This is a live mirror of the Perl 5 development currently hosted at https://github.com/perl/perl5
perlfunc.pod: ioctl.ph
[perl5.git] / pod / perlfunc.pod
CommitLineData
a0d0e21e
LW
1=head1 NAME
2
3perlfunc - Perl builtin functions
4
5=head1 DESCRIPTION
6
7The functions in this section can serve as terms in an expression.
8They fall into two major categories: list operators and named unary
9operators. These differ in their precedence relationship with a
10following comma. (See the precedence table in L<perlop>.) List
11operators take more than one argument, while unary operators can never
12take more than one argument. Thus, a comma terminates the argument of
13a unary operator, but merely separates the arguments of a list
14operator. A unary operator generally provides a scalar context to its
2b5ab1e7 15argument, while a list operator may provide either scalar or list
a0d0e21e 16contexts for its arguments. If it does both, the scalar arguments will
5f05dabc 17be first, and the list argument will follow. (Note that there can ever
0f31cffe 18be only one such list argument.) For instance, splice() has three scalar
2b5ab1e7
TC
19arguments followed by a list, whereas gethostbyname() has four scalar
20arguments.
a0d0e21e
LW
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
19799a22 33surprising) rule is this: It I<looks> like a function, therefore it I<is> a
a0d0e21e
LW
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
68dc0745
PP
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.
a0d0e21e
LW
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
2b5ab1e7
TC
51A few functions take no arguments at all, and therefore work as neither
52unary nor list operators. These include such functions as C<time>
53and C<endpwent>. For example, C<time+86_400> always means
54C<time() + 86_400>.
55
a0d0e21e 56For functions that can be used in either a scalar or list context,
54310121 57nonabortive failure is generally indicated in a scalar context by
a0d0e21e
LW
58returning the undefined value, and in a list context by returning the
59null list.
60
5a964f20
TC
61Remember the following important rule: There is B<no rule> that relates
62the behavior of an expression in list context to its behavior in scalar
63context, or vice versa. It might do two totally different things.
a0d0e21e 64Each operator and function decides which sort of value it would be most
2b5ab1e7 65appropriate to return in scalar context. Some operators return the
5a964f20 66length of the list that would have been returned in list context. Some
a0d0e21e
LW
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
d1be9408 72A named array in scalar context is quite different from what would at
5a964f20
TC
73first glance appear to be a list in scalar context. You can't get a list
74like C<(1,2,3)> into being in scalar context, because the compiler knows
75the context at compile time. It would generate the scalar comma operator
76there, not the list construction version of the comma. That means it
77was never a list to start with.
78
79In general, functions in Perl that serve as wrappers for system calls
f86cebdf 80of the same name (like chown(2), fork(2), closedir(2), etc.) all return
5a964f20
TC
81true when they succeed and C<undef> otherwise, as is usually mentioned
82in the descriptions below. This is different from the C interfaces,
19799a22
GS
83which return C<-1> on failure. Exceptions to this rule are C<wait>,
84C<waitpid>, and C<syscall>. System calls also set the special C<$!>
5a964f20
TC
85variable on failure. Other functions do not, except accidentally.
86
cb1a09d0
AD
87=head2 Perl Functions by Category
88
89Here are Perl's functions (including things that look like
5a964f20 90functions, like some keywords and named operators)
cb1a09d0
AD
91arranged by category. Some functions appear in more
92than one place.
93
13a2d996 94=over 4
cb1a09d0
AD
95
96=item Functions for SCALARs or strings
97
22fae026 98C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>,
945c54fd
JH
99C<length>, C<oct>, C<ord>, C<pack>, C<q/STRING/>, C<qq/STRING/>, C<reverse>,
100C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///>
cb1a09d0
AD
101
102=item Regular expressions and pattern matching
103
ab4f32c2 104C<m//>, C<pos>, C<quotemeta>, C<s///>, C<split>, C<study>, C<qr//>
cb1a09d0
AD
105
106=item Numeric functions
107
22fae026
TM
108C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>,
109C<sin>, C<sqrt>, C<srand>
cb1a09d0
AD
110
111=item Functions for real @ARRAYs
112
22fae026 113C<pop>, C<push>, C<shift>, C<splice>, C<unshift>
cb1a09d0
AD
114
115=item Functions for list data
116
ab4f32c2 117C<grep>, C<join>, C<map>, C<qw/STRING/>, C<reverse>, C<sort>, C<unpack>
cb1a09d0
AD
118
119=item Functions for real %HASHes
120
22fae026 121C<delete>, C<each>, C<exists>, C<keys>, C<values>
cb1a09d0
AD
122
123=item Input and output functions
124
22fae026
TM
125C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
126C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
127C<readdir>, C<rewinddir>, C<seek>, C<seekdir>, C<select>, C<syscall>,
128C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>,
129C<warn>, C<write>
cb1a09d0
AD
130
131=item Functions for fixed length data or records
132
22fae026 133C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec>
cb1a09d0
AD
134
135=item Functions for filehandles, files, or directories
136
22fae026 137C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>,
5ff3f7a4 138C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>,
1e278fd9
JH
139C<readlink>, C<rename>, C<rmdir>, C<stat>, C<symlink>, C<sysopen>,
140C<umask>, C<unlink>, C<utime>
cb1a09d0
AD
141
142=item Keywords related to the control flow of your perl program
143
98293880
JH
144C<caller>, C<continue>, C<die>, C<do>, C<dump>, C<eval>, C<exit>,
145C<goto>, C<last>, C<next>, C<redo>, C<return>, C<sub>, C<wantarray>
cb1a09d0 146
54310121 147=item Keywords related to scoping
cb1a09d0 148
4375e838 149C<caller>, C<import>, C<local>, C<my>, C<our>, C<package>, C<use>
cb1a09d0
AD
150
151=item Miscellaneous functions
152
4375e838 153C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<our>, C<reset>,
22fae026 154C<scalar>, C<undef>, C<wantarray>
cb1a09d0
AD
155
156=item Functions for processes and process groups
157
22fae026 158C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
ab4f32c2 159C<pipe>, C<qx/STRING/>, C<setpgrp>, C<setpriority>, C<sleep>, C<system>,
22fae026 160C<times>, C<wait>, C<waitpid>
cb1a09d0
AD
161
162=item Keywords related to perl modules
163
22fae026 164C<do>, C<import>, C<no>, C<package>, C<require>, C<use>
cb1a09d0
AD
165
166=item Keywords related to classes and object-orientedness
167
22fae026
TM
168C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
169C<untie>, C<use>
cb1a09d0
AD
170
171=item Low-level socket functions
172
22fae026
TM
173C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
174C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
737dd4b4 175C<socket>, C<socketpair>
cb1a09d0
AD
176
177=item System V interprocess communication functions
178
22fae026
TM
179C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
180C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>
cb1a09d0
AD
181
182=item Fetching user and group info
183
22fae026
TM
184C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
185C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
186C<getpwuid>, C<setgrent>, C<setpwent>
cb1a09d0
AD
187
188=item Fetching network info
189
22fae026
TM
190C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
191C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
192C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
193C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
194C<setnetent>, C<setprotoent>, C<setservent>
cb1a09d0
AD
195
196=item Time-related functions
197
22fae026 198C<gmtime>, C<localtime>, C<time>, C<times>
cb1a09d0 199
37798a01
PP
200=item Functions new in perl5
201
22fae026 202C<abs>, C<bless>, C<chomp>, C<chr>, C<exists>, C<formline>, C<glob>,
b76cc8ba 203C<import>, C<lc>, C<lcfirst>, C<map>, C<my>, C<no>, C<our>, C<prototype>,
4375e838 204C<qx>, C<qw>, C<readline>, C<readpipe>, C<ref>, C<sub*>, C<sysopen>, C<tie>,
22fae026 205C<tied>, C<uc>, C<ucfirst>, C<untie>, C<use>
37798a01
PP
206
207* - C<sub> was a keyword in perl4, but in perl5 it is an
5a964f20 208operator, which can be used in expressions.
37798a01
PP
209
210=item Functions obsoleted in perl5
211
22fae026 212C<dbmclose>, C<dbmopen>
37798a01 213
cb1a09d0
AD
214=back
215
60f9f73c
JH
216=head2 Portability
217
2b5ab1e7
TC
218Perl was born in Unix and can therefore access all common Unix
219system calls. In non-Unix environments, the functionality of some
220Unix system calls may not be available, or details of the available
221functionality may differ slightly. The Perl functions affected
60f9f73c
JH
222by this are:
223
224C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>,
225C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>,
226C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>,
ef5a6dd7
JH
227C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostbyname>,
228C<gethostent>, C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
60f9f73c
JH
229C<getppid>, C<getprgp>, C<getpriority>, C<getprotobynumber>,
230C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>,
231C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>,
232C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>,
2b5ab1e7 233C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<rename>, C<select>, C<semctl>,
60f9f73c
JH
234C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>,
235C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>,
236C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>,
737dd4b4 237C<shmwrite>, C<socket>, C<socketpair>,
80cbd5ad
JH
238C<stat>, C<symlink>, C<syscall>, C<sysopen>, C<system>,
239C<times>, C<truncate>, C<umask>, C<unlink>,
2b5ab1e7 240C<utime>, C<wait>, C<waitpid>
60f9f73c
JH
241
242For more information about the portability of these functions, see
243L<perlport> and other available platform-specific documentation.
244
cb1a09d0
AD
245=head2 Alphabetical Listing of Perl Functions
246
a0d0e21e
LW
247=over 8
248
5b3c99c0 249=item -X FILEHANDLE
a0d0e21e 250
5b3c99c0 251=item -X EXPR
a0d0e21e 252
5b3c99c0 253=item -X
a0d0e21e
LW
254
255A file test, where X is one of the letters listed below. This unary
256operator takes one argument, either a filename or a filehandle, and
257tests the associated file to see if something is true about it. If the
7660c0ab 258argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
19799a22 259Unless otherwise documented, it returns C<1> for true and C<''> for false, or
a0d0e21e
LW
260the undefined value if the file doesn't exist. Despite the funny
261names, precedence is the same as any other named unary operator, and
262the argument may be parenthesized like any other unary operator. The
263operator may be any of:
7e778d91
IZ
264X<-r>X<-w>X<-x>X<-o>X<-R>X<-W>X<-X>X<-O>X<-e>X<-z>X<-s>X<-f>X<-d>X<-l>X<-p>
265X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
a0d0e21e
LW
266
267 -r File is readable by effective uid/gid.
268 -w File is writable by effective uid/gid.
269 -x File is executable by effective uid/gid.
270 -o File is owned by effective uid.
271
272 -R File is readable by real uid/gid.
273 -W File is writable by real uid/gid.
274 -X File is executable by real uid/gid.
275 -O File is owned by real uid.
276
277 -e File exists.
8e7e0aa8
MJD
278 -z File has zero size (is empty).
279 -s File has nonzero size (returns size in bytes).
a0d0e21e
LW
280
281 -f File is a plain file.
282 -d File is a directory.
283 -l File is a symbolic link.
9c4d0f16 284 -p File is a named pipe (FIFO), or Filehandle is a pipe.
a0d0e21e
LW
285 -S File is a socket.
286 -b File is a block special file.
287 -c File is a character special file.
288 -t Filehandle is opened to a tty.
289
290 -u File has setuid bit set.
291 -g File has setgid bit set.
292 -k File has sticky bit set.
293
121910a4 294 -T File is an ASCII text file (heuristic guess).
2cdbc966 295 -B File is a "binary" file (opposite of -T).
a0d0e21e 296
95a3fe12 297 -M Script start time minus file modification time, in days.
a0d0e21e 298 -A Same for access time.
95a3fe12 299 -C Same for inode change time (Unix, may differ for other platforms)
a0d0e21e 300
a0d0e21e
LW
301Example:
302
303 while (<>) {
5b3eff12 304 chomp;
a0d0e21e 305 next unless -f $_; # ignore specials
5a964f20 306 #...
a0d0e21e
LW
307 }
308
5ff3f7a4
GS
309The interpretation of the file permission operators C<-r>, C<-R>,
310C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
311of the file and the uids and gids of the user. There may be other
312reasons you can't actually read, write, or execute the file. Such
313reasons may be for example network filesystem access controls, ACLs
314(access control lists), read-only filesystems, and unrecognized
315executable formats.
316
2b5ab1e7
TC
317Also note that, for the superuser on the local filesystems, the C<-r>,
318C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
5ff3f7a4
GS
319if any execute bit is set in the mode. Scripts run by the superuser
320may thus need to do a stat() to determine the actual mode of the file,
2b5ab1e7 321or temporarily set their effective uid to something else.
5ff3f7a4
GS
322
323If you are using ACLs, there is a pragma called C<filetest> that may
324produce more accurate results than the bare stat() mode bits.
5ff3f7a4
GS
325When under the C<use filetest 'access'> the above-mentioned filetests
326will test whether the permission can (not) be granted using the
468541a8 327access() family of system calls. Also note that the C<-x> and C<-X> may
5ff3f7a4
GS
328under this pragma return true even if there are no execute permission
329bits set (nor any extra execute permission ACLs). This strangeness is
330due to the underlying system calls' definitions. Read the
331documentation for the C<filetest> pragma for more information.
332
a0d0e21e
LW
333Note that C<-s/a/b/> does not do a negated substitution. Saying
334C<-exp($foo)> still works as expected, however--only single letters
335following a minus are interpreted as file tests.
336
337The C<-T> and C<-B> switches work as follows. The first block or so of the
338file is examined for odd characters such as strange control codes or
61eff3bc 339characters with the high bit set. If too many strange characters (>30%)
a0d0e21e
LW
340are found, it's a C<-B> file, otherwise it's a C<-T> file. Also, any file
341containing null in the first block is considered a binary file. If C<-T>
9124316e 342or C<-B> is used on a filehandle, the current IO buffer is examined
19799a22 343rather than the first block. Both C<-T> and C<-B> return true on a null
54310121 344file, or a file at EOF when testing a filehandle. Because you have to
4633a7c4
LW
345read a file to do the C<-T> test, on most occasions you want to use a C<-f>
346against the file first, as in C<next unless -f $file && -T $file>.
a0d0e21e 347
19799a22 348If any of the file tests (or either the C<stat> or C<lstat> operators) are given
28757baa 349the special filehandle consisting of a solitary underline, then the stat
a0d0e21e
LW
350structure of the previous file test (or stat operator) is used, saving
351a system call. (This doesn't work with C<-t>, and you need to remember
352that lstat() and C<-l> will leave values in the stat structure for the
5c9aa243
RGS
353symbolic link, not the real file.) (Also, if the stat buffer was filled by
354a C<lstat> call, C<-T> and C<-B> will reset it with the results of C<stat _>).
355Example:
a0d0e21e
LW
356
357 print "Can do.\n" if -r $a || -w _ || -x _;
358
359 stat($filename);
360 print "Readable\n" if -r _;
361 print "Writable\n" if -w _;
362 print "Executable\n" if -x _;
363 print "Setuid\n" if -u _;
364 print "Setgid\n" if -g _;
365 print "Sticky\n" if -k _;
366 print "Text\n" if -T _;
367 print "Binary\n" if -B _;
368
fbb0b3b3
RGS
369As of Perl 5.9.1, as a form of purely syntactic sugar, you can stack file
370test operators, in a way that C<-f -w -x $file> is equivalent to
371C<-x $file && -w _ && -f _>. (This is only syntax fancy : if you use
372the return value of C<-f $file> as an argument to another filetest
373operator, no special magic will happen.)
374
a0d0e21e
LW
375=item abs VALUE
376
54310121 377=item abs
bbce6d69 378
a0d0e21e 379Returns the absolute value of its argument.
7660c0ab 380If VALUE is omitted, uses C<$_>.
a0d0e21e
LW
381
382=item accept NEWSOCKET,GENERICSOCKET
383
f86cebdf 384Accepts an incoming socket connect, just as the accept(2) system call
19799a22 385does. Returns the packed address if it succeeded, false otherwise.
2b5ab1e7 386See the example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 387
8d2a6795
GS
388On systems that support a close-on-exec flag on files, the flag will
389be set for the newly opened file descriptor, as determined by the
390value of $^F. See L<perlvar/$^F>.
391
a0d0e21e
LW
392=item alarm SECONDS
393
54310121 394=item alarm
bbce6d69 395
a0d0e21e 396Arranges to have a SIGALRM delivered to this process after the
d400eac8
JH
397specified number of wallclock seconds have elapsed. If SECONDS is not
398specified, the value stored in C<$_> is used. (On some machines,
399unfortunately, the elapsed time may be up to one second less or more
400than you specified because of how seconds are counted, and process
401scheduling may delay the delivery of the signal even further.)
402
403Only one timer may be counting at once. Each call disables the
404previous timer, and an argument of C<0> may be supplied to cancel the
405previous timer without starting a new one. The returned value is the
406amount of time remaining on the previous timer.
a0d0e21e 407
4633a7c4 408For delays of finer granularity than one second, you may use Perl's
19799a22
GS
409four-argument version of select() leaving the first three arguments
410undefined, or you might be able to use the C<syscall> interface to
83df6a1d
JH
411access setitimer(2) if your system supports it. The Time::HiRes
412module (from CPAN, and starting from Perl 5.8 part of the standard
413distribution) may also prove useful.
2b5ab1e7 414
68f8bed4
JH
415It is usually a mistake to intermix C<alarm> and C<sleep> calls.
416(C<sleep> may be internally implemented in your system with C<alarm>)
a0d0e21e 417
19799a22
GS
418If you want to use C<alarm> to time out a system call you need to use an
419C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
f86cebdf 420fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
19799a22 421restart system calls on some systems. Using C<eval>/C<die> always works,
5a964f20 422modulo the caveats given in L<perlipc/"Signals">.
ff68c719
PP
423
424 eval {
f86cebdf 425 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
36477c24 426 alarm $timeout;
ff68c719 427 $nread = sysread SOCKET, $buffer, $size;
36477c24 428 alarm 0;
ff68c719 429 };
ff68c719 430 if ($@) {
f86cebdf 431 die unless $@ eq "alarm\n"; # propagate unexpected errors
ff68c719
PP
432 # timed out
433 }
434 else {
435 # didn't
436 }
437
91d81acc
JH
438For more information see L<perlipc>.
439
a0d0e21e
LW
440=item atan2 Y,X
441
442Returns the arctangent of Y/X in the range -PI to PI.
443
ca6e1c26 444For the tangent operation, you may use the C<Math::Trig::tan>
28757baa
PP
445function, or use the familiar relation:
446
447 sub tan { sin($_[0]) / cos($_[0]) }
448
a0d0e21e
LW
449=item bind SOCKET,NAME
450
451Binds a network address to a socket, just as the bind system call
19799a22 452does. Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
453packed address of the appropriate type for the socket. See the examples in
454L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 455
fae2c0fb 456=item binmode FILEHANDLE, LAYER
1c1fc3ea 457
a0d0e21e
LW
458=item binmode FILEHANDLE
459
1cbfc93d
NIS
460Arranges for FILEHANDLE to be read or written in "binary" or "text"
461mode on systems where the run-time libraries distinguish between
462binary and text files. If FILEHANDLE is an expression, the value is
463taken as the name of the filehandle. Returns true on success,
b5fe5ca2 464otherwise it returns C<undef> and sets C<$!> (errno).
1cbfc93d 465
d807c6f4
JH
466On some systems (in general, DOS and Windows-based systems) binmode()
467is necessary when you're not working with a text file. For the sake
468of portability it is a good idea to always use it when appropriate,
469and to never use it when it isn't appropriate. Also, people can
470set their I/O to be by default UTF-8 encoded Unicode, not bytes.
471
472In other words: regardless of platform, use binmode() on binary data,
473like for example images.
474
475If LAYER is present it is a single string, but may contain multiple
476directives. The directives alter the behaviour of the file handle.
477When LAYER is present using binmode on text file makes sense.
478
fae2c0fb 479If LAYER is omitted or specified as C<:raw> the filehandle is made
0226bbdb
NIS
480suitable for passing binary data. This includes turning off possible CRLF
481translation and marking it as bytes (as opposed to Unicode characters).
749683d2
YST
482Note that, despite what may be implied in I<"Programming Perl"> (the
483Camel) or elsewhere, C<:raw> is I<not> the simply inverse of C<:crlf>
fae2c0fb 484-- other layers which would affect binary nature of the stream are
0226bbdb
NIS
485I<also> disabled. See L<PerlIO>, L<perlrun> and the discussion about the
486PERLIO environment variable.
01e6739c 487
d807c6f4
JH
488The C<:bytes>, C<:crlf>, and C<:utf8>, and any other directives of the
489form C<:...>, are called I/O I<layers>. The C<open> pragma can be used to
490establish default I/O layers. See L<open>.
491
fae2c0fb
RGS
492I<The LAYER parameter of the binmode() function is described as "DISCIPLINE"
493in "Programming Perl, 3rd Edition". However, since the publishing of this
494book, by many known as "Camel III", the consensus of the naming of this
495functionality has moved from "discipline" to "layer". All documentation
496of this version of Perl therefore refers to "layers" rather than to
497"disciplines". Now back to the regularly scheduled documentation...>
498
01e6739c 499To mark FILEHANDLE as UTF-8, use C<:utf8>.
1cbfc93d 500
ed53a2bb 501In general, binmode() should be called after open() but before any I/O
01e6739c
NIS
502is done on the filehandle. Calling binmode() will normally flush any
503pending buffered output data (and perhaps pending input data) on the
fae2c0fb 504handle. An exception to this is the C<:encoding> layer that
01e6739c 505changes the default character encoding of the handle, see L<open>.
fae2c0fb 506The C<:encoding> layer sometimes needs to be called in
3874323d
JH
507mid-stream, and it doesn't flush the stream. The C<:encoding>
508also implicitly pushes on top of itself the C<:utf8> layer because
509internally Perl will operate on UTF-8 encoded Unicode characters.
16fe6d59 510
19799a22 511The operating system, device drivers, C libraries, and Perl run-time
30168b04
GS
512system all work together to let the programmer treat a single
513character (C<\n>) as the line terminator, irrespective of the external
514representation. On many operating systems, the native text file
515representation matches the internal representation, but on some
516platforms the external representation of C<\n> is made up of more than
517one character.
518
68bd7414
NIS
519Mac OS, all variants of Unix, and Stream_LF files on VMS use a single
520character to end each line in the external representation of text (even
5e12dbfa 521though that single character is CARRIAGE RETURN on Mac OS and LINE FEED
01e6739c
NIS
522on Unix and most VMS files). In other systems like OS/2, DOS and the
523various flavors of MS-Windows your program sees a C<\n> as a simple C<\cJ>,
524but what's stored in text files are the two characters C<\cM\cJ>. That
525means that, if you don't use binmode() on these systems, C<\cM\cJ>
526sequences on disk will be converted to C<\n> on input, and any C<\n> in
527your program will be converted back to C<\cM\cJ> on output. This is what
528you want for text files, but it can be disastrous for binary files.
30168b04
GS
529
530Another consequence of using binmode() (on some systems) is that
531special end-of-file markers will be seen as part of the data stream.
532For systems from the Microsoft family this means that if your binary
4375e838 533data contains C<\cZ>, the I/O subsystem will regard it as the end of
30168b04
GS
534the file, unless you use binmode().
535
536binmode() is not only important for readline() and print() operations,
537but also when using read(), seek(), sysread(), syswrite() and tell()
538(see L<perlport> for more details). See the C<$/> and C<$\> variables
539in L<perlvar> for how to manually set your input and output
540line-termination sequences.
a0d0e21e 541
4633a7c4 542=item bless REF,CLASSNAME
a0d0e21e
LW
543
544=item bless REF
545
2b5ab1e7
TC
546This function tells the thingy referenced by REF that it is now an object
547in the CLASSNAME package. If CLASSNAME is omitted, the current package
19799a22 548is used. Because a C<bless> is often the last thing in a constructor,
2b5ab1e7
TC
549it returns the reference for convenience. Always use the two-argument
550version if the function doing the blessing might be inherited by a
551derived class. See L<perltoot> and L<perlobj> for more about the blessing
552(and blessings) of objects.
a0d0e21e 553
57668c4d 554Consider always blessing objects in CLASSNAMEs that are mixed case.
2b5ab1e7
TC
555Namespaces with all lowercase names are considered reserved for
556Perl pragmata. Builtin types have all uppercase names, so to prevent
557confusion, you may wish to avoid such package names as well. Make sure
558that CLASSNAME is a true value.
60ad88b8
GS
559
560See L<perlmod/"Perl Modules">.
561
a0d0e21e
LW
562=item caller EXPR
563
564=item caller
565
5a964f20 566Returns the context of the current subroutine call. In scalar context,
28757baa 567returns the caller's package name if there is a caller, that is, if
19799a22 568we're in a subroutine or C<eval> or C<require>, and the undefined value
5a964f20 569otherwise. In list context, returns
a0d0e21e 570
748a9306 571 ($package, $filename, $line) = caller;
a0d0e21e
LW
572
573With EXPR, it returns some extra information that the debugger uses to
574print a stack trace. The value of EXPR indicates how many call frames
575to go back before the current one.
576
f3aa04c2 577 ($package, $filename, $line, $subroutine, $hasargs,
e476b1b5 578 $wantarray, $evaltext, $is_require, $hints, $bitmask) = caller($i);
e7ea3e70 579
951ba7fe 580Here $subroutine may be C<(eval)> if the frame is not a subroutine
19799a22 581call, but an C<eval>. In such a case additional elements $evaltext and
7660c0ab 582C<$is_require> are set: C<$is_require> is true if the frame is created by a
19799a22 583C<require> or C<use> statement, $evaltext contains the text of the
277ddfaf 584C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
951ba7fe 585$filename is C<(eval)>, but $evaltext is undefined. (Note also that
0fc9dec4
RGS
586each C<use> statement creates a C<require> frame inside an C<eval EXPR>
587frame.) $subroutine may also be C<(unknown)> if this particular
588subroutine happens to have been deleted from the symbol table.
589C<$hasargs> is true if a new instance of C<@_> was set up for the frame.
590C<$hints> and C<$bitmask> contain pragmatic hints that the caller was
591compiled with. The C<$hints> and C<$bitmask> values are subject to change
592between versions of Perl, and are not meant for external use.
748a9306
LW
593
594Furthermore, when called from within the DB package, caller returns more
7660c0ab 595detailed information: it sets the list variable C<@DB::args> to be the
54310121 596arguments with which the subroutine was invoked.
748a9306 597
7660c0ab 598Be aware that the optimizer might have optimized call frames away before
19799a22 599C<caller> had a chance to get the information. That means that C<caller(N)>
7660c0ab 600might not return information about the call frame you expect it do, for
b76cc8ba 601C<< N > 1 >>. In particular, C<@DB::args> might have information from the
19799a22 602previous time C<caller> was called.
7660c0ab 603
a0d0e21e
LW
604=item chdir EXPR
605
ffce7b87 606Changes the working directory to EXPR, if possible. If EXPR is omitted,
0bfc1ec4 607changes to the directory specified by C<$ENV{HOME}>, if set; if not,
ffce7b87 608changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
b4ad75f0
AMS
609variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
610neither is set, C<chdir> does nothing. It returns true upon success,
611false otherwise. See the example under C<die>.
a0d0e21e
LW
612
613=item chmod LIST
614
615Changes the permissions of a list of files. The first element of the
4633a7c4 616list must be the numerical mode, which should probably be an octal
4ad40acf 617number, and which definitely should I<not> be a string of octal digits:
2f9daede 618C<0644> is okay, C<'0644'> is not. Returns the number of files
dc848c6f 619successfully changed. See also L</oct>, if all you have is a string.
a0d0e21e
LW
620
621 $cnt = chmod 0755, 'foo', 'bar';
622 chmod 0755, @executables;
f86cebdf
GS
623 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
624 # --w----r-T
2f9daede
TPG
625 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
626 $mode = 0644; chmod $mode, 'foo'; # this is best
a0d0e21e 627
ca6e1c26
JH
628You can also import the symbolic C<S_I*> constants from the Fcntl
629module:
630
631 use Fcntl ':mode';
632
633 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
634 # This is identical to the chmod 0755 of the above example.
635
a0d0e21e
LW
636=item chomp VARIABLE
637
313c9f5c 638=item chomp( LIST )
a0d0e21e
LW
639
640=item chomp
641
2b5ab1e7
TC
642This safer version of L</chop> removes any trailing string
643that corresponds to the current value of C<$/> (also known as
28757baa
PP
644$INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
645number of characters removed from all its arguments. It's often used to
646remove the newline from the end of an input record when you're worried
2b5ab1e7
TC
647that the final record may be missing its newline. When in paragraph
648mode (C<$/ = "">), it removes all trailing newlines from the string.
4c5a6083
GS
649When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
650a reference to an integer or the like, see L<perlvar>) chomp() won't
b76cc8ba 651remove anything.
19799a22 652If VARIABLE is omitted, it chomps C<$_>. Example:
a0d0e21e
LW
653
654 while (<>) {
655 chomp; # avoid \n on last field
656 @array = split(/:/);
5a964f20 657 # ...
a0d0e21e
LW
658 }
659
4bf21a6d
RD
660If VARIABLE is a hash, it chomps the hash's values, but not its keys.
661
a0d0e21e
LW
662You can actually chomp anything that's an lvalue, including an assignment:
663
664 chomp($cwd = `pwd`);
665 chomp($answer = <STDIN>);
666
667If you chomp a list, each element is chomped, and the total number of
668characters removed is returned.
669
442a8c12
NC
670If the C<encoding> pragma is in scope then the lengths returned are
671calculated from the length of C<$/> in Unicode characters, which is not
672always the same as the length of C<$/> in the native encoding.
673
15e44fd8
RGS
674Note that parentheses are necessary when you're chomping anything
675that is not a simple variable. This is because C<chomp $cwd = `pwd`;>
676is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as
677C<chomp( $cwd = `pwd` )> which you might expect. Similarly,
678C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than
679as C<chomp($a, $b)>.
680
a0d0e21e
LW
681=item chop VARIABLE
682
313c9f5c 683=item chop( LIST )
a0d0e21e
LW
684
685=item chop
686
687Chops off the last character of a string and returns the character
5b3eff12 688chopped. It is much more efficient than C<s/.$//s> because it neither
7660c0ab 689scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
4bf21a6d
RD
690If VARIABLE is a hash, it chops the hash's values, but not its keys.
691
5b3eff12 692You can actually chop anything that's an lvalue, including an assignment.
a0d0e21e
LW
693
694If you chop a list, each element is chopped. Only the value of the
19799a22 695last C<chop> is returned.
a0d0e21e 696
19799a22 697Note that C<chop> returns the last character. To return all but the last
748a9306
LW
698character, use C<substr($string, 0, -1)>.
699
15e44fd8
RGS
700See also L</chomp>.
701
a0d0e21e
LW
702=item chown LIST
703
704Changes the owner (and group) of a list of files. The first two
19799a22
GS
705elements of the list must be the I<numeric> uid and gid, in that
706order. A value of -1 in either position is interpreted by most
707systems to leave that value unchanged. Returns the number of files
708successfully changed.
a0d0e21e
LW
709
710 $cnt = chown $uid, $gid, 'foo', 'bar';
711 chown $uid, $gid, @filenames;
712
54310121 713Here's an example that looks up nonnumeric uids in the passwd file:
a0d0e21e
LW
714
715 print "User: ";
19799a22 716 chomp($user = <STDIN>);
5a964f20 717 print "Files: ";
19799a22 718 chomp($pattern = <STDIN>);
a0d0e21e
LW
719
720 ($login,$pass,$uid,$gid) = getpwnam($user)
721 or die "$user not in passwd file";
722
5a964f20 723 @ary = glob($pattern); # expand filenames
a0d0e21e
LW
724 chown $uid, $gid, @ary;
725
54310121 726On most systems, you are not allowed to change the ownership of the
4633a7c4
LW
727file unless you're the superuser, although you should be able to change
728the group to any of your secondary groups. On insecure systems, these
729restrictions may be relaxed, but this is not a portable assumption.
19799a22
GS
730On POSIX systems, you can detect this condition this way:
731
732 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
733 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
4633a7c4 734
a0d0e21e
LW
735=item chr NUMBER
736
54310121 737=item chr
bbce6d69 738
a0d0e21e 739Returns the character represented by that NUMBER in the character set.
a0ed51b3 740For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
1e54db1a
JH
741chr(0x263a) is a Unicode smiley face. Note that characters from 128
742to 255 (inclusive) are by default not encoded in UTF-8 Unicode for
743backward compatibility reasons (but see L<encoding>).
aaa68c4a 744
8a064bd6
JH
745Negative values give the Unicode replacement character (chr(0xfffd)),
746except under the L</bytes> pragma, where low eight bits of the value
747(truncated to an integer) are used.
748
974da8e5
JH
749If NUMBER is omitted, uses C<$_>.
750
b76cc8ba 751For the reverse, use L</ord>.
a0d0e21e 752
974da8e5
JH
753Note that under the C<bytes> pragma the NUMBER is masked to
754the low eight bits.
755
756See L<perlunicode> and L<encoding> for more about Unicode.
bbce6d69 757
a0d0e21e
LW
758=item chroot FILENAME
759
54310121 760=item chroot
bbce6d69 761
5a964f20 762This function works like the system call by the same name: it makes the
4633a7c4 763named directory the new root directory for all further pathnames that
951ba7fe 764begin with a C</> by your process and all its children. (It doesn't
28757baa 765change your current working directory, which is unaffected.) For security
4633a7c4 766reasons, this call is restricted to the superuser. If FILENAME is
19799a22 767omitted, does a C<chroot> to C<$_>.
a0d0e21e
LW
768
769=item close FILEHANDLE
770
6a518fbc
TP
771=item close
772
9124316e
JH
773Closes the file or pipe associated with the file handle, returning
774true only if IO buffers are successfully flushed and closes the system
775file descriptor. Closes the currently selected filehandle if the
776argument is omitted.
fb73857a
PP
777
778You don't have to close FILEHANDLE if you are immediately going to do
19799a22
GS
779another C<open> on it, because C<open> will close it for you. (See
780C<open>.) However, an explicit C<close> on an input file resets the line
781counter (C<$.>), while the implicit close done by C<open> does not.
fb73857a 782
dede8123
RGS
783If the file handle came from a piped open, C<close> will additionally
784return false if one of the other system calls involved fails, or if the
fb73857a 785program exits with non-zero status. (If the only problem was that the
dede8123 786program exited non-zero, C<$!> will be set to C<0>.) Closing a pipe
2b5ab1e7 787also waits for the process executing on the pipe to complete, in case you
b76cc8ba 788want to look at the output of the pipe afterwards, and
e5218da5
GA
789implicitly puts the exit status value of that command into C<$?> and
790C<${^CHILD_ERROR_NATIVE}>.
5a964f20 791
73689b13
GS
792Prematurely closing the read end of a pipe (i.e. before the process
793writing to it at the other end has closed it) will result in a
794SIGPIPE being delivered to the writer. If the other end can't
795handle that, be sure to read all the data before closing the pipe.
796
fb73857a 797Example:
a0d0e21e 798
fb73857a
PP
799 open(OUTPUT, '|sort >foo') # pipe to sort
800 or die "Can't start sort: $!";
5a964f20 801 #... # print stuff to output
fb73857a
PP
802 close OUTPUT # wait for sort to finish
803 or warn $! ? "Error closing sort pipe: $!"
804 : "Exit status $? from sort";
805 open(INPUT, 'foo') # get sort's results
806 or die "Can't open 'foo' for input: $!";
a0d0e21e 807
5a964f20
TC
808FILEHANDLE may be an expression whose value can be used as an indirect
809filehandle, usually the real filehandle name.
a0d0e21e
LW
810
811=item closedir DIRHANDLE
812
19799a22 813Closes a directory opened by C<opendir> and returns the success of that
5a964f20
TC
814system call.
815
a0d0e21e
LW
816=item connect SOCKET,NAME
817
818Attempts to connect to a remote socket, just as the connect system call
19799a22 819does. Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
820packed address of the appropriate type for the socket. See the examples in
821L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 822
cb1a09d0
AD
823=item continue BLOCK
824
825Actually a flow control statement rather than a function. If there is a
98293880
JH
826C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
827C<foreach>), it is always executed just before the conditional is about to
828be evaluated again, just like the third part of a C<for> loop in C. Thus
cb1a09d0
AD
829it can be used to increment a loop variable, even when the loop has been
830continued via the C<next> statement (which is similar to the C C<continue>
831statement).
832
98293880 833C<last>, C<next>, or C<redo> may appear within a C<continue>
19799a22
GS
834block. C<last> and C<redo> will behave as if they had been executed within
835the main block. So will C<next>, but since it will execute a C<continue>
1d2dff63
GS
836block, it may be more entertaining.
837
838 while (EXPR) {
839 ### redo always comes here
840 do_something;
841 } continue {
842 ### next always comes here
843 do_something_else;
844 # then back the top to re-check EXPR
845 }
846 ### last always comes here
847
848Omitting the C<continue> section is semantically equivalent to using an
19799a22 849empty one, logically enough. In that case, C<next> goes directly back
1d2dff63
GS
850to check the condition at the top of the loop.
851
a0d0e21e
LW
852=item cos EXPR
853
d6217f1e
GS
854=item cos
855
5a964f20 856Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
7660c0ab 857takes cosine of C<$_>.
a0d0e21e 858
ca6e1c26 859For the inverse cosine operation, you may use the C<Math::Trig::acos()>
28757baa
PP
860function, or use this relation:
861
862 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
863
a0d0e21e
LW
864=item crypt PLAINTEXT,SALT
865
f86cebdf 866Encrypts a string exactly like the crypt(3) function in the C library
4633a7c4
LW
867(assuming that you actually have a version there that has not been
868extirpated as a potential munition). This can prove useful for checking
869the password file for lousy passwords, amongst other things. Only the
870guys wearing white hats should do this.
a0d0e21e 871
a6d05634 872Note that L<crypt|/crypt> is intended to be a one-way function, much like
85c16d83
JH
873breaking eggs to make an omelette. There is no (known) corresponding
874decrypt function (in other words, the crypt() is a one-way hash
875function). As a result, this function isn't all that useful for
11155c91 876cryptography. (For that, see your nearby CPAN mirror.)
2f9daede 877
85c16d83
JH
878When verifying an existing encrypted string you should use the
879encrypted text as the salt (like C<crypt($plain, $crypted) eq
8e2ffcbe 880$crypted>). This allows your code to work with the standard L<crypt|/crypt>
85c16d83
JH
881and with more exotic implementations. In other words, do not assume
882anything about the returned string itself, or how many bytes in
883the encrypted string matter.
884
885Traditionally the result is a string of 13 bytes: two first bytes of
886the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
887the first eight bytes of the encrypted string mattered, but
888alternative hashing schemes (like MD5), higher level security schemes
889(like C2), and implementations on non-UNIX platforms may produce
890different strings.
891
892When choosing a new salt create a random two character string whose
893characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
d3989d75
CW
894'/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of
895characters is just a recommendation; the characters allowed in
896the salt depend solely on your system's crypt library, and Perl can't
897restrict what salts C<crypt()> accepts.
e71965be 898
a0d0e21e
LW
899Here's an example that makes sure that whoever runs this program knows
900their own password:
901
902 $pwd = (getpwuid($<))[1];
a0d0e21e
LW
903
904 system "stty -echo";
905 print "Password: ";
e71965be 906 chomp($word = <STDIN>);
a0d0e21e
LW
907 print "\n";
908 system "stty echo";
909
e71965be 910 if (crypt($word, $pwd) ne $pwd) {
a0d0e21e
LW
911 die "Sorry...\n";
912 } else {
913 print "ok\n";
54310121 914 }
a0d0e21e 915
9f8f0c9d 916Of course, typing in your own password to whoever asks you
748a9306 917for it is unwise.
a0d0e21e 918
8e2ffcbe 919The L<crypt|/crypt> function is unsuitable for encrypting large quantities
19799a22
GS
920of data, not least of all because you can't get the information
921back. Look at the F<by-module/Crypt> and F<by-module/PGP> directories
922on your favorite CPAN mirror for a slew of potentially useful
923modules.
924
f2791508
JH
925If using crypt() on a Unicode string (which I<potentially> has
926characters with codepoints above 255), Perl tries to make sense
927of the situation by trying to downgrade (a copy of the string)
928the string back to an eight-bit byte string before calling crypt()
929(on that copy). If that works, good. If not, crypt() dies with
930C<Wide character in crypt>.
85c16d83 931
aa689395 932=item dbmclose HASH
a0d0e21e 933
19799a22 934[This function has been largely superseded by the C<untie> function.]
a0d0e21e 935
aa689395 936Breaks the binding between a DBM file and a hash.
a0d0e21e 937
19799a22 938=item dbmopen HASH,DBNAME,MASK
a0d0e21e 939
19799a22 940[This function has been largely superseded by the C<tie> function.]
a0d0e21e 941
7b8d334a 942This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
19799a22
GS
943hash. HASH is the name of the hash. (Unlike normal C<open>, the first
944argument is I<not> a filehandle, even though it looks like one). DBNAME
aa689395
PP
945is the name of the database (without the F<.dir> or F<.pag> extension if
946any). If the database does not exist, it is created with protection
19799a22
GS
947specified by MASK (as modified by the C<umask>). If your system supports
948only the older DBM functions, you may perform only one C<dbmopen> in your
aa689395 949program. In older versions of Perl, if your system had neither DBM nor
19799a22 950ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
aa689395
PP
951sdbm(3).
952
953If you don't have write access to the DBM file, you can only read hash
954variables, not set them. If you want to test whether you can write,
19799a22 955either use file tests or try setting a dummy hash entry inside an C<eval>,
aa689395 956which will trap the error.
a0d0e21e 957
19799a22
GS
958Note that functions such as C<keys> and C<values> may return huge lists
959when used on large DBM files. You may prefer to use the C<each>
a0d0e21e
LW
960function to iterate over large DBM files. Example:
961
962 # print out history file offsets
963 dbmopen(%HIST,'/usr/lib/news/history',0666);
964 while (($key,$val) = each %HIST) {
965 print $key, ' = ', unpack('L',$val), "\n";
966 }
967 dbmclose(%HIST);
968
cb1a09d0 969See also L<AnyDBM_File> for a more general description of the pros and
184e9718 970cons of the various dbm approaches, as well as L<DB_File> for a particularly
cb1a09d0 971rich implementation.
4633a7c4 972
2b5ab1e7
TC
973You can control which DBM library you use by loading that library
974before you call dbmopen():
975
976 use DB_File;
977 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
978 or die "Can't open netscape history file: $!";
979
a0d0e21e
LW
980=item defined EXPR
981
54310121 982=item defined
bbce6d69 983
2f9daede
TPG
984Returns a Boolean value telling whether EXPR has a value other than
985the undefined value C<undef>. If EXPR is not present, C<$_> will be
986checked.
987
988Many operations return C<undef> to indicate failure, end of file,
989system error, uninitialized variable, and other exceptional
990conditions. This function allows you to distinguish C<undef> from
991other values. (A simple Boolean test will not distinguish among
7660c0ab 992C<undef>, zero, the empty string, and C<"0">, which are all equally
2f9daede 993false.) Note that since C<undef> is a valid scalar, its presence
19799a22 994doesn't I<necessarily> indicate an exceptional condition: C<pop>
2f9daede
TPG
995returns C<undef> when its argument is an empty array, I<or> when the
996element to return happens to be C<undef>.
997
f10b0346
GS
998You may also use C<defined(&func)> to check whether subroutine C<&func>
999has ever been defined. The return value is unaffected by any forward
04891299 1000declarations of C<&func>. Note that a subroutine which is not defined
847c7ebe
DD
1001may still be callable: its package may have an C<AUTOLOAD> method that
1002makes it spring into existence the first time that it is called -- see
1003L<perlsub>.
f10b0346
GS
1004
1005Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
1006used to report whether memory for that aggregate has ever been
1007allocated. This behavior may disappear in future versions of Perl.
1008You should instead use a simple test for size:
1009
1010 if (@an_array) { print "has array elements\n" }
1011 if (%a_hash) { print "has hash members\n" }
2f9daede
TPG
1012
1013When used on a hash element, it tells you whether the value is defined,
dc848c6f 1014not whether the key exists in the hash. Use L</exists> for the latter
2f9daede 1015purpose.
a0d0e21e
LW
1016
1017Examples:
1018
1019 print if defined $switch{'D'};
1020 print "$val\n" while defined($val = pop(@ary));
1021 die "Can't readlink $sym: $!"
1022 unless defined($value = readlink $sym);
a0d0e21e 1023 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
2f9daede 1024 $debugging = 0 unless defined $debugging;
a0d0e21e 1025
19799a22 1026Note: Many folks tend to overuse C<defined>, and then are surprised to
7660c0ab 1027discover that the number C<0> and C<""> (the zero-length string) are, in fact,
2f9daede 1028defined values. For example, if you say
a5f75d66
AD
1029
1030 "ab" =~ /a(.*)b/;
1031
7660c0ab 1032The pattern match succeeds, and C<$1> is defined, despite the fact that it
a5f75d66 1033matched "nothing". But it didn't really match nothing--rather, it
2b5ab1e7 1034matched something that happened to be zero characters long. This is all
a5f75d66 1035very above-board and honest. When a function returns an undefined value,
2f9daede 1036it's an admission that it couldn't give you an honest answer. So you
19799a22 1037should use C<defined> only when you're questioning the integrity of what
7660c0ab 1038you're trying to do. At other times, a simple comparison to C<0> or C<""> is
2f9daede
TPG
1039what you want.
1040
dc848c6f 1041See also L</undef>, L</exists>, L</ref>.
2f9daede 1042
a0d0e21e
LW
1043=item delete EXPR
1044
01020589
GS
1045Given an expression that specifies a hash element, array element, hash slice,
1046or array slice, deletes the specified element(s) from the hash or array.
8216c1fd 1047In the case of an array, if the array elements happen to be at the end,
b76cc8ba 1048the size of the array will shrink to the highest element that tests
8216c1fd 1049true for exists() (or 0 if no such element exists).
a0d0e21e 1050
eba0920a
EM
1051Returns a list with the same number of elements as the number of elements
1052for which deletion was attempted. Each element of that list consists of
1053either the value of the element deleted, or the undefined value. In scalar
1054context, this means that you get the value of the last element deleted (or
1055the undefined value if that element did not exist).
1056
1057 %hash = (foo => 11, bar => 22, baz => 33);
1058 $scalar = delete $hash{foo}; # $scalar is 11
1059 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
1060 @array = delete @hash{qw(foo bar baz)}; # @array is (undef,undef,33)
1061
1062Deleting from C<%ENV> modifies the environment. Deleting from
01020589
GS
1063a hash tied to a DBM file deletes the entry from the DBM file. Deleting
1064from a C<tie>d hash or array may not necessarily return anything.
1065
8ea97a1e
GS
1066Deleting an array element effectively returns that position of the array
1067to its initial, uninitialized state. Subsequently testing for the same
8216c1fd
GS
1068element with exists() will return false. Note that deleting array
1069elements in the middle of an array will not shift the index of the ones
1070after them down--use splice() for that. See L</exists>.
8ea97a1e 1071
01020589 1072The following (inefficiently) deletes all the values of %HASH and @ARRAY:
a0d0e21e 1073
5f05dabc
PP
1074 foreach $key (keys %HASH) {
1075 delete $HASH{$key};
a0d0e21e
LW
1076 }
1077
01020589
GS
1078 foreach $index (0 .. $#ARRAY) {
1079 delete $ARRAY[$index];
1080 }
1081
1082And so do these:
5f05dabc 1083
01020589
GS
1084 delete @HASH{keys %HASH};
1085
9740c838 1086 delete @ARRAY[0 .. $#ARRAY];
5f05dabc 1087
2b5ab1e7 1088But both of these are slower than just assigning the empty list
01020589
GS
1089or undefining %HASH or @ARRAY:
1090
1091 %HASH = (); # completely empty %HASH
1092 undef %HASH; # forget %HASH ever existed
2b5ab1e7 1093
01020589
GS
1094 @ARRAY = (); # completely empty @ARRAY
1095 undef @ARRAY; # forget @ARRAY ever existed
2b5ab1e7
TC
1096
1097Note that the EXPR can be arbitrarily complicated as long as the final
01020589
GS
1098operation is a hash element, array element, hash slice, or array slice
1099lookup:
a0d0e21e
LW
1100
1101 delete $ref->[$x][$y]{$key};
5f05dabc 1102 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
a0d0e21e 1103
01020589
GS
1104 delete $ref->[$x][$y][$index];
1105 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1106
a0d0e21e
LW
1107=item die LIST
1108
19799a22
GS
1109Outside an C<eval>, prints the value of LIST to C<STDERR> and
1110exits with the current value of C<$!> (errno). If C<$!> is C<0>,
61eff3bc
JH
1111exits with the value of C<<< ($? >> 8) >>> (backtick `command`
1112status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside
19799a22
GS
1113an C<eval(),> the error message is stuffed into C<$@> and the
1114C<eval> is terminated with the undefined value. This makes
1115C<die> the way to raise an exception.
a0d0e21e
LW
1116
1117Equivalent examples:
1118
1119 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
54310121 1120 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
a0d0e21e 1121
ccac6780 1122If the last element of LIST does not end in a newline, the current
df37ec69
WW
1123script line number and input line number (if any) are also printed,
1124and a newline is supplied. Note that the "input line number" (also
1125known as "chunk") is subject to whatever notion of "line" happens to
1126be currently in effect, and is also available as the special variable
1127C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1128
1129Hint: sometimes appending C<", stopped"> to your message will cause it
1130to make better sense when the string C<"at foo line 123"> is appended.
1131Suppose you are running script "canasta".
a0d0e21e
LW
1132
1133 die "/etc/games is no good";
1134 die "/etc/games is no good, stopped";
1135
1136produce, respectively
1137
1138 /etc/games is no good at canasta line 123.
1139 /etc/games is no good, stopped at canasta line 123.
1140
2b5ab1e7 1141See also exit(), warn(), and the Carp module.
a0d0e21e 1142
7660c0ab
A
1143If LIST is empty and C<$@> already contains a value (typically from a
1144previous eval) that value is reused after appending C<"\t...propagated">.
fb73857a
PP
1145This is useful for propagating exceptions:
1146
1147 eval { ... };
1148 die unless $@ =~ /Expected exception/;
1149
ad216e65
JH
1150If LIST is empty and C<$@> contains an object reference that has a
1151C<PROPAGATE> method, that method will be called with additional file
1152and line number parameters. The return value replaces the value in
16869676 1153C<$@>. ie. as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >>
ad216e65
JH
1154were called.
1155
7660c0ab 1156If C<$@> is empty then the string C<"Died"> is used.
fb73857a 1157
52531d10
GS
1158die() can also be called with a reference argument. If this happens to be
1159trapped within an eval(), $@ contains the reference. This behavior permits
1160a more elaborate exception handling implementation using objects that
4375e838 1161maintain arbitrary state about the nature of the exception. Such a scheme
52531d10
GS
1162is sometimes preferable to matching particular string values of $@ using
1163regular expressions. Here's an example:
1164
1165 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1166 if ($@) {
1167 if (ref($@) && UNIVERSAL::isa($@,"Some::Module::Exception")) {
1168 # handle Some::Module::Exception
1169 }
1170 else {
1171 # handle all other possible exceptions
1172 }
1173 }
1174
19799a22 1175Because perl will stringify uncaught exception messages before displaying
52531d10
GS
1176them, you may want to overload stringification operations on such custom
1177exception objects. See L<overload> for details about that.
1178
19799a22
GS
1179You can arrange for a callback to be run just before the C<die>
1180does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1181handler will be called with the error text and can change the error
1182message, if it sees fit, by calling C<die> again. See
1183L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
1184L<"eval BLOCK"> for some examples. Although this feature was meant
1185to be run only right before your program was to exit, this is not
1186currently the case--the C<$SIG{__DIE__}> hook is currently called
1187even inside eval()ed blocks/strings! If one wants the hook to do
1188nothing in such situations, put
fb73857a
PP
1189
1190 die @_ if $^S;
1191
19799a22
GS
1192as the first line of the handler (see L<perlvar/$^S>). Because
1193this promotes strange action at a distance, this counterintuitive
b76cc8ba 1194behavior may be fixed in a future release.
774d564b 1195
a0d0e21e
LW
1196=item do BLOCK
1197
1198Not really a function. Returns the value of the last command in the
1199sequence of commands indicated by BLOCK. When modified by a loop
98293880
JH
1200modifier, executes the BLOCK once before testing the loop condition.
1201(On other statements the loop modifiers test the conditional first.)
a0d0e21e 1202
4968c1e4 1203C<do BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7
TC
1204C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1205See L<perlsyn> for alternative strategies.
4968c1e4 1206
a0d0e21e
LW
1207=item do SUBROUTINE(LIST)
1208
1209A deprecated form of subroutine call. See L<perlsub>.
1210
1211=item do EXPR
1212
1213Uses the value of EXPR as a filename and executes the contents of the
ea63ef19 1214file as a Perl script.
a0d0e21e
LW
1215
1216 do 'stat.pl';
1217
1218is just like
1219
986b19de 1220 eval `cat stat.pl`;
a0d0e21e 1221
2b5ab1e7 1222except that it's more efficient and concise, keeps track of the current
ea63ef19 1223filename for error messages, searches the @INC directories, and updates
2b5ab1e7
TC
1224C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1225variables. It also differs in that code evaluated with C<do FILENAME>
1226cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1227same, however, in that it does reparse the file every time you call it,
1228so you probably don't want to do this inside a loop.
a0d0e21e 1229
8e30cc93 1230If C<do> cannot read the file, it returns undef and sets C<$!> to the
2b5ab1e7 1231error. If C<do> can read the file but cannot compile it, it
8e30cc93
G
1232returns undef and sets an error message in C<$@>. If the file is
1233successfully compiled, C<do> returns the value of the last expression
1234evaluated.
1235
a0d0e21e 1236Note that inclusion of library modules is better done with the
19799a22 1237C<use> and C<require> operators, which also do automatic error checking
4633a7c4 1238and raise an exception if there's a problem.
a0d0e21e 1239
5a964f20
TC
1240You might like to use C<do> to read in a program configuration
1241file. Manual error checking can be done this way:
1242
b76cc8ba 1243 # read in config files: system first, then user
f86cebdf 1244 for $file ("/share/prog/defaults.rc",
b76cc8ba 1245 "$ENV{HOME}/.someprogrc")
2b5ab1e7 1246 {
5a964f20 1247 unless ($return = do $file) {
f86cebdf
GS
1248 warn "couldn't parse $file: $@" if $@;
1249 warn "couldn't do $file: $!" unless defined $return;
1250 warn "couldn't run $file" unless $return;
5a964f20
TC
1251 }
1252 }
1253
a0d0e21e
LW
1254=item dump LABEL
1255
1614b0e3
JD
1256=item dump
1257
19799a22
GS
1258This function causes an immediate core dump. See also the B<-u>
1259command-line switch in L<perlrun>, which does the same thing.
1260Primarily this is so that you can use the B<undump> program (not
1261supplied) to turn your core dump into an executable binary after
1262having initialized all your variables at the beginning of the
1263program. When the new binary is executed it will begin by executing
1264a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1265Think of it as a goto with an intervening core dump and reincarnation.
1266If C<LABEL> is omitted, restarts the program from the top.
1267
1268B<WARNING>: Any files opened at the time of the dump will I<not>
1269be open any more when the program is reincarnated, with possible
b76cc8ba 1270resulting confusion on the part of Perl.
19799a22
GS
1271
1272This function is now largely obsolete, partly because it's very
1273hard to convert a core file into an executable, and because the
1274real compiler backends for generating portable bytecode and compilable
ac206dc8
RGS
1275C code have superseded it. That's why you should now invoke it as
1276C<CORE::dump()>, if you don't want to be warned against a possible
1277typo.
19799a22
GS
1278
1279If you're looking to use L<dump> to speed up your program, consider
1280generating bytecode or native C code as described in L<perlcc>. If
1281you're just trying to accelerate a CGI script, consider using the
210b36aa 1282C<mod_perl> extension to B<Apache>, or the CPAN module, CGI::Fast.
19799a22 1283You might also consider autoloading or selfloading, which at least
b76cc8ba 1284make your program I<appear> to run faster.
5a964f20 1285
aa689395
PP
1286=item each HASH
1287
5a964f20 1288When called in list context, returns a 2-element list consisting of the
aa689395 1289key and value for the next element of a hash, so that you can iterate over
74fc8b5f 1290it. When called in scalar context, returns only the key for the next
e902a979 1291element in the hash.
2f9daede 1292
ab192400 1293Entries are returned in an apparently random order. The actual random
504f80c1
JH
1294order is subject to change in future versions of perl, but it is
1295guaranteed to be in the same order as either the C<keys> or C<values>
4546b9e6
JH
1296function would produce on the same (unmodified) hash. Since Perl
12975.8.1 the ordering is different even between different runs of Perl
1298for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
ab192400
GS
1299
1300When the hash is entirely read, a null array is returned in list context
19799a22
GS
1301(which when assigned produces a false (C<0>) value), and C<undef> in
1302scalar context. The next call to C<each> after that will start iterating
1303again. There is a single iterator for each hash, shared by all C<each>,
1304C<keys>, and C<values> function calls in the program; it can be reset by
2f9daede
TPG
1305reading all the elements from the hash, or by evaluating C<keys HASH> or
1306C<values HASH>. If you add or delete elements of a hash while you're
74fc8b5f
MJD
1307iterating over it, you may get entries skipped or duplicated, so
1308don't. Exception: It is always safe to delete the item most recently
1309returned by C<each()>, which means that the following code will work:
1310
1311 while (($key, $value) = each %hash) {
1312 print $key, "\n";
1313 delete $hash{$key}; # This is safe
1314 }
aa689395 1315
f86cebdf 1316The following prints out your environment like the printenv(1) program,
aa689395 1317only in a different order:
a0d0e21e
LW
1318
1319 while (($key,$value) = each %ENV) {
1320 print "$key=$value\n";
1321 }
1322
19799a22 1323See also C<keys>, C<values> and C<sort>.
a0d0e21e
LW
1324
1325=item eof FILEHANDLE
1326
4633a7c4
LW
1327=item eof ()
1328
a0d0e21e
LW
1329=item eof
1330
1331Returns 1 if the next read on FILEHANDLE will return end of file, or if
1332FILEHANDLE is not open. FILEHANDLE may be an expression whose value
5a964f20 1333gives the real filehandle. (Note that this function actually
19799a22 1334reads a character and then C<ungetc>s it, so isn't very useful in an
748a9306 1335interactive context.) Do not read from a terminal file (or call
19799a22 1336C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
748a9306
LW
1337as terminals may lose the end-of-file condition if you do.
1338
820475bd
GS
1339An C<eof> without an argument uses the last file read. Using C<eof()>
1340with empty parentheses is very different. It refers to the pseudo file
1341formed from the files listed on the command line and accessed via the
61eff3bc
JH
1342C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1343as a normal filehandle is, an C<eof()> before C<< <> >> has been
820475bd 1344used will cause C<@ARGV> to be examined to determine if input is
67408cae 1345available. Similarly, an C<eof()> after C<< <> >> has returned
efdd0218
RB
1346end-of-file will assume you are processing another C<@ARGV> list,
1347and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1348see L<perlop/"I/O Operators">.
820475bd 1349
61eff3bc 1350In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
820475bd
GS
1351detect the end of each file, C<eof()> will only detect the end of the
1352last file. Examples:
a0d0e21e 1353
748a9306
LW
1354 # reset line numbering on each input file
1355 while (<>) {
b76cc8ba 1356 next if /^\s*#/; # skip comments
748a9306 1357 print "$.\t$_";
5a964f20
TC
1358 } continue {
1359 close ARGV if eof; # Not eof()!
748a9306
LW
1360 }
1361
a0d0e21e
LW
1362 # insert dashes just before last line of last file
1363 while (<>) {
6ac88b13 1364 if (eof()) { # check for end of last file
a0d0e21e
LW
1365 print "--------------\n";
1366 }
1367 print;
6ac88b13 1368 last if eof(); # needed if we're reading from a terminal
a0d0e21e
LW
1369 }
1370
a0d0e21e 1371Practical hint: you almost never need to use C<eof> in Perl, because the
3ce0d271
GS
1372input operators typically return C<undef> when they run out of data, or if
1373there was an error.
a0d0e21e
LW
1374
1375=item eval EXPR
1376
1377=item eval BLOCK
1378
c7cc6f1c
GS
1379In the first form, the return value of EXPR is parsed and executed as if it
1380were a little Perl program. The value of the expression (which is itself
5a964f20 1381determined within scalar context) is first parsed, and if there weren't any
be3174d2
GS
1382errors, executed in the lexical context of the current Perl program, so
1383that any variable settings or subroutine and format definitions remain
1384afterwards. Note that the value is parsed every time the eval executes.
1385If EXPR is omitted, evaluates C<$_>. This form is typically used to
1386delay parsing and subsequent execution of the text of EXPR until run time.
c7cc6f1c
GS
1387
1388In the second form, the code within the BLOCK is parsed only once--at the
1389same time the code surrounding the eval itself was parsed--and executed
1390within the context of the current Perl program. This form is typically
1391used to trap exceptions more efficiently than the first (see below), while
1392also providing the benefit of checking the code within BLOCK at compile
1393time.
1394
1395The final semicolon, if any, may be omitted from the value of EXPR or within
1396the BLOCK.
1397
1398In both forms, the value returned is the value of the last expression
5a964f20 1399evaluated inside the mini-program; a return statement may be also used, just
c7cc6f1c 1400as with subroutines. The expression providing the return value is evaluated
5a964f20 1401in void, scalar, or list context, depending on the context of the eval itself.
c7cc6f1c 1402See L</wantarray> for more on how the evaluation context can be determined.
a0d0e21e 1403
19799a22
GS
1404If there is a syntax error or runtime error, or a C<die> statement is
1405executed, an undefined value is returned by C<eval>, and C<$@> is set to the
a0d0e21e 1406error message. If there was no error, C<$@> is guaranteed to be a null
19799a22 1407string. Beware that using C<eval> neither silences perl from printing
c7cc6f1c 1408warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
d9984052
A
1409To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1410turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1411See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
a0d0e21e 1412
19799a22
GS
1413Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1414determining whether a particular feature (such as C<socket> or C<symlink>)
a0d0e21e
LW
1415is implemented. It is also Perl's exception trapping mechanism, where
1416the die operator is used to raise exceptions.
1417
1418If the code to be executed doesn't vary, you may use the eval-BLOCK
1419form to trap run-time errors without incurring the penalty of
1420recompiling each time. The error, if any, is still returned in C<$@>.
1421Examples:
1422
54310121 1423 # make divide-by-zero nonfatal
a0d0e21e
LW
1424 eval { $answer = $a / $b; }; warn $@ if $@;
1425
1426 # same thing, but less efficient
1427 eval '$answer = $a / $b'; warn $@ if $@;
1428
1429 # a compile-time error
5a964f20 1430 eval { $answer = }; # WRONG
a0d0e21e
LW
1431
1432 # a run-time error
1433 eval '$answer ='; # sets $@
1434
2b5ab1e7
TC
1435Due to the current arguably broken state of C<__DIE__> hooks, when using
1436the C<eval{}> form as an exception trap in libraries, you may wish not
1437to trigger any C<__DIE__> hooks that user code may have installed.
1438You can use the C<local $SIG{__DIE__}> construct for this purpose,
1439as shown in this example:
774d564b
PP
1440
1441 # a very private exception trap for divide-by-zero
f86cebdf
GS
1442 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1443 warn $@ if $@;
774d564b
PP
1444
1445This is especially significant, given that C<__DIE__> hooks can call
19799a22 1446C<die> again, which has the effect of changing their error messages:
774d564b
PP
1447
1448 # __DIE__ hooks may modify error messages
1449 {
f86cebdf
GS
1450 local $SIG{'__DIE__'} =
1451 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
c7cc6f1c
GS
1452 eval { die "foo lives here" };
1453 print $@ if $@; # prints "bar lives here"
774d564b
PP
1454 }
1455
19799a22 1456Because this promotes action at a distance, this counterintuitive behavior
2b5ab1e7
TC
1457may be fixed in a future release.
1458
19799a22 1459With an C<eval>, you should be especially careful to remember what's
a0d0e21e
LW
1460being looked at when:
1461
1462 eval $x; # CASE 1
1463 eval "$x"; # CASE 2
1464
1465 eval '$x'; # CASE 3
1466 eval { $x }; # CASE 4
1467
5a964f20 1468 eval "\$$x++"; # CASE 5
a0d0e21e
LW
1469 $$x++; # CASE 6
1470
2f9daede 1471Cases 1 and 2 above behave identically: they run the code contained in
19799a22 1472the variable $x. (Although case 2 has misleading double quotes making
2f9daede 1473the reader wonder what else might be happening (nothing is).) Cases 3
7660c0ab 1474and 4 likewise behave in the same way: they run the code C<'$x'>, which
19799a22 1475does nothing but return the value of $x. (Case 4 is preferred for
2f9daede
TPG
1476purely visual reasons, but it also has the advantage of compiling at
1477compile-time instead of at run-time.) Case 5 is a place where
19799a22 1478normally you I<would> like to use double quotes, except that in this
2f9daede
TPG
1479particular situation, you can just use symbolic references instead, as
1480in case 6.
a0d0e21e 1481
4968c1e4 1482C<eval BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7 1483C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
4968c1e4 1484
d819b83a
DM
1485Note that as a very special case, an C<eval ''> executed within the C<DB>
1486package doesn't see the usual surrounding lexical scope, but rather the
1487scope of the first non-DB piece of code that called it. You don't normally
1488need to worry about this unless you are writing a Perl debugger.
1489
a0d0e21e
LW
1490=item exec LIST
1491
8bf3b016
GS
1492=item exec PROGRAM LIST
1493
19799a22
GS
1494The C<exec> function executes a system command I<and never returns>--
1495use C<system> instead of C<exec> if you want it to return. It fails and
1496returns false only if the command does not exist I<and> it is executed
fb73857a 1497directly instead of via your system's command shell (see below).
a0d0e21e 1498
19799a22
GS
1499Since it's a common mistake to use C<exec> instead of C<system>, Perl
1500warns you if there is a following statement which isn't C<die>, C<warn>,
1501or C<exit> (if C<-w> is set - but you always do that). If you
1502I<really> want to follow an C<exec> with some other statement, you
55d729e4
GS
1503can use one of these styles to avoid the warning:
1504
5a964f20
TC
1505 exec ('foo') or print STDERR "couldn't exec foo: $!";
1506 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
55d729e4 1507
5a964f20 1508If there is more than one argument in LIST, or if LIST is an array
f86cebdf 1509with more than one value, calls execvp(3) with the arguments in LIST.
5a964f20
TC
1510If there is only one scalar argument or an array with one element in it,
1511the argument is checked for shell metacharacters, and if there are any,
1512the entire argument is passed to the system's command shell for parsing
1513(this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1514If there are no shell metacharacters in the argument, it is split into
b76cc8ba 1515words and passed directly to C<execvp>, which is more efficient.
19799a22 1516Examples:
a0d0e21e 1517
19799a22
GS
1518 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1519 exec "sort $outfile | uniq";
a0d0e21e
LW
1520
1521If you don't really want to execute the first argument, but want to lie
1522to the program you are executing about its own name, you can specify
1523the program you actually want to run as an "indirect object" (without a
1524comma) in front of the LIST. (This always forces interpretation of the
54310121 1525LIST as a multivalued list, even if there is only a single scalar in
a0d0e21e
LW
1526the list.) Example:
1527
1528 $shell = '/bin/csh';
1529 exec $shell '-sh'; # pretend it's a login shell
1530
1531or, more directly,
1532
1533 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1534
bb32b41a
GS
1535When the arguments get executed via the system shell, results will
1536be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1537for details.
1538
19799a22
GS
1539Using an indirect object with C<exec> or C<system> is also more
1540secure. This usage (which also works fine with system()) forces
1541interpretation of the arguments as a multivalued list, even if the
1542list had just one argument. That way you're safe from the shell
1543expanding wildcards or splitting up words with whitespace in them.
5a964f20
TC
1544
1545 @args = ( "echo surprise" );
1546
2b5ab1e7 1547 exec @args; # subject to shell escapes
f86cebdf 1548 # if @args == 1
2b5ab1e7 1549 exec { $args[0] } @args; # safe even with one-arg list
5a964f20
TC
1550
1551The first version, the one without the indirect object, ran the I<echo>
1552program, passing it C<"surprise"> an argument. The second version
1553didn't--it tried to run a program literally called I<"echo surprise">,
1554didn't find it, and set C<$?> to a non-zero value indicating failure.
1555
0f897271
GS
1556Beginning with v5.6.0, Perl will attempt to flush all files opened for
1557output before the exec, but this may not be supported on some platforms
1558(see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1559in English) or call the C<autoflush()> method of C<IO::Handle> on any
1560open handles in order to avoid lost output.
1561
19799a22 1562Note that C<exec> will not call your C<END> blocks, nor will it call
7660c0ab
A
1563any C<DESTROY> methods in your objects.
1564
a0d0e21e
LW
1565=item exists EXPR
1566
01020589 1567Given an expression that specifies a hash element or array element,
8ea97a1e
GS
1568returns true if the specified element in the hash or array has ever
1569been initialized, even if the corresponding value is undefined. The
1570element is not autovivified if it doesn't exist.
a0d0e21e 1571
01020589
GS
1572 print "Exists\n" if exists $hash{$key};
1573 print "Defined\n" if defined $hash{$key};
1574 print "True\n" if $hash{$key};
1575
1576 print "Exists\n" if exists $array[$index];
1577 print "Defined\n" if defined $array[$index];
1578 print "True\n" if $array[$index];
a0d0e21e 1579
8ea97a1e 1580A hash or array element can be true only if it's defined, and defined if
a0d0e21e
LW
1581it exists, but the reverse doesn't necessarily hold true.
1582
afebc493
GS
1583Given an expression that specifies the name of a subroutine,
1584returns true if the specified subroutine has ever been declared, even
1585if it is undefined. Mentioning a subroutine name for exists or defined
847c7ebe
DD
1586does not count as declaring it. Note that a subroutine which does not
1587exist may still be callable: its package may have an C<AUTOLOAD>
1588method that makes it spring into existence the first time that it is
1589called -- see L<perlsub>.
afebc493
GS
1590
1591 print "Exists\n" if exists &subroutine;
1592 print "Defined\n" if defined &subroutine;
1593
a0d0e21e 1594Note that the EXPR can be arbitrarily complicated as long as the final
afebc493 1595operation is a hash or array key lookup or subroutine name:
a0d0e21e 1596
2b5ab1e7
TC
1597 if (exists $ref->{A}->{B}->{$key}) { }
1598 if (exists $hash{A}{B}{$key}) { }
1599
01020589
GS
1600 if (exists $ref->{A}->{B}->[$ix]) { }
1601 if (exists $hash{A}{B}[$ix]) { }
1602
afebc493
GS
1603 if (exists &{$ref->{A}{B}{$key}}) { }
1604
01020589
GS
1605Although the deepest nested array or hash will not spring into existence
1606just because its existence was tested, any intervening ones will.
61eff3bc 1607Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
01020589
GS
1608into existence due to the existence test for the $key element above.
1609This happens anywhere the arrow operator is used, including even:
5a964f20 1610
2b5ab1e7
TC
1611 undef $ref;
1612 if (exists $ref->{"Some key"}) { }
1613 print $ref; # prints HASH(0x80d3d5c)
1614
1615This surprising autovivification in what does not at first--or even
1616second--glance appear to be an lvalue context may be fixed in a future
5a964f20 1617release.
a0d0e21e 1618
afebc493
GS
1619Use of a subroutine call, rather than a subroutine name, as an argument
1620to exists() is an error.
1621
1622 exists &sub; # OK
1623 exists &sub(); # Error
1624
a0d0e21e
LW
1625=item exit EXPR
1626
2b5ab1e7 1627Evaluates EXPR and exits immediately with that value. Example:
a0d0e21e
LW
1628
1629 $ans = <STDIN>;
1630 exit 0 if $ans =~ /^[Xx]/;
1631
19799a22 1632See also C<die>. If EXPR is omitted, exits with C<0> status. The only
2b5ab1e7
TC
1633universally recognized values for EXPR are C<0> for success and C<1>
1634for error; other values are subject to interpretation depending on the
1635environment in which the Perl program is running. For example, exiting
163669 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1637the mailer to return the item undelivered, but that's not true everywhere.
a0d0e21e 1638
19799a22
GS
1639Don't use C<exit> to abort a subroutine if there's any chance that
1640someone might want to trap whatever error happened. Use C<die> instead,
1641which can be trapped by an C<eval>.
28757baa 1642
19799a22 1643The exit() function does not always exit immediately. It calls any
2b5ab1e7 1644defined C<END> routines first, but these C<END> routines may not
19799a22 1645themselves abort the exit. Likewise any object destructors that need to
2b5ab1e7
TC
1646be called are called before the real exit. If this is a problem, you
1647can call C<POSIX:_exit($status)> to avoid END and destructor processing.
87275199 1648See L<perlmod> for details.
5a964f20 1649
a0d0e21e
LW
1650=item exp EXPR
1651
54310121 1652=item exp
bbce6d69 1653
b76cc8ba 1654Returns I<e> (the natural logarithm base) to the power of EXPR.
a0d0e21e
LW
1655If EXPR is omitted, gives C<exp($_)>.
1656
1657=item fcntl FILEHANDLE,FUNCTION,SCALAR
1658
f86cebdf 1659Implements the fcntl(2) function. You'll probably have to say
a0d0e21e
LW
1660
1661 use Fcntl;
1662
0ade1984 1663first to get the correct constant definitions. Argument processing and
b76cc8ba 1664value return works just like C<ioctl> below.
a0d0e21e
LW
1665For example:
1666
1667 use Fcntl;
5a964f20
TC
1668 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1669 or die "can't fcntl F_GETFL: $!";
1670
554ad1fc 1671You don't have to check for C<defined> on the return from C<fcntl>.
951ba7fe
GS
1672Like C<ioctl>, it maps a C<0> return from the system call into
1673C<"0 but true"> in Perl. This string is true in boolean context and C<0>
2b5ab1e7
TC
1674in numeric context. It is also exempt from the normal B<-w> warnings
1675on improper numeric conversions.
5a964f20 1676
19799a22 1677Note that C<fcntl> will produce a fatal error if used on a machine that
2b5ab1e7
TC
1678doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1679manpage to learn what functions are available on your system.
a0d0e21e 1680
be2f7487 1681Here's an example of setting a filehandle named C<REMOTE> to be
1682non-blocking at the system level. You'll have to negotiate C<$|>
1683on your own, though.
1684
1685 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
1686
1687 $flags = fcntl(REMOTE, F_GETFL, 0)
1688 or die "Can't get flags for the socket: $!\n";
1689
1690 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
1691 or die "Can't set flags for the socket: $!\n";
1692
a0d0e21e
LW
1693=item fileno FILEHANDLE
1694
2b5ab1e7
TC
1695Returns the file descriptor for a filehandle, or undefined if the
1696filehandle is not open. This is mainly useful for constructing
19799a22 1697bitmaps for C<select> and low-level POSIX tty-handling operations.
2b5ab1e7
TC
1698If FILEHANDLE is an expression, the value is taken as an indirect
1699filehandle, generally its name.
5a964f20 1700
b76cc8ba 1701You can use this to find out whether two handles refer to the
5a964f20
TC
1702same underlying descriptor:
1703
1704 if (fileno(THIS) == fileno(THAT)) {
1705 print "THIS and THAT are dups\n";
b76cc8ba
NIS
1706 }
1707
1708(Filehandles connected to memory objects via new features of C<open> may
1709return undefined even though they are open.)
1710
a0d0e21e
LW
1711
1712=item flock FILEHANDLE,OPERATION
1713
19799a22
GS
1714Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1715for success, false on failure. Produces a fatal error if used on a
2b5ab1e7 1716machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
19799a22 1717C<flock> is Perl's portable file locking interface, although it locks
2b5ab1e7
TC
1718only entire files, not records.
1719
1720Two potentially non-obvious but traditional C<flock> semantics are
1721that it waits indefinitely until the lock is granted, and that its locks
1722B<merely advisory>. Such discretionary locks are more flexible, but offer
19799a22
GS
1723fewer guarantees. This means that files locked with C<flock> may be
1724modified by programs that do not also use C<flock>. See L<perlport>,
2b5ab1e7
TC
1725your port's specific documentation, or your system-specific local manpages
1726for details. It's best to assume traditional behavior if you're writing
1727portable programs. (But if you're not, you should as always feel perfectly
1728free to write for your own system's idiosyncrasies (sometimes called
1729"features"). Slavish adherence to portability concerns shouldn't get
1730in the way of your getting your job done.)
a3cb178b 1731
8ebc5c01
PP
1732OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1733LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
ea3105be 1734you can use the symbolic names if you import them from the Fcntl module,
68dc0745
PP
1735either individually, or as a group using the ':flock' tag. LOCK_SH
1736requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
ea3105be
GS
1737releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1738LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
68dc0745
PP
1739waiting for the lock (check the return status to see if you got it).
1740
2b5ab1e7
TC
1741To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1742before locking or unlocking it.
8ebc5c01 1743
f86cebdf 1744Note that the emulation built with lockf(3) doesn't provide shared
8ebc5c01 1745locks, and it requires that FILEHANDLE be open with write intent. These
2b5ab1e7 1746are the semantics that lockf(3) implements. Most if not all systems
f86cebdf 1747implement lockf(3) in terms of fcntl(2) locking, though, so the
8ebc5c01
PP
1748differing semantics shouldn't bite too many people.
1749
becacb53
TM
1750Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
1751be open with read intent to use LOCK_SH and requires that it be open
1752with write intent to use LOCK_EX.
1753
19799a22
GS
1754Note also that some versions of C<flock> cannot lock things over the
1755network; you would need to use the more system-specific C<fcntl> for
f86cebdf
GS
1756that. If you like you can force Perl to ignore your system's flock(2)
1757function, and so provide its own fcntl(2)-based emulation, by passing
8ebc5c01
PP
1758the switch C<-Ud_flock> to the F<Configure> program when you configure
1759perl.
4633a7c4
LW
1760
1761Here's a mailbox appender for BSD systems.
a0d0e21e 1762
7e1af8bc 1763 use Fcntl ':flock'; # import LOCK_* constants
a0d0e21e
LW
1764
1765 sub lock {
7e1af8bc 1766 flock(MBOX,LOCK_EX);
a0d0e21e
LW
1767 # and, in case someone appended
1768 # while we were waiting...
1769 seek(MBOX, 0, 2);
1770 }
1771
1772 sub unlock {
7e1af8bc 1773 flock(MBOX,LOCK_UN);
a0d0e21e
LW
1774 }
1775
1776 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1777 or die "Can't open mailbox: $!";
1778
1779 lock();
1780 print MBOX $msg,"\n\n";
1781 unlock();
1782
2b5ab1e7
TC
1783On systems that support a real flock(), locks are inherited across fork()
1784calls, whereas those that must resort to the more capricious fcntl()
1785function lose the locks, making it harder to write servers.
1786
cb1a09d0 1787See also L<DB_File> for other flock() examples.
a0d0e21e
LW
1788
1789=item fork
1790
2b5ab1e7
TC
1791Does a fork(2) system call to create a new process running the
1792same program at the same point. It returns the child pid to the
1793parent process, C<0> to the child process, or C<undef> if the fork is
1794unsuccessful. File descriptors (and sometimes locks on those descriptors)
1795are shared, while everything else is copied. On most systems supporting
1796fork(), great care has gone into making it extremely efficient (for
1797example, using copy-on-write technology on data pages), making it the
1798dominant paradigm for multitasking over the last few decades.
5a964f20 1799
0f897271
GS
1800Beginning with v5.6.0, Perl will attempt to flush all files opened for
1801output before forking the child process, but this may not be supported
1802on some platforms (see L<perlport>). To be safe, you may need to set
1803C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1804C<IO::Handle> on any open handles in order to avoid duplicate output.
a0d0e21e 1805
19799a22 1806If you C<fork> without ever waiting on your children, you will
2b5ab1e7
TC
1807accumulate zombies. On some systems, you can avoid this by setting
1808C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1809forking and reaping moribund children.
cb1a09d0 1810
28757baa
PP
1811Note that if your forked child inherits system file descriptors like
1812STDIN and STDOUT that are actually connected by a pipe or socket, even
2b5ab1e7 1813if you exit, then the remote server (such as, say, a CGI script or a
19799a22 1814backgrounded job launched from a remote shell) won't think you're done.
2b5ab1e7 1815You should reopen those to F</dev/null> if it's any issue.
28757baa 1816
cb1a09d0
AD
1817=item format
1818
19799a22 1819Declare a picture format for use by the C<write> function. For
cb1a09d0
AD
1820example:
1821
54310121 1822 format Something =
cb1a09d0
AD
1823 Test: @<<<<<<<< @||||| @>>>>>
1824 $str, $%, '$' . int($num)
1825 .
1826
1827 $str = "widget";
184e9718 1828 $num = $cost/$quantity;
cb1a09d0
AD
1829 $~ = 'Something';
1830 write;
1831
1832See L<perlform> for many details and examples.
1833
8903cb82 1834=item formline PICTURE,LIST
a0d0e21e 1835
5a964f20 1836This is an internal function used by C<format>s, though you may call it,
a0d0e21e
LW
1837too. It formats (see L<perlform>) a list of values according to the
1838contents of PICTURE, placing the output into the format output
7660c0ab 1839accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
19799a22 1840Eventually, when a C<write> is done, the contents of
a0d0e21e 1841C<$^A> are written to some filehandle, but you could also read C<$^A>
7660c0ab 1842yourself and then set C<$^A> back to C<"">. Note that a format typically
19799a22 1843does one C<formline> per line of form, but the C<formline> function itself
748a9306 1844doesn't care how many newlines are embedded in the PICTURE. This means
4633a7c4 1845that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
748a9306
LW
1846You may therefore need to use multiple formlines to implement a single
1847record format, just like the format compiler.
1848
19799a22 1849Be careful if you put double quotes around the picture, because an C<@>
748a9306 1850character may be taken to mean the beginning of an array name.
19799a22 1851C<formline> always returns true. See L<perlform> for other examples.
a0d0e21e
LW
1852
1853=item getc FILEHANDLE
1854
1855=item getc
1856
1857Returns the next character from the input file attached to FILEHANDLE,
b5fe5ca2
SR
1858or the undefined value at end of file, or if there was an error (in
1859the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
1860STDIN. This is not particularly efficient. However, it cannot be
1861used by itself to fetch single characters without waiting for the user
1862to hit enter. For that, try something more like:
4633a7c4
LW
1863
1864 if ($BSD_STYLE) {
1865 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1866 }
1867 else {
54310121 1868 system "stty", '-icanon', 'eol', "\001";
4633a7c4
LW
1869 }
1870
1871 $key = getc(STDIN);
1872
1873 if ($BSD_STYLE) {
1874 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1875 }
1876 else {
5f05dabc 1877 system "stty", 'icanon', 'eol', '^@'; # ASCII null
4633a7c4
LW
1878 }
1879 print "\n";
1880
54310121
PP
1881Determination of whether $BSD_STYLE should be set
1882is left as an exercise to the reader.
cb1a09d0 1883
19799a22 1884The C<POSIX::getattr> function can do this more portably on
2b5ab1e7
TC
1885systems purporting POSIX compliance. See also the C<Term::ReadKey>
1886module from your nearest CPAN site; details on CPAN can be found on
1887L<perlmodlib/CPAN>.
a0d0e21e
LW
1888
1889=item getlogin
1890
5a964f20
TC
1891Implements the C library function of the same name, which on most
1892systems returns the current login from F</etc/utmp>, if any. If null,
19799a22 1893use C<getpwuid>.
a0d0e21e 1894
f86702cc 1895 $login = getlogin || getpwuid($<) || "Kilroy";
a0d0e21e 1896
19799a22
GS
1897Do not consider C<getlogin> for authentication: it is not as
1898secure as C<getpwuid>.
4633a7c4 1899
a0d0e21e
LW
1900=item getpeername SOCKET
1901
1902Returns the packed sockaddr address of other end of the SOCKET connection.
1903
4633a7c4
LW
1904 use Socket;
1905 $hersockaddr = getpeername(SOCK);
19799a22 1906 ($port, $iaddr) = sockaddr_in($hersockaddr);
4633a7c4
LW
1907 $herhostname = gethostbyaddr($iaddr, AF_INET);
1908 $herstraddr = inet_ntoa($iaddr);
a0d0e21e
LW
1909
1910=item getpgrp PID
1911
47e29363 1912Returns the current process group for the specified PID. Use
7660c0ab 1913a PID of C<0> to get the current process group for the
4633a7c4 1914current process. Will raise an exception if used on a machine that
f86cebdf 1915doesn't implement getpgrp(2). If PID is omitted, returns process
19799a22 1916group of current process. Note that the POSIX version of C<getpgrp>
7660c0ab 1917does not accept a PID argument, so only C<PID==0> is truly portable.
a0d0e21e
LW
1918
1919=item getppid
1920
1921Returns the process id of the parent process.
1922
4d76a344
RGS
1923Note for Linux users: on Linux, the C functions C<getpid()> and
1924C<getppid()> return different values from different threads. In order to
1925be portable, this behavior is not reflected by the perl-level function
1926C<getppid()>, that returns a consistent value across threads. If you want
e3256f86
RGS
1927to call the underlying C<getppid()>, you may use the CPAN module
1928C<Linux::Pid>.
4d76a344 1929
a0d0e21e
LW
1930=item getpriority WHICH,WHO
1931
4633a7c4
LW
1932Returns the current priority for a process, a process group, or a user.
1933(See L<getpriority(2)>.) Will raise a fatal exception if used on a
f86cebdf 1934machine that doesn't implement getpriority(2).
a0d0e21e
LW
1935
1936=item getpwnam NAME
1937
1938=item getgrnam NAME
1939
1940=item gethostbyname NAME
1941
1942=item getnetbyname NAME
1943
1944=item getprotobyname NAME
1945
1946=item getpwuid UID
1947
1948=item getgrgid GID
1949
1950=item getservbyname NAME,PROTO
1951
1952=item gethostbyaddr ADDR,ADDRTYPE
1953
1954=item getnetbyaddr ADDR,ADDRTYPE
1955
1956=item getprotobynumber NUMBER
1957
1958=item getservbyport PORT,PROTO
1959
1960=item getpwent
1961
1962=item getgrent
1963
1964=item gethostent
1965
1966=item getnetent
1967
1968=item getprotoent
1969
1970=item getservent
1971
1972=item setpwent
1973
1974=item setgrent
1975
1976=item sethostent STAYOPEN
1977
1978=item setnetent STAYOPEN
1979
1980=item setprotoent STAYOPEN
1981
1982=item setservent STAYOPEN
1983
1984=item endpwent
1985
1986=item endgrent
1987
1988=item endhostent
1989
1990=item endnetent
1991
1992=item endprotoent
1993
1994=item endservent
1995
1996These routines perform the same functions as their counterparts in the
5a964f20 1997system library. In list context, the return values from the
a0d0e21e
LW
1998various get routines are as follows:
1999
2000 ($name,$passwd,$uid,$gid,
6ee623d5 2001 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
a0d0e21e
LW
2002 ($name,$passwd,$gid,$members) = getgr*
2003 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
2004 ($name,$aliases,$addrtype,$net) = getnet*
2005 ($name,$aliases,$proto) = getproto*
2006 ($name,$aliases,$port,$proto) = getserv*
2007
2008(If the entry doesn't exist you get a null list.)
2009
4602f195
JH
2010The exact meaning of the $gcos field varies but it usually contains
2011the real name of the user (as opposed to the login name) and other
2012information pertaining to the user. Beware, however, that in many
2013system users are able to change this information and therefore it
106325ad 2014cannot be trusted and therefore the $gcos is tainted (see
2959b6e3
JH
2015L<perlsec>). The $passwd and $shell, user's encrypted password and
2016login shell, are also tainted, because of the same reason.
4602f195 2017
5a964f20 2018In scalar context, you get the name, unless the function was a
a0d0e21e
LW
2019lookup by name, in which case you get the other thing, whatever it is.
2020(If the entry doesn't exist you get the undefined value.) For example:
2021
5a964f20
TC
2022 $uid = getpwnam($name);
2023 $name = getpwuid($num);
2024 $name = getpwent();
2025 $gid = getgrnam($name);
08a33e13 2026 $name = getgrgid($num);
5a964f20
TC
2027 $name = getgrent();
2028 #etc.
a0d0e21e 2029
4602f195
JH
2030In I<getpw*()> the fields $quota, $comment, and $expire are special
2031cases in the sense that in many systems they are unsupported. If the
2032$quota is unsupported, it is an empty scalar. If it is supported, it
2033usually encodes the disk quota. If the $comment field is unsupported,
2034it is an empty scalar. If it is supported it usually encodes some
2035administrative comment about the user. In some systems the $quota
2036field may be $change or $age, fields that have to do with password
2037aging. In some systems the $comment field may be $class. The $expire
2038field, if present, encodes the expiration period of the account or the
2039password. For the availability and the exact meaning of these fields
2040in your system, please consult your getpwnam(3) documentation and your
2041F<pwd.h> file. You can also find out from within Perl what your
2042$quota and $comment fields mean and whether you have the $expire field
2043by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2044C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
2045files are only supported if your vendor has implemented them in the
2046intuitive fashion that calling the regular C library routines gets the
5d3a0a3b
GS
2047shadow versions if you're running under privilege or if there exists
2048the shadow(3) functions as found in System V ( this includes Solaris
2049and Linux.) Those systems which implement a proprietary shadow password
2050facility are unlikely to be supported.
6ee623d5 2051
19799a22 2052The $members value returned by I<getgr*()> is a space separated list of
a0d0e21e
LW
2053the login names of the members of the group.
2054
2055For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2056C, it will be returned to you via C<$?> if the function call fails. The
7660c0ab 2057C<@addrs> value returned by a successful call is a list of the raw
a0d0e21e
LW
2058addresses returned by the corresponding system library call. In the
2059Internet domain, each address is four bytes long and you can unpack it
2060by saying something like:
2061
f337b084 2062 ($a,$b,$c,$d) = unpack('W4',$addr[0]);
a0d0e21e 2063
2b5ab1e7
TC
2064The Socket library makes this slightly easier:
2065
2066 use Socket;
2067 $iaddr = inet_aton("127.1"); # or whatever address
2068 $name = gethostbyaddr($iaddr, AF_INET);
2069
2070 # or going the other way
19799a22 2071 $straddr = inet_ntoa($iaddr);
2b5ab1e7 2072
19799a22
GS
2073If you get tired of remembering which element of the return list
2074contains which return value, by-name interfaces are provided
2075in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2076C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2077and C<User::grent>. These override the normal built-ins, supplying
2078versions that return objects with the appropriate names
2079for each field. For example:
5a964f20
TC
2080
2081 use File::stat;
2082 use User::pwent;
2083 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2084
b76cc8ba
NIS
2085Even though it looks like they're the same method calls (uid),
2086they aren't, because a C<File::stat> object is different from
19799a22 2087a C<User::pwent> object.
5a964f20 2088
a0d0e21e
LW
2089=item getsockname SOCKET
2090
19799a22
GS
2091Returns the packed sockaddr address of this end of the SOCKET connection,
2092in case you don't know the address because you have several different
2093IPs that the connection might have come in on.
a0d0e21e 2094
4633a7c4
LW
2095 use Socket;
2096 $mysockaddr = getsockname(SOCK);
19799a22 2097 ($port, $myaddr) = sockaddr_in($mysockaddr);
b76cc8ba 2098 printf "Connect to %s [%s]\n",
19799a22
GS
2099 scalar gethostbyaddr($myaddr, AF_INET),
2100 inet_ntoa($myaddr);
a0d0e21e
LW
2101
2102=item getsockopt SOCKET,LEVEL,OPTNAME
2103
636e6b1f
TH
2104Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
2105Options may exist at multiple protocol levels depending on the socket
2106type, but at least the uppermost socket level SOL_SOCKET (defined in the
2107C<Socket> module) will exist. To query options at another level the
2108protocol number of the appropriate protocol controlling the option
2109should be supplied. For example, to indicate that an option is to be
2110interpreted by the TCP protocol, LEVEL should be set to the protocol
2111number of TCP, which you can get using getprotobyname.
2112
2113The call returns a packed string representing the requested socket option,
2114or C<undef> if there is an error (the error reason will be in $!). What
2115exactly is in the packed string depends in the LEVEL and OPTNAME, consult
2116your system documentation for details. A very common case however is that
2117the option is an integer, in which case the result will be an packed
2118integer which you can decode using unpack with the C<i> (or C<I>) format.
2119
2120An example testing if Nagle's algorithm is turned on on a socket:
2121
4852725b 2122 use Socket qw(:all);
636e6b1f
TH
2123
2124 defined(my $tcp = getprotobyname("tcp"))
2125 or die "Could not determine the protocol number for tcp";
4852725b
DD
2126 # my $tcp = IPPROTO_TCP; # Alternative
2127 my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
2128 or die "Could not query TCP_NODELAY socket option: $!";
636e6b1f
TH
2129 my $nodelay = unpack("I", $packed);
2130 print "Nagle's algorithm is turned ", $nodelay ? "off\n" : "on\n";
2131
a0d0e21e
LW
2132
2133=item glob EXPR
2134
0a753a76
PP
2135=item glob
2136
d9a9d457
JL
2137In list context, returns a (possibly empty) list of filename expansions on
2138the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
2139scalar context, glob iterates through such filename expansions, returning
2140undef when the list is exhausted. This is the internal function
2141implementing the C<< <*.c> >> operator, but you can use it directly. If
2142EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2143more detail in L<perlop/"I/O Operators">.
a0d0e21e 2144
3a4b19e4
GS
2145Beginning with v5.6.0, this operator is implemented using the standard
2146C<File::Glob> extension. See L<File::Glob> for details.
2147
a0d0e21e
LW
2148=item gmtime EXPR
2149
d1be9408 2150Converts a time as returned by the time function to an 8-element list
54310121 2151with the time localized for the standard Greenwich time zone.
4633a7c4 2152Typically used as follows:
a0d0e21e 2153
b76cc8ba 2154 # 0 1 2 3 4 5 6 7
48a26b3a 2155 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday) =
a0d0e21e
LW
2156 gmtime(time);
2157
48a26b3a
GS
2158All list elements are numeric, and come straight out of the C `struct
2159tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2160specified time. $mday is the day of the month, and $mon is the month
2161itself, in the range C<0..11> with 0 indicating January and 11
2162indicating December. $year is the number of years since 1900. That
2163is, $year is C<123> in year 2023. $wday is the day of the week, with
21640 indicating Sunday and 3 indicating Wednesday. $yday is the day of
b76cc8ba 2165the year, in the range C<0..364> (or C<0..365> in leap years.)
48a26b3a
GS
2166
2167Note that the $year element is I<not> simply the last two digits of
2168the year. If you assume it is, then you create non-Y2K-compliant
2169programs--and you wouldn't want to do that, would you?
2f9daede 2170
abd75f24
GS
2171The proper way to get a complete 4-digit year is simply:
2172
2173 $year += 1900;
2174
2175And to get the last two digits of the year (e.g., '01' in 2001) do:
2176
2177 $year = sprintf("%02d", $year % 100);
2178
48a26b3a 2179If EXPR is omitted, C<gmtime()> uses the current time (C<gmtime(time)>).
a0d0e21e 2180
48a26b3a 2181In scalar context, C<gmtime()> returns the ctime(3) value:
0a753a76
PP
2182
2183 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
2184
fe86afc2
NC
2185If you need local time instead of GMT use the L</localtime> builtin.
2186See also the C<timegm> function provided by the C<Time::Local> module,
2187and the strftime(3) and mktime(3) functions available via the L<POSIX> module.
7660c0ab 2188
fe86afc2
NC
2189This scalar value is B<not> locale dependent (see L<perllocale>), but is
2190instead a Perl builtin. To get somewhat similar but locale dependent date
2191strings, see the example in L</localtime>.
0a753a76 2192
62aa5637
MS
2193See L<perlport/gmtime> for portability concerns.
2194
a0d0e21e
LW
2195=item goto LABEL
2196
748a9306
LW
2197=item goto EXPR
2198
a0d0e21e
LW
2199=item goto &NAME
2200
7660c0ab 2201The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
a0d0e21e 2202execution there. It may not be used to go into any construct that
7660c0ab 2203requires initialization, such as a subroutine or a C<foreach> loop. It
0a753a76 2204also can't be used to go into a construct that is optimized away,
19799a22 2205or to get out of a block or subroutine given to C<sort>.
0a753a76 2206It can be used to go almost anywhere else within the dynamic scope,
a0d0e21e 2207including out of subroutines, but it's usually better to use some other
19799a22 2208construct such as C<last> or C<die>. The author of Perl has never felt the
7660c0ab 2209need to use this form of C<goto> (in Perl, that is--C is another matter).
1b6921cb
BT
2210(The difference being that C does not offer named loops combined with
2211loop control. Perl does, and this replaces most structured uses of C<goto>
2212in other languages.)
a0d0e21e 2213
7660c0ab
A
2214The C<goto-EXPR> form expects a label name, whose scope will be resolved
2215dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
748a9306
LW
2216necessarily recommended if you're optimizing for maintainability:
2217
2218 goto ("FOO", "BAR", "GLARCH")[$i];
2219
1b6921cb
BT
2220The C<goto-&NAME> form is quite different from the other forms of
2221C<goto>. In fact, it isn't a goto in the normal sense at all, and
2222doesn't have the stigma associated with other gotos. Instead, it
2223exits the current subroutine (losing any changes set by local()) and
2224immediately calls in its place the named subroutine using the current
2225value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2226load another subroutine and then pretend that the other subroutine had
2227been called in the first place (except that any modifications to C<@_>
6cb9131c
GS
2228in the current subroutine are propagated to the other subroutine.)
2229After the C<goto>, not even C<caller> will be able to tell that this
2230routine was called first.
2231
2232NAME needn't be the name of a subroutine; it can be a scalar variable
2233containing a code reference, or a block which evaluates to a code
2234reference.
a0d0e21e
LW
2235
2236=item grep BLOCK LIST
2237
2238=item grep EXPR,LIST
2239
2b5ab1e7
TC
2240This is similar in spirit to, but not the same as, grep(1) and its
2241relatives. In particular, it is not limited to using regular expressions.
2f9daede 2242
a0d0e21e 2243Evaluates the BLOCK or EXPR for each element of LIST (locally setting
7660c0ab 2244C<$_> to each element) and returns the list value consisting of those
19799a22
GS
2245elements for which the expression evaluated to true. In scalar
2246context, returns the number of times the expression was true.
a0d0e21e
LW
2247
2248 @foo = grep(!/^#/, @bar); # weed out comments
2249
2250or equivalently,
2251
2252 @foo = grep {!/^#/} @bar; # weed out comments
2253
be3174d2
GS
2254Note that C<$_> is an alias to the list value, so it can be used to
2255modify the elements of the LIST. While this is useful and supported,
2256it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
2257Similarly, grep returns aliases into the original list, much as a for
2258loop's index variable aliases the list elements. That is, modifying an
19799a22
GS
2259element of a list returned by grep (for example, in a C<foreach>, C<map>
2260or another C<grep>) actually modifies the element in the original list.
2b5ab1e7 2261This is usually something to be avoided when writing clear code.
a0d0e21e 2262
a4fb8298
RGS
2263If C<$_> is lexical in the scope where the C<grep> appears (because it has
2264been declared with C<my $_>) then, in addition the be locally aliased to
2265the list elements, C<$_> keeps being lexical inside the block; i.e. it
2266can't be seen from the outside, avoiding any potential side-effects.
2267
19799a22 2268See also L</map> for a list composed of the results of the BLOCK or EXPR.
38325410 2269
a0d0e21e
LW
2270=item hex EXPR
2271
54310121 2272=item hex
bbce6d69 2273
2b5ab1e7 2274Interprets EXPR as a hex string and returns the corresponding value.
38366c11 2275(To convert strings that might start with either C<0>, C<0x>, or C<0b>, see
2b5ab1e7 2276L</oct>.) If EXPR is omitted, uses C<$_>.
2f9daede
TPG
2277
2278 print hex '0xAf'; # prints '175'
2279 print hex 'aF'; # same
a0d0e21e 2280
19799a22 2281Hex strings may only represent integers. Strings that would cause
53305cf1 2282integer overflow trigger a warning. Leading whitespace is not stripped,
38366c11
DN
2283unlike oct(). To present something as hex, look into L</printf>,
2284L</sprintf>, or L</unpack>.
19799a22 2285
a0d0e21e
LW
2286=item import
2287
19799a22 2288There is no builtin C<import> function. It is just an ordinary
4633a7c4 2289method (subroutine) defined (or inherited) by modules that wish to export
19799a22 2290names to another module. The C<use> function calls the C<import> method
cea6626f 2291for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
a0d0e21e
LW
2292
2293=item index STR,SUBSTR,POSITION
2294
2295=item index STR,SUBSTR
2296
2b5ab1e7
TC
2297The index function searches for one string within another, but without
2298the wildcard-like behavior of a full regular-expression pattern match.
2299It returns the position of the first occurrence of SUBSTR in STR at
2300or after POSITION. If POSITION is omitted, starts searching from the
2301beginning of the string. The return value is based at C<0> (or whatever
2302you've set the C<$[> variable to--but don't do that). If the substring
2303is not found, returns one less than the base, ordinarily C<-1>.
a0d0e21e
LW
2304
2305=item int EXPR
2306
54310121 2307=item int
bbce6d69 2308
7660c0ab 2309Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2b5ab1e7
TC
2310You should not use this function for rounding: one because it truncates
2311towards C<0>, and two because machine representations of floating point
2312numbers can sometimes produce counterintuitive results. For example,
2313C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2314because it's really more like -268.99999999999994315658 instead. Usually,
19799a22 2315the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2b5ab1e7 2316functions will serve you better than will int().
a0d0e21e
LW
2317
2318=item ioctl FILEHANDLE,FUNCTION,SCALAR
2319
2b5ab1e7 2320Implements the ioctl(2) function. You'll probably first have to say
a0d0e21e 2321
a11c483f 2322 require "sys/ioctl.ph"; # probably in $Config{archlib}/ioctl.ph
a0d0e21e 2323
a11c483f 2324to get the correct function definitions. If F<sys/ioctl.ph> doesn't
a0d0e21e 2325exist or doesn't have the correct definitions you'll have to roll your
61eff3bc 2326own, based on your C header files such as F<< <sys/ioctl.h> >>.
5a964f20 2327(There is a Perl script called B<h2ph> that comes with the Perl kit that
54310121 2328may help you in this, but it's nontrivial.) SCALAR will be read and/or
4633a7c4 2329written depending on the FUNCTION--a pointer to the string value of SCALAR
19799a22 2330will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
4633a7c4
LW
2331has no string value but does have a numeric value, that value will be
2332passed rather than a pointer to the string value. To guarantee this to be
19799a22
GS
2333true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2334functions may be needed to manipulate the values of structures used by
b76cc8ba 2335C<ioctl>.
a0d0e21e 2336
19799a22 2337The return value of C<ioctl> (and C<fcntl>) is as follows:
a0d0e21e
LW
2338
2339 if OS returns: then Perl returns:
2340 -1 undefined value
2341 0 string "0 but true"
2342 anything else that number
2343
19799a22 2344Thus Perl returns true on success and false on failure, yet you can
a0d0e21e
LW
2345still easily determine the actual value returned by the operating
2346system:
2347
2b5ab1e7 2348 $retval = ioctl(...) || -1;
a0d0e21e
LW
2349 printf "System returned %d\n", $retval;
2350
be2f7487 2351The special string C<"0 but true"> is exempt from B<-w> complaints
5a964f20
TC
2352about improper numeric conversions.
2353
a0d0e21e
LW
2354=item join EXPR,LIST
2355
2b5ab1e7
TC
2356Joins the separate strings of LIST into a single string with fields
2357separated by the value of EXPR, and returns that new string. Example:
a0d0e21e 2358
2b5ab1e7 2359 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
a0d0e21e 2360
eb6e2d6f
GS
2361Beware that unlike C<split>, C<join> doesn't take a pattern as its
2362first argument. Compare L</split>.
a0d0e21e 2363
aa689395
PP
2364=item keys HASH
2365
504f80c1
JH
2366Returns a list consisting of all the keys of the named hash.
2367(In scalar context, returns the number of keys.)
2368
2369The keys are returned in an apparently random order. The actual
2370random order is subject to change in future versions of perl, but it
2371is guaranteed to be the same order as either the C<values> or C<each>
4546b9e6
JH
2372function produces (given that the hash has not been modified). Since
2373Perl 5.8.1 the ordering is different even between different runs of
2374Perl for security reasons (see L<perlsec/"Algorithmic Complexity
d6df3700 2375Attacks">).
504f80c1
JH
2376
2377As a side effect, calling keys() resets the HASH's internal iterator,
2f65b2f0
RGS
2378see L</each>. (In particular, calling keys() in void context resets
2379the iterator with no other overhead.)
a0d0e21e 2380
aa689395 2381Here is yet another way to print your environment:
a0d0e21e
LW
2382
2383 @keys = keys %ENV;
2384 @values = values %ENV;
b76cc8ba 2385 while (@keys) {
a0d0e21e
LW
2386 print pop(@keys), '=', pop(@values), "\n";
2387 }
2388
2389or how about sorted by key:
2390
2391 foreach $key (sort(keys %ENV)) {
2392 print $key, '=', $ENV{$key}, "\n";
2393 }
2394
8ea1e5d4
GS
2395The returned values are copies of the original keys in the hash, so
2396modifying them will not affect the original hash. Compare L</values>.
2397
19799a22 2398To sort a hash by value, you'll need to use a C<sort> function.
aa689395 2399Here's a descending numeric sort of a hash by its values:
4633a7c4 2400
5a964f20 2401 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
4633a7c4
LW
2402 printf "%4d %s\n", $hash{$key}, $key;
2403 }
2404
19799a22 2405As an lvalue C<keys> allows you to increase the number of hash buckets
aa689395
PP
2406allocated for the given hash. This can gain you a measure of efficiency if
2407you know the hash is going to get big. (This is similar to pre-extending
2408an array by assigning a larger number to $#array.) If you say
55497cff
PP
2409
2410 keys %hash = 200;
2411
ab192400
GS
2412then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2413in fact, since it rounds up to the next power of two. These
55497cff
PP
2414buckets will be retained even if you do C<%hash = ()>, use C<undef
2415%hash> if you want to free the storage while C<%hash> is still in scope.
2416You can't shrink the number of buckets allocated for the hash using
19799a22 2417C<keys> in this way (but you needn't worry about doing this by accident,
55497cff
PP
2418as trying has no effect).
2419
19799a22 2420See also C<each>, C<values> and C<sort>.
ab192400 2421
b350dd2f 2422=item kill SIGNAL, LIST
a0d0e21e 2423
b350dd2f 2424Sends a signal to a list of processes. Returns the number of
517db077
GS
2425processes successfully signaled (which is not necessarily the
2426same as the number actually killed).
a0d0e21e
LW
2427
2428 $cnt = kill 1, $child1, $child2;
2429 kill 9, @goners;
2430
b350dd2f 2431If SIGNAL is zero, no signal is sent to the process. This is a
1e9c1022 2432useful way to check that a child process is alive and hasn't changed
b350dd2f
GS
2433its UID. See L<perlport> for notes on the portability of this
2434construct.
2435
2436Unlike in the shell, if SIGNAL is negative, it kills
4633a7c4
LW
2437process groups instead of processes. (On System V, a negative I<PROCESS>
2438number will also kill process groups, but that's not portable.) That
2439means you usually want to use positive not negative signals. You may also
1e9c1022
JL
2440use a signal name in quotes.
2441
2442See L<perlipc/"Signals"> for more details.
a0d0e21e
LW
2443
2444=item last LABEL
2445
2446=item last
2447
2448The C<last> command is like the C<break> statement in C (as used in
2449loops); it immediately exits the loop in question. If the LABEL is
2450omitted, the command refers to the innermost enclosing loop. The
2451C<continue> block, if any, is not executed:
2452
4633a7c4
LW
2453 LINE: while (<STDIN>) {
2454 last LINE if /^$/; # exit when done with header
5a964f20 2455 #...
a0d0e21e
LW
2456 }
2457
4968c1e4 2458C<last> cannot be used to exit a block which returns a value such as
2b5ab1e7
TC
2459C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2460a grep() or map() operation.
4968c1e4 2461
6c1372ed
GS
2462Note that a block by itself is semantically identical to a loop
2463that executes once. Thus C<last> can be used to effect an early
2464exit out of such a block.
2465
98293880
JH
2466See also L</continue> for an illustration of how C<last>, C<next>, and
2467C<redo> work.
1d2dff63 2468
a0d0e21e
LW
2469=item lc EXPR
2470
54310121 2471=item lc
bbce6d69 2472
d1be9408 2473Returns a lowercased version of EXPR. This is the internal function
ad0029c4
JH
2474implementing the C<\L> escape in double-quoted strings. Respects
2475current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
983ffd37 2476and L<perlunicode> for more details about locale and Unicode support.
a0d0e21e 2477
7660c0ab 2478If EXPR is omitted, uses C<$_>.
bbce6d69 2479
a0d0e21e
LW
2480=item lcfirst EXPR
2481
54310121 2482=item lcfirst
bbce6d69 2483
ad0029c4
JH
2484Returns the value of EXPR with the first character lowercased. This
2485is the internal function implementing the C<\l> escape in
2486double-quoted strings. Respects current LC_CTYPE locale if C<use
983ffd37
JH
2487locale> in force. See L<perllocale> and L<perlunicode> for more
2488details about locale and Unicode support.
a0d0e21e 2489
7660c0ab 2490If EXPR is omitted, uses C<$_>.
bbce6d69 2491
a0d0e21e
LW
2492=item length EXPR
2493
54310121 2494=item length
bbce6d69 2495
974da8e5 2496Returns the length in I<characters> of the value of EXPR. If EXPR is
b76cc8ba 2497omitted, returns length of C<$_>. Note that this cannot be used on
2b5ab1e7
TC
2498an entire array or hash to find out how many elements these have.
2499For that, use C<scalar @array> and C<scalar keys %hash> respectively.
a0d0e21e 2500
974da8e5
JH
2501Note the I<characters>: if the EXPR is in Unicode, you will get the
2502number of characters, not the number of bytes. To get the length
2503in bytes, use C<do { use bytes; length(EXPR) }>, see L<bytes>.
2504
a0d0e21e
LW
2505=item link OLDFILE,NEWFILE
2506
19799a22 2507Creates a new filename linked to the old filename. Returns true for
b76cc8ba 2508success, false otherwise.
a0d0e21e
LW
2509
2510=item listen SOCKET,QUEUESIZE
2511
19799a22 2512Does the same thing that the listen system call does. Returns true if
b76cc8ba 2513it succeeded, false otherwise. See the example in
cea6626f 2514L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
2515
2516=item local EXPR
2517
19799a22 2518You really probably want to be using C<my> instead, because C<local> isn't
b76cc8ba 2519what most people think of as "local". See
13a2d996 2520L<perlsub/"Private Variables via my()"> for details.
2b5ab1e7 2521
5a964f20
TC
2522A local modifies the listed variables to be local to the enclosing
2523block, file, or eval. If more than one value is listed, the list must
2524be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2525for details, including issues with tied arrays and hashes.
a0d0e21e 2526
a0d0e21e
LW
2527=item localtime EXPR
2528
ba053783
AL
2529=item localtime
2530
19799a22 2531Converts a time as returned by the time function to a 9-element list
5f05dabc 2532with the time analyzed for the local time zone. Typically used as
a0d0e21e
LW
2533follows:
2534
54310121 2535 # 0 1 2 3 4 5 6 7 8
a0d0e21e 2536 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
ba053783 2537 localtime(time);
a0d0e21e 2538
48a26b3a 2539All list elements are numeric, and come straight out of the C `struct
ba053783
AL
2540tm'. C<$sec>, C<$min>, and C<$hour> are the seconds, minutes, and hours
2541of the specified time.
48a26b3a 2542
ba053783
AL
2543C<$mday> is the day of the month, and C<$mon> is the month itself, in
2544the range C<0..11> with 0 indicating January and 11 indicating December.
2545This makes it easy to get a month name from a list:
54310121 2546
ba053783
AL
2547 my @abbr = qw( Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec );
2548 print "$abbr[$mon] $mday";
2549 # $mon=9, $mday=18 gives "Oct 18"
abd75f24 2550
ba053783
AL
2551C<$year> is the number of years since 1900, not just the last two digits
2552of the year. That is, C<$year> is C<123> in year 2023. The proper way
2553to get a complete 4-digit year is simply:
abd75f24 2554
ba053783 2555 $year += 1900;
abd75f24 2556
ba053783
AL
2557To get the last two digits of the year (e.g., '01' in 2001) do:
2558
2559 $year = sprintf("%02d", $year % 100);
2560
2561C<$wday> is the day of the week, with 0 indicating Sunday and 3 indicating
2562Wednesday. C<$yday> is the day of the year, in the range C<0..364>
2563(or C<0..365> in leap years.)
2564
2565C<$isdst> is true if the specified time occurs during Daylight Saving
2566Time, false otherwise.
abd75f24 2567
48a26b3a 2568If EXPR is omitted, C<localtime()> uses the current time (C<localtime(time)>).
a0d0e21e 2569
48a26b3a 2570In scalar context, C<localtime()> returns the ctime(3) value:
a0d0e21e 2571
5f05dabc 2572 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
a0d0e21e 2573
fe86afc2
NC
2574This scalar value is B<not> locale dependent but is a Perl builtin. For GMT
2575instead of local time use the L</gmtime> builtin. See also the
2576C<Time::Local> module (to convert the second, minutes, hours, ... back to
2577the integer value returned by time()), and the L<POSIX> module's strftime(3)
2578and mktime(3) functions.
2579
2580To get somewhat similar but locale dependent date strings, set up your
2581locale environment variables appropriately (please see L<perllocale>) and
2582try for example:
a3cb178b 2583
5a964f20 2584 use POSIX qw(strftime);
2b5ab1e7 2585 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
fe86afc2
NC
2586 # or for GMT formatted appropriately for your locale:
2587 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
a3cb178b
GS
2588
2589Note that the C<%a> and C<%b>, the short forms of the day of the week
2590and the month of the year, may not necessarily be three characters wide.
a0d0e21e 2591
62aa5637
MS
2592See L<perlport/localtime> for portability concerns.
2593
07698885 2594=item lock THING
19799a22 2595
01e6739c 2596This function places an advisory lock on a shared variable, or referenced
03730085 2597object contained in I<THING> until the lock goes out of scope.
a6d5524e 2598
f3a23afb 2599lock() is a "weak keyword" : this means that if you've defined a function
67408cae 2600by this name (before any calls to it), that function will be called
03730085
AB
2601instead. (However, if you've said C<use threads>, lock() is always a
2602keyword.) See L<threads>.
19799a22 2603
a0d0e21e
LW
2604=item log EXPR
2605
54310121 2606=item log
bbce6d69 2607
2b5ab1e7
TC
2608Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2609returns log of C<$_>. To get the log of another base, use basic algebra:
19799a22 2610The base-N log of a number is equal to the natural log of that number
2b5ab1e7
TC
2611divided by the natural log of N. For example:
2612
2613 sub log10 {
2614 my $n = shift;
2615 return log($n)/log(10);
b76cc8ba 2616 }
2b5ab1e7
TC
2617
2618See also L</exp> for the inverse operation.
a0d0e21e 2619
a0d0e21e
LW
2620=item lstat EXPR
2621
54310121 2622=item lstat
bbce6d69 2623
19799a22 2624Does the same thing as the C<stat> function (including setting the
5a964f20
TC
2625special C<_> filehandle) but stats a symbolic link instead of the file
2626the symbolic link points to. If symbolic links are unimplemented on
c837d5b4
DP
2627your system, a normal C<stat> is done. For much more detailed
2628information, please see the documentation for C<stat>.
a0d0e21e 2629
7660c0ab 2630If EXPR is omitted, stats C<$_>.
bbce6d69 2631
a0d0e21e
LW
2632=item m//
2633
2634The match operator. See L<perlop>.
2635
2636=item map BLOCK LIST
2637
2638=item map EXPR,LIST
2639
19799a22
GS
2640Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2641C<$_> to each element) and returns the list value composed of the
2642results of each such evaluation. In scalar context, returns the
2643total number of elements so generated. Evaluates BLOCK or EXPR in
2644list context, so each element of LIST may produce zero, one, or
2645more elements in the returned value.
dd99ebda 2646
a0d0e21e
LW
2647 @chars = map(chr, @nums);
2648
2649translates a list of numbers to the corresponding characters. And
2650
4633a7c4 2651 %hash = map { getkey($_) => $_ } @array;
a0d0e21e
LW
2652
2653is just a funny way to write
2654
2655 %hash = ();
2656 foreach $_ (@array) {
4633a7c4 2657 $hash{getkey($_)} = $_;
a0d0e21e
LW
2658 }
2659
be3174d2
GS
2660Note that C<$_> is an alias to the list value, so it can be used to
2661modify the elements of the LIST. While this is useful and supported,
2662it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
2663Using a regular C<foreach> loop for this purpose would be clearer in
2664most cases. See also L</grep> for an array composed of those items of
2665the original list for which the BLOCK or EXPR evaluates to true.
fb73857a 2666
a4fb8298
RGS
2667If C<$_> is lexical in the scope where the C<map> appears (because it has
2668been declared with C<my $_>) then, in addition the be locally aliased to
2669the list elements, C<$_> keeps being lexical inside the block; i.e. it
2670can't be seen from the outside, avoiding any potential side-effects.
2671
205fdb4d
NC
2672C<{> starts both hash references and blocks, so C<map { ...> could be either
2673the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look
2674ahead for the closing C<}> it has to take a guess at which its dealing with
2675based what it finds just after the C<{>. Usually it gets it right, but if it
2676doesn't it won't realize something is wrong until it gets to the C<}> and
2677encounters the missing (or unexpected) comma. The syntax error will be
2678reported close to the C<}> but you'll need to change something near the C<{>
2679such as using a unary C<+> to give perl some help:
2680
2681 %hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong
2682 %hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right
2683 %hash = map { ("\L$_", 1) } @array # this also works
2684 %hash = map { lc($_), 1 } @array # as does this.
2685 %hash = map +( lc($_), 1 ), @array # this is EXPR and works!
cea6626f 2686
205fdb4d
NC
2687 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
2688
2689or to force an anon hash constructor use C<+{>
2690
2691 @hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end
2692
2693and you get list of anonymous hashes each with only 1 entry.
2694
19799a22 2695=item mkdir FILENAME,MASK
a0d0e21e 2696
5a211162
GS
2697=item mkdir FILENAME
2698
491873e5
RGS
2699=item mkdir
2700
0591cd52 2701Creates the directory specified by FILENAME, with permissions
19799a22
GS
2702specified by MASK (as modified by C<umask>). If it succeeds it
2703returns true, otherwise it returns false and sets C<$!> (errno).
491873e5
RGS
2704If omitted, MASK defaults to 0777. If omitted, FILENAME defaults
2705to C<$_>.
0591cd52 2706
19799a22 2707In general, it is better to create directories with permissive MASK,
0591cd52 2708and let the user modify that with their C<umask>, than it is to supply
19799a22 2709a restrictive MASK and give the user no way to be more permissive.
0591cd52
NT
2710The exceptions to this rule are when the file or directory should be
2711kept private (mail files, for instance). The perlfunc(1) entry on
19799a22 2712C<umask> discusses the choice of MASK in more detail.
a0d0e21e 2713
cc1852e8
JH
2714Note that according to the POSIX 1003.1-1996 the FILENAME may have any
2715number of trailing slashes. Some operating and filesystems do not get
2716this right, so Perl automatically removes all trailing slashes to keep
2717everyone happy.
2718
a0d0e21e
LW
2719=item msgctl ID,CMD,ARG
2720
f86cebdf 2721Calls the System V IPC function msgctl(2). You'll probably have to say
0ade1984
JH
2722
2723 use IPC::SysV;
2724
7660c0ab
A
2725first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2726then ARG must be a variable which will hold the returned C<msqid_ds>
951ba7fe
GS
2727structure. Returns like C<ioctl>: the undefined value for error,
2728C<"0 but true"> for zero, or the actual return value otherwise. See also
4755096e 2729L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation.
a0d0e21e
LW
2730
2731=item msgget KEY,FLAGS
2732
f86cebdf 2733Calls the System V IPC function msgget(2). Returns the message queue
4755096e
GS
2734id, or the undefined value if there is an error. See also
2735L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation.
a0d0e21e 2736
a0d0e21e
LW
2737=item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2738
2739Calls the System V IPC function msgrcv to receive a message from
2740message queue ID into variable VAR with a maximum message size of
41d6edb2
JH
2741SIZE. Note that when a message is received, the message type as a
2742native long integer will be the first thing in VAR, followed by the
2743actual message. This packing may be opened with C<unpack("l! a*")>.
2744Taints the variable. Returns true if successful, or false if there is
4755096e
GS
2745an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and
2746C<IPC::SysV::Msg> documentation.
41d6edb2
JH
2747
2748=item msgsnd ID,MSG,FLAGS
2749
2750Calls the System V IPC function msgsnd to send the message MSG to the
2751message queue ID. MSG must begin with the native long integer message
2752type, and be followed by the length of the actual message, and finally
2753the message itself. This kind of packing can be achieved with
2754C<pack("l! a*", $type, $message)>. Returns true if successful,
2755or false if there is an error. See also C<IPC::SysV>
2756and C<IPC::SysV::Msg> documentation.
a0d0e21e
LW
2757
2758=item my EXPR
2759
307ea6df
JH
2760=item my TYPE EXPR
2761
1d2de774 2762=item my EXPR : ATTRS
09bef843 2763
1d2de774 2764=item my TYPE EXPR : ATTRS
307ea6df 2765
19799a22 2766A C<my> declares the listed variables to be local (lexically) to the
1d2de774
JH
2767enclosing block, file, or C<eval>. If more than one value is listed,
2768the list must be placed in parentheses.
307ea6df 2769
1d2de774
JH
2770The exact semantics and interface of TYPE and ATTRS are still
2771evolving. TYPE is currently bound to the use of C<fields> pragma,
307ea6df
JH
2772and attributes are handled using the C<attributes> pragma, or starting
2773from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
2774L<perlsub/"Private Variables via my()"> for details, and L<fields>,
2775L<attributes>, and L<Attribute::Handlers>.
4633a7c4 2776
a0d0e21e
LW
2777=item next LABEL
2778
2779=item next
2780
2781The C<next> command is like the C<continue> statement in C; it starts
2782the next iteration of the loop:
2783
4633a7c4
LW
2784 LINE: while (<STDIN>) {
2785 next LINE if /^#/; # discard comments
5a964f20 2786 #...
a0d0e21e
LW
2787 }
2788
2789Note that if there were a C<continue> block on the above, it would get
2790executed even on discarded lines. If the LABEL is omitted, the command
2791refers to the innermost enclosing loop.
2792
4968c1e4 2793C<next> cannot be used to exit a block which returns a value such as
2b5ab1e7
TC
2794C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2795a grep() or map() operation.
4968c1e4 2796
6c1372ed
GS
2797Note that a block by itself is semantically identical to a loop
2798that executes once. Thus C<next> will exit such a block early.
2799
98293880
JH
2800See also L</continue> for an illustration of how C<last>, C<next>, and
2801C<redo> work.
1d2dff63 2802
4a66ea5a
RGS
2803=item no Module VERSION LIST
2804
2805=item no Module VERSION
2806
a0d0e21e
LW
2807=item no Module LIST
2808
4a66ea5a
RGS
2809=item no Module
2810
593b9c14 2811See the C<use> function, of which C<no> is the opposite.
a0d0e21e
LW
2812
2813=item oct EXPR
2814
54310121 2815=item oct
bbce6d69 2816
4633a7c4 2817Interprets EXPR as an octal string and returns the corresponding
4f19785b
WSI
2818value. (If EXPR happens to start off with C<0x>, interprets it as a
2819hex string. If EXPR starts off with C<0b>, it is interpreted as a
53305cf1
NC
2820binary string. Leading whitespace is ignored in all three cases.)
2821The following will handle decimal, binary, octal, and hex in the standard
2822Perl or C notation:
a0d0e21e
LW
2823
2824 $val = oct($val) if $val =~ /^0/;
2825
19799a22
GS
2826If EXPR is omitted, uses C<$_>. To go the other way (produce a number
2827in octal), use sprintf() or printf():
2828
2829 $perms = (stat("filename"))[2] & 07777;
2830 $oct_perms = sprintf "%lo", $perms;
2831
2832The oct() function is commonly used when a string such as C<644> needs
2833to be converted into a file mode, for example. (Although perl will
2834automatically convert strings into numbers as needed, this automatic
2835conversion assumes base 10.)
a0d0e21e
LW
2836
2837=item open FILEHANDLE,EXPR
2838
68bd7414
NIS
2839=item open FILEHANDLE,MODE,EXPR
2840
2841=item open FILEHANDLE,MODE,EXPR,LIST
2842
ba964c95
T
2843=item open FILEHANDLE,MODE,REFERENCE
2844
a0d0e21e
LW
2845=item open FILEHANDLE
2846
2847Opens the file whose filename is given by EXPR, and associates it with
ed53a2bb
JH
2848FILEHANDLE.
2849
2850(The following is a comprehensive reference to open(): for a gentler
2851introduction you may consider L<perlopentut>.)
2852
a28cd5c9
NT
2853If FILEHANDLE is an undefined scalar variable (or array or hash element)
2854the variable is assigned a reference to a new anonymous filehandle,
2855otherwise if FILEHANDLE is an expression, its value is used as the name of
2856the real filehandle wanted. (This is considered a symbolic reference, so
2857C<use strict 'refs'> should I<not> be in effect.)
ed53a2bb
JH
2858
2859If EXPR is omitted, the scalar variable of the same name as the
2860FILEHANDLE contains the filename. (Note that lexical variables--those
2861declared with C<my>--will not work for this purpose; so if you're
67408cae 2862using C<my>, specify EXPR in your call to open.)
ed53a2bb
JH
2863
2864If three or more arguments are specified then the mode of opening and
2865the file name are separate. If MODE is C<< '<' >> or nothing, the file
2866is opened for input. If MODE is C<< '>' >>, the file is truncated and
2867opened for output, being created if necessary. If MODE is C<<< '>>' >>>,
b76cc8ba 2868the file is opened for appending, again being created if necessary.
5a964f20 2869
ed53a2bb
JH
2870You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to
2871indicate that you want both read and write access to the file; thus
2872C<< '+<' >> is almost always preferred for read/write updates--the C<<
2873'+>' >> mode would clobber the file first. You can't usually use
2874either read-write mode for updating textfiles, since they have
2875variable length records. See the B<-i> switch in L<perlrun> for a
2876better approach. The file is created with permissions of C<0666>
2877modified by the process' C<umask> value.
2878
2879These various prefixes correspond to the fopen(3) modes of C<'r'>,
2880C<'r+'>, C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
5f05dabc 2881
6170680b
IZ
2882In the 2-arguments (and 1-argument) form of the call the mode and
2883filename should be concatenated (in this order), possibly separated by
68bd7414
NIS
2884spaces. It is possible to omit the mode in these forms if the mode is
2885C<< '<' >>.
6170680b 2886
7660c0ab 2887If the filename begins with C<'|'>, the filename is interpreted as a
5a964f20 2888command to which output is to be piped, and if the filename ends with a
f244e06d
GS
2889C<'|'>, the filename is interpreted as a command which pipes output to
2890us. See L<perlipc/"Using open() for IPC">
19799a22 2891for more examples of this. (You are not allowed to C<open> to a command
5a964f20 2892that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
4a4eefd0
GS
2893and L<perlipc/"Bidirectional Communication with Another Process">
2894for alternatives.)
cb1a09d0 2895
ed53a2bb
JH
2896For three or more arguments if MODE is C<'|-'>, the filename is
2897interpreted as a command to which output is to be piped, and if MODE
2898is C<'-|'>, the filename is interpreted as a command which pipes
2899output to us. In the 2-arguments (and 1-argument) form one should
2900replace dash (C<'-'>) with the command.
2901See L<perlipc/"Using open() for IPC"> for more examples of this.
2902(You are not allowed to C<open> to a command that pipes both in I<and>
2903out, but see L<IPC::Open2>, L<IPC::Open3>, and
2904L<perlipc/"Bidirectional Communication"> for alternatives.)
2905
2906In the three-or-more argument form of pipe opens, if LIST is specified
2907(extra arguments after the command name) then LIST becomes arguments
2908to the command invoked if the platform supports it. The meaning of
2909C<open> with more than three arguments for non-pipe modes is not yet
2910specified. Experimental "layers" may give extra LIST arguments
2911meaning.
6170680b
IZ
2912
2913In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
b76cc8ba 2914and opening C<< '>-' >> opens STDOUT.
6170680b 2915
fae2c0fb
RGS
2916You may use the three-argument form of open to specify IO "layers"
2917(sometimes also referred to as "disciplines") to be applied to the handle
2918that affect how the input and output are processed (see L<open> and
2919L<PerlIO> for more details). For example
7207e29d 2920
9124316e
JH
2921 open(FH, "<:utf8", "file")
2922
2923will open the UTF-8 encoded file containing Unicode characters,
fae2c0fb
RGS
2924see L<perluniintro>. (Note that if layers are specified in the
2925three-arg form then default layers set by the C<open> pragma are
01e6739c 2926ignored.)
ed53a2bb
JH
2927
2928Open returns nonzero upon success, the undefined value otherwise. If
2929the C<open> involved a pipe, the return value happens to be the pid of
2930the subprocess.
cb1a09d0 2931
ed53a2bb
JH
2932If you're running Perl on a system that distinguishes between text
2933files and binary files, then you should check out L</binmode> for tips
2934for dealing with this. The key distinction between systems that need
2935C<binmode> and those that don't is their text file formats. Systems
8939ba94 2936like Unix, Mac OS, and Plan 9, which delimit lines with a single
ed53a2bb
JH
2937character, and which encode that character in C as C<"\n">, do not
2938need C<binmode>. The rest need it.
cb1a09d0 2939
fb73857a 2940When opening a file, it's usually a bad idea to continue normal execution
19799a22
GS
2941if the request failed, so C<open> is frequently used in connection with
2942C<die>. Even if C<die> won't do what you want (say, in a CGI script,
fb73857a 2943where you want to make a nicely formatted error message (but there are
5a964f20 2944modules that can help with that problem)) you should always check
19799a22 2945the return value from opening a file. The infrequent exception is when
fb73857a
PP
2946working with an unopened filehandle is actually what you want to do.
2947
ed53a2bb
JH
2948As a special case the 3 arg form with a read/write mode and the third
2949argument being C<undef>:
b76cc8ba
NIS
2950
2951 open(TMP, "+>", undef) or die ...
2952
f253e835
JH
2953opens a filehandle to an anonymous temporary file. Also using "+<"
2954works for symmetry, but you really should consider writing something
2955to the temporary file first. You will need to seek() to do the
2956reading.
b76cc8ba 2957
2ce64696
JC
2958Since v5.8.0, perl has built using PerlIO by default. Unless you've
2959changed this (ie Configure -Uuseperlio), you can open file handles to
2960"in memory" files held in Perl scalars via:
ba964c95 2961
b996200f
SB
2962 open($fh, '>', \$variable) || ..
2963
2964Though if you try to re-open C<STDOUT> or C<STDERR> as an "in memory"
2965file, you have to close it first:
2966
2967 close STDOUT;
2968 open STDOUT, '>', \$variable or die "Can't open STDOUT: $!";
ba964c95 2969
cb1a09d0 2970Examples:
a0d0e21e
LW
2971
2972 $ARTICLE = 100;
2973 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2974 while (<ARTICLE>) {...
2975
6170680b 2976 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
fb73857a 2977 # if the open fails, output is discarded
a0d0e21e 2978
6170680b 2979 open(DBASE, '+<', 'dbase.mine') # open for update
fb73857a 2980 or die "Can't open 'dbase.mine' for update: $!";
cb1a09d0 2981
6170680b
IZ
2982 open(DBASE, '+<dbase.mine') # ditto
2983 or die "Can't open 'dbase.mine' for update: $!";
2984
2985 open(ARTICLE, '-|', "caesar <$article") # decrypt article
fb73857a 2986 or die "Can't start caesar: $!";
a0d0e21e 2987
6170680b
IZ
2988 open(ARTICLE, "caesar <$article |") # ditto
2989 or die "Can't start caesar: $!";
2990
2359510d 2991 open(EXTRACT, "|sort >Tmp$$") # $$ is our process id
fb73857a 2992 or die "Can't start sort: $!";
a0d0e21e 2993
ba964c95
T
2994 # in memory files
2995 open(MEMORY,'>', \$var)
2996 or die "Can't open memory file: $!";
2997 print MEMORY "foo!\n"; # output will end up in $var
2998
a0d0e21e
LW
2999 # process argument list of files along with any includes
3000
3001 foreach $file (@ARGV) {
3002 process($file, 'fh00');
3003 }
3004
3005 sub process {
5a964f20 3006 my($filename, $input) = @_;
a0d0e21e
LW
3007 $input++; # this is a string increment
3008 unless (open($input, $filename)) {
3009 print STDERR "Can't open $filename: $!\n";
3010 return;
3011 }
3012
5a964f20 3013 local $_;
a0d0e21e
LW
3014 while (<$input>) { # note use of indirection
3015 if (/^#include "(.*)"/) {
3016 process($1, $input);
3017 next;
3018 }
5a964f20 3019 #... # whatever
a0d0e21e
LW
3020 }
3021 }
3022
ae4c5402 3023See L<perliol> for detailed info on PerlIO.
2ce64696 3024
a0d0e21e 3025You may also, in the Bourne shell tradition, specify an EXPR beginning
00cafafa
JH
3026with C<< '>&' >>, in which case the rest of the string is interpreted
3027as the name of a filehandle (or file descriptor, if numeric) to be
3028duped (as L<dup(2)>) and opened. You may use C<&> after C<< > >>,
3029C<<< >> >>>, C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>.
3030The mode you specify should match the mode of the original filehandle.
3031(Duping a filehandle does not take into account any existing contents
3032of IO buffers.) If you use the 3 arg form then you can pass either a
3033number, the name of a filehandle or the normal "reference to a glob".
6170680b 3034
eae1b76b
SB
3035Here is a script that saves, redirects, and restores C<STDOUT> and
3036C<STDERR> using various methods:
a0d0e21e
LW
3037
3038 #!/usr/bin/perl
eae1b76b
SB
3039 open my $oldout, ">&STDOUT" or die "Can't dup STDOUT: $!";
3040 open OLDERR, ">&", \*STDERR or die "Can't dup STDERR: $!";
818c4caa 3041
eae1b76b
SB
3042 open STDOUT, '>', "foo.out" or die "Can't redirect STDOUT: $!";
3043 open STDERR, ">&STDOUT" or die "Can't dup STDOUT: $!";
a0d0e21e 3044
eae1b76b
SB
3045 select STDERR; $| = 1; # make unbuffered
3046 select STDOUT; $| = 1; # make unbuffered
a0d0e21e
LW
3047
3048 print STDOUT "stdout 1\n"; # this works for
3049 print STDERR "stderr 1\n"; # subprocesses too
3050
eae1b76b
SB
3051 open STDOUT, ">&", $oldout or die "Can't dup \$oldout: $!";
3052 open STDERR, ">&OLDERR" or die "Can't dup OLDERR: $!";
a0d0e21e
LW
3053
3054 print STDOUT "stdout 2\n";
3055 print STDERR "stderr 2\n";
3056
ef8b303f
JH
3057If you specify C<< '<&=X' >>, where C<X> is a file descriptor number
3058or a filehandle, then Perl will do an equivalent of C's C<fdopen> of
3059that file descriptor (and not call L<dup(2)>); this is more
3060parsimonious of file descriptors. For example:
a0d0e21e 3061
00cafafa 3062 # open for input, reusing the fileno of $fd
a0d0e21e 3063 open(FILEHANDLE, "<&=$fd")
df632fdf 3064
b76cc8ba 3065or
df632fdf 3066
b76cc8ba 3067 open(FILEHANDLE, "<&=", $fd)
a0d0e21e 3068
00cafafa
JH
3069or
3070
3071 # open for append, using the fileno of OLDFH
3072 open(FH, ">>&=", OLDFH)
3073
3074or
3075
3076 open(FH, ">>&=OLDFH")
3077
ef8b303f
JH
3078Being parsimonious on filehandles is also useful (besides being
3079parsimonious) for example when something is dependent on file
3080descriptors, like for example locking using flock(). If you do just
3081C<< open(A, '>>&B') >>, the filehandle A will not have the same file
3082descriptor as B, and therefore flock(A) will not flock(B), and vice
3083versa. But with C<< open(A, '>>&=B') >> the filehandles will share
3084the same file descriptor.
3085
3086Note that if you are using Perls older than 5.8.0, Perl will be using
3087the standard C libraries' fdopen() to implement the "=" functionality.
3088On many UNIX systems fdopen() fails when file descriptors exceed a
3089certain value, typically 255. For Perls 5.8.0 and later, PerlIO is
3090most often the default.
4af147f6 3091
df632fdf
JH
3092You can see whether Perl has been compiled with PerlIO or not by
3093running C<perl -V> and looking for C<useperlio=> line. If C<useperlio>
3094is C<define>, you have PerlIO, otherwise you don't.
3095
6170680b
IZ
3096If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
3097with 2-arguments (or 1-argument) form of open(), then
a0d0e21e 3098there is an implicit fork done, and the return value of open is the pid
7660c0ab 3099of the child within the parent process, and C<0> within the child
184e9718 3100process. (Use C<defined($pid)> to determine whether the open was successful.)
a0d0e21e
LW
3101The filehandle behaves normally for the parent, but i/o to that
3102filehandle is piped from/to the STDOUT/STDIN of the child process.
3103In the child process the filehandle isn't opened--i/o happens from/to
3104the new STDOUT or STDIN. Typically this is used like the normal
3105piped open when you want to exercise more control over just how the
3106pipe command gets executed, such as when you are running setuid, and
54310121 3107don't want to have to scan shell commands for metacharacters.
6170680b 3108The following triples are more or less equivalent:
a0d0e21e
LW
3109
3110 open(FOO, "|tr '[a-z]' '[A-Z]'");
6170680b
IZ
3111 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
3112 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
b76cc8ba 3113 open(FOO, '|-', "tr", '[a-z]', '[A-Z]');
a0d0e21e
LW
3114
3115 open(FOO, "cat -n '$file'|");
6170680b
IZ
3116 open(FOO, '-|', "cat -n '$file'");
3117 open(FOO, '-|') || exec 'cat', '-n', $file;
b76cc8ba
NIS
3118 open(FOO, '-|', "cat", '-n', $file);
3119
3120The last example in each block shows the pipe as "list form", which is
64da03b2
JH
3121not yet supported on all platforms. A good rule of thumb is that if
3122your platform has true C<fork()> (in other words, if your platform is
3123UNIX) you can use the list form.
a0d0e21e 3124
4633a7c4
LW
3125See L<perlipc/"Safe Pipe Opens"> for more examples of this.
3126
0f897271
GS
3127Beginning with v5.6.0, Perl will attempt to flush all files opened for
3128output before any operation that may do a fork, but this may not be
3129supported on some platforms (see L<perlport>). To be safe, you may need
3130to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
3131of C<IO::Handle> on any open handles.
3132
ed53a2bb
JH
3133On systems that support a close-on-exec flag on files, the flag will
3134be set for the newly opened file descriptor as determined by the value
3135of $^F. See L<perlvar/$^F>.
a0d0e21e 3136
0dccf244 3137Closing any piped filehandle causes the parent process to wait for the
e5218da5
GA
3138child to finish, and returns the status value in C<$?> and
3139C<${^CHILD_ERROR_NATIVE}>.
0dccf244 3140
ed53a2bb
JH
3141The filename passed to 2-argument (or 1-argument) form of open() will
3142have leading and trailing whitespace deleted, and the normal
3143redirection characters honored. This property, known as "magic open",
5a964f20 3144can often be used to good effect. A user could specify a filename of
7660c0ab 3145F<"rsh cat file |">, or you could change certain filenames as needed:
5a964f20
TC
3146
3147 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
3148 open(FH, $filename) or die "Can't open $filename: $!";
3149
6170680b
IZ
3150Use 3-argument form to open a file with arbitrary weird characters in it,
3151
3152 open(FOO, '<', $file);
3153
3154otherwise it's necessary to protect any leading and trailing whitespace:
5a964f20
TC
3155
3156 $file =~ s#^(\s)#./$1#;
3157 open(FOO, "< $file\0");
3158
a31a806a 3159(this may not work on some bizarre filesystems). One should
106325ad 3160conscientiously choose between the I<magic> and 3-arguments form
6170680b
IZ
3161of open():
3162
3163 open IN, $ARGV[0];
3164
3165will allow the user to specify an argument of the form C<"rsh cat file |">,
3166but will not work on a filename which happens to have a trailing space, while
3167
3168 open IN, '<', $ARGV[0];
3169
3170will have exactly the opposite restrictions.
3171
19799a22 3172If you want a "real" C C<open> (see L<open(2)> on your system), then you
6170680b
IZ
3173should use the C<sysopen> function, which involves no such magic (but
3174may use subtly different filemodes than Perl open(), which is mapped
3175to C fopen()). This is
5a964f20
TC
3176another way to protect your filenames from interpretation. For example:
3177
3178 use IO::Handle;
3179 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
3180 or die "sysopen $path: $!";
3181 $oldfh = select(HANDLE); $| = 1; select($oldfh);
38762f02 3182 print HANDLE "stuff $$\n";
5a964f20
TC
3183 seek(HANDLE, 0, 0);
3184 print "File contains: ", <HANDLE>;
3185
7660c0ab
A
3186Using the constructor from the C<IO::Handle> package (or one of its
3187subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
5a964f20
TC
3188filehandles that have the scope of whatever variables hold references to
3189them, and automatically close whenever and however you leave that scope:
c07a80fd 3190
5f05dabc 3191 use IO::File;
5a964f20 3192 #...
c07a80fd
PP
3193 sub read_myfile_munged {
3194 my $ALL = shift;
5f05dabc 3195 my $handle = new IO::File;
c07a80fd
PP
3196 open($handle, "myfile") or die "myfile: $!";
3197 $first = <$handle>
3198 or return (); # Automatically closed here.
3199 mung $first or die "mung failed"; # Or here.
3200 return $first, <$handle> if $ALL; # Or here.
3201 $first; # Or here.
3202 }
3203
b687b08b 3204See L</seek> for some details about mixing reading and writing.
a0d0e21e
LW
3205
3206=item opendir DIRHANDLE,EXPR
3207
19799a22
GS
3208Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
3209C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
a28cd5c9
NT
3210DIRHANDLE may be an expression whose value can be used as an indirect
3211dirhandle, usually the real dirhandle name. If DIRHANDLE is an undefined
3212scalar variable (or array or hash element), the variable is assigned a
3213reference to a new anonymous dirhandle.
a0d0e21e
LW
3214DIRHANDLEs have their own namespace separate from FILEHANDLEs.
3215
3216=item ord EXPR
3217
54310121 3218=item ord
bbce6d69 3219
121910a4
JH
3220Returns the numeric (the native 8-bit encoding, like ASCII or EBCDIC,
3221or Unicode) value of the first character of EXPR. If EXPR is omitted,
3222uses C<$_>.
3223
3224For the reverse, see L</chr>.
3225See L<perlunicode> and L<encoding> for more about Unicode.
a0d0e21e 3226
77ca0c92
LW
3227=item our EXPR
3228
307ea6df
JH
3229=item our EXPR TYPE
3230
1d2de774 3231=item our EXPR : ATTRS
9969eac4 3232
1d2de774 3233=item our TYPE EXPR : ATTRS
307ea6df 3234
77ca0c92
LW
3235An C<our> declares the listed variables to be valid globals within
3236the enclosing block, file, or C<eval>. That is, it has the same
3237scoping rules as a "my" declaration, but does not create a local
3238variable. If more than one value is listed, the list must be placed
3239in parentheses. The C<our> declaration has no semantic effect unless
3240"use strict vars" is in effect, in which case it lets you use the
3241declared global variable without qualifying it with a package name.
3242(But only within the lexical scope of the C<our> declaration. In this
3243it differs from "use vars", which is package scoped.)
3244
f472eb5c
GS
3245An C<our> declaration declares a global variable that will be visible
3246across its entire lexical scope, even across package boundaries. The
3247package in which the variable is entered is determined at the point
3248of the declaration, not at the point of use. This means the following
3249behavior holds:
3250
3251 package Foo;
3252 our $bar; # declares $Foo::bar for rest of lexical scope
3253 $bar = 20;
3254
3255 package Bar;
3256 print $bar; # prints 20
3257
3258Multiple C<our> declarations in the same lexical scope are allowed