This is a live mirror of the Perl 5 development currently hosted at https://github.com/perl/perl5
Fix typo, change "an array" to "a hash"
[perl5.git] / pod / perlfunc.pod
CommitLineData
a0d0e21e
LW
1=head1 NAME
2
3perlfunc - Perl builtin functions
4
5=head1 DESCRIPTION
6
7The functions in this section can serve as terms in an expression.
8They fall into two major categories: list operators and named unary
9operators. These differ in their precedence relationship with a
10following comma. (See the precedence table in L<perlop>.) List
11operators take more than one argument, while unary operators can never
12take more than one argument. Thus, a comma terminates the argument of
13a unary operator, but merely separates the arguments of a list
14operator. A unary operator generally provides a scalar context to its
15argument, while a list operator may provide either scalar and list
16contexts for its arguments. If it does both, the scalar arguments will
5f05dabc
PP
17be first, and the list argument will follow. (Note that there can ever
18be only one list argument.) For instance, splice() has three scalar
a0d0e21e
LW
19arguments followed by a list.
20
21In the syntax descriptions that follow, list operators that expect a
22list (and provide list context for the elements of the list) are shown
23with LIST as an argument. Such a list may consist of any combination
24of scalar arguments or list values; the list values will be included
25in the list as if each individual element were interpolated at that
26point in the list, forming a longer single-dimensional list value.
27Elements of the LIST should be separated by commas.
28
29Any function in the list below may be used either with or without
30parentheses around its arguments. (The syntax descriptions omit the
5f05dabc 31parentheses.) If you use the parentheses, the simple (but occasionally
a0d0e21e
LW
32surprising) rule is this: It I<LOOKS> like a function, therefore it I<IS> a
33function, and precedence doesn't matter. Otherwise it's a list
34operator or unary operator, and precedence does matter. And whitespace
35between the function and left parenthesis doesn't count--so you need to
36be careful sometimes:
37
68dc0745
PP
38 print 1+2+4; # Prints 7.
39 print(1+2) + 4; # Prints 3.
40 print (1+2)+4; # Also prints 3!
41 print +(1+2)+4; # Prints 7.
42 print ((1+2)+4); # Prints 7.
a0d0e21e
LW
43
44If you run Perl with the B<-w> switch it can warn you about this. For
45example, the third line above produces:
46
47 print (...) interpreted as function at - line 1.
48 Useless use of integer addition in void context at - line 1.
49
50For functions that can be used in either a scalar or list context,
54310121 51nonabortive failure is generally indicated in a scalar context by
a0d0e21e
LW
52returning the undefined value, and in a list context by returning the
53null list.
54
5a964f20
TC
55Remember the following important rule: There is B<no rule> that relates
56the behavior of an expression in list context to its behavior in scalar
57context, or vice versa. It might do two totally different things.
a0d0e21e
LW
58Each operator and function decides which sort of value it would be most
59appropriate to return in a scalar context. Some operators return the
5a964f20 60length of the list that would have been returned in list context. Some
a0d0e21e
LW
61operators return the first value in the list. Some operators return the
62last value in the list. Some operators return a count of successful
63operations. In general, they do what you want, unless you want
64consistency.
65
5a964f20
TC
66An named array in scalar context is quite different from what would at
67first glance appear to be a list in scalar context. You can't get a list
68like C<(1,2,3)> into being in scalar context, because the compiler knows
69the context at compile time. It would generate the scalar comma operator
70there, not the list construction version of the comma. That means it
71was never a list to start with.
72
73In general, functions in Perl that serve as wrappers for system calls
f86cebdf 74of the same name (like chown(2), fork(2), closedir(2), etc.) all return
5a964f20
TC
75true when they succeed and C<undef> otherwise, as is usually mentioned
76in the descriptions below. This is different from the C interfaces,
7660c0ab
A
77which return C<-1> on failure. Exceptions to this rule are C<wait()>,
78C<waitpid()>, and C<syscall()>. System calls also set the special C<$!>
5a964f20
TC
79variable on failure. Other functions do not, except accidentally.
80
cb1a09d0
AD
81=head2 Perl Functions by Category
82
83Here are Perl's functions (including things that look like
5a964f20 84functions, like some keywords and named operators)
cb1a09d0
AD
85arranged by category. Some functions appear in more
86than one place.
87
88=over
89
90=item Functions for SCALARs or strings
91
22fae026 92C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>,
ab4f32c2
A
93C<length>, C<oct>, C<ord>, C<pack>, C<q/STRING/>, C<qq/STRING/>, C<reverse>,
94C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///>
cb1a09d0
AD
95
96=item Regular expressions and pattern matching
97
ab4f32c2 98C<m//>, C<pos>, C<quotemeta>, C<s///>, C<split>, C<study>, C<qr//>
cb1a09d0
AD
99
100=item Numeric functions
101
22fae026
TM
102C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>,
103C<sin>, C<sqrt>, C<srand>
cb1a09d0
AD
104
105=item Functions for real @ARRAYs
106
22fae026 107C<pop>, C<push>, C<shift>, C<splice>, C<unshift>
cb1a09d0
AD
108
109=item Functions for list data
110
ab4f32c2 111C<grep>, C<join>, C<map>, C<qw/STRING/>, C<reverse>, C<sort>, C<unpack>
cb1a09d0
AD
112
113=item Functions for real %HASHes
114
22fae026 115C<delete>, C<each>, C<exists>, C<keys>, C<values>
cb1a09d0
AD
116
117=item Input and output functions
118
22fae026
TM
119C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
120C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
121C<readdir>, C<rewinddir>, C<seek>, C<seekdir>, C<select>, C<syscall>,
122C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>,
123C<warn>, C<write>
cb1a09d0
AD
124
125=item Functions for fixed length data or records
126
22fae026 127C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec>
cb1a09d0
AD
128
129=item Functions for filehandles, files, or directories
130
22fae026
TM
131C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>,
132C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>, C<readlink>,
133C<rename>, C<rmdir>, C<stat>, C<symlink>, C<umask>, C<unlink>, C<utime>
cb1a09d0
AD
134
135=item Keywords related to the control flow of your perl program
136
22fae026
TM
137C<caller>, C<continue>, C<die>, C<do>, C<dump>, C<eval>, C<exit>,
138C<goto>, C<last>, C<next>, C<redo>, C<return>, C<sub>, C<wantarray>
cb1a09d0 139
54310121 140=item Keywords related to scoping
cb1a09d0 141
22fae026 142C<caller>, C<import>, C<local>, C<my>, C<package>, C<use>
cb1a09d0
AD
143
144=item Miscellaneous functions
145
22fae026
TM
146C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<reset>,
147C<scalar>, C<undef>, C<wantarray>
cb1a09d0
AD
148
149=item Functions for processes and process groups
150
22fae026 151C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
ab4f32c2 152C<pipe>, C<qx/STRING/>, C<setpgrp>, C<setpriority>, C<sleep>, C<system>,
22fae026 153C<times>, C<wait>, C<waitpid>
cb1a09d0
AD
154
155=item Keywords related to perl modules
156
22fae026 157C<do>, C<import>, C<no>, C<package>, C<require>, C<use>
cb1a09d0
AD
158
159=item Keywords related to classes and object-orientedness
160
22fae026
TM
161C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
162C<untie>, C<use>
cb1a09d0
AD
163
164=item Low-level socket functions
165
22fae026
TM
166C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
167C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
168C<socket>, C<socketpair>
cb1a09d0
AD
169
170=item System V interprocess communication functions
171
22fae026
TM
172C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
173C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>
cb1a09d0
AD
174
175=item Fetching user and group info
176
22fae026
TM
177C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
178C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
179C<getpwuid>, C<setgrent>, C<setpwent>
cb1a09d0
AD
180
181=item Fetching network info
182
22fae026
TM
183C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
184C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
185C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
186C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
187C<setnetent>, C<setprotoent>, C<setservent>
cb1a09d0
AD
188
189=item Time-related functions
190
22fae026 191C<gmtime>, C<localtime>, C<time>, C<times>
cb1a09d0 192
37798a01
PP
193=item Functions new in perl5
194
22fae026
TM
195C<abs>, C<bless>, C<chomp>, C<chr>, C<exists>, C<formline>, C<glob>,
196C<import>, C<lc>, C<lcfirst>, C<map>, C<my>, C<no>, C<prototype>, C<qx>,
197C<qw>, C<readline>, C<readpipe>, C<ref>, C<sub*>, C<sysopen>, C<tie>,
198C<tied>, C<uc>, C<ucfirst>, C<untie>, C<use>
37798a01
PP
199
200* - C<sub> was a keyword in perl4, but in perl5 it is an
5a964f20 201operator, which can be used in expressions.
37798a01
PP
202
203=item Functions obsoleted in perl5
204
22fae026 205C<dbmclose>, C<dbmopen>
37798a01 206
cb1a09d0
AD
207=back
208
209=head2 Alphabetical Listing of Perl Functions
210
a0d0e21e
LW
211=over 8
212
22fae026 213=item I<-X> FILEHANDLE
a0d0e21e 214
22fae026 215=item I<-X> EXPR
a0d0e21e 216
22fae026 217=item I<-X>
a0d0e21e
LW
218
219A file test, where X is one of the letters listed below. This unary
220operator takes one argument, either a filename or a filehandle, and
221tests the associated file to see if something is true about it. If the
7660c0ab 222argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
a0d0e21e
LW
223Unless otherwise documented, it returns C<1> for TRUE and C<''> for FALSE, or
224the undefined value if the file doesn't exist. Despite the funny
225names, precedence is the same as any other named unary operator, and
226the argument may be parenthesized like any other unary operator. The
227operator may be any of:
7e778d91
IZ
228X<-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>
229X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
a0d0e21e
LW
230
231 -r File is readable by effective uid/gid.
232 -w File is writable by effective uid/gid.
233 -x File is executable by effective uid/gid.
234 -o File is owned by effective uid.
235
236 -R File is readable by real uid/gid.
237 -W File is writable by real uid/gid.
238 -X File is executable by real uid/gid.
239 -O File is owned by real uid.
240
241 -e File exists.
242 -z File has zero size.
54310121 243 -s File has nonzero size (returns size).
a0d0e21e
LW
244
245 -f File is a plain file.
246 -d File is a directory.
247 -l File is a symbolic link.
9c4d0f16 248 -p File is a named pipe (FIFO), or Filehandle is a pipe.
a0d0e21e
LW
249 -S File is a socket.
250 -b File is a block special file.
251 -c File is a character special file.
252 -t Filehandle is opened to a tty.
253
254 -u File has setuid bit set.
255 -g File has setgid bit set.
256 -k File has sticky bit set.
257
258 -T File is a text file.
259 -B File is a binary file (opposite of -T).
260
261 -M Age of file in days when script started.
262 -A Same for access time.
263 -C Same for inode change time.
264
265The interpretation of the file permission operators C<-r>, C<-R>, C<-w>,
5f05dabc 266C<-W>, C<-x>, and C<-X> is based solely on the mode of the file and the
a0d0e21e 267uids and gids of the user. There may be other reasons you can't actually
5a964f20 268read, write, or execute the file, such as AFS access control lists. Also note that, for the superuser,
7660c0ab
A
269C<-r>, C<-R>, C<-w>, and C<-W> always return C<1>, and C<-x> and C<-X> return
270C<1> if any execute bit is set in the mode. Scripts run by the superuser may
271thus need to do a C<stat()> to determine the actual mode of the
a0d0e21e
LW
272file, or temporarily set the uid to something else.
273
274Example:
275
276 while (<>) {
277 chop;
278 next unless -f $_; # ignore specials
5a964f20 279 #...
a0d0e21e
LW
280 }
281
282Note that C<-s/a/b/> does not do a negated substitution. Saying
283C<-exp($foo)> still works as expected, however--only single letters
284following a minus are interpreted as file tests.
285
286The C<-T> and C<-B> switches work as follows. The first block or so of the
287file is examined for odd characters such as strange control codes or
5a964f20 288characters with the high bit set. If too many strange characters (E<gt>30%)
a0d0e21e
LW
289are found, it's a C<-B> file, otherwise it's a C<-T> file. Also, any file
290containing null in the first block is considered a binary file. If C<-T>
291or C<-B> is used on a filehandle, the current stdio buffer is examined
292rather than the first block. Both C<-T> and C<-B> return TRUE on a null
54310121 293file, or a file at EOF when testing a filehandle. Because you have to
4633a7c4
LW
294read a file to do the C<-T> test, on most occasions you want to use a C<-f>
295against the file first, as in C<next unless -f $file && -T $file>.
a0d0e21e 296
7660c0ab 297If any of the file tests (or either the C<stat()> or C<lstat()> operators) are given
28757baa 298the special filehandle consisting of a solitary underline, then the stat
a0d0e21e
LW
299structure of the previous file test (or stat operator) is used, saving
300a system call. (This doesn't work with C<-t>, and you need to remember
301that lstat() and C<-l> will leave values in the stat structure for the
302symbolic link, not the real file.) Example:
303
304 print "Can do.\n" if -r $a || -w _ || -x _;
305
306 stat($filename);
307 print "Readable\n" if -r _;
308 print "Writable\n" if -w _;
309 print "Executable\n" if -x _;
310 print "Setuid\n" if -u _;
311 print "Setgid\n" if -g _;
312 print "Sticky\n" if -k _;
313 print "Text\n" if -T _;
314 print "Binary\n" if -B _;
315
316=item abs VALUE
317
54310121 318=item abs
bbce6d69 319
a0d0e21e 320Returns the absolute value of its argument.
7660c0ab 321If VALUE is omitted, uses C<$_>.
a0d0e21e
LW
322
323=item accept NEWSOCKET,GENERICSOCKET
324
f86cebdf 325Accepts an incoming socket connect, just as the accept(2) system call
a0d0e21e 326does. Returns the packed address if it succeeded, FALSE otherwise.
4633a7c4 327See example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
328
329=item alarm SECONDS
330
54310121 331=item alarm
bbce6d69 332
a0d0e21e 333Arranges to have a SIGALRM delivered to this process after the
bbce6d69 334specified number of seconds have elapsed. If SECONDS is not specified,
7660c0ab 335the value stored in C<$_> is used. (On some machines,
a0d0e21e
LW
336unfortunately, the elapsed time may be up to one second less than you
337specified because of how seconds are counted.) Only one timer may be
338counting at once. Each call disables the previous timer, and an
7660c0ab 339argument of C<0> may be supplied to cancel the previous timer without
a0d0e21e
LW
340starting a new one. The returned value is the amount of time remaining
341on the previous timer.
342
4633a7c4 343For delays of finer granularity than one second, you may use Perl's
f86cebdf 344C<syscall()> interface to access setitimer(2) if your system supports it,
7660c0ab
A
345or else see L</select()>. It is usually a mistake to intermix C<alarm()>
346and C<sleep()> calls.
a0d0e21e 347
7660c0ab 348If you want to use C<alarm()> to time out a system call you need to use an
ab4f32c2 349C<eval()>/C<die()> pair. You can't rely on the alarm causing the system call to
f86cebdf 350fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
ab4f32c2 351restart system calls on some systems. Using C<eval()>/C<die()> always works,
5a964f20 352modulo the caveats given in L<perlipc/"Signals">.
ff68c719
PP
353
354 eval {
f86cebdf 355 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
36477c24 356 alarm $timeout;
ff68c719 357 $nread = sysread SOCKET, $buffer, $size;
36477c24 358 alarm 0;
ff68c719 359 };
ff68c719 360 if ($@) {
f86cebdf 361 die unless $@ eq "alarm\n"; # propagate unexpected errors
ff68c719
PP
362 # timed out
363 }
364 else {
365 # didn't
366 }
367
a0d0e21e
LW
368=item atan2 Y,X
369
370Returns the arctangent of Y/X in the range -PI to PI.
371
7660c0ab 372For the tangent operation, you may use the C<POSIX::tan()>
28757baa
PP
373function, or use the familiar relation:
374
375 sub tan { sin($_[0]) / cos($_[0]) }
376
a0d0e21e
LW
377=item bind SOCKET,NAME
378
379Binds a network address to a socket, just as the bind system call
380does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
4633a7c4
LW
381packed address of the appropriate type for the socket. See the examples in
382L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
383
384=item binmode FILEHANDLE
385
cb1a09d0
AD
386Arranges for the file to be read or written in "binary" mode in operating
387systems that distinguish between binary and text files. Files that are
388not in binary mode have CR LF sequences translated to LF on input and LF
54310121 389translated to CR LF on output. Binmode has no effect under Unix; in MS-DOS
cb1a09d0 390and similarly archaic systems, it may be imperative--otherwise your
54310121 391MS-DOS-damaged C library may mangle your file. The key distinction between
ab4f32c2 392systems that need C<binmode()> and those that don't is their text file
5a964f20 393formats. Systems like Unix, MacOS, and Plan9 that delimit lines with a single
7660c0ab 394character, and that encode that character in C as C<"\n">, do not need
ab4f32c2 395C<binmode()>. The rest need it. If FILEHANDLE is an expression, the value
cb1a09d0 396is taken as the name of the filehandle.
a0d0e21e 397
4633a7c4 398=item bless REF,CLASSNAME
a0d0e21e
LW
399
400=item bless REF
401
28757baa 402This function tells the thingy referenced by REF that it is now
4633a7c4
LW
403an object in the CLASSNAME package--or the current package if no CLASSNAME
404is specified, which is often the case. It returns the reference for
7660c0ab 405convenience, because a C<bless()> is often the last thing in a constructor.
4633a7c4 406Always use the two-argument version if the function doing the blessing
5a964f20
TC
407might be inherited by a derived class. See L<perltoot> and L<perlobj>
408for more about the blessing (and blessings) of objects.
a0d0e21e
LW
409
410=item caller EXPR
411
412=item caller
413
5a964f20 414Returns the context of the current subroutine call. In scalar context,
28757baa 415returns the caller's package name if there is a caller, that is, if
7660c0ab 416we're in a subroutine or C<eval()> or C<require()>, and the undefined value
5a964f20 417otherwise. In list context, returns
a0d0e21e 418
748a9306 419 ($package, $filename, $line) = caller;
a0d0e21e
LW
420
421With EXPR, it returns some extra information that the debugger uses to
422print a stack trace. The value of EXPR indicates how many call frames
423to go back before the current one.
424
54310121 425 ($package, $filename, $line, $subroutine,
e7ea3e70
IZ
426 $hasargs, $wantarray, $evaltext, $is_require) = caller($i);
427
7660c0ab 428Here C<$subroutine> may be C<"(eval)"> if the frame is not a subroutine
ab4f32c2 429call, but an C<eval()>. In such a case additional elements C<$evaltext> and
7660c0ab
A
430C<$is_require> are set: C<$is_require> is true if the frame is created by a
431C<require> or C<use> statement, C<$evaltext> contains the text of the
dc848c6f 432C<eval EXPR> statement. In particular, for a C<eval BLOCK> statement,
7660c0ab 433C<$filename> is C<"(eval)">, but C<$evaltext> is undefined. (Note also that
dc848c6f
PP
434each C<use> statement creates a C<require> frame inside an C<eval EXPR>)
435frame.
748a9306
LW
436
437Furthermore, when called from within the DB package, caller returns more
7660c0ab 438detailed information: it sets the list variable C<@DB::args> to be the
54310121 439arguments with which the subroutine was invoked.
748a9306 440
7660c0ab 441Be aware that the optimizer might have optimized call frames away before
ab4f32c2 442C<caller()> had a chance to get the information. That means that C<caller(N)>
7660c0ab
A
443might not return information about the call frame you expect it do, for
444C<N E<gt> 1>. In particular, C<@DB::args> might have information from the
445previous time C<caller()> was called.
446
a0d0e21e
LW
447=item chdir EXPR
448
449Changes the working directory to EXPR, if possible. If EXPR is
450omitted, changes to home directory. Returns TRUE upon success, FALSE
7660c0ab 451otherwise. See example under C<die()>.
a0d0e21e
LW
452
453=item chmod LIST
454
455Changes the permissions of a list of files. The first element of the
4633a7c4 456list must be the numerical mode, which should probably be an octal
2f9daede
TPG
457number, and which definitely should I<not> a string of octal digits:
458C<0644> is okay, C<'0644'> is not. Returns the number of files
dc848c6f 459successfully changed. See also L</oct>, if all you have is a string.
a0d0e21e
LW
460
461 $cnt = chmod 0755, 'foo', 'bar';
462 chmod 0755, @executables;
f86cebdf
GS
463 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
464 # --w----r-T
2f9daede
TPG
465 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
466 $mode = 0644; chmod $mode, 'foo'; # this is best
a0d0e21e
LW
467
468=item chomp VARIABLE
469
470=item chomp LIST
471
472=item chomp
473
3e3baf6d 474This is a slightly safer version of L</chop>. It removes any
a0d0e21e 475line ending that corresponds to the current value of C<$/> (also known as
28757baa
PP
476$INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
477number of characters removed from all its arguments. It's often used to
478remove the newline from the end of an input record when you're worried
479that the final record may be missing its newline. When in paragraph mode
480(C<$/ = "">), it removes all trailing newlines from the string. If
7660c0ab 481VARIABLE is omitted, it chomps C<$_>. Example:
a0d0e21e
LW
482
483 while (<>) {
484 chomp; # avoid \n on last field
485 @array = split(/:/);
5a964f20 486 # ...
a0d0e21e
LW
487 }
488
489You can actually chomp anything that's an lvalue, including an assignment:
490
491 chomp($cwd = `pwd`);
492 chomp($answer = <STDIN>);
493
494If you chomp a list, each element is chomped, and the total number of
495characters removed is returned.
496
497=item chop VARIABLE
498
499=item chop LIST
500
501=item chop
502
503Chops off the last character of a string and returns the character
504chopped. It's used primarily to remove the newline from the end of an
505input record, but is much more efficient than C<s/\n//> because it neither
7660c0ab 506scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
a0d0e21e
LW
507Example:
508
509 while (<>) {
510 chop; # avoid \n on last field
511 @array = split(/:/);
5a964f20 512 #...
a0d0e21e
LW
513 }
514
515You can actually chop anything that's an lvalue, including an assignment:
516
517 chop($cwd = `pwd`);
518 chop($answer = <STDIN>);
519
520If you chop a list, each element is chopped. Only the value of the
ab4f32c2 521last C<chop()> is returned.
a0d0e21e 522
ab4f32c2 523Note that C<chop()> returns the last character. To return all but the last
748a9306
LW
524character, use C<substr($string, 0, -1)>.
525
a0d0e21e
LW
526=item chown LIST
527
528Changes the owner (and group) of a list of files. The first two
529elements of the list must be the I<NUMERICAL> uid and gid, in that order.
530Returns the number of files successfully changed.
531
532 $cnt = chown $uid, $gid, 'foo', 'bar';
533 chown $uid, $gid, @filenames;
534
54310121 535Here's an example that looks up nonnumeric uids in the passwd file:
a0d0e21e
LW
536
537 print "User: ";
538 chop($user = <STDIN>);
5a964f20 539 print "Files: ";
a0d0e21e
LW
540 chop($pattern = <STDIN>);
541
542 ($login,$pass,$uid,$gid) = getpwnam($user)
543 or die "$user not in passwd file";
544
5a964f20 545 @ary = glob($pattern); # expand filenames
a0d0e21e
LW
546 chown $uid, $gid, @ary;
547
54310121 548On most systems, you are not allowed to change the ownership of the
4633a7c4
LW
549file unless you're the superuser, although you should be able to change
550the group to any of your secondary groups. On insecure systems, these
551restrictions may be relaxed, but this is not a portable assumption.
552
a0d0e21e
LW
553=item chr NUMBER
554
54310121 555=item chr
bbce6d69 556
a0d0e21e 557Returns the character represented by that NUMBER in the character set.
7660c0ab 558For example, C<chr(65)> is C<"A"> in ASCII. For the reverse, use L</ord>.
a0d0e21e 559
7660c0ab 560If NUMBER is omitted, uses C<$_>.
bbce6d69 561
a0d0e21e
LW
562=item chroot FILENAME
563
54310121 564=item chroot
bbce6d69 565
5a964f20 566This function works like the system call by the same name: it makes the
4633a7c4 567named directory the new root directory for all further pathnames that
7660c0ab 568begin with a C<"/"> by your process and all its children. (It doesn't
28757baa 569change your current working directory, which is unaffected.) For security
4633a7c4 570reasons, this call is restricted to the superuser. If FILENAME is
ab4f32c2 571omitted, does a C<chroot()> to C<$_>.
a0d0e21e
LW
572
573=item close FILEHANDLE
574
6a518fbc
TP
575=item close
576
a0d0e21e
LW
577Closes the file or pipe associated with the file handle, returning TRUE
578only if stdio successfully flushes buffers and closes the system file
6a518fbc
TP
579descriptor. Closes the currently selected filehandle if the argument
580is omitted.
fb73857a
PP
581
582You don't have to close FILEHANDLE if you are immediately going to do
7660c0ab 583another C<open()> on it, because C<open()> will close it for you. (See
ab4f32c2 584C<open()>.) However, an explicit C<close()> on an input file resets the line
7660c0ab 585counter (C<$.>), while the implicit close done by C<open()> does not.
fb73857a 586
ab4f32c2 587If the file handle came from a piped open C<close()> will additionally
fb73857a
PP
588return FALSE if one of the other system calls involved fails or if the
589program exits with non-zero status. (If the only problem was that the
7660c0ab 590program exited non-zero C<$!> will be set to C<0>.) Also, closing a pipe
5a964f20 591waits for the process executing on the pipe to complete, in case you
fb73857a
PP
592want to look at the output of the pipe afterwards. Closing a pipe
593explicitly also puts the exit status value of the command into C<$?>.
5a964f20 594
fb73857a 595Example:
a0d0e21e 596
fb73857a
PP
597 open(OUTPUT, '|sort >foo') # pipe to sort
598 or die "Can't start sort: $!";
5a964f20 599 #... # print stuff to output
fb73857a
PP
600 close OUTPUT # wait for sort to finish
601 or warn $! ? "Error closing sort pipe: $!"
602 : "Exit status $? from sort";
603 open(INPUT, 'foo') # get sort's results
604 or die "Can't open 'foo' for input: $!";
a0d0e21e 605
5a964f20
TC
606FILEHANDLE may be an expression whose value can be used as an indirect
607filehandle, usually the real filehandle name.
a0d0e21e
LW
608
609=item closedir DIRHANDLE
610
7660c0ab 611Closes a directory opened by C<opendir()> and returns the success of that
5a964f20
TC
612system call.
613
614DIRHANDLE may be an expression whose value can be used as an indirect
615dirhandle, usually the real dirhandle name.
a0d0e21e
LW
616
617=item connect SOCKET,NAME
618
619Attempts to connect to a remote socket, just as the connect system call
620does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
4633a7c4
LW
621packed address of the appropriate type for the socket. See the examples in
622L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 623
cb1a09d0
AD
624=item continue BLOCK
625
626Actually a flow control statement rather than a function. If there is a
627C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
628C<foreach>), it is always executed just before the conditional is about to
629be evaluated again, just like the third part of a C<for> loop in C. Thus
630it can be used to increment a loop variable, even when the loop has been
631continued via the C<next> statement (which is similar to the C C<continue>
632statement).
633
1d2dff63
GS
634C<last>, C<next>, or C<redo> may appear within a C<continue>
635block. C<last> and C<redo> will behave as if they had been executed within
636the main block. So will C<next>, but since it will execute a C<continue>
637block, it may be more entertaining.
638
639 while (EXPR) {
640 ### redo always comes here
641 do_something;
642 } continue {
643 ### next always comes here
644 do_something_else;
645 # then back the top to re-check EXPR
646 }
647 ### last always comes here
648
649Omitting the C<continue> section is semantically equivalent to using an
650empty one, logically enough. In that case, C<next> goes directly back
651to check the condition at the top of the loop.
652
a0d0e21e
LW
653=item cos EXPR
654
5a964f20 655Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
7660c0ab 656takes cosine of C<$_>.
a0d0e21e 657
7660c0ab 658For the inverse cosine operation, you may use the C<POSIX::acos()>
28757baa
PP
659function, or use this relation:
660
661 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
662
a0d0e21e
LW
663=item crypt PLAINTEXT,SALT
664
f86cebdf 665Encrypts a string exactly like the crypt(3) function in the C library
4633a7c4
LW
666(assuming that you actually have a version there that has not been
667extirpated as a potential munition). This can prove useful for checking
668the password file for lousy passwords, amongst other things. Only the
669guys wearing white hats should do this.
a0d0e21e 670
7660c0ab 671Note that C<crypt()> is intended to be a one-way function, much like breaking
11155c91
CS
672eggs to make an omelette. There is no (known) corresponding decrypt
673function. As a result, this function isn't all that useful for
674cryptography. (For that, see your nearby CPAN mirror.)
2f9daede 675
a0d0e21e
LW
676Here's an example that makes sure that whoever runs this program knows
677their own password:
678
679 $pwd = (getpwuid($<))[1];
680 $salt = substr($pwd, 0, 2);
681
682 system "stty -echo";
683 print "Password: ";
684 chop($word = <STDIN>);
685 print "\n";
686 system "stty echo";
687
688 if (crypt($word, $salt) ne $pwd) {
689 die "Sorry...\n";
690 } else {
691 print "ok\n";
54310121 692 }
a0d0e21e 693
9f8f0c9d 694Of course, typing in your own password to whoever asks you
748a9306 695for it is unwise.
a0d0e21e 696
aa689395 697=item dbmclose HASH
a0d0e21e 698
7660c0ab 699[This function has been superseded by the C<untie()> function.]
a0d0e21e 700
aa689395 701Breaks the binding between a DBM file and a hash.
a0d0e21e 702
aa689395 703=item dbmopen HASH,DBNAME,MODE
a0d0e21e 704
7660c0ab 705[This function has been superseded by the C<tie()> function.]
a0d0e21e 706
7b8d334a 707This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
ab4f32c2 708hash. HASH is the name of the hash. (Unlike normal C<open()>, the first
aa689395
PP
709argument is I<NOT> a filehandle, even though it looks like one). DBNAME
710is the name of the database (without the F<.dir> or F<.pag> extension if
711any). If the database does not exist, it is created with protection
7660c0ab
A
712specified by MODE (as modified by the C<umask()>). If your system supports
713only the older DBM functions, you may perform only one C<dbmopen()> in your
aa689395 714program. In older versions of Perl, if your system had neither DBM nor
7660c0ab 715ndbm, calling C<dbmopen()> produced a fatal error; it now falls back to
aa689395
PP
716sdbm(3).
717
718If you don't have write access to the DBM file, you can only read hash
719variables, not set them. If you want to test whether you can write,
7660c0ab 720either use file tests or try setting a dummy hash entry inside an C<eval()>,
aa689395 721which will trap the error.
a0d0e21e 722
7660c0ab
A
723Note that functions such as C<keys()> and C<values()> may return huge lists
724when used on large DBM files. You may prefer to use the C<each()>
a0d0e21e
LW
725function to iterate over large DBM files. Example:
726
727 # print out history file offsets
728 dbmopen(%HIST,'/usr/lib/news/history',0666);
729 while (($key,$val) = each %HIST) {
730 print $key, ' = ', unpack('L',$val), "\n";
731 }
732 dbmclose(%HIST);
733
cb1a09d0 734See also L<AnyDBM_File> for a more general description of the pros and
184e9718 735cons of the various dbm approaches, as well as L<DB_File> for a particularly
cb1a09d0 736rich implementation.
4633a7c4 737
a0d0e21e
LW
738=item defined EXPR
739
54310121 740=item defined
bbce6d69 741
2f9daede
TPG
742Returns a Boolean value telling whether EXPR has a value other than
743the undefined value C<undef>. If EXPR is not present, C<$_> will be
744checked.
745
746Many operations return C<undef> to indicate failure, end of file,
747system error, uninitialized variable, and other exceptional
748conditions. This function allows you to distinguish C<undef> from
749other values. (A simple Boolean test will not distinguish among
7660c0ab 750C<undef>, zero, the empty string, and C<"0">, which are all equally
2f9daede 751false.) Note that since C<undef> is a valid scalar, its presence
7660c0ab 752doesn't I<necessarily> indicate an exceptional condition: C<pop()>
2f9daede
TPG
753returns C<undef> when its argument is an empty array, I<or> when the
754element to return happens to be C<undef>.
755
7660c0ab 756You may also use C<defined()> to check whether a subroutine exists, by
5a964f20 757saying C<defined &func> without parentheses. On the other hand, use
7660c0ab 758of C<defined()> upon aggregates (hashes and arrays) is not guaranteed to
5a964f20 759produce intuitive results, and should probably be avoided.
2f9daede
TPG
760
761When used on a hash element, it tells you whether the value is defined,
dc848c6f 762not whether the key exists in the hash. Use L</exists> for the latter
2f9daede 763purpose.
a0d0e21e
LW
764
765Examples:
766
767 print if defined $switch{'D'};
768 print "$val\n" while defined($val = pop(@ary));
769 die "Can't readlink $sym: $!"
770 unless defined($value = readlink $sym);
a0d0e21e 771 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
2f9daede 772 $debugging = 0 unless defined $debugging;
a0d0e21e 773
7660c0ab
A
774Note: Many folks tend to overuse C<defined()>, and then are surprised to
775discover that the number C<0> and C<""> (the zero-length string) are, in fact,
2f9daede 776defined values. For example, if you say
a5f75d66
AD
777
778 "ab" =~ /a(.*)b/;
779
7660c0ab 780The pattern match succeeds, and C<$1> is defined, despite the fact that it
a5f75d66 781matched "nothing". But it didn't really match nothing--rather, it
7660c0ab 782matched something that happened to be C<0> characters long. This is all
a5f75d66 783very above-board and honest. When a function returns an undefined value,
2f9daede 784it's an admission that it couldn't give you an honest answer. So you
7660c0ab
A
785should use C<defined()> only when you're questioning the integrity of what
786you're trying to do. At other times, a simple comparison to C<0> or C<""> is
2f9daede
TPG
787what you want.
788
7660c0ab 789Currently, using C<defined()> on an entire array or hash reports whether
2f9daede
TPG
790memory for that aggregate has ever been allocated. So an array you set
791to the empty list appears undefined initially, and one that once was full
792and that you then set to the empty list still appears defined. You
793should instead use a simple test for size:
28757baa
PP
794
795 if (@an_array) { print "has array elements\n" }
796 if (%a_hash) { print "has hash members\n" }
797
7660c0ab 798Using C<undef()> on these, however, does clear their memory and then report
5a964f20 799them as not defined anymore, but you shouldn't do that unless you don't
28757baa 800plan to use them again, because it saves time when you load them up
5a964f20
TC
801again to have memory already ready to be filled. The normal way to
802free up space used by an aggregate is to assign the empty list.
28757baa 803
7660c0ab 804This counterintuitive behavior of C<defined()> on aggregates may be
28757baa
PP
805changed, fixed, or broken in a future release of Perl.
806
dc848c6f 807See also L</undef>, L</exists>, L</ref>.
2f9daede 808
a0d0e21e
LW
809=item delete EXPR
810
aa689395
PP
811Deletes the specified key(s) and their associated values from a hash.
812For each key, returns the deleted value associated with that key, or
813the undefined value if there was no such key. Deleting from C<$ENV{}>
814modifies the environment. Deleting from a hash tied to a DBM file
7660c0ab 815deletes the entry from the DBM file. (But deleting from a C<tie()>d hash
5f05dabc 816doesn't necessarily return anything.)
a0d0e21e 817
aa689395 818The following deletes all the values of a hash:
a0d0e21e 819
5f05dabc
PP
820 foreach $key (keys %HASH) {
821 delete $HASH{$key};
a0d0e21e
LW
822 }
823
5f05dabc
PP
824And so does this:
825
826 delete @HASH{keys %HASH}
827
5a964f20 828(But both of these are slower than just assigning the empty list, or
7660c0ab 829using C<undef()>.) Note that the EXPR can be arbitrarily complicated as
5a964f20 830long as the final operation is a hash element lookup or hash slice:
a0d0e21e
LW
831
832 delete $ref->[$x][$y]{$key};
5f05dabc 833 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
a0d0e21e
LW
834
835=item die LIST
836
7660c0ab
A
837Outside an C<eval()>, prints the value of LIST to C<STDERR> and exits with
838the current value of C<$!> (errno). If C<$!> is C<0>, exits with the value of
54310121 839C<($? E<gt>E<gt> 8)> (backtick `command` status). If C<($? E<gt>E<gt> 8)>
7660c0ab
A
840is C<0>, exits with C<255>. Inside an C<eval(),> the error message is stuffed into
841C<$@> and the C<eval()> is terminated with the undefined value. This makes
842C<die()> the way to raise an exception.
a0d0e21e
LW
843
844Equivalent examples:
845
846 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
54310121 847 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
a0d0e21e
LW
848
849If the value of EXPR does not end in a newline, the current script line
850number and input line number (if any) are also printed, and a newline
7660c0ab
A
851is supplied. Hint: sometimes appending C<", stopped"> to your message
852will cause it to make better sense when the string C<"at foo line 123"> is
a0d0e21e
LW
853appended. Suppose you are running script "canasta".
854
855 die "/etc/games is no good";
856 die "/etc/games is no good, stopped";
857
858produce, respectively
859
860 /etc/games is no good at canasta line 123.
861 /etc/games is no good, stopped at canasta line 123.
862
7660c0ab 863See also C<exit()> and C<warn()>.
a0d0e21e 864
7660c0ab
A
865If LIST is empty and C<$@> already contains a value (typically from a
866previous eval) that value is reused after appending C<"\t...propagated">.
fb73857a
PP
867This is useful for propagating exceptions:
868
869 eval { ... };
870 die unless $@ =~ /Expected exception/;
871
7660c0ab 872If C<$@> is empty then the string C<"Died"> is used.
fb73857a 873
7660c0ab 874You can arrange for a callback to be run just before the C<die()> does
774d564b
PP
875its deed, by setting the C<$SIG{__DIE__}> hook. The associated handler
876will be called with the error text and can change the error message, if
7660c0ab 877it sees fit, by calling C<die()> again. See L<perlvar/$SIG{expr}> for details on
fb73857a
PP
878setting C<%SIG> entries, and L<"eval BLOCK"> for some examples.
879
880Note that the C<$SIG{__DIE__}> hook is called even inside eval()ed
881blocks/strings. If one wants the hook to do nothing in such
882situations, put
883
884 die @_ if $^S;
885
886as the first line of the handler (see L<perlvar/$^S>).
774d564b 887
a0d0e21e
LW
888=item do BLOCK
889
890Not really a function. Returns the value of the last command in the
891sequence of commands indicated by BLOCK. When modified by a loop
892modifier, executes the BLOCK once before testing the loop condition.
893(On other statements the loop modifiers test the conditional first.)
894
895=item do SUBROUTINE(LIST)
896
897A deprecated form of subroutine call. See L<perlsub>.
898
899=item do EXPR
900
901Uses the value of EXPR as a filename and executes the contents of the
902file as a Perl script. Its primary use is to include subroutines
903from a Perl subroutine library.
904
905 do 'stat.pl';
906
907is just like
908
fb73857a 909 scalar eval `cat stat.pl`;
a0d0e21e 910
5a964f20 911except that it's more efficient and concise, keeps track of the
a0d0e21e
LW
912current filename for error messages, and searches all the B<-I>
913libraries if the file isn't in the current directory (see also the @INC
dc1be6b5
GS
914array in L<perlvar/Predefined Names>). It is also different in how
915code evaluated with C<do FILENAME> doesn't see lexicals in the enclosing
916scope like C<eval STRING> does. It's the same, however, in that it does
54310121 917reparse the file every time you call it, so you probably don't want to
a0d0e21e
LW
918do this inside a loop.
919
8e30cc93
G
920If C<do> cannot read the file, it returns undef and sets C<$!> to the
921error. If C<do> can read the file but cannot compile it, it
922returns undef and sets an error message in C<$@>. If the file is
923successfully compiled, C<do> returns the value of the last expression
924evaluated.
925
a0d0e21e 926Note that inclusion of library modules is better done with the
7660c0ab 927C<use()> and C<require()> operators, which also do automatic error checking
4633a7c4 928and raise an exception if there's a problem.
a0d0e21e 929
5a964f20
TC
930You might like to use C<do> to read in a program configuration
931file. Manual error checking can be done this way:
932
933 # read in config files: system first, then user
f86cebdf
GS
934 for $file ("/share/prog/defaults.rc",
935 "$ENV{HOME}/.someprogrc") {
5a964f20 936 unless ($return = do $file) {
f86cebdf
GS
937 warn "couldn't parse $file: $@" if $@;
938 warn "couldn't do $file: $!" unless defined $return;
939 warn "couldn't run $file" unless $return;
5a964f20
TC
940 }
941 }
942
a0d0e21e
LW
943=item dump LABEL
944
945This causes an immediate core dump. Primarily this is so that you can
946use the B<undump> program to turn your core dump into an executable binary
947after having initialized all your variables at the beginning of the
948program. When the new binary is executed it will begin by executing a
949C<goto LABEL> (with all the restrictions that C<goto> suffers). Think of
f86cebdf 950it as a goto with an intervening core dump and reincarnation. If C<LABEL>
5a964f20 951is omitted, restarts the program from the top. WARNING: Any files
a0d0e21e
LW
952opened at the time of the dump will NOT be open any more when the
953program is reincarnated, with possible resulting confusion on the part
954of Perl. See also B<-u> option in L<perlrun>.
955
956Example:
957
958 #!/usr/bin/perl
959 require 'getopt.pl';
960 require 'stat.pl';
961 %days = (
962 'Sun' => 1,
963 'Mon' => 2,
964 'Tue' => 3,
965 'Wed' => 4,
966 'Thu' => 5,
967 'Fri' => 6,
968 'Sat' => 7,
969 );
970
971 dump QUICKSTART if $ARGV[0] eq '-d';
972
973 QUICKSTART:
974 Getopt('f');
975
5a964f20
TC
976This operator is largely obsolete, partly because it's very hard to
977convert a core file into an executable, and because the real perl-to-C
978compiler has superseded it.
979
aa689395
PP
980=item each HASH
981
5a964f20 982When called in list context, returns a 2-element list consisting of the
aa689395 983key and value for the next element of a hash, so that you can iterate over
5a964f20 984it. When called in scalar context, returns the key for only the "next"
7660c0ab 985element in the hash. (Note: Keys may be C<"0"> or C<"">, which are logically
2f9daede
TPG
986false; you may wish to avoid constructs like C<while ($k = each %foo) {}>
987for this reason.)
988
989Entries are returned in an apparently random order. When the hash is
990entirely read, a null array is returned in list context (which when
7660c0ab
A
991assigned produces a FALSE (C<0>) value), and C<undef> in
992scalar context. The next call to C<each()> after that will start iterating
993again. There is a single iterator for each hash, shared by all C<each()>,
994C<keys()>, and C<values()> function calls in the program; it can be reset by
2f9daede
TPG
995reading all the elements from the hash, or by evaluating C<keys HASH> or
996C<values HASH>. If you add or delete elements of a hash while you're
997iterating over it, you may get entries skipped or duplicated, so don't.
aa689395 998
f86cebdf 999The following prints out your environment like the printenv(1) program,
aa689395 1000only in a different order:
a0d0e21e
LW
1001
1002 while (($key,$value) = each %ENV) {
1003 print "$key=$value\n";
1004 }
1005
7660c0ab 1006See also C<keys()> and C<values()>.
a0d0e21e
LW
1007
1008=item eof FILEHANDLE
1009
4633a7c4
LW
1010=item eof ()
1011
a0d0e21e
LW
1012=item eof
1013
1014Returns 1 if the next read on FILEHANDLE will return end of file, or if
1015FILEHANDLE is not open. FILEHANDLE may be an expression whose value
5a964f20 1016gives the real filehandle. (Note that this function actually
7660c0ab 1017reads a character and then C<ungetc()>s it, so isn't very useful in an
748a9306
LW
1018interactive context.) Do not read from a terminal file (or call
1019C<eof(FILEHANDLE)> on it) after end-of-file is reached. Filetypes such
1020as terminals may lose the end-of-file condition if you do.
1021
1022An C<eof> without an argument uses the last file read as argument.
5a964f20 1023Using C<eof()> with empty parentheses is very different. It indicates the pseudo file formed of
2f9daede
TPG
1024the files listed on the command line, i.e., C<eof()> is reasonable to
1025use inside a C<while (E<lt>E<gt>)> loop to detect the end of only the
1026last file. Use C<eof(ARGV)> or eof without the parentheses to test
1027I<EACH> file in a while (E<lt>E<gt>) loop. Examples:
a0d0e21e 1028
748a9306
LW
1029 # reset line numbering on each input file
1030 while (<>) {
5a964f20 1031 next if /^\s*#/; # skip comments
748a9306 1032 print "$.\t$_";
5a964f20
TC
1033 } continue {
1034 close ARGV if eof; # Not eof()!
748a9306
LW
1035 }
1036
a0d0e21e
LW
1037 # insert dashes just before last line of last file
1038 while (<>) {
5a964f20 1039 if (eof()) { # check for end of current file
a0d0e21e 1040 print "--------------\n";
748a9306
LW
1041 close(ARGV); # close or break; is needed if we
1042 # are reading from the terminal
a0d0e21e
LW
1043 }
1044 print;
1045 }
1046
a0d0e21e 1047Practical hint: you almost never need to use C<eof> in Perl, because the
3b02c43c
GS
1048input operators return false values when they run out of data, or if there
1049was an error.
a0d0e21e
LW
1050
1051=item eval EXPR
1052
1053=item eval BLOCK
1054
c7cc6f1c
GS
1055In the first form, the return value of EXPR is parsed and executed as if it
1056were a little Perl program. The value of the expression (which is itself
5a964f20 1057determined within scalar context) is first parsed, and if there weren't any
c7cc6f1c 1058errors, executed in the context of the current Perl program, so that any
5f05dabc 1059variable settings or subroutine and format definitions remain afterwards.
c7cc6f1c
GS
1060Note that the value is parsed every time the eval executes. If EXPR is
1061omitted, evaluates C<$_>. This form is typically used to delay parsing
1062and subsequent execution of the text of EXPR until run time.
1063
1064In the second form, the code within the BLOCK is parsed only once--at the
1065same time the code surrounding the eval itself was parsed--and executed
1066within the context of the current Perl program. This form is typically
1067used to trap exceptions more efficiently than the first (see below), while
1068also providing the benefit of checking the code within BLOCK at compile
1069time.
1070
1071The final semicolon, if any, may be omitted from the value of EXPR or within
1072the BLOCK.
1073
1074In both forms, the value returned is the value of the last expression
5a964f20 1075evaluated inside the mini-program; a return statement may be also used, just
c7cc6f1c 1076as with subroutines. The expression providing the return value is evaluated
5a964f20 1077in void, scalar, or list context, depending on the context of the eval itself.
c7cc6f1c 1078See L</wantarray> for more on how the evaluation context can be determined.
a0d0e21e 1079
7660c0ab
A
1080If there is a syntax error or runtime error, or a C<die()> statement is
1081executed, an undefined value is returned by C<eval()>, and C<$@> is set to the
a0d0e21e 1082error message. If there was no error, C<$@> is guaranteed to be a null
7660c0ab 1083string. Beware that using C<eval()> neither silences perl from printing
c7cc6f1c
GS
1084warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1085To do either of those, you have to use the C<$SIG{__WARN__}> facility. See
1086L</warn> and L<perlvar>.
a0d0e21e 1087
7660c0ab
A
1088Note that, because C<eval()> traps otherwise-fatal errors, it is useful for
1089determining whether a particular feature (such as C<socket()> or C<symlink()>)
a0d0e21e
LW
1090is implemented. It is also Perl's exception trapping mechanism, where
1091the die operator is used to raise exceptions.
1092
1093If the code to be executed doesn't vary, you may use the eval-BLOCK
1094form to trap run-time errors without incurring the penalty of
1095recompiling each time. The error, if any, is still returned in C<$@>.
1096Examples:
1097
54310121 1098 # make divide-by-zero nonfatal
a0d0e21e
LW
1099 eval { $answer = $a / $b; }; warn $@ if $@;
1100
1101 # same thing, but less efficient
1102 eval '$answer = $a / $b'; warn $@ if $@;
1103
1104 # a compile-time error
5a964f20 1105 eval { $answer = }; # WRONG
a0d0e21e
LW
1106
1107 # a run-time error
1108 eval '$answer ='; # sets $@
1109
7660c0ab 1110When using the C<eval{}> form as an exception trap in libraries, you may
774d564b
PP
1111wish not to trigger any C<__DIE__> hooks that user code may have
1112installed. You can use the C<local $SIG{__DIE__}> construct for this
1113purpose, as shown in this example:
1114
1115 # a very private exception trap for divide-by-zero
f86cebdf
GS
1116 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1117 warn $@ if $@;
774d564b
PP
1118
1119This is especially significant, given that C<__DIE__> hooks can call
7660c0ab 1120C<die()> again, which has the effect of changing their error messages:
774d564b
PP
1121
1122 # __DIE__ hooks may modify error messages
1123 {
f86cebdf
GS
1124 local $SIG{'__DIE__'} =
1125 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
c7cc6f1c
GS
1126 eval { die "foo lives here" };
1127 print $@ if $@; # prints "bar lives here"
774d564b
PP
1128 }
1129
7660c0ab 1130With an C<eval()>, you should be especially careful to remember what's
a0d0e21e
LW
1131being looked at when:
1132
1133 eval $x; # CASE 1
1134 eval "$x"; # CASE 2
1135
1136 eval '$x'; # CASE 3
1137 eval { $x }; # CASE 4
1138
5a964f20 1139 eval "\$$x++"; # CASE 5
a0d0e21e
LW
1140 $$x++; # CASE 6
1141
2f9daede 1142Cases 1 and 2 above behave identically: they run the code contained in
7660c0ab 1143the variable C<$x>. (Although case 2 has misleading double quotes making
2f9daede 1144the reader wonder what else might be happening (nothing is).) Cases 3
7660c0ab 1145and 4 likewise behave in the same way: they run the code C<'$x'>, which
2f9daede
TPG
1146does nothing but return the value of C<$x>. (Case 4 is preferred for
1147purely visual reasons, but it also has the advantage of compiling at
1148compile-time instead of at run-time.) Case 5 is a place where
54310121 1149normally you I<WOULD> like to use double quotes, except that in this
2f9daede
TPG
1150particular situation, you can just use symbolic references instead, as
1151in case 6.
a0d0e21e
LW
1152
1153=item exec LIST
1154
8bf3b016
GS
1155=item exec PROGRAM LIST
1156
7660c0ab
A
1157The C<exec()> function executes a system command I<AND NEVER RETURNS> -
1158use C<system()> instead of C<exec()> if you want it to return. It fails and
fb73857a
PP
1159returns FALSE only if the command does not exist I<and> it is executed
1160directly instead of via your system's command shell (see below).
a0d0e21e 1161
7660c0ab
A
1162Since it's a common mistake to use C<exec()> instead of C<system()>, Perl
1163warns you if there is a following statement which isn't C<die()>, C<warn()>,
1164or C<exit()> (if C<-w> is set - but you always do that). If you
1165I<really> want to follow an C<exec()> with some other statement, you
55d729e4
GS
1166can use one of these styles to avoid the warning:
1167
5a964f20
TC
1168 exec ('foo') or print STDERR "couldn't exec foo: $!";
1169 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
55d729e4 1170
5a964f20 1171If there is more than one argument in LIST, or if LIST is an array
f86cebdf 1172with more than one value, calls execvp(3) with the arguments in LIST.
5a964f20
TC
1173If there is only one scalar argument or an array with one element in it,
1174the argument is checked for shell metacharacters, and if there are any,
1175the entire argument is passed to the system's command shell for parsing
1176(this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1177If there are no shell metacharacters in the argument, it is split into
7660c0ab
A
1178words and passed directly to C<execvp()>, which is more efficient. Note:
1179C<exec()> and C<system()> do not flush your output buffer, so you may need to
5a964f20 1180set C<$|> to avoid lost output. Examples:
a0d0e21e
LW
1181
1182 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1183 exec "sort $outfile | uniq";
1184
1185If you don't really want to execute the first argument, but want to lie
1186to the program you are executing about its own name, you can specify
1187the program you actually want to run as an "indirect object" (without a
1188comma) in front of the LIST. (This always forces interpretation of the
54310121 1189LIST as a multivalued list, even if there is only a single scalar in
a0d0e21e
LW
1190the list.) Example:
1191
1192 $shell = '/bin/csh';
1193 exec $shell '-sh'; # pretend it's a login shell
1194
1195or, more directly,
1196
1197 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1198
bb32b41a
GS
1199When the arguments get executed via the system shell, results will
1200be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1201for details.
1202
ab4f32c2 1203Using an indirect object with C<exec()> or C<system()> is also more secure.
5a964f20
TC
1204This usage forces interpretation of the arguments as a multivalued list,
1205even if the list had just one argument. That way you're safe from the
1206shell expanding wildcards or splitting up words with whitespace in them.
1207
1208 @args = ( "echo surprise" );
1209
f86cebdf
GS
1210 system @args; # subject to shell escapes
1211 # if @args == 1
5a964f20
TC
1212 system { $args[0] } @args; # safe even with one-arg list
1213
1214The first version, the one without the indirect object, ran the I<echo>
1215program, passing it C<"surprise"> an argument. The second version
1216didn't--it tried to run a program literally called I<"echo surprise">,
1217didn't find it, and set C<$?> to a non-zero value indicating failure.
1218
ab4f32c2 1219Note that C<exec()> will not call your C<END> blocks, nor will it call
7660c0ab
A
1220any C<DESTROY> methods in your objects.
1221
a0d0e21e
LW
1222=item exists EXPR
1223
1224Returns TRUE if the specified hash key exists in its hash array, even
1225if the corresponding value is undefined.
1226
1227 print "Exists\n" if exists $array{$key};
1228 print "Defined\n" if defined $array{$key};
1229 print "True\n" if $array{$key};
1230
5f05dabc 1231A hash element can be TRUE only if it's defined, and defined if
a0d0e21e
LW
1232it exists, but the reverse doesn't necessarily hold true.
1233
1234Note that the EXPR can be arbitrarily complicated as long as the final
1235operation is a hash key lookup:
1236
5a964f20
TC
1237 if (exists $ref->{"A"}{"B"}{$key}) { ... }
1238
1239Although the last element will not spring into existence just because its
1240existence was tested, intervening ones will. Thus C<$ref-E<gt>{"A"}>
1241C<$ref-E<gt>{"B"}> will spring into existence due to the existence
1242test for a $key element. This autovivification may be fixed in a later
1243release.
a0d0e21e
LW
1244
1245=item exit EXPR
1246
1247Evaluates EXPR and exits immediately with that value. (Actually, it
1248calls any defined C<END> routines first, but the C<END> routines may not
1249abort the exit. Likewise any object destructors that need to be called
1250are called before exit.) Example:
1251
1252 $ans = <STDIN>;
1253 exit 0 if $ans =~ /^[Xx]/;
1254
7660c0ab
A
1255See also C<die()>. If EXPR is omitted, exits with C<0> status. The only
1256universally portable values for EXPR are C<0> for success and C<1> for error;
f86702cc
PP
1257all other values are subject to unpredictable interpretation depending
1258on the environment in which the Perl program is running.
a0d0e21e 1259
7660c0ab
A
1260You shouldn't use C<exit()> to abort a subroutine if there's any chance that
1261someone might want to trap whatever error happened. Use C<die()> instead,
1262which can be trapped by an C<eval()>.
28757baa 1263
5a964f20
TC
1264All C<END{}> blocks are run at exit time. See L<perlsub> for details.
1265
a0d0e21e
LW
1266=item exp EXPR
1267
54310121 1268=item exp
bbce6d69 1269
54310121 1270Returns I<e> (the natural logarithm base) to the power of EXPR.
a0d0e21e
LW
1271If EXPR is omitted, gives C<exp($_)>.
1272
1273=item fcntl FILEHANDLE,FUNCTION,SCALAR
1274
f86cebdf 1275Implements the fcntl(2) function. You'll probably have to say
a0d0e21e
LW
1276
1277 use Fcntl;
1278
0ade1984 1279first to get the correct constant definitions. Argument processing and
7660c0ab 1280value return works just like C<ioctl()> below.
a0d0e21e
LW
1281For example:
1282
1283 use Fcntl;
5a964f20
TC
1284 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1285 or die "can't fcntl F_GETFL: $!";
1286
ab4f32c2
A
1287You don't have to check for C<defined()> on the return from
1288C<fnctl()>. Like C<ioctl()>, it maps a C<0> return from the system
7660c0ab
A
1289call into "C<0> but true" in Perl. This string is true in
1290boolean context and C<0> in numeric context. It is also
5a964f20
TC
1291exempt from the normal B<-w> warnings on improper numeric
1292conversions.
1293
7660c0ab 1294Note that C<fcntl()> will produce a fatal error if used on a machine that
f86cebdf 1295doesn't implement fcntl(2).
a0d0e21e
LW
1296
1297=item fileno FILEHANDLE
1298
1299Returns the file descriptor for a filehandle. This is useful for
7660c0ab 1300constructing bitmaps for C<select()> and low-level POSIX tty-handling
5a964f20
TC
1301operations. If FILEHANDLE is an expression, the value is taken as
1302an indirect filehandle, generally its name.
1303
1304You can use this to find out whether two handles refer to the
1305same underlying descriptor:
1306
1307 if (fileno(THIS) == fileno(THAT)) {
1308 print "THIS and THAT are dups\n";
1309 }
a0d0e21e
LW
1310
1311=item flock FILEHANDLE,OPERATION
1312
f86cebdf 1313Calls flock(2), or an emulation of it, on FILEHANDLE. Returns TRUE for
68dc0745 1314success, FALSE on failure. Produces a fatal error if used on a machine
f86cebdf 1315that doesn't implement flock(2), fcntl(2) locking, or lockf(3). C<flock()>
68dc0745
PP
1316is Perl's portable file locking interface, although it locks only entire
1317files, not records.
8ebc5c01 1318
a3cb178b 1319On many platforms (including most versions or clones of Unix), locks
7660c0ab 1320established by C<flock()> are B<merely advisory>. Such discretionary locks
5a964f20 1321are more flexible, but offer fewer guarantees. This means that files
7660c0ab
A
1322locked with C<flock()> may be modified by programs that do not also use
1323C<flock()>. Windows NT and OS/2 are among the platforms which
5a964f20 1324enforce mandatory locking. See your local documentation for details.
a3cb178b 1325
8ebc5c01
PP
1326OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1327LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
68dc0745
PP
1328you can use the symbolic names if import them from the Fcntl module,
1329either individually, or as a group using the ':flock' tag. LOCK_SH
1330requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1331releases a previously requested lock. If LOCK_NB is added to LOCK_SH or
7660c0ab 1332LOCK_EX then C<flock()> will return immediately rather than blocking
68dc0745
PP
1333waiting for the lock (check the return status to see if you got it).
1334
1335To avoid the possibility of mis-coordination, Perl flushes FILEHANDLE
1336before (un)locking it.
8ebc5c01 1337
f86cebdf 1338Note that the emulation built with lockf(3) doesn't provide shared
8ebc5c01 1339locks, and it requires that FILEHANDLE be open with write intent. These
f86cebdf
GS
1340are the semantics that lockf(3) implements. Most (all?) systems
1341implement lockf(3) in terms of fcntl(2) locking, though, so the
8ebc5c01
PP
1342differing semantics shouldn't bite too many people.
1343
7660c0ab
A
1344Note also that some versions of C<flock()> cannot lock things over the
1345network; you would need to use the more system-specific C<fcntl()> for
f86cebdf
GS
1346that. If you like you can force Perl to ignore your system's flock(2)
1347function, and so provide its own fcntl(2)-based emulation, by passing
8ebc5c01
PP
1348the switch C<-Ud_flock> to the F<Configure> program when you configure
1349perl.
4633a7c4
LW
1350
1351Here's a mailbox appender for BSD systems.
a0d0e21e 1352
7e1af8bc 1353 use Fcntl ':flock'; # import LOCK_* constants
a0d0e21e
LW
1354
1355 sub lock {
7e1af8bc 1356 flock(MBOX,LOCK_EX);
a0d0e21e
LW
1357 # and, in case someone appended
1358 # while we were waiting...
1359 seek(MBOX, 0, 2);
1360 }
1361
1362 sub unlock {
7e1af8bc 1363 flock(MBOX,LOCK_UN);
a0d0e21e
LW
1364 }
1365
1366 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1367 or die "Can't open mailbox: $!";
1368
1369 lock();
1370 print MBOX $msg,"\n\n";
1371 unlock();
1372
cb1a09d0 1373See also L<DB_File> for other flock() examples.
a0d0e21e
LW
1374
1375=item fork
1376
f86cebdf 1377Does a fork(2) system call. Returns the child pid to the parent process,
7660c0ab 1378C<0> to the child process, or C<undef> if the fork is unsuccessful.
5a964f20 1379
a0d0e21e 1380Note: unflushed buffers remain unflushed in both processes, which means
7660c0ab
A
1381you may need to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()>
1382method of C<IO::Handle> to avoid duplicate output.
a0d0e21e 1383
7660c0ab 1384If you C<fork()> without ever waiting on your children, you will accumulate
a0d0e21e
LW
1385zombies:
1386
4633a7c4 1387 $SIG{CHLD} = sub { wait };
a0d0e21e 1388
54310121 1389There's also the double-fork trick (error checking on
7660c0ab 1390C<fork()> returns omitted);
a0d0e21e
LW
1391
1392 unless ($pid = fork) {
1393 unless (fork) {
1394 exec "what you really wanna do";
1395 die "no exec";
1396 # ... or ...
4633a7c4 1397 ## (some_perl_code_here)
a0d0e21e
LW
1398 exit 0;
1399 }
1400 exit 0;
1401 }
1402 waitpid($pid,0);
1403
cb1a09d0
AD
1404See also L<perlipc> for more examples of forking and reaping
1405moribund children.
1406
28757baa
PP
1407Note that if your forked child inherits system file descriptors like
1408STDIN and STDOUT that are actually connected by a pipe or socket, even
5a964f20 1409if you exit, then the remote server (such as, say, httpd or rsh) won't think
7660c0ab 1410you're done. You should reopen those to F</dev/null> if it's any issue.
28757baa 1411
cb1a09d0
AD
1412=item format
1413
7660c0ab 1414Declare a picture format for use by the C<write()> function. For
cb1a09d0
AD
1415example:
1416
54310121 1417 format Something =
cb1a09d0
AD
1418 Test: @<<<<<<<< @||||| @>>>>>
1419 $str, $%, '$' . int($num)
1420 .
1421
1422 $str = "widget";
184e9718 1423 $num = $cost/$quantity;
cb1a09d0
AD
1424 $~ = 'Something';
1425 write;
1426
1427See L<perlform> for many details and examples.
1428
8903cb82 1429=item formline PICTURE,LIST
a0d0e21e 1430
5a964f20 1431This is an internal function used by C<format>s, though you may call it,
a0d0e21e
LW
1432too. It formats (see L<perlform>) a list of values according to the
1433contents of PICTURE, placing the output into the format output
7660c0ab
A
1434accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
1435Eventually, when a C<write()> is done, the contents of
a0d0e21e 1436C<$^A> are written to some filehandle, but you could also read C<$^A>
7660c0ab
A
1437yourself and then set C<$^A> back to C<"">. Note that a format typically
1438does one C<formline()> per line of form, but the C<formline()> function itself
748a9306 1439doesn't care how many newlines are embedded in the PICTURE. This means
4633a7c4 1440that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
748a9306
LW
1441You may therefore need to use multiple formlines to implement a single
1442record format, just like the format compiler.
1443
5f05dabc 1444Be careful if you put double quotes around the picture, because an "C<@>"
748a9306 1445character may be taken to mean the beginning of an array name.
7660c0ab 1446C<formline()> always returns TRUE. See L<perlform> for other examples.
a0d0e21e
LW
1447
1448=item getc FILEHANDLE
1449
1450=item getc
1451
1452Returns the next character from the input file attached to FILEHANDLE,
9bc64814 1453or the undefined value at end of file, or if there was an error. If
3b02c43c
GS
1454FILEHANDLE is omitted, reads from STDIN. This is not particularly
1455efficient. It cannot be used to get unbuffered single-characters,
1456however. For that, try something more like:
4633a7c4
LW
1457
1458 if ($BSD_STYLE) {
1459 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1460 }
1461 else {
54310121 1462 system "stty", '-icanon', 'eol', "\001";
4633a7c4
LW
1463 }
1464
1465 $key = getc(STDIN);
1466
1467 if ($BSD_STYLE) {
1468 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1469 }
1470 else {
5f05dabc 1471 system "stty", 'icanon', 'eol', '^@'; # ASCII null
4633a7c4
LW
1472 }
1473 print "\n";
1474
54310121
PP
1475Determination of whether $BSD_STYLE should be set
1476is left as an exercise to the reader.
cb1a09d0 1477
7660c0ab 1478The C<POSIX::getattr()> function can do this more portably on systems
5a964f20 1479purporting POSIX compliance.
cb1a09d0 1480See also the C<Term::ReadKey> module from your nearest CPAN site;
54310121 1481details on CPAN can be found on L<perlmod/CPAN>.
a0d0e21e
LW
1482
1483=item getlogin
1484
5a964f20
TC
1485Implements the C library function of the same name, which on most
1486systems returns the current login from F</etc/utmp>, if any. If null,
7660c0ab 1487use C<getpwuid()>.
a0d0e21e 1488
f86702cc 1489 $login = getlogin || getpwuid($<) || "Kilroy";
a0d0e21e 1490
7660c0ab
A
1491Do not consider C<getlogin()> for authentication: it is not as
1492secure as C<getpwuid()>.
4633a7c4 1493
a0d0e21e
LW
1494=item getpeername SOCKET
1495
1496Returns the packed sockaddr address of other end of the SOCKET connection.
1497
4633a7c4
LW
1498 use Socket;
1499 $hersockaddr = getpeername(SOCK);
1500 ($port, $iaddr) = unpack_sockaddr_in($hersockaddr);
1501 $herhostname = gethostbyaddr($iaddr, AF_INET);
1502 $herstraddr = inet_ntoa($iaddr);
a0d0e21e
LW
1503
1504=item getpgrp PID
1505
47e29363 1506Returns the current process group for the specified PID. Use
7660c0ab 1507a PID of C<0> to get the current process group for the
4633a7c4 1508current process. Will raise an exception if used on a machine that
f86cebdf 1509doesn't implement getpgrp(2). If PID is omitted, returns process
7660c0ab
A
1510group of current process. Note that the POSIX version of C<getpgrp()>
1511does not accept a PID argument, so only C<PID==0> is truly portable.
a0d0e21e
LW
1512
1513=item getppid
1514
1515Returns the process id of the parent process.
1516
1517=item getpriority WHICH,WHO
1518
4633a7c4
LW
1519Returns the current priority for a process, a process group, or a user.
1520(See L<getpriority(2)>.) Will raise a fatal exception if used on a
f86cebdf 1521machine that doesn't implement getpriority(2).
a0d0e21e
LW
1522
1523=item getpwnam NAME
1524
1525=item getgrnam NAME
1526
1527=item gethostbyname NAME
1528
1529=item getnetbyname NAME
1530
1531=item getprotobyname NAME
1532
1533=item getpwuid UID
1534
1535=item getgrgid GID
1536
1537=item getservbyname NAME,PROTO
1538
1539=item gethostbyaddr ADDR,ADDRTYPE
1540
1541=item getnetbyaddr ADDR,ADDRTYPE
1542
1543=item getprotobynumber NUMBER
1544
1545=item getservbyport PORT,PROTO
1546
1547=item getpwent
1548
1549=item getgrent
1550
1551=item gethostent
1552
1553=item getnetent
1554
1555=item getprotoent
1556
1557=item getservent
1558
1559=item setpwent
1560
1561=item setgrent
1562
1563=item sethostent STAYOPEN
1564
1565=item setnetent STAYOPEN
1566
1567=item setprotoent STAYOPEN
1568
1569=item setservent STAYOPEN
1570
1571=item endpwent
1572
1573=item endgrent
1574
1575=item endhostent
1576
1577=item endnetent
1578
1579=item endprotoent
1580
1581=item endservent
1582
1583These routines perform the same functions as their counterparts in the
5a964f20 1584system library. In list context, the return values from the
a0d0e21e
LW
1585various get routines are as follows:
1586
1587 ($name,$passwd,$uid,$gid,
6ee623d5 1588 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
a0d0e21e
LW
1589 ($name,$passwd,$gid,$members) = getgr*
1590 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1591 ($name,$aliases,$addrtype,$net) = getnet*
1592 ($name,$aliases,$proto) = getproto*
1593 ($name,$aliases,$port,$proto) = getserv*
1594
1595(If the entry doesn't exist you get a null list.)
1596
5a964f20 1597In scalar context, you get the name, unless the function was a
a0d0e21e
LW
1598lookup by name, in which case you get the other thing, whatever it is.
1599(If the entry doesn't exist you get the undefined value.) For example:
1600
5a964f20
TC
1601 $uid = getpwnam($name);
1602 $name = getpwuid($num);
1603 $name = getpwent();
1604 $gid = getgrnam($name);
1605 $name = getgrgid($num;
1606 $name = getgrent();
1607 #etc.
a0d0e21e 1608
7660c0ab 1609In I<getpw*()> the fields C<$quota>, C<$comment>, and C<$expire> are special
6ee623d5 1610cases in the sense that in many systems they are unsupported. If the
7660c0ab
A
1611C<$quota> is unsupported, it is an empty scalar. If it is supported, it
1612usually encodes the disk quota. If the C<$comment> field is unsupported,
6ee623d5
GS
1613it is an empty scalar. If it is supported it usually encodes some
1614administrative comment about the user. In some systems the $quota
7660c0ab
A
1615field may be C<$change> or C<$age>, fields that have to do with password
1616aging. In some systems the C<$comment> field may be C<$class>. The C<$expire>
6ee623d5
GS
1617field, if present, encodes the expiration period of the account or the
1618password. For the availability and the exact meaning of these fields
f86cebdf 1619in your system, please consult your getpwnam(3) documentation and your
7660c0ab
A
1620F<pwd.h> file. You can also find out from within Perl which meaning
1621your C<$quota> and C<$comment> fields have and whether you have the C<$expire>
1622field by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
1623C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>.
6ee623d5 1624
7660c0ab 1625The C<$members> value returned by I<getgr*()> is a space separated list of
a0d0e21e
LW
1626the login names of the members of the group.
1627
1628For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1629C, it will be returned to you via C<$?> if the function call fails. The
7660c0ab 1630C<@addrs> value returned by a successful call is a list of the raw
a0d0e21e
LW
1631addresses returned by the corresponding system library call. In the
1632Internet domain, each address is four bytes long and you can unpack it
1633by saying something like:
1634
1635 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1636
5a964f20
TC
1637If you get tired of remembering which element of the return list contains
1638which return value, by-name interfaces are also provided in modules:
7660c0ab
A
1639C<File::stat>, C<Net::hostent>, C<Net::netent>, C<Net::protoent>, C<Net::servent>,
1640C<Time::gmtime>, C<Time::localtime>, and C<User::grent>. These override the
5a964f20
TC
1641normal built-in, replacing them with versions that return objects with
1642the appropriate names for each field. For example:
1643
1644 use File::stat;
1645 use User::pwent;
1646 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
1647
1648Even though it looks like they're the same method calls (uid),
7660c0ab 1649they aren't, because a C<File::stat> object is different from a C<User::pwent> object.
5a964f20 1650
a0d0e21e
LW
1651=item getsockname SOCKET
1652
1653Returns the packed sockaddr address of this end of the SOCKET connection.
1654
4633a7c4
LW
1655 use Socket;
1656 $mysockaddr = getsockname(SOCK);
1657 ($port, $myaddr) = unpack_sockaddr_in($mysockaddr);
a0d0e21e
LW
1658
1659=item getsockopt SOCKET,LEVEL,OPTNAME
1660
5a964f20 1661Returns the socket option requested, or undef if there is an error.
a0d0e21e
LW
1662
1663=item glob EXPR
1664
0a753a76
PP
1665=item glob
1666
7660c0ab 1667Returns the value of EXPR with filename expansions such as the standard Unix shell F</bin/sh> would
68dc0745 1668do. This is the internal function implementing the C<E<lt>*.cE<gt>>
7660c0ab 1669operator, but you can use it directly. If EXPR is omitted, C<$_> is used.
68dc0745
PP
1670The C<E<lt>*.cE<gt>> operator is discussed in more detail in
1671L<perlop/"I/O Operators">.
a0d0e21e
LW
1672
1673=item gmtime EXPR
1674
1675Converts a time as returned by the time function to a 9-element array
54310121 1676with the time localized for the standard Greenwich time zone.
4633a7c4 1677Typically used as follows:
a0d0e21e 1678
54310121 1679 # 0 1 2 3 4 5 6 7 8
a0d0e21e
LW
1680 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1681 gmtime(time);
1682
1683All array elements are numeric, and come straight out of a struct tm.
7660c0ab
A
1684In particular this means that C<$mon> has the range C<0..11> and C<$wday> has
1685the range C<0..6> with sunday as day C<0>. Also, C<$year> is the number of
1686years since 1900, that is, C<$year> is C<123> in year 2023, I<not> simply the last two digits of the year.
2f9daede
TPG
1687
1688If EXPR is omitted, does C<gmtime(time())>.
a0d0e21e 1689
f86cebdf 1690In scalar context, returns the ctime(3) value:
0a753a76
PP
1691
1692 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
1693
7660c0ab 1694Also see the C<timegm()> function provided by the C<Time::Local> module,
f86cebdf 1695and the strftime(3) function available via the POSIX module.
7660c0ab
A
1696
1697This scalar value is B<not> locale dependent, see L<perllocale>, but
1698instead a Perl builtin. Also see the C<Time::Local> module, and the
f86cebdf 1699strftime(3) and mktime(3) function available via the POSIX module. To
7660c0ab
A
1700get somewhat similar but locale dependent date strings, set up your
1701locale environment variables appropriately (please see L<perllocale>)
1702and try for example:
1703
1704 use POSIX qw(strftime);
1705 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
1706
1707Note that the C<%a> and C<%b>, the short forms of the day of the week
1708and the month of the year, may not necessarily be three characters wide.
0a753a76 1709
a0d0e21e
LW
1710=item goto LABEL
1711
748a9306
LW
1712=item goto EXPR
1713
a0d0e21e
LW
1714=item goto &NAME
1715
7660c0ab 1716The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
a0d0e21e 1717execution there. It may not be used to go into any construct that
7660c0ab 1718requires initialization, such as a subroutine or a C<foreach> loop. It
0a753a76 1719also can't be used to go into a construct that is optimized away,
7660c0ab 1720or to get out of a block or subroutine given to C<sort()>.
0a753a76 1721It can be used to go almost anywhere else within the dynamic scope,
a0d0e21e 1722including out of subroutines, but it's usually better to use some other
ab4f32c2 1723construct such as C<last> or C<die()>. The author of Perl has never felt the
7660c0ab 1724need to use this form of C<goto> (in Perl, that is--C is another matter).
a0d0e21e 1725
7660c0ab
A
1726The C<goto-EXPR> form expects a label name, whose scope will be resolved
1727dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
748a9306
LW
1728necessarily recommended if you're optimizing for maintainability:
1729
1730 goto ("FOO", "BAR", "GLARCH")[$i];
1731
7660c0ab 1732The C<goto-&NAME> form is highly magical, and substitutes a call to the
a0d0e21e 1733named subroutine for the currently running subroutine. This is used by
7660c0ab 1734C<AUTOLOAD> subroutines that wish to load another subroutine and then
a0d0e21e 1735pretend that the other subroutine had been called in the first place
7660c0ab
A
1736(except that any modifications to C<@_> in the current subroutine are
1737propagated to the other subroutine.) After the C<goto>, not even C<caller()>
a0d0e21e
LW
1738will be able to tell that this routine was called first.
1739
1740=item grep BLOCK LIST
1741
1742=item grep EXPR,LIST
1743
f86cebdf 1744This is similar in spirit to, but not the same as, grep(1)
2f9daede
TPG
1745and its relatives. In particular, it is not limited to using
1746regular expressions.
1747
a0d0e21e 1748Evaluates the BLOCK or EXPR for each element of LIST (locally setting
7660c0ab 1749C<$_> to each element) and returns the list value consisting of those
a0d0e21e
LW
1750elements for which the expression evaluated to TRUE. In a scalar
1751context, returns the number of times the expression was TRUE.
1752
1753 @foo = grep(!/^#/, @bar); # weed out comments
1754
1755or equivalently,
1756
1757 @foo = grep {!/^#/} @bar; # weed out comments
1758
7660c0ab 1759Note that, because C<$_> is a reference into the list value, it can be used
a0d0e21e
LW
1760to modify the elements of the array. While this is useful and
1761supported, it can cause bizarre results if the LIST is not a named
2f9daede 1762array. Similarly, grep returns aliases into the original list,
f86cebdf 1763much like the way that a for loop's index variable aliases the list
2f9daede 1764elements. That is, modifying an element of a list returned by grep
ab4f32c2 1765(for example, in a C<foreach>, C<map()> or another C<grep()>)
2f9daede 1766actually modifies the element in the original list.
a0d0e21e 1767
fb73857a 1768See also L</map> for an array composed of the results of the BLOCK or EXPR.
38325410 1769
a0d0e21e
LW
1770=item hex EXPR
1771
54310121 1772=item hex
bbce6d69 1773
54310121 1774Interprets EXPR as a hex string and returns the corresponding
2f9daede 1775value. (To convert strings that might start with either 0 or 0x
7660c0ab 1776see L</oct>.) If EXPR is omitted, uses C<$_>.
2f9daede
TPG
1777
1778 print hex '0xAf'; # prints '175'
1779 print hex 'aF'; # same
a0d0e21e
LW
1780
1781=item import
1782
7660c0ab 1783There is no builtin C<import()> function. It is just an ordinary
4633a7c4 1784method (subroutine) defined (or inherited) by modules that wish to export
7660c0ab 1785names to another module. The C<use()> function calls the C<import()> method
54310121 1786for the package used. See also L</use()>, L<perlmod>, and L<Exporter>.
a0d0e21e
LW
1787
1788=item index STR,SUBSTR,POSITION
1789
1790=item index STR,SUBSTR
1791
4633a7c4
LW
1792Returns the position of the first occurrence of SUBSTR in STR at or after
1793POSITION. If POSITION is omitted, starts searching from the beginning of
7660c0ab 1794the string. The return value is based at C<0> (or whatever you've set the C<$[>
4633a7c4 1795variable to--but don't do that). If the substring is not found, returns
7660c0ab 1796one less than the base, ordinarily C<-1>.
a0d0e21e
LW
1797
1798=item int EXPR
1799
54310121 1800=item int
bbce6d69 1801
7660c0ab 1802Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
5a964f20 1803You should not use this for rounding, because it truncates
7660c0ab
A
1804towards C<0>, and because machine representations of floating point
1805numbers can sometimes produce counterintuitive results. Usually C<sprintf()> or C<printf()>,
1806or the C<POSIX::floor> or C<POSIX::ceil> functions, would serve you better.
a0d0e21e
LW
1807
1808=item ioctl FILEHANDLE,FUNCTION,SCALAR
1809
f86cebdf 1810Implements the ioctl(2) function. You'll probably have to say
a0d0e21e 1811
4633a7c4 1812 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
a0d0e21e 1813
4633a7c4 1814first to get the correct function definitions. If F<ioctl.ph> doesn't
a0d0e21e 1815exist or doesn't have the correct definitions you'll have to roll your
4633a7c4 1816own, based on your C header files such as F<E<lt>sys/ioctl.hE<gt>>.
5a964f20 1817(There is a Perl script called B<h2ph> that comes with the Perl kit that
54310121 1818may help you in this, but it's nontrivial.) SCALAR will be read and/or
4633a7c4 1819written depending on the FUNCTION--a pointer to the string value of SCALAR
ab4f32c2 1820will be passed as the third argument of the actual C<ioctl()> call. (If SCALAR
4633a7c4
LW
1821has no string value but does have a numeric value, that value will be
1822passed rather than a pointer to the string value. To guarantee this to be
7660c0ab 1823TRUE, add a C<0> to the scalar before using it.) The C<pack()> and C<unpack()>
4633a7c4 1824functions are useful for manipulating the values of structures used by
7660c0ab 1825C<ioctl()>. The following example sets the erase character to DEL.
a0d0e21e
LW
1826
1827 require 'ioctl.ph';
4633a7c4
LW
1828 $getp = &TIOCGETP;
1829 die "NO TIOCGETP" if $@ || !$getp;
a0d0e21e 1830 $sgttyb_t = "ccccs"; # 4 chars and a short
4633a7c4 1831 if (ioctl(STDIN,$getp,$sgttyb)) {
a0d0e21e
LW
1832 @ary = unpack($sgttyb_t,$sgttyb);
1833 $ary[2] = 127;
1834 $sgttyb = pack($sgttyb_t,@ary);
4633a7c4 1835 ioctl(STDIN,&TIOCSETP,$sgttyb)
a0d0e21e
LW
1836 || die "Can't ioctl: $!";
1837 }
1838
ab4f32c2 1839The return value of C<ioctl()> (and C<fcntl()>) is as follows:
a0d0e21e
LW
1840
1841 if OS returns: then Perl returns:
1842 -1 undefined value
1843 0 string "0 but true"
1844 anything else that number
1845
1846Thus Perl returns TRUE on success and FALSE on failure, yet you can
1847still easily determine the actual value returned by the operating
1848system:
1849
1850 ($retval = ioctl(...)) || ($retval = -1);
1851 printf "System returned %d\n", $retval;
1852
7660c0ab 1853The special string "C<0> but true" is excempt from B<-w> complaints
5a964f20
TC
1854about improper numeric conversions.
1855
a0d0e21e
LW
1856=item join EXPR,LIST
1857
54310121 1858Joins the separate strings of LIST into a single string with
a0d0e21e
LW
1859fields separated by the value of EXPR, and returns the string.
1860Example:
1861
1862 $_ = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
1863
7660c0ab 1864See L</split>.
a0d0e21e 1865
aa689395
PP
1866=item keys HASH
1867
1d2dff63
GS
1868Returns a list consisting of all the keys of the named hash. (In a
1869scalar context, returns the number of keys.) The keys are returned in
aa689395 1870an apparently random order, but it is the same order as either the
7660c0ab 1871C<values()> or C<each()> function produces (given that the hash has not been
aa689395 1872modified). As a side effect, it resets HASH's iterator.
a0d0e21e 1873
aa689395 1874Here is yet another way to print your environment:
a0d0e21e
LW
1875
1876 @keys = keys %ENV;
1877 @values = values %ENV;
1878 while ($#keys >= 0) {
1879 print pop(@keys), '=', pop(@values), "\n";
1880 }
1881
1882or how about sorted by key:
1883
1884 foreach $key (sort(keys %ENV)) {
1885 print $key, '=', $ENV{$key}, "\n";
1886 }
1887
aca803df 1888To sort a hash by value, you'll need to use a C<sort()> function.
aa689395 1889Here's a descending numeric sort of a hash by its values:
4633a7c4 1890
5a964f20 1891 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
4633a7c4
LW
1892 printf "%4d %s\n", $hash{$key}, $key;
1893 }
1894
ab4f32c2 1895As an lvalue C<keys()> allows you to increase the number of hash buckets
aa689395
PP
1896allocated for the given hash. This can gain you a measure of efficiency if
1897you know the hash is going to get big. (This is similar to pre-extending
1898an array by assigning a larger number to $#array.) If you say
55497cff
PP
1899
1900 keys %hash = 200;
1901
5a964f20
TC
1902then C<%hash> will have at least 200 buckets allocated for it--256 of them, in fact, since
1903it rounds up to the next power of two. These
55497cff
PP
1904buckets will be retained even if you do C<%hash = ()>, use C<undef
1905%hash> if you want to free the storage while C<%hash> is still in scope.
1906You can't shrink the number of buckets allocated for the hash using
ab4f32c2 1907C<keys()> in this way (but you needn't worry about doing this by accident,
55497cff
PP
1908as trying has no effect).
1909
a0d0e21e
LW
1910=item kill LIST
1911
54310121
PP
1912Sends a signal to a list of processes. The first element of
1913the list must be the signal to send. Returns the number of
4633a7c4 1914processes successfully signaled.
a0d0e21e
LW
1915
1916 $cnt = kill 1, $child1, $child2;
1917 kill 9, @goners;
1918
4633a7c4
LW
1919Unlike in the shell, in Perl if the I<SIGNAL> is negative, it kills
1920process groups instead of processes. (On System V, a negative I<PROCESS>
1921number will also kill process groups, but that's not portable.) That
1922means you usually want to use positive not negative signals. You may also
da0045b7 1923use a signal name in quotes. See L<perlipc/"Signals"> for details.
a0d0e21e
LW
1924
1925=item last LABEL
1926
1927=item last
1928
1929The C<last> command is like the C<break> statement in C (as used in
1930loops); it immediately exits the loop in question. If the LABEL is
1931omitted, the command refers to the innermost enclosing loop. The
1932C<continue> block, if any, is not executed:
1933
4633a7c4
LW
1934 LINE: while (<STDIN>) {
1935 last LINE if /^$/; # exit when done with header
5a964f20 1936 #...
a0d0e21e
LW
1937 }
1938
1d2dff63
GS
1939See also L</continue> for an illustration of how C<last>, C<next>, and
1940C<redo> work.
1941
a0d0e21e
LW
1942=item lc EXPR
1943
54310121 1944=item lc
bbce6d69 1945
a0d0e21e 1946Returns an lowercased version of EXPR. This is the internal function
7660c0ab
A
1947implementing the C<\L> escape in double-quoted strings.
1948Respects current C<LC_CTYPE> locale if C<use locale> in force. See L<perllocale>.
a0d0e21e 1949
7660c0ab 1950If EXPR is omitted, uses C<$_>.
bbce6d69 1951
a0d0e21e
LW
1952=item lcfirst EXPR
1953
54310121 1954=item lcfirst
bbce6d69 1955
a0d0e21e 1956Returns the value of EXPR with the first character lowercased. This is
7660c0ab
A
1957the internal function implementing the C<\l> escape in double-quoted strings.
1958Respects current C<LC_CTYPE> locale if C<use locale> in force. See L<perllocale>.
a0d0e21e 1959
7660c0ab 1960If EXPR is omitted, uses C<$_>.
bbce6d69 1961
a0d0e21e
LW
1962=item length EXPR
1963
54310121 1964=item length
bbce6d69 1965
5a964f20 1966Returns the length in bytes of the value of EXPR. If EXPR is
7660c0ab 1967omitted, returns length of C<$_>.
a0d0e21e
LW
1968
1969=item link OLDFILE,NEWFILE
1970
5a964f20
TC
1971Creates a new filename linked to the old filename. Returns TRUE for
1972success, FALSE otherwise.
a0d0e21e
LW
1973
1974=item listen SOCKET,QUEUESIZE
1975
1976Does the same thing that the listen system call does. Returns TRUE if
4633a7c4 1977it succeeded, FALSE otherwise. See example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
1978
1979=item local EXPR
1980
5a964f20
TC
1981A local modifies the listed variables to be local to the enclosing
1982block, file, or eval. If more than one value is listed, the list must
1983be placed in parentheses. See L<perlsub/"Temporary Values via local()">
1984for details, including issues with tied arrays and hashes.
a0d0e21e 1985
7660c0ab 1986You really probably want to be using C<my()> instead, because C<local()> isn't
7b8d334a 1987what most people think of as "local". See L<perlsub/"Private Variables
cb1a09d0 1988via my()"> for details.
a0d0e21e
LW
1989
1990=item localtime EXPR
1991
1992Converts a time as returned by the time function to a 9-element array
5f05dabc 1993with the time analyzed for the local time zone. Typically used as
a0d0e21e
LW
1994follows:
1995
54310121 1996 # 0 1 2 3 4 5 6 7 8
a0d0e21e
LW
1997 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1998 localtime(time);
1999
2000All array elements are numeric, and come straight out of a struct tm.
7660c0ab
A
2001In particular this means that C<$mon> has the range C<0..11> and C<$wday> has
2002the range C<0..6> with sunday as day C<0>. Also, C<$year> is the number of
2003years since 1900, that is, C<$year> is C<123> in year 2023, and I<not> simply the last two digits of the year.
54310121
PP
2004
2005If EXPR is omitted, uses the current time (C<localtime(time)>).
a0d0e21e 2006
f86cebdf 2007In scalar context, returns the ctime(3) value:
a0d0e21e 2008
5f05dabc 2009 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
a0d0e21e 2010
a3cb178b 2011This scalar value is B<not> locale dependent, see L<perllocale>, but
7660c0ab 2012instead a Perl builtin. Also see the C<Time::Local> module, and the
f86cebdf 2013strftime(3) and mktime(3) function available via the POSIX module. To
a3cb178b
GS
2014get somewhat similar but locale dependent date strings, set up your
2015locale environment variables appropriately (please see L<perllocale>)
5a964f20 2016and try for example:
a3cb178b 2017
5a964f20 2018 use POSIX qw(strftime);
a3cb178b
GS
2019 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
2020
2021Note that the C<%a> and C<%b>, the short forms of the day of the week
2022and the month of the year, may not necessarily be three characters wide.
a0d0e21e
LW
2023
2024=item log EXPR
2025
54310121 2026=item log
bbce6d69 2027
5a964f20 2028Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted, returns log
7660c0ab 2029of C<$_>.
a0d0e21e
LW
2030
2031=item lstat FILEHANDLE
2032
2033=item lstat EXPR
2034
54310121 2035=item lstat
bbce6d69 2036
7660c0ab 2037Does the same thing as the C<stat()> function (including setting the
5a964f20
TC
2038special C<_> filehandle) but stats a symbolic link instead of the file
2039the symbolic link points to. If symbolic links are unimplemented on
7660c0ab 2040your system, a normal C<stat()> is done.
a0d0e21e 2041
7660c0ab 2042If EXPR is omitted, stats C<$_>.
bbce6d69 2043
a0d0e21e
LW
2044=item m//
2045
2046The match operator. See L<perlop>.
2047
2048=item map BLOCK LIST
2049
2050=item map EXPR,LIST
2051
7660c0ab 2052Evaluates the BLOCK or EXPR for each element of LIST (locally setting C<$_> to each
a0d0e21e
LW
2053element) and returns the list value composed of the results of each such
2054evaluation. Evaluates BLOCK or EXPR in a list context, so each element of LIST
2055may produce zero, one, or more elements in the returned value.
2056
2057 @chars = map(chr, @nums);
2058
2059translates a list of numbers to the corresponding characters. And
2060
4633a7c4 2061 %hash = map { getkey($_) => $_ } @array;
a0d0e21e
LW
2062
2063is just a funny way to write
2064
2065 %hash = ();
2066 foreach $_ (@array) {
4633a7c4 2067 $hash{getkey($_)} = $_;
a0d0e21e
LW
2068 }
2069
7660c0ab 2070Note that, because C<$_> is a reference into the list value, it can be used
fb73857a
PP
2071to modify the elements of the array. While this is useful and
2072supported, it can cause bizarre results if the LIST is not a named
2073array. See also L</grep> for an array composed of those items of the
2074original list for which the BLOCK or EXPR evaluates to true.
2075
a0d0e21e
LW
2076=item mkdir FILENAME,MODE
2077
2078Creates the directory specified by FILENAME, with permissions specified
5a964f20
TC
2079by MODE (as modified by umask). If it succeeds it returns TRUE, otherwise
2080it returns FALSE and sets C<$!> (errno).
a0d0e21e
LW
2081
2082=item msgctl ID,CMD,ARG
2083
f86cebdf 2084Calls the System V IPC function msgctl(2). You'll probably have to say
0ade1984
JH
2085
2086 use IPC::SysV;
2087
7660c0ab
A
2088first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2089then ARG must be a variable which will hold the returned C<msqid_ds>
ab4f32c2 2090structure. Returns like C<ioctl()>: the undefined value for error, "C<0> but
0ade1984 2091true" for zero, or the actual return value otherwise. See also
7660c0ab 2092C<IPC::SysV> and C<IPC::Semaphore::Msg> documentation.
a0d0e21e
LW
2093
2094=item msgget KEY,FLAGS
2095
f86cebdf 2096Calls the System V IPC function msgget(2). Returns the message queue
7660c0ab
A
2097id, or the undefined value if there is an error. See also C<IPC::SysV>
2098and C<IPC::SysV::Msg> documentation.
a0d0e21e
LW
2099
2100=item msgsnd ID,MSG,FLAGS
2101
2102Calls the System V IPC function msgsnd to send the message MSG to the
2103message queue ID. MSG must begin with the long integer message type,
c07a80fd 2104which may be created with C<pack("l", $type)>. Returns TRUE if
7660c0ab
A
2105successful, or FALSE if there is an error. See also C<IPC::SysV>
2106and C<IPC::SysV::Msg> documentation.
a0d0e21e
LW
2107
2108=item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2109
2110Calls the System V IPC function msgrcv to receive a message from
2111message queue ID into variable VAR with a maximum message size of
0ade1984
JH
2112SIZE. Note that if a message is received, the message type will be
2113the first thing in VAR, and the maximum length of VAR is SIZE plus the
2114size of the message type. Returns TRUE if successful, or FALSE if
7660c0ab 2115there is an error. See also C<IPC::SysV> and C<IPC::SysV::Msg> documentation.
a0d0e21e
LW
2116
2117=item my EXPR
2118
ab4f32c2
A
2119A C<my()> declares the listed variables to be local (lexically) to the
2120enclosing block, file, or C<eval()>. If
5f05dabc 2121more than one value is listed, the list must be placed in parentheses. See
cb1a09d0 2122L<perlsub/"Private Variables via my()"> for details.
4633a7c4 2123
a0d0e21e
LW
2124=item next LABEL
2125
2126=item next
2127
2128The C<next> command is like the C<continue> statement in C; it starts
2129the next iteration of the loop:
2130
4633a7c4
LW
2131 LINE: while (<STDIN>) {
2132 next LINE if /^#/; # discard comments
5a964f20 2133 #...
a0d0e21e
LW
2134 }
2135
2136Note that if there were a C<continue> block on the above, it would get
2137executed even on discarded lines. If the LABEL is omitted, the command
2138refers to the innermost enclosing loop.
2139
1d2dff63
GS
2140See also L</continue> for an illustration of how C<last>, C<next>, and
2141C<redo> work.
2142
a0d0e21e
LW
2143=item no Module LIST
2144
7660c0ab 2145See the L</use> function, which C<no> is the opposite of.
a0d0e21e
LW
2146
2147=item oct EXPR
2148
54310121 2149=item oct
bbce6d69 2150
4633a7c4 2151Interprets EXPR as an octal string and returns the corresponding
7660c0ab 2152value. (If EXPR happens to start off with C<0x>, interprets it as
4633a7c4
LW
2153a hex string instead.) The following will handle decimal, octal, and
2154hex in the standard Perl or C notation:
a0d0e21e
LW
2155
2156 $val = oct($val) if $val =~ /^0/;
2157
7660c0ab
A
2158If EXPR is omitted, uses C<$_>. This function is commonly used when
2159a string such as C<644> needs to be converted into a file mode, for
2f9daede
TPG
2160example. (Although perl will automatically convert strings into
2161numbers as needed, this automatic conversion assumes base 10.)
a0d0e21e
LW
2162
2163=item open FILEHANDLE,EXPR
2164
2165=item open FILEHANDLE
2166
2167Opens the file whose filename is given by EXPR, and associates it with
5f05dabc
PP
2168FILEHANDLE. If FILEHANDLE is an expression, its value is used as the
2169name of the real filehandle wanted. If EXPR is omitted, the scalar
2170variable of the same name as the FILEHANDLE contains the filename.
ab4f32c2
A
2171(Note that lexical variables--those declared with C<my()>--will not work
2172for this purpose; so if you're using C<my()>, specify EXPR in your call
5f05dabc
PP
2173to open.)
2174
7660c0ab
A
2175If the filename begins with C<'E<lt>'> or nothing, the file is opened for input.
2176If the filename begins with C<'E<gt>'>, the file is truncated and opened for
2177output, being created if necessary. If the filename begins with C<'E<gt>E<gt>'>,
fbb426e4 2178the file is opened for appending, again being created if necessary.
7660c0ab
A
2179You can put a C<'+'> in front of the C<'E<gt>'> or C<'E<lt>'> to indicate that
2180you want both read and write access to the file; thus C<'+E<lt>'> is almost
2181always preferred for read/write updates--the C<'+E<gt>'> mode would clobber the
5a964f20
TC
2182file first. You can't usually use either read-write mode for updating
2183textfiles, since they have variable length records. See the B<-i>
2184switch in L<perlrun> for a better approach.
2185
2186The prefix and the filename may be separated with spaces.
f86cebdf 2187These various prefixes correspond to the fopen(3) modes of C<'r'>, C<'r+'>, C<'w'>,
7660c0ab 2188C<'w+'>, C<'a'>, and C<'a+'>.
5f05dabc 2189
7660c0ab 2190If the filename begins with C<'|'>, the filename is interpreted as a
5a964f20 2191command to which output is to be piped, and if the filename ends with a
7660c0ab
A
2192C<'|'>, the filename is interpreted See L<perlipc/"Using open() for IPC">
2193for more examples of this. (You are not allowed to C<open()> to a command
5a964f20
TC
2194that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2195and L<perlipc/"Bidirectional Communication"> for alternatives.)
cb1a09d0 2196
7660c0ab 2197Opening C<'-'> opens STDIN and opening C<'E<gt>-'> opens STDOUT. Open returns
ab4f32c2 2198nonzero upon success, the undefined value otherwise. If the C<open()>
4633a7c4 2199involved a pipe, the return value happens to be the pid of the
54310121 2200subprocess.
cb1a09d0
AD
2201
2202If you're unfortunate enough to be running Perl on a system that
2203distinguishes between text files and binary files (modern operating
2204systems don't care), then you should check out L</binmode> for tips for
ab4f32c2 2205dealing with this. The key distinction between systems that need C<binmode()>
5a964f20
TC
2206and those that don't is their text file formats. Systems like Unix, MacOS, and
2207Plan9, which delimit lines with a single character, and which encode that
ab4f32c2 2208character in C as C<"\n">, do not need C<binmode()>. The rest need it.
cb1a09d0 2209
fb73857a 2210When opening a file, it's usually a bad idea to continue normal execution
ab4f32c2
A
2211if the request failed, so C<open()> is frequently used in connection with
2212C<die()>. Even if C<die()> won't do what you want (say, in a CGI script,
fb73857a 2213where you want to make a nicely formatted error message (but there are
5a964f20 2214modules that can help with that problem)) you should always check
fb73857a
PP
2215the return value from opening a file. The infrequent exception is when
2216working with an unopened filehandle is actually what you want to do.
2217
cb1a09d0 2218Examples:
a0d0e21e
LW
2219
2220 $ARTICLE = 100;
2221 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2222 while (<ARTICLE>) {...
2223
2224 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
fb73857a 2225 # if the open fails, output is discarded
a0d0e21e 2226
fb73857a
PP
2227 open(DBASE, '+<dbase.mine') # open for update
2228 or die "Can't open 'dbase.mine' for update: $!";
cb1a09d0 2229
fb73857a
PP
2230 open(ARTICLE, "caesar <$article |") # decrypt article
2231 or die "Can't start caesar: $!";
a0d0e21e 2232
fb73857a
PP
2233 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
2234 or die "Can't start sort: $!";
a0d0e21e
LW
2235
2236 # process argument list of files along with any includes
2237
2238 foreach $file (@ARGV) {
2239 process($file, 'fh00');
2240 }
2241
2242 sub process {
5a964f20 2243 my($filename, $input) = @_;
a0d0e21e
LW
2244 $input++; # this is a string increment
2245 unless (open($input, $filename)) {
2246 print STDERR "Can't open $filename: $!\n";
2247 return;
2248 }
2249
5a964f20 2250 local $_;
a0d0e21e
LW
2251 while (<$input>) { # note use of indirection
2252 if (/^#include "(.*)"/) {
2253 process($1, $input);
2254 next;
2255 }
5a964f20 2256 #... # whatever
a0d0e21e
LW
2257 }
2258 }
2259
2260You may also, in the Bourne shell tradition, specify an EXPR beginning
7660c0ab 2261with C<'E<gt>&'>, in which case the rest of the string is interpreted as the
5a964f20 2262name of a filehandle (or file descriptor, if numeric) to be
7660c0ab
A
2263duped and opened. You may use C<&> after C<E<gt>>, C<E<gt>E<gt>>, C<E<lt>>, C<+E<gt>>,
2264C<+E<gt>E<gt>>, and C<+E<lt>>. The
a0d0e21e 2265mode you specify should match the mode of the original filehandle.
184e9718 2266(Duping a filehandle does not take into account any existing contents of
cb1a09d0 2267stdio buffers.)
a0d0e21e
LW
2268Here is a script that saves, redirects, and restores STDOUT and
2269STDERR:
2270
2271 #!/usr/bin/perl
5a964f20
TC
2272 open(OLDOUT, ">&STDOUT");
2273 open(OLDERR, ">&STDERR");
a0d0e21e
LW
2274
2275 open(STDOUT, ">foo.out") || die "Can't redirect stdout";
2276 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
2277
2278 select(STDERR); $| = 1; # make unbuffered
2279 select(STDOUT); $| = 1; # make unbuffered
2280
2281 print STDOUT "stdout 1\n"; # this works for
2282 print STDERR "stderr 1\n"; # subprocesses too
2283
2284 close(STDOUT);
2285 close(STDERR);
2286
5a964f20
TC
2287 open(STDOUT, ">&OLDOUT");
2288 open(STDERR, ">&OLDERR");
a0d0e21e
LW
2289
2290 print STDOUT "stdout 2\n";
2291 print STDERR "stderr 2\n";
2292
2293
7660c0ab
A
2294If you specify C<'E<lt>&=N'>, where C<N> is a number, then Perl will do an
2295equivalent of C's C<fdopen()> of that file descriptor; this is more
4633a7c4 2296parsimonious of file descriptors. For example:
a0d0e21e
LW
2297
2298 open(FILEHANDLE, "<&=$fd")
2299
7660c0ab 2300If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>, then
a0d0e21e 2301there is an implicit fork done, and the return value of open is the pid
7660c0ab 2302of the child within the parent process, and C<0> within the child
184e9718 2303process. (Use C<defined($pid)> to determine whether the open was successful.)
a0d0e21e
LW
2304The filehandle behaves normally for the parent, but i/o to that
2305filehandle is piped from/to the STDOUT/STDIN of the child process.
2306In the child process the filehandle isn't opened--i/o happens from/to
2307the new STDOUT or STDIN. Typically this is used like the normal
2308piped open when you want to exercise more control over just how the
2309pipe command gets executed, such as when you are running setuid, and
54310121 2310don't want to have to scan shell commands for metacharacters.
4633a7c4 2311The following pairs are more or less equivalent:
a0d0e21e
LW
2312
2313 open(FOO, "|tr '[a-z]' '[A-Z]'");
2314 open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
2315
2316 open(FOO, "cat -n '$file'|");
2317 open(FOO, "-|") || exec 'cat', '-n', $file;
2318
4633a7c4
LW
2319See L<perlipc/"Safe Pipe Opens"> for more examples of this.
2320
5a964f20 2321NOTE: On any operation that may do a fork, any unflushed buffers remain
184e9718 2322unflushed in both processes, which means you may need to set C<$|> to
a0d0e21e
LW
2323avoid duplicate output.
2324
0dccf244
CS
2325Closing any piped filehandle causes the parent process to wait for the
2326child to finish, and returns the status value in C<$?>.
2327
5a964f20 2328The filename passed to open will have leading and trailing
f86cebdf 2329whitespace deleted, and the normal redirection characters
5a964f20
TC
2330honored. This property, known as "magic open",
2331can often be used to good effect. A user could specify a filename of
7660c0ab 2332F<"rsh cat file |">, or you could change certain filenames as needed:
5a964f20
TC
2333
2334 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
2335 open(FH, $filename) or die "Can't open $filename: $!";
2336
2337However, to open a file with arbitrary weird characters in it, it's
2338necessary to protect any leading and trailing whitespace:
2339
2340 $file =~ s#^(\s)#./$1#;
2341 open(FOO, "< $file\0");
2342
7660c0ab
A
2343If you want a "real" C C<open()> (see L<open(2)> on your system), then you
2344should use the C<sysopen()> function, which involves no such magic. This is
5a964f20
TC
2345another way to protect your filenames from interpretation. For example:
2346
2347 use IO::Handle;
2348 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
2349 or die "sysopen $path: $!";
2350 $oldfh = select(HANDLE); $| = 1; select($oldfh);
2351 print HANDLE "stuff $$\n");
2352 seek(HANDLE, 0, 0);
2353 print "File contains: ", <HANDLE>;
2354
7660c0ab
A
2355Using the constructor from the C<IO::Handle> package (or one of its
2356subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
5a964f20
TC
2357filehandles that have the scope of whatever variables hold references to
2358them, and automatically close whenever and however you leave that scope:
c07a80fd 2359
5f05dabc 2360 use IO::File;
5a964f20 2361 #...
c07a80fd
PP
2362 sub read_myfile_munged {
2363 my $ALL = shift;
5f05dabc 2364 my $handle = new IO::File;
c07a80fd
PP
2365 open($handle, "myfile") or die "myfile: $!";
2366 $first = <$handle>
2367 or return (); # Automatically closed here.
2368 mung $first or die "mung failed"; # Or here.
2369 return $first, <$handle> if $ALL; # Or here.
2370 $first; # Or here.
2371 }
2372
cb1a09d0 2373See L</seek()> for some details about mixing reading and writing.
a0d0e21e
LW
2374
2375=item opendir DIRHANDLE,EXPR
2376
7660c0ab
A
2377Opens a directory named EXPR for processing by C<readdir()>, C<telldir()>,
2378C<seekdir()>, C<rewinddir()>, and C<closedir()>. Returns TRUE if successful.
a0d0e21e
LW
2379DIRHANDLEs have their own namespace separate from FILEHANDLEs.
2380
2381=item ord EXPR
2382
54310121 2383=item ord
bbce6d69 2384
a0d0e21e 2385Returns the numeric ascii value of the first character of EXPR. If
7660c0ab 2386EXPR is omitted, uses C<$_>. For the reverse, see L</chr>.
a0d0e21e
LW
2387
2388=item pack TEMPLATE,LIST
2389
2390Takes an array or list of values and packs it into a binary structure,
2391returning the string containing the structure. The TEMPLATE is a
2392sequence of characters that give the order and type of values, as
2393follows:
2394
2395 A An ascii string, will be space padded.
2396 a An ascii string, will be null padded.
2397 b A bit string (ascending bit order, like vec()).
2398 B A bit string (descending bit order).
2399 h A hex string (low nybble first).
2400 H A hex string (high nybble first).
2401
2402 c A signed char value.
2403 C An unsigned char value.
96e4d5b1 2404
a0d0e21e
LW
2405 s A signed short value.
2406 S An unsigned short value.
96e4d5b1
PP
2407 (This 'short' is _exactly_ 16 bits, which may differ from
2408 what a local C compiler calls 'short'.)
2409
a0d0e21e
LW
2410 i A signed integer value.
2411 I An unsigned integer value.
f86cebdf
GS
2412 (This 'integer' is _at_least_ 32 bits wide. Its exact
2413 size depends on what a local C compiler calls 'int',
2414 and may even be larger than the 'long' described in
2415 the next item.)
96e4d5b1 2416
a0d0e21e
LW
2417 l A signed long value.
2418 L An unsigned long value.
96e4d5b1
PP
2419 (This 'long' is _exactly_ 32 bits, which may differ from
2420 what a local C compiler calls 'long'.)
a0d0e21e 2421
96e4d5b1
PP
2422 n A short in "network" (big-endian) order.
2423 N A long in "network" (big-endian) order.
a0d0e21e
LW
2424 v A short in "VAX" (little-endian) order.
2425 V A long in "VAX" (little-endian) order.
96e4d5b1
PP
2426 (These 'shorts' and 'longs' are _exactly_ 16 bits and
2427 _exactly_ 32 bits, respectively.)
a0d0e21e
LW
2428
2429 f A single-precision float in the native format.
2430 d A double-precision float in the native format.
2431
2432 p A pointer to a null-terminated string.
2433 P A pointer to a structure (fixed-length string).
2434
2435 u A uuencoded string.
2436
96e4d5b1 2437 w A BER compressed integer. Its bytes represent an unsigned
f86cebdf
GS
2438 integer in base 128, most significant digit first, with as
2439 few digits as possible. Bit eight (the high bit) is set
2440 on each byte except the last.
def98dd4 2441
a0d0e21e
LW
2442 x A null byte.
2443 X Back up a byte.
2444 @ Null fill to absolute position.
2445
5a964f20 2446Each letter may optionally be followed by a number giving a repeat
7660c0ab
A
2447count. With all types except C<"a">, C<"A">, C<"b">, C<"B">, C<"h">, C<"H">, and C<"P"> the
2448pack function will gobble up that many values from the LIST. A C<*> for the
2449repeat count means to use however many items are left. The C<"a"> and C<"A">
a0d0e21e 2450types gobble just one value, but pack it as a string of length count,
7660c0ab
A
2451padding with nulls or spaces as necessary. (When unpacking, C<"A"> strips
2452trailing spaces and nulls, but C<"a"> does not.) Likewise, the C<"b"> and C<"B">
2453fields pack a string that many bits long. The C<"h"> and C<"H"> fields pack a
2454string that many nybbles long. The C<"p"> type packs a pointer to a null-
84902520
TB
2455terminated string. You are responsible for ensuring the string is not a
2456temporary value (which can potentially get deallocated before you get
7660c0ab 2457around to using the packed result). The C<"P"> packs a pointer to a structure
61167c6f 2458of the size indicated by the length. A NULL pointer is created if the
7660c0ab 2459corresponding value for C<"p"> or C<"P"> is C<undef>.
61167c6f 2460Real numbers (floats and doubles) are
a0d0e21e
LW
2461in the native machine format only; due to the multiplicity of floating
2462formats around, and the lack of a standard "network" representation, no
2463facility for interchange has been made. This means that packed floating
2464point data written on one machine may not be readable on another - even if
2465both use IEEE floating point arithmetic (as the endian-ness of the memory
2466representation is not part of the IEEE spec). Note that Perl uses doubles
2467internally for all numeric calculation, and converting from double into
5f05dabc 2468float and thence back to double again will lose precision (i.e.,
7660c0ab 2469C<unpack("f", pack("f", $foo)>) will not in general equal C<$foo>).
a0d0e21e
LW
2470
2471Examples:
2472
2473 $foo = pack("cccc",65,66,67,68);
2474 # foo eq "ABCD"
2475 $foo = pack("c4",65,66,67,68);
2476 # same thing
2477
2478 $foo = pack("ccxxcc",65,66,67,68);
2479 # foo eq "AB\0\0CD"
2480
2481 $foo = pack("s2",1,2);
2482 # "\1\0\2\0" on little-endian
2483 # "\0\1\0\2" on big-endian
2484
2485 $foo = pack("a4","abcd","x","y","z");
2486 # "abcd"
2487
2488 $foo = pack("aaaa","abcd","x","y","z");
2489 # "axyz"
2490
2491 $foo = pack("a14","abcdefg");
2492 # "abcdefg\0\0\0\0\0\0\0"
2493
2494 $foo = pack("i9pl", gmtime);
2495 # a real struct tm (on my system anyway)
2496
2497 sub bintodec {
2498 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
2499 }
2500
2501The same template may generally also be used in the unpack function.
2502
5a964f20
TC
2503=item package
2504
cb1a09d0
AD
2505=item package NAMESPACE
2506
2507Declares the compilation unit as being in the given namespace. The scope
2508of the package declaration is from the declaration itself through the end of
7660c0ab 2509the enclosing block (the same scope as the C<local()> operator). All further
cb1a09d0 2510unqualified dynamic identifiers will be in this namespace. A package
5f05dabc 2511statement affects only dynamic variables--including those you've used
7660c0ab 2512C<local()> on--but I<not> lexical variables created with C<my()>. Typically it
cb1a09d0
AD
2513would be the first declaration in a file to be included by the C<require>
2514or C<use> operator. You can switch into a package in more than one place;
5a964f20 2515it merely influences which symbol table is used by the compiler for the
cb1a09d0
AD
2516rest of that block. You can refer to variables and filehandles in other
2517packages by prefixing the identifier with the package name and a double
2518colon: C<$Package::Variable>. If the package name is null, the C<main>
2519package as assumed. That is, C<$::sail> is equivalent to C<$main::sail>.
2520
5a964f20
TC
2521If NAMESPACE is omitted, then there is no current package, and all
2522identifiers must be fully qualified or lexicals. This is stricter
2523than C<use strict>, since it also extends to function names.
2524
cb1a09d0
AD
2525See L<perlmod/"Packages"> for more information about packages, modules,
2526and classes. See L<perlsub> for other scoping issues.
2527
a0d0e21e
LW
2528=item pipe READHANDLE,WRITEHANDLE
2529
2530Opens a pair of connected pipes like the corresponding system call.
2531Note that if you set up a loop of piped processes, deadlock can occur
2532unless you are very careful. In addition, note that Perl's pipes use
184e9718 2533stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE
a0d0e21e
LW
2534after each command, depending on the application.
2535
7e1af8bc 2536See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
4633a7c4
LW
2537for examples of such things.
2538
a0d0e21e
LW
2539=item pop ARRAY
2540
54310121 2541=item pop
28757baa 2542
a0d0e21e
LW
2543Pops and returns the last value of the array, shortening the array by
25441. Has a similar effect to
2545
2546 $tmp = $ARRAY[$#ARRAY--];
2547
2548If there are no elements in the array, returns the undefined value.
cb1a09d0 2549If ARRAY is omitted, pops the
7660c0ab
A
2550C<@ARGV> array in the main program, and the C<@_> array in subroutines, just
2551like C<shift()>.
a0d0e21e
LW
2552
2553=item pos SCALAR
2554
54310121 2555=item pos
bbce6d69 2556
4633a7c4 2557Returns the offset of where the last C<m//g> search left off for the variable
7660c0ab 2558is in question (C<$_> is used when the variable is not specified). May be
44a8e56a
PP
2559modified to change that offset. Such modification will also influence
2560the C<\G> zero-width assertion in regular expressions. See L<perlre> and
2561L<perlop>.
a0d0e21e
LW
2562
2563=item print FILEHANDLE LIST
2564
2565=item print LIST
2566
2567=item print
2568
cb1a09d0 2569Prints a string or a comma-separated list of strings. Returns TRUE
a0d0e21e 2570if successful. FILEHANDLE may be a scalar variable name, in which case
cb1a09d0 2571the variable contains the name of or a reference to the filehandle, thus introducing one
a0d0e21e
LW
2572level of indirection. (NOTE: If FILEHANDLE is a variable and the next
2573token is a term, it may be misinterpreted as an operator unless you
7660c0ab 2574interpose a C<+> or put parentheses around the arguments.) If FILEHANDLE is
a0d0e21e 2575omitted, prints by default to standard output (or to the last selected
7660c0ab 2576output channel--see L</select>). If LIST is also omitted, prints C<$_> to
5a964f20 2577the currently selected output channel. To set the default output channel to something other than
a0d0e21e 2578STDOUT use the select operation. Note that, because print takes a
5a964f20 2579LIST, anything in the LIST is evaluated in list context, and any
a0d0e21e 2580subroutine that you call will have one or more of its expressions
5a964f20 2581evaluated in list context. Also be careful not to follow the print
a0d0e21e 2582keyword with a left parenthesis unless you want the corresponding right
7660c0ab 2583parenthesis to terminate the arguments to the print--interpose a C<+> or
5f05dabc 2584put parentheses around all the arguments.
a0d0e21e 2585
4633a7c4 2586Note that if you're storing FILEHANDLES in an array or other expression,
da0045b7 2587you will have to use a block returning its value instead:
4633a7c4
LW
2588
2589 print { $files[$i] } "stuff\n";
2590 print { $OK ? STDOUT : STDERR } "stuff\n";
2591
5f05dabc 2592=item printf FILEHANDLE FORMAT, LIST
a0d0e21e 2593
5f05dabc 2594=item printf FORMAT, LIST
a0d0e21e 2595
7660c0ab 2596Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
a3cb178b 2597(the output record separator) is not appended. The first argument
ab4f32c2 2598of the list will be interpreted as the C<printf()> format. If C<use locale> is
a034a98d
DD
2599in effect, the character used for the decimal point in formatted real numbers
2600is affected by the LC_NUMERIC locale. See L<perllocale>.
a0d0e21e 2601
7660c0ab
A
2602Don't fall into the trap of using a C<printf()> when a simple
2603C<print()> would do. The C<print()> is more efficient and less
28757baa
PP
2604error prone.
2605
da0045b7
PP
2606=item prototype FUNCTION
2607
2608Returns the prototype of a function as a string (or C<undef> if the
5f05dabc
PP
2609function has no prototype). FUNCTION is a reference to, or the name of,
2610the function whose prototype you want to retrieve.
da0045b7 2611
b6c543e3
IZ
2612If FUNCTION is a string starting with C<CORE::>, the rest is taken as
2613a name for Perl builtin. If builtin is not I<overridable> (such as
ab4f32c2
A
2614C<qw//>) or its arguments cannot be expressed by a prototype (such as
2615C<system()>) - in other words, the builtin does not behave like a Perl
b6c543e3
IZ
2616function - returns C<undef>. Otherwise, the string describing the
2617equivalent prototype is returned.
2618
a0d0e21e
LW
2619=item push ARRAY,LIST
2620
2621Treats ARRAY as a stack, and pushes the values of LIST
2622onto the end of ARRAY. The length of ARRAY increases by the length of
2623LIST. Has the same effect as
2624
2625 for $value (LIST) {
2626 $ARRAY[++$#ARRAY] = $value;
2627 }
2628
2629but is more efficient. Returns the new number of elements in the array.
2630
2631=item q/STRING/
2632
2633=item qq/STRING/
2634
8782bef2
GB
2635=item qr/STRING/
2636
a0d0e21e
LW
2637=item qx/STRING/
2638
2639=item qw/STRING/
2640
2641Generalized quotes. See L<perlop>.
2642
2643=item quotemeta EXPR
2644
54310121 2645=item quotemeta
bbce6d69 2646
68dc0745 2647Returns the value of EXPR with all non-alphanumeric
a034a98d
DD
2648characters backslashed. (That is, all characters not matching
2649C</[A-Za-z_0-9]/> will be preceded by a backslash in the
2650returned string, regardless of any locale settings.)
2651This is the internal function implementing
7660c0ab 2652the C<\Q> escape in double-quoted strings.
a0d0e21e 2653
7660c0ab 2654If EXPR is omitted, uses C<$_>.
bbce6d69 2655
a0d0e21e
LW
2656=item rand EXPR
2657
2658=item rand
2659
7660c0ab 2660Returns a random fractional number greater than or equal to C<0> and less
3e3baf6d 2661than the value of EXPR. (EXPR should be positive.) If EXPR is
7660c0ab
A
2662omitted, the value C<1> is used. Automatically calls C<srand()> unless
2663C<srand()> has already been called. See also C<srand()>.
a0d0e21e 2664
2f9daede 2665(Note: If your rand function consistently returns numbers that are too
a0d0e21e 2666large or too small, then your version of Perl was probably compiled
2f9daede 2667with the wrong number of RANDBITS.)
a0d0e21e
LW
2668
2669=item read FILEHANDLE,SCALAR,LENGTH,OFFSET
2670
2671=item read FILEHANDLE,SCALAR,LENGTH
2672
2673Attempts to read LENGTH bytes of data into variable SCALAR from the
3b02c43c
GS
2674specified FILEHANDLE. Returns the number of bytes actually read,
2675C<0> at end of file, or undef if there was an error. SCALAR will be grown
2676or shrunk to the length actually read. An OFFSET may be specified to
2677place the read data at some other place than the beginning of the
f86cebdf
GS
2678string. This call is actually implemented in terms of stdio's fread(3)
2679call. To get a true read(2) system call, see C<sysread()>.
a0d0e21e
LW
2680
2681=item readdir DIRHANDLE
2682
7660c0ab 2683Returns the next directory entry for a directory opened by C<opendir()>.
5a964f20 2684If used in list context, returns all the rest of the entries in the
a0d0e21e 2685directory. If there are no more entries, returns an undefined value in
5a964f20 2686scalar context or a null list in list context.
a0d0e21e 2687
7660c0ab 2688If you're planning to filetest the return values out of a C<readdir()>, you'd
5f05dabc 2689better prepend the directory in question. Otherwise, because we didn't
7660c0ab 2690C<chdir()> there, it would have been testing the wrong file.
cb1a09d0
AD
2691
2692 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
2693 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
2694 closedir DIR;
2695
84902520
TB
2696=item readline EXPR
2697
5a964f20 2698Reads from the filehandle whose typeglob is contained in EXPR. In scalar context, a single line
84902520
TB
2699is read and returned. In list context, reads until end-of-file is
2700reached and returns a list of lines (however you've defined lines
7660c0ab 2701with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
84902520
TB
2702This is the internal function implementing the C<E<lt>EXPRE<gt>>
2703operator, but you can use it directly. The C<E<lt>EXPRE<gt>>
2704operator is discussed in more detail in L<perlop/"I/O Operators">.
2705
5a964f20
TC
2706 $line = <STDIN>;
2707 $line = readline(*STDIN); # same thing
2708
a0d0e21e
LW
2709=item readlink EXPR
2710
54310121 2711=item readlink
bbce6d69 2712
a0d0e21e
LW
2713Returns the value of a symbolic link, if symbolic links are
2714implemented. If not, gives a fatal error. If there is some system
184e9718 2715error, returns the undefined value and sets C<$!> (errno). If EXPR is
7660c0ab 2716omitted, uses C<$_>.
a0d0e21e 2717
84902520
TB
2718=item readpipe EXPR
2719
5a964f20 2720EXPR is executed as a system command.
84902520
TB
2721The collected standard output of the command is returned.
2722In scalar context, it comes back as a single (potentially
2723multi-line) string. In list context, returns a list of lines
7660c0ab 2724(however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
84902520
TB
2725This is the internal function implementing the C<qx/EXPR/>
2726operator, but you can use it directly. The C<qx/EXPR/>
2727operator is discussed in more detail in L<perlop/"I/O Operators">.
2728
a0d0e21e
LW
2729=item recv SOCKET,SCALAR,LEN,FLAGS
2730
2731Receives a message on a socket. Attempts to receive LENGTH bytes of
2732data into variable SCALAR from the specified SOCKET filehandle.
7660c0ab 2733Actually does a C C<recvfrom()>, so that it can return the address of the
a0d0e21e
LW
2734sender. Returns the undefined value if there's an error. SCALAR will
2735be grown or shrunk to the length actually read. Takes the same flags
54310121 2736as the system call of the same name.
4633a7c4 2737See L<perlipc/"UDP: Message Passing"> for examples.
a0d0e21e
LW
2738
2739=item redo LABEL
2740
2741=item redo
2742
2743The C<redo> command restarts the loop block without evaluating the
2744conditional again. The C<continue> block, if any, is not executed. If
2745the LABEL is omitted, the command refers to the innermost enclosing
2746loop. This command is normally used by programs that want to lie to
2747themselves about what was just input:
2748
2749 # a simpleminded Pascal comment stripper
2750 # (warning: assumes no { or } in strings)
4633a7c4 2751 LINE: while (<STDIN>) {
a0d0e21e
LW
2752 while (s|({.*}.*){.*}|$1 |) {}
2753 s|{.*}| |;
2754 if (s|{.*| |) {
2755 $front = $_;
2756 while (<STDIN>) {
2757 if (/}/) { # end of comment?
5a964f20 2758 s|^|$front\{|;
4633a7c4 2759 redo LINE;
a0d0e21e
LW
2760 }
2761 }
2762 }
2763 print;
2764 }
2765
1d2dff63
GS
2766See also L</continue> for an illustration of how C<last>, C<next>, and
2767C<redo> work.
2768
a0d0e21e
LW
2769=item ref EXPR
2770
54310121 2771=item ref
bbce6d69 2772
2f9daede 2773Returns a TRUE value if EXPR is a reference, FALSE otherwise. If EXPR
7660c0ab 2774is not specified, C<$_> will be used. The value returned depends on the
bbce6d69 2775type of thing the reference is a reference to.
a0d0e21e
LW
2776Builtin types include:
2777
2778 REF
2779 SCALAR
2780 ARRAY
2781 HASH
2782 CODE
2783 GLOB
2784
54310121 2785If the referenced object has been blessed into a package, then that package
7660c0ab 2786name is returned instead. You can think of C<ref()> as a C<typeof()> operator.
a0d0e21e
LW
2787
2788 if (ref($r) eq "HASH") {
aa689395 2789 print "r is a reference to a hash.\n";
54310121 2790 }
5a964f20 2791 if (!ref($r)) {
a0d0e21e 2792 print "r is not a reference at all.\n";
54310121 2793 }
a0d0e21e
LW
2794
2795See also L<perlref>.
2796
2797=item rename OLDNAME,NEWNAME
2798
7660c0ab 2799Changes the name of a file. Returns C<1> for success, C<0> otherwise. Will
5f05dabc 2800not work across file system boundaries.
a0d0e21e
LW
2801
2802=item require EXPR
2803
2804=item require
2805
7660c0ab 2806Demands some semantics specified by EXPR, or by C<$_> if EXPR is not
a0d0e21e 2807supplied. If EXPR is numeric, demands that the current version of Perl
184e9718 2808(C<$]> or $PERL_VERSION) be equal or greater than EXPR.
a0d0e21e
LW
2809
2810Otherwise, demands that a library file be included if it hasn't already
2811been included. The file is included via the do-FILE mechanism, which is
7660c0ab 2812essentially just a variety of C<eval()>. Has semantics similar to the following
a0d0e21e
LW
2813subroutine:
2814
2815 sub require {
5a964f20 2816 my($filename) = @_;
a0d0e21e 2817 return 1 if $INC{$filename};
5a964f20 2818 my($realfilename,$result);
a0d0e21e
LW
2819 ITER: {
2820 foreach $prefix (@INC) {
2821 $realfilename = "$prefix/$filename";
2822 if (-f $realfilename) {
2823 $result = do $realfilename;
2824 last ITER;
2825 }
2826 }
2827 die "Can't find $filename in \@INC";
2828 }
2829 die $@ if $@;
2830 die "$filename did not return true value" unless $result;
2831 $INC{$filename} = $realfilename;
5a964f20 2832 return $result;
a0d0e21e
LW
2833 }
2834
2835Note that the file will not be included twice under the same specified
2836name. The file must return TRUE as the last statement to indicate
2837successful execution of any initialization code, so it's customary to
7660c0ab 2838end such a file with "C<1;>" unless you're sure it'll return TRUE
a0d0e21e
LW
2839otherwise. But it's better just to put the "C<1;>", in case you add more
2840statements.
2841
54310121 2842If EXPR is a bareword, the require assumes a "F<.pm>" extension and
da0045b7 2843replaces "F<::>" with "F</>" in the filename for you,
54310121 2844to make it easy to load standard modules. This form of loading of
a0d0e21e
LW
2845modules does not risk altering your namespace.
2846
ee580363
GS
2847In other words, if you try this:
2848
f86cebdf 2849 require Foo::Bar; # a splendid bareword
ee580363 2850
7660c0ab
A
2851The require function will actually look for the "F<Foo/Bar.pm>" file in the
2852directories specified in the C<@INC> array.
ee580363 2853
5a964f20 2854But if you try this:
ee580363
GS
2855
2856 $class = 'Foo::Bar';
f86cebdf 2857 require $class; # $class is not a bareword
5a964f20 2858 #or
f86cebdf 2859 require "Foo::Bar"; # not a bareword because of the ""
ee580363 2860
7660c0ab
A
2861The require function will look for the "F<Foo::Bar>" file in the @INC array and
2862will complain about not finding "F<Foo::Bar>" there. In this case you can do:
ee580363
GS
2863
2864 eval "require $class";
2865
2866For a yet-more-powerful import facility, see L</use> and L<perlmod>.
a0d0e21e
LW
2867
2868=item reset EXPR
2869
2870=item reset
2871
2872Generally used in a C<continue> block at the end of a loop to clear
7660c0ab 2873variables and reset C<??> searches so that they work again. The
a0d0e21e
LW
2874expression is interpreted as a list of single characters (hyphens
2875allowed for ranges). All variables and arrays beginning with one of
2876those letters are reset to their pristine state. If the expression is
7660c0ab 2877omitted, one-match searches (C<?pattern?>) are reset to match again. Resets
5f05dabc 2878only variables or searches in the current package. Always returns
a0d0e21e
LW
28791. Examples:
2880
2881 reset 'X'; # reset all X variables
2882 reset 'a-z'; # reset lower case variables
2883 reset; # just reset ?? searches
2884
7660c0ab
A
2885Resetting C<"A-Z"> is not recommended because you'll wipe out your
2886C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package variables--lexical variables
a0d0e21e 2887are unaffected, but they clean themselves up on scope exit anyway,
da0045b7 2888so you'll probably want to use them instead. See L</my>.
a0d0e21e 2889
54310121
PP
2890=item return EXPR
2891
2892=item return
2893
7660c0ab 2894Returns from a subroutine, C<eval()>, or C<do FILE> with the value
5a964f20 2895given in EXPR. Evaluation of EXPR may be in list, scalar, or void
54310121 2896context, depending on how the return value will be used, and the context
7660c0ab 2897may vary from one execution to the next (see C<wantarray()>). If no EXPR
5a964f20
TC
2898is given, returns an empty list in list context, an undefined value in
2899scalar context, or nothing in a void context.
a0d0e21e 2900
68dc0745
PP
2901(Note that in the absence of a return, a subroutine, eval, or do FILE
2902will automatically return the value of the last expression evaluated.)
a0d0e21e
LW
2903
2904=item reverse LIST
2905
5a964f20
TC
2906In list context, returns a list value consisting of the elements
2907of LIST in the opposite order. In scalar context, concatenates the
2f9daede
TPG
2908elements of LIST, and returns a string value consisting of those bytes,
2909but in the opposite order.
4633a7c4 2910
2f9daede 2911 print reverse <>; # line tac, last line first
4633a7c4 2912
2f9daede
TPG
2913 undef $/; # for efficiency of <>
2914 print scalar reverse <>; # byte tac, last line tsrif
2915
2916This operator is also handy for inverting a hash, although there are some
2917caveats. If a value is duplicated in the original hash, only one of those
2918can be represented as a key in the inverted hash. Also, this has to
2919unwind one hash and build a whole new one, which may take some time
2920on a large hash.
2921
2922 %by_name = reverse %by_address; # Invert the hash
a0d0e21e
LW
2923
2924=item rewinddir DIRHANDLE
2925
2926Sets the current position to the beginning of the directory for the
7660c0ab 2927C<readdir()> routine on DIRHANDLE.
a0d0e21e
LW
2928
2929=item rindex STR,SUBSTR,POSITION
2930
2931=item rindex STR,SUBSTR
2932
2933Works just like index except that it returns the position of the LAST
2934occurrence of SUBSTR in STR. If POSITION is specified, returns the
2935last occurrence at or before that position.
2936
2937=item rmdir FILENAME
2938
54310121 2939=item rmdir
bbce6d69 2940
5a964f20
TC
2941Deletes the directory specified by FILENAME if that directory is empty. If it
2942succeeds it returns TRUE, otherwise it returns FALSE and sets C<$!> (errno). If
7660c0ab 2943FILENAME is omitted, uses C<$_>.
a0d0e21e
LW
2944
2945=item s///
2946
2947The substitution operator. See L<perlop>.
2948
2949=item scalar EXPR
2950
5a964f20 2951Forces EXPR to be interpreted in scalar context and returns the value
54310121 2952of EXPR.
cb1a09d0
AD
2953
2954 @counts = ( scalar @a, scalar @b, scalar @c );
2955
54310121 2956There is no equivalent operator to force an expression to
5a964f20 2957be interpolated in list context because it's in practice never
cb1a09d0
AD
2958needed. If you really wanted to do so, however, you could use
2959the construction C<@{[ (some expression) ]}>, but usually a simple
2960C<(some expression)> suffices.
a0d0e21e
LW
2961
2962=item seek FILEHANDLE,POSITION,WHENCE
2963
ab4f32c2 2964Sets FILEHANDLE's position, just like the C<fseek()> call of C<stdio()>.
8903cb82 2965FILEHANDLE may be an expression whose value gives the name of the
7660c0ab
A
2966filehandle. The values for WHENCE are C<0> to set the new position to
2967POSITION, C<1> to set it to the current position plus POSITION, and C<2> to
8903cb82 2968set it to EOF plus POSITION (typically negative). For WHENCE you may
7660c0ab
A
2969use the constants C<SEEK_SET>, C<SEEK_CUR>, and C<SEEK_END> from either the
2970C<IO::Seekable> or the POSIX module. Returns C<1> upon success, C<0> otherwise.
8903cb82 2971
7660c0ab
A
2972If you want to position file for C<sysread()> or C<syswrite()>, don't use
2973C<seek()> -- buffering makes its effect on the file's system position
2974unpredictable and non-portable. Use C<sysseek()> instead.
a0d0e21e 2975
cb1a09d0
AD
2976On some systems you have to do a seek whenever you switch between reading
2977and writing. Amongst other things, this may have the effect of calling
f86cebdf 2978stdio's clearerr(3). A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving
8903cb82 2979the file position:
cb1a09d0
AD
2980
2981 seek(TEST,0,1);
2982
2983This is also useful for applications emulating C<tail -f>. Once you hit
2984EOF on your read, and then sleep for a while, you might have to stick in a
7660c0ab 2985seek() to reset things. The C<seek()> doesn't change the current position,
8903cb82
PP
2986but it I<does> clear the end-of-file condition on the handle, so that the
2987next C<E<lt>FILEE<gt>> makes Perl try again to read something. We hope.
cb1a09d0
AD
2988
2989If that doesn't work (some stdios are particularly cantankerous), then
2990you may need something more like this:
2991
2992 for (;;) {
f86cebdf
GS
2993 for ($curpos = tell(FILE); $_ = <FILE>;
2994 $curpos = tell(FILE)) {
cb1a09d0
AD
2995 # search for some stuff and put it into files
2996 }
2997 sleep($for_a_while);
2998 seek(FILE, $curpos, 0);
2999 }
3000
a0d0e21e
LW
3001=item seekdir DIRHANDLE,POS
3002
7660c0ab
A
3003Sets the current position for the C<readdir()> routine on DIRHANDLE. POS
3004must be a value returned by C<telldir()>. Has the same caveats about
a0d0e21e
LW
3005possible directory compaction as the corresponding system library
3006routine.
3007
3008=item select FILEHANDLE
3009
3010=item select
3011
3012Returns the currently selected filehandle. Sets the current default
3013filehandle for output, if FILEHANDLE is supplied. This has two
ab4f32c2 3014effects: first, a C<write()> or a C<print()> without a filehandle will
a0d0e21e
LW
3015default to this FILEHANDLE. Second, references to variables related to
3016output will refer to this output channel. For example, if you have to
3017set the top of form format for more than one output channel, you might
3018do the following:
3019
3020 select(REPORT1);
3021 $^ = 'report1_top';
3022 select(REPORT2);
3023 $^ = 'report2_top';
3024
3025FILEHANDLE may be an expression whose value gives the name of the
3026actual filehandle. Thus:
3027
3028 $oldfh = select(STDERR); $| = 1; select($oldfh);
3029
4633a7c4
LW
3030Some programmers may prefer to think of filehandles as objects with
3031methods, preferring to write the last example as:
a0d0e21e 3032
28757baa 3033 use IO::Handle;
a0d0e21e
LW
3034 STDERR->autoflush(1);
3035
3036=item select RBITS,WBITS,EBITS,TIMEOUT
3037
f86cebdf 3038This calls the select(2) system call with the bit masks specified, which
7660c0ab 3039can be constructed using C<fileno()> and C<vec()>, along these lines:
a0d0e21e
LW
3040
3041 $rin = $win = $ein = '';
3042 vec($rin,fileno(STDIN),1) = 1;
3043 vec($win,fileno(STDOUT),1) = 1;
3044 $ein = $rin | $win;
3045
3046If you want to select on many filehandles you might wish to write a
3047subroutine:
3048
3049 sub fhbits {
5a964f20
TC
3050 my(@fhlist) = split(' ',$_[0]);
3051 my($bits);
a0d0e21e
LW
3052 for (@fhlist) {
3053 vec($bits,fileno($_),1) = 1;
3054 }
3055 $bits;
3056 }
4633a7c4 3057 $rin = fhbits('STDIN TTY SOCK');
a0d0e21e
LW
3058
3059The usual idiom is:
3060
3061 ($nfound,$timeleft) =
3062 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
3063
54310121 3064or to block until something becomes ready just do this
a0d0e21e
LW
3065
3066 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
3067
7660c0ab
A
3068Most systems do not bother to return anything useful in C<$timeleft>, so
3069calling select() in scalar context just returns C<$nfound>.
c07a80fd 3070
5f05dabc 3071Any of the bit masks can also be undef. The timeout, if specified, is
a0d0e21e 3072in seconds, which may be fractional. Note: not all implementations are
7660c0ab
A
3073capable of returning theC<$timeleft>. If not, they always return
3074C<$timeleft> equal to the supplied C<$timeout>.
a0d0e21e 3075
ff68c719 3076You can effect a sleep of 250 milliseconds this way:
a0d0e21e
LW
3077
3078 select(undef, undef, undef, 0.25);
3079
7660c0ab
A
3080B<WARNING>: One should not attempt to mix buffered I/O (like C<read()>
3081or E<lt>FHE<gt>) with C<select()>, except as permitted by POSIX, and even
3082then only on POSIX systems. You have to use C<sysread()> instead.
a0d0e21e
LW
3083
3084=item semctl ID,SEMNUM,CMD,ARG
3085
ab4f32c2 3086Calls the System V IPC function C<semctl()>. You'll probably have to say
0ade1984
JH
3087
3088 use IPC::SysV;
3089
3090first to get the correct constant definitions. If CMD is IPC_STAT or
3091GETALL, then ARG must be a variable which will hold the returned
ab4f32c2 3092semid_ds structure or semaphore value array. Returns like C<ioctl()>: the
7660c0ab
A
3093undefined value for error, "C<0> but true" for zero, or the actual return
3094value otherwise. See also C<IPC::SysV> and C<IPC::Semaphore> documentation.
a0d0e21e
LW
3095
3096=item semget KEY,NSEMS,FLAGS
3097
3098Calls the System V IPC function semget. Returns the semaphore id, or
7660c0ab
A
3099the undefined value if there is an error. See also C<IPC::SysV> and
3100C<IPC::SysV::Semaphore> documentation.
a0d0e21e
LW
3101
3102=item semop KEY,OPSTRING
3103
3104Calls the System V IPC function semop to perform semaphore operations
3105such as signaling and waiting. OPSTRING must be a packed array of
3106semop structures. Each semop structure can be generated with
3107C<pack("sss", $semnum, $semop, $semflag)>. The number of semaphore
3108operations is implied by the length of OPSTRING. Returns TRUE if
3109successful, or FALSE if there is an error. As an example, the
7660c0ab 3110following code waits on semaphore C<$semnum> of semaphore id C<$semid>:
a0d0e21e
LW
3111
3112 $semop = pack("sss", $semnum, -1, 0);
3113 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
3114
7660c0ab
A
3115To signal the semaphore, replace C<-1> with C<1>. See also C<IPC::SysV>
3116and C<IPC::SysV::Semaphore> documentation.
a0d0e21e
LW
3117
3118=item send SOCKET,MSG,FLAGS,TO
3119
3120=item send SOCKET,MSG,FLAGS
3121
3122Sends a message on a socket. Takes the same flags as the system call
3123of the same name. On unconnected sockets you must specify a
7660c0ab 3124destination to send TO, in which case it does a C C<sendto()>. Returns
a0d0e21e
LW
3125the number of characters sent, or the undefined value if there is an
3126error.
4633a7c4 3127See L<perlipc/"UDP: Message Passing"> for examples.
a0d0e21e
LW
3128
3129=item setpgrp PID,PGRP
3130
7660c0ab 3131Sets the current process group for the specified PID, C<0> for the current
a0d0e21e 3132process. Will produce a fatal error if used on a machine that doesn't
f86cebdf 3133implement setpgrp(2). If the arguments are omitted, it defaults to
7660c0ab
A
3134C<0,0>. Note that the POSIX version of C<setpgrp()> does not accept any
3135arguments, so only setpgrp C<0,0> is portable.
a0d0e21e
LW
3136
3137=item setpriority WHICH,WHO,PRIORITY
3138
3139Sets the current priority for a process, a process group, or a user.
f86cebdf
GS
3140(See setpriority(2).) Will produce a fatal error if used on a machine
3141that doesn't implement setpriority(2).
a0d0e21e
LW
3142
3143=item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
3144
3145Sets the socket option requested. Returns undefined if there is an
7660c0ab 3146error. OPTVAL may be specified as C<undef> if you don't want to pass an
a0d0e21e
LW
3147argument.
3148
3149=item shift ARRAY
3150
3151=item shift
3152
3153Shifts the first value of the array off and returns it, shortening the
3154array by 1 and moving everything down. If there are no elements in the
3155array, returns the undefined value. If ARRAY is omitted, shifts the
7660c0ab
A
3156C<@_> array within the lexical scope of subroutines and formats, and the
3157C<@ARGV> array at file scopes or within the lexical scopes established by
977336f5 3158the C<eval ''>, C<BEGIN {}>, C<END {}>, and C<INIT {}> constructs.
7660c0ab
A
3159See also C<unshift()>, C<push()>, and C<pop()>. C<Shift()> and C<unshift()> do the
3160same thing to the left end of an array that C<pop()> and C<push()> do to the
977336f5 3161right end.
a0d0e21e
LW
3162
3163=item shmctl ID,CMD,ARG
3164
0ade1984
JH
3165Calls the System V IPC function shmctl. You'll probably have to say
3166
3167 use IPC::SysV;
3168
7660c0ab
A
3169first to get the correct constant definitions. If CMD is C<IPC_STAT>,
3170then ARG must be a variable which will hold the returned C<shmid_ds>
3171structure. Returns like ioctl: the undefined value for error, "C<0> but
0ade1984 3172true" for zero, or the actual return value otherwise.
7660c0ab 3173See also C<IPC::SysV> documentation.
a0d0e21e
LW
3174
3175=item shmget KEY,SIZE,FLAGS
3176
3177Calls the System V IPC function shmget. Returns the shared memory
3178segment id, or the undefined value if there is an error.
7660c0ab 3179See also C<IPC::SysV> documentation.
a0d0e21e
LW
3180
3181=item shmread ID,VAR,POS,SIZE
3182
3183=item shmwrite ID,STRING,POS,SIZE
3184
3185Reads or writes the System V shared memory segment ID starting at
3186position POS for size SIZE by attaching to it, copying in/out, and
5a964f20 3187detaching from it. When reading, VAR must be a variable that will
a0d0e21e
LW
3188hold the data read. When writing, if STRING is too long, only SIZE
3189bytes are used; if STRING is too short, nulls are written to fill out
3190SIZE bytes. Return TRUE if successful, or FALSE if there is an error.
7660c0ab 3191See also C<IPC::SysV> documentation.
a0d0e21e
LW
3192
3193=item shutdown SOCKET,HOW
3194
3195Shuts down a socket connection in the manner indicated by HOW, which
3196has the same interpretation as in the system call of the same name.
3197
f86cebdf
GS
3198 shutdown(SOCKET, 0); # I/we have stopped reading data
3199 shutdown(SOCKET, 1); # I/we have stopped writing data
3200 shutdown(SOCKET, 2); # I/we have stopped using this socket
5a964f20
TC
3201
3202This is useful with sockets when you want to tell the other
3203side you're done writing but not done reading, or vice versa.
3204It's also a more insistent form of close because it also
3205disables the filedescriptor in any forked copies in other
3206processes.
3207
a0d0e21e
LW
3208=item sin EXPR
3209
54310121 3210=item sin
bbce6d69 3211
a0d0e21e 3212Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
7660c0ab 3213returns sine of C<$_>.
a0d0e21e 3214
7660c0ab 3215For the inverse sine operation, you may use the C<POSIX::asin()>
28757baa
PP
3216function, or use this relation:
3217
3218 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
3219
a0d0e21e
LW
3220=item sleep EXPR
3221
3222=item sleep
3223
3224Causes the script to sleep for EXPR seconds, or forever if no EXPR.
7660c0ab 3225May be interrupted if the process receives a signal such as C<SIGALRM>.
1d3434b8 3226Returns the number of seconds actually slept. You probably cannot
7660c0ab
A
3227mix C<alarm()> and C<sleep()> calls, because C<sleep()> is often implemented
3228using C<alarm()>.
a0d0e21e
LW
3229
3230On some older systems, it may sleep up to a full second less than what
3231you requested, depending on how it counts seconds. Most modern systems
5a964f20
TC
3232always sleep the full amount. They may appear to sleep longer than that,
3233however, because your process might not be scheduled right away in a
3234busy multitasking system.
a0d0e21e 3235
cb1a09d0 3236For delays of finer granularity than one second, you may use Perl's
f86cebdf 3237C<syscall()> interface to access setitimer(2) if your system supports it,
7b8d334a 3238or else see L</select()> above.
cb1a09d0 3239
7660c0ab 3240See also the POSIX module's C<sigpause()> function.
5f05dabc 3241
a0d0e21e
LW
3242=item socket SOCKET,DOMAIN,TYPE,PROTOCOL
3243
3244Opens a socket of the specified kind and attaches it to filehandle
5f05dabc 3245SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for the
7660c0ab 3246system call of the same name. You should "C<use Socket;>" first to get
4633a7c4 3247the proper definitions imported. See the example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
3248
3249=item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
3250
3251Creates an unnamed pair of sockets in the specified domain, of the
5f05dabc 3252specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
a0d0e21e
LW
3253for the system call of the same name. If unimplemented, yields a fatal
3254error. Returns TRUE if successful.
3255
ab4f32c2 3256Some systems defined C<pipe()> in terms of C<socketpair()>, in which a call
5a964f20
TC
3257to C<pipe(Rdr, Wtr)> is essentially:
3258
3259 use Socket;
3260 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
3261 shutdown(Rdr, 1); # no more writing for reader
3262 shutdown(Wtr, 0); # no more reading for writer
3263
3264See L<perlipc> for an example of socketpair use.
3265
a0d0e21e
LW
3266=item sort SUBNAME LIST
3267
3268=item sort BLOCK LIST
3269
3270=item sort LIST
3271
2f9daede 3272Sorts the LIST and returns the sorted list value. If SUBNAME or BLOCK
ab4f32c2 3273is omitted, C<sort()>s in standard string comparison order. If SUBNAME is
2f9daede 3274specified, it gives the name of a subroutine that returns an integer
7660c0ab 3275less than, equal to, or greater than C<0>, depending on how the elements
2f9daede
TPG
3276of the array are to be ordered. (The C<E<lt>=E<gt>> and C<cmp>
3277operators are extremely useful in such routines.) SUBNAME may be a
1d3434b8
GS
3278scalar variable name (unsubscripted), in which case the value provides
3279the name of (or a reference to) the actual subroutine to use. In place
3280of a SUBNAME, you can provide a BLOCK as an anonymous, in-line sort
3281subroutine.
a0d0e21e 3282
cb1a09d0
AD
3283In the interests of efficiency the normal calling code for subroutines is
3284bypassed, with the following effects: the subroutine may not be a
3285recursive subroutine, and the two elements to be compared are passed into
7660c0ab
A
3286the subroutine not via C<@_> but as the package global variables C<$a> and
3287C<$b> (see example below). They are passed by reference, so don't
3288modify C<$a> and C<$b>. And don't try to declare them as lexicals either.
a0d0e21e 3289
0a753a76 3290You also cannot exit out of the sort block or subroutine using any of the
7660c0ab 3291loop control operators described in L<perlsyn> or with C<goto()>.