4 perlfunc - Perl builtin functions
8 The functions in this section can serve as terms in an expression.
9 They fall into two major categories: list operators and named unary
10 operators. These differ in their precedence relationship with a
11 following comma. (See the precedence table in L<perlop>.) List
12 operators take more than one argument, while unary operators can never
13 take more than one argument. Thus, a comma terminates the argument of
14 a unary operator, but merely separates the arguments of a list
15 operator. A unary operator generally provides scalar context to its
16 argument, while a list operator may provide either scalar or list
17 contexts for its arguments. If it does both, scalar arguments
18 come first and list argument follow, and there can only ever
19 be one such list argument. For instance,
20 L<C<splice>|/splice ARRAY,OFFSET,LENGTH,LIST> has three scalar arguments
21 followed by a list, whereas L<C<gethostbyname>|/gethostbyname NAME> has
22 four scalar arguments.
24 In the syntax descriptions that follow, list operators that expect a
25 list (and provide list context for elements of the list) are shown
26 with LIST as an argument. Such a list may consist of any combination
27 of scalar arguments or list values; the list values will be included
28 in the list as if each individual element were interpolated at that
29 point in the list, forming a longer single-dimensional list value.
30 Commas should separate literal elements of the LIST.
32 Any function in the list below may be used either with or without
33 parentheses around its arguments. (The syntax descriptions omit the
34 parentheses.) If you use parentheses, the simple but occasionally
35 surprising rule is this: It I<looks> like a function, therefore it I<is> a
36 function, and precedence doesn't matter. Otherwise it's a list
37 operator or unary operator, and precedence does matter. Whitespace
38 between the function and left parenthesis doesn't count, so sometimes
39 you need to be careful:
41 print 1+2+4; # Prints 7.
42 print(1+2) + 4; # Prints 3.
43 print (1+2)+4; # Also prints 3!
44 print +(1+2)+4; # Prints 7.
45 print ((1+2)+4); # Prints 7.
47 If you run Perl with the L<C<use warnings>|warnings> pragma, it can warn
48 you about this. For example, the third line above produces:
50 print (...) interpreted as function at - line 1.
51 Useless use of integer addition in void context at - line 1.
53 A few functions take no arguments at all, and therefore work as neither
54 unary nor list operators. These include such functions as
55 L<C<time>|/time> and L<C<endpwent>|/endpwent>. For example,
56 C<time+86_400> always means C<time() + 86_400>.
58 For functions that can be used in either a scalar or list context,
59 nonabortive failure is generally indicated in scalar context by
60 returning the undefined value, and in list context by returning the
63 Remember the following important rule: There is B<no rule> that relates
64 the behavior of an expression in list context to its behavior in scalar
65 context, or vice versa. It might do two totally different things.
66 Each operator and function decides which sort of value would be most
67 appropriate to return in scalar context. Some operators return the
68 length of the list that would have been returned in list context. Some
69 operators return the first value in the list. Some operators return the
70 last value in the list. Some operators return a count of successful
71 operations. In general, they do what you want, unless you want
75 A named array in scalar context is quite different from what would at
76 first glance appear to be a list in scalar context. You can't get a list
77 like C<(1,2,3)> into being in scalar context, because the compiler knows
78 the context at compile time. It would generate the scalar comma operator
79 there, not the list concatenation version of the comma. That means it
80 was never a list to start with.
82 In general, functions in Perl that serve as wrappers for system calls
83 ("syscalls") of the same name (like L<chown(2)>, L<fork(2)>,
84 L<closedir(2)>, etc.) return true when they succeed and
85 L<C<undef>|/undef EXPR> otherwise, as is usually mentioned in the
86 descriptions below. This is different from the C interfaces, which
87 return C<-1> on failure. Exceptions to this rule include
88 L<C<wait>|/wait>, L<C<waitpid>|/waitpid PID,FLAGS>, and
89 L<C<syscall>|/syscall NUMBER, LIST>. System calls also set the special
90 L<C<$!>|perlvar/$!> variable on failure. Other functions do not, except
93 Extension modules can also hook into the Perl parser to define new
94 kinds of keyword-headed expression. These may look like functions, but
95 may also look completely different. The syntax following the keyword
96 is defined entirely by the extension. If you are an implementor, see
97 L<perlapi/PL_keyword_plugin> for the mechanism. If you are using such
98 a module, see the module's documentation for details of the syntax that
101 =head2 Perl Functions by Category
104 Here are Perl's functions (including things that look like
105 functions, like some keywords and named operators)
106 arranged by category. Some functions appear in more
111 =item Functions for SCALARs or strings
112 X<scalar> X<string> X<character>
114 =for Pod::Functions =String
116 L<C<chomp>|/chomp VARIABLE>, L<C<chop>|/chop VARIABLE>,
117 L<C<chr>|/chr NUMBER>, L<C<crypt>|/crypt PLAINTEXT,SALT>,
118 L<C<fc>|/fc EXPR>, L<C<hex>|/hex EXPR>,
119 L<C<index>|/index STR,SUBSTR,POSITION>, L<C<lc>|/lc EXPR>,
120 L<C<lcfirst>|/lcfirst EXPR>, L<C<length>|/length EXPR>,
121 L<C<oct>|/oct EXPR>, L<C<ord>|/ord EXPR>,
122 L<C<pack>|/pack TEMPLATE,LIST>,
123 L<C<qE<sol>E<sol>>|/qE<sol>STRINGE<sol>>,
124 L<C<qqE<sol>E<sol>>|/qqE<sol>STRINGE<sol>>, L<C<reverse>|/reverse LIST>,
125 L<C<rindex>|/rindex STR,SUBSTR,POSITION>,
126 L<C<sprintf>|/sprintf FORMAT, LIST>,
127 L<C<substr>|/substr EXPR,OFFSET,LENGTH,REPLACEMENT>,
128 L<C<trE<sol>E<sol>E<sol>>|/trE<sol>E<sol>E<sol>>, L<C<uc>|/uc EXPR>,
129 L<C<ucfirst>|/ucfirst EXPR>,
130 L<C<yE<sol>E<sol>E<sol>>|/yE<sol>E<sol>E<sol>>
132 L<C<fc>|/fc EXPR> is available only if the
133 L<C<"fc"> feature|feature/The 'fc' feature> is enabled or if it is
134 prefixed with C<CORE::>. The
135 L<C<"fc"> feature|feature/The 'fc' feature> is enabled automatically
136 with a C<use v5.16> (or higher) declaration in the current scope.
138 =item Regular expressions and pattern matching
139 X<regular expression> X<regex> X<regexp>
141 =for Pod::Functions =Regexp
143 L<C<mE<sol>E<sol>>|/mE<sol>E<sol>>, L<C<pos>|/pos SCALAR>,
144 L<C<qrE<sol>E<sol>>|/qrE<sol>STRINGE<sol>>,
145 L<C<quotemeta>|/quotemeta EXPR>,
146 L<C<sE<sol>E<sol>E<sol>>|/sE<sol>E<sol>E<sol>>,
147 L<C<split>|/split E<sol>PATTERNE<sol>,EXPR,LIMIT>,
148 L<C<study>|/study SCALAR>
150 =item Numeric functions
151 X<numeric> X<number> X<trigonometric> X<trigonometry>
153 =for Pod::Functions =Math
155 L<C<abs>|/abs VALUE>, L<C<atan2>|/atan2 Y,X>, L<C<cos>|/cos EXPR>,
156 L<C<exp>|/exp EXPR>, L<C<hex>|/hex EXPR>, L<C<int>|/int EXPR>,
157 L<C<log>|/log EXPR>, L<C<oct>|/oct EXPR>, L<C<rand>|/rand EXPR>,
158 L<C<sin>|/sin EXPR>, L<C<sqrt>|/sqrt EXPR>, L<C<srand>|/srand EXPR>
160 =item Functions for real @ARRAYs
163 =for Pod::Functions =ARRAY
165 L<C<each>|/each HASH>, L<C<keys>|/keys HASH>, L<C<pop>|/pop ARRAY>,
166 L<C<push>|/push ARRAY,LIST>, L<C<shift>|/shift ARRAY>,
167 L<C<splice>|/splice ARRAY,OFFSET,LENGTH,LIST>,
168 L<C<unshift>|/unshift ARRAY,LIST>, L<C<values>|/values HASH>
170 =item Functions for list data
173 =for Pod::Functions =LIST
175 L<C<grep>|/grep BLOCK LIST>, L<C<join>|/join EXPR,LIST>,
176 L<C<map>|/map BLOCK LIST>, L<C<qwE<sol>E<sol>>|/qwE<sol>STRINGE<sol>>,
177 L<C<reverse>|/reverse LIST>, L<C<sort>|/sort SUBNAME LIST>,
178 L<C<unpack>|/unpack TEMPLATE,EXPR>
180 =item Functions for real %HASHes
183 =for Pod::Functions =HASH
185 L<C<delete>|/delete EXPR>, L<C<each>|/each HASH>,
186 L<C<exists>|/exists EXPR>, L<C<keys>|/keys HASH>,
187 L<C<values>|/values HASH>
189 =item Input and output functions
190 X<I/O> X<input> X<output> X<dbm>
192 =for Pod::Functions =I/O
194 L<C<binmode>|/binmode FILEHANDLE, LAYER>, L<C<close>|/close FILEHANDLE>,
195 L<C<closedir>|/closedir DIRHANDLE>, L<C<dbmclose>|/dbmclose HASH>,
196 L<C<dbmopen>|/dbmopen HASH,DBNAME,MASK>, L<C<die>|/die LIST>,
197 L<C<eof>|/eof FILEHANDLE>, L<C<fileno>|/fileno FILEHANDLE>,
198 L<C<flock>|/flock FILEHANDLE,OPERATION>, L<C<format>|/format>,
199 L<C<getc>|/getc FILEHANDLE>, L<C<print>|/print FILEHANDLE LIST>,
200 L<C<printf>|/printf FILEHANDLE FORMAT, LIST>,
201 L<C<read>|/read FILEHANDLE,SCALAR,LENGTH,OFFSET>,
202 L<C<readdir>|/readdir DIRHANDLE>, L<C<readline>|/readline EXPR>,
203 L<C<rewinddir>|/rewinddir DIRHANDLE>, L<C<say>|/say FILEHANDLE LIST>,
204 L<C<seek>|/seek FILEHANDLE,POSITION,WHENCE>,
205 L<C<seekdir>|/seekdir DIRHANDLE,POS>,
206 L<C<select>|/select RBITS,WBITS,EBITS,TIMEOUT>,
207 L<C<syscall>|/syscall NUMBER, LIST>,
208 L<C<sysread>|/sysread FILEHANDLE,SCALAR,LENGTH,OFFSET>,
209 L<C<sysseek>|/sysseek FILEHANDLE,POSITION,WHENCE>,
210 L<C<syswrite>|/syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET>,
211 L<C<tell>|/tell FILEHANDLE>, L<C<telldir>|/telldir DIRHANDLE>,
212 L<C<truncate>|/truncate FILEHANDLE,LENGTH>, L<C<warn>|/warn LIST>,
213 L<C<write>|/write FILEHANDLE>
215 L<C<say>|/say FILEHANDLE LIST> is available only if the
216 L<C<"say"> feature|feature/The 'say' feature> is enabled or if it is
217 prefixed with C<CORE::>. The
218 L<C<"say"> feature|feature/The 'say' feature> is enabled automatically
219 with a C<use v5.10> (or higher) declaration in the current scope.
221 =item Functions for fixed-length data or records
223 =for Pod::Functions =Binary
225 L<C<pack>|/pack TEMPLATE,LIST>,
226 L<C<read>|/read FILEHANDLE,SCALAR,LENGTH,OFFSET>,
227 L<C<syscall>|/syscall NUMBER, LIST>,
228 L<C<sysread>|/sysread FILEHANDLE,SCALAR,LENGTH,OFFSET>,
229 L<C<sysseek>|/sysseek FILEHANDLE,POSITION,WHENCE>,
230 L<C<syswrite>|/syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET>,
231 L<C<unpack>|/unpack TEMPLATE,EXPR>, L<C<vec>|/vec EXPR,OFFSET,BITS>
233 =item Functions for filehandles, files, or directories
234 X<file> X<filehandle> X<directory> X<pipe> X<link> X<symlink>
236 =for Pod::Functions =File
238 L<C<-I<X>>|/-X FILEHANDLE>, L<C<chdir>|/chdir EXPR>,
239 L<C<chmod>|/chmod LIST>, L<C<chown>|/chown LIST>,
240 L<C<chroot>|/chroot FILENAME>,
241 L<C<fcntl>|/fcntl FILEHANDLE,FUNCTION,SCALAR>, L<C<glob>|/glob EXPR>,
242 L<C<ioctl>|/ioctl FILEHANDLE,FUNCTION,SCALAR>,
243 L<C<link>|/link OLDFILE,NEWFILE>, L<C<lstat>|/lstat FILEHANDLE>,
244 L<C<mkdir>|/mkdir FILENAME,MASK>, L<C<open>|/open FILEHANDLE,EXPR>,
245 L<C<opendir>|/opendir DIRHANDLE,EXPR>, L<C<readlink>|/readlink EXPR>,
246 L<C<rename>|/rename OLDNAME,NEWNAME>, L<C<rmdir>|/rmdir FILENAME>,
247 L<C<select>|/select FILEHANDLE>, L<C<stat>|/stat FILEHANDLE>,
248 L<C<symlink>|/symlink OLDFILE,NEWFILE>,
249 L<C<sysopen>|/sysopen FILEHANDLE,FILENAME,MODE>,
250 L<C<umask>|/umask EXPR>, L<C<unlink>|/unlink LIST>,
251 L<C<utime>|/utime LIST>
253 =item Keywords related to the control flow of your Perl program
256 =for Pod::Functions =Flow
258 L<C<break>|/break>, L<C<caller>|/caller EXPR>,
259 L<C<continue>|/continue BLOCK>, L<C<die>|/die LIST>, L<C<do>|/do BLOCK>,
260 L<C<dump>|/dump LABEL>, L<C<eval>|/eval EXPR>,
261 L<C<evalbytes>|/evalbytes EXPR>, L<C<exit>|/exit EXPR>,
262 L<C<__FILE__>|/__FILE__>, L<C<goto>|/goto LABEL>,
263 L<C<last>|/last LABEL>, L<C<__LINE__>|/__LINE__>,
264 L<C<next>|/next LABEL>, L<C<__PACKAGE__>|/__PACKAGE__>,
265 L<C<redo>|/redo LABEL>, L<C<return>|/return EXPR>,
266 L<C<sub>|/sub NAME BLOCK>, L<C<__SUB__>|/__SUB__>,
267 L<C<wantarray>|/wantarray>
269 L<C<break>|/break> is available only if you enable the experimental
270 L<C<"switch"> feature|feature/The 'switch' feature> or use the C<CORE::>
271 prefix. The L<C<"switch"> feature|feature/The 'switch' feature> also
272 enables the C<default>, C<given> and C<when> statements, which are
273 documented in L<perlsyn/"Switch Statements">.
274 The L<C<"switch"> feature|feature/The 'switch' feature> is enabled
275 automatically with a C<use v5.10> (or higher) declaration in the current
276 scope. In Perl v5.14 and earlier, L<C<continue>|/continue BLOCK>
277 required the L<C<"switch"> feature|feature/The 'switch' feature>, like
280 L<C<evalbytes>|/evalbytes EXPR> is only available with the
281 L<C<"evalbytes"> feature|feature/The 'unicode_eval' and 'evalbytes' features>
282 (see L<feature>) or if prefixed with C<CORE::>. L<C<__SUB__>|/__SUB__>
283 is only available with the
284 L<C<"current_sub"> feature|feature/The 'current_sub' feature> or if
285 prefixed with C<CORE::>. Both the
286 L<C<"evalbytes">|feature/The 'unicode_eval' and 'evalbytes' features>
287 and L<C<"current_sub">|feature/The 'current_sub' feature> features are
288 enabled automatically with a C<use v5.16> (or higher) declaration in the
291 =item Keywords related to scoping
293 =for Pod::Functions =Namespace
295 L<C<caller>|/caller EXPR>, L<C<import>|/import LIST>,
296 L<C<local>|/local EXPR>, L<C<my>|/my VARLIST>, L<C<our>|/our VARLIST>,
297 L<C<package>|/package NAMESPACE>, L<C<state>|/state VARLIST>,
298 L<C<use>|/use Module VERSION LIST>
300 L<C<state>|/state VARLIST> is available only if the
301 L<C<"state"> feature|feature/The 'state' feature> is enabled or if it is
302 prefixed with C<CORE::>. The
303 L<C<"state"> feature|feature/The 'state' feature> is enabled
304 automatically with a C<use v5.10> (or higher) declaration in the current
307 =item Miscellaneous functions
309 =for Pod::Functions =Misc
311 L<C<defined>|/defined EXPR>, L<C<formline>|/formline PICTURE,LIST>,
312 L<C<lock>|/lock THING>, L<C<prototype>|/prototype FUNCTION>,
313 L<C<reset>|/reset EXPR>, L<C<scalar>|/scalar EXPR>,
314 L<C<undef>|/undef EXPR>
316 =item Functions for processes and process groups
317 X<process> X<pid> X<process id>
319 =for Pod::Functions =Process
321 L<C<alarm>|/alarm SECONDS>, L<C<exec>|/exec LIST>, L<C<fork>|/fork>,
322 L<C<getpgrp>|/getpgrp PID>, L<C<getppid>|/getppid>,
323 L<C<getpriority>|/getpriority WHICH,WHO>, L<C<kill>|/kill SIGNAL, LIST>,
324 L<C<pipe>|/pipe READHANDLE,WRITEHANDLE>,
325 L<C<qxE<sol>E<sol>>|/qxE<sol>STRINGE<sol>>,
326 L<C<readpipe>|/readpipe EXPR>, L<C<setpgrp>|/setpgrp PID,PGRP>,
327 L<C<setpriority>|/setpriority WHICH,WHO,PRIORITY>,
328 L<C<sleep>|/sleep EXPR>, L<C<system>|/system LIST>, L<C<times>|/times>,
329 L<C<wait>|/wait>, L<C<waitpid>|/waitpid PID,FLAGS>
331 =item Keywords related to Perl modules
334 =for Pod::Functions =Modules
336 L<C<do>|/do EXPR>, L<C<import>|/import LIST>,
337 L<C<no>|/no MODULE VERSION LIST>, L<C<package>|/package NAMESPACE>,
338 L<C<require>|/require VERSION>, L<C<use>|/use Module VERSION LIST>
340 =item Keywords related to classes and object-orientation
341 X<object> X<class> X<package>
343 =for Pod::Functions =Objects
345 L<C<bless>|/bless REF,CLASSNAME>, L<C<dbmclose>|/dbmclose HASH>,
346 L<C<dbmopen>|/dbmopen HASH,DBNAME,MASK>,
347 L<C<package>|/package NAMESPACE>, L<C<ref>|/ref EXPR>,
348 L<C<tie>|/tie VARIABLE,CLASSNAME,LIST>, L<C<tied>|/tied VARIABLE>,
349 L<C<untie>|/untie VARIABLE>, L<C<use>|/use Module VERSION LIST>
351 =item Low-level socket functions
354 =for Pod::Functions =Socket
356 L<C<accept>|/accept NEWSOCKET,GENERICSOCKET>,
357 L<C<bind>|/bind SOCKET,NAME>, L<C<connect>|/connect SOCKET,NAME>,
358 L<C<getpeername>|/getpeername SOCKET>,
359 L<C<getsockname>|/getsockname SOCKET>,
360 L<C<getsockopt>|/getsockopt SOCKET,LEVEL,OPTNAME>,
361 L<C<listen>|/listen SOCKET,QUEUESIZE>,
362 L<C<recv>|/recv SOCKET,SCALAR,LENGTH,FLAGS>,
363 L<C<send>|/send SOCKET,MSG,FLAGS,TO>,
364 L<C<setsockopt>|/setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL>,
365 L<C<shutdown>|/shutdown SOCKET,HOW>,
366 L<C<socket>|/socket SOCKET,DOMAIN,TYPE,PROTOCOL>,
367 L<C<socketpair>|/socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL>
369 =item System V interprocess communication functions
370 X<IPC> X<System V> X<semaphore> X<shared memory> X<memory> X<message>
372 =for Pod::Functions =SysV
374 L<C<msgctl>|/msgctl ID,CMD,ARG>, L<C<msgget>|/msgget KEY,FLAGS>,
375 L<C<msgrcv>|/msgrcv ID,VAR,SIZE,TYPE,FLAGS>,
376 L<C<msgsnd>|/msgsnd ID,MSG,FLAGS>,
377 L<C<semctl>|/semctl ID,SEMNUM,CMD,ARG>,
378 L<C<semget>|/semget KEY,NSEMS,FLAGS>, L<C<semop>|/semop KEY,OPSTRING>,
379 L<C<shmctl>|/shmctl ID,CMD,ARG>, L<C<shmget>|/shmget KEY,SIZE,FLAGS>,
380 L<C<shmread>|/shmread ID,VAR,POS,SIZE>,
381 L<C<shmwrite>|/shmwrite ID,STRING,POS,SIZE>
383 =item Fetching user and group info
384 X<user> X<group> X<password> X<uid> X<gid> X<passwd> X</etc/passwd>
386 =for Pod::Functions =User
388 L<C<endgrent>|/endgrent>, L<C<endhostent>|/endhostent>,
389 L<C<endnetent>|/endnetent>, L<C<endpwent>|/endpwent>,
390 L<C<getgrent>|/getgrent>, L<C<getgrgid>|/getgrgid GID>,
391 L<C<getgrnam>|/getgrnam NAME>, L<C<getlogin>|/getlogin>,
392 L<C<getpwent>|/getpwent>, L<C<getpwnam>|/getpwnam NAME>,
393 L<C<getpwuid>|/getpwuid UID>, L<C<setgrent>|/setgrent>,
394 L<C<setpwent>|/setpwent>
396 =item Fetching network info
397 X<network> X<protocol> X<host> X<hostname> X<IP> X<address> X<service>
399 =for Pod::Functions =Network
401 L<C<endprotoent>|/endprotoent>, L<C<endservent>|/endservent>,
402 L<C<gethostbyaddr>|/gethostbyaddr ADDR,ADDRTYPE>,
403 L<C<gethostbyname>|/gethostbyname NAME>, L<C<gethostent>|/gethostent>,
404 L<C<getnetbyaddr>|/getnetbyaddr ADDR,ADDRTYPE>,
405 L<C<getnetbyname>|/getnetbyname NAME>, L<C<getnetent>|/getnetent>,
406 L<C<getprotobyname>|/getprotobyname NAME>,
407 L<C<getprotobynumber>|/getprotobynumber NUMBER>,
408 L<C<getprotoent>|/getprotoent>,
409 L<C<getservbyname>|/getservbyname NAME,PROTO>,
410 L<C<getservbyport>|/getservbyport PORT,PROTO>,
411 L<C<getservent>|/getservent>, L<C<sethostent>|/sethostent STAYOPEN>,
412 L<C<setnetent>|/setnetent STAYOPEN>,
413 L<C<setprotoent>|/setprotoent STAYOPEN>,
414 L<C<setservent>|/setservent STAYOPEN>
416 =item Time-related functions
419 =for Pod::Functions =Time
421 L<C<gmtime>|/gmtime EXPR>, L<C<localtime>|/localtime EXPR>,
422 L<C<time>|/time>, L<C<times>|/times>
424 =item Non-function keywords
426 =for Pod::Functions =!Non-functions
428 C<and>, C<AUTOLOAD>, C<BEGIN>, C<CHECK>, C<cmp>, C<CORE>, C<__DATA__>,
429 C<default>, C<DESTROY>, C<else>, C<elseif>, C<elsif>, C<END>, C<__END__>,
430 C<eq>, C<for>, C<foreach>, C<ge>, C<given>, C<gt>, C<if>, C<INIT>, C<le>,
431 C<lt>, C<ne>, C<not>, C<or>, C<UNITCHECK>, C<unless>, C<until>, C<when>,
432 C<while>, C<x>, C<xor>
437 X<portability> X<Unix> X<portable>
439 Perl was born in Unix and can therefore access all common Unix
440 system calls. In non-Unix environments, the functionality of some
441 Unix system calls may not be available or details of the available
442 functionality may differ slightly. The Perl functions affected
445 L<C<-I<X>>|/-X FILEHANDLE>, L<C<binmode>|/binmode FILEHANDLE, LAYER>,
446 L<C<chmod>|/chmod LIST>, L<C<chown>|/chown LIST>,
447 L<C<chroot>|/chroot FILENAME>, L<C<crypt>|/crypt PLAINTEXT,SALT>,
448 L<C<dbmclose>|/dbmclose HASH>, L<C<dbmopen>|/dbmopen HASH,DBNAME,MASK>,
449 L<C<dump>|/dump LABEL>, L<C<endgrent>|/endgrent>,
450 L<C<endhostent>|/endhostent>, L<C<endnetent>|/endnetent>,
451 L<C<endprotoent>|/endprotoent>, L<C<endpwent>|/endpwent>,
452 L<C<endservent>|/endservent>, L<C<exec>|/exec LIST>,
453 L<C<fcntl>|/fcntl FILEHANDLE,FUNCTION,SCALAR>,
454 L<C<flock>|/flock FILEHANDLE,OPERATION>, L<C<fork>|/fork>,
455 L<C<getgrent>|/getgrent>, L<C<getgrgid>|/getgrgid GID>,
456 L<C<gethostbyname>|/gethostbyname NAME>, L<C<gethostent>|/gethostent>,
457 L<C<getlogin>|/getlogin>,
458 L<C<getnetbyaddr>|/getnetbyaddr ADDR,ADDRTYPE>,
459 L<C<getnetbyname>|/getnetbyname NAME>, L<C<getnetent>|/getnetent>,
460 L<C<getppid>|/getppid>, L<C<getpgrp>|/getpgrp PID>,
461 L<C<getpriority>|/getpriority WHICH,WHO>,
462 L<C<getprotobynumber>|/getprotobynumber NUMBER>,
463 L<C<getprotoent>|/getprotoent>, L<C<getpwent>|/getpwent>,
464 L<C<getpwnam>|/getpwnam NAME>, L<C<getpwuid>|/getpwuid UID>,
465 L<C<getservbyport>|/getservbyport PORT,PROTO>,
466 L<C<getservent>|/getservent>,
467 L<C<getsockopt>|/getsockopt SOCKET,LEVEL,OPTNAME>,
468 L<C<glob>|/glob EXPR>, L<C<ioctl>|/ioctl FILEHANDLE,FUNCTION,SCALAR>,
469 L<C<kill>|/kill SIGNAL, LIST>, L<C<link>|/link OLDFILE,NEWFILE>,
470 L<C<lstat>|/lstat FILEHANDLE>, L<C<msgctl>|/msgctl ID,CMD,ARG>,
471 L<C<msgget>|/msgget KEY,FLAGS>,
472 L<C<msgrcv>|/msgrcv ID,VAR,SIZE,TYPE,FLAGS>,
473 L<C<msgsnd>|/msgsnd ID,MSG,FLAGS>, L<C<open>|/open FILEHANDLE,EXPR>,
474 L<C<pipe>|/pipe READHANDLE,WRITEHANDLE>, L<C<readlink>|/readlink EXPR>,
475 L<C<rename>|/rename OLDNAME,NEWNAME>,
476 L<C<select>|/select RBITS,WBITS,EBITS,TIMEOUT>,
477 L<C<semctl>|/semctl ID,SEMNUM,CMD,ARG>,
478 L<C<semget>|/semget KEY,NSEMS,FLAGS>, L<C<semop>|/semop KEY,OPSTRING>,
479 L<C<setgrent>|/setgrent>, L<C<sethostent>|/sethostent STAYOPEN>,
480 L<C<setnetent>|/setnetent STAYOPEN>, L<C<setpgrp>|/setpgrp PID,PGRP>,
481 L<C<setpriority>|/setpriority WHICH,WHO,PRIORITY>,
482 L<C<setprotoent>|/setprotoent STAYOPEN>, L<C<setpwent>|/setpwent>,
483 L<C<setservent>|/setservent STAYOPEN>,
484 L<C<setsockopt>|/setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL>,
485 L<C<shmctl>|/shmctl ID,CMD,ARG>, L<C<shmget>|/shmget KEY,SIZE,FLAGS>,
486 L<C<shmread>|/shmread ID,VAR,POS,SIZE>,
487 L<C<shmwrite>|/shmwrite ID,STRING,POS,SIZE>,
488 L<C<socket>|/socket SOCKET,DOMAIN,TYPE,PROTOCOL>,
489 L<C<socketpair>|/socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL>,
490 L<C<stat>|/stat FILEHANDLE>, L<C<symlink>|/symlink OLDFILE,NEWFILE>,
491 L<C<syscall>|/syscall NUMBER, LIST>,
492 L<C<sysopen>|/sysopen FILEHANDLE,FILENAME,MODE>,
493 L<C<system>|/system LIST>, L<C<times>|/times>,
494 L<C<truncate>|/truncate FILEHANDLE,LENGTH>, L<C<umask>|/umask EXPR>,
495 L<C<unlink>|/unlink LIST>, L<C<utime>|/utime LIST>, L<C<wait>|/wait>,
496 L<C<waitpid>|/waitpid PID,FLAGS>
498 For more information about the portability of these functions, see
499 L<perlport> and other available platform-specific documentation.
501 =head2 Alphabetical Listing of Perl Functions
506 X<-r>X<-w>X<-x>X<-o>X<-R>X<-W>X<-X>X<-O>X<-e>X<-z>X<-s>X<-f>X<-d>X<-l>X<-p>
507 X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
515 =for Pod::Functions a file test (-r, -x, etc)
517 A file test, where X is one of the letters listed below. This unary
518 operator takes one argument, either a filename, a filehandle, or a dirhandle,
519 and tests the associated file to see if something is true about it. If the
520 argument is omitted, tests L<C<$_>|perlvar/$_>, except for C<-t>, which
521 tests STDIN. Unless otherwise documented, it returns C<1> for true and
522 C<''> for false. If the file doesn't exist or can't be examined, it
523 returns L<C<undef>|/undef EXPR> and sets L<C<$!>|perlvar/$!> (errno).
524 Despite the funny names, precedence is the same as any other named unary
525 operator. The operator may be any of:
527 -r File is readable by effective uid/gid.
528 -w File is writable by effective uid/gid.
529 -x File is executable by effective uid/gid.
530 -o File is owned by effective uid.
532 -R File is readable by real uid/gid.
533 -W File is writable by real uid/gid.
534 -X File is executable by real uid/gid.
535 -O File is owned by real uid.
538 -z File has zero size (is empty).
539 -s File has nonzero size (returns size in bytes).
541 -f File is a plain file.
542 -d File is a directory.
543 -l File is a symbolic link (false if symlinks aren't
544 supported by the file system).
545 -p File is a named pipe (FIFO), or Filehandle is a pipe.
547 -b File is a block special file.
548 -c File is a character special file.
549 -t Filehandle is opened to a tty.
551 -u File has setuid bit set.
552 -g File has setgid bit set.
553 -k File has sticky bit set.
555 -T File is an ASCII or UTF-8 text file (heuristic guess).
556 -B File is a "binary" file (opposite of -T).
558 -M Script start time minus file modification time, in days.
559 -A Same for access time.
560 -C Same for inode change time (Unix, may differ for other
567 next unless -f $_; # ignore specials
571 Note that C<-s/a/b/> does not do a negated substitution. Saying
572 C<-exp($foo)> still works as expected, however: only single letters
573 following a minus are interpreted as file tests.
575 These operators are exempt from the "looks like a function rule" described
576 above. That is, an opening parenthesis after the operator does not affect
577 how much of the following code constitutes the argument. Put the opening
578 parentheses before the operator to separate it from code that follows (this
579 applies only to operators with higher precedence than unary operators, of
582 -s($file) + 1024 # probably wrong; same as -s($file + 1024)
583 (-s $file) + 1024 # correct
585 The interpretation of the file permission operators C<-r>, C<-R>,
586 C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
587 of the file and the uids and gids of the user. There may be other
588 reasons you can't actually read, write, or execute the file: for
589 example network filesystem access controls, ACLs (access control lists),
590 read-only filesystems, and unrecognized executable formats. Note
591 that the use of these six specific operators to verify if some operation
592 is possible is usually a mistake, because it may be open to race
595 Also note that, for the superuser on the local filesystems, the C<-r>,
596 C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
597 if any execute bit is set in the mode. Scripts run by the superuser
598 may thus need to do a L<C<stat>|/stat FILEHANDLE> to determine the
599 actual mode of the file, or temporarily set their effective uid to
602 If you are using ACLs, there is a pragma called L<C<filetest>|filetest>
603 that may produce more accurate results than the bare
604 L<C<stat>|/stat FILEHANDLE> mode bits.
605 When under C<use filetest 'access'>, the above-mentioned filetests
606 test whether the permission can(not) be granted using the L<access(2)>
607 family of system calls. Also note that the C<-x> and C<-X> tests may
608 under this pragma return true even if there are no execute permission
609 bits set (nor any extra execute permission ACLs). This strangeness is
610 due to the underlying system calls' definitions. Note also that, due to
611 the implementation of C<use filetest 'access'>, the C<_> special
612 filehandle won't cache the results of the file tests when this pragma is
613 in effect. Read the documentation for the L<C<filetest>|filetest>
614 pragma for more information.
616 The C<-T> and C<-B> tests work as follows. The first block or so of
617 the file is examined to see if it is valid UTF-8 that includes non-ASCII
618 characters. If so, it's a C<-T> file. Otherwise, that same portion of
619 the file is examined for odd characters such as strange control codes or
620 characters with the high bit set. If more than a third of the
621 characters are strange, it's a C<-B> file; otherwise it's a C<-T> file.
622 Also, any file containing a zero byte in the examined portion is
623 considered a binary file. (If executed within the scope of a L<S<use
624 locale>|perllocale> which includes C<LC_CTYPE>, odd characters are
625 anything that isn't a printable nor space in the current locale.) If
626 C<-T> or C<-B> is used on a filehandle, the current IO buffer is
628 rather than the first block. Both C<-T> and C<-B> return true on an empty
629 file, or a file at EOF when testing a filehandle. Because you have to
630 read a file to do the C<-T> test, on most occasions you want to use a C<-f>
631 against the file first, as in C<next unless -f $file && -T $file>.
633 If any of the file tests (or either the L<C<stat>|/stat FILEHANDLE> or
634 L<C<lstat>|/lstat FILEHANDLE> operator) is given the special filehandle
635 consisting of a solitary underline, then the stat structure of the
636 previous file test (or L<C<stat>|/stat FILEHANDLE> operator) is used,
637 saving a system call. (This doesn't work with C<-t>, and you need to
638 remember that L<C<lstat>|/lstat FILEHANDLE> and C<-l> leave values in
639 the stat structure for the symbolic link, not the real file.) (Also, if
640 the stat buffer was filled by an L<C<lstat>|/lstat FILEHANDLE> call,
641 C<-T> and C<-B> will reset it with the results of C<stat _>).
644 print "Can do.\n" if -r $a || -w _ || -x _;
647 print "Readable\n" if -r _;
648 print "Writable\n" if -w _;
649 print "Executable\n" if -x _;
650 print "Setuid\n" if -u _;
651 print "Setgid\n" if -g _;
652 print "Sticky\n" if -k _;
653 print "Text\n" if -T _;
654 print "Binary\n" if -B _;
656 As of Perl 5.10.0, as a form of purely syntactic sugar, you can stack file
657 test operators, in a way that C<-f -w -x $file> is equivalent to
658 C<-x $file && -w _ && -f _>. (This is only fancy syntax: if you use
659 the return value of C<-f $file> as an argument to another filetest
660 operator, no special magic will happen.)
662 Portability issues: L<perlport/-X>.
664 To avoid confusing would-be users of your code with mysterious
665 syntax errors, put something like this at the top of your script:
667 use 5.010; # so filetest ops can stack
674 =for Pod::Functions absolute value function
676 Returns the absolute value of its argument.
677 If VALUE is omitted, uses L<C<$_>|perlvar/$_>.
679 =item accept NEWSOCKET,GENERICSOCKET
682 =for Pod::Functions accept an incoming socket connect
684 Accepts an incoming socket connect, just as L<accept(2)>
685 does. Returns the packed address if it succeeded, false otherwise.
686 See the example in L<perlipc/"Sockets: Client/Server Communication">.
688 On systems that support a close-on-exec flag on files, the flag will
689 be set for the newly opened file descriptor, as determined by the
690 value of L<C<$^F>|perlvar/$^F>. See L<perlvar/$^F>.
699 =for Pod::Functions schedule a SIGALRM
701 Arranges to have a SIGALRM delivered to this process after the
702 specified number of wallclock seconds has elapsed. If SECONDS is not
703 specified, the value stored in L<C<$_>|perlvar/$_> is used. (On some
704 machines, unfortunately, the elapsed time may be up to one second less
705 or more than you specified because of how seconds are counted, and
706 process scheduling may delay the delivery of the signal even further.)
708 Only one timer may be counting at once. Each call disables the
709 previous timer, and an argument of C<0> may be supplied to cancel the
710 previous timer without starting a new one. The returned value is the
711 amount of time remaining on the previous timer.
713 For delays of finer granularity than one second, the L<Time::HiRes> module
714 (from CPAN, and starting from Perl 5.8 part of the standard
715 distribution) provides
716 L<C<ualarm>|Time::HiRes/ualarm ( $useconds [, $interval_useconds ] )>.
717 You may also use Perl's four-argument version of
718 L<C<select>|/select RBITS,WBITS,EBITS,TIMEOUT> leaving the first three
719 arguments undefined, or you might be able to use the
720 L<C<syscall>|/syscall NUMBER, LIST> interface to access L<setitimer(2)>
721 if your system supports it. See L<perlfaq8> for details.
723 It is usually a mistake to intermix L<C<alarm>|/alarm SECONDS> and
724 L<C<sleep>|/sleep EXPR> calls, because L<C<sleep>|/sleep EXPR> may be
725 internally implemented on your system with L<C<alarm>|/alarm SECONDS>.
727 If you want to use L<C<alarm>|/alarm SECONDS> to time out a system call
728 you need to use an L<C<eval>|/eval EXPR>/L<C<die>|/die LIST> pair. You
729 can't rely on the alarm causing the system call to fail with
730 L<C<$!>|perlvar/$!> set to C<EINTR> because Perl sets up signal handlers
731 to restart system calls on some systems. Using
732 L<C<eval>|/eval EXPR>/L<C<die>|/die LIST> always works, modulo the
733 caveats given in L<perlipc/"Signals">.
736 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
738 my $nread = sysread $socket, $buffer, $size;
742 die unless $@ eq "alarm\n"; # propagate unexpected errors
749 For more information see L<perlipc>.
751 Portability issues: L<perlport/alarm>.
754 X<atan2> X<arctangent> X<tan> X<tangent>
756 =for Pod::Functions arctangent of Y/X in the range -PI to PI
758 Returns the arctangent of Y/X in the range -PI to PI.
760 For the tangent operation, you may use the
761 L<C<Math::Trig::tan>|Math::Trig/B<tan>> function, or use the familiar
764 sub tan { sin($_[0]) / cos($_[0]) }
766 The return value for C<atan2(0,0)> is implementation-defined; consult
767 your L<atan2(3)> manpage for more information.
769 Portability issues: L<perlport/atan2>.
771 =item bind SOCKET,NAME
774 =for Pod::Functions binds an address to a socket
776 Binds a network address to a socket, just as L<bind(2)>
777 does. Returns true if it succeeded, false otherwise. NAME should be a
778 packed address of the appropriate type for the socket. See the examples in
779 L<perlipc/"Sockets: Client/Server Communication">.
781 =item binmode FILEHANDLE, LAYER
782 X<binmode> X<binary> X<text> X<DOS> X<Windows>
784 =item binmode FILEHANDLE
786 =for Pod::Functions prepare binary files for I/O
788 Arranges for FILEHANDLE to be read or written in "binary" or "text"
789 mode on systems where the run-time libraries distinguish between
790 binary and text files. If FILEHANDLE is an expression, the value is
791 taken as the name of the filehandle. Returns true on success,
792 otherwise it returns L<C<undef>|/undef EXPR> and sets
793 L<C<$!>|perlvar/$!> (errno).
795 On some systems (in general, DOS- and Windows-based systems)
796 L<C<binmode>|/binmode FILEHANDLE, LAYER> is necessary when you're not
797 working with a text file. For the sake of portability it is a good idea
798 always to use it when appropriate, and never to use it when it isn't
799 appropriate. Also, people can set their I/O to be by default
800 UTF8-encoded Unicode, not bytes.
802 In other words: regardless of platform, use
803 L<C<binmode>|/binmode FILEHANDLE, LAYER> on binary data, like images,
806 If LAYER is present it is a single string, but may contain multiple
807 directives. The directives alter the behaviour of the filehandle.
808 When LAYER is present, using binmode on a text file makes sense.
810 If LAYER is omitted or specified as C<:raw> the filehandle is made
811 suitable for passing binary data. This includes turning off possible CRLF
812 translation and marking it as bytes (as opposed to Unicode characters).
813 Note that, despite what may be implied in I<"Programming Perl"> (the
814 Camel, 3rd edition) or elsewhere, C<:raw> is I<not> simply the inverse of C<:crlf>.
815 Other layers that would affect the binary nature of the stream are
816 I<also> disabled. See L<PerlIO>, L<perlrun>, and the discussion about the
817 PERLIO environment variable.
819 The C<:bytes>, C<:crlf>, C<:utf8>, and any other directives of the
820 form C<:...>, are called I/O I<layers>. The L<open> pragma can be used to
821 establish default I/O layers.
823 I<The LAYER parameter of the L<C<binmode>|/binmode FILEHANDLE, LAYER>
824 function is described as "DISCIPLINE" in "Programming Perl, 3rd
825 Edition". However, since the publishing of this book, by many known as
826 "Camel III", the consensus of the naming of this functionality has moved
827 from "discipline" to "layer". All documentation of this version of Perl
828 therefore refers to "layers" rather than to "disciplines". Now back to
829 the regularly scheduled documentation...>
831 To mark FILEHANDLE as UTF-8, use C<:utf8> or C<:encoding(UTF-8)>.
832 C<:utf8> just marks the data as UTF-8 without further checking,
833 while C<:encoding(UTF-8)> checks the data for actually being valid
834 UTF-8. More details can be found in L<PerlIO::encoding>.
836 In general, L<C<binmode>|/binmode FILEHANDLE, LAYER> should be called
837 after L<C<open>|/open FILEHANDLE,EXPR> but before any I/O is done on the
838 filehandle. Calling L<C<binmode>|/binmode FILEHANDLE, LAYER> normally
839 flushes any pending buffered output data (and perhaps pending input
840 data) on the handle. An exception to this is the C<:encoding> layer
841 that changes the default character encoding of the handle.
842 The C<:encoding> layer sometimes needs to be called in
843 mid-stream, and it doesn't flush the stream. C<:encoding>
844 also implicitly pushes on top of itself the C<:utf8> layer because
845 internally Perl operates on UTF8-encoded Unicode characters.
847 The operating system, device drivers, C libraries, and Perl run-time
848 system all conspire to let the programmer treat a single
849 character (C<\n>) as the line terminator, irrespective of external
850 representation. On many operating systems, the native text file
851 representation matches the internal representation, but on some
852 platforms the external representation of C<\n> is made up of more than
855 All variants of Unix, Mac OS (old and new), and Stream_LF files on VMS use
856 a single character to end each line in the external representation of text
857 (even though that single character is CARRIAGE RETURN on old, pre-Darwin
858 flavors of Mac OS, and is LINE FEED on Unix and most VMS files). In other
859 systems like OS/2, DOS, and the various flavors of MS-Windows, your program
860 sees a C<\n> as a simple C<\cJ>, but what's stored in text files are the
861 two characters C<\cM\cJ>. That means that if you don't use
862 L<C<binmode>|/binmode FILEHANDLE, LAYER> on these systems, C<\cM\cJ>
863 sequences on disk will be converted to C<\n> on input, and any C<\n> in
864 your program will be converted back to C<\cM\cJ> on output. This is
865 what you want for text files, but it can be disastrous for binary files.
867 Another consequence of using L<C<binmode>|/binmode FILEHANDLE, LAYER>
868 (on some systems) is that special end-of-file markers will be seen as
869 part of the data stream. For systems from the Microsoft family this
870 means that, if your binary data contain C<\cZ>, the I/O subsystem will
871 regard it as the end of the file, unless you use
872 L<C<binmode>|/binmode FILEHANDLE, LAYER>.
874 L<C<binmode>|/binmode FILEHANDLE, LAYER> is important not only for
875 L<C<readline>|/readline EXPR> and L<C<print>|/print FILEHANDLE LIST>
876 operations, but also when using
877 L<C<read>|/read FILEHANDLE,SCALAR,LENGTH,OFFSET>,
878 L<C<seek>|/seek FILEHANDLE,POSITION,WHENCE>,
879 L<C<sysread>|/sysread FILEHANDLE,SCALAR,LENGTH,OFFSET>,
880 L<C<syswrite>|/syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET> and
881 L<C<tell>|/tell FILEHANDLE> (see L<perlport> for more details). See the
882 L<C<$E<sol>>|perlvar/$E<sol>> and L<C<$\>|perlvar/$\> variables in
883 L<perlvar> for how to manually set your input and output
884 line-termination sequences.
886 Portability issues: L<perlport/binmode>.
888 =item bless REF,CLASSNAME
893 =for Pod::Functions create an object
895 This function tells the thingy referenced by REF that it is now an object
896 in the CLASSNAME package. If CLASSNAME is omitted, the current package
897 is used. Because a L<C<bless>|/bless REF,CLASSNAME> is often the last
898 thing in a constructor, it returns the reference for convenience.
899 Always use the two-argument version if a derived class might inherit the
900 method doing the blessing. See L<perlobj> for more about the blessing
901 (and blessings) of objects.
903 Consider always blessing objects in CLASSNAMEs that are mixed case.
904 Namespaces with all lowercase names are considered reserved for
905 Perl pragmas. Builtin types have all uppercase names. To prevent
906 confusion, you may wish to avoid such package names as well. Make sure
907 that CLASSNAME is a true value.
909 See L<perlmod/"Perl Modules">.
913 =for Pod::Functions +switch break out of a C<given> block
915 Break out of a C<given> block.
917 L<C<break>|/break> is available only if the
918 L<C<"switch"> feature|feature/The 'switch' feature> is enabled or if it
919 is prefixed with C<CORE::>. The
920 L<C<"switch"> feature|feature/The 'switch' feature> is enabled
921 automatically with a C<use v5.10> (or higher) declaration in the current
925 X<caller> X<call stack> X<stack> X<stack trace>
929 =for Pod::Functions get context of the current subroutine call
931 Returns the context of the current pure perl subroutine call. In scalar
932 context, returns the caller's package name if there I<is> a caller (that is, if
933 we're in a subroutine or L<C<eval>|/eval EXPR> or
934 L<C<require>|/require VERSION>) and the undefined value otherwise.
935 caller never returns XS subs and they are skipped. The next pure perl
936 sub will appear instead of the XS sub in caller's return values. In
937 list context, caller returns
940 my ($package, $filename, $line) = caller;
942 With EXPR, it returns some extra information that the debugger uses to
943 print a stack trace. The value of EXPR indicates how many call frames
944 to go back before the current one.
947 my ($package, $filename, $line, $subroutine, $hasargs,
950 $wantarray, $evaltext, $is_require, $hints, $bitmask, $hinthash)
953 Here, $subroutine is the function that the caller called (rather than the
954 function containing the caller). Note that $subroutine may be C<(eval)> if
955 the frame is not a subroutine call, but an L<C<eval>|/eval EXPR>. In
956 such a case additional elements $evaltext and C<$is_require> are set:
957 C<$is_require> is true if the frame is created by a
958 L<C<require>|/require VERSION> or L<C<use>|/use Module VERSION LIST>
959 statement, $evaltext contains the text of the C<eval EXPR> statement.
960 In particular, for an C<eval BLOCK> statement, $subroutine is C<(eval)>,
961 but $evaltext is undefined. (Note also that each
962 L<C<use>|/use Module VERSION LIST> statement creates a
963 L<C<require>|/require VERSION> frame inside an C<eval EXPR> frame.)
964 $subroutine may also be C<(unknown)> if this particular subroutine
965 happens to have been deleted from the symbol table. C<$hasargs> is true
966 if a new instance of L<C<@_>|perlvar/@_> was set up for the frame.
967 C<$hints> and C<$bitmask> contain pragmatic hints that the caller was
968 compiled with. C<$hints> corresponds to L<C<$^H>|perlvar/$^H>, and
969 C<$bitmask> corresponds to
970 L<C<${^WARNING_BITS}>|perlvar/${^WARNING_BITS}>. The C<$hints> and
971 C<$bitmask> values are subject to change between versions of Perl, and
972 are not meant for external use.
974 C<$hinthash> is a reference to a hash containing the value of
975 L<C<%^H>|perlvar/%^H> when the caller was compiled, or
976 L<C<undef>|/undef EXPR> if L<C<%^H>|perlvar/%^H> was empty. Do not
977 modify the values of this hash, as they are the actual values stored in
980 Furthermore, when called from within the DB package in
981 list context, and with an argument, caller returns more
982 detailed information: it sets the list variable C<@DB::args> to be the
983 arguments with which the subroutine was invoked.
985 Be aware that the optimizer might have optimized call frames away before
986 L<C<caller>|/caller EXPR> had a chance to get the information. That
987 means that C<caller(N)> might not return information about the call
988 frame you expect it to, for C<< N > 1 >>. In particular, C<@DB::args>
989 might have information from the previous time L<C<caller>|/caller EXPR>
992 Be aware that setting C<@DB::args> is I<best effort>, intended for
993 debugging or generating backtraces, and should not be relied upon. In
994 particular, as L<C<@_>|perlvar/@_> contains aliases to the caller's
995 arguments, Perl does not take a copy of L<C<@_>|perlvar/@_>, so
996 C<@DB::args> will contain modifications the subroutine makes to
997 L<C<@_>|perlvar/@_> or its contents, not the original values at call
998 time. C<@DB::args>, like L<C<@_>|perlvar/@_>, does not hold explicit
999 references to its elements, so under certain cases its elements may have
1000 become freed and reallocated for other variables or temporary values.
1001 Finally, a side effect of the current implementation is that the effects
1002 of C<shift @_> can I<normally> be undone (but not C<pop @_> or other
1003 splicing, I<and> not if a reference to L<C<@_>|perlvar/@_> has been
1004 taken, I<and> subject to the caveat about reallocated elements), so
1005 C<@DB::args> is actually a hybrid of the current state and initial state
1006 of L<C<@_>|perlvar/@_>. Buyer beware.
1011 X<directory, change>
1013 =item chdir FILEHANDLE
1015 =item chdir DIRHANDLE
1019 =for Pod::Functions change your current working directory
1021 Changes the working directory to EXPR, if possible. If EXPR is omitted,
1022 changes to the directory specified by C<$ENV{HOME}>, if set; if not,
1023 changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
1024 variable C<$ENV{'SYS$LOGIN'}> is also checked, and used if it is set.) If
1025 neither is set, L<C<chdir>|/chdir EXPR> does nothing and fails. It
1026 returns true on success, false otherwise. See the example under
1027 L<C<die>|/die LIST>.
1029 On systems that support L<fchdir(2)>, you may pass a filehandle or
1030 directory handle as the argument. On systems that don't support L<fchdir(2)>,
1031 passing handles raises an exception.
1034 X<chmod> X<permission> X<mode>
1036 =for Pod::Functions changes the permissions on a list of files
1038 Changes the permissions of a list of files. The first element of the
1039 list must be the numeric mode, which should probably be an octal
1040 number, and which definitely should I<not> be a string of octal digits:
1041 C<0644> is okay, but C<"0644"> is not. Returns the number of files
1042 successfully changed. See also L<C<oct>|/oct EXPR> if all you have is a
1045 my $cnt = chmod 0755, "foo", "bar";
1046 chmod 0755, @executables;
1047 my $mode = "0644"; chmod $mode, "foo"; # !!! sets mode to
1049 my $mode = "0644"; chmod oct($mode), "foo"; # this is better
1050 my $mode = 0644; chmod $mode, "foo"; # this is best
1052 On systems that support L<fchmod(2)>, you may pass filehandles among the
1053 files. On systems that don't support L<fchmod(2)>, passing filehandles raises
1054 an exception. Filehandles must be passed as globs or glob references to be
1055 recognized; barewords are considered filenames.
1057 open(my $fh, "<", "foo");
1058 my $perm = (stat $fh)[2] & 07777;
1059 chmod($perm | 0600, $fh);
1061 You can also import the symbolic C<S_I*> constants from the
1062 L<C<Fcntl>|Fcntl> module:
1064 use Fcntl qw( :mode );
1065 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
1066 # Identical to the chmod 0755 of the example above.
1068 Portability issues: L<perlport/chmod>.
1070 =item chomp VARIABLE
1071 X<chomp> X<INPUT_RECORD_SEPARATOR> X<$/> X<newline> X<eol>
1077 =for Pod::Functions remove a trailing record separator from a string
1079 This safer version of L<C<chop>|/chop VARIABLE> removes any trailing
1080 string that corresponds to the current value of
1081 L<C<$E<sol>>|perlvar/$E<sol>> (also known as C<$INPUT_RECORD_SEPARATOR>
1082 in the L<C<English>|English> module). It returns the total
1083 number of characters removed from all its arguments. It's often used to
1084 remove the newline from the end of an input record when you're worried
1085 that the final record may be missing its newline. When in paragraph
1086 mode (C<$/ = ''>), it removes all trailing newlines from the string.
1087 When in slurp mode (C<$/ = undef>) or fixed-length record mode
1088 (L<C<$E<sol>>|perlvar/$E<sol>> is a reference to an integer or the like;
1089 see L<perlvar>), L<C<chomp>|/chomp VARIABLE> won't remove anything.
1090 If VARIABLE is omitted, it chomps L<C<$_>|perlvar/$_>. Example:
1093 chomp; # avoid \n on last field
1094 my @array = split(/:/);
1098 If VARIABLE is a hash, it chomps the hash's values, but not its keys,
1099 resetting the L<C<each>|/each HASH> iterator in the process.
1101 You can actually chomp anything that's an lvalue, including an assignment:
1103 chomp(my $cwd = `pwd`);
1104 chomp(my $answer = <STDIN>);
1106 If you chomp a list, each element is chomped, and the total number of
1107 characters removed is returned.
1109 Note that parentheses are necessary when you're chomping anything
1110 that is not a simple variable. This is because C<chomp $cwd = `pwd`;>
1111 is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as
1112 C<chomp( $cwd = `pwd` )> which you might expect. Similarly,
1113 C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than
1114 as C<chomp($a, $b)>.
1123 =for Pod::Functions remove the last character from a string
1125 Chops off the last character of a string and returns the character
1126 chopped. It is much more efficient than C<s/.$//s> because it neither
1127 scans nor copies the string. If VARIABLE is omitted, chops
1128 L<C<$_>|perlvar/$_>.
1129 If VARIABLE is a hash, it chops the hash's values, but not its keys,
1130 resetting the L<C<each>|/each HASH> iterator in the process.
1132 You can actually chop anything that's an lvalue, including an assignment.
1134 If you chop a list, each element is chopped. Only the value of the
1135 last L<C<chop>|/chop VARIABLE> is returned.
1137 Note that L<C<chop>|/chop VARIABLE> returns the last character. To
1138 return all but the last character, use C<substr($string, 0, -1)>.
1140 See also L<C<chomp>|/chomp VARIABLE>.
1143 X<chown> X<owner> X<user> X<group>
1145 =for Pod::Functions change the ownership on a list of files
1147 Changes the owner (and group) of a list of files. The first two
1148 elements of the list must be the I<numeric> uid and gid, in that
1149 order. A value of -1 in either position is interpreted by most
1150 systems to leave that value unchanged. Returns the number of files
1151 successfully changed.
1153 my $cnt = chown $uid, $gid, 'foo', 'bar';
1154 chown $uid, $gid, @filenames;
1156 On systems that support L<fchown(2)>, you may pass filehandles among the
1157 files. On systems that don't support L<fchown(2)>, passing filehandles raises
1158 an exception. Filehandles must be passed as globs or glob references to be
1159 recognized; barewords are considered filenames.
1161 Here's an example that looks up nonnumeric uids in the passwd file:
1164 chomp(my $user = <STDIN>);
1166 chomp(my $pattern = <STDIN>);
1168 my ($login,$pass,$uid,$gid) = getpwnam($user)
1169 or die "$user not in passwd file";
1171 my @ary = glob($pattern); # expand filenames
1172 chown $uid, $gid, @ary;
1174 On most systems, you are not allowed to change the ownership of the
1175 file unless you're the superuser, although you should be able to change
1176 the group to any of your secondary groups. On insecure systems, these
1177 restrictions may be relaxed, but this is not a portable assumption.
1178 On POSIX systems, you can detect this condition this way:
1180 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
1181 my $can_chown_giveaway = ! sysconf(_PC_CHOWN_RESTRICTED);
1183 Portability issues: L<perlport/chown>.
1186 X<chr> X<character> X<ASCII> X<Unicode>
1190 =for Pod::Functions get character this number represents
1192 Returns the character represented by that NUMBER in the character set.
1193 For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
1194 chr(0x263a) is a Unicode smiley face.
1196 Negative values give the Unicode replacement character (chr(0xfffd)),
1197 except under the L<bytes> pragma, where the low eight bits of the value
1198 (truncated to an integer) are used.
1200 If NUMBER is omitted, uses L<C<$_>|perlvar/$_>.
1202 For the reverse, use L<C<ord>|/ord EXPR>.
1204 Note that characters from 128 to 255 (inclusive) are by default
1205 internally not encoded as UTF-8 for backward compatibility reasons.
1207 See L<perlunicode> for more about Unicode.
1209 =item chroot FILENAME
1214 =for Pod::Functions make directory new root for path lookups
1216 This function works like the system call by the same name: it makes the
1217 named directory the new root directory for all further pathnames that
1218 begin with a C</> by your process and all its children. (It doesn't
1219 change your current working directory, which is unaffected.) For security
1220 reasons, this call is restricted to the superuser. If FILENAME is
1221 omitted, does a L<C<chroot>|/chroot FILENAME> to L<C<$_>|perlvar/$_>.
1223 B<NOTE:> It is good security practice to do C<chdir("/")>
1224 (L<C<chdir>|/chdir EXPR> to the root directory) immediately after a
1225 L<C<chroot>|/chroot FILENAME>.
1227 Portability issues: L<perlport/chroot>.
1229 =item close FILEHANDLE
1234 =for Pod::Functions close file (or pipe or socket) handle
1236 Closes the file or pipe associated with the filehandle, flushes the IO
1237 buffers, and closes the system file descriptor. Returns true if those
1238 operations succeed and if no error was reported by any PerlIO
1239 layer. Closes the currently selected filehandle if the argument is
1242 You don't have to close FILEHANDLE if you are immediately going to do
1243 another L<C<open>|/open FILEHANDLE,EXPR> on it, because
1244 L<C<open>|/open FILEHANDLE,EXPR> closes it for you. (See
1245 L<C<open>|/open FILEHANDLE,EXPR>.) However, an explicit
1246 L<C<close>|/close FILEHANDLE> on an input file resets the line counter
1247 (L<C<$.>|perlvar/$.>), while the implicit close done by
1248 L<C<open>|/open FILEHANDLE,EXPR> does not.
1250 If the filehandle came from a piped open, L<C<close>|/close FILEHANDLE>
1251 returns false if one of the other syscalls involved fails or if its
1252 program exits with non-zero status. If the only problem was that the
1253 program exited non-zero, L<C<$!>|perlvar/$!> will be set to C<0>.
1254 Closing a pipe also waits for the process executing on the pipe to
1255 exit--in case you wish to look at the output of the pipe afterwards--and
1256 implicitly puts the exit status value of that command into
1257 L<C<$?>|perlvar/$?> and
1258 L<C<${^CHILD_ERROR_NATIVE}>|perlvar/${^CHILD_ERROR_NATIVE}>.
1260 If there are multiple threads running, L<C<close>|/close FILEHANDLE> on
1261 a filehandle from a piped open returns true without waiting for the
1262 child process to terminate, if the filehandle is still open in another
1265 Closing the read end of a pipe before the process writing to it at the
1266 other end is done writing results in the writer receiving a SIGPIPE. If
1267 the other end can't handle that, be sure to read all the data before
1272 open(OUTPUT, '|sort >foo') # pipe to sort
1273 or die "Can't start sort: $!";
1274 #... # print stuff to output
1275 close OUTPUT # wait for sort to finish
1276 or warn $! ? "Error closing sort pipe: $!"
1277 : "Exit status $? from sort";
1278 open(INPUT, 'foo') # get sort's results
1279 or die "Can't open 'foo' for input: $!";
1281 FILEHANDLE may be an expression whose value can be used as an indirect
1282 filehandle, usually the real filehandle name or an autovivified handle.
1284 =item closedir DIRHANDLE
1287 =for Pod::Functions close directory handle
1289 Closes a directory opened by L<C<opendir>|/opendir DIRHANDLE,EXPR> and
1290 returns the success of that system call.
1292 =item connect SOCKET,NAME
1295 =for Pod::Functions connect to a remote socket
1297 Attempts to connect to a remote socket, just like L<connect(2)>.
1298 Returns true if it succeeded, false otherwise. NAME should be a
1299 packed address of the appropriate type for the socket. See the examples in
1300 L<perlipc/"Sockets: Client/Server Communication">.
1302 =item continue BLOCK
1307 =for Pod::Functions optional trailing block in a while or foreach
1309 When followed by a BLOCK, L<C<continue>|/continue BLOCK> is actually a
1310 flow control statement rather than a function. If there is a
1311 L<C<continue>|/continue BLOCK> BLOCK attached to a BLOCK (typically in a
1312 C<while> or C<foreach>), it is always executed just before the
1313 conditional is about to be evaluated again, just like the third part of
1314 a C<for> loop in C. Thus it can be used to increment a loop variable,
1315 even when the loop has been continued via the L<C<next>|/next LABEL>
1316 statement (which is similar to the C L<C<continue>|/continue BLOCK>
1319 L<C<last>|/last LABEL>, L<C<next>|/next LABEL>, or
1320 L<C<redo>|/redo LABEL> may appear within a
1321 L<C<continue>|/continue BLOCK> block; L<C<last>|/last LABEL> and
1322 L<C<redo>|/redo LABEL> behave as if they had been executed within the
1323 main block. So will L<C<next>|/next LABEL>, but since it will execute a
1324 L<C<continue>|/continue BLOCK> block, it may be more entertaining.
1327 ### redo always comes here
1330 ### next always comes here
1332 # then back the top to re-check EXPR
1334 ### last always comes here
1336 Omitting the L<C<continue>|/continue BLOCK> section is equivalent to
1337 using an empty one, logically enough, so L<C<next>|/next LABEL> goes
1338 directly back to check the condition at the top of the loop.
1340 When there is no BLOCK, L<C<continue>|/continue BLOCK> is a function
1341 that falls through the current C<when> or C<default> block instead of
1342 iterating a dynamically enclosing C<foreach> or exiting a lexically
1343 enclosing C<given>. In Perl 5.14 and earlier, this form of
1344 L<C<continue>|/continue BLOCK> was only available when the
1345 L<C<"switch"> feature|feature/The 'switch' feature> was enabled. See
1346 L<feature> and L<perlsyn/"Switch Statements"> for more information.
1349 X<cos> X<cosine> X<acos> X<arccosine>
1353 =for Pod::Functions cosine function
1355 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
1356 takes the cosine of L<C<$_>|perlvar/$_>.
1358 For the inverse cosine operation, you may use the
1359 L<C<Math::Trig::acos>|Math::Trig> function, or use this relation:
1361 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
1363 =item crypt PLAINTEXT,SALT
1364 X<crypt> X<digest> X<hash> X<salt> X<plaintext> X<password>
1365 X<decrypt> X<cryptography> X<passwd> X<encrypt>
1367 =for Pod::Functions one-way passwd-style encryption
1369 Creates a digest string exactly like the L<crypt(3)> function in the C
1370 library (assuming that you actually have a version there that has not
1371 been extirpated as a potential munition).
1373 L<C<crypt>|/crypt PLAINTEXT,SALT> is a one-way hash function. The
1374 PLAINTEXT and SALT are turned
1375 into a short string, called a digest, which is returned. The same
1376 PLAINTEXT and SALT will always return the same string, but there is no
1377 (known) way to get the original PLAINTEXT from the hash. Small
1378 changes in the PLAINTEXT or SALT will result in large changes in the
1381 There is no decrypt function. This function isn't all that useful for
1382 cryptography (for that, look for F<Crypt> modules on your nearby CPAN
1383 mirror) and the name "crypt" is a bit of a misnomer. Instead it is
1384 primarily used to check if two pieces of text are the same without
1385 having to transmit or store the text itself. An example is checking
1386 if a correct password is given. The digest of the password is stored,
1387 not the password itself. The user types in a password that is
1388 L<C<crypt>|/crypt PLAINTEXT,SALT>'d with the same salt as the stored
1389 digest. If the two digests match, the password is correct.
1391 When verifying an existing digest string you should use the digest as
1392 the salt (like C<crypt($plain, $digest) eq $digest>). The SALT used
1393 to create the digest is visible as part of the digest. This ensures
1394 L<C<crypt>|/crypt PLAINTEXT,SALT> will hash the new string with the same
1395 salt as the digest. This allows your code to work with the standard
1396 L<C<crypt>|/crypt PLAINTEXT,SALT> and with more exotic implementations.
1397 In other words, assume nothing about the returned string itself nor
1398 about how many bytes of SALT may matter.
1400 Traditionally the result is a string of 13 bytes: two first bytes of
1401 the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
1402 the first eight bytes of PLAINTEXT mattered. But alternative
1403 hashing schemes (like MD5), higher level security schemes (like C2),
1404 and implementations on non-Unix platforms may produce different
1407 When choosing a new salt create a random two character string whose
1408 characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
1409 '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of
1410 characters is just a recommendation; the characters allowed in
1411 the salt depend solely on your system's crypt library, and Perl can't
1412 restrict what salts L<C<crypt>|/crypt PLAINTEXT,SALT> accepts.
1414 Here's an example that makes sure that whoever runs this program knows
1417 my $pwd = (getpwuid($<))[1];
1419 system "stty -echo";
1421 chomp(my $word = <STDIN>);
1425 if (crypt($word, $pwd) ne $pwd) {
1431 Of course, typing in your own password to whoever asks you
1434 The L<C<crypt>|/crypt PLAINTEXT,SALT> function is unsuitable for hashing
1435 large quantities of data, not least of all because you can't get the
1436 information back. Look at the L<Digest> module for more robust
1439 If using L<C<crypt>|/crypt PLAINTEXT,SALT> on a Unicode string (which
1440 I<potentially> has characters with codepoints above 255), Perl tries to
1441 make sense of the situation by trying to downgrade (a copy of) the
1442 string back to an eight-bit byte string before calling
1443 L<C<crypt>|/crypt PLAINTEXT,SALT> (on that copy). If that works, good.
1444 If not, L<C<crypt>|/crypt PLAINTEXT,SALT> dies with
1445 L<C<Wide character in crypt>|perldiag/Wide character in %s>.
1447 Portability issues: L<perlport/crypt>.
1452 =for Pod::Functions breaks binding on a tied dbm file
1454 [This function has been largely superseded by the
1455 L<C<untie>|/untie VARIABLE> function.]
1457 Breaks the binding between a DBM file and a hash.
1459 Portability issues: L<perlport/dbmclose>.
1461 =item dbmopen HASH,DBNAME,MASK
1462 X<dbmopen> X<dbm> X<ndbm> X<sdbm> X<gdbm>
1464 =for Pod::Functions create binding on a tied dbm file
1466 [This function has been largely superseded by the
1467 L<C<tie>|/tie VARIABLE,CLASSNAME,LIST> function.]
1469 This binds a L<dbm(3)>, L<ndbm(3)>, L<sdbm(3)>, L<gdbm(3)>, or Berkeley
1470 DB file to a hash. HASH is the name of the hash. (Unlike normal
1471 L<C<open>|/open FILEHANDLE,EXPR>, the first argument is I<not> a
1472 filehandle, even though it looks like one). DBNAME is the name of the
1473 database (without the F<.dir> or F<.pag> extension if any). If the
1474 database does not exist, it is created with protection specified by MASK
1475 (as modified by the L<C<umask>|/umask EXPR>). To prevent creation of
1476 the database if it doesn't exist, you may specify a MODE of 0, and the
1477 function will return a false value if it can't find an existing
1478 database. If your system supports only the older DBM functions, you may
1479 make only one L<C<dbmopen>|/dbmopen HASH,DBNAME,MASK> call in your
1480 program. In older versions of Perl, if your system had neither DBM nor
1481 ndbm, calling L<C<dbmopen>|/dbmopen HASH,DBNAME,MASK> produced a fatal
1482 error; it now falls back to L<sdbm(3)>.
1484 If you don't have write access to the DBM file, you can only read hash
1485 variables, not set them. If you want to test whether you can write,
1486 either use file tests or try setting a dummy hash entry inside an
1487 L<C<eval>|/eval EXPR> to trap the error.
1489 Note that functions such as L<C<keys>|/keys HASH> and
1490 L<C<values>|/values HASH> may return huge lists when used on large DBM
1491 files. You may prefer to use the L<C<each>|/each HASH> function to
1492 iterate over large DBM files. Example:
1494 # print out history file offsets
1495 dbmopen(%HIST,'/usr/lib/news/history',0666);
1496 while (($key,$val) = each %HIST) {
1497 print $key, ' = ', unpack('L',$val), "\n";
1501 See also L<AnyDBM_File> for a more general description of the pros and
1502 cons of the various dbm approaches, as well as L<DB_File> for a particularly
1503 rich implementation.
1505 You can control which DBM library you use by loading that library
1506 before you call L<C<dbmopen>|/dbmopen HASH,DBNAME,MASK>:
1509 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
1510 or die "Can't open netscape history file: $!";
1512 Portability issues: L<perlport/dbmopen>.
1515 X<defined> X<undef> X<undefined>
1519 =for Pod::Functions test whether a value, variable, or function is defined
1521 Returns a Boolean value telling whether EXPR has a value other than the
1522 undefined value L<C<undef>|/undef EXPR>. If EXPR is not present,
1523 L<C<$_>|perlvar/$_> is checked.
1525 Many operations return L<C<undef>|/undef EXPR> to indicate failure, end
1526 of file, system error, uninitialized variable, and other exceptional
1527 conditions. This function allows you to distinguish
1528 L<C<undef>|/undef EXPR> from other values. (A simple Boolean test will
1529 not distinguish among L<C<undef>|/undef EXPR>, zero, the empty string,
1530 and C<"0">, which are all equally false.) Note that since
1531 L<C<undef>|/undef EXPR> is a valid scalar, its presence doesn't
1532 I<necessarily> indicate an exceptional condition: L<C<pop>|/pop ARRAY>
1533 returns L<C<undef>|/undef EXPR> when its argument is an empty array,
1534 I<or> when the element to return happens to be L<C<undef>|/undef EXPR>.
1536 You may also use C<defined(&func)> to check whether subroutine C<func>
1537 has ever been defined. The return value is unaffected by any forward
1538 declarations of C<func>. A subroutine that is not defined
1539 may still be callable: its package may have an C<AUTOLOAD> method that
1540 makes it spring into existence the first time that it is called; see
1543 Use of L<C<defined>|/defined EXPR> on aggregates (hashes and arrays) is
1544 no longer supported. It used to report whether memory for that
1545 aggregate had ever been allocated. You should instead use a simple
1548 if (@an_array) { print "has array elements\n" }
1549 if (%a_hash) { print "has hash members\n" }
1551 When used on a hash element, it tells you whether the value is defined,
1552 not whether the key exists in the hash. Use L<C<exists>|/exists EXPR>
1553 for the latter purpose.
1557 print if defined $switch{D};
1558 print "$val\n" while defined($val = pop(@ary));
1559 die "Can't readlink $sym: $!"
1560 unless defined($value = readlink $sym);
1561 sub foo { defined &$bar ? $bar->(@_) : die "No bar"; }
1562 $debugging = 0 unless defined $debugging;
1564 Note: Many folks tend to overuse L<C<defined>|/defined EXPR> and are
1565 then surprised to discover that the number C<0> and C<""> (the
1566 zero-length string) are, in fact, defined values. For example, if you
1571 The pattern match succeeds and C<$1> is defined, although it
1572 matched "nothing". It didn't really fail to match anything. Rather, it
1573 matched something that happened to be zero characters long. This is all
1574 very above-board and honest. When a function returns an undefined value,
1575 it's an admission that it couldn't give you an honest answer. So you
1576 should use L<C<defined>|/defined EXPR> only when questioning the
1577 integrity of what you're trying to do. At other times, a simple
1578 comparison to C<0> or C<""> is what you want.
1580 See also L<C<undef>|/undef EXPR>, L<C<exists>|/exists EXPR>,
1581 L<C<ref>|/ref EXPR>.
1586 =for Pod::Functions deletes a value from a hash
1588 Given an expression that specifies an element or slice of a hash,
1589 L<C<delete>|/delete EXPR> deletes the specified elements from that hash
1590 so that L<C<exists>|/exists EXPR> on that element no longer returns
1591 true. Setting a hash element to the undefined value does not remove its
1592 key, but deleting it does; see L<C<exists>|/exists EXPR>.
1594 In list context, returns the value or values deleted, or the last such
1595 element in scalar context. The return list's length always matches that of
1596 the argument list: deleting non-existent elements returns the undefined value
1597 in their corresponding positions.
1599 L<C<delete>|/delete EXPR> may also be used on arrays and array slices,
1600 but its behavior is less straightforward. Although
1601 L<C<exists>|/exists EXPR> will return false for deleted entries,
1602 deleting array elements never changes indices of existing values; use
1603 L<C<shift>|/shift ARRAY> or L<C<splice>|/splice
1604 ARRAY,OFFSET,LENGTH,LIST> for that. However, if any deleted elements
1605 fall at the end of an array, the array's size shrinks to the position of
1606 the highest element that still tests true for L<C<exists>|/exists EXPR>,
1607 or to 0 if none do. In other words, an array won't have trailing
1608 nonexistent elements after a delete.
1610 B<WARNING:> Calling L<C<delete>|/delete EXPR> on array values is
1611 strongly discouraged. The
1612 notion of deleting or checking the existence of Perl array elements is not
1613 conceptually coherent, and can lead to surprising behavior.
1615 Deleting from L<C<%ENV>|perlvar/%ENV> modifies the environment.
1616 Deleting from a hash tied to a DBM file deletes the entry from the DBM
1617 file. Deleting from a L<C<tied>|/tied VARIABLE> hash or array may not
1618 necessarily return anything; it depends on the implementation of the
1619 L<C<tied>|/tied VARIABLE> package's DELETE method, which may do whatever
1622 The C<delete local EXPR> construct localizes the deletion to the current
1623 block at run time. Until the block exits, elements locally deleted
1624 temporarily no longer exist. See L<perlsub/"Localized deletion of elements
1625 of composite types">.
1627 my %hash = (foo => 11, bar => 22, baz => 33);
1628 my $scalar = delete $hash{foo}; # $scalar is 11
1629 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
1630 my @array = delete @hash{qw(foo baz)}; # @array is (undef,33)
1632 The following (inefficiently) deletes all the values of %HASH and @ARRAY:
1634 foreach my $key (keys %HASH) {
1638 foreach my $index (0 .. $#ARRAY) {
1639 delete $ARRAY[$index];
1644 delete @HASH{keys %HASH};
1646 delete @ARRAY[0 .. $#ARRAY];
1648 But both are slower than assigning the empty list
1649 or undefining %HASH or @ARRAY, which is the customary
1650 way to empty out an aggregate:
1652 %HASH = (); # completely empty %HASH
1653 undef %HASH; # forget %HASH ever existed
1655 @ARRAY = (); # completely empty @ARRAY
1656 undef @ARRAY; # forget @ARRAY ever existed
1658 The EXPR can be arbitrarily complicated provided its
1659 final operation is an element or slice of an aggregate:
1661 delete $ref->[$x][$y]{$key};
1662 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
1664 delete $ref->[$x][$y][$index];
1665 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1668 X<die> X<throw> X<exception> X<raise> X<$@> X<abort>
1670 =for Pod::Functions raise an exception or bail out
1672 L<C<die>|/die LIST> raises an exception. Inside an
1673 L<C<eval>|/eval EXPR> the error message is stuffed into
1674 L<C<$@>|perlvar/$@> and the L<C<eval>|/eval EXPR> is terminated with the
1675 undefined value. If the exception is outside of all enclosing
1676 L<C<eval>|/eval EXPR>s, then the uncaught exception prints LIST to
1677 C<STDERR> and exits with a non-zero value. If you need to exit the
1678 process with a specific exit code, see L<C<exit>|/exit EXPR>.
1680 Equivalent examples:
1682 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
1683 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
1685 If the last element of LIST does not end in a newline, the current
1686 script line number and input line number (if any) are also printed,
1687 and a newline is supplied. Note that the "input line number" (also
1688 known as "chunk") is subject to whatever notion of "line" happens to
1689 be currently in effect, and is also available as the special variable
1690 L<C<$.>|perlvar/$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1692 Hint: sometimes appending C<", stopped"> to your message will cause it
1693 to make better sense when the string C<"at foo line 123"> is appended.
1694 Suppose you are running script "canasta".
1696 die "/etc/games is no good";
1697 die "/etc/games is no good, stopped";
1699 produce, respectively
1701 /etc/games is no good at canasta line 123.
1702 /etc/games is no good, stopped at canasta line 123.
1704 If the output is empty and L<C<$@>|perlvar/$@> already contains a value
1705 (typically from a previous L<C<eval>|/eval EXPR>) that value is reused after
1706 appending C<"\t...propagated">. This is useful for propagating exceptions:
1709 die unless $@ =~ /Expected exception/;
1711 If the output is empty and L<C<$@>|perlvar/$@> contains an object
1712 reference that has a C<PROPAGATE> method, that method will be called
1713 with additional file and line number parameters. The return value
1714 replaces the value in L<C<$@>|perlvar/$@>; i.e., as if
1715 C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >> were called.
1717 If L<C<$@>|perlvar/$@> is empty, then the string C<"Died"> is used.
1719 If an uncaught exception results in interpreter exit, the exit code is
1720 determined from the values of L<C<$!>|perlvar/$!> and
1721 L<C<$?>|perlvar/$?> with this pseudocode:
1723 exit $! if $!; # errno
1724 exit $? >> 8 if $? >> 8; # child exit status
1725 exit 255; # last resort
1727 As with L<C<exit>|/exit EXPR>, L<C<$?>|perlvar/$?> is set prior to
1728 unwinding the call stack; any C<DESTROY> or C<END> handlers can then
1729 alter this value, and thus Perl's exit code.
1731 The intent is to squeeze as much possible information about the likely cause
1732 into the limited space of the system exit code. However, as
1733 L<C<$!>|perlvar/$!> is the value of C's C<errno>, which can be set by
1734 any system call, this means that the value of the exit code used by
1735 L<C<die>|/die LIST> can be non-predictable, so should not be relied
1736 upon, other than to be non-zero.
1738 You can also call L<C<die>|/die LIST> with a reference argument, and if
1739 this is trapped within an L<C<eval>|/eval EXPR>, L<C<$@>|perlvar/$@>
1740 contains that reference. This permits more elaborate exception handling
1741 using objects that maintain arbitrary state about the exception. Such a
1742 scheme is sometimes preferable to matching particular string values of
1743 L<C<$@>|perlvar/$@> with regular expressions. Because
1744 L<C<$@>|perlvar/$@> is a global variable and L<C<eval>|/eval EXPR> may
1745 be used within object implementations, be careful that analyzing the
1746 error object doesn't replace the reference in the global variable. It's
1747 easiest to make a local copy of the reference before any manipulations.
1750 use Scalar::Util "blessed";
1752 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1753 if (my $ev_err = $@) {
1754 if (blessed($ev_err)
1755 && $ev_err->isa("Some::Module::Exception")) {
1756 # handle Some::Module::Exception
1759 # handle all other possible exceptions
1763 Because Perl stringifies uncaught exception messages before display,
1764 you'll probably want to overload stringification operations on
1765 exception objects. See L<overload> for details about that.
1767 You can arrange for a callback to be run just before the
1768 L<C<die>|/die LIST> does its deed, by setting the
1769 L<C<$SIG{__DIE__}>|perlvar/%SIG> hook. The associated handler is called
1770 with the error text and can change the error message, if it sees fit, by
1771 calling L<C<die>|/die LIST> again. See L<perlvar/%SIG> for details on
1772 setting L<C<%SIG>|perlvar/%SIG> entries, and L<C<eval>|/eval EXPR> for some
1773 examples. Although this feature was to be run only right before your
1774 program was to exit, this is not currently so: the
1775 L<C<$SIG{__DIE__}>|perlvar/%SIG> hook is currently called even inside
1776 L<C<eval>|/eval EXPR>ed blocks/strings! If one wants the hook to do
1777 nothing in such situations, put
1781 as the first line of the handler (see L<perlvar/$^S>). Because
1782 this promotes strange action at a distance, this counterintuitive
1783 behavior may be fixed in a future release.
1785 See also L<C<exit>|/exit EXPR>, L<C<warn>|/warn LIST>, and the L<Carp>
1791 =for Pod::Functions turn a BLOCK into a TERM
1793 Not really a function. Returns the value of the last command in the
1794 sequence of commands indicated by BLOCK. When modified by the C<while> or
1795 C<until> loop modifier, executes the BLOCK once before testing the loop
1796 condition. (On other statements the loop modifiers test the conditional
1799 C<do BLOCK> does I<not> count as a loop, so the loop control statements
1800 L<C<next>|/next LABEL>, L<C<last>|/last LABEL>, or
1801 L<C<redo>|/redo LABEL> cannot be used to leave or restart the block.
1802 See L<perlsyn> for alternative strategies.
1807 Uses the value of EXPR as a filename and executes the contents of the
1808 file as a Perl script:
1810 # load the exact specified file (./ and ../ special-cased)
1813 do '../foo/stat.pl';
1815 # search for the named file within @INC
1819 C<do './stat.pl'> is largely like
1823 except that it's more concise, runs no external processes, and keeps
1824 track of the current filename for error messages. It also differs in that
1825 code evaluated with C<do FILE> cannot see lexicals in the enclosing
1826 scope; C<eval STRING> does. It's the same, however, in that it does
1827 reparse the file every time you call it, so you probably don't want
1828 to do this inside a loop.
1830 Using C<do> with a relative path (except for F<./> and F<../>), like
1834 will search the L<C<@INC>|perlvar/@INC> directories, and update
1835 L<C<%INC>|perlvar/%INC> if the file is found. See L<perlvar/@INC>
1836 and L<perlvar/%INC> for these variables. In particular, note that
1837 whilst historically L<C<@INC>|perlvar/@INC> contained '.' (the
1838 current directory) making these two cases equivalent, that is no
1839 longer necessarily the case, as '.' is not included in C<@INC> by default
1840 in perl versions 5.26.0 onwards. Instead, perl will now warn:
1842 do "stat.pl" failed, '.' is no longer in @INC;
1843 did you mean do "./stat.pl"?
1845 If L<C<do>|/do EXPR> can read the file but cannot compile it, it
1846 returns L<C<undef>|/undef EXPR> and sets an error message in
1847 L<C<$@>|perlvar/$@>. If L<C<do>|/do EXPR> cannot read the file, it
1848 returns undef and sets L<C<$!>|perlvar/$!> to the error. Always check
1849 L<C<$@>|perlvar/$@> first, as compilation could fail in a way that also
1850 sets L<C<$!>|perlvar/$!>. If the file is successfully compiled,
1851 L<C<do>|/do EXPR> returns the value of the last expression evaluated.
1853 Inclusion of library modules is better done with the
1854 L<C<use>|/use Module VERSION LIST> and L<C<require>|/require VERSION>
1855 operators, which also do automatic error checking and raise an exception
1856 if there's a problem.
1858 You might like to use L<C<do>|/do EXPR> to read in a program
1859 configuration file. Manual error checking can be done this way:
1861 # Read in config files: system first, then user.
1862 # Beware of using relative pathnames here.
1863 for $file ("/share/prog/defaults.rc",
1864 "$ENV{HOME}/.someprogrc")
1866 unless ($return = do $file) {
1867 warn "couldn't parse $file: $@" if $@;
1868 warn "couldn't do $file: $!" unless defined $return;
1869 warn "couldn't run $file" unless $return;
1874 X<dump> X<core> X<undump>
1880 =for Pod::Functions create an immediate core dump
1882 This function causes an immediate core dump. See also the B<-u>
1883 command-line switch in L<perlrun>, which does the same thing.
1884 Primarily this is so that you can use the B<undump> program (not
1885 supplied) to turn your core dump into an executable binary after
1886 having initialized all your variables at the beginning of the
1887 program. When the new binary is executed it will begin by executing
1888 a C<goto LABEL> (with all the restrictions that L<C<goto>|/goto LABEL>
1890 Think of it as a goto with an intervening core dump and reincarnation.
1891 If C<LABEL> is omitted, restarts the program from the top. The
1892 C<dump EXPR> form, available starting in Perl 5.18.0, allows a name to be
1893 computed at run time, being otherwise identical to C<dump LABEL>.
1895 B<WARNING>: Any files opened at the time of the dump will I<not>
1896 be open any more when the program is reincarnated, with possible
1897 resulting confusion by Perl.
1899 This function is now largely obsolete, mostly because it's very hard to
1900 convert a core file into an executable. That's why you should now invoke
1901 it as C<CORE::dump()> if you don't want to be warned against a possible
1904 Unlike most named operators, this has the same precedence as assignment.
1905 It is also exempt from the looks-like-a-function rule, so
1906 C<dump ("foo")."bar"> will cause "bar" to be part of the argument to
1907 L<C<dump>|/dump LABEL>.
1909 Portability issues: L<perlport/dump>.
1912 X<each> X<hash, iterator>
1917 =for Pod::Functions retrieve the next key/value pair from a hash
1919 When called on a hash in list context, returns a 2-element list
1920 consisting of the key and value for the next element of a hash. In Perl
1921 5.12 and later only, it will also return the index and value for the next
1922 element of an array so that you can iterate over it; older Perls consider
1923 this a syntax error. When called in scalar context, returns only the key
1924 (not the value) in a hash, or the index in an array.
1926 Hash entries are returned in an apparently random order. The actual random
1927 order is specific to a given hash; the exact same series of operations
1928 on two hashes may result in a different order for each hash. Any insertion
1929 into the hash may change the order, as will any deletion, with the exception
1930 that the most recent key returned by L<C<each>|/each HASH> or
1931 L<C<keys>|/keys HASH> may be deleted without changing the order. So
1932 long as a given hash is unmodified you may rely on
1933 L<C<keys>|/keys HASH>, L<C<values>|/values HASH> and
1934 L<C<each>|/each HASH> to repeatedly return the same order
1935 as each other. See L<perlsec/"Algorithmic Complexity Attacks"> for
1936 details on why hash order is randomized. Aside from the guarantees
1937 provided here the exact details of Perl's hash algorithm and the hash
1938 traversal order are subject to change in any release of Perl.
1940 After L<C<each>|/each HASH> has returned all entries from the hash or
1941 array, the next call to L<C<each>|/each HASH> returns the empty list in
1942 list context and L<C<undef>|/undef EXPR> in scalar context; the next
1943 call following I<that> one restarts iteration. Each hash or array has
1944 its own internal iterator, accessed by L<C<each>|/each HASH>,
1945 L<C<keys>|/keys HASH>, and L<C<values>|/values HASH>. The iterator is
1946 implicitly reset when L<C<each>|/each HASH> has reached the end as just
1947 described; it can be explicitly reset by calling L<C<keys>|/keys HASH>
1948 or L<C<values>|/values HASH> on the hash or array. If you add or delete
1949 a hash's elements while iterating over it, the effect on the iterator is
1950 unspecified; for example, entries may be skipped or duplicated--so don't
1951 do that. Exception: It is always safe to delete the item most recently
1952 returned by L<C<each>|/each HASH>, so the following code works properly:
1954 while (my ($key, $value) = each %hash) {
1956 delete $hash{$key}; # This is safe
1959 Tied hashes may have a different ordering behaviour to perl's hash
1962 This prints out your environment like the L<printenv(1)> program,
1963 but in a different order:
1965 while (my ($key,$value) = each %ENV) {
1966 print "$key=$value\n";
1969 Starting with Perl 5.14, an experimental feature allowed
1970 L<C<each>|/each HASH> to take a scalar expression. This experiment has
1971 been deemed unsuccessful, and was removed as of Perl 5.24.
1973 As of Perl 5.18 you can use a bare L<C<each>|/each HASH> in a C<while>
1974 loop, which will set L<C<$_>|perlvar/$_> on every iteration.
1977 print "$_=$ENV{$_}\n";
1980 To avoid confusing would-be users of your code who are running earlier
1981 versions of Perl with mysterious syntax errors, put this sort of thing at
1982 the top of your file to signal that your code will work I<only> on Perls of
1985 use 5.012; # so keys/values/each work on arrays
1986 use 5.018; # so each assigns to $_ in a lone while test
1988 See also L<C<keys>|/keys HASH>, L<C<values>|/values HASH>, and
1989 L<C<sort>|/sort SUBNAME LIST>.
1991 =item eof FILEHANDLE
2000 =for Pod::Functions test a filehandle for its end
2002 Returns 1 if the next read on FILEHANDLE will return end of file I<or> if
2003 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
2004 gives the real filehandle. (Note that this function actually
2005 reads a character and then C<ungetc>s it, so isn't useful in an
2006 interactive context.) Do not read from a terminal file (or call
2007 C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
2008 as terminals may lose the end-of-file condition if you do.
2010 An L<C<eof>|/eof FILEHANDLE> without an argument uses the last file
2011 read. Using L<C<eof()>|/eof FILEHANDLE> with empty parentheses is
2012 different. It refers to the pseudo file formed from the files listed on
2013 the command line and accessed via the C<< <> >> operator. Since
2014 C<< <> >> isn't explicitly opened, as a normal filehandle is, an
2015 L<C<eof()>|/eof FILEHANDLE> before C<< <> >> has been used will cause
2016 L<C<@ARGV>|perlvar/@ARGV> to be examined to determine if input is
2017 available. Similarly, an L<C<eof()>|/eof FILEHANDLE> after C<< <> >>
2018 has returned end-of-file will assume you are processing another
2019 L<C<@ARGV>|perlvar/@ARGV> list, and if you haven't set
2020 L<C<@ARGV>|perlvar/@ARGV>, will read input from C<STDIN>; see
2021 L<perlop/"I/O Operators">.
2023 In a C<< while (<>) >> loop, L<C<eof>|/eof FILEHANDLE> or C<eof(ARGV)>
2024 can be used to detect the end of each file, whereas
2025 L<C<eof()>|/eof FILEHANDLE> will detect the end of the very last file
2028 # reset line numbering on each input file
2030 next if /^\s*#/; # skip comments
2033 close ARGV if eof; # Not eof()!
2036 # insert dashes just before last line of last file
2038 if (eof()) { # check for end of last file
2039 print "--------------\n";
2042 last if eof(); # needed if we're reading from a terminal
2045 Practical hint: you almost never need to use L<C<eof>|/eof FILEHANDLE>
2046 in Perl, because the input operators typically return L<C<undef>|/undef
2047 EXPR> when they run out of data or encounter an error.
2050 X<eval> X<try> X<catch> X<evaluate> X<parse> X<execute>
2051 X<error, handling> X<exception, handling>
2057 =for Pod::Functions catch exceptions or compile and run code
2059 C<eval> in all its forms is used to execute a little Perl program,
2060 trapping any errors encountered so they don't crash the calling program.
2062 Plain C<eval> with no argument is just C<eval EXPR>, where the
2063 expression is understood to be contained in L<C<$_>|perlvar/$_>. Thus
2064 there are only two real C<eval> forms; the one with an EXPR is often
2065 called "string eval". In a string eval, the value of the expression
2066 (which is itself determined within scalar context) is first parsed, and
2067 if there were no errors, executed as a block within the lexical context
2068 of the current Perl program. This form is typically used to delay
2069 parsing and subsequent execution of the text of EXPR until run time.
2070 Note that the value is parsed every time the C<eval> executes.
2072 The other form is called "block eval". It is less general than string
2073 eval, but the code within the BLOCK is parsed only once (at the same
2074 time the code surrounding the C<eval> itself was parsed) and executed
2075 within the context of the current Perl program. This form is typically
2076 used to trap exceptions more efficiently than the first, while also
2077 providing the benefit of checking the code within BLOCK at compile time.
2078 BLOCK is parsed and compiled just once. Since errors are trapped, it
2079 often is used to check if a given feature is available.
2081 In both forms, the value returned is the value of the last expression
2082 evaluated inside the mini-program; a return statement may also be used, just
2083 as with subroutines. The expression providing the return value is evaluated
2084 in void, scalar, or list context, depending on the context of the
2085 C<eval> itself. See L<C<wantarray>|/wantarray> for more
2086 on how the evaluation context can be determined.
2088 If there is a syntax error or runtime error, or a L<C<die>|/die LIST>
2089 statement is executed, C<eval> returns
2090 L<C<undef>|/undef EXPR> in scalar context, or an empty list in list
2091 context, and L<C<$@>|perlvar/$@> is set to the error message. (Prior to
2092 5.16, a bug caused L<C<undef>|/undef EXPR> to be returned in list
2093 context for syntax errors, but not for runtime errors.) If there was no
2094 error, L<C<$@>|perlvar/$@> is set to the empty string. A control flow
2095 operator like L<C<last>|/last LABEL> or L<C<goto>|/goto LABEL> can
2096 bypass the setting of L<C<$@>|perlvar/$@>. Beware that using
2097 C<eval> neither silences Perl from printing warnings to
2098 STDERR, nor does it stuff the text of warning messages into
2099 L<C<$@>|perlvar/$@>. To do either of those, you have to use the
2100 L<C<$SIG{__WARN__}>|perlvar/%SIG> facility, or turn off warnings inside
2101 the BLOCK or EXPR using S<C<no warnings 'all'>>. See
2102 L<C<warn>|/warn LIST>, L<perlvar>, and L<warnings>.
2104 Note that, because C<eval> traps otherwise-fatal errors,
2105 it is useful for determining whether a particular feature (such as
2106 L<C<socket>|/socket SOCKET,DOMAIN,TYPE,PROTOCOL> or
2107 L<C<symlink>|/symlink OLDFILE,NEWFILE>) is implemented. It is also
2108 Perl's exception-trapping mechanism, where the L<C<die>|/die LIST>
2109 operator is used to raise exceptions.
2111 Before Perl 5.14, the assignment to L<C<$@>|perlvar/$@> occurred before
2113 of localized variables, which means that for your code to run on older
2114 versions, a temporary is required if you want to mask some, but not all
2117 # alter $@ on nefarious repugnancy only
2121 local $@; # protect existing $@
2122 eval { test_repugnancy() };
2123 # $@ =~ /nefarious/ and die $@; # Perl 5.14 and higher only
2124 $@ =~ /nefarious/ and $e = $@;
2126 die $e if defined $e
2129 There are some different considerations for each form:
2135 Since the return value of EXPR is executed as a block within the lexical
2136 context of the current Perl program, any outer lexical variables are
2137 visible to it, and any package variable settings or subroutine and
2138 format definitions remain afterwards.
2142 =item Under the L<C<"unicode_eval"> feature|feature/The 'unicode_eval' and 'evalbytes' features>
2144 If this feature is enabled (which is the default under a C<use 5.16> or
2145 higher declaration), EXPR is considered to be
2146 in the same encoding as the surrounding program. Thus if
2147 S<L<C<use utf8>|utf8>> is in effect, the string will be treated as being
2148 UTF-8 encoded. Otherwise, the string is considered to be a sequence of
2149 independent bytes. Bytes that correspond to ASCII-range code points
2150 will have their normal meanings for operators in the string. The
2151 treatment of the other bytes depends on if the
2152 L<C<'unicode_strings"> feature|feature/The 'unicode_strings' feature> is
2155 In a plain C<eval> without an EXPR argument, being in S<C<use utf8>> or
2156 not is irrelevant; the UTF-8ness of C<$_> itself determines the
2159 Any S<C<use utf8>> or S<C<no utf8>> declarations within the string have
2160 no effect, and source filters are forbidden. (C<unicode_strings>,
2161 however, can appear within the string.) See also the
2162 L<C<evalbytes>|/evalbytes EXPR> operator, which works properly with
2165 Variables defined outside the C<eval> and used inside it retain their
2166 original UTF-8ness. Everything inside the string follows the normal
2167 rules for a Perl program with the given state of S<C<use utf8>>.
2169 =item Outside the C<"unicode_eval"> feature
2171 In this case, the behavior is problematic and is not so easily
2172 described. Here are two bugs that cannot easily be fixed without
2173 breaking existing programs:
2179 It can lose track of whether something should be encoded as UTF-8 or
2184 Source filters activated within C<eval> leak out into whichever file
2185 scope is currently being compiled. To give an example with the CPAN module
2186 L<Semi::Semicolons>:
2188 BEGIN { eval "use Semi::Semicolons; # not filtered" }
2191 L<C<evalbytes>|/evalbytes EXPR> fixes that to work the way one would
2194 use feature "evalbytes";
2195 BEGIN { evalbytes "use Semi::Semicolons; # filtered" }
2202 Problems can arise if the string expands a scalar containing a floating
2203 point number. That scalar can expand to letters, such as C<"NaN"> or
2204 C<"Infinity">; or, within the scope of a L<C<use locale>|locale>, the
2205 decimal point character may be something other than a dot (such as a
2206 comma). None of these are likely to parse as you are likely expecting.
2208 You should be especially careful to remember what's being looked at
2215 eval { $x }; # CASE 4
2217 eval "\$$x++"; # CASE 5
2220 Cases 1 and 2 above behave identically: they run the code contained in
2221 the variable $x. (Although case 2 has misleading double quotes making
2222 the reader wonder what else might be happening (nothing is).) Cases 3
2223 and 4 likewise behave in the same way: they run the code C<'$x'>, which
2224 does nothing but return the value of $x. (Case 4 is preferred for
2225 purely visual reasons, but it also has the advantage of compiling at
2226 compile-time instead of at run-time.) Case 5 is a place where
2227 normally you I<would> like to use double quotes, except that in this
2228 particular situation, you can just use symbolic references instead, as
2231 An C<eval ''> executed within a subroutine defined
2232 in the C<DB> package doesn't see the usual
2233 surrounding lexical scope, but rather the scope of the first non-DB piece
2234 of code that called it. You don't normally need to worry about this unless
2235 you are writing a Perl debugger.
2237 The final semicolon, if any, may be omitted from the value of EXPR.
2241 If the code to be executed doesn't vary, you may use the eval-BLOCK
2242 form to trap run-time errors without incurring the penalty of
2243 recompiling each time. The error, if any, is still returned in
2244 L<C<$@>|perlvar/$@>.
2247 # make divide-by-zero nonfatal
2248 eval { $answer = $a / $b; }; warn $@ if $@;
2250 # same thing, but less efficient
2251 eval '$answer = $a / $b'; warn $@ if $@;
2253 # a compile-time error
2254 eval { $answer = }; # WRONG
2257 eval '$answer ='; # sets $@
2259 If you want to trap errors when loading an XS module, some problems with
2260 the binary interface (such as Perl version skew) may be fatal even with
2261 C<eval> unless C<$ENV{PERL_DL_NONLAZY}> is set. See
2264 Using the C<eval {}> form as an exception trap in libraries does have some
2265 issues. Due to the current arguably broken state of C<__DIE__> hooks, you
2266 may wish not to trigger any C<__DIE__> hooks that user code may have installed.
2267 You can use the C<local $SIG{__DIE__}> construct for this purpose,
2268 as this example shows:
2270 # a private exception trap for divide-by-zero
2271 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
2274 This is especially significant, given that C<__DIE__> hooks can call
2275 L<C<die>|/die LIST> again, which has the effect of changing their error
2278 # __DIE__ hooks may modify error messages
2280 local $SIG{'__DIE__'} =
2281 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
2282 eval { die "foo lives here" };
2283 print $@ if $@; # prints "bar lives here"
2286 Because this promotes action at a distance, this counterintuitive behavior
2287 may be fixed in a future release.
2289 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
2290 L<C<next>|/next LABEL>, L<C<last>|/last LABEL>, or
2291 L<C<redo>|/redo LABEL> cannot be used to leave or restart the block.
2293 The final semicolon, if any, may be omitted from within the BLOCK.
2297 =item evalbytes EXPR
2302 =for Pod::Functions +evalbytes similar to string eval, but intend to parse a bytestream
2304 This function is similar to a L<string eval|/eval EXPR>, except it
2305 always parses its argument (or L<C<$_>|perlvar/$_> if EXPR is omitted)
2306 as a string of independent bytes.
2308 If called when S<C<use utf8>> is in effect, the string will be assumed
2309 to be encoded in UTF-8, and C<evalbytes> will make a temporary copy to
2310 work from, downgraded to non-UTF-8. If this is not possible
2311 (because one or more characters in it require UTF-8), the C<evalbytes>
2312 will fail with the error stored in C<$@>.
2314 Bytes that correspond to ASCII-range code points will have their normal
2315 meanings for operators in the string. The treatment of the other bytes
2316 depends on if the L<C<'unicode_strings"> feature|feature/The
2317 'unicode_strings' feature> is in effect.
2319 Of course, variables that are UTF-8 and are referred to in the string
2323 evalbytes 'print ord $a, "\n"';
2331 Source filters activated within the evaluated code apply to the code
2334 L<C<evalbytes>|/evalbytes EXPR> is available starting in Perl v5.16. To
2335 access it, you must say C<CORE::evalbytes>, but you can omit the
2337 L<C<"evalbytes"> feature|feature/The 'unicode_eval' and 'evalbytes' features>
2338 is enabled. This is enabled automatically with a C<use v5.16> (or
2339 higher) declaration in the current scope.
2344 =item exec PROGRAM LIST
2346 =for Pod::Functions abandon this program to run another
2348 The L<C<exec>|/exec LIST> function executes a system command I<and never
2349 returns>; use L<C<system>|/system LIST> instead of L<C<exec>|/exec LIST>
2350 if you want it to return. It fails and
2351 returns false only if the command does not exist I<and> it is executed
2352 directly instead of via your system's command shell (see below).
2354 Since it's a common mistake to use L<C<exec>|/exec LIST> instead of
2355 L<C<system>|/system LIST>, Perl warns you if L<C<exec>|/exec LIST> is
2356 called in void context and if there is a following statement that isn't
2357 L<C<die>|/die LIST>, L<C<warn>|/warn LIST>, or L<C<exit>|/exit EXPR> (if
2358 L<warnings> are enabled--but you always do that, right?). If you
2359 I<really> want to follow an L<C<exec>|/exec LIST> with some other
2360 statement, you can use one of these styles to avoid the warning:
2362 exec ('foo') or print STDERR "couldn't exec foo: $!";
2363 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
2365 If there is more than one argument in LIST, this calls L<execvp(3)> with the
2366 arguments in LIST. If there is only one element in LIST, the argument is
2367 checked for shell metacharacters, and if there are any, the entire
2368 argument is passed to the system's command shell for parsing (this is
2369 C</bin/sh -c> on Unix platforms, but varies on other platforms). If
2370 there are no shell metacharacters in the argument, it is split into words
2371 and passed directly to C<execvp>, which is more efficient. Examples:
2373 exec '/bin/echo', 'Your arguments are: ', @ARGV;
2374 exec "sort $outfile | uniq";
2376 If you don't really want to execute the first argument, but want to lie
2377 to the program you are executing about its own name, you can specify
2378 the program you actually want to run as an "indirect object" (without a
2379 comma) in front of the LIST, as in C<exec PROGRAM LIST>. (This always
2380 forces interpretation of the LIST as a multivalued list, even if there
2381 is only a single scalar in the list.) Example:
2383 my $shell = '/bin/csh';
2384 exec $shell '-sh'; # pretend it's a login shell
2388 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
2390 When the arguments get executed via the system shell, results are
2391 subject to its quirks and capabilities. See L<perlop/"`STRING`">
2394 Using an indirect object with L<C<exec>|/exec LIST> or
2395 L<C<system>|/system LIST> is also more secure. This usage (which also
2396 works fine with L<C<system>|/system LIST>) forces
2397 interpretation of the arguments as a multivalued list, even if the
2398 list had just one argument. That way you're safe from the shell
2399 expanding wildcards or splitting up words with whitespace in them.
2401 my @args = ( "echo surprise" );
2403 exec @args; # subject to shell escapes
2405 exec { $args[0] } @args; # safe even with one-arg list
2407 The first version, the one without the indirect object, ran the I<echo>
2408 program, passing it C<"surprise"> an argument. The second version didn't;
2409 it tried to run a program named I<"echo surprise">, didn't find it, and set
2410 L<C<$?>|perlvar/$?> to a non-zero value indicating failure.
2412 On Windows, only the C<exec PROGRAM LIST> indirect object syntax will
2413 reliably avoid using the shell; C<exec LIST>, even with more than one
2414 element, will fall back to the shell if the first spawn fails.
2416 Perl attempts to flush all files opened for output before the exec,
2417 but this may not be supported on some platforms (see L<perlport>).
2418 To be safe, you may need to set L<C<$E<verbar>>|perlvar/$E<verbar>>
2419 (C<$AUTOFLUSH> in L<English>) or call the C<autoflush> method of
2420 L<C<IO::Handle>|IO::Handle/METHODS> on any open handles to avoid lost
2423 Note that L<C<exec>|/exec LIST> will not call your C<END> blocks, nor
2424 will it invoke C<DESTROY> methods on your objects.
2426 Portability issues: L<perlport/exec>.
2429 X<exists> X<autovivification>
2431 =for Pod::Functions test whether a hash key is present
2433 Given an expression that specifies an element of a hash, returns true if the
2434 specified element in the hash has ever been initialized, even if the
2435 corresponding value is undefined.
2437 print "Exists\n" if exists $hash{$key};
2438 print "Defined\n" if defined $hash{$key};
2439 print "True\n" if $hash{$key};
2441 exists may also be called on array elements, but its behavior is much less
2442 obvious and is strongly tied to the use of L<C<delete>|/delete EXPR> on
2445 B<WARNING:> Calling L<C<exists>|/exists EXPR> on array values is
2446 strongly discouraged. The
2447 notion of deleting or checking the existence of Perl array elements is not
2448 conceptually coherent, and can lead to surprising behavior.
2450 print "Exists\n" if exists $array[$index];
2451 print "Defined\n" if defined $array[$index];
2452 print "True\n" if $array[$index];
2454 A hash or array element can be true only if it's defined and defined only if
2455 it exists, but the reverse doesn't necessarily hold true.
2457 Given an expression that specifies the name of a subroutine,
2458 returns true if the specified subroutine has ever been declared, even
2459 if it is undefined. Mentioning a subroutine name for exists or defined
2460 does not count as declaring it. Note that a subroutine that does not
2461 exist may still be callable: its package may have an C<AUTOLOAD>
2462 method that makes it spring into existence the first time that it is
2463 called; see L<perlsub>.
2465 print "Exists\n" if exists &subroutine;
2466 print "Defined\n" if defined &subroutine;
2468 Note that the EXPR can be arbitrarily complicated as long as the final
2469 operation is a hash or array key lookup or subroutine name:
2471 if (exists $ref->{A}->{B}->{$key}) { }
2472 if (exists $hash{A}{B}{$key}) { }
2474 if (exists $ref->{A}->{B}->[$ix]) { }
2475 if (exists $hash{A}{B}[$ix]) { }
2477 if (exists &{$ref->{A}{B}{$key}}) { }
2479 Although the most deeply nested array or hash element will not spring into
2480 existence just because its existence was tested, any intervening ones will.
2481 Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
2482 into existence due to the existence test for the C<$key> element above.
2483 This happens anywhere the arrow operator is used, including even here:
2486 if (exists $ref->{"Some key"}) { }
2487 print $ref; # prints HASH(0x80d3d5c)
2489 This surprising autovivification in what does not at first--or even
2490 second--glance appear to be an lvalue context may be fixed in a future
2493 Use of a subroutine call, rather than a subroutine name, as an argument
2494 to L<C<exists>|/exists EXPR> is an error.
2497 exists &sub(); # Error
2500 X<exit> X<terminate> X<abort>
2504 =for Pod::Functions terminate this program
2506 Evaluates EXPR and exits immediately with that value. Example:
2509 exit 0 if $ans =~ /^[Xx]/;
2511 See also L<C<die>|/die LIST>. If EXPR is omitted, exits with C<0>
2513 universally recognized values for EXPR are C<0> for success and C<1>
2514 for error; other values are subject to interpretation depending on the
2515 environment in which the Perl program is running. For example, exiting
2516 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
2517 the mailer to return the item undelivered, but that's not true everywhere.
2519 Don't use L<C<exit>|/exit EXPR> to abort a subroutine if there's any
2520 chance that someone might want to trap whatever error happened. Use
2521 L<C<die>|/die LIST> instead, which can be trapped by an
2522 L<C<eval>|/eval EXPR>.
2524 The L<C<exit>|/exit EXPR> function does not always exit immediately. It
2525 calls any defined C<END> routines first, but these C<END> routines may
2526 not themselves abort the exit. Likewise any object destructors that
2527 need to be called are called before the real exit. C<END> routines and
2528 destructors can change the exit status by modifying L<C<$?>|perlvar/$?>.
2529 If this is a problem, you can call
2530 L<C<POSIX::_exit($status)>|POSIX/C<_exit>> to avoid C<END> and destructor
2531 processing. See L<perlmod> for details.
2533 Portability issues: L<perlport/exit>.
2536 X<exp> X<exponential> X<antilog> X<antilogarithm> X<e>
2540 =for Pod::Functions raise I<e> to a power
2542 Returns I<e> (the natural logarithm base) to the power of EXPR.
2543 If EXPR is omitted, gives C<exp($_)>.
2546 X<fc> X<foldcase> X<casefold> X<fold-case> X<case-fold>
2550 =for Pod::Functions +fc return casefolded version of a string
2552 Returns the casefolded version of EXPR. This is the internal function
2553 implementing the C<\F> escape in double-quoted strings.
2555 Casefolding is the process of mapping strings to a form where case
2556 differences are erased; comparing two strings in their casefolded
2557 form is effectively a way of asking if two strings are equal,
2560 Roughly, if you ever found yourself writing this
2562 lc($this) eq lc($that) # Wrong!
2564 uc($this) eq uc($that) # Also wrong!
2566 $this =~ /^\Q$that\E\z/i # Right!
2570 fc($this) eq fc($that)
2572 And get the correct results.
2574 Perl only implements the full form of casefolding, but you can access
2575 the simple folds using L<Unicode::UCD/B<casefold()>> and
2576 L<Unicode::UCD/B<prop_invmap()>>.
2577 For further information on casefolding, refer to
2578 the Unicode Standard, specifically sections 3.13 C<Default Case Operations>,
2579 4.2 C<Case-Normative>, and 5.18 C<Case Mappings>,
2580 available at L<http://www.unicode.org/versions/latest/>, as well as the
2581 Case Charts available at L<http://www.unicode.org/charts/case/>.
2583 If EXPR is omitted, uses L<C<$_>|perlvar/$_>.
2585 This function behaves the same way under various pragmas, such as within
2586 L<S<C<"use feature 'unicode_strings">>|feature/The 'unicode_strings' feature>,
2587 as L<C<lc>|/lc EXPR> does, with the single exception of
2588 L<C<fc>|/fc EXPR> of I<LATIN CAPITAL LETTER SHARP S> (U+1E9E) within the
2589 scope of L<S<C<use locale>>|locale>. The foldcase of this character
2590 would normally be C<"ss">, but as explained in the L<C<lc>|/lc EXPR>
2592 changes that cross the 255/256 boundary are problematic under locales,
2593 and are hence prohibited. Therefore, this function under locale returns
2594 instead the string C<"\x{17F}\x{17F}">, which is the I<LATIN SMALL LETTER
2595 LONG S>. Since that character itself folds to C<"s">, the string of two
2596 of them together should be equivalent to a single U+1E9E when foldcased.
2598 While the Unicode Standard defines two additional forms of casefolding,
2599 one for Turkic languages and one that never maps one character into multiple
2600 characters, these are not provided by the Perl core. However, the CPAN module
2601 L<C<Unicode::Casing>|Unicode::Casing> may be used to provide an implementation.
2603 L<C<fc>|/fc EXPR> is available only if the
2604 L<C<"fc"> feature|feature/The 'fc' feature> is enabled or if it is
2605 prefixed with C<CORE::>. The
2606 L<C<"fc"> feature|feature/The 'fc' feature> is enabled automatically
2607 with a C<use v5.16> (or higher) declaration in the current scope.
2609 =item fcntl FILEHANDLE,FUNCTION,SCALAR
2612 =for Pod::Functions file control system call
2614 Implements the L<fcntl(2)> function. You'll probably have to say
2618 first to get the correct constant definitions. Argument processing and
2619 value returned work just like L<C<ioctl>|/ioctl
2620 FILEHANDLE,FUNCTION,SCALAR> below. For example:
2623 my $flags = fcntl($filehandle, F_GETFL, 0)
2624 or die "Can't fcntl F_GETFL: $!";
2626 You don't have to check for L<C<defined>|/defined EXPR> on the return
2627 from L<C<fcntl>|/fcntl FILEHANDLE,FUNCTION,SCALAR>. Like
2628 L<C<ioctl>|/ioctl FILEHANDLE,FUNCTION,SCALAR>, it maps a C<0> return
2629 from the system call into C<"0 but true"> in Perl. This string is true
2630 in boolean context and C<0> in numeric context. It is also exempt from
2632 L<C<Argument "..." isn't numeric>|perldiag/Argument "%s" isn't numeric%s>
2633 L<warnings> on improper numeric conversions.
2635 Note that L<C<fcntl>|/fcntl FILEHANDLE,FUNCTION,SCALAR> raises an
2636 exception if used on a machine that doesn't implement L<fcntl(2)>. See
2637 the L<Fcntl> module or your L<fcntl(2)> manpage to learn what functions
2638 are available on your system.
2640 Here's an example of setting a filehandle named C<$REMOTE> to be
2641 non-blocking at the system level. You'll have to negotiate
2642 L<C<$E<verbar>>|perlvar/$E<verbar>> on your own, though.
2644 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
2646 my $flags = fcntl($REMOTE, F_GETFL, 0)
2647 or die "Can't get flags for the socket: $!\n";
2649 fcntl($REMOTE, F_SETFL, $flags | O_NONBLOCK)
2650 or die "Can't set flags for the socket: $!\n";
2652 Portability issues: L<perlport/fcntl>.
2657 =for Pod::Functions the name of the current source file
2659 A special token that returns the name of the file in which it occurs.
2661 =item fileno FILEHANDLE
2664 =item fileno DIRHANDLE
2666 =for Pod::Functions return file descriptor from filehandle
2668 Returns the file descriptor for a filehandle or directory handle,
2670 filehandle is not open. If there is no real file descriptor at the OS
2671 level, as can happen with filehandles connected to memory objects via
2672 L<C<open>|/open FILEHANDLE,EXPR> with a reference for the third
2673 argument, -1 is returned.
2675 This is mainly useful for constructing bitmaps for
2676 L<C<select>|/select RBITS,WBITS,EBITS,TIMEOUT> and low-level POSIX
2677 tty-handling operations.
2678 If FILEHANDLE is an expression, the value is taken as an indirect
2679 filehandle, generally its name.
2681 You can use this to find out whether two handles refer to the
2682 same underlying descriptor:
2684 if (fileno($this) != -1 && fileno($this) == fileno($that)) {
2685 print "\$this and \$that are dups\n";
2686 } elsif (fileno($this) != -1 && fileno($that) != -1) {
2687 print "\$this and \$that have different " .
2688 "underlying file descriptors\n";
2690 print "At least one of \$this and \$that does " .
2691 "not have a real file descriptor\n";
2694 The behavior of L<C<fileno>|/fileno FILEHANDLE> on a directory handle
2695 depends on the operating system. On a system with L<dirfd(3)> or
2696 similar, L<C<fileno>|/fileno FILEHANDLE> on a directory
2697 handle returns the underlying file descriptor associated with the
2698 handle; on systems with no such support, it returns the undefined value,
2699 and sets L<C<$!>|perlvar/$!> (errno).
2701 =item flock FILEHANDLE,OPERATION
2702 X<flock> X<lock> X<locking>
2704 =for Pod::Functions lock an entire file with an advisory lock
2706 Calls L<flock(2)>, or an emulation of it, on FILEHANDLE. Returns true
2707 for success, false on failure. Produces a fatal error if used on a
2708 machine that doesn't implement L<flock(2)>, L<fcntl(2)> locking, or
2709 L<lockf(3)>. L<C<flock>|/flock FILEHANDLE,OPERATION> is Perl's portable
2710 file-locking interface, although it locks entire files only, not
2713 Two potentially non-obvious but traditional L<C<flock>|/flock
2714 FILEHANDLE,OPERATION> semantics are
2715 that it waits indefinitely until the lock is granted, and that its locks
2716 are B<merely advisory>. Such discretionary locks are more flexible, but
2717 offer fewer guarantees. This means that programs that do not also use
2718 L<C<flock>|/flock FILEHANDLE,OPERATION> may modify files locked with
2719 L<C<flock>|/flock FILEHANDLE,OPERATION>. See L<perlport>,
2720 your port's specific documentation, and your system-specific local manpages
2721 for details. It's best to assume traditional behavior if you're writing
2722 portable programs. (But if you're not, you should as always feel perfectly
2723 free to write for your own system's idiosyncrasies (sometimes called
2724 "features"). Slavish adherence to portability concerns shouldn't get
2725 in the way of your getting your job done.)
2727 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
2728 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
2729 you can use the symbolic names if you import them from the L<Fcntl> module,
2730 either individually, or as a group using the C<:flock> tag. LOCK_SH
2731 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
2732 releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
2733 LOCK_SH or LOCK_EX, then L<C<flock>|/flock FILEHANDLE,OPERATION> returns
2734 immediately rather than blocking waiting for the lock; check the return
2735 status to see if you got it.
2737 To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
2738 before locking or unlocking it.
2740 Note that the emulation built with L<lockf(3)> doesn't provide shared
2741 locks, and it requires that FILEHANDLE be open with write intent. These
2742 are the semantics that L<lockf(3)> implements. Most if not all systems
2743 implement L<lockf(3)> in terms of L<fcntl(2)> locking, though, so the
2744 differing semantics shouldn't bite too many people.
2746 Note that the L<fcntl(2)> emulation of L<flock(3)> requires that FILEHANDLE
2747 be open with read intent to use LOCK_SH and requires that it be open
2748 with write intent to use LOCK_EX.
2750 Note also that some versions of L<C<flock>|/flock FILEHANDLE,OPERATION>
2751 cannot lock things over the network; you would need to use the more
2752 system-specific L<C<fcntl>|/fcntl FILEHANDLE,FUNCTION,SCALAR> for
2753 that. If you like you can force Perl to ignore your system's L<flock(2)>
2754 function, and so provide its own L<fcntl(2)>-based emulation, by passing
2755 the switch C<-Ud_flock> to the F<Configure> program when you configure
2756 and build a new Perl.
2758 Here's a mailbox appender for BSD systems.
2760 # import LOCK_* and SEEK_END constants
2761 use Fcntl qw(:flock SEEK_END);
2765 flock($fh, LOCK_EX) or die "Cannot lock mailbox - $!\n";
2766 # and, in case we're running on a very old UNIX
2767 # variant without the modern O_APPEND semantics...
2768 seek($fh, 0, SEEK_END) or die "Cannot seek - $!\n";
2773 flock($fh, LOCK_UN) or die "Cannot unlock mailbox - $!\n";
2776 open(my $mbox, ">>", "/usr/spool/mail/$ENV{'USER'}")
2777 or die "Can't open mailbox: $!";
2780 print $mbox $msg,"\n\n";
2783 On systems that support a real L<flock(2)>, locks are inherited across
2784 L<C<fork>|/fork> calls, whereas those that must resort to the more
2785 capricious L<fcntl(2)> function lose their locks, making it seriously
2786 harder to write servers.
2788 See also L<DB_File> for other L<C<flock>|/flock FILEHANDLE,OPERATION>
2791 Portability issues: L<perlport/flock>.
2794 X<fork> X<child> X<parent>
2796 =for Pod::Functions create a new process just like this one
2798 Does a L<fork(2)> system call to create a new process running the
2799 same program at the same point. It returns the child pid to the
2800 parent process, C<0> to the child process, or L<C<undef>|/undef EXPR> if
2802 unsuccessful. File descriptors (and sometimes locks on those descriptors)
2803 are shared, while everything else is copied. On most systems supporting
2804 L<fork(2)>, great care has gone into making it extremely efficient (for
2805 example, using copy-on-write technology on data pages), making it the
2806 dominant paradigm for multitasking over the last few decades.
2808 Perl attempts to flush all files opened for output before forking the
2809 child process, but this may not be supported on some platforms (see
2810 L<perlport>). To be safe, you may need to set
2811 L<C<$E<verbar>>|perlvar/$E<verbar>> (C<$AUTOFLUSH> in L<English>) or
2812 call the C<autoflush> method of L<C<IO::Handle>|IO::Handle/METHODS> on
2813 any open handles to avoid duplicate output.
2815 If you L<C<fork>|/fork> without ever waiting on your children, you will
2816 accumulate zombies. On some systems, you can avoid this by setting
2817 L<C<$SIG{CHLD}>|perlvar/%SIG> to C<"IGNORE">. See also L<perlipc> for
2818 more examples of forking and reaping moribund children.
2820 Note that if your forked child inherits system file descriptors like
2821 STDIN and STDOUT that are actually connected by a pipe or socket, even
2822 if you exit, then the remote server (such as, say, a CGI script or a
2823 backgrounded job launched from a remote shell) won't think you're done.
2824 You should reopen those to F</dev/null> if it's any issue.
2826 On some platforms such as Windows, where the L<fork(2)> system call is
2827 not available, Perl can be built to emulate L<C<fork>|/fork> in the Perl
2828 interpreter. The emulation is designed, at the level of the Perl
2829 program, to be as compatible as possible with the "Unix" L<fork(2)>.
2830 However it has limitations that have to be considered in code intended
2831 to be portable. See L<perlfork> for more details.
2833 Portability issues: L<perlport/fork>.
2838 =for Pod::Functions declare a picture format with use by the write() function
2840 Declare a picture format for use by the L<C<write>|/write FILEHANDLE>
2841 function. For example:
2844 Test: @<<<<<<<< @||||| @>>>>>
2845 $str, $%, '$' . int($num)
2849 $num = $cost/$quantity;
2853 See L<perlform> for many details and examples.
2855 =item formline PICTURE,LIST
2858 =for Pod::Functions internal function used for formats
2860 This is an internal function used by L<C<format>|/format>s, though you
2861 may call it, too. It formats (see L<perlform>) a list of values
2862 according to the contents of PICTURE, placing the output into the format
2863 output accumulator, L<C<$^A>|perlvar/$^A> (or C<$ACCUMULATOR> in
2864 L<English>). Eventually, when a L<C<write>|/write FILEHANDLE> is done,
2865 the contents of L<C<$^A>|perlvar/$^A> are written to some filehandle.
2866 You could also read L<C<$^A>|perlvar/$^A> and then set
2867 L<C<$^A>|perlvar/$^A> back to C<"">. Note that a format typically does
2868 one L<C<formline>|/formline PICTURE,LIST> per line of form, but the
2869 L<C<formline>|/formline PICTURE,LIST> function itself doesn't care how
2870 many newlines are embedded in the PICTURE. This means that the C<~> and
2871 C<~~> tokens treat the entire PICTURE as a single line. You may
2872 therefore need to use multiple formlines to implement a single record
2873 format, just like the L<C<format>|/format> compiler.
2875 Be careful if you put double quotes around the picture, because an C<@>
2876 character may be taken to mean the beginning of an array name.
2877 L<C<formline>|/formline PICTURE,LIST> always returns true. See
2878 L<perlform> for other examples.
2880 If you are trying to use this instead of L<C<write>|/write FILEHANDLE>
2881 to capture the output, you may find it easier to open a filehandle to a
2882 scalar (C<< open my $fh, ">", \$output >>) and write to that instead.
2884 =item getc FILEHANDLE
2885 X<getc> X<getchar> X<character> X<file, read>
2889 =for Pod::Functions get the next character from the filehandle
2891 Returns the next character from the input file attached to FILEHANDLE,
2892 or the undefined value at end of file or if there was an error (in
2893 the latter case L<C<$!>|perlvar/$!> is set). If FILEHANDLE is omitted,
2895 STDIN. This is not particularly efficient. However, it cannot be
2896 used by itself to fetch single characters without waiting for the user
2897 to hit enter. For that, try something more like:
2900 system "stty cbreak </dev/tty >/dev/tty 2>&1";
2903 system "stty", '-icanon', 'eol', "\001";
2906 my $key = getc(STDIN);
2909 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
2912 system 'stty', 'icanon', 'eol', '^@'; # ASCII NUL
2916 Determination of whether C<$BSD_STYLE> should be set is left as an
2917 exercise to the reader.
2919 The L<C<POSIX::getattr>|POSIX/C<getattr>> function can do this more
2920 portably on systems purporting POSIX compliance. See also the
2921 L<C<Term::ReadKey>|Term::ReadKey> module on CPAN.
2924 X<getlogin> X<login>
2926 =for Pod::Functions return who logged in at this tty
2928 This implements the C library function of the same name, which on most
2929 systems returns the current login from F</etc/utmp>, if any. If it
2930 returns the empty string, use L<C<getpwuid>|/getpwuid UID>.
2932 my $login = getlogin || getpwuid($<) || "Kilroy";
2934 Do not consider L<C<getlogin>|/getlogin> for authentication: it is not
2935 as secure as L<C<getpwuid>|/getpwuid UID>.
2937 Portability issues: L<perlport/getlogin>.
2939 =item getpeername SOCKET
2940 X<getpeername> X<peer>
2942 =for Pod::Functions find the other end of a socket connection
2944 Returns the packed sockaddr address of the other end of the SOCKET
2948 my $hersockaddr = getpeername($sock);
2949 my ($port, $iaddr) = sockaddr_in($hersockaddr);
2950 my $herhostname = gethostbyaddr($iaddr, AF_INET);
2951 my $herstraddr = inet_ntoa($iaddr);
2956 =for Pod::Functions get process group
2958 Returns the current process group for the specified PID. Use
2959 a PID of C<0> to get the current process group for the
2960 current process. Will raise an exception if used on a machine that
2961 doesn't implement L<getpgrp(2)>. If PID is omitted, returns the process
2962 group of the current process. Note that the POSIX version of
2963 L<C<getpgrp>|/getpgrp PID> does not accept a PID argument, so only
2964 C<PID==0> is truly portable.
2966 Portability issues: L<perlport/getpgrp>.
2969 X<getppid> X<parent> X<pid>
2971 =for Pod::Functions get parent process ID
2973 Returns the process id of the parent process.
2975 Note for Linux users: Between v5.8.1 and v5.16.0 Perl would work
2976 around non-POSIX thread semantics the minority of Linux systems (and
2977 Debian GNU/kFreeBSD systems) that used LinuxThreads, this emulation
2978 has since been removed. See the documentation for L<$$|perlvar/$$> for
2981 Portability issues: L<perlport/getppid>.
2983 =item getpriority WHICH,WHO
2984 X<getpriority> X<priority> X<nice>
2986 =for Pod::Functions get current nice value
2988 Returns the current priority for a process, a process group, or a user.
2989 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
2990 machine that doesn't implement L<getpriority(2)>.
2992 C<WHICH> can be any of C<PRIO_PROCESS>, C<PRIO_PGRP> or C<PRIO_USER>
2993 imported from L<POSIX/RESOURCE CONSTANTS>.
2995 Portability issues: L<perlport/getpriority>.
2998 X<getpwnam> X<getgrnam> X<gethostbyname> X<getnetbyname> X<getprotobyname>
2999 X<getpwuid> X<getgrgid> X<getservbyname> X<gethostbyaddr> X<getnetbyaddr>
3000 X<getprotobynumber> X<getservbyport> X<getpwent> X<getgrent> X<gethostent>
3001 X<getnetent> X<getprotoent> X<getservent> X<setpwent> X<setgrent> X<sethostent>
3002 X<setnetent> X<setprotoent> X<setservent> X<endpwent> X<endgrent> X<endhostent>
3003 X<endnetent> X<endprotoent> X<endservent>
3005 =for Pod::Functions get passwd record given user login name
3009 =for Pod::Functions get group record given group name
3011 =item gethostbyname NAME
3013 =for Pod::Functions get host record given name
3015 =item getnetbyname NAME
3017 =for Pod::Functions get networks record given name
3019 =item getprotobyname NAME
3021 =for Pod::Functions get protocol record given name
3025 =for Pod::Functions get passwd record given user ID
3029 =for Pod::Functions get group record given group user ID
3031 =item getservbyname NAME,PROTO
3033 =for Pod::Functions get services record given its name
3035 =item gethostbyaddr ADDR,ADDRTYPE
3037 =for Pod::Functions get host record given its address
3039 =item getnetbyaddr ADDR,ADDRTYPE
3041 =for Pod::Functions get network record given its address
3043 =item getprotobynumber NUMBER
3045 =for Pod::Functions get protocol record numeric protocol
3047 =item getservbyport PORT,PROTO
3049 =for Pod::Functions get services record given numeric port
3053 =for Pod::Functions get next passwd record
3057 =for Pod::Functions get next group record
3061 =for Pod::Functions get next hosts record
3065 =for Pod::Functions get next networks record
3069 =for Pod::Functions get next protocols record
3073 =for Pod::Functions get next services record
3077 =for Pod::Functions prepare passwd file for use
3081 =for Pod::Functions prepare group file for use
3083 =item sethostent STAYOPEN
3085 =for Pod::Functions prepare hosts file for use
3087 =item setnetent STAYOPEN
3089 =for Pod::Functions prepare networks file for use
3091 =item setprotoent STAYOPEN
3093 =for Pod::Functions prepare protocols file for use
3095 =item setservent STAYOPEN
3097 =for Pod::Functions prepare services file for use
3101 =for Pod::Functions be done using passwd file
3105 =for Pod::Functions be done using group file
3109 =for Pod::Functions be done using hosts file
3113 =for Pod::Functions be done using networks file
3117 =for Pod::Functions be done using protocols file
3121 =for Pod::Functions be done using services file
3123 These routines are the same as their counterparts in the
3124 system C library. In list context, the return values from the
3125 various get routines are as follows:
3128 my ( $name, $passwd, $gid, $members ) = getgr*
3129 my ( $name, $aliases, $addrtype, $net ) = getnet*
3130 my ( $name, $aliases, $port, $proto ) = getserv*
3131 my ( $name, $aliases, $proto ) = getproto*
3132 my ( $name, $aliases, $addrtype, $length, @addrs ) = gethost*
3133 my ( $name, $passwd, $uid, $gid, $quota,
3134 $comment, $gcos, $dir, $shell, $expire ) = getpw*
3137 (If the entry doesn't exist, the return value is a single meaningless true
3140 The exact meaning of the $gcos field varies but it usually contains
3141 the real name of the user (as opposed to the login name) and other
3142 information pertaining to the user. Beware, however, that in many
3143 system users are able to change this information and therefore it
3144 cannot be trusted and therefore the $gcos is tainted (see
3145 L<perlsec>). The $passwd and $shell, user's encrypted password and
3146 login shell, are also tainted, for the same reason.
3148 In scalar context, you get the name, unless the function was a
3149 lookup by name, in which case you get the other thing, whatever it is.
3150 (If the entry doesn't exist you get the undefined value.) For example:
3152 my $uid = getpwnam($name);
3153 my $name = getpwuid($num);
3154 my $name = getpwent();
3155 my $gid = getgrnam($name);
3156 my $name = getgrgid($num);
3157 my $name = getgrent();
3160 In I<getpw*()> the fields $quota, $comment, and $expire are special
3161 in that they are unsupported on many systems. If the
3162 $quota is unsupported, it is an empty scalar. If it is supported, it
3163 usually encodes the disk quota. If the $comment field is unsupported,
3164 it is an empty scalar. If it is supported it usually encodes some
3165 administrative comment about the user. In some systems the $quota
3166 field may be $change or $age, fields that have to do with password
3167 aging. In some systems the $comment field may be $class. The $expire
3168 field, if present, encodes the expiration period of the account or the
3169 password. For the availability and the exact meaning of these fields
3170 in your system, please consult L<getpwnam(3)> and your system's
3171 F<pwd.h> file. You can also find out from within Perl what your
3172 $quota and $comment fields mean and whether you have the $expire field
3173 by using the L<C<Config>|Config> module and the values C<d_pwquota>, C<d_pwage>,
3174 C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
3175 files are supported only if your vendor has implemented them in the
3176 intuitive fashion that calling the regular C library routines gets the
3177 shadow versions if you're running under privilege or if there exists
3178 the L<shadow(3)> functions as found in System V (this includes Solaris
3179 and Linux). Those systems that implement a proprietary shadow password
3180 facility are unlikely to be supported.
3182 The $members value returned by I<getgr*()> is a space-separated list of
3183 the login names of the members of the group.
3185 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
3186 C, it will be returned to you via L<C<$?>|perlvar/$?> if the function
3188 C<@addrs> value returned by a successful call is a list of raw
3189 addresses returned by the corresponding library call. In the
3190 Internet domain, each address is four bytes long; you can unpack it
3191 by saying something like:
3193 my ($w,$x,$y,$z) = unpack('W4',$addr[0]);
3195 The Socket library makes this slightly easier:
3198 my $iaddr = inet_aton("127.1"); # or whatever address
3199 my $name = gethostbyaddr($iaddr, AF_INET);
3201 # or going the other way
3202 my $straddr = inet_ntoa($iaddr);
3204 In the opposite way, to resolve a hostname to the IP address
3208 my $packed_ip = gethostbyname("www.perl.org");
3210 if (defined $packed_ip) {
3211 $ip_address = inet_ntoa($packed_ip);
3214 Make sure L<C<gethostbyname>|/gethostbyname NAME> is called in SCALAR
3215 context and that its return value is checked for definedness.
3217 The L<C<getprotobynumber>|/getprotobynumber NUMBER> function, even
3218 though it only takes one argument, has the precedence of a list
3219 operator, so beware:
3221 getprotobynumber $number eq 'icmp' # WRONG
3222 getprotobynumber($number eq 'icmp') # actually means this
3223 getprotobynumber($number) eq 'icmp' # better this way
3225 If you get tired of remembering which element of the return list
3226 contains which return value, by-name interfaces are provided in standard
3227 modules: L<C<File::stat>|File::stat>, L<C<Net::hostent>|Net::hostent>,
3228 L<C<Net::netent>|Net::netent>, L<C<Net::protoent>|Net::protoent>,
3229 L<C<Net::servent>|Net::servent>, L<C<Time::gmtime>|Time::gmtime>,
3230 L<C<Time::localtime>|Time::localtime>, and
3231 L<C<User::grent>|User::grent>. These override the normal built-ins,
3232 supplying versions that return objects with the appropriate names for
3233 each field. For example:
3237 my $is_his = (stat($filename)->uid == pwent($whoever)->uid);
3239 Even though it looks as though they're the same method calls (uid),
3240 they aren't, because a C<File::stat> object is different from
3241 a C<User::pwent> object.
3243 Portability issues: L<perlport/getpwnam> to L<perlport/endservent>.
3245 =item getsockname SOCKET
3248 =for Pod::Functions retrieve the sockaddr for a given socket
3250 Returns the packed sockaddr address of this end of the SOCKET connection,
3251 in case you don't know the address because you have several different
3252 IPs that the connection might have come in on.
3255 my $mysockaddr = getsockname($sock);
3256 my ($port, $myaddr) = sockaddr_in($mysockaddr);
3257 printf "Connect to %s [%s]\n",
3258 scalar gethostbyaddr($myaddr, AF_INET),
3261 =item getsockopt SOCKET,LEVEL,OPTNAME
3264 =for Pod::Functions get socket options on a given socket
3266 Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
3267 Options may exist at multiple protocol levels depending on the socket
3268 type, but at least the uppermost socket level SOL_SOCKET (defined in the
3269 L<C<Socket>|Socket> module) will exist. To query options at another
3270 level the protocol number of the appropriate protocol controlling the
3271 option should be supplied. For example, to indicate that an option is
3272 to be interpreted by the TCP protocol, LEVEL should be set to the
3273 protocol number of TCP, which you can get using
3274 L<C<getprotobyname>|/getprotobyname NAME>.
3276 The function returns a packed string representing the requested socket
3277 option, or L<C<undef>|/undef EXPR> on error, with the reason for the
3278 error placed in L<C<$!>|perlvar/$!>. Just what is in the packed string
3279 depends on LEVEL and OPTNAME; consult L<getsockopt(2)> for details. A
3280 common case is that the option is an integer, in which case the result
3281 is a packed integer, which you can decode using
3282 L<C<unpack>|/unpack TEMPLATE,EXPR> with the C<i> (or C<I>) format.
3284 Here's an example to test whether Nagle's algorithm is enabled on a socket:
3286 use Socket qw(:all);
3288 defined(my $tcp = getprotobyname("tcp"))
3289 or die "Could not determine the protocol number for tcp";
3290 # my $tcp = IPPROTO_TCP; # Alternative
3291 my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
3292 or die "getsockopt TCP_NODELAY: $!";
3293 my $nodelay = unpack("I", $packed);
3294 print "Nagle's algorithm is turned ",
3295 $nodelay ? "off\n" : "on\n";
3297 Portability issues: L<perlport/getsockopt>.
3300 X<glob> X<wildcard> X<filename, expansion> X<expand>
3304 =for Pod::Functions expand filenames using wildcards
3306 In list context, returns a (possibly empty) list of filename expansions on
3307 the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
3308 scalar context, glob iterates through such filename expansions, returning
3309 undef when the list is exhausted. This is the internal function
3310 implementing the C<< <*.c> >> operator, but you can use it directly. If
3311 EXPR is omitted, L<C<$_>|perlvar/$_> is used. The C<< <*.c> >> operator
3312 is discussed in more detail in L<perlop/"I/O Operators">.
3314 Note that L<C<glob>|/glob EXPR> splits its arguments on whitespace and
3316 each segment as separate pattern. As such, C<glob("*.c *.h")>
3317 matches all files with a F<.c> or F<.h> extension. The expression
3318 C<glob(".* *")> matches all files in the current working directory.
3319 If you want to glob filenames that might contain whitespace, you'll
3320 have to use extra quotes around the spacey filename to protect it.
3321 For example, to glob filenames that have an C<e> followed by a space
3322 followed by an C<f>, use one of:
3324 my @spacies = <"*e f*">;
3325 my @spacies = glob '"*e f*"';
3326 my @spacies = glob q("*e f*");
3328 If you had to get a variable through, you could do this:
3330 my @spacies = glob "'*${var}e f*'";
3331 my @spacies = glob qq("*${var}e f*");
3333 If non-empty braces are the only wildcard characters used in the
3334 L<C<glob>|/glob EXPR>, no filenames are matched, but potentially many
3335 strings are returned. For example, this produces nine strings, one for
3336 each pairing of fruits and colors:
3338 my @many = glob "{apple,tomato,cherry}={green,yellow,red}";
3340 This operator is implemented using the standard C<File::Glob> extension.
3341 See L<File::Glob> for details, including
3342 L<C<bsd_glob>|File::Glob/C<bsd_glob>>, which does not treat whitespace
3343 as a pattern separator.
3345 Portability issues: L<perlport/glob>.
3348 X<gmtime> X<UTC> X<Greenwich>
3352 =for Pod::Functions convert UNIX time into record or string using Greenwich time
3354 Works just like L<C<localtime>|/localtime EXPR> but the returned values
3355 are localized for the standard Greenwich time zone.
3357 Note: When called in list context, $isdst, the last value
3358 returned by gmtime, is always C<0>. There is no
3359 Daylight Saving Time in GMT.
3361 Portability issues: L<perlport/gmtime>.
3364 X<goto> X<jump> X<jmp>
3370 =for Pod::Functions create spaghetti code
3372 The C<goto LABEL> form finds the statement labeled with LABEL and
3373 resumes execution there. It can't be used to get out of a block or
3374 subroutine given to L<C<sort>|/sort SUBNAME LIST>. It can be used to go
3375 almost anywhere else within the dynamic scope, including out of
3376 subroutines, but it's usually better to use some other construct such as
3377 L<C<last>|/last LABEL> or L<C<die>|/die LIST>. The author of Perl has
3378 never felt the need to use this form of L<C<goto>|/goto LABEL> (in Perl,
3379 that is; C is another matter). (The difference is that C does not offer
3380 named loops combined with loop control. Perl does, and this replaces
3381 most structured uses of L<C<goto>|/goto LABEL> in other languages.)
3383 The C<goto EXPR> form expects to evaluate C<EXPR> to a code reference or
3384 a label name. If it evaluates to a code reference, it will be handled
3385 like C<goto &NAME>, below. This is especially useful for implementing
3386 tail recursion via C<goto __SUB__>.
3388 If the expression evaluates to a label name, its scope will be resolved
3389 dynamically. This allows for computed L<C<goto>|/goto LABEL>s per
3390 FORTRAN, but isn't necessarily recommended if you're optimizing for
3393 goto ("FOO", "BAR", "GLARCH")[$i];
3395 As shown in this example, C<goto EXPR> is exempt from the "looks like a
3396 function" rule. A pair of parentheses following it does not (necessarily)
3397 delimit its argument. C<goto("NE")."XT"> is equivalent to C<goto NEXT>.
3398 Also, unlike most named operators, this has the same precedence as
3401 Use of C<goto LABEL> or C<goto EXPR> to jump into a construct is
3402 deprecated and will issue a warning. Even then, it may not be used to
3403 go into any construct that requires initialization, such as a
3404 subroutine or a C<foreach> loop. It also can't be used to go into a
3405 construct that is optimized away.
3407 The C<goto &NAME> form is quite different from the other forms of
3408 L<C<goto>|/goto LABEL>. In fact, it isn't a goto in the normal sense at
3409 all, and doesn't have the stigma associated with other gotos. Instead,
3410 it exits the current subroutine (losing any changes set by
3411 L<C<local>|/local EXPR>) and immediately calls in its place the named
3412 subroutine using the current value of L<C<@_>|perlvar/@_>. This is used
3413 by C<AUTOLOAD> subroutines that wish to load another subroutine and then
3414 pretend that the other subroutine had been called in the first place
3415 (except that any modifications to L<C<@_>|perlvar/@_> in the current
3416 subroutine are propagated to the other subroutine.) After the
3417 L<C<goto>|/goto LABEL>, not even L<C<caller>|/caller EXPR> will be able
3418 to tell that this routine was called first.
3420 NAME needn't be the name of a subroutine; it can be a scalar variable
3421 containing a code reference or a block that evaluates to a code
3424 =item grep BLOCK LIST
3427 =item grep EXPR,LIST
3429 =for Pod::Functions locate elements in a list test true against a given criterion
3431 This is similar in spirit to, but not the same as, L<grep(1)> and its
3432 relatives. In particular, it is not limited to using regular expressions.
3434 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
3435 L<C<$_>|perlvar/$_> to each element) and returns the list value
3437 elements for which the expression evaluated to true. In scalar
3438 context, returns the number of times the expression was true.
3440 my @foo = grep(!/^#/, @bar); # weed out comments
3444 my @foo = grep {!/^#/} @bar; # weed out comments
3446 Note that L<C<$_>|perlvar/$_> is an alias to the list value, so it can
3448 modify the elements of the LIST. While this is useful and supported,
3449 it can cause bizarre results if the elements of LIST are not variables.
3450 Similarly, grep returns aliases into the original list, much as a for
3451 loop's index variable aliases the list elements. That is, modifying an
3452 element of a list returned by grep (for example, in a C<foreach>,
3453 L<C<map>|/map BLOCK LIST> or another L<C<grep>|/grep BLOCK LIST>)
3454 actually modifies the element in the original list.
3455 This is usually something to be avoided when writing clear code.
3457 See also L<C<map>|/map BLOCK LIST> for a list composed of the results of
3461 X<hex> X<hexadecimal>
3465 =for Pod::Functions convert a hexadecimal string to a number
3467 Interprets EXPR as a hex string and returns the corresponding numeric value.
3468 If EXPR is omitted, uses L<C<$_>|perlvar/$_>.
3470 print hex '0xAf'; # prints '175'
3471 print hex 'aF'; # same
3472 $valid_input =~ /\A(?:0?[xX])?(?:_?[0-9a-fA-F])*\z/
3474 A hex string consists of hex digits and an optional C<0x> or C<x> prefix.
3475 Each hex digit may be preceded by a single underscore, which will be ignored.
3476 Any other character triggers a warning and causes the rest of the string
3477 to be ignored (even leading whitespace, unlike L<C<oct>|/oct EXPR>).
3478 Only integers can be represented, and integer overflow triggers a warning.
3480 To convert strings that might start with any of C<0>, C<0x>, or C<0b>,
3481 see L<C<oct>|/oct EXPR>. To present something as hex, look into
3482 L<C<printf>|/printf FILEHANDLE FORMAT, LIST>,
3483 L<C<sprintf>|/sprintf FORMAT, LIST>, and
3484 L<C<unpack>|/unpack TEMPLATE,EXPR>.
3489 =for Pod::Functions patch a module's namespace into your own
3491 There is no builtin L<C<import>|/import LIST> function. It is just an
3492 ordinary method (subroutine) defined (or inherited) by modules that wish
3493 to export names to another module. The
3494 L<C<use>|/use Module VERSION LIST> function calls the
3495 L<C<import>|/import LIST> method for the package used. See also
3496 L<C<use>|/use Module VERSION LIST>, L<perlmod>, and L<Exporter>.
3498 =item index STR,SUBSTR,POSITION
3499 X<index> X<indexOf> X<InStr>
3501 =item index STR,SUBSTR
3503 =for Pod::Functions find a substring within a string
3505 The index function searches for one string within another, but without
3506 the wildcard-like behavior of a full regular-expression pattern match.
3507 It returns the position of the first occurrence of SUBSTR in STR at
3508 or after POSITION. If POSITION is omitted, starts searching from the
3509 beginning of the string. POSITION before the beginning of the string
3510 or after its end is treated as if it were the beginning or the end,
3511 respectively. POSITION and the return value are based at zero.
3512 If the substring is not found, L<C<index>|/index STR,SUBSTR,POSITION>
3516 X<int> X<integer> X<truncate> X<trunc> X<floor>
3520 =for Pod::Functions get the integer portion of a number
3522 Returns the integer portion of EXPR. If EXPR is omitted, uses
3523 L<C<$_>|perlvar/$_>.
3524 You should not use this function for rounding: one because it truncates
3525 towards C<0>, and two because machine representations of floating-point
3526 numbers can sometimes produce counterintuitive results. For example,
3527 C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
3528 because it's really more like -268.99999999999994315658 instead. Usually,
3529 the L<C<sprintf>|/sprintf FORMAT, LIST>,
3530 L<C<printf>|/printf FILEHANDLE FORMAT, LIST>, or the
3531 L<C<POSIX::floor>|POSIX/C<floor>> and L<C<POSIX::ceil>|POSIX/C<ceil>>
3532 functions will serve you better than will L<C<int>|/int EXPR>.
3534 =item ioctl FILEHANDLE,FUNCTION,SCALAR
3537 =for Pod::Functions system-dependent device control system call
3539 Implements the L<ioctl(2)> function. You'll probably first have to say
3541 require "sys/ioctl.ph"; # probably in
3542 # $Config{archlib}/sys/ioctl.ph
3544 to get the correct function definitions. If F<sys/ioctl.ph> doesn't
3545 exist or doesn't have the correct definitions you'll have to roll your
3546 own, based on your C header files such as F<< <sys/ioctl.h> >>.
3547 (There is a Perl script called B<h2ph> that comes with the Perl kit that
3548 may help you in this, but it's nontrivial.) SCALAR will be read and/or
3549 written depending on the FUNCTION; a C pointer to the string value of SCALAR
3550 will be passed as the third argument of the actual
3551 L<C<ioctl>|/ioctl FILEHANDLE,FUNCTION,SCALAR> call. (If SCALAR
3552 has no string value but does have a numeric value, that value will be
3553 passed rather than a pointer to the string value. To guarantee this to be
3554 true, add a C<0> to the scalar before using it.) The
3555 L<C<pack>|/pack TEMPLATE,LIST> and L<C<unpack>|/unpack TEMPLATE,EXPR>
3556 functions may be needed to manipulate the values of structures used by
3557 L<C<ioctl>|/ioctl FILEHANDLE,FUNCTION,SCALAR>.
3559 The return value of L<C<ioctl>|/ioctl FILEHANDLE,FUNCTION,SCALAR> (and
3560 L<C<fcntl>|/fcntl FILEHANDLE,FUNCTION,SCALAR>) is as follows:
3562 if OS returns: then Perl returns:
3564 0 string "0 but true"
3565 anything else that number
3567 Thus Perl returns true on success and false on failure, yet you can
3568 still easily determine the actual value returned by the operating
3571 my $retval = ioctl(...) || -1;
3572 printf "System returned %d\n", $retval;
3574 The special string C<"0 but true"> is exempt from
3575 L<C<Argument "..." isn't numeric>|perldiag/Argument "%s" isn't numeric%s>
3576 L<warnings> on improper numeric conversions.
3578 Portability issues: L<perlport/ioctl>.
3580 =item join EXPR,LIST
3583 =for Pod::Functions join a list into a string using a separator
3585 Joins the separate strings of LIST into a single string with fields
3586 separated by the value of EXPR, and returns that new string. Example:
3588 my $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
3590 Beware that unlike L<C<split>|/split E<sol>PATTERNE<sol>,EXPR,LIMIT>,
3591 L<C<join>|/join EXPR,LIST> doesn't take a pattern as its first argument.
3592 Compare L<C<split>|/split E<sol>PATTERNE<sol>,EXPR,LIMIT>.
3599 =for Pod::Functions retrieve list of indices from a hash
3601 Called in list context, returns a list consisting of all the keys of the
3602 named hash, or in Perl 5.12 or later only, the indices of an array. Perl
3603 releases prior to 5.12 will produce a syntax error if you try to use an
3604 array argument. In scalar context, returns the number of keys or indices.
3606 Hash entries are returned in an apparently random order. The actual random
3607 order is specific to a given hash; the exact same series of operations
3608 on two hashes may result in a different order for each hash. Any insertion
3609 into the hash may change the order, as will any deletion, with the exception
3610 that the most recent key returned by L<C<each>|/each HASH> or
3611 L<C<keys>|/keys HASH> may be deleted without changing the order. So
3612 long as a given hash is unmodified you may rely on
3613 L<C<keys>|/keys HASH>, L<C<values>|/values HASH> and L<C<each>|/each
3614 HASH> to repeatedly return the same order
3615 as each other. See L<perlsec/"Algorithmic Complexity Attacks"> for
3616 details on why hash order is randomized. Aside from the guarantees
3617 provided here the exact details of Perl's hash algorithm and the hash
3618 traversal order are subject to change in any release of Perl. Tied hashes
3619 may behave differently to Perl's hashes with respect to changes in order on
3620 insertion and deletion of items.
3622 As a side effect, calling L<C<keys>|/keys HASH> resets the internal
3623 iterator of the HASH or ARRAY (see L<C<each>|/each HASH>). In
3624 particular, calling L<C<keys>|/keys HASH> in void context resets the
3625 iterator with no other overhead.
3627 Here is yet another way to print your environment:
3629 my @keys = keys %ENV;
3630 my @values = values %ENV;
3632 print pop(@keys), '=', pop(@values), "\n";
3635 or how about sorted by key:
3637 foreach my $key (sort(keys %ENV)) {
3638 print $key, '=', $ENV{$key}, "\n";
3641 The returned values are copies of the original keys in the hash, so
3642 modifying them will not affect the original hash. Compare
3643 L<C<values>|/values HASH>.
3645 To sort a hash by value, you'll need to use a
3646 L<C<sort>|/sort SUBNAME LIST> function. Here's a descending numeric
3647 sort of a hash by its values:
3649 foreach my $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
3650 printf "%4d %s\n", $hash{$key}, $key;
3653 Used as an lvalue, L<C<keys>|/keys HASH> allows you to increase the
3654 number of hash buckets
3655 allocated for the given hash. This can gain you a measure of efficiency if
3656 you know the hash is going to get big. (This is similar to pre-extending
3657 an array by assigning a larger number to $#array.) If you say
3661 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
3662 in fact, since it rounds up to the next power of two. These
3663 buckets will be retained even if you do C<%hash = ()>, use C<undef
3664 %hash> if you want to free the storage while C<%hash> is still in scope.
3665 You can't shrink the number of buckets allocated for the hash using
3666 L<C<keys>|/keys HASH> in this way (but you needn't worry about doing
3667 this by accident, as trying has no effect). C<keys @array> in an lvalue
3668 context is a syntax error.
3670 Starting with Perl 5.14, an experimental feature allowed
3671 L<C<keys>|/keys HASH> to take a scalar expression. This experiment has
3672 been deemed unsuccessful, and was removed as of Perl 5.24.
3674 To avoid confusing would-be users of your code who are running earlier
3675 versions of Perl with mysterious syntax errors, put this sort of thing at
3676 the top of your file to signal that your code will work I<only> on Perls of
3679 use 5.012; # so keys/values/each work on arrays
3681 See also L<C<each>|/each HASH>, L<C<values>|/values HASH>, and
3682 L<C<sort>|/sort SUBNAME LIST>.
3684 =item kill SIGNAL, LIST
3689 =for Pod::Functions send a signal to a process or process group
3691 Sends a signal to a list of processes. Returns the number of arguments
3692 that were successfully used to signal (which is not necessarily the same
3693 as the number of processes actually killed, e.g. where a process group is
3696 my $cnt = kill 'HUP', $child1, $child2;
3697 kill 'KILL', @goners;
3699 SIGNAL may be either a signal name (a string) or a signal number. A signal
3700 name may start with a C<SIG> prefix, thus C<FOO> and C<SIGFOO> refer to the
3701 same signal. The string form of SIGNAL is recommended for portability because
3702 the same signal may have different numbers in different operating systems.
3704 A list of signal names supported by the current platform can be found in
3705 C<$Config{sig_name}>, which is provided by the L<C<Config>|Config>
3706 module. See L<Config> for more details.
3708 A negative signal name is the same as a negative signal number, killing process
3709 groups instead of processes. For example, C<kill '-KILL', $pgrp> and
3710 C<kill -9, $pgrp> will send C<SIGKILL> to
3711 the entire process group specified. That
3712 means you usually want to use positive not negative signals.
3714 If SIGNAL is either the number 0 or the string C<ZERO> (or C<SIGZERO>),
3715 no signal is sent to the process, but L<C<kill>|/kill SIGNAL, LIST>
3716 checks whether it's I<possible> to send a signal to it
3717 (that means, to be brief, that the process is owned by the same user, or we are
3718 the super-user). This is useful to check that a child process is still
3719 alive (even if only as a zombie) and hasn't changed its UID. See
3720 L<perlport> for notes on the portability of this construct.
3722 The behavior of kill when a I<PROCESS> number is zero or negative depends on
3723 the operating system. For example, on POSIX-conforming systems, zero will
3724 signal the current process group, -1 will signal all processes, and any
3725 other negative PROCESS number will act as a negative signal number and
3726 kill the entire process group specified.
3728 If both the SIGNAL and the PROCESS are negative, the results are undefined.
3729 A warning may be produced in a future version.
3731 See L<perlipc/"Signals"> for more details.
3733 On some platforms such as Windows where the L<fork(2)> system call is not
3734 available, Perl can be built to emulate L<C<fork>|/fork> at the
3736 This emulation has limitations related to kill that have to be considered,
3737 for code running on Windows and in code intended to be portable.
3739 See L<perlfork> for more details.
3741 If there is no I<LIST> of processes, no signal is sent, and the return
3742 value is 0. This form is sometimes used, however, because it causes
3743 tainting checks to be run. But see
3744 L<perlsec/Laundering and Detecting Tainted Data>.
3746 Portability issues: L<perlport/kill>.
3755 =for Pod::Functions exit a block prematurely
3757 The L<C<last>|/last LABEL> command is like the C<break> statement in C
3759 loops); it immediately exits the loop in question. If the LABEL is
3760 omitted, the command refers to the innermost enclosing
3761 loop. The C<last EXPR> form, available starting in Perl
3762 5.18.0, allows a label name to be computed at run time,
3763 and is otherwise identical to C<last LABEL>. The
3764 L<C<continue>|/continue BLOCK> block, if any, is not executed:
3766 LINE: while (<STDIN>) {
3767 last LINE if /^$/; # exit when done with header
3771 L<C<last>|/last LABEL> cannot be used to exit a block that returns a
3772 value such as C<eval {}>, C<sub {}>, or C<do {}>, and should not be used
3773 to exit a L<C<grep>|/grep BLOCK LIST> or L<C<map>|/map BLOCK LIST>
3776 Note that a block by itself is semantically identical to a loop
3777 that executes once. Thus L<C<last>|/last LABEL> can be used to effect
3778 an early exit out of such a block.
3780 See also L<C<continue>|/continue BLOCK> for an illustration of how
3781 L<C<last>|/last LABEL>, L<C<next>|/next LABEL>, and
3782 L<C<redo>|/redo LABEL> work.
3784 Unlike most named operators, this has the same precedence as assignment.
3785 It is also exempt from the looks-like-a-function rule, so
3786 C<last ("foo")."bar"> will cause "bar" to be part of the argument to
3787 L<C<last>|/last LABEL>.
3794 =for Pod::Functions return lower-case version of a string
3796 Returns a lowercased version of EXPR. This is the internal function
3797 implementing the C<\L> escape in double-quoted strings.
3799 If EXPR is omitted, uses L<C<$_>|perlvar/$_>.
3801 What gets returned depends on several factors:
3805 =item If C<use bytes> is in effect:
3807 The results follow ASCII rules. Only the characters C<A-Z> change,
3808 to C<a-z> respectively.
3810 =item Otherwise, if C<use locale> for C<LC_CTYPE> is in effect:
3812 Respects current C<LC_CTYPE> locale for code points < 256; and uses Unicode
3813 rules for the remaining code points (this last can only happen if
3814 the UTF8 flag is also set). See L<perllocale>.
3816 Starting in v5.20, Perl uses full Unicode rules if the locale is
3817 UTF-8. Otherwise, there is a deficiency in this scheme, which is that
3818 case changes that cross the 255/256
3819 boundary are not well-defined. For example, the lower case of LATIN CAPITAL
3820 LETTER SHARP S (U+1E9E) in Unicode rules is U+00DF (on ASCII
3821 platforms). But under C<use locale> (prior to v5.20 or not a UTF-8
3822 locale), the lower case of U+1E9E is
3823 itself, because 0xDF may not be LATIN SMALL LETTER SHARP S in the
3824 current locale, and Perl has no way of knowing if that character even
3825 exists in the locale, much less what code point it is. Perl returns
3826 a result that is above 255 (almost always the input character unchanged),
3827 for all instances (and there aren't many) where the 255/256 boundary
3828 would otherwise be crossed; and starting in v5.22, it raises a
3829 L<locale|perldiag/Can't do %s("%s") on non-UTF-8 locale; resolved to "%s".> warning.
3831 =item Otherwise, If EXPR has the UTF8 flag set:
3833 Unicode rules are used for the case change.
3835 =item Otherwise, if C<use feature 'unicode_strings'> or C<use locale ':not_characters'> is in effect:
3837 Unicode rules are used for the case change.
3841 ASCII rules are used for the case change. The lowercase of any character
3842 outside the ASCII range is the character itself.
3847 X<lcfirst> X<lowercase>
3851 =for Pod::Functions return a string with just the next letter in lower case
3853 Returns the value of EXPR with the first character lowercased. This
3854 is the internal function implementing the C<\l> escape in
3855 double-quoted strings.
3857 If EXPR is omitted, uses L<C<$_>|perlvar/$_>.
3859 This function behaves the same way under various pragmas, such as in a locale,
3860 as L<C<lc>|/lc EXPR> does.
3867 =for Pod::Functions return the number of characters in a string
3869 Returns the length in I<characters> of the value of EXPR. If EXPR is
3870 omitted, returns the length of L<C<$_>|perlvar/$_>. If EXPR is
3871 undefined, returns L<C<undef>|/undef EXPR>.
3873 This function cannot be used on an entire array or hash to find out how
3874 many elements these have. For that, use C<scalar @array> and C<scalar keys
3875 %hash>, respectively.
3877 Like all Perl character operations, L<C<length>|/length EXPR> normally
3879 characters, not physical bytes. For how many bytes a string encoded as
3880 UTF-8 would take up, use C<length(Encode::encode('UTF-8', EXPR))>
3881 (you'll have to C<use Encode> first). See L<Encode> and L<perlunicode>.
3886 =for Pod::Functions the current source line number
3888 A special token that compiles to the current line number.
3890 =item link OLDFILE,NEWFILE
3893 =for Pod::Functions create a hard link in the filesystem
3895 Creates a new filename linked to the old filename. Returns true for
3896 success, false otherwise.
3898 Portability issues: L<perlport/link>.
3900 =item listen SOCKET,QUEUESIZE
3903 =for Pod::Functions register your socket as a server
3905 Does the same thing that the L<listen(2)> system call does. Returns true if
3906 it succeeded, false otherwise. See the example in
3907 L<perlipc/"Sockets: Client/Server Communication">.
3912 =for Pod::Functions create a temporary value for a global variable (dynamic scoping)
3914 You really probably want to be using L<C<my>|/my VARLIST> instead,
3915 because L<C<local>|/local EXPR> isn't what most people think of as
3916 "local". See L<perlsub/"Private Variables via my()"> for details.
3918 A local modifies the listed variables to be local to the enclosing
3919 block, file, or eval. If more than one value is listed, the list must
3920 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
3921 for details, including issues with tied arrays and hashes.
3923 The C<delete local EXPR> construct can also be used to localize the deletion
3924 of array/hash elements to the current block.
3925 See L<perlsub/"Localized deletion of elements of composite types">.
3927 =item localtime EXPR
3928 X<localtime> X<ctime>
3932 =for Pod::Functions convert UNIX time into record or string using local time
3934 Converts a time as returned by the time function to a 9-element list
3935 with the time analyzed for the local time zone. Typically used as
3939 my ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
3942 All list elements are numeric and come straight out of the C `struct
3943 tm'. C<$sec>, C<$min>, and C<$hour> are the seconds, minutes, and hours
3944 of the specified time.
3946 C<$mday> is the day of the month and C<$mon> the month in
3947 the range C<0..11>, with 0 indicating January and 11 indicating December.
3948 This makes it easy to get a month name from a list:
3950 my @abbr = qw(Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec);
3951 print "$abbr[$mon] $mday";
3952 # $mon=9, $mday=18 gives "Oct 18"
3954 C<$year> contains the number of years since 1900. To get a 4-digit
3959 To get the last two digits of the year (e.g., "01" in 2001) do:
3961 $year = sprintf("%02d", $year % 100);
3963 C<$wday> is the day of the week, with 0 indicating Sunday and 3 indicating
3964 Wednesday. C<$yday> is the day of the year, in the range C<0..364>
3965 (or C<0..365> in leap years.)
3967 C<$isdst> is true if the specified time occurs during Daylight Saving
3968 Time, false otherwise.
3970 If EXPR is omitted, L<C<localtime>|/localtime EXPR> uses the current
3971 time (as returned by L<C<time>|/time>).
3973 In scalar context, L<C<localtime>|/localtime EXPR> returns the
3976 my $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
3978 The format of this scalar value is B<not> locale-dependent but built
3979 into Perl. For GMT instead of local time use the
3980 L<C<gmtime>|/gmtime EXPR> builtin. See also the
3981 L<C<Time::Local>|Time::Local> module (for converting seconds, minutes,
3982 hours, and such back to the integer value returned by L<C<time>|/time>),
3983 and the L<POSIX> module's L<C<strftime>|POSIX/C<strftime>> and
3984 L<C<mktime>|POSIX/C<mktime>> functions.
3986 To get somewhat similar but locale-dependent date strings, set up your
3987 locale environment variables appropriately (please see L<perllocale>) and
3990 use POSIX qw(strftime);
3991 my $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
3992 # or for GMT formatted appropriately for your locale:
3993 my $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
3995 Note that C<%a> and C<%b>, the short forms of the day of the week
3996 and the month of the year, may not necessarily be three characters wide.
3998 The L<Time::gmtime> and L<Time::localtime> modules provide a convenient,
3999 by-name access mechanism to the L<C<gmtime>|/gmtime EXPR> and
4000 L<C<localtime>|/localtime EXPR> functions, respectively.
4002 For a comprehensive date and time representation look at the
4003 L<DateTime> module on CPAN.
4005 Portability issues: L<perlport/localtime>.
4010 =for Pod::Functions +5.005 get a thread lock on a variable, subroutine, or method
4012 This function places an advisory lock on a shared variable or referenced
4013 object contained in I<THING> until the lock goes out of scope.
4015 The value returned is the scalar itself, if the argument is a scalar, or a
4016 reference, if the argument is a hash, array or subroutine.
4018 L<C<lock>|/lock THING> is a "weak keyword"; this means that if you've
4020 by this name (before any calls to it), that function will be called
4021 instead. If you are not under C<use threads::shared> this does nothing.
4022 See L<threads::shared>.
4025 X<log> X<logarithm> X<e> X<ln> X<base>
4029 =for Pod::Functions retrieve the natural logarithm for a number
4031 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
4032 returns the log of L<C<$_>|perlvar/$_>. To get the
4033 log of another base, use basic algebra:
4034 The base-N log of a number is equal to the natural log of that number
4035 divided by the natural log of N. For example:
4039 return log($n)/log(10);
4042 See also L<C<exp>|/exp EXPR> for the inverse operation.
4044 =item lstat FILEHANDLE
4049 =item lstat DIRHANDLE
4053 =for Pod::Functions stat a symbolic link
4055 Does the same thing as the L<C<stat>|/stat FILEHANDLE> function
4056 (including setting the special C<_> filehandle) but stats a symbolic
4057 link instead of the file the symbolic link points to. If symbolic links
4058 are unimplemented on your system, a normal L<C<stat>|/stat FILEHANDLE>
4059 is done. For much more detailed information, please see the
4060 documentation for L<C<stat>|/stat FILEHANDLE>.
4062 If EXPR is omitted, stats L<C<$_>|perlvar/$_>.
4064 Portability issues: L<perlport/lstat>.
4068 =for Pod::Functions match a string with a regular expression pattern
4070 The match operator. See L<perlop/"Regexp Quote-Like Operators">.
4072 =item map BLOCK LIST
4077 =for Pod::Functions apply a change to a list to get back a new list with the changes
4079 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
4080 L<C<$_>|perlvar/$_> to each element) and composes a list of the results of
4081 each such evaluation. Each element of LIST may produce zero, one, or more
4082 elements in the generated list, so the number of elements in the generated
4083 list may differ from that in LIST. In scalar context, returns the total
4084 number of elements so generated. In list context, returns the generated list.
4086 my @chars = map(chr, @numbers);
4088 translates a list of numbers to the corresponding characters.
4090 my @squares = map { $_ * $_ } @numbers;
4092 translates a list of numbers to their squared values.
4094 my @squares = map { $_ > 5 ? ($_ * $_) : () } @numbers;
4096 shows that number of returned elements can differ from the number of
4097 input elements. To omit an element, return an empty list ().
4098 This could also be achieved by writing
4100 my @squares = map { $_ * $_ } grep { $_ > 5 } @numbers;
4102 which makes the intention more clear.
4104 Map always returns a list, which can be
4105 assigned to a hash such that the elements
4106 become key/value pairs. See L<perldata> for more details.
4108 my %hash = map { get_a_key_for($_) => $_ } @array;
4110 is just a funny way to write
4114 $hash{get_a_key_for($_)} = $_;
4117 Note that L<C<$_>|perlvar/$_> is an alias to the list value, so it can
4118 be used to modify the elements of the LIST. While this is useful and
4119 supported, it can cause bizarre results if the elements of LIST are not
4120 variables. Using a regular C<foreach> loop for this purpose would be
4121 clearer in most cases. See also L<C<grep>|/grep BLOCK LIST> for a
4122 list composed of those items of the original list for which the BLOCK
4123 or EXPR evaluates to true.
4125 C<{> starts both hash references and blocks, so C<map { ...> could be either
4126 the start of map BLOCK LIST or map EXPR, LIST. Because Perl doesn't look
4127 ahead for the closing C<}> it has to take a guess at which it's dealing with
4128 based on what it finds just after the
4129 C<{>. Usually it gets it right, but if it
4130 doesn't it won't realize something is wrong until it gets to the C<}> and
4131 encounters the missing (or unexpected) comma. The syntax error will be
4132 reported close to the C<}>, but you'll need to change something near the C<{>
4133 such as using a unary C<+> or semicolon to give Perl some help:
4135 my %hash = map { "\L$_" => 1 } @array # perl guesses EXPR. wrong
4136 my %hash = map { +"\L$_" => 1 } @array # perl guesses BLOCK. right
4137 my %hash = map {; "\L$_" => 1 } @array # this also works
4138 my %hash = map { ("\L$_" => 1) } @array # as does this
4139 my %hash = map { lc($_) => 1 } @array # and this.
4140 my %hash = map +( lc($_) => 1 ), @array # this is EXPR and works!
4142 my %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
4144 or to force an anon hash constructor use C<+{>:
4146 my @hashes = map +{ lc($_) => 1 }, @array # EXPR, so needs
4149 to get a list of anonymous hashes each with only one entry apiece.
4151 =item mkdir FILENAME,MASK
4152 X<mkdir> X<md> X<directory, create>
4154 =item mkdir FILENAME
4158 =for Pod::Functions create a directory
4160 Creates the directory specified by FILENAME, with permissions
4161 specified by MASK (as modified by L<C<umask>|/umask EXPR>). If it
4162 succeeds it returns true; otherwise it returns false and sets
4163 L<C<$!>|perlvar/$!> (errno).
4164 MASK defaults to 0777 if omitted, and FILENAME defaults
4165 to L<C<$_>|perlvar/$_> if omitted.
4167 In general, it is better to create directories with a permissive MASK
4168 and let the user modify that with their L<C<umask>|/umask EXPR> than it
4170 a restrictive MASK and give the user no way to be more permissive.
4171 The exceptions to this rule are when the file or directory should be
4172 kept private (mail files, for instance). The documentation for
4173 L<C<umask>|/umask EXPR> discusses the choice of MASK in more detail.
4175 Note that according to the POSIX 1003.1-1996 the FILENAME may have any
4176 number of trailing slashes. Some operating and filesystems do not get
4177 this right, so Perl automatically removes all trailing slashes to keep
4180 To recursively create a directory structure, look at
4181 the L<C<make_path>|File::Path/make_path( $dir1, $dir2, .... )> function
4182 of the L<File::Path> module.
4184 =item msgctl ID,CMD,ARG
4187 =for Pod::Functions SysV IPC message control operations
4189 Calls the System V IPC function L<msgctl(2)>. You'll probably have to say
4193 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
4194 then ARG must be a variable that will hold the returned C<msqid_ds>
4195 structure. Returns like L<C<ioctl>|/ioctl FILEHANDLE,FUNCTION,SCALAR>:
4196 the undefined value for error, C<"0 but true"> for zero, or the actual
4197 return value otherwise. See also L<perlipc/"SysV IPC"> and the
4198 documentation for L<C<IPC::SysV>|IPC::SysV> and
4199 L<C<IPC::Semaphore>|IPC::Semaphore>.
4201 Portability issues: L<perlport/msgctl>.
4203 =item msgget KEY,FLAGS
4206 =for Pod::Functions get SysV IPC message queue
4208 Calls the System V IPC function L<msgget(2)>. Returns the message queue
4209 id, or L<C<undef>|/undef EXPR> on error. See also L<perlipc/"SysV IPC">
4210 and the documentation for L<C<IPC::SysV>|IPC::SysV> and
4211 L<C<IPC::Msg>|IPC::Msg>.
4213 Portability issues: L<perlport/msgget>.
4215 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
4218 =for Pod::Functions receive a SysV IPC message from a message queue
4220 Calls the System V IPC function msgrcv to receive a message from
4221 message queue ID into variable VAR with a maximum message size of
4222 SIZE. Note that when a message is received, the message type as a
4223 native long integer will be the first thing in VAR, followed by the
4224 actual message. This packing may be opened with C<unpack("l! a*")>.
4225 Taints the variable. Returns true if successful, false
4226 on error. See also L<perlipc/"SysV IPC"> and the documentation for
4227 L<C<IPC::SysV>|IPC::SysV> and L<C<IPC::Msg>|IPC::Msg>.
4229 Portability issues: L<perlport/msgrcv>.
4231 =item msgsnd ID,MSG,FLAGS
4234 =for Pod::Functions send a SysV IPC message to a message queue
4236 Calls the System V IPC function msgsnd to send the message MSG to the
4237 message queue ID. MSG must begin with the native long integer message
4238 type, be followed by the length of the actual message, and then finally
4239 the message itself. This kind of packing can be achieved with
4240 C<pack("l! a*", $type, $message)>. Returns true if successful,
4241 false on error. See also L<perlipc/"SysV IPC"> and the documentation
4242 for L<C<IPC::SysV>|IPC::SysV> and L<C<IPC::Msg>|IPC::Msg>.
4244 Portability issues: L<perlport/msgsnd>.
4249 =item my TYPE VARLIST
4251 =item my VARLIST : ATTRS
4253 =item my TYPE VARLIST : ATTRS
4255 =for Pod::Functions declare and assign a local variable (lexical scoping)
4257 A L<C<my>|/my VARLIST> declares the listed variables to be local
4258 (lexically) to the enclosing block, file, or L<C<eval>|/eval EXPR>. If
4259 more than one variable is listed, the list must be placed in
4262 The exact semantics and interface of TYPE and ATTRS are still
4263 evolving. TYPE may be a bareword, a constant declared
4264 with L<C<use constant>|constant>, or L<C<__PACKAGE__>|/__PACKAGE__>. It
4266 currently bound to the use of the L<fields> pragma,
4267 and attributes are handled using the L<attributes> pragma, or starting
4268 from Perl 5.8.0 also via the L<Attribute::Handlers> module. See
4269 L<perlsub/"Private Variables via my()"> for details.
4271 Note that with a parenthesised list, L<C<undef>|/undef EXPR> can be used
4272 as a dummy placeholder, for example to skip assignment of initial
4275 my ( undef, $min, $hour ) = localtime;
4284 =for Pod::Functions iterate a block prematurely
4286 The L<C<next>|/next LABEL> command is like the C<continue> statement in
4287 C; it starts the next iteration of the loop:
4289 LINE: while (<STDIN>) {
4290 next LINE if /^#/; # discard comments
4294 Note that if there were a L<C<continue>|/continue BLOCK> block on the
4296 executed even on discarded lines. If LABEL is omitted, the command
4297 refers to the innermost enclosing loop. The C<next EXPR> form, available
4298 as of Perl 5.18.0, allows a label name to be computed at run time, being
4299 otherwise identical to C<next LABEL>.
4301 L<C<next>|/next LABEL> cannot be used to exit a block which returns a
4302 value such as C<eval {}>, C<sub {}>, or C<do {}>, and should not be used
4303 to exit a L<C<grep>|/grep BLOCK LIST> or L<C<map>|/map BLOCK LIST>
4306 Note that a block by itself is semantically identical to a loop
4307 that executes once. Thus L<C<next>|/next LABEL> will exit such a block
4310 See also L<C<continue>|/continue BLOCK> for an illustration of how
4311 L<C<last>|/last LABEL>, L<C<next>|/next LABEL>, and
4312 L<C<redo>|/redo LABEL> work.
4314 Unlike most named operators, this has the same precedence as assignment.
4315 It is also exempt from the looks-like-a-function rule, so
4316 C<next ("foo")."bar"> will cause "bar" to be part of the argument to
4317 L<C<next>|/next LABEL>.
4319 =item no MODULE VERSION LIST
4323 =item no MODULE VERSION
4325 =item no MODULE LIST
4331 =for Pod::Functions unimport some module symbols or semantics at compile time
4333 See the L<C<use>|/use Module VERSION LIST> function, of which
4334 L<C<no>|/no MODULE VERSION LIST> is the opposite.
4337 X<oct> X<octal> X<hex> X<hexadecimal> X<binary> X<bin>
4341 =for Pod::Functions convert a string to an octal number
4343 Interprets EXPR as an octal string and returns the corresponding
4344 value. (If EXPR happens to start off with C<0x>, interprets it as a
4345 hex string. If EXPR starts off with C<0b>, it is interpreted as a
4346 binary string. Leading whitespace is ignored in all three cases.)
4347 The following will handle decimal, binary, octal, and hex in standard
4350 $val = oct($val) if $val =~ /^0/;
4352 If EXPR is omitted, uses L<C<$_>|perlvar/$_>. To go the other way
4353 (produce a number in octal), use L<C<sprintf>|/sprintf FORMAT, LIST> or
4354 L<C<printf>|/printf FILEHANDLE FORMAT, LIST>:
4356 my $dec_perms = (stat("filename"))[2] & 07777;
4357 my $oct_perm_str = sprintf "%o", $perms;
4359 The L<C<oct>|/oct EXPR> function is commonly used when a string such as
4361 to be converted into a file mode, for example. Although Perl
4362 automatically converts strings into numbers as needed, this automatic
4363 conversion assumes base 10.
4365 Leading white space is ignored without warning, as too are any trailing
4366 non-digits, such as a decimal point (L<C<oct>|/oct EXPR> only handles
4367 non-negative integers, not negative integers or floating point).
4369 =item open FILEHANDLE,EXPR
4370 X<open> X<pipe> X<file, open> X<fopen>
4372 =item open FILEHANDLE,MODE,EXPR
4374 =item open FILEHANDLE,MODE,EXPR,LIST
4376 =item open FILEHANDLE,MODE,REFERENCE
4378 =item open FILEHANDLE
4380 =for Pod::Functions open a file, pipe, or descriptor
4382 Opens the file whose filename is given by EXPR, and associates it with
4385 Simple examples to open a file for reading:
4387 open(my $fh, "<", "input.txt")
4388 or die "Can't open < input.txt: $!";
4392 open(my $fh, ">", "output.txt")
4393 or die "Can't open > output.txt: $!";
4395 (The following is a comprehensive reference to
4396 L<C<open>|/open FILEHANDLE,EXPR>: for a gentler introduction you may
4397 consider L<perlopentut>.)
4399 If FILEHANDLE is an undefined scalar variable (or array or hash element), a
4400 new filehandle is autovivified, meaning that the variable is assigned a
4401 reference to a newly allocated anonymous filehandle. Otherwise if
4402 FILEHANDLE is an expression, its value is the real filehandle. (This is
4403 considered a symbolic reference, so C<use strict "refs"> should I<not> be
4406 If three (or more) arguments are specified, the open mode (including
4407 optional encoding) in the second argument are distinct from the filename in
4408 the third. If MODE is C<< < >> or nothing, the file is opened for input.
4409 If MODE is C<< > >>, the file is opened for output, with existing files
4410 first being truncated ("clobbered") and nonexisting files newly created.
4411 If MODE is C<<< >> >>>, the file is opened for appending, again being
4412 created if necessary.
4414 You can put a C<+> in front of the C<< > >> or C<< < >> to
4415 indicate that you want both read and write access to the file; thus
4416 C<< +< >> is almost always preferred for read/write updates--the
4417 C<< +> >> mode would clobber the file first. You can't usually use
4418 either read-write mode for updating textfiles, since they have
4419 variable-length records. See the B<-i> switch in L<perlrun> for a
4420 better approach. The file is created with permissions of C<0666>
4421 modified by the process's L<C<umask>|/umask EXPR> value.
4423 These various prefixes correspond to the L<fopen(3)> modes of C<r>,
4424 C<r+>, C<w>, C<w+>, C<a>, and C<a+>.
4426 In the one- and two-argument forms of the call, the mode and filename
4427 should be concatenated (in that order), preferably separated by white
4428 space. You can--but shouldn't--omit the mode in these forms when that mode
4429 is C<< < >>. It is safe to use the two-argument form of
4430 L<C<open>|/open FILEHANDLE,EXPR> if the filename argument is a known literal.
4432 For three or more arguments if MODE is C<|->, the filename is
4433 interpreted as a command to which output is to be piped, and if MODE
4434 is C<-|>, the filename is interpreted as a command that pipes
4435 output to us. In the two-argument (and one-argument) form, one should
4436 replace dash (C<->) with the command.
4437 See L<perlipc/"Using open() for IPC"> for more examples of this.
4438 (You are not allowed to L<C<open>|/open FILEHANDLE,EXPR> to a command
4439 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>, and
4440 L<perlipc/"Bidirectional Communication with Another Process"> for
4443 In the form of pipe opens taking three or more arguments, if LIST is specified
4444 (extra arguments after the command name) then LIST becomes arguments
4445 to the command invoked if the platform supports it. The meaning of
4446 L<C<open>|/open FILEHANDLE,EXPR> with more than three arguments for
4447 non-pipe modes is not yet defined, but experimental "layers" may give
4448 extra LIST arguments meaning.
4450 In the two-argument (and one-argument) form, opening C<< <- >>
4451 or C<-> opens STDIN and opening C<< >- >> opens STDOUT.
4453 You may (and usually should) use the three-argument form of open to specify
4454 I/O layers (sometimes referred to as "disciplines") to apply to the handle
4455 that affect how the input and output are processed (see L<open> and
4456 L<PerlIO> for more details). For example:
4458 open(my $fh, "<:encoding(UTF-8)", $filename)
4459 || die "Can't open UTF-8 encoded $filename: $!";
4461 opens the UTF8-encoded file containing Unicode characters;
4462 see L<perluniintro>. Note that if layers are specified in the
4463 three-argument form, then default layers stored in ${^OPEN} (see L<perlvar>;
4464 usually set by the L<open> pragma or the switch C<-CioD>) are ignored.
4465 Those layers will also be ignored if you specify a colon with no name
4466 following it. In that case the default layer for the operating system
4467 (:raw on Unix, :crlf on Windows) is used.
4469 Open returns nonzero on success, the undefined value otherwise. If
4470 the L<C<open>|/open FILEHANDLE,EXPR> involved a pipe, the return value
4471 happens to be the pid of the subprocess.
4473 On some systems (in general, DOS- and Windows-based systems)
4474 L<C<binmode>|/binmode FILEHANDLE, LAYER> is necessary when you're not
4475 working with a text file. For the sake of portability it is a good idea
4476 always to use it when appropriate, and never to use it when it isn't
4477 appropriate. Also, people can set their I/O to be by default
4478 UTF8-encoded Unicode, not bytes.
4480 When opening a file, it's seldom a good idea to continue
4481 if the request failed, so L<C<open>|/open FILEHANDLE,EXPR> is frequently
4482 used with L<C<die>|/die LIST>. Even if L<C<die>|/die LIST> won't do
4483 what you want (say, in a CGI script,
4484 where you want to format a suitable error message (but there are
4485 modules that can help with that problem)) always check
4486 the return value from opening a file.
4488 The filehandle will be closed when its reference count reaches zero.
4489 If it is a lexically scoped variable declared with L<C<my>|/my VARLIST>,
4491 means the end of the enclosing scope. However, this automatic close
4492 does not check for errors, so it is better to explicitly close
4493 filehandles, especially those used for writing:
4496 || warn "close failed: $!";
4498 An older style is to use a bareword as the filehandle, as
4500 open(FH, "<", "input.txt")
4501 or die "Can't open < input.txt: $!";
4503 Then you can use C<FH> as the filehandle, in C<< close FH >> and C<<
4504 <FH> >> and so on. Note that it's a global variable, so this form is
4505 not recommended in new code.
4507 As a shortcut a one-argument call takes the filename from the global
4508 scalar variable of the same name as the filehandle:
4511 open(ARTICLE) or die "Can't find article $ARTICLE: $!\n";
4513 Here C<$ARTICLE> must be a global (package) scalar variable - not one
4514 declared with L<C<my>|/my VARLIST> or L<C<state>|/state VARLIST>.
4516 As a special case the three-argument form with a read/write mode and the third
4517 argument being L<C<undef>|/undef EXPR>:
4519 open(my $tmp, "+>", undef) or die ...
4521 opens a filehandle to a newly created empty anonymous temporary file.
4522 (This happens under any mode, which makes C<< +> >> the only useful and
4523 sensible mode to use.) You will need to
4524 L<C<seek>|/seek FILEHANDLE,POSITION,WHENCE> to do the reading.
4526 Perl is built using PerlIO by default. Unless you've
4527 changed this (such as building Perl with C<Configure -Uuseperlio>), you can
4528 open filehandles directly to Perl scalars via:
4530 open(my $fh, ">", \$variable) || ..
4532 To (re)open C<STDOUT> or C<STDERR> as an in-memory file, close it first:
4535 open(STDOUT, ">", \$variable)
4536 or die "Can't open STDOUT: $!";
4538 The scalars for in-memory files are treated as octet strings: unless
4539 the file is being opened with truncation the scalar may not contain
4540 any code points over 0xFF.
4542 Opening in-memory files I<can> fail for a variety of reasons. As with
4543 any other C<open>, check the return value for success.
4545 See L<perliol> for detailed info on PerlIO.
4549 open(my $log, ">>", "/usr/spool/news/twitlog");
4550 # if the open fails, output is discarded
4552 open(my $dbase, "+<", "dbase.mine") # open for update
4553 or die "Can't open 'dbase.mine' for update: $!";
4555 open(my $dbase, "+<dbase.mine") # ditto
4556 or die "Can't open 'dbase.mine' for update: $!";
4558 open(my $article_fh, "-|", "caesar <$article") # decrypt
4560 or die "Can't start caesar: $!";
4562 open(my $article_fh, "caesar <$article |") # ditto
4563 or die "Can't start caesar: $!";
4565 open(my $out_fh, "|-", "sort >Tmp$$") # $$ is our process id
4566 or die "Can't start sort: $!";
4569 open(my $memory, ">", \$var)
4570 or die "Can't open memory file: $!";
4571 print $memory "foo!\n"; # output will appear in $var
4573 You may also, in the Bourne shell tradition, specify an EXPR beginning
4574 with C<< >& >>, in which case the rest of the string is interpreted
4575 as the name of a filehandle (or file descriptor, if numeric) to be
4576 duped (as in L<dup(2)>) and opened. You may use C<&> after C<< > >>,
4577 C<<< >> >>>, C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>.
4578 The mode you specify should match the mode of the original filehandle.
4579 (Duping a filehandle does not take into account any existing contents
4580 of IO buffers.) If you use the three-argument
4581 form, then you can pass either a
4582 number, the name of a filehandle, or the normal "reference to a glob".
4584 Here is a script that saves, redirects, and restores C<STDOUT> and
4585 C<STDERR> using various methods:
4588 open(my $oldout, ">&STDOUT") or die "Can't dup STDOUT: $!";
4589 open(OLDERR, ">&", \*STDERR) or die "Can't dup STDERR: $!";
4591 open(STDOUT, '>', "foo.out") or die "Can't redirect STDOUT: $!";
4592 open(STDERR, ">&STDOUT") or die "Can't dup STDOUT: $!";
4594 select STDERR; $| = 1; # make unbuffered
4595 select STDOUT; $| = 1; # make unbuffered
4597 print STDOUT "stdout 1\n"; # this works for
4598 print STDERR "stderr 1\n"; # subprocesses too
4600 open(STDOUT, ">&", $oldout) or die "Can't dup \$oldout: $!";
4601 open(STDERR, ">&OLDERR") or die "Can't dup OLDERR: $!";
4603 print STDOUT "stdout 2\n";
4604 print STDERR "stderr 2\n";
4606 If you specify C<< '<&=X' >>, where C<X> is a file descriptor number
4607 or a filehandle, then Perl will do an equivalent of C's L<fdopen(3)> of
4608 that file descriptor (and not call L<dup(2)>); this is more
4609 parsimonious of file descriptors. For example:
4611 # open for input, reusing the fileno of $fd
4612 open(my $fh, "<&=", $fd)
4616 open(my $fh, "<&=$fd")
4620 # open for append, using the fileno of $oldfh
4621 open(my $fh, ">>&=", $oldfh)
4623 Being parsimonious on filehandles is also useful (besides being
4624 parsimonious) for example when something is dependent on file
4625 descriptors, like for example locking using
4626 L<C<flock>|/flock FILEHANDLE,OPERATION>. If you do just
4627 C<< open(my $A, ">>&", $B) >>, the filehandle C<$A> will not have the
4628 same file descriptor as C<$B>, and therefore C<flock($A)> will not
4629 C<flock($B)> nor vice versa. But with C<< open(my $A, ">>&=", $B) >>,
4630 the filehandles will share the same underlying system file descriptor.
4632 Note that under Perls older than 5.8.0, Perl uses the standard C library's'
4633 L<fdopen(3)> to implement the C<=> functionality. On many Unix systems,
4634 L<fdopen(3)> fails when file descriptors exceed a certain value, typically 255.
4635 For Perls 5.8.0 and later, PerlIO is (most often) the default.
4637 You can see whether your Perl was built with PerlIO by running
4638 C<perl -V:useperlio>. If it says C<'define'>, you have PerlIO;
4639 otherwise you don't.
4641 If you open a pipe on the command C<-> (that is, specify either C<|-> or C<-|>
4642 with the one- or two-argument forms of
4643 L<C<open>|/open FILEHANDLE,EXPR>), an implicit L<C<fork>|/fork> is done,
4644 so L<C<open>|/open FILEHANDLE,EXPR> returns twice: in the parent process
4646 of the child process, and in the child process it returns (a defined) C<0>.
4647 Use C<defined($pid)> or C<//> to determine whether the open was successful.
4649 For example, use either
4651 my $child_pid = open(my $from_kid, "-|") // die "Can't fork: $!";
4655 my $child_pid = open(my $to_kid, "|-") // die "Can't fork: $!";
4661 # either write $to_kid or else read $from_kid
4663 waitpid $child_pid, 0;
4665 # am the child; use STDIN/STDOUT normally
4670 The filehandle behaves normally for the parent, but I/O to that
4671 filehandle is piped from/to the STDOUT/STDIN of the child process.
4672 In the child process, the filehandle isn't opened--I/O happens from/to
4673 the new STDOUT/STDIN. Typically this is used like the normal
4674 piped open when you want to exercise more control over just how the
4675 pipe command gets executed, such as when running setuid and
4676 you don't want to have to scan shell commands for metacharacters.
4678 The following blocks are more or less equivalent:
4680 open(my $fh, "|tr '[a-z]' '[A-Z]'");
4681 open(my $fh, "|-", "tr '[a-z]' '[A-Z]'");
4682 open(my $fh, "|-") || exec 'tr', '[a-z]', '[A-Z]';
4683 open(my $fh, "|-", "tr", '[a-z]', '[A-Z]');