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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
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164C<caller>, C<continue>, C<die>, C<do>, C<dump>, C<eval>, C<exit>,
165C<goto>, C<last>, C<next>, C<redo>, C<return>, C<sub>, C<wantarray>
cb1a09d0 166
8f1da26d 167=item Keywords related to the switch feature
0d863452 168
8f1da26d 169C<break>, C<continue>, C<default, >C<given>, C<when>
0d863452 170
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171These are available only if you enable the C<"switch"> feature.
172See L<feature> and L<perlsyn/"Switch statements">.
173Alternately, include a C<use v5.10> or later to the current scope.
0d863452 174
54310121 175=item Keywords related to scoping
cb1a09d0 176
8f1da26d 177C<caller>, C<import>, C<local>, C<my>, C<our>, C<package>, C<state>, C<use>
36fb85f3 178
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179C<state> is available only if the C<"state"> feature is enabled. See
180L<feature>. Alternately, include a C<use v5.10> or later to the current scope.
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181
182=item Miscellaneous functions
183
36fb85f3 184C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<our>,
834df1c5 185C<reset>, C<scalar>, C<state>, C<undef>, C<wantarray>
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186
187=item Functions for processes and process groups
d74e8afc 188X<process> X<pid> X<process id>
cb1a09d0 189
22fae026 190C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
1dc8ecb8 191C<pipe>, C<qx//>, C<setpgrp>, C<setpriority>, C<sleep>, C<system>,
22fae026 192C<times>, C<wait>, C<waitpid>
cb1a09d0 193
3b10bc60 194=item Keywords related to Perl modules
d74e8afc 195X<module>
cb1a09d0 196
22fae026 197C<do>, C<import>, C<no>, C<package>, C<require>, C<use>
cb1a09d0 198
353c6505 199=item Keywords related to classes and object-orientation
d74e8afc 200X<object> X<class> X<package>
cb1a09d0 201
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202C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
203C<untie>, C<use>
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204
205=item Low-level socket functions
d74e8afc 206X<socket> X<sock>
cb1a09d0 207
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208C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
209C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
737dd4b4 210C<socket>, C<socketpair>
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211
212=item System V interprocess communication functions
d74e8afc 213X<IPC> X<System V> X<semaphore> X<shared memory> X<memory> X<message>
cb1a09d0 214
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215C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
216C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>
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217
218=item Fetching user and group info
d74e8afc 219X<user> X<group> X<password> X<uid> X<gid> X<passwd> X</etc/passwd>
cb1a09d0 220
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221C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
222C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
223C<getpwuid>, C<setgrent>, C<setpwent>
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224
225=item Fetching network info
d74e8afc 226X<network> X<protocol> X<host> X<hostname> X<IP> X<address> X<service>
cb1a09d0 227
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228C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
229C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
230C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
231C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
232C<setnetent>, C<setprotoent>, C<setservent>
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233
234=item Time-related functions
d74e8afc 235X<time> X<date>
cb1a09d0 236
22fae026 237C<gmtime>, C<localtime>, C<time>, C<times>
cb1a09d0 238
37798a01 239=item Functions new in perl5
d74e8afc 240X<perl5>
37798a01 241
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242C<abs>, C<bless>, C<break>, C<chomp>, C<chr>, C<continue>, C<default>,
243C<exists>, C<formline>, C<given>, C<glob>, C<import>, C<lc>, C<lcfirst>,
1dc8ecb8 244C<lock>, C<map>, C<my>, C<no>, C<our>, C<prototype>, C<qr//>, C<qw//>, C<qx//>,
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245C<readline>, C<readpipe>, C<ref>, C<sub>*, C<sysopen>, C<tie>, C<tied>, C<uc>,
246C<ucfirst>, C<untie>, C<use>, C<when>
37798a01 247
3b10bc60 248* C<sub> was a keyword in Perl 4, but in Perl 5 it is an
5a964f20 249operator, which can be used in expressions.
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250
251=item Functions obsoleted in perl5
252
22fae026 253C<dbmclose>, C<dbmopen>
37798a01 254
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255=back
256
60f9f73c 257=head2 Portability
d74e8afc 258X<portability> X<Unix> X<portable>
60f9f73c 259
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260Perl was born in Unix and can therefore access all common Unix
261system calls. In non-Unix environments, the functionality of some
8f1da26d 262Unix system calls may not be available or details of the available
2b5ab1e7 263functionality may differ slightly. The Perl functions affected
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264by this are:
265
266C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>,
267C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>,
268C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>,
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269C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostbyname>,
270C<gethostent>, C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
54d7b083 271C<getppid>, C<getpgrp>, C<getpriority>, C<getprotobynumber>,
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272C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>,
273C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>,
274C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>,
2b5ab1e7 275C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<rename>, C<select>, C<semctl>,
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276C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>,
277C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>,
278C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>,
737dd4b4 279C<shmwrite>, C<socket>, C<socketpair>,
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280C<stat>, C<symlink>, C<syscall>, C<sysopen>, C<system>,
281C<times>, C<truncate>, C<umask>, C<unlink>,
2b5ab1e7 282C<utime>, C<wait>, C<waitpid>
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283
284For more information about the portability of these functions, see
285L<perlport> and other available platform-specific documentation.
286
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287=head2 Alphabetical Listing of Perl Functions
288
3b10bc60 289=over
a0d0e21e 290
5b3c99c0 291=item -X FILEHANDLE
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292X<-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>
293X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
a0d0e21e 294
5b3c99c0 295=item -X EXPR
a0d0e21e 296
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297=item -X DIRHANDLE
298
5b3c99c0 299=item -X
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300
301A file test, where X is one of the letters listed below. This unary
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302operator takes one argument, either a filename, a filehandle, or a dirhandle,
303and tests the associated file to see if something is true about it. If the
7660c0ab 304argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
19799a22 305Unless otherwise documented, it returns C<1> for true and C<''> for false, or
a0d0e21e 306the undefined value if the file doesn't exist. Despite the funny
d0821a6a 307names, precedence is the same as any other named unary operator. The
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308operator may be any of:
309
5ed4f2ec 310 -r File is readable by effective uid/gid.
311 -w File is writable by effective uid/gid.
312 -x File is executable by effective uid/gid.
313 -o File is owned by effective uid.
a0d0e21e 314
5ed4f2ec 315 -R File is readable by real uid/gid.
316 -W File is writable by real uid/gid.
317 -X File is executable by real uid/gid.
318 -O File is owned by real uid.
a0d0e21e 319
5ed4f2ec 320 -e File exists.
321 -z File has zero size (is empty).
322 -s File has nonzero size (returns size in bytes).
a0d0e21e 323
5ed4f2ec 324 -f File is a plain file.
325 -d File is a directory.
326 -l File is a symbolic link.
327 -p File is a named pipe (FIFO), or Filehandle is a pipe.
328 -S File is a socket.
329 -b File is a block special file.
330 -c File is a character special file.
331 -t Filehandle is opened to a tty.
a0d0e21e 332
5ed4f2ec 333 -u File has setuid bit set.
334 -g File has setgid bit set.
335 -k File has sticky bit set.
a0d0e21e 336
5ed4f2ec 337 -T File is an ASCII text file (heuristic guess).
338 -B File is a "binary" file (opposite of -T).
a0d0e21e 339
5ed4f2ec 340 -M Script start time minus file modification time, in days.
341 -A Same for access time.
342 -C Same for inode change time (Unix, may differ for other platforms)
a0d0e21e 343
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344Example:
345
346 while (<>) {
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347 chomp;
348 next unless -f $_; # ignore specials
349 #...
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350 }
351
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352Note that C<-s/a/b/> does not do a negated substitution. Saying
353C<-exp($foo)> still works as expected, however: only single letters
354following a minus are interpreted as file tests.
355
356These operators are exempt from the "looks like a function rule" described
357above. That is, an opening parenthesis after the operator does not affect
358how much of the following code constitutes the argument. Put the opening
359parentheses before the operator to separate it from code that follows (this
360applies only to operators with higher precedence than unary operators, of
361course):
362
363 -s($file) + 1024 # probably wrong; same as -s($file + 1024)
364 (-s $file) + 1024 # correct
365
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366The interpretation of the file permission operators C<-r>, C<-R>,
367C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
368of the file and the uids and gids of the user. There may be other
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369reasons you can't actually read, write, or execute the file: for
370example network filesystem access controls, ACLs (access control lists),
371read-only filesystems, and unrecognized executable formats. Note
372that the use of these six specific operators to verify if some operation
373is possible is usually a mistake, because it may be open to race
374conditions.
5ff3f7a4 375
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376Also note that, for the superuser on the local filesystems, the C<-r>,
377C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
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378if any execute bit is set in the mode. Scripts run by the superuser
379may thus need to do a stat() to determine the actual mode of the file,
2b5ab1e7 380or temporarily set their effective uid to something else.
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381
382If you are using ACLs, there is a pragma called C<filetest> that may
383produce more accurate results than the bare stat() mode bits.
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384When under C<use filetest 'access'> the above-mentioned filetests
385test whether the permission can(not) be granted using the
3b10bc60 386access(2) family of system calls. Also note that the C<-x> and C<-X> may
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387under this pragma return true even if there are no execute permission
388bits set (nor any extra execute permission ACLs). This strangeness is
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389due to the underlying system calls' definitions. Note also that, due to
390the implementation of C<use filetest 'access'>, the C<_> special
391filehandle won't cache the results of the file tests when this pragma is
392in effect. Read the documentation for the C<filetest> pragma for more
393information.
5ff3f7a4 394
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395The C<-T> and C<-B> switches work as follows. The first block or so of the
396file is examined for odd characters such as strange control codes or
61eff3bc 397characters with the high bit set. If too many strange characters (>30%)
cf264981 398are found, it's a C<-B> file; otherwise it's a C<-T> file. Also, any file
3b10bc60 399containing a zero byte in the first block is considered a binary file. If C<-T>
9124316e 400or C<-B> is used on a filehandle, the current IO buffer is examined
3b10bc60 401rather than the first block. Both C<-T> and C<-B> return true on an empty
54310121 402file, or a file at EOF when testing a filehandle. Because you have to
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403read a file to do the C<-T> test, on most occasions you want to use a C<-f>
404against the file first, as in C<next unless -f $file && -T $file>.
a0d0e21e 405
5dac7880 406If any of the file tests (or either the C<stat> or C<lstat> operator) is given
28757baa 407the special filehandle consisting of a solitary underline, then the stat
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408structure of the previous file test (or stat operator) is used, saving
409a system call. (This doesn't work with C<-t>, and you need to remember
3b10bc60 410that lstat() and C<-l> leave values in the stat structure for the
5c9aa243 411symbolic link, not the real file.) (Also, if the stat buffer was filled by
cf264981 412an C<lstat> call, C<-T> and C<-B> will reset it with the results of C<stat _>).
5c9aa243 413Example:
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414
415 print "Can do.\n" if -r $a || -w _ || -x _;
416
417 stat($filename);
418 print "Readable\n" if -r _;
419 print "Writable\n" if -w _;
420 print "Executable\n" if -x _;
421 print "Setuid\n" if -u _;
422 print "Setgid\n" if -g _;
423 print "Sticky\n" if -k _;
424 print "Text\n" if -T _;
425 print "Binary\n" if -B _;
426
fbb0b3b3
RGS
427As of Perl 5.9.1, as a form of purely syntactic sugar, you can stack file
428test operators, in a way that C<-f -w -x $file> is equivalent to
3b10bc60 429C<-x $file && -w _ && -f _>. (This is only fancy fancy: if you use
fbb0b3b3
RGS
430the return value of C<-f $file> as an argument to another filetest
431operator, no special magic will happen.)
432
bee96257 433Portability issues: L<perlport/-X>.
ea9eb35a 434
a0d0e21e 435=item abs VALUE
d74e8afc 436X<abs> X<absolute>
a0d0e21e 437
54310121 438=item abs
bbce6d69 439
a0d0e21e 440Returns the absolute value of its argument.
7660c0ab 441If VALUE is omitted, uses C<$_>.
a0d0e21e
LW
442
443=item accept NEWSOCKET,GENERICSOCKET
d74e8afc 444X<accept>
a0d0e21e 445
3b10bc60 446Accepts an incoming socket connect, just as accept(2)
19799a22 447does. Returns the packed address if it succeeded, false otherwise.
2b5ab1e7 448See the example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 449
8d2a6795
GS
450On systems that support a close-on-exec flag on files, the flag will
451be set for the newly opened file descriptor, as determined by the
452value of $^F. See L<perlvar/$^F>.
453
a0d0e21e 454=item alarm SECONDS
d74e8afc
ITB
455X<alarm>
456X<SIGALRM>
457X<timer>
a0d0e21e 458
54310121 459=item alarm
bbce6d69 460
a0d0e21e 461Arranges to have a SIGALRM delivered to this process after the
cf264981 462specified number of wallclock seconds has elapsed. If SECONDS is not
d400eac8
JH
463specified, the value stored in C<$_> is used. (On some machines,
464unfortunately, the elapsed time may be up to one second less or more
465than you specified because of how seconds are counted, and process
466scheduling may delay the delivery of the signal even further.)
467
468Only one timer may be counting at once. Each call disables the
469previous timer, and an argument of C<0> may be supplied to cancel the
470previous timer without starting a new one. The returned value is the
471amount of time remaining on the previous timer.
a0d0e21e 472
2bc69794
BS
473For delays of finer granularity than one second, the Time::HiRes module
474(from CPAN, and starting from Perl 5.8 part of the standard
475distribution) provides ualarm(). You may also use Perl's four-argument
476version of select() leaving the first three arguments undefined, or you
477might be able to use the C<syscall> interface to access setitimer(2) if
478your system supports it. See L<perlfaq8> for details.
2b5ab1e7 479
80d38338
TC
480It is usually a mistake to intermix C<alarm> and C<sleep> calls, because
481C<sleep> may be internally implemented on your system with C<alarm>.
a0d0e21e 482
19799a22
GS
483If you want to use C<alarm> to time out a system call you need to use an
484C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
f86cebdf 485fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
19799a22 486restart system calls on some systems. Using C<eval>/C<die> always works,
5a964f20 487modulo the caveats given in L<perlipc/"Signals">.
ff68c719
PP
488
489 eval {
a9a5a0dc
VP
490 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
491 alarm $timeout;
492 $nread = sysread SOCKET, $buffer, $size;
493 alarm 0;
ff68c719 494 };
ff68c719 495 if ($@) {
a9a5a0dc 496 die unless $@ eq "alarm\n"; # propagate unexpected errors
5ed4f2ec 497 # timed out
ff68c719
PP
498 }
499 else {
5ed4f2ec 500 # didn't
ff68c719
PP
501 }
502
91d81acc
JH
503For more information see L<perlipc>.
504
ea9eb35a 505Portability issues: L<perlport/alarm>.
506
a0d0e21e 507=item atan2 Y,X
d74e8afc 508X<atan2> X<arctangent> X<tan> X<tangent>
a0d0e21e
LW
509
510Returns the arctangent of Y/X in the range -PI to PI.
511
ca6e1c26 512For the tangent operation, you may use the C<Math::Trig::tan>
28757baa
PP
513function, or use the familiar relation:
514
515 sub tan { sin($_[0]) / cos($_[0]) }
516
a1021d57
RGS
517The return value for C<atan2(0,0)> is implementation-defined; consult
518your atan2(3) manpage for more information.
bf5f1b4c 519
ea9eb35a 520Portability issues: L<perlport/atan2>.
521
a0d0e21e 522=item bind SOCKET,NAME
d74e8afc 523X<bind>
a0d0e21e 524
3b10bc60 525Binds a network address to a socket, just as bind(2)
19799a22 526does. Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
527packed address of the appropriate type for the socket. See the examples in
528L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 529
fae2c0fb 530=item binmode FILEHANDLE, LAYER
d74e8afc 531X<binmode> X<binary> X<text> X<DOS> X<Windows>
1c1fc3ea 532
a0d0e21e
LW
533=item binmode FILEHANDLE
534
1cbfc93d
NIS
535Arranges for FILEHANDLE to be read or written in "binary" or "text"
536mode on systems where the run-time libraries distinguish between
537binary and text files. If FILEHANDLE is an expression, the value is
538taken as the name of the filehandle. Returns true on success,
b5fe5ca2 539otherwise it returns C<undef> and sets C<$!> (errno).
1cbfc93d 540
8f1da26d 541On some systems (in general, DOS- and Windows-based systems) binmode()
d807c6f4 542is necessary when you're not working with a text file. For the sake
d7a0d798
FC
543of portability it is a good idea always to use it when appropriate,
544and never to use it when it isn't appropriate. Also, people can
8f1da26d 545set their I/O to be by default UTF8-encoded Unicode, not bytes.
d807c6f4
JH
546
547In other words: regardless of platform, use binmode() on binary data,
d7a0d798 548like images, for example.
d807c6f4
JH
549
550If LAYER is present it is a single string, but may contain multiple
3b10bc60 551directives. The directives alter the behaviour of the filehandle.
d7a0d798 552When LAYER is present, using binmode on a text file makes sense.
d807c6f4 553
fae2c0fb 554If LAYER is omitted or specified as C<:raw> the filehandle is made
0226bbdb
NIS
555suitable for passing binary data. This includes turning off possible CRLF
556translation and marking it as bytes (as opposed to Unicode characters).
749683d2 557Note that, despite what may be implied in I<"Programming Perl"> (the
3b10bc60 558Camel, 3rd edition) or elsewhere, C<:raw> is I<not> simply the inverse of C<:crlf>.
559Other layers that would affect the binary nature of the stream are
560I<also> disabled. See L<PerlIO>, L<perlrun>, and the discussion about the
0226bbdb 561PERLIO environment variable.
01e6739c 562
3b10bc60 563The C<:bytes>, C<:crlf>, C<:utf8>, and any other directives of the
d807c6f4
JH
564form C<:...>, are called I/O I<layers>. The C<open> pragma can be used to
565establish default I/O layers. See L<open>.
566
fae2c0fb
RGS
567I<The LAYER parameter of the binmode() function is described as "DISCIPLINE"
568in "Programming Perl, 3rd Edition". However, since the publishing of this
569book, by many known as "Camel III", the consensus of the naming of this
570functionality has moved from "discipline" to "layer". All documentation
571of this version of Perl therefore refers to "layers" rather than to
572"disciplines". Now back to the regularly scheduled documentation...>
573
8f1da26d 574To mark FILEHANDLE as UTF-8, use C<:utf8> or C<:encoding(UTF-8)>.
6902c96a 575C<:utf8> just marks the data as UTF-8 without further checking,
8f1da26d 576while C<:encoding(UTF-8)> checks the data for actually being valid
6902c96a 577UTF-8. More details can be found in L<PerlIO::encoding>.
1cbfc93d 578
ed53a2bb 579In general, binmode() should be called after open() but before any I/O
3b10bc60 580is done on the filehandle. Calling binmode() normally flushes any
01e6739c 581pending buffered output data (and perhaps pending input data) on the
fae2c0fb 582handle. An exception to this is the C<:encoding> layer that
d7a0d798 583changes the default character encoding of the handle; see L</open>.
fae2c0fb 584The C<:encoding> layer sometimes needs to be called in
3874323d
JH
585mid-stream, and it doesn't flush the stream. The C<:encoding>
586also implicitly pushes on top of itself the C<:utf8> layer because
3b10bc60 587internally Perl operates on UTF8-encoded Unicode characters.
16fe6d59 588
19799a22 589The operating system, device drivers, C libraries, and Perl run-time
8f1da26d
TC
590system all conspire to let the programmer treat a single
591character (C<\n>) as the line terminator, irrespective of external
30168b04
GS
592representation. On many operating systems, the native text file
593representation matches the internal representation, but on some
594platforms the external representation of C<\n> is made up of more than
595one character.
596
8f1da26d
TC
597All variants of Unix, Mac OS (old and new), and Stream_LF files on VMS use
598a single character to end each line in the external representation of text
599(even though that single character is CARRIAGE RETURN on old, pre-Darwin
600flavors of Mac OS, and is LINE FEED on Unix and most VMS files). In other
601systems like OS/2, DOS, and the various flavors of MS-Windows, your program
602sees a C<\n> as a simple C<\cJ>, but what's stored in text files are the
603two characters C<\cM\cJ>. That means that if you don't use binmode() on
604these systems, C<\cM\cJ> sequences on disk will be converted to C<\n> on
605input, and any C<\n> in your program will be converted back to C<\cM\cJ> on
606output. This is what you want for text files, but it can be disastrous for
607binary files.
30168b04
GS
608
609Another consequence of using binmode() (on some systems) is that
610special end-of-file markers will be seen as part of the data stream.
d7a0d798
FC
611For systems from the Microsoft family this means that, if your binary
612data contain C<\cZ>, the I/O subsystem will regard it as the end of
30168b04
GS
613the file, unless you use binmode().
614
3b10bc60 615binmode() is important not only for readline() and print() operations,
30168b04
GS
616but also when using read(), seek(), sysread(), syswrite() and tell()
617(see L<perlport> for more details). See the C<$/> and C<$\> variables
618in L<perlvar> for how to manually set your input and output
619line-termination sequences.
a0d0e21e 620
ea9eb35a 621Portability issues: L<perlport/binmode>.
622
4633a7c4 623=item bless REF,CLASSNAME
d74e8afc 624X<bless>
a0d0e21e
LW
625
626=item bless REF
627
2b5ab1e7
TC
628This function tells the thingy referenced by REF that it is now an object
629in the CLASSNAME package. If CLASSNAME is omitted, the current package
19799a22 630is used. Because a C<bless> is often the last thing in a constructor,
2b5ab1e7 631it returns the reference for convenience. Always use the two-argument
cf264981
SP
632version if a derived class might inherit the function doing the blessing.
633See L<perltoot> and L<perlobj> for more about the blessing (and blessings)
634of objects.
a0d0e21e 635
57668c4d 636Consider always blessing objects in CLASSNAMEs that are mixed case.
2b5ab1e7 637Namespaces with all lowercase names are considered reserved for
cf264981 638Perl pragmata. Builtin types have all uppercase names. To prevent
2b5ab1e7
TC
639confusion, you may wish to avoid such package names as well. Make sure
640that CLASSNAME is a true value.
60ad88b8
GS
641
642See L<perlmod/"Perl Modules">.
643
0d863452
RH
644=item break
645
646Break out of a C<given()> block.
647
8f1da26d
TC
648This keyword is enabled by the C<"switch"> feature: see
649L<feature> for more information. Alternately, include a C<use
650v5.10> or later to the current scope.
0d863452 651
a0d0e21e 652=item caller EXPR
d74e8afc 653X<caller> X<call stack> X<stack> X<stack trace>
a0d0e21e
LW
654
655=item caller
656
5a964f20 657Returns the context of the current subroutine call. In scalar context,
80d38338
TC
658returns the caller's package name if there I<is> a caller (that is, if
659we're in a subroutine or C<eval> or C<require>) and the undefined value
5a964f20 660otherwise. In list context, returns
a0d0e21e 661
ee6b43cc 662 # 0 1 2
748a9306 663 ($package, $filename, $line) = caller;
a0d0e21e
LW
664
665With EXPR, it returns some extra information that the debugger uses to
666print a stack trace. The value of EXPR indicates how many call frames
667to go back before the current one.
668
ee6b43cc 669 # 0 1 2 3 4
f3aa04c2 670 ($package, $filename, $line, $subroutine, $hasargs,
ee6b43cc 671
672 # 5 6 7 8 9 10
b3ca2e83 673 $wantarray, $evaltext, $is_require, $hints, $bitmask, $hinthash)
ee6b43cc 674 = caller($i);
e7ea3e70 675
951ba7fe 676Here $subroutine may be C<(eval)> if the frame is not a subroutine
19799a22 677call, but an C<eval>. In such a case additional elements $evaltext and
7660c0ab 678C<$is_require> are set: C<$is_require> is true if the frame is created by a
19799a22 679C<require> or C<use> statement, $evaltext contains the text of the
277ddfaf 680C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
cc1c2e42 681$subroutine is C<(eval)>, but $evaltext is undefined. (Note also that
0fc9dec4
RGS
682each C<use> statement creates a C<require> frame inside an C<eval EXPR>
683frame.) $subroutine may also be C<(unknown)> if this particular
684subroutine happens to have been deleted from the symbol table.
685C<$hasargs> is true if a new instance of C<@_> was set up for the frame.
686C<$hints> and C<$bitmask> contain pragmatic hints that the caller was
687compiled with. The C<$hints> and C<$bitmask> values are subject to change
688between versions of Perl, and are not meant for external use.
748a9306 689
b3ca2e83
NC
690C<$hinthash> is a reference to a hash containing the value of C<%^H> when the
691caller was compiled, or C<undef> if C<%^H> was empty. Do not modify the values
692of this hash, as they are the actual values stored in the optree.
693
748a9306 694Furthermore, when called from within the DB package, caller returns more
7660c0ab 695detailed information: it sets the list variable C<@DB::args> to be the
54310121 696arguments with which the subroutine was invoked.
748a9306 697
7660c0ab 698Be aware that the optimizer might have optimized call frames away before
19799a22 699C<caller> had a chance to get the information. That means that C<caller(N)>
80d38338 700might not return information about the call frame you expect it to, for
b76cc8ba 701C<< N > 1 >>. In particular, C<@DB::args> might have information from the
19799a22 702previous time C<caller> was called.
7660c0ab 703
8f1da26d 704Be aware that setting C<@DB::args> is I<best effort>, intended for
ca9f0cb5
NC
705debugging or generating backtraces, and should not be relied upon. In
706particular, as C<@_> contains aliases to the caller's arguments, Perl does
707not take a copy of C<@_>, so C<@DB::args> will contain modifications the
708subroutine makes to C<@_> or its contents, not the original values at call
709time. C<@DB::args>, like C<@_>, does not hold explicit references to its
710elements, so under certain cases its elements may have become freed and
711reallocated for other variables or temporary values. Finally, a side effect
d7a0d798 712of the current implementation is that the effects of C<shift @_> can
8f1da26d
TC
713I<normally> be undone (but not C<pop @_> or other splicing, I<and> not if a
714reference to C<@_> has been taken, I<and> subject to the caveat about reallocated
ca9f0cb5
NC
715elements), so C<@DB::args> is actually a hybrid of the current state and
716initial state of C<@_>. Buyer beware.
717
a0d0e21e 718=item chdir EXPR
d74e8afc
ITB
719X<chdir>
720X<cd>
f723aae1 721X<directory, change>
a0d0e21e 722
c4aca7d0
GA
723=item chdir FILEHANDLE
724
725=item chdir DIRHANDLE
726
ce2984c3
PF
727=item chdir
728
ffce7b87 729Changes the working directory to EXPR, if possible. If EXPR is omitted,
0bfc1ec4 730changes to the directory specified by C<$ENV{HOME}>, if set; if not,
ffce7b87 731changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
b4ad75f0 732variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
80d38338 733neither is set, C<chdir> does nothing. It returns true on success,
b4ad75f0 734false otherwise. See the example under C<die>.
a0d0e21e 735
3b10bc60 736On systems that support fchdir(2), you may pass a filehandle or
34169887 737directory handle as the argument. On systems that don't support fchdir(2),
3b10bc60 738passing handles raises an exception.
c4aca7d0 739
a0d0e21e 740=item chmod LIST
d74e8afc 741X<chmod> X<permission> X<mode>
a0d0e21e
LW
742
743Changes the permissions of a list of files. The first element of the
8f1da26d 744list must be the numeric mode, which should probably be an octal
4ad40acf 745number, and which definitely should I<not> be a string of octal digits:
3b10bc60 746C<0644> is okay, but C<"0644"> is not. Returns the number of files
8f1da26d 747successfully changed. See also L</oct> if all you have is a string.
a0d0e21e 748
3b10bc60 749 $cnt = chmod 0755, "foo", "bar";
a0d0e21e 750 chmod 0755, @executables;
3b10bc60 751 $mode = "0644"; chmod $mode, "foo"; # !!! sets mode to
f86cebdf 752 # --w----r-T
3b10bc60 753 $mode = "0644"; chmod oct($mode), "foo"; # this is better
754 $mode = 0644; chmod $mode, "foo"; # this is best
a0d0e21e 755
3b10bc60 756On systems that support fchmod(2), you may pass filehandles among the
757files. On systems that don't support fchmod(2), passing filehandles raises
758an exception. Filehandles must be passed as globs or glob references to be
759recognized; barewords are considered filenames.
c4aca7d0
GA
760
761 open(my $fh, "<", "foo");
762 my $perm = (stat $fh)[2] & 07777;
763 chmod($perm | 0600, $fh);
764
3b10bc60 765You can also import the symbolic C<S_I*> constants from the C<Fcntl>
ca6e1c26
JH
766module:
767
3b10bc60 768 use Fcntl qw( :mode );
ca6e1c26 769 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
3b10bc60 770 # Identical to the chmod 0755 of the example above.
ca6e1c26 771
ea9eb35a 772Portability issues: L<perlport/chmod>.
773
a0d0e21e 774=item chomp VARIABLE
d74e8afc 775X<chomp> X<INPUT_RECORD_SEPARATOR> X<$/> X<newline> X<eol>
a0d0e21e 776
313c9f5c 777=item chomp( LIST )
a0d0e21e
LW
778
779=item chomp
780
2b5ab1e7
TC
781This safer version of L</chop> removes any trailing string
782that corresponds to the current value of C<$/> (also known as
28757baa
PP
783$INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
784number of characters removed from all its arguments. It's often used to
785remove the newline from the end of an input record when you're worried
2b5ab1e7
TC
786that the final record may be missing its newline. When in paragraph
787mode (C<$/ = "">), it removes all trailing newlines from the string.
4c5a6083 788When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
34169887 789a reference to an integer or the like; see L<perlvar>) chomp() won't
b76cc8ba 790remove anything.
19799a22 791If VARIABLE is omitted, it chomps C<$_>. Example:
a0d0e21e
LW
792
793 while (<>) {
a9a5a0dc
VP
794 chomp; # avoid \n on last field
795 @array = split(/:/);
796 # ...
a0d0e21e
LW
797 }
798
4bf21a6d
RD
799If VARIABLE is a hash, it chomps the hash's values, but not its keys.
800
a0d0e21e
LW
801You can actually chomp anything that's an lvalue, including an assignment:
802
803 chomp($cwd = `pwd`);
804 chomp($answer = <STDIN>);
805
806If you chomp a list, each element is chomped, and the total number of
807characters removed is returned.
808
15e44fd8
RGS
809Note that parentheses are necessary when you're chomping anything
810that is not a simple variable. This is because C<chomp $cwd = `pwd`;>
811is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as
812C<chomp( $cwd = `pwd` )> which you might expect. Similarly,
813C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than
814as C<chomp($a, $b)>.
815
a0d0e21e 816=item chop VARIABLE
d74e8afc 817X<chop>
a0d0e21e 818
313c9f5c 819=item chop( LIST )
a0d0e21e
LW
820
821=item chop
822
823Chops off the last character of a string and returns the character
5b3eff12 824chopped. It is much more efficient than C<s/.$//s> because it neither
7660c0ab 825scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
4bf21a6d
RD
826If VARIABLE is a hash, it chops the hash's values, but not its keys.
827
5b3eff12 828You can actually chop anything that's an lvalue, including an assignment.
a0d0e21e
LW
829
830If you chop a list, each element is chopped. Only the value of the
19799a22 831last C<chop> is returned.
a0d0e21e 832
19799a22 833Note that C<chop> returns the last character. To return all but the last
748a9306
LW
834character, use C<substr($string, 0, -1)>.
835
15e44fd8
RGS
836See also L</chomp>.
837
a0d0e21e 838=item chown LIST
d74e8afc 839X<chown> X<owner> X<user> X<group>
a0d0e21e
LW
840
841Changes the owner (and group) of a list of files. The first two
19799a22
GS
842elements of the list must be the I<numeric> uid and gid, in that
843order. A value of -1 in either position is interpreted by most
844systems to leave that value unchanged. Returns the number of files
845successfully changed.
a0d0e21e
LW
846
847 $cnt = chown $uid, $gid, 'foo', 'bar';
848 chown $uid, $gid, @filenames;
849
3b10bc60 850On systems that support fchown(2), you may pass filehandles among the
851files. On systems that don't support fchown(2), passing filehandles raises
852an exception. Filehandles must be passed as globs or glob references to be
853recognized; barewords are considered filenames.
c4aca7d0 854
54310121 855Here's an example that looks up nonnumeric uids in the passwd file:
a0d0e21e
LW
856
857 print "User: ";
19799a22 858 chomp($user = <STDIN>);
5a964f20 859 print "Files: ";
19799a22 860 chomp($pattern = <STDIN>);
a0d0e21e
LW
861
862 ($login,$pass,$uid,$gid) = getpwnam($user)
a9a5a0dc 863 or die "$user not in passwd file";
a0d0e21e 864
5ed4f2ec 865 @ary = glob($pattern); # expand filenames
a0d0e21e
LW
866 chown $uid, $gid, @ary;
867
54310121 868On most systems, you are not allowed to change the ownership of the
4633a7c4
LW
869file unless you're the superuser, although you should be able to change
870the group to any of your secondary groups. On insecure systems, these
871restrictions may be relaxed, but this is not a portable assumption.
19799a22
GS
872On POSIX systems, you can detect this condition this way:
873
874 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
875 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
4633a7c4 876
ea9eb35a 877Portability issues: L<perlport/chmod>.
878
a0d0e21e 879=item chr NUMBER
d74e8afc 880X<chr> X<character> X<ASCII> X<Unicode>
a0d0e21e 881
54310121 882=item chr
bbce6d69 883
a0d0e21e 884Returns the character represented by that NUMBER in the character set.
a0ed51b3 885For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
2575c402 886chr(0x263a) is a Unicode smiley face.
aaa68c4a 887
8a064bd6 888Negative values give the Unicode replacement character (chr(0xfffd)),
80d38338 889except under the L<bytes> pragma, where the low eight bits of the value
8a064bd6
JH
890(truncated to an integer) are used.
891
974da8e5
JH
892If NUMBER is omitted, uses C<$_>.
893
b76cc8ba 894For the reverse, use L</ord>.
a0d0e21e 895
2575c402
JW
896Note that characters from 128 to 255 (inclusive) are by default
897internally not encoded as UTF-8 for backward compatibility reasons.
974da8e5 898
2575c402 899See L<perlunicode> for more about Unicode.
bbce6d69 900
a0d0e21e 901=item chroot FILENAME
d74e8afc 902X<chroot> X<root>
a0d0e21e 903
54310121 904=item chroot
bbce6d69 905
5a964f20 906This function works like the system call by the same name: it makes the
4633a7c4 907named directory the new root directory for all further pathnames that
951ba7fe 908begin with a C</> by your process and all its children. (It doesn't
28757baa 909change your current working directory, which is unaffected.) For security
4633a7c4 910reasons, this call is restricted to the superuser. If FILENAME is
19799a22 911omitted, does a C<chroot> to C<$_>.
a0d0e21e 912
ea9eb35a 913Portability issues: L<perlport/chroot>.
914
a0d0e21e 915=item close FILEHANDLE
d74e8afc 916X<close>
a0d0e21e 917
6a518fbc
TP
918=item close
919
3b10bc60 920Closes the file or pipe associated with the filehandle, flushes the IO
e0f13c26 921buffers, and closes the system file descriptor. Returns true if those
8f1da26d 922operations succeed and if no error was reported by any PerlIO
e0f13c26
RGS
923layer. Closes the currently selected filehandle if the argument is
924omitted.
fb73857a
PP
925
926You don't have to close FILEHANDLE if you are immediately going to do
3b10bc60 927another C<open> on it, because C<open> closes it for you. (See
19799a22
GS
928C<open>.) However, an explicit C<close> on an input file resets the line
929counter (C<$.>), while the implicit close done by C<open> does not.
fb73857a 930
3b10bc60 931If the filehandle came from a piped open, C<close> returns false if one of
932the other syscalls involved fails or if its program exits with non-zero
933status. If the only problem was that the program exited non-zero, C<$!>
934will be set to C<0>. Closing a pipe also waits for the process executing
935on the pipe to exit--in case you wish to look at the output of the pipe
936afterwards--and implicitly puts the exit status value of that command into
937C<$?> and C<${^CHILD_ERROR_NATIVE}>.
5a964f20 938
2e0cfa16
FC
939If there are multiple threads running, C<close> on a filehandle from a
940piped open returns true without waiting for the child process to terminate,
941if the filehandle is still open in another thread.
942
80d38338
TC
943Closing the read end of a pipe before the process writing to it at the
944other end is done writing results in the writer receiving a SIGPIPE. If
945the other end can't handle that, be sure to read all the data before
946closing the pipe.
73689b13 947
fb73857a 948Example:
a0d0e21e 949
fb73857a
PP
950 open(OUTPUT, '|sort >foo') # pipe to sort
951 or die "Can't start sort: $!";
5ed4f2ec 952 #... # print stuff to output
953 close OUTPUT # wait for sort to finish
fb73857a
PP
954 or warn $! ? "Error closing sort pipe: $!"
955 : "Exit status $? from sort";
5ed4f2ec 956 open(INPUT, 'foo') # get sort's results
fb73857a 957 or die "Can't open 'foo' for input: $!";
a0d0e21e 958
5a964f20 959FILEHANDLE may be an expression whose value can be used as an indirect
8f1da26d 960filehandle, usually the real filehandle name or an autovivified handle.
a0d0e21e
LW
961
962=item closedir DIRHANDLE
d74e8afc 963X<closedir>
a0d0e21e 964
19799a22 965Closes a directory opened by C<opendir> and returns the success of that
5a964f20
TC
966system call.
967
a0d0e21e 968=item connect SOCKET,NAME
d74e8afc 969X<connect>
a0d0e21e 970
80d38338
TC
971Attempts to connect to a remote socket, just like connect(2).
972Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
973packed address of the appropriate type for the socket. See the examples in
974L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 975
cb1a09d0 976=item continue BLOCK
d74e8afc 977X<continue>
cb1a09d0 978
0d863452
RH
979=item continue
980
cf264981
SP
981C<continue> is actually a flow control statement rather than a function. If
982there is a C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
98293880
JH
983C<foreach>), it is always executed just before the conditional is about to
984be evaluated again, just like the third part of a C<for> loop in C. Thus
cb1a09d0
AD
985it can be used to increment a loop variable, even when the loop has been
986continued via the C<next> statement (which is similar to the C C<continue>
987statement).
988
98293880 989C<last>, C<next>, or C<redo> may appear within a C<continue>
3b10bc60 990block; C<last> and C<redo> behave as if they had been executed within
19799a22 991the main block. So will C<next>, but since it will execute a C<continue>
1d2dff63
GS
992block, it may be more entertaining.
993
994 while (EXPR) {
a9a5a0dc
VP
995 ### redo always comes here
996 do_something;
1d2dff63 997 } continue {
a9a5a0dc
VP
998 ### next always comes here
999 do_something_else;
1000 # then back the top to re-check EXPR
1d2dff63
GS
1001 }
1002 ### last always comes here
1003
3b10bc60 1004Omitting the C<continue> section is equivalent to using an
1005empty one, logically enough, so C<next> goes directly back
1d2dff63
GS
1006to check the condition at the top of the loop.
1007
8f1da26d
TC
1008If the C<"switch"> feature is enabled, C<continue> is also a function that
1009falls through the current C<when> or C<default> block instead of iterating
1010a dynamically enclosing C<foreach> or exiting a lexically enclosing C<given>.
1011See L<feature> and L<perlsyn/"Switch statements"> for more
1012information.
0d863452 1013
a0d0e21e 1014=item cos EXPR
d74e8afc 1015X<cos> X<cosine> X<acos> X<arccosine>
a0d0e21e 1016
d6217f1e
GS
1017=item cos
1018
5a964f20 1019Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
34169887 1020takes the cosine of C<$_>.
a0d0e21e 1021
ca6e1c26 1022For the inverse cosine operation, you may use the C<Math::Trig::acos()>
28757baa
PP
1023function, or use this relation:
1024
1025 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
1026
a0d0e21e 1027=item crypt PLAINTEXT,SALT
d74e8afc 1028X<crypt> X<digest> X<hash> X<salt> X<plaintext> X<password>
f723aae1 1029X<decrypt> X<cryptography> X<passwd> X<encrypt>
a0d0e21e 1030
ef2e6798
MS
1031Creates a digest string exactly like the crypt(3) function in the C
1032library (assuming that you actually have a version there that has not
bb23f8d1 1033been extirpated as a potential munition).
ef2e6798 1034
34169887 1035crypt() is a one-way hash function. The PLAINTEXT and SALT are turned
ef2e6798
MS
1036into a short string, called a digest, which is returned. The same
1037PLAINTEXT and SALT will always return the same string, but there is no
1038(known) way to get the original PLAINTEXT from the hash. Small
1039changes in the PLAINTEXT or SALT will result in large changes in the
1040digest.
1041
1042There is no decrypt function. This function isn't all that useful for
1043cryptography (for that, look for F<Crypt> modules on your nearby CPAN
1044mirror) and the name "crypt" is a bit of a misnomer. Instead it is
1045primarily used to check if two pieces of text are the same without
1046having to transmit or store the text itself. An example is checking
1047if a correct password is given. The digest of the password is stored,
cf264981 1048not the password itself. The user types in a password that is
ef2e6798 1049crypt()'d with the same salt as the stored digest. If the two digests
34169887 1050match, the password is correct.
ef2e6798
MS
1051
1052When verifying an existing digest string you should use the digest as
1053the salt (like C<crypt($plain, $digest) eq $digest>). The SALT used
cf264981 1054to create the digest is visible as part of the digest. This ensures
ef2e6798
MS
1055crypt() will hash the new string with the same salt as the digest.
1056This allows your code to work with the standard L<crypt|/crypt> and
8f1da26d
TC
1057with more exotic implementations. In other words, assume
1058nothing about the returned string itself nor about how many bytes
1059of SALT may matter.
85c16d83
JH
1060
1061Traditionally the result is a string of 13 bytes: two first bytes of
1062the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
bb23f8d1 1063the first eight bytes of PLAINTEXT mattered. But alternative
ef2e6798 1064hashing schemes (like MD5), higher level security schemes (like C2),
e1020413 1065and implementations on non-Unix platforms may produce different
ef2e6798 1066strings.
85c16d83
JH
1067
1068When choosing a new salt create a random two character string whose
1069characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
d3989d75
CW
1070'/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of
1071characters is just a recommendation; the characters allowed in
1072the salt depend solely on your system's crypt library, and Perl can't
1073restrict what salts C<crypt()> accepts.
e71965be 1074
a0d0e21e 1075Here's an example that makes sure that whoever runs this program knows
cf264981 1076their password:
a0d0e21e
LW
1077
1078 $pwd = (getpwuid($<))[1];
a0d0e21e
LW
1079
1080 system "stty -echo";
1081 print "Password: ";
e71965be 1082 chomp($word = <STDIN>);
a0d0e21e
LW
1083 print "\n";
1084 system "stty echo";
1085
e71965be 1086 if (crypt($word, $pwd) ne $pwd) {
a9a5a0dc 1087 die "Sorry...\n";
a0d0e21e 1088 } else {
a9a5a0dc 1089 print "ok\n";
54310121 1090 }
a0d0e21e 1091
9f8f0c9d 1092Of course, typing in your own password to whoever asks you
748a9306 1093for it is unwise.
a0d0e21e 1094
ef2e6798 1095The L<crypt|/crypt> function is unsuitable for hashing large quantities
19799a22 1096of data, not least of all because you can't get the information
ef2e6798 1097back. Look at the L<Digest> module for more robust algorithms.
19799a22 1098
f2791508
JH
1099If using crypt() on a Unicode string (which I<potentially> has
1100characters with codepoints above 255), Perl tries to make sense
34169887 1101of the situation by trying to downgrade (a copy of)
f2791508
JH
1102the string back to an eight-bit byte string before calling crypt()
1103(on that copy). If that works, good. If not, crypt() dies with
1104C<Wide character in crypt>.
85c16d83 1105
ea9eb35a 1106Portability issues: L<perlport/crypt>.
1107
aa689395 1108=item dbmclose HASH
d74e8afc 1109X<dbmclose>
a0d0e21e 1110
19799a22 1111[This function has been largely superseded by the C<untie> function.]
a0d0e21e 1112
aa689395 1113Breaks the binding between a DBM file and a hash.
a0d0e21e 1114
ea9eb35a 1115Portability issues: L<perlport/dbmclose>.
1116
19799a22 1117=item dbmopen HASH,DBNAME,MASK
d74e8afc 1118X<dbmopen> X<dbm> X<ndbm> X<sdbm> X<gdbm>
a0d0e21e 1119
19799a22 1120[This function has been largely superseded by the C<tie> function.]
a0d0e21e 1121
7b8d334a 1122This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
19799a22
GS
1123hash. HASH is the name of the hash. (Unlike normal C<open>, the first
1124argument is I<not> a filehandle, even though it looks like one). DBNAME
aa689395
PP
1125is the name of the database (without the F<.dir> or F<.pag> extension if
1126any). If the database does not exist, it is created with protection
19799a22 1127specified by MASK (as modified by the C<umask>). If your system supports
80d38338 1128only the older DBM functions, you may make only one C<dbmopen> call in your
aa689395 1129program. In older versions of Perl, if your system had neither DBM nor
19799a22 1130ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
aa689395
PP
1131sdbm(3).
1132
1133If you don't have write access to the DBM file, you can only read hash
1134variables, not set them. If you want to test whether you can write,
3b10bc60 1135either use file tests or try setting a dummy hash entry inside an C<eval>
1136to trap the error.
a0d0e21e 1137
19799a22
GS
1138Note that functions such as C<keys> and C<values> may return huge lists
1139when used on large DBM files. You may prefer to use the C<each>
a0d0e21e
LW
1140function to iterate over large DBM files. Example:
1141
1142 # print out history file offsets
1143 dbmopen(%HIST,'/usr/lib/news/history',0666);
1144 while (($key,$val) = each %HIST) {
a9a5a0dc 1145 print $key, ' = ', unpack('L',$val), "\n";
a0d0e21e
LW
1146 }
1147 dbmclose(%HIST);
1148
cb1a09d0 1149See also L<AnyDBM_File> for a more general description of the pros and
184e9718 1150cons of the various dbm approaches, as well as L<DB_File> for a particularly
cb1a09d0 1151rich implementation.
4633a7c4 1152
2b5ab1e7
TC
1153You can control which DBM library you use by loading that library
1154before you call dbmopen():
1155
1156 use DB_File;
1157 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
a9a5a0dc 1158 or die "Can't open netscape history file: $!";
2b5ab1e7 1159
ea9eb35a 1160Portability issues: L<perlport/dbmopen>.
1161
8f1da26d
TC
1162=item default BLOCK
1163
1164Within a C<foreach> or a C<given>, a C<default> BLOCK acts like a C<when>
1165that's always true. Only available after Perl 5.10, and only if the
1166C<switch> feature has been requested. See L</when>.
1167
a0d0e21e 1168=item defined EXPR
d74e8afc 1169X<defined> X<undef> X<undefined>
a0d0e21e 1170
54310121 1171=item defined
bbce6d69 1172
2f9daede 1173Returns a Boolean value telling whether EXPR has a value other than
3b10bc60 1174the undefined value C<undef>. If EXPR is not present, C<$_> is
2f9daede
TPG
1175checked.
1176
1177Many operations return C<undef> to indicate failure, end of file,
1178system error, uninitialized variable, and other exceptional
1179conditions. This function allows you to distinguish C<undef> from
1180other values. (A simple Boolean test will not distinguish among
7660c0ab 1181C<undef>, zero, the empty string, and C<"0">, which are all equally
2f9daede 1182false.) Note that since C<undef> is a valid scalar, its presence
19799a22 1183doesn't I<necessarily> indicate an exceptional condition: C<pop>
2f9daede
TPG
1184returns C<undef> when its argument is an empty array, I<or> when the
1185element to return happens to be C<undef>.
1186
f10b0346
GS
1187You may also use C<defined(&func)> to check whether subroutine C<&func>
1188has ever been defined. The return value is unaffected by any forward
80d38338 1189declarations of C<&func>. A subroutine that is not defined
847c7ebe 1190may still be callable: its package may have an C<AUTOLOAD> method that
3b10bc60 1191makes it spring into existence the first time that it is called; see
847c7ebe 1192L<perlsub>.
f10b0346
GS
1193
1194Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
34169887 1195used to report whether memory for that aggregate had ever been
f10b0346
GS
1196allocated. This behavior may disappear in future versions of Perl.
1197You should instead use a simple test for size:
1198
1199 if (@an_array) { print "has array elements\n" }
1200 if (%a_hash) { print "has hash members\n" }
2f9daede
TPG
1201
1202When used on a hash element, it tells you whether the value is defined,
dc848c6f 1203not whether the key exists in the hash. Use L</exists> for the latter
2f9daede 1204purpose.
a0d0e21e
LW
1205
1206Examples:
1207
8f1da26d 1208 print if defined $switch{D};
a0d0e21e
LW
1209 print "$val\n" while defined($val = pop(@ary));
1210 die "Can't readlink $sym: $!"
a9a5a0dc 1211 unless defined($value = readlink $sym);
a0d0e21e 1212 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
2f9daede 1213 $debugging = 0 unless defined $debugging;
a0d0e21e 1214
8f1da26d 1215Note: Many folks tend to overuse C<defined> and are then surprised to
7660c0ab 1216discover that the number C<0> and C<""> (the zero-length string) are, in fact,
2f9daede 1217defined values. For example, if you say
a5f75d66
AD
1218
1219 "ab" =~ /a(.*)b/;
1220
80d38338 1221The pattern match succeeds and C<$1> is defined, although it
cf264981 1222matched "nothing". It didn't really fail to match anything. Rather, it
2b5ab1e7 1223matched something that happened to be zero characters long. This is all
a5f75d66 1224very above-board and honest. When a function returns an undefined value,
2f9daede 1225it's an admission that it couldn't give you an honest answer. So you
3b10bc60 1226should use C<defined> only when questioning the integrity of what
7660c0ab 1227you're trying to do. At other times, a simple comparison to C<0> or C<""> is
2f9daede
TPG
1228what you want.
1229
dc848c6f 1230See also L</undef>, L</exists>, L</ref>.
2f9daede 1231
a0d0e21e 1232=item delete EXPR
d74e8afc 1233X<delete>
a0d0e21e 1234
d0a76353
RS
1235Given an expression that specifies an element or slice of a hash, C<delete>
1236deletes the specified elements from that hash so that exists() on that element
1237no longer returns true. Setting a hash element to the undefined value does
1238not remove its key, but deleting it does; see L</exists>.
80d38338 1239
8f1da26d 1240In list context, returns the value or values deleted, or the last such
80d38338 1241element in scalar context. The return list's length always matches that of
d0a76353
RS
1242the argument list: deleting non-existent elements returns the undefined value
1243in their corresponding positions.
80d38338 1244
d0a76353
RS
1245delete() may also be used on arrays and array slices, but its behavior is less
1246straightforward. Although exists() will return false for deleted entries,
1247deleting array elements never changes indices of existing values; use shift()
1248or splice() for that. However, if all deleted elements fall at the end of an
1249array, the array's size shrinks to the position of the highest element that
1250still tests true for exists(), or to 0 if none do.
1251
8f1da26d 1252B<WARNING:> Calling delete on array values is deprecated and likely to
d0a76353 1253be removed in a future version of Perl.
80d38338
TC
1254
1255Deleting from C<%ENV> modifies the environment. Deleting from a hash tied to
1256a DBM file deletes the entry from the DBM file. Deleting from a C<tied> hash
1257or array may not necessarily return anything; it depends on the implementation
1258of the C<tied> package's DELETE method, which may do whatever it pleases.
a0d0e21e 1259
80d38338
TC
1260The C<delete local EXPR> construct localizes the deletion to the current
1261block at run time. Until the block exits, elements locally deleted
1262temporarily no longer exist. See L<perlsub/"Localized deletion of elements
1263of composite types">.
eba0920a
EM
1264
1265 %hash = (foo => 11, bar => 22, baz => 33);
1266 $scalar = delete $hash{foo}; # $scalar is 11
1267 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
1268 @array = delete @hash{qw(foo bar baz)}; # @array is (undef,undef,33)
1269
01020589 1270The following (inefficiently) deletes all the values of %HASH and @ARRAY:
a0d0e21e 1271
5f05dabc 1272 foreach $key (keys %HASH) {
a9a5a0dc 1273 delete $HASH{$key};
a0d0e21e
LW
1274 }
1275
01020589 1276 foreach $index (0 .. $#ARRAY) {
a9a5a0dc 1277 delete $ARRAY[$index];
01020589
GS
1278 }
1279
1280And so do these:
5f05dabc 1281
01020589
GS
1282 delete @HASH{keys %HASH};
1283
9740c838 1284 delete @ARRAY[0 .. $#ARRAY];
5f05dabc 1285
80d38338
TC
1286But both are slower than assigning the empty list
1287or undefining %HASH or @ARRAY, which is the customary
1288way to empty out an aggregate:
01020589 1289
5ed4f2ec 1290 %HASH = (); # completely empty %HASH
1291 undef %HASH; # forget %HASH ever existed
2b5ab1e7 1292
5ed4f2ec 1293 @ARRAY = (); # completely empty @ARRAY
1294 undef @ARRAY; # forget @ARRAY ever existed
2b5ab1e7 1295
80d38338
TC
1296The EXPR can be arbitrarily complicated provided its
1297final operation is an element or slice of an aggregate:
a0d0e21e
LW
1298
1299 delete $ref->[$x][$y]{$key};
5f05dabc 1300 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
a0d0e21e 1301
01020589
GS
1302 delete $ref->[$x][$y][$index];
1303 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1304
a0d0e21e 1305=item die LIST
d74e8afc 1306X<die> X<throw> X<exception> X<raise> X<$@> X<abort>
a0d0e21e 1307
4c050ad5
NC
1308C<die> raises an exception. Inside an C<eval> the error message is stuffed
1309into C<$@> and the C<eval> is terminated with the undefined value.
1310If the exception is outside of all enclosing C<eval>s, then the uncaught
1311exception prints LIST to C<STDERR> and exits with a non-zero value. If you
96090e4f 1312need to exit the process with a specific exit code, see L</exit>.
a0d0e21e
LW
1313
1314Equivalent examples:
1315
1316 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
54310121 1317 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
a0d0e21e 1318
ccac6780 1319If the last element of LIST does not end in a newline, the current
df37ec69
WW
1320script line number and input line number (if any) are also printed,
1321and a newline is supplied. Note that the "input line number" (also
1322known as "chunk") is subject to whatever notion of "line" happens to
1323be currently in effect, and is also available as the special variable
1324C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1325
1326Hint: sometimes appending C<", stopped"> to your message will cause it
1327to make better sense when the string C<"at foo line 123"> is appended.
1328Suppose you are running script "canasta".
a0d0e21e
LW
1329
1330 die "/etc/games is no good";
1331 die "/etc/games is no good, stopped";
1332
1333produce, respectively
1334
1335 /etc/games is no good at canasta line 123.
1336 /etc/games is no good, stopped at canasta line 123.
1337
a96d0188 1338If the output is empty and C<$@> already contains a value (typically from a
7660c0ab 1339previous eval) that value is reused after appending C<"\t...propagated">.
fb73857a
PP
1340This is useful for propagating exceptions:
1341
1342 eval { ... };
1343 die unless $@ =~ /Expected exception/;
1344
a96d0188 1345If the output is empty and C<$@> contains an object reference that has a
ad216e65
JH
1346C<PROPAGATE> method, that method will be called with additional file
1347and line number parameters. The return value replaces the value in
34169887 1348C<$@>; i.e., as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >>
ad216e65
JH
1349were called.
1350
7660c0ab 1351If C<$@> is empty then the string C<"Died"> is used.
fb73857a 1352
4c050ad5
NC
1353If an uncaught exception results in interpreter exit, the exit code is
1354determined from the values of C<$!> and C<$?> with this pseudocode:
1355
1356 exit $! if $!; # errno
1357 exit $? >> 8 if $? >> 8; # child exit status
1358 exit 255; # last resort
1359
1360The intent is to squeeze as much possible information about the likely cause
1361into the limited space of the system exit code. However, as C<$!> is the value
1362of C's C<errno>, which can be set by any system call, this means that the value
1363of the exit code used by C<die> can be non-predictable, so should not be relied
1364upon, other than to be non-zero.
1365
80d38338
TC
1366You can also call C<die> with a reference argument, and if this is trapped
1367within an C<eval>, C<$@> contains that reference. This permits more
1368elaborate exception handling using objects that maintain arbitrary state
1369about the exception. Such a scheme is sometimes preferable to matching
1370particular string values of C<$@> with regular expressions. Because C<$@>
1371is a global variable and C<eval> may be used within object implementations,
1372be careful that analyzing the error object doesn't replace the reference in
1373the global variable. It's easiest to make a local copy of the reference
1374before any manipulations. Here's an example:
52531d10 1375
80d38338 1376 use Scalar::Util "blessed";
da279afe 1377
52531d10 1378 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
746d7dd7
GL
1379 if (my $ev_err = $@) {
1380 if (blessed($ev_err) && $ev_err->isa("Some::Module::Exception")) {
52531d10
GS
1381 # handle Some::Module::Exception
1382 }
1383 else {
1384 # handle all other possible exceptions
1385 }
1386 }
1387
3b10bc60 1388Because Perl stringifies uncaught exception messages before display,
80d38338 1389you'll probably want to overload stringification operations on
52531d10
GS
1390exception objects. See L<overload> for details about that.
1391
19799a22
GS
1392You can arrange for a callback to be run just before the C<die>
1393does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
3b10bc60 1394handler is called with the error text and can change the error
19799a22 1395message, if it sees fit, by calling C<die> again. See
96090e4f 1396L<perlvar/%SIG> for details on setting C<%SIG> entries, and
cf264981 1397L<"eval BLOCK"> for some examples. Although this feature was
19799a22 1398to be run only right before your program was to exit, this is not
3b10bc60 1399currently so: the C<$SIG{__DIE__}> hook is currently called
19799a22
GS
1400even inside eval()ed blocks/strings! If one wants the hook to do
1401nothing in such situations, put
fb73857a 1402
5ed4f2ec 1403 die @_ if $^S;
fb73857a 1404
19799a22
GS
1405as the first line of the handler (see L<perlvar/$^S>). Because
1406this promotes strange action at a distance, this counterintuitive
b76cc8ba 1407behavior may be fixed in a future release.
774d564b 1408
4c050ad5
NC
1409See also exit(), warn(), and the Carp module.
1410
a0d0e21e 1411=item do BLOCK
d74e8afc 1412X<do> X<block>
a0d0e21e
LW
1413
1414Not really a function. Returns the value of the last command in the
6b275a1f
RGS
1415sequence of commands indicated by BLOCK. When modified by the C<while> or
1416C<until> loop modifier, executes the BLOCK once before testing the loop
1417condition. (On other statements the loop modifiers test the conditional
1418first.)
a0d0e21e 1419
4968c1e4 1420C<do BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7
TC
1421C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1422See L<perlsyn> for alternative strategies.
4968c1e4 1423
a0d0e21e 1424=item do SUBROUTINE(LIST)
d74e8afc 1425X<do>
a0d0e21e 1426
51124b83
FC
1427This form of subroutine call is deprecated. SUBROUTINE can be a bareword,
1428a scalar variable or a subroutine beginning with C<&>.
a0d0e21e
LW
1429
1430=item do EXPR
d74e8afc 1431X<do>
a0d0e21e
LW
1432
1433Uses the value of EXPR as a filename and executes the contents of the
ea63ef19 1434file as a Perl script.
a0d0e21e
LW
1435
1436 do 'stat.pl';
1437
1438is just like
1439
986b19de 1440 eval `cat stat.pl`;
a0d0e21e 1441
2b5ab1e7 1442except that it's more efficient and concise, keeps track of the current
96090e4f
A
1443filename for error messages, searches the C<@INC> directories, and updates
1444C<%INC> if the file is found. See L<perlvar/@INC> and L<perlvar/%INC> for
1445these variables. It also differs in that code evaluated with C<do FILENAME>
2b5ab1e7
TC
1446cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1447same, however, in that it does reparse the file every time you call it,
1448so you probably don't want to do this inside a loop.
a0d0e21e 1449
8f1da26d 1450If C<do> can read the file but cannot compile it, it returns C<undef> and sets
9dc513c5
DG
1451an error message in C<$@>. If C<do> cannot read the file, it returns undef
1452and sets C<$!> to the error. Always check C<$@> first, as compilation
1453could fail in a way that also sets C<$!>. If the file is successfully
1454compiled, C<do> returns the value of the last expression evaluated.
8e30cc93 1455
80d38338 1456Inclusion of library modules is better done with the
19799a22 1457C<use> and C<require> operators, which also do automatic error checking
4633a7c4 1458and raise an exception if there's a problem.
a0d0e21e 1459
5a964f20
TC
1460You might like to use C<do> to read in a program configuration
1461file. Manual error checking can be done this way:
1462
b76cc8ba 1463 # read in config files: system first, then user
f86cebdf 1464 for $file ("/share/prog/defaults.rc",
b76cc8ba 1465 "$ENV{HOME}/.someprogrc")
a9a5a0dc
VP
1466 {
1467 unless ($return = do $file) {
1468 warn "couldn't parse $file: $@" if $@;
1469 warn "couldn't do $file: $!" unless defined $return;
1470 warn "couldn't run $file" unless $return;
1471 }
5a964f20
TC
1472 }
1473
a0d0e21e 1474=item dump LABEL
d74e8afc 1475X<dump> X<core> X<undump>
a0d0e21e 1476
1614b0e3
JD
1477=item dump
1478
19799a22
GS
1479This function causes an immediate core dump. See also the B<-u>
1480command-line switch in L<perlrun>, which does the same thing.
1481Primarily this is so that you can use the B<undump> program (not
1482supplied) to turn your core dump into an executable binary after
1483having initialized all your variables at the beginning of the
1484program. When the new binary is executed it will begin by executing
1485a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1486Think of it as a goto with an intervening core dump and reincarnation.
1487If C<LABEL> is omitted, restarts the program from the top.
1488
1489B<WARNING>: Any files opened at the time of the dump will I<not>
1490be open any more when the program is reincarnated, with possible
80d38338 1491resulting confusion by Perl.
19799a22 1492
59f521f4
RGS
1493This function is now largely obsolete, mostly because it's very hard to
1494convert a core file into an executable. That's why you should now invoke
1495it as C<CORE::dump()>, if you don't want to be warned against a possible
ac206dc8 1496typo.
19799a22 1497
ea9eb35a 1498Portability issues: L<perlport/dump>.
1499
532eee96 1500=item each HASH
d74e8afc 1501X<each> X<hash, iterator>
aa689395 1502
532eee96 1503=item each ARRAY
aeedbbed
NC
1504X<array, iterator>
1505
f5a93a43
TC
1506=item each EXPR
1507
80d38338
TC
1508When called in list context, returns a 2-element list consisting of the key
1509and value for the next element of a hash, or the index and value for the
1510next element of an array, so that you can iterate over it. When called in
1511scalar context, returns only the key (not the value) in a hash, or the index
1512in an array.
2f9daede 1513
aeedbbed 1514Hash entries are returned in an apparently random order. The actual random
3b10bc60 1515order is subject to change in future versions of Perl, but it is
504f80c1 1516guaranteed to be in the same order as either the C<keys> or C<values>
4546b9e6 1517function would produce on the same (unmodified) hash. Since Perl
22883ac5 15185.8.2 the ordering can be different even between different runs of Perl
4546b9e6 1519for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
ab192400 1520
80d38338
TC
1521After C<each> has returned all entries from the hash or array, the next
1522call to C<each> returns the empty list in list context and C<undef> in
1523scalar context. The next call following that one restarts iteration. Each
1524hash or array has its own internal iterator, accessed by C<each>, C<keys>,
1525and C<values>. The iterator is implicitly reset when C<each> has reached
1526the end as just described; it can be explicitly reset by calling C<keys> or
1527C<values> on the hash or array. If you add or delete a hash's elements
1528while iterating over it, entries may be skipped or duplicated--so don't do
1529that. Exception: It is always safe to delete the item most recently
3b10bc60 1530returned by C<each()>, so the following code works properly:
74fc8b5f
MJD
1531
1532 while (($key, $value) = each %hash) {
1533 print $key, "\n";
1534 delete $hash{$key}; # This is safe
1535 }
aa689395 1536
80d38338 1537This prints out your environment like the printenv(1) program,
3b10bc60 1538but in a different order:
a0d0e21e
LW
1539
1540 while (($key,$value) = each %ENV) {
a9a5a0dc 1541 print "$key=$value\n";
a0d0e21e
LW
1542 }
1543
f5a93a43
TC
1544Starting with Perl 5.14, C<each> can take a scalar EXPR, which must hold
1545reference to an unblessed hash or array. The argument will be dereferenced
1546automatically. This aspect of C<each> is considered highly experimental.
1547The exact behaviour may change in a future version of Perl.
cba5a3b0
DG
1548
1549 while (($key,$value) = each $hashref) { ... }
1550
8f1da26d 1551See also C<keys>, C<values>, and C<sort>.
a0d0e21e
LW
1552
1553=item eof FILEHANDLE
d74e8afc
ITB
1554X<eof>
1555X<end of file>
1556X<end-of-file>
a0d0e21e 1557
4633a7c4
LW
1558=item eof ()
1559
a0d0e21e
LW
1560=item eof
1561
8f1da26d 1562Returns 1 if the next read on FILEHANDLE will return end of file I<or> if
a0d0e21e 1563FILEHANDLE is not open. FILEHANDLE may be an expression whose value
5a964f20 1564gives the real filehandle. (Note that this function actually
80d38338 1565reads a character and then C<ungetc>s it, so isn't useful in an
748a9306 1566interactive context.) Do not read from a terminal file (or call
19799a22 1567C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
748a9306
LW
1568as terminals may lose the end-of-file condition if you do.
1569
820475bd 1570An C<eof> without an argument uses the last file read. Using C<eof()>
80d38338 1571with empty parentheses is different. It refers to the pseudo file
820475bd 1572formed from the files listed on the command line and accessed via the
61eff3bc
JH
1573C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1574as a normal filehandle is, an C<eof()> before C<< <> >> has been
820475bd 1575used will cause C<@ARGV> to be examined to determine if input is
67408cae 1576available. Similarly, an C<eof()> after C<< <> >> has returned
efdd0218
RB
1577end-of-file will assume you are processing another C<@ARGV> list,
1578and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1579see L<perlop/"I/O Operators">.
820475bd 1580
61eff3bc 1581In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
8f1da26d
TC
1582detect the end of each file, whereas C<eof()> will detect the end
1583of the very last file only. Examples:
a0d0e21e 1584
748a9306
LW
1585 # reset line numbering on each input file
1586 while (<>) {
a9a5a0dc
VP
1587 next if /^\s*#/; # skip comments
1588 print "$.\t$_";
5a964f20 1589 } continue {
a9a5a0dc 1590 close ARGV if eof; # Not eof()!
748a9306
LW
1591 }
1592
a0d0e21e
LW
1593 # insert dashes just before last line of last file
1594 while (<>) {
a9a5a0dc
VP
1595 if (eof()) { # check for end of last file
1596 print "--------------\n";
1597 }
1598 print;
1599 last if eof(); # needed if we're reading from a terminal
a0d0e21e
LW
1600 }
1601
a0d0e21e 1602Practical hint: you almost never need to use C<eof> in Perl, because the
8f1da26d
TC
1603input operators typically return C<undef> when they run out of data or
1604encounter an error.
a0d0e21e
LW
1605
1606=item eval EXPR
d74e8afc 1607X<eval> X<try> X<catch> X<evaluate> X<parse> X<execute>
f723aae1 1608X<error, handling> X<exception, handling>
a0d0e21e
LW
1609
1610=item eval BLOCK
1611
ce2984c3
PF
1612=item eval
1613
c7cc6f1c
GS
1614In the first form, the return value of EXPR is parsed and executed as if it
1615were a little Perl program. The value of the expression (which is itself
8f1da26d 1616determined within scalar context) is first parsed, and if there were no
2341804c
DM
1617errors, executed as a block within the lexical context of the current Perl
1618program. This means, that in particular, any outer lexical variables are
1619visible to it, and any package variable settings or subroutine and format
1620definitions remain afterwards.
1621
1622Note that the value is parsed every time the C<eval> executes.
be3174d2
GS
1623If EXPR is omitted, evaluates C<$_>. This form is typically used to
1624delay parsing and subsequent execution of the text of EXPR until run time.
c7cc6f1c
GS
1625
1626In the second form, the code within the BLOCK is parsed only once--at the
cf264981 1627same time the code surrounding the C<eval> itself was parsed--and executed
c7cc6f1c
GS
1628within the context of the current Perl program. This form is typically
1629used to trap exceptions more efficiently than the first (see below), while
1630also providing the benefit of checking the code within BLOCK at compile
1631time.
1632
1633The final semicolon, if any, may be omitted from the value of EXPR or within
1634the BLOCK.
1635
1636In both forms, the value returned is the value of the last expression
5a964f20 1637evaluated inside the mini-program; a return statement may be also used, just
c7cc6f1c 1638as with subroutines. The expression providing the return value is evaluated
cf264981
SP
1639in void, scalar, or list context, depending on the context of the C<eval>
1640itself. See L</wantarray> for more on how the evaluation context can be
1641determined.
a0d0e21e 1642
19799a22 1643If there is a syntax error or runtime error, or a C<die> statement is
8f1da26d 1644executed, C<eval> returns C<undef> in scalar context
1f0d8f98
FC
1645or an empty list--or, for syntax errors, a list containing a single
1646undefined value--in list context, and C<$@> is set to the error
1647message. The discrepancy in the return values in list context is
1648considered a bug by some, and will probably be fixed in a future
1649release. If there was no error, C<$@> is guaranteed to be the empty
3b10bc60 1650string. Beware that using C<eval> neither silences Perl from printing
c7cc6f1c 1651warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
d9984052
A
1652To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1653turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1654See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
a0d0e21e 1655
19799a22
GS
1656Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1657determining whether a particular feature (such as C<socket> or C<symlink>)
82bcec1b 1658is implemented. It is also Perl's exception-trapping mechanism, where
a0d0e21e
LW
1659the die operator is used to raise exceptions.
1660
5f1da31c
NT
1661If you want to trap errors when loading an XS module, some problems with
1662the binary interface (such as Perl version skew) may be fatal even with
1663C<eval> unless C<$ENV{PERL_DL_NONLAZY}> is set. See L<perlrun>.
1664
a0d0e21e
LW
1665If the code to be executed doesn't vary, you may use the eval-BLOCK
1666form to trap run-time errors without incurring the penalty of
1667recompiling each time. The error, if any, is still returned in C<$@>.
1668Examples:
1669
54310121 1670 # make divide-by-zero nonfatal
a0d0e21e
LW
1671 eval { $answer = $a / $b; }; warn $@ if $@;
1672
1673 # same thing, but less efficient
1674 eval '$answer = $a / $b'; warn $@ if $@;
1675
1676 # a compile-time error
5ed4f2ec 1677 eval { $answer = }; # WRONG
a0d0e21e
LW
1678
1679 # a run-time error
5ed4f2ec 1680 eval '$answer ='; # sets $@
a0d0e21e 1681
cf264981
SP
1682Using the C<eval{}> form as an exception trap in libraries does have some
1683issues. Due to the current arguably broken state of C<__DIE__> hooks, you
1684may wish not to trigger any C<__DIE__> hooks that user code may have installed.
2b5ab1e7 1685You can use the C<local $SIG{__DIE__}> construct for this purpose,
80d38338 1686as this example shows:
774d564b 1687
80d38338 1688 # a private exception trap for divide-by-zero
f86cebdf
GS
1689 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1690 warn $@ if $@;
774d564b
PP
1691
1692This is especially significant, given that C<__DIE__> hooks can call
19799a22 1693C<die> again, which has the effect of changing their error messages:
774d564b
PP
1694
1695 # __DIE__ hooks may modify error messages
1696 {
f86cebdf
GS
1697 local $SIG{'__DIE__'} =
1698 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
c7cc6f1c
GS
1699 eval { die "foo lives here" };
1700 print $@ if $@; # prints "bar lives here"
774d564b
PP
1701 }
1702
19799a22 1703Because this promotes action at a distance, this counterintuitive behavior
2b5ab1e7
TC
1704may be fixed in a future release.
1705
19799a22 1706With an C<eval>, you should be especially careful to remember what's
a0d0e21e
LW
1707being looked at when:
1708
5ed4f2ec 1709 eval $x; # CASE 1
1710 eval "$x"; # CASE 2
a0d0e21e 1711
5ed4f2ec 1712 eval '$x'; # CASE 3
1713 eval { $x }; # CASE 4
a0d0e21e 1714
5ed4f2ec 1715 eval "\$$x++"; # CASE 5
1716 $$x++; # CASE 6
a0d0e21e 1717
2f9daede 1718Cases 1 and 2 above behave identically: they run the code contained in
19799a22 1719the variable $x. (Although case 2 has misleading double quotes making
2f9daede 1720the reader wonder what else might be happening (nothing is).) Cases 3
7660c0ab 1721and 4 likewise behave in the same way: they run the code C<'$x'>, which
19799a22 1722does nothing but return the value of $x. (Case 4 is preferred for
2f9daede
TPG
1723purely visual reasons, but it also has the advantage of compiling at
1724compile-time instead of at run-time.) Case 5 is a place where
19799a22 1725normally you I<would> like to use double quotes, except that in this
2f9daede
TPG
1726particular situation, you can just use symbolic references instead, as
1727in case 6.
a0d0e21e 1728
b6538e4f
TC
1729Before Perl 5.14, the assignment to C<$@> occurred before restoration
1730of localised variables, which means that for your code to run on older
b208c909 1731versions, a temporary is required if you want to mask some but not all
8a5a710d
DN
1732errors:
1733
1734 # alter $@ on nefarious repugnancy only
1735 {
1736 my $e;
1737 {
1738 local $@; # protect existing $@
1739 eval { test_repugnancy() };
b208c909 1740 # $@ =~ /nefarious/ and die $@; # Perl 5.14 and higher only
8a5a710d
DN
1741 $@ =~ /nefarious/ and $e = $@;
1742 }
1743 die $e if defined $e
1744 }
1745
4968c1e4 1746C<eval BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7 1747C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
4968c1e4 1748
3b10bc60 1749An C<eval ''> executed within the C<DB> package doesn't see the usual
1750surrounding lexical scope, but rather the scope of the first non-DB piece
1751of code that called it. You don't normally need to worry about this unless
1752you are writing a Perl debugger.
d819b83a 1753
a0d0e21e 1754=item exec LIST
d74e8afc 1755X<exec> X<execute>
a0d0e21e 1756
8bf3b016
GS
1757=item exec PROGRAM LIST
1758
3b10bc60 1759The C<exec> function executes a system command I<and never returns>;
19799a22
GS
1760use C<system> instead of C<exec> if you want it to return. It fails and
1761returns false only if the command does not exist I<and> it is executed
fb73857a 1762directly instead of via your system's command shell (see below).
a0d0e21e 1763
19799a22 1764Since it's a common mistake to use C<exec> instead of C<system>, Perl
80d38338 1765warns you if there is a following statement that isn't C<die>, C<warn>,
3b10bc60 1766or C<exit> (if C<-w> is set--but you always do that, right?). If you
19799a22 1767I<really> want to follow an C<exec> with some other statement, you
55d729e4
GS
1768can use one of these styles to avoid the warning:
1769
5a964f20
TC
1770 exec ('foo') or print STDERR "couldn't exec foo: $!";
1771 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
55d729e4 1772
5a964f20 1773If there is more than one argument in LIST, or if LIST is an array
f86cebdf 1774with more than one value, calls execvp(3) with the arguments in LIST.
5a964f20
TC
1775If there is only one scalar argument or an array with one element in it,
1776the argument is checked for shell metacharacters, and if there are any,
1777the entire argument is passed to the system's command shell for parsing
1778(this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1779If there are no shell metacharacters in the argument, it is split into
b76cc8ba 1780words and passed directly to C<execvp>, which is more efficient.
19799a22 1781Examples:
a0d0e21e 1782
19799a22
GS
1783 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1784 exec "sort $outfile | uniq";
a0d0e21e
LW
1785
1786If you don't really want to execute the first argument, but want to lie
1787to the program you are executing about its own name, you can specify
1788the program you actually want to run as an "indirect object" (without a
1789comma) in front of the LIST. (This always forces interpretation of the
54310121 1790LIST as a multivalued list, even if there is only a single scalar in
a0d0e21e
LW
1791the list.) Example:
1792
1793 $shell = '/bin/csh';
5ed4f2ec 1794 exec $shell '-sh'; # pretend it's a login shell
a0d0e21e
LW
1795
1796or, more directly,
1797
5ed4f2ec 1798 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
a0d0e21e 1799
3b10bc60 1800When the arguments get executed via the system shell, results are
1801subject to its quirks and capabilities. See L<perlop/"`STRING`">
bb32b41a
GS
1802for details.
1803
19799a22
GS
1804Using an indirect object with C<exec> or C<system> is also more
1805secure. This usage (which also works fine with system()) forces
1806interpretation of the arguments as a multivalued list, even if the
1807list had just one argument. That way you're safe from the shell
1808expanding wildcards or splitting up words with whitespace in them.
5a964f20
TC
1809
1810 @args = ( "echo surprise" );
1811
2b5ab1e7 1812 exec @args; # subject to shell escapes
f86cebdf 1813 # if @args == 1
2b5ab1e7 1814 exec { $args[0] } @args; # safe even with one-arg list
5a964f20
TC
1815
1816The first version, the one without the indirect object, ran the I<echo>
80d38338
TC
1817program, passing it C<"surprise"> an argument. The second version didn't;
1818it tried to run a program named I<"echo surprise">, didn't find it, and set
1819C<$?> to a non-zero value indicating failure.
5a964f20 1820
80d38338 1821Beginning with v5.6.0, Perl attempts to flush all files opened for
0f897271
GS
1822output before the exec, but this may not be supported on some platforms
1823(see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1824in English) or call the C<autoflush()> method of C<IO::Handle> on any
80d38338 1825open handles to avoid lost output.
0f897271 1826
80d38338
TC
1827Note that C<exec> will not call your C<END> blocks, nor will it invoke
1828C<DESTROY> methods on your objects.
7660c0ab 1829
ea9eb35a 1830Portability issues: L<perlport/exec>.
1831
a0d0e21e 1832=item exists EXPR
d74e8afc 1833X<exists> X<autovivification>
a0d0e21e 1834
d0a76353
RS
1835Given an expression that specifies an element of a hash, returns true if the
1836specified element in the hash has ever been initialized, even if the
1837corresponding value is undefined.
a0d0e21e 1838
5ed4f2ec 1839 print "Exists\n" if exists $hash{$key};
1840 print "Defined\n" if defined $hash{$key};
01020589
GS
1841 print "True\n" if $hash{$key};
1842
d0a76353 1843exists may also be called on array elements, but its behavior is much less
8f1da26d 1844obvious and is strongly tied to the use of L</delete> on arrays. B<Be aware>
d0a76353
RS
1845that calling exists on array values is deprecated and likely to be removed in
1846a future version of Perl.
1847
5ed4f2ec 1848 print "Exists\n" if exists $array[$index];
1849 print "Defined\n" if defined $array[$index];
01020589 1850 print "True\n" if $array[$index];
a0d0e21e 1851
8f1da26d 1852A hash or array element can be true only if it's defined and defined only if
a0d0e21e
LW
1853it exists, but the reverse doesn't necessarily hold true.
1854
afebc493
GS
1855Given an expression that specifies the name of a subroutine,
1856returns true if the specified subroutine has ever been declared, even
1857if it is undefined. Mentioning a subroutine name for exists or defined
80d38338 1858does not count as declaring it. Note that a subroutine that does not
847c7ebe
DD
1859exist may still be callable: its package may have an C<AUTOLOAD>
1860method that makes it spring into existence the first time that it is
3b10bc60 1861called; see L<perlsub>.
afebc493 1862
5ed4f2ec 1863 print "Exists\n" if exists &subroutine;
1864 print "Defined\n" if defined &subroutine;
afebc493 1865
a0d0e21e 1866Note that the EXPR can be arbitrarily complicated as long as the final
afebc493 1867operation is a hash or array key lookup or subroutine name:
a0d0e21e 1868
5ed4f2ec 1869 if (exists $ref->{A}->{B}->{$key}) { }
1870 if (exists $hash{A}{B}{$key}) { }
2b5ab1e7 1871
5ed4f2ec 1872 if (exists $ref->{A}->{B}->[$ix]) { }
1873 if (exists $hash{A}{B}[$ix]) { }
01020589 1874
afebc493
GS
1875 if (exists &{$ref->{A}{B}{$key}}) { }
1876
3b10bc60 1877Although the mostly deeply nested array or hash will not spring into
1878existence just because its existence was tested, any intervening ones will.
61eff3bc 1879Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
01020589 1880into existence due to the existence test for the $key element above.
3b10bc60 1881This happens anywhere the arrow operator is used, including even here:
5a964f20 1882
2b5ab1e7 1883 undef $ref;
5ed4f2ec 1884 if (exists $ref->{"Some key"}) { }
1885 print $ref; # prints HASH(0x80d3d5c)
2b5ab1e7
TC
1886
1887This surprising autovivification in what does not at first--or even
1888second--glance appear to be an lvalue context may be fixed in a future
5a964f20 1889release.
a0d0e21e 1890
afebc493
GS
1891Use of a subroutine call, rather than a subroutine name, as an argument
1892to exists() is an error.
1893
5ed4f2ec 1894 exists &sub; # OK
1895 exists &sub(); # Error
afebc493 1896
a0d0e21e 1897=item exit EXPR
d74e8afc 1898X<exit> X<terminate> X<abort>
a0d0e21e 1899
ce2984c3
PF
1900=item exit
1901
2b5ab1e7 1902Evaluates EXPR and exits immediately with that value. Example:
a0d0e21e
LW
1903
1904 $ans = <STDIN>;
1905 exit 0 if $ans =~ /^[Xx]/;
1906
19799a22 1907See also C<die>. If EXPR is omitted, exits with C<0> status. The only
2b5ab1e7
TC
1908universally recognized values for EXPR are C<0> for success and C<1>
1909for error; other values are subject to interpretation depending on the
1910environment in which the Perl program is running. For example, exiting
191169 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1912the mailer to return the item undelivered, but that's not true everywhere.
a0d0e21e 1913
19799a22
GS
1914Don't use C<exit> to abort a subroutine if there's any chance that
1915someone might want to trap whatever error happened. Use C<die> instead,
1916which can be trapped by an C<eval>.
28757baa 1917
19799a22 1918The exit() function does not always exit immediately. It calls any
2b5ab1e7 1919defined C<END> routines first, but these C<END> routines may not
19799a22 1920themselves abort the exit. Likewise any object destructors that need to
60275626
FC
1921be called are called before the real exit. C<END> routines and destructors
1922can change the exit status by modifying C<$?>. If this is a problem, you
2b5ab1e7 1923can call C<POSIX:_exit($status)> to avoid END and destructor processing.
87275199 1924See L<perlmod> for details.
5a964f20 1925
ea9eb35a 1926Portability issues: L<perlport/exit>.
1927
a0d0e21e 1928=item exp EXPR
d74e8afc 1929X<exp> X<exponential> X<antilog> X<antilogarithm> X<e>
a0d0e21e 1930
54310121 1931=item exp
bbce6d69 1932
b76cc8ba 1933Returns I<e> (the natural logarithm base) to the power of EXPR.
a0d0e21e
LW
1934If EXPR is omitted, gives C<exp($_)>.
1935
1936=item fcntl FILEHANDLE,FUNCTION,SCALAR
d74e8afc 1937X<fcntl>
a0d0e21e 1938
f86cebdf 1939Implements the fcntl(2) function. You'll probably have to say
a0d0e21e
LW
1940
1941 use Fcntl;
1942
0ade1984 1943first to get the correct constant definitions. Argument processing and
3b10bc60 1944value returned work just like C<ioctl> below.
a0d0e21e
LW
1945For example:
1946
1947 use Fcntl;
5a964f20 1948 fcntl($filehandle, F_GETFL, $packed_return_buffer)
a9a5a0dc 1949 or die "can't fcntl F_GETFL: $!";
5a964f20 1950
554ad1fc 1951You don't have to check for C<defined> on the return from C<fcntl>.
951ba7fe
GS
1952Like C<ioctl>, it maps a C<0> return from the system call into
1953C<"0 but true"> in Perl. This string is true in boolean context and C<0>
2b5ab1e7
TC
1954in numeric context. It is also exempt from the normal B<-w> warnings
1955on improper numeric conversions.
5a964f20 1956
3b10bc60 1957Note that C<fcntl> raises an exception if used on a machine that
2b5ab1e7
TC
1958doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1959manpage to learn what functions are available on your system.
a0d0e21e 1960
be2f7487 1961Here's an example of setting a filehandle named C<REMOTE> to be
1962non-blocking at the system level. You'll have to negotiate C<$|>
1963on your own, though.
1964
1965 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
1966
1967 $flags = fcntl(REMOTE, F_GETFL, 0)
1968 or die "Can't get flags for the socket: $!\n";
1969
1970 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
1971 or die "Can't set flags for the socket: $!\n";
1972
ea9eb35a 1973Portability issues: L<perlport/fcntl>.
1974
a0d0e21e 1975=item fileno FILEHANDLE
d74e8afc 1976X<fileno>
a0d0e21e 1977
2b5ab1e7 1978Returns the file descriptor for a filehandle, or undefined if the
a7c1632d
FC
1979filehandle is not open. If there is no real file descriptor at the OS
1980level, as can happen with filehandles connected to memory objects via
1981C<open> with a reference for the third argument, -1 is returned.
1982
1983This is mainly useful for constructing
19799a22 1984bitmaps for C<select> and low-level POSIX tty-handling operations.
2b5ab1e7
TC
1985If FILEHANDLE is an expression, the value is taken as an indirect
1986filehandle, generally its name.
5a964f20 1987
b76cc8ba 1988You can use this to find out whether two handles refer to the
5a964f20
TC
1989same underlying descriptor:
1990
1991 if (fileno(THIS) == fileno(THAT)) {
a9a5a0dc 1992 print "THIS and THAT are dups\n";
b76cc8ba
NIS
1993 }
1994
a0d0e21e 1995=item flock FILEHANDLE,OPERATION
d74e8afc 1996X<flock> X<lock> X<locking>
a0d0e21e 1997
19799a22
GS
1998Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1999for success, false on failure. Produces a fatal error if used on a
2b5ab1e7 2000machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
dbfe1e81 2001C<flock> is Perl's portable file-locking interface, although it locks
3b10bc60 2002entire files only, not records.
2b5ab1e7
TC
2003
2004Two potentially non-obvious but traditional C<flock> semantics are
2005that it waits indefinitely until the lock is granted, and that its locks
dbfe1e81
FC
2006are B<merely advisory>. Such discretionary locks are more flexible, but
2007offer fewer guarantees. This means that programs that do not also use
2008C<flock> may modify files locked with C<flock>. See L<perlport>,
8f1da26d 2009your port's specific documentation, and your system-specific local manpages
2b5ab1e7
TC
2010for details. It's best to assume traditional behavior if you're writing
2011portable programs. (But if you're not, you should as always feel perfectly
2012free to write for your own system's idiosyncrasies (sometimes called
2013"features"). Slavish adherence to portability concerns shouldn't get
2014in the way of your getting your job done.)
a3cb178b 2015
8ebc5c01
PP
2016OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
2017LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
8f1da26d
TC
2018you can use the symbolic names if you import them from the L<Fcntl> module,
2019either individually, or as a group using the C<:flock> tag. LOCK_SH
68dc0745 2020requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
ea3105be 2021releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
8f1da26d 2022LOCK_SH or LOCK_EX, then C<flock> returns immediately rather than blocking
3b10bc60 2023waiting for the lock; check the return status to see if you got it.
68dc0745 2024
2b5ab1e7
TC
2025To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
2026before locking or unlocking it.
8ebc5c01 2027
f86cebdf 2028Note that the emulation built with lockf(3) doesn't provide shared
8ebc5c01 2029locks, and it requires that FILEHANDLE be open with write intent. These
2b5ab1e7 2030are the semantics that lockf(3) implements. Most if not all systems
f86cebdf 2031implement lockf(3) in terms of fcntl(2) locking, though, so the
8ebc5c01
PP
2032differing semantics shouldn't bite too many people.
2033
becacb53
TM
2034Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
2035be open with read intent to use LOCK_SH and requires that it be open
2036with write intent to use LOCK_EX.
2037
19799a22
GS
2038Note also that some versions of C<flock> cannot lock things over the
2039network; you would need to use the more system-specific C<fcntl> for
f86cebdf
GS
2040that. If you like you can force Perl to ignore your system's flock(2)
2041function, and so provide its own fcntl(2)-based emulation, by passing
8ebc5c01 2042the switch C<-Ud_flock> to the F<Configure> program when you configure
8f1da26d 2043and build a new Perl.
4633a7c4
LW
2044
2045Here's a mailbox appender for BSD systems.
a0d0e21e 2046
7ed5353d 2047 use Fcntl qw(:flock SEEK_END); # import LOCK_* and SEEK_END constants
a0d0e21e
LW
2048
2049 sub lock {
a9a5a0dc
VP
2050 my ($fh) = @_;
2051 flock($fh, LOCK_EX) or die "Cannot lock mailbox - $!\n";
7ed5353d 2052
a9a5a0dc
VP
2053 # and, in case someone appended while we were waiting...
2054 seek($fh, 0, SEEK_END) or die "Cannot seek - $!\n";
a0d0e21e
LW
2055 }
2056
2057 sub unlock {
a9a5a0dc
VP
2058 my ($fh) = @_;
2059 flock($fh, LOCK_UN) or die "Cannot unlock mailbox - $!\n";
a0d0e21e
LW
2060 }
2061
b0169937 2062 open(my $mbox, ">>", "/usr/spool/mail/$ENV{'USER'}")
5ed4f2ec 2063 or die "Can't open mailbox: $!";
a0d0e21e 2064
7ed5353d 2065 lock($mbox);
b0169937 2066 print $mbox $msg,"\n\n";
7ed5353d 2067 unlock($mbox);
a0d0e21e 2068
3b10bc60 2069On systems that support a real flock(2), locks are inherited across fork()
2070calls, whereas those that must resort to the more capricious fcntl(2)
2071function lose their locks, making it seriously harder to write servers.
2b5ab1e7 2072
cb1a09d0 2073See also L<DB_File> for other flock() examples.
a0d0e21e 2074
ea9eb35a 2075Portability issues: L<perlport/flock>.
2076
a0d0e21e 2077=item fork
d74e8afc 2078X<fork> X<child> X<parent>
a0d0e21e 2079
2b5ab1e7
TC
2080Does a fork(2) system call to create a new process running the
2081same program at the same point. It returns the child pid to the
2082parent process, C<0> to the child process, or C<undef> if the fork is
2083unsuccessful. File descriptors (and sometimes locks on those descriptors)
2084are shared, while everything else is copied. On most systems supporting
2085fork(), great care has gone into making it extremely efficient (for
2086example, using copy-on-write technology on data pages), making it the
2087dominant paradigm for multitasking over the last few decades.
5a964f20 2088
80d38338 2089Beginning with v5.6.0, Perl attempts to flush all files opened for
0f897271
GS
2090output before forking the child process, but this may not be supported
2091on some platforms (see L<perlport>). To be safe, you may need to set
2092C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
80d38338 2093C<IO::Handle> on any open handles to avoid duplicate output.
a0d0e21e 2094
19799a22 2095If you C<fork> without ever waiting on your children, you will
2b5ab1e7
TC
2096accumulate zombies. On some systems, you can avoid this by setting
2097C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
2098forking and reaping moribund children.
cb1a09d0 2099
28757baa
PP
2100Note that if your forked child inherits system file descriptors like
2101STDIN and STDOUT that are actually connected by a pipe or socket, even
2b5ab1e7 2102if you exit, then the remote server (such as, say, a CGI script or a
19799a22 2103backgrounded job launched from a remote shell) won't think you're done.
2b5ab1e7 2104You should reopen those to F</dev/null> if it's any issue.
28757baa 2105
ea9eb35a 2106On some platforms such as Windows, where the fork() system call is not available,
2107Perl can be built to emulate fork() in the Perl interpreter. The emulation is designed to,
2108at the level of the Perl program, be as compatible as possible with the "Unix" fork().
2109However it has limitation that has to be considered in code intended to be portable.
2110See L<perlfork> for more details.
2111
2112Portability issues: L<perlport/fork>.
2113
cb1a09d0 2114=item format
d74e8afc 2115X<format>
cb1a09d0 2116
19799a22 2117Declare a picture format for use by the C<write> function. For
cb1a09d0
AD
2118example:
2119
54310121 2120 format Something =
a9a5a0dc
VP
2121 Test: @<<<<<<<< @||||| @>>>>>
2122 $str, $%, '$' . int($num)
cb1a09d0
AD
2123 .
2124
2125 $str = "widget";
184e9718 2126 $num = $cost/$quantity;
cb1a09d0
AD
2127 $~ = 'Something';
2128 write;
2129
2130See L<perlform> for many details and examples.
2131
8903cb82 2132=item formline PICTURE,LIST
d74e8afc 2133X<formline>
a0d0e21e 2134
5a964f20 2135This is an internal function used by C<format>s, though you may call it,
a0d0e21e
LW
2136too. It formats (see L<perlform>) a list of values according to the
2137contents of PICTURE, placing the output into the format output
7660c0ab 2138accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
19799a22 2139Eventually, when a C<write> is done, the contents of
cf264981
SP
2140C<$^A> are written to some filehandle. You could also read C<$^A>
2141and then set C<$^A> back to C<"">. Note that a format typically
19799a22 2142does one C<formline> per line of form, but the C<formline> function itself
748a9306 2143doesn't care how many newlines are embedded in the PICTURE. This means
3b10bc60 2144that the C<~> and C<~~> tokens treat the entire PICTURE as a single line.
748a9306 2145You may therefore need to use multiple formlines to implement a single
3b10bc60 2146record format, just like the C<format> compiler.
748a9306 2147
19799a22 2148Be careful if you put double quotes around the picture, because an C<@>
748a9306 2149character may be taken to mean the beginning of an array name.
19799a22 2150C<formline> always returns true. See L<perlform> for other examples.
a0d0e21e 2151
445b09e5
FC
2152If you are trying to use this instead of C<write> to capture the output,
2153you may find it easier to open a filehandle to a scalar
2154(C<< open $fh, ">", \$output >>) and write to that instead.
2155
a0d0e21e 2156=item getc FILEHANDLE
f723aae1 2157X<getc> X<getchar> X<character> X<file, read>
a0d0e21e
LW
2158
2159=item getc
2160
2161Returns the next character from the input file attached to FILEHANDLE,
3b10bc60 2162or the undefined value at end of file or if there was an error (in
b5fe5ca2
SR
2163the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
2164STDIN. This is not particularly efficient. However, it cannot be
2165used by itself to fetch single characters without waiting for the user
2166to hit enter. For that, try something more like:
4633a7c4
LW
2167
2168 if ($BSD_STYLE) {
a9a5a0dc 2169 system "stty cbreak </dev/tty >/dev/tty 2>&1";
4633a7c4
LW
2170 }
2171 else {
a9a5a0dc 2172 system "stty", '-icanon', 'eol', "\001";
4633a7c4
LW
2173 }
2174
2175 $key = getc(STDIN);
2176
2177 if ($BSD_STYLE) {
a9a5a0dc 2178 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
4633a7c4
LW
2179 }
2180 else {
3b10bc60 2181 system 'stty', 'icanon', 'eol', '^@'; # ASCII NUL
4633a7c4
LW
2182 }
2183 print "\n";
2184
54310121
PP
2185Determination of whether $BSD_STYLE should be set
2186is left as an exercise to the reader.
cb1a09d0 2187
19799a22 2188The C<POSIX::getattr> function can do this more portably on
2b5ab1e7 2189systems purporting POSIX compliance. See also the C<Term::ReadKey>
a3390c9f 2190module from your nearest CPAN site; details on CPAN can be found under
2b5ab1e7 2191L<perlmodlib/CPAN>.
a0d0e21e
LW
2192
2193=item getlogin
d74e8afc 2194X<getlogin> X<login>
a0d0e21e 2195
cf264981 2196This implements the C library function of the same name, which on most
3b10bc60 2197systems returns the current login from F</etc/utmp>, if any. If it
2198returns the empty string, use C<getpwuid>.
a0d0e21e 2199
f86702cc 2200 $login = getlogin || getpwuid($<) || "Kilroy";
a0d0e21e 2201
19799a22
GS
2202Do not consider C<getlogin> for authentication: it is not as
2203secure as C<getpwuid>.
4633a7c4 2204
ea9eb35a 2205Portability issues: L<perlport/getlogin>.
2206
a0d0e21e 2207=item getpeername SOCKET
d74e8afc 2208X<getpeername> X<peer>
a0d0e21e 2209
a3390c9f
FC
2210Returns the packed sockaddr address of the other end of the SOCKET
2211connection.
a0d0e21e 2212
4633a7c4
LW
2213 use Socket;
2214 $hersockaddr = getpeername(SOCK);
19799a22 2215 ($port, $iaddr) = sockaddr_in($hersockaddr);
4633a7c4
LW
2216 $herhostname = gethostbyaddr($iaddr, AF_INET);
2217 $herstraddr = inet_ntoa($iaddr);
a0d0e21e
LW
2218
2219=item getpgrp PID
d74e8afc 2220X<getpgrp> X<group>
a0d0e21e 2221
47e29363 2222Returns the current process group for the specified PID. Use
7660c0ab 2223a PID of C<0> to get the current process group for the
4633a7c4 2224current process. Will raise an exception if used on a machine that
a3390c9f
FC
2225doesn't implement getpgrp(2). If PID is omitted, returns the process
2226group of the current process. Note that the POSIX version of C<getpgrp>
7660c0ab 2227does not accept a PID argument, so only C<PID==0> is truly portable.
a0d0e21e 2228
ea9eb35a 2229Portability issues: L<perlport/getpgrp>.
2230
a0d0e21e 2231=item getppid
d74e8afc 2232X<getppid> X<parent> X<pid>
a0d0e21e
LW
2233
2234Returns the process id of the parent process.
2235
4d76a344
RGS
2236Note for Linux users: on Linux, the C functions C<getpid()> and
2237C<getppid()> return different values from different threads. In order to
3b10bc60 2238be portable, this behavior is not reflected by the Perl-level function
4d76a344 2239C<getppid()>, that returns a consistent value across threads. If you want
e3256f86
RGS
2240to call the underlying C<getppid()>, you may use the CPAN module
2241C<Linux::Pid>.
4d76a344 2242
ea9eb35a 2243Portability issues: L<perlport/getppid>.
2244
a0d0e21e 2245=item getpriority WHICH,WHO
d74e8afc 2246X<getpriority> X<priority> X<nice>
a0d0e21e 2247
4633a7c4 2248Returns the current priority for a process, a process group, or a user.
f4084e39 2249(See C<getpriority(2)>.) Will raise a fatal exception if used on a
f86cebdf 2250machine that doesn't implement getpriority(2).
a0d0e21e 2251
ea9eb35a 2252Portability issues: L<perlport/getpriority>.
2253
a0d0e21e 2254=item getpwnam NAME
d74e8afc
ITB
2255X<getpwnam> X<getgrnam> X<gethostbyname> X<getnetbyname> X<getprotobyname>
2256X<getpwuid> X<getgrgid> X<getservbyname> X<gethostbyaddr> X<getnetbyaddr>
2257X<getprotobynumber> X<getservbyport> X<getpwent> X<getgrent> X<gethostent>
2258X<getnetent> X<getprotoent> X<getservent> X<setpwent> X<setgrent> X<sethostent>
2259X<setnetent> X<setprotoent> X<setservent> X<endpwent> X<endgrent> X<endhostent>
2260X<endnetent> X<endprotoent> X<endservent>
a0d0e21e
LW
2261
2262=item getgrnam NAME
2263
2264=item gethostbyname NAME
2265
2266=item getnetbyname NAME
2267
2268=item getprotobyname NAME
2269
2270=item getpwuid UID
2271
2272=item getgrgid GID
2273
2274=item getservbyname NAME,PROTO
2275
2276=item gethostbyaddr ADDR,ADDRTYPE
2277
2278=item getnetbyaddr ADDR,ADDRTYPE
2279
2280=item getprotobynumber NUMBER
2281
2282=item getservbyport PORT,PROTO
2283
2284=item getpwent
2285
2286=item getgrent
2287
2288=item gethostent
2289
2290=item getnetent
2291
2292=item getprotoent
2293
2294=item getservent
2295
2296=item setpwent
2297
2298=item setgrent
2299
2300=item sethostent STAYOPEN
2301
2302=item setnetent STAYOPEN
2303
2304=item setprotoent STAYOPEN
2305
2306=item setservent STAYOPEN
2307
2308=item endpwent
2309
2310=item endgrent
2311
2312=item endhostent
2313
2314=item endnetent
2315
2316=item endprotoent
2317
2318=item endservent
2319
80d38338
TC
2320These routines are the same as their counterparts in the
2321system C library. In list context, the return values from the
a0d0e21e
LW
2322various get routines are as follows:
2323
2324 ($name,$passwd,$uid,$gid,
6ee623d5 2325 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
a0d0e21e
LW
2326 ($name,$passwd,$gid,$members) = getgr*
2327 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
2328 ($name,$aliases,$addrtype,$net) = getnet*
2329 ($name,$aliases,$proto) = getproto*
2330 ($name,$aliases,$port,$proto) = getserv*
2331
3b10bc60 2332(If the entry doesn't exist you get an empty list.)
a0d0e21e 2333
4602f195
JH
2334The exact meaning of the $gcos field varies but it usually contains
2335the real name of the user (as opposed to the login name) and other
2336information pertaining to the user. Beware, however, that in many
2337system users are able to change this information and therefore it
106325ad 2338cannot be trusted and therefore the $gcos is tainted (see
2959b6e3 2339L<perlsec>). The $passwd and $shell, user's encrypted password and
a3390c9f 2340login shell, are also tainted, for the same reason.
4602f195 2341
5a964f20 2342In scalar context, you get the name, unless the function was a
a0d0e21e
LW
2343lookup by name, in which case you get the other thing, whatever it is.
2344(If the entry doesn't exist you get the undefined value.) For example:
2345
5a964f20
TC
2346 $uid = getpwnam($name);
2347 $name = getpwuid($num);
2348 $name = getpwent();
2349 $gid = getgrnam($name);
08a33e13 2350 $name = getgrgid($num);
5a964f20
TC
2351 $name = getgrent();
2352 #etc.
a0d0e21e 2353
4602f195 2354In I<getpw*()> the fields $quota, $comment, and $expire are special
80d38338 2355in that they are unsupported on many systems. If the
4602f195
JH
2356$quota is unsupported, it is an empty scalar. If it is supported, it
2357usually encodes the disk quota. If the $comment field is unsupported,
2358it is an empty scalar. If it is supported it usually encodes some
2359administrative comment about the user. In some systems the $quota
2360field may be $change or $age, fields that have to do with password
2361aging. In some systems the $comment field may be $class. The $expire
2362field, if present, encodes the expiration period of the account or the
2363password. For the availability and the exact meaning of these fields
8f1da26d 2364in your system, please consult getpwnam(3) and your system's
4602f195
JH
2365F<pwd.h> file. You can also find out from within Perl what your
2366$quota and $comment fields mean and whether you have the $expire field
2367by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2368C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
3b10bc60 2369files are supported only if your vendor has implemented them in the
4602f195 2370intuitive fashion that calling the regular C library routines gets the
5d3a0a3b 2371shadow versions if you're running under privilege or if there exists
cf264981 2372the shadow(3) functions as found in System V (this includes Solaris
a3390c9f 2373and Linux). Those systems that implement a proprietary shadow password
5d3a0a3b 2374facility are unlikely to be supported.
6ee623d5 2375
a3390c9f 2376The $members value returned by I<getgr*()> is a space-separated list of
a0d0e21e
LW
2377the login names of the members of the group.
2378
2379For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2380C, it will be returned to you via C<$?> if the function call fails. The
3b10bc60 2381C<@addrs> value returned by a successful call is a list of raw
2382addresses returned by the corresponding library call. In the
2383Internet domain, each address is four bytes long; you can unpack it
a0d0e21e
LW
2384by saying something like:
2385
f337b084 2386 ($a,$b,$c,$d) = unpack('W4',$addr[0]);
a0d0e21e 2387
2b5ab1e7
TC
2388The Socket library makes this slightly easier:
2389
2390 use Socket;
2391 $iaddr = inet_aton("127.1"); # or whatever address
2392 $name = gethostbyaddr($iaddr, AF_INET);
2393
2394 # or going the other way
19799a22 2395 $straddr = inet_ntoa($iaddr);
2b5ab1e7 2396
d760c846
GS
2397In the opposite way, to resolve a hostname to the IP address
2398you can write this:
2399
2400 use Socket;
2401 $packed_ip = gethostbyname("www.perl.org");
2402 if (defined $packed_ip) {
2403 $ip_address = inet_ntoa($packed_ip);
2404 }
2405
2406Make sure <gethostbyname()> is called in SCALAR context and that
2407its return value is checked for definedness.
2408
19799a22
GS
2409If you get tired of remembering which element of the return list
2410contains which return value, by-name interfaces are provided
2411in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2412C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2413and C<User::grent>. These override the normal built-ins, supplying
2414versions that return objects with the appropriate names
2415for each field. For example:
5a964f20
TC
2416
2417 use File::stat;
2418 use User::pwent;
2419 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2420
a3390c9f 2421Even though it looks as though they're the same method calls (uid),
b76cc8ba 2422they aren't, because a C<File::stat> object is different from
19799a22 2423a C<User::pwent> object.
5a964f20 2424
ea9eb35a 2425Portability issues: L<perlport/getpwnam> to L<perlport/endservent>.
2426
a0d0e21e 2427=item getsockname SOCKET
d74e8afc 2428X<getsockname>
a0d0e21e 2429
19799a22
GS
2430Returns the packed sockaddr address of this end of the SOCKET connection,
2431in case you don't know the address because you have several different
2432IPs that the connection might have come in on.
a0d0e21e 2433
4633a7c4
LW
2434 use Socket;
2435 $mysockaddr = getsockname(SOCK);
19799a22 2436 ($port, $myaddr) = sockaddr_in($mysockaddr);
b76cc8ba 2437 printf "Connect to %s [%s]\n",
19799a22
GS
2438 scalar gethostbyaddr($myaddr, AF_INET),
2439 inet_ntoa($myaddr);
a0d0e21e
LW
2440
2441=item getsockopt SOCKET,LEVEL,OPTNAME
d74e8afc 2442X<getsockopt>
a0d0e21e 2443
636e6b1f
TH
2444Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
2445Options may exist at multiple protocol levels depending on the socket
2446type, but at least the uppermost socket level SOL_SOCKET (defined in the
2447C<Socket> module) will exist. To query options at another level the
2448protocol number of the appropriate protocol controlling the option
2449should be supplied. For example, to indicate that an option is to be
2450interpreted by the TCP protocol, LEVEL should be set to the protocol
80d38338 2451number of TCP, which you can get using C<getprotobyname>.
636e6b1f 2452
80d38338 2453The function returns a packed string representing the requested socket
3b10bc60 2454option, or C<undef> on error, with the reason for the error placed in
a3390c9f 2455C<$!>. Just what is in the packed string depends on LEVEL and OPTNAME;
80d38338
TC
2456consult getsockopt(2) for details. A common case is that the option is an
2457integer, in which case the result is a packed integer, which you can decode
2458using C<unpack> with the C<i> (or C<I>) format.
636e6b1f 2459
8f1da26d 2460Here's an example to test whether Nagle's algorithm is enabled on a socket:
636e6b1f 2461
4852725b 2462 use Socket qw(:all);
636e6b1f
TH
2463
2464 defined(my $tcp = getprotobyname("tcp"))
a9a5a0dc 2465 or die "Could not determine the protocol number for tcp";
4852725b
DD
2466 # my $tcp = IPPROTO_TCP; # Alternative
2467 my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
80d38338 2468 or die "getsockopt TCP_NODELAY: $!";
636e6b1f
TH
2469 my $nodelay = unpack("I", $packed);
2470 print "Nagle's algorithm is turned ", $nodelay ? "off\n" : "on\n";
2471
ea9eb35a 2472Portability issues: L<perlport/getsockopt>.
a0d0e21e 2473
15a348aa 2474=item given EXPR BLOCK
2475X<given>
2476
2477=item given BLOCK
2478
2479C<given> is analogous to the C<switch> keyword in other languages. C<given>
2480and C<when> are used in Perl to implement C<switch>/C<case> like statements.
8f1da26d 2481Only available after Perl 5.10. For example:
15a348aa 2482
8f1da26d 2483 use v5.10;
15a348aa 2484 given ($fruit) {
2485 when (/apples?/) {
2486 print "I like apples."
2487 }
2488 when (/oranges?/) {
2489 print "I don't like oranges."
2490 }
2491 default {
2492 print "I don't like anything"
2493 }
2494 }
2495
2496See L<perlsyn/"Switch statements"> for detailed information.
2497
a0d0e21e 2498=item glob EXPR
d74e8afc 2499X<glob> X<wildcard> X<filename, expansion> X<expand>
a0d0e21e 2500
0a753a76
PP
2501=item glob
2502
d9a9d457
JL
2503In list context, returns a (possibly empty) list of filename expansions on
2504the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
2505scalar context, glob iterates through such filename expansions, returning
2506undef when the list is exhausted. This is the internal function
2507implementing the C<< <*.c> >> operator, but you can use it directly. If
2508EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2509more detail in L<perlop/"I/O Operators">.
a0d0e21e 2510
80d38338
TC
2511Note that C<glob> splits its arguments on whitespace and treats
2512each segment as separate pattern. As such, C<glob("*.c *.h")>
2513matches all files with a F<.c> or F<.h> extension. The expression
b474a1b1 2514C<glob(".* *")> matches all files in the current working directory.
80d38338
TC
2515
2516If non-empty braces are the only wildcard characters used in the
2517C<glob>, no filenames are matched, but potentially many strings
2518are returned. For example, this produces nine strings, one for
2519each pairing of fruits and colors:
2520
2521 @many = glob "{apple,tomato,cherry}={green,yellow,red}";
5c0c9249 2522
3a4b19e4 2523Beginning with v5.6.0, this operator is implemented using the standard
5c0c9249
PF
2524C<File::Glob> extension. See L<File::Glob> for details, including
2525C<bsd_glob> which does not treat whitespace as a pattern separator.
3a4b19e4 2526
ea9eb35a 2527Portability issues: L<perlport/glob>.
2528
a0d0e21e 2529=item gmtime EXPR
d74e8afc 2530X<gmtime> X<UTC> X<Greenwich>
a0d0e21e 2531
ce2984c3
PF
2532=item gmtime
2533
4509d391 2534Works just like L</localtime> but the returned values are
435fbc73 2535localized for the standard Greenwich time zone.
a0d0e21e 2536
a3390c9f
FC
2537Note: When called in list context, $isdst, the last value
2538returned by gmtime, is always C<0>. There is no
435fbc73 2539Daylight Saving Time in GMT.
0a753a76 2540
ea9eb35a 2541Portability issues: L<perlport/gmtime>.
62aa5637 2542
a0d0e21e 2543=item goto LABEL
d74e8afc 2544X<goto> X<jump> X<jmp>
a0d0e21e 2545
748a9306
LW
2546=item goto EXPR
2547
a0d0e21e
LW
2548=item goto &NAME
2549
b500e03b
GG
2550The C<goto-LABEL> form finds the statement labeled with LABEL and
2551resumes execution there. It can't be used to get out of a block or
2552subroutine given to C<sort>. It can be used to go almost anywhere
2553else within the dynamic scope, including out of subroutines, but it's
2554usually better to use some other construct such as C<last> or C<die>.
2555The author of Perl has never felt the need to use this form of C<goto>
3b10bc60 2556(in Perl, that is; C is another matter). (The difference is that C
b500e03b
GG
2557does not offer named loops combined with loop control. Perl does, and
2558this replaces most structured uses of C<goto> in other languages.)
a0d0e21e 2559
7660c0ab
A
2560The C<goto-EXPR> form expects a label name, whose scope will be resolved
2561dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
748a9306
LW
2562necessarily recommended if you're optimizing for maintainability:
2563
2564 goto ("FOO", "BAR", "GLARCH")[$i];
2565
887d89fd
FC
2566As shown in this example, C<goto-EXPR> is exempt from the "looks like a
2567function" rule. A pair of parentheses following it does not (necessarily)
2568delimit its argument. C<goto("NE")."XT"> is equivalent to C<goto NEXT>.
2569
b500e03b 2570Use of C<goto-LABEL> or C<goto-EXPR> to jump into a construct is
0b98bec9 2571deprecated and will issue a warning. Even then, it may not be used to
b500e03b
GG
2572go into any construct that requires initialization, such as a
2573subroutine or a C<foreach> loop. It also can't be used to go into a
0b98bec9 2574construct that is optimized away.
b500e03b 2575
1b6921cb
BT
2576The C<goto-&NAME> form is quite different from the other forms of
2577C<goto>. In fact, it isn't a goto in the normal sense at all, and
2578doesn't have the stigma associated with other gotos. Instead, it
2579exits the current subroutine (losing any changes set by local()) and
2580immediately calls in its place the named subroutine using the current
2581value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2582load another subroutine and then pretend that the other subroutine had
2583been called in the first place (except that any modifications to C<@_>
6cb9131c
GS
2584in the current subroutine are propagated to the other subroutine.)
2585After the C<goto>, not even C<caller> will be able to tell that this
2586routine was called first.
2587
2588NAME needn't be the name of a subroutine; it can be a scalar variable
8f1da26d 2589containing a code reference or a block that evaluates to a code
6cb9131c 2590reference.
a0d0e21e
LW
2591
2592=item grep BLOCK LIST
d74e8afc 2593X<grep>
a0d0e21e
LW
2594
2595=item grep EXPR,LIST
2596
2b5ab1e7
TC
2597This is similar in spirit to, but not the same as, grep(1) and its
2598relatives. In particular, it is not limited to using regular expressions.
2f9daede 2599
a0d0e21e 2600Evaluates the BLOCK or EXPR for each element of LIST (locally setting
7660c0ab 2601C<$_> to each element) and returns the list value consisting of those
19799a22
GS
2602elements for which the expression evaluated to true. In scalar
2603context, returns the number of times the expression was true.
a0d0e21e
LW
2604
2605 @foo = grep(!/^#/, @bar); # weed out comments
2606
2607or equivalently,
2608
2609 @foo = grep {!/^#/} @bar; # weed out comments
2610
be3174d2
GS
2611Note that C<$_> is an alias to the list value, so it can be used to
2612modify the elements of the LIST. While this is useful and supported,
2613it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
2614Similarly, grep returns aliases into the original list, much as a for
2615loop's index variable aliases the list elements. That is, modifying an
19799a22
GS
2616element of a list returned by grep (for example, in a C<foreach>, C<map>
2617or another C<grep>) actually modifies the element in the original list.
2b5ab1e7 2618This is usually something to be avoided when writing clear code.
a0d0e21e 2619
a4fb8298 2620If C<$_> is lexical in the scope where the C<grep> appears (because it has
cf264981 2621been declared with C<my $_>) then, in addition to being locally aliased to
80d38338 2622the list elements, C<$_> keeps being lexical inside the block; i.e., it
a4fb8298
RGS
2623can't be seen from the outside, avoiding any potential side-effects.
2624
19799a22 2625See also L</map> for a list composed of the results of the BLOCK or EXPR.
38325410 2626
a0d0e21e 2627=item hex EXPR
d74e8afc 2628X<hex> X<hexadecimal>
a0d0e21e 2629
54310121 2630=item hex
bbce6d69 2631
2b5ab1e7 2632Interprets EXPR as a hex string and returns the corresponding value.
38366c11 2633(To convert strings that might start with either C<0>, C<0x>, or C<0b>, see
2b5ab1e7 2634L</oct>.) If EXPR is omitted, uses C<$_>.
2f9daede
TPG
2635
2636 print hex '0xAf'; # prints '175'
2637 print hex 'aF'; # same
a0d0e21e 2638
19799a22 2639Hex strings may only represent integers. Strings that would cause
53305cf1 2640integer overflow trigger a warning. Leading whitespace is not stripped,
38366c11 2641unlike oct(). To present something as hex, look into L</printf>,
8f1da26d 2642L</sprintf>, and L</unpack>.
19799a22 2643
ce2984c3 2644=item import LIST
d74e8afc 2645X<import>
a0d0e21e 2646
19799a22 2647There is no builtin C<import> function. It is just an ordinary
4633a7c4 2648method (subroutine) defined (or inherited) by modules that wish to export
19799a22 2649names to another module. The C<use> function calls the C<import> method
cea6626f 2650for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
a0d0e21e
LW
2651
2652=item index STR,SUBSTR,POSITION
d74e8afc 2653X<index> X<indexOf> X<InStr>
a0d0e21e
LW
2654
2655=item index STR,SUBSTR
2656
2b5ab1e7
TC
2657The index function searches for one string within another, but without
2658the wildcard-like behavior of a full regular-expression pattern match.
2659It returns the position of the first occurrence of SUBSTR in STR at
2660or after POSITION. If POSITION is omitted, starts searching from the
26f149de
YST
2661beginning of the string. POSITION before the beginning of the string
2662or after its end is treated as if it were the beginning or the end,
2663respectively. POSITION and the return value are based at C<0> (or whatever
2b5ab1e7 2664you've set the C<$[> variable to--but don't do that). If the substring
cf264981 2665is not found, C<index> returns one less than the base, ordinarily C<-1>.
a0d0e21e
LW
2666
2667=item int EXPR
f723aae1 2668X<int> X<integer> X<truncate> X<trunc> X<floor>
a0d0e21e 2669
54310121 2670=item int
bbce6d69 2671
7660c0ab 2672Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2b5ab1e7 2673You should not use this function for rounding: one because it truncates
3b10bc60 2674towards C<0>, and two because machine representations of floating-point
2b5ab1e7
TC
2675numbers can sometimes produce counterintuitive results. For example,
2676C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2677because it's really more like -268.99999999999994315658 instead. Usually,
19799a22 2678the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2b5ab1e7 2679functions will serve you better than will int().
a0d0e21e
LW
2680
2681=item ioctl FILEHANDLE,FUNCTION,SCALAR
d74e8afc 2682X<ioctl>
a0d0e21e 2683
2b5ab1e7 2684Implements the ioctl(2) function. You'll probably first have to say
a0d0e21e 2685
5ed4f2ec 2686 require "sys/ioctl.ph"; # probably in $Config{archlib}/sys/ioctl.ph
a0d0e21e 2687
a11c483f 2688to get the correct function definitions. If F<sys/ioctl.ph> doesn't
a0d0e21e 2689exist or doesn't have the correct definitions you'll have to roll your
61eff3bc 2690own, based on your C header files such as F<< <sys/ioctl.h> >>.
5a964f20 2691(There is a Perl script called B<h2ph> that comes with the Perl kit that
54310121 2692may help you in this, but it's nontrivial.) SCALAR will be read and/or
3b10bc60 2693written depending on the FUNCTION; a C pointer to the string value of SCALAR
19799a22 2694will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
4633a7c4
LW
2695has no string value but does have a numeric value, that value will be
2696passed rather than a pointer to the string value. To guarantee this to be
19799a22
GS
2697true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2698functions may be needed to manipulate the values of structures used by
b76cc8ba 2699C<ioctl>.
a0d0e21e 2700
19799a22 2701The return value of C<ioctl> (and C<fcntl>) is as follows:
a0d0e21e 2702
5ed4f2ec 2703 if OS returns: then Perl returns:
2704 -1 undefined value
2705 0 string "0 but true"
2706 anything else that number
a0d0e21e 2707
19799a22 2708Thus Perl returns true on success and false on failure, yet you can
a0d0e21e
LW
2709still easily determine the actual value returned by the operating
2710system:
2711
2b5ab1e7 2712 $retval = ioctl(...) || -1;
a0d0e21e
LW
2713 printf "System returned %d\n", $retval;
2714
be2f7487 2715The special string C<"0 but true"> is exempt from B<-w> complaints
5a964f20
TC
2716about improper numeric conversions.
2717
ea9eb35a 2718Portability issues: L<perlport/ioctl>.
2719
a0d0e21e 2720=item join EXPR,LIST
d74e8afc 2721X<join>
a0d0e21e 2722
2b5ab1e7
TC
2723Joins the separate strings of LIST into a single string with fields
2724separated by the value of EXPR, and returns that new string. Example:
a0d0e21e 2725
2b5ab1e7 2726 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
a0d0e21e 2727
eb6e2d6f
GS
2728Beware that unlike C<split>, C<join> doesn't take a pattern as its
2729first argument. Compare L</split>.
a0d0e21e 2730
532eee96 2731=item keys HASH
d74e8afc 2732X<keys> X<key>
aa689395 2733
532eee96 2734=item keys ARRAY
aeedbbed 2735
f5a93a43
TC
2736=item keys EXPR
2737
aeedbbed
NC
2738Returns a list consisting of all the keys of the named hash, or the indices
2739of an array. (In scalar context, returns the number of keys or indices.)
504f80c1 2740
aeedbbed 2741The keys of a hash are returned in an apparently random order. The actual
3b10bc60 2742random order is subject to change in future versions of Perl, but it
504f80c1 2743is guaranteed to be the same order as either the C<values> or C<each>
4546b9e6 2744function produces (given that the hash has not been modified). Since
c5f61d2f 2745Perl 5.8.1 the ordering can be different even between different runs of
4546b9e6 2746Perl for security reasons (see L<perlsec/"Algorithmic Complexity
d6df3700 2747Attacks">).
504f80c1 2748
8f1da26d 2749As a side effect, calling keys() resets the internal interator of the HASH or ARRAY
cf264981
SP
2750(see L</each>). In particular, calling keys() in void context resets
2751the iterator with no other overhead.
a0d0e21e 2752
aa689395 2753Here is yet another way to print your environment:
a0d0e21e
LW
2754
2755 @keys = keys %ENV;
2756 @values = values %ENV;
b76cc8ba 2757 while (@keys) {
a9a5a0dc 2758 print pop(@keys), '=', pop(@values), "\n";
a0d0e21e
LW
2759 }
2760
2761or how about sorted by key:
2762
2763 foreach $key (sort(keys %ENV)) {
a9a5a0dc 2764 print $key, '=', $ENV{$key}, "\n";
a0d0e21e
LW
2765 }
2766
8ea1e5d4
GS
2767The returned values are copies of the original keys in the hash, so
2768modifying them will not affect the original hash. Compare L</values>.
2769
19799a22 2770To sort a hash by value, you'll need to use a C<sort> function.
aa689395 2771Here's a descending numeric sort of a hash by its values:
4633a7c4 2772
5a964f20 2773 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
a9a5a0dc 2774 printf "%4d %s\n", $hash{$key}, $key;
4633a7c4
LW
2775 }
2776
3b10bc60 2777Used as an lvalue, C<keys> allows you to increase the number of hash buckets
aa689395
PP
2778allocated for the given hash. This can gain you a measure of efficiency if
2779you know the hash is going to get big. (This is similar to pre-extending
2780an array by assigning a larger number to $#array.) If you say
55497cff
PP
2781
2782 keys %hash = 200;
2783
ab192400
GS
2784then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2785in fact, since it rounds up to the next power of two. These
55497cff
PP
2786buckets will be retained even if you do C<%hash = ()>, use C<undef
2787%hash> if you want to free the storage while C<%hash> is still in scope.
2788You can't shrink the number of buckets allocated for the hash using
19799a22 2789C<keys> in this way (but you needn't worry about doing this by accident,
aeedbbed
NC
2790as trying has no effect). C<keys @array> in an lvalue context is a syntax
2791error.
55497cff 2792
f5a93a43
TC
2793Starting with Perl 5.14, C<keys> can take a scalar EXPR, which must contain
2794a reference to an unblessed hash or array. The argument will be
2795dereferenced automatically. This aspect of C<keys> is considered highly
2796experimental. The exact behaviour may change in a future version of Perl.
cba5a3b0
DG
2797
2798 for (keys $hashref) { ... }
2799 for (keys $obj->get_arrayref) { ... }
2800
8f1da26d 2801See also C<each>, C<values>, and C<sort>.
ab192400 2802
b350dd2f 2803=item kill SIGNAL, LIST
d74e8afc 2804X<kill> X<signal>
a0d0e21e 2805
b350dd2f 2806Sends a signal to a list of processes. Returns the number of
517db077
GS
2807processes successfully signaled (which is not necessarily the
2808same as the number actually killed).
a0d0e21e
LW
2809
2810 $cnt = kill 1, $child1, $child2;
2811 kill 9, @goners;
2812
3b10bc60 2813If SIGNAL is zero, no signal is sent to the process, but C<kill>
2814checks whether it's I<possible> to send a signal to it (that
70fb64f6 2815means, to be brief, that the process is owned by the same user, or we are
3b10bc60 2816the super-user). This is useful to check that a child process is still
81fd35db
DN
2817alive (even if only as a zombie) and hasn't changed its UID. See
2818L<perlport> for notes on the portability of this construct.
b350dd2f 2819
e2c0f81f
DG
2820Unlike in the shell, if SIGNAL is negative, it kills process groups instead
2821of processes. That means you usually want to use positive not negative signals.
2822You may also use a signal name in quotes.
2823
2824The behavior of kill when a I<PROCESS> number is zero or negative depends on
2825the operating system. For example, on POSIX-conforming systems, zero will
2826signal the current process group and -1 will signal all processes.
1e9c1022
JL
2827
2828See L<perlipc/"Signals"> for more details.
a0d0e21e 2829
ea9eb35a 2830On some platforms such as Windows where the fork() system call is not available.
2831Perl can be built to emulate fork() at the interpreter level.
2832This emulation has limitation related to kill that has to be considered,
2833for code running on Windows and in code intended to be portable.
2834
2835See L<perlfork> for more details.
2836
2837Portability issues: L<perlport/kill>.
2838
a0d0e21e 2839=item last LABEL
d74e8afc 2840X<last> X<break>
a0d0e21e
LW
2841
2842=item last
2843
2844The C<last> command is like the C<break> statement in C (as used in
2845loops); it immediately exits the loop in question. If the LABEL is
2846omitted, the command refers to the innermost enclosing loop. The
2847C<continue> block, if any, is not executed:
2848
4633a7c4 2849 LINE: while (<STDIN>) {
a9a5a0dc
VP
2850 last LINE if /^$/; # exit when done with header
2851 #...
a0d0e21e
LW
2852 }
2853
80d38338 2854C<last> cannot be used to exit a block that returns a value such as
8f1da26d 2855C<eval {}>, C<sub {}>, or C<do {}>, and should not be used to exit
2b5ab1e7 2856a grep() or map() operation.
4968c1e4 2857
6c1372ed
GS
2858Note that a block by itself is semantically identical to a loop
2859that executes once. Thus C<last> can be used to effect an early
2860exit out of such a block.
2861
98293880
JH
2862See also L</continue> for an illustration of how C<last>, C<next>, and
2863C<redo> work.
1d2dff63 2864
a0d0e21e 2865=item lc EXPR
d74e8afc 2866X<lc> X<lowercase>
a0d0e21e 2867
54310121 2868=item lc
bbce6d69 2869
d1be9408 2870Returns a lowercased version of EXPR. This is the internal function
3980dc9c 2871implementing the C<\L> escape in double-quoted strings.
a0d0e21e 2872
7660c0ab 2873If EXPR is omitted, uses C<$_>.
bbce6d69 2874
3980dc9c
KW
2875What gets returned depends on several factors:
2876
2877=over
2878
2879=item If C<use bytes> is in effect:
2880
2881=over
2882
2883=item On EBCDIC platforms
2884
2885The results are what the C language system call C<tolower()> returns.
2886
2887=item On ASCII platforms
2888
2889The results follow ASCII semantics. Only characters C<A-Z> change, to C<a-z>
2890respectively.
2891
2892=back
2893
2894=item Otherwise, If EXPR has the UTF8 flag set
2895
2896If the current package has a subroutine named C<ToLower>, it will be used to
96090e4f
A
2897change the case
2898(See L<perlunicode/"User-Defined Case Mappings (for serious hackers only)">.)
3980dc9c
KW
2899Otherwise Unicode semantics are used for the case change.
2900
2901=item Otherwise, if C<use locale> is in effect
2902
2903Respects current LC_CTYPE locale. See L<perllocale>.
2904
2905=item Otherwise, if C<use feature 'unicode_strings'> is in effect:
2906
2907Unicode semantics are used for the case change. Any subroutine named
15dbbbab 2908C<ToLower> will be ignored.
3980dc9c
KW
2909
2910=item Otherwise:
2911
2912=over
2913
2914=item On EBCDIC platforms
2915
2916The results are what the C language system call C<tolower()> returns.
2917
2918=item On ASCII platforms
2919
2920ASCII semantics are used for the case change. The lowercase of any character
2921outside the ASCII range is the character itself.
2922
2923=back
2924
2925=back
2926
a0d0e21e 2927=item lcfirst EXPR
d74e8afc 2928X<lcfirst> X<lowercase>
a0d0e21e 2929
54310121 2930=item lcfirst
bbce6d69 2931
ad0029c4
JH
2932Returns the value of EXPR with the first character lowercased. This
2933is the internal function implementing the C<\l> escape in
3980dc9c 2934double-quoted strings.
a0d0e21e 2935
7660c0ab 2936If EXPR is omitted, uses C<$_>.
bbce6d69 2937
15dbbbab 2938This function behaves the same way under various pragmata, such as in a locale,
3980dc9c
KW
2939as L</lc> does.
2940
a0d0e21e 2941=item length EXPR
d74e8afc 2942X<length> X<size>
a0d0e21e 2943
54310121 2944=item length
bbce6d69 2945
974da8e5 2946Returns the length in I<characters> of the value of EXPR. If EXPR is
15dbbbab
FC
2947omitted, returns the length of C<$_>. If EXPR is undefined, returns
2948C<undef>.
3b10bc60 2949
2950This function cannot be used on an entire array or hash to find out how
2951many elements these have. For that, use C<scalar @array> and C<scalar keys
2952%hash>, respectively.
2953
2954Like all Perl character operations, length() normally deals in logical
2955characters, not physical bytes. For how many bytes a string encoded as
2956UTF-8 would take up, use C<length(Encode::encode_utf8(EXPR))> (you'll have
2957to C<use Encode> first). See L<Encode> and L<perlunicode>.
974da8e5 2958
a0d0e21e 2959=item link OLDFILE,NEWFILE
d74e8afc 2960X<link>
a0d0e21e 2961
19799a22 2962Creates a new filename linked to the old filename. Returns true for
b76cc8ba 2963success, false otherwise.
a0d0e21e 2964
ea9eb35a 2965Portability issues: L<perlport/link>.
2966
a0d0e21e 2967=item listen SOCKET,QUEUESIZE
d74e8afc 2968X<listen>
a0d0e21e 2969
3b10bc60 2970Does the same thing that the listen(2) system call does. Returns true if
b76cc8ba 2971it succeeded, false otherwise. See the example in
cea6626f 2972L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
2973
2974=item local EXPR
d74e8afc 2975X<local>
a0d0e21e 2976
19799a22 2977You really probably want to be using C<my> instead, because C<local> isn't
b76cc8ba 2978what most people think of as "local". See
13a2d996 2979L<perlsub/"Private Variables via my()"> for details.
2b5ab1e7 2980
5a964f20
TC
2981A local modifies the listed variables to be local to the enclosing
2982block, file, or eval. If more than one value is listed, the list must
2983be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2984for details, including issues with tied arrays and hashes.
a0d0e21e 2985
d361fafa
VP
2986The C<delete local EXPR> construct can also be used to localize the deletion
2987of array/hash elements to the current block.
2988See L<perlsub/"Localized deletion of elements of composite types">.
2989
a0d0e21e 2990=item localtime EXPR
435fbc73 2991X<localtime> X<ctime>
a0d0e21e 2992
ba053783
AL
2993=item localtime
2994
19799a22 2995Converts a time as returned by the time function to a 9-element list
5f05dabc 2996with the time analyzed for the local time zone. Typically used as
a0d0e21e
LW
2997follows:
2998
54310121 2999 # 0 1 2 3 4 5 6 7 8
a0d0e21e 3000 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
ba053783 3001 localtime(time);
a0d0e21e 3002
8f1da26d 3003All list elements are numeric and come straight out of the C `struct
ba053783
AL
3004tm'. C<$sec>, C<$min>, and C<$hour> are the seconds, minutes, and hours
3005of the specified time.
48a26b3a 3006
8f1da26d
TC
3007C<$mday> is the day of the month and C<$mon> the month in
3008the range C<0..11>, with 0 indicating January and 11 indicating December.
ba053783 3009This makes it easy to get a month name from a list:
54310121 3010
ba053783
AL
3011 my @abbr = qw( Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec );
3012 print "$abbr[$mon] $mday";
3013 # $mon=9, $mday=18 gives "Oct 18"
abd75f24 3014
8f1da26d 3015C<$year> is the number of years since 1900, B<not> just the last two digits
ba053783 3016of the year. That is, C<$year> is C<123> in year 2023. The proper way
80d38338 3017to get a 4-digit year is simply:
abd75f24 3018
ba053783 3019 $year += 1900;
abd75f24 3020
435fbc73
GS
3021Otherwise you create non-Y2K-compliant programs--and you wouldn't want
3022to do that, would you?
3023
8f1da26d 3024To get the last two digits of the year (e.g., "01" in 2001) do:
ba053783
AL
3025
3026 $year = sprintf("%02d", $year % 100);
3027
3028C<$wday> is the day of the week, with 0 indicating Sunday and 3 indicating
3029Wednesday. C<$yday> is the day of the year, in the range C<0..364>
3030(or C<0..365> in leap years.)
3031
3032C<$isdst> is true if the specified time occurs during Daylight Saving
3033Time, false otherwise.
abd75f24 3034
e1998452 3035If EXPR is omitted, C<localtime()> uses the current time (as returned
e3176d09 3036by time(3)).
a0d0e21e 3037
48a26b3a 3038In scalar context, C<localtime()> returns the ctime(3) value:
a0d0e21e 3039
5f05dabc 3040 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
a0d0e21e 3041
15dbbbab 3042This scalar value is B<not> locale-dependent but is a Perl builtin. For GMT
fe86afc2 3043instead of local time use the L</gmtime> builtin. See also the
8f1da26d 3044C<Time::Local> module (for converting seconds, minutes, hours, and such back to
fe86afc2
NC
3045the integer value returned by time()), and the L<POSIX> module's strftime(3)
3046and mktime(3) functions.
3047
15dbbbab 3048To get somewhat similar but locale-dependent date strings, set up your
fe86afc2
NC
3049locale environment variables appropriately (please see L<perllocale>) and
3050try for example:
a3cb178b 3051
5a964f20 3052 use POSIX qw(strftime);
2b5ab1e7 3053 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
fe86afc2
NC
3054 # or for GMT formatted appropriately for your locale:
3055 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
a3cb178b
GS
3056
3057Note that the C<%a> and C<%b>, the short forms of the day of the week
3058and the month of the year, may not necessarily be three characters wide.
a0d0e21e 3059
15dbbbab 3060The L<Time::gmtime> and L<Time::localtime> modules provide a convenient,
435fbc73
GS
3061by-name access mechanism to the gmtime() and localtime() functions,
3062respectively.
3063
3064For a comprehensive date and time representation look at the
3065L<DateTime> module on CPAN.
3066
ea9eb35a 3067Portability issues: L<perlport/localtime>.
3068
07698885 3069=item lock THING
d74e8afc 3070X<lock>