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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
17be first, and the list argument will follow. (Note that there can only
18ever be one list argument.) For instance, splice() has three scalar
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
31parens.) If you use the parens, the simple (but occasionally
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
38 print 1+2+3; # Prints 6.
39 print(1+2) + 3; # Prints 3.
40 print (1+2)+3; # Also prints 3!
41 print +(1+2)+3; # Prints 6.
42 print ((1+2)+3); # Prints 6.
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,
51non-abortive failure is generally indicated in a scalar context by
52returning the undefined value, and in a list context by returning the
53null list.
54
55Remember the following rule:
56
cb1a09d0 57=over 8
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cb1a09d0 59=item
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60
61I<THERE IS NO GENERAL RULE FOR CONVERTING A LIST INTO A SCALAR!>
62
63=back
64
65Each operator and function decides which sort of value it would be most
66appropriate to return in a scalar context. Some operators return the
67length of the list that would have been returned in a list context. Some
68operators return the first value in the list. Some operators return the
69last value in the list. Some operators return a count of successful
70operations. In general, they do what you want, unless you want
71consistency.
72
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73=head2 Perl Functions by Category
74
75Here are Perl's functions (including things that look like
76functions, like some of the keywords and named operators)
77arranged by category. Some functions appear in more
78than one place.
79
80=over
81
82=item Functions for SCALARs or strings
83
84chomp, chop, chr, crypt, hex, index, lc, lcfirst, length,
85oct, ord, pack, q/STRING/, qq/STRING/, reverse, rindex,
86sprintf, substr, tr///, uc, ucfirst, y///
87
88=item Regular expressions and pattern matching
89
90m//, pos, quotemeta, s///, split, study
91
92=item Numeric functions
93
94abs, atan2, cos, exp, hex, int, log, oct, rand, sin, sqrt,
95srand
96
97=item Functions for real @ARRAYs
98
99pop, push, shift, splice, unshift
100
101=item Functions for list data
102
103grep, join, map, qw/STRING/, reverse, sort, unpack
104
105=item Functions for real %HASHes
106
107delete, each, exists, keys, values
108
109=item Input and output functions
110
111binmode, close, closedir, dbmclose, dbmopen, die, eof,
112fileno, flock, format, getc, print, printf, read, readdir,
113rewinddir, seek, seekdir, select, syscall, sysread,
114syswrite, tell, telldir, truncate, warn, write
115
116=item Functions for fixed length data or records
117
118pack, read, syscall, sysread, syswrite, unpack, vec
119
120=item Functions for filehandles, files, or directories
121
c07a80fd 122-X, chdir, chmod, chown, chroot, fcntl, glob, ioctl, link,
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123lstat, mkdir, open, opendir, readlink, rename, rmdir,
124stat, symlink, umask, unlink, utime
125
126=item Keywords related to the control flow of your perl program
127
128caller, continue, die, do, dump, eval, exit, goto, last,
129next, redo, return, sub, wantarray
130
131=item Keywords related to scoping
132
133caller, import, local, my, package, use
134
135=item Miscellaneous functions
136
137defined, dump, eval, formline, local, my, reset, scalar,
138undef, wantarray
139
140=item Functions for processes and process groups
141
142alarm, exec, fork, getpgrp, getppid, getpriority, kill,
143pipe, qx/STRING/, setpgrp, setpriority, sleep, system,
144times, wait, waitpid
145
146=item Keywords related to perl modules
147
148do, import, no, package, require, use
149
150=item Keywords related to classes and object-orientedness
151
f3cbc334 152bless, dbmclose, dbmopen, package, ref, tie, tied, untie, use
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153
154=item Low-level socket functions
155
156accept, bind, connect, getpeername, getsockname,
157getsockopt, listen, recv, send, setsockopt, shutdown,
158socket, socketpair
159
160=item System V interprocess communication functions
161
162msgctl, msgget, msgrcv, msgsnd, semctl, semget, semop,
163shmctl, shmget, shmread, shmwrite
164
165=item Fetching user and group info
166
167endgrent, endhostent, endnetent, endpwent, getgrent,
168getgrgid, getgrnam, getlogin, getpwent, getpwnam,
169getpwuid, setgrent, setpwent
170
171=item Fetching network info
172
173endprotoent, endservent, gethostbyaddr, gethostbyname,
174gethostent, getnetbyaddr, getnetbyname, getnetent,
175getprotobyname, getprotobynumber, getprotoent,
176getservbyname, getservbyport, getservent, sethostent,
177setnetent, setprotoent, setservent
178
179=item Time-related functions
180
181gmtime, localtime, time, times
182
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183=item Functions new in perl5
184
185abs, bless, chomp, chr, exists, formline, glob, import, lc,
186lcfirst, map, my, no, qx, qw, ref, sub*, sysopen, tie, tied, uc,
187ucfirst, untie, use
188
189* - C<sub> was a keyword in perl4, but in perl5 it is an
190operator which can be used in expressions.
191
192=item Functions obsoleted in perl5
193
194dbmclose, dbmopen
195
196
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197=back
198
199=head2 Alphabetical Listing of Perl Functions
200
201
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202=over 8
203
204=item -X FILEHANDLE
205
206=item -X EXPR
207
208=item -X
209
210A file test, where X is one of the letters listed below. This unary
211operator takes one argument, either a filename or a filehandle, and
212tests the associated file to see if something is true about it. If the
213argument is omitted, tests $_, except for C<-t>, which tests STDIN.
214Unless otherwise documented, it returns C<1> for TRUE and C<''> for FALSE, or
215the undefined value if the file doesn't exist. Despite the funny
216names, precedence is the same as any other named unary operator, and
217the argument may be parenthesized like any other unary operator. The
218operator may be any of:
219
220 -r File is readable by effective uid/gid.
221 -w File is writable by effective uid/gid.
222 -x File is executable by effective uid/gid.
223 -o File is owned by effective uid.
224
225 -R File is readable by real uid/gid.
226 -W File is writable by real uid/gid.
227 -X File is executable by real uid/gid.
228 -O File is owned by real uid.
229
230 -e File exists.
231 -z File has zero size.
232 -s File has non-zero size (returns size).
233
234 -f File is a plain file.
235 -d File is a directory.
236 -l File is a symbolic link.
237 -p File is a named pipe (FIFO).
238 -S File is a socket.
239 -b File is a block special file.
240 -c File is a character special file.
241 -t Filehandle is opened to a tty.
242
243 -u File has setuid bit set.
244 -g File has setgid bit set.
245 -k File has sticky bit set.
246
247 -T File is a text file.
248 -B File is a binary file (opposite of -T).
249
250 -M Age of file in days when script started.
251 -A Same for access time.
252 -C Same for inode change time.
253
254The interpretation of the file permission operators C<-r>, C<-R>, C<-w>,
255C<-W>, C<-x> and C<-X> is based solely on the mode of the file and the
256uids and gids of the user. There may be other reasons you can't actually
257read, write or execute the file. Also note that, for the superuser,
258C<-r>, C<-R>, C<-w> and C<-W> always return 1, and C<-x> and C<-X> return
2591 if any execute bit is set in the mode. Scripts run by the superuser may
260thus need to do a stat() in order to determine the actual mode of the
261file, or temporarily set the uid to something else.
262
263Example:
264
265 while (<>) {
266 chop;
267 next unless -f $_; # ignore specials
268 ...
269 }
270
271Note that C<-s/a/b/> does not do a negated substitution. Saying
272C<-exp($foo)> still works as expected, however--only single letters
273following a minus are interpreted as file tests.
274
275The C<-T> and C<-B> switches work as follows. The first block or so of the
276file is examined for odd characters such as strange control codes or
277characters with the high bit set. If too many odd characters (>30%)
278are found, it's a C<-B> file, otherwise it's a C<-T> file. Also, any file
279containing null in the first block is considered a binary file. If C<-T>
280or C<-B> is used on a filehandle, the current stdio buffer is examined
281rather than the first block. Both C<-T> and C<-B> return TRUE on a null
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282file, or a file at EOF when testing a filehandle. Because you have to
283read a file to do the C<-T> test, on most occasions you want to use a C<-f>
284against the file first, as in C<next unless -f $file && -T $file>.
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285
286If any of the file tests (or either the stat() or lstat() operators) are given the
287special filehandle consisting of a solitary underline, then the stat
288structure of the previous file test (or stat operator) is used, saving
289a system call. (This doesn't work with C<-t>, and you need to remember
290that lstat() and C<-l> will leave values in the stat structure for the
291symbolic link, not the real file.) Example:
292
293 print "Can do.\n" if -r $a || -w _ || -x _;
294
295 stat($filename);
296 print "Readable\n" if -r _;
297 print "Writable\n" if -w _;
298 print "Executable\n" if -x _;
299 print "Setuid\n" if -u _;
300 print "Setgid\n" if -g _;
301 print "Sticky\n" if -k _;
302 print "Text\n" if -T _;
303 print "Binary\n" if -B _;
304
305=item abs VALUE
306
307Returns the absolute value of its argument.
308
309=item accept NEWSOCKET,GENERICSOCKET
310
311Accepts an incoming socket connect, just as the accept(2) system call
312does. Returns the packed address if it succeeded, FALSE otherwise.
4633a7c4 313See example in L<perlipc/"Sockets: Client/Server Communication">.
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314
315=item alarm SECONDS
316
317Arranges to have a SIGALRM delivered to this process after the
318specified number of seconds have elapsed. (On some machines,
319unfortunately, the elapsed time may be up to one second less than you
320specified because of how seconds are counted.) Only one timer may be
321counting at once. Each call disables the previous timer, and an
322argument of 0 may be supplied to cancel the previous timer without
323starting a new one. The returned value is the amount of time remaining
324on the previous timer.
325
4633a7c4 326For delays of finer granularity than one second, you may use Perl's
a0d0e21e 327syscall() interface to access setitimer(2) if your system supports it,
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328or else see L</select()> below. It is not advised to intermix alarm()
329and sleep() calls.
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330
331=item atan2 Y,X
332
333Returns the arctangent of Y/X in the range -PI to PI.
334
335=item bind SOCKET,NAME
336
337Binds a network address to a socket, just as the bind system call
338does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
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339packed address of the appropriate type for the socket. See the examples in
340L<perlipc/"Sockets: Client/Server Communication">.
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341
342=item binmode FILEHANDLE
343
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344Arranges for the file to be read or written in "binary" mode in operating
345systems that distinguish between binary and text files. Files that are
346not in binary mode have CR LF sequences translated to LF on input and LF
347translated to CR LF on output. Binmode has no effect under Unix; in DOS
348and similarly archaic systems, it may be imperative--otherwise your
349DOS-damaged C library may mangle your file. The key distinction between
350systems that need binmode and those that don't is their text file
351formats. Systems like Unix and Plan9 that delimit lines with a single
352character, and that encode that character in C as '\n', do not need
353C<binmode>. The rest need it. If FILEHANDLE is an expression, the value
354is taken as the name of the filehandle.
a0d0e21e 355
4633a7c4 356=item bless REF,CLASSNAME
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357
358=item bless REF
359
360This function tells the referenced object (passed as REF) that it is now
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361an object in the CLASSNAME package--or the current package if no CLASSNAME
362is specified, which is often the case. It returns the reference for
363convenience, since a bless() is often the last thing in a constructor.
364Always use the two-argument version if the function doing the blessing
365might be inherited by a derived class. See L<perlobj> for more about the
366blessing (and blessings) of objects.
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367
368=item caller EXPR
369
370=item caller
371
372Returns the context of the current subroutine call. In a scalar context,
373returns TRUE if there is a caller, that is, if we're in a subroutine or
374eval() or require(), and FALSE otherwise. In a list context, returns
375
748a9306 376 ($package, $filename, $line) = caller;
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377
378With EXPR, it returns some extra information that the debugger uses to
379print a stack trace. The value of EXPR indicates how many call frames
380to go back before the current one.
381
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382 ($package, $filename, $line,
383 $subroutine, $hasargs, $wantargs) = caller($i);
384
385Furthermore, when called from within the DB package, caller returns more
4633a7c4 386detailed information: it sets the list variable @DB::args to be the
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387arguments with which that subroutine was invoked.
388
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389=item chdir EXPR
390
391Changes the working directory to EXPR, if possible. If EXPR is
392omitted, changes to home directory. Returns TRUE upon success, FALSE
393otherwise. See example under die().
394
395=item chmod LIST
396
397Changes the permissions of a list of files. The first element of the
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398list must be the numerical mode, which should probably be an octal
399number. Returns the number of files successfully changed.
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400
401 $cnt = chmod 0755, 'foo', 'bar';
402 chmod 0755, @executables;
403
404=item chomp VARIABLE
405
406=item chomp LIST
407
408=item chomp
409
410This is a slightly safer version of chop (see below). It removes any
411line ending that corresponds to the current value of C<$/> (also known as
412$INPUT_RECORD_SEPARATOR in the C<English> module). It returns the number
413of characters removed. It's often used to remove the newline from the
414end of an input record when you're worried that the final record may be
415missing its newline. When in paragraph mode (C<$/ = "">), it removes all
416trailing newlines from the string. If VARIABLE is omitted, it chomps
417$_. Example:
418
419 while (<>) {
420 chomp; # avoid \n on last field
421 @array = split(/:/);
422 ...
423 }
424
425You can actually chomp anything that's an lvalue, including an assignment:
426
427 chomp($cwd = `pwd`);
428 chomp($answer = <STDIN>);
429
430If you chomp a list, each element is chomped, and the total number of
431characters removed is returned.
432
433=item chop VARIABLE
434
435=item chop LIST
436
437=item chop
438
439Chops off the last character of a string and returns the character
440chopped. It's used primarily to remove the newline from the end of an
441input record, but is much more efficient than C<s/\n//> because it neither
442scans nor copies the string. If VARIABLE is omitted, chops $_.
443Example:
444
445 while (<>) {
446 chop; # avoid \n on last field
447 @array = split(/:/);
448 ...
449 }
450
451You can actually chop anything that's an lvalue, including an assignment:
452
453 chop($cwd = `pwd`);
454 chop($answer = <STDIN>);
455
456If you chop a list, each element is chopped. Only the value of the
457last chop is returned.
458
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459Note that chop returns the last character. To return all but the last
460character, use C<substr($string, 0, -1)>.
461
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462=item chown LIST
463
464Changes the owner (and group) of a list of files. The first two
465elements of the list must be the I<NUMERICAL> uid and gid, in that order.
466Returns the number of files successfully changed.
467
468 $cnt = chown $uid, $gid, 'foo', 'bar';
469 chown $uid, $gid, @filenames;
470
471Here's an example that looks up non-numeric uids in the passwd file:
472
473 print "User: ";
474 chop($user = <STDIN>);
475 print "Files: "
476 chop($pattern = <STDIN>);
477
478 ($login,$pass,$uid,$gid) = getpwnam($user)
479 or die "$user not in passwd file";
480
481 @ary = <${pattern}>; # expand filenames
482 chown $uid, $gid, @ary;
483
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484On most systems, you are not allowed to change the ownership of the
485file unless you're the superuser, although you should be able to change
486the group to any of your secondary groups. On insecure systems, these
487restrictions may be relaxed, but this is not a portable assumption.
488
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489=item chr NUMBER
490
491Returns the character represented by that NUMBER in the character set.
492For example, C<chr(65)> is "A" in ASCII.
493
494=item chroot FILENAME
495
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496This function works as the system call by the same name: it makes the
497named directory the new root directory for all further pathnames that
498begin with a "/" by your process and all of its children. (It doesn't
499change your current working directory is unaffected.) For security
500reasons, this call is restricted to the superuser. If FILENAME is
501omitted, does chroot to $_.
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502
503=item close FILEHANDLE
504
505Closes the file or pipe associated with the file handle, returning TRUE
506only if stdio successfully flushes buffers and closes the system file
507descriptor. You don't have to close FILEHANDLE if you are immediately
4633a7c4 508going to do another open() on it, since open() will close it for you. (See
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509open().) However, an explicit close on an input file resets the line
510counter ($.), while the implicit close done by open() does not. Also,
511closing a pipe will wait for the process executing on the pipe to
512complete, in case you want to look at the output of the pipe
513afterwards. Closing a pipe explicitly also puts the status value of
514the command into C<$?>. Example:
515
516 open(OUTPUT, '|sort >foo'); # pipe to sort
517 ... # print stuff to output
518 close OUTPUT; # wait for sort to finish
519 open(INPUT, 'foo'); # get sort's results
520
521FILEHANDLE may be an expression whose value gives the real filehandle name.
522
523=item closedir DIRHANDLE
524
525Closes a directory opened by opendir().
526
527=item connect SOCKET,NAME
528
529Attempts to connect to a remote socket, just as the connect system call
530does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
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531packed address of the appropriate type for the socket. See the examples in
532L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 533
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534=item continue BLOCK
535
536Actually a flow control statement rather than a function. If there is a
537C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
538C<foreach>), it is always executed just before the conditional is about to
539be evaluated again, just like the third part of a C<for> loop in C. Thus
540it can be used to increment a loop variable, even when the loop has been
541continued via the C<next> statement (which is similar to the C C<continue>
542statement).
543
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544=item cos EXPR
545
546Returns the cosine of EXPR (expressed in radians). If EXPR is omitted
547takes cosine of $_.
548
549=item crypt PLAINTEXT,SALT
550
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551Encrypts a string exactly like the crypt(3) function in the C library
552(assuming that you actually have a version there that has not been
553extirpated as a potential munition). This can prove useful for checking
554the password file for lousy passwords, amongst other things. Only the
555guys wearing white hats should do this.
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556
557Here's an example that makes sure that whoever runs this program knows
558their own password:
559
560 $pwd = (getpwuid($<))[1];
561 $salt = substr($pwd, 0, 2);
562
563 system "stty -echo";
564 print "Password: ";
565 chop($word = <STDIN>);
566 print "\n";
567 system "stty echo";
568
569 if (crypt($word, $salt) ne $pwd) {
570 die "Sorry...\n";
571 } else {
572 print "ok\n";
573 }
574
575Of course, typing in your own password to whoever asks you
748a9306 576for it is unwise.
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577
578=item dbmclose ASSOC_ARRAY
579
580[This function has been superseded by the untie() function.]
581
582Breaks the binding between a DBM file and an associative array.
583
584=item dbmopen ASSOC,DBNAME,MODE
585
586[This function has been superseded by the tie() function.]
587
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588This binds a dbm(3), ndbm(3), sdbm(3), gdbm(), or Berkeley DB file to an
589associative array. ASSOC is the name of the associative array. (Unlike
590normal open, the first argument is I<NOT> a filehandle, even though it
591looks like one). DBNAME is the name of the database (without the F<.dir>
592or F<.pag> extension if any). If the database does not exist, it is
593created with protection specified by MODE (as modified by the umask()).
594If your system only supports the older DBM functions, you may perform only
595one dbmopen() in your program. In older versions of Perl, if your system
596had neither DBM nor ndbm, calling dbmopen() produced a fatal error; it now
597falls back to sdbm(3).
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598
599If you don't have write access to the DBM file, you can only read
600associative array variables, not set them. If you want to test whether
601you can write, either use file tests or try setting a dummy array entry
602inside an eval(), which will trap the error.
603
604Note that functions such as keys() and values() may return huge array
605values when used on large DBM files. You may prefer to use the each()
606function to iterate over large DBM files. Example:
607
608 # print out history file offsets
609 dbmopen(%HIST,'/usr/lib/news/history',0666);
610 while (($key,$val) = each %HIST) {
611 print $key, ' = ', unpack('L',$val), "\n";
612 }
613 dbmclose(%HIST);
614
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615See also L<AnyDBM_File> for a more general description of the pros and
616cons of the various dbm apparoches, as well as L<DB_File> for a particularly
617rich implementation.
4633a7c4 618
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619=item defined EXPR
620
cb1a09d0 621Returns a boolean value saying whether EXPR has a real value
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622or not. Many operations return the undefined value under exceptional
623conditions, such as end of file, uninitialized variable, system error
624and such. This function allows you to distinguish between an undefined
625null scalar and a defined null scalar with operations that might return
626a real null string, such as referencing elements of an array. You may
627also check to see if arrays or subroutines exist. Use of defined on
628predefined variables is not guaranteed to produce intuitive results.
629
630When used on a hash array element, it tells you whether the value
631is defined, not whether the key exists in the hash. Use exists() for that.
632
633Examples:
634
635 print if defined $switch{'D'};
636 print "$val\n" while defined($val = pop(@ary));
637 die "Can't readlink $sym: $!"
638 unless defined($value = readlink $sym);
639 eval '@foo = ()' if defined(@foo);
640 die "No XYZ package defined" unless defined %_XYZ;
641 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
642
643See also undef().
644
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645Note: many folks tend to overuse defined(), and then are surprised to
646discover that the number 0 and the null string are, in fact, defined
647concepts. For example, if you say
648
649 "ab" =~ /a(.*)b/;
650
651the pattern match succeeds, and $1 is defined, despite the fact that it
652matched "nothing". But it didn't really match nothing--rather, it
653matched something that happened to be 0 characters long. This is all
654very above-board and honest. When a function returns an undefined value,
655it's an admission that it couldn't give you an honest answer. So
656you should only use defined() when you're questioning the integrity
657of what you're trying to do. At other times, a simple comparison to
6580 or "" is what you want.
659
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660=item delete EXPR
661
662Deletes the specified value from its hash array. Returns the deleted
663value, or the undefined value if nothing was deleted. Deleting from
664C<$ENV{}> modifies the environment. Deleting from an array tied to a DBM
665file deletes the entry from the DBM file. (But deleting from a tie()d
666hash doesn't necessarily return anything.)
667
668The following deletes all the values of an associative array:
669
670 foreach $key (keys %ARRAY) {
671 delete $ARRAY{$key};
672 }
673
674(But it would be faster to use the undef() command.) Note that the
675EXPR can be arbitrarily complicated as long as the final operation is
676a hash key lookup:
677
678 delete $ref->[$x][$y]{$key};
679
680=item die LIST
681
682Outside of an eval(), prints the value of LIST to C<STDERR> and exits with
4633a7c4 683the current value of $! (errno). If $! is 0, exits with the value of
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684C<($? E<gt>E<gt> 8)> (backtick `command` status). If C<($? E<gt>E<gt> 8)> is 0,
685exits with 255. Inside an eval(), the error message is stuffed into C<$@>,
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686and the eval() is terminated with the undefined value; this makes die()
687the way to raise an exception.
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688
689Equivalent examples:
690
691 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
692 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
693
694If the value of EXPR does not end in a newline, the current script line
695number and input line number (if any) are also printed, and a newline
696is supplied. Hint: sometimes appending ", stopped" to your message
697will cause it to make better sense when the string "at foo line 123" is
698appended. Suppose you are running script "canasta".
699
700 die "/etc/games is no good";
701 die "/etc/games is no good, stopped";
702
703produce, respectively
704
705 /etc/games is no good at canasta line 123.
706 /etc/games is no good, stopped at canasta line 123.
707
708See also exit() and warn().
709
710=item do BLOCK
711
712Not really a function. Returns the value of the last command in the
713sequence of commands indicated by BLOCK. When modified by a loop
714modifier, executes the BLOCK once before testing the loop condition.
715(On other statements the loop modifiers test the conditional first.)
716
717=item do SUBROUTINE(LIST)
718
719A deprecated form of subroutine call. See L<perlsub>.
720
721=item do EXPR
722
723Uses the value of EXPR as a filename and executes the contents of the
724file as a Perl script. Its primary use is to include subroutines
725from a Perl subroutine library.
726
727 do 'stat.pl';
728
729is just like
730
731 eval `cat stat.pl`;
732
733except that it's more efficient, more concise, keeps track of the
734current filename for error messages, and searches all the B<-I>
735libraries if the file isn't in the current directory (see also the @INC
736array in L<perlvar/Predefined Names>). It's the same, however, in that it does
737reparse the file every time you call it, so you probably don't want to
738do this inside a loop.
739
740Note that inclusion of library modules is better done with the
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741use() and require() operators, which also do error checking
742and raise an exception if there's a problem.
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743
744=item dump LABEL
745
746This causes an immediate core dump. Primarily this is so that you can
747use the B<undump> program to turn your core dump into an executable binary
748after having initialized all your variables at the beginning of the
749program. When the new binary is executed it will begin by executing a
750C<goto LABEL> (with all the restrictions that C<goto> suffers). Think of
751it as a goto with an intervening core dump and reincarnation. If LABEL
752is omitted, restarts the program from the top. WARNING: any files
753opened at the time of the dump will NOT be open any more when the
754program is reincarnated, with possible resulting confusion on the part
755of Perl. See also B<-u> option in L<perlrun>.
756
757Example:
758
759 #!/usr/bin/perl
760 require 'getopt.pl';
761 require 'stat.pl';
762 %days = (
763 'Sun' => 1,
764 'Mon' => 2,
765 'Tue' => 3,
766 'Wed' => 4,
767 'Thu' => 5,
768 'Fri' => 6,
769 'Sat' => 7,
770 );
771
772 dump QUICKSTART if $ARGV[0] eq '-d';
773
774 QUICKSTART:
775 Getopt('f');
776
777=item each ASSOC_ARRAY
778
4633a7c4 779Returns a 2-element array consisting of the key and value for the next
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780value of an associative array, so that you can iterate over it.
781Entries are returned in an apparently random order. When the array is
782entirely read, a null array is returned (which when assigned produces a
783FALSE (0) value). The next call to each() after that will start
784iterating again. The iterator can be reset only by reading all the
785elements from the array. You should not add elements to an array while
786you're iterating over it. There is a single iterator for each
787associative array, shared by all each(), keys() and values() function
788calls in the program. The following prints out your environment like
789the printenv(1) program, only in a different order:
790
791 while (($key,$value) = each %ENV) {
792 print "$key=$value\n";
793 }
794
795See also keys() and values().
796
797=item eof FILEHANDLE
798
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799=item eof ()
800
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801=item eof
802
803Returns 1 if the next read on FILEHANDLE will return end of file, or if
804FILEHANDLE is not open. FILEHANDLE may be an expression whose value
805gives the real filehandle name. (Note that this function actually
806reads a character and then ungetc()s it, so it is not very useful in an
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807interactive context.) Do not read from a terminal file (or call
808C<eof(FILEHANDLE)> on it) after end-of-file is reached. Filetypes such
809as terminals may lose the end-of-file condition if you do.
810
811An C<eof> without an argument uses the last file read as argument.
812Empty parentheses () may be used to indicate
4633a7c4 813the pseudofile formed of the files listed on the command line, i.e.
37798a01 814C<eof()> is reasonable to use inside a while (E<lt>E<gt>) loop to detect the end
a0d0e21e 815of only the last file. Use C<eof(ARGV)> or eof without the parentheses to
37798a01 816test I<EACH> file in a while (E<lt>E<gt>) loop. Examples:
a0d0e21e 817
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818 # reset line numbering on each input file
819 while (<>) {
820 print "$.\t$_";
821 close(ARGV) if (eof); # Not eof().
822 }
823
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824 # insert dashes just before last line of last file
825 while (<>) {
826 if (eof()) {
827 print "--------------\n";
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828 close(ARGV); # close or break; is needed if we
829 # are reading from the terminal
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830 }
831 print;
832 }
833
a0d0e21e 834Practical hint: you almost never need to use C<eof> in Perl, because the
37798a01 835input operators return undef when they run out of data.
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836
837=item eval EXPR
838
839=item eval BLOCK
840
841EXPR is parsed and executed as if it were a little Perl program. It
842is executed in the context of the current Perl program, so that any
843variable settings, subroutine or format definitions remain afterwards.
844The value returned is the value of the last expression evaluated, or a
845return statement may be used, just as with subroutines.
846
847If there is a syntax error or runtime error, or a die() statement is
848executed, an undefined value is returned by eval(), and C<$@> is set to the
849error message. If there was no error, C<$@> is guaranteed to be a null
850string. If EXPR is omitted, evaluates $_. The final semicolon, if
851any, may be omitted from the expression.
852
853Note that, since eval() traps otherwise-fatal errors, it is useful for
4633a7c4 854determining whether a particular feature (such as socket() or symlink())
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855is implemented. It is also Perl's exception trapping mechanism, where
856the die operator is used to raise exceptions.
857
858If the code to be executed doesn't vary, you may use the eval-BLOCK
859form to trap run-time errors without incurring the penalty of
860recompiling each time. The error, if any, is still returned in C<$@>.
861Examples:
862
863 # make divide-by-zero non-fatal
864 eval { $answer = $a / $b; }; warn $@ if $@;
865
866 # same thing, but less efficient
867 eval '$answer = $a / $b'; warn $@ if $@;
868
869 # a compile-time error
870 eval { $answer = };
871
872 # a run-time error
873 eval '$answer ='; # sets $@
874
875With an eval(), you should be especially careful to remember what's
876being looked at when:
877
878 eval $x; # CASE 1
879 eval "$x"; # CASE 2
880
881 eval '$x'; # CASE 3
882 eval { $x }; # CASE 4
883
884 eval "\$$x++" # CASE 5
885 $$x++; # CASE 6
886
887Cases 1 and 2 above behave identically: they run the code contained in the
888variable $x. (Although case 2 has misleading double quotes making the
889reader wonder what else might be happening (nothing is).) Cases 3 and 4
890likewise behave in the same way: they run the code <$x>, which does
891nothing at all. (Case 4 is preferred for purely visual reasons.) Case 5
892is a place where normally you I<WOULD> like to use double quotes, except
cb1a09d0 893that in that particular situation, you can just use symbolic references
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894instead, as in case 6.
895
896=item exec LIST
897
898The exec() function executes a system command I<AND NEVER RETURNS>. Use
899the system() function if you want it to return.
900
901If there is more than one argument in LIST, or if LIST is an array with
902more than one value, calls execvp(3) with the arguments in LIST. If
903there is only one scalar argument, the argument is checked for shell
904metacharacters. If there are any, the entire argument is passed to
905C</bin/sh -c> for parsing. If there are none, the argument is split
906into words and passed directly to execvp(), which is more efficient.
37798a01 907Note: exec() and system() do not flush your output buffer, so you may
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908need to set C<$|> to avoid lost output. Examples:
909
910 exec '/bin/echo', 'Your arguments are: ', @ARGV;
911 exec "sort $outfile | uniq";
912
913If you don't really want to execute the first argument, but want to lie
914to the program you are executing about its own name, you can specify
915the program you actually want to run as an "indirect object" (without a
916comma) in front of the LIST. (This always forces interpretation of the
917LIST as a multi-valued list, even if there is only a single scalar in
918the list.) Example:
919
920 $shell = '/bin/csh';
921 exec $shell '-sh'; # pretend it's a login shell
922
923or, more directly,
924
925 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
926
927=item exists EXPR
928
929Returns TRUE if the specified hash key exists in its hash array, even
930if the corresponding value is undefined.
931
932 print "Exists\n" if exists $array{$key};
933 print "Defined\n" if defined $array{$key};
934 print "True\n" if $array{$key};
935
936A hash element can only be TRUE if it's defined, and defined if
937it exists, but the reverse doesn't necessarily hold true.
938
939Note that the EXPR can be arbitrarily complicated as long as the final
940operation is a hash key lookup:
941
942 if (exists $ref->[$x][$y]{$key}) { ... }
943
944=item exit EXPR
945
946Evaluates EXPR and exits immediately with that value. (Actually, it
947calls any defined C<END> routines first, but the C<END> routines may not
948abort the exit. Likewise any object destructors that need to be called
949are called before exit.) Example:
950
951 $ans = <STDIN>;
952 exit 0 if $ans =~ /^[Xx]/;
953
954See also die(). If EXPR is omitted, exits with 0 status.
955
956=item exp EXPR
957
958Returns I<e> (the natural logarithm base) to the power of EXPR.
959If EXPR is omitted, gives C<exp($_)>.
960
961=item fcntl FILEHANDLE,FUNCTION,SCALAR
962
963Implements the fcntl(2) function. You'll probably have to say
964
965 use Fcntl;
966
967first to get the correct function definitions. Argument processing and
968value return works just like ioctl() below. Note that fcntl() will produce
969a fatal error if used on a machine that doesn't implement fcntl(2).
970For example:
971
972 use Fcntl;
973 fcntl($filehandle, F_GETLK, $packed_return_buffer);
974
975=item fileno FILEHANDLE
976
977Returns the file descriptor for a filehandle. This is useful for
978constructing bitmaps for select(). If FILEHANDLE is an expression, the
979value is taken as the name of the filehandle.
980
981=item flock FILEHANDLE,OPERATION
982
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983Calls flock(2) on FILEHANDLE. See L<flock(2)> for definition of
984OPERATION. Returns TRUE for success, FALSE on failure. Will produce a
985fatal error if used on a machine that doesn't implement either flock(2) or
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986fcntl(2). The fcntl(2) system call will be automatically used if flock(2)
987is missing from your system. This makes flock() the portable file locking
988strategy, although it will only lock entire files, not records. Note also
989that some versions of flock() cannot lock things over the network; you
990would need to use the more system-specific fcntl() for that.
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991
992Here's a mailbox appender for BSD systems.
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993
994 $LOCK_SH = 1;
995 $LOCK_EX = 2;
996 $LOCK_NB = 4;
997 $LOCK_UN = 8;
998
999 sub lock {
1000 flock(MBOX,$LOCK_EX);
1001 # and, in case someone appended
1002 # while we were waiting...
1003 seek(MBOX, 0, 2);
1004 }
1005
1006 sub unlock {
1007 flock(MBOX,$LOCK_UN);
1008 }
1009
1010 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1011 or die "Can't open mailbox: $!";
1012
1013 lock();
1014 print MBOX $msg,"\n\n";
1015 unlock();
1016
cb1a09d0 1017See also L<DB_File> for other flock() examples.
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1018
1019=item fork
1020
1021Does a fork(2) system call. Returns the child pid to the parent process
4633a7c4 1022and 0 to the child process, or C<undef> if the fork is unsuccessful.
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1023Note: unflushed buffers remain unflushed in both processes, which means
1024you may need to set C<$|> ($AUTOFLUSH in English) or call the
1025autoflush() FileHandle method to avoid duplicate output.
1026
1027If you fork() without ever waiting on your children, you will accumulate
1028zombies:
1029
4633a7c4 1030 $SIG{CHLD} = sub { wait };
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1031
1032There's also the double-fork trick (error checking on
1033fork() returns omitted);
1034
1035 unless ($pid = fork) {
1036 unless (fork) {
1037 exec "what you really wanna do";
1038 die "no exec";
1039 # ... or ...
4633a7c4 1040 ## (some_perl_code_here)
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1041 exit 0;
1042 }
1043 exit 0;
1044 }
1045 waitpid($pid,0);
1046
cb1a09d0
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1047See also L<perlipc> for more examples of forking and reaping
1048moribund children.
1049
1050=item format
1051
1052Declare a picture format with use by the write() function. For
1053example:
1054
1055 format Something =
1056 Test: @<<<<<<<< @||||| @>>>>>
1057 $str, $%, '$' . int($num)
1058 .
1059
1060 $str = "widget";
1061 $num = $cost/$quantiy;
1062 $~ = 'Something';
1063 write;
1064
1065See L<perlform> for many details and examples.
1066
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1067
1068=item formline PICTURE, LIST
1069
4633a7c4 1070This is an internal function used by C<format>s, though you may call it
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1071too. It formats (see L<perlform>) a list of values according to the
1072contents of PICTURE, placing the output into the format output
4633a7c4
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1073accumulator, C<$^A> (or $ACCUMULATOR in English).
1074Eventually, when a write() is done, the contents of
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1075C<$^A> are written to some filehandle, but you could also read C<$^A>
1076yourself and then set C<$^A> back to "". Note that a format typically
1077does one formline() per line of form, but the formline() function itself
748a9306 1078doesn't care how many newlines are embedded in the PICTURE. This means
4633a7c4 1079that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
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1080You may therefore need to use multiple formlines to implement a single
1081record format, just like the format compiler.
1082
1083Be careful if you put double quotes around the picture, since an "C<@>"
1084character may be taken to mean the beginning of an array name.
4633a7c4 1085formline() always returns TRUE. See L<perlform> for other examples.
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1086
1087=item getc FILEHANDLE
1088
1089=item getc
1090
1091Returns the next character from the input file attached to FILEHANDLE,
1092or a null string at end of file. If FILEHANDLE is omitted, reads from STDIN.
4633a7c4 1093This is not particularly efficient. It cannot be used to get unbuffered
cb1a09d0 1094single-characters, however. For that, try something more like:
4633a7c4
LW
1095
1096 if ($BSD_STYLE) {
1097 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1098 }
1099 else {
cb1a09d0 1100 system "stty", '-icanon', 'eol', "\001";
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1101 }
1102
1103 $key = getc(STDIN);
1104
1105 if ($BSD_STYLE) {
1106 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1107 }
1108 else {
cb1a09d0 1109 system "stty", 'icanon', 'eol', '^@'; # ascii null
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1110 }
1111 print "\n";
1112
1113Determination of whether to whether $BSD_STYLE should be set
cb1a09d0
AD
1114is left as an exercise to the reader.
1115
1116See also the C<Term::ReadKey> module from your nearest CPAN site;
1117details on CPAN can be found on L<perlmod/CPAN>
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1118
1119=item getlogin
1120
1121Returns the current login from F</etc/utmp>, if any. If null, use
4633a7c4 1122getpwuid().
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1123
1124 $login = getlogin || (getpwuid($<))[0] || "Kilroy";
1125
4633a7c4
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1126Do not consider getlogin() for authorentication: it is not as
1127secure as getpwuid().
1128
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1129=item getpeername SOCKET
1130
1131Returns the packed sockaddr address of other end of the SOCKET connection.
1132
4633a7c4
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1133 use Socket;
1134 $hersockaddr = getpeername(SOCK);
1135 ($port, $iaddr) = unpack_sockaddr_in($hersockaddr);
1136 $herhostname = gethostbyaddr($iaddr, AF_INET);
1137 $herstraddr = inet_ntoa($iaddr);
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1138
1139=item getpgrp PID
1140
1141Returns the current process group for the specified PID, 0 for the
4633a7c4 1142current process. Will raise an exception if used on a machine that
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1143doesn't implement getpgrp(2). If PID is omitted, returns process
1144group of current process.
1145
1146=item getppid
1147
1148Returns the process id of the parent process.
1149
1150=item getpriority WHICH,WHO
1151
4633a7c4
LW
1152Returns the current priority for a process, a process group, or a user.
1153(See L<getpriority(2)>.) Will raise a fatal exception if used on a
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1154machine that doesn't implement getpriority(2).
1155
1156=item getpwnam NAME
1157
1158=item getgrnam NAME
1159
1160=item gethostbyname NAME
1161
1162=item getnetbyname NAME
1163
1164=item getprotobyname NAME
1165
1166=item getpwuid UID
1167
1168=item getgrgid GID
1169
1170=item getservbyname NAME,PROTO
1171
1172=item gethostbyaddr ADDR,ADDRTYPE
1173
1174=item getnetbyaddr ADDR,ADDRTYPE
1175
1176=item getprotobynumber NUMBER
1177
1178=item getservbyport PORT,PROTO
1179
1180=item getpwent
1181
1182=item getgrent
1183
1184=item gethostent
1185
1186=item getnetent
1187
1188=item getprotoent
1189
1190=item getservent
1191
1192=item setpwent
1193
1194=item setgrent
1195
1196=item sethostent STAYOPEN
1197
1198=item setnetent STAYOPEN
1199
1200=item setprotoent STAYOPEN
1201
1202=item setservent STAYOPEN
1203
1204=item endpwent
1205
1206=item endgrent
1207
1208=item endhostent
1209
1210=item endnetent
1211
1212=item endprotoent
1213
1214=item endservent
1215
1216These routines perform the same functions as their counterparts in the
1217system library. Within a list context, the return values from the
1218various get routines are as follows:
1219
1220 ($name,$passwd,$uid,$gid,
1221 $quota,$comment,$gcos,$dir,$shell) = getpw*
1222 ($name,$passwd,$gid,$members) = getgr*
1223 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1224 ($name,$aliases,$addrtype,$net) = getnet*
1225 ($name,$aliases,$proto) = getproto*
1226 ($name,$aliases,$port,$proto) = getserv*
1227
1228(If the entry doesn't exist you get a null list.)
1229
1230Within a scalar context, you get the name, unless the function was a
1231lookup by name, in which case you get the other thing, whatever it is.
1232(If the entry doesn't exist you get the undefined value.) For example:
1233
1234 $uid = getpwnam
1235 $name = getpwuid
1236 $name = getpwent
1237 $gid = getgrnam
1238 $name = getgrgid
1239 $name = getgrent
1240 etc.
1241
1242The $members value returned by I<getgr*()> is a space separated list of
1243the login names of the members of the group.
1244
1245For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1246C, it will be returned to you via C<$?> if the function call fails. The
1247@addrs value returned by a successful call is a list of the raw
1248addresses returned by the corresponding system library call. In the
1249Internet domain, each address is four bytes long and you can unpack it
1250by saying something like:
1251
1252 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1253
1254=item getsockname SOCKET
1255
1256Returns the packed sockaddr address of this end of the SOCKET connection.
1257
4633a7c4
LW
1258 use Socket;
1259 $mysockaddr = getsockname(SOCK);
1260 ($port, $myaddr) = unpack_sockaddr_in($mysockaddr);
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1261
1262=item getsockopt SOCKET,LEVEL,OPTNAME
1263
1264Returns the socket option requested, or undefined if there is an error.
1265
1266=item glob EXPR
1267
1268Returns the value of EXPR with filename expansions such as a shell
1269would do. This is the internal function implementing the <*.*>
4633a7c4 1270operator, except it's easier to use.
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1271
1272=item gmtime EXPR
1273
1274Converts a time as returned by the time function to a 9-element array
4633a7c4
LW
1275with the time localized for the standard Greenwich timezone.
1276Typically used as follows:
a0d0e21e
LW
1277
1278
1279 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1280 gmtime(time);
1281
1282All array elements are numeric, and come straight out of a struct tm.
1283In particular this means that $mon has the range 0..11 and $wday has
1284the range 0..6. If EXPR is omitted, does C<gmtime(time())>.
1285
1286=item goto LABEL
1287
748a9306
LW
1288=item goto EXPR
1289
a0d0e21e
LW
1290=item goto &NAME
1291
1292The goto-LABEL form finds the statement labeled with LABEL and resumes
1293execution there. It may not be used to go into any construct that
1294requires initialization, such as a subroutine or a foreach loop. It
1295also can't be used to go into a construct that is optimized away. It
1296can be used to go almost anywhere else within the dynamic scope,
1297including out of subroutines, but it's usually better to use some other
1298construct such as last or die. The author of Perl has never felt the
1299need to use this form of goto (in Perl, that is--C is another matter).
1300
748a9306
LW
1301The goto-EXPR form expects a label name, whose scope will be resolved
1302dynamically. This allows for computed gotos per FORTRAN, but isn't
1303necessarily recommended if you're optimizing for maintainability:
1304
1305 goto ("FOO", "BAR", "GLARCH")[$i];
1306
a0d0e21e
LW
1307The goto-&NAME form is highly magical, and substitutes a call to the
1308named subroutine for the currently running subroutine. This is used by
1309AUTOLOAD subroutines that wish to load another subroutine and then
1310pretend that the other subroutine had been called in the first place
1311(except that any modifications to @_ in the current subroutine are
1312propagated to the other subroutine.) After the goto, not even caller()
1313will be able to tell that this routine was called first.
1314
1315=item grep BLOCK LIST
1316
1317=item grep EXPR,LIST
1318
1319Evaluates the BLOCK or EXPR for each element of LIST (locally setting
1320$_ to each element) and returns the list value consisting of those
1321elements for which the expression evaluated to TRUE. In a scalar
1322context, returns the number of times the expression was TRUE.
1323
1324 @foo = grep(!/^#/, @bar); # weed out comments
1325
1326or equivalently,
1327
1328 @foo = grep {!/^#/} @bar; # weed out comments
1329
1330Note that, since $_ is a reference into the list value, it can be used
1331to modify the elements of the array. While this is useful and
1332supported, it can cause bizarre results if the LIST is not a named
1333array.
1334
1335=item hex EXPR
1336
4633a7c4
LW
1337Interprets EXPR as a hex string and returns the corresponding decimal
1338value. (To convert strings that might start with 0 or 0x see
1339oct().) If EXPR is omitted, uses $_.
a0d0e21e
LW
1340
1341=item import
1342
1343There is no built-in import() function. It is merely an ordinary
4633a7c4 1344method (subroutine) defined (or inherited) by modules that wish to export
a0d0e21e 1345names to another module. The use() function calls the import() method
4633a7c4 1346for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
a0d0e21e
LW
1347
1348=item index STR,SUBSTR,POSITION
1349
1350=item index STR,SUBSTR
1351
4633a7c4
LW
1352Returns the position of the first occurrence of SUBSTR in STR at or after
1353POSITION. If POSITION is omitted, starts searching from the beginning of
1354the string. The return value is based at 0 (or whatever you've set the $[
1355variable to--but don't do that). If the substring is not found, returns
a0d0e21e
LW
1356one less than the base, ordinarily -1.
1357
1358=item int EXPR
1359
1360Returns the integer portion of EXPR. If EXPR is omitted, uses $_.
1361
1362=item ioctl FILEHANDLE,FUNCTION,SCALAR
1363
1364Implements the ioctl(2) function. You'll probably have to say
1365
4633a7c4 1366 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
a0d0e21e 1367
4633a7c4 1368first to get the correct function definitions. If F<ioctl.ph> doesn't
a0d0e21e 1369exist or doesn't have the correct definitions you'll have to roll your
4633a7c4
LW
1370own, based on your C header files such as F<E<lt>sys/ioctl.hE<gt>>.
1371(There is a Perl script called B<h2ph> that comes with the Perl kit which
1372may help you in this, but it's non-trivial.) SCALAR will be read and/or
1373written depending on the FUNCTION--a pointer to the string value of SCALAR
1374will be passed as the third argument of the actual ioctl call. (If SCALAR
1375has no string value but does have a numeric value, that value will be
1376passed rather than a pointer to the string value. To guarantee this to be
1377TRUE, add a 0 to the scalar before using it.) The pack() and unpack()
1378functions are useful for manipulating the values of structures used by
1379ioctl(). The following example sets the erase character to DEL.
a0d0e21e
LW
1380
1381 require 'ioctl.ph';
4633a7c4
LW
1382 $getp = &TIOCGETP;
1383 die "NO TIOCGETP" if $@ || !$getp;
a0d0e21e 1384 $sgttyb_t = "ccccs"; # 4 chars and a short
4633a7c4 1385 if (ioctl(STDIN,$getp,$sgttyb)) {
a0d0e21e
LW
1386 @ary = unpack($sgttyb_t,$sgttyb);
1387 $ary[2] = 127;
1388 $sgttyb = pack($sgttyb_t,@ary);
4633a7c4 1389 ioctl(STDIN,&TIOCSETP,$sgttyb)
a0d0e21e
LW
1390 || die "Can't ioctl: $!";
1391 }
1392
1393The return value of ioctl (and fcntl) is as follows:
1394
1395 if OS returns: then Perl returns:
1396 -1 undefined value
1397 0 string "0 but true"
1398 anything else that number
1399
1400Thus Perl returns TRUE on success and FALSE on failure, yet you can
1401still easily determine the actual value returned by the operating
1402system:
1403
1404 ($retval = ioctl(...)) || ($retval = -1);
1405 printf "System returned %d\n", $retval;
1406
1407=item join EXPR,LIST
1408
1409Joins the separate strings of LIST or ARRAY into a single string with
1410fields separated by the value of EXPR, and returns the string.
1411Example:
1412
1413 $_ = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
1414
1415See L<perlfunc/split>.
1416
1417=item keys ASSOC_ARRAY
1418
1419Returns a normal array consisting of all the keys of the named
1420associative array. (In a scalar context, returns the number of keys.)
1421The keys are returned in an apparently random order, but it is the same
1422order as either the values() or each() function produces (given that
1423the associative array has not been modified). Here is yet another way
1424to print your environment:
1425
1426 @keys = keys %ENV;
1427 @values = values %ENV;
1428 while ($#keys >= 0) {
1429 print pop(@keys), '=', pop(@values), "\n";
1430 }
1431
1432or how about sorted by key:
1433
1434 foreach $key (sort(keys %ENV)) {
1435 print $key, '=', $ENV{$key}, "\n";
1436 }
1437
4633a7c4 1438To sort an array by value, you'll need to use a C<sort{}>
cb1a09d0 1439function. Here's a descending numeric sort of a hash by its values:
4633a7c4
LW
1440
1441 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash)) {
1442 printf "%4d %s\n", $hash{$key}, $key;
1443 }
1444
a0d0e21e
LW
1445=item kill LIST
1446
4633a7c4
LW
1447Sends a signal to a list of processes. The first element of
1448the list must be the signal to send. Returns the number of
1449processes successfully signaled.
a0d0e21e
LW
1450
1451 $cnt = kill 1, $child1, $child2;
1452 kill 9, @goners;
1453
4633a7c4
LW
1454Unlike in the shell, in Perl if the I<SIGNAL> is negative, it kills
1455process groups instead of processes. (On System V, a negative I<PROCESS>
1456number will also kill process groups, but that's not portable.) That
1457means you usually want to use positive not negative signals. You may also
1458use a signal name in quotes. See the L<perlipc/"Signals"> man page for details.
a0d0e21e
LW
1459
1460=item last LABEL
1461
1462=item last
1463
1464The C<last> command is like the C<break> statement in C (as used in
1465loops); it immediately exits the loop in question. If the LABEL is
1466omitted, the command refers to the innermost enclosing loop. The
1467C<continue> block, if any, is not executed:
1468
4633a7c4
LW
1469 LINE: while (<STDIN>) {
1470 last LINE if /^$/; # exit when done with header
a0d0e21e
LW
1471 ...
1472 }
1473
1474=item lc EXPR
1475
1476Returns an lowercased version of EXPR. This is the internal function
4633a7c4
LW
1477implementing the \L escape in double-quoted strings.
1478Should respect any POSIX setlocale() settings.
a0d0e21e
LW
1479
1480=item lcfirst EXPR
1481
1482Returns the value of EXPR with the first character lowercased. This is
1483the internal function implementing the \l escape in double-quoted strings.
4633a7c4 1484Should respect any POSIX setlocale() settings.
a0d0e21e
LW
1485
1486=item length EXPR
1487
1488Returns the length in characters of the value of EXPR. If EXPR is
1489omitted, returns length of $_.
1490
1491=item link OLDFILE,NEWFILE
1492
1493Creates a new filename linked to the old filename. Returns 1 for
1494success, 0 otherwise.
1495
1496=item listen SOCKET,QUEUESIZE
1497
1498Does the same thing that the listen system call does. Returns TRUE if
4633a7c4 1499it succeeded, FALSE otherwise. See example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
1500
1501=item local EXPR
1502
a0d0e21e 1503A local modifies the listed variables to be local to the enclosing block,
cb1a09d0
AD
1504subroutine, C<eval{}> or C<do>. If more than one value is listed, the
1505list must be placed in parens. See L<perlsub/"Temporary Values via
1506local()"> for details.
a0d0e21e 1507
cb1a09d0
AD
1508But you really probably want to be using my() instead, because local() isn't
1509what most people think of as "local"). See L<perlsub/"Private Variables
1510via my()"> for details.
a0d0e21e
LW
1511
1512=item localtime EXPR
1513
1514Converts a time as returned by the time function to a 9-element array
1515with the time analyzed for the local timezone. Typically used as
1516follows:
1517
1518 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1519 localtime(time);
1520
1521All array elements are numeric, and come straight out of a struct tm.
1522In particular this means that $mon has the range 0..11 and $wday has
1523the range 0..6. If EXPR is omitted, does localtime(time).
1524
1525In a scalar context, prints out the ctime(3) value:
1526
1527 $now_string = localtime; # e.g. "Thu Oct 13 04:54:34 1994"
1528
37798a01 1529Also see the F<timelocal.pl> library, and the strftime(3) function available
a0d0e21e
LW
1530via the POSIX modulie.
1531
1532=item log EXPR
1533
1534Returns logarithm (base I<e>) of EXPR. If EXPR is omitted, returns log
1535of $_.
1536
1537=item lstat FILEHANDLE
1538
1539=item lstat EXPR
1540
1541Does the same thing as the stat() function, but stats a symbolic link
1542instead of the file the symbolic link points to. If symbolic links are
1543unimplemented on your system, a normal stat() is done.
1544
1545=item m//
1546
1547The match operator. See L<perlop>.
1548
1549=item map BLOCK LIST
1550
1551=item map EXPR,LIST
1552
1553Evaluates the BLOCK or EXPR for each element of LIST (locally setting $_ to each
1554element) and returns the list value composed of the results of each such
1555evaluation. Evaluates BLOCK or EXPR in a list context, so each element of LIST
1556may produce zero, one, or more elements in the returned value.
1557
1558 @chars = map(chr, @nums);
1559
1560translates a list of numbers to the corresponding characters. And
1561
4633a7c4 1562 %hash = map { getkey($_) => $_ } @array;
a0d0e21e
LW
1563
1564is just a funny way to write
1565
1566 %hash = ();
1567 foreach $_ (@array) {
4633a7c4 1568 $hash{getkey($_)} = $_;
a0d0e21e
LW
1569 }
1570
1571=item mkdir FILENAME,MODE
1572
1573Creates the directory specified by FILENAME, with permissions specified
1574by MODE (as modified by umask). If it succeeds it returns 1, otherwise
1575it returns 0 and sets $! (errno).
1576
1577=item msgctl ID,CMD,ARG
1578
4633a7c4 1579Calls the System V IPC function msgctl(2). If CMD is &IPC_STAT, then ARG
a0d0e21e
LW
1580must be a variable which will hold the returned msqid_ds structure.
1581Returns like ioctl: the undefined value for error, "0 but true" for
1582zero, or the actual return value otherwise.
1583
1584=item msgget KEY,FLAGS
1585
4633a7c4 1586Calls the System V IPC function msgget(2). Returns the message queue id,
a0d0e21e
LW
1587or the undefined value if there is an error.
1588
1589=item msgsnd ID,MSG,FLAGS
1590
1591Calls the System V IPC function msgsnd to send the message MSG to the
1592message queue ID. MSG must begin with the long integer message type,
c07a80fd 1593which may be created with C<pack("l", $type)>. Returns TRUE if
a0d0e21e
LW
1594successful, or FALSE if there is an error.
1595
1596=item msgrcv ID,VAR,SIZE,TYPE,FLAGS
1597
1598Calls the System V IPC function msgrcv to receive a message from
1599message queue ID into variable VAR with a maximum message size of
1600SIZE. Note that if a message is received, the message type will be the
1601first thing in VAR, and the maximum length of VAR is SIZE plus the size
1602of the message type. Returns TRUE if successful, or FALSE if there is
1603an error.
1604
1605=item my EXPR
1606
1607A "my" declares the listed variables to be local (lexically) to the
cb1a09d0
AD
1608enclosing block, subroutine, C<eval>, or C<do/require/use>'d file. If
1609more than one value is listed, the list must be placed in parens. See
1610L<perlsub/"Private Variables via my()"> for details.
4633a7c4 1611
a0d0e21e
LW
1612=item next LABEL
1613
1614=item next
1615
1616The C<next> command is like the C<continue> statement in C; it starts
1617the next iteration of the loop:
1618
4633a7c4
LW
1619 LINE: while (<STDIN>) {
1620 next LINE if /^#/; # discard comments
a0d0e21e
LW
1621 ...
1622 }
1623
1624Note that if there were a C<continue> block on the above, it would get
1625executed even on discarded lines. If the LABEL is omitted, the command
1626refers to the innermost enclosing loop.
1627
1628=item no Module LIST
1629
1630See the "use" function, which "no" is the opposite of.
1631
1632=item oct EXPR
1633
4633a7c4
LW
1634Interprets EXPR as an octal string and returns the corresponding
1635decimal value. (If EXPR happens to start off with 0x, interprets it as
1636a hex string instead.) The following will handle decimal, octal, and
1637hex in the standard Perl or C notation:
a0d0e21e
LW
1638
1639 $val = oct($val) if $val =~ /^0/;
1640
1641If EXPR is omitted, uses $_.
1642
1643=item open FILEHANDLE,EXPR
1644
1645=item open FILEHANDLE
1646
1647Opens the file whose filename is given by EXPR, and associates it with
4633a7c4
LW
1648FILEHANDLE. If FILEHANDLE is an expression, its value is used as the name
1649of the real filehandle wanted. If EXPR is omitted, the scalar variable of
1650the same name as the FILEHANDLE contains the filename. If the filename
1651begins with "<" or nothing, the file is opened for input. If the filename
1652begins with ">", the file is opened for output. If the filename begins
cb1a09d0 1653with ">>", the file is opened for appending. You can put a '+' in front
4633a7c4 1654of the '>' or '<' to indicate that you want both read and write access to
cb1a09d0
AD
1655the file; thus '+<' is usually preferred for read/write updates--the '+>'
1656mode would clobber the file first. These correspond to the fopen(3) modes
1657of 'r', 'r+', 'w', 'w+', 'a', and 'a+'.
1658
1659If the filename begins with "|", the filename is interpreted
4633a7c4
LW
1660as a command to which output is to be piped, and if the filename ends with
1661a "|", the filename is interpreted See L<perlipc/"Using open() for IPC">
1662for more examples of this. as command which pipes input to us. (You may
cb1a09d0 1663not have a raw open() to a command that pipes both in I<and> out, but see See L<open2>,
4633a7c4 1664L<open3>, and L<perlipc/"Bidirectional Communication"> for alternatives.)
cb1a09d0 1665
4633a7c4
LW
1666Opening '-' opens STDIN and opening '>-' opens STDOUT. Open returns
1667non-zero upon success, the undefined value otherwise. If the open
1668involved a pipe, the return value happens to be the pid of the
cb1a09d0
AD
1669subprocess.
1670
1671If you're unfortunate enough to be running Perl on a system that
1672distinguishes between text files and binary files (modern operating
1673systems don't care), then you should check out L</binmode> for tips for
1674dealing with this. The key distinction between systems that need binmode
1675and those that don't is their text file formats. Systems like Unix and
1676Plan9 that delimit lines with a single character, and that encode that
1677character in C as '\n', do not need C<binmode>. The rest need it.
1678
cb1a09d0 1679Examples:
a0d0e21e
LW
1680
1681 $ARTICLE = 100;
1682 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
1683 while (<ARTICLE>) {...
1684
1685 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
1686
cb1a09d0
AD
1687 open(DBASE, '+<dbase.mine'); # open for update
1688
4633a7c4 1689 open(ARTICLE, "caesar <$article |"); # decrypt article
a0d0e21e 1690
4633a7c4 1691 open(EXTRACT, "|sort >/tmp/Tmp$$"); # $$ is our process id
a0d0e21e
LW
1692
1693 # process argument list of files along with any includes
1694
1695 foreach $file (@ARGV) {
1696 process($file, 'fh00');
1697 }
1698
1699 sub process {
1700 local($filename, $input) = @_;
1701 $input++; # this is a string increment
1702 unless (open($input, $filename)) {
1703 print STDERR "Can't open $filename: $!\n";
1704 return;
1705 }
1706
1707 while (<$input>) { # note use of indirection
1708 if (/^#include "(.*)"/) {
1709 process($1, $input);
1710 next;
1711 }
1712 ... # whatever
1713 }
1714 }
1715
1716You may also, in the Bourne shell tradition, specify an EXPR beginning
1717with ">&", in which case the rest of the string is interpreted as the
1718name of a filehandle (or file descriptor, if numeric) which is to be
1719duped and opened. You may use & after >, >>, <, +>, +>> and +<. The
1720mode you specify should match the mode of the original filehandle.
cb1a09d0
AD
1721(Duping a filehandle does not take into acount any existing contents of
1722stdio buffers.)
a0d0e21e
LW
1723Here is a script that saves, redirects, and restores STDOUT and
1724STDERR:
1725
1726 #!/usr/bin/perl
1727 open(SAVEOUT, ">&STDOUT");
1728 open(SAVEERR, ">&STDERR");
1729
1730 open(STDOUT, ">foo.out") || die "Can't redirect stdout";
1731 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
1732
1733 select(STDERR); $| = 1; # make unbuffered
1734 select(STDOUT); $| = 1; # make unbuffered
1735
1736 print STDOUT "stdout 1\n"; # this works for
1737 print STDERR "stderr 1\n"; # subprocesses too
1738
1739 close(STDOUT);
1740 close(STDERR);
1741
1742 open(STDOUT, ">&SAVEOUT");
1743 open(STDERR, ">&SAVEERR");
1744
1745 print STDOUT "stdout 2\n";
1746 print STDERR "stderr 2\n";
1747
1748
1749If you specify "<&=N", where N is a number, then Perl will do an
4633a7c4
LW
1750equivalent of C's fdopen() of that file descriptor; this is more
1751parsimonious of file descriptors. For example:
a0d0e21e
LW
1752
1753 open(FILEHANDLE, "<&=$fd")
1754
1755If you open a pipe on the command "-", i.e. either "|-" or "-|", then
1756there is an implicit fork done, and the return value of open is the pid
1757of the child within the parent process, and 0 within the child
1758process. (Use defined($pid) to determine whether the open was successful.)
1759The filehandle behaves normally for the parent, but i/o to that
1760filehandle is piped from/to the STDOUT/STDIN of the child process.
1761In the child process the filehandle isn't opened--i/o happens from/to
1762the new STDOUT or STDIN. Typically this is used like the normal
1763piped open when you want to exercise more control over just how the
1764pipe command gets executed, such as when you are running setuid, and
4633a7c4
LW
1765don't want to have to scan shell commands for metacharacters.
1766The following pairs are more or less equivalent:
a0d0e21e
LW
1767
1768 open(FOO, "|tr '[a-z]' '[A-Z]'");
1769 open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
1770
1771 open(FOO, "cat -n '$file'|");
1772 open(FOO, "-|") || exec 'cat', '-n', $file;
1773
4633a7c4
LW
1774See L<perlipc/"Safe Pipe Opens"> for more examples of this.
1775
a0d0e21e
LW
1776Explicitly closing any piped filehandle causes the parent process to
1777wait for the child to finish, and returns the status value in $?.
1778Note: on any operation which may do a fork, unflushed buffers remain
1779unflushed in both processes, which means you may need to set $| to
1780avoid duplicate output.
1781
c07a80fd
PP
1782Using the FileHandle constructor from the FileHandle package,
1783you can generate anonymous filehandles which have the scope of whatever
1784variables hold references to them, and automatically close whenever
1785and however you leave that scope:
1786
1787 use FileHandle;
1788 ...
1789 sub read_myfile_munged {
1790 my $ALL = shift;
1791 my $handle = new FileHandle;
1792 open($handle, "myfile") or die "myfile: $!";
1793 $first = <$handle>
1794 or return (); # Automatically closed here.
1795 mung $first or die "mung failed"; # Or here.
1796 return $first, <$handle> if $ALL; # Or here.
1797 $first; # Or here.
1798 }
1799
a0d0e21e
LW
1800The filename that is passed to open will have leading and trailing
1801whitespace deleted. In order to open a file with arbitrary weird
1802characters in it, it's necessary to protect any leading and trailing
1803whitespace thusly:
1804
cb1a09d0
AD
1805 $file =~ s#^(\s)#./$1#;
1806 open(FOO, "< $file\0");
1807
c07a80fd
PP
1808If you want a "real" C open() (see L<open(2)> on your system), then
1809you should use the sysopen() function. This is another way to
1810protect your filenames from interpretation. For example:
cb1a09d0
AD
1811
1812 use FileHandle;
c07a80fd
PP
1813 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL, 0700)
1814 or die "sysopen $path: $!";
1815 HANDLE->autoflush(1);
1816 HANDLE->print("stuff $$\n");
1817 seek(HANDLE, 0, 0);
1818 print "File contains: ", <HANDLE>;
cb1a09d0
AD
1819
1820See L</seek()> for some details about mixing reading and writing.
a0d0e21e
LW
1821
1822=item opendir DIRHANDLE,EXPR
1823
1824Opens a directory named EXPR for processing by readdir(), telldir(),
1825seekdir(), rewinddir() and closedir(). Returns TRUE if successful.
1826DIRHANDLEs have their own namespace separate from FILEHANDLEs.
1827
1828=item ord EXPR
1829
1830Returns the numeric ascii value of the first character of EXPR. If
1831EXPR is omitted, uses $_.
1832
1833=item pack TEMPLATE,LIST
1834
1835Takes an array or list of values and packs it into a binary structure,
1836returning the string containing the structure. The TEMPLATE is a
1837sequence of characters that give the order and type of values, as
1838follows:
1839
1840 A An ascii string, will be space padded.
1841 a An ascii string, will be null padded.
1842 b A bit string (ascending bit order, like vec()).
1843 B A bit string (descending bit order).
1844 h A hex string (low nybble first).
1845 H A hex string (high nybble first).
1846
1847 c A signed char value.
1848 C An unsigned char value.
1849 s A signed short value.
1850 S An unsigned short value.
1851 i A signed integer value.
1852 I An unsigned integer value.
1853 l A signed long value.
1854 L An unsigned long value.
1855
1856 n A short in "network" order.
1857 N A long in "network" order.
1858 v A short in "VAX" (little-endian) order.
1859 V A long in "VAX" (little-endian) order.
1860
1861 f A single-precision float in the native format.
1862 d A double-precision float in the native format.
1863
1864 p A pointer to a null-terminated string.
1865 P A pointer to a structure (fixed-length string).
1866
1867 u A uuencoded string.
1868
1869 x A null byte.
1870 X Back up a byte.
1871 @ Null fill to absolute position.
1872
1873Each letter may optionally be followed by a number which gives a repeat
1874count. With all types except "a", "A", "b", "B", "h" and "H", and "P" the
1875pack function will gobble up that many values from the LIST. A * for the
1876repeat count means to use however many items are left. The "a" and "A"
1877types gobble just one value, but pack it as a string of length count,
1878padding with nulls or spaces as necessary. (When unpacking, "A" strips
1879trailing spaces and nulls, but "a" does not.) Likewise, the "b" and "B"
1880fields pack a string that many bits long. The "h" and "H" fields pack a
1881string that many nybbles long. The "P" packs a pointer to a structure of
1882the size indicated by the length. Real numbers (floats and doubles) are
1883in the native machine format only; due to the multiplicity of floating
1884formats around, and the lack of a standard "network" representation, no
1885facility for interchange has been made. This means that packed floating
1886point data written on one machine may not be readable on another - even if
1887both use IEEE floating point arithmetic (as the endian-ness of the memory
1888representation is not part of the IEEE spec). Note that Perl uses doubles
1889internally for all numeric calculation, and converting from double into
1890float and thence back to double again will lose precision (i.e.
1891C<unpack("f", pack("f", $foo)>) will not in general equal $foo).
1892
1893Examples:
1894
1895 $foo = pack("cccc",65,66,67,68);
1896 # foo eq "ABCD"
1897 $foo = pack("c4",65,66,67,68);
1898 # same thing
1899
1900 $foo = pack("ccxxcc",65,66,67,68);
1901 # foo eq "AB\0\0CD"
1902
1903 $foo = pack("s2",1,2);
1904 # "\1\0\2\0" on little-endian
1905 # "\0\1\0\2" on big-endian
1906
1907 $foo = pack("a4","abcd","x","y","z");
1908 # "abcd"
1909
1910 $foo = pack("aaaa","abcd","x","y","z");
1911 # "axyz"
1912
1913 $foo = pack("a14","abcdefg");
1914 # "abcdefg\0\0\0\0\0\0\0"
1915
1916 $foo = pack("i9pl", gmtime);
1917 # a real struct tm (on my system anyway)
1918
1919 sub bintodec {
1920 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
1921 }
1922
1923The same template may generally also be used in the unpack function.
1924
cb1a09d0
AD
1925=item package NAMESPACE
1926
1927Declares the compilation unit as being in the given namespace. The scope
1928of the package declaration is from the declaration itself through the end of
1929the enclosing block (the same scope as the local() operator). All further
1930unqualified dynamic identifiers will be in this namespace. A package
1931statement only affects dynamic variables--including those you've used
1932local() on--but I<not> lexical variables created with my(). Typically it
1933would be the first declaration in a file to be included by the C<require>
1934or C<use> operator. You can switch into a package in more than one place;
1935it merely influences which symbol table is used by the compiler for the
1936rest of that block. You can refer to variables and filehandles in other
1937packages by prefixing the identifier with the package name and a double
1938colon: C<$Package::Variable>. If the package name is null, the C<main>
1939package as assumed. That is, C<$::sail> is equivalent to C<$main::sail>.
1940
1941See L<perlmod/"Packages"> for more information about packages, modules,
1942and classes. See L<perlsub> for other scoping issues.
1943
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1944=item pipe READHANDLE,WRITEHANDLE
1945
1946Opens a pair of connected pipes like the corresponding system call.
1947Note that if you set up a loop of piped processes, deadlock can occur
1948unless you are very careful. In addition, note that Perl's pipes use
1949stdio buffering, so you may need to set $| to flush your WRITEHANDLE
1950after each command, depending on the application.
1951
4633a7c4
LW
1952See L<open2>, L<open3>, and L<perlipc/"Bidirectional Communication">
1953for examples of such things.
1954
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LW
1955=item pop ARRAY
1956
1957Pops and returns the last value of the array, shortening the array by
19581. Has a similar effect to
1959
1960 $tmp = $ARRAY[$#ARRAY--];
1961
1962If there are no elements in the array, returns the undefined value.
cb1a09d0
AD
1963If ARRAY is omitted, pops the
1964@ARGV array in the main program, and the @_ array in subroutines, just
1965like shift().
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LW
1966
1967=item pos SCALAR
1968
4633a7c4 1969Returns the offset of where the last C<m//g> search left off for the variable
a0d0e21e
LW
1970in question. May be modified to change that offset.
1971
1972=item print FILEHANDLE LIST
1973
1974=item print LIST
1975
1976=item print
1977
cb1a09d0 1978Prints a string or a comma-separated list of strings. Returns TRUE
a0d0e21e 1979if successful. FILEHANDLE may be a scalar variable name, in which case
cb1a09d0 1980the variable contains the name of or a reference to the filehandle, thus introducing one
a0d0e21e
LW
1981level of indirection. (NOTE: If FILEHANDLE is a variable and the next
1982token is a term, it may be misinterpreted as an operator unless you
1983interpose a + or put parens around the arguments.) If FILEHANDLE is
1984omitted, prints by default to standard output (or to the last selected
1985output channel--see select()). If LIST is also omitted, prints $_ to
1986STDOUT. To set the default output channel to something other than
1987STDOUT use the select operation. Note that, because print takes a
1988LIST, anything in the LIST is evaluated in a list context, and any
1989subroutine that you call will have one or more of its expressions
1990evaluated in a list context. Also be careful not to follow the print
1991keyword with a left parenthesis unless you want the corresponding right
1992parenthesis to terminate the arguments to the print--interpose a + or
1993put parens around all the arguments.
1994
4633a7c4
LW
1995Note that if you're storing FILEHANDLES in an array or other expression,
1996you will have to use a block returning its value instead
1997
1998 print { $files[$i] } "stuff\n";
1999 print { $OK ? STDOUT : STDERR } "stuff\n";
2000
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LW
2001=item printf FILEHANDLE LIST
2002
2003=item printf LIST
2004
2005Equivalent to a "print FILEHANDLE sprintf(LIST)". The first argument
2006of the list will be interpreted as the printf format.
2007
2008=item push ARRAY,LIST
2009
2010Treats ARRAY as a stack, and pushes the values of LIST
2011onto the end of ARRAY. The length of ARRAY increases by the length of
2012LIST. Has the same effect as
2013
2014 for $value (LIST) {
2015 $ARRAY[++$#ARRAY] = $value;
2016 }
2017
2018but is more efficient. Returns the new number of elements in the array.
2019
2020=item q/STRING/
2021
2022=item qq/STRING/
2023
2024=item qx/STRING/
2025
2026=item qw/STRING/
2027
2028Generalized quotes. See L<perlop>.
2029
2030=item quotemeta EXPR
2031
2032Returns the value of EXPR with with all regular expression
2033metacharacters backslashed. This is the internal function implementing
2034the \Q escape in double-quoted strings.
2035
2036=item rand EXPR
2037
2038=item rand
2039
2040Returns a random fractional number between 0 and the value of EXPR.
2041(EXPR should be positive.) If EXPR is omitted, returns a value between
20420 and 1. This function produces repeatable sequences unless srand()
2043is invoked. See also srand().
2044
2045(Note: if your rand function consistently returns numbers that are too
2046large or too small, then your version of Perl was probably compiled
2047with the wrong number of RANDBITS. As a workaround, you can usually
2048multiply EXPR by the correct power of 2 to get the range you want.
2049This will make your script unportable, however. It's better to recompile
2050if you can.)
2051
2052=item read FILEHANDLE,SCALAR,LENGTH,OFFSET
2053
2054=item read FILEHANDLE,SCALAR,LENGTH
2055
2056Attempts to read LENGTH bytes of data into variable SCALAR from the
2057specified FILEHANDLE. Returns the number of bytes actually read, or
2058undef if there was an error. SCALAR will be grown or shrunk to the
2059length actually read. An OFFSET may be specified to place the read
2060data at some other place than the beginning of the string. This call
2061is actually implemented in terms of stdio's fread call. To get a true
2062read system call, see sysread().
2063
2064=item readdir DIRHANDLE
2065
2066Returns the next directory entry for a directory opened by opendir().
2067If used in a list context, returns all the rest of the entries in the
2068directory. If there are no more entries, returns an undefined value in
2069a scalar context or a null list in a list context.
2070
cb1a09d0
AD
2071If you're planning to filetest the return values out of a readdir(), you'd
2072better prepend the directory in question. Otherwise, since we didn't
2073chdir() there, it would have been testing the wrong file.
2074
2075 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
2076 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
2077 closedir DIR;
2078
a0d0e21e
LW
2079=item readlink EXPR
2080
2081Returns the value of a symbolic link, if symbolic links are
2082implemented. If not, gives a fatal error. If there is some system
2083error, returns the undefined value and sets $! (errno). If EXPR is
2084omitted, uses $_.
2085
2086=item recv SOCKET,SCALAR,LEN,FLAGS
2087
2088Receives a message on a socket. Attempts to receive LENGTH bytes of
2089data into variable SCALAR from the specified SOCKET filehandle.
2090Actually does a C recvfrom(), so that it can returns the address of the
2091sender. Returns the undefined value if there's an error. SCALAR will
2092be grown or shrunk to the length actually read. Takes the same flags
4633a7c4
LW
2093as the system call of the same name.
2094See L<perlipc/"UDP: Message Passing"> for examples.
a0d0e21e
LW
2095
2096=item redo LABEL
2097
2098=item redo
2099
2100The C<redo> command restarts the loop block without evaluating the
2101conditional again. The C<continue> block, if any, is not executed. If
2102the LABEL is omitted, the command refers to the innermost enclosing
2103loop. This command is normally used by programs that want to lie to
2104themselves about what was just input:
2105
2106 # a simpleminded Pascal comment stripper
2107 # (warning: assumes no { or } in strings)
4633a7c4 2108 LINE: while (<STDIN>) {
a0d0e21e
LW
2109 while (s|({.*}.*){.*}|$1 |) {}
2110 s|{.*}| |;
2111 if (s|{.*| |) {
2112 $front = $_;
2113 while (<STDIN>) {
2114 if (/}/) { # end of comment?
2115 s|^|$front{|;
4633a7c4 2116 redo LINE;
a0d0e21e
LW
2117 }
2118 }
2119 }
2120 print;
2121 }
2122
2123=item ref EXPR
2124
2125Returns a TRUE value if EXPR is a reference, FALSE otherwise. The value
2126returned depends on the type of thing the reference is a reference to.
2127Builtin types include:
2128
2129 REF
2130 SCALAR
2131 ARRAY
2132 HASH
2133 CODE
2134 GLOB
2135
2136If the referenced object has been blessed into a package, then that package
2137name is returned instead. You can think of ref() as a typeof() operator.
2138
2139 if (ref($r) eq "HASH") {
2140 print "r is a reference to an associative array.\n";
2141 }
2142 if (!ref ($r) {
2143 print "r is not a reference at all.\n";
2144 }
2145
2146See also L<perlref>.
2147
2148=item rename OLDNAME,NEWNAME
2149
2150Changes the name of a file. Returns 1 for success, 0 otherwise. Will
2151not work across filesystem boundaries.
2152
2153=item require EXPR
2154
2155=item require
2156
2157Demands some semantics specified by EXPR, or by $_ if EXPR is not
2158supplied. If EXPR is numeric, demands that the current version of Perl
2159($] or $PERL_VERSION) be equal or greater than EXPR.
2160
2161Otherwise, demands that a library file be included if it hasn't already
2162been included. The file is included via the do-FILE mechanism, which is
2163essentially just a variety of eval(). Has semantics similar to the following
2164subroutine:
2165
2166 sub require {
2167 local($filename) = @_;
2168 return 1 if $INC{$filename};
2169 local($realfilename,$result);
2170 ITER: {
2171 foreach $prefix (@INC) {
2172 $realfilename = "$prefix/$filename";
2173 if (-f $realfilename) {
2174 $result = do $realfilename;
2175 last ITER;
2176 }
2177 }
2178 die "Can't find $filename in \@INC";
2179 }
2180 die $@ if $@;
2181 die "$filename did not return true value" unless $result;
2182 $INC{$filename} = $realfilename;
2183 $result;
2184 }
2185
2186Note that the file will not be included twice under the same specified
2187name. The file must return TRUE as the last statement to indicate
2188successful execution of any initialization code, so it's customary to
2189end such a file with "1;" unless you're sure it'll return TRUE
2190otherwise. But it's better just to put the "C<1;>", in case you add more
2191statements.
2192
2193If EXPR is a bare word, the require assumes a "F<.pm>" extension for you,
2194to make it easy to load standard modules. This form of loading of
2195modules does not risk altering your namespace.
2196
748a9306
LW
2197For a yet-more-powerful import facility, see the L</use()> and
2198L<perlmod>.
a0d0e21e
LW
2199
2200=item reset EXPR
2201
2202=item reset
2203
2204Generally used in a C<continue> block at the end of a loop to clear
2205variables and reset ?? searches so that they work again. The
2206expression is interpreted as a list of single characters (hyphens
2207allowed for ranges). All variables and arrays beginning with one of
2208those letters are reset to their pristine state. If the expression is
2209omitted, one-match searches (?pattern?) are reset to match again. Only
2210resets variables or searches in the current package. Always returns
22111. Examples:
2212
2213 reset 'X'; # reset all X variables
2214 reset 'a-z'; # reset lower case variables
2215 reset; # just reset ?? searches
2216
2217Resetting "A-Z" is not recommended since you'll wipe out your
2218ARGV and ENV arrays. Only resets package variables--lexical variables
2219are unaffected, but they clean themselves up on scope exit anyway,
2220so anymore you probably want to use them instead. See L</my>.
2221
2222=item return LIST
2223
2224Returns from a subroutine or eval with the value specified. (Note that
4633a7c4 2225in the absence of a return a subroutine or eval() will automatically
a0d0e21e
LW
2226return the value of the last expression evaluated.)
2227
2228=item reverse LIST
2229
2230In a list context, returns a list value consisting of the elements
2231of LIST in the opposite order. In a scalar context, returns a string
2232value consisting of the bytes of the first element of LIST in the
4633a7c4
LW
2233opposite order.
2234
2235 print reverse <>; # line tac
2236
2237 undef $/;
2238 print scalar reverse scalar <>; # byte tac
a0d0e21e
LW
2239
2240=item rewinddir DIRHANDLE
2241
2242Sets the current position to the beginning of the directory for the
2243readdir() routine on DIRHANDLE.
2244
2245=item rindex STR,SUBSTR,POSITION
2246
2247=item rindex STR,SUBSTR
2248
2249Works just like index except that it returns the position of the LAST
2250occurrence of SUBSTR in STR. If POSITION is specified, returns the
2251last occurrence at or before that position.
2252
2253=item rmdir FILENAME
2254
2255Deletes the directory specified by FILENAME if it is empty. If it
2256succeeds it returns 1, otherwise it returns 0 and sets $! (errno). If
2257FILENAME is omitted, uses $_.
2258
2259=item s///
2260
2261The substitution operator. See L<perlop>.
2262
2263=item scalar EXPR
2264
2265Forces EXPR to be interpreted in a scalar context and returns the value
cb1a09d0
AD
2266of EXPR.
2267
2268 @counts = ( scalar @a, scalar @b, scalar @c );
2269
2270There is no equivalent operator to force an expression to
2271be interpolated in a list context because it's in practice never
2272needed. If you really wanted to do so, however, you could use
2273the construction C<@{[ (some expression) ]}>, but usually a simple
2274C<(some expression)> suffices.
a0d0e21e
LW
2275
2276=item seek FILEHANDLE,POSITION,WHENCE
2277
2278Randomly positions the file pointer for FILEHANDLE, just like the fseek()
2279call of stdio. FILEHANDLE may be an expression whose value gives the name
2280of the filehandle. The values for WHENCE are 0 to set the file pointer to
2281POSITION, 1 to set the it to current plus POSITION, and 2 to set it to EOF
2282plus offset. You may use the values SEEK_SET, SEEK_CUR, and SEEK_END for
4633a7c4 2283this from POSIX module. Returns 1 upon success, 0 otherwise.
a0d0e21e 2284
cb1a09d0
AD
2285On some systems you have to do a seek whenever you switch between reading
2286and writing. Amongst other things, this may have the effect of calling
2287stdio's clearerr(3). A "whence" of 1 (SEEK_CUR) is useful for not moving
2288the file pointer:
2289
2290 seek(TEST,0,1);
2291
2292This is also useful for applications emulating C<tail -f>. Once you hit
2293EOF on your read, and then sleep for a while, you might have to stick in a
2294seek() to reset things. First the simple trick listed above to clear the
2295filepointer. The seek() doesn't change the current position, but it
2296I<does> clear the end-of-file condition on the handle, so that the next
37798a01 2297C<E<lt>FILEE<gt>> makes Perl try again to read something. Hopefully.
cb1a09d0
AD
2298
2299If that doesn't work (some stdios are particularly cantankerous), then
2300you may need something more like this:
2301
2302 for (;;) {
2303 for ($curpos = tell(FILE); $_ = <FILE>; $curpos = tell(FILE)) {
2304 # search for some stuff and put it into files
2305 }
2306 sleep($for_a_while);
2307 seek(FILE, $curpos, 0);
2308 }
2309
a0d0e21e
LW
2310=item seekdir DIRHANDLE,POS
2311
2312Sets the current position for the readdir() routine on DIRHANDLE. POS
2313must be a value returned by telldir(). Has the same caveats about
2314possible directory compaction as the corresponding system library
2315routine.
2316
2317=item select FILEHANDLE
2318
2319=item select
2320
2321Returns the currently selected filehandle. Sets the current default
2322filehandle for output, if FILEHANDLE is supplied. This has two
2323effects: first, a C<write> or a C<print> without a filehandle will
2324default to this FILEHANDLE. Second, references to variables related to
2325output will refer to this output channel. For example, if you have to
2326set the top of form format for more than one output channel, you might
2327do the following:
2328
2329 select(REPORT1);
2330 $^ = 'report1_top';
2331 select(REPORT2);
2332 $^ = 'report2_top';
2333
2334FILEHANDLE may be an expression whose value gives the name of the
2335actual filehandle. Thus:
2336
2337 $oldfh = select(STDERR); $| = 1; select($oldfh);
2338
4633a7c4
LW
2339Some programmers may prefer to think of filehandles as objects with
2340methods, preferring to write the last example as:
a0d0e21e
LW
2341
2342 use FileHandle;
2343 STDERR->autoflush(1);
2344
2345=item select RBITS,WBITS,EBITS,TIMEOUT
2346
4633a7c4 2347This calls the select(2) system call with the bitmasks specified, which
a0d0e21e
LW
2348can be constructed using fileno() and vec(), along these lines:
2349
2350 $rin = $win = $ein = '';
2351 vec($rin,fileno(STDIN),1) = 1;
2352 vec($win,fileno(STDOUT),1) = 1;
2353 $ein = $rin | $win;
2354
2355If you want to select on many filehandles you might wish to write a
2356subroutine:
2357
2358 sub fhbits {
2359 local(@fhlist) = split(' ',$_[0]);
2360 local($bits);
2361 for (@fhlist) {
2362 vec($bits,fileno($_),1) = 1;
2363 }
2364 $bits;
2365 }
4633a7c4 2366 $rin = fhbits('STDIN TTY SOCK');
a0d0e21e
LW
2367
2368The usual idiom is:
2369
2370 ($nfound,$timeleft) =
2371 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
2372
c07a80fd 2373or to block until something becomes ready just do this
a0d0e21e
LW
2374
2375 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
2376
c07a80fd
PP
2377Most systems do not both to return anything useful in $timeleft, so
2378calling select() in a scalar context just returns $nfound.
2379
a0d0e21e
LW
2380Any of the bitmasks can also be undef. The timeout, if specified, is
2381in seconds, which may be fractional. Note: not all implementations are
2382capable of returning the $timeleft. If not, they always return
2383$timeleft equal to the supplied $timeout.
2384
4633a7c4 2385You can effect a 250-microsecond sleep this way:
a0d0e21e
LW
2386
2387 select(undef, undef, undef, 0.25);
2388
cb1a09d0
AD
2389B<WARNING>: Do not attempt to mix buffered I/O (like read() or <FH>)
2390with select(). You have to use sysread() instead.
a0d0e21e
LW
2391
2392=item semctl ID,SEMNUM,CMD,ARG
2393
2394Calls the System V IPC function semctl. If CMD is &IPC_STAT or
2395&GETALL, then ARG must be a variable which will hold the returned
2396semid_ds structure or semaphore value array. Returns like ioctl: the
2397undefined value for error, "0 but true" for zero, or the actual return
2398value otherwise.
2399
2400=item semget KEY,NSEMS,FLAGS
2401
2402Calls the System V IPC function semget. Returns the semaphore id, or
2403the undefined value if there is an error.
2404
2405=item semop KEY,OPSTRING
2406
2407Calls the System V IPC function semop to perform semaphore operations
2408such as signaling and waiting. OPSTRING must be a packed array of
2409semop structures. Each semop structure can be generated with
2410C<pack("sss", $semnum, $semop, $semflag)>. The number of semaphore
2411operations is implied by the length of OPSTRING. Returns TRUE if
2412successful, or FALSE if there is an error. As an example, the
2413following code waits on semaphore $semnum of semaphore id $semid:
2414
2415 $semop = pack("sss", $semnum, -1, 0);
2416 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
2417
2418To signal the semaphore, replace "-1" with "1".
2419
2420=item send SOCKET,MSG,FLAGS,TO
2421
2422=item send SOCKET,MSG,FLAGS
2423
2424Sends a message on a socket. Takes the same flags as the system call
2425of the same name. On unconnected sockets you must specify a
2426destination to send TO, in which case it does a C sendto(). Returns
2427the number of characters sent, or the undefined value if there is an
2428error.
4633a7c4 2429See L<perlipc/"UDP: Message Passing"> for examples.
a0d0e21e
LW
2430
2431=item setpgrp PID,PGRP
2432
2433Sets the current process group for the specified PID, 0 for the current
2434process. Will produce a fatal error if used on a machine that doesn't
2435implement setpgrp(2).
2436
2437=item setpriority WHICH,WHO,PRIORITY
2438
2439Sets the current priority for a process, a process group, or a user.
748a9306 2440(See setpriority(2).) Will produce a fatal error if used on a machine
a0d0e21e
LW
2441that doesn't implement setpriority(2).
2442
2443=item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
2444
2445Sets the socket option requested. Returns undefined if there is an
2446error. OPTVAL may be specified as undef if you don't want to pass an
2447argument.
2448
2449=item shift ARRAY
2450
2451=item shift
2452
2453Shifts the first value of the array off and returns it, shortening the
2454array by 1 and moving everything down. If there are no elements in the
2455array, returns the undefined value. If ARRAY is omitted, shifts the
2456@ARGV array in the main program, and the @_ array in subroutines.
2457(This is determined lexically.) See also unshift(), push(), and pop().
2458Shift() and unshift() do the same thing to the left end of an array
2459that push() and pop() do to the right end.
2460
2461=item shmctl ID,CMD,ARG
2462
2463Calls the System V IPC function shmctl. If CMD is &IPC_STAT, then ARG
2464must be a variable which will hold the returned shmid_ds structure.
2465Returns like ioctl: the undefined value for error, "0 but true" for
2466zero, or the actual return value otherwise.
2467
2468=item shmget KEY,SIZE,FLAGS
2469
2470Calls the System V IPC function shmget. Returns the shared memory
2471segment id, or the undefined value if there is an error.
2472
2473=item shmread ID,VAR,POS,SIZE
2474
2475=item shmwrite ID,STRING,POS,SIZE
2476
2477Reads or writes the System V shared memory segment ID starting at
2478position POS for size SIZE by attaching to it, copying in/out, and
2479detaching from it. When reading, VAR must be a variable which will
2480hold the data read. When writing, if STRING is too long, only SIZE
2481bytes are used; if STRING is too short, nulls are written to fill out
2482SIZE bytes. Return TRUE if successful, or FALSE if there is an error.
2483
2484=item shutdown SOCKET,HOW
2485
2486Shuts down a socket connection in the manner indicated by HOW, which
2487has the same interpretation as in the system call of the same name.
2488
2489=item sin EXPR
2490
2491Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
2492returns sine of $_.
2493
2494=item sleep EXPR
2495
2496=item sleep
2497
2498Causes the script to sleep for EXPR seconds, or forever if no EXPR.
2499May be interrupted by sending the process a SIGALRM. Returns the
2500number of seconds actually slept. You probably cannot mix alarm() and
2501sleep() calls, since sleep() is often implemented using alarm().
2502
2503On some older systems, it may sleep up to a full second less than what
2504you requested, depending on how it counts seconds. Most modern systems
2505always sleep the full amount.
2506
cb1a09d0
AD
2507For delays of finer granularity than one second, you may use Perl's
2508syscall() interface to access setitimer(2) if your system supports it,
2509or else see L</select()> below.
2510
a0d0e21e
LW
2511=item socket SOCKET,DOMAIN,TYPE,PROTOCOL
2512
2513Opens a socket of the specified kind and attaches it to filehandle
2514SOCKET. DOMAIN, TYPE and PROTOCOL are specified the same as for the
2515system call of the same name. You should "use Socket;" first to get
4633a7c4 2516the proper definitions imported. See the example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
2517
2518=item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
2519
2520Creates an unnamed pair of sockets in the specified domain, of the
2521specified type. DOMAIN, TYPE and PROTOCOL are specified the same as
2522for the system call of the same name. If unimplemented, yields a fatal
2523error. Returns TRUE if successful.
2524
2525=item sort SUBNAME LIST
2526
2527=item sort BLOCK LIST
2528
2529=item sort LIST
2530
2531Sorts the LIST and returns the sorted list value. Nonexistent values
2532of arrays are stripped out. If SUBNAME or BLOCK is omitted, sorts
2533in standard string comparison order. If SUBNAME is specified, it
2534gives the name of a subroutine that returns an integer less than, equal
2535to, or greater than 0, depending on how the elements of the array are
2536to be ordered. (The <=> and cmp operators are extremely useful in such
2537routines.) SUBNAME may be a scalar variable name, in which case the
2538value provides the name of the subroutine to use. In place of a
2539SUBNAME, you can provide a BLOCK as an anonymous, in-line sort
2540subroutine.
2541
cb1a09d0
AD
2542In the interests of efficiency the normal calling code for subroutines is
2543bypassed, with the following effects: the subroutine may not be a
2544recursive subroutine, and the two elements to be compared are passed into
2545the subroutine not via @_ but as the package global variables $a and
2546$b (see example below). They are passed by reference, so don't
2547modify $a and $b. And don't try to declare them as lexicals either.
a0d0e21e
LW
2548
2549Examples:
2550
2551 # sort lexically
2552 @articles = sort @files;
2553
2554 # same thing, but with explicit sort routine
2555 @articles = sort {$a cmp $b} @files;
2556
cb1a09d0
AD
2557 # now case-insensitively
2558 @articles = sort { uc($a) cmp uc($b)} @files;
2559
a0d0e21e
LW
2560 # same thing in reversed order
2561 @articles = sort {$b cmp $a} @files;
2562
2563 # sort numerically ascending
2564 @articles = sort {$a <=> $b} @files;
2565
2566 # sort numerically descending
2567 @articles = sort {$b <=> $a} @files;
2568
2569 # sort using explicit subroutine name
2570 sub byage {
2571 $age{$a} <=> $age{$b}; # presuming integers
2572 }
2573 @sortedclass = sort byage @class;
2574
c07a80fd
PP
2575 # this sorts the %age associative arrays by value
2576 # instead of key using an inline function
2577 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
2578
a0d0e21e
LW
2579 sub backwards { $b cmp $a; }
2580 @harry = ('dog','cat','x','Cain','Abel');
2581 @george = ('gone','chased','yz','Punished','Axed');
2582 print sort @harry;
2583 # prints AbelCaincatdogx
2584 print sort backwards @harry;
2585 # prints xdogcatCainAbel
2586 print sort @george, 'to', @harry;
2587 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
2588
cb1a09d0
AD
2589 # inefficiently sort by descending numeric compare using
2590 # the first integer after the first = sign, or the
2591 # whole record case-insensitively otherwise
2592
2593 @new = sort {
2594 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
2595 ||
2596 uc($a) cmp uc($b)
2597 } @old;
2598
2599 # same thing, but much more efficiently;
2600 # we'll build auxiliary indices instead
2601 # for speed
2602 @nums = @caps = ();
2603 for (@old) {
2604 push @nums, /=(\d+)/;
2605 push @caps, uc($_);
2606 }
2607
2608 @new = @old[ sort {
2609 $nums[$b] <=> $nums[$a]
2610 ||
2611 $caps[$a] cmp $caps[$b]
2612 } 0..$#old
2613 ];
2614
2615 # same thing using a Schwartzian Transform (no temps)
2616 @new = map { $_->[0] }
2617 sort { $b->[1] <=> $a->[1]
2618 ||
2619 $a->[2] cmp $b->[2]
2620 } map { [$_, /=(\d+)/, uc($_)] } @old;
2621
2622If you're and using strict, you I<MUST NOT> declare $a
2623and $b as lexicals. They are package globals. That means
2624if you're in the C<main> package, it's
2625
2626 @articles = sort {$main::b <=> $main::a} @files;
2627
2628or just
2629
2630 @articles = sort {$::b <=> $::a} @files;
2631
2632but if you're in the C<FooPack> package, it's
2633
2634 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
2635
a0d0e21e
LW
2636=item splice ARRAY,OFFSET,LENGTH,LIST
2637
2638=item splice ARRAY,OFFSET,LENGTH
2639
2640=item splice ARRAY,OFFSET
2641
2642Removes the elements designated by OFFSET and LENGTH from an array, and
2643replaces them with the elements of LIST, if any. Returns the elements
2644removed from the array. The array grows or shrinks as necessary. If
2645LENGTH is omitted, removes everything from OFFSET onward. The
2646following equivalencies hold (assuming $[ == 0):
2647
2648 push(@a,$x,$y) splice(@a,$#a+1,0,$x,$y)
2649 pop(@a) splice(@a,-1)
2650 shift(@a) splice(@a,0,1)
2651 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
2652 $a[$x] = $y splice(@a,$x,1,$y);
2653
2654Example, assuming array lengths are passed before arrays:
2655
2656 sub aeq { # compare two list values
2657 local(@a) = splice(@_,0,shift);
2658 local(@b) = splice(@_,0,shift);
2659 return 0 unless @a == @b; # same len?
2660 while (@a) {
2661 return 0 if pop(@a) ne pop(@b);
2662 }
2663 return 1;
2664 }
2665 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
2666
2667=item split /PATTERN/,EXPR,LIMIT
2668
2669=item split /PATTERN/,EXPR
2670
2671=item split /PATTERN/
2672
2673=item split
2674
2675Splits a string into an array of strings, and returns it.
2676
2677If not in a list context, returns the number of fields found and splits into
2678the @_ array. (In a list context, you can force the split into @_ by
2679using C<??> as the pattern delimiters, but it still returns the array
2680value.) The use of implicit split to @_ is deprecated, however.
2681
2682If EXPR is omitted, splits the $_ string. If PATTERN is also omitted,
4633a7c4
LW
2683splits on whitespace (after skipping any leading whitespace). Anything
2684matching PATTERN is taken to be a delimiter separating the fields. (Note
2685that the delimiter may be longer than one character.) If LIMIT is
2686specified and is not negative, splits into no more than that many fields
2687(though it may split into fewer). If LIMIT is unspecified, trailing null
2688fields are stripped (which potential users of pop() would do well to
2689remember). If LIMIT is negative, it is treated as if an arbitrarily large
2690LIMIT had been specified.
a0d0e21e
LW
2691
2692A pattern matching the null string (not to be confused with
748a9306 2693a null pattern C<//>, which is just one member of the set of patterns
a0d0e21e
LW
2694matching a null string) will split the value of EXPR into separate
2695characters at each point it matches that way. For example:
2696
2697 print join(':', split(/ */, 'hi there'));
2698
2699produces the output 'h:i:t:h:e:r:e'.
2700
2701The LIMIT parameter can be used to partially split a line
2702
2703 ($login, $passwd, $remainder) = split(/:/, $_, 3);
2704
2705When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT
2706one larger than the number of variables in the list, to avoid
2707unnecessary work. For the list above LIMIT would have been 4 by
2708default. In time critical applications it behooves you not to split
2709into more fields than you really need.
2710
2711If the PATTERN contains parentheses, additional array elements are
2712created from each matching substring in the delimiter.
2713
2714 split(/([,-])/, "1-10,20");
2715
2716produces the list value
2717
2718 (1, '-', 10, ',', 20)
2719
4633a7c4
LW
2720If you had the entire header of a normal Unix email message in $header,
2721you could split it up into fields and their values this way:
2722
2723 $header =~ s/\n\s+/ /g; # fix continuation lines
2724 %hdrs = (UNIX_FROM => split /^(.*?):\s*/m, $header);
2725
a0d0e21e
LW
2726The pattern C</PATTERN/> may be replaced with an expression to specify
2727patterns that vary at runtime. (To do runtime compilation only once,
748a9306
LW
2728use C</$variable/o>.)
2729
2730As a special case, specifying a PATTERN of space (C<' '>) will split on
2731white space just as split with no arguments does. Thus, split(' ') can
2732be used to emulate B<awk>'s default behavior, whereas C<split(/ /)>
2733will give you as many null initial fields as there are leading spaces.
2734A split on /\s+/ is like a split(' ') except that any leading
2735whitespace produces a null first field. A split with no arguments
2736really does a C<split(' ', $_)> internally.
a0d0e21e
LW
2737
2738Example:
2739
2740 open(passwd, '/etc/passwd');
2741 while (<passwd>) {
748a9306
LW
2742 ($login, $passwd, $uid, $gid, $gcos,
2743 $home, $shell) = split(/:/);
a0d0e21e
LW
2744 ...
2745 }
2746
2747(Note that $shell above will still have a newline on it. See L</chop>,
2748L</chomp>, and L</join>.)
2749
2750=item sprintf FORMAT,LIST
2751
2752Returns a string formatted by the usual printf conventions of the C
cb1a09d0
AD
2753language. See L<sprintf(3)> or L<printf(3)> on your system for details.
2754(The * character for an indirectly specified length is not
a0d0e21e 2755supported, but you can get the same effect by interpolating a variable
cb1a09d0
AD
2756into the pattern.) Some C libraries' implementations of sprintf() can
2757dump core when fed ludicrous arguments.
a0d0e21e
LW
2758
2759=item sqrt EXPR
2760
2761Return the square root of EXPR. If EXPR is omitted, returns square
2762root of $_.
2763
2764=item srand EXPR
2765
cb1a09d0
AD
2766Sets the random number seed for the C<rand> operator. If EXPR is omitted,
2767does C<srand(time)>. Many folks use an explicit C<srand(time ^ $$)>
2768instead. Of course, you'd need something much more random than that for
2769cryptographic purposes, since it's easy to guess the current time.
2770Checksumming the compressed output of rapidly changing operating system
2771status programs is the usual method. Examples are posted regularly to
2772the comp.security.unix newsgroup.
a0d0e21e
LW
2773
2774=item stat FILEHANDLE
2775
2776=item stat EXPR
2777
2778Returns a 13-element array giving the status info for a file, either the
2779file opened via FILEHANDLE, or named by EXPR. Returns a null list if
2780the stat fails. Typically used as follows:
2781
2782 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
2783 $atime,$mtime,$ctime,$blksize,$blocks)
2784 = stat($filename);
2785
c07a80fd
PP
2786Not all fields are supported on all filesystem types. Here are the
2787meaning of the fields:
2788
2789 dev device number of filesystem
2790 ino inode number
2791 mode file mode (type and permissions)
2792 nlink number of (hard) links to the file
2793 uid numeric user ID of file's owner
2794 gid numer group ID of file's owner
2795 rdev the device identifier (special files only)
2796 size total size of file, in bytes
2797 atime last access time since the epoch
2798 mtime last modify time since the epoch
2799 ctime inode change time (NOT creation type!) since the epoch
2800 blksize preferred blocksize for file system I/O
2801 blocks actual number of blocks allocated
2802
2803(The epoch was at 00:00 January 1, 1970 GMT.)
2804
a0d0e21e
LW
2805If stat is passed the special filehandle consisting of an underline, no
2806stat is done, but the current contents of the stat structure from the
2807last stat or filetest are returned. Example:
2808
2809 if (-x $file && (($d) = stat(_)) && $d < 0) {
2810 print "$file is executable NFS file\n";
2811 }
2812
2813(This only works on machines for which the device number is negative under NFS.)
2814
2815=item study SCALAR
2816
2817=item study
2818
2819Takes extra time to study SCALAR ($_ if unspecified) in anticipation of
2820doing many pattern matches on the string before it is next modified.
2821This may or may not save time, depending on the nature and number of
2822patterns you are searching on, and on the distribution of character
2823frequencies in the string to be searched--you probably want to compare
2824runtimes with and without it to see which runs faster. Those loops
2825which scan for many short constant strings (including the constant
2826parts of more complex patterns) will benefit most. You may have only
2827one study active at a time--if you study a different scalar the first
2828is "unstudied". (The way study works is this: a linked list of every
2829character in the string to be searched is made, so we know, for
2830example, where all the 'k' characters are. From each search string,
2831the rarest character is selected, based on some static frequency tables
2832constructed from some C programs and English text. Only those places
2833that contain this "rarest" character are examined.)
2834
2835For example, here is a loop which inserts index producing entries
2836before any line containing a certain pattern:
2837
2838 while (<>) {
2839 study;
2840 print ".IX foo\n" if /\bfoo\b/;
2841 print ".IX bar\n" if /\bbar\b/;
2842 print ".IX blurfl\n" if /\bblurfl\b/;
2843 ...
2844 print;
2845 }
2846
2847In searching for /\bfoo\b/, only those locations in $_ that contain "f"
2848will be looked at, because "f" is rarer than "o". In general, this is
2849a big win except in pathological cases. The only question is whether
2850it saves you more time than it took to build the linked list in the
2851first place.
2852
2853Note that if you have to look for strings that you don't know till
2854runtime, you can build an entire loop as a string and eval that to
2855avoid recompiling all your patterns all the time. Together with
2856undefining $/ to input entire files as one record, this can be very
2857fast, often faster than specialized programs like fgrep(1). The following
2858scans a list of files (@files) for a list of words (@words), and prints
2859out the names of those files that contain a match:
2860
2861 $search = 'while (<>) { study;';
2862 foreach $word (@words) {
2863 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
2864 }
2865 $search .= "}";
2866 @ARGV = @files;
2867 undef $/;
2868 eval $search; # this screams
2869 $/ = "\n"; # put back to normal input delim
2870 foreach $file (sort keys(%seen)) {
2871 print $file, "\n";
2872 }
2873
cb1a09d0
AD
2874=item sub BLOCK
2875
2876=item sub NAME
2877
2878=item sub NAME BLOCK
2879
2880This is subroutine definition, not a real function I<per se>. With just a
2881NAME (and possibly prototypes), it's just a forward declaration. Without
2882a NAME, it's an anonymous function declaration, and does actually return a
2883value: the CODE ref of the closure you just created. See L<perlsub> and
2884L<perlref> for details.
2885
a0d0e21e
LW
2886=item substr EXPR,OFFSET,LEN
2887
2888=item substr EXPR,OFFSET
2889
2890Extracts a substring out of EXPR and returns it. First character is at
2891offset 0, or whatever you've set $[ to. If OFFSET is negative, starts
2892that far from the end of the string. If LEN is omitted, returns
748a9306
LW
2893everything to the end of the string. If LEN is negative, leaves that
2894many characters off the end of the string.
2895
2896You can use the substr() function
a0d0e21e
LW
2897as an lvalue, in which case EXPR must be an lvalue. If you assign
2898something shorter than LEN, the string will shrink, and if you assign
2899something longer than LEN, the string will grow to accommodate it. To
2900keep the string the same length you may need to pad or chop your value
2901using sprintf().
2902
2903=item symlink OLDFILE,NEWFILE
2904
2905Creates a new filename symbolically linked to the old filename.
2906Returns 1 for success, 0 otherwise. On systems that don't support
2907symbolic links, produces a fatal error at run time. To check for that,
2908use eval:
2909
2910 $symlink_exists = (eval 'symlink("","");', $@ eq '');
2911
2912=item syscall LIST
2913
2914Calls the system call specified as the first element of the list,
2915passing the remaining elements as arguments to the system call. If
2916unimplemented, produces a fatal error. The arguments are interpreted
2917as follows: if a given argument is numeric, the argument is passed as
2918an int. If not, the pointer to the string value is passed. You are
2919responsible to make sure a string is pre-extended long enough to
2920receive any result that might be written into a string. If your
2921integer arguments are not literals and have never been interpreted in a
2922numeric context, you may need to add 0 to them to force them to look
2923like numbers.
2924
2925 require 'syscall.ph'; # may need to run h2ph
2926 syscall(&SYS_write, fileno(STDOUT), "hi there\n", 9);
2927
2928Note that Perl only supports passing of up to 14 arguments to your system call,
2929which in practice should usually suffice.
2930
c07a80fd
PP
2931=item sysopen FILEHANDLE,FILENAME,MODE
2932
2933=item sysopen FILEHANDLE,FILENAME,MODE,PERMS
2934
2935Opens the file whose filename is given by FILENAME, and associates it
2936with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
2937the name of the real filehandle wanted. This function calls the
2938underlying operating system's C<open> function with the parameters
2939FILENAME, MODE, PERMS.
2940
2941The possible values and flag bits of the MODE parameter are
2942system-dependent; they are available via the standard module C<Fcntl>.
2943However, for historical reasons, some values are universal: zero means
2944read-only, one means write-only, and two means read/write.
2945
2946If the file named by FILENAME does not exist and the C<open> call
2947creates it (typically because MODE includes the O_CREAT flag), then
2948the value of PERMS specifies the permissions of the newly created
2949file. If PERMS is omitted, the default value is 0666, which allows
2950read and write for all. This default is reasonable: see C<umask>.
2951
a0d0e21e
LW
2952=item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
2953
2954=item sysread FILEHANDLE,SCALAR,LENGTH
2955
2956Attempts to read LENGTH bytes of data into variable SCALAR from the
2957specified FILEHANDLE, using the system call read(2). It bypasses
2958stdio, so mixing this with other kinds of reads may cause confusion.
2959Returns the number of bytes actually read, or undef if there was an
2960error. SCALAR will be grown or shrunk to the length actually read. An
2961OFFSET may be specified to place the read data at some other place than
2962the beginning of the string.
2963
2964=item system LIST
2965
2966Does exactly the same thing as "exec LIST" except that a fork is done
2967first, and the parent process waits for the child process to complete.
2968Note that argument processing varies depending on the number of
2969arguments. The return value is the exit status of the program as
2970returned by the wait() call. To get the actual exit value divide by
cb1a09d0
AD
2971256. See also L</exec>. This is I<NOT> what you want to use to capture
2972the output from a command, for that you should merely use backticks, as
2973described in L<perlop/"`STRING`">.
a0d0e21e
LW
2974
2975=item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
2976
2977=item syswrite FILEHANDLE,SCALAR,LENGTH
2978
2979Attempts to write LENGTH bytes of data from variable SCALAR to the
2980specified FILEHANDLE, using the system call write(2). It bypasses
2981stdio, so mixing this with prints may cause confusion. Returns the
2982number of bytes actually written, or undef if there was an error. An
22dc801b 2983OFFSET may be specified to get the write data from some other place than
a0d0e21e
LW
2984the beginning of the string.
2985
2986=item tell FILEHANDLE
2987
2988=item tell
2989
2990Returns the current file position for FILEHANDLE. FILEHANDLE may be an
2991expression whose value gives the name of the actual filehandle. If
2992FILEHANDLE is omitted, assumes the file last read.
2993
2994=item telldir DIRHANDLE
2995
2996Returns the current position of the readdir() routines on DIRHANDLE.
2997Value may be given to seekdir() to access a particular location in a
2998directory. Has the same caveats about possible directory compaction as
2999the corresponding system library routine.
3000
4633a7c4 3001=item tie VARIABLE,CLASSNAME,LIST
a0d0e21e 3002
4633a7c4
LW
3003This function binds a variable to a package class that will provide the
3004implementation for the variable. VARIABLE is the name of the variable
3005to be enchanted. CLASSNAME is the name of a class implementing objects
3006of correct type. Any additional arguments are passed to the "new"
3007method of the class (meaning TIESCALAR, TIEARRAY, or TIEHASH).
3008Typically these are arguments such as might be passed to the dbm_open()
cb1a09d0
AD
3009function of C. The object returned by the "new" method is also
3010returned by the tie() function, which would be useful if you want to
4633a7c4 3011access other methods in CLASSNAME.
a0d0e21e
LW
3012
3013Note that functions such as keys() and values() may return huge array
748a9306
LW
3014values when used on large objects, like DBM files. You may prefer to
3015use the each() function to iterate over such. Example:
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LW
3016
3017 # print out history file offsets
4633a7c4 3018 use NDBM_File;
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LW
3019 tie(%HIST, NDBM_File, '/usr/lib/news/history', 1, 0);
3020 while (($key,$val) = each %HIST) {
3021 print $key, ' = ', unpack('L',$val), "\n";
3022 }
3023 untie(%HIST);
3024
4633a7c4 3025A class implementing an associative array should have the following
a0d0e21e
LW
3026methods:
3027
4633a7c4 3028 TIEHASH classname, LIST
a0d0e21e
LW
3029 DESTROY this
3030 FETCH this, key
3031 STORE this, key, value
3032 DELETE this, key
3033 EXISTS this, key
3034 FIRSTKEY this
3035 NEXTKEY this, lastkey
3036
4633a7c4 3037A class implementing an ordinary array should have the following methods:
a0d0e21e 3038
4633a7c4 3039 TIEARRAY classname, LIST
a0d0e21e
LW
3040 DESTROY this
3041 FETCH this, key
3042 STORE this, key, value
3043 [others TBD]
3044
4633a7c4 3045A class implementing a scalar should have the following methods:
a0d0e21e 3046
4633a7c4 3047 TIESCALAR classname, LIST
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LW
3048 DESTROY this
3049 FETCH this,
3050 STORE this, value
3051
4633a7c4
LW
3052Unlike dbmopen(), the tie() function will not use or require a module
3053for you--you need to do that explicitly yourself. See L<DB_File>
3054or the F<Config> module for interesting tie() implementations.
3055
f3cbc334
RS
3056=item tied VARIABLE
3057
3058Returns a reference to the object underlying VARIABLE (the same value
3059that was originally returned by the tie() call which bound the variable
3060to a package.) Returns the undefined value if VARIABLE isn't tied to a
3061package.
3062
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LW
3063=item time
3064
3065Returns the number of non-leap seconds since 00:00:00 UTC, January 1,
30661970. Suitable for feeding to gmtime() and localtime().
3067
3068=item times
3069
3070Returns a four-element array giving the user and system times, in
3071seconds, for this process and the children of this process.
3072
3073 ($user,$system,$cuser,$csystem) = times;
3074
3075=item tr///
3076
3077The translation operator. See L<perlop>.
3078
3079=item truncate FILEHANDLE,LENGTH
3080
3081=item truncate EXPR,LENGTH
3082
3083Truncates the file opened on FILEHANDLE, or named by EXPR, to the
3084specified length. Produces a fatal error if truncate isn't implemented
3085on your system.
3086
3087=item uc EXPR
3088
3089Returns an uppercased version of EXPR. This is the internal function
3090implementing the \U escape in double-quoted strings.
4633a7c4 3091Should respect any POSIX setlocale() settings.
a0d0e21e
LW
3092
3093=item ucfirst EXPR
3094
3095Returns the value of EXPR with the first character uppercased. This is
3096the internal function implementing the \u escape in double-quoted strings.
4633a7c4 3097Should respect any POSIX setlocale() settings.
a0d0e21e
LW
3098
3099=item umask EXPR
3100
3101=item umask
3102
3103Sets the umask for the process and returns the old one. If EXPR is
3104omitted, merely returns current umask.
3105
3106=item undef EXPR
3107
3108=item undef
3109
3110Undefines the value of EXPR, which must be an lvalue. Use only on a
3111scalar value, an entire array, or a subroutine name (using "&"). (Using undef()
3112will probably not do what you expect on most predefined variables or
3113DBM list values, so don't do that.) Always returns the undefined value. You can omit
3114the EXPR, in which case nothing is undefined, but you still get an
3115undefined value that you could, for instance, return from a
3116subroutine. Examples:
3117
3118 undef $foo;
3119 undef $bar{'blurfl'};
3120 undef @ary;
3121 undef %assoc;
3122 undef &mysub;
3123 return (wantarray ? () : undef) if $they_blew_it;
3124
3125=item unlink LIST
3126
3127Deletes a list of files. Returns the number of files successfully
3128deleted.
3129
3130 $cnt = unlink 'a', 'b', 'c';
3131 unlink @goners;
3132 unlink <*.bak>;
3133
3134Note: unlink will not delete directories unless you are superuser and
3135the B<-U> flag is supplied to Perl. Even if these conditions are
3136met, be warned that unlinking a directory can inflict damage on your
3137filesystem. Use rmdir instead.
3138
3139=item unpack TEMPLATE,EXPR
3140
3141Unpack does the reverse of pack: it takes a string representing a
3142structure and expands it out into a list value, returning the array
3143value. (In a scalar context, it merely returns the first value
3144produced.) The TEMPLATE has the same format as in the pack function.
3145Here's a subroutine that does substring:
3146
3147 sub substr {
3148 local($what,$where,$howmuch) = @_;
3149 unpack("x$where a$howmuch", $what);
3150 }
3151
3152and then there's
3153
3154 sub ordinal { unpack("c",$_[0]); } # same as ord()
3155
3156In addition, you may prefix a field with a %<number> to indicate that
3157you want a <number>-bit checksum of the items instead of the items
3158themselves. Default is a 16-bit checksum. For example, the following
3159computes the same number as the System V sum program:
3160
3161 while (<>) {
3162 $checksum += unpack("%16C*", $_);
3163 }
3164 $checksum %= 65536;
3165
3166The following efficiently counts the number of set bits in a bit vector:
3167
3168 $setbits = unpack("%32b*", $selectmask);
3169
3170=item untie VARIABLE
3171
3172Breaks the binding between a variable and a package. (See tie().)
3173
3174=item unshift ARRAY,LIST
3175
3176Does the opposite of a C<shift>. Or the opposite of a C<push>,
3177depending on how you look at it. Prepends list to the front of the
3178array, and returns the new number of elements in the array.
3179
3180 unshift(ARGV, '-e') unless $ARGV[0] =~ /^-/;
3181
3182Note the LIST is prepended whole, not one element at a time, so the
3183prepended elements stay in the same order. Use reverse to do the
3184reverse.
3185
3186=item use Module LIST
3187
3188=item use Module
3189
3190Imports some semantics into the current package from the named module,
3191generally by aliasing certain subroutine or variable names into your
3192package. It is exactly equivalent to
3193
3194 BEGIN { require Module; import Module LIST; }
3195
a0d0e21e
LW
3196The BEGIN forces the require and import to happen at compile time. The
3197require makes sure the module is loaded into memory if it hasn't been
3198yet. The import is not a builtin--it's just an ordinary static method
3199call into the "Module" package to tell the module to import the list of
3200features back into the current package. The module can implement its
3201import method any way it likes, though most modules just choose to
3202derive their import method via inheritance from the Exporter class that
cb1a09d0
AD
3203is defined in the Exporter module. See L<Exporter>.
3204
3205If you don't want your namespace altered, explicitly supply an empty list:
3206
3207 use Module ();
3208
3209That is exactly equivalent to
3210
3211 BEGIN { require Module; }
a0d0e21e
LW
3212
3213Because this is a wide-open interface, pragmas (compiler directives)
3214are also implemented this way. Currently implemented pragmas are:
3215
3216 use integer;
4633a7c4 3217 use diagnostics;
a0d0e21e
LW
3218 use sigtrap qw(SEGV BUS);
3219 use strict qw(subs vars refs);
3220 use subs qw(afunc blurfl);
3221
3222These pseudomodules import semantics into the current block scope, unlike
3223ordinary modules, which import symbols into the current package (which are
3224effective through the end of the file).
3225
3226There's a corresponding "no" command that unimports meanings imported
3227by use.
3228
3229 no integer;
3230 no strict 'refs';
3231
3232See L<perlmod> for a list of standard modules and pragmas.
3233
3234=item utime LIST
3235
3236Changes the access and modification times on each file of a list of
3237files. The first two elements of the list must be the NUMERICAL access
3238and modification times, in that order. Returns the number of files
3239successfully changed. The inode modification time of each file is set
3240to the current time. Example of a "touch" command:
3241
3242 #!/usr/bin/perl
3243 $now = time;
3244 utime $now, $now, @ARGV;
3245
3246=item values ASSOC_ARRAY
3247
3248Returns a normal array consisting of all the values of the named
3249associative array. (In a scalar context, returns the number of
3250values.) The values are returned in an apparently random order, but it
3251is the same order as either the keys() or each() function would produce
c07a80fd 3252on the same array. See also keys(), each(), and sort().
a0d0e21e
LW
3253
3254=item vec EXPR,OFFSET,BITS
3255
22dc801b
PP
3256Treats the string in EXPR as a vector of unsigned integers, and
3257returns the value of the bitfield specified by OFFSET. BITS specifies
3258the number of bits that are reserved for each entry in the bit
3259vector. This must be a power of two from 1 to 32. vec() may also be
3260assigned to, in which case parens are needed to give the expression
3261the correct precedence as in
3262
3263 vec($image, $max_x * $x + $y, 8) = 3;
a0d0e21e
LW
3264
3265Vectors created with vec() can also be manipulated with the logical
3266operators |, & and ^, which will assume a bit vector operation is
3267desired when both operands are strings.
3268
3269To transform a bit vector into a string or array of 0's and 1's, use these:
3270
3271 $bits = unpack("b*", $vector);
3272 @bits = split(//, unpack("b*", $vector));
3273
3274If you know the exact length in bits, it can be used in place of the *.
3275
3276=item wait
3277
3278Waits for a child process to terminate and returns the pid of the
3279deceased process, or -1 if there are no child processes. The status is
3280returned in $?.
3281
3282=item waitpid PID,FLAGS
3283
3284Waits for a particular child process to terminate and returns the pid
3285of the deceased process, or -1 if there is no such child process. The
3286status is returned in $?. If you say
3287
3288 use POSIX "wait_h";
3289 ...
3290 waitpid(-1,&WNOHANG);
3291
3292then you can do a non-blocking wait for any process. Non-blocking wait
3293is only available on machines supporting either the waitpid(2) or
3294wait4(2) system calls. However, waiting for a particular pid with
3295FLAGS of 0 is implemented everywhere. (Perl emulates the system call
3296by remembering the status values of processes that have exited but have
3297not been harvested by the Perl script yet.)
3298
3299=item wantarray
3300
3301Returns TRUE if the context of the currently executing subroutine is
3302looking for a list value. Returns FALSE if the context is looking
3303for a scalar.
3304
3305 return wantarray ? () : undef;
3306
3307=item warn LIST
3308
3309Produces a message on STDERR just like die(), but doesn't exit or
4633a7c4 3310on an exception.
a0d0e21e
LW
3311
3312=item write FILEHANDLE
3313
3314=item write EXPR
3315
3316=item write
3317
3318Writes a formatted record (possibly multi-line) to the specified file,
3319using the format associated with that file. By default the format for
3320a file is the one having the same name is the filehandle, but the
3321format for the current output channel (see the select() function) may be set
3322explicitly by assigning the name of the format to the $~ variable.
3323
3324Top of form processing is handled automatically: if there is
3325insufficient room on the current page for the formatted record, the
3326page is advanced by writing a form feed, a special top-of-page format
3327is used to format the new page header, and then the record is written.
3328By default the top-of-page format is the name of the filehandle with
3329"_TOP" appended, but it may be dynamically set to the format of your
3330choice by assigning the name to the $^ variable while the filehandle is
3331selected. The number of lines remaining on the current page is in
3332variable $-, which can be set to 0 to force a new page.
3333
3334If FILEHANDLE is unspecified, output goes to the current default output
3335channel, which starts out as STDOUT but may be changed by the
3336C<select> operator. If the FILEHANDLE is an EXPR, then the expression
3337is evaluated and the resulting string is used to look up the name of
3338the FILEHANDLE at run time. For more on formats, see L<perlform>.
3339
3340Note that write is I<NOT> the opposite of read. Unfortunately.
3341
3342=item y///
3343
37798a01 3344The translation operator. See L<perlop>.
a0d0e21e
LW
3345
3346=back