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
111 =item Functions for SCALARs or strings
112 X<scalar> X<string> X<character>
114 =for Pod::Functions =String
116 L<C<chomp>|/chomp VARIABLE>, L<C<chop>|/chop VARIABLE>,
117 L<C<chr>|/chr NUMBER>, L<C<crypt>|/crypt PLAINTEXT,SALT>,
118 L<C<fc>|/fc EXPR>, L<C<hex>|/hex EXPR>,
119 L<C<index>|/index STR,SUBSTR,POSITION>, L<C<lc>|/lc EXPR>,
120 L<C<lcfirst>|/lcfirst EXPR>, L<C<length>|/length EXPR>,
121 L<C<oct>|/oct EXPR>, L<C<ord>|/ord EXPR>,
122 L<C<pack>|/pack TEMPLATE,LIST>,
123 L<C<qE<sol>E<sol>>|/qE<sol>STRINGE<sol>>,
124 L<C<qqE<sol>E<sol>>|/qqE<sol>STRINGE<sol>>, L<C<reverse>|/reverse LIST>,
125 L<C<rindex>|/rindex STR,SUBSTR,POSITION>,
126 L<C<sprintf>|/sprintf FORMAT, LIST>,
127 L<C<substr>|/substr EXPR,OFFSET,LENGTH,REPLACEMENT>,
128 L<C<trE<sol>E<sol>E<sol>>|/trE<sol>E<sol>E<sol>>, L<C<uc>|/uc EXPR>,
129 L<C<ucfirst>|/ucfirst EXPR>,
130 L<C<yE<sol>E<sol>E<sol>>|/yE<sol>E<sol>E<sol>>
132 L<C<fc>|/fc EXPR> is available only if the
133 L<C<"fc"> feature|feature/The 'fc' feature> is enabled or if it is
134 prefixed with C<CORE::>. The
135 L<C<"fc"> feature|feature/The 'fc' feature> is enabled automatically
136 with a C<use v5.16> (or higher) declaration in the current scope.
138 =item Regular expressions and pattern matching
139 X<regular expression> X<regex> X<regexp>
141 =for Pod::Functions =Regexp
143 L<C<mE<sol>E<sol>>|/mE<sol>E<sol>>, L<C<pos>|/pos SCALAR>,
144 L<C<qrE<sol>E<sol>>|/qrE<sol>STRINGE<sol>>,
145 L<C<quotemeta>|/quotemeta EXPR>,
146 L<C<sE<sol>E<sol>E<sol>>|/sE<sol>E<sol>E<sol>>,
147 L<C<split>|/split E<sol>PATTERNE<sol>,EXPR,LIMIT>,
148 L<C<study>|/study SCALAR>
150 =item Numeric functions
151 X<numeric> X<number> X<trigonometric> X<trigonometry>
153 =for Pod::Functions =Math
155 L<C<abs>|/abs VALUE>, L<C<atan2>|/atan2 Y,X>, L<C<cos>|/cos EXPR>,
156 L<C<exp>|/exp EXPR>, L<C<hex>|/hex EXPR>, L<C<int>|/int EXPR>,
157 L<C<log>|/log EXPR>, L<C<oct>|/oct EXPR>, L<C<rand>|/rand EXPR>,
158 L<C<sin>|/sin EXPR>, L<C<sqrt>|/sqrt EXPR>, L<C<srand>|/srand EXPR>
160 =item Functions for real @ARRAYs
163 =for Pod::Functions =ARRAY
165 L<C<each>|/each HASH>, L<C<keys>|/keys HASH>, L<C<pop>|/pop ARRAY>,
166 L<C<push>|/push ARRAY,LIST>, L<C<shift>|/shift ARRAY>,
167 L<C<splice>|/splice ARRAY,OFFSET,LENGTH,LIST>,
168 L<C<unshift>|/unshift ARRAY,LIST>, L<C<values>|/values HASH>
170 =item Functions for list data
173 =for Pod::Functions =LIST
175 L<C<grep>|/grep BLOCK LIST>, L<C<join>|/join EXPR,LIST>,
176 L<C<map>|/map BLOCK LIST>, L<C<qwE<sol>E<sol>>|/qwE<sol>STRINGE<sol>>,
177 L<C<reverse>|/reverse LIST>, L<C<sort>|/sort SUBNAME LIST>,
178 L<C<unpack>|/unpack TEMPLATE,EXPR>
180 =item Functions for real %HASHes
183 =for Pod::Functions =HASH
185 L<C<delete>|/delete EXPR>, L<C<each>|/each HASH>,
186 L<C<exists>|/exists EXPR>, L<C<keys>|/keys HASH>,
187 L<C<values>|/values HASH>
189 =item Input and output functions
190 X<I/O> X<input> X<output> X<dbm>
192 =for Pod::Functions =I/O
194 L<C<binmode>|/binmode FILEHANDLE, LAYER>, L<C<close>|/close FILEHANDLE>,
195 L<C<closedir>|/closedir DIRHANDLE>, L<C<dbmclose>|/dbmclose HASH>,
196 L<C<dbmopen>|/dbmopen HASH,DBNAME,MASK>, L<C<die>|/die LIST>,
197 L<C<eof>|/eof FILEHANDLE>, L<C<fileno>|/fileno FILEHANDLE>,
198 L<C<flock>|/flock FILEHANDLE,OPERATION>, L<C<format>|/format>,
199 L<C<getc>|/getc FILEHANDLE>, L<C<print>|/print FILEHANDLE LIST>,
200 L<C<printf>|/printf FILEHANDLE FORMAT, LIST>,
201 L<C<read>|/read FILEHANDLE,SCALAR,LENGTH,OFFSET>,
202 L<C<readdir>|/readdir DIRHANDLE>, L<C<readline>|/readline EXPR>
203 L<C<rewinddir>|/rewinddir DIRHANDLE>, L<C<say>|/say FILEHANDLE LIST>,
204 L<C<seek>|/seek FILEHANDLE,POSITION,WHENCE>,
205 L<C<seekdir>|/seekdir DIRHANDLE,POS>,
206 L<C<select>|/select RBITS,WBITS,EBITS,TIMEOUT>,
207 L<C<syscall>|/syscall NUMBER, LIST>,
208 L<C<sysread>|/sysread FILEHANDLE,SCALAR,LENGTH,OFFSET>,
209 L<C<sysseek>|/sysseek FILEHANDLE,POSITION,WHENCE>,
210 L<C<syswrite>|/syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET>,
211 L<C<tell>|/tell FILEHANDLE>, L<C<telldir>|/telldir DIRHANDLE>,
212 L<C<truncate>|/truncate FILEHANDLE,LENGTH>, L<C<warn>|/warn LIST>,
213 L<C<write>|/write FILEHANDLE>
215 L<C<say>|/say FILEHANDLE LIST> is available only if the
216 L<C<"say"> feature|feature/The 'say' feature> is enabled or if it is
217 prefixed with C<CORE::>. The
218 L<C<"say"> feature|feature/The 'say' feature> is enabled automatically
219 with a C<use v5.10> (or higher) declaration in the current scope.
221 =item Functions for fixed-length data or records
223 =for Pod::Functions =Binary
225 L<C<pack>|/pack TEMPLATE,LIST>,
226 L<C<read>|/read FILEHANDLE,SCALAR,LENGTH,OFFSET>,
227 L<C<syscall>|/syscall NUMBER, LIST>,
228 L<C<sysread>|/sysread FILEHANDLE,SCALAR,LENGTH,OFFSET>,
229 L<C<sysseek>|/sysseek FILEHANDLE,POSITION,WHENCE>,
230 L<C<syswrite>|/syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET>,
231 L<C<unpack>|/unpack TEMPLATE,EXPR>, L<C<vec>|/vec EXPR,OFFSET,BITS>
233 =item Functions for filehandles, files, or directories
234 X<file> X<filehandle> X<directory> X<pipe> X<link> X<symlink>
236 =for Pod::Functions =File
238 L<C<-I<X>>|/-X FILEHANDLE>, L<C<chdir>|/chdir EXPR>,
239 L<C<chmod>|/chmod LIST>, L<C<chown>|/chown LIST>,
240 L<C<chroot>|/chroot FILENAME>,
241 L<C<fcntl>|/fcntl FILEHANDLE,FUNCTION,SCALAR>, L<C<glob>|/glob EXPR>,
242 L<C<ioctl>|/ioctl FILEHANDLE,FUNCTION,SCALAR>,
243 L<C<link>|/link OLDFILE,NEWFILE>, L<C<lstat>|/lstat FILEHANDLE>,
244 L<C<mkdir>|/mkdir FILENAME,MASK>, L<C<open>|/open FILEHANDLE,EXPR>,
245 L<C<opendir>|/opendir DIRHANDLE,EXPR>, L<C<readlink>|/readlink EXPR>,
246 L<C<rename>|/rename OLDNAME,NEWNAME>, L<C<rmdir>|/rmdir FILENAME>,
247 L<C<select>|/select FILEHANDLE>, L<C<stat>|/stat FILEHANDLE>,
248 L<C<symlink>|/symlink OLDFILE,NEWFILE>,
249 L<C<sysopen>|/sysopen FILEHANDLE,FILENAME,MODE>,
250 L<C<umask>|/umask EXPR>, L<C<unlink>|/unlink LIST>,
251 L<C<utime>|/utime LIST>
253 =item Keywords related to the control flow of your Perl program
256 =for Pod::Functions =Flow
258 L<C<break>|/break>, L<C<caller>|/caller EXPR>,
259 L<C<continue>|/continue BLOCK>, L<C<die>|/die LIST>, L<C<do>|/do BLOCK>,
260 L<C<dump>|/dump LABEL>, L<C<eval>|/eval EXPR>,
261 L<C<evalbytes>|/evalbytes EXPR> L<C<exit>|/exit EXPR>,
262 L<C<__FILE__>|/__FILE__>, L<C<goto>|/goto LABEL>,
263 L<C<last>|/last LABEL>, L<C<__LINE__>|/__LINE__>,
264 L<C<next>|/next LABEL>, L<C<__PACKAGE__>|/__PACKAGE__>,
265 L<C<redo>|/redo LABEL>, L<C<return>|/return EXPR>,
266 L<C<sub>|/sub NAME BLOCK>, L<C<__SUB__>|/__SUB__>,
267 L<C<wantarray>|/wantarray>
269 L<C<break>|/break> is available only if you enable the experimental
270 L<C<"switch"> feature|feature/The 'switch' feature> or use the C<CORE::>
271 prefix. The L<C<"switch"> feature|feature/The 'switch' feature> also
272 enables the C<default>, C<given> and C<when> statements, which are
273 documented in L<perlsyn/"Switch Statements">.
274 The L<C<"switch"> feature|feature/The 'switch' feature> is enabled
275 automatically with a C<use v5.10> (or higher) declaration in the current
276 scope. In Perl v5.14 and earlier, L<C<continue>|/continue BLOCK>
277 required the L<C<"switch"> feature|feature/The 'switch' feature>, like
280 L<C<evalbytes>|/evalbytes EXPR> is only available with the
281 L<C<"evalbytes"> feature|feature/The 'unicode_eval' and 'evalbytes' features>
282 (see L<feature>) or if prefixed with C<CORE::>. L<C<__SUB__>|/__SUB__>
283 is only available with the
284 L<C<"current_sub"> feature|feature/The 'current_sub' feature> or if
285 prefixed with C<CORE::>. Both the
286 L<C<"evalbytes">|feature/The 'unicode_eval' and 'evalbytes' features>
287 and L<C<"current_sub">|feature/The 'current_sub' feature> features are
288 enabled automatically with a C<use v5.16> (or higher) declaration in the
291 =item Keywords related to scoping
293 =for Pod::Functions =Namespace
295 L<C<caller>|/caller EXPR>, L<C<import>|/import LIST>,
296 L<C<local>|/local EXPR>, L<C<my>|/my VARLIST>, L<C<our>|/our VARLIST>,
297 L<C<package>|/package NAMESPACE>, L<C<state>|/state VARLIST>,
298 L<C<use>|/use Module VERSION LIST>
300 L<C<state>|/state VARLIST> is available only if the
301 L<C<"state"> feature|feature/The 'state' feature> is enabled or if it is
302 prefixed with C<CORE::>. The
303 L<C<"state"> feature|feature/The 'state' feature> is enabled
304 automatically with a C<use v5.10> (or higher) declaration in the current
307 =item Miscellaneous functions
309 =for Pod::Functions =Misc
311 L<C<defined>|/defined EXPR>, L<C<formline>|/formline PICTURE,LIST>,
312 L<C<lock>|/lock THING>, L<C<prototype>|/prototype FUNCTION>,
313 L<C<reset>|/reset EXPR>, L<C<scalar>|/scalar EXPR>,
314 L<C<undef>|/undef EXPR>
316 =item Functions for processes and process groups
317 X<process> X<pid> X<process id>
319 =for Pod::Functions =Process
321 L<C<alarm>|/alarm SECONDS>, L<C<exec>|/exec LIST>, L<C<fork>|/fork>,
322 L<C<getpgrp>|/getpgrp PID>, L<C<getppid>|/getppid>,
323 L<C<getpriority>|/getpriority WHICH,WHO>, L<C<kill>|/kill SIGNAL, LIST>,
324 L<C<pipe>|/pipe READHANDLE,WRITEHANDLE>,
325 L<C<qxE<sol>E<sol>>|/qxE<sol>STRINGE<sol>>,
326 L<C<readpipe>|/readpipe EXPR>, L<C<setpgrp>|/setpgrp PID,PGRP>,
327 L<C<setpriority>|/setpriority WHICH,WHO,PRIORITY>,
328 L<C<sleep>|/sleep EXPR>, L<C<system>|/system LIST>, L<C<times>|/times>,
329 L<C<wait>|/wait>, L<C<waitpid>|/waitpid PID,FLAGS>
331 =item Keywords related to Perl modules
334 =for Pod::Functions =Modules
336 L<C<do>|/do EXPR>, L<C<import>|/import LIST>,
337 L<C<no>|/no MODULE VERSION LIST>, L<C<package>|/package NAMESPACE>,
338 L<C<require>|/require VERSION>, L<C<use>|/use Module VERSION LIST>
340 =item Keywords related to classes and object-orientation
341 X<object> X<class> X<package>
343 =for Pod::Functions =Objects
345 L<C<bless>|/bless REF,CLASSNAME>, L<C<dbmclose>|/dbmclose HASH>,
346 L<C<dbmopen>|/dbmopen HASH,DBNAME,MASK>,
347 L<C<package>|/package NAMESPACE>, L<C<ref>|/ref EXPR>,
348 L<C<tie>|/tie VARIABLE,CLASSNAME,LIST>, L<C<tied>|/tied VARIABLE>,
349 L<C<untie>|/untie VARIABLE>, L<C<use>|/use Module VERSION LIST>
351 =item Low-level socket functions
354 =for Pod::Functions =Socket
356 L<C<accept>|/accept NEWSOCKET,GENERICSOCKET>,
357 L<C<bind>|/bind SOCKET,NAME>, L<C<connect>|/connect SOCKET,NAME>,
358 L<C<getpeername>|/getpeername SOCKET>,
359 L<C<getsockname>|/getsockname SOCKET>,
360 L<C<getsockopt>|/getsockopt SOCKET,LEVEL,OPTNAME>,
361 L<C<listen>|/listen SOCKET,QUEUESIZE>,
362 L<C<recv>|/recv SOCKET,SCALAR,LENGTH,FLAGS>,
363 L<C<send>|/send SOCKET,MSG,FLAGS,TO>,
364 L<C<setsockopt>|/setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL>,
365 L<C<shutdown>|/shutdown SOCKET,HOW>,
366 L<C<socket>|/socket SOCKET,DOMAIN,TYPE,PROTOCOL>,
367 L<C<socketpair>|/socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL>
369 =item System V interprocess communication functions
370 X<IPC> X<System V> X<semaphore> X<shared memory> X<memory> X<message>
372 =for Pod::Functions =SysV
374 L<C<msgctl>|/msgctl ID,CMD,ARG>, L<C<msgget>|/msgget KEY,FLAGS>,
375 L<C<msgrcv>|/msgrcv ID,VAR,SIZE,TYPE,FLAGS>,
376 L<C<msgsnd>|/msgsnd ID,MSG,FLAGS>,
377 L<C<semctl>|/semctl ID,SEMNUM,CMD,ARG>,
378 L<C<semget>|/semget KEY,NSEMS,FLAGS>, L<C<semop>|/semop KEY,OPSTRING>,
379 L<C<shmctl>|/shmctl ID,CMD,ARG>, L<C<shmget>|/shmget KEY,SIZE,FLAGS>,
380 L<C<shmread>|/shmread ID,VAR,POS,SIZE>,
381 L<C<shmwrite>|/shmwrite ID,STRING,POS,SIZE>
383 =item Fetching user and group info
384 X<user> X<group> X<password> X<uid> X<gid> X<passwd> X</etc/passwd>
386 =for Pod::Functions =User
388 L<C<endgrent>|/endgrent>, L<C<endhostent>|/endhostent>,
389 L<C<endnetent>|/endnetent>, L<C<endpwent>|/endpwent>,
390 L<C<getgrent>|/getgrent>, L<C<getgrgid>|/getgrgid GID>,
391 L<C<getgrnam>|/getgrnam NAME>, L<C<getlogin>|/getlogin>,
392 L<C<getpwent>|/getpwent>, L<C<getpwnam>|/getpwnam NAME>,
393 L<C<getpwuid>|/getpwuid UID>, L<C<setgrent>|/setgrent>,
394 L<C<setpwent>|/setpwent>
396 =item Fetching network info
397 X<network> X<protocol> X<host> X<hostname> X<IP> X<address> X<service>
399 =for Pod::Functions =Network
401 L<C<endprotoent>|/endprotoent>, L<C<endservent>|/endservent>,
402 L<C<gethostbyaddr>|/gethostbyaddr ADDR,ADDRTYPE>,
403 L<C<gethostbyname>|/gethostbyname NAME>, L<C<gethostent>|/gethostent>,
404 L<C<getnetbyaddr>|/getnetbyaddr ADDR,ADDRTYPE>,
405 L<C<getnetbyname>|/getnetbyname NAME>, L<C<getnetent>|/getnetent>,
406 L<C<getprotobyname>|/getprotobyname NAME>,
407 L<C<getprotobynumber>|/getprotobynumber NUMBER>,
408 L<C<getprotoent>|/getprotoent>,
409 L<C<getservbyname>|/getservbyname NAME,PROTO>,
410 L<C<getservbyport>|/getservbyport PORT,PROTO>,
411 L<C<getservent>|/getservent>, L<C<sethostent>|/sethostent STAYOPEN>,
412 L<C<setnetent>|/setnetent STAYOPEN>,
413 L<C<setprotoent>|/setprotoent STAYOPEN>,
414 L<C<setservent>|/setservent STAYOPEN>
416 =item Time-related functions
419 =for Pod::Functions =Time
421 L<C<gmtime>|/gmtime EXPR>, L<C<localtime>|/localtime EXPR>,
422 L<C<time>|/time>, L<C<times>|/times>
424 =item Non-function keywords
426 =for Pod::Functions =!Non-functions
428 C<and>, C<AUTOLOAD>, C<BEGIN>, C<CHECK>, C<cmp>, C<CORE>, C<__DATA__>,
429 C<default>, C<DESTROY>, C<else>, C<elseif>, C<elsif>, C<END>, C<__END__>,
430 C<eq>, C<for>, C<foreach>, C<ge>, C<given>, C<gt>, C<if>, C<INIT>, C<le>,
431 C<lt>, C<ne>, C<not>, C<or>, C<UNITCHECK>, C<unless>, C<until>, C<when>,
432 C<while>, C<x>, C<xor>
437 X<portability> X<Unix> X<portable>
439 Perl was born in Unix and can therefore access all common Unix
440 system calls. In non-Unix environments, the functionality of some
441 Unix system calls may not be available or details of the available
442 functionality may differ slightly. The Perl functions affected
445 L<C<-I<X>>|/-X FILEHANDLE>, L<C<binmode>|/binmode FILEHANDLE, LAYER>,
446 L<C<chmod>|/chmod LIST>, L<C<chown>|/chown LIST>,
447 L<C<chroot>|/chroot FILENAME>, L<C<crypt>|/crypt PLAINTEXT,SALT>,
448 L<C<dbmclose>|/dbmclose HASH>, L<C<dbmopen>|/dbmopen HASH,DBNAME,MASK>,
449 L<C<dump>|/dump LABEL>, L<C<endgrent>|/endgrent>,
450 L<C<endhostent>|/endhostent>, L<C<endnetent>|/endnetent>,
451 L<C<endprotoent>|/endprotoent>, L<C<endpwent>|/endpwent>,
452 L<C<endservent>|/endservent>, L<C<exec>|/exec LIST>,
453 L<C<fcntl>|/fcntl FILEHANDLE,FUNCTION,SCALAR>,
454 L<C<flock>|/flock FILEHANDLE,OPERATION>, L<C<fork>|/fork>,
455 L<C<getgrent>|/getgrent>, L<C<getgrgid>|/getgrgid GID>,
456 L<C<gethostbyname>|/gethostbyname NAME>, L<C<gethostent>|/gethostent>,
457 L<C<getlogin>|/getlogin>,
458 L<C<getnetbyaddr>|/getnetbyaddr ADDR,ADDRTYPE>,
459 L<C<getnetbyname>|/getnetbyname NAME>, L<C<getnetent>|/getnetent>,
460 L<C<getppid>|/getppid>, L<C<getpgrp>|/getpgrp PID>,
461 L<C<getpriority>|/getpriority WHICH,WHO>,
462 L<C<getprotobynumber>|/getprotobynumber NUMBER>,
463 L<C<getprotoent>|/getprotoent>, L<C<getpwent>|/getpwent>,
464 L<C<getpwnam>|/getpwnam NAME>, L<C<getpwuid>|/getpwuid UID>,
465 L<C<getservbyport>|/getservbyport PORT,PROTO>,
466 L<C<getservent>|/getservent>,
467 L<C<getsockopt>|/getsockopt SOCKET,LEVEL,OPTNAME>,
468 L<C<glob>|/glob EXPR>, L<C<ioctl>|/ioctl FILEHANDLE,FUNCTION,SCALAR>,
469 L<C<kill>|/kill SIGNAL, LIST>, L<C<link>|/link OLDFILE,NEWFILE>,
470 L<C<lstat>|/lstat FILEHANDLE>, L<C<msgctl>|/msgctl ID,CMD,ARG>,
471 L<C<msgget>|/msgget KEY,FLAGS>,
472 L<C<msgrcv>|/msgrcv ID,VAR,SIZE,TYPE,FLAGS>,
473 L<C<msgsnd>|/msgsnd ID,MSG,FLAGS>, L<C<open>|/open FILEHANDLE,EXPR>,
474 L<C<pipe>|/pipe READHANDLE,WRITEHANDLE>, L<C<readlink>|/readlink EXPR>,
475 L<C<rename>|/rename OLDNAME,NEWNAME>,
476 L<C<select>|/select RBITS,WBITS,EBITS,TIMEOUT>,
477 L<C<semctl>|/semctl ID,SEMNUM,CMD,ARG>,
478 L<C<semget>|/semget KEY,NSEMS,FLAGS>, L<C<semop>|/semop KEY,OPSTRING>,
479 L<C<setgrent>|/setgrent>, L<C<sethostent>|/sethostent STAYOPEN>,
480 L<C<setnetent>|/setnetent STAYOPEN>, L<C<setpgrp>|/setpgrp PID,PGRP>,
481 L<C<setpriority>|/setpriority WHICH,WHO,PRIORITY>,
482 L<C<setprotoent>|/setprotoent STAYOPEN>, L<C<setpwent>|/setpwent>,
483 L<C<setservent>|/setservent STAYOPEN>,
484 L<C<setsockopt>|/setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL>,
485 L<C<shmctl>|/shmctl ID,CMD,ARG>, L<C<shmget>|/shmget KEY,SIZE,FLAGS>,
486 L<C<shmread>|/shmread ID,VAR,POS,SIZE>,
487 L<C<shmwrite>|/shmwrite ID,STRING,POS,SIZE>,
488 L<C<socket>|/socket SOCKET,DOMAIN,TYPE,PROTOCOL>,
489 L<C<socketpair>|/socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL>,
490 L<C<stat>|/stat FILEHANDLE>, L<C<symlink>|/symlink OLDFILE,NEWFILE>,
491 L<C<syscall>|/syscall NUMBER, LIST>,
492 L<C<sysopen>|/sysopen FILEHANDLE,FILENAME,MODE>,
493 L<C<system>|/system LIST>, L<C<times>|/times>,
494 L<C<truncate>|/truncate FILEHANDLE,LENGTH>, L<C<umask>|/umask EXPR>,
495 L<C<unlink>|/unlink LIST>, L<C<utime>|/utime LIST>, L<C<wait>|/wait>,
496 L<C<waitpid>|/waitpid PID,FLAGS>
498 For more information about the portability of these functions, see
499 L<perlport> and other available platform-specific documentation.
501 =head2 Alphabetical Listing of Perl Functions
506 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>
507 X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
515 =for Pod::Functions a file test (-r, -x, etc)
517 A file test, where X is one of the letters listed below. This unary
518 operator takes one argument, either a filename, a filehandle, or a dirhandle,
519 and tests the associated file to see if something is true about it. If the
520 argument is omitted, tests L<C<$_>|perlvar/$_>, except for C<-t>, which
521 tests STDIN. Unless otherwise documented, it returns C<1> for true and
522 C<''> for false. If the file doesn't exist or can't be examined, it
523 returns L<C<undef>|/undef EXPR> and sets L<C<$!>|perlvar/$!> (errno).
524 Despite the funny names, precedence is the same as any other named unary
525 operator. The operator may be any of:
527 -r File is readable by effective uid/gid.
528 -w File is writable by effective uid/gid.
529 -x File is executable by effective uid/gid.
530 -o File is owned by effective uid.
532 -R File is readable by real uid/gid.
533 -W File is writable by real uid/gid.
534 -X File is executable by real uid/gid.
535 -O File is owned by real uid.
538 -z File has zero size (is empty).
539 -s File has nonzero size (returns size in bytes).
541 -f File is a plain file.
542 -d File is a directory.
543 -l File is a symbolic link (false if symlinks aren't
544 supported by the file system).
545 -p File is a named pipe (FIFO), or Filehandle is a pipe.
547 -b File is a block special file.
548 -c File is a character special file.
549 -t Filehandle is opened to a tty.
551 -u File has setuid bit set.
552 -g File has setgid bit set.
553 -k File has sticky bit set.
555 -T File is an ASCII or UTF-8 text file (heuristic guess).
556 -B File is a "binary" file (opposite of -T).
558 -M Script start time minus file modification time, in days.
559 -A Same for access time.
560 -C Same for inode change time (Unix, may differ for other
567 next unless -f $_; # ignore specials
571 Note that C<-s/a/b/> does not do a negated substitution. Saying
572 C<-exp($foo)> still works as expected, however: only single letters
573 following a minus are interpreted as file tests.
575 These operators are exempt from the "looks like a function rule" described
576 above. That is, an opening parenthesis after the operator does not affect
577 how much of the following code constitutes the argument. Put the opening
578 parentheses before the operator to separate it from code that follows (this
579 applies only to operators with higher precedence than unary operators, of
582 -s($file) + 1024 # probably wrong; same as -s($file + 1024)
583 (-s $file) + 1024 # correct
585 The interpretation of the file permission operators C<-r>, C<-R>,
586 C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
587 of the file and the uids and gids of the user. There may be other
588 reasons you can't actually read, write, or execute the file: for
589 example network filesystem access controls, ACLs (access control lists),
590 read-only filesystems, and unrecognized executable formats. Note
591 that the use of these six specific operators to verify if some operation
592 is possible is usually a mistake, because it may be open to race
595 Also note that, for the superuser on the local filesystems, the C<-r>,
596 C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
597 if any execute bit is set in the mode. Scripts run by the superuser
598 may thus need to do a L<C<stat>|/stat FILEHANDLE> to determine the
599 actual mode of the file, or temporarily set their effective uid to
602 If you are using ACLs, there is a pragma called L<C<filetest>|filetest>
603 that may produce more accurate results than the bare
604 L<C<stat>|/stat FILEHANDLE> mode bits.
605 When under C<use filetest 'access'>, the above-mentioned filetests
606 test whether the permission can(not) be granted using the L<access(2)>
607 family of system calls. Also note that the C<-x> and C<-X> tests may
608 under this pragma return true even if there are no execute permission
609 bits set (nor any extra execute permission ACLs). This strangeness is
610 due to the underlying system calls' definitions. Note also that, due to
611 the implementation of C<use filetest 'access'>, the C<_> special
612 filehandle won't cache the results of the file tests when this pragma is
613 in effect. Read the documentation for the L<C<filetest>|filetest>
614 pragma for more information.
616 The C<-T> and C<-B> tests work as follows. The first block or so of
617 the file is examined to see if it is valid UTF-8 that includes non-ASCII
618 characters. If so, it's a C<-T> file. Otherwise, that same portion of
619 the file is examined for odd characters such as strange control codes or
620 characters with the high bit set. If more than a third of the
621 characters are strange, it's a C<-B> file; otherwise it's a C<-T> file.
622 Also, any file containing a zero byte in the examined portion is
623 considered a binary file. (If executed within the scope of a L<S<use
624 locale>|perllocale> which includes C<LC_CTYPE>, odd characters are
625 anything that isn't a printable nor space in the current locale.) If
626 C<-T> or C<-B> is used on a filehandle, the current IO buffer is
628 rather than the first block. Both C<-T> and C<-B> return true on an empty
629 file, or a file at EOF when testing a filehandle. Because you have to
630 read a file to do the C<-T> test, on most occasions you want to use a C<-f>
631 against the file first, as in C<next unless -f $file && -T $file>.
633 If any of the file tests (or either the L<C<stat>|/stat FILEHANDLE> or
634 L<C<lstat>|/lstat FILEHANDLE> operator) is given the special filehandle
635 consisting of a solitary underline, then the stat structure of the
636 previous file test (or L<C<stat>|/stat FILEHANDLE> operator) is used,
637 saving a system call. (This doesn't work with C<-t>, and you need to
638 remember that L<C<lstat>|/lstat FILEHANDLE> and C<-l> leave values in
639 the stat structure for the symbolic link, not the real file.) (Also, if
640 the stat buffer was filled by an L<C<lstat>|/lstat FILEHANDLE> call,
641 C<-T> and C<-B> will reset it with the results of C<stat _>).
644 print "Can do.\n" if -r $a || -w _ || -x _;
647 print "Readable\n" if -r _;
648 print "Writable\n" if -w _;
649 print "Executable\n" if -x _;
650 print "Setuid\n" if -u _;
651 print "Setgid\n" if -g _;
652 print "Sticky\n" if -k _;
653 print "Text\n" if -T _;
654 print "Binary\n" if -B _;
656 As of Perl 5.10.0, as a form of purely syntactic sugar, you can stack file
657 test operators, in a way that C<-f -w -x $file> is equivalent to
658 C<-x $file && -w _ && -f _>. (This is only fancy syntax: if you use
659 the return value of C<-f $file> as an argument to another filetest
660 operator, no special magic will happen.)
662 Portability issues: L<perlport/-X>.
664 To avoid confusing would-be users of your code with mysterious
665 syntax errors, put something like this at the top of your script:
667 use 5.010; # so filetest ops can stack
674 =for Pod::Functions absolute value function
676 Returns the absolute value of its argument.
677 If VALUE is omitted, uses L<C<$_>|perlvar/$_>.
679 =item accept NEWSOCKET,GENERICSOCKET
682 =for Pod::Functions accept an incoming socket connect
684 Accepts an incoming socket connect, just as L<accept(2)>
685 does. Returns the packed address if it succeeded, false otherwise.
686 See the example in L<perlipc/"Sockets: Client/Server Communication">.
688 On systems that support a close-on-exec flag on files, the flag will
689 be set for the newly opened file descriptor, as determined by the
690 value of L<C<$^F>|perlvar/$^F>. See L<perlvar/$^F>.
699 =for Pod::Functions schedule a SIGALRM
701 Arranges to have a SIGALRM delivered to this process after the
702 specified number of wallclock seconds has elapsed. If SECONDS is not
703 specified, the value stored in L<C<$_>|perlvar/$_> is used. (On some
704 machines, unfortunately, the elapsed time may be up to one second less
705 or more than you specified because of how seconds are counted, and
706 process scheduling may delay the delivery of the signal even further.)
708 Only one timer may be counting at once. Each call disables the
709 previous timer, and an argument of C<0> may be supplied to cancel the
710 previous timer without starting a new one. The returned value is the
711 amount of time remaining on the previous timer.
713 For delays of finer granularity than one second, the L<Time::HiRes> module
714 (from CPAN, and starting from Perl 5.8 part of the standard
715 distribution) provides
716 L<C<ualarm>|Time::HiRes/ualarm ( $useconds [, $interval_useconds ] )>.
717 You may also use Perl's four-argument version of
718 L<C<select>|/select RBITS,WBITS,EBITS,TIMEOUT> leaving the first three
719 arguments undefined, or you might be able to use the
720 L<C<syscall>|/syscall NUMBER, LIST> interface to access L<setitimer(2)>
721 if your system supports it. See L<perlfaq8> for details.
723 It is usually a mistake to intermix L<C<alarm>|/alarm SECONDS> and
724 L<C<sleep>|/sleep EXPR> calls, because L<C<sleep>|/sleep EXPR> may be
725 internally implemented on your system with L<C<alarm>|/alarm SECONDS>.
727 If you want to use L<C<alarm>|/alarm SECONDS> to time out a system call
728 you need to use an L<C<eval>|/eval EXPR>/L<C<die>|/die LIST> pair. You
729 can't rely on the alarm causing the system call to fail with
730 L<C<$!>|perlvar/$!> set to C<EINTR> because Perl sets up signal handlers
731 to restart system calls on some systems. Using
732 L<C<eval>|/eval EXPR>/L<C<die>|/die LIST> always works, modulo the
733 caveats given in L<perlipc/"Signals">.
736 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
738 my $nread = sysread $socket, $buffer, $size;
742 die unless $@ eq "alarm\n"; # propagate unexpected errors
749 For more information see L<perlipc>.
751 Portability issues: L<perlport/alarm>.
754 X<atan2> X<arctangent> X<tan> X<tangent>
756 =for Pod::Functions arctangent of Y/X in the range -PI to PI
758 Returns the arctangent of Y/X in the range -PI to PI.
760 For the tangent operation, you may use the
761 L<C<Math::Trig::tan>|Math::Trig/B<tan>> function, or use the familiar
764 sub tan { sin($_[0]) / cos($_[0]) }
766 The return value for C<atan2(0,0)> is implementation-defined; consult
767 your L<atan2(3)> manpage for more information.
769 Portability issues: L<perlport/atan2>.
771 =item bind SOCKET,NAME
774 =for Pod::Functions binds an address to a socket
776 Binds a network address to a socket, just as L<bind(2)>
777 does. Returns true if it succeeded, false otherwise. NAME should be a
778 packed address of the appropriate type for the socket. See the examples in
779 L<perlipc/"Sockets: Client/Server Communication">.
781 =item binmode FILEHANDLE, LAYER
782 X<binmode> X<binary> X<text> X<DOS> X<Windows>
784 =item binmode FILEHANDLE
786 =for Pod::Functions prepare binary files for I/O
788 Arranges for FILEHANDLE to be read or written in "binary" or "text"
789 mode on systems where the run-time libraries distinguish between
790 binary and text files. If FILEHANDLE is an expression, the value is
791 taken as the name of the filehandle. Returns true on success,
792 otherwise it returns L<C<undef>|/undef EXPR> and sets
793 L<C<$!>|perlvar/$!> (errno).
795 On some systems (in general, DOS- and Windows-based systems)
796 L<C<binmode>|/binmode FILEHANDLE, LAYER> is necessary when you're not
797 working with a text file. For the sake of portability it is a good idea
798 always to use it when appropriate, and never to use it when it isn't
799 appropriate. Also, people can set their I/O to be by default
800 UTF8-encoded Unicode, not bytes.
802 In other words: regardless of platform, use
803 L<C<binmode>|/binmode FILEHANDLE, LAYER> on binary data, like images,
806 If LAYER is present it is a single string, but may contain multiple
807 directives. The directives alter the behaviour of the filehandle.
808 When LAYER is present, using binmode on a text file makes sense.
810 If LAYER is omitted or specified as C<:raw> the filehandle is made
811 suitable for passing binary data. This includes turning off possible CRLF
812 translation and marking it as bytes (as opposed to Unicode characters).
813 Note that, despite what may be implied in I<"Programming Perl"> (the
814 Camel, 3rd edition) or elsewhere, C<:raw> is I<not> simply the inverse of C<:crlf>.
815 Other layers that would affect the binary nature of the stream are
816 I<also> disabled. See L<PerlIO>, L<perlrun>, and the discussion about the
817 PERLIO environment variable.
819 The C<:bytes>, C<:crlf>, C<:utf8>, and any other directives of the
820 form C<:...>, are called I/O I<layers>. The L<open> pragma can be used to
821 establish default I/O layers.
823 I<The LAYER parameter of the L<C<binmode>|/binmode FILEHANDLE, LAYER>
824 function is described as "DISCIPLINE" in "Programming Perl, 3rd
825 Edition". However, since the publishing of this book, by many known as
826 "Camel III", the consensus of the naming of this functionality has moved
827 from "discipline" to "layer". All documentation of this version of Perl
828 therefore refers to "layers" rather than to "disciplines". Now back to
829 the regularly scheduled documentation...>
831 To mark FILEHANDLE as UTF-8, use C<:utf8> or C<:encoding(UTF-8)>.
832 C<:utf8> just marks the data as UTF-8 without further checking,
833 while C<:encoding(UTF-8)> checks the data for actually being valid
834 UTF-8. More details can be found in L<PerlIO::encoding>.
836 In general, L<C<binmode>|/binmode FILEHANDLE, LAYER> should be called
837 after L<C<open>|/open FILEHANDLE,EXPR> but before any I/O is done on the
838 filehandle. Calling L<C<binmode>|/binmode FILEHANDLE, LAYER> normally
839 flushes any pending buffered output data (and perhaps pending input
840 data) on the handle. An exception to this is the C<:encoding> layer
841 that changes the default character encoding of the handle.
842 The C<:encoding> layer sometimes needs to be called in
843 mid-stream, and it doesn't flush the stream. C<:encoding>
844 also implicitly pushes on top of itself the C<:utf8> layer because
845 internally Perl operates on UTF8-encoded Unicode characters.
847 The operating system, device drivers, C libraries, and Perl run-time
848 system all conspire to let the programmer treat a single
849 character (C<\n>) as the line terminator, irrespective of external
850 representation. On many operating systems, the native text file
851 representation matches the internal representation, but on some
852 platforms the external representation of C<\n> is made up of more than
855 All variants of Unix, Mac OS (old and new), and Stream_LF files on VMS use
856 a single character to end each line in the external representation of text
857 (even though that single character is CARRIAGE RETURN on old, pre-Darwin
858 flavors of Mac OS, and is LINE FEED on Unix and most VMS files). In other
859 systems like OS/2, DOS, and the various flavors of MS-Windows, your program
860 sees a C<\n> as a simple C<\cJ>, but what's stored in text files are the
861 two characters C<\cM\cJ>. That means that if you don't use
862 L<C<binmode>|/binmode FILEHANDLE, LAYER> on these systems, C<\cM\cJ>
863 sequences on disk will be converted to C<\n> on input, and any C<\n> in
864 your program will be converted back to C<\cM\cJ> on output. This is
865 what you want for text files, but it can be disastrous for binary files.
867 Another consequence of using L<C<binmode>|/binmode FILEHANDLE, LAYER>
868 (on some systems) is that special end-of-file markers will be seen as
869 part of the data stream. For systems from the Microsoft family this
870 means that, if your binary data contain C<\cZ>, the I/O subsystem will
871 regard it as the end of the file, unless you use
872 L<C<binmode>|/binmode FILEHANDLE, LAYER>.
874 L<C<binmode>|/binmode FILEHANDLE, LAYER> is important not only for
875 L<C<readline>|/readline EXPR> and L<C<print>|/print FILEHANDLE LIST>
876 operations, but also when using
877 L<C<read>|/read FILEHANDLE,SCALAR,LENGTH,OFFSET>,
878 L<C<seek>|/seek FILEHANDLE,POSITION,WHENCE>,
879 L<C<sysread>|/sysread FILEHANDLE,SCALAR,LENGTH,OFFSET>,
880 L<C<syswrite>|/syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET> and
881 L<C<tell>|/tell FILEHANDLE> (see L<perlport> for more details). See the
882 L<C<$E<sol>>|perlvar/$E<sol>> and L<C<$\>|perlvar/$\> variables in
883 L<perlvar> for how to manually set your input and output
884 line-termination sequences.
886 Portability issues: L<perlport/binmode>.
888 =item bless REF,CLASSNAME
893 =for Pod::Functions create an object
895 This function tells the thingy referenced by REF that it is now an object
896 in the CLASSNAME package. If CLASSNAME is omitted, the current package
897 is used. Because a L<C<bless>|/bless REF,CLASSNAME> is often the last
898 thing in a constructor, it returns the reference for convenience.
899 Always use the two-argument version if a derived class might inherit the
900 method doing the blessing. See L<perlobj> for more about the blessing
901 (and blessings) of objects.
903 Consider always blessing objects in CLASSNAMEs that are mixed case.
904 Namespaces with all lowercase names are considered reserved for
905 Perl pragmas. Builtin types have all uppercase names. To prevent
906 confusion, you may wish to avoid such package names as well. Make sure
907 that CLASSNAME is a true value.
909 See L<perlmod/"Perl Modules">.
913 =for Pod::Functions +switch break out of a C<given> block
915 Break out of a C<given> block.
917 L<C<break>|/break> is available only if the
918 L<C<"switch"> feature|feature/The 'switch' feature> is enabled or if it
919 is prefixed with C<CORE::>. The
920 L<C<"switch"> feature|feature/The 'switch' feature> is enabled
921 automatically with a C<use v5.10> (or higher) declaration in the current
925 X<caller> X<call stack> X<stack> X<stack trace>
929 =for Pod::Functions get context of the current subroutine call
931 Returns the context of the current pure perl subroutine call. In scalar
932 context, returns the caller's package name if there I<is> a caller (that is, if
933 we're in a subroutine or L<C<eval>|/eval EXPR> or
934 L<C<require>|/require VERSION>) and the undefined value otherwise.
935 caller never returns XS subs and they are skipped. The next pure perl
936 sub will appear instead of the XS sub in caller's return values. In
937 list context, caller returns
940 my ($package, $filename, $line) = caller;
942 With EXPR, it returns some extra information that the debugger uses to
943 print a stack trace. The value of EXPR indicates how many call frames
944 to go back before the current one.
947 my ($package, $filename, $line, $subroutine, $hasargs,
950 $wantarray, $evaltext, $is_require, $hints, $bitmask, $hinthash)
953 Here, $subroutine is the function that the caller called (rather than the
954 function containing the caller). Note that $subroutine may be C<(eval)> if
955 the frame is not a subroutine call, but an L<C<eval>|/eval EXPR>. In
956 such a case additional elements $evaltext and C<$is_require> are set:
957 C<$is_require> is true if the frame is created by a
958 L<C<require>|/require VERSION> or L<C<use>|/use Module VERSION LIST>
959 statement, $evaltext contains the text of the C<eval EXPR> statement.
960 In particular, for an C<eval BLOCK> statement, $subroutine is C<(eval)>,
961 but $evaltext is undefined. (Note also that each
962 L<C<use>|/use Module VERSION LIST> statement creates a
963 L<C<require>|/require VERSION> frame inside an C<eval EXPR> frame.)
964 $subroutine may also be C<(unknown)> if this particular subroutine
965 happens to have been deleted from the symbol table. C<$hasargs> is true
966 if a new instance of L<C<@_>|perlvar/@_> was set up for the frame.
967 C<$hints> and C<$bitmask> contain pragmatic hints that the caller was
968 compiled with. C<$hints> corresponds to L<C<$^H>|perlvar/$^H>, and
969 C<$bitmask> corresponds to
970 L<C<${^WARNING_BITS}>|perlvar/${^WARNING_BITS}>. The C<$hints> and
971 C<$bitmask> values are subject to change between versions of Perl, and
972 are not meant for external use.
974 C<$hinthash> is a reference to a hash containing the value of
975 L<C<%^H>|perlvar/%^H> when the caller was compiled, or
976 L<C<undef>|/undef EXPR> if L<C<%^H>|perlvar/%^H> was empty. Do not
977 modify the values of this hash, as they are the actual values stored in
980 Furthermore, when called from within the DB package in
981 list context, and with an argument, caller returns more
982 detailed information: it sets the list variable C<@DB::args> to be the
983 arguments with which the subroutine was invoked.
985 Be aware that the optimizer might have optimized call frames away before
986 L<C<caller>|/caller EXPR> had a chance to get the information. That
987 means that C<caller(N)> might not return information about the call
988 frame you expect it to, for C<< N > 1 >>. In particular, C<@DB::args>
989 might have information from the previous time L<C<caller>|/caller EXPR>
992 Be aware that setting C<@DB::args> is I<best effort>, intended for
993 debugging or generating backtraces, and should not be relied upon. In
994 particular, as L<C<@_>|perlvar/@_> contains aliases to the caller's
995 arguments, Perl does not take a copy of L<C<@_>|perlvar/@_>, so
996 C<@DB::args> will contain modifications the subroutine makes to
997 L<C<@_>|perlvar/@_> or its contents, not the original values at call
998 time. C<@DB::args>, like L<C<@_>|perlvar/@_>, does not hold explicit
999 references to its elements, so under certain cases its elements may have
1000 become freed and reallocated for other variables or temporary values.
1001 Finally, a side effect of the current implementation is that the effects
1002 of C<shift @_> can I<normally> be undone (but not C<pop @_> or other
1003 splicing, I<and> not if a reference to L<C<@_>|perlvar/@_> has been
1004 taken, I<and> subject to the caveat about reallocated elements), so
1005 C<@DB::args> is actually a hybrid of the current state and initial state
1006 of L<C<@_>|perlvar/@_>. Buyer beware.
1011 X<directory, change>
1013 =item chdir FILEHANDLE
1015 =item chdir DIRHANDLE
1019 =for Pod::Functions change your current working directory
1021 Changes the working directory to EXPR, if possible. If EXPR is omitted,
1022 changes to the directory specified by C<$ENV{HOME}>, if set; if not,
1023 changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
1024 variable C<$ENV{'SYS$LOGIN'}> is also checked, and used if it is set.) If
1025 neither is set, L<C<chdir>|/chdir EXPR> does nothing and fails. It
1026 returns true on success, false otherwise. See the example under
1027 L<C<die>|/die LIST>.
1029 On systems that support L<fchdir(2)>, you may pass a filehandle or
1030 directory handle as the argument. On systems that don't support L<fchdir(2)>,
1031 passing handles raises an exception.
1034 X<chmod> X<permission> X<mode>
1036 =for Pod::Functions changes the permissions on a list of files
1038 Changes the permissions of a list of files. The first element of the
1039 list must be the numeric mode, which should probably be an octal
1040 number, and which definitely should I<not> be a string of octal digits:
1041 C<0644> is okay, but C<"0644"> is not. Returns the number of files
1042 successfully changed. See also L<C<oct>|/oct EXPR> if all you have is a
1045 my $cnt = chmod 0755, "foo", "bar";
1046 chmod 0755, @executables;
1047 my $mode = "0644"; chmod $mode, "foo"; # !!! sets mode to
1049 my $mode = "0644"; chmod oct($mode), "foo"; # this is better
1050 my $mode = 0644; chmod $mode, "foo"; # this is best
1052 On systems that support L<fchmod(2)>, you may pass filehandles among the
1053 files. On systems that don't support L<fchmod(2)>, passing filehandles raises
1054 an exception. Filehandles must be passed as globs or glob references to be
1055 recognized; barewords are considered filenames.
1057 open(my $fh, "<", "foo");
1058 my $perm = (stat $fh)[2] & 07777;
1059 chmod($perm | 0600, $fh);
1061 You can also import the symbolic C<S_I*> constants from the
1062 L<C<Fcntl>|Fcntl> module:
1064 use Fcntl qw( :mode );
1065 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
1066 # Identical to the chmod 0755 of the example above.
1068 Portability issues: L<perlport/chmod>.
1070 =item chomp VARIABLE
1071 X<chomp> X<INPUT_RECORD_SEPARATOR> X<$/> X<newline> X<eol>
1077 =for Pod::Functions remove a trailing record separator from a string
1079 This safer version of L<C<chop>|/chop VARIABLE> removes any trailing
1080 string that corresponds to the current value of
1081 L<C<$E<sol>>|perlvar/$E<sol>> (also known as C<$INPUT_RECORD_SEPARATOR>
1082 in the L<C<English>|English> module). It returns the total
1083 number of characters removed from all its arguments. It's often used to
1084 remove the newline from the end of an input record when you're worried
1085 that the final record may be missing its newline. When in paragraph
1086 mode (C<$/ = ''>), it removes all trailing newlines from the string.
1087 When in slurp mode (C<$/ = undef>) or fixed-length record mode
1088 (L<C<$E<sol>>|perlvar/$E<sol>> is a reference to an integer or the like;
1089 see L<perlvar>), L<C<chomp>|/chomp VARIABLE> won't remove anything.
1090 If VARIABLE is omitted, it chomps L<C<$_>|perlvar/$_>. Example:
1093 chomp; # avoid \n on last field
1094 my @array = split(/:/);
1098 If VARIABLE is a hash, it chomps the hash's values, but not its keys,
1099 resetting the L<C<each>|/each HASH> iterator in the process.
1101 You can actually chomp anything that's an lvalue, including an assignment:
1103 chomp(my $cwd = `pwd`);
1104 chomp(my $answer = <STDIN>);
1106 If you chomp a list, each element is chomped, and the total number of
1107 characters removed is returned.
1109 Note that parentheses are necessary when you're chomping anything
1110 that is not a simple variable. This is because C<chomp $cwd = `pwd`;>
1111 is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as
1112 C<chomp( $cwd = `pwd` )> which you might expect. Similarly,
1113 C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than
1114 as C<chomp($a, $b)>.
1123 =for Pod::Functions remove the last character from a string
1125 Chops off the last character of a string and returns the character
1126 chopped. It is much more efficient than C<s/.$//s> because it neither
1127 scans nor copies the string. If VARIABLE is omitted, chops
1128 L<C<$_>|perlvar/$_>.
1129 If VARIABLE is a hash, it chops the hash's values, but not its keys,
1130 resetting the L<C<each>|/each HASH> iterator in the process.
1132 You can actually chop anything that's an lvalue, including an assignment.
1134 If you chop a list, each element is chopped. Only the value of the
1135 last L<C<chop>|/chop VARIABLE> is returned.
1137 Note that L<C<chop>|/chop VARIABLE> returns the last character. To
1138 return all but the last character, use C<substr($string, 0, -1)>.
1140 See also L<C<chomp>|/chomp VARIABLE>.
1143 X<chown> X<owner> X<user> X<group>
1145 =for Pod::Functions change the ownership on a list of files
1147 Changes the owner (and group) of a list of files. The first two
1148 elements of the list must be the I<numeric> uid and gid, in that
1149 order. A value of -1 in either position is interpreted by most
1150 systems to leave that value unchanged. Returns the number of files
1151 successfully changed.
1153 my $cnt = chown $uid, $gid, 'foo', 'bar';
1154 chown $uid, $gid, @filenames;
1156 On systems that support L<fchown(2)>, you may pass filehandles among the
1157 files. On systems that don't support L<fchown(2)>, passing filehandles raises
1158 an exception. Filehandles must be passed as globs or glob references to be
1159 recognized; barewords are considered filenames.
1161 Here's an example that looks up nonnumeric uids in the passwd file:
1164 chomp(my $user = <STDIN>);
1166 chomp(my $pattern = <STDIN>);
1168 my ($login,$pass,$uid,$gid) = getpwnam($user)
1169 or die "$user not in passwd file";
1171 my @ary = glob($pattern); # expand filenames
1172 chown $uid, $gid, @ary;
1174 On most systems, you are not allowed to change the ownership of the
1175 file unless you're the superuser, although you should be able to change
1176 the group to any of your secondary groups. On insecure systems, these
1177 restrictions may be relaxed, but this is not a portable assumption.
1178 On POSIX systems, you can detect this condition this way:
1180 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
1181 my $can_chown_giveaway = ! sysconf(_PC_CHOWN_RESTRICTED);
1183 Portability issues: L<perlport/chown>.
1186 X<chr> X<character> X<ASCII> X<Unicode>
1190 =for Pod::Functions get character this number represents
1192 Returns the character represented by that NUMBER in the character set.
1193 For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
1194 chr(0x263a) is a Unicode smiley face.
1196 Negative values give the Unicode replacement character (chr(0xfffd)),
1197 except under the L<bytes> pragma, where the low eight bits of the value
1198 (truncated to an integer) are used.
1200 If NUMBER is omitted, uses L<C<$_>|perlvar/$_>.
1202 For the reverse, use L<C<ord>|/ord EXPR>.
1204 Note that characters from 128 to 255 (inclusive) are by default
1205 internally not encoded as UTF-8 for backward compatibility reasons.
1207 See L<perlunicode> for more about Unicode.
1209 =item chroot FILENAME
1214 =for Pod::Functions make directory new root for path lookups
1216 This function works like the system call by the same name: it makes the
1217 named directory the new root directory for all further pathnames that
1218 begin with a C</> by your process and all its children. (It doesn't
1219 change your current working directory, which is unaffected.) For security
1220 reasons, this call is restricted to the superuser. If FILENAME is
1221 omitted, does a L<C<chroot>|/chroot FILENAME> to L<C<$_>|perlvar/$_>.
1223 B<NOTE:> It is good security practice to do C<chdir("/")>
1224 (L<C<chdir>|/chdir EXPR> to the root directory) immediately after a
1225 L<C<chroot>|/chroot FILENAME>.
1227 Portability issues: L<perlport/chroot>.
1229 =item close FILEHANDLE
1234 =for Pod::Functions close file (or pipe or socket) handle
1236 Closes the file or pipe associated with the filehandle, flushes the IO
1237 buffers, and closes the system file descriptor. Returns true if those
1238 operations succeed and if no error was reported by any PerlIO
1239 layer. Closes the currently selected filehandle if the argument is
1242 You don't have to close FILEHANDLE if you are immediately going to do
1243 another L<C<open>|/open FILEHANDLE,EXPR> on it, because
1244 L<C<open>|/open FILEHANDLE,EXPR> closes it for you. (See
1245 L<C<open>|/open FILEHANDLE,EXPR>.) However, an explicit
1246 L<C<close>|/close FILEHANDLE> on an input file resets the line counter
1247 (L<C<$.>|perlvar/$.>), while the implicit close done by
1248 L<C<open>|/open FILEHANDLE,EXPR> does not.
1250 If the filehandle came from a piped open, L<C<close>|/close FILEHANDLE>
1251 returns false if one of the other syscalls involved fails or if its
1252 program exits with non-zero status. If the only problem was that the
1253 program exited non-zero, L<C<$!>|perlvar/$!> will be set to C<0>.
1254 Closing a pipe also waits for the process executing on the pipe to
1255 exit--in case you wish to look at the output of the pipe afterwards--and
1256 implicitly puts the exit status value of that command into
1257 L<C<$?>|perlvar/$?> and
1258 L<C<${^CHILD_ERROR_NATIVE}>|perlvar/${^CHILD_ERROR_NATIVE}>.
1260 If there are multiple threads running, L<C<close>|/close FILEHANDLE> on
1261 a filehandle from a piped open returns true without waiting for the
1262 child process to terminate, if the filehandle is still open in another
1265 Closing the read end of a pipe before the process writing to it at the
1266 other end is done writing results in the writer receiving a SIGPIPE. If
1267 the other end can't handle that, be sure to read all the data before
1272 open(OUTPUT, '|sort >foo') # pipe to sort
1273 or die "Can't start sort: $!";
1274 #... # print stuff to output
1275 close OUTPUT # wait for sort to finish
1276 or warn $! ? "Error closing sort pipe: $!"
1277 : "Exit status $? from sort";
1278 open(INPUT, 'foo') # get sort's results
1279 or die "Can't open 'foo' for input: $!";
1281 FILEHANDLE may be an expression whose value can be used as an indirect
1282 filehandle, usually the real filehandle name or an autovivified handle.
1284 =item closedir DIRHANDLE
1287 =for Pod::Functions close directory handle
1289 Closes a directory opened by L<C<opendir>|/opendir DIRHANDLE,EXPR> and
1290 returns the success of that system call.
1292 =item connect SOCKET,NAME
1295 =for Pod::Functions connect to a remote socket
1297 Attempts to connect to a remote socket, just like L<connect(2)>.
1298 Returns true if it succeeded, false otherwise. NAME should be a
1299 packed address of the appropriate type for the socket. See the examples in
1300 L<perlipc/"Sockets: Client/Server Communication">.
1302 =item continue BLOCK
1307 =for Pod::Functions optional trailing block in a while or foreach
1309 When followed by a BLOCK, L<C<continue>|/continue BLOCK> is actually a
1310 flow control statement rather than a function. If there is a
1311 L<C<continue>|/continue BLOCK> BLOCK attached to a BLOCK (typically in a
1312 C<while> or C<foreach>), it is always executed just before the
1313 conditional is about to be evaluated again, just like the third part of
1314 a C<for> loop in C. Thus it can be used to increment a loop variable,
1315 even when the loop has been continued via the L<C<next>|/next LABEL>
1316 statement (which is similar to the C L<C<continue>|/continue BLOCK>
1319 L<C<last>|/last LABEL>, L<C<next>|/next LABEL>, or
1320 L<C<redo>|/redo LABEL> may appear within a
1321 L<C<continue>|/continue BLOCK> block; L<C<last>|/last LABEL> and
1322 L<C<redo>|/redo LABEL> behave as if they had been executed within the
1323 main block. So will L<C<next>|/next LABEL>, but since it will execute a
1324 L<C<continue>|/continue BLOCK> block, it may be more entertaining.
1327 ### redo always comes here
1330 ### next always comes here
1332 # then back the top to re-check EXPR
1334 ### last always comes here
1336 Omitting the L<C<continue>|/continue BLOCK> section is equivalent to
1337 using an empty one, logically enough, so L<C<next>|/next LABEL> goes
1338 directly back to check the condition at the top of the loop.
1340 When there is no BLOCK, L<C<continue>|/continue BLOCK> is a function
1341 that falls through the current C<when> or C<default> block instead of
1342 iterating a dynamically enclosing C<foreach> or exiting a lexically
1343 enclosing C<given>. In Perl 5.14 and earlier, this form of
1344 L<C<continue>|/continue BLOCK> was only available when the
1345 L<C<"switch"> feature|feature/The 'switch' feature> was enabled. See
1346 L<feature> and L<perlsyn/"Switch Statements"> for more information.
1349 X<cos> X<cosine> X<acos> X<arccosine>
1353 =for Pod::Functions cosine function
1355 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
1356 takes the cosine of L<C<$_>|perlvar/$_>.
1358 For the inverse cosine operation, you may use the
1359 L<C<Math::Trig::acos>|Math::Trig> function, or use this relation:
1361 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
1363 =item crypt PLAINTEXT,SALT
1364 X<crypt> X<digest> X<hash> X<salt> X<plaintext> X<password>
1365 X<decrypt> X<cryptography> X<passwd> X<encrypt>
1367 =for Pod::Functions one-way passwd-style encryption
1369 Creates a digest string exactly like the L<crypt(3)> function in the C
1370 library (assuming that you actually have a version there that has not
1371 been extirpated as a potential munition).
1373 L<C<crypt>|/crypt PLAINTEXT,SALT> is a one-way hash function. The
1374 PLAINTEXT and SALT are turned
1375 into a short string, called a digest, which is returned. The same
1376 PLAINTEXT and SALT will always return the same string, but there is no
1377 (known) way to get the original PLAINTEXT from the hash. Small
1378 changes in the PLAINTEXT or SALT will result in large changes in the
1381 There is no decrypt function. This function isn't all that useful for
1382 cryptography (for that, look for F<Crypt> modules on your nearby CPAN
1383 mirror) and the name "crypt" is a bit of a misnomer. Instead it is
1384 primarily used to check if two pieces of text are the same without
1385 having to transmit or store the text itself. An example is checking
1386 if a correct password is given. The digest of the password is stored,
1387 not the password itself. The user types in a password that is
1388 L<C<crypt>|/crypt PLAINTEXT,SALT>'d with the same salt as the stored
1389 digest. If the two digests match, the password is correct.
1391 When verifying an existing digest string you should use the digest as
1392 the salt (like C<crypt($plain, $digest) eq $digest>). The SALT used
1393 to create the digest is visible as part of the digest. This ensures
1394 L<C<crypt>|/crypt PLAINTEXT,SALT> will hash the new string with the same
1395 salt as the digest. This allows your code to work with the standard
1396 L<C<crypt>|/crypt PLAINTEXT,SALT> and with more exotic implementations.
1397 In other words, assume nothing about the returned string itself nor
1398 about how many bytes of SALT may matter.
1400 Traditionally the result is a string of 13 bytes: two first bytes of
1401 the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
1402 the first eight bytes of PLAINTEXT mattered. But alternative
1403 hashing schemes (like MD5), higher level security schemes (like C2),
1404 and implementations on non-Unix platforms may produce different
1407 When choosing a new salt create a random two character string whose
1408 characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
1409 '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of
1410 characters is just a recommendation; the characters allowed in
1411 the salt depend solely on your system's crypt library, and Perl can't
1412 restrict what salts L<C<crypt>|/crypt PLAINTEXT,SALT> accepts.
1414 Here's an example that makes sure that whoever runs this program knows
1417 my $pwd = (getpwuid($<))[1];
1419 system "stty -echo";
1421 chomp(my $word = <STDIN>);
1425 if (crypt($word, $pwd) ne $pwd) {
1431 Of course, typing in your own password to whoever asks you
1434 The L<C<crypt>|/crypt PLAINTEXT,SALT> function is unsuitable for hashing
1435 large quantities of data, not least of all because you can't get the
1436 information back. Look at the L<Digest> module for more robust
1439 If using L<C<crypt>|/crypt PLAINTEXT,SALT> on a Unicode string (which
1440 I<potentially> has characters with codepoints above 255), Perl tries to
1441 make sense of the situation by trying to downgrade (a copy of) the
1442 string back to an eight-bit byte string before calling
1443 L<C<crypt>|/crypt PLAINTEXT,SALT> (on that copy). If that works, good.
1444 If not, L<C<crypt>|/crypt PLAINTEXT,SALT> dies with
1445 L<C<Wide character in crypt>|perldiag/Wide character in %s>.
1447 Portability issues: L<perlport/crypt>.
1452 =for Pod::Functions breaks binding on a tied dbm file
1454 [This function has been largely superseded by the
1455 L<C<untie>|/untie VARIABLE> function.]
1457 Breaks the binding between a DBM file and a hash.
1459 Portability issues: L<perlport/dbmclose>.
1461 =item dbmopen HASH,DBNAME,MASK
1462 X<dbmopen> X<dbm> X<ndbm> X<sdbm> X<gdbm>
1464 =for Pod::Functions create binding on a tied dbm file
1466 [This function has been largely superseded by the
1467 L<C<tie>|/tie VARIABLE,CLASSNAME,LIST> function.]
1469 This binds a L<dbm(3)>, L<ndbm(3)>, L<sdbm(3)>, L<gdbm(3)>, or Berkeley
1470 DB file to a hash. HASH is the name of the hash. (Unlike normal
1471 L<C<open>|/open FILEHANDLE,EXPR>, the first argument is I<not> a
1472 filehandle, even though it looks like one). DBNAME is the name of the
1473 database (without the F<.dir> or F<.pag> extension if any). If the
1474 database does not exist, it is created with protection specified by MASK
1475 (as modified by the L<C<umask>|/umask EXPR>). To prevent creation of
1476 the database if it doesn't exist, you may specify a MODE of 0, and the
1477 function will return a false value if it can't find an existing
1478 database. If your system supports only the older DBM functions, you may
1479 make only one L<C<dbmopen>|/dbmopen HASH,DBNAME,MASK> call in your
1480 program. In older versions of Perl, if your system had neither DBM nor
1481 ndbm, calling L<C<dbmopen>|/dbmopen HASH,DBNAME,MASK> produced a fatal
1482 error; it now falls back to L<sdbm(3)>.
1484 If you don't have write access to the DBM file, you can only read hash
1485 variables, not set them. If you want to test whether you can write,
1486 either use file tests or try setting a dummy hash entry inside an
1487 L<C<eval>|/eval EXPR> to trap the error.
1489 Note that functions such as L<C<keys>|/keys HASH> and
1490 L<C<values>|/values HASH> may return huge lists when used on large DBM
1491 files. You may prefer to use the L<C<each>|/each HASH> function to
1492 iterate over large DBM files. Example:
1494 # print out history file offsets
1495 dbmopen(%HIST,'/usr/lib/news/history',0666);
1496 while (($key,$val) = each %HIST) {
1497 print $key, ' = ', unpack('L',$val), "\n";
1501 See also L<AnyDBM_File> for a more general description of the pros and
1502 cons of the various dbm approaches, as well as L<DB_File> for a particularly
1503 rich implementation.
1505 You can control which DBM library you use by loading that library
1506 before you call L<C<dbmopen>|/dbmopen HASH,DBNAME,MASK>:
1509 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
1510 or die "Can't open netscape history file: $!";
1512 Portability issues: L<perlport/dbmopen>.
1515 X<defined> X<undef> X<undefined>
1519 =for Pod::Functions test whether a value, variable, or function is defined
1521 Returns a Boolean value telling whether EXPR has a value other than the
1522 undefined value L<C<undef>|/undef EXPR>. If EXPR is not present,
1523 L<C<$_>|perlvar/$_> is checked.
1525 Many operations return L<C<undef>|/undef EXPR> to indicate failure, end
1526 of file, system error, uninitialized variable, and other exceptional
1527 conditions. This function allows you to distinguish
1528 L<C<undef>|/undef EXPR> from other values. (A simple Boolean test will
1529 not distinguish among L<C<undef>|/undef EXPR>, zero, the empty string,
1530 and C<"0">, which are all equally false.) Note that since
1531 L<C<undef>|/undef EXPR> is a valid scalar, its presence doesn't
1532 I<necessarily> indicate an exceptional condition: L<C<pop>|/pop ARRAY>
1533 returns L<C<undef>|/undef EXPR> when its argument is an empty array,
1534 I<or> when the element to return happens to be L<C<undef>|/undef EXPR>.
1536 You may also use C<defined(&func)> to check whether subroutine C<func>
1537 has ever been defined. The return value is unaffected by any forward
1538 declarations of C<func>. A subroutine that is not defined
1539 may still be callable: its package may have an C<AUTOLOAD> method that
1540 makes it spring into existence the first time that it is called; see
1543 Use of L<C<defined>|/defined EXPR> on aggregates (hashes and arrays) is
1544 no longer supported. It used to report whether memory for that
1545 aggregate had ever been allocated. You should instead use a simple
1548 if (@an_array) { print "has array elements\n" }
1549 if (%a_hash) { print "has hash members\n" }
1551 When used on a hash element, it tells you whether the value is defined,
1552 not whether the key exists in the hash. Use L<C<exists>|/exists EXPR>
1553 for the latter purpose.
1557 print if defined $switch{D};
1558 print "$val\n" while defined($val = pop(@ary));
1559 die "Can't readlink $sym: $!"
1560 unless defined($value = readlink $sym);
1561 sub foo { defined &$bar ? $bar->(@_) : die "No bar"; }
1562 $debugging = 0 unless defined $debugging;
1564 Note: Many folks tend to overuse L<C<defined>|/defined EXPR> and are
1565 then surprised to discover that the number C<0> and C<""> (the
1566 zero-length string) are, in fact, defined values. For example, if you
1571 The pattern match succeeds and C<$1> is defined, although it
1572 matched "nothing". It didn't really fail to match anything. Rather, it
1573 matched something that happened to be zero characters long. This is all
1574 very above-board and honest. When a function returns an undefined value,
1575 it's an admission that it couldn't give you an honest answer. So you
1576 should use L<C<defined>|/defined EXPR> only when questioning the
1577 integrity of what you're trying to do. At other times, a simple
1578 comparison to C<0> or C<""> is what you want.
1580 See also L<C<undef>|/undef EXPR>, L<C<exists>|/exists EXPR>,
1581 L<C<ref>|/ref EXPR>.
1586 =for Pod::Functions deletes a value from a hash
1588 Given an expression that specifies an element or slice of a hash,
1589 L<C<delete>|/delete EXPR> deletes the specified elements from that hash
1590 so that L<C<exists>|/exists EXPR> on that element no longer returns
1591 true. Setting a hash element to the undefined value does not remove its
1592 key, but deleting it does; see L<C<exists>|/exists EXPR>.
1594 In list context, returns the value or values deleted, or the last such
1595 element in scalar context. The return list's length always matches that of
1596 the argument list: deleting non-existent elements returns the undefined value
1597 in their corresponding positions.
1599 L<C<delete>|/delete EXPR> may also be used on arrays and array slices,
1600 but its behavior is less straightforward. Although
1601 L<C<exists>|/exists EXPR> will return false for deleted entries,
1602 deleting array elements never changes indices of existing values; use
1603 L<C<shift>|/shift ARRAY> or L<C<splice>|/splice
1604 ARRAY,OFFSET,LENGTH,LIST> for that. However, if any deleted elements
1605 fall at the end of an array, the array's size shrinks to the position of
1606 the highest element that still tests true for L<C<exists>|/exists EXPR>,
1607 or to 0 if none do. In other words, an array won't have trailing
1608 nonexistent elements after a delete.
1610 B<WARNING:> Calling L<C<delete>|/delete EXPR> on array values is
1611 strongly discouraged. The
1612 notion of deleting or checking the existence of Perl array elements is not
1613 conceptually coherent, and can lead to surprising behavior.
1615 Deleting from L<C<%ENV>|perlvar/%ENV> modifies the environment.
1616 Deleting from a hash tied to a DBM file deletes the entry from the DBM
1617 file. Deleting from a L<C<tied>|/tied VARIABLE> hash or array may not
1618 necessarily return anything; it depends on the implementation of the
1619 L<C<tied>|/tied VARIABLE> package's DELETE method, which may do whatever
1622 The C<delete local EXPR> construct localizes the deletion to the current
1623 block at run time. Until the block exits, elements locally deleted
1624 temporarily no longer exist. See L<perlsub/"Localized deletion of elements
1625 of composite types">.
1627 my %hash = (foo => 11, bar => 22, baz => 33);
1628 my $scalar = delete $hash{foo}; # $scalar is 11
1629 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
1630 my @array = delete @hash{qw(foo baz)}; # @array is (undef,33)
1632 The following (inefficiently) deletes all the values of %HASH and @ARRAY:
1634 foreach my $key (keys %HASH) {
1638 foreach my $index (0 .. $#ARRAY) {
1639 delete $ARRAY[$index];
1644 delete @HASH{keys %HASH};
1646 delete @ARRAY[0 .. $#ARRAY];
1648 But both are slower than assigning the empty list
1649 or undefining %HASH or @ARRAY, which is the customary
1650 way to empty out an aggregate:
1652 %HASH = (); # completely empty %HASH
1653 undef %HASH; # forget %HASH ever existed
1655 @ARRAY = (); # completely empty @ARRAY
1656 undef @ARRAY; # forget @ARRAY ever existed
1658 The EXPR can be arbitrarily complicated provided its
1659 final operation is an element or slice of an aggregate:
1661 delete $ref->[$x][$y]{$key};
1662 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
1664 delete $ref->[$x][$y][$index];
1665 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1668 X<die> X<throw> X<exception> X<raise> X<$@> X<abort>
1670 =for Pod::Functions raise an exception or bail out
1672 L<C<die>|/die LIST> raises an exception. Inside an
1673 L<C<eval>|/eval EXPR> the error message is stuffed into
1674 L<C<$@>|perlvar/$@> and the L<C<eval>|/eval EXPR> is terminated with the
1675 undefined value. If the exception is outside of all enclosing
1676 L<C<eval>|/eval EXPR>s, then the uncaught exception prints LIST to
1677 C<STDERR> and exits with a non-zero value. If you need to exit the
1678 process with a specific exit code, see L<C<exit>|/exit EXPR>.
1680 Equivalent examples:
1682 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
1683 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
1685 If the last element of LIST does not end in a newline, the current
1686 script line number and input line number (if any) are also printed,
1687 and a newline is supplied. Note that the "input line number" (also
1688 known as "chunk") is subject to whatever notion of "line" happens to
1689 be currently in effect, and is also available as the special variable
1690 L<C<$.>|perlvar/$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1692 Hint: sometimes appending C<", stopped"> to your message will cause it
1693 to make better sense when the string C<"at foo line 123"> is appended.
1694 Suppose you are running script "canasta".
1696 die "/etc/games is no good";
1697 die "/etc/games is no good, stopped";
1699 produce, respectively
1701 /etc/games is no good at canasta line 123.
1702 /etc/games is no good, stopped at canasta line 123.
1704 If the output is empty and L<C<$@>|perlvar/$@> already contains a value
1705 (typically from a previous L<C<eval>|/eval EXPR>) that value is reused after
1706 appending C<"\t...propagated">. This is useful for propagating exceptions:
1709 die unless $@ =~ /Expected exception/;
1711 If the output is empty and L<C<$@>|perlvar/$@> contains an object
1712 reference that has a C<PROPAGATE> method, that method will be called
1713 with additional file and line number parameters. The return value
1714 replaces the value in L<C<$@>|perlvar/$@>; i.e., as if
1715 C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >> were called.
1717 If L<C<$@>|perlvar/$@> is empty, then the string C<"Died"> is used.
1719 If an uncaught exception results in interpreter exit, the exit code is
1720 determined from the values of L<C<$!>|perlvar/$!> and
1721 L<C<$?>|perlvar/$?> with this pseudocode:
1723 exit $! if $!; # errno
1724 exit $? >> 8 if $? >> 8; # child exit status
1725 exit 255; # last resort
1727 As with L<C<exit>|/exit EXPR>, L<C<$?>|perlvar/$?> is set prior to
1728 unwinding the call stack; any C<DESTROY> or C<END> handlers can then
1729 alter this value, and thus Perl's exit code.
1731 The intent is to squeeze as much possible information about the likely cause
1732 into the limited space of the system exit code. However, as
1733 L<C<$!>|perlvar/$!> is the value of C's C<errno>, which can be set by
1734 any system call, this means that the value of the exit code used by
1735 L<C<die>|/die LIST> can be non-predictable, so should not be relied
1736 upon, other than to be non-zero.
1738 You can also call L<C<die>|/die LIST> with a reference argument, and if
1739 this is trapped within an L<C<eval>|/eval EXPR>, L<C<$@>|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/$@> with regular expressions. Because
1744 L<C<$@>|perlvar/$@> is a global variable and L<C<eval>|/eval EXPR> may
1745 be used within object implementations, be careful that analyzing the
1746 error object doesn't replace the reference in the global variable. It's
1747 easiest to make a local copy of the reference before any manipulations.
1750 use Scalar::Util "blessed";
1752 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1753 if (my $ev_err = $@) {
1754 if (blessed($ev_err)
1755 && $ev_err->isa("Some::Module::Exception")) {
1756 # handle Some::Module::Exception
1759 # handle all other possible exceptions
1763 Because Perl stringifies uncaught exception messages before display,
1764 you'll probably want to overload stringification operations on
1765 exception objects. See L<overload> for details about that.
1767 You can arrange for a callback to be run just before the
1768 L<C<die>|/die LIST> does its deed, by setting the
1769 L<C<$SIG{__DIE__}>|perlvar/%SIG> hook. The associated handler is called
1770 with the error text and can change the error message, if it sees fit, by
1771 calling L<C<die>|/die LIST> again. See L<perlvar/%SIG> for details on
1772 setting L<C<%SIG>|perlvar/%SIG> entries, and L<C<eval>|/eval EXPR> for some
1773 examples. Although this feature was to be run only right before your
1774 program was to exit, this is not currently so: the
1775 L<C<$SIG{__DIE__}>|perlvar/%SIG> hook is currently called even inside
1776 L<C<eval>|/eval EXPR>ed blocks/strings! If one wants the hook to do
1777 nothing in such situations, put
1781 as the first line of the handler (see L<perlvar/$^S>). Because
1782 this promotes strange action at a distance, this counterintuitive
1783 behavior may be fixed in a future release.
1785 See also L<C<exit>|/exit EXPR>, L<C<warn>|/warn LIST>, and the L<Carp>
1791 =for Pod::Functions turn a BLOCK into a TERM
1793 Not really a function. Returns the value of the last command in the
1794 sequence of commands indicated by BLOCK. When modified by the C<while> or
1795 C<until> loop modifier, executes the BLOCK once before testing the loop
1796 condition. (On other statements the loop modifiers test the conditional
1799 C<do BLOCK> does I<not> count as a loop, so the loop control statements
1800 L<C<next>|/next LABEL>, L<C<last>|/last LABEL>, or
1801 L<C<redo>|/redo LABEL> cannot be used to leave or restart the block.
1802 See L<perlsyn> for alternative strategies.
1807 Uses the value of EXPR as a filename and executes the contents of the
1808 file as a Perl script.
1816 except that it's more concise, runs no external processes, keeps track of
1817 the current filename for error messages, searches the
1818 L<C<@INC>|perlvar/@INC> directories, and updates L<C<%INC>|perlvar/%INC>
1819 if the file is found. See L<perlvar/@INC> and L<perlvar/%INC> for these
1820 variables. It also differs in that code evaluated with C<do FILE>
1821 cannot see lexicals in the enclosing scope; C<eval STRING> does. It's
1822 the same, however, in that it does reparse the file every time you call
1823 it, so you probably don't want to do this inside a loop.
1825 If L<C<do>|/do EXPR> can read the file but cannot compile it, it
1826 returns L<C<undef>|/undef EXPR> and sets an error message in
1827 L<C<$@>|perlvar/$@>. If L<C<do>|/do EXPR> cannot read the file, it
1828 returns undef and sets L<C<$!>|perlvar/$!> to the error. Always check
1829 L<C<$@>|perlvar/$@> first, as compilation could fail in a way that also
1830 sets L<C<$!>|perlvar/$!>. If the file is successfully compiled,
1831 L<C<do>|/do EXPR> returns the value of the last expression evaluated.
1833 Inclusion of library modules is better done with the
1834 L<C<use>|/use Module VERSION LIST> and L<C<require>|/require VERSION>
1835 operators, which also do automatic error checking and raise an exception
1836 if there's a problem.
1838 You might like to use L<C<do>|/do EXPR> to read in a program
1839 configuration file. Manual error checking can be done this way:
1841 # read in config files: system first, then user
1842 for $file ("/share/prog/defaults.rc",
1843 "$ENV{HOME}/.someprogrc")
1845 unless ($return = do $file) {
1846 warn "couldn't parse $file: $@" if $@;
1847 warn "couldn't do $file: $!" unless defined $return;
1848 warn "couldn't run $file" unless $return;
1853 X<dump> X<core> X<undump>
1859 =for Pod::Functions create an immediate core dump
1861 This function causes an immediate core dump. See also the B<-u>
1862 command-line switch in L<perlrun>, which does the same thing.
1863 Primarily this is so that you can use the B<undump> program (not
1864 supplied) to turn your core dump into an executable binary after
1865 having initialized all your variables at the beginning of the
1866 program. When the new binary is executed it will begin by executing
1867 a C<goto LABEL> (with all the restrictions that L<C<goto>|/goto LABEL>
1869 Think of it as a goto with an intervening core dump and reincarnation.
1870 If C<LABEL> is omitted, restarts the program from the top. The
1871 C<dump EXPR> form, available starting in Perl 5.18.0, allows a name to be
1872 computed at run time, being otherwise identical to C<dump LABEL>.
1874 B<WARNING>: Any files opened at the time of the dump will I<not>
1875 be open any more when the program is reincarnated, with possible
1876 resulting confusion by Perl.
1878 This function is now largely obsolete, mostly because it's very hard to
1879 convert a core file into an executable. That's why you should now invoke
1880 it as C<CORE::dump()> if you don't want to be warned against a possible
1883 Unlike most named operators, this has the same precedence as assignment.
1884 It is also exempt from the looks-like-a-function rule, so
1885 C<dump ("foo")."bar"> will cause "bar" to be part of the argument to
1886 L<C<dump>|/dump LABEL>.
1888 Portability issues: L<perlport/dump>.
1891 X<each> X<hash, iterator>
1896 =for Pod::Functions retrieve the next key/value pair from a hash
1898 When called on a hash in list context, returns a 2-element list
1899 consisting of the key and value for the next element of a hash. In Perl
1900 5.12 and later only, it will also return the index and value for the next
1901 element of an array so that you can iterate over it; older Perls consider
1902 this a syntax error. When called in scalar context, returns only the key
1903 (not the value) in a hash, or the index in an array.
1905 Hash entries are returned in an apparently random order. The actual random
1906 order is specific to a given hash; the exact same series of operations
1907 on two hashes may result in a different order for each hash. Any insertion
1908 into the hash may change the order, as will any deletion, with the exception
1909 that the most recent key returned by L<C<each>|/each HASH> or
1910 L<C<keys>|/keys HASH> may be deleted without changing the order. So
1911 long as a given hash is unmodified you may rely on
1912 L<C<keys>|/keys HASH>, L<C<values>|/values HASH> and
1913 L<C<each>|/each HASH> to repeatedly return the same order
1914 as each other. See L<perlsec/"Algorithmic Complexity Attacks"> for
1915 details on why hash order is randomized. Aside from the guarantees
1916 provided here the exact details of Perl's hash algorithm and the hash
1917 traversal order are subject to change in any release of Perl.
1919 After L<C<each>|/each HASH> has returned all entries from the hash or
1920 array, the next call to L<C<each>|/each HASH> returns the empty list in
1921 list context and L<C<undef>|/undef EXPR> in scalar context; the next
1922 call following I<that> one restarts iteration. Each hash or array has
1923 its own internal iterator, accessed by L<C<each>|/each HASH>,
1924 L<C<keys>|/keys HASH>, and L<C<values>|/values HASH>. The iterator is
1925 implicitly reset when L<C<each>|/each HASH> has reached the end as just
1926 described; it can be explicitly reset by calling L<C<keys>|/keys HASH>
1927 or L<C<values>|/values HASH> on the hash or array. If you add or delete
1928 a hash's elements while iterating over it, the effect on the iterator is
1929 unspecified; for example, entries may be skipped or duplicated--so don't
1930 do that. Exception: It is always safe to delete the item most recently
1931 returned by L<C<each>|/each HASH>, so the following code works properly:
1933 while (my ($key, $value) = each %hash) {
1935 delete $hash{$key}; # This is safe
1938 Tied hashes may have a different ordering behaviour to perl's hash
1941 This prints out your environment like the L<printenv(1)> program,
1942 but in a different order:
1944 while (my ($key,$value) = each %ENV) {
1945 print "$key=$value\n";
1948 Starting with Perl 5.14, an experimental feature allowed
1949 L<C<each>|/each HASH> to take a scalar expression. This experiment has
1950 been deemed unsuccessful, and was removed as of Perl 5.24.
1952 As of Perl 5.18 you can use a bare L<C<each>|/each HASH> in a C<while>
1953 loop, which will set L<C<$_>|perlvar/$_> on every iteration.
1956 print "$_=$ENV{$_}\n";
1959 To avoid confusing would-be users of your code who are running earlier
1960 versions of Perl with mysterious syntax errors, put this sort of thing at
1961 the top of your file to signal that your code will work I<only> on Perls of
1964 use 5.012; # so keys/values/each work on arrays
1965 use 5.018; # so each assigns to $_ in a lone while test
1967 See also L<C<keys>|/keys HASH>, L<C<values>|/values HASH>, and
1968 L<C<sort>|/sort SUBNAME LIST>.
1970 =item eof FILEHANDLE
1979 =for Pod::Functions test a filehandle for its end
1981 Returns 1 if the next read on FILEHANDLE will return end of file I<or> if
1982 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1983 gives the real filehandle. (Note that this function actually
1984 reads a character and then C<ungetc>s it, so isn't useful in an
1985 interactive context.) Do not read from a terminal file (or call
1986 C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
1987 as terminals may lose the end-of-file condition if you do.
1989 An L<C<eof>|/eof FILEHANDLE> without an argument uses the last file
1990 read. Using L<C<eof()>|/eof FILEHANDLE> with empty parentheses is
1991 different. It refers to the pseudo file formed from the files listed on
1992 the command line and accessed via the C<< <> >> operator. Since
1993 C<< <> >> isn't explicitly opened, as a normal filehandle is, an
1994 L<C<eof()>|/eof FILEHANDLE> before C<< <> >> has been used will cause
1995 L<C<@ARGV>|perlvar/@ARGV> to be examined to determine if input is
1996 available. Similarly, an L<C<eof()>|/eof FILEHANDLE> after C<< <> >>
1997 has returned end-of-file will assume you are processing another
1998 L<C<@ARGV>|perlvar/@ARGV> list, and if you haven't set
1999 L<C<@ARGV>|perlvar/@ARGV>, will read input from C<STDIN>; see
2000 L<perlop/"I/O Operators">.
2002 In a C<< while (<>) >> loop, L<C<eof>|/eof FILEHANDLE> or C<eof(ARGV)>
2003 can be used to detect the end of each file, whereas
2004 L<C<eof()>|/eof FILEHANDLE> will detect the end of the very last file
2007 # reset line numbering on each input file
2009 next if /^\s*#/; # skip comments
2012 close ARGV if eof; # Not eof()!
2015 # insert dashes just before last line of last file
2017 if (eof()) { # check for end of last file
2018 print "--------------\n";
2021 last if eof(); # needed if we're reading from a terminal
2024 Practical hint: you almost never need to use L<C<eof>|/eof FILEHANDLE>
2025 in Perl, because the input operators typically return L<C<undef>|/undef
2026 EXPR> when they run out of data or encounter an error.
2029 X<eval> X<try> X<catch> X<evaluate> X<parse> X<execute>
2030 X<error, handling> X<exception, handling>
2036 =for Pod::Functions catch exceptions or compile and run code
2038 In the first form, often referred to as a "string eval", the return
2039 value of EXPR is parsed and executed as if it
2040 were a little Perl program. The value of the expression (which is itself
2041 determined within scalar context) is first parsed, and if there were no
2042 errors, executed as a block within the lexical context of the current Perl
2043 program. This means, that in particular, any outer lexical variables are
2044 visible to it, and any package variable settings or subroutine and format
2045 definitions remain afterwards.
2047 Note that the value is parsed every time the L<C<eval>|/eval EXPR>
2048 executes. If EXPR is omitted, evaluates L<C<$_>|perlvar/$_>. This form
2049 is typically used to delay parsing and subsequent execution of the text
2050 of EXPR until run time.
2053 L<C<"unicode_eval"> feature|feature/The 'unicode_eval' and 'evalbytes' features>
2054 is enabled (which is the default under a
2055 C<use 5.16> or higher declaration), EXPR or L<C<$_>|perlvar/$_> is
2056 treated as a string of characters, so L<C<use utf8>|utf8> declarations
2057 have no effect, and source filters are forbidden. In the absence of the
2058 L<C<"unicode_eval"> feature|feature/The 'unicode_eval' and 'evalbytes' features>,
2059 will sometimes be treated as characters and sometimes as bytes,
2060 depending on the internal encoding, and source filters activated within
2061 the L<C<eval>|/eval EXPR> exhibit the erratic, but historical, behaviour
2062 of affecting some outer file scope that is still compiling. See also
2063 the L<C<evalbytes>|/evalbytes EXPR> operator, which always treats its
2064 input as a byte stream and works properly with source filters, and the
2067 Problems can arise if the string expands a scalar containing a floating
2068 point number. That scalar can expand to letters, such as C<"NaN"> or
2069 C<"Infinity">; or, within the scope of a L<C<use locale>|locale>, the
2070 decimal point character may be something other than a dot (such as a
2071 comma). None of these are likely to parse as you are likely expecting.
2073 In the second form, the code within the BLOCK is parsed only once--at the
2074 same time the code surrounding the L<C<eval>|/eval EXPR> itself was
2075 parsed--and executed
2076 within the context of the current Perl program. This form is typically
2077 used to trap exceptions more efficiently than the first (see below), while
2078 also providing the benefit of checking the code within BLOCK at compile
2081 The final semicolon, if any, may be omitted from the value of EXPR or within
2084 In both forms, the value returned is the value of the last expression
2085 evaluated inside the mini-program; a return statement may be also used, just
2086 as with subroutines. The expression providing the return value is evaluated
2087 in void, scalar, or list context, depending on the context of the
2088 L<C<eval>|/eval EXPR> itself. See L<C<wantarray>|/wantarray> for more
2089 on how the evaluation context can be determined.
2091 If there is a syntax error or runtime error, or a L<C<die>|/die LIST>
2092 statement is executed, L<C<eval>|/eval EXPR> returns
2093 L<C<undef>|/undef EXPR> in scalar context or an empty list in list
2094 context, and L<C<$@>|perlvar/$@> is set to the error message. (Prior to
2095 5.16, a bug caused L<C<undef>|/undef EXPR> to be returned in list
2096 context for syntax errors, but not for runtime errors.) If there was no
2097 error, L<C<$@>|perlvar/$@> is set to the empty string. A control flow
2098 operator like L<C<last>|/last LABEL> or L<C<goto>|/goto LABEL> can
2099 bypass the setting of L<C<$@>|perlvar/$@>. Beware that using
2100 L<C<eval>|/eval EXPR> neither silences Perl from printing warnings to
2101 STDERR, nor does it stuff the text of warning messages into
2102 L<C<$@>|perlvar/$@>. To do either of those, you have to use the
2103 L<C<$SIG{__WARN__}>|perlvar/%SIG> facility, or turn off warnings inside
2104 the BLOCK or EXPR using S<C<no warnings 'all'>>. See
2105 L<C<warn>|/warn LIST>, L<perlvar>, and L<warnings>.
2107 Note that, because L<C<eval>|/eval EXPR> traps otherwise-fatal errors,
2108 it is useful for determining whether a particular feature (such as
2109 L<C<socket>|/socket SOCKET,DOMAIN,TYPE,PROTOCOL> or
2110 L<C<symlink>|/symlink OLDFILE,NEWFILE>) is implemented. It is also
2111 Perl's exception-trapping mechanism, where the L<C<die>|/die LIST>
2112 operator is used to raise exceptions.
2114 If you want to trap errors when loading an XS module, some problems with
2115 the binary interface (such as Perl version skew) may be fatal even with
2116 L<C<eval>|/eval EXPR> unless C<$ENV{PERL_DL_NONLAZY}> is set. See
2119 If the code to be executed doesn't vary, you may use the eval-BLOCK
2120 form to trap run-time errors without incurring the penalty of
2121 recompiling each time. The error, if any, is still returned in
2122 L<C<$@>|perlvar/$@>.
2125 # make divide-by-zero nonfatal
2126 eval { $answer = $a / $b; }; warn $@ if $@;
2128 # same thing, but less efficient
2129 eval '$answer = $a / $b'; warn $@ if $@;
2131 # a compile-time error
2132 eval { $answer = }; # WRONG
2135 eval '$answer ='; # sets $@
2137 Using the C<eval {}> form as an exception trap in libraries does have some
2138 issues. Due to the current arguably broken state of C<__DIE__> hooks, you
2139 may wish not to trigger any C<__DIE__> hooks that user code may have installed.
2140 You can use the C<local $SIG{__DIE__}> construct for this purpose,
2141 as this example shows:
2143 # a private exception trap for divide-by-zero
2144 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
2147 This is especially significant, given that C<__DIE__> hooks can call
2148 L<C<die>|/die LIST> again, which has the effect of changing their error
2151 # __DIE__ hooks may modify error messages
2153 local $SIG{'__DIE__'} =
2154 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
2155 eval { die "foo lives here" };
2156 print $@ if $@; # prints "bar lives here"
2159 Because this promotes action at a distance, this counterintuitive behavior
2160 may be fixed in a future release.
2162 With an L<C<eval>|/eval EXPR>, you should be especially careful to
2163 remember what's being looked at when:
2169 eval { $x }; # CASE 4
2171 eval "\$$x++"; # CASE 5
2174 Cases 1 and 2 above behave identically: they run the code contained in
2175 the variable $x. (Although case 2 has misleading double quotes making
2176 the reader wonder what else might be happening (nothing is).) Cases 3
2177 and 4 likewise behave in the same way: they run the code C<'$x'>, which
2178 does nothing but return the value of $x. (Case 4 is preferred for
2179 purely visual reasons, but it also has the advantage of compiling at
2180 compile-time instead of at run-time.) Case 5 is a place where
2181 normally you I<would> like to use double quotes, except that in this
2182 particular situation, you can just use symbolic references instead, as
2185 Before Perl 5.14, the assignment to L<C<$@>|perlvar/$@> occurred before
2187 of localized variables, which means that for your code to run on older
2188 versions, a temporary is required if you want to mask some but not all
2191 # alter $@ on nefarious repugnancy only
2195 local $@; # protect existing $@
2196 eval { test_repugnancy() };
2197 # $@ =~ /nefarious/ and die $@; # Perl 5.14 and higher only
2198 $@ =~ /nefarious/ and $e = $@;
2200 die $e if defined $e
2203 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
2204 L<C<next>|/next LABEL>, L<C<last>|/last LABEL>, or
2205 L<C<redo>|/redo LABEL> cannot be used to leave or restart the block.
2207 An C<eval ''> executed within a subroutine defined
2208 in the C<DB> package doesn't see the usual
2209 surrounding lexical scope, but rather the scope of the first non-DB piece
2210 of code that called it. You don't normally need to worry about this unless
2211 you are writing a Perl debugger.
2213 =item evalbytes EXPR
2218 =for Pod::Functions +evalbytes similar to string eval, but intend to parse a bytestream
2220 This function is like L<C<eval>|/eval EXPR> with a string argument,
2221 except it always parses its argument, or L<C<$_>|perlvar/$_> if EXPR is
2222 omitted, as a string of bytes. A string containing characters whose
2223 ordinal value exceeds 255 results in an error. Source filters activated
2224 within the evaluated code apply to the code itself.
2226 L<C<evalbytes>|/evalbytes EXPR> is available only if the
2227 L<C<"evalbytes"> feature|feature/The 'unicode_eval' and 'evalbytes' features>
2228 is enabled or if it is prefixed with C<CORE::>. The
2229 L<C<"evalbytes"> feature|feature/The 'unicode_eval' and 'evalbytes' features>
2230 is enabled automatically with a C<use v5.16> (or higher) declaration in
2236 =item exec PROGRAM LIST
2238 =for Pod::Functions abandon this program to run another
2240 The L<C<exec>|/exec LIST> function executes a system command I<and never
2241 returns>; use L<C<system>|/system LIST> instead of L<C<exec>|/exec LIST>
2242 if you want it to return. It fails and
2243 returns false only if the command does not exist I<and> it is executed
2244 directly instead of via your system's command shell (see below).
2246 Since it's a common mistake to use L<C<exec>|/exec LIST> instead of
2247 L<C<system>|/system LIST>, Perl warns you if L<C<exec>|/exec LIST> is
2248 called in void context and if there is a following statement that isn't
2249 L<C<die>|/die LIST>, L<C<warn>|/warn LIST>, or L<C<exit>|/exit EXPR> (if
2250 L<warnings> are enabled--but you always do that, right?). If you
2251 I<really> want to follow an L<C<exec>|/exec LIST> with some other
2252 statement, you can use one of these styles to avoid the warning:
2254 exec ('foo') or print STDERR "couldn't exec foo: $!";
2255 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
2257 If there is more than one argument in LIST, this calls L<execvp(3)> with the
2258 arguments in LIST. If there is only one element in LIST, the argument is
2259 checked for shell metacharacters, and if there are any, the entire
2260 argument is passed to the system's command shell for parsing (this is
2261 C</bin/sh -c> on Unix platforms, but varies on other platforms). If
2262 there are no shell metacharacters in the argument, it is split into words
2263 and passed directly to C<execvp>, which is more efficient. Examples:
2265 exec '/bin/echo', 'Your arguments are: ', @ARGV;
2266 exec "sort $outfile | uniq";
2268 If you don't really want to execute the first argument, but want to lie
2269 to the program you are executing about its own name, you can specify
2270 the program you actually want to run as an "indirect object" (without a
2271 comma) in front of the LIST, as in C<exec PROGRAM LIST>. (This always
2272 forces interpretation of the LIST as a multivalued list, even if there
2273 is only a single scalar in the list.) Example:
2275 my $shell = '/bin/csh';
2276 exec $shell '-sh'; # pretend it's a login shell
2280 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
2282 When the arguments get executed via the system shell, results are
2283 subject to its quirks and capabilities. See L<perlop/"`STRING`">
2286 Using an indirect object with L<C<exec>|/exec LIST> or
2287 L<C<system>|/system LIST> is also more secure. This usage (which also
2288 works fine with L<C<system>|/system LIST>) forces
2289 interpretation of the arguments as a multivalued list, even if the
2290 list had just one argument. That way you're safe from the shell
2291 expanding wildcards or splitting up words with whitespace in them.
2293 my @args = ( "echo surprise" );
2295 exec @args; # subject to shell escapes
2297 exec { $args[0] } @args; # safe even with one-arg list
2299 The first version, the one without the indirect object, ran the I<echo>
2300 program, passing it C<"surprise"> an argument. The second version didn't;
2301 it tried to run a program named I<"echo surprise">, didn't find it, and set
2302 L<C<$?>|perlvar/$?> to a non-zero value indicating failure.
2304 On Windows, only the C<exec PROGRAM LIST> indirect object syntax will
2305 reliably avoid using the shell; C<exec LIST>, even with more than one
2306 element, will fall back to the shell if the first spawn fails.
2308 Perl attempts to flush all files opened for output before the exec,
2309 but this may not be supported on some platforms (see L<perlport>).
2310 To be safe, you may need to set L<C<$E<verbar>>|perlvar/$E<verbar>>
2311 (C<$AUTOFLUSH> in L<English>) or call the C<autoflush> method of
2312 L<C<IO::Handle>|IO::Handle/METHODS> on any open handles to avoid lost
2315 Note that L<C<exec>|/exec LIST> will not call your C<END> blocks, nor
2316 will it invoke C<DESTROY> methods on your objects.
2318 Portability issues: L<perlport/exec>.
2321 X<exists> X<autovivification>
2323 =for Pod::Functions test whether a hash key is present
2325 Given an expression that specifies an element of a hash, returns true if the
2326 specified element in the hash has ever been initialized, even if the
2327 corresponding value is undefined.
2329 print "Exists\n" if exists $hash{$key};
2330 print "Defined\n" if defined $hash{$key};
2331 print "True\n" if $hash{$key};
2333 exists may also be called on array elements, but its behavior is much less
2334 obvious and is strongly tied to the use of L<C<delete>|/delete EXPR> on
2337 B<WARNING:> Calling L<C<exists>|/exists EXPR> on array values is
2338 strongly discouraged. The
2339 notion of deleting or checking the existence of Perl array elements is not
2340 conceptually coherent, and can lead to surprising behavior.
2342 print "Exists\n" if exists $array[$index];
2343 print "Defined\n" if defined $array[$index];
2344 print "True\n" if $array[$index];
2346 A hash or array element can be true only if it's defined and defined only if
2347 it exists, but the reverse doesn't necessarily hold true.
2349 Given an expression that specifies the name of a subroutine,
2350 returns true if the specified subroutine has ever been declared, even
2351 if it is undefined. Mentioning a subroutine name for exists or defined
2352 does not count as declaring it. Note that a subroutine that does not
2353 exist may still be callable: its package may have an C<AUTOLOAD>
2354 method that makes it spring into existence the first time that it is
2355 called; see L<perlsub>.
2357 print "Exists\n" if exists &subroutine;
2358 print "Defined\n" if defined &subroutine;
2360 Note that the EXPR can be arbitrarily complicated as long as the final
2361 operation is a hash or array key lookup or subroutine name:
2363 if (exists $ref->{A}->{B}->{$key}) { }
2364 if (exists $hash{A}{B}{$key}) { }
2366 if (exists $ref->{A}->{B}->[$ix]) { }
2367 if (exists $hash{A}{B}[$ix]) { }
2369 if (exists &{$ref->{A}{B}{$key}}) { }
2371 Although the most deeply nested array or hash element will not spring into
2372 existence just because its existence was tested, any intervening ones will.
2373 Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
2374 into existence due to the existence test for the C<$key> element above.
2375 This happens anywhere the arrow operator is used, including even here:
2378 if (exists $ref->{"Some key"}) { }
2379 print $ref; # prints HASH(0x80d3d5c)
2381 This surprising autovivification in what does not at first--or even
2382 second--glance appear to be an lvalue context may be fixed in a future
2385 Use of a subroutine call, rather than a subroutine name, as an argument
2386 to L<C<exists>|/exists EXPR> is an error.
2389 exists &sub(); # Error
2392 X<exit> X<terminate> X<abort>
2396 =for Pod::Functions terminate this program
2398 Evaluates EXPR and exits immediately with that value. Example:
2401 exit 0 if $ans =~ /^[Xx]/;
2403 See also L<C<die>|/die LIST>. If EXPR is omitted, exits with C<0>
2405 universally recognized values for EXPR are C<0> for success and C<1>
2406 for error; other values are subject to interpretation depending on the
2407 environment in which the Perl program is running. For example, exiting
2408 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
2409 the mailer to return the item undelivered, but that's not true everywhere.
2411 Don't use L<C<exit>|/exit EXPR> to abort a subroutine if there's any
2412 chance that someone might want to trap whatever error happened. Use
2413 L<C<die>|/die LIST> instead, which can be trapped by an
2414 L<C<eval>|/eval EXPR>.
2416 The L<C<exit>|/exit EXPR> function does not always exit immediately. It
2417 calls any defined C<END> routines first, but these C<END> routines may
2418 not themselves abort the exit. Likewise any object destructors that
2419 need to be called are called before the real exit. C<END> routines and
2420 destructors can change the exit status by modifying L<C<$?>|perlvar/$?>.
2421 If this is a problem, you can call
2422 L<C<POSIX::_exit($status)>|POSIX/C<_exit>> to avoid C<END> and destructor
2423 processing. See L<perlmod> for details.
2425 Portability issues: L<perlport/exit>.
2428 X<exp> X<exponential> X<antilog> X<antilogarithm> X<e>
2432 =for Pod::Functions raise I<e> to a power
2434 Returns I<e> (the natural logarithm base) to the power of EXPR.
2435 If EXPR is omitted, gives C<exp($_)>.
2438 X<fc> X<foldcase> X<casefold> X<fold-case> X<case-fold>
2442 =for Pod::Functions +fc return casefolded version of a string
2444 Returns the casefolded version of EXPR. This is the internal function
2445 implementing the C<\F> escape in double-quoted strings.
2447 Casefolding is the process of mapping strings to a form where case
2448 differences are erased; comparing two strings in their casefolded
2449 form is effectively a way of asking if two strings are equal,
2452 Roughly, if you ever found yourself writing this
2454 lc($this) eq lc($that) # Wrong!
2456 uc($this) eq uc($that) # Also wrong!
2458 $this =~ /^\Q$that\E\z/i # Right!
2462 fc($this) eq fc($that)
2464 And get the correct results.
2466 Perl only implements the full form of casefolding, but you can access
2467 the simple folds using L<Unicode::UCD/B<casefold()>> and
2468 L<Unicode::UCD/B<prop_invmap()>>.
2469 For further information on casefolding, refer to
2470 the Unicode Standard, specifically sections 3.13 C<Default Case Operations>,
2471 4.2 C<Case-Normative>, and 5.18 C<Case Mappings>,
2472 available at L<http://www.unicode.org/versions/latest/>, as well as the
2473 Case Charts available at L<http://www.unicode.org/charts/case/>.
2475 If EXPR is omitted, uses L<C<$_>|perlvar/$_>.
2477 This function behaves the same way under various pragmas, such as within
2478 L<S<C<"use feature 'unicode_strings">>|feature/The 'unicode_strings' feature>,
2479 as L<C<lc>|/lc EXPR> does, with the single exception of
2480 L<C<fc>|/fc EXPR> of I<LATIN CAPITAL LETTER SHARP S> (U+1E9E) within the
2481 scope of L<S<C<use locale>>|locale>. The foldcase of this character
2482 would normally be C<"ss">, but as explained in the L<C<lc>|/lc EXPR>
2484 changes that cross the 255/256 boundary are problematic under locales,
2485 and are hence prohibited. Therefore, this function under locale returns
2486 instead the string C<"\x{17F}\x{17F}">, which is the I<LATIN SMALL LETTER
2487 LONG S>. Since that character itself folds to C<"s">, the string of two
2488 of them together should be equivalent to a single U+1E9E when foldcased.
2490 While the Unicode Standard defines two additional forms of casefolding,
2491 one for Turkic languages and one that never maps one character into multiple
2492 characters, these are not provided by the Perl core. However, the CPAN module
2493 L<C<Unicode::Casing>|Unicode::Casing> may be used to provide an implementation.
2495 L<C<fc>|/fc EXPR> is available only if the
2496 L<C<"fc"> feature|feature/The 'fc' feature> is enabled or if it is
2497 prefixed with C<CORE::>. The
2498 L<C<"fc"> feature|feature/The 'fc' feature> is enabled automatically
2499 with a C<use v5.16> (or higher) declaration in the current scope.
2501 =item fcntl FILEHANDLE,FUNCTION,SCALAR
2504 =for Pod::Functions file control system call
2506 Implements the L<fcntl(2)> function. You'll probably have to say
2510 first to get the correct constant definitions. Argument processing and
2511 value returned work just like L<C<ioctl>|/ioctl
2512 FILEHANDLE,FUNCTION,SCALAR> below. For example:
2515 my $flags = fcntl($filehandle, F_GETFL, 0)
2516 or die "Can't fcntl F_GETFL: $!";
2518 You don't have to check for L<C<defined>|/defined EXPR> on the return
2519 from L<C<fcntl>|/fcntl FILEHANDLE,FUNCTION,SCALAR>. Like
2520 L<C<ioctl>|/ioctl FILEHANDLE,FUNCTION,SCALAR>, it maps a C<0> return
2521 from the system call into C<"0 but true"> in Perl. This string is true
2522 in boolean context and C<0> in numeric context. It is also exempt from
2524 L<C<Argument "..." isn't numeric>|perldiag/Argument "%s" isn't numeric%s>
2525 L<warnings> on improper numeric conversions.
2527 Note that L<C<fcntl>|/fcntl FILEHANDLE,FUNCTION,SCALAR> raises an
2528 exception if used on a machine that doesn't implement L<fcntl(2)>. See
2529 the L<Fcntl> module or your L<fcntl(2)> manpage to learn what functions
2530 are available on your system.
2532 Here's an example of setting a filehandle named C<$REMOTE> to be
2533 non-blocking at the system level. You'll have to negotiate
2534 L<C<$E<verbar>>|perlvar/$E<verbar>> on your own, though.
2536 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
2538 my $flags = fcntl($REMOTE, F_GETFL, 0)
2539 or die "Can't get flags for the socket: $!\n";
2541 fcntl($REMOTE, F_SETFL, $flags | O_NONBLOCK)
2542 or die "Can't set flags for the socket: $!\n";
2544 Portability issues: L<perlport/fcntl>.
2549 =for Pod::Functions the name of the current source file
2551 A special token that returns the name of the file in which it occurs.
2553 =item fileno FILEHANDLE
2556 =for Pod::Functions return file descriptor from filehandle
2558 Returns the file descriptor for a filehandle, or undefined if the
2559 filehandle is not open. If there is no real file descriptor at the OS
2560 level, as can happen with filehandles connected to memory objects via
2561 L<C<open>|/open FILEHANDLE,EXPR> with a reference for the third
2562 argument, -1 is returned.
2564 This is mainly useful for constructing bitmaps for
2565 L<C<select>|/select RBITS,WBITS,EBITS,TIMEOUT> and low-level POSIX
2566 tty-handling operations.
2567 If FILEHANDLE is an expression, the value is taken as an indirect
2568 filehandle, generally its name.
2570 You can use this to find out whether two handles refer to the
2571 same underlying descriptor:
2573 if (fileno($this) != -1 && fileno($this) == fileno($that)) {
2574 print "\$this and \$that are dups\n";
2575 } elsif (fileno($this) != -1 && fileno($that) != -1) {
2576 print "\$this and \$that have different " .
2577 "underlying file descriptors\n";
2579 print "At least one of \$this and \$that does " .
2580 "not have a real file descriptor\n";
2583 The behavior of L<C<fileno>|/fileno FILEHANDLE> on a directory handle
2584 depends on the operating system. On a system with L<dirfd(3)> or
2585 similar, L<C<fileno>|/fileno FILEHANDLE> on a directory
2586 handle returns the underlying file descriptor associated with the
2587 handle; on systems with no such support, it returns the undefined value,
2588 and sets L<C<$!>|perlvar/$!> (errno).
2590 =item flock FILEHANDLE,OPERATION
2591 X<flock> X<lock> X<locking>
2593 =for Pod::Functions lock an entire file with an advisory lock
2595 Calls L<flock(2)>, or an emulation of it, on FILEHANDLE. Returns true
2596 for success, false on failure. Produces a fatal error if used on a
2597 machine that doesn't implement L<flock(2)>, L<fcntl(2)> locking, or
2598 L<lockf(3)>. L<C<flock>|/flock FILEHANDLE,OPERATION> is Perl's portable
2599 file-locking interface, although it locks entire files only, not
2602 Two potentially non-obvious but traditional L<C<flock>|/flock
2603 FILEHANDLE,OPERATION> semantics are
2604 that it waits indefinitely until the lock is granted, and that its locks
2605 are B<merely advisory>. Such discretionary locks are more flexible, but
2606 offer fewer guarantees. This means that programs that do not also use
2607 L<C<flock>|/flock FILEHANDLE,OPERATION> may modify files locked with
2608 L<C<flock>|/flock FILEHANDLE,OPERATION>. See L<perlport>,
2609 your port's specific documentation, and your system-specific local manpages
2610 for details. It's best to assume traditional behavior if you're writing
2611 portable programs. (But if you're not, you should as always feel perfectly
2612 free to write for your own system's idiosyncrasies (sometimes called
2613 "features"). Slavish adherence to portability concerns shouldn't get
2614 in the way of your getting your job done.)
2616 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
2617 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
2618 you can use the symbolic names if you import them from the L<Fcntl> module,
2619 either individually, or as a group using the C<:flock> tag. LOCK_SH
2620 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
2621 releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
2622 LOCK_SH or LOCK_EX, then L<C<flock>|/flock FILEHANDLE,OPERATION> returns
2623 immediately rather than blocking waiting for the lock; check the return
2624 status to see if you got it.
2626 To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
2627 before locking or unlocking it.
2629 Note that the emulation built with L<lockf(3)> doesn't provide shared
2630 locks, and it requires that FILEHANDLE be open with write intent. These
2631 are the semantics that L<lockf(3)> implements. Most if not all systems
2632 implement L<lockf(3)> in terms of L<fcntl(2)> locking, though, so the
2633 differing semantics shouldn't bite too many people.
2635 Note that the L<fcntl(2)> emulation of L<flock(3)> requires that FILEHANDLE
2636 be open with read intent to use LOCK_SH and requires that it be open
2637 with write intent to use LOCK_EX.
2639 Note also that some versions of L<C<flock>|/flock FILEHANDLE,OPERATION>
2640 cannot lock things over the network; you would need to use the more
2641 system-specific L<C<fcntl>|/fcntl FILEHANDLE,FUNCTION,SCALAR> for
2642 that. If you like you can force Perl to ignore your system's L<flock(2)>
2643 function, and so provide its own L<fcntl(2)>-based emulation, by passing
2644 the switch C<-Ud_flock> to the F<Configure> program when you configure
2645 and build a new Perl.
2647 Here's a mailbox appender for BSD systems.
2649 # import LOCK_* and SEEK_END constants
2650 use Fcntl qw(:flock SEEK_END);
2654 flock($fh, LOCK_EX) or die "Cannot lock mailbox - $!\n";
2656 # and, in case someone appended while we were waiting...
2657 seek($fh, 0, SEEK_END) or die "Cannot seek - $!\n";
2662 flock($fh, LOCK_UN) or die "Cannot unlock mailbox - $!\n";
2665 open(my $mbox, ">>", "/usr/spool/mail/$ENV{'USER'}")
2666 or die "Can't open mailbox: $!";
2669 print $mbox $msg,"\n\n";
2672 On systems that support a real L<flock(2)>, locks are inherited across
2673 L<C<fork>|/fork> calls, whereas those that must resort to the more
2674 capricious L<fcntl(2)> function lose their locks, making it seriously
2675 harder to write servers.
2677 See also L<DB_File> for other L<C<flock>|/flock FILEHANDLE,OPERATION>
2680 Portability issues: L<perlport/flock>.
2683 X<fork> X<child> X<parent>
2685 =for Pod::Functions create a new process just like this one
2687 Does a L<fork(2)> system call to create a new process running the
2688 same program at the same point. It returns the child pid to the
2689 parent process, C<0> to the child process, or L<C<undef>|/undef EXPR> if
2691 unsuccessful. File descriptors (and sometimes locks on those descriptors)
2692 are shared, while everything else is copied. On most systems supporting
2693 L<fork(2)>, great care has gone into making it extremely efficient (for
2694 example, using copy-on-write technology on data pages), making it the
2695 dominant paradigm for multitasking over the last few decades.
2697 Perl attempts to flush all files opened for output before forking the
2698 child process, but this may not be supported on some platforms (see
2699 L<perlport>). To be safe, you may need to set
2700 L<C<$E<verbar>>|perlvar/$E<verbar>> (C<$AUTOFLUSH> in L<English>) or
2701 call the C<autoflush> method of L<C<IO::Handle>|IO::Handle/METHODS> on
2702 any open handles to avoid duplicate output.
2704 If you L<C<fork>|/fork> without ever waiting on your children, you will
2705 accumulate zombies. On some systems, you can avoid this by setting
2706 L<C<$SIG{CHLD}>|perlvar/%SIG> to C<"IGNORE">. See also L<perlipc> for
2707 more examples of forking and reaping moribund children.
2709 Note that if your forked child inherits system file descriptors like
2710 STDIN and STDOUT that are actually connected by a pipe or socket, even
2711 if you exit, then the remote server (such as, say, a CGI script or a
2712 backgrounded job launched from a remote shell) won't think you're done.
2713 You should reopen those to F</dev/null> if it's any issue.
2715 On some platforms such as Windows, where the L<fork(2)> system call is
2716 not available, Perl can be built to emulate L<C<fork>|/fork> in the Perl
2717 interpreter. The emulation is designed, at the level of the Perl
2718 program, to be as compatible as possible with the "Unix" L<fork(2)>.
2719 However it has limitations that have to be considered in code intended
2720 to be portable. See L<perlfork> for more details.
2722 Portability issues: L<perlport/fork>.
2727 =for Pod::Functions declare a picture format with use by the write() function
2729 Declare a picture format for use by the L<C<write>|/write FILEHANDLE>
2730 function. For example:
2733 Test: @<<<<<<<< @||||| @>>>>>
2734 $str, $%, '$' . int($num)
2738 $num = $cost/$quantity;
2742 See L<perlform> for many details and examples.
2744 =item formline PICTURE,LIST
2747 =for Pod::Functions internal function used for formats
2749 This is an internal function used by L<C<format>|/format>s, though you
2750 may call it, too. It formats (see L<perlform>) a list of values
2751 according to the contents of PICTURE, placing the output into the format
2752 output accumulator, L<C<$^A>|perlvar/$^A> (or C<$ACCUMULATOR> in
2753 L<English>). Eventually, when a L<C<write>|/write FILEHANDLE> is done,
2754 the contents of L<C<$^A>|perlvar/$^A> are written to some filehandle.
2755 You could also read L<C<$^A>|perlvar/$^A> and then set
2756 L<C<$^A>|perlvar/$^A> back to C<"">. Note that a format typically does
2757 one L<C<formline>|/formline PICTURE,LIST> per line of form, but the
2758 L<C<formline>|/formline PICTURE,LIST> function itself doesn't care how
2759 many newlines are embedded in the PICTURE. This means that the C<~> and
2760 C<~~> tokens treat the entire PICTURE as a single line. You may
2761 therefore need to use multiple formlines to implement a single record
2762 format, just like the L<C<format>|/format> compiler.
2764 Be careful if you put double quotes around the picture, because an C<@>
2765 character may be taken to mean the beginning of an array name.
2766 L<C<formline>|/formline PICTURE,LIST> always returns true. See
2767 L<perlform> for other examples.
2769 If you are trying to use this instead of L<C<write>|/write FILEHANDLE>
2770 to capture the output, you may find it easier to open a filehandle to a
2771 scalar (C<< open my $fh, ">", \$output >>) and write to that instead.
2773 =item getc FILEHANDLE
2774 X<getc> X<getchar> X<character> X<file, read>
2778 =for Pod::Functions get the next character from the filehandle
2780 Returns the next character from the input file attached to FILEHANDLE,
2781 or the undefined value at end of file or if there was an error (in
2782 the latter case L<C<$!>|perlvar/$!> is set). If FILEHANDLE is omitted,
2784 STDIN. This is not particularly efficient. However, it cannot be
2785 used by itself to fetch single characters without waiting for the user
2786 to hit enter. For that, try something more like:
2789 system "stty cbreak </dev/tty >/dev/tty 2>&1";
2792 system "stty", '-icanon', 'eol', "\001";
2795 my $key = getc(STDIN);
2798 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
2801 system 'stty', 'icanon', 'eol', '^@'; # ASCII NUL
2805 Determination of whether C<$BSD_STYLE> should be set is left as an
2806 exercise to the reader.
2808 The L<C<POSIX::getattr>|POSIX/C<getattr>> function can do this more
2809 portably on systems purporting POSIX compliance. See also the
2810 L<C<Term::ReadKey>|Term::ReadKey> module on CPAN.
2813 X<getlogin> X<login>
2815 =for Pod::Functions return who logged in at this tty
2817 This implements the C library function of the same name, which on most
2818 systems returns the current login from F</etc/utmp>, if any. If it
2819 returns the empty string, use L<C<getpwuid>|/getpwuid UID>.
2821 my $login = getlogin || getpwuid($<) || "Kilroy";
2823 Do not consider L<C<getlogin>|/getlogin> for authentication: it is not
2824 as secure as L<C<getpwuid>|/getpwuid UID>.
2826 Portability issues: L<perlport/getlogin>.
2828 =item getpeername SOCKET
2829 X<getpeername> X<peer>
2831 =for Pod::Functions find the other end of a socket connection
2833 Returns the packed sockaddr address of the other end of the SOCKET
2837 my $hersockaddr = getpeername($sock);
2838 my ($port, $iaddr) = sockaddr_in($hersockaddr);
2839 my $herhostname = gethostbyaddr($iaddr, AF_INET);
2840 my $herstraddr = inet_ntoa($iaddr);
2845 =for Pod::Functions get process group
2847 Returns the current process group for the specified PID. Use
2848 a PID of C<0> to get the current process group for the
2849 current process. Will raise an exception if used on a machine that
2850 doesn't implement L<getpgrp(2)>. If PID is omitted, returns the process
2851 group of the current process. Note that the POSIX version of
2852 L<C<getpgrp>|/getpgrp PID> does not accept a PID argument, so only
2853 C<PID==0> is truly portable.
2855 Portability issues: L<perlport/getpgrp>.
2858 X<getppid> X<parent> X<pid>
2860 =for Pod::Functions get parent process ID
2862 Returns the process id of the parent process.
2864 Note for Linux users: Between v5.8.1 and v5.16.0 Perl would work
2865 around non-POSIX thread semantics the minority of Linux systems (and
2866 Debian GNU/kFreeBSD systems) that used LinuxThreads, this emulation
2867 has since been removed. See the documentation for L<$$|perlvar/$$> for
2870 Portability issues: L<perlport/getppid>.
2872 =item getpriority WHICH,WHO
2873 X<getpriority> X<priority> X<nice>
2875 =for Pod::Functions get current nice value
2877 Returns the current priority for a process, a process group, or a user.
2878 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
2879 machine that doesn't implement L<getpriority(2)>.
2881 Portability issues: L<perlport/getpriority>.
2884 X<getpwnam> X<getgrnam> X<gethostbyname> X<getnetbyname> X<getprotobyname>
2885 X<getpwuid> X<getgrgid> X<getservbyname> X<gethostbyaddr> X<getnetbyaddr>
2886 X<getprotobynumber> X<getservbyport> X<getpwent> X<getgrent> X<gethostent>
2887 X<getnetent> X<getprotoent> X<getservent> X<setpwent> X<setgrent> X<sethostent>
2888 X<setnetent> X<setprotoent> X<setservent> X<endpwent> X<endgrent> X<endhostent>
2889 X<endnetent> X<endprotoent> X<endservent>
2891 =for Pod::Functions get passwd record given user login name
2895 =for Pod::Functions get group record given group name
2897 =item gethostbyname NAME
2899 =for Pod::Functions get host record given name
2901 =item getnetbyname NAME
2903 =for Pod::Functions get networks record given name
2905 =item getprotobyname NAME
2907 =for Pod::Functions get protocol record given name
2911 =for Pod::Functions get passwd record given user ID
2915 =for Pod::Functions get group record given group user ID
2917 =item getservbyname NAME,PROTO
2919 =for Pod::Functions get services record given its name
2921 =item gethostbyaddr ADDR,ADDRTYPE
2923 =for Pod::Functions get host record given its address
2925 =item getnetbyaddr ADDR,ADDRTYPE
2927 =for Pod::Functions get network record given its address
2929 =item getprotobynumber NUMBER
2931 =for Pod::Functions get protocol record numeric protocol
2933 =item getservbyport PORT,PROTO
2935 =for Pod::Functions get services record given numeric port
2939 =for Pod::Functions get next passwd record
2943 =for Pod::Functions get next group record
2947 =for Pod::Functions get next hosts record
2951 =for Pod::Functions get next networks record
2955 =for Pod::Functions get next protocols record
2959 =for Pod::Functions get next services record
2963 =for Pod::Functions prepare passwd file for use
2967 =for Pod::Functions prepare group file for use
2969 =item sethostent STAYOPEN
2971 =for Pod::Functions prepare hosts file for use
2973 =item setnetent STAYOPEN
2975 =for Pod::Functions prepare networks file for use
2977 =item setprotoent STAYOPEN
2979 =for Pod::Functions prepare protocols file for use
2981 =item setservent STAYOPEN
2983 =for Pod::Functions prepare services file for use
2987 =for Pod::Functions be done using passwd file
2991 =for Pod::Functions be done using group file
2995 =for Pod::Functions be done using hosts file
2999 =for Pod::Functions be done using networks file
3003 =for Pod::Functions be done using protocols file
3007 =for Pod::Functions be done using services file
3009 These routines are the same as their counterparts in the
3010 system C library. In list context, the return values from the
3011 various get routines are as follows:
3014 my ( $name, $passwd, $gid, $members ) = getgr*
3015 my ( $name, $aliases, $addrtype, $net ) = getnet*
3016 my ( $name, $aliases, $port, $proto ) = getserv*
3017 my ( $name, $aliases, $proto ) = getproto*
3018 my ( $name, $aliases, $addrtype, $length, @addrs ) = gethost*
3019 my ( $name, $passwd, $uid, $gid, $quota,
3020 $comment, $gcos, $dir, $shell, $expire ) = getpw*
3023 (If the entry doesn't exist, the return value is a single meaningless true
3026 The exact meaning of the $gcos field varies but it usually contains
3027 the real name of the user (as opposed to the login name) and other
3028 information pertaining to the user. Beware, however, that in many
3029 system users are able to change this information and therefore it
3030 cannot be trusted and therefore the $gcos is tainted (see
3031 L<perlsec>). The $passwd and $shell, user's encrypted password and
3032 login shell, are also tainted, for the same reason.
3034 In scalar context, you get the name, unless the function was a
3035 lookup by name, in which case you get the other thing, whatever it is.
3036 (If the entry doesn't exist you get the undefined value.) For example:
3038 my $uid = getpwnam($name);
3039 my $name = getpwuid($num);
3040 my $name = getpwent();
3041 my $gid = getgrnam($name);
3042 my $name = getgrgid($num);
3043 my $name = getgrent();
3046 In I<getpw*()> the fields $quota, $comment, and $expire are special
3047 in that they are unsupported on many systems. If the
3048 $quota is unsupported, it is an empty scalar. If it is supported, it
3049 usually encodes the disk quota. If the $comment field is unsupported,
3050 it is an empty scalar. If it is supported it usually encodes some
3051 administrative comment about the user. In some systems the $quota
3052 field may be $change or $age, fields that have to do with password
3053 aging. In some systems the $comment field may be $class. The $expire
3054 field, if present, encodes the expiration period of the account or the
3055 password. For the availability and the exact meaning of these fields
3056 in your system, please consult L<getpwnam(3)> and your system's
3057 F<pwd.h> file. You can also find out from within Perl what your
3058 $quota and $comment fields mean and whether you have the $expire field
3059 by using the L<C<Config>|Config> module and the values C<d_pwquota>, C<d_pwage>,
3060 C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
3061 files are supported only if your vendor has implemented them in the
3062 intuitive fashion that calling the regular C library routines gets the
3063 shadow versions if you're running under privilege or if there exists
3064 the L<shadow(3)> functions as found in System V (this includes Solaris
3065 and Linux). Those systems that implement a proprietary shadow password
3066 facility are unlikely to be supported.
3068 The $members value returned by I<getgr*()> is a space-separated list of
3069 the login names of the members of the group.
3071 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
3072 C, it will be returned to you via L<C<$?>|perlvar/$?> if the function
3074 C<@addrs> value returned by a successful call is a list of raw
3075 addresses returned by the corresponding library call. In the
3076 Internet domain, each address is four bytes long; you can unpack it
3077 by saying something like:
3079 my ($w,$x,$y,$z) = unpack('W4',$addr[0]);
3081 The Socket library makes this slightly easier:
3084 my $iaddr = inet_aton("127.1"); # or whatever address
3085 my $name = gethostbyaddr($iaddr, AF_INET);
3087 # or going the other way
3088 my $straddr = inet_ntoa($iaddr);
3090 In the opposite way, to resolve a hostname to the IP address
3094 my $packed_ip = gethostbyname("www.perl.org");
3096 if (defined $packed_ip) {
3097 $ip_address = inet_ntoa($packed_ip);
3100 Make sure L<C<gethostbyname>|/gethostbyname NAME> is called in SCALAR
3101 context and that its return value is checked for definedness.
3103 The L<C<getprotobynumber>|/getprotobynumber NUMBER> function, even
3104 though it only takes one argument, has the precedence of a list
3105 operator, so beware:
3107 getprotobynumber $number eq 'icmp' # WRONG
3108 getprotobynumber($number eq 'icmp') # actually means this
3109 getprotobynumber($number) eq 'icmp' # better this way
3111 If you get tired of remembering which element of the return list
3112 contains which return value, by-name interfaces are provided in standard
3113 modules: L<C<File::stat>|File::stat>, L<C<Net::hostent>|Net::hostent>,
3114 L<C<Net::netent>|Net::netent>, L<C<Net::protoent>|Net::protoent>,
3115 L<C<Net::servent>|Net::servent>, L<C<Time::gmtime>|Time::gmtime>,
3116 L<C<Time::localtime>|Time::localtime>, and
3117 L<C<User::grent>|User::grent>. These override the normal built-ins,
3118 supplying versions that return objects with the appropriate names for
3119 each field. For example:
3123 my $is_his = (stat($filename)->uid == pwent($whoever)->uid);
3125 Even though it looks as though they're the same method calls (uid),
3126 they aren't, because a C<File::stat> object is different from
3127 a C<User::pwent> object.
3129 Portability issues: L<perlport/getpwnam> to L<perlport/endservent>.
3131 =item getsockname SOCKET
3134 =for Pod::Functions retrieve the sockaddr for a given socket
3136 Returns the packed sockaddr address of this end of the SOCKET connection,
3137 in case you don't know the address because you have several different
3138 IPs that the connection might have come in on.
3141 my $mysockaddr = getsockname($sock);
3142 my ($port, $myaddr) = sockaddr_in($mysockaddr);
3143 printf "Connect to %s [%s]\n",
3144 scalar gethostbyaddr($myaddr, AF_INET),
3147 =item getsockopt SOCKET,LEVEL,OPTNAME
3150 =for Pod::Functions get socket options on a given socket
3152 Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
3153 Options may exist at multiple protocol levels depending on the socket
3154 type, but at least the uppermost socket level SOL_SOCKET (defined in the
3155 L<C<Socket>|Socket> module) will exist. To query options at another
3156 level the protocol number of the appropriate protocol controlling the
3157 option should be supplied. For example, to indicate that an option is
3158 to be interpreted by the TCP protocol, LEVEL should be set to the
3159 protocol number of TCP, which you can get using
3160 L<C<getprotobyname>|/getprotobyname NAME>.
3162 The function returns a packed string representing the requested socket
3163 option, or L<C<undef>|/undef EXPR> on error, with the reason for the
3164 error placed in L<C<$!>|perlvar/$!>. Just what is in the packed string
3165 depends on LEVEL and OPTNAME; consult L<getsockopt(2)> for details. A
3166 common case is that the option is an integer, in which case the result
3167 is a packed integer, which you can decode using
3168 L<C<unpack>|/unpack TEMPLATE,EXPR> with the C<i> (or C<I>) format.
3170 Here's an example to test whether Nagle's algorithm is enabled on a socket:
3172 use Socket qw(:all);
3174 defined(my $tcp = getprotobyname("tcp"))
3175 or die "Could not determine the protocol number for tcp";
3176 # my $tcp = IPPROTO_TCP; # Alternative
3177 my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
3178 or die "getsockopt TCP_NODELAY: $!";
3179 my $nodelay = unpack("I", $packed);
3180 print "Nagle's algorithm is turned ",
3181 $nodelay ? "off\n" : "on\n";
3183 Portability issues: L<perlport/getsockopt>.
3186 X<glob> X<wildcard> X<filename, expansion> X<expand>
3190 =for Pod::Functions expand filenames using wildcards
3192 In list context, returns a (possibly empty) list of filename expansions on
3193 the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
3194 scalar context, glob iterates through such filename expansions, returning
3195 undef when the list is exhausted. This is the internal function
3196 implementing the C<< <*.c> >> operator, but you can use it directly. If
3197 EXPR is omitted, L<C<$_>|perlvar/$_> is used. The C<< <*.c> >> operator
3198 is discussed in more detail in L<perlop/"I/O Operators">.
3200 Note that L<C<glob>|/glob EXPR> splits its arguments on whitespace and
3202 each segment as separate pattern. As such, C<glob("*.c *.h")>
3203 matches all files with a F<.c> or F<.h> extension. The expression
3204 C<glob(".* *")> matches all files in the current working directory.
3205 If you want to glob filenames that might contain whitespace, you'll
3206 have to use extra quotes around the spacey filename to protect it.
3207 For example, to glob filenames that have an C<e> followed by a space
3208 followed by an C<f>, use one of:
3210 my @spacies = <"*e f*">;
3211 my @spacies = glob '"*e f*"';
3212 my @spacies = glob q("*e f*");
3214 If you had to get a variable through, you could do this:
3216 my @spacies = glob "'*${var}e f*'";
3217 my @spacies = glob qq("*${var}e f*");
3219 If non-empty braces are the only wildcard characters used in the
3220 L<C<glob>|/glob EXPR>, no filenames are matched, but potentially many
3221 strings are returned. For example, this produces nine strings, one for
3222 each pairing of fruits and colors:
3224 my @many = glob "{apple,tomato,cherry}={green,yellow,red}";
3226 This operator is implemented using the standard C<File::Glob> extension.
3227 See L<File::Glob> for details, including
3228 L<C<bsd_glob>|File::Glob/C<bsd_glob>>, which does not treat whitespace
3229 as a pattern separator.
3231 Portability issues: L<perlport/glob>.
3234 X<gmtime> X<UTC> X<Greenwich>
3238 =for Pod::Functions convert UNIX time into record or string using Greenwich time
3240 Works just like L<C<localtime>|/localtime EXPR> but the returned values
3241 are localized for the standard Greenwich time zone.
3243 Note: When called in list context, $isdst, the last value
3244 returned by gmtime, is always C<0>. There is no
3245 Daylight Saving Time in GMT.
3247 Portability issues: L<perlport/gmtime>.
3250 X<goto> X<jump> X<jmp>
3256 =for Pod::Functions create spaghetti code
3258 The C<goto LABEL> form finds the statement labeled with LABEL and
3259 resumes execution there. It can't be used to get out of a block or
3260 subroutine given to L<C<sort>|/sort SUBNAME LIST>. It can be used to go
3261 almost anywhere else within the dynamic scope, including out of
3262 subroutines, but it's usually better to use some other construct such as
3263 L<C<last>|/last LABEL> or L<C<die>|/die LIST>. The author of Perl has
3264 never felt the need to use this form of L<C<goto>|/goto LABEL> (in Perl,
3265 that is; C is another matter). (The difference is that C does not offer
3266 named loops combined with loop control. Perl does, and this replaces
3267 most structured uses of L<C<goto>|/goto LABEL> in other languages.)
3269 The C<goto EXPR> form expects to evaluate C<EXPR> to a code reference or
3270 a label name. If it evaluates to a code reference, it will be handled
3271 like C<goto &NAME>, below. This is especially useful for implementing
3272 tail recursion via C<goto __SUB__>.
3274 If the expression evaluates to a label name, its scope will be resolved
3275 dynamically. This allows for computed L<C<goto>|/goto LABEL>s per
3276 FORTRAN, but isn't necessarily recommended if you're optimizing for
3279 goto ("FOO", "BAR", "GLARCH")[$i];
3281 As shown in this example, C<goto EXPR> is exempt from the "looks like a
3282 function" rule. A pair of parentheses following it does not (necessarily)
3283 delimit its argument. C<goto("NE")."XT"> is equivalent to C<goto NEXT>.
3284 Also, unlike most named operators, this has the same precedence as
3287 Use of C<goto LABEL> or C<goto EXPR> to jump into a construct is
3288 deprecated and will issue a warning. Even then, it may not be used to
3289 go into any construct that requires initialization, such as a
3290 subroutine or a C<foreach> loop. It also can't be used to go into a
3291 construct that is optimized away.
3293 The C<goto &NAME> form is quite different from the other forms of
3294 L<C<goto>|/goto LABEL>. In fact, it isn't a goto in the normal sense at
3295 all, and doesn't have the stigma associated with other gotos. Instead,
3296 it exits the current subroutine (losing any changes set by
3297 L<C<local>|/local EXPR>) and immediately calls in its place the named
3298 subroutine using the current value of L<C<@_>|perlvar/@_>. This is used
3299 by C<AUTOLOAD> subroutines that wish to load another subroutine and then
3300 pretend that the other subroutine had been called in the first place
3301 (except that any modifications to L<C<@_>|perlvar/@_> in the current
3302 subroutine are propagated to the other subroutine.) After the
3303 L<C<goto>|/goto LABEL>, not even L<C<caller>|/caller EXPR> will be able
3304 to tell that this routine was called first.
3306 NAME needn't be the name of a subroutine; it can be a scalar variable
3307 containing a code reference or a block that evaluates to a code
3310 =item grep BLOCK LIST
3313 =item grep EXPR,LIST
3315 =for Pod::Functions locate elements in a list test true against a given criterion
3317 This is similar in spirit to, but not the same as, L<grep(1)> and its
3318 relatives. In particular, it is not limited to using regular expressions.
3320 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
3321 L<C<$_>|perlvar/$_> to each element) and returns the list value
3323 elements for which the expression evaluated to true. In scalar
3324 context, returns the number of times the expression was true.
3326 my @foo = grep(!/^#/, @bar); # weed out comments
3330 my @foo = grep {!/^#/} @bar; # weed out comments
3332 Note that L<C<$_>|perlvar/$_> is an alias to the list value, so it can
3334 modify the elements of the LIST. While this is useful and supported,
3335 it can cause bizarre results if the elements of LIST are not variables.
3336 Similarly, grep returns aliases into the original list, much as a for
3337 loop's index variable aliases the list elements. That is, modifying an
3338 element of a list returned by grep (for example, in a C<foreach>,
3339 L<C<map>|/map BLOCK LIST> or another L<C<grep>|/grep BLOCK LIST>)
3340 actually modifies the element in the original list.
3341 This is usually something to be avoided when writing clear code.
3343 See also L<C<map>|/map BLOCK LIST> for a list composed of the results of
3347 X<hex> X<hexadecimal>
3351 =for Pod::Functions convert a hexadecimal string to a number
3353 Interprets EXPR as a hex string and returns the corresponding numeric value.
3354 If EXPR is omitted, uses L<C<$_>|perlvar/$_>.
3356 print hex '0xAf'; # prints '175'
3357 print hex 'aF'; # same
3358 $valid_input =~ /\A(?:0?[xX])?(?:_?[0-9a-fA-F])*\z/
3360 A hex string consists of hex digits and an optional C<0x> or C<x> prefix.
3361 Each hex digit may be preceded by a single underscore, which will be ignored.
3362 Any other character triggers a warning and causes the rest of the string
3363 to be ignored (even leading whitespace, unlike L<C<oct>|/oct EXPR>).
3364 Only integers can be represented, and integer overflow triggers a warning.
3366 To convert strings that might start with any of C<0>, C<0x>, or C<0b>,
3367 see L<C<oct>|/oct EXPR>. To present something as hex, look into
3368 L<C<printf>|/printf FILEHANDLE FORMAT, LIST>,
3369 L<C<sprintf>|/sprintf FORMAT, LIST>, and
3370 L<C<unpack>|/unpack TEMPLATE,EXPR>.
3375 =for Pod::Functions patch a module's namespace into your own
3377 There is no builtin L<C<import>|/import LIST> function. It is just an
3378 ordinary method (subroutine) defined (or inherited) by modules that wish
3379 to export names to another module. The
3380 L<C<use>|/use Module VERSION LIST> function calls the
3381 L<C<import>|/import LIST> method for the package used. See also
3382 L<C<use>|/use Module VERSION LIST>, L<perlmod>, and L<Exporter>.
3384 =item index STR,SUBSTR,POSITION
3385 X<index> X<indexOf> X<InStr>
3387 =item index STR,SUBSTR
3389 =for Pod::Functions find a substring within a string
3391 The index function searches for one string within another, but without
3392 the wildcard-like behavior of a full regular-expression pattern match.
3393 It returns the position of the first occurrence of SUBSTR in STR at
3394 or after POSITION. If POSITION is omitted, starts searching from the
3395 beginning of the string. POSITION before the beginning of the string
3396 or after its end is treated as if it were the beginning or the end,
3397 respectively. POSITION and the return value are based at zero.
3398 If the substring is not found, L<C<index>|/index STR,SUBSTR,POSITION>
3402 X<int> X<integer> X<truncate> X<trunc> X<floor>
3406 =for Pod::Functions get the integer portion of a number
3408 Returns the integer portion of EXPR. If EXPR is omitted, uses
3409 L<C<$_>|perlvar/$_>.
3410 You should not use this function for rounding: one because it truncates
3411 towards C<0>, and two because machine representations of floating-point
3412 numbers can sometimes produce counterintuitive results. For example,
3413 C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
3414 because it's really more like -268.99999999999994315658 instead. Usually,
3415 the L<C<sprintf>|/sprintf FORMAT, LIST>,
3416 L<C<printf>|/printf FILEHANDLE FORMAT, LIST>, or the
3417 L<C<POSIX::floor>|POSIX/C<floor>> and L<C<POSIX::ceil>|POSIX/C<ceil>>
3418 functions will serve you better than will L<C<int>|/int EXPR>.
3420 =item ioctl FILEHANDLE,FUNCTION,SCALAR
3423 =for Pod::Functions system-dependent device control system call
3425 Implements the L<ioctl(2)> function. You'll probably first have to say
3427 require "sys/ioctl.ph"; # probably in
3428 # $Config{archlib}/sys/ioctl.ph
3430 to get the correct function definitions. If F<sys/ioctl.ph> doesn't
3431 exist or doesn't have the correct definitions you'll have to roll your
3432 own, based on your C header files such as F<< <sys/ioctl.h> >>.
3433 (There is a Perl script called B<h2ph> that comes with the Perl kit that
3434 may help you in this, but it's nontrivial.) SCALAR will be read and/or
3435 written depending on the FUNCTION; a C pointer to the string value of SCALAR
3436 will be passed as the third argument of the actual
3437 L<C<ioctl>|/ioctl FILEHANDLE,FUNCTION,SCALAR> call. (If SCALAR
3438 has no string value but does have a numeric value, that value will be
3439 passed rather than a pointer to the string value. To guarantee this to be
3440 true, add a C<0> to the scalar before using it.) The
3441 L<C<pack>|/pack TEMPLATE,LIST> and L<C<unpack>|/unpack TEMPLATE,EXPR>
3442 functions may be needed to manipulate the values of structures used by
3443 L<C<ioctl>|/ioctl FILEHANDLE,FUNCTION,SCALAR>.
3445 The return value of L<C<ioctl>|/ioctl FILEHANDLE,FUNCTION,SCALAR> (and
3446 L<C<fcntl>|/fcntl FILEHANDLE,FUNCTION,SCALAR>) is as follows:
3448 if OS returns: then Perl returns:
3450 0 string "0 but true"
3451 anything else that number
3453 Thus Perl returns true on success and false on failure, yet you can
3454 still easily determine the actual value returned by the operating
3457 my $retval = ioctl(...) || -1;
3458 printf "System returned %d\n", $retval;
3460 The special string C<"0 but true"> is exempt from
3461 L<C<Argument "..." isn't numeric>|perldiag/Argument "%s" isn't numeric%s>
3462 L<warnings> on improper numeric conversions.
3464 Portability issues: L<perlport/ioctl>.
3466 =item join EXPR,LIST
3469 =for Pod::Functions join a list into a string using a separator
3471 Joins the separate strings of LIST into a single string with fields
3472 separated by the value of EXPR, and returns that new string. Example:
3474 my $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
3476 Beware that unlike L<C<split>|/split E<sol>PATTERNE<sol>,EXPR,LIMIT>,
3477 L<C<join>|/join EXPR,LIST> doesn't take a pattern as its first argument.
3478 Compare L<C<split>|/split E<sol>PATTERNE<sol>,EXPR,LIMIT>.
3485 =for Pod::Functions retrieve list of indices from a hash
3487 Called in list context, returns a list consisting of all the keys of the
3488 named hash, or in Perl 5.12 or later only, the indices of an array. Perl
3489 releases prior to 5.12 will produce a syntax error if you try to use an
3490 array argument. In scalar context, returns the number of keys or indices.
3492 Hash entries are returned in an apparently random order. The actual random
3493 order is specific to a given hash; the exact same series of operations
3494 on two hashes may result in a different order for each hash. Any insertion
3495 into the hash may change the order, as will any deletion, with the exception
3496 that the most recent key returned by L<C<each>|/each HASH> or
3497 L<C<keys>|/keys HASH> may be deleted without changing the order. So
3498 long as a given hash is unmodified you may rely on
3499 L<C<keys>|/keys HASH>, L<C<values>|/values HASH> and L<C<each>|/each
3500 HASH> to repeatedly return the same order
3501 as each other. See L<perlsec/"Algorithmic Complexity Attacks"> for
3502 details on why hash order is randomized. Aside from the guarantees
3503 provided here the exact details of Perl's hash algorithm and the hash
3504 traversal order are subject to change in any release of Perl. Tied hashes
3505 may behave differently to Perl's hashes with respect to changes in order on
3506 insertion and deletion of items.
3508 As a side effect, calling L<C<keys>|/keys HASH> resets the internal
3509 iterator of the HASH or ARRAY (see L<C<each>|/each HASH>). In
3510 particular, calling L<C<keys>|/keys HASH> in void context resets the
3511 iterator with no other overhead.
3513 Here is yet another way to print your environment:
3515 my @keys = keys %ENV;
3516 my @values = values %ENV;
3518 print pop(@keys), '=', pop(@values), "\n";
3521 or how about sorted by key:
3523 foreach my $key (sort(keys %ENV)) {
3524 print $key, '=', $ENV{$key}, "\n";
3527 The returned values are copies of the original keys in the hash, so
3528 modifying them will not affect the original hash. Compare
3529 L<C<values>|/values HASH>.
3531 To sort a hash by value, you'll need to use a
3532 L<C<sort>|/sort SUBNAME LIST> function. Here's a descending numeric
3533 sort of a hash by its values:
3535 foreach my $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
3536 printf "%4d %s\n", $hash{$key}, $key;
3539 Used as an lvalue, L<C<keys>|/keys HASH> allows you to increase the
3540 number of hash buckets
3541 allocated for the given hash. This can gain you a measure of efficiency if
3542 you know the hash is going to get big. (This is similar to pre-extending
3543 an array by assigning a larger number to $#array.) If you say
3547 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
3548 in fact, since it rounds up to the next power of two. These
3549 buckets will be retained even if you do C<%hash = ()>, use C<undef
3550 %hash> if you want to free the storage while C<%hash> is still in scope.
3551 You can't shrink the number of buckets allocated for the hash using
3552 L<C<keys>|/keys HASH> in this way (but you needn't worry about doing
3553 this by accident, as trying has no effect). C<keys @array> in an lvalue
3554 context is a syntax error.
3556 Starting with Perl 5.14, an experimental feature allowed
3557 L<C<keys>|/keys HASH> to take a scalar expression. This experiment has
3558 been deemed unsuccessful, and was removed as of Perl 5.24.
3560 To avoid confusing would-be users of your code who are running earlier
3561 versions of Perl with mysterious syntax errors, put this sort of thing at
3562 the top of your file to signal that your code will work I<only> on Perls of
3565 use 5.012; # so keys/values/each work on arrays
3567 See also L<C<each>|/each HASH>, L<C<values>|/values HASH>, and
3568 L<C<sort>|/sort SUBNAME LIST>.
3570 =item kill SIGNAL, LIST
3575 =for Pod::Functions send a signal to a process or process group
3577 Sends a signal to a list of processes. Returns the number of arguments
3578 that were successfully used to signal (which is not necessarily the same
3579 as the number of processes actually killed, e.g. where a process group is
3582 my $cnt = kill 'HUP', $child1, $child2;
3583 kill 'KILL', @goners;
3585 SIGNAL may be either a signal name (a string) or a signal number. A signal
3586 name may start with a C<SIG> prefix, thus C<FOO> and C<SIGFOO> refer to the
3587 same signal. The string form of SIGNAL is recommended for portability because
3588 the same signal may have different numbers in different operating systems.
3590 A list of signal names supported by the current platform can be found in
3591 C<$Config{sig_name}>, which is provided by the L<C<Config>|Config>
3592 module. See L<Config> for more details.
3594 A negative signal name is the same as a negative signal number, killing process
3595 groups instead of processes. For example, C<kill '-KILL', $pgrp> and
3596 C<kill -9, $pgrp> will send C<SIGKILL> to
3597 the entire process group specified. That
3598 means you usually want to use positive not negative signals.
3600 If SIGNAL is either the number 0 or the string C<ZERO> (or C<SIGZERO>),
3601 no signal is sent to the process, but L<C<kill>|/kill SIGNAL, LIST>
3602 checks whether it's I<possible> to send a signal to it
3603 (that means, to be brief, that the process is owned by the same user, or we are
3604 the super-user). This is useful to check that a child process is still
3605 alive (even if only as a zombie) and hasn't changed its UID. See
3606 L<perlport> for notes on the portability of this construct.
3608 The behavior of kill when a I<PROCESS> number is zero or negative depends on
3609 the operating system. For example, on POSIX-conforming systems, zero will
3610 signal the current process group, -1 will signal all processes, and any
3611 other negative PROCESS number will act as a negative signal number and
3612 kill the entire process group specified.
3614 If both the SIGNAL and the PROCESS are negative, the results are undefined.
3615 A warning may be produced in a future version.
3617 See L<perlipc/"Signals"> for more details.
3619 On some platforms such as Windows where the L<fork(2)> system call is not
3620 available, Perl can be built to emulate L<C<fork>|/fork> at the
3622 This emulation has limitations related to kill that have to be considered,
3623 for code running on Windows and in code intended to be portable.
3625 See L<perlfork> for more details.
3627 If there is no I<LIST> of processes, no signal is sent, and the return
3628 value is 0. This form is sometimes used, however, because it causes
3629 tainting checks to be run. But see
3630 L<perlsec/Laundering and Detecting Tainted Data>.
3632 Portability issues: L<perlport/kill>.
3641 =for Pod::Functions exit a block prematurely
3643 The L<C<last>|/last LABEL> command is like the C<break> statement in C
3645 loops); it immediately exits the loop in question. If the LABEL is
3646 omitted, the command refers to the innermost enclosing
3647 loop. The C<last EXPR> form, available starting in Perl
3648 5.18.0, allows a label name to be computed at run time,
3649 and is otherwise identical to C<last LABEL>. The
3650 L<C<continue>|/continue BLOCK> block, if any, is not executed:
3652 LINE: while (<STDIN>) {
3653 last LINE if /^$/; # exit when done with header
3657 L<C<last>|/last LABEL> cannot be used to exit a block that returns a
3658 value such as C<eval {}>, C<sub {}>, or C<do {}>, and should not be used
3659 to exit a L<C<grep>|/grep BLOCK LIST> or L<C<map>|/map BLOCK LIST>
3662 Note that a block by itself is semantically identical to a loop
3663 that executes once. Thus L<C<last>|/last LABEL> can be used to effect
3664 an early exit out of such a block.
3666 See also L<C<continue>|/continue BLOCK> for an illustration of how
3667 L<C<last>|/last LABEL>, L<C<next>|/next LABEL>, and
3668 L<C<redo>|/redo LABEL> work.
3670 Unlike most named operators, this has the same precedence as assignment.
3671 It is also exempt from the looks-like-a-function rule, so
3672 C<last ("foo")."bar"> will cause "bar" to be part of the argument to
3673 L<C<last>|/last LABEL>.
3680 =for Pod::Functions return lower-case version of a string
3682 Returns a lowercased version of EXPR. This is the internal function
3683 implementing the C<\L> escape in double-quoted strings.
3685 If EXPR is omitted, uses L<C<$_>|perlvar/$_>.
3687 What gets returned depends on several factors:
3691 =item If C<use bytes> is in effect:
3693 The results follow ASCII rules. Only the characters C<A-Z> change,
3694 to C<a-z> respectively.
3696 =item Otherwise, if C<use locale> for C<LC_CTYPE> is in effect:
3698 Respects current C<LC_CTYPE> locale for code points < 256; and uses Unicode
3699 rules for the remaining code points (this last can only happen if
3700 the UTF8 flag is also set). See L<perllocale>.
3702 Starting in v5.20, Perl uses full Unicode rules if the locale is
3703 UTF-8. Otherwise, there is a deficiency in this scheme, which is that
3704 case changes that cross the 255/256
3705 boundary are not well-defined. For example, the lower case of LATIN CAPITAL
3706 LETTER SHARP S (U+1E9E) in Unicode rules is U+00DF (on ASCII
3707 platforms). But under C<use locale> (prior to v5.20 or not a UTF-8
3708 locale), the lower case of U+1E9E is
3709 itself, because 0xDF may not be LATIN SMALL LETTER SHARP S in the
3710 current locale, and Perl has no way of knowing if that character even
3711 exists in the locale, much less what code point it is. Perl returns
3712 a result that is above 255 (almost always the input character unchanged),
3713 for all instances (and there aren't many) where the 255/256 boundary
3714 would otherwise be crossed; and starting in v5.22, it raises a
3715 L<locale|perldiag/Can't do %s("%s") on non-UTF-8 locale; resolved to "%s".> warning.
3717 =item Otherwise, If EXPR has the UTF8 flag set:
3719 Unicode rules are used for the case change.
3721 =item Otherwise, if C<use feature 'unicode_strings'> or C<use locale ':not_characters'> is in effect:
3723 Unicode rules are used for the case change.
3727 ASCII rules are used for the case change. The lowercase of any character
3728 outside the ASCII range is the character itself.
3733 X<lcfirst> X<lowercase>
3737 =for Pod::Functions return a string with just the next letter in lower case
3739 Returns the value of EXPR with the first character lowercased. This
3740 is the internal function implementing the C<\l> escape in
3741 double-quoted strings.
3743 If EXPR is omitted, uses L<C<$_>|perlvar/$_>.
3745 This function behaves the same way under various pragmas, such as in a locale,
3746 as L<C<lc>|/lc EXPR> does.
3753 =for Pod::Functions return the number of characters in a string
3755 Returns the length in I<characters> of the value of EXPR. If EXPR is
3756 omitted, returns the length of L<C<$_>|perlvar/$_>. If EXPR is
3757 undefined, returns L<C<undef>|/undef EXPR>.
3759 This function cannot be used on an entire array or hash to find out how
3760 many elements these have. For that, use C<scalar @array> and C<scalar keys
3761 %hash>, respectively.
3763 Like all Perl character operations, L<C<length>|/length EXPR> normally
3765 characters, not physical bytes. For how many bytes a string encoded as
3766 UTF-8 would take up, use C<length(Encode::encode_utf8(EXPR))> (you'll have
3767 to C<use Encode> first). See L<Encode> and L<perlunicode>.
3772 =for Pod::Functions the current source line number
3774 A special token that compiles to the current line number.
3776 =item link OLDFILE,NEWFILE
3779 =for Pod::Functions create a hard link in the filesystem
3781 Creates a new filename linked to the old filename. Returns true for
3782 success, false otherwise.
3784 Portability issues: L<perlport/link>.
3786 =item listen SOCKET,QUEUESIZE
3789 =for Pod::Functions register your socket as a server
3791 Does the same thing that the L<listen(2)> system call does. Returns true if
3792 it succeeded, false otherwise. See the example in
3793 L<perlipc/"Sockets: Client/Server Communication">.
3798 =for Pod::Functions create a temporary value for a global variable (dynamic scoping)
3800 You really probably want to be using L<C<my>|/my VARLIST> instead,
3801 because L<C<local>|/local EXPR> isn't what most people think of as
3802 "local". See L<perlsub/"Private Variables via my()"> for details.
3804 A local modifies the listed variables to be local to the enclosing
3805 block, file, or eval. If more than one value is listed, the list must
3806 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
3807 for details, including issues with tied arrays and hashes.
3809 The C<delete local EXPR> construct can also be used to localize the deletion
3810 of array/hash elements to the current block.
3811 See L<perlsub/"Localized deletion of elements of composite types">.
3813 =item localtime EXPR
3814 X<localtime> X<ctime>
3818 =for Pod::Functions convert UNIX time into record or string using local time
3820 Converts a time as returned by the time function to a 9-element list
3821 with the time analyzed for the local time zone. Typically used as
3825 my ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
3828 All list elements are numeric and come straight out of the C `struct
3829 tm'. C<$sec>, C<$min>, and C<$hour> are the seconds, minutes, and hours
3830 of the specified time.
3832 C<$mday> is the day of the month and C<$mon> the month in
3833 the range C<0..11>, with 0 indicating January and 11 indicating December.
3834 This makes it easy to get a month name from a list:
3836 my @abbr = qw(Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec);
3837 print "$abbr[$mon] $mday";
3838 # $mon=9, $mday=18 gives "Oct 18"
3840 C<$year> contains the number of years since 1900. To get a 4-digit
3845 To get the last two digits of the year (e.g., "01" in 2001) do:
3847 $year = sprintf("%02d", $year % 100);
3849 C<$wday> is the day of the week, with 0 indicating Sunday and 3 indicating
3850 Wednesday. C<$yday> is the day of the year, in the range C<0..364>
3851 (or C<0..365> in leap years.)
3853 C<$isdst> is true if the specified time occurs during Daylight Saving
3854 Time, false otherwise.
3856 If EXPR is omitted, L<C<localtime>|/localtime EXPR> uses the current
3857 time (as returned by L<C<time>|/time>).
3859 In scalar context, L<C<localtime>|/localtime EXPR> returns the
3862 my $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
3864 The format of this scalar value is B<not> locale-dependent but built
3865 into Perl. For GMT instead of local time use the
3866 L<C<gmtime>|/gmtime EXPR> builtin. See also the
3867 L<C<Time::Local>|Time::Local> module (for converting seconds, minutes,
3868 hours, and such back to the integer value returned by L<C<time>|/time>),
3869 and the L<POSIX> module's L<C<strftime>|POSIX/C<strftime>> and
3870 L<C<mktime>|POSIX/C<mktime>> functions.
3872 To get somewhat similar but locale-dependent date strings, set up your
3873 locale environment variables appropriately (please see L<perllocale>) and
3876 use POSIX qw(strftime);
3877 my $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
3878 # or for GMT formatted appropriately for your locale:
3879 my $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
3881 Note that C<%a> and C<%b>, the short forms of the day of the week
3882 and the month of the year, may not necessarily be three characters wide.
3884 The L<Time::gmtime> and L<Time::localtime> modules provide a convenient,
3885 by-name access mechanism to the L<C<gmtime>|/gmtime EXPR> and
3886 L<C<localtime>|/localtime EXPR> functions, respectively.
3888 For a comprehensive date and time representation look at the
3889 L<DateTime> module on CPAN.
3891 Portability issues: L<perlport/localtime>.
3896 =for Pod::Functions +5.005 get a thread lock on a variable, subroutine, or method
3898 This function places an advisory lock on a shared variable or referenced
3899 object contained in I<THING> until the lock goes out of scope.
3901 The value returned is the scalar itself, if the argument is a scalar, or a
3902 reference, if the argument is a hash, array or subroutine.
3904 L<C<lock>|/lock THING> is a "weak keyword"; this means that if you've
3906 by this name (before any calls to it), that function will be called
3907 instead. If you are not under C<use threads::shared> this does nothing.
3908 See L<threads::shared>.
3911 X<log> X<logarithm> X<e> X<ln> X<base>
3915 =for Pod::Functions retrieve the natural logarithm for a number
3917 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
3918 returns the log of L<C<$_>|perlvar/$_>. To get the
3919 log of another base, use basic algebra:
3920 The base-N log of a number is equal to the natural log of that number
3921 divided by the natural log of N. For example:
3925 return log($n)/log(10);
3928 See also L<C<exp>|/exp EXPR> for the inverse operation.
3930 =item lstat FILEHANDLE
3935 =item lstat DIRHANDLE
3939 =for Pod::Functions stat a symbolic link
3941 Does the same thing as the L<C<stat>|/stat FILEHANDLE> function
3942 (including setting the special C<_> filehandle) but stats a symbolic
3943 link instead of the file the symbolic link points to. If symbolic links
3944 are unimplemented on your system, a normal L<C<stat>|/stat FILEHANDLE>
3945 is done. For much more detailed information, please see the
3946 documentation for L<C<stat>|/stat FILEHANDLE>.
3948 If EXPR is omitted, stats L<C<$_>|perlvar/$_>.
3950 Portability issues: L<perlport/lstat>.
3954 =for Pod::Functions match a string with a regular expression pattern
3956 The match operator. See L<perlop/"Regexp Quote-Like Operators">.
3958 =item map BLOCK LIST
3963 =for Pod::Functions apply a change to a list to get back a new list with the changes
3965 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
3966 L<C<$_>|perlvar/$_> to each element) and returns the list value composed
3968 results of each such evaluation. In scalar context, returns the
3969 total number of elements so generated. Evaluates BLOCK or EXPR in
3970 list context, so each element of LIST may produce zero, one, or
3971 more elements in the returned value.
3973 my @chars = map(chr, @numbers);
3975 translates a list of numbers to the corresponding characters.
3977 my @squares = map { $_ * $_ } @numbers;
3979 translates a list of numbers to their squared values.
3981 my @squares = map { $_ > 5 ? ($_ * $_) : () } @numbers;
3983 shows that number of returned elements can differ from the number of
3984 input elements. To omit an element, return an empty list ().
3985 This could also be achieved by writing
3987 my @squares = map { $_ * $_ } grep { $_ > 5 } @numbers;
3989 which makes the intention more clear.
3991 Map always returns a list, which can be
3992 assigned to a hash such that the elements
3993 become key/value pairs. See L<perldata> for more details.
3995 my %hash = map { get_a_key_for($_) => $_ } @array;
3997 is just a funny way to write
4001 $hash{get_a_key_for($_)} = $_;
4004 Note that L<C<$_>|perlvar/$_> is an alias to the list value, so it can
4005 be used to modify the elements of the LIST. While this is useful and
4006 supported, it can cause bizarre results if the elements of LIST are not
4007 variables. Using a regular C<foreach> loop for this purpose would be
4008 clearer in most cases. See also L<C<grep>|/grep BLOCK LIST> for a
4009 list composed of those items of the original list for which the BLOCK
4010 or EXPR evaluates to true.
4012 C<{> starts both hash references and blocks, so C<map { ...> could be either
4013 the start of map BLOCK LIST or map EXPR, LIST. Because Perl doesn't look
4014 ahead for the closing C<}> it has to take a guess at which it's dealing with
4015 based on what it finds just after the
4016 C<{>. Usually it gets it right, but if it
4017 doesn't it won't realize something is wrong until it gets to the C<}> and
4018 encounters the missing (or unexpected) comma. The syntax error will be
4019 reported close to the C<}>, but you'll need to change something near the C<{>
4020 such as using a unary C<+> or semicolon to give Perl some help:
4022 my %hash = map { "\L$_" => 1 } @array # perl guesses EXPR. wrong
4023 my %hash = map { +"\L$_" => 1 } @array # perl guesses BLOCK. right
4024 my %hash = map {; "\L$_" => 1 } @array # this also works
4025 my %hash = map { ("\L$_" => 1) } @array # as does this
4026 my %hash = map { lc($_) => 1 } @array # and this.
4027 my %hash = map +( lc($_) => 1 ), @array # this is EXPR and works!
4029 my %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
4031 or to force an anon hash constructor use C<+{>:
4033 my @hashes = map +{ lc($_) => 1 }, @array # EXPR, so needs
4036 to get a list of anonymous hashes each with only one entry apiece.
4038 =item mkdir FILENAME,MASK
4039 X<mkdir> X<md> X<directory, create>
4041 =item mkdir FILENAME
4045 =for Pod::Functions create a directory
4047 Creates the directory specified by FILENAME, with permissions
4048 specified by MASK (as modified by L<C<umask>|/umask EXPR>). If it
4049 succeeds it returns true; otherwise it returns false and sets
4050 L<C<$!>|perlvar/$!> (errno).
4051 MASK defaults to 0777 if omitted, and FILENAME defaults
4052 to L<C<$_>|perlvar/$_> if omitted.
4054 In general, it is better to create directories with a permissive MASK
4055 and let the user modify that with their L<C<umask>|/umask EXPR> than it
4057 a restrictive MASK and give the user no way to be more permissive.
4058 The exceptions to this rule are when the file or directory should be
4059 kept private (mail files, for instance). The documentation for
4060 L<C<umask>|/umask EXPR> discusses the choice of MASK in more detail.
4062 Note that according to the POSIX 1003.1-1996 the FILENAME may have any
4063 number of trailing slashes. Some operating and filesystems do not get
4064 this right, so Perl automatically removes all trailing slashes to keep
4067 To recursively create a directory structure, look at
4068 the L<C<make_path>|File::Path/make_path( $dir1, $dir2, .... )> function
4069 of the L<File::Path> module.
4071 =item msgctl ID,CMD,ARG
4074 =for Pod::Functions SysV IPC message control operations
4076 Calls the System V IPC function L<msgctl(2)>. You'll probably have to say
4080 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
4081 then ARG must be a variable that will hold the returned C<msqid_ds>
4082 structure. Returns like L<C<ioctl>|/ioctl FILEHANDLE,FUNCTION,SCALAR>:
4083 the undefined value for error, C<"0 but true"> for zero, or the actual
4084 return value otherwise. See also L<perlipc/"SysV IPC"> and the
4085 documentation for L<C<IPC::SysV>|IPC::SysV> and
4086 L<C<IPC::Semaphore>|IPC::Semaphore>.
4088 Portability issues: L<perlport/msgctl>.
4090 =item msgget KEY,FLAGS
4093 =for Pod::Functions get SysV IPC message queue
4095 Calls the System V IPC function L<msgget(2)>. Returns the message queue
4096 id, or L<C<undef>|/undef EXPR> on error. See also L<perlipc/"SysV IPC">
4097 and the documentation for L<C<IPC::SysV>|IPC::SysV> and
4098 L<C<IPC::Msg>|IPC::Msg>.
4100 Portability issues: L<perlport/msgget>.
4102 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
4105 =for Pod::Functions receive a SysV IPC message from a message queue
4107 Calls the System V IPC function msgrcv to receive a message from
4108 message queue ID into variable VAR with a maximum message size of
4109 SIZE. Note that when a message is received, the message type as a
4110 native long integer will be the first thing in VAR, followed by the
4111 actual message. This packing may be opened with C<unpack("l! a*")>.
4112 Taints the variable. Returns true if successful, false
4113 on error. See also L<perlipc/"SysV IPC"> and the documentation for
4114 L<C<IPC::SysV>|IPC::SysV> and L<C<IPC::Msg>|IPC::Msg>.
4116 Portability issues: L<perlport/msgrcv>.
4118 =item msgsnd ID,MSG,FLAGS
4121 =for Pod::Functions send a SysV IPC message to a message queue
4123 Calls the System V IPC function msgsnd to send the message MSG to the
4124 message queue ID. MSG must begin with the native long integer message
4125 type, be followed by the length of the actual message, and then finally
4126 the message itself. This kind of packing can be achieved with
4127 C<pack("l! a*", $type, $message)>. Returns true if successful,
4128 false on error. See also L<perlipc/"SysV IPC"> and the documentation
4129 for L<C<IPC::SysV>|IPC::SysV> and L<C<IPC::Msg>|IPC::Msg>.
4131 Portability issues: L<perlport/msgsnd>.
4136 =item my TYPE VARLIST
4138 =item my VARLIST : ATTRS
4140 =item my TYPE VARLIST : ATTRS
4142 =for Pod::Functions declare and assign a local variable (lexical scoping)
4144 A L<C<my>|/my VARLIST> declares the listed variables to be local
4145 (lexically) to the enclosing block, file, or L<C<eval>|/eval EXPR>. If
4146 more than one variable is listed, the list must be placed in
4149 The exact semantics and interface of TYPE and ATTRS are still
4150 evolving. TYPE may be a bareword, a constant declared
4151 with L<C<use constant>|constant>, or L<C<__PACKAGE__>|/__PACKAGE__>. It
4153 currently bound to the use of the L<fields> pragma,
4154 and attributes are handled using the L<attributes> pragma, or starting
4155 from Perl 5.8.0 also via the L<Attribute::Handlers> module. See
4156 L<perlsub/"Private Variables via my()"> for details.
4158 Note that with a parenthesised list, L<C<undef>|/undef EXPR> can be used
4159 as a dummy placeholder, for example to skip assignment of initial
4162 my ( undef, $min, $hour ) = localtime;
4171 =for Pod::Functions iterate a block prematurely
4173 The L<C<next>|/next LABEL> command is like the C<continue> statement in
4174 C; it starts the next iteration of the loop:
4176 LINE: while (<STDIN>) {
4177 next LINE if /^#/; # discard comments
4181 Note that if there were a L<C<continue>|/continue BLOCK> block on the
4183 executed even on discarded lines. If LABEL is omitted, the command
4184 refers to the innermost enclosing loop. The C<next EXPR> form, available
4185 as of Perl 5.18.0, allows a label name to be computed at run time, being
4186 otherwise identical to C<next LABEL>.
4188 L<C<next>|/next LABEL> cannot be used to exit a block which returns a
4189 value such as C<eval {}>, C<sub {}>, or C<do {}>, and should not be used
4190 to exit a L<C<grep>|/grep BLOCK LIST> or L<C<map>|/map BLOCK LIST>
4193 Note that a block by itself is semantically identical to a loop
4194 that executes once. Thus L<C<next>|/next LABEL> will exit such a block
4197 See also L<C<continue>|/continue BLOCK> for an illustration of how
4198 L<C<last>|/last LABEL>, L<C<next>|/next LABEL>, and
4199 L<C<redo>|/redo LABEL> work.
4201 Unlike most named operators, this has the same precedence as assignment.
4202 It is also exempt from the looks-like-a-function rule, so
4203 C<next ("foo")."bar"> will cause "bar" to be part of the argument to
4204 L<C<next>|/next LABEL>.
4206 =item no MODULE VERSION LIST
4210 =item no MODULE VERSION
4212 =item no MODULE LIST
4218 =for Pod::Functions unimport some module symbols or semantics at compile time
4220 See the L<C<use>|/use Module VERSION LIST> function, of which
4221 L<C<no>|/no MODULE VERSION LIST> is the opposite.
4224 X<oct> X<octal> X<hex> X<hexadecimal> X<binary> X<bin>
4228 =for Pod::Functions convert a string to an octal number
4230 Interprets EXPR as an octal string and returns the corresponding
4231 value. (If EXPR happens to start off with C<0x>, interprets it as a
4232 hex string. If EXPR starts off with C<0b>, it is interpreted as a
4233 binary string. Leading whitespace is ignored in all three cases.)
4234 The following will handle decimal, binary, octal, and hex in standard
4237 $val = oct($val) if $val =~ /^0/;
4239 If EXPR is omitted, uses L<C<$_>|perlvar/$_>. To go the other way
4240 (produce a number in octal), use L<C<sprintf>|/sprintf FORMAT, LIST> or
4241 L<C<printf>|/printf FILEHANDLE FORMAT, LIST>:
4243 my $dec_perms = (stat("filename"))[2] & 07777;
4244 my $oct_perm_str = sprintf "%o", $perms;
4246 The L<C<oct>|/oct EXPR> function is commonly used when a string such as
4248 to be converted into a file mode, for example. Although Perl
4249 automatically converts strings into numbers as needed, this automatic
4250 conversion assumes base 10.
4252 Leading white space is ignored without warning, as too are any trailing
4253 non-digits, such as a decimal point (L<C<oct>|/oct EXPR> only handles
4254 non-negative integers, not negative integers or floating point).
4256 =item open FILEHANDLE,EXPR
4257 X<open> X<pipe> X<file, open> X<fopen>
4259 =item open FILEHANDLE,MODE,EXPR
4261 =item open FILEHANDLE,MODE,EXPR,LIST
4263 =item open FILEHANDLE,MODE,REFERENCE
4265 =item open FILEHANDLE
4267 =for Pod::Functions open a file, pipe, or descriptor
4269 Opens the file whose filename is given by EXPR, and associates it with
4272 Simple examples to open a file for reading:
4274 open(my $fh, "<", "input.txt")
4275 or die "Can't open < input.txt: $!";
4279 open(my $fh, ">", "output.txt")
4280 or die "Can't open > output.txt: $!";
4282 (The following is a comprehensive reference to
4283 L<C<open>|/open FILEHANDLE,EXPR>: for a gentler introduction you may
4284 consider L<perlopentut>.)
4286 If FILEHANDLE is an undefined scalar variable (or array or hash element), a
4287 new filehandle is autovivified, meaning that the variable is assigned a
4288 reference to a newly allocated anonymous filehandle. Otherwise if
4289 FILEHANDLE is an expression, its value is the real filehandle. (This is
4290 considered a symbolic reference, so C<use strict "refs"> should I<not> be
4293 If three (or more) arguments are specified, the open mode (including
4294 optional encoding) in the second argument are distinct from the filename in
4295 the third. If MODE is C<< < >> or nothing, the file is opened for input.
4296 If MODE is C<< > >>, the file is opened for output, with existing files
4297 first being truncated ("clobbered") and nonexisting files newly created.
4298 If MODE is C<<< >> >>>, the file is opened for appending, again being
4299 created if necessary.
4301 You can put a C<+> in front of the C<< > >> or C<< < >> to
4302 indicate that you want both read and write access to the file; thus
4303 C<< +< >> is almost always preferred for read/write updates--the
4304 C<< +> >> mode would clobber the file first. You can't usually use
4305 either read-write mode for updating textfiles, since they have
4306 variable-length records. See the B<-i> switch in L<perlrun> for a
4307 better approach. The file is created with permissions of C<0666>
4308 modified by the process's L<C<umask>|/umask EXPR> value.
4310 These various prefixes correspond to the L<fopen(3)> modes of C<r>,
4311 C<r+>, C<w>, C<w+>, C<a>, and C<a+>.
4313 In the one- and two-argument forms of the call, the mode and filename
4314 should be concatenated (in that order), preferably separated by white
4315 space. You can--but shouldn't--omit the mode in these forms when that mode
4316 is C<< < >>. It is safe to use the two-argument form of
4317 L<C<open>|/open FILEHANDLE,EXPR> if the filename argument is a known literal.
4319 For three or more arguments if MODE is C<|->, the filename is
4320 interpreted as a command to which output is to be piped, and if MODE
4321 is C<-|>, the filename is interpreted as a command that pipes
4322 output to us. In the two-argument (and one-argument) form, one should
4323 replace dash (C<->) with the command.
4324 See L<perlipc/"Using open() for IPC"> for more examples of this.
4325 (You are not allowed to L<C<open>|/open FILEHANDLE,EXPR> to a command
4326 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>, and
4327 L<perlipc/"Bidirectional Communication with Another Process"> for
4330 In the form of pipe opens taking three or more arguments, if LIST is specified
4331 (extra arguments after the command name) then LIST becomes arguments
4332 to the command invoked if the platform supports it. The meaning of
4333 L<C<open>|/open FILEHANDLE,EXPR> with more than three arguments for
4334 non-pipe modes is not yet defined, but experimental "layers" may give
4335 extra LIST arguments meaning.
4337 In the two-argument (and one-argument) form, opening C<< <- >>
4338 or C<-> opens STDIN and opening C<< >- >> opens STDOUT.
4340 You may (and usually should) use the three-argument form of open to specify
4341 I/O layers (sometimes referred to as "disciplines") to apply to the handle
4342 that affect how the input and output are processed (see L<open> and
4343 L<PerlIO> for more details). For example:
4345 open(my $fh, "<:encoding(UTF-8)", $filename)
4346 || die "Can't open UTF-8 encoded $filename: $!";
4348 opens the UTF8-encoded file containing Unicode characters;
4349 see L<perluniintro>. Note that if layers are specified in the
4350 three-argument form, then default layers stored in ${^OPEN} (see L<perlvar>;
4351 usually set by the L<open> pragma or the switch C<-CioD>) are ignored.
4352 Those layers will also be ignored if you specify a colon with no name
4353 following it. In that case the default layer for the operating system
4354 (:raw on Unix, :crlf on Windows) is used.
4356 Open returns nonzero on success, the undefined value otherwise. If
4357 the L<C<open>|/open FILEHANDLE,EXPR> involved a pipe, the return value
4358 happens to be the pid of the subprocess.
4360 On some systems (in general, DOS- and Windows-based systems)
4361 L<C<binmode>|/binmode FILEHANDLE, LAYER> is necessary when you're not
4362 working with a text file. For the sake of portability it is a good idea
4363 always to use it when appropriate, and never to use it when it isn't
4364 appropriate. Also, people can set their I/O to be by default
4365 UTF8-encoded Unicode, not bytes.
4367 When opening a file, it's seldom a good idea to continue
4368 if the request failed, so L<C<open>|/open FILEHANDLE,EXPR> is frequently
4369 used with L<C<die>|/die LIST>. Even if L<C<die>|/die LIST> won't do
4370 what you want (say, in a CGI script,
4371 where you want to format a suitable error message (but there are
4372 modules that can help with that problem)) always check
4373 the return value from opening a file.
4375 The filehandle will be closed when its reference count reaches zero.
4376 If it is a lexically scoped variable declared with L<C<my>|/my VARLIST>,
4378 means the end of the enclosing scope. However, this automatic close
4379 does not check for errors, so it is better to explicitly close
4380 filehandles, especially those used for writing:
4383 || warn "close failed: $!";
4385 An older style is to use a bareword as the filehandle, as
4387 open(FH, "<", "input.txt")
4388 or die "Can't open < input.txt: $!";
4390 Then you can use C<FH> as the filehandle, in C<< close FH >> and C<<
4391 <FH> >> and so on. Note that it's a global variable, so this form is
4392 not recommended in new code.
4394 As a shortcut a one-argument call takes the filename from the global
4395 scalar variable of the same name as the filehandle:
4398 open(ARTICLE) or die "Can't find article $ARTICLE: $!\n";
4400 Here C<$ARTICLE> must be a global (package) scalar variable - not one
4401 declared with L<C<my>|/my VARLIST> or L<C<state>|/state VARLIST>.
4403 As a special case the three-argument form with a read/write mode and the third
4404 argument being L<C<undef>|/undef EXPR>:
4406 open(my $tmp, "+>", undef) or die ...
4408 opens a filehandle to a newly created empty anonymous temporary file.
4409 (This happens under any mode, which makes C<< +> >> the only useful and
4410 sensible mode to use.) You will need to
4411 L<C<seek>|/seek FILEHANDLE,POSITION,WHENCE> to do the reading.
4413 Perl is built using PerlIO by default. Unless you've
4414 changed this (such as building Perl with C<Configure -Uuseperlio>), you can
4415 open filehandles directly to Perl scalars via:
4417 open(my $fh, ">", \$variable) || ..
4419 To (re)open C<STDOUT> or C<STDERR> as an in-memory file, close it first:
4422 open(STDOUT, ">", \$variable)
4423 or die "Can't open STDOUT: $!";
4425 See L<perliol> for detailed info on PerlIO.
4429 open(my $log, ">>", "/usr/spool/news/twitlog");
4430 # if the open fails, output is discarded
4432 open(my $dbase, "+<", "dbase.mine") # open for update
4433 or die "Can't open 'dbase.mine' for update: $!";
4435 open(my $dbase, "+<dbase.mine") # ditto
4436 or die "Can't open 'dbase.mine' for update: $!";
4438 open(my $article_fh, "-|", "caesar <$article") # decrypt
4440 or die "Can't start caesar: $!";
4442 open(my $article_fh, "caesar <$article |") # ditto
4443 or die "Can't start caesar: $!";
4445 open(my $out_fh, "|-", "sort >Tmp$$") # $$ is our process id
4446 or die "Can't start sort: $!";
4449 open(my $memory, ">", \$var)
4450 or die "Can't open memory file: $!";
4451 print $memory "foo!\n"; # output will appear in $var
4453 You may also, in the Bourne shell tradition, specify an EXPR beginning
4454 with C<< >& >>, in which case the rest of the string is interpreted
4455 as the name of a filehandle (or file descriptor, if numeric) to be
4456 duped (as in L<dup(2)>) and opened. You may use C<&> after C<< > >>,
4457 C<<< >> >>>, C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>.
4458 The mode you specify should match the mode of the original filehandle.
4459 (Duping a filehandle does not take into account any existing contents
4460 of IO buffers.) If you use the three-argument
4461 form, then you can pass either a
4462 number, the name of a filehandle, or the normal "reference to a glob".
4464 Here is a script that saves, redirects, and restores C<STDOUT> and
4465 C<STDERR> using various methods:
4468 open(my $oldout, ">&STDOUT") or die "Can't dup STDOUT: $!";
4469 open(OLDERR, ">&", \*STDERR) or die "Can't dup STDERR: $!";
4471 open(STDOUT, '>', "foo.out") or die "Can't redirect STDOUT: $!";
4472 open(STDERR, ">&STDOUT") or die "Can't dup STDOUT: $!";
4474 select STDERR; $| = 1; # make unbuffered
4475 select STDOUT; $| = 1; # make unbuffered
4477 print STDOUT "stdout 1\n"; # this works for
4478 print STDERR "stderr 1\n"; # subprocesses too
4480 open(STDOUT, ">&", $oldout) or die "Can't dup \$oldout: $!";
4481 open(STDERR, ">&OLDERR") or die "Can't dup OLDERR: $!";
4483 print STDOUT "stdout 2\n";
4484 print STDERR "stderr 2\n";
4486 If you specify C<< '<&=X' >>, where C<X> is a file descriptor number
4487 or a filehandle, then Perl will do an equivalent of C's L<fdopen(3)> of
4488 that file descriptor (and not call L<dup(2)>); this is more
4489 parsimonious of file descriptors. For example:
4491 # open for input, reusing the fileno of $fd
4492 open(my $fh, "<&=", $fd)
4496 open(my $fh, "<&=$fd")
4500 # open for append, using the fileno of $oldfh
4501 open(my $fh, ">>&=", $oldfh)
4503 Being parsimonious on filehandles is also useful (besides being
4504 parsimonious) for example when something is dependent on file
4505 descriptors, like for example locking using
4506 L<C<flock>|/flock FILEHANDLE,OPERATION>. If you do just
4507 C<< open(my $A, ">>&", $B) >>, the filehandle C<$A> will not have the
4508 same file descriptor as C<$B>, and therefore C<flock($A)> will not
4509 C<flock($B)> nor vice versa. But with C<< open(my $A, ">>&=", $B) >>,
4510 the filehandles will share the same underlying system file descriptor.
4512 Note that under Perls older than 5.8.0, Perl uses the standard C library's'
4513 L<fdopen(3)> to implement the C<=> functionality. On many Unix systems,
4514 L<fdopen(3)> fails when file descriptors exceed a certain value, typically 255.
4515 For Perls 5.8.0 and later, PerlIO is (most often) the default.
4517 You can see whether your Perl was built with PerlIO by running
4518 C<perl -V:useperlio>. If it says C<'define'>, you have PerlIO;
4519 otherwise you don't.
4521 If you open a pipe on the command C<-> (that is, specify either C<|-> or C<-|>
4522 with the one- or two-argument forms of
4523 L<C<open>|/open FILEHANDLE,EXPR>), an implicit L<C<fork>|/fork> is done,
4524 so L<C<open>|/open FILEHANDLE,EXPR> returns twice: in the parent process
4526 of the child process, and in the child process it returns (a defined) C<0>.
4527 Use C<defined($pid)> or C<//> to determine whether the open was successful.
4529 For example, use either
4531 my $child_pid = open(my $from_kid, "-|") // die "Can't fork: $!";
4535 my $child_pid = open(my $to_kid, "|-") // die "Can't fork: $!";
4541 # either write $to_kid or else read $from_kid
4543 waitpid $child_pid, 0;
4545 # am the child; use STDIN/STDOUT normally
4550 The filehandle behaves normally for the parent, but I/O to that
4551 filehandle is piped from/to the STDOUT/STDIN of the child process.
4552 In the child process, the filehandle isn't opened--I/O happens from/to
4553 the new STDOUT/STDIN. Typically this is used like the normal
4554 piped open when you want to exercise more control over just how the
4555 pipe command gets executed, such as when running setuid and
4556 you don't want to have to scan shell commands for metacharacters.
4558 The following blocks are more or less equivalent:
4560 open(my $fh, "|tr '[a-z]' '[A-Z]'");
4561 open(my $fh, "|-", "tr '[a-z]' '[A-Z]'");
4562 open(my $fh, "|-") || exec 'tr', '[a-z]', '[A-Z]';
4563 open(my $fh, "|-", "tr", '[a-z]', '[A-Z]');
4565 open(my $fh, "cat -n '$file'|");
4566 open(my $fh, "-|", "cat -n '$file'");
4567 open(my $fh, "-|") || exec "cat", "-n", $file;
4568 open(my $fh, "-|", "cat", "-n", $file);
4570 The last two examples in each block show the pipe as "list form", which is
4571 not yet supported on all platforms. A good rule of thumb is that if
4572 your platform has a real L<C<fork>|/fork> (in other words, if your platform is
4573 Unix, including Linux and MacOS X), you can use the list form. You would
4574 want to use the list form of the pipe so you can pass literal arguments
4575 to the command without risk of the shell interpreting any shell metacharacters
4576 in them. However, this also bars you from opening pipes to commands
4577 that intentionally contain shell metacharacters, such as:
4579 open(my $fh, "|cat -n | expand -4 | lpr")
4580 || die "Can't open pipeline to lpr: $!";
4582 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
4584 Perl will attempt to flush all files opened for
4585 output before any operation that may do a fork, but this may not be
4586 supported on some platforms (see L<perlport>). To be safe, you may need
4587 to set L<C<$E<verbar>>|perlvar/$E<verbar>> (C<$AUTOFLUSH> in L<English>)
4588 or call the C<autoflush> method of L<C<IO::Handle>|IO::Handle/METHODS>
4589 on any open handles.
4591 On systems that support a close-on-exec flag on files, the flag will
4592 be set for the newly opened file descriptor as determined by the value
4593 of L<C<$^F>|perlvar/$^F>. See L<perlvar/$^F>.
4595 Closing any piped filehandle causes the parent process to wait for the
4596 child to finish, then returns the status value in L<C<$?>|perlvar/$?> and
4597 L<C<${^CHILD_ERROR_NATIVE}>|perlvar/${^CHILD_ERROR_NATIVE}>.
4599 The filename passed to the one- and two-argument forms of
4600 L<C<open>|/open FILEHANDLE,EXPR> will
4601 have leading and trailing whitespace deleted and normal
4602 redirection characters honored. This property, known as "magic open",
4603 can often be used to good effect. A user could specify a filename of
4604 F<"rsh cat file |">, or you could change certain filenames as needed:
4606 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
4607 open(my $fh, $filename) or die "Can't open $filename: $!";
4609 Use the three-argument form to open a file with arbitrary weird characters in it,
4611 open(my $fh, "<", $file)
4612 || die "Can't open $file: $!";
4614 otherwise it's necessary to protect any leading and trailing whitespace:
4616 $file =~ s#^(\s)#./$1#;
4617 open(my $fh, "< $file\0")
4618 || die "Can't open $file: $!";
4620 (this may not work on some bizarre filesystems). One should
4621 conscientiously choose between the I<magic> and I<three-argument> form
4622 of L<C<open>|/open FILEHANDLE,EXPR>:
4624 open(my $in, $ARGV[0]) || die "Can't open $ARGV[0]: $!";
4626 will allow the user to specify an argument of the form C<"rsh cat file |">,
4627 but will not work on a filename that happens to have a trailing space, while
4629 open(my $in, "<", $ARGV[0])
4630 || die "Can't open $ARGV[0]: $!";
4632 will have exactly the opposite restrictions. (However, some shells
4633 support the syntax C<< perl your_program.pl <( rsh cat file ) >>, which
4634 produces a filename that can be opened normally.)
4636 If you want a "real" C L<open(2)>, then you should use the
4637 L<C<sysopen>|/sysopen FILEHANDLE,FILENAME,MODE> function, which involves
4638 no such magic (but uses different filemodes than Perl
4639 L<C<open>|/open FILEHANDLE,EXPR>, which corresponds to C L<fopen(3)>).
4640 This is another way to protect your filenames from interpretation. For
4644 sysopen(my $fh, $path, O_RDWR|O_CREAT|O_EXCL)
4645 or die "Can't open $path: $!";
4647 print $fh "stuff $$\n";
4649 print "File contains: ", readline($fh);
4651 See L<C<seek>|/seek FILEHANDLE,POSITION,WHENCE> for some details about
4652 mixing reading and writing.
4654 Portability issues: L<perlport/open>.
4656 =item opendir DIRHANDLE,EXPR
4659 =for Pod::Functions open a directory
4661 Opens a directory named EXPR for processing by
4662 L<C<readdir>|/readdir DIRHANDLE>, L<C<telldir>|/telldir DIRHANDLE>,
4663 L<C<seekdir>|/seekdir DIRHANDLE,POS>,
4664 L<C<rewinddir>|/rewinddir DIRHANDLE>, and
4665 L<C<closedir>|/closedir DIRHANDLE>. Returns true if successful.
4666 DIRHANDLE may be an expression whose value can be used as an indirect
4667 dirhandle, usually the real dirhandle name. If DIRHANDLE is an undefined
4668 scalar variable (or array or hash element), the variable is assigned a
4669 reference to a new anonymous dirhandle; that is, it's autovivified.
4670 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
4672 See the example at L<C<readdir>|/readdir DIRHANDLE>.
4679 =for Pod::Functions find a character's numeric representation
4681 Returns the numeric value of the first character of EXPR.
4682 If EXPR is an empty string, returns 0. If EXPR is omitted, uses
4683 L<C<$_>|perlvar/$_>.
4684 (Note I<character>, not byte.)
4686 For the reverse, see L<C<chr>|/chr NUMBER>.
4687 See L<perlunicode> for more about Unicode.
4692 =item our TYPE VARLIST
4694 =item our VARLIST : ATTRS
4696 =item our TYPE VARLIST : ATTRS
4698 =for Pod::Functions +5.6.0 declare and assign a package variable (lexical scoping)
4700 L<C<our>|/our VARLIST> makes a lexical alias to a package (i.e. global)
4701 variable of the same name in the current package for use within the
4702 current lexical scope.
4704 L<C<our>|/our VARLIST> has the same scoping rules as
4705 L<C<my>|/my VARLIST> or L<C<state>|/state VARLIST>, meaning that it is
4706 only valid within a lexical scope. Unlike L<C<my>|/my VARLIST> and
4707 L<C<state>|/state VARLIST>, which both declare new (lexical) variables,
4708 L<C<our>|/our VARLIST> only creates an alias to an existing variable: a
4709 package variable of the same name.
4711 This means that when C<use strict 'vars'> is in effect, L<C<our>|/our
4712 VARLIST> lets you use a package variable without qualifying it with the
4713 package name, but only within the lexical scope of the
4714 L<C<our>|/our VARLIST> declaration. This applies immediately--even
4715 within the same statement.
4723 our $foo; # alias to $Foo::foo
4724 print $foo; # prints 23
4727 print $Foo::foo; # prints 23
4729 print $foo; # ERROR: requires explicit package name
4731 This works even if the package variable has not been used before, as
4732 package variables spring into existence when first used.
4737 our $foo = 23; # just like $Foo::foo = 23
4739 print $Foo::foo; # prints 23
4741 Because the variable becomes legal immediately under C<use strict 'vars'>, so
4742 long as there is no variable with that name is already in scope, you can then
4743 reference the package variable again even within the same statement.
4748 my $foo = $foo; # error, undeclared $foo on right-hand side
4749 our $foo = $foo; # no errors
4751 If more than one variable is listed, the list must be placed
4756 An L<C<our>|/our VARLIST> declaration declares an alias for a package
4757 variable that will be visible
4758 across its entire lexical scope, even across package boundaries. The
4759 package in which the variable is entered is determined at the point
4760 of the declaration, not at the point of use. This means the following
4764 our $bar; # declares $Foo::bar for rest of lexical scope
4768 print $bar; # prints 20, as it refers to $Foo::bar
4770 Multiple L<C<our>|/our VARLIST> declarations with the same name in the
4772 scope are allowed if they are in different packages. If they happen
4773 to be in the same package, Perl will emit warnings if you have asked
4774 for them, just like multiple L<C<my>|/my VARLIST> declarations. Unlike
4775 a second L<C<my>|/my VARLIST> declaration, which will bind the name to a
4776 fresh variable, a second L<C<our>|/our VARLIST> declaration in the same
4777 package, in the same scope, is merely redundant.
4781 our $bar; # declares $Foo::bar for rest of lexical scope
4785 our $bar = 30; # declares $Bar::bar for rest of lexical scope
4786 print $bar; # prints 30
4788 our $bar; # emits warning but has no other effect
4789 print $bar; # still prints 30
4791 An L<C<our>|/our VARLIST> declaration may also have a list of attributes
4794 The exact semantics and interface of TYPE and ATTRS are still
4795 evolving. TYPE is currently bound to the use of the L<fields> pragma,
4796 and attributes are handled using the L<attributes> pragma, or, starting
4797 from Perl 5.8.0, also via the L<Attribute::Handlers> module. See
4798 L<perlsub/"Private Variables via my()"> for details.
4800 Note that with a parenthesised list, L<C<undef>|/undef EXPR> can be used
4801 as a dummy placeholder, for example to skip assignment of initial
4804 our ( undef, $min, $hour ) = localtime;
4806 L<C<our>|/our VARLIST> differs from L<C<use vars>|vars>, which allows
4807 use of an unqualified name I<only> within the affected package, but
4810 =item pack TEMPLATE,LIST
4813 =for Pod::Functions convert a list into a binary representation
4815 Takes a LIST of values and converts it into a string using the rules
4816 given by the TEMPLATE. The resulting string is the concatenation of
4817 the converted values. Typically, each converted value looks
4818 like its machine-level representation. For example, on 32-bit machines
4819 an integer may be represented by a sequence of 4 bytes, which will in
4820 Perl be presented as a string that's 4 characters long.
4822 See L<perlpacktut> for an introduction to this function.
4824 The TEMPLATE is a sequence of characters that give the order and type
4825 of values, as follows:
4827 a A string with arbitrary binary data, will be null padded.
4828 A A text (ASCII) string, will be space padded.
4829 Z A null-terminated (ASCIZ) string, will be null padded.
4831 b A bit string (ascending bit order inside each byte,
4833 B A bit string (descending bit order inside each byte).
4834 h A hex string (low nybble first).
4835 H A hex string (high nybble first).
4837 c A signed char (8-bit) value.
4838 C An unsigned char (octet) value.
4839 W An unsigned char value (can be greater than 255).
4841 s A signed short (16-bit) value.
4842 S An unsigned short value.
4844 l A signed long (32-bit) value.
4845 L An unsigned long value.
4847 q A signed quad (64-bit) value.
4848 Q An unsigned quad value.
4849 (Quads are available only if your system supports 64-bit
4850 integer values _and_ if Perl has been compiled to support
4851 those. Raises an exception otherwise.)
4853 i A signed integer value.
4854 I A unsigned integer value.
4855 (This 'integer' is _at_least_ 32 bits wide. Its exact
4856 size depends on what a local C compiler calls 'int'.)
4858 n An unsigned short (16-bit) in "network" (big-endian) order.
4859 N An unsigned long (32-bit) in "network" (big-endian) order.
4860 v An unsigned short (16-bit) in "VAX" (little-endian) order.
4861 V An unsigned long (32-bit) in "VAX" (little-endian) order.
4863 j A Perl internal signed integer value (IV).
4864 J A Perl internal unsigned integer value (UV).
4866 f A single-precision float in native format.
4867 d A double-precision float in native format.
4869 F A Perl internal floating-point value (NV) in native format
4870 D A float of long-double precision in native format.
4871 (Long doubles are available only if your system supports
4872 long double values _and_ if Perl has been compiled to
4873 support those. Raises an exception otherwise.
4874 Note that there are different long double formats.)
4876 p A pointer to a null-terminated string.
4877 P A pointer to a structure (fixed-length string).
4879 u A uuencoded string.
4880 U A Unicode character number. Encodes to a character in char-
4881 acter mode and UTF-8 (or UTF-EBCDIC in EBCDIC platforms) in
4884 w A BER compressed integer (not an ASN.1 BER, see perlpacktut
4885 for details). Its bytes represent an unsigned integer in
4886 base 128, most significant digit first, with as few digits
4887 as possible. Bit eight (the high bit) is set on each byte
4890 x A null byte (a.k.a ASCII NUL, "\000", chr(0))
4892 @ Null-fill or truncate to absolute position, counted from the
4893 start of the innermost ()-group.
4894 . Null-fill or truncate to absolute position specified by
4896 ( Start of a ()-group.
4898 One or more modifiers below may optionally follow certain letters in the
4899 TEMPLATE (the second column lists letters for which the modifier is valid):
4901 ! sSlLiI Forces native (short, long, int) sizes instead
4902 of fixed (16-/32-bit) sizes.
4904 ! xX Make x and X act as alignment commands.
4906 ! nNvV Treat integers as signed instead of unsigned.
4908 ! @. Specify position as byte offset in the internal
4909 representation of the packed string. Efficient
4912 > sSiIlLqQ Force big-endian byte-order on the type.
4913 jJfFdDpP (The "big end" touches the construct.)
4915 < sSiIlLqQ Force little-endian byte-order on the type.
4916 jJfFdDpP (The "little end" touches the construct.)
4918 The C<< > >> and C<< < >> modifiers can also be used on C<()> groups
4919 to force a particular byte-order on all components in that group,
4920 including all its subgroups.
4924 Larry recalls that the hex and bit string formats (H, h, B, b) were added to
4925 pack for processing data from NASA's Magellan probe. Magellan was in an
4926 elliptical orbit, using the antenna for the radar mapping when close to
4927 Venus and for communicating data back to Earth for the rest of the orbit.
4928 There were two transmission units, but one of these failed, and then the
4929 other developed a fault whereby it would randomly flip the sense of all the
4930 bits. It was easy to automatically detect complete records with the correct
4931 sense, and complete records with all the bits flipped. However, this didn't
4932 recover the records where the sense flipped midway. A colleague of Larry's
4933 was able to pretty much eyeball where the records flipped, so they wrote an
4934 editor named kybble (a pun on the dog food Kibbles 'n Bits) to enable him to
4935 manually correct the records and recover the data. For this purpose pack
4936 gained the hex and bit string format specifiers.
4938 git shows that they were added to perl 3.0 in patch #44 (Jan 1991, commit
4939 27e2fb84680b9cc1), but the patch description makes no mention of their
4940 addition, let alone the story behind them.
4944 The following rules apply:
4950 Each letter may optionally be followed by a number indicating the repeat
4951 count. A numeric repeat count may optionally be enclosed in brackets, as
4952 in C<pack("C[80]", @arr)>. The repeat count gobbles that many values from
4953 the LIST when used with all format types other than C<a>, C<A>, C<Z>, C<b>,
4954 C<B>, C<h>, C<H>, C<@>, C<.>, C<x>, C<X>, and C<P>, where it means
4955 something else, described below. Supplying a C<*> for the repeat count
4956 instead of a number means to use however many items are left, except for:
4962 C<@>, C<x>, and C<X>, where it is equivalent to C<0>.
4966 <.>, where it means relative to the start of the string.
4970 C<u>, where it is equivalent to 1 (or 45, which here is equivalent).
4974 One can replace a numeric repeat count with a template letter enclosed in
4975 brackets to use the packed byte length of the bracketed template for the
4978 For example, the template C<x[L]> skips as many bytes as in a packed long,
4979 and the template C<"$t X[$t] $t"> unpacks twice whatever $t (when
4980 variable-expanded) unpacks. If the template in brackets contains alignment
4981 commands (such as C<x![d]>), its packed length is calculated as if the
4982 start of the template had the maximal possible alignment.
4984 When used with C<Z>, a C<*> as the repeat count is guaranteed to add a
4985 trailing null byte, so the resulting string is always one byte longer than
4986 the byte length of the item itself.
4988 When used with C<@>, the repeat count represents an offset from the start
4989 of the innermost C<()> group.
4991 When used with C<.>, the repeat count determines the starting position to
4992 calculate the value offset as follows:
4998 If the repeat count is C<0>, it's relative to the current position.
5002 If the repeat count is C<*>, the offset is relative to the start of the
5007 And if it's an integer I<n>, the offset is relative to the start of the
5008 I<n>th innermost C<( )> group, or to the start of the string if I<n> is
5009 bigger then the group level.
5013 The repeat count for C<u> is interpreted as the maximal number of bytes
5014 to encode per line of output, with 0, 1 and 2 replaced by 45. The repeat
5015 count should not be more than 65.
5019 The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
5020 string of length count, padding with nulls or spaces as needed. When
5021 unpacking, C<A> strips trailing whitespace and nulls, C<Z> strips everything
5022 after the first null, and C<a> returns data with no stripping at all.
5024 If the value to pack is too long, the result is truncated. If it's too
5025 long and an explicit count is provided, C<Z> packs only C<$count-1> bytes,
5026 followed by a null byte. Thus C<Z> always packs a trailing null, except
5027 when the count is 0.
5031 Likewise, the C<b> and C<B> formats pack a string that's that many bits long.
5032 Each such format generates 1 bit of the result. These are typically followed
5033 by a repeat count like C<B8> or C<B64>.
5035 Each result bit is based on the least-significant bit of the corresponding
5036 input character, i.e., on C<ord($char)%2>. In particular, characters C<"0">
5037 and C<"1"> generate bits 0 and 1, as do characters C<"\000"> and C<"\001">.
5039 Starting from the beginning of the input string, each 8-tuple
5040 of characters is converted to 1 character of output. With format C<b>,
5041 the first character of the 8-tuple determines the least-significant bit of a
5042 character; with format C<B>, it determines the most-significant bit of
5045 If the length of the input string is not evenly divisible by 8, the
5046 remainder is packed as if the input string were padded by null characters
5047 at the end. Similarly during unpacking, "extra" bits are ignored.
5049 If the input string is longer than needed, remaining characters are ignored.
5051 A C<*> for the repeat count uses all characters of the input field.
5052 On unpacking, bits are converted to a string of C<0>s and C<1>s.
5056 The C<h> and C<H> formats pack a string that many nybbles (4-bit groups,
5057 representable as hexadecimal digits, C<"0".."9"> C<"a".."f">) long.
5059 For each such format, L<C<pack>|/pack TEMPLATE,LIST> generates 4 bits of result.
5060 With non-alphabetical characters, the result is based on the 4 least-significant
5061 bits of the input character, i.e., on C<ord($char)%16>. In particular,
5062 characters C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
5063 C<"\000"> and C<"\001">. For characters C<"a".."f"> and C<"A".."F">, the result
5064 is compatible with the usual hexadecimal digits, so that C<"a"> and
5065 C<"A"> both generate the nybble C<0xA==10>. Use only these specific hex
5066 characters with this format.
5068 Starting from the beginning of the template to
5069 L<C<pack>|/pack TEMPLATE,LIST>, each pair
5070 of characters is converted to 1 character of output. With format C<h>, the
5071 first character of the pair determines the least-significant nybble of the
5072 output character; with format C<H>, it determines the most-significant
5075 If the length of the input string is not even, it behaves as if padded by
5076 a null character at the end. Similarly, "extra" nybbles are ignored during
5079 If the input string is longer than needed, extra characters are ignored.
5081 A C<*> for the repeat count uses all characters of the input field. For
5082 L<C<unpack>|/unpack TEMPLATE,EXPR>, nybbles are converted to a string of
5087 The C<p> format packs a pointer to a null-terminated string. You are
5088 responsible for ensuring that the string is not a temporary value, as that
5089 could potentially get deallocated before you got around to using the packed
5090 result. The C<P> format packs a pointer to a structure of the size indicated
5091 by the length. A null pointer is created if the corresponding value for
5092 C<p> or C<P> is L<C<undef>|/undef EXPR>; similarly with
5093 L<C<unpack>|/unpack TEMPLATE,EXPR>, where a null pointer unpacks into
5094 L<C<undef>|/undef EXPR>.
5096 If your system has a strange pointer size--meaning a pointer is neither as
5097 big as an int nor as big as a long--it may not be possible to pack or
5098 unpack pointers in big- or little-endian byte order. Attempting to do
5099 so raises an exception.
5103 The C</> template character allows packing and unpacking of a sequence of
5104 items where the packed structure contains a packed item count followed by
5105 the packed items themselves. This is useful when the structure you're
5106 unpacking has encoded the sizes or repeat counts for some of its fields
5107 within the structure itself as separate fields.
5109 For L<C<pack>|/pack TEMPLATE,LIST>, you write
5110 I<length-item>C</>I<sequence-item>, and the
5111 I<length-item> describes how the length value is packed. Formats likely
5112 to be of most use are integer-packing ones like C<n> for Java strings,
5113 C<w> for ASN.1 or SNMP, and C<N> for Sun XDR.
5115 For L<C<pack>|/pack TEMPLATE,LIST>, I<sequence-item> may have a repeat
5116 count, in which case
5117 the minimum of that and the number of available items is used as the argument
5118 for I<length-item>. If it has no repeat count or uses a '*', the number
5119 of available items is used.
5121 For L<C<unpack>|/unpack TEMPLATE,EXPR>, an internal stack of integer
5122 arguments unpacked so far is
5123 used. You write C</>I<sequence-item> and the repeat count is obtained by
5124 popping off the last element from the stack. The I<sequence-item> must not
5125 have a repeat count.
5127 If I<sequence-item> refers to a string type (C<"A">, C<"a">, or C<"Z">),
5128 the I<length-item> is the string length, not the number of strings. With
5129 an explicit repeat count for pack, the packed string is adjusted to that
5130 length. For example:
5132 This code: gives this result:
5134 unpack("W/a", "\004Gurusamy") ("Guru")
5135 unpack("a3/A A*", "007 Bond J ") (" Bond", "J")
5136 unpack("a3 x2 /A A*", "007: Bond, J.") ("Bond, J", ".")
5138 pack("n/a* w/a","hello,","world") "\000\006hello,\005world"
5139 pack("a/W2", ord("a") .. ord("z")) "2ab"
5141 The I<length-item> is not returned explicitly from
5142 L<C<unpack>|/unpack TEMPLATE,EXPR>.
5144 Supplying a count to the I<length-item> format letter is only useful with
5145 C<A>, C<a>, or C<Z>. Packing with a I<length-item> of C<a> or C<Z> may
5146 introduce C<"\000"> characters, which Perl does not regard as legal in
5151 The integer types C<s>, C<S>, C<l>, and C<L> may be
5152 followed by a C<!> modifier to specify native shorts or
5153 longs. As shown in the example above, a bare C<l> means
5154 exactly 32 bits, although the native C<long> as seen by the local C compiler
5155 may be larger. This is mainly an issue on 64-bit platforms. You can
5156 see whether using C<!> makes any difference this way:
5158 printf "format s is %d, s! is %d\n",
5159 length pack("s"), length pack("s!");
5161 printf "format l is %d, l! is %d\n",
5162 length pack("l"), length pack("l!");
5165 C<i!> and C<I!> are also allowed, but only for completeness' sake:
5166 they are identical to C<i> and C<I>.
5168 The actual sizes (in bytes) of native shorts, ints, longs, and long
5169 longs on the platform where Perl was built are also available from
5172 $ perl -V:{short,int,long{,long}}size
5178 or programmatically via the L<C<Config>|Config> module:
5181 print $Config{shortsize}, "\n";
5182 print $Config{intsize}, "\n";
5183 print $Config{longsize}, "\n";
5184 print $Config{longlongsize}, "\n";
5186 C<$Config{longlongsize}> is undefined on systems without
5191 The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, C<L>, C<j>, and C<J> are
5192 inherently non-portable between processors and operating systems because
5193 they obey native byteorder and endianness. For example, a 4-byte integer
5194 0x12345678 (305419896 decimal) would be ordered natively (arranged in and
5195 handled by the CPU registers) into bytes as
5197 0x12 0x34 0x56 0x78 # big-endian
5198 0x78 0x56 0x34 0x12 # little-endian
5200 Basically, Intel and VAX CPUs are little-endian, while everybody else,
5201 including Motorola m68k/88k, PPC, Sparc, HP PA, Power, and Cray, are
5202 big-endian. Alpha and MIPS can be either: Digital/Compaq uses (well, used)
5203 them in little-endian mode, but SGI/Cray uses them in big-endian mode.
5205 The names I<big-endian> and I<little-endian> are comic references to the
5206 egg-eating habits of the little-endian Lilliputians and the big-endian
5207 Blefuscudians from the classic Jonathan Swift satire, I<Gulliver's Travels>.
5208 This entered computer lingo via the paper "On Holy Wars and a Plea for
5209 Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980.
5211 Some systems may have even weirder byte orders such as
5216 These are called mid-endian, middle-endian, mixed-endian, or just weird.
5218 You can determine your system endianness with this incantation:
5220 printf("%#02x ", $_) for unpack("W*", pack L=>0x12345678);
5222 The byteorder on the platform where Perl was built is also available
5226 print "$Config{byteorder}\n";
5228 or from the command line:
5232 Byteorders C<"1234"> and C<"12345678"> are little-endian; C<"4321">
5233 and C<"87654321"> are big-endian. Systems with multiarchitecture binaries
5234 will have C<"ffff">, signifying that static information doesn't work,
5235 one must use runtime probing.
5237 For portably packed integers, either use the formats C<n>, C<N>, C<v>,
5238 and C<V> or else use the C<< > >> and C<< < >> modifiers described
5239 immediately below. See also L<perlport>.
5243 Also floating point numbers have endianness. Usually (but not always)
5244 this agrees with the integer endianness. Even though most platforms
5245 these days use the IEEE 754 binary format, there are differences,
5246 especially if the long doubles are involved. You can see the
5247 C<Config> variables C<doublekind> and C<longdblkind> (also C<doublesize>,
5248 C<longdblsize>): the "kind" values are enums, unlike C<byteorder>.
5250 Portability-wise the best option is probably to keep to the IEEE 754
5251 64-bit doubles, and of agreed-upon endianness. Another possibility
5252 is the C<"%a">) format of L<C<printf>|/printf FILEHANDLE FORMAT, LIST>.
5256 Starting with Perl 5.10.0, integer and floating-point formats, along with
5257 the C<p> and C<P> formats and C<()> groups, may all be followed by the
5258 C<< > >> or C<< < >> endianness modifiers to respectively enforce big-
5259 or little-endian byte-order. These modifiers are especially useful
5260 given how C<n>, C<N>, C<v>, and C<V> don't cover signed integers,
5261 64-bit integers, or floating-point values.
5263 Here are some concerns to keep in mind when using an endianness modifier:
5269 Exchanging signed integers between different platforms works only
5270 when all platforms store them in the same format. Most platforms store
5271 signed integers in two's-complement notation, so usually this is not an issue.
5275 The C<< > >> or C<< < >> modifiers can only be used on floating-point
5276 formats on big- or little-endian machines. Otherwise, attempting to
5277 use them raises an exception.
5281 Forcing big- or little-endian byte-order on floating-point values for
5282 data exchange can work only if all platforms use the same
5283 binary representation such as IEEE floating-point. Even if all
5284 platforms are using IEEE, there may still be subtle differences. Being able
5285 to use C<< > >> or C<< < >> on floating-point values can be useful,
5286 but also dangerous if you don't know exactly what you're doing.
5287 It is not a general way to portably store floating-point values.
5291 When using C<< > >> or C<< < >> on a C<()> group, this affects
5292 all types inside the group that accept byte-order modifiers,
5293 including all subgroups. It is silently ignored for all other
5294 types. You are not allowed to override the byte-order within a group
5295 that already has a byte-order modifier suffix.
5301 Real numbers (floats and doubles) are in native machine format only.
5302 Due to the multiplicity of floating-point formats and the lack of a
5303 standard "network" representation for them, no facility for interchange has been
5304 made. This means that packed floating-point data written on one machine
5305 may not be readable on another, even if both use IEEE floating-point
5306 arithmetic (because the endianness of the memory representation is not part
5307 of the IEEE spec). See also L<perlport>.
5309 If you know I<exactly> what you're doing, you can use the C<< > >> or C<< < >>
5310 modifiers to force big- or little-endian byte-order on floating-point values.
5312 Because Perl uses doubles (or long doubles, if configured) internally for
5313 all numeric calculation, converting from double into float and thence
5314 to double again loses precision, so C<unpack("f", pack("f", $foo)>)
5315 will not in general equal $foo.
5319 Pack and unpack can operate in two modes: character mode (C<C0> mode) where
5320 the packed string is processed per character, and UTF-8 byte mode (C<U0> mode)
5321 where the packed string is processed in its UTF-8-encoded Unicode form on
5322 a byte-by-byte basis. Character mode is the default
5323 unless the format string starts with C<U>. You
5324 can always switch mode mid-format with an explicit
5325 C<C0> or C<U0> in the format. This mode remains in effect until the next
5326 mode change, or until the end of the C<()> group it (directly) applies to.
5328 Using C<C0> to get Unicode characters while using C<U0> to get I<non>-Unicode
5329 bytes is not necessarily obvious. Probably only the first of these
5332 $ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
5333 perl -CS -ne 'printf "%v04X\n", $_ for unpack("C0A*", $_)'
5335 $ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
5336 perl -CS -ne 'printf "%v02X\n", $_ for unpack("U0A*", $_)'
5338 $ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
5339 perl -C0 -ne 'printf "%v02X\n", $_ for unpack("C0A*", $_)'
5341 $ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
5342 perl -C0 -ne 'printf "%v02X\n", $_ for unpack("U0A*", $_)'
5343 C3.8E.C2.B1.C3.8F.C2.89
5345 Those examples also illustrate that you should not try to use
5346 L<C<pack>|/pack TEMPLATE,LIST>/L<C<unpack>|/unpack TEMPLATE,EXPR> as a
5347 substitute for the L<Encode> module.
5351 You must yourself do any alignment or padding by inserting, for example,
5352 enough C<"x">es while packing. There is no way for
5353 L<C<pack>|/pack TEMPLATE,LIST> and L<C<unpack>|/unpack TEMPLATE,EXPR>
5354 to know where characters are going to or coming from, so they
5355 handle their output and input as flat sequences of characters.
5359 A C<()> group is a sub-TEMPLATE enclosed in parentheses. A group may
5360 take a repeat count either as postfix, or for
5361 L<C<unpack>|/unpack TEMPLATE,EXPR>, also via the C</>
5362 template character. Within each repetition of a group, positioning with
5363 C<@> starts over at 0. Therefore, the result of
5365 pack("@1A((@2A)@3A)", qw[X Y Z])
5367 is the string C<"\0X\0\0YZ">.
5371 C<x> and C<X> accept the C<!> modifier to act as alignment commands: they
5372 jump forward or back to the closest position aligned at a multiple of C<count>
5373 characters. For example, to L<C<pack>|/pack TEMPLATE,LIST> or
5374 L<C<unpack>|/unpack TEMPLATE,EXPR> a C structure like
5377 char c; /* one signed, 8-bit character */
5382 one may need to use the template C<c x![d] d c[2]>. This assumes that
5383 doubles must be aligned to the size of double.
5385 For alignment commands, a C<count> of 0 is equivalent to a C<count> of 1;
5390 C<n>, C<N>, C<v> and C<V> accept the C<!> modifier to
5391 represent signed 16-/32-bit integers in big-/little-endian order.
5392 This is portable only when all platforms sharing packed data use the
5393 same binary representation for signed integers; for example, when all
5394 platforms use two's-complement representation.
5398 Comments can be embedded in a TEMPLATE using C<#> through the end of line.
5399 White space can separate pack codes from each other, but modifiers and
5400 repeat counts must follow immediately. Breaking complex templates into
5401 individual line-by-line components, suitably annotated, can do as much to
5402 improve legibility and maintainability of pack/unpack formats as C</x> can
5403 for complicated pattern matches.
5407 If TEMPLATE requires more arguments than L<C<pack>|/pack TEMPLATE,LIST>
5408 is given, L<C<pack>|/pack TEMPLATE,LIST>
5409 assumes additional C<""> arguments. If TEMPLATE requires fewer arguments
5410 than given, extra arguments are ignored.
5414 Attempting to pack the special floating point values C<Inf> and C<NaN>
5415 (infinity, also in negative, and not-a-number) into packed integer values
5416 (like C<"L">) is a fatal error. The reason for this is that there simply
5417 isn't any sensible mapping for these special values into integers.
5423 $foo = pack("WWWW",65,66,67,68);
5425 $foo = pack("W4",65,66,67,68);
5427 $foo = pack("W4",0x24b6,0x24b7,0x24b8,0x24b9);
5428 # same thing with Unicode circled letters.
5429 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
5430 # same thing with Unicode circled letters. You don't get the
5431 # UTF-8 bytes because the U at the start of the format caused
5432 # a switch to U0-mode, so the UTF-8 bytes get joined into
5434 $foo = pack("C0U4",0x24b6,0x24b7,0x24b8,0x24b9);
5435 # foo eq "\xe2\x92\xb6\xe2\x92\xb7\xe2\x92\xb8\xe2\x92\xb9"
5436 # This is the UTF-8 encoding of the string in the
5439 $foo = pack("ccxxcc",65,66,67,68);
5442 # NOTE: The examples above featuring "W" and "c" are true
5443 # only on ASCII and ASCII-derived systems such as ISO Latin 1
5444 # and UTF-8. On EBCDIC systems, the first example would be
5445 # $foo = pack("WWWW",193,194,195,196);
5447 $foo = pack("s2",1,2);
5448 # "\001\000\002\000" on little-endian
5449 # "\000\001\000\002" on big-endian
5451 $foo = pack("a4","abcd","x","y","z");
5454 $foo = pack("aaaa","abcd","x","y","z");
5457 $foo = pack("a14","abcdefg");
5458 # "abcdefg\0\0\0\0\0\0\0"
5460 $foo = pack("i9pl", gmtime);
5461 # a real struct tm (on my system anyway)
5463 $utmp_template = "Z8 Z8 Z16 L";
5464 $utmp = pack($utmp_template, @utmp1);
5465 # a struct utmp (BSDish)
5467 @utmp2 = unpack($utmp_template, $utmp);
5468 # "@utmp1" eq "@utmp2"
5471 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
5474 $foo = pack('sx2l', 12, 34);
5475 # short 12, two zero bytes padding, long 34
5476 $bar = pack('s@4l', 12, 34);
5477 # short 12, zero fill to position 4, long 34
5479 $baz = pack('s.l', 12, 4, 34);
5480 # short 12, zero fill to position 4, long 34
5482 $foo = pack('nN', 42, 4711);
5483 # pack big-endian 16- and 32-bit unsigned integers
5484 $foo = pack('S>L>', 42, 4711);
5486 $foo = pack('s<l<', -42, 4711);
5487 # pack little-endian 16- and 32-bit signed integers
5488 $foo = pack('(sl)<', -42, 4711);
5491 The same template may generally also be used in
5492 L<C<unpack>|/unpack TEMPLATE,EXPR>.
5494 =item package NAMESPACE
5496 =item package NAMESPACE VERSION
5497 X<package> X<module> X<namespace> X<version>
5499 =item package NAMESPACE BLOCK
5501 =item package NAMESPACE VERSION BLOCK
5502 X<package> X<module> X<namespace> X<version>
5504 =for Pod::Functions declare a separate global namespace
5506 Declares the BLOCK or the rest of the compilation unit as being in the
5507 given namespace. The scope of the package declaration is either the
5508 supplied code BLOCK or, in the absence of a BLOCK, from the declaration
5509 itself through the end of current scope (the enclosing block, file, or
5510 L<C<eval>|/eval EXPR>). That is, the forms without a BLOCK are
5511 operative through the end of the current scope, just like the
5512 L<C<my>|/my VARLIST>, L<C<state>|/state VARLIST>, and
5513 L<C<our>|/our VARLIST> operators. All unqualified dynamic identifiers
5514 in this scope will be in the given namespace, except where overridden by
5515 another L<C<package>|/package NAMESPACE> declaration or
5516 when they're one of the special identifiers that qualify into C<main::>,
5517 like C<STDOUT>, C<ARGV>, C<ENV>, and the punctuation variables.
5519 A package statement affects dynamic variables only, including those
5520 you've used L<C<local>|/local EXPR> on, but I<not> lexically-scoped
5521 variables, which are created with L<C<my>|/my VARLIST>,
5522 L<C<state>|/state VARLIST>, or L<C<our>|/our VARLIST>. Typically it
5523 would be the first declaration in a file included by
5524 L<C<require>|/require VERSION> or L<C<use>|/use Module VERSION LIST>.
5525 You can switch into a
5526 package in more than one place, since this only determines which default
5527 symbol table the compiler uses for the rest of that block. You can refer to
5528 identifiers in other packages than the current one by prefixing the identifier
5529 with the package name and a double colon, as in C<$SomePack::var>
5530 or C<ThatPack::INPUT_HANDLE>. If package name is omitted, the C<main>
5531 package as assumed. That is, C<$::sail> is equivalent to
5532 C<$main::sail> (as well as to C<$main'sail>, still seen in ancient
5533 code, mostly from Perl 4).
5535 If VERSION is provided, L<C<package>|/package NAMESPACE> sets the
5536 C<$VERSION> variable in the given
5537 namespace to a L<version> object with the VERSION provided. VERSION must be a
5538 "strict" style version number as defined by the L<version> module: a positive
5539 decimal number (integer or decimal-fraction) without exponentiation or else a
5540 dotted-decimal v-string with a leading 'v' character and at least three
5541 components. You should set C<$VERSION> only once per package.
5543 See L<perlmod/"Packages"> for more information about packages, modules,
5544 and classes. See L<perlsub> for other scoping issues.
5549 =for Pod::Functions +5.004 the current package
5551 A special token that returns the name of the package in which it occurs.
5553 =item pipe READHANDLE,WRITEHANDLE
5556 =for Pod::Functions open a pair of connected filehandles
5558 Opens a pair of connected pipes like the corresponding system call.
5559 Note that if you set up a loop of piped processes, deadlock can occur
5560 unless you are very careful. In addition, note that Perl's pipes use
5561 IO buffering, so you may need to set L<C<$E<verbar>>|perlvar/$E<verbar>>
5562 to flush your WRITEHANDLE after each command, depending on the
5565 Returns true on success.
5567 See L<IPC::Open2>, L<IPC::Open3>, and
5568 L<perlipc/"Bidirectional Communication with Another Process">
5569 for examples of such things.
5571 On systems that support a close-on-exec flag on files, that flag is set
5572 on all newly opened file descriptors whose
5573 L<C<fileno>|/fileno FILEHANDLE>s are I<higher> than the current value of
5574 L<C<$^F>|perlvar/$^F> (by default 2 for C<STDERR>). See L<perlvar/$^F>.
5581 =for Pod::Functions remove the last element from an array and return it
5583 Pops and returns the last value of the array, shortening the array by
5586 Returns the undefined value if the array is empty, although this may
5587 also happen at other times. If ARRAY is omitted, pops the
5588 L<C<@ARGV>|perlvar/@ARGV> array in the main program, but the
5589 L<C<@_>|perlvar/@_> array in subroutines, just like
5590 L<C<shift>|/shift ARRAY>.
5592 Starting with Perl 5.14, an experimental feature allowed
5593 L<C<pop>|/pop ARRAY> to take a
5594 scalar expression. This experiment has been deemed unsuccessful, and was
5595 removed as of Perl 5.24.
5598 X<pos> X<match, position>
5602 =for Pod::Functions find or set the offset for the last/next m//g search
5604 Returns the offset of where the last C<m//g> search left off for the
5605 variable in question (L<C<$_>|perlvar/$_> is used when the variable is not
5606 specified). This offset is in characters unless the
5607 (no-longer-recommended) L<C<use bytes>|bytes> pragma is in effect, in
5608 which case the offset is in bytes. Note that 0 is a valid match offset.
5609 L<C<undef>|/undef EXPR> indicates
5610 that the search position is reset (usually due to match failure, but
5611 can also be because no match has yet been run on the scalar).
5613 L<C<pos>|/pos SCALAR> directly accesses the location used by the regexp
5614 engine to store the offset, so assigning to L<C<pos>|/pos SCALAR> will
5615 change that offset, and so will also influence the C<\G> zero-width
5616 assertion in regular expressions. Both of these effects take place for
5617 the next match, so you can't affect the position with
5618 L<C<pos>|/pos SCALAR> during the current match, such as in
5619 C<(?{pos() = 5})> or C<s//pos() = 5/e>.
5621 Setting L<C<pos>|/pos SCALAR> also resets the I<matched with
5622 zero-length> flag, described
5623 under L<perlre/"Repeated Patterns Matching a Zero-length Substring">.
5625 Because a failed C<m//gc> match doesn't reset the offset, the return
5626 from L<C<pos>|/pos SCALAR> won't change either in this case. See
5627 L<perlre> and L<perlop>.
5629 =item print FILEHANDLE LIST
5632 =item print FILEHANDLE
5638 =for Pod::Functions output a list to a filehandle
5640 Prints a string or a list of strings. Returns true if successful.
5641 FILEHANDLE may be a scalar variable containing the name of or a reference
5642 to the filehandle, thus introducing one level of indirection. (NOTE: If
5643 FILEHANDLE is a variable and the next token is a term, it may be
5644 misinterpreted as an operator unless you interpose a C<+> or put
5645 parentheses around the arguments.) If FILEHANDLE is omitted, prints to the
5646 last selected (see L<C<select>|/select FILEHANDLE>) output handle. If
5647 LIST is omitted, prints L<C<$_>|perlvar/$_> to the currently selected
5648 output handle. To use FILEHANDLE alone to print the content of
5649 L<C<$_>|perlvar/$_> to it, you must use a bareword filehandle like
5650 C<FH>, not an indirect one like C<$fh>. To set the default output handle
5651 to something other than STDOUT, use the select operation.
5653 The current value of L<C<$,>|perlvar/$,> (if any) is printed between
5654 each LIST item. The current value of L<C<$\>|perlvar/$\> (if any) is
5655 printed after the entire LIST has been printed. Because print takes a
5656 LIST, anything in the LIST is evaluated in list context, including any
5657 subroutines whose return lists you pass to
5658 L<C<print>|/print FILEHANDLE LIST>. Be careful not to follow the print
5660 parenthesis unless you want the corresponding right parenthesis to
5661 terminate the arguments to the print; put parentheses around all arguments
5662 (or interpose a C<+>, but that doesn't look as good).
5664 If you're storing handles in an array or hash, or in general whenever
5665 you're using any expression more complex than a bareword handle or a plain,
5666 unsubscripted scalar variable to retrieve it, you will have to use a block
5667 returning the filehandle value instead, in which case the LIST may not be
5670 print { $files[$i] } "stuff\n";
5671 print { $OK ? STDOUT : STDERR } "stuff\n";
5673 Printing to a closed pipe or socket will generate a SIGPIPE signal. See
5674 L<perlipc> for more on signal handling.
5676 =item printf FILEHANDLE FORMAT, LIST
5679 =item printf FILEHANDLE
5681 =item printf FORMAT, LIST
5685 =for Pod::Functions output a formatted list to a filehandle
5687 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that
5688 L<C<$\>|perlvar/$\> (the output record separator) is not appended. The
5689 FORMAT and the LIST are actually parsed as a single list. The first
5690 argument of the list will be interpreted as the
5691 L<C<printf>|/printf FILEHANDLE FORMAT, LIST> format. This means that
5692 C<printf(@_)> will use C<$_[0]> as the format. See
5693 L<sprintf|/sprintf FORMAT, LIST> for an explanation of the format
5694 argument. If C<use locale> (including C<use locale ':not_characters'>)
5695 is in effect and L<C<POSIX::setlocale>|POSIX/C<setlocale>> has been
5696 called, the character used for the decimal separator in formatted
5697 floating-point numbers is affected by the C<LC_NUMERIC> locale setting.
5698 See L<perllocale> and L<POSIX>.
5700 For historical reasons, if you omit the list, L<C<$_>|perlvar/$_> is
5702 to use FILEHANDLE without a list, you must use a bareword filehandle like
5703 C<FH>, not an indirect one like C<$fh>. However, this will rarely do what
5704 you want; if L<C<$_>|perlvar/$_> contains formatting codes, they will be
5705 replaced with the empty string and a warning will be emitted if
5706 L<warnings> are enabled. Just use L<C<print>|/print FILEHANDLE LIST> if
5707 you want to print the contents of L<C<$_>|perlvar/$_>.
5709 Don't fall into the trap of using a
5710 L<C<printf>|/printf FILEHANDLE FORMAT, LIST> when a simple
5711 L<C<print>|/print FILEHANDLE LIST> would do. The
5712 L<C<print>|/print FILEHANDLE LIST> is more efficient and less error
5715 =item prototype FUNCTION
5720 =for Pod::Functions +5.002 get the prototype (if any) of a subroutine
5722 Returns the prototype of a function as a string (or
5723 L<C<undef>|/undef EXPR> if the
5724 function has no prototype). FUNCTION is a reference to, or the name of,
5725 the function whose prototype you want to retrieve. If FUNCTION is omitted,
5726 L<C<$_>|perlvar/$_> is used.
5728 If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
5729 name for a Perl builtin. If the builtin's arguments
5730 cannot be adequately expressed by a prototype
5731 (such as L<C<system>|/system LIST>), L<C<prototype>|/prototype FUNCTION>
5732 returns L<C<undef>|/undef EXPR>, because the builtin
5733 does not really behave like a Perl function. Otherwise, the string
5734 describing the equivalent prototype is returned.
5736 =item push ARRAY,LIST
5739 =for Pod::Functions append one or more elements to an array
5741 Treats ARRAY as a stack by appending the values of LIST to the end of
5742 ARRAY. The length of ARRAY increases by the length of LIST. Has the same
5745 for my $value (LIST) {
5746 $ARRAY[++$#ARRAY] = $value;
5749 but is more efficient. Returns the number of elements in the array following
5750 the completed L<C<push>|/push ARRAY,LIST>.
5752 Starting with Perl 5.14, an experimental feature allowed
5753 L<C<push>|/push ARRAY,LIST> to take a
5754 scalar expression. This experiment has been deemed unsuccessful, and was
5755 removed as of Perl 5.24.
5759 =for Pod::Functions singly quote a string
5763 =for Pod::Functions doubly quote a string
5767 =for Pod::Functions quote a list of words
5771 =for Pod::Functions backquote quote a string
5773 Generalized quotes. See L<perlop/"Quote-Like Operators">.
5777 =for Pod::Functions +5.005 compile pattern
5779 Regexp-like quote. See L<perlop/"Regexp Quote-Like Operators">.
5781 =item quotemeta EXPR
5782 X<quotemeta> X<metacharacter>
5786 =for Pod::Functions quote regular expression magic characters
5788 Returns the value of EXPR with all the ASCII non-"word"
5789 characters backslashed. (That is, all ASCII characters not matching
5790 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
5791 returned string, regardless of any locale settings.)
5792 This is the internal function implementing
5793 the C<\Q> escape in double-quoted strings.
5794 (See below for the behavior on non-ASCII code points.)
5796 If EXPR is omitted, uses L<C<$_>|perlvar/$_>.
5798 quotemeta (and C<\Q> ... C<\E>) are useful when interpolating strings into
5799 regular expressions, because by default an interpolated variable will be
5800 considered a mini-regular expression. For example:
5802 my $sentence = 'The quick brown fox jumped over the lazy dog';
5803 my $substring = 'quick.*?fox';
5804 $sentence =~ s{$substring}{big bad wolf};
5806 Will cause C<$sentence> to become C<'The big bad wolf jumped over...'>.
5810 my $sentence = 'The quick brown fox jumped over the lazy dog';
5811 my $substring = 'quick.*?fox';
5812 $sentence =~ s{\Q$substring\E}{big bad wolf};
5816 my $sentence = 'The quick brown fox jumped over the lazy dog';
5817 my $substring = 'quick.*?fox';
5818 my $quoted_substring = quotemeta($substring);
5819 $sentence =~ s{$quoted_substring}{big bad wolf};
5821 Will both leave the sentence as is.
5822 Normally, when accepting literal string input from the user,
5823 L<C<quotemeta>|/quotemeta EXPR> or C<\Q> must be used.
5825 In Perl v5.14, all non-ASCII characters are quoted in non-UTF-8-encoded
5826 strings, but not quoted in UTF-8 strings.
5828 Starting in Perl v5.16, Perl adopted a Unicode-defined strategy for
5829 quoting non-ASCII characters; the quoting of ASCII characters is
5832 Also unchanged is the quoting of non-UTF-8 strings when outside the
5834 L<C<use feature 'unicode_strings'>|feature/The 'unicode_strings' feature>,
5835 which is to quote all
5836 characters in the upper Latin1 range. This provides complete backwards
5837 compatibility for old programs which do not use Unicode. (Note that
5838 C<unicode_strings> is automatically enabled within the scope of a
5839 S<C<use v5.12>> or greater.)
5841 Within the scope of L<C<use locale>|locale>, all non-ASCII Latin1 code
5843 are quoted whether the string is encoded as UTF-8 or not. As mentioned
5844 above, locale does not affect the quoting of ASCII-range characters.
5845 This protects against those locales where characters such as C<"|"> are
5846 considered to be word characters.
5848 Otherwise, Perl quotes non-ASCII characters using an adaptation from
5849 Unicode (see L<http://www.unicode.org/reports/tr31/>).
5850 The only code points that are quoted are those that have any of the
5851 Unicode properties: Pattern_Syntax, Pattern_White_Space, White_Space,
5852 Default_Ignorable_Code_Point, or General_Category=Control.
5854 Of these properties, the two important ones are Pattern_Syntax and
5855 Pattern_White_Space. They have been set up by Unicode for exactly this
5856 purpose of deciding which characters in a regular expression pattern
5857 should be quoted. No character that can be in an identifier has these
5860 Perl promises, that if we ever add regular expression pattern
5861 metacharacters to the dozen already defined
5862 (C<\ E<verbar> ( ) [ { ^ $ * + ? .>), that we will only use ones that have the
5863 Pattern_Syntax property. Perl also promises, that if we ever add
5864 characters that are considered to be white space in regular expressions
5865 (currently mostly affected by C</x>), they will all have the
5866 Pattern_White_Space property.
5868 Unicode promises that the set of code points that have these two
5869 properties will never change, so something that is not quoted in v5.16
5870 will never need to be quoted in any future Perl release. (Not all the
5871 code points that match Pattern_Syntax have actually had characters
5872 assigned to them; so there is room to grow, but they are quoted
5873 whether assigned or not. Perl, of course, would never use an
5874 unassigned code point as an actual metacharacter.)
5876 Quoting characters that have the other 3 properties is done to enhance
5877 the readability of the regular expression and not because they actually
5878 need to be quoted for regular expression purposes (characters with the
5879 White_Space property are likely to be indistinguishable on the page or
5880 screen from those with the Pattern_White_Space property; and the other
5881 two properties contain non-printing characters).
5888 =for Pod::Functions retrieve the next pseudorandom number
5890 Returns a random fractional number greater than or equal to C<0> and less
5891 than the value of EXPR. (EXPR should be positive.) If EXPR is
5892 omitted, the value C<1> is used. Currently EXPR with the value C<0> is
5893 also special-cased as C<1> (this was undocumented before Perl 5.8.0
5894 and is subject to change in future versions of Perl). Automatically calls
5895 L<C<srand>|/srand EXPR> unless L<C<srand>|/srand EXPR> has already been
5896 called. See also L<C<srand>|/srand EXPR>.
5898 Apply L<C<int>|/int EXPR> to the value returned by L<C<rand>|/rand EXPR>
5899 if you want random integers instead of random fractional numbers. For
5904 returns a random integer between C<0> and C<9>, inclusive.
5906 (Note: If your rand function consistently returns numbers that are too
5907 large or too small, then your version of Perl was probably compiled
5908 with the wrong number of RANDBITS.)
5910 B<L<C<rand>|/rand EXPR> is not cryptographically secure. You should not rely
5911 on it in security-sensitive situations.> As of this writing, a
5912 number of third-party CPAN modules offer random number generators
5913 intended by their authors to be cryptographically secure,
5914 including: L<Data::Entropy>, L<Crypt::Random>, L<Math::Random::Secure>,
5915 and L<Math::TrulyRandom>.
5917 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
5918 X<read> X<file, read>
5920 =item read FILEHANDLE,SCALAR,LENGTH
5922 =for Pod::Functions fixed-length buffered input from a filehandle
5924 Attempts to read LENGTH I<characters> of data into variable SCALAR
5925 from the specified FILEHANDLE. Returns the number of characters
5926 actually read, C<0> at end of file, or undef if there was an error (in
5927 the latter case L<C<$!>|perlvar/$!> is also set). SCALAR will be grown
5929 so that the last character actually read is the last character of the
5930 scalar after the read.
5932 An OFFSET may be specified to place the read data at some place in the
5933 string other than the beginning. A negative OFFSET specifies
5934 placement at that many characters counting backwards from the end of
5935 the string. A positive OFFSET greater than the length of SCALAR
5936 results in the string being padded to the required size with C<"\0">
5937 bytes before the result of the read is appended.
5939 The call is implemented in terms of either Perl's or your system's native
5940 L<fread(3)> library function. To get a true L<read(2)> system call, see
5941 L<sysread|/sysread FILEHANDLE,SCALAR,LENGTH,OFFSET>.
5943 Note the I<characters>: depending on the status of the filehandle,
5944 either (8-bit) bytes or characters are read. By default, all
5945 filehandles operate on bytes, but for example if the filehandle has
5946 been opened with the C<:utf8> I/O layer (see
5947 L<C<open>|/open FILEHANDLE,EXPR>, and the L<open>
5948 pragma), the I/O will operate on UTF8-encoded Unicode
5949 characters, not bytes. Similarly for the C<:encoding> layer:
5950 in that case pretty much any characters can be read.
5952 =item readdir DIRHANDLE
5955 =for Pod::Functions get a directory from a directory handle
5957 Returns the next directory entry for a directory opened by
5958 L<C<opendir>|/opendir DIRHANDLE,EXPR>.
5959 If used in list context, returns all the rest of the entries in the
5960 directory. If there are no more entries, returns the undefined value in
5961 scalar context and the empty list in list context.
5963 If you're planning to filetest the return values out of a
5964 L<C<readdir>|/readdir DIRHANDLE>, you'd better prepend the directory in
5965 question. Otherwise, because we didn't L<C<chdir>|/chdir EXPR> there,
5966 it would have been testing the wrong file.
5968 opendir(my $dh, $some_dir) || die "Can't opendir $some_dir: $!";
5969 my @dots = grep { /^\./ && -f "$some_dir/$_" } readdir($dh);
5972 As of Perl 5.12 you can use a bare L<C<readdir>|/readdir DIRHANDLE> in a
5973 C<while> loop, which will set L<C<$_>|perlvar/$_> on every iteration.
5975 opendir(my $dh, $some_dir) || die "Can't open $some_dir: $!";
5976 while (readdir $dh) {
5977 print "$some_dir/$_\n";
5981 To avoid confusing would-be users of your code who are running earlier
5982 versions of Perl with mysterious failures, put this sort of thing at the
5983 top of your file to signal that your code will work I<only> on Perls of a
5986 use 5.012; # so readdir assigns to $_ in a lone while test
5991 X<readline> X<gets> X<fgets>
5993 =for Pod::Functions fetch a record from a file
5995 Reads from the filehandle whose typeglob is contained in EXPR (or from
5996 C<*ARGV> if EXPR is not provided). In scalar context, each call reads and
5997 returns the next line until end-of-file is reached, whereupon the
5998 subsequent call returns L<C<undef>|/undef EXPR>. In list context, reads
5999 until end-of-file is reached and returns a list of lines. Note that the
6000 notion of "line" used here is whatever you may have defined with
6001 L<C<$E<sol>>|perlvar/$E<sol>> (or C<$INPUT_RECORD_SEPARATOR> in
6002 L<English>). See L<perlvar/"$/">.
6004 When L<C<$E<sol>>|perlvar/$E<sol>> is set to L<C<undef>|/undef EXPR>,
6005 when L<C<readline>|/readline EXPR> is in scalar context (i.e., file
6006 slurp mode), and when an empty file is read, it returns C<''> the first
6007 time, followed by L<C<undef>|/undef EXPR> subsequently.
6009 This is the internal function implementing the C<< <EXPR> >>
6010 operator, but you can use it directly. The C<< <EXPR> >>
6011 operator is discussed in more detail in L<perlop/"I/O Operators">.
6014 my $line = readline(STDIN); # same thing
6016 If L<C<readline>|/readline EXPR> encounters an operating system error,
6017 L<C<$!>|perlvar/$!> will be set with the corresponding error message.
6018 It can be helpful to check L<C<$!>|perlvar/$!> when you are reading from
6019 filehandles you don't trust, such as a tty or a socket. The following
6020 example uses the operator form of L<C<readline>|/readline EXPR> and dies
6021 if the result is not defined.
6023 while ( ! eof($fh) ) {
6024 defined( $_ = readline $fh ) or die "readline failed: $!";
6028 Note that you have can't handle L<C<readline>|/readline EXPR> errors
6029 that way with the C<ARGV> filehandle. In that case, you have to open
6030 each element of L<C<@ARGV>|perlvar/@ARGV> yourself since
6031 L<C<eof>|/eof FILEHANDLE> handles C<ARGV> differently.
6033 foreach my $arg (@ARGV) {
6034 open(my $fh, $arg) or warn "Can't open $arg: $!";
6036 while ( ! eof($fh) ) {
6037 defined( $_ = readline $fh )
6038 or die "readline failed for $arg: $!";
6048 =for Pod::Functions determine where a symbolic link is pointing
6050 Returns the value of a symbolic link, if symbolic links are
6051 implemented. If not, raises an exception. If there is a system
6052 error, returns the undefined value and sets L<C<$!>|perlvar/$!> (errno).
6053 If EXPR is omitted, uses L<C<$_>|perlvar/$_>.
6055 Portability issues: L<perlport/readlink>.
6062 =for Pod::Functions execute a system command and collect standard output
6064 EXPR is executed as a system command.
6065 The collected standard output of the command is returned.
6066 In scalar context, it comes back as a single (potentially
6067 multi-line) string. In list context, returns a list of lines
6068 (however you've defined lines with L<C<$E<sol>>|perlvar/$E<sol>> (or
6069 C<$INPUT_RECORD_SEPARATOR> in L<English>)).
6070 This is the internal function implementing the C<qx/EXPR/>
6071 operator, but you can use it directly. The C<qx/EXPR/>
6072 operator is discussed in more detail in L<perlop/"I/O Operators">.
6073 If EXPR is omitted, uses L<C<$_>|perlvar/$_>.
6075 =item recv SOCKET,SCALAR,LENGTH,FLAGS
6078 =for Pod::Functions receive a message over a Socket
6080 Receives a message on a socket. Attempts to receive LENGTH characters
6081 of data into variable SCALAR from the specified SOCKET filehandle.
6082 SCALAR will be grown or shrunk to the length actually read. Takes the
6083 same flags as the system call of the same name. Returns the address
6084 of the sender if SOCKET's protocol supports this; returns an empty
6085 string otherwise. If there's an error, returns the undefined value.
6086 This call is actually implemented in terms of the L<recvfrom(2)> system call.
6087 See L<perlipc/"UDP: Message Passing"> for examples.
6089 Note the I<characters>: depending on the status of the socket, either
6090 (8-bit) bytes or characters are received. By default all sockets
6091 operate on bytes, but for example if the socket has been changed using
6092 L<C<binmode>|/binmode FILEHANDLE, LAYER> to operate with the
6093 C<:encoding(utf8)> I/O layer (see the L<open> pragma), the I/O will
6094 operate on UTF8-encoded Unicode
6095 characters, not bytes. Similarly for the C<:encoding> layer: in that
6096 case pretty much any characters can be read.
6105 =for Pod::Functions start this loop iteration over again
6107 The L<C<redo>|/redo LABEL> command restarts the loop block without
6108 evaluating the conditional again. The L<C<continue>|/continue BLOCK>
6109 block, if any, is not executed. If
6110 the LABEL is omitted, the command refers to the innermost enclosing
6111 loop. The C<redo EXPR> form, available starting in Perl 5.18.0, allows a
6112 label name to be computed at run time, and is otherwise identical to C<redo
6113 LABEL>. Programs that want to lie to themselves about what was just input
6114 normally use this command:
6116 # a simpleminded Pascal comment stripper
6117 # (warning: assumes no { or } in strings)
6118 LINE: while (<STDIN>) {
6119 while (s|({.*}.*){.*}|$1 |) {}
6124 if (/}/) { # end of comment?
6133 L<C<redo>|/redo LABEL> cannot be used to retry a block that returns a
6134 value such as C<eval {}>, C<sub {}>, or C<do {}>, and should not be used
6135 to exit a L<C<grep>|/grep BLOCK LIST> or L<C<map>|/map BLOCK LIST>
6138 Note that a block by itself is semantically identical to a loop
6139 that executes once. Thus L<C<redo>|/redo LABEL> inside such a block
6140 will effectively turn it into a looping construct.
6142 See also L<C<continue>|/continue BLOCK> for an illustration of how
6143 L<C<last>|/last LABEL>, L<C<next>|/next LABEL>, and
6144 L<C<redo>|/redo LABEL> work.
6146 Unlike most named operators, this has the same precedence as assignment.
6147 It is also exempt from the looks-like-a-function rule, so
6148 C<redo ("foo")."bar"> will cause "bar" to be part of the argument to
6149 L<C<redo>|/redo LABEL>.
6156 =for Pod::Functions find out the type of thing being referenced
6158 Returns a non-empty string if EXPR is a reference, the empty
6159 string otherwise. If EXPR is not specified, L<C<$_>|perlvar/$_> will be
6160 used. The value returned depends on the type of thing the reference is
6163 Builtin types include:
6177 You can think of L<C<ref>|/ref EXPR> as a C<typeof> operator.
6179 if (ref($r) eq "HASH") {
6180 print "r is a reference to a hash.\n";
6183 print "r is not a reference at all.\n";
6186 The return value C<LVALUE> indicates a reference to an lvalue that is not
6187 a variable. You get this from taking the reference of function calls like
6188 L<C<pos>|/pos SCALAR> or
6189 L<C<substr>|/substr EXPR,OFFSET,LENGTH,REPLACEMENT>. C<VSTRING> is
6190 returned if the reference points to a
6191 L<version string|perldata/"Version Strings">.
6193 The result C<Regexp> indicates that the argument is a regular expression
6194 resulting from L<C<qrE<sol>E<sol>>|/qrE<sol>STRINGE<sol>>.
6196 If the referenced object has been blessed into a package, then that package
6197 name is returned instead. But don't use that, as it's now considered
6198 "bad practice". For one reason, an object could be using a class called
6199 C<Regexp> or C<IO>, or even C<HASH>. Also, L<C<ref>|/ref EXPR> doesn't
6200 take into account subclasses, like
6201 L<C<isa>|UNIVERSAL/C<< $obj->isa( TYPE ) >>> does.
6203 Instead, use L<C<blessed>|Scalar::Util/blessed> (in the L<Scalar::Util>
6204 module) for boolean checks, L<C<isa>|UNIVERSAL/C<< $obj->isa( TYPE ) >>>
6205 for specific class checks and L<C<reftype>|Scalar::Util/reftype> (also
6206 from L<Scalar::Util>) for type checks. (See L<perlobj> for details and
6207 a L<C<blessed>|Scalar::Util/blessed>/L<C<isa>|UNIVERSAL/C<< $obj->isa( TYPE ) >>>
6210 See also L<perlref>.
6212 =item rename OLDNAME,NEWNAME
6213 X<rename> X<move> X<mv> X<ren>
6215 =for Pod::Functions change a filename
6217 Changes the name of a file; an existing file NEWNAME will be
6218 clobbered. Returns true for success, false otherwise.
6220 Behavior of this function varies wildly depending on your system
6221 implementation. For example, it will usually not work across file system
6222 boundaries, even though the system I<mv> command sometimes compensates
6223 for this. Other restrictions include whether it works on directories,
6224 open files, or pre-existing files. Check L<perlport> and either the
6225 L<rename(2)> manpage or equivalent system documentation for details.
6227 For a platform independent L<C<move>|File::Copy/move> function look at
6228 the L<File::Copy> module.
6230 Portability issues: L<perlport/rename>.
6232 =item require VERSION
6239 =for Pod::Functions load in external functions from a library at runtime
6241 Demands a version of Perl specified by VERSION, or demands some semantics
6242 specified by EXPR or by L<C<$_>|perlvar/$_> if EXPR is not supplied.
6244 VERSION may be either a numeric argument such as 5.006, which will be
6245 compared to L<C<$]>|perlvar/$]>, or a literal of the form v5.6.1, which
6246 will be compared to L<C<$^V>|perlvar/$^V> (or C<$PERL_VERSION> in
6247 L<English>). An exception is raised if VERSION is greater than the
6248 version of the current Perl interpreter. Compare with
6249 L<C<use>|/use Module VERSION LIST>, which can do a similar check at
6252 Specifying VERSION as a literal of the form v5.6.1 should generally be
6253 avoided, because it leads to misleading error messages under earlier
6254 versions of Perl that do not support this syntax. The equivalent numeric
6255 version should be used instead.
6257 require v5.6.1; # run time version check
6258 require 5.6.1; # ditto
6259 require 5.006_001; # ditto; preferred for backwards
6262 Otherwise, L<C<require>|/require VERSION> demands that a library file be
6263 included if it hasn't already been included. The file is included via
6264 the do-FILE mechanism, which is essentially just a variety of
6265 L<C<eval>|/eval EXPR> with the
6266 caveat that lexical variables in the invoking script will be invisible
6267 to the included code. If it were implemented in pure Perl, it
6268 would have semantics similar to the following:
6274 my ($filename) = @_;
6275 if ( my $version = eval { version->parse($filename) } ) {
6276 if ( $version > $^V ) {
6277 my $vn = $version->normal;
6278 croak "Perl $vn required--this is only $^V, stopped";
6283 if (exists $INC{$filename}) {
6284 return 1 if $INC{$filename};
6285 croak "Compilation failed in require";
6288 foreach $prefix (@INC) {
6290 #... do other stuff - see text below ....
6292 # (see text below about possible appending of .pmc
6293 # suffix to $filename)
6294 my $realfilename = "$prefix/$filename";
6295 next if ! -e $realfilename || -d _ || -b _;
6296 $INC{$filename} = $realfilename;
6297 my $result = do($realfilename);
6298 # but run in caller's namespace
6300 if (!defined $result) {
6301 $INC{$filename} = undef;
6302 croak $@ ? "$@Compilation failed in require"
6303 : "Can't locate $filename: $!\n";
6306 delete $INC{$filename};
6307 croak "$filename did not return true value";
6312 croak "Can't locate $filename in \@INC ...";
6315 Note that the file will not be included twice under the same specified
6318 The file must return true as the last statement to indicate
6319 successful execution of any initialization code, so it's customary to
6320 end such a file with C<1;> unless you're sure it'll return true
6321 otherwise. But it's better just to put the C<1;>, in case you add more
6324 If EXPR is a bareword, L<C<require>|/require VERSION> assumes a F<.pm>
6325 extension and replaces C<::> with C</> in the filename for you,
6326 to make it easy to load standard modules. This form of loading of
6327 modules does not risk altering your namespace.
6329 In other words, if you try this:
6331 require Foo::Bar; # a splendid bareword
6333 The require function will actually look for the F<Foo/Bar.pm> file in the
6334 directories specified in the L<C<@INC>|perlvar/@INC> array.
6336 But if you try this:
6338 my $class = 'Foo::Bar';
6339 require $class; # $class is not a bareword
6341 require "Foo::Bar"; # not a bareword because of the ""
6343 The require function will look for the F<Foo::Bar> file in the
6344 L<C<@INC>|perlvar/@INC> array and
6345 will complain about not finding F<Foo::Bar> there. In this case you can do:
6347 eval "require $class";
6349 Now that you understand how L<C<require>|/require VERSION> looks for
6350 files with a bareword argument, there is a little extra functionality
6351 going on behind the scenes. Before L<C<require>|/require VERSION> looks
6352 for a F<.pm> extension, it will first look for a similar filename with a
6353 F<.pmc> extension. If this file is found, it will be loaded in place of
6354 any file ending in a F<.pm> extension.
6356 You can also insert hooks into the import facility by putting Perl code
6357 directly into the L<C<@INC>|perlvar/@INC> array. There are three forms
6358 of hooks: subroutine references, array references, and blessed objects.
6360 Subroutine references are the simplest case. When the inclusion system
6361 walks through L<C<@INC>|perlvar/@INC> and encounters a subroutine, this
6362 subroutine gets called with two parameters, the first a reference to
6363 itself, and the second the name of the file to be included (e.g.,
6364 F<Foo/Bar.pm>). The subroutine should return either nothing or else a
6365 list of up to four values in the following order:
6371 A reference to a scalar, containing any initial source code to prepend to
6372 the file or generator output.
6376 A filehandle, from which the file will be read.
6380 A reference to a subroutine. If there is no filehandle (previous item),
6381 then this subroutine is expected to generate one line of source code per
6382 call, writing the line into L<C<$_>|perlvar/$_> and returning 1, then
6383 finally at end of file returning 0. If there is a filehandle, then the
6384 subroutine will be called to act as a simple source filter, with the
6385 line as read in L<C<$_>|perlvar/$_>.
6386 Again, return 1 for each valid line, and 0 after all lines have been
6391 Optional state for the subroutine. The state is passed in as C<$_[1]>. A
6392 reference to the subroutine itself is passed in as C<$_[0]>.
6396 If an empty list, L<C<undef>|/undef EXPR>, or nothing that matches the
6397 first 3 values above is returned, then L<C<require>|/require VERSION>
6398 looks at the remaining elements of L<C<@INC>|perlvar/@INC>.
6399 Note that this filehandle must be a real filehandle (strictly a typeglob
6400 or reference to a typeglob, whether blessed or unblessed); tied filehandles
6401 will be ignored and processing will stop there.
6403 If the hook is an array reference, its first element must be a subroutine
6404 reference. This subroutine is called as above, but the first parameter is
6405 the array reference. This lets you indirectly pass arguments to
6408 In other words, you can write:
6410 push @INC, \&my_sub;
6412 my ($coderef, $filename) = @_; # $coderef is \&my_sub
6418 push @INC, [ \&my_sub, $x, $y, ... ];
6420 my ($arrayref, $filename) = @_;
6421 # Retrieve $x, $y, ...
6422 my (undef, @parameters) = @$arrayref;
6426 If the hook is an object, it must provide an C<INC> method that will be
6427 called as above, the first parameter being the object itself. (Note that
6428 you must fully qualify the sub's name, as unqualified C<INC> is always forced
6429 into package C<main>.) Here is a typical code layout:
6435 my ($self, $filename) = @_;
6439 # In the main program
6440 push @INC, Foo->new(...);
6442 These hooks are also permitted to set the L<C<%INC>|perlvar/%INC> entry
6443 corresponding to the files they have loaded. See L<perlvar/%INC>.
6445 For a yet-more-powerful import facility, see
6446 L<C<use>|/use Module VERSION LIST> and L<perlmod>.
6453 =for Pod::Functions clear all variables of a given name
6455 Generally used in a L<C<continue>|/continue BLOCK> block at the end of a
6456 loop to clear variables and reset C<m?pattern?> searches so that they
6458 expression is interpreted as a list of single characters (hyphens
6459 allowed for ranges). All variables and arrays beginning with one of
6460 those letters are reset to their pristine state. If the expression is
6461 omitted, one-match searches (C<m?pattern?>) are reset to match again.
6462 Only resets variables or searches in the current package. Always returns
6465 reset 'X'; # reset all X variables
6466 reset 'a-z'; # reset lower case variables
6467 reset; # just reset m?one-time? searches
6469 Resetting C<"A-Z"> is not recommended because you'll wipe out your
6470 L<C<@ARGV>|perlvar/@ARGV> and L<C<@INC>|perlvar/@INC> arrays and your
6471 L<C<%ENV>|perlvar/%ENV> hash.
6472 Resets only package variables; lexical variables are unaffected, but
6473 they clean themselves up on scope exit anyway, so you'll probably want
6474 to use them instead. See L<C<my>|/my VARLIST>.
6481 =for Pod::Functions get out of a function early
6483 Returns from a subroutine, L<C<eval>|/eval EXPR>,
6484 L<C<do FILE>|/do EXPR>, L<C<sort>|/sort SUBNAME LIST> block or regex
6485 eval block (but not a L<C<grep>|/grep BLOCK LIST> or
6486 L<C<map>|/map BLOCK LIST> block) with the value
6487 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
6488 context, depending on how the return value will be used, and the context
6489 may vary from one execution to the next (see
6490 L<C<wantarray>|/wantarray>). If no EXPR
6491 is given, returns an empty list in list context, the undefined value in
6492 scalar context, and (of course) nothing at all in void context.
6494 (In the absence of an explicit L<C<return>|/return EXPR>, a subroutine,
6495 L<C<eval>|/eval EXPR>,
6496 or L<C<do FILE>|/do EXPR> automatically returns the value of the last expression
6499 Unlike most named operators, this is also exempt from the
6500 looks-like-a-function rule, so C<return ("foo")."bar"> will
6501 cause C<"bar"> to be part of the argument to L<C<return>|/return EXPR>.
6504 X<reverse> X<rev> X<invert>
6506 =for Pod::Functions flip a string or a list
6508 In list context, returns a list value consisting of the elements
6509 of LIST in the opposite order. In scalar context, concatenates the
6510 elements of LIST and returns a string value with all characters
6511 in the opposite order.
6513 print join(", ", reverse "world", "Hello"); # Hello, world
6515 print scalar reverse "dlrow ,", "olleH"; # Hello, world
6517 Used without arguments in scalar context, L<C<reverse>|/reverse LIST>
6518 reverses L<C<$_>|perlvar/$_>.
6520 $_ = "dlrow ,olleH";
6521 print reverse; # No output, list context
6522 print scalar reverse; # Hello, world
6524 Note that reversing an array to itself (as in C<@a = reverse @a>) will
6525 preserve non-existent elements whenever possible; i.e., for non-magical
6526 arrays or for tied arrays with C<EXISTS> and C<DELETE> methods.
6528 This operator is also handy for inverting a hash, although there are some
6529 caveats. If a value is duplicated in the original hash, only one of those
6530 can be represented as a key in the inverted hash. Also, this has to
6531 unwind one hash and build a whole new one, which may take some time
6532 on a large hash, such as from a DBM file.
6534 my %by_name = reverse %by_address; # Invert the hash
6536 =item rewinddir DIRHANDLE
6539 =for Pod::Functions reset directory handle
6541 Sets the current position to the beginning of the directory for the
6542 L<C<readdir>|/readdir DIRHANDLE> routine on DIRHANDLE.
6544 Portability issues: L<perlport/rewinddir>.
6546 =item rindex STR,SUBSTR,POSITION
6549 =item rindex STR,SUBSTR
6551 =for Pod::Functions right-to-left substring search
6553 Works just like L<C<index>|/index STR,SUBSTR,POSITION> except that it
6554 returns the position of the I<last>
6555 occurrence of SUBSTR in STR. If POSITION is specified, returns the
6556 last occurrence beginning at or before that position.
6558 =item rmdir FILENAME
6559 X<rmdir> X<rd> X<directory, remove>
6563 =for Pod::Functions remove a directory
6565 Deletes the directory specified by FILENAME if that directory is
6566 empty. If it succeeds it returns true; otherwise it returns false and
6567 sets L<C<$!>|perlvar/$!> (errno). If FILENAME is omitted, uses
6568 L<C<$_>|perlvar/$_>.
6570 To remove a directory tree recursively (C<rm -rf> on Unix) look at
6571 the L<C<rmtree>|File::Path/rmtree( $dir )> function of the L<File::Path>
6576 =for Pod::Functions replace a pattern with a string
6578 The substitution operator. See L<perlop/"Regexp Quote-Like Operators">.
6580 =item say FILEHANDLE LIST
6583 =item say FILEHANDLE
6589 =for Pod::Functions +say output a list to a filehandle, appending a newline
6591 Just like L<C<print>|/print FILEHANDLE LIST>, but implicitly appends a
6592 newline. C<say LIST> is simply an abbreviation for
6593 C<{ local $\ = "\n"; print LIST }>. To use FILEHANDLE without a LIST to
6594 print the contents of L<C<$_>|perlvar/$_> to it, you must use a bareword
6595 filehandle like C<FH>, not an indirect one like C<$fh>.
6597 L<C<say>|/say FILEHANDLE LIST> is available only if the
6598 L<C<"say"> feature|feature/The 'say' feature> is enabled or if it is
6599 prefixed with C<CORE::>. The
6600 L<C<"say"> feature|feature/The 'say' feature> is enabled automatically
6601 with a C<use v5.10> (or higher) declaration in the current scope.
6604 X<scalar> X<context>
6606 =for Pod::Functions force a scalar context
6608 Forces EXPR to be interpreted in scalar context and returns the value
6611 my @counts = ( scalar @a, scalar @b, scalar @c );
6613 There is no equivalent operator to force an expression to
6614 be interpolated in list context because in practice, this is never
6615 needed. If you really wanted to do so, however, you could use
6616 the construction C<@{[ (some expression) ]}>, but usually a simple
6617 C<(some expression)> suffices.
6619 Because L<C<scalar>|/scalar EXPR> is a unary operator, if you
6621 parenthesized list for the EXPR, this behaves as a scalar comma expression,
6622 evaluating all but the last element in void context and returning the final
6623 element evaluated in scalar context. This is seldom what you want.
6625 The following single statement:
6627 print uc(scalar(foo(), $bar)), $baz;
6629 is the moral equivalent of these two:
6632 print(uc($bar), $baz);
6634 See L<perlop> for more details on unary operators and the comma operator,
6635 and L<perldata> for details on evaluating a hash in scalar contex.
6637 =item seek FILEHANDLE,POSITION,WHENCE
6638 X<seek> X<fseek> X<filehandle, position>
6640 =for Pod::Functions reposition file pointer for random-access I/O
6642 Sets FILEHANDLE's position, just like the L<fseek(3)> call of C C<stdio>.
6643 FILEHANDLE may be an expression whose value gives the name of the
6644 filehandle. The values for WHENCE are C<0> to set the new position
6645 I<in bytes> to POSITION; C<1> to set it to the current position plus
6646 POSITION; and C<2> to set it to EOF plus POSITION, typically
6647 negative. For WHENCE you may use the constants C<SEEK_SET>,
6648 C<SEEK_CUR>, and C<SEEK_END> (start of the file, current position, end
6649 of the file) from the L<Fcntl> module. Returns C<1> on success, false
6652 Note the emphasis on bytes: even if the filehandle has been set to operate
6653 on characters (for example using the C<:encoding(utf8)> I/O layer), the
6654 L<C<seek>|/seek FILEHANDLE,POSITION,WHENCE>,
6655 L<C<tell>|/tell FILEHANDLE>, and
6656 L<C<sysseek>|/sysseek FILEHANDLE,POSITION,WHENCE>
6657 family of functions use byte offsets, not character offsets,
6658 because seeking to a character offset would be very slow in a UTF-8 file.
6660 If you want to position the file for
6661 L<C<sysread>|/sysread FILEHANDLE,SCALAR,LENGTH,OFFSET> or
6662 L<C<syswrite>|/syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET>, don't use
6663 L<C<seek>|/seek FILEHANDLE,POSITION,WHENCE>, because buffering makes its
6664 effect on the file's read-write position unpredictable and non-portable.
6665 Use L<C<sysseek>|/sysseek FILEHANDLE,POSITION,WHENCE> instead.
6667 Due to the rules and rigors of ANSI C, on some systems you have to do a
6668 seek whenever you switch between reading and writing. Amongst other
6669 things, this may have the effect of calling stdio's L<clearerr(3)>.
6670 A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position:
6674 This is also useful for applications emulating C<tail -f>. Once you hit
6675 EOF on your read and then sleep for a while, you (probably) have to stick in a
6676 dummy L<C<seek>|/seek FILEHANDLE,POSITION,WHENCE> to reset things. The
6677 L<C<seek>|/seek FILEHANDLE,POSITION,WHENCE> doesn't change the position,
6678 but it I<does> clear the end-of-file condition on the handle, so that the
6679 next C<readline FILE> makes Perl try again to read something. (We hope.)
6681 If that doesn't work (some I/O implementations are particularly
6682 cantankerous), you might need something like this:
6685 for ($curpos = tell($fh); $_ = readline($fh);
6686 $curpos = tell($fh)) {
6687 # search for some stuff and put it into files
6689 sleep($for_a_while);
6690 seek($fh, $curpos, 0);
6693 =item seekdir DIRHANDLE,POS
6696 =for Pod::Functions reposition directory pointer
6698 Sets the current position for the L<C<readdir>|/readdir DIRHANDLE>
6699 routine on DIRHANDLE. POS must be a value returned by
6700 L<C<telldir>|/telldir DIRHANDLE>. L<C<seekdir>|/seekdir DIRHANDLE,POS>
6701 also has the same caveats about possible directory compaction as the
6702 corresponding system library routine.
6704 =item select FILEHANDLE
6705 X<select> X<filehandle, default>
6709 =for Pod::Functions reset default output or do I/O multiplexing
6711 Returns the currently selected filehandle. If FILEHANDLE is supplied,
6712 sets the new current default filehandle for output. This has two
6713 effects: first, a L<C<write>|/write FILEHANDLE> or a L<C<print>|/print
6714 FILEHANDLE LIST> without a filehandle
6715 default to this FILEHANDLE. Second, references to variables related to
6716 output will refer to this output channel.
6718 For example, to set the top-of-form format for more than one
6719 output channel, you might do the following:
6726 FILEHANDLE may be an expression whose value gives the name of the
6727 actual filehandle. Thus:
6729 my $oldfh = select(STDERR); $| = 1; select($oldfh);
6731 Some programmers may prefer to think of filehandles as objects with
6732 methods, preferring to write the last example as:
6734 STDERR->autoflush(1);
6736 (Prior to Perl version 5.14, you have to C<use IO::Handle;> explicitly
6739 Portability issues: L<perlport/select>.
6741 =item select RBITS,WBITS,EBITS,TIMEOUT
6744 This calls the L<select(2)> syscall with the bit masks specified, which
6745 can be constructed using L<C<fileno>|/fileno FILEHANDLE> and
6746 L<C<vec>|/vec EXPR,OFFSET,BITS>, along these lines:
6748 my $rin = my $win = my $ein = '';
6749 vec($rin, fileno(STDIN), 1) = 1;
6750 vec($win, fileno(STDOUT), 1) = 1;
6753 If you want to select on many filehandles, you may wish to write a
6754 subroutine like this:
6759 for my $fh (@fhlist) {
6760 vec($bits, fileno($fh), 1) = 1;
6764 my $rin = fhbits(\*STDIN, $tty, $mysock);
6768 my ($nfound, $timeleft) =
6769 select(my $rout = $rin, my $wout = $win, my $eout = $ein,
6772 or to block until something becomes ready just do this
6775 select(my $rout = $rin, my $wout = $win, my $eout = $ein, undef);
6777 Most systems do not bother to return anything useful in C<$timeleft>, so
6778 calling L<C<select>|/select RBITS,WBITS,EBITS,TIMEOUT> in scalar context
6779 just returns C<$nfound>.
6781 Any of the bit masks can also be L<C<undef>|/undef EXPR>. The timeout,
6783 in seconds, which may be fractional. Note: not all implementations are
6784 capable of returning the C<$timeleft>. If not, they always return
6785 C<$timeleft> equal to the supplied C<$timeout>.
6787 You can effect a sleep of 250 milliseconds this way:
6789 select(undef, undef, undef, 0.25);
6791 Note that whether L<C<select>|/select RBITS,WBITS,EBITS,TIMEOUT> gets
6792 restarted after signals (say, SIGALRM) is implementation-dependent. See
6793 also L<perlport> for notes on the portability of
6794 L<C<select>|/select RBITS,WBITS,EBITS,TIMEOUT>.
6796 On error, L<C<select>|/select RBITS,WBITS,EBITS,TIMEOUT> behaves just
6797 like L<select(2)>: it returns C<-1> and sets L<C<$!>|perlvar/$!>.
6799 On some Unixes, L<select(2)> may report a socket file descriptor as
6800 "ready for reading" even when no data is available, and thus any
6801 subsequent L<C<read>|/read FILEHANDLE,SCALAR,LENGTH,OFFSET> would block.
6802 This can be avoided if you always use C<O_NONBLOCK> on the socket. See
6803 L<select(2)> and L<fcntl(2)> for further details.
6805 The standard L<C<IO::Select>|IO::Select> module provides a
6806 user-friendlier interface to
6807 L<C<select>|/select RBITS,WBITS,EBITS,TIMEOUT>, mostly because it does
6808 all the bit-mask work for you.
6810 B<WARNING>: One should not attempt to mix buffered I/O (like
6811 L<C<read>|/read FILEHANDLE,SCALAR,LENGTH,OFFSET> or
6812 L<C<readline>|/readline EXPR>) with
6813 L<C<select>|/select RBITS,WBITS,EBITS,TIMEOUT>, except as permitted by
6814 POSIX, and even then only on POSIX systems. You have to use
6815 L<C<sysread>|/sysread FILEHANDLE,SCALAR,LENGTH,OFFSET> instead.
6817 Portability issues: L<perlport/select>.
6819 =item semctl ID,SEMNUM,CMD,ARG
6822 =for Pod::Functions SysV semaphore control operations
6824 Calls the System V IPC function L<semctl(2)>. You'll probably have to say
6828 first to get the correct constant definitions. If CMD is IPC_STAT or
6829 GETALL, then ARG must be a variable that will hold the returned
6830 semid_ds structure or semaphore value array. Returns like
6831 L<C<ioctl>|/ioctl FILEHANDLE,FUNCTION,SCALAR>:
6832 the undefined value for error, "C<0 but true>" for zero, or the actual
6833 return value otherwise. The ARG must consist of a vector of native
6834 short integers, which may be created with C<pack("s!",(0)x$nsem)>.
6835 See also L<perlipc/"SysV IPC"> and the documentation for
6836 L<C<IPC::SysV>|IPC::SysV> and L<C<IPC::Semaphore>|IPC::Semaphore>.
6838 Portability issues: L<perlport/semctl>.
6840 =item semget KEY,NSEMS,FLAGS
6843 =for Pod::Functions get set of SysV semaphores
6845 Calls the System V IPC function L<semget(2)>. Returns the semaphore id, or
6846 the undefined value on error. See also
6847 L<perlipc/"SysV IPC"> and the documentation for
6848 L<C<IPC::SysV>|IPC::SysV> and L<C<IPC::Semaphore>|IPC::Semaphore>.
6850 Portability issues: L<perlport/semget>.
6852 =item semop KEY,OPSTRING
6855 =for Pod::Functions SysV semaphore operations
6857 Calls the System V IPC function L<semop(2)> for semaphore operations
6858 such as signalling and waiting. OPSTRING must be a packed array of
6859 semop structures. Each semop structure can be generated with
6860 C<pack("s!3", $semnum, $semop, $semflag)>. The length of OPSTRING
6861 implies the number of semaphore operations. Returns true if
6862 successful, false on error. As an example, the
6863 following code waits on semaphore $semnum of semaphore id $semid:
6865 my $semop = pack("s!3", $semnum, -1, 0);
6866 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
6868 To signal the semaphore, replace C<-1> with C<1>. See also
6869 L<perlipc/"SysV IPC"> and the documentation for
6870 L<C<IPC::SysV>|IPC::SysV> and L<C<IPC::Semaphore>|IPC::Semaphore>.
6872 Portability issues: L<perlport/semop>.
6874 =item send SOCKET,MSG,FLAGS,TO
6877 =item send SOCKET,MSG,FLAGS
6879 =for Pod::Functions send a message over a socket
6881 Sends a message on a socket. Attempts to send the scalar MSG to the SOCKET
6882 filehandle. Takes the same flags as the system call of the same name. On
6883 unconnected sockets, you must specify a destination to I<send to>, in which
6884 case it does a L<sendto(2)> syscall. Returns the number of characters sent,
6885 or the undefined value on error. The L<sendmsg(2)> syscall is currently
6886 unimplemented. See L<perlipc/"UDP: Message Passing"> for examples.
6888 Note the I<characters>: depending on the status of the socket, either
6889 (8-bit) bytes or characters are sent. By default all sockets operate
6890 on bytes, but for example if the socket has been changed using
6891 L<C<binmode>|/binmode FILEHANDLE, LAYER> to operate with the
6892 C<:encoding(utf8)> I/O layer (see L<C<open>|/open FILEHANDLE,EXPR>, or
6893 the L<open> pragma), the I/O will operate on UTF-8
6894 encoded Unicode characters, not bytes. Similarly for the C<:encoding>
6895 layer: in that case pretty much any characters can be sent.
6897 =item setpgrp PID,PGRP
6900 =for Pod::Functions set the process group of a process
6902 Sets the current process group for the specified PID, C<0> for the current
6903 process. Raises an exception when used on a machine that doesn't
6904 implement POSIX L<setpgid(2)> or BSD L<setpgrp(2)>. If the arguments
6905 are omitted, it defaults to C<0,0>. Note that the BSD 4.2 version of
6906 L<C<setpgrp>|/setpgrp PID,PGRP> does not accept any arguments, so only
6907 C<setpgrp(0,0)> is portable. See also
6908 L<C<POSIX::setsid()>|POSIX/C<setsid>>.
6910 Portability issues: L<perlport/setpgrp>.
6912 =item setpriority WHICH,WHO,PRIORITY
6913 X<setpriority> X<priority> X<nice> X<renice>
6915 =for Pod::Functions set a process's nice value
6917 Sets the current priority for a process, a process group, or a user.
6918 (See L<setpriority(2)>.) Raises an exception when used on a machine
6919 that doesn't implement L<setpriority(2)>.
6921 Portability issues: L<perlport/setpriority>.
6923 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
6926 =for Pod::Functions set some socket options
6928 Sets the socket option requested. Returns L<C<undef>|/undef EXPR> on
6929 error. Use integer constants provided by the L<C<Socket>|Socket> module
6931 LEVEL and OPNAME. Values for LEVEL can also be obtained from
6932 getprotobyname. OPTVAL might either be a packed string or an integer.
6933 An integer OPTVAL is shorthand for pack("i", OPTVAL).
6935 An example disabling Nagle's algorithm on a socket:
6937 use Socket qw(IPPROTO_TCP TCP_NODELAY);
6938 setsockopt($socket, IPPROTO_TCP, TCP_NODELAY, 1);
6940 Portability issues: L<perlport/setsockopt>.
6947 =for Pod::Functions remove the first element of an array, and return it
6949 Shifts the first value of the array off and returns it, shortening the
6950 array by 1 and moving everything down. If there are no elements in the
6951 array, returns the undefined value. If ARRAY is omitted, shifts the
6952 L<C<@_>|perlvar/@_> array within the lexical scope of subroutines and
6953 formats, and the L<C<@ARGV>|perlvar/@ARGV> array outside a subroutine
6954 and also within the lexical scopes
6955 established by the C<eval STRING>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>,
6956 C<UNITCHECK {}>, and C<END {}> constructs.
6958 Starting with Perl 5.14, an experimental feature allowed
6959 L<C<shift>|/shift ARRAY> to take a
6960 scalar expression. This experiment has been deemed unsuccessful, and was
6961 removed as of Perl 5.24.
6963 See also L<C<unshift>|/unshift ARRAY,LIST>, L<C<push>|/push ARRAY,LIST>,
6964 and L<C<pop>|/pop ARRAY>. L<C<shift>|/shift ARRAY> and
6965 L<C<unshift>|/unshift ARRAY,LIST> do the same thing to the left end of
6966 an array that L<C<pop>|/pop ARRAY> and L<C<push>|/push ARRAY,LIST> do to
6969 =item shmctl ID,CMD,ARG
6972 =for Pod::Functions SysV shared memory operations
6974 Calls the System V IPC function shmctl. You'll probably have to say
6978 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
6979 then ARG must be a variable that will hold the returned C<shmid_ds>
6980 structure. Returns like ioctl: L<C<undef>|/undef EXPR> for error; "C<0>
6981 but true" for zero; and the actual return value otherwise.
6982 See also L<perlipc/"SysV IPC"> and the documentation for
6983 L<C<IPC::SysV>|IPC::SysV>.
6985 Portability issues: L<perlport/shmctl>.
6987 =item shmget KEY,SIZE,FLAGS
6990 =for Pod::Functions get SysV shared memory segment identifier
6992 Calls the System V IPC function shmget. Returns the shared memory
6993 segment id, or L<C<undef>|/undef EXPR> on error.
6994 See also L<perlipc/"SysV IPC"> and the documentation for
6995 L<C<IPC::SysV>|IPC::SysV>.
6997 Portability issues: L<perlport/shmget>.
6999 =item shmread ID,VAR,POS,SIZE
7003 =for Pod::Functions read SysV shared memory
7005 =item shmwrite ID,STRING,POS,SIZE
7007 =for Pod::Functions write SysV shared memory
7009 Reads or writes the System V shared memory segment ID starting at
7010 position POS for size SIZE by attaching to it, copying in/out, and
7011 detaching from it. When reading, VAR must be a variable that will
7012 hold the data read. When writing, if STRING is too long, only SIZE
7013 bytes are used; if STRING is too short, nulls are written to fill out
7014 SIZE bytes. Return true if successful, false on error.
7015 L<C<shmread>|/shmread ID,VAR,POS,SIZE> taints the variable. See also
7016 L<perlipc/"SysV IPC"> and the documentation for
7017 L<C<IPC::SysV>|IPC::SysV> and the L<C<IPC::Shareable>|IPC::Shareable>
7020 Portability issues: L<perlport/shmread> and L<perlport/shmwrite>.
7022 =item shutdown SOCKET,HOW
7025 =for Pod::Functions close down just half of a socket connection
7027 Shuts down a socket connection in the manner indicated by HOW, which
7028 has the same interpretation as in the syscall of the same name.
7030 shutdown($socket, 0); # I/we have stopped reading data
7031 shutdown($socket, 1); # I/we have stopped writing data
7032 shutdown($socket, 2); # I/we have stopped using this socket
7034 This is useful with sockets when you want to tell the other
7035 side you're done writing but not done reading, or vice versa.
7036 It's also a more insistent form of close because it also
7037 disables the file descriptor in any forked copies in other
7040 Returns C<1> for success; on error, returns L<C<undef>|/undef EXPR> if
7041 the first argument is not a valid filehandle, or returns C<0> and sets
7042 L<C<$!>|perlvar/$!> for any other failure.
7045 X<sin> X<sine> X<asin> X<arcsine>
7049 =for Pod::Functions return the sine of a number
7051 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
7052 returns sine of L<C<$_>|perlvar/$_>.
7054 For the inverse sine operation, you may use the C<Math::Trig::asin>
7055 function, or use this relation:
7057 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
7064 =for Pod::Functions block for some number of seconds
7066 Causes the script to sleep for (integer) EXPR seconds, or forever if no
7067 argument is given. Returns the integer number of seconds actually slept.
7069 May be interrupted if the process receives a signal such as C<SIGALRM>.
7072 local $SIG{ALRM} = sub { die "Alarm!\n" };
7075 die $@ unless $@ eq "Alarm!\n";
7077 You probably cannot mix L<C<alarm>|/alarm SECONDS> and
7078 L<C<sleep>|/sleep EXPR> calls, because L<C<sleep>|/sleep EXPR> is often
7079 implemented using L<C<alarm>|/alarm SECONDS>.
7081 On some older systems, it may sleep up to a full second less than what
7082 you requested, depending on how it counts seconds. Most modern systems
7083 always sleep the full amount. They may appear to sleep longer than that,
7084 however, because your process might not be scheduled right away in a
7085 busy multitasking system.
7087 For delays of finer granularity than one second, the L<Time::HiRes>
7088 module (from CPAN, and starting from Perl 5.8 part of the standard
7089 distribution) provides L<C<usleep>|Time::HiRes/usleep ( $useconds )>.
7090 You may also use Perl's four-argument
7091 version of L<C<select>|/select RBITS,WBITS,EBITS,TIMEOUT> leaving the
7092 first three arguments undefined, or you might be able to use the
7093 L<C<syscall>|/syscall NUMBER, LIST> interface to access L<setitimer(2)>
7094 if your system supports it. See L<perlfaq8> for details.
7096 See also the L<POSIX> module's L<C<pause>|POSIX/C<pause>> function.
7098 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
7101 =for Pod::Functions create a socket
7103 Opens a socket of the specified kind and attaches it to filehandle
7104 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for
7105 the syscall of the same name. You should C<use Socket> first
7106 to get the proper definitions imported. See the examples in
7107 L<perlipc/"Sockets: Client/Server Communication">.
7109 On systems that support a close-on-exec flag on files, the flag will
7110 be set for the newly opened file descriptor, as determined by the
7111 value of L<C<$^F>|perlvar/$^F>. See L<perlvar/$^F>.
7113 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
7116 =for Pod::Functions create a pair of sockets
7118 Creates an unnamed pair of sockets in the specified domain, of the
7119 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
7120 for the syscall of the same name. If unimplemented, raises an exception.
7121 Returns true if successful.
7123 On systems that support a close-on-exec flag on files, the flag will
7124 be set for the newly opened file descriptors, as determined by the value
7125 of L<C<$^F>|perlvar/$^F>. See L<perlvar/$^F>.
7127 Some systems define L<C<pipe>|/pipe READHANDLE,WRITEHANDLE> in terms of
7128 L<C<socketpair>|/socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL>, in
7129 which a call to C<pipe($rdr, $wtr)> is essentially:
7132 socketpair(my $rdr, my $wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
7133 shutdown($rdr, 1); # no more writing for reader
7134 shutdown($wtr, 0); # no more reading for writer
7136 See L<perlipc> for an example of socketpair use. Perl 5.8 and later will
7137 emulate socketpair using IP sockets to localhost if your system implements
7138 sockets but not socketpair.
7140 Portability issues: L<perlport/socketpair>.
7142 =item sort SUBNAME LIST
7143 X<sort> X<qsort> X<quicksort> X<mergesort>
7145 =item sort BLOCK LIST
7149 =for Pod::Functions sort a list of values
7151 In list context, this sorts the LIST and returns the sorted list value.
7152 In scalar context, the behaviour of L<C<sort>|/sort SUBNAME LIST> is
7155 If SUBNAME or BLOCK is omitted, L<C<sort>|/sort SUBNAME LIST>s in
7156 standard string comparison
7157 order. If SUBNAME is specified, it gives the name of a subroutine
7158 that returns an integer less than, equal to, or greater than C<0>,
7159 depending on how the elements of the list are to be ordered. (The
7160 C<< <=> >> and C<cmp> operators are extremely useful in such routines.)
7161 SUBNAME may be a scalar variable name (unsubscripted), in which case
7162 the value provides the name of (or a reference to) the actual
7163 subroutine to use. In place of a SUBNAME, you can provide a BLOCK as
7164 an anonymous, in-line sort subroutine.
7166 If the subroutine's prototype is C<($$)>, the elements to be compared are
7167 passed by reference in L<C<@_>|perlvar/@_>, as for a normal subroutine.
7168 This is slower than unprototyped subroutines, where the elements to be
7169 compared are passed into the subroutine as the package global variables
7170 C<$a> and C<$b> (see example below). Note that in the latter case, it
7171 is usually highly counter-productive to declare C<$a> and C<$b> as
7174 If the subroutine is an XSUB, the elements to be compared are pushed on
7175 to the stack, the way arguments are usually passed to XSUBs. C<$a> and
7178 The values to be compared are always passed by reference and should not
7181 You also cannot exit out of the sort block or subroutine using any of the
7182 loop control operators described in L<perlsyn> or with
7183 L<C<goto>|/goto LABEL>.
7185 When L<C<use locale>|locale> (but not C<use locale ':not_characters'>)
7186 is in effect, C<sort LIST> sorts LIST according to the
7187 current collation locale. See L<perllocale>.
7189 L<C<sort>|/sort SUBNAME LIST> returns aliases into the original list,
7190 much as a for loop's index variable aliases the list elements. That is,
7191 modifying an element of a list returned by L<C<sort>|/sort SUBNAME LIST>
7192 (for example, in a C<foreach>, L<C<map>|/map BLOCK LIST> or
7193 L<C<grep>|/grep BLOCK LIST>)
7194 actually modifies the element in the original list. This is usually
7195 something to be avoided when writing clear code.
7197 Perl 5.6 and earlier used a quicksort algorithm to implement sort.
7198 That algorithm was not stable and I<could> go quadratic. (A I<stable> sort
7199 preserves the input order of elements that compare equal. Although
7200 quicksort's run time is O(NlogN) when averaged over all arrays of
7201 length N, the time can be O(N**2), I<quadratic> behavior, for some
7202 inputs.) In 5.7, the quicksort implementation was replaced with
7203 a stable mergesort algorithm whose worst-case behavior is O(NlogN).
7204 But benchmarks indicated that for some inputs, on some platforms,
7205 the original quicksort was faster. 5.8 has a L<sort> pragma for
7206 limited control of the sort. Its rather blunt control of the
7207 underlying algorithm may not persist into future Perls, but the
7208 ability to characterize the input or output in implementation
7209 independent ways quite probably will.
7214 my @articles = sort @files;
7216 # same thing, but with explicit sort routine
7217 my @articles = sort {$a cmp $b} @files;
7219 # now case-insensitively
7220 my @articles = sort {fc($a) cmp fc($b)} @files;
7222 # same thing in reversed order
7223 my @articles = sort {$b cmp $a} @files;
7225 # sort numerically ascending
7226 my @articles = sort {$a <=> $b} @files;
7228 # sort numerically descending
7229 my @articles = sort {$b <=> $a} @files;
7231 # this sorts the %age hash by value instead of key
7232 # using an in-line function
7233 my @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
7235 # sort using explicit subroutine name
7237 $age{$a} <=> $age{$b}; # presuming numeric
7239 my @sortedclass = sort byage @class;
7241 sub backwards { $b cmp $a }
7242 my @harry = qw(dog cat x Cain Abel);
7243 my @george = qw(gone chased yz Punished Axed);
7245 # prints AbelCaincatdogx
7246 print sort backwards @harry;
7247 # prints xdogcatCainAbel
7248 print sort @george, 'to', @harry;
7249 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
7251 # inefficiently sort by descending numeric compare using
7252 # the first integer after the first = sign, or the
7253 # whole record case-insensitively otherwise
7256 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
7261 # same thing, but much more efficiently;
7262 # we'll build auxiliary indices instead
7266 push @nums, ( /=(\d+)/ ? $1 : undef );
7270 my @new = @old[ sort {
7271 $nums[$b] <=> $nums[$a]
7273 $caps[$a] cmp $caps[$b]
7277 # same thing, but without any temps
7278 my @new = map { $_->[0] }
7279 sort { $b->[1] <=> $a->[1]
7282 } map { [$_, /=(\d+)/, fc($_)] } @old;
7284 # using a prototype allows you to use any comparison subroutine
7285 # as a sort subroutine (including other package's subroutines)
7287 sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are
7290 my @new = sort Other::backwards @old;
7292 # guarantee stability, regardless of algorithm
7294 my @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
7296 # force use of mergesort (not portable outside Perl 5.8)
7297 use sort '_mergesort'; # note discouraging _
7298 my @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
7300 Warning: syntactical care is required when sorting the list returned from
7301 a function. If you want to sort the list returned by the function call
7302 C<find_records(@key)>, you can use:
7304 my @contact = sort { $a cmp $b } find_records @key;
7305 my @contact = sort +find_records(@key);
7306 my @contact = sort &find_records(@key);
7307 my @contact = sort(find_records(@key));
7309 If instead you want to sort the array C<@key> with the comparison routine
7310 C<find_records()> then you can use:
7312 my @contact = sort { find_records() } @key;
7313 my @contact = sort find_records(@key);
7314 my @contact = sort(find_records @key);
7315 my @contact = sort(find_records (@key));
7317 You I<must not> declare C<$a>
7318 and C<$b> as lexicals. They are package globals. That means
7319 that if you're in the C<main> package and type
7321 my @articles = sort {$b <=> $a} @files;
7323 then C<$a> and C<$b> are C<$main::a> and C<$main::b> (or C<$::a> and C<$::b>),
7324 but if you're in the C<FooPack> package, it's the same as typing
7326 my @articles = sort {$FooPack::b <=> $FooPack::a} @files;
7328 The comparison function is required to behave. If it returns
7329 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
7330 sometimes saying the opposite, for example) the results are not
7333 Because C<< <=> >> returns L<C<undef>|/undef EXPR> when either operand
7334 is C<NaN> (not-a-number), be careful when sorting with a
7335 comparison function like C<< $a <=> $b >> any lists that might contain a
7336 C<NaN>. The following example takes advantage that C<NaN != NaN> to
7337 eliminate any C<NaN>s from the input list.
7339 my @result = sort { $a <=> $b } grep { $_ == $_ } @input;
7341 =item splice ARRAY,OFFSET,LENGTH,LIST
7344 =item splice ARRAY,OFFSET,LENGTH
7346 =item splice ARRAY,OFFSET
7350 =for Pod::Functions add or remove elements anywhere in an array
7352 Removes the elements designated by OFFSET and LENGTH from an array, and
7353 replaces them with the elements of LIST, if any. In list context,
7354 returns the elements removed from the array. In scalar context,
7355 returns the last element removed, or L<C<undef>|/undef EXPR> if no
7357 removed. The array grows or shrinks as necessary.
7358 If OFFSET is negative then it starts that far from the end of the array.
7359 If LENGTH is omitted, removes everything from OFFSET onward.
7360 If LENGTH is negative, removes the elements from OFFSET onward
7361 except for -LENGTH elements at the end of the array.
7362 If both OFFSET and LENGTH are omitted, removes everything. If OFFSET is
7363 past the end of the array and a LENGTH was provided, Perl issues a warning,
7364 and splices at the end of the array.
7366 The following equivalences hold (assuming C<< $#a >= $i >> )
7368 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
7369 pop(@a) splice(@a,-1)
7370 shift(@a) splice(@a,0,1)
7371 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
7372 $a[$i] = $y splice(@a,$i,1,$y)
7374 L<C<splice>|/splice ARRAY,OFFSET,LENGTH,LIST> can be used, for example,
7375 to implement n-ary queue processing:
7379 while (my @next_n = splice @_, 0, $n) {
7380 say join q{ -- }, @next_n;
7384 nary_print(3, qw(a b c d e f g h));
7390 Starting with Perl 5.14, an experimental feature allowed
7391 L<C<splice>|/splice ARRAY,OFFSET,LENGTH,LIST> to take a
7392 scalar expression. This experiment has been deemed unsuccessful, and was
7393 removed as of Perl 5.24.
7395 =item split /PATTERN/,EXPR,LIMIT
7398 =item split /PATTERN/,EXPR
7400 =item split /PATTERN/
7404 =for Pod::Functions split up a string using a regexp delimiter
7406 Splits the string EXPR into a list of strings and returns the
7407 list in list context, or the size of the list in scalar context.
7408 (Prior to Perl 5.11, it also overwrote C<@_> with the list in
7409 void and scalar context. If you target old perls, beware.)
7411 If only PATTERN is given, EXPR defaults to L<C<$_>|perlvar/$_>.
7413 Anything in EXPR that matches PATTERN is taken to be a separator
7414 that separates the EXPR into substrings (called "I<fields>") that
7415 do B<not> include the separator. Note that a separator may be
7416 longer than one character or even have no characters at all (the
7417 empty string, which is a zero-width match).
7419 The PATTERN need not be constant; an expression may be used
7420 to specify a pattern that varies at runtime.
7422 If PATTERN matches the empty string, the EXPR is split at the match
7423 position (between characters). As an example, the following:
7425 print join(':', split(/b/, 'abc')), "\n";
7427 uses the C<b> in C<'abc'> as a separator to produce the output C<a:c>.
7430 print join(':', split(//, 'abc')), "\n";
7432 uses empty string matches as separators to produce the output
7433 C<a:b:c>; thus, the empty string may be used to split EXPR into a
7434 list of its component characters.
7436 As a special case for L<C<split>|/split E<sol>PATTERNE<sol>,EXPR,LIMIT>,
7437 the empty pattern given in
7438 L<match operator|perlop/"m/PATTERN/msixpodualngc"> syntax (C<//>)
7439 specifically matches the empty string, which is contrary to its usual
7440 interpretation as the last successful match.
7442 If PATTERN is C</^/>, then it is treated as if it used the
7443 L<multiline modifier|perlreref/OPERATORS> (C</^/m>), since it
7444 isn't much use otherwise.
7446 As another special case,
7447 L<C<split>|/split E<sol>PATTERNE<sol>,EXPR,LIMIT> emulates the default
7449 command line tool B<awk> when the PATTERN is either omitted or a
7450 string composed of a single space character (such as S<C<' '>> or
7451 S<C<"\x20">>, but not e.g. S<C</ />>). In this case, any leading
7452 whitespace in EXPR is removed before splitting occurs, and the PATTERN is
7453 instead treated as if it were C</\s+/>; in particular, this means that
7454 I<any> contiguous whitespace (not just a single space character) is used as
7455 a separator. However, this special treatment can be avoided by specifying
7456 the pattern S<C</ />> instead of the string S<C<" ">>, thereby allowing
7457 only a single space character to be a separator. In earlier Perls this
7458 special case was restricted to the use of a plain S<C<" ">> as the
7459 pattern argument to split; in Perl 5.18.0 and later this special case is
7460 triggered by any expression which evaluates to the simple string S<C<" ">>.
7462 If omitted, PATTERN defaults to a single space, S<C<" ">>, triggering
7463 the previously described I<awk> emulation.
7465 If LIMIT is specified and positive, it represents the maximum number
7466 of fields into which the EXPR may be split; in other words, LIMIT is
7467 one greater than the maximum number of times EXPR may be split. Thus,
7468 the LIMIT value C<1> means that EXPR may be split a maximum of zero
7469 times, producing a maximum of one field (namely, the entire value of
7470 EXPR). For instance:
7472 print join(':', split(//, 'abc', 1)), "\n";
7474 produces the output C<abc>, and this:
7476 print join(':', split(//, 'abc', 2)), "\n";
7478 produces the output C<a:bc>, and each of these:
7480 print join(':', split(//, 'abc', 3)), "\n";
7481 print join(':', split(//, 'abc', 4)), "\n";
7483 produces the output C<a:b:c>.
7485 If LIMIT is negative, it is treated as if it were instead arbitrarily
7486 large; as many fields as possible are produced.
7488 If LIMIT is omitted (or, equivalently, zero), then it is usually
7489 treated as if it were instead negative but with the exception that
7490 trailing empty fields are stripped (empty leading fields are always
7491 preserved); if all fields are empty, then all fields are considered to
7492 be trailing (and are thus stripped in this case). Thus, the following:
7494 print join(':', split(/,/, 'a,b,c,,,')), "\n";
7496 produces the output C<a:b:c>, but the following:
7498 print join(':', split(/,/, 'a,b,c,,,', -1)), "\n";
7500 produces the output C<a:b:c:::>.
7502 In time-critical applications, it is worthwhile to avoid splitting
7503 into more fields than necessary. Thus, when assigning to a list,
7504 if LIMIT is omitted (or zero), then LIMIT is treated as though it
7505 were one larger than the number of variables in the list; for the
7506 following, LIMIT is implicitly 3:
7508 my ($login, $passwd) = split(/:/);
7510 Note that splitting an EXPR that evaluates to the empty string always
7511 produces zero fields, regardless of the LIMIT specified.
7513 An empty leading field is produced when there is a positive-width
7514 match at the beginning of EXPR. For instance:
7516 print join(':', split(/ /, ' abc')), "\n";
7518 produces the output C<:abc>. However, a zero-width match at the
7519 beginning of EXPR never produces an empty field, so that:
7521 print join(':', split(//, ' abc'));
7523 produces the output S<C< :a:b:c>> (rather than S<C<: :a:b:c>>).
7525 An empty trailing field, on the other hand, is produced when there is a
7526 match at the end of EXPR, regardless of the length of the match
7527 (of course, unless a non-zero LIMIT is given explicitly, such fields are
7528 removed, as in the last example). Thus:
7530 print join(':', split(//, ' abc', -1)), "\n";
7532 produces the output S<C< :a:b:c:>>.
7534 If the PATTERN contains
7535 L<capturing groups|perlretut/Grouping things and hierarchical matching>,
7536 then for each separator, an additional field is produced for each substring
7537 captured by a group (in the order in which the groups are specified,
7538 as per L<backreferences|perlretut/Backreferences>); if any group does not
7539 match, then it captures the L<C<undef>|/undef EXPR> value instead of a
7541 note that any such additional field is produced whenever there is a
7542 separator (that is, whenever a split occurs), and such an additional field
7543 does B<not> count towards the LIMIT. Consider the following expressions
7544 evaluated in list context (each returned list is provided in the associated
7547 split(/-|,/, "1-10,20", 3)
7550 split(/(-|,)/, "1-10,20", 3)
7551 # ('1', '-', '10', ',', '20')
7553 split(/-|(,)/, "1-10,20", 3)
7554 # ('1', undef, '10', ',', '20')
7556 split(/(-)|,/, "1-10,20", 3)
7557 # ('1', '-', '10', undef, '20')
7559 split(/(-)|(,)/, "1-10,20", 3)
7560 # ('1', '-', undef, '10', undef, ',', '20')
7562 =item sprintf FORMAT, LIST
7565 =for Pod::Functions formatted print into a string
7567 Returns a string formatted by the usual
7568 L<C<printf>|/printf FILEHANDLE FORMAT, LIST> conventions of the C
7569 library function L<C<sprintf>|/sprintf FORMAT, LIST>. See below for
7570 more details and see L<sprintf(3)> or L<printf(3)> on your system for an
7571 explanation of the general principles.
7575 # Format number with up to 8 leading zeroes
7576 my $result = sprintf("%08d", $number);
7578 # Round number to 3 digits after decimal point
7579 my $rounded = sprintf("%.3f", $number);
7581 Perl does its own L<C<sprintf>|/sprintf FORMAT, LIST> formatting: it
7583 function L<sprintf(3)>, but doesn't use it except for floating-point
7584 numbers, and even then only standard modifiers are allowed.
7585 Non-standard extensions in your local L<sprintf(3)> are
7586 therefore unavailable from Perl.
7588 Unlike L<C<printf>|/printf FILEHANDLE FORMAT, LIST>,
7589 L<C<sprintf>|/sprintf FORMAT, LIST> does not do what you probably mean
7590 when you pass it an array as your first argument.
7591 The array is given scalar context,
7592 and instead of using the 0th element of the array as the format, Perl will
7593 use the count of elements in the array as the format, which is almost never
7596 Perl's L<C<sprintf>|/sprintf FORMAT, LIST> permits the following
7597 universally-known conversions:
7600 %c a character with the given number
7602 %d a signed integer, in decimal
7603 %u an unsigned integer, in decimal
7604 %o an unsigned integer, in octal
7605 %x an unsigned integer, in hexadecimal
7606 %e a floating-point number, in scientific notation
7607 %f a floating-point number, in fixed decimal notation
7608 %g a floating-point number, in %e or %f notation
7610 In addition, Perl permits the following widely-supported conversions:
7612 %X like %x, but using upper-case letters
7613 %E like %e, but using an upper-case "E"
7614 %G like %g, but with an upper-case "E" (if applicable)
7615 %b an unsigned integer, in binary
7616 %B like %b, but using an upper-case "B" with the # flag
7617 %p a pointer (outputs the Perl value's address in hexadecimal)
7618 %n special: *stores* the number of characters output so far
7619 into the next argument in the parameter list
7620 %a hexadecimal floating point
7621 %A like %a, but using upper-case letters
7623 Finally, for backward (and we do mean "backward") compatibility, Perl
7624 permits these unnecessary but widely-supported conversions:
7627 %D a synonym for %ld
7628 %U a synonym for %lu
7629 %O a synonym for %lo
7632 Note that the number of exponent digits in the scientific notation produced
7633 by C<%e>, C<%E>, C<%g> and C<%G> for numbers with the modulus of the
7634 exponent less than 100 is system-dependent: it may be three or less
7635 (zero-padded as necessary). In other words, 1.23 times ten to the
7636 99th may be either "1.23e99" or "1.23e099". Similarly for C<%a> and C<%A>:
7637 the exponent or the hexadecimal digits may float: especially the
7638 "long doubles" Perl configuration option may cause surprises.
7640 Between the C<%> and the format letter, you may specify several
7641 additional attributes controlling the interpretation of the format.
7642 In order, these are:
7646 =item format parameter index
7648 An explicit format parameter index, such as C<2$>. By default sprintf
7649 will format the next unused argument in the list, but this allows you
7650 to take the arguments out of order:
7652 printf '%2$d %1$d', 12, 34; # prints "34 12"
7653 printf '%3$d %d %1$d', 1, 2, 3; # prints "3 1 1"
7659 space prefix non-negative number with a space
7660 + prefix non-negative number with a plus sign
7661 - left-justify within the field
7662 0 use zeros, not spaces, to right-justify
7663 # ensure the leading "0" for any octal,
7664 prefix non-zero hexadecimal with "0x" or "0X",
7665 prefix non-zero binary with "0b" or "0B"
7669 printf '<% d>', 12; # prints "< 12>"
7670 printf '<% d>', 0; # prints "< 0>"
7671 printf '<% d>', -12; # prints "<-12>"
7672 printf '<%+d>', 12; # prints "<+12>"
7673 printf '<%+d>', 0; # prints "<+0>"
7674 printf '<%+d>', -12; # prints "<-12>"
7675 printf '<%6s>', 12; # prints "< 12>"
7676 printf '<%-6s>', 12; # prints "<12 >"
7677 printf '<%06s>', 12; # prints "<000012>"
7678 printf '<%#o>', 12; # prints "<014>"
7679 printf '<%#x>', 12; # prints "<0xc>"
7680 printf '<%#X>', 12; # prints "<0XC>"
7681 printf '<%#b>', 12; # prints "<0b1100>"
7682 printf '<%#B>', 12; # prints "<0B1100>"
7684 When a space and a plus sign are given as the flags at once,
7685 the space is ignored.
7687 printf '<%+ d>', 12; # prints "<+12>"
7688 printf '<% +d>', 12; # prints "<+12>"
7690 When the # flag and a precision are given in the %o conversion,
7691 the precision is incremented if it's necessary for the leading "0".
7693 printf '<%#.5o>', 012; # prints "<00012>"
7694 printf '<%#.5o>', 012345; # prints "<012345>"
7695 printf '<%#.0o>', 0; # prints "<0>"
7699 This flag tells Perl to interpret the supplied string as a vector of
7700 integers, one for each character in the string. Perl applies the format to
7701 each integer in turn, then joins the resulting strings with a separator (a
7702 dot C<.> by default). This can be useful for displaying ordinal values of
7703 characters in arbitrary strings:
7705 printf "%vd", "AB\x{100}"; # prints "65.66.256"
7706 printf "version is v%vd\n", $^V; # Perl's version
7708 Put an asterisk C<*> before the C<v> to override the string to
7709 use to separate the numbers:
7711 printf "address is %*vX\n", ":", $addr; # IPv6 address
7712 printf "bits are %0*v8b\n", " ", $bits; # random bitstring
7714 You can also explicitly specify the argument number to use for
7715 the join string using something like C<*2$v>; for example:
7717 printf '%*4$vX %*4$vX %*4$vX', # 3 IPv6 addresses
7720 =item (minimum) width
7722 Arguments are usually formatted to be only as wide as required to
7723 display the given value. You can override the width by putting
7724 a number here, or get the width from the next argument (with C<*>)
7725 or from a specified argument (e.g., with C<*2$>):
7727 printf "<%s>", "a"; # prints "<a>"
7728 printf "<%6s>", "a"; # prints "< a>"
7729 printf "<%*s>", 6, "a"; # prints "< a>"
7730 printf '<%*2$s>', "a", 6; # prints "< a>"
7731 printf "<%2s>", "long"; # prints "<long>" (does not truncate)
7733 If a field width obtained through C<*> is negative, it has the same
7734 effect as the C<-> flag: left-justification.
7736 =item precision, or maximum width
7739 You can specify a precision (for numeric conversions) or a maximum
7740 width (for string conversions) by specifying a C<.> followed by a number.
7741 For floating-point formats except C<g> and C<G>, this specifies
7742 how many places right of the decimal point to show (the default being 6).
7745 # these examples are subject to system-specific variation
7746 printf '<%f>', 1; # prints "<1.000000>"
7747 printf '<%.1f>', 1; # prints "<1.0>"
7748 printf '<%.0f>', 1; # prints "<1>"
7749 printf '<%e>', 10; # prints "<1.000000e+01>"
7750 printf '<%.1e>', 10; # prints "<1.0e+01>"
7752 For "g" and "G", this specifies the maximum number of significant digits to
7755 # These examples are subject to system-specific variation.
7756 printf '<%g>', 1; # prints "<1>"
7757 printf '<%.10g>', 1; # prints "<1>"
7758 printf '<%g>', 100; # prints "<100>"
7759 printf '<%.1g>', 100; # prints "<1e+02>"
7760 printf '<%.2g>', 100.01; # prints "<1e+02>"
7761 printf '<%.5g>', 100.01; # prints "<100.01>"
7762 printf '<%.4g>', 100.01; # prints "<100>"
7763 printf '<%.1g>', 0.0111; # prints "<0.01>"
7764 printf '<%.2g>', 0.0111; # prints "<0.011>"
7765 printf '<%.3g>', 0.0111; # prints "<0.0111>"
7767 For integer conversions, specifying a precision implies that the
7768 output of the number itself should be zero-padded to this width,
7769 where the 0 flag is ignored:
7771 printf '<%.6d>', 1; # prints "<000001>"
7772 printf '<%+.6d>', 1; # prints "<+000001>"
7773 printf '<%-10.6d>', 1; # prints "<000001 >"
7774 printf '<%10.6d>', 1; # prints "< 000001>"
7775 printf '<%010.6d>', 1; # prints "< 000001>"
7776 printf '<%+10.6d>', 1; # prints "< +000001>"
7778 printf '<%.6x>', 1; # prints "<000001>"
7779 printf '<%#.6x>', 1; # prints "<0x000001>"
7780 printf '<%-10.6x>', 1; # prints "<000001 >"
7781 printf '<%10.6x>', 1; # prints "< 000001>"
7782 printf '<%010.6x>', 1; # prints "< 000001>"
7783 printf '<%#10.6x>', 1; # prints "< 0x000001>"
7785 For string conversions, specifying a precision truncates the string
7786 to fit the specified width:
7788 printf '<%.5s>', "truncated"; # prints "<trunc>"
7789 printf '<%10.5s>', "truncated"; # prints "< trunc>"
7791 You can also get the precision from the next argument using C<.*>, or from a
7792 specified argument (e.g., with C<.*2$>):
7794 printf '<%.6x>', 1; # prints "<000001>"
7795 printf '<%.*x>', 6, 1; # prints "<000001>"
7797 printf '<%.*2$x>', 1, 6; # prints "<000001>"
7799 printf '<%6.*2$x>', 1, 4; # prints "< 0001>"
7801 If a precision obtained through C<*> is negative, it counts
7802 as having no precision at all.
7804 printf '<%.*s>', 7, "string"; # prints "<string>"
7805 printf '<%.*s>', 3, "string"; # prints "<str>"
7806 printf '<%.*s>', 0, "string"; # prints "<>"
7807 printf '<%.*s>', -1, "string"; # prints "<string>"
7809 printf '<%.*d>', 1, 0; # prints "<0>"
7810 printf '<%.*d>', 0, 0; # prints "<>"
7811 printf '<%.*d>', -1, 0; # prints "<0>"
7815 For numeric conversions, you can specify the size to interpret the
7816 number as using C<l>, C<h>, C<V>, C<q>, C<L>, or C<ll>. For integer
7817 conversions (C<d u o x X b i D U O>), numbers are usually assumed to be
7818 whatever the default integer size is on your platform (usually 32 or 64
7819 bits), but you can override this to use instead one of the standard C types,
7820 as supported by the compiler used to build Perl:
7822 hh interpret integer as C type "char" or "unsigned
7823 char" on Perl 5.14 or later
7824 h interpret integer as C type "short" or
7826 j interpret integer as C type "intmax_t" on Perl
7827 5.14 or later, and only with a C99 compiler
7829 l interpret integer as C type "long" or
7831 q, L, or ll interpret integer as C type "long long",
7832 "unsigned long long", or "quad" (typically
7834 t interpret integer as C type "ptrdiff_t" on Perl
7836 z interpret integer as C type "size_t" on Perl 5.14
7839 As of 5.14, none of these raises an exception if they are not supported on
7840 your platform. However, if warnings are enabled, a warning of the
7841 L<C<printf>|warnings> warning class is issued on an unsupported
7842 conversion flag. Should you instead prefer an exception, do this:
7844 use warnings FATAL => "printf";
7846 If you would like to know about a version dependency before you
7847 start running the program, put something like this at its top:
7849 use 5.014; # for hh/j/t/z/ printf modifiers
7851 You can find out whether your Perl supports quads via L<Config>:
7854 if ($Config{use64bitint} eq "define"
7855 || $Config{longsize} >= 8) {
7856 print "Nice quads!\n";
7859 For floating-point conversions (C<e f g E F G>), numbers are usually assumed
7860 to be the default floating-point size on your platform (double or long double),
7861 but you can force "long double" with C<q>, C<L>, or C<ll> if your
7862 platform supports them. You can find out whether your Perl supports long
7863 doubles via L<Config>:
7866 print "long doubles\n" if $Config{d_longdbl} eq "define";
7868 You can find out whether Perl considers "long double" to be the default
7869 floating-point size to use on your platform via L<Config>:
7872 if ($Config{uselongdouble} eq "define") {
7873 print "long doubles by default\n";
7876 It can also be that long doubles and doubles are the same thing:
7879 ($Config{doublesize} == $Config{longdblsize}) &&
7880 print "doubles are long doubles\n";
7882 The size specifier C<V> has no effect for Perl code, but is supported for
7883 compatibility with XS code. It means "use the standard size for a Perl
7884 integer or floating-point number", which is the default.
7886 =item order of arguments
7888 Normally, L<C<sprintf>|/sprintf FORMAT, LIST> takes the next unused
7889 argument as the value to
7890 format for each format specification. If the format specification
7891 uses C<*> to require additional arguments, these are consumed from
7892 the argument list in the order they appear in the format
7893 specification I<before> the value to format. Where an argument is
7894 specified by an explicit index, this does not affect the normal
7895 order for the arguments, even when the explicitly specified index
7896 would have been the next argument.
7900 printf "<%*.*s>", $a, $b, $c;
7902 uses C<$a> for the width, C<$b> for the precision, and C<$c>
7903 as the value to format; while:
7905 printf '<%*1$.*s>', $a, $b;
7907 would use C<$a> for the width and precision, and C<$b> as the
7910 Here are some more examples; be aware that when using an explicit
7911 index, the C<$> may need escaping:
7913 printf "%2\$d %d\n", 12, 34; # will print "34 12\n"
7914 printf "%2\$d %d %d\n", 12, 34; # will print "34 12 34\n"
7915 printf "%3\$d %d %d\n", 12, 34, 56; # will print "56 12 34\n"
7916 printf "%2\$*3\$d %d\n", 12, 34, 3; # will print " 34 12\n"
7917 printf "%*1\$.*f\n", 4, 5, 10; # will print "5.0000\n"
7921 If L<C<use locale>|locale> (including C<use locale ':not_characters'>)
7922 is in effect and L<C<POSIX::setlocale>|POSIX/C<setlocale>> has been
7924 the character used for the decimal separator in formatted floating-point
7925 numbers is affected by the C<LC_NUMERIC> locale. See L<perllocale>
7929 X<sqrt> X<root> X<square root>
7933 =for Pod::Functions square root function
7935 Return the positive square root of EXPR. If EXPR is omitted, uses
7936 L<C<$_>|perlvar/$_>. Works only for non-negative operands unless you've
7937 loaded the L<C<Math::Complex>|Math::Complex> module.
7940 print sqrt(-4); # prints 2i
7943 X<srand> X<seed> X<randseed>
7947 =for Pod::Functions seed the random number generator
7949 Sets and returns the random number seed for the L<C<rand>|/rand EXPR>
7952 The point of the function is to "seed" the L<C<rand>|/rand EXPR>
7953 function so that L<C<rand>|/rand EXPR> can produce a different sequence
7954 each time you run your program. When called with a parameter,
7955 L<C<srand>|/srand EXPR> uses that for the seed; otherwise it
7956 (semi-)randomly chooses a seed. In either case, starting with Perl 5.14,
7957 it returns the seed. To signal that your code will work I<only> on Perls
7958 of a recent vintage:
7960 use 5.014; # so srand returns the seed
7962 If L<C<srand>|/srand EXPR> is not called explicitly, it is called
7963 implicitly without a parameter at the first use of the
7964 L<C<rand>|/rand EXPR> operator. However, there are a few situations
7965 where programs are likely to want to call L<C<srand>|/srand EXPR>. One
7966 is for generating predictable results, generally for testing or
7967 debugging. There, you use C<srand($seed)>, with the same C<$seed> each
7968 time. Another case is that you may want to call L<C<srand>|/srand EXPR>
7969 after a L<C<fork>|/fork> to avoid child processes sharing the same seed
7970 value as the parent (and consequently each other).
7972 Do B<not> call C<srand()> (i.e., without an argument) more than once per
7973 process. The internal state of the random number generator should
7974 contain more entropy than can be provided by any seed, so calling
7975 L<C<srand>|/srand EXPR> again actually I<loses> randomness.
7977 Most implementations of L<C<srand>|/srand EXPR> take an integer and will
7979 truncate decimal numbers. This means C<srand(42)> will usually
7980 produce the same results as C<srand(42.1)>. To be safe, always pass
7981 L<C<srand>|/srand EXPR> an integer.
7983 A typical use of the returned seed is for a test program which has too many
7984 combinations to test comprehensively in the time available to it each run. It
7985 can test a random subset each time, and should there be a failure, log the seed
7986 used for that run so that it can later be used to reproduce the same results.
7988 B<L<C<rand>|/rand EXPR> is not cryptographically secure. You should not rely
7989 on it in security-sensitive situations.> As of this writing, a
7990 number of third-party CPAN modules offer random number generators
7991 intended by their authors to be cryptographically secure,
7992 including: L<Data::Entropy>, L<Crypt::Random>, L<Math::Random::Secure>,
7993 and L<Math::TrulyRandom>.
7995 =item stat FILEHANDLE
7996 X<stat> X<file, status> X<ctime>
8000 =item stat DIRHANDLE
8004 =for Pod::Functions get a file's status information
8006 Returns a 13-element list giving the status info for a file, either
8007 the file opened via FILEHANDLE or DIRHANDLE, or named by EXPR. If EXPR is
8008 omitted, it stats L<C<$_>|perlvar/$_> (not C<_>!). Returns the empty
8009 list if L<C<stat>|/stat FILEHANDLE> fails. Typically
8012 my ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
8013 $atime,$mtime,$ctime,$blksize,$blocks)
8016 Not all fields are supported on all filesystem types. Here are the
8017 meanings of the fields:
8019 0 dev device number of filesystem
8021 2 mode file mode (type and permissions)
8022 3 nlink number of (hard) links to the file
8023 4 uid numeric user ID of file's owner
8024 5 gid numeric group ID of file's owner
8025 6 rdev the device identifier (special files only)
8026 7 size total size of file, in bytes
8027 8 atime last access time in seconds since the epoch
8028 9 mtime last modify time in seconds since the epoch
8029 10 ctime inode change time in seconds since the epoch (*)
8030 11 blksize preferred I/O size in bytes for interacting with the
8031 file (may vary from file to file)
8032 12 blocks actual number of system-specific blocks allocated
8033 on disk (often, but not always, 512 bytes each)
8035 (The epoch was at 00:00 January 1, 1970 GMT.)
8037 (*) Not all fields are supported on all filesystem types. Notably, the
8038 ctime field is non-portable. In particular, you cannot expect it to be a
8039 "creation time"; see L<perlport/"Files and Filesystems"> for details.
8041 If L<C<stat>|/stat FILEHANDLE> is passed the special filehandle
8042 consisting of an underline, no stat is done, but the current contents of
8043 the stat structure from the last L<C<stat>|/stat FILEHANDLE>,
8044 L<C<lstat>|/lstat FILEHANDLE>, or filetest are returned. Example:
8046 if (-x $file && (($d) = stat(_)) && $d < 0) {
8047 print "$file is executable NFS file\n";
8050 (This works on machines only for which the device number is negative
8053 Because the mode contains both the file type and its permissions, you
8054 should mask off the file type portion and (s)printf using a C<"%o">
8055 if you want to see the real permissions.
8057 my $mode = (stat($filename))[2];
8058 printf "Permissions are %04o\n", $mode & 07777;
8060 In scalar context, L<C<stat>|/stat FILEHANDLE> returns a boolean value
8062 or failure, and, if successful, sets the information associated with
8063 the special filehandle C<_>.
8065 The L<File::stat> module provides a convenient, by-name access mechanism:
8068 my $sb = stat($filename);
8069 printf "File is %s, size is %s, perm %04o, mtime %s\n",
8070 $filename, $sb->size, $sb->mode & 07777,
8071 scalar localtime $sb->mtime;
8073 You can import symbolic mode constants (C<S_IF*>) and functions
8074 (C<S_IS*>) from the L<Fcntl> module:
8078 my $mode = (stat($filename))[2];
8080 my $user_rwx = ($mode & S_IRWXU) >> 6;
8081 my $group_read = ($mode & S_IRGRP) >> 3;
8082 my $other_execute = $mode & S_IXOTH;
8084 printf "Permissions are %04o\n", S_IMODE($mode), "\n";
8086 my $is_setuid = $mode & S_ISUID;
8087 my $is_directory = S_ISDIR($mode);
8089 You could write the last two using the C<-u> and C<-d> operators.
8090 Commonly available C<S_IF*> constants are:
8092 # Permissions: read, write, execute, for user, group, others.
8094 S_IRWXU S_IRUSR S_IWUSR S_IXUSR
8095 S_IRWXG S_IRGRP S_IWGRP S_IXGRP
8096 S_IRWXO S_IROTH S_IWOTH S_IXOTH
8098 # Setuid/Setgid/Stickiness/SaveText.
8099 # Note that the exact meaning of these is system-dependent.
8101 S_ISUID S_ISGID S_ISVTX S_ISTXT
8103 # File types. Not all are necessarily available on
8106 S_IFREG S_IFDIR S_IFLNK S_IFBLK S_IFCHR
8107 S_IFIFO S_IFSOCK S_IFWHT S_ENFMT
8109 # The following are compatibility aliases for S_IRUSR,
8110 # S_IWUSR, and S_IXUSR.
8112 S_IREAD S_IWRITE S_IEXEC
8114 and the C<S_IF*> functions are
8116 S_IMODE($mode) the part of $mode containing the permission
8117 bits and the setuid/setgid/sticky bits
8119 S_IFMT($mode) the part of $mode containing the file type
8120 which can be bit-anded with (for example)
8121 S_IFREG or with the following functions
8123 # The operators -f, -d, -l, -b, -c, -p, and -S.
8125 S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
8126 S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)
8128 # No direct -X operator counterpart, but for the first one
8129 # the -g operator is often equivalent. The ENFMT stands for
8130 # record flocking enforcement, a platform-dependent feature.
8132 S_ISENFMT($mode) S_ISWHT($mode)
8134 See your native L<chmod(2)> and L<stat(2)> documentation for more details
8135 about the C<S_*> constants. To get status info for a symbolic link
8136 instead of the target file behind the link, use the
8137 L<C<lstat>|/lstat FILEHANDLE> function.
8139 Portability issues: L<perlport/stat>.
8144 =item state TYPE VARLIST
8146 =item state VARLIST : ATTRS
8148 =item state TYPE VARLIST : ATTRS
8150 =for Pod::Functions +state declare and assign a persistent lexical variable
8152 L<C<state>|/state VARLIST> declares a lexically scoped variable, just
8153 like L<C<my>|/my VARLIST>.
8154 However, those variables will never be reinitialized, contrary to
8155 lexical variables that are reinitialized each time their enclosing block
8157 See L<perlsub/"Persistent Private Variables"> for details.
8159 If more than one variable is listed, the list must be placed in
8160 parentheses. With a parenthesised list, L<C<undef>|/undef EXPR> can be
8162 dummy placeholder. However, since initialization of state variables in
8163 list context is currently not possible this would serve no purpose.
8165 L<C<state>|/state VARLIST> is available only if the
8166 L<C<"state"> feature|feature/The 'state' feature> is enabled or if it is
8167 prefixed with C<CORE::>. The
8168 L<C<"state"> feature|feature/The 'state' feature> is enabled
8169 automatically with a C<use v5.10> (or higher) declaration in the current
8178 =for Pod::Functions no-op, formerly optimized input data for repeated searches
8180 At this time, C<study> does nothing. This may change in the future.
8182 Prior to Perl version 5.16, it would create an inverted index of all characters
8183 that occurred in the given SCALAR (or L<C<$_>|perlvar/$_> if unspecified). When
8184 matching a pattern, the rarest character from the pattern would be looked up in
8185 this index. Rarity was based on some static frequency tables constructed from
8186 some C programs and English text.
8189 =item sub NAME BLOCK
8192 =item sub NAME (PROTO) BLOCK
8194 =item sub NAME : ATTRS BLOCK
8196 =item sub NAME (PROTO) : ATTRS BLOCK
8198 =for Pod::Functions declare a subroutine, possibly anonymously
8200 This is subroutine definition, not a real function I<per se>. Without a
8201 BLOCK it's just a forward declaration. Without a NAME, it's an anonymous
8202 function declaration, so does return a value: the CODE ref of the closure
8205 See L<perlsub> and L<perlref> for details about subroutines and
8206 references; see L<attributes> and L<Attribute::Handlers> for more
8207 information about attributes.
8212 =for Pod::Functions +current_sub the current subroutine, or C<undef> if not in a subroutine
8214 A special token that returns a reference to the current subroutine, or
8215 L<C<undef>|/undef EXPR> outside of a subroutine.
8217 The behaviour of L<C<__SUB__>|/__SUB__> within a regex code block (such
8218 as C</(?{...})/>) is subject to change.
8220 This token is only available under C<use v5.16> or the
8221 L<C<"current_sub"> feature|feature/The 'current_sub' feature>.
8224 =item substr EXPR,OFFSET,LENGTH,REPLACEMENT
8225 X<substr> X<substring> X<mid> X<left> X<right>
8227 =item substr EXPR,OFFSET,LENGTH
8229 =item substr EXPR,OFFSET
8231 =for Pod::Functions get or alter a portion of a string
8233 Extracts a substring out of EXPR and returns it. First character is at
8234 offset zero. If OFFSET is negative, starts
8235 that far back from the end of the string. If LENGTH is omitted, returns
8236 everything through the end of the string. If LENGTH is negative, leaves that
8237 many characters off the end of the string.
8239 my $s = "The black cat climbed the green tree";
8240 my $color = substr $s, 4, 5; # black
8241 my $middle = substr $s, 4, -11; # black cat climbed the
8242 my $end = substr $s, 14; # climbed the green tree
8243 my $tail = substr $s, -4; # tree
8244 my $z = substr $s, -4, 2; # tr
8246 You can use the L<C<substr>|/substr EXPR,OFFSET,LENGTH,REPLACEMENT>
8247 function as an lvalue, in which case EXPR
8248 must itself be an lvalue. If you assign something shorter than LENGTH,
8249 the string will shrink, and if you assign something longer than LENGTH,
8250 the string will grow to accommodate it. To keep the string the same
8251 length, you may need to pad or chop your value using
8252 L<C<sprintf>|/sprintf FORMAT, LIST>.
8254 If OFFSET and LENGTH specify a substring that is partly outside the
8255 string, only the part within the string is returned. If the substring
8256 is beyond either end of the string,
8257 L<C<substr>|/substr EXPR,OFFSET,LENGTH,REPLACEMENT> returns the undefined
8258 value and produces a warning. When used as an lvalue, specifying a
8259 substring that is entirely outside the string raises an exception.
8260 Here's an example showing the behavior for boundary cases:
8263 substr($name, 4) = 'dy'; # $name is now 'freddy'
8264 my $null = substr $name, 6, 2; # returns "" (no warning)
8265 my $oops = substr $name, 7; # returns undef, with warning
8266 substr($name, 7) = 'gap'; # raises an exception
8268 An alternative to using
8269 L<C<substr>|/substr EXPR,OFFSET,LENGTH,REPLACEMENT> as an lvalue is to
8271 replacement string as the 4th argument. This allows you to replace
8272 parts of the EXPR and return what was there before in one operation,
8273 just as you can with
8274 L<C<splice>|/splice ARRAY,OFFSET,LENGTH,LIST>.
8276 my $s = "The black cat climbed the green tree";
8277 my $z = substr $s, 14, 7, "jumped from"; # climbed
8278 # $s is now "The black cat jumped from the green tree"
8280 Note that the lvalue returned by the three-argument version of
8281 L<C<substr>|/substr EXPR,OFFSET,LENGTH,REPLACEMENT> acts as
8282 a 'magic bullet'; each time it is assigned to, it remembers which part
8283 of the original string is being modified; for example:
8286 for (substr($x,1,2)) {
8287 $_ = 'a'; print $x,"\n"; # prints 1a4
8288 $_ = 'xyz'; print $x,"\n"; # prints 1xyz4
8290 $_ = 'pq'; print $x,"\n"; # prints 5pq9
8293 With negative offsets, it remembers its position from the end of the string
8294 when the target string is modified:
8297 for (substr($x, -3, 2)) {
8298 $_ = 'a'; print $x,"\n"; # prints 1a4, as above
8300 print $_,"\n"; # prints f
8303 Prior to Perl version 5.10, the result of using an lvalue multiple times was
8304 unspecified. Prior to 5.16, the result with negative offsets was
8307 =item symlink OLDFILE,NEWFILE
8308 X<symlink> X<link> X<symbolic link> X<link, symbolic>
8310 =for Pod::Functions create a symbolic link to a file
8312 Creates a new filename symbolically linked to the old filename.
8313 Returns C<1> for success, C<0> otherwise. On systems that don't support
8314 symbolic links, raises an exception. To check for that,
8317 my $symlink_exists = eval { symlink("",""); 1 };
8319 Portability issues: L<perlport/symlink>.
8321 =item syscall NUMBER, LIST
8322 X<syscall> X<system call>
8324 =for Pod::Functions execute an arbitrary system call
8326 Calls the system call specified as the first element of the list,
8327 passing the remaining elements as arguments to the system call. If
8328 unimplemented, raises an exception. The arguments are interpreted
8329 as follows: if a given argument is numeric, the argument is passed as
8330 an int. If not, the pointer to the string value is passed. You are
8331 responsible to make sure a string is pre-extended long enough to
8332 receive any result that might be written into a string. You can't use a
8333 string literal (or other read-only string) as an argument to
8334 L<C<syscall>|/syscall NUMBER, LIST> because Perl has to assume that any
8335 string pointer might be written through. If your
8336 integer arguments are not literals and have never been interpreted in a
8337 numeric context, you may need to add C<0> to them to force them to look
8338 like numbers. This emulates the
8339 L<C<syswrite>|/syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET> function (or
8342 require 'syscall.ph'; # may need to run h2ph
8343 my $s = "hi there\n";
8344 syscall(SYS_write(), fileno(STDOUT), $s, length $s);
8346 Note that Perl supports passing of up to only 14 arguments to your syscall,
8347 which in practice should (usually) suffice.
8349 Syscall returns whatever value returned by the system call it calls.
8350 If the system call fails, L<C<syscall>|/syscall NUMBER, LIST> returns
8351 C<-1> and sets L<C<$!>|perlvar/$!> (errno).
8352 Note that some system calls I<can> legitimately return C<-1>. The proper
8353 way to handle such calls is to assign C<$! = 0> before the call, then
8354 check the value of L<C<$!>|perlvar/$!> if
8355 L<C<syscall>|/syscall NUMBER, LIST> returns C<-1>.
8357 There's a problem with C<syscall(SYS_pipe())>: it returns the file
8358 number of the read end of the pipe it creates, but there is no way
8359 to retrieve the file number of the other end. You can avoid this
8360 problem by using L<C<pipe>|/pipe READHANDLE,WRITEHANDLE> instead.
8362 Portability issues: L<perlport/syscall>.
8364 =item sysopen FILEHANDLE,FILENAME,MODE
8367 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
8369 =for Pod::Functions +5.002 open a file, pipe, or descriptor
8371 Opens the file whose filename is given by FILENAME, and associates it with
8372 FILEHANDLE. If FILEHANDLE is an expression, its value is used as the real
8373 filehandle wanted; an undefined scalar will be suitably autovivified. This
8374 function calls the underlying operating system's L<open(2)> function with the
8375 parameters FILENAME, MODE, and PERMS.
8377 Returns true on success and L<C<undef>|/undef EXPR> otherwise.
8379 The possible values and flag bits of the MODE parameter are
8380 system-dependent; they are available via the standard module
8381 L<C<Fcntl>|Fcntl>. See the documentation of your operating system's
8382 L<open(2)> syscall to see
8383 which values and flag bits are available. You may combine several flags
8384 using the C<|>-operator.
8386 Some of the most common values are C<O_RDONLY> for opening the file in
8387 read-only mode, C<O_WRONLY> for opening the file in write-only mode,
8388 and C<O_RDWR> for opening the file in read-write mode.
8389 X<O_RDONLY> X<O_RDWR> X<O_WRONLY>
8391 For historical reasons, some values work on almost every system
8392 supported by Perl: 0 means read-only, 1 means write-only, and 2
8393 means read/write. We know that these values do I<not> work under
8394 OS/390 and on the Macintosh; you probably don't want to
8395 use them in new code.
8397 If the file named by FILENAME does not exist and the
8398 L<C<open>|/open FILEHANDLE,EXPR> call creates
8399 it (typically because MODE includes the C<O_CREAT> flag), then the value of
8400 PERMS specifies the permissions of the newly created file. If you omit
8401 the PERMS argument to L<C<sysopen>|/sysopen FILEHANDLE,FILENAME,MODE>,
8402 Perl uses the octal value C<0666>.
8403 These permission values need to be in octal, and are modified by your
8404 process's current L<C<umask>|/umask EXPR>.
8407 In many systems the C<O_EXCL> flag is available for opening files in
8408 exclusive mode. This is B<not> locking: exclusiveness means here that
8409 if the file already exists,
8410 L<C<sysopen>|/sysopen FILEHANDLE,FILENAME,MODE> fails. C<O_EXCL> may
8412 on network filesystems, and has no effect unless the C<O_CREAT> flag
8413 is set as well. Setting C<O_CREAT|O_EXCL> prevents the file from
8414 being opened if it is a symbolic link. It does not protect against
8415 symbolic links in the file's path.
8418 Sometimes you may want to truncate an already-existing file. This
8419 can be done using the C<O_TRUNC> flag. The behavior of
8420 C<O_TRUNC> with C<O_RDONLY> is undefined.
8423 You should seldom if ever use C<0644> as argument to
8424 L<C<sysopen>|/sysopen FILEHANDLE,FILENAME,MODE>, because
8425 that takes away the user's option to have a more permissive umask.
8426 Better to omit it. See L<C<umask>|/umask EXPR> for more on this.
8428 Note that under Perls older than 5.8.0,
8429 L<C<sysopen>|/sysopen FILEHANDLE,FILENAME,MODE> depends on the
8430 L<fdopen(3)> C library function. On many Unix systems, L<fdopen(3)> is known
8431 to fail when file descriptors exceed a certain value, typically 255. If
8432 you need more file descriptors than that, consider using the
8433 L<C<POSIX::open>|POSIX/C<open>> function. For Perls 5.8.0 and later,
8434 PerlIO is (most often) the default.
8436 See L<perlopentut> for a kinder, gentler explanation of opening files.
8438 Portability issues: L<perlport/sysopen>.
8440 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
8443 =item sysread FILEHANDLE,SCALAR,LENGTH
8445 =for Pod::Functions fixed-length unbuffered input from a filehandle
8447 Attempts to read LENGTH bytes of data into variable SCALAR from the
8448 specified FILEHANDLE, using L<read(2)>. It bypasses
8449 buffered IO, so mixing this with other kinds of reads,
8450 L<C<print>|/print FILEHANDLE LIST>, L<C<write>|/write FILEHANDLE>,
8451 L<C<seek>|/seek FILEHANDLE,POSITION,WHENCE>,
8452 L<C<tell>|/tell FILEHANDLE>, or L<C<eof>|/eof FILEHANDLE> can cause
8453 confusion because the
8454 perlio or stdio layers usually buffer data. Returns the number of
8455 bytes actually read, C<0> at end of file, or undef if there was an
8456 error (in the latter case L<C<$!>|perlvar/$!> is also set). SCALAR will
8458 shrunk so that the last byte actually read is the last byte of the
8459 scalar after the read.
8461 An OFFSET may be specified to place the read data at some place in the
8462 string other than the beginning. A negative OFFSET specifies
8463 placement at that many characters counting backwards from the end of
8464 the string. A positive OFFSET greater than the length of SCALAR
8465 results in the string being padded to the required size with C<"\0">
8466 bytes before the result of the read is appended.
8468 There is no syseof() function, which is ok, since
8469 L<C<eof>|/eof FILEHANDLE> doesn't work well on device files (like ttys)
8470 anyway. Use L<C<sysread>|/sysread FILEHANDLE,SCALAR,LENGTH,OFFSET> and
8471 check for a return value for 0 to decide whether you're done.
8473 Note that if the filehandle has been marked as C<:utf8>, Unicode
8474 characters are read instead of bytes (the LENGTH, OFFSET, and the
8475 return value of L<C<sysread>|/sysread FILEHANDLE,SCALAR,LENGTH,OFFSET>
8476 are in Unicode characters). The C<:encoding(...)> layer implicitly
8477 introduces the C<:utf8> layer. See
8478 L<C<binmode>|/binmode FILEHANDLE, LAYER>,
8479 L<C<open>|/open FILEHANDLE,EXPR>, and the L<open> pragma.
8481 =item sysseek FILEHANDLE,POSITION,WHENCE
8484 =for Pod::Functions +5.004 position I/O pointer on handle used with sysread and syswrite
8486 Sets FILEHANDLE's system position I<in bytes> using L<lseek(2)>. FILEHANDLE may
8487 be an expression whose value gives the name of the filehandle. The values
8488 for WHENCE are C<0> to set the new position to POSITION; C<1> to set the it
8489 to the current position plus POSITION; and C<2> to set it to EOF plus
8490 POSITION, typically negative.
8492 Note the emphasis on bytes: even if the filehandle has been set to operate
8493 on characters (for example using the C<:encoding(utf8)> I/O layer), the
8494 L<C<seek>|/seek FILEHANDLE,POSITION,WHENCE>,
8495 L<C<tell>|/tell FILEHANDLE>, and
8496 L<C<sysseek>|/sysseek FILEHANDLE,POSITION,WHENCE>
8497 family of functions use byte offsets, not character offsets,
8498 because seeking to a character offset would be very slow in a UTF-8 file.
8500 L<C<sysseek>|/sysseek FILEHANDLE,POSITION,WHENCE> bypasses normal
8501 buffered IO, so mixing it with reads other than
8502 L<C<sysread>|/sysread FILEHANDLE,SCALAR,LENGTH,OFFSET> (for example
8503 L<C<readline>|/readline EXPR> or
8504 L<C<read>|/read FILEHANDLE,SCALAR,LENGTH,OFFSET>),
8505 L<C<print>|/print FILEHANDLE LIST>, L<C<write>|/write FILEHANDLE>,
8506 L<C<seek>|/seek FILEHANDLE,POSITION,WHENCE>,
8507 L<C<tell>|/tell FILEHANDLE>, or L<C<eof>|/eof FILEHANDLE> may cause
8510 For WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>,
8511 and C<SEEK_END> (start of the file, current position, end of the file)
8512 from the L<Fcntl> module. Use of the constants is also more portable
8513 than relying on 0, 1, and 2. For example to define a "systell" function:
8515 use Fcntl 'SEEK_CUR';
8516 sub systell { sysseek($_[0], 0, SEEK_CUR) }
8518 Returns the new position, or the undefined value on failure. A position
8519 of zero is returned as the string C<"0 but true">; thus
8520 L<C<sysseek>|/sysseek FILEHANDLE,POSITION,WHENCE> returns
8521 true on success and false on failure, yet you can still easily determine
8527 =item system PROGRAM LIST
8529 =for Pod::Functions run a separate program
8531 Does exactly the same thing as L<C<exec>|/exec LIST>, except that a fork is
8532 done first and the parent process waits for the child process to
8533 exit. Note that argument processing varies depending on the
8534 number of arguments. If there is more than one argument in LIST,
8535 or if LIST is an array with more than one value, starts the program
8536 given by the first element of the list with arguments given by the
8537 rest of the list. If there is only one scalar argument, the argument
8538 is checked for shell metacharacters, and if there are any, the
8539 entire argument is passed to the system's command shell for parsing
8540 (this is C</bin/sh -c> on Unix platforms, but varies on other
8541 platforms). If there are no shell metacharacters in the argument,
8542 it is split into words and passed directly to C<execvp>, which is
8543 more efficient. On Windows, only the C<system PROGRAM LIST> syntax will
8544 reliably avoid using the shell; C<system LIST>, even with more than one
8545 element, will fall back to the shell if the first spawn fails.
8547 Perl will attempt to flush all files opened for
8548 output before any operation that may do a fork, but this may not be
8549 supported on some platforms (see L<perlport>). To be safe, you may need
8550 to set L<C<$E<verbar>>|perlvar/$E<verbar>> (C<$AUTOFLUSH> in L<English>)
8551 or call the C<autoflush> method of L<C<IO::Handle>|IO::Handle/METHODS>
8552 on any open handles.
8554 The return value is the exit status of the program as returned by the
8555 L<C<wait>|/wait> call. To get the actual exit value, shift right by
8556 eight (see below). See also L<C<exec>|/exec LIST>. This is I<not> what
8557 you want to use to capture the output from a command; for that you
8558 should use merely backticks or
8559 L<C<qxE<sol>E<sol>>|/qxE<sol>STRINGE<sol>>, as described in
8560 L<perlop/"`STRING`">. Return value of -1 indicates a failure to start
8561 the program or an error of the L<wait(2)> system call (inspect
8562 L<C<$!>|perlvar/$!> for the reason).
8564 If you'd like to make L<C<system>|/system LIST> (and many other bits of
8565 Perl) die on error, have a look at the L<autodie> pragma.
8567 Like L<C<exec>|/exec LIST>, L<C<system>|/system LIST> allows you to lie
8568 to a program about its name if you use the C<system PROGRAM LIST>
8569 syntax. Again, see L<C<exec>|/exec LIST>.
8571 Since C<SIGINT> and C<SIGQUIT> are ignored during the execution of
8572 L<C<system>|/system LIST>, if you expect your program to terminate on
8573 receipt of these signals you will need to arrange to do so yourself
8574 based on the return value.
8576 my @args = ("command", "arg1", "arg2");
8578 or die "system @args failed: $?";
8580 If you'd like to manually inspect L<C<system>|/system LIST>'s failure,
8581 you can check all possible failure modes by inspecting
8582 L<C<$?>|perlvar/$?> like this:
8585 print "failed to execute: $!\n";
8588 printf "child died with signal %d, %s coredump\n",
8589 ($? & 127), ($? & 128) ? 'with' : 'without';
8592 printf "child exited with value %d\n", $? >> 8;
8595 Alternatively, you may inspect the value of
8596 L<C<${^CHILD_ERROR_NATIVE}>|perlvar/${^CHILD_ERROR_NATIVE}> with the
8597 L<C<W*()>|POSIX/C<WIFEXITED>> calls from the L<POSIX> module.
8599 When L<C<system>|/system LIST>'s arguments are executed indirectly by
8600 the shell, results and return codes are subject to its quirks.
8601 See L<perlop/"`STRING`"> and L<C<exec>|/exec LIST> for details.
8603 Since L<C<system>|/system LIST> does a L<C<fork>|/fork> and
8604 L<C<wait>|/wait> it may affect a C<SIGCHLD> handler. See L<perlipc> for
8607 Portability issues: L<perlport/system>.
8609 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
8612 =item syswrite FILEHANDLE,SCALAR,LENGTH
8614 =item syswrite FILEHANDLE,SCALAR
8616 =for Pod::Functions fixed-length unbuffered output to a filehandle
8618 Attempts to write LENGTH bytes of data from variable SCALAR to the
8619 specified FILEHANDLE, using L<write(2)>. If LENGTH is
8620 not specified, writes whole SCALAR. It bypasses buffered IO, so
8621 mixing this with reads (other than C<sysread)>),
8622 L<C<print>|/print FILEHANDLE LIST>, L<C<write>|/write FILEHANDLE>,
8623 L<C<seek>|/seek FILEHANDLE,POSITION,WHENCE>,
8624 L<C<tell>|/tell FILEHANDLE>, or L<C<eof>|/eof FILEHANDLE> may cause
8625 confusion because the perlio and stdio layers usually buffer data.
8626 Returns the number of bytes actually written, or L<C<undef>|/undef EXPR>
8627 if there was an error (in this case the errno variable
8628 L<C<$!>|perlvar/$!> is also set). If the LENGTH is greater than the
8629 data available in the SCALAR after the OFFSET, only as much data as is
8630 available will be written.
8632 An OFFSET may be specified to write the data from some part of the
8633 string other than the beginning. A negative OFFSET specifies writing
8634 that many characters counting backwards from the end of the string.
8635 If SCALAR is of length zero, you can only use an OFFSET of 0.
8637 B<WARNING>: If the filehandle is marked C<:utf8>, Unicode characters
8638 encoded in UTF-8 are written instead of bytes, and the LENGTH, OFFSET, and
8639 return value of L<C<syswrite>|/syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET>
8640 are in (UTF8-encoded Unicode) characters.
8641 The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer.
8642 Alternately, if the handle is not marked with an encoding but you
8643 attempt to write characters with code points over 255, raises an exception.
8644 See L<C<binmode>|/binmode FILEHANDLE, LAYER>,
8645 L<C<open>|/open FILEHANDLE,EXPR>, and the L<open> pragma.
8647 =item tell FILEHANDLE
8652 =for Pod::Functions get current seekpointer on a filehandle
8654 Returns the current position I<in bytes> for FILEHANDLE, or -1 on
8655 error. FILEHANDLE may be an expression whose value gives the name of
8656 the actual filehandle. If FILEHANDLE is omitted, assumes the file
8659 Note the emphasis on bytes: even if the filehandle has been set to operate
8660 on characters (for example using the C<:encoding(utf8)> I/O layer), the
8661 L<C<seek>|/seek FILEHANDLE,POSITION,WHENCE>,
8662 L<C<tell>|/tell FILEHANDLE>, and
8663 L<C<sysseek>|/sysseek FILEHANDLE,POSITION,WHENCE>
8664 family of functions use byte offsets, not character offsets,
8665 because seeking to a character offset would be very slow in a UTF-8 file.
8667 The return value of L<C<tell>|/tell FILEHANDLE> for the standard streams
8668 like the STDIN depends on the operating system: it may return -1 or
8669 something else. L<C<tell>|/tell FILEHANDLE> on pipes, fifos, and
8670 sockets usually returns -1.
8672 There is no C<systell> function. Use
8673 L<C<sysseek($fh, 0, 1)>|/sysseek FILEHANDLE,POSITION,WHENCE> for that.
8675 Do not use L<C<tell>|/tell FILEHANDLE> (or other buffered I/O
8676 operations) on a filehandle that has been manipulated by
8677 L<C<sysread>|/sysread FILEHANDLE,SCALAR,LENGTH,OFFSET>,
8678 L<C<syswrite>|/syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET>, or
8679 L<C<sysseek>|/sysseek FILEHANDLE,POSITION,WHENCE>. Those functions
8680 ignore the buffering, while L<C<tell>|/tell FILEHANDLE> does not.
8682 =item telldir DIRHANDLE
8685 =for Pod::Functions get current seekpointer on a directory handle
8687 Returns the current position of the L<C<readdir>|/readdir DIRHANDLE>
8688 routines on DIRHANDLE. Value may be given to
8689 L<C<seekdir>|/seekdir DIRHANDLE,POS> to access a particular location in
8690 a directory. L<C<telldir>|/telldir DIRHANDLE> has the same caveats
8691 about possible directory compaction as the corresponding system library
8694 =item tie VARIABLE,CLASSNAME,LIST
8697 =for Pod::Functions +5.002 bind a variable to an object class
8699 This function binds a variable to a package class that will provide the
8700 implementation for the variable. VARIABLE is the name of the variable
8701 to be enchanted. CLASSNAME is the name of a class implementing objects
8702 of correct type. Any additional arguments are passed to the
8703 appropriate constructor
8704 method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
8705 or C<TIEHASH>). Typically these are arguments such as might be passed
8706 to the L<dbm_open(3)> function of C. The object returned by the
8707 constructor is also returned by the
8708 L<C<tie>|/tie VARIABLE,CLASSNAME,LIST> function, which would be useful
8709 if you want to access other methods in CLASSNAME.
8711 Note that functions such as L<C<keys>|/keys HASH> and
8712 L<C<values>|/values HASH> may return huge lists when used on large
8713 objects, like DBM files. You may prefer to use the L<C<each>|/each
8714 HASH> function to iterate over such. Example:
8716 # print out history file offsets
8718 tie(my %HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
8719 while (my ($key,$val) = each %HIST) {
8720 print $key, ' = ', unpack('L', $val), "\n";
8723 A class implementing a hash should have the following methods:
8725 TIEHASH classname, LIST
8727 STORE this, key, value
8732 NEXTKEY this, lastkey
8737 A class implementing an ordinary array should have the following methods:
8739 TIEARRAY classname, LIST
8741 STORE this, key, value
8743 STORESIZE this, count
8749 SPLICE this, offset, length, LIST
8756 A class implementing a filehandle should have the following methods:
8758 TIEHANDLE classname, LIST
8759 READ this, scalar, length, offset
8762 WRITE this, scalar, length, offset
8764 PRINTF this, format, LIST
8768 SEEK this, position, whence
8770 OPEN this, mode, LIST
8775 A class implementing a scalar should have the following methods:
8777 TIESCALAR classname, LIST
8783 Not all methods indicated above need be implemented. See L<perltie>,
8784 L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>.
8786 Unlike L<C<dbmopen>|/dbmopen HASH,DBNAME,MASK>, the
8787 L<C<tie>|/tie VARIABLE,CLASSNAME,LIST> function will not
8788 L<C<use>|/use Module VERSION LIST> or L<C<require>|/require VERSION> a
8789 module for you; you need to do that explicitly yourself. See L<DB_File>
8790 or the L<Config> module for interesting
8791 L<C<tie>|/tie VARIABLE,CLASSNAME,LIST> implementations.
8793 For further details see L<perltie>, L<C<tied>|/tied VARIABLE>.
8798 =for Pod::Functions get a reference to the object underlying a tied variable
8800 Returns a reference to the object underlying VARIABLE (the same value
8801 that was originally returned by the
8802 L<C<tie>|/tie VARIABLE,CLASSNAME,LIST> call that bound the variable
8803 to a package.) Returns the undefined value if VARIABLE isn't tied to a
8809 =for Pod::Functions return number of seconds since 1970
8811 Returns the number of non-leap seconds since whatever time the system
8812 considers to be the epoch, suitable for feeding to
8813 L<C<gmtime>|/gmtime EXPR> and L<C<localtime>|/localtime EXPR>. On most
8814 systems the epoch is 00:00:00 UTC, January 1, 1970;
8815 a prominent exception being Mac OS Classic which uses 00:00:00, January 1,
8816 1904 in the current local time zone for its epoch.
8818 For measuring time in better granularity than one second, use the
8819 L<Time::HiRes> module from Perl 5.8 onwards (or from CPAN before then), or,
8820 if you have L<gettimeofday(2)>, you may be able to use the
8821 L<C<syscall>|/syscall NUMBER, LIST> interface of Perl. See L<perlfaq8>
8824 For date and time processing look at the many related modules on CPAN.
8825 For a comprehensive date and time representation look at the
8831 =for Pod::Functions return elapsed time for self and child processes
8833 Returns a four-element list giving the user and system times in
8834 seconds for this process and any exited children of this process.
8836 my ($user,$system,$cuser,$csystem) = times;
8838 In scalar context, L<C<times>|/times> returns C<$user>.
8840 Children's times are only included for terminated children.
8842 Portability issues: L<perlport/times>.
8846 =for Pod::Functions transliterate a string
8848 The transliteration operator. Same as
8849 L<C<yE<sol>E<sol>E<sol>>|/yE<sol>E<sol>E<sol>>. See
8850 L<perlop/"Quote-Like Operators">.
8852 =item truncate FILEHANDLE,LENGTH
8855 =item truncate EXPR,LENGTH
8857 =for Pod::Functions shorten a file
8859 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
8860 specified length. Raises an exception if truncate isn't implemented
8861 on your system. Returns true if successful, L<C<undef>|/undef EXPR> on
8864 The behavior is undefined if LENGTH is greater than the length of the
8867 The position in the file of FILEHANDLE is left unchanged. You may want to
8868 call L<seek|/"seek FILEHANDLE,POSITION,WHENCE"> before writing to the
8871 Portability issues: L<perlport/truncate>.
8874 X<uc> X<uppercase> X<toupper>
8878 =for Pod::Functions return upper-case version of a string
8880 Returns an uppercased version of EXPR. This is the internal function
8881 implementing the C<\U> escape in double-quoted strings.
8882 It does not attempt to do titlecase mapping on initial letters. See
8883 L<C<ucfirst>|/ucfirst EXPR> for that.
8885 If EXPR is omitted, uses L<C<$_>|perlvar/$_>.
8887 This function behaves the same way under various pragmas, such as in a locale,
8888 as L<C<lc>|/lc EXPR> does.
8891 X<ucfirst> X<uppercase>
8895 =for Pod::Functions return a string with just the next letter in upper case
8897 Returns the value of EXPR with the first character in uppercase
8898 (titlecase in Unicode). This is the internal function implementing
8899 the C<\u> escape in double-quoted strings.
8901 If EXPR is omitted, uses L<C<$_>|perlvar/$_>.
8903 This function behaves the same way under various pragmas, such as in a locale,
8904 as L<C<lc>|/lc EXPR> does.
8911 =for Pod::Functions set file creation mode mask
8913 Sets the umask for the process to EXPR and returns the previous value.
8914 If EXPR is omitted, merely returns the current umask.
8916 The Unix permission C<rwxr-x---> is represented as three sets of three
8917 bits, or three octal digits: C<0750> (the leading 0 indicates octal
8918 and isn't one of the digits). The L<C<umask>|/umask EXPR> value is such
8919 a number representing disabled permissions bits. The permission (or
8920 "mode") values you pass L<C<mkdir>|/mkdir FILENAME,MASK> or
8921 L<C<sysopen>|/sysopen FILEHANDLE,FILENAME,MODE> are modified by your
8922 umask, so even if you tell
8923 L<C<sysopen>|/sysopen FILEHANDLE,FILENAME,MODE> to create a file with
8924 permissions C<0777>, if your umask is C<0022>, then the file will
8925 actually be created with permissions C<0755>. If your
8926 L<C<umask>|/umask EXPR> were C<0027> (group can't write; others can't
8927 read, write, or execute), then passing
8928 L<C<sysopen>|/sysopen FILEHANDLE,FILENAME,MODE> C<0666> would create a
8929 file with mode C<0640> (because C<0666 &~ 027> is C<0640>).
8931 Here's some advice: supply a creation mode of C<0666> for regular
8932 files (in L<C<sysopen>|/sysopen FILEHANDLE,FILENAME,MODE>) and one of
8933 C<0777> for directories (in L<C<mkdir>|/mkdir FILENAME,MASK>) and
8934 executable files. This gives users the freedom of
8935 choice: if they want protected files, they might choose process umasks
8936 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
8937 Programs should rarely if ever make policy decisions better left to
8938 the user. The exception to this is when writing files that should be
8939 kept private: mail files, web browser cookies, F<.rhosts> files, and
8942 If L<umask(2)> is not implemented on your system and you are trying to
8943 restrict access for I<yourself> (i.e., C<< (EXPR & 0700) > 0 >>),
8944 raises an exception. If L<umask(2)> is not implemented and you are
8945 not trying to restrict access for yourself, returns
8946 L<C<undef>|/undef EXPR>.
8948 Remember that a umask is a number, usually given in octal; it is I<not> a
8949 string of octal digits. See also L<C<oct>|/oct EXPR>, if all you have
8952 Portability issues: L<perlport/umask>.
8955 X<undef> X<undefine>
8959 =for Pod::Functions remove a variable or function definition
8961 Undefines the value of EXPR, which must be an lvalue. Use only on a
8962 scalar value, an array (using C<@>), a hash (using C<%>), a subroutine
8963 (using C<&>), or a typeglob (using C<*>). Saying C<undef $hash{$key}>
8964 will probably not do what you expect on most predefined variables or
8965 DBM list values, so don't do that; see L<C<delete>|/delete EXPR>.
8966 Always returns the undefined value.
8967 You can omit the EXPR, in which case nothing is
8968 undefined, but you still get an undefined value that you could, for
8969 instance, return from a subroutine, assign to a variable, or pass as a
8970 parameter. Examples:
8973 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
8977 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
8978 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
8979 select undef, undef, undef, 0.25;
8980 my ($x, $y, undef, $z) = foo(); # Ignore third value returned
8982 Note that this is a unary operator, not a list operator.
8985 X<unlink> X<delete> X<remove> X<rm> X<del>
8989 =for Pod::Functions remove one link to a file
8991 Deletes a list of files. On success, it returns the number of files
8992 it successfully deleted. On failure, it returns false and sets
8993 L<C<$!>|perlvar/$!> (errno):
8995 my $unlinked = unlink 'a', 'b', 'c';
8997 unlink glob "*.bak";
8999 On error, L<C<unlink>|/unlink LIST> will not tell you which files it
9001 If you want to know which files you could not remove, try them one
9004 foreach my $file ( @goners ) {
9005 unlink $file or warn "Could not unlink $file: $!";
9008 Note: L<C<unlink>|/unlink LIST> will not attempt to delete directories
9010 superuser and the B<-U> flag is supplied to Perl. Even if these
9011 conditions are met, be warned that unlinking a directory can inflict
9012 damage on your filesystem. Finally, using L<C<unlink>|/unlink LIST> on
9013 directories is not supported on many operating systems. Use
9014 L<C<rmdir>|/rmdir FILENAME> instead.
9016 If LIST is omitted, L<C<unlink>|/unlink LIST> uses L<C<$_>|perlvar/$_>.
9018 =item unpack TEMPLATE,EXPR
9021 =item unpack TEMPLATE
9023 =for Pod::Functions convert binary structure into normal perl variables
9025 L<C<unpack>|/unpack TEMPLATE,EXPR> does the reverse of
9026 L<C<pack>|/pack TEMPLATE,LIST>: it takes a string
9027 and expands it out into a list of values.
9028 (In scalar context, it returns merely the first value produced.)
9030 If EXPR is omitted, unpacks the L<C<$_>|perlvar/$_> string.
9031 See L<perlpacktut> for an introduction to this function.
9033 The string is broken into chunks described by the TEMPLATE. Each chunk
9034 is converted separately to a value. Typically, either the string is a result
9035 of L<C<pack>|/pack TEMPLATE,LIST>, or the characters of the string
9036 represent a C structure of some kind.
9038 The TEMPLATE has the same format as in the
9039 L<C<pack>|/pack TEMPLATE,LIST> function.
9040 Here's a subroutine that does substring:
9043 my ($what, $where, $howmuch) = @_;
9044 unpack("x$where a$howmuch", $what);
9049 sub ordinal { unpack("W",$_[0]); } # same as ord()
9051 In addition to fields allowed in L<C<pack>|/pack TEMPLATE,LIST>, you may
9052 prefix a field with a %<number> to indicate that
9053 you want a <number>-bit checksum of the items instead of the items
9054 themselves. Default is a 16-bit checksum. The checksum is calculated by
9055 summing numeric values of expanded values (for string fields the sum of
9056 C<ord($char)> is taken; for bit fields the sum of zeroes and ones).
9058 For example, the following
9059 computes the same number as the System V sum program:
9063 unpack("%32W*", readline) % 65535;
9066 The following efficiently counts the number of set bits in a bit vector:
9068 my $setbits = unpack("%32b*", $selectmask);
9070 The C<p> and C<P> formats should be used with care. Since Perl
9071 has no way of checking whether the value passed to
9072 L<C<unpack>|/unpack TEMPLATE,EXPR>
9073 corresponds to a valid memory location, passing a pointer value that's
9074 not known to be valid is likely to have disastrous consequences.
9076 If there are more pack codes or if the repeat count of a field or a group
9077 is larger than what the remainder of the input string allows, the result
9078 is not well defined: the repeat count may be decreased, or
9079 L<C<unpack>|/unpack TEMPLATE,EXPR> may produce empty strings or zeros,
9080 or it may raise an exception.
9081 If the input string is longer than one described by the TEMPLATE,
9082 the remainder of that input string is ignored.
9084 See L<C<pack>|/pack TEMPLATE,LIST> for more examples and notes.
9086 =item unshift ARRAY,LIST
9089 =for Pod::Functions prepend more elements to the beginning of a list
9091 Does the opposite of a L<C<shift>|/shift ARRAY>. Or the opposite of a
9092 L<C<push>|/push ARRAY,LIST>,
9093 depending on how you look at it. Prepends list to the front of the
9094 array and returns the new number of elements in the array.
9096 unshift(@ARGV, '-e') unless $ARGV[0] =~ /^-/;
9098 Note the LIST is prepended whole, not one element at a time, so the
9099 prepended elements stay in the same order. Use
9100 L<C<reverse>|/reverse LIST> to do the reverse.
9102 Starting with Perl 5.14, an experimental feature allowed
9103 L<C<unshift>|/unshift ARRAY,LIST> to take
9104 a scalar expression. This experiment has been deemed unsuccessful, and was
9105 removed as of Perl 5.24.
9107 =item untie VARIABLE
9110 =for Pod::Functions break a tie binding to a variable
9112 Breaks the binding between a variable and a package.
9113 (See L<tie|/tie VARIABLE,CLASSNAME,LIST>.)
9114 Has no effect if the variable is not tied.
9116 =item use Module VERSION LIST
9117 X<use> X<module> X<import>
9119 =item use Module VERSION
9121 =item use Module LIST
9127 =for Pod::Functions load in a module at compile time and import its namespace
9129 Imports some semantics into the current package from the named module,
9130 generally by aliasing certain subroutine or variable names into your
9131 package. It is exactly equivalent to
9133 BEGIN { require Module; Module->import( LIST ); }
9135 except that Module I<must> be a bareword.
9136 The importation can be made conditional by using the L<if> module.
9138 In the peculiar C<use VERSION> form, VERSION may be either a positive
9139 decimal fraction such as 5.006, which will be compared to
9140 L<C<$]>|perlvar/$]>, or a v-string of the form v5.6.1, which will be
9141 compared to L<C<$^V>|perlvar/$^V> (aka $PERL_VERSION). An
9142 exception is raised if VERSION is greater than the version of the
9143 current Perl interpreter; Perl will not attempt to parse the rest of the
9144 file. Compare with L<C<require>|/require VERSION>, which can do a
9145 similar check at run time.
9146 Symmetrically, C<no VERSION> allows you to specify that you want a version
9147 of Perl older than the specified one.
9149 Specifying VERSION as a literal of the form v5.6.1 should generally be
9150 avoided, because it leads to misleading error messages under earlier
9151 versions of Perl (that is, prior to 5.6.0) that do not support this
9152 syntax. The equivalent numeric version should be used instead.
9154 use v5.6.1; # compile time version check
9156 use 5.006_001; # ditto; preferred for backwards compatibility
9158 This is often useful if you need to check the current Perl version before
9159 L<C<use>|/use Module VERSION LIST>ing library modules that won't work
9160 with older versions of Perl.
9161 (We try not to do this more than we have to.)
9163 C<use VERSION> also lexically enables all features available in the requested
9164 version as defined by the L<feature> pragma, disabling any features
9165 not in the requested version's feature bundle. See L<feature>.
9166 Similarly, if the specified Perl version is greater than or equal to
9167 5.12.0, strictures are enabled lexically as
9168 with L<C<use strict>|strict>. Any explicit use of
9169 C<use strict> or C<no strict> overrides C<use VERSION>, even if it comes
9170 before it. Later use of C<use VERSION>
9171 will override all behavior of a previous
9172 C<use VERSION>, possibly removing the C<strict> and C<feature> added by
9173 C<use VERSION>. C<use VERSION> does not
9174 load the F<feature.pm> or F<strict.pm>
9177 The C<BEGIN> forces the L<C<require>|/require VERSION> and
9178 L<C<import>|/import LIST> to happen at compile time. The
9179 L<C<require>|/require VERSION> makes sure the module is loaded into
9180 memory if it hasn't been yet. The L<C<import>|/import LIST> is not a
9181 builtin; it's just an ordinary static method
9182 call into the C<Module> package to tell the module to import the list of
9183 features back into the current package. The module can implement its
9184 L<C<import>|/import LIST> method any way it likes, though most modules
9185 just choose to derive their L<C<import>|/import LIST> method via
9186 inheritance from the C<Exporter> class that is defined in the
9187 L<C<Exporter>|Exporter> module. See L<Exporter>. If no
9188 L<C<import>|/import LIST> method can be found, then the call is skipped,
9189 even if there is an AUTOLOAD method.
9191 If you do not want to call the package's L<C<import>|/import LIST>
9192 method (for instance,
9193 to stop your namespace from being altered), explicitly supply the empty list:
9197 That is exactly equivalent to
9199 BEGIN { require Module }
9201 If the VERSION argument is present between Module and LIST, then the
9202 L<C<use>|/use Module VERSION LIST> will call the C<VERSION> method in
9203 class Module with the given version as an argument:
9209 BEGIN { require Module; Module->VERSION(12.34) }
9211 The L<default C<VERSION> method|UNIVERSAL/C<VERSION ( [ REQUIRE ] )>>,
9212 inherited from the L<C<UNIVERSAL>|UNIVERSAL> class, croaks if the given
9213 version is larger than the value of the variable C<$Module::VERSION>.
9215 Again, there is a distinction between omitting LIST (L<C<import>|/import
9216 LIST> called with no arguments) and an explicit empty LIST C<()>
9217 (L<C<import>|/import LIST> not called). Note that there is no comma
9220 Because this is a wide-open interface, pragmas (compiler directives)
9221 are also implemented this way. Some of the currently implemented
9227 use sigtrap qw(SEGV BUS);
9228 use strict qw(subs vars refs);
9229 use subs qw(afunc blurfl);
9230 use warnings qw(all);
9231 use sort qw(stable _quicksort _mergesort);
9233 Some of these pseudo-modules import semantics into the current
9234 block scope (like L<C<strict>|strict> or L<C<integer>|integer>, unlike
9235 ordinary modules, which import symbols into the current package (which
9236 are effective through the end of the file).
9238 Because L<C<use>|/use Module VERSION LIST> takes effect at compile time,
9239 it doesn't respect the ordinary flow control of the code being compiled.
9240 In particular, putting a L<C<use>|/use Module VERSION LIST> inside the
9241 false branch of a conditional doesn't prevent it
9242 from being processed. If a module or pragma only needs to be loaded
9243 conditionally, this can be done using the L<if> pragma:
9245 use if $] < 5.008, "utf8";
9246 use if WANT_WARNINGS, warnings => qw(all);
9248 There's a corresponding L<C<no>|/no MODULE VERSION LIST> declaration
9249 that unimports meanings imported by L<C<use>|/use Module VERSION LIST>,
9250 i.e., it calls C<< Module->unimport(LIST) >> instead of
9251 L<C<import>|/import LIST>. It behaves just as L<C<import>|/import LIST>
9252 does with VERSION, an omitted or empty LIST,
9253 or no unimport method being found.
9259 Care should be taken when using the C<no VERSION> form of L<C<no>|/no
9260 MODULE VERSION LIST>. It is
9261 I<only> meant to be used to assert that the running Perl is of a earlier
9262 version than its argument and I<not> to undo the feature-enabling side effects
9265 See L<perlmodlib> for a list of standard modules and pragmas. See L<perlrun>
9266 for the C<-M> and C<-m> command-line options to Perl that give
9267 L<C<use>|/use Module VERSION LIST> functionality from the command-line.
9272 =for Pod::Functions set a file's last access and modify times
9274 Changes the access and modification times on each file of a list of
9275 files. The first two elements of the list must be the NUMERIC access
9276 and modification times, in that order. Returns the number of files
9277 successfully changed. The inode change time of each file is set
9278 to the current time. For example, this code has the same effect as the
9279 Unix L<touch(1)> command when the files I<already exist> and belong to
9280 the user running the program:
9283 my $atime = my $mtime = time;
9284 utime $atime, $mtime, @ARGV;
9286 Since Perl 5.8.0, if the first two elements of the list are
9287 L<C<undef>|/undef EXPR>,
9288 the L<utime(2)> syscall from your C library is called with a null second
9289 argument. On most systems, this will set the file's access and
9290 modification times to the current time (i.e., equivalent to the example
9291 above) and will work even on files you don't own provided you have write
9294 for my $file (@ARGV) {
9295 utime(undef, undef, $file)
9296 || warn "Couldn't touch $file: $!";
9299 Under NFS this will use the time of the NFS server, not the time of
9300 the local machine. If there is a time synchronization problem, the
9301 NFS server and local machine will have different times. The Unix
9302 L<touch(1)> command will in fact normally use this form instead of the
9303 one shown in the first example.
9305 Passing only one of the first two elements as L<C<undef>|/undef EXPR> is
9306 equivalent to passing a 0 and will not have the effect described when
9307 both are L<C<undef>|/undef EXPR>. This also triggers an
9308 uninitialized warning.
9310 On systems that support L<futimes(2)>, you may pass filehandles among the
9311 files. On systems that don't support L<futimes(2)>, passing filehandles raises
9312 an exception. Filehandles must be passed as globs or glob references to be
9313 recognized; barewords are considered filenames.
9315 Portability issues: L<perlport/utime>.
9322 =for Pod::Functions return a list of the values in a hash
9324 In list context, returns a list consisting of all the values of the named
9325 hash. In Perl 5.12 or later only, will also return a list of the values of
9326 an array; prior to that release, attempting to use an array argument will
9327 produce a syntax error. In scalar context, returns the number of values.
9329 Hash entries are returned in an apparently random order. The actual random
9330 order is specific to a given hash; the exact same series of operations
9331 on two hashes may result in a different order for each hash. Any insertion
9332 into the hash may change the order, as will any deletion, with the exception
9333 that the most recent key returned by L<C<each>|/each HASH> or
9334 L<C<keys>|/keys HASH> may be deleted without changing the order. So
9335 long as a given hash is unmodified you may rely on
9336 L<C<keys>|/keys HASH>, L<C<values>|/values HASH> and
9337 L<C<each>|/each HASH> to repeatedly return the same order
9338 as each other. See L<perlsec/"Algorithmic Complexity Attacks"> for
9339 details on why hash order is randomized. Aside from the guarantees
9340 provided here the exact details of Perl's hash algorithm and the hash
9341 traversal order are subject to change in any release of Perl. Tied hashes
9342 may behave differently to Perl's hashes with respect to changes in order on
9343 insertion and deletion of items.
9345 As a side effect, calling L<C<values>|/values HASH> resets the HASH or
9346 ARRAY's internal iterator, see L<C<each>|/each HASH>. (In particular,
9347 calling L<C<values>|/values HASH> in void context resets the iterator
9348 with no other overhead. Apart from resetting the iterator,
9349 C<values @array> in list context is the same as plain C<@array>.
9350 (We recommend that you use void context C<keys @array> for this, but
9351 reasoned that taking C<values @array> out would require more
9352 documentation than leaving it in.)
9354 Note that the values are not copied, which means modifying them will
9355 modify the contents of the hash:
9357 for (values %hash) { s/foo/bar/g } # modifies %hash values
9358 for (@hash{keys %hash}) { s/foo/bar/g } # same
9360 Starting with Perl 5.14, an experimental feature allowed
9361 L<C<values>|/values HASH> to take a
9362 scalar expression. This experiment has been deemed unsuccessful, and was
9363 removed as of Perl 5.24.
9365 To avoid confusing would-be users of your code who are running earlier
9366 versions of Perl with mysterious syntax errors, put this sort of thing at
9367 the top of your file to signal that your code will work I<only> on Perls of
9370 use 5.012; # so keys/values/each work on arrays
9372 See also L<C<keys>|/keys HASH>, L<C<each>|/each HASH>, and
9373 L<C<sort>|/sort SUBNAME LIST>.
9375 =item vec EXPR,OFFSET,BITS
9376 X<vec> X<bit> X<bit vector>
9378 =for Pod::Functions test or set particular bits in a string
9380 Treats the string in EXPR as a bit vector made up of elements of
9381 width BITS and returns the value of the element specified by OFFSET
9382 as an unsigned integer. BITS therefore specifies the number of bits
9383 that are reserved for each element in the bit vector. This must
9384 be a power of two from 1 to 32 (or 64, if your platform supports
9387 If BITS is 8, "elements" coincide with bytes of the input string.
9389 If BITS is 16 or more, bytes of the input string are grouped into chunks
9390 of size BITS/8, and each group is converted to a number as with
9391 L<C<pack>|/pack TEMPLATE,LIST>/L<C<unpack>|/unpack TEMPLATE,EXPR> with
9392 big-endian formats C<n>/C<N> (and analogously for BITS==64). See
9393 L<C<pack>|/pack TEMPLATE,LIST> for details.
9395 If bits is 4 or less, the string is broken into bytes, then the bits
9396 of each byte are broken into 8/BITS groups. Bits of a byte are
9397 numbered in a little-endian-ish way, as in C<0x01>, C<0x02>,
9398 C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example,
9399 breaking the single input byte C<chr(0x36)> into two groups gives a list
9400 C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>.
9402 L<C<vec>|/vec EXPR,OFFSET,BITS> may also be assigned to, in which case
9403 parentheses are needed
9404 to give the expression the correct precedence as in
9406 vec($image, $max_x * $x + $y, 8) = 3;
9408 If the selected element is outside the string, the value 0 is returned.
9409 If an element off the end of the string is written to, Perl will first
9410 extend the string with sufficiently many zero bytes. It is an error
9411 to try to write off the beginning of the string (i.e., negative OFFSET).
9413 If the string happens to be encoded as UTF-8 internally (and thus has
9414 the UTF8 flag set), this is ignored by L<C<vec>|/vec EXPR,OFFSET,BITS>,
9415 and it operates on the
9416 internal byte string, not the conceptual character string, even if you
9417 only have characters with values less than 256.
9419 Strings created with L<C<vec>|/vec EXPR,OFFSET,BITS> can also be
9420 manipulated with the logical
9421 operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit
9422 vector operation is desired when both operands are strings.
9423 See L<perlop/"Bitwise String Operators">.
9425 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
9426 The comments show the string after each step. Note that this code works
9427 in the same way on big-endian or little-endian machines.
9430 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
9432 # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
9433 print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P')
9435 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
9436 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
9437 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
9438 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
9439 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
9440 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
9442 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
9443 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
9444 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
9447 To transform a bit vector into a string or list of 0's and 1's, use these:
9449 my $bits = unpack("b*", $vector);
9450 my @bits = split(//, unpack("b*", $vector));
9452 If you know the exact length in bits, it can be used in place of the C<*>.
9454 Here is an example to illustrate how the bits actually fall in place:
9460 unpack("V",$_) 01234567890123456789012345678901
9461 ------------------------------------------------------------------
9466 for ($shift=0; $shift < $width; ++$shift) {
9467 for ($off=0; $off < 32/$width; ++$off) {
9468 $str = pack("B*", "0"x32);
9469 $bits = (1<<$shift);
9470 vec($str, $off, $width) = $bits;
9471 $res = unpack("b*",$str);
9472 $val = unpack("V", $str);
9479 vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
9480 $off, $width, $bits, $val, $res
9484 Regardless of the machine architecture on which it runs, the
9485 example above should print the following table:
9488 unpack("V",$_) 01234567890123456789012345678901
9489 ------------------------------------------------------------------
9490 vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000
9491 vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000
9492 vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000
9493 vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000
9494 vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000
9495 vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000
9496 vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000
9497 vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000
9498 vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000
9499 vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000
9500 vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000
9501 vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000
9502 vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000
9503 vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000
9504 vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000
9505 vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000
9506 vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000
9507 vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000
9508 vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000
9509 vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000
9510 vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000
9511 vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000
9512 vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000
9513 vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000
9514 vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000
9515 vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000
9516 vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000
9517 vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000
9518 vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000
9519 vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100
9520 vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010
9521 vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001
9522 vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000
9523 vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000
9524 vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000
9525 vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000
9526 vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000
9527 vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000
9528 vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000
9529 vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000
9530 vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000
9531 vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000
9532 vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000
9533 vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000
9534 vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000
9535 vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000
9536 vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000
9537 vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010
9538 vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000
9539 vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000
9540 vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000
9541 vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000
9542 vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000
9543 vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000
9544 vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000
9545 vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000
9546 vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000
9547 vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000
9548 vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000
9549 vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000
9550 vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000
9551 vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000
9552 vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100
9553 vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001
9554 vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000
9555 vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000
9556 vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000
9557 vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000
9558 vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000
9559 vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000
9560 vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000
9561 vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000
9562 vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000
9563 vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000
9564 vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000
9565 vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000
9566 vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000
9567 vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000
9568 vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000
9569 vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100
9570 vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000
9571 vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000
9572 vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000
9573 vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000
9574 vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000
9575 vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000
9576 vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000
9577 vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010
9578 vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000
9579 vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000
9580 vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000
9581 vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000
9582 vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000
9583 vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000
9584 vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000
9585 vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001
9586 vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000
9587 vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000
9588 vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000
9589 vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000
9590 vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000
9591 vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000
9592 vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000
9593 vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000
9594 vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000
9595 vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000
9596 vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000
9597 vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000
9598 vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000
9599 vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000
9600 vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000
9601 vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000
9602 vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000
9603 vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000
9604 vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000
9605 vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000
9606 vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000
9607 vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000
9608 vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000
9609 vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100
9610 vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000
9611 vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000
9612 vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000
9613 vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010
9614 vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000
9615 vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000
9616 vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000
9617 vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
9622 =for Pod::Functions wait for any child process to die
9624 Behaves like L<wait(2)> on your system: it waits for a child
9625 process to terminate and returns the pid of the deceased process, or
9626 C<-1> if there are no child processes. The status is returned in
9627 L<C<$?>|perlvar/$?> and
9628 L<C<${^CHILD_ERROR_NATIVE}>|perlvar/${^CHILD_ERROR_NATIVE}>.
9629 Note that a return value of C<-1> could mean that child processes are
9630 being automatically reaped, as described in L<perlipc>.
9632 If you use L<C<wait>|/wait> in your handler for
9633 L<C<$SIG{CHLD}>|perlvar/%SIG>, it may accidentally wait for the child
9634 created by L<C<qx>|/qxE<sol>STRINGE<sol>> or L<C<system>|/system LIST>.
9635 See L<perlipc> for details.
9637 Portability issues: L<perlport/wait>.
9639 =item waitpid PID,FLAGS
9642 =for Pod::Functions wait for a particular child process to die
9644 Waits for a particular child process to terminate and returns the pid of
9645 the deceased process, or C<-1> if there is no such child process. A
9646 non-blocking wait (with L<WNOHANG|POSIX/C<WNOHANG>> in FLAGS) can return 0 if
9647 there are child processes matching PID but none have terminated yet.
9648 The status is returned in L<C<$?>|perlvar/$?> and
9649 L<C<${^CHILD_ERROR_NATIVE}>|perlvar/${^CHILD_ERROR_NATIVE}>.
9651 A PID of C<0> indicates to wait for any child process whose process group ID is
9652 equal to that of the current process. A PID of less than C<-1> indicates to
9653 wait for any child process whose process group ID is equal to -PID. A PID of
9654 C<-1> indicates to wait for any child process.
9658 use POSIX ":sys_wait_h";
9662 $kid = waitpid(-1, WNOHANG);
9667 1 while waitpid(-1, WNOHANG) > 0;
9669 then you can do a non-blocking wait for all pending zombie processes (see
9671 Non-blocking wait is available on machines supporting either the
9672 L<waitpid(2)> or L<wait4(2)> syscalls. However, waiting for a particular
9673 pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the
9674 system call by remembering the status values of processes that have
9675 exited but have not been harvested by the Perl script yet.)
9677 Note that on some systems, a return value of C<-1> could mean that child
9678 processes are being automatically reaped. See L<perlipc> for details,
9679 and for other examples.
9681 Portability issues: L<perlport/waitpid>.
9684 X<wantarray> X<context>
9686 =for Pod::Functions get void vs scalar vs list context of current subroutine call
9688 Returns true if the context of the currently executing subroutine or
9689 L<C<eval>|/eval EXPR> is looking for a list value. Returns false if the
9691 looking for a scalar. Returns the undefined value if the context is
9692 looking for no value (void context).
9694 return unless defined wantarray; # don't bother doing more
9695 my @a = complex_calculation();
9696 return wantarray ? @a : "@a";
9698 L<C<wantarray>|/wantarray>'s result is unspecified in the top level of a file,
9699 in a C<BEGIN>, C<UNITCHECK>, C<CHECK>, C<INIT> or C<END> block, or
9700 in a C<DESTROY> method.
9702 This function should have been named wantlist() instead.
9705 X<warn> X<warning> X<STDERR>
9707 =for Pod::Functions print debugging info
9709 Prints the value of LIST to STDERR. If the last element of LIST does
9710 not end in a newline, it appends the same file/line number text as
9711 L<C<die>|/die LIST> does.
9713 If the output is empty and L<C<$@>|perlvar/$@> already contains a value
9714 (typically from a previous eval) that value is used after appending
9715 C<"\t...caught"> to L<C<$@>|perlvar/$@>. This is useful for staying
9716 almost, but not entirely similar to L<C<die>|/die LIST>.
9718 If L<C<$@>|perlvar/$@> is empty, then the string
9719 C<"Warning: Something's wrong"> is used.
9721 No message is printed if there is a L<C<$SIG{__WARN__}>|perlvar/%SIG>
9723 installed. It is the handler's responsibility to deal with the message
9724 as it sees fit (like, for instance, converting it into a
9725 L<C<die>|/die LIST>). Most
9726 handlers must therefore arrange to actually display the
9727 warnings that they are not prepared to deal with, by calling
9728 L<C<warn>|/warn LIST>
9729 again in the handler. Note that this is quite safe and will not
9730 produce an endless loop, since C<__WARN__> hooks are not called from
9733 You will find this behavior is slightly different from that of
9734 L<C<$SIG{__DIE__}>|perlvar/%SIG> handlers (which don't suppress the
9735 error text, but can instead call L<C<die>|/die LIST> again to change
9738 Using a C<__WARN__> handler provides a powerful way to silence all
9739 warnings (even the so-called mandatory ones). An example:
9741 # wipe out *all* compile-time warnings
9742 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
9744 my $foo = 20; # no warning about duplicate my $foo,
9745 # but hey, you asked for it!
9746 # no compile-time or run-time warnings before here
9749 # run-time warnings enabled after here
9750 warn "\$foo is alive and $foo!"; # does show up
9752 See L<perlvar> for details on setting L<C<%SIG>|perlvar/%SIG> entries
9754 examples. See the L<Carp> module for other kinds of warnings using its
9755 C<carp> and C<cluck> functions.
9757 =item write FILEHANDLE
9764 =for Pod::Functions print a picture record
9766 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
9767 using the format associated with that file. By default the format for
9768 a file is the one having the same name as the filehandle, but the
9769 format for the current output channel (see the
9770 L<C<select>|/select FILEHANDLE> function) may be set explicitly by
9771 assigning the name of the format to the L<C<$~>|perlvar/$~> variable.
9773 Top of form processing is handled automatically: if there is insufficient
9774 room on the current page for the formatted record, the page is advanced by
9775 writing a form feed and a special top-of-page
9776 format is used to format the new
9777 page header before the record is written. By default, the top-of-page
9778 format is the name of the filehandle with C<_TOP> appended, or C<top>
9779 in the current package if the former does not exist. This would be a
9780 problem with autovivified filehandles, but it may be dynamically set to the
9781 format of your choice by assigning the name to the L<C<$^>|perlvar/$^>
9782 variable while that filehandle is selected. The number of lines
9783 remaining on the current page is in variable L<C<$->|perlvar/$->, which
9784 can be set to C<0> to force a new page.
9786 If FILEHANDLE is unspecified, output goes to the current default output
9787 channel, which starts out as STDOUT but may be changed by the
9788 L<C<select>|/select FILEHANDLE> operator. If the FILEHANDLE is an EXPR,
9790 is evaluated and the resulting string is used to look up the name of
9791 the FILEHANDLE at run time. For more on formats, see L<perlform>.
9793 Note that write is I<not> the opposite of
9794 L<C<read>|/read FILEHANDLE,SCALAR,LENGTH,OFFSET>. Unfortunately.
9798 =for Pod::Functions transliterate a string
9800 The transliteration operator. Same as
9801 L<C<trE<sol>E<sol>E<sol>>|/trE<sol>E<sol>E<sol>>. See
9802 L<perlop/"Quote-Like Operators">.
9806 =head2 Non-function Keywords by Cross-reference
9816 These keywords are documented in L<perldata/"Special Literals">.
9834 These compile phase keywords are documented in L<perlmod/"BEGIN, UNITCHECK, CHECK, INIT and END">.
9844 This method keyword is documented in L<perlobj/"Destructors">.
9876 These operators are documented in L<perlop>.
9886 This keyword is documented in L<perlsub/"Autoloading">.
9910 These flow-control keywords are documented in L<perlsyn/"Compound Statements">.
9914 The "else if" keyword is spelled C<elsif> in Perl. There's no C<elif>
9915 or C<else if> either. It does parse C<elseif>, but only to warn you
9918 See the documentation for flow-control keywords in L<perlsyn/"Compound
9931 These flow-control keywords related to the experimental switch feature are
9932 documented in L<perlsyn/"Switch Statements">.