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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
5f05dabc 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
68dc0745 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
37798a01 181=item Functions new in perl5
182
183abs, bless, chomp, chr, exists, formline, glob, import, lc,
da0045b7 184lcfirst, map, my, no, prototype, qx, qw, readline, readpipe,
185ref, sub*, sysopen, tie, tied, uc, ucfirst, untie, use
37798a01 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
28757baa 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
bbce6d69 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
54310121 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
ff68c719 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;
ff68c719 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
28757baa 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,
28757baa 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
dc848c6f 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
28757baa 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
bbce6d69 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
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595=item cos EXPR
596
597Returns the cosine of EXPR (expressed in radians). If EXPR is omitted
598takes cosine of $_.
599
28757baa 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
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605=item crypt PLAINTEXT,SALT
606
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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
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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 $pwd) {
631 die "Sorry...\n";
632 } else {
633 print "ok\n";
54310121 634 }
a0d0e21e 635
54310121 636Of course, typing in your own password to whomever asks you
748a9306 637for it is unwise.
a0d0e21e 638
aa689395 639=item dbmclose HASH
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640
641[This function has been superseded by the untie() function.]
642
aa689395 643Breaks the binding between a DBM file and a hash.
a0d0e21e 644
aa689395 645=item dbmopen HASH,DBNAME,MODE
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646
647[This function has been superseded by the tie() function.]
648
aa689395 649This binds a dbm(3), ndbm(3), sdbm(3), gdbm(), or Berkeley DB file to a
650hash. HASH is the name of the hash. (Unlike normal open, the first
651argument is I<NOT> a filehandle, even though it looks like one). DBNAME
652is the name of the database (without the F<.dir> or F<.pag> extension if
653any). If the database does not exist, it is created with protection
654specified by MODE (as modified by the umask()). If your system supports
655only the older DBM functions, you may perform only one dbmopen() in your
656program. In older versions of Perl, if your system had neither DBM nor
657ndbm, calling dbmopen() produced a fatal error; it now falls back to
658sdbm(3).
659
660If you don't have write access to the DBM file, you can only read hash
661variables, not set them. If you want to test whether you can write,
662either use file tests or try setting a dummy hash entry inside an eval(),
663which will trap the error.
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664
665Note that functions such as keys() and values() may return huge array
666values when used on large DBM files. You may prefer to use the each()
667function to iterate over large DBM files. Example:
668
669 # print out history file offsets
670 dbmopen(%HIST,'/usr/lib/news/history',0666);
671 while (($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
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680=item defined EXPR
681
54310121 682=item defined
bbce6d69 683
2f9daede
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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.
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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
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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 $1 is defined, despite the fact that it
723matched "nothing". But it didn't really match nothing--rather, it
724matched something that happened to be 0 characters long. This is all
725very above-board and honest. When a function returns an undefined value,
2f9daede
TP
726it's an admission that it couldn't give you an honest answer. So you
727should use defined() only when you're questioning the integrity of what
728you're trying to do. At other times, a simple comparison to 0 or "" is
729what you want.
730
731Currently, using defined() on an entire array or hash reports whether
732memory for that aggregate has ever been allocated. So an array you set
733to the empty list appears undefined initially, and one that once was full
734and that you then set to the empty list still appears defined. You
735should instead use a simple test for size:
28757baa 736
737 if (@an_array) { print "has array elements\n" }
738 if (%a_hash) { print "has hash members\n" }
739
740Using undef() on these, however, does clear their memory and then report
741them as not defined anymore, but you shoudln't do that unless you don't
742plan to use them again, because it saves time when you load them up
743again to have memory already ready to be filled.
744
54310121 745This counterintuitive behaviour of defined() on aggregates may be
28757baa 746changed, fixed, or broken in a future release of Perl.
747
dc848c6f 748See also L</undef>, L</exists>, L</ref>.
2f9daede 749
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750=item delete EXPR
751
aa689395 752Deletes the specified key(s) and their associated values from a hash.
753For each key, returns the deleted value associated with that key, or
754the undefined value if there was no such key. Deleting from C<$ENV{}>
755modifies the environment. Deleting from a hash tied to a DBM file
5f05dabc 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 761 foreach $key (keys %HASH) {
762 delete $HASH{$key};
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LW
763 }
764
5f05dabc 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:
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LW
772
773 delete $ref->[$x][$y]{$key};
5f05dabc 774 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
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775
776=item die LIST
777
778Outside of an eval(), prints the value of LIST to C<STDERR> and exits with
184e9718 779the current value of C<$!> (errno). If C<$!> is 0, exits with the value of
54310121 780C<($? E<gt>E<gt> 8)> (backtick `command` status). If C<($? E<gt>E<gt> 8)>
28757baa 781is 0, exits with 255. Inside an eval(), the error message is stuffed into
782C<$@>, 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: $!\n"
a0d0e21e
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789
790If the value of EXPR does not end in a newline, the current script line
791number and input line number (if any) are also printed, and a newline
792is supplied. Hint: sometimes appending ", stopped" to your message
793will cause it to make better sense when the string "at foo line 123" is
794appended. Suppose you are running script "canasta".
795
796 die "/etc/games is no good";
797 die "/etc/games is no good, stopped";
798
799produce, respectively
800
801 /etc/games is no good at canasta line 123.
802 /etc/games is no good, stopped at canasta line 123.
803
804See also exit() and warn().
805
774d564b 806You can arrange for a callback to be called just before the die() does
807its deed, by setting the C<$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
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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 $ARGV[0] eq '-d';
875
876 QUICKSTART:
877 Getopt('f');
878
aa689395 879=item each HASH
880
881When called in a list context, returns a 2-element array consisting of the
882key and value for the next element of a hash, so that you can iterate over
883it. When called in a scalar context, returns the key for only the next
2f9daede
TP
884element in the hash. (Note: Keys may be "0" or "", which are logically
885false; you may wish to avoid constructs like C<while ($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 897
898The following prints out your environment like the printenv(1) program,
899only in a different order:
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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
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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
TP
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 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 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 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
TP
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
TP
1028particular situation, you can just use symbolic references instead, as
1029in case 6.
a0d0e21e
LW
1030
1031=item exec LIST
1032
55497cff 1033The exec() function executes a system command I<AND NEVER RETURNS>,
1034unless the command does not exist and is executed directly instead of
1035via C</bin/sh -c> (see below). Use system() instead of exec() if you
1036want 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
1041metacharacters. If there are any, the entire argument is passed to
1042C</bin/sh -c> for parsing. If there are none, the argument is split
1043into words and passed directly to execvp(), which is more efficient.
37798a01 1044Note: exec() and system() do not flush your output buffer, so you may
a0d0e21e
LW
1045need to set C<$|> to avoid lost output. Examples:
1046
1047 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1048 exec "sort $outfile | uniq";
1049
1050If you don't really want to execute the first argument, but want to lie
1051to the program you are executing about its own name, you can specify
1052the program you actually want to run as an "indirect object" (without a
1053comma) in front of the LIST. (This always forces interpretation of the
54310121 1054LIST as a multivalued list, even if there is only a single scalar in
a0d0e21e
LW
1055the list.) Example:
1056
1057 $shell = '/bin/csh';
1058 exec $shell '-sh'; # pretend it's a login shell
1059
1060or, more directly,
1061
1062 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1063
1064=item exists EXPR
1065
1066Returns TRUE if the specified hash key exists in its hash array, even
1067if the corresponding value is undefined.
1068
1069 print "Exists\n" if exists $array{$key};
1070 print "Defined\n" if defined $array{$key};
1071 print "True\n" if $array{$key};
1072
5f05dabc 1073A hash element can be TRUE only if it's defined, and defined if
a0d0e21e
LW
1074it exists, but the reverse doesn't necessarily hold true.
1075
1076Note that the EXPR can be arbitrarily complicated as long as the final
1077operation is a hash key lookup:
1078
1079 if (exists $ref->[$x][$y]{$key}) { ... }
1080
1081=item exit EXPR
1082
1083Evaluates EXPR and exits immediately with that value. (Actually, it
1084calls any defined C<END> routines first, but the C<END> routines may not
1085abort the exit. Likewise any object destructors that need to be called
1086are called before exit.) Example:
1087
1088 $ans = <STDIN>;
1089 exit 0 if $ans =~ /^[Xx]/;
1090
f86702cc 1091See also die(). If EXPR is omitted, exits with 0 status. The only
54310121 1092universally portable values for EXPR are 0 for success and 1 for error;
f86702cc 1093all other values are subject to unpredictable interpretation depending
1094on the environment in which the Perl program is running.
a0d0e21e 1095
28757baa 1096You shouldn't use exit() to abort a subroutine if there's any chance that
1097someone might want to trap whatever error happened. Use die() instead,
1098which can be trapped by an eval().
1099
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LW
1100=item exp EXPR
1101
54310121 1102=item exp
bbce6d69 1103
54310121 1104Returns I<e> (the natural logarithm base) to the power of EXPR.
a0d0e21e
LW
1105If EXPR is omitted, gives C<exp($_)>.
1106
1107=item fcntl FILEHANDLE,FUNCTION,SCALAR
1108
1109Implements the fcntl(2) function. You'll probably have to say
1110
1111 use Fcntl;
1112
1113first to get the correct function definitions. Argument processing and
1114value return works just like ioctl() below. Note that fcntl() will produce
1115a fatal error if used on a machine that doesn't implement fcntl(2).
1116For example:
1117
1118 use Fcntl;
1119 fcntl($filehandle, F_GETLK, $packed_return_buffer);
1120
1121=item fileno FILEHANDLE
1122
1123Returns the file descriptor for a filehandle. This is useful for
1124constructing bitmaps for select(). If FILEHANDLE is an expression, the
1125value is taken as the name of the filehandle.
1126
1127=item flock FILEHANDLE,OPERATION
1128
8ebc5c01 1129Calls flock(2), or an emulation of it, on FILEHANDLE. Returns TRUE for
68dc0745 1130success, FALSE on failure. Produces a fatal error if used on a machine
1131that doesn't implement flock(2), fcntl(2) locking, or lockf(3). flock()
1132is Perl's portable file locking interface, although it locks only entire
1133files, not records.
8ebc5c01 1134
1135OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1136LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
68dc0745 1137you can use the symbolic names if import them from the Fcntl module,
1138either individually, or as a group using the ':flock' tag. LOCK_SH
1139requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1140releases a previously requested lock. If LOCK_NB is added to LOCK_SH or
1141LOCK_EX then flock() will return immediately rather than blocking
1142waiting for the lock (check the return status to see if you got it).
1143
1144To avoid the possibility of mis-coordination, Perl flushes FILEHANDLE
1145before (un)locking it.
8ebc5c01 1146
1147Note that the emulation built with lockf(3) doesn't provide shared
1148locks, and it requires that FILEHANDLE be open with write intent. These
1149are the semantics that lockf(3) implements. Most (all?) systems
1150implement lockf(3) in terms of fcntl(2) locking, though, so the
1151differing semantics shouldn't bite too many people.
1152
1153Note also that some versions of flock() cannot lock things over the
1154network; you would need to use the more system-specific fcntl() for
1155that. If you like you can force Perl to ignore your system's flock(2)
1156function, and so provide its own fcntl(2)-based emulation, by passing
1157the switch C<-Ud_flock> to the F<Configure> program when you configure
1158perl.
4633a7c4
LW
1159
1160Here's a mailbox appender for BSD systems.
a0d0e21e 1161
7e1af8bc 1162 use Fcntl ':flock'; # import LOCK_* constants
a0d0e21e
LW
1163
1164 sub lock {
7e1af8bc 1165 flock(MBOX,LOCK_EX);
a0d0e21e
LW
1166 # and, in case someone appended
1167 # while we were waiting...
1168 seek(MBOX, 0, 2);
1169 }
1170
1171 sub unlock {
7e1af8bc 1172 flock(MBOX,LOCK_UN);
a0d0e21e
LW
1173 }
1174
1175 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1176 or die "Can't open mailbox: $!";
1177
1178 lock();
1179 print MBOX $msg,"\n\n";
1180 unlock();
1181
cb1a09d0 1182See also L<DB_File> for other flock() examples.
a0d0e21e
LW
1183
1184=item fork
1185
1186Does a fork(2) system call. Returns the child pid to the parent process
4633a7c4 1187and 0 to the child process, or C<undef> if the fork is unsuccessful.
a0d0e21e 1188Note: unflushed buffers remain unflushed in both processes, which means
28757baa 1189you may need to set C<$|> ($AUTOFLUSH in English) or call the autoflush()
1190method of IO::Handle to avoid duplicate output.
a0d0e21e
LW
1191
1192If you fork() without ever waiting on your children, you will accumulate
1193zombies:
1194
4633a7c4 1195 $SIG{CHLD} = sub { wait };
a0d0e21e 1196
54310121 1197There's also the double-fork trick (error checking on
a0d0e21e
LW
1198fork() returns omitted);
1199
1200 unless ($pid = fork) {
1201 unless (fork) {
1202 exec "what you really wanna do";
1203 die "no exec";
1204 # ... or ...
4633a7c4 1205 ## (some_perl_code_here)
a0d0e21e
LW
1206 exit 0;
1207 }
1208 exit 0;
1209 }
1210 waitpid($pid,0);
1211
cb1a09d0
AD
1212See also L<perlipc> for more examples of forking and reaping
1213moribund children.
1214
28757baa 1215Note that if your forked child inherits system file descriptors like
1216STDIN and STDOUT that are actually connected by a pipe or socket, even
1217if you exit, the remote server (such as, say, httpd or rsh) won't think
1218you're done. You should reopen those to /dev/null if it's any issue.
1219
cb1a09d0
AD
1220=item format
1221
1222Declare a picture format with use by the write() function. For
1223example:
1224
54310121 1225 format Something =
cb1a09d0
AD
1226 Test: @<<<<<<<< @||||| @>>>>>
1227 $str, $%, '$' . int($num)
1228 .
1229
1230 $str = "widget";
184e9718 1231 $num = $cost/$quantity;
cb1a09d0
AD
1232 $~ = 'Something';
1233 write;
1234
1235See L<perlform> for many details and examples.
1236
a0d0e21e 1237
8903cb82 1238=item formline PICTURE,LIST
a0d0e21e 1239
4633a7c4 1240This is an internal function used by C<format>s, though you may call it
a0d0e21e
LW
1241too. It formats (see L<perlform>) a list of values according to the
1242contents of PICTURE, placing the output into the format output
4633a7c4
LW
1243accumulator, C<$^A> (or $ACCUMULATOR in English).
1244Eventually, when a write() is done, the contents of
a0d0e21e
LW
1245C<$^A> are written to some filehandle, but you could also read C<$^A>
1246yourself and then set C<$^A> back to "". Note that a format typically
1247does one formline() per line of form, but the formline() function itself
748a9306 1248doesn't care how many newlines are embedded in the PICTURE. This means
4633a7c4 1249that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
748a9306
LW
1250You may therefore need to use multiple formlines to implement a single
1251record format, just like the format compiler.
1252
5f05dabc 1253Be careful if you put double quotes around the picture, because an "C<@>"
748a9306 1254character may be taken to mean the beginning of an array name.
4633a7c4 1255formline() always returns TRUE. See L<perlform> for other examples.
a0d0e21e
LW
1256
1257=item getc FILEHANDLE
1258
1259=item getc
1260
1261Returns the next character from the input file attached to FILEHANDLE,
1262or a null string at end of file. If FILEHANDLE is omitted, reads from STDIN.
4633a7c4 1263This is not particularly efficient. It cannot be used to get unbuffered
cb1a09d0 1264single-characters, however. For that, try something more like:
4633a7c4
LW
1265
1266 if ($BSD_STYLE) {
1267 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1268 }
1269 else {
54310121 1270 system "stty", '-icanon', 'eol', "\001";
4633a7c4
LW
1271 }
1272
1273 $key = getc(STDIN);
1274
1275 if ($BSD_STYLE) {
1276 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1277 }
1278 else {
5f05dabc 1279 system "stty", 'icanon', 'eol', '^@'; # ASCII null
4633a7c4
LW
1280 }
1281 print "\n";
1282
54310121 1283Determination of whether $BSD_STYLE should be set
1284is left as an exercise to the reader.
cb1a09d0 1285
28757baa 1286The POSIX::getattr() function can do this more portably on systems
1287alleging POSIX compliance.
cb1a09d0 1288See also the C<Term::ReadKey> module from your nearest CPAN site;
54310121 1289details on CPAN can be found on L<perlmod/CPAN>.
a0d0e21e
LW
1290
1291=item getlogin
1292
1293Returns the current login from F</etc/utmp>, if any. If null, use
54310121 1294getpwuid().
a0d0e21e 1295
f86702cc 1296 $login = getlogin || getpwuid($<) || "Kilroy";
a0d0e21e 1297
da0045b7 1298Do not consider getlogin() for authentication: it is not as
4633a7c4
LW
1299secure as getpwuid().
1300
a0d0e21e
LW
1301=item getpeername SOCKET
1302
1303Returns the packed sockaddr address of other end of the SOCKET connection.
1304
4633a7c4
LW
1305 use Socket;
1306 $hersockaddr = getpeername(SOCK);
1307 ($port, $iaddr) = unpack_sockaddr_in($hersockaddr);
1308 $herhostname = gethostbyaddr($iaddr, AF_INET);
1309 $herstraddr = inet_ntoa($iaddr);
a0d0e21e
LW
1310
1311=item getpgrp PID
1312
47e29363 1313Returns the current process group for the specified PID. Use
1314a PID of 0 to get the current process group for the
4633a7c4 1315current process. Will raise an exception if used on a machine that
a0d0e21e 1316doesn't implement getpgrp(2). If PID is omitted, returns process
47e29363 1317group of current process. Note that the POSIX version of getpgrp()
1318does not accept a PID argument, so only PID==0 is truly portable.
a0d0e21e
LW
1319
1320=item getppid
1321
1322Returns the process id of the parent process.
1323
1324=item getpriority WHICH,WHO
1325
4633a7c4
LW
1326Returns the current priority for a process, a process group, or a user.
1327(See L<getpriority(2)>.) Will raise a fatal exception if used on a
a0d0e21e
LW
1328machine that doesn't implement getpriority(2).
1329
1330=item getpwnam NAME
1331
1332=item getgrnam NAME
1333
1334=item gethostbyname NAME
1335
1336=item getnetbyname NAME
1337
1338=item getprotobyname NAME
1339
1340=item getpwuid UID
1341
1342=item getgrgid GID
1343
1344=item getservbyname NAME,PROTO
1345
1346=item gethostbyaddr ADDR,ADDRTYPE
1347
1348=item getnetbyaddr ADDR,ADDRTYPE
1349
1350=item getprotobynumber NUMBER
1351
1352=item getservbyport PORT,PROTO
1353
1354=item getpwent
1355
1356=item getgrent
1357
1358=item gethostent
1359
1360=item getnetent
1361
1362=item getprotoent
1363
1364=item getservent
1365
1366=item setpwent
1367
1368=item setgrent
1369
1370=item sethostent STAYOPEN
1371
1372=item setnetent STAYOPEN
1373
1374=item setprotoent STAYOPEN
1375
1376=item setservent STAYOPEN
1377
1378=item endpwent
1379
1380=item endgrent
1381
1382=item endhostent
1383
1384=item endnetent
1385
1386=item endprotoent
1387
1388=item endservent
1389
1390These routines perform the same functions as their counterparts in the
1391system library. Within a list context, the return values from the
1392various get routines are as follows:
1393
1394 ($name,$passwd,$uid,$gid,
1395 $quota,$comment,$gcos,$dir,$shell) = getpw*
1396 ($name,$passwd,$gid,$members) = getgr*
1397 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1398 ($name,$aliases,$addrtype,$net) = getnet*
1399 ($name,$aliases,$proto) = getproto*
1400 ($name,$aliases,$port,$proto) = getserv*
1401
1402(If the entry doesn't exist you get a null list.)
1403
1404Within a scalar context, you get the name, unless the function was a
1405lookup by name, in which case you get the other thing, whatever it is.
1406(If the entry doesn't exist you get the undefined value.) For example:
1407
1408 $uid = getpwnam
1409 $name = getpwuid
1410 $name = getpwent
1411 $gid = getgrnam
1412 $name = getgrgid
1413 $name = getgrent
1414 etc.
1415
1416The $members value returned by I<getgr*()> is a space separated list of
1417the login names of the members of the group.
1418
1419For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1420C, it will be returned to you via C<$?> if the function call fails. The
1421@addrs value returned by a successful call is a list of the raw
1422addresses returned by the corresponding system library call. In the
1423Internet domain, each address is four bytes long and you can unpack it
1424by saying something like:
1425
1426 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1427
1428=item getsockname SOCKET
1429
1430Returns the packed sockaddr address of this end of the SOCKET connection.
1431
4633a7c4
LW
1432 use Socket;
1433 $mysockaddr = getsockname(SOCK);
1434 ($port, $myaddr) = unpack_sockaddr_in($mysockaddr);
a0d0e21e
LW
1435
1436=item getsockopt SOCKET,LEVEL,OPTNAME
1437
1438Returns the socket option requested, or undefined if there is an error.
1439
1440=item glob EXPR
1441
0a753a76 1442=item glob
1443
68dc0745 1444Returns the value of EXPR with filename expansions such as a shell would
1445do. This is the internal function implementing the C<E<lt>*.cE<gt>>
1446operator, but you can use it directly. If EXPR is omitted, $_ is used.
1447The C<E<lt>*.cE<gt>> operator is discussed in more detail in
1448L<perlop/"I/O Operators">.
a0d0e21e
LW
1449
1450=item gmtime EXPR
1451
1452Converts a time as returned by the time function to a 9-element array
54310121 1453with the time localized for the standard Greenwich time zone.
4633a7c4 1454Typically used as follows:
a0d0e21e 1455
54310121 1456 # 0 1 2 3 4 5 6 7 8
a0d0e21e
LW
1457 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1458 gmtime(time);
1459
1460All array elements are numeric, and come straight out of a struct tm.
1461In particular this means that $mon has the range 0..11 and $wday has
54310121 1462the range 0..6 with sunday as day 0. Also, $year is the number of
1463years since 1900, I<not> simply the last two digits of the year.
2f9daede
TP
1464
1465If EXPR is omitted, does C<gmtime(time())>.
a0d0e21e 1466
54310121 1467In a scalar context, returns the ctime(3) value:
0a753a76 1468
1469 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
1470
54310121 1471Also see the timegm() function provided by the Time::Local module,
1472and the strftime(3) function available via the POSIX module.
0a753a76 1473
a0d0e21e
LW
1474=item goto LABEL
1475
748a9306
LW
1476=item goto EXPR
1477
a0d0e21e
LW
1478=item goto &NAME
1479
1480The goto-LABEL form finds the statement labeled with LABEL and resumes
1481execution there. It may not be used to go into any construct that
1482requires initialization, such as a subroutine or a foreach loop. It
0a753a76 1483also can't be used to go into a construct that is optimized away,
1484or to get out of a block or subroutine given to sort().
1485It can be used to go almost anywhere else within the dynamic scope,
a0d0e21e
LW
1486including out of subroutines, but it's usually better to use some other
1487construct such as last or die. The author of Perl has never felt the
1488need to use this form of goto (in Perl, that is--C is another matter).
1489
748a9306
LW
1490The goto-EXPR form expects a label name, whose scope will be resolved
1491dynamically. This allows for computed gotos per FORTRAN, but isn't
1492necessarily recommended if you're optimizing for maintainability:
1493
1494 goto ("FOO", "BAR", "GLARCH")[$i];
1495
a0d0e21e
LW
1496The goto-&NAME form is highly magical, and substitutes a call to the
1497named subroutine for the currently running subroutine. This is used by
1498AUTOLOAD subroutines that wish to load another subroutine and then
1499pretend that the other subroutine had been called in the first place
1500(except that any modifications to @_ in the current subroutine are
1501propagated to the other subroutine.) After the goto, not even caller()
1502will be able to tell that this routine was called first.
1503
1504=item grep BLOCK LIST
1505
1506=item grep EXPR,LIST
1507
54310121 1508This is similar in spirit to, but not the same as, grep(1)
2f9daede
TP
1509and its relatives. In particular, it is not limited to using
1510regular expressions.
1511
a0d0e21e
LW
1512Evaluates the BLOCK or EXPR for each element of LIST (locally setting
1513$_ to each element) and returns the list value consisting of those
1514elements for which the expression evaluated to TRUE. In a scalar
1515context, returns the number of times the expression was TRUE.
1516
1517 @foo = grep(!/^#/, @bar); # weed out comments
1518
1519or equivalently,
1520
1521 @foo = grep {!/^#/} @bar; # weed out comments
1522
5f05dabc 1523Note that, because $_ is a reference into the list value, it can be used
a0d0e21e
LW
1524to modify the elements of the array. While this is useful and
1525supported, it can cause bizarre results if the LIST is not a named
2f9daede 1526array. Similarly, grep returns aliases into the original list,
2ae324a7 1527much like the way that L<Foreach Loops>'s index variable aliases the list
2f9daede
TP
1528elements. That is, modifying an element of a list returned by grep
1529actually modifies the element in the original list.
a0d0e21e
LW
1530
1531=item hex EXPR
1532
54310121 1533=item hex
bbce6d69 1534
54310121 1535Interprets EXPR as a hex string and returns the corresponding
2f9daede 1536value. (To convert strings that might start with either 0 or 0x
dc848c6f 1537see L</oct>.) If EXPR is omitted, uses $_.
2f9daede
TP
1538
1539 print hex '0xAf'; # prints '175'
1540 print hex 'aF'; # same
a0d0e21e
LW
1541
1542=item import
1543
54310121 1544There is no builtin import() function. It is merely an ordinary
4633a7c4 1545method (subroutine) defined (or inherited) by modules that wish to export
a0d0e21e 1546names to another module. The use() function calls the import() method
54310121 1547for the package used. See also L</use()>, L<perlmod>, and L<Exporter>.
a0d0e21e
LW
1548
1549=item index STR,SUBSTR,POSITION
1550
1551=item index STR,SUBSTR
1552
4633a7c4
LW
1553Returns the position of the first occurrence of SUBSTR in STR at or after
1554POSITION. If POSITION is omitted, starts searching from the beginning of
184e9718 1555the string. The return value is based at 0 (or whatever you've set the C<$[>
4633a7c4 1556variable to--but don't do that). If the substring is not found, returns
a0d0e21e
LW
1557one less than the base, ordinarily -1.
1558
1559=item int EXPR
1560
54310121 1561=item int
bbce6d69 1562
a0d0e21e
LW
1563Returns the integer portion of EXPR. If EXPR is omitted, uses $_.
1564
1565=item ioctl FILEHANDLE,FUNCTION,SCALAR
1566
1567Implements the ioctl(2) function. You'll probably have to say
1568
4633a7c4 1569 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
a0d0e21e 1570
4633a7c4 1571first to get the correct function definitions. If F<ioctl.ph> doesn't
a0d0e21e 1572exist or doesn't have the correct definitions you'll have to roll your
4633a7c4
LW
1573own, based on your C header files such as F<E<lt>sys/ioctl.hE<gt>>.
1574(There is a Perl script called B<h2ph> that comes with the Perl kit which
54310121 1575may help you in this, but it's nontrivial.) SCALAR will be read and/or
4633a7c4
LW
1576written depending on the FUNCTION--a pointer to the string value of SCALAR
1577will be passed as the third argument of the actual ioctl call. (If SCALAR
1578has no string value but does have a numeric value, that value will be
1579passed rather than a pointer to the string value. To guarantee this to be
1580TRUE, add a 0 to the scalar before using it.) The pack() and unpack()
1581functions are useful for manipulating the values of structures used by
1582ioctl(). The following example sets the erase character to DEL.
a0d0e21e
LW
1583
1584 require 'ioctl.ph';
4633a7c4
LW
1585 $getp = &TIOCGETP;
1586 die "NO TIOCGETP" if $@ || !$getp;
a0d0e21e 1587 $sgttyb_t = "ccccs"; # 4 chars and a short
4633a7c4 1588 if (ioctl(STDIN,$getp,$sgttyb)) {
a0d0e21e
LW
1589 @ary = unpack($sgttyb_t,$sgttyb);
1590 $ary[2] = 127;
1591 $sgttyb = pack($sgttyb_t,@ary);
4633a7c4 1592 ioctl(STDIN,&TIOCSETP,$sgttyb)
a0d0e21e
LW
1593 || die "Can't ioctl: $!";
1594 }
1595
1596The return value of ioctl (and fcntl) is as follows:
1597
1598 if OS returns: then Perl returns:
1599 -1 undefined value
1600 0 string "0 but true"
1601 anything else that number
1602
1603Thus Perl returns TRUE on success and FALSE on failure, yet you can
1604still easily determine the actual value returned by the operating
1605system:
1606
1607 ($retval = ioctl(...)) || ($retval = -1);
1608 printf "System returned %d\n", $retval;
1609
1610=item join EXPR,LIST
1611
54310121 1612Joins the separate strings of LIST into a single string with
a0d0e21e
LW
1613fields separated by the value of EXPR, and returns the string.
1614Example:
1615
1616 $_ = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
1617
1618See L<perlfunc/split>.
1619
aa689395 1620=item keys HASH
1621
1622Returns a normal array consisting of all the keys of the named hash. (In
1623a scalar context, returns the number of keys.) The keys are returned in
1624an apparently random order, but it is the same order as either the
1625values() or each() function produces (given that the hash has not been
1626modified). As a side effect, it resets HASH's iterator.
a0d0e21e 1627
aa689395 1628Here is yet another way to print your environment:
a0d0e21e
LW
1629
1630 @keys = keys %ENV;
1631 @values = values %ENV;
1632 while ($#keys >= 0) {
1633 print pop(@keys), '=', pop(@values), "\n";
1634 }
1635
1636or how about sorted by key:
1637
1638 foreach $key (sort(keys %ENV)) {
1639 print $key, '=', $ENV{$key}, "\n";
1640 }
1641
54310121 1642To sort an array by value, you'll need to use a C<sort> function.
aa689395 1643Here's a descending numeric sort of a hash by its values:
4633a7c4
LW
1644
1645 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash)) {
1646 printf "%4d %s\n", $hash{$key}, $key;
1647 }
1648
55497cff 1649As an lvalue C<keys> allows you to increase the number of hash buckets
aa689395 1650allocated for the given hash. This can gain you a measure of efficiency if
1651you know the hash is going to get big. (This is similar to pre-extending
1652an array by assigning a larger number to $#array.) If you say
55497cff 1653
1654 keys %hash = 200;
1655
1656then C<%hash> will have at least 200 buckets allocated for it. These
1657buckets will be retained even if you do C<%hash = ()>, use C<undef
1658%hash> if you want to free the storage while C<%hash> is still in scope.
1659You can't shrink the number of buckets allocated for the hash using
1660C<keys> in this way (but you needn't worry about doing this by accident,
1661as trying has no effect).
1662
a0d0e21e
LW
1663=item kill LIST
1664
54310121 1665Sends a signal to a list of processes. The first element of
1666the list must be the signal to send. Returns the number of
4633a7c4 1667processes successfully signaled.
a0d0e21e
LW
1668
1669 $cnt = kill 1, $child1, $child2;
1670 kill 9, @goners;
1671
4633a7c4
LW
1672Unlike in the shell, in Perl if the I<SIGNAL> is negative, it kills
1673process groups instead of processes. (On System V, a negative I<PROCESS>
1674number will also kill process groups, but that's not portable.) That
1675means you usually want to use positive not negative signals. You may also
da0045b7 1676use a signal name in quotes. See L<perlipc/"Signals"> for details.
a0d0e21e
LW
1677
1678=item last LABEL
1679
1680=item last
1681
1682The C<last> command is like the C<break> statement in C (as used in
1683loops); it immediately exits the loop in question. If the LABEL is
1684omitted, the command refers to the innermost enclosing loop. The
1685C<continue> block, if any, is not executed:
1686
4633a7c4
LW
1687 LINE: while (<STDIN>) {
1688 last LINE if /^$/; # exit when done with header
a0d0e21e
LW
1689 ...
1690 }
1691
1692=item lc EXPR
1693
54310121 1694=item lc
bbce6d69 1695
a0d0e21e 1696Returns an lowercased version of EXPR. This is the internal function
54310121 1697implementing the \L escape in double-quoted strings.
a034a98d 1698Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
a0d0e21e 1699
bbce6d69 1700If EXPR is omitted, uses $_.
1701
a0d0e21e
LW
1702=item lcfirst EXPR
1703
54310121 1704=item lcfirst
bbce6d69 1705
a0d0e21e
LW
1706Returns the value of EXPR with the first character lowercased. This is
1707the internal function implementing the \l escape in double-quoted strings.
a034a98d 1708Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
a0d0e21e 1709
bbce6d69 1710If EXPR is omitted, uses $_.
1711
a0d0e21e
LW
1712=item length EXPR
1713
54310121 1714=item length
bbce6d69 1715
a0d0e21e
LW
1716Returns the length in characters of the value of EXPR. If EXPR is
1717omitted, returns length of $_.
1718
1719=item link OLDFILE,NEWFILE
1720
1721Creates a new filename linked to the old filename. Returns 1 for
1722success, 0 otherwise.
1723
1724=item listen SOCKET,QUEUESIZE
1725
1726Does the same thing that the listen system call does. Returns TRUE if
4633a7c4 1727it succeeded, FALSE otherwise. See example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
1728
1729=item local EXPR
1730
a0d0e21e 1731A local modifies the listed variables to be local to the enclosing block,
5f05dabc 1732subroutine, C<eval{}>, or C<do>. If more than one value is listed, the
1733list must be placed in parentheses. See L<perlsub/"Temporary Values via
3e3baf6d 1734local()"> for details, including issues with tied arrays and hashes.
a0d0e21e 1735
cb1a09d0
AD
1736But you really probably want to be using my() instead, because local() isn't
1737what most people think of as "local"). See L<perlsub/"Private Variables
1738via my()"> for details.
a0d0e21e
LW
1739
1740=item localtime EXPR
1741
1742Converts a time as returned by the time function to a 9-element array
5f05dabc 1743with the time analyzed for the local time zone. Typically used as
a0d0e21e
LW
1744follows:
1745
54310121 1746 # 0 1 2 3 4 5 6 7 8
a0d0e21e
LW
1747 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1748 localtime(time);
1749
1750All array elements are numeric, and come straight out of a struct tm.
1751In particular this means that $mon has the range 0..11 and $wday has
54310121 1752the range 0..6 with sunday as day 0. Also, $year is the number of
1753years since 1900, that is, $year is 123 in year 2023.
1754
1755If EXPR is omitted, uses the current time (C<localtime(time)>).
a0d0e21e 1756
0a753a76 1757In a scalar context, returns the ctime(3) value:
a0d0e21e 1758
5f05dabc 1759 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
a0d0e21e 1760
54310121 1761Also see the Time::Local module, and the strftime(3) and mktime(3)
1762function available via the POSIX module.
a0d0e21e
LW
1763
1764=item log EXPR
1765
54310121 1766=item log
bbce6d69 1767
a0d0e21e
LW
1768Returns logarithm (base I<e>) of EXPR. If EXPR is omitted, returns log
1769of $_.
1770
1771=item lstat FILEHANDLE
1772
1773=item lstat EXPR
1774
54310121 1775=item lstat
bbce6d69 1776
a0d0e21e
LW
1777Does the same thing as the stat() function, but stats a symbolic link
1778instead of the file the symbolic link points to. If symbolic links are
1779unimplemented on your system, a normal stat() is done.
1780
bbce6d69 1781If EXPR is omitted, stats $_.
1782
a0d0e21e
LW
1783=item m//
1784
1785The match operator. See L<perlop>.
1786
1787=item map BLOCK LIST
1788
1789=item map EXPR,LIST
1790
1791Evaluates the BLOCK or EXPR for each element of LIST (locally setting $_ to each
1792element) and returns the list value composed of the results of each such
1793evaluation. Evaluates BLOCK or EXPR in a list context, so each element of LIST
1794may produce zero, one, or more elements in the returned value.
1795
1796 @chars = map(chr, @nums);
1797
1798translates a list of numbers to the corresponding characters. And
1799
4633a7c4 1800 %hash = map { getkey($_) => $_ } @array;
a0d0e21e
LW
1801
1802is just a funny way to write
1803
1804 %hash = ();
1805 foreach $_ (@array) {
4633a7c4 1806 $hash{getkey($_)} = $_;
a0d0e21e
LW
1807 }
1808
1809=item mkdir FILENAME,MODE
1810
1811Creates the directory specified by FILENAME, with permissions specified
1812by MODE (as modified by umask). If it succeeds it returns 1, otherwise
184e9718 1813it returns 0 and sets C<$!> (errno).
a0d0e21e
LW
1814
1815=item msgctl ID,CMD,ARG
1816
4633a7c4 1817Calls the System V IPC function msgctl(2). If CMD is &IPC_STAT, then ARG
a0d0e21e
LW
1818must be a variable which will hold the returned msqid_ds structure.
1819Returns like ioctl: the undefined value for error, "0 but true" for
1820zero, or the actual return value otherwise.
1821
1822=item msgget KEY,FLAGS
1823
4633a7c4 1824Calls the System V IPC function msgget(2). Returns the message queue id,
a0d0e21e
LW
1825or the undefined value if there is an error.
1826
1827=item msgsnd ID,MSG,FLAGS
1828
1829Calls the System V IPC function msgsnd to send the message MSG to the
1830message queue ID. MSG must begin with the long integer message type,
c07a80fd 1831which may be created with C<pack("l", $type)>. Returns TRUE if
a0d0e21e
LW
1832successful, or FALSE if there is an error.
1833
1834=item msgrcv ID,VAR,SIZE,TYPE,FLAGS
1835
1836Calls the System V IPC function msgrcv to receive a message from
1837message queue ID into variable VAR with a maximum message size of
1838SIZE. Note that if a message is received, the message type will be the
1839first thing in VAR, and the maximum length of VAR is SIZE plus the size
1840of the message type. Returns TRUE if successful, or FALSE if there is
1841an error.
1842
1843=item my EXPR
1844
1845A "my" declares the listed variables to be local (lexically) to the
cb1a09d0 1846enclosing block, subroutine, C<eval>, or C<do/require/use>'d file. If
5f05dabc 1847more than one value is listed, the list must be placed in parentheses. See
cb1a09d0 1848L<perlsub/"Private Variables via my()"> for details.
4633a7c4 1849
a0d0e21e
LW
1850=item next LABEL
1851
1852=item next
1853
1854The C<next> command is like the C<continue> statement in C; it starts
1855the next iteration of the loop:
1856
4633a7c4
LW
1857 LINE: while (<STDIN>) {
1858 next LINE if /^#/; # discard comments
a0d0e21e
LW
1859 ...
1860 }
1861
1862Note that if there were a C<continue> block on the above, it would get
1863executed even on discarded lines. If the LABEL is omitted, the command
1864refers to the innermost enclosing loop.
1865
1866=item no Module LIST
1867
1868See the "use" function, which "no" is the opposite of.
1869
1870=item oct EXPR
1871
54310121 1872=item oct
bbce6d69 1873
4633a7c4 1874Interprets EXPR as an octal string and returns the corresponding
2f9daede 1875value. (If EXPR happens to start off with 0x, interprets it as
4633a7c4
LW
1876a hex string instead.) The following will handle decimal, octal, and
1877hex in the standard Perl or C notation:
a0d0e21e
LW
1878
1879 $val = oct($val) if $val =~ /^0/;
1880
2f9daede
TP
1881If EXPR is omitted, uses $_. This function is commonly used when
1882a string such as "644" needs to be converted into a file mode, for
1883example. (Although perl will automatically convert strings into
1884numbers as needed, this automatic conversion assumes base 10.)
a0d0e21e
LW
1885
1886=item open FILEHANDLE,EXPR
1887
1888=item open FILEHANDLE
1889
1890Opens the file whose filename is given by EXPR, and associates it with
5f05dabc 1891FILEHANDLE. If FILEHANDLE is an expression, its value is used as the
1892name of the real filehandle wanted. If EXPR is omitted, the scalar
1893variable of the same name as the FILEHANDLE contains the filename.
1894(Note that lexical variables--those declared with C<my>--will not work
1895for this purpose; so if you're using C<my>, specify EXPR in your call
1896to open.)
1897
1898If the filename begins with '<' or nothing, the file is opened for input.
1899If the filename begins with '>', the file is truncated and opened for
1900output. If the filename begins with '>>', the file is opened for
1901appending. You can put a '+' in front of the '>' or '<' to indicate that
1902you want both read and write access to the file; thus '+<' is almost
1903always preferred for read/write updates--the '+>' mode would clobber the
1904file first. The prefix and the filename may be separated with spaces.
1905These various prefixes correspond to the fopen(3) modes of 'r', 'r+', 'w',
1906'w+', 'a', and 'a+'.
1907
1908If the filename begins with "|", the filename is interpreted as a command
1909to which output is to be piped, and if the filename ends with a "|", the
1910filename is interpreted See L<perlipc/"Using open() for IPC"> for more
1911examples of this. as command which pipes input to us. (You may not have
7e1af8bc 1912a raw open() to a command that pipes both in I<and> out, but see
1913L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
1914for alternatives.)
cb1a09d0 1915
184e9718 1916Opening '-' opens STDIN and opening 'E<gt>-' opens STDOUT. Open returns
54310121 1917nonzero upon success, the undefined value otherwise. If the open
4633a7c4 1918involved a pipe, the return value happens to be the pid of the
54310121 1919subprocess.
cb1a09d0
AD
1920
1921If you're unfortunate enough to be running Perl on a system that
1922distinguishes between text files and binary files (modern operating
1923systems don't care), then you should check out L</binmode> for tips for
1924dealing with this. The key distinction between systems that need binmode
1925and those that don't is their text file formats. Systems like Unix and
1926Plan9 that delimit lines with a single character, and that encode that
1927character in C as '\n', do not need C<binmode>. The rest need it.
1928
cb1a09d0 1929Examples:
a0d0e21e
LW
1930
1931 $ARTICLE = 100;
1932 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
1933 while (<ARTICLE>) {...
1934
1935 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
1936
cb1a09d0
AD
1937 open(DBASE, '+<dbase.mine'); # open for update
1938
4633a7c4 1939 open(ARTICLE, "caesar <$article |"); # decrypt article
a0d0e21e 1940
4633a7c4 1941 open(EXTRACT, "|sort >/tmp/Tmp$$"); # $$ is our process id
a0d0e21e
LW
1942
1943 # process argument list of files along with any includes
1944
1945 foreach $file (@ARGV) {
1946 process($file, 'fh00');
1947 }
1948
1949 sub process {
1950 local($filename, $input) = @_;
1951 $input++; # this is a string increment
1952 unless (open($input, $filename)) {
1953 print STDERR "Can't open $filename: $!\n";
1954 return;
1955 }
1956
1957 while (<$input>) { # note use of indirection
1958 if (/^#include "(.*)"/) {
1959 process($1, $input);
1960 next;
1961 }
1962 ... # whatever
1963 }
1964 }
1965
1966You may also, in the Bourne shell tradition, specify an EXPR beginning
184e9718 1967with "E<gt>&", in which case the rest of the string is interpreted as the
a0d0e21e 1968name of a filehandle (or file descriptor, if numeric) which is to be
184e9718 1969duped and opened. You may use & after E<gt>, E<gt>E<gt>, E<lt>, +E<gt>,
5f05dabc 1970+E<gt>E<gt>, and +E<lt>. The
a0d0e21e 1971mode you specify should match the mode of the original filehandle.
184e9718 1972(Duping a filehandle does not take into account any existing contents of
cb1a09d0 1973stdio buffers.)
a0d0e21e
LW
1974Here is a script that saves, redirects, and restores STDOUT and
1975STDERR:
1976
1977 #!/usr/bin/perl
1978 open(SAVEOUT, ">&STDOUT");
1979 open(SAVEERR, ">&STDERR");
1980
1981 open(STDOUT, ">foo.out") || die "Can't redirect stdout";
1982 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
1983
1984 select(STDERR); $| = 1; # make unbuffered
1985 select(STDOUT); $| = 1; # make unbuffered
1986
1987 print STDOUT "stdout 1\n"; # this works for
1988 print STDERR "stderr 1\n"; # subprocesses too
1989
1990 close(STDOUT);
1991 close(STDERR);
1992
1993 open(STDOUT, ">&SAVEOUT");
1994 open(STDERR, ">&SAVEERR");
1995
1996 print STDOUT "stdout 2\n";
1997 print STDERR "stderr 2\n";
1998
1999
184e9718 2000If you specify "E<lt>&=N", where N is a number, then Perl will do an
4633a7c4
LW
2001equivalent of C's fdopen() of that file descriptor; this is more
2002parsimonious of file descriptors. For example:
a0d0e21e
LW
2003
2004 open(FILEHANDLE, "<&=$fd")
2005
5f05dabc 2006If you open a pipe on the command "-", i.e., either "|-" or "-|", then
a0d0e21e
LW
2007there is an implicit fork done, and the return value of open is the pid
2008of the child within the parent process, and 0 within the child
184e9718 2009process. (Use C<defined($pid)> to determine whether the open was successful.)
a0d0e21e
LW
2010The filehandle behaves normally for the parent, but i/o to that
2011filehandle is piped from/to the STDOUT/STDIN of the child process.
2012In the child process the filehandle isn't opened--i/o happens from/to
2013the new STDOUT or STDIN. Typically this is used like the normal
2014piped open when you want to exercise more control over just how the
2015pipe command gets executed, such as when you are running setuid, and
54310121 2016don't want to have to scan shell commands for metacharacters.
4633a7c4 2017The following pairs are more or less equivalent:
a0d0e21e
LW
2018
2019 open(FOO, "|tr '[a-z]' '[A-Z]'");
2020 open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
2021
2022 open(FOO, "cat -n '$file'|");
2023 open(FOO, "-|") || exec 'cat', '-n', $file;
2024
4633a7c4
LW
2025See L<perlipc/"Safe Pipe Opens"> for more examples of this.
2026
0dccf244 2027NOTE: On any operation which may do a fork, unflushed buffers remain
184e9718 2028unflushed in both processes, which means you may need to set C<$|> to
a0d0e21e
LW
2029avoid duplicate output.
2030
0dccf244
CS
2031Closing any piped filehandle causes the parent process to wait for the
2032child to finish, and returns the status value in C<$?>.
2033
5f05dabc 2034Using the constructor from the IO::Handle package (or one of its
2035subclasses, such as IO::File or IO::Socket),
c07a80fd 2036you can generate anonymous filehandles which have the scope of whatever
2037variables hold references to them, and automatically close whenever
2038and however you leave that scope:
2039
5f05dabc 2040 use IO::File;
c07a80fd 2041 ...
2042 sub read_myfile_munged {
2043 my $ALL = shift;
5f05dabc 2044 my $handle = new IO::File;
c07a80fd 2045 open($handle, "myfile") or die "myfile: $!";
2046 $first = <$handle>
2047 or return (); # Automatically closed here.
2048 mung $first or die "mung failed"; # Or here.
2049 return $first, <$handle> if $ALL; # Or here.
2050 $first; # Or here.
2051 }
2052
a0d0e21e 2053The filename that is passed to open will have leading and trailing
5f05dabc 2054whitespace deleted. To open a file with arbitrary weird
a0d0e21e
LW
2055characters in it, it's necessary to protect any leading and trailing
2056whitespace thusly:
2057
cb1a09d0
AD
2058 $file =~ s#^(\s)#./$1#;
2059 open(FOO, "< $file\0");
2060
c07a80fd 2061If you want a "real" C open() (see L<open(2)> on your system), then
2062you should use the sysopen() function. This is another way to
2063protect your filenames from interpretation. For example:
cb1a09d0 2064
28757baa 2065 use IO::Handle;
c07a80fd 2066 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL, 0700)
2067 or die "sysopen $path: $!";
2068 HANDLE->autoflush(1);
2069 HANDLE->print("stuff $$\n");
2070 seek(HANDLE, 0, 0);
2071 print "File contains: ", <HANDLE>;
cb1a09d0
AD
2072
2073See L</seek()> for some details about mixing reading and writing.
a0d0e21e
LW
2074
2075=item opendir DIRHANDLE,EXPR
2076
2077Opens a directory named EXPR for processing by readdir(), telldir(),
5f05dabc 2078seekdir(), rewinddir(), and closedir(). Returns TRUE if successful.
a0d0e21e
LW
2079DIRHANDLEs have their own namespace separate from FILEHANDLEs.
2080
2081=item ord EXPR
2082
54310121 2083=item ord
bbce6d69 2084
a0d0e21e 2085Returns the numeric ascii value of the first character of EXPR. If
dc848c6f 2086EXPR is omitted, uses $_. For the reverse, see L</chr>.
a0d0e21e
LW
2087
2088=item pack TEMPLATE,LIST
2089
2090Takes an array or list of values and packs it into a binary structure,
2091returning the string containing the structure. The TEMPLATE is a
2092sequence of characters that give the order and type of values, as
2093follows:
2094
2095 A An ascii string, will be space padded.
2096 a An ascii string, will be null padded.
2097 b A bit string (ascending bit order, like vec()).
2098 B A bit string (descending bit order).
2099 h A hex string (low nybble first).
2100 H A hex string (high nybble first).
2101
2102 c A signed char value.
2103 C An unsigned char value.
96e4d5b1 2104
a0d0e21e
LW
2105 s A signed short value.
2106 S An unsigned short value.
96e4d5b1 2107 (This 'short' is _exactly_ 16 bits, which may differ from
2108 what a local C compiler calls 'short'.)
2109
a0d0e21e
LW
2110 i A signed integer value.
2111 I An unsigned integer value.
96e4d5b1 2112 (This 'integer' is _at_least_ 32 bits wide. Its exact size
2113 depends on what a local C compiler calls 'int', and may
2114 even be larger than the 'long' described in the next item.)
2115
a0d0e21e
LW
2116 l A signed long value.
2117 L An unsigned long value.
96e4d5b1 2118 (This 'long' is _exactly_ 32 bits, which may differ from
2119 what a local C compiler calls 'long'.)
a0d0e21e 2120
96e4d5b1 2121 n A short in "network" (big-endian) order.
2122 N A long in "network" (big-endian) order.
a0d0e21e
LW
2123 v A short in "VAX" (little-endian) order.
2124 V A long in "VAX" (little-endian) order.
96e4d5b1 2125 (These 'shorts' and 'longs' are _exactly_ 16 bits and
2126 _exactly_ 32 bits, respectively.)
a0d0e21e
LW
2127
2128 f A single-precision float in the native format.
2129 d A double-precision float in the native format.
2130
2131 p A pointer to a null-terminated string.
2132 P A pointer to a structure (fixed-length string).
2133
2134 u A uuencoded string.
2135
96e4d5b1 2136 w A BER compressed integer. Its bytes represent an unsigned
2137 integer in base 128, most significant digit first, with as few
2138 digits as possible. Bit eight (the high bit) is set on each
2139 byte except the last.
def98dd4 2140
a0d0e21e
LW
2141 x A null byte.
2142 X Back up a byte.
2143 @ Null fill to absolute position.
2144
2145Each letter may optionally be followed by a number which gives a repeat
5f05dabc 2146count. With all types except "a", "A", "b", "B", "h", "H", and "P" the
a0d0e21e
LW
2147pack function will gobble up that many values from the LIST. A * for the
2148repeat count means to use however many items are left. The "a" and "A"
2149types gobble just one value, but pack it as a string of length count,
2150padding with nulls or spaces as necessary. (When unpacking, "A" strips
2151trailing spaces and nulls, but "a" does not.) Likewise, the "b" and "B"
2152fields pack a string that many bits long. The "h" and "H" fields pack a
2153string that many nybbles long. The "P" packs a pointer to a structure of
2154the size indicated by the length. Real numbers (floats and doubles) are
2155in the native machine format only; due to the multiplicity of floating
2156formats around, and the lack of a standard "network" representation, no
2157facility for interchange has been made. This means that packed floating
2158point data written on one machine may not be readable on another - even if
2159both use IEEE floating point arithmetic (as the endian-ness of the memory
2160representation is not part of the IEEE spec). Note that Perl uses doubles
2161internally for all numeric calculation, and converting from double into
5f05dabc 2162float and thence back to double again will lose precision (i.e.,
a0d0e21e
LW
2163C<unpack("f", pack("f", $foo)>) will not in general equal $foo).
2164
2165Examples:
2166
2167 $foo = pack("cccc",65,66,67,68);
2168 # foo eq "ABCD"
2169 $foo = pack("c4",65,66,67,68);
2170 # same thing
2171
2172 $foo = pack("ccxxcc",65,66,67,68);
2173 # foo eq "AB\0\0CD"
2174
2175 $foo = pack("s2",1,2);
2176 # "\1\0\2\0" on little-endian
2177 # "\0\1\0\2" on big-endian
2178
2179 $foo = pack("a4","abcd","x","y","z");
2180 # "abcd"
2181
2182 $foo = pack("aaaa","abcd","x","y","z");
2183 # "axyz"
2184
2185 $foo = pack("a14","abcdefg");
2186 # "abcdefg\0\0\0\0\0\0\0"
2187
2188 $foo = pack("i9pl", gmtime);
2189 # a real struct tm (on my system anyway)
2190
2191 sub bintodec {
2192 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
2193 }
2194
2195The same template may generally also be used in the unpack function.
2196
cb1a09d0
AD
2197=item package NAMESPACE
2198
2199Declares the compilation unit as being in the given namespace. The scope
2200of the package declaration is from the declaration itself through the end of
2201the enclosing block (the same scope as the local() operator). All further
2202unqualified dynamic identifiers will be in this namespace. A package
5f05dabc 2203statement affects only dynamic variables--including those you've used
cb1a09d0
AD
2204local() on--but I<not> lexical variables created with my(). Typically it
2205would be the first declaration in a file to be included by the C<require>
2206or C<use> operator. You can switch into a package in more than one place;
5f05dabc 2207it influences merely which symbol table is used by the compiler for the
cb1a09d0
AD
2208rest of that block. You can refer to variables and filehandles in other
2209packages by prefixing the identifier with the package name and a double
2210colon: C<$Package::Variable>. If the package name is null, the C<main>
2211package as assumed. That is, C<$::sail> is equivalent to C<$main::sail>.
2212
2213See L<perlmod/"Packages"> for more information about packages, modules,
2214and classes. See L<perlsub> for other scoping issues.
2215
a0d0e21e
LW
2216=item pipe READHANDLE,WRITEHANDLE
2217
2218Opens a pair of connected pipes like the corresponding system call.
2219Note that if you set up a loop of piped processes, deadlock can occur
2220unless you are very careful. In addition, note that Perl's pipes use
184e9718 2221stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE
a0d0e21e
LW
2222after each command, depending on the application.
2223
7e1af8bc 2224See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
4633a7c4
LW
2225for examples of such things.
2226
a0d0e21e
LW
2227=item pop ARRAY
2228
54310121 2229=item pop
28757baa 2230
a0d0e21e
LW
2231Pops and returns the last value of the array, shortening the array by
22321. Has a similar effect to
2233
2234 $tmp = $ARRAY[$#ARRAY--];
2235
2236If there are no elements in the array, returns the undefined value.
cb1a09d0
AD
2237If ARRAY is omitted, pops the
2238@ARGV array in the main program, and the @_ array in subroutines, just
2239like shift().
a0d0e21e
LW
2240
2241=item pos SCALAR
2242
54310121 2243=item pos
bbce6d69 2244
4633a7c4 2245Returns the offset of where the last C<m//g> search left off for the variable
2f9daede 2246is in question ($_ is used when the variable is not specified). May be
44a8e56a 2247modified to change that offset. Such modification will also influence
2248the C<\G> zero-width assertion in regular expressions. See L<perlre> and
2249L<perlop>.
a0d0e21e
LW
2250
2251=item print FILEHANDLE LIST
2252
2253=item print LIST
2254
2255=item print
2256
cb1a09d0 2257Prints a string or a comma-separated list of strings. Returns TRUE
a0d0e21e 2258if successful. FILEHANDLE may be a scalar variable name, in which case
cb1a09d0 2259the variable contains the name of or a reference to the filehandle, thus introducing one
a0d0e21e
LW
2260level of indirection. (NOTE: If FILEHANDLE is a variable and the next
2261token is a term, it may be misinterpreted as an operator unless you
5f05dabc 2262interpose a + or put parentheses around the arguments.) If FILEHANDLE is
a0d0e21e 2263omitted, prints by default to standard output (or to the last selected
da0045b7 2264output channel--see L</select>). If LIST is also omitted, prints $_ to
a0d0e21e
LW
2265STDOUT. To set the default output channel to something other than
2266STDOUT use the select operation. Note that, because print takes a
2267LIST, anything in the LIST is evaluated in a list context, and any
2268subroutine that you call will have one or more of its expressions
2269evaluated in a list context. Also be careful not to follow the print
2270keyword with a left parenthesis unless you want the corresponding right
2271parenthesis to terminate the arguments to the print--interpose a + or
5f05dabc 2272put parentheses around all the arguments.
a0d0e21e 2273
4633a7c4 2274Note that if you're storing FILEHANDLES in an array or other expression,
da0045b7 2275you will have to use a block returning its value instead:
4633a7c4
LW
2276
2277 print { $files[$i] } "stuff\n";
2278 print { $OK ? STDOUT : STDERR } "stuff\n";
2279
5f05dabc 2280=item printf FILEHANDLE FORMAT, LIST
a0d0e21e 2281
5f05dabc 2282=item printf FORMAT, LIST
a0d0e21e 2283
a034a98d
DD
2284Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>. The first argument
2285of the list will be interpreted as the printf format. If C<use locale> is
2286in effect, the character used for the decimal point in formatted real numbers
2287is affected by the LC_NUMERIC locale. See L<perllocale>.
a0d0e21e 2288
28757baa 2289Don't fall into the trap of using a printf() when a simple
2290print() would do. The print() is more efficient, and less
2291error prone.
2292
da0045b7 2293=item prototype FUNCTION
2294
2295Returns the prototype of a function as a string (or C<undef> if the
5f05dabc 2296function has no prototype). FUNCTION is a reference to, or the name of,
2297the function whose prototype you want to retrieve.
da0045b7 2298
a0d0e21e
LW
2299=item push ARRAY,LIST
2300
2301Treats ARRAY as a stack, and pushes the values of LIST
2302onto the end of ARRAY. The length of ARRAY increases by the length of
2303LIST. Has the same effect as
2304
2305 for $value (LIST) {
2306 $ARRAY[++$#ARRAY] = $value;
2307 }
2308
2309but is more efficient. Returns the new number of elements in the array.
2310
2311=item q/STRING/
2312
2313=item qq/STRING/
2314
2315=item qx/STRING/
2316
2317=item qw/STRING/
2318
2319Generalized quotes. See L<perlop>.
2320
2321=item quotemeta EXPR
2322
54310121 2323=item quotemeta
bbce6d69 2324
68dc0745 2325Returns the value of EXPR with all non-alphanumeric
a034a98d
DD
2326characters backslashed. (That is, all characters not matching
2327C</[A-Za-z_0-9]/> will be preceded by a backslash in the
2328returned string, regardless of any locale settings.)
2329This is the internal function implementing
a0d0e21e
LW
2330the \Q escape in double-quoted strings.
2331
bbce6d69 2332If EXPR is omitted, uses $_.
2333
a0d0e21e
LW
2334=item rand EXPR
2335
2336=item rand
2337
3e3baf6d
TB
2338Returns a random fractional number greater than or equal to 0 and less
2339than the value of EXPR. (EXPR should be positive.) If EXPR is
2340omitted, the value 1 is used. Automatically calls srand() unless
2341srand() has already been called. See also srand().
a0d0e21e 2342
2f9daede 2343(Note: If your rand function consistently returns numbers that are too
a0d0e21e 2344large or too small, then your version of Perl was probably compiled
2f9daede 2345with the wrong number of RANDBITS.)
a0d0e21e
LW
2346
2347=item read FILEHANDLE,SCALAR,LENGTH,OFFSET
2348
2349=item read FILEHANDLE,SCALAR,LENGTH
2350
2351Attempts to read LENGTH bytes of data into variable SCALAR from the
2352specified FILEHANDLE. Returns the number of bytes actually read, or
2353undef if there was an error. SCALAR will be grown or shrunk to the
2354length actually read. An OFFSET may be specified to place the read
2355data at some other place than the beginning of the string. This call
2356is actually implemented in terms of stdio's fread call. To get a true
2357read system call, see sysread().
2358
2359=item readdir DIRHANDLE
2360
2361Returns the next directory entry for a directory opened by opendir().
2362If used in a list context, returns all the rest of the entries in the
2363directory. If there are no more entries, returns an undefined value in
2364a scalar context or a null list in a list context.
2365
cb1a09d0 2366If you're planning to filetest the return values out of a readdir(), you'd
5f05dabc 2367better prepend the directory in question. Otherwise, because we didn't
cb1a09d0
AD
2368chdir() there, it would have been testing the wrong file.
2369
2370 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
2371 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
2372 closedir DIR;
2373
a0d0e21e
LW
2374=item readlink EXPR
2375
54310121 2376=item readlink
bbce6d69 2377
a0d0e21e
LW
2378Returns the value of a symbolic link, if symbolic links are
2379implemented. If not, gives a fatal error. If there is some system
184e9718 2380error, returns the undefined value and sets C<$!> (errno). If EXPR is
a0d0e21e
LW
2381omitted, uses $_.
2382
2383=item recv SOCKET,SCALAR,LEN,FLAGS
2384
2385Receives a message on a socket. Attempts to receive LENGTH bytes of
2386data into variable SCALAR from the specified SOCKET filehandle.
2387Actually does a C recvfrom(), so that it can returns the address of the
2388sender. Returns the undefined value if there's an error. SCALAR will
2389be grown or shrunk to the length actually read. Takes the same flags
54310121 2390as the system call of the same name.
4633a7c4 2391See L<perlipc/"UDP: Message Passing"> for examples.
a0d0e21e
LW
2392
2393=item redo LABEL
2394
2395=item redo
2396
2397The C<redo> command restarts the loop block without evaluating the
2398conditional again. The C<continue> block, if any, is not executed. If
2399the LABEL is omitted, the command refers to the innermost enclosing
2400loop. This command is normally used by programs that want to lie to
2401themselves about what was just input:
2402
2403 # a simpleminded Pascal comment stripper
2404 # (warning: assumes no { or } in strings)
4633a7c4 2405 LINE: while (<STDIN>) {
a0d0e21e
LW
2406 while (s|({.*}.*){.*}|$1 |) {}
2407 s|{.*}| |;
2408 if (s|{.*| |) {
2409 $front = $_;
2410 while (<STDIN>) {
2411 if (/}/) { # end of comment?
2412 s|^|$front{|;
4633a7c4 2413 redo LINE;
a0d0e21e
LW
2414 }
2415 }
2416 }
2417 print;
2418 }
2419
2420=item ref EXPR
2421
54310121 2422=item ref
bbce6d69 2423
2f9daede
TP
2424Returns a TRUE value if EXPR is a reference, FALSE otherwise. If EXPR
2425is not specified, $_ will be used. The value returned depends on the
bbce6d69 2426type of thing the reference is a reference to.
a0d0e21e
LW
2427Builtin types include:
2428
2429 REF
2430 SCALAR
2431 ARRAY
2432 HASH
2433 CODE
2434 GLOB
2435
54310121 2436If the referenced object has been blessed into a package, then that package
a0d0e21e
LW
2437name is returned instead. You can think of ref() as a typeof() operator.
2438
2439 if (ref($r) eq "HASH") {
aa689395 2440 print "r is a reference to a hash.\n";
54310121 2441 }
a0d0e21e
LW
2442 if (!ref ($r) {
2443 print "r is not a reference at all.\n";
54310121 2444 }
a0d0e21e
LW
2445
2446See also L<perlref>.
2447
2448=item rename OLDNAME,NEWNAME
2449
2450Changes the name of a file. Returns 1 for success, 0 otherwise. Will
5f05dabc 2451not work across file system boundaries.
a0d0e21e
LW
2452
2453=item require EXPR
2454
2455=item require
2456
2457Demands some semantics specified by EXPR, or by $_ if EXPR is not
2458supplied. If EXPR is numeric, demands that the current version of Perl
184e9718 2459(C<$]> or $PERL_VERSION) be equal or greater than EXPR.
a0d0e21e
LW
2460
2461Otherwise, demands that a library file be included if it hasn't already
2462been included. The file is included via the do-FILE mechanism, which is
2463essentially just a variety of eval(). Has semantics similar to the following
2464subroutine:
2465
2466 sub require {
2467 local($filename) = @_;
2468 return 1 if $INC{$filename};
2469 local($realfilename,$result);
2470 ITER: {
2471 foreach $prefix (@INC) {
2472 $realfilename = "$prefix/$filename";
2473 if (-f $realfilename) {
2474 $result = do $realfilename;
2475 last ITER;
2476 }
2477 }
2478 die "Can't find $filename in \@INC";
2479 }
2480 die $@ if $@;
2481 die "$filename did not return true value" unless $result;
2482 $INC{$filename} = $realfilename;
2483 $result;
2484 }
2485
2486Note that the file will not be included twice under the same specified
2487name. The file must return TRUE as the last statement to indicate
2488successful execution of any initialization code, so it's customary to
2489end such a file with "1;" unless you're sure it'll return TRUE
2490otherwise. But it's better just to put the "C<1;>", in case you add more
2491statements.
2492
54310121 2493If EXPR is a bareword, the require assumes a "F<.pm>" extension and
da0045b7 2494replaces "F<::>" with "F</>" in the filename for you,
54310121 2495to make it easy to load standard modules. This form of loading of
a0d0e21e
LW
2496modules does not risk altering your namespace.
2497
54310121 2498For a yet-more-powerful import facility, see L</use> and
748a9306 2499L<perlmod>.
a0d0e21e
LW
2500
2501=item reset EXPR
2502
2503=item reset
2504
2505Generally used in a C<continue> block at the end of a loop to clear
2506variables and reset ?? searches so that they work again. The
2507expression is interpreted as a list of single characters (hyphens
2508allowed for ranges). All variables and arrays beginning with one of
2509those letters are reset to their pristine state. If the expression is
5f05dabc 2510omitted, one-match searches (?pattern?) are reset to match again. Resets
2511only variables or searches in the current package. Always returns
a0d0e21e
LW
25121. Examples:
2513
2514 reset 'X'; # reset all X variables
2515 reset 'a-z'; # reset lower case variables
2516 reset; # just reset ?? searches
2517
5f05dabc 2518Resetting "A-Z" is not recommended because you'll wipe out your
2519ARGV and ENV arrays. Resets only package variables--lexical variables
a0d0e21e 2520are unaffected, but they clean themselves up on scope exit anyway,
da0045b7 2521so you'll probably want to use them instead. See L</my>.
a0d0e21e 2522
54310121 2523=item return EXPR
2524
2525=item return
2526
2527Returns from a subroutine, eval(), or do FILE with the value of the
2528given EXPR. Evaluation of EXPR may be in a list, scalar, or void
2529context, depending on how the return value will be used, and the context
2530may vary from one execution to the next (see wantarray()). If no EXPR
2531is given, returns an empty list in a list context, an undefined value in
2532a scalar context, or nothing in a void context.
a0d0e21e 2533
68dc0745 2534(Note that in the absence of a return, a subroutine, eval, or do FILE
2535will automatically return the value of the last expression evaluated.)
a0d0e21e
LW
2536
2537=item reverse LIST
2538
2539In a list context, returns a list value consisting of the elements
2f9daede
TP
2540of LIST in the opposite order. In a scalar context, concatenates the
2541elements of LIST, and returns a string value consisting of those bytes,
2542but in the opposite order.
4633a7c4 2543
2f9daede 2544 print reverse <>; # line tac, last line first
4633a7c4 2545
2f9daede
TP
2546 undef $/; # for efficiency of <>
2547 print scalar reverse <>; # byte tac, last line tsrif
2548
2549This operator is also handy for inverting a hash, although there are some
2550caveats. If a value is duplicated in the original hash, only one of those
2551can be represented as a key in the inverted hash. Also, this has to
2552unwind one hash and build a whole new one, which may take some time
2553on a large hash.
2554
2555 %by_name = reverse %by_address; # Invert the hash
a0d0e21e
LW
2556
2557=item rewinddir DIRHANDLE
2558
2559Sets the current position to the beginning of the directory for the
2560readdir() routine on DIRHANDLE.
2561
2562=item rindex STR,SUBSTR,POSITION
2563
2564=item rindex STR,SUBSTR
2565
2566Works just like index except that it returns the position of the LAST
2567occurrence of SUBSTR in STR. If POSITION is specified, returns the
2568last occurrence at or before that position.
2569
2570=item rmdir FILENAME
2571
54310121 2572=item rmdir
bbce6d69 2573
a0d0e21e 2574Deletes the directory specified by FILENAME if it is empty. If it
184e9718 2575succeeds it returns 1, otherwise it returns 0 and sets C<$!> (errno). If
a0d0e21e
LW
2576FILENAME is omitted, uses $_.
2577
2578=item s///
2579
2580The substitution operator. See L<perlop>.
2581
2582=item scalar EXPR
2583
2584Forces EXPR to be interpreted in a scalar context and returns the value
54310121 2585of EXPR.
cb1a09d0
AD
2586
2587 @counts = ( scalar @a, scalar @b, scalar @c );
2588
54310121 2589There is no equivalent operator to force an expression to
cb1a09d0
AD
2590be interpolated in a list context because it's in practice never
2591needed. If you really wanted to do so, however, you could use
2592the construction C<@{[ (some expression) ]}>, but usually a simple
2593C<(some expression)> suffices.
a0d0e21e
LW
2594
2595=item seek FILEHANDLE,POSITION,WHENCE
2596
8903cb82 2597Sets FILEHANDLE's position, just like the fseek() call of stdio.
2598FILEHANDLE may be an expression whose value gives the name of the
2599filehandle. The values for WHENCE are 0 to set the new position to
2600POSITION, 1 to set it to the current position plus POSITION, and 2 to
2601set it to EOF plus POSITION (typically negative). For WHENCE you may
2602use the constants SEEK_SET, SEEK_CUR, and SEEK_END from either the
2603IO::Seekable or the POSIX module. Returns 1 upon success, 0 otherwise.
2604
2605If you want to position file for sysread() or syswrite(), don't use
2606seek() -- buffering makes its effect on the file's system position
137443ea 2607unpredictable and non-portable. Use sysseek() instead.
a0d0e21e 2608
cb1a09d0
AD
2609On some systems you have to do a seek whenever you switch between reading
2610and writing. Amongst other things, this may have the effect of calling
8903cb82 2611stdio's clearerr(3). A WHENCE of 1 (SEEK_CUR) is useful for not moving
2612the file position:
cb1a09d0
AD
2613
2614 seek(TEST,0,1);
2615
2616This is also useful for applications emulating C<tail -f>. Once you hit
2617EOF on your read, and then sleep for a while, you might have to stick in a
8903cb82 2618seek() to reset things. The seek() doesn't change the current position,
2619but it I<does> clear the end-of-file condition on the handle, so that the
2620next C<E<lt>FILEE<gt>> makes Perl try again to read something. We hope.
cb1a09d0
AD
2621
2622If that doesn't work (some stdios are particularly cantankerous), then
2623you may need something more like this:
2624
2625 for (;;) {
2626 for ($curpos = tell(FILE); $_ = <FILE>; $curpos = tell(FILE)) {
2627 # search for some stuff and put it into files
2628 }
2629 sleep($for_a_while);
2630 seek(FILE, $curpos, 0);
2631 }
2632
a0d0e21e
LW
2633=item seekdir DIRHANDLE,POS
2634
2635Sets the current position for the readdir() routine on DIRHANDLE. POS
2636must be a value returned by telldir(). Has the same caveats about
2637possible directory compaction as the corresponding system library
2638routine.
2639
2640=item select FILEHANDLE
2641
2642=item select
2643
2644Returns the currently selected filehandle. Sets the current default
2645filehandle for output, if FILEHANDLE is supplied. This has two
2646effects: first, a C<write> or a C<print> without a filehandle will
2647default to this FILEHANDLE. Second, references to variables related to
2648output will refer to this output channel. For example, if you have to
2649set the top of form format for more than one output channel, you might
2650do the following:
2651
2652 select(REPORT1);
2653 $^ = 'report1_top';
2654 select(REPORT2);
2655 $^ = 'report2_top';
2656
2657FILEHANDLE may be an expression whose value gives the name of the
2658actual filehandle. Thus:
2659
2660 $oldfh = select(STDERR); $| = 1; select($oldfh);
2661
4633a7c4
LW
2662Some programmers may prefer to think of filehandles as objects with
2663methods, preferring to write the last example as:
a0d0e21e 2664
28757baa 2665 use IO::Handle;
a0d0e21e
LW
2666 STDERR->autoflush(1);
2667
2668=item select RBITS,WBITS,EBITS,TIMEOUT
2669
5f05dabc 2670This calls the select(2) system call with the bit masks specified, which
a0d0e21e
LW
2671can be constructed using fileno() and vec(), along these lines:
2672
2673 $rin = $win = $ein = '';
2674 vec($rin,fileno(STDIN),1) = 1;
2675 vec($win,fileno(STDOUT),1) = 1;
2676 $ein = $rin | $win;
2677
2678If you want to select on many filehandles you might wish to write a
2679subroutine:
2680
2681 sub fhbits {
2682 local(@fhlist) = split(' ',$_[0]);
2683 local($bits);
2684 for (@fhlist) {
2685 vec($bits,fileno($_),1) = 1;
2686 }
2687 $bits;
2688 }
4633a7c4 2689 $rin = fhbits('STDIN TTY SOCK');
a0d0e21e
LW
2690
2691The usual idiom is:
2692
2693 ($nfound,$timeleft) =
2694 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
2695
54310121 2696or to block until something becomes ready just do this
a0d0e21e
LW
2697
2698 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
2699
5f05dabc 2700Most systems do not bother to return anything useful in $timeleft, so
c07a80fd 2701calling select() in a scalar context just returns $nfound.
2702
5f05dabc 2703Any of the bit masks can also be undef. The timeout, if specified, is
a0d0e21e
LW
2704in seconds, which may be fractional. Note: not all implementations are
2705capable of returning the $timeleft. If not, they always return
2706$timeleft equal to the supplied $timeout.
2707
ff68c719 2708You can effect a sleep of 250 milliseconds this way:
a0d0e21e
LW
2709
2710 select(undef, undef, undef, 0.25);
2711
184e9718 2712B<WARNING>: Do not attempt to mix buffered I/O (like read() or E<lt>FHE<gt>)
cb1a09d0 2713with select(). You have to use sysread() instead.
a0d0e21e
LW
2714
2715=item semctl ID,SEMNUM,CMD,ARG
2716
2717Calls the System V IPC function semctl. If CMD is &IPC_STAT or
2718&GETALL, then ARG must be a variable which will hold the returned
2719semid_ds structure or semaphore value array. Returns like ioctl: the
2720undefined value for error, "0 but true" for zero, or the actual return
2721value otherwise.
2722
2723=item semget KEY,NSEMS,FLAGS
2724
2725Calls the System V IPC function semget. Returns the semaphore id, or
2726the undefined value if there is an error.
2727
2728=item semop KEY,OPSTRING
2729
2730Calls the System V IPC function semop to perform semaphore operations
2731such as signaling and waiting. OPSTRING must be a packed array of
2732semop structures. Each semop structure can be generated with
2733C<pack("sss", $semnum, $semop, $semflag)>. The number of semaphore
2734operations is implied by the length of OPSTRING. Returns TRUE if
2735successful, or FALSE if there is an error. As an example, the
2736following code waits on semaphore $semnum of semaphore id $semid:
2737
2738 $semop = pack("sss", $semnum, -1, 0);
2739 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
2740
2741To signal the semaphore, replace "-1" with "1".
2742
2743=item send SOCKET,MSG,FLAGS,TO
2744
2745=item send SOCKET,MSG,FLAGS
2746
2747Sends a message on a socket. Takes the same flags as the system call
2748of the same name. On unconnected sockets you must specify a
2749destination to send TO, in which case it does a C sendto(). Returns
2750the number of characters sent, or the undefined value if there is an
2751error.
4633a7c4 2752See L<perlipc/"UDP: Message Passing"> for examples.
a0d0e21e
LW
2753
2754=item setpgrp PID,PGRP
2755
2756Sets the current process group for the specified PID, 0 for the current
2757process. Will produce a fatal error if used on a machine that doesn't
5f05dabc 2758implement setpgrp(2). If the arguments are omitted, it defaults to
47e29363 27590,0. Note that the POSIX version of setpgrp() does not accept any
2760arguments, so only setpgrp 0,0 is portable.
a0d0e21e
LW
2761
2762=item setpriority WHICH,WHO,PRIORITY
2763
2764Sets the current priority for a process, a process group, or a user.
748a9306 2765(See setpriority(2).) Will produce a fatal error if used on a machine
a0d0e21e
LW
2766that doesn't implement setpriority(2).
2767
2768=item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
2769
2770Sets the socket option requested. Returns undefined if there is an
2771error. OPTVAL may be specified as undef if you don't want to pass an
2772argument.
2773
2774=item shift ARRAY
2775
2776=item shift
2777
2778Shifts the first value of the array off and returns it, shortening the
2779array by 1 and moving everything down. If there are no elements in the
2780array, returns the undefined value. If ARRAY is omitted, shifts the
2781@ARGV array in the main program, and the @_ array in subroutines.
2782(This is determined lexically.) See also unshift(), push(), and pop().
2783Shift() and unshift() do the same thing to the left end of an array
2f9daede 2784that pop() and push() do to the right end.
a0d0e21e
LW
2785
2786=item shmctl ID,CMD,ARG
2787
2788Calls the System V IPC function shmctl. If CMD is &IPC_STAT, then ARG
2789must be a variable which will hold the returned shmid_ds structure.
2790Returns like ioctl: the undefined value for error, "0 but true" for
2791zero, or the actual return value otherwise.
2792
2793=item shmget KEY,SIZE,FLAGS
2794
2795Calls the System V IPC function shmget. Returns the shared memory
2796segment id, or the undefined value if there is an error.
2797
2798=item shmread ID,VAR,POS,SIZE
2799
2800=item shmwrite ID,STRING,POS,SIZE
2801
2802Reads or writes the System V shared memory segment ID starting at
2803position POS for size SIZE by attaching to it, copying in/out, and
2804detaching from it. When reading, VAR must be a variable which will
2805hold the data read. When writing, if STRING is too long, only SIZE
2806bytes are used; if STRING is too short, nulls are written to fill out
2807SIZE bytes. Return TRUE if successful, or FALSE if there is an error.
2808
2809=item shutdown SOCKET,HOW
2810
2811Shuts down a socket connection in the manner indicated by HOW, which
2812has the same interpretation as in the system call of the same name.
2813
2814=item sin EXPR
2815
54310121 2816=item sin
bbce6d69 2817
a0d0e21e
LW
2818Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
2819returns sine of $_.
2820
54310121 2821For the inverse sine operation, you may use the POSIX::asin()
28757baa 2822function, or use this relation:
2823
2824 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
2825
a0d0e21e
LW
2826=item sleep EXPR
2827
2828=item sleep
2829
2830Causes the script to sleep for EXPR seconds, or forever if no EXPR.
2831May be interrupted by sending the process a SIGALRM. Returns the
2832number of seconds actually slept. You probably cannot mix alarm() and
5f05dabc 2833sleep() calls, because sleep() is often implemented using alarm().
a0d0e21e
LW
2834
2835On some older systems, it may sleep up to a full second less than what
2836you requested, depending on how it counts seconds. Most modern systems
2837always sleep the full amount.
2838
cb1a09d0 2839For delays of finer granularity than one second, you may use Perl's
54310121 2840syscall() interface to access setitimer(2) if your system supports it,
2841or else see L</select()> below.
cb1a09d0 2842
5f05dabc 2843See also the POSIX module's sigpause() function.
2844
a0d0e21e
LW
2845=item socket SOCKET,DOMAIN,TYPE,PROTOCOL
2846
2847Opens a socket of the specified kind and attaches it to filehandle
5f05dabc 2848SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for the
a0d0e21e 2849system call of the same name. You should "use Socket;" first to get
4633a7c4 2850the proper definitions imported. See the example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
2851
2852=item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
2853
2854Creates an unnamed pair of sockets in the specified domain, of the
5f05dabc 2855specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
a0d0e21e
LW
2856for the system call of the same name. If unimplemented, yields a fatal
2857error. Returns TRUE if successful.
2858
2859=item sort SUBNAME LIST
2860
2861=item sort BLOCK LIST
2862
2863=item sort LIST
2864
2f9daede
TP
2865Sorts the LIST and returns the sorted list value. If SUBNAME or BLOCK
2866is omitted, sorts in standard string comparison order. If SUBNAME is
2867specified, it gives the name of a subroutine that returns an integer
2868less than, equal to, or greater than 0, depending on how the elements
2869of the array are to be ordered. (The C<E<lt>=E<gt>> and C<cmp>
2870operators are extremely useful in such routines.) SUBNAME may be a
2871scalar variable name, in which case the value provides the name of the
2872subroutine to use. In place of a SUBNAME, you can provide a BLOCK as
2873an anonymous, in-line sort subroutine.
a0d0e21e 2874
cb1a09d0
AD
2875In the interests of efficiency the normal calling code for subroutines is
2876bypassed, with the following effects: the subroutine may not be a
2877recursive subroutine, and the two elements to be compared are passed into
2878the subroutine not via @_ but as the package global variables $a and
2879$b (see example below). They are passed by reference, so don't
2880modify $a and $b. And don't try to declare them as lexicals either.
a0d0e21e 2881
0a753a76 2882You also cannot exit out of the sort block or subroutine using any of the
2883loop control operators described in L<perlsyn> or with goto().
2884
a034a98d
DD
2885When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
2886current collation locale. See L<perllocale>.
2887
a0d0e21e
LW
2888Examples:
2889
2890 # sort lexically
2891 @articles = sort @files;
2892
2893 # same thing, but with explicit sort routine
2894 @articles = sort {$a cmp $b} @files;
2895
cb1a09d0 2896 # now case-insensitively
54310121 2897 @articles = sort {uc($a) cmp uc($b)} @files;
cb1a09d0 2898
a0d0e21e
LW
2899 # same thing in reversed order
2900 @articles = sort {$b cmp $a} @files;
2901
2902 # sort numerically ascending
2903 @articles = sort {$a <=> $b} @files;
2904
2905 # sort numerically descending
2906 @articles = sort {$b <=> $a} @files;
2907
2908 # sort using explicit subroutine name
2909 sub byage {
2f9daede 2910 $age{$a} <=> $age{$b}; # presuming numeric
a0d0e21e
LW
2911 }
2912 @sortedclass = sort byage @class;
2913
aa689395 2914 # this sorts the %age hash by value instead of key
2915 # using an in-line function
c07a80fd 2916 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
2917
a0d0e21e
LW
2918 sub backwards { $b cmp $a; }
2919 @harry = ('dog','cat','x','Cain','Abel');
2920 @george = ('gone','chased','yz','Punished','Axed');
2921 print sort @harry;
2922 # prints AbelCaincatdogx
2923 print sort backwards @harry;
2924 # prints xdogcatCainAbel
2925 print sort @george, 'to', @harry;
2926 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
2927
54310121 2928 # inefficiently sort by descending numeric compare using
2929 # the first integer after the first = sign, or the
cb1a09d0
AD
2930 # whole record case-insensitively otherwise
2931
2932 @new = sort {
2933 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
2934 ||
2935 uc($a) cmp uc($b)
2936 } @old;
2937
2938 # same thing, but much more efficiently;
2939 # we'll build auxiliary indices instead
2940 # for speed
2941 @nums = @caps = ();
54310121 2942 for (@old) {
cb1a09d0
AD
2943 push @nums, /=(\d+)/;
2944 push @caps, uc($_);
54310121 2945 }
cb1a09d0
AD
2946
2947 @new = @old[ sort {
2948 $nums[$b] <=> $nums[$a]
2949 ||
2950 $caps[$a] cmp $caps[$b]
2951 } 0..$#old
2952 ];
2953
2954 # same thing using a Schwartzian Transform (no temps)
2955 @new = map { $_->[0] }
2956 sort { $b->[1] <=> $a->[1]
2957 ||
2958 $a->[2] cmp $b->[2]
2959 } map { [$_, /=(\d+)/, uc($_)] } @old;
2960
184e9718 2961If you're using strict, you I<MUST NOT> declare $a
cb1a09d0
AD
2962and $b as lexicals. They are package globals. That means
2963if you're in the C<main> package, it's
2964
2965 @articles = sort {$main::b <=> $main::a} @files;
2966
2967or just
2968
2969 @articles = sort {$::b <=> $::a} @files;
2970
2971but if you're in the C<FooPack> package, it's
2972
2973 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
2974
55497cff 2975The comparison function is required to behave. If it returns
2976inconsistent results (sometimes saying $x[1] is less than $x[2] and
2977sometimes saying the opposite, for example) the Perl interpreter will
2978probably crash and dump core. This is entirely due to and dependent
2979upon your system's qsort(3) library routine; this routine often avoids
2980sanity checks in the interest of speed.
2981
a0d0e21e
LW
2982=item splice ARRAY,OFFSET,LENGTH,LIST
2983
2984=item splice ARRAY,OFFSET,LENGTH
2985
2986=item splice ARRAY,OFFSET
2987
2988Removes the elements designated by OFFSET and LENGTH from an array, and
2989replaces them with the elements of LIST, if any. Returns the elements
2990removed from the array. The array grows or shrinks as necessary. If
2991LENGTH is omitted, removes everything from OFFSET onward. The
5f05dabc 2992following equivalences hold (assuming C<$[ == 0>):
a0d0e21e
LW
2993
2994 push(@a,$x,$y) splice(@a,$#a+1,0,$x,$y)
2995 pop(@a) splice(@a,-1)
2996 shift(@a) splice(@a,0,1)
2997 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
2998 $a[$x] = $y splice(@a,$x,1,$y);
2999
3000Example, assuming array lengths are passed before arrays:
3001
3002 sub aeq { # compare two list values
3003 local(@a) = splice(@_,0,shift);
3004 local(@b) = splice(@_,0,shift);
3005 return 0 unless @a == @b; # same len?
3006 while (@a) {
3007 return 0 if pop(@a) ne pop(@b);
3008 }
3009 return 1;
3010 }
3011 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
3012
3013=item split /PATTERN/,EXPR,LIMIT
3014
3015=item split /PATTERN/,EXPR
3016
3017=item split /PATTERN/
3018
3019=item split
3020
3021Splits a string into an array of strings, and returns it.
3022
3023If not in a list context, returns the number of fields found and splits into
3024the @_ array. (In a list context, you can force the split into @_ by
3025using C<??> as the pattern delimiters, but it still returns the array
3026value.) The use of implicit split to @_ is deprecated, however.
3027
3028If EXPR is omitted, splits the $_ string. If PATTERN is also omitted,
4633a7c4
LW
3029splits on whitespace (after skipping any leading whitespace). Anything
3030matching PATTERN is taken to be a delimiter separating the fields. (Note
3031that the delimiter may be longer than one character.) If LIMIT is
3032specified and is not negative, splits into no more than that many fields
3033(though it may split into fewer). If LIMIT is unspecified, trailing null
3034fields are stripped (which potential users of pop() would do well to
3035remember). If LIMIT is negative, it is treated as if an arbitrarily large
3036LIMIT had been specified.
a0d0e21e
LW
3037
3038A pattern matching the null string (not to be confused with
748a9306 3039a null pattern C<//>, which is just one member of the set of patterns
a0d0e21e
LW
3040matching a null string) will split the value of EXPR into separate
3041characters at each point it matches that way. For example:
3042
3043 print join(':', split(/ */, 'hi there'));
3044
3045produces the output 'h:i:t:h:e:r:e'.
3046
5f05dabc 3047The LIMIT parameter can be used to split a line partially
a0d0e21e
LW
3048
3049 ($login, $passwd, $remainder) = split(/:/, $_, 3);
3050
3051When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT
3052one larger than the number of variables in the list, to avoid
3053unnecessary work. For the list above LIMIT would have been 4 by
3054default. In time critical applications it behooves you not to split
3055into more fields than you really need.
3056
3057If the PATTERN contains parentheses, additional array elements are
3058created from each matching substring in the delimiter.
3059
da0045b7 3060 split(/([,-])/, "1-10,20", 3);
a0d0e21e
LW
3061
3062produces the list value
3063
3064 (1, '-', 10, ',', 20)
3065
54310121 3066If you had the entire header of a normal Unix email message in $header,
4633a7c4
LW
3067you could split it up into fields and their values this way:
3068
3069 $header =~ s/\n\s+/ /g; # fix continuation lines
3070 %hdrs = (UNIX_FROM => split /^(.*?):\s*/m, $header);
3071
a0d0e21e
LW
3072The pattern C</PATTERN/> may be replaced with an expression to specify
3073patterns that vary at runtime. (To do runtime compilation only once,
748a9306
LW
3074use C</$variable/o>.)
3075
3076As a special case, specifying a PATTERN of space (C<' '>) will split on
3077white space just as split with no arguments does. Thus, split(' ') can
3078be used to emulate B<awk>'s default behavior, whereas C<split(/ /)>
3079will give you as many null initial fields as there are leading spaces.
3080A split on /\s+/ is like a split(' ') except that any leading
3081whitespace produces a null first field. A split with no arguments
3082really does a C<split(' ', $_)> internally.
a0d0e21e
LW
3083
3084Example:
3085
3086 open(passwd, '/etc/passwd');
3087 while (<passwd>) {
54310121 3088 ($login, $passwd, $uid, $gid, $gcos,
748a9306 3089 $home, $shell) = split(/:/);
a0d0e21e
LW
3090 ...
3091 }
3092
54310121 3093(Note that $shell above will still have a newline on it. See L</chop>,
a0d0e21e
LW
3094L</chomp>, and L</join>.)
3095
5f05dabc 3096=item sprintf FORMAT, LIST
a0d0e21e 3097
74a77017
CS
3098Returns a string formatted by the usual printf conventions of the
3099C library function sprintf(). See L<sprintf(3)> or L<printf(3)>
3100on your system for an explanation of the general principles.
3101
3102Perl does all of its own sprintf() formatting -- it emulates the C
3103function sprintf(), but it doesn't use it (except for floating-point
3104numbers, and even then only the standard modifiers are allowed). As a
3105result, any non-standard extensions in your local sprintf() are not
3106available from Perl.
3107
3108Perl's sprintf() permits the following universally-known conversions:
3109
3110 %% a percent sign
3111 %c a character with the given number
3112 %s a string
3113 %d a signed integer, in decimal
3114 %u an unsigned integer, in decimal
3115 %o an unsigned integer, in octal
3116 %x an unsigned integer, in hexadecimal
3117 %e a floating-point number, in scientific notation
3118 %f a floating-point number, in fixed decimal notation
3119 %g a floating-point number, in %e or %f notation
3120
1b3f7d21 3121In addition, Perl permits the following widely-supported conversions:
74a77017 3122
74a77017
CS
3123 %X like %x, but using upper-case letters
3124 %E like %e, but using an upper-case "E"
3125 %G like %g, but with an upper-case "E" (if applicable)
3126 %p a pointer (outputs the Perl value's address in hexadecimal)
1b3f7d21
CS
3127 %n special: *stores* the number of characters output so far
3128 into the next variable in the parameter list
74a77017 3129
1b3f7d21
CS
3130Finally, for backward (and we do mean "backward") compatibility, Perl
3131permits these unnecessary but widely-supported conversions:
74a77017 3132
1b3f7d21 3133 %i a synonym for %d
74a77017
CS
3134 %D a synonym for %ld
3135 %U a synonym for %lu
3136 %O a synonym for %lo
3137 %F a synonym for %f
3138
3139Perl permits the following universally-known flags between the C<%>
3140and the conversion letter:
3141
3142 space prefix positive number with a space
3143 + prefix positive number with a plus sign
3144 - left-justify within the field
3145 0 use zeros, not spaces, to right-justify
1b3f7d21 3146 # prefix octal with "0", hex with "0x"
74a77017
CS
3147 number minimum field width
3148 .number "precision": digits after decimal point for floating-point,
3149 max length for string, minimum length for integer
3150 l interpret integer as C type "long" or "unsigned long"
74a77017
CS
3151 h interpret integer as C type "short" or "unsigned short"
3152
1b3f7d21 3153There is also one Perl-specific flag:
74a77017
CS
3154
3155 V interpret integer as Perl's standard integer type
3156
3157Where a number would appear in the flags, an asterisk ("*") may be
3158used instead, in which case Perl uses the next item in the parameter
3159list as the given number (that is, as the field width or precision).
3160If a field width obtained through "*" is negative, it has the same
3161effect as the '-' flag: left-justification.
3162
3163If C<use locale> is in effect, the character used for the decimal
3164point in formatted real numbers is affected by the LC_NUMERIC locale.
3165See L<perllocale>.
a0d0e21e
LW
3166
3167=item sqrt EXPR
3168
54310121 3169=item sqrt
bbce6d69 3170
a0d0e21e
LW
3171Return the square root of EXPR. If EXPR is omitted, returns square
3172root of $_.
3173
3174=item srand EXPR
3175
93dc8474
CS
3176=item srand
3177
3178Sets the random number seed for the C<rand> operator. If EXPR is
3179omitted, uses a semi-random value based on the current time and process
3180ID, among other things. In versions of Perl prior to 5.004 the default
3181seed was just the current time(). This isn't a particularly good seed,
3182so many old programs supply their own seed value (often C<time ^ $$> or
3183C<time ^ ($$ + ($$ << 15))>), but that isn't necessary any more.
3184
3185In fact, it's usually not necessary to call srand() at all, because if
3186it is not called explicitly, it is called implicitly at the first use of
2f9daede
TP
3187the C<rand> operator. However, this was not the case in version of Perl
3188before 5.004, so if your script will run under older Perl versions, it
3189should call srand().
93dc8474 3190
2f9daede
TP
3191Note that you need something much more random than the default seed for
3192cryptographic purposes. Checksumming the compressed output of one or more
3193rapidly changing operating system status programs is the usual method. For
3194example:
28757baa 3195
3196 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
3197
0078ec44
RS
3198If you're particularly concerned with this, see the Math::TrulyRandom
3199module in CPAN.
3200
3201Do I<not> call srand() multiple times in your program unless you know
28757baa 3202exactly what you're doing and why you're doing it. The point of the
3203function is to "seed" the rand() function so that rand() can produce
3204a different sequence each time you run your program. Just do it once at the
3205top of your program, or you I<won't> get random numbers out of rand()!
3206
54310121 3207Frequently called programs (like CGI scripts) that simply use
28757baa 3208
3209 time ^ $$
3210
54310121 3211for a seed can fall prey to the mathematical property that
28757baa 3212
3213 a^b == (a+1)^(b+1)
3214
0078ec44 3215one-third of the time. So don't do that.
f86702cc 3216
a0d0e21e
LW
3217=item stat FILEHANDLE
3218
3219=item stat EXPR
3220
54310121 3221=item stat
bbce6d69 3222
a0d0e21e 3223Returns a 13-element array giving the status info for a file, either the
2f9daede 3224file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted, it
bbce6d69 3225stats $_. Returns a null list if the stat fails. Typically used as
3226follows:
3227
a0d0e21e
LW
3228
3229 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
3230 $atime,$mtime,$ctime,$blksize,$blocks)
3231 = stat($filename);
3232
54310121 3233Not all fields are supported on all filesystem types. Here are the
c07a80fd 3234meaning of the fields:
3235
54310121 3236 0 dev device number of filesystem
3237 1 ino inode number
3238 2 mode file mode (type and permissions)
3239 3 nlink number of (hard) links to the file
3240 4 uid numeric user ID of file's owner
3241 5 gid numeric group ID of file's owner
3242 6 rdev the device identifier (special files only)
3243 7 size total size of file, in bytes
3244 8 atime last access time since the epoch
3245 9 mtime last modify time since the epoch
3246 10 ctime inode change time (NOT creation time!) since the epoch
3247 11 blksize preferred block size for file system I/O
3248 12 blocks actual number of blocks allocated
c07a80fd 3249
3250(The epoch was at 00:00 January 1, 1970 GMT.)
3251
a0d0e21e
LW
3252If stat is passed the special filehandle consisting of an underline, no
3253stat is done, but the current contents of the stat structure from the
3254last stat or filetest are returned. Example:
3255
3256 if (-x $file && (($d) = stat(_)) && $d < 0) {
3257 print "$file is executable NFS file\n";
3258 }
3259
5f05dabc 3260(This works on machines only for which the device number is negative under NFS.)
a0d0e21e
LW
3261
3262=item study SCALAR
3263
3264=item study
3265
184e9718 3266Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
a0d0e21e
LW
3267doing many pattern matches on the string before it is next modified.
3268This may or may not save time, depending on the nature and number of
3269patterns you are searching on, and on the distribution of character
54310121 3270frequencies in the string to be searched -- you probably want to compare
5f05dabc 3271run times with and without it to see which runs faster. Those loops
a0d0e21e
LW
3272which scan for many short constant strings (including the constant
3273parts of more complex patterns) will benefit most. You may have only
54310121 3274one study active at a time -- if you study a different scalar the first
a0d0e21e
LW
3275is "unstudied". (The way study works is this: a linked list of every
3276character in the string to be searched is made, so we know, for
3277example, where all the 'k' characters are. From each search string,
3278the rarest character is selected, based on some static frequency tables
3279constructed from some C programs and English text. Only those places
3280that contain this "rarest" character are examined.)
3281
3282For example, here is a loop which inserts index producing entries
3283before any line containing a certain pattern:
3284
3285 while (<>) {
3286 study;
3287 print ".IX foo\n" if /\bfoo\b/;
3288 print ".IX bar\n" if /\bbar\b/;
3289 print ".IX blurfl\n" if /\bblurfl\b/;
3290 ...
3291 print;
3292 }
3293
3294In searching for /\bfoo\b/, only those locations in $_ that contain "f"
3295will be looked at, because "f" is rarer than "o". In general, this is
3296a big win except in pathological cases. The only question is whether
3297it saves you more time than it took to build the linked list in the
3298first place.
3299
3300Note that if you have to look for strings that you don't know till
3301runtime, you can build an entire loop as a string and eval that to
3302avoid recompiling all your patterns all the time. Together with
3303undefining $/ to input entire files as one record, this can be very
3304fast, often faster than specialized programs like fgrep(1). The following
184e9718 3305scans a list of files (C<@files>) for a list of words (C<@words>), and prints
a0d0e21e
LW
3306out the names of those files that contain a match:
3307
3308 $search = 'while (<>) { study;';
3309 foreach $word (@words) {
3310 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
3311 }
3312 $search .= "}";
3313 @ARGV = @files;
3314 undef $/;
3315 eval $search; # this screams
5f05dabc 3316 $/ = "\n"; # put back to normal input delimiter
a0d0e21e
LW
3317 foreach $file (sort keys(%seen)) {
3318 print $file, "\n";
3319 }
3320
cb1a09d0
AD
3321=item sub BLOCK
3322
3323=item sub NAME
3324
3325=item sub NAME BLOCK
3326
3327This is subroutine definition, not a real function I<per se>. With just a
3328NAME (and possibly prototypes), it's just a forward declaration. Without
3329a NAME, it's an anonymous function declaration, and does actually return a
2f9daede 3330value: the CODE ref of the closure you just created. See L<perlsub> and
cb1a09d0
AD
3331L<perlref> for details.
3332
a0d0e21e
LW
3333=item substr EXPR,OFFSET,LEN
3334
3335=item substr EXPR,OFFSET
3336
3337Extracts a substring out of EXPR and returns it. First character is at
2f9daede
TP
3338offset 0, or whatever you've set C<$[> to (but don't do that).
3339If OFFSET is negative, starts
a0d0e21e 3340that far from the end of the string. If LEN is omitted, returns
748a9306
LW
3341everything to the end of the string. If LEN is negative, leaves that
3342many characters off the end of the string.
3343
3344You can use the substr() function
a0d0e21e
LW
3345as an lvalue, in which case EXPR must be an lvalue. If you assign
3346something shorter than LEN, the string will shrink, and if you assign
3347something longer than LEN, the string will grow to accommodate it. To
3348keep the string the same length you may need to pad or chop your value
3349using sprintf().
3350
3351=item symlink OLDFILE,NEWFILE
3352
3353Creates a new filename symbolically linked to the old filename.
3354Returns 1 for success, 0 otherwise. On systems that don't support
3355symbolic links, produces a fatal error at run time. To check for that,
3356use eval:
3357
54310121 3358 $symlink_exists = (eval {symlink("","")};, $@ eq '');
a0d0e21e
LW
3359
3360=item syscall LIST
3361
3362Calls the system call specified as the first element of the list,
3363passing the remaining elements as arguments to the system call. If
3364unimplemented, produces a fatal error. The arguments are interpreted
3365as follows: if a given argument is numeric, the argument is passed as
3366an int. If not, the pointer to the string value is passed. You are
3367responsible to make sure a string is pre-extended long enough to
3368receive any result that might be written into a string. If your
3369integer arguments are not literals and have never been interpreted in a
3370numeric context, you may need to add 0 to them to force them to look
3371like numbers.
3372
3373 require 'syscall.ph'; # may need to run h2ph
3374 syscall(&SYS_write, fileno(STDOUT), "hi there\n", 9);
3375
5f05dabc 3376Note that Perl supports passing of up to only 14 arguments to your system call,
a0d0e21e
LW
3377which in practice should usually suffice.
3378
c07a80fd 3379=item sysopen FILEHANDLE,FILENAME,MODE
3380
3381=item sysopen FILEHANDLE,FILENAME,MODE,PERMS
3382
3383Opens the file whose filename is given by FILENAME, and associates it
3384with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
3385the name of the real filehandle wanted. This function calls the
3386underlying operating system's C<open> function with the parameters
3387FILENAME, MODE, PERMS.
3388
3389The possible values and flag bits of the MODE parameter are
3390system-dependent; they are available via the standard module C<Fcntl>.
3391However, for historical reasons, some values are universal: zero means
3392read-only, one means write-only, and two means read/write.
3393
3394If the file named by FILENAME does not exist and the C<open> call
3395creates it (typically because MODE includes the O_CREAT flag), then
3396the value of PERMS specifies the permissions of the newly created
3397file. If PERMS is omitted, the default value is 0666, which allows
3398read and write for all. This default is reasonable: see C<umask>.
3399
28757baa 3400The IO::File module provides a more object-oriented approach, if you're
3401into that kind of thing.
3402
a0d0e21e
LW
3403=item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
3404
3405=item sysread FILEHANDLE,SCALAR,LENGTH
3406
3407Attempts to read LENGTH bytes of data into variable SCALAR from the
96e4d5b1 3408specified FILEHANDLE, using the system call read(2). It bypasses stdio,
3409so mixing this with other kinds of reads, print(), write(), seek(), or
3410tell() can cause confusion. Returns the number of bytes actually read,
3411or undef if there was an error. SCALAR will be grown or shrunk so that
3412the last byte actually read is the last byte of the scalar after the
3413read.
ff68c719 3414
3415An OFFSET may be specified to place the read data at some place in the
3416string other than the beginning. A negative OFFSET specifies
3417placement at that many bytes counting backwards from the end of the
3418string. A positive OFFSET greater than the length of SCALAR results
3419in the string being padded to the required size with "\0" bytes before
3420the result of the read is appended.
a0d0e21e 3421
137443ea 3422=item sysseek FILEHANDLE,POSITION,WHENCE
3423
8903cb82 3424Sets FILEHANDLE's system position using the system call lseek(2). It
3425bypasses stdio, so mixing this with reads (other than sysread()),
3426print(), write(), seek(), or tell() may cause confusion. FILEHANDLE may
3427be an expression whose value gives the name of the filehandle. The
3428values for WHENCE are 0 to set the new position to POSITION, 1 to set
3429the it to the current position plus POSITION, and 2 to set it to EOF
3430plus POSITION (typically negative). For WHENCE, you may use the
3431constants SEEK_SET, SEEK_CUR, and SEEK_END from either the IO::Seekable
3432or the POSIX module.
3433
3434Returns the new position, or the undefined value on failure. A position
3435of zero is returned as the string "0 but true"; thus sysseek() returns
3436TRUE on success and FALSE on failure, yet you can still easily determine
3437the new position.
137443ea 3438
a0d0e21e
LW
3439=item system LIST
3440
3441Does exactly the same thing as "exec LIST" except that a fork is done
3442first, and the parent process waits for the child process to complete.
3443Note that argument processing varies depending on the number of
3444arguments. The return value is the exit status of the program as
3445returned by the wait() call. To get the actual exit value divide by
54310121 3446256. See also L</exec>. This is I<NOT> what you want to use to capture
3447the output from a command, for that you should use merely backticks or
28757baa 3448qx//, as described in L<perlop/"`STRING`">.
a0d0e21e 3449
54310121 3450Because system() and backticks block SIGINT and SIGQUIT, killing the
28757baa 3451program they're running doesn't actually interrupt your program.
3452
3453 @args = ("command", "arg1", "arg2");
54310121 3454 system(@args) == 0
3455 or die "system @args failed: $?"
28757baa 3456
3457Here's a more elaborate example of analysing the return value from
54310121 3458system() on a Unix system to check for all possibilities, including for
3459signals and core dumps.
28757baa 3460
3461 $rc = 0xffff & system @args;
3462 printf "system(%s) returned %#04x: ", "@args", $rc;
3463 if ($rc == 0) {
3464 print "ran with normal exit\n";
54310121 3465 }
28757baa 3466 elsif ($rc == 0xff00) {
3467 print "command failed: $!\n";
54310121 3468 }
28757baa 3469 elsif ($rc > 0x80) {
3470 $rc >>= 8;
3471 print "ran with non-zero exit status $rc\n";
54310121 3472 }
28757baa 3473 else {
3474 print "ran with ";
3475 if ($rc & 0x80) {
3476 $rc &= ~0x80;
54310121 3477 print "core dump from ";
3478 }
28757baa 3479 print "signal $rc\n"
54310121 3480 }
28757baa 3481 $ok = ($rc != 0);
f86702cc 3482
a0d0e21e
LW
3483=item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
3484
3485=item syswrite FILEHANDLE,SCALAR,LENGTH
3486
3487Attempts to write LENGTH bytes of data from variable SCALAR to the
3488specified FILEHANDLE, using the system call write(2). It bypasses
96e4d5b1 3489stdio, so mixing this with reads (other than sysread()), print(),
3490write(), seek(), or tell() may cause confusion. Returns the number of
3491bytes actually written, or undef if there was an error. If the length
3492is greater than the available data, only as much data as is available
3493will be written.
ff68c719 3494
3495An OFFSET may be specified to write the data from some part of the
3496string other than the beginning. A negative OFFSET specifies writing
54310121 3497that many bytes counting backwards from the end of the string.
a0d0e21e
LW
3498
3499=item tell FILEHANDLE
3500
3501=item tell
3502
8903cb82 3503Returns the current position for FILEHANDLE. FILEHANDLE may be an
a0d0e21e
LW
3504expression whose value gives the name of the actual filehandle. If
3505FILEHANDLE is omitted, assumes the file last read.
3506
3507=item telldir DIRHANDLE
3508
3509Returns the current position of the readdir() routines on DIRHANDLE.
3510Value may be given to seekdir() to access a particular location in a
3511directory. Has the same caveats about possible directory compaction as
3512the corresponding system library routine.
3513
4633a7c4 3514=item tie VARIABLE,CLASSNAME,LIST
a0d0e21e 3515
4633a7c4
LW
3516This function binds a variable to a package class that will provide the
3517implementation for the variable. VARIABLE is the name of the variable
3518to be enchanted. CLASSNAME is the name of a class implementing objects
3519of correct type. Any additional arguments are passed to the "new"
3520method of the class (meaning TIESCALAR, TIEARRAY, or TIEHASH).
3521Typically these are arguments such as might be passed to the dbm_open()
cb1a09d0
AD
3522function of C. The object returned by the "new" method is also
3523returned by the tie() function, which would be useful if you want to
4633a7c4 3524access other methods in CLASSNAME.
a0d0e21e
LW
3525
3526Note that functions such as keys() and values() may return huge array
748a9306
LW
3527values when used on large objects, like DBM files. You may prefer to
3528use the each() function to iterate over such. Example:
a0d0e21e
LW
3529
3530 # print out history file offsets
4633a7c4 3531 use NDBM_File;
da0045b7 3532 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
a0d0e21e
LW
3533 while (($key,$val) = each %HIST) {
3534 print $key, ' = ', unpack('L',$val), "\n";
3535 }
3536 untie(%HIST);
3537
aa689395 3538A class implementing a hash should have the following methods:
a0d0e21e 3539
4633a7c4 3540 TIEHASH classname, LIST
a0d0e21e
LW
3541 DESTROY this
3542 FETCH this, key
3543 STORE this, key, value
3544 DELETE this, key
3545 EXISTS this, key
3546 FIRSTKEY this
3547 NEXTKEY this, lastkey
3548
4633a7c4 3549A class implementing an ordinary array should have the following methods:
a0d0e21e 3550
4633a7c4 3551 TIEARRAY classname, LIST
a0d0e21e
LW
3552 DESTROY this
3553 FETCH this, key
3554 STORE this, key, value
3555 [others TBD]
3556
4633a7c4 3557A class implementing a scalar should have the following methods:
a0d0e21e 3558
4633a7c4 3559 TIESCALAR classname, LIST
a0d0e21e 3560 DESTROY this
54310121 3561 FETCH this,
a0d0e21e
LW
3562 STORE this, value
3563
4633a7c4
LW
3564Unlike dbmopen(), the tie() function will not use or require a module
3565for you--you need to do that explicitly yourself. See L<DB_File>
3566or the F<Config> module for interesting tie() implementations.
3567
f3cbc334
RS
3568=item tied VARIABLE
3569
3570Returns a reference to the object underlying VARIABLE (the same value
3571that was originally returned by the tie() call which bound the variable
3572to a package.) Returns the undefined value if VARIABLE isn't tied to a
3573package.
3574
a0d0e21e
LW
3575=item time
3576
da0045b7 3577Returns the number of non-leap seconds since whatever time the system
3578considers to be the epoch (that's 00:00:00, January 1, 1904 for MacOS,
3579and 00:00:00 UTC, January 1, 1970 for most other systems).
3580Suitable for feeding to gmtime() and localtime().
a0d0e21e
LW
3581
3582=item times
3583
3584Returns a four-element array giving the user and system times, in
3585seconds, for this process and the children of this process.
3586
3587 ($user,$system,$cuser,$csystem) = times;
3588
3589=item tr///
3590
54310121 3591The translation operator. Same as y///. See L<perlop>.
a0d0e21e
LW
3592
3593=item truncate FILEHANDLE,LENGTH
3594
3595=item truncate EXPR,LENGTH
3596
3597Truncates the file opened on FILEHANDLE, or named by EXPR, to the
3598specified length. Produces a fatal error if truncate isn't implemented
3599on your system.
3600
3601=item uc EXPR
3602
54310121 3603=item uc
bbce6d69 3604
a0d0e21e
LW
3605Returns an uppercased version of EXPR. This is the internal function
3606implementing the \U escape in double-quoted strings.
a034a98d 3607Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
a0d0e21e 3608
bbce6d69 3609If EXPR is omitted, uses $_.
3610
a0d0e21e
LW
3611=item ucfirst EXPR
3612
54310121 3613=item ucfirst
bbce6d69 3614
a0d0e21e
LW
3615Returns the value of EXPR with the first character uppercased. This is
3616the internal function implementing 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 3619If EXPR is omitted, uses $_.
3620
a0d0e21e
LW
3621=item umask EXPR
3622
3623=item umask
3624
2f9daede
TP
3625Sets the umask for the process to EXPR and returns the previous value.
3626If EXPR is omitted, merely returns the current umask. Remember that a
3627umask is a number, usually given in octal; it is I<not> a string of octal
dc848c6f 3628digits. See also L</oct>, if all you have is a string.
a0d0e21e
LW
3629
3630=item undef EXPR
3631
3632=item undef
3633
54310121 3634Undefines the value of EXPR, which must be an lvalue. Use only on a
3635scalar value, an entire array, an entire hash, or a subroutine name (using
2f9daede
TP
3636"&"). (Using undef() will probably not do what you expect on most
3637predefined variables or DBM list values, so don't do that.) Always
3638returns the undefined value. You can omit the EXPR, in which case
3639nothing is undefined, but you still get an undefined value that you
3640could, for instance, return from a subroutine, assign to a variable or
3641pass as a parameter. Examples:
a0d0e21e
LW
3642
3643 undef $foo;
2f9daede 3644 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
a0d0e21e 3645 undef @ary;
aa689395 3646 undef %hash;
a0d0e21e 3647 undef &mysub;
54310121 3648 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
2f9daede
TP
3649 select undef, undef, undef, 0.25;
3650 ($a, $b, undef, $c) = &foo; # Ignore third value returned
a0d0e21e
LW
3651
3652=item unlink LIST
3653
54310121 3654=item unlink
bbce6d69 3655
a0d0e21e
LW
3656Deletes a list of files. Returns the number of files successfully
3657deleted.
3658
3659 $cnt = unlink 'a', 'b', 'c';
3660 unlink @goners;
3661 unlink <*.bak>;
3662
3663Note: unlink will not delete directories unless you are superuser and
3664the B<-U> flag is supplied to Perl. Even if these conditions are
3665met, be warned that unlinking a directory can inflict damage on your
3666filesystem. Use rmdir instead.
3667