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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
866array in L<perlvar/Predefined Names>). It's the same, however, in that it does
54310121 867reparse the file every time you call it, so you probably don't want to
a0d0e21e
LW
868do this inside a loop.
869
870Note that inclusion of library modules is better done with the
4633a7c4
LW
871use() and require() operators, which also do error checking
872and raise an exception if there's a problem.
a0d0e21e
LW
873
874=item dump LABEL
875
876This causes an immediate core dump. Primarily this is so that you can
877use the B<undump> program to turn your core dump into an executable binary
878after having initialized all your variables at the beginning of the
879program. When the new binary is executed it will begin by executing a
880C<goto LABEL> (with all the restrictions that C<goto> suffers). Think of
881it as a goto with an intervening core dump and reincarnation. If LABEL
882is omitted, restarts the program from the top. WARNING: any files
883opened at the time of the dump will NOT be open any more when the
884program is reincarnated, with possible resulting confusion on the part
885of Perl. See also B<-u> option in L<perlrun>.
886
887Example:
888
889 #!/usr/bin/perl
890 require 'getopt.pl';
891 require 'stat.pl';
892 %days = (
893 'Sun' => 1,
894 'Mon' => 2,
895 'Tue' => 3,
896 'Wed' => 4,
897 'Thu' => 5,
898 'Fri' => 6,
899 'Sat' => 7,
900 );
901
902 dump QUICKSTART if $ARGV[0] eq '-d';
903
904 QUICKSTART:
905 Getopt('f');
906
aa689395
PP
907=item each HASH
908
909When called in a list context, returns a 2-element array consisting of the
910key and value for the next element of a hash, so that you can iterate over
911it. When called in a scalar context, returns the key for only the next
2f9daede
TPG
912element in the hash. (Note: Keys may be "0" or "", which are logically
913false; you may wish to avoid constructs like C<while ($k = each %foo) {}>
914for this reason.)
915
916Entries are returned in an apparently random order. When the hash is
917entirely read, a null array is returned in list context (which when
918assigned produces a FALSE (0) value), and C<undef> is returned in a
919scalar context. The next call to each() after that will start iterating
920again. There is a single iterator for each hash, shared by all each(),
921keys(), and values() function calls in the program; it can be reset by
922reading all the elements from the hash, or by evaluating C<keys HASH> or
923C<values HASH>. If you add or delete elements of a hash while you're
924iterating over it, you may get entries skipped or duplicated, so don't.
aa689395
PP
925
926The following prints out your environment like the printenv(1) program,
927only in a different order:
a0d0e21e
LW
928
929 while (($key,$value) = each %ENV) {
930 print "$key=$value\n";
931 }
932
933See also keys() and values().
934
935=item eof FILEHANDLE
936
4633a7c4
LW
937=item eof ()
938
a0d0e21e
LW
939=item eof
940
941Returns 1 if the next read on FILEHANDLE will return end of file, or if
942FILEHANDLE is not open. FILEHANDLE may be an expression whose value
943gives the real filehandle name. (Note that this function actually
944reads a character and then ungetc()s it, so it is not very useful in an
748a9306
LW
945interactive context.) Do not read from a terminal file (or call
946C<eof(FILEHANDLE)> on it) after end-of-file is reached. Filetypes such
947as terminals may lose the end-of-file condition if you do.
948
949An C<eof> without an argument uses the last file read as argument.
2f9daede
TPG
950Empty parentheses () may be used to indicate the pseudo file formed of
951the files listed on the command line, i.e., C<eof()> is reasonable to
952use inside a C<while (E<lt>E<gt>)> loop to detect the end of only the
953last file. Use C<eof(ARGV)> or eof without the parentheses to test
954I<EACH> file in a while (E<lt>E<gt>) loop. Examples:
a0d0e21e 955
748a9306
LW
956 # reset line numbering on each input file
957 while (<>) {
958 print "$.\t$_";
959 close(ARGV) if (eof); # Not eof().
960 }
961
a0d0e21e
LW
962 # insert dashes just before last line of last file
963 while (<>) {
964 if (eof()) {
965 print "--------------\n";
748a9306
LW
966 close(ARGV); # close or break; is needed if we
967 # are reading from the terminal
a0d0e21e
LW
968 }
969 print;
970 }
971
a0d0e21e 972Practical hint: you almost never need to use C<eof> in Perl, because the
54310121 973input operators return undef when they run out of data.
a0d0e21e
LW
974
975=item eval EXPR
976
977=item eval BLOCK
978
c7cc6f1c
GS
979In the first form, the return value of EXPR is parsed and executed as if it
980were a little Perl program. The value of the expression (which is itself
981determined within a scalar context) is first parsed, and if there are no
982errors, executed in the context of the current Perl program, so that any
5f05dabc 983variable settings or subroutine and format definitions remain afterwards.
c7cc6f1c
GS
984Note that the value is parsed every time the eval executes. If EXPR is
985omitted, evaluates C<$_>. This form is typically used to delay parsing
986and subsequent execution of the text of EXPR until run time.
987
988In the second form, the code within the BLOCK is parsed only once--at the
989same time the code surrounding the eval itself was parsed--and executed
990within the context of the current Perl program. This form is typically
991used to trap exceptions more efficiently than the first (see below), while
992also providing the benefit of checking the code within BLOCK at compile
993time.
994
995The final semicolon, if any, may be omitted from the value of EXPR or within
996the BLOCK.
997
998In both forms, the value returned is the value of the last expression
999evaluated inside the mini-program, or a return statement may be used, just
1000as with subroutines. The expression providing the return value is evaluated
1001in void, scalar or array context, depending on the context of the eval itself.
1002See L</wantarray> for more on how the evaluation context can be determined.
a0d0e21e
LW
1003
1004If there is a syntax error or runtime error, or a die() statement is
1005executed, an undefined value is returned by eval(), and C<$@> is set to the
1006error message. If there was no error, C<$@> is guaranteed to be a null
c7cc6f1c
GS
1007string. Beware that using eval() neither silences perl from printing
1008warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1009To do either of those, you have to use the C<$SIG{__WARN__}> facility. See
1010L</warn> and L<perlvar>.
a0d0e21e 1011
5f05dabc 1012Note that, because eval() traps otherwise-fatal errors, it is useful for
4633a7c4 1013determining whether a particular feature (such as socket() or symlink())
a0d0e21e
LW
1014is implemented. It is also Perl's exception trapping mechanism, where
1015the die operator is used to raise exceptions.
1016
1017If the code to be executed doesn't vary, you may use the eval-BLOCK
1018form to trap run-time errors without incurring the penalty of
1019recompiling each time. The error, if any, is still returned in C<$@>.
1020Examples:
1021
54310121 1022 # make divide-by-zero nonfatal
a0d0e21e
LW
1023 eval { $answer = $a / $b; }; warn $@ if $@;
1024
1025 # same thing, but less efficient
1026 eval '$answer = $a / $b'; warn $@ if $@;
1027
1028 # a compile-time error
1029 eval { $answer = };
1030
1031 # a run-time error
1032 eval '$answer ='; # sets $@
1033
774d564b
PP
1034When using the eval{} form as an exception trap in libraries, you may
1035wish not to trigger any C<__DIE__> hooks that user code may have
1036installed. You can use the C<local $SIG{__DIE__}> construct for this
1037purpose, as shown in this example:
1038
1039 # a very private exception trap for divide-by-zero
1040 eval { local $SIG{'__DIE__'}; $answer = $a / $b; }; warn $@ if $@;
1041
1042This is especially significant, given that C<__DIE__> hooks can call
1043die() again, which has the effect of changing their error messages:
1044
1045 # __DIE__ hooks may modify error messages
1046 {
1047 local $SIG{'__DIE__'} = sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
c7cc6f1c
GS
1048 eval { die "foo lives here" };
1049 print $@ if $@; # prints "bar lives here"
774d564b
PP
1050 }
1051
54310121 1052With an eval(), you should be especially careful to remember what's
a0d0e21e
LW
1053being looked at when:
1054
1055 eval $x; # CASE 1
1056 eval "$x"; # CASE 2
1057
1058 eval '$x'; # CASE 3
1059 eval { $x }; # CASE 4
1060
1061 eval "\$$x++" # CASE 5
1062 $$x++; # CASE 6
1063
2f9daede
TPG
1064Cases 1 and 2 above behave identically: they run the code contained in
1065the variable $x. (Although case 2 has misleading double quotes making
1066the reader wonder what else might be happening (nothing is).) Cases 3
1067and 4 likewise behave in the same way: they run the code '$x', which
1068does nothing but return the value of C<$x>. (Case 4 is preferred for
1069purely visual reasons, but it also has the advantage of compiling at
1070compile-time instead of at run-time.) Case 5 is a place where
54310121 1071normally you I<WOULD> like to use double quotes, except that in this
2f9daede
TPG
1072particular situation, you can just use symbolic references instead, as
1073in case 6.
a0d0e21e
LW
1074
1075=item exec LIST
1076
fb73857a
PP
1077The exec() function executes a system command I<AND NEVER RETURNS> -
1078use system() instead of exec() if you want it to return. It fails and
1079returns FALSE only if the command does not exist I<and> it is executed
1080directly instead of via your system's command shell (see below).
a0d0e21e
LW
1081
1082If there is more than one argument in LIST, or if LIST is an array with
1083more than one value, calls execvp(3) with the arguments in LIST. If
1084there is only one scalar argument, the argument is checked for shell
bb32b41a
GS
1085metacharacters, and if there are any, the entire argument is passed to
1086the system's command shell for parsing (this is C</bin/sh -c> on Unix
1087platforms, but varies on other platforms). If there are no shell
1088metacharacters in the argument, it is split into words and passed
1089directly to execvp(), which is more efficient. Note: exec() and
1090system() do not flush your output buffer, so you may need to set C<$|>
1091to avoid lost output. Examples:
a0d0e21e
LW
1092
1093 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1094 exec "sort $outfile | uniq";
1095
1096If you don't really want to execute the first argument, but want to lie
1097to the program you are executing about its own name, you can specify
1098the program you actually want to run as an "indirect object" (without a
1099comma) in front of the LIST. (This always forces interpretation of the
54310121 1100LIST as a multivalued list, even if there is only a single scalar in
a0d0e21e
LW
1101the list.) Example:
1102
1103 $shell = '/bin/csh';
1104 exec $shell '-sh'; # pretend it's a login shell
1105
1106or, more directly,
1107
1108 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1109
bb32b41a
GS
1110When the arguments get executed via the system shell, results will
1111be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1112for details.
1113
a0d0e21e
LW
1114=item exists EXPR
1115
1116Returns TRUE if the specified hash key exists in its hash array, even
1117if the corresponding value is undefined.
1118
1119 print "Exists\n" if exists $array{$key};
1120 print "Defined\n" if defined $array{$key};
1121 print "True\n" if $array{$key};
1122
5f05dabc 1123A hash element can be TRUE only if it's defined, and defined if
a0d0e21e
LW
1124it exists, but the reverse doesn't necessarily hold true.
1125
1126Note that the EXPR can be arbitrarily complicated as long as the final
1127operation is a hash key lookup:
1128
1129 if (exists $ref->[$x][$y]{$key}) { ... }
1130
1131=item exit EXPR
1132
1133Evaluates EXPR and exits immediately with that value. (Actually, it
1134calls any defined C<END> routines first, but the C<END> routines may not
1135abort the exit. Likewise any object destructors that need to be called
1136are called before exit.) Example:
1137
1138 $ans = <STDIN>;
1139 exit 0 if $ans =~ /^[Xx]/;
1140
f86702cc 1141See also die(). If EXPR is omitted, exits with 0 status. The only
54310121 1142universally portable values for EXPR are 0 for success and 1 for error;
f86702cc
PP
1143all other values are subject to unpredictable interpretation depending
1144on the environment in which the Perl program is running.
a0d0e21e 1145
28757baa
PP
1146You shouldn't use exit() to abort a subroutine if there's any chance that
1147someone might want to trap whatever error happened. Use die() instead,
1148which can be trapped by an eval().
1149
a0d0e21e
LW
1150=item exp EXPR
1151
54310121 1152=item exp
bbce6d69 1153
54310121 1154Returns I<e> (the natural logarithm base) to the power of EXPR.
a0d0e21e
LW
1155If EXPR is omitted, gives C<exp($_)>.
1156
1157=item fcntl FILEHANDLE,FUNCTION,SCALAR
1158
1159Implements the fcntl(2) function. You'll probably have to say
1160
1161 use Fcntl;
1162
1163first to get the correct function definitions. Argument processing and
1164value return works just like ioctl() below. Note that fcntl() will produce
1165a fatal error if used on a machine that doesn't implement fcntl(2).
1166For example:
1167
1168 use Fcntl;
1169 fcntl($filehandle, F_GETLK, $packed_return_buffer);
1170
1171=item fileno FILEHANDLE
1172
1173Returns the file descriptor for a filehandle. This is useful for
1174constructing bitmaps for select(). If FILEHANDLE is an expression, the
1175value is taken as the name of the filehandle.
1176
1177=item flock FILEHANDLE,OPERATION
1178
8ebc5c01 1179Calls flock(2), or an emulation of it, on FILEHANDLE. Returns TRUE for
68dc0745
PP
1180success, FALSE on failure. Produces a fatal error if used on a machine
1181that doesn't implement flock(2), fcntl(2) locking, or lockf(3). flock()
1182is Perl's portable file locking interface, although it locks only entire
1183files, not records.
8ebc5c01
PP
1184
1185OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1186LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
68dc0745
PP
1187you can use the symbolic names if import them from the Fcntl module,
1188either individually, or as a group using the ':flock' tag. LOCK_SH
1189requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1190releases a previously requested lock. If LOCK_NB is added to LOCK_SH or
1191LOCK_EX then flock() will return immediately rather than blocking
1192waiting for the lock (check the return status to see if you got it).
1193
1194To avoid the possibility of mis-coordination, Perl flushes FILEHANDLE
1195before (un)locking it.
8ebc5c01
PP
1196
1197Note that the emulation built with lockf(3) doesn't provide shared
1198locks, and it requires that FILEHANDLE be open with write intent. These
1199are the semantics that lockf(3) implements. Most (all?) systems
1200implement lockf(3) in terms of fcntl(2) locking, though, so the
1201differing semantics shouldn't bite too many people.
1202
1203Note also that some versions of flock() cannot lock things over the
1204network; you would need to use the more system-specific fcntl() for
1205that. If you like you can force Perl to ignore your system's flock(2)
1206function, and so provide its own fcntl(2)-based emulation, by passing
1207the switch C<-Ud_flock> to the F<Configure> program when you configure
1208perl.
4633a7c4
LW
1209
1210Here's a mailbox appender for BSD systems.
a0d0e21e 1211
7e1af8bc 1212 use Fcntl ':flock'; # import LOCK_* constants
a0d0e21e
LW
1213
1214 sub lock {
7e1af8bc 1215 flock(MBOX,LOCK_EX);
a0d0e21e
LW
1216 # and, in case someone appended
1217 # while we were waiting...
1218 seek(MBOX, 0, 2);
1219 }
1220
1221 sub unlock {
7e1af8bc 1222 flock(MBOX,LOCK_UN);
a0d0e21e
LW
1223 }
1224
1225 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1226 or die "Can't open mailbox: $!";
1227
1228 lock();
1229 print MBOX $msg,"\n\n";
1230 unlock();
1231
cb1a09d0 1232See also L<DB_File> for other flock() examples.
a0d0e21e
LW
1233
1234=item fork
1235
1236Does a fork(2) system call. Returns the child pid to the parent process
4633a7c4 1237and 0 to the child process, or C<undef> if the fork is unsuccessful.
a0d0e21e 1238Note: unflushed buffers remain unflushed in both processes, which means
28757baa
PP
1239you may need to set C<$|> ($AUTOFLUSH in English) or call the autoflush()
1240method of IO::Handle to avoid duplicate output.
a0d0e21e
LW
1241
1242If you fork() without ever waiting on your children, you will accumulate
1243zombies:
1244
4633a7c4 1245 $SIG{CHLD} = sub { wait };
a0d0e21e 1246
54310121 1247There's also the double-fork trick (error checking on
a0d0e21e
LW
1248fork() returns omitted);
1249
1250 unless ($pid = fork) {
1251 unless (fork) {
1252 exec "what you really wanna do";
1253 die "no exec";
1254 # ... or ...
4633a7c4 1255 ## (some_perl_code_here)
a0d0e21e
LW
1256 exit 0;
1257 }
1258 exit 0;
1259 }
1260 waitpid($pid,0);
1261
cb1a09d0
AD
1262See also L<perlipc> for more examples of forking and reaping
1263moribund children.
1264
28757baa
PP
1265Note that if your forked child inherits system file descriptors like
1266STDIN and STDOUT that are actually connected by a pipe or socket, even
1267if you exit, the remote server (such as, say, httpd or rsh) won't think
1268you're done. You should reopen those to /dev/null if it's any issue.
1269
cb1a09d0
AD
1270=item format
1271
1272Declare a picture format with use by the write() function. For
1273example:
1274
54310121 1275 format Something =
cb1a09d0
AD
1276 Test: @<<<<<<<< @||||| @>>>>>
1277 $str, $%, '$' . int($num)
1278 .
1279
1280 $str = "widget";
184e9718 1281 $num = $cost/$quantity;
cb1a09d0
AD
1282 $~ = 'Something';
1283 write;
1284
1285See L<perlform> for many details and examples.
1286
a0d0e21e 1287
8903cb82 1288=item formline PICTURE,LIST
a0d0e21e 1289
4633a7c4 1290This is an internal function used by C<format>s, though you may call it
a0d0e21e
LW
1291too. It formats (see L<perlform>) a list of values according to the
1292contents of PICTURE, placing the output into the format output
4633a7c4
LW
1293accumulator, C<$^A> (or $ACCUMULATOR in English).
1294Eventually, when a write() is done, the contents of
a0d0e21e
LW
1295C<$^A> are written to some filehandle, but you could also read C<$^A>
1296yourself and then set C<$^A> back to "". Note that a format typically
1297does one formline() per line of form, but the formline() function itself
748a9306 1298doesn't care how many newlines are embedded in the PICTURE. This means
4633a7c4 1299that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
748a9306
LW
1300You may therefore need to use multiple formlines to implement a single
1301record format, just like the format compiler.
1302
5f05dabc 1303Be careful if you put double quotes around the picture, because an "C<@>"
748a9306 1304character may be taken to mean the beginning of an array name.
4633a7c4 1305formline() always returns TRUE. See L<perlform> for other examples.
a0d0e21e
LW
1306
1307=item getc FILEHANDLE
1308
1309=item getc
1310
1311Returns the next character from the input file attached to FILEHANDLE,
1312or a null string at end of file. If FILEHANDLE is omitted, reads from STDIN.
4633a7c4 1313This is not particularly efficient. It cannot be used to get unbuffered
cb1a09d0 1314single-characters, however. For that, try something more like:
4633a7c4
LW
1315
1316 if ($BSD_STYLE) {
1317 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1318 }
1319 else {
54310121 1320 system "stty", '-icanon', 'eol', "\001";
4633a7c4
LW
1321 }
1322
1323 $key = getc(STDIN);
1324
1325 if ($BSD_STYLE) {
1326 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1327 }
1328 else {
5f05dabc 1329 system "stty", 'icanon', 'eol', '^@'; # ASCII null
4633a7c4
LW
1330 }
1331 print "\n";
1332
54310121
PP
1333Determination of whether $BSD_STYLE should be set
1334is left as an exercise to the reader.
cb1a09d0 1335
28757baa
PP
1336The POSIX::getattr() function can do this more portably on systems
1337alleging POSIX compliance.
cb1a09d0 1338See also the C<Term::ReadKey> module from your nearest CPAN site;
54310121 1339details on CPAN can be found on L<perlmod/CPAN>.
a0d0e21e
LW
1340
1341=item getlogin
1342
1343Returns the current login from F</etc/utmp>, if any. If null, use
54310121 1344getpwuid().
a0d0e21e 1345
f86702cc 1346 $login = getlogin || getpwuid($<) || "Kilroy";
a0d0e21e 1347
da0045b7 1348Do not consider getlogin() for authentication: it is not as
4633a7c4
LW
1349secure as getpwuid().
1350
a0d0e21e
LW
1351=item getpeername SOCKET
1352
1353Returns the packed sockaddr address of other end of the SOCKET connection.
1354
4633a7c4
LW
1355 use Socket;
1356 $hersockaddr = getpeername(SOCK);
1357 ($port, $iaddr) = unpack_sockaddr_in($hersockaddr);
1358 $herhostname = gethostbyaddr($iaddr, AF_INET);
1359 $herstraddr = inet_ntoa($iaddr);
a0d0e21e
LW
1360
1361=item getpgrp PID
1362
47e29363
PP
1363Returns the current process group for the specified PID. Use
1364a PID of 0 to get the current process group for the
4633a7c4 1365current process. Will raise an exception if used on a machine that
a0d0e21e 1366doesn't implement getpgrp(2). If PID is omitted, returns process
47e29363
PP
1367group of current process. Note that the POSIX version of getpgrp()
1368does not accept a PID argument, so only PID==0 is truly portable.
a0d0e21e
LW
1369
1370=item getppid
1371
1372Returns the process id of the parent process.
1373
1374=item getpriority WHICH,WHO
1375
4633a7c4
LW
1376Returns the current priority for a process, a process group, or a user.
1377(See L<getpriority(2)>.) Will raise a fatal exception if used on a
a0d0e21e
LW
1378machine that doesn't implement getpriority(2).
1379
1380=item getpwnam NAME
1381
1382=item getgrnam NAME
1383
1384=item gethostbyname NAME
1385
1386=item getnetbyname NAME
1387
1388=item getprotobyname NAME
1389
1390=item getpwuid UID
1391
1392=item getgrgid GID
1393
1394=item getservbyname NAME,PROTO
1395
1396=item gethostbyaddr ADDR,ADDRTYPE
1397
1398=item getnetbyaddr ADDR,ADDRTYPE
1399
1400=item getprotobynumber NUMBER
1401
1402=item getservbyport PORT,PROTO
1403
1404=item getpwent
1405
1406=item getgrent
1407
1408=item gethostent
1409
1410=item getnetent
1411
1412=item getprotoent
1413
1414=item getservent
1415
1416=item setpwent
1417
1418=item setgrent
1419
1420=item sethostent STAYOPEN
1421
1422=item setnetent STAYOPEN
1423
1424=item setprotoent STAYOPEN
1425
1426=item setservent STAYOPEN
1427
1428=item endpwent
1429
1430=item endgrent
1431
1432=item endhostent
1433
1434=item endnetent
1435
1436=item endprotoent
1437
1438=item endservent
1439
1440These routines perform the same functions as their counterparts in the
1441system library. Within a list context, the return values from the
1442various get routines are as follows:
1443
1444 ($name,$passwd,$uid,$gid,
1445 $quota,$comment,$gcos,$dir,$shell) = getpw*
1446 ($name,$passwd,$gid,$members) = getgr*
1447 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1448 ($name,$aliases,$addrtype,$net) = getnet*
1449 ($name,$aliases,$proto) = getproto*
1450 ($name,$aliases,$port,$proto) = getserv*
1451
1452(If the entry doesn't exist you get a null list.)
1453
1454Within a scalar context, you get the name, unless the function was a
1455lookup by name, in which case you get the other thing, whatever it is.
1456(If the entry doesn't exist you get the undefined value.) For example:
1457
1458 $uid = getpwnam
1459 $name = getpwuid
1460 $name = getpwent
1461 $gid = getgrnam
1462 $name = getgrgid
1463 $name = getgrent
1464 etc.
1465
1466The $members value returned by I<getgr*()> is a space separated list of
1467the login names of the members of the group.
1468
1469For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1470C, it will be returned to you via C<$?> if the function call fails. The
1471@addrs value returned by a successful call is a list of the raw
1472addresses returned by the corresponding system library call. In the
1473Internet domain, each address is four bytes long and you can unpack it
1474by saying something like:
1475
1476 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1477
1478=item getsockname SOCKET
1479
1480Returns the packed sockaddr address of this end of the SOCKET connection.
1481
4633a7c4
LW
1482 use Socket;
1483 $mysockaddr = getsockname(SOCK);
1484 ($port, $myaddr) = unpack_sockaddr_in($mysockaddr);
a0d0e21e
LW
1485
1486=item getsockopt SOCKET,LEVEL,OPTNAME
1487
1488Returns the socket option requested, or undefined if there is an error.
1489
1490=item glob EXPR
1491
0a753a76
PP
1492=item glob
1493
68dc0745
PP
1494Returns the value of EXPR with filename expansions such as a shell would
1495do. This is the internal function implementing the C<E<lt>*.cE<gt>>
1496operator, but you can use it directly. If EXPR is omitted, $_ is used.
1497The C<E<lt>*.cE<gt>> operator is discussed in more detail in
1498L<perlop/"I/O Operators">.
a0d0e21e
LW
1499
1500=item gmtime EXPR
1501
1502Converts a time as returned by the time function to a 9-element array
54310121 1503with the time localized for the standard Greenwich time zone.
4633a7c4 1504Typically used as follows:
a0d0e21e 1505
54310121 1506 # 0 1 2 3 4 5 6 7 8
a0d0e21e
LW
1507 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1508 gmtime(time);
1509
1510All array elements are numeric, and come straight out of a struct tm.
1511In particular this means that $mon has the range 0..11 and $wday has
54310121
PP
1512the range 0..6 with sunday as day 0. Also, $year is the number of
1513years since 1900, I<not> simply the last two digits of the year.
2f9daede
TPG
1514
1515If EXPR is omitted, does C<gmtime(time())>.
a0d0e21e 1516
54310121 1517In a scalar context, returns the ctime(3) value:
0a753a76
PP
1518
1519 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
1520
54310121
PP
1521Also see the timegm() function provided by the Time::Local module,
1522and the strftime(3) function available via the POSIX module.
0a753a76 1523
a0d0e21e
LW
1524=item goto LABEL
1525
748a9306
LW
1526=item goto EXPR
1527
a0d0e21e
LW
1528=item goto &NAME
1529
1530The goto-LABEL form finds the statement labeled with LABEL and resumes
1531execution there. It may not be used to go into any construct that
1532requires initialization, such as a subroutine or a foreach loop. It
0a753a76
PP
1533also can't be used to go into a construct that is optimized away,
1534or to get out of a block or subroutine given to sort().
1535It can be used to go almost anywhere else within the dynamic scope,
a0d0e21e
LW
1536including out of subroutines, but it's usually better to use some other
1537construct such as last or die. The author of Perl has never felt the
1538need to use this form of goto (in Perl, that is--C is another matter).
1539
748a9306
LW
1540The goto-EXPR form expects a label name, whose scope will be resolved
1541dynamically. This allows for computed gotos per FORTRAN, but isn't
1542necessarily recommended if you're optimizing for maintainability:
1543
1544 goto ("FOO", "BAR", "GLARCH")[$i];
1545
a0d0e21e
LW
1546The goto-&NAME form is highly magical, and substitutes a call to the
1547named subroutine for the currently running subroutine. This is used by
1548AUTOLOAD subroutines that wish to load another subroutine and then
1549pretend that the other subroutine had been called in the first place
1550(except that any modifications to @_ in the current subroutine are
1551propagated to the other subroutine.) After the goto, not even caller()
1552will be able to tell that this routine was called first.
1553
1554=item grep BLOCK LIST
1555
1556=item grep EXPR,LIST
1557
54310121 1558This is similar in spirit to, but not the same as, grep(1)
2f9daede
TPG
1559and its relatives. In particular, it is not limited to using
1560regular expressions.
1561
a0d0e21e
LW
1562Evaluates the BLOCK or EXPR for each element of LIST (locally setting
1563$_ to each element) and returns the list value consisting of those
1564elements for which the expression evaluated to TRUE. In a scalar
1565context, returns the number of times the expression was TRUE.
1566
1567 @foo = grep(!/^#/, @bar); # weed out comments
1568
1569or equivalently,
1570
1571 @foo = grep {!/^#/} @bar; # weed out comments
1572
5f05dabc 1573Note that, because $_ is a reference into the list value, it can be used
a0d0e21e
LW
1574to modify the elements of the array. While this is useful and
1575supported, it can cause bizarre results if the LIST is not a named
2f9daede 1576array. Similarly, grep returns aliases into the original list,
2ae324a7 1577much like the way that L<Foreach Loops>'s index variable aliases the list
2f9daede 1578elements. That is, modifying an element of a list returned by grep
fb73857a 1579(for example, in a C<foreach>, C<map> or another C<grep>)
2f9daede 1580actually modifies the element in the original list.
a0d0e21e 1581
fb73857a 1582See also L</map> for an array composed of the results of the BLOCK or EXPR.
38325410 1583
a0d0e21e
LW
1584=item hex EXPR
1585
54310121 1586=item hex
bbce6d69 1587
54310121 1588Interprets EXPR as a hex string and returns the corresponding
2f9daede 1589value. (To convert strings that might start with either 0 or 0x
dc848c6f 1590see L</oct>.) If EXPR is omitted, uses $_.
2f9daede
TPG
1591
1592 print hex '0xAf'; # prints '175'
1593 print hex 'aF'; # same
a0d0e21e
LW
1594
1595=item import
1596
54310121 1597There is no builtin import() function. It is merely an ordinary
4633a7c4 1598method (subroutine) defined (or inherited) by modules that wish to export
a0d0e21e 1599names to another module. The use() function calls the import() method
54310121 1600for the package used. See also L</use()>, L<perlmod>, and L<Exporter>.
a0d0e21e
LW
1601
1602=item index STR,SUBSTR,POSITION
1603
1604=item index STR,SUBSTR
1605
4633a7c4
LW
1606Returns the position of the first occurrence of SUBSTR in STR at or after
1607POSITION. If POSITION is omitted, starts searching from the beginning of
184e9718 1608the string. The return value is based at 0 (or whatever you've set the C<$[>
4633a7c4 1609variable to--but don't do that). If the substring is not found, returns
a0d0e21e
LW
1610one less than the base, ordinarily -1.
1611
1612=item int EXPR
1613
54310121 1614=item int
bbce6d69 1615
a0d0e21e
LW
1616Returns the integer portion of EXPR. If EXPR is omitted, uses $_.
1617
1618=item ioctl FILEHANDLE,FUNCTION,SCALAR
1619
1620Implements the ioctl(2) function. You'll probably have to say
1621
4633a7c4 1622 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
a0d0e21e 1623
4633a7c4 1624first to get the correct function definitions. If F<ioctl.ph> doesn't
a0d0e21e 1625exist or doesn't have the correct definitions you'll have to roll your
4633a7c4
LW
1626own, based on your C header files such as F<E<lt>sys/ioctl.hE<gt>>.
1627(There is a Perl script called B<h2ph> that comes with the Perl kit which
54310121 1628may help you in this, but it's nontrivial.) SCALAR will be read and/or
4633a7c4
LW
1629written depending on the FUNCTION--a pointer to the string value of SCALAR
1630will be passed as the third argument of the actual ioctl call. (If SCALAR
1631has no string value but does have a numeric value, that value will be
1632passed rather than a pointer to the string value. To guarantee this to be
1633TRUE, add a 0 to the scalar before using it.) The pack() and unpack()
1634functions are useful for manipulating the values of structures used by
1635ioctl(). The following example sets the erase character to DEL.
a0d0e21e
LW
1636
1637 require 'ioctl.ph';
4633a7c4
LW
1638 $getp = &TIOCGETP;
1639 die "NO TIOCGETP" if $@ || !$getp;
a0d0e21e 1640 $sgttyb_t = "ccccs"; # 4 chars and a short
4633a7c4 1641 if (ioctl(STDIN,$getp,$sgttyb)) {
a0d0e21e
LW
1642 @ary = unpack($sgttyb_t,$sgttyb);
1643 $ary[2] = 127;
1644 $sgttyb = pack($sgttyb_t,@ary);
4633a7c4 1645 ioctl(STDIN,&TIOCSETP,$sgttyb)
a0d0e21e
LW
1646 || die "Can't ioctl: $!";
1647 }
1648
1649The return value of ioctl (and fcntl) is as follows:
1650
1651 if OS returns: then Perl returns:
1652 -1 undefined value
1653 0 string "0 but true"
1654 anything else that number
1655
1656Thus Perl returns TRUE on success and FALSE on failure, yet you can
1657still easily determine the actual value returned by the operating
1658system:
1659
1660 ($retval = ioctl(...)) || ($retval = -1);
1661 printf "System returned %d\n", $retval;
1662
1663=item join EXPR,LIST
1664
54310121 1665Joins the separate strings of LIST into a single string with
a0d0e21e
LW
1666fields separated by the value of EXPR, and returns the string.
1667Example:
1668
1669 $_ = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
1670
1671See L<perlfunc/split>.
1672
aa689395
PP
1673=item keys HASH
1674
1675Returns a normal array consisting of all the keys of the named hash. (In
1676a scalar context, returns the number of keys.) The keys are returned in
1677an apparently random order, but it is the same order as either the
1678values() or each() function produces (given that the hash has not been
1679modified). As a side effect, it resets HASH's iterator.
a0d0e21e 1680
aa689395 1681Here is yet another way to print your environment:
a0d0e21e
LW
1682
1683 @keys = keys %ENV;
1684 @values = values %ENV;
1685 while ($#keys >= 0) {
1686 print pop(@keys), '=', pop(@values), "\n";
1687 }
1688
1689or how about sorted by key:
1690
1691 foreach $key (sort(keys %ENV)) {
1692 print $key, '=', $ENV{$key}, "\n";
1693 }
1694
54310121 1695To sort an array by value, you'll need to use a C<sort> function.
aa689395 1696Here's a descending numeric sort of a hash by its values:
4633a7c4
LW
1697
1698 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash)) {
1699 printf "%4d %s\n", $hash{$key}, $key;
1700 }
1701
55497cff 1702As an lvalue C<keys> allows you to increase the number of hash buckets
aa689395
PP
1703allocated for the given hash. This can gain you a measure of efficiency if
1704you know the hash is going to get big. (This is similar to pre-extending
1705an array by assigning a larger number to $#array.) If you say
55497cff
PP
1706
1707 keys %hash = 200;
1708
1709then C<%hash> will have at least 200 buckets allocated for it. These
1710buckets will be retained even if you do C<%hash = ()>, use C<undef
1711%hash> if you want to free the storage while C<%hash> is still in scope.
1712You can't shrink the number of buckets allocated for the hash using
1713C<keys> in this way (but you needn't worry about doing this by accident,
1714as trying has no effect).
1715
a0d0e21e
LW
1716=item kill LIST
1717
54310121
PP
1718Sends a signal to a list of processes. The first element of
1719the list must be the signal to send. Returns the number of
4633a7c4 1720processes successfully signaled.
a0d0e21e
LW
1721
1722 $cnt = kill 1, $child1, $child2;
1723 kill 9, @goners;
1724
4633a7c4
LW
1725Unlike in the shell, in Perl if the I<SIGNAL> is negative, it kills
1726process groups instead of processes. (On System V, a negative I<PROCESS>
1727number will also kill process groups, but that's not portable.) That
1728means you usually want to use positive not negative signals. You may also
da0045b7 1729use a signal name in quotes. See L<perlipc/"Signals"> for details.
a0d0e21e
LW
1730
1731=item last LABEL
1732
1733=item last
1734
1735The C<last> command is like the C<break> statement in C (as used in
1736loops); it immediately exits the loop in question. If the LABEL is
1737omitted, the command refers to the innermost enclosing loop. The
1738C<continue> block, if any, is not executed:
1739
4633a7c4
LW
1740 LINE: while (<STDIN>) {
1741 last LINE if /^$/; # exit when done with header
a0d0e21e
LW
1742 ...
1743 }
1744
1745=item lc EXPR
1746
54310121 1747=item lc
bbce6d69 1748
a0d0e21e 1749Returns an lowercased version of EXPR. This is the internal function
54310121 1750implementing the \L escape in double-quoted strings.
a034a98d 1751Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
a0d0e21e 1752
bbce6d69
PP
1753If EXPR is omitted, uses $_.
1754
a0d0e21e
LW
1755=item lcfirst EXPR
1756
54310121 1757=item lcfirst
bbce6d69 1758
a0d0e21e
LW
1759Returns the value of EXPR with the first character lowercased. This is
1760the internal function implementing 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 length EXPR
1766
54310121 1767=item length
bbce6d69 1768
a0d0e21e
LW
1769Returns the length in characters of the value of EXPR. If EXPR is
1770omitted, returns length of $_.
1771
1772=item link OLDFILE,NEWFILE
1773
1774Creates a new filename linked to the old filename. Returns 1 for
1775success, 0 otherwise.
1776
1777=item listen SOCKET,QUEUESIZE
1778
1779Does the same thing that the listen system call does. Returns TRUE if
4633a7c4 1780it succeeded, FALSE otherwise. See example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
1781
1782=item local EXPR
1783
a0d0e21e 1784A local modifies the listed variables to be local to the enclosing block,
5f05dabc
PP
1785subroutine, C<eval{}>, or C<do>. If more than one value is listed, the
1786list must be placed in parentheses. See L<perlsub/"Temporary Values via
3e3baf6d 1787local()"> for details, including issues with tied arrays and hashes.
a0d0e21e 1788
cb1a09d0
AD
1789But you really probably want to be using my() instead, because local() isn't
1790what most people think of as "local"). See L<perlsub/"Private Variables
1791via my()"> for details.
a0d0e21e
LW
1792
1793=item localtime EXPR
1794
1795Converts a time as returned by the time function to a 9-element array
5f05dabc 1796with the time analyzed for the local time zone. Typically used as
a0d0e21e
LW
1797follows:
1798
54310121 1799 # 0 1 2 3 4 5 6 7 8
a0d0e21e
LW
1800 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1801 localtime(time);
1802
1803All array elements are numeric, and come straight out of a struct tm.
1804In particular this means that $mon has the range 0..11 and $wday has
54310121
PP
1805the range 0..6 with sunday as day 0. Also, $year is the number of
1806years since 1900, that is, $year is 123 in year 2023.
1807
1808If EXPR is omitted, uses the current time (C<localtime(time)>).
a0d0e21e 1809
0a753a76 1810In a scalar context, returns the ctime(3) value:
a0d0e21e 1811
5f05dabc 1812 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
a0d0e21e 1813
fb73857a
PP
1814This scalar value is B<not> locale dependent, see L<perllocale>,
1815but instead a Perl builtin.
54310121
PP
1816Also see the Time::Local module, and the strftime(3) and mktime(3)
1817function available via the POSIX module.
a0d0e21e
LW
1818
1819=item log EXPR
1820
54310121 1821=item log
bbce6d69 1822
a0d0e21e
LW
1823Returns logarithm (base I<e>) of EXPR. If EXPR is omitted, returns log
1824of $_.
1825
1826=item lstat FILEHANDLE
1827
1828=item lstat EXPR
1829
54310121 1830=item lstat
bbce6d69 1831
a0d0e21e
LW
1832Does the same thing as the stat() function, but stats a symbolic link
1833instead of the file the symbolic link points to. If symbolic links are
1834unimplemented on your system, a normal stat() is done.
1835
bbce6d69
PP
1836If EXPR is omitted, stats $_.
1837
a0d0e21e
LW
1838=item m//
1839
1840The match operator. See L<perlop>.
1841
1842=item map BLOCK LIST
1843
1844=item map EXPR,LIST
1845
1846Evaluates the BLOCK or EXPR for each element of LIST (locally setting $_ to each
1847element) and returns the list value composed of the results of each such
1848evaluation. Evaluates BLOCK or EXPR in a list context, so each element of LIST
1849may produce zero, one, or more elements in the returned value.
1850
1851 @chars = map(chr, @nums);
1852
1853translates a list of numbers to the corresponding characters. And
1854
4633a7c4 1855 %hash = map { getkey($_) => $_ } @array;
a0d0e21e
LW
1856
1857is just a funny way to write
1858
1859 %hash = ();
1860 foreach $_ (@array) {
4633a7c4 1861 $hash{getkey($_)} = $_;
a0d0e21e
LW
1862 }
1863
fb73857a
PP
1864Note that, because $_ is a reference into the list value, it can be used
1865to modify the elements of the array. While this is useful and
1866supported, it can cause bizarre results if the LIST is not a named
1867array. See also L</grep> for an array composed of those items of the
1868original list for which the BLOCK or EXPR evaluates to true.
1869
a0d0e21e
LW
1870=item mkdir FILENAME,MODE
1871
1872Creates the directory specified by FILENAME, with permissions specified
1873by MODE (as modified by umask). If it succeeds it returns 1, otherwise
184e9718 1874it returns 0 and sets C<$!> (errno).
a0d0e21e
LW
1875
1876=item msgctl ID,CMD,ARG
1877
4633a7c4 1878Calls the System V IPC function msgctl(2). If CMD is &IPC_STAT, then ARG
a0d0e21e
LW
1879must be a variable which will hold the returned msqid_ds structure.
1880Returns like ioctl: the undefined value for error, "0 but true" for
1881zero, or the actual return value otherwise.
1882
1883=item msgget KEY,FLAGS
1884
4633a7c4 1885Calls the System V IPC function msgget(2). Returns the message queue id,
a0d0e21e
LW
1886or the undefined value if there is an error.
1887
1888=item msgsnd ID,MSG,FLAGS
1889
1890Calls the System V IPC function msgsnd to send the message MSG to the
1891message queue ID. MSG must begin with the long integer message type,
c07a80fd 1892which may be created with C<pack("l", $type)>. Returns TRUE if
a0d0e21e
LW
1893successful, or FALSE if there is an error.
1894
1895=item msgrcv ID,VAR,SIZE,TYPE,FLAGS
1896
1897Calls the System V IPC function msgrcv to receive a message from
1898message queue ID into variable VAR with a maximum message size of
1899SIZE. Note that if a message is received, the message type will be the
1900first thing in VAR, and the maximum length of VAR is SIZE plus the size
1901of the message type. Returns TRUE if successful, or FALSE if there is
1902an error.
1903
1904=item my EXPR
1905
1906A "my" declares the listed variables to be local (lexically) to the
cb1a09d0 1907enclosing block, subroutine, C<eval>, or C<do/require/use>'d file. If
5f05dabc 1908more than one value is listed, the list must be placed in parentheses. See
cb1a09d0 1909L<perlsub/"Private Variables via my()"> for details.
4633a7c4 1910
a0d0e21e
LW
1911=item next LABEL
1912
1913=item next
1914
1915The C<next> command is like the C<continue> statement in C; it starts
1916the next iteration of the loop:
1917
4633a7c4
LW
1918 LINE: while (<STDIN>) {
1919 next LINE if /^#/; # discard comments
a0d0e21e
LW
1920 ...
1921 }
1922
1923Note that if there were a C<continue> block on the above, it would get
1924executed even on discarded lines. If the LABEL is omitted, the command
1925refers to the innermost enclosing loop.
1926
1927=item no Module LIST
1928
1929See the "use" function, which "no" is the opposite of.
1930
1931=item oct EXPR
1932
54310121 1933=item oct
bbce6d69 1934
4633a7c4 1935Interprets EXPR as an octal string and returns the corresponding
2f9daede 1936value. (If EXPR happens to start off with 0x, interprets it as
4633a7c4
LW
1937a hex string instead.) The following will handle decimal, octal, and
1938hex in the standard Perl or C notation:
a0d0e21e
LW
1939
1940 $val = oct($val) if $val =~ /^0/;
1941
2f9daede
TPG
1942If EXPR is omitted, uses $_. This function is commonly used when
1943a string such as "644" needs to be converted into a file mode, for
1944example. (Although perl will automatically convert strings into
1945numbers as needed, this automatic conversion assumes base 10.)
a0d0e21e
LW
1946
1947=item open FILEHANDLE,EXPR
1948
1949=item open FILEHANDLE
1950
1951Opens the file whose filename is given by EXPR, and associates it with
5f05dabc
PP
1952FILEHANDLE. If FILEHANDLE is an expression, its value is used as the
1953name of the real filehandle wanted. If EXPR is omitted, the scalar
1954variable of the same name as the FILEHANDLE contains the filename.
1955(Note that lexical variables--those declared with C<my>--will not work
1956for this purpose; so if you're using C<my>, specify EXPR in your call
1957to open.)
1958
1959If the filename begins with '<' or nothing, the file is opened for input.
1960If the filename begins with '>', the file is truncated and opened for
1961output. If the filename begins with '>>', the file is opened for
1962appending. You can put a '+' in front of the '>' or '<' to indicate that
1963you want both read and write access to the file; thus '+<' is almost
1964always preferred for read/write updates--the '+>' mode would clobber the
1965file first. The prefix and the filename may be separated with spaces.
1966These various prefixes correspond to the fopen(3) modes of 'r', 'r+', 'w',
1967'w+', 'a', and 'a+'.
1968
1969If the filename begins with "|", the filename is interpreted as a command
1970to which output is to be piped, and if the filename ends with a "|", the
1971filename is interpreted See L<perlipc/"Using open() for IPC"> for more
1972examples of this. as command which pipes input to us. (You may not have
7e1af8bc
PP
1973a raw open() to a command that pipes both in I<and> out, but see
1974L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
1975for alternatives.)
cb1a09d0 1976
184e9718 1977Opening '-' opens STDIN and opening 'E<gt>-' opens STDOUT. Open returns
54310121 1978nonzero upon success, the undefined value otherwise. If the open
4633a7c4 1979involved a pipe, the return value happens to be the pid of the
54310121 1980subprocess.
cb1a09d0
AD
1981
1982If you're unfortunate enough to be running Perl on a system that
1983distinguishes between text files and binary files (modern operating
1984systems don't care), then you should check out L</binmode> for tips for
1985dealing with this. The key distinction between systems that need binmode
1986and those that don't is their text file formats. Systems like Unix and
1987Plan9 that delimit lines with a single character, and that encode that
1988character in C as '\n', do not need C<binmode>. The rest need it.
1989
fb73857a
PP
1990When opening a file, it's usually a bad idea to continue normal execution
1991if the request failed, so C<open> is frequently used in connection with
1992C<die>. Even if C<die> won't do what you want (say, in a CGI script,
1993where you want to make a nicely formatted error message (but there are
1994modules which can help with that problem)) you should always check
1995the return value from opening a file. The infrequent exception is when
1996working with an unopened filehandle is actually what you want to do.
1997
cb1a09d0 1998Examples:
a0d0e21e
LW
1999
2000 $ARTICLE = 100;
2001 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2002 while (<ARTICLE>) {...
2003
2004 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
fb73857a 2005 # if the open fails, output is discarded
a0d0e21e 2006
fb73857a
PP
2007 open(DBASE, '+<dbase.mine') # open for update
2008 or die "Can't open 'dbase.mine' for update: $!";
cb1a09d0 2009
fb73857a
PP
2010 open(ARTICLE, "caesar <$article |") # decrypt article
2011 or die "Can't start caesar: $!";
a0d0e21e 2012
fb73857a
PP
2013 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
2014 or die "Can't start sort: $!";
a0d0e21e
LW
2015
2016 # process argument list of files along with any includes
2017
2018 foreach $file (@ARGV) {
2019 process($file, 'fh00');
2020 }
2021
2022 sub process {
2023 local($filename, $input) = @_;
2024 $input++; # this is a string increment
2025 unless (open($input, $filename)) {
2026 print STDERR "Can't open $filename: $!\n";
2027 return;
2028 }
2029
2030 while (<$input>) { # note use of indirection
2031 if (/^#include "(.*)"/) {
2032 process($1, $input);
2033 next;
2034 }
2035 ... # whatever
2036 }
2037 }
2038
2039You may also, in the Bourne shell tradition, specify an EXPR beginning
184e9718 2040with "E<gt>&", in which case the rest of the string is interpreted as the
a0d0e21e 2041name of a filehandle (or file descriptor, if numeric) which is to be
184e9718 2042duped and opened. You may use & after E<gt>, E<gt>E<gt>, E<lt>, +E<gt>,
5f05dabc 2043+E<gt>E<gt>, and +E<lt>. The
a0d0e21e 2044mode you specify should match the mode of the original filehandle.
184e9718 2045(Duping a filehandle does not take into account any existing contents of
cb1a09d0 2046stdio buffers.)
a0d0e21e
LW
2047Here is a script that saves, redirects, and restores STDOUT and
2048STDERR:
2049
2050 #!/usr/bin/perl
2051 open(SAVEOUT, ">&STDOUT");
2052 open(SAVEERR, ">&STDERR");
2053
2054 open(STDOUT, ">foo.out") || die "Can't redirect stdout";
2055 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
2056
2057 select(STDERR); $| = 1; # make unbuffered
2058 select(STDOUT); $| = 1; # make unbuffered
2059
2060 print STDOUT "stdout 1\n"; # this works for
2061 print STDERR "stderr 1\n"; # subprocesses too
2062
2063 close(STDOUT);
2064 close(STDERR);
2065
2066 open(STDOUT, ">&SAVEOUT");
2067 open(STDERR, ">&SAVEERR");
2068
2069 print STDOUT "stdout 2\n";
2070 print STDERR "stderr 2\n";
2071
2072
184e9718 2073If you specify "E<lt>&=N", where N is a number, then Perl will do an
4633a7c4
LW
2074equivalent of C's fdopen() of that file descriptor; this is more
2075parsimonious of file descriptors. For example:
a0d0e21e
LW
2076
2077 open(FILEHANDLE, "<&=$fd")
2078
5f05dabc 2079If you open a pipe on the command "-", i.e., either "|-" or "-|", then
a0d0e21e
LW
2080there is an implicit fork done, and the return value of open is the pid
2081of the child within the parent process, and 0 within the child
184e9718 2082process. (Use C<defined($pid)> to determine whether the open was successful.)
a0d0e21e
LW
2083The filehandle behaves normally for the parent, but i/o to that
2084filehandle is piped from/to the STDOUT/STDIN of the child process.
2085In the child process the filehandle isn't opened--i/o happens from/to
2086the new STDOUT or STDIN. Typically this is used like the normal
2087piped open when you want to exercise more control over just how the
2088pipe command gets executed, such as when you are running setuid, and
54310121 2089don't want to have to scan shell commands for metacharacters.
4633a7c4 2090The following pairs are more or less equivalent:
a0d0e21e
LW
2091
2092 open(FOO, "|tr '[a-z]' '[A-Z]'");
2093 open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
2094
2095 open(FOO, "cat -n '$file'|");
2096 open(FOO, "-|") || exec 'cat', '-n', $file;
2097
4633a7c4
LW
2098See L<perlipc/"Safe Pipe Opens"> for more examples of this.
2099
0dccf244 2100NOTE: On any operation which may do a fork, unflushed buffers remain
184e9718 2101unflushed in both processes, which means you may need to set C<$|> to
a0d0e21e
LW
2102avoid duplicate output.
2103
0dccf244
CS
2104Closing any piped filehandle causes the parent process to wait for the
2105child to finish, and returns the status value in C<$?>.
2106
5f05dabc
PP
2107Using the constructor from the IO::Handle package (or one of its
2108subclasses, such as IO::File or IO::Socket),
c07a80fd
PP
2109you can generate anonymous filehandles which have the scope of whatever
2110variables hold references to them, and automatically close whenever
2111and however you leave that scope:
2112
5f05dabc 2113 use IO::File;
c07a80fd
PP
2114 ...
2115 sub read_myfile_munged {
2116 my $ALL = shift;
5f05dabc 2117 my $handle = new IO::File;
c07a80fd
PP
2118 open($handle, "myfile") or die "myfile: $!";
2119 $first = <$handle>
2120 or return (); # Automatically closed here.
2121 mung $first or die "mung failed"; # Or here.
2122 return $first, <$handle> if $ALL; # Or here.
2123 $first; # Or here.
2124 }
2125
a0d0e21e 2126The filename that is passed to open will have leading and trailing
5f05dabc 2127whitespace deleted. To open a file with arbitrary weird
a0d0e21e
LW
2128characters in it, it's necessary to protect any leading and trailing
2129whitespace thusly:
2130
cb1a09d0
AD
2131 $file =~ s#^(\s)#./$1#;
2132 open(FOO, "< $file\0");
2133
c07a80fd
PP
2134If you want a "real" C open() (see L<open(2)> on your system), then
2135you should use the sysopen() function. This is another way to
2136protect your filenames from interpretation. For example:
cb1a09d0 2137
28757baa 2138 use IO::Handle;
c07a80fd
PP
2139 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL, 0700)
2140 or die "sysopen $path: $!";
2141 HANDLE->autoflush(1);
2142 HANDLE->print("stuff $$\n");
2143 seek(HANDLE, 0, 0);
2144 print "File contains: ", <HANDLE>;
cb1a09d0
AD
2145
2146See L</seek()> for some details about mixing reading and writing.
a0d0e21e
LW
2147
2148=item opendir DIRHANDLE,EXPR
2149
2150Opens a directory named EXPR for processing by readdir(), telldir(),
5f05dabc 2151seekdir(), rewinddir(), and closedir(). Returns TRUE if successful.
a0d0e21e
LW
2152DIRHANDLEs have their own namespace separate from FILEHANDLEs.
2153
2154=item ord EXPR
2155
54310121 2156=item ord
bbce6d69 2157
a0d0e21e 2158Returns the numeric ascii value of the first character of EXPR. If
dc848c6f 2159EXPR is omitted, uses $_. For the reverse, see L</chr>.
a0d0e21e
LW
2160
2161=item pack TEMPLATE,LIST
2162
2163Takes an array or list of values and packs it into a binary structure,
2164returning the string containing the structure. The TEMPLATE is a
2165sequence of characters that give the order and type of values, as
2166follows:
2167
2168 A An ascii string, will be space padded.
2169 a An ascii string, will be null padded.
2170 b A bit string (ascending bit order, like vec()).
2171 B A bit string (descending bit order).
2172 h A hex string (low nybble first).
2173 H A hex string (high nybble first).
2174
2175 c A signed char value.
2176 C An unsigned char value.
96e4d5b1 2177
a0d0e21e
LW
2178 s A signed short value.
2179 S An unsigned short value.
96e4d5b1
PP
2180 (This 'short' is _exactly_ 16 bits, which may differ from
2181 what a local C compiler calls 'short'.)
2182
a0d0e21e
LW
2183 i A signed integer value.
2184 I An unsigned integer value.
96e4d5b1
PP
2185 (This 'integer' is _at_least_ 32 bits wide. Its exact size
2186 depends on what a local C compiler calls 'int', and may
2187 even be larger than the 'long' described in the next item.)
2188
a0d0e21e
LW
2189 l A signed long value.
2190 L An unsigned long value.
96e4d5b1
PP
2191 (This 'long' is _exactly_ 32 bits, which may differ from
2192 what a local C compiler calls 'long'.)
a0d0e21e 2193
96e4d5b1
PP
2194 n A short in "network" (big-endian) order.
2195 N A long in "network" (big-endian) order.
a0d0e21e
LW
2196 v A short in "VAX" (little-endian) order.
2197 V A long in "VAX" (little-endian) order.
96e4d5b1
PP
2198 (These 'shorts' and 'longs' are _exactly_ 16 bits and
2199 _exactly_ 32 bits, respectively.)
a0d0e21e
LW
2200
2201 f A single-precision float in the native format.
2202 d A double-precision float in the native format.
2203
2204 p A pointer to a null-terminated string.
2205 P A pointer to a structure (fixed-length string).
2206
2207 u A uuencoded string.
2208
96e4d5b1
PP
2209 w A BER compressed integer. Its bytes represent an unsigned
2210 integer in base 128, most significant digit first, with as few
2211 digits as possible. Bit eight (the high bit) is set on each
2212 byte except the last.
def98dd4 2213
a0d0e21e
LW
2214 x A null byte.
2215 X Back up a byte.
2216 @ Null fill to absolute position.
2217
2218Each letter may optionally be followed by a number which gives a repeat
5f05dabc 2219count. With all types except "a", "A", "b", "B", "h", "H", and "P" the
a0d0e21e
LW
2220pack function will gobble up that many values from the LIST. A * for the
2221repeat count means to use however many items are left. The "a" and "A"
2222types gobble just one value, but pack it as a string of length count,
2223padding with nulls or spaces as necessary. (When unpacking, "A" strips
2224trailing spaces and nulls, but "a" does not.) Likewise, the "b" and "B"
2225fields pack a string that many bits long. The "h" and "H" fields pack a
84902520
TB
2226string that many nybbles long. The "p" type packs a pointer to a null-
2227terminated string. You are responsible for ensuring the string is not a
2228temporary value (which can potentially get deallocated before you get
2229around to using the packed result). The "P" packs a pointer to a structure
61167c6f
PM
2230of the size indicated by the length. A NULL pointer is created if the
2231corresponding value for "p" or "P" is C<undef>.
2232Real numbers (floats and doubles) are
a0d0e21e
LW
2233in the native machine format only; due to the multiplicity of floating
2234formats around, and the lack of a standard "network" representation, no
2235facility for interchange has been made. This means that packed floating
2236point data written on one machine may not be readable on another - even if
2237both use IEEE floating point arithmetic (as the endian-ness of the memory
2238representation is not part of the IEEE spec). Note that Perl uses doubles
2239internally for all numeric calculation, and converting from double into
5f05dabc 2240float and thence back to double again will lose precision (i.e.,
a0d0e21e
LW
2241C<unpack("f", pack("f", $foo)>) will not in general equal $foo).
2242
2243Examples:
2244
2245 $foo = pack("cccc",65,66,67,68);
2246 # foo eq "ABCD"
2247 $foo = pack("c4",65,66,67,68);
2248 # same thing
2249
2250 $foo = pack("ccxxcc",65,66,67,68);
2251 # foo eq "AB\0\0CD"
2252
2253 $foo = pack("s2",1,2);
2254 # "\1\0\2\0" on little-endian
2255 # "\0\1\0\2" on big-endian
2256
2257 $foo = pack("a4","abcd","x","y","z");
2258 # "abcd"
2259
2260 $foo = pack("aaaa","abcd","x","y","z");
2261 # "axyz"
2262
2263 $foo = pack("a14","abcdefg");
2264 # "abcdefg\0\0\0\0\0\0\0"
2265
2266 $foo = pack("i9pl", gmtime);
2267 # a real struct tm (on my system anyway)
2268
2269 sub bintodec {
2270 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
2271 }
2272
2273The same template may generally also be used in the unpack function.
2274
cb1a09d0
AD
2275=item package NAMESPACE
2276
2277Declares the compilation unit as being in the given namespace. The scope
2278of the package declaration is from the declaration itself through the end of
2279the enclosing block (the same scope as the local() operator). All further
2280unqualified dynamic identifiers will be in this namespace. A package
5f05dabc 2281statement affects only dynamic variables--including those you've used
cb1a09d0
AD
2282local() on--but I<not> lexical variables created with my(). Typically it
2283would be the first declaration in a file to be included by the C<require>
2284or C<use> operator. You can switch into a package in more than one place;
5f05dabc 2285it influences merely which symbol table is used by the compiler for the
cb1a09d0
AD
2286rest of that block. You can refer to variables and filehandles in other
2287packages by prefixing the identifier with the package name and a double
2288colon: C<$Package::Variable>. If the package name is null, the C<main>
2289package as assumed. That is, C<$::sail> is equivalent to C<$main::sail>.
2290
2291See L<perlmod/"Packages"> for more information about packages, modules,
2292and classes. See L<perlsub> for other scoping issues.
2293
a0d0e21e
LW
2294=item pipe READHANDLE,WRITEHANDLE
2295
2296Opens a pair of connected pipes like the corresponding system call.
2297Note that if you set up a loop of piped processes, deadlock can occur
2298unless you are very careful. In addition, note that Perl's pipes use
184e9718 2299stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE
a0d0e21e
LW
2300after each command, depending on the application.
2301
7e1af8bc 2302See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
4633a7c4
LW
2303for examples of such things.
2304
a0d0e21e
LW
2305=item pop ARRAY
2306
54310121 2307=item pop
28757baa 2308
a0d0e21e
LW
2309Pops and returns the last value of the array, shortening the array by
23101. Has a similar effect to
2311
2312 $tmp = $ARRAY[$#ARRAY--];
2313
2314If there are no elements in the array, returns the undefined value.
cb1a09d0
AD
2315If ARRAY is omitted, pops the
2316@ARGV array in the main program, and the @_ array in subroutines, just
2317like shift().
a0d0e21e
LW
2318
2319=item pos SCALAR
2320
54310121 2321=item pos
bbce6d69 2322
4633a7c4 2323Returns the offset of where the last C<m//g> search left off for the variable
2f9daede 2324is in question ($_ is used when the variable is not specified). May be
44a8e56a
PP
2325modified to change that offset. Such modification will also influence
2326the C<\G> zero-width assertion in regular expressions. See L<perlre> and
2327L<perlop>.
a0d0e21e
LW
2328
2329=item print FILEHANDLE LIST
2330
2331=item print LIST
2332
2333=item print
2334
cb1a09d0 2335Prints a string or a comma-separated list of strings. Returns TRUE
a0d0e21e 2336if successful. FILEHANDLE may be a scalar variable name, in which case
cb1a09d0 2337the variable contains the name of or a reference to the filehandle, thus introducing one
a0d0e21e
LW
2338level of indirection. (NOTE: If FILEHANDLE is a variable and the next
2339token is a term, it may be misinterpreted as an operator unless you
5f05dabc 2340interpose a + or put parentheses around the arguments.) If FILEHANDLE is
a0d0e21e 2341omitted, prints by default to standard output (or to the last selected
da0045b7 2342output channel--see L</select>). If LIST is also omitted, prints $_ to
a0d0e21e
LW
2343STDOUT. To set the default output channel to something other than
2344STDOUT use the select operation. Note that, because print takes a
2345LIST, anything in the LIST is evaluated in a list context, and any
2346subroutine that you call will have one or more of its expressions
2347evaluated in a list context. Also be careful not to follow the print
2348keyword with a left parenthesis unless you want the corresponding right
2349parenthesis to terminate the arguments to the print--interpose a + or
5f05dabc 2350put parentheses around all the arguments.
a0d0e21e 2351
4633a7c4 2352Note that if you're storing FILEHANDLES in an array or other expression,
da0045b7 2353you will have to use a block returning its value instead:
4633a7c4
LW
2354
2355 print { $files[$i] } "stuff\n";
2356 print { $OK ? STDOUT : STDERR } "stuff\n";
2357
5f05dabc 2358=item printf FILEHANDLE FORMAT, LIST
a0d0e21e 2359
5f05dabc 2360=item printf FORMAT, LIST
a0d0e21e 2361
a034a98d
DD
2362Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>. The first argument
2363of the list will be interpreted as the printf format. If C<use locale> is
2364in effect, the character used for the decimal point in formatted real numbers
2365is affected by the LC_NUMERIC locale. See L<perllocale>.
a0d0e21e 2366
28757baa
PP
2367Don't fall into the trap of using a printf() when a simple
2368print() would do. The print() is more efficient, and less
2369error prone.
2370
da0045b7
PP
2371=item prototype FUNCTION
2372
2373Returns the prototype of a function as a string (or C<undef> if the
5f05dabc
PP
2374function has no prototype). FUNCTION is a reference to, or the name of,
2375the function whose prototype you want to retrieve.
da0045b7 2376
b6c543e3
IZ
2377If FUNCTION is a string starting with C<CORE::>, the rest is taken as
2378a name for Perl builtin. If builtin is not I<overridable> (such as
2379C<qw>) or its arguments cannot be expressed by a prototype (such as
2380C<system>) - in other words, the builtin does not behave like a Perl
2381function - returns C<undef>. Otherwise, the string describing the
2382equivalent prototype is returned.
2383
a0d0e21e
LW
2384=item push ARRAY,LIST
2385
2386Treats ARRAY as a stack, and pushes the values of LIST
2387onto the end of ARRAY. The length of ARRAY increases by the length of
2388LIST. Has the same effect as
2389
2390 for $value (LIST) {
2391 $ARRAY[++$#ARRAY] = $value;
2392 }
2393
2394but is more efficient. Returns the new number of elements in the array.
2395
2396=item q/STRING/
2397
2398=item qq/STRING/
2399
2400=item qx/STRING/
2401
2402=item qw/STRING/
2403
2404Generalized quotes. See L<perlop>.
2405
2406=item quotemeta EXPR
2407
54310121 2408=item quotemeta
bbce6d69 2409
68dc0745 2410Returns the value of EXPR with all non-alphanumeric
a034a98d
DD
2411characters backslashed. (That is, all characters not matching
2412C</[A-Za-z_0-9]/> will be preceded by a backslash in the
2413returned string, regardless of any locale settings.)
2414This is the internal function implementing
a0d0e21e
LW
2415the \Q escape in double-quoted strings.
2416
bbce6d69
PP
2417If EXPR is omitted, uses $_.
2418
a0d0e21e
LW
2419=item rand EXPR
2420
2421=item rand
2422
3e3baf6d
TB
2423Returns a random fractional number greater than or equal to 0 and less
2424than the value of EXPR. (EXPR should be positive.) If EXPR is
2425omitted, the value 1 is used. Automatically calls srand() unless
2426srand() has already been called. See also srand().
a0d0e21e 2427
2f9daede 2428(Note: If your rand function consistently returns numbers that are too
a0d0e21e 2429large or too small, then your version of Perl was probably compiled
2f9daede 2430with the wrong number of RANDBITS.)
a0d0e21e
LW
2431
2432=item read FILEHANDLE,SCALAR,LENGTH,OFFSET
2433
2434=item read FILEHANDLE,SCALAR,LENGTH
2435
2436Attempts to read LENGTH bytes of data into variable SCALAR from the
2437specified FILEHANDLE. Returns the number of bytes actually read, or
2438undef if there was an error. SCALAR will be grown or shrunk to the
2439length actually read. An OFFSET may be specified to place the read
2440data at some other place than the beginning of the string. This call
2441is actually implemented in terms of stdio's fread call. To get a true
2442read system call, see sysread().
2443
2444=item readdir DIRHANDLE
2445
2446Returns the next directory entry for a directory opened by opendir().
2447If used in a list context, returns all the rest of the entries in the
2448directory. If there are no more entries, returns an undefined value in
2449a scalar context or a null list in a list context.
2450
cb1a09d0 2451If you're planning to filetest the return values out of a readdir(), you'd
5f05dabc 2452better prepend the directory in question. Otherwise, because we didn't
cb1a09d0
AD
2453chdir() there, it would have been testing the wrong file.
2454
2455 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
2456 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
2457 closedir DIR;
2458
84902520
TB
2459=item readline EXPR
2460
2461Reads from the file handle EXPR. In scalar context, a single line
2462is read and returned. In list context, reads until end-of-file is
2463reached and returns a list of lines (however you've defined lines
2464with $/ or $INPUT_RECORD_SEPARATOR).
2465This is the internal function implementing the C<E<lt>EXPRE<gt>>
2466operator, but you can use it directly. The C<E<lt>EXPRE<gt>>
2467operator is discussed in more detail in L<perlop/"I/O Operators">.
2468
a0d0e21e
LW
2469=item readlink EXPR
2470
54310121 2471=item readlink
bbce6d69 2472
a0d0e21e
LW
2473Returns the value of a symbolic link, if symbolic links are
2474implemented. If not, gives a fatal error. If there is some system
184e9718 2475error, returns the undefined value and sets C<$!> (errno). If EXPR is
a0d0e21e
LW
2476omitted, uses $_.
2477
84902520
TB
2478=item readpipe EXPR
2479
2480EXPR is interpolated and then executed as a system command.
2481The collected standard output of the command is returned.
2482In scalar context, it comes back as a single (potentially
2483multi-line) string. In list context, returns a list of lines
2484(however you've defined lines with $/ or $INPUT_RECORD_SEPARATOR).
2485This is the internal function implementing the C<qx/EXPR/>
2486operator, but you can use it directly. The C<qx/EXPR/>
2487operator is discussed in more detail in L<perlop/"I/O Operators">.
2488
a0d0e21e
LW
2489=item recv SOCKET,SCALAR,LEN,FLAGS
2490
2491Receives a message on a socket. Attempts to receive LENGTH bytes of
2492data into variable SCALAR from the specified SOCKET filehandle.
2493Actually does a C recvfrom(), so that it can returns the address of the
2494sender. Returns the undefined value if there's an error. SCALAR will
2495be grown or shrunk to the length actually read. Takes the same flags
54310121 2496as the system call of the same name.
4633a7c4 2497See L<perlipc/"UDP: Message Passing"> for examples.
a0d0e21e
LW
2498
2499=item redo LABEL
2500
2501=item redo
2502
2503The C<redo> command restarts the loop block without evaluating the
2504conditional again. The C<continue> block, if any, is not executed. If
2505the LABEL is omitted, the command refers to the innermost enclosing
2506loop. This command is normally used by programs that want to lie to
2507themselves about what was just input:
2508
2509 # a simpleminded Pascal comment stripper
2510 # (warning: assumes no { or } in strings)
4633a7c4 2511 LINE: while (<STDIN>) {
a0d0e21e
LW
2512 while (s|({.*}.*){.*}|$1 |) {}
2513 s|{.*}| |;
2514 if (s|{.*| |) {
2515 $front = $_;
2516 while (<STDIN>) {
2517 if (/}/) { # end of comment?
2518 s|^|$front{|;
4633a7c4 2519 redo LINE;
a0d0e21e
LW
2520 }
2521 }
2522 }
2523 print;
2524 }
2525
2526=item ref EXPR
2527
54310121 2528=item ref
bbce6d69 2529
2f9daede
TPG
2530Returns a TRUE value if EXPR is a reference, FALSE otherwise. If EXPR
2531is not specified, $_ will be used. The value returned depends on the
bbce6d69 2532type of thing the reference is a reference to.
a0d0e21e
LW
2533Builtin types include:
2534
2535 REF
2536 SCALAR
2537 ARRAY
2538 HASH
2539 CODE
2540 GLOB
2541
54310121 2542If the referenced object has been blessed into a package, then that package
a0d0e21e
LW
2543name is returned instead. You can think of ref() as a typeof() operator.
2544
2545 if (ref($r) eq "HASH") {
aa689395 2546 print "r is a reference to a hash.\n";
54310121 2547 }
a0d0e21e
LW
2548 if (!ref ($r) {
2549 print "r is not a reference at all.\n";
54310121 2550 }
a0d0e21e
LW
2551
2552See also L<perlref>.
2553
2554=item rename OLDNAME,NEWNAME
2555
2556Changes the name of a file. Returns 1 for success, 0 otherwise. Will
5f05dabc 2557not work across file system boundaries.
a0d0e21e
LW
2558
2559=item require EXPR
2560
2561=item require
2562
2563Demands some semantics specified by EXPR, or by $_ if EXPR is not
2564supplied. If EXPR is numeric, demands that the current version of Perl
184e9718 2565(C<$]> or $PERL_VERSION) be equal or greater than EXPR.
a0d0e21e
LW
2566
2567Otherwise, demands that a library file be included if it hasn't already
2568been included. The file is included via the do-FILE mechanism, which is
2569essentially just a variety of eval(). Has semantics similar to the following
2570subroutine:
2571
2572 sub require {
2573 local($filename) = @_;
2574 return 1 if $INC{$filename};
2575 local($realfilename,$result);
2576 ITER: {
2577 foreach $prefix (@INC) {
2578 $realfilename = "$prefix/$filename";
2579 if (-f $realfilename) {
2580 $result = do $realfilename;
2581 last ITER;
2582 }
2583 }
2584 die "Can't find $filename in \@INC";
2585 }
2586 die $@ if $@;
2587 die "$filename did not return true value" unless $result;
2588 $INC{$filename} = $realfilename;
2589 $result;
2590 }
2591
2592Note that the file will not be included twice under the same specified
2593name. The file must return TRUE as the last statement to indicate
2594successful execution of any initialization code, so it's customary to
2595end such a file with "1;" unless you're sure it'll return TRUE
2596otherwise. But it's better just to put the "C<1;>", in case you add more
2597statements.
2598
54310121 2599If EXPR is a bareword, the require assumes a "F<.pm>" extension and
da0045b7 2600replaces "F<::>" with "F</>" in the filename for you,
54310121 2601to make it easy to load standard modules. This form of loading of
a0d0e21e
LW
2602modules does not risk altering your namespace.
2603
54310121 2604For a yet-more-powerful import facility, see L</use> and
748a9306 2605L<perlmod>.
a0d0e21e
LW
2606
2607=item reset EXPR
2608
2609=item reset
2610
2611Generally used in a C<continue> block at the end of a loop to clear
2612variables and reset ?? searches so that they work again. The
2613expression is interpreted as a list of single characters (hyphens
2614allowed for ranges). All variables and arrays beginning with one of
2615those letters are reset to their pristine state. If the expression is
5f05dabc
PP
2616omitted, one-match searches (?pattern?) are reset to match again. Resets
2617only variables or searches in the current package. Always returns
a0d0e21e
LW
26181. Examples:
2619
2620 reset 'X'; # reset all X variables
2621 reset 'a-z'; # reset lower case variables
2622 reset; # just reset ?? searches
2623
5f05dabc
PP
2624Resetting "A-Z" is not recommended because you'll wipe out your
2625ARGV and ENV arrays. Resets only package variables--lexical variables
a0d0e21e 2626are unaffected, but they clean themselves up on scope exit anyway,
da0045b7 2627so you'll probably want to use them instead. See L</my>.
a0d0e21e 2628
54310121
PP
2629=item return EXPR
2630
2631=item return
2632
2633Returns from a subroutine, eval(), or do FILE with the value of the
2634given EXPR. Evaluation of EXPR may be in a list, scalar, or void
2635context, depending on how the return value will be used, and the context
2636may vary from one execution to the next (see wantarray()). If no EXPR
2637is given, returns an empty list in a list context, an undefined value in
2638a scalar context, or nothing in a void context.
a0d0e21e 2639
68dc0745
PP
2640(Note that in the absence of a return, a subroutine, eval, or do FILE
2641will automatically return the value of the last expression evaluated.)
a0d0e21e
LW
2642
2643=item reverse LIST
2644
2645In a list context, returns a list value consisting of the elements
2f9daede
TPG
2646of LIST in the opposite order. In a scalar context, concatenates the
2647elements of LIST, and returns a string value consisting of those bytes,
2648but in the opposite order.
4633a7c4 2649
2f9daede 2650 print reverse <>; # line tac, last line first
4633a7c4 2651
2f9daede
TPG
2652 undef $/; # for efficiency of <>
2653 print scalar reverse <>; # byte tac, last line tsrif
2654
2655This operator is also handy for inverting a hash, although there are some
2656caveats. If a value is duplicated in the original hash, only one of those
2657can be represented as a key in the inverted hash. Also, this has to
2658unwind one hash and build a whole new one, which may take some time
2659on a large hash.
2660
2661 %by_name = reverse %by_address; # Invert the hash
a0d0e21e
LW
2662
2663=item rewinddir DIRHANDLE
2664
2665Sets the current position to the beginning of the directory for the
2666readdir() routine on DIRHANDLE.
2667
2668=item rindex STR,SUBSTR,POSITION
2669
2670=item rindex STR,SUBSTR
2671
2672Works just like index except that it returns the position of the LAST
2673occurrence of SUBSTR in STR. If POSITION is specified, returns the
2674last occurrence at or before that position.
2675
2676=item rmdir FILENAME
2677
54310121 2678=item rmdir
bbce6d69 2679
a0d0e21e 2680Deletes the directory specified by FILENAME if it is empty. If it
184e9718 2681succeeds it returns 1, otherwise it returns 0 and sets C<$!> (errno). If
a0d0e21e
LW
2682FILENAME is omitted, uses $_.
2683
2684=item s///
2685
2686The substitution operator. See L<perlop>.
2687
2688=item scalar EXPR
2689
2690Forces EXPR to be interpreted in a scalar context and returns the value
54310121 2691of EXPR.
cb1a09d0
AD
2692
2693 @counts = ( scalar @a, scalar @b, scalar @c );
2694
54310121 2695There is no equivalent operator to force an expression to
cb1a09d0
AD
2696be interpolated in a list context because it's in practice never
2697needed. If you really wanted to do so, however, you could use
2698the construction C<@{[ (some expression) ]}>, but usually a simple
2699C<(some expression)> suffices.
a0d0e21e
LW
2700
2701=item seek FILEHANDLE,POSITION,WHENCE
2702
8903cb82
PP
2703Sets FILEHANDLE's position, just like the fseek() call of stdio.
2704FILEHANDLE may be an expression whose value gives the name of the
2705filehandle. The values for WHENCE are 0 to set the new position to
2706POSITION, 1 to set it to the current position plus POSITION, and 2 to
2707set it to EOF plus POSITION (typically negative). For WHENCE you may
2708use the constants SEEK_SET, SEEK_CUR, and SEEK_END from either the
2709IO::Seekable or the POSIX module. Returns 1 upon success, 0 otherwise.
2710
2711If you want to position file for sysread() or syswrite(), don't use
2712seek() -- buffering makes its effect on the file's system position
137443ea 2713unpredictable and non-portable. Use sysseek() instead.
a0d0e21e 2714
cb1a09d0
AD
2715On some systems you have to do a seek whenever you switch between reading
2716and writing. Amongst other things, this may have the effect of calling
8903cb82
PP
2717stdio's clearerr(3). A WHENCE of 1 (SEEK_CUR) is useful for not moving
2718the file position:
cb1a09d0
AD
2719
2720 seek(TEST,0,1);
2721
2722This is also useful for applications emulating C<tail -f>. Once you hit
2723EOF on your read, and then sleep for a while, you might have to stick in a
8903cb82
PP
2724seek() to reset things. The seek() doesn't change the current position,
2725but it I<does> clear the end-of-file condition on the handle, so that the
2726next C<E<lt>FILEE<gt>> makes Perl try again to read something. We hope.
cb1a09d0
AD
2727
2728If that doesn't work (some stdios are particularly cantankerous), then
2729you may need something more like this:
2730
2731 for (;;) {
2732 for ($curpos = tell(FILE); $_ = <FILE>; $curpos = tell(FILE)) {
2733 # search for some stuff and put it into files
2734 }
2735 sleep($for_a_while);
2736 seek(FILE, $curpos, 0);
2737 }
2738
a0d0e21e
LW
2739=item seekdir DIRHANDLE,POS
2740
2741Sets the current position for the readdir() routine on DIRHANDLE. POS
2742must be a value returned by telldir(). Has the same caveats about
2743possible directory compaction as the corresponding system library
2744routine.
2745
2746=item select FILEHANDLE
2747
2748=item select
2749
2750Returns the currently selected filehandle. Sets the current default
2751filehandle for output, if FILEHANDLE is supplied. This has two
2752effects: first, a C<write> or a C<print> without a filehandle will
2753default to this FILEHANDLE. Second, references to variables related to
2754output will refer to this output channel. For example, if you have to
2755set the top of form format for more than one output channel, you might
2756do the following:
2757
2758 select(REPORT1);
2759 $^ = 'report1_top';
2760 select(REPORT2);
2761 $^ = 'report2_top';
2762
2763FILEHANDLE may be an expression whose value gives the name of the
2764actual filehandle. Thus:
2765
2766 $oldfh = select(STDERR); $| = 1; select($oldfh);
2767
4633a7c4
LW
2768Some programmers may prefer to think of filehandles as objects with
2769methods, preferring to write the last example as:
a0d0e21e 2770
28757baa 2771 use IO::Handle;
a0d0e21e
LW
2772 STDERR->autoflush(1);
2773
2774=item select RBITS,WBITS,EBITS,TIMEOUT
2775
5f05dabc 2776This calls the select(2) system call with the bit masks specified, which
a0d0e21e
LW
2777can be constructed using fileno() and vec(), along these lines:
2778
2779 $rin = $win = $ein = '';
2780 vec($rin,fileno(STDIN),1) = 1;
2781 vec($win,fileno(STDOUT),1) = 1;
2782 $ein = $rin | $win;
2783
2784If you want to select on many filehandles you might wish to write a
2785subroutine:
2786
2787 sub fhbits {
2788 local(@fhlist) = split(' ',$_[0]);
2789 local($bits);
2790 for (@fhlist) {
2791 vec($bits,fileno($_),1) = 1;
2792 }
2793 $bits;
2794 }
4633a7c4 2795 $rin = fhbits('STDIN TTY SOCK');
a0d0e21e
LW
2796
2797The usual idiom is:
2798
2799 ($nfound,$timeleft) =
2800 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
2801
54310121 2802or to block until something becomes ready just do this
a0d0e21e
LW
2803
2804 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
2805
5f05dabc 2806Most systems do not bother to return anything useful in $timeleft, so
c07a80fd
PP
2807calling select() in a scalar context just returns $nfound.
2808
5f05dabc 2809Any of the bit masks can also be undef. The timeout, if specified, is
a0d0e21e
LW
2810in seconds, which may be fractional. Note: not all implementations are
2811capable of returning the $timeleft. If not, they always return
2812$timeleft equal to the supplied $timeout.
2813
ff68c719 2814You can effect a sleep of 250 milliseconds this way:
a0d0e21e
LW
2815
2816 select(undef, undef, undef, 0.25);
2817
184e9718 2818B<WARNING>: Do not attempt to mix buffered I/O (like read() or E<lt>FHE<gt>)
cb1a09d0 2819with select(). You have to use sysread() instead.
a0d0e21e
LW
2820
2821=item semctl ID,SEMNUM,CMD,ARG
2822
2823Calls the System V IPC function semctl. If CMD is &IPC_STAT or
2824&GETALL, then ARG must be a variable which will hold the returned
2825semid_ds structure or semaphore value array. Returns like ioctl: the
2826undefined value for error, "0 but true" for zero, or the actual return
2827value otherwise.
2828
2829=item semget KEY,NSEMS,FLAGS
2830
2831Calls the System V IPC function semget. Returns the semaphore id, or
2832the undefined value if there is an error.
2833
2834=item semop KEY,OPSTRING
2835
2836Calls the System V IPC function semop to perform semaphore operations
2837such as signaling and waiting. OPSTRING must be a packed array of
2838semop structures. Each semop structure can be generated with
2839C<pack("sss", $semnum, $semop, $semflag)>. The number of semaphore
2840operations is implied by the length of OPSTRING. Returns TRUE if
2841successful, or FALSE if there is an error. As an example, the
2842following code waits on semaphore $semnum of semaphore id $semid:
2843
2844 $semop = pack("sss", $semnum, -1, 0);
2845 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
2846
2847To signal the semaphore, replace "-1" with "1".
2848
2849=item send SOCKET,MSG,FLAGS,TO
2850
2851=item send SOCKET,MSG,FLAGS
2852
2853Sends a message on a socket. Takes the same flags as the system call
2854of the same name. On unconnected sockets you must specify a
2855destination to send TO, in which case it does a C sendto(). Returns
2856the number of characters sent, or the undefined value if there is an
2857error.
4633a7c4 2858See L<perlipc/"UDP: Message Passing"> for examples.
a0d0e21e
LW
2859
2860=item setpgrp PID,PGRP
2861
2862Sets the current process group for the specified PID, 0 for the current
2863process. Will produce a fatal error if used on a machine that doesn't
5f05dabc 2864implement setpgrp(2). If the arguments are omitted, it defaults to
47e29363
PP
28650,0. Note that the POSIX version of setpgrp() does not accept any
2866arguments, so only setpgrp 0,0 is portable.
a0d0e21e
LW
2867
2868=item setpriority WHICH,WHO,PRIORITY
2869
2870Sets the current priority for a process, a process group, or a user.
748a9306 2871(See setpriority(2).) Will produce a fatal error if used on a machine
a0d0e21e
LW
2872that doesn't implement setpriority(2).
2873
2874=item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
2875
2876Sets the socket option requested. Returns undefined if there is an
2877error. OPTVAL may be specified as undef if you don't want to pass an
2878argument.
2879
2880=item shift ARRAY
2881
2882=item shift
2883
2884Shifts the first value of the array off and returns it, shortening the
2885array by 1 and moving everything down. If there are no elements in the
2886array, returns the undefined value. If ARRAY is omitted, shifts the
977336f5
GS
2887@_ array within the lexical scope of subroutines and formats, and the
2888@ARGV array at file scopes or within the lexical scopes established by
2889the C<eval ''>, C<BEGIN {}>, C<END {}>, and C<INIT {}> constructs.
2890See also unshift(), push(), and pop(). Shift() and unshift() do the
2891same thing to the left end of an array that pop() and push() do to the
2892right end.
a0d0e21e
LW
2893
2894=item shmctl ID,CMD,ARG
2895
2896Calls the System V IPC function shmctl. If CMD is &IPC_STAT, then ARG
2897must be a variable which will hold the returned shmid_ds structure.
2898Returns like ioctl: the undefined value for error, "0 but true" for
2899zero, or the actual return value otherwise.
2900
2901=item shmget KEY,SIZE,FLAGS
2902
2903Calls the System V IPC function shmget. Returns the shared memory
2904segment id, or the undefined value if there is an error.
2905
2906=item shmread ID,VAR,POS,SIZE
2907
2908=item shmwrite ID,STRING,POS,SIZE
2909
2910Reads or writes the System V shared memory segment ID starting at
2911position POS for size SIZE by attaching to it, copying in/out, and
2912detaching from it. When reading, VAR must be a variable which will
2913hold the data read. When writing, if STRING is too long, only SIZE
2914bytes are used; if STRING is too short, nulls are written to fill out
2915SIZE bytes. Return TRUE if successful, or FALSE if there is an error.
2916
2917=item shutdown SOCKET,HOW
2918
2919Shuts down a socket connection in the manner indicated by HOW, which
2920has the same interpretation as in the system call of the same name.
2921
2922=item sin EXPR
2923
54310121 2924=item sin
bbce6d69 2925
a0d0e21e
LW
2926Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
2927returns sine of $_.
2928
54310121 2929For the inverse sine operation, you may use the POSIX::asin()
28757baa
PP
2930function, or use this relation:
2931
2932 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
2933
a0d0e21e
LW
2934=item sleep EXPR
2935
2936=item sleep
2937
2938Causes the script to sleep for EXPR seconds, or forever if no EXPR.
2939May be interrupted by sending the process a SIGALRM. Returns the
2940number of seconds actually slept. You probably cannot mix alarm() and
5f05dabc 2941sleep() calls, because sleep() is often implemented using alarm().
a0d0e21e
LW
2942
2943On some older systems, it may sleep up to a full second less than what
2944you requested, depending on how it counts seconds. Most modern systems
2945always sleep the full amount.
2946
cb1a09d0 2947For delays of finer granularity than one second, you may use Perl's
54310121
PP
2948syscall() interface to access setitimer(2) if your system supports it,
2949or else see L</select()> below.
cb1a09d0 2950
5f05dabc
PP
2951See also the POSIX module's sigpause() function.
2952
a0d0e21e
LW
2953=item socket SOCKET,DOMAIN,TYPE,PROTOCOL
2954
2955Opens a socket of the specified kind and attaches it to filehandle
5f05dabc 2956SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for the
a0d0e21e 2957system call of the same name. You should "use Socket;" first to get
4633a7c4 2958the proper definitions imported. See the example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
2959
2960=item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
2961
2962Creates an unnamed pair of sockets in the specified domain, of the
5f05dabc 2963specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
a0d0e21e
LW
2964for the system call of the same name. If unimplemented, yields a fatal
2965error. Returns TRUE if successful.
2966
2967=item sort SUBNAME LIST
2968
2969=item sort BLOCK LIST
2970
2971=item sort LIST
2972
2f9daede
TPG
2973Sorts the LIST and returns the sorted list value. If SUBNAME or BLOCK
2974is omitted, sorts in standard string comparison order. If SUBNAME is
2975specified, it gives the name of a subroutine that returns an integer
2976less than, equal to, or greater than 0, depending on how the elements
2977of the array are to be ordered. (The C<E<lt>=E<gt>> and C<cmp>
2978operators are extremely useful in such routines.) SUBNAME may be a
2979scalar variable name, in which case the value provides the name of the
2980subroutine to use. In place of a SUBNAME, you can provide a BLOCK as
2981an anonymous, in-line sort subroutine.
a0d0e21e 2982
cb1a09d0
AD
2983In the interests of efficiency the normal calling code for subroutines is
2984bypassed, with the following effects: the subroutine may not be a
2985recursive subroutine, and the two elements to be compared are passed into
2986the subroutine not via @_ but as the package global variables $a and
2987$b (see example below). They are passed by reference, so don't
2988modify $a and $b. And don't try to declare them as lexicals either.
a0d0e21e 2989
0a753a76
PP
2990You also cannot exit out of the sort block or subroutine using any of the
2991loop control operators described in L<perlsyn> or with goto().
2992
a034a98d
DD
2993When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
2994current collation locale. See L<perllocale>.
2995
a0d0e21e
LW
2996Examples:
2997
2998 # sort lexically
2999 @articles = sort @files;
3000
3001 # same thing, but with explicit sort routine
3002 @articles = sort {$a cmp $b} @files;
3003
cb1a09d0 3004 # now case-insensitively
54310121 3005 @articles = sort {uc($a) cmp uc($b)} @files;
cb1a09d0 3006
a0d0e21e
LW
3007 # same thing in reversed order
3008 @articles = sort {$b cmp $a} @files;
3009
3010 # sort numerically ascending
3011 @articles = sort {$a <=> $b} @files;
3012
3013 # sort numerically descending
3014 @articles = sort {$b <=> $a} @files;
3015
3016 # sort using explicit subroutine name
3017 sub byage {
2f9daede 3018 $age{$a} <=> $age{$b}; # presuming numeric
a0d0e21e
LW
3019 }
3020 @sortedclass = sort byage @class;
3021
aa689395
PP
3022 # this sorts the %age hash by value instead of key
3023 # using an in-line function
c07a80fd
PP
3024 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
3025
a0d0e21e
LW
3026 sub backwards { $b cmp $a; }
3027 @harry = ('dog','cat','x','Cain','Abel');
3028 @george = ('gone','chased','yz','Punished','Axed');
3029 print sort @harry;
3030 # prints AbelCaincatdogx
3031 print sort backwards @harry;
3032 # prints xdogcatCainAbel
3033 print sort @george, 'to', @harry;
3034 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
3035
54310121
PP
3036 # inefficiently sort by descending numeric compare using
3037 # the first integer after the first = sign, or the
cb1a09d0
AD
3038 # whole record case-insensitively otherwise
3039
3040 @new = sort {
3041 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
3042 ||
3043 uc($a) cmp uc($b)
3044 } @old;
3045
3046 # same thing, but much more efficiently;
3047 # we'll build auxiliary indices instead
3048 # for speed
3049 @nums = @caps = ();
54310121 3050 for (@old) {
cb1a09d0
AD
3051 push @nums, /=(\d+)/;
3052 push @caps, uc($_);
54310121 3053 }
cb1a09d0
AD
3054
3055 @new = @old[ sort {
3056 $nums[$b] <=> $nums[$a]
3057 ||
3058 $caps[$a] cmp $caps[$b]
3059 } 0..$#old
3060 ];
3061
3062 # same thing using a Schwartzian Transform (no temps)
3063 @new = map { $_->[0] }
3064 sort { $b->[1] <=> $a->[1]
3065 ||
3066 $a->[2] cmp $b->[2]
3067 } map { [$_, /=(\d+)/, uc($_)] } @old;
3068
184e9718 3069If you're using strict, you I<MUST NOT> declare $a
cb1a09d0
AD
3070and $b as lexicals. They are package globals. That means
3071if you're in the C<main> package, it's
3072
3073 @articles = sort {$main::b <=> $main::a} @files;
3074
3075or just
3076
3077 @articles = sort {$::b <=> $::a} @files;
3078
3079but if you're in the C<FooPack> package, it's
3080
3081 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
3082
55497cff
PP
3083The comparison function is required to behave. If it returns
3084inconsistent results (sometimes saying $x[1] is less than $x[2] and
3085sometimes saying the opposite, for example) the Perl interpreter will
3086probably crash and dump core. This is entirely due to and dependent
3087upon your system's qsort(3) library routine; this routine often avoids
3088sanity checks in the interest of speed.
3089
a0d0e21e
LW
3090=item splice ARRAY,OFFSET,LENGTH,LIST
3091
3092=item splice ARRAY,OFFSET,LENGTH
3093
3094=item splice ARRAY,OFFSET
3095
3096Removes the elements designated by OFFSET and LENGTH from an array, and
3097replaces them with the elements of LIST, if any. Returns the elements
3098removed from the array. The array grows or shrinks as necessary. If
3099LENGTH is omitted, removes everything from OFFSET onward. The
5f05dabc 3100following equivalences hold (assuming C<$[ == 0>):
a0d0e21e
LW
3101
3102 push(@a,$x,$y) splice(@a,$#a+1,0,$x,$y)
3103 pop(@a) splice(@a,-1)
3104 shift(@a) splice(@a,0,1)
3105 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
3106 $a[$x] = $y splice(@a,$x,1,$y);
3107
3108Example, assuming array lengths are passed before arrays:
3109
3110 sub aeq { # compare two list values
3111 local(@a) = splice(@_,0,shift);
3112 local(@b) = splice(@_,0,shift);
3113 return 0 unless @a == @b; # same len?
3114 while (@a) {
3115 return 0 if pop(@a) ne pop(@b);
3116 }
3117 return 1;
3118 }
3119 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
3120
3121=item split /PATTERN/,EXPR,LIMIT
3122
3123=item split /PATTERN/,EXPR
3124
3125=item split /PATTERN/
3126
3127=item split
3128
3129Splits a string into an array of strings, and returns it.
3130
3131If not in a list context, returns the number of fields found and splits into
3132the @_ array. (In a list context, you can force the split into @_ by
3133using C<??> as the pattern delimiters, but it still returns the array
3134value.) The use of implicit split to @_ is deprecated, however.
3135
3136If EXPR is omitted, splits the $_ string. If PATTERN is also omitted,
4633a7c4
LW
3137splits on whitespace (after skipping any leading whitespace). Anything
3138matching PATTERN is taken to be a delimiter separating the fields. (Note
fb73857a
PP
3139that the delimiter may be longer than one character.)
3140
3141If LIMIT is specified and is not negative, splits into no more than
3142that many fields (though it may split into fewer). If LIMIT is
3143unspecified, trailing null fields are stripped (which potential users
3144of pop() would do well to remember). If LIMIT is negative, it is
3145treated as if an arbitrarily large LIMIT had been specified.
a0d0e21e
LW
3146
3147A pattern matching the null string (not to be confused with
748a9306 3148a null pattern C<//>, which is just one member of the set of patterns
a0d0e21e
LW
3149matching a null string) will split the value of EXPR into separate
3150characters at each point it matches that way. For example:
3151
3152 print join(':', split(/ */, 'hi there'));
3153
3154produces the output 'h:i:t:h:e:r:e'.
3155
5f05dabc 3156The LIMIT parameter can be used to split a line partially
a0d0e21e
LW
3157
3158 ($login, $passwd, $remainder) = split(/:/, $_, 3);
3159
3160When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT
3161one larger than the number of variables in the list, to avoid
3162unnecessary work. For the list above LIMIT would have been 4 by
3163default. In time critical applications it behooves you not to split
3164into more fields than you really need.
3165
3166If the PATTERN contains parentheses, additional array elements are
3167created from each matching substring in the delimiter.
3168
da0045b7 3169 split(/([,-])/, "1-10,20", 3);
a0d0e21e
LW
3170
3171produces the list value
3172
3173 (1, '-', 10, ',', 20)
3174
54310121 3175If you had the entire header of a normal Unix email message in $header,
4633a7c4
LW
3176you could split it up into fields and their values this way:
3177
3178 $header =~ s/\n\s+/ /g; # fix continuation lines
fb73857a 3179 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
4633a7c4 3180
a0d0e21e
LW
3181The pattern C</PATTERN/> may be replaced with an expression to specify
3182patterns that vary at runtime. (To do runtime compilation only once,
748a9306
LW
3183use C</$variable/o>.)
3184
3185As a special case, specifying a PATTERN of space (C<' '>) will split on
3186white space just as split with no arguments does. Thus, split(' ') can
3187be used to emulate B<awk>'s default behavior, whereas C<split(/ /)>
3188will give you as many null initial fields as there are leading spaces.
3189A split on /\s+/ is like a split(' ') except that any leading
3190whitespace produces a null first field. A split with no arguments
3191really does a C<split(' ', $_)> internally.
a0d0e21e
LW
3192
3193Example:
3194
3195 open(passwd, '/etc/passwd');
3196 while (<passwd>) {
54310121 3197 ($login, $passwd, $uid, $gid, $gcos,
748a9306 3198 $home, $shell) = split(/:/);
a0d0e21e
LW
3199 ...
3200 }
3201
54310121 3202(Note that $shell above will still have a newline on it. See L</chop>,
a0d0e21e
LW
3203L</chomp>, and L</join>.)
3204
5f05dabc 3205=item sprintf FORMAT, LIST
a0d0e21e 3206
74a77017
CS
3207Returns a string formatted by the usual printf conventions of the
3208C library function sprintf(). See L<sprintf(3)> or L<printf(3)>
3209on your system for an explanation of the general principles.
3210
3211Perl does all of its own sprintf() formatting -- it emulates the C
3212function sprintf(), but it doesn't use it (except for floating-point
3213numbers, and even then only the standard modifiers are allowed). As a
3214result, any non-standard extensions in your local sprintf() are not
3215available from Perl.
3216
3217Perl's sprintf() permits the following universally-known conversions:
3218
3219 %% a percent sign
3220 %c a character with the given number
3221 %s a string
3222 %d a signed integer, in decimal
3223 %u an unsigned integer, in decimal
3224 %o an unsigned integer, in octal
3225 %x an unsigned integer, in hexadecimal
3226 %e a floating-point number, in scientific notation
3227 %f a floating-point number, in fixed decimal notation
3228 %g a floating-point number, in %e or %f notation
3229
1b3f7d21 3230In addition, Perl permits the following widely-supported conversions:
74a77017 3231
74a77017
CS
3232 %X like %x, but using upper-case letters
3233 %E like %e, but using an upper-case "E"
3234 %G like %g, but with an upper-case "E" (if applicable)
3235 %p a pointer (outputs the Perl value's address in hexadecimal)
1b3f7d21
CS
3236 %n special: *stores* the number of characters output so far
3237 into the next variable in the parameter list
74a77017 3238
1b3f7d21
CS
3239Finally, for backward (and we do mean "backward") compatibility, Perl
3240permits these unnecessary but widely-supported conversions:
74a77017 3241
1b3f7d21 3242 %i a synonym for %d
74a77017
CS
3243 %D a synonym for %ld
3244 %U a synonym for %lu
3245 %O a synonym for %lo
3246 %F a synonym for %f
3247
3248Perl permits the following universally-known flags between the C<%>
3249and the conversion letter:
3250
3251 space prefix positive number with a space
3252 + prefix positive number with a plus sign
3253 - left-justify within the field
3254 0 use zeros, not spaces, to right-justify
1b3f7d21 3255 # prefix octal with "0", hex with "0x"
74a77017
CS
3256 number minimum field width
3257 .number "precision": digits after decimal point for floating-point,
3258 max length for string, minimum length for integer
3259 l interpret integer as C type "long" or "unsigned long"
74a77017
CS
3260 h interpret integer as C type "short" or "unsigned short"
3261
1b3f7d21 3262There is also one Perl-specific flag:
74a77017
CS
3263
3264 V interpret integer as Perl's standard integer type
3265
3266Where a number would appear in the flags, an asterisk ("*") may be
3267used instead, in which case Perl uses the next item in the parameter
3268list as the given number (that is, as the field width or precision).
3269If a field width obtained through "*" is negative, it has the same
3270effect as the '-' flag: left-justification.
3271
3272If C<use locale> is in effect, the character used for the decimal
3273point in formatted real numbers is affected by the LC_NUMERIC locale.
3274See L<perllocale>.
a0d0e21e
LW
3275
3276=item sqrt EXPR
3277
54310121 3278=item sqrt
bbce6d69 3279
a0d0e21e
LW
3280Return the square root of EXPR. If EXPR is omitted, returns square
3281root of $_.
3282
3283=item srand EXPR
3284
93dc8474
CS
3285=item srand
3286
3287Sets the random number seed for the C<rand> operator. If EXPR is
3288omitted, uses a semi-random value based on the current time and process
3289ID, among other things. In versions of Perl prior to 5.004 the default
3290seed was just the current time(). This isn't a particularly good seed,
3291so many old programs supply their own seed value (often C<time ^ $$> or
3292C<time ^ ($$ + ($$ << 15))>), but that isn't necessary any more.
3293
3294In fact, it's usually not necessary to call srand() at all, because if
3295it is not called explicitly, it is called implicitly at the first use of
2f9daede
TPG
3296the C<rand> operator. However, this was not the case in version of Perl
3297before 5.004, so if your script will run under older Perl versions, it
3298should call srand().
93dc8474 3299
2f9daede
TPG
3300Note that you need something much more random than the default seed for
3301cryptographic purposes. Checksumming the compressed output of one or more
3302rapidly changing operating system status programs is the usual method. For
3303example:
28757baa
PP
3304
3305 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
3306
0078ec44
RS
3307If you're particularly concerned with this, see the Math::TrulyRandom
3308module in CPAN.
3309
3310Do I<not> call srand() multiple times in your program unless you know
28757baa
PP
3311exactly what you're doing and why you're doing it. The point of the
3312function is to "seed" the rand() function so that rand() can produce
3313a different sequence each time you run your program. Just do it once at the
3314top of your program, or you I<won't> get random numbers out of rand()!
3315
54310121 3316Frequently called programs (like CGI scripts) that simply use
28757baa
PP
3317
3318 time ^ $$
3319
54310121 3320for a seed can fall prey to the mathematical property that
28757baa
PP
3321
3322 a^b == (a+1)^(b+1)
3323
0078ec44 3324one-third of the time. So don't do that.
f86702cc 3325
a0d0e21e
LW
3326=item stat FILEHANDLE
3327
3328=item stat EXPR
3329
54310121 3330=item stat
bbce6d69 3331
a0d0e21e 3332Returns a 13-element array giving the status info for a file, either the
2f9daede 3333file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted, it
bbce6d69
PP
3334stats $_. Returns a null list if the stat fails. Typically used as
3335follows:
3336
a0d0e21e
LW
3337
3338 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
3339 $atime,$mtime,$ctime,$blksize,$blocks)
3340 = stat($filename);
3341
54310121 3342Not all fields are supported on all filesystem types. Here are the
c07a80fd
PP
3343meaning of the fields:
3344
54310121
PP
3345 0 dev device number of filesystem
3346 1 ino inode number
3347 2 mode file mode (type and permissions)
3348 3 nlink number of (hard) links to the file
3349 4 uid numeric user ID of file's owner
3350 5 gid numeric group ID of file's owner
3351 6 rdev the device identifier (special files only)
3352 7 size total size of file, in bytes
3353 8 atime last access time since the epoch
3354 9 mtime last modify time since the epoch
3355 10 ctime inode change time (NOT creation time!) since the epoch
3356 11 blksize preferred block size for file system I/O
3357 12 blocks actual number of blocks allocated
c07a80fd
PP
3358
3359(The epoch was at 00:00 January 1, 1970 GMT.)
3360
a0d0e21e
LW
3361If stat is passed the special filehandle consisting of an underline, no
3362stat is done, but the current contents of the stat structure from the
3363last stat or filetest are returned. Example:
3364
3365 if (-x $file && (($d) = stat(_)) && $d < 0) {
3366 print "$file is executable NFS file\n";
3367 }
3368
5f05dabc 3369(This works on machines only for which the device number is negative under NFS.)
a0d0e21e
LW
3370
3371=item study SCALAR
3372
3373=item study
3374
184e9718 3375Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
a0d0e21e
LW
3376doing many pattern matches on the string before it is next modified.
3377This may or may not save time, depending on the nature and number of
3378patterns you are searching on, and on the distribution of character
54310121 3379frequencies in the string to be searched -- you probably want to compare
5f05dabc 3380run times with and without it to see which runs faster. Those loops
a0d0e21e
LW
3381which scan for many short constant strings (including the constant
3382parts of more complex patterns) will benefit most. You may have only
54310121 3383one study active at a time -- if you study a different scalar the first
a0d0e21e
LW
3384is "unstudied". (The way study works is this: a linked list of every
3385character in the string to be searched is made, so we know, for
3386example, where all the 'k' characters are. From each search string,
3387the rarest character is selected, based on some static frequency tables
3388constructed from some C programs and English text. Only those places
3389that contain this "rarest" character are examined.)
3390
3391For example, here is a loop which inserts index producing entries
3392before any line containing a certain pattern:
3393
3394 while (<>) {
3395 study;
3396 print ".IX foo\n" if /\bfoo\b/;
3397 print ".IX bar\n" if /\bbar\b/;
3398 print ".IX blurfl\n" if /\bblurfl\b/;
3399 ...
3400 print;
3401 }
3402
3403In searching for /\bfoo\b/, only those locations in $_ that contain "f"
3404will be looked at, because "f" is rarer than "o". In general, this is
3405a big win except in pathological cases. The only question is whether
3406it saves you more time than it took to build the linked list in the
3407first place.
3408
3409Note that if you have to look for strings that you don't know till
3410runtime, you can build an entire loop as a string and eval that to
3411avoid recompiling all your patterns all the time. Together with
3412undefining $/ to input entire files as one record, this can be very
3413fast, often faster than specialized programs like fgrep(1). The following
184e9718 3414scans a list of files (C<@files>) for a list of words (C<@words>), and prints
a0d0e21e
LW
3415out the names of those files that contain a match:
3416
3417 $search = 'while (<>) { study;';
3418 foreach $word (@words) {
3419 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
3420 }
3421 $search .= "}";
3422 @ARGV = @files;
3423 undef $/;
3424 eval $search; # this screams
5f05dabc 3425 $/ = "\n"; # put back to normal input delimiter
a0d0e21e
LW
3426 foreach $file (sort keys(%seen)) {
3427 print $file, "\n";
3428 }
3429
cb1a09d0
AD
3430=item sub BLOCK
3431
3432=item sub NAME
3433
3434=item sub NAME BLOCK
3435
3436This is subroutine definition, not a real function I<per se>. With just a
3437NAME (and possibly prototypes), it's just a forward declaration. Without
3438a NAME, it's an anonymous function declaration, and does actually return a
2f9daede 3439value: the CODE ref of the closure you just created. See L<perlsub> and
cb1a09d0
AD
3440L<perlref> for details.
3441
a0d0e21e
LW
3442=item substr EXPR,OFFSET,LEN
3443
3444=item substr EXPR,OFFSET
3445
3446Extracts a substring out of EXPR and returns it. First character is at
2f9daede 3447offset 0, or whatever you've set C<$[> to (but don't do that).
84902520 3448If OFFSET is negative (or more precisely, less than C<$[>), starts
a0d0e21e 3449that far from the end of the string. If LEN is omitted, returns
748a9306
LW
3450everything to the end of the string. If LEN is negative, leaves that
3451many characters off the end of the string.
3452
84902520
TB
3453If you specify a substring which is partly outside the string, the part
3454within the string is returned. If the substring is totally outside
3455the string a warning is produced.
3456
748a9306 3457You can use the substr() function
a0d0e21e
LW
3458as an lvalue, in which case EXPR must be an lvalue. If you assign
3459something shorter than LEN, the string will shrink, and if you assign
3460something longer than LEN, the string will grow to accommodate it. To
3461keep the string the same length you may need to pad or chop your value
3462using sprintf().
3463
3464=item symlink OLDFILE,NEWFILE
3465
3466Creates a new filename symbolically linked to the old filename.
3467Returns 1 for success, 0 otherwise. On systems that don't support
3468symbolic links, produces a fatal error at run time. To check for that,
3469use eval:
3470
54310121 3471 $symlink_exists = (eval {symlink("","")};, $@ eq '');
a0d0e21e
LW
3472
3473=item syscall LIST
3474
3475Calls the system call specified as the first element of the list,
3476passing the remaining elements as arguments to the system call. If
3477unimplemented, produces a fatal error. The arguments are interpreted
3478as follows: if a given argument is numeric, the argument is passed as
3479an int. If not, the pointer to the string value is passed. You are
3480responsible to make sure a string is pre-extended long enough to
3481receive any result that might be written into a string. If your
3482integer arguments are not literals and have never been interpreted in a
3483numeric context, you may need to add 0 to them to force them to look
3484like numbers.
3485
3486 require 'syscall.ph'; # may need to run h2ph
3487 syscall(&SYS_write, fileno(STDOUT), "hi there\n", 9);
3488
5f05dabc 3489Note that Perl supports passing of up to only 14 arguments to your system call,
a0d0e21e
LW
3490which in practice should usually suffice.
3491
fb73857a
PP
3492Syscall returns whatever value returned by the system call it calls.
3493If the system call fails, syscall returns -1 and sets C<$!> (errno).
3494Note that some system calls can legitimately return -1. The proper
3495way to handle such calls is to assign C<$!=0;> before the call and
3496check the value of <$!> if syscall returns -1.
3497
3498There's a problem with C<syscall(&SYS_pipe)>: it returns the file
3499number of the read end of the pipe it creates. There is no way
3500to retrieve the file number of the other end. You can avoid this
3501problem by using C<pipe> instead.
3502
c07a80fd
PP
3503=item sysopen FILEHANDLE,FILENAME,MODE
3504
3505=item sysopen FILEHANDLE,FILENAME,MODE,PERMS
3506
3507Opens the file whose filename is given by FILENAME, and associates it
3508with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
3509the name of the real filehandle wanted. This function calls the
3510underlying operating system's C<open> function with the parameters
3511FILENAME, MODE, PERMS.
3512
3513The possible values and flag bits of the MODE parameter are
3514system-dependent; they are available via the standard module C<Fcntl>.
3515However, for historical reasons, some values are universal: zero means
3516read-only, one means write-only, and two means read/write.
3517
3518If the file named by FILENAME does not exist and the C<open> call
3519creates it (typically because MODE includes the O_CREAT flag), then
3520the value of PERMS specifies the permissions of the newly created
3521file. If PERMS is omitted, the default value is 0666, which allows
3522read and write for all. This default is reasonable: see C<umask>.
3523
28757baa
PP
3524The IO::File module provides a more object-oriented approach, if you're
3525into that kind of thing.
3526
a0d0e21e
LW
3527=item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
3528
3529=item sysread FILEHANDLE,SCALAR,LENGTH
3530
3531Attempts to read LENGTH bytes of data into variable SCALAR from the
fb73857a
PP
3532specified FILEHANDLE, using the system call read(2). It bypasses
3533stdio, so mixing this with other kinds of reads, print(), write(),
3534seek(), or tell() can cause confusion because stdio usually buffers
3535data. Returns the number of bytes actually read, or undef if there
3536was an error. SCALAR will be grown or shrunk so that the last byte
3537actually read is the last byte of the scalar after the read.
ff68c719
PP
3538
3539An OFFSET may be specified to place the read data at some place in the
3540string other than the beginning. A negative OFFSET specifies
3541placement at that many bytes counting backwards from the end of the
3542string. A positive OFFSET greater than the length of SCALAR results
3543in the string being padded to the required size with "\0" bytes before
3544the result of the read is appended.
a0d0e21e 3545
137443ea
PP
3546=item sysseek FILEHANDLE,POSITION,WHENCE
3547
8903cb82
PP
3548Sets FILEHANDLE's system position using the system call lseek(2). It
3549bypasses stdio, so mixing this with reads (other than sysread()),
3550print(), write(), seek(), or tell() may cause confusion. FILEHANDLE may
3551be an expression whose value gives the name of the filehandle. The
3552values for WHENCE are 0 to set the new position to POSITION, 1 to set
3553the it to the current position plus POSITION, and 2 to set it to EOF
3554plus POSITION (typically negative). For WHENCE, you may use the
3555constants SEEK_SET, SEEK_CUR, and SEEK_END from either the IO::Seekable
3556or the POSIX module.
3557
3558Returns the new position, or the undefined value on failure. A position
3559of zero is returned as the string "0 but true"; thus sysseek() returns
3560TRUE on success and FALSE on failure, yet you can still easily determine
3561the new position.
137443ea 3562
a0d0e21e
LW
3563=item system LIST
3564
3565Does exactly the same thing as "exec LIST" except that a fork is done
3566first, and the parent process waits for the child process to complete.
3567Note that argument processing varies depending on the number of
3568arguments. The return value is the exit status of the program as
3569returned by the wait() call. To get the actual exit value divide by
54310121
PP
3570256. See also L</exec>. This is I<NOT> what you want to use to capture
3571the output from a command, for that you should use merely backticks or
28757baa 3572qx//, as described in L<perlop/"`STRING`">.
a0d0e21e 3573
54310121 3574Because system() and backticks block SIGINT and SIGQUIT, killing the
28757baa
PP
3575program they're running doesn't actually interrupt your program.
3576
3577 @args = ("command", "arg1", "arg2");
54310121
PP
3578 system(@args) == 0
3579 or die "system @args failed: $?"
28757baa
PP
3580
3581Here's a more elaborate example of analysing the return value from
54310121
PP
3582system() on a Unix system to check for all possibilities, including for
3583signals and core dumps.
28757baa
PP
3584
3585 $rc = 0xffff & system @args;
3586 printf "system(%s) returned %#04x: ", "@args", $rc;
3587 if ($rc == 0) {
3588 print "ran with normal exit\n";
54310121 3589 }
28757baa
PP
3590 elsif ($rc == 0xff00) {
3591 print "command failed: $!\n";
54310121 3592 }
28757baa
PP
3593 elsif ($rc > 0x80) {
3594 $rc >>= 8;
3595 print "ran with non-zero exit status $rc\n";
54310121 3596 }
28757baa
PP
3597 else {
3598 print "ran with ";
3599 if ($rc & 0x80) {
3600 $rc &= ~0x80;
54310121
PP
3601 print "core dump from ";
3602 }
28757baa 3603 print "signal $rc\n"
54310121 3604 }
28757baa 3605 $ok = ($rc != 0);
f86702cc 3606
bb32b41a
GS
3607When the arguments get executed via the system shell, results will
3608be subject to its quirks and capabilities. See L<perlop/"`STRING`">
3609for details.
3610
a0d0e21e
LW
3611=item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
3612
3613=item syswrite FILEHANDLE,SCALAR,LENGTH
3614
3615Attempts to write LENGTH bytes of data from variable SCALAR to the
3616specified FILEHANDLE, using the system call write(2). It bypasses
96e4d5b1 3617stdio, so mixing this with reads (other than sysread()), print(),
fb73857a
PP
3618write(), seek(), or tell() may cause confusion because stdio usually
3619buffers data. Returns the number of bytes actually written, or undef