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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
a0d0e21e
LW
606=item cos EXPR
607
608Returns the cosine of EXPR (expressed in radians). If EXPR is omitted
609takes cosine of $_.
610
28757baa
PP
611For the inverse cosine operation, you may use the POSIX::acos()
612function, or use this relation:
613
614 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
615
a0d0e21e
LW
616=item crypt PLAINTEXT,SALT
617
4633a7c4
LW
618Encrypts a string exactly like the crypt(3) function in the C library
619(assuming that you actually have a version there that has not been
620extirpated as a potential munition). This can prove useful for checking
621the password file for lousy passwords, amongst other things. Only the
622guys wearing white hats should do this.
a0d0e21e 623
11155c91
CS
624Note that crypt is intended to be a one-way function, much like breaking
625eggs to make an omelette. There is no (known) corresponding decrypt
626function. As a result, this function isn't all that useful for
627cryptography. (For that, see your nearby CPAN mirror.)
2f9daede 628
a0d0e21e
LW
629Here's an example that makes sure that whoever runs this program knows
630their own password:
631
632 $pwd = (getpwuid($<))[1];
633 $salt = substr($pwd, 0, 2);
634
635 system "stty -echo";
636 print "Password: ";
637 chop($word = <STDIN>);
638 print "\n";
639 system "stty echo";
640
641 if (crypt($word, $salt) ne $pwd) {
642 die "Sorry...\n";
643 } else {
644 print "ok\n";
54310121 645 }
a0d0e21e 646
9f8f0c9d 647Of course, typing in your own password to whoever asks you
748a9306 648for it is unwise.
a0d0e21e 649
aa689395 650=item dbmclose HASH
a0d0e21e
LW
651
652[This function has been superseded by the untie() function.]
653
aa689395 654Breaks the binding between a DBM file and a hash.
a0d0e21e 655
aa689395 656=item dbmopen HASH,DBNAME,MODE
a0d0e21e
LW
657
658[This function has been superseded by the tie() function.]
659
aa689395
PP
660This binds a dbm(3), ndbm(3), sdbm(3), gdbm(), or Berkeley DB file to a
661hash. HASH is the name of the hash. (Unlike normal open, the first
662argument is I<NOT> a filehandle, even though it looks like one). DBNAME
663is the name of the database (without the F<.dir> or F<.pag> extension if
664any). If the database does not exist, it is created with protection
665specified by MODE (as modified by the umask()). If your system supports
666only the older DBM functions, you may perform only one dbmopen() in your
667program. In older versions of Perl, if your system had neither DBM nor
668ndbm, calling dbmopen() produced a fatal error; it now falls back to
669sdbm(3).
670
671If you don't have write access to the DBM file, you can only read hash
672variables, not set them. If you want to test whether you can write,
673either use file tests or try setting a dummy hash entry inside an eval(),
674which will trap the error.
a0d0e21e
LW
675
676Note that functions such as keys() and values() may return huge array
677values when used on large DBM files. You may prefer to use the each()
678function to iterate over large DBM files. Example:
679
680 # print out history file offsets
681 dbmopen(%HIST,'/usr/lib/news/history',0666);
682 while (($key,$val) = each %HIST) {
683 print $key, ' = ', unpack('L',$val), "\n";
684 }
685 dbmclose(%HIST);
686
cb1a09d0 687See also L<AnyDBM_File> for a more general description of the pros and
184e9718 688cons of the various dbm approaches, as well as L<DB_File> for a particularly
cb1a09d0 689rich implementation.
4633a7c4 690
a0d0e21e
LW
691=item defined EXPR
692
54310121 693=item defined
bbce6d69 694
2f9daede
TPG
695Returns a Boolean value telling whether EXPR has a value other than
696the undefined value C<undef>. If EXPR is not present, C<$_> will be
697checked.
698
699Many operations return C<undef> to indicate failure, end of file,
700system error, uninitialized variable, and other exceptional
701conditions. This function allows you to distinguish C<undef> from
702other values. (A simple Boolean test will not distinguish among
703C<undef>, zero, the empty string, and "0", which are all equally
704false.) Note that since C<undef> is a valid scalar, its presence
705doesn't I<necessarily> indicate an exceptional condition: pop()
706returns C<undef> when its argument is an empty array, I<or> when the
707element to return happens to be C<undef>.
708
709You may also use defined() to check whether a subroutine exists. On
710the other hand, use of defined() upon aggregates (hashes and arrays)
711is not guaranteed to produce intuitive results, and should probably be
712avoided.
713
714When used on a hash element, it tells you whether the value is defined,
dc848c6f 715not whether the key exists in the hash. Use L</exists> for the latter
2f9daede 716purpose.
a0d0e21e
LW
717
718Examples:
719
720 print if defined $switch{'D'};
721 print "$val\n" while defined($val = pop(@ary));
722 die "Can't readlink $sym: $!"
723 unless defined($value = readlink $sym);
a0d0e21e 724 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
2f9daede 725 $debugging = 0 unless defined $debugging;
a0d0e21e 726
2f9daede
TPG
727Note: Many folks tend to overuse defined(), and then are surprised to
728discover that the number 0 and "" (the zero-length string) are, in fact,
729defined values. For example, if you say
a5f75d66
AD
730
731 "ab" =~ /a(.*)b/;
732
733the pattern match succeeds, and $1 is defined, despite the fact that it
734matched "nothing". But it didn't really match nothing--rather, it
735matched something that happened to be 0 characters long. This is all
736very above-board and honest. When a function returns an undefined value,
2f9daede
TPG
737it's an admission that it couldn't give you an honest answer. So you
738should use defined() only when you're questioning the integrity of what
739you're trying to do. At other times, a simple comparison to 0 or "" is
740what you want.
741
742Currently, using defined() on an entire array or hash reports whether
743memory for that aggregate has ever been allocated. So an array you set
744to the empty list appears undefined initially, and one that once was full
745and that you then set to the empty list still appears defined. You
746should instead use a simple test for size:
28757baa
PP
747
748 if (@an_array) { print "has array elements\n" }
749 if (%a_hash) { print "has hash members\n" }
750
751Using undef() on these, however, does clear their memory and then report
752them as not defined anymore, but you shoudln't do that unless you don't
753plan to use them again, because it saves time when you load them up
754again to have memory already ready to be filled.
755
54310121 756This counterintuitive behaviour of defined() on aggregates may be
28757baa
PP
757changed, fixed, or broken in a future release of Perl.
758
dc848c6f 759See also L</undef>, L</exists>, L</ref>.
2f9daede 760
a0d0e21e
LW
761=item delete EXPR
762
aa689395
PP
763Deletes the specified key(s) and their associated values from a hash.
764For each key, returns the deleted value associated with that key, or
765the undefined value if there was no such key. Deleting from C<$ENV{}>
766modifies the environment. Deleting from a hash tied to a DBM file
5f05dabc
PP
767deletes the entry from the DBM file. (But deleting from a tie()d hash
768doesn't necessarily return anything.)
a0d0e21e 769
aa689395 770The following deletes all the values of a hash:
a0d0e21e 771
5f05dabc
PP
772 foreach $key (keys %HASH) {
773 delete $HASH{$key};
a0d0e21e
LW
774 }
775
5f05dabc
PP
776And so does this:
777
778 delete @HASH{keys %HASH}
779
780(But both of these are slower than the undef() command.) Note that the
781EXPR can be arbitrarily complicated as long as the final operation is a
782hash element lookup or hash slice:
a0d0e21e
LW
783
784 delete $ref->[$x][$y]{$key};
5f05dabc 785 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
a0d0e21e
LW
786
787=item die LIST
788
789Outside of an eval(), prints the value of LIST to C<STDERR> and exits with
184e9718 790the current value of C<$!> (errno). If C<$!> is 0, exits with the value of
54310121 791C<($? E<gt>E<gt> 8)> (backtick `command` status). If C<($? E<gt>E<gt> 8)>
28757baa
PP
792is 0, exits with 255. Inside an eval(), the error message is stuffed into
793C<$@>, and the eval() is terminated with the undefined value; this makes
794die() the way to raise an exception.
a0d0e21e
LW
795
796Equivalent examples:
797
798 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
54310121 799 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
a0d0e21e
LW
800
801If the value of EXPR does not end in a newline, the current script line
802number and input line number (if any) are also printed, and a newline
803is supplied. Hint: sometimes appending ", stopped" to your message
804will cause it to make better sense when the string "at foo line 123" is
805appended. Suppose you are running script "canasta".
806
807 die "/etc/games is no good";
808 die "/etc/games is no good, stopped";
809
810produce, respectively
811
812 /etc/games is no good at canasta line 123.
813 /etc/games is no good, stopped at canasta line 123.
814
815See also exit() and warn().
816
fb73857a
PP
817If LIST is empty and $@ already contains a value (typically from a
818previous eval) that value is reused after appending "\t...propagated".
819This is useful for propagating exceptions:
820
821 eval { ... };
822 die unless $@ =~ /Expected exception/;
823
824If $@ is empty then the string "Died" is used.
825
774d564b
PP
826You can arrange for a callback to be called just before the die() does
827its deed, by setting the C<$SIG{__DIE__}> hook. The associated handler
828will be called with the error text and can change the error message, if
fb73857a
PP
829it sees fit, by calling die() again. See L<perlvar/$SIG{expr}> for details on
830setting C<%SIG> entries, and L<"eval BLOCK"> for some examples.
831
832Note that the C<$SIG{__DIE__}> hook is called even inside eval()ed
833blocks/strings. If one wants the hook to do nothing in such
834situations, put
835
836 die @_ if $^S;
837
838as the first line of the handler (see L<perlvar/$^S>).
774d564b 839
a0d0e21e
LW
840=item do BLOCK
841
842Not really a function. Returns the value of the last command in the
843sequence of commands indicated by BLOCK. When modified by a loop
844modifier, executes the BLOCK once before testing the loop condition.
845(On other statements the loop modifiers test the conditional first.)
846
847=item do SUBROUTINE(LIST)
848
849A deprecated form of subroutine call. See L<perlsub>.
850
851=item do EXPR
852
853Uses the value of EXPR as a filename and executes the contents of the
854file as a Perl script. Its primary use is to include subroutines
855from a Perl subroutine library.
856
857 do 'stat.pl';
858
859is just like
860
fb73857a 861 scalar eval `cat stat.pl`;
a0d0e21e
LW
862
863except that it's more efficient, more concise, keeps track of the
864current filename for error messages, and searches all the B<-I>
865libraries if the file isn't in the current directory (see also the @INC
dc1be6b5
GS
866array in L<perlvar/Predefined Names>). It is also different in how
867code evaluated with C<do FILENAME> doesn't see lexicals in the enclosing
868scope like C<eval STRING> does. It's the same, however, in that it does
54310121 869reparse the file every time you call it, so you probably don't want to
a0d0e21e
LW
870do this inside a loop.
871
872Note that inclusion of library modules is better done with the
4633a7c4
LW
873use() and require() operators, which also do error checking
874and raise an exception if there's a problem.
a0d0e21e
LW
875
876=item dump LABEL
877
878This causes an immediate core dump. Primarily this is so that you can
879use the B<undump> program to turn your core dump into an executable binary
880after having initialized all your variables at the beginning of the
881program. When the new binary is executed it will begin by executing a
882C<goto LABEL> (with all the restrictions that C<goto> suffers). Think of
883it as a goto with an intervening core dump and reincarnation. If LABEL
884is omitted, restarts the program from the top. WARNING: any files
885opened at the time of the dump will NOT be open any more when the
886program is reincarnated, with possible resulting confusion on the part
887of Perl. See also B<-u> option in L<perlrun>.
888
889Example:
890
891 #!/usr/bin/perl
892 require 'getopt.pl';
893 require 'stat.pl';
894 %days = (
895 'Sun' => 1,
896 'Mon' => 2,
897 'Tue' => 3,
898 'Wed' => 4,
899 'Thu' => 5,
900 'Fri' => 6,
901 'Sat' => 7,
902 );
903
904 dump QUICKSTART if $ARGV[0] eq '-d';
905
906 QUICKSTART:
907 Getopt('f');
908
aa689395
PP
909=item each HASH
910
911When called in a list context, returns a 2-element array consisting of the
912key and value for the next element of a hash, so that you can iterate over
913it. When called in a scalar context, returns the key for only the next
2f9daede
TPG
914element in the hash. (Note: Keys may be "0" or "", which are logically
915false; you may wish to avoid constructs like C<while ($k = each %foo) {}>
916for this reason.)
917
918Entries are returned in an apparently random order. When the hash is
919entirely read, a null array is returned in list context (which when
920assigned produces a FALSE (0) value), and C<undef> is returned in a
921scalar context. The next call to each() after that will start iterating
922again. There is a single iterator for each hash, shared by all each(),
923keys(), and values() function calls in the program; it can be reset by
924reading all the elements from the hash, or by evaluating C<keys HASH> or
925C<values HASH>. If you add or delete elements of a hash while you're
926iterating over it, you may get entries skipped or duplicated, so don't.
aa689395
PP
927
928The following prints out your environment like the printenv(1) program,
929only in a different order:
a0d0e21e
LW
930
931 while (($key,$value) = each %ENV) {
932 print "$key=$value\n";
933 }
934
935See also keys() and values().
936
937=item eof FILEHANDLE
938
4633a7c4
LW
939=item eof ()
940
a0d0e21e
LW
941=item eof
942
943Returns 1 if the next read on FILEHANDLE will return end of file, or if
944FILEHANDLE is not open. FILEHANDLE may be an expression whose value
945gives the real filehandle name. (Note that this function actually
946reads a character and then ungetc()s it, so it is not very useful in an
748a9306
LW
947interactive context.) Do not read from a terminal file (or call
948C<eof(FILEHANDLE)> on it) after end-of-file is reached. Filetypes such
949as terminals may lose the end-of-file condition if you do.
950
951An C<eof> without an argument uses the last file read as argument.
2f9daede
TPG
952Empty parentheses () may be used to indicate the pseudo file formed of
953the files listed on the command line, i.e., C<eof()> is reasonable to
954use inside a C<while (E<lt>E<gt>)> loop to detect the end of only the
955last file. Use C<eof(ARGV)> or eof without the parentheses to test
956I<EACH> file in a while (E<lt>E<gt>) loop. Examples:
a0d0e21e 957
748a9306
LW
958 # reset line numbering on each input file
959 while (<>) {
960 print "$.\t$_";
961 close(ARGV) if (eof); # Not eof().
962 }
963
a0d0e21e
LW
964 # insert dashes just before last line of last file
965 while (<>) {
966 if (eof()) {
967 print "--------------\n";
748a9306
LW
968 close(ARGV); # close or break; is needed if we
969 # are reading from the terminal
a0d0e21e
LW
970 }
971 print;
972 }
973
a0d0e21e 974Practical hint: you almost never need to use C<eof> in Perl, because the
54310121 975input operators return undef when they run out of data.
a0d0e21e
LW
976
977=item eval EXPR
978
979=item eval BLOCK
980
c7cc6f1c
GS
981In the first form, the return value of EXPR is parsed and executed as if it
982were a little Perl program. The value of the expression (which is itself
983determined within a scalar context) is first parsed, and if there are no
984errors, executed in the context of the current Perl program, so that any
5f05dabc 985variable settings or subroutine and format definitions remain afterwards.
c7cc6f1c
GS
986Note that the value is parsed every time the eval executes. If EXPR is
987omitted, evaluates C<$_>. This form is typically used to delay parsing
988and subsequent execution of the text of EXPR until run time.
989
990In the second form, the code within the BLOCK is parsed only once--at the
991same time the code surrounding the eval itself was parsed--and executed
992within the context of the current Perl program. This form is typically
993used to trap exceptions more efficiently than the first (see below), while
994also providing the benefit of checking the code within BLOCK at compile
995time.
996
997The final semicolon, if any, may be omitted from the value of EXPR or within
998the BLOCK.
999
1000In both forms, the value returned is the value of the last expression
1001evaluated inside the mini-program, or a return statement may be used, just
1002as with subroutines. The expression providing the return value is evaluated
1003in void, scalar or array context, depending on the context of the eval itself.
1004See L</wantarray> for more on how the evaluation context can be determined.
a0d0e21e
LW
1005
1006If there is a syntax error or runtime error, or a die() statement is
1007executed, an undefined value is returned by eval(), and C<$@> is set to the
1008error message. If there was no error, C<$@> is guaranteed to be a null
c7cc6f1c
GS
1009string. Beware that using eval() neither silences perl from printing
1010warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1011To do either of those, you have to use the C<$SIG{__WARN__}> facility. See
1012L</warn> and L<perlvar>.
a0d0e21e 1013
5f05dabc 1014Note that, because eval() traps otherwise-fatal errors, it is useful for
4633a7c4 1015determining whether a particular feature (such as socket() or symlink())
a0d0e21e
LW
1016is implemented. It is also Perl's exception trapping mechanism, where
1017the die operator is used to raise exceptions.
1018
1019If the code to be executed doesn't vary, you may use the eval-BLOCK
1020form to trap run-time errors without incurring the penalty of
1021recompiling each time. The error, if any, is still returned in C<$@>.
1022Examples:
1023
54310121 1024 # make divide-by-zero nonfatal
a0d0e21e
LW
1025 eval { $answer = $a / $b; }; warn $@ if $@;
1026
1027 # same thing, but less efficient
1028 eval '$answer = $a / $b'; warn $@ if $@;
1029
1030 # a compile-time error
1031 eval { $answer = };
1032
1033 # a run-time error
1034 eval '$answer ='; # sets $@
1035
774d564b
PP
1036When using the eval{} form as an exception trap in libraries, you may
1037wish not to trigger any C<__DIE__> hooks that user code may have
1038installed. You can use the C<local $SIG{__DIE__}> construct for this
1039purpose, as shown in this example:
1040
1041 # a very private exception trap for divide-by-zero
1042 eval { local $SIG{'__DIE__'}; $answer = $a / $b; }; warn $@ if $@;
1043
1044This is especially significant, given that C<__DIE__> hooks can call
1045die() again, which has the effect of changing their error messages:
1046
1047 # __DIE__ hooks may modify error messages
1048 {
1049 local $SIG{'__DIE__'} = sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
c7cc6f1c
GS
1050 eval { die "foo lives here" };
1051 print $@ if $@; # prints "bar lives here"
774d564b
PP
1052 }
1053
54310121 1054With an eval(), you should be especially careful to remember what's
a0d0e21e
LW
1055being looked at when:
1056
1057 eval $x; # CASE 1
1058 eval "$x"; # CASE 2
1059
1060 eval '$x'; # CASE 3
1061 eval { $x }; # CASE 4
1062
1063 eval "\$$x++" # CASE 5
1064 $$x++; # CASE 6
1065
2f9daede
TPG
1066Cases 1 and 2 above behave identically: they run the code contained in
1067the variable $x. (Although case 2 has misleading double quotes making
1068the reader wonder what else might be happening (nothing is).) Cases 3
1069and 4 likewise behave in the same way: they run the code '$x', which
1070does nothing but return the value of C<$x>. (Case 4 is preferred for
1071purely visual reasons, but it also has the advantage of compiling at
1072compile-time instead of at run-time.) Case 5 is a place where
54310121 1073normally you I<WOULD> like to use double quotes, except that in this
2f9daede
TPG
1074particular situation, you can just use symbolic references instead, as
1075in case 6.
a0d0e21e
LW
1076
1077=item exec LIST
1078
8bf3b016
GS
1079=item exec PROGRAM LIST
1080
fb73857a
PP
1081The exec() function executes a system command I<AND NEVER RETURNS> -
1082use system() instead of exec() if you want it to return. It fails and
1083returns FALSE only if the command does not exist I<and> it is executed
1084directly instead of via your system's command shell (see below).
a0d0e21e
LW
1085
1086If there is more than one argument in LIST, or if LIST is an array with
1087more than one value, calls execvp(3) with the arguments in LIST. If
1088there is only one scalar argument, the argument is checked for shell
bb32b41a
GS
1089metacharacters, and if there are any, the entire argument is passed to
1090the system's command shell for parsing (this is C</bin/sh -c> on Unix
1091platforms, but varies on other platforms). If there are no shell
1092metacharacters in the argument, it is split into words and passed
1093directly to execvp(), which is more efficient. Note: exec() and
1094system() do not flush your output buffer, so you may need to set C<$|>
1095to avoid lost output. Examples:
a0d0e21e
LW
1096
1097 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1098 exec "sort $outfile | uniq";
1099
1100If you don't really want to execute the first argument, but want to lie
1101to the program you are executing about its own name, you can specify
1102the program you actually want to run as an "indirect object" (without a
1103comma) in front of the LIST. (This always forces interpretation of the
54310121 1104LIST as a multivalued list, even if there is only a single scalar in
a0d0e21e
LW
1105the list.) Example:
1106
1107 $shell = '/bin/csh';
1108 exec $shell '-sh'; # pretend it's a login shell
1109
1110or, more directly,
1111
1112 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1113
bb32b41a
GS
1114When the arguments get executed via the system shell, results will
1115be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1116for details.
1117
a0d0e21e
LW
1118=item exists EXPR
1119
1120Returns TRUE if the specified hash key exists in its hash array, even
1121if the corresponding value is undefined.
1122
1123 print "Exists\n" if exists $array{$key};
1124 print "Defined\n" if defined $array{$key};
1125 print "True\n" if $array{$key};
1126
5f05dabc 1127A hash element can be TRUE only if it's defined, and defined if
a0d0e21e
LW
1128it exists, but the reverse doesn't necessarily hold true.
1129
1130Note that the EXPR can be arbitrarily complicated as long as the final
1131operation is a hash key lookup:
1132
1133 if (exists $ref->[$x][$y]{$key}) { ... }
1134
1135=item exit EXPR
1136
1137Evaluates EXPR and exits immediately with that value. (Actually, it
1138calls any defined C<END> routines first, but the C<END> routines may not
1139abort the exit. Likewise any object destructors that need to be called
1140are called before exit.) Example:
1141
1142 $ans = <STDIN>;
1143 exit 0 if $ans =~ /^[Xx]/;
1144
f86702cc 1145See also die(). If EXPR is omitted, exits with 0 status. The only
54310121 1146universally portable values for EXPR are 0 for success and 1 for error;
f86702cc
PP
1147all other values are subject to unpredictable interpretation depending
1148on the environment in which the Perl program is running.
a0d0e21e 1149
28757baa
PP
1150You shouldn't use exit() to abort a subroutine if there's any chance that
1151someone might want to trap whatever error happened. Use die() instead,
1152which can be trapped by an eval().
1153
a0d0e21e
LW
1154=item exp EXPR
1155
54310121 1156=item exp
bbce6d69 1157
54310121 1158Returns I<e> (the natural logarithm base) to the power of EXPR.
a0d0e21e
LW
1159If EXPR is omitted, gives C<exp($_)>.
1160
1161=item fcntl FILEHANDLE,FUNCTION,SCALAR
1162
1163Implements the fcntl(2) function. You'll probably have to say
1164
1165 use Fcntl;
1166
1167first to get the correct function definitions. Argument processing and
1168value return works just like ioctl() below. Note that fcntl() will produce
1169a fatal error if used on a machine that doesn't implement fcntl(2).
1170For example:
1171
1172 use Fcntl;
1173 fcntl($filehandle, F_GETLK, $packed_return_buffer);
1174
1175=item fileno FILEHANDLE
1176
1177Returns the file descriptor for a filehandle. This is useful for
1178constructing bitmaps for select(). If FILEHANDLE is an expression, the
1179value is taken as the name of the filehandle.
1180
1181=item flock FILEHANDLE,OPERATION
1182
8ebc5c01 1183Calls flock(2), or an emulation of it, on FILEHANDLE. Returns TRUE for
68dc0745
PP
1184success, FALSE on failure. Produces a fatal error if used on a machine
1185that doesn't implement flock(2), fcntl(2) locking, or lockf(3). flock()
1186is Perl's portable file locking interface, although it locks only entire
1187files, not records.
8ebc5c01 1188
a3cb178b
GS
1189On many platforms (including most versions or clones of Unix), locks
1190established by flock() are B<merely advisory>. This means that files
1191locked with flock() may be modified by programs which do not also use
1192flock(). Windows NT and OS/2, however, are among the platforms which
1193supply mandatory locking. See your local documentation for details.
1194
8ebc5c01
PP
1195OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1196LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
68dc0745
PP
1197you can use the symbolic names if import them from the Fcntl module,
1198either individually, or as a group using the ':flock' tag. LOCK_SH
1199requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1200releases a previously requested lock. If LOCK_NB is added to LOCK_SH or
1201LOCK_EX then flock() will return immediately rather than blocking
1202waiting for the lock (check the return status to see if you got it).
1203
1204To avoid the possibility of mis-coordination, Perl flushes FILEHANDLE
1205before (un)locking it.
8ebc5c01
PP
1206
1207Note that the emulation built with lockf(3) doesn't provide shared
1208locks, and it requires that FILEHANDLE be open with write intent. These
1209are the semantics that lockf(3) implements. Most (all?) systems
1210implement lockf(3) in terms of fcntl(2) locking, though, so the
1211differing semantics shouldn't bite too many people.
1212
1213Note also that some versions of flock() cannot lock things over the
1214network; you would need to use the more system-specific fcntl() for
1215that. If you like you can force Perl to ignore your system's flock(2)
1216function, and so provide its own fcntl(2)-based emulation, by passing
1217the switch C<-Ud_flock> to the F<Configure> program when you configure
1218perl.
4633a7c4
LW
1219
1220Here's a mailbox appender for BSD systems.
a0d0e21e 1221
7e1af8bc 1222 use Fcntl ':flock'; # import LOCK_* constants
a0d0e21e
LW
1223
1224 sub lock {
7e1af8bc 1225 flock(MBOX,LOCK_EX);
a0d0e21e
LW
1226 # and, in case someone appended
1227 # while we were waiting...
1228 seek(MBOX, 0, 2);
1229 }
1230
1231 sub unlock {
7e1af8bc 1232 flock(MBOX,LOCK_UN);
a0d0e21e
LW
1233 }
1234
1235 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1236 or die "Can't open mailbox: $!";
1237
1238 lock();
1239 print MBOX $msg,"\n\n";
1240 unlock();
1241
cb1a09d0 1242See also L<DB_File> for other flock() examples.
a0d0e21e
LW
1243
1244=item fork
1245
1246Does a fork(2) system call. Returns the child pid to the parent process
4633a7c4 1247and 0 to the child process, or C<undef> if the fork is unsuccessful.
a0d0e21e 1248Note: unflushed buffers remain unflushed in both processes, which means
28757baa
PP
1249you may need to set C<$|> ($AUTOFLUSH in English) or call the autoflush()
1250method of IO::Handle to avoid duplicate output.
a0d0e21e
LW
1251
1252If you fork() without ever waiting on your children, you will accumulate
1253zombies:
1254
4633a7c4 1255 $SIG{CHLD} = sub { wait };
a0d0e21e 1256
54310121 1257There's also the double-fork trick (error checking on
a0d0e21e
LW
1258fork() returns omitted);
1259
1260 unless ($pid = fork) {
1261 unless (fork) {
1262 exec "what you really wanna do";
1263 die "no exec";
1264 # ... or ...
4633a7c4 1265 ## (some_perl_code_here)
a0d0e21e
LW
1266 exit 0;
1267 }
1268 exit 0;
1269 }
1270 waitpid($pid,0);
1271
cb1a09d0
AD
1272See also L<perlipc> for more examples of forking and reaping
1273moribund children.
1274
28757baa
PP
1275Note that if your forked child inherits system file descriptors like
1276STDIN and STDOUT that are actually connected by a pipe or socket, even
1277if you exit, the remote server (such as, say, httpd or rsh) won't think
1278you're done. You should reopen those to /dev/null if it's any issue.
1279
cb1a09d0
AD
1280=item format
1281
1282Declare a picture format with use by the write() function. For
1283example:
1284
54310121 1285 format Something =
cb1a09d0
AD
1286 Test: @<<<<<<<< @||||| @>>>>>
1287 $str, $%, '$' . int($num)
1288 .
1289
1290 $str = "widget";
184e9718 1291 $num = $cost/$quantity;
cb1a09d0
AD
1292 $~ = 'Something';
1293 write;
1294
1295See L<perlform> for many details and examples.
1296
a0d0e21e 1297
8903cb82 1298=item formline PICTURE,LIST
a0d0e21e 1299
4633a7c4 1300This is an internal function used by C<format>s, though you may call it
a0d0e21e
LW
1301too. It formats (see L<perlform>) a list of values according to the
1302contents of PICTURE, placing the output into the format output
4633a7c4
LW
1303accumulator, C<$^A> (or $ACCUMULATOR in English).
1304Eventually, when a write() is done, the contents of
a0d0e21e
LW
1305C<$^A> are written to some filehandle, but you could also read C<$^A>
1306yourself and then set C<$^A> back to "". Note that a format typically
1307does one formline() per line of form, but the formline() function itself
748a9306 1308doesn't care how many newlines are embedded in the PICTURE. This means
4633a7c4 1309that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
748a9306
LW
1310You may therefore need to use multiple formlines to implement a single
1311record format, just like the format compiler.
1312
5f05dabc 1313Be careful if you put double quotes around the picture, because an "C<@>"
748a9306 1314character may be taken to mean the beginning of an array name.
4633a7c4 1315formline() always returns TRUE. See L<perlform> for other examples.
a0d0e21e
LW
1316
1317=item getc FILEHANDLE
1318
1319=item getc
1320
1321Returns the next character from the input file attached to FILEHANDLE,
1322or a null string at end of file. If FILEHANDLE is omitted, reads from STDIN.
4633a7c4 1323This is not particularly efficient. It cannot be used to get unbuffered
cb1a09d0 1324single-characters, however. For that, try something more like:
4633a7c4
LW
1325
1326 if ($BSD_STYLE) {
1327 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1328 }
1329 else {
54310121 1330 system "stty", '-icanon', 'eol', "\001";
4633a7c4
LW
1331 }
1332
1333 $key = getc(STDIN);
1334
1335 if ($BSD_STYLE) {
1336 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1337 }
1338 else {
5f05dabc 1339 system "stty", 'icanon', 'eol', '^@'; # ASCII null
4633a7c4
LW
1340 }
1341 print "\n";
1342
54310121
PP
1343Determination of whether $BSD_STYLE should be set
1344is left as an exercise to the reader.
cb1a09d0 1345
28757baa
PP
1346The POSIX::getattr() function can do this more portably on systems
1347alleging POSIX compliance.
cb1a09d0 1348See also the C<Term::ReadKey> module from your nearest CPAN site;
54310121 1349details on CPAN can be found on L<perlmod/CPAN>.
a0d0e21e
LW
1350
1351=item getlogin
1352
1353Returns the current login from F</etc/utmp>, if any. If null, use
54310121 1354getpwuid().
a0d0e21e 1355
f86702cc 1356 $login = getlogin || getpwuid($<) || "Kilroy";
a0d0e21e 1357
da0045b7 1358Do not consider getlogin() for authentication: it is not as
4633a7c4
LW
1359secure as getpwuid().
1360
a0d0e21e
LW
1361=item getpeername SOCKET
1362
1363Returns the packed sockaddr address of other end of the SOCKET connection.
1364
4633a7c4
LW
1365 use Socket;
1366 $hersockaddr = getpeername(SOCK);
1367 ($port, $iaddr) = unpack_sockaddr_in($hersockaddr);
1368 $herhostname = gethostbyaddr($iaddr, AF_INET);
1369 $herstraddr = inet_ntoa($iaddr);
a0d0e21e
LW
1370
1371=item getpgrp PID
1372
47e29363
PP
1373Returns the current process group for the specified PID. Use
1374a PID of 0 to get the current process group for the
4633a7c4 1375current process. Will raise an exception if used on a machine that
a0d0e21e 1376doesn't implement getpgrp(2). If PID is omitted, returns process
47e29363
PP
1377group of current process. Note that the POSIX version of getpgrp()
1378does not accept a PID argument, so only PID==0 is truly portable.
a0d0e21e
LW
1379
1380=item getppid
1381
1382Returns the process id of the parent process.
1383
1384=item getpriority WHICH,WHO
1385
4633a7c4
LW
1386Returns the current priority for a process, a process group, or a user.
1387(See L<getpriority(2)>.) Will raise a fatal exception if used on a
a0d0e21e
LW
1388machine that doesn't implement getpriority(2).
1389
1390=item getpwnam NAME
1391
1392=item getgrnam NAME
1393
1394=item gethostbyname NAME
1395
1396=item getnetbyname NAME
1397
1398=item getprotobyname NAME
1399
1400=item getpwuid UID
1401
1402=item getgrgid GID
1403
1404=item getservbyname NAME,PROTO
1405
1406=item gethostbyaddr ADDR,ADDRTYPE
1407
1408=item getnetbyaddr ADDR,ADDRTYPE
1409
1410=item getprotobynumber NUMBER
1411
1412=item getservbyport PORT,PROTO
1413
1414=item getpwent
1415
1416=item getgrent
1417
1418=item gethostent
1419
1420=item getnetent
1421
1422=item getprotoent
1423
1424=item getservent
1425
1426=item setpwent
1427
1428=item setgrent
1429
1430=item sethostent STAYOPEN
1431
1432=item setnetent STAYOPEN
1433
1434=item setprotoent STAYOPEN
1435
1436=item setservent STAYOPEN
1437
1438=item endpwent
1439
1440=item endgrent
1441
1442=item endhostent
1443
1444=item endnetent
1445
1446=item endprotoent
1447
1448=item endservent
1449
1450These routines perform the same functions as their counterparts in the
1451system library. Within a list context, the return values from the
1452various get routines are as follows:
1453
1454 ($name,$passwd,$uid,$gid,
1455 $quota,$comment,$gcos,$dir,$shell) = getpw*
1456 ($name,$passwd,$gid,$members) = getgr*
1457 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1458 ($name,$aliases,$addrtype,$net) = getnet*
1459 ($name,$aliases,$proto) = getproto*
1460 ($name,$aliases,$port,$proto) = getserv*
1461
1462(If the entry doesn't exist you get a null list.)
1463
1464Within a scalar context, you get the name, unless the function was a
1465lookup by name, in which case you get the other thing, whatever it is.
1466(If the entry doesn't exist you get the undefined value.) For example:
1467
1468 $uid = getpwnam
1469 $name = getpwuid
1470 $name = getpwent
1471 $gid = getgrnam
1472 $name = getgrgid
1473 $name = getgrent
1474 etc.
1475
1476The $members value returned by I<getgr*()> is a space separated list of
1477the login names of the members of the group.
1478
1479For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1480C, it will be returned to you via C<$?> if the function call fails. The
1481@addrs value returned by a successful call is a list of the raw
1482addresses returned by the corresponding system library call. In the
1483Internet domain, each address is four bytes long and you can unpack it
1484by saying something like:
1485
1486 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1487
1488=item getsockname SOCKET
1489
1490Returns the packed sockaddr address of this end of the SOCKET connection.
1491
4633a7c4
LW
1492 use Socket;
1493 $mysockaddr = getsockname(SOCK);
1494 ($port, $myaddr) = unpack_sockaddr_in($mysockaddr);
a0d0e21e
LW
1495
1496=item getsockopt SOCKET,LEVEL,OPTNAME
1497
1498Returns the socket option requested, or undefined if there is an error.
1499
1500=item glob EXPR
1501
0a753a76
PP
1502=item glob
1503
68dc0745
PP
1504Returns the value of EXPR with filename expansions such as a shell would
1505do. This is the internal function implementing the C<E<lt>*.cE<gt>>
1506operator, but you can use it directly. If EXPR is omitted, $_ is used.
1507The C<E<lt>*.cE<gt>> operator is discussed in more detail in
1508L<perlop/"I/O Operators">.
a0d0e21e
LW
1509
1510=item gmtime EXPR
1511
1512Converts a time as returned by the time function to a 9-element array
54310121 1513with the time localized for the standard Greenwich time zone.
4633a7c4 1514Typically used as follows:
a0d0e21e 1515
54310121 1516 # 0 1 2 3 4 5 6 7 8
a0d0e21e
LW
1517 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1518 gmtime(time);
1519
1520All array elements are numeric, and come straight out of a struct tm.
1521In particular this means that $mon has the range 0..11 and $wday has
54310121
PP
1522the range 0..6 with sunday as day 0. Also, $year is the number of
1523years since 1900, I<not> simply the last two digits of the year.
2f9daede
TPG
1524
1525If EXPR is omitted, does C<gmtime(time())>.
a0d0e21e 1526
54310121 1527In a scalar context, returns the ctime(3) value:
0a753a76
PP
1528
1529 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
1530
54310121
PP
1531Also see the timegm() function provided by the Time::Local module,
1532and the strftime(3) function available via the POSIX module.
0a753a76 1533
a0d0e21e
LW
1534=item goto LABEL
1535
748a9306
LW
1536=item goto EXPR
1537
a0d0e21e
LW
1538=item goto &NAME
1539
1540The goto-LABEL form finds the statement labeled with LABEL and resumes
1541execution there. It may not be used to go into any construct that
1542requires initialization, such as a subroutine or a foreach loop. It
0a753a76
PP
1543also can't be used to go into a construct that is optimized away,
1544or to get out of a block or subroutine given to sort().
1545It can be used to go almost anywhere else within the dynamic scope,
a0d0e21e
LW
1546including out of subroutines, but it's usually better to use some other
1547construct such as last or die. The author of Perl has never felt the
1548need to use this form of goto (in Perl, that is--C is another matter).
1549
748a9306
LW
1550The goto-EXPR form expects a label name, whose scope will be resolved
1551dynamically. This allows for computed gotos per FORTRAN, but isn't
1552necessarily recommended if you're optimizing for maintainability:
1553
1554 goto ("FOO", "BAR", "GLARCH")[$i];
1555
a0d0e21e
LW
1556The goto-&NAME form is highly magical, and substitutes a call to the
1557named subroutine for the currently running subroutine. This is used by
1558AUTOLOAD subroutines that wish to load another subroutine and then
1559pretend that the other subroutine had been called in the first place
1560(except that any modifications to @_ in the current subroutine are
1561propagated to the other subroutine.) After the goto, not even caller()
1562will be able to tell that this routine was called first.
1563
1564=item grep BLOCK LIST
1565
1566=item grep EXPR,LIST
1567
54310121 1568This is similar in spirit to, but not the same as, grep(1)
2f9daede
TPG
1569and its relatives. In particular, it is not limited to using
1570regular expressions.
1571
a0d0e21e
LW
1572Evaluates the BLOCK or EXPR for each element of LIST (locally setting
1573$_ to each element) and returns the list value consisting of those
1574elements for which the expression evaluated to TRUE. In a scalar
1575context, returns the number of times the expression was TRUE.
1576
1577 @foo = grep(!/^#/, @bar); # weed out comments
1578
1579or equivalently,
1580
1581 @foo = grep {!/^#/} @bar; # weed out comments
1582
5f05dabc 1583Note that, because $_ is a reference into the list value, it can be used
a0d0e21e
LW
1584to modify the elements of the array. While this is useful and
1585supported, it can cause bizarre results if the LIST is not a named
2f9daede 1586array. Similarly, grep returns aliases into the original list,
2ae324a7 1587much like the way that L<Foreach Loops>'s index variable aliases the list
2f9daede 1588elements. That is, modifying an element of a list returned by grep
fb73857a 1589(for example, in a C<foreach>, C<map> or another C<grep>)
2f9daede 1590actually modifies the element in the original list.
a0d0e21e 1591
fb73857a 1592See also L</map> for an array composed of the results of the BLOCK or EXPR.
38325410 1593
a0d0e21e
LW
1594=item hex EXPR
1595
54310121 1596=item hex
bbce6d69 1597
54310121 1598Interprets EXPR as a hex string and returns the corresponding
2f9daede 1599value. (To convert strings that might start with either 0 or 0x
dc848c6f 1600see L</oct>.) If EXPR is omitted, uses $_.
2f9daede
TPG
1601
1602 print hex '0xAf'; # prints '175'
1603 print hex 'aF'; # same
a0d0e21e
LW
1604
1605=item import
1606
54310121 1607There is no builtin import() function. It is merely an ordinary
4633a7c4 1608method (subroutine) defined (or inherited) by modules that wish to export
a0d0e21e 1609names to another module. The use() function calls the import() method
54310121 1610for the package used. See also L</use()>, L<perlmod>, and L<Exporter>.
a0d0e21e
LW
1611
1612=item index STR,SUBSTR,POSITION
1613
1614=item index STR,SUBSTR
1615
4633a7c4
LW
1616Returns the position of the first occurrence of SUBSTR in STR at or after
1617POSITION. If POSITION is omitted, starts searching from the beginning of
184e9718 1618the string. The return value is based at 0 (or whatever you've set the C<$[>
4633a7c4 1619variable to--but don't do that). If the substring is not found, returns
a0d0e21e
LW
1620one less than the base, ordinarily -1.
1621
1622=item int EXPR
1623
54310121 1624=item int
bbce6d69 1625
a0d0e21e
LW
1626Returns the integer portion of EXPR. If EXPR is omitted, uses $_.
1627
1628=item ioctl FILEHANDLE,FUNCTION,SCALAR
1629
1630Implements the ioctl(2) function. You'll probably have to say
1631
4633a7c4 1632 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
a0d0e21e 1633
4633a7c4 1634first to get the correct function definitions. If F<ioctl.ph> doesn't
a0d0e21e 1635exist or doesn't have the correct definitions you'll have to roll your
4633a7c4
LW
1636own, based on your C header files such as F<E<lt>sys/ioctl.hE<gt>>.
1637(There is a Perl script called B<h2ph> that comes with the Perl kit which
54310121 1638may help you in this, but it's nontrivial.) SCALAR will be read and/or
4633a7c4
LW
1639written depending on the FUNCTION--a pointer to the string value of SCALAR
1640will be passed as the third argument of the actual ioctl call. (If SCALAR
1641has no string value but does have a numeric value, that value will be
1642passed rather than a pointer to the string value. To guarantee this to be
1643TRUE, add a 0 to the scalar before using it.) The pack() and unpack()
1644functions are useful for manipulating the values of structures used by
1645ioctl(). The following example sets the erase character to DEL.
a0d0e21e
LW
1646
1647 require 'ioctl.ph';
4633a7c4
LW
1648 $getp = &TIOCGETP;
1649 die "NO TIOCGETP" if $@ || !$getp;
a0d0e21e 1650 $sgttyb_t = "ccccs"; # 4 chars and a short
4633a7c4 1651 if (ioctl(STDIN,$getp,$sgttyb)) {
a0d0e21e
LW
1652 @ary = unpack($sgttyb_t,$sgttyb);
1653 $ary[2] = 127;
1654 $sgttyb = pack($sgttyb_t,@ary);
4633a7c4 1655 ioctl(STDIN,&TIOCSETP,$sgttyb)
a0d0e21e
LW
1656 || die "Can't ioctl: $!";
1657 }
1658
1659The return value of ioctl (and fcntl) is as follows:
1660
1661 if OS returns: then Perl returns:
1662 -1 undefined value
1663 0 string "0 but true"
1664 anything else that number
1665
1666Thus Perl returns TRUE on success and FALSE on failure, yet you can
1667still easily determine the actual value returned by the operating
1668system:
1669
1670 ($retval = ioctl(...)) || ($retval = -1);
1671 printf "System returned %d\n", $retval;
1672
1673=item join EXPR,LIST
1674
54310121 1675Joins the separate strings of LIST into a single string with
a0d0e21e
LW
1676fields separated by the value of EXPR, and returns the string.
1677Example:
1678
1679 $_ = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
1680
1681See L<perlfunc/split>.
1682
aa689395
PP
1683=item keys HASH
1684
1685Returns a normal array consisting of all the keys of the named hash. (In
1686a scalar context, returns the number of keys.) The keys are returned in
1687an apparently random order, but it is the same order as either the
1688values() or each() function produces (given that the hash has not been
1689modified). As a side effect, it resets HASH's iterator.
a0d0e21e 1690
aa689395 1691Here is yet another way to print your environment:
a0d0e21e
LW
1692
1693 @keys = keys %ENV;
1694 @values = values %ENV;
1695 while ($#keys >= 0) {
1696 print pop(@keys), '=', pop(@values), "\n";
1697 }
1698
1699or how about sorted by key:
1700
1701 foreach $key (sort(keys %ENV)) {
1702 print $key, '=', $ENV{$key}, "\n";
1703 }
1704
54310121 1705To sort an array by value, you'll need to use a C<sort> function.
aa689395 1706Here's a descending numeric sort of a hash by its values:
4633a7c4
LW
1707
1708 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash)) {
1709 printf "%4d %s\n", $hash{$key}, $key;
1710 }
1711
55497cff 1712As an lvalue C<keys> allows you to increase the number of hash buckets
aa689395
PP
1713allocated for the given hash. This can gain you a measure of efficiency if
1714you know the hash is going to get big. (This is similar to pre-extending
1715an array by assigning a larger number to $#array.) If you say
55497cff
PP
1716
1717 keys %hash = 200;
1718
1719then C<%hash> will have at least 200 buckets allocated for it. These
1720buckets will be retained even if you do C<%hash = ()>, use C<undef
1721%hash> if you want to free the storage while C<%hash> is still in scope.
1722You can't shrink the number of buckets allocated for the hash using
1723C<keys> in this way (but you needn't worry about doing this by accident,
1724as trying has no effect).
1725
a0d0e21e
LW
1726=item kill LIST
1727
54310121
PP
1728Sends a signal to a list of processes. The first element of
1729the list must be the signal to send. Returns the number of
4633a7c4 1730processes successfully signaled.
a0d0e21e
LW
1731
1732 $cnt = kill 1, $child1, $child2;
1733 kill 9, @goners;
1734
4633a7c4
LW
1735Unlike in the shell, in Perl if the I<SIGNAL> is negative, it kills
1736process groups instead of processes. (On System V, a negative I<PROCESS>
1737number will also kill process groups, but that's not portable.) That
1738means you usually want to use positive not negative signals. You may also
da0045b7 1739use a signal name in quotes. See L<perlipc/"Signals"> for details.
a0d0e21e
LW
1740
1741=item last LABEL
1742
1743=item last
1744
1745The C<last> command is like the C<break> statement in C (as used in
1746loops); it immediately exits the loop in question. If the LABEL is
1747omitted, the command refers to the innermost enclosing loop. The
1748C<continue> block, if any, is not executed:
1749
4633a7c4
LW
1750 LINE: while (<STDIN>) {
1751 last LINE if /^$/; # exit when done with header
a0d0e21e
LW
1752 ...
1753 }
1754
1755=item lc EXPR
1756
54310121 1757=item lc
bbce6d69 1758
a0d0e21e 1759Returns an lowercased version of EXPR. This is the internal function
54310121 1760implementing the \L escape in double-quoted strings.
a034a98d 1761Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
a0d0e21e 1762
bbce6d69
PP
1763If EXPR is omitted, uses $_.
1764
a0d0e21e
LW
1765=item lcfirst EXPR
1766
54310121 1767=item lcfirst
bbce6d69 1768
a0d0e21e
LW
1769Returns the value of EXPR with the first character lowercased. This is
1770the internal function implementing the \l escape in double-quoted strings.
a034a98d 1771Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
a0d0e21e 1772
bbce6d69
PP
1773If EXPR is omitted, uses $_.
1774
a0d0e21e
LW
1775=item length EXPR
1776
54310121 1777=item length
bbce6d69 1778
a0d0e21e
LW
1779Returns the length in characters of the value of EXPR. If EXPR is
1780omitted, returns length of $_.
1781
1782=item link OLDFILE,NEWFILE
1783
1784Creates a new filename linked to the old filename. Returns 1 for
1785success, 0 otherwise.
1786
1787=item listen SOCKET,QUEUESIZE
1788
1789Does the same thing that the listen system call does. Returns TRUE if
4633a7c4 1790it succeeded, FALSE otherwise. See example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
1791
1792=item local EXPR
1793
a0d0e21e 1794A local modifies the listed variables to be local to the enclosing block,
5f05dabc
PP
1795subroutine, C<eval{}>, or C<do>. If more than one value is listed, the
1796list must be placed in parentheses. See L<perlsub/"Temporary Values via
3e3baf6d 1797local()"> for details, including issues with tied arrays and hashes.
a0d0e21e 1798
cb1a09d0
AD
1799But you really probably want to be using my() instead, because local() isn't
1800what most people think of as "local"). See L<perlsub/"Private Variables
1801via my()"> for details.
a0d0e21e
LW
1802
1803=item localtime EXPR
1804
1805Converts a time as returned by the time function to a 9-element array
5f05dabc 1806with the time analyzed for the local time zone. Typically used as
a0d0e21e
LW
1807follows:
1808
54310121 1809 # 0 1 2 3 4 5 6 7 8
a0d0e21e
LW
1810 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1811 localtime(time);
1812
1813All array elements are numeric, and come straight out of a struct tm.
1814In particular this means that $mon has the range 0..11 and $wday has
54310121
PP
1815the range 0..6 with sunday as day 0. Also, $year is the number of
1816years since 1900, that is, $year is 123 in year 2023.
1817
1818If EXPR is omitted, uses the current time (C<localtime(time)>).
a0d0e21e 1819
0a753a76 1820In a scalar context, returns the ctime(3) value:
a0d0e21e 1821
5f05dabc 1822 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
a0d0e21e 1823
a3cb178b
GS
1824This scalar value is B<not> locale dependent, see L<perllocale>, but
1825instead a Perl builtin. Also see the Time::Local module, and the
1826strftime(3) and mktime(3) function available via the POSIX module. To
1827get somewhat similar but locale dependent date strings, set up your
1828locale environment variables appropriately (please see L<perllocale>)
1829and try for example
1830
1831 use POSIX qw(strftime)
1832 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
1833
1834Note that the C<%a> and C<%b>, the short forms of the day of the week
1835and the month of the year, may not necessarily be three characters wide.
a0d0e21e
LW
1836
1837=item log EXPR
1838
54310121 1839=item log
bbce6d69 1840
a0d0e21e
LW
1841Returns logarithm (base I<e>) of EXPR. If EXPR is omitted, returns log
1842of $_.
1843
1844=item lstat FILEHANDLE
1845
1846=item lstat EXPR
1847
54310121 1848=item lstat
bbce6d69 1849
a0d0e21e
LW
1850Does the same thing as the stat() function, but stats a symbolic link
1851instead of the file the symbolic link points to. If symbolic links are
1852unimplemented on your system, a normal stat() is done.
1853
bbce6d69
PP
1854If EXPR is omitted, stats $_.
1855
a0d0e21e
LW
1856=item m//
1857
1858The match operator. See L<perlop>.
1859
1860=item map BLOCK LIST
1861
1862=item map EXPR,LIST
1863
1864Evaluates the BLOCK or EXPR for each element of LIST (locally setting $_ to each
1865element) and returns the list value composed of the results of each such
1866evaluation. Evaluates BLOCK or EXPR in a list context, so each element of LIST
1867may produce zero, one, or more elements in the returned value.
1868
1869 @chars = map(chr, @nums);
1870
1871translates a list of numbers to the corresponding characters. And
1872
4633a7c4 1873 %hash = map { getkey($_) => $_ } @array;
a0d0e21e
LW
1874
1875is just a funny way to write
1876
1877 %hash = ();
1878 foreach $_ (@array) {
4633a7c4 1879 $hash{getkey($_)} = $_;
a0d0e21e
LW
1880 }
1881
fb73857a
PP
1882Note that, because $_ is a reference into the list value, it can be used
1883to modify the elements of the array. While this is useful and
1884supported, it can cause bizarre results if the LIST is not a named
1885array. See also L</grep> for an array composed of those items of the
1886original list for which the BLOCK or EXPR evaluates to true.
1887
a0d0e21e
LW
1888=item mkdir FILENAME,MODE
1889
1890Creates the directory specified by FILENAME, with permissions specified
1891by MODE (as modified by umask). If it succeeds it returns 1, otherwise
184e9718 1892it returns 0 and sets C<$!> (errno).
a0d0e21e
LW
1893
1894=item msgctl ID,CMD,ARG
1895
4633a7c4 1896Calls the System V IPC function msgctl(2). If CMD is &IPC_STAT, then ARG
a0d0e21e
LW
1897must be a variable which will hold the returned msqid_ds structure.
1898Returns like ioctl: the undefined value for error, "0 but true" for
1899zero, or the actual return value otherwise.
1900
1901=item msgget KEY,FLAGS
1902
4633a7c4 1903Calls the System V IPC function msgget(2). Returns the message queue id,
a0d0e21e
LW
1904or the undefined value if there is an error.
1905
1906=item msgsnd ID,MSG,FLAGS
1907
1908Calls the System V IPC function msgsnd to send the message MSG to the
1909message queue ID. MSG must begin with the long integer message type,
c07a80fd 1910which may be created with C<pack("l", $type)>. Returns TRUE if
a0d0e21e
LW
1911successful, or FALSE if there is an error.
1912
1913=item msgrcv ID,VAR,SIZE,TYPE,FLAGS
1914
1915Calls the System V IPC function msgrcv to receive a message from
1916message queue ID into variable VAR with a maximum message size of
1917SIZE. Note that if a message is received, the message type will be the
1918first thing in VAR, and the maximum length of VAR is SIZE plus the size
1919of the message type. Returns TRUE if successful, or FALSE if there is
1920an error.
1921
1922=item my EXPR
1923
1924A "my" declares the listed variables to be local (lexically) to the
cb1a09d0 1925enclosing block, subroutine, C<eval>, or C<do/require/use>'d file. If
5f05dabc 1926more than one value is listed, the list must be placed in parentheses. See
cb1a09d0 1927L<perlsub/"Private Variables via my()"> for details.
4633a7c4 1928
a0d0e21e
LW
1929=item next LABEL
1930
1931=item next
1932
1933The C<next> command is like the C<continue> statement in C; it starts
1934the next iteration of the loop:
1935
4633a7c4
LW
1936 LINE: while (<STDIN>) {
1937 next LINE if /^#/; # discard comments
a0d0e21e
LW
1938 ...
1939 }
1940
1941Note that if there were a C<continue> block on the above, it would get
1942executed even on discarded lines. If the LABEL is omitted, the command
1943refers to the innermost enclosing loop.
1944
1945=item no Module LIST
1946
1947See the "use" function, which "no" is the opposite of.
1948
1949=item oct EXPR
1950
54310121 1951=item oct
bbce6d69 1952
4633a7c4 1953Interprets EXPR as an octal string and returns the corresponding
2f9daede 1954value. (If EXPR happens to start off with 0x, interprets it as
4633a7c4
LW
1955a hex string instead.) The following will handle decimal, octal, and
1956hex in the standard Perl or C notation:
a0d0e21e
LW
1957
1958 $val = oct($val) if $val =~ /^0/;
1959
2f9daede
TPG
1960If EXPR is omitted, uses $_. This function is commonly used when
1961a string such as "644" needs to be converted into a file mode, for
1962example. (Although perl will automatically convert strings into
1963numbers as needed, this automatic conversion assumes base 10.)
a0d0e21e
LW
1964
1965=item open FILEHANDLE,EXPR
1966
1967=item open FILEHANDLE
1968
1969Opens the file whose filename is given by EXPR, and associates it with
5f05dabc
PP
1970FILEHANDLE. If FILEHANDLE is an expression, its value is used as the
1971name of the real filehandle wanted. If EXPR is omitted, the scalar
1972variable of the same name as the FILEHANDLE contains the filename.
1973(Note that lexical variables--those declared with C<my>--will not work
1974for this purpose; so if you're using C<my>, specify EXPR in your call
1975to open.)
1976
1977If the filename begins with '<' or nothing, the file is opened for input.
1978If the filename begins with '>', the file is truncated and opened for
1979output. If the filename begins with '>>', the file is opened for
1980appending. You can put a '+' in front of the '>' or '<' to indicate that
1981you want both read and write access to the file; thus '+<' is almost
1982always preferred for read/write updates--the '+>' mode would clobber the
1983file first. The prefix and the filename may be separated with spaces.
1984These various prefixes correspond to the fopen(3) modes of 'r', 'r+', 'w',
1985'w+', 'a', and 'a+'.
1986
1987If the filename begins with "|", the filename is interpreted as a command
1988to which output is to be piped, and if the filename ends with a "|", the
1989filename is interpreted See L<perlipc/"Using open() for IPC"> for more
1990examples of this. as command which pipes input to us. (You may not have
7e1af8bc
PP
1991a raw open() to a command that pipes both in I<and> out, but see
1992L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
1993for alternatives.)
cb1a09d0 1994
184e9718 1995Opening '-' opens STDIN and opening 'E<gt>-' opens STDOUT. Open returns
54310121 1996nonzero upon success, the undefined value otherwise. If the open
4633a7c4 1997involved a pipe, the return value happens to be the pid of the
54310121 1998subprocess.
cb1a09d0
AD
1999
2000If you're unfortunate enough to be running Perl on a system that
2001distinguishes between text files and binary files (modern operating
2002systems don't care), then you should check out L</binmode> for tips for
2003dealing with this. The key distinction between systems that need binmode
2004and those that don't is their text file formats. Systems like Unix and
2005Plan9 that delimit lines with a single character, and that encode that
2006character in C as '\n', do not need C<binmode>. The rest need it.
2007
fb73857a
PP
2008When opening a file, it's usually a bad idea to continue normal execution
2009if the request failed, so C<open> is frequently used in connection with
2010C<die>. Even if C<die> won't do what you want (say, in a CGI script,
2011where you want to make a nicely formatted error message (but there are
2012modules which can help with that problem)) you should always check
2013the return value from opening a file. The infrequent exception is when
2014working with an unopened filehandle is actually what you want to do.
2015
cb1a09d0 2016Examples:
a0d0e21e
LW
2017
2018 $ARTICLE = 100;
2019 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2020 while (<ARTICLE>) {...
2021
2022 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
fb73857a 2023 # if the open fails, output is discarded
a0d0e21e 2024
fb73857a
PP
2025 open(DBASE, '+<dbase.mine') # open for update
2026 or die "Can't open 'dbase.mine' for update: $!";
cb1a09d0 2027
fb73857a
PP
2028 open(ARTICLE, "caesar <$article |") # decrypt article
2029 or die "Can't start caesar: $!";
a0d0e21e 2030
fb73857a
PP
2031 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
2032 or die "Can't start sort: $!";
a0d0e21e
LW
2033
2034 # process argument list of files along with any includes
2035
2036 foreach $file (@ARGV) {
2037 process($file, 'fh00');
2038 }
2039
2040 sub process {
2041 local($filename, $input) = @_;
2042 $input++; # this is a string increment
2043 unless (open($input, $filename)) {
2044 print STDERR "Can't open $filename: $!\n";
2045 return;
2046 }
2047
2048 while (<$input>) { # note use of indirection
2049 if (/^#include "(.*)"/) {
2050 process($1, $input);
2051 next;
2052 }
2053 ... # whatever
2054 }
2055 }
2056
2057You may also, in the Bourne shell tradition, specify an EXPR beginning
184e9718 2058with "E<gt>&", in which case the rest of the string is interpreted as the
a0d0e21e 2059name of a filehandle (or file descriptor, if numeric) which is to be
184e9718 2060duped and opened. You may use & after E<gt>, E<gt>E<gt>, E<lt>, +E<gt>,
5f05dabc 2061+E<gt>E<gt>, and +E<lt>. The
a0d0e21e 2062mode you specify should match the mode of the original filehandle.
184e9718 2063(Duping a filehandle does not take into account any existing contents of
cb1a09d0 2064stdio buffers.)
a0d0e21e
LW
2065Here is a script that saves, redirects, and restores STDOUT and
2066STDERR:
2067
2068 #!/usr/bin/perl
2069 open(SAVEOUT, ">&STDOUT");
2070 open(SAVEERR, ">&STDERR");
2071
2072 open(STDOUT, ">foo.out") || die "Can't redirect stdout";
2073 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
2074
2075 select(STDERR); $| = 1; # make unbuffered
2076 select(STDOUT); $| = 1; # make unbuffered
2077
2078 print STDOUT "stdout 1\n"; # this works for
2079 print STDERR "stderr 1\n"; # subprocesses too
2080
2081 close(STDOUT);
2082 close(STDERR);
2083
2084 open(STDOUT, ">&SAVEOUT");
2085 open(STDERR, ">&SAVEERR");
2086
2087 print STDOUT "stdout 2\n";
2088 print STDERR "stderr 2\n";
2089
2090
184e9718 2091If you specify "E<lt>&=N", where N is a number, then Perl will do an
4633a7c4
LW
2092equivalent of C's fdopen() of that file descriptor; this is more
2093parsimonious of file descriptors. For example:
a0d0e21e
LW
2094
2095 open(FILEHANDLE, "<&=$fd")
2096
5f05dabc 2097If you open a pipe on the command "-", i.e., either "|-" or "-|", then
a0d0e21e
LW
2098there is an implicit fork done, and the return value of open is the pid
2099of the child within the parent process, and 0 within the child
184e9718 2100process. (Use C<defined($pid)> to determine whether the open was successful.)
a0d0e21e
LW
2101The filehandle behaves normally for the parent, but i/o to that
2102filehandle is piped from/to the STDOUT/STDIN of the child process.
2103In the child process the filehandle isn't opened--i/o happens from/to
2104the new STDOUT or STDIN. Typically this is used like the normal
2105piped open when you want to exercise more control over just how the
2106pipe command gets executed, such as when you are running setuid, and
54310121 2107don't want to have to scan shell commands for metacharacters.
4633a7c4 2108The following pairs are more or less equivalent:
a0d0e21e
LW
2109
2110 open(FOO, "|tr '[a-z]' '[A-Z]'");
2111 open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
2112
2113 open(FOO, "cat -n '$file'|");
2114 open(FOO, "-|") || exec 'cat', '-n', $file;
2115
4633a7c4
LW
2116See L<perlipc/"Safe Pipe Opens"> for more examples of this.
2117
0dccf244 2118NOTE: On any operation which may do a fork, unflushed buffers remain
184e9718 2119unflushed in both processes, which means you may need to set C<$|> to
a0d0e21e
LW
2120avoid duplicate output.
2121
0dccf244
CS
2122Closing any piped filehandle causes the parent process to wait for the
2123child to finish, and returns the status value in C<$?>.
2124
5f05dabc
PP
2125Using the constructor from the IO::Handle package (or one of its
2126subclasses, such as IO::File or IO::Socket),
c07a80fd
PP
2127you can generate anonymous filehandles which have the scope of whatever
2128variables hold references to them, and automatically close whenever
2129and however you leave that scope:
2130
5f05dabc 2131 use IO::File;
c07a80fd
PP
2132 ...
2133 sub read_myfile_munged {
2134 my $ALL = shift;
5f05dabc 2135 my $handle = new IO::File;
c07a80fd
PP
2136 open($handle, "myfile") or die "myfile: $!";
2137 $first = <$handle>
2138 or return (); # Automatically closed here.
2139 mung $first or die "mung failed"; # Or here.
2140 return $first, <$handle> if $ALL; # Or here.
2141 $first; # Or here.
2142 }
2143
a0d0e21e 2144The filename that is passed to open will have leading and trailing
5f05dabc 2145whitespace deleted. To open a file with arbitrary weird
a0d0e21e
LW
2146characters in it, it's necessary to protect any leading and trailing
2147whitespace thusly:
2148
cb1a09d0
AD
2149 $file =~ s#^(\s)#./$1#;
2150 open(FOO, "< $file\0");
2151
c07a80fd
PP
2152If you want a "real" C open() (see L<open(2)> on your system), then
2153you should use the sysopen() function. This is another way to
2154protect your filenames from interpretation. For example:
cb1a09d0 2155
28757baa 2156 use IO::Handle;
c07a80fd
PP
2157 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL, 0700)
2158 or die "sysopen $path: $!";
2159 HANDLE->autoflush(1);
2160 HANDLE->print("stuff $$\n");
2161 seek(HANDLE, 0, 0);
2162 print "File contains: ", <HANDLE>;
cb1a09d0
AD
2163
2164See L</seek()> for some details about mixing reading and writing.
a0d0e21e
LW
2165
2166=item opendir DIRHANDLE,EXPR
2167
2168Opens a directory named EXPR for processing by readdir(), telldir(),
5f05dabc 2169seekdir(), rewinddir(), and closedir(). Returns TRUE if successful.
a0d0e21e
LW
2170DIRHANDLEs have their own namespace separate from FILEHANDLEs.
2171
2172=item ord EXPR
2173
54310121 2174=item ord
bbce6d69 2175
a0d0e21e 2176Returns the numeric ascii value of the first character of EXPR. If
dc848c6f 2177EXPR is omitted, uses $_. For the reverse, see L</chr>.
a0d0e21e
LW
2178
2179=item pack TEMPLATE,LIST
2180
2181Takes an array or list of values and packs it into a binary structure,
2182returning the string containing the structure. The TEMPLATE is a
2183sequence of characters that give the order and type of values, as
2184follows:
2185
2186 A An ascii string, will be space padded.
2187 a An ascii string, will be null padded.
2188 b A bit string (ascending bit order, like vec()).
2189 B A bit string (descending bit order).
2190 h A hex string (low nybble first).
2191 H A hex string (high nybble first).
2192
2193 c A signed char value.
2194 C An unsigned char value.
96e4d5b1 2195
a0d0e21e
LW
2196 s A signed short value.
2197 S An unsigned short value.
96e4d5b1
PP
2198 (This 'short' is _exactly_ 16 bits, which may differ from
2199 what a local C compiler calls 'short'.)
2200
a0d0e21e
LW
2201 i A signed integer value.
2202 I An unsigned integer value.
96e4d5b1
PP
2203 (This 'integer' is _at_least_ 32 bits wide. Its exact size
2204 depends on what a local C compiler calls 'int', and may
2205 even be larger than the 'long' described in the next item.)
2206
a0d0e21e
LW
2207 l A signed long value.
2208 L An unsigned long value.
96e4d5b1
PP
2209 (This 'long' is _exactly_ 32 bits, which may differ from
2210 what a local C compiler calls 'long'.)
a0d0e21e 2211
96e4d5b1
PP
2212 n A short in "network" (big-endian) order.
2213 N A long in "network" (big-endian) order.
a0d0e21e
LW
2214 v A short in "VAX" (little-endian) order.
2215 V A long in "VAX" (little-endian) order.
96e4d5b1
PP
2216 (These 'shorts' and 'longs' are _exactly_ 16 bits and
2217 _exactly_ 32 bits, respectively.)
a0d0e21e
LW
2218
2219 f A single-precision float in the native format.
2220 d A double-precision float in the native format.
2221
2222 p A pointer to a null-terminated string.
2223 P A pointer to a structure (fixed-length string).
2224
2225 u A uuencoded string.
2226
96e4d5b1
PP
2227 w A BER compressed integer. Its bytes represent an unsigned
2228 integer in base 128, most significant digit first, with as few
2229 digits as possible. Bit eight (the high bit) is set on each
2230 byte except the last.
def98dd4 2231
a0d0e21e
LW
2232 x A null byte.
2233 X Back up a byte.
2234 @ Null fill to absolute position.
2235
2236Each letter may optionally be followed by a number which gives a repeat
5f05dabc 2237count. With all types except "a", "A", "b", "B", "h", "H", and "P" the
a0d0e21e
LW
2238pack function will gobble up that many values from the LIST. A * for the
2239repeat count means to use however many items are left. The "a" and "A"
2240types gobble just one value, but pack it as a string of length count,
2241padding with nulls or spaces as necessary. (When unpacking, "A" strips
2242trailing spaces and nulls, but "a" does not.) Likewise, the "b" and "B"
2243fields pack a string that many bits long. The "h" and "H" fields pack a
84902520
TB
2244string that many nybbles long. The "p" type packs a pointer to a null-
2245terminated string. You are responsible for ensuring the string is not a
2246temporary value (which can potentially get deallocated before you get
2247around to using the packed result). The "P" packs a pointer to a structure
61167c6f
PM
2248of the size indicated by the length. A NULL pointer is created if the
2249corresponding value for "p" or "P" is C<undef>.
2250Real numbers (floats and doubles) are
a0d0e21e
LW
2251in the native machine format only; due to the multiplicity of floating
2252formats around, and the lack of a standard "network" representation, no
2253facility for interchange has been made. This means that packed floating
2254point data written on one machine may not be readable on another - even if
2255both use IEEE floating point arithmetic (as the endian-ness of the memory
2256representation is not part of the IEEE spec). Note that Perl uses doubles
2257internally for all numeric calculation, and converting from double into
5f05dabc 2258float and thence back to double again will lose precision (i.e.,
a0d0e21e
LW
2259C<unpack("f", pack("f", $foo)>) will not in general equal $foo).
2260
2261Examples:
2262
2263 $foo = pack("cccc",65,66,67,68);
2264 # foo eq "ABCD"
2265 $foo = pack("c4",65,66,67,68);
2266 # same thing
2267
2268 $foo = pack("ccxxcc",65,66,67,68);
2269 # foo eq "AB\0\0CD"
2270
2271 $foo = pack("s2",1,2);
2272 # "\1\0\2\0" on little-endian
2273 # "\0\1\0\2" on big-endian
2274
2275 $foo = pack("a4","abcd","x","y","z");
2276 # "abcd"
2277
2278 $foo = pack("aaaa","abcd","x","y","z");
2279 # "axyz"
2280
2281 $foo = pack("a14","abcdefg");
2282 # "abcdefg\0\0\0\0\0\0\0"
2283
2284 $foo = pack("i9pl", gmtime);
2285 # a real struct tm (on my system anyway)
2286
2287 sub bintodec {
2288 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
2289 }
2290
2291The same template may generally also be used in the unpack function.
2292
cb1a09d0
AD
2293=item package NAMESPACE
2294
2295Declares the compilation unit as being in the given namespace. The scope
2296of the package declaration is from the declaration itself through the end of
2297the enclosing block (the same scope as the local() operator). All further
2298unqualified dynamic identifiers will be in this namespace. A package
5f05dabc 2299statement affects only dynamic variables--including those you've used
cb1a09d0
AD
2300local() on--but I<not> lexical variables created with my(). Typically it
2301would be the first declaration in a file to be included by the C<require>
2302or C<use> operator. You can switch into a package in more than one place;
5f05dabc 2303it influences merely which symbol table is used by the compiler for the
cb1a09d0
AD
2304rest of that block. You can refer to variables and filehandles in other
2305packages by prefixing the identifier with the package name and a double
2306colon: C<$Package::Variable>. If the package name is null, the C<main>
2307package as assumed. That is, C<$::sail> is equivalent to C<$main::sail>.
2308
2309See L<perlmod/"Packages"> for more information about packages, modules,
2310and classes. See L<perlsub> for other scoping issues.
2311
a0d0e21e
LW
2312=item pipe READHANDLE,WRITEHANDLE
2313
2314Opens a pair of connected pipes like the corresponding system call.
2315Note that if you set up a loop of piped processes, deadlock can occur
2316unless you are very careful. In addition, note that Perl's pipes use
184e9718 2317stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE
a0d0e21e
LW
2318after each command, depending on the application.
2319
7e1af8bc 2320See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
4633a7c4
LW
2321for examples of such things.
2322
a0d0e21e
LW
2323=item pop ARRAY
2324
54310121 2325=item pop
28757baa 2326
a0d0e21e
LW
2327Pops and returns the last value of the array, shortening the array by
23281. Has a similar effect to
2329
2330 $tmp = $ARRAY[$#ARRAY--];
2331
2332If there are no elements in the array, returns the undefined value.
cb1a09d0
AD
2333If ARRAY is omitted, pops the
2334@ARGV array in the main program, and the @_ array in subroutines, just
2335like shift().
a0d0e21e
LW
2336
2337=item pos SCALAR
2338
54310121 2339=item pos
bbce6d69 2340
4633a7c4 2341Returns the offset of where the last C<m//g> search left off for the variable
2f9daede 2342is in question ($_ is used when the variable is not specified). May be
44a8e56a
PP
2343modified to change that offset. Such modification will also influence
2344the C<\G> zero-width assertion in regular expressions. See L<perlre> and
2345L<perlop>.
a0d0e21e
LW
2346
2347=item print FILEHANDLE LIST
2348
2349=item print LIST
2350
2351=item print
2352
cb1a09d0 2353Prints a string or a comma-separated list of strings. Returns TRUE
a0d0e21e 2354if successful. FILEHANDLE may be a scalar variable name, in which case
cb1a09d0 2355the variable contains the name of or a reference to the filehandle, thus introducing one
a0d0e21e
LW
2356level of indirection. (NOTE: If FILEHANDLE is a variable and the next
2357token is a term, it may be misinterpreted as an operator unless you
5f05dabc 2358interpose a + or put parentheses around the arguments.) If FILEHANDLE is
a0d0e21e 2359omitted, prints by default to standard output (or to the last selected
da0045b7 2360output channel--see L</select>). If LIST is also omitted, prints $_ to
a0d0e21e
LW
2361STDOUT. To set the default output channel to something other than
2362STDOUT use the select operation. Note that, because print takes a
2363LIST, anything in the LIST is evaluated in a list context, and any
2364subroutine that you call will have one or more of its expressions
2365evaluated in a list context. Also be careful not to follow the print
2366keyword with a left parenthesis unless you want the corresponding right
2367parenthesis to terminate the arguments to the print--interpose a + or
5f05dabc 2368put parentheses around all the arguments.
a0d0e21e 2369
4633a7c4 2370Note that if you're storing FILEHANDLES in an array or other expression,
da0045b7 2371you will have to use a block returning its value instead:
4633a7c4
LW
2372
2373 print { $files[$i] } "stuff\n";
2374 print { $OK ? STDOUT : STDERR } "stuff\n";
2375
5f05dabc 2376=item printf FILEHANDLE FORMAT, LIST
a0d0e21e 2377
5f05dabc 2378=item printf FORMAT, LIST
a0d0e21e 2379
a3cb178b
GS
2380Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that $\
2381(the output record separator) is not appended. The first argument
a034a98d
DD
2382of the list will be interpreted as the printf format. If C<use locale> is
2383in effect, the character used for the decimal point in formatted real numbers
2384is affected by the LC_NUMERIC locale. See L<perllocale>.
a0d0e21e 2385
28757baa
PP
2386Don't fall into the trap of using a printf() when a simple
2387print() would do. The print() is more efficient, and less
2388error prone.
2389
da0045b7
PP
2390=item prototype FUNCTION
2391
2392Returns the prototype of a function as a string (or C<undef> if the
5f05dabc
PP
2393function has no prototype). FUNCTION is a reference to, or the name of,
2394the function whose prototype you want to retrieve.
da0045b7 2395
b6c543e3
IZ
2396If FUNCTION is a string starting with C<CORE::>, the rest is taken as
2397a name for Perl builtin. If builtin is not I<overridable> (such as
2398C<qw>) or its arguments cannot be expressed by a prototype (such as
2399C<system>) - in other words, the builtin does not behave like a Perl
2400function - returns C<undef>. Otherwise, the string describing the
2401equivalent prototype is returned.
2402
a0d0e21e
LW
2403=item push ARRAY,LIST
2404
2405Treats ARRAY as a stack, and pushes the values of LIST
2406onto the end of ARRAY. The length of ARRAY increases by the length of
2407LIST. Has the same effect as
2408
2409 for $value (LIST) {
2410 $ARRAY[++$#ARRAY] = $value;
2411 }
2412
2413but is more efficient. Returns the new number of elements in the array.
2414
2415=item q/STRING/
2416
2417=item qq/STRING/
2418
2419=item qx/STRING/
2420
2421=item qw/STRING/
2422
2423Generalized quotes. See L<perlop>.
2424
2425=item quotemeta EXPR
2426
54310121 2427=item quotemeta
bbce6d69 2428
68dc0745 2429Returns the value of EXPR with all non-alphanumeric
a034a98d
DD
2430characters backslashed. (That is, all characters not matching
2431C</[A-Za-z_0-9]/> will be preceded by a backslash in the
2432returned string, regardless of any locale settings.)
2433This is the internal function implementing
a0d0e21e
LW
2434the \Q escape in double-quoted strings.
2435
bbce6d69
PP
2436If EXPR is omitted, uses $_.
2437
a0d0e21e
LW
2438=item rand EXPR
2439
2440=item rand
2441
3e3baf6d
TB
2442Returns a random fractional number greater than or equal to 0 and less
2443than the value of EXPR. (EXPR should be positive.) If EXPR is
2444omitted, the value 1 is used. Automatically calls srand() unless
2445srand() has already been called. See also srand().
a0d0e21e 2446
2f9daede 2447(Note: If your rand function consistently returns numbers that are too
a0d0e21e 2448large or too small, then your version of Perl was probably compiled
2f9daede 2449with the wrong number of RANDBITS.)
a0d0e21e
LW
2450
2451=item read FILEHANDLE,SCALAR,LENGTH,OFFSET
2452
2453=item read FILEHANDLE,SCALAR,LENGTH
2454
2455Attempts to read LENGTH bytes of data into variable SCALAR from the
2456specified FILEHANDLE. Returns the number of bytes actually read, or
2457undef if there was an error. SCALAR will be grown or shrunk to the
2458length actually read. An OFFSET may be specified to place the read
2459data at some other place than the beginning of the string. This call
2460is actually implemented in terms of stdio's fread call. To get a true
2461read system call, see sysread().
2462
2463=item readdir DIRHANDLE
2464
2465Returns the next directory entry for a directory opened by opendir().
2466If used in a list context, returns all the rest of the entries in the
2467directory. If there are no more entries, returns an undefined value in
2468a scalar context or a null list in a list context.
2469
cb1a09d0 2470If you're planning to filetest the return values out of a readdir(), you'd
5f05dabc 2471better prepend the directory in question. Otherwise, because we didn't
cb1a09d0
AD
2472chdir() there, it would have been testing the wrong file.
2473
2474 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
2475 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
2476 closedir DIR;
2477
84902520
TB
2478=item readline EXPR
2479
2480Reads from the file handle EXPR. In scalar context, a single line
2481is read and returned. In list context, reads until end-of-file is
2482reached and returns a list of lines (however you've defined lines
2483with $/ or $INPUT_RECORD_SEPARATOR).
2484This is the internal function implementing the C<E<lt>EXPRE<gt>>
2485operator, but you can use it directly. The C<E<lt>EXPRE<gt>>
2486operator is discussed in more detail in L<perlop/"I/O Operators">.
2487
a0d0e21e
LW
2488=item readlink EXPR
2489
54310121 2490=item readlink
bbce6d69 2491
a0d0e21e
LW
2492Returns the value of a symbolic link, if symbolic links are
2493implemented. If not, gives a fatal error. If there is some system
184e9718 2494error, returns the undefined value and sets C<$!> (errno). If EXPR is
a0d0e21e
LW
2495omitted, uses $_.
2496
84902520
TB
2497=item readpipe EXPR
2498
2499EXPR is interpolated and then executed as a system command.
2500The collected standard output of the command is returned.
2501In scalar context, it comes back as a single (potentially
2502multi-line) string. In list context, returns a list of lines
2503(however you've defined lines with $/ or $INPUT_RECORD_SEPARATOR).
2504This is the internal function implementing the C<qx/EXPR/>
2505operator, but you can use it directly. The C<qx/EXPR/>
2506operator is discussed in more detail in L<perlop/"I/O Operators">.
2507
a0d0e21e
LW
2508=item recv SOCKET,SCALAR,LEN,FLAGS
2509
2510Receives a message on a socket. Attempts to receive LENGTH bytes of
2511data into variable SCALAR from the specified SOCKET filehandle.
a3cb178b 2512Actually does a C recvfrom(), so that it can return the address of the
a0d0e21e
LW
2513sender. Returns the undefined value if there's an error. SCALAR will
2514be grown or shrunk to the length actually read. Takes the same flags
54310121 2515as the system call of the same name.
4633a7c4 2516See L<perlipc/"UDP: Message Passing"> for examples.
a0d0e21e
LW
2517
2518=item redo LABEL
2519
2520=item redo
2521
2522The C<redo> command restarts the loop block without evaluating the
2523conditional again. The C<continue> block, if any, is not executed. If
2524the LABEL is omitted, the command refers to the innermost enclosing
2525loop. This command is normally used by programs that want to lie to
2526themselves about what was just input:
2527
2528 # a simpleminded Pascal comment stripper
2529 # (warning: assumes no { or } in strings)
4633a7c4 2530 LINE: while (<STDIN>) {
a0d0e21e
LW
2531 while (s|({.*}.*){.*}|$1 |) {}
2532 s|{.*}| |;
2533 if (s|{.*| |) {
2534 $front = $_;
2535 while (<STDIN>) {
2536 if (/}/) { # end of comment?
2537 s|^|$front{|;
4633a7c4 2538 redo LINE;
a0d0e21e
LW
2539 }
2540 }
2541 }
2542 print;
2543 }
2544
2545=item ref EXPR
2546
54310121 2547=item ref
bbce6d69 2548
2f9daede
TPG
2549Returns a TRUE value if EXPR is a reference, FALSE otherwise. If EXPR
2550is not specified, $_ will be used. The value returned depends on the
bbce6d69 2551type of thing the reference is a reference to.
a0d0e21e
LW
2552Builtin types include:
2553
2554 REF
2555 SCALAR
2556 ARRAY
2557 HASH
2558 CODE
2559 GLOB
2560
54310121 2561If the referenced object has been blessed into a package, then that package
a0d0e21e
LW
2562name is returned instead. You can think of ref() as a typeof() operator.
2563
2564 if (ref($r) eq "HASH") {
aa689395 2565 print "r is a reference to a hash.\n";
54310121 2566 }
a0d0e21e
LW
2567 if (!ref ($r) {
2568 print "r is not a reference at all.\n";
54310121 2569 }
a0d0e21e
LW
2570
2571See also L<perlref>.
2572
2573=item rename OLDNAME,NEWNAME
2574
2575Changes the name of a file. Returns 1 for success, 0 otherwise. Will
5f05dabc 2576not work across file system boundaries.
a0d0e21e
LW
2577
2578=item require EXPR
2579
2580=item require
2581
2582Demands some semantics specified by EXPR, or by $_ if EXPR is not
2583supplied. If EXPR is numeric, demands that the current version of Perl
184e9718 2584(C<$]> or $PERL_VERSION) be equal or greater than EXPR.
a0d0e21e
LW
2585
2586Otherwise, demands that a library file be included if it hasn't already
2587been included. The file is included via the do-FILE mechanism, which is
2588essentially just a variety of eval(). Has semantics similar to the following
2589subroutine:
2590
2591 sub require {
2592 local($filename) = @_;
2593 return 1 if $INC{$filename};
2594 local($realfilename,$result);
2595 ITER: {
2596 foreach $prefix (@INC) {
2597 $realfilename = "$prefix/$filename";
2598 if (-f $realfilename) {
2599 $result = do $realfilename;
2600 last ITER;
2601 }
2602 }
2603 die "Can't find $filename in \@INC";
2604 }
2605 die $@ if $@;
2606 die "$filename did not return true value" unless $result;
2607 $INC{$filename} = $realfilename;
2608 $result;
2609 }
2610
2611Note that the file will not be included twice under the same specified
2612name. The file must return TRUE as the last statement to indicate
2613successful execution of any initialization code, so it's customary to
2614end such a file with "1;" unless you're sure it'll return TRUE
2615otherwise. But it's better just to put the "C<1;>", in case you add more
2616statements.
2617
54310121 2618If EXPR is a bareword, the require assumes a "F<.pm>" extension and
da0045b7 2619replaces "F<::>" with "F</>" in the filename for you,
54310121 2620to make it easy to load standard modules. This form of loading of
a0d0e21e
LW
2621modules does not risk altering your namespace.
2622
54310121 2623For a yet-more-powerful import facility, see L</use> and
748a9306 2624L<perlmod>.
a0d0e21e
LW
2625
2626=item reset EXPR
2627
2628=item reset
2629
2630Generally used in a C<continue> block at the end of a loop to clear
2631variables and reset ?? searches so that they work again. The
2632expression is interpreted as a list of single characters (hyphens
2633allowed for ranges). All variables and arrays beginning with one of
2634those letters are reset to their pristine state. If the expression is
5f05dabc
PP
2635omitted, one-match searches (?pattern?) are reset to match again. Resets
2636only variables or searches in the current package. Always returns
a0d0e21e
LW
26371. Examples:
2638
2639 reset 'X'; # reset all X variables
2640 reset 'a-z'; # reset lower case variables
2641 reset; # just reset ?? searches
2642
5f05dabc
PP
2643Resetting "A-Z" is not recommended because you'll wipe out your
2644ARGV and ENV arrays. Resets only package variables--lexical variables
a0d0e21e 2645are unaffected, but they clean themselves up on scope exit anyway,
da0045b7 2646so you'll probably want to use them instead. See L</my>.
a0d0e21e 2647
54310121
PP
2648=item return EXPR
2649
2650=item return
2651
2652Returns from a subroutine, eval(), or do FILE with the value of the
2653given EXPR. Evaluation of EXPR may be in a list, scalar, or void
2654context, depending on how the return value will be used, and the context
2655may vary from one execution to the next (see wantarray()). If no EXPR
2656is given, returns an empty list in a list context, an undefined value in
2657a scalar context, or nothing in a void context.
a0d0e21e 2658
68dc0745
PP
2659(Note that in the absence of a return, a subroutine, eval, or do FILE
2660will automatically return the value of the last expression evaluated.)
a0d0e21e
LW
2661
2662=item reverse LIST
2663
2664In a list context, returns a list value consisting of the elements
2f9daede
TPG
2665of LIST in the opposite order. In a scalar context, concatenates the
2666elements of LIST, and returns a string value consisting of those bytes,
2667but in the opposite order.
4633a7c4 2668
2f9daede 2669 print reverse <>; # line tac, last line first
4633a7c4 2670
2f9daede
TPG
2671 undef $/; # for efficiency of <>
2672 print scalar reverse <>; # byte tac, last line tsrif
2673
2674This operator is also handy for inverting a hash, although there are some
2675caveats. If a value is duplicated in the original hash, only one of those
2676can be represented as a key in the inverted hash. Also, this has to
2677unwind one hash and build a whole new one, which may take some time
2678on a large hash.
2679
2680 %by_name = reverse %by_address; # Invert the hash
a0d0e21e
LW
2681
2682=item rewinddir DIRHANDLE
2683
2684Sets the current position to the beginning of the directory for the
2685readdir() routine on DIRHANDLE.
2686
2687=item rindex STR,SUBSTR,POSITION
2688
2689=item rindex STR,SUBSTR
2690
2691Works just like index except that it returns the position of the LAST
2692occurrence of SUBSTR in STR. If POSITION is specified, returns the
2693last occurrence at or before that position.
2694
2695=item rmdir FILENAME
2696
54310121 2697=item rmdir
bbce6d69 2698
a0d0e21e 2699Deletes the directory specified by FILENAME if it is empty. If it
184e9718 2700succeeds it returns 1, otherwise it returns 0 and sets C<$!> (errno). If
a0d0e21e
LW
2701FILENAME is omitted, uses $_.
2702
2703=item s///
2704
2705The substitution operator. See L<perlop>.
2706
2707=item scalar EXPR
2708
2709Forces EXPR to be interpreted in a scalar context and returns the value
54310121 2710of EXPR.
cb1a09d0
AD
2711
2712 @counts = ( scalar @a, scalar @b, scalar @c );
2713
54310121 2714There is no equivalent operator to force an expression to
cb1a09d0
AD
2715be interpolated in a list context because it's in practice never
2716needed. If you really wanted to do so, however, you could use
2717the construction C<@{[ (some expression) ]}>, but usually a simple
2718C<(some expression)> suffices.
a0d0e21e
LW
2719
2720=item seek FILEHANDLE,POSITION,WHENCE
2721
8903cb82
PP
2722Sets FILEHANDLE's position, just like the fseek() call of stdio.
2723FILEHANDLE may be an expression whose value gives the name of the
2724filehandle. The values for WHENCE are 0 to set the new position to
2725POSITION, 1 to set it to the current position plus POSITION, and 2 to
2726set it to EOF plus POSITION (typically negative). For WHENCE you may
2727use the constants SEEK_SET, SEEK_CUR, and SEEK_END from either the
2728IO::Seekable or the POSIX module. Returns 1 upon success, 0 otherwise.
2729
2730If you want to position file for sysread() or syswrite(), don't use
2731seek() -- buffering makes its effect on the file's system position
137443ea 2732unpredictable and non-portable. Use sysseek() instead.
a0d0e21e 2733
cb1a09d0
AD
2734On some systems you have to do a seek whenever you switch between reading
2735and writing. Amongst other things, this may have the effect of calling
8903cb82
PP
2736stdio's clearerr(3). A WHENCE of 1 (SEEK_CUR) is useful for not moving
2737the file position:
cb1a09d0
AD
2738
2739 seek(TEST,0,1);
2740
2741This is also useful for applications emulating C<tail -f>. Once you hit
2742EOF on your read, and then sleep for a while, you might have to stick in a
8903cb82
PP
2743seek() to reset things. The seek() doesn't change the current position,
2744but it I<does> clear the end-of-file condition on the handle, so that the
2745next C<E<lt>FILEE<gt>> makes Perl try again to read something. We hope.
cb1a09d0
AD
2746
2747If that doesn't work (some stdios are particularly cantankerous), then
2748you may need something more like this:
2749
2750 for (;;) {
2751 for ($curpos = tell(FILE); $_ = <FILE>; $curpos = tell(FILE)) {
2752 # search for some stuff and put it into files
2753 }
2754 sleep($for_a_while);
2755 seek(FILE, $curpos, 0);
2756 }
2757
a0d0e21e
LW
2758=item seekdir DIRHANDLE,POS
2759
2760Sets the current position for the readdir() routine on DIRHANDLE. POS
2761must be a value returned by telldir(). Has the same caveats about
2762possible directory compaction as the corresponding system library
2763routine.
2764
2765=item select FILEHANDLE
2766
2767=item select
2768
2769Returns the currently selected filehandle. Sets the current default
2770filehandle for output, if FILEHANDLE is supplied. This has two
2771effects: first, a C<write> or a C<print> without a filehandle will
2772default to this FILEHANDLE. Second, references to variables related to
2773output will refer to this output channel. For example, if you have to
2774set the top of form format for more than one output channel, you might
2775do the following:
2776
2777 select(REPORT1);
2778 $^ = 'report1_top';
2779 select(REPORT2);
2780 $^ = 'report2_top';
2781
2782FILEHANDLE may be an expression whose value gives the name of the
2783actual filehandle. Thus:
2784
2785 $oldfh = select(STDERR); $| = 1; select($oldfh);
2786
4633a7c4
LW
2787Some programmers may prefer to think of filehandles as objects with
2788methods, preferring to write the last example as:
a0d0e21e 2789
28757baa 2790 use IO::Handle;
a0d0e21e
LW
2791 STDERR->autoflush(1);
2792
2793=item select RBITS,WBITS,EBITS,TIMEOUT
2794
5f05dabc 2795This calls the select(2) system call with the bit masks specified, which
a0d0e21e
LW
2796can be constructed using fileno() and vec(), along these lines:
2797
2798 $rin = $win = $ein = '';
2799 vec($rin,fileno(STDIN),1) = 1;
2800 vec($win,fileno(STDOUT),1) = 1;
2801 $ein = $rin | $win;
2802
2803If you want to select on many filehandles you might wish to write a
2804subroutine:
2805
2806 sub fhbits {
2807 local(@fhlist) = split(' ',$_[0]);
2808 local($bits);
2809 for (@fhlist) {
2810 vec($bits,fileno($_),1) = 1;
2811 }
2812 $bits;
2813 }
4633a7c4 2814 $rin = fhbits('STDIN TTY SOCK');
a0d0e21e
LW
2815
2816The usual idiom is:
2817
2818 ($nfound,$timeleft) =
2819 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
2820
54310121 2821or to block until something becomes ready just do this
a0d0e21e
LW
2822
2823 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
2824
5f05dabc 2825Most systems do not bother to return anything useful in $timeleft, so
c07a80fd
PP
2826calling select() in a scalar context just returns $nfound.
2827
5f05dabc 2828Any of the bit masks can also be undef. The timeout, if specified, is
a0d0e21e
LW
2829in seconds, which may be fractional. Note: not all implementations are
2830capable of returning the $timeleft. If not, they always return
2831$timeleft equal to the supplied $timeout.
2832
ff68c719 2833You can effect a sleep of 250 milliseconds this way:
a0d0e21e
LW
2834
2835 select(undef, undef, undef, 0.25);
2836
184e9718 2837B<WARNING>: Do not attempt to mix buffered I/O (like read() or E<lt>FHE<gt>)
cb1a09d0 2838with select(). You have to use sysread() instead.
a0d0e21e
LW
2839
2840=item semctl ID,SEMNUM,CMD,ARG
2841
2842Calls the System V IPC function semctl. If CMD is &IPC_STAT or
2843&GETALL, then ARG must be a variable which will hold the returned
2844semid_ds structure or semaphore value array. Returns like ioctl: the
2845undefined value for error, "0 but true" for zero, or the actual return
2846value otherwise.
2847
2848=item semget KEY,NSEMS,FLAGS
2849
2850Calls the System V IPC function semget. Returns the semaphore id, or
2851the undefined value if there is an error.
2852
2853=item semop KEY,OPSTRING
2854
2855Calls the System V IPC function semop to perform semaphore operations
2856such as signaling and waiting. OPSTRING must be a packed array of
2857semop structures. Each semop structure can be generated with
2858C<pack("sss", $semnum, $semop, $semflag)>. The number of semaphore
2859operations is implied by the length of OPSTRING. Returns TRUE if
2860successful, or FALSE if there is an error. As an example, the
2861following code waits on semaphore $semnum of semaphore id $semid:
2862
2863 $semop = pack("sss", $semnum, -1, 0);
2864 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
2865
2866To signal the semaphore, replace "-1" with "1".
2867
2868=item send SOCKET,MSG,FLAGS,TO
2869
2870=item send SOCKET,MSG,FLAGS
2871
2872Sends a message on a socket. Takes the same flags as the system call
2873of the same name. On unconnected sockets you must specify a
2874destination to send TO, in which case it does a C sendto(). Returns
2875the number of characters sent, or the undefined value if there is an
2876error.
4633a7c4 2877See L<perlipc/"UDP: Message Passing"> for examples.
a0d0e21e
LW
2878
2879=item setpgrp PID,PGRP
2880
2881Sets the current process group for the specified PID, 0 for the current
2882process. Will produce a fatal error if used on a machine that doesn't
5f05dabc 2883implement setpgrp(2). If the arguments are omitted, it defaults to
47e29363
PP
28840,0. Note that the POSIX version of setpgrp() does not accept any
2885arguments, so only setpgrp 0,0 is portable.
a0d0e21e
LW
2886
2887=item setpriority WHICH,WHO,PRIORITY
2888
2889Sets the current priority for a process, a process group, or a user.
748a9306 2890(See setpriority(2).) Will produce a fatal error if used on a machine
a0d0e21e
LW
2891that doesn't implement setpriority(2).
2892
2893=item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
2894
2895Sets the socket option requested. Returns undefined if there is an
2896error. OPTVAL may be specified as undef if you don't want to pass an
2897argument.
2898
2899=item shift ARRAY
2900
2901=item shift
2902
2903Shifts the first value of the array off and returns it, shortening the
2904array by 1 and moving everything down. If there are no elements in the
2905array, returns the undefined value. If ARRAY is omitted, shifts the
977336f5
GS
2906@_ array within the lexical scope of subroutines and formats, and the
2907@ARGV array at file scopes or within the lexical scopes established by
2908the C<eval ''>, C<BEGIN {}>, C<END {}>, and C<INIT {}> constructs.
2909See also unshift(), push(), and pop(). Shift() and unshift() do the
2910same thing to the left end of an array that pop() and push() do to the
2911right end.
a0d0e21e
LW
2912
2913=item shmctl ID,CMD,ARG
2914
2915Calls the System V IPC function shmctl. If CMD is &IPC_STAT, then ARG
2916must be a variable which will hold the returned shmid_ds structure.
2917Returns like ioctl: the undefined value for error, "0 but true" for
2918zero, or the actual return value otherwise.
2919
2920=item shmget KEY,SIZE,FLAGS
2921
2922Calls the System V IPC function shmget. Returns the shared memory
2923segment id, or the undefined value if there is an error.
2924
2925=item shmread ID,VAR,POS,SIZE
2926
2927=item shmwrite ID,STRING,POS,SIZE
2928
2929Reads or writes the System V shared memory segment ID starting at
2930position POS for size SIZE by attaching to it, copying in/out, and
2931detaching from it. When reading, VAR must be a variable which will
2932hold the data read. When writing, if STRING is too long, only SIZE
2933bytes are used; if STRING is too short, nulls are written to fill out
2934SIZE bytes. Return TRUE if successful, or FALSE if there is an error.
2935
2936=item shutdown SOCKET,HOW
2937
2938Shuts down a socket connection in the manner indicated by HOW, which
2939has the same interpretation as in the system call of the same name.
2940
2941=item sin EXPR
2942
54310121 2943=item sin
bbce6d69 2944
a0d0e21e
LW
2945Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
2946returns sine of $_.
2947
54310121 2948For the inverse sine operation, you may use the POSIX::asin()
28757baa
PP
2949function, or use this relation:
2950
2951 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
2952
a0d0e21e
LW
2953=item sleep EXPR
2954
2955=item sleep
2956
2957Causes the script to sleep for EXPR seconds, or forever if no EXPR.
2958May be interrupted by sending the process a SIGALRM. Returns the
2959number of seconds actually slept. You probably cannot mix alarm() and
5f05dabc 2960sleep() calls, because sleep() is often implemented using alarm().
a0d0e21e
LW
2961
2962On some older systems, it may sleep up to a full second less than what
2963you requested, depending on how it counts seconds. Most modern systems
2964always sleep the full amount.
2965
cb1a09d0 2966For delays of finer granularity than one second, you may use Perl's
54310121
PP
2967syscall() interface to access setitimer(2) if your system supports it,
2968or else see L</select()> below.
cb1a09d0 2969
5f05dabc
PP
2970See also the POSIX module's sigpause() function.
2971
a0d0e21e
LW
2972=item socket SOCKET,DOMAIN,TYPE,PROTOCOL
2973
2974Opens a socket of the specified kind and attaches it to filehandle
5f05dabc 2975SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for the
a0d0e21e 2976system call of the same name. You should "use Socket;" first to get
4633a7c4 2977the proper definitions imported. See the example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
2978
2979=item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
2980
2981Creates an unnamed pair of sockets in the specified domain, of the
5f05dabc 2982specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
a0d0e21e
LW
2983for the system call of the same name. If unimplemented, yields a fatal
2984error. Returns TRUE if successful.
2985
2986=item sort SUBNAME LIST
2987
2988=item sort BLOCK LIST
2989
2990=item sort LIST
2991
2f9daede
TPG
2992Sorts the LIST and returns the sorted list value. If SUBNAME or BLOCK
2993is omitted, sorts in standard string comparison order. If SUBNAME is
2994specified, it gives the name of a subroutine that returns an integer
2995less than, equal to, or greater than 0, depending on how the elements
2996of the array are to be ordered. (The C<E<lt>=E<gt>> and C<cmp>
2997operators are extremely useful in such routines.) SUBNAME may be a
2998scalar variable name, in which case the value provides the name of the
2999subroutine to use. In place of a SUBNAME, you can provide a BLOCK as
3000an anonymous, in-line sort subroutine.
a0d0e21e 3001
cb1a09d0
AD
3002In the interests of efficiency the normal calling code for subroutines is
3003bypassed, with the following effects: the subroutine may not be a
3004recursive subroutine, and the two elements to be compared are passed into
3005the subroutine not via @_ but as the package global variables $a and
3006$b (see example below). They are passed by reference, so don't
3007modify $a and $b. And don't try to declare them as lexicals either.
a0d0e21e 3008
0a753a76
PP
3009You also cannot exit out of the sort block or subroutine using any of the
3010loop control operators described in L<perlsyn> or with goto().
3011
a034a98d
DD
3012When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
3013current collation locale. See L<perllocale>.
3014
a0d0e21e
LW
3015Examples:
3016
3017 # sort lexically
3018 @articles = sort @files;
3019
3020 # same thing, but with explicit sort routine
3021 @articles = sort {$a cmp $b} @files;
3022
cb1a09d0 3023 # now case-insensitively
54310121 3024 @articles = sort {uc($a) cmp uc($b)} @files;
cb1a09d0 3025
a0d0e21e
LW
3026 # same thing in reversed order
3027 @articles = sort {$b cmp $a} @files;
3028
3029 # sort numerically ascending
3030 @articles = sort {$a <=> $b} @files;
3031
3032 # sort numerically descending
3033 @articles = sort {$b <=> $a} @files;
3034
3035 # sort using explicit subroutine name
3036 sub byage {
2f9daede 3037 $age{$a} <=> $age{$b}; # presuming numeric
a0d0e21e
LW
3038 }
3039 @sortedclass = sort byage @class;
3040
aa689395
PP
3041 # this sorts the %age hash by value instead of key
3042 # using an in-line function
c07a80fd
PP
3043 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
3044
a0d0e21e
LW
3045 sub backwards { $b cmp $a; }
3046 @harry = ('dog','cat','x','Cain','Abel');
3047 @george = ('gone','chased','yz','Punished','Axed');
3048 print sort @harry;
3049 # prints AbelCaincatdogx
3050 print sort backwards @harry;
3051 # prints xdogcatCainAbel
3052 print sort @george, 'to', @harry;
3053 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
3054
54310121
PP
3055 # inefficiently sort by descending numeric compare using
3056 # the first integer after the first = sign, or the
cb1a09d0
AD
3057 # whole record case-insensitively otherwise
3058
3059 @new = sort {
3060 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
3061 ||
3062 uc($a) cmp uc($b)
3063 } @old;
3064
3065 # same thing, but much more efficiently;
3066 # we'll build auxiliary indices instead
3067 # for speed
3068 @nums = @caps = ();
54310121 3069 for (@old) {
cb1a09d0
AD
3070 push @nums, /=(\d+)/;
3071 push @caps, uc($_);
54310121 3072 }
cb1a09d0
AD
3073
3074 @new = @old[ sort {
3075 $nums[$b] <=> $nums[$a]
3076 ||
3077 $caps[$a] cmp $caps[$b]
3078 } 0..$#old
3079 ];
3080
3081 # same thing using a Schwartzian Transform (no temps)
3082 @new = map { $_->[0] }
3083 sort { $b->[1] <=> $a->[1]
3084 ||
3085 $a->[2] cmp $b->[2]
3086 } map { [$_, /=(\d+)/, uc($_)] } @old;
3087
184e9718 3088If you're using strict, you I<MUST NOT> declare $a
cb1a09d0
AD
3089and $b as lexicals. They are package globals. That means
3090if you're in the C<main> package, it's
3091
3092 @articles = sort {$main::b <=> $main::a} @files;
3093
3094or just
3095
3096 @articles = sort {$::b <=> $::a} @files;
3097
3098but if you're in the C<FooPack> package, it's
3099
3100 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
3101
55497cff
PP
3102The comparison function is required to behave. If it returns
3103inconsistent results (sometimes saying $x[1] is less than $x[2] and
3104sometimes saying the opposite, for example) the Perl interpreter will
3105probably crash and dump core. This is entirely due to and dependent
3106upon your system's qsort(3) library routine; this routine often avoids
3107sanity checks in the interest of speed.
3108
a0d0e21e
LW
3109=item splice ARRAY,OFFSET,LENGTH,LIST
3110
3111=item splice ARRAY,OFFSET,LENGTH
3112
3113=item splice ARRAY,OFFSET
3114
3115Removes the elements designated by OFFSET and LENGTH from an array, and
43051805
GS
3116replaces them with the elements of LIST, if any. In a list context,
3117returns the elements removed from the array. In a scalar context,
3118returns the last element removed, or C<undef> if no elements are
3119removed. The array grows or shrinks as necessary. If LENGTH is
3120omitted, removes everything from OFFSET onward. The following
3121equivalences hold (assuming C<$[ == 0>):
a0d0e21e
LW
3122
3123 push(@a,$x,$y) splice(@a,$#a+1,0,$x,$y)
3124 pop(@a) splice(@a,-1)
3125 shift(@a) splice(@a,0,1)
3126 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
3127 $a[$x] = $y splice(@a,$x,1,$y);
3128
3129Example, assuming array lengths are passed before arrays:
3130
3131 sub aeq { # compare two list values
3132 local(@a) = splice(@_,0,shift);
3133 local(@b) = splice(@_,0,shift);
3134 return 0 unless @a == @b; # same len?
3135 while (@a) {
3136 return 0 if pop(@a) ne pop(@b);
3137 }
3138 return 1;
3139 }
3140 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
3141
3142=item split /PATTERN/,EXPR,LIMIT
3143
3144=item split /PATTERN/,EXPR
3145
3146=item split /PATTERN/
3147
3148=item split
3149
3150Splits a string into an array of strings, and returns it.
3151
3152If not in a list context, returns the number of fields found and splits into
3153the @_ array. (In a list context, you can force the split into @_ by
3154using C<??> as the pattern delimiters, but it still returns the array
3155value.) The use of implicit split to @_ is deprecated, however.
3156
3157If EXPR is omitted, splits the $_ string. If PATTERN is also omitted,
4633a7c4
LW
3158splits on whitespace (after skipping any leading whitespace). Anything
3159matching PATTERN is taken to be a delimiter separating the fields. (Note
fb73857a
PP
3160that the delimiter may be longer than one character.)
3161
3162If LIMIT is specified and is not negative, splits into no more than
3163that many fields (though it may split into fewer). If LIMIT is
3164unspecified, trailing null fields are stripped (which potential users
3165of pop() would do well to remember). If LIMIT is negative, it is
3166treated as if an arbitrarily large LIMIT had been specified.
a0d0e21e
LW
3167
3168A pattern matching the null string (not to be confused with
748a9306 3169a null pattern C<//>, which is just one member of the set of patterns
a0d0e21e
LW
3170matching a null string) will split the value of EXPR into separate
3171characters at each point it matches that way. For example:
3172
3173 print join(':', split(/ */, 'hi there'));
3174
3175produces the output 'h:i:t:h:e:r:e'.
3176
5f05dabc 3177The LIMIT parameter can be used to split a line partially
a0d0e21e
LW
3178
3179 ($login, $passwd, $remainder) = split(/:/, $_, 3);
3180
3181When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT
3182one larger than the number of variables in the list, to avoid
3183unnecessary work. For the list above LIMIT would have been 4 by
3184default. In time critical applications it behooves you not to split
3185into more fields than you really need.
3186
3187If the PATTERN contains parentheses, additional array elements are
3188created from each matching substring in the delimiter.
3189
da0045b7 3190 split(/([,-])/, "1-10,20", 3);
a0d0e21e
LW
3191
3192produces the list value
3193
3194 (1, '-', 10, ',', 20)
3195
54310121 3196If you had the entire header of a normal Unix email message in $header,
4633a7c4
LW
3197you could split it up into fields and their values this way:
3198
3199 $header =~ s/\n\s+/ /g; # fix continuation lines
fb73857a 3200 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
4633a7c4 3201
a0d0e21e
LW
3202The pattern C</PATTERN/> may be replaced with an expression to specify
3203patterns that vary at runtime. (To do runtime compilation only once,
748a9306
LW
3204use C</$variable/o>.)
3205
3206As a special case, specifying a PATTERN of space (C<' '>) will split on
3207white space just as split with no arguments does. Thus, split(' ') can
3208be used to emulate B<awk>'s default behavior, whereas C<split(/ /)>
3209will give you as many null initial fields as there are leading spaces.
3210A split on /\s+/ is like a split(' ') except that any leading
3211whitespace produces a null first field. A split with no arguments
3212really does a C<split(' ', $_)> internally.
a0d0e21e
LW
3213
3214Example:
3215
3216 open(passwd, '/etc/passwd');
3217 while (<passwd>) {
54310121 3218 ($login, $passwd, $uid, $gid, $gcos,
748a9306 3219 $home, $shell) = split(/:/);
a0d0e21e
LW
3220 ...
3221 }
3222
54310121 3223(Note that $shell above will still have a newline on it. See L</chop>,
a0d0e21e
LW
3224L</chomp>, and L</join>.)
3225
5f05dabc 3226=item sprintf FORMAT, LIST
a0d0e21e 3227
74a77017
CS
3228Returns a string formatted by the usual printf conventions of the
3229C library function sprintf(). See L<sprintf(3)> or L<printf(3)>
3230on your system for an explanation of the general principles.
3231
3232Perl does all of its own sprintf() formatting -- it emulates the C
3233function sprintf(), but it doesn't use it (except for floating-point
3234numbers, and even then only the standard modifiers are allowed). As a
3235result, any non-standard extensions in your local sprintf() are not
3236available from Perl.
3237
3238Perl's sprintf() permits the following universally-known conversions:
3239
3240 %% a percent sign
3241 %c a character with the given number
3242 %s a string
3243 %d a signed integer, in decimal
3244 %u an unsigned integer, in decimal
3245 %o an unsigned integer, in octal
3246 %x an unsigned integer, in hexadecimal
3247 %e a floating-point number, in scientific notation
3248 %f a floating-point number, in fixed decimal notation
3249 %g a floating-point number, in %e or %f notation
3250
1b3f7d21 3251In addition, Perl permits the following widely-supported conversions:
74a77017 3252
74a77017
CS
3253 %X like %x, but using upper-case letters
3254 %E like %e, but using an upper-case "E"
3255 %G like %g, but with an upper-case "E" (if applicable)
3256 %p a pointer (outputs the Perl value's address in hexadecimal)
1b3f7d21
CS
3257 %n special: *stores* the number of characters output so far
3258 into the next variable in the parameter list
74a77017 3259
1b3f7d21
CS
3260Finally, for backward (and we do mean "backward") compatibility, Perl
3261permits these unnecessary but widely-supported conversions:
74a77017 3262
1b3f7d21 3263 %i a synonym for %d
74a77017
CS
3264 %D a synonym for %ld
3265 %U a synonym for %lu
3266 %O a synonym for %lo
3267 %F a synonym for %f
3268
3269Perl permits the following universally-known flags between the C<%>
3270and the conversion letter:
3271
3272 space prefix positive number with a space
3273 + prefix positive number with a plus sign
3274 - left-justify within the field
3275 0 use zeros, not spaces, to right-justify
a3cb178b 3276 # prefix non-zero octal with "0", non-zero hex with "0x"
74a77017
CS
3277 number minimum field width
3278 .number "precision": digits after decimal point for floating-point,
3279 max length for string, minimum length for integer
3280 l interpret integer as C type "long" or "unsigned long"
74a77017
CS
3281 h interpret integer as C type "short" or "unsigned short"
3282
1b3f7d21 3283There is also one Perl-specific flag:
74a77017
CS
3284
3285 V interpret integer as Perl's standard integer type
3286
3287Where a number would appear in the flags, an asterisk ("*") may be
3288used instead, in which case Perl uses the next item in the parameter
3289list as the given number (that is, as the field width or precision).
3290If a field width obtained through "*" is negative, it has the same
3291effect as the '-' flag: left-justification.
3292
3293If C<use locale> is in effect, the character used for the decimal
3294point in formatted real numbers is affected by the LC_NUMERIC locale.
3295See L<perllocale>.
a0d0e21e
LW
3296
3297=item sqrt EXPR
3298
54310121 3299=item sqrt
bbce6d69 3300
a0d0e21e
LW
3301Return the square root of EXPR. If EXPR is omitted, returns square
3302root of $_.
3303
3304=item srand EXPR
3305
93dc8474
CS
3306=item srand
3307
3308Sets the random number seed for the C<rand> operator. If EXPR is
3309omitted, uses a semi-random value based on the current time and process
3310ID, among other things. In versions of Perl prior to 5.004 the default
3311seed was just the current time(). This isn't a particularly good seed,
3312so many old programs supply their own seed value (often C<time ^ $$> or
3313C<time ^ ($$ + ($$ << 15))>), but that isn't necessary any more.
3314
3315In fact, it's usually not necessary to call srand() at all, because if
3316it is not called explicitly, it is called implicitly at the first use of
2f9daede
TPG
3317the C<rand> operator. However, this was not the case in version of Perl
3318before 5.004, so if your script will run under older Perl versions, it
3319should call srand().
93dc8474 3320
2f9daede
TPG
3321Note that you need something much more random than the default seed for
3322cryptographic purposes. Checksumming the compressed output of one or more
3323rapidly changing operating system status programs is the usual method. For
3324example:
28757baa
PP
3325
3326 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
3327
0078ec44
RS
3328If you're particularly concerned with this, see the Math::TrulyRandom
3329module in CPAN.
3330
3331Do I<not> call srand() multiple times in your program unless you know
28757baa
PP
3332exactly what you're doing and why you're doing it. The point of the
3333function is to "seed" the rand() function so that rand() can produce
3334a different sequence each time you run your program. Just do it once at the
3335top of your program, or you I<won't> get random numbers out of rand()!
3336
54310121 3337Frequently called programs (like CGI scripts) that simply use
28757baa
PP
3338
3339 time ^ $$
3340
54310121 3341for a seed can fall prey to the mathematical property that
28757baa
PP
3342
3343 a^b == (a+1)^(b+1)
3344
0078ec44 3345one-third of the time. So don't do that.
f86702cc 3346
a0d0e21e
LW
3347=item stat FILEHANDLE
3348
3349=item stat EXPR
3350
54310121 3351=item stat
bbce6d69 3352
a0d0e21e 3353Returns a 13-element array giving the status info for a file, either the
2f9daede 3354file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted, it
bbce6d69
PP
3355stats $_. Returns a null list if the stat fails. Typically used as
3356follows:
3357
a0d0e21e
LW
3358
3359 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
3360 $atime,$mtime,$ctime,$blksize,$blocks)
3361 = stat($filename);
3362
54310121 3363Not all fields are supported on all filesystem types. Here are the
c07a80fd
PP
3364meaning of the fields:
3365
54310121
PP
3366 0 dev device number of filesystem
3367 1 ino inode number
3368 2 mode file mode (type and permissions)
3369 3 nlink number of (hard) links to the file
3370 4 uid numeric user ID of file's owner
3371 5 gid numeric group ID of file's owner
3372 6 rdev the device identifier (special files only)
3373 7 size total size of file, in bytes
3374 8 atime last access time since the epoch
3375 9 mtime last modify time since the epoch
3376 10 ctime inode change time (NOT creation time!) since the epoch
3377 11 blksize preferred block size for file system I/O
3378 12 blocks actual number of blocks allocated
c07a80fd
PP
3379
3380(The epoch was at 00:00 January 1, 1970 GMT.)
3381
a0d0e21e
LW
3382If stat is passed the special filehandle consisting of an underline, no
3383stat is done, but the current contents of the stat structure from the
3384last stat or filetest are returned. Example:
3385
3386 if (-x $file && (($d) = stat(_)) && $d < 0) {
3387 print "$file is executable NFS file\n";
3388 }
3389
5f05dabc 3390(This works on machines only for which the device number is negative under NFS.)
a0d0e21e
LW
3391
3392=item study SCALAR
3393
3394=item study
3395
184e9718 3396Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
a0d0e21e
LW
3397doing many pattern matches on the string before it is next modified.
3398This may or may not save time, depending on the nature and number of
3399patterns you are searching on, and on the distribution of character
54310121 3400frequencies in the string to be searched -- you probably want to compare
5f05dabc 3401run times with and without it to see which runs faster. Those loops
a0d0e21e
LW
3402which scan for many short constant strings (including the constant
3403parts of more complex patterns) will benefit most. You may have only
54310121 3404one study active at a time -- if you study a different scalar the first
a0d0e21e
LW
3405is "unstudied". (The way study works is this: a linked list of every
3406character in the string to be searched is made, so we know, for
3407example, where all the 'k' characters are. From each search string,
3408the rarest character is selected, based on some static frequency tables
3409constructed from some C programs and English text. Only those places
3410that contain this "rarest" character are examined.)
3411
3412For example, here is a loop which inserts index producing entries
3413before any line containing a certain pattern:
3414
3415 while (<>) {
3416 study;
3417 print ".IX foo\n" if /\bfoo\b/;
3418 print ".IX bar\n" if /\bbar\b/;
3419 print ".IX blurfl\n" if /\bblurfl\b/;
3420 ...
3421 print;
3422 }
3423
3424In searching for /\bfoo\b/, only those locations in $_ that contain "f"
3425will be looked at, because "f" is rarer than "o". In general, this is
3426a big win except in pathological cases. The only question is whether
3427it saves you more time than it took to build the linked list in the
3428first place.
3429
3430Note that if you have to look for strings that you don't know till
3431runtime, you can build an entire loop as a string and eval that to
3432avoid recompiling all your patterns all the time. Together with
3433undefining $/ to input entire files as one record, this can be very
3434fast, often faster than specialized programs like fgrep(1). The following
184e9718 3435scans a list of files (C<@files>) for a list of words (C<@words>), and prints
a0d0e21e
LW
3436out the names of those files that contain a match:
3437
3438 $search = 'while (<>) { study;';
3439 foreach $word (@words) {
3440 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
3441 }
3442 $search .= "}";
3443 @ARGV = @files;
3444 undef $/;
3445 eval $search; # this screams
5f05dabc 3446 $/ = "\n"; # put back to normal input delimiter
a0d0e21e
LW
3447 foreach $file (sort keys(%seen)) {
3448 print $file, "\n";
3449 }
3450
cb1a09d0
AD
3451=item sub BLOCK
3452
3453=item sub NAME
3454
3455=item sub NAME BLOCK
3456
3457This is subroutine definition, not a real function I<per se>. With just a
3458NAME (and possibly prototypes), it's just a forward declaration. Without
3459a NAME, it's an anonymous function declaration, and does actually return a
2f9daede 3460value: the CODE ref of the closure you just created. See L<perlsub> and
cb1a09d0
AD
3461L<perlref> for details.
3462
a0d0e21e
LW
3463=item substr EXPR,OFFSET,LEN
3464
3465=item substr EXPR,OFFSET
3466
3467Extracts a substring out of EXPR and returns it. First character is at
2f9daede 3468offset 0, or whatever you've set C<$[> to (but don't do that).
84902520 3469If OFFSET is negative (or more precisely, less than C<$[>), starts
a0d0e21e 3470that far from the end of the string. If LEN is omitted, returns
748a9306
LW
3471everything to the end of the string. If LEN is negative, leaves that
3472many characters off the end of the string.
3473
84902520
TB
3474If you specify a substring which is partly outside the string, the part
3475within the string is returned. If the substring is totally outside
3476the string a warning is produced.
3477
748a9306 3478You can use the substr() function
a0d0e21e
LW
3479as an lvalue, in which case EXPR must be an lvalue. If you assign
3480something shorter than LEN, the string will shrink, and if you assign
3481something longer than LEN, the string will grow to accommodate it. To
3482keep the string the same length you may need to pad or chop your value
3483using sprintf().
3484
3485=item symlink OLDFILE,NEWFILE
3486
3487Creates a new filename symbolically linked to the old filename.
3488Returns 1 for success, 0 otherwise. On systems that don't support
3489symbolic links, produces a fatal error at run time. To check for that,
3490use eval:
3491
54310121 3492 $symlink_exists = (eval {symlink("","")};, $@ eq '');
a0d0e21e
LW
3493
3494=item syscall LIST
3495
3496Calls the system call specified as the first element of the list,
3497passing the remaining elements as arguments to the system call. If
3498unimplemented, produces a fatal error. The arguments are interpreted
3499as follows: if a given argument is numeric, the argument is passed as
3500an int. If not, the pointer to the string value is passed. You are
3501responsible to make sure a string is pre-extended long enough to
a3cb178b
GS
3502receive any result that might be written into a string. You can't use a
3503string literal (or other read-only string) as an argument to syscall()
3504because Perl has to assume that any string pointer might be written
3505through. If your
a0d0e21e
LW
3506integer arguments are not literals and have never been interpreted in a
3507numeric context, you may need to add 0 to them to force them to look
3508like numbers.
3509
3510 require 'syscall.ph'; # may need to run h2ph
a3cb178b
GS
3511 $s = "hi there\n";
3512 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
a0d0e21e 3513
5f05dabc 3514Note that Perl supports passing of up to only 14 arguments to your system call,
a0d0e21e
LW
3515which in practice should usually suffice.
3516
fb73857a
PP
3517Syscall returns whatever value returned by the system call it calls.
3518If the system call fails, syscall returns -1 and sets C<$!> (errno).
3519Note that some system calls can legitimately return -1. The proper
3520way to handle such calls is to assign C<$!=0;> before the call and
3521check the value of <$!> if syscall returns -1.
3522
3523There's a problem with C<syscall(&SYS_pipe)>: it returns the file
3524number of the read end of the pipe it creates. There is no way
3525to retrieve the file number of the other end. You can avoid this
3526problem by using C<pipe> instead.
3527
c07a80fd
PP
3528=item sysopen FILEHANDLE,FILENAME,MODE
3529
3530=item sysopen FILEHANDLE,FILENAME,MODE,PERMS
3531
3532Opens the file whose filename is given by FILENAME, and associates it
3533with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
3534the name of the real filehandle wanted. This function calls the
3535underlying operating system's C<open> function with the parameters
3536FILENAME, MODE, PERMS.
3537
3538The possible values and flag bits of the MODE parameter are
3539system-dependent; they are available via the standard module C<Fcntl>.
3540However, for historical reasons, some values are universal: zero means
3541read-only, one means write-only, and two means read/write.
3542
3543If the file named by FILENAME does not exist and the C<open> call
3544creates it (typically because MODE includes the O_CREAT flag), then
3545the value of PERMS specifies the permissions of the newly created
3546file. If PERMS is omitted, the default value is 0666, which allows
3547read and write for all. This default is reasonable: see C<umask>.
3548
28757baa
PP
3549The IO::File module provides a more object-oriented approach, if you're
3550into that kind of thing.
3551
a0d0e21e
LW
3552=item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
3553
3554=item sysread FILEHANDLE,SCALAR,LENGTH
3555
3556Attempts to read LENGTH bytes of data into variable SCALAR from the
fb73857a
PP
3557specified FILEHANDLE, using the system call read(2). It bypasses
3558stdio, so mixing this with other kinds of reads, print(), write(),
3559seek(), or tell() can cause confusion because stdio usually buffers
3560data. Returns the number of bytes actually read, or undef if there
3561was an error. SCALAR will be grown or shrunk so that the last byte
3562actually read is the last byte of the scalar after the read.
ff68c719
PP
3563
3564An OFFSET may be specified to place the read data at some place in the
3565string other than the beginning. A negative OFFSET specifies
3566placement at that many bytes counting backwards from the end of the
3567string. A positive OFFSET greater than the length of SCALAR results
3568in the string being padded to the required size with "\0" bytes before
3569the result of the read is appended.
a0d0e21e 3570
137443ea
PP
3571=item sysseek FILEHANDLE,POSITION,WHENCE
3572
8903cb82
PP
3573Sets FILEHANDLE's system position using the system call lseek(2). It
3574bypasses stdio, so mixing this with reads (other than sysread()),
3575print(), write(), seek(), or tell() may cause confusion. FILEHANDLE may
3576be an expression whose value gives the name of the filehandle. The
3577values for WHENCE are 0 to set the new position to POSITION, 1 to set
3578the it to the current position plus POSITION, and 2 to set it to EOF
3579plus POSITION (typically negative). For WHENCE, you may use the
3580constants SEEK_SET, SEEK_CUR, and SEEK_END from either the IO::Seekable
3581or the POSIX module.
3582
3583Returns the new position, or the undefined value on failure. A position
3584of zero is returned as the string "0 but true"; thus sysseek() returns
3585TRUE on success and FALSE on failure, yet you can still easily determine
3586the new position.
137443ea 3587
a0d0e21e
LW
3588=item system LIST
3589
8bf3b016
GS
3590=item system PROGRAM LIST
3591
a0d0e21e
LW
3592Does exactly the same thing as "exec LIST" except that a fork is done
3593first, and the parent process waits for the child process to complete.
3594Note that argument processing varies depending on the number of
3595arguments. The return value is the exit status of the program as
3596returned by the wait() call. To get the actual exit value divide by
54310121
PP
3597256. See also L</exec>. This is I<NOT> what you want to use to capture
3598the output from a command, for that you should use merely backticks or
28757baa 3599qx//, as described in L<perlop/"`STRING`">.
a0d0e21e 3600
8bf3b016
GS
3601Like exec(), system() allows you to lie to a program about its name if
3602you use the "system PROGRAM LIST" syntax. Again, see L</exec>.
3603
54310121 3604Because system() and backticks block SIGINT and SIGQUIT, killing the
28757baa
PP
3605program they're running doesn't actually interrupt your program.
3606
3607 @args = ("command", "arg1", "arg2");
54310121
PP
3608 system(@args) == 0
3609 or die "system @args failed: $?"
28757baa
PP
3610
3611Here's a more elaborate example of analysing the return value from
54310121
PP
3612system() on a Unix system to check for all possibilities, including for
3613signals and core dumps.
28757baa
PP
3614
3615 $rc = 0xffff & system @args;
3616 printf "system(%s) returned %#04x: ", "@args", $rc;
3617 if ($rc == 0) {
3618 print "ran with normal exit\n";
54310121