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
1545 used to report whether memory for that aggregate had ever been
1546 allocated. This behavior may disappear in future versions of Perl.
1547 You should instead use a simple test for size:
1549 if (@an_array) { print "has array elements\n" }
1550 if (%a_hash) { print "has hash members\n" }
1552 When used on a hash element, it tells you whether the value is defined,
1553 not whether the key exists in the hash. Use L<C<exists>|/exists EXPR>
1554 for the latter purpose.
1558 print if defined $switch{D};
1559 print "$val\n" while defined($val = pop(@ary));
1560 die "Can't readlink $sym: $!"
1561 unless defined($value = readlink $sym);
1562 sub foo { defined &$bar ? $bar->(@_) : die "No bar"; }
1563 $debugging = 0 unless defined $debugging;
1565 Note: Many folks tend to overuse L<C<defined>|/defined EXPR> and are
1566 then surprised to discover that the number C<0> and C<""> (the
1567 zero-length string) are, in fact, defined values. For example, if you
1572 The pattern match succeeds and C<$1> is defined, although it
1573 matched "nothing". It didn't really fail to match anything. Rather, it
1574 matched something that happened to be zero characters long. This is all
1575 very above-board and honest. When a function returns an undefined value,
1576 it's an admission that it couldn't give you an honest answer. So you
1577 should use L<C<defined>|/defined EXPR> only when questioning the
1578 integrity of what you're trying to do. At other times, a simple
1579 comparison to C<0> or C<""> is what you want.
1581 See also L<C<undef>|/undef EXPR>, L<C<exists>|/exists EXPR>,
1582 L<C<ref>|/ref EXPR>.
1587 =for Pod::Functions deletes a value from a hash
1589 Given an expression that specifies an element or slice of a hash,
1590 L<C<delete>|/delete EXPR> deletes the specified elements from that hash
1591 so that L<C<exists>|/exists EXPR> on that element no longer returns
1592 true. Setting a hash element to the undefined value does not remove its
1593 key, but deleting it does; see L<C<exists>|/exists EXPR>.
1595 In list context, returns the value or values deleted, or the last such
1596 element in scalar context. The return list's length always matches that of
1597 the argument list: deleting non-existent elements returns the undefined value
1598 in their corresponding positions.
1600 L<C<delete>|/delete EXPR> may also be used on arrays and array slices,
1601 but its behavior is less straightforward. Although
1602 L<C<exists>|/exists EXPR> will return false for deleted entries,
1603 deleting array elements never changes indices of existing values; use
1604 L<C<shift>|/shift ARRAY> or L<C<splice>|/splice
1605 ARRAY,OFFSET,LENGTH,LIST> for that. However, if any deleted elements
1606 fall at the end of an array, the array's size shrinks to the position of
1607 the highest element that still tests true for L<C<exists>|/exists EXPR>,
1608 or to 0 if none do. In other words, an array won't have trailing
1609 nonexistent elements after a delete.
1611 B<WARNING:> Calling L<C<delete>|/delete EXPR> on array values is
1612 strongly discouraged. The
1613 notion of deleting or checking the existence of Perl array elements is not
1614 conceptually coherent, and can lead to surprising behavior.
1616 Deleting from L<C<%ENV>|perlvar/%ENV> modifies the environment.
1617 Deleting from a hash tied to a DBM file deletes the entry from the DBM
1618 file. Deleting from a L<C<tied>|/tied VARIABLE> hash or array may not
1619 necessarily return anything; it depends on the implementation of the
1620 L<C<tied>|/tied VARIABLE> package's DELETE method, which may do whatever
1623 The C<delete local EXPR> construct localizes the deletion to the current
1624 block at run time. Until the block exits, elements locally deleted
1625 temporarily no longer exist. See L<perlsub/"Localized deletion of elements
1626 of composite types">.
1628 my %hash = (foo => 11, bar => 22, baz => 33);
1629 my $scalar = delete $hash{foo}; # $scalar is 11
1630 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
1631 my @array = delete @hash{qw(foo baz)}; # @array is (undef,33)
1633 The following (inefficiently) deletes all the values of %HASH and @ARRAY:
1635 foreach my $key (keys %HASH) {
1639 foreach my $index (0 .. $#ARRAY) {
1640 delete $ARRAY[$index];
1645 delete @HASH{keys %HASH};
1647 delete @ARRAY[0 .. $#ARRAY];
1649 But both are slower than assigning the empty list
1650 or undefining %HASH or @ARRAY, which is the customary
1651 way to empty out an aggregate:
1653 %HASH = (); # completely empty %HASH
1654 undef %HASH; # forget %HASH ever existed
1656 @ARRAY = (); # completely empty @ARRAY
1657 undef @ARRAY; # forget @ARRAY ever existed
1659 The EXPR can be arbitrarily complicated provided its
1660 final operation is an element or slice of an aggregate:
1662 delete $ref->[$x][$y]{$key};
1663 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
1665 delete $ref->[$x][$y][$index];
1666 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1669 X<die> X<throw> X<exception> X<raise> X<$@> X<abort>
1671 =for Pod::Functions raise an exception or bail out
1673 L<C<die>|/die LIST> raises an exception. Inside an
1674 L<C<eval>|/eval EXPR> the error message is stuffed into
1675 L<C<$@>|perlvar/$@> and the L<C<eval>|/eval EXPR> is terminated with the
1676 undefined value. If the exception is outside of all enclosing
1677 L<C<eval>|/eval EXPR>s, then the uncaught exception prints LIST to
1678 C<STDERR> and exits with a non-zero value. If you need to exit the
1679 process with a specific exit code, see L<C<exit>|/exit EXPR>.
1681 Equivalent examples:
1683 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
1684 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
1686 If the last element of LIST does not end in a newline, the current
1687 script line number and input line number (if any) are also printed,
1688 and a newline is supplied. Note that the "input line number" (also
1689 known as "chunk") is subject to whatever notion of "line" happens to
1690 be currently in effect, and is also available as the special variable
1691 L<C<$.>|perlvar/$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1693 Hint: sometimes appending C<", stopped"> to your message will cause it
1694 to make better sense when the string C<"at foo line 123"> is appended.
1695 Suppose you are running script "canasta".
1697 die "/etc/games is no good";
1698 die "/etc/games is no good, stopped";
1700 produce, respectively
1702 /etc/games is no good at canasta line 123.
1703 /etc/games is no good, stopped at canasta line 123.
1705 If the output is empty and L<C<$@>|perlvar/$@> already contains a value
1706 (typically from a previous L<C<eval>|/eval EXPR>) that value is reused after
1707 appending C<"\t...propagated">. This is useful for propagating exceptions:
1710 die unless $@ =~ /Expected exception/;
1712 If the output is empty and L<C<$@>|perlvar/$@> contains an object
1713 reference that has a C<PROPAGATE> method, that method will be called
1714 with additional file and line number parameters. The return value
1715 replaces the value in L<C<$@>|perlvar/$@>; i.e., as if
1716 C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >> were called.
1718 If L<C<$@>|perlvar/$@> is empty, then the string C<"Died"> is used.
1720 If an uncaught exception results in interpreter exit, the exit code is
1721 determined from the values of L<C<$!>|perlvar/$!> and
1722 L<C<$?>|perlvar/$?> with this pseudocode:
1724 exit $! if $!; # errno
1725 exit $? >> 8 if $? >> 8; # child exit status
1726 exit 255; # last resort
1728 As with L<C<exit>|/exit EXPR>, L<C<$?>|perlvar/$?> is set prior to
1729 unwinding the call stack; any C<DESTROY> or C<END> handlers can then
1730 alter this value, and thus Perl's exit code.
1732 The intent is to squeeze as much possible information about the likely cause
1733 into the limited space of the system exit code. However, as
1734 L<C<$!>|perlvar/$!> is the value of C's C<errno>, which can be set by
1735 any system call, this means that the value of the exit code used by
1736 L<C<die>|/die LIST> can be non-predictable, so should not be relied
1737 upon, other than to be non-zero.
1739 You can also call L<C<die>|/die LIST> with a reference argument, and if
1740 this is trapped within an L<C<eval>|/eval EXPR>, L<C<$@>|perlvar/$@>
1741 contains that reference. This permits more elaborate exception handling
1742 using objects that maintain arbitrary state about the exception. Such a
1743 scheme is sometimes preferable to matching particular string values of
1744 L<C<$@>|perlvar/$@> with regular expressions. Because
1745 L<C<$@>|perlvar/$@> is a global variable and L<C<eval>|/eval EXPR> may
1746 be used within object implementations, be careful that analyzing the
1747 error object doesn't replace the reference in the global variable. It's
1748 easiest to make a local copy of the reference before any manipulations.
1751 use Scalar::Util "blessed";
1753 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1754 if (my $ev_err = $@) {
1755 if (blessed($ev_err)
1756 && $ev_err->isa("Some::Module::Exception")) {
1757 # handle Some::Module::Exception
1760 # handle all other possible exceptions
1764 Because Perl stringifies uncaught exception messages before display,
1765 you'll probably want to overload stringification operations on
1766 exception objects. See L<overload> for details about that.
1768 You can arrange for a callback to be run just before the
1769 L<C<die>|/die LIST> does its deed, by setting the
1770 L<C<$SIG{__DIE__}>|perlvar/%SIG> hook. The associated handler is called
1771 with the error text and can change the error message, if it sees fit, by
1772 calling L<C<die>|/die LIST> again. See L<perlvar/%SIG> for details on
1773 setting L<C<%SIG>|perlvar/%SIG> entries, and L<C<eval>|/eval EXPR> for some
1774 examples. Although this feature was to be run only right before your
1775 program was to exit, this is not currently so: the
1776 L<C<$SIG{__DIE__}>|perlvar/%SIG> hook is currently called even inside
1777 L<C<eval>|/eval EXPR>ed blocks/strings! If one wants the hook to do
1778 nothing in such situations, put
1782 as the first line of the handler (see L<perlvar/$^S>). Because
1783 this promotes strange action at a distance, this counterintuitive
1784 behavior may be fixed in a future release.
1786 See also L<C<exit>|/exit EXPR>, L<C<warn>|/warn LIST>, and the L<Carp>
1792 =for Pod::Functions turn a BLOCK into a TERM
1794 Not really a function. Returns the value of the last command in the
1795 sequence of commands indicated by BLOCK. When modified by the C<while> or
1796 C<until> loop modifier, executes the BLOCK once before testing the loop
1797 condition. (On other statements the loop modifiers test the conditional
1800 C<do BLOCK> does I<not> count as a loop, so the loop control statements
1801 L<C<next>|/next LABEL>, L<C<last>|/last LABEL>, or
1802 L<C<redo>|/redo LABEL> cannot be used to leave or restart the block.
1803 See L<perlsyn> for alternative strategies.
1808 Uses the value of EXPR as a filename and executes the contents of the
1809 file as a Perl script.
1817 except that it's more concise, runs no external processes, keeps track of
1818 the current filename for error messages, searches the
1819 L<C<@INC>|perlvar/@INC> directories, and updates L<C<%INC>|perlvar/%INC>
1820 if the file is found. See L<perlvar/@INC> and L<perlvar/%INC> for these
1821 variables. It also differs in that code evaluated with C<do FILE>
1822 cannot see lexicals in the enclosing scope; C<eval STRING> does. It's
1823 the same, however, in that it does reparse the file every time you call
1824 it, so you probably don't want to do this inside a loop.
1826 If L<C<do>|/do EXPR> can read the file but cannot compile it, it
1827 returns L<C<undef>|/undef EXPR> and sets an error message in
1828 L<C<$@>|perlvar/$@>. If L<C<do>|/do EXPR> cannot read the file, it
1829 returns undef and sets L<C<$!>|perlvar/$!> to the error. Always check
1830 L<C<$@>|perlvar/$@> first, as compilation could fail in a way that also
1831 sets L<C<$!>|perlvar/$!>. If the file is successfully compiled,
1832 L<C<do>|/do EXPR> returns the value of the last expression evaluated.
1834 Inclusion of library modules is better done with the
1835 L<C<use>|/use Module VERSION LIST> and L<C<require>|/require VERSION>
1836 operators, which also do automatic error checking and raise an exception
1837 if there's a problem.
1839 You might like to use L<C<do>|/do EXPR> to read in a program
1840 configuration file. Manual error checking can be done this way:
1842 # read in config files: system first, then user
1843 for $file ("/share/prog/defaults.rc",
1844 "$ENV{HOME}/.someprogrc")
1846 unless ($return = do $file) {
1847 warn "couldn't parse $file: $@" if $@;
1848 warn "couldn't do $file: $!" unless defined $return;
1849 warn "couldn't run $file" unless $return;
1854 X<dump> X<core> X<undump>
1860 =for Pod::Functions create an immediate core dump
1862 This function causes an immediate core dump. See also the B<-u>
1863 command-line switch in L<perlrun>, which does the same thing.
1864 Primarily this is so that you can use the B<undump> program (not
1865 supplied) to turn your core dump into an executable binary after
1866 having initialized all your variables at the beginning of the
1867 program. When the new binary is executed it will begin by executing
1868 a C<goto LABEL> (with all the restrictions that L<C<goto>|/goto LABEL>
1870 Think of it as a goto with an intervening core dump and reincarnation.
1871 If C<LABEL> is omitted, restarts the program from the top. The
1872 C<dump EXPR> form, available starting in Perl 5.18.0, allows a name to be
1873 computed at run time, being otherwise identical to C<dump LABEL>.
1875 B<WARNING>: Any files opened at the time of the dump will I<not>
1876 be open any more when the program is reincarnated, with possible
1877 resulting confusion by Perl.
1879 This function is now largely obsolete, mostly because it's very hard to
1880 convert a core file into an executable. That's why you should now invoke
1881 it as C<CORE::dump()> if you don't want to be warned against a possible
1884 Unlike most named operators, this has the same precedence as assignment.
1885 It is also exempt from the looks-like-a-function rule, so
1886 C<dump ("foo")."bar"> will cause "bar" to be part of the argument to
1887 L<C<dump>|/dump LABEL>.
1889 Portability issues: L<perlport/dump>.
1892 X<each> X<hash, iterator>
1897 =for Pod::Functions retrieve the next key/value pair from a hash
1899 When called on a hash in list context, returns a 2-element list
1900 consisting of the key and value for the next element of a hash. In Perl
1901 5.12 and later only, it will also return the index and value for the next
1902 element of an array so that you can iterate over it; older Perls consider
1903 this a syntax error. When called in scalar context, returns only the key
1904 (not the value) in a hash, or the index in an array.
1906 Hash entries are returned in an apparently random order. The actual random
1907 order is specific to a given hash; the exact same series of operations
1908 on two hashes may result in a different order for each hash. Any insertion
1909 into the hash may change the order, as will any deletion, with the exception
1910 that the most recent key returned by L<C<each>|/each HASH> or
1911 L<C<keys>|/keys HASH> may be deleted without changing the order. So
1912 long as a given hash is unmodified you may rely on
1913 L<C<keys>|/keys HASH>, L<C<values>|/values HASH> and
1914 L<C<each>|/each HASH> to repeatedly return the same order
1915 as each other. See L<perlsec/"Algorithmic Complexity Attacks"> for
1916 details on why hash order is randomized. Aside from the guarantees
1917 provided here the exact details of Perl's hash algorithm and the hash
1918 traversal order are subject to change in any release of Perl.
1920 After L<C<each>|/each HASH> has returned all entries from the hash or
1921 array, the next call to L<C<each>|/each HASH> returns the empty list in
1922 list context and L<C<undef>|/undef EXPR> in scalar context; the next
1923 call following I<that> one restarts iteration. Each hash or array has
1924 its own internal iterator, accessed by L<C<each>|/each HASH>,
1925 L<C<keys>|/keys HASH>, and L<C<values>|/values HASH>. The iterator is
1926 implicitly reset when L<C<each>|/each HASH> has reached the end as just
1927 described; it can be explicitly reset by calling L<C<keys>|/keys HASH>
1928 or L<C<values>|/values HASH> on the hash or array. If you add or delete
1929 a hash's elements while iterating over it, the effect on the iterator is
1930 unspecified; for example, entries may be skipped or duplicated--so don't
1931 do that. Exception: It is always safe to delete the item most recently
1932 returned by L<C<each>|/each HASH>, so the following code works properly:
1934 while (my ($key, $value) = each %hash) {
1936 delete $hash{$key}; # This is safe
1939 Tied hashes may have a different ordering behaviour to perl's hash
1942 This prints out your environment like the L<printenv(1)> program,
1943 but in a different order:
1945 while (my ($key,$value) = each %ENV) {
1946 print "$key=$value\n";
1949 Starting with Perl 5.14, an experimental feature allowed
1950 L<C<each>|/each HASH> to take a scalar expression. This experiment has
1951 been deemed unsuccessful, and was removed as of Perl 5.24.
1953 As of Perl 5.18 you can use a bare L<C<each>|/each HASH> in a C<while>
1954 loop, which will set L<C<$_>|perlvar/$_> on every iteration.
1957 print "$_=$ENV{$_}\n";
1960 To avoid confusing would-be users of your code who are running earlier
1961 versions of Perl with mysterious syntax errors, put this sort of thing at
1962 the top of your file to signal that your code will work I<only> on Perls of
1965 use 5.012; # so keys/values/each work on arrays
1966 use 5.018; # so each assigns to $_ in a lone while test
1968 See also L<C<keys>|/keys HASH>, L<C<values>|/values HASH>, and
1969 L<C<sort>|/sort SUBNAME LIST>.
1971 =item eof FILEHANDLE
1980 =for Pod::Functions test a filehandle for its end
1982 Returns 1 if the next read on FILEHANDLE will return end of file I<or> if
1983 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1984 gives the real filehandle. (Note that this function actually
1985 reads a character and then C<ungetc>s it, so isn't useful in an
1986 interactive context.) Do not read from a terminal file (or call
1987 C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
1988 as terminals may lose the end-of-file condition if you do.
1990 An L<C<eof>|/eof FILEHANDLE> without an argument uses the last file
1991 read. Using L<C<eof()>|/eof FILEHANDLE> with empty parentheses is
1992 different. It refers to the pseudo file formed from the files listed on
1993 the command line and accessed via the C<< <> >> operator. Since
1994 C<< <> >> isn't explicitly opened, as a normal filehandle is, an
1995 L<C<eof()>|/eof FILEHANDLE> before C<< <> >> has been used will cause
1996 L<C<@ARGV>|perlvar/@ARGV> to be examined to determine if input is
1997 available. Similarly, an L<C<eof()>|/eof FILEHANDLE> after C<< <> >>
1998 has returned end-of-file will assume you are processing another
1999 L<C<@ARGV>|perlvar/@ARGV> list, and if you haven't set
2000 L<C<@ARGV>|perlvar/@ARGV>, will read input from C<STDIN>; see
2001 L<perlop/"I/O Operators">.
2003 In a C<< while (<>) >> loop, L<C<eof>|/eof FILEHANDLE> or C<eof(ARGV)>
2004 can be used to detect the end of each file, whereas
2005 L<C<eof()>|/eof FILEHANDLE> will detect the end of the very last file
2008 # reset line numbering on each input file
2010 next if /^\s*#/; # skip comments
2013 close ARGV if eof; # Not eof()!
2016 # insert dashes just before last line of last file
2018 if (eof()) { # check for end of last file
2019 print "--------------\n";
2022 last if eof(); # needed if we're reading from a terminal
2025 Practical hint: you almost never need to use L<C<eof>|/eof FILEHANDLE>
2026 in Perl, because the input operators typically return L<C<undef>|/undef
2027 EXPR> when they run out of data or encounter an error.
2030 X<eval> X<try> X<catch> X<evaluate> X<parse> X<execute>
2031 X<error, handling> X<exception, handling>
2037 =for Pod::Functions catch exceptions or compile and run code
2039 In the first form, often referred to as a "string eval", the return
2040 value of EXPR is parsed and executed as if it
2041 were a little Perl program. The value of the expression (which is itself
2042 determined within scalar context) is first parsed, and if there were no
2043 errors, executed as a block within the lexical context of the current Perl
2044 program. This means, that in particular, any outer lexical variables are
2045 visible to it, and any package variable settings or subroutine and format
2046 definitions remain afterwards.
2048 Note that the value is parsed every time the L<C<eval>|/eval EXPR>
2049 executes. If EXPR is omitted, evaluates L<C<$_>|perlvar/$_>. This form
2050 is typically used to delay parsing and subsequent execution of the text
2051 of EXPR until run time.
2054 L<C<"unicode_eval"> feature|feature/The 'unicode_eval' and 'evalbytes' features>
2055 is enabled (which is the default under a
2056 C<use 5.16> or higher declaration), EXPR or L<C<$_>|perlvar/$_> is
2057 treated as a string of characters, so L<C<use utf8>|utf8> declarations
2058 have no effect, and source filters are forbidden. In the absence of the
2059 L<C<"unicode_eval"> feature|feature/The 'unicode_eval' and 'evalbytes' features>,
2060 will sometimes be treated as characters and sometimes as bytes,
2061 depending on the internal encoding, and source filters activated within
2062 the L<C<eval>|/eval EXPR> exhibit the erratic, but historical, behaviour
2063 of affecting some outer file scope that is still compiling. See also
2064 the L<C<evalbytes>|/evalbytes EXPR> operator, which always treats its
2065 input as a byte stream and works properly with source filters, and the
2068 Problems can arise if the string expands a scalar containing a floating
2069 point number. That scalar can expand to letters, such as C<"NaN"> or
2070 C<"Infinity">; or, within the scope of a L<C<use locale>|locale>, the
2071 decimal point character may be something other than a dot (such as a
2072 comma). None of these are likely to parse as you are likely expecting.
2074 In the second form, the code within the BLOCK is parsed only once--at the
2075 same time the code surrounding the L<C<eval>|/eval EXPR> itself was
2076 parsed--and executed
2077 within the context of the current Perl program. This form is typically
2078 used to trap exceptions more efficiently than the first (see below), while
2079 also providing the benefit of checking the code within BLOCK at compile
2082 The final semicolon, if any, may be omitted from the value of EXPR or within
2085 In both forms, the value returned is the value of the last expression
2086 evaluated inside the mini-program; a return statement may be also used, just
2087 as with subroutines. The expression providing the return value is evaluated
2088 in void, scalar, or list context, depending on the context of the
2089 L<C<eval>|/eval EXPR> itself. See L<C<wantarray>|/wantarray> for more
2090 on how the evaluation context can be determined.
2092 If there is a syntax error or runtime error, or a L<C<die>|/die LIST>
2093 statement is executed, L<C<eval>|/eval EXPR> returns
2094 L<C<undef>|/undef EXPR> in scalar context or an empty list in list
2095 context, and L<C<$@>|perlvar/$@> is set to the error message. (Prior to
2096 5.16, a bug caused L<C<undef>|/undef EXPR> to be returned in list
2097 context for syntax errors, but not for runtime errors.) If there was no
2098 error, L<C<$@>|perlvar/$@> is set to the empty string. A control flow
2099 operator like L<C<last>|/last LABEL> or L<C<goto>|/goto LABEL> can
2100 bypass the setting of L<C<$@>|perlvar/$@>. Beware that using
2101 L<C<eval>|/eval EXPR> neither silences Perl from printing warnings to
2102 STDERR, nor does it stuff the text of warning messages into
2103 L<C<$@>|perlvar/$@>. To do either of those, you have to use the
2104 L<C<$SIG{__WARN__}>|perlvar/%SIG> facility, or turn off warnings inside
2105 the BLOCK or EXPR using S<C<no warnings 'all'>>. See
2106 L<C<warn>|/warn LIST>, L<perlvar>, and L<warnings>.
2108 Note that, because L<C<eval>|/eval EXPR> traps otherwise-fatal errors,
2109 it is useful for determining whether a particular feature (such as
2110 L<C<socket>|/socket SOCKET,DOMAIN,TYPE,PROTOCOL> or
2111 L<C<symlink>|/symlink OLDFILE,NEWFILE>) is implemented. It is also
2112 Perl's exception-trapping mechanism, where the L<C<die>|/die LIST>
2113 operator is used to raise exceptions.
2115 If you want to trap errors when loading an XS module, some problems with
2116 the binary interface (such as Perl version skew) may be fatal even with
2117 L<C<eval>|/eval EXPR> unless C<$ENV{PERL_DL_NONLAZY}> is set. See
2120 If the code to be executed doesn't vary, you may use the eval-BLOCK
2121 form to trap run-time errors without incurring the penalty of
2122 recompiling each time. The error, if any, is still returned in
2123 L<C<$@>|perlvar/$@>.
2126 # make divide-by-zero nonfatal
2127 eval { $answer = $a / $b; }; warn $@ if $@;
2129 # same thing, but less efficient
2130 eval '$answer = $a / $b'; warn $@ if $@;
2132 # a compile-time error
2133 eval { $answer = }; # WRONG
2136 eval '$answer ='; # sets $@
2138 Using the C<eval {}> form as an exception trap in libraries does have some
2139 issues. Due to the current arguably broken state of C<__DIE__> hooks, you
2140 may wish not to trigger any C<__DIE__> hooks that user code may have installed.
2141 You can use the C<local $SIG{__DIE__}> construct for this purpose,
2142 as this example shows:
2144 # a private exception trap for divide-by-zero
2145 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
2148 This is especially significant, given that C<__DIE__> hooks can call
2149 L<C<die>|/die LIST> again, which has the effect of changing their error
2152 # __DIE__ hooks may modify error messages
2154 local $SIG{'__DIE__'} =
2155 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
2156 eval { die "foo lives here" };
2157 print $@ if $@; # prints "bar lives here"
2160 Because this promotes action at a distance, this counterintuitive behavior
2161 may be fixed in a future release.
2163 With an L<C<eval>|/eval EXPR>, you should be especially careful to
2164 remember what's being looked at when:
2170 eval { $x }; # CASE 4
2172 eval "\$$x++"; # CASE 5
2175 Cases 1 and 2 above behave identically: they run the code contained in
2176 the variable $x. (Although case 2 has misleading double quotes making
2177 the reader wonder what else might be happening (nothing is).) Cases 3
2178 and 4 likewise behave in the same way: they run the code C<'$x'>, which
2179 does nothing but return the value of $x. (Case 4 is preferred for
2180 purely visual reasons, but it also has the advantage of compiling at
2181 compile-time instead of at run-time.) Case 5 is a place where
2182 normally you I<would> like to use double quotes, except that in this
2183 particular situation, you can just use symbolic references instead, as
2186 Before Perl 5.14, the assignment to L<C<$@>|perlvar/$@> occurred before
2188 of localized variables, which means that for your code to run on older
2189 versions, a temporary is required if you want to mask some but not all
2192 # alter $@ on nefarious repugnancy only
2196 local $@; # protect existing $@
2197 eval { test_repugnancy() };
2198 # $@ =~ /nefarious/ and die $@; # Perl 5.14 and higher only
2199 $@ =~ /nefarious/ and $e = $@;
2201 die $e if defined $e
2204 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
2205 L<C<next>|/next LABEL>, L<C<last>|/last LABEL>, or
2206 L<C<redo>|/redo LABEL> cannot be used to leave or restart the block.
2208 An C<eval ''> executed within a subroutine defined
2209 in the C<DB> package doesn't see the usual
2210 surrounding lexical scope, but rather the scope of the first non-DB piece
2211 of code that called it. You don't normally need to worry about this unless
2212 you are writing a Perl debugger.
2214 =item evalbytes EXPR
2219 =for Pod::Functions +evalbytes similar to string eval, but intend to parse a bytestream
2221 This function is like L<C<eval>|/eval EXPR> with a string argument,
2222 except it always parses its argument, or L<C<$_>|perlvar/$_> if EXPR is
2223 omitted, as a string of bytes. A string containing characters whose
2224 ordinal value exceeds 255 results in an error. Source filters activated
2225 within the evaluated code apply to the code itself.
2227 L<C<evalbytes>|/evalbytes EXPR> is available only if the
2228 L<C<"evalbytes"> feature|feature/The 'unicode_eval' and 'evalbytes' features>
2229 is enabled or if it is prefixed with C<CORE::>. The
2230 L<C<"evalbytes"> feature|feature/The 'unicode_eval' and 'evalbytes' features>
2231 is enabled automatically with a C<use v5.16> (or higher) declaration in
2237 =item exec PROGRAM LIST
2239 =for Pod::Functions abandon this program to run another
2241 The L<C<exec>|/exec LIST> function executes a system command I<and never
2242 returns>; use L<C<system>|/system LIST> instead of L<C<exec>|/exec LIST>
2243 if you want it to return. It fails and
2244 returns false only if the command does not exist I<and> it is executed
2245 directly instead of via your system's command shell (see below).
2247 Since it's a common mistake to use L<C<exec>|/exec LIST> instead of
2248 L<C<system>|/system LIST>, Perl warns you if L<C<exec>|/exec LIST> is
2249 called in void context and if there is a following statement that isn't
2250 L<C<die>|/die LIST>, L<C<warn>|/warn LIST>, or L<C<exit>|/exit EXPR> (if
2251 L<warnings> are enabled--but you always do that, right?). If you
2252 I<really> want to follow an L<C<exec>|/exec LIST> with some other
2253 statement, you can use one of these styles to avoid the warning:
2255 exec ('foo') or print STDERR "couldn't exec foo: $!";
2256 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
2258 If there is more than one argument in LIST, this calls L<execvp(3)> with the
2259 arguments in LIST. If there is only one element in LIST, the argument is
2260 checked for shell metacharacters, and if there are any, the entire
2261 argument is passed to the system's command shell for parsing (this is
2262 C</bin/sh -c> on Unix platforms, but varies on other platforms). If
2263 there are no shell metacharacters in the argument, it is split into words
2264 and passed directly to C<execvp>, which is more efficient. Examples:
2266 exec '/bin/echo', 'Your arguments are: ', @ARGV;
2267 exec "sort $outfile | uniq";
2269 If you don't really want to execute the first argument, but want to lie
2270 to the program you are executing about its own name, you can specify
2271 the program you actually want to run as an "indirect object" (without a
2272 comma) in front of the LIST, as in C<exec PROGRAM LIST>. (This always
2273 forces interpretation of the LIST as a multivalued list, even if there
2274 is only a single scalar in the list.) Example:
2276 my $shell = '/bin/csh';
2277 exec $shell '-sh'; # pretend it's a login shell
2281 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
2283 When the arguments get executed via the system shell, results are
2284 subject to its quirks and capabilities. See L<perlop/"`STRING`">
2287 Using an indirect object with L<C<exec>|/exec LIST> or
2288 L<C<system>|/system LIST> is also more secure. This usage (which also
2289 works fine with L<C<system>|/system LIST>) forces
2290 interpretation of the arguments as a multivalued list, even if the
2291 list had just one argument. That way you're safe from the shell
2292 expanding wildcards or splitting up words with whitespace in them.
2294 my @args = ( "echo surprise" );
2296 exec @args; # subject to shell escapes
2298 exec { $args[0] } @args; # safe even with one-arg list
2300 The first version, the one without the indirect object, ran the I<echo>
2301 program, passing it C<"surprise"> an argument. The second version didn't;
2302 it tried to run a program named I<"echo surprise">, didn't find it, and set
2303 L<C<$?>|perlvar/$?> to a non-zero value indicating failure.
2305 On Windows, only the C<exec PROGRAM LIST> indirect object syntax will
2306 reliably avoid using the shell; C<exec LIST>, even with more than one
2307 element, will fall back to the shell if the first spawn fails.
2309 Perl attempts to flush all files opened for output before the exec,
2310 but this may not be supported on some platforms (see L<perlport>).
2311 To be safe, you may need to set L<C<$E<verbar>>|perlvar/$E<verbar>>
2312 (C<$AUTOFLUSH> in L<English>) or call the C<autoflush> method of
2313 L<C<IO::Handle>|IO::Handle/METHODS> on any open handles to avoid lost
2316 Note that L<C<exec>|/exec LIST> will not call your C<END> blocks, nor
2317 will it invoke C<DESTROY> methods on your objects.
2319 Portability issues: L<perlport/exec>.
2322 X<exists> X<autovivification>
2324 =for Pod::Functions test whether a hash key is present
2326 Given an expression that specifies an element of a hash, returns true if the
2327 specified element in the hash has ever been initialized, even if the
2328 corresponding value is undefined.
2330 print "Exists\n" if exists $hash{$key};
2331 print "Defined\n" if defined $hash{$key};
2332 print "True\n" if $hash{$key};
2334 exists may also be called on array elements, but its behavior is much less
2335 obvious and is strongly tied to the use of L<C<delete>|/delete EXPR> on
2338 B<WARNING:> Calling L<C<exists>|/exists EXPR> on array values is
2339 strongly discouraged. The
2340 notion of deleting or checking the existence of Perl array elements is not
2341 conceptually coherent, and can lead to surprising behavior.
2343 print "Exists\n" if exists $array[$index];
2344 print "Defined\n" if defined $array[$index];
2345 print "True\n" if $array[$index];
2347 A hash or array element can be true only if it's defined and defined only if
2348 it exists, but the reverse doesn't necessarily hold true.
2350 Given an expression that specifies the name of a subroutine,
2351 returns true if the specified subroutine has ever been declared, even
2352 if it is undefined. Mentioning a subroutine name for exists or defined
2353 does not count as declaring it. Note that a subroutine that does not
2354 exist may still be callable: its package may have an C<AUTOLOAD>
2355 method that makes it spring into existence the first time that it is
2356 called; see L<perlsub>.
2358 print "Exists\n" if exists &subroutine;
2359 print "Defined\n" if defined &subroutine;
2361 Note that the EXPR can be arbitrarily complicated as long as the final
2362 operation is a hash or array key lookup or subroutine name:
2364 if (exists $ref->{A}->{B}->{$key}) { }
2365 if (exists $hash{A}{B}{$key}) { }
2367 if (exists $ref->{A}->{B}->[$ix]) { }
2368 if (exists $hash{A}{B}[$ix]) { }
2370 if (exists &{$ref->{A}{B}{$key}}) { }
2372 Although the most deeply nested array or hash element will not spring into
2373 existence just because its existence was tested, any intervening ones will.
2374 Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
2375 into existence due to the existence test for the C<$key> element above.
2376 This happens anywhere the arrow operator is used, including even here:
2379 if (exists $ref->{"Some key"}) { }
2380 print $ref; # prints HASH(0x80d3d5c)
2382 This surprising autovivification in what does not at first--or even
2383 second--glance appear to be an lvalue context may be fixed in a future
2386 Use of a subroutine call, rather than a subroutine name, as an argument
2387 to L<C<exists>|/exists EXPR> is an error.
2390 exists &sub(); # Error
2393 X<exit> X<terminate> X<abort>
2397 =for Pod::Functions terminate this program
2399 Evaluates EXPR and exits immediately with that value. Example:
2402 exit 0 if $ans =~ /^[Xx]/;
2404 See also L<C<die>|/die LIST>. If EXPR is omitted, exits with C<0>
2406 universally recognized values for EXPR are C<0> for success and C<1>
2407 for error; other values are subject to interpretation depending on the
2408 environment in which the Perl program is running. For example, exiting
2409 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
2410 the mailer to return the item undelivered, but that's not true everywhere.
2412 Don't use L<C<exit>|/exit EXPR> to abort a subroutine if there's any
2413 chance that someone might want to trap whatever error happened. Use
2414 L<C<die>|/die LIST> instead, which can be trapped by an
2415 L<C<eval>|/eval EXPR>.
2417 The L<C<exit>|/exit EXPR> function does not always exit immediately. It
2418 calls any defined C<END> routines first, but these C<END> routines may
2419 not themselves abort the exit. Likewise any object destructors that
2420 need to be called are called before the real exit. C<END> routines and
2421 destructors can change the exit status by modifying L<C<$?>|perlvar/$?>.
2422 If this is a problem, you can call
2423 L<C<POSIX::_exit($status)>|POSIX/C<_exit>> to avoid C<END> and destructor
2424 processing. See L<perlmod> for details.
2426 Portability issues: L<perlport/exit>.
2429 X<exp> X<exponential> X<antilog> X<antilogarithm> X<e>
2433 =for Pod::Functions raise I<e> to a power
2435 Returns I<e> (the natural logarithm base) to the power of EXPR.
2436 If EXPR is omitted, gives C<exp($_)>.
2439 X<fc> X<foldcase> X<casefold> X<fold-case> X<case-fold>
2443 =for Pod::Functions +fc return casefolded version of a string
2445 Returns the casefolded version of EXPR. This is the internal function
2446 implementing the C<\F> escape in double-quoted strings.
2448 Casefolding is the process of mapping strings to a form where case
2449 differences are erased; comparing two strings in their casefolded
2450 form is effectively a way of asking if two strings are equal,
2453 Roughly, if you ever found yourself writing this
2455 lc($this) eq lc($that) # Wrong!
2457 uc($this) eq uc($that) # Also wrong!
2459 $this =~ /^\Q$that\E\z/i # Right!
2463 fc($this) eq fc($that)
2465 And get the correct results.
2467 Perl only implements the full form of casefolding, but you can access
2468 the simple folds using L<Unicode::UCD/B<casefold()>> and
2469 L<Unicode::UCD/B<prop_invmap()>>.
2470 For further information on casefolding, refer to
2471 the Unicode Standard, specifically sections 3.13 C<Default Case Operations>,
2472 4.2 C<Case-Normative>, and 5.18 C<Case Mappings>,
2473 available at L<http://www.unicode.org/versions/latest/>, as well as the
2474 Case Charts available at L<http://www.unicode.org/charts/case/>.
2476 If EXPR is omitted, uses L<C<$_>|perlvar/$_>.
2478 This function behaves the same way under various pragmas, such as within
2479 L<S<C<"use feature 'unicode_strings">>|feature/The 'unicode_strings' feature>,
2480 as L<C<lc>|/lc EXPR> does, with the single exception of
2481 L<C<fc>|/fc EXPR> of I<LATIN CAPITAL LETTER SHARP S> (U+1E9E) within the
2482 scope of L<S<C<use locale>>|locale>. The foldcase of this character
2483 would normally be C<"ss">, but as explained in the L<C<lc>|/lc EXPR>
2485 changes that cross the 255/256 boundary are problematic under locales,
2486 and are hence prohibited. Therefore, this function under locale returns
2487 instead the string C<"\x{17F}\x{17F}">, which is the I<LATIN SMALL LETTER
2488 LONG S>. Since that character itself folds to C<"s">, the string of two
2489 of them together should be equivalent to a single U+1E9E when foldcased.
2491 While the Unicode Standard defines two additional forms of casefolding,
2492 one for Turkic languages and one that never maps one character into multiple
2493 characters, these are not provided by the Perl core. However, the CPAN module
2494 L<C<Unicode::Casing>|Unicode::Casing> may be used to provide an implementation.
2496 L<C<fc>|/fc EXPR> is available only if the
2497 L<C<"fc"> feature|feature/The 'fc' feature> is enabled or if it is
2498 prefixed with C<CORE::>. The
2499 L<C<"fc"> feature|feature/The 'fc' feature> is enabled automatically
2500 with a C<use v5.16> (or higher) declaration in the current scope.
2502 =item fcntl FILEHANDLE,FUNCTION,SCALAR
2505 =for Pod::Functions file control system call
2507 Implements the L<fcntl(2)> function. You'll probably have to say
2511 first to get the correct constant definitions. Argument processing and
2512 value returned work just like L<C<ioctl>|/ioctl
2513 FILEHANDLE,FUNCTION,SCALAR> below. For example:
2516 my $flags = fcntl($filehandle, F_GETFL, 0)
2517 or die "Can't fcntl F_GETFL: $!";
2519 You don't have to check for L<C<defined>|/defined EXPR> on the return
2520 from L<C<fcntl>|/fcntl FILEHANDLE,FUNCTION,SCALAR>. Like
2521 L<C<ioctl>|/ioctl FILEHANDLE,FUNCTION,SCALAR>, it maps a C<0> return
2522 from the system call into C<"0 but true"> in Perl. This string is true
2523 in boolean context and C<0> in numeric context. It is also exempt from
2525 L<C<Argument "..." isn't numeric>|perldiag/Argument "%s" isn't numeric%s>
2526 L<warnings> on improper numeric conversions.
2528 Note that L<C<fcntl>|/fcntl FILEHANDLE,FUNCTION,SCALAR> raises an
2529 exception if used on a machine that doesn't implement L<fcntl(2)>. See
2530 the L<Fcntl> module or your L<fcntl(2)> manpage to learn what functions
2531 are available on your system.
2533 Here's an example of setting a filehandle named C<$REMOTE> to be
2534 non-blocking at the system level. You'll have to negotiate
2535 L<C<$E<verbar>>|perlvar/$E<verbar>> on your own, though.
2537 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
2539 my $flags = fcntl($REMOTE, F_GETFL, 0)
2540 or die "Can't get flags for the socket: $!\n";
2542 fcntl($REMOTE, F_SETFL, $flags | O_NONBLOCK)
2543 or die "Can't set flags for the socket: $!\n";
2545 Portability issues: L<perlport/fcntl>.
2550 =for Pod::Functions the name of the current source file
2552 A special token that returns the name of the file in which it occurs.
2554 =item fileno FILEHANDLE
2557 =for Pod::Functions return file descriptor from filehandle
2559 Returns the file descriptor for a filehandle, or undefined if the
2560 filehandle is not open. If there is no real file descriptor at the OS
2561 level, as can happen with filehandles connected to memory objects via
2562 L<C<open>|/open FILEHANDLE,EXPR> with a reference for the third
2563 argument, -1 is returned.
2565 This is mainly useful for constructing bitmaps for
2566 L<C<select>|/select RBITS,WBITS,EBITS,TIMEOUT> and low-level POSIX
2567 tty-handling operations.
2568 If FILEHANDLE is an expression, the value is taken as an indirect
2569 filehandle, generally its name.
2571 You can use this to find out whether two handles refer to the
2572 same underlying descriptor:
2574 if (fileno($this) != -1 && fileno($this) == fileno($that)) {
2575 print "\$this and \$that are dups\n";
2576 } elsif (fileno($this) != -1 && fileno($that) != -1) {
2577 print "\$this and \$that have different " .
2578 "underlying file descriptors\n";
2580 print "At least one of \$this and \$that does " .
2581 "not have a real file descriptor\n";
2584 The behavior of L<C<fileno>|/fileno FILEHANDLE> on a directory handle
2585 depends on the operating system. On a system with L<dirfd(3)> or
2586 similar, L<C<fileno>|/fileno FILEHANDLE> on a directory
2587 handle returns the underlying file descriptor associated with the
2588 handle; on systems with no such support, it returns the undefined value,
2589 and sets L<C<$!>|perlvar/$!> (errno).
2591 =item flock FILEHANDLE,OPERATION
2592 X<flock> X<lock> X<locking>
2594 =for Pod::Functions lock an entire file with an advisory lock
2596 Calls L<flock(2)>, or an emulation of it, on FILEHANDLE. Returns true
2597 for success, false on failure. Produces a fatal error if used on a
2598 machine that doesn't implement L<flock(2)>, L<fcntl(2)> locking, or
2599 L<lockf(3)>. L<C<flock>|/flock FILEHANDLE,OPERATION> is Perl's portable
2600 file-locking interface, although it locks entire files only, not
2603 Two potentially non-obvious but traditional L<C<flock>|/flock
2604 FILEHANDLE,OPERATION> semantics are
2605 that it waits indefinitely until the lock is granted, and that its locks
2606 are B<merely advisory>. Such discretionary locks are more flexible, but
2607 offer fewer guarantees. This means that programs that do not also use
2608 L<C<flock>|/flock FILEHANDLE,OPERATION> may modify files locked with
2609 L<C<flock>|/flock FILEHANDLE,OPERATION>. See L<perlport>,
2610 your port's specific documentation, and your system-specific local manpages
2611 for details. It's best to assume traditional behavior if you're writing
2612 portable programs. (But if you're not, you should as always feel perfectly
2613 free to write for your own system's idiosyncrasies (sometimes called
2614 "features"). Slavish adherence to portability concerns shouldn't get
2615 in the way of your getting your job done.)
2617 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
2618 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
2619 you can use the symbolic names if you import them from the L<Fcntl> module,
2620 either individually, or as a group using the C<:flock> tag. LOCK_SH
2621 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
2622 releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
2623 LOCK_SH or LOCK_EX, then L<C<flock>|/flock FILEHANDLE,OPERATION> returns
2624 immediately rather than blocking waiting for the lock; check the return
2625 status to see if you got it.
2627 To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
2628 before locking or unlocking it.
2630 Note that the emulation built with L<lockf(3)> doesn't provide shared
2631 locks, and it requires that FILEHANDLE be open with write intent. These
2632 are the semantics that L<lockf(3)> implements. Most if not all systems
2633 implement L<lockf(3)> in terms of L<fcntl(2)> locking, though, so the
2634 differing semantics shouldn't bite too many people.
2636 Note that the L<fcntl(2)> emulation of L<flock(3)> requires that FILEHANDLE
2637 be open with read intent to use LOCK_SH and requires that it be open
2638 with write intent to use LOCK_EX.
2640 Note also that some versions of L<C<flock>|/flock FILEHANDLE,OPERATION>
2641 cannot lock things over the network; you would need to use the more
2642 system-specific L<C<fcntl>|/fcntl FILEHANDLE,FUNCTION,SCALAR> for
2643 that. If you like you can force Perl to ignore your system's L<flock(2)>
2644 function, and so provide its own L<fcntl(2)>-based emulation, by passing
2645 the switch C<-Ud_flock> to the F<Configure> program when you configure
2646 and build a new Perl.
2648 Here's a mailbox appender for BSD systems.
2650 # import LOCK_* and SEEK_END constants
2651 use Fcntl qw(:flock SEEK_END);
2655 flock($fh, LOCK_EX) or die "Cannot lock mailbox - $!\n";
2657 # and, in case someone appended while we were waiting...
2658 seek($fh, 0, SEEK_END) or die "Cannot seek - $!\n";
2663 flock($fh, LOCK_UN) or die "Cannot unlock mailbox - $!\n";
2666 open(my $mbox, ">>", "/usr/spool/mail/$ENV{'USER'}")
2667 or die "Can't open mailbox: $!";
2670 print $mbox $msg,"\n\n";
2673 On systems that support a real L<flock(2)>, locks are inherited across
2674 L<C<fork>|/fork> calls, whereas those that must resort to the more
2675 capricious L<fcntl(2)> function lose their locks, making it seriously
2676 harder to write servers.
2678 See also L<DB_File> for other L<C<flock>|/flock FILEHANDLE,OPERATION>
2681 Portability issues: L<perlport/flock>.
2684 X<fork> X<child> X<parent>
2686 =for Pod::Functions create a new process just like this one
2688 Does a L<fork(2)> system call to create a new process running the
2689 same program at the same point. It returns the child pid to the
2690 parent process, C<0> to the child process, or L<C<undef>|/undef EXPR> if
2692 unsuccessful. File descriptors (and sometimes locks on those descriptors)
2693 are shared, while everything else is copied. On most systems supporting
2694 L<fork(2)>, great care has gone into making it extremely efficient (for
2695 example, using copy-on-write technology on data pages), making it the
2696 dominant paradigm for multitasking over the last few decades.
2698 Perl attempts to flush all files opened for output before forking the
2699 child process, but this may not be supported on some platforms (see
2700 L<perlport>). To be safe, you may need to set
2701 L<C<$E<verbar>>|perlvar/$E<verbar>> (C<$AUTOFLUSH> in L<English>) or
2702 call the C<autoflush> method of L<C<IO::Handle>|IO::Handle/METHODS> on
2703 any open handles to avoid duplicate output.
2705 If you L<C<fork>|/fork> without ever waiting on your children, you will
2706 accumulate zombies. On some systems, you can avoid this by setting
2707 L<C<$SIG{CHLD}>|perlvar/%SIG> to C<"IGNORE">. See also L<perlipc> for
2708 more examples of forking and reaping moribund children.
2710 Note that if your forked child inherits system file descriptors like
2711 STDIN and STDOUT that are actually connected by a pipe or socket, even
2712 if you exit, then the remote server (such as, say, a CGI script or a
2713 backgrounded job launched from a remote shell) won't think you're done.
2714 You should reopen those to F</dev/null> if it's any issue.
2716 On some platforms such as Windows, where the L<fork(2)> system call is
2717 not available, Perl can be built to emulate L<C<fork>|/fork> in the Perl
2718 interpreter. The emulation is designed, at the level of the Perl
2719 program, to be as compatible as possible with the "Unix" L<fork(2)>.
2720 However it has limitations that have to be considered in code intended
2721 to be portable. See L<perlfork> for more details.
2723 Portability issues: L<perlport/fork>.
2728 =for Pod::Functions declare a picture format with use by the write() function
2730 Declare a picture format for use by the L<C<write>|/write FILEHANDLE>
2731 function. For example:
2734 Test: @<<<<<<<< @||||| @>>>>>
2735 $str, $%, '$' . int($num)
2739 $num = $cost/$quantity;
2743 See L<perlform> for many details and examples.
2745 =item formline PICTURE,LIST
2748 =for Pod::Functions internal function used for formats
2750 This is an internal function used by L<C<format>|/format>s, though you
2751 may call it, too. It formats (see L<perlform>) a list of values
2752 according to the contents of PICTURE, placing the output into the format
2753 output accumulator, L<C<$^A>|perlvar/$^A> (or C<$ACCUMULATOR> in
2754 L<English>). Eventually, when a L<C<write>|/write FILEHANDLE> is done,
2755 the contents of L<C<$^A>|perlvar/$^A> are written to some filehandle.
2756 You could also read L<C<$^A>|perlvar/$^A> and then set
2757 L<C<$^A>|perlvar/$^A> back to C<"">. Note that a format typically does
2758 one L<C<formline>|/formline PICTURE,LIST> per line of form, but the
2759 L<C<formline>|/formline PICTURE,LIST> function itself doesn't care how
2760 many newlines are embedded in the PICTURE. This means that the C<~> and
2761 C<~~> tokens treat the entire PICTURE as a single line. You may
2762 therefore need to use multiple formlines to implement a single record
2763 format, just like the L<C<format>|/format> compiler.
2765 Be careful if you put double quotes around the picture, because an C<@>
2766 character may be taken to mean the beginning of an array name.
2767 L<C<formline>|/formline PICTURE,LIST> always returns true. See
2768 L<perlform> for other examples.
2770 If you are trying to use this instead of L<C<write>|/write FILEHANDLE>
2771 to capture the output, you may find it easier to open a filehandle to a
2772 scalar (C<< open my $fh, ">", \$output >>) and write to that instead.
2774 =item getc FILEHANDLE
2775 X<getc> X<getchar> X<character> X<file, read>
2779 =for Pod::Functions get the next character from the filehandle
2781 Returns the next character from the input file attached to FILEHANDLE,
2782 or the undefined value at end of file or if there was an error (in
2783 the latter case L<C<$!>|perlvar/$!> is set). If FILEHANDLE is omitted,
2785 STDIN. This is not particularly efficient. However, it cannot be
2786 used by itself to fetch single characters without waiting for the user
2787 to hit enter. For that, try something more like:
2790 system "stty cbreak </dev/tty >/dev/tty 2>&1";
2793 system "stty", '-icanon', 'eol', "\001";
2796 my $key = getc(STDIN);
2799 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
2802 system 'stty', 'icanon', 'eol', '^@'; # ASCII NUL
2806 Determination of whether C<$BSD_STYLE> should be set is left as an
2807 exercise to the reader.
2809 The L<C<POSIX::getattr>|POSIX/C<getattr>> function can do this more
2810 portably on systems purporting POSIX compliance. See also the
2811 L<C<Term::ReadKey>|Term::ReadKey> module on CPAN.
2814 X<getlogin> X<login>
2816 =for Pod::Functions return who logged in at this tty
2818 This implements the C library function of the same name, which on most
2819 systems returns the current login from F</etc/utmp>, if any. If it
2820 returns the empty string, use L<C<getpwuid>|/getpwuid UID>.
2822 my $login = getlogin || getpwuid($<) || "Kilroy";
2824 Do not consider L<C<getlogin>|/getlogin> for authentication: it is not
2825 as secure as L<C<getpwuid>|/getpwuid UID>.
2827 Portability issues: L<perlport/getlogin>.
2829 =item getpeername SOCKET
2830 X<getpeername> X<peer>
2832 =for Pod::Functions find the other end of a socket connection
2834 Returns the packed sockaddr address of the other end of the SOCKET
2838 my $hersockaddr = getpeername($sock);
2839 my ($port, $iaddr) = sockaddr_in($hersockaddr);
2840 my $herhostname = gethostbyaddr($iaddr, AF_INET);
2841 my $herstraddr = inet_ntoa($iaddr);
2846 =for Pod::Functions get process group
2848 Returns the current process group for the specified PID. Use
2849 a PID of C<0> to get the current process group for the
2850 current process. Will raise an exception if used on a machine that
2851 doesn't implement L<getpgrp(2)>. If PID is omitted, returns the process
2852 group of the current process. Note that the POSIX version of
2853 L<C<getpgrp>|/getpgrp PID> does not accept a PID argument, so only
2854 C<PID==0> is truly portable.
2856 Portability issues: L<perlport/getpgrp>.
2859 X<getppid> X<parent> X<pid>
2861 =for Pod::Functions get parent process ID
2863 Returns the process id of the parent process.
2865 Note for Linux users: Between v5.8.1 and v5.16.0 Perl would work
2866 around non-POSIX thread semantics the minority of Linux systems (and
2867 Debian GNU/kFreeBSD systems) that used LinuxThreads, this emulation
2868 has since been removed. See the documentation for L<$$|perlvar/$$> for
2871 Portability issues: L<perlport/getppid>.
2873 =item getpriority WHICH,WHO
2874 X<getpriority> X<priority> X<nice>
2876 =for Pod::Functions get current nice value
2878 Returns the current priority for a process, a process group, or a user.
2879 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
2880 machine that doesn't implement L<getpriority(2)>.
2882 Portability issues: L<perlport/getpriority>.
2885 X<getpwnam> X<getgrnam> X<gethostbyname> X<getnetbyname> X<getprotobyname>
2886 X<getpwuid> X<getgrgid> X<getservbyname> X<gethostbyaddr> X<getnetbyaddr>
2887 X<getprotobynumber> X<getservbyport> X<getpwent> X<getgrent> X<gethostent>
2888 X<getnetent> X<getprotoent> X<getservent> X<setpwent> X<setgrent> X<sethostent>
2889 X<setnetent> X<setprotoent> X<setservent> X<endpwent> X<endgrent> X<endhostent>
2890 X<endnetent> X<endprotoent> X<endservent>
2892 =for Pod::Functions get passwd record given user login name
2896 =for Pod::Functions get group record given group name
2898 =item gethostbyname NAME
2900 =for Pod::Functions get host record given name
2902 =item getnetbyname NAME
2904 =for Pod::Functions get networks record given name
2906 =item getprotobyname NAME
2908 =for Pod::Functions get protocol record given name
2912 =for Pod::Functions get passwd record given user ID
2916 =for Pod::Functions get group record given group user ID
2918 =item getservbyname NAME,PROTO
2920 =for Pod::Functions get services record given its name
2922 =item gethostbyaddr ADDR,ADDRTYPE
2924 =for Pod::Functions get host record given its address
2926 =item getnetbyaddr ADDR,ADDRTYPE
2928 =for Pod::Functions get network record given its address
2930 =item getprotobynumber NUMBER
2932 =for Pod::Functions get protocol record numeric protocol
2934 =item getservbyport PORT,PROTO
2936 =for Pod::Functions get services record given numeric port
2940 =for Pod::Functions get next passwd record
2944 =for Pod::Functions get next group record
2948 =for Pod::Functions get next hosts record
2952 =for Pod::Functions get next networks record
2956 =for Pod::Functions get next protocols record
2960 =for Pod::Functions get next services record
2964 =for Pod::Functions prepare passwd file for use
2968 =for Pod::Functions prepare group file for use
2970 =item sethostent STAYOPEN
2972 =for Pod::Functions prepare hosts file for use
2974 =item setnetent STAYOPEN
2976 =for Pod::Functions prepare networks file for use
2978 =item setprotoent STAYOPEN
2980 =for Pod::Functions prepare protocols file for use
2982 =item setservent STAYOPEN
2984 =for Pod::Functions prepare services file for use
2988 =for Pod::Functions be done using passwd file
2992 =for Pod::Functions be done using group file
2996 =for Pod::Functions be done using hosts file
3000 =for Pod::Functions be done using networks file
3004 =for Pod::Functions be done using protocols file
3008 =for Pod::Functions be done using services file
3010 These routines are the same as their counterparts in the
3011 system C library. In list context, the return values from the
3012 various get routines are as follows:
3015 my ( $name, $passwd, $gid, $members ) = getgr*
3016 my ( $name, $aliases, $addrtype, $net ) = getnet*
3017 my ( $name, $aliases, $port, $proto ) = getserv*
3018 my ( $name, $aliases, $proto ) = getproto*
3019 my ( $name, $aliases, $addrtype, $length, @addrs ) = gethost*
3020 my ( $name, $passwd, $uid, $gid, $quota,
3021 $comment, $gcos, $dir, $shell, $expire ) = getpw*
3024 (If the entry doesn't exist, the return value is a single meaningless true
3027 The exact meaning of the $gcos field varies but it usually contains
3028 the real name of the user (as opposed to the login name) and other
3029 information pertaining to the user. Beware, however, that in many
3030 system users are able to change this information and therefore it
3031 cannot be trusted and therefore the $gcos is tainted (see
3032 L<perlsec>). The $passwd and $shell, user's encrypted password and
3033 login shell, are also tainted, for the same reason.
3035 In scalar context, you get the name, unless the function was a
3036 lookup by name, in which case you get the other thing, whatever it is.
3037 (If the entry doesn't exist you get the undefined value.) For example:
3039 my $uid = getpwnam($name);
3040 my $name = getpwuid($num);
3041 my $name = getpwent();
3042 my $gid = getgrnam($name);
3043 my $name = getgrgid($num);
3044 my $name = getgrent();
3047 In I<getpw*()> the fields $quota, $comment, and $expire are special
3048 in that they are unsupported on many systems. If the
3049 $quota is unsupported, it is an empty scalar. If it is supported, it
3050 usually encodes the disk quota. If the $comment field is unsupported,
3051 it is an empty scalar. If it is supported it usually encodes some
3052 administrative comment about the user. In some systems the $quota
3053 field may be $change or $age, fields that have to do with password
3054 aging. In some systems the $comment field may be $class. The $expire
3055 field, if present, encodes the expiration period of the account or the
3056 password. For the availability and the exact meaning of these fields
3057 in your system, please consult L<getpwnam(3)> and your system's
3058 F<pwd.h> file. You can also find out from within Perl what your
3059 $quota and $comment fields mean and whether you have the $expire field
3060 by using the L<C<Config>|Config> module and the values C<d_pwquota>, C<d_pwage>,
3061 C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
3062 files are supported only if your vendor has implemented them in the
3063 intuitive fashion that calling the regular C library routines gets the
3064 shadow versions if you're running under privilege or if there exists
3065 the L<shadow(3)> functions as found in System V (this includes Solaris
3066 and Linux). Those systems that implement a proprietary shadow password
3067 facility are unlikely to be supported.
3069 The $members value returned by I<getgr*()> is a space-separated list of
3070 the login names of the members of the group.
3072 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
3073 C, it will be returned to you via L<C<$?>|perlvar/$?> if the function
3075 C<@addrs> value returned by a successful call is a list of raw
3076 addresses returned by the corresponding library call. In the
3077 Internet domain, each address is four bytes long; you can unpack it
3078 by saying something like:
3080 my ($w,$x,$y,$z) = unpack('W4',$addr[0]);
3082 The Socket library makes this slightly easier:
3085 my $iaddr = inet_aton("127.1"); # or whatever address
3086 my $name = gethostbyaddr($iaddr, AF_INET);
3088 # or going the other way
3089 my $straddr = inet_ntoa($iaddr);
3091 In the opposite way, to resolve a hostname to the IP address
3095 my $packed_ip = gethostbyname("www.perl.org");
3097 if (defined $packed_ip) {
3098 $ip_address = inet_ntoa($packed_ip);
3101 Make sure L<C<gethostbyname>|/gethostbyname NAME> is called in SCALAR
3102 context and that its return value is checked for definedness.
3104 The L<C<getprotobynumber>|/getprotobynumber NUMBER> function, even
3105 though it only takes one argument, has the precedence of a list
3106 operator, so beware:
3108 getprotobynumber $number eq 'icmp' # WRONG
3109 getprotobynumber($number eq 'icmp') # actually means this
3110 getprotobynumber($number) eq 'icmp' # better this way
3112 If you get tired of remembering which element of the return list
3113 contains which return value, by-name interfaces are provided in standard
3114 modules: L<C<File::stat>|File::stat>, L<C<Net::hostent>|Net::hostent>,
3115 L<C<Net::netent>|Net::netent>, L<C<Net::protoent>|Net::protoent>,
3116 L<C<Net::servent>|Net::servent>, L<C<Time::gmtime>|Time::gmtime>,
3117 L<C<Time::localtime>|Time::localtime>, and
3118 L<C<User::grent>|User::grent>. These override the normal built-ins,
3119 supplying versions that return objects with the appropriate names for
3120 each field. For example:
3124 my $is_his = (stat($filename)->uid == pwent($whoever)->uid);
3126 Even though it looks as though they're the same method calls (uid),
3127 they aren't, because a C<File::stat> object is different from
3128 a C<User::pwent> object.
3130 Portability issues: L<perlport/getpwnam> to L<perlport/endservent>.
3132 =item getsockname SOCKET
3135 =for Pod::Functions retrieve the sockaddr for a given socket
3137 Returns the packed sockaddr address of this end of the SOCKET connection,
3138 in case you don't know the address because you have several different
3139 IPs that the connection might have come in on.
3142 my $mysockaddr = getsockname($sock);
3143 my ($port, $myaddr) = sockaddr_in($mysockaddr);
3144 printf "Connect to %s [%s]\n",
3145 scalar gethostbyaddr($myaddr, AF_INET),
3148 =item getsockopt SOCKET,LEVEL,OPTNAME
3151 =for Pod::Functions get socket options on a given socket
3153 Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
3154 Options may exist at multiple protocol levels depending on the socket
3155 type, but at least the uppermost socket level SOL_SOCKET (defined in the
3156 L<C<Socket>|Socket> module) will exist. To query options at another
3157 level the protocol number of the appropriate protocol controlling the
3158 option should be supplied. For example, to indicate that an option is
3159 to be interpreted by the TCP protocol, LEVEL should be set to the
3160 protocol number of TCP, which you can get using
3161 L<C<getprotobyname>|/getprotobyname NAME>.
3163 The function returns a packed string representing the requested socket
3164 option, or L<C<undef>|/undef EXPR> on error, with the reason for the
3165 error placed in L<C<$!>|perlvar/$!>. Just what is in the packed string
3166 depends on LEVEL and OPTNAME; consult L<getsockopt(2)> for details. A
3167 common case is that the option is an integer, in which case the result
3168 is a packed integer, which you can decode using
3169 L<C<unpack>|/unpack TEMPLATE,EXPR> with the C<i> (or C<I>) format.
3171 Here's an example to test whether Nagle's algorithm is enabled on a socket:
3173 use Socket qw(:all);
3175 defined(my $tcp = getprotobyname("tcp"))
3176 or die "Could not determine the protocol number for tcp";
3177 # my $tcp = IPPROTO_TCP; # Alternative
3178 my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
3179 or die "getsockopt TCP_NODELAY: $!";
3180 my $nodelay = unpack("I", $packed);
3181 print "Nagle's algorithm is turned ",
3182 $nodelay ? "off\n" : "on\n";
3184 Portability issues: L<perlport/getsockopt>.
3187 X<glob> X<wildcard> X<filename, expansion> X<expand>
3191 =for Pod::Functions expand filenames using wildcards
3193 In list context, returns a (possibly empty) list of filename expansions on
3194 the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
3195 scalar context, glob iterates through such filename expansions, returning
3196 undef when the list is exhausted. This is the internal function
3197 implementing the C<< <*.c> >> operator, but you can use it directly. If
3198 EXPR is omitted, L<C<$_>|perlvar/$_> is used. The C<< <*.c> >> operator
3199 is discussed in more detail in L<perlop/"I/O Operators">.
3201 Note that L<C<glob>|/glob EXPR> splits its arguments on whitespace and
3203 each segment as separate pattern. As such, C<glob("*.c *.h")>
3204 matches all files with a F<.c> or F<.h> extension. The expression
3205 C<glob(".* *")> matches all files in the current working directory.
3206 If you want to glob filenames that might contain whitespace, you'll
3207 have to use extra quotes around the spacey filename to protect it.
3208 For example, to glob filenames that have an C<e> followed by a space
3209 followed by an C<f>, use one of:
3211 my @spacies = <"*e f*">;
3212 my @spacies = glob '"*e f*"';
3213 my @spacies = glob q("*e f*");
3215 If you had to get a variable through, you could do this:
3217 my @spacies = glob "'*${var}e f*'";
3218 my @spacies = glob qq("*${var}e f*");
3220 If non-empty braces are the only wildcard characters used in the
3221 L<C<glob>|/glob EXPR>, no filenames are matched, but potentially many
3222 strings are returned. For example, this produces nine strings, one for
3223 each pairing of fruits and colors:
3225 my @many = glob "{apple,tomato,cherry}={green,yellow,red}";
3227 This operator is implemented using the standard C<File::Glob> extension.
3228 See L<File::Glob> for details, including
3229 L<C<bsd_glob>|File::Glob/C<bsd_glob>>, which does not treat whitespace
3230 as a pattern separator.
3232 Portability issues: L<perlport/glob>.
3235 X<gmtime> X<UTC> X<Greenwich>
3239 =for Pod::Functions convert UNIX time into record or string using Greenwich time
3241 Works just like L<C<localtime>|/localtime EXPR> but the returned values
3242 are localized for the standard Greenwich time zone.
3244 Note: When called in list context, $isdst, the last value
3245 returned by gmtime, is always C<0>. There is no
3246 Daylight Saving Time in GMT.
3248 Portability issues: L<perlport/gmtime>.
3251 X<goto> X<jump> X<jmp>
3257 =for Pod::Functions create spaghetti code
3259 The C<goto LABEL> form finds the statement labeled with LABEL and
3260 resumes execution there. It can't be used to get out of a block or
3261 subroutine given to L<C<sort>|/sort SUBNAME LIST>. It can be used to go
3262 almost anywhere else within the dynamic scope, including out of
3263 subroutines, but it's usually better to use some other construct such as
3264 L<C<last>|/last LABEL> or L<C<die>|/die LIST>. The author of Perl has
3265 never felt the need to use this form of L<C<goto>|/goto LABEL> (in Perl,
3266 that is; C is another matter). (The difference is that C does not offer
3267 named loops combined with loop control. Perl does, and this replaces
3268 most structured uses of L<C<goto>|/goto LABEL> in other languages.)
3270 The C<goto EXPR> form expects to evaluate C<EXPR> to a code reference or
3271 a label name. If it evaluates to a code reference, it will be handled
3272 like C<goto &NAME>, below. This is especially useful for implementing
3273 tail recursion via C<goto __SUB__>.
3275 If the expression evaluates to a label name, its scope will be resolved
3276 dynamically. This allows for computed L<C<goto>|/goto LABEL>s per
3277 FORTRAN, but isn't necessarily recommended if you're optimizing for
3280 goto ("FOO", "BAR", "GLARCH")[$i];
3282 As shown in this example, C<goto EXPR> is exempt from the "looks like a
3283 function" rule. A pair of parentheses following it does not (necessarily)
3284 delimit its argument. C<goto("NE")."XT"> is equivalent to C<goto NEXT>.
3285 Also, unlike most named operators, this has the same precedence as
3288 Use of C<goto LABEL> or C<goto EXPR> to jump into a construct is
3289 deprecated and will issue a warning. Even then, it may not be used to
3290 go into any construct that requires initialization, such as a
3291 subroutine or a C<foreach> loop. It also can't be used to go into a
3292 construct that is optimized away.
3294 The C<goto &NAME> form is quite different from the other forms of
3295 L<C<goto>|/goto LABEL>. In fact, it isn't a goto in the normal sense at
3296 all, and doesn't have the stigma associated with other gotos. Instead,
3297 it exits the current subroutine (losing any changes set by
3298 L<C<local>|/local EXPR>) and immediately calls in its place the named
3299 subroutine using the current value of L<C<@_>|perlvar/@_>. This is used
3300 by C<AUTOLOAD> subroutines that wish to load another subroutine and then
3301 pretend that the other subroutine had been called in the first place
3302 (except that any modifications to L<C<@_>|perlvar/@_> in the current
3303 subroutine are propagated to the other subroutine.) After the
3304 L<C<goto>|/goto LABEL>, not even L<C<caller>|/caller EXPR> will be able
3305 to tell that this routine was called first.
3307 NAME needn't be the name of a subroutine; it can be a scalar variable
3308 containing a code reference or a block that evaluates to a code
3311 =item grep BLOCK LIST
3314 =item grep EXPR,LIST
3316 =for Pod::Functions locate elements in a list test true against a given criterion
3318 This is similar in spirit to, but not the same as, L<grep(1)> and its
3319 relatives. In particular, it is not limited to using regular expressions.
3321 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
3322 L<C<$_>|perlvar/$_> to each element) and returns the list value
3324 elements for which the expression evaluated to true. In scalar
3325 context, returns the number of times the expression was true.
3327 my @foo = grep(!/^#/, @bar); # weed out comments
3331 my @foo = grep {!/^#/} @bar; # weed out comments
3333 Note that L<C<$_>|perlvar/$_> is an alias to the list value, so it can
3335 modify the elements of the LIST. While this is useful and supported,
3336 it can cause bizarre results if the elements of LIST are not variables.
3337 Similarly, grep returns aliases into the original list, much as a for
3338 loop's index variable aliases the list elements. That is, modifying an
3339 element of a list returned by grep (for example, in a C<foreach>,
3340 L<C<map>|/map BLOCK LIST> or another L<C<grep>|/grep BLOCK LIST>)
3341 actually modifies the element in the original list.
3342 This is usually something to be avoided when writing clear code.
3344 See also L<C<map>|/map BLOCK LIST> for a list composed of the results of
3348 X<hex> X<hexadecimal>
3352 =for Pod::Functions convert a hexadecimal string to a number
3354 Interprets EXPR as a hex string and returns the corresponding numeric value.
3355 If EXPR is omitted, uses L<C<$_>|perlvar/$_>.
3357 print hex '0xAf'; # prints '175'
3358 print hex 'aF'; # same
3359 $valid_input =~ /\A(?:0?[xX])?(?:_?[0-9a-fA-F])*\z/
3361 A hex string consists of hex digits and an optional C<0x> or C<x> prefix.
3362 Each hex digit may be preceded by a single underscore, which will be ignored.
3363 Any other character triggers a warning and causes the rest of the string
3364 to be ignored (even leading whitespace, unlike L<C<oct>|/oct EXPR>).
3365 Only integers can be represented, and integer overflow triggers a warning.
3367 To convert strings that might start with any of C<0>, C<0x>, or C<0b>,
3368 see L<C<oct>|/oct EXPR>. To present something as hex, look into
3369 L<C<printf>|/printf FILEHANDLE FORMAT, LIST>,
3370 L<C<sprintf>|/sprintf FORMAT, LIST>, and
3371 L<C<unpack>|/unpack TEMPLATE,EXPR>.
3376 =for Pod::Functions patch a module's namespace into your own
3378 There is no builtin L<C<import>|/import LIST> function. It is just an
3379 ordinary method (subroutine) defined (or inherited) by modules that wish
3380 to export names to another module. The
3381 L<C<use>|/use Module VERSION LIST> function calls the
3382 L<C<import>|/import LIST> method for the package used. See also
3383 L<C<use>|/use Module VERSION LIST>, L<perlmod>, and L<Exporter>.
3385 =item index STR,SUBSTR,POSITION
3386 X<index> X<indexOf> X<InStr>
3388 =item index STR,SUBSTR
3390 =for Pod::Functions find a substring within a string
3392 The index function searches for one string within another, but without
3393 the wildcard-like behavior of a full regular-expression pattern match.
3394 It returns the position of the first occurrence of SUBSTR in STR at
3395 or after POSITION. If POSITION is omitted, starts searching from the
3396 beginning of the string. POSITION before the beginning of the string
3397 or after its end is treated as if it were the beginning or the end,
3398 respectively. POSITION and the return value are based at zero.
3399 If the substring is not found, L<C<index>|/index STR,SUBSTR,POSITION>
3403 X<int> X<integer> X<truncate> X<trunc> X<floor>
3407 =for Pod::Functions get the integer portion of a number
3409 Returns the integer portion of EXPR. If EXPR is omitted, uses
3410 L<C<$_>|perlvar/$_>.
3411 You should not use this function for rounding: one because it truncates
3412 towards C<0>, and two because machine representations of floating-point
3413 numbers can sometimes produce counterintuitive results. For example,
3414 C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
3415 because it's really more like -268.99999999999994315658 instead. Usually,
3416 the L<C<sprintf>|/sprintf FORMAT, LIST>,
3417 L<C<printf>|/printf FILEHANDLE FORMAT, LIST>, or the
3418 L<C<POSIX::floor>|POSIX/C<floor>> and L<C<POSIX::ceil>|POSIX/C<ceil>>
3419 functions will serve you better than will L<C<int>|/int EXPR>.
3421 =item ioctl FILEHANDLE,FUNCTION,SCALAR
3424 =for Pod::Functions system-dependent device control system call
3426 Implements the L<ioctl(2)> function. You'll probably first have to say
3428 require "sys/ioctl.ph"; # probably in
3429 # $Config{archlib}/sys/ioctl.ph
3431 to get the correct function definitions. If F<sys/ioctl.ph> doesn't
3432 exist or doesn't have the correct definitions you'll have to roll your
3433 own, based on your C header files such as F<< <sys/ioctl.h> >>.
3434 (There is a Perl script called B<h2ph> that comes with the Perl kit that
3435 may help you in this, but it's nontrivial.) SCALAR will be read and/or
3436 written depending on the FUNCTION; a C pointer to the string value of SCALAR
3437 will be passed as the third argument of the actual
3438 L<C<ioctl>|/ioctl FILEHANDLE,FUNCTION,SCALAR> call. (If SCALAR
3439 has no string value but does have a numeric value, that value will be
3440 passed rather than a pointer to the string value. To guarantee this to be
3441 true, add a C<0> to the scalar before using it.) The
3442 L<C<pack>|/pack TEMPLATE,LIST> and L<C<unpack>|/unpack TEMPLATE,EXPR>
3443 functions may be needed to manipulate the values of structures used by
3444 L<C<ioctl>|/ioctl FILEHANDLE,FUNCTION,SCALAR>.
3446 The return value of L<C<ioctl>|/ioctl FILEHANDLE,FUNCTION,SCALAR> (and
3447 L<C<fcntl>|/fcntl FILEHANDLE,FUNCTION,SCALAR>) is as follows:
3449 if OS returns: then Perl returns:
3451 0 string "0 but true"
3452 anything else that number
3454 Thus Perl returns true on success and false on failure, yet you can
3455 still easily determine the actual value returned by the operating
3458 my $retval = ioctl(...) || -1;
3459 printf "System returned %d\n", $retval;
3461 The special string C<"0 but true"> is exempt from
3462 L<C<Argument "..." isn't numeric>|perldiag/Argument "%s" isn't numeric%s>
3463 L<warnings> on improper numeric conversions.
3465 Portability issues: L<perlport/ioctl>.
3467 =item join EXPR,LIST
3470 =for Pod::Functions join a list into a string using a separator
3472 Joins the separate strings of LIST into a single string with fields
3473 separated by the value of EXPR, and returns that new string. Example:
3475 my $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
3477 Beware that unlike L<C<split>|/split E<sol>PATTERNE<sol>,EXPR,LIMIT>,
3478 L<C<join>|/join EXPR,LIST> doesn't take a pattern as its first argument.
3479 Compare L<C<split>|/split E<sol>PATTERNE<sol>,EXPR,LIMIT>.
3486 =for Pod::Functions retrieve list of indices from a hash
3488 Called in list context, returns a list consisting of all the keys of the
3489 named hash, or in Perl 5.12 or later only, the indices of an array. Perl
3490 releases prior to 5.12 will produce a syntax error if you try to use an
3491 array argument. In scalar context, returns the number of keys or indices.
3493 Hash entries are returned in an apparently random order. The actual random
3494 order is specific to a given hash; the exact same series of operations
3495 on two hashes may result in a different order for each hash. Any insertion
3496 into the hash may change the order, as will any deletion, with the exception
3497 that the most recent key returned by L<C<each>|/each HASH> or
3498 L<C<keys>|/keys HASH> may be deleted without changing the order. So
3499 long as a given hash is unmodified you may rely on
3500 L<C<keys>|/keys HASH>, L<C<values>|/values HASH> and L<C<each>|/each
3501 HASH> to repeatedly return the same order
3502 as each other. See L<perlsec/"Algorithmic Complexity Attacks"> for
3503 details on why hash order is randomized. Aside from the guarantees
3504 provided here the exact details of Perl's hash algorithm and the hash
3505 traversal order are subject to change in any release of Perl. Tied hashes
3506 may behave differently to Perl's hashes with respect to changes in order on
3507 insertion and deletion of items.
3509 As a side effect, calling L<C<keys>|/keys HASH> resets the internal
3510 iterator of the HASH or ARRAY (see L<C<each>|/each HASH>). In
3511 particular, calling L<C<keys>|/keys HASH> in void context resets the
3512 iterator with no other overhead.
3514 Here is yet another way to print your environment:
3516 my @keys = keys %ENV;
3517 my @values = values %ENV;
3519 print pop(@keys), '=', pop(@values), "\n";
3522 or how about sorted by key:
3524 foreach my $key (sort(keys %ENV)) {
3525 print $key, '=', $ENV{$key}, "\n";
3528 The returned values are copies of the original keys in the hash, so
3529 modifying them will not affect the original hash. Compare
3530 L<C<values>|/values HASH>.
3532 To sort a hash by value, you'll need to use a
3533 L<C<sort>|/sort SUBNAME LIST> function. Here's a descending numeric
3534 sort of a hash by its values:
3536 foreach my $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
3537 printf "%4d %s\n", $hash{$key}, $key;
3540 Used as an lvalue, L<C<keys>|/keys HASH> allows you to increase the
3541 number of hash buckets
3542 allocated for the given hash. This can gain you a measure of efficiency if
3543 you know the hash is going to get big. (This is similar to pre-extending
3544 an array by assigning a larger number to $#array.) If you say
3548 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
3549 in fact, since it rounds up to the next power of two. These
3550 buckets will be retained even if you do C<%hash = ()>, use C<undef
3551 %hash> if you want to free the storage while C<%hash> is still in scope.
3552 You can't shrink the number of buckets allocated for the hash using
3553 L<C<keys>|/keys HASH> in this way (but you needn't worry about doing
3554 this by accident, as trying has no effect). C<keys @array> in an lvalue
3555 context is a syntax error.
3557 Starting with Perl 5.14, an experimental feature allowed
3558 L<C<keys>|/keys HASH> to take a scalar expression. This experiment has
3559 been deemed unsuccessful, and was removed as of Perl 5.24.
3561 To avoid confusing would-be users of your code who are running earlier
3562 versions of Perl with mysterious syntax errors, put this sort of thing at
3563 the top of your file to signal that your code will work I<only> on Perls of
3566 use 5.012; # so keys/values/each work on arrays
3568 See also L<C<each>|/each HASH>, L<C<values>|/values HASH>, and
3569 L<C<sort>|/sort SUBNAME LIST>.
3571 =item kill SIGNAL, LIST
3576 =for Pod::Functions send a signal to a process or process group
3578 Sends a signal to a list of processes. Returns the number of arguments
3579 that were successfully used to signal (which is not necessarily the same
3580 as the number of processes actually killed, e.g. where a process group is
3583 my $cnt = kill 'HUP', $child1, $child2;
3584 kill 'KILL', @goners;
3586 SIGNAL may be either a signal name (a string) or a signal number. A signal
3587 name may start with a C<SIG> prefix, thus C<FOO> and C<SIGFOO> refer to the
3588 same signal. The string form of SIGNAL is recommended for portability because
3589 the same signal may have different numbers in different operating systems.
3591 A list of signal names supported by the current platform can be found in
3592 C<$Config{sig_name}>, which is provided by the L<C<Config>|Config>
3593 module. See L<Config> for more details.
3595 A negative signal name is the same as a negative signal number, killing process
3596 groups instead of processes. For example, C<kill '-KILL', $pgrp> and
3597 C<kill -9, $pgrp> will send C<SIGKILL> to
3598 the entire process group specified. That
3599 means you usually want to use positive not negative signals.
3601 If SIGNAL is either the number 0 or the string C<ZERO> (or C<SIGZERO>),
3602 no signal is sent to the process, but L<C<kill>|/kill SIGNAL, LIST>
3603 checks whether it's I<possible> to send a signal to it
3604 (that means, to be brief, that the process is owned by the same user, or we are
3605 the super-user). This is useful to check that a child process is still
3606 alive (even if only as a zombie) and hasn't changed its UID. See
3607 L<perlport> for notes on the portability of this construct.
3609 The behavior of kill when a I<PROCESS> number is zero or negative depends on
3610 the operating system. For example, on POSIX-conforming systems, zero will
3611 signal the current process group, -1 will signal all processes, and any
3612 other negative PROCESS number will act as a negative signal number and
3613 kill the entire process group specified.
3615 If both the SIGNAL and the PROCESS are negative, the results are undefined.
3616 A warning may be produced in a future version.
3618 See L<perlipc/"Signals"> for more details.
3620 On some platforms such as Windows where the L<fork(2)> system call is not
3621 available, Perl can be built to emulate L<C<fork>|/fork> at the
3623 This emulation has limitations related to kill that have to be considered,
3624 for code running on Windows and in code intended to be portable.
3626 See L<perlfork> for more details.
3628 If there is no I<LIST> of processes, no signal is sent, and the return
3629 value is 0. This form is sometimes used, however, because it causes
3630 tainting checks to be run. But see
3631 L<perlsec/Laundering and Detecting Tainted Data>.
3633 Portability issues: L<perlport/kill>.
3642 =for Pod::Functions exit a block prematurely
3644 The L<C<last>|/last LABEL> command is like the C<break> statement in C
3646 loops); it immediately exits the loop in question. If the LABEL is
3647 omitted, the command refers to the innermost enclosing
3648 loop. The C<last EXPR> form, available starting in Perl
3649 5.18.0, allows a label name to be computed at run time,
3650 and is otherwise identical to C<last LABEL>. The
3651 L<C<continue>|/continue BLOCK> block, if any, is not executed:
3653 LINE: while (<STDIN>) {
3654 last LINE if /^$/; # exit when done with header
3658 L<C<last>|/last LABEL> cannot be used to exit a block that returns a
3659 value such as C<eval {}>, C<sub {}>, or C<do {}>, and should not be used
3660 to exit a L<C<grep>|/grep BLOCK LIST> or L<C<map>|/map BLOCK LIST>
3663 Note that a block by itself is semantically identical to a loop
3664 that executes once. Thus L<C<last>|/last LABEL> can be used to effect
3665 an early exit out of such a block.
3667 See also L<C<continue>|/continue BLOCK> for an illustration of how
3668 L<C<last>|/last LABEL>, L<C<next>|/next LABEL>, and
3669 L<C<redo>|/redo LABEL> work.
3671 Unlike most named operators, this has the same precedence as assignment.
3672 It is also exempt from the looks-like-a-function rule, so
3673 C<last ("foo")."bar"> will cause "bar" to be part of the argument to
3674 L<C<last>|/last LABEL>.
3681 =for Pod::Functions return lower-case version of a string
3683 Returns a lowercased version of EXPR. This is the internal function
3684 implementing the C<\L> escape in double-quoted strings.
3686 If EXPR is omitted, uses L<C<$_>|perlvar/$_>.
3688 What gets returned depends on several factors:
3692 =item If C<use bytes> is in effect:
3694 The results follow ASCII rules. Only the characters C<A-Z> change,
3695 to C<a-z> respectively.
3697 =item Otherwise, if C<use locale> for C<LC_CTYPE> is in effect:
3699 Respects current C<LC_CTYPE> locale for code points < 256; and uses Unicode
3700 rules for the remaining code points (this last can only happen if
3701 the UTF8 flag is also set). See L<perllocale>.
3703 Starting in v5.20, Perl uses full Unicode rules if the locale is
3704 UTF-8. Otherwise, there is a deficiency in this scheme, which is that
3705 case changes that cross the 255/256
3706 boundary are not well-defined. For example, the lower case of LATIN CAPITAL
3707 LETTER SHARP S (U+1E9E) in Unicode rules is U+00DF (on ASCII
3708 platforms). But under C<use locale> (prior to v5.20 or not a UTF-8
3709 locale), the lower case of U+1E9E is
3710 itself, because 0xDF may not be LATIN SMALL LETTER SHARP S in the
3711 current locale, and Perl has no way of knowing if that character even
3712 exists in the locale, much less what code point it is. Perl returns
3713 a result that is above 255 (almost always the input character unchanged),
3714 for all instances (and there aren't many) where the 255/256 boundary
3715 would otherwise be crossed; and starting in v5.22, it raises a
3716 L<locale|perldiag/Can't do %s("%s") on non-UTF-8 locale; resolved to "%s".> warning.
3718 =item Otherwise, If EXPR has the UTF8 flag set:
3720 Unicode rules are used for the case change.
3722 =item Otherwise, if C<use feature 'unicode_strings'> or C<use locale ':not_characters'> is in effect:
3724 Unicode rules are used for the case change.
3728 ASCII rules are used for the case change. The lowercase of any character
3729 outside the ASCII range is the character itself.
3734 X<lcfirst> X<lowercase>
3738 =for Pod::Functions return a string with just the next letter in lower case
3740 Returns the value of EXPR with the first character lowercased. This
3741 is the internal function implementing the C<\l> escape in
3742 double-quoted strings.
3744 If EXPR is omitted, uses L<C<$_>|perlvar/$_>.
3746 This function behaves the same way under various pragmas, such as in a locale,
3747 as L<C<lc>|/lc EXPR> does.
3754 =for Pod::Functions return the number of characters in a string
3756 Returns the length in I<characters> of the value of EXPR. If EXPR is
3757 omitted, returns the length of L<C<$_>|perlvar/$_>. If EXPR is
3758 undefined, returns L<C<undef>|/undef EXPR>.
3760 This function cannot be used on an entire array or hash to find out how
3761 many elements these have. For that, use C<scalar @array> and C<scalar keys
3762 %hash>, respectively.
3764 Like all Perl character operations, L<C<length>|/length EXPR> normally
3766 characters, not physical bytes. For how many bytes a string encoded as
3767 UTF-8 would take up, use C<length(Encode::encode_utf8(EXPR))> (you'll have
3768 to C<use Encode> first). See L<Encode> and L<perlunicode>.
3773 =for Pod::Functions the current source line number
3775 A special token that compiles to the current line number.
3777 =item link OLDFILE,NEWFILE
3780 =for Pod::Functions create a hard link in the filesystem
3782 Creates a new filename linked to the old filename. Returns true for
3783 success, false otherwise.
3785 Portability issues: L<perlport/link>.
3787 =item listen SOCKET,QUEUESIZE
3790 =for Pod::Functions register your socket as a server
3792 Does the same thing that the L<listen(2)> system call does. Returns true if
3793 it succeeded, false otherwise. See the example in
3794 L<perlipc/"Sockets: Client/Server Communication">.
3799 =for Pod::Functions create a temporary value for a global variable (dynamic scoping)
3801 You really probably want to be using L<C<my>|/my VARLIST> instead,
3802 because L<C<local>|/local EXPR> isn't what most people think of as
3803 "local". See L<perlsub/"Private Variables via my()"> for details.
3805 A local modifies the listed variables to be local to the enclosing
3806 block, file, or eval. If more than one value is listed, the list must
3807 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
3808 for details, including issues with tied arrays and hashes.
3810 The C<delete local EXPR> construct can also be used to localize the deletion
3811 of array/hash elements to the current block.
3812 See L<perlsub/"Localized deletion of elements of composite types">.
3814 =item localtime EXPR
3815 X<localtime> X<ctime>
3819 =for Pod::Functions convert UNIX time into record or string using local time
3821 Converts a time as returned by the time function to a 9-element list
3822 with the time analyzed for the local time zone. Typically used as
3826 my ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
3829 All list elements are numeric and come straight out of the C `struct
3830 tm'. C<$sec>, C<$min>, and C<$hour> are the seconds, minutes, and hours
3831 of the specified time.
3833 C<$mday> is the day of the month and C<$mon> the month in
3834 the range C<0..11>, with 0 indicating January and 11 indicating December.
3835 This makes it easy to get a month name from a list:
3837 my @abbr = qw(Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec);
3838 print "$abbr[$mon] $mday";
3839 # $mon=9, $mday=18 gives "Oct 18"
3841 C<$year> contains the number of years since 1900. To get a 4-digit
3846 To get the last two digits of the year (e.g., "01" in 2001) do:
3848 $year = sprintf("%02d", $year % 100);
3850 C<$wday> is the day of the week, with 0 indicating Sunday and 3 indicating
3851 Wednesday. C<$yday> is the day of the year, in the range C<0..364>
3852 (or C<0..365> in leap years.)
3854 C<$isdst> is true if the specified time occurs during Daylight Saving
3855 Time, false otherwise.
3857 If EXPR is omitted, L<C<localtime>|/localtime EXPR> uses the current
3858 time (as returned by L<C<time>|/time>).
3860 In scalar context, L<C<localtime>|/localtime EXPR> returns the
3863 my $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
3865 The format of this scalar value is B<not> locale-dependent but built
3866 into Perl. For GMT instead of local time use the
3867 L<C<gmtime>|/gmtime EXPR> builtin. See also the
3868 L<C<Time::Local>|Time::Local> module (for converting seconds, minutes,
3869 hours, and such back to the integer value returned by L<C<time>|/time>),
3870 and the L<POSIX> module's L<C<strftime>|POSIX/C<strftime>> and
3871 L<C<mktime>|POSIX/C<mktime>> functions.
3873 To get somewhat similar but locale-dependent date strings, set up your
3874 locale environment variables appropriately (please see L<perllocale>) and
3877 use POSIX qw(strftime);
3878 my $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
3879 # or for GMT formatted appropriately for your locale:
3880 my $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
3882 Note that C<%a> and C<%b>, the short forms of the day of the week
3883 and the month of the year, may not necessarily be three characters wide.
3885 The L<Time::gmtime> and L<Time::localtime> modules provide a convenient,
3886 by-name access mechanism to the L<C<gmtime>|/gmtime EXPR> and
3887 L<C<localtime>|/localtime EXPR> functions, respectively.
3889 For a comprehensive date and time representation look at the
3890 L<DateTime> module on CPAN.
3892 Portability issues: L<perlport/localtime>.
3897 =for Pod::Functions +5.005 get a thread lock on a variable, subroutine, or method
3899 This function places an advisory lock on a shared variable or referenced
3900 object contained in I<THING> until the lock goes out of scope.
3902 The value returned is the scalar itself, if the argument is a scalar, or a
3903 reference, if the argument is a hash, array or subroutine.
3905 L<C<lock>|/lock THING> is a "weak keyword"; this means that if you've
3907 by this name (before any calls to it), that function will be called
3908 instead. If you are not under C<use threads::shared> this does nothing.
3909 See L<threads::shared>.
3912 X<log> X<logarithm> X<e> X<ln> X<base>
3916 =for Pod::Functions retrieve the natural logarithm for a number
3918 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
3919 returns the log of L<C<$_>|perlvar/$_>. To get the
3920 log of another base, use basic algebra:
3921 The base-N log of a number is equal to the natural log of that number
3922 divided by the natural log of N. For example:
3926 return log($n)/log(10);
3929 See also L<C<exp>|/exp EXPR> for the inverse operation.
3931 =item lstat FILEHANDLE
3936 =item lstat DIRHANDLE
3940 =for Pod::Functions stat a symbolic link
3942 Does the same thing as the L<C<stat>|/stat FILEHANDLE> function
3943 (including setting the special C<_> filehandle) but stats a symbolic
3944 link instead of the file the symbolic link points to. If symbolic links
3945 are unimplemented on your system, a normal L<C<stat>|/stat FILEHANDLE>
3946 is done. For much more detailed information, please see the
3947 documentation for L<C<stat>|/stat FILEHANDLE>.
3949 If EXPR is omitted, stats L<C<$_>|perlvar/$_>.
3951 Portability issues: L<perlport/lstat>.
3955 =for Pod::Functions match a string with a regular expression pattern
3957 The match operator. See L<perlop/"Regexp Quote-Like Operators">.
3959 =item map BLOCK LIST
3964 =for Pod::Functions apply a change to a list to get back a new list with the changes
3966 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
3967 L<C<$_>|perlvar/$_> to each element) and returns the list value composed
3969 results of each such evaluation. In scalar context, returns the
3970 total number of elements so generated. Evaluates BLOCK or EXPR in
3971 list context, so each element of LIST may produce zero, one, or
3972 more elements in the returned value.
3974 my @chars = map(chr, @numbers);
3976 translates a list of numbers to the corresponding characters.
3978 my @squares = map { $_ * $_ } @numbers;
3980 translates a list of numbers to their squared values.
3982 my @squares = map { $_ > 5 ? ($_ * $_) : () } @numbers;
3984 shows that number of returned elements can differ from the number of
3985 input elements. To omit an element, return an empty list ().
3986 This could also be achieved by writing
3988 my @squares = map { $_ * $_ } grep { $_ > 5 } @numbers;
3990 which makes the intention more clear.
3992 Map always returns a list, which can be
3993 assigned to a hash such that the elements
3994 become key/value pairs. See L<perldata> for more details.
3996 my %hash = map { get_a_key_for($_) => $_ } @array;
3998 is just a funny way to write
4002 $hash{get_a_key_for($_)} = $_;
4005 Note that L<C<$_>|perlvar/$_> is an alias to the list value, so it can
4006 be used to modify the elements of the LIST. While this is useful and
4007 supported, it can cause bizarre results if the elements of LIST are not
4008 variables. Using a regular C<foreach> loop for this purpose would be
4009 clearer in most cases. See also L<C<grep>|/grep BLOCK LIST> for a
4010 list composed of those items of the original list for which the BLOCK
4011 or EXPR evaluates to true.
4013 C<{> starts both hash references and blocks, so C<map { ...> could be either
4014 the start of map BLOCK LIST or map EXPR, LIST. Because Perl doesn't look
4015 ahead for the closing C<}> it has to take a guess at which it's dealing with
4016 based on what it finds just after the
4017 C<{>. Usually it gets it right, but if it
4018 doesn't it won't realize something is wrong until it gets to the C<}> and
4019 encounters the missing (or unexpected) comma. The syntax error will be
4020 reported close to the C<}>, but you'll need to change something near the C<{>
4021 such as using a unary C<+> or semicolon to give Perl some help:
4023 my %hash = map { "\L$_" => 1 } @array # perl guesses EXPR. wrong
4024 my %hash = map { +"\L$_" => 1 } @array # perl guesses BLOCK. right
4025 my %hash = map {; "\L$_" => 1 } @array # this also works
4026 my %hash = map { ("\L$_" => 1) } @array # as does this
4027 my %hash = map { lc($_) => 1 } @array # and this.
4028 my %hash = map +( lc($_) => 1 ), @array # this is EXPR and works!
4030 my %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
4032 or to force an anon hash constructor use C<+{>:
4034 my @hashes = map +{ lc($_) => 1 }, @array # EXPR, so needs
4037 to get a list of anonymous hashes each with only one entry apiece.
4039 =item mkdir FILENAME,MASK
4040 X<mkdir> X<md> X<directory, create>
4042 =item mkdir FILENAME
4046 =for Pod::Functions create a directory
4048 Creates the directory specified by FILENAME, with permissions
4049 specified by MASK (as modified by L<C<umask>|/umask EXPR>). If it
4050 succeeds it returns true; otherwise it returns false and sets
4051 L<C<$!>|perlvar/$!> (errno).
4052 MASK defaults to 0777 if omitted, and FILENAME defaults
4053 to L<C<$_>|perlvar/$_> if omitted.
4055 In general, it is better to create directories with a permissive MASK
4056 and let the user modify that with their L<C<umask>|/umask EXPR> than it
4058 a restrictive MASK and give the user no way to be more permissive.
4059 The exceptions to this rule are when the file or directory should be
4060 kept private (mail files, for instance). The documentation for
4061 L<C<umask>|/umask EXPR> discusses the choice of MASK in more detail.
4063 Note that according to the POSIX 1003.1-1996 the FILENAME may have any
4064 number of trailing slashes. Some operating and filesystems do not get
4065 this right, so Perl automatically removes all trailing slashes to keep
4068 To recursively create a directory structure, look at
4069 the L<C<make_path>|File::Path/make_path( $dir1, $dir2, .... )> function
4070 of the L<File::Path> module.
4072 =item msgctl ID,CMD,ARG
4075 =for Pod::Functions SysV IPC message control operations
4077 Calls the System V IPC function L<msgctl(2)>. You'll probably have to say
4081 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
4082 then ARG must be a variable that will hold the returned C<msqid_ds>
4083 structure. Returns like L<C<ioctl>|/ioctl FILEHANDLE,FUNCTION,SCALAR>:
4084 the undefined value for error, C<"0 but true"> for zero, or the actual
4085 return value otherwise. See also L<perlipc/"SysV IPC"> and the
4086 documentation for L<C<IPC::SysV>|IPC::SysV> and
4087 L<C<IPC::Semaphore>|IPC::Semaphore>.
4089 Portability issues: L<perlport/msgctl>.
4091 =item msgget KEY,FLAGS
4094 =for Pod::Functions get SysV IPC message queue
4096 Calls the System V IPC function L<msgget(2)>. Returns the message queue
4097 id, or L<C<undef>|/undef EXPR> on error. See also L<perlipc/"SysV IPC">
4098 and the documentation for L<C<IPC::SysV>|IPC::SysV> and
4099 L<C<IPC::Msg>|IPC::Msg>.
4101 Portability issues: L<perlport/msgget>.
4103 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
4106 =for Pod::Functions receive a SysV IPC message from a message queue
4108 Calls the System V IPC function msgrcv to receive a message from
4109 message queue ID into variable VAR with a maximum message size of
4110 SIZE. Note that when a message is received, the message type as a
4111 native long integer will be the first thing in VAR, followed by the
4112 actual message. This packing may be opened with C<unpack("l! a*")>.
4113 Taints the variable. Returns true if successful, false
4114 on error. See also L<perlipc/"SysV IPC"> and the documentation for
4115 L<C<IPC::SysV>|IPC::SysV> and L<C<IPC::Msg>|IPC::Msg>.
4117 Portability issues: L<perlport/msgrcv>.
4119 =item msgsnd ID,MSG,FLAGS
4122 =for Pod::Functions send a SysV IPC message to a message queue
4124 Calls the System V IPC function msgsnd to send the message MSG to the
4125 message queue ID. MSG must begin with the native long integer message
4126 type, be followed by the length of the actual message, and then finally
4127 the message itself. This kind of packing can be achieved with
4128 C<pack("l! a*", $type, $message)>. Returns true if successful,
4129 false on error. See also L<perlipc/"SysV IPC"> and the documentation
4130 for L<C<IPC::SysV>|IPC::SysV> and L<C<IPC::Msg>|IPC::Msg>.
4132 Portability issues: L<perlport/msgsnd>.
4137 =item my TYPE VARLIST
4139 =item my VARLIST : ATTRS
4141 =item my TYPE VARLIST : ATTRS
4143 =for Pod::Functions declare and assign a local variable (lexical scoping)
4145 A L<C<my>|/my VARLIST> declares the listed variables to be local
4146 (lexically) to the enclosing block, file, or L<C<eval>|/eval EXPR>. If
4147 more than one variable is listed, the list must be placed in
4150 The exact semantics and interface of TYPE and ATTRS are still
4151 evolving. TYPE may be a bareword, a constant declared
4152 with L<C<use constant>|constant>, or L<C<__PACKAGE__>|/__PACKAGE__>. It
4154 currently bound to the use of the L<fields> pragma,
4155 and attributes are handled using the L<attributes> pragma, or starting
4156 from Perl 5.8.0 also via the L<Attribute::Handlers> module. See
4157 L<perlsub/"Private Variables via my()"> for details.
4159 Note that with a parenthesised list, L<C<undef>|/undef EXPR> can be used
4160 as a dummy placeholder, for example to skip assignment of initial
4163 my ( undef, $min, $hour ) = localtime;
4172 =for Pod::Functions iterate a block prematurely
4174 The L<C<next>|/next LABEL> command is like the C<continue> statement in
4175 C; it starts the next iteration of the loop:
4177 LINE: while (<STDIN>) {
4178 next LINE if /^#/; # discard comments
4182 Note that if there were a L<C<continue>|/continue BLOCK> block on the
4184 executed even on discarded lines. If LABEL is omitted, the command
4185 refers to the innermost enclosing loop. The C<next EXPR> form, available
4186 as of Perl 5.18.0, allows a label name to be computed at run time, being
4187 otherwise identical to C<next LABEL>.
4189 L<C<next>|/next LABEL> cannot be used to exit a block which returns a
4190 value such as C<eval {}>, C<sub {}>, or C<do {}>, and should not be used
4191 to exit a L<C<grep>|/grep BLOCK LIST> or L<C<map>|/map BLOCK LIST>
4194 Note that a block by itself is semantically identical to a loop
4195 that executes once. Thus L<C<next>|/next LABEL> will exit such a block
4198 See also L<C<continue>|/continue BLOCK> for an illustration of how
4199 L<C<last>|/last LABEL>, L<C<next>|/next LABEL>, and
4200 L<C<redo>|/redo LABEL> work.
4202 Unlike most named operators, this has the same precedence as assignment.
4203 It is also exempt from the looks-like-a-function rule, so
4204 C<next ("foo")."bar"> will cause "bar" to be part of the argument to
4205 L<C<next>|/next LABEL>.
4207 =item no MODULE VERSION LIST
4211 =item no MODULE VERSION
4213 =item no MODULE LIST
4219 =for Pod::Functions unimport some module symbols or semantics at compile time
4221 See the L<C<use>|/use Module VERSION LIST> function, of which
4222 L<C<no>|/no MODULE VERSION LIST> is the opposite.
4225 X<oct> X<octal> X<hex> X<hexadecimal> X<binary> X<bin>
4229 =for Pod::Functions convert a string to an octal number
4231 Interprets EXPR as an octal string and returns the corresponding
4232 value. (If EXPR happens to start off with C<0x>, interprets it as a
4233 hex string. If EXPR starts off with C<0b>, it is interpreted as a
4234 binary string. Leading whitespace is ignored in all three cases.)
4235 The following will handle decimal, binary, octal, and hex in standard
4238 $val = oct($val) if $val =~ /^0/;
4240 If EXPR is omitted, uses L<C<$_>|perlvar/$_>. To go the other way
4241 (produce a number in octal), use L<C<sprintf>|/sprintf FORMAT, LIST> or
4242 L<C<printf>|/printf FILEHANDLE FORMAT, LIST>:
4244 my $dec_perms = (stat("filename"))[2] & 07777;
4245 my $oct_perm_str = sprintf "%o", $perms;
4247 The L<C<oct>|/oct EXPR> function is commonly used when a string such as
4249 to be converted into a file mode, for example. Although Perl
4250 automatically converts strings into numbers as needed, this automatic
4251 conversion assumes base 10.
4253 Leading white space is ignored without warning, as too are any trailing
4254 non-digits, such as a decimal point (L<C<oct>|/oct EXPR> only handles
4255 non-negative integers, not negative integers or floating point).
4257 =item open FILEHANDLE,EXPR
4258 X<open> X<pipe> X<file, open> X<fopen>
4260 =item open FILEHANDLE,MODE,EXPR
4262 =item open FILEHANDLE,MODE,EXPR,LIST
4264 =item open FILEHANDLE,MODE,REFERENCE
4266 =item open FILEHANDLE
4268 =for Pod::Functions open a file, pipe, or descriptor
4270 Opens the file whose filename is given by EXPR, and associates it with
4273 Simple examples to open a file for reading:
4275 open(my $fh, "<", "input.txt")
4276 or die "Can't open < input.txt: $!";
4280 open(my $fh, ">", "output.txt")
4281 or die "Can't open > output.txt: $!";
4283 (The following is a comprehensive reference to
4284 L<C<open>|/open FILEHANDLE,EXPR>: for a gentler introduction you may
4285 consider L<perlopentut>.)
4287 If FILEHANDLE is an undefined scalar variable (or array or hash element), a
4288 new filehandle is autovivified, meaning that the variable is assigned a
4289 reference to a newly allocated anonymous filehandle. Otherwise if
4290 FILEHANDLE is an expression, its value is the real filehandle. (This is
4291 considered a symbolic reference, so C<use strict "refs"> should I<not> be
4294 If three (or more) arguments are specified, the open mode (including
4295 optional encoding) in the second argument are distinct from the filename in
4296 the third. If MODE is C<< < >> or nothing, the file is opened for input.
4297 If MODE is C<< > >>, the file is opened for output, with existing files
4298 first being truncated ("clobbered") and nonexisting files newly created.
4299 If MODE is C<<< >> >>>, the file is opened for appending, again being
4300 created if necessary.
4302 You can put a C<+> in front of the C<< > >> or C<< < >> to
4303 indicate that you want both read and write access to the file; thus
4304 C<< +< >> is almost always preferred for read/write updates--the
4305 C<< +> >> mode would clobber the file first. You can't usually use
4306 either read-write mode for updating textfiles, since they have
4307 variable-length records. See the B<-i> switch in L<perlrun> for a
4308 better approach. The file is created with permissions of C<0666>
4309 modified by the process's L<C<umask>|/umask EXPR> value.
4311 These various prefixes correspond to the L<fopen(3)> modes of C<r>,
4312 C<r+>, C<w>, C<w+>, C<a>, and C<a+>.
4314 In the one- and two-argument forms of the call, the mode and filename
4315 should be concatenated (in that order), preferably separated by white
4316 space. You can--but shouldn't--omit the mode in these forms when that mode
4317 is C<< < >>. It is safe to use the two-argument form of
4318 L<C<open>|/open FILEHANDLE,EXPR> if the filename argument is a known literal.
4320 For three or more arguments if MODE is C<|->, the filename is
4321 interpreted as a command to which output is to be piped, and if MODE
4322 is C<-|>, the filename is interpreted as a command that pipes
4323 output to us. In the two-argument (and one-argument) form, one should
4324 replace dash (C<->) with the command.
4325 See L<perlipc/"Using open() for IPC"> for more examples of this.
4326 (You are not allowed to L<C<open>|/open FILEHANDLE,EXPR> to a command
4327 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>, and
4328 L<perlipc/"Bidirectional Communication with Another Process"> for
4331 In the form of pipe opens taking three or more arguments, if LIST is specified
4332 (extra arguments after the command name) then LIST becomes arguments
4333 to the command invoked if the platform supports it. The meaning of
4334 L<C<open>|/open FILEHANDLE,EXPR> with more than three arguments for
4335 non-pipe modes is not yet defined, but experimental "layers" may give
4336 extra LIST arguments meaning.
4338 In the two-argument (and one-argument) form, opening C<< <- >>
4339 or C<-> opens STDIN and opening C<< >- >> opens STDOUT.
4341 You may (and usually should) use the three-argument form of open to specify
4342 I/O layers (sometimes referred to as "disciplines") to apply to the handle
4343 that affect how the input and output are processed (see L<open> and
4344 L<PerlIO> for more details). For example:
4346 open(my $fh, "<:encoding(UTF-8)", $filename)
4347 || die "Can't open UTF-8 encoded $filename: $!";
4349 opens the UTF8-encoded file containing Unicode characters;
4350 see L<perluniintro>. Note that if layers are specified in the
4351 three-argument form, then default layers stored in ${^OPEN} (see L<perlvar>;
4352 usually set by the L<open> pragma or the switch C<-CioD>) are ignored.
4353 Those layers will also be ignored if you specify a colon with no name
4354 following it. In that case the default layer for the operating system
4355 (:raw on Unix, :crlf on Windows) is used.
4357 Open returns nonzero on success, the undefined value otherwise. If
4358 the L<C<open>|/open FILEHANDLE,EXPR> involved a pipe, the return value
4359 happens to be the pid of the subprocess.
4361 On some systems (in general, DOS- and Windows-based systems)
4362 L<C<binmode>|/binmode FILEHANDLE, LAYER> is necessary when you're not
4363 working with a text file. For the sake of portability it is a good idea
4364 always to use it when appropriate, and never to use it when it isn't
4365 appropriate. Also, people can set their I/O to be by default
4366 UTF8-encoded Unicode, not bytes.
4368 When opening a file, it's seldom a good idea to continue
4369 if the request failed, so L<C<open>|/open FILEHANDLE,EXPR> is frequently
4370 used with L<C<die>|/die LIST>. Even if L<C<die>|/die LIST> won't do
4371 what you want (say, in a CGI script,
4372 where you want to format a suitable error message (but there are
4373 modules that can help with that problem)) always check
4374 the return value from opening a file.
4376 The filehandle will be closed when its reference count reaches zero.
4377 If it is a lexically scoped variable declared with L<C<my>|/my VARLIST>,
4379 means the end of the enclosing scope. However, this automatic close
4380 does not check for errors, so it is better to explicitly close
4381 filehandles, especially those used for writing:
4384 || warn "close failed: $!";
4386 An older style is to use a bareword as the filehandle, as
4388 open(FH, "<", "input.txt")
4389 or die "Can't open < input.txt: $!";
4391 Then you can use C<FH> as the filehandle, in C<< close FH >> and C<<
4392 <FH> >> and so on. Note that it's a global variable, so this form is
4393 not recommended in new code.
4395 As a shortcut a one-argument call takes the filename from the global
4396 scalar variable of the same name as the filehandle:
4399 open(ARTICLE) or die "Can't find article $ARTICLE: $!\n";
4401 Here C<$ARTICLE> must be a global (package) scalar variable - not one
4402 declared with L<C<my>|/my VARLIST> or L<C<state>|/state VARLIST>.
4404 As a special case the three-argument form with a read/write mode and the third
4405 argument being L<C<undef>|/undef EXPR>:
4407 open(my $tmp, "+>", undef) or die ...
4409 opens a filehandle to an anonymous temporary file. Also using C<< +< >>
4410 works for symmetry, but you really should consider writing something
4411 to the temporary file first. You will need to
4412 L<C<seek>|/seek FILEHANDLE,POSITION,WHENCE> to do the reading.
4414 Perl is built using PerlIO by default. Unless you've
4415 changed this (such as building Perl with C<Configure -Uuseperlio>), you can
4416 open filehandles directly to Perl scalars via:
4418 open(my $fh, ">", \$variable) || ..
4420 To (re)open C<STDOUT> or C<STDERR> as an in-memory file, close it first:
4423 open(STDOUT, ">", \$variable)
4424 or die "Can't open STDOUT: $!";
4426 See L<perliol> for detailed info on PerlIO.
4430 open(my $log, ">>", "/usr/spool/news/twitlog");
4431 # if the open fails, output is discarded
4433 open(my $dbase, "+<", "dbase.mine") # open for update
4434 or die "Can't open 'dbase.mine' for update: $!";
4436 open(my $dbase, "+<dbase.mine") # ditto
4437 or die "Can't open 'dbase.mine' for update: $!";
4439 open(my $article_fh, "-|", "caesar <$article") # decrypt
4441 or die "Can't start caesar: $!";
4443 open(my $article_fh, "caesar <$article |") # ditto
4444 or die "Can't start caesar: $!";
4446 open(my $out_fh, "|-", "sort >Tmp$$") # $$ is our process id
4447 or die "Can't start sort: $!";
4450 open(my $memory, ">", \$var)
4451 or die "Can't open memory file: $!";
4452 print $memory "foo!\n"; # output will appear in $var
4454 You may also, in the Bourne shell tradition, specify an EXPR beginning
4455 with C<< >& >>, in which case the rest of the string is interpreted
4456 as the name of a filehandle (or file descriptor, if numeric) to be
4457 duped (as in L<dup(2)>) and opened. You may use C<&> after C<< > >>,
4458 C<<< >> >>>, C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>.
4459 The mode you specify should match the mode of the original filehandle.
4460 (Duping a filehandle does not take into account any existing contents
4461 of IO buffers.) If you use the three-argument
4462 form, then you can pass either a
4463 number, the name of a filehandle, or the normal "reference to a glob".
4465 Here is a script that saves, redirects, and restores C<STDOUT> and
4466 C<STDERR> using various methods:
4469 open(my $oldout, ">&STDOUT") or die "Can't dup STDOUT: $!";
4470 open(OLDERR, ">&", \*STDERR) or die "Can't dup STDERR: $!";
4472 open(STDOUT, '>', "foo.out") or die "Can't redirect STDOUT: $!";
4473 open(STDERR, ">&STDOUT") or die "Can't dup STDOUT: $!";
4475 select STDERR; $| = 1; # make unbuffered
4476 select STDOUT; $| = 1; # make unbuffered
4478 print STDOUT "stdout 1\n"; # this works for
4479 print STDERR "stderr 1\n"; # subprocesses too
4481 open(STDOUT, ">&", $oldout) or die "Can't dup \$oldout: $!";
4482 open(STDERR, ">&OLDERR") or die "Can't dup OLDERR: $!";
4484 print STDOUT "stdout 2\n";
4485 print STDERR "stderr 2\n";
4487 If you specify C<< '<&=X' >>, where C<X> is a file descriptor number
4488 or a filehandle, then Perl will do an equivalent of C's L<fdopen(3)> of
4489 that file descriptor (and not call L<dup(2)>); this is more
4490 parsimonious of file descriptors. For example:
4492 # open for input, reusing the fileno of $fd
4493 open(my $fh, "<&=", $fd)
4497 open(my $fh, "<&=$fd")
4501 # open for append, using the fileno of $oldfh
4502 open(my $fh, ">>&=", $oldfh)
4504 Being parsimonious on filehandles is also useful (besides being
4505 parsimonious) for example when something is dependent on file
4506 descriptors, like for example locking using
4507 L<C<flock>|/flock FILEHANDLE,OPERATION>. If you do just
4508 C<< open(my $A, ">>&", $B) >>, the filehandle C<$A> will not have the
4509 same file descriptor as C<$B>, and therefore C<flock($A)> will not
4510 C<flock($B)> nor vice versa. But with C<< open(my $A, ">>&=", $B) >>,
4511 the filehandles will share the same underlying system file descriptor.
4513 Note that under Perls older than 5.8.0, Perl uses the standard C library's'
4514 L<fdopen(3)> to implement the C<=> functionality. On many Unix systems,
4515 L<fdopen(3)> fails when file descriptors exceed a certain value, typically 255.
4516 For Perls 5.8.0 and later, PerlIO is (most often) the default.
4518 You can see whether your Perl was built with PerlIO by running
4519 C<perl -V:useperlio>. If it says C<'define'>, you have PerlIO;
4520 otherwise you don't.
4522 If you open a pipe on the command C<-> (that is, specify either C<|-> or C<-|>
4523 with the one- or two-argument forms of
4524 L<C<open>|/open FILEHANDLE,EXPR>), an implicit L<C<fork>|/fork> is done,
4525 so L<C<open>|/open FILEHANDLE,EXPR> returns twice: in the parent process
4527 of the child process, and in the child process it returns (a defined) C<0>.
4528 Use C<defined($pid)> or C<//> to determine whether the open was successful.
4530 For example, use either
4532 my $child_pid = open(my $from_kid, "-|") // die "Can't fork: $!";
4536 my $child_pid = open(my $to_kid, "|-") // die "Can't fork: $!";
4542 # either write $to_kid or else read $from_kid
4544 waitpid $child_pid, 0;
4546 # am the child; use STDIN/STDOUT normally
4551 The filehandle behaves normally for the parent, but I/O to that
4552 filehandle is piped from/to the STDOUT/STDIN of the child process.
4553 In the child process, the filehandle isn't opened--I/O happens from/to
4554 the new STDOUT/STDIN. Typically this is used like the normal
4555 piped open when you want to exercise more control over just how the
4556 pipe command gets executed, such as when running setuid and
4557 you don't want to have to scan shell commands for metacharacters.
4559 The following blocks are more or less equivalent:
4561 open(my $fh, "|tr '[a-z]' '[A-Z]'");
4562 open(my $fh, "|-", "tr '[a-z]' '[A-Z]'");
4563 open(my $fh, "|-") || exec 'tr', '[a-z]', '[A-Z]';
4564 open(my $fh, "|-", "tr", '[a-z]', '[A-Z]');
4566 open(my $fh, "cat -n '$file'|");
4567 open(my $fh, "-|", "cat -n '$file'");
4568 open(my $fh, "-|") || exec "cat", "-n", $file;
4569 open(my $fh, "-|", "cat", "-n", $file);
4571 The last two examples in each block show the pipe as "list form", which is
4572 not yet supported on all platforms. A good rule of thumb is that if
4573 your platform has a real L<C<fork>|/fork> (in other words, if your platform is
4574 Unix, including Linux and MacOS X), you can use the list form. You would
4575 want to use the list form of the pipe so you can pass literal arguments
4576 to the command without risk of the shell interpreting any shell metacharacters
4577 in them. However, this also bars you from opening pipes to commands
4578 that intentionally contain shell metacharacters, such as:
4580 open(my $fh, "|cat -n | expand -4 | lpr")
4581 || die "Can't open pipeline to lpr: $!";
4583 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
4585 Perl will attempt to flush all files opened for
4586 output before any operation that may do a fork, but this may not be
4587 supported on some platforms (see L<perlport>). To be safe, you may need
4588 to set L<C<$E<verbar>>|perlvar/$E<verbar>> (C<$AUTOFLUSH> in L<English>)
4589 or call the C<autoflush> method of L<C<IO::Handle>|IO::Handle/METHODS>
4590 on any open handles.
4592 On systems that support a close-on-exec flag on files, the flag will
4593 be set for the newly opened file descriptor as determined by the value
4594 of L<C<$^F>|perlvar/$^F>. See L<perlvar/$^F>.
4596 Closing any piped filehandle causes the parent process to wait for the
4597 child to finish, then returns the status value in L<C<$?>|perlvar/$?> and
4598 L<C<${^CHILD_ERROR_NATIVE}>|perlvar/${^CHILD_ERROR_NATIVE}>.
4600 The filename passed to the one- and two-argument forms of
4601 L<C<open>|/open FILEHANDLE,EXPR> will
4602 have leading and trailing whitespace deleted and normal
4603 redirection characters honored. This property, known as "magic open",
4604 can often be used to good effect. A user could specify a filename of
4605 F<"rsh cat file |">, or you could change certain filenames as needed:
4607 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
4608 open(my $fh, $filename) or die "Can't open $filename: $!";
4610 Use the three-argument form to open a file with arbitrary weird characters in it,
4612 open(my $fh, "<", $file)
4613 || die "Can't open $file: $!";
4615 otherwise it's necessary to protect any leading and trailing whitespace:
4617 $file =~ s#^(\s)#./$1#;
4618 open(my $fh, "< $file\0")
4619 || die "Can't open $file: $!";
4621 (this may not work on some bizarre filesystems). One should
4622 conscientiously choose between the I<magic> and I<three-argument> form
4623 of L<C<open>|/open FILEHANDLE,EXPR>:
4625 open(my $in, $ARGV[0]) || die "Can't open $ARGV[0]: $!";
4627 will allow the user to specify an argument of the form C<"rsh cat file |">,
4628 but will not work on a filename that happens to have a trailing space, while
4630 open(my $in, "<", $ARGV[0])
4631 || die "Can't open $ARGV[0]: $!";
4633 will have exactly the opposite restrictions. (However, some shells
4634 support the syntax C<< perl your_program.pl <( rsh cat file ) >>, which
4635 produces a filename that can be opened normally.)
4637 If you want a "real" C L<open(2)>, then you should use the
4638 L<C<sysopen>|/sysopen FILEHANDLE,FILENAME,MODE> function, which involves
4639 no such magic (but uses different filemodes than Perl
4640 L<C<open>|/open FILEHANDLE,EXPR>, which corresponds to C L<fopen(3)>).
4641 This is another way to protect your filenames from interpretation. For
4645 sysopen(my $fh, $path, O_RDWR|O_CREAT|O_EXCL)
4646 or die "Can't open $path: $!";
4648 print $fh "stuff $$\n";
4650 print "File contains: ", readline($fh);
4652 See L<C<seek>|/seek FILEHANDLE,POSITION,WHENCE> for some details about
4653 mixing reading and writing.
4655 Portability issues: L<perlport/open>.
4657 =item opendir DIRHANDLE,EXPR
4660 =for Pod::Functions open a directory
4662 Opens a directory named EXPR for processing by
4663 L<C<readdir>|/readdir DIRHANDLE>, L<C<telldir>|/telldir DIRHANDLE>,
4664 L<C<seekdir>|/seekdir DIRHANDLE,POS>,
4665 L<C<rewinddir>|/rewinddir DIRHANDLE>, and
4666 L<C<closedir>|/closedir DIRHANDLE>. Returns true if successful.
4667 DIRHANDLE may be an expression whose value can be used as an indirect
4668 dirhandle, usually the real dirhandle name. If DIRHANDLE is an undefined
4669 scalar variable (or array or hash element), the variable is assigned a
4670 reference to a new anonymous dirhandle; that is, it's autovivified.
4671 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
4673 See the example at L<C<readdir>|/readdir DIRHANDLE>.
4680 =for Pod::Functions find a character's numeric representation
4682 Returns the numeric value of the first character of EXPR.
4683 If EXPR is an empty string, returns 0. If EXPR is omitted, uses
4684 L<C<$_>|perlvar/$_>.
4685 (Note I<character>, not byte.)
4687 For the reverse, see L<C<chr>|/chr NUMBER>.
4688 See L<perlunicode> for more about Unicode.
4693 =item our TYPE VARLIST
4695 =item our VARLIST : ATTRS
4697 =item our TYPE VARLIST : ATTRS
4699 =for Pod::Functions +5.6.0 declare and assign a package variable (lexical scoping)
4701 L<C<our>|/our VARLIST> makes a lexical alias to a package (i.e. global)
4702 variable of the same name in the current package for use within the
4703 current lexical scope.
4705 L<C<our>|/our VARLIST> has the same scoping rules as
4706 L<C<my>|/my VARLIST> or L<C<state>|/state VARLIST>, meaning that it is
4707 only valid within a lexical scope. Unlike L<C<my>|/my VARLIST> and
4708 L<C<state>|/state VARLIST>, which both declare new (lexical) variables,
4709 L<C<our>|/our VARLIST> only creates an alias to an existing variable: a
4710 package variable of the same name.
4712 This means that when C<use strict 'vars'> is in effect, L<C<our>|/our
4713 VARLIST> lets you use a package variable without qualifying it with the
4714 package name, but only within the lexical scope of the
4715 L<C<our>|/our VARLIST> declaration. This applies immediately--even
4716 within the same statement.
4724 our $foo; # alias to $Foo::foo
4725 print $foo; # prints 23
4728 print $Foo::foo; # prints 23
4730 print $foo; # ERROR: requires explicit package name
4732 This works even if the package variable has not been used before, as
4733 package variables spring into existence when first used.
4738 our $foo = 23; # just like $Foo::foo = 23
4740 print $Foo::foo; # prints 23
4742 Because the variable becomes legal immediately under C<use strict 'vars'>, so
4743 long as there is no variable with that name is already in scope, you can then
4744 reference the package variable again even within the same statement.
4749 my $foo = $foo; # error, undeclared $foo on right-hand side
4750 our $foo = $foo; # no errors
4752 If more than one variable is listed, the list must be placed
4757 An L<C<our>|/our VARLIST> declaration declares an alias for a package
4758 variable that will be visible
4759 across its entire lexical scope, even across package boundaries. The
4760 package in which the variable is entered is determined at the point
4761 of the declaration, not at the point of use. This means the following
4765 our $bar; # declares $Foo::bar for rest of lexical scope
4769 print $bar; # prints 20, as it refers to $Foo::bar
4771 Multiple L<C<our>|/our VARLIST> declarations with the same name in the
4773 scope are allowed if they are in different packages. If they happen
4774 to be in the same package, Perl will emit warnings if you have asked
4775 for them, just like multiple L<C<my>|/my VARLIST> declarations. Unlike
4776 a second L<C<my>|/my VARLIST> declaration, which will bind the name to a
4777 fresh variable, a second L<C<our>|/our VARLIST> declaration in the same
4778 package, in the same scope, is merely redundant.
4782 our $bar; # declares $Foo::bar for rest of lexical scope
4786 our $bar = 30; # declares $Bar::bar for rest of lexical scope
4787 print $bar; # prints 30
4789 our $bar; # emits warning but has no other effect
4790 print $bar; # still prints 30
4792 An L<C<our>|/our VARLIST> declaration may also have a list of attributes
4795 The exact semantics and interface of TYPE and ATTRS are still
4796 evolving. TYPE is currently bound to the use of the L<fields> pragma,
4797 and attributes are handled using the L<attributes> pragma, or, starting
4798 from Perl 5.8.0, also via the L<Attribute::Handlers> module. See
4799 L<perlsub/"Private Variables via my()"> for details.
4801 Note that with a parenthesised list, L<C<undef>|/undef EXPR> can be used
4802 as a dummy placeholder, for example to skip assignment of initial
4805 our ( undef, $min, $hour ) = localtime;
4807 L<C<our>|/our VARLIST> differs from L<C<use vars>|vars>, which allows
4808 use of an unqualified name I<only> within the affected package, but
4811 =item pack TEMPLATE,LIST
4814 =for Pod::Functions convert a list into a binary representation
4816 Takes a LIST of values and converts it into a string using the rules
4817 given by the TEMPLATE. The resulting string is the concatenation of
4818 the converted values. Typically, each converted value looks
4819 like its machine-level representation. For example, on 32-bit machines
4820 an integer may be represented by a sequence of 4 bytes, which will in
4821 Perl be presented as a string that's 4 characters long.
4823 See L<perlpacktut> for an introduction to this function.
4825 The TEMPLATE is a sequence of characters that give the order and type
4826 of values, as follows:
4828 a A string with arbitrary binary data, will be null padded.
4829 A A text (ASCII) string, will be space padded.
4830 Z A null-terminated (ASCIZ) string, will be null padded.
4832 b A bit string (ascending bit order inside each byte,
4834 B A bit string (descending bit order inside each byte).
4835 h A hex string (low nybble first).
4836 H A hex string (high nybble first).
4838 c A signed char (8-bit) value.
4839 C An unsigned char (octet) value.
4840 W An unsigned char value (can be greater than 255).
4842 s A signed short (16-bit) value.
4843 S An unsigned short value.
4845 l A signed long (32-bit) value.
4846 L An unsigned long value.
4848 q A signed quad (64-bit) value.
4849 Q An unsigned quad value.
4850 (Quads are available only if your system supports 64-bit
4851 integer values _and_ if Perl has been compiled to support
4852 those. Raises an exception otherwise.)
4854 i A signed integer value.
4855 I A unsigned integer value.
4856 (This 'integer' is _at_least_ 32 bits wide. Its exact
4857 size depends on what a local C compiler calls 'int'.)
4859 n An unsigned short (16-bit) in "network" (big-endian) order.
4860 N An unsigned long (32-bit) in "network" (big-endian) order.
4861 v An unsigned short (16-bit) in "VAX" (little-endian) order.
4862 V An unsigned long (32-bit) in "VAX" (little-endian) order.
4864 j A Perl internal signed integer value (IV).
4865 J A Perl internal unsigned integer value (UV).
4867 f A single-precision float in native format.
4868 d A double-precision float in native format.
4870 F A Perl internal floating-point value (NV) in native format
4871 D A float of long-double precision in native format.
4872 (Long doubles are available only if your system supports
4873 long double values _and_ if Perl has been compiled to
4874 support those. Raises an exception otherwise.
4875 Note that there are different long double formats.)
4877 p A pointer to a null-terminated string.
4878 P A pointer to a structure (fixed-length string).
4880 u A uuencoded string.
4881 U A Unicode character number. Encodes to a character in char-
4882 acter mode and UTF-8 (or UTF-EBCDIC in EBCDIC platforms) in
4885 w A BER compressed integer (not an ASN.1 BER, see perlpacktut
4886 for details). Its bytes represent an unsigned integer in
4887 base 128, most significant digit first, with as few digits
4888 as possible. Bit eight (the high bit) is set on each byte
4891 x A null byte (a.k.a ASCII NUL, "\000", chr(0))
4893 @ Null-fill or truncate to absolute position, counted from the
4894 start of the innermost ()-group.
4895 . Null-fill or truncate to absolute position specified by
4897 ( Start of a ()-group.
4899 One or more modifiers below may optionally follow certain letters in the
4900 TEMPLATE (the second column lists letters for which the modifier is valid):
4902 ! sSlLiI Forces native (short, long, int) sizes instead
4903 of fixed (16-/32-bit) sizes.
4905 ! xX Make x and X act as alignment commands.
4907 ! nNvV Treat integers as signed instead of unsigned.
4909 ! @. Specify position as byte offset in the internal
4910 representation of the packed string. Efficient
4913 > sSiIlLqQ Force big-endian byte-order on the type.
4914 jJfFdDpP (The "big end" touches the construct.)
4916 < sSiIlLqQ Force little-endian byte-order on the type.
4917 jJfFdDpP (The "little end" touches the construct.)
4919 The C<< > >> and C<< < >> modifiers can also be used on C<()> groups
4920 to force a particular byte-order on all components in that group,
4921 including all its subgroups.
4925 Larry recalls that the hex and bit string formats (H, h, B, b) were added to
4926 pack for processing data from NASA's Magellan probe. Magellan was in an
4927 elliptical orbit, using the antenna for the radar mapping when close to
4928 Venus and for communicating data back to Earth for the rest of the orbit.
4929 There were two transmission units, but one of these failed, and then the
4930 other developed a fault whereby it would randomly flip the sense of all the
4931 bits. It was easy to automatically detect complete records with the correct
4932 sense, and complete records with all the bits flipped. However, this didn't
4933 recover the records where the sense flipped midway. A colleague of Larry's
4934 was able to pretty much eyeball where the records flipped, so they wrote an
4935 editor named kybble (a pun on the dog food Kibbles 'n Bits) to enable him to
4936 manually correct the records and recover the data. For this purpose pack
4937 gained the hex and bit string format specifiers.
4939 git shows that they were added to perl 3.0 in patch #44 (Jan 1991, commit
4940 27e2fb84680b9cc1), but the patch description makes no mention of their
4941 addition, let alone the story behind them.
4945 The following rules apply:
4951 Each letter may optionally be followed by a number indicating the repeat
4952 count. A numeric repeat count may optionally be enclosed in brackets, as
4953 in C<pack("C[80]", @arr)>. The repeat count gobbles that many values from
4954 the LIST when used with all format types other than C<a>, C<A>, C<Z>, C<b>,
4955 C<B>, C<h>, C<H>, C<@>, C<.>, C<x>, C<X>, and C<P>, where it means
4956 something else, described below. Supplying a C<*> for the repeat count
4957 instead of a number means to use however many items are left, except for:
4963 C<@>, C<x>, and C<X>, where it is equivalent to C<0>.
4967 <.>, where it means relative to the start of the string.
4971 C<u>, where it is equivalent to 1 (or 45, which here is equivalent).
4975 One can replace a numeric repeat count with a template letter enclosed in
4976 brackets to use the packed byte length of the bracketed template for the
4979 For example, the template C<x[L]> skips as many bytes as in a packed long,
4980 and the template C<"$t X[$t] $t"> unpacks twice whatever $t (when
4981 variable-expanded) unpacks. If the template in brackets contains alignment
4982 commands (such as C<x![d]>), its packed length is calculated as if the
4983 start of the template had the maximal possible alignment.
4985 When used with C<Z>, a C<*> as the repeat count is guaranteed to add a
4986 trailing null byte, so the resulting string is always one byte longer than
4987 the byte length of the item itself.
4989 When used with C<@>, the repeat count represents an offset from the start
4990 of the innermost C<()> group.
4992 When used with C<.>, the repeat count determines the starting position to
4993 calculate the value offset as follows:
4999 If the repeat count is C<0>, it's relative to the current position.
5003 If the repeat count is C<*>, the offset is relative to the start of the
5008 And if it's an integer I<n>, the offset is relative to the start of the
5009 I<n>th innermost C<( )> group, or to the start of the string if I<n> is
5010 bigger then the group level.
5014 The repeat count for C<u> is interpreted as the maximal number of bytes
5015 to encode per line of output, with 0, 1 and 2 replaced by 45. The repeat
5016 count should not be more than 65.
5020 The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
5021 string of length count, padding with nulls or spaces as needed. When
5022 unpacking, C<A> strips trailing whitespace and nulls, C<Z> strips everything
5023 after the first null, and C<a> returns data with no stripping at all.
5025 If the value to pack is too long, the result is truncated. If it's too
5026 long and an explicit count is provided, C<Z> packs only C<$count-1> bytes,
5027 followed by a null byte. Thus C<Z> always packs a trailing null, except
5028 when the count is 0.
5032 Likewise, the C<b> and C<B> formats pack a string that's that many bits long.
5033 Each such format generates 1 bit of the result. These are typically followed
5034 by a repeat count like C<B8> or C<B64>.
5036 Each result bit is based on the least-significant bit of the corresponding
5037 input character, i.e., on C<ord($char)%2>. In particular, characters C<"0">
5038 and C<"1"> generate bits 0 and 1, as do characters C<"\000"> and C<"\001">.
5040 Starting from the beginning of the input string, each 8-tuple
5041 of characters is converted to 1 character of output. With format C<b>,
5042 the first character of the 8-tuple determines the least-significant bit of a
5043 character; with format C<B>, it determines the most-significant bit of
5046 If the length of the input string is not evenly divisible by 8, the
5047 remainder is packed as if the input string were padded by null characters
5048 at the end. Similarly during unpacking, "extra" bits are ignored.
5050 If the input string is longer than needed, remaining characters are ignored.
5052 A C<*> for the repeat count uses all characters of the input field.
5053 On unpacking, bits are converted to a string of C<0>s and C<1>s.
5057 The C<h> and C<H> formats pack a string that many nybbles (4-bit groups,
5058 representable as hexadecimal digits, C<"0".."9"> C<"a".."f">) long.
5060 For each such format, L<C<pack>|/pack TEMPLATE,LIST> generates 4 bits of result.
5061 With non-alphabetical characters, the result is based on the 4 least-significant
5062 bits of the input character, i.e., on C<ord($char)%16>. In particular,
5063 characters C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
5064 C<"\000"> and C<"\001">. For characters C<"a".."f"> and C<"A".."F">, the result
5065 is compatible with the usual hexadecimal digits, so that C<"a"> and
5066 C<"A"> both generate the nybble C<0xA==10>. Use only these specific hex
5067 characters with this format.
5069 Starting from the beginning of the template to
5070 L<C<pack>|/pack TEMPLATE,LIST>, each pair
5071 of characters is converted to 1 character of output. With format C<h>, the
5072 first character of the pair determines the least-significant nybble of the
5073 output character; with format C<H>, it determines the most-significant
5076 If the length of the input string is not even, it behaves as if padded by
5077 a null character at the end. Similarly, "extra" nybbles are ignored during
5080 If the input string is longer than needed, extra characters are ignored.
5082 A C<*> for the repeat count uses all characters of the input field. For
5083 L<C<unpack>|/unpack TEMPLATE,EXPR>, nybbles are converted to a string of
5088 The C<p> format packs a pointer to a null-terminated string. You are
5089 responsible for ensuring that the string is not a temporary value, as that
5090 could potentially get deallocated before you got around to using the packed
5091 result. The C<P> format packs a pointer to a structure of the size indicated
5092 by the length. A null pointer is created if the corresponding value for
5093 C<p> or C<P> is L<C<undef>|/undef EXPR>; similarly with
5094 L<C<unpack>|/unpack TEMPLATE,EXPR>, where a null pointer unpacks into
5095 L<C<undef>|/undef EXPR>.
5097 If your system has a strange pointer size--meaning a pointer is neither as
5098 big as an int nor as big as a long--it may not be possible to pack or
5099 unpack pointers in big- or little-endian byte order. Attempting to do
5100 so raises an exception.
5104 The C</> template character allows packing and unpacking of a sequence of
5105 items where the packed structure contains a packed item count followed by
5106 the packed items themselves. This is useful when the structure you're
5107 unpacking has encoded the sizes or repeat counts for some of its fields
5108 within the structure itself as separate fields.
5110 For L<C<pack>|/pack TEMPLATE,LIST>, you write
5111 I<length-item>C</>I<sequence-item>, and the
5112 I<length-item> describes how the length value is packed. Formats likely
5113 to be of most use are integer-packing ones like C<n> for Java strings,
5114 C<w> for ASN.1 or SNMP, and C<N> for Sun XDR.
5116 For L<C<pack>|/pack TEMPLATE,LIST>, I<sequence-item> may have a repeat
5117 count, in which case
5118 the minimum of that and the number of available items is used as the argument
5119 for I<length-item>. If it has no repeat count or uses a '*', the number
5120 of available items is used.
5122 For L<C<unpack>|/unpack TEMPLATE,EXPR>, an internal stack of integer
5123 arguments unpacked so far is
5124 used. You write C</>I<sequence-item> and the repeat count is obtained by
5125 popping off the last element from the stack. The I<sequence-item> must not
5126 have a repeat count.
5128 If I<sequence-item> refers to a string type (C<"A">, C<"a">, or C<"Z">),
5129 the I<length-item> is the string length, not the number of strings. With
5130 an explicit repeat count for pack, the packed string is adjusted to that
5131 length. For example:
5133 This code: gives this result:
5135 unpack("W/a", "\004Gurusamy") ("Guru")
5136 unpack("a3/A A*", "007 Bond J ") (" Bond", "J")
5137 unpack("a3 x2 /A A*", "007: Bond, J.") ("Bond, J", ".")
5139 pack("n/a* w/a","hello,","world") "\000\006hello,\005world"
5140 pack("a/W2", ord("a") .. ord("z")) "2ab"
5142 The I<length-item> is not returned explicitly from
5143 L<C<unpack>|/unpack TEMPLATE,EXPR>.
5145 Supplying a count to the I<length-item> format letter is only useful with
5146 C<A>, C<a>, or C<Z>. Packing with a I<length-item> of C<a> or C<Z> may
5147 introduce C<"\000"> characters, which Perl does not regard as legal in
5152 The integer types C<s>, C<S>, C<l>, and C<L> may be
5153 followed by a C<!> modifier to specify native shorts or
5154 longs. As shown in the example above, a bare C<l> means
5155 exactly 32 bits, although the native C<long> as seen by the local C compiler
5156 may be larger. This is mainly an issue on 64-bit platforms. You can
5157 see whether using C<!> makes any difference this way:
5159 printf "format s is %d, s! is %d\n",
5160 length pack("s"), length pack("s!");
5162 printf "format l is %d, l! is %d\n",
5163 length pack("l"), length pack("l!");
5166 C<i!> and C<I!> are also allowed, but only for completeness' sake:
5167 they are identical to C<i> and C<I>.
5169 The actual sizes (in bytes) of native shorts, ints, longs, and long
5170 longs on the platform where Perl was built are also available from
5173 $ perl -V:{short,int,long{,long}}size
5179 or programmatically via the L<C<Config>|Config> module:
5182 print $Config{shortsize}, "\n";
5183 print $Config{intsize}, "\n";
5184 print $Config{longsize}, "\n";
5185 print $Config{longlongsize}, "\n";
5187 C<$Config{longlongsize}> is undefined on systems without
5192 The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, C<L>, C<j>, and C<J> are
5193 inherently non-portable between processors and operating systems because
5194 they obey native byteorder and endianness. For example, a 4-byte integer
5195 0x12345678 (305419896 decimal) would be ordered natively (arranged in and
5196 handled by the CPU registers) into bytes as
5198 0x12 0x34 0x56 0x78 # big-endian
5199 0x78 0x56 0x34 0x12 # little-endian
5201 Basically, Intel and VAX CPUs are little-endian, while everybody else,
5202 including Motorola m68k/88k, PPC, Sparc, HP PA, Power, and Cray, are
5203 big-endian. Alpha and MIPS can be either: Digital/Compaq uses (well, used)
5204 them in little-endian mode, but SGI/Cray uses them in big-endian mode.
5206 The names I<big-endian> and I<little-endian> are comic references to the
5207 egg-eating habits of the little-endian Lilliputians and the big-endian
5208 Blefuscudians from the classic Jonathan Swift satire, I<Gulliver's Travels>.
5209 This entered computer lingo via the paper "On Holy Wars and a Plea for
5210 Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980.
5212 Some systems may have even weirder byte orders such as
5217 These are called mid-endian, middle-endian, mixed-endian, or just weird.
5219 You can determine your system endianness with this incantation:
5221 printf("%#02x ", $_) for unpack("W*", pack L=>0x12345678);
5223 The byteorder on the platform where Perl was built is also available
5227 print "$Config{byteorder}\n";
5229 or from the command line:
5233 Byteorders C<"1234"> and C<"12345678"> are little-endian; C<"4321">
5234 and C<"87654321"> are big-endian. Systems with multiarchitecture binaries
5235 will have C<"ffff">, signifying that static information doesn't work,
5236 one must use runtime probing.
5238 For portably packed integers, either use the formats C<n>, C<N>, C<v>,
5239 and C<V> or else use the C<< > >> and C<< < >> modifiers described
5240 immediately below. See also L<perlport>.
5244 Also floating point numbers have endianness. Usually (but not always)
5245 this agrees with the integer endianness. Even though most platforms
5246 these days use the IEEE 754 binary format, there are differences,
5247 especially if the long doubles are involved. You can see the
5248 C<Config> variables C<doublekind> and C<longdblkind> (also C<doublesize>,
5249 C<longdblsize>): the "kind" values are enums, unlike C<byteorder>.
5251 Portability-wise the best option is probably to keep to the IEEE 754
5252 64-bit doubles, and of agreed-upon endianness. Another possibility
5253 is the C<"%a">) format of L<C<printf>|/printf FILEHANDLE FORMAT, LIST>.
5257 Starting with Perl 5.10.0, integer and floating-point formats, along with
5258 the C<p> and C<P> formats and C<()> groups, may all be followed by the
5259 C<< > >> or C<< < >> endianness modifiers to respectively enforce big-
5260 or little-endian byte-order. These modifiers are especially useful
5261 given how C<n>, C<N>, C<v>, and C<V> don't cover signed integers,
5262 64-bit integers, or floating-point values.
5264 Here are some concerns to keep in mind when using an endianness modifier:
5270 Exchanging signed integers between different platforms works only
5271 when all platforms store them in the same format. Most platforms store
5272 signed integers in two's-complement notation, so usually this is not an issue.
5276 The C<< > >> or C<< < >> modifiers can only be used on floating-point
5277 formats on big- or little-endian machines. Otherwise, attempting to
5278 use them raises an exception.
5282 Forcing big- or little-endian byte-order on floating-point values for
5283 data exchange can work only if all platforms use the same
5284 binary representation such as IEEE floating-point. Even if all
5285 platforms are using IEEE, there may still be subtle differences. Being able
5286 to use C<< > >> or C<< < >> on floating-point values can be useful,
5287 but also dangerous if you don't know exactly what you're doing.
5288 It is not a general way to portably store floating-point values.
5292 When using C<< > >> or C<< < >> on a C<()> group, this affects
5293 all types inside the group that accept byte-order modifiers,
5294 including all subgroups. It is silently ignored for all other
5295 types. You are not allowed to override the byte-order within a group
5296 that already has a byte-order modifier suffix.
5302 Real numbers (floats and doubles) are in native machine format only.
5303 Due to the multiplicity of floating-point formats and the lack of a
5304 standard "network" representation for them, no facility for interchange has been
5305 made. This means that packed floating-point data written on one machine
5306 may not be readable on another, even if both use IEEE floating-point
5307 arithmetic (because the endianness of the memory representation is not part
5308 of the IEEE spec). See also L<perlport>.
5310 If you know I<exactly> what you're doing, you can use the C<< > >> or C<< < >>
5311 modifiers to force big- or little-endian byte-order on floating-point values.
5313 Because Perl uses doubles (or long doubles, if configured) internally for
5314 all numeric calculation, converting from double into float and thence
5315 to double again loses precision, so C<unpack("f", pack("f", $foo)>)
5316 will not in general equal $foo.
5320 Pack and unpack can operate in two modes: character mode (C<C0> mode) where
5321 the packed string is processed per character, and UTF-8 byte mode (C<U0> mode)
5322 where the packed string is processed in its UTF-8-encoded Unicode form on
5323 a byte-by-byte basis. Character mode is the default
5324 unless the format string starts with C<U>. You
5325 can always switch mode mid-format with an explicit
5326 C<C0> or C<U0> in the format. This mode remains in effect until the next
5327 mode change, or until the end of the C<()> group it (directly) applies to.
5329 Using C<C0> to get Unicode characters while using C<U0> to get I<non>-Unicode
5330 bytes is not necessarily obvious. Probably only the first of these
5333 $ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
5334 perl -CS -ne 'printf "%v04X\n", $_ for unpack("C0A*", $_)'
5336 $ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
5337 perl -CS -ne 'printf "%v02X\n", $_ for unpack("U0A*", $_)'
5339 $ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
5340 perl -C0 -ne 'printf "%v02X\n", $_ for unpack("C0A*", $_)'
5342 $ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
5343 perl -C0 -ne 'printf "%v02X\n", $_ for unpack("U0A*", $_)'
5344 C3.8E.C2.B1.C3.8F.C2.89
5346 Those examples also illustrate that you should not try to use
5347 L<C<pack>|/pack TEMPLATE,LIST>/L<C<unpack>|/unpack TEMPLATE,EXPR> as a
5348 substitute for the L<Encode> module.
5352 You must yourself do any alignment or padding by inserting, for example,
5353 enough C<"x">es while packing. There is no way for
5354 L<C<pack>|/pack TEMPLATE,LIST> and L<C<unpack>|/unpack TEMPLATE,EXPR>
5355 to know where characters are going to or coming from, so they
5356 handle their output and input as flat sequences of characters.
5360 A C<()> group is a sub-TEMPLATE enclosed in parentheses. A group may
5361 take a repeat count either as postfix, or for
5362 L<C<unpack>|/unpack TEMPLATE,EXPR>, also via the C</>
5363 template character. Within each repetition of a group, positioning with
5364 C<@> starts over at 0. Therefore, the result of
5366 pack("@1A((@2A)@3A)", qw[X Y Z])
5368 is the string C<"\0X\0\0YZ">.
5372 C<x> and C<X> accept the C<!> modifier to act as alignment commands: they
5373 jump forward or back to the closest position aligned at a multiple of C<count>
5374 characters. For example, to L<C<pack>|/pack TEMPLATE,LIST> or
5375 L<C<unpack>|/unpack TEMPLATE,EXPR> a C structure like
5378 char c; /* one signed, 8-bit character */
5383 one may need to use the template C<c x![d] d c[2]>. This assumes that
5384 doubles must be aligned to the size of double.
5386 For alignment commands, a C<count> of 0 is equivalent to a C<count> of 1;
5391 C<n>, C<N>, C<v> and C<V> accept the C<!> modifier to
5392 represent signed 16-/32-bit integers in big-/little-endian order.
5393 This is portable only when all platforms sharing packed data use the
5394 same binary representation for signed integers; for example, when all
5395 platforms use two's-complement representation.
5399 Comments can be embedded in a TEMPLATE using C<#> through the end of line.
5400 White space can separate pack codes from each other, but modifiers and
5401 repeat counts must follow immediately. Breaking complex templates into
5402 individual line-by-line components, suitably annotated, can do as much to
5403 improve legibility and maintainability of pack/unpack formats as C</x> can
5404 for complicated pattern matches.
5408 If TEMPLATE requires more arguments than L<C<pack>|/pack TEMPLATE,LIST>
5409 is given, L<C<pack>|/pack TEMPLATE,LIST>
5410 assumes additional C<""> arguments. If TEMPLATE requires fewer arguments
5411 than given, extra arguments are ignored.
5415 Attempting to pack the special floating point values C<Inf> and C<NaN>
5416 (infinity, also in negative, and not-a-number) into packed integer values
5417 (like C<"L">) is a fatal error. The reason for this is that there simply
5418 isn't any sensible mapping for these special values into integers.
5424 $foo = pack("WWWW",65,66,67,68);
5426 $foo = pack("W4",65,66,67,68);
5428 $foo = pack("W4",0x24b6,0x24b7,0x24b8,0x24b9);
5429 # same thing with Unicode circled letters.
5430 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
5431 # same thing with Unicode circled letters. You don't get the
5432 # UTF-8 bytes because the U at the start of the format caused
5433 # a switch to U0-mode, so the UTF-8 bytes get joined into
5435 $foo = pack("C0U4",0x24b6,0x24b7,0x24b8,0x24b9);
5436 # foo eq "\xe2\x92\xb6\xe2\x92\xb7\xe2\x92\xb8\xe2\x92\xb9"
5437 # This is the UTF-8 encoding of the string in the
5440 $foo = pack("ccxxcc",65,66,67,68);
5443 # NOTE: The examples above featuring "W" and "c" are true
5444 # only on ASCII and ASCII-derived systems such as ISO Latin 1
5445 # and UTF-8. On EBCDIC systems, the first example would be
5446 # $foo = pack("WWWW",193,194,195,196);
5448 $foo = pack("s2",1,2);
5449 # "\001\000\002\000" on little-endian
5450 # "\000\001\000\002" on big-endian
5452 $foo = pack("a4","abcd","x","y","z");
5455 $foo = pack("aaaa","abcd","x","y","z");
5458 $foo = pack("a14","abcdefg");
5459 # "abcdefg\0\0\0\0\0\0\0"
5461 $foo = pack("i9pl", gmtime);
5462 # a real struct tm (on my system anyway)
5464 $utmp_template = "Z8 Z8 Z16 L";
5465 $utmp = pack($utmp_template, @utmp1);
5466 # a struct utmp (BSDish)
5468 @utmp2 = unpack($utmp_template, $utmp);
5469 # "@utmp1" eq "@utmp2"
5472 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
5475 $foo = pack('sx2l', 12, 34);
5476 # short 12, two zero bytes padding, long 34
5477 $bar = pack('s@4l', 12, 34);
5478 # short 12, zero fill to position 4, long 34
5480 $baz = pack('s.l', 12, 4, 34);
5481 # short 12, zero fill to position 4, long 34
5483 $foo = pack('nN', 42, 4711);
5484 # pack big-endian 16- and 32-bit unsigned integers
5485 $foo = pack('S>L>', 42, 4711);
5487 $foo = pack('s<l<', -42, 4711);
5488 # pack little-endian 16- and 32-bit signed integers
5489 $foo = pack('(sl)<', -42, 4711);
5492 The same template may generally also be used in
5493 L<C<unpack>|/unpack TEMPLATE,EXPR>.
5495 =item package NAMESPACE
5497 =item package NAMESPACE VERSION
5498 X<package> X<module> X<namespace> X<version>
5500 =item package NAMESPACE BLOCK
5502 =item package NAMESPACE VERSION BLOCK
5503 X<package> X<module> X<namespace> X<version>
5505 =for Pod::Functions declare a separate global namespace
5507 Declares the BLOCK or the rest of the compilation unit as being in the
5508 given namespace. The scope of the package declaration is either the
5509 supplied code BLOCK or, in the absence of a BLOCK, from the declaration
5510 itself through the end of current scope (the enclosing block, file, or
5511 L<C<eval>|/eval EXPR>). That is, the forms without a BLOCK are
5512 operative through the end of the current scope, just like the
5513 L<C<my>|/my VARLIST>, L<C<state>|/state VARLIST>, and
5514 L<C<our>|/our VARLIST> operators. All unqualified dynamic identifiers
5515 in this scope will be in the given namespace, except where overridden by
5516 another L<C<package>|/package NAMESPACE> declaration or
5517 when they're one of the special identifiers that qualify into C<main::>,
5518 like C<STDOUT>, C<ARGV>, C<ENV>, and the punctuation variables.
5520 A package statement affects dynamic variables only, including those
5521 you've used L<C<local>|/local EXPR> on, but I<not> lexically-scoped
5522 variables, which are created with L<C<my>|/my VARLIST>,
5523 L<C<state>|/state VARLIST>, or L<C<our>|/our VARLIST>. Typically it
5524 would be the first declaration in a file included by
5525 L<C<require>|/require VERSION> or L<C<use>|/use Module VERSION LIST>.
5526 You can switch into a
5527 package in more than one place, since this only determines which default
5528 symbol table the compiler uses for the rest of that block. You can refer to
5529 identifiers in other packages than the current one by prefixing the identifier
5530 with the package name and a double colon, as in C<$SomePack::var>
5531 or C<ThatPack::INPUT_HANDLE>. If package name is omitted, the C<main>
5532 package as assumed. That is, C<$::sail> is equivalent to
5533 C<$main::sail> (as well as to C<$main'sail>, still seen in ancient
5534 code, mostly from Perl 4).
5536 If VERSION is provided, L<C<package>|/package NAMESPACE> sets the
5537 C<$VERSION> variable in the given
5538 namespace to a L<version> object with the VERSION provided. VERSION must be a
5539 "strict" style version number as defined by the L<version> module: a positive
5540 decimal number (integer or decimal-fraction) without exponentiation or else a
5541 dotted-decimal v-string with a leading 'v' character and at least three
5542 components. You should set C<$VERSION> only once per package.
5544 See L<perlmod/"Packages"> for more information about packages, modules,
5545 and classes. See L<perlsub> for other scoping issues.
5550 =for Pod::Functions +5.004 the current package
5552 A special token that returns the name of the package in which it occurs.
5554 =item pipe READHANDLE,WRITEHANDLE
5557 =for Pod::Functions open a pair of connected filehandles
5559 Opens a pair of connected pipes like the corresponding system call.
5560 Note that if you set up a loop of piped processes, deadlock can occur
5561 unless you are very careful. In addition, note that Perl's pipes use
5562 IO buffering, so you may need to set L<C<$E<verbar>>|perlvar/$E<verbar>>
5563 to flush your WRITEHANDLE after each command, depending on the
5566 Returns true on success.
5568 See L<IPC::Open2>, L<IPC::Open3>, and
5569 L<perlipc/"Bidirectional Communication with Another Process">
5570 for examples of such things.
5572 On systems that support a close-on-exec flag on files, that flag is set
5573 on all newly opened file descriptors whose
5574 L<C<fileno>|/fileno FILEHANDLE>s are I<higher> than the current value of
5575 L<C<$^F>|perlvar/$^F> (by default 2 for C<STDERR>). See L<perlvar/$^F>.
5582 =for Pod::Functions remove the last element from an array and return it
5584 Pops and returns the last value of the array, shortening the array by
5587 Returns the undefined value if the array is empty, although this may
5588 also happen at other times. If ARRAY is omitted, pops the
5589 L<C<@ARGV>|perlvar/@ARGV> array in the main program, but the
5590 L<C<@_>|perlvar/@_> array in subroutines, just like
5591 L<C<shift>|/shift ARRAY>.
5593 Starting with Perl 5.14, an experimental feature allowed
5594 L<C<pop>|/pop ARRAY> to take a
5595 scalar expression. This experiment has been deemed unsuccessful, and was
5596 removed as of Perl 5.24.
5599 X<pos> X<match, position>
5603 =for Pod::Functions find or set the offset for the last/next m//g search
5605 Returns the offset of where the last C<m//g> search left off for the
5606 variable in question (L<C<$_>|perlvar/$_> is used when the variable is not
5607 specified). This offset is in characters unless the
5608 (no-longer-recommended) L<C<use bytes>|bytes> pragma is in effect, in
5609 which case the offset is in bytes. Note that 0 is a valid match offset.
5610 L<C<undef>|/undef EXPR> indicates
5611 that the search position is reset (usually due to match failure, but
5612 can also be because no match has yet been run on the scalar).
5614 L<C<pos>|/pos SCALAR> directly accesses the location used by the regexp
5615 engine to store the offset, so assigning to L<C<pos>|/pos SCALAR> will
5616 change that offset, and so will also influence the C<\G> zero-width
5617 assertion in regular expressions. Both of these effects take place for
5618 the next match, so you can't affect the position with
5619 L<C<pos>|/pos SCALAR> during the current match, such as in
5620 C<(?{pos() = 5})> or C<s//pos() = 5/e>.
5622 Setting L<C<pos>|/pos SCALAR> also resets the I<matched with
5623 zero-length> flag, described
5624 under L<perlre/"Repeated Patterns Matching a Zero-length Substring">.
5626 Because a failed C<m//gc> match doesn't reset the offset, the return
5627 from L<C<pos>|/pos SCALAR> won't change either in this case. See
5628 L<perlre> and L<perlop>.
5630 =item print FILEHANDLE LIST
5633 =item print FILEHANDLE
5639 =for Pod::Functions output a list to a filehandle
5641 Prints a string or a list of strings. Returns true if successful.
5642 FILEHANDLE may be a scalar variable containing the name of or a reference
5643 to the filehandle, thus introducing one level of indirection. (NOTE: If
5644 FILEHANDLE is a variable and the next token is a term, it may be
5645 misinterpreted as an operator unless you interpose a C<+> or put
5646 parentheses around the arguments.) If FILEHANDLE is omitted, prints to the
5647 last selected (see L<C<select>|/select FILEHANDLE>) output handle. If
5648 LIST is omitted, prints L<C<$_>|perlvar/$_> to the currently selected
5649 output handle. To use FILEHANDLE alone to print the content of
5650 L<C<$_>|perlvar/$_> to it, you must use a bareword filehandle like
5651 C<FH>, not an indirect one like C<$fh>. To set the default output handle
5652 to something other than STDOUT, use the select operation.
5654 The current value of L<C<$,>|perlvar/$,> (if any) is printed between
5655 each LIST item. The current value of L<C<$\>|perlvar/$\> (if any) is
5656 printed after the entire LIST has been printed. Because print takes a
5657 LIST, anything in the LIST is evaluated in list context, including any
5658 subroutines whose return lists you pass to
5659 L<C<print>|/print FILEHANDLE LIST>. Be careful not to follow the print
5661 parenthesis unless you want the corresponding right parenthesis to
5662 terminate the arguments to the print; put parentheses around all arguments
5663 (or interpose a C<+>, but that doesn't look as good).
5665 If you're storing handles in an array or hash, or in general whenever
5666 you're using any expression more complex than a bareword handle or a plain,
5667 unsubscripted scalar variable to retrieve it, you will have to use a block
5668 returning the filehandle value instead, in which case the LIST may not be
5671 print { $files[$i] } "stuff\n";
5672 print { $OK ? STDOUT : STDERR } "stuff\n";
5674 Printing to a closed pipe or socket will generate a SIGPIPE signal. See
5675 L<perlipc> for more on signal handling.
5677 =item printf FILEHANDLE FORMAT, LIST
5680 =item printf FILEHANDLE
5682 =item printf FORMAT, LIST
5686 =for Pod::Functions output a formatted list to a filehandle
5688 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that
5689 L<C<$\>|perlvar/$\> (the output record separator) is not appended. The
5690 FORMAT and the LIST are actually parsed as a single list. The first
5691 argument of the list will be interpreted as the
5692 L<C<printf>|/printf FILEHANDLE FORMAT, LIST> format. This means that
5693 C<printf(@_)> will use C<$_[0]> as the format. See
5694 L<sprintf|/sprintf FORMAT, LIST> for an explanation of the format
5695 argument. If C<use locale> (including C<use locale ':not_characters'>)
5696 is in effect and L<C<POSIX::setlocale>|POSIX/C<setlocale>> has been
5697 called, the character used for the decimal separator in formatted
5698 floating-point numbers is affected by the C<LC_NUMERIC> locale setting.
5699 See L<perllocale> and L<POSIX>.
5701 For historical reasons, if you omit the list, L<C<$_>|perlvar/$_> is
5703 to use FILEHANDLE without a list, you must use a bareword filehandle like
5704 C<FH>, not an indirect one like C<$fh>. However, this will rarely do what
5705 you want; if L<C<$_>|perlvar/$_> contains formatting codes, they will be
5706 replaced with the empty string and a warning will be emitted if
5707 L<warnings> are enabled. Just use L<C<print>|/print FILEHANDLE LIST> if
5708 you want to print the contents of L<C<$_>|perlvar/$_>.
5710 Don't fall into the trap of using a
5711 L<C<printf>|/printf FILEHANDLE FORMAT, LIST> when a simple
5712 L<C<print>|/print FILEHANDLE LIST> would do. The
5713 L<C<print>|/print FILEHANDLE LIST> is more efficient and less error
5716 =item prototype FUNCTION
5721 =for Pod::Functions +5.002 get the prototype (if any) of a subroutine
5723 Returns the prototype of a function as a string (or
5724 L<C<undef>|/undef EXPR> if the
5725 function has no prototype). FUNCTION is a reference to, or the name of,
5726 the function whose prototype you want to retrieve. If FUNCTION is omitted,
5727 L<C<$_>|perlvar/$_> is used.
5729 If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
5730 name for a Perl builtin. If the builtin's arguments
5731 cannot be adequately expressed by a prototype
5732 (such as L<C<system>|/system LIST>), L<C<prototype>|/prototype FUNCTION>
5733 returns L<C<undef>|/undef EXPR>, because the builtin
5734 does not really behave like a Perl function. Otherwise, the string
5735 describing the equivalent prototype is returned.
5737 =item push ARRAY,LIST
5740 =for Pod::Functions append one or more elements to an array
5742 Treats ARRAY as a stack by appending the values of LIST to the end of
5743 ARRAY. The length of ARRAY increases by the length of LIST. Has the same
5746 for my $value (LIST) {
5747 $ARRAY[++$#ARRAY] = $value;
5750 but is more efficient. Returns the number of elements in the array following
5751 the completed L<C<push>|/push ARRAY,LIST>.
5753 Starting with Perl 5.14, an experimental feature allowed
5754 L<C<push>|/push ARRAY,LIST> to take a
5755 scalar expression. This experiment has been deemed unsuccessful, and was
5756 removed as of Perl 5.24.
5760 =for Pod::Functions singly quote a string
5764 =for Pod::Functions doubly quote a string
5768 =for Pod::Functions quote a list of words
5772 =for Pod::Functions backquote quote a string
5774 Generalized quotes. See L<perlop/"Quote-Like Operators">.
5778 =for Pod::Functions +5.005 compile pattern
5780 Regexp-like quote. See L<perlop/"Regexp Quote-Like Operators">.
5782 =item quotemeta EXPR
5783 X<quotemeta> X<metacharacter>
5787 =for Pod::Functions quote regular expression magic characters
5789 Returns the value of EXPR with all the ASCII non-"word"
5790 characters backslashed. (That is, all ASCII characters not matching
5791 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
5792 returned string, regardless of any locale settings.)
5793 This is the internal function implementing
5794 the C<\Q> escape in double-quoted strings.
5795 (See below for the behavior on non-ASCII code points.)
5797 If EXPR is omitted, uses L<C<$_>|perlvar/$_>.
5799 quotemeta (and C<\Q> ... C<\E>) are useful when interpolating strings into
5800 regular expressions, because by default an interpolated variable will be
5801 considered a mini-regular expression. For example:
5803 my $sentence = 'The quick brown fox jumped over the lazy dog';
5804 my $substring = 'quick.*?fox';
5805 $sentence =~ s{$substring}{big bad wolf};
5807 Will cause C<$sentence> to become C<'The big bad wolf jumped over...'>.
5811 my $sentence = 'The quick brown fox jumped over the lazy dog';
5812 my $substring = 'quick.*?fox';
5813 $sentence =~ s{\Q$substring\E}{big bad wolf};
5817 my $sentence = 'The quick brown fox jumped over the lazy dog';
5818 my $substring = 'quick.*?fox';
5819 my $quoted_substring = quotemeta($substring);
5820 $sentence =~ s{$quoted_substring}{big bad wolf};
5822 Will both leave the sentence as is.
5823 Normally, when accepting literal string input from the user,
5824 L<C<quotemeta>|/quotemeta EXPR> or C<\Q> must be used.
5826 In Perl v5.14, all non-ASCII characters are quoted in non-UTF-8-encoded
5827 strings, but not quoted in UTF-8 strings.
5829 Starting in Perl v5.16, Perl adopted a Unicode-defined strategy for
5830 quoting non-ASCII characters; the quoting of ASCII characters is
5833 Also unchanged is the quoting of non-UTF-8 strings when outside the
5835 L<C<use feature 'unicode_strings'>|feature/The 'unicode_strings' feature>,
5836 which is to quote all
5837 characters in the upper Latin1 range. This provides complete backwards
5838 compatibility for old programs which do not use Unicode. (Note that
5839 C<unicode_strings> is automatically enabled within the scope of a
5840 S<C<use v5.12>> or greater.)
5842 Within the scope of L<C<use locale>|locale>, all non-ASCII Latin1 code
5844 are quoted whether the string is encoded as UTF-8 or not. As mentioned
5845 above, locale does not affect the quoting of ASCII-range characters.
5846 This protects against those locales where characters such as C<"|"> are
5847 considered to be word characters.
5849 Otherwise, Perl quotes non-ASCII characters using an adaptation from
5850 Unicode (see L<http://www.unicode.org/reports/tr31/>).
5851 The only code points that are quoted are those that have any of the
5852 Unicode properties: Pattern_Syntax, Pattern_White_Space, White_Space,
5853 Default_Ignorable_Code_Point, or General_Category=Control.
5855 Of these properties, the two important ones are Pattern_Syntax and
5856 Pattern_White_Space. They have been set up by Unicode for exactly this
5857 purpose of deciding which characters in a regular expression pattern
5858 should be quoted. No character that can be in an identifier has these
5861 Perl promises, that if we ever add regular expression pattern
5862 metacharacters to the dozen already defined
5863 (C<\ E<verbar> ( ) [ { ^ $ * + ? .>), that we will only use ones that have the
5864 Pattern_Syntax property. Perl also promises, that if we ever add
5865 characters that are considered to be white space in regular expressions
5866 (currently mostly affected by C</x>), they will all have the
5867 Pattern_White_Space property.
5869 Unicode promises that the set of code points that have these two
5870 properties will never change, so something that is not quoted in v5.16
5871 will never need to be quoted in any future Perl release. (Not all the
5872 code points that match Pattern_Syntax have actually had characters
5873 assigned to them; so there is room to grow, but they are quoted
5874 whether assigned or not. Perl, of course, would never use an
5875 unassigned code point as an actual metacharacter.)
5877 Quoting characters that have the other 3 properties is done to enhance
5878 the readability of the regular expression and not because they actually
5879 need to be quoted for regular expression purposes (characters with the
5880 White_Space property are likely to be indistinguishable on the page or
5881 screen from those with the Pattern_White_Space property; and the other
5882 two properties contain non-printing characters).
5889 =for Pod::Functions retrieve the next pseudorandom number
5891 Returns a random fractional number greater than or equal to C<0> and less
5892 than the value of EXPR. (EXPR should be positive.) If EXPR is
5893 omitted, the value C<1> is used. Currently EXPR with the value C<0> is
5894 also special-cased as C<1> (this was undocumented before Perl 5.8.0
5895 and is subject to change in future versions of Perl). Automatically calls
5896 L<C<srand>|/srand EXPR> unless L<C<srand>|/srand EXPR> has already been
5897 called. See also L<C<srand>|/srand EXPR>.
5899 Apply L<C<int>|/int EXPR> to the value returned by L<C<rand>|/rand EXPR>
5900 if you want random integers instead of random fractional numbers. For
5905 returns a random integer between C<0> and C<9>, inclusive.
5907 (Note: If your rand function consistently returns numbers that are too
5908 large or too small, then your version of Perl was probably compiled
5909 with the wrong number of RANDBITS.)
5911 B<L<C<rand>|/rand EXPR> is not cryptographically secure. You should not rely
5912 on it in security-sensitive situations.> As of this writing, a
5913 number of third-party CPAN modules offer random number generators
5914 intended by their authors to be cryptographically secure,
5915 including: L<Data::Entropy>, L<Crypt::Random>, L<Math::Random::Secure>,
5916 and L<Math::TrulyRandom>.
5918 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
5919 X<read> X<file, read>
5921 =item read FILEHANDLE,SCALAR,LENGTH
5923 =for Pod::Functions fixed-length buffered input from a filehandle
5925 Attempts to read LENGTH I<characters> of data into variable SCALAR
5926 from the specified FILEHANDLE. Returns the number of characters
5927 actually read, C<0> at end of file, or undef if there was an error (in
5928 the latter case L<C<$!>|perlvar/$!> is also set). SCALAR will be grown
5930 so that the last character actually read is the last character of the
5931 scalar after the read.
5933 An OFFSET may be specified to place the read data at some place in the
5934 string other than the beginning. A negative OFFSET specifies
5935 placement at that many characters counting backwards from the end of
5936 the string. A positive OFFSET greater than the length of SCALAR
5937 results in the string being padded to the required size with C<"\0">
5938 bytes before the result of the read is appended.
5940 The call is implemented in terms of either Perl's or your system's native
5941 L<fread(3)> library function. To get a true L<read(2)> system call, see
5942 L<sysread|/sysread FILEHANDLE,SCALAR,LENGTH,OFFSET>.
5944 Note the I<characters>: depending on the status of the filehandle,
5945 either (8-bit) bytes or characters are read. By default, all
5946 filehandles operate on bytes, but for example if the filehandle has
5947 been opened with the C<:utf8> I/O layer (see
5948 L<C<open>|/open FILEHANDLE,EXPR>, and the L<open>
5949 pragma), the I/O will operate on UTF8-encoded Unicode
5950 characters, not bytes. Similarly for the C<:encoding> layer:
5951 in that case pretty much any characters can be read.
5953 =item readdir DIRHANDLE
5956 =for Pod::Functions get a directory from a directory handle
5958 Returns the next directory entry for a directory opened by
5959 L<C<opendir>|/opendir DIRHANDLE,EXPR>.
5960 If used in list context, returns all the rest of the entries in the
5961 directory. If there are no more entries, returns the undefined value in
5962 scalar context and the empty list in list context.
5964 If you're planning to filetest the return values out of a
5965 L<C<readdir>|/readdir DIRHANDLE>, you'd better prepend the directory in
5966 question. Otherwise, because we didn't L<C<chdir>|/chdir EXPR> there,
5967 it would have been testing the wrong file.
5969 opendir(my $dh, $some_dir) || die "Can't opendir $some_dir: $!";
5970 my @dots = grep { /^\./ && -f "$some_dir/$_" } readdir($dh);
5973 As of Perl 5.12 you can use a bare L<C<readdir>|/readdir DIRHANDLE> in a
5974 C<while> loop, which will set L<C<$_>|perlvar/$_> on every iteration.
5976 opendir(my $dh, $some_dir) || die "Can't open $some_dir: $!";
5977 while (readdir $dh) {
5978 print "$some_dir/$_\n";
5982 To avoid confusing would-be users of your code who are running earlier
5983 versions of Perl with mysterious failures, put this sort of thing at the
5984 top of your file to signal that your code will work I<only> on Perls of a
5987 use 5.012; # so readdir assigns to $_ in a lone while test
5992 X<readline> X<gets> X<fgets>
5994 =for Pod::Functions fetch a record from a file
5996 Reads from the filehandle whose typeglob is contained in EXPR (or from
5997 C<*ARGV> if EXPR is not provided). In scalar context, each call reads and
5998 returns the next line until end-of-file is reached, whereupon the
5999 subsequent call returns L<C<undef>|/undef EXPR>. In list context, reads
6000 until end-of-file is reached and returns a list of lines. Note that the
6001 notion of "line" used here is whatever you may have defined with
6002 L<C<$E<sol>>|perlvar/$E<sol>> (or C<$INPUT_RECORD_SEPARATOR> in
6003 L<English>). See L<perlvar/"$/">.
6005 When L<C<$E<sol>>|perlvar/$E<sol>> is set to L<C<undef>|/undef EXPR>,
6006 when L<C<readline>|/readline EXPR> is in scalar context (i.e., file
6007 slurp mode), and when an empty file is read, it returns C<''> the first
6008 time, followed by L<C<undef>|/undef EXPR> subsequently.
6010 This is the internal function implementing the C<< <EXPR> >>
6011 operator, but you can use it directly. The C<< <EXPR> >>
6012 operator is discussed in more detail in L<perlop/"I/O Operators">.
6015 my $line = readline(STDIN); # same thing
6017 If L<C<readline>|/readline EXPR> encounters an operating system error,
6018 L<C<$!>|perlvar/$!> will be set with the corresponding error message.
6019 It can be helpful to check L<C<$!>|perlvar/$!> when you are reading from
6020 filehandles you don't trust, such as a tty or a socket. The following
6021 example uses the operator form of L<C<readline>|/readline EXPR> and dies
6022 if the result is not defined.
6024 while ( ! eof($fh) ) {
6025 defined( $_ = readline $fh ) or die "readline failed: $!";
6029 Note that you have can't handle L<C<readline>|/readline EXPR> errors
6030 that way with the C<ARGV> filehandle. In that case, you have to open
6031 each element of L<C<@ARGV>|perlvar/@ARGV> yourself since
6032 L<C<eof>|/eof FILEHANDLE> handles C<ARGV> differently.
6034 foreach my $arg (@ARGV) {
6035 open(my $fh, $arg) or warn "Can't open $arg: $!";
6037 while ( ! eof($fh) ) {
6038 defined( $_ = readline $fh )
6039 or die "readline failed for $arg: $!";
6049 =for Pod::Functions determine where a symbolic link is pointing
6051 Returns the value of a symbolic link, if symbolic links are
6052 implemented. If not, raises an exception. If there is a system
6053 error, returns the undefined value and sets L<C<$!>|perlvar/$!> (errno).
6054 If EXPR is omitted, uses L<C<$_>|perlvar/$_>.
6056 Portability issues: L<perlport/readlink>.
6063 =for Pod::Functions execute a system command and collect standard output
6065 EXPR is executed as a system command.
6066 The collected standard output of the command is returned.
6067 In scalar context, it comes back as a single (potentially
6068 multi-line) string. In list context, returns a list of lines
6069 (however you've defined lines with L<C<$E<sol>>|perlvar/$E<sol>> (or
6070 C<$INPUT_RECORD_SEPARATOR> in L<English>)).
6071 This is the internal function implementing the C<qx/EXPR/>
6072 operator, but you can use it directly. The C<qx/EXPR/>
6073 operator is discussed in more detail in L<perlop/"I/O Operators">.
6074 If EXPR is omitted, uses L<C<$_>|perlvar/$_>.
6076 =item recv SOCKET,SCALAR,LENGTH,FLAGS
6079 =for Pod::Functions receive a message over a Socket
6081 Receives a message on a socket. Attempts to receive LENGTH characters
6082 of data into variable SCALAR from the specified SOCKET filehandle.
6083 SCALAR will be grown or shrunk to the length actually read. Takes the
6084 same flags as the system call of the same name. Returns the address
6085 of the sender if SOCKET's protocol supports this; returns an empty
6086 string otherwise. If there's an error, returns the undefined value.
6087 This call is actually implemented in terms of the L<recvfrom(2)> system call.
6088 See L<perlipc/"UDP: Message Passing"> for examples.
6090 Note the I<characters>: depending on the status of the socket, either
6091 (8-bit) bytes or characters are received. By default all sockets
6092 operate on bytes, but for example if the socket has been changed using
6093 L<C<binmode>|/binmode FILEHANDLE, LAYER> to operate with the
6094 C<:encoding(utf8)> I/O layer (see the L<open> pragma), the I/O will
6095 operate on UTF8-encoded Unicode
6096 characters, not bytes. Similarly for the C<:encoding> layer: in that
6097 case pretty much any characters can be read.
6106 =for Pod::Functions start this loop iteration over again
6108 The L<C<redo>|/redo LABEL> command restarts the loop block without
6109 evaluating the conditional again. The L<C<continue>|/continue BLOCK>
6110 block, if any, is not executed. If
6111 the LABEL is omitted, the command refers to the innermost enclosing
6112 loop. The C<redo EXPR> form, available starting in Perl 5.18.0, allows a
6113 label name to be computed at run time, and is otherwise identical to C<redo
6114 LABEL>. Programs that want to lie to themselves about what was just input
6115 normally use this command:
6117 # a simpleminded Pascal comment stripper
6118 # (warning: assumes no { or } in strings)
6119 LINE: while (<STDIN>) {
6120 while (s|({.*}.*){.*}|$1 |) {}
6125 if (/}/) { # end of comment?
6134 L<C<redo>|/redo LABEL> cannot be used to retry a block that returns a
6135 value such as C<eval {}>, C<sub {}>, or C<do {}>, and should not be used
6136 to exit a L<C<grep>|/grep BLOCK LIST> or L<C<map>|/map BLOCK LIST>
6139 Note that a block by itself is semantically identical to a loop
6140 that executes once. Thus L<C<redo>|/redo LABEL> inside such a block
6141 will effectively turn it into a looping construct.
6143 See also L<C<continue>|/continue BLOCK> for an illustration of how
6144 L<C<last>|/last LABEL>, L<C<next>|/next LABEL>, and
6145 L<C<redo>|/redo LABEL> work.
6147 Unlike most named operators, this has the same precedence as assignment.
6148 It is also exempt from the looks-like-a-function rule, so
6149 C<redo ("foo")."bar"> will cause "bar" to be part of the argument to
6150 L<C<redo>|/redo LABEL>.
6157 =for Pod::Functions find out the type of thing being referenced
6159 Returns a non-empty string if EXPR is a reference, the empty
6160 string otherwise. If EXPR is not specified, L<C<$_>|perlvar/$_> will be
6161 used. The value returned depends on the type of thing the reference is
6164 Builtin types include:
6178 You can think of L<C<ref>|/ref EXPR> as a C<typeof> operator.
6180 if (ref($r) eq "HASH") {
6181 print "r is a reference to a hash.\n";
6184 print "r is not a reference at all.\n";
6187 The return value C<LVALUE> indicates a reference to an lvalue that is not
6188 a variable. You get this from taking the reference of function calls like
6189 L<C<pos>|/pos SCALAR> or
6190 L<C<substr>|/substr EXPR,OFFSET,LENGTH,REPLACEMENT>. C<VSTRING> is
6191 returned if the reference points to a
6192 L<version string|perldata/"Version Strings">.
6194 The result C<Regexp> indicates that the argument is a regular expression
6195 resulting from L<C<qrE<sol>E<sol>>|/qrE<sol>STRINGE<sol>>.
6197 If the referenced object has been blessed into a package, then that package
6198 name is returned instead. But don't use that, as it's now considered
6199 "bad practice". For one reason, an object could be using a class called
6200 C<Regexp> or C<IO>, or even C<HASH>. Also, L<C<ref>|/ref EXPR> doesn't
6201 take into account subclasses, like
6202 L<C<isa>|UNIVERSAL/C<< $obj->isa( TYPE ) >>> does.
6204 Instead, use L<C<blessed>|Scalar::Util/blessed> (in the L<Scalar::Util>
6205 module) for boolean checks, L<C<isa>|UNIVERSAL/C<< $obj->isa( TYPE ) >>>
6206 for specific class checks and L<C<reftype>|Scalar::Util/reftype> (also
6207 from L<Scalar::Util>) for type checks. (See L<perlobj> for details and
6208 a L<C<blessed>|Scalar::Util/blessed>/L<C<isa>|UNIVERSAL/C<< $obj->isa( TYPE ) >>>
6211 See also L<perlref>.
6213 =item rename OLDNAME,NEWNAME
6214 X<rename> X<move> X<mv> X<ren>
6216 =for Pod::Functions change a filename
6218 Changes the name of a file; an existing file NEWNAME will be
6219 clobbered. Returns true for success, false otherwise.
6221 Behavior of this function varies wildly depending on your system
6222 implementation. For example, it will usually not work across file system
6223 boundaries, even though the system I<mv> command sometimes compensates
6224 for this. Other restrictions include whether it works on directories,
6225 open files, or pre-existing files. Check L<perlport> and either the
6226 L<rename(2)> manpage or equivalent system documentation for details.
6228 For a platform independent L<C<move>|File::Copy/move> function look at
6229 the L<File::Copy> module.
6231 Portability issues: L<perlport/rename>.
6233 =item require VERSION
6240 =for Pod::Functions load in external functions from a library at runtime
6242 Demands a version of Perl specified by VERSION, or demands some semantics
6243 specified by EXPR or by L<C<$_>|perlvar/$_> if EXPR is not supplied.
6245 VERSION may be either a numeric argument such as 5.006, which will be
6246 compared to L<C<$]>|perlvar/$]>, or a literal of the form v5.6.1, which
6247 will be compared to L<C<$^V>|perlvar/$^V> (or C<$PERL_VERSION> in
6248 L<English>). An exception is raised if VERSION is greater than the
6249 version of the current Perl interpreter. Compare with
6250 L<C<use>|/use Module VERSION LIST>, which can do a similar check at
6253 Specifying VERSION as a literal of the form v5.6.1 should generally be
6254 avoided, because it leads to misleading error messages under earlier
6255 versions of Perl that do not support this syntax. The equivalent numeric
6256 version should be used instead.
6258 require v5.6.1; # run time version check
6259 require 5.6.1; # ditto
6260 require 5.006_001; # ditto; preferred for backwards
6263 Otherwise, L<C<require>|/require VERSION> demands that a library file be
6264 included if it hasn't already been included. The file is included via
6265 the do-FILE mechanism, which is essentially just a variety of
6266 L<C<eval>|/eval EXPR> with the
6267 caveat that lexical variables in the invoking script will be invisible
6268 to the included code. If it were implemented in pure Perl, it
6269 would have semantics similar to the following:
6275 my ($filename) = @_;
6276 if ( my $version = eval { version->parse($filename) } ) {
6277 if ( $version > $^V ) {
6278 my $vn = $version->normal;
6279 croak "Perl $vn required--this is only $^V, stopped";
6284 if (exists $INC{$filename}) {
6285 return 1 if $INC{$filename};
6286 croak "Compilation failed in require";
6289 foreach $prefix (@INC) {
6291 #... do other stuff - see text below ....
6293 # (see text below about possible appending of .pmc
6294 # suffix to $filename)
6295 my $realfilename = "$prefix/$filename";
6296 next if ! -e $realfilename || -d _ || -b _;
6297 $INC{$filename} = $realfilename;
6298 my $result = do($realfilename);
6299 # but run in caller's namespace
6301 if (!defined $result) {
6302 $INC{$filename} = undef;
6303 croak $@ ? "$@Compilation failed in require"
6304 : "Can't locate $filename: $!\n";
6307 delete $INC{$filename};
6308 croak "$filename did not return true value";
6313 croak "Can't locate $filename in \@INC ...";
6316 Note that the file will not be included twice under the same specified
6319 The file must return true as the last statement to indicate
6320 successful execution of any initialization code, so it's customary to
6321 end such a file with C<1;> unless you're sure it'll return true
6322 otherwise. But it's better just to put the C<1;>, in case you add more
6325 If EXPR is a bareword, L<C<require>|/require VERSION> assumes a F<.pm>
6326 extension and replaces C<::> with C</> in the filename for you,
6327 to make it easy to load standard modules. This form of loading of
6328 modules does not risk altering your namespace.
6330 In other words, if you try this:
6332 require Foo::Bar; # a splendid bareword
6334 The require function will actually look for the F<Foo/Bar.pm> file in the
6335 directories specified in the L<C<@INC>|perlvar/@INC> array.
6337 But if you try this:
6339 my $class = 'Foo::Bar';
6340 require $class; # $class is not a bareword
6342 require "Foo::Bar"; # not a bareword because of the ""
6344 The require function will look for the F<Foo::Bar> file in the
6345 L<C<@INC>|perlvar/@INC> array and
6346 will complain about not finding F<Foo::Bar> there. In this case you can do:
6348 eval "require $class";
6350 Now that you understand how L<C<require>|/require VERSION> looks for
6351 files with a bareword argument, there is a little extra functionality
6352 going on behind the scenes. Before L<C<require>|/require VERSION> looks
6353 for a F<.pm> extension, it will first look for a similar filename with a
6354 F<.pmc> extension. If this file is found, it will be loaded in place of
6355 any file ending in a F<.pm> extension.
6357 You can also insert hooks into the import facility by putting Perl code
6358 directly into the L<C<@INC>|perlvar/@INC> array. There are three forms
6359 of hooks: subroutine references, array references, and blessed objects.
6361 Subroutine references are the simplest case. When the inclusion system
6362 walks through L<C<@INC>|perlvar/@INC> and encounters a subroutine, this
6363 subroutine gets called with two parameters, the first a reference to
6364 itself, and the second the name of the file to be included (e.g.,
6365 F<Foo/Bar.pm>). The subroutine should return either nothing or else a
6366 list of up to four values in the following order:
6372 A reference to a scalar, containing any initial source code to prepend to
6373 the file or generator output.
6377 A filehandle, from which the file will be read.
6381 A reference to a subroutine. If there is no filehandle (previous item),
6382 then this subroutine is expected to generate one line of source code per
6383 call, writing the line into L<C<$_>|perlvar/$_> and returning 1, then
6384 finally at end of file returning 0. If there is a filehandle, then the
6385 subroutine will be called to act as a simple source filter, with the
6386 line as read in L<C<$_>|perlvar/$_>.
6387 Again, return 1 for each valid line, and 0 after all lines have been
6392 Optional state for the subroutine. The state is passed in as C<$_[1]>. A
6393 reference to the subroutine itself is passed in as C<$_[0]>.
6397 If an empty list, L<C<undef>|/undef EXPR>, or nothing that matches the
6398 first 3 values above is returned, then L<C<require>|/require VERSION>
6399 looks at the remaining elements of L<C<@INC>|perlvar/@INC>.
6400 Note that this filehandle must be a real filehandle (strictly a typeglob
6401 or reference to a typeglob, whether blessed or unblessed); tied filehandles
6402 will be ignored and processing will stop there.
6404 If the hook is an array reference, its first element must be a subroutine
6405 reference. This subroutine is called as above, but the first parameter is
6406 the array reference. This lets you indirectly pass arguments to
6409 In other words, you can write:
6411 push @INC, \&my_sub;
6413 my ($coderef, $filename) = @_; # $coderef is \&my_sub
6419 push @INC, [ \&my_sub, $x, $y, ... ];
6421 my ($arrayref, $filename) = @_;
6422 # Retrieve $x, $y, ...
6423 my (undef, @parameters) = @$arrayref;
6427 If the hook is an object, it must provide an C<INC> method that will be
6428 called as above, the first parameter being the object itself. (Note that
6429 you must fully qualify the sub's name, as unqualified C<INC> is always forced
6430 into package C<main>.) Here is a typical code layout:
6436 my ($self, $filename) = @_;
6440 # In the main program
6441 push @INC, Foo->new(...);
6443 These hooks are also permitted to set the L<C<%INC>|perlvar/%INC> entry
6444 corresponding to the files they have loaded. See L<perlvar/%INC>.
6446 For a yet-more-powerful import facility, see
6447 L<C<use>|/use Module VERSION LIST> and L<perlmod>.
6454 =for Pod::Functions clear all variables of a given name
6456 Generally used in a L<C<continue>|/continue BLOCK> block at the end of a
6457 loop to clear variables and reset C<m?pattern?> searches so that they
6459 expression is interpreted as a list of single characters (hyphens
6460 allowed for ranges). All variables and arrays beginning with one of
6461 those letters are reset to their pristine state. If the expression is
6462 omitted, one-match searches (C<m?pattern?>) are reset to match again.
6463 Only resets variables or searches in the current package. Always returns
6466 reset 'X'; # reset all X variables
6467 reset 'a-z'; # reset lower case variables
6468 reset; # just reset m?one-time? searches
6470 Resetting C<"A-Z"> is not recommended because you'll wipe out your
6471 L<C<@ARGV>|perlvar/@ARGV> and L<C<@INC>|perlvar/@INC> arrays and your
6472 L<C<%ENV>|perlvar/%ENV> hash.
6473 Resets only package variables; lexical variables are unaffected, but
6474 they clean themselves up on scope exit anyway, so you'll probably want
6475 to use them instead. See L<C<my>|/my VARLIST>.
6482 =for Pod::Functions get out of a function early
6484 Returns from a subroutine, L<C<eval>|/eval EXPR>,
6485 L<C<do FILE>|/do EXPR>, L<C<sort>|/sort SUBNAME LIST> block or regex
6486 eval block (but not a L<C<grep>|/grep BLOCK LIST> or
6487 L<C<map>|/map BLOCK LIST> block) with the value
6488 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
6489 context, depending on how the return value will be used, and the context
6490 may vary from one execution to the next (see
6491 L<C<wantarray>|/wantarray>). If no EXPR
6492 is given, returns an empty list in list context, the undefined value in
6493 scalar context, and (of course) nothing at all in void context.
6495 (In the absence of an explicit L<C<return>|/return EXPR>, a subroutine,
6496 L<C<eval>|/eval EXPR>,
6497 or L<C<do FILE>|/do EXPR> automatically returns the value of the last expression
6500 Unlike most named operators, this is also exempt from the
6501 looks-like-a-function rule, so C<return ("foo")."bar"> will
6502 cause C<"bar"> to be part of the argument to L<C<return>|/return EXPR>.
6505 X<reverse> X<rev> X<invert>
6507 =for Pod::Functions flip a string or a list
6509 In list context, returns a list value consisting of the elements
6510 of LIST in the opposite order. In scalar context, concatenates the
6511 elements of LIST and returns a string value with all characters
6512 in the opposite order.
6514 print join(", ", reverse "world", "Hello"); # Hello, world
6516 print scalar reverse "dlrow ,", "olleH"; # Hello, world
6518 Used without arguments in scalar context, L<C<reverse>|/reverse LIST>
6519 reverses L<C<$_>|perlvar/$_>.
6521 $_ = "dlrow ,olleH";
6522 print reverse; # No output, list context
6523 print scalar reverse; # Hello, world
6525 Note that reversing an array to itself (as in C<@a = reverse @a>) will
6526 preserve non-existent elements whenever possible; i.e., for non-magical
6527 arrays or for tied arrays with C<EXISTS> and C<DELETE> methods.
6529 This operator is also handy for inverting a hash, although there are some
6530 caveats. If a value is duplicated in the original hash, only one of those
6531 can be represented as a key in the inverted hash. Also, this has to
6532 unwind one hash and build a whole new one, which may take some time
6533 on a large hash, such as from a DBM file.
6535 my %by_name = reverse %by_address; # Invert the hash
6537 =item rewinddir DIRHANDLE
6540 =for Pod::Functions reset directory handle
6542 Sets the current position to the beginning of the directory for the
6543 L<C<readdir>|/readdir DIRHANDLE> routine on DIRHANDLE.
6545 Portability issues: L<perlport/rewinddir>.
6547 =item rindex STR,SUBSTR,POSITION
6550 =item rindex STR,SUBSTR
6552 =for Pod::Functions right-to-left substring search
6554 Works just like L<C<index>|/index STR,SUBSTR,POSITION> except that it
6555 returns the position of the I<last>
6556 occurrence of SUBSTR in STR. If POSITION is specified, returns the
6557 last occurrence beginning at or before that position.
6559 =item rmdir FILENAME
6560 X<rmdir> X<rd> X<directory, remove>
6564 =for Pod::Functions remove a directory
6566 Deletes the directory specified by FILENAME if that directory is
6567 empty. If it succeeds it returns true; otherwise it returns false and
6568 sets L<C<$!>|perlvar/$!> (errno). If FILENAME is omitted, uses
6569 L<C<$_>|perlvar/$_>.
6571 To remove a directory tree recursively (C<rm -rf> on Unix) look at
6572 the L<C<rmtree>|File::Path/rmtree( $dir )> function of the L<File::Path>
6577 =for Pod::Functions replace a pattern with a string
6579 The substitution operator. See L<perlop/"Regexp Quote-Like Operators">.
6581 =item say FILEHANDLE LIST
6584 =item say FILEHANDLE
6590 =for Pod::Functions +say output a list to a filehandle, appending a newline
6592 Just like L<C<print>|/print FILEHANDLE LIST>, but implicitly appends a
6593 newline. C<say LIST> is simply an abbreviation for
6594 C<{ local $\ = "\n"; print LIST }>. To use FILEHANDLE without a LIST to
6595 print the contents of L<C<$_>|perlvar/$_> to it, you must use a bareword
6596 filehandle like C<FH>, not an indirect one like C<$fh>.
6598 L<C<say>|/say FILEHANDLE LIST> is available only if the
6599 L<C<"say"> feature|feature/The 'say' feature> is enabled or if it is
6600 prefixed with C<CORE::>. The
6601 L<C<"say"> feature|feature/The 'say' feature> is enabled automatically
6602 with a C<use v5.10> (or higher) declaration in the current scope.
6605 X<scalar> X<context>
6607 =for Pod::Functions force a scalar context
6609 Forces EXPR to be interpreted in scalar context and returns the value
6612 my @counts = ( scalar @a, scalar @b, scalar @c );
6614 There is no equivalent operator to force an expression to
6615 be interpolated in list context because in practice, this is never
6616 needed. If you really wanted to do so, however, you could use
6617 the construction C<@{[ (some expression) ]}>, but usually a simple
6618 C<(some expression)> suffices.
6620 Because L<C<scalar>|/scalar EXPR> is a unary operator, if you
6622 parenthesized list for the EXPR, this behaves as a scalar comma expression,
6623 evaluating all but the last element in void context and returning the final
6624 element evaluated in scalar context. This is seldom what you want.
6626 The following single statement:
6628 print uc(scalar(foo(), $bar)), $baz;
6630 is the moral equivalent of these two:
6633 print(uc($bar), $baz);
6635 See L<perlop> for more details on unary operators and the comma operator,
6636 and L<perldata> for details on evaluating a hash in scalar contex.
6638 =item seek FILEHANDLE,POSITION,WHENCE
6639 X<seek> X<fseek> X<filehandle, position>
6641 =for Pod::Functions reposition file pointer for random-access I/O
6643 Sets FILEHANDLE's position, just like the L<fseek(3)> call of C C<stdio>.
6644 FILEHANDLE may be an expression whose value gives the name of the
6645 filehandle. The values for WHENCE are C<0> to set the new position
6646 I<in bytes> to POSITION; C<1> to set it to the current position plus
6647 POSITION; and C<2> to set it to EOF plus POSITION, typically
6648 negative. For WHENCE you may use the constants C<SEEK_SET>,
6649 C<SEEK_CUR>, and C<SEEK_END> (start of the file, current position, end
6650 of the file) from the L<Fcntl> module. Returns C<1> on success, false
6653 Note the emphasis on bytes: even if the filehandle has been set to operate
6654 on characters (for example using the C<:encoding(utf8)> I/O layer), the
6655 L<C<seek>|/seek FILEHANDLE,POSITION,WHENCE>,
6656 L<C<tell>|/tell FILEHANDLE>, and
6657 L<C<sysseek>|/sysseek FILEHANDLE,POSITION,WHENCE>
6658 family of functions use byte offsets, not character offsets,
6659 because seeking to a character offset would be very slow in a UTF-8 file.
6661 If you want to position the file for
6662 L<C<sysread>|/sysread FILEHANDLE,SCALAR,LENGTH,OFFSET> or
6663 L<C<syswrite>|/syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET>, don't use
6664 L<C<seek>|/seek FILEHANDLE,POSITION,WHENCE>, because buffering makes its
6665 effect on the file's read-write position unpredictable and non-portable.
6666 Use L<C<sysseek>|/sysseek FILEHANDLE,POSITION,WHENCE> instead.
6668 Due to the rules and rigors of ANSI C, on some systems you have to do a
6669 seek whenever you switch between reading and writing. Amongst other
6670 things, this may have the effect of calling stdio's L<clearerr(3)>.
6671 A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position:
6675 This is also useful for applications emulating C<tail -f>. Once you hit
6676 EOF on your read and then sleep for a while, you (probably) have to stick in a
6677 dummy L<C<seek>|/seek FILEHANDLE,POSITION,WHENCE> to reset things. The
6678 L<C<seek>|/seek FILEHANDLE,POSITION,WHENCE> doesn't change the position,
6679 but it I<does> clear the end-of-file condition on the handle, so that the
6680 next C<readline FILE> makes Perl try again to read something. (We hope.)
6682 If that doesn't work (some I/O implementations are particularly
6683 cantankerous), you might need something like this:
6686 for ($curpos = tell($fh); $_ = readline($fh);
6687 $curpos = tell($fh)) {
6688 # search for some stuff and put it into files
6690 sleep($for_a_while);
6691 seek($fh, $curpos, 0);
6694 =item seekdir DIRHANDLE,POS
6697 =for Pod::Functions reposition directory pointer
6699 Sets the current position for the L<C<readdir>|/readdir DIRHANDLE>
6700 routine on DIRHANDLE. POS must be a value returned by
6701 L<C<telldir>|/telldir DIRHANDLE>. L<C<seekdir>|/seekdir DIRHANDLE,POS>
6702 also has the same caveats about possible directory compaction as the
6703 corresponding system library routine.
6705 =item select FILEHANDLE
6706 X<select> X<filehandle, default>
6710 =for Pod::Functions reset default output or do I/O multiplexing
6712 Returns the currently selected filehandle. If FILEHANDLE is supplied,
6713 sets the new current default filehandle for output. This has two
6714 effects: first, a L<C<write>|/write FILEHANDLE> or a L<C<print>|/print
6715 FILEHANDLE LIST> without a filehandle
6716 default to this FILEHANDLE. Second, references to variables related to
6717 output will refer to this output channel.
6719 For example, to set the top-of-form format for more than one
6720 output channel, you might do the following:
6727 FILEHANDLE may be an expression whose value gives the name of the
6728 actual filehandle. Thus:
6730 my $oldfh = select(STDERR); $| = 1; select($oldfh);
6732 Some programmers may prefer to think of filehandles as objects with
6733 methods, preferring to write the last example as:
6735 STDERR->autoflush(1);
6737 (Prior to Perl version 5.14, you have to C<use IO::Handle;> explicitly
6740 Portability issues: L<perlport/select>.
6742 =item select RBITS,WBITS,EBITS,TIMEOUT
6745 This calls the L<select(2)> syscall with the bit masks specified, which
6746 can be constructed using L<C<fileno>|/fileno FILEHANDLE> and
6747 L<C<vec>|/vec EXPR,OFFSET,BITS>, along these lines:
6749 my $rin = my $win = my $ein = '';
6750 vec($rin, fileno(STDIN), 1) = 1;
6751 vec($win, fileno(STDOUT), 1) = 1;
6754 If you want to select on many filehandles, you may wish to write a
6755 subroutine like this:
6760 for my $fh (@fhlist) {
6761 vec($bits, fileno($fh), 1) = 1;
6765 my $rin = fhbits(\*STDIN, $tty, $mysock);
6769 my ($nfound, $timeleft) =
6770 select(my $rout = $rin, my $wout = $win, my $eout = $ein,
6773 or to block until something becomes ready just do this
6776 select(my $rout = $rin, my $wout = $win, my $eout = $ein, undef);
6778 Most systems do not bother to return anything useful in C<$timeleft>, so
6779 calling L<C<select>|/select RBITS,WBITS,EBITS,TIMEOUT> in scalar context
6780 just returns C<$nfound>.
6782 Any of the bit masks can also be L<C<undef>|/undef EXPR>. The timeout,
6784 in seconds, which may be fractional. Note: not all implementations are
6785 capable of returning the C<$timeleft>. If not, they always return
6786 C<$timeleft> equal to the supplied C<$timeout>.
6788 You can effect a sleep of 250 milliseconds this way:
6790 select(undef, undef, undef, 0.25);
6792 Note that whether L<C<select>|/select RBITS,WBITS,EBITS,TIMEOUT> gets
6793 restarted after signals (say, SIGALRM) is implementation-dependent. See
6794 also L<perlport> for notes on the portability of
6795 L<C<select>|/select RBITS,WBITS,EBITS,TIMEOUT>.
6797 On error, L<C<select>|/select RBITS,WBITS,EBITS,TIMEOUT> behaves just
6798 like L<select(2)>: it returns C<-1> and sets L<C<$!>|perlvar/$!>.
6800 On some Unixes, L<select(2)> may report a socket file descriptor as
6801 "ready for reading" even when no data is available, and thus any
6802 subsequent L<C<read>|/read FILEHANDLE,SCALAR,LENGTH,OFFSET> would block.
6803 This can be avoided if you always use C<O_NONBLOCK> on the socket. See
6804 L<select(2)> and L<fcntl(2)> for further details.
6806 The standard L<C<IO::Select>|IO::Select> module provides a
6807 user-friendlier interface to
6808 L<C<select>|/select RBITS,WBITS,EBITS,TIMEOUT>, mostly because it does
6809 all the bit-mask work for you.
6811 B<WARNING>: One should not attempt to mix buffered I/O (like
6812 L<C<read>|/read FILEHANDLE,SCALAR,LENGTH,OFFSET> or
6813 L<C<readline>|/readline EXPR>) with
6814 L<C<select>|/select RBITS,WBITS,EBITS,TIMEOUT>, except as permitted by
6815 POSIX, and even then only on POSIX systems. You have to use
6816 L<C<sysread>|/sysread FILEHANDLE,SCALAR,LENGTH,OFFSET> instead.
6818 Portability issues: L<perlport/select>.
6820 =item semctl ID,SEMNUM,CMD,ARG
6823 =for Pod::Functions SysV semaphore control operations
6825 Calls the System V IPC function L<semctl(2)>. You'll probably have to say
6829 first to get the correct constant definitions. If CMD is IPC_STAT or
6830 GETALL, then ARG must be a variable that will hold the returned
6831 semid_ds structure or semaphore value array. Returns like
6832 L<C<ioctl>|/ioctl FILEHANDLE,FUNCTION,SCALAR>:
6833 the undefined value for error, "C<0 but true>" for zero, or the actual
6834 return value otherwise. The ARG must consist of a vector of native
6835 short integers, which may be created with C<pack("s!",(0)x$nsem)>.
6836 See also L<perlipc/"SysV IPC"> and the documentation for
6837 L<C<IPC::SysV>|IPC::SysV> and L<C<IPC::Semaphore>|IPC::Semaphore>.
6839 Portability issues: L<perlport/semctl>.
6841 =item semget KEY,NSEMS,FLAGS
6844 =for Pod::Functions get set of SysV semaphores
6846 Calls the System V IPC function L<semget(2)>. Returns the semaphore id, or
6847 the undefined value on error. See also
6848 L<perlipc/"SysV IPC"> and the documentation for
6849 L<C<IPC::SysV>|IPC::SysV> and L<C<IPC::Semaphore>|IPC::Semaphore>.
6851 Portability issues: L<perlport/semget>.
6853 =item semop KEY,OPSTRING
6856 =for Pod::Functions SysV semaphore operations
6858 Calls the System V IPC function L<semop(2)> for semaphore operations
6859 such as signalling and waiting. OPSTRING must be a packed array of
6860 semop structures. Each semop structure can be generated with
6861 C<pack("s!3", $semnum, $semop, $semflag)>. The length of OPSTRING
6862 implies the number of semaphore operations. Returns true if
6863 successful, false on error. As an example, the
6864 following code waits on semaphore $semnum of semaphore id $semid:
6866 my $semop = pack("s!3", $semnum, -1, 0);
6867 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
6869 To signal the semaphore, replace C<-1> with C<1>. See also
6870 L<perlipc/"SysV IPC"> and the documentation for
6871 L<C<IPC::SysV>|IPC::SysV> and L<C<IPC::Semaphore>|IPC::Semaphore>.
6873 Portability issues: L<perlport/semop>.
6875 =item send SOCKET,MSG,FLAGS,TO
6878 =item send SOCKET,MSG,FLAGS
6880 =for Pod::Functions send a message over a socket
6882 Sends a message on a socket. Attempts to send the scalar MSG to the SOCKET
6883 filehandle. Takes the same flags as the system call of the same name. On
6884 unconnected sockets, you must specify a destination to I<send to>, in which
6885 case it does a L<sendto(2)> syscall. Returns the number of characters sent,
6886 or the undefined value on error. The L<sendmsg(2)> syscall is currently
6887 unimplemented. See L<perlipc/"UDP: Message Passing"> for examples.
6889 Note the I<characters>: depending on the status of the socket, either
6890 (8-bit) bytes or characters are sent. By default all sockets operate
6891 on bytes, but for example if the socket has been changed using
6892 L<C<binmode>|/binmode FILEHANDLE, LAYER> to operate with the
6893 C<:encoding(utf8)> I/O layer (see L<C<open>|/open FILEHANDLE,EXPR>, or
6894 the L<open> pragma), the I/O will operate on UTF-8
6895 encoded Unicode characters, not bytes. Similarly for the C<:encoding>
6896 layer: in that case pretty much any characters can be sent.
6898 =item setpgrp PID,PGRP
6901 =for Pod::Functions set the process group of a process
6903 Sets the current process group for the specified PID, C<0> for the current
6904 process. Raises an exception when used on a machine that doesn't
6905 implement POSIX L<setpgid(2)> or BSD L<setpgrp(2)>. If the arguments
6906 are omitted, it defaults to C<0,0>. Note that the BSD 4.2 version of
6907 L<C<setpgrp>|/setpgrp PID,PGRP> does not accept any arguments, so only
6908 C<setpgrp(0,0)> is portable. See also
6909 L<C<POSIX::setsid()>|POSIX/C<setsid>>.
6911 Portability issues: L<perlport/setpgrp>.
6913 =item setpriority WHICH,WHO,PRIORITY
6914 X<setpriority> X<priority> X<nice> X<renice>
6916 =for Pod::Functions set a process's nice value
6918 Sets the current priority for a process, a process group, or a user.
6919 (See L<setpriority(2)>.) Raises an exception when used on a machine
6920 that doesn't implement L<setpriority(2)>.
6922 Portability issues: L<perlport/setpriority>.
6924 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
6927 =for Pod::Functions set some socket options
6929 Sets the socket option requested. Returns L<C<undef>|/undef EXPR> on
6930 error. Use integer constants provided by the L<C<Socket>|Socket> module
6932 LEVEL and OPNAME. Values for LEVEL can also be obtained from
6933 getprotobyname. OPTVAL might either be a packed string or an integer.
6934 An integer OPTVAL is shorthand for pack("i", OPTVAL).
6936 An example disabling Nagle's algorithm on a socket:
6938 use Socket qw(IPPROTO_TCP TCP_NODELAY);
6939 setsockopt($socket, IPPROTO_TCP, TCP_NODELAY, 1);
6941 Portability issues: L<perlport/setsockopt>.
6948 =for Pod::Functions remove the first element of an array, and return it
6950 Shifts the first value of the array off and returns it, shortening the
6951 array by 1 and moving everything down. If there are no elements in the
6952 array, returns the undefined value. If ARRAY is omitted, shifts the
6953 L<C<@_>|perlvar/@_> array within the lexical scope of subroutines and
6954 formats, and the L<C<@ARGV>|perlvar/@ARGV> array outside a subroutine
6955 and also within the lexical scopes
6956 established by the C<eval STRING>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>,
6957 C<UNITCHECK {}>, and C<END {}> constructs.
6959 Starting with Perl 5.14, an experimental feature allowed
6960 L<C<shift>|/shift ARRAY> to take a
6961 scalar expression. This experiment has been deemed unsuccessful, and was
6962 removed as of Perl 5.24.
6964 See also L<C<unshift>|/unshift ARRAY,LIST>, L<C<push>|/push ARRAY,LIST>,
6965 and L<C<pop>|/pop ARRAY>. L<C<shift>|/shift ARRAY> and
6966 L<C<unshift>|/unshift ARRAY,LIST> do the same thing to the left end of
6967 an array that L<C<pop>|/pop ARRAY> and L<C<push>|/push ARRAY,LIST> do to
6970 =item shmctl ID,CMD,ARG
6973 =for Pod::Functions SysV shared memory operations
6975 Calls the System V IPC function shmctl. You'll probably have to say
6979 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
6980 then ARG must be a variable that will hold the returned C<shmid_ds>
6981 structure. Returns like ioctl: L<C<undef>|/undef EXPR> for error; "C<0>
6982 but true" for zero; and the actual return value otherwise.
6983 See also L<perlipc/"SysV IPC"> and the documentation for
6984 L<C<IPC::SysV>|IPC::SysV>.
6986 Portability issues: L<perlport/shmctl>.
6988 =item shmget KEY,SIZE,FLAGS
6991 =for Pod::Functions get SysV shared memory segment identifier
6993 Calls the System V IPC function shmget. Returns the shared memory
6994 segment id, or L<C<undef>|/undef EXPR> on error.
6995 See also L<perlipc/"SysV IPC"> and the documentation for
6996 L<C<IPC::SysV>|IPC::SysV>.
6998 Portability issues: L<perlport/shmget>.
7000 =item shmread ID,VAR,POS,SIZE
7004 =for Pod::Functions read SysV shared memory
7006 =item shmwrite ID,STRING,POS,SIZE
7008 =for Pod::Functions write SysV shared memory
7010 Reads or writes the System V shared memory segment ID starting at
7011 position POS for size SIZE by attaching to it, copying in/out, and
7012 detaching from it. When reading, VAR must be a variable that will
7013 hold the data read. When writing, if STRING is too long, only SIZE
7014 bytes are used; if STRING is too short, nulls are written to fill out
7015 SIZE bytes. Return true if successful, false on error.
7016 L<C<shmread>|/shmread ID,VAR,POS,SIZE> taints the variable. See also
7017 L<perlipc/"SysV IPC"> and the documentation for
7018 L<C<IPC::SysV>|IPC::SysV> and the L<C<IPC::Shareable>|IPC::Shareable>
7021 Portability issues: L<perlport/shmread> and L<perlport/shmwrite>.
7023 =item shutdown SOCKET,HOW
7026 =for Pod::Functions close down just half of a socket connection
7028 Shuts down a socket connection in the manner indicated by HOW, which
7029 has the same interpretation as in the syscall of the same name.
7031 shutdown($socket, 0); # I/we have stopped reading data
7032 shutdown($socket, 1); # I/we have stopped writing data
7033 shutdown($socket, 2); # I/we have stopped using this socket
7035 This is useful with sockets when you want to tell the other
7036 side you're done writing but not done reading, or vice versa.
7037 It's also a more insistent form of close because it also
7038 disables the file descriptor in any forked copies in other
7041 Returns C<1> for success; on error, returns L<C<undef>|/undef EXPR> if
7042 the first argument is not a valid filehandle, or returns C<0> and sets
7043 L<C<$!>|perlvar/$!> for any other failure.
7046 X<sin> X<sine> X<asin> X<arcsine>
7050 =for Pod::Functions return the sine of a number
7052 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
7053 returns sine of L<C<$_>|perlvar/$_>.
7055 For the inverse sine operation, you may use the C<Math::Trig::asin>
7056 function, or use this relation:
7058 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
7065 =for Pod::Functions block for some number of seconds
7067 Causes the script to sleep for (integer) EXPR seconds, or forever if no
7068 argument is given. Returns the integer number of seconds actually slept.
7070 May be interrupted if the process receives a signal such as C<SIGALRM>.
7073 local $SIG{ALRM} = sub { die "Alarm!\n" };
7076 die $@ unless $@ eq "Alarm!\n";
7078 You probably cannot mix L<C<alarm>|/alarm SECONDS> and
7079 L<C<sleep>|/sleep EXPR> calls, because L<C<sleep>|/sleep EXPR> is often
7080 implemented using L<C<alarm>|/alarm SECONDS>.
7082 On some older systems, it may sleep up to a full second less than what
7083 you requested, depending on how it counts seconds. Most modern systems
7084 always sleep the full amount. They may appear to sleep longer than that,
7085 however, because your process might not be scheduled right away in a
7086 busy multitasking system.
7088 For delays of finer granularity than one second, the L<Time::HiRes>
7089 module (from CPAN, and starting from Perl 5.8 part of the standard
7090 distribution) provides L<C<usleep>|Time::HiRes/usleep ( $useconds )>.
7091 You may also use Perl's four-argument
7092 version of L<C<select>|/select RBITS,WBITS,EBITS,TIMEOUT> leaving the
7093 first three arguments undefined, or you might be able to use the
7094 L<C<syscall>|/syscall NUMBER, LIST> interface to access L<setitimer(2)>
7095 if your system supports it. See L<perlfaq8> for details.
7097 See also the L<POSIX> module's L<C<pause>|POSIX/C<pause>> function.
7099 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
7102 =for Pod::Functions create a socket
7104 Opens a socket of the specified kind and attaches it to filehandle
7105 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for
7106 the syscall of the same name. You should C<use Socket> first
7107 to get the proper definitions imported. See the examples in
7108 L<perlipc/"Sockets: Client/Server Communication">.
7110 On systems that support a close-on-exec flag on files, the flag will
7111 be set for the newly opened file descriptor, as determined by the
7112 value of L<C<$^F>|perlvar/$^F>. See L<perlvar/$^F>.
7114 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
7117 =for Pod::Functions create a pair of sockets
7119 Creates an unnamed pair of sockets in the specified domain, of the
7120 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
7121 for the syscall of the same name. If unimplemented, raises an exception.
7122 Returns true if successful.
7124 On systems that support a close-on-exec flag on files, the flag will
7125 be set for the newly opened file descriptors, as determined by the value
7126 of L<C<$^F>|perlvar/$^F>. See L<perlvar/$^F>.
7128 Some systems define L<C<pipe>|/pipe READHANDLE,WRITEHANDLE> in terms of
7129 L<C<socketpair>|/socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL>, in
7130 which a call to C<pipe($rdr, $wtr)> is essentially:
7133 socketpair(my $rdr, my $wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
7134 shutdown($rdr, 1); # no more writing for reader
7135 shutdown($wtr, 0); # no more reading for writer
7137 See L<perlipc> for an example of socketpair use. Perl 5.8 and later will
7138 emulate socketpair using IP sockets to localhost if your system implements
7139 sockets but not socketpair.
7141 Portability issues: L<perlport/socketpair>.
7143 =item sort SUBNAME LIST
7144 X<sort> X<qsort> X<quicksort> X<mergesort>
7146 =item sort BLOCK LIST
7150 =for Pod::Functions sort a list of values
7152 In list context, this sorts the LIST and returns the sorted list value.
7153 In scalar context, the behaviour of L<C<sort>|/sort SUBNAME LIST> is
7156 If SUBNAME or BLOCK is omitted, L<C<sort>|/sort SUBNAME LIST>s in
7157 standard string comparison
7158 order. If SUBNAME is specified, it gives the name of a subroutine
7159 that returns an integer less than, equal to, or greater than C<0>,
7160 depending on how the elements of the list are to be ordered. (The
7161 C<< <=> >> and C<cmp> operators are extremely useful in such routines.)
7162 SUBNAME may be a scalar variable name (unsubscripted), in which case
7163 the value provides the name of (or a reference to) the actual
7164 subroutine to use. In place of a SUBNAME, you can provide a BLOCK as
7165 an anonymous, in-line sort subroutine.
7167 If the subroutine's prototype is C<($$)>, the elements to be compared are
7168 passed by reference in L<C<@_>|perlvar/@_>, as for a normal subroutine.
7169 This is slower than unprototyped subroutines, where the elements to be
7170 compared are passed into the subroutine as the package global variables
7171 C<$a> and C<$b> (see example below). Note that in the latter case, it
7172 is usually highly counter-productive to declare C<$a> and C<$b> as
7175 If the subroutine is an XSUB, the elements to be compared are pushed on
7176 to the stack, the way arguments are usually passed to XSUBs. C<$a> and
7179 The values to be compared are always passed by reference and should not
7182 You also cannot exit out of the sort block or subroutine using any of the
7183 loop control operators described in L<perlsyn> or with
7184 L<C<goto>|/goto LABEL>.
7186 When L<C<use locale>|locale> (but not C<use locale ':not_characters'>)
7187 is in effect, C<sort LIST> sorts LIST according to the
7188 current collation locale. See L<perllocale>.
7190 L<C<sort>|/sort SUBNAME LIST> returns aliases into the original list,
7191 much as a for loop's index variable aliases the list elements. That is,
7192 modifying an element of a list returned by L<C<sort>|/sort SUBNAME LIST>
7193 (for example, in a C<foreach>, L<C<map>|/map BLOCK LIST> or
7194 L<C<grep>|/grep BLOCK LIST>)
7195 actually modifies the element in the original list. This is usually
7196 something to be avoided when writing clear code.
7198 Perl 5.6 and earlier used a quicksort algorithm to implement sort.
7199 That algorithm was not stable and I<could> go quadratic. (A I<stable> sort
7200 preserves the input order of elements that compare equal. Although
7201 quicksort's run time is O(NlogN) when averaged over all arrays of
7202 length N, the time can be O(N**2), I<quadratic> behavior, for some
7203 inputs.) In 5.7, the quicksort implementation was replaced with
7204 a stable mergesort algorithm whose worst-case behavior is O(NlogN).
7205 But benchmarks indicated that for some inputs, on some platforms,
7206 the original quicksort was faster. 5.8 has a L<sort> pragma for
7207 limited control of the sort. Its rather blunt control of the
7208 underlying algorithm may not persist into future Perls, but the
7209 ability to characterize the input or output in implementation
7210 independent ways quite probably will.
7215 my @articles = sort @files;
7217 # same thing, but with explicit sort routine
7218 my @articles = sort {$a cmp $b} @files;
7220 # now case-insensitively
7221 my @articles = sort {fc($a) cmp fc($b)} @files;
7223 # same thing in reversed order
7224 my @articles = sort {$b cmp $a} @files;
7226 # sort numerically ascending
7227 my @articles = sort {$a <=> $b} @files;
7229 # sort numerically descending
7230 my @articles = sort {$b <=> $a} @files;
7232 # this sorts the %age hash by value instead of key
7233 # using an in-line function
7234 my @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
7236 # sort using explicit subroutine name
7238 $age{$a} <=> $age{$b}; # presuming numeric
7240 my @sortedclass = sort byage @class;
7242 sub backwards { $b cmp $a }
7243 my @harry = qw(dog cat x Cain Abel);
7244 my @george = qw(gone chased yz Punished Axed);
7246 # prints AbelCaincatdogx
7247 print sort backwards @harry;
7248 # prints xdogcatCainAbel
7249 print sort @george, 'to', @harry;
7250 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
7252 # inefficiently sort by descending numeric compare using
7253 # the first integer after the first = sign, or the
7254 # whole record case-insensitively otherwise
7257 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
7262 # same thing, but much more efficiently;
7263 # we'll build auxiliary indices instead
7267 push @nums, ( /=(\d+)/ ? $1 : undef );
7271 my @new = @old[ sort {
7272 $nums[$b] <=> $nums[$a]
7274 $caps[$a] cmp $caps[$b]
7278 # same thing, but without any temps
7279 my @new = map { $_->[0] }
7280 sort { $b->[1] <=> $a->[1]
7283 } map { [$_, /=(\d+)/, fc($_)] } @old;
7285 # using a prototype allows you to use any comparison subroutine
7286 # as a sort subroutine (including other package's subroutines)
7288 sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are
7291 my @new = sort Other::backwards @old;
7293 # guarantee stability, regardless of algorithm
7295 my @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
7297 # force use of mergesort (not portable outside Perl 5.8)
7298 use sort '_mergesort'; # note discouraging _
7299 my @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
7301 Warning: syntactical care is required when sorting the list returned from
7302 a function. If you want to sort the list returned by the function call
7303 C<find_records(@key)>, you can use:
7305 my @contact = sort { $a cmp $b } find_records @key;
7306 my @contact = sort +find_records(@key);
7307 my @contact = sort &find_records(@key);
7308 my @contact = sort(find_records(@key));
7310 If instead you want to sort the array C<@key> with the comparison routine
7311 C<find_records()> then you can use:
7313 my @contact = sort { find_records() } @key;
7314 my @contact = sort find_records(@key);
7315 my @contact = sort(find_records @key);
7316 my @contact = sort(find_records (@key));
7318 You I<must not> declare C<$a>
7319 and C<$b> as lexicals. They are package globals. That means
7320 that if you're in the C<main> package and type
7322 my @articles = sort {$b <=> $a} @files;
7324 then C<$a> and C<$b> are C<$main::a> and C<$main::b> (or C<$::a> and C<$::b>),
7325 but if you're in the C<FooPack> package, it's the same as typing
7327 my @articles = sort {$FooPack::b <=> $FooPack::a} @files;
7329 The comparison function is required to behave. If it returns
7330 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
7331 sometimes saying the opposite, for example) the results are not
7334 Because C<< <=> >> returns L<C<undef>|/undef EXPR> when either operand
7335 is C<NaN> (not-a-number), be careful when sorting with a
7336 comparison function like C<< $a <=> $b >> any lists that might contain a
7337 C<NaN>. The following example takes advantage that C<NaN != NaN> to
7338 eliminate any C<NaN>s from the input list.
7340 my @result = sort { $a <=> $b } grep { $_ == $_ } @input;
7342 =item splice ARRAY,OFFSET,LENGTH,LIST
7345 =item splice ARRAY,OFFSET,LENGTH
7347 =item splice ARRAY,OFFSET
7351 =for Pod::Functions add or remove elements anywhere in an array
7353 Removes the elements designated by OFFSET and LENGTH from an array, and
7354 replaces them with the elements of LIST, if any. In list context,
7355 returns the elements removed from the array. In scalar context,
7356 returns the last element removed, or L<C<undef>|/undef EXPR> if no
7358 removed. The array grows or shrinks as necessary.
7359 If OFFSET is negative then it starts that far from the end of the array.
7360 If LENGTH is omitted, removes everything from OFFSET onward.
7361 If LENGTH is negative, removes the elements from OFFSET onward
7362 except for -LENGTH elements at the end of the array.
7363 If both OFFSET and LENGTH are omitted, removes everything. If OFFSET is
7364 past the end of the array and a LENGTH was provided, Perl issues a warning,
7365 and splices at the end of the array.
7367 The following equivalences hold (assuming C<< $#a >= $i >> )
7369 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
7370 pop(@a) splice(@a,-1)
7371 shift(@a) splice(@a,0,1)
7372 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
7373 $a[$i] = $y splice(@a,$i,1,$y)
7375 L<C<splice>|/splice ARRAY,OFFSET,LENGTH,LIST> can be used, for example,
7376 to implement n-ary queue processing:
7380 while (my @next_n = splice @_, 0, $n) {
7381 say join q{ -- }, @next_n;
7385 nary_print(3, qw(a b c d e f g h));
7391 Starting with Perl 5.14, an experimental feature allowed
7392 L<C<splice>|/splice ARRAY,OFFSET,LENGTH,LIST> to take a
7393 scalar expression. This experiment has been deemed unsuccessful, and was
7394 removed as of Perl 5.24.
7396 =item split /PATTERN/,EXPR,LIMIT
7399 =item split /PATTERN/,EXPR
7401 =item split /PATTERN/
7405 =for Pod::Functions split up a string using a regexp delimiter
7407 Splits the string EXPR into a list of strings and returns the
7408 list in list context, or the size of the list in scalar context.
7410 If only PATTERN is given, EXPR defaults to L<C<$_>|perlvar/$_>.
7412 Anything in EXPR that matches PATTERN is taken to be a separator
7413 that separates the EXPR into substrings (called "I<fields>") that
7414 do B<not> include the separator. Note that a separator may be
7415 longer than one character or even have no characters at all (the
7416 empty string, which is a zero-width match).
7418 The PATTERN need not be constant; an expression may be used
7419 to specify a pattern that varies at runtime.
7421 If PATTERN matches the empty string, the EXPR is split at the match
7422 position (between characters). As an example, the following:
7424 print join(':', split(/b/, 'abc')), "\n";
7426 uses the C<b> in C<'abc'> as a separator to produce the output C<a:c>.
7429 print join(':', split(//, 'abc')), "\n";
7431 uses empty string matches as separators to produce the output
7432 C<a:b:c>; thus, the empty string may be used to split EXPR into a
7433 list of its component characters.
7435 As a special case for L<C<split>|/split E<sol>PATTERNE<sol>,EXPR,LIMIT>,
7436 the empty pattern given in
7437 L<match operator|perlop/"m/PATTERN/msixpodualngc"> syntax (C<//>)
7438 specifically matches the empty string, which is contrary to its usual
7439 interpretation as the last successful match.
7441 If PATTERN is C</^/>, then it is treated as if it used the
7442 L<multiline modifier|perlreref/OPERATORS> (C</^/m>), since it
7443 isn't much use otherwise.
7445 As another special case,
7446 L<C<split>|/split E<sol>PATTERNE<sol>,EXPR,LIMIT> emulates the default
7448 command line tool B<awk> when the PATTERN is either omitted or a
7449 string composed of a single space character (such as S<C<' '>> or
7450 S<C<"\x20">>, but not e.g. S<C</ />>). In this case, any leading
7451 whitespace in EXPR is removed before splitting occurs, and the PATTERN is
7452 instead treated as if it were C</\s+/>; in particular, this means that
7453 I<any> contiguous whitespace (not just a single space character) is used as
7454 a separator. However, this special treatment can be avoided by specifying
7455 the pattern S<C</ />> instead of the string S<C<" ">>, thereby allowing
7456 only a single space character to be a separator. In earlier Perls this
7457 special case was restricted to the use of a plain S<C<" ">> as the
7458 pattern argument to split; in Perl 5.18.0 and later this special case is
7459 triggered by any expression which evaluates to the simple string S<C<" ">>.
7461 If omitted, PATTERN defaults to a single space, S<C<" ">>, triggering
7462 the previously described I<awk> emulation.
7464 If LIMIT is specified and positive, it represents the maximum number
7465 of fields into which the EXPR may be split; in other words, LIMIT is
7466 one greater than the maximum number of times EXPR may be split. Thus,
7467 the LIMIT value C<1> means that EXPR may be split a maximum of zero
7468 times, producing a maximum of one field (namely, the entire value of
7469 EXPR). For instance:
7471 print join(':', split(//, 'abc', 1)), "\n";
7473 produces the output C<abc>, and this:
7475 print join(':', split(//, 'abc', 2)), "\n";
7477 produces the output C<a:bc>, and each of these:
7479 print join(':', split(//, 'abc', 3)), "\n";
7480 print join(':', split(//, 'abc', 4)), "\n";
7482 produces the output C<a:b:c>.
7484 If LIMIT is negative, it is treated as if it were instead arbitrarily
7485 large; as many fields as possible are produced.
7487 If LIMIT is omitted (or, equivalently, zero), then it is usually
7488 treated as if it were instead negative but with the exception that
7489 trailing empty fields are stripped (empty leading fields are always
7490 preserved); if all fields are empty, then all fields are considered to
7491 be trailing (and are thus stripped in this case). Thus, the following:
7493 print join(':', split(/,/, 'a,b,c,,,')), "\n";
7495 produces the output C<a:b:c>, but the following:
7497 print join(':', split(/,/, 'a,b,c,,,', -1)), "\n";
7499 produces the output C<a:b:c:::>.
7501 In time-critical applications, it is worthwhile to avoid splitting
7502 into more fields than necessary. Thus, when assigning to a list,
7503 if LIMIT is omitted (or zero), then LIMIT is treated as though it
7504 were one larger than the number of variables in the list; for the
7505 following, LIMIT is implicitly 3:
7507 my ($login, $passwd) = split(/:/);
7509 Note that splitting an EXPR that evaluates to the empty string always
7510 produces zero fields, regardless of the LIMIT specified.
7512 An empty leading field is produced when there is a positive-width
7513 match at the beginning of EXPR. For instance:
7515 print join(':', split(/ /, ' abc')), "\n";
7517 produces the output C<:abc>. However, a zero-width match at the
7518 beginning of EXPR never produces an empty field, so that:
7520 print join(':', split(//, ' abc'));
7522 produces the output S<C< :a:b:c>> (rather than S<C<: :a:b:c>>).
7524 An empty trailing field, on the other hand, is produced when there is a
7525 match at the end of EXPR, regardless of the length of the match
7526 (of course, unless a non-zero LIMIT is given explicitly, such fields are
7527 removed, as in the last example). Thus:
7529 print join(':', split(//, ' abc', -1)), "\n";
7531 produces the output S<C< :a:b:c:>>.
7533 If the PATTERN contains
7534 L<capturing groups|perlretut/Grouping things and hierarchical matching>,
7535 then for each separator, an additional field is produced for each substring
7536 captured by a group (in the order in which the groups are specified,
7537 as per L<backreferences|perlretut/Backreferences>); if any group does not
7538 match, then it captures the L<C<undef>|/undef EXPR> value instead of a
7540 note that any such additional field is produced whenever there is a
7541 separator (that is, whenever a split occurs), and such an additional field
7542 does B<not> count towards the LIMIT. Consider the following expressions
7543 evaluated in list context (each returned list is provided in the associated
7546 split(/-|,/, "1-10,20", 3)
7549 split(/(-|,)/, "1-10,20", 3)
7550 # ('1', '-', '10', ',', '20')
7552 split(/-|(,)/, "1-10,20", 3)
7553 # ('1', undef, '10', ',', '20')
7555 split(/(-)|,/, "1-10,20", 3)
7556 # ('1', '-', '10', undef, '20')
7558 split(/(-)|(,)/, "1-10,20", 3)
7559 # ('1', '-', undef, '10', undef, ',', '20')
7561 =item sprintf FORMAT, LIST
7564 =for Pod::Functions formatted print into a string
7566 Returns a string formatted by the usual
7567 L<C<printf>|/printf FILEHANDLE FORMAT, LIST> conventions of the C
7568 library function L<C<sprintf>|/sprintf FORMAT, LIST>. See below for
7569 more details and see L<sprintf(3)> or L<printf(3)> on your system for an
7570 explanation of the general principles.
7574 # Format number with up to 8 leading zeroes
7575 my $result = sprintf("%08d", $number);
7577 # Round number to 3 digits after decimal point
7578 my $rounded = sprintf("%.3f", $number);
7580 Perl does its own L<C<sprintf>|/sprintf FORMAT, LIST> formatting: it
7582 function L<sprintf(3)>, but doesn't use it except for floating-point
7583 numbers, and even then only standard modifiers are allowed.
7584 Non-standard extensions in your local L<sprintf(3)> are
7585 therefore unavailable from Perl.
7587 Unlike L<C<printf>|/printf FILEHANDLE FORMAT, LIST>,
7588 L<C<sprintf>|/sprintf FORMAT, LIST> does not do what you probably mean
7589 when you pass it an array as your first argument.
7590 The array is given scalar context,
7591 and instead of using the 0th element of the array as the format, Perl will
7592 use the count of elements in the array as the format, which is almost never
7595 Perl's L<C<sprintf>|/sprintf FORMAT, LIST> permits the following
7596 universally-known conversions:
7599 %c a character with the given number
7601 %d a signed integer, in decimal
7602 %u an unsigned integer, in decimal
7603 %o an unsigned integer, in octal
7604 %x an unsigned integer, in hexadecimal
7605 %e a floating-point number, in scientific notation
7606 %f a floating-point number, in fixed decimal notation
7607 %g a floating-point number, in %e or %f notation
7609 In addition, Perl permits the following widely-supported conversions:
7611 %X like %x, but using upper-case letters
7612 %E like %e, but using an upper-case "E"
7613 %G like %g, but with an upper-case "E" (if applicable)
7614 %b an unsigned integer, in binary
7615 %B like %b, but using an upper-case "B" with the # flag
7616 %p a pointer (outputs the Perl value's address in hexadecimal)
7617 %n special: *stores* the number of characters output so far
7618 into the next argument in the parameter list
7619 %a hexadecimal floating point
7620 %A like %a, but using upper-case letters
7622 Finally, for backward (and we do mean "backward") compatibility, Perl
7623 permits these unnecessary but widely-supported conversions:
7626 %D a synonym for %ld
7627 %U a synonym for %lu
7628 %O a synonym for %lo
7631 Note that the number of exponent digits in the scientific notation produced
7632 by C<%e>, C<%E>, C<%g> and C<%G> for numbers with the modulus of the
7633 exponent less than 100 is system-dependent: it may be three or less
7634 (zero-padded as necessary). In other words, 1.23 times ten to the
7635 99th may be either "1.23e99" or "1.23e099". Similarly for C<%a> and C<%A>:
7636 the exponent or the hexadecimal digits may float: especially the
7637 "long doubles" Perl configuration option may cause surprises.
7639 Between the C<%> and the format letter, you may specify several
7640 additional attributes controlling the interpretation of the format.
7641 In order, these are:
7645 =item format parameter index
7647 An explicit format parameter index, such as C<2$>. By default sprintf
7648 will format the next unused argument in the list, but this allows you
7649 to take the arguments out of order:
7651 printf '%2$d %1$d', 12, 34; # prints "34 12"
7652 printf '%3$d %d %1$d', 1, 2, 3; # prints "3 1 1"
7658 space prefix non-negative number with a space
7659 + prefix non-negative number with a plus sign
7660 - left-justify within the field
7661 0 use zeros, not spaces, to right-justify
7662 # ensure the leading "0" for any octal,
7663 prefix non-zero hexadecimal with "0x" or "0X",
7664 prefix non-zero binary with "0b" or "0B"
7668 printf '<% d>', 12; # prints "< 12>"
7669 printf '<% d>', 0; # prints "< 0>"
7670 printf '<% d>', -12; # prints "<-12>"
7671 printf '<%+d>', 12; # prints "<+12>"
7672 printf '<%+d>', 0; # prints "<+0>"
7673 printf '<%+d>', -12; # prints "<-12>"
7674 printf '<%6s>', 12; # prints "< 12>"
7675 printf '<%-6s>', 12; # prints "<12 >"
7676 printf '<%06s>', 12; # prints "<000012>"
7677 printf '<%#o>', 12; # prints "<014>"
7678 printf '<%#x>', 12; # prints "<0xc>"
7679 printf '<%#X>', 12; # prints "<0XC>"
7680 printf '<%#b>', 12; # prints "<0b1100>"
7681 printf '<%#B>', 12; # prints "<0B1100>"
7683 When a space and a plus sign are given as the flags at once,
7684 the space is ignored.
7686 printf '<%+ d>', 12; # prints "<+12>"
7687 printf '<% +d>', 12; # prints "<+12>"
7689 When the # flag and a precision are given in the %o conversion,
7690 the precision is incremented if it's necessary for the leading "0".
7692 printf '<%#.5o>', 012; # prints "<00012>"
7693 printf '<%#.5o>', 012345; # prints "<012345>"
7694 printf '<%#.0o>', 0; # prints "<0>"
7698 This flag tells Perl to interpret the supplied string as a vector of
7699 integers, one for each character in the string. Perl applies the format to
7700 each integer in turn, then joins the resulting strings with a separator (a
7701 dot C<.> by default). This can be useful for displaying ordinal values of
7702 characters in arbitrary strings:
7704 printf "%vd", "AB\x{100}"; # prints "65.66.256"
7705 printf "version is v%vd\n", $^V; # Perl's version
7707 Put an asterisk C<*> before the C<v> to override the string to
7708 use to separate the numbers:
7710 printf "address is %*vX\n", ":", $addr; # IPv6 address
7711 printf "bits are %0*v8b\n", " ", $bits; # random bitstring
7713 You can also explicitly specify the argument number to use for
7714 the join string using something like C<*2$v>; for example:
7716 printf '%*4$vX %*4$vX %*4$vX', # 3 IPv6 addresses
7719 =item (minimum) width
7721 Arguments are usually formatted to be only as wide as required to
7722 display the given value. You can override the width by putting
7723 a number here, or get the width from the next argument (with C<*>)
7724 or from a specified argument (e.g., with C<*2$>):
7726 printf "<%s>", "a"; # prints "<a>"
7727 printf "<%6s>", "a"; # prints "< a>"
7728 printf "<%*s>", 6, "a"; # prints "< a>"
7729 printf '<%*2$s>', "a", 6; # prints "< a>"
7730 printf "<%2s>", "long"; # prints "<long>" (does not truncate)
7732 If a field width obtained through C<*> is negative, it has the same
7733 effect as the C<-> flag: left-justification.
7735 =item precision, or maximum width
7738 You can specify a precision (for numeric conversions) or a maximum
7739 width (for string conversions) by specifying a C<.> followed by a number.
7740 For floating-point formats except C<g> and C<G>, this specifies
7741 how many places right of the decimal point to show (the default being 6).
7744 # these examples are subject to system-specific variation
7745 printf '<%f>', 1; # prints "<1.000000>"
7746 printf '<%.1f>', 1; # prints "<1.0>"
7747 printf '<%.0f>', 1; # prints "<1>"
7748 printf '<%e>', 10; # prints "<1.000000e+01>"
7749 printf '<%.1e>', 10; # prints "<1.0e+01>"
7751 For "g" and "G", this specifies the maximum number of significant digits to
7754 # These examples are subject to system-specific variation.
7755 printf '<%g>', 1; # prints "<1>"
7756 printf '<%.10g>', 1; # prints "<1>"
7757 printf '<%g>', 100; # prints "<100>"
7758 printf '<%.1g>', 100; # prints "<1e+02>"
7759 printf '<%.2g>', 100.01; # prints "<1e+02>"
7760 printf '<%.5g>', 100.01; # prints "<100.01>"
7761 printf '<%.4g>', 100.01; # prints "<100>"
7762 printf '<%.1g>', 0.0111; # prints "<0.01>"
7763 printf '<%.2g>', 0.0111; # prints "<0.011>"
7764 printf '<%.3g>', 0.0111; # prints "<0.0111>"
7766 For integer conversions, specifying a precision implies that the
7767 output of the number itself should be zero-padded to this width,
7768 where the 0 flag is ignored:
7770 printf '<%.6d>', 1; # prints "<000001>"
7771 printf '<%+.6d>', 1; # prints "<+000001>"
7772 printf '<%-10.6d>', 1; # prints "<000001 >"
7773 printf '<%10.6d>', 1; # prints "< 000001>"
7774 printf '<%010.6d>', 1; # prints "< 000001>"
7775 printf '<%+10.6d>', 1; # prints "< +000001>"
7777 printf '<%.6x>', 1; # prints "<000001>"
7778 printf '<%#.6x>', 1; # prints "<0x000001>"
7779 printf '<%-10.6x>', 1; # prints "<000001 >"
7780 printf '<%10.6x>', 1; # prints "< 000001>"
7781 printf '<%010.6x>', 1; # prints "< 000001>"
7782 printf '<%#10.6x>', 1; # prints "< 0x000001>"
7784 For string conversions, specifying a precision truncates the string
7785 to fit the specified width:
7787 printf '<%.5s>', "truncated"; # prints "<trunc>"
7788 printf '<%10.5s>', "truncated"; # prints "< trunc>"
7790 You can also get the precision from the next argument using C<.*>, or from a
7791 specified argument (e.g., with C<.*2$>):
7793 printf '<%.6x>', 1; # prints "<000001>"
7794 printf '<%.*x>', 6, 1; # prints "<000001>"
7796 printf '<%.*2$x>', 1, 6; # prints "<000001>"
7798 printf '<%6.*2$x>', 1, 4; # prints "< 0001>"
7800 If a precision obtained through C<*> is negative, it counts
7801 as having no precision at all.
7803 printf '<%.*s>', 7, "string"; # prints "<string>"
7804 printf '<%.*s>', 3, "string"; # prints "<str>"
7805 printf '<%.*s>', 0, "string"; # prints "<>"
7806 printf '<%.*s>', -1, "string"; # prints "<string>"
7808 printf '<%.*d>', 1, 0; # prints "<0>"
7809 printf '<%.*d>', 0, 0; # prints "<>"
7810 printf '<%.*d>', -1, 0; # prints "<0>"
7814 For numeric conversions, you can specify the size to interpret the
7815 number as using C<l>, C<h>, C<V>, C<q>, C<L>, or C<ll>. For integer
7816 conversions (C<d u o x X b i D U O>), numbers are usually assumed to be
7817 whatever the default integer size is on your platform (usually 32 or 64
7818 bits), but you can override this to use instead one of the standard C types,
7819 as supported by the compiler used to build Perl:
7821 hh interpret integer as C type "char" or "unsigned
7822 char" on Perl 5.14 or later
7823 h interpret integer as C type "short" or
7825 j interpret integer as C type "intmax_t" on Perl
7826 5.14 or later, and only with a C99 compiler
7828 l interpret integer as C type "long" or
7830 q, L, or ll interpret integer as C type "long long",
7831 "unsigned long long", or "quad" (typically
7833 t interpret integer as C type "ptrdiff_t" on Perl
7835 z interpret integer as C type "size_t" on Perl 5.14
7838 As of 5.14, none of these raises an exception if they are not supported on
7839 your platform. However, if warnings are enabled, a warning of the
7840 L<C<printf>|warnings> warning class is issued on an unsupported
7841 conversion flag. Should you instead prefer an exception, do this:
7843 use warnings FATAL => "printf";
7845 If you would like to know about a version dependency before you
7846 start running the program, put something like this at its top:
7848 use 5.014; # for hh/j/t/z/ printf modifiers
7850 You can find out whether your Perl supports quads via L<Config>:
7853 if ($Config{use64bitint} eq "define"
7854 || $Config{longsize} >= 8) {
7855 print "Nice quads!\n";
7858 For floating-point conversions (C<e f g E F G>), numbers are usually assumed
7859 to be the default floating-point size on your platform (double or long double),
7860 but you can force "long double" with C<q>, C<L>, or C<ll> if your
7861 platform supports them. You can find out whether your Perl supports long
7862 doubles via L<Config>:
7865 print "long doubles\n" if $Config{d_longdbl} eq "define";
7867 You can find out whether Perl considers "long double" to be the default
7868 floating-point size to use on your platform via L<Config>:
7871 if ($Config{uselongdouble} eq "define") {
7872 print "long doubles by default\n";
7875 It can also be that long doubles and doubles are the same thing:
7878 ($Config{doublesize} == $Config{longdblsize}) &&
7879 print "doubles are long doubles\n";
7881 The size specifier C<V> has no effect for Perl code, but is supported for
7882 compatibility with XS code. It means "use the standard size for a Perl
7883 integer or floating-point number", which is the default.
7885 =item order of arguments
7887 Normally, L<C<sprintf>|/sprintf FORMAT, LIST> takes the next unused
7888 argument as the value to
7889 format for each format specification. If the format specification
7890 uses C<*> to require additional arguments, these are consumed from
7891 the argument list in the order they appear in the format
7892 specification I<before> the value to format. Where an argument is
7893 specified by an explicit index, this does not affect the normal
7894 order for the arguments, even when the explicitly specified index
7895 would have been the next argument.
7899 printf "<%*.*s>", $a, $b, $c;
7901 uses C<$a> for the width, C<$b> for the precision, and C<$c>
7902 as the value to format; while:
7904 printf '<%*1$.*s>', $a, $b;
7906 would use C<$a> for the width and precision, and C<$b> as the
7909 Here are some more examples; be aware that when using an explicit
7910 index, the C<$> may need escaping:
7912 printf "%2\$d %d\n", 12, 34; # will print "34 12\n"
7913 printf "%2\$d %d %d\n", 12, 34; # will print "34 12 34\n"
7914 printf "%3\$d %d %d\n", 12, 34, 56; # will print "56 12 34\n"
7915 printf "%2\$*3\$d %d\n", 12, 34, 3; # will print " 34 12\n"
7916 printf "%*1\$.*f\n", 4, 5, 10; # will print "5.0000\n"
7920 If L<C<use locale>|locale> (including C<use locale ':not_characters'>)
7921 is in effect and L<C<POSIX::setlocale>|POSIX/C<setlocale>> has been
7923 the character used for the decimal separator in formatted floating-point
7924 numbers is affected by the C<LC_NUMERIC> locale. See L<perllocale>
7928 X<sqrt> X<root> X<square root>
7932 =for Pod::Functions square root function
7934 Return the positive square root of EXPR. If EXPR is omitted, uses
7935 L<C<$_>|perlvar/$_>. Works only for non-negative operands unless you've
7936 loaded the L<C<Math::Complex>|Math::Complex> module.
7939 print sqrt(-4); # prints 2i
7942 X<srand> X<seed> X<randseed>
7946 =for Pod::Functions seed the random number generator
7948 Sets and returns the random number seed for the L<C<rand>|/rand EXPR>
7951 The point of the function is to "seed" the L<C<rand>|/rand EXPR>
7952 function so that L<C<rand>|/rand EXPR> can produce a different sequence
7953 each time you run your program. When called with a parameter,
7954 L<C<srand>|/srand EXPR> uses that for the seed; otherwise it
7955 (semi-)randomly chooses a seed. In either case, starting with Perl 5.14,
7956 it returns the seed. To signal that your code will work I<only> on Perls
7957 of a recent vintage:
7959 use 5.014; # so srand returns the seed
7961 If L<C<srand>|/srand EXPR> is not called explicitly, it is called
7962 implicitly without a parameter at the first use of the
7963 L<C<rand>|/rand EXPR> operator. However, there are a few situations
7964 where programs are likely to want to call L<C<srand>|/srand EXPR>. One
7965 is for generating predictable results, generally for testing or
7966 debugging. There, you use C<srand($seed)>, with the same C<$seed> each
7967 time. Another case is that you may want to call L<C<srand>|/srand EXPR>
7968 after a L<C<fork>|/fork> to avoid child processes sharing the same seed
7969 value as the parent (and consequently each other).
7971 Do B<not> call C<srand()> (i.e., without an argument) more than once per
7972 process. The internal state of the random number generator should
7973 contain more entropy than can be provided by any seed, so calling
7974 L<C<srand>|/srand EXPR> again actually I<loses> randomness.
7976 Most implementations of L<C<srand>|/srand EXPR> take an integer and will
7978 truncate decimal numbers. This means C<srand(42)> will usually
7979 produce the same results as C<srand(42.1)>. To be safe, always pass
7980 L<C<srand>|/srand EXPR> an integer.
7982 A typical use of the returned seed is for a test program which has too many
7983 combinations to test comprehensively in the time available to it each run. It
7984 can test a random subset each time, and should there be a failure, log the seed
7985 used for that run so that it can later be used to reproduce the same results.
7987 B<L<C<rand>|/rand EXPR> is not cryptographically secure. You should not rely
7988 on it in security-sensitive situations.> As of this writing, a
7989 number of third-party CPAN modules offer random number generators
7990 intended by their authors to be cryptographically secure,
7991 including: L<Data::Entropy>, L<Crypt::Random>, L<Math::Random::Secure>,
7992 and L<Math::TrulyRandom>.
7994 =item stat FILEHANDLE
7995 X<stat> X<file, status> X<ctime>
7999 =item stat DIRHANDLE
8003 =for Pod::Functions get a file's status information
8005 Returns a 13-element list giving the status info for a file, either
8006 the file opened via FILEHANDLE or DIRHANDLE, or named by EXPR. If EXPR is
8007 omitted, it stats L<C<$_>|perlvar/$_> (not C<_>!). Returns the empty
8008 list if L<C<stat>|/stat FILEHANDLE> fails. Typically
8011 my ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
8012 $atime,$mtime,$ctime,$blksize,$blocks)
8015 Not all fields are supported on all filesystem types. Here are the
8016 meanings of the fields:
8018 0 dev device number of filesystem
8020 2 mode file mode (type and permissions)
8021 3 nlink number of (hard) links to the file
8022 4 uid numeric user ID of file's owner
8023 5 gid numeric group ID of file's owner
8024 6 rdev the device identifier (special files only)
8025 7 size total size of file, in bytes
8026 8 atime last access time in seconds since the epoch
8027 9 mtime last modify time in seconds since the epoch
8028 10 ctime inode change time in seconds since the epoch (*)
8029 11 blksize preferred I/O size in bytes for interacting with the
8030 file (may vary from file to file)
8031 12 blocks actual number of system-specific blocks allocated
8032 on disk (often, but not always, 512 bytes each)
8034 (The epoch was at 00:00 January 1, 1970 GMT.)
8036 (*) Not all fields are supported on all filesystem types. Notably, the
8037 ctime field is non-portable. In particular, you cannot expect it to be a
8038 "creation time"; see L<perlport/"Files and Filesystems"> for details.
8040 If L<C<stat>|/stat FILEHANDLE> is passed the special filehandle
8041 consisting of an underline, no stat is done, but the current contents of
8042 the stat structure from the last L<C<stat>|/stat FILEHANDLE>,
8043 L<C<lstat>|/lstat FILEHANDLE>, or filetest are returned. Example:
8045 if (-x $file && (($d) = stat(_)) && $d < 0) {
8046 print "$file is executable NFS file\n";
8049 (This works on machines only for which the device number is negative
8052 Because the mode contains both the file type and its permissions, you
8053 should mask off the file type portion and (s)printf using a C<"%o">
8054 if you want to see the real permissions.
8056 my $mode = (stat($filename))[2];
8057 printf "Permissions are %04o\n", $mode & 07777;
8059 In scalar context, L<C<stat>|/stat FILEHANDLE> returns a boolean value
8061 or failure, and, if successful, sets the information associated with
8062 the special filehandle C<_>.
8064 The L<File::stat> module provides a convenient, by-name access mechanism:
8067 my $sb = stat($filename);
8068 printf "File is %s, size is %s, perm %04o, mtime %s\n",
8069 $filename, $sb->size, $sb->mode & 07777,
8070 scalar localtime $sb->mtime;
8072 You can import symbolic mode constants (C<S_IF*>) and functions
8073 (C<S_IS*>) from the L<Fcntl> module:
8077 my $mode = (stat($filename))[2];
8079 my $user_rwx = ($mode & S_IRWXU) >> 6;
8080 my $group_read = ($mode & S_IRGRP) >> 3;
8081 my $other_execute = $mode & S_IXOTH;
8083 printf "Permissions are %04o\n", S_IMODE($mode), "\n";
8085 my $is_setuid = $mode & S_ISUID;
8086 my $is_directory = S_ISDIR($mode);
8088 You could write the last two using the C<-u> and C<-d> operators.
8089 Commonly available C<S_IF*> constants are:
8091 # Permissions: read, write, execute, for user, group, others.
8093 S_IRWXU S_IRUSR S_IWUSR S_IXUSR
8094 S_IRWXG S_IRGRP S_IWGRP S_IXGRP
8095 S_IRWXO S_IROTH S_IWOTH S_IXOTH
8097 # Setuid/Setgid/Stickiness/SaveText.
8098 # Note that the exact meaning of these is system-dependent.
8100 S_ISUID S_ISGID S_ISVTX S_ISTXT
8102 # File types. Not all are necessarily available on
8105 S_IFREG S_IFDIR S_IFLNK S_IFBLK S_IFCHR
8106 S_IFIFO S_IFSOCK S_IFWHT S_ENFMT
8108 # The following are compatibility aliases for S_IRUSR,
8109 # S_IWUSR, and S_IXUSR.
8111 S_IREAD S_IWRITE S_IEXEC
8113 and the C<S_IF*> functions are
8115 S_IMODE($mode) the part of $mode containing the permission
8116 bits and the setuid/setgid/sticky bits
8118 S_IFMT($mode) the part of $mode containing the file type
8119 which can be bit-anded with (for example)
8120 S_IFREG or with the following functions
8122 # The operators -f, -d, -l, -b, -c, -p, and -S.
8124 S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
8125 S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)
8127 # No direct -X operator counterpart, but for the first one
8128 # the -g operator is often equivalent. The ENFMT stands for
8129 # record flocking enforcement, a platform-dependent feature.
8131 S_ISENFMT($mode) S_ISWHT($mode)
8133 See your native L<chmod(2)> and L<stat(2)> documentation for more details
8134 about the C<S_*> constants. To get status info for a symbolic link
8135 instead of the target file behind the link, use the
8136 L<C<lstat>|/lstat FILEHANDLE> function.
8138 Portability issues: L<perlport/stat>.
8143 =item state TYPE VARLIST
8145 =item state VARLIST : ATTRS
8147 =item state TYPE VARLIST : ATTRS
8149 =for Pod::Functions +state declare and assign a persistent lexical variable
8151 L<C<state>|/state VARLIST> declares a lexically scoped variable, just
8152 like L<C<my>|/my VARLIST>.
8153 However, those variables will never be reinitialized, contrary to
8154 lexical variables that are reinitialized each time their enclosing block
8156 See L<perlsub/"Persistent Private Variables"> for details.
8158 If more than one variable is listed, the list must be placed in
8159 parentheses. With a parenthesised list, L<C<undef>|/undef EXPR> can be
8161 dummy placeholder. However, since initialization of state variables in
8162 list context is currently not possible this would serve no purpose.
8164 L<C<state>|/state VARLIST> is available only if the
8165 L<C<"state"> feature|feature/The 'state' feature> is enabled or if it is
8166 prefixed with C<CORE::>. The
8167 L<C<"state"> feature|feature/The 'state' feature> is enabled
8168 automatically with a C<use v5.10> (or higher) declaration in the current
8177 =for Pod::Functions no-op, formerly optimized input data for repeated searches
8179 At this time, C<study> does nothing. This may change in the future.
8181 Prior to Perl version 5.16, it would create an inverted index of all characters
8182 that occurred in the given SCALAR (or L<C<$_>|perlvar/$_> if unspecified). When
8183 matching a pattern, the rarest character from the pattern would be looked up in
8184 this index. Rarity was based on some static frequency tables constructed from
8185 some C programs and English text.
8188 =item sub NAME BLOCK
8191 =item sub NAME (PROTO) BLOCK
8193 =item sub NAME : ATTRS BLOCK
8195 =item sub NAME (PROTO) : ATTRS BLOCK
8197 =for Pod::Functions declare a subroutine, possibly anonymously
8199 This is subroutine definition, not a real function I<per se>. Without a
8200 BLOCK it's just a forward declaration. Without a NAME, it's an anonymous
8201 function declaration, so does return a value: the CODE ref of the closure
8204 See L<perlsub> and L<perlref> for details about subroutines and
8205 references; see L<attributes> and L<Attribute::Handlers> for more
8206 information about attributes.
8211 =for Pod::Functions +current_sub the current subroutine, or C<undef> if not in a subroutine
8213 A special token that returns a reference to the current subroutine, or
8214 L<C<undef>|/undef EXPR> outside of a subroutine.
8216 The behaviour of L<C<__SUB__>|/__SUB__> within a regex code block (such
8217 as C</(?{...})/>) is subject to change.
8219 This token is only available under C<use v5.16> or the
8220 L<C<"current_sub"> feature|feature/The 'current_sub' feature>.
8223 =item substr EXPR,OFFSET,LENGTH,REPLACEMENT
8224 X<substr> X<substring> X<mid> X<left> X<right>
8226 =item substr EXPR,OFFSET,LENGTH
8228 =item substr EXPR,OFFSET
8230 =for Pod::Functions get or alter a portion of a string
8232 Extracts a substring out of EXPR and returns it. First character is at
8233 offset zero. If OFFSET is negative, starts
8234 that far back from the end of the string. If LENGTH is omitted, returns
8235 everything through the end of the string. If LENGTH is negative, leaves that
8236 many characters off the end of the string.
8238 my $s = "The black cat climbed the green tree";
8239 my $color = substr $s, 4, 5; # black
8240 my $middle = substr $s, 4, -11; # black cat climbed the
8241 my $end = substr $s, 14; # climbed the green tree
8242 my $tail = substr $s, -4; # tree
8243 my $z = substr $s, -4, 2; # tr
8245 You can use the L<C<substr>|/substr EXPR,OFFSET,LENGTH,REPLACEMENT>
8246 function as an lvalue, in which case EXPR
8247 must itself be an lvalue. If you assign something shorter than LENGTH,
8248 the string will shrink, and if you assign something longer than LENGTH,
8249 the string will grow to accommodate it. To keep the string the same
8250 length, you may need to pad or chop your value using
8251 L<C<sprintf>|/sprintf FORMAT, LIST>.
8253 If OFFSET and LENGTH specify a substring that is partly outside the
8254 string, only the part within the string is returned. If the substring
8255 is beyond either end of the string,
8256 L<C<substr>|/substr EXPR,OFFSET,LENGTH,REPLACEMENT> returns the undefined
8257 value and produces a warning. When used as an lvalue, specifying a
8258 substring that is entirely outside the string raises an exception.
8259 Here's an example showing the behavior for boundary cases:
8262 substr($name, 4) = 'dy'; # $name is now 'freddy'
8263 my $null = substr $name, 6, 2; # returns "" (no warning)
8264 my $oops = substr $name, 7; # returns undef, with warning
8265 substr($name, 7) = 'gap'; # raises an exception
8267 An alternative to using
8268 L<C<substr>|/substr EXPR,OFFSET,LENGTH,REPLACEMENT> as an lvalue is to
8270 replacement string as the 4th argument. This allows you to replace
8271 parts of the EXPR and return what was there before in one operation,
8272 just as you can with
8273 L<C<splice>|/splice ARRAY,OFFSET,LENGTH,LIST>.
8275 my $s = "The black cat climbed the green tree";
8276 my $z = substr $s, 14, 7, "jumped from"; # climbed
8277 # $s is now "The black cat jumped from the green tree"
8279 Note that the lvalue returned by the three-argument version of
8280 L<C<substr>|/substr EXPR,OFFSET,LENGTH,REPLACEMENT> acts as
8281 a 'magic bullet'; each time it is assigned to, it remembers which part
8282 of the original string is being modified; for example:
8285 for (substr($x,1,2)) {
8286 $_ = 'a'; print $x,"\n"; # prints 1a4
8287 $_ = 'xyz'; print $x,"\n"; # prints 1xyz4
8289 $_ = 'pq'; print $x,"\n"; # prints 5pq9
8292 With negative offsets, it remembers its position from the end of the string
8293 when the target string is modified:
8296 for (substr($x, -3, 2)) {
8297 $_ = 'a'; print $x,"\n"; # prints 1a4, as above
8299 print $_,"\n"; # prints f
8302 Prior to Perl version 5.10, the result of using an lvalue multiple times was
8303 unspecified. Prior to 5.16, the result with negative offsets was
8306 =item symlink OLDFILE,NEWFILE
8307 X<symlink> X<link> X<symbolic link> X<link, symbolic>
8309 =for Pod::Functions create a symbolic link to a file
8311 Creates a new filename symbolically linked to the old filename.
8312 Returns C<1> for success, C<0> otherwise. On systems that don't support
8313 symbolic links, raises an exception. To check for that,
8316 my $symlink_exists = eval { symlink("",""); 1 };
8318 Portability issues: L<perlport/symlink>.
8320 =item syscall NUMBER, LIST
8321 X<syscall> X<system call>
8323 =for Pod::Functions execute an arbitrary system call
8325 Calls the system call specified as the first element of the list,
8326 passing the remaining elements as arguments to the system call. If
8327 unimplemented, raises an exception. The arguments are interpreted
8328 as follows: if a given argument is numeric, the argument is passed as
8329 an int. If not, the pointer to the string value is passed. You are
8330 responsible to make sure a string is pre-extended long enough to
8331 receive any result that might be written into a string. You can't use a
8332 string literal (or other read-only string) as an argument to
8333 L<C<syscall>|/syscall NUMBER, LIST> because Perl has to assume that any
8334 string pointer might be written through. If your
8335 integer arguments are not literals and have never been interpreted in a
8336 numeric context, you may need to add C<0> to them to force them to look
8337 like numbers. This emulates the
8338 L<C<syswrite>|/syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET> function (or
8341 require 'syscall.ph'; # may need to run h2ph
8342 my $s = "hi there\n";
8343 syscall(SYS_write(), fileno(STDOUT), $s, length $s);
8345 Note that Perl supports passing of up to only 14 arguments to your syscall,
8346 which in practice should (usually) suffice.
8348 Syscall returns whatever value returned by the system call it calls.
8349 If the system call fails, L<C<syscall>|/syscall NUMBER, LIST> returns
8350 C<-1> and sets L<C<$!>|perlvar/$!> (errno).
8351 Note that some system calls I<can> legitimately return C<-1>. The proper
8352 way to handle such calls is to assign C<$! = 0> before the call, then
8353 check the value of L<C<$!>|perlvar/$!> if
8354 L<C<syscall>|/syscall NUMBER, LIST> returns C<-1>.
8356 There's a problem with C<syscall(SYS_pipe())>: it returns the file
8357 number of the read end of the pipe it creates, but there is no way
8358 to retrieve the file number of the other end. You can avoid this
8359 problem by using L<C<pipe>|/pipe READHANDLE,WRITEHANDLE> instead.
8361 Portability issues: L<perlport/syscall>.
8363 =item sysopen FILEHANDLE,FILENAME,MODE
8366 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
8368 =for Pod::Functions +5.002 open a file, pipe, or descriptor
8370 Opens the file whose filename is given by FILENAME, and associates it with
8371 FILEHANDLE. If FILEHANDLE is an expression, its value is used as the real
8372 filehandle wanted; an undefined scalar will be suitably autovivified. This
8373 function calls the underlying operating system's L<open(2)> function with the
8374 parameters FILENAME, MODE, and PERMS.
8376 Returns true on success and L<C<undef>|/undef EXPR> otherwise.
8378 The possible values and flag bits of the MODE parameter are
8379 system-dependent; they are available via the standard module
8380 L<C<Fcntl>|Fcntl>. See the documentation of your operating system's
8381 L<open(2)> syscall to see
8382 which values and flag bits are available. You may combine several flags
8383 using the C<|>-operator.
8385 Some of the most common values are C<O_RDONLY> for opening the file in
8386 read-only mode, C<O_WRONLY> for opening the file in write-only mode,
8387 and C<O_RDWR> for opening the file in read-write mode.
8388 X<O_RDONLY> X<O_RDWR> X<O_WRONLY>
8390 For historical reasons, some values work on almost every system
8391 supported by Perl: 0 means read-only, 1 means write-only, and 2
8392 means read/write. We know that these values do I<not> work under
8393 OS/390 and on the Macintosh; you probably don't want to
8394 use them in new code.
8396 If the file named by FILENAME does not exist and the
8397 L<C<open>|/open FILEHANDLE,EXPR> call creates
8398 it (typically because MODE includes the C<O_CREAT> flag), then the value of
8399 PERMS specifies the permissions of the newly created file. If you omit
8400 the PERMS argument to L<C<sysopen>|/sysopen FILEHANDLE,FILENAME,MODE>,
8401 Perl uses the octal value C<0666>.
8402 These permission values need to be in octal, and are modified by your
8403 process's current L<C<umask>|/umask EXPR>.
8406 In many systems the C<O_EXCL> flag is available for opening files in
8407 exclusive mode. This is B<not> locking: exclusiveness means here that
8408 if the file already exists,
8409 L<C<sysopen>|/sysopen FILEHANDLE,FILENAME,MODE> fails. C<O_EXCL> may
8411 on network filesystems, and has no effect unless the C<O_CREAT> flag
8412 is set as well. Setting C<O_CREAT|O_EXCL> prevents the file from
8413 being opened if it is a symbolic link. It does not protect against
8414 symbolic links in the file's path.
8417 Sometimes you may want to truncate an already-existing file. This
8418 can be done using the C<O_TRUNC> flag. The behavior of
8419 C<O_TRUNC> with C<O_RDONLY> is undefined.
8422 You should seldom if ever use C<0644> as argument to
8423 L<C<sysopen>|/sysopen FILEHANDLE,FILENAME,MODE>, because
8424 that takes away the user's option to have a more permissive umask.
8425 Better to omit it. See L<C<umask>|/umask EXPR> for more on this.
8427 Note that under Perls older than 5.8.0,
8428 L<C<sysopen>|/sysopen FILEHANDLE,FILENAME,MODE> depends on the
8429 L<fdopen(3)> C library function. On many Unix systems, L<fdopen(3)> is known
8430 to fail when file descriptors exceed a certain value, typically 255. If
8431 you need more file descriptors than that, consider using the
8432 L<C<POSIX::open>|POSIX/C<open>> function. For Perls 5.8.0 and later,
8433 PerlIO is (most often) the default.
8435 See L<perlopentut> for a kinder, gentler explanation of opening files.
8437 Portability issues: L<perlport/sysopen>.
8439 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
8442 =item sysread FILEHANDLE,SCALAR,LENGTH
8444 =for Pod::Functions fixed-length unbuffered input from a filehandle
8446 Attempts to read LENGTH bytes of data into variable SCALAR from the
8447 specified FILEHANDLE, using L<read(2)>. It bypasses
8448 buffered IO, so mixing this with other kinds of reads,
8449 L<C<print>|/print FILEHANDLE LIST>, L<C<write>|/write FILEHANDLE>,
8450 L<C<seek>|/seek FILEHANDLE,POSITION,WHENCE>,
8451 L<C<tell>|/tell FILEHANDLE>, or L<C<eof>|/eof FILEHANDLE> can cause
8452 confusion because the
8453 perlio or stdio layers usually buffer data. Returns the number of
8454 bytes actually read, C<0> at end of file, or undef if there was an
8455 error (in the latter case L<C<$!>|perlvar/$!> is also set). SCALAR will
8457 shrunk so that the last byte actually read is the last byte of the
8458 scalar after the read.
8460 An OFFSET may be specified to place the read data at some place in the
8461 string other than the beginning. A negative OFFSET specifies
8462 placement at that many characters counting backwards from the end of
8463 the string. A positive OFFSET greater than the length of SCALAR
8464 results in the string being padded to the required size with C<"\0">
8465 bytes before the result of the read is appended.
8467 There is no syseof() function, which is ok, since
8468 L<C<eof>|/eof FILEHANDLE> doesn't work well on device files (like ttys)
8469 anyway. Use L<C<sysread>|/sysread FILEHANDLE,SCALAR,LENGTH,OFFSET> and
8470 check for a return value for 0 to decide whether you're done.
8472 Note that if the filehandle has been marked as C<:utf8>, Unicode
8473 characters are read instead of bytes (the LENGTH, OFFSET, and the
8474 return value of L<C<sysread>|/sysread FILEHANDLE,SCALAR,LENGTH,OFFSET>
8475 are in Unicode characters). The C<:encoding(...)> layer implicitly
8476 introduces the C<:utf8> layer. See
8477 L<C<binmode>|/binmode FILEHANDLE, LAYER>,
8478 L<C<open>|/open FILEHANDLE,EXPR>, and the L<open> pragma.
8480 =item sysseek FILEHANDLE,POSITION,WHENCE
8483 =for Pod::Functions +5.004 position I/O pointer on handle used with sysread and syswrite
8485 Sets FILEHANDLE's system position I<in bytes> using L<lseek(2)>. FILEHANDLE may
8486 be an expression whose value gives the name of the filehandle. The values
8487 for WHENCE are C<0> to set the new position to POSITION; C<1> to set the it
8488 to the current position plus POSITION; and C<2> to set it to EOF plus
8489 POSITION, typically negative.
8491 Note the emphasis on bytes: even if the filehandle has been set to operate
8492 on characters (for example using the C<:encoding(utf8)> I/O layer), the
8493 L<C<seek>|/seek FILEHANDLE,POSITION,WHENCE>,
8494 L<C<tell>|/tell FILEHANDLE>, and
8495 L<C<sysseek>|/sysseek FILEHANDLE,POSITION,WHENCE>
8496 family of functions use byte offsets, not character offsets,
8497 because seeking to a character offset would be very slow in a UTF-8 file.
8499 L<C<sysseek>|/sysseek FILEHANDLE,POSITION,WHENCE> bypasses normal
8500 buffered IO, so mixing it with reads other than
8501 L<C<sysread>|/sysread FILEHANDLE,SCALAR,LENGTH,OFFSET> (for example
8502 L<C<readline>|/readline EXPR> or
8503 L<C<read>|/read FILEHANDLE,SCALAR,LENGTH,OFFSET>),
8504 L<C<print>|/print FILEHANDLE LIST>, L<C<write>|/write FILEHANDLE>,
8505 L<C<seek>|/seek FILEHANDLE,POSITION,WHENCE>,
8506 L<C<tell>|/tell FILEHANDLE>, or L<C<eof>|/eof FILEHANDLE> may cause
8509 For WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>,
8510 and C<SEEK_END> (start of the file, current position, end of the file)
8511 from the L<Fcntl> module. Use of the constants is also more portable
8512 than relying on 0, 1, and 2. For example to define a "systell" function:
8514 use Fcntl 'SEEK_CUR';
8515 sub systell { sysseek($_[0], 0, SEEK_CUR) }
8517 Returns the new position, or the undefined value on failure. A position
8518 of zero is returned as the string C<"0 but true">; thus
8519 L<C<sysseek>|/sysseek FILEHANDLE,POSITION,WHENCE> returns
8520 true on success and false on failure, yet you can still easily determine
8526 =item system PROGRAM LIST
8528 =for Pod::Functions run a separate program
8530 Does exactly the same thing as L<C<exec>|/exec LIST>, except that a fork is
8531 done first and the parent process waits for the child process to
8532 exit. Note that argument processing varies depending on the
8533 number of arguments. If there is more than one argument in LIST,
8534 or if LIST is an array with more than one value, starts the program
8535 given by the first element of the list with arguments given by the
8536 rest of the list. If there is only one scalar argument, the argument
8537 is checked for shell metacharacters, and if there are any, the
8538 entire argument is passed to the system's command shell for parsing
8539 (this is C</bin/sh -c> on Unix platforms, but varies on other
8540 platforms). If there are no shell metacharacters in the argument,
8541 it is split into words and passed directly to C<execvp>, which is
8542 more efficient. On Windows, only the C<system PROGRAM LIST> syntax will
8543 reliably avoid using the shell; C<system LIST>, even with more than one
8544 element, will fall back to the shell if the first spawn fails.
8546 Perl will attempt to flush all files opened for
8547 output before any operation that may do a fork, but this may not be
8548 supported on some platforms (see L<perlport>). To be safe, you may need
8549 to set L<C<$E<verbar>>|perlvar/$E<verbar>> (C<$AUTOFLUSH> in L<English>)
8550 or call the C<autoflush> method of L<C<IO::Handle>|IO::Handle/METHODS>
8551 on any open handles.
8553 The return value is the exit status of the program as returned by the
8554 L<C<wait>|/wait> call. To get the actual exit value, shift right by
8555 eight (see below). See also L<C<exec>|/exec LIST>. This is I<not> what
8556 you want to use to capture the output from a command; for that you
8557 should use merely backticks or
8558 L<C<qxE<sol>E<sol>>|/qxE<sol>STRINGE<sol>>, as described in
8559 L<perlop/"`STRING`">. Return value of -1 indicates a failure to start
8560 the program or an error of the L<wait(2)> system call (inspect
8561 L<C<$!>|perlvar/$!> for the reason).
8563 If you'd like to make L<C<system>|/system LIST> (and many other bits of
8564 Perl) die on error, have a look at the L<autodie> pragma.
8566 Like L<C<exec>|/exec LIST>, L<C<system>|/system LIST> allows you to lie
8567 to a program about its name if you use the C<system PROGRAM LIST>
8568 syntax. Again, see L<C<exec>|/exec LIST>.
8570 Since C<SIGINT> and C<SIGQUIT> are ignored during the execution of
8571 L<C<system>|/system LIST>, if you expect your program to terminate on
8572 receipt of these signals you will need to arrange to do so yourself
8573 based on the return value.
8575 my @args = ("command", "arg1", "arg2");
8577 or die "system @args failed: $?";
8579 If you'd like to manually inspect L<C<system>|/system LIST>'s failure,
8580 you can check all possible failure modes by inspecting
8581 L<C<$?>|perlvar/$?> like this:
8584 print "failed to execute: $!\n";
8587 printf "child died with signal %d, %s coredump\n",
8588 ($? & 127), ($? & 128) ? 'with' : 'without';
8591 printf "child exited with value %d\n", $? >> 8;
8594 Alternatively, you may inspect the value of
8595 L<C<${^CHILD_ERROR_NATIVE}>|perlvar/${^CHILD_ERROR_NATIVE}> with the
8596 L<C<W*()>|POSIX/C<WIFEXITED>> calls from the L<POSIX> module.
8598 When L<C<system>|/system LIST>'s arguments are executed indirectly by
8599 the shell, results and return codes are subject to its quirks.
8600 See L<perlop/"`STRING`"> and L<C<exec>|/exec LIST> for details.
8602 Since L<C<system>|/system LIST> does a L<C<fork>|/fork> and
8603 L<C<wait>|/wait> it may affect a C<SIGCHLD> handler. See L<perlipc> for
8606 Portability issues: L<perlport/system>.
8608 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
8611 =item syswrite FILEHANDLE,SCALAR,LENGTH
8613 =item syswrite FILEHANDLE,SCALAR
8615 =for Pod::Functions fixed-length unbuffered output to a filehandle
8617 Attempts to write LENGTH bytes of data from variable SCALAR to the
8618 specified FILEHANDLE, using L<write(2)>. If LENGTH is
8619 not specified, writes whole SCALAR. It bypasses buffered IO, so
8620 mixing this with reads (other than C<sysread)>),
8621 L<C<print>|/print FILEHANDLE LIST>, L<C<write>|/write FILEHANDLE>,
8622 L<C<seek>|/seek FILEHANDLE,POSITION,WHENCE>,
8623 L<C<tell>|/tell FILEHANDLE>, or L<C<eof>|/eof FILEHANDLE> may cause
8624 confusion because the perlio and stdio layers usually buffer data.
8625 Returns the number of bytes actually written, or L<C<undef>|/undef EXPR>
8626 if there was an error (in this case the errno variable
8627 L<C<$!>|perlvar/$!> is also set). If the LENGTH is greater than the
8628 data available in the SCALAR after the OFFSET, only as much data as is
8629 available will be written.
8631 An OFFSET may be specified to write the data from some part of the
8632 string other than the beginning. A negative OFFSET specifies writing
8633 that many characters counting backwards from the end of the string.
8634 If SCALAR is of length zero, you can only use an OFFSET of 0.
8636 B<WARNING>: If the filehandle is marked C<:utf8>, Unicode characters
8637 encoded in UTF-8 are written instead of bytes, and the LENGTH, OFFSET, and
8638 return value of L<C<syswrite>|/syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET>
8639 are in (UTF8-encoded Unicode) characters.
8640 The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer.
8641 Alternately, if the handle is not marked with an encoding but you
8642 attempt to write characters with code points over 255, raises an exception.
8643 See L<C<binmode>|/binmode FILEHANDLE, LAYER>,
8644 L<C<open>|/open FILEHANDLE,EXPR>, and the L<open> pragma.
8646 =item tell FILEHANDLE
8651 =for Pod::Functions get current seekpointer on a filehandle
8653 Returns the current position I<in bytes> for FILEHANDLE, or -1 on
8654 error. FILEHANDLE may be an expression whose value gives the name of
8655 the actual filehandle. If FILEHANDLE is omitted, assumes the file
8658 Note the emphasis on bytes: even if the filehandle has been set to operate
8659 on characters (for example using the C<:encoding(utf8)> I/O layer), the
8660 L<C<seek>|/seek FILEHANDLE,POSITION,WHENCE>,
8661 L<C<tell>|/tell FILEHANDLE>, and
8662 L<C<sysseek>|/sysseek FILEHANDLE,POSITION,WHENCE>
8663 family of functions use byte offsets, not character offsets,
8664 because seeking to a character offset would be very slow in a UTF-8 file.
8666 The return value of L<C<tell>|/tell FILEHANDLE> for the standard streams
8667 like the STDIN depends on the operating system: it may return -1 or
8668 something else. L<C<tell>|/tell FILEHANDLE> on pipes, fifos, and
8669 sockets usually returns -1.
8671 There is no C<systell> function. Use
8672 L<C<sysseek($fh, 0, 1)>|/sysseek FILEHANDLE,POSITION,WHENCE> for that.
8674 Do not use L<C<tell>|/tell FILEHANDLE> (or other buffered I/O
8675 operations) on a filehandle that has been manipulated by
8676 L<C<sysread>|/sysread FILEHANDLE,SCALAR,LENGTH,OFFSET>,
8677 L<C<syswrite>|/syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET>, or
8678 L<C<sysseek>|/sysseek FILEHANDLE,POSITION,WHENCE>. Those functions
8679 ignore the buffering, while L<C<tell>|/tell FILEHANDLE> does not.
8681 =item telldir DIRHANDLE
8684 =for Pod::Functions get current seekpointer on a directory handle
8686 Returns the current position of the L<C<readdir>|/readdir DIRHANDLE>
8687 routines on DIRHANDLE. Value may be given to
8688 L<C<seekdir>|/seekdir DIRHANDLE,POS> to access a particular location in
8689 a directory. L<C<telldir>|/telldir DIRHANDLE> has the same caveats
8690 about possible directory compaction as the corresponding system library
8693 =item tie VARIABLE,CLASSNAME,LIST
8696 =for Pod::Functions +5.002 bind a variable to an object class
8698 This function binds a variable to a package class that will provide the
8699 implementation for the variable. VARIABLE is the name of the variable
8700 to be enchanted. CLASSNAME is the name of a class implementing objects
8701 of correct type. Any additional arguments are passed to the
8702 appropriate constructor
8703 method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
8704 or C<TIEHASH>). Typically these are arguments such as might be passed
8705 to the L<dbm_open(3)> function of C. The object returned by the
8706 constructor is also returned by the
8707 L<C<tie>|/tie VARIABLE,CLASSNAME,LIST> function, which would be useful
8708 if you want to access other methods in CLASSNAME.
8710 Note that functions such as L<C<keys>|/keys HASH> and
8711 L<C<values>|/values HASH> may return huge lists when used on large
8712 objects, like DBM files. You may prefer to use the L<C<each>|/each
8713 HASH> function to iterate over such. Example:
8715 # print out history file offsets
8717 tie(my %HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
8718 while (my ($key,$val) = each %HIST) {
8719 print $key, ' = ', unpack('L', $val), "\n";
8722 A class implementing a hash should have the following methods:
8724 TIEHASH classname, LIST
8726 STORE this, key, value
8731 NEXTKEY this, lastkey
8736 A class implementing an ordinary array should have the following methods:
8738 TIEARRAY classname, LIST
8740 STORE this, key, value
8742 STORESIZE this, count
8748 SPLICE this, offset, length, LIST
8755 A class implementing a filehandle should have the following methods:
8757 TIEHANDLE classname, LIST
8758 READ this, scalar, length, offset
8761 WRITE this, scalar, length, offset
8763 PRINTF this, format, LIST
8767 SEEK this, position, whence
8769 OPEN this, mode, LIST
8774 A class implementing a scalar should have the following methods:
8776 TIESCALAR classname, LIST
8782 Not all methods indicated above need be implemented. See L<perltie>,
8783 L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>.
8785 Unlike L<C<dbmopen>|/dbmopen HASH,DBNAME,MASK>, the
8786 L<C<tie>|/tie VARIABLE,CLASSNAME,LIST> function will not
8787 L<C<use>|/use Module VERSION LIST> or L<C<require>|/require VERSION> a
8788 module for you; you need to do that explicitly yourself. See L<DB_File>
8789 or the L<Config> module for interesting
8790 L<C<tie>|/tie VARIABLE,CLASSNAME,LIST> implementations.
8792 For further details see L<perltie>, L<C<tied>|/tied VARIABLE>.
8797 =for Pod::Functions get a reference to the object underlying a tied variable
8799 Returns a reference to the object underlying VARIABLE (the same value
8800 that was originally returned by the
8801 L<C<tie>|/tie VARIABLE,CLASSNAME,LIST> call that bound the variable
8802 to a package.) Returns the undefined value if VARIABLE isn't tied to a
8808 =for Pod::Functions return number of seconds since 1970
8810 Returns the number of non-leap seconds since whatever time the system
8811 considers to be the epoch, suitable for feeding to
8812 L<C<gmtime>|/gmtime EXPR> and L<C<localtime>|/localtime EXPR>. On most
8813 systems the epoch is 00:00:00 UTC, January 1, 1970;
8814 a prominent exception being Mac OS Classic which uses 00:00:00, January 1,
8815 1904 in the current local time zone for its epoch.
8817 For measuring time in better granularity than one second, use the
8818 L<Time::HiRes> module from Perl 5.8 onwards (or from CPAN before then), or,
8819 if you have L<gettimeofday(2)>, you may be able to use the
8820 L<C<syscall>|/syscall NUMBER, LIST> interface of Perl. See L<perlfaq8>
8823 For date and time processing look at the many related modules on CPAN.
8824 For a comprehensive date and time representation look at the
8830 =for Pod::Functions return elapsed time for self and child processes
8832 Returns a four-element list giving the user and system times in
8833 seconds for this process and any exited children of this process.
8835 my ($user,$system,$cuser,$csystem) = times;
8837 In scalar context, L<C<times>|/times> returns C<$user>.
8839 Children's times are only included for terminated children.
8841 Portability issues: L<perlport/times>.
8845 =for Pod::Functions transliterate a string
8847 The transliteration operator. Same as
8848 L<C<yE<sol>E<sol>E<sol>>|/yE<sol>E<sol>E<sol>>. See
8849 L<perlop/"Quote-Like Operators">.
8851 =item truncate FILEHANDLE,LENGTH
8854 =item truncate EXPR,LENGTH
8856 =for Pod::Functions shorten a file
8858 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
8859 specified length. Raises an exception if truncate isn't implemented
8860 on your system. Returns true if successful, L<C<undef>|/undef EXPR> on
8863 The behavior is undefined if LENGTH is greater than the length of the
8866 The position in the file of FILEHANDLE is left unchanged. You may want to
8867 call L<seek|/"seek FILEHANDLE,POSITION,WHENCE"> before writing to the
8870 Portability issues: L<perlport/truncate>.
8873 X<uc> X<uppercase> X<toupper>
8877 =for Pod::Functions return upper-case version of a string
8879 Returns an uppercased version of EXPR. This is the internal function
8880 implementing the C<\U> escape in double-quoted strings.
8881 It does not attempt to do titlecase mapping on initial letters. See
8882 L<C<ucfirst>|/ucfirst EXPR> for that.
8884 If EXPR is omitted, uses L<C<$_>|perlvar/$_>.
8886 This function behaves the same way under various pragmas, such as in a locale,
8887 as L<C<lc>|/lc EXPR> does.
8890 X<ucfirst> X<uppercase>
8894 =for Pod::Functions return a string with just the next letter in upper case
8896 Returns the value of EXPR with the first character in uppercase
8897 (titlecase in Unicode). This is the internal function implementing
8898 the C<\u> escape in double-quoted strings.
8900 If EXPR is omitted, uses L<C<$_>|perlvar/$_>.
8902 This function behaves the same way under various pragmas, such as in a locale,
8903 as L<C<lc>|/lc EXPR> does.
8910 =for Pod::Functions set file creation mode mask
8912 Sets the umask for the process to EXPR and returns the previous value.
8913 If EXPR is omitted, merely returns the current umask.
8915 The Unix permission C<rwxr-x---> is represented as three sets of three
8916 bits, or three octal digits: C<0750> (the leading 0 indicates octal
8917 and isn't one of the digits). The L<C<umask>|/umask EXPR> value is such
8918 a number representing disabled permissions bits. The permission (or
8919 "mode") values you pass L<C<mkdir>|/mkdir FILENAME,MASK> or
8920 L<C<sysopen>|/sysopen FILEHANDLE,FILENAME,MODE> are modified by your
8921 umask, so even if you tell
8922 L<C<sysopen>|/sysopen FILEHANDLE,FILENAME,MODE> to create a file with
8923 permissions C<0777>, if your umask is C<0022>, then the file will
8924 actually be created with permissions C<0755>. If your
8925 L<C<umask>|/umask EXPR> were C<0027> (group can't write; others can't
8926 read, write, or execute), then passing
8927 L<C<sysopen>|/sysopen FILEHANDLE,FILENAME,MODE> C<0666> would create a
8928 file with mode C<0640> (because C<0666 &~ 027> is C<0640>).
8930 Here's some advice: supply a creation mode of C<0666> for regular
8931 files (in L<C<sysopen>|/sysopen FILEHANDLE,FILENAME,MODE>) and one of
8932 C<0777> for directories (in L<C<mkdir>|/mkdir FILENAME,MASK>) and
8933 executable files. This gives users the freedom of
8934 choice: if they want protected files, they might choose process umasks
8935 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
8936 Programs should rarely if ever make policy decisions better left to
8937 the user. The exception to this is when writing files that should be
8938 kept private: mail files, web browser cookies, F<.rhosts> files, and
8941 If L<umask(2)> is not implemented on your system and you are trying to
8942 restrict access for I<yourself> (i.e., C<< (EXPR & 0700) > 0 >>),
8943 raises an exception. If L<umask(2)> is not implemented and you are
8944 not trying to restrict access for yourself, returns
8945 L<C<undef>|/undef EXPR>.
8947 Remember that a umask is a number, usually given in octal; it is I<not> a
8948 string of octal digits. See also L<C<oct>|/oct EXPR>, if all you have
8951 Portability issues: L<perlport/umask>.
8954 X<undef> X<undefine>
8958 =for Pod::Functions remove a variable or function definition
8960 Undefines the value of EXPR, which must be an lvalue. Use only on a
8961 scalar value, an array (using C<@>), a hash (using C<%>), a subroutine
8962 (using C<&>), or a typeglob (using C<*>). Saying C<undef $hash{$key}>
8963 will probably not do what you expect on most predefined variables or
8964 DBM list values, so don't do that; see L<C<delete>|/delete EXPR>.
8965 Always returns the undefined value.
8966 You can omit the EXPR, in which case nothing is
8967 undefined, but you still get an undefined value that you could, for
8968 instance, return from a subroutine, assign to a variable, or pass as a
8969 parameter. Examples:
8972 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
8976 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
8977 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
8978 select undef, undef, undef, 0.25;
8979 my ($x, $y, undef, $z) = foo(); # Ignore third value returned
8981 Note that this is a unary operator, not a list operator.
8984 X<unlink> X<delete> X<remove> X<rm> X<del>
8988 =for Pod::Functions remove one link to a file
8990 Deletes a list of files. On success, it returns the number of files
8991 it successfully deleted. On failure, it returns false and sets
8992 L<C<$!>|perlvar/$!> (errno):
8994 my $unlinked = unlink 'a', 'b', 'c';
8996 unlink glob "*.bak";
8998 On error, L<C<unlink>|/unlink LIST> will not tell you which files it
9000 If you want to know which files you could not remove, try them one
9003 foreach my $file ( @goners ) {
9004 unlink $file or warn "Could not unlink $file: $!";
9007 Note: L<C<unlink>|/unlink LIST> will not attempt to delete directories
9009 superuser and the B<-U> flag is supplied to Perl. Even if these
9010 conditions are met, be warned that unlinking a directory can inflict
9011 damage on your filesystem. Finally, using L<C<unlink>|/unlink LIST> on
9012 directories is not supported on many operating systems. Use
9013 L<C<rmdir>|/rmdir FILENAME> instead.
9015 If LIST is omitted, L<C<unlink>|/unlink LIST> uses L<C<$_>|perlvar/$_>.
9017 =item unpack TEMPLATE,EXPR
9020 =item unpack TEMPLATE
9022 =for Pod::Functions convert binary structure into normal perl variables
9024 L<C<unpack>|/unpack TEMPLATE,EXPR> does the reverse of
9025 L<C<pack>|/pack TEMPLATE,LIST>: it takes a string
9026 and expands it out into a list of values.
9027 (In scalar context, it returns merely the first value produced.)
9029 If EXPR is omitted, unpacks the L<C<$_>|perlvar/$_> string.
9030 See L<perlpacktut> for an introduction to this function.
9032 The string is broken into chunks described by the TEMPLATE. Each chunk
9033 is converted separately to a value. Typically, either the string is a result
9034 of L<C<pack>|/pack TEMPLATE,LIST>, or the characters of the string
9035 represent a C structure of some kind.
9037 The TEMPLATE has the same format as in the
9038 L<C<pack>|/pack TEMPLATE,LIST> function.
9039 Here's a subroutine that does substring:
9042 my ($what, $where, $howmuch) = @_;
9043 unpack("x$where a$howmuch", $what);
9048 sub ordinal { unpack("W",$_[0]); } # same as ord()
9050 In addition to fields allowed in L<C<pack>|/pack TEMPLATE,LIST>, you may
9051 prefix a field with a %<number> to indicate that
9052 you want a <number>-bit checksum of the items instead of the items
9053 themselves. Default is a 16-bit checksum. The checksum is calculated by
9054 summing numeric values of expanded values (for string fields the sum of
9055 C<ord($char)> is taken; for bit fields the sum of zeroes and ones).
9057 For example, the following
9058 computes the same number as the System V sum program:
9062 unpack("%32W*", readline) % 65535;
9065 The following efficiently counts the number of set bits in a bit vector:
9067 my $setbits = unpack("%32b*", $selectmask);
9069 The C<p> and C<P> formats should be used with care. Since Perl
9070 has no way of checking whether the value passed to
9071 L<C<unpack>|/unpack TEMPLATE,EXPR>
9072 corresponds to a valid memory location, passing a pointer value that's
9073 not known to be valid is likely to have disastrous consequences.
9075 If there are more pack codes or if the repeat count of a field or a group
9076 is larger than what the remainder of the input string allows, the result
9077 is not well defined: the repeat count may be decreased, or
9078 L<C<unpack>|/unpack TEMPLATE,EXPR> may produce empty strings or zeros,
9079 or it may raise an exception.
9080 If the input string is longer than one described by the TEMPLATE,
9081 the remainder of that input string is ignored.
9083 See L<C<pack>|/pack TEMPLATE,LIST> for more examples and notes.
9085 =item unshift ARRAY,LIST
9088 =for Pod::Functions prepend more elements to the beginning of a list
9090 Does the opposite of a L<C<shift>|/shift ARRAY>. Or the opposite of a
9091 L<C<push>|/push ARRAY,LIST>,
9092 depending on how you look at it. Prepends list to the front of the
9093 array and returns the new number of elements in the array.
9095 unshift(@ARGV, '-e') unless $ARGV[0] =~ /^-/;
9097 Note the LIST is prepended whole, not one element at a time, so the
9098 prepended elements stay in the same order. Use
9099 L<C<reverse>|/reverse LIST> to do the reverse.
9101 Starting with Perl 5.14, an experimental feature allowed
9102 L<C<unshift>|/unshift ARRAY,LIST> to take
9103 a scalar expression. This experiment has been deemed unsuccessful, and was
9104 removed as of Perl 5.24.
9106 =item untie VARIABLE
9109 =for Pod::Functions break a tie binding to a variable
9111 Breaks the binding between a variable and a package.
9112 (See L<tie|/tie VARIABLE,CLASSNAME,LIST>.)
9113 Has no effect if the variable is not tied.
9115 =item use Module VERSION LIST
9116 X<use> X<module> X<import>
9118 =item use Module VERSION
9120 =item use Module LIST
9126 =for Pod::Functions load in a module at compile time and import its namespace
9128 Imports some semantics into the current package from the named module,
9129 generally by aliasing certain subroutine or variable names into your
9130 package. It is exactly equivalent to
9132 BEGIN { require Module; Module->import( LIST ); }
9134 except that Module I<must> be a bareword.
9135 The importation can be made conditional by using the L<if> module.
9137 In the peculiar C<use VERSION> form, VERSION may be either a positive
9138 decimal fraction such as 5.006, which will be compared to
9139 L<C<$]>|perlvar/$]>, or a v-string of the form v5.6.1, which will be
9140 compared to L<C<$^V>|perlvar/$^V> (aka $PERL_VERSION). An
9141 exception is raised if VERSION is greater than the version of the
9142 current Perl interpreter; Perl will not attempt to parse the rest of the
9143 file. Compare with L<C<require>|/require VERSION>, which can do a
9144 similar check at run time.
9145 Symmetrically, C<no VERSION> allows you to specify that you want a version
9146 of Perl older than the specified one.
9148 Specifying VERSION as a literal of the form v5.6.1 should generally be
9149 avoided, because it leads to misleading error messages under earlier
9150 versions of Perl (that is, prior to 5.6.0) that do not support this
9151 syntax. The equivalent numeric version should be used instead.
9153 use v5.6.1; # compile time version check
9155 use 5.006_001; # ditto; preferred for backwards compatibility
9157 This is often useful if you need to check the current Perl version before
9158 L<C<use>|/use Module VERSION LIST>ing library modules that won't work
9159 with older versions of Perl.
9160 (We try not to do this more than we have to.)
9162 C<use VERSION> also lexically enables all features available in the requested
9163 version as defined by the L<feature> pragma, disabling any features
9164 not in the requested version's feature bundle. See L<feature>.
9165 Similarly, if the specified Perl version is greater than or equal to
9166 5.12.0, strictures are enabled lexically as
9167 with L<C<use strict>|strict>. Any explicit use of
9168 C<use strict> or C<no strict> overrides C<use VERSION>, even if it comes
9169 before it. Later use of C<use VERSION>
9170 will override all behavior of a previous
9171 C<use VERSION>, possibly removing the C<strict> and C<feature> added by
9172 C<use VERSION>. C<use VERSION> does not
9173 load the F<feature.pm> or F<strict.pm>
9176 The C<BEGIN> forces the L<C<require>|/require VERSION> and
9177 L<C<import>|/import LIST> to happen at compile time. The
9178 L<C<require>|/require VERSION> makes sure the module is loaded into
9179 memory if it hasn't been yet. The L<C<import>|/import LIST> is not a
9180 builtin; it's just an ordinary static method
9181 call into the C<Module> package to tell the module to import the list of
9182 features back into the current package. The module can implement its
9183 L<C<import>|/import LIST> method any way it likes, though most modules
9184 just choose to derive their L<C<import>|/import LIST> method via
9185 inheritance from the C<Exporter> class that is defined in the
9186 L<C<Exporter>|Exporter> module. See L<Exporter>. If no
9187 L<C<import>|/import LIST> method can be found, then the call is skipped,
9188 even if there is an AUTOLOAD method.
9190 If you do not want to call the package's L<C<import>|/import LIST>
9191 method (for instance,
9192 to stop your namespace from being altered), explicitly supply the empty list:
9196 That is exactly equivalent to
9198 BEGIN { require Module }
9200 If the VERSION argument is present between Module and LIST, then the
9201 L<C<use>|/use Module VERSION LIST> will call the C<VERSION> method in
9202 class Module with the given version as an argument:
9208 BEGIN { require Module; Module->VERSION(12.34) }
9210 The L<default C<VERSION> method|UNIVERSAL/C<VERSION ( [ REQUIRE ] )>>,
9211 inherited from the L<C<UNIVERSAL>|UNIVERSAL> class, croaks if the given
9212 version is larger than the value of the variable C<$Module::VERSION>.
9214 Again, there is a distinction between omitting LIST (L<C<import>|/import
9215 LIST> called with no arguments) and an explicit empty LIST C<()>
9216 (L<C<import>|/import LIST> not called). Note that there is no comma
9219 Because this is a wide-open interface, pragmas (compiler directives)
9220 are also implemented this way. Some of the currently implemented
9226 use sigtrap qw(SEGV BUS);
9227 use strict qw(subs vars refs);
9228 use subs qw(afunc blurfl);
9229 use warnings qw(all);
9230 use sort qw(stable _quicksort _mergesort);
9232 Some of these pseudo-modules import semantics into the current
9233 block scope (like L<C<strict>|strict> or L<C<integer>|integer>, unlike
9234 ordinary modules, which import symbols into the current package (which
9235 are effective through the end of the file).
9237 Because L<C<use>|/use Module VERSION LIST> takes effect at compile time,
9238 it doesn't respect the ordinary flow control of the code being compiled.
9239 In particular, putting a L<C<use>|/use Module VERSION LIST> inside the
9240 false branch of a conditional doesn't prevent it
9241 from being processed. If a module or pragma only needs to be loaded
9242 conditionally, this can be done using the L<if> pragma:
9244 use if $] < 5.008, "utf8";
9245 use if WANT_WARNINGS, warnings => qw(all);
9247 There's a corresponding L<C<no>|/no MODULE VERSION LIST> declaration
9248 that unimports meanings imported by L<C<use>|/use Module VERSION LIST>,
9249 i.e., it calls C<< Module->unimport(LIST) >> instead of
9250 L<C<import>|/import LIST>. It behaves just as L<C<import>|/import LIST>
9251 does with VERSION, an omitted or empty LIST,
9252 or no unimport method being found.
9258 Care should be taken when using the C<no VERSION> form of L<C<no>|/no
9259 MODULE VERSION LIST>. It is
9260 I<only> meant to be used to assert that the running Perl is of a earlier
9261 version than its argument and I<not> to undo the feature-enabling side effects
9264 See L<perlmodlib> for a list of standard modules and pragmas. See L<perlrun>
9265 for the C<-M> and C<-m> command-line options to Perl that give
9266 L<C<use>|/use Module VERSION LIST> functionality from the command-line.
9271 =for Pod::Functions set a file's last access and modify times
9273 Changes the access and modification times on each file of a list of
9274 files. The first two elements of the list must be the NUMERIC access
9275 and modification times, in that order. Returns the number of files
9276 successfully changed. The inode change time of each file is set
9277 to the current time. For example, this code has the same effect as the
9278 Unix L<touch(1)> command when the files I<already exist> and belong to
9279 the user running the program:
9282 my $atime = my $mtime = time;
9283 utime $atime, $mtime, @ARGV;
9285 Since Perl 5.8.0, if the first two elements of the list are
9286 L<C<undef>|/undef EXPR>,
9287 the L<utime(2)> syscall from your C library is called with a null second
9288 argument. On most systems, this will set the file's access and
9289 modification times to the current time (i.e., equivalent to the example
9290 above) and will work even on files you don't own provided you have write
9293 for my $file (@ARGV) {
9294 utime(undef, undef, $file)
9295 || warn "Couldn't touch $file: $!";
9298 Under NFS this will use the time of the NFS server, not the time of
9299 the local machine. If there is a time synchronization problem, the
9300 NFS server and local machine will have different times. The Unix
9301 L<touch(1)> command will in fact normally use this form instead of the
9302 one shown in the first example.
9304 Passing only one of the first two elements as L<C<undef>|/undef EXPR> is
9305 equivalent to passing a 0 and will not have the effect described when
9306 both are L<C<undef>|/undef EXPR>. This also triggers an
9307 uninitialized warning.
9309 On systems that support L<futimes(2)>, you may pass filehandles among the
9310 files. On systems that don't support L<futimes(2)>, passing filehandles raises
9311 an exception. Filehandles must be passed as globs or glob references to be
9312 recognized; barewords are considered filenames.
9314 Portability issues: L<perlport/utime>.
9321 =for Pod::Functions return a list of the values in a hash
9323 In list context, returns a list consisting of all the values of the named
9324 hash. In Perl 5.12 or later only, will also return a list of the values of
9325 an array; prior to that release, attempting to use an array argument will
9326 produce a syntax error. In scalar context, returns the number of values.
9328 Hash entries are returned in an apparently random order. The actual random
9329 order is specific to a given hash; the exact same series of operations
9330 on two hashes may result in a different order for each hash. Any insertion
9331 into the hash may change the order, as will any deletion, with the exception
9332 that the most recent key returned by L<C<each>|/each HASH> or
9333 L<C<keys>|/keys HASH> may be deleted without changing the order. So
9334 long as a given hash is unmodified you may rely on
9335 L<C<keys>|/keys HASH>, L<C<values>|/values HASH> and
9336 L<C<each>|/each HASH> to repeatedly return the same order
9337 as each other. See L<perlsec/"Algorithmic Complexity Attacks"> for
9338 details on why hash order is randomized. Aside from the guarantees
9339 provided here the exact details of Perl's hash algorithm and the hash
9340 traversal order are subject to change in any release of Perl. Tied hashes
9341 may behave differently to Perl's hashes with respect to changes in order on
9342 insertion and deletion of items.
9344 As a side effect, calling L<C<values>|/values HASH> resets the HASH or
9345 ARRAY's internal iterator, see L<C<each>|/each HASH>. (In particular,
9346 calling L<C<values>|/values HASH> in void context resets the iterator
9347 with no other overhead. Apart from resetting the iterator,
9348 C<values @array> in list context is the same as plain C<@array>.
9349 (We recommend that you use void context C<keys @array> for this, but
9350 reasoned that taking C<values @array> out would require more
9351 documentation than leaving it in.)
9353 Note that the values are not copied, which means modifying them will
9354 modify the contents of the hash:
9356 for (values %hash) { s/foo/bar/g } # modifies %hash values
9357 for (@hash{keys %hash}) { s/foo/bar/g } # same
9359 Starting with Perl 5.14, an experimental feature allowed
9360 L<C<values>|/values HASH> to take a
9361 scalar expression. This experiment has been deemed unsuccessful, and was
9362 removed as of Perl 5.24.
9364 To avoid confusing would-be users of your code who are running earlier
9365 versions of Perl with mysterious syntax errors, put this sort of thing at
9366 the top of your file to signal that your code will work I<only> on Perls of
9369 use 5.012; # so keys/values/each work on arrays
9371 See also L<C<keys>|/keys HASH>, L<C<each>|/each HASH>, and
9372 L<C<sort>|/sort SUBNAME LIST>.
9374 =item vec EXPR,OFFSET,BITS
9375 X<vec> X<bit> X<bit vector>
9377 =for Pod::Functions test or set particular bits in a string
9379 Treats the string in EXPR as a bit vector made up of elements of
9380 width BITS and returns the value of the element specified by OFFSET
9381 as an unsigned integer. BITS therefore specifies the number of bits
9382 that are reserved for each element in the bit vector. This must
9383 be a power of two from 1 to 32 (or 64, if your platform supports
9386 If BITS is 8, "elements" coincide with bytes of the input string.
9388 If BITS is 16 or more, bytes of the input string are grouped into chunks
9389 of size BITS/8, and each group is converted to a number as with
9390 L<C<pack>|/pack TEMPLATE,LIST>/L<C<unpack>|/unpack TEMPLATE,EXPR> with
9391 big-endian formats C<n>/C<N> (and analogously for BITS==64). See
9392 L<C<pack>|/pack TEMPLATE,LIST> for details.
9394 If bits is 4 or less, the string is broken into bytes, then the bits
9395 of each byte are broken into 8/BITS groups. Bits of a byte are
9396 numbered in a little-endian-ish way, as in C<0x01>, C<0x02>,
9397 C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example,
9398 breaking the single input byte C<chr(0x36)> into two groups gives a list
9399 C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>.
9401 L<C<vec>|/vec EXPR,OFFSET,BITS> may also be assigned to, in which case
9402 parentheses are needed
9403 to give the expression the correct precedence as in
9405 vec($image, $max_x * $x + $y, 8) = 3;
9407 If the selected element is outside the string, the value 0 is returned.
9408 If an element off the end of the string is written to, Perl will first
9409 extend the string with sufficiently many zero bytes. It is an error
9410 to try to write off the beginning of the string (i.e., negative OFFSET).
9412 If the string happens to be encoded as UTF-8 internally (and thus has
9413 the UTF8 flag set), this is ignored by L<C<vec>|/vec EXPR,OFFSET,BITS>,
9414 and it operates on the
9415 internal byte string, not the conceptual character string, even if you
9416 only have characters with values less than 256.
9418 Strings created with L<C<vec>|/vec EXPR,OFFSET,BITS> can also be
9419 manipulated with the logical
9420 operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit
9421 vector operation is desired when both operands are strings.
9422 See L<perlop/"Bitwise String Operators">.
9424 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
9425 The comments show the string after each step. Note that this code works
9426 in the same way on big-endian or little-endian machines.
9429 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
9431 # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
9432 print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P')
9434 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
9435 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
9436 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
9437 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
9438 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
9439 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
9441 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
9442 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
9443 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
9446 To transform a bit vector into a string or list of 0's and 1's, use these:
9448 my $bits = unpack("b*", $vector);
9449 my @bits = split(//, unpack("b*", $vector));
9451 If you know the exact length in bits, it can be used in place of the C<*>.
9453 Here is an example to illustrate how the bits actually fall in place:
9459 unpack("V",$_) 01234567890123456789012345678901
9460 ------------------------------------------------------------------
9465 for ($shift=0; $shift < $width; ++$shift) {
9466 for ($off=0; $off < 32/$width; ++$off) {
9467 $str = pack("B*", "0"x32);
9468 $bits = (1<<$shift);
9469 vec($str, $off, $width) = $bits;
9470 $res = unpack("b*",$str);
9471 $val = unpack("V", $str);
9478 vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
9479 $off, $width, $bits, $val, $res
9483 Regardless of the machine architecture on which it runs, the
9484 example above should print the following table:
9487 unpack("V",$_) 01234567890123456789012345678901
9488 ------------------------------------------------------------------
9489 vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000
9490 vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000
9491 vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000
9492 vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000
9493 vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000
9494 vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000
9495 vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000
9496 vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000
9497 vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000
9498 vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000
9499 vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000
9500 vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000
9501 vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000
9502 vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000
9503 vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000
9504 vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000
9505 vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000
9506 vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000
9507 vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000
9508 vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000
9509 vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000
9510 vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000
9511 vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000
9512 vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000
9513 vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000
9514 vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000
9515 vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000
9516 vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000
9517 vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000
9518 vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100
9519 vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010
9520 vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001
9521 vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000
9522 vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000
9523 vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000
9524 vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000
9525 vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000
9526 vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000
9527 vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000
9528 vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000
9529 vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000
9530 vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000
9531 vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000
9532 vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000
9533 vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000
9534 vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000
9535 vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000
9536 vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010
9537 vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000
9538 vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000
9539 vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000
9540 vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000
9541 vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000
9542 vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000
9543 vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000
9544 vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000
9545 vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000
9546 vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000
9547 vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000
9548 vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000
9549 vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000
9550 vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000
9551 vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100
9552 vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001
9553 vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000
9554 vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000
9555 vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000
9556 vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000
9557 vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000
9558 vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000
9559 vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000
9560 vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000
9561 vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000
9562 vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000
9563 vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000
9564 vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000
9565 vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000
9566 vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000
9567 vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000
9568 vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100
9569 vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000
9570 vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000
9571 vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000
9572 vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000
9573 vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000
9574 vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000
9575 vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000
9576 vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010
9577 vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000
9578 vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000
9579 vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000
9580 vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000
9581 vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000
9582 vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000
9583 vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000
9584 vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001
9585 vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000
9586 vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000
9587 vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000
9588 vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000
9589 vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000
9590 vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000
9591 vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000
9592 vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000
9593 vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000
9594 vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000
9595 vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000
9596 vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000
9597 vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000
9598 vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000
9599 vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000
9600 vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000
9601 vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000
9602 vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000
9603 vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000
9604 vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000
9605 vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000
9606 vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000
9607 vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000
9608 vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100
9609 vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000
9610 vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000
9611 vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000
9612 vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010
9613 vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000
9614 vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000
9615 vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000
9616 vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
9621 =for Pod::Functions wait for any child process to die
9623 Behaves like L<wait(2)> on your system: it waits for a child
9624 process to terminate and returns the pid of the deceased process, or
9625 C<-1> if there are no child processes. The status is returned in
9626 L<C<$?>|perlvar/$?> and
9627 L<C<${^CHILD_ERROR_NATIVE}>|perlvar/${^CHILD_ERROR_NATIVE}>.
9628 Note that a return value of C<-1> could mean that child processes are
9629 being automatically reaped, as described in L<perlipc>.
9631 If you use L<C<wait>|/wait> in your handler for
9632 L<C<$SIG{CHLD}>|perlvar/%SIG>, it may accidentally wait for the child
9633 created by L<C<qx>|/qxE<sol>STRINGE<sol>> or L<C<system>|/system LIST>.
9634 See L<perlipc> for details.
9636 Portability issues: L<perlport/wait>.
9638 =item waitpid PID,FLAGS
9641 =for Pod::Functions wait for a particular child process to die
9643 Waits for a particular child process to terminate and returns the pid of
9644 the deceased process, or C<-1> if there is no such child process. A
9645 non-blocking wait (with L<WNOHANG|POSIX/C<WNOHANG>> in FLAGS) can return 0 if
9646 there are child processes matching PID but none have terminated yet.
9647 The status is returned in L<C<$?>|perlvar/$?> and
9648 L<C<${^CHILD_ERROR_NATIVE}>|perlvar/${^CHILD_ERROR_NATIVE}>.
9650 A PID of C<0> indicates to wait for any child process whose process group ID is
9651 equal to that of the current process. A PID of less than C<-1> indicates to
9652 wait for any child process whose process group ID is equal to -PID. A PID of
9653 C<-1> indicates to wait for any child process.
9657 use POSIX ":sys_wait_h";
9661 $kid = waitpid(-1, WNOHANG);
9666 1 while waitpid(-1, WNOHANG) > 0;
9668 then you can do a non-blocking wait for all pending zombie processes (see
9670 Non-blocking wait is available on machines supporting either the
9671 L<waitpid(2)> or L<wait4(2)> syscalls. However, waiting for a particular
9672 pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the
9673 system call by remembering the status values of processes that have
9674 exited but have not been harvested by the Perl script yet.)
9676 Note that on some systems, a return value of C<-1> could mean that child
9677 processes are being automatically reaped. See L<perlipc> for details,
9678 and for other examples.
9680 Portability issues: L<perlport/waitpid>.
9683 X<wantarray> X<context>
9685 =for Pod::Functions get void vs scalar vs list context of current subroutine call
9687 Returns true if the context of the currently executing subroutine or
9688 L<C<eval>|/eval EXPR> is looking for a list value. Returns false if the
9690 looking for a scalar. Returns the undefined value if the context is
9691 looking for no value (void context).
9693 return unless defined wantarray; # don't bother doing more
9694 my @a = complex_calculation();
9695 return wantarray ? @a : "@a";
9697 L<C<wantarray>|/wantarray>'s result is unspecified in the top level of a file,
9698 in a C<BEGIN>, C<UNITCHECK>, C<CHECK>, C<INIT> or C<END> block, or
9699 in a C<DESTROY> method.
9701 This function should have been named wantlist() instead.
9704 X<warn> X<warning> X<STDERR>
9706 =for Pod::Functions print debugging info
9708 Prints the value of LIST to STDERR. If the last element of LIST does
9709 not end in a newline, it appends the same file/line number text as
9710 L<C<die>|/die LIST> does.
9712 If the output is empty and L<C<$@>|perlvar/$@> already contains a value
9713 (typically from a previous eval) that value is used after appending
9714 C<"\t...caught"> to L<C<$@>|perlvar/$@>. This is useful for staying
9715 almost, but not entirely similar to L<C<die>|/die LIST>.
9717 If L<C<$@>|perlvar/$@> is empty, then the string
9718 C<"Warning: Something's wrong"> is used.
9720 No message is printed if there is a L<C<$SIG{__WARN__}>|perlvar/%SIG>
9722 installed. It is the handler's responsibility to deal with the message
9723 as it sees fit (like, for instance, converting it into a
9724 L<C<die>|/die LIST>). Most
9725 handlers must therefore arrange to actually display the
9726 warnings that they are not prepared to deal with, by calling
9727 L<C<warn>|/warn LIST>
9728 again in the handler. Note that this is quite safe and will not
9729 produce an endless loop, since C<__WARN__> hooks are not called from
9732 You will find this behavior is slightly different from that of
9733 L<C<$SIG{__DIE__}>|perlvar/%SIG> handlers (which don't suppress the
9734 error text, but can instead call L<C<die>|/die LIST> again to change
9737 Using a C<__WARN__> handler provides a powerful way to silence all
9738 warnings (even the so-called mandatory ones). An example:
9740 # wipe out *all* compile-time warnings
9741 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
9743 my $foo = 20; # no warning about duplicate my $foo,
9744 # but hey, you asked for it!
9745 # no compile-time or run-time warnings before here
9748 # run-time warnings enabled after here
9749 warn "\$foo is alive and $foo!"; # does show up
9751 See L<perlvar> for details on setting L<C<%SIG>|perlvar/%SIG> entries
9753 examples. See the L<Carp> module for other kinds of warnings using its
9754 C<carp> and C<cluck> functions.
9756 =item write FILEHANDLE
9763 =for Pod::Functions print a picture record
9765 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
9766 using the format associated with that file. By default the format for
9767 a file is the one having the same name as the filehandle, but the
9768 format for the current output channel (see the
9769 L<C<select>|/select FILEHANDLE> function) may be set explicitly by
9770 assigning the name of the format to the L<C<$~>|perlvar/$~> variable.
9772 Top of form processing is handled automatically: if there is insufficient
9773 room on the current page for the formatted record, the page is advanced by
9774 writing a form feed and a special top-of-page
9775 format is used to format the new
9776 page header before the record is written. By default, the top-of-page
9777 format is the name of the filehandle with C<_TOP> appended, or C<top>
9778 in the current package if the former does not exist. This would be a
9779 problem with autovivified filehandles, but it may be dynamically set to the
9780 format of your choice by assigning the name to the L<C<$^>|perlvar/$^>
9781 variable while that filehandle is selected. The number of lines
9782 remaining on the current page is in variable L<C<$->|perlvar/$->, which
9783 can be set to C<0> to force a new page.
9785 If FILEHANDLE is unspecified, output goes to the current default output
9786 channel, which starts out as STDOUT but may be changed by the
9787 L<C<select>|/select FILEHANDLE> operator. If the FILEHANDLE is an EXPR,
9789 is evaluated and the resulting string is used to look up the name of
9790 the FILEHANDLE at run time. For more on formats, see L<perlform>.
9792 Note that write is I<not> the opposite of
9793 L<C<read>|/read FILEHANDLE,SCALAR,LENGTH,OFFSET>. Unfortunately.
9797 =for Pod::Functions transliterate a string
9799 The transliteration operator. Same as
9800 L<C<trE<sol>E<sol>E<sol>>|/trE<sol>E<sol>E<sol>>. See
9801 L<perlop/"Quote-Like Operators">.
9805 =head2 Non-function Keywords by Cross-reference
9815 These keywords are documented in L<perldata/"Special Literals">.
9833 These compile phase keywords are documented in L<perlmod/"BEGIN, UNITCHECK, CHECK, INIT and END">.
9843 This method keyword is documented in L<perlobj/"Destructors">.
9875 These operators are documented in L<perlop>.
9885 This keyword is documented in L<perlsub/"Autoloading">.
9909 These flow-control keywords are documented in L<perlsyn/"Compound Statements">.
9913 The "else if" keyword is spelled C<elsif> in Perl. There's no C<elif>
9914 or C<else if> either. It does parse C<elseif>, but only to warn you
9917 See the documentation for flow-control keywords in L<perlsyn/"Compound
9930 These flow-control keywords related to the experimental switch feature are
9931 documented in L<perlsyn/"Switch Statements">.