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2=head1 NAME
3
4perlfunc - Perl builtin functions
5
6=head1 DESCRIPTION
7
8The functions in this section can serve as terms in an expression.
9They fall into two major categories: list operators and named unary
10operators. These differ in their precedence relationship with a
11following comma. (See the precedence table in L<perlop>.) List
12operators take more than one argument, while unary operators can never
13take more than one argument. Thus, a comma terminates the argument of
14a unary operator, but merely separates the arguments of a list
15operator. A unary operator generally provides a scalar context to its
16argument, while a list operator may provide either scalar and list
17contexts for its arguments. If it does both, the scalar arguments will
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18be first, and the list argument will follow. (Note that there can ever
19be only one list argument.) For instance, splice() has three scalar
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20arguments followed by a list.
21
22In the syntax descriptions that follow, list operators that expect a
23list (and provide list context for the elements of the list) are shown
24with LIST as an argument. Such a list may consist of any combination
25of scalar arguments or list values; the list values will be included
26in the list as if each individual element were interpolated at that
27point in the list, forming a longer single-dimensional list value.
28Elements of the LIST should be separated by commas.
29
30Any function in the list below may be used either with or without
31parentheses around its arguments. (The syntax descriptions omit the
5f05dabc 32parentheses.) If you use the parentheses, the simple (but occasionally
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33surprising) rule is this: It I<LOOKS> like a function, therefore it I<IS> a
34function, and precedence doesn't matter. Otherwise it's a list
35operator or unary operator, and precedence does matter. And whitespace
36between the function and left parenthesis doesn't count--so you need to
37be careful sometimes:
38
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39 print 1+2+4; # Prints 7.
40 print(1+2) + 4; # Prints 3.
41 print (1+2)+4; # Also prints 3!
42 print +(1+2)+4; # Prints 7.
43 print ((1+2)+4); # Prints 7.
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44
45If you run Perl with the B<-w> switch it can warn you about this. For
46example, the third line above produces:
47
48 print (...) interpreted as function at - line 1.
49 Useless use of integer addition in void context at - line 1.
50
51For functions that can be used in either a scalar or list context,
54310121 52nonabortive failure is generally indicated in a scalar context by
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53returning the undefined value, and in a list context by returning the
54null list.
55
56Remember the following rule:
57
cb1a09d0 58=over 8
a0d0e21e 59
8ebc5c01 60=item I<THERE IS NO GENERAL RULE FOR CONVERTING A LIST INTO A SCALAR!>
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61
62=back
63
64Each operator and function decides which sort of value it would be most
65appropriate to return in a scalar context. Some operators return the
66length of the list that would have been returned in a list context. Some
67operators return the first value in the list. Some operators return the
68last value in the list. Some operators return a count of successful
69operations. In general, they do what you want, unless you want
70consistency.
71
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72=head2 Perl Functions by Category
73
74Here are Perl's functions (including things that look like
75functions, like some of the keywords and named operators)
76arranged by category. Some functions appear in more
77than one place.
78
79=over
80
81=item Functions for SCALARs or strings
82
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83C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>,
84C<length>, C<oct>, C<ord>, C<pack>, C<q>/STRING/, C<qq>/STRING/, C<reverse>,
85C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y>///
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86
87=item Regular expressions and pattern matching
88
22fae026 89C<m>//, C<pos>, C<quotemeta>, C<s>///, C<split>, C<study>
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90
91=item Numeric functions
92
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93C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>,
94C<sin>, C<sqrt>, C<srand>
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95
96=item Functions for real @ARRAYs
97
22fae026 98C<pop>, C<push>, C<shift>, C<splice>, C<unshift>
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99
100=item Functions for list data
101
22fae026 102C<grep>, C<join>, C<map>, C<qw>/STRING/, C<reverse>, C<sort>, C<unpack>
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103
104=item Functions for real %HASHes
105
22fae026 106C<delete>, C<each>, C<exists>, C<keys>, C<values>
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107
108=item Input and output functions
109
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110C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
111C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
112C<readdir>, C<rewinddir>, C<seek>, C<seekdir>, C<select>, C<syscall>,
113C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>,
114C<warn>, C<write>
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115
116=item Functions for fixed length data or records
117
22fae026 118C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec>
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119
120=item Functions for filehandles, files, or directories
121
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122C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>,
123C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>, C<readlink>,
124C<rename>, C<rmdir>, C<stat>, C<symlink>, C<umask>, C<unlink>, C<utime>
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125
126=item Keywords related to the control flow of your perl program
127
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128C<caller>, C<continue>, C<die>, C<do>, C<dump>, C<eval>, C<exit>,
129C<goto>, C<last>, C<next>, C<redo>, C<return>, C<sub>, C<wantarray>
cb1a09d0 130
54310121 131=item Keywords related to scoping
cb1a09d0 132
22fae026 133C<caller>, C<import>, C<local>, C<my>, C<package>, C<use>
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134
135=item Miscellaneous functions
136
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137C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<reset>,
138C<scalar>, C<undef>, C<wantarray>
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139
140=item Functions for processes and process groups
141
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142C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
143C<pipe>, C<qx>/STRING/, C<setpgrp>, C<setpriority>, C<sleep>, C<system>,
144C<times>, C<wait>, C<waitpid>
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145
146=item Keywords related to perl modules
147
22fae026 148C<do>, C<import>, C<no>, C<package>, C<require>, C<use>
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149
150=item Keywords related to classes and object-orientedness
151
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152C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
153C<untie>, C<use>
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154
155=item Low-level socket functions
156
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157C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
158C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
159C<socket>, C<socketpair>
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160
161=item System V interprocess communication functions
162
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163C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
164C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>
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165
166=item Fetching user and group info
167
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168C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
169C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
170C<getpwuid>, C<setgrent>, C<setpwent>
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171
172=item Fetching network info
173
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174C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
175C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
176C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
177C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
178C<setnetent>, C<setprotoent>, C<setservent>
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179
180=item Time-related functions
181
22fae026 182C<gmtime>, C<localtime>, C<time>, C<times>
cb1a09d0 183
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184=item Functions new in perl5
185
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186C<abs>, C<bless>, C<chomp>, C<chr>, C<exists>, C<formline>, C<glob>,
187C<import>, C<lc>, C<lcfirst>, C<map>, C<my>, C<no>, C<prototype>, C<qx>,
188C<qw>, C<readline>, C<readpipe>, C<ref>, C<sub*>, C<sysopen>, C<tie>,
189C<tied>, C<uc>, C<ucfirst>, C<untie>, C<use>
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190
191* - C<sub> was a keyword in perl4, but in perl5 it is an
192operator which can be used in expressions.
193
194=item Functions obsoleted in perl5
195
22fae026 196C<dbmclose>, C<dbmopen>
37798a01 197
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198=back
199
200=head2 Alphabetical Listing of Perl Functions
201
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202=over 8
203
22fae026 204=item I<-X> FILEHANDLE
a0d0e21e 205
22fae026 206=item I<-X> EXPR
a0d0e21e 207
22fae026 208=item I<-X>
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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.
54310121 232 -s File has nonzero size (returns size).
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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>,
5f05dabc 255C<-W>, C<-x>, and C<-X> is based solely on the mode of the file and the
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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,
5f05dabc 258C<-r>, C<-R>, C<-w>, and C<-W> always return 1, and C<-x> and C<-X> return
a0d0e21e 2591 if any execute bit is set in the mode. Scripts run by the superuser may
5f05dabc 260thus need to do a stat() to determine the actual mode of the
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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
184e9718 277characters with the high bit set. If too many odd characters (E<gt>30%)
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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
54310121 282file, or a file at EOF when testing a filehandle. Because you have to
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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>.
a0d0e21e 285
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286If any of the file tests (or either the stat() or lstat() operators) are given
287the special filehandle consisting of a solitary underline, then the stat
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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
54310121 307=item abs
bbce6d69 308
a0d0e21e 309Returns the absolute value of its argument.
bbce6d69 310If VALUE is omitted, uses $_.
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311
312=item accept NEWSOCKET,GENERICSOCKET
313
314Accepts an incoming socket connect, just as the accept(2) system call
315does. Returns the packed address if it succeeded, FALSE otherwise.
4633a7c4 316See example in L<perlipc/"Sockets: Client/Server Communication">.
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317
318=item alarm SECONDS
319
54310121 320=item alarm
bbce6d69 321
a0d0e21e 322Arranges to have a SIGALRM delivered to this process after the
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323specified number of seconds have elapsed. If SECONDS is not specified,
324the value stored in $_ is used. (On some machines,
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325unfortunately, the elapsed time may be up to one second less than you
326specified because of how seconds are counted.) Only one timer may be
327counting at once. Each call disables the previous timer, and an
328argument of 0 may be supplied to cancel the previous timer without
329starting a new one. The returned value is the amount of time remaining
330on the previous timer.
331
4633a7c4 332For delays of finer granularity than one second, you may use Perl's
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333syscall() interface to access setitimer(2) if your system supports it,
334or else see L</select()>. It is usually a mistake to intermix alarm()
4633a7c4 335and sleep() calls.
a0d0e21e 336
ff68c719 337If you want to use alarm() to time out a system call you need to use an
2f9daede 338eval/die pair. You can't rely on the alarm causing the system call to
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339fail with $! set to EINTR because Perl sets up signal handlers to
340restart system calls on some systems. Using eval/die always works.
341
342 eval {
28757baa 343 local $SIG{ALRM} = sub { die "alarm\n" }; # NB \n required
36477c24 344 alarm $timeout;
ff68c719 345 $nread = sysread SOCKET, $buffer, $size;
36477c24 346 alarm 0;
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347 };
348 die if $@ && $@ ne "alarm\n"; # propagate errors
349 if ($@) {
350 # timed out
351 }
352 else {
353 # didn't
354 }
355
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356=item atan2 Y,X
357
358Returns the arctangent of Y/X in the range -PI to PI.
359
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360For the tangent operation, you may use the POSIX::tan()
361function, or use the familiar relation:
362
363 sub tan { sin($_[0]) / cos($_[0]) }
364
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365=item bind SOCKET,NAME
366
367Binds a network address to a socket, just as the bind system call
368does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
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369packed address of the appropriate type for the socket. See the examples in
370L<perlipc/"Sockets: Client/Server Communication">.
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371
372=item binmode FILEHANDLE
373
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374Arranges for the file to be read or written in "binary" mode in operating
375systems that distinguish between binary and text files. Files that are
376not in binary mode have CR LF sequences translated to LF on input and LF
54310121 377translated to CR LF on output. Binmode has no effect under Unix; in MS-DOS
cb1a09d0 378and similarly archaic systems, it may be imperative--otherwise your
54310121 379MS-DOS-damaged C library may mangle your file. The key distinction between
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380systems that need binmode and those that don't is their text file
381formats. Systems like Unix and Plan9 that delimit lines with a single
382character, and that encode that character in C as '\n', do not need
383C<binmode>. The rest need it. If FILEHANDLE is an expression, the value
384is taken as the name of the filehandle.
a0d0e21e 385
4633a7c4 386=item bless REF,CLASSNAME
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387
388=item bless REF
389
28757baa 390This function tells the thingy referenced by REF that it is now
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391an object in the CLASSNAME package--or the current package if no CLASSNAME
392is specified, which is often the case. It returns the reference for
5f05dabc 393convenience, because a bless() is often the last thing in a constructor.
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394Always use the two-argument version if the function doing the blessing
395might be inherited by a derived class. See L<perlobj> for more about the
396blessing (and blessings) of objects.
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397
398=item caller EXPR
399
400=item caller
401
402Returns the context of the current subroutine call. In a scalar context,
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403returns the caller's package name if there is a caller, that is, if
404we're in a subroutine or eval() or require(), and the undefined value
405otherwise. In a list context, returns
a0d0e21e 406
748a9306 407 ($package, $filename, $line) = caller;
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408
409With EXPR, it returns some extra information that the debugger uses to
410print a stack trace. The value of EXPR indicates how many call frames
411to go back before the current one.
412
54310121 413 ($package, $filename, $line, $subroutine,
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414 $hasargs, $wantarray, $evaltext, $is_require) = caller($i);
415
416Here $subroutine may be C<"(eval)"> if the frame is not a subroutine
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417call, but an C<eval>. In such a case additional elements $evaltext and
418$is_require are set: $is_require is true if the frame is created by a
419C<require> or C<use> statement, $evaltext contains the text of the
420C<eval EXPR> statement. In particular, for a C<eval BLOCK> statement,
421$filename is C<"(eval)">, but $evaltext is undefined. (Note also that
422each C<use> statement creates a C<require> frame inside an C<eval EXPR>)
423frame.
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424
425Furthermore, when called from within the DB package, caller returns more
4633a7c4 426detailed information: it sets the list variable @DB::args to be the
54310121 427arguments with which the subroutine was invoked.
748a9306 428
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429=item chdir EXPR
430
431Changes the working directory to EXPR, if possible. If EXPR is
432omitted, changes to home directory. Returns TRUE upon success, FALSE
433otherwise. See example under die().
434
435=item chmod LIST
436
437Changes the permissions of a list of files. The first element of the
4633a7c4 438list must be the numerical mode, which should probably be an octal
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439number, and which definitely should I<not> a string of octal digits:
440C<0644> is okay, C<'0644'> is not. Returns the number of files
dc848c6f 441successfully changed. See also L</oct>, if all you have is a string.
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442
443 $cnt = chmod 0755, 'foo', 'bar';
444 chmod 0755, @executables;
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445 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to --w----r-T
446 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
447 $mode = 0644; chmod $mode, 'foo'; # this is best
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448
449=item chomp VARIABLE
450
451=item chomp LIST
452
453=item chomp
454
3e3baf6d 455This is a slightly safer version of L</chop>. It removes any
a0d0e21e 456line ending that corresponds to the current value of C<$/> (also known as
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457$INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
458number of characters removed from all its arguments. It's often used to
459remove the newline from the end of an input record when you're worried
460that the final record may be missing its newline. When in paragraph mode
461(C<$/ = "">), it removes all trailing newlines from the string. If
462VARIABLE is omitted, it chomps $_. Example:
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463
464 while (<>) {
465 chomp; # avoid \n on last field
466 @array = split(/:/);
467 ...
468 }
469
470You can actually chomp anything that's an lvalue, including an assignment:
471
472 chomp($cwd = `pwd`);
473 chomp($answer = <STDIN>);
474
475If you chomp a list, each element is chomped, and the total number of
476characters removed is returned.
477
478=item chop VARIABLE
479
480=item chop LIST
481
482=item chop
483
484Chops off the last character of a string and returns the character
485chopped. It's used primarily to remove the newline from the end of an
486input record, but is much more efficient than C<s/\n//> because it neither
487scans nor copies the string. If VARIABLE is omitted, chops $_.
488Example:
489
490 while (<>) {
491 chop; # avoid \n on last field
492 @array = split(/:/);
493 ...
494 }
495
496You can actually chop anything that's an lvalue, including an assignment:
497
498 chop($cwd = `pwd`);
499 chop($answer = <STDIN>);
500
501If you chop a list, each element is chopped. Only the value of the
502last chop is returned.
503
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504Note that chop returns the last character. To return all but the last
505character, use C<substr($string, 0, -1)>.
506
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507=item chown LIST
508
509Changes the owner (and group) of a list of files. The first two
510elements of the list must be the I<NUMERICAL> uid and gid, in that order.
511Returns the number of files successfully changed.
512
513 $cnt = chown $uid, $gid, 'foo', 'bar';
514 chown $uid, $gid, @filenames;
515
54310121 516Here's an example that looks up nonnumeric uids in the passwd file:
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517
518 print "User: ";
519 chop($user = <STDIN>);
520 print "Files: "
521 chop($pattern = <STDIN>);
522
523 ($login,$pass,$uid,$gid) = getpwnam($user)
524 or die "$user not in passwd file";
525
526 @ary = <${pattern}>; # expand filenames
527 chown $uid, $gid, @ary;
528
54310121 529On most systems, you are not allowed to change the ownership of the
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530file unless you're the superuser, although you should be able to change
531the group to any of your secondary groups. On insecure systems, these
532restrictions may be relaxed, but this is not a portable assumption.
533
a0d0e21e
LW
534=item chr NUMBER
535
54310121 536=item chr
bbce6d69 537
a0d0e21e 538Returns the character represented by that NUMBER in the character set.
dc848c6f 539For example, C<chr(65)> is "A" in ASCII. For the reverse, use L</ord>.
a0d0e21e 540
bbce6d69
PP
541If NUMBER is omitted, uses $_.
542
a0d0e21e
LW
543=item chroot FILENAME
544
54310121 545=item chroot
bbce6d69 546
4633a7c4
LW
547This function works as the system call by the same name: it makes the
548named directory the new root directory for all further pathnames that
549begin with a "/" by your process and all of its children. (It doesn't
28757baa 550change your current working directory, which is unaffected.) For security
4633a7c4
LW
551reasons, this call is restricted to the superuser. If FILENAME is
552omitted, does chroot to $_.
a0d0e21e
LW
553
554=item close FILEHANDLE
555
556Closes the file or pipe associated with the file handle, returning TRUE
557only if stdio successfully flushes buffers and closes the system file
fb73857a
PP
558descriptor.
559
560You don't have to close FILEHANDLE if you are immediately going to do
561another open() on it, because open() will close it for you. (See
a0d0e21e 562open().) However, an explicit close on an input file resets the line
fb73857a
PP
563counter ($.), while the implicit close done by open() does not.
564
565If the file handle came from a piped open C<close> will additionally
566return FALSE if one of the other system calls involved fails or if the
567program exits with non-zero status. (If the only problem was that the
568program exited non-zero $! will be set to 0.) Also, closing a pipe will
569wait for the process executing on the pipe to complete, in case you
570want to look at the output of the pipe afterwards. Closing a pipe
571explicitly also puts the exit status value of the command into C<$?>.
572Example:
a0d0e21e 573
fb73857a
PP
574 open(OUTPUT, '|sort >foo') # pipe to sort
575 or die "Can't start sort: $!";
a0d0e21e 576 ... # print stuff to output
fb73857a
PP
577 close OUTPUT # wait for sort to finish
578 or warn $! ? "Error closing sort pipe: $!"
579 : "Exit status $? from sort";
580 open(INPUT, 'foo') # get sort's results
581 or die "Can't open 'foo' for input: $!";
a0d0e21e
LW
582
583FILEHANDLE may be an expression whose value gives the real filehandle name.
584
585=item closedir DIRHANDLE
586
587Closes a directory opened by opendir().
588
589=item connect SOCKET,NAME
590
591Attempts to connect to a remote socket, just as the connect system call
592does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
4633a7c4
LW
593packed address of the appropriate type for the socket. See the examples in
594L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 595
cb1a09d0
AD
596=item continue BLOCK
597
598Actually a flow control statement rather than a function. If there is a
599C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
600C<foreach>), it is always executed just before the conditional is about to
601be evaluated again, just like the third part of a C<for> loop in C. Thus
602it can be used to increment a loop variable, even when the loop has been
603continued via the C<next> statement (which is similar to the C C<continue>
604statement).
605
1d2dff63
GS
606C<last>, C<next>, or C<redo> may appear within a C<continue>
607block. C<last> and C<redo> will behave as if they had been executed within
608the main block. So will C<next>, but since it will execute a C<continue>
609block, it may be more entertaining.
610
611 while (EXPR) {
612 ### redo always comes here
613 do_something;
614 } continue {
615 ### next always comes here
616 do_something_else;
617 # then back the top to re-check EXPR
618 }
619 ### last always comes here
620
621Omitting the C<continue> section is semantically equivalent to using an
622empty one, logically enough. In that case, C<next> goes directly back
623to check the condition at the top of the loop.
624
a0d0e21e
LW
625=item cos EXPR
626
627Returns the cosine of EXPR (expressed in radians). If EXPR is omitted
628takes cosine of $_.
629
28757baa
PP
630For the inverse cosine operation, you may use the POSIX::acos()
631function, or use this relation:
632
633 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
634
a0d0e21e
LW
635=item crypt PLAINTEXT,SALT
636
4633a7c4
LW
637Encrypts a string exactly like the crypt(3) function in the C library
638(assuming that you actually have a version there that has not been
639extirpated as a potential munition). This can prove useful for checking
640the password file for lousy passwords, amongst other things. Only the
641guys wearing white hats should do this.
a0d0e21e 642
11155c91
CS
643Note that crypt is intended to be a one-way function, much like breaking
644eggs to make an omelette. There is no (known) corresponding decrypt
645function. As a result, this function isn't all that useful for
646cryptography. (For that, see your nearby CPAN mirror.)
2f9daede 647
a0d0e21e
LW
648Here's an example that makes sure that whoever runs this program knows
649their own password:
650
651 $pwd = (getpwuid($<))[1];
652 $salt = substr($pwd, 0, 2);
653
654 system "stty -echo";
655 print "Password: ";
656 chop($word = <STDIN>);
657 print "\n";
658 system "stty echo";
659
660 if (crypt($word, $salt) ne $pwd) {
661 die "Sorry...\n";
662 } else {
663 print "ok\n";
54310121 664 }
a0d0e21e 665
9f8f0c9d 666Of course, typing in your own password to whoever asks you
748a9306 667for it is unwise.
a0d0e21e 668
aa689395 669=item dbmclose HASH
a0d0e21e
LW
670
671[This function has been superseded by the untie() function.]
672
aa689395 673Breaks the binding between a DBM file and a hash.
a0d0e21e 674
aa689395 675=item dbmopen HASH,DBNAME,MODE
a0d0e21e
LW
676
677[This function has been superseded by the tie() function.]
678
7b8d334a 679This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
aa689395
PP
680hash. HASH is the name of the hash. (Unlike normal open, the first
681argument is I<NOT> a filehandle, even though it looks like one). DBNAME
682is the name of the database (without the F<.dir> or F<.pag> extension if
683any). If the database does not exist, it is created with protection
684specified by MODE (as modified by the umask()). If your system supports
685only the older DBM functions, you may perform only one dbmopen() in your
686program. In older versions of Perl, if your system had neither DBM nor
687ndbm, calling dbmopen() produced a fatal error; it now falls back to
688sdbm(3).
689
690If you don't have write access to the DBM file, you can only read hash
691variables, not set them. If you want to test whether you can write,
692either use file tests or try setting a dummy hash entry inside an eval(),
693which will trap the error.
a0d0e21e 694
1d2dff63
GS
695Note that functions such as keys() and values() may return huge lists
696when used on large DBM files. You may prefer to use the each()
a0d0e21e
LW
697function to iterate over large DBM files. Example:
698
699 # print out history file offsets
700 dbmopen(%HIST,'/usr/lib/news/history',0666);
701 while (($key,$val) = each %HIST) {
702 print $key, ' = ', unpack('L',$val), "\n";
703 }
704 dbmclose(%HIST);
705
cb1a09d0 706See also L<AnyDBM_File> for a more general description of the pros and
184e9718 707cons of the various dbm approaches, as well as L<DB_File> for a particularly
cb1a09d0 708rich implementation.
4633a7c4 709
a0d0e21e
LW
710=item defined EXPR
711
54310121 712=item defined
bbce6d69 713
2f9daede
TPG
714Returns a Boolean value telling whether EXPR has a value other than
715the undefined value C<undef>. If EXPR is not present, C<$_> will be
716checked.
717
718Many operations return C<undef> to indicate failure, end of file,
719system error, uninitialized variable, and other exceptional
720conditions. This function allows you to distinguish C<undef> from
721other values. (A simple Boolean test will not distinguish among
722C<undef>, zero, the empty string, and "0", which are all equally
723false.) Note that since C<undef> is a valid scalar, its presence
724doesn't I<necessarily> indicate an exceptional condition: pop()
725returns C<undef> when its argument is an empty array, I<or> when the
726element to return happens to be C<undef>.
727
728You may also use defined() to check whether a subroutine exists. On
729the other hand, use of defined() upon aggregates (hashes and arrays)
730is not guaranteed to produce intuitive results, and should probably be
731avoided.
732
733When used on a hash element, it tells you whether the value is defined,
dc848c6f 734not whether the key exists in the hash. Use L</exists> for the latter
2f9daede 735purpose.
a0d0e21e
LW
736
737Examples:
738
739 print if defined $switch{'D'};
740 print "$val\n" while defined($val = pop(@ary));
741 die "Can't readlink $sym: $!"
742 unless defined($value = readlink $sym);
a0d0e21e 743 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
2f9daede 744 $debugging = 0 unless defined $debugging;
a0d0e21e 745
2f9daede
TPG
746Note: Many folks tend to overuse defined(), and then are surprised to
747discover that the number 0 and "" (the zero-length string) are, in fact,
748defined values. For example, if you say
a5f75d66
AD
749
750 "ab" =~ /a(.*)b/;
751
752the pattern match succeeds, and $1 is defined, despite the fact that it
753matched "nothing". But it didn't really match nothing--rather, it
754matched something that happened to be 0 characters long. This is all
755very above-board and honest. When a function returns an undefined value,
2f9daede
TPG
756it's an admission that it couldn't give you an honest answer. So you
757should use defined() only when you're questioning the integrity of what
758you're trying to do. At other times, a simple comparison to 0 or "" is
759what you want.
760
761Currently, using defined() on an entire array or hash reports whether
762memory for that aggregate has ever been allocated. So an array you set
763to the empty list appears undefined initially, and one that once was full
764and that you then set to the empty list still appears defined. You
765should instead use a simple test for size:
28757baa
PP
766
767 if (@an_array) { print "has array elements\n" }
768 if (%a_hash) { print "has hash members\n" }
769
770Using undef() on these, however, does clear their memory and then report
771them as not defined anymore, but you shoudln't do that unless you don't
772plan to use them again, because it saves time when you load them up
773again to have memory already ready to be filled.
774
54310121 775This counterintuitive behaviour of defined() on aggregates may be
28757baa
PP
776changed, fixed, or broken in a future release of Perl.
777
dc848c6f 778See also L</undef>, L</exists>, L</ref>.
2f9daede 779
a0d0e21e
LW
780=item delete EXPR
781
aa689395
PP
782Deletes the specified key(s) and their associated values from a hash.
783For each key, returns the deleted value associated with that key, or
784the undefined value if there was no such key. Deleting from C<$ENV{}>
785modifies the environment. Deleting from a hash tied to a DBM file
5f05dabc
PP
786deletes the entry from the DBM file. (But deleting from a tie()d hash
787doesn't necessarily return anything.)
a0d0e21e 788
aa689395 789The following deletes all the values of a hash:
a0d0e21e 790
5f05dabc
PP
791 foreach $key (keys %HASH) {
792 delete $HASH{$key};
a0d0e21e
LW
793 }
794
5f05dabc
PP
795And so does this:
796
797 delete @HASH{keys %HASH}
798
799(But both of these are slower than the undef() command.) Note that the
800EXPR can be arbitrarily complicated as long as the final operation is a
801hash element lookup or hash slice:
a0d0e21e
LW
802
803 delete $ref->[$x][$y]{$key};
5f05dabc 804 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
a0d0e21e
LW
805
806=item die LIST
807
808Outside of an eval(), prints the value of LIST to C<STDERR> and exits with
184e9718 809the current value of C<$!> (errno). If C<$!> is 0, exits with the value of
54310121 810C<($? E<gt>E<gt> 8)> (backtick `command` status). If C<($? E<gt>E<gt> 8)>
28757baa
PP
811is 0, exits with 255. Inside an eval(), the error message is stuffed into
812C<$@>, and the eval() is terminated with the undefined value; this makes
813die() the way to raise an exception.
a0d0e21e
LW
814
815Equivalent examples:
816
817 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
54310121 818 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
a0d0e21e
LW
819
820If the value of EXPR does not end in a newline, the current script line
821number and input line number (if any) are also printed, and a newline
822is supplied. Hint: sometimes appending ", stopped" to your message
823will cause it to make better sense when the string "at foo line 123" is
824appended. Suppose you are running script "canasta".
825
826 die "/etc/games is no good";
827 die "/etc/games is no good, stopped";
828
829produce, respectively
830
831 /etc/games is no good at canasta line 123.
832 /etc/games is no good, stopped at canasta line 123.
833
834See also exit() and warn().
835
fb73857a
PP
836If LIST is empty and $@ already contains a value (typically from a
837previous eval) that value is reused after appending "\t...propagated".
838This is useful for propagating exceptions:
839
840 eval { ... };
841 die unless $@ =~ /Expected exception/;
842
843If $@ is empty then the string "Died" is used.
844
774d564b
PP
845You can arrange for a callback to be called just before the die() does
846its deed, by setting the C<$SIG{__DIE__}> hook. The associated handler
847will be called with the error text and can change the error message, if
fb73857a
PP
848it sees fit, by calling die() again. See L<perlvar/$SIG{expr}> for details on
849setting C<%SIG> entries, and L<"eval BLOCK"> for some examples.
850
851Note that the C<$SIG{__DIE__}> hook is called even inside eval()ed
852blocks/strings. If one wants the hook to do nothing in such
853situations, put
854
855 die @_ if $^S;
856
857as the first line of the handler (see L<perlvar/$^S>).
774d564b 858
a0d0e21e
LW
859=item do BLOCK
860
861Not really a function. Returns the value of the last command in the
862sequence of commands indicated by BLOCK. When modified by a loop
863modifier, executes the BLOCK once before testing the loop condition.
864(On other statements the loop modifiers test the conditional first.)
865
866=item do SUBROUTINE(LIST)
867
868A deprecated form of subroutine call. See L<perlsub>.
869
870=item do EXPR
871
872Uses the value of EXPR as a filename and executes the contents of the
873file as a Perl script. Its primary use is to include subroutines
874from a Perl subroutine library.
875
876 do 'stat.pl';
877
878is just like
879
fb73857a 880 scalar eval `cat stat.pl`;
a0d0e21e
LW
881
882except that it's more efficient, more concise, keeps track of the
883current filename for error messages, and searches all the B<-I>
884libraries if the file isn't in the current directory (see also the @INC
dc1be6b5
GS
885array in L<perlvar/Predefined Names>). It is also different in how
886code evaluated with C<do FILENAME> doesn't see lexicals in the enclosing
887scope like C<eval STRING> does. It's the same, however, in that it does
54310121 888reparse the file every time you call it, so you probably don't want to
a0d0e21e
LW
889do this inside a loop.
890
891Note that inclusion of library modules is better done with the
4633a7c4
LW
892use() and require() operators, which also do error checking
893and raise an exception if there's a problem.
a0d0e21e
LW
894
895=item dump LABEL
896
897This causes an immediate core dump. Primarily this is so that you can
898use the B<undump> program to turn your core dump into an executable binary
899after having initialized all your variables at the beginning of the
900program. When the new binary is executed it will begin by executing a
901C<goto LABEL> (with all the restrictions that C<goto> suffers). Think of
902it as a goto with an intervening core dump and reincarnation. If LABEL
903is omitted, restarts the program from the top. WARNING: any files
904opened at the time of the dump will NOT be open any more when the
905program is reincarnated, with possible resulting confusion on the part
906of Perl. See also B<-u> option in L<perlrun>.
907
908Example:
909
910 #!/usr/bin/perl
911 require 'getopt.pl';
912 require 'stat.pl';
913 %days = (
914 'Sun' => 1,
915 'Mon' => 2,
916 'Tue' => 3,
917 'Wed' => 4,
918 'Thu' => 5,
919 'Fri' => 6,
920 'Sat' => 7,
921 );
922
923 dump QUICKSTART if $ARGV[0] eq '-d';
924
925 QUICKSTART:
926 Getopt('f');
927
aa689395
PP
928=item each HASH
929
1d2dff63 930When called in a list context, returns a 2-element list consisting of the
aa689395
PP
931key and value for the next element of a hash, so that you can iterate over
932it. When called in a scalar context, returns the key for only the next
2f9daede
TPG
933element in the hash. (Note: Keys may be "0" or "", which are logically
934false; you may wish to avoid constructs like C<while ($k = each %foo) {}>
935for this reason.)
936
937Entries are returned in an apparently random order. When the hash is
938entirely read, a null array is returned in list context (which when
939assigned produces a FALSE (0) value), and C<undef> is returned in a
940scalar context. The next call to each() after that will start iterating
941again. There is a single iterator for each hash, shared by all each(),
942keys(), and values() function calls in the program; it can be reset by
943reading all the elements from the hash, or by evaluating C<keys HASH> or
944C<values HASH>. If you add or delete elements of a hash while you're
945iterating over it, you may get entries skipped or duplicated, so don't.
aa689395
PP
946
947The following prints out your environment like the printenv(1) program,
948only in a different order:
a0d0e21e
LW
949
950 while (($key,$value) = each %ENV) {
951 print "$key=$value\n";
952 }
953
954See also keys() and values().
955
956=item eof FILEHANDLE
957
4633a7c4
LW
958=item eof ()
959
a0d0e21e
LW
960=item eof
961
962Returns 1 if the next read on FILEHANDLE will return end of file, or if
963FILEHANDLE is not open. FILEHANDLE may be an expression whose value
964gives the real filehandle name. (Note that this function actually
965reads a character and then ungetc()s it, so it is not very useful in an
748a9306
LW
966interactive context.) Do not read from a terminal file (or call
967C<eof(FILEHANDLE)> on it) after end-of-file is reached. Filetypes such
968as terminals may lose the end-of-file condition if you do.
969
970An C<eof> without an argument uses the last file read as argument.
2f9daede
TPG
971Empty parentheses () may be used to indicate the pseudo file formed of
972the files listed on the command line, i.e., C<eof()> is reasonable to
973use inside a C<while (E<lt>E<gt>)> loop to detect the end of only the
974last file. Use C<eof(ARGV)> or eof without the parentheses to test
975I<EACH> file in a while (E<lt>E<gt>) loop. Examples:
a0d0e21e 976
748a9306
LW
977 # reset line numbering on each input file
978 while (<>) {
979 print "$.\t$_";
980 close(ARGV) if (eof); # Not eof().
981 }
982
a0d0e21e
LW
983 # insert dashes just before last line of last file
984 while (<>) {
985 if (eof()) {
986 print "--------------\n";
748a9306
LW
987 close(ARGV); # close or break; is needed if we
988 # are reading from the terminal
a0d0e21e
LW
989 }
990 print;
991 }
992
a0d0e21e 993Practical hint: you almost never need to use C<eof> in Perl, because the
54310121 994input operators return undef when they run out of data.
a0d0e21e
LW
995
996=item eval EXPR
997
998=item eval BLOCK
999
c7cc6f1c
GS
1000In the first form, the return value of EXPR is parsed and executed as if it
1001were a little Perl program. The value of the expression (which is itself
1002determined within a scalar context) is first parsed, and if there are no
1003errors, executed in the context of the current Perl program, so that any
5f05dabc 1004variable settings or subroutine and format definitions remain afterwards.
c7cc6f1c
GS
1005Note that the value is parsed every time the eval executes. If EXPR is
1006omitted, evaluates C<$_>. This form is typically used to delay parsing
1007and subsequent execution of the text of EXPR until run time.
1008
1009In the second form, the code within the BLOCK is parsed only once--at the
1010same time the code surrounding the eval itself was parsed--and executed
1011within the context of the current Perl program. This form is typically
1012used to trap exceptions more efficiently than the first (see below), while
1013also providing the benefit of checking the code within BLOCK at compile
1014time.
1015
1016The final semicolon, if any, may be omitted from the value of EXPR or within
1017the BLOCK.
1018
1019In both forms, the value returned is the value of the last expression
1020evaluated inside the mini-program, or a return statement may be used, just
1021as with subroutines. The expression providing the return value is evaluated
1022in void, scalar or array context, depending on the context of the eval itself.
1023See L</wantarray> for more on how the evaluation context can be determined.
a0d0e21e
LW
1024
1025If there is a syntax error or runtime error, or a die() statement is
1026executed, an undefined value is returned by eval(), and C<$@> is set to the
1027error message. If there was no error, C<$@> is guaranteed to be a null
c7cc6f1c
GS
1028string. Beware that using eval() neither silences perl from printing
1029warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1030To do either of those, you have to use the C<$SIG{__WARN__}> facility. See
1031L</warn> and L<perlvar>.
a0d0e21e 1032
5f05dabc 1033Note that, because eval() traps otherwise-fatal errors, it is useful for
4633a7c4 1034determining whether a particular feature (such as socket() or symlink())
a0d0e21e
LW
1035is implemented. It is also Perl's exception trapping mechanism, where
1036the die operator is used to raise exceptions.
1037
1038If the code to be executed doesn't vary, you may use the eval-BLOCK
1039form to trap run-time errors without incurring the penalty of
1040recompiling each time. The error, if any, is still returned in C<$@>.
1041Examples:
1042
54310121 1043 # make divide-by-zero nonfatal
a0d0e21e
LW
1044 eval { $answer = $a / $b; }; warn $@ if $@;
1045
1046 # same thing, but less efficient
1047 eval '$answer = $a / $b'; warn $@ if $@;
1048
1049 # a compile-time error
1050 eval { $answer = };
1051
1052 # a run-time error
1053 eval '$answer ='; # sets $@
1054
774d564b
PP
1055When using the eval{} form as an exception trap in libraries, you may
1056wish not to trigger any C<__DIE__> hooks that user code may have
1057installed. You can use the C<local $SIG{__DIE__}> construct for this
1058purpose, as shown in this example:
1059
1060 # a very private exception trap for divide-by-zero
1061 eval { local $SIG{'__DIE__'}; $answer = $a / $b; }; warn $@ if $@;
1062
1063This is especially significant, given that C<__DIE__> hooks can call
1064die() again, which has the effect of changing their error messages:
1065
1066 # __DIE__ hooks may modify error messages
1067 {
1068 local $SIG{'__DIE__'} = sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
c7cc6f1c
GS
1069 eval { die "foo lives here" };
1070 print $@ if $@; # prints "bar lives here"
774d564b
PP
1071 }
1072
54310121 1073With an eval(), you should be especially careful to remember what's
a0d0e21e
LW
1074being looked at when:
1075
1076 eval $x; # CASE 1
1077 eval "$x"; # CASE 2
1078
1079 eval '$x'; # CASE 3
1080 eval { $x }; # CASE 4
1081
1082 eval "\$$x++" # CASE 5
1083 $$x++; # CASE 6
1084
2f9daede
TPG
1085Cases 1 and 2 above behave identically: they run the code contained in
1086the variable $x. (Although case 2 has misleading double quotes making
1087the reader wonder what else might be happening (nothing is).) Cases 3
1088and 4 likewise behave in the same way: they run the code '$x', which
1089does nothing but return the value of C<$x>. (Case 4 is preferred for
1090purely visual reasons, but it also has the advantage of compiling at
1091compile-time instead of at run-time.) Case 5 is a place where
54310121 1092normally you I<WOULD> like to use double quotes, except that in this
2f9daede
TPG
1093particular situation, you can just use symbolic references instead, as
1094in case 6.
a0d0e21e
LW
1095
1096=item exec LIST
1097
8bf3b016
GS
1098=item exec PROGRAM LIST
1099
fb73857a
PP
1100The exec() function executes a system command I<AND NEVER RETURNS> -
1101use system() instead of exec() if you want it to return. It fails and
1102returns FALSE only if the command does not exist I<and> it is executed
1103directly instead of via your system's command shell (see below).
a0d0e21e 1104
55d729e4
GS
1105Since it's a common mistake to use system() instead of exec(), Perl
1106warns you if there is a following statement which isn't die(), warn()
1107or exit() (if C<-w> is set - but you always do that). If you
1108I<really> want to follow an exec() with some other statement, you
1109can use one of these styles to avoid the warning:
1110
1111 exec ('foo') or print STDERR "couldn't exec foo";
1112 { exec ('foo') }; print STDERR "couldn't exec foo";
1113
a0d0e21e
LW
1114If there is more than one argument in LIST, or if LIST is an array with
1115more than one value, calls execvp(3) with the arguments in LIST. If
1116there is only one scalar argument, the argument is checked for shell
bb32b41a
GS
1117metacharacters, and if there are any, the entire argument is passed to
1118the system's command shell for parsing (this is C</bin/sh -c> on Unix
1119platforms, but varies on other platforms). If there are no shell
1120metacharacters in the argument, it is split into words and passed
1121directly to execvp(), which is more efficient. Note: exec() and
1122system() do not flush your output buffer, so you may need to set C<$|>
1123to avoid lost output. Examples:
a0d0e21e
LW
1124
1125 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1126 exec "sort $outfile | uniq";
1127
1128If you don't really want to execute the first argument, but want to lie
1129to the program you are executing about its own name, you can specify
1130the program you actually want to run as an "indirect object" (without a
1131comma) in front of the LIST. (This always forces interpretation of the
54310121 1132LIST as a multivalued list, even if there is only a single scalar in
a0d0e21e
LW
1133the list.) Example:
1134
1135 $shell = '/bin/csh';
1136 exec $shell '-sh'; # pretend it's a login shell
1137
1138or, more directly,
1139
1140 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1141
bb32b41a
GS
1142When the arguments get executed via the system shell, results will
1143be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1144for details.
1145
a0d0e21e
LW
1146=item exists EXPR
1147
1148Returns TRUE if the specified hash key exists in its hash array, even
1149if the corresponding value is undefined.
1150
1151 print "Exists\n" if exists $array{$key};
1152 print "Defined\n" if defined $array{$key};
1153 print "True\n" if $array{$key};
1154
5f05dabc 1155A hash element can be TRUE only if it's defined, and defined if
a0d0e21e
LW
1156it exists, but the reverse doesn't necessarily hold true.
1157
1158Note that the EXPR can be arbitrarily complicated as long as the final
1159operation is a hash key lookup:
1160
1161 if (exists $ref->[$x][$y]{$key}) { ... }
1162
1163=item exit EXPR
1164
1165Evaluates EXPR and exits immediately with that value. (Actually, it
1166calls any defined C<END> routines first, but the C<END> routines may not
1167abort the exit. Likewise any object destructors that need to be called
1168are called before exit.) Example:
1169
1170 $ans = <STDIN>;
1171 exit 0 if $ans =~ /^[Xx]/;
1172
f86702cc 1173See also die(). If EXPR is omitted, exits with 0 status. The only
54310121 1174universally portable values for EXPR are 0 for success and 1 for error;
f86702cc
PP
1175all other values are subject to unpredictable interpretation depending
1176on the environment in which the Perl program is running.
a0d0e21e 1177
28757baa
PP
1178You shouldn't use exit() to abort a subroutine if there's any chance that
1179someone might want to trap whatever error happened. Use die() instead,
1180which can be trapped by an eval().
1181
a0d0e21e
LW
1182=item exp EXPR
1183
54310121 1184=item exp
bbce6d69 1185
54310121 1186Returns I<e> (the natural logarithm base) to the power of EXPR.
a0d0e21e
LW
1187If EXPR is omitted, gives C<exp($_)>.
1188
1189=item fcntl FILEHANDLE,FUNCTION,SCALAR
1190
1191Implements the fcntl(2) function. You'll probably have to say
1192
1193 use Fcntl;
1194
1195first to get the correct function definitions. Argument processing and
1196value return works just like ioctl() below. Note that fcntl() will produce
1197a fatal error if used on a machine that doesn't implement fcntl(2).
1198For example:
1199
1200 use Fcntl;
1201 fcntl($filehandle, F_GETLK, $packed_return_buffer);
1202
1203=item fileno FILEHANDLE
1204
1205Returns the file descriptor for a filehandle. This is useful for
1206constructing bitmaps for select(). If FILEHANDLE is an expression, the
1207value is taken as the name of the filehandle.
1208
1209=item flock FILEHANDLE,OPERATION
1210
8ebc5c01 1211Calls flock(2), or an emulation of it, on FILEHANDLE. Returns TRUE for
68dc0745
PP
1212success, FALSE on failure. Produces a fatal error if used on a machine
1213that doesn't implement flock(2), fcntl(2) locking, or lockf(3). flock()
1214is Perl's portable file locking interface, although it locks only entire
1215files, not records.
8ebc5c01 1216
a3cb178b
GS
1217On many platforms (including most versions or clones of Unix), locks
1218established by flock() are B<merely advisory>. This means that files
1219locked with flock() may be modified by programs which do not also use
1220flock(). Windows NT and OS/2, however, are among the platforms which
1221supply mandatory locking. See your local documentation for details.
1222
8ebc5c01
PP
1223OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1224LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
68dc0745
PP
1225you can use the symbolic names if import them from the Fcntl module,
1226either individually, or as a group using the ':flock' tag. LOCK_SH
1227requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1228releases a previously requested lock. If LOCK_NB is added to LOCK_SH or
1229LOCK_EX then flock() will return immediately rather than blocking
1230waiting for the lock (check the return status to see if you got it).
1231
1232To avoid the possibility of mis-coordination, Perl flushes FILEHANDLE
1233before (un)locking it.
8ebc5c01
PP
1234
1235Note that the emulation built with lockf(3) doesn't provide shared
1236locks, and it requires that FILEHANDLE be open with write intent. These
1237are the semantics that lockf(3) implements. Most (all?) systems
1238implement lockf(3) in terms of fcntl(2) locking, though, so the
1239differing semantics shouldn't bite too many people.
1240
1241Note also that some versions of flock() cannot lock things over the
1242network; you would need to use the more system-specific fcntl() for
1243that. If you like you can force Perl to ignore your system's flock(2)
1244function, and so provide its own fcntl(2)-based emulation, by passing
1245the switch C<-Ud_flock> to the F<Configure> program when you configure
1246perl.
4633a7c4
LW
1247
1248Here's a mailbox appender for BSD systems.
a0d0e21e 1249
7e1af8bc 1250 use Fcntl ':flock'; # import LOCK_* constants
a0d0e21e
LW
1251
1252 sub lock {
7e1af8bc 1253 flock(MBOX,LOCK_EX);
a0d0e21e
LW
1254 # and, in case someone appended
1255 # while we were waiting...
1256 seek(MBOX, 0, 2);
1257 }
1258
1259 sub unlock {
7e1af8bc 1260 flock(MBOX,LOCK_UN);
a0d0e21e
LW
1261 }
1262
1263 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1264 or die "Can't open mailbox: $!";
1265
1266 lock();
1267 print MBOX $msg,"\n\n";
1268 unlock();
1269
cb1a09d0 1270See also L<DB_File> for other flock() examples.
a0d0e21e
LW
1271
1272=item fork
1273
1274Does a fork(2) system call. Returns the child pid to the parent process
4633a7c4 1275and 0 to the child process, or C<undef> if the fork is unsuccessful.
a0d0e21e 1276Note: unflushed buffers remain unflushed in both processes, which means
28757baa
PP
1277you may need to set C<$|> ($AUTOFLUSH in English) or call the autoflush()
1278method of IO::Handle to avoid duplicate output.
a0d0e21e
LW
1279
1280If you fork() without ever waiting on your children, you will accumulate
1281zombies:
1282
4633a7c4 1283 $SIG{CHLD} = sub { wait };
a0d0e21e 1284
54310121 1285There's also the double-fork trick (error checking on
a0d0e21e
LW
1286fork() returns omitted);
1287
1288 unless ($pid = fork) {
1289 unless (fork) {
1290 exec "what you really wanna do";
1291 die "no exec";
1292 # ... or ...
4633a7c4 1293 ## (some_perl_code_here)
a0d0e21e
LW
1294 exit 0;
1295 }
1296 exit 0;
1297 }
1298 waitpid($pid,0);
1299
cb1a09d0
AD
1300See also L<perlipc> for more examples of forking and reaping
1301moribund children.
1302
28757baa
PP
1303Note that if your forked child inherits system file descriptors like
1304STDIN and STDOUT that are actually connected by a pipe or socket, even
1305if you exit, the remote server (such as, say, httpd or rsh) won't think
1306you're done. You should reopen those to /dev/null if it's any issue.
1307
cb1a09d0
AD
1308=item format
1309
7b8d334a 1310Declare a picture format for use by the write() function. For
cb1a09d0
AD
1311example:
1312
54310121 1313 format Something =
cb1a09d0
AD
1314 Test: @<<<<<<<< @||||| @>>>>>
1315 $str, $%, '$' . int($num)
1316 .
1317
1318 $str = "widget";
184e9718 1319 $num = $cost/$quantity;
cb1a09d0
AD
1320 $~ = 'Something';
1321 write;
1322
1323See L<perlform> for many details and examples.
1324
a0d0e21e 1325
8903cb82 1326=item formline PICTURE,LIST
a0d0e21e 1327
4633a7c4 1328This is an internal function used by C<format>s, though you may call it
a0d0e21e
LW
1329too. It formats (see L<perlform>) a list of values according to the
1330contents of PICTURE, placing the output into the format output
4633a7c4
LW
1331accumulator, C<$^A> (or $ACCUMULATOR in English).
1332Eventually, when a write() is done, the contents of
a0d0e21e
LW
1333C<$^A> are written to some filehandle, but you could also read C<$^A>
1334yourself and then set C<$^A> back to "". Note that a format typically
1335does one formline() per line of form, but the formline() function itself
748a9306 1336doesn't care how many newlines are embedded in the PICTURE. This means
4633a7c4 1337that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
748a9306
LW
1338You may therefore need to use multiple formlines to implement a single
1339record format, just like the format compiler.
1340
5f05dabc 1341Be careful if you put double quotes around the picture, because an "C<@>"
748a9306 1342character may be taken to mean the beginning of an array name.
4633a7c4 1343formline() always returns TRUE. See L<perlform> for other examples.
a0d0e21e
LW
1344
1345=item getc FILEHANDLE
1346
1347=item getc
1348
1349Returns the next character from the input file attached to FILEHANDLE,
1350or a null string at end of file. If FILEHANDLE is omitted, reads from STDIN.
4633a7c4 1351This is not particularly efficient. It cannot be used to get unbuffered
cb1a09d0 1352single-characters, however. For that, try something more like:
4633a7c4
LW
1353
1354 if ($BSD_STYLE) {
1355 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1356 }
1357 else {
54310121 1358 system "stty", '-icanon', 'eol', "\001";
4633a7c4
LW
1359 }
1360
1361 $key = getc(STDIN);
1362
1363 if ($BSD_STYLE) {
1364 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1365 }
1366 else {
5f05dabc 1367 system "stty", 'icanon', 'eol', '^@'; # ASCII null
4633a7c4
LW
1368 }
1369 print "\n";
1370
54310121
PP
1371Determination of whether $BSD_STYLE should be set
1372is left as an exercise to the reader.
cb1a09d0 1373
28757baa
PP
1374The POSIX::getattr() function can do this more portably on systems
1375alleging POSIX compliance.
cb1a09d0 1376See also the C<Term::ReadKey> module from your nearest CPAN site;
54310121 1377details on CPAN can be found on L<perlmod/CPAN>.
a0d0e21e
LW
1378
1379=item getlogin
1380
1381Returns the current login from F</etc/utmp>, if any. If null, use
54310121 1382getpwuid().
a0d0e21e 1383
f86702cc 1384 $login = getlogin || getpwuid($<) || "Kilroy";
a0d0e21e 1385
da0045b7 1386Do not consider getlogin() for authentication: it is not as
4633a7c4
LW
1387secure as getpwuid().
1388
a0d0e21e
LW
1389=item getpeername SOCKET
1390
1391Returns the packed sockaddr address of other end of the SOCKET connection.
1392
4633a7c4
LW
1393 use Socket;
1394 $hersockaddr = getpeername(SOCK);
1395 ($port, $iaddr) = unpack_sockaddr_in($hersockaddr);
1396 $herhostname = gethostbyaddr($iaddr, AF_INET);
1397 $herstraddr = inet_ntoa($iaddr);
a0d0e21e
LW
1398
1399=item getpgrp PID
1400
47e29363
PP
1401Returns the current process group for the specified PID. Use
1402a PID of 0 to get the current process group for the
4633a7c4 1403current process. Will raise an exception if used on a machine that
a0d0e21e 1404doesn't implement getpgrp(2). If PID is omitted, returns process
47e29363
PP
1405group of current process. Note that the POSIX version of getpgrp()
1406does not accept a PID argument, so only PID==0 is truly portable.
a0d0e21e
LW
1407
1408=item getppid
1409
1410Returns the process id of the parent process.
1411
1412=item getpriority WHICH,WHO
1413
4633a7c4
LW
1414Returns the current priority for a process, a process group, or a user.
1415(See L<getpriority(2)>.) Will raise a fatal exception if used on a
a0d0e21e
LW
1416machine that doesn't implement getpriority(2).
1417
1418=item getpwnam NAME
1419
1420=item getgrnam NAME
1421
1422=item gethostbyname NAME
1423
1424=item getnetbyname NAME
1425
1426=item getprotobyname NAME
1427
1428=item getpwuid UID
1429
1430=item getgrgid GID
1431
1432=item getservbyname NAME,PROTO
1433
1434=item gethostbyaddr ADDR,ADDRTYPE
1435
1436=item getnetbyaddr ADDR,ADDRTYPE
1437
1438=item getprotobynumber NUMBER
1439
1440=item getservbyport PORT,PROTO
1441
1442=item getpwent
1443
1444=item getgrent
1445
1446=item gethostent
1447
1448=item getnetent
1449
1450=item getprotoent
1451
1452=item getservent
1453
1454=item setpwent
1455
1456=item setgrent
1457
1458=item sethostent STAYOPEN
1459
1460=item setnetent STAYOPEN
1461
1462=item setprotoent STAYOPEN
1463
1464=item setservent STAYOPEN
1465
1466=item endpwent
1467
1468=item endgrent
1469
1470=item endhostent
1471
1472=item endnetent
1473
1474=item endprotoent
1475
1476=item endservent
1477
1478These routines perform the same functions as their counterparts in the
1479system library. Within a list context, the return values from the
1480various get routines are as follows:
1481
1482 ($name,$passwd,$uid,$gid,
6ee623d5 1483 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
a0d0e21e
LW
1484 ($name,$passwd,$gid,$members) = getgr*
1485 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1486 ($name,$aliases,$addrtype,$net) = getnet*
1487 ($name,$aliases,$proto) = getproto*
1488 ($name,$aliases,$port,$proto) = getserv*
1489
1490(If the entry doesn't exist you get a null list.)
1491
1492Within a scalar context, you get the name, unless the function was a
1493lookup by name, in which case you get the other thing, whatever it is.
1494(If the entry doesn't exist you get the undefined value.) For example:
1495
1496 $uid = getpwnam
1497 $name = getpwuid
1498 $name = getpwent
1499 $gid = getgrnam
1500 $name = getgrgid
1501 $name = getgrent
1502 etc.
1503
6ee623d5
GS
1504In I<getpw*()> the fields $quota, $comment, and $expire are special
1505cases in the sense that in many systems they are unsupported. If the
1506$quota is unsupported, it is an empty scalar. If it is supported, it
1507usually encodes the disk quota. If the $comment field is unsupported,
1508it is an empty scalar. If it is supported it usually encodes some
1509administrative comment about the user. In some systems the $quota
1510field may be $change or $age, fields that have to do with password
1511aging. In some systems the $comment field may be $class. The $expire
1512field, if present, encodes the expiration period of the account or the
1513password. For the availability and the exact meaning of these fields
1514in your system, please consult your getpwnam(3) documentation and your
1515<pwd.h> file. You can also find out from within Perl which meaning
1516your $quota and $comment fields have and whether you have the $expire
1517field by using the Config module and the values d_pwquota, d_pwage,
1518d_pwchange, d_pwcomment, and d_pwexpire.
1519
a0d0e21e
LW
1520The $members value returned by I<getgr*()> is a space separated list of
1521the login names of the members of the group.
1522
1523For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1524C, it will be returned to you via C<$?> if the function call fails. The
1525@addrs value returned by a successful call is a list of the raw
1526addresses returned by the corresponding system library call. In the
1527Internet domain, each address is four bytes long and you can unpack it
1528by saying something like:
1529
1530 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1531
1532=item getsockname SOCKET
1533
1534Returns the packed sockaddr address of this end of the SOCKET connection.
1535
4633a7c4
LW
1536 use Socket;
1537 $mysockaddr = getsockname(SOCK);
1538 ($port, $myaddr) = unpack_sockaddr_in($mysockaddr);
a0d0e21e
LW
1539
1540=item getsockopt SOCKET,LEVEL,OPTNAME
1541
1542Returns the socket option requested, or undefined if there is an error.
1543
1544=item glob EXPR
1545
0a753a76
PP
1546=item glob
1547
68dc0745
PP
1548Returns the value of EXPR with filename expansions such as a shell would
1549do. This is the internal function implementing the C<E<lt>*.cE<gt>>
1550operator, but you can use it directly. If EXPR is omitted, $_ is used.
1551The C<E<lt>*.cE<gt>> operator is discussed in more detail in
1552L<perlop/"I/O Operators">.
a0d0e21e
LW
1553
1554=item gmtime EXPR
1555
1556Converts a time as returned by the time function to a 9-element array
54310121 1557with the time localized for the standard Greenwich time zone.
4633a7c4 1558Typically used as follows:
a0d0e21e 1559
54310121 1560 # 0 1 2 3 4 5 6 7 8
a0d0e21e
LW
1561 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1562 gmtime(time);
1563
1564All array elements are numeric, and come straight out of a struct tm.
1565In particular this means that $mon has the range 0..11 and $wday has
54310121
PP
1566the range 0..6 with sunday as day 0. Also, $year is the number of
1567years since 1900, I<not> simply the last two digits of the year.
2f9daede
TPG
1568
1569If EXPR is omitted, does C<gmtime(time())>.
a0d0e21e 1570
54310121 1571In a scalar context, returns the ctime(3) value:
0a753a76
PP
1572
1573 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
1574
54310121
PP
1575Also see the timegm() function provided by the Time::Local module,
1576and the strftime(3) function available via the POSIX module.
0a753a76 1577
a0d0e21e
LW
1578=item goto LABEL
1579
748a9306
LW
1580=item goto EXPR
1581
a0d0e21e
LW
1582=item goto &NAME
1583
1584The goto-LABEL form finds the statement labeled with LABEL and resumes
1585execution there. It may not be used to go into any construct that
1586requires initialization, such as a subroutine or a foreach loop. It
0a753a76
PP
1587also can't be used to go into a construct that is optimized away,
1588or to get out of a block or subroutine given to sort().
1589It can be used to go almost anywhere else within the dynamic scope,
a0d0e21e
LW
1590including out of subroutines, but it's usually better to use some other
1591construct such as last or die. The author of Perl has never felt the
1592need to use this form of goto (in Perl, that is--C is another matter).
1593
748a9306
LW
1594The goto-EXPR form expects a label name, whose scope will be resolved
1595dynamically. This allows for computed gotos per FORTRAN, but isn't
1596necessarily recommended if you're optimizing for maintainability:
1597
1598 goto ("FOO", "BAR", "GLARCH")[$i];
1599
a0d0e21e
LW
1600The goto-&NAME form is highly magical, and substitutes a call to the
1601named subroutine for the currently running subroutine. This is used by
1602AUTOLOAD subroutines that wish to load another subroutine and then
1603pretend that the other subroutine had been called in the first place
1604(except that any modifications to @_ in the current subroutine are
1605propagated to the other subroutine.) After the goto, not even caller()
1606will be able to tell that this routine was called first.
1607
1608=item grep BLOCK LIST
1609
1610=item grep EXPR,LIST
1611
54310121 1612This is similar in spirit to, but not the same as, grep(1)
2f9daede
TPG
1613and its relatives. In particular, it is not limited to using
1614regular expressions.
1615
a0d0e21e
LW
1616Evaluates the BLOCK or EXPR for each element of LIST (locally setting
1617$_ to each element) and returns the list value consisting of those
1618elements for which the expression evaluated to TRUE. In a scalar
1619context, returns the number of times the expression was TRUE.
1620
1621 @foo = grep(!/^#/, @bar); # weed out comments
1622
1623or equivalently,
1624
1625 @foo = grep {!/^#/} @bar; # weed out comments
1626
5f05dabc 1627Note that, because $_ is a reference into the list value, it can be used
a0d0e21e
LW
1628to modify the elements of the array. While this is useful and
1629supported, it can cause bizarre results if the LIST is not a named
2f9daede 1630array. Similarly, grep returns aliases into the original list,
7b8d334a 1631much like the way that a for loops's index variable aliases the list
2f9daede 1632elements. That is, modifying an element of a list returned by grep
fb73857a 1633(for example, in a C<foreach>, C<map> or another C<grep>)
2f9daede 1634actually modifies the element in the original list.
a0d0e21e 1635
fb73857a 1636See also L</map> for an array composed of the results of the BLOCK or EXPR.
38325410 1637
a0d0e21e
LW
1638=item hex EXPR
1639
54310121 1640=item hex
bbce6d69 1641
54310121 1642Interprets EXPR as a hex string and returns the corresponding
2f9daede 1643value. (To convert strings that might start with either 0 or 0x
dc848c6f 1644see L</oct>.) If EXPR is omitted, uses $_.
2f9daede
TPG
1645
1646 print hex '0xAf'; # prints '175'
1647 print hex 'aF'; # same
a0d0e21e
LW
1648
1649=item import
1650
54310121 1651There is no builtin import() function. It is merely an ordinary
4633a7c4 1652method (subroutine) defined (or inherited) by modules that wish to export
a0d0e21e 1653names to another module. The use() function calls the import() method
54310121 1654for the package used. See also L</use()>, L<perlmod>, and L<Exporter>.
a0d0e21e
LW
1655
1656=item index STR,SUBSTR,POSITION
1657
1658=item index STR,SUBSTR
1659
4633a7c4
LW
1660Returns the position of the first occurrence of SUBSTR in STR at or after
1661POSITION. If POSITION is omitted, starts searching from the beginning of
184e9718 1662the string. The return value is based at 0 (or whatever you've set the C<$[>
4633a7c4 1663variable to--but don't do that). If the substring is not found, returns
a0d0e21e
LW
1664one less than the base, ordinarily -1.
1665
1666=item int EXPR
1667
54310121 1668=item int
bbce6d69 1669
a0d0e21e
LW
1670Returns the integer portion of EXPR. If EXPR is omitted, uses $_.
1671
1672=item ioctl FILEHANDLE,FUNCTION,SCALAR
1673
1674Implements the ioctl(2) function. You'll probably have to say
1675
4633a7c4 1676 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
a0d0e21e 1677
4633a7c4 1678first to get the correct function definitions. If F<ioctl.ph> doesn't
a0d0e21e 1679exist or doesn't have the correct definitions you'll have to roll your
4633a7c4
LW
1680own, based on your C header files such as F<E<lt>sys/ioctl.hE<gt>>.
1681(There is a Perl script called B<h2ph> that comes with the Perl kit which
54310121 1682may help you in this, but it's nontrivial.) SCALAR will be read and/or
4633a7c4
LW
1683written depending on the FUNCTION--a pointer to the string value of SCALAR
1684will be passed as the third argument of the actual ioctl call. (If SCALAR
1685has no string value but does have a numeric value, that value will be
1686passed rather than a pointer to the string value. To guarantee this to be
1687TRUE, add a 0 to the scalar before using it.) The pack() and unpack()
1688functions are useful for manipulating the values of structures used by
1689ioctl(). The following example sets the erase character to DEL.
a0d0e21e
LW
1690
1691 require 'ioctl.ph';
4633a7c4
LW
1692 $getp = &TIOCGETP;
1693 die "NO TIOCGETP" if $@ || !$getp;
a0d0e21e 1694 $sgttyb_t = "ccccs"; # 4 chars and a short
4633a7c4 1695 if (ioctl(STDIN,$getp,$sgttyb)) {
a0d0e21e
LW
1696 @ary = unpack($sgttyb_t,$sgttyb);
1697 $ary[2] = 127;
1698 $sgttyb = pack($sgttyb_t,@ary);
4633a7c4 1699 ioctl(STDIN,&TIOCSETP,$sgttyb)
a0d0e21e
LW
1700 || die "Can't ioctl: $!";
1701 }
1702
1703The return value of ioctl (and fcntl) is as follows:
1704
1705 if OS returns: then Perl returns:
1706 -1 undefined value
1707 0 string "0 but true"
1708 anything else that number
1709
1710Thus Perl returns TRUE on success and FALSE on failure, yet you can
1711still easily determine the actual value returned by the operating
1712system:
1713
1714 ($retval = ioctl(...)) || ($retval = -1);
1715 printf "System returned %d\n", $retval;
1716
1717=item join EXPR,LIST
1718
54310121 1719Joins the separate strings of LIST into a single string with
a0d0e21e
LW
1720fields separated by the value of EXPR, and returns the string.
1721Example:
1722
1723 $_ = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
1724
1725See L<perlfunc/split>.
1726
aa689395
PP
1727=item keys HASH
1728
1d2dff63
GS
1729Returns a list consisting of all the keys of the named hash. (In a
1730scalar context, returns the number of keys.) The keys are returned in
aa689395
PP
1731an apparently random order, but it is the same order as either the
1732values() or each() function produces (given that the hash has not been
1733modified). As a side effect, it resets HASH's iterator.
a0d0e21e 1734
aa689395 1735Here is yet another way to print your environment:
a0d0e21e
LW
1736
1737 @keys = keys %ENV;
1738 @values = values %ENV;
1739 while ($#keys >= 0) {
1740 print pop(@keys), '=', pop(@values), "\n";
1741 }
1742
1743or how about sorted by key:
1744
1745 foreach $key (sort(keys %ENV)) {
1746 print $key, '=', $ENV{$key}, "\n";
1747 }
1748
54310121 1749To sort an array by value, you'll need to use a C<sort> function.
aa689395 1750Here's a descending numeric sort of a hash by its values:
4633a7c4
LW
1751
1752 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash)) {
1753 printf "%4d %s\n", $hash{$key}, $key;
1754 }
1755
55497cff 1756As an lvalue C<keys> allows you to increase the number of hash buckets
aa689395
PP
1757allocated for the given hash. This can gain you a measure of efficiency if
1758you know the hash is going to get big. (This is similar to pre-extending
1759an array by assigning a larger number to $#array.) If you say
55497cff
PP
1760
1761 keys %hash = 200;
1762
1763then C<%hash> will have at least 200 buckets allocated for it. These
1764buckets will be retained even if you do C<%hash = ()>, use C<undef
1765%hash> if you want to free the storage while C<%hash> is still in scope.
1766You can't shrink the number of buckets allocated for the hash using
1767C<keys> in this way (but you needn't worry about doing this by accident,
1768as trying has no effect).
1769
a0d0e21e
LW
1770=item kill LIST
1771
54310121
PP
1772Sends a signal to a list of processes. The first element of
1773the list must be the signal to send. Returns the number of
4633a7c4 1774processes successfully signaled.
a0d0e21e
LW
1775
1776 $cnt = kill 1, $child1, $child2;
1777 kill 9, @goners;
1778
4633a7c4
LW
1779Unlike in the shell, in Perl if the I<SIGNAL> is negative, it kills
1780process groups instead of processes. (On System V, a negative I<PROCESS>
1781number will also kill process groups, but that's not portable.) That
1782means you usually want to use positive not negative signals. You may also
da0045b7 1783use a signal name in quotes. See L<perlipc/"Signals"> for details.
a0d0e21e
LW
1784
1785=item last LABEL
1786
1787=item last
1788
1789The C<last> command is like the C<break> statement in C (as used in
1790loops); it immediately exits the loop in question. If the LABEL is
1791omitted, the command refers to the innermost enclosing loop. The
1792C<continue> block, if any, is not executed:
1793
4633a7c4
LW
1794 LINE: while (<STDIN>) {
1795 last LINE if /^$/; # exit when done with header
a0d0e21e
LW
1796 ...
1797 }
1798
1d2dff63
GS
1799See also L</continue> for an illustration of how C<last>, C<next>, and
1800C<redo> work.
1801
a0d0e21e
LW
1802=item lc EXPR
1803
54310121 1804=item lc
bbce6d69 1805
a0d0e21e 1806Returns an lowercased version of EXPR. This is the internal function
54310121 1807implementing the \L escape in double-quoted strings.
a034a98d 1808Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
a0d0e21e 1809
bbce6d69
PP
1810If EXPR is omitted, uses $_.
1811
a0d0e21e
LW
1812=item lcfirst EXPR
1813
54310121 1814=item lcfirst
bbce6d69 1815
a0d0e21e
LW
1816Returns the value of EXPR with the first character lowercased. This is
1817the internal function implementing the \l escape in double-quoted strings.
a034a98d 1818Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
a0d0e21e 1819
bbce6d69
PP
1820If EXPR is omitted, uses $_.
1821
a0d0e21e
LW
1822=item length EXPR
1823
54310121 1824=item length
bbce6d69 1825
a0d0e21e
LW
1826Returns the length in characters of the value of EXPR. If EXPR is
1827omitted, returns length of $_.
1828
1829=item link OLDFILE,NEWFILE
1830
1831Creates a new filename linked to the old filename. Returns 1 for
1832success, 0 otherwise.
1833
1834=item listen SOCKET,QUEUESIZE
1835
1836Does the same thing that the listen system call does. Returns TRUE if
4633a7c4 1837it succeeded, FALSE otherwise. See example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
1838
1839=item local EXPR
1840
a0d0e21e 1841A local modifies the listed variables to be local to the enclosing block,
5f05dabc
PP
1842subroutine, C<eval{}>, or C<do>. If more than one value is listed, the
1843list must be placed in parentheses. See L<perlsub/"Temporary Values via
3e3baf6d 1844local()"> for details, including issues with tied arrays and hashes.
a0d0e21e 1845
7b8d334a
GS
1846You really probably want to be using my() instead, because local() isn't
1847what most people think of as "local". See L<perlsub/"Private Variables
cb1a09d0 1848via my()"> for details.
a0d0e21e
LW
1849
1850=item localtime EXPR
1851
1852Converts a time as returned by the time function to a 9-element array
5f05dabc 1853with the time analyzed for the local time zone. Typically used as
a0d0e21e
LW
1854follows:
1855
54310121 1856 # 0 1 2 3 4 5 6 7 8
a0d0e21e
LW
1857 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1858 localtime(time);
1859
1860All array elements are numeric, and come straight out of a struct tm.
1861In particular this means that $mon has the range 0..11 and $wday has
54310121
PP
1862the range 0..6 with sunday as day 0. Also, $year is the number of
1863years since 1900, that is, $year is 123 in year 2023.
1864
1865If EXPR is omitted, uses the current time (C<localtime(time)>).
a0d0e21e 1866
0a753a76 1867In a scalar context, returns the ctime(3) value:
a0d0e21e 1868
5f05dabc 1869 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
a0d0e21e 1870
a3cb178b
GS
1871This scalar value is B<not> locale dependent, see L<perllocale>, but
1872instead a Perl builtin. Also see the Time::Local module, and the
1873strftime(3) and mktime(3) function available via the POSIX module. To
1874get somewhat similar but locale dependent date strings, set up your
1875locale environment variables appropriately (please see L<perllocale>)
1876and try for example
1877
1878 use POSIX qw(strftime)
1879 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
1880
1881Note that the C<%a> and C<%b>, the short forms of the day of the week
1882and the month of the year, may not necessarily be three characters wide.
a0d0e21e
LW
1883
1884=item log EXPR
1885
54310121 1886=item log
bbce6d69 1887
a0d0e21e
LW
1888Returns logarithm (base I<e>) of EXPR. If EXPR is omitted, returns log
1889of $_.
1890
1891=item lstat FILEHANDLE
1892
1893=item lstat EXPR
1894
54310121 1895=item lstat
bbce6d69 1896
a0d0e21e
LW
1897Does the same thing as the stat() function, but stats a symbolic link
1898instead of the file the symbolic link points to. If symbolic links are
1899unimplemented on your system, a normal stat() is done.
1900
bbce6d69
PP
1901If EXPR is omitted, stats $_.
1902
a0d0e21e
LW
1903=item m//
1904
1905The match operator. See L<perlop>.
1906
1907=item map BLOCK LIST
1908
1909=item map EXPR,LIST
1910
1911Evaluates the BLOCK or EXPR for each element of LIST (locally setting $_ to each
1912element) and returns the list value composed of the results of each such
1913evaluation. Evaluates BLOCK or EXPR in a list context, so each element of LIST
1914may produce zero, one, or more elements in the returned value.
1915
1916 @chars = map(chr, @nums);
1917
1918translates a list of numbers to the corresponding characters. And
1919
4633a7c4 1920 %hash = map { getkey($_) => $_ } @array;
a0d0e21e
LW
1921
1922is just a funny way to write
1923
1924 %hash = ();
1925 foreach $_ (@array) {
4633a7c4 1926 $hash{getkey($_)} = $_;
a0d0e21e
LW
1927 }
1928
fb73857a
PP
1929Note that, because $_ is a reference into the list value, it can be used
1930to modify the elements of the array. While this is useful and
1931supported, it can cause bizarre results if the LIST is not a named
1932array. See also L</grep> for an array composed of those items of the
1933original list for which the BLOCK or EXPR evaluates to true.
1934
a0d0e21e
LW
1935=item mkdir FILENAME,MODE
1936
1937Creates the directory specified by FILENAME, with permissions specified
1938by MODE (as modified by umask). If it succeeds it returns 1, otherwise
184e9718 1939it returns 0 and sets C<$!> (errno).
a0d0e21e
LW
1940
1941=item msgctl ID,CMD,ARG
1942
4633a7c4 1943Calls the System V IPC function msgctl(2). If CMD is &IPC_STAT, then ARG
a0d0e21e
LW
1944must be a variable which will hold the returned msqid_ds structure.
1945Returns like ioctl: the undefined value for error, "0 but true" for
1946zero, or the actual return value otherwise.
1947
1948=item msgget KEY,FLAGS
1949
4633a7c4 1950Calls the System V IPC function msgget(2). Returns the message queue id,
a0d0e21e
LW
1951or the undefined value if there is an error.
1952
1953=item msgsnd ID,MSG,FLAGS
1954
1955Calls the System V IPC function msgsnd to send the message MSG to the
1956message queue ID. MSG must begin with the long integer message type,
c07a80fd 1957which may be created with C<pack("l", $type)>. Returns TRUE if
a0d0e21e
LW
1958successful, or FALSE if there is an error.
1959
1960=item msgrcv ID,VAR,SIZE,TYPE,FLAGS
1961
1962Calls the System V IPC function msgrcv to receive a message from
1963message queue ID into variable VAR with a maximum message size of
1964SIZE. Note that if a message is received, the message type will be the
1965first thing in VAR, and the maximum length of VAR is SIZE plus the size
1966of the message type. Returns TRUE if successful, or FALSE if there is
1967an error.
1968
1969=item my EXPR
1970
1971A "my" declares the listed variables to be local (lexically) to the
cb1a09d0 1972enclosing block, subroutine, C<eval>, or C<do/require/use>'d file. If
5f05dabc 1973more than one value is listed, the list must be placed in parentheses. See
cb1a09d0 1974L<perlsub/"Private Variables via my()"> for details.
4633a7c4 1975
a0d0e21e
LW
1976=item next LABEL
1977
1978=item next
1979
1980The C<next> command is like the C<continue> statement in C; it starts
1981the next iteration of the loop:
1982
4633a7c4
LW
1983 LINE: while (<STDIN>) {
1984 next LINE if /^#/; # discard comments
a0d0e21e
LW
1985 ...
1986 }
1987
1988Note that if there were a C<continue> block on the above, it would get
1989executed even on discarded lines. If the LABEL is omitted, the command
1990refers to the innermost enclosing loop.
1991
1d2dff63
GS
1992See also L</continue> for an illustration of how C<last>, C<next>, and
1993C<redo> work.
1994
a0d0e21e
LW
1995=item no Module LIST
1996
1997See the "use" function, which "no" is the opposite of.
1998
1999=item oct EXPR
2000
54310121 2001=item oct
bbce6d69 2002
4633a7c4 2003Interprets EXPR as an octal string and returns the corresponding
2f9daede 2004value. (If EXPR happens to start off with 0x, interprets it as
4633a7c4
LW
2005a hex string instead.) The following will handle decimal, octal, and
2006hex in the standard Perl or C notation:
a0d0e21e
LW
2007
2008 $val = oct($val) if $val =~ /^0/;
2009
2f9daede
TPG
2010If EXPR is omitted, uses $_. This function is commonly used when
2011a string such as "644" needs to be converted into a file mode, for
2012example. (Although perl will automatically convert strings into
2013numbers as needed, this automatic conversion assumes base 10.)
a0d0e21e
LW
2014
2015=item open FILEHANDLE,EXPR
2016
2017=item open FILEHANDLE
2018
2019Opens the file whose filename is given by EXPR, and associates it with
5f05dabc
PP
2020FILEHANDLE. If FILEHANDLE is an expression, its value is used as the
2021name of the real filehandle wanted. If EXPR is omitted, the scalar
2022variable of the same name as the FILEHANDLE contains the filename.
2023(Note that lexical variables--those declared with C<my>--will not work
2024for this purpose; so if you're using C<my>, specify EXPR in your call
2025to open.)
2026
2027If the filename begins with '<' or nothing, the file is opened for input.
2028If the filename begins with '>', the file is truncated and opened for
2029output. If the filename begins with '>>', the file is opened for
2030appending. You can put a '+' in front of the '>' or '<' to indicate that
2031you want both read and write access to the file; thus '+<' is almost
2032always preferred for read/write updates--the '+>' mode would clobber the
2033file first. The prefix and the filename may be separated with spaces.
2034These various prefixes correspond to the fopen(3) modes of 'r', 'r+', 'w',
2035'w+', 'a', and 'a+'.
2036
2037If the filename begins with "|", the filename is interpreted as a command
2038to which output is to be piped, and if the filename ends with a "|", the
2039filename is interpreted See L<perlipc/"Using open() for IPC"> for more
2040examples of this. as command which pipes input to us. (You may not have
7e1af8bc
PP
2041a raw open() to a command that pipes both in I<and> out, but see
2042L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
2043for alternatives.)
cb1a09d0 2044
184e9718 2045Opening '-' opens STDIN and opening 'E<gt>-' opens STDOUT. Open returns
54310121 2046nonzero upon success, the undefined value otherwise. If the open
4633a7c4 2047involved a pipe, the return value happens to be the pid of the
54310121 2048subprocess.
cb1a09d0
AD
2049
2050If you're unfortunate enough to be running Perl on a system that
2051distinguishes between text files and binary files (modern operating
2052systems don't care), then you should check out L</binmode> for tips for
2053dealing with this. The key distinction between systems that need binmode
2054and those that don't is their text file formats. Systems like Unix and
2055Plan9 that delimit lines with a single character, and that encode that
2056character in C as '\n', do not need C<binmode>. The rest need it.
2057
fb73857a
PP
2058When opening a file, it's usually a bad idea to continue normal execution
2059if the request failed, so C<open> is frequently used in connection with
2060C<die>. Even if C<die> won't do what you want (say, in a CGI script,
2061where you want to make a nicely formatted error message (but there are
2062modules which can help with that problem)) you should always check
2063the return value from opening a file. The infrequent exception is when
2064working with an unopened filehandle is actually what you want to do.
2065
cb1a09d0 2066Examples:
a0d0e21e
LW
2067
2068 $ARTICLE = 100;
2069 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2070 while (<ARTICLE>) {...
2071
2072 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
fb73857a 2073 # if the open fails, output is discarded
a0d0e21e 2074
fb73857a
PP
2075 open(DBASE, '+<dbase.mine') # open for update
2076 or die "Can't open 'dbase.mine' for update: $!";
cb1a09d0 2077
fb73857a
PP
2078 open(ARTICLE, "caesar <$article |") # decrypt article
2079 or die "Can't start caesar: $!";
a0d0e21e 2080
fb73857a
PP
2081 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
2082 or die "Can't start sort: $!";
a0d0e21e
LW
2083
2084 # process argument list of files along with any includes
2085
2086 foreach $file (@ARGV) {
2087 process($file, 'fh00');
2088 }
2089
2090 sub process {
2091 local($filename, $input) = @_;
2092 $input++; # this is a string increment
2093 unless (open($input, $filename)) {
2094 print STDERR "Can't open $filename: $!\n";
2095 return;
2096 }
2097
2098 while (<$input>) { # note use of indirection
2099 if (/^#include "(.*)"/) {
2100 process($1, $input);
2101 next;
2102 }
2103 ... # whatever
2104 }
2105 }
2106
2107You may also, in the Bourne shell tradition, specify an EXPR beginning
184e9718 2108with "E<gt>&", in which case the rest of the string is interpreted as the
a0d0e21e 2109name of a filehandle (or file descriptor, if numeric) which is to be
184e9718 2110duped and opened. You may use & after E<gt>, E<gt>E<gt>, E<lt>, +E<gt>,
5f05dabc 2111+E<gt>E<gt>, and +E<lt>. The
a0d0e21e 2112mode you specify should match the mode of the original filehandle.
184e9718 2113(Duping a filehandle does not take into account any existing contents of
cb1a09d0 2114stdio buffers.)
a0d0e21e
LW
2115Here is a script that saves, redirects, and restores STDOUT and
2116STDERR:
2117
2118 #!/usr/bin/perl
2119 open(SAVEOUT, ">&STDOUT");
2120 open(SAVEERR, ">&STDERR");
2121
2122 open(STDOUT, ">foo.out") || die "Can't redirect stdout";
2123 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
2124
2125 select(STDERR); $| = 1; # make unbuffered
2126 select(STDOUT); $| = 1; # make unbuffered
2127
2128 print STDOUT "stdout 1\n"; # this works for
2129 print STDERR "stderr 1\n"; # subprocesses too
2130
2131 close(STDOUT);
2132 close(STDERR);
2133
2134 open(STDOUT, ">&SAVEOUT");
2135 open(STDERR, ">&SAVEERR");
2136
2137 print STDOUT "stdout 2\n";
2138 print STDERR "stderr 2\n";
2139
2140
184e9718 2141If you specify "E<lt>&=N", where N is a number, then Perl will do an
4633a7c4
LW
2142equivalent of C's fdopen() of that file descriptor; this is more
2143parsimonious of file descriptors. For example:
a0d0e21e
LW
2144
2145 open(FILEHANDLE, "<&=$fd")
2146
5f05dabc 2147If you open a pipe on the command "-", i.e., either "|-" or "-|", then
a0d0e21e
LW
2148there is an implicit fork done, and the return value of open is the pid
2149of the child within the parent process, and 0 within the child
184e9718 2150process. (Use C<defined($pid)> to determine whether the open was successful.)
a0d0e21e
LW
2151The filehandle behaves normally for the parent, but i/o to that
2152filehandle is piped from/to the STDOUT/STDIN of the child process.
2153In the child process the filehandle isn't opened--i/o happens from/to
2154the new STDOUT or STDIN. Typically this is used like the normal
2155piped open when you want to exercise more control over just how the
2156pipe command gets executed, such as when you are running setuid, and
54310121 2157don't want to have to scan shell commands for metacharacters.
4633a7c4 2158The following pairs are more or less equivalent:
a0d0e21e
LW
2159
2160 open(FOO, "|tr '[a-z]' '[A-Z]'");
2161 open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
2162
2163 open(FOO, "cat -n '$file'|");
2164 open(FOO, "-|") || exec 'cat', '-n', $file;
2165
4633a7c4
LW
2166See L<perlipc/"Safe Pipe Opens"> for more examples of this.
2167
0dccf244 2168NOTE: On any operation which may do a fork, unflushed buffers remain
184e9718 2169unflushed in both processes, which means you may need to set C<$|> to
a0d0e21e
LW
2170avoid duplicate output.
2171
0dccf244
CS
2172Closing any piped filehandle causes the parent process to wait for the
2173child to finish, and returns the status value in C<$?>.
2174
5f05dabc
PP
2175Using the constructor from the IO::Handle package (or one of its
2176subclasses, such as IO::File or IO::Socket),
c07a80fd
PP
2177you can generate anonymous filehandles which have the scope of whatever
2178variables hold references to them, and automatically close whenever
2179and however you leave that scope:
2180
5f05dabc 2181 use IO::File;
c07a80fd
PP
2182 ...
2183 sub read_myfile_munged {
2184 my $ALL = shift;
5f05dabc 2185 my $handle = new IO::File;
c07a80fd
PP
2186 open($handle, "myfile") or die "myfile: $!";
2187 $first = <$handle>
2188 or return (); # Automatically closed here.
2189 mung $first or die "mung failed"; # Or here.
2190 return $first, <$handle> if $ALL; # Or here.
2191 $first; # Or here.
2192 }
2193
a0d0e21e 2194The filename that is passed to open will have leading and trailing
5f05dabc 2195whitespace deleted. To open a file with arbitrary weird
a0d0e21e
LW
2196characters in it, it's necessary to protect any leading and trailing
2197whitespace thusly:
2198
cb1a09d0
AD
2199 $file =~ s#^(\s)#./$1#;
2200 open(FOO, "< $file\0");
2201
c07a80fd
PP
2202If you want a "real" C open() (see L<open(2)> on your system), then
2203you should use the sysopen() function. This is another way to
2204protect your filenames from interpretation. For example:
cb1a09d0 2205
28757baa 2206 use IO::Handle;
c07a80fd
PP
2207 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL, 0700)
2208 or die "sysopen $path: $!";
2209 HANDLE->autoflush(1);
2210 HANDLE->print("stuff $$\n");
2211 seek(HANDLE, 0, 0);
2212 print "File contains: ", <HANDLE>;
cb1a09d0
AD
2213
2214See L</seek()> for some details about mixing reading and writing.
a0d0e21e
LW
2215
2216=item opendir DIRHANDLE,EXPR
2217
2218Opens a directory named EXPR for processing by readdir(), telldir(),
5f05dabc 2219seekdir(), rewinddir(), and closedir(). Returns TRUE if successful.
a0d0e21e
LW
2220DIRHANDLEs have their own namespace separate from FILEHANDLEs.
2221
2222=item ord EXPR
2223
54310121 2224=item ord
bbce6d69 2225
a0d0e21e 2226Returns the numeric ascii value of the first character of EXPR. If
dc848c6f 2227EXPR is omitted, uses $_. For the reverse, see L</chr>.
a0d0e21e
LW
2228
2229=item pack TEMPLATE,LIST
2230
2231Takes an array or list of values and packs it into a binary structure,
2232returning the string containing the structure. The TEMPLATE is a
2233sequence of characters that give the order and type of values, as
2234follows:
2235
2236 A An ascii string, will be space padded.
2237 a An ascii string, will be null padded.
2238 b A bit string (ascending bit order, like vec()).
2239 B A bit string (descending bit order).
2240 h A hex string (low nybble first).
2241 H A hex string (high nybble first).
2242
2243 c A signed char value.
2244 C An unsigned char value.
96e4d5b1 2245
a0d0e21e
LW
2246 s A signed short value.
2247 S An unsigned short value.
96e4d5b1
PP
2248 (This 'short' is _exactly_ 16 bits, which may differ from
2249 what a local C compiler calls 'short'.)
2250
a0d0e21e
LW
2251 i A signed integer value.
2252 I An unsigned integer value.
96e4d5b1
PP
2253 (This 'integer' is _at_least_ 32 bits wide. Its exact size
2254 depends on what a local C compiler calls 'int', and may
2255 even be larger than the 'long' described in the next item.)
2256
a0d0e21e
LW
2257 l A signed long value.
2258 L An unsigned long value.
96e4d5b1
PP
2259 (This 'long' is _exactly_ 32 bits, which may differ from
2260 what a local C compiler calls 'long'.)
a0d0e21e 2261
96e4d5b1
PP
2262 n A short in "network" (big-endian) order.
2263 N A long in "network" (big-endian) order.
a0d0e21e
LW
2264 v A short in "VAX" (little-endian) order.
2265 V A long in "VAX" (little-endian) order.
96e4d5b1
PP
2266 (These 'shorts' and 'longs' are _exactly_ 16 bits and
2267 _exactly_ 32 bits, respectively.)
a0d0e21e
LW
2268
2269 f A single-precision float in the native format.
2270 d A double-precision float in the native format.
2271
2272 p A pointer to a null-terminated string.
2273 P A pointer to a structure (fixed-length string).
2274
2275 u A uuencoded string.
2276
96e4d5b1
PP
2277 w A BER compressed integer. Its bytes represent an unsigned
2278 integer in base 128, most significant digit first, with as few
2279 digits as possible. Bit eight (the high bit) is set on each
2280 byte except the last.
def98dd4 2281
a0d0e21e
LW
2282 x A null byte.
2283 X Back up a byte.
2284 @ Null fill to absolute position.
2285
2286Each letter may optionally be followed by a number which gives a repeat
5f05dabc 2287count. With all types except "a", "A", "b", "B", "h", "H", and "P" the
a0d0e21e
LW
2288pack function will gobble up that many values from the LIST. A * for the
2289repeat count means to use however many items are left. The "a" and "A"
2290types gobble just one value, but pack it as a string of length count,
2291padding with nulls or spaces as necessary. (When unpacking, "A" strips
2292trailing spaces and nulls, but "a" does not.) Likewise, the "b" and "B"
2293fields pack a string that many bits long. The "h" and "H" fields pack a
84902520
TB
2294string that many nybbles long. The "p" type packs a pointer to a null-
2295terminated string. You are responsible for ensuring the string is not a
2296temporary value (which can potentially get deallocated before you get
2297around to using the packed result). The "P" packs a pointer to a structure
61167c6f
PM
2298of the size indicated by the length. A NULL pointer is created if the
2299corresponding value for "p" or "P" is C<undef>.
2300Real numbers (floats and doubles) are
a0d0e21e
LW
2301in the native machine format only; due to the multiplicity of floating
2302formats around, and the lack of a standard "network" representation, no
2303facility for interchange has been made. This means that packed floating
2304point data written on one machine may not be readable on another - even if
2305both use IEEE floating point arithmetic (as the endian-ness of the memory
2306representation is not part of the IEEE spec). Note that Perl uses doubles
2307internally for all numeric calculation, and converting from double into
5f05dabc 2308float and thence back to double again will lose precision (i.e.,
a0d0e21e
LW
2309C<unpack("f", pack("f", $foo)>) will not in general equal $foo).
2310
2311Examples:
2312
2313 $foo = pack("cccc",65,66,67,68);
2314 # foo eq "ABCD"
2315 $foo = pack("c4",65,66,67,68);
2316 # same thing
2317
2318 $foo = pack("ccxxcc",65,66,67,68);
2319 # foo eq "AB\0\0CD"
2320
2321 $foo = pack("s2",1,2);
2322 # "\1\0\2\0" on little-endian
2323 # "\0\1\0\2" on big-endian
2324
2325 $foo = pack("a4","abcd","x","y","z");
2326 # "abcd"
2327
2328 $foo = pack("aaaa","abcd","x","y","z");
2329 # "axyz"
2330
2331 $foo = pack("a14","abcdefg");
2332 # "abcdefg\0\0\0\0\0\0\0"
2333
2334 $foo = pack("i9pl", gmtime);
2335 # a real struct tm (on my system anyway)
2336
2337 sub bintodec {
2338 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
2339 }
2340
2341The same template may generally also be used in the unpack function.
2342
cb1a09d0
AD
2343=item package NAMESPACE
2344
2345Declares the compilation unit as being in the given namespace. The scope
2346of the package declaration is from the declaration itself through the end of
2347the enclosing block (the same scope as the local() operator). All further
2348unqualified dynamic identifiers will be in this namespace. A package
5f05dabc 2349statement affects only dynamic variables--including those you've used
cb1a09d0
AD
2350local() on--but I<not> lexical variables created with my(). Typically it
2351would be the first declaration in a file to be included by the C<require>
2352or C<use> operator. You can switch into a package in more than one place;
5f05dabc 2353it influences merely which symbol table is used by the compiler for the
cb1a09d0
AD
2354rest of that block. You can refer to variables and filehandles in other
2355packages by prefixing the identifier with the package name and a double
2356colon: C<$Package::Variable>. If the package name is null, the C<main>
2357package as assumed. That is, C<$::sail> is equivalent to C<$main::sail>.
2358
2359See L<perlmod/"Packages"> for more information about packages, modules,
2360and classes. See L<perlsub> for other scoping issues.
2361
a0d0e21e
LW
2362=item pipe READHANDLE,WRITEHANDLE
2363
2364Opens a pair of connected pipes like the corresponding system call.
2365Note that if you set up a loop of piped processes, deadlock can occur
2366unless you are very careful. In addition, note that Perl's pipes use
184e9718 2367stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE
a0d0e21e
LW
2368after each command, depending on the application.
2369
7e1af8bc 2370See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
4633a7c4
LW
2371for examples of such things.
2372
a0d0e21e
LW
2373=item pop ARRAY
2374
54310121 2375=item pop
28757baa 2376
a0d0e21e
LW
2377Pops and returns the last value of the array, shortening the array by
23781. Has a similar effect to
2379
2380 $tmp = $ARRAY[$#ARRAY--];
2381
2382If there are no elements in the array, returns the undefined value.
cb1a09d0
AD
2383If ARRAY is omitted, pops the
2384@ARGV array in the main program, and the @_ array in subroutines, just
2385like shift().
a0d0e21e
LW
2386
2387=item pos SCALAR
2388
54310121 2389=item pos
bbce6d69 2390
4633a7c4 2391Returns the offset of where the last C<m//g> search left off for the variable
2f9daede 2392is in question ($_ is used when the variable is not specified). May be
44a8e56a
PP
2393modified to change that offset. Such modification will also influence
2394the C<\G> zero-width assertion in regular expressions. See L<perlre> and
2395L<perlop>.
a0d0e21e
LW
2396
2397=item print FILEHANDLE LIST
2398
2399=item print LIST
2400
2401=item print
2402
cb1a09d0 2403Prints a string or a comma-separated list of strings. Returns TRUE
a0d0e21e 2404if successful. FILEHANDLE may be a scalar variable name, in which case
cb1a09d0 2405the variable contains the name of or a reference to the filehandle, thus introducing one
a0d0e21e
LW
2406level of indirection. (NOTE: If FILEHANDLE is a variable and the next
2407token is a term, it may be misinterpreted as an operator unless you
5f05dabc 2408interpose a + or put parentheses around the arguments.) If FILEHANDLE is
a0d0e21e 2409omitted, prints by default to standard output (or to the last selected
da0045b7 2410output channel--see L</select>). If LIST is also omitted, prints $_ to
a0d0e21e
LW
2411STDOUT. To set the default output channel to something other than
2412STDOUT use the select operation. Note that, because print takes a
2413LIST, anything in the LIST is evaluated in a list context, and any
2414subroutine that you call will have one or more of its expressions
2415evaluated in a list context. Also be careful not to follow the print
2416keyword with a left parenthesis unless you want the corresponding right
2417parenthesis to terminate the arguments to the print--interpose a + or
5f05dabc 2418put parentheses around all the arguments.
a0d0e21e 2419
4633a7c4 2420Note that if you're storing FILEHANDLES in an array or other expression,
da0045b7 2421you will have to use a block returning its value instead:
4633a7c4
LW
2422
2423 print { $files[$i] } "stuff\n";
2424 print { $OK ? STDOUT : STDERR } "stuff\n";
2425
5f05dabc 2426=item printf FILEHANDLE FORMAT, LIST
a0d0e21e 2427
5f05dabc 2428=item printf FORMAT, LIST
a0d0e21e 2429
a3cb178b
GS
2430Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that $\
2431(the output record separator) is not appended. The first argument
a034a98d
DD
2432of the list will be interpreted as the printf format. If C<use locale> is
2433in effect, the character used for the decimal point in formatted real numbers
2434is affected by the LC_NUMERIC locale. See L<perllocale>.
a0d0e21e 2435
28757baa
PP
2436Don't fall into the trap of using a printf() when a simple
2437print() would do. The print() is more efficient, and less
2438error prone.
2439
da0045b7
PP
2440=item prototype FUNCTION
2441
2442Returns the prototype of a function as a string (or C<undef> if the
5f05dabc
PP
2443function has no prototype). FUNCTION is a reference to, or the name of,
2444the function whose prototype you want to retrieve.
da0045b7 2445
b6c543e3
IZ
2446If FUNCTION is a string starting with C<CORE::>, the rest is taken as
2447a name for Perl builtin. If builtin is not I<overridable> (such as
2448C<qw>) or its arguments cannot be expressed by a prototype (such as
2449C<system>) - in other words, the builtin does not behave like a Perl
2450function - returns C<undef>. Otherwise, the string describing the
2451equivalent prototype is returned.
2452
a0d0e21e
LW
2453=item push ARRAY,LIST
2454
2455Treats ARRAY as a stack, and pushes the values of LIST
2456onto the end of ARRAY. The length of ARRAY increases by the length of
2457LIST. Has the same effect as
2458
2459 for $value (LIST) {
2460 $ARRAY[++$#ARRAY] = $value;
2461 }
2462
2463but is more efficient. Returns the new number of elements in the array.
2464
2465=item q/STRING/
2466
2467=item qq/STRING/
2468
2469=item qx/STRING/
2470
2471=item qw/STRING/
2472
2473Generalized quotes. See L<perlop>.
2474
2475=item quotemeta EXPR
2476
54310121 2477=item quotemeta
bbce6d69 2478
68dc0745 2479Returns the value of EXPR with all non-alphanumeric
a034a98d
DD
2480characters backslashed. (That is, all characters not matching
2481C</[A-Za-z_0-9]/> will be preceded by a backslash in the
2482returned string, regardless of any locale settings.)
2483This is the internal function implementing
a0d0e21e
LW
2484the \Q escape in double-quoted strings.
2485
bbce6d69
PP
2486If EXPR is omitted, uses $_.
2487
a0d0e21e
LW
2488=item rand EXPR
2489
2490=item rand
2491
3e3baf6d
TB
2492Returns a random fractional number greater than or equal to 0 and less
2493than the value of EXPR. (EXPR should be positive.) If EXPR is
2494omitted, the value 1 is used. Automatically calls srand() unless
2495srand() has already been called. See also srand().
a0d0e21e 2496
2f9daede 2497(Note: If your rand function consistently returns numbers that are too
a0d0e21e 2498large or too small, then your version of Perl was probably compiled
2f9daede 2499with the wrong number of RANDBITS.)
a0d0e21e
LW
2500
2501=item read FILEHANDLE,SCALAR,LENGTH,OFFSET
2502
2503=item read FILEHANDLE,SCALAR,LENGTH
2504
2505Attempts to read LENGTH bytes of data into variable SCALAR from the
2506specified FILEHANDLE. Returns the number of bytes actually read, or
2507undef if there was an error. SCALAR will be grown or shrunk to the
2508length actually read. An OFFSET may be specified to place the read
2509data at some other place than the beginning of the string. This call
2510is actually implemented in terms of stdio's fread call. To get a true
2511read system call, see sysread().
2512
2513=item readdir DIRHANDLE
2514
2515Returns the next directory entry for a directory opened by opendir().
2516If used in a list context, returns all the rest of the entries in the
2517directory. If there are no more entries, returns an undefined value in
2518a scalar context or a null list in a list context.
2519
cb1a09d0 2520If you're planning to filetest the return values out of a readdir(), you'd
5f05dabc 2521better prepend the directory in question. Otherwise, because we didn't
cb1a09d0
AD
2522chdir() there, it would have been testing the wrong file.
2523
2524 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
2525 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
2526 closedir DIR;
2527
84902520
TB
2528=item readline EXPR
2529
2530Reads from the file handle EXPR. In scalar context, a single line
2531is read and returned. In list context, reads until end-of-file is
2532reached and returns a list of lines (however you've defined lines
2533with $/ or $INPUT_RECORD_SEPARATOR).
2534This is the internal function implementing the C<E<lt>EXPRE<gt>>
2535operator, but you can use it directly. The C<E<lt>EXPRE<gt>>
2536operator is discussed in more detail in L<perlop/"I/O Operators">.
2537
a0d0e21e
LW
2538=item readlink EXPR
2539
54310121 2540=item readlink
bbce6d69 2541
a0d0e21e
LW
2542Returns the value of a symbolic link, if symbolic links are
2543implemented. If not, gives a fatal error. If there is some system
184e9718 2544error, returns the undefined value and sets C<$!> (errno). If EXPR is
a0d0e21e
LW
2545omitted, uses $_.
2546
84902520
TB
2547=item readpipe EXPR
2548
2549EXPR is interpolated and then executed as a system command.
2550The collected standard output of the command is returned.
2551In scalar context, it comes back as a single (potentially
2552multi-line) string. In list context, returns a list of lines
2553(however you've defined lines with $/ or $INPUT_RECORD_SEPARATOR).
2554This is the internal function implementing the C<qx/EXPR/>
2555operator, but you can use it directly. The C<qx/EXPR/>
2556operator is discussed in more detail in L<perlop/"I/O Operators">.
2557
a0d0e21e
LW
2558=item recv SOCKET,SCALAR,LEN,FLAGS
2559
2560Receives a message on a socket. Attempts to receive LENGTH bytes of
2561data into variable SCALAR from the specified SOCKET filehandle.
a3cb178b 2562Actually does a C recvfrom(), so that it can return the address of the
a0d0e21e
LW
2563sender. Returns the undefined value if there's an error. SCALAR will
2564be grown or shrunk to the length actually read. Takes the same flags
54310121 2565as the system call of the same name.
4633a7c4 2566See L<perlipc/"UDP: Message Passing"> for examples.
a0d0e21e
LW
2567
2568=item redo LABEL
2569
2570=item redo
2571
2572The C<redo> command restarts the loop block without evaluating the
2573conditional again. The C<continue> block, if any, is not executed. If
2574the LABEL is omitted, the command refers to the innermost enclosing
2575loop. This command is normally used by programs that want to lie to
2576themselves about what was just input:
2577
2578 # a simpleminded Pascal comment stripper
2579 # (warning: assumes no { or } in strings)
4633a7c4 2580 LINE: while (<STDIN>) {
a0d0e21e
LW
2581 while (s|({.*}.*){.*}|$1 |) {}
2582 s|{.*}| |;
2583 if (s|{.*| |) {
2584 $front = $_;
2585 while (<STDIN>) {
2586 if (/}/) { # end of comment?
2587 s|^|$front{|;
4633a7c4 2588 redo LINE;
a0d0e21e
LW
2589 }
2590 }
2591 }
2592 print;
2593 }
2594
1d2dff63
GS
2595See also L</continue> for an illustration of how C<last>, C<next>, and
2596C<redo> work.
2597
a0d0e21e
LW
2598=item ref EXPR
2599
54310121 2600=item ref
bbce6d69 2601
2f9daede
TPG
2602Returns a TRUE value if EXPR is a reference, FALSE otherwise. If EXPR
2603is not specified, $_ will be used. The value returned depends on the
bbce6d69 2604type of thing the reference is a reference to.
a0d0e21e
LW
2605Builtin types include:
2606
2607 REF
2608 SCALAR
2609 ARRAY
2610 HASH
2611 CODE
2612 GLOB
2613
54310121 2614If the referenced object has been blessed into a package, then that package
a0d0e21e
LW
2615name is returned instead. You can think of ref() as a typeof() operator.
2616
2617 if (ref($r) eq "HASH") {
aa689395 2618 print "r is a reference to a hash.\n";
54310121 2619 }
a0d0e21e
LW
2620 if (!ref ($r) {
2621 print "r is not a reference at all.\n";
54310121 2622 }
a0d0e21e
LW
2623
2624See also L<perlref>.
2625
2626=item rename OLDNAME,NEWNAME
2627
2628Changes the name of a file. Returns 1 for success, 0 otherwise. Will
5f05dabc 2629not work across file system boundaries.
a0d0e21e
LW
2630
2631=item require EXPR
2632
2633=item require
2634
2635Demands some semantics specified by EXPR, or by $_ if EXPR is not
2636supplied. If EXPR is numeric, demands that the current version of Perl
184e9718 2637(C<$]> or $PERL_VERSION) be equal or greater than EXPR.
a0d0e21e
LW
2638
2639Otherwise, demands that a library file be included if it hasn't already
2640been included. The file is included via the do-FILE mechanism, which is
2641essentially just a variety of eval(). Has semantics similar to the following
2642subroutine:
2643
2644 sub require {
2645 local($filename) = @_;
2646 return 1 if $INC{$filename};
2647 local($realfilename,$result);
2648 ITER: {
2649 foreach $prefix (@INC) {
2650 $realfilename = "$prefix/$filename";
2651 if (-f $realfilename) {
2652 $result = do $realfilename;
2653 last ITER;
2654 }
2655 }
2656 die "Can't find $filename in \@INC";
2657 }
2658 die $@ if $@;
2659 die "$filename did not return true value" unless $result;
2660 $INC{$filename} = $realfilename;
2661 $result;
2662 }
2663
2664Note that the file will not be included twice under the same specified
2665name. The file must return TRUE as the last statement to indicate
2666successful execution of any initialization code, so it's customary to
2667end such a file with "1;" unless you're sure it'll return TRUE
2668otherwise. But it's better just to put the "C<1;>", in case you add more
2669statements.
2670
54310121 2671If EXPR is a bareword, the require assumes a "F<.pm>" extension and
da0045b7 2672replaces "F<::>" with "F</>" in the filename for you,
54310121 2673to make it easy to load standard modules. This form of loading of
a0d0e21e
LW
2674modules does not risk altering your namespace.
2675
ee580363
GS
2676In other words, if you try this:
2677
2678 require Foo::Bar ; # a splendid bareword
2679
2680The require function will actually look for the "Foo/Bar.pm" file in the
2681directories specified in the @INC array.
2682
2683But if you try this :
2684
2685 $class = 'Foo::Bar';
2686 require $class ; # $class is not a bareword
2687or
2688 require "Foo::Bar" ; # not a bareword because of the ""
2689
2690The require function will look for the "Foo::Bar" file in the @INC array and
2691will complain about not finding "Foo::Bar" there. In this case you can do :
2692
2693 eval "require $class";
2694
2695For a yet-more-powerful import facility, see L</use> and L<perlmod>.
a0d0e21e
LW
2696
2697=item reset EXPR
2698
2699=item reset
2700
2701Generally used in a C<continue> block at the end of a loop to clear
2702variables and reset ?? searches so that they work again. The
2703expression is interpreted as a list of single characters (hyphens
2704allowed for ranges). All variables and arrays beginning with one of
2705those letters are reset to their pristine state. If the expression is
5f05dabc
PP
2706omitted, one-match searches (?pattern?) are reset to match again. Resets
2707only variables or searches in the current package. Always returns
a0d0e21e
LW
27081. Examples:
2709
2710 reset 'X'; # reset all X variables
2711 reset 'a-z'; # reset lower case variables
2712 reset; # just reset ?? searches
2713
5f05dabc
PP
2714Resetting "A-Z" is not recommended because you'll wipe out your
2715ARGV and ENV arrays. Resets only package variables--lexical variables
a0d0e21e 2716are unaffected, but they clean themselves up on scope exit anyway,
da0045b7 2717so you'll probably want to use them instead. See L</my>.
a0d0e21e 2718
54310121
PP
2719=item return EXPR
2720
2721=item return
2722
2723Returns from a subroutine, eval(), or do FILE with the value of the
2724given EXPR. Evaluation of EXPR may be in a list, scalar, or void
2725context, depending on how the return value will be used, and the context
2726may vary from one execution to the next (see wantarray()). If no EXPR
2727is given, returns an empty list in a list context, an undefined value in
2728a scalar context, or nothing in a void context.
a0d0e21e 2729
68dc0745
PP
2730(Note that in the absence of a return, a subroutine, eval, or do FILE
2731will automatically return the value of the last expression evaluated.)
a0d0e21e
LW
2732
2733=item reverse LIST
2734
2735In a list context, returns a list value consisting of the elements
2f9daede
TPG
2736of LIST in the opposite order. In a scalar context, concatenates the
2737elements of LIST, and returns a string value consisting of those bytes,
2738but in the opposite order.
4633a7c4 2739
2f9daede 2740 print reverse <>; # line tac, last line first
4633a7c4 2741
2f9daede
TPG
2742 undef $/; # for efficiency of <>
2743 print scalar reverse <>; # byte tac, last line tsrif
2744
2745This operator is also handy for inverting a hash, although there are some
2746caveats. If a value is duplicated in the original hash, only one of those
2747can be represented as a key in the inverted hash. Also, this has to
2748unwind one hash and build a whole new one, which may take some time
2749on a large hash.
2750
2751 %by_name = reverse %by_address; # Invert the hash
a0d0e21e
LW
2752
2753=item rewinddir DIRHANDLE
2754
2755Sets the current position to the beginning of the directory for the
2756readdir() routine on DIRHANDLE.
2757
2758=item rindex STR,SUBSTR,POSITION
2759
2760=item rindex STR,SUBSTR
2761
2762Works just like index except that it returns the position of the LAST
2763occurrence of SUBSTR in STR. If POSITION is specified, returns the
2764last occurrence at or before that position.
2765
2766=item rmdir FILENAME
2767
54310121 2768=item rmdir
bbce6d69 2769
a0d0e21e 2770Deletes the directory specified by FILENAME if it is empty. If it
184e9718 2771succeeds it returns 1, otherwise it returns 0 and sets C<$!> (errno). If
a0d0e21e
LW
2772FILENAME is omitted, uses $_.
2773
2774=item s///
2775
2776The substitution operator. See L<perlop>.
2777
2778=item scalar EXPR
2779
2780Forces EXPR to be interpreted in a scalar context and returns the value
54310121 2781of EXPR.
cb1a09d0
AD
2782
2783 @counts = ( scalar @a, scalar @b, scalar @c );
2784
54310121 2785There is no equivalent operator to force an expression to
cb1a09d0
AD
2786be interpolated in a list context because it's in practice never
2787needed. If you really wanted to do so, however, you could use
2788the construction C<@{[ (some expression) ]}>, but usually a simple
2789C<(some expression)> suffices.
a0d0e21e
LW
2790
2791=item seek FILEHANDLE,POSITION,WHENCE
2792
8903cb82
PP
2793Sets FILEHANDLE's position, just like the fseek() call of stdio.
2794FILEHANDLE may be an expression whose value gives the name of the
2795filehandle. The values for WHENCE are 0 to set the new position to
2796POSITION, 1 to set it to the current position plus POSITION, and 2 to
2797set it to EOF plus POSITION (typically negative). For WHENCE you may
2798use the constants SEEK_SET, SEEK_CUR, and SEEK_END from either the
2799IO::Seekable or the POSIX module. Returns 1 upon success, 0 otherwise.
2800
2801If you want to position file for sysread() or syswrite(), don't use
2802seek() -- buffering makes its effect on the file's system position
137443ea 2803unpredictable and non-portable. Use sysseek() instead.
a0d0e21e 2804
cb1a09d0
AD
2805On some systems you have to do a seek whenever you switch between reading
2806and writing. Amongst other things, this may have the effect of calling
8903cb82
PP
2807stdio's clearerr(3). A WHENCE of 1 (SEEK_CUR) is useful for not moving
2808the file position:
cb1a09d0
AD
2809
2810 seek(TEST,0,1);
2811
2812This is also useful for applications emulating C<tail -f>. Once you hit
2813EOF on your read, and then sleep for a while, you might have to stick in a
8903cb82
PP
2814seek() to reset things. The seek() doesn't change the current position,
2815but it I<does> clear the end-of-file condition on the handle, so that the
2816next C<E<lt>FILEE<gt>> makes Perl try again to read something. We hope.
cb1a09d0
AD
2817
2818If that doesn't work (some stdios are particularly cantankerous), then
2819you may need something more like this:
2820
2821 for (;;) {
2822 for ($curpos = tell(FILE); $_ = <FILE>; $curpos = tell(FILE)) {
2823 # search for some stuff and put it into files
2824 }
2825 sleep($for_a_while);
2826 seek(FILE, $curpos, 0);
2827 }
2828
a0d0e21e
LW
2829=item seekdir DIRHANDLE,POS
2830
2831Sets the current position for the readdir() routine on DIRHANDLE. POS
2832must be a value returned by telldir(). Has the same caveats about
2833possible directory compaction as the corresponding system library
2834routine.
2835
2836=item select FILEHANDLE
2837
2838=item select
2839
2840Returns the currently selected filehandle. Sets the current default
2841filehandle for output, if FILEHANDLE is supplied. This has two
2842effects: first, a C<write> or a C<print> without a filehandle will
2843default to this FILEHANDLE. Second, references to variables related to
2844output will refer to this output channel. For example, if you have to
2845set the top of form format for more than one output channel, you might
2846do the following:
2847
2848 select(REPORT1);
2849 $^ = 'report1_top';
2850 select(REPORT2);
2851 $^ = 'report2_top';
2852
2853FILEHANDLE may be an expression whose value gives the name of the
2854actual filehandle. Thus:
2855
2856 $oldfh = select(STDERR); $| = 1; select($oldfh);
2857
4633a7c4
LW
2858Some programmers may prefer to think of filehandles as objects with
2859methods, preferring to write the last example as:
a0d0e21e 2860
28757baa 2861 use IO::Handle;
a0d0e21e
LW
2862 STDERR->autoflush(1);
2863
2864=item select RBITS,WBITS,EBITS,TIMEOUT
2865
5f05dabc 2866This calls the select(2) system call with the bit masks specified, which
a0d0e21e
LW
2867can be constructed using fileno() and vec(), along these lines:
2868
2869 $rin = $win = $ein = '';
2870 vec($rin,fileno(STDIN),1) = 1;
2871 vec($win,fileno(STDOUT),1) = 1;
2872 $ein = $rin | $win;
2873
2874If you want to select on many filehandles you might wish to write a
2875subroutine:
2876
2877 sub fhbits {
2878 local(@fhlist) = split(' ',$_[0]);
2879 local($bits);
2880 for (@fhlist) {
2881 vec($bits,fileno($_),1) = 1;
2882 }
2883 $bits;
2884 }
4633a7c4 2885 $rin = fhbits('STDIN TTY SOCK');
a0d0e21e
LW
2886
2887The usual idiom is:
2888
2889 ($nfound,$timeleft) =
2890 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
2891
54310121 2892or to block until something becomes ready just do this
a0d0e21e
LW
2893
2894 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
2895
5f05dabc 2896Most systems do not bother to return anything useful in $timeleft, so
c07a80fd
PP
2897calling select() in a scalar context just returns $nfound.
2898
5f05dabc 2899Any of the bit masks can also be undef. The timeout, if specified, is
a0d0e21e
LW
2900in seconds, which may be fractional. Note: not all implementations are
2901capable of returning the $timeleft. If not, they always return
2902$timeleft equal to the supplied $timeout.
2903
ff68c719 2904You can effect a sleep of 250 milliseconds this way:
a0d0e21e
LW
2905
2906 select(undef, undef, undef, 0.25);
2907
184e9718 2908B<WARNING>: Do not attempt to mix buffered I/O (like read() or E<lt>FHE<gt>)
cb1a09d0 2909with select(). You have to use sysread() instead.
a0d0e21e
LW
2910
2911=item semctl ID,SEMNUM,CMD,ARG
2912
2913Calls the System V IPC function semctl. If CMD is &IPC_STAT or
2914&GETALL, then ARG must be a variable which will hold the returned
2915semid_ds structure or semaphore value array. Returns like ioctl: the
2916undefined value for error, "0 but true" for zero, or the actual return
2917value otherwise.
2918
2919=item semget KEY,NSEMS,FLAGS
2920
2921Calls the System V IPC function semget. Returns the semaphore id, or
2922the undefined value if there is an error.
2923
2924=item semop KEY,OPSTRING
2925
2926Calls the System V IPC function semop to perform semaphore operations
2927such as signaling and waiting. OPSTRING must be a packed array of
2928semop structures. Each semop structure can be generated with
2929C<pack("sss", $semnum, $semop, $semflag)>. The number of semaphore
2930operations is implied by the length of OPSTRING. Returns TRUE if
2931successful, or FALSE if there is an error. As an example, the
2932following code waits on semaphore $semnum of semaphore id $semid:
2933
2934 $semop = pack("sss", $semnum, -1, 0);
2935 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
2936
2937To signal the semaphore, replace "-1" with "1".
2938
2939=item send SOCKET,MSG,FLAGS,TO
2940
2941=item send SOCKET,MSG,FLAGS
2942
2943Sends a message on a socket. Takes the same flags as the system call
2944of the same name. On unconnected sockets you must specify a
2945destination to send TO, in which case it does a C sendto(). Returns
2946the number of characters sent, or the undefined value if there is an
2947error.
4633a7c4 2948See L<perlipc/"UDP: Message Passing"> for examples.
a0d0e21e
LW
2949
2950=item setpgrp PID,PGRP
2951
2952Sets the current process group for the specified PID, 0 for the current
2953process. Will produce a fatal error if used on a machine that doesn't
5f05dabc 2954implement setpgrp(2). If the arguments are omitted, it defaults to
47e29363
PP
29550,0. Note that the POSIX version of setpgrp() does not accept any
2956arguments, so only setpgrp 0,0 is portable.
a0d0e21e
LW
2957
2958=item setpriority WHICH,WHO,PRIORITY
2959
2960Sets the current priority for a process, a process group, or a user.
748a9306 2961(See setpriority(2).) Will produce a fatal error if used on a machine
a0d0e21e
LW
2962that doesn't implement setpriority(2).
2963
2964=item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
2965
2966Sets the socket option requested. Returns undefined if there is an
2967error. OPTVAL may be specified as undef if you don't want to pass an
2968argument.
2969
2970=item shift ARRAY
2971
2972=item shift
2973
2974Shifts the first value of the array off and returns it, shortening the
2975array by 1 and moving everything down. If there are no elements in the
2976array, returns the undefined value. If ARRAY is omitted, shifts the
977336f5
GS
2977@_ array within the lexical scope of subroutines and formats, and the
2978@ARGV array at file scopes or within the lexical scopes established by
2979the C<eval ''>, C<BEGIN {}>, C<END {}>, and C<INIT {}> constructs.
2980See also unshift(), push(), and pop(). Shift() and unshift() do the
2981same thing to the left end of an array that pop() and push() do to the
2982right end.
a0d0e21e
LW
2983
2984=item shmctl ID,CMD,ARG
2985
2986Calls the System V IPC function shmctl. If CMD is &IPC_STAT, then ARG
2987must be a variable which will hold the returned shmid_ds structure.
2988Returns like ioctl: the undefined value for error, "0 but true" for
2989zero, or the actual return value otherwise.
2990
2991=item shmget KEY,SIZE,FLAGS
2992
2993Calls the System V IPC function shmget. Returns the shared memory
2994segment id, or the undefined value if there is an error.
2995
2996=item shmread ID,VAR,POS,SIZE
2997
2998=item shmwrite ID,STRING,POS,SIZE
2999
3000Reads or writes the System V shared memory segment ID starting at
3001position POS for size SIZE by attaching to it, copying in/out, and
3002detaching from it. When reading, VAR must be a variable which will
3003hold the data read. When writing, if STRING is too long, only SIZE
3004bytes are used; if STRING is too short, nulls are written to fill out
3005SIZE bytes. Return TRUE if successful, or FALSE if there is an error.
3006
3007=item shutdown SOCKET,HOW
3008
3009Shuts down a socket connection in the manner indicated by HOW, which
3010has the same interpretation as in the system call of the same name.
3011
3012=item sin EXPR
3013
54310121 3014=item sin
bbce6d69 3015
a0d0e21e
LW
3016Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
3017returns sine of $_.
3018
54310121 3019For the inverse sine operation, you may use the POSIX::asin()
28757baa
PP
3020function, or use this relation:
3021
3022 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
3023
a0d0e21e
LW
3024=item sleep EXPR
3025
3026=item sleep
3027
3028Causes the script to sleep for EXPR seconds, or forever if no EXPR.
1d3434b8
GS
3029May be interrupted if the process receives a signal such as SIGALRM.
3030Returns the number of seconds actually slept. You probably cannot
3031mix alarm() and sleep() calls, because sleep() is often implemented
3032using alarm().
a0d0e21e
LW
3033
3034On some older systems, it may sleep up to a full second less than what
3035you requested, depending on how it counts seconds. Most modern systems
3036always sleep the full amount.
3037
cb1a09d0 3038For delays of finer granularity than one second, you may use Perl's
54310121 3039syscall() interface to access setitimer(2) if your system supports it,
7b8d334a 3040or else see L</select()> above.
cb1a09d0 3041
5f05dabc
PP
3042See also the POSIX module's sigpause() function.
3043
a0d0e21e
LW
3044=item socket SOCKET,DOMAIN,TYPE,PROTOCOL
3045
3046Opens a socket of the specified kind and attaches it to filehandle
5f05dabc 3047SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for the
a0d0e21e 3048system call of the same name. You should "use Socket;" first to get
4633a7c4 3049the proper definitions imported. See the example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
3050
3051=item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
3052
3053Creates an unnamed pair of sockets in the specified domain, of the
5f05dabc 3054specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
a0d0e21e
LW
3055for the system call of the same name. If unimplemented, yields a fatal
3056error. Returns TRUE if successful.
3057
3058=item sort SUBNAME LIST
3059
3060=item sort BLOCK LIST
3061
3062=item sort LIST
3063
2f9daede
TPG
3064Sorts the LIST and returns the sorted list value. If SUBNAME or BLOCK
3065is omitted, sorts in standard string comparison order. If SUBNAME is
3066specified, it gives the name of a subroutine that returns an integer
3067less than, equal to, or greater than 0, depending on how the elements
3068of the array are to be ordered. (The C<E<lt>=E<gt>> and C<cmp>
3069operators are extremely useful in such routines.) SUBNAME may be a
1d3434b8
GS
3070scalar variable name (unsubscripted), in which case the value provides
3071the name of (or a reference to) the actual subroutine to use. In place
3072of a SUBNAME, you can provide a BLOCK as an anonymous, in-line sort
3073subroutine.
a0d0e21e 3074
cb1a09d0
AD
3075In the interests of efficiency the normal calling code for subroutines is
3076bypassed, with the following effects: the subroutine may not be a
3077recursive subroutine, and the two elements to be compared are passed into
3078the subroutine not via @_ but as the package global variables $a and
3079$b (see example below). They are passed by reference, so don't
3080modify $a and $b. And don't try to declare them as lexicals either.
a0d0e21e 3081
0a753a76
PP
3082You also cannot exit out of the sort block or subroutine using any of the
3083loop control operators described in L<perlsyn> or with goto().
3084
a034a98d
DD
3085When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
3086current collation locale. See L<perllocale>.
3087
a0d0e21e
LW
3088Examples:
3089
3090 # sort lexically
3091 @articles = sort @files;
3092
3093 # same thing, but with explicit sort routine
3094 @articles = sort {$a cmp $b} @files;
3095
cb1a09d0 3096 # now case-insensitively
54310121 3097 @articles = sort {uc($a) cmp uc($b)} @files;
cb1a09d0 3098
a0d0e21e
LW
3099 # same thing in reversed order
3100 @articles = sort {$b cmp $a} @files;
3101
3102 # sort numerically ascending
3103 @articles = sort {$a <=> $b} @files;
3104
3105 # sort numerically descending
3106 @articles = sort {$b <=> $a} @files;
3107
3108 # sort using explicit subroutine name
3109 sub byage {
2f9daede 3110 $age{$a} <=> $age{$b}; # presuming numeric
a0d0e21e
LW
3111 }
3112 @sortedclass = sort byage @class;
3113
aa689395
PP
3114 # this sorts the %age hash by value instead of key
3115 # using an in-line function
c07a80fd
PP
3116 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
3117
a0d0e21e
LW
3118 sub backwards { $b cmp $a; }
3119 @harry = ('dog','cat','x','Cain','Abel');
3120 @george = ('gone','chased','yz','Punished','Axed');
3121 print sort @harry;
3122 # prints AbelCaincatdogx
3123 print sort backwards @harry;
3124 # prints xdogcatCainAbel
3125 print sort @george, 'to', @harry;
3126 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
3127
54310121
PP
3128 # inefficiently sort by descending numeric compare using
3129 # the first integer after the first = sign, or the
cb1a09d0
AD
3130 # whole record case-insensitively otherwise
3131
3132 @new = sort {
3133 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
3134 ||
3135 uc($a) cmp uc($b)
3136 } @old;
3137
3138 # same thing, but much more efficiently;
3139 # we'll build auxiliary indices instead
3140 # for speed
3141 @nums = @caps = ();
54310121 3142 for (@old) {
cb1a09d0
AD
3143 push @nums, /=(\d+)/;
3144 push @caps, uc($_);
54310121 3145 }
cb1a09d0
AD
3146
3147 @new = @old[ sort {
3148 $nums[$b] <=> $nums[$a]
3149 ||
3150 $caps[$a] cmp $caps[$b]
3151 } 0..$#old
3152 ];
3153
3154 # same thing using a Schwartzian Transform (no temps)
3155 @new = map { $_->[0] }
3156 sort { $b->[1] <=> $a->[1]
3157 ||
3158 $a->[2] cmp $b->[2]
3159 } map { [$_, /=(\d+)/, uc($_)] } @old;
3160
184e9718 3161If you're using strict, you I<MUST NOT> declare $a
cb1a09d0
AD
3162and $b as lexicals. They are package globals. That means
3163if you're in the C<main> package, it's
3164
3165 @articles = sort {$main::b <=> $main::a} @files;
3166
3167or just
3168
3169 @articles = sort {$::b <=> $::a} @files;
3170
3171but if you're in the C<FooPack> package, it's
3172
3173 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
3174
55497cff
PP
3175The comparison function is required to behave. If it returns
3176inconsistent results (sometimes saying $x[1] is less than $x[2] and
3177sometimes saying the opposite, for example) the Perl interpreter will
3178probably crash and dump core. This is entirely due to and dependent
3179upon your system's qsort(3) library routine; this routine often avoids
3180sanity checks in the interest of speed.
3181
a0d0e21e
LW
3182=item splice ARRAY,OFFSET,LENGTH,LIST
3183
3184=item splice ARRAY,OFFSET,LENGTH
3185
3186=item splice ARRAY,OFFSET
3187
3188Removes the elements designated by OFFSET and LENGTH from an array, and
43051805
GS
3189replaces them with the elements of LIST, if any. In a list context,
3190returns the elements removed from the array. In a scalar context,
3191returns the last element removed, or C<undef> if no elements are
3192removed. The array grows or shrinks as necessary. If LENGTH is
3193omitted, removes everything from OFFSET onward. The following
3194equivalences hold (assuming C<$[ == 0>):
a0d0e21e
LW
3195
3196 push(@a,$x,$y) splice(@a,$#a+1,0,$x,$y)
3197 pop(@a) splice(@a,-1)
3198 shift(@a) splice(@a,0,1)
3199 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
3200 $a[$x] = $y splice(@a,$x,1,$y);
3201
3202Example, assuming array lengths are passed before arrays:
3203
3204 sub aeq { # compare two list values
3205 local(@a) = splice(@_,0,shift);
3206 local(@b) = splice(@_,0,shift);
3207 return 0 unless @a == @b; # same len?
3208 while (@a) {
3209 return 0 if pop(@a) ne pop(@b);
3210 }
3211 return 1;
3212 }
3213 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
3214
3215=item split /PATTERN/,EXPR,LIMIT
3216
3217=item split /PATTERN/,EXPR
3218
3219=item split /PATTERN/
3220
3221=item split
3222
3223Splits a string into an array of strings, and returns it.
3224
3225If not in a list context, returns the number of fields found and splits into
3226the @_ array. (In a list context, you can force the split into @_ by
1d2dff63 3227using C<??> as the pattern delimiters, but it still returns the list
a0d0e21e
LW
3228value.) The use of implicit split to @_ is deprecated, however.
3229
3230If EXPR is omitted, splits the $_ string. If PATTERN is also omitted,
4633a7c4
LW
3231splits on whitespace (after skipping any leading whitespace). Anything
3232matching PATTERN is taken to be a delimiter separating the fields. (Note
fb73857a
PP
3233that the delimiter may be longer than one character.)
3234
7b8d334a
GS
3235If LIMIT is specified and is positive, splits into no more than that
3236many fields (though it may split into fewer). If LIMIT is unspecified
3237or zero, trailing null fields are stripped (which potential users
fb73857a
PP
3238of pop() would do well to remember). If LIMIT is negative, it is
3239treated as if an arbitrarily large LIMIT had been specified.
a0d0e21e
LW
3240
3241A pattern matching the null string (not to be confused with
748a9306 3242a null pattern C<//>, which is just one member of the set of patterns
a0d0e21e
LW
3243matching a null string) will split the value of EXPR into separate
3244characters at each point it matches that way. For example:
3245
3246 print join(':', split(/ */, 'hi there'));
3247
3248produces the output 'h:i:t:h:e:r:e'.
3249
5f05dabc 3250The LIMIT parameter can be used to split a line partially
a0d0e21e
LW
3251
3252 ($login, $passwd, $remainder) = split(/:/, $_, 3);
3253
3254When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT
3255one larger than the number of variables in the list, to avoid
3256unnecessary work. For the list above LIMIT would have been 4 by
3257default. In time critical applications it behooves you not to split
3258into more fields than you really need.
3259
3260If the PATTERN contains parentheses, additional array elements are
3261created from each matching substring in the delimiter.
3262
da0045b7 3263 split(/([,-])/, "1-10,20", 3);
a0d0e21e
LW
3264
3265produces the list value
3266
3267 (1, '-', 10, ',', 20)
3268
54310121 3269If you had the entire header of a normal Unix email message in $header,
4633a7c4
LW
3270you could split it up into fields and their values this way:
3271
3272 $header =~ s/\n\s+/ /g; # fix continuation lines
fb73857a 3273 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
4633a7c4 3274
a0d0e21e
LW
3275The pattern C</PATTERN/> may be replaced with an expression to specify
3276patterns that vary at runtime. (To do runtime compilation only once,
748a9306
LW
3277use C</$variable/o>.)
3278
3279As a special case, specifying a PATTERN of space (C<' '>) will split on
3280white space just as split with no arguments does. Thus, split(' ') can
3281be used to emulate B<awk>'s default behavior, whereas C<split(/ /)>
3282will give you as many null initial fields as there are leading spaces.
3283A split on /\s+/ is like a split(' ') except that any leading
3284whitespace produces a null first field. A split with no arguments
3285really does a C<split(' ', $_)> internally.
a0d0e21e
LW
3286
3287Example:
3288
3289 open(passwd, '/etc/passwd');
3290 while (<passwd>) {
54310121 3291 ($login, $passwd, $uid, $gid, $gcos,
748a9306 3292 $home, $shell) = split(/:/);
a0d0e21e
LW
3293 ...
3294 }
3295
54310121 3296(Note that $shell above will still have a newline on it. See L</chop>,
a0d0e21e
LW
3297L</chomp>, and L</join>.)
3298
5f05dabc 3299=item sprintf FORMAT, LIST
a0d0e21e 3300
74a77017
CS
3301Returns a string formatted by the usual printf conventions of the
3302C library function sprintf(). See L<sprintf(3)> or L<printf(3)>
3303on your system for an explanation of the general principles.
3304
3305Perl does all of its own sprintf() formatting -- it emulates the C
3306function sprintf(), but it doesn't use it (except for floating-point
3307numbers, and even then only the standard modifiers are allowed). As a
3308result, any non-standard extensions in your local sprintf() are not
3309available from Perl.
3310
3311Perl's sprintf() permits the following universally-known conversions:
3312
3313 %% a percent sign
3314 %c a character with the given number
3315 %s a string
3316 %d a signed integer, in decimal
3317 %u an unsigned integer, in decimal
3318 %o an unsigned integer, in octal
3319 %x an unsigned integer, in hexadecimal
3320 %e a floating-point number, in scientific notation
3321 %f a floating-point number, in fixed decimal notation
3322 %g a floating-point number, in %e or %f notation
3323
1b3f7d21 3324In addition, Perl permits the following widely-supported conversions:
74a77017 3325
74a77017
CS
3326 %X like %x, but using upper-case letters
3327 %E like %e, but using an upper-case "E"
3328 %G like %g, but with an upper-case "E" (if applicable)
3329 %p a pointer (outputs the Perl value's address in hexadecimal)
1b3f7d21
CS
3330 %n special: *stores* the number of characters output so far
3331 into the next variable in the parameter list
74a77017 3332
1b3f7d21
CS
3333Finally, for backward (and we do mean "backward") compatibility, Perl
3334permits these unnecessary but widely-supported conversions:
74a77017 3335
1b3f7d21 3336 %i a synonym for %d
74a77017
CS
3337 %D a synonym for %ld
3338 %U a synonym for %lu
3339 %O a synonym for %lo
3340 %F a synonym for %f
3341
3342Perl permits the following universally-known flags between the C<%>
3343and the conversion letter:
3344
3345 space prefix positive number with a space
3346 + prefix positive number with a plus sign
3347 - left-justify within the field
3348 0 use zeros, not spaces, to right-justify
a3cb178b 3349 # prefix non-zero octal with "0", non-zero hex with "0x"
74a77017
CS
3350 number minimum field width
3351 .number "precision": digits after decimal point for floating-point,
3352 max length for string, minimum length for integer
3353 l interpret integer as C type "long" or "unsigned long"
74a77017
CS
3354 h interpret integer as C type "short" or "unsigned short"
3355
1b3f7d21 3356There is also one Perl-specific flag:
74a77017
CS
3357
3358 V interpret integer as Perl's standard integer type
3359
3360Where a number would appear in the flags, an asterisk ("*") may be
3361used instead, in which case Perl uses the next item in the parameter
3362list as the given number (that is, as the field width or precision).
3363If a field width obtained through "*" is negative, it has the same
3364effect as the '-' flag: left-justification.
3365
3366If C<use locale> is in effect, the character used for the decimal
3367point in formatted real numbers is affected by the LC_NUMERIC locale.
3368See L<perllocale>.
a0d0e21e
LW
3369
3370=item sqrt EXPR
3371
54310121 3372=item sqrt
bbce6d69 3373
a0d0e21e
LW
3374Return the square root of EXPR. If EXPR is omitted, returns square
3375root of $_.
3376
3377=item srand EXPR
3378
93dc8474
CS
3379=item srand
3380
3381Sets the random number seed for the C<rand> operator. If EXPR is
3382omitted, uses a semi-random value based on the current time and process
3383ID, among other things. In versions of Perl prior to 5.004 the default
3384seed was just the current time(). This isn't a particularly good seed,
3385so many old programs supply their own seed value (often C<time ^ $$> or
7b8d334a 3386C<time ^ ($$ + ($$ E<lt>E<lt> 15))>), but that isn't necessary any more.
93dc8474
CS
3387
3388In fact, it's usually not necessary to call srand() at all, because if
3389it is not called explicitly, it is called implicitly at the first use of
2f9daede
TPG
3390the C<rand> operator. However, this was not the case in version of Perl
3391before 5.004, so if your script will run under older Perl versions, it
3392should call srand().
93dc8474 3393
2f9daede
TPG
3394Note that you need something much more random than the default seed for
3395cryptographic purposes. Checksumming the compressed output of one or more
3396rapidly changing operating system status programs is the usual method. For
3397example:
28757baa
PP
3398
3399 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
3400
0078ec44
RS
3401If you're particularly concerned with this, see the Math::TrulyRandom
3402module in CPAN.
3403
3404Do I<not> call srand() multiple times in your program unless you know
28757baa
PP
3405exactly what you're doing and why you're doing it. The point of the
3406function is to "seed" the rand() function so that rand() can produce
3407a different sequence each time you run your program. Just do it once at the
3408top of your program, or you I<won't> get random numbers out of rand()!
3409
54310121 3410Frequently called programs (like CGI scripts) that simply use
28757baa
PP
3411
3412 time ^ $$
3413
54310121 3414for a seed can fall prey to the mathematical property that
28757baa
PP
3415
3416 a^b == (a+1)^(b+1)
3417
0078ec44 3418one-third of the time. So don't do that.
f86702cc 3419
a0d0e21e
LW
3420=item stat FILEHANDLE
3421
3422=item stat EXPR
3423
54310121 3424=item stat
bbce6d69 3425
1d2dff63
GS
3426Returns a 13-element list giving the status info for a file, either
3427the file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted,
3428it stats $_. Returns a null list if the stat fails. Typically used
3429as follows:
a0d0e21e
LW
3430
3431 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
3432 $atime,$mtime,$ctime,$blksize,$blocks)
3433 = stat($filename);
3434
54310121 3435Not all fields are supported on all filesystem types. Here are the
c07a80fd
PP
3436meaning of the fields:
3437
54310121
PP
3438 0 dev device number of filesystem
3439 1 ino inode number
3440 2 mode file mode (type and permissions)
3441 3 nlink number of (hard) links to the file
3442 4 uid numeric user ID of file's owner
3443 5 gid numeric group ID of file's owner
3444 6 rdev the device identifier (special files only)
3445 7 size total size of file, in bytes
3446 8 atime last access time since the epoch
3447 9 mtime last modify time since the epoch
3448 10 ctime inode change time (NOT creation time!) since the epoch
3449 11 blksize preferred block size for file system I/O
3450 12 blocks actual number of blocks allocated
c07a80fd
PP
3451
3452(The epoch was at 00:00 January 1, 1970 GMT.)
3453
a0d0e21e
LW
3454If stat is passed the special filehandle consisting of an underline, no
3455stat is done, but the current contents of the stat structure from the
3456last stat or filetest are returned. Example:
3457
3458 if (-x $file && (($d) = stat(_)) && $d < 0) {
3459 print "$file is executable NFS file\n";
3460 }
3461
5f05dabc 3462(This works on machines only for which the device number is negative under NFS.)
a0d0e21e 3463
1d2dff63
GS
3464In scalar context, C<stat> returns a boolean value indicating success
3465or failure, and, if successful, sets the information associated with
3466the special filehandle C<_>.
3467
a0d0e21e
LW
3468=item study SCALAR
3469
3470=item study
3471
184e9718 3472Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
a0d0e21e
LW
3473doing many pattern matches on the string before it is next modified.
3474This may or may not save time, depending on the nature and number of
3475patterns you are searching on, and on the distribution of character
54310121 3476frequencies in the string to be searched -- you probably want to compare
5f05dabc 3477run times with and without it to see which runs faster. Those loops
a0d0e21e
LW
3478which scan for many short constant strings (including the constant
3479parts of more complex patterns) will benefit most. You may have only
54310121 3480one study active at a time -- if you study a different scalar the first
a0d0e21e
LW
3481is "unstudied". (The way study works is this: a linked list of every
3482character in the string to be searched is made, so we know, for
3483example, where all the 'k' characters are. From each search string,
3484the rarest character is selected, based on some static frequency tables
3485constructed from some C programs and English text. Only those places
3486that contain this "rarest" character are examined.)
3487
3488For example, here is a loop which inserts index producing entries
3489before any line containing a certain pattern:
3490
3491 while (<>) {
3492 study;
3493 print ".IX foo\n" if /\bfoo\b/;
3494 print ".IX bar\n" if /\bbar\b/;
3495 print ".IX blurfl\n" if /\bblurfl\b/;
3496 ...
3497 print;
3498 }
3499
3500In searching for /\bfoo\b/, only those locations in $_ that contain "f"
3501will be looked at, because "f" is rarer than "o". In general, this is
3502a big win except in pathological cases. The only question is whether
3503it saves you more time than it took to build the linked list in the
3504first place.
3505
3506Note that if you have to look for strings that you don't know till
3507runtime, you can build an entire loop as a string and eval that to
3508avoid recompiling all your patterns all the time. Together with
3509undefining $/ to input entire files as one record, this can be very
3510fast, often faster than specialized programs like fgrep(1). The following
184e9718 3511scans a list of files (C<@files>) for a list of words (C<@words>), and prints
a0d0e21e
LW
3512out the names of those files that contain a match:
3513
3514 $search = 'while (<>) { study;';
3515 foreach $word (@words) {
3516 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
3517 }
3518 $search .= "}";
3519 @ARGV = @files;
3520 undef $/;
3521 eval $search; # this screams
5f05dabc 3522 $/ = "\n"; # put back to normal input delimiter
a0d0e21e
LW
3523 foreach $file (sort keys(%seen)) {
3524 print $file, "\n";
3525 }
3526
cb1a09d0
AD
3527=item sub BLOCK
3528
3529=item sub NAME
3530
3531=item sub NAME BLOCK
3532
3533This is subroutine definition, not a real function I<per se>. With just a
3534NAME (and possibly prototypes), it's just a forward declaration. Without
3535a NAME, it's an anonymous function declaration, and does actually return a
2f9daede 3536value: the CODE ref of the closure you just created. See L<perlsub> and
cb1a09d0
AD
3537L<perlref> for details.
3538
7b8d334a
GS
3539=item substr EXPR,OFFSET,LEN,REPLACEMENT
3540
a0d0e21e
LW
3541=item substr EXPR,OFFSET,LEN
3542
3543=item substr EXPR,OFFSET
3544
3545Extracts a substring out of EXPR and returns it. First character is at
2f9daede 3546offset 0, or whatever you've set C<$[> to (but don't do that).
84902520 3547If OFFSET is negative (or more precisely, less than C<$[>), starts
a0d0e21e 3548that far from the end of the string. If LEN is omitted, returns
748a9306
LW
3549everything to the end of the string. If LEN is negative, leaves that
3550many characters off the end of the string.
3551
84902520
TB
3552If you specify a substring which is partly outside the string, the part
3553within the string is returned. If the substring is totally outside
3554the string a warning is produced.
3555
748a9306 3556You can use the substr() function
a0d0e21e
LW
3557as an lvalue, in which case EXPR must be an lvalue. If you assign
3558something shorter than LEN, the string will shrink, and if you assign
3559something longer than LEN, the string will grow to accommodate it. To
3560keep the string the same length you may need to pad or chop your value
3561using sprintf().
3562
7b8d334a
GS
3563An alternative to using substr() as an lvalue is to specify the
3564replacement string as the 4th argument. This allows you to replace
3565parts of the EXPR and return what was there before in one operation.
3566In this case LEN can be C<undef> if you want to affect everything to
3567the end of the string.
3568
a0d0e21e
LW
3569=item symlink OLDFILE,NEWFILE
3570
3571Creates a new filename symbolically linked to the old filename.
3572Returns 1 for success, 0 otherwise. On systems that don't support
3573symbolic links, produces a fatal error at run time. To check for that,
3574use eval:
3575
54310121 3576 $symlink_exists = (eval {symlink("","")};, $@ eq '');
a0d0e21e
LW
3577
3578=item syscall LIST
3579
3580Calls the system call specified as the first element of the list,
3581passing the remaining elements as arguments to the system call. If
3582unimplemented, produces a fatal error. The arguments are interpreted
3583as follows: if a given argument is numeric, the argument is passed as
3584an int. If not, the pointer to the string value is passed. You are
3585responsible to make sure a string is pre-extended long enough to
a3cb178b
GS
3586receive any result that might be written into a string. You can't use a
3587string literal (or other read-only string) as an argument to syscall()
3588because Perl has to assume that any string pointer might be written
3589through. If your
a0d0e21e
LW
3590integer arguments are not literals and have never been interpreted in a
3591numeric context, you may need to add 0 to them to force them to look
3592like numbers.
3593
3594 require 'syscall.ph'; # may need to run h2ph
a3cb178b
GS
3595 $s = "hi there\n";
3596 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
a0d0e21e 3597
5f05dabc 3598Note that Perl supports passing of up to only 14 arguments to your system call,
a0d0e21e
LW
3599which in practice should usually suffice.
3600
fb73857a
PP
3601Syscall returns whatever value returned by the system call it calls.
3602If the system call fails, syscall returns -1 and sets C<$!> (errno).
3603Note that some system calls can legitimately return -1. The proper
3604way to handle such calls is to assign C<$!=0;> before the call and
7b8d334a 3605check the value of C<$!> if syscall returns -1.
fb73857a
PP
3606
3607There's a problem with C<syscall(&SYS_pipe)>: it returns the file
3608number of the read end of the pipe it creates. There is no way
3609to retrieve the file number of the other end. You can avoid this
3610problem by using C<pipe> instead.