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a0d0e21e 1=head1 NAME
d74e8afc 2X<function>
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3
4perlfunc - Perl builtin functions
5
6=head1 DESCRIPTION
7
8The functions in this section can serve as terms in an expression.
9They fall into two major categories: list operators and named unary
10operators. These differ in their precedence relationship with a
11following comma. (See the precedence table in L<perlop>.) List
12operators take more than one argument, while unary operators can never
13take more than one argument. Thus, a comma terminates the argument of
14a unary operator, but merely separates the arguments of a list
8f1da26d 15operator. A unary operator generally provides scalar context to its
2b5ab1e7 16argument, while a list operator may provide either scalar or list
3b10bc60 17contexts for its arguments. If it does both, scalar arguments
18come first and list argument follow, and there can only ever
19be one such list argument. For instance, splice() has three scalar
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20arguments followed by a list, whereas gethostbyname() has four scalar
21arguments.
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22
23In the syntax descriptions that follow, list operators that expect a
3b10bc60 24list (and provide list context for elements of the list) are shown
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25with LIST as an argument. Such a list may consist of any combination
26of scalar arguments or list values; the list values will be included
27in the list as if each individual element were interpolated at that
28point in the list, forming a longer single-dimensional list value.
8bdbc703 29Commas should separate literal elements of the LIST.
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30
31Any function in the list below may be used either with or without
32parentheses around its arguments. (The syntax descriptions omit the
3b10bc60 33parentheses.) If you use parentheses, the simple but occasionally
34surprising rule is this: It I<looks> like a function, therefore it I<is> a
a0d0e21e 35function, and precedence doesn't matter. Otherwise it's a list
3b10bc60 36operator or unary operator, and precedence does matter. Whitespace
37between the function and left parenthesis doesn't count, so sometimes
38you need to be careful:
a0d0e21e 39
5ed4f2ec 40 print 1+2+4; # Prints 7.
41 print(1+2) + 4; # Prints 3.
42 print (1+2)+4; # Also prints 3!
43 print +(1+2)+4; # Prints 7.
44 print ((1+2)+4); # Prints 7.
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45
46If you run Perl with the B<-w> switch it can warn you about this. For
47example, the third line above produces:
48
49 print (...) interpreted as function at - line 1.
50 Useless use of integer addition in void context at - line 1.
51
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52A few functions take no arguments at all, and therefore work as neither
53unary nor list operators. These include such functions as C<time>
54and C<endpwent>. For example, C<time+86_400> always means
55C<time() + 86_400>.
56
a0d0e21e 57For functions that can be used in either a scalar or list context,
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58nonabortive failure is generally indicated in scalar context by
59returning the undefined value, and in list context by returning the
3b10bc60 60empty list.
a0d0e21e 61
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62Remember the following important rule: There is B<no rule> that relates
63the behavior of an expression in list context to its behavior in scalar
64context, or vice versa. It might do two totally different things.
80d38338 65Each operator and function decides which sort of value would be most
2b5ab1e7 66appropriate to return in scalar context. Some operators return the
5a964f20 67length of the list that would have been returned in list context. Some
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68operators return the first value in the list. Some operators return the
69last value in the list. Some operators return a count of successful
70operations. In general, they do what you want, unless you want
71consistency.
d74e8afc 72X<context>
a0d0e21e 73
d1be9408 74A named array in scalar context is quite different from what would at
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75first glance appear to be a list in scalar context. You can't get a list
76like C<(1,2,3)> into being in scalar context, because the compiler knows
77the context at compile time. It would generate the scalar comma operator
78there, not the list construction version of the comma. That means it
79was never a list to start with.
80
3b10bc60 81In general, functions in Perl that serve as wrappers for system calls ("syscalls")
5dac7880 82of the same name (like chown(2), fork(2), closedir(2), etc.) return
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83true when they succeed and C<undef> otherwise, as is usually mentioned
84in the descriptions below. This is different from the C interfaces,
5dac7880 85which return C<-1> on failure. Exceptions to this rule include C<wait>,
19799a22 86C<waitpid>, and C<syscall>. System calls also set the special C<$!>
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87variable on failure. Other functions do not, except accidentally.
88
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89Extension modules can also hook into the Perl parser to define new
90kinds of keyword-headed expression. These may look like functions, but
91may also look completely different. The syntax following the keyword
92is defined entirely by the extension. If you are an implementor, see
93L<perlapi/PL_keyword_plugin> for the mechanism. If you are using such
94a module, see the module's documentation for details of the syntax that
95it defines.
96
cb1a09d0 97=head2 Perl Functions by Category
d74e8afc 98X<function>
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99
100Here are Perl's functions (including things that look like
5a964f20 101functions, like some keywords and named operators)
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102arranged by category. Some functions appear in more
103than one place.
104
13a2d996 105=over 4
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106
107=item Functions for SCALARs or strings
d74e8afc 108X<scalar> X<string> X<character>
cb1a09d0 109
22fae026 110C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>,
1dc8ecb8 111C<length>, C<oct>, C<ord>, C<pack>, C<q//>, C<qq//>, C<reverse>,
945c54fd 112C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///>
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113
114=item Regular expressions and pattern matching
d74e8afc 115X<regular expression> X<regex> X<regexp>
cb1a09d0 116
ab4f32c2 117C<m//>, C<pos>, C<quotemeta>, C<s///>, C<split>, C<study>, C<qr//>
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118
119=item Numeric functions
d74e8afc 120X<numeric> X<number> X<trigonometric> X<trigonometry>
cb1a09d0 121
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122C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>,
123C<sin>, C<sqrt>, C<srand>
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124
125=item Functions for real @ARRAYs
d74e8afc 126X<array>
cb1a09d0 127
a5ce339c 128C<each>, C<keys>, C<pop>, C<push>, C<shift>, C<splice>, C<unshift>, C<values>
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129
130=item Functions for list data
d74e8afc 131X<list>
cb1a09d0 132
1dc8ecb8 133C<grep>, C<join>, C<map>, C<qw//>, C<reverse>, C<sort>, C<unpack>
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134
135=item Functions for real %HASHes
d74e8afc 136X<hash>
cb1a09d0 137
22fae026 138C<delete>, C<each>, C<exists>, C<keys>, C<values>
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139
140=item Input and output functions
d74e8afc 141X<I/O> X<input> X<output> X<dbm>
cb1a09d0 142
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143C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
144C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
0d863452 145C<readdir>, C<rewinddir>, C<say>, C<seek>, C<seekdir>, C<select>, C<syscall>,
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146C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>,
147C<warn>, C<write>
cb1a09d0 148
5dac7880 149=item Functions for fixed-length data or records
cb1a09d0 150
22fae026 151C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec>
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152
153=item Functions for filehandles, files, or directories
d74e8afc 154X<file> X<filehandle> X<directory> X<pipe> X<link> X<symlink>
cb1a09d0 155
22fae026 156C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>,
5ff3f7a4 157C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>,
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158C<readlink>, C<rename>, C<rmdir>, C<stat>, C<symlink>, C<sysopen>,
159C<umask>, C<unlink>, C<utime>
cb1a09d0 160
cf264981 161=item Keywords related to the control flow of your Perl program
d74e8afc 162X<control flow>
cb1a09d0 163
98293880 164C<caller>, C<continue>, C<die>, C<do>, C<dump>, C<eval>, C<exit>,
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165C<__FILE__>, C<goto>, C<last>, C<__LINE__>, C<next>, C<__PACKAGE__>,
166C<redo>, C<return>, C<sub>, C<wantarray>,
cb1a09d0 167
8f1da26d 168=item Keywords related to the switch feature
0d863452 169
4a904372 170C<break>, C<continue>, C<default>, C<given>, C<when>
0d863452 171
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172Except for C<continue>, these are available only if you enable the
173C<"switch"> feature or use the C<CORE::> prefix.
8f1da26d 174See L<feature> and L<perlsyn/"Switch statements">.
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175Alternately, include a C<use v5.10> or later to the current scope. In Perl
1765.14 and earlier, C<continue> required the C<"switch"> feature, like the
177other keywords.
0d863452 178
54310121 179=item Keywords related to scoping
cb1a09d0 180
8f1da26d 181C<caller>, C<import>, C<local>, C<my>, C<our>, C<package>, C<state>, C<use>
36fb85f3 182
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183C<state> is available only if the C<"state"> feature
184is enabled or if it is prefixed with C<CORE::>. See
8f1da26d 185L<feature>. Alternately, include a C<use v5.10> or later to the current scope.
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186
187=item Miscellaneous functions
188
36fb85f3 189C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<our>,
834df1c5 190C<reset>, C<scalar>, C<state>, C<undef>, C<wantarray>
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191
192=item Functions for processes and process groups
d74e8afc 193X<process> X<pid> X<process id>
cb1a09d0 194
22fae026 195C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
1dc8ecb8 196C<pipe>, C<qx//>, C<setpgrp>, C<setpriority>, C<sleep>, C<system>,
22fae026 197C<times>, C<wait>, C<waitpid>
cb1a09d0 198
3b10bc60 199=item Keywords related to Perl modules
d74e8afc 200X<module>
cb1a09d0 201
22fae026 202C<do>, C<import>, C<no>, C<package>, C<require>, C<use>
cb1a09d0 203
353c6505 204=item Keywords related to classes and object-orientation
d74e8afc 205X<object> X<class> X<package>
cb1a09d0 206
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207C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
208C<untie>, C<use>
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209
210=item Low-level socket functions
d74e8afc 211X<socket> X<sock>
cb1a09d0 212
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213C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
214C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
737dd4b4 215C<socket>, C<socketpair>
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216
217=item System V interprocess communication functions
d74e8afc 218X<IPC> X<System V> X<semaphore> X<shared memory> X<memory> X<message>
cb1a09d0 219
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220C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
221C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>
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222
223=item Fetching user and group info
d74e8afc 224X<user> X<group> X<password> X<uid> X<gid> X<passwd> X</etc/passwd>
cb1a09d0 225
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226C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
227C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
228C<getpwuid>, C<setgrent>, C<setpwent>
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229
230=item Fetching network info
d74e8afc 231X<network> X<protocol> X<host> X<hostname> X<IP> X<address> X<service>
cb1a09d0 232
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233C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
234C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
235C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
236C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
237C<setnetent>, C<setprotoent>, C<setservent>
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238
239=item Time-related functions
d74e8afc 240X<time> X<date>
cb1a09d0 241
22fae026 242C<gmtime>, C<localtime>, C<time>, C<times>
cb1a09d0 243
37798a01 244=item Functions new in perl5
d74e8afc 245X<perl5>
37798a01 246
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247C<abs>, C<bless>, C<break>, C<chomp>, C<chr>, C<continue>, C<default>,
248C<exists>, C<formline>, C<given>, C<glob>, C<import>, C<lc>, C<lcfirst>,
1dc8ecb8 249C<lock>, C<map>, C<my>, C<no>, C<our>, C<prototype>, C<qr//>, C<qw//>, C<qx//>,
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250C<readline>, C<readpipe>, C<ref>, C<sub>*, C<sysopen>, C<tie>, C<tied>, C<uc>,
251C<ucfirst>, C<untie>, C<use>, C<when>
37798a01 252
3b10bc60 253* C<sub> was a keyword in Perl 4, but in Perl 5 it is an
5a964f20 254operator, which can be used in expressions.
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255
256=item Functions obsoleted in perl5
257
22fae026 258C<dbmclose>, C<dbmopen>
37798a01 259
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260=back
261
60f9f73c 262=head2 Portability
d74e8afc 263X<portability> X<Unix> X<portable>
60f9f73c 264
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265Perl was born in Unix and can therefore access all common Unix
266system calls. In non-Unix environments, the functionality of some
8f1da26d 267Unix system calls may not be available or details of the available
2b5ab1e7 268functionality may differ slightly. The Perl functions affected
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269by this are:
270
271C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>,
272C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>,
273C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>,
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274C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostbyname>,
275C<gethostent>, C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
54d7b083 276C<getppid>, C<getpgrp>, C<getpriority>, C<getprotobynumber>,
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277C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>,
278C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>,
279C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>,
2b5ab1e7 280C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<rename>, C<select>, C<semctl>,
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281C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>,
282C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>,
283C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>,
737dd4b4 284C<shmwrite>, C<socket>, C<socketpair>,
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285C<stat>, C<symlink>, C<syscall>, C<sysopen>, C<system>,
286C<times>, C<truncate>, C<umask>, C<unlink>,
2b5ab1e7 287C<utime>, C<wait>, C<waitpid>
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288
289For more information about the portability of these functions, see
290L<perlport> and other available platform-specific documentation.
291
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292=head2 Alphabetical Listing of Perl Functions
293
3b10bc60 294=over
a0d0e21e 295
5b3c99c0 296=item -X FILEHANDLE
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297X<-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>
298X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
a0d0e21e 299
5b3c99c0 300=item -X EXPR
a0d0e21e 301
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302=item -X DIRHANDLE
303
5b3c99c0 304=item -X
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305
306A file test, where X is one of the letters listed below. This unary
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307operator takes one argument, either a filename, a filehandle, or a dirhandle,
308and tests the associated file to see if something is true about it. If the
7660c0ab 309argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
19799a22 310Unless otherwise documented, it returns C<1> for true and C<''> for false, or
a0d0e21e 311the undefined value if the file doesn't exist. Despite the funny
d0821a6a 312names, precedence is the same as any other named unary operator. The
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313operator may be any of:
314
5ed4f2ec 315 -r File is readable by effective uid/gid.
316 -w File is writable by effective uid/gid.
317 -x File is executable by effective uid/gid.
318 -o File is owned by effective uid.
a0d0e21e 319
5ed4f2ec 320 -R File is readable by real uid/gid.
321 -W File is writable by real uid/gid.
322 -X File is executable by real uid/gid.
323 -O File is owned by real uid.
a0d0e21e 324
5ed4f2ec 325 -e File exists.
326 -z File has zero size (is empty).
327 -s File has nonzero size (returns size in bytes).
a0d0e21e 328
5ed4f2ec 329 -f File is a plain file.
330 -d File is a directory.
331 -l File is a symbolic link.
332 -p File is a named pipe (FIFO), or Filehandle is a pipe.
333 -S File is a socket.
334 -b File is a block special file.
335 -c File is a character special file.
336 -t Filehandle is opened to a tty.
a0d0e21e 337
5ed4f2ec 338 -u File has setuid bit set.
339 -g File has setgid bit set.
340 -k File has sticky bit set.
a0d0e21e 341
5ed4f2ec 342 -T File is an ASCII text file (heuristic guess).
343 -B File is a "binary" file (opposite of -T).
a0d0e21e 344
5ed4f2ec 345 -M Script start time minus file modification time, in days.
346 -A Same for access time.
347 -C Same for inode change time (Unix, may differ for other platforms)
a0d0e21e 348
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349Example:
350
351 while (<>) {
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352 chomp;
353 next unless -f $_; # ignore specials
354 #...
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355 }
356
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357Note that C<-s/a/b/> does not do a negated substitution. Saying
358C<-exp($foo)> still works as expected, however: only single letters
359following a minus are interpreted as file tests.
360
361These operators are exempt from the "looks like a function rule" described
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FC
362above. That is, an opening parenthesis after the operator does not affect
363how much of the following code constitutes the argument. Put the opening
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FC
364parentheses before the operator to separate it from code that follows (this
365applies only to operators with higher precedence than unary operators, of
366course):
367
368 -s($file) + 1024 # probably wrong; same as -s($file + 1024)
369 (-s $file) + 1024 # correct
370
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371The interpretation of the file permission operators C<-r>, C<-R>,
372C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
373of the file and the uids and gids of the user. There may be other
ecae030f
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374reasons you can't actually read, write, or execute the file: for
375example network filesystem access controls, ACLs (access control lists),
376read-only filesystems, and unrecognized executable formats. Note
377that the use of these six specific operators to verify if some operation
378is possible is usually a mistake, because it may be open to race
379conditions.
5ff3f7a4 380
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381Also note that, for the superuser on the local filesystems, the C<-r>,
382C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
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383if any execute bit is set in the mode. Scripts run by the superuser
384may thus need to do a stat() to determine the actual mode of the file,
2b5ab1e7 385or temporarily set their effective uid to something else.
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386
387If you are using ACLs, there is a pragma called C<filetest> that may
388produce more accurate results than the bare stat() mode bits.
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389When under C<use filetest 'access'> the above-mentioned filetests
390test whether the permission can(not) be granted using the
3b10bc60 391access(2) family of system calls. Also note that the C<-x> and C<-X> may
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392under this pragma return true even if there are no execute permission
393bits set (nor any extra execute permission ACLs). This strangeness is
ecae030f
MO
394due to the underlying system calls' definitions. Note also that, due to
395the implementation of C<use filetest 'access'>, the C<_> special
396filehandle won't cache the results of the file tests when this pragma is
397in effect. Read the documentation for the C<filetest> pragma for more
398information.
5ff3f7a4 399
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400The C<-T> and C<-B> switches work as follows. The first block or so of the
401file is examined for odd characters such as strange control codes or
61eff3bc 402characters with the high bit set. If too many strange characters (>30%)
cf264981 403are found, it's a C<-B> file; otherwise it's a C<-T> file. Also, any file
3b10bc60 404containing a zero byte in the first block is considered a binary file. If C<-T>
9124316e 405or C<-B> is used on a filehandle, the current IO buffer is examined
3b10bc60 406rather than the first block. Both C<-T> and C<-B> return true on an empty
54310121 407file, or a file at EOF when testing a filehandle. Because you have to
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408read a file to do the C<-T> test, on most occasions you want to use a C<-f>
409against the file first, as in C<next unless -f $file && -T $file>.
a0d0e21e 410
5dac7880 411If any of the file tests (or either the C<stat> or C<lstat> operator) is given
28757baa 412the special filehandle consisting of a solitary underline, then the stat
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413structure of the previous file test (or stat operator) is used, saving
414a system call. (This doesn't work with C<-t>, and you need to remember
3b10bc60 415that lstat() and C<-l> leave values in the stat structure for the
5c9aa243 416symbolic link, not the real file.) (Also, if the stat buffer was filled by
cf264981 417an C<lstat> call, C<-T> and C<-B> will reset it with the results of C<stat _>).
5c9aa243 418Example:
a0d0e21e
LW
419
420 print "Can do.\n" if -r $a || -w _ || -x _;
421
422 stat($filename);
423 print "Readable\n" if -r _;
424 print "Writable\n" if -w _;
425 print "Executable\n" if -x _;
426 print "Setuid\n" if -u _;
427 print "Setgid\n" if -g _;
428 print "Sticky\n" if -k _;
429 print "Text\n" if -T _;
430 print "Binary\n" if -B _;
431
fbb0b3b3
RGS
432As of Perl 5.9.1, as a form of purely syntactic sugar, you can stack file
433test operators, in a way that C<-f -w -x $file> is equivalent to
3b10bc60 434C<-x $file && -w _ && -f _>. (This is only fancy fancy: if you use
fbb0b3b3
RGS
435the return value of C<-f $file> as an argument to another filetest
436operator, no special magic will happen.)
437
bee96257 438Portability issues: L<perlport/-X>.
ea9eb35a 439
a0d0e21e 440=item abs VALUE
d74e8afc 441X<abs> X<absolute>
a0d0e21e 442
54310121 443=item abs
bbce6d69 444
a0d0e21e 445Returns the absolute value of its argument.
7660c0ab 446If VALUE is omitted, uses C<$_>.
a0d0e21e
LW
447
448=item accept NEWSOCKET,GENERICSOCKET
d74e8afc 449X<accept>
a0d0e21e 450
3b10bc60 451Accepts an incoming socket connect, just as accept(2)
19799a22 452does. Returns the packed address if it succeeded, false otherwise.
2b5ab1e7 453See the example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 454
8d2a6795
GS
455On systems that support a close-on-exec flag on files, the flag will
456be set for the newly opened file descriptor, as determined by the
457value of $^F. See L<perlvar/$^F>.
458
a0d0e21e 459=item alarm SECONDS
d74e8afc
ITB
460X<alarm>
461X<SIGALRM>
462X<timer>
a0d0e21e 463
54310121 464=item alarm
bbce6d69 465
a0d0e21e 466Arranges to have a SIGALRM delivered to this process after the
cf264981 467specified number of wallclock seconds has elapsed. If SECONDS is not
d400eac8
JH
468specified, the value stored in C<$_> is used. (On some machines,
469unfortunately, the elapsed time may be up to one second less or more
470than you specified because of how seconds are counted, and process
471scheduling may delay the delivery of the signal even further.)
472
473Only one timer may be counting at once. Each call disables the
474previous timer, and an argument of C<0> may be supplied to cancel the
475previous timer without starting a new one. The returned value is the
476amount of time remaining on the previous timer.
a0d0e21e 477
2bc69794
BS
478For delays of finer granularity than one second, the Time::HiRes module
479(from CPAN, and starting from Perl 5.8 part of the standard
480distribution) provides ualarm(). You may also use Perl's four-argument
481version of select() leaving the first three arguments undefined, or you
482might be able to use the C<syscall> interface to access setitimer(2) if
483your system supports it. See L<perlfaq8> for details.
2b5ab1e7 484
80d38338
TC
485It is usually a mistake to intermix C<alarm> and C<sleep> calls, because
486C<sleep> may be internally implemented on your system with C<alarm>.
a0d0e21e 487
19799a22
GS
488If you want to use C<alarm> to time out a system call you need to use an
489C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
f86cebdf 490fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
19799a22 491restart system calls on some systems. Using C<eval>/C<die> always works,
5a964f20 492modulo the caveats given in L<perlipc/"Signals">.
ff68c719
PP
493
494 eval {
a9a5a0dc
VP
495 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
496 alarm $timeout;
497 $nread = sysread SOCKET, $buffer, $size;
498 alarm 0;
ff68c719 499 };
ff68c719 500 if ($@) {
a9a5a0dc 501 die unless $@ eq "alarm\n"; # propagate unexpected errors
5ed4f2ec 502 # timed out
ff68c719
PP
503 }
504 else {
5ed4f2ec 505 # didn't
ff68c719
PP
506 }
507
91d81acc
JH
508For more information see L<perlipc>.
509
ea9eb35a 510Portability issues: L<perlport/alarm>.
511
a0d0e21e 512=item atan2 Y,X
d74e8afc 513X<atan2> X<arctangent> X<tan> X<tangent>
a0d0e21e
LW
514
515Returns the arctangent of Y/X in the range -PI to PI.
516
ca6e1c26 517For the tangent operation, you may use the C<Math::Trig::tan>
28757baa
PP
518function, or use the familiar relation:
519
520 sub tan { sin($_[0]) / cos($_[0]) }
521
a1021d57
RGS
522The return value for C<atan2(0,0)> is implementation-defined; consult
523your atan2(3) manpage for more information.
bf5f1b4c 524
ea9eb35a 525Portability issues: L<perlport/atan2>.
526
a0d0e21e 527=item bind SOCKET,NAME
d74e8afc 528X<bind>
a0d0e21e 529
3b10bc60 530Binds a network address to a socket, just as bind(2)
19799a22 531does. Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
532packed address of the appropriate type for the socket. See the examples in
533L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 534
fae2c0fb 535=item binmode FILEHANDLE, LAYER
d74e8afc 536X<binmode> X<binary> X<text> X<DOS> X<Windows>
1c1fc3ea 537
a0d0e21e
LW
538=item binmode FILEHANDLE
539
1cbfc93d
NIS
540Arranges for FILEHANDLE to be read or written in "binary" or "text"
541mode on systems where the run-time libraries distinguish between
542binary and text files. If FILEHANDLE is an expression, the value is
543taken as the name of the filehandle. Returns true on success,
b5fe5ca2 544otherwise it returns C<undef> and sets C<$!> (errno).
1cbfc93d 545
8f1da26d 546On some systems (in general, DOS- and Windows-based systems) binmode()
d807c6f4 547is necessary when you're not working with a text file. For the sake
d7a0d798
FC
548of portability it is a good idea always to use it when appropriate,
549and never to use it when it isn't appropriate. Also, people can
8f1da26d 550set their I/O to be by default UTF8-encoded Unicode, not bytes.
d807c6f4
JH
551
552In other words: regardless of platform, use binmode() on binary data,
d7a0d798 553like images, for example.
d807c6f4
JH
554
555If LAYER is present it is a single string, but may contain multiple
3b10bc60 556directives. The directives alter the behaviour of the filehandle.
d7a0d798 557When LAYER is present, using binmode on a text file makes sense.
d807c6f4 558
fae2c0fb 559If LAYER is omitted or specified as C<:raw> the filehandle is made
0226bbdb
NIS
560suitable for passing binary data. This includes turning off possible CRLF
561translation and marking it as bytes (as opposed to Unicode characters).
749683d2 562Note that, despite what may be implied in I<"Programming Perl"> (the
3b10bc60 563Camel, 3rd edition) or elsewhere, C<:raw> is I<not> simply the inverse of C<:crlf>.
564Other layers that would affect the binary nature of the stream are
565I<also> disabled. See L<PerlIO>, L<perlrun>, and the discussion about the
0226bbdb 566PERLIO environment variable.
01e6739c 567
3b10bc60 568The C<:bytes>, C<:crlf>, C<:utf8>, and any other directives of the
d807c6f4
JH
569form C<:...>, are called I/O I<layers>. The C<open> pragma can be used to
570establish default I/O layers. See L<open>.
571
fae2c0fb
RGS
572I<The LAYER parameter of the binmode() function is described as "DISCIPLINE"
573in "Programming Perl, 3rd Edition". However, since the publishing of this
574book, by many known as "Camel III", the consensus of the naming of this
575functionality has moved from "discipline" to "layer". All documentation
576of this version of Perl therefore refers to "layers" rather than to
577"disciplines". Now back to the regularly scheduled documentation...>
578
8f1da26d 579To mark FILEHANDLE as UTF-8, use C<:utf8> or C<:encoding(UTF-8)>.
6902c96a 580C<:utf8> just marks the data as UTF-8 without further checking,
8f1da26d 581while C<:encoding(UTF-8)> checks the data for actually being valid
6902c96a 582UTF-8. More details can be found in L<PerlIO::encoding>.
1cbfc93d 583
ed53a2bb 584In general, binmode() should be called after open() but before any I/O
3b10bc60 585is done on the filehandle. Calling binmode() normally flushes any
01e6739c 586pending buffered output data (and perhaps pending input data) on the
fae2c0fb 587handle. An exception to this is the C<:encoding> layer that
d7a0d798 588changes the default character encoding of the handle; see L</open>.
fae2c0fb 589The C<:encoding> layer sometimes needs to be called in
3874323d
JH
590mid-stream, and it doesn't flush the stream. The C<:encoding>
591also implicitly pushes on top of itself the C<:utf8> layer because
3b10bc60 592internally Perl operates on UTF8-encoded Unicode characters.
16fe6d59 593
19799a22 594The operating system, device drivers, C libraries, and Perl run-time
8f1da26d
TC
595system all conspire to let the programmer treat a single
596character (C<\n>) as the line terminator, irrespective of external
30168b04
GS
597representation. On many operating systems, the native text file
598representation matches the internal representation, but on some
599platforms the external representation of C<\n> is made up of more than
600one character.
601
8f1da26d
TC
602All variants of Unix, Mac OS (old and new), and Stream_LF files on VMS use
603a single character to end each line in the external representation of text
604(even though that single character is CARRIAGE RETURN on old, pre-Darwin
605flavors of Mac OS, and is LINE FEED on Unix and most VMS files). In other
606systems like OS/2, DOS, and the various flavors of MS-Windows, your program
607sees a C<\n> as a simple C<\cJ>, but what's stored in text files are the
608two characters C<\cM\cJ>. That means that if you don't use binmode() on
609these systems, C<\cM\cJ> sequences on disk will be converted to C<\n> on
610input, and any C<\n> in your program will be converted back to C<\cM\cJ> on
611output. This is what you want for text files, but it can be disastrous for
612binary files.
30168b04
GS
613
614Another consequence of using binmode() (on some systems) is that
615special end-of-file markers will be seen as part of the data stream.
d7a0d798
FC
616For systems from the Microsoft family this means that, if your binary
617data contain C<\cZ>, the I/O subsystem will regard it as the end of
30168b04
GS
618the file, unless you use binmode().
619
3b10bc60 620binmode() is important not only for readline() and print() operations,
30168b04
GS
621but also when using read(), seek(), sysread(), syswrite() and tell()
622(see L<perlport> for more details). See the C<$/> and C<$\> variables
623in L<perlvar> for how to manually set your input and output
624line-termination sequences.
a0d0e21e 625
ea9eb35a 626Portability issues: L<perlport/binmode>.
627
4633a7c4 628=item bless REF,CLASSNAME
d74e8afc 629X<bless>
a0d0e21e
LW
630
631=item bless REF
632
2b5ab1e7
TC
633This function tells the thingy referenced by REF that it is now an object
634in the CLASSNAME package. If CLASSNAME is omitted, the current package
19799a22 635is used. Because a C<bless> is often the last thing in a constructor,
2b5ab1e7 636it returns the reference for convenience. Always use the two-argument
cf264981 637version if a derived class might inherit the function doing the blessing.
82e1c0d9 638SeeL<perlobj> for more about the blessing (and blessings) of objects.
a0d0e21e 639
57668c4d 640Consider always blessing objects in CLASSNAMEs that are mixed case.
2b5ab1e7 641Namespaces with all lowercase names are considered reserved for
cf264981 642Perl pragmata. Builtin types have all uppercase names. To prevent
2b5ab1e7
TC
643confusion, you may wish to avoid such package names as well. Make sure
644that CLASSNAME is a true value.
60ad88b8
GS
645
646See L<perlmod/"Perl Modules">.
647
0d863452
RH
648=item break
649
650Break out of a C<given()> block.
651
8f1da26d 652This keyword is enabled by the C<"switch"> feature: see
4a904372
FC
653L<feature> for more information. You can also access it by
654prefixing it with C<CORE::>. Alternately, include a C<use
8f1da26d 655v5.10> or later to the current scope.
0d863452 656
a0d0e21e 657=item caller EXPR
d74e8afc 658X<caller> X<call stack> X<stack> X<stack trace>
a0d0e21e
LW
659
660=item caller
661
5a964f20 662Returns the context of the current subroutine call. In scalar context,
80d38338
TC
663returns the caller's package name if there I<is> a caller (that is, if
664we're in a subroutine or C<eval> or C<require>) and the undefined value
5a964f20 665otherwise. In list context, returns
a0d0e21e 666
ee6b43cc 667 # 0 1 2
748a9306 668 ($package, $filename, $line) = caller;
a0d0e21e
LW
669
670With EXPR, it returns some extra information that the debugger uses to
671print a stack trace. The value of EXPR indicates how many call frames
672to go back before the current one.
673
ee6b43cc 674 # 0 1 2 3 4
f3aa04c2 675 ($package, $filename, $line, $subroutine, $hasargs,
ee6b43cc 676
677 # 5 6 7 8 9 10
b3ca2e83 678 $wantarray, $evaltext, $is_require, $hints, $bitmask, $hinthash)
ee6b43cc 679 = caller($i);
e7ea3e70 680
951ba7fe 681Here $subroutine may be C<(eval)> if the frame is not a subroutine
19799a22 682call, but an C<eval>. In such a case additional elements $evaltext and
7660c0ab 683C<$is_require> are set: C<$is_require> is true if the frame is created by a
19799a22 684C<require> or C<use> statement, $evaltext contains the text of the
277ddfaf 685C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
cc1c2e42 686$subroutine is C<(eval)>, but $evaltext is undefined. (Note also that
0fc9dec4
RGS
687each C<use> statement creates a C<require> frame inside an C<eval EXPR>
688frame.) $subroutine may also be C<(unknown)> if this particular
689subroutine happens to have been deleted from the symbol table.
690C<$hasargs> is true if a new instance of C<@_> was set up for the frame.
691C<$hints> and C<$bitmask> contain pragmatic hints that the caller was
692compiled with. The C<$hints> and C<$bitmask> values are subject to change
693between versions of Perl, and are not meant for external use.
748a9306 694
b3ca2e83
NC
695C<$hinthash> is a reference to a hash containing the value of C<%^H> when the
696caller was compiled, or C<undef> if C<%^H> was empty. Do not modify the values
697of this hash, as they are the actual values stored in the optree.
698
748a9306 699Furthermore, when called from within the DB package, caller returns more
7660c0ab 700detailed information: it sets the list variable C<@DB::args> to be the
54310121 701arguments with which the subroutine was invoked.
748a9306 702
7660c0ab 703Be aware that the optimizer might have optimized call frames away before
19799a22 704C<caller> had a chance to get the information. That means that C<caller(N)>
80d38338 705might not return information about the call frame you expect it to, for
b76cc8ba 706C<< N > 1 >>. In particular, C<@DB::args> might have information from the
19799a22 707previous time C<caller> was called.
7660c0ab 708
8f1da26d 709Be aware that setting C<@DB::args> is I<best effort>, intended for
ca9f0cb5
NC
710debugging or generating backtraces, and should not be relied upon. In
711particular, as C<@_> contains aliases to the caller's arguments, Perl does
712not take a copy of C<@_>, so C<@DB::args> will contain modifications the
713subroutine makes to C<@_> or its contents, not the original values at call
714time. C<@DB::args>, like C<@_>, does not hold explicit references to its
715elements, so under certain cases its elements may have become freed and
716reallocated for other variables or temporary values. Finally, a side effect
d7a0d798 717of the current implementation is that the effects of C<shift @_> can
8f1da26d
TC
718I<normally> be undone (but not C<pop @_> or other splicing, I<and> not if a
719reference to C<@_> has been taken, I<and> subject to the caveat about reallocated
ca9f0cb5
NC
720elements), so C<@DB::args> is actually a hybrid of the current state and
721initial state of C<@_>. Buyer beware.
722
a0d0e21e 723=item chdir EXPR
d74e8afc
ITB
724X<chdir>
725X<cd>
f723aae1 726X<directory, change>
a0d0e21e 727
c4aca7d0
GA
728=item chdir FILEHANDLE
729
730=item chdir DIRHANDLE
731
ce2984c3
PF
732=item chdir
733
ffce7b87 734Changes the working directory to EXPR, if possible. If EXPR is omitted,
0bfc1ec4 735changes to the directory specified by C<$ENV{HOME}>, if set; if not,
ffce7b87 736changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
b4ad75f0 737variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
80d38338 738neither is set, C<chdir> does nothing. It returns true on success,
b4ad75f0 739false otherwise. See the example under C<die>.
a0d0e21e 740
3b10bc60 741On systems that support fchdir(2), you may pass a filehandle or
34169887 742directory handle as the argument. On systems that don't support fchdir(2),
3b10bc60 743passing handles raises an exception.
c4aca7d0 744
a0d0e21e 745=item chmod LIST
d74e8afc 746X<chmod> X<permission> X<mode>
a0d0e21e
LW
747
748Changes the permissions of a list of files. The first element of the
8f1da26d 749list must be the numeric mode, which should probably be an octal
4ad40acf 750number, and which definitely should I<not> be a string of octal digits:
3b10bc60 751C<0644> is okay, but C<"0644"> is not. Returns the number of files
8f1da26d 752successfully changed. See also L</oct> if all you have is a string.
a0d0e21e 753
3b10bc60 754 $cnt = chmod 0755, "foo", "bar";
a0d0e21e 755 chmod 0755, @executables;
3b10bc60 756 $mode = "0644"; chmod $mode, "foo"; # !!! sets mode to
f86cebdf 757 # --w----r-T
3b10bc60 758 $mode = "0644"; chmod oct($mode), "foo"; # this is better
759 $mode = 0644; chmod $mode, "foo"; # this is best
a0d0e21e 760
3b10bc60 761On systems that support fchmod(2), you may pass filehandles among the
762files. On systems that don't support fchmod(2), passing filehandles raises
763an exception. Filehandles must be passed as globs or glob references to be
764recognized; barewords are considered filenames.
c4aca7d0
GA
765
766 open(my $fh, "<", "foo");
767 my $perm = (stat $fh)[2] & 07777;
768 chmod($perm | 0600, $fh);
769
3b10bc60 770You can also import the symbolic C<S_I*> constants from the C<Fcntl>
ca6e1c26
JH
771module:
772
3b10bc60 773 use Fcntl qw( :mode );
ca6e1c26 774 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
3b10bc60 775 # Identical to the chmod 0755 of the example above.
ca6e1c26 776
ea9eb35a 777Portability issues: L<perlport/chmod>.
778
a0d0e21e 779=item chomp VARIABLE
d74e8afc 780X<chomp> X<INPUT_RECORD_SEPARATOR> X<$/> X<newline> X<eol>
a0d0e21e 781
313c9f5c 782=item chomp( LIST )
a0d0e21e
LW
783
784=item chomp
785
2b5ab1e7
TC
786This safer version of L</chop> removes any trailing string
787that corresponds to the current value of C<$/> (also known as
28757baa
PP
788$INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
789number of characters removed from all its arguments. It's often used to
790remove the newline from the end of an input record when you're worried
2b5ab1e7
TC
791that the final record may be missing its newline. When in paragraph
792mode (C<$/ = "">), it removes all trailing newlines from the string.
4c5a6083 793When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
34169887 794a reference to an integer or the like; see L<perlvar>) chomp() won't
b76cc8ba 795remove anything.
19799a22 796If VARIABLE is omitted, it chomps C<$_>. Example:
a0d0e21e
LW
797
798 while (<>) {
a9a5a0dc
VP
799 chomp; # avoid \n on last field
800 @array = split(/:/);
801 # ...
a0d0e21e
LW
802 }
803
4bf21a6d
RD
804If VARIABLE is a hash, it chomps the hash's values, but not its keys.
805
a0d0e21e
LW
806You can actually chomp anything that's an lvalue, including an assignment:
807
808 chomp($cwd = `pwd`);
809 chomp($answer = <STDIN>);
810
811If you chomp a list, each element is chomped, and the total number of
812characters removed is returned.
813
15e44fd8
RGS
814Note that parentheses are necessary when you're chomping anything
815that is not a simple variable. This is because C<chomp $cwd = `pwd`;>
816is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as
817C<chomp( $cwd = `pwd` )> which you might expect. Similarly,
818C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than
819as C<chomp($a, $b)>.
820
a0d0e21e 821=item chop VARIABLE
d74e8afc 822X<chop>
a0d0e21e 823
313c9f5c 824=item chop( LIST )
a0d0e21e
LW
825
826=item chop
827
828Chops off the last character of a string and returns the character
5b3eff12 829chopped. It is much more efficient than C<s/.$//s> because it neither
7660c0ab 830scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
4bf21a6d
RD
831If VARIABLE is a hash, it chops the hash's values, but not its keys.
832
5b3eff12 833You can actually chop anything that's an lvalue, including an assignment.
a0d0e21e
LW
834
835If you chop a list, each element is chopped. Only the value of the
19799a22 836last C<chop> is returned.
a0d0e21e 837
19799a22 838Note that C<chop> returns the last character. To return all but the last
748a9306
LW
839character, use C<substr($string, 0, -1)>.
840
15e44fd8
RGS
841See also L</chomp>.
842
a0d0e21e 843=item chown LIST
d74e8afc 844X<chown> X<owner> X<user> X<group>
a0d0e21e
LW
845
846Changes the owner (and group) of a list of files. The first two
19799a22
GS
847elements of the list must be the I<numeric> uid and gid, in that
848order. A value of -1 in either position is interpreted by most
849systems to leave that value unchanged. Returns the number of files
850successfully changed.
a0d0e21e
LW
851
852 $cnt = chown $uid, $gid, 'foo', 'bar';
853 chown $uid, $gid, @filenames;
854
3b10bc60 855On systems that support fchown(2), you may pass filehandles among the
856files. On systems that don't support fchown(2), passing filehandles raises
857an exception. Filehandles must be passed as globs or glob references to be
858recognized; barewords are considered filenames.
c4aca7d0 859
54310121 860Here's an example that looks up nonnumeric uids in the passwd file:
a0d0e21e
LW
861
862 print "User: ";
19799a22 863 chomp($user = <STDIN>);
5a964f20 864 print "Files: ";
19799a22 865 chomp($pattern = <STDIN>);
a0d0e21e
LW
866
867 ($login,$pass,$uid,$gid) = getpwnam($user)
a9a5a0dc 868 or die "$user not in passwd file";
a0d0e21e 869
5ed4f2ec 870 @ary = glob($pattern); # expand filenames
a0d0e21e
LW
871 chown $uid, $gid, @ary;
872
54310121 873On most systems, you are not allowed to change the ownership of the
4633a7c4
LW
874file unless you're the superuser, although you should be able to change
875the group to any of your secondary groups. On insecure systems, these
876restrictions may be relaxed, but this is not a portable assumption.
19799a22
GS
877On POSIX systems, you can detect this condition this way:
878
879 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
880 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
4633a7c4 881
ea9eb35a 882Portability issues: L<perlport/chmod>.
883
a0d0e21e 884=item chr NUMBER
d74e8afc 885X<chr> X<character> X<ASCII> X<Unicode>
a0d0e21e 886
54310121 887=item chr
bbce6d69 888
a0d0e21e 889Returns the character represented by that NUMBER in the character set.
a0ed51b3 890For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
2575c402 891chr(0x263a) is a Unicode smiley face.
aaa68c4a 892
8a064bd6 893Negative values give the Unicode replacement character (chr(0xfffd)),
80d38338 894except under the L<bytes> pragma, where the low eight bits of the value
8a064bd6
JH
895(truncated to an integer) are used.
896
974da8e5
JH
897If NUMBER is omitted, uses C<$_>.
898
b76cc8ba 899For the reverse, use L</ord>.
a0d0e21e 900
2575c402
JW
901Note that characters from 128 to 255 (inclusive) are by default
902internally not encoded as UTF-8 for backward compatibility reasons.
974da8e5 903
2575c402 904See L<perlunicode> for more about Unicode.
bbce6d69 905
a0d0e21e 906=item chroot FILENAME
d74e8afc 907X<chroot> X<root>
a0d0e21e 908
54310121 909=item chroot
bbce6d69 910
5a964f20 911This function works like the system call by the same name: it makes the
4633a7c4 912named directory the new root directory for all further pathnames that
951ba7fe 913begin with a C</> by your process and all its children. (It doesn't
28757baa 914change your current working directory, which is unaffected.) For security
4633a7c4 915reasons, this call is restricted to the superuser. If FILENAME is
19799a22 916omitted, does a C<chroot> to C<$_>.
a0d0e21e 917
ea9eb35a 918Portability issues: L<perlport/chroot>.
919
a0d0e21e 920=item close FILEHANDLE
d74e8afc 921X<close>
a0d0e21e 922
6a518fbc
TP
923=item close
924
3b10bc60 925Closes the file or pipe associated with the filehandle, flushes the IO
e0f13c26 926buffers, and closes the system file descriptor. Returns true if those
8f1da26d 927operations succeed and if no error was reported by any PerlIO
e0f13c26
RGS
928layer. Closes the currently selected filehandle if the argument is
929omitted.
fb73857a
PP
930
931You don't have to close FILEHANDLE if you are immediately going to do
3b10bc60 932another C<open> on it, because C<open> closes it for you. (See
01aa884e 933L<open|/open FILEHANDLE>.) However, an explicit C<close> on an input file resets the line
19799a22 934counter (C<$.>), while the implicit close done by C<open> does not.
fb73857a 935
3b10bc60 936If the filehandle came from a piped open, C<close> returns false if one of
937the other syscalls involved fails or if its program exits with non-zero
938status. If the only problem was that the program exited non-zero, C<$!>
939will be set to C<0>. Closing a pipe also waits for the process executing
940on the pipe to exit--in case you wish to look at the output of the pipe
941afterwards--and implicitly puts the exit status value of that command into
942C<$?> and C<${^CHILD_ERROR_NATIVE}>.
5a964f20 943
2e0cfa16
FC
944If there are multiple threads running, C<close> on a filehandle from a
945piped open returns true without waiting for the child process to terminate,
946if the filehandle is still open in another thread.
947
80d38338
TC
948Closing the read end of a pipe before the process writing to it at the
949other end is done writing results in the writer receiving a SIGPIPE. If
950the other end can't handle that, be sure to read all the data before
951closing the pipe.
73689b13 952
fb73857a 953Example:
a0d0e21e 954
fb73857a
PP
955 open(OUTPUT, '|sort >foo') # pipe to sort
956 or die "Can't start sort: $!";
5ed4f2ec 957 #... # print stuff to output
958 close OUTPUT # wait for sort to finish
fb73857a
PP
959 or warn $! ? "Error closing sort pipe: $!"
960 : "Exit status $? from sort";
5ed4f2ec 961 open(INPUT, 'foo') # get sort's results
fb73857a 962 or die "Can't open 'foo' for input: $!";
a0d0e21e 963
5a964f20 964FILEHANDLE may be an expression whose value can be used as an indirect
8f1da26d 965filehandle, usually the real filehandle name or an autovivified handle.
a0d0e21e
LW
966
967=item closedir DIRHANDLE
d74e8afc 968X<closedir>
a0d0e21e 969
19799a22 970Closes a directory opened by C<opendir> and returns the success of that
5a964f20
TC
971system call.
972
a0d0e21e 973=item connect SOCKET,NAME
d74e8afc 974X<connect>
a0d0e21e 975
80d38338
TC
976Attempts to connect to a remote socket, just like connect(2).
977Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
978packed address of the appropriate type for the socket. See the examples in
979L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 980
cb1a09d0 981=item continue BLOCK
d74e8afc 982X<continue>
cb1a09d0 983
0d863452
RH
984=item continue
985
4a904372
FC
986When followed by a BLOCK, C<continue> is actually a
987flow control statement rather than a function. If
cf264981 988there is a C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
98293880
JH
989C<foreach>), it is always executed just before the conditional is about to
990be evaluated again, just like the third part of a C<for> loop in C. Thus
cb1a09d0
AD
991it can be used to increment a loop variable, even when the loop has been
992continued via the C<next> statement (which is similar to the C C<continue>
993statement).
994
98293880 995C<last>, C<next>, or C<redo> may appear within a C<continue>
3b10bc60 996block; C<last> and C<redo> behave as if they had been executed within
19799a22 997the main block. So will C<next>, but since it will execute a C<continue>
1d2dff63
GS
998block, it may be more entertaining.
999
1000 while (EXPR) {
a9a5a0dc
VP
1001 ### redo always comes here
1002 do_something;
1d2dff63 1003 } continue {
a9a5a0dc
VP
1004 ### next always comes here
1005 do_something_else;
1006 # then back the top to re-check EXPR
1d2dff63
GS
1007 }
1008 ### last always comes here
1009
3b10bc60 1010Omitting the C<continue> section is equivalent to using an
1011empty one, logically enough, so C<next> goes directly back
1d2dff63
GS
1012to check the condition at the top of the loop.
1013
4a904372 1014When there is no BLOCK, C<continue> is a function that
8f1da26d
TC
1015falls through the current C<when> or C<default> block instead of iterating
1016a dynamically enclosing C<foreach> or exiting a lexically enclosing C<given>.
4a904372
FC
1017In Perl 5.14 and earlier, this form of C<continue> was
1018only available when the C<"switch"> feature was enabled.
8f1da26d
TC
1019See L<feature> and L<perlsyn/"Switch statements"> for more
1020information.
0d863452 1021
a0d0e21e 1022=item cos EXPR
d74e8afc 1023X<cos> X<cosine> X<acos> X<arccosine>
a0d0e21e 1024
d6217f1e
GS
1025=item cos
1026
5a964f20 1027Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
34169887 1028takes the cosine of C<$_>.
a0d0e21e 1029
ca6e1c26 1030For the inverse cosine operation, you may use the C<Math::Trig::acos()>
28757baa
PP
1031function, or use this relation:
1032
1033 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
1034
a0d0e21e 1035=item crypt PLAINTEXT,SALT
d74e8afc 1036X<crypt> X<digest> X<hash> X<salt> X<plaintext> X<password>
f723aae1 1037X<decrypt> X<cryptography> X<passwd> X<encrypt>
a0d0e21e 1038
ef2e6798
MS
1039Creates a digest string exactly like the crypt(3) function in the C
1040library (assuming that you actually have a version there that has not
bb23f8d1 1041been extirpated as a potential munition).
ef2e6798 1042
34169887 1043crypt() is a one-way hash function. The PLAINTEXT and SALT are turned
ef2e6798
MS
1044into a short string, called a digest, which is returned. The same
1045PLAINTEXT and SALT will always return the same string, but there is no
1046(known) way to get the original PLAINTEXT from the hash. Small
1047changes in the PLAINTEXT or SALT will result in large changes in the
1048digest.
1049
1050There is no decrypt function. This function isn't all that useful for
1051cryptography (for that, look for F<Crypt> modules on your nearby CPAN
1052mirror) and the name "crypt" is a bit of a misnomer. Instead it is
1053primarily used to check if two pieces of text are the same without
1054having to transmit or store the text itself. An example is checking
1055if a correct password is given. The digest of the password is stored,
cf264981 1056not the password itself. The user types in a password that is
ef2e6798 1057crypt()'d with the same salt as the stored digest. If the two digests
34169887 1058match, the password is correct.
ef2e6798
MS
1059
1060When verifying an existing digest string you should use the digest as
1061the salt (like C<crypt($plain, $digest) eq $digest>). The SALT used
cf264981 1062to create the digest is visible as part of the digest. This ensures
ef2e6798
MS
1063crypt() will hash the new string with the same salt as the digest.
1064This allows your code to work with the standard L<crypt|/crypt> and
8f1da26d
TC
1065with more exotic implementations. In other words, assume
1066nothing about the returned string itself nor about how many bytes
1067of SALT may matter.
85c16d83
JH
1068
1069Traditionally the result is a string of 13 bytes: two first bytes of
1070the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
bb23f8d1 1071the first eight bytes of PLAINTEXT mattered. But alternative
ef2e6798 1072hashing schemes (like MD5), higher level security schemes (like C2),
e1020413 1073and implementations on non-Unix platforms may produce different
ef2e6798 1074strings.
85c16d83
JH
1075
1076When choosing a new salt create a random two character string whose
1077characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
d3989d75
CW
1078'/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of
1079characters is just a recommendation; the characters allowed in
1080the salt depend solely on your system's crypt library, and Perl can't
1081restrict what salts C<crypt()> accepts.
e71965be 1082
a0d0e21e 1083Here's an example that makes sure that whoever runs this program knows
cf264981 1084their password:
a0d0e21e
LW
1085
1086 $pwd = (getpwuid($<))[1];
a0d0e21e
LW
1087
1088 system "stty -echo";
1089 print "Password: ";
e71965be 1090 chomp($word = <STDIN>);
a0d0e21e
LW
1091 print "\n";
1092 system "stty echo";
1093
e71965be 1094 if (crypt($word, $pwd) ne $pwd) {
a9a5a0dc 1095 die "Sorry...\n";
a0d0e21e 1096 } else {
a9a5a0dc 1097 print "ok\n";
54310121 1098 }
a0d0e21e 1099
9f8f0c9d 1100Of course, typing in your own password to whoever asks you
748a9306 1101for it is unwise.
a0d0e21e 1102
ef2e6798 1103The L<crypt|/crypt> function is unsuitable for hashing large quantities
19799a22 1104of data, not least of all because you can't get the information
ef2e6798 1105back. Look at the L<Digest> module for more robust algorithms.
19799a22 1106
f2791508
JH
1107If using crypt() on a Unicode string (which I<potentially> has
1108characters with codepoints above 255), Perl tries to make sense
34169887 1109of the situation by trying to downgrade (a copy of)
f2791508
JH
1110the string back to an eight-bit byte string before calling crypt()
1111(on that copy). If that works, good. If not, crypt() dies with
1112C<Wide character in crypt>.
85c16d83 1113
ea9eb35a 1114Portability issues: L<perlport/crypt>.
1115
aa689395 1116=item dbmclose HASH
d74e8afc 1117X<dbmclose>
a0d0e21e 1118
19799a22 1119[This function has been largely superseded by the C<untie> function.]
a0d0e21e 1120
aa689395 1121Breaks the binding between a DBM file and a hash.
a0d0e21e 1122
ea9eb35a 1123Portability issues: L<perlport/dbmclose>.
1124
19799a22 1125=item dbmopen HASH,DBNAME,MASK
d74e8afc 1126X<dbmopen> X<dbm> X<ndbm> X<sdbm> X<gdbm>
a0d0e21e 1127
01aa884e
KW
1128[This function has been largely superseded by the
1129L<tie|/tie VARIABLE,CLASSNAME,LIST> function.]
a0d0e21e 1130
7b8d334a 1131This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
19799a22
GS
1132hash. HASH is the name of the hash. (Unlike normal C<open>, the first
1133argument is I<not> a filehandle, even though it looks like one). DBNAME
aa689395
PP
1134is the name of the database (without the F<.dir> or F<.pag> extension if
1135any). If the database does not exist, it is created with protection
19799a22 1136specified by MASK (as modified by the C<umask>). If your system supports
80d38338 1137only the older DBM functions, you may make only one C<dbmopen> call in your
aa689395 1138program. In older versions of Perl, if your system had neither DBM nor
19799a22 1139ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
aa689395
PP
1140sdbm(3).
1141
1142If you don't have write access to the DBM file, you can only read hash
1143variables, not set them. If you want to test whether you can write,
3b10bc60 1144either use file tests or try setting a dummy hash entry inside an C<eval>
1145to trap the error.
a0d0e21e 1146
19799a22
GS
1147Note that functions such as C<keys> and C<values> may return huge lists
1148when used on large DBM files. You may prefer to use the C<each>
a0d0e21e
LW
1149function to iterate over large DBM files. Example:
1150
1151 # print out history file offsets
1152 dbmopen(%HIST,'/usr/lib/news/history',0666);
1153 while (($key,$val) = each %HIST) {
a9a5a0dc 1154 print $key, ' = ', unpack('L',$val), "\n";
a0d0e21e
LW
1155 }
1156 dbmclose(%HIST);
1157
cb1a09d0 1158See also L<AnyDBM_File> for a more general description of the pros and
184e9718 1159cons of the various dbm approaches, as well as L<DB_File> for a particularly
cb1a09d0 1160rich implementation.
4633a7c4 1161
2b5ab1e7
TC
1162You can control which DBM library you use by loading that library
1163before you call dbmopen():
1164
1165 use DB_File;
1166 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
a9a5a0dc 1167 or die "Can't open netscape history file: $!";
2b5ab1e7 1168
ea9eb35a 1169Portability issues: L<perlport/dbmopen>.
1170
8f1da26d
TC
1171=item default BLOCK
1172
1173Within a C<foreach> or a C<given>, a C<default> BLOCK acts like a C<when>
1174that's always true. Only available after Perl 5.10, and only if the
4a904372
FC
1175C<switch> feature has been requested or if the keyword is prefixed with
1176C<CORE::>. See L</when>.
8f1da26d 1177
a0d0e21e 1178=item defined EXPR
d74e8afc 1179X<defined> X<undef> X<undefined>
a0d0e21e 1180
54310121 1181=item defined
bbce6d69 1182
2f9daede 1183Returns a Boolean value telling whether EXPR has a value other than
3b10bc60 1184the undefined value C<undef>. If EXPR is not present, C<$_> is
2f9daede
TPG
1185checked.
1186
1187Many operations return C<undef> to indicate failure, end of file,
1188system error, uninitialized variable, and other exceptional
1189conditions. This function allows you to distinguish C<undef> from
1190other values. (A simple Boolean test will not distinguish among
7660c0ab 1191C<undef>, zero, the empty string, and C<"0">, which are all equally
2f9daede 1192false.) Note that since C<undef> is a valid scalar, its presence
19799a22 1193doesn't I<necessarily> indicate an exceptional condition: C<pop>
2f9daede
TPG
1194returns C<undef> when its argument is an empty array, I<or> when the
1195element to return happens to be C<undef>.
1196
f10b0346
GS
1197You may also use C<defined(&func)> to check whether subroutine C<&func>
1198has ever been defined. The return value is unaffected by any forward
80d38338 1199declarations of C<&func>. A subroutine that is not defined
847c7ebe 1200may still be callable: its package may have an C<AUTOLOAD> method that
3b10bc60 1201makes it spring into existence the first time that it is called; see
847c7ebe 1202L<perlsub>.
f10b0346
GS
1203
1204Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
34169887 1205used to report whether memory for that aggregate had ever been
f10b0346
GS
1206allocated. This behavior may disappear in future versions of Perl.
1207You should instead use a simple test for size:
1208
1209 if (@an_array) { print "has array elements\n" }
1210 if (%a_hash) { print "has hash members\n" }
2f9daede
TPG
1211
1212When used on a hash element, it tells you whether the value is defined,
dc848c6f 1213not whether the key exists in the hash. Use L</exists> for the latter
2f9daede 1214purpose.
a0d0e21e
LW
1215
1216Examples:
1217
8f1da26d 1218 print if defined $switch{D};
a0d0e21e
LW
1219 print "$val\n" while defined($val = pop(@ary));
1220 die "Can't readlink $sym: $!"
a9a5a0dc 1221 unless defined($value = readlink $sym);
a0d0e21e 1222 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
2f9daede 1223 $debugging = 0 unless defined $debugging;
a0d0e21e 1224
8f1da26d 1225Note: Many folks tend to overuse C<defined> and are then surprised to
7660c0ab 1226discover that the number C<0> and C<""> (the zero-length string) are, in fact,
2f9daede 1227defined values. For example, if you say
a5f75d66
AD
1228
1229 "ab" =~ /a(.*)b/;
1230
80d38338 1231The pattern match succeeds and C<$1> is defined, although it
cf264981 1232matched "nothing". It didn't really fail to match anything. Rather, it
2b5ab1e7 1233matched something that happened to be zero characters long. This is all
a5f75d66 1234very above-board and honest. When a function returns an undefined value,
2f9daede 1235it's an admission that it couldn't give you an honest answer. So you
3b10bc60 1236should use C<defined> only when questioning the integrity of what
7660c0ab 1237you're trying to do. At other times, a simple comparison to C<0> or C<""> is
2f9daede
TPG
1238what you want.
1239
dc848c6f 1240See also L</undef>, L</exists>, L</ref>.
2f9daede 1241
a0d0e21e 1242=item delete EXPR
d74e8afc 1243X<delete>
a0d0e21e 1244
d0a76353
RS
1245Given an expression that specifies an element or slice of a hash, C<delete>
1246deletes the specified elements from that hash so that exists() on that element
1247no longer returns true. Setting a hash element to the undefined value does
1248not remove its key, but deleting it does; see L</exists>.
80d38338 1249
8f1da26d 1250In list context, returns the value or values deleted, or the last such
80d38338 1251element in scalar context. The return list's length always matches that of
d0a76353
RS
1252the argument list: deleting non-existent elements returns the undefined value
1253in their corresponding positions.
80d38338 1254
d0a76353
RS
1255delete() may also be used on arrays and array slices, but its behavior is less
1256straightforward. Although exists() will return false for deleted entries,
1257deleting array elements never changes indices of existing values; use shift()
1258or splice() for that. However, if all deleted elements fall at the end of an
1259array, the array's size shrinks to the position of the highest element that
1260still tests true for exists(), or to 0 if none do.
1261
8f1da26d 1262B<WARNING:> Calling delete on array values is deprecated and likely to
d0a76353 1263be removed in a future version of Perl.
80d38338
TC
1264
1265Deleting from C<%ENV> modifies the environment. Deleting from a hash tied to
1266a DBM file deletes the entry from the DBM file. Deleting from a C<tied> hash
1267or array may not necessarily return anything; it depends on the implementation
1268of the C<tied> package's DELETE method, which may do whatever it pleases.
a0d0e21e 1269
80d38338
TC
1270The C<delete local EXPR> construct localizes the deletion to the current
1271block at run time. Until the block exits, elements locally deleted
1272temporarily no longer exist. See L<perlsub/"Localized deletion of elements
1273of composite types">.
eba0920a
EM
1274
1275 %hash = (foo => 11, bar => 22, baz => 33);
1276 $scalar = delete $hash{foo}; # $scalar is 11
1277 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
1278 @array = delete @hash{qw(foo bar baz)}; # @array is (undef,undef,33)
1279
01020589 1280The following (inefficiently) deletes all the values of %HASH and @ARRAY:
a0d0e21e 1281
5f05dabc 1282 foreach $key (keys %HASH) {
a9a5a0dc 1283 delete $HASH{$key};
a0d0e21e
LW
1284 }
1285
01020589 1286 foreach $index (0 .. $#ARRAY) {
a9a5a0dc 1287 delete $ARRAY[$index];
01020589
GS
1288 }
1289
1290And so do these:
5f05dabc 1291
01020589
GS
1292 delete @HASH{keys %HASH};
1293
9740c838 1294 delete @ARRAY[0 .. $#ARRAY];
5f05dabc 1295
80d38338
TC
1296But both are slower than assigning the empty list
1297or undefining %HASH or @ARRAY, which is the customary
1298way to empty out an aggregate:
01020589 1299
5ed4f2ec 1300 %HASH = (); # completely empty %HASH
1301 undef %HASH; # forget %HASH ever existed
2b5ab1e7 1302
5ed4f2ec 1303 @ARRAY = (); # completely empty @ARRAY
1304 undef @ARRAY; # forget @ARRAY ever existed
2b5ab1e7 1305
80d38338
TC
1306The EXPR can be arbitrarily complicated provided its
1307final operation is an element or slice of an aggregate:
a0d0e21e
LW
1308
1309 delete $ref->[$x][$y]{$key};
5f05dabc 1310 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
a0d0e21e 1311
01020589
GS
1312 delete $ref->[$x][$y][$index];
1313 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1314
a0d0e21e 1315=item die LIST
d74e8afc 1316X<die> X<throw> X<exception> X<raise> X<$@> X<abort>
a0d0e21e 1317
4c050ad5
NC
1318C<die> raises an exception. Inside an C<eval> the error message is stuffed
1319into C<$@> and the C<eval> is terminated with the undefined value.
1320If the exception is outside of all enclosing C<eval>s, then the uncaught
1321exception prints LIST to C<STDERR> and exits with a non-zero value. If you
96090e4f 1322need to exit the process with a specific exit code, see L</exit>.
a0d0e21e
LW
1323
1324Equivalent examples:
1325
1326 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
54310121 1327 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
a0d0e21e 1328
ccac6780 1329If the last element of LIST does not end in a newline, the current
df37ec69
WW
1330script line number and input line number (if any) are also printed,
1331and a newline is supplied. Note that the "input line number" (also
1332known as "chunk") is subject to whatever notion of "line" happens to
1333be currently in effect, and is also available as the special variable
1334C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1335
1336Hint: sometimes appending C<", stopped"> to your message will cause it
1337to make better sense when the string C<"at foo line 123"> is appended.
1338Suppose you are running script "canasta".
a0d0e21e
LW
1339
1340 die "/etc/games is no good";
1341 die "/etc/games is no good, stopped";
1342
1343produce, respectively
1344
1345 /etc/games is no good at canasta line 123.
1346 /etc/games is no good, stopped at canasta line 123.
1347
a96d0188 1348If the output is empty and C<$@> already contains a value (typically from a
7660c0ab 1349previous eval) that value is reused after appending C<"\t...propagated">.
fb73857a
PP
1350This is useful for propagating exceptions:
1351
1352 eval { ... };
1353 die unless $@ =~ /Expected exception/;
1354
a96d0188 1355If the output is empty and C<$@> contains an object reference that has a
ad216e65
JH
1356C<PROPAGATE> method, that method will be called with additional file
1357and line number parameters. The return value replaces the value in
34169887 1358C<$@>; i.e., as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >>
ad216e65
JH
1359were called.
1360
7660c0ab 1361If C<$@> is empty then the string C<"Died"> is used.
fb73857a 1362
4c050ad5
NC
1363If an uncaught exception results in interpreter exit, the exit code is
1364determined from the values of C<$!> and C<$?> with this pseudocode:
1365
1366 exit $! if $!; # errno
1367 exit $? >> 8 if $? >> 8; # child exit status
1368 exit 255; # last resort
1369
1370The intent is to squeeze as much possible information about the likely cause
1371into the limited space of the system exit code. However, as C<$!> is the value
1372of C's C<errno>, which can be set by any system call, this means that the value
1373of the exit code used by C<die> can be non-predictable, so should not be relied
1374upon, other than to be non-zero.
1375
80d38338
TC
1376You can also call C<die> with a reference argument, and if this is trapped
1377within an C<eval>, C<$@> contains that reference. This permits more
1378elaborate exception handling using objects that maintain arbitrary state
1379about the exception. Such a scheme is sometimes preferable to matching
1380particular string values of C<$@> with regular expressions. Because C<$@>
1381is a global variable and C<eval> may be used within object implementations,
1382be careful that analyzing the error object doesn't replace the reference in
1383the global variable. It's easiest to make a local copy of the reference
1384before any manipulations. Here's an example:
52531d10 1385
80d38338 1386 use Scalar::Util "blessed";
da279afe 1387
52531d10 1388 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
746d7dd7
GL
1389 if (my $ev_err = $@) {
1390 if (blessed($ev_err) && $ev_err->isa("Some::Module::Exception")) {
52531d10
GS
1391 # handle Some::Module::Exception
1392 }
1393 else {
1394 # handle all other possible exceptions
1395 }
1396 }
1397
3b10bc60 1398Because Perl stringifies uncaught exception messages before display,
80d38338 1399you'll probably want to overload stringification operations on
52531d10
GS
1400exception objects. See L<overload> for details about that.
1401
19799a22
GS
1402You can arrange for a callback to be run just before the C<die>
1403does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
3b10bc60 1404handler is called with the error text and can change the error
19799a22 1405message, if it sees fit, by calling C<die> again. See
96090e4f 1406L<perlvar/%SIG> for details on setting C<%SIG> entries, and
cf264981 1407L<"eval BLOCK"> for some examples. Although this feature was
19799a22 1408to be run only right before your program was to exit, this is not
3b10bc60 1409currently so: the C<$SIG{__DIE__}> hook is currently called
19799a22
GS
1410even inside eval()ed blocks/strings! If one wants the hook to do
1411nothing in such situations, put
fb73857a 1412
5ed4f2ec 1413 die @_ if $^S;
fb73857a 1414
19799a22
GS
1415as the first line of the handler (see L<perlvar/$^S>). Because
1416this promotes strange action at a distance, this counterintuitive
b76cc8ba 1417behavior may be fixed in a future release.
774d564b 1418
4c050ad5
NC
1419See also exit(), warn(), and the Carp module.
1420
a0d0e21e 1421=item do BLOCK
d74e8afc 1422X<do> X<block>
a0d0e21e
LW
1423
1424Not really a function. Returns the value of the last command in the
6b275a1f
RGS
1425sequence of commands indicated by BLOCK. When modified by the C<while> or
1426C<until> loop modifier, executes the BLOCK once before testing the loop
1427condition. (On other statements the loop modifiers test the conditional
1428first.)
a0d0e21e 1429
4968c1e4 1430C<do BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7
TC
1431C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1432See L<perlsyn> for alternative strategies.
4968c1e4 1433
a0d0e21e 1434=item do SUBROUTINE(LIST)
d74e8afc 1435X<do>
a0d0e21e 1436
51124b83
FC
1437This form of subroutine call is deprecated. SUBROUTINE can be a bareword,
1438a scalar variable or a subroutine beginning with C<&>.
a0d0e21e
LW
1439
1440=item do EXPR
d74e8afc 1441X<do>
a0d0e21e
LW
1442
1443Uses the value of EXPR as a filename and executes the contents of the
ea63ef19 1444file as a Perl script.
a0d0e21e
LW
1445
1446 do 'stat.pl';
1447
1448is just like
1449
986b19de 1450 eval `cat stat.pl`;
a0d0e21e 1451
2b5ab1e7 1452except that it's more efficient and concise, keeps track of the current
96090e4f
A
1453filename for error messages, searches the C<@INC> directories, and updates
1454C<%INC> if the file is found. See L<perlvar/@INC> and L<perlvar/%INC> for
1455these variables. It also differs in that code evaluated with C<do FILENAME>
2b5ab1e7
TC
1456cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1457same, however, in that it does reparse the file every time you call it,
1458so you probably don't want to do this inside a loop.
a0d0e21e 1459
8f1da26d 1460If C<do> can read the file but cannot compile it, it returns C<undef> and sets
9dc513c5
DG
1461an error message in C<$@>. If C<do> cannot read the file, it returns undef
1462and sets C<$!> to the error. Always check C<$@> first, as compilation
1463could fail in a way that also sets C<$!>. If the file is successfully
1464compiled, C<do> returns the value of the last expression evaluated.
8e30cc93 1465
80d38338 1466Inclusion of library modules is better done with the
19799a22 1467C<use> and C<require> operators, which also do automatic error checking
4633a7c4 1468and raise an exception if there's a problem.
a0d0e21e 1469
5a964f20
TC
1470You might like to use C<do> to read in a program configuration
1471file. Manual error checking can be done this way:
1472
b76cc8ba 1473 # read in config files: system first, then user
f86cebdf 1474 for $file ("/share/prog/defaults.rc",
b76cc8ba 1475 "$ENV{HOME}/.someprogrc")
a9a5a0dc
VP
1476 {
1477 unless ($return = do $file) {
1478 warn "couldn't parse $file: $@" if $@;
1479 warn "couldn't do $file: $!" unless defined $return;
1480 warn "couldn't run $file" unless $return;
1481 }
5a964f20
TC
1482 }
1483
a0d0e21e 1484=item dump LABEL
d74e8afc 1485X<dump> X<core> X<undump>
a0d0e21e 1486
1614b0e3
JD
1487=item dump
1488
19799a22
GS
1489This function causes an immediate core dump. See also the B<-u>
1490command-line switch in L<perlrun>, which does the same thing.
1491Primarily this is so that you can use the B<undump> program (not
1492supplied) to turn your core dump into an executable binary after
1493having initialized all your variables at the beginning of the
1494program. When the new binary is executed it will begin by executing
1495a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1496Think of it as a goto with an intervening core dump and reincarnation.
1497If C<LABEL> is omitted, restarts the program from the top.
1498
1499B<WARNING>: Any files opened at the time of the dump will I<not>
1500be open any more when the program is reincarnated, with possible
80d38338 1501resulting confusion by Perl.
19799a22 1502
59f521f4
RGS
1503This function is now largely obsolete, mostly because it's very hard to
1504convert a core file into an executable. That's why you should now invoke
1505it as C<CORE::dump()>, if you don't want to be warned against a possible
ac206dc8 1506typo.
19799a22 1507
ea9eb35a 1508Portability issues: L<perlport/dump>.
1509
532eee96 1510=item each HASH
d74e8afc 1511X<each> X<hash, iterator>
aa689395 1512
532eee96 1513=item each ARRAY
aeedbbed
NC
1514X<array, iterator>
1515
f5a93a43
TC
1516=item each EXPR
1517
80d38338
TC
1518When called in list context, returns a 2-element list consisting of the key
1519and value for the next element of a hash, or the index and value for the
1520next element of an array, so that you can iterate over it. When called in
1521scalar context, returns only the key (not the value) in a hash, or the index
1522in an array.
2f9daede 1523
aeedbbed 1524Hash entries are returned in an apparently random order. The actual random
3b10bc60 1525order is subject to change in future versions of Perl, but it is
504f80c1 1526guaranteed to be in the same order as either the C<keys> or C<values>
4546b9e6 1527function would produce on the same (unmodified) hash. Since Perl
22883ac5 15285.8.2 the ordering can be different even between different runs of Perl
4546b9e6 1529for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
ab192400 1530
80d38338
TC
1531After C<each> has returned all entries from the hash or array, the next
1532call to C<each> returns the empty list in list context and C<undef> in
1533scalar context. The next call following that one restarts iteration. Each
1534hash or array has its own internal iterator, accessed by C<each>, C<keys>,
1535and C<values>. The iterator is implicitly reset when C<each> has reached
1536the end as just described; it can be explicitly reset by calling C<keys> or
1537C<values> on the hash or array. If you add or delete a hash's elements
1538while iterating over it, entries may be skipped or duplicated--so don't do
1539that. Exception: It is always safe to delete the item most recently
3b10bc60 1540returned by C<each()>, so the following code works properly:
74fc8b5f
MJD
1541
1542 while (($key, $value) = each %hash) {
1543 print $key, "\n";
1544 delete $hash{$key}; # This is safe
1545 }
aa689395 1546
80d38338 1547This prints out your environment like the printenv(1) program,
3b10bc60 1548but in a different order:
a0d0e21e
LW
1549
1550 while (($key,$value) = each %ENV) {
a9a5a0dc 1551 print "$key=$value\n";
a0d0e21e
LW
1552 }
1553
f5a93a43
TC
1554Starting with Perl 5.14, C<each> can take a scalar EXPR, which must hold
1555reference to an unblessed hash or array. The argument will be dereferenced
1556automatically. This aspect of C<each> is considered highly experimental.
1557The exact behaviour may change in a future version of Perl.
cba5a3b0
DG
1558
1559 while (($key,$value) = each $hashref) { ... }
1560
8f1da26d 1561See also C<keys>, C<values>, and C<sort>.
a0d0e21e
LW
1562
1563=item eof FILEHANDLE
d74e8afc
ITB
1564X<eof>
1565X<end of file>
1566X<end-of-file>
a0d0e21e 1567
4633a7c4
LW
1568=item eof ()
1569
a0d0e21e
LW
1570=item eof
1571
8f1da26d 1572Returns 1 if the next read on FILEHANDLE will return end of file I<or> if
a0d0e21e 1573FILEHANDLE is not open. FILEHANDLE may be an expression whose value
5a964f20 1574gives the real filehandle. (Note that this function actually
80d38338 1575reads a character and then C<ungetc>s it, so isn't useful in an
748a9306 1576interactive context.) Do not read from a terminal file (or call
19799a22 1577C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
748a9306
LW
1578as terminals may lose the end-of-file condition if you do.
1579
820475bd 1580An C<eof> without an argument uses the last file read. Using C<eof()>
80d38338 1581with empty parentheses is different. It refers to the pseudo file
820475bd 1582formed from the files listed on the command line and accessed via the
61eff3bc
JH
1583C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1584as a normal filehandle is, an C<eof()> before C<< <> >> has been
820475bd 1585used will cause C<@ARGV> to be examined to determine if input is
67408cae 1586available. Similarly, an C<eof()> after C<< <> >> has returned
efdd0218
RB
1587end-of-file will assume you are processing another C<@ARGV> list,
1588and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1589see L<perlop/"I/O Operators">.
820475bd 1590
61eff3bc 1591In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
8f1da26d
TC
1592detect the end of each file, whereas C<eof()> will detect the end
1593of the very last file only. Examples:
a0d0e21e 1594
748a9306
LW
1595 # reset line numbering on each input file
1596 while (<>) {
a9a5a0dc
VP
1597 next if /^\s*#/; # skip comments
1598 print "$.\t$_";
5a964f20 1599 } continue {
a9a5a0dc 1600 close ARGV if eof; # Not eof()!
748a9306
LW
1601 }
1602
a0d0e21e
LW
1603 # insert dashes just before last line of last file
1604 while (<>) {
a9a5a0dc
VP
1605 if (eof()) { # check for end of last file
1606 print "--------------\n";
1607 }
1608 print;
4d0444a3 1609 last if eof(); # needed if we're reading from a terminal
a0d0e21e
LW
1610 }
1611
a0d0e21e 1612Practical hint: you almost never need to use C<eof> in Perl, because the
8f1da26d
TC
1613input operators typically return C<undef> when they run out of data or
1614encounter an error.
a0d0e21e
LW
1615
1616=item eval EXPR
d74e8afc 1617X<eval> X<try> X<catch> X<evaluate> X<parse> X<execute>
f723aae1 1618X<error, handling> X<exception, handling>
a0d0e21e
LW
1619
1620=item eval BLOCK
1621
ce2984c3
PF
1622=item eval
1623
c7cc6f1c
GS
1624In the first form, the return value of EXPR is parsed and executed as if it
1625were a little Perl program. The value of the expression (which is itself
8f1da26d 1626determined within scalar context) is first parsed, and if there were no
2341804c 1627errors, executed as a block within the lexical context of the current Perl
df4833a8 1628program. This means, that in particular, any outer lexical variables are
2341804c
DM
1629visible to it, and any package variable settings or subroutine and format
1630definitions remain afterwards.
1631
1632Note that the value is parsed every time the C<eval> executes.
be3174d2
GS
1633If EXPR is omitted, evaluates C<$_>. This form is typically used to
1634delay parsing and subsequent execution of the text of EXPR until run time.
c7cc6f1c
GS
1635
1636In the second form, the code within the BLOCK is parsed only once--at the
cf264981 1637same time the code surrounding the C<eval> itself was parsed--and executed
c7cc6f1c
GS
1638within the context of the current Perl program. This form is typically
1639used to trap exceptions more efficiently than the first (see below), while
1640also providing the benefit of checking the code within BLOCK at compile
1641time.
1642
1643The final semicolon, if any, may be omitted from the value of EXPR or within
1644the BLOCK.
1645
1646In both forms, the value returned is the value of the last expression
5a964f20 1647evaluated inside the mini-program; a return statement may be also used, just
c7cc6f1c 1648as with subroutines. The expression providing the return value is evaluated
cf264981
SP
1649in void, scalar, or list context, depending on the context of the C<eval>
1650itself. See L</wantarray> for more on how the evaluation context can be
1651determined.
a0d0e21e 1652
19799a22 1653If there is a syntax error or runtime error, or a C<die> statement is
8f1da26d 1654executed, C<eval> returns C<undef> in scalar context
1f0d8f98
FC
1655or an empty list--or, for syntax errors, a list containing a single
1656undefined value--in list context, and C<$@> is set to the error
1657message. The discrepancy in the return values in list context is
1658considered a bug by some, and will probably be fixed in a future
9cc672d4
FC
1659release. If there was no error, C<$@> is set to the empty string. A
1660control flow operator like C<last> or C<goto> can bypass the setting of
1661C<$@>. Beware that using C<eval> neither silences Perl from printing
c7cc6f1c 1662warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
d9984052
A
1663To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1664turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1665See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
a0d0e21e 1666
19799a22
GS
1667Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1668determining whether a particular feature (such as C<socket> or C<symlink>)
82bcec1b 1669is implemented. It is also Perl's exception-trapping mechanism, where
a0d0e21e
LW
1670the die operator is used to raise exceptions.
1671
5f1da31c
NT
1672If you want to trap errors when loading an XS module, some problems with
1673the binary interface (such as Perl version skew) may be fatal even with
df4833a8 1674C<eval> unless C<$ENV{PERL_DL_NONLAZY}> is set. See L<perlrun>.
5f1da31c 1675
a0d0e21e
LW
1676If the code to be executed doesn't vary, you may use the eval-BLOCK
1677form to trap run-time errors without incurring the penalty of
1678recompiling each time. The error, if any, is still returned in C<$@>.
1679Examples:
1680
54310121 1681 # make divide-by-zero nonfatal
a0d0e21e
LW
1682 eval { $answer = $a / $b; }; warn $@ if $@;
1683
1684 # same thing, but less efficient
1685 eval '$answer = $a / $b'; warn $@ if $@;
1686
1687 # a compile-time error
5ed4f2ec 1688 eval { $answer = }; # WRONG
a0d0e21e
LW
1689
1690 # a run-time error
5ed4f2ec 1691 eval '$answer ='; # sets $@
a0d0e21e 1692
cf264981
SP
1693Using the C<eval{}> form as an exception trap in libraries does have some
1694issues. Due to the current arguably broken state of C<__DIE__> hooks, you
1695may wish not to trigger any C<__DIE__> hooks that user code may have installed.
2b5ab1e7 1696You can use the C<local $SIG{__DIE__}> construct for this purpose,
80d38338 1697as this example shows:
774d564b 1698
80d38338 1699 # a private exception trap for divide-by-zero
f86cebdf
GS
1700 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1701 warn $@ if $@;
774d564b
PP
1702
1703This is especially significant, given that C<__DIE__> hooks can call
19799a22 1704C<die> again, which has the effect of changing their error messages:
774d564b
PP
1705
1706 # __DIE__ hooks may modify error messages
1707 {
f86cebdf
GS
1708 local $SIG{'__DIE__'} =
1709 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
c7cc6f1c
GS
1710 eval { die "foo lives here" };
1711 print $@ if $@; # prints "bar lives here"
774d564b
PP
1712 }
1713
19799a22 1714Because this promotes action at a distance, this counterintuitive behavior
2b5ab1e7
TC
1715may be fixed in a future release.
1716
19799a22 1717With an C<eval>, you should be especially careful to remember what's
a0d0e21e
LW
1718being looked at when:
1719
5ed4f2ec 1720 eval $x; # CASE 1
1721 eval "$x"; # CASE 2
a0d0e21e 1722
5ed4f2ec 1723 eval '$x'; # CASE 3
1724 eval { $x }; # CASE 4
a0d0e21e 1725
5ed4f2ec 1726 eval "\$$x++"; # CASE 5
1727 $$x++; # CASE 6
a0d0e21e 1728
2f9daede 1729Cases 1 and 2 above behave identically: they run the code contained in
19799a22 1730the variable $x. (Although case 2 has misleading double quotes making
2f9daede 1731the reader wonder what else might be happening (nothing is).) Cases 3
7660c0ab 1732and 4 likewise behave in the same way: they run the code C<'$x'>, which
19799a22 1733does nothing but return the value of $x. (Case 4 is preferred for
2f9daede
TPG
1734purely visual reasons, but it also has the advantage of compiling at
1735compile-time instead of at run-time.) Case 5 is a place where
19799a22 1736normally you I<would> like to use double quotes, except that in this
2f9daede
TPG
1737particular situation, you can just use symbolic references instead, as
1738in case 6.
a0d0e21e 1739
b6538e4f
TC
1740Before Perl 5.14, the assignment to C<$@> occurred before restoration
1741of localised variables, which means that for your code to run on older
b208c909 1742versions, a temporary is required if you want to mask some but not all
8a5a710d
DN
1743errors:
1744
1745 # alter $@ on nefarious repugnancy only
1746 {
1747 my $e;
1748 {
1749 local $@; # protect existing $@
1750 eval { test_repugnancy() };
b208c909 1751 # $@ =~ /nefarious/ and die $@; # Perl 5.14 and higher only
8a5a710d
DN
1752 $@ =~ /nefarious/ and $e = $@;
1753 }
1754 die $e if defined $e
1755 }
1756
4968c1e4 1757C<eval BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7 1758C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
4968c1e4 1759
3b10bc60 1760An C<eval ''> executed within the C<DB> package doesn't see the usual
1761surrounding lexical scope, but rather the scope of the first non-DB piece
df4833a8 1762of code that called it. You don't normally need to worry about this unless
3b10bc60 1763you are writing a Perl debugger.
d819b83a 1764
a0d0e21e 1765=item exec LIST
d74e8afc 1766X<exec> X<execute>
a0d0e21e 1767
8bf3b016
GS
1768=item exec PROGRAM LIST
1769
3b10bc60 1770The C<exec> function executes a system command I<and never returns>;
19799a22
GS
1771use C<system> instead of C<exec> if you want it to return. It fails and
1772returns false only if the command does not exist I<and> it is executed
fb73857a 1773directly instead of via your system's command shell (see below).
a0d0e21e 1774
19799a22 1775Since it's a common mistake to use C<exec> instead of C<system>, Perl
80d38338 1776warns you if there is a following statement that isn't C<die>, C<warn>,
3b10bc60 1777or C<exit> (if C<-w> is set--but you always do that, right?). If you
19799a22 1778I<really> want to follow an C<exec> with some other statement, you
55d729e4
GS
1779can use one of these styles to avoid the warning:
1780
5a964f20
TC
1781 exec ('foo') or print STDERR "couldn't exec foo: $!";
1782 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
55d729e4 1783
5a964f20 1784If there is more than one argument in LIST, or if LIST is an array
f86cebdf 1785with more than one value, calls execvp(3) with the arguments in LIST.
5a964f20
TC
1786If there is only one scalar argument or an array with one element in it,
1787the argument is checked for shell metacharacters, and if there are any,
1788the entire argument is passed to the system's command shell for parsing
1789(this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1790If there are no shell metacharacters in the argument, it is split into
b76cc8ba 1791words and passed directly to C<execvp>, which is more efficient.
19799a22 1792Examples:
a0d0e21e 1793
19799a22
GS
1794 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1795 exec "sort $outfile | uniq";
a0d0e21e
LW
1796
1797If you don't really want to execute the first argument, but want to lie
1798to the program you are executing about its own name, you can specify
1799the program you actually want to run as an "indirect object" (without a
1800comma) in front of the LIST. (This always forces interpretation of the
54310121 1801LIST as a multivalued list, even if there is only a single scalar in
a0d0e21e
LW
1802the list.) Example:
1803
1804 $shell = '/bin/csh';
5ed4f2ec 1805 exec $shell '-sh'; # pretend it's a login shell
a0d0e21e
LW
1806
1807or, more directly,
1808
5ed4f2ec 1809 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
a0d0e21e 1810
3b10bc60 1811When the arguments get executed via the system shell, results are
1812subject to its quirks and capabilities. See L<perlop/"`STRING`">
bb32b41a
GS
1813for details.
1814
19799a22
GS
1815Using an indirect object with C<exec> or C<system> is also more
1816secure. This usage (which also works fine with system()) forces
1817interpretation of the arguments as a multivalued list, even if the
1818list had just one argument. That way you're safe from the shell
1819expanding wildcards or splitting up words with whitespace in them.
5a964f20
TC
1820
1821 @args = ( "echo surprise" );
1822
2b5ab1e7 1823 exec @args; # subject to shell escapes
f86cebdf 1824 # if @args == 1
2b5ab1e7 1825 exec { $args[0] } @args; # safe even with one-arg list
5a964f20
TC
1826
1827The first version, the one without the indirect object, ran the I<echo>
80d38338
TC
1828program, passing it C<"surprise"> an argument. The second version didn't;
1829it tried to run a program named I<"echo surprise">, didn't find it, and set
1830C<$?> to a non-zero value indicating failure.
5a964f20 1831
80d38338 1832Beginning with v5.6.0, Perl attempts to flush all files opened for
0f897271
GS
1833output before the exec, but this may not be supported on some platforms
1834(see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1835in English) or call the C<autoflush()> method of C<IO::Handle> on any
80d38338 1836open handles to avoid lost output.
0f897271 1837
80d38338
TC
1838Note that C<exec> will not call your C<END> blocks, nor will it invoke
1839C<DESTROY> methods on your objects.
7660c0ab 1840
ea9eb35a 1841Portability issues: L<perlport/exec>.
1842
a0d0e21e 1843=item exists EXPR
d74e8afc 1844X<exists> X<autovivification>
a0d0e21e 1845
d0a76353
RS
1846Given an expression that specifies an element of a hash, returns true if the
1847specified element in the hash has ever been initialized, even if the
1848corresponding value is undefined.
a0d0e21e 1849
5ed4f2ec 1850 print "Exists\n" if exists $hash{$key};
1851 print "Defined\n" if defined $hash{$key};
01020589
GS
1852 print "True\n" if $hash{$key};
1853
d0a76353 1854exists may also be called on array elements, but its behavior is much less
8f1da26d 1855obvious and is strongly tied to the use of L</delete> on arrays. B<Be aware>
d0a76353
RS
1856that calling exists on array values is deprecated and likely to be removed in
1857a future version of Perl.
1858
5ed4f2ec 1859 print "Exists\n" if exists $array[$index];
1860 print "Defined\n" if defined $array[$index];
01020589 1861 print "True\n" if $array[$index];
a0d0e21e 1862
8f1da26d 1863A hash or array element can be true only if it's defined and defined only if
a0d0e21e
LW
1864it exists, but the reverse doesn't necessarily hold true.
1865
afebc493
GS
1866Given an expression that specifies the name of a subroutine,
1867returns true if the specified subroutine has ever been declared, even
1868if it is undefined. Mentioning a subroutine name for exists or defined
80d38338 1869does not count as declaring it. Note that a subroutine that does not
847c7ebe
DD
1870exist may still be callable: its package may have an C<AUTOLOAD>
1871method that makes it spring into existence the first time that it is
3b10bc60 1872called; see L<perlsub>.
afebc493 1873
5ed4f2ec 1874 print "Exists\n" if exists &subroutine;
1875 print "Defined\n" if defined &subroutine;
afebc493 1876
a0d0e21e 1877Note that the EXPR can be arbitrarily complicated as long as the final
afebc493 1878operation is a hash or array key lookup or subroutine name:
a0d0e21e 1879
5ed4f2ec 1880 if (exists $ref->{A}->{B}->{$key}) { }
1881 if (exists $hash{A}{B}{$key}) { }
2b5ab1e7 1882
5ed4f2ec 1883 if (exists $ref->{A}->{B}->[$ix]) { }
1884 if (exists $hash{A}{B}[$ix]) { }
01020589 1885
afebc493
GS
1886 if (exists &{$ref->{A}{B}{$key}}) { }
1887
3b10bc60 1888Although the mostly deeply nested array or hash will not spring into
1889existence just because its existence was tested, any intervening ones will.
61eff3bc 1890Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
01020589 1891into existence due to the existence test for the $key element above.
3b10bc60 1892This happens anywhere the arrow operator is used, including even here:
5a964f20 1893
2b5ab1e7 1894 undef $ref;
5ed4f2ec 1895 if (exists $ref->{"Some key"}) { }
1896 print $ref; # prints HASH(0x80d3d5c)
2b5ab1e7
TC
1897
1898This surprising autovivification in what does not at first--or even
1899second--glance appear to be an lvalue context may be fixed in a future
5a964f20 1900release.
a0d0e21e 1901
afebc493
GS
1902Use of a subroutine call, rather than a subroutine name, as an argument
1903to exists() is an error.
1904
5ed4f2ec 1905 exists &sub; # OK
1906 exists &sub(); # Error
afebc493 1907
a0d0e21e 1908=item exit EXPR
d74e8afc 1909X<exit> X<terminate> X<abort>
a0d0e21e 1910
ce2984c3
PF
1911=item exit
1912
2b5ab1e7 1913Evaluates EXPR and exits immediately with that value. Example:
a0d0e21e
LW
1914
1915 $ans = <STDIN>;
1916 exit 0 if $ans =~ /^[Xx]/;
1917
19799a22 1918See also C<die>. If EXPR is omitted, exits with C<0> status. The only
2b5ab1e7
TC
1919universally recognized values for EXPR are C<0> for success and C<1>
1920for error; other values are subject to interpretation depending on the
1921environment in which the Perl program is running. For example, exiting
192269 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1923the mailer to return the item undelivered, but that's not true everywhere.
a0d0e21e 1924
19799a22
GS
1925Don't use C<exit> to abort a subroutine if there's any chance that
1926someone might want to trap whatever error happened. Use C<die> instead,
1927which can be trapped by an C<eval>.
28757baa 1928
19799a22 1929The exit() function does not always exit immediately. It calls any
2b5ab1e7 1930defined C<END> routines first, but these C<END> routines may not
19799a22 1931themselves abort the exit. Likewise any object destructors that need to
60275626
FC
1932be called are called before the real exit. C<END> routines and destructors
1933can change the exit status by modifying C<$?>. If this is a problem, you
2b5ab1e7 1934can call C<POSIX:_exit($status)> to avoid END and destructor processing.
87275199 1935See L<perlmod> for details.
5a964f20 1936
ea9eb35a 1937Portability issues: L<perlport/exit>.
1938
a0d0e21e 1939=item exp EXPR
d74e8afc 1940X<exp> X<exponential> X<antilog> X<antilogarithm> X<e>
a0d0e21e 1941
54310121 1942=item exp
bbce6d69 1943
b76cc8ba 1944Returns I<e> (the natural logarithm base) to the power of EXPR.
a0d0e21e
LW
1945If EXPR is omitted, gives C<exp($_)>.
1946
1947=item fcntl FILEHANDLE,FUNCTION,SCALAR
d74e8afc 1948X<fcntl>
a0d0e21e 1949
f86cebdf 1950Implements the fcntl(2) function. You'll probably have to say
a0d0e21e
LW
1951
1952 use Fcntl;
1953
0ade1984 1954first to get the correct constant definitions. Argument processing and
3b10bc60 1955value returned work just like C<ioctl> below.
a0d0e21e
LW
1956For example:
1957
1958 use Fcntl;
5a964f20 1959 fcntl($filehandle, F_GETFL, $packed_return_buffer)
a9a5a0dc 1960 or die "can't fcntl F_GETFL: $!";
5a964f20 1961
554ad1fc 1962You don't have to check for C<defined> on the return from C<fcntl>.
951ba7fe
GS
1963Like C<ioctl>, it maps a C<0> return from the system call into
1964C<"0 but true"> in Perl. This string is true in boolean context and C<0>
2b5ab1e7
TC
1965in numeric context. It is also exempt from the normal B<-w> warnings
1966on improper numeric conversions.
5a964f20 1967
3b10bc60 1968Note that C<fcntl> raises an exception if used on a machine that
2b5ab1e7
TC
1969doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1970manpage to learn what functions are available on your system.
a0d0e21e 1971
be2f7487 1972Here's an example of setting a filehandle named C<REMOTE> to be
1973non-blocking at the system level. You'll have to negotiate C<$|>
1974on your own, though.
1975
1976 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
1977
1978 $flags = fcntl(REMOTE, F_GETFL, 0)
1979 or die "Can't get flags for the socket: $!\n";
1980
1981 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
1982 or die "Can't set flags for the socket: $!\n";
1983
ea9eb35a 1984Portability issues: L<perlport/fcntl>.
1985
cfa52385
FC
1986=item __FILE__
1987X<__FILE__>
1988
1989A special token that returns the name of the file in which it occurs.
1990
a0d0e21e 1991=item fileno FILEHANDLE
d74e8afc 1992X<fileno>
a0d0e21e 1993
2b5ab1e7 1994Returns the file descriptor for a filehandle, or undefined if the
a7c1632d
FC
1995filehandle is not open. If there is no real file descriptor at the OS
1996level, as can happen with filehandles connected to memory objects via
1997C<open> with a reference for the third argument, -1 is returned.
1998
1999This is mainly useful for constructing
19799a22 2000bitmaps for C<select> and low-level POSIX tty-handling operations.
2b5ab1e7
TC
2001If FILEHANDLE is an expression, the value is taken as an indirect
2002filehandle, generally its name.
5a964f20 2003
b76cc8ba 2004You can use this to find out whether two handles refer to the
5a964f20
TC
2005same underlying descriptor:
2006
2007 if (fileno(THIS) == fileno(THAT)) {
a9a5a0dc 2008 print "THIS and THAT are dups\n";
b76cc8ba
NIS
2009 }
2010
a0d0e21e 2011=item flock FILEHANDLE,OPERATION
d74e8afc 2012X<flock> X<lock> X<locking>
a0d0e21e 2013
19799a22
GS
2014Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
2015for success, false on failure. Produces a fatal error if used on a
2b5ab1e7 2016machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
dbfe1e81 2017C<flock> is Perl's portable file-locking interface, although it locks
3b10bc60 2018entire files only, not records.
2b5ab1e7
TC
2019
2020Two potentially non-obvious but traditional C<flock> semantics are
2021that it waits indefinitely until the lock is granted, and that its locks
dbfe1e81
FC
2022are B<merely advisory>. Such discretionary locks are more flexible, but
2023offer fewer guarantees. This means that programs that do not also use
2024C<flock> may modify files locked with C<flock>. See L<perlport>,
8f1da26d 2025your port's specific documentation, and your system-specific local manpages
2b5ab1e7
TC
2026for details. It's best to assume traditional behavior if you're writing
2027portable programs. (But if you're not, you should as always feel perfectly
2028free to write for your own system's idiosyncrasies (sometimes called
2029"features"). Slavish adherence to portability concerns shouldn't get
2030in the way of your getting your job done.)
a3cb178b 2031
8ebc5c01
PP
2032OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
2033LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
8f1da26d
TC
2034you can use the symbolic names if you import them from the L<Fcntl> module,
2035either individually, or as a group using the C<:flock> tag. LOCK_SH
68dc0745 2036requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
ea3105be 2037releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
8f1da26d 2038LOCK_SH or LOCK_EX, then C<flock> returns immediately rather than blocking
3b10bc60 2039waiting for the lock; check the return status to see if you got it.
68dc0745 2040
2b5ab1e7
TC
2041To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
2042before locking or unlocking it.
8ebc5c01 2043
f86cebdf 2044Note that the emulation built with lockf(3) doesn't provide shared
8ebc5c01 2045locks, and it requires that FILEHANDLE be open with write intent. These
2b5ab1e7 2046are the semantics that lockf(3) implements. Most if not all systems
f86cebdf 2047implement lockf(3) in terms of fcntl(2) locking, though, so the
8ebc5c01
PP
2048differing semantics shouldn't bite too many people.
2049
becacb53
TM
2050Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
2051be open with read intent to use LOCK_SH and requires that it be open
2052with write intent to use LOCK_EX.
2053
19799a22
GS
2054Note also that some versions of C<flock> cannot lock things over the
2055network; you would need to use the more system-specific C<fcntl> for
f86cebdf
GS
2056that. If you like you can force Perl to ignore your system's flock(2)
2057function, and so provide its own fcntl(2)-based emulation, by passing
8ebc5c01 2058the switch C<-Ud_flock> to the F<Configure> program when you configure
8f1da26d 2059and build a new Perl.
4633a7c4
LW
2060
2061Here's a mailbox appender for BSD systems.
a0d0e21e 2062
7ed5353d 2063 use Fcntl qw(:flock SEEK_END); # import LOCK_* and SEEK_END constants
a0d0e21e
LW
2064
2065 sub lock {
a9a5a0dc
VP
2066 my ($fh) = @_;
2067 flock($fh, LOCK_EX) or die "Cannot lock mailbox - $!\n";
7ed5353d 2068
a9a5a0dc
VP
2069 # and, in case someone appended while we were waiting...
2070 seek($fh, 0, SEEK_END) or die "Cannot seek - $!\n";
a0d0e21e
LW
2071 }
2072
2073 sub unlock {
a9a5a0dc
VP
2074 my ($fh) = @_;
2075 flock($fh, LOCK_UN) or die "Cannot unlock mailbox - $!\n";
a0d0e21e
LW
2076 }
2077
b0169937 2078 open(my $mbox, ">>", "/usr/spool/mail/$ENV{'USER'}")
5ed4f2ec 2079 or die "Can't open mailbox: $!";
a0d0e21e 2080
7ed5353d 2081 lock($mbox);
b0169937 2082 print $mbox $msg,"\n\n";
7ed5353d 2083 unlock($mbox);
a0d0e21e 2084
3b10bc60 2085On systems that support a real flock(2), locks are inherited across fork()
2086calls, whereas those that must resort to the more capricious fcntl(2)
2087function lose their locks, making it seriously harder to write servers.
2b5ab1e7 2088
cb1a09d0 2089See also L<DB_File> for other flock() examples.
a0d0e21e 2090
ea9eb35a 2091Portability issues: L<perlport/flock>.
2092
a0d0e21e 2093=item fork
d74e8afc 2094X<fork> X<child> X<parent>
a0d0e21e 2095
2b5ab1e7
TC
2096Does a fork(2) system call to create a new process running the
2097same program at the same point. It returns the child pid to the
2098parent process, C<0> to the child process, or C<undef> if the fork is
2099unsuccessful. File descriptors (and sometimes locks on those descriptors)
2100are shared, while everything else is copied. On most systems supporting
2101fork(), great care has gone into making it extremely efficient (for
2102example, using copy-on-write technology on data pages), making it the
2103dominant paradigm for multitasking over the last few decades.
5a964f20 2104
80d38338 2105Beginning with v5.6.0, Perl attempts to flush all files opened for
0f897271
GS
2106output before forking the child process, but this may not be supported
2107on some platforms (see L<perlport>). To be safe, you may need to set
2108C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
80d38338 2109C<IO::Handle> on any open handles to avoid duplicate output.
a0d0e21e 2110
19799a22 2111If you C<fork> without ever waiting on your children, you will
2b5ab1e7
TC
2112accumulate zombies. On some systems, you can avoid this by setting
2113C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
2114forking and reaping moribund children.
cb1a09d0 2115
28757baa
PP
2116Note that if your forked child inherits system file descriptors like
2117STDIN and STDOUT that are actually connected by a pipe or socket, even
2b5ab1e7 2118if you exit, then the remote server (such as, say, a CGI script or a
19799a22 2119backgrounded job launched from a remote shell) won't think you're done.
2b5ab1e7 2120You should reopen those to F</dev/null> if it's any issue.
28757baa 2121
ea9eb35a 2122On some platforms such as Windows, where the fork() system call is not available,
2123Perl can be built to emulate fork() in the Perl interpreter. The emulation is designed to,
2124at the level of the Perl program, be as compatible as possible with the "Unix" fork().
6d17f725 2125However it has limitations that have to be considered in code intended to be portable.
ea9eb35a 2126See L<perlfork> for more details.
2127
2128Portability issues: L<perlport/fork>.
2129
cb1a09d0 2130=item format
d74e8afc 2131X<format>
cb1a09d0 2132
19799a22 2133Declare a picture format for use by the C<write> function. For
cb1a09d0
AD
2134example:
2135
54310121 2136 format Something =
a9a5a0dc
VP
2137 Test: @<<<<<<<< @||||| @>>>>>
2138 $str, $%, '$' . int($num)
cb1a09d0
AD
2139 .
2140
2141 $str = "widget";
184e9718 2142 $num = $cost/$quantity;
cb1a09d0
AD
2143 $~ = 'Something';
2144 write;
2145
2146See L<perlform> for many details and examples.
2147
8903cb82 2148=item formline PICTURE,LIST
d74e8afc 2149X<formline>
a0d0e21e 2150
5a964f20 2151This is an internal function used by C<format>s, though you may call it,
a0d0e21e
LW
2152too. It formats (see L<perlform>) a list of values according to the
2153contents of PICTURE, placing the output into the format output
7660c0ab 2154accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
19799a22 2155Eventually, when a C<write> is done, the contents of
cf264981
SP
2156C<$^A> are written to some filehandle. You could also read C<$^A>
2157and then set C<$^A> back to C<"">. Note that a format typically
19799a22 2158does one C<formline> per line of form, but the C<formline> function itself
748a9306 2159doesn't care how many newlines are embedded in the PICTURE. This means
3b10bc60 2160that the C<~> and C<~~> tokens treat the entire PICTURE as a single line.
748a9306 2161You may therefore need to use multiple formlines to implement a single
3b10bc60 2162record format, just like the C<format> compiler.
748a9306 2163
19799a22 2164Be careful if you put double quotes around the picture, because an C<@>
748a9306 2165character may be taken to mean the beginning of an array name.
19799a22 2166C<formline> always returns true. See L<perlform> for other examples.
a0d0e21e 2167
445b09e5
FC
2168If you are trying to use this instead of C<write> to capture the output,
2169you may find it easier to open a filehandle to a scalar
2170(C<< open $fh, ">", \$output >>) and write to that instead.
2171
a0d0e21e 2172=item getc FILEHANDLE
f723aae1 2173X<getc> X<getchar> X<character> X<file, read>
a0d0e21e
LW
2174
2175=item getc
2176
2177Returns the next character from the input file attached to FILEHANDLE,
3b10bc60 2178or the undefined value at end of file or if there was an error (in
b5fe5ca2
SR
2179the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
2180STDIN. This is not particularly efficient. However, it cannot be
2181used by itself to fetch single characters without waiting for the user
2182to hit enter. For that, try something more like:
4633a7c4
LW
2183
2184 if ($BSD_STYLE) {
a9a5a0dc 2185 system "stty cbreak </dev/tty >/dev/tty 2>&1";
4633a7c4
LW
2186 }
2187 else {
a9a5a0dc 2188 system "stty", '-icanon', 'eol', "\001";
4633a7c4
LW
2189 }
2190
2191 $key = getc(STDIN);
2192
2193 if ($BSD_STYLE) {
a9a5a0dc 2194 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
4633a7c4
LW
2195 }
2196 else {
3b10bc60 2197 system 'stty', 'icanon', 'eol', '^@'; # ASCII NUL
4633a7c4
LW
2198 }
2199 print "\n";
2200
54310121
PP
2201Determination of whether $BSD_STYLE should be set
2202is left as an exercise to the reader.
cb1a09d0 2203
19799a22 2204The C<POSIX::getattr> function can do this more portably on
2b5ab1e7 2205systems purporting POSIX compliance. See also the C<Term::ReadKey>
a3390c9f 2206module from your nearest CPAN site; details on CPAN can be found under
2b5ab1e7 2207L<perlmodlib/CPAN>.
a0d0e21e
LW
2208
2209=item getlogin
d74e8afc 2210X<getlogin> X<login>
a0d0e21e 2211
cf264981 2212This implements the C library function of the same name, which on most
3b10bc60 2213systems returns the current login from F</etc/utmp>, if any. If it
2214returns the empty string, use C<getpwuid>.
a0d0e21e 2215
f86702cc 2216 $login = getlogin || getpwuid($<) || "Kilroy";
a0d0e21e 2217
19799a22
GS
2218Do not consider C<getlogin> for authentication: it is not as
2219secure as C<getpwuid>.
4633a7c4 2220
ea9eb35a 2221Portability issues: L<perlport/getlogin>.
2222
a0d0e21e 2223=item getpeername SOCKET
d74e8afc 2224X<getpeername> X<peer>
a0d0e21e 2225
a3390c9f
FC
2226Returns the packed sockaddr address of the other end of the SOCKET
2227connection.
a0d0e21e 2228
4633a7c4
LW
2229 use Socket;
2230 $hersockaddr = getpeername(SOCK);
19799a22 2231 ($port, $iaddr) = sockaddr_in($hersockaddr);
4633a7c4
LW
2232 $herhostname = gethostbyaddr($iaddr, AF_INET);
2233 $herstraddr = inet_ntoa($iaddr);
a0d0e21e
LW
2234
2235=item getpgrp PID
d74e8afc 2236X<getpgrp> X<group>
a0d0e21e 2237
47e29363 2238Returns the current process group for the specified PID. Use
7660c0ab 2239a PID of C<0> to get the current process group for the
4633a7c4 2240current process. Will raise an exception if used on a machine that
a3390c9f
FC
2241doesn't implement getpgrp(2). If PID is omitted, returns the process
2242group of the current process. Note that the POSIX version of C<getpgrp>
7660c0ab 2243does not accept a PID argument, so only C<PID==0> is truly portable.
a0d0e21e 2244
ea9eb35a 2245Portability issues: L<perlport/getpgrp>.
2246
a0d0e21e 2247=item getppid
d74e8afc 2248X<getppid> X<parent> X<pid>
a0d0e21e
LW
2249
2250Returns the process id of the parent process.
2251
4d76a344
RGS
2252Note for Linux users: on Linux, the C functions C<getpid()> and
2253C<getppid()> return different values from different threads. In order to
3b10bc60 2254be portable, this behavior is not reflected by the Perl-level function
4d76a344 2255C<getppid()>, that returns a consistent value across threads. If you want
e3256f86
RGS
2256to call the underlying C<getppid()>, you may use the CPAN module
2257C<Linux::Pid>.
4d76a344 2258
ea9eb35a 2259Portability issues: L<perlport/getppid>.
2260
a0d0e21e 2261=item getpriority WHICH,WHO
d74e8afc 2262X<getpriority> X<priority> X<nice>
a0d0e21e 2263
4633a7c4 2264Returns the current priority for a process, a process group, or a user.
01aa884e 2265(See L<getpriority(2)>.) Will raise a fatal exception if used on a
f86cebdf 2266machine that doesn't implement getpriority(2).
a0d0e21e 2267
ea9eb35a 2268Portability issues: L<perlport/getpriority>.
2269
a0d0e21e 2270=item getpwnam NAME
d74e8afc
ITB
2271X<getpwnam> X<getgrnam> X<gethostbyname> X<getnetbyname> X<getprotobyname>
2272X<getpwuid> X<getgrgid> X<getservbyname> X<gethostbyaddr> X<getnetbyaddr>
2273X<getprotobynumber> X<getservbyport> X<getpwent> X<getgrent> X<gethostent>
2274X<getnetent> X<getprotoent> X<getservent> X<setpwent> X<setgrent> X<sethostent>
2275X<setnetent> X<setprotoent> X<setservent> X<endpwent> X<endgrent> X<endhostent>
2276X<endnetent> X<endprotoent> X<endservent>
a0d0e21e
LW
2277
2278=item getgrnam NAME
2279
2280=item gethostbyname NAME
2281
2282=item getnetbyname NAME
2283
2284=item getprotobyname NAME
2285
2286=item getpwuid UID
2287
2288=item getgrgid GID
2289
2290=item getservbyname NAME,PROTO
2291
2292=item gethostbyaddr ADDR,ADDRTYPE
2293
2294=item getnetbyaddr ADDR,ADDRTYPE
2295
2296=item getprotobynumber NUMBER
2297
2298=item getservbyport PORT,PROTO
2299
2300=item getpwent
2301
2302=item getgrent
2303
2304=item gethostent
2305
2306=item getnetent
2307
2308=item getprotoent
2309
2310=item getservent
2311
2312=item setpwent
2313
2314=item setgrent
2315
2316=item sethostent STAYOPEN
2317
2318=item setnetent STAYOPEN
2319
2320=item setprotoent STAYOPEN
2321
2322=item setservent STAYOPEN
2323
2324=item endpwent
2325
2326=item endgrent
2327
2328=item endhostent
2329
2330=item endnetent
2331
2332=item endprotoent
2333
2334=item endservent
2335
80d38338
TC
2336These routines are the same as their counterparts in the
2337system C library. In list context, the return values from the
a0d0e21e
LW
2338various get routines are as follows:
2339
2340 ($name,$passwd,$uid,$gid,
6ee623d5 2341 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
a0d0e21e
LW
2342 ($name,$passwd,$gid,$members) = getgr*
2343 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
2344 ($name,$aliases,$addrtype,$net) = getnet*
2345 ($name,$aliases,$proto) = getproto*
2346 ($name,$aliases,$port,$proto) = getserv*
2347
3b10bc60 2348(If the entry doesn't exist you get an empty list.)
a0d0e21e 2349
4602f195
JH
2350The exact meaning of the $gcos field varies but it usually contains
2351the real name of the user (as opposed to the login name) and other
2352information pertaining to the user. Beware, however, that in many
2353system users are able to change this information and therefore it
106325ad 2354cannot be trusted and therefore the $gcos is tainted (see
2959b6e3 2355L<perlsec>). The $passwd and $shell, user's encrypted password and
a3390c9f 2356login shell, are also tainted, for the same reason.
4602f195 2357
5a964f20 2358In scalar context, you get the name, unless the function was a
a0d0e21e
LW
2359lookup by name, in which case you get the other thing, whatever it is.
2360(If the entry doesn't exist you get the undefined value.) For example:
2361
5a964f20
TC
2362 $uid = getpwnam($name);
2363 $name = getpwuid($num);
2364 $name = getpwent();
2365 $gid = getgrnam($name);
08a33e13 2366 $name = getgrgid($num);
5a964f20
TC
2367 $name = getgrent();
2368 #etc.
a0d0e21e 2369
4602f195 2370In I<getpw*()> the fields $quota, $comment, and $expire are special
80d38338 2371in that they are unsupported on many systems. If the
4602f195
JH
2372$quota is unsupported, it is an empty scalar. If it is supported, it
2373usually encodes the disk quota. If the $comment field is unsupported,
2374it is an empty scalar. If it is supported it usually encodes some
2375administrative comment about the user. In some systems the $quota
2376field may be $change or $age, fields that have to do with password
2377aging. In some systems the $comment field may be $class. The $expire
2378field, if present, encodes the expiration period of the account or the
2379password. For the availability and the exact meaning of these fields
8f1da26d 2380in your system, please consult getpwnam(3) and your system's
4602f195
JH
2381F<pwd.h> file. You can also find out from within Perl what your
2382$quota and $comment fields mean and whether you have the $expire field
2383by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2384C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
3b10bc60 2385files are supported only if your vendor has implemented them in the
4602f195 2386intuitive fashion that calling the regular C library routines gets the
5d3a0a3b 2387shadow versions if you're running under privilege or if there exists
cf264981 2388the shadow(3) functions as found in System V (this includes Solaris
a3390c9f 2389and Linux). Those systems that implement a proprietary shadow password
5d3a0a3b 2390facility are unlikely to be supported.
6ee623d5 2391
a3390c9f 2392The $members value returned by I<getgr*()> is a space-separated list of
a0d0e21e
LW
2393the login names of the members of the group.
2394
2395For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2396C, it will be returned to you via C<$?> if the function call fails. The
3b10bc60 2397C<@addrs> value returned by a successful call is a list of raw
2398addresses returned by the corresponding library call. In the
2399Internet domain, each address is four bytes long; you can unpack it
a0d0e21e
LW
2400by saying something like:
2401
f337b084 2402 ($a,$b,$c,$d) = unpack('W4',$addr[0]);
a0d0e21e 2403
2b5ab1e7
TC
2404The Socket library makes this slightly easier:
2405
2406 use Socket;
2407 $iaddr = inet_aton("127.1"); # or whatever address
2408 $name = gethostbyaddr($iaddr, AF_INET);
2409
2410 # or going the other way
19799a22 2411 $straddr = inet_ntoa($iaddr);
2b5ab1e7 2412
d760c846
GS
2413In the opposite way, to resolve a hostname to the IP address
2414you can write this:
2415
2416 use Socket;
2417 $packed_ip = gethostbyname("www.perl.org");
2418 if (defined $packed_ip) {
2419 $ip_address = inet_ntoa($packed_ip);
2420 }
2421
b018eaf1 2422Make sure C<gethostbyname()> is called in SCALAR context and that
d760c846
GS
2423its return value is checked for definedness.
2424
0d043efa
FC
2425The C<getprotobynumber> function, even though it only takes one argument,
2426has the precedence of a list operator, so beware:
2427
2428 getprotobynumber $number eq 'icmp' # WRONG
2429 getprotobynumber($number eq 'icmp') # actually means this
2430 getprotobynumber($number) eq 'icmp' # better this way
2431
19799a22
GS
2432If you get tired of remembering which element of the return list
2433contains which return value, by-name interfaces are provided
2434in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2435C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2436and C<User::grent>. These override the normal built-ins, supplying
2437versions that return objects with the appropriate names
2438for each field. For example:
5a964f20
TC
2439
2440 use File::stat;
2441 use User::pwent;
2442 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2443
a3390c9f 2444Even though it looks as though they're the same method calls (uid),
b76cc8ba 2445they aren't, because a C<File::stat> object is different from
19799a22 2446a C<User::pwent> object.
5a964f20 2447
ea9eb35a 2448Portability issues: L<perlport/getpwnam> to L<perlport/endservent>.
2449
a0d0e21e 2450=item getsockname SOCKET
d74e8afc 2451X<getsockname>
a0d0e21e 2452
19799a22
GS
2453Returns the packed sockaddr address of this end of the SOCKET connection,
2454in case you don't know the address because you have several different
2455IPs that the connection might have come in on.
a0d0e21e 2456
4633a7c4
LW
2457 use Socket;
2458 $mysockaddr = getsockname(SOCK);
19799a22 2459 ($port, $myaddr) = sockaddr_in($mysockaddr);
b76cc8ba 2460 printf "Connect to %s [%s]\n",
19799a22
GS
2461 scalar gethostbyaddr($myaddr, AF_INET),
2462 inet_ntoa($myaddr);
a0d0e21e
LW
2463
2464=item getsockopt SOCKET,LEVEL,OPTNAME
d74e8afc 2465X<getsockopt>
a0d0e21e 2466
636e6b1f
TH
2467Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
2468Options may exist at multiple protocol levels depending on the socket
2469type, but at least the uppermost socket level SOL_SOCKET (defined in the
2470C<Socket> module) will exist. To query options at another level the
2471protocol number of the appropriate protocol controlling the option
2472should be supplied. For example, to indicate that an option is to be
2473interpreted by the TCP protocol, LEVEL should be set to the protocol
80d38338 2474number of TCP, which you can get using C<getprotobyname>.
636e6b1f 2475
80d38338 2476The function returns a packed string representing the requested socket
3b10bc60 2477option, or C<undef> on error, with the reason for the error placed in
a3390c9f 2478C<$!>. Just what is in the packed string depends on LEVEL and OPTNAME;
80d38338
TC
2479consult getsockopt(2) for details. A common case is that the option is an
2480integer, in which case the result is a packed integer, which you can decode
2481using C<unpack> with the C<i> (or C<I>) format.
636e6b1f 2482
8f1da26d 2483Here's an example to test whether Nagle's algorithm is enabled on a socket:
636e6b1f 2484
4852725b 2485 use Socket qw(:all);
636e6b1f
TH
2486
2487 defined(my $tcp = getprotobyname("tcp"))
a9a5a0dc 2488 or die "Could not determine the protocol number for tcp";
4852725b
DD
2489 # my $tcp = IPPROTO_TCP; # Alternative
2490 my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
80d38338 2491 or die "getsockopt TCP_NODELAY: $!";
636e6b1f
TH
2492 my $nodelay = unpack("I", $packed);
2493 print "Nagle's algorithm is turned ", $nodelay ? "off\n" : "on\n";
2494
ea9eb35a 2495Portability issues: L<perlport/getsockopt>.
a0d0e21e 2496
15a348aa 2497=item given EXPR BLOCK
2498X<given>
2499
2500=item given BLOCK
2501
2502C<given> is analogous to the C<switch> keyword in other languages. C<given>
2503and C<when> are used in Perl to implement C<switch>/C<case> like statements.
8f1da26d 2504Only available after Perl 5.10. For example:
15a348aa 2505
8f1da26d 2506 use v5.10;
15a348aa 2507 given ($fruit) {
2508 when (/apples?/) {
2509 print "I like apples."
2510 }
2511 when (/oranges?/) {
2512 print "I don't like oranges."
2513 }
2514 default {
2515 print "I don't like anything"
2516 }
2517 }
2518
2519See L<perlsyn/"Switch statements"> for detailed information.
2520
a0d0e21e 2521=item glob EXPR
d74e8afc 2522X<glob> X<wildcard> X<filename, expansion> X<expand>
a0d0e21e 2523
0a753a76
PP
2524=item glob
2525
d9a9d457
JL
2526In list context, returns a (possibly empty) list of filename expansions on
2527the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
2528scalar context, glob iterates through such filename expansions, returning
2529undef when the list is exhausted. This is the internal function
2530implementing the C<< <*.c> >> operator, but you can use it directly. If
2531EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2532more detail in L<perlop/"I/O Operators">.
a0d0e21e 2533
80d38338
TC
2534Note that C<glob> splits its arguments on whitespace and treats
2535each segment as separate pattern. As such, C<glob("*.c *.h")>
2536matches all files with a F<.c> or F<.h> extension. The expression
b474a1b1 2537C<glob(".* *")> matches all files in the current working directory.
80d38338
TC
2538
2539If non-empty braces are the only wildcard characters used in the
2540C<glob>, no filenames are matched, but potentially many strings
2541are returned. For example, this produces nine strings, one for
2542each pairing of fruits and colors:
2543
2544 @many = glob "{apple,tomato,cherry}={green,yellow,red}";
5c0c9249 2545
3a4b19e4 2546Beginning with v5.6.0, this operator is implemented using the standard
5c0c9249
PF
2547C<File::Glob> extension. See L<File::Glob> for details, including
2548C<bsd_glob> which does not treat whitespace as a pattern separator.
3a4b19e4 2549
ea9eb35a 2550Portability issues: L<perlport/glob>.
2551
a0d0e21e 2552=item gmtime EXPR
d74e8afc 2553X<gmtime> X<UTC> X<Greenwich>
a0d0e21e 2554
ce2984c3
PF
2555=item gmtime
2556
4509d391 2557Works just like L</localtime> but the returned values are
435fbc73 2558localized for the standard Greenwich time zone.
a0d0e21e 2559
a3390c9f
FC
2560Note: When called in list context, $isdst, the last value
2561returned by gmtime, is always C<0>. There is no
435fbc73 2562Daylight Saving Time in GMT.
0a753a76 2563
ea9eb35a 2564Portability issues: L<perlport/gmtime>.
62aa5637 2565
a0d0e21e 2566=item goto LABEL
d74e8afc 2567X<goto> X<jump> X<jmp>
a0d0e21e 2568
748a9306
LW
2569=item goto EXPR
2570
a0d0e21e
LW
2571=item goto &NAME
2572
b500e03b
GG
2573The C<goto-LABEL> form finds the statement labeled with LABEL and
2574resumes execution there. It can't be used to get out of a block or
2575subroutine given to C<sort>. It can be used to go almost anywhere
2576else within the dynamic scope, including out of subroutines, but it's
2577usually better to use some other construct such as C<last> or C<die>.
2578The author of Perl has never felt the need to use this form of C<goto>
3b10bc60 2579(in Perl, that is; C is another matter). (The difference is that C
b500e03b
GG
2580does not offer named loops combined with loop control. Perl does, and
2581this replaces most structured uses of C<goto> in other languages.)
a0d0e21e 2582
7660c0ab
A
2583The C<goto-EXPR> form expects a label name, whose scope will be resolved
2584dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
748a9306
LW
2585necessarily recommended if you're optimizing for maintainability:
2586
2587 goto ("FOO", "BAR", "GLARCH")[$i];
2588
887d89fd
FC
2589As shown in this example, C<goto-EXPR> is exempt from the "looks like a
2590function" rule. A pair of parentheses following it does not (necessarily)
2591delimit its argument. C<goto("NE")."XT"> is equivalent to C<goto NEXT>.
2592
b500e03b 2593Use of C<goto-LABEL> or C<goto-EXPR> to jump into a construct is
0b98bec9 2594deprecated and will issue a warning. Even then, it may not be used to
b500e03b
GG
2595go into any construct that requires initialization, such as a
2596subroutine or a C<foreach> loop. It also can't be used to go into a
0b98bec9 2597construct that is optimized away.
b500e03b 2598
1b6921cb
BT
2599The C<goto-&NAME> form is quite different from the other forms of
2600C<goto>. In fact, it isn't a goto in the normal sense at all, and
2601doesn't have the stigma associated with other gotos. Instead, it
2602exits the current subroutine (losing any changes set by local()) and
2603immediately calls in its place the named subroutine using the current
2604value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2605load another subroutine and then pretend that the other subroutine had
2606been called in the first place (except that any modifications to C<@_>
6cb9131c
GS
2607in the current subroutine are propagated to the other subroutine.)
2608After the C<goto>, not even C<caller> will be able to tell that this
2609routine was called first.
2610
2611NAME needn't be the name of a subroutine; it can be a scalar variable
8f1da26d 2612containing a code reference or a block that evaluates to a code
6cb9131c 2613reference.
a0d0e21e
LW
2614
2615=item grep BLOCK LIST
d74e8afc 2616X<grep>
a0d0e21e
LW
2617
2618=item grep EXPR,LIST
2619
2b5ab1e7
TC
2620This is similar in spirit to, but not the same as, grep(1) and its
2621relatives. In particular, it is not limited to using regular expressions.
2f9daede 2622
a0d0e21e 2623Evaluates the BLOCK or EXPR for each element of LIST (locally setting
7660c0ab 2624C<$_> to each element) and returns the list value consisting of those
19799a22
GS
2625elements for which the expression evaluated to true. In scalar
2626context, returns the number of times the expression was true.
a0d0e21e
LW
2627
2628 @foo = grep(!/^#/, @bar); # weed out comments
2629
2630or equivalently,
2631
2632 @foo = grep {!/^#/} @bar; # weed out comments
2633
be3174d2
GS
2634Note that C<$_> is an alias to the list value, so it can be used to
2635modify the elements of the LIST. While this is useful and supported,
2636it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
2637Similarly, grep returns aliases into the original list, much as a for
2638loop's index variable aliases the list elements. That is, modifying an
19799a22
GS
2639element of a list returned by grep (for example, in a C<foreach>, C<map>
2640or another C<grep>) actually modifies the element in the original list.
2b5ab1e7 2641This is usually something to be avoided when writing clear code.
a0d0e21e 2642
a4fb8298 2643If C<$_> is lexical in the scope where the C<grep> appears (because it has
cf264981 2644been declared with C<my $_>) then, in addition to being locally aliased to
80d38338 2645the list elements, C<$_> keeps being lexical inside the block; i.e., it
a4fb8298
RGS
2646can't be seen from the outside, avoiding any potential side-effects.
2647
19799a22 2648See also L</map> for a list composed of the results of the BLOCK or EXPR.
38325410 2649
a0d0e21e 2650=item hex EXPR
d74e8afc 2651X<hex> X<hexadecimal>
a0d0e21e 2652
54310121 2653=item hex
bbce6d69 2654
2b5ab1e7 2655Interprets EXPR as a hex string and returns the corresponding value.
38366c11 2656(To convert strings that might start with either C<0>, C<0x>, or C<0b>, see
2b5ab1e7 2657L</oct>.) If EXPR is omitted, uses C<$_>.
2f9daede
TPG
2658
2659 print hex '0xAf'; # prints '175'
2660 print hex 'aF'; # same
a0d0e21e 2661
19799a22 2662Hex strings may only represent integers. Strings that would cause
53305cf1 2663integer overflow trigger a warning. Leading whitespace is not stripped,
38366c11 2664unlike oct(). To present something as hex, look into L</printf>,
8f1da26d 2665L</sprintf>, and L</unpack>.
19799a22 2666
ce2984c3 2667=item import LIST
d74e8afc 2668X<import>
a0d0e21e 2669
19799a22 2670There is no builtin C<import> function. It is just an ordinary
4633a7c4 2671method (subroutine) defined (or inherited) by modules that wish to export
19799a22 2672names to another module. The C<use> function calls the C<import> method
cea6626f 2673for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
a0d0e21e
LW
2674
2675=item index STR,SUBSTR,POSITION
d74e8afc 2676X<index> X<indexOf> X<InStr>
a0d0e21e
LW
2677
2678=item index STR,SUBSTR
2679
2b5ab1e7
TC
2680The index function searches for one string within another, but without
2681the wildcard-like behavior of a full regular-expression pattern match.
2682It returns the position of the first occurrence of SUBSTR in STR at
2683or after POSITION. If POSITION is omitted, starts searching from the
26f149de
YST
2684beginning of the string. POSITION before the beginning of the string
2685or after its end is treated as if it were the beginning or the end,
e1dccc0d
Z
2686respectively. POSITION and the return value are based at zero.
2687If the substring is not found, C<index> returns -1.
a0d0e21e
LW
2688
2689=item int EXPR
f723aae1 2690X<int> X<integer> X<truncate> X<trunc> X<floor>
a0d0e21e 2691
54310121 2692=item int
bbce6d69 2693
7660c0ab 2694Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2b5ab1e7 2695You should not use this function for rounding: one because it truncates
3b10bc60 2696towards C<0>, and two because machine representations of floating-point
2b5ab1e7
TC
2697numbers can sometimes produce counterintuitive results. For example,
2698C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2699because it's really more like -268.99999999999994315658 instead. Usually,
19799a22 2700the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2b5ab1e7 2701functions will serve you better than will int().
a0d0e21e
LW
2702
2703=item ioctl FILEHANDLE,FUNCTION,SCALAR
d74e8afc 2704X<ioctl>
a0d0e21e 2705
2b5ab1e7 2706Implements the ioctl(2) function. You'll probably first have to say
a0d0e21e 2707
5ed4f2ec 2708 require "sys/ioctl.ph"; # probably in $Config{archlib}/sys/ioctl.ph
a0d0e21e 2709
a11c483f 2710to get the correct function definitions. If F<sys/ioctl.ph> doesn't
a0d0e21e 2711exist or doesn't have the correct definitions you'll have to roll your
61eff3bc 2712own, based on your C header files such as F<< <sys/ioctl.h> >>.
5a964f20 2713(There is a Perl script called B<h2ph> that comes with the Perl kit that
54310121 2714may help you in this, but it's nontrivial.) SCALAR will be read and/or
3b10bc60 2715written depending on the FUNCTION; a C pointer to the string value of SCALAR
19799a22 2716will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
4633a7c4
LW
2717has no string value but does have a numeric value, that value will be
2718passed rather than a pointer to the string value. To guarantee this to be
19799a22
GS
2719true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2720functions may be needed to manipulate the values of structures used by
b76cc8ba 2721C<ioctl>.
a0d0e21e 2722
19799a22 2723The return value of C<ioctl> (and C<fcntl>) is as follows:
a0d0e21e 2724
5ed4f2ec 2725 if OS returns: then Perl returns:
2726 -1 undefined value
2727 0 string "0 but true"
2728 anything else that number
a0d0e21e 2729
19799a22 2730Thus Perl returns true on success and false on failure, yet you can
a0d0e21e
LW
2731still easily determine the actual value returned by the operating
2732system:
2733
2b5ab1e7 2734 $retval = ioctl(...) || -1;
a0d0e21e
LW
2735 printf "System returned %d\n", $retval;
2736
be2f7487 2737The special string C<"0 but true"> is exempt from B<-w> complaints
5a964f20
TC
2738about improper numeric conversions.
2739
ea9eb35a 2740Portability issues: L<perlport/ioctl>.
2741
a0d0e21e 2742=item join EXPR,LIST
d74e8afc 2743X<join>
a0d0e21e 2744
2b5ab1e7
TC
2745Joins the separate strings of LIST into a single string with fields
2746separated by the value of EXPR, and returns that new string. Example:
a0d0e21e 2747
2b5ab1e7 2748 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
a0d0e21e 2749
eb6e2d6f
GS
2750Beware that unlike C<split>, C<join> doesn't take a pattern as its
2751first argument. Compare L</split>.
a0d0e21e 2752
532eee96 2753=item keys HASH
d74e8afc 2754X<keys> X<key>
aa689395 2755
532eee96 2756=item keys ARRAY
aeedbbed 2757
f5a93a43
TC
2758=item keys EXPR
2759
aeedbbed
NC
2760Returns a list consisting of all the keys of the named hash, or the indices
2761of an array. (In scalar context, returns the number of keys or indices.)
504f80c1 2762
aeedbbed 2763The keys of a hash are returned in an apparently random order. The actual
3b10bc60 2764random order is subject to change in future versions of Perl, but it
504f80c1 2765is guaranteed to be the same order as either the C<values> or C<each>
4546b9e6 2766function produces (given that the hash has not been modified). Since
c5f61d2f 2767Perl 5.8.1 the ordering can be different even between different runs of
4546b9e6 2768Perl for security reasons (see L<perlsec/"Algorithmic Complexity
d6df3700 2769Attacks">).
504f80c1 2770
8f1da26d 2771As a side effect, calling keys() resets the internal interator of the HASH or ARRAY
cf264981
SP
2772(see L</each>). In particular, calling keys() in void context resets
2773the iterator with no other overhead.
a0d0e21e 2774
aa689395 2775Here is yet another way to print your environment:
a0d0e21e
LW
2776
2777 @keys = keys %ENV;
2778 @values = values %ENV;
b76cc8ba 2779 while (@keys) {
a9a5a0dc 2780 print pop(@keys), '=', pop(@values), "\n";
a0d0e21e
LW
2781 }
2782
2783or how about sorted by key:
2784
2785 foreach $key (sort(keys %ENV)) {
a9a5a0dc 2786 print $key, '=', $ENV{$key}, "\n";
a0d0e21e
LW
2787 }
2788
8ea1e5d4
GS
2789The returned values are copies of the original keys in the hash, so
2790modifying them will not affect the original hash. Compare L</values>.
2791
19799a22 2792To sort a hash by value, you'll need to use a C<sort> function.
aa689395 2793Here's a descending numeric sort of a hash by its values:
4633a7c4 2794
5a964f20 2795 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
a9a5a0dc 2796 printf "%4d %s\n", $hash{$key}, $key;
4633a7c4
LW
2797 }
2798
3b10bc60 2799Used as an lvalue, C<keys> allows you to increase the number of hash buckets
aa689395
PP
2800allocated for the given hash. This can gain you a measure of efficiency if
2801you know the hash is going to get big. (This is similar to pre-extending
2802an array by assigning a larger number to $#array.) If you say
55497cff
PP
2803
2804 keys %hash = 200;
2805
ab192400
GS
2806then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2807in fact, since it rounds up to the next power of two. These
55497cff
PP
2808buckets will be retained even if you do C<%hash = ()>, use C<undef
2809%hash> if you want to free the storage while C<%hash> is still in scope.
2810You can't shrink the number of buckets allocated for the hash using
19799a22 2811C<keys> in this way (but you needn't worry about doing this by accident,
aeedbbed
NC
2812as trying has no effect). C<keys @array> in an lvalue context is a syntax
2813error.
55497cff 2814
f5a93a43
TC
2815Starting with Perl 5.14, C<keys> can take a scalar EXPR, which must contain
2816a reference to an unblessed hash or array. The argument will be
2817dereferenced automatically. This aspect of C<keys> is considered highly
2818experimental. The exact behaviour may change in a future version of Perl.
cba5a3b0
DG
2819
2820 for (keys $hashref) { ... }
2821 for (keys $obj->get_arrayref) { ... }
2822
8f1da26d 2823See also C<each>, C<values>, and C<sort>.
ab192400 2824
b350dd2f 2825=item kill SIGNAL, LIST
d74e8afc 2826X<kill> X<signal>
a0d0e21e 2827
b350dd2f 2828Sends a signal to a list of processes. Returns the number of
517db077
GS
2829processes successfully signaled (which is not necessarily the
2830same as the number actually killed).
a0d0e21e
LW
2831
2832 $cnt = kill 1, $child1, $child2;
2833 kill 9, @goners;
2834
3b10bc60 2835If SIGNAL is zero, no signal is sent to the process, but C<kill>
2836checks whether it's I<possible> to send a signal to it (that
70fb64f6 2837means, to be brief, that the process is owned by the same user, or we are
3b10bc60 2838the super-user). This is useful to check that a child process is still
81fd35db
DN
2839alive (even if only as a zombie) and hasn't changed its UID. See
2840L<perlport> for notes on the portability of this construct.
b350dd2f 2841
e2c0f81f
DG
2842Unlike in the shell, if SIGNAL is negative, it kills process groups instead
2843of processes. That means you usually want to use positive not negative signals.
2844You may also use a signal name in quotes.
2845
2846The behavior of kill when a I<PROCESS> number is zero or negative depends on
2847the operating system. For example, on POSIX-conforming systems, zero will
2848signal the current process group and -1 will signal all processes.
1e9c1022
JL
2849
2850See L<perlipc/"Signals"> for more details.
a0d0e21e 2851
ea9eb35a 2852On some platforms such as Windows where the fork() system call is not available.
2853Perl can be built to emulate fork() at the interpreter level.
6d17f725 2854This emulation has limitations related to kill that have to be considered,
ea9eb35a 2855for code running on Windows and in code intended to be portable.
2856
2857See L<perlfork> for more details.
2858
2859Portability issues: L<perlport/kill>.
2860
a0d0e21e 2861=item last LABEL
d74e8afc 2862X<last> X<break>
a0d0e21e
LW
2863
2864=item last
2865
2866The C<last> command is like the C<break> statement in C (as used in
2867loops); it immediately exits the loop in question. If the LABEL is
2868omitted, the command refers to the innermost enclosing loop. The
2869C<continue> block, if any, is not executed:
2870
4633a7c4 2871 LINE: while (<STDIN>) {
a9a5a0dc
VP
2872 last LINE if /^$/; # exit when done with header
2873 #...
a0d0e21e
LW
2874 }
2875
80d38338 2876C<last> cannot be used to exit a block that returns a value such as
8f1da26d 2877C<eval {}>, C<sub {}>, or C<do {}>, and should not be used to exit
2b5ab1e7 2878a grep() or map() operation.
4968c1e4 2879
6c1372ed
GS
2880Note that a block by itself is semantically identical to a loop
2881that executes once. Thus C<last> can be used to effect an early
2882exit out of such a block.
2883
98293880
JH
2884See also L</continue> for an illustration of how C<last>, C<next>, and
2885C<redo> work.
1d2dff63 2886
a0d0e21e 2887=item lc EXPR
d74e8afc 2888X<lc> X<lowercase>
a0d0e21e 2889
54310121 2890=item lc
bbce6d69 2891
d1be9408 2892Returns a lowercased version of EXPR. This is the internal function
3980dc9c 2893implementing the C<\L> escape in double-quoted strings.
a0d0e21e 2894
7660c0ab 2895If EXPR is omitted, uses C<$_>.
bbce6d69 2896
3980dc9c
KW
2897What gets returned depends on several factors:
2898
2899=over
2900
2901=item If C<use bytes> is in effect:
2902
2903=over
2904
2905=item On EBCDIC platforms
2906
2907The results are what the C language system call C<tolower()> returns.
2908
2909=item On ASCII platforms
2910
2911The results follow ASCII semantics. Only characters C<A-Z> change, to C<a-z>
2912respectively.
2913
2914=back
2915
2916=item Otherwise, If EXPR has the UTF8 flag set
2917
5d1892be 2918Unicode semantics are used for the case change.
3980dc9c
KW
2919
2920=item Otherwise, if C<use locale> is in effect
2921
2922Respects current LC_CTYPE locale. See L<perllocale>.
2923
2924=item Otherwise, if C<use feature 'unicode_strings'> is in effect:
2925
5d1892be 2926Unicode semantics are used for the case change.
3980dc9c
KW
2927
2928=item Otherwise:
2929
2930=over
2931
2932=item On EBCDIC platforms
2933
2934The results are what the C language system call C<tolower()> returns.
2935
2936=item On ASCII platforms
2937
2938ASCII semantics are used for the case change. The lowercase of any character
2939outside the ASCII range is the character itself.
2940
2941=back
2942
2943=back
2944
a0d0e21e 2945=item lcfirst EXPR
d74e8afc 2946X<lcfirst> X<lowercase>
a0d0e21e 2947
54310121 2948=item lcfirst
bbce6d69 2949
ad0029c4
JH
2950Returns the value of EXPR with the first character lowercased. This
2951is the internal function implementing the C<\l> escape in
3980dc9c 2952double-quoted strings.
a0d0e21e 2953
7660c0ab 2954If EXPR is omitted, uses C<$_>.
bbce6d69 2955
15dbbbab 2956This function behaves the same way under various pragmata, such as in a locale,
3980dc9c
KW
2957as L</lc> does.
2958
a0d0e21e 2959=item length EXPR
d74e8afc 2960X<length> X<size>
a0d0e21e 2961
54310121 2962=item length
bbce6d69 2963
974da8e5 2964Returns the length in I<characters> of the value of EXPR. If EXPR is
15dbbbab
FC
2965omitted, returns the length of C<$_>. If EXPR is undefined, returns
2966C<undef>.
3b10bc60 2967
2968This function cannot be used on an entire array or hash to find out how
2969many elements these have. For that, use C<scalar @array> and C<scalar keys
2970%hash>, respectively.
2971
2972Like all Perl character operations, length() normally deals in logical
2973characters, not physical bytes. For how many bytes a string encoded as
2974UTF-8 would take up, use C<length(Encode::encode_utf8(EXPR))> (you'll have
2975to C<use Encode> first). See L<Encode> and L<perlunicode>.
974da8e5 2976
cfa52385
FC
2977=item __LINE__
2978X<__LINE__>
2979
2980A special token that compiles to the current line number.
2981
a0d0e21e 2982=item link OLDFILE,NEWFILE
d74e8afc 2983X<link>
a0d0e21e 2984
19799a22 2985Creates a new filename linked to the old filename. Returns true for
b76cc8ba 2986success, false otherwise.
a0d0e21e 2987
ea9eb35a 2988Portability issues: L<perlport/link>.
2989
a0d0e21e 2990=item listen SOCKET,QUEUESIZE
d74e8afc 2991X<listen>
a0d0e21e 2992
3b10bc60 2993Does the same thing that the listen(2) system call does. Returns true if
b76cc8ba 2994it succeeded, false otherwise. See the example in
cea6626f 2995L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
2996
2997=item local EXPR
d74e8afc 2998X<local>
a0d0e21e 2999
19799a22 3000You really probably want to be using C<my> instead, because C<local> isn't
b76cc8ba 3001what most people think of as "local". See
13a2d996 3002L<perlsub/"Private Variables via my()"> for details.
2b5ab1e7 3003
5a964f20
TC
3004A local modifies the listed variables to be local to the enclosing
3005block, file, or eval. If more than one value is listed, the list must
3006be placed in parentheses. See L<perlsub/"Temporary Values via local()">
3007for details, including issues with tied arrays and hashes.
a0d0e21e 3008
d361fafa
VP
3009The C<delete local EXPR> construct can also be used to localize the deletion
3010of array/hash elements to the current block.
3011See L<perlsub/"Localized deletion of elements of composite types">.
3012
a0d0e21e 3013=item localtime EXPR
435fbc73 3014X<localtime> X<ctime>
a0d0e21e 3015
ba053783
AL
3016=item localtime
3017
19799a22 3018Converts a time as returned by the time function to a 9-element list
5f05dabc 3019with the time analyzed for the local time zone. Typically used as
a0d0e21e
LW
3020follows:
3021
54310121 3022 # 0 1 2 3 4 5 6 7 8
a0d0e21e 3023 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
ba053783 3024 localtime(time);
a0d0e21e 3025
8f1da26d 3026All list elements are numeric and come straight out of the C `struct
ba053783
AL
3027tm'. C<$sec>, C<$min>, and C<$hour> are the seconds, minutes, and hours
3028of the specified time.
48a26b3a 3029
8f1da26d
TC
3030C<$mday> is the day of the month and C<$mon> the month in
3031the range C<0..11>, with 0 indicating January and 11 indicating December.
ba053783 3032This makes it easy to get a month name from a list:
54310121 3033
ba053783
AL
3034 my @abbr = qw( Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec );
3035 print "$abbr[$mon] $mday";
3036 # $mon=9, $mday=18 gives "Oct 18"
abd75f24 3037
8f1da26d 3038C<$year> is the number of years since 1900, B<not> just the last two digits
ba053783 3039of the year. That is, C<$year> is C<123> in year 2023. The proper way
80d38338 3040to get a 4-digit year is simply:
abd75f24 3041
ba053783 3042 $year += 1900;
abd75f24 3043
435fbc73
GS
3044Otherwise you create non-Y2K-compliant programs--and you wouldn't want
3045to do that, would you?
3046
8f1da26d 3047To get the last two digits of the year (e.g., "01" in 2001) do:
ba053783
AL
3048
3049 $year = sprintf("%02d", $year % 100);
3050
3051C<$wday> is the day of the week, with 0 indicating Sunday and 3 indicating
3052Wednesday. C<$yday> is the day of the year, in the range C<0..364>
3053(or C<0..365> in leap years.)
3054
3055C<$isdst> is true if the specified time occurs during Daylight Saving
3056Time, false otherwise.
abd75f24 3057
e1998452 3058If EXPR is omitted, C<localtime()> uses the current time (as returned
e3176d09 3059by time(3)).
a0d0e21e 3060
48a26b3a 3061In scalar context, C<localtime()> returns the ctime(3) value:
a0d0e21e