This is a live mirror of the Perl 5 development currently hosted at https://github.com/perl/perl5
Accidental interpolation of $@ in Pod::Html [PATCH]
[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.
cf264981 28Commas should separate elements of the LIST.
a0d0e21e
LW
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 141
cf264981 142=item Keywords related to the control flow of your Perl program
cb1a09d0 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>,
54d7b083 229C<getppid>, C<getpgrp>, C<getpriority>, C<getprotobynumber>,
60f9f73c
JH
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%)
cf264981 340are found, it's a C<-B> file; otherwise it's a C<-T> file. Also, any file
a0d0e21e 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 353symbolic link, not the real file.) (Also, if the stat buffer was filled by
cf264981 354an C<lstat> call, C<-T> and C<-B> will reset it with the results of C<stat _>).
5c9aa243 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
cf264981 371C<-x $file && -w _ && -f _>. (This is only syntax fancy: if you use
fbb0b3b3
RGS
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
cf264981 397specified number of wallclock seconds has elapsed. If SECONDS is not
d400eac8
JH
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($_[0]) }
448
bf5f1b4c
JH
449Note that atan2(0, 0) is not well-defined.
450
a0d0e21e
LW
451=item bind SOCKET,NAME
452
453Binds a network address to a socket, just as the bind system call
19799a22 454does. Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
455packed address of the appropriate type for the socket. See the examples in
456L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 457
fae2c0fb 458=item binmode FILEHANDLE, LAYER
1c1fc3ea 459
a0d0e21e
LW
460=item binmode FILEHANDLE
461
1cbfc93d
NIS
462Arranges for FILEHANDLE to be read or written in "binary" or "text"
463mode on systems where the run-time libraries distinguish between
464binary and text files. If FILEHANDLE is an expression, the value is
465taken as the name of the filehandle. Returns true on success,
b5fe5ca2 466otherwise it returns C<undef> and sets C<$!> (errno).
1cbfc93d 467
d807c6f4
JH
468On some systems (in general, DOS and Windows-based systems) binmode()
469is necessary when you're not working with a text file. For the sake
470of portability it is a good idea to always use it when appropriate,
471and to never use it when it isn't appropriate. Also, people can
472set their I/O to be by default UTF-8 encoded Unicode, not bytes.
473
474In other words: regardless of platform, use binmode() on binary data,
475like for example images.
476
477If LAYER is present it is a single string, but may contain multiple
478directives. The directives alter the behaviour of the file handle.
479When LAYER is present using binmode on text file makes sense.
480
fae2c0fb 481If LAYER is omitted or specified as C<:raw> the filehandle is made
0226bbdb
NIS
482suitable for passing binary data. This includes turning off possible CRLF
483translation and marking it as bytes (as opposed to Unicode characters).
749683d2
YST
484Note that, despite what may be implied in I<"Programming Perl"> (the
485Camel) or elsewhere, C<:raw> is I<not> the simply inverse of C<:crlf>
fae2c0fb 486-- other layers which would affect binary nature of the stream are
0226bbdb
NIS
487I<also> disabled. See L<PerlIO>, L<perlrun> and the discussion about the
488PERLIO environment variable.
01e6739c 489
d807c6f4
JH
490The C<:bytes>, C<:crlf>, and C<:utf8>, and any other directives of the
491form C<:...>, are called I/O I<layers>. The C<open> pragma can be used to
492establish default I/O layers. See L<open>.
493
fae2c0fb
RGS
494I<The LAYER parameter of the binmode() function is described as "DISCIPLINE"
495in "Programming Perl, 3rd Edition". However, since the publishing of this
496book, by many known as "Camel III", the consensus of the naming of this
497functionality has moved from "discipline" to "layer". All documentation
498of this version of Perl therefore refers to "layers" rather than to
499"disciplines". Now back to the regularly scheduled documentation...>
500
01e6739c 501To mark FILEHANDLE as UTF-8, use C<:utf8>.
1cbfc93d 502
ed53a2bb 503In general, binmode() should be called after open() but before any I/O
01e6739c
NIS
504is done on the filehandle. Calling binmode() will normally flush any
505pending buffered output data (and perhaps pending input data) on the
fae2c0fb 506handle. An exception to this is the C<:encoding> layer that
01e6739c 507changes the default character encoding of the handle, see L<open>.
fae2c0fb 508The C<:encoding> layer sometimes needs to be called in
3874323d
JH
509mid-stream, and it doesn't flush the stream. The C<:encoding>
510also implicitly pushes on top of itself the C<:utf8> layer because
511internally Perl will operate on UTF-8 encoded Unicode characters.
16fe6d59 512
19799a22 513The operating system, device drivers, C libraries, and Perl run-time
30168b04
GS
514system all work together to let the programmer treat a single
515character (C<\n>) as the line terminator, irrespective of the external
516representation. On many operating systems, the native text file
517representation matches the internal representation, but on some
518platforms the external representation of C<\n> is made up of more than
519one character.
520
68bd7414
NIS
521Mac OS, all variants of Unix, and Stream_LF files on VMS use a single
522character to end each line in the external representation of text (even
5e12dbfa 523though that single character is CARRIAGE RETURN on Mac OS and LINE FEED
01e6739c
NIS
524on Unix and most VMS files). In other systems like OS/2, DOS and the
525various flavors of MS-Windows your program sees a C<\n> as a simple C<\cJ>,
526but what's stored in text files are the two characters C<\cM\cJ>. That
527means that, if you don't use binmode() on these systems, C<\cM\cJ>
528sequences on disk will be converted to C<\n> on input, and any C<\n> in
529your program will be converted back to C<\cM\cJ> on output. This is what
530you want for text files, but it can be disastrous for binary files.
30168b04
GS
531
532Another consequence of using binmode() (on some systems) is that
533special end-of-file markers will be seen as part of the data stream.
534For systems from the Microsoft family this means that if your binary
4375e838 535data contains C<\cZ>, the I/O subsystem will regard it as the end of
30168b04
GS
536the file, unless you use binmode().
537
538binmode() is not only important for readline() and print() operations,
539but also when using read(), seek(), sysread(), syswrite() and tell()
540(see L<perlport> for more details). See the C<$/> and C<$\> variables
541in L<perlvar> for how to manually set your input and output
542line-termination sequences.
a0d0e21e 543
4633a7c4 544=item bless REF,CLASSNAME
a0d0e21e
LW
545
546=item bless REF
547
2b5ab1e7
TC
548This function tells the thingy referenced by REF that it is now an object
549in the CLASSNAME package. If CLASSNAME is omitted, the current package
19799a22 550is used. Because a C<bless> is often the last thing in a constructor,
2b5ab1e7 551it returns the reference for convenience. Always use the two-argument
cf264981
SP
552version if a derived class might inherit the function doing the blessing.
553See L<perltoot> and L<perlobj> for more about the blessing (and blessings)
554of objects.
a0d0e21e 555
57668c4d 556Consider always blessing objects in CLASSNAMEs that are mixed case.
2b5ab1e7 557Namespaces with all lowercase names are considered reserved for
cf264981 558Perl pragmata. Builtin types have all uppercase names. To prevent
2b5ab1e7
TC
559confusion, you may wish to avoid such package names as well. Make sure
560that CLASSNAME is a true value.
60ad88b8
GS
561
562See L<perlmod/"Perl Modules">.
563
a0d0e21e
LW
564=item caller EXPR
565
566=item caller
567
5a964f20 568Returns the context of the current subroutine call. In scalar context,
28757baa 569returns the caller's package name if there is a caller, that is, if
19799a22 570we're in a subroutine or C<eval> or C<require>, and the undefined value
5a964f20 571otherwise. In list context, returns
a0d0e21e 572
748a9306 573 ($package, $filename, $line) = caller;
a0d0e21e
LW
574
575With EXPR, it returns some extra information that the debugger uses to
576print a stack trace. The value of EXPR indicates how many call frames
577to go back before the current one.
578
f3aa04c2 579 ($package, $filename, $line, $subroutine, $hasargs,
e476b1b5 580 $wantarray, $evaltext, $is_require, $hints, $bitmask) = caller($i);
e7ea3e70 581
951ba7fe 582Here $subroutine may be C<(eval)> if the frame is not a subroutine
19799a22 583call, but an C<eval>. In such a case additional elements $evaltext and
7660c0ab 584C<$is_require> are set: C<$is_require> is true if the frame is created by a
19799a22 585C<require> or C<use> statement, $evaltext contains the text of the
277ddfaf 586C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
951ba7fe 587$filename is C<(eval)>, but $evaltext is undefined. (Note also that
0fc9dec4
RGS
588each C<use> statement creates a C<require> frame inside an C<eval EXPR>
589frame.) $subroutine may also be C<(unknown)> if this particular
590subroutine happens to have been deleted from the symbol table.
591C<$hasargs> is true if a new instance of C<@_> was set up for the frame.
592C<$hints> and C<$bitmask> contain pragmatic hints that the caller was
593compiled with. The C<$hints> and C<$bitmask> values are subject to change
594between versions of Perl, and are not meant for external use.
748a9306
LW
595
596Furthermore, when called from within the DB package, caller returns more
7660c0ab 597detailed information: it sets the list variable C<@DB::args> to be the
54310121 598arguments with which the subroutine was invoked.
748a9306 599
7660c0ab 600Be aware that the optimizer might have optimized call frames away before
19799a22 601C<caller> had a chance to get the information. That means that C<caller(N)>
7660c0ab 602might not return information about the call frame you expect it do, for
b76cc8ba 603C<< N > 1 >>. In particular, C<@DB::args> might have information from the
19799a22 604previous time C<caller> was called.
7660c0ab 605
a0d0e21e
LW
606=item chdir EXPR
607
c4aca7d0
GA
608=item chdir FILEHANDLE
609
610=item chdir DIRHANDLE
611
ce2984c3
PF
612=item chdir
613
ffce7b87 614Changes the working directory to EXPR, if possible. If EXPR is omitted,
0bfc1ec4 615changes to the directory specified by C<$ENV{HOME}>, if set; if not,
ffce7b87 616changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
b4ad75f0
AMS
617variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
618neither is set, C<chdir> does nothing. It returns true upon success,
619false otherwise. See the example under C<die>.
a0d0e21e 620
c4aca7d0
GA
621On systems that support fchdir, you might pass a file handle or
622directory handle as argument. On systems that don't support fchdir,
623passing handles produces a fatal error at run time.
624
a0d0e21e
LW
625=item chmod LIST
626
627Changes the permissions of a list of files. The first element of the
4633a7c4 628list must be the numerical mode, which should probably be an octal
4ad40acf 629number, and which definitely should I<not> be a string of octal digits:
2f9daede 630C<0644> is okay, C<'0644'> is not. Returns the number of files
dc848c6f 631successfully changed. See also L</oct>, if all you have is a string.
a0d0e21e
LW
632
633 $cnt = chmod 0755, 'foo', 'bar';
634 chmod 0755, @executables;
f86cebdf
GS
635 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
636 # --w----r-T
2f9daede
TPG
637 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
638 $mode = 0644; chmod $mode, 'foo'; # this is best
a0d0e21e 639
c4aca7d0
GA
640On systems that support fchmod, you might pass file handles among the
641files. On systems that don't support fchmod, passing file handles
642produces a fatal error at run time.
643
644 open(my $fh, "<", "foo");
645 my $perm = (stat $fh)[2] & 07777;
646 chmod($perm | 0600, $fh);
647
ca6e1c26
JH
648You can also import the symbolic C<S_I*> constants from the Fcntl
649module:
650
651 use Fcntl ':mode';
652
653 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
654 # This is identical to the chmod 0755 of the above example.
655
a0d0e21e
LW
656=item chomp VARIABLE
657
313c9f5c 658=item chomp( LIST )
a0d0e21e
LW
659
660=item chomp
661
2b5ab1e7
TC
662This safer version of L</chop> removes any trailing string
663that corresponds to the current value of C<$/> (also known as
28757baa
PP
664$INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
665number of characters removed from all its arguments. It's often used to
666remove the newline from the end of an input record when you're worried
2b5ab1e7
TC
667that the final record may be missing its newline. When in paragraph
668mode (C<$/ = "">), it removes all trailing newlines from the string.
4c5a6083
GS
669When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
670a reference to an integer or the like, see L<perlvar>) chomp() won't
b76cc8ba 671remove anything.
19799a22 672If VARIABLE is omitted, it chomps C<$_>. Example:
a0d0e21e
LW
673
674 while (<>) {
675 chomp; # avoid \n on last field
676 @array = split(/:/);
5a964f20 677 # ...
a0d0e21e
LW
678 }
679
4bf21a6d
RD
680If VARIABLE is a hash, it chomps the hash's values, but not its keys.
681
a0d0e21e
LW
682You can actually chomp anything that's an lvalue, including an assignment:
683
684 chomp($cwd = `pwd`);
685 chomp($answer = <STDIN>);
686
687If you chomp a list, each element is chomped, and the total number of
688characters removed is returned.
689
442a8c12
NC
690If the C<encoding> pragma is in scope then the lengths returned are
691calculated from the length of C<$/> in Unicode characters, which is not
692always the same as the length of C<$/> in the native encoding.
693
15e44fd8
RGS
694Note that parentheses are necessary when you're chomping anything
695that is not a simple variable. This is because C<chomp $cwd = `pwd`;>
696is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as
697C<chomp( $cwd = `pwd` )> which you might expect. Similarly,
698C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than
699as C<chomp($a, $b)>.
700
a0d0e21e
LW
701=item chop VARIABLE
702
313c9f5c 703=item chop( LIST )
a0d0e21e
LW
704
705=item chop
706
707Chops off the last character of a string and returns the character
5b3eff12 708chopped. It is much more efficient than C<s/.$//s> because it neither
7660c0ab 709scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
4bf21a6d
RD
710If VARIABLE is a hash, it chops the hash's values, but not its keys.
711
5b3eff12 712You can actually chop anything that's an lvalue, including an assignment.
a0d0e21e
LW
713
714If you chop a list, each element is chopped. Only the value of the
19799a22 715last C<chop> is returned.
a0d0e21e 716
19799a22 717Note that C<chop> returns the last character. To return all but the last
748a9306
LW
718character, use C<substr($string, 0, -1)>.
719
15e44fd8
RGS
720See also L</chomp>.
721
a0d0e21e
LW
722=item chown LIST
723
724Changes the owner (and group) of a list of files. The first two
19799a22
GS
725elements of the list must be the I<numeric> uid and gid, in that
726order. A value of -1 in either position is interpreted by most
727systems to leave that value unchanged. Returns the number of files
728successfully changed.
a0d0e21e
LW
729
730 $cnt = chown $uid, $gid, 'foo', 'bar';
731 chown $uid, $gid, @filenames;
732
c4aca7d0
GA
733On systems that support fchown, you might pass file handles among the
734files. On systems that don't support fchown, passing file handles
735produces a fatal error at run time.
736
54310121 737Here's an example that looks up nonnumeric uids in the passwd file:
a0d0e21e
LW
738
739 print "User: ";
19799a22 740 chomp($user = <STDIN>);
5a964f20 741 print "Files: ";
19799a22 742 chomp($pattern = <STDIN>);
a0d0e21e
LW
743
744 ($login,$pass,$uid,$gid) = getpwnam($user)
745 or die "$user not in passwd file";
746
5a964f20 747 @ary = glob($pattern); # expand filenames
a0d0e21e
LW
748 chown $uid, $gid, @ary;
749
54310121 750On most systems, you are not allowed to change the ownership of the
4633a7c4
LW
751file unless you're the superuser, although you should be able to change
752the group to any of your secondary groups. On insecure systems, these
753restrictions may be relaxed, but this is not a portable assumption.
19799a22
GS
754On POSIX systems, you can detect this condition this way:
755
756 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
757 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
4633a7c4 758
a0d0e21e
LW
759=item chr NUMBER
760
54310121 761=item chr
bbce6d69 762
a0d0e21e 763Returns the character represented by that NUMBER in the character set.
a0ed51b3 764For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
1e54db1a
JH
765chr(0x263a) is a Unicode smiley face. Note that characters from 128
766to 255 (inclusive) are by default not encoded in UTF-8 Unicode for
767backward compatibility reasons (but see L<encoding>).
aaa68c4a 768
8a064bd6 769Negative values give the Unicode replacement character (chr(0xfffd)),
5f0135eb 770except under the L<bytes> pragma, where low eight bits of the value
8a064bd6
JH
771(truncated to an integer) are used.
772
974da8e5
JH
773If NUMBER is omitted, uses C<$_>.
774
b76cc8ba 775For the reverse, use L</ord>.
a0d0e21e 776
974da8e5
JH
777Note that under the C<bytes> pragma the NUMBER is masked to
778the low eight bits.
779
780See L<perlunicode> and L<encoding> for more about Unicode.
bbce6d69 781
a0d0e21e
LW
782=item chroot FILENAME
783
54310121 784=item chroot
bbce6d69 785
5a964f20 786This function works like the system call by the same name: it makes the
4633a7c4 787named directory the new root directory for all further pathnames that
951ba7fe 788begin with a C</> by your process and all its children. (It doesn't
28757baa 789change your current working directory, which is unaffected.) For security
4633a7c4 790reasons, this call is restricted to the superuser. If FILENAME is
19799a22 791omitted, does a C<chroot> to C<$_>.
a0d0e21e
LW
792
793=item close FILEHANDLE
794
6a518fbc
TP
795=item close
796
9124316e
JH
797Closes the file or pipe associated with the file handle, returning
798true only if IO buffers are successfully flushed and closes the system
799file descriptor. Closes the currently selected filehandle if the
800argument is omitted.
fb73857a
PP
801
802You don't have to close FILEHANDLE if you are immediately going to do
19799a22
GS
803another C<open> on it, because C<open> will close it for you. (See
804C<open>.) However, an explicit C<close> on an input file resets the line
805counter (C<$.>), while the implicit close done by C<open> does not.
fb73857a 806
dede8123
RGS
807If the file handle came from a piped open, C<close> will additionally
808return false if one of the other system calls involved fails, or if the
fb73857a 809program exits with non-zero status. (If the only problem was that the
dede8123 810program exited non-zero, C<$!> will be set to C<0>.) Closing a pipe
2b5ab1e7 811also waits for the process executing on the pipe to complete, in case you
b76cc8ba 812want to look at the output of the pipe afterwards, and
e5218da5
GA
813implicitly puts the exit status value of that command into C<$?> and
814C<${^CHILD_ERROR_NATIVE}>.
5a964f20 815
73689b13
GS
816Prematurely closing the read end of a pipe (i.e. before the process
817writing to it at the other end has closed it) will result in a
818SIGPIPE being delivered to the writer. If the other end can't
819handle that, be sure to read all the data before closing the pipe.
820
fb73857a 821Example:
a0d0e21e 822
fb73857a
PP
823 open(OUTPUT, '|sort >foo') # pipe to sort
824 or die "Can't start sort: $!";
5a964f20 825 #... # print stuff to output
fb73857a
PP
826 close OUTPUT # wait for sort to finish
827 or warn $! ? "Error closing sort pipe: $!"
828 : "Exit status $? from sort";
829 open(INPUT, 'foo') # get sort's results
830 or die "Can't open 'foo' for input: $!";
a0d0e21e 831
5a964f20
TC
832FILEHANDLE may be an expression whose value can be used as an indirect
833filehandle, usually the real filehandle name.
a0d0e21e
LW
834
835=item closedir DIRHANDLE
836
19799a22 837Closes a directory opened by C<opendir> and returns the success of that
5a964f20
TC
838system call.
839
a0d0e21e
LW
840=item connect SOCKET,NAME
841
842Attempts to connect to a remote socket, just as the connect system call
19799a22 843does. Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
844packed address of the appropriate type for the socket. See the examples in
845L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 846
cb1a09d0
AD
847=item continue BLOCK
848
cf264981
SP
849C<continue> is actually a flow control statement rather than a function. If
850there is a C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
98293880
JH
851C<foreach>), it is always executed just before the conditional is about to
852be evaluated again, just like the third part of a C<for> loop in C. Thus
cb1a09d0
AD
853it can be used to increment a loop variable, even when the loop has been
854continued via the C<next> statement (which is similar to the C C<continue>
855statement).
856
98293880 857C<last>, C<next>, or C<redo> may appear within a C<continue>
19799a22
GS
858block. C<last> and C<redo> will behave as if they had been executed within
859the main block. So will C<next>, but since it will execute a C<continue>
1d2dff63
GS
860block, it may be more entertaining.
861
862 while (EXPR) {
863 ### redo always comes here
864 do_something;
865 } continue {
866 ### next always comes here
867 do_something_else;
868 # then back the top to re-check EXPR
869 }
870 ### last always comes here
871
872Omitting the C<continue> section is semantically equivalent to using an
19799a22 873empty one, logically enough. In that case, C<next> goes directly back
1d2dff63
GS
874to check the condition at the top of the loop.
875
a0d0e21e
LW
876=item cos EXPR
877
d6217f1e
GS
878=item cos
879
5a964f20 880Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
7660c0ab 881takes cosine of C<$_>.
a0d0e21e 882
ca6e1c26 883For the inverse cosine operation, you may use the C<Math::Trig::acos()>
28757baa
PP
884function, or use this relation:
885
886 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
887
a0d0e21e
LW
888=item crypt PLAINTEXT,SALT
889
ef2e6798
MS
890Creates a digest string exactly like the crypt(3) function in the C
891library (assuming that you actually have a version there that has not
cf264981 892been extirpated as a potential munitions).
ef2e6798
MS
893
894crypt() is a one-way hash function. The PLAINTEXT and SALT is turned
895into a short string, called a digest, which is returned. The same
896PLAINTEXT and SALT will always return the same string, but there is no
897(known) way to get the original PLAINTEXT from the hash. Small
898changes in the PLAINTEXT or SALT will result in large changes in the
899digest.
900
901There is no decrypt function. This function isn't all that useful for
902cryptography (for that, look for F<Crypt> modules on your nearby CPAN
903mirror) and the name "crypt" is a bit of a misnomer. Instead it is
904primarily used to check if two pieces of text are the same without
905having to transmit or store the text itself. An example is checking
906if a correct password is given. The digest of the password is stored,
cf264981 907not the password itself. The user types in a password that is
ef2e6798
MS
908crypt()'d with the same salt as the stored digest. If the two digests
909match the password is correct.
910
911When verifying an existing digest string you should use the digest as
912the salt (like C<crypt($plain, $digest) eq $digest>). The SALT used
cf264981 913to create the digest is visible as part of the digest. This ensures
ef2e6798
MS
914crypt() will hash the new string with the same salt as the digest.
915This allows your code to work with the standard L<crypt|/crypt> and
916with more exotic implementations. In other words, do not assume
917anything about the returned string itself, or how many bytes in the
918digest matter.
85c16d83
JH
919
920Traditionally the result is a string of 13 bytes: two first bytes of
921the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
ef2e6798
MS
922the first eight bytes of the digest string mattered, but alternative
923hashing schemes (like MD5), higher level security schemes (like C2),
924and implementations on non-UNIX platforms may produce different
925strings.
85c16d83
JH
926
927When choosing a new salt create a random two character string whose
928characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
d3989d75
CW
929'/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of
930characters is just a recommendation; the characters allowed in
931the salt depend solely on your system's crypt library, and Perl can't
932restrict what salts C<crypt()> accepts.
e71965be 933
a0d0e21e 934Here's an example that makes sure that whoever runs this program knows
cf264981 935their password:
a0d0e21e
LW
936
937 $pwd = (getpwuid($<))[1];
a0d0e21e
LW
938
939 system "stty -echo";
940 print "Password: ";
e71965be 941 chomp($word = <STDIN>);
a0d0e21e
LW
942 print "\n";
943 system "stty echo";
944
e71965be 945 if (crypt($word, $pwd) ne $pwd) {
a0d0e21e
LW
946 die "Sorry...\n";
947 } else {
948 print "ok\n";
54310121 949 }
a0d0e21e 950
9f8f0c9d 951Of course, typing in your own password to whoever asks you
748a9306 952for it is unwise.
a0d0e21e 953
ef2e6798 954The L<crypt|/crypt> function is unsuitable for hashing large quantities
19799a22 955of data, not least of all because you can't get the information
ef2e6798 956back. Look at the L<Digest> module for more robust algorithms.
19799a22 957
f2791508
JH
958If using crypt() on a Unicode string (which I<potentially> has
959characters with codepoints above 255), Perl tries to make sense
960of the situation by trying to downgrade (a copy of the string)
961the string back to an eight-bit byte string before calling crypt()
962(on that copy). If that works, good. If not, crypt() dies with
963C<Wide character in crypt>.
85c16d83 964
aa689395 965=item dbmclose HASH
a0d0e21e 966
19799a22 967[This function has been largely superseded by the C<untie> function.]
a0d0e21e 968
aa689395 969Breaks the binding between a DBM file and a hash.
a0d0e21e 970
19799a22 971=item dbmopen HASH,DBNAME,MASK
a0d0e21e 972
19799a22 973[This function has been largely superseded by the C<tie> function.]
a0d0e21e 974
7b8d334a 975This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
19799a22
GS
976hash. HASH is the name of the hash. (Unlike normal C<open>, the first
977argument is I<not> a filehandle, even though it looks like one). DBNAME
aa689395
PP
978is the name of the database (without the F<.dir> or F<.pag> extension if
979any). If the database does not exist, it is created with protection
19799a22
GS
980specified by MASK (as modified by the C<umask>). If your system supports
981only the older DBM functions, you may perform only one C<dbmopen> in your
aa689395 982program. In older versions of Perl, if your system had neither DBM nor
19799a22 983ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
aa689395
PP
984sdbm(3).
985
986If you don't have write access to the DBM file, you can only read hash
987variables, not set them. If you want to test whether you can write,
19799a22 988either use file tests or try setting a dummy hash entry inside an C<eval>,
aa689395 989which will trap the error.
a0d0e21e 990
19799a22
GS
991Note that functions such as C<keys> and C<values> may return huge lists
992when used on large DBM files. You may prefer to use the C<each>
a0d0e21e
LW
993function to iterate over large DBM files. Example:
994
995 # print out history file offsets
996 dbmopen(%HIST,'/usr/lib/news/history',0666);
997 while (($key,$val) = each %HIST) {
998 print $key, ' = ', unpack('L',$val), "\n";
999 }
1000 dbmclose(%HIST);
1001
cb1a09d0 1002See also L<AnyDBM_File> for a more general description of the pros and
184e9718 1003cons of the various dbm approaches, as well as L<DB_File> for a particularly
cb1a09d0 1004rich implementation.
4633a7c4 1005
2b5ab1e7
TC
1006You can control which DBM library you use by loading that library
1007before you call dbmopen():
1008
1009 use DB_File;
1010 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
1011 or die "Can't open netscape history file: $!";
1012
a0d0e21e
LW
1013=item defined EXPR
1014
54310121 1015=item defined
bbce6d69 1016
2f9daede
TPG
1017Returns a Boolean value telling whether EXPR has a value other than
1018the undefined value C<undef>. If EXPR is not present, C<$_> will be
1019checked.
1020
1021Many operations return C<undef> to indicate failure, end of file,
1022system error, uninitialized variable, and other exceptional
1023conditions. This function allows you to distinguish C<undef> from
1024other values. (A simple Boolean test will not distinguish among
7660c0ab 1025C<undef>, zero, the empty string, and C<"0">, which are all equally
2f9daede 1026false.) Note that since C<undef> is a valid scalar, its presence
19799a22 1027doesn't I<necessarily> indicate an exceptional condition: C<pop>
2f9daede
TPG
1028returns C<undef> when its argument is an empty array, I<or> when the
1029element to return happens to be C<undef>.
1030
f10b0346
GS
1031You may also use C<defined(&func)> to check whether subroutine C<&func>
1032has ever been defined. The return value is unaffected by any forward
04891299 1033declarations of C<&func>. Note that a subroutine which is not defined
847c7ebe
DD
1034may still be callable: its package may have an C<AUTOLOAD> method that
1035makes it spring into existence the first time that it is called -- see
1036L<perlsub>.
f10b0346
GS
1037
1038Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
1039used to report whether memory for that aggregate has ever been
1040allocated. This behavior may disappear in future versions of Perl.
1041You should instead use a simple test for size:
1042
1043 if (@an_array) { print "has array elements\n" }
1044 if (%a_hash) { print "has hash members\n" }
2f9daede
TPG
1045
1046When used on a hash element, it tells you whether the value is defined,
dc848c6f 1047not whether the key exists in the hash. Use L</exists> for the latter
2f9daede 1048purpose.
a0d0e21e
LW
1049
1050Examples:
1051
1052 print if defined $switch{'D'};
1053 print "$val\n" while defined($val = pop(@ary));
1054 die "Can't readlink $sym: $!"
1055 unless defined($value = readlink $sym);
a0d0e21e 1056 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
2f9daede 1057 $debugging = 0 unless defined $debugging;
a0d0e21e 1058
19799a22 1059Note: Many folks tend to overuse C<defined>, and then are surprised to
7660c0ab 1060discover that the number C<0> and C<""> (the zero-length string) are, in fact,
2f9daede 1061defined values. For example, if you say
a5f75d66
AD
1062
1063 "ab" =~ /a(.*)b/;
1064
7660c0ab 1065The pattern match succeeds, and C<$1> is defined, despite the fact that it
cf264981 1066matched "nothing". It didn't really fail to match anything. Rather, it
2b5ab1e7 1067matched something that happened to be zero characters long. This is all
a5f75d66 1068very above-board and honest. When a function returns an undefined value,
2f9daede 1069it's an admission that it couldn't give you an honest answer. So you
19799a22 1070should use C<defined> only when you're questioning the integrity of what
7660c0ab 1071you're trying to do. At other times, a simple comparison to C<0> or C<""> is
2f9daede
TPG
1072what you want.
1073
dc848c6f 1074See also L</undef>, L</exists>, L</ref>.
2f9daede 1075
a0d0e21e
LW
1076=item delete EXPR
1077
01020589
GS
1078Given an expression that specifies a hash element, array element, hash slice,
1079or array slice, deletes the specified element(s) from the hash or array.
8216c1fd 1080In the case of an array, if the array elements happen to be at the end,
b76cc8ba 1081the size of the array will shrink to the highest element that tests
8216c1fd 1082true for exists() (or 0 if no such element exists).
a0d0e21e 1083
eba0920a
EM
1084Returns a list with the same number of elements as the number of elements
1085for which deletion was attempted. Each element of that list consists of
1086either the value of the element deleted, or the undefined value. In scalar
1087context, this means that you get the value of the last element deleted (or
1088the undefined value if that element did not exist).
1089
1090 %hash = (foo => 11, bar => 22, baz => 33);
1091 $scalar = delete $hash{foo}; # $scalar is 11
1092 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
1093 @array = delete @hash{qw(foo bar baz)}; # @array is (undef,undef,33)
1094
1095Deleting from C<%ENV> modifies the environment. Deleting from
01020589
GS
1096a hash tied to a DBM file deletes the entry from the DBM file. Deleting
1097from a C<tie>d hash or array may not necessarily return anything.
1098
8ea97a1e
GS
1099Deleting an array element effectively returns that position of the array
1100to its initial, uninitialized state. Subsequently testing for the same
cf264981
SP
1101element with exists() will return false. Also, deleting array elements
1102in the middle of an array will not shift the index of the elements
1103after them down. Use splice() for that. See L</exists>.
8ea97a1e 1104
01020589 1105The following (inefficiently) deletes all the values of %HASH and @ARRAY:
a0d0e21e 1106
5f05dabc
PP
1107 foreach $key (keys %HASH) {
1108 delete $HASH{$key};
a0d0e21e
LW
1109 }
1110
01020589
GS
1111 foreach $index (0 .. $#ARRAY) {
1112 delete $ARRAY[$index];
1113 }
1114
1115And so do these:
5f05dabc 1116
01020589
GS
1117 delete @HASH{keys %HASH};
1118
9740c838 1119 delete @ARRAY[0 .. $#ARRAY];
5f05dabc 1120
2b5ab1e7 1121But both of these are slower than just assigning the empty list
01020589
GS
1122or undefining %HASH or @ARRAY:
1123
1124 %HASH = (); # completely empty %HASH
1125 undef %HASH; # forget %HASH ever existed
2b5ab1e7 1126
01020589
GS
1127 @ARRAY = (); # completely empty @ARRAY
1128 undef @ARRAY; # forget @ARRAY ever existed
2b5ab1e7
TC
1129
1130Note that the EXPR can be arbitrarily complicated as long as the final
01020589
GS
1131operation is a hash element, array element, hash slice, or array slice
1132lookup:
a0d0e21e
LW
1133
1134 delete $ref->[$x][$y]{$key};
5f05dabc 1135 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
a0d0e21e 1136
01020589
GS
1137 delete $ref->[$x][$y][$index];
1138 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1139
a0d0e21e
LW
1140=item die LIST
1141
19799a22
GS
1142Outside an C<eval>, prints the value of LIST to C<STDERR> and
1143exits with the current value of C<$!> (errno). If C<$!> is C<0>,
61eff3bc
JH
1144exits with the value of C<<< ($? >> 8) >>> (backtick `command`
1145status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside
19799a22
GS
1146an C<eval(),> the error message is stuffed into C<$@> and the
1147C<eval> is terminated with the undefined value. This makes
1148C<die> the way to raise an exception.
a0d0e21e
LW
1149
1150Equivalent examples:
1151
1152 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
54310121 1153 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
a0d0e21e 1154
ccac6780 1155If the last element of LIST does not end in a newline, the current
df37ec69
WW
1156script line number and input line number (if any) are also printed,
1157and a newline is supplied. Note that the "input line number" (also
1158known as "chunk") is subject to whatever notion of "line" happens to
1159be currently in effect, and is also available as the special variable
1160C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1161
1162Hint: sometimes appending C<", stopped"> to your message will cause it
1163to make better sense when the string C<"at foo line 123"> is appended.
1164Suppose you are running script "canasta".
a0d0e21e
LW
1165
1166 die "/etc/games is no good";
1167 die "/etc/games is no good, stopped";
1168
1169produce, respectively
1170
1171 /etc/games is no good at canasta line 123.
1172 /etc/games is no good, stopped at canasta line 123.
1173
2b5ab1e7 1174See also exit(), warn(), and the Carp module.
a0d0e21e 1175
7660c0ab
A
1176If LIST is empty and C<$@> already contains a value (typically from a
1177previous eval) that value is reused after appending C<"\t...propagated">.
fb73857a
PP
1178This is useful for propagating exceptions:
1179
1180 eval { ... };
1181 die unless $@ =~ /Expected exception/;
1182
ad216e65
JH
1183If LIST is empty and C<$@> contains an object reference that has a
1184C<PROPAGATE> method, that method will be called with additional file
1185and line number parameters. The return value replaces the value in
28a5cf3b 1186C<$@>. i.e. as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >>
ad216e65
JH
1187were called.
1188
7660c0ab 1189If C<$@> is empty then the string C<"Died"> is used.
fb73857a 1190
52531d10
GS
1191die() can also be called with a reference argument. If this happens to be
1192trapped within an eval(), $@ contains the reference. This behavior permits
1193a more elaborate exception handling implementation using objects that
4375e838 1194maintain arbitrary state about the nature of the exception. Such a scheme
52531d10
GS
1195is sometimes preferable to matching particular string values of $@ using
1196regular expressions. Here's an example:
1197
da279afe 1198 use Scalar::Util 'blessed';
1199
52531d10
GS
1200 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1201 if ($@) {
da279afe 1202 if (blessed($@) && $@->isa("Some::Module::Exception")) {
52531d10
GS
1203 # handle Some::Module::Exception
1204 }
1205 else {
1206 # handle all other possible exceptions
1207 }
1208 }
1209
19799a22 1210Because perl will stringify uncaught exception messages before displaying
52531d10
GS
1211them, you may want to overload stringification operations on such custom
1212exception objects. See L<overload> for details about that.
1213
19799a22
GS
1214You can arrange for a callback to be run just before the C<die>
1215does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1216handler will be called with the error text and can change the error
1217message, if it sees fit, by calling C<die> again. See
1218L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
cf264981 1219L<"eval BLOCK"> for some examples. Although this feature was
19799a22
GS
1220to be run only right before your program was to exit, this is not
1221currently the case--the C<$SIG{__DIE__}> hook is currently called
1222even inside eval()ed blocks/strings! If one wants the hook to do
1223nothing in such situations, put
fb73857a
PP
1224
1225 die @_ if $^S;
1226
19799a22
GS
1227as the first line of the handler (see L<perlvar/$^S>). Because
1228this promotes strange action at a distance, this counterintuitive
b76cc8ba 1229behavior may be fixed in a future release.
774d564b 1230
a0d0e21e
LW
1231=item do BLOCK
1232
1233Not really a function. Returns the value of the last command in the
6b275a1f
RGS
1234sequence of commands indicated by BLOCK. When modified by the C<while> or
1235C<until> loop modifier, executes the BLOCK once before testing the loop
1236condition. (On other statements the loop modifiers test the conditional
1237first.)
a0d0e21e 1238
4968c1e4 1239C<do BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7
TC
1240C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1241See L<perlsyn> for alternative strategies.
4968c1e4 1242
a0d0e21e
LW
1243=item do SUBROUTINE(LIST)
1244
cf264981 1245This form of subroutine call is deprecated. See L<perlsub>.
a0d0e21e
LW
1246
1247=item do EXPR
1248
1249Uses the value of EXPR as a filename and executes the contents of the
ea63ef19 1250file as a Perl script.
a0d0e21e
LW
1251
1252 do 'stat.pl';
1253
1254is just like
1255
986b19de 1256 eval `cat stat.pl`;
a0d0e21e 1257
2b5ab1e7 1258except that it's more efficient and concise, keeps track of the current
ea63ef19 1259filename for error messages, searches the @INC directories, and updates
2b5ab1e7
TC
1260C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1261variables. It also differs in that code evaluated with C<do FILENAME>
1262cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1263same, however, in that it does reparse the file every time you call it,
1264so you probably don't want to do this inside a loop.
a0d0e21e 1265
8e30cc93 1266If C<do> cannot read the file, it returns undef and sets C<$!> to the
2b5ab1e7 1267error. If C<do> can read the file but cannot compile it, it
8e30cc93
G
1268returns undef and sets an error message in C<$@>. If the file is
1269successfully compiled, C<do> returns the value of the last expression
1270evaluated.
1271
a0d0e21e 1272Note that inclusion of library modules is better done with the
19799a22 1273C<use> and C<require> operators, which also do automatic error checking
4633a7c4 1274and raise an exception if there's a problem.
a0d0e21e 1275
5a964f20
TC
1276You might like to use C<do> to read in a program configuration
1277file. Manual error checking can be done this way:
1278
b76cc8ba 1279 # read in config files: system first, then user
f86cebdf 1280 for $file ("/share/prog/defaults.rc",
b76cc8ba 1281 "$ENV{HOME}/.someprogrc")
2b5ab1e7 1282 {
5a964f20 1283 unless ($return = do $file) {
f86cebdf
GS
1284 warn "couldn't parse $file: $@" if $@;
1285 warn "couldn't do $file: $!" unless defined $return;
1286 warn "couldn't run $file" unless $return;
5a964f20
TC
1287 }
1288 }
1289
a0d0e21e
LW
1290=item dump LABEL
1291
1614b0e3
JD
1292=item dump
1293
19799a22
GS
1294This function causes an immediate core dump. See also the B<-u>
1295command-line switch in L<perlrun>, which does the same thing.
1296Primarily this is so that you can use the B<undump> program (not
1297supplied) to turn your core dump into an executable binary after
1298having initialized all your variables at the beginning of the
1299program. When the new binary is executed it will begin by executing
1300a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1301Think of it as a goto with an intervening core dump and reincarnation.
1302If C<LABEL> is omitted, restarts the program from the top.
1303
1304B<WARNING>: Any files opened at the time of the dump will I<not>
1305be open any more when the program is reincarnated, with possible
b76cc8ba 1306resulting confusion on the part of Perl.
19799a22
GS
1307
1308This function is now largely obsolete, partly because it's very
1309hard to convert a core file into an executable, and because the
1310real compiler backends for generating portable bytecode and compilable
ac206dc8
RGS
1311C code have superseded it. That's why you should now invoke it as
1312C<CORE::dump()>, if you don't want to be warned against a possible
1313typo.
19799a22
GS
1314
1315If you're looking to use L<dump> to speed up your program, consider
1316generating bytecode or native C code as described in L<perlcc>. If
1317you're just trying to accelerate a CGI script, consider using the
210b36aa 1318C<mod_perl> extension to B<Apache>, or the CPAN module, CGI::Fast.
19799a22 1319You might also consider autoloading or selfloading, which at least
b76cc8ba 1320make your program I<appear> to run faster.
5a964f20 1321
aa689395
PP
1322=item each HASH
1323
5a964f20 1324When called in list context, returns a 2-element list consisting of the
aa689395 1325key and value for the next element of a hash, so that you can iterate over
74fc8b5f 1326it. When called in scalar context, returns only the key for the next
e902a979 1327element in the hash.
2f9daede 1328
ab192400 1329Entries are returned in an apparently random order. The actual random
504f80c1
JH
1330order is subject to change in future versions of perl, but it is
1331guaranteed to be in the same order as either the C<keys> or C<values>
4546b9e6
JH
1332function would produce on the same (unmodified) hash. Since Perl
13335.8.1 the ordering is different even between different runs of Perl
1334for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
ab192400
GS
1335
1336When the hash is entirely read, a null array is returned in list context
19799a22
GS
1337(which when assigned produces a false (C<0>) value), and C<undef> in
1338scalar context. The next call to C<each> after that will start iterating
1339again. There is a single iterator for each hash, shared by all C<each>,
1340C<keys>, and C<values> function calls in the program; it can be reset by
2f9daede
TPG
1341reading all the elements from the hash, or by evaluating C<keys HASH> or
1342C<values HASH>. If you add or delete elements of a hash while you're
74fc8b5f
MJD
1343iterating over it, you may get entries skipped or duplicated, so
1344don't. Exception: It is always safe to delete the item most recently
1345returned by C<each()>, which means that the following code will work:
1346
1347 while (($key, $value) = each %hash) {
1348 print $key, "\n";
1349 delete $hash{$key}; # This is safe
1350 }
aa689395 1351
f86cebdf 1352The following prints out your environment like the printenv(1) program,
aa689395 1353only in a different order:
a0d0e21e
LW
1354
1355 while (($key,$value) = each %ENV) {
1356 print "$key=$value\n";
1357 }
1358
19799a22 1359See also C<keys>, C<values> and C<sort>.
a0d0e21e
LW
1360
1361=item eof FILEHANDLE
1362
4633a7c4
LW
1363=item eof ()
1364
a0d0e21e
LW
1365=item eof
1366
1367Returns 1 if the next read on FILEHANDLE will return end of file, or if
1368FILEHANDLE is not open. FILEHANDLE may be an expression whose value
5a964f20 1369gives the real filehandle. (Note that this function actually
19799a22 1370reads a character and then C<ungetc>s it, so isn't very useful in an
748a9306 1371interactive context.) Do not read from a terminal file (or call
19799a22 1372C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
748a9306
LW
1373as terminals may lose the end-of-file condition if you do.
1374
820475bd
GS
1375An C<eof> without an argument uses the last file read. Using C<eof()>
1376with empty parentheses is very different. It refers to the pseudo file
1377formed from the files listed on the command line and accessed via the
61eff3bc
JH
1378C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1379as a normal filehandle is, an C<eof()> before C<< <> >> has been
820475bd 1380used will cause C<@ARGV> to be examined to determine if input is
67408cae 1381available. Similarly, an C<eof()> after C<< <> >> has returned
efdd0218
RB
1382end-of-file will assume you are processing another C<@ARGV> list,
1383and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1384see L<perlop/"I/O Operators">.
820475bd 1385
61eff3bc 1386In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
820475bd
GS
1387detect the end of each file, C<eof()> will only detect the end of the
1388last file. Examples:
a0d0e21e 1389
748a9306
LW
1390 # reset line numbering on each input file
1391 while (<>) {
b76cc8ba 1392 next if /^\s*#/; # skip comments
748a9306 1393 print "$.\t$_";
5a964f20
TC
1394 } continue {
1395 close ARGV if eof; # Not eof()!
748a9306
LW
1396 }
1397
a0d0e21e
LW
1398 # insert dashes just before last line of last file
1399 while (<>) {
6ac88b13 1400 if (eof()) { # check for end of last file
a0d0e21e
LW
1401 print "--------------\n";
1402 }
1403 print;
6ac88b13 1404 last if eof(); # needed if we're reading from a terminal
a0d0e21e
LW
1405 }
1406
a0d0e21e 1407Practical hint: you almost never need to use C<eof> in Perl, because the
3ce0d271
GS
1408input operators typically return C<undef> when they run out of data, or if
1409there was an error.
a0d0e21e
LW
1410
1411=item eval EXPR
1412
1413=item eval BLOCK
1414
ce2984c3
PF
1415=item eval
1416
c7cc6f1c
GS
1417In the first form, the return value of EXPR is parsed and executed as if it
1418were a little Perl program. The value of the expression (which is itself
5a964f20 1419determined within scalar context) is first parsed, and if there weren't any
be3174d2
GS
1420errors, executed in the lexical context of the current Perl program, so
1421that any variable settings or subroutine and format definitions remain
cf264981 1422afterwards. Note that the value is parsed every time the C<eval> executes.
be3174d2
GS
1423If EXPR is omitted, evaluates C<$_>. This form is typically used to
1424delay parsing and subsequent execution of the text of EXPR until run time.
c7cc6f1c
GS
1425
1426In the second form, the code within the BLOCK is parsed only once--at the
cf264981 1427same time the code surrounding the C<eval> itself was parsed--and executed
c7cc6f1c
GS
1428within the context of the current Perl program. This form is typically
1429used to trap exceptions more efficiently than the first (see below), while
1430also providing the benefit of checking the code within BLOCK at compile
1431time.
1432
1433The final semicolon, if any, may be omitted from the value of EXPR or within
1434the BLOCK.
1435
1436In both forms, the value returned is the value of the last expression
5a964f20 1437evaluated inside the mini-program; a return statement may be also used, just
c7cc6f1c 1438as with subroutines. The expression providing the return value is evaluated
cf264981
SP
1439in void, scalar, or list context, depending on the context of the C<eval>
1440itself. See L</wantarray> for more on how the evaluation context can be
1441determined.
a0d0e21e 1442
19799a22
GS
1443If there is a syntax error or runtime error, or a C<die> statement is
1444executed, an undefined value is returned by C<eval>, and C<$@> is set to the
a0d0e21e 1445error message. If there was no error, C<$@> is guaranteed to be a null
19799a22 1446string. Beware that using C<eval> neither silences perl from printing
c7cc6f1c 1447warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
d9984052
A
1448To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1449turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1450See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
a0d0e21e 1451
19799a22
GS
1452Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1453determining whether a particular feature (such as C<socket> or C<symlink>)
a0d0e21e
LW
1454is implemented. It is also Perl's exception trapping mechanism, where
1455the die operator is used to raise exceptions.
1456
1457If the code to be executed doesn't vary, you may use the eval-BLOCK
1458form to trap run-time errors without incurring the penalty of
1459recompiling each time. The error, if any, is still returned in C<$@>.
1460Examples:
1461
54310121 1462 # make divide-by-zero nonfatal
a0d0e21e
LW
1463 eval { $answer = $a / $b; }; warn $@ if $@;
1464
1465 # same thing, but less efficient
1466 eval '$answer = $a / $b'; warn $@ if $@;
1467
1468 # a compile-time error
5a964f20 1469 eval { $answer = }; # WRONG
a0d0e21e
LW
1470
1471 # a run-time error
1472 eval '$answer ='; # sets $@
1473
cf264981
SP
1474Using the C<eval{}> form as an exception trap in libraries does have some
1475issues. Due to the current arguably broken state of C<__DIE__> hooks, you
1476may wish not to trigger any C<__DIE__> hooks that user code may have installed.
2b5ab1e7
TC
1477You can use the C<local $SIG{__DIE__}> construct for this purpose,
1478as shown in this example:
774d564b
PP
1479
1480 # a very private exception trap for divide-by-zero
f86cebdf
GS
1481 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1482 warn $@ if $@;
774d564b
PP
1483
1484This is especially significant, given that C<__DIE__> hooks can call
19799a22 1485C<die> again, which has the effect of changing their error messages:
774d564b
PP
1486
1487 # __DIE__ hooks may modify error messages
1488 {
f86cebdf
GS
1489 local $SIG{'__DIE__'} =
1490 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
c7cc6f1c
GS
1491 eval { die "foo lives here" };
1492 print $@ if $@; # prints "bar lives here"
774d564b
PP
1493 }
1494
19799a22 1495Because this promotes action at a distance, this counterintuitive behavior
2b5ab1e7
TC
1496may be fixed in a future release.
1497
19799a22 1498With an C<eval>, you should be especially careful to remember what's
a0d0e21e
LW
1499being looked at when:
1500
1501 eval $x; # CASE 1
1502 eval "$x"; # CASE 2
1503
1504 eval '$x'; # CASE 3
1505 eval { $x }; # CASE 4
1506
5a964f20 1507 eval "\$$x++"; # CASE 5
a0d0e21e
LW
1508 $$x++; # CASE 6
1509
2f9daede 1510Cases 1 and 2 above behave identically: they run the code contained in
19799a22 1511the variable $x. (Although case 2 has misleading double quotes making
2f9daede 1512the reader wonder what else might be happening (nothing is).) Cases 3
7660c0ab 1513and 4 likewise behave in the same way: they run the code C<'$x'>, which
19799a22 1514does nothing but return the value of $x. (Case 4 is preferred for
2f9daede
TPG
1515purely visual reasons, but it also has the advantage of compiling at
1516compile-time instead of at run-time.) Case 5 is a place where
19799a22 1517normally you I<would> like to use double quotes, except that in this
2f9daede
TPG
1518particular situation, you can just use symbolic references instead, as
1519in case 6.
a0d0e21e 1520
4968c1e4 1521C<eval BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7 1522C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
4968c1e4 1523
d819b83a
DM
1524Note that as a very special case, an C<eval ''> executed within the C<DB>
1525package doesn't see the usual surrounding lexical scope, but rather the
1526scope of the first non-DB piece of code that called it. You don't normally
1527need to worry about this unless you are writing a Perl debugger.
1528
a0d0e21e
LW
1529=item exec LIST
1530
8bf3b016
GS
1531=item exec PROGRAM LIST
1532
19799a22
GS
1533The C<exec> function executes a system command I<and never returns>--
1534use C<system> instead of C<exec> if you want it to return. It fails and
1535returns false only if the command does not exist I<and> it is executed
fb73857a 1536directly instead of via your system's command shell (see below).
a0d0e21e 1537
19799a22
GS
1538Since it's a common mistake to use C<exec> instead of C<system>, Perl
1539warns you if there is a following statement which isn't C<die>, C<warn>,
1540or C<exit> (if C<-w> is set - but you always do that). If you
1541I<really> want to follow an C<exec> with some other statement, you
55d729e4
GS
1542can use one of these styles to avoid the warning:
1543
5a964f20
TC
1544 exec ('foo') or print STDERR "couldn't exec foo: $!";
1545 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
55d729e4 1546
5a964f20 1547If there is more than one argument in LIST, or if LIST is an array
f86cebdf 1548with more than one value, calls execvp(3) with the arguments in LIST.
5a964f20
TC
1549If there is only one scalar argument or an array with one element in it,
1550the argument is checked for shell metacharacters, and if there are any,
1551the entire argument is passed to the system's command shell for parsing
1552(this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1553If there are no shell metacharacters in the argument, it is split into
b76cc8ba 1554words and passed directly to C<execvp>, which is more efficient.
19799a22 1555Examples:
a0d0e21e 1556
19799a22
GS
1557 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1558 exec "sort $outfile | uniq";
a0d0e21e
LW
1559
1560If you don't really want to execute the first argument, but want to lie
1561to the program you are executing about its own name, you can specify
1562the program you actually want to run as an "indirect object" (without a
1563comma) in front of the LIST. (This always forces interpretation of the
54310121 1564LIST as a multivalued list, even if there is only a single scalar in
a0d0e21e
LW
1565the list.) Example:
1566
1567 $shell = '/bin/csh';
1568 exec $shell '-sh'; # pretend it's a login shell
1569
1570or, more directly,
1571
1572 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1573
bb32b41a
GS
1574When the arguments get executed via the system shell, results will
1575be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1576for details.
1577
19799a22
GS
1578Using an indirect object with C<exec> or C<system> is also more
1579secure. This usage (which also works fine with system()) forces
1580interpretation of the arguments as a multivalued list, even if the
1581list had just one argument. That way you're safe from the shell
1582expanding wildcards or splitting up words with whitespace in them.
5a964f20
TC
1583
1584 @args = ( "echo surprise" );
1585
2b5ab1e7 1586 exec @args; # subject to shell escapes
f86cebdf 1587 # if @args == 1
2b5ab1e7 1588 exec { $args[0] } @args; # safe even with one-arg list
5a964f20
TC
1589
1590The first version, the one without the indirect object, ran the I<echo>
1591program, passing it C<"surprise"> an argument. The second version
1592didn't--it tried to run a program literally called I<"echo surprise">,
1593didn't find it, and set C<$?> to a non-zero value indicating failure.
1594
0f897271
GS
1595Beginning with v5.6.0, Perl will attempt to flush all files opened for
1596output before the exec, but this may not be supported on some platforms
1597(see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1598in English) or call the C<autoflush()> method of C<IO::Handle> on any
1599open handles in order to avoid lost output.
1600
19799a22 1601Note that C<exec> will not call your C<END> blocks, nor will it call
7660c0ab
A
1602any C<DESTROY> methods in your objects.
1603
a0d0e21e
LW
1604=item exists EXPR
1605
01020589 1606Given an expression that specifies a hash element or array element,
8ea97a1e
GS
1607returns true if the specified element in the hash or array has ever
1608been initialized, even if the corresponding value is undefined. The
1609element is not autovivified if it doesn't exist.
a0d0e21e 1610
01020589
GS
1611 print "Exists\n" if exists $hash{$key};
1612 print "Defined\n" if defined $hash{$key};
1613 print "True\n" if $hash{$key};
1614
1615 print "Exists\n" if exists $array[$index];
1616 print "Defined\n" if defined $array[$index];
1617 print "True\n" if $array[$index];
a0d0e21e 1618
8ea97a1e 1619A hash or array element can be true only if it's defined, and defined if
a0d0e21e
LW
1620it exists, but the reverse doesn't necessarily hold true.
1621
afebc493
GS
1622Given an expression that specifies the name of a subroutine,
1623returns true if the specified subroutine has ever been declared, even
1624if it is undefined. Mentioning a subroutine name for exists or defined
847c7ebe
DD
1625does not count as declaring it. Note that a subroutine which does not
1626exist may still be callable: its package may have an C<AUTOLOAD>
1627method that makes it spring into existence the first time that it is
1628called -- see L<perlsub>.
afebc493
GS
1629
1630 print "Exists\n" if exists &subroutine;
1631 print "Defined\n" if defined &subroutine;
1632
a0d0e21e 1633Note that the EXPR can be arbitrarily complicated as long as the final
afebc493 1634operation is a hash or array key lookup or subroutine name:
a0d0e21e 1635
2b5ab1e7
TC
1636 if (exists $ref->{A}->{B}->{$key}) { }
1637 if (exists $hash{A}{B}{$key}) { }
1638
01020589
GS
1639 if (exists $ref->{A}->{B}->[$ix]) { }
1640 if (exists $hash{A}{B}[$ix]) { }
1641
afebc493
GS
1642 if (exists &{$ref->{A}{B}{$key}}) { }
1643
01020589
GS
1644Although the deepest nested array or hash will not spring into existence
1645just because its existence was tested, any intervening ones will.
61eff3bc 1646Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
01020589
GS
1647into existence due to the existence test for the $key element above.
1648This happens anywhere the arrow operator is used, including even:
5a964f20 1649
2b5ab1e7
TC
1650 undef $ref;
1651 if (exists $ref->{"Some key"}) { }
1652 print $ref; # prints HASH(0x80d3d5c)
1653
1654This surprising autovivification in what does not at first--or even
1655second--glance appear to be an lvalue context may be fixed in a future
5a964f20 1656release.
a0d0e21e 1657
afebc493
GS
1658Use of a subroutine call, rather than a subroutine name, as an argument
1659to exists() is an error.
1660
1661 exists &sub; # OK
1662 exists &sub(); # Error
1663
a0d0e21e
LW
1664=item exit EXPR
1665
ce2984c3
PF
1666=item exit
1667
2b5ab1e7 1668Evaluates EXPR and exits immediately with that value. Example:
a0d0e21e
LW
1669
1670 $ans = <STDIN>;
1671 exit 0 if $ans =~ /^[Xx]/;
1672
19799a22 1673See also C<die>. If EXPR is omitted, exits with C<0> status. The only
2b5ab1e7
TC
1674universally recognized values for EXPR are C<0> for success and C<1>
1675for error; other values are subject to interpretation depending on the
1676environment in which the Perl program is running. For example, exiting
167769 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1678the mailer to return the item undelivered, but that's not true everywhere.
a0d0e21e 1679
19799a22
GS
1680Don't use C<exit> to abort a subroutine if there's any chance that
1681someone might want to trap whatever error happened. Use C<die> instead,
1682which can be trapped by an C<eval>.
28757baa 1683
19799a22 1684The exit() function does not always exit immediately. It calls any
2b5ab1e7 1685defined C<END> routines first, but these C<END> routines may not
19799a22 1686themselves abort the exit. Likewise any object destructors that need to
2b5ab1e7
TC
1687be called are called before the real exit. If this is a problem, you
1688can call C<POSIX:_exit($status)> to avoid END and destructor processing.
87275199 1689See L<perlmod> for details.
5a964f20 1690
a0d0e21e
LW
1691=item exp EXPR
1692
54310121 1693=item exp
bbce6d69 1694
b76cc8ba 1695Returns I<e> (the natural logarithm base) to the power of EXPR.
a0d0e21e
LW
1696If EXPR is omitted, gives C<exp($_)>.
1697
1698=item fcntl FILEHANDLE,FUNCTION,SCALAR
1699
f86cebdf 1700Implements the fcntl(2) function. You'll probably have to say
a0d0e21e
LW
1701
1702 use Fcntl;
1703
0ade1984 1704first to get the correct constant definitions. Argument processing and
b76cc8ba 1705value return works just like C<ioctl> below.
a0d0e21e
LW
1706For example:
1707
1708 use Fcntl;
5a964f20
TC
1709 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1710 or die "can't fcntl F_GETFL: $!";
1711
554ad1fc 1712You don't have to check for C<defined> on the return from C<fcntl>.
951ba7fe
GS
1713Like C<ioctl>, it maps a C<0> return from the system call into
1714C<"0 but true"> in Perl. This string is true in boolean context and C<0>
2b5ab1e7
TC
1715in numeric context. It is also exempt from the normal B<-w> warnings
1716on improper numeric conversions.
5a964f20 1717
19799a22 1718Note that C<fcntl> will produce a fatal error if used on a machine that
2b5ab1e7
TC
1719doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1720manpage to learn what functions are available on your system.
a0d0e21e 1721
be2f7487 1722Here's an example of setting a filehandle named C<REMOTE> to be
1723non-blocking at the system level. You'll have to negotiate C<$|>
1724on your own, though.
1725
1726 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
1727
1728 $flags = fcntl(REMOTE, F_GETFL, 0)
1729 or die "Can't get flags for the socket: $!\n";
1730
1731 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
1732 or die "Can't set flags for the socket: $!\n";
1733
a0d0e21e
LW
1734=item fileno FILEHANDLE
1735
2b5ab1e7
TC
1736Returns the file descriptor for a filehandle, or undefined if the
1737filehandle is not open. This is mainly useful for constructing
19799a22 1738bitmaps for C<select> and low-level POSIX tty-handling operations.
2b5ab1e7
TC
1739If FILEHANDLE is an expression, the value is taken as an indirect
1740filehandle, generally its name.
5a964f20 1741
b76cc8ba 1742You can use this to find out whether two handles refer to the
5a964f20
TC
1743same underlying descriptor:
1744
1745 if (fileno(THIS) == fileno(THAT)) {
1746 print "THIS and THAT are dups\n";
b76cc8ba
NIS
1747 }
1748
1749(Filehandles connected to memory objects via new features of C<open> may
1750return undefined even though they are open.)
1751
a0d0e21e
LW
1752
1753=item flock FILEHANDLE,OPERATION
1754
19799a22
GS
1755Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1756for success, false on failure. Produces a fatal error if used on a
2b5ab1e7 1757machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
19799a22 1758C<flock> is Perl's portable file locking interface, although it locks
2b5ab1e7
TC
1759only entire files, not records.
1760
1761Two potentially non-obvious but traditional C<flock> semantics are
1762that it waits indefinitely until the lock is granted, and that its locks
1763B<merely advisory>. Such discretionary locks are more flexible, but offer
cf264981
SP
1764fewer guarantees. This means that programs that do not also use C<flock>
1765may modify files locked with C<flock>. See L<perlport>,
2b5ab1e7
TC
1766your port's specific documentation, or your system-specific local manpages
1767for details. It's best to assume traditional behavior if you're writing
1768portable programs. (But if you're not, you should as always feel perfectly
1769free to write for your own system's idiosyncrasies (sometimes called
1770"features"). Slavish adherence to portability concerns shouldn't get
1771in the way of your getting your job done.)
a3cb178b 1772
8ebc5c01
PP
1773OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1774LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
ea3105be 1775you can use the symbolic names if you import them from the Fcntl module,
68dc0745
PP
1776either individually, or as a group using the ':flock' tag. LOCK_SH
1777requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
ea3105be
GS
1778releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1779LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
68dc0745
PP
1780waiting for the lock (check the return status to see if you got it).
1781
2b5ab1e7
TC
1782To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1783before locking or unlocking it.
8ebc5c01 1784
f86cebdf 1785Note that the emulation built with lockf(3) doesn't provide shared
8ebc5c01 1786locks, and it requires that FILEHANDLE be open with write intent. These
2b5ab1e7 1787are the semantics that lockf(3) implements. Most if not all systems
f86cebdf 1788implement lockf(3) in terms of fcntl(2) locking, though, so the
8ebc5c01
PP
1789differing semantics shouldn't bite too many people.
1790
becacb53
TM
1791Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
1792be open with read intent to use LOCK_SH and requires that it be open
1793with write intent to use LOCK_EX.
1794
19799a22
GS
1795Note also that some versions of C<flock> cannot lock things over the
1796network; you would need to use the more system-specific C<fcntl> for
f86cebdf
GS
1797that. If you like you can force Perl to ignore your system's flock(2)
1798function, and so provide its own fcntl(2)-based emulation, by passing
8ebc5c01
PP
1799the switch C<-Ud_flock> to the F<Configure> program when you configure
1800perl.
4633a7c4
LW
1801
1802Here's a mailbox appender for BSD systems.
a0d0e21e 1803
7e1af8bc 1804 use Fcntl ':flock'; # import LOCK_* constants
a0d0e21e
LW
1805
1806 sub lock {
7e1af8bc 1807 flock(MBOX,LOCK_EX);
a0d0e21e
LW
1808 # and, in case someone appended
1809 # while we were waiting...
1810 seek(MBOX, 0, 2);
1811 }
1812
1813 sub unlock {
7e1af8bc 1814 flock(MBOX,LOCK_UN);
a0d0e21e
LW
1815 }
1816
1817 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1818 or die "Can't open mailbox: $!";
1819
1820 lock();
1821 print MBOX $msg,"\n\n";
1822 unlock();
1823
2b5ab1e7
TC
1824On systems that support a real flock(), locks are inherited across fork()
1825calls, whereas those that must resort to the more capricious fcntl()
1826function lose the locks, making it harder to write servers.
1827
cb1a09d0 1828See also L<DB_File> for other flock() examples.
a0d0e21e
LW
1829
1830=item fork
1831
2b5ab1e7
TC
1832Does a fork(2) system call to create a new process running the
1833same program at the same point. It returns the child pid to the
1834parent process, C<0> to the child process, or C<undef> if the fork is
1835unsuccessful. File descriptors (and sometimes locks on those descriptors)
1836are shared, while everything else is copied. On most systems supporting
1837fork(), great care has gone into making it extremely efficient (for
1838example, using copy-on-write technology on data pages), making it the
1839dominant paradigm for multitasking over the last few decades.
5a964f20 1840
0f897271
GS
1841Beginning with v5.6.0, Perl will attempt to flush all files opened for
1842output before forking the child process, but this may not be supported
1843on some platforms (see L<perlport>). To be safe, you may need to set
1844C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1845C<IO::Handle> on any open handles in order to avoid duplicate output.
a0d0e21e 1846
19799a22 1847If you C<fork> without ever waiting on your children, you will
2b5ab1e7
TC
1848accumulate zombies. On some systems, you can avoid this by setting
1849C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1850forking and reaping moribund children.
cb1a09d0 1851
28757baa
PP
1852Note that if your forked child inherits system file descriptors like
1853STDIN and STDOUT that are actually connected by a pipe or socket, even
2b5ab1e7 1854if you exit, then the remote server (such as, say, a CGI script or a
19799a22 1855backgrounded job launched from a remote shell) won't think you're done.
2b5ab1e7 1856You should reopen those to F</dev/null> if it's any issue.
28757baa 1857
cb1a09d0
AD
1858=item format
1859
19799a22 1860Declare a picture format for use by the C<write> function. For
cb1a09d0
AD
1861example:
1862
54310121 1863 format Something =
cb1a09d0
AD
1864 Test: @<<<<<<<< @||||| @>>>>>
1865 $str, $%, '$' . int($num)
1866 .
1867
1868 $str = "widget";
184e9718 1869 $num = $cost/$quantity;
cb1a09d0
AD
1870 $~ = 'Something';
1871 write;
1872
1873See L<perlform> for many details and examples.
1874
8903cb82 1875=item formline PICTURE,LIST
a0d0e21e 1876
5a964f20 1877This is an internal function used by C<format>s, though you may call it,
a0d0e21e
LW
1878too. It formats (see L<perlform>) a list of values according to the
1879contents of PICTURE, placing the output into the format output
7660c0ab 1880accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
19799a22 1881Eventually, when a C<write> is done, the contents of
cf264981
SP
1882C<$^A> are written to some filehandle. You could also read C<$^A>
1883and then set C<$^A> back to C<"">. Note that a format typically
19799a22 1884does one C<formline> per line of form, but the C<formline> function itself
748a9306 1885doesn't care how many newlines are embedded in the PICTURE. This means
4633a7c4 1886that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
748a9306
LW
1887You may therefore need to use multiple formlines to implement a single
1888record format, just like the format compiler.
1889
19799a22 1890Be careful if you put double quotes around the picture, because an C<@>
748a9306 1891character may be taken to mean the beginning of an array name.
19799a22 1892C<formline> always returns true. See L<perlform> for other examples.
a0d0e21e
LW
1893
1894=item getc FILEHANDLE
1895
1896=item getc
1897
1898Returns the next character from the input file attached to FILEHANDLE,
b5fe5ca2
SR
1899or the undefined value at end of file, or if there was an error (in
1900the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
1901STDIN. This is not particularly efficient. However, it cannot be
1902used by itself to fetch single characters without waiting for the user
1903to hit enter. For that, try something more like:
4633a7c4
LW
1904
1905 if ($BSD_STYLE) {
1906 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1907 }
1908 else {
54310121 1909 system "stty", '-icanon', 'eol', "\001";
4633a7c4
LW
1910 }
1911
1912 $key = getc(STDIN);
1913
1914 if ($BSD_STYLE) {
1915 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1916 }
1917 else {
5f05dabc 1918 system "stty", 'icanon', 'eol', '^@'; # ASCII null
4633a7c4
LW
1919 }
1920 print "\n";
1921
54310121
PP
1922Determination of whether $BSD_STYLE should be set
1923is left as an exercise to the reader.
cb1a09d0 1924
19799a22 1925The C<POSIX::getattr> function can do this more portably on
2b5ab1e7
TC
1926systems purporting POSIX compliance. See also the C<Term::ReadKey>
1927module from your nearest CPAN site; details on CPAN can be found on
1928L<perlmodlib/CPAN>.
a0d0e21e
LW
1929
1930=item getlogin
1931
cf264981 1932This implements the C library function of the same name, which on most
5a964f20 1933systems returns the current login from F</etc/utmp>, if any. If null,
19799a22 1934use C<getpwuid>.
a0d0e21e 1935
f86702cc 1936 $login = getlogin || getpwuid($<) || "Kilroy";
a0d0e21e 1937
19799a22
GS
1938Do not consider C<getlogin> for authentication: it is not as
1939secure as C<getpwuid>.
4633a7c4 1940
a0d0e21e
LW
1941=item getpeername SOCKET
1942
1943Returns the packed sockaddr address of other end of the SOCKET connection.
1944
4633a7c4
LW
1945 use Socket;
1946 $hersockaddr = getpeername(SOCK);
19799a22 1947 ($port, $iaddr) = sockaddr_in($hersockaddr);
4633a7c4
LW
1948 $herhostname = gethostbyaddr($iaddr, AF_INET);
1949 $herstraddr = inet_ntoa($iaddr);
a0d0e21e
LW
1950
1951=item getpgrp PID
1952
47e29363 1953Returns the current process group for the specified PID. Use
7660c0ab 1954a PID of C<0> to get the current process group for the
4633a7c4 1955current process. Will raise an exception if used on a machine that
f86cebdf 1956doesn't implement getpgrp(2). If PID is omitted, returns process
19799a22 1957group of current process. Note that the POSIX version of C<getpgrp>
7660c0ab 1958does not accept a PID argument, so only C<PID==0> is truly portable.
a0d0e21e
LW
1959
1960=item getppid
1961
1962Returns the process id of the parent process.
1963
4d76a344
RGS
1964Note for Linux users: on Linux, the C functions C<getpid()> and
1965C<getppid()> return different values from different threads. In order to
1966be portable, this behavior is not reflected by the perl-level function
1967C<getppid()>, that returns a consistent value across threads. If you want
e3256f86
RGS
1968to call the underlying C<getppid()>, you may use the CPAN module
1969C<Linux::Pid>.
4d76a344 1970
a0d0e21e
LW
1971=item getpriority WHICH,WHO
1972
4633a7c4
LW
1973Returns the current priority for a process, a process group, or a user.
1974(See L<getpriority(2)>.) Will raise a fatal exception if used on a
f86cebdf 1975machine that doesn't implement getpriority(2).
a0d0e21e
LW
1976
1977=item getpwnam NAME
1978
1979=item getgrnam NAME
1980
1981=item gethostbyname NAME
1982
1983=item getnetbyname NAME
1984
1985=item getprotobyname NAME
1986
1987=item getpwuid UID
1988
1989=item getgrgid GID
1990
1991=item getservbyname NAME,PROTO
1992
1993=item gethostbyaddr ADDR,ADDRTYPE
1994
1995=item getnetbyaddr ADDR,ADDRTYPE
1996
1997=item getprotobynumber NUMBER
1998
1999=item getservbyport PORT,PROTO
2000
2001=item getpwent
2002
2003=item getgrent
2004
2005=item gethostent
2006
2007=item getnetent
2008
2009=item getprotoent
2010
2011=item getservent
2012
2013=item setpwent
2014
2015=item setgrent
2016
2017=item sethostent STAYOPEN
2018
2019=item setnetent STAYOPEN
2020
2021=item setprotoent STAYOPEN
2022
2023=item setservent STAYOPEN
2024
2025=item endpwent
2026
2027=item endgrent
2028
2029=item endhostent
2030
2031=item endnetent
2032
2033=item endprotoent
2034
2035=item endservent
2036
2037These routines perform the same functions as their counterparts in the
5a964f20 2038system library. In list context, the return values from the
a0d0e21e
LW
2039various get routines are as follows:
2040
2041 ($name,$passwd,$uid,$gid,
6ee623d5 2042 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
a0d0e21e
LW
2043 ($name,$passwd,$gid,$members) = getgr*
2044 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
2045 ($name,$aliases,$addrtype,$net) = getnet*
2046 ($name,$aliases,$proto) = getproto*
2047 ($name,$aliases,$port,$proto) = getserv*
2048
2049(If the entry doesn't exist you get a null list.)
2050
4602f195
JH
2051The exact meaning of the $gcos field varies but it usually contains
2052the real name of the user (as opposed to the login name) and other
2053information pertaining to the user. Beware, however, that in many
2054system users are able to change this information and therefore it
106325ad 2055cannot be trusted and therefore the $gcos is tainted (see
2959b6e3
JH
2056L<perlsec>). The $passwd and $shell, user's encrypted password and
2057login shell, are also tainted, because of the same reason.
4602f195 2058
5a964f20 2059In scalar context, you get the name, unless the function was a
a0d0e21e
LW
2060lookup by name, in which case you get the other thing, whatever it is.
2061(If the entry doesn't exist you get the undefined value.) For example:
2062
5a964f20
TC
2063 $uid = getpwnam($name);
2064 $name = getpwuid($num);
2065 $name = getpwent();
2066 $gid = getgrnam($name);
08a33e13 2067 $name = getgrgid($num);
5a964f20
TC
2068 $name = getgrent();
2069 #etc.
a0d0e21e 2070
4602f195
JH
2071In I<getpw*()> the fields $quota, $comment, and $expire are special
2072cases in the sense that in many systems they are unsupported. If the
2073$quota is unsupported, it is an empty scalar. If it is supported, it
2074usually encodes the disk quota. If the $comment field is unsupported,
2075it is an empty scalar. If it is supported it usually encodes some
2076administrative comment about the user. In some systems the $quota
2077field may be $change or $age, fields that have to do with password
2078aging. In some systems the $comment field may be $class. The $expire
2079field, if present, encodes the expiration period of the account or the
2080password. For the availability and the exact meaning of these fields
2081in your system, please consult your getpwnam(3) documentation and your
2082F<pwd.h> file. You can also find out from within Perl what your
2083$quota and $comment fields mean and whether you have the $expire field
2084by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2085C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
2086files are only supported if your vendor has implemented them in the
2087intuitive fashion that calling the regular C library routines gets the
5d3a0a3b 2088shadow versions if you're running under privilege or if there exists
cf264981
SP
2089the shadow(3) functions as found in System V (this includes Solaris
2090and Linux.) Those systems that implement a proprietary shadow password
5d3a0a3b 2091facility are unlikely to be supported.
6ee623d5 2092
19799a22 2093The $members value returned by I<getgr*()> is a space separated list of
a0d0e21e
LW
2094the login names of the members of the group.
2095
2096For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2097C, it will be returned to you via C<$?> if the function call fails. The
7660c0ab 2098C<@addrs> value returned by a successful call is a list of the raw
a0d0e21e
LW
2099addresses returned by the corresponding system library call. In the
2100Internet domain, each address is four bytes long and you can unpack it
2101by saying something like:
2102
f337b084 2103 ($a,$b,$c,$d) = unpack('W4',$addr[0]);
a0d0e21e 2104
2b5ab1e7
TC
2105The Socket library makes this slightly easier:
2106
2107 use Socket;
2108 $iaddr = inet_aton("127.1"); # or whatever address
2109 $name = gethostbyaddr($iaddr, AF_INET);
2110
2111 # or going the other way
19799a22 2112 $straddr = inet_ntoa($iaddr);
2b5ab1e7 2113
19799a22
GS
2114If you get tired of remembering which element of the return list
2115contains which return value, by-name interfaces are provided
2116in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2117C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2118and C<User::grent>. These override the normal built-ins, supplying
2119versions that return objects with the appropriate names
2120for each field. For example:
5a964f20
TC
2121
2122 use File::stat;
2123 use User::pwent;
2124 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2125
b76cc8ba
NIS
2126Even though it looks like they're the same method calls (uid),
2127they aren't, because a C<File::stat> object is different from
19799a22 2128a C<User::pwent> object.
5a964f20 2129
a0d0e21e
LW
2130=item getsockname SOCKET
2131
19799a22
GS
2132Returns the packed sockaddr address of this end of the SOCKET connection,
2133in case you don't know the address because you have several different
2134IPs that the connection might have come in on.
a0d0e21e 2135
4633a7c4
LW
2136 use Socket;
2137 $mysockaddr = getsockname(SOCK);
19799a22 2138 ($port, $myaddr) = sockaddr_in($mysockaddr);
b76cc8ba 2139 printf "Connect to %s [%s]\n",
19799a22
GS
2140 scalar gethostbyaddr($myaddr, AF_INET),
2141 inet_ntoa($myaddr);
a0d0e21e
LW
2142
2143=item getsockopt SOCKET,LEVEL,OPTNAME
2144
636e6b1f
TH
2145Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
2146Options may exist at multiple protocol levels depending on the socket
2147type, but at least the uppermost socket level SOL_SOCKET (defined in the
2148C<Socket> module) will exist. To query options at another level the
2149protocol number of the appropriate protocol controlling the option
2150should be supplied. For example, to indicate that an option is to be
2151interpreted by the TCP protocol, LEVEL should be set to the protocol
2152number of TCP, which you can get using getprotobyname.
2153
2154The call returns a packed string representing the requested socket option,
2155or C<undef> if there is an error (the error reason will be in $!). What
2156exactly is in the packed string depends in the LEVEL and OPTNAME, consult
2157your system documentation for details. A very common case however is that
cf264981 2158the option is an integer, in which case the result will be a packed
636e6b1f
TH
2159integer which you can decode using unpack with the C<i> (or C<I>) format.
2160
2161An example testing if Nagle's algorithm is turned on on a socket:
2162
4852725b 2163 use Socket qw(:all);
636e6b1f
TH
2164
2165 defined(my $tcp = getprotobyname("tcp"))
2166 or die "Could not determine the protocol number for tcp";
4852725b
DD
2167 # my $tcp = IPPROTO_TCP; # Alternative
2168 my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
2169 or die "Could not query TCP_NODELAY socket option: $!";
636e6b1f
TH
2170 my $nodelay = unpack("I", $packed);
2171 print "Nagle's algorithm is turned ", $nodelay ? "off\n" : "on\n";
2172
a0d0e21e
LW
2173
2174=item glob EXPR
2175
0a753a76
PP
2176=item glob
2177
d9a9d457
JL
2178In list context, returns a (possibly empty) list of filename expansions on
2179the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
2180scalar context, glob iterates through such filename expansions, returning
2181undef when the list is exhausted. This is the internal function
2182implementing the C<< <*.c> >> operator, but you can use it directly. If
2183EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2184more detail in L<perlop/"I/O Operators">.
a0d0e21e 2185
3a4b19e4
GS
2186Beginning with v5.6.0, this operator is implemented using the standard
2187C<File::Glob> extension. See L<File::Glob> for details.
2188
a0d0e21e
LW
2189=item gmtime EXPR
2190
ce2984c3
PF
2191=item gmtime
2192
d1be9408 2193Converts a time as returned by the time function to an 8-element list
54310121 2194with the time localized for the standard Greenwich time zone.
4633a7c4 2195Typically used as follows:
a0d0e21e 2196
b76cc8ba 2197 # 0 1 2 3 4 5 6 7
48a26b3a 2198 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday) =
a0d0e21e
LW
2199 gmtime(time);
2200
48a26b3a
GS
2201All list elements are numeric, and come straight out of the C `struct
2202tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2203specified time. $mday is the day of the month, and $mon is the month
2204itself, in the range C<0..11> with 0 indicating January and 11
2205indicating December. $year is the number of years since 1900. That
2206is, $year is C<123> in year 2023. $wday is the day of the week, with
22070 indicating Sunday and 3 indicating Wednesday. $yday is the day of
b76cc8ba 2208the year, in the range C<0..364> (or C<0..365> in leap years.)
48a26b3a
GS
2209
2210Note that the $year element is I<not> simply the last two digits of
cf264981 2211the year. If you assume it is then you create non-Y2K-compliant
48a26b3a 2212programs--and you wouldn't want to do that, would you?
2f9daede 2213
abd75f24
GS
2214The proper way to get a complete 4-digit year is simply:
2215
2216 $year += 1900;
2217
2218And to get the last two digits of the year (e.g., '01' in 2001) do:
2219
2220 $year = sprintf("%02d", $year % 100);
2221
48a26b3a 2222If EXPR is omitted, C<gmtime()> uses the current time (C<gmtime(time)>).
a0d0e21e 2223
48a26b3a 2224In scalar context, C<gmtime()> returns the ctime(3) value:
0a753a76
PP
2225
2226 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
2227
fe86afc2
NC
2228If you need local time instead of GMT use the L</localtime> builtin.
2229See also the C<timegm> function provided by the C<Time::Local> module,
2230and the strftime(3) and mktime(3) functions available via the L<POSIX> module.
7660c0ab 2231
fe86afc2
NC
2232This scalar value is B<not> locale dependent (see L<perllocale>), but is
2233instead a Perl builtin. To get somewhat similar but locale dependent date
2234strings, see the example in L</localtime>.
0a753a76 2235
62aa5637
MS
2236See L<perlport/gmtime> for portability concerns.
2237
a0d0e21e
LW
2238=item goto LABEL
2239
748a9306
LW
2240=item goto EXPR
2241
a0d0e21e
LW
2242=item goto &NAME
2243
7660c0ab 2244The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
a0d0e21e 2245execution there. It may not be used to go into any construct that
7660c0ab 2246requires initialization, such as a subroutine or a C<foreach> loop. It
0a753a76 2247also can't be used to go into a construct that is optimized away,
19799a22 2248or to get out of a block or subroutine given to C<sort>.
0a753a76 2249It can be used to go almost anywhere else within the dynamic scope,
a0d0e21e 2250including out of subroutines, but it's usually better to use some other
19799a22 2251construct such as C<last> or C<die>. The author of Perl has never felt the
7660c0ab 2252need to use this form of C<goto> (in Perl, that is--C is another matter).
1b6921cb
BT
2253(The difference being that C does not offer named loops combined with
2254loop control. Perl does, and this replaces most structured uses of C<goto>
2255in other languages.)
a0d0e21e 2256
7660c0ab
A
2257The C<goto-EXPR> form expects a label name, whose scope will be resolved
2258dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
748a9306
LW
2259necessarily recommended if you're optimizing for maintainability:
2260
2261 goto ("FOO", "BAR", "GLARCH")[$i];
2262
1b6921cb
BT
2263The C<goto-&NAME> form is quite different from the other forms of
2264C<goto>. In fact, it isn't a goto in the normal sense at all, and
2265doesn't have the stigma associated with other gotos. Instead, it
2266exits the current subroutine (losing any changes set by local()) and
2267immediately calls in its place the named subroutine using the current
2268value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2269load another subroutine and then pretend that the other subroutine had
2270been called in the first place (except that any modifications to C<@_>
6cb9131c
GS
2271in the current subroutine are propagated to the other subroutine.)
2272After the C<goto>, not even C<caller> will be able to tell that this
2273routine was called first.
2274
2275NAME needn't be the name of a subroutine; it can be a scalar variable
cf264981 2276containing a code reference, or a block that evaluates to a code
6cb9131c 2277reference.
a0d0e21e
LW
2278
2279=item grep BLOCK LIST
2280
2281=item grep EXPR,LIST
2282
2b5ab1e7
TC
2283This is similar in spirit to, but not the same as, grep(1) and its
2284relatives. In particular, it is not limited to using regular expressions.
2f9daede 2285
a0d0e21e 2286Evaluates the BLOCK or EXPR for each element of LIST (locally setting
7660c0ab 2287C<$_> to each element) and returns the list value consisting of those
19799a22
GS
2288elements for which the expression evaluated to true. In scalar
2289context, returns the number of times the expression was true.
a0d0e21e
LW
2290
2291 @foo = grep(!/^#/, @bar); # weed out comments
2292
2293or equivalently,
2294
2295 @foo = grep {!/^#/} @bar; # weed out comments
2296
be3174d2
GS
2297Note that C<$_> is an alias to the list value, so it can be used to
2298modify the elements of the LIST. While this is useful and supported,
2299it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
2300Similarly, grep returns aliases into the original list, much as a for
2301loop's index variable aliases the list elements. That is, modifying an
19799a22
GS
2302element of a list returned by grep (for example, in a C<foreach>, C<map>
2303or another C<grep>) actually modifies the element in the original list.
2b5ab1e7 2304This is usually something to be avoided when writing clear code.
a0d0e21e 2305
a4fb8298 2306If C<$_> is lexical in the scope where the C<grep> appears (because it has
cf264981 2307been declared with C<my $_>) then, in addition to being locally aliased to
a4fb8298
RGS
2308the list elements, C<$_> keeps being lexical inside the block; i.e. it
2309can't be seen from the outside, avoiding any potential side-effects.
2310
19799a22 2311See also L</map> for a list composed of the results of the BLOCK or EXPR.
38325410 2312
a0d0e21e
LW
2313=item hex EXPR
2314
54310121 2315=item hex
bbce6d69 2316
2b5ab1e7 2317Interprets EXPR as a hex string and returns the corresponding value.
38366c11 2318(To convert strings that might start with either C<0>, C<0x>, or C<0b>, see
2b5ab1e7 2319L</oct>.) If EXPR is omitted, uses C<$_>.
2f9daede
TPG
2320
2321 print hex '0xAf'; # prints '175'
2322 print hex 'aF'; # same
a0d0e21e 2323
19799a22 2324Hex strings may only represent integers. Strings that would cause
53305cf1 2325integer overflow trigger a warning. Leading whitespace is not stripped,
38366c11
DN
2326unlike oct(). To present something as hex, look into L</printf>,
2327L</sprintf>, or L</unpack>.
19799a22 2328
ce2984c3 2329=item import LIST
a0d0e21e 2330
19799a22 2331There is no builtin C<import> function. It is just an ordinary
4633a7c4 2332method (subroutine) defined (or inherited) by modules that wish to export
19799a22 2333names to another module. The C<use> function calls the C<import> method
cea6626f 2334for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
a0d0e21e
LW
2335
2336=item index STR,SUBSTR,POSITION
2337
2338=item index STR,SUBSTR
2339
2b5ab1e7
TC
2340The index function searches for one string within another, but without
2341the wildcard-like behavior of a full regular-expression pattern match.
2342It returns the position of the first occurrence of SUBSTR in STR at
2343or after POSITION. If POSITION is omitted, starts searching from the
26f149de
YST
2344beginning of the string. POSITION before the beginning of the string
2345or after its end is treated as if it were the beginning or the end,
2346respectively. POSITION and the return value are based at C<0> (or whatever
2b5ab1e7 2347you've set the C<$[> variable to--but don't do that). If the substring
cf264981 2348is not found, C<index> returns one less than the base, ordinarily C<-1>.
a0d0e21e
LW
2349
2350=item int EXPR
2351
54310121 2352=item int
bbce6d69 2353
7660c0ab 2354Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2b5ab1e7
TC
2355You should not use this function for rounding: one because it truncates
2356towards C<0>, and two because machine representations of floating point
2357numbers can sometimes produce counterintuitive results. For example,
2358C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2359because it's really more like -268.99999999999994315658 instead. Usually,
19799a22 2360the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2b5ab1e7 2361functions will serve you better than will int().
a0d0e21e
LW
2362
2363=item ioctl FILEHANDLE,FUNCTION,SCALAR
2364
2b5ab1e7 2365Implements the ioctl(2) function. You'll probably first have to say
a0d0e21e 2366
a11c483f 2367 require "sys/ioctl.ph"; # probably in $Config{archlib}/ioctl.ph
a0d0e21e 2368
a11c483f 2369to get the correct function definitions. If F<sys/ioctl.ph> doesn't
a0d0e21e 2370exist or doesn't have the correct definitions you'll have to roll your
61eff3bc 2371own, based on your C header files such as F<< <sys/ioctl.h> >>.
5a964f20 2372(There is a Perl script called B<h2ph> that comes with the Perl kit that
54310121 2373may help you in this, but it's nontrivial.) SCALAR will be read and/or
4633a7c4 2374written depending on the FUNCTION--a pointer to the string value of SCALAR
19799a22 2375will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
4633a7c4
LW
2376has no string value but does have a numeric value, that value will be
2377passed rather than a pointer to the string value. To guarantee this to be
19799a22
GS
2378true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2379functions may be needed to manipulate the values of structures used by
b76cc8ba 2380C<ioctl>.
a0d0e21e 2381
19799a22 2382The return value of C<ioctl> (and C<fcntl>) is as follows:
a0d0e21e
LW
2383
2384 if OS returns: then Perl returns:
2385 -1 undefined value
2386 0 string "0 but true"
2387 anything else that number
2388
19799a22 2389Thus Perl returns true on success and false on failure, yet you can
a0d0e21e
LW
2390still easily determine the actual value returned by the operating
2391system:
2392
2b5ab1e7 2393 $retval = ioctl(...) || -1;
a0d0e21e
LW
2394 printf "System returned %d\n", $retval;
2395
be2f7487 2396The special string C<"0 but true"> is exempt from B<-w> complaints
5a964f20
TC
2397about improper numeric conversions.
2398
a0d0e21e
LW
2399=item join EXPR,LIST
2400
2b5ab1e7
TC
2401Joins the separate strings of LIST into a single string with fields
2402separated by the value of EXPR, and returns that new string. Example:
a0d0e21e 2403
2b5ab1e7 2404 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
a0d0e21e 2405
eb6e2d6f
GS
2406Beware that unlike C<split>, C<join> doesn't take a pattern as its
2407first argument. Compare L</split>.
a0d0e21e 2408
aa689395
PP
2409=item keys HASH
2410
504f80c1
JH
2411Returns a list consisting of all the keys of the named hash.
2412(In scalar context, returns the number of keys.)
2413
2414The keys are returned in an apparently random order. The actual
2415random order is subject to change in future versions of perl, but it
2416is guaranteed to be the same order as either the C<values> or C<each>
4546b9e6
JH
2417function produces (given that the hash has not been modified). Since
2418Perl 5.8.1 the ordering is different even between different runs of
2419Perl for security reasons (see L<perlsec/"Algorithmic Complexity
d6df3700 2420Attacks">).
504f80c1 2421
cf264981
SP
2422As a side effect, calling keys() resets the HASH's internal iterator
2423(see L</each>). In particular, calling keys() in void context resets
2424the iterator with no other overhead.
a0d0e21e 2425
aa689395 2426Here is yet another way to print your environment:
a0d0e21e
LW
2427
2428 @keys = keys %ENV;
2429 @values = values %ENV;
b76cc8ba 2430 while (@keys) {
a0d0e21e
LW
2431 print pop(@keys), '=', pop(@values), "\n";
2432 }
2433
2434or how about sorted by key:
2435
2436 foreach $key (sort(keys %ENV)) {
2437 print $key, '=', $ENV{$key}, "\n";
2438 }
2439
8ea1e5d4
GS
2440The returned values are copies of the original keys in the hash, so
2441modifying them will not affect the original hash. Compare L</values>.
2442
19799a22 2443To sort a hash by value, you'll need to use a C<sort> function.
aa689395 2444Here's a descending numeric sort of a hash by its values:
4633a7c4 2445
5a964f20 2446 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
4633a7c4
LW
2447 printf "%4d %s\n", $hash{$key}, $key;
2448 }
2449
19799a22 2450As an lvalue C<keys> allows you to increase the number of hash buckets
aa689395
PP
2451allocated for the given hash. This can gain you a measure of efficiency if
2452you know the hash is going to get big. (This is similar to pre-extending
2453an array by assigning a larger number to $#array.) If you say
55497cff
PP
2454
2455 keys %hash = 200;
2456
ab192400
GS
2457then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2458in fact, since it rounds up to the next power of two. These
55497cff
PP
2459buckets will be retained even if you do C<%hash = ()>, use C<undef
2460%hash> if you want to free the storage while C<%hash> is still in scope.
2461You can't shrink the number of buckets allocated for the hash using
19799a22 2462C<keys> in this way (but you needn't worry about doing this by accident,
55497cff
PP
2463as trying has no effect).
2464
19799a22 2465See also C<each>, C<values> and C<sort>.
ab192400 2466
b350dd2f 2467=item kill SIGNAL, LIST
a0d0e21e 2468
b350dd2f 2469Sends a signal to a list of processes. Returns the number of
517db077
GS
2470processes successfully signaled (which is not necessarily the
2471same as the number actually killed).
a0d0e21e
LW
2472
2473 $cnt = kill 1, $child1, $child2;
2474 kill 9, @goners;
2475
b350dd2f 2476If SIGNAL is zero, no signal is sent to the process. This is a
1e9c1022 2477useful way to check that a child process is alive and hasn't changed
b350dd2f
GS
2478its UID. See L<perlport> for notes on the portability of this
2479construct.
2480
2481Unlike in the shell, if SIGNAL is negative, it kills
4633a7c4
LW
2482process groups instead of processes. (On System V, a negative I<PROCESS>
2483number will also kill process groups, but that's not portable.) That
2484means you usually want to use positive not negative signals. You may also
1e9c1022
JL
2485use a signal name in quotes.
2486
2487See L<perlipc/"Signals"> for more details.
a0d0e21e
LW
2488
2489=item last LABEL
2490
2491=item last
2492
2493The C<last> command is like the C<break> statement in C (as used in
2494loops); it immediately exits the loop in question. If the LABEL is
2495omitted, the command refers to the innermost enclosing loop. The
2496C<continue> block, if any, is not executed:
2497
4633a7c4
LW
2498 LINE: while (<STDIN>) {
2499 last LINE if /^$/; # exit when done with header
5a964f20 2500 #...
a0d0e21e
LW
2501 }
2502
4968c1e4 2503C<last> cannot be used to exit a block which returns a value such as
2b5ab1e7
TC
2504C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2505a grep() or map() operation.
4968c1e4 2506
6c1372ed
GS
2507Note that a block by itself is semantically identical to a loop
2508that executes once. Thus C<last> can be used to effect an early
2509exit out of such a block.
2510
98293880
JH
2511See also L</continue> for an illustration of how C<last>, C<next>, and
2512C<redo> work.
1d2dff63 2513
a0d0e21e
LW
2514=item lc EXPR
2515
54310121 2516=item lc
bbce6d69 2517
d1be9408 2518Returns a lowercased version of EXPR. This is the internal function
ad0029c4
JH
2519implementing the C<\L> escape in double-quoted strings. Respects
2520current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
983ffd37 2521and L<perlunicode> for more details about locale and Unicode support.
a0d0e21e 2522
7660c0ab 2523If EXPR is omitted, uses C<$_>.
bbce6d69 2524
a0d0e21e
LW
2525=item lcfirst EXPR
2526
54310121 2527=item lcfirst
bbce6d69 2528
ad0029c4
JH
2529Returns the value of EXPR with the first character lowercased. This
2530is the internal function implementing the C<\l> escape in
2531double-quoted strings. Respects current LC_CTYPE locale if C<use
983ffd37
JH
2532locale> in force. See L<perllocale> and L<perlunicode> for more
2533details about locale and Unicode support.
a0d0e21e 2534
7660c0ab 2535If EXPR is omitted, uses C<$_>.
bbce6d69 2536
a0d0e21e
LW
2537=item length EXPR
2538
54310121 2539=item length
bbce6d69 2540
974da8e5 2541Returns the length in I<characters> of the value of EXPR. If EXPR is
b76cc8ba 2542omitted, returns length of C<$_>. Note that this cannot be used on
2b5ab1e7
TC
2543an entire array or hash to find out how many elements these have.
2544For that, use C<scalar @array> and C<scalar keys %hash> respectively.
a0d0e21e 2545
974da8e5
JH
2546Note the I<characters>: if the EXPR is in Unicode, you will get the
2547number of characters, not the number of bytes. To get the length
2548in bytes, use C<do { use bytes; length(EXPR) }>, see L<bytes>.
2549
a0d0e21e
LW
2550=item link OLDFILE,NEWFILE
2551
19799a22 2552Creates a new filename linked to the old filename. Returns true for
b76cc8ba 2553success, false otherwise.
a0d0e21e
LW
2554
2555=item listen SOCKET,QUEUESIZE
2556
19799a22 2557Does the same thing that the listen system call does. Returns true if
b76cc8ba 2558it succeeded, false otherwise. See the example in
cea6626f 2559L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
2560
2561=item local EXPR
2562
19799a22 2563You really probably want to be using C<my> instead, because C<local> isn't
b76cc8ba 2564what most people think of as "local". See
13a2d996 2565L<perlsub/"Private Variables via my()"> for details.
2b5ab1e7 2566
5a964f20
TC
2567A local modifies the listed variables to be local to the enclosing
2568block, file, or eval. If more than one value is listed, the list must
2569be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2570for details, including issues with tied arrays and hashes.
a0d0e21e 2571
a0d0e21e
LW
2572=item localtime EXPR
2573
ba053783
AL
2574=item localtime
2575
19799a22 2576Converts a time as returned by the time function to a 9-element list
5f05dabc 2577with the time analyzed for the local time zone. Typically used as
a0d0e21e
LW
2578follows:
2579
54310121 2580 # 0 1 2 3 4 5 6 7 8
a0d0e21e 2581 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
ba053783 2582 localtime(time);
a0d0e21e 2583
48a26b3a 2584All list elements are numeric, and come straight out of the C `struct
ba053783
AL
2585tm'. C<$sec>, C<$min>, and C<$hour> are the seconds, minutes, and hours
2586of the specified time.
48a26b3a 2587
ba053783
AL
2588C<$mday> is the day of the month, and C<$mon> is the month itself, in
2589the range C<0..11> with 0 indicating January and 11 indicating December.
2590This makes it easy to get a month name from a list:
54310121 2591
ba053783
AL
2592 my @abbr = qw( Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec );
2593 print "$abbr[$mon] $mday";
2594 # $mon=9, $mday=18 gives "Oct 18"
abd75f24 2595
ba053783
AL
2596C<$year> is the number of years since 1900, not just the last two digits
2597of the year. That is, C<$year> is C<123> in year 2023. The proper way
2598to get a complete 4-digit year is simply:
abd75f24 2599
ba053783 2600 $year += 1900;
abd75f24 2601
ba053783
AL
2602To get the last two digits of the year (e.g., '01' in 2001) do:
2603
2604 $year = sprintf("%02d", $year % 100);
2605
2606C<$wday> is the day of the week, with 0 indicating Sunday and 3 indicating
2607Wednesday. C<$yday> is the day of the year, in the range C<0..364>
2608(or C<0..365> in leap years.)
2609
2610C<$isdst> is true if the specified time occurs during Daylight Saving
2611Time, false otherwise.
abd75f24 2612
48a26b3a 2613If EXPR is omitted, C<localtime()> uses the current time (C<localtime(time)>).
a0d0e21e 2614
48a26b3a 2615In scalar context, C<localtime()> returns the ctime(3) value:
a0d0e21e 2616
5f05dabc 2617 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
a0d0e21e 2618
fe86afc2
NC
2619This scalar value is B<not> locale dependent but is a Perl builtin. For GMT
2620instead of local time use the L</gmtime> builtin. See also the
2621C<Time::Local> module (to convert the second, minutes, hours, ... back to
2622the integer value returned by time()), and the L<POSIX> module's strftime(3)
2623and mktime(3) functions.
2624
2625To get somewhat similar but locale dependent date strings, set up your
2626locale environment variables appropriately (please see L<perllocale>) and
2627try for example:
a3cb178b 2628
5a964f20 2629 use POSIX qw(strftime);
2b5ab1e7 2630 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
fe86afc2
NC
2631 # or for GMT formatted appropriately for your locale:
2632 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
a3cb178b
GS
2633
2634Note that the C<%a> and C<%b>, the short forms of the day of the week
2635and the month of the year, may not necessarily be three characters wide.
a0d0e21e 2636
62aa5637
MS
2637See L<perlport/localtime> for portability concerns.
2638
07698885 2639=item lock THING
19799a22 2640
01e6739c 2641This function places an advisory lock on a shared variable, or referenced
03730085 2642object contained in I<THING> until the lock goes out of scope.
a6d5524e 2643
f3a23afb 2644lock() is a "weak keyword" : this means that if you've defined a function
67408cae 2645by this name (before any calls to it), that function will be called
03730085
AB
2646instead. (However, if you've said C<use threads>, lock() is always a
2647keyword.) See L<threads>.
19799a22 2648
a0d0e21e
LW
2649=item log EXPR
2650
54310121 2651=item log
bbce6d69 2652
2b5ab1e7
TC
2653Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2654returns log of C<$_>. To get the log of another base, use basic algebra:
19799a22 2655The base-N log of a number is equal to the natural log of that number
2b5ab1e7
TC
2656divided by the natural log of N. For example:
2657
2658 sub log10 {
2659 my $n = shift;
2660 return log($n)/log(10);
b76cc8ba 2661 }
2b5ab1e7
TC
2662
2663See also L</exp> for the inverse operation.
a0d0e21e 2664
a0d0e21e
LW
2665=item lstat EXPR
2666
54310121 2667=item lstat
bbce6d69 2668
19799a22 2669Does the same thing as the C<stat> function (including setting the
5a964f20
TC
2670special C<_> filehandle) but stats a symbolic link instead of the file
2671the symbolic link points to. If symbolic links are unimplemented on
c837d5b4
DP
2672your system, a normal C<stat> is done. For much more detailed
2673information, please see the documentation for C<stat>.
a0d0e21e 2674
7660c0ab 2675If EXPR is omitted, stats C<$_>.
bbce6d69 2676
a0d0e21e
LW
2677=item m//
2678
2679The match operator. See L<perlop>.
2680
2681=item map BLOCK LIST
2682
2683=item map EXPR,LIST
2684
19799a22
GS
2685Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2686C<$_> to each element) and returns the list value composed of the
2687results of each such evaluation. In scalar context, returns the
2688total number of elements so generated. Evaluates BLOCK or EXPR in
2689list context, so each element of LIST may produce zero, one, or
2690more elements in the returned value.
dd99ebda 2691
a0d0e21e
LW
2692 @chars = map(chr, @nums);
2693
2694translates a list of numbers to the corresponding characters. And
2695
4633a7c4 2696 %hash = map { getkey($_) => $_ } @array;
a0d0e21e
LW
2697
2698is just a funny way to write
2699
2700 %hash = ();
2701 foreach $_ (@array) {
4633a7c4 2702 $hash{getkey($_)} = $_;
a0d0e21e
LW
2703 }
2704
be3174d2
GS
2705Note that C<$_> is an alias to the list value, so it can be used to
2706modify the elements of the LIST. While this is useful and supported,
2707it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
2708Using a regular C<foreach> loop for this purpose would be clearer in
2709most cases. See also L</grep> for an array composed of those items of
2710the original list for which the BLOCK or EXPR evaluates to true.
fb73857a 2711
a4fb8298 2712If C<$_> is lexical in the scope where the C<map> appears (because it has
cf264981 2713been declared with C<my $_>) then, in addition to being locally aliased to
a4fb8298
RGS
2714the list elements, C<$_> keeps being lexical inside the block; i.e. it
2715can't be seen from the outside, avoiding any potential side-effects.
2716
205fdb4d
NC
2717C<{> starts both hash references and blocks, so C<map { ...> could be either
2718the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look
2719ahead for the closing C<}> it has to take a guess at which its dealing with
2720based what it finds just after the C<{>. Usually it gets it right, but if it
2721doesn't it won't realize something is wrong until it gets to the C<}> and
2722encounters the missing (or unexpected) comma. The syntax error will be
2723reported close to the C<}> but you'll need to change something near the C<{>
2724such as using a unary C<+> to give perl some help:
2725
2726 %hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong
2727 %hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right
2728 %hash = map { ("\L$_", 1) } @array # this also works
2729 %hash = map { lc($_), 1 } @array # as does this.
2730 %hash = map +( lc($_), 1 ), @array # this is EXPR and works!
cea6626f 2731
205fdb4d
NC
2732 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
2733
2734or to force an anon hash constructor use C<+{>
2735
2736 @hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end
2737
2738and you get list of anonymous hashes each with only 1 entry.
2739
19799a22 2740=item mkdir FILENAME,MASK
a0d0e21e 2741
5a211162
GS
2742=item mkdir FILENAME
2743
491873e5
RGS
2744=item mkdir
2745
0591cd52 2746Creates the directory specified by FILENAME, with permissions
19799a22
GS
2747specified by MASK (as modified by C<umask>). If it succeeds it
2748returns true, otherwise it returns false and sets C<$!> (errno).
491873e5
RGS
2749If omitted, MASK defaults to 0777. If omitted, FILENAME defaults
2750to C<$_>.
0591cd52 2751
19799a22 2752In general, it is better to create directories with permissive MASK,
0591cd52 2753and let the user modify that with their C<umask>, than it is to supply
19799a22 2754a restrictive MASK and give the user no way to be more permissive.
0591cd52
NT
2755The exceptions to this rule are when the file or directory should be
2756kept private (mail files, for instance). The perlfunc(1) entry on
19799a22 2757C<umask> discusses the choice of MASK in more detail.
a0d0e21e 2758
cc1852e8
JH
2759Note that according to the POSIX 1003.1-1996 the FILENAME may have any
2760number of trailing slashes. Some operating and filesystems do not get
2761this right, so Perl automatically removes all trailing slashes to keep
2762everyone happy.
2763
a0d0e21e
LW
2764=item msgctl ID,CMD,ARG
2765
f86cebdf 2766Calls the System V IPC function msgctl(2). You'll probably have to say
0ade1984
JH
2767
2768 use IPC::SysV;
2769
7660c0ab 2770first to get the correct constant definitions. If CMD is C<IPC_STAT>,
cf264981 2771then ARG must be a variable that will hold the returned C<msqid_ds>
951ba7fe
GS
2772structure. Returns like C<ioctl>: the undefined value for error,
2773C<"0 but true"> for zero, or the actual return value otherwise. See also
4755096e 2774L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation.
a0d0e21e
LW
2775
2776=item msgget KEY,FLAGS
2777
f86cebdf 2778Calls the System V IPC function msgget(2). Returns the message queue
4755096e
GS
2779id, or the undefined value if there is an error. See also
2780L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation.
a0d0e21e 2781
a0d0e21e
LW
2782=item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2783
2784Calls the System V IPC function msgrcv to receive a message from
2785message queue ID into variable VAR with a maximum message size of
41d6edb2
JH
2786SIZE. Note that when a message is received, the message type as a
2787native long integer will be the first thing in VAR, followed by the
2788actual message. This packing may be opened with C<unpack("l! a*")>.
2789Taints the variable. Returns true if successful, or false if there is
4755096e
GS
2790an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and
2791C<IPC::SysV::Msg> documentation.
41d6edb2
JH
2792
2793=item msgsnd ID,MSG,FLAGS
2794
2795Calls the System V IPC function msgsnd to send the message MSG to the
2796message queue ID. MSG must begin with the native long integer message
2797type, and be followed by the length of the actual message, and finally
2798the message itself. This kind of packing can be achieved with
2799C<pack("l! a*", $type, $message)>. Returns true if successful,
2800or false if there is an error. See also C<IPC::SysV>
2801and C<IPC::SysV::Msg> documentation.
a0d0e21e
LW
2802
2803=item my EXPR
2804
307ea6df
JH
2805=item my TYPE EXPR
2806
1d2de774 2807=item my EXPR : ATTRS
09bef843 2808
1d2de774 2809=item my TYPE EXPR : ATTRS
307ea6df 2810
19799a22 2811A C<my> declares the listed variables to be local (lexically) to the
1d2de774
JH
2812enclosing block, file, or C<eval>. If more than one value is listed,
2813the list must be placed in parentheses.
307ea6df 2814
1d2de774
JH
2815The exact semantics and interface of TYPE and ATTRS are still
2816evolving. TYPE is currently bound to the use of C<fields> pragma,
307ea6df
JH
2817and attributes are handled using the C<attributes> pragma, or starting
2818from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
2819L<perlsub/"Private Variables via my()"> for details, and L<fields>,
2820L<attributes>, and L<Attribute::Handlers>.
4633a7c4 2821
a0d0e21e
LW
2822=item next LABEL
2823
2824=item next
2825
2826The C<next> command is like the C<continue> statement in C; it starts
2827the next iteration of the loop:
2828
4633a7c4
LW
2829 LINE: while (<STDIN>) {
2830 next LINE if /^#/; # discard comments
5a964f20 2831 #...
a0d0e21e
LW
2832 }
2833
2834Note that if there were a C<continue> block on the above, it would get
2835executed even on discarded lines. If the LABEL is omitted, the command
2836refers to the innermost enclosing loop.
2837
4968c1e4 2838C<next> cannot be used to exit a block which returns a value such as
2b5ab1e7
TC
2839C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2840a grep() or map() operation.
4968c1e4 2841
6c1372ed
GS
2842Note that a block by itself is semantically identical to a loop
2843that executes once. Thus C<next> will exit such a block early.
2844
98293880
JH
2845See also L</continue> for an illustration of how C<last>, C<next>, and
2846C<redo> work.
1d2dff63 2847
4a66ea5a
RGS
2848=item no Module VERSION LIST
2849
2850=item no Module VERSION
2851
a0d0e21e
LW
2852=item no Module LIST
2853
4a66ea5a
RGS
2854=item no Module
2855
593b9c14 2856See the C<use> function, of which C<no> is the opposite.
a0d0e21e
LW
2857
2858=item oct EXPR
2859
54310121 2860=item oct
bbce6d69 2861
4633a7c4 2862Interprets EXPR as an octal string and returns the corresponding
4f19785b
WSI
2863value. (If EXPR happens to start off with C<0x>, interprets it as a
2864hex string. If EXPR starts off with C<0b>, it is interpreted as a
53305cf1
NC
2865binary string. Leading whitespace is ignored in all three cases.)
2866The following will handle decimal, binary, octal, and hex in the standard
2867Perl or C notation:
a0d0e21e
LW
2868
2869 $val = oct($val) if $val =~ /^0/;
2870
19799a22
GS
2871If EXPR is omitted, uses C<$_>. To go the other way (produce a number
2872in octal), use sprintf() or printf():
2873
2874 $perms = (stat("filename"))[2] & 07777;
2875 $oct_perms = sprintf "%lo", $perms;
2876
2877The oct() function is commonly used when a string such as C<644> needs
2878to be converted into a file mode, for example. (Although perl will
2879automatically convert strings into numbers as needed, this automatic
2880conversion assumes base 10.)
a0d0e21e
LW
2881
2882=item open FILEHANDLE,EXPR
2883
68bd7414
NIS
2884=item open FILEHANDLE,MODE,EXPR
2885
2886=item open FILEHANDLE,MODE,EXPR,LIST
2887
ba964c95
T
2888=item open FILEHANDLE,MODE,REFERENCE
2889
a0d0e21e
LW
2890=item open FILEHANDLE
2891
2892Opens the file whose filename is given by EXPR, and associates it with
ed53a2bb
JH
2893FILEHANDLE.
2894
2895(The following is a comprehensive reference to open(): for a gentler
2896introduction you may consider L<perlopentut>.)
2897
a28cd5c9
NT
2898If FILEHANDLE is an undefined scalar variable (or array or hash element)
2899the variable is assigned a reference to a new anonymous filehandle,
2900otherwise if FILEHANDLE is an expression, its value is used as the name of
2901the real filehandle wanted. (This is considered a symbolic reference, so
2902C<use strict 'refs'> should I<not> be in effect.)
ed53a2bb
JH
2903
2904If EXPR is omitted, the scalar variable of the same name as the
2905FILEHANDLE contains the filename. (Note that lexical variables--those
2906declared with C<my>--will not work for this purpose; so if you're
67408cae 2907using C<my>, specify EXPR in your call to open.)
ed53a2bb
JH
2908
2909If three or more arguments are specified then the mode of opening and
2910the file name are separate. If MODE is C<< '<' >> or nothing, the file
2911is opened for input. If MODE is C<< '>' >>, the file is truncated and
2912opened for output, being created if necessary. If MODE is C<<< '>>' >>>,
b76cc8ba 2913the file is opened for appending, again being created if necessary.
5a964f20 2914
ed53a2bb
JH
2915You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to
2916indicate that you want both read and write access to the file; thus
2917C<< '+<' >> is almost always preferred for read/write updates--the C<<
2918'+>' >> mode would clobber the file first. You can't usually use
2919either read-write mode for updating textfiles, since they have
2920variable length records. See the B<-i> switch in L<perlrun> for a
2921better approach. The file is created with permissions of C<0666>
2922modified by the process' C<umask> value.
2923
2924These various prefixes correspond to the fopen(3) modes of C<'r'>,
2925C<'r+'>, C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
5f05dabc 2926
6170680b
IZ
2927In the 2-arguments (and 1-argument) form of the call the mode and
2928filename should be concatenated (in this order), possibly separated by
68bd7414
NIS
2929spaces. It is possible to omit the mode in these forms if the mode is
2930C<< '<' >>.
6170680b 2931
7660c0ab 2932If the filename begins with C<'|'>, the filename is interpreted as a
5a964f20 2933command to which output is to be piped, and if the filename ends with a
f244e06d
GS
2934C<'|'>, the filename is interpreted as a command which pipes output to
2935us. See L<perlipc/"Using open() for IPC">
19799a22 2936for more examples of this. (You are not allowed to C<open> to a command
5a964f20 2937that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
4a4eefd0
GS
2938and L<perlipc/"Bidirectional Communication with Another Process">
2939for alternatives.)
cb1a09d0 2940
ed53a2bb
JH
2941For three or more arguments if MODE is C<'|-'>, the filename is
2942interpreted as a command to which output is to be piped, and if MODE
2943is C<'-|'>, the filename is interpreted as a command which pipes
2944output to us. In the 2-arguments (and 1-argument) form one should
2945replace dash (C<'-'>) with the command.
2946See L<perlipc/"Using open() for IPC"> for more examples of this.
2947(You are not allowed to C<open> to a command that pipes both in I<and>
2948out, but see L<IPC::Open2>, L<IPC::Open3>, and
2949L<perlipc/"Bidirectional Communication"> for alternatives.)
2950
2951In the three-or-more argument form of pipe opens, if LIST is specified
2952(extra arguments after the command name) then LIST becomes arguments
2953to the command invoked if the platform supports it. The meaning of
2954C<open> with more than three arguments for non-pipe modes is not yet
2955specified. Experimental "layers" may give extra LIST arguments
2956meaning.
6170680b
IZ
2957
2958In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
b76cc8ba 2959and opening C<< '>-' >> opens STDOUT.
6170680b 2960
fae2c0fb
RGS
2961You may use the three-argument form of open to specify IO "layers"
2962(sometimes also referred to as "disciplines") to be applied to the handle
2963that affect how the input and output are processed (see L<open> and
2964L<PerlIO> for more details). For example
7207e29d 2965
9124316e
JH
2966 open(FH, "<:utf8", "file")
2967
2968will open the UTF-8 encoded file containing Unicode characters,
fae2c0fb
RGS
2969see L<perluniintro>. (Note that if layers are specified in the
2970three-arg form then default layers set by the C<open> pragma are
01e6739c 2971ignored.)
ed53a2bb
JH
2972
2973Open returns nonzero upon success, the undefined value otherwise. If
2974the C<open> involved a pipe, the return value happens to be the pid of
2975the subprocess.
cb1a09d0 2976
ed53a2bb
JH
2977If you're running Perl on a system that distinguishes between text
2978files and binary files, then you should check out L</binmode> for tips
2979for dealing with this. The key distinction between systems that need
2980C<binmode> and those that don't is their text file formats. Systems
8939ba94 2981like Unix, Mac OS, and Plan 9, which delimit lines with a single
ed53a2bb
JH
2982character, and which encode that character in C as C<"\n">, do not
2983need C<binmode>. The rest need it.
cb1a09d0 2984
fb73857a 2985When opening a file, it's usually a bad idea to continue normal execution
19799a22
GS
2986if the request failed, so C<open> is frequently used in connection with
2987C<die>. Even if C<die> won't do what you want (say, in a CGI script,
fb73857a 2988where you want to make a nicely formatted error message (but there are
5a964f20 2989modules that can help with that problem)) you should always check
19799a22 2990the return value from opening a file. The infrequent exception is when
fb73857a
PP
2991working with an unopened filehandle is actually what you want to do.
2992
cf264981 2993As a special case the 3-arg form with a read/write mode and the third
ed53a2bb 2994argument being C<undef>:
b76cc8ba
NIS
2995
2996 open(TMP, "+>", undef) or die ...
2997
f253e835
JH
2998opens a filehandle to an anonymous temporary file. Also using "+<"
2999works for symmetry, but you really should consider writing something
3000to the temporary file first. You will need to seek() to do the
3001reading.
b76cc8ba 3002
2ce64696 3003Since v5.8.0, perl has built using PerlIO by default. Unless you've
28a5cf3b 3004changed this (i.e. Configure -Uuseperlio), you can open file handles to
2ce64696 3005"in memory" files held in Perl scalars via:
ba964c95 3006
b996200f
SB
3007 open($fh, '>', \$variable) || ..
3008
3009Though if you try to re-open C<STDOUT> or C<STDERR> as an "in memory"
3010file, you have to close it first:
3011
3012 close STDOUT;
3013 open STDOUT, '>', \$variable or die "Can't open STDOUT: $!";
ba964c95 3014
cb1a09d0 3015Examples:
a0d0e21e
LW
3016
3017 $ARTICLE = 100;
3018 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
3019 while (<ARTICLE>) {...
3020
6170680b 3021 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
fb73857a 3022 # if the open fails, output is discarded
a0d0e21e 3023
6170680b 3024 open(DBASE, '+<', 'dbase.mine') # open for update
fb73857a 3025 or die "Can't open 'dbase.mine' for update: $!";
cb1a09d0 3026
6170680b
IZ
3027 open(DBASE, '+<dbase.mine') # ditto
3028 or die "Can't open 'dbase.mine' for update: $!";
3029
3030 open(ARTICLE, '-|', "caesar <$article") # decrypt article
fb73857a 3031 or die "Can't start caesar: $!";
a0d0e21e 3032
6170680b
IZ
3033 open(ARTICLE, "caesar <$article |") # ditto
3034 or die "Can't start caesar: $!";
3035
2359510d 3036 open(EXTRACT, "|sort >Tmp$$") # $$ is our process id
fb73857a 3037 or die "Can't start sort: $!";
a0d0e21e 3038
ba964c95
T
3039 # in memory files
3040 open(MEMORY,'>', \$var)
3041 or die "Can't open memory file: $!";
3042 print MEMORY "foo!\n"; # output will end up in $var
3043
a0d0e21e
LW
3044 # process argument list of files along with any includes
3045
3046 foreach $file (@ARGV) {
3047 process($file, 'fh00');
3048 }
3049
3050 sub process {
5a964f20 3051 my($filename, $input) = @_;
a0d0e21e
LW
3052 $input++; # this is a string increment
3053 unless (open($input, $filename)) {
3054 print STDERR "Can't open $filename: $!\n";
3055 return;
3056 }
3057
5a964f20 3058 local $_;
a0d0e21e
LW
3059 while (<$input>) { # note use of indirection
3060 if (/^#include "(.*)"/) {
3061 process($1, $input);
3062 next;
3063 }
5a964f20 3064 #... # whatever
a0d0e21e
LW
3065 }
3066 }
3067
ae4c5402 3068See L<perliol> for detailed info on PerlIO.
2ce64696 3069
a0d0e21e 3070You may also, in the Bourne shell tradition, specify an EXPR beginning
00cafafa
JH
3071with C<< '>&' >>, in which case the rest of the string is interpreted
3072as the name of a filehandle (or file descriptor, if numeric) to be
3073duped (as L<dup(2)>) and opened. You may use C<&> after C<< > >>,
3074C<<< >> >>>, C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>.
3075The mode you specify should match the mode of the original filehandle.
3076(Duping a filehandle does not take into account any existing contents
cf264981 3077of IO buffers.) If you use the 3-arg form then you can pass either a
00cafafa 3078number, the name of a filehandle or the normal "reference to a glob".
6170680b 3079
eae1b76b
SB
3080Here is a script that saves, redirects, and restores C<STDOUT> and
3081C<STDERR> using various methods:
a0d0e21e
LW
3082
3083 #!/usr/bin/perl
eae1b76b
SB
3084 open my $oldout, ">&STDOUT" or die "Can't dup STDOUT: $!";
3085 open OLDERR, ">&", \*STDERR or die "Can't dup STDERR: $!";
818c4caa 3086
eae1b76b
SB
3087 open STDOUT, '>', "foo.out" or die "Can't redirect STDOUT: $!";
3088 open STDERR, ">&STDOUT" or die "Can't dup STDOUT: $!";
a0d0e21e 3089
eae1b76b
SB
3090 select STDERR; $| = 1; # make unbuffered
3091 select STDOUT; $| = 1; # make unbuffered
a0d0e21e
LW
3092
3093 print STDOUT "stdout 1\n"; # this works for
3094 print STDERR "stderr 1\n"; # subprocesses too
3095
eae1b76b
SB
3096 open STDOUT, ">&", $oldout or die "Can't dup \$oldout: $!";
3097 open STDERR, ">&OLDERR" or die "Can't dup OLDERR: $!";
a0d0e21e
LW
3098
3099 print STDOUT "stdout 2\n";
3100 print STDERR "stderr 2\n";
3101
ef8b303f
JH
3102If you specify C<< '<&=X' >>, where C<X> is a file descriptor number
3103or a filehandle, then Perl will do an equivalent of C's C<fdopen> of
3104that file descriptor (and not call L<dup(2)>); this is more
3105parsimonious of file descriptors. For example:
a0d0e21e 3106
00cafafa 3107 # open for input, reusing the fileno of $fd
a0d0e21e 3108 open(FILEHANDLE, "<&=$fd")
df632fdf 3109
b76cc8ba 3110or
df632fdf 3111
b76cc8ba 3112 open(FILEHANDLE, "<&=", $fd)
a0d0e21e 3113
00cafafa
JH
3114or
3115
3116 # open for append, using the fileno of OLDFH
3117 open(FH, ">>&=", OLDFH)
3118
3119or
3120
3121 open(FH, ">>&=OLDFH")
3122
ef8b303f
JH
3123Being parsimonious on filehandles is also useful (besides being
3124parsimonious) for example when something is dependent on file
3125descriptors, like for example locking using flock(). If you do just
3126C<< open(A, '>>&B') >>, the filehandle A will not have the same file
3127descriptor as B, and therefore flock(A) will not flock(B), and vice
3128versa. But with C<< open(A, '>>&=B') >> the filehandles will share
3129the same file descriptor.
3130
3131Note that if you are using Perls older than 5.8.0, Perl will be using
3132the standard C libraries' fdopen() to implement the "=" functionality.
3133On many UNIX systems fdopen() fails when file descriptors exceed a
3134certain value, typically 255. For Perls 5.8.0 and later, PerlIO is
3135most often the default.
4af147f6 3136
df632fdf
JH
3137You can see whether Perl has been compiled with PerlIO or not by
3138running C<perl -V> and looking for C<useperlio=> line. If C<useperlio>
3139is C<define>, you have PerlIO, otherwise you don't.
3140
6170680b
IZ
3141If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
3142with 2-arguments (or 1-argument) form of open(), then
a0d0e21e 3143there is an implicit fork done, and the return value of open is the pid
7660c0ab 3144of the child within the parent process, and C<0> within the child
184e9718 3145process. (Use C<defined($pid)> to determine whether the open was successful.)
a0d0e21e
LW
3146The filehandle behaves normally for the parent, but i/o to that
3147filehandle is piped from/to the STDOUT/STDIN of the child process.
3148In the child process the filehandle isn't opened--i/o happens from/to
3149the new STDOUT or STDIN. Typically this is used like the normal
3150piped open when you want to exercise more control over just how the
3151pipe command gets executed, such as when you are running setuid, and
54310121 3152don't want to have to scan shell commands for metacharacters.
6170680b 3153The following triples are more or less equivalent:
a0d0e21e
LW
3154
3155 open(FOO, "|tr '[a-z]' '[A-Z]'");
6170680b
IZ
3156 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
3157 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
b76cc8ba 3158 open(FOO, '|-', "tr", '[a-z]', '[A-Z]');
a0d0e21e
LW
3159
3160 open(FOO, "cat -n '$file'|");
6170680b
IZ
3161 open(FOO, '-|', "cat -n '$file'");
3162 open(FOO, '-|') || exec 'cat', '-n', $file;
b76cc8ba
NIS
3163 open(FOO, '-|', "cat", '-n', $file);
3164
3165The last example in each block shows the pipe as "list form", which is
64da03b2
JH
3166not yet supported on all platforms. A good rule of thumb is that if
3167your platform has true C<fork()> (in other words, if your platform is
3168UNIX) you can use the list form.
a0d0e21e 3169
4633a7c4
LW
3170See L<perlipc/"Safe Pipe Opens"> for more examples of this.
3171
0f897271
GS
3172Beginning with v5.6.0, Perl will attempt to flush all files opened for
3173output before any operation that may do a fork, but this may not be
3174supported on some platforms (see L<perlport>). To be safe, you may need
3175to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
3176of C<IO::Handle> on any open handles.
3177
ed53a2bb
JH
3178On systems that support a close-on-exec flag on files, the flag will
3179be set for the newly opened file descriptor as determined by the value
3180of $^F. See L<perlvar/$^F>.
a0d0e21e 3181
0dccf244 3182Closing any piped filehandle causes the parent process to wait for the
e5218da5
GA
3183child to finish, and returns the status value in C<$?> and
3184C<${^CHILD_ERROR_NATIVE}>.
0dccf244 3185
ed53a2bb
JH
3186The filename passed to 2-argument (or 1-argument) form of open() will
3187have leading and trailing whitespace deleted, and the normal
3188redirection characters honored. This property, known as "magic open",
5a964f20 3189can often be used to good effect. A user could specify a filename of
7660c0ab 3190F<"rsh cat file |">, or you could change certain filenames as needed:
5a964f20
TC
3191
3192 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
3193 open(FH, $filename) or die "Can't open $filename: $!";
3194
6170680b
IZ
3195Use 3-argument form to open a file with arbitrary weird characters in it,
3196
3197 open(FOO, '<', $file);
3198
3199otherwise it's necessary to protect any leading and trailing whitespace:
5a964f20
TC
3200
3201 $file =~ s#^(\s)#./$1#;
3202 open(FOO, "< $file\0");
3203
a31a806a 3204(this may not work on some bizarre filesystems). One should
106325ad 3205conscientiously choose between the I<magic> and 3-arguments form
6170680b
IZ
3206of open():
3207
3208 open IN, $ARGV[0];
3209
3210will allow the user to specify an argument of the form C<"rsh cat file |">,
3211but will not work on a filename which happens to have a trailing space, while
3212
3213 open IN, '<', $ARGV[0];
3214
3215will have exactly the opposite restrictions.
3216
19799a22 3217If you want a "real" C C<open> (see L<open(2)> on your system), then you
6170680b
IZ
3218should use the C<sysopen> function, which involves no such magic (but
3219may use subtly different filemodes than Perl open(), which is mapped
3220to C fopen()). This is
5a964f20
TC
3221another way to protect your filenames from interpretation. For example:
3222
3223 use IO::Handle;
3224 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
3225 or die "sysopen $path: $!";
3226 $oldfh = select(HANDLE); $| = 1; select($oldfh);
38762f02 3227 print HANDLE "stuff $$\n";
5a964f20
TC
3228 seek(HANDLE, 0, 0);
3229 print "File contains: ", <HANDLE>;
3230