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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
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17be first, and the list argument will follow. (Note that there can ever
18be only one list argument.) For instance, splice() has three scalar
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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
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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
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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.
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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
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52returning the undefined value, and in a list context by returning the
53null list.
54
55Remember the following rule:
56
cb1a09d0 57=over 8
a0d0e21e 58
8ebc5c01 59=item I<THERE IS NO GENERAL RULE FOR CONVERTING A LIST INTO A SCALAR!>
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60
61=back
62
63Each operator and function decides which sort of value it would be most
64appropriate to return in a scalar context. Some operators return the
65length of the list that would have been returned in a list context. Some
66operators return the first value in the list. Some operators return the
67last value in the list. Some operators return a count of successful
68operations. In general, they do what you want, unless you want
69consistency.
70
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71=head2 Perl Functions by Category
72
73Here are Perl's functions (including things that look like
74functions, like some of the keywords and named operators)
75arranged by category. Some functions appear in more
76than one place.
77
78=over
79
80=item Functions for SCALARs or strings
81
82chomp, chop, chr, crypt, hex, index, lc, lcfirst, length,
83oct, ord, pack, q/STRING/, qq/STRING/, reverse, rindex,
84sprintf, substr, tr///, uc, ucfirst, y///
85
86=item Regular expressions and pattern matching
87
88m//, pos, quotemeta, s///, split, study
89
90=item Numeric functions
91
92abs, atan2, cos, exp, hex, int, log, oct, rand, sin, sqrt,
93srand
94
95=item Functions for real @ARRAYs
96
97pop, push, shift, splice, unshift
98
99=item Functions for list data
100
101grep, join, map, qw/STRING/, reverse, sort, unpack
102
103=item Functions for real %HASHes
104
105delete, each, exists, keys, values
106
107=item Input and output functions
108
109binmode, close, closedir, dbmclose, dbmopen, die, eof,
110fileno, flock, format, getc, print, printf, read, readdir,
96e4d5b1 111rewinddir, seek, seekdir, select, syscall, sysread, sysseek,
8903cb82 112syswrite, tell, telldir, truncate, warn, write
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113
114=item Functions for fixed length data or records
115
116pack, read, syscall, sysread, syswrite, unpack, vec
117
118=item Functions for filehandles, files, or directories
119
da0045b7 120I<-X>, chdir, chmod, chown, chroot, fcntl, glob, ioctl, link,
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121lstat, mkdir, open, opendir, readlink, rename, rmdir,
122stat, symlink, umask, unlink, utime
123
124=item Keywords related to the control flow of your perl program
125
126caller, continue, die, do, dump, eval, exit, goto, last,
127next, redo, return, sub, wantarray
128
54310121 129=item Keywords related to scoping
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130
131caller, import, local, my, package, use
132
133=item Miscellaneous functions
134
135defined, dump, eval, formline, local, my, reset, scalar,
136undef, wantarray
137
138=item Functions for processes and process groups
139
140alarm, exec, fork, getpgrp, getppid, getpriority, kill,
141pipe, qx/STRING/, setpgrp, setpriority, sleep, system,
142times, wait, waitpid
143
144=item Keywords related to perl modules
145
146do, import, no, package, require, use
147
148=item Keywords related to classes and object-orientedness
149
f3cbc334 150bless, dbmclose, dbmopen, package, ref, tie, tied, untie, use
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151
152=item Low-level socket functions
153
154accept, bind, connect, getpeername, getsockname,
155getsockopt, listen, recv, send, setsockopt, shutdown,
156socket, socketpair
157
158=item System V interprocess communication functions
159
160msgctl, msgget, msgrcv, msgsnd, semctl, semget, semop,
161shmctl, shmget, shmread, shmwrite
162
163=item Fetching user and group info
164
165endgrent, endhostent, endnetent, endpwent, getgrent,
166getgrgid, getgrnam, getlogin, getpwent, getpwnam,
167getpwuid, setgrent, setpwent
168
169=item Fetching network info
170
171endprotoent, endservent, gethostbyaddr, gethostbyname,
172gethostent, getnetbyaddr, getnetbyname, getnetent,
173getprotobyname, getprotobynumber, getprotoent,
174getservbyname, getservbyport, getservent, sethostent,
175setnetent, setprotoent, setservent
176
177=item Time-related functions
178
179gmtime, localtime, time, times
180
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181=item Functions new in perl5
182
183abs, bless, chomp, chr, exists, formline, glob, import, lc,
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184lcfirst, map, my, no, prototype, qx, qw, readline, readpipe,
185ref, sub*, sysopen, tie, tied, uc, ucfirst, untie, use
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186
187* - C<sub> was a keyword in perl4, but in perl5 it is an
188operator which can be used in expressions.
189
190=item Functions obsoleted in perl5
191
192dbmclose, dbmopen
193
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194=back
195
196=head2 Alphabetical Listing of Perl Functions
197
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198=over 8
199
200=item -X FILEHANDLE
201
202=item -X EXPR
203
204=item -X
205
206A file test, where X is one of the letters listed below. This unary
207operator takes one argument, either a filename or a filehandle, and
208tests the associated file to see if something is true about it. If the
209argument is omitted, tests $_, except for C<-t>, which tests STDIN.
210Unless otherwise documented, it returns C<1> for TRUE and C<''> for FALSE, or
211the undefined value if the file doesn't exist. Despite the funny
212names, precedence is the same as any other named unary operator, and
213the argument may be parenthesized like any other unary operator. The
214operator may be any of:
215
216 -r File is readable by effective uid/gid.
217 -w File is writable by effective uid/gid.
218 -x File is executable by effective uid/gid.
219 -o File is owned by effective uid.
220
221 -R File is readable by real uid/gid.
222 -W File is writable by real uid/gid.
223 -X File is executable by real uid/gid.
224 -O File is owned by real uid.
225
226 -e File exists.
227 -z File has zero size.
54310121 228 -s File has nonzero size (returns size).
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229
230 -f File is a plain file.
231 -d File is a directory.
232 -l File is a symbolic link.
233 -p File is a named pipe (FIFO).
234 -S File is a socket.
235 -b File is a block special file.
236 -c File is a character special file.
237 -t Filehandle is opened to a tty.
238
239 -u File has setuid bit set.
240 -g File has setgid bit set.
241 -k File has sticky bit set.
242
243 -T File is a text file.
244 -B File is a binary file (opposite of -T).
245
246 -M Age of file in days when script started.
247 -A Same for access time.
248 -C Same for inode change time.
249
250The interpretation of the file permission operators C<-r>, C<-R>, C<-w>,
5f05dabc 251C<-W>, C<-x>, and C<-X> is based solely on the mode of the file and the
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252uids and gids of the user. There may be other reasons you can't actually
253read, write or execute the file. Also note that, for the superuser,
5f05dabc 254C<-r>, C<-R>, C<-w>, and C<-W> always return 1, and C<-x> and C<-X> return
a0d0e21e 2551 if any execute bit is set in the mode. Scripts run by the superuser may
5f05dabc 256thus need to do a stat() to determine the actual mode of the
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257file, or temporarily set the uid to something else.
258
259Example:
260
261 while (<>) {
262 chop;
263 next unless -f $_; # ignore specials
264 ...
265 }
266
267Note that C<-s/a/b/> does not do a negated substitution. Saying
268C<-exp($foo)> still works as expected, however--only single letters
269following a minus are interpreted as file tests.
270
271The C<-T> and C<-B> switches work as follows. The first block or so of the
272file is examined for odd characters such as strange control codes or
184e9718 273characters with the high bit set. If too many odd characters (E<gt>30%)
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274are found, it's a C<-B> file, otherwise it's a C<-T> file. Also, any file
275containing null in the first block is considered a binary file. If C<-T>
276or C<-B> is used on a filehandle, the current stdio buffer is examined
277rather than the first block. Both C<-T> and C<-B> return TRUE on a null
54310121 278file, or a file at EOF when testing a filehandle. Because you have to
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279read a file to do the C<-T> test, on most occasions you want to use a C<-f>
280against the file first, as in C<next unless -f $file && -T $file>.
a0d0e21e 281
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282If any of the file tests (or either the stat() or lstat() operators) are given
283the special filehandle consisting of a solitary underline, then the stat
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284structure of the previous file test (or stat operator) is used, saving
285a system call. (This doesn't work with C<-t>, and you need to remember
286that lstat() and C<-l> will leave values in the stat structure for the
287symbolic link, not the real file.) Example:
288
289 print "Can do.\n" if -r $a || -w _ || -x _;
290
291 stat($filename);
292 print "Readable\n" if -r _;
293 print "Writable\n" if -w _;
294 print "Executable\n" if -x _;
295 print "Setuid\n" if -u _;
296 print "Setgid\n" if -g _;
297 print "Sticky\n" if -k _;
298 print "Text\n" if -T _;
299 print "Binary\n" if -B _;
300
301=item abs VALUE
302
54310121 303=item abs
bbce6d69 304
a0d0e21e 305Returns the absolute value of its argument.
bbce6d69 306If VALUE is omitted, uses $_.
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307
308=item accept NEWSOCKET,GENERICSOCKET
309
310Accepts an incoming socket connect, just as the accept(2) system call
311does. Returns the packed address if it succeeded, FALSE otherwise.
4633a7c4 312See example in L<perlipc/"Sockets: Client/Server Communication">.
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313
314=item alarm SECONDS
315
54310121 316=item alarm
bbce6d69 317
a0d0e21e 318Arranges to have a SIGALRM delivered to this process after the
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319specified number of seconds have elapsed. If SECONDS is not specified,
320the value stored in $_ is used. (On some machines,
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321unfortunately, the elapsed time may be up to one second less than you
322specified because of how seconds are counted.) Only one timer may be
323counting at once. Each call disables the previous timer, and an
324argument of 0 may be supplied to cancel the previous timer without
325starting a new one. The returned value is the amount of time remaining
326on the previous timer.
327
4633a7c4 328For delays of finer granularity than one second, you may use Perl's
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329syscall() interface to access setitimer(2) if your system supports it,
330or else see L</select()>. It is usually a mistake to intermix alarm()
4633a7c4 331and sleep() calls.
a0d0e21e 332
ff68c719 333If you want to use alarm() to time out a system call you need to use an
2f9daede 334eval/die pair. You can't rely on the alarm causing the system call to
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335fail with $! set to EINTR because Perl sets up signal handlers to
336restart system calls on some systems. Using eval/die always works.
337
338 eval {
28757baa 339 local $SIG{ALRM} = sub { die "alarm\n" }; # NB \n required
36477c24 340 alarm $timeout;
ff68c719 341 $nread = sysread SOCKET, $buffer, $size;
36477c24 342 alarm 0;
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343 };
344 die if $@ && $@ ne "alarm\n"; # propagate errors
345 if ($@) {
346 # timed out
347 }
348 else {
349 # didn't
350 }
351
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352=item atan2 Y,X
353
354Returns the arctangent of Y/X in the range -PI to PI.
355
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356For the tangent operation, you may use the POSIX::tan()
357function, or use the familiar relation:
358
359 sub tan { sin($_[0]) / cos($_[0]) }
360
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361=item bind SOCKET,NAME
362
363Binds a network address to a socket, just as the bind system call
364does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
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365packed address of the appropriate type for the socket. See the examples in
366L<perlipc/"Sockets: Client/Server Communication">.
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367
368=item binmode FILEHANDLE
369
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370Arranges for the file to be read or written in "binary" mode in operating
371systems that distinguish between binary and text files. Files that are
372not in binary mode have CR LF sequences translated to LF on input and LF
54310121 373translated to CR LF on output. Binmode has no effect under Unix; in MS-DOS
cb1a09d0 374and similarly archaic systems, it may be imperative--otherwise your
54310121 375MS-DOS-damaged C library may mangle your file. The key distinction between
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376systems that need binmode and those that don't is their text file
377formats. Systems like Unix and Plan9 that delimit lines with a single
378character, and that encode that character in C as '\n', do not need
379C<binmode>. The rest need it. If FILEHANDLE is an expression, the value
380is taken as the name of the filehandle.
a0d0e21e 381
4633a7c4 382=item bless REF,CLASSNAME
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383
384=item bless REF
385
28757baa 386This function tells the thingy referenced by REF that it is now
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387an object in the CLASSNAME package--or the current package if no CLASSNAME
388is specified, which is often the case. It returns the reference for
5f05dabc 389convenience, because a bless() is often the last thing in a constructor.
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390Always use the two-argument version if the function doing the blessing
391might be inherited by a derived class. See L<perlobj> for more about the
392blessing (and blessings) of objects.
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393
394=item caller EXPR
395
396=item caller
397
398Returns the context of the current subroutine call. In a scalar context,
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399returns the caller's package name if there is a caller, that is, if
400we're in a subroutine or eval() or require(), and the undefined value
401otherwise. In a list context, returns
a0d0e21e 402
748a9306 403 ($package, $filename, $line) = caller;
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404
405With EXPR, it returns some extra information that the debugger uses to
406print a stack trace. The value of EXPR indicates how many call frames
407to go back before the current one.
408
54310121 409 ($package, $filename, $line, $subroutine,
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410 $hasargs, $wantarray, $evaltext, $is_require) = caller($i);
411
412Here $subroutine may be C<"(eval)"> if the frame is not a subroutine
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413call, but an C<eval>. In such a case additional elements $evaltext and
414$is_require are set: $is_require is true if the frame is created by a
415C<require> or C<use> statement, $evaltext contains the text of the
416C<eval EXPR> statement. In particular, for a C<eval BLOCK> statement,
417$filename is C<"(eval)">, but $evaltext is undefined. (Note also that
418each C<use> statement creates a C<require> frame inside an C<eval EXPR>)
419frame.
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420
421Furthermore, when called from within the DB package, caller returns more
4633a7c4 422detailed information: it sets the list variable @DB::args to be the
54310121 423arguments with which the subroutine was invoked.
748a9306 424
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425=item chdir EXPR
426
427Changes the working directory to EXPR, if possible. If EXPR is
428omitted, changes to home directory. Returns TRUE upon success, FALSE
429otherwise. See example under die().
430
431=item chmod LIST
432
433Changes the permissions of a list of files. The first element of the
4633a7c4 434list must be the numerical mode, which should probably be an octal
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435number, and which definitely should I<not> a string of octal digits:
436C<0644> is okay, C<'0644'> is not. Returns the number of files
dc848c6f 437successfully changed. See also L</oct>, if all you have is a string.
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438
439 $cnt = chmod 0755, 'foo', 'bar';
440 chmod 0755, @executables;
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441 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to --w----r-T
442 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
443 $mode = 0644; chmod $mode, 'foo'; # this is best
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444
445=item chomp VARIABLE
446
447=item chomp LIST
448
449=item chomp
450
3e3baf6d 451This is a slightly safer version of L</chop>. It removes any
a0d0e21e 452line ending that corresponds to the current value of C<$/> (also known as
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453$INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
454number of characters removed from all its arguments. It's often used to
455remove the newline from the end of an input record when you're worried
456that the final record may be missing its newline. When in paragraph mode
457(C<$/ = "">), it removes all trailing newlines from the string. If
458VARIABLE is omitted, it chomps $_. Example:
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459
460 while (<>) {
461 chomp; # avoid \n on last field
462 @array = split(/:/);
463 ...
464 }
465
466You can actually chomp anything that's an lvalue, including an assignment:
467
468 chomp($cwd = `pwd`);
469 chomp($answer = <STDIN>);
470
471If you chomp a list, each element is chomped, and the total number of
472characters removed is returned.
473
474=item chop VARIABLE
475
476=item chop LIST
477
478=item chop
479
480Chops off the last character of a string and returns the character
481chopped. It's used primarily to remove the newline from the end of an
482input record, but is much more efficient than C<s/\n//> because it neither
483scans nor copies the string. If VARIABLE is omitted, chops $_.
484Example:
485
486 while (<>) {
487 chop; # avoid \n on last field
488 @array = split(/:/);
489 ...
490 }
491
492You can actually chop anything that's an lvalue, including an assignment:
493
494 chop($cwd = `pwd`);
495 chop($answer = <STDIN>);
496
497If you chop a list, each element is chopped. Only the value of the
498last chop is returned.
499
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500Note that chop returns the last character. To return all but the last
501character, use C<substr($string, 0, -1)>.
502
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503=item chown LIST
504
505Changes the owner (and group) of a list of files. The first two
506elements of the list must be the I<NUMERICAL> uid and gid, in that order.
507Returns the number of files successfully changed.
508
509 $cnt = chown $uid, $gid, 'foo', 'bar';
510 chown $uid, $gid, @filenames;
511
54310121 512Here's an example that looks up nonnumeric uids in the passwd file:
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513
514 print "User: ";
515 chop($user = <STDIN>);
516 print "Files: "
517 chop($pattern = <STDIN>);
518
519 ($login,$pass,$uid,$gid) = getpwnam($user)
520 or die "$user not in passwd file";
521
522 @ary = <${pattern}>; # expand filenames
523 chown $uid, $gid, @ary;
524
54310121 525On most systems, you are not allowed to change the ownership of the
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526file unless you're the superuser, although you should be able to change
527the group to any of your secondary groups. On insecure systems, these
528restrictions may be relaxed, but this is not a portable assumption.
529
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530=item chr NUMBER
531
54310121 532=item chr
bbce6d69 533
a0d0e21e 534Returns the character represented by that NUMBER in the character set.
dc848c6f 535For example, C<chr(65)> is "A" in ASCII. For the reverse, use L</ord>.
a0d0e21e 536
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537If NUMBER is omitted, uses $_.
538
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539=item chroot FILENAME
540
54310121 541=item chroot
bbce6d69 542
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543This function works as the system call by the same name: it makes the
544named directory the new root directory for all further pathnames that
545begin with a "/" by your process and all of its children. (It doesn't
28757baa 546change your current working directory, which is unaffected.) For security
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547reasons, this call is restricted to the superuser. If FILENAME is
548omitted, does chroot to $_.
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549
550=item close FILEHANDLE
551
552Closes the file or pipe associated with the file handle, returning TRUE
553only if stdio successfully flushes buffers and closes the system file
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554descriptor. If the file handle came from a piped open C<close> will
555additionally return FALSE if one of the other system calls involved
556fails or if the program exits with non-zero status. (If the problem was
557that the program exited non-zero $! will be set to 0.)
558You don't have to close FILEHANDLE if you are immediately
5f05dabc 559going to do another open() on it, because open() will close it for you. (See
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560open().) However, an explicit close on an input file resets the line
561counter ($.), while the implicit close done by open() does not. Also,
562closing a pipe will wait for the process executing on the pipe to
563complete, in case you want to look at the output of the pipe
564afterwards. Closing a pipe explicitly also puts the status value of
565the command into C<$?>. Example:
566
567 open(OUTPUT, '|sort >foo'); # pipe to sort
568 ... # print stuff to output
569 close OUTPUT; # wait for sort to finish
570 open(INPUT, 'foo'); # get sort's results
571
572FILEHANDLE may be an expression whose value gives the real filehandle name.
573
574=item closedir DIRHANDLE
575
576Closes a directory opened by opendir().
577
578=item connect SOCKET,NAME
579
580Attempts to connect to a remote socket, just as the connect system call
581does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
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582packed address of the appropriate type for the socket. See the examples in
583L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 584
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585=item continue BLOCK
586
587Actually a flow control statement rather than a function. If there is a
588C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
589C<foreach>), it is always executed just before the conditional is about to
590be evaluated again, just like the third part of a C<for> loop in C. Thus
591it can be used to increment a loop variable, even when the loop has been
592continued via the C<next> statement (which is similar to the C C<continue>
593statement).
594
a0d0e21e
LW
595=item cos EXPR
596
597Returns the cosine of EXPR (expressed in radians). If EXPR is omitted
598takes cosine of $_.
599
28757baa
PP
600For the inverse cosine operation, you may use the POSIX::acos()
601function, or use this relation:
602
603 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
604
a0d0e21e
LW
605=item crypt PLAINTEXT,SALT
606
4633a7c4
LW
607Encrypts a string exactly like the crypt(3) function in the C library
608(assuming that you actually have a version there that has not been
609extirpated as a potential munition). This can prove useful for checking
610the password file for lousy passwords, amongst other things. Only the
611guys wearing white hats should do this.
a0d0e21e 612
11155c91
CS
613Note that crypt is intended to be a one-way function, much like breaking
614eggs to make an omelette. There is no (known) corresponding decrypt
615function. As a result, this function isn't all that useful for
616cryptography. (For that, see your nearby CPAN mirror.)
2f9daede 617
a0d0e21e
LW
618Here's an example that makes sure that whoever runs this program knows
619their own password:
620
621 $pwd = (getpwuid($<))[1];
622 $salt = substr($pwd, 0, 2);
623
624 system "stty -echo";
625 print "Password: ";
626 chop($word = <STDIN>);
627 print "\n";
628 system "stty echo";
629
630 if (crypt($word, $salt) ne $pwd) {
631 die "Sorry...\n";
632 } else {
633 print "ok\n";
54310121 634 }
a0d0e21e 635
54310121 636Of course, typing in your own password to whomever asks you
748a9306 637for it is unwise.
a0d0e21e 638
aa689395 639=item dbmclose HASH
a0d0e21e
LW
640
641[This function has been superseded by the untie() function.]
642
aa689395 643Breaks the binding between a DBM file and a hash.
a0d0e21e 644
aa689395 645=item dbmopen HASH,DBNAME,MODE
a0d0e21e
LW
646
647[This function has been superseded by the tie() function.]
648
aa689395
PP
649This binds a dbm(3), ndbm(3), sdbm(3), gdbm(), or Berkeley DB file to a
650hash. HASH is the name of the hash. (Unlike normal open, the first
651argument is I<NOT> a filehandle, even though it looks like one). DBNAME
652is the name of the database (without the F<.dir> or F<.pag> extension if
653any). If the database does not exist, it is created with protection
654specified by MODE (as modified by the umask()). If your system supports
655only the older DBM functions, you may perform only one dbmopen() in your
656program. In older versions of Perl, if your system had neither DBM nor
657ndbm, calling dbmopen() produced a fatal error; it now falls back to
658sdbm(3).
659
660If you don't have write access to the DBM file, you can only read hash
661variables, not set them. If you want to test whether you can write,
662either use file tests or try setting a dummy hash entry inside an eval(),
663which will trap the error.
a0d0e21e
LW
664
665Note that functions such as keys() and values() may return huge array
666values when used on large DBM files. You may prefer to use the each()
667function to iterate over large DBM files. Example:
668
669 # print out history file offsets
670 dbmopen(%HIST,'/usr/lib/news/history',0666);
671 while (($key,$val) = each %HIST) {
672 print $key, ' = ', unpack('L',$val), "\n";
673 }
674 dbmclose(%HIST);
675
cb1a09d0 676See also L<AnyDBM_File> for a more general description of the pros and
184e9718 677cons of the various dbm approaches, as well as L<DB_File> for a particularly
cb1a09d0 678rich implementation.
4633a7c4 679
a0d0e21e
LW
680=item defined EXPR
681
54310121 682=item defined
bbce6d69 683
2f9daede
TPG
684Returns a Boolean value telling whether EXPR has a value other than
685the undefined value C<undef>. If EXPR is not present, C<$_> will be
686checked.
687
688Many operations return C<undef> to indicate failure, end of file,
689system error, uninitialized variable, and other exceptional
690conditions. This function allows you to distinguish C<undef> from
691other values. (A simple Boolean test will not distinguish among
692C<undef>, zero, the empty string, and "0", which are all equally
693false.) Note that since C<undef> is a valid scalar, its presence
694doesn't I<necessarily> indicate an exceptional condition: pop()
695returns C<undef> when its argument is an empty array, I<or> when the
696element to return happens to be C<undef>.
697
698You may also use defined() to check whether a subroutine exists. On
699the other hand, use of defined() upon aggregates (hashes and arrays)
700is not guaranteed to produce intuitive results, and should probably be
701avoided.
702
703When used on a hash element, it tells you whether the value is defined,
dc848c6f 704not whether the key exists in the hash. Use L</exists> for the latter
2f9daede 705purpose.
a0d0e21e
LW
706
707Examples:
708
709 print if defined $switch{'D'};
710 print "$val\n" while defined($val = pop(@ary));
711 die "Can't readlink $sym: $!"
712 unless defined($value = readlink $sym);
a0d0e21e 713 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
2f9daede 714 $debugging = 0 unless defined $debugging;
a0d0e21e 715
2f9daede
TPG
716Note: Many folks tend to overuse defined(), and then are surprised to
717discover that the number 0 and "" (the zero-length string) are, in fact,
718defined values. For example, if you say
a5f75d66
AD
719
720 "ab" =~ /a(.*)b/;
721
722the pattern match succeeds, and $1 is defined, despite the fact that it
723matched "nothing". But it didn't really match nothing--rather, it
724matched something that happened to be 0 characters long. This is all
725very above-board and honest. When a function returns an undefined value,
2f9daede
TPG
726it's an admission that it couldn't give you an honest answer. So you
727should use defined() only when you're questioning the integrity of what
728you're trying to do. At other times, a simple comparison to 0 or "" is
729what you want.
730
731Currently, using defined() on an entire array or hash reports whether
732memory for that aggregate has ever been allocated. So an array you set
733to the empty list appears undefined initially, and one that once was full
734and that you then set to the empty list still appears defined. You
735should instead use a simple test for size:
28757baa
PP
736
737 if (@an_array) { print "has array elements\n" }
738 if (%a_hash) { print "has hash members\n" }
739
740Using undef() on these, however, does clear their memory and then report
741them as not defined anymore, but you shoudln't do that unless you don't
742plan to use them again, because it saves time when you load them up
743again to have memory already ready to be filled.
744
54310121 745This counterintuitive behaviour of defined() on aggregates may be
28757baa
PP
746changed, fixed, or broken in a future release of Perl.
747
dc848c6f 748See also L</undef>, L</exists>, L</ref>.
2f9daede 749
a0d0e21e
LW
750=item delete EXPR
751
aa689395
PP
752Deletes the specified key(s) and their associated values from a hash.
753For each key, returns the deleted value associated with that key, or
754the undefined value if there was no such key. Deleting from C<$ENV{}>
755modifies the environment. Deleting from a hash tied to a DBM file
5f05dabc
PP
756deletes the entry from the DBM file. (But deleting from a tie()d hash
757doesn't necessarily return anything.)
a0d0e21e 758
aa689395 759The following deletes all the values of a hash:
a0d0e21e 760
5f05dabc
PP
761 foreach $key (keys %HASH) {
762 delete $HASH{$key};
a0d0e21e
LW
763 }
764
5f05dabc
PP
765And so does this:
766
767 delete @HASH{keys %HASH}
768
769(But both of these are slower than the undef() command.) Note that the
770EXPR can be arbitrarily complicated as long as the final operation is a
771hash element lookup or hash slice:
a0d0e21e
LW
772
773 delete $ref->[$x][$y]{$key};
5f05dabc 774 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
a0d0e21e
LW
775
776=item die LIST
777
778Outside of an eval(), prints the value of LIST to C<STDERR> and exits with
184e9718 779the current value of C<$!> (errno). If C<$!> is 0, exits with the value of
54310121 780C<($? E<gt>E<gt> 8)> (backtick `command` status). If C<($? E<gt>E<gt> 8)>
28757baa
PP
781is 0, exits with 255. Inside an eval(), the error message is stuffed into
782C<$@>, and the eval() is terminated with the undefined value; this makes
783die() the way to raise an exception.
a0d0e21e
LW
784
785Equivalent examples:
786
787 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
54310121 788 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
a0d0e21e
LW
789
790If the value of EXPR does not end in a newline, the current script line
791number and input line number (if any) are also printed, and a newline
792is supplied. Hint: sometimes appending ", stopped" to your message
793will cause it to make better sense when the string "at foo line 123" is
794appended. Suppose you are running script "canasta".
795
796 die "/etc/games is no good";
797 die "/etc/games is no good, stopped";
798
799produce, respectively
800
801 /etc/games is no good at canasta line 123.
802 /etc/games is no good, stopped at canasta line 123.
803
804See also exit() and warn().
805
774d564b
PP
806You can arrange for a callback to be called just before the die() does
807its deed, by setting the C<$SIG{__DIE__}> hook. The associated handler
808will be called with the error text and can change the error message, if
809it sees fit, by calling die() again. See L<perlvar> for details on
810setting C<%SIG> entries, and eval() for some examples.
811
a0d0e21e
LW
812=item do BLOCK
813
814Not really a function. Returns the value of the last command in the
815sequence of commands indicated by BLOCK. When modified by a loop
816modifier, executes the BLOCK once before testing the loop condition.
817(On other statements the loop modifiers test the conditional first.)
818
819=item do SUBROUTINE(LIST)
820
821A deprecated form of subroutine call. See L<perlsub>.
822
823=item do EXPR
824
825Uses the value of EXPR as a filename and executes the contents of the
826file as a Perl script. Its primary use is to include subroutines
827from a Perl subroutine library.
828
829 do 'stat.pl';
830
831is just like
832
833 eval `cat stat.pl`;
834
835except that it's more efficient, more concise, keeps track of the
836current filename for error messages, and searches all the B<-I>
837libraries if the file isn't in the current directory (see also the @INC
838array in L<perlvar/Predefined Names>). It's the same, however, in that it does
54310121 839reparse the file every time you call it, so you probably don't want to
a0d0e21e
LW
840do this inside a loop.
841
842Note that inclusion of library modules is better done with the
4633a7c4
LW
843use() and require() operators, which also do error checking
844and raise an exception if there's a problem.
a0d0e21e
LW
845
846=item dump LABEL
847
848This causes an immediate core dump. Primarily this is so that you can
849use the B<undump> program to turn your core dump into an executable binary
850after having initialized all your variables at the beginning of the
851program. When the new binary is executed it will begin by executing a
852C<goto LABEL> (with all the restrictions that C<goto> suffers). Think of
853it as a goto with an intervening core dump and reincarnation. If LABEL
854is omitted, restarts the program from the top. WARNING: any files
855opened at the time of the dump will NOT be open any more when the
856program is reincarnated, with possible resulting confusion on the part
857of Perl. See also B<-u> option in L<perlrun>.
858
859Example:
860
861 #!/usr/bin/perl
862 require 'getopt.pl';
863 require 'stat.pl';
864 %days = (
865 'Sun' => 1,
866 'Mon' => 2,
867 'Tue' => 3,
868 'Wed' => 4,
869 'Thu' => 5,
870 'Fri' => 6,
871 'Sat' => 7,
872 );
873
874 dump QUICKSTART if $ARGV[0] eq '-d';
875
876 QUICKSTART:
877 Getopt('f');
878
aa689395
PP
879=item each HASH
880
881When called in a list context, returns a 2-element array consisting of the
882key and value for the next element of a hash, so that you can iterate over
883it. When called in a scalar context, returns the key for only the next
2f9daede
TPG
884element in the hash. (Note: Keys may be "0" or "", which are logically
885false; you may wish to avoid constructs like C<while ($k = each %foo) {}>
886for this reason.)
887
888Entries are returned in an apparently random order. When the hash is
889entirely read, a null array is returned in list context (which when
890assigned produces a FALSE (0) value), and C<undef> is returned in a
891scalar context. The next call to each() after that will start iterating
892again. There is a single iterator for each hash, shared by all each(),
893keys(), and values() function calls in the program; it can be reset by
894reading all the elements from the hash, or by evaluating C<keys HASH> or
895C<values HASH>. If you add or delete elements of a hash while you're
896iterating over it, you may get entries skipped or duplicated, so don't.
aa689395
PP
897
898The following prints out your environment like the printenv(1) program,
899only in a different order:
a0d0e21e
LW
900
901 while (($key,$value) = each %ENV) {
902 print "$key=$value\n";
903 }
904
905See also keys() and values().
906
907=item eof FILEHANDLE
908
4633a7c4
LW
909=item eof ()
910
a0d0e21e
LW
911=item eof
912
913Returns 1 if the next read on FILEHANDLE will return end of file, or if
914FILEHANDLE is not open. FILEHANDLE may be an expression whose value
915gives the real filehandle name. (Note that this function actually
916reads a character and then ungetc()s it, so it is not very useful in an
748a9306
LW
917interactive context.) Do not read from a terminal file (or call
918C<eof(FILEHANDLE)> on it) after end-of-file is reached. Filetypes such
919as terminals may lose the end-of-file condition if you do.
920
921An C<eof> without an argument uses the last file read as argument.
2f9daede
TPG
922Empty parentheses () may be used to indicate the pseudo file formed of
923the files listed on the command line, i.e., C<eof()> is reasonable to
924use inside a C<while (E<lt>E<gt>)> loop to detect the end of only the
925last file. Use C<eof(ARGV)> or eof without the parentheses to test
926I<EACH> file in a while (E<lt>E<gt>) loop. Examples:
a0d0e21e 927
748a9306
LW
928 # reset line numbering on each input file
929 while (<>) {
930 print "$.\t$_";
931 close(ARGV) if (eof); # Not eof().
932 }
933
a0d0e21e
LW
934 # insert dashes just before last line of last file
935 while (<>) {
936 if (eof()) {
937 print "--------------\n";
748a9306
LW
938 close(ARGV); # close or break; is needed if we
939 # are reading from the terminal
a0d0e21e
LW
940 }
941 print;
942 }
943
a0d0e21e 944Practical hint: you almost never need to use C<eof> in Perl, because the
54310121 945input operators return undef when they run out of data.
a0d0e21e
LW
946
947=item eval EXPR
948
949=item eval BLOCK
950
951EXPR is parsed and executed as if it were a little Perl program. It
952is executed in the context of the current Perl program, so that any
5f05dabc 953variable settings or subroutine and format definitions remain afterwards.
a0d0e21e 954The value returned is the value of the last expression evaluated, or a
55497cff
PP
955return statement may be used, just as with subroutines. The last
956expression is evaluated in scalar or array context, depending on the
957context of the eval.
a0d0e21e
LW
958
959If there is a syntax error or runtime error, or a die() statement is
960executed, an undefined value is returned by eval(), and C<$@> is set to the
961error message. If there was no error, C<$@> is guaranteed to be a null
774d564b
PP
962string. If EXPR is omitted, evaluates C<$_>. The final semicolon, if
963any, may be omitted from the expression. Beware that using eval()
964neither silences perl from printing warnings to STDERR, nor does it
965stuff the text of warning messages into C<$@>. To do either of those,
966you have to use the C<$SIG{__WARN__}> facility. See warn() and L<perlvar>.
a0d0e21e 967
5f05dabc 968Note that, because eval() traps otherwise-fatal errors, it is useful for
4633a7c4 969determining whether a particular feature (such as socket() or symlink())
a0d0e21e
LW
970is implemented. It is also Perl's exception trapping mechanism, where
971the die operator is used to raise exceptions.
972
973If the code to be executed doesn't vary, you may use the eval-BLOCK
974form to trap run-time errors without incurring the penalty of
975recompiling each time. The error, if any, is still returned in C<$@>.
976Examples:
977
54310121 978 # make divide-by-zero nonfatal
a0d0e21e
LW
979 eval { $answer = $a / $b; }; warn $@ if $@;
980
981 # same thing, but less efficient
982 eval '$answer = $a / $b'; warn $@ if $@;
983
984 # a compile-time error
985 eval { $answer = };
986
987 # a run-time error
988 eval '$answer ='; # sets $@
989
774d564b
PP
990When using the eval{} form as an exception trap in libraries, you may
991wish not to trigger any C<__DIE__> hooks that user code may have
992installed. You can use the C<local $SIG{__DIE__}> construct for this
993purpose, as shown in this example:
994
995 # a very private exception trap for divide-by-zero
996 eval { local $SIG{'__DIE__'}; $answer = $a / $b; }; warn $@ if $@;
997
998This is especially significant, given that C<__DIE__> hooks can call
999die() again, which has the effect of changing their error messages:
1000
1001 # __DIE__ hooks may modify error messages
1002 {
1003 local $SIG{'__DIE__'} = sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1004 eval { die "foo foofs here" };
1005 print $@ if $@; # prints "bar barfs here"
1006 }
1007
54310121 1008With an eval(), you should be especially careful to remember what's
a0d0e21e
LW
1009being looked at when:
1010
1011 eval $x; # CASE 1
1012 eval "$x"; # CASE 2
1013
1014 eval '$x'; # CASE 3
1015 eval { $x }; # CASE 4
1016
1017 eval "\$$x++" # CASE 5
1018 $$x++; # CASE 6
1019
2f9daede
TPG
1020Cases 1 and 2 above behave identically: they run the code contained in
1021the variable $x. (Although case 2 has misleading double quotes making
1022the reader wonder what else might be happening (nothing is).) Cases 3
1023and 4 likewise behave in the same way: they run the code '$x', which
1024does nothing but return the value of C<$x>. (Case 4 is preferred for
1025purely visual reasons, but it also has the advantage of compiling at
1026compile-time instead of at run-time.) Case 5 is a place where
54310121 1027normally you I<WOULD> like to use double quotes, except that in this
2f9daede
TPG
1028particular situation, you can just use symbolic references instead, as
1029in case 6.
a0d0e21e
LW
1030
1031=item exec LIST
1032
55497cff
PP
1033The exec() function executes a system command I<AND NEVER RETURNS>,
1034unless the command does not exist and is executed directly instead of
bb32b41a
GS
1035via your system's command shell (see below). Use system() instead of
1036exec() if you want it to return.
a0d0e21e
LW
1037
1038If there is more than one argument in LIST, or if LIST is an array with
1039more than one value, calls execvp(3) with the arguments in LIST. If
1040there is only one scalar argument, the argument is checked for shell
bb32b41a
GS
1041metacharacters, and if there are any, the entire argument is passed to
1042the system's command shell for parsing (this is C</bin/sh -c> on Unix
1043platforms, but varies on other platforms). If there are no shell
1044metacharacters in the argument, it is split into words and passed
1045directly to execvp(), which is more efficient. Note: exec() and
1046system() do not flush your output buffer, so you may need to set C<$|>
1047to avoid lost output. Examples:
a0d0e21e
LW
1048
1049 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1050 exec "sort $outfile | uniq";
1051
1052If you don't really want to execute the first argument, but want to lie
1053to the program you are executing about its own name, you can specify
1054the program you actually want to run as an "indirect object" (without a
1055comma) in front of the LIST. (This always forces interpretation of the
54310121 1056LIST as a multivalued list, even if there is only a single scalar in
a0d0e21e
LW
1057the list.) Example:
1058
1059 $shell = '/bin/csh';
1060 exec $shell '-sh'; # pretend it's a login shell
1061
1062or, more directly,
1063
1064 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1065
bb32b41a
GS
1066When the arguments get executed via the system shell, results will
1067be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1068for details.
1069
a0d0e21e
LW
1070=item exists EXPR
1071
1072Returns TRUE if the specified hash key exists in its hash array, even
1073if the corresponding value is undefined.
1074
1075 print "Exists\n" if exists $array{$key};
1076 print "Defined\n" if defined $array{$key};
1077 print "True\n" if $array{$key};
1078
5f05dabc 1079A hash element can be TRUE only if it's defined, and defined if
a0d0e21e
LW
1080it exists, but the reverse doesn't necessarily hold true.
1081
1082Note that the EXPR can be arbitrarily complicated as long as the final
1083operation is a hash key lookup:
1084
1085 if (exists $ref->[$x][$y]{$key}) { ... }
1086
1087=item exit EXPR
1088
1089Evaluates EXPR and exits immediately with that value. (Actually, it
1090calls any defined C<END> routines first, but the C<END> routines may not
1091abort the exit. Likewise any object destructors that need to be called
1092are called before exit.) Example:
1093
1094 $ans = <STDIN>;
1095 exit 0 if $ans =~ /^[Xx]/;
1096
f86702cc 1097See also die(). If EXPR is omitted, exits with 0 status. The only
54310121 1098universally portable values for EXPR are 0 for success and 1 for error;
f86702cc
PP
1099all other values are subject to unpredictable interpretation depending
1100on the environment in which the Perl program is running.
a0d0e21e 1101
28757baa
PP
1102You shouldn't use exit() to abort a subroutine if there's any chance that
1103someone might want to trap whatever error happened. Use die() instead,
1104which can be trapped by an eval().
1105
a0d0e21e
LW
1106=item exp EXPR
1107
54310121 1108=item exp
bbce6d69 1109
54310121 1110Returns I<e> (the natural logarithm base) to the power of EXPR.
a0d0e21e
LW
1111If EXPR is omitted, gives C<exp($_)>.
1112
1113=item fcntl FILEHANDLE,FUNCTION,SCALAR
1114
1115Implements the fcntl(2) function. You'll probably have to say
1116
1117 use Fcntl;
1118
1119first to get the correct function definitions. Argument processing and
1120value return works just like ioctl() below. Note that fcntl() will produce
1121a fatal error if used on a machine that doesn't implement fcntl(2).
1122For example:
1123
1124 use Fcntl;
1125 fcntl($filehandle, F_GETLK, $packed_return_buffer);
1126
1127=item fileno FILEHANDLE
1128
1129Returns the file descriptor for a filehandle. This is useful for
1130constructing bitmaps for select(). If FILEHANDLE is an expression, the
1131value is taken as the name of the filehandle.
1132
1133=item flock FILEHANDLE,OPERATION
1134
8ebc5c01 1135Calls flock(2), or an emulation of it, on FILEHANDLE. Returns TRUE for
68dc0745
PP
1136success, FALSE on failure. Produces a fatal error if used on a machine
1137that doesn't implement flock(2), fcntl(2) locking, or lockf(3). flock()
1138is Perl's portable file locking interface, although it locks only entire
1139files, not records.
8ebc5c01
PP
1140
1141OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1142LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
68dc0745
PP
1143you can use the symbolic names if import them from the Fcntl module,
1144either individually, or as a group using the ':flock' tag. LOCK_SH
1145requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1146releases a previously requested lock. If LOCK_NB is added to LOCK_SH or
1147LOCK_EX then flock() will return immediately rather than blocking
1148waiting for the lock (check the return status to see if you got it).
1149
1150To avoid the possibility of mis-coordination, Perl flushes FILEHANDLE
1151before (un)locking it.
8ebc5c01
PP
1152
1153Note that the emulation built with lockf(3) doesn't provide shared
1154locks, and it requires that FILEHANDLE be open with write intent. These
1155are the semantics that lockf(3) implements. Most (all?) systems
1156implement lockf(3) in terms of fcntl(2) locking, though, so the
1157differing semantics shouldn't bite too many people.
1158
1159Note also that some versions of flock() cannot lock things over the
1160network; you would need to use the more system-specific fcntl() for
1161that. If you like you can force Perl to ignore your system's flock(2)
1162function, and so provide its own fcntl(2)-based emulation, by passing
1163the switch C<-Ud_flock> to the F<Configure> program when you configure
1164perl.
4633a7c4
LW
1165
1166Here's a mailbox appender for BSD systems.
a0d0e21e 1167
7e1af8bc 1168 use Fcntl ':flock'; # import LOCK_* constants
a0d0e21e
LW
1169
1170 sub lock {
7e1af8bc 1171 flock(MBOX,LOCK_EX);
a0d0e21e
LW
1172 # and, in case someone appended
1173 # while we were waiting...
1174 seek(MBOX, 0, 2);
1175 }
1176
1177 sub unlock {
7e1af8bc 1178 flock(MBOX,LOCK_UN);
a0d0e21e
LW
1179 }
1180
1181 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1182 or die "Can't open mailbox: $!";
1183
1184 lock();
1185 print MBOX $msg,"\n\n";
1186 unlock();
1187
cb1a09d0 1188See also L<DB_File> for other flock() examples.
a0d0e21e
LW
1189
1190=item fork
1191
1192Does a fork(2) system call. Returns the child pid to the parent process
4633a7c4 1193and 0 to the child process, or C<undef> if the fork is unsuccessful.
a0d0e21e 1194Note: unflushed buffers remain unflushed in both processes, which means
28757baa
PP
1195you may need to set C<$|> ($AUTOFLUSH in English) or call the autoflush()
1196method of IO::Handle to avoid duplicate output.
a0d0e21e
LW
1197
1198If you fork() without ever waiting on your children, you will accumulate
1199zombies:
1200
4633a7c4 1201 $SIG{CHLD} = sub { wait };
a0d0e21e 1202
54310121 1203There's also the double-fork trick (error checking on
a0d0e21e
LW
1204fork() returns omitted);
1205
1206 unless ($pid = fork) {
1207 unless (fork) {
1208 exec "what you really wanna do";
1209 die "no exec";
1210 # ... or ...
4633a7c4 1211 ## (some_perl_code_here)
a0d0e21e
LW
1212 exit 0;
1213 }
1214 exit 0;
1215 }
1216 waitpid($pid,0);
1217
cb1a09d0
AD
1218See also L<perlipc> for more examples of forking and reaping
1219moribund children.
1220
28757baa
PP
1221Note that if your forked child inherits system file descriptors like
1222STDIN and STDOUT that are actually connected by a pipe or socket, even
1223if you exit, the remote server (such as, say, httpd or rsh) won't think
1224you're done. You should reopen those to /dev/null if it's any issue.
1225
cb1a09d0
AD
1226=item format
1227
1228Declare a picture format with use by the write() function. For
1229example:
1230
54310121 1231 format Something =
cb1a09d0
AD
1232 Test: @<<<<<<<< @||||| @>>>>>
1233 $str, $%, '$' . int($num)
1234 .
1235
1236 $str = "widget";
184e9718 1237 $num = $cost/$quantity;
cb1a09d0
AD
1238 $~ = 'Something';
1239 write;
1240
1241See L<perlform> for many details and examples.
1242
a0d0e21e 1243
8903cb82 1244=item formline PICTURE,LIST
a0d0e21e 1245
4633a7c4 1246This is an internal function used by C<format>s, though you may call it
a0d0e21e
LW
1247too. It formats (see L<perlform>) a list of values according to the
1248contents of PICTURE, placing the output into the format output
4633a7c4
LW
1249accumulator, C<$^A> (or $ACCUMULATOR in English).
1250Eventually, when a write() is done, the contents of
a0d0e21e
LW
1251C<$^A> are written to some filehandle, but you could also read C<$^A>
1252yourself and then set C<$^A> back to "". Note that a format typically
1253does one formline() per line of form, but the formline() function itself
748a9306 1254doesn't care how many newlines are embedded in the PICTURE. This means
4633a7c4 1255that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
748a9306
LW
1256You may therefore need to use multiple formlines to implement a single
1257record format, just like the format compiler.
1258
5f05dabc 1259Be careful if you put double quotes around the picture, because an "C<@>"
748a9306 1260character may be taken to mean the beginning of an array name.
4633a7c4 1261formline() always returns TRUE. See L<perlform> for other examples.
a0d0e21e
LW
1262
1263=item getc FILEHANDLE
1264
1265=item getc
1266
1267Returns the next character from the input file attached to FILEHANDLE,
1268or a null string at end of file. If FILEHANDLE is omitted, reads from STDIN.
4633a7c4 1269This is not particularly efficient. It cannot be used to get unbuffered
cb1a09d0 1270single-characters, however. For that, try something more like:
4633a7c4
LW
1271
1272 if ($BSD_STYLE) {
1273 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1274 }
1275 else {
54310121 1276 system "stty", '-icanon', 'eol', "\001";
4633a7c4
LW
1277 }
1278
1279 $key = getc(STDIN);
1280
1281 if ($BSD_STYLE) {
1282 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1283 }
1284 else {
5f05dabc 1285 system "stty", 'icanon', 'eol', '^@'; # ASCII null
4633a7c4
LW
1286 }
1287 print "\n";
1288
54310121
PP
1289Determination of whether $BSD_STYLE should be set
1290is left as an exercise to the reader.
cb1a09d0 1291
28757baa
PP
1292The POSIX::getattr() function can do this more portably on systems
1293alleging POSIX compliance.
cb1a09d0 1294See also the C<Term::ReadKey> module from your nearest CPAN site;
54310121 1295details on CPAN can be found on L<perlmod/CPAN>.
a0d0e21e
LW
1296
1297=item getlogin
1298
1299Returns the current login from F</etc/utmp>, if any. If null, use
54310121 1300getpwuid().
a0d0e21e 1301
f86702cc 1302 $login = getlogin || getpwuid($<) || "Kilroy";
a0d0e21e 1303
da0045b7 1304Do not consider getlogin() for authentication: it is not as
4633a7c4
LW
1305secure as getpwuid().
1306
a0d0e21e
LW
1307=item getpeername SOCKET
1308
1309Returns the packed sockaddr address of other end of the SOCKET connection.
1310
4633a7c4
LW
1311 use Socket;
1312 $hersockaddr = getpeername(SOCK);
1313 ($port, $iaddr) = unpack_sockaddr_in($hersockaddr);
1314 $herhostname = gethostbyaddr($iaddr, AF_INET);
1315 $herstraddr = inet_ntoa($iaddr);
a0d0e21e
LW
1316
1317=item getpgrp PID
1318
47e29363
PP
1319Returns the current process group for the specified PID. Use
1320a PID of 0 to get the current process group for the
4633a7c4 1321current process. Will raise an exception if used on a machine that
a0d0e21e 1322doesn't implement getpgrp(2). If PID is omitted, returns process
47e29363
PP
1323group of current process. Note that the POSIX version of getpgrp()
1324does not accept a PID argument, so only PID==0 is truly portable.
a0d0e21e
LW
1325
1326=item getppid
1327
1328Returns the process id of the parent process.
1329
1330=item getpriority WHICH,WHO
1331
4633a7c4
LW
1332Returns the current priority for a process, a process group, or a user.
1333(See L<getpriority(2)>.) Will raise a fatal exception if used on a
a0d0e21e
LW
1334machine that doesn't implement getpriority(2).
1335
1336=item getpwnam NAME
1337
1338=item getgrnam NAME
1339
1340=item gethostbyname NAME
1341
1342=item getnetbyname NAME
1343
1344=item getprotobyname NAME
1345
1346=item getpwuid UID
1347
1348=item getgrgid GID
1349
1350=item getservbyname NAME,PROTO
1351
1352=item gethostbyaddr ADDR,ADDRTYPE
1353
1354=item getnetbyaddr ADDR,ADDRTYPE
1355
1356=item getprotobynumber NUMBER
1357
1358=item getservbyport PORT,PROTO
1359
1360=item getpwent
1361
1362=item getgrent
1363
1364=item gethostent
1365
1366=item getnetent
1367
1368=item getprotoent
1369
1370=item getservent
1371
1372=item setpwent
1373
1374=item setgrent
1375
1376=item sethostent STAYOPEN
1377
1378=item setnetent STAYOPEN
1379
1380=item setprotoent STAYOPEN
1381
1382=item setservent STAYOPEN
1383
1384=item endpwent
1385
1386=item endgrent
1387
1388=item endhostent
1389
1390=item endnetent
1391
1392=item endprotoent
1393
1394=item endservent
1395
1396These routines perform the same functions as their counterparts in the
1397system library. Within a list context, the return values from the
1398various get routines are as follows:
1399
1400 ($name,$passwd,$uid,$gid,
1401 $quota,$comment,$gcos,$dir,$shell) = getpw*
1402 ($name,$passwd,$gid,$members) = getgr*
1403 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1404 ($name,$aliases,$addrtype,$net) = getnet*
1405 ($name,$aliases,$proto) = getproto*
1406 ($name,$aliases,$port,$proto) = getserv*
1407
1408(If the entry doesn't exist you get a null list.)
1409
1410Within a scalar context, you get the name, unless the function was a
1411lookup by name, in which case you get the other thing, whatever it is.
1412(If the entry doesn't exist you get the undefined value.) For example:
1413
1414 $uid = getpwnam
1415 $name = getpwuid
1416 $name = getpwent
1417 $gid = getgrnam
1418 $name = getgrgid
1419 $name = getgrent
1420 etc.
1421
1422The $members value returned by I<getgr*()> is a space separated list of
1423the login names of the members of the group.
1424
1425For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1426C, it will be returned to you via C<$?> if the function call fails. The
1427@addrs value returned by a successful call is a list of the raw
1428addresses returned by the corresponding system library call. In the
1429Internet domain, each address is four bytes long and you can unpack it
1430by saying something like:
1431
1432 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1433
1434=item getsockname SOCKET
1435
1436Returns the packed sockaddr address of this end of the SOCKET connection.
1437
4633a7c4
LW
1438 use Socket;
1439 $mysockaddr = getsockname(SOCK);
1440 ($port, $myaddr) = unpack_sockaddr_in($mysockaddr);
a0d0e21e
LW
1441
1442=item getsockopt SOCKET,LEVEL,OPTNAME
1443
1444Returns the socket option requested, or undefined if there is an error.
1445
1446=item glob EXPR
1447
0a753a76
PP
1448=item glob
1449
68dc0745
PP
1450Returns the value of EXPR with filename expansions such as a shell would
1451do. This is the internal function implementing the C<E<lt>*.cE<gt>>
1452operator, but you can use it directly. If EXPR is omitted, $_ is used.
1453The C<E<lt>*.cE<gt>> operator is discussed in more detail in
1454L<perlop/"I/O Operators">.
a0d0e21e
LW
1455
1456=item gmtime EXPR
1457
1458Converts a time as returned by the time function to a 9-element array
54310121 1459with the time localized for the standard Greenwich time zone.
4633a7c4 1460Typically used as follows:
a0d0e21e 1461
54310121 1462 # 0 1 2 3 4 5 6 7 8
a0d0e21e
LW
1463 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1464 gmtime(time);
1465
1466All array elements are numeric, and come straight out of a struct tm.
1467In particular this means that $mon has the range 0..11 and $wday has
54310121
PP
1468the range 0..6 with sunday as day 0. Also, $year is the number of
1469years since 1900, I<not> simply the last two digits of the year.
2f9daede
TPG
1470
1471If EXPR is omitted, does C<gmtime(time())>.
a0d0e21e 1472
54310121 1473In a scalar context, returns the ctime(3) value:
0a753a76
PP
1474
1475 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
1476
54310121
PP
1477Also see the timegm() function provided by the Time::Local module,
1478and the strftime(3) function available via the POSIX module.
0a753a76 1479
a0d0e21e
LW
1480=item goto LABEL
1481
748a9306
LW
1482=item goto EXPR
1483
a0d0e21e
LW
1484=item goto &NAME
1485
1486The goto-LABEL form finds the statement labeled with LABEL and resumes
1487execution there. It may not be used to go into any construct that
1488requires initialization, such as a subroutine or a foreach loop. It
0a753a76
PP
1489also can't be used to go into a construct that is optimized away,
1490or to get out of a block or subroutine given to sort().
1491It can be used to go almost anywhere else within the dynamic scope,
a0d0e21e
LW
1492including out of subroutines, but it's usually better to use some other
1493construct such as last or die. The author of Perl has never felt the
1494need to use this form of goto (in Perl, that is--C is another matter).
1495
748a9306
LW
1496The goto-EXPR form expects a label name, whose scope will be resolved
1497dynamically. This allows for computed gotos per FORTRAN, but isn't
1498necessarily recommended if you're optimizing for maintainability:
1499
1500 goto ("FOO", "BAR", "GLARCH")[$i];
1501
a0d0e21e
LW
1502The goto-&NAME form is highly magical, and substitutes a call to the
1503named subroutine for the currently running subroutine. This is used by
1504AUTOLOAD subroutines that wish to load another subroutine and then
1505pretend that the other subroutine had been called in the first place
1506(except that any modifications to @_ in the current subroutine are
1507propagated to the other subroutine.) After the goto, not even caller()
1508will be able to tell that this routine was called first.
1509
1510=item grep BLOCK LIST
1511
1512=item grep EXPR,LIST
1513
54310121 1514This is similar in spirit to, but not the same as, grep(1)
2f9daede
TPG
1515and its relatives. In particular, it is not limited to using
1516regular expressions.
1517
a0d0e21e
LW
1518Evaluates the BLOCK or EXPR for each element of LIST (locally setting
1519$_ to each element) and returns the list value consisting of those
1520elements for which the expression evaluated to TRUE. In a scalar
1521context, returns the number of times the expression was TRUE.
1522
1523 @foo = grep(!/^#/, @bar); # weed out comments
1524
1525or equivalently,
1526
1527 @foo = grep {!/^#/} @bar; # weed out comments
1528
5f05dabc 1529Note that, because $_ is a reference into the list value, it can be used
a0d0e21e
LW
1530to modify the elements of the array. While this is useful and
1531supported, it can cause bizarre results if the LIST is not a named
2f9daede 1532array. Similarly, grep returns aliases into the original list,
2ae324a7 1533much like the way that L<Foreach Loops>'s index variable aliases the list
2f9daede
TPG
1534elements. That is, modifying an element of a list returned by grep
1535actually modifies the element in the original list.
a0d0e21e
LW
1536
1537=item hex EXPR
1538
54310121 1539=item hex
bbce6d69 1540
54310121 1541Interprets EXPR as a hex string and returns the corresponding
2f9daede 1542value. (To convert strings that might start with either 0 or 0x
dc848c6f 1543see L</oct>.) If EXPR is omitted, uses $_.
2f9daede
TPG
1544
1545 print hex '0xAf'; # prints '175'
1546 print hex 'aF'; # same
a0d0e21e
LW
1547
1548=item import
1549
54310121 1550There is no builtin import() function. It is merely an ordinary
4633a7c4 1551method (subroutine) defined (or inherited) by modules that wish to export
a0d0e21e 1552names to another module. The use() function calls the import() method
54310121 1553for the package used. See also L</use()>, L<perlmod>, and L<Exporter>.
a0d0e21e
LW
1554
1555=item index STR,SUBSTR,POSITION
1556
1557=item index STR,SUBSTR
1558
4633a7c4
LW
1559Returns the position of the first occurrence of SUBSTR in STR at or after
1560POSITION. If POSITION is omitted, starts searching from the beginning of
184e9718 1561the string. The return value is based at 0 (or whatever you've set the C<$[>
4633a7c4 1562variable to--but don't do that). If the substring is not found, returns
a0d0e21e
LW
1563one less than the base, ordinarily -1.
1564
1565=item int EXPR
1566
54310121 1567=item int
bbce6d69 1568
a0d0e21e
LW
1569Returns the integer portion of EXPR. If EXPR is omitted, uses $_.
1570
1571=item ioctl FILEHANDLE,FUNCTION,SCALAR
1572
1573Implements the ioctl(2) function. You'll probably have to say
1574
4633a7c4 1575 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
a0d0e21e 1576
4633a7c4 1577first to get the correct function definitions. If F<ioctl.ph> doesn't
a0d0e21e 1578exist or doesn't have the correct definitions you'll have to roll your
4633a7c4
LW
1579own, based on your C header files such as F<E<lt>sys/ioctl.hE<gt>>.
1580(There is a Perl script called B<h2ph> that comes with the Perl kit which
54310121 1581may help you in this, but it's nontrivial.) SCALAR will be read and/or
4633a7c4
LW
1582written depending on the FUNCTION--a pointer to the string value of SCALAR
1583will be passed as the third argument of the actual ioctl call. (If SCALAR
1584has no string value but does have a numeric value, that value will be
1585passed rather than a pointer to the string value. To guarantee this to be
1586TRUE, add a 0 to the scalar before using it.) The pack() and unpack()
1587functions are useful for manipulating the values of structures used by
1588ioctl(). The following example sets the erase character to DEL.
a0d0e21e
LW
1589
1590 require 'ioctl.ph';
4633a7c4
LW
1591 $getp = &TIOCGETP;
1592 die "NO TIOCGETP" if $@ || !$getp;
a0d0e21e 1593 $sgttyb_t = "ccccs"; # 4 chars and a short
4633a7c4 1594 if (ioctl(STDIN,$getp,$sgttyb)) {
a0d0e21e
LW
1595 @ary = unpack($sgttyb_t,$sgttyb);
1596 $ary[2] = 127;
1597 $sgttyb = pack($sgttyb_t,@ary);
4633a7c4 1598 ioctl(STDIN,&TIOCSETP,$sgttyb)
a0d0e21e
LW
1599 || die "Can't ioctl: $!";
1600 }
1601
1602The return value of ioctl (and fcntl) is as follows:
1603
1604 if OS returns: then Perl returns:
1605 -1 undefined value
1606 0 string "0 but true"
1607 anything else that number
1608
1609Thus Perl returns TRUE on success and FALSE on failure, yet you can
1610still easily determine the actual value returned by the operating
1611system:
1612
1613 ($retval = ioctl(...)) || ($retval = -1);
1614 printf "System returned %d\n", $retval;
1615
1616=item join EXPR,LIST
1617
54310121 1618Joins the separate strings of LIST into a single string with
a0d0e21e
LW
1619fields separated by the value of EXPR, and returns the string.
1620Example:
1621
1622 $_ = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
1623
1624See L<perlfunc/split>.
1625
aa689395
PP
1626=item keys HASH
1627
1628Returns a normal array consisting of all the keys of the named hash. (In
1629a scalar context, returns the number of keys.) The keys are returned in
1630an apparently random order, but it is the same order as either the
1631values() or each() function produces (given that the hash has not been
1632modified). As a side effect, it resets HASH's iterator.
a0d0e21e 1633
aa689395 1634Here is yet another way to print your environment:
a0d0e21e
LW
1635
1636 @keys = keys %ENV;
1637 @values = values %ENV;
1638 while ($#keys >= 0) {
1639 print pop(@keys), '=', pop(@values), "\n";
1640 }
1641
1642or how about sorted by key:
1643
1644 foreach $key (sort(keys %ENV)) {
1645 print $key, '=', $ENV{$key}, "\n";
1646 }
1647
54310121 1648To sort an array by value, you'll need to use a C<sort> function.
aa689395 1649Here's a descending numeric sort of a hash by its values:
4633a7c4
LW
1650
1651 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash)) {
1652 printf "%4d %s\n", $hash{$key}, $key;
1653 }
1654
55497cff 1655As an lvalue C<keys> allows you to increase the number of hash buckets
aa689395
PP
1656allocated for the given hash. This can gain you a measure of efficiency if
1657you know the hash is going to get big. (This is similar to pre-extending
1658an array by assigning a larger number to $#array.) If you say
55497cff
PP
1659
1660 keys %hash = 200;
1661
1662then C<%hash> will have at least 200 buckets allocated for it. These
1663buckets will be retained even if you do C<%hash = ()>, use C<undef
1664%hash> if you want to free the storage while C<%hash> is still in scope.
1665You can't shrink the number of buckets allocated for the hash using
1666C<keys> in this way (but you needn't worry about doing this by accident,
1667as trying has no effect).
1668
a0d0e21e
LW
1669=item kill LIST
1670
54310121
PP
1671Sends a signal to a list of processes. The first element of
1672the list must be the signal to send. Returns the number of
4633a7c4 1673processes successfully signaled.
a0d0e21e
LW
1674
1675 $cnt = kill 1, $child1, $child2;
1676 kill 9, @goners;
1677
4633a7c4
LW
1678Unlike in the shell, in Perl if the I<SIGNAL> is negative, it kills
1679process groups instead of processes. (On System V, a negative I<PROCESS>
1680number will also kill process groups, but that's not portable.) That
1681means you usually want to use positive not negative signals. You may also
da0045b7 1682use a signal name in quotes. See L<perlipc/"Signals"> for details.
a0d0e21e
LW
1683
1684=item last LABEL
1685
1686=item last
1687
1688The C<last> command is like the C<break> statement in C (as used in
1689loops); it immediately exits the loop in question. If the LABEL is
1690omitted, the command refers to the innermost enclosing loop. The
1691C<continue> block, if any, is not executed:
1692
4633a7c4
LW
1693 LINE: while (<STDIN>) {
1694 last LINE if /^$/; # exit when done with header
a0d0e21e
LW
1695 ...
1696 }
1697
1698=item lc EXPR
1699
54310121 1700=item lc
bbce6d69 1701
a0d0e21e 1702Returns an lowercased version of EXPR. This is the internal function
54310121 1703implementing the \L escape in double-quoted strings.
a034a98d 1704Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
a0d0e21e 1705
bbce6d69
PP
1706If EXPR is omitted, uses $_.
1707
a0d0e21e
LW
1708=item lcfirst EXPR
1709
54310121 1710=item lcfirst
bbce6d69 1711
a0d0e21e
LW
1712Returns the value of EXPR with the first character lowercased. This is
1713the internal function implementing the \l escape in double-quoted strings.
a034a98d 1714Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
a0d0e21e 1715
bbce6d69
PP
1716If EXPR is omitted, uses $_.
1717
a0d0e21e
LW
1718=item length EXPR
1719
54310121 1720=item length
bbce6d69 1721
a0d0e21e
LW
1722Returns the length in characters of the value of EXPR. If EXPR is
1723omitted, returns length of $_.
1724
1725=item link OLDFILE,NEWFILE
1726
1727Creates a new filename linked to the old filename. Returns 1 for
1728success, 0 otherwise.
1729
1730=item listen SOCKET,QUEUESIZE
1731
1732Does the same thing that the listen system call does. Returns TRUE if
4633a7c4 1733it succeeded, FALSE otherwise. See example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
1734
1735=item local EXPR
1736
a0d0e21e 1737A local modifies the listed variables to be local to the enclosing block,
5f05dabc
PP
1738subroutine, C<eval{}>, or C<do>. If more than one value is listed, the
1739list must be placed in parentheses. See L<perlsub/"Temporary Values via
3e3baf6d 1740local()"> for details, including issues with tied arrays and hashes.
a0d0e21e 1741
cb1a09d0
AD
1742But you really probably want to be using my() instead, because local() isn't
1743what most people think of as "local"). See L<perlsub/"Private Variables
1744via my()"> for details.
a0d0e21e
LW
1745
1746=item localtime EXPR
1747
1748Converts a time as returned by the time function to a 9-element array
5f05dabc 1749with the time analyzed for the local time zone. Typically used as
a0d0e21e
LW
1750follows:
1751
54310121 1752 # 0 1 2 3 4 5 6 7 8
a0d0e21e
LW
1753 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1754 localtime(time);
1755
1756All array elements are numeric, and come straight out of a struct tm.
1757In particular this means that $mon has the range 0..11 and $wday has
54310121
PP
1758the range 0..6 with sunday as day 0. Also, $year is the number of
1759years since 1900, that is, $year is 123 in year 2023.
1760
1761If EXPR is omitted, uses the current time (C<localtime(time)>).
a0d0e21e 1762
0a753a76 1763In a scalar context, returns the ctime(3) value:
a0d0e21e 1764
5f05dabc 1765 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
a0d0e21e 1766
54310121
PP
1767Also see the Time::Local module, and the strftime(3) and mktime(3)
1768function available via the POSIX module.
a0d0e21e
LW
1769
1770=item log EXPR
1771
54310121 1772=item log
bbce6d69 1773
a0d0e21e
LW
1774Returns logarithm (base I<e>) of EXPR. If EXPR is omitted, returns log
1775of $_.
1776
1777=item lstat FILEHANDLE
1778
1779=item lstat EXPR
1780
54310121 1781=item lstat
bbce6d69 1782
a0d0e21e
LW
1783Does the same thing as the stat() function, but stats a symbolic link
1784instead of the file the symbolic link points to. If symbolic links are
1785unimplemented on your system, a normal stat() is done.
1786
bbce6d69
PP
1787If EXPR is omitted, stats $_.
1788
a0d0e21e
LW
1789=item m//
1790
1791The match operator. See L<perlop>.
1792
1793=item map BLOCK LIST
1794
1795=item map EXPR,LIST
1796
1797Evaluates the BLOCK or EXPR for each element of LIST (locally setting $_ to each
1798element) and returns the list value composed of the results of each such
1799evaluation. Evaluates BLOCK or EXPR in a list context, so each element of LIST
1800may produce zero, one, or more elements in the returned value.
1801
1802 @chars = map(chr, @nums);
1803
1804translates a list of numbers to the corresponding characters. And
1805
4633a7c4 1806 %hash = map { getkey($_) => $_ } @array;
a0d0e21e
LW
1807
1808is just a funny way to write
1809
1810 %hash = ();
1811 foreach $_ (@array) {
4633a7c4 1812 $hash{getkey($_)} = $_;
a0d0e21e
LW
1813 }
1814
1815=item mkdir FILENAME,MODE
1816
1817Creates the directory specified by FILENAME, with permissions specified
1818by MODE (as modified by umask). If it succeeds it returns 1, otherwise
184e9718 1819it returns 0 and sets C<$!> (errno).
a0d0e21e
LW
1820
1821=item msgctl ID,CMD,ARG
1822
4633a7c4 1823Calls the System V IPC function msgctl(2). If CMD is &IPC_STAT, then ARG
a0d0e21e
LW
1824must be a variable which will hold the returned msqid_ds structure.
1825Returns like ioctl: the undefined value for error, "0 but true" for
1826zero, or the actual return value otherwise.
1827
1828=item msgget KEY,FLAGS
1829
4633a7c4 1830Calls the System V IPC function msgget(2). Returns the message queue id,
a0d0e21e
LW
1831or the undefined value if there is an error.
1832
1833=item msgsnd ID,MSG,FLAGS
1834
1835Calls the System V IPC function msgsnd to send the message MSG to the
1836message queue ID. MSG must begin with the long integer message type,
c07a80fd 1837which may be created with C<pack("l", $type)>. Returns TRUE if
a0d0e21e
LW
1838successful, or FALSE if there is an error.
1839
1840=item msgrcv ID,VAR,SIZE,TYPE,FLAGS
1841
1842Calls the System V IPC function msgrcv to receive a message from
1843message queue ID into variable VAR with a maximum message size of
1844SIZE. Note that if a message is received, the message type will be the
1845first thing in VAR, and the maximum length of VAR is SIZE plus the size
1846of the message type. Returns TRUE if successful, or FALSE if there is
1847an error.
1848
1849=item my EXPR
1850
1851A "my" declares the listed variables to be local (lexically) to the
cb1a09d0 1852enclosing block, subroutine, C<eval>, or C<do/require/use>'d file. If
5f05dabc 1853more than one value is listed, the list must be placed in parentheses. See
cb1a09d0 1854L<perlsub/"Private Variables via my()"> for details.
4633a7c4 1855
a0d0e21e
LW
1856=item next LABEL
1857
1858=item next
1859
1860The C<next> command is like the C<continue> statement in C; it starts
1861the next iteration of the loop:
1862
4633a7c4
LW
1863 LINE: while (<STDIN>) {
1864 next LINE if /^#/; # discard comments
a0d0e21e
LW
1865 ...
1866 }
1867
1868Note that if there were a C<continue> block on the above, it would get
1869executed even on discarded lines. If the LABEL is omitted, the command
1870refers to the innermost enclosing loop.
1871
1872=item no Module LIST
1873
1874See the "use" function, which "no" is the opposite of.
1875
1876=item oct EXPR
1877
54310121 1878=item oct
bbce6d69 1879
4633a7c4 1880Interprets EXPR as an octal string and returns the corresponding
2f9daede 1881value. (If EXPR happens to start off with 0x, interprets it as
4633a7c4
LW
1882a hex string instead.) The following will handle decimal, octal, and
1883hex in the standard Perl or C notation:
a0d0e21e
LW
1884
1885 $val = oct($val) if $val =~ /^0/;
1886
2f9daede
TPG
1887If EXPR is omitted, uses $_. This function is commonly used when
1888a string such as "644" needs to be converted into a file mode, for
1889example. (Although perl will automatically convert strings into
1890numbers as needed, this automatic conversion assumes base 10.)
a0d0e21e
LW
1891
1892=item open FILEHANDLE,EXPR
1893
1894=item open FILEHANDLE
1895
1896Opens the file whose filename is given by EXPR, and associates it with
5f05dabc
PP
1897FILEHANDLE. If FILEHANDLE is an expression, its value is used as the
1898name of the real filehandle wanted. If EXPR is omitted, the scalar
1899variable of the same name as the FILEHANDLE contains the filename.
1900(Note that lexical variables--those declared with C<my>--will not work
1901for this purpose; so if you're using C<my>, specify EXPR in your call
1902to open.)
1903
1904If the filename begins with '<' or nothing, the file is opened for input.
1905If the filename begins with '>', the file is truncated and opened for
1906output. If the filename begins with '>>', the file is opened for
1907appending. You can put a '+' in front of the '>' or '<' to indicate that
1908you want both read and write access to the file; thus '+<' is almost
1909always preferred for read/write updates--the '+>' mode would clobber the
1910file first. The prefix and the filename may be separated with spaces.
1911These various prefixes correspond to the fopen(3) modes of 'r', 'r+', 'w',
1912'w+', 'a', and 'a+'.
1913
1914If the filename begins with "|", the filename is interpreted as a command
1915to which output is to be piped, and if the filename ends with a "|", the
1916filename is interpreted See L<perlipc/"Using open() for IPC"> for more
1917examples of this. as command which pipes input to us. (You may not have
7e1af8bc
PP
1918a raw open() to a command that pipes both in I<and> out, but see
1919L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
1920for alternatives.)
cb1a09d0 1921
184e9718 1922Opening '-' opens STDIN and opening 'E<gt>-' opens STDOUT. Open returns
54310121 1923nonzero upon success, the undefined value otherwise. If the open
4633a7c4 1924involved a pipe, the return value happens to be the pid of the
54310121 1925subprocess.
cb1a09d0
AD
1926
1927If you're unfortunate enough to be running Perl on a system that
1928distinguishes between text files and binary files (modern operating
1929systems don't care), then you should check out L</binmode> for tips for
1930dealing with this. The key distinction between systems that need binmode
1931and those that don't is their text file formats. Systems like Unix and
1932Plan9 that delimit lines with a single character, and that encode that
1933character in C as '\n', do not need C<binmode>. The rest need it.
1934
cb1a09d0 1935Examples:
a0d0e21e
LW
1936
1937 $ARTICLE = 100;
1938 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
1939 while (<ARTICLE>) {...
1940
1941 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
1942
cb1a09d0
AD
1943 open(DBASE, '+<dbase.mine'); # open for update
1944
4633a7c4 1945 open(ARTICLE, "caesar <$article |"); # decrypt article
a0d0e21e 1946
4633a7c4 1947 open(EXTRACT, "|sort >/tmp/Tmp$$"); # $$ is our process id
a0d0e21e
LW
1948
1949 # process argument list of files along with any includes
1950
1951 foreach $file (@ARGV) {
1952 process($file, 'fh00');
1953 }
1954
1955 sub process {
1956 local($filename, $input) = @_;
1957 $input++; # this is a string increment
1958 unless (open($input, $filename)) {
1959 print STDERR "Can't open $filename: $!\n";
1960 return;
1961 }
1962
1963 while (<$input>) { # note use of indirection
1964 if (/^#include "(.*)"/) {
1965 process($1, $input);
1966 next;
1967 }
1968 ... # whatever
1969 }
1970 }
1971
1972You may also, in the Bourne shell tradition, specify an EXPR beginning
184e9718 1973with "E<gt>&", in which case the rest of the string is interpreted as the
a0d0e21e 1974name of a filehandle (or file descriptor, if numeric) which is to be
184e9718 1975duped and opened. You may use & after E<gt>, E<gt>E<gt>, E<lt>, +E<gt>,
5f05dabc 1976+E<gt>E<gt>, and +E<lt>. The
a0d0e21e 1977mode you specify should match the mode of the original filehandle.
184e9718 1978(Duping a filehandle does not take into account any existing contents of
cb1a09d0 1979stdio buffers.)
a0d0e21e
LW
1980Here is a script that saves, redirects, and restores STDOUT and
1981STDERR:
1982
1983 #!/usr/bin/perl
1984 open(SAVEOUT, ">&STDOUT");
1985 open(SAVEERR, ">&STDERR");
1986
1987 open(STDOUT, ">foo.out") || die "Can't redirect stdout";
1988 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
1989
1990 select(STDERR); $| = 1; # make unbuffered
1991 select(STDOUT); $| = 1; # make unbuffered
1992
1993 print STDOUT "stdout 1\n"; # this works for
1994 print STDERR "stderr 1\n"; # subprocesses too
1995
1996 close(STDOUT);
1997 close(STDERR);
1998
1999 open(STDOUT, ">&SAVEOUT");
2000 open(STDERR, ">&SAVEERR");
2001
2002 print STDOUT "stdout 2\n";
2003 print STDERR "stderr 2\n";
2004
2005
184e9718 2006If you specify "E<lt>&=N", where N is a number, then Perl will do an
4633a7c4
LW
2007equivalent of C's fdopen() of that file descriptor; this is more
2008parsimonious of file descriptors. For example:
a0d0e21e
LW
2009
2010 open(FILEHANDLE, "<&=$fd")
2011
5f05dabc 2012If you open a pipe on the command "-", i.e., either "|-" or "-|", then
a0d0e21e
LW
2013there is an implicit fork done, and the return value of open is the pid
2014of the child within the parent process, and 0 within the child
184e9718 2015process. (Use C<defined($pid)> to determine whether the open was successful.)
a0d0e21e
LW
2016The filehandle behaves normally for the parent, but i/o to that
2017filehandle is piped from/to the STDOUT/STDIN of the child process.
2018In the child process the filehandle isn't opened--i/o happens from/to
2019the new STDOUT or STDIN. Typically this is used like the normal
2020piped open when you want to exercise more control over just how the
2021pipe command gets executed, such as when you are running setuid, and
54310121 2022don't want to have to scan shell commands for metacharacters.
4633a7c4 2023The following pairs are more or less equivalent:
a0d0e21e
LW
2024
2025 open(FOO, "|tr '[a-z]' '[A-Z]'");
2026 open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
2027
2028 open(FOO, "cat -n '$file'|");
2029 open(FOO, "-|") || exec 'cat', '-n', $file;
2030
4633a7c4
LW
2031See L<perlipc/"Safe Pipe Opens"> for more examples of this.
2032
0dccf244 2033NOTE: On any operation which may do a fork, unflushed buffers remain
184e9718 2034unflushed in both processes, which means you may need to set C<$|> to
a0d0e21e
LW
2035avoid duplicate output.
2036
0dccf244
CS
2037Closing any piped filehandle causes the parent process to wait for the
2038child to finish, and returns the status value in C<$?>.
2039
5f05dabc
PP
2040Using the constructor from the IO::Handle package (or one of its
2041subclasses, such as IO::File or IO::Socket),
c07a80fd
PP
2042you can generate anonymous filehandles which have the scope of whatever
2043variables hold references to them, and automatically close whenever
2044and however you leave that scope:
2045
5f05dabc 2046 use IO::File;
c07a80fd
PP
2047 ...
2048 sub read_myfile_munged {
2049 my $ALL = shift;
5f05dabc 2050 my $handle = new IO::File;
c07a80fd
PP
2051 open($handle, "myfile") or die "myfile: $!";
2052 $first = <$handle>
2053 or return (); # Automatically closed here.
2054 mung $first or die "mung failed"; # Or here.
2055 return $first, <$handle> if $ALL; # Or here.
2056 $first; # Or here.
2057 }
2058
a0d0e21e 2059The filename that is passed to open will have leading and trailing
5f05dabc 2060whitespace deleted. To open a file with arbitrary weird
a0d0e21e
LW
2061characters in it, it's necessary to protect any leading and trailing
2062whitespace thusly:
2063
cb1a09d0
AD
2064 $file =~ s#^(\s)#./$1#;
2065 open(FOO, "< $file\0");
2066
c07a80fd
PP
2067If you want a "real" C open() (see L<open(2)> on your system), then
2068you should use the sysopen() function. This is another way to
2069protect your filenames from interpretation. For example:
cb1a09d0 2070
28757baa 2071 use IO::Handle;
c07a80fd
PP
2072 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL, 0700)
2073 or die "sysopen $path: $!";
2074 HANDLE->autoflush(1);
2075 HANDLE->print("stuff $$\n");
2076 seek(HANDLE, 0, 0);
2077 print "File contains: ", <HANDLE>;
cb1a09d0
AD
2078
2079See L</seek()> for some details about mixing reading and writing.
a0d0e21e
LW
2080
2081=item opendir DIRHANDLE,EXPR
2082
2083Opens a directory named EXPR for processing by readdir(), telldir(),
5f05dabc 2084seekdir(), rewinddir(), and closedir(). Returns TRUE if successful.
a0d0e21e
LW
2085DIRHANDLEs have their own namespace separate from FILEHANDLEs.
2086
2087=item ord EXPR
2088
54310121 2089=item ord
bbce6d69 2090
a0d0e21e 2091Returns the numeric ascii value of the first character of EXPR. If
dc848c6f 2092EXPR is omitted, uses $_. For the reverse, see L</chr>.
a0d0e21e
LW
2093
2094=item pack TEMPLATE,LIST
2095
2096Takes an array or list of values and packs it into a binary structure,
2097returning the string containing the structure. The TEMPLATE is a
2098sequence of characters that give the order and type of values, as
2099follows:
2100
2101 A An ascii string, will be space padded.
2102 a An ascii string, will be null padded.
2103 b A bit string (ascending bit order, like vec()).
2104 B A bit string (descending bit order).
2105 h A hex string (low nybble first).
2106 H A hex string (high nybble first).
2107
2108 c A signed char value.
2109 C An unsigned char value.
96e4d5b1 2110
a0d0e21e
LW
2111 s A signed short value.
2112 S An unsigned short value.
96e4d5b1
PP
2113 (This 'short' is _exactly_ 16 bits, which may differ from
2114 what a local C compiler calls 'short'.)
2115
a0d0e21e
LW
2116 i A signed integer value.
2117 I An unsigned integer value.
96e4d5b1
PP
2118 (This 'integer' is _at_least_ 32 bits wide. Its exact size
2119 depends on what a local C compiler calls 'int', and may
2120 even be larger than the 'long' described in the next item.)
2121
a0d0e21e
LW
2122 l A signed long value.
2123 L An unsigned long value.
96e4d5b1
PP
2124 (This 'long' is _exactly_ 32 bits, which may differ from
2125 what a local C compiler calls 'long'.)
a0d0e21e 2126
96e4d5b1
PP
2127 n A short in "network" (big-endian) order.
2128 N A long in "network" (big-endian) order.
a0d0e21e
LW
2129 v A short in "VAX" (little-endian) order.
2130 V A long in "VAX" (little-endian) order.
96e4d5b1
PP
2131 (These 'shorts' and 'longs' are _exactly_ 16 bits and
2132 _exactly_ 32 bits, respectively.)
a0d0e21e
LW
2133
2134 f A single-precision float in the native format.
2135 d A double-precision float in the native format.
2136
2137 p A pointer to a null-terminated string.
2138 P A pointer to a structure (fixed-length string).
2139
2140 u A uuencoded string.
2141
96e4d5b1
PP
2142 w A BER compressed integer. Its bytes represent an unsigned
2143 integer in base 128, most significant digit first, with as few
2144 digits as possible. Bit eight (the high bit) is set on each
2145 byte except the last.
def98dd4 2146
a0d0e21e
LW
2147 x A null byte.
2148 X Back up a byte.
2149 @ Null fill to absolute position.
2150
2151Each letter may optionally be followed by a number which gives a repeat
5f05dabc 2152count. With all types except "a", "A", "b", "B", "h", "H", and "P" the
a0d0e21e
LW
2153pack function will gobble up that many values from the LIST. A * for the
2154repeat count means to use however many items are left. The "a" and "A"
2155types gobble just one value, but pack it as a string of length count,
2156padding with nulls or spaces as necessary. (When unpacking, "A" strips
2157trailing spaces and nulls, but "a" does not.) Likewise, the "b" and "B"
2158fields pack a string that many bits long. The "h" and "H" fields pack a
2159string that many nybbles long. The "P" packs a pointer to a structure of
2160the size indicated by the length. Real numbers (floats and doubles) are
2161in the native machine format only; due to the multiplicity of floating
2162formats around, and the lack of a standard "network" representation, no
2163facility for interchange has been made. This means that packed floating
2164point data written on one machine may not be readable on another - even if
2165both use IEEE floating point arithmetic (as the endian-ness of the memory
2166representation is not part of the IEEE spec). Note that Perl uses doubles
2167internally for all numeric calculation, and converting from double into
5f05dabc 2168float and thence back to double again will lose precision (i.e.,
a0d0e21e
LW
2169C<unpack("f", pack("f", $foo)>) will not in general equal $foo).
2170
2171Examples:
2172
2173 $foo = pack("cccc",65,66,67,68);
2174 # foo eq "ABCD"
2175 $foo = pack("c4",65,66,67,68);
2176 # same thing
2177
2178 $foo = pack("ccxxcc",65,66,67,68);
2179 # foo eq "AB\0\0CD"
2180
2181 $foo = pack("s2",1,2);
2182 # "\1\0\2\0" on little-endian
2183 # "\0\1\0\2" on big-endian
2184
2185 $foo = pack("a4","abcd","x","y","z");
2186 # "abcd"
2187
2188 $foo = pack("aaaa","abcd","x","y","z");
2189 # "axyz"
2190
2191 $foo = pack("a14","abcdefg");
2192 # "abcdefg\0\0\0\0\0\0\0"
2193
2194 $foo = pack("i9pl", gmtime);
2195 # a real struct tm (on my system anyway)
2196
2197 sub bintodec {
2198 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
2199 }
2200
2201The same template may generally also be used in the unpack function.
2202
cb1a09d0
AD
2203=item package NAMESPACE
2204
2205Declares the compilation unit as being in the given namespace. The scope
2206of the package declaration is from the declaration itself through the end of
2207the enclosing block (the same scope as the local() operator). All further
2208unqualified dynamic identifiers will be in this namespace. A package
5f05dabc 2209statement affects only dynamic variables--including those you've used
cb1a09d0
AD
2210local() on--but I<not> lexical variables created with my(). Typically it
2211would be the first declaration in a file to be included by the C<require>
2212or C<use> operator. You can switch into a package in more than one place;
5f05dabc 2213it influences merely which symbol table is used by the compiler for the
cb1a09d0
AD
2214rest of that block. You can refer to variables and filehandles in other
2215packages by prefixing the identifier with the package name and a double
2216colon: C<$Package::Variable>. If the package name is null, the C<main>
2217package as assumed. That is, C<$::sail> is equivalent to C<$main::sail>.
2218
2219See L<perlmod/"Packages"> for more information about packages, modules,
2220and classes. See L<perlsub> for other scoping issues.
2221
a0d0e21e
LW
2222=item pipe READHANDLE,WRITEHANDLE
2223
2224Opens a pair of connected pipes like the corresponding system call.
2225Note that if you set up a loop of piped processes, deadlock can occur
2226unless you are very careful. In addition, note that Perl's pipes use
184e9718 2227stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE
a0d0e21e
LW
2228after each command, depending on the application.
2229
7e1af8bc 2230See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
4633a7c4
LW
2231for examples of such things.
2232
a0d0e21e
LW
2233=item pop ARRAY
2234
54310121 2235=item pop
28757baa 2236
a0d0e21e
LW
2237Pops and returns the last value of the array, shortening the array by
22381. Has a similar effect to
2239
2240 $tmp = $ARRAY[$#ARRAY--];
2241
2242If there are no elements in the array, returns the undefined value.
cb1a09d0
AD
2243If ARRAY is omitted, pops the
2244@ARGV array in the main program, and the @_ array in subroutines, just
2245like shift().
a0d0e21e
LW
2246
2247=item pos SCALAR
2248
54310121 2249=item pos
bbce6d69 2250
4633a7c4 2251Returns the offset of where the last C<m//g> search left off for the variable
2f9daede 2252is in question ($_ is used when the variable is not specified). May be
44a8e56a
PP
2253modified to change that offset. Such modification will also influence
2254the C<\G> zero-width assertion in regular expressions. See L<perlre> and
2255L<perlop>.
a0d0e21e
LW
2256
2257=item print FILEHANDLE LIST
2258
2259=item print LIST
2260
2261=item print
2262
cb1a09d0 2263Prints a string or a comma-separated list of strings. Returns TRUE
a0d0e21e 2264if successful. FILEHANDLE may be a scalar variable name, in which case
cb1a09d0 2265the variable contains the name of or a reference to the filehandle, thus introducing one
a0d0e21e
LW
2266level of indirection. (NOTE: If FILEHANDLE is a variable and the next
2267token is a term, it may be misinterpreted as an operator unless you
5f05dabc 2268interpose a + or put parentheses around the arguments.) If FILEHANDLE is
a0d0e21e 2269omitted, prints by default to standard output (or to the last selected
da0045b7 2270output channel--see L</select>). If LIST is also omitted, prints $_ to
a0d0e21e
LW
2271STDOUT. To set the default output channel to something other than
2272STDOUT use the select operation. Note that, because print takes a
2273LIST, anything in the LIST is evaluated in a list context, and any
2274subroutine that you call will have one or more of its expressions
2275evaluated in a list context. Also be careful not to follow the print
2276keyword with a left parenthesis unless you want the corresponding right
2277parenthesis to terminate the arguments to the print--interpose a + or
5f05dabc 2278put parentheses around all the arguments.
a0d0e21e 2279
4633a7c4 2280Note that if you're storing FILEHANDLES in an array or other expression,
da0045b7 2281you will have to use a block returning its value instead:
4633a7c4
LW
2282
2283 print { $files[$i] } "stuff\n";
2284 print { $OK ? STDOUT : STDERR } "stuff\n";
2285
5f05dabc 2286=item printf FILEHANDLE FORMAT, LIST
a0d0e21e 2287
5f05dabc 2288=item printf FORMAT, LIST
a0d0e21e 2289
a034a98d
DD
2290Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>. The first argument
2291of the list will be interpreted as the printf format. If C<use locale> is
2292in effect, the character used for the decimal point in formatted real numbers
2293is affected by the LC_NUMERIC locale. See L<perllocale>.
a0d0e21e 2294
28757baa
PP
2295Don't fall into the trap of using a printf() when a simple
2296print() would do. The print() is more efficient, and less
2297error prone.
2298
da0045b7
PP
2299=item prototype FUNCTION
2300
2301Returns the prototype of a function as a string (or C<undef> if the
5f05dabc
PP
2302function has no prototype). FUNCTION is a reference to, or the name of,
2303the function whose prototype you want to retrieve.
da0045b7 2304
a0d0e21e
LW
2305=item push ARRAY,LIST
2306
2307Treats ARRAY as a stack, and pushes the values of LIST
2308onto the end of ARRAY. The length of ARRAY increases by the length of
2309LIST. Has the same effect as
2310
2311 for $value (LIST) {
2312 $ARRAY[++$#ARRAY] = $value;
2313 }
2314
2315but is more efficient. Returns the new number of elements in the array.
2316
2317=item q/STRING/
2318
2319=item qq/STRING/
2320
2321=item qx/STRING/
2322
2323=item qw/STRING/
2324
2325Generalized quotes. See L<perlop>.
2326
2327=item quotemeta EXPR
2328
54310121 2329=item quotemeta
bbce6d69 2330
68dc0745 2331Returns the value of EXPR with all non-alphanumeric
a034a98d
DD
2332characters backslashed. (That is, all characters not matching
2333C</[A-Za-z_0-9]/> will be preceded by a backslash in the
2334returned string, regardless of any locale settings.)
2335This is the internal function implementing
a0d0e21e
LW
2336the \Q escape in double-quoted strings.
2337
bbce6d69
PP
2338If EXPR is omitted, uses $_.
2339
a0d0e21e
LW
2340=item rand EXPR
2341
2342=item rand
2343
3e3baf6d
TB
2344Returns a random fractional number greater than or equal to 0 and less
2345than the value of EXPR. (EXPR should be positive.) If EXPR is
2346omitted, the value 1 is used. Automatically calls srand() unless
2347srand() has already been called. See also srand().
a0d0e21e 2348
2f9daede 2349(Note: If your rand function consistently returns numbers that are too
a0d0e21e 2350large or too small, then your version of Perl was probably compiled
2f9daede 2351with the wrong number of RANDBITS.)
a0d0e21e
LW
2352
2353=item read FILEHANDLE,SCALAR,LENGTH,OFFSET
2354
2355=item read FILEHANDLE,SCALAR,LENGTH
2356
2357Attempts to read LENGTH bytes of data into variable SCALAR from the
2358specified FILEHANDLE. Returns the number of bytes actually read, or
2359undef if there was an error. SCALAR will be grown or shrunk to the
2360length actually read. An OFFSET may be specified to place the read
2361data at some other place than the beginning of the string. This call
2362is actually implemented in terms of stdio's fread call. To get a true
2363read system call, see sysread().
2364
2365=item readdir DIRHANDLE
2366
2367Returns the next directory entry for a directory opened by opendir().
2368If used in a list context, returns all the rest of the entries in the
2369directory. If there are no more entries, returns an undefined value in
2370a scalar context or a null list in a list context.
2371
cb1a09d0 2372If you're planning to filetest the return values out of a readdir(), you'd
5f05dabc 2373better prepend the directory in question. Otherwise, because we didn't
cb1a09d0
AD
2374chdir() there, it would have been testing the wrong file.
2375
2376 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
2377 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
2378 closedir DIR;
2379
a0d0e21e
LW
2380=item readlink EXPR
2381
54310121 2382=item readlink
bbce6d69 2383
a0d0e21e
LW
2384Returns the value of a symbolic link, if symbolic links are
2385implemented. If not, gives a fatal error. If there is some system
184e9718 2386error, returns the undefined value and sets C<$!> (errno). If EXPR is
a0d0e21e
LW
2387omitted, uses $_.
2388
2389=item recv SOCKET,SCALAR,LEN,FLAGS
2390
2391Receives a message on a socket. Attempts to receive LENGTH bytes of
2392data into variable SCALAR from the specified SOCKET filehandle.
2393Actually does a C recvfrom(), so that it can returns the address of the
2394sender. Returns the undefined value if there's an error. SCALAR will
2395be grown or shrunk to the length actually read. Takes the same flags
54310121 2396as the system call of the same name.
4633a7c4 2397See L<perlipc/"UDP: Message Passing"> for examples.
a0d0e21e
LW
2398
2399=item redo LABEL
2400
2401=item redo
2402
2403The C<redo> command restarts the loop block without evaluating the
2404conditional again. The C<continue> block, if any, is not executed. If
2405the LABEL is omitted, the command refers to the innermost enclosing
2406loop. This command is normally used by programs that want to lie to
2407themselves about what was just input:
2408
2409 # a simpleminded Pascal comment stripper
2410 # (warning: assumes no { or } in strings)
4633a7c4 2411 LINE: while (<STDIN>) {
a0d0e21e
LW
2412 while (s|({.*}.*){.*}|$1 |) {}
2413 s|{.*}| |;
2414 if (s|{.*| |) {
2415 $front = $_;
2416 while (<STDIN>) {
2417 if (/}/) { # end of comment?
2418 s|^|$front{|;
4633a7c4 2419 redo LINE;
a0d0e21e
LW
2420 }
2421 }
2422 }
2423 print;
2424 }
2425
2426=item ref EXPR
2427
54310121 2428=item ref
bbce6d69 2429
2f9daede
TPG
2430Returns a TRUE value if EXPR is a reference, FALSE otherwise. If EXPR
2431is not specified, $_ will be used. The value returned depends on the
bbce6d69 2432type of thing the reference is a reference to.
a0d0e21e
LW
2433Builtin types include:
2434
2435 REF
2436 SCALAR
2437 ARRAY
2438 HASH
2439 CODE
2440 GLOB
2441
54310121 2442If the referenced object has been blessed into a package, then that package
a0d0e21e
LW
2443name is returned instead. You can think of ref() as a typeof() operator.
2444
2445 if (ref($r) eq "HASH") {
aa689395 2446 print "r is a reference to a hash.\n";
54310121 2447 }
a0d0e21e
LW
2448 if (!ref ($r) {
2449 print "r is not a reference at all.\n";
54310121 2450 }
a0d0e21e
LW
2451
2452See also L<perlref>.
2453
2454=item rename OLDNAME,NEWNAME
2455
2456Changes the name of a file. Returns 1 for success, 0 otherwise. Will
5f05dabc 2457not work across file system boundaries.
a0d0e21e
LW
2458
2459=item require EXPR
2460
2461=item require
2462
2463Demands some semantics specified by EXPR, or by $_ if EXPR is not
2464supplied. If EXPR is numeric, demands that the current version of Perl
184e9718 2465(C<$]> or $PERL_VERSION) be equal or greater than EXPR.
a0d0e21e
LW
2466
2467Otherwise, demands that a library file be included if it hasn't already
2468been included. The file is included via the do-FILE mechanism, which is
2469essentially just a variety of eval(). Has semantics similar to the following
2470subroutine:
2471
2472 sub require {
2473 local($filename) = @_;
2474 return 1 if $INC{$filename};
2475 local($realfilename,$result);
2476 ITER: {
2477 foreach $prefix (@INC) {
2478 $realfilename = "$prefix/$filename";
2479 if (-f $realfilename) {
2480 $result = do $realfilename;
2481 last ITER;
2482 }
2483 }
2484 die "Can't find $filename in \@INC";
2485 }
2486 die $@ if $@;
2487 die "$filename did not return true value" unless $result;
2488 $INC{$filename} = $realfilename;
2489 $result;
2490 }
2491
2492Note that the file will not be included twice under the same specified
2493name. The file must return TRUE as the last statement to indicate
2494successful execution of any initialization code, so it's customary to
2495end such a file with "1;" unless you're sure it'll return TRUE
2496otherwise. But it's better just to put the "C<1;>", in case you add more
2497statements.
2498
54310121 2499If EXPR is a bareword, the require assumes a "F<.pm>" extension and
da0045b7 2500replaces "F<::>" with "F</>" in the filename for you,
54310121 2501to make it easy to load standard modules. This form of loading of
a0d0e21e
LW
2502modules does not risk altering your namespace.
2503
54310121 2504For a yet-more-powerful import facility, see L</use> and
748a9306 2505L<perlmod>.
a0d0e21e
LW
2506
2507=item reset EXPR
2508
2509=item reset
2510
2511Generally used in a C<continue> block at the end of a loop to clear
2512variables and reset ?? searches so that they work again. The
2513expression is interpreted as a list of single characters (hyphens
2514allowed for ranges). All variables and arrays beginning with one of
2515those letters are reset to their pristine state. If the expression is
5f05dabc
PP
2516omitted, one-match searches (?pattern?) are reset to match again. Resets
2517only variables or searches in the current package. Always returns
a0d0e21e
LW
25181. Examples:
2519
2520 reset 'X'; # reset all X variables
2521 reset 'a-z'; # reset lower case variables
2522 reset; # just reset ?? searches
2523
5f05dabc
PP
2524Resetting "A-Z" is not recommended because you'll wipe out your
2525ARGV and ENV arrays. Resets only package variables--lexical variables
a0d0e21e 2526are unaffected, but they clean themselves up on scope exit anyway,
da0045b7 2527so you'll probably want to use them instead. See L</my>.
a0d0e21e 2528
54310121
PP
2529=item return EXPR
2530
2531=item return
2532
2533Returns from a subroutine, eval(), or do FILE with the value of the
2534given EXPR. Evaluation of EXPR may be in a list, scalar, or void
2535context, depending on how the return value will be used, and the context
2536may vary from one execution to the next (see wantarray()). If no EXPR
2537is given, returns an empty list in a list context, an undefined value in
2538a scalar context, or nothing in a void context.
a0d0e21e 2539
68dc0745
PP
2540(Note that in the absence of a return, a subroutine, eval, or do FILE
2541will automatically return the value of the last expression evaluated.)
a0d0e21e
LW
2542
2543=item reverse LIST
2544
2545In a list context, returns a list value consisting of the elements
2f9daede
TPG
2546of LIST in the opposite order. In a scalar context, concatenates the
2547elements of LIST, and returns a string value consisting of those bytes,
2548but in the opposite order.
4633a7c4 2549
2f9daede 2550 print reverse <>; # line tac, last line first
4633a7c4 2551
2f9daede
TPG
2552 undef $/; # for efficiency of <>
2553 print scalar reverse <>; # byte tac, last line tsrif
2554
2555This operator is also handy for inverting a hash, although there are some
2556caveats. If a value is duplicated in the original hash, only one of those
2557can be represented as a key in the inverted hash. Also, this has to
2558unwind one hash and build a whole new one, which may take some time
2559on a large hash.
2560
2561 %by_name = reverse %by_address; # Invert the hash
a0d0e21e
LW
2562
2563=item rewinddir DIRHANDLE
2564
2565Sets the current position to the beginning of the directory for the
2566readdir() routine on DIRHANDLE.
2567
2568=item rindex STR,SUBSTR,POSITION
2569
2570=item rindex STR,SUBSTR
2571
2572Works just like index except that it returns the position of the LAST
2573occurrence of SUBSTR in STR. If POSITION is specified, returns the
2574last occurrence at or before that position.
2575
2576=item rmdir FILENAME
2577
54310121 2578=item rmdir
bbce6d69 2579
a0d0e21e 2580Deletes the directory specified by FILENAME if it is empty. If it
184e9718 2581succeeds it returns 1, otherwise it returns 0 and sets C<$!> (errno). If
a0d0e21e
LW
2582FILENAME is omitted, uses $_.
2583
2584=item s///
2585
2586The substitution operator. See L<perlop>.
2587
2588=item scalar EXPR
2589
2590Forces EXPR to be interpreted in a scalar context and returns the value
54310121 2591of EXPR.
cb1a09d0
AD
2592
2593 @counts = ( scalar @a, scalar @b, scalar @c );
2594
54310121 2595There is no equivalent operator to force an expression to
cb1a09d0
AD
2596be interpolated in a list context because it's in practice never
2597needed. If you really wanted to do so, however, you could use
2598the construction C<@{[ (some expression) ]}>, but usually a simple
2599C<(some expression)> suffices.
a0d0e21e
LW
2600
2601=item seek FILEHANDLE,POSITION,WHENCE
2602
8903cb82
PP
2603Sets FILEHANDLE's position, just like the fseek() call of stdio.
2604FILEHANDLE may be an expression whose value gives the name of the
2605filehandle. The values for WHENCE are 0 to set the new position to
2606POSITION, 1 to set it to the current position plus POSITION, and 2 to
2607set it to EOF plus POSITION (typically negative). For WHENCE you may
2608use the constants SEEK_SET, SEEK_CUR, and SEEK_END from either the
2609IO::Seekable or the POSIX module. Returns 1 upon success, 0 otherwise.
2610
2611If you want to position file for sysread() or syswrite(), don't use
2612seek() -- buffering makes its effect on the file's system position
137443ea 2613unpredictable and non-portable. Use sysseek() instead.
a0d0e21e 2614
cb1a09d0
AD
2615On some systems you have to do a seek whenever you switch between reading
2616and writing. Amongst other things, this may have the effect of calling
8903cb82
PP
2617stdio's clearerr(3). A WHENCE of 1 (SEEK_CUR) is useful for not moving
2618the file position:
cb1a09d0
AD
2619
2620 seek(TEST,0,1);
2621
2622This is also useful for applications emulating C<tail -f>. Once you hit
2623EOF on your read, and then sleep for a while, you might have to stick in a
8903cb82
PP
2624seek() to reset things. The seek() doesn't change the current position,
2625but it I<does> clear the end-of-file condition on the handle, so that the
2626next C<E<lt>FILEE<gt>> makes Perl try again to read something. We hope.
cb1a09d0
AD
2627
2628If that doesn't work (some stdios are particularly cantankerous), then
2629you may need something more like this:
2630
2631 for (;;) {
2632 for ($curpos = tell(FILE); $_ = <FILE>; $curpos = tell(FILE)) {
2633 # search for some stuff and put it into files
2634 }
2635 sleep($for_a_while);
2636 seek(FILE, $curpos, 0);
2637 }
2638
a0d0e21e
LW
2639=item seekdir DIRHANDLE,POS
2640
2641Sets the current position for the readdir() routine on DIRHANDLE. POS
2642must be a value returned by telldir(). Has the same caveats about
2643possible directory compaction as the corresponding system library
2644routine.
2645
2646=item select FILEHANDLE
2647
2648=item select
2649
2650Returns the currently selected filehandle. Sets the current default
2651filehandle for output, if FILEHANDLE is supplied. This has two
2652effects: first, a C<write> or a C<print> without a filehandle will
2653default to this FILEHANDLE. Second, references to variables related to
2654output will refer to this output channel. For example, if you have to
2655set the top of form format for more than one output channel, you might
2656do the following:
2657
2658 select(REPORT1);
2659 $^ = 'report1_top';
2660 select(REPORT2);
2661 $^ = 'report2_top';
2662
2663FILEHANDLE may be an expression whose value gives the name of the
2664actual filehandle. Thus:
2665
2666 $oldfh = select(STDERR); $| = 1; select($oldfh);
2667
4633a7c4
LW
2668Some programmers may prefer to think of filehandles as objects with
2669methods, preferring to write the last example as:
a0d0e21e 2670
28757baa 2671 use IO::Handle;
a0d0e21e
LW
2672 STDERR->autoflush(1);
2673
2674=item select RBITS,WBITS,EBITS,TIMEOUT
2675
5f05dabc 2676This calls the select(2) system call with the bit masks specified, which
a0d0e21e
LW
2677can be constructed using fileno() and vec(), along these lines:
2678
2679 $rin = $win = $ein = '';
2680 vec($rin,fileno(STDIN),1) = 1;
2681 vec($win,fileno(STDOUT),1) = 1;
2682 $ein = $rin | $win;
2683
2684If you want to select on many filehandles you might wish to write a
2685subroutine:
2686
2687 sub fhbits {
2688 local(@fhlist) = split(' ',$_[0]);
2689 local($bits);
2690 for (@fhlist) {
2691 vec($bits,fileno($_),1) = 1;
2692 }
2693 $bits;
2694 }
4633a7c4 2695 $rin = fhbits('STDIN TTY SOCK');
a0d0e21e
LW
2696
2697The usual idiom is:
2698
2699 ($nfound,$timeleft) =
2700 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
2701
54310121 2702or to block until something becomes ready just do this
a0d0e21e
LW
2703
2704 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
2705
5f05dabc 2706Most systems do not bother to return anything useful in $timeleft, so
c07a80fd
PP
2707calling select() in a scalar context just returns $nfound.
2708
5f05dabc 2709Any of the bit masks can also be undef. The timeout, if specified, is
a0d0e21e
LW
2710in seconds, which may be fractional. Note: not all implementations are
2711capable of returning the $timeleft. If not, they always return
2712$timeleft equal to the supplied $timeout.
2713
ff68c719 2714You can effect a sleep of 250 milliseconds this way:
a0d0e21e
LW
2715
2716 select(undef, undef, undef, 0.25);
2717
184e9718 2718B<WARNING>: Do not attempt to mix buffered I/O (like read() or E<lt>FHE<gt>)
cb1a09d0 2719with select(). You have to use sysread() instead.
a0d0e21e
LW
2720
2721=item semctl ID,SEMNUM,CMD,ARG
2722
2723Calls the System V IPC function semctl. If CMD is &IPC_STAT or
2724&GETALL, then ARG must be a variable which will hold the returned
2725semid_ds structure or semaphore value array. Returns like ioctl: the
2726undefined value for error, "0 but true" for zero, or the actual return
2727value otherwise.
2728
2729=item semget KEY,NSEMS,FLAGS
2730
2731Calls the System V IPC function semget. Returns the semaphore id, or
2732the undefined value if there is an error.
2733
2734=item semop KEY,OPSTRING
2735
2736Calls the System V IPC function semop to perform semaphore operations
2737such as signaling and waiting. OPSTRING must be a packed array of
2738semop structures. Each semop structure can be generated with
2739C<pack("sss", $semnum, $semop, $semflag)>. The number of semaphore
2740operations is implied by the length of OPSTRING. Returns TRUE if
2741successful, or FALSE if there is an error. As an example, the
2742following code waits on semaphore $semnum of semaphore id $semid:
2743
2744 $semop = pack("sss", $semnum, -1, 0);
2745 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
2746
2747To signal the semaphore, replace "-1" with "1".
2748
2749=item send SOCKET,MSG,FLAGS,TO
2750
2751=item send SOCKET,MSG,FLAGS
2752
2753Sends a message on a socket. Takes the same flags as the system call
2754of the same name. On unconnected sockets you must specify a
2755destination to send TO, in which case it does a C sendto(). Returns
2756the number of characters sent, or the undefined value if there is an
2757error.
4633a7c4 2758See L<perlipc/"UDP: Message Passing"> for examples.
a0d0e21e
LW
2759
2760=item setpgrp PID,PGRP
2761
2762Sets the current process group for the specified PID, 0 for the current
2763process. Will produce a fatal error if used on a machine that doesn't
5f05dabc 2764implement setpgrp(2). If the arguments are omitted, it defaults to
47e29363
PP
27650,0. Note that the POSIX version of setpgrp() does not accept any
2766arguments, so only setpgrp 0,0 is portable.
a0d0e21e
LW
2767
2768=item setpriority WHICH,WHO,PRIORITY
2769
2770Sets the current priority for a process, a process group, or a user.
748a9306 2771(See setpriority(2).) Will produce a fatal error if used on a machine
a0d0e21e
LW
2772that doesn't implement setpriority(2).
2773
2774=item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
2775
2776Sets the socket option requested. Returns undefined if there is an
2777error. OPTVAL may be specified as undef if you don't want to pass an
2778argument.
2779
2780=item shift ARRAY
2781
2782=item shift
2783
2784Shifts the first value of the array off and returns it, shortening the
2785array by 1 and moving everything down. If there are no elements in the
2786array, returns the undefined value. If ARRAY is omitted, shifts the
2787@ARGV array in the main program, and the @_ array in subroutines.
2788(This is determined lexically.) See also unshift(), push(), and pop().
2789Shift() and unshift() do the same thing to the left end of an array
2f9daede 2790that pop() and push() do to the right end.
a0d0e21e
LW
2791
2792=item shmctl ID,CMD,ARG
2793
2794Calls the System V IPC function shmctl. If CMD is &IPC_STAT, then ARG
2795must be a variable which will hold the returned shmid_ds structure.
2796Returns like ioctl: the undefined value for error, "0 but true" for
2797zero, or the actual return value otherwise.
2798
2799=item shmget KEY,SIZE,FLAGS
2800
2801Calls the System V IPC function shmget. Returns the shared memory
2802segment id, or the undefined value if there is an error.
2803
2804=item shmread ID,VAR,POS,SIZE
2805
2806=item shmwrite ID,STRING,POS,SIZE
2807
2808Reads or writes the System V shared memory segment ID starting at
2809position POS for size SIZE by attaching to it, copying in/out, and
2810detaching from it. When reading, VAR must be a variable which will
2811hold the data read. When writing, if STRING is too long, only SIZE
2812bytes are used; if STRING is too short, nulls are written to fill out
2813SIZE bytes. Return TRUE if successful, or FALSE if there is an error.
2814
2815=item shutdown SOCKET,HOW
2816
2817Shuts down a socket connection in the manner indicated by HOW, which
2818has the same interpretation as in the system call of the same name.
2819
2820=item sin EXPR
2821
54310121 2822=item sin
bbce6d69 2823
a0d0e21e
LW
2824Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
2825returns sine of $_.
2826
54310121 2827For the inverse sine operation, you may use the POSIX::asin()
28757baa
PP
2828function, or use this relation:
2829
2830 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
2831
a0d0e21e
LW
2832=item sleep EXPR
2833
2834=item sleep
2835
2836Causes the script to sleep for EXPR seconds, or forever if no EXPR.
2837May be interrupted by sending the process a SIGALRM. Returns the
2838number of seconds actually slept. You probably cannot mix alarm() and
5f05dabc 2839sleep() calls, because sleep() is often implemented using alarm().
a0d0e21e
LW
2840
2841On some older systems, it may sleep up to a full second less than what
2842you requested, depending on how it counts seconds. Most modern systems
2843always sleep the full amount.
2844
cb1a09d0 2845For delays of finer granularity than one second, you may use Perl's
54310121
PP
2846syscall() interface to access setitimer(2) if your system supports it,
2847or else see L</select()> below.
cb1a09d0 2848
5f05dabc
PP
2849See also the POSIX module's sigpause() function.
2850
a0d0e21e
LW
2851=item socket SOCKET,DOMAIN,TYPE,PROTOCOL
2852
2853Opens a socket of the specified kind and attaches it to filehandle
5f05dabc 2854SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for the
a0d0e21e 2855system call of the same name. You should "use Socket;" first to get
4633a7c4 2856the proper definitions imported. See the example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
2857
2858=item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
2859
2860Creates an unnamed pair of sockets in the specified domain, of the
5f05dabc 2861specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
a0d0e21e
LW
2862for the system call of the same name. If unimplemented, yields a fatal
2863error. Returns TRUE if successful.
2864
2865=item sort SUBNAME LIST
2866
2867=item sort BLOCK LIST
2868
2869=item sort LIST
2870
2f9daede
TPG
2871Sorts the LIST and returns the sorted list value. If SUBNAME or BLOCK
2872is omitted, sorts in standard string comparison order. If SUBNAME is
2873specified, it gives the name of a subroutine that returns an integer
2874less than, equal to, or greater than 0, depending on how the elements
2875of the array are to be ordered. (The C<E<lt>=E<gt>> and C<cmp>
2876operators are extremely useful in such routines.) SUBNAME may be a
2877scalar variable name, in which case the value provides the name of the
2878subroutine to use. In place of a SUBNAME, you can provide a BLOCK as
2879an anonymous, in-line sort subroutine.
a0d0e21e 2880
cb1a09d0
AD
2881In the interests of efficiency the normal calling code for subroutines is
2882bypassed, with the following effects: the subroutine may not be a
2883recursive subroutine, and the two elements to be compared are passed into
2884the subroutine not via @_ but as the package global variables $a and
2885$b (see example below). They are passed by reference, so don't
2886modify $a and $b. And don't try to declare them as lexicals either.
a0d0e21e 2887
0a753a76
PP
2888You also cannot exit out of the sort block or subroutine using any of the
2889loop control operators described in L<perlsyn> or with goto().
2890
a034a98d
DD
2891When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
2892current collation locale. See L<perllocale>.
2893
a0d0e21e
LW
2894Examples:
2895
2896 # sort lexically
2897 @articles = sort @files;
2898
2899 # same thing, but with explicit sort routine
2900 @articles = sort {$a cmp $b} @files;
2901
cb1a09d0 2902 # now case-insensitively
54310121 2903 @articles = sort {uc($a) cmp uc($b)} @files;
cb1a09d0 2904
a0d0e21e
LW
2905 # same thing in reversed order
2906 @articles = sort {$b cmp $a} @files;
2907
2908 # sort numerically ascending
2909 @articles = sort {$a <=> $b} @files;
2910
2911 # sort numerically descending
2912 @articles = sort {$b <=> $a} @files;
2913
2914 # sort using explicit subroutine name
2915 sub byage {
2f9daede 2916 $age{$a} <=> $age{$b}; # presuming numeric
a0d0e21e
LW
2917 }
2918 @sortedclass = sort byage @class;
2919
aa689395
PP
2920 # this sorts the %age hash by value instead of key
2921 # using an in-line function
c07a80fd
PP
2922 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
2923
a0d0e21e
LW
2924 sub backwards { $b cmp $a; }
2925 @harry = ('dog','cat','x','Cain','Abel');
2926 @george = ('gone','chased','yz','Punished','Axed');
2927 print sort @harry;
2928 # prints AbelCaincatdogx
2929 print sort backwards @harry;
2930 # prints xdogcatCainAbel
2931 print sort @george, 'to', @harry;
2932 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
2933
54310121
PP
2934 # inefficiently sort by descending numeric compare using
2935 # the first integer after the first = sign, or the
cb1a09d0
AD
2936 # whole record case-insensitively otherwise
2937
2938 @new = sort {
2939 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
2940 ||
2941 uc($a) cmp uc($b)
2942 } @old;
2943
2944 # same thing, but much more efficiently;
2945 # we'll build auxiliary indices instead
2946 # for speed
2947 @nums = @caps = ();
54310121 2948 for (@old) {
cb1a09d0
AD
2949 push @nums, /=(\d+)/;
2950 push @caps, uc($_);
54310121 2951 }
cb1a09d0
AD
2952
2953 @new = @old[ sort {
2954 $nums[$b] <=> $nums[$a]
2955 ||
2956 $caps[$a] cmp $caps[$b]
2957 } 0..$#old
2958 ];
2959
2960 # same thing using a Schwartzian Transform (no temps)
2961 @new = map { $_->[0] }
2962 sort { $b->[1] <=> $a->[1]
2963 ||
2964 $a->[2] cmp $b->[2]
2965 } map { [$_, /=(\d+)/, uc($_)] } @old;
2966
184e9718 2967If you're using strict, you I<MUST NOT> declare $a
cb1a09d0
AD
2968and $b as lexicals. They are package globals. That means
2969if you're in the C<main> package, it's
2970
2971 @articles = sort {$main::b <=> $main::a} @files;
2972
2973or just
2974
2975 @articles = sort {$::b <=> $::a} @files;
2976
2977but if you're in the C<FooPack> package, it's
2978
2979 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
2980
55497cff
PP
2981The comparison function is required to behave. If it returns
2982inconsistent results (sometimes saying $x[1] is less than $x[2] and
2983sometimes saying the opposite, for example) the Perl interpreter will
2984probably crash and dump core. This is entirely due to and dependent
2985upon your system's qsort(3) library routine; this routine often avoids
2986sanity checks in the interest of speed.
2987
a0d0e21e
LW
2988=item splice ARRAY,OFFSET,LENGTH,LIST
2989
2990=item splice ARRAY,OFFSET,LENGTH
2991
2992=item splice ARRAY,OFFSET
2993
2994Removes the elements designated by OFFSET and LENGTH from an array, and
2995replaces them with the elements of LIST, if any. Returns the elements
2996removed from the array. The array grows or shrinks as necessary. If
2997LENGTH is omitted, removes everything from OFFSET onward. The
5f05dabc 2998following equivalences hold (assuming C<$[ == 0>):
a0d0e21e
LW
2999
3000 push(@a,$x,$y) splice(@a,$#a+1,0,$x,$y)
3001 pop(@a) splice(@a,-1)
3002 shift(@a) splice(@a,0,1)
3003 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
3004 $a[$x] = $y splice(@a,$x,1,$y);
3005
3006Example, assuming array lengths are passed before arrays:
3007
3008 sub aeq { # compare two list values
3009 local(@a) = splice(@_,0,shift);
3010 local(@b) = splice(@_,0,shift);
3011 return 0 unless @a == @b; # same len?
3012 while (@a) {
3013 return 0 if pop(@a) ne pop(@b);
3014 }
3015 return 1;
3016 }
3017 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
3018
3019=item split /PATTERN/,EXPR,LIMIT
3020
3021=item split /PATTERN/,EXPR
3022
3023=item split /PATTERN/
3024
3025=item split
3026
3027Splits a string into an array of strings, and returns it.
3028
3029If not in a list context, returns the number of fields found and splits into
3030the @_ array. (In a list context, you can force the split into @_ by
3031using C<??> as the pattern delimiters, but it still returns the array
3032value.) The use of implicit split to @_ is deprecated, however.
3033
3034If EXPR is omitted, splits the $_ string. If PATTERN is also omitted,
4633a7c4
LW
3035splits on whitespace (after skipping any leading whitespace). Anything
3036matching PATTERN is taken to be a delimiter separating the fields. (Note
3037that the delimiter may be longer than one character.) If LIMIT is
3038specified and is not negative, splits into no more than that many fields
3039(though it may split into fewer). If LIMIT is unspecified, trailing null
3040fields are stripped (which potential users of pop() would do well to
3041remember). If LIMIT is negative, it is treated as if an arbitrarily large
3042LIMIT had been specified.
a0d0e21e
LW
3043
3044A pattern matching the null string (not to be confused with
748a9306 3045a null pattern C<//>, which is just one member of the set of patterns
a0d0e21e
LW
3046matching a null string) will split the value of EXPR into separate
3047characters at each point it matches that way. For example:
3048
3049 print join(':', split(/ */, 'hi there'));
3050
3051produces the output 'h:i:t:h:e:r:e'.
3052
5f05dabc 3053The LIMIT parameter can be used to split a line partially
a0d0e21e
LW
3054
3055 ($login, $passwd, $remainder) = split(/:/, $_, 3);
3056
3057When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT
3058one larger than the number of variables in the list, to avoid
3059unnecessary work. For the list above LIMIT would have been 4 by
3060default. In time critical applications it behooves you not to split
3061into more fields than you really need.
3062
3063If the PATTERN contains parentheses, additional array elements are
3064created from each matching substring in the delimiter.
3065
da0045b7 3066 split(/([,-])/, "1-10,20", 3);
a0d0e21e
LW
3067
3068produces the list value
3069
3070 (1, '-', 10, ',', 20)
3071
54310121 3072If you had the entire header of a normal Unix email message in $header,
4633a7c4
LW
3073you could split it up into fields and their values this way:
3074
3075 $header =~ s/\n\s+/ /g; # fix continuation lines
3076 %hdrs = (UNIX_FROM => split /^(.*?):\s*/m, $header);
3077
a0d0e21e
LW
3078The pattern C</PATTERN/> may be replaced with an expression to specify
3079patterns that vary at runtime. (To do runtime compilation only once,
748a9306
LW
3080use C</$variable/o>.)
3081
3082As a special case, specifying a PATTERN of space (C<' '>) will split on
3083white space just as split with no arguments does. Thus, split(' ') can
3084be used to emulate B<awk>'s default behavior, whereas C<split(/ /)>
3085will give you as many null initial fields as there are leading spaces.
3086A split on /\s+/ is like a split(' ') except that any leading
3087whitespace produces a null first field. A split with no arguments
3088really does a C<split(' ', $_)> internally.
a0d0e21e
LW
3089
3090Example:
3091
3092 open(passwd, '/etc/passwd');
3093 while (<passwd>) {
54310121 3094 ($login, $passwd, $uid, $gid, $gcos,
748a9306 3095 $home, $shell) = split(/:/);
a0d0e21e
LW
3096 ...
3097 }
3098
54310121 3099(Note that $shell above will still have a newline on it. See L</chop>,
a0d0e21e
LW
3100L</chomp>, and L</join>.)
3101
5f05dabc 3102=item sprintf FORMAT, LIST
a0d0e21e 3103
74a77017
CS
3104Returns a string formatted by the usual printf conventions of the
3105C library function sprintf(). See L<sprintf(3)> or L<printf(3)>
3106on your system for an explanation of the general principles.
3107
3108Perl does all of its own sprintf() formatting -- it emulates the C
3109function sprintf(), but it doesn't use it (except for floating-point
3110numbers, and even then only the standard modifiers are allowed). As a
3111result, any non-standard extensions in your local sprintf() are not
3112available from Perl.
3113
3114Perl's sprintf() permits the following universally-known conversions:
3115
3116 %% a percent sign
3117 %c a character with the given number
3118 %s a string
3119 %d a signed integer, in decimal
3120 %u an unsigned integer, in decimal
3121 %o an unsigned integer, in octal
3122 %x an unsigned integer, in hexadecimal
3123 %e a floating-point number, in scientific notation
3124 %f a floating-point number, in fixed decimal notation
3125 %g a floating-point number, in %e or %f notation
3126
1b3f7d21 3127In addition, Perl permits the following widely-supported conversions:
74a77017 3128
74a77017
CS
3129 %X like %x, but using upper-case letters
3130 %E like %e, but using an upper-case "E"
3131 %G like %g, but with an upper-case "E" (if applicable)
3132 %p a pointer (outputs the Perl value's address in hexadecimal)
1b3f7d21
CS
3133 %n special: *stores* the number of characters output so far
3134 into the next variable in the parameter list
74a77017 3135
1b3f7d21
CS
3136Finally, for backward (and we do mean "backward") compatibility, Perl
3137permits these unnecessary but widely-supported conversions:
74a77017 3138
1b3f7d21 3139 %i a synonym for %d
74a77017
CS
3140 %D a synonym for %ld
3141 %U a synonym for %lu
3142 %O a synonym for %lo
3143 %F a synonym for %f
3144
3145Perl permits the following universally-known flags between the C<%>
3146and the conversion letter:
3147
3148 space prefix positive number with a space
3149 + prefix positive number with a plus sign
3150 - left-justify within the field
3151 0 use zeros, not spaces, to right-justify
1b3f7d21 3152 # prefix octal with "0", hex with "0x"
74a77017
CS
3153 number minimum field width
3154 .number "precision": digits after decimal point for floating-point,
3155 max length for string, minimum length for integer
3156 l interpret integer as C type "long" or "unsigned long"
74a77017
CS
3157 h interpret integer as C type "short" or "unsigned short"
3158
1b3f7d21 3159There is also one Perl-specific flag:
74a77017
CS
3160
3161 V interpret integer as Perl's standard integer type
3162
3163Where a number would appear in the flags, an asterisk ("*") may be
3164used instead, in which case Perl uses the next item in the parameter
3165list as the given number (that is, as the field width or precision).
3166If a field width obtained through "*" is negative, it has the same
3167effect as the '-' flag: left-justification.
3168
3169If C<use locale> is in effect, the character used for the decimal
3170point in formatted real numbers is affected by the LC_NUMERIC locale.
3171See L<perllocale>.
a0d0e21e
LW
3172
3173=item sqrt EXPR
3174
54310121 3175=item sqrt
bbce6d69 3176
a0d0e21e
LW
3177Return the square root of EXPR. If EXPR is omitted, returns square
3178root of $_.
3179
3180=item srand EXPR
3181
93dc8474
CS
3182=item srand
3183
3184Sets the random number seed for the C<rand> operator. If EXPR is
3185omitted, uses a semi-random value based on the current time and process
3186ID, among other things. In versions of Perl prior to 5.004 the default
3187seed was just the current time(). This isn't a particularly good seed,
3188so many old programs supply their own seed value (often C<time ^ $$> or
3189C<time ^ ($$ + ($$ << 15))>), but that isn't necessary any more.
3190
3191In fact, it's usually not necessary to call srand() at all, because if
3192it is not called explicitly, it is called implicitly at the first use of
2f9daede
TPG
3193the C<rand> operator. However, this was not the case in version of Perl
3194before 5.004, so if your script will run under older Perl versions, it
3195should call srand().
93dc8474 3196
2f9daede
TPG
3197Note that you need something much more random than the default seed for
3198cryptographic purposes. Checksumming the compressed output of one or more
3199rapidly changing operating system status programs is the usual method. For
3200example:
28757baa
PP
3201
3202 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
3203
0078ec44
RS
3204If you're particularly concerned with this, see the Math::TrulyRandom
3205module in CPAN.
3206
3207Do I<not> call srand() multiple times in your program unless you know
28757baa
PP
3208exactly what you're doing and why you're doing it. The point of the
3209function is to "seed" the rand() function so that rand() can produce
3210a different sequence each time you run your program. Just do it once at the
3211top of your program, or you I<won't> get random numbers out of rand()!
3212
54310121 3213Frequently called programs (like CGI scripts) that simply use
28757baa
PP
3214
3215 time ^ $$
3216
54310121 3217for a seed can fall prey to the mathematical property that
28757baa
PP
3218
3219 a^b == (a+1)^(b+1)
3220
0078ec44 3221one-third of the time. So don't do that.
f86702cc 3222
a0d0e21e
LW
3223=item stat FILEHANDLE
3224
3225=item stat EXPR
3226
54310121 3227=item stat
bbce6d69 3228
a0d0e21e 3229Returns a 13-element array giving the status info for a file, either the
2f9daede 3230file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted, it
bbce6d69
PP
3231stats $_. Returns a null list if the stat fails. Typically used as
3232follows:
3233
a0d0e21e
LW
3234
3235 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
3236 $atime,$mtime,$ctime,$blksize,$blocks)
3237 = stat($filename);
3238
54310121 3239Not all fields are supported on all filesystem types. Here are the
c07a80fd
PP
3240meaning of the fields:
3241
54310121
PP
3242 0 dev device number of filesystem
3243 1 ino inode number
3244 2 mode file mode (type and permissions)
3245 3 nlink number of (hard) links to the file
3246 4 uid numeric user ID of file's owner
3247 5 gid numeric group ID of file's owner
3248 6 rdev the device identifier (special files only)
3249 7 size total size of file, in bytes
3250 8 atime last access time since the epoch
3251 9 mtime last modify time since the epoch
3252 10 ctime inode change time (NOT creation time!) since the epoch
3253 11 blksize preferred block size for file system I/O
3254 12 blocks actual number of blocks allocated
c07a80fd
PP
3255
3256(The epoch was at 00:00 January 1, 1970 GMT.)
3257
a0d0e21e
LW
3258If stat is passed the special filehandle consisting of an underline, no
3259stat is done, but the current contents of the stat structure from the
3260last stat or filetest are returned. Example:
3261
3262 if (-x $file && (($d) = stat(_)) && $d < 0) {
3263 print "$file is executable NFS file\n";
3264 }
3265
5f05dabc 3266(This works on machines only for which the device number is negative under NFS.)
a0d0e21e
LW
3267
3268=item study SCALAR
3269
3270=item study
3271
184e9718 3272Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
a0d0e21e
LW
3273doing many pattern matches on the string before it is next modified.
3274This may or may not save time, depending on the nature and number of
3275patterns you are searching on, and on the distribution of character
54310121 3276frequencies in the string to be searched -- you probably want to compare
5f05dabc 3277run times with and without it to see which runs faster. Those loops
a0d0e21e
LW
3278which scan for many short constant strings (including the constant
3279parts of more complex patterns) will benefit most. You may have only
54310121 3280one study active at a time -- if you study a different scalar the first
a0d0e21e
LW
3281is "unstudied". (The way study works is this: a linked list of every
3282character in the string to be searched is made, so we know, for
3283example, where all the 'k' characters are. From each search string,
3284the rarest character is selected, based on some static frequency tables
3285constructed from some C programs and English text. Only those places
3286that contain this "rarest" character are examined.)
3287
3288For example, here is a loop which inserts index producing entries
3289before any line containing a certain pattern:
3290
3291 while (<>) {
3292 study;
3293 print ".IX foo\n" if /\bfoo\b/;
3294 print ".IX bar\n" if /\bbar\b/;
3295 print ".IX blurfl\n" if /\bblurfl\b/;
3296 ...
3297 print;
3298 }
3299
3300In searching for /\bfoo\b/, only those locations in $_ that contain "f"
3301will be looked at, because "f" is rarer than "o". In general, this is
3302a big win except in pathological cases. The only question is whether
3303it saves you more time than it took to build the linked list in the
3304first place.
3305
3306Note that if you have to look for strings that you don't know till
3307runtime, you can build an entire loop as a string and eval that to
3308avoid recompiling all your patterns all the time. Together with
3309undefining $/ to input entire files as one record, this can be very
3310fast, often faster than specialized programs like fgrep(1). The following
184e9718 3311scans a list of files (C<@files>) for a list of words (C<@words>), and prints
a0d0e21e
LW
3312out the names of those files that contain a match:
3313
3314 $search = 'while (<>) { study;';
3315 foreach $word (@words) {
3316 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
3317 }
3318 $search .= "}";
3319 @ARGV = @files;
3320 undef $/;
3321 eval $search; # this screams
5f05dabc 3322 $/ = "\n"; # put back to normal input delimiter
a0d0e21e
LW
3323 foreach $file (sort keys(%seen)) {
3324 print $file, "\n";
3325 }
3326
cb1a09d0
AD
3327=item sub BLOCK
3328
3329=item sub NAME
3330
3331=item sub NAME BLOCK
3332
3333This is subroutine definition, not a real function I<per se>. With just a
3334NAME (and possibly prototypes), it's just a forward declaration. Without
3335a NAME, it's an anonymous function declaration, and does actually return a
2f9daede 3336value: the CODE ref of the closure you just created. See L<perlsub> and
cb1a09d0
AD
3337L<perlref> for details.
3338
a0d0e21e
LW
3339=item substr EXPR,OFFSET,LEN
3340
3341=item substr EXPR,OFFSET
3342
3343Extracts a substring out of EXPR and returns it. First character is at
2f9daede
TPG
3344offset 0, or whatever you've set C<$[> to (but don't do that).
3345If OFFSET is negative, starts
a0d0e21e 3346that far from the end of the string. If LEN is omitted, returns
748a9306
LW
3347everything to the end of the string. If LEN is negative, leaves that
3348many characters off the end of the string.
3349
3350You can use the substr() function
a0d0e21e
LW
3351as an lvalue, in which case EXPR must be an lvalue. If you assign
3352something shorter than LEN, the string will shrink, and if you assign
3353something longer than LEN, the string will grow to accommodate it. To
3354keep the string the same length you may need to pad or chop your value
3355using sprintf().
3356
3357=item symlink OLDFILE,NEWFILE
3358
3359Creates a new filename symbolically linked to the old filename.
3360Returns 1 for success, 0 otherwise. On systems that don't support
3361symbolic links, produces a fatal error at run time. To check for that,
3362use eval:
3363
54310121 3364 $symlink_exists = (eval {symlink("","")};, $@ eq '');
a0d0e21e
LW
3365
3366=item syscall LIST
3367
3368Calls the system call specified as the first element of the list,
3369passing the remaining elements as arguments to the system call. If
3370unimplemented, produces a fatal error. The arguments are interpreted
3371as follows: if a given argument is numeric, the argument is passed as
3372an int. If not, the pointer to the string value is passed. You are
3373responsible to make sure a string is pre-extended long enough to
3374receive any result that might be written into a string. If your
3375integer arguments are not literals and have never been interpreted in a
3376numeric context, you may need to add 0 to them to force them to look
3377like numbers.
3378
3379 require 'syscall.ph'; # may need to run h2ph
3380 syscall(&SYS_write, fileno(STDOUT), "hi there\n", 9);
3381
5f05dabc 3382Note that Perl supports passing of up to only 14 arguments to your system call,
a0d0e21e
LW
3383which in practice should usually suffice.
3384
c07a80fd
PP
3385=item sysopen FILEHANDLE,FILENAME,MODE
3386
3387=item sysopen FILEHANDLE,FILENAME,MODE,PERMS
3388
3389Opens the file whose filename is given by FILENAME, and associates it
3390with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
3391the name of the real filehandle wanted. This function calls the
3392underlying operating system's C<open> function with the parameters
3393FILENAME, MODE, PERMS.
3394
3395The possible values and flag bits of the MODE parameter are
3396system-dependent; they are available via the standard module C<Fcntl>.
3397However, for historical reasons, some values are universal: zero means
3398read-only, one means write-only, and two means read/write.
3399
3400If the file named by FILENAME does not exist and the C<open> call
3401creates it (typically because MODE includes the O_CREAT flag), then
3402the value of PERMS specifies the permissions of the newly created
3403file. If PERMS is omitted, the default value is 0666, which allows
3404read and write for all. This default is reasonable: see C<umask>.
3405
28757baa
PP
3406The IO::File module provides a more object-oriented approach, if you're
3407into that kind of thing.
3408
a0d0e21e
LW
3409=item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
3410
3411=item sysread FILEHANDLE,SCALAR,LENGTH
3412
3413Attempts to read LENGTH bytes of data into variable SCALAR from the
96e4d5b1
PP
3414specified FILEHANDLE, using the system call read(2). It bypasses stdio,
3415so mixing this with other kinds of reads, print(), write(), seek(), or
3416tell() can cause confusion. Returns the number of bytes actually read,
3417or undef if there was an error. SCALAR will be grown or shrunk so that
3418the last byte actually read is the last byte of the scalar after the
3419read.
ff68c719
PP
3420
3421An OFFSET may be specified to place the read data at some place in the
3422string other than the beginning. A negative OFFSET specifies
3423placement at that many bytes counting backwards from the end of the
3424string. A positive OFFSET greater than the length of SCALAR results
3425in the string being padded to the required size with "\0" bytes before
3426the result of the read is appended.
a0d0e21e 3427
137443ea
PP
3428=item sysseek FILEHANDLE,POSITION,WHENCE
3429
8903cb82
PP
3430Sets FILEHANDLE's system position using the system call lseek(2). It
3431bypasses stdio, so mixing this with reads (other than sysread()),
3432print(), write(), seek(), or tell() may cause confusion. FILEHANDLE may
3433be an expression whose value gives the name of the filehandle. The
3434values for WHENCE are 0 to set the new position to POSITION, 1 to set
3435the it to the current position plus POSITION, and 2 to set it to EOF
3436plus POSITION (typically negative). For WHENCE, you may use the
3437constants SEEK_SET, SEEK_CUR, and SEEK_END from either the IO::Seekable
3438or the POSIX module.
3439
3440Returns the new position, or the undefined value on failure. A position
3441of zero is returned as the string "0 but true"; thus sysseek() returns
3442TRUE on success and FALSE on failure, yet you can still easily determine
3443the new position.
137443ea 3444
a0d0e21e
LW
3445=item system LIST
3446
3447Does exactly the same thing as "exec LIST" except that a fork is done
3448first, and the parent process waits for the child process to complete.
3449Note that argument processing varies depending on the number of
3450arguments. The return value is the exit status of the program as
3451returned by the wait() call. To get the actual exit value divide by
54310121
PP
3452256. See also L</exec>. This is I<NOT> what you want to use to capture
3453the output from a command, for that you should use merely backticks or
28757baa 3454qx//, as described in L<perlop/"`STRING`">.
a0d0e21e 3455
54310121 3456Because system() and backticks block SIGINT and SIGQUIT, killing the
28757baa
PP
3457program they're running doesn't actually interrupt your program.
3458
3459 @args = ("command", "arg1", "arg2");
54310121
PP
3460 system(@args) == 0
3461 or die "system @args failed: $?"
28757baa
PP
3462
3463Here's a more elaborate example of analysing the return value from
54310121
PP
3464system() on a Unix system to check for all possibilities, including for
3465signals and core dumps.
28757baa
PP
3466
3467 $rc = 0xffff & system @args;
3468 printf "system(%s) returned %#04x: ", "@args", $rc;
3469 if ($rc == 0) {
3470 print "ran with normal exit\n";
54310121 3471 }
28757baa
PP
3472 elsif ($rc == 0xff00) {
3473 print "command failed: $!\n";
54310121 3474 }
28757baa
PP
3475 elsif ($rc > 0x80) {
3476 $rc >>= 8;
3477 print "ran with non-zero exit status $rc\n";
54310121 3478 }
28757baa
PP
3479 else {
3480 print "ran with ";
3481 if ($rc & 0x80) {
3482 $rc &= ~0x80;
54310121
PP
3483 print "core dump from ";
3484 }
28757baa 3485 print "signal $rc\n"
54310121 3486 }
28757baa 3487 $ok = ($rc != 0);
f86702cc 3488
bb32b41a
GS
3489When the arguments get executed via the system shell, results will
3490be subject to its quirks and capabilities. See L<perlop/"`STRING`">
3491for details.
3492
a0d0e21e
LW
3493=item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
3494
3495=item syswrite FILEHANDLE,SCALAR,LENGTH
3496
3497Attempts to write LENGTH bytes of data from variable SCALAR to the
3498specified FILEHANDLE, using the system call write(2). It bypasses
96e4d5b1
PP
3499stdio, so mixing this with reads (other than sysread()), print(),
3500write(), seek(), or tell() may cause confusion. Returns the number of
3501bytes actually written, or undef if there was an error. If the length
3502is greater than the available data, only as much data as is available
3503will be written.
ff68c719
PP
3504
3505An OFFSET may be specified to write the data from some part of the
3506string other than the beginning. A negative OFFSET specifies writing
54310121 3507that many bytes counting backwards from the end of the string.
a0d0e21e
LW
3508
3509=item tell FILEHANDLE
3510
3511=item tell
3512
8903cb82 3513Returns the current position for FILEHANDLE. FILEHANDLE may be an
a0d0e21e
LW
3514expression whose value gives the name of the actual filehandle. If
3515FILEHANDLE is omitted, assumes the file last read.
3516
3517=item telldir DIRHANDLE
3518
3519Returns the current position of the readdir() routines on DIRHANDLE.
3520Value may be given to seekdir() to access a particular location in a
3521directory. Has the same caveats about possible directory compaction as
3522the corresponding system library routine.
3523
4633a7c4 3524=item tie VARIABLE,CLASSNAME,LIST
a0d0e21e 3525
4633a7c4
LW
3526This function binds a variable to a package class that will provide the
3527implementation for the variable. VARIABLE is the name of the variable
3528to be enchanted. CLASSNAME is the name of a class implementing objects
3529of correct type. Any additional arguments are passed to the "new"
3530method of the class (meaning TIESCALAR, TIEARRAY, or TIEHASH).
3531Typically these are arguments such as might be passed to the dbm_open()
cb1a09d0
AD
3532function of C. The object returned by the "new" method is also
3533returned by the tie() function, which would be useful if you want to
4633a7c4 3534access other methods in CLASSNAME.
a0d0e21e
LW
3535
3536Note that functions such as keys() and values() may return huge array
748a9306
LW
3537values when used on large objects, like DBM files. You may prefer to
3538use the each() function to iterate over such. Example:
a0d0e21e
LW
3539
3540 # print out history file offsets
4633a7c4 3541 use NDBM_File;
da0045b7 3542 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
a0d0e21e
LW
3543 while (($key,$val) = each %HIST) {
3544 print $key, ' = ', unpack('L',$val), "\n";
3545 }
3546 untie(%HIST);
3547
aa689395 3548A class implementing a hash should have the following methods:
a0d0e21e 3549
4633a7c4 3550 TIEHASH classname, LIST
a0d0e21e
LW
3551 DESTROY this
3552 FETCH this, key
3553 STORE this, key, value
3554 DELETE this, key
3555 EXISTS this, key
3556 FIRSTKEY this
3557 NEXTKEY this, lastkey
3558
4633a7c4 3559A class implementing an ordinary array should have the following methods:
a0d0e21e 3560
4633a7c4 3561 TIEARRAY classname, LIST
a0d0e21e
LW
3562 DESTROY this
3563 FETCH this, key
3564 STORE this, key, value
3565 [others TBD]
3566
4633a7c4 3567A class implementing a scalar should have the following methods:
a0d0e21e 3568
4633a7c4 3569 TIESCALAR classname, LIST
a0d0e21e 3570 DESTROY this
54310121 3571 FETCH this,
a0d0e21e
LW
3572 STORE this, value
3573
4633a7c4
LW
3574Unlike dbmopen(), the tie() function will not use or require a module
3575for you--you need to do that explicitly yourself. See L<DB_File>
3576or the F<Config> module for interesting tie() implementations.
3577
f3cbc334
RS
3578=item tied VARIABLE
3579
3580Returns a reference to the object underlying VARIABLE (the same value
3581that was originally returned by the tie() call which bound the variable
3582to a package.) Returns the undefined value if VARIABLE isn't tied to a
3583package.
3584
a0d0e21e
LW
3585=item time
3586
da0045b7
PP
3587Returns the number of non-leap seconds since whatever time the system
3588considers to be the epoch (that's 00:00:00, January 1, 1904 for MacOS,
3589and 00:00:00 UTC, January 1, 1970 for most other systems).
3590Suitable for feeding to gmtime() and localtime().
a0d0e21e
LW
3591
3592=item times
3593
3594Returns a four-element array giving the user and system times, in
3595seconds, for this process and the children of this process.
3596
3597 ($user,$system,$cuser,$csystem) = times;
3598
3599=item tr///
3600
54310121 3601The translation operator. Same as y///. See L<perlop>.
a0d0e21e
LW
3602
3603=item truncate FILEHANDLE,LENGTH
3604
3605=item truncate EXPR,LENGTH
3606
3607Truncates the file opened on FILEHANDLE, or named by EXPR, to the
3608specified length. Produces a fatal error if truncate isn't implemented
3609on your system.
3610
3611=item uc EXPR
3612
54310121 3613=item uc
bbce6d69 3614
a0d0e21e
LW
3615Returns an uppercased version of EXPR. This is the internal function
3616implementing the \U escape in double-quoted strings.
a034a98d 3617Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
a0d0e21e 3618
bbce6d69
PP
3619If EXPR is omitted, uses $_.
3620
a0d0e21e
LW
3621=item ucfirst EXPR
3622
54310121 3623=item ucfirst
bbce6d69 3624
a0d0e21e
LW
3625Returns the value of EXPR with the first character uppercased. This is
3626the internal function implementing the \u escape in double-quoted strings.
a034a98d 3627Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
a0d0e21e 3628
bbce6d69
PP
3629If EXPR is omitted, uses $_.
3630
a0d0e21e
LW
3631=item umask EXPR
3632
3633=item umask
3634
2f9daede
TPG
3635Sets the umask for the process to EXPR and returns the previous value.
3636If EXPR is omitted, merely returns the current umask. Remember that a
3637umask is a number, usually given in octal; it is I<not> a string of octal
dc848c6f 3638digits. See also L</oct>, if all you have is a string.
a0d0e21e
LW
3639
3640=item undef EXPR
3641
3642=item undef
3643
54310121
PP
3644Undefines the value of EXPR, which must be an lvalue. Use only on a
3645scalar value, an entire array, an entire hash, or a subroutine name (using
2f9daede
TPG
3646"&"). (Using undef() will probably not do what you expect on most
3647predefined variables or DBM list values, so don't do that.) Always
3648returns the undefined value. You can omit the EXPR, in which case
3649nothing is undefined, but you still get an undefined value that you
3650could, for instance, return from a subroutine, assign to a variable or
3651pass as a parameter. Examples: