
8523 just as you can with
8524 L<C<splice>|/splice ARRAY,OFFSET,LENGTH,LIST>.
8526 my $s = "The black cat climbed the green tree";
8527 my $z = substr $s, 14, 7, "jumped from"; # climbed
8528 # $s is now "The black cat jumped from the green tree"
8530 Note that the lvalue returned by the three-argument version of
8531 L<C<substr>|/substr EXPR,OFFSET,LENGTH,REPLACEMENT> acts as
8532 a 'magic bullet'; each time it is assigned to, it remembers which part
8533 of the original string is being modified; for example:
8536 for (substr($x,1,2)) {
8537 $_ = 'a'; print $x,"\n"; # prints 1a4
8538 $_ = 'xyz'; print $x,"\n"; # prints 1xyz4
8540 $_ = 'pq'; print $x,"\n"; # prints 5pq9
8543 With negative offsets, it remembers its position from the end of the string
8544 when the target string is modified:
8547 for (substr($x, -3, 2)) {
8548 $_ = 'a'; print $x,"\n"; # prints 1a4, as above
8550 print $_,"\n"; # prints f
8553 Prior to Perl version 5.10, the result of using an lvalue multiple times was
8554 unspecified. Prior to 5.16, the result with negative offsets was
8557 =item symlink OLDFILE,NEWFILE
8558 X<symlink> X<link> X<symbolic link> X<link, symbolic>
8560 =for Pod::Functions create a symbolic link to a file
8562 Creates a new filename symbolically linked to the old filename.
8563 Returns C<1> for success, C<0> otherwise. On systems that don't support
8564 symbolic links, raises an exception. To check for that,
8567 my $symlink_exists = eval { symlink("",""); 1 };
8569 Portability issues: L<perlport/symlink>.
8571 =item syscall NUMBER, LIST
8572 X<syscall> X<system call>
8574 =for Pod::Functions execute an arbitrary system call
8576 Calls the system call specified as the first element of the list,
8577 passing the remaining elements as arguments to the system call. If
8578 unimplemented, raises an exception. The arguments are interpreted
8579 as follows: if a given argument is numeric, the argument is passed as
8580 an int. If not, the pointer to the string value is passed. You are
8581 responsible to make sure a string is pre-extended long enough to
8582 receive any result that might be written into a string. You can't use a
8583 string literal (or other read-only string) as an argument to
8584 L<C<syscall>|/syscall NUMBER, LIST> because Perl has to assume that any
8585 string pointer might be written through. If your
8586 integer arguments are not literals and have never been interpreted in a
8587 numeric context, you may need to add C<0> to them to force them to look
8588 like numbers. This emulates the
8589 L<C<syswrite>|/syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET> function (or
8592 require 'syscall.ph'; # may need to run h2ph
8593 my $s = "hi there\n";
8594 syscall(SYS_write(), fileno(STDOUT), $s, length $s);
8596 Note that Perl supports passing of up to only 14 arguments to your syscall,
8597 which in practice should (usually) suffice.
8599 Syscall returns whatever value returned by the system call it calls.
8600 If the system call fails, L<C<syscall>|/syscall NUMBER, LIST> returns
8601 C<-1> and sets L<C<$!>|perlvar/$!> (errno).
8602 Note that some system calls I<can> legitimately return C<-1>. The proper
8603 way to handle such calls is to assign C<$! = 0> before the call, then
8604 check the value of L<C<$!>|perlvar/$!> if
8605 L<C<syscall>|/syscall NUMBER, LIST> returns C<-1>.
8607 There's a problem with C<syscall(SYS_pipe())>: it returns the file
8608 number of the read end of the pipe it creates, but there is no way
8609 to retrieve the file number of the other end. You can avoid this
8610 problem by using L<C<pipe>|/pipe READHANDLE,WRITEHANDLE> instead.
8612 Portability issues: L<perlport/syscall>.
8614 =item sysopen FILEHANDLE,FILENAME,MODE
8617 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
8619 =for Pod::Functions +5.002 open a file, pipe, or descriptor
8621 Opens the file whose filename is given by FILENAME, and associates it with
8622 FILEHANDLE. If FILEHANDLE is an expression, its value is used as the real
8623 filehandle wanted; an undefined scalar will be suitably autovivified. This
8624 function calls the underlying operating system's L<open(2)> function with the
8625 parameters FILENAME, MODE, and PERMS.
8627 Returns true on success and L<C<undef>|/undef EXPR> otherwise.
8629 The possible values and flag bits of the MODE parameter are
8630 system-dependent; they are available via the standard module
8631 L<C<Fcntl>|Fcntl>. See the documentation of your operating system's
8632 L<open(2)> syscall to see
8633 which values and flag bits are available. You may combine several flags
8634 using the C<|>-operator.
8636 Some of the most common values are C<O_RDONLY> for opening the file in
8637 read-only mode, C<O_WRONLY> for opening the file in write-only mode,
8638 and C<O_RDWR> for opening the file in read-write mode.
8639 X<O_RDONLY> X<O_RDWR> X<O_WRONLY>
8641 For historical reasons, some values work on almost every system
8642 supported by Perl: 0 means read-only, 1 means write-only, and 2
8643 means read/write. We know that these values do I<not> work under
8644 OS/390 and on the Macintosh; you probably don't want to
8645 use them in new code.
8647 If the file named by FILENAME does not exist and the
8648 L<C<open>|/open FILEHANDLE,EXPR> call creates
8649 it (typically because MODE includes the C<O_CREAT> flag), then the value of
8650 PERMS specifies the permissions of the newly created file. If you omit
8651 the PERMS argument to L<C<sysopen>|/sysopen FILEHANDLE,FILENAME,MODE>,
8652 Perl uses the octal value C<0666>.
8653 These permission values need to be in octal, and are modified by your
8654 process's current L<C<umask>|/umask EXPR>.
8657 In many systems the C<O_EXCL> flag is available for opening files in
8658 exclusive mode. This is B<not> locking: exclusiveness means here that
8659 if the file already exists,
8660 L<C<sysopen>|/sysopen FILEHANDLE,FILENAME,MODE> fails. C<O_EXCL> may
8662 on network filesystems, and has no effect unless the C<O_CREAT> flag
8663 is set as well. Setting C<O_CREAT|O_EXCL> prevents the file from
8664 being opened if it is a symbolic link. It does not protect against
8665 symbolic links in the file's path.
8668 Sometimes you may want to truncate an already-existing file. This
8669 can be done using the C<O_TRUNC> flag. The behavior of
8670 C<O_TRUNC> with C<O_RDONLY> is undefined.
8673 You should seldom if ever use C<0644> as argument to
8674 L<C<sysopen>|/sysopen FILEHANDLE,FILENAME,MODE>, because
8675 that takes away the user's option to have a more permissive umask.
8676 Better to omit it. See L<C<umask>|/umask EXPR> for more on this.
8678 Note that under Perls older than 5.8.0,
8679 L<C<sysopen>|/sysopen FILEHANDLE,FILENAME,MODE> depends on the
8680 L<fdopen(3)> C library function. On many Unix systems, L<fdopen(3)> is known
8681 to fail when file descriptors exceed a certain value, typically 255. If
8682 you need more file descriptors than that, consider using the
8683 L<C<POSIX::open>|POSIX/C<open>> function. For Perls 5.8.0 and later,
8684 PerlIO is (most often) the default.
8686 See L<perlopentut> for a kinder, gentler explanation of opening files.
8688 Portability issues: L<perlport/sysopen>.
8690 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
8693 =item sysread FILEHANDLE,SCALAR,LENGTH
8695 =for Pod::Functions fixed-length unbuffered input from a filehandle
8697 Attempts to read LENGTH bytes of data into variable SCALAR from the
8698 specified FILEHANDLE, using L<read(2)>. It bypasses
8699 buffered IO, so mixing this with other kinds of reads,
8700 L<C<print>|/print FILEHANDLE LIST>, L<C<write>|/write FILEHANDLE>,
8701 L<C<seek>|/seek FILEHANDLE,POSITION,WHENCE>,
8702 L<C<tell>|/tell FILEHANDLE>, or L<C<eof>|/eof FILEHANDLE> can cause
8703 confusion because the
8704 perlio or stdio layers usually buffer data. Returns the number of
8705 bytes actually read, C<0> at end of file, or undef if there was an
8706 error (in the latter case L<C<$!>|perlvar/$!> is also set). SCALAR will
8708 shrunk so that the last byte actually read is the last byte of the
8709 scalar after the read.
8711 An OFFSET may be specified to place the read data at some place in the
8712 string other than the beginning. A negative OFFSET specifies
8713 placement at that many characters counting backwards from the end of
8714 the string. A positive OFFSET greater than the length of SCALAR
8715 results in the string being padded to the required size with C<"\0">
8716 bytes before the result of the read is appended.
8718 There is no syseof() function, which is ok, since
8719 L<C<eof>|/eof FILEHANDLE> doesn't work well on device files (like ttys)
8720 anyway. Use L<C<sysread>|/sysread FILEHANDLE,SCALAR,LENGTH,OFFSET> and
8721 check for a return value of 0 to decide whether you're done.
8723 Note that if the filehandle has been marked as C<:utf8>, C<sysread> will
8724 throw an exception. The C<:encoding(...)> layer implicitly
8725 introduces the C<:utf8> layer. See
8726 L<C<binmode>|/binmode FILEHANDLE, LAYER>,
8727 L<C<open>|/open FILEHANDLE,EXPR>, and the L<open> pragma.
8729 =item sysseek FILEHANDLE,POSITION,WHENCE
8732 =for Pod::Functions +5.004 position I/O pointer on handle used with sysread and syswrite
8734 Sets FILEHANDLE's system position I<in bytes> using L<lseek(2)>. FILEHANDLE may
8735 be an expression whose value gives the name of the filehandle. The values
8736 for WHENCE are C<0> to set the new position to POSITION; C<1> to set it
8737 to the current position plus POSITION; and C<2> to set it to EOF plus
8738 POSITION, typically negative.
8740 Note the emphasis on bytes: even if the filehandle has been set to operate
8741 on characters (for example using the C<:encoding(UTF-8)> I/O layer), the
8742 L<C<seek>|/seek FILEHANDLE,POSITION,WHENCE>,
8743 L<C<tell>|/tell FILEHANDLE>, and
8744 L<C<sysseek>|/sysseek FILEHANDLE,POSITION,WHENCE>
8745 family of functions use byte offsets, not character offsets,
8746 because seeking to a character offset would be very slow in a UTF-8 file.
8748 L<C<sysseek>|/sysseek FILEHANDLE,POSITION,WHENCE> bypasses normal
8749 buffered IO, so mixing it with reads other than
8750 L<C<sysread>|/sysread FILEHANDLE,SCALAR,LENGTH,OFFSET> (for example
8751 L<C<readline>|/readline EXPR> or
8752 L<C<read>|/read FILEHANDLE,SCALAR,LENGTH,OFFSET>),
8753 L<C<print>|/print FILEHANDLE LIST>, L<C<write>|/write FILEHANDLE>,
8754 L<C<seek>|/seek FILEHANDLE,POSITION,WHENCE>,
8755 L<C<tell>|/tell FILEHANDLE>, or L<C<eof>|/eof FILEHANDLE> may cause
8758 For WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>,
8759 and C<SEEK_END> (start of the file, current position, end of the file)
8760 from the L<Fcntl> module. Use of the constants is also more portable
8761 than relying on 0, 1, and 2. For example to define a "systell" function:
8763 use Fcntl 'SEEK_CUR';
8764 sub systell { sysseek($_[0], 0, SEEK_CUR) }
8766 Returns the new position, or the undefined value on failure. A position
8767 of zero is returned as the string C<"0 but true">; thus
8768 L<C<sysseek>|/sysseek FILEHANDLE,POSITION,WHENCE> returns
8769 true on success and false on failure, yet you can still easily determine
8775 =item system PROGRAM LIST
8777 =for Pod::Functions run a separate program
8779 Does exactly the same thing as L<C<exec>|/exec LIST>, except that a fork is
8780 done first and the parent process waits for the child process to
8781 exit. Note that argument processing varies depending on the
8782 number of arguments. If there is more than one argument in LIST,
8783 or if LIST is an array with more than one value, starts the program
8784 given by the first element of the list with arguments given by the
8785 rest of the list. If there is only one scalar argument, the argument
8786 is checked for shell metacharacters, and if there are any, the
8787 entire argument is passed to the system's command shell for parsing
8788 (this is C</bin/sh -c> on Unix platforms, but varies on other
8789 platforms). If there are no shell metacharacters in the argument,
8790 it is split into words and passed directly to C<execvp>, which is
8791 more efficient. On Windows, only the C<system PROGRAM LIST> syntax will
8792 reliably avoid using the shell; C<system LIST>, even with more than one
8793 element, will fall back to the shell if the first spawn fails.
8795 Perl will attempt to flush all files opened for
8796 output before any operation that may do a fork, but this may not be
8797 supported on some platforms (see L<perlport>). To be safe, you may need
8798 to set L<C<$E<verbar>>|perlvar/$E<verbar>> (C<$AUTOFLUSH> in L<English>)
8799 or call the C<autoflush> method of L<C<IO::Handle>|IO::Handle/METHODS>
8800 on any open handles.
8802 The return value is the exit status of the program as returned by the
8803 L<C<wait>|/wait> call. To get the actual exit value, shift right by
8804 eight (see below). See also L<C<exec>|/exec LIST>. This is I<not> what
8805 you want to use to capture the output from a command; for that you
8806 should use merely backticks or
8807 L<C<qxE<sol>E<sol>>|/qxE<sol>STRINGE<sol>>, as described in
8808 L<perlop/"`STRING`">. Return value of -1 indicates a failure to start
8809 the program or an error of the L<wait(2)> system call (inspect
8810 L<C<$!>|perlvar/$!> for the reason).
8812 If you'd like to make L<C<system>|/system LIST> (and many other bits of
8813 Perl) die on error, have a look at the L<autodie> pragma.
8815 Like L<C<exec>|/exec LIST>, L<C<system>|/system LIST> allows you to lie
8816 to a program about its name if you use the C<system PROGRAM LIST>
8817 syntax. Again, see L<C<exec>|/exec LIST>.
8819 Since C<SIGINT> and C<SIGQUIT> are ignored during the execution of
8820 L<C<system>|/system LIST>, if you expect your program to terminate on
8821 receipt of these signals you will need to arrange to do so yourself
8822 based on the return value.
8824 my @args = ("command", "arg1", "arg2");
8826 or die "system @args failed: $?";
8828 If you'd like to manually inspect L<C<system>|/system LIST>'s failure,
8829 you can check all possible failure modes by inspecting
8830 L<C<$?>|perlvar/$?> like this:
8833 print "failed to execute: $!\n";
8836 printf "child died with signal %d, %s coredump\n",
8837 ($? & 127), ($? & 128) ? 'with' : 'without';
8840 printf "child exited with value %d\n", $? >> 8;
8843 Alternatively, you may inspect the value of
8844 L<C<${^CHILD_ERROR_NATIVE}>|perlvar/${^CHILD_ERROR_NATIVE}> with the
8845 L<C<W*()>|POSIX/C<WIFEXITED>> calls from the L<POSIX> module.
8847 When L<C<system>|/system LIST>'s arguments are executed indirectly by
8848 the shell, results and return codes are subject to its quirks.
8849 See L<perlop/"`STRING`"> and L<C<exec>|/exec LIST> for details.
8851 Since L<C<system>|/system LIST> does a L<C<fork>|/fork> and
8852 L<C<wait>|/wait> it may affect a C<SIGCHLD> handler. See L<perlipc> for
8855 Portability issues: L<perlport/system>.
8857 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
8860 =item syswrite FILEHANDLE,SCALAR,LENGTH
8862 =item syswrite FILEHANDLE,SCALAR
8864 =for Pod::Functions fixed-length unbuffered output to a filehandle
8866 Attempts to write LENGTH bytes of data from variable SCALAR to the
8867 specified FILEHANDLE, using L<write(2)>. If LENGTH is
8868 not specified, writes whole SCALAR. It bypasses buffered IO, so
8869 mixing this with reads (other than C<sysread)>),
8870 L<C<print>|/print FILEHANDLE LIST>, L<C<write>|/write FILEHANDLE>,
8871 L<C<seek>|/seek FILEHANDLE,POSITION,WHENCE>,
8872 L<C<tell>|/tell FILEHANDLE>, or L<C<eof>|/eof FILEHANDLE> may cause
8873 confusion because the perlio and stdio layers usually buffer data.
8874 Returns the number of bytes actually written, or L<C<undef>|/undef EXPR>
8875 if there was an error (in this case the errno variable
8876 L<C<$!>|perlvar/$!> is also set). If the LENGTH is greater than the
8877 data available in the SCALAR after the OFFSET, only as much data as is
8878 available will be written.
8880 An OFFSET may be specified to write the data from some part of the
8881 string other than the beginning. A negative OFFSET specifies writing
8882 that many characters counting backwards from the end of the string.
8883 If SCALAR is of length zero, you can only use an OFFSET of 0.
8885 B<WARNING>: If the filehandle is marked C<:utf8>, C<syswrite> will raise an exception.
8886 The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer.
8887 Alternately, if the handle is not marked with an encoding but you
8888 attempt to write characters with code points over 255, raises an exception.
8889 See L<C<binmode>|/binmode FILEHANDLE, LAYER>,
8890 L<C<open>|/open FILEHANDLE,EXPR>, and the L<open> pragma.
8892 =item tell FILEHANDLE
8897 =for Pod::Functions get current seekpointer on a filehandle
8899 Returns the current position I<in bytes> for FILEHANDLE, or -1 on
8900 error. FILEHANDLE may be an expression whose value gives the name of
8901 the actual filehandle. If FILEHANDLE is omitted, assumes the file
8904 Note the emphasis on bytes: even if the filehandle has been set to operate
8905 on characters (for example using the C<:encoding(UTF-8)> I/O layer), the
8906 L<C<seek>|/seek FILEHANDLE,POSITION,WHENCE>,
8907 L<C<tell>|/tell FILEHANDLE>, and
8908 L<C<sysseek>|/sysseek FILEHANDLE,POSITION,WHENCE>
8909 family of functions use byte offsets, not character offsets,
8910 because seeking to a character offset would be very slow in a UTF-8 file.
8912 The return value of L<C<tell>|/tell FILEHANDLE> for the standard streams
8913 like the STDIN depends on the operating system: it may return -1 or
8914 something else. L<C<tell>|/tell FILEHANDLE> on pipes, fifos, and
8915 sockets usually returns -1.
8917 There is no C<systell> function. Use
8918 L<C<sysseek($fh, 0, 1)>|/sysseek FILEHANDLE,POSITION,WHENCE> for that.
8920 Do not use L<C<tell>|/tell FILEHANDLE> (or other buffered I/O
8921 operations) on a filehandle that has been manipulated by
8922 L<C<sysread>|/sysread FILEHANDLE,SCALAR,LENGTH,OFFSET>,
8923 L<C<syswrite>|/syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET>, or
8924 L<C<sysseek>|/sysseek FILEHANDLE,POSITION,WHENCE>. Those functions
8925 ignore the buffering, while L<C<tell>|/tell FILEHANDLE> does not.
8927 =item telldir DIRHANDLE
8930 =for Pod::Functions get current seekpointer on a directory handle
8932 Returns the current position of the L<C<readdir>|/readdir DIRHANDLE>
8933 routines on DIRHANDLE. Value may be given to
8934 L<C<seekdir>|/seekdir DIRHANDLE,POS> to access a particular location in
8935 a directory. L<C<telldir>|/telldir DIRHANDLE> has the same caveats
8936 about possible directory compaction as the corresponding system library
8939 =item tie VARIABLE,CLASSNAME,LIST
8942 =for Pod::Functions +5.002 bind a variable to an object class
8944 This function binds a variable to a package class that will provide the
8945 implementation for the variable. VARIABLE is the name of the variable
8946 to be enchanted. CLASSNAME is the name of a class implementing objects
8947 of correct type. Any additional arguments are passed to the
8948 appropriate constructor
8949 method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
8950 or C<TIEHASH>). Typically these are arguments such as might be passed
8951 to the L<dbm_open(3)> function of C. The object returned by the
8952 constructor is also returned by the
8953 L<C<tie>|/tie VARIABLE,CLASSNAME,LIST> function, which would be useful
8954 if you want to access other methods in CLASSNAME.
8956 Note that functions such as L<C<keys>|/keys HASH> and
8957 L<C<values>|/values HASH> may return huge lists when used on large
8958 objects, like DBM files. You may prefer to use the L<C<each>|/each
8959 HASH> function to iterate over such. Example:
8961 # print out history file offsets
8963 tie(my %HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
8964 while (my ($key,$val) = each %HIST) {
8965 print $key, ' = ', unpack('L', $val), "\n";
8968 A class implementing a hash should have the following methods:
8970 TIEHASH classname, LIST
8972 STORE this, key, value
8977 NEXTKEY this, lastkey
8982 A class implementing an ordinary array should have the following methods:
8984 TIEARRAY classname, LIST
8986 STORE this, key, value
8988 STORESIZE this, count
8994 SPLICE this, offset, length, LIST
9001 A class implementing a filehandle should have the following methods:
9003 TIEHANDLE classname, LIST
9004 READ this, scalar, length, offset
9007 WRITE this, scalar, length, offset
9009 PRINTF this, format, LIST
9013 SEEK this, position, whence
9015 OPEN this, mode, LIST
9020 A class implementing a scalar should have the following methods:
9022 TIESCALAR classname, LIST
9028 Not all methods indicated above need be implemented. See L<perltie>,
9029 L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>.
9031 Unlike L<C<dbmopen>|/dbmopen HASH,DBNAME,MASK>, the
9032 L<C<tie>|/tie VARIABLE,CLASSNAME,LIST> function will not
9033 L<C<use>|/use Module VERSION LIST> or L<C<require>|/require VERSION> a
9034 module for you; you need to do that explicitly yourself. See L<DB_File>
9035 or the L<Config> module for interesting
9036 L<C<tie>|/tie VARIABLE,CLASSNAME,LIST> implementations.
9038 For further details see L<perltie>, L<C<tied>|/tied VARIABLE>.
9043 =for Pod::Functions get a reference to the object underlying a tied variable
9045 Returns a reference to the object underlying VARIABLE (the same value
9046 that was originally returned by the
9047 L<C<tie>|/tie VARIABLE,CLASSNAME,LIST> call that bound the variable
9048 to a package.) Returns the undefined value if VARIABLE isn't tied to a
9054 =for Pod::Functions return number of seconds since 1970
9056 Returns the number of non-leap seconds since whatever time the system
9057 considers to be the epoch, suitable for feeding to
9058 L<C<gmtime>|/gmtime EXPR> and L<C<localtime>|/localtime EXPR>. On most
9059 systems the epoch is 00:00:00 UTC, January 1, 1970;
9060 a prominent exception being Mac OS Classic which uses 00:00:00, January 1,
9061 1904 in the current local time zone for its epoch.
9063 For measuring time in better granularity than one second, use the
9064 L<Time::HiRes> module from Perl 5.8 onwards (or from CPAN before then), or,
9065 if you have L<gettimeofday(2)>, you may be able to use the
9066 L<C<syscall>|/syscall NUMBER, LIST> interface of Perl. See L<perlfaq8>
9069 For date and time processing look at the many related modules on CPAN.
9070 For a comprehensive date and time representation look at the
9076 =for Pod::Functions return elapsed time for self and child processes
9078 Returns a four-element list giving the user and system times in
9079 seconds for this process and any exited children of this process.
9081 my ($user,$system,$cuser,$csystem) = times;
9083 In scalar context, L<C<times>|/times> returns C<$user>.
9085 Children's times are only included for terminated children.
9087 Portability issues: L<perlport/times>.
9091 =for Pod::Functions transliterate a string
9093 The transliteration operator. Same as
9094 L<C<yE<sol>E<sol>E<sol>>|/yE<sol>E<sol>E<sol>>. See
9095 L<perlop/"Quote-Like Operators">.
9097 =item truncate FILEHANDLE,LENGTH
9100 =item truncate EXPR,LENGTH
9102 =for Pod::Functions shorten a file
9104 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
9105 specified length. Raises an exception if truncate isn't implemented
9106 on your system. Returns true if successful, L<C<undef>|/undef EXPR> on
9109 The behavior is undefined if LENGTH is greater than the length of the
9112 The position in the file of FILEHANDLE is left unchanged. You may want to
9113 call L<seek|/"seek FILEHANDLE,POSITION,WHENCE"> before writing to the
9116 Portability issues: L<perlport/truncate>.
9119 X<uc> X<uppercase> X<toupper>
9123 =for Pod::Functions return upper-case version of a string
9125 Returns an uppercased version of EXPR. This is the internal function
9126 implementing the C<\U> escape in double-quoted strings.
9127 It does not attempt to do titlecase mapping on initial letters. See
9128 L<C<ucfirst>|/ucfirst EXPR> for that.
9130 If EXPR is omitted, uses L<C<$_>|perlvar/$_>.
9132 This function behaves the same way under various pragmas, such as in a locale,
9133 as L<C<lc>|/lc EXPR> does.
9136 X<ucfirst> X<uppercase>
9140 =for Pod::Functions return a string with just the next letter in upper case
9142 Returns the value of EXPR with the first character in uppercase
9143 (titlecase in Unicode). This is the internal function implementing
9144 the C<\u> escape in double-quoted strings.
9146 If EXPR is omitted, uses L<C<$_>|perlvar/$_>.
9148 This function behaves the same way under various pragmas, such as in a locale,
9149 as L<C<lc>|/lc EXPR> does.
9156 =for Pod::Functions set file creation mode mask
9158 Sets the umask for the process to EXPR and returns the previous value.
9159 If EXPR is omitted, merely returns the current umask.
9161 The Unix permission C<rwxr-x---> is represented as three sets of three
9162 bits, or three octal digits: C<0750> (the leading 0 indicates octal
9163 and isn't one of the digits). The L<C<umask>|/umask EXPR> value is such
9164 a number representing disabled permissions bits. The permission (or
9165 "mode") values you pass L<C<mkdir>|/mkdir FILENAME,MODE> or
9166 L<C<sysopen>|/sysopen FILEHANDLE,FILENAME,MODE> are modified by your
9167 umask, so even if you tell
9168 L<C<sysopen>|/sysopen FILEHANDLE,FILENAME,MODE> to create a file with
9169 permissions C<0777>, if your umask is C<0022>, then the file will
9170 actually be created with permissions C<0755>. If your
9171 L<C<umask>|/umask EXPR> were C<0027> (group can't write; others can't
9172 read, write, or execute), then passing
9173 L<C<sysopen>|/sysopen FILEHANDLE,FILENAME,MODE> C<0666> would create a
9174 file with mode C<0640> (because C<0666 &~ 027> is C<0640>).
9176 Here's some advice: supply a creation mode of C<0666> for regular
9177 files (in L<C<sysopen>|/sysopen FILEHANDLE,FILENAME,MODE>) and one of
9178 C<0777> for directories (in L<C<mkdir>|/mkdir FILENAME,MODE>) and
9179 executable files. This gives users the freedom of
9180 choice: if they want protected files, they might choose process umasks
9181 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
9182 Programs should rarely if ever make policy decisions better left to
9183 the user. The exception to this is when writing files that should be
9184 kept private: mail files, web browser cookies, F<.rhosts> files, and
9187 If L<umask(2)> is not implemented on your system and you are trying to
9188 restrict access for I<yourself> (i.e., C<< (EXPR & 0700) > 0 >>),
9189 raises an exception. If L<umask(2)> is not implemented and you are
9190 not trying to restrict access for yourself, returns
9191 L<C<undef>|/undef EXPR>.
9193 Remember that a umask is a number, usually given in octal; it is I<not> a
9194 string of octal digits. See also L<C<oct>|/oct EXPR>, if all you have
9197 Portability issues: L<perlport/umask>.
9200 X<undef> X<undefine>
9204 =for Pod::Functions remove a variable or function definition
9206 Undefines the value of EXPR, which must be an lvalue. Use only on a
9207 scalar value, an array (using C<@>), a hash (using C<%>), a subroutine
9208 (using C<&>), or a typeglob (using C<*>). Saying C<undef $hash{$key}>
9209 will probably not do what you expect on most predefined variables or
9210 DBM list values, so don't do that; see L<C<delete>|/delete EXPR>.
9211 Always returns the undefined value.
9212 You can omit the EXPR, in which case nothing is
9213 undefined, but you still get an undefined value that you could, for
9214 instance, return from a subroutine, assign to a variable, or pass as a
9215 parameter. Examples:
9218 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
9222 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
9223 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
9224 select undef, undef, undef, 0.25;
9225 my ($x, $y, undef, $z) = foo(); # Ignore third value returned
9227 Note that this is a unary operator, not a list operator.
9230 X<unlink> X<delete> X<remove> X<rm> X<del>
9234 =for Pod::Functions remove one link to a file
9236 Deletes a list of files. On success, it returns the number of files
9237 it successfully deleted. On failure, it returns false and sets
9238 L<C<$!>|perlvar/$!> (errno):
9240 my $unlinked = unlink 'a', 'b', 'c';
9242 unlink glob "*.bak";
9244 On error, L<C<unlink>|/unlink LIST> will not tell you which files it
9246 If you want to know which files you could not remove, try them one
9249 foreach my $file ( @goners ) {
9250 unlink $file or warn "Could not unlink $file: $!";
9253 Note: L<C<unlink>|/unlink LIST> will not attempt to delete directories
9255 superuser and the B<-U> flag is supplied to Perl. Even if these
9256 conditions are met, be warned that unlinking a directory can inflict
9257 damage on your filesystem. Finally, using L<C<unlink>|/unlink LIST> on
9258 directories is not supported on many operating systems. Use
9259 L<C<rmdir>|/rmdir FILENAME> instead.
9261 If LIST is omitted, L<C<unlink>|/unlink LIST> uses L<C<$_>|perlvar/$_>.
9263 =item unpack TEMPLATE,EXPR
9266 =item unpack TEMPLATE
9268 =for Pod::Functions convert binary structure into normal perl variables
9270 L<C<unpack>|/unpack TEMPLATE,EXPR> does the reverse of
9271 L<C<pack>|/pack TEMPLATE,LIST>: it takes a string
9272 and expands it out into a list of values.
9273 (In scalar context, it returns merely the first value produced.)
9275 If EXPR is omitted, unpacks the L<C<$_>|perlvar/$_> string.
9276 See L<perlpacktut> for an introduction to this function.
9278 The string is broken into chunks described by the TEMPLATE. Each chunk
9279 is converted separately to a value. Typically, either the string is a result
9280 of L<C<pack>|/pack TEMPLATE,LIST>, or the characters of the string
9281 represent a C structure of some kind.
9283 The TEMPLATE has the same format as in the
9284 L<C<pack>|/pack TEMPLATE,LIST> function.
9285 Here's a subroutine that does substring:
9288 my ($what, $where, $howmuch) = @_;
9289 unpack("x$where a$howmuch", $what);
9294 sub ordinal { unpack("W",$_[0]); } # same as ord()
9296 In addition to fields allowed in L<C<pack>|/pack TEMPLATE,LIST>, you may
9297 prefix a field with a %<number> to indicate that
9298 you want a <number>-bit checksum of the items instead of the items
9299 themselves. Default is a 16-bit checksum. The checksum is calculated by
9300 summing numeric values of expanded values (for string fields the sum of
9301 C<ord($char)> is taken; for bit fields the sum of zeroes and ones).
9303 For example, the following
9304 computes the same number as the System V sum program:
9308 unpack("%32W*", readline) % 65535;
9311 The following efficiently counts the number of set bits in a bit vector:
9313 my $setbits = unpack("%32b*", $selectmask);
9315 The C<p> and C<P> formats should be used with care. Since Perl
9316 has no way of checking whether the value passed to
9317 L<C<unpack>|/unpack TEMPLATE,EXPR>
9318 corresponds to a valid memory location, passing a pointer value that's
9319 not known to be valid is likely to have disastrous consequences.
9321 If there are more pack codes or if the repeat count of a field or a group
9322 is larger than what the remainder of the input string allows, the result
9323 is not well defined: the repeat count may be decreased, or
9324 L<C<unpack>|/unpack TEMPLATE,EXPR> may produce empty strings or zeros,
9325 or it may raise an exception.
9326 If the input string is longer than one described by the TEMPLATE,
9327 the remainder of that input string is ignored.
9329 See L<C<pack>|/pack TEMPLATE,LIST> for more examples and notes.
9331 =item unshift ARRAY,LIST
9334 =for Pod::Functions prepend more elements to the beginning of a list
9336 Does the opposite of a L<C<shift>|/shift ARRAY>. Or the opposite of a
9337 L<C<push>|/push ARRAY,LIST>,
9338 depending on how you look at it. Prepends list to the front of the
9339 array and returns the new number of elements in the array.
9341 unshift(@ARGV, '-e') unless $ARGV[0] =~ /^-/;
9343 Note the LIST is prepended whole, not one element at a time, so the
9344 prepended elements stay in the same order. Use
9345 L<C<reverse>|/reverse LIST> to do the reverse.
9347 Starting with Perl 5.14, an experimental feature allowed
9348 L<C<unshift>|/unshift ARRAY,LIST> to take
9349 a scalar expression. This experiment has been deemed unsuccessful, and was
9350 removed as of Perl 5.24.
9352 =item untie VARIABLE
9355 =for Pod::Functions break a tie binding to a variable
9357 Breaks the binding between a variable and a package.
9358 (See L<tie|/tie VARIABLE,CLASSNAME,LIST>.)
9359 Has no effect if the variable is not tied.
9361 =item use Module VERSION LIST
9362 X<use> X<module> X<import>
9364 =item use Module VERSION
9366 =item use Module LIST
9372 =for Pod::Functions load in a module at compile time and import its namespace
9374 Imports some semantics into the current package from the named module,
9375 generally by aliasing certain subroutine or variable names into your
9376 package. It is exactly equivalent to
9378 BEGIN { require Module; Module->import( LIST ); }
9380 except that Module I<must> be a bareword.
9381 The importation can be made conditional by using the L<if> module.
9383 In the C<use VERSION> form, VERSION may be either a v-string such as
9384 v5.24.1, which will be compared to L<C<$^V>|perlvar/$^V> (aka
9385 $PERL_VERSION), or a numeric argument of the form 5.024001, which will
9386 be compared to L<C<$]>|perlvar/$]>. An exception is raised if VERSION
9387 is greater than the version of the current Perl interpreter; Perl will
9388 not attempt to parse the rest of the file. Compare with
9389 L<C<require>|/require VERSION>, which can do a similar check at run
9390 time. Symmetrically, C<no VERSION> allows you to specify that you
9391 want a version of Perl older than the specified one.
9393 Specifying VERSION as a numeric argument of the form 5.024001 should
9394 generally be avoided as older less readable syntax compared to
9395 v5.24.1. Before perl 5.8.0 released in 2002 the more verbose numeric
9396 form was the only supported syntax, which is why you might see it in
9398 use v5.24.1; # compile time version check
9400 use 5.024_001; # ditto; older syntax compatible with perl 5.6
9402 This is often useful if you need to check the current Perl version before
9403 L<C<use>|/use Module VERSION LIST>ing library modules that won't work
9404 with older versions of Perl.
9405 (We try not to do this more than we have to.)
9407 C<use VERSION> also lexically enables all features available in the requested
9408 version as defined by the L<feature> pragma, disabling any features
9409 not in the requested version's feature bundle. See L<feature>.
9410 Similarly, if the specified Perl version is greater than or equal to
9411 5.12.0, strictures are enabled lexically as
9412 with L<C<use strict>|strict>. Any explicit use of
9413 C<use strict> or C<no strict> overrides C<use VERSION>, even if it comes
9414 before it. Later use of C<use VERSION>
9415 will override all behavior of a previous
9416 C<use VERSION>, possibly removing the C<strict> and C<feature> added by
9417 C<use VERSION>. C<use VERSION> does not
9418 load the F<feature.pm> or F<strict.pm>
9421 The C<BEGIN> forces the L<C<require>|/require VERSION> and
9422 L<C<import>|/import LIST> to happen at compile time. The
9423 L<C<require>|/require VERSION> makes sure the module is loaded into
9424 memory if it hasn't been yet. The L<C<import>|/import LIST> is not a
9425 builtin; it's just an ordinary static method
9426 call into the C<Module> package to tell the module to import the list of
9427 features back into the current package. The module can implement its
9428 L<C<import>|/import LIST> method any way it likes, though most modules
9429 just choose to derive their L<C<import>|/import LIST> method via
9430 inheritance from the C<Exporter> class that is defined in the
9431 L<C<Exporter>|Exporter> module. See L<Exporter>. If no
9432 L<C<import>|/import LIST> method can be found, then the call is skipped,
9433 even if there is an AUTOLOAD method.
9435 If you do not want to call the package's L<C<import>|/import LIST>
9436 method (for instance,
9437 to stop your namespace from being altered), explicitly supply the empty list:
9441 That is exactly equivalent to
9443 BEGIN { require Module }
9445 If the VERSION argument is present between Module and LIST, then the
9446 L<C<use>|/use Module VERSION LIST> will call the C<VERSION> method in
9447 class Module with the given version as an argument:
9453 BEGIN { require Module; Module->VERSION(12.34) }
9455 The L<default C<VERSION> method|UNIVERSAL/C<VERSION ( [ REQUIRE ] )>>,
9456 inherited from the L<C<UNIVERSAL>|UNIVERSAL> class, croaks if the given
9457 version is larger than the value of the variable C<$Module::VERSION>.
9459 The VERSION argument cannot be an arbitrary expression. It only counts
9460 as a VERSION argument if it is a version number literal, starting with
9461 either a digit or C<v> followed by a digit. Anything that doesn't
9462 look like a version literal will be parsed as the start of the LIST.
9463 Nevertheless, many attempts to use an arbitrary expression as a VERSION
9464 argument will appear to work, because L<Exporter>'s C<import> method
9465 handles numeric arguments specially, performing version checks rather
9466 than treating them as things to export.
9468 Again, there is a distinction between omitting LIST (L<C<import>|/import
9469 LIST> called with no arguments) and an explicit empty LIST C<()>
9470 (L<C<import>|/import LIST> not called). Note that there is no comma
9473 Because this is a wide-open interface, pragmas (compiler directives)
9474 are also implemented this way. Some of the currently implemented
9480 use sigtrap qw(SEGV BUS);
9481 use strict qw(subs vars refs);
9482 use subs qw(afunc blurfl);
9483 use warnings qw(all);
9484 use sort qw(stable);
9486 Some of these pseudo-modules import semantics into the current
9487 block scope (like L<C<strict>|strict> or L<C<integer>|integer>, unlike
9488 ordinary modules, which import symbols into the current package (which
9489 are effective through the end of the file).
9491 Because L<C<use>|/use Module VERSION LIST> takes effect at compile time,
9492 it doesn't respect the ordinary flow control of the code being compiled.
9493 In particular, putting a L<C<use>|/use Module VERSION LIST> inside the
9494 false branch of a conditional doesn't prevent it
9495 from being processed. If a module or pragma only needs to be loaded
9496 conditionally, this can be done using the L<if> pragma:
9498 use if $] < 5.008, "utf8";
9499 use if WANT_WARNINGS, warnings => qw(all);
9501 There's a corresponding L<C<no>|/no MODULE VERSION LIST> declaration
9502 that unimports meanings imported by L<C<use>|/use Module VERSION LIST>,
9503 i.e., it calls C<< Module->unimport(LIST) >> instead of
9504 L<C<import>|/import LIST>. It behaves just as L<C<import>|/import LIST>
9505 does with VERSION, an omitted or empty LIST,
9506 or no unimport method being found.
9512 Care should be taken when using the C<no VERSION> form of L<C<no>|/no
9513 MODULE VERSION LIST>. It is
9514 I<only> meant to be used to assert that the running Perl is of a earlier
9515 version than its argument and I<not> to undo the feature-enabling side effects
9518 See L<perlmodlib> for a list of standard modules and pragmas. See L<perlrun>
9519 for the C<-M> and C<-m> command-line options to Perl that give
9520 L<C<use>|/use Module VERSION LIST> functionality from the command-line.
9525 =for Pod::Functions set a file's last access and modify times
9527 Changes the access and modification times on each file of a list of
9528 files. The first two elements of the list must be the NUMERIC access
9529 and modification times, in that order. Returns the number of files
9530 successfully changed. The inode change time of each file is set
9531 to the current time. For example, this code has the same effect as the
9532 Unix L<touch(1)> command when the files I<already exist> and belong to
9533 the user running the program:
9536 my $atime = my $mtime = time;
9537 utime $atime, $mtime, @ARGV;
9539 Since Perl 5.8.0, if the first two elements of the list are
9540 L<C<undef>|/undef EXPR>,
9541 the L<utime(2)> syscall from your C library is called with a null second
9542 argument. On most systems, this will set the file's access and
9543 modification times to the current time (i.e., equivalent to the example
9544 above) and will work even on files you don't own provided you have write
9547 for my $file (@ARGV) {
9548 utime(undef, undef, $file)
9549 || warn "Couldn't touch $file: $!";
9552 Under NFS this will use the time of the NFS server, not the time of
9553 the local machine. If there is a time synchronization problem, the
9554 NFS server and local machine will have different times. The Unix
9555 L<touch(1)> command will in fact normally use this form instead of the
9556 one shown in the first example.
9558 Passing only one of the first two elements as L<C<undef>|/undef EXPR> is
9559 equivalent to passing a 0 and will not have the effect described when
9560 both are L<C<undef>|/undef EXPR>. This also triggers an
9561 uninitialized warning.
9563 On systems that support L<futimes(2)>, you may pass filehandles among the
9564 files. On systems that don't support L<futimes(2)>, passing filehandles raises
9565 an exception. Filehandles must be passed as globs or glob references to be
9566 recognized; barewords are considered filenames.
9568 Portability issues: L<perlport/utime>.
9575 =for Pod::Functions return a list of the values in a hash
9577 In list context, returns a list consisting of all the values of the named
9578 hash. In Perl 5.12 or later only, will also return a list of the values of
9579 an array; prior to that release, attempting to use an array argument will
9580 produce a syntax error. In scalar context, returns the number of values.
9582 Hash entries are returned in an apparently random order. The actual random
9583 order is specific to a given hash; the exact same series of operations
9584 on two hashes may result in a different order for each hash. Any insertion
9585 into the hash may change the order, as will any deletion, with the exception
9586 that the most recent key returned by L<C<each>|/each HASH> or
9587 L<C<keys>|/keys HASH> may be deleted without changing the order. So
9588 long as a given hash is unmodified you may rely on
9589 L<C<keys>|/keys HASH>, L<C<values>|/values HASH> and
9590 L<C<each>|/each HASH> to repeatedly return the same order
9591 as each other. See L<perlsec/"Algorithmic Complexity Attacks"> for
9592 details on why hash order is randomized. Aside from the guarantees
9593 provided here the exact details of Perl's hash algorithm and the hash
9594 traversal order are subject to change in any release of Perl. Tied hashes
9595 may behave differently to Perl's hashes with respect to changes in order on
9596 insertion and deletion of items.
9598 As a side effect, calling L<C<values>|/values HASH> resets the HASH or
9599 ARRAY's internal iterator (see L<C<each>|/each HASH>) before yielding the
9600 values. In particular,
9601 calling L<C<values>|/values HASH> in void context resets the iterator
9602 with no other overhead.
9604 Apart from resetting the iterator,
9605 C<values @array> in list context is the same as plain C<@array>.
9606 (We recommend that you use void context C<keys @array> for this, but
9607 reasoned that taking C<values @array> out would require more
9608 documentation than leaving it in.)
9610 Note that the values are not copied, which means modifying them will
9611 modify the contents of the hash:
9613 for (values %hash) { s/foo/bar/g } # modifies %hash values
9614 for (@hash{keys %hash}) { s/foo/bar/g } # same
9616 Starting with Perl 5.14, an experimental feature allowed
9617 L<C<values>|/values HASH> to take a
9618 scalar expression. This experiment has been deemed unsuccessful, and was
9619 removed as of Perl 5.24.
9621 To avoid confusing would-be users of your code who are running earlier
9622 versions of Perl with mysterious syntax errors, put this sort of thing at
9623 the top of your file to signal that your code will work I<only> on Perls of
9626 use 5.012; # so keys/values/each work on arrays
9628 See also L<C<keys>|/keys HASH>, L<C<each>|/each HASH>, and
9629 L<C<sort>|/sort SUBNAME LIST>.
9631 =item vec EXPR,OFFSET,BITS
9632 X<vec> X<bit> X<bit vector>
9634 =for Pod::Functions test or set particular bits in a string
9636 Treats the string in EXPR as a bit vector made up of elements of
9637 width BITS and returns the value of the element specified by OFFSET
9638 as an unsigned integer. BITS therefore specifies the number of bits
9639 that are reserved for each element in the bit vector. This must
9640 be a power of two from 1 to 32 (or 64, if your platform supports
9643 If BITS is 8, "elements" coincide with bytes of the input string.
9645 If BITS is 16 or more, bytes of the input string are grouped into chunks
9646 of size BITS/8, and each group is converted to a number as with
9647 L<C<pack>|/pack TEMPLATE,LIST>/L<C<unpack>|/unpack TEMPLATE,EXPR> with
9648 big-endian formats C<n>/C<N> (and analogously for BITS==64). See
9649 L<C<pack>|/pack TEMPLATE,LIST> for details.
9651 If bits is 4 or less, the string is broken into bytes, then the bits
9652 of each byte are broken into 8/BITS groups. Bits of a byte are
9653 numbered in a little-endian-ish way, as in C<0x01>, C<0x02>,
9654 C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example,
9655 breaking the single input byte C<chr(0x36)> into two groups gives a list
9656 C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>.
9658 L<C<vec>|/vec EXPR,OFFSET,BITS> may also be assigned to, in which case
9659 parentheses are needed
9660 to give the expression the correct precedence as in
9662 vec($image, $max_x * $x + $y, 8) = 3;
9664 If the selected element is outside the string, the value 0 is returned.
9665 If an element off the end of the string is written to, Perl will first
9666 extend the string with sufficiently many zero bytes. It is an error
9667 to try to write off the beginning of the string (i.e., negative OFFSET).
9669 If the string happens to be encoded as UTF-8 internally (and thus has
9670 the UTF8 flag set), L<C<vec>|/vec EXPR,OFFSET,BITS> tries to convert it
9671 to use a one-byte-per-character internal representation. However, if the
9672 string contains characters with values of 256 or higher, a fatal error
9675 Strings created with L<C<vec>|/vec EXPR,OFFSET,BITS> can also be
9676 manipulated with the logical
9677 operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit
9678 vector operation is desired when both operands are strings.
9679 See L<perlop/"Bitwise String Operators">.
9681 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
9682 The comments show the string after each step. Note that this code works
9683 in the same way on big-endian or little-endian machines.
9686 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
9688 # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
9689 print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P')
9691 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
9692 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
9693 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
9694 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
9695 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
9696 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
9698 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
9699 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
9700 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
9703 To transform a bit vector into a string or list of 0's and 1's, use these:
9705 my $bits = unpack("b*", $vector);
9706 my @bits = split(//, unpack("b*", $vector));
9708 If you know the exact length in bits, it can be used in place of the C<*>.
9710 Here is an example to illustrate how the bits actually fall in place:
9716 unpack("V",$_) 01234567890123456789012345678901
9717 ------------------------------------------------------------------
9722 for ($shift=0; $shift < $width; ++$shift) {
9723 for ($off=0; $off < 32/$width; ++$off) {
9724 $str = pack("B*", "0"x32);
9725 $bits = (1<<$shift);
9726 vec($str, $off, $width) = $bits;
9727 $res = unpack("b*",$str);
9728 $val = unpack("V", $str);
9735 vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
9736 $off, $width, $bits, $val, $res
9740 Regardless of the machine architecture on which it runs, the
9741 example above should print the following table:
9744 unpack("V",$_) 01234567890123456789012345678901
9745 ------------------------------------------------------------------
9746 vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000
9747 vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000
9748 vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000
9749 vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000
9750 vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000
9751 vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000
9752 vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000
9753 vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000
9754 vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000
9755 vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000
9756 vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000
9757 vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000
9758 vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000
9759 vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000
9760 vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000
9761 vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000
9762 vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000
9763 vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000
9764 vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000
9765 vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000
9766 vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000
9767 vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000
9768 vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000
9769 vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000
9770 vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000
9771 vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000
9772 vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000
9773 vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000
9774 vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000
9775 vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100
9776 vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010
9777 vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001
9778 vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000
9779 vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000
9780 vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000
9781 vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000
9782 vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000
9783 vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000
9784 vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000
9785 vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000
9786 vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000
9787 vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000
9788 vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000
9789 vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000
9790 vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000
9791 vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000
9792 vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000
9793 vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010
9794 vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000
9795 vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000
9796 vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000
9797 vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000
9798 vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000
9799 vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000
9800 vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000
9801 vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000
9802 vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000
9803 vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000
9804 vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000
9805 vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000
9806 vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000
9807 vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000
9808 vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100
9809 vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001
9810 vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000
9811 vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000
9812 vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000
9813 vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000
9814 vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000
9815 vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000
9816 vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000
9817 vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000
9818 vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000
9819 vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000
9820 vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000
9821 vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000
9822 vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000
9823 vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000
9824 vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000
9825 vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100
9826 vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000
9827 vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000
9828 vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000
9829 vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000
9830 vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000
9831 vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000
9832 vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000
9833 vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010
9834 vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000
9835 vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000
9836 vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000
9837 vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000
9838 vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000
9839 vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000
9840 vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000
9841 vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001
9842 vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000
9843 vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000
9844 vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000
9845 vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000
9846 vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000
9847 vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000
9848 vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000
9849 vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000
9850 vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000
9851 vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000
9852 vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000
9853 vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000
9854 vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000
9855 vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000
9856 vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000
9857 vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000
9858 vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000
9859 vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000
9860 vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000
9861 vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000
9862 vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000
9863 vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000
9864 vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000
9865 vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100
9866 vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000
9867 vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000
9868 vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000
9869 vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010
9870 vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000
9871 vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000
9872 vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000
9873 vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
9878 =for Pod::Functions wait for any child process to die
9880 Behaves like L<wait(2)> on your system: it waits for a child
9881 process to terminate and returns the pid of the deceased process, or
9882 C<-1> if there are no child processes. The status is returned in
9883 L<C<$?>|perlvar/$?> and
9884 L<C<${^CHILD_ERROR_NATIVE}>|perlvar/${^CHILD_ERROR_NATIVE}>.
9885 Note that a return value of C<-1> could mean that child processes are
9886 being automatically reaped, as described in L<perlipc>.
9888 If you use L<C<wait>|/wait> in your handler for
9889 L<C<$SIG{CHLD}>|perlvar/%SIG>, it may accidentally wait for the child
9890 created by L<C<qx>|/qxE<sol>STRINGE<sol>> or L<C<system>|/system LIST>.
9891 See L<perlipc> for details.
9893 Portability issues: L<perlport/wait>.
9895 =item waitpid PID,FLAGS
9898 =for Pod::Functions wait for a particular child process to die
9900 Waits for a particular child process to terminate and returns the pid of
9901 the deceased process, or C<-1> if there is no such child process. A
9902 non-blocking wait (with L<WNOHANG|POSIX/C<WNOHANG>> in FLAGS) can return 0 if
9903 there are child processes matching PID but none have terminated yet.
9904 The status is returned in L<C<$?>|perlvar/$?> and
9905 L<C<${^CHILD_ERROR_NATIVE}>|perlvar/${^CHILD_ERROR_NATIVE}>.
9907 A PID of C<0> indicates to wait for any child process whose process group ID is
9908 equal to that of the current process. A PID of less than C<-1> indicates to
9909 wait for any child process whose process group ID is equal to -PID. A PID of
9910 C<-1> indicates to wait for any child process.
9914 use POSIX ":sys_wait_h";
9918 $kid = waitpid(-1, WNOHANG);
9923 1 while waitpid(-1, WNOHANG) > 0;
9925 then you can do a non-blocking wait for all pending zombie processes (see
9927 Non-blocking wait is available on machines supporting either the
9928 L<waitpid(2)> or L<wait4(2)> syscalls. However, waiting for a particular
9929 pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the
9930 system call by remembering the status values of processes that have
9931 exited but have not been harvested by the Perl script yet.)
9933 Note that on some systems, a return value of C<-1> could mean that child
9934 processes are being automatically reaped. See L<perlipc> for details,
9935 and for other examples.
9937 Portability issues: L<perlport/waitpid>.
9940 X<wantarray> X<context>
9942 =for Pod::Functions get void vs scalar vs list context of current subroutine call
9944 Returns true if the context of the currently executing subroutine or
9945 L<C<eval>|/eval EXPR> is looking for a list value. Returns false if the
9947 looking for a scalar. Returns the undefined value if the context is
9948 looking for no value (void context).
9950 return unless defined wantarray; # don't bother doing more
9951 my @a = complex_calculation();
9952 return wantarray ? @a : "@a";
9954 L<C<wantarray>|/wantarray>'s result is unspecified in the top level of a file,
9955 in a C<BEGIN>, C<UNITCHECK>, C<CHECK>, C<INIT> or C<END> block, or
9956 in a C<DESTROY> method.
9958 This function should have been named wantlist() instead.
9961 X<warn> X<warning> X<STDERR>
9963 =for Pod::Functions print debugging info
9965 Emits a warning, usually by printing it to C<STDERR>. C<warn> interprets
9966 its operand LIST in the same way as C<die>, but is slightly different
9967 in what it defaults to when LIST is empty or makes an empty string.
9968 If it is empty and L<C<$@>|perlvar/$@> already contains an exception
9969 value then that value is used after appending C<"\t...caught">. If it
9970 is empty and C<$@> is also empty then the string C<"Warning: Something's
9973 By default, the exception derived from the operand LIST is stringified
9974 and printed to C<STDERR>. This behaviour can be altered by installing
9975 a L<C<$SIG{__WARN__}>|perlvar/%SIG> handler. If there is such a
9976 handler then no message is automatically printed; it is the handler's
9977 responsibility to deal with the exception
9978 as it sees fit (like, for instance, converting it into a
9979 L<C<die>|/die LIST>). Most
9980 handlers must therefore arrange to actually display the
9981 warnings that they are not prepared to deal with, by calling
9982 L<C<warn>|/warn LIST>
9983 again in the handler. Note that this is quite safe and will not
9984 produce an endless loop, since C<__WARN__> hooks are not called from
9987 You will find this behavior is slightly different from that of
9988 L<C<$SIG{__DIE__}>|perlvar/%SIG> handlers (which don't suppress the
9989 error text, but can instead call L<C<die>|/die LIST> again to change
9992 Using a C<__WARN__> handler provides a powerful way to silence all
9993 warnings (even the so-called mandatory ones). An example:
9995 # wipe out *all* compile-time warnings
9996 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
9998 my $foo = 20; # no warning about duplicate my $foo,
9999 # but hey, you asked for it!
10000 # no compile-time or run-time warnings before here
10003 # run-time warnings enabled after here
10004 warn "\$foo is alive and $foo!"; # does show up
10006 See L<perlvar> for details on setting L<C<%SIG>|perlvar/%SIG> entries
10008 examples. See the L<Carp> module for other kinds of warnings using its
10009 C<carp> and C<cluck> functions.
10011 =item write FILEHANDLE
10018 =for Pod::Functions print a picture record
10020 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
10021 using the format associated with that file. By default the format for
10022 a file is the one having the same name as the filehandle, but the
10023 format for the current output channel (see the
10024 L<C<select>|/select FILEHANDLE> function) may be set explicitly by
10025 assigning the name of the format to the L<C<$~>|perlvar/$~> variable.
10027 Top of form processing is handled automatically: if there is insufficient
10028 room on the current page for the formatted record, the page is advanced by
10029 writing a form feed and a special top-of-page
10030 format is used to format the new
10031 page header before the record is written. By default, the top-of-page
10032 format is the name of the filehandle with C<_TOP> appended, or C<top>
10033 in the current package if the former does not exist. This would be a
10034 problem with autovivified filehandles, but it may be dynamically set to the
10035 format of your choice by assigning the name to the L<C<$^>|perlvar/$^>
10036 variable while that filehandle is selected. The number of lines
10037 remaining on the current page is in variable L<C<$->|perlvar/$->, which
10038 can be set to C<0> to force a new page.
10040 If FILEHANDLE is unspecified, output goes to the current default output
10041 channel, which starts out as STDOUT but may be changed by the
10042 L<C<select>|/select FILEHANDLE> operator. If the FILEHANDLE is an EXPR,
10043 then the expression
10044 is evaluated and the resulting string is used to look up the name of
10045 the FILEHANDLE at run time. For more on formats, see L<perlform>.
10047 Note that write is I<not> the opposite of
10048 L<C<read>|/read FILEHANDLE,SCALAR,LENGTH,OFFSET>. Unfortunately.
10052 =for Pod::Functions transliterate a string
10054 The transliteration operator. Same as
10055 L<C<trE<sol>E<sol>E<sol>>|/trE<sol>E<sol>E<sol>>. See
10056 L<perlop/"Quote-Like Operators">.
10060 =head2 Non-function Keywords by Cross-reference
10070 These keywords are documented in L<perldata/"Special Literals">.
10088 These compile phase keywords are documented in L<perlmod/"BEGIN, UNITCHECK, CHECK, INIT and END">.
10098 This method keyword is documented in L<perlobj/"Destructors">.
10130 These operators are documented in L<perlop>.
10140 This keyword is documented in L<perlsub/"Autoloading">.
10164 These flow-control keywords are documented in L<perlsyn/"Compound Statements">.
10168 The "else if" keyword is spelled C<elsif> in Perl. There's no C<elif>
10169 or C<else if> either. It does parse C<elseif>, but only to warn you
10170 about not using it.
10172 See the documentation for flow-control keywords in L<perlsyn/"Compound
10185 These flow-control keywords related to the experimental switch feature are
10186 documented in L<perlsyn/"Switch Statements">.