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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
15argument, while a list operator may provide either scalar and list
16contexts for its arguments. If it does both, the scalar arguments will
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17be first, and the list argument will follow. (Note that there can ever
18be only one list argument.) For instance, splice() has three scalar
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19arguments followed by a list.
20
21In the syntax descriptions that follow, list operators that expect a
22list (and provide list context for the elements of the list) are shown
23with LIST as an argument. Such a list may consist of any combination
24of scalar arguments or list values; the list values will be included
25in the list as if each individual element were interpolated at that
26point in the list, forming a longer single-dimensional list value.
27Elements of the LIST should be separated by commas.
28
29Any function in the list below may be used either with or without
30parentheses around its arguments. (The syntax descriptions omit the
5f05dabc 31parentheses.) If you use the parentheses, the simple (but occasionally
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32surprising) rule is this: It I<LOOKS> like a function, therefore it I<IS> a
33function, and precedence doesn't matter. Otherwise it's a list
34operator or unary operator, and precedence does matter. And whitespace
35between the function and left parenthesis doesn't count--so you need to
36be careful sometimes:
37
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38 print 1+2+4; # Prints 7.
39 print(1+2) + 4; # Prints 3.
40 print (1+2)+4; # Also prints 3!
41 print +(1+2)+4; # Prints 7.
42 print ((1+2)+4); # Prints 7.
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43
44If you run Perl with the B<-w> switch it can warn you about this. For
45example, the third line above produces:
46
47 print (...) interpreted as function at - line 1.
48 Useless use of integer addition in void context at - line 1.
49
50For functions that can be used in either a scalar or list context,
54310121 51nonabortive failure is generally indicated in a scalar context by
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52returning the undefined value, and in a list context by returning the
53null list.
54
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55Remember the following important rule: There is B<no rule> that relates
56the behavior of an expression in list context to its behavior in scalar
57context, or vice versa. It might do two totally different things.
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58Each operator and function decides which sort of value it would be most
59appropriate to return in a scalar context. Some operators return the
5a964f20 60length of the list that would have been returned in list context. Some
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61operators return the first value in the list. Some operators return the
62last value in the list. Some operators return a count of successful
63operations. In general, they do what you want, unless you want
64consistency.
65
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66An named array in scalar context is quite different from what would at
67first glance appear to be a list in scalar context. You can't get a list
68like C<(1,2,3)> into being in scalar context, because the compiler knows
69the context at compile time. It would generate the scalar comma operator
70there, not the list construction version of the comma. That means it
71was never a list to start with.
72
73In general, functions in Perl that serve as wrappers for system calls
f86cebdf 74of the same name (like chown(2), fork(2), closedir(2), etc.) all return
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75true when they succeed and C<undef> otherwise, as is usually mentioned
76in the descriptions below. This is different from the C interfaces,
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77which return C<-1> on failure. Exceptions to this rule are C<wait()>,
78C<waitpid()>, and C<syscall()>. System calls also set the special C<$!>
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79variable on failure. Other functions do not, except accidentally.
80
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81=head2 Perl Functions by Category
82
83Here are Perl's functions (including things that look like
5a964f20 84functions, like some keywords and named operators)
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85arranged by category. Some functions appear in more
86than one place.
87
88=over
89
90=item Functions for SCALARs or strings
91
22fae026 92C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>,
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93C<length>, C<oct>, C<ord>, C<pack>, C<q/STRING/>, C<qq/STRING/>, C<reverse>,
94C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///>
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95
96=item Regular expressions and pattern matching
97
ab4f32c2 98C<m//>, C<pos>, C<quotemeta>, C<s///>, C<split>, C<study>, C<qr//>
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99
100=item Numeric functions
101
22fae026
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102C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>,
103C<sin>, C<sqrt>, C<srand>
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104
105=item Functions for real @ARRAYs
106
22fae026 107C<pop>, C<push>, C<shift>, C<splice>, C<unshift>
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108
109=item Functions for list data
110
ab4f32c2 111C<grep>, C<join>, C<map>, C<qw/STRING/>, C<reverse>, C<sort>, C<unpack>
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112
113=item Functions for real %HASHes
114
22fae026 115C<delete>, C<each>, C<exists>, C<keys>, C<values>
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116
117=item Input and output functions
118
22fae026
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119C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
120C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
121C<readdir>, C<rewinddir>, C<seek>, C<seekdir>, C<select>, C<syscall>,
122C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>,
123C<warn>, C<write>
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124
125=item Functions for fixed length data or records
126
22fae026 127C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec>
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128
129=item Functions for filehandles, files, or directories
130
22fae026 131C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>,
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132C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>,
133C<readlink>, C<rename>, C<rmdir>, C<stat>, C<symlink>, C<umask>,
134C<unlink>, C<utime>
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135
136=item Keywords related to the control flow of your perl program
137
22fae026
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138C<caller>, C<continue>, C<die>, C<do>, C<dump>, C<eval>, C<exit>,
139C<goto>, C<last>, C<next>, C<redo>, C<return>, C<sub>, C<wantarray>
cb1a09d0 140
54310121 141=item Keywords related to scoping
cb1a09d0 142
22fae026 143C<caller>, C<import>, C<local>, C<my>, C<package>, C<use>
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144
145=item Miscellaneous functions
146
22fae026
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147C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<reset>,
148C<scalar>, C<undef>, C<wantarray>
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149
150=item Functions for processes and process groups
151
22fae026 152C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
ab4f32c2 153C<pipe>, C<qx/STRING/>, C<setpgrp>, C<setpriority>, C<sleep>, C<system>,
22fae026 154C<times>, C<wait>, C<waitpid>
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155
156=item Keywords related to perl modules
157
22fae026 158C<do>, C<import>, C<no>, C<package>, C<require>, C<use>
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159
160=item Keywords related to classes and object-orientedness
161
22fae026
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162C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
163C<untie>, C<use>
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164
165=item Low-level socket functions
166
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167C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
168C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
169C<socket>, C<socketpair>
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170
171=item System V interprocess communication functions
172
22fae026
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173C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
174C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>
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175
176=item Fetching user and group info
177
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178C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
179C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
180C<getpwuid>, C<setgrent>, C<setpwent>
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181
182=item Fetching network info
183
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184C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
185C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
186C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
187C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
188C<setnetent>, C<setprotoent>, C<setservent>
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189
190=item Time-related functions
191
22fae026 192C<gmtime>, C<localtime>, C<time>, C<times>
cb1a09d0 193
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194=item Functions new in perl5
195
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196C<abs>, C<bless>, C<chomp>, C<chr>, C<exists>, C<formline>, C<glob>,
197C<import>, C<lc>, C<lcfirst>, C<map>, C<my>, C<no>, C<prototype>, C<qx>,
198C<qw>, C<readline>, C<readpipe>, C<ref>, C<sub*>, C<sysopen>, C<tie>,
199C<tied>, C<uc>, C<ucfirst>, C<untie>, C<use>
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200
201* - C<sub> was a keyword in perl4, but in perl5 it is an
5a964f20 202operator, which can be used in expressions.
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203
204=item Functions obsoleted in perl5
205
22fae026 206C<dbmclose>, C<dbmopen>
37798a01 207
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208=back
209
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210=head2 Portability
211
212Perl was born in UNIX and therefore it can access all the common UNIX
213system calls. In non-UNIX environments the functionality of many
214UNIX system calls may not be available or the details of the available
215functionality may be slightly different. The Perl functions affected
216by this are:
217
218C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>,
219C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>,
220C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>,
221C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostent>,
222C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
223C<getppid>, C<getprgp>, C<getpriority>, C<getprotobynumber>,
224C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>,
225C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>,
226C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>,
227C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<select>, C<semctl>,
228C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>,
229C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>,
230C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>,
231C<shmwrite>, C<socketpair>, C<stat>, C<symlink>, C<syscall>,
232C<sysopen>, C<system>, C<times>, C<truncate>, C<umask>, C<utime>,
233C<wait>, C<waitpid>
234
235For more information about the portability of these functions, see
236L<perlport> and other available platform-specific documentation.
237
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238=head2 Alphabetical Listing of Perl Functions
239
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240=over 8
241
22fae026 242=item I<-X> FILEHANDLE
a0d0e21e 243
22fae026 244=item I<-X> EXPR
a0d0e21e 245
22fae026 246=item I<-X>
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247
248A file test, where X is one of the letters listed below. This unary
249operator takes one argument, either a filename or a filehandle, and
250tests the associated file to see if something is true about it. If the
7660c0ab 251argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
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252Unless otherwise documented, it returns C<1> for TRUE and C<''> for FALSE, or
253the undefined value if the file doesn't exist. Despite the funny
254names, precedence is the same as any other named unary operator, and
255the argument may be parenthesized like any other unary operator. The
256operator may be any of:
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257X<-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>
258X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
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259
260 -r File is readable by effective uid/gid.
261 -w File is writable by effective uid/gid.
262 -x File is executable by effective uid/gid.
263 -o File is owned by effective uid.
264
265 -R File is readable by real uid/gid.
266 -W File is writable by real uid/gid.
267 -X File is executable by real uid/gid.
268 -O File is owned by real uid.
269
270 -e File exists.
271 -z File has zero size.
54310121 272 -s File has nonzero size (returns size).
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273
274 -f File is a plain file.
275 -d File is a directory.
276 -l File is a symbolic link.
9c4d0f16 277 -p File is a named pipe (FIFO), or Filehandle is a pipe.
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278 -S File is a socket.
279 -b File is a block special file.
280 -c File is a character special file.
281 -t Filehandle is opened to a tty.
282
283 -u File has setuid bit set.
284 -g File has setgid bit set.
285 -k File has sticky bit set.
286
287 -T File is a text file.
288 -B File is a binary file (opposite of -T).
289
290 -M Age of file in days when script started.
291 -A Same for access time.
292 -C Same for inode change time.
293
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294Example:
295
296 while (<>) {
297 chop;
298 next unless -f $_; # ignore specials
5a964f20 299 #...
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300 }
301
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302The interpretation of the file permission operators C<-r>, C<-R>,
303C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
304of the file and the uids and gids of the user. There may be other
305reasons you can't actually read, write, or execute the file. Such
306reasons may be for example network filesystem access controls, ACLs
307(access control lists), read-only filesystems, and unrecognized
308executable formats.
309
310Also note that, for the superuser on the local filesystems, C<-r>,
311C<-R>, C<-w>, and C<-W> always return 1, and C<-x> and C<-X> return 1
312if any execute bit is set in the mode. Scripts run by the superuser
313may thus need to do a stat() to determine the actual mode of the file,
314or temporarily set the uid to something else.
315
316If you are using ACLs, there is a pragma called C<filetest> that may
317produce more accurate results than the bare stat() mode bits.
318
319When under the C<use filetest 'access'> the above-mentioned filetests
320will test whether the permission can (not) be granted using the
321access() family of system calls. Also note that the -x and -X may
322under this pragma return true even if there are no execute permission
323bits set (nor any extra execute permission ACLs). This strangeness is
324due to the underlying system calls' definitions. Read the
325documentation for the C<filetest> pragma for more information.
326
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327Note that C<-s/a/b/> does not do a negated substitution. Saying
328C<-exp($foo)> still works as expected, however--only single letters
329following a minus are interpreted as file tests.
330
331The C<-T> and C<-B> switches work as follows. The first block or so of the
332file is examined for odd characters such as strange control codes or
5a964f20 333characters with the high bit set. If too many strange characters (E<gt>30%)
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334are found, it's a C<-B> file, otherwise it's a C<-T> file. Also, any file
335containing null in the first block is considered a binary file. If C<-T>
336or C<-B> is used on a filehandle, the current stdio buffer is examined
337rather than the first block. Both C<-T> and C<-B> return TRUE on a null
54310121 338file, or a file at EOF when testing a filehandle. Because you have to
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339read a file to do the C<-T> test, on most occasions you want to use a C<-f>
340against the file first, as in C<next unless -f $file && -T $file>.
a0d0e21e 341
7660c0ab 342If any of the file tests (or either the C<stat()> or C<lstat()> operators) are given
28757baa 343the special filehandle consisting of a solitary underline, then the stat
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344structure of the previous file test (or stat operator) is used, saving
345a system call. (This doesn't work with C<-t>, and you need to remember
346that lstat() and C<-l> will leave values in the stat structure for the
347symbolic link, not the real file.) Example:
348
349 print "Can do.\n" if -r $a || -w _ || -x _;
350
351 stat($filename);
352 print "Readable\n" if -r _;
353 print "Writable\n" if -w _;
354 print "Executable\n" if -x _;
355 print "Setuid\n" if -u _;
356 print "Setgid\n" if -g _;
357 print "Sticky\n" if -k _;
358 print "Text\n" if -T _;
359 print "Binary\n" if -B _;
360
361=item abs VALUE
362
54310121 363=item abs
bbce6d69 364
a0d0e21e 365Returns the absolute value of its argument.
7660c0ab 366If VALUE is omitted, uses C<$_>.
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367
368=item accept NEWSOCKET,GENERICSOCKET
369
f86cebdf 370Accepts an incoming socket connect, just as the accept(2) system call
a0d0e21e 371does. Returns the packed address if it succeeded, FALSE otherwise.
4633a7c4 372See example in L<perlipc/"Sockets: Client/Server Communication">.
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373
374=item alarm SECONDS
375
54310121 376=item alarm
bbce6d69 377
a0d0e21e 378Arranges to have a SIGALRM delivered to this process after the
bbce6d69 379specified number of seconds have elapsed. If SECONDS is not specified,
7660c0ab 380the value stored in C<$_> is used. (On some machines,
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381unfortunately, the elapsed time may be up to one second less than you
382specified because of how seconds are counted.) Only one timer may be
383counting at once. Each call disables the previous timer, and an
7660c0ab 384argument of C<0> may be supplied to cancel the previous timer without
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385starting a new one. The returned value is the amount of time remaining
386on the previous timer.
387
4633a7c4 388For delays of finer granularity than one second, you may use Perl's
f86cebdf 389C<syscall()> interface to access setitimer(2) if your system supports it,
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390or else see L</select()>. It is usually a mistake to intermix C<alarm()>
391and C<sleep()> calls.
a0d0e21e 392
7660c0ab 393If you want to use C<alarm()> to time out a system call you need to use an
ab4f32c2 394C<eval()>/C<die()> pair. You can't rely on the alarm causing the system call to
f86cebdf 395fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
ab4f32c2 396restart system calls on some systems. Using C<eval()>/C<die()> always works,
5a964f20 397modulo the caveats given in L<perlipc/"Signals">.
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398
399 eval {
f86cebdf 400 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
36477c24 401 alarm $timeout;
ff68c719 402 $nread = sysread SOCKET, $buffer, $size;
36477c24 403 alarm 0;
ff68c719 404 };
ff68c719 405 if ($@) {
f86cebdf 406 die unless $@ eq "alarm\n"; # propagate unexpected errors
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407 # timed out
408 }
409 else {
410 # didn't
411 }
412
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413=item atan2 Y,X
414
415Returns the arctangent of Y/X in the range -PI to PI.
416
7660c0ab 417For the tangent operation, you may use the C<POSIX::tan()>
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418function, or use the familiar relation:
419
420 sub tan { sin($_[0]) / cos($_[0]) }
421
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422=item bind SOCKET,NAME
423
424Binds a network address to a socket, just as the bind system call
425does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
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426packed address of the appropriate type for the socket. See the examples in
427L<perlipc/"Sockets: Client/Server Communication">.
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428
429=item binmode FILEHANDLE
430
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431Arranges for the file to be read or written in "binary" mode in operating
432systems that distinguish between binary and text files. Files that are
433not in binary mode have CR LF sequences translated to LF on input and LF
54310121 434translated to CR LF on output. Binmode has no effect under Unix; in MS-DOS
cb1a09d0 435and similarly archaic systems, it may be imperative--otherwise your
54310121 436MS-DOS-damaged C library may mangle your file. The key distinction between
ab4f32c2 437systems that need C<binmode()> and those that don't is their text file
5a964f20 438formats. Systems like Unix, MacOS, and Plan9 that delimit lines with a single
7660c0ab 439character, and that encode that character in C as C<"\n">, do not need
ab4f32c2 440C<binmode()>. The rest need it. If FILEHANDLE is an expression, the value
cb1a09d0 441is taken as the name of the filehandle.
a0d0e21e 442
4633a7c4 443=item bless REF,CLASSNAME
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444
445=item bless REF
446
28757baa 447This function tells the thingy referenced by REF that it is now
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448an object in the CLASSNAME package--or the current package if no CLASSNAME
449is specified, which is often the case. It returns the reference for
7660c0ab 450convenience, because a C<bless()> is often the last thing in a constructor.
4633a7c4 451Always use the two-argument version if the function doing the blessing
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452might be inherited by a derived class. See L<perltoot> and L<perlobj>
453for more about the blessing (and blessings) of objects.
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454
455=item caller EXPR
456
457=item caller
458
5a964f20 459Returns the context of the current subroutine call. In scalar context,
28757baa 460returns the caller's package name if there is a caller, that is, if
7660c0ab 461we're in a subroutine or C<eval()> or C<require()>, and the undefined value
5a964f20 462otherwise. In list context, returns
a0d0e21e 463
748a9306 464 ($package, $filename, $line) = caller;
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465
466With EXPR, it returns some extra information that the debugger uses to
467print a stack trace. The value of EXPR indicates how many call frames
468to go back before the current one.
469
54310121 470 ($package, $filename, $line, $subroutine,
e7ea3e70
IZ
471 $hasargs, $wantarray, $evaltext, $is_require) = caller($i);
472
7660c0ab 473Here C<$subroutine> may be C<"(eval)"> if the frame is not a subroutine
ab4f32c2 474call, but an C<eval()>. In such a case additional elements C<$evaltext> and
7660c0ab
A
475C<$is_require> are set: C<$is_require> is true if the frame is created by a
476C<require> or C<use> statement, C<$evaltext> contains the text of the
dc848c6f 477C<eval EXPR> statement. In particular, for a C<eval BLOCK> statement,
7660c0ab 478C<$filename> is C<"(eval)">, but C<$evaltext> is undefined. (Note also that
dc848c6f
PP
479each C<use> statement creates a C<require> frame inside an C<eval EXPR>)
480frame.
748a9306
LW
481
482Furthermore, when called from within the DB package, caller returns more
7660c0ab 483detailed information: it sets the list variable C<@DB::args> to be the
54310121 484arguments with which the subroutine was invoked.
748a9306 485
7660c0ab 486Be aware that the optimizer might have optimized call frames away before
ab4f32c2 487C<caller()> had a chance to get the information. That means that C<caller(N)>
7660c0ab
A
488might not return information about the call frame you expect it do, for
489C<N E<gt> 1>. In particular, C<@DB::args> might have information from the
490previous time C<caller()> was called.
491
a0d0e21e
LW
492=item chdir EXPR
493
494Changes the working directory to EXPR, if possible. If EXPR is
495omitted, changes to home directory. Returns TRUE upon success, FALSE
7660c0ab 496otherwise. See example under C<die()>.
a0d0e21e
LW
497
498=item chmod LIST
499
500Changes the permissions of a list of files. The first element of the
4633a7c4 501list must be the numerical mode, which should probably be an octal
2f9daede
TPG
502number, and which definitely should I<not> a string of octal digits:
503C<0644> is okay, C<'0644'> is not. Returns the number of files
dc848c6f 504successfully changed. See also L</oct>, if all you have is a string.
a0d0e21e
LW
505
506 $cnt = chmod 0755, 'foo', 'bar';
507 chmod 0755, @executables;
f86cebdf
GS
508 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
509 # --w----r-T
2f9daede
TPG
510 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
511 $mode = 0644; chmod $mode, 'foo'; # this is best
a0d0e21e
LW
512
513=item chomp VARIABLE
514
515=item chomp LIST
516
517=item chomp
518
3e3baf6d 519This is a slightly safer version of L</chop>. It removes any
a0d0e21e 520line ending that corresponds to the current value of C<$/> (also known as
28757baa
PP
521$INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
522number of characters removed from all its arguments. It's often used to
523remove the newline from the end of an input record when you're worried
524that the final record may be missing its newline. When in paragraph mode
525(C<$/ = "">), it removes all trailing newlines from the string. If
7660c0ab 526VARIABLE is omitted, it chomps C<$_>. Example:
a0d0e21e
LW
527
528 while (<>) {
529 chomp; # avoid \n on last field
530 @array = split(/:/);
5a964f20 531 # ...
a0d0e21e
LW
532 }
533
534You can actually chomp anything that's an lvalue, including an assignment:
535
536 chomp($cwd = `pwd`);
537 chomp($answer = <STDIN>);
538
539If you chomp a list, each element is chomped, and the total number of
540characters removed is returned.
541
542=item chop VARIABLE
543
544=item chop LIST
545
546=item chop
547
548Chops off the last character of a string and returns the character
549chopped. It's used primarily to remove the newline from the end of an
550input record, but is much more efficient than C<s/\n//> because it neither
7660c0ab 551scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
a0d0e21e
LW
552Example:
553
554 while (<>) {
555 chop; # avoid \n on last field
556 @array = split(/:/);
5a964f20 557 #...
a0d0e21e
LW
558 }
559
560You can actually chop anything that's an lvalue, including an assignment:
561
562 chop($cwd = `pwd`);
563 chop($answer = <STDIN>);
564
565If you chop a list, each element is chopped. Only the value of the
ab4f32c2 566last C<chop()> is returned.
a0d0e21e 567
ab4f32c2 568Note that C<chop()> returns the last character. To return all but the last
748a9306
LW
569character, use C<substr($string, 0, -1)>.
570
a0d0e21e
LW
571=item chown LIST
572
573Changes the owner (and group) of a list of files. The first two
574elements of the list must be the I<NUMERICAL> uid and gid, in that order.
575Returns the number of files successfully changed.
576
577 $cnt = chown $uid, $gid, 'foo', 'bar';
578 chown $uid, $gid, @filenames;
579
54310121 580Here's an example that looks up nonnumeric uids in the passwd file:
a0d0e21e
LW
581
582 print "User: ";
583 chop($user = <STDIN>);
5a964f20 584 print "Files: ";
a0d0e21e
LW
585 chop($pattern = <STDIN>);
586
587 ($login,$pass,$uid,$gid) = getpwnam($user)
588 or die "$user not in passwd file";
589
5a964f20 590 @ary = glob($pattern); # expand filenames
a0d0e21e
LW
591 chown $uid, $gid, @ary;
592
54310121 593On most systems, you are not allowed to change the ownership of the
4633a7c4
LW
594file unless you're the superuser, although you should be able to change
595the group to any of your secondary groups. On insecure systems, these
596restrictions may be relaxed, but this is not a portable assumption.
597
a0d0e21e
LW
598=item chr NUMBER
599
54310121 600=item chr
bbce6d69 601
a0d0e21e 602Returns the character represented by that NUMBER in the character set.
a0ed51b3
LW
603For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
604chr(0x263a) is a Unicode smiley face (but only within the scope of a
605C<use utf8>). For the reverse, use L</ord>.
a0d0e21e 606
7660c0ab 607If NUMBER is omitted, uses C<$_>.
bbce6d69 608
a0d0e21e
LW
609=item chroot FILENAME
610
54310121 611=item chroot
bbce6d69 612
5a964f20 613This function works like the system call by the same name: it makes the
4633a7c4 614named directory the new root directory for all further pathnames that
7660c0ab 615begin with a C<"/"> by your process and all its children. (It doesn't
28757baa 616change your current working directory, which is unaffected.) For security
4633a7c4 617reasons, this call is restricted to the superuser. If FILENAME is
ab4f32c2 618omitted, does a C<chroot()> to C<$_>.
a0d0e21e
LW
619
620=item close FILEHANDLE
621
6a518fbc
TP
622=item close
623
a0d0e21e
LW
624Closes the file or pipe associated with the file handle, returning TRUE
625only if stdio successfully flushes buffers and closes the system file
6a518fbc
TP
626descriptor. Closes the currently selected filehandle if the argument
627is omitted.
fb73857a
PP
628
629You don't have to close FILEHANDLE if you are immediately going to do
7660c0ab 630another C<open()> on it, because C<open()> will close it for you. (See
ab4f32c2 631C<open()>.) However, an explicit C<close()> on an input file resets the line
7660c0ab 632counter (C<$.>), while the implicit close done by C<open()> does not.
fb73857a 633
ab4f32c2 634If the file handle came from a piped open C<close()> will additionally
fb73857a
PP
635return FALSE if one of the other system calls involved fails or if the
636program exits with non-zero status. (If the only problem was that the
7660c0ab 637program exited non-zero C<$!> will be set to C<0>.) Also, closing a pipe
5a964f20 638waits for the process executing on the pipe to complete, in case you
fb73857a
PP
639want to look at the output of the pipe afterwards. Closing a pipe
640explicitly also puts the exit status value of the command into C<$?>.
5a964f20 641
fb73857a 642Example:
a0d0e21e 643
fb73857a
PP
644 open(OUTPUT, '|sort >foo') # pipe to sort
645 or die "Can't start sort: $!";
5a964f20 646 #... # print stuff to output
fb73857a
PP
647 close OUTPUT # wait for sort to finish
648 or warn $! ? "Error closing sort pipe: $!"
649 : "Exit status $? from sort";
650 open(INPUT, 'foo') # get sort's results
651 or die "Can't open 'foo' for input: $!";
a0d0e21e 652
5a964f20
TC
653FILEHANDLE may be an expression whose value can be used as an indirect
654filehandle, usually the real filehandle name.
a0d0e21e
LW
655
656=item closedir DIRHANDLE
657
7660c0ab 658Closes a directory opened by C<opendir()> and returns the success of that
5a964f20
TC
659system call.
660
661DIRHANDLE may be an expression whose value can be used as an indirect
662dirhandle, usually the real dirhandle name.
a0d0e21e
LW
663
664=item connect SOCKET,NAME
665
666Attempts to connect to a remote socket, just as the connect system call
667does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
4633a7c4
LW
668packed address of the appropriate type for the socket. See the examples in
669L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 670
cb1a09d0
AD
671=item continue BLOCK
672
673Actually a flow control statement rather than a function. If there is a
674C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
675C<foreach>), it is always executed just before the conditional is about to
676be evaluated again, just like the third part of a C<for> loop in C. Thus
677it can be used to increment a loop variable, even when the loop has been
678continued via the C<next> statement (which is similar to the C C<continue>
679statement).
680
1d2dff63
GS
681C<last>, C<next>, or C<redo> may appear within a C<continue>
682block. C<last> and C<redo> will behave as if they had been executed within
683the main block. So will C<next>, but since it will execute a C<continue>
684block, it may be more entertaining.
685
686 while (EXPR) {
687 ### redo always comes here
688 do_something;
689 } continue {
690 ### next always comes here
691 do_something_else;
692 # then back the top to re-check EXPR
693 }
694 ### last always comes here
695
696Omitting the C<continue> section is semantically equivalent to using an
697empty one, logically enough. In that case, C<next> goes directly back
698to check the condition at the top of the loop.
699
a0d0e21e
LW
700=item cos EXPR
701
5a964f20 702Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
7660c0ab 703takes cosine of C<$_>.
a0d0e21e 704
7660c0ab 705For the inverse cosine operation, you may use the C<POSIX::acos()>
28757baa
PP
706function, or use this relation:
707
708 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
709
a0d0e21e
LW
710=item crypt PLAINTEXT,SALT
711
f86cebdf 712Encrypts a string exactly like the crypt(3) function in the C library
4633a7c4
LW
713(assuming that you actually have a version there that has not been
714extirpated as a potential munition). This can prove useful for checking
715the password file for lousy passwords, amongst other things. Only the
716guys wearing white hats should do this.
a0d0e21e 717
7660c0ab 718Note that C<crypt()> is intended to be a one-way function, much like breaking
11155c91
CS
719eggs to make an omelette. There is no (known) corresponding decrypt
720function. As a result, this function isn't all that useful for
721cryptography. (For that, see your nearby CPAN mirror.)
2f9daede 722
e71965be
RS
723When verifying an existing encrypted string you should use the encrypted
724text as the salt (like C<crypt($plain, $crypted) eq $crypted>). This
725allows your code to work with the standard C<crypt()> and with more
726exotic implementations. When choosing a new salt create a random two
727character string whose characters come from the set C<[./0-9A-Za-z]>
728(like C<join '', ('.', '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>).
729
a0d0e21e
LW
730Here's an example that makes sure that whoever runs this program knows
731their own password:
732
733 $pwd = (getpwuid($<))[1];
a0d0e21e
LW
734
735 system "stty -echo";
736 print "Password: ";
e71965be 737 chomp($word = <STDIN>);
a0d0e21e
LW
738 print "\n";
739 system "stty echo";
740
e71965be 741 if (crypt($word, $pwd) ne $pwd) {
a0d0e21e
LW
742 die "Sorry...\n";
743 } else {
744 print "ok\n";
54310121 745 }
a0d0e21e 746
9f8f0c9d 747Of course, typing in your own password to whoever asks you
748a9306 748for it is unwise.
a0d0e21e 749
aa689395 750=item dbmclose HASH
a0d0e21e 751
7660c0ab 752[This function has been superseded by the C<untie()> function.]
a0d0e21e 753
aa689395 754Breaks the binding between a DBM file and a hash.
a0d0e21e 755
aa689395 756=item dbmopen HASH,DBNAME,MODE
a0d0e21e 757
7660c0ab 758[This function has been superseded by the C<tie()> function.]
a0d0e21e 759
7b8d334a 760This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
ab4f32c2 761hash. HASH is the name of the hash. (Unlike normal C<open()>, the first
aa689395
PP
762argument is I<NOT> a filehandle, even though it looks like one). DBNAME
763is the name of the database (without the F<.dir> or F<.pag> extension if
764any). If the database does not exist, it is created with protection
7660c0ab
A
765specified by MODE (as modified by the C<umask()>). If your system supports
766only the older DBM functions, you may perform only one C<dbmopen()> in your
aa689395 767program. In older versions of Perl, if your system had neither DBM nor
7660c0ab 768ndbm, calling C<dbmopen()> produced a fatal error; it now falls back to
aa689395
PP
769sdbm(3).
770
771If you don't have write access to the DBM file, you can only read hash
772variables, not set them. If you want to test whether you can write,
7660c0ab 773either use file tests or try setting a dummy hash entry inside an C<eval()>,
aa689395 774which will trap the error.
a0d0e21e 775
7660c0ab
A
776Note that functions such as C<keys()> and C<values()> may return huge lists
777when used on large DBM files. You may prefer to use the C<each()>
a0d0e21e
LW
778function to iterate over large DBM files. Example:
779
780 # print out history file offsets
781 dbmopen(%HIST,'/usr/lib/news/history',0666);
782 while (($key,$val) = each %HIST) {
783 print $key, ' = ', unpack('L',$val), "\n";
784 }
785 dbmclose(%HIST);
786
cb1a09d0 787See also L<AnyDBM_File> for a more general description of the pros and
184e9718 788cons of the various dbm approaches, as well as L<DB_File> for a particularly
cb1a09d0 789rich implementation.
4633a7c4 790
a0d0e21e
LW
791=item defined EXPR
792
54310121 793=item defined
bbce6d69 794
2f9daede
TPG
795Returns a Boolean value telling whether EXPR has a value other than
796the undefined value C<undef>. If EXPR is not present, C<$_> will be
797checked.
798
799Many operations return C<undef> to indicate failure, end of file,
800system error, uninitialized variable, and other exceptional
801conditions. This function allows you to distinguish C<undef> from
802other values. (A simple Boolean test will not distinguish among
7660c0ab 803C<undef>, zero, the empty string, and C<"0">, which are all equally
2f9daede 804false.) Note that since C<undef> is a valid scalar, its presence
7660c0ab 805doesn't I<necessarily> indicate an exceptional condition: C<pop()>
2f9daede
TPG
806returns C<undef> when its argument is an empty array, I<or> when the
807element to return happens to be C<undef>.
808
7660c0ab 809You may also use C<defined()> to check whether a subroutine exists, by
5a964f20 810saying C<defined &func> without parentheses. On the other hand, use
7660c0ab 811of C<defined()> upon aggregates (hashes and arrays) is not guaranteed to
5a964f20 812produce intuitive results, and should probably be avoided.
2f9daede
TPG
813
814When used on a hash element, it tells you whether the value is defined,
dc848c6f 815not whether the key exists in the hash. Use L</exists> for the latter
2f9daede 816purpose.
a0d0e21e
LW
817
818Examples:
819
820 print if defined $switch{'D'};
821 print "$val\n" while defined($val = pop(@ary));
822 die "Can't readlink $sym: $!"
823 unless defined($value = readlink $sym);
a0d0e21e 824 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
2f9daede 825 $debugging = 0 unless defined $debugging;
a0d0e21e 826
7660c0ab
A
827Note: Many folks tend to overuse C<defined()>, and then are surprised to
828discover that the number C<0> and C<""> (the zero-length string) are, in fact,
2f9daede 829defined values. For example, if you say
a5f75d66
AD
830
831 "ab" =~ /a(.*)b/;
832
7660c0ab 833The pattern match succeeds, and C<$1> is defined, despite the fact that it
a5f75d66 834matched "nothing". But it didn't really match nothing--rather, it
7660c0ab 835matched something that happened to be C<0> characters long. This is all
a5f75d66 836very above-board and honest. When a function returns an undefined value,
2f9daede 837it's an admission that it couldn't give you an honest answer. So you
7660c0ab
A
838should use C<defined()> only when you're questioning the integrity of what
839you're trying to do. At other times, a simple comparison to C<0> or C<""> is
2f9daede
TPG
840what you want.
841
7660c0ab 842Currently, using C<defined()> on an entire array or hash reports whether
2f9daede
TPG
843memory for that aggregate has ever been allocated. So an array you set
844to the empty list appears undefined initially, and one that once was full
845and that you then set to the empty list still appears defined. You
846should instead use a simple test for size:
28757baa
PP
847
848 if (@an_array) { print "has array elements\n" }
849 if (%a_hash) { print "has hash members\n" }
850
7660c0ab 851Using C<undef()> on these, however, does clear their memory and then report
5a964f20 852them as not defined anymore, but you shouldn't do that unless you don't
28757baa 853plan to use them again, because it saves time when you load them up
5a964f20
TC
854again to have memory already ready to be filled. The normal way to
855free up space used by an aggregate is to assign the empty list.
28757baa 856
7660c0ab 857This counterintuitive behavior of C<defined()> on aggregates may be
28757baa
PP
858changed, fixed, or broken in a future release of Perl.
859
dc848c6f 860See also L</undef>, L</exists>, L</ref>.
2f9daede 861
a0d0e21e
LW
862=item delete EXPR
863
aa689395
PP
864Deletes the specified key(s) and their associated values from a hash.
865For each key, returns the deleted value associated with that key, or
866the undefined value if there was no such key. Deleting from C<$ENV{}>
867modifies the environment. Deleting from a hash tied to a DBM file
7660c0ab 868deletes the entry from the DBM file. (But deleting from a C<tie()>d hash
5f05dabc 869doesn't necessarily return anything.)
a0d0e21e 870
aa689395 871The following deletes all the values of a hash:
a0d0e21e 872
5f05dabc
PP
873 foreach $key (keys %HASH) {
874 delete $HASH{$key};
a0d0e21e
LW
875 }
876
5f05dabc
PP
877And so does this:
878
879 delete @HASH{keys %HASH}
880
5a964f20 881(But both of these are slower than just assigning the empty list, or
7660c0ab 882using C<undef()>.) Note that the EXPR can be arbitrarily complicated as
5a964f20 883long as the final operation is a hash element lookup or hash slice:
a0d0e21e
LW
884
885 delete $ref->[$x][$y]{$key};
5f05dabc 886 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
a0d0e21e
LW
887
888=item die LIST
889
7660c0ab
A
890Outside an C<eval()>, prints the value of LIST to C<STDERR> and exits with
891the current value of C<$!> (errno). If C<$!> is C<0>, exits with the value of
54310121 892C<($? E<gt>E<gt> 8)> (backtick `command` status). If C<($? E<gt>E<gt> 8)>
7660c0ab
A
893is C<0>, exits with C<255>. Inside an C<eval(),> the error message is stuffed into
894C<$@> and the C<eval()> is terminated with the undefined value. This makes
895C<die()> the way to raise an exception.
a0d0e21e
LW
896
897Equivalent examples:
898
899 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
54310121 900 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
a0d0e21e
LW
901
902If the value of EXPR does not end in a newline, the current script line
903number and input line number (if any) are also printed, and a newline
7660c0ab
A
904is supplied. Hint: sometimes appending C<", stopped"> to your message
905will cause it to make better sense when the string C<"at foo line 123"> is
a0d0e21e
LW
906appended. Suppose you are running script "canasta".
907
908 die "/etc/games is no good";
909 die "/etc/games is no good, stopped";
910
911produce, respectively
912
913 /etc/games is no good at canasta line 123.
914 /etc/games is no good, stopped at canasta line 123.
915
7660c0ab 916See also C<exit()> and C<warn()>.
a0d0e21e 917
7660c0ab
A
918If LIST is empty and C<$@> already contains a value (typically from a
919previous eval) that value is reused after appending C<"\t...propagated">.
fb73857a
PP
920This is useful for propagating exceptions:
921
922 eval { ... };
923 die unless $@ =~ /Expected exception/;
924
7660c0ab 925If C<$@> is empty then the string C<"Died"> is used.
fb73857a 926
7660c0ab 927You can arrange for a callback to be run just before the C<die()> does
774d564b
PP
928its deed, by setting the C<$SIG{__DIE__}> hook. The associated handler
929will be called with the error text and can change the error message, if
7660c0ab 930it sees fit, by calling C<die()> again. See L<perlvar/$SIG{expr}> for details on
fb73857a
PP
931setting C<%SIG> entries, and L<"eval BLOCK"> for some examples.
932
933Note that the C<$SIG{__DIE__}> hook is called even inside eval()ed
934blocks/strings. If one wants the hook to do nothing in such
935situations, put
936
937 die @_ if $^S;
938
939as the first line of the handler (see L<perlvar/$^S>).
774d564b 940
a0d0e21e
LW
941=item do BLOCK
942
943Not really a function. Returns the value of the last command in the
944sequence of commands indicated by BLOCK. When modified by a loop
945modifier, executes the BLOCK once before testing the loop condition.
946(On other statements the loop modifiers test the conditional first.)
947
4968c1e4
G
948C<do BLOCK> does I<not> count as a loop, so the loop control statements
949C<next>, C<last> or C<redo> cannot be used to leave or restart the block.
950
a0d0e21e
LW
951=item do SUBROUTINE(LIST)
952
953A deprecated form of subroutine call. See L<perlsub>.
954
955=item do EXPR
956
957Uses the value of EXPR as a filename and executes the contents of the
958file as a Perl script. Its primary use is to include subroutines
959from a Perl subroutine library.
960
961 do 'stat.pl';
962
963is just like
964
fb73857a 965 scalar eval `cat stat.pl`;
a0d0e21e 966
5a964f20 967except that it's more efficient and concise, keeps track of the
a0d0e21e
LW
968current filename for error messages, and searches all the B<-I>
969libraries if the file isn't in the current directory (see also the @INC
dc1be6b5
GS
970array in L<perlvar/Predefined Names>). It is also different in how
971code evaluated with C<do FILENAME> doesn't see lexicals in the enclosing
972scope like C<eval STRING> does. It's the same, however, in that it does
54310121 973reparse the file every time you call it, so you probably don't want to
a0d0e21e
LW
974do this inside a loop.
975
8e30cc93
G
976If C<do> cannot read the file, it returns undef and sets C<$!> to the
977error. If C<do> can read the file but cannot compile it, it
978returns undef and sets an error message in C<$@>. If the file is
979successfully compiled, C<do> returns the value of the last expression
980evaluated.
981
a0d0e21e 982Note that inclusion of library modules is better done with the
7660c0ab 983C<use()> and C<require()> operators, which also do automatic error checking
4633a7c4 984and raise an exception if there's a problem.
a0d0e21e 985
5a964f20
TC
986You might like to use C<do> to read in a program configuration
987file. Manual error checking can be done this way:
988
989 # read in config files: system first, then user
f86cebdf
GS
990 for $file ("/share/prog/defaults.rc",
991 "$ENV{HOME}/.someprogrc") {
5a964f20 992 unless ($return = do $file) {
f86cebdf
GS
993 warn "couldn't parse $file: $@" if $@;
994 warn "couldn't do $file: $!" unless defined $return;
995 warn "couldn't run $file" unless $return;
5a964f20
TC
996 }
997 }
998
a0d0e21e
LW
999=item dump LABEL
1000
1001This causes an immediate core dump. Primarily this is so that you can
1002use the B<undump> program to turn your core dump into an executable binary
1003after having initialized all your variables at the beginning of the
1004program. When the new binary is executed it will begin by executing a
1005C<goto LABEL> (with all the restrictions that C<goto> suffers). Think of
f86cebdf 1006it as a goto with an intervening core dump and reincarnation. If C<LABEL>
5a964f20 1007is omitted, restarts the program from the top. WARNING: Any files
a0d0e21e
LW
1008opened at the time of the dump will NOT be open any more when the
1009program is reincarnated, with possible resulting confusion on the part
1010of Perl. See also B<-u> option in L<perlrun>.
1011
1012Example:
1013
1014 #!/usr/bin/perl
1015 require 'getopt.pl';
1016 require 'stat.pl';
1017 %days = (
1018 'Sun' => 1,
1019 'Mon' => 2,
1020 'Tue' => 3,
1021 'Wed' => 4,
1022 'Thu' => 5,
1023 'Fri' => 6,
1024 'Sat' => 7,
1025 );
1026
1027 dump QUICKSTART if $ARGV[0] eq '-d';
1028
1029 QUICKSTART:
1030 Getopt('f');
1031
5a964f20
TC
1032This operator is largely obsolete, partly because it's very hard to
1033convert a core file into an executable, and because the real perl-to-C
1034compiler has superseded it.
1035
aa689395
PP
1036=item each HASH
1037
5a964f20 1038When called in list context, returns a 2-element list consisting of the
aa689395 1039key and value for the next element of a hash, so that you can iterate over
5a964f20 1040it. When called in scalar context, returns the key for only the "next"
7660c0ab 1041element in the hash. (Note: Keys may be C<"0"> or C<"">, which are logically
2f9daede
TPG
1042false; you may wish to avoid constructs like C<while ($k = each %foo) {}>
1043for this reason.)
1044
1045Entries are returned in an apparently random order. When the hash is
1046entirely read, a null array is returned in list context (which when
7660c0ab
A
1047assigned produces a FALSE (C<0>) value), and C<undef> in
1048scalar context. The next call to C<each()> after that will start iterating
1049again. There is a single iterator for each hash, shared by all C<each()>,
1050C<keys()>, and C<values()> function calls in the program; it can be reset by
2f9daede
TPG
1051reading all the elements from the hash, or by evaluating C<keys HASH> or
1052C<values HASH>. If you add or delete elements of a hash while you're
1053iterating over it, you may get entries skipped or duplicated, so don't.
aa689395 1054
f86cebdf 1055The following prints out your environment like the printenv(1) program,
aa689395 1056only in a different order:
a0d0e21e
LW
1057
1058 while (($key,$value) = each %ENV) {
1059 print "$key=$value\n";
1060 }
1061
7660c0ab 1062See also C<keys()> and C<values()>.
a0d0e21e
LW
1063
1064=item eof FILEHANDLE
1065
4633a7c4
LW
1066=item eof ()
1067
a0d0e21e
LW
1068=item eof
1069
1070Returns 1 if the next read on FILEHANDLE will return end of file, or if
1071FILEHANDLE is not open. FILEHANDLE may be an expression whose value
5a964f20 1072gives the real filehandle. (Note that this function actually
7660c0ab 1073reads a character and then C<ungetc()>s it, so isn't very useful in an
748a9306
LW
1074interactive context.) Do not read from a terminal file (or call
1075C<eof(FILEHANDLE)> on it) after end-of-file is reached. Filetypes such
1076as terminals may lose the end-of-file condition if you do.
1077
1078An C<eof> without an argument uses the last file read as argument.
5a964f20 1079Using C<eof()> with empty parentheses is very different. It indicates the pseudo file formed of
2f9daede
TPG
1080the files listed on the command line, i.e., C<eof()> is reasonable to
1081use inside a C<while (E<lt>E<gt>)> loop to detect the end of only the
1082last file. Use C<eof(ARGV)> or eof without the parentheses to test
1083I<EACH> file in a while (E<lt>E<gt>) loop. Examples:
a0d0e21e 1084
748a9306
LW
1085 # reset line numbering on each input file
1086 while (<>) {
5a964f20 1087 next if /^\s*#/; # skip comments
748a9306 1088 print "$.\t$_";
5a964f20
TC
1089 } continue {
1090 close ARGV if eof; # Not eof()!
748a9306
LW
1091 }
1092
a0d0e21e
LW
1093 # insert dashes just before last line of last file
1094 while (<>) {
5a964f20 1095 if (eof()) { # check for end of current file
a0d0e21e 1096 print "--------------\n";
748a9306
LW
1097 close(ARGV); # close or break; is needed if we
1098 # are reading from the terminal
a0d0e21e
LW
1099 }
1100 print;
1101 }
1102
a0d0e21e 1103Practical hint: you almost never need to use C<eof> in Perl, because the
3b02c43c
GS
1104input operators return false values when they run out of data, or if there
1105was an error.
a0d0e21e
LW
1106
1107=item eval EXPR
1108
1109=item eval BLOCK
1110
c7cc6f1c
GS
1111In the first form, the return value of EXPR is parsed and executed as if it
1112were a little Perl program. The value of the expression (which is itself
5a964f20 1113determined within scalar context) is first parsed, and if there weren't any
c7cc6f1c 1114errors, executed in the context of the current Perl program, so that any
5f05dabc 1115variable settings or subroutine and format definitions remain afterwards.
c7cc6f1c
GS
1116Note that the value is parsed every time the eval executes. If EXPR is
1117omitted, evaluates C<$_>. This form is typically used to delay parsing
1118and subsequent execution of the text of EXPR until run time.
1119
1120In the second form, the code within the BLOCK is parsed only once--at the
1121same time the code surrounding the eval itself was parsed--and executed
1122within the context of the current Perl program. This form is typically
1123used to trap exceptions more efficiently than the first (see below), while
1124also providing the benefit of checking the code within BLOCK at compile
1125time.
1126
1127The final semicolon, if any, may be omitted from the value of EXPR or within
1128the BLOCK.
1129
1130In both forms, the value returned is the value of the last expression
5a964f20 1131evaluated inside the mini-program; a return statement may be also used, just
c7cc6f1c 1132as with subroutines. The expression providing the return value is evaluated
5a964f20 1133in void, scalar, or list context, depending on the context of the eval itself.
c7cc6f1c 1134See L</wantarray> for more on how the evaluation context can be determined.
a0d0e21e 1135
7660c0ab
A
1136If there is a syntax error or runtime error, or a C<die()> statement is
1137executed, an undefined value is returned by C<eval()>, and C<$@> is set to the
a0d0e21e 1138error message. If there was no error, C<$@> is guaranteed to be a null
7660c0ab 1139string. Beware that using C<eval()> neither silences perl from printing
c7cc6f1c
GS
1140warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1141To do either of those, you have to use the C<$SIG{__WARN__}> facility. See
1142L</warn> and L<perlvar>.
a0d0e21e 1143
7660c0ab
A
1144Note that, because C<eval()> traps otherwise-fatal errors, it is useful for
1145determining whether a particular feature (such as C<socket()> or C<symlink()>)
a0d0e21e
LW
1146is implemented. It is also Perl's exception trapping mechanism, where
1147the die operator is used to raise exceptions.
1148
1149If the code to be executed doesn't vary, you may use the eval-BLOCK
1150form to trap run-time errors without incurring the penalty of
1151recompiling each time. The error, if any, is still returned in C<$@>.
1152Examples:
1153
54310121 1154 # make divide-by-zero nonfatal
a0d0e21e
LW
1155 eval { $answer = $a / $b; }; warn $@ if $@;
1156
1157 # same thing, but less efficient
1158 eval '$answer = $a / $b'; warn $@ if $@;
1159
1160 # a compile-time error
5a964f20 1161 eval { $answer = }; # WRONG
a0d0e21e
LW
1162
1163 # a run-time error
1164 eval '$answer ='; # sets $@
1165
7660c0ab 1166When using the C<eval{}> form as an exception trap in libraries, you may
774d564b
PP
1167wish not to trigger any C<__DIE__> hooks that user code may have
1168installed. You can use the C<local $SIG{__DIE__}> construct for this
1169purpose, as shown in this example:
1170
1171 # a very private exception trap for divide-by-zero
f86cebdf
GS
1172 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1173 warn $@ if $@;
774d564b
PP
1174
1175This is especially significant, given that C<__DIE__> hooks can call
7660c0ab 1176C<die()> again, which has the effect of changing their error messages:
774d564b
PP
1177
1178 # __DIE__ hooks may modify error messages
1179 {
f86cebdf
GS
1180 local $SIG{'__DIE__'} =
1181 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
c7cc6f1c
GS
1182 eval { die "foo lives here" };
1183 print $@ if $@; # prints "bar lives here"
774d564b
PP
1184 }
1185
7660c0ab 1186With an C<eval()>, you should be especially careful to remember what's
a0d0e21e
LW
1187being looked at when:
1188
1189 eval $x; # CASE 1
1190 eval "$x"; # CASE 2
1191
1192 eval '$x'; # CASE 3
1193 eval { $x }; # CASE 4
1194
5a964f20 1195 eval "\$$x++"; # CASE 5
a0d0e21e
LW
1196 $$x++; # CASE 6
1197
2f9daede 1198Cases 1 and 2 above behave identically: they run the code contained in
7660c0ab 1199the variable C<$x>. (Although case 2 has misleading double quotes making
2f9daede 1200the reader wonder what else might be happening (nothing is).) Cases 3
7660c0ab 1201and 4 likewise behave in the same way: they run the code C<'$x'>, which
2f9daede
TPG
1202does nothing but return the value of C<$x>. (Case 4 is preferred for
1203purely visual reasons, but it also has the advantage of compiling at
1204compile-time instead of at run-time.) Case 5 is a place where
54310121 1205normally you I<WOULD> like to use double quotes, except that in this
2f9daede
TPG
1206particular situation, you can just use symbolic references instead, as
1207in case 6.
a0d0e21e 1208
4968c1e4
G
1209C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1210C<next>, C<last> or C<redo> cannot be used to leave or restart the block.
1211
1212
a0d0e21e
LW
1213=item exec LIST
1214
8bf3b016
GS
1215=item exec PROGRAM LIST
1216
7660c0ab
A
1217The C<exec()> function executes a system command I<AND NEVER RETURNS> -
1218use C<system()> instead of C<exec()> if you want it to return. It fails and
fb73857a
PP
1219returns FALSE only if the command does not exist I<and> it is executed
1220directly instead of via your system's command shell (see below).
a0d0e21e 1221
7660c0ab
A
1222Since it's a common mistake to use C<exec()> instead of C<system()>, Perl
1223warns you if there is a following statement which isn't C<die()>, C<warn()>,
1224or C<exit()> (if C<-w> is set - but you always do that). If you
1225I<really> want to follow an C<exec()> with some other statement, you
55d729e4
GS
1226can use one of these styles to avoid the warning:
1227
5a964f20
TC
1228 exec ('foo') or print STDERR "couldn't exec foo: $!";
1229 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
55d729e4 1230
5a964f20 1231If there is more than one argument in LIST, or if LIST is an array
f86cebdf 1232with more than one value, calls execvp(3) with the arguments in LIST.
5a964f20
TC
1233If there is only one scalar argument or an array with one element in it,
1234the argument is checked for shell metacharacters, and if there are any,
1235the entire argument is passed to the system's command shell for parsing
1236(this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1237If there are no shell metacharacters in the argument, it is split into
7660c0ab
A
1238words and passed directly to C<execvp()>, which is more efficient. Note:
1239C<exec()> and C<system()> do not flush your output buffer, so you may need to
5a964f20 1240set C<$|> to avoid lost output. Examples:
a0d0e21e
LW
1241
1242 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1243 exec "sort $outfile | uniq";
1244
1245If you don't really want to execute the first argument, but want to lie
1246to the program you are executing about its own name, you can specify
1247the program you actually want to run as an "indirect object" (without a
1248comma) in front of the LIST. (This always forces interpretation of the
54310121 1249LIST as a multivalued list, even if there is only a single scalar in
a0d0e21e
LW
1250the list.) Example:
1251
1252 $shell = '/bin/csh';
1253 exec $shell '-sh'; # pretend it's a login shell
1254
1255or, more directly,
1256
1257 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1258
bb32b41a
GS
1259When the arguments get executed via the system shell, results will
1260be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1261for details.
1262
ab4f32c2 1263Using an indirect object with C<exec()> or C<system()> is also more secure.
5a964f20
TC
1264This usage forces interpretation of the arguments as a multivalued list,
1265even if the list had just one argument. That way you're safe from the
1266shell expanding wildcards or splitting up words with whitespace in them.
1267
1268 @args = ( "echo surprise" );
1269
f86cebdf
GS
1270 system @args; # subject to shell escapes
1271 # if @args == 1
5a964f20
TC
1272 system { $args[0] } @args; # safe even with one-arg list
1273
1274The first version, the one without the indirect object, ran the I<echo>
1275program, passing it C<"surprise"> an argument. The second version
1276didn't--it tried to run a program literally called I<"echo surprise">,
1277didn't find it, and set C<$?> to a non-zero value indicating failure.
1278
ab4f32c2 1279Note that C<exec()> will not call your C<END> blocks, nor will it call
7660c0ab
A
1280any C<DESTROY> methods in your objects.
1281
a0d0e21e
LW
1282=item exists EXPR
1283
1284Returns TRUE if the specified hash key exists in its hash array, even
1285if the corresponding value is undefined.
1286
1287 print "Exists\n" if exists $array{$key};
1288 print "Defined\n" if defined $array{$key};
1289 print "True\n" if $array{$key};
1290
5f05dabc 1291A hash element can be TRUE only if it's defined, and defined if
a0d0e21e
LW
1292it exists, but the reverse doesn't necessarily hold true.
1293
1294Note that the EXPR can be arbitrarily complicated as long as the final
1295operation is a hash key lookup:
1296
5a964f20
TC
1297 if (exists $ref->{"A"}{"B"}{$key}) { ... }
1298
1299Although the last element will not spring into existence just because its
1300existence was tested, intervening ones will. Thus C<$ref-E<gt>{"A"}>
1301C<$ref-E<gt>{"B"}> will spring into existence due to the existence
1302test for a $key element. This autovivification may be fixed in a later
1303release.
a0d0e21e
LW
1304
1305=item exit EXPR
1306
1307Evaluates EXPR and exits immediately with that value. (Actually, it
1308calls any defined C<END> routines first, but the C<END> routines may not
1309abort the exit. Likewise any object destructors that need to be called
1310are called before exit.) Example:
1311
1312 $ans = <STDIN>;
1313 exit 0 if $ans =~ /^[Xx]/;
1314
7660c0ab
A
1315See also C<die()>. If EXPR is omitted, exits with C<0> status. The only
1316universally portable values for EXPR are C<0> for success and C<1> for error;
f86702cc
PP
1317all other values are subject to unpredictable interpretation depending
1318on the environment in which the Perl program is running.
a0d0e21e 1319
7660c0ab
A
1320You shouldn't use C<exit()> to abort a subroutine if there's any chance that
1321someone might want to trap whatever error happened. Use C<die()> instead,
1322which can be trapped by an C<eval()>.
28757baa 1323
5a964f20
TC
1324All C<END{}> blocks are run at exit time. See L<perlsub> for details.
1325
a0d0e21e
LW
1326=item exp EXPR
1327
54310121 1328=item exp
bbce6d69 1329
54310121 1330Returns I<e> (the natural logarithm base) to the power of EXPR.
a0d0e21e
LW
1331If EXPR is omitted, gives C<exp($_)>.
1332
1333=item fcntl FILEHANDLE,FUNCTION,SCALAR
1334
f86cebdf 1335Implements the fcntl(2) function. You'll probably have to say
a0d0e21e
LW
1336
1337 use Fcntl;
1338
0ade1984 1339first to get the correct constant definitions. Argument processing and
7660c0ab 1340value return works just like C<ioctl()> below.
a0d0e21e
LW
1341For example:
1342
1343 use Fcntl;
5a964f20
TC
1344 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1345 or die "can't fcntl F_GETFL: $!";
1346
ab4f32c2
A
1347You don't have to check for C<defined()> on the return from
1348C<fnctl()>. Like C<ioctl()>, it maps a C<0> return from the system
7660c0ab
A
1349call into "C<0> but true" in Perl. This string is true in
1350boolean context and C<0> in numeric context. It is also
5a964f20
TC
1351exempt from the normal B<-w> warnings on improper numeric
1352conversions.
1353
7660c0ab 1354Note that C<fcntl()> will produce a fatal error if used on a machine that
f86cebdf 1355doesn't implement fcntl(2).
a0d0e21e
LW
1356
1357=item fileno FILEHANDLE
1358
1359Returns the file descriptor for a filehandle. This is useful for
7660c0ab 1360constructing bitmaps for C<select()> and low-level POSIX tty-handling
5a964f20
TC
1361operations. If FILEHANDLE is an expression, the value is taken as
1362an indirect filehandle, generally its name.
1363
1364You can use this to find out whether two handles refer to the
1365same underlying descriptor:
1366
1367 if (fileno(THIS) == fileno(THAT)) {
1368 print "THIS and THAT are dups\n";
1369 }
a0d0e21e
LW
1370
1371=item flock FILEHANDLE,OPERATION
1372
f86cebdf 1373Calls flock(2), or an emulation of it, on FILEHANDLE. Returns TRUE for
68dc0745 1374success, FALSE on failure. Produces a fatal error if used on a machine
f86cebdf 1375that doesn't implement flock(2), fcntl(2) locking, or lockf(3). C<flock()>
68dc0745
PP
1376is Perl's portable file locking interface, although it locks only entire
1377files, not records.
8ebc5c01 1378
a3cb178b 1379On many platforms (including most versions or clones of Unix), locks
7660c0ab 1380established by C<flock()> are B<merely advisory>. Such discretionary locks
5a964f20 1381are more flexible, but offer fewer guarantees. This means that files
7660c0ab
A
1382locked with C<flock()> may be modified by programs that do not also use
1383C<flock()>. Windows NT and OS/2 are among the platforms which
5a964f20 1384enforce mandatory locking. See your local documentation for details.
a3cb178b 1385
8ebc5c01
PP
1386OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1387LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
68dc0745
PP
1388you can use the symbolic names if import them from the Fcntl module,
1389either individually, or as a group using the ':flock' tag. LOCK_SH
1390requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1391releases a previously requested lock. If LOCK_NB is added to LOCK_SH or
7660c0ab 1392LOCK_EX then C<flock()> will return immediately rather than blocking
68dc0745
PP
1393waiting for the lock (check the return status to see if you got it).
1394
1395To avoid the possibility of mis-coordination, Perl flushes FILEHANDLE
1396before (un)locking it.
8ebc5c01 1397
f86cebdf 1398Note that the emulation built with lockf(3) doesn't provide shared
8ebc5c01 1399locks, and it requires that FILEHANDLE be open with write intent. These
f86cebdf
GS
1400are the semantics that lockf(3) implements. Most (all?) systems
1401implement lockf(3) in terms of fcntl(2) locking, though, so the
8ebc5c01
PP
1402differing semantics shouldn't bite too many people.
1403
7660c0ab
A
1404Note also that some versions of C<flock()> cannot lock things over the
1405network; you would need to use the more system-specific C<fcntl()> for
f86cebdf
GS
1406that. If you like you can force Perl to ignore your system's flock(2)
1407function, and so provide its own fcntl(2)-based emulation, by passing
8ebc5c01
PP
1408the switch C<-Ud_flock> to the F<Configure> program when you configure
1409perl.
4633a7c4
LW
1410
1411Here's a mailbox appender for BSD systems.
a0d0e21e 1412
7e1af8bc 1413 use Fcntl ':flock'; # import LOCK_* constants
a0d0e21e
LW
1414
1415 sub lock {
7e1af8bc 1416 flock(MBOX,LOCK_EX);
a0d0e21e
LW
1417 # and, in case someone appended
1418 # while we were waiting...
1419 seek(MBOX, 0, 2);
1420 }
1421
1422 sub unlock {
7e1af8bc 1423 flock(MBOX,LOCK_UN);
a0d0e21e
LW
1424 }
1425
1426 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1427 or die "Can't open mailbox: $!";
1428
1429 lock();
1430 print MBOX $msg,"\n\n";
1431 unlock();
1432
cb1a09d0 1433See also L<DB_File> for other flock() examples.
a0d0e21e
LW
1434
1435=item fork
1436
f86cebdf 1437Does a fork(2) system call. Returns the child pid to the parent process,
7660c0ab 1438C<0> to the child process, or C<undef> if the fork is unsuccessful.
5a964f20 1439
a0d0e21e 1440Note: unflushed buffers remain unflushed in both processes, which means
7660c0ab
A
1441you may need to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()>
1442method of C<IO::Handle> to avoid duplicate output.
a0d0e21e 1443
7660c0ab 1444If you C<fork()> without ever waiting on your children, you will accumulate
a0d0e21e
LW
1445zombies:
1446
4633a7c4 1447 $SIG{CHLD} = sub { wait };
a0d0e21e 1448
54310121 1449There's also the double-fork trick (error checking on
7660c0ab 1450C<fork()> returns omitted);
a0d0e21e
LW
1451
1452 unless ($pid = fork) {
1453 unless (fork) {
1454 exec "what you really wanna do";
1455 die "no exec";
1456 # ... or ...
4633a7c4 1457 ## (some_perl_code_here)
a0d0e21e
LW
1458 exit 0;
1459 }
1460 exit 0;
1461 }
1462 waitpid($pid,0);
1463
cb1a09d0
AD
1464See also L<perlipc> for more examples of forking and reaping
1465moribund children.
1466
28757baa
PP
1467Note that if your forked child inherits system file descriptors like
1468STDIN and STDOUT that are actually connected by a pipe or socket, even
5a964f20 1469if you exit, then the remote server (such as, say, httpd or rsh) won't think
7660c0ab 1470you're done. You should reopen those to F</dev/null> if it's any issue.
28757baa 1471
cb1a09d0
AD
1472=item format
1473
7660c0ab 1474Declare a picture format for use by the C<write()> function. For
cb1a09d0
AD
1475example:
1476
54310121 1477 format Something =
cb1a09d0
AD
1478 Test: @<<<<<<<< @||||| @>>>>>
1479 $str, $%, '$' . int($num)
1480 .
1481
1482 $str = "widget";
184e9718 1483 $num = $cost/$quantity;
cb1a09d0
AD
1484 $~ = 'Something';
1485 write;
1486
1487See L<perlform> for many details and examples.
1488
8903cb82 1489=item formline PICTURE,LIST
a0d0e21e 1490
5a964f20 1491This is an internal function used by C<format>s, though you may call it,
a0d0e21e
LW
1492too. It formats (see L<perlform>) a list of values according to the
1493contents of PICTURE, placing the output into the format output
7660c0ab
A
1494accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
1495Eventually, when a C<write()> is done, the contents of
a0d0e21e 1496C<$^A> are written to some filehandle, but you could also read C<$^A>
7660c0ab
A
1497yourself and then set C<$^A> back to C<"">. Note that a format typically
1498does one C<formline()> per line of form, but the C<formline()> function itself
748a9306 1499doesn't care how many newlines are embedded in the PICTURE. This means
4633a7c4 1500that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
748a9306
LW
1501You may therefore need to use multiple formlines to implement a single
1502record format, just like the format compiler.
1503
5f05dabc 1504Be careful if you put double quotes around the picture, because an "C<@>"
748a9306 1505character may be taken to mean the beginning of an array name.
7660c0ab 1506C<formline()> always returns TRUE. See L<perlform> for other examples.
a0d0e21e
LW
1507
1508=item getc FILEHANDLE
1509
1510=item getc
1511
1512Returns the next character from the input file attached to FILEHANDLE,
9bc64814 1513or the undefined value at end of file, or if there was an error. If
3b02c43c
GS
1514FILEHANDLE is omitted, reads from STDIN. This is not particularly
1515efficient. It cannot be used to get unbuffered single-characters,
1516however. For that, try something more like:
4633a7c4
LW
1517
1518 if ($BSD_STYLE) {
1519 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1520 }
1521 else {
54310121 1522 system "stty", '-icanon', 'eol', "\001";
4633a7c4
LW
1523 }
1524
1525 $key = getc(STDIN);
1526
1527 if ($BSD_STYLE) {
1528 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1529 }
1530 else {
5f05dabc 1531 system "stty", 'icanon', 'eol', '^@'; # ASCII null
4633a7c4
LW
1532 }
1533 print "\n";
1534
54310121
PP
1535Determination of whether $BSD_STYLE should be set
1536is left as an exercise to the reader.
cb1a09d0 1537
7660c0ab 1538The C<POSIX::getattr()> function can do this more portably on systems
5a964f20 1539purporting POSIX compliance.
cb1a09d0 1540See also the C<Term::ReadKey> module from your nearest CPAN site;
54310121 1541details on CPAN can be found on L<perlmod/CPAN>.
a0d0e21e
LW
1542
1543=item getlogin
1544
5a964f20
TC
1545Implements the C library function of the same name, which on most
1546systems returns the current login from F</etc/utmp>, if any. If null,
7660c0ab 1547use C<getpwuid()>.
a0d0e21e 1548
f86702cc 1549 $login = getlogin || getpwuid($<) || "Kilroy";
a0d0e21e 1550
7660c0ab
A
1551Do not consider C<getlogin()> for authentication: it is not as
1552secure as C<getpwuid()>.
4633a7c4 1553
a0d0e21e
LW
1554=item getpeername SOCKET
1555
1556Returns the packed sockaddr address of other end of the SOCKET connection.
1557
4633a7c4
LW
1558 use Socket;
1559 $hersockaddr = getpeername(SOCK);
1560 ($port, $iaddr) = unpack_sockaddr_in($hersockaddr);
1561 $herhostname = gethostbyaddr($iaddr, AF_INET);
1562 $herstraddr = inet_ntoa($iaddr);
a0d0e21e
LW
1563
1564=item getpgrp PID
1565
47e29363 1566Returns the current process group for the specified PID. Use
7660c0ab 1567a PID of C<0> to get the current process group for the
4633a7c4 1568current process. Will raise an exception if used on a machine that
f86cebdf 1569doesn't implement getpgrp(2). If PID is omitted, returns process
7660c0ab
A
1570group of current process. Note that the POSIX version of C<getpgrp()>
1571does not accept a PID argument, so only C<PID==0> is truly portable.
a0d0e21e
LW
1572
1573=item getppid
1574
1575Returns the process id of the parent process.
1576
1577=item getpriority WHICH,WHO
1578
4633a7c4
LW
1579Returns the current priority for a process, a process group, or a user.
1580(See L<getpriority(2)>.) Will raise a fatal exception if used on a
f86cebdf 1581machine that doesn't implement getpriority(2).
a0d0e21e
LW
1582
1583=item getpwnam NAME
1584
1585=item getgrnam NAME
1586
1587=item gethostbyname NAME
1588
1589=item getnetbyname NAME
1590
1591=item getprotobyname NAME
1592
1593=item getpwuid UID
1594
1595=item getgrgid GID
1596
1597=item getservbyname NAME,PROTO
1598
1599=item gethostbyaddr ADDR,ADDRTYPE
1600
1601=item getnetbyaddr ADDR,ADDRTYPE
1602
1603=item getprotobynumber NUMBER
1604
1605=item getservbyport PORT,PROTO
1606
1607=item getpwent
1608
1609=item getgrent
1610
1611=item gethostent
1612
1613=item getnetent
1614
1615=item getprotoent
1616
1617=item getservent
1618
1619=item setpwent
1620
1621=item setgrent
1622
1623=item sethostent STAYOPEN
1624
1625=item setnetent STAYOPEN
1626
1627=item setprotoent STAYOPEN
1628
1629=item setservent STAYOPEN
1630
1631=item endpwent
1632
1633=item endgrent
1634
1635=item endhostent
1636
1637=item endnetent
1638
1639=item endprotoent
1640
1641=item endservent
1642
1643These routines perform the same functions as their counterparts in the
5a964f20 1644system library. In list context, the return values from the
a0d0e21e
LW
1645various get routines are as follows:
1646
1647 ($name,$passwd,$uid,$gid,
6ee623d5 1648 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
a0d0e21e
LW
1649 ($name,$passwd,$gid,$members) = getgr*
1650 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1651 ($name,$aliases,$addrtype,$net) = getnet*
1652 ($name,$aliases,$proto) = getproto*
1653 ($name,$aliases,$port,$proto) = getserv*
1654
1655(If the entry doesn't exist you get a null list.)
1656
5a964f20 1657In scalar context, you get the name, unless the function was a
a0d0e21e
LW
1658lookup by name, in which case you get the other thing, whatever it is.
1659(If the entry doesn't exist you get the undefined value.) For example:
1660
5a964f20
TC
1661 $uid = getpwnam($name);
1662 $name = getpwuid($num);
1663 $name = getpwent();
1664 $gid = getgrnam($name);
1665 $name = getgrgid($num;
1666 $name = getgrent();
1667 #etc.
a0d0e21e 1668
7660c0ab 1669In I<getpw*()> the fields C<$quota>, C<$comment>, and C<$expire> are special
6ee623d5 1670cases in the sense that in many systems they are unsupported. If the
7660c0ab
A
1671C<$quota> is unsupported, it is an empty scalar. If it is supported, it
1672usually encodes the disk quota. If the C<$comment> field is unsupported,
6ee623d5
GS
1673it is an empty scalar. If it is supported it usually encodes some
1674administrative comment about the user. In some systems the $quota
7660c0ab
A
1675field may be C<$change> or C<$age>, fields that have to do with password
1676aging. In some systems the C<$comment> field may be C<$class>. The C<$expire>
6ee623d5
GS
1677field, if present, encodes the expiration period of the account or the
1678password. For the availability and the exact meaning of these fields
f86cebdf 1679in your system, please consult your getpwnam(3) documentation and your
7660c0ab
A
1680F<pwd.h> file. You can also find out from within Perl which meaning
1681your C<$quota> and C<$comment> fields have and whether you have the C<$expire>
1682field by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
1683C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>.
6ee623d5 1684
7660c0ab 1685The C<$members> value returned by I<getgr*()> is a space separated list of
a0d0e21e
LW
1686the login names of the members of the group.
1687
1688For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1689C, it will be returned to you via C<$?> if the function call fails. The
7660c0ab 1690C<@addrs> value returned by a successful call is a list of the raw
a0d0e21e
LW
1691addresses returned by the corresponding system library call. In the
1692Internet domain, each address is four bytes long and you can unpack it
1693by saying something like:
1694
1695 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1696
5a964f20
TC
1697If you get tired of remembering which element of the return list contains
1698which return value, by-name interfaces are also provided in modules:
7660c0ab
A
1699C<File::stat>, C<Net::hostent>, C<Net::netent>, C<Net::protoent>, C<Net::servent>,
1700C<Time::gmtime>, C<Time::localtime>, and C<User::grent>. These override the
5a964f20
TC
1701normal built-in, replacing them with versions that return objects with
1702the appropriate names for each field. For example:
1703
1704 use File::stat;
1705 use User::pwent;
1706 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
1707
1708Even though it looks like they're the same method calls (uid),
7660c0ab 1709they aren't, because a C<File::stat> object is different from a C<User::pwent> object.
5a964f20 1710
a0d0e21e
LW
1711=item getsockname SOCKET
1712
1713Returns the packed sockaddr address of this end of the SOCKET connection.
1714
4633a7c4
LW
1715 use Socket;
1716 $mysockaddr = getsockname(SOCK);
1717 ($port, $myaddr) = unpack_sockaddr_in($mysockaddr);
a0d0e21e
LW
1718
1719=item getsockopt SOCKET,LEVEL,OPTNAME
1720
5a964f20 1721Returns the socket option requested, or undef if there is an error.
a0d0e21e
LW
1722
1723=item glob EXPR
1724
0a753a76
PP
1725=item glob
1726
7660c0ab 1727Returns the value of EXPR with filename expansions such as the standard Unix shell F</bin/sh> would
68dc0745 1728do. This is the internal function implementing the C<E<lt>*.cE<gt>>
7660c0ab 1729operator, but you can use it directly. If EXPR is omitted, C<$_> is used.
68dc0745
PP
1730The C<E<lt>*.cE<gt>> operator is discussed in more detail in
1731L<perlop/"I/O Operators">.
a0d0e21e
LW
1732
1733=item gmtime EXPR
1734
1735Converts a time as returned by the time function to a 9-element array
54310121 1736with the time localized for the standard Greenwich time zone.
4633a7c4 1737Typically used as follows:
a0d0e21e 1738
54310121 1739 # 0 1 2 3 4 5 6 7 8
a0d0e21e
LW
1740 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1741 gmtime(time);
1742
1743All array elements are numeric, and come straight out of a struct tm.
7660c0ab
A
1744In particular this means that C<$mon> has the range C<0..11> and C<$wday> has
1745the range C<0..6> with sunday as day C<0>. Also, C<$year> is the number of
1746years since 1900, that is, C<$year> is C<123> in year 2023, I<not> simply the last two digits of the year.
2f9daede
TPG
1747
1748If EXPR is omitted, does C<gmtime(time())>.
a0d0e21e 1749
f86cebdf 1750In scalar context, returns the ctime(3) value:
0a753a76
PP
1751
1752 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
1753
7660c0ab 1754Also see the C<timegm()> function provided by the C<Time::Local> module,
f86cebdf 1755and the strftime(3) function available via the POSIX module.
7660c0ab
A
1756
1757This scalar value is B<not> locale dependent, see L<perllocale>, but
1758instead a Perl builtin. Also see the C<Time::Local> module, and the
f86cebdf 1759strftime(3) and mktime(3) function available via the POSIX module. To
7660c0ab
A
1760get somewhat similar but locale dependent date strings, set up your
1761locale environment variables appropriately (please see L<perllocale>)
1762and try for example:
1763
1764 use POSIX qw(strftime);
1765 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
1766
1767Note that the C<%a> and C<%b>, the short forms of the day of the week
1768and the month of the year, may not necessarily be three characters wide.
0a753a76 1769
a0d0e21e
LW
1770=item goto LABEL
1771
748a9306
LW
1772=item goto EXPR
1773
a0d0e21e
LW
1774=item goto &NAME
1775
7660c0ab 1776The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
a0d0e21e 1777execution there. It may not be used to go into any construct that
7660c0ab 1778requires initialization, such as a subroutine or a C<foreach> loop. It
0a753a76 1779also can't be used to go into a construct that is optimized away,
7660c0ab 1780or to get out of a block or subroutine given to C<sort()>.
0a753a76 1781It can be used to go almost anywhere else within the dynamic scope,
a0d0e21e 1782including out of subroutines, but it's usually better to use some other
ab4f32c2 1783construct such as C<last> or C<die()>. The author of Perl has never felt the
7660c0ab 1784need to use this form of C<goto> (in Perl, that is--C is another matter).
a0d0e21e 1785
7660c0ab
A
1786The C<goto-EXPR> form expects a label name, whose scope will be resolved
1787dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
748a9306
LW
1788necessarily recommended if you're optimizing for maintainability:
1789
1790 goto ("FOO", "BAR", "GLARCH")[$i];
1791
7660c0ab 1792The C<goto-&NAME> form is highly magical, and substitutes a call to the
a0d0e21e 1793named subroutine for the currently running subroutine. This is used by
7660c0ab 1794C<AUTOLOAD> subroutines that wish to load another subroutine and then
a0d0e21e 1795pretend that the other subroutine had been called in the first place
7660c0ab
A
1796(except that any modifications to C<@_> in the current subroutine are
1797propagated to the other subroutine.) After the C<goto>, not even C<caller()>
a0d0e21e
LW
1798will be able to tell that this routine was called first.
1799
1800=item grep BLOCK LIST
1801
1802=item grep EXPR,LIST
1803
f86cebdf 1804This is similar in spirit to, but not the same as, grep(1)
2f9daede
TPG
1805and its relatives. In particular, it is not limited to using
1806regular expressions.
1807
a0d0e21e 1808Evaluates the BLOCK or EXPR for each element of LIST (locally setting
7660c0ab 1809C<$_> to each element) and returns the list value consisting of those
a0d0e21e
LW
1810elements for which the expression evaluated to TRUE. In a scalar
1811context, returns the number of times the expression was TRUE.
1812
1813 @foo = grep(!/^#/, @bar); # weed out comments
1814
1815or equivalently,
1816
1817 @foo = grep {!/^#/} @bar; # weed out comments
1818
7660c0ab 1819Note that, because C<$_> is a reference into the list value, it can be used
a0d0e21e
LW
1820to modify the elements of the array. While this is useful and
1821supported, it can cause bizarre results if the LIST is not a named
2f9daede 1822array. Similarly, grep returns aliases into the original list,
f86cebdf 1823much like the way that a for loop's index variable aliases the list
2f9daede 1824elements. That is, modifying an element of a list returned by grep
ab4f32c2 1825(for example, in a C<foreach>, C<map()> or another C<grep()>)
2f9daede 1826actually modifies the element in the original list.
a0d0e21e 1827
fb73857a 1828See also L</map> for an array composed of the results of the BLOCK or EXPR.
38325410 1829
a0d0e21e
LW
1830=item hex EXPR
1831
54310121 1832=item hex
bbce6d69 1833
54310121 1834Interprets EXPR as a hex string and returns the corresponding
2f9daede 1835value. (To convert strings that might start with either 0 or 0x
7660c0ab 1836see L</oct>.) If EXPR is omitted, uses C<$_>.
2f9daede
TPG
1837
1838 print hex '0xAf'; # prints '175'
1839 print hex 'aF'; # same
a0d0e21e
LW
1840
1841=item import
1842
7660c0ab 1843There is no builtin C<import()> function. It is just an ordinary
4633a7c4 1844method (subroutine) defined (or inherited) by modules that wish to export
7660c0ab 1845names to another module. The C<use()> function calls the C<import()> method
54310121 1846for the package used. See also L</use()>, L<perlmod>, and L<Exporter>.
a0d0e21e
LW
1847
1848=item index STR,SUBSTR,POSITION
1849
1850=item index STR,SUBSTR
1851
4633a7c4
LW
1852Returns the position of the first occurrence of SUBSTR in STR at or after
1853POSITION. If POSITION is omitted, starts searching from the beginning of
7660c0ab 1854the string. The return value is based at C<0> (or whatever you've set the C<$[>
4633a7c4 1855variable to--but don't do that). If the substring is not found, returns
7660c0ab 1856one less than the base, ordinarily C<-1>.
a0d0e21e
LW
1857
1858=item int EXPR
1859
54310121 1860=item int
bbce6d69 1861
7660c0ab 1862Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
5a964f20 1863You should not use this for rounding, because it truncates
7660c0ab
A
1864towards C<0>, and because machine representations of floating point
1865numbers can sometimes produce counterintuitive results. Usually C<sprintf()> or C<printf()>,
1866or the C<POSIX::floor> or C<POSIX::ceil> functions, would serve you better.
a0d0e21e
LW
1867
1868=item ioctl FILEHANDLE,FUNCTION,SCALAR
1869
f86cebdf 1870Implements the ioctl(2) function. You'll probably have to say
a0d0e21e 1871
4633a7c4 1872 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
a0d0e21e 1873
4633a7c4 1874first to get the correct function definitions. If F<ioctl.ph> doesn't
a0d0e21e 1875exist or doesn't have the correct definitions you'll have to roll your
4633a7c4 1876own, based on your C header files such as F<E<lt>sys/ioctl.hE<gt>>.
5a964f20 1877(There is a Perl script called B<h2ph> that comes with the Perl kit that
54310121 1878may help you in this, but it's nontrivial.) SCALAR will be read and/or
4633a7c4 1879written depending on the FUNCTION--a pointer to the string value of SCALAR
ab4f32c2 1880will be passed as the third argument of the actual C<ioctl()> call. (If SCALAR
4633a7c4
LW
1881has no string value but does have a numeric value, that value will be
1882passed rather than a pointer to the string value. To guarantee this to be
7660c0ab 1883TRUE, add a C<0> to the scalar before using it.) The C<pack()> and C<unpack()>
4633a7c4 1884functions are useful for manipulating the values of structures used by
7660c0ab 1885C<ioctl()>. The following example sets the erase character to DEL.
a0d0e21e
LW
1886
1887 require 'ioctl.ph';
4633a7c4
LW
1888 $getp = &TIOCGETP;
1889 die "NO TIOCGETP" if $@ || !$getp;
a0d0e21e 1890 $sgttyb_t = "ccccs"; # 4 chars and a short
4633a7c4 1891 if (ioctl(STDIN,$getp,$sgttyb)) {
a0d0e21e
LW
1892 @ary = unpack($sgttyb_t,$sgttyb);
1893 $ary[2] = 127;
1894 $sgttyb = pack($sgttyb_t,@ary);
4633a7c4 1895 ioctl(STDIN,&TIOCSETP,$sgttyb)
a0d0e21e
LW
1896 || die "Can't ioctl: $!";
1897 }
1898
ab4f32c2 1899The return value of C<ioctl()> (and C<fcntl()>) is as follows:
a0d0e21e
LW
1900
1901 if OS returns: then Perl returns:
1902 -1 undefined value
1903 0 string "0 but true"
1904 anything else that number
1905
1906Thus Perl returns TRUE on success and FALSE on failure, yet you can
1907still easily determine the actual value returned by the operating
1908system:
1909
1910 ($retval = ioctl(...)) || ($retval = -1);
1911 printf "System returned %d\n", $retval;
1912
7660c0ab 1913The special string "C<0> but true" is excempt from B<-w> complaints
5a964f20
TC
1914about improper numeric conversions.
1915
a0d0e21e
LW
1916=item join EXPR,LIST
1917
54310121 1918Joins the separate strings of LIST into a single string with
a0d0e21e
LW
1919fields separated by the value of EXPR, and returns the string.
1920Example:
1921
1922 $_ = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
1923
7660c0ab 1924See L</split>.
a0d0e21e 1925
aa689395
PP
1926=item keys HASH
1927
1d2dff63
GS
1928Returns a list consisting of all the keys of the named hash. (In a
1929scalar context, returns the number of keys.) The keys are returned in
aa689395 1930an apparently random order, but it is the same order as either the
7660c0ab 1931C<values()> or C<each()> function produces (given that the hash has not been
aa689395 1932modified). As a side effect, it resets HASH's iterator.
a0d0e21e 1933
aa689395 1934Here is yet another way to print your environment:
a0d0e21e
LW
1935
1936 @keys = keys %ENV;
1937 @values = values %ENV;
1938 while ($#keys >= 0) {
1939 print pop(@keys), '=', pop(@values), "\n";
1940 }
1941
1942or how about sorted by key:
1943
1944 foreach $key (sort(keys %ENV)) {
1945 print $key, '=', $ENV{$key}, "\n";
1946 }
1947
aca803df 1948To sort a hash by value, you'll need to use a C<sort()> function.
aa689395 1949Here's a descending numeric sort of a hash by its values:
4633a7c4 1950
5a964f20 1951 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
4633a7c4
LW
1952 printf "%4d %s\n", $hash{$key}, $key;
1953 }
1954
ab4f32c2 1955As an lvalue C<keys()> allows you to increase the number of hash buckets
aa689395
PP
1956allocated for the given hash. This can gain you a measure of efficiency if
1957you know the hash is going to get big. (This is similar to pre-extending
1958an array by assigning a larger number to $#array.) If you say
55497cff
PP
1959
1960 keys %hash = 200;
1961
5a964f20
TC
1962then C<%hash> will have at least 200 buckets allocated for it--256 of them, in fact, since
1963it rounds up to the next power of two. These
55497cff
PP
1964buckets will be retained even if you do C<%hash = ()>, use C<undef
1965%hash> if you want to free the storage while C<%hash> is still in scope.
1966You can't shrink the number of buckets allocated for the hash using
ab4f32c2 1967C<keys()> in this way (but you needn't worry about doing this by accident,
55497cff
PP
1968as trying has no effect).
1969
a0d0e21e
LW
1970=item kill LIST
1971
54310121
PP
1972Sends a signal to a list of processes. The first element of
1973the list must be the signal to send. Returns the number of
4633a7c4 1974processes successfully signaled.
a0d0e21e
LW
1975
1976 $cnt = kill 1, $child1, $child2;
1977 kill 9, @goners;
1978
4633a7c4
LW
1979Unlike in the shell, in Perl if the I<SIGNAL> is negative, it kills
1980process groups instead of processes. (On System V, a negative I<PROCESS>
1981number will also kill process groups, but that's not portable.) That
1982means you usually want to use positive not negative signals. You may also
da0045b7 1983use a signal name in quotes. See L<perlipc/"Signals"> for details.
a0d0e21e
LW
1984
1985=item last LABEL
1986
1987=item last
1988
1989The C<last> command is like the C<break> statement in C (as used in
1990loops); it immediately exits the loop in question. If the LABEL is
1991omitted, the command refers to the innermost enclosing loop. The
1992C<continue> block, if any, is not executed:
1993
4633a7c4
LW
1994 LINE: while (<STDIN>) {
1995 last LINE if /^$/; # exit when done with header
5a964f20 1996 #...
a0d0e21e
LW
1997 }
1998
4968c1e4
G
1999C<last> cannot be used to exit a block which returns a value such as
2000C<eval {}>, C<sub {}> or C<do {}>.
2001
1d2dff63
GS
2002See also L</continue> for an illustration of how C<last>, C<next>, and
2003C<redo> work.
2004
a0d0e21e
LW
2005=item lc EXPR
2006
54310121 2007=item lc
bbce6d69 2008
a0d0e21e 2009Returns an lowercased version of EXPR. This is the internal function
7660c0ab 2010implementing the C<\L> escape in double-quoted strings.
a0ed51b3 2011Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
a0d0e21e 2012
7660c0ab 2013If EXPR is omitted, uses C<$_>.
bbce6d69 2014
a0d0e21e
LW
2015=item lcfirst EXPR
2016
54310121 2017=item lcfirst
bbce6d69 2018
a0d0e21e 2019Returns the value of EXPR with the first character lowercased. This is
7660c0ab 2020the internal function implementing the C<\l> escape in double-quoted strings.
a0ed51b3 2021Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
a0d0e21e 2022
7660c0ab 2023If EXPR is omitted, uses C<$_>.
bbce6d69 2024
a0d0e21e
LW
2025=item length EXPR
2026
54310121 2027=item length
bbce6d69 2028
a0ed51b3 2029Returns the length in characters of the value of EXPR. If EXPR is
7660c0ab 2030omitted, returns length of C<$_>.
a0d0e21e
LW
2031
2032=item link OLDFILE,NEWFILE
2033
5a964f20
TC
2034Creates a new filename linked to the old filename. Returns TRUE for
2035success, FALSE otherwise.
a0d0e21e
LW
2036
2037=item listen SOCKET,QUEUESIZE
2038
2039Does the same thing that the listen system call does. Returns TRUE if
4633a7c4 2040it succeeded, FALSE otherwise. See example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
2041
2042=item local EXPR
2043
5a964f20
TC
2044A local modifies the listed variables to be local to the enclosing
2045block, file, or eval. If more than one value is listed, the list must
2046be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2047for details, including issues with tied arrays and hashes.
a0d0e21e 2048
7660c0ab 2049You really probably want to be using C<my()> instead, because C<local()> isn't
7b8d334a 2050what most people think of as "local". See L<perlsub/"Private Variables
cb1a09d0 2051via my()"> for details.
a0d0e21e
LW
2052
2053=item localtime EXPR
2054
2055Converts a time as returned by the time function to a 9-element array
5f05dabc 2056with the time analyzed for the local time zone. Typically used as
a0d0e21e
LW
2057follows:
2058
54310121 2059 # 0 1 2 3 4 5 6 7 8
a0d0e21e
LW
2060 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2061 localtime(time);
2062
2063All array elements are numeric, and come straight out of a struct tm.
7660c0ab
A
2064In particular this means that C<$mon> has the range C<0..11> and C<$wday> has
2065the range C<0..6> with sunday as day C<0>. Also, C<$year> is the number of
2066years since 1900, that is, C<$year> is C<123> in year 2023, and I<not> simply the last two digits of the year.
54310121
PP
2067
2068If EXPR is omitted, uses the current time (C<localtime(time)>).
a0d0e21e 2069
f86cebdf 2070In scalar context, returns the ctime(3) value:
a0d0e21e 2071
5f05dabc 2072 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
a0d0e21e 2073
a3cb178b 2074This scalar value is B<not> locale dependent, see L<perllocale>, but
7660c0ab 2075instead a Perl builtin. Also see the C<Time::Local> module, and the
f86cebdf 2076strftime(3) and mktime(3) function available via the POSIX module. To
a3cb178b
GS
2077get somewhat similar but locale dependent date strings, set up your
2078locale environment variables appropriately (please see L<perllocale>)
5a964f20 2079and try for example:
a3cb178b 2080
5a964f20 2081 use POSIX qw(strftime);
a3cb178b
GS
2082 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
2083
2084Note that the C<%a> and C<%b>, the short forms of the day of the week
2085and the month of the year, may not necessarily be three characters wide.
a0d0e21e
LW
2086
2087=item log EXPR
2088
54310121 2089=item log
bbce6d69 2090
5a964f20 2091Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted, returns log
7660c0ab 2092of C<$_>.
a0d0e21e
LW
2093
2094=item lstat FILEHANDLE
2095
2096=item lstat EXPR
2097
54310121 2098=item lstat
bbce6d69 2099
7660c0ab 2100Does the same thing as the C<stat()> function (including setting the
5a964f20
TC
2101special C<_> filehandle) but stats a symbolic link instead of the file
2102the symbolic link points to. If symbolic links are unimplemented on
7660c0ab 2103your system, a normal C<stat()> is done.
a0d0e21e 2104
7660c0ab 2105If EXPR is omitted, stats C<$_>.
bbce6d69 2106
a0d0e21e
LW
2107=item m//
2108
2109The match operator. See L<perlop>.
2110
2111=item map BLOCK LIST
2112
2113=item map EXPR,LIST
2114
7660c0ab 2115Evaluates the BLOCK or EXPR for each element of LIST (locally setting C<$_> to each
a0d0e21e
LW
2116element) and returns the list value composed of the results of each such
2117evaluation. Evaluates BLOCK or EXPR in a list context, so each element of LIST
2118may produce zero, one, or more elements in the returned value.
2119
2120 @chars = map(chr, @nums);
2121
2122translates a list of numbers to the corresponding characters. And
2123
4633a7c4 2124 %hash = map { getkey($_) => $_ } @array;
a0d0e21e
LW
2125
2126is just a funny way to write
2127
2128 %hash = ();
2129 foreach $_ (@array) {
4633a7c4 2130 $hash{getkey($_)} = $_;
a0d0e21e
LW
2131 }
2132
7660c0ab 2133Note that, because C<$_> is a reference into the list value, it can be used
fb73857a
PP
2134to modify the elements of the array. While this is useful and
2135supported, it can cause bizarre results if the LIST is not a named
2136array. See also L</grep> for an array composed of those items of the
2137original list for which the BLOCK or EXPR evaluates to true.
2138
a0d0e21e
LW
2139=item mkdir FILENAME,MODE
2140
0591cd52
NT
2141Creates the directory specified by FILENAME, with permissions
2142specified by MODE (as modified by C<umask>). If it succeeds it
2143returns TRUE, otherwise it returns FALSE and sets C<$!> (errno).
2144
2145In general, it is better to create directories with permissive MODEs,
2146and let the user modify that with their C<umask>, than it is to supply
2147a restrictive MODE and give the user no way to be more permissive.
2148The exceptions to this rule are when the file or directory should be
2149kept private (mail files, for instance). The perlfunc(1) entry on
2150C<umask> discusses the choice of MODE in more detail.
a0d0e21e
LW
2151
2152=item msgctl ID,CMD,ARG
2153
f86cebdf 2154Calls the System V IPC function msgctl(2). You'll probably have to say
0ade1984
JH
2155
2156 use IPC::SysV;
2157
7660c0ab
A
2158first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2159then ARG must be a variable which will hold the returned C<msqid_ds>
ab4f32c2 2160structure. Returns like C<ioctl()>: the undefined value for error, "C<0> but
0ade1984 2161true" for zero, or the actual return value otherwise. See also
7660c0ab 2162C<IPC::SysV> and C<IPC::Semaphore::Msg> documentation.
a0d0e21e
LW
2163
2164=item msgget KEY,FLAGS
2165
f86cebdf 2166Calls the System V IPC function msgget(2). Returns the message queue
7660c0ab
A
2167id, or the undefined value if there is an error. See also C<IPC::SysV>
2168and C<IPC::SysV::Msg> documentation.
a0d0e21e
LW
2169
2170=item msgsnd ID,MSG,FLAGS
2171
2172Calls the System V IPC function msgsnd to send the message MSG to the
2173message queue ID. MSG must begin with the long integer message type,
c07a80fd 2174which may be created with C<pack("l", $type)>. Returns TRUE if
7660c0ab
A
2175successful, or FALSE if there is an error. See also C<IPC::SysV>
2176and C<IPC::SysV::Msg> documentation.
a0d0e21e
LW
2177
2178=item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2179
2180Calls the System V IPC function msgrcv to receive a message from
2181message queue ID into variable VAR with a maximum message size of
0ade1984
JH
2182SIZE. Note that if a message is received, the message type will be
2183the first thing in VAR, and the maximum length of VAR is SIZE plus the
2184size of the message type. Returns TRUE if successful, or FALSE if
7660c0ab 2185there is an error. See also C<IPC::SysV> and C<IPC::SysV::Msg> documentation.
a0d0e21e
LW
2186
2187=item my EXPR
2188
ab4f32c2
A
2189A C<my()> declares the listed variables to be local (lexically) to the
2190enclosing block, file, or C<eval()>. If
5f05dabc 2191more than one value is listed, the list must be placed in parentheses. See
cb1a09d0 2192L<perlsub/"Private Variables via my()"> for details.
4633a7c4 2193
a0d0e21e
LW
2194=item next LABEL
2195
2196=item next
2197
2198The C<next> command is like the C<continue> statement in C; it starts
2199the next iteration of the loop:
2200
4633a7c4
LW
2201 LINE: while (<STDIN>) {
2202 next LINE if /^#/; # discard comments
5a964f20 2203 #...
a0d0e21e
LW
2204 }
2205
2206Note that if there were a C<continue> block on the above, it would get
2207executed even on discarded lines. If the LABEL is omitted, the command
2208refers to the innermost enclosing loop.
2209
4968c1e4
G
2210C<next> cannot be used to exit a block which returns a value such as
2211C<eval {}>, C<sub {}> or C<do {}>.
2212
1d2dff63
GS
2213See also L</continue> for an illustration of how C<last>, C<next>, and
2214C<redo> work.
2215
a0d0e21e
LW
2216=item no Module LIST
2217
7660c0ab 2218See the L</use> function, which C<no> is the opposite of.
a0d0e21e
LW
2219
2220=item oct EXPR
2221
54310121 2222=item oct
bbce6d69 2223
4633a7c4 2224Interprets EXPR as an octal string and returns the corresponding
7660c0ab 2225value. (If EXPR happens to start off with C<0x>, interprets it as
4633a7c4
LW
2226a hex string instead.) The following will handle decimal, octal, and
2227hex in the standard Perl or C notation:
a0d0e21e
LW
2228
2229 $val = oct($val) if $val =~ /^0/;
2230
7660c0ab
A
2231If EXPR is omitted, uses C<$_>. This function is commonly used when
2232a string such as C<644> needs to be converted into a file mode, for
2f9daede
TPG
2233example. (Although perl will automatically convert strings into
2234numbers as needed, this automatic conversion assumes base 10.)
a0d0e21e
LW
2235
2236=item open FILEHANDLE,EXPR
2237
2238=item open FILEHANDLE
2239
2240Opens the file whose filename is given by EXPR, and associates it with
5f05dabc
PP
2241FILEHANDLE. If FILEHANDLE is an expression, its value is used as the
2242name of the real filehandle wanted. If EXPR is omitted, the scalar
2243variable of the same name as the FILEHANDLE contains the filename.
ab4f32c2
A
2244(Note that lexical variables--those declared with C<my()>--will not work
2245for this purpose; so if you're using C<my()>, specify EXPR in your call
5f05dabc
PP
2246to open.)
2247
7660c0ab
A
2248If the filename begins with C<'E<lt>'> or nothing, the file is opened for input.
2249If the filename begins with C<'E<gt>'>, the file is truncated and opened for
2250output, being created if necessary. If the filename begins with C<'E<gt>E<gt>'>,
fbb426e4 2251the file is opened for appending, again being created if necessary.
7660c0ab
A
2252You can put a C<'+'> in front of the C<'E<gt>'> or C<'E<lt>'> to indicate that
2253you want both read and write access to the file; thus C<'+E<lt>'> is almost
2254always preferred for read/write updates--the C<'+E<gt>'> mode would clobber the
5a964f20
TC
2255file first. You can't usually use either read-write mode for updating
2256textfiles, since they have variable length records. See the B<-i>
0591cd52
NT
2257switch in L<perlrun> for a better approach. The file is created with
2258permissions of C<0666> modified by the process' C<umask> value.
5a964f20
TC
2259
2260The prefix and the filename may be separated with spaces.
f86cebdf 2261These various prefixes correspond to the fopen(3) modes of C<'r'>, C<'r+'>, C<'w'>,
7660c0ab 2262C<'w+'>, C<'a'>, and C<'a+'>.
5f05dabc 2263
7660c0ab 2264If the filename begins with C<'|'>, the filename is interpreted as a
5a964f20 2265command to which output is to be piped, and if the filename ends with a
7660c0ab
A
2266C<'|'>, the filename is interpreted See L<perlipc/"Using open() for IPC">
2267for more examples of this. (You are not allowed to C<open()> to a command
5a964f20
TC
2268that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2269and L<perlipc/"Bidirectional Communication"> for alternatives.)
cb1a09d0 2270
7660c0ab 2271Opening C<'-'> opens STDIN and opening C<'E<gt>-'> opens STDOUT. Open returns
ab4f32c2 2272nonzero upon success, the undefined value otherwise. If the C<open()>
4633a7c4 2273involved a pipe, the return value happens to be the pid of the
54310121 2274subprocess.
cb1a09d0
AD
2275
2276If you're unfortunate enough to be running Perl on a system that
2277distinguishes between text files and binary files (modern operating
2278systems don't care), then you should check out L</binmode> for tips for
ab4f32c2 2279dealing with this. The key distinction between systems that need C<binmode()>
5a964f20
TC
2280and those that don't is their text file formats. Systems like Unix, MacOS, and
2281Plan9, which delimit lines with a single character, and which encode that
ab4f32c2 2282character in C as C<"\n">, do not need C<binmode()>. The rest need it.
cb1a09d0 2283
fb73857a 2284When opening a file, it's usually a bad idea to continue normal execution
ab4f32c2
A
2285if the request failed, so C<open()> is frequently used in connection with
2286C<die()>. Even if C<die()> won't do what you want (say, in a CGI script,
fb73857a 2287where you want to make a nicely formatted error message (but there are
5a964f20 2288modules that can help with that problem)) you should always check
fb73857a
PP
2289the return value from opening a file. The infrequent exception is when
2290working with an unopened filehandle is actually what you want to do.
2291
cb1a09d0 2292Examples:
a0d0e21e
LW
2293
2294 $ARTICLE = 100;
2295 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2296 while (<ARTICLE>) {...
2297
2298 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
fb73857a 2299 # if the open fails, output is discarded
a0d0e21e 2300
fb73857a
PP
2301 open(DBASE, '+<dbase.mine') # open for update
2302 or die "Can't open 'dbase.mine' for update: $!";
cb1a09d0 2303
fb73857a
PP
2304 open(ARTICLE, "caesar <$article |") # decrypt article
2305 or die "Can't start caesar: $!";
a0d0e21e 2306
fb73857a
PP
2307 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
2308 or die "Can't start sort: $!";
a0d0e21e
LW
2309
2310 # process argument list of files along with any includes
2311
2312 foreach $file (@ARGV) {
2313 process($file, 'fh00');
2314 }
2315
2316 sub process {
5a964f20 2317 my($filename, $input) = @_;
a0d0e21e
LW
2318 $input++; # this is a string increment
2319 unless (open($input, $filename)) {
2320 print STDERR "Can't open $filename: $!\n";
2321 return;
2322 }
2323
5a964f20 2324 local $_;
a0d0e21e
LW
2325 while (<$input>) { # note use of indirection
2326 if (/^#include "(.*)"/) {
2327 process($1, $input);
2328 next;
2329 }
5a964f20 2330 #... # whatever
a0d0e21e
LW
2331 }
2332 }
2333
2334You may also, in the Bourne shell tradition, specify an EXPR beginning
7660c0ab 2335with C<'E<gt>&'>, in which case the rest of the string is interpreted as the
5a964f20 2336name of a filehandle (or file descriptor, if numeric) to be
7660c0ab
A
2337duped and opened. You may use C<&> after C<E<gt>>, C<E<gt>E<gt>>, C<E<lt>>, C<+E<gt>>,
2338C<+E<gt>E<gt>>, and C<+E<lt>>. The
a0d0e21e 2339mode you specify should match the mode of the original filehandle.
184e9718 2340(Duping a filehandle does not take into account any existing contents of
cb1a09d0 2341stdio buffers.)
a0d0e21e
LW
2342Here is a script that saves, redirects, and restores STDOUT and
2343STDERR:
2344
2345 #!/usr/bin/perl
5a964f20
TC
2346 open(OLDOUT, ">&STDOUT");
2347 open(OLDERR, ">&STDERR");
a0d0e21e
LW
2348
2349 open(STDOUT, ">foo.out") || die "Can't redirect stdout";
2350 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
2351
2352 select(STDERR); $| = 1; # make unbuffered
2353 select(STDOUT); $| = 1; # make unbuffered
2354
2355 print STDOUT "stdout 1\n"; # this works for
2356 print STDERR "stderr 1\n"; # subprocesses too
2357
2358 close(STDOUT);
2359 close(STDERR);
2360
5a964f20
TC
2361 open(STDOUT, ">&OLDOUT");
2362 open(STDERR, ">&OLDERR");
a0d0e21e
LW
2363
2364 print STDOUT "stdout 2\n";
2365 print STDERR "stderr 2\n";
2366
2367
7660c0ab
A
2368If you specify C<'E<lt>&=N'>, where C<N> is a number, then Perl will do an
2369equivalent of C's C<fdopen()> of that file descriptor; this is more
4633a7c4 2370parsimonious of file descriptors. For example:
a0d0e21e
LW
2371
2372 open(FILEHANDLE, "<&=$fd")
2373
7660c0ab 2374If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>, then
a0d0e21e 2375there is an implicit fork done, and the return value of open is the pid
7660c0ab 2376of the child within the parent process, and C<0> within the child
184e9718 2377process. (Use C<defined($pid)> to determine whether the open was successful.)
a0d0e21e
LW
2378The filehandle behaves normally for the parent, but i/o to that
2379filehandle is piped from/to the STDOUT/STDIN of the child process.
2380In the child process the filehandle isn't opened--i/o happens from/to
2381the new STDOUT or STDIN. Typically this is used like the normal
2382piped open when you want to exercise more control over just how the
2383pipe command gets executed, such as when you are running setuid, and
54310121 2384don't want to have to scan shell commands for metacharacters.
4633a7c4 2385The following pairs are more or less equivalent:
a0d0e21e
LW
2386
2387 open(FOO, "|tr '[a-z]' '[A-Z]'");
2388 open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
2389
2390 open(FOO, "cat -n '$file'|");
2391 open(FOO, "-|") || exec 'cat', '-n', $file;
2392
4633a7c4
LW
2393See L<perlipc/"Safe Pipe Opens"> for more examples of this.
2394
5a964f20 2395NOTE: On any operation that may do a fork, any unflushed buffers remain
184e9718 2396unflushed in both processes, which means you may need to set C<$|> to
a0d0e21e
LW
2397avoid duplicate output.
2398
0dccf244
CS
2399Closing any piped filehandle causes the parent process to wait for the
2400child to finish, and returns the status value in C<$?>.
2401
5a964f20 2402The filename passed to open will have leading and trailing
f86cebdf 2403whitespace deleted, and the normal redirection characters
5a964f20
TC
2404honored. This property, known as "magic open",
2405can often be used to good effect. A user could specify a filename of
7660c0ab 2406F<"rsh cat file |">, or you could change certain filenames as needed:
5a964f20
TC
2407
2408 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
2409 open(FH, $filename) or die "Can't open $filename: $!";
2410
2411However, to open a file with arbitrary weird characters in it, it's
2412necessary to protect any leading and trailing whitespace:
2413
2414 $file =~ s#^(\s)#./$1#;
2415 open(FOO, "< $file\0");
2416
7660c0ab
A
2417If you want a "real" C C<open()> (see L<open(2)> on your system), then you
2418should use the C<sysopen()> function, which involves no such magic. This is
5a964f20
TC
2419another way to protect your filenames from interpretation. For example:
2420
2421 use IO::Handle;
2422 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
2423 or die "sysopen $path: $!";
2424 $oldfh = select(HANDLE); $| = 1; select($oldfh);
2425 print HANDLE "stuff $$\n");
2426 seek(HANDLE, 0, 0);
2427 print "File contains: ", <HANDLE>;
2428
7660c0ab
A
2429Using the constructor from the C<IO::Handle> package (or one of its
2430subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
5a964f20
TC
2431filehandles that have the scope of whatever variables hold references to
2432them, and automatically close whenever and however you leave that scope:
c07a80fd 2433
5f05dabc 2434 use IO::File;
5a964f20 2435 #...
c07a80fd
PP
2436 sub read_myfile_munged {
2437 my $ALL = shift;
5f05dabc 2438 my $handle = new IO::File;
c07a80fd
PP
2439 open($handle, "myfile") or die "myfile: $!";
2440 $first = <$handle>
2441 or return (); # Automatically closed here.
2442 mung $first or die "mung failed"; # Or here.
2443 return $first, <$handle> if $ALL; # Or here.
2444 $first; # Or here.
2445 }
2446
cb1a09d0 2447See L</seek()> for some details about mixing reading and writing.
a0d0e21e
LW
2448
2449=item opendir DIRHANDLE,EXPR
2450
7660c0ab
A
2451Opens a directory named EXPR for processing by C<readdir()>, C<telldir()>,
2452C<seekdir()>, C<rewinddir()>, and C<closedir()>. Returns TRUE if successful.
a0d0e21e
LW
2453DIRHANDLEs have their own namespace separate from FILEHANDLEs.
2454
2455=item ord EXPR
2456
54310121 2457=item ord
bbce6d69 2458
a0ed51b3 2459Returns the numeric (ASCII or Unicode) value of the first character of EXPR. If
7660c0ab 2460EXPR is omitted, uses C<$_>. For the reverse, see L</chr>.
a0d0e21e
LW
2461
2462=item pack TEMPLATE,LIST
2463
2464Takes an array or list of values and packs it into a binary structure,
2465returning the string containing the structure. The TEMPLATE is a
2466sequence of characters that give the order and type of values, as
2467follows:
2468
2469 A An ascii string, will be space padded.
2470 a An ascii string, will be null padded.
2471 b A bit string (ascending bit order, like vec()).
2472 B A bit string (descending bit order).
2473 h A hex string (low nybble first).
2474 H A hex string (high nybble first).
2475
2476 c A signed char value.
a0ed51b3 2477 C An unsigned char value. Only does bytes. See U for Unicode.
96e4d5b1 2478
a0d0e21e
LW
2479 s A signed short value.
2480 S An unsigned short value.
96e4d5b1
PP
2481 (This 'short' is _exactly_ 16 bits, which may differ from
2482 what a local C compiler calls 'short'.)
2483
a0d0e21e
LW
2484 i A signed integer value.
2485 I An unsigned integer value.
f86cebdf
GS
2486 (This 'integer' is _at_least_ 32 bits wide. Its exact
2487 size depends on what a local C compiler calls 'int',
2488 and may even be larger than the 'long' described in
2489 the next item.)
96e4d5b1 2490
a0d0e21e
LW
2491 l A signed long value.
2492 L An unsigned long value.
96e4d5b1
PP
2493 (This 'long' is _exactly_ 32 bits, which may differ from
2494 what a local C compiler calls 'long'.)
a0d0e21e 2495
96e4d5b1
PP
2496 n A short in "network" (big-endian) order.
2497 N A long in "network" (big-endian) order.
a0d0e21e
LW
2498 v A short in "VAX" (little-endian) order.
2499 V A long in "VAX" (little-endian) order.
96e4d5b1
PP
2500 (These 'shorts' and 'longs' are _exactly_ 16 bits and
2501 _exactly_ 32 bits, respectively.)
a0d0e21e 2502
dae0da7a
JH
2503 q A signed quad (64-bit) value.
2504 Q An unsigned quad value.
2505 (Available only if your system supports 64-bit integer values
2506 _and_ if Perl has been compiled to support those.
2507 Causes a fatal error otherwise.)
2508
a0d0e21e
LW
2509 f A single-precision float in the native format.
2510 d A double-precision float in the native format.
2511
2512 p A pointer to a null-terminated string.
2513 P A pointer to a structure (fixed-length string).
2514
2515 u A uuencoded string.
a0ed51b3
LW
2516 U A Unicode character number. Encodes to UTF-8 internally.
2517 Works even if C<use utf8> is not in effect.
a0d0e21e 2518
96e4d5b1 2519 w A BER compressed integer. Its bytes represent an unsigned
f86cebdf
GS
2520 integer in base 128, most significant digit first, with as
2521 few digits as possible. Bit eight (the high bit) is set
2522 on each byte except the last.
def98dd4 2523
a0d0e21e
LW
2524 x A null byte.
2525 X Back up a byte.
2526 @ Null fill to absolute position.
2527
5a964f20 2528Each letter may optionally be followed by a number giving a repeat
7660c0ab
A
2529count. With all types except C<"a">, C<"A">, C<"b">, C<"B">, C<"h">, C<"H">, and C<"P"> the
2530pack function will gobble up that many values from the LIST. A C<*> for the
2531repeat count means to use however many items are left. The C<"a"> and C<"A">
a0d0e21e 2532types gobble just one value, but pack it as a string of length count,
7660c0ab
A
2533padding with nulls or spaces as necessary. (When unpacking, C<"A"> strips
2534trailing spaces and nulls, but C<"a"> does not.) Likewise, the C<"b"> and C<"B">
2535fields pack a string that many bits long. The C<"h"> and C<"H"> fields pack a
2536string that many nybbles long. The C<"p"> type packs a pointer to a null-
84902520
TB
2537terminated string. You are responsible for ensuring the string is not a
2538temporary value (which can potentially get deallocated before you get
7660c0ab 2539around to using the packed result). The C<"P"> packs a pointer to a structure
61167c6f 2540of the size indicated by the length. A NULL pointer is created if the
7660c0ab 2541corresponding value for C<"p"> or C<"P"> is C<undef>.
61167c6f 2542Real numbers (floats and doubles) are
a0d0e21e
LW
2543in the native machine format only; due to the multiplicity of floating
2544formats around, and the lack of a standard "network" representation, no
2545facility for interchange has been made. This means that packed floating
2546point data written on one machine may not be readable on another - even if
2547both use IEEE floating point arithmetic (as the endian-ness of the memory
2548representation is not part of the IEEE spec). Note that Perl uses doubles
2549internally for all numeric calculation, and converting from double into
5f05dabc 2550float and thence back to double again will lose precision (i.e.,
7660c0ab 2551C<unpack("f", pack("f", $foo)>) will not in general equal C<$foo>).
a0d0e21e
LW
2552
2553Examples:
2554
a0ed51b3 2555 $foo = pack("CCCC",65,66,67,68);
a0d0e21e 2556 # foo eq "ABCD"
a0ed51b3 2557 $foo = pack("C4",65,66,67,68);
a0d0e21e 2558 # same thing
a0ed51b3
LW
2559 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
2560 # same thing with Unicode circled letters
a0d0e21e
LW
2561
2562 $foo = pack("ccxxcc",65,66,67,68);
2563 # foo eq "AB\0\0CD"
2564
2565 $foo = pack("s2",1,2);
2566 # "\1\0\2\0" on little-endian
2567 # "\0\1\0\2" on big-endian
2568
2569 $foo = pack("a4","abcd","x","y","z");
2570 # "abcd"
2571
2572 $foo = pack("aaaa","abcd","x","y","z");
2573 # "axyz"
2574
2575 $foo = pack("a14","abcdefg");
2576 # "abcdefg\0\0\0\0\0\0\0"
2577
2578 $foo = pack("i9pl", gmtime);
2579 # a real struct tm (on my system anyway)
2580
2581 sub bintodec {
2582 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
2583 }
2584
2585The same template may generally also be used in the unpack function.
2586
5a964f20
TC
2587=item package
2588
cb1a09d0
AD
2589=item package NAMESPACE
2590
2591Declares the compilation unit as being in the given namespace. The scope
2592of the package declaration is from the declaration itself through the end of
7660c0ab 2593the enclosing block (the same scope as the C<local()> operator). All further
cb1a09d0 2594unqualified dynamic identifiers will be in this namespace. A package
5f05dabc 2595statement affects only dynamic variables--including those you've used
7660c0ab 2596C<local()> on--but I<not> lexical variables created with C<my()>. Typically it
cb1a09d0
AD
2597would be the first declaration in a file to be included by the C<require>
2598or C<use> operator. You can switch into a package in more than one place;
5a964f20 2599it merely influences which symbol table is used by the compiler for the
cb1a09d0
AD
2600rest of that block. You can refer to variables and filehandles in other
2601packages by prefixing the identifier with the package name and a double
2602colon: C<$Package::Variable>. If the package name is null, the C<main>
2603package as assumed. That is, C<$::sail> is equivalent to C<$main::sail>.
2604
5a964f20
TC
2605If NAMESPACE is omitted, then there is no current package, and all
2606identifiers must be fully qualified or lexicals. This is stricter
2607than C<use strict>, since it also extends to function names.
2608
cb1a09d0
AD
2609See L<perlmod/"Packages"> for more information about packages, modules,
2610and classes. See L<perlsub> for other scoping issues.
2611
a0d0e21e
LW
2612=item pipe READHANDLE,WRITEHANDLE
2613
2614Opens a pair of connected pipes like the corresponding system call.
2615Note that if you set up a loop of piped processes, deadlock can occur
2616unless you are very careful. In addition, note that Perl's pipes use
184e9718 2617stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE
a0d0e21e
LW
2618after each command, depending on the application.
2619
7e1af8bc 2620See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
4633a7c4
LW
2621for examples of such things.
2622
a0d0e21e
LW
2623=item pop ARRAY
2624
54310121 2625=item pop
28757baa 2626
a0d0e21e
LW
2627Pops and returns the last value of the array, shortening the array by
26281. Has a similar effect to
2629
2630 $tmp = $ARRAY[$#ARRAY--];
2631
2632If there are no elements in the array, returns the undefined value.
cb1a09d0 2633If ARRAY is omitted, pops the
7660c0ab
A
2634C<@ARGV> array in the main program, and the C<@_> array in subroutines, just
2635like C<shift()>.
a0d0e21e
LW
2636
2637=item pos SCALAR
2638
54310121 2639=item pos
bbce6d69 2640
4633a7c4 2641Returns the offset of where the last C<m//g> search left off for the variable
7660c0ab 2642is in question (C<$_> is used when the variable is not specified). May be
44a8e56a
PP
2643modified to change that offset. Such modification will also influence
2644the C<\G> zero-width assertion in regular expressions. See L<perlre> and
2645L<perlop>.
a0d0e21e
LW
2646
2647=item print FILEHANDLE LIST
2648
2649=item print LIST
2650
2651=item print
2652
cb1a09d0 2653Prints a string or a comma-separated list of strings. Returns TRUE
a0d0e21e 2654if successful. FILEHANDLE may be a scalar variable name, in which case
cb1a09d0 2655the variable contains the name of or a reference to the filehandle, thus introducing one
a0d0e21e
LW
2656level of indirection. (NOTE: If FILEHANDLE is a variable and the next
2657token is a term, it may be misinterpreted as an operator unless you
7660c0ab 2658interpose a C<+> or put parentheses around the arguments.) If FILEHANDLE is
a0d0e21e 2659omitted, prints by default to standard output (or to the last selected
7660c0ab 2660output channel--see L</select>). If LIST is also omitted, prints C<$_> to
5a964f20 2661the currently selected output channel. To set the default output channel to something other than
a0d0e21e 2662STDOUT use the select operation. Note that, because print takes a
5a964f20 2663LIST, anything in the LIST is evaluated in list context, and any
a0d0e21e 2664subroutine that you call will have one or more of its expressions
5a964f20 2665evaluated in list context. Also be careful not to follow the print
a0d0e21e 2666keyword with a left parenthesis unless you want the corresponding right
7660c0ab 2667parenthesis to terminate the arguments to the print--interpose a C<+> or
5f05dabc 2668put parentheses around all the arguments.
a0d0e21e 2669
4633a7c4 2670Note that if you're storing FILEHANDLES in an array or other expression,
da0045b7 2671you will have to use a block returning its value instead:
4633a7c4
LW
2672
2673 print { $files[$i] } "stuff\n";
2674 print { $OK ? STDOUT : STDERR } "stuff\n";
2675
5f05dabc 2676=item printf FILEHANDLE FORMAT, LIST
a0d0e21e 2677
5f05dabc 2678=item printf FORMAT, LIST
a0d0e21e 2679
7660c0ab 2680Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
a3cb178b 2681(the output record separator) is not appended. The first argument
ab4f32c2 2682of the list will be interpreted as the C<printf()> format. If C<use locale> is
a034a98d
DD
2683in effect, the character used for the decimal point in formatted real numbers
2684is affected by the LC_NUMERIC locale. See L<perllocale>.
a0d0e21e 2685
7660c0ab
A
2686Don't fall into the trap of using a C<printf()> when a simple
2687C<print()> would do. The C<print()> is more efficient and less
28757baa
PP
2688error prone.
2689
da0045b7
PP
2690=item prototype FUNCTION
2691
2692Returns the prototype of a function as a string (or C<undef> if the
5f05dabc
PP
2693function has no prototype). FUNCTION is a reference to, or the name of,
2694the function whose prototype you want to retrieve.
da0045b7 2695
b6c543e3
IZ
2696If FUNCTION is a string starting with C<CORE::>, the rest is taken as
2697a name for Perl builtin. If builtin is not I<overridable> (such as
ab4f32c2
A
2698C<qw//>) or its arguments cannot be expressed by a prototype (such as
2699C<system()>) - in other words, the builtin does not behave like a Perl
b6c543e3
IZ
2700function - returns C<undef>. Otherwise, the string describing the
2701equivalent prototype is returned.
2702
a0d0e21e
LW
2703=item push ARRAY,LIST
2704
2705Treats ARRAY as a stack, and pushes the values of LIST
2706onto the end of ARRAY. The length of ARRAY increases by the length of
2707LIST. Has the same effect as
2708
2709 for $value (LIST) {
2710 $ARRAY[++$#ARRAY] = $value;
2711 }
2712
2713but is more efficient. Returns the new number of elements in the array.
2714
2715=item q/STRING/
2716
2717=item qq/STRING/
2718
8782bef2
GB
2719=item qr/STRING/
2720
a0d0e21e
LW
2721=item qx/STRING/
2722
2723=item qw/STRING/
2724
2725Generalized quotes. See L<perlop>.
2726
2727=item quotemeta EXPR
2728
54310121 2729=item quotemeta
bbce6d69 2730
68dc0745 2731Returns the value of EXPR with all non-alphanumeric
a034a98d
DD
2732characters backslashed. (That is, all characters not matching
2733C</[A-Za-z_0-9]/> will be preceded by a backslash in the
2734returned string, regardless of any locale settings.)
2735This is the internal function implementing
7660c0ab 2736the C<\Q> escape in double-quoted strings.
a0d0e21e 2737
7660c0ab 2738If EXPR is omitted, uses C<$_>.
bbce6d69 2739
a0d0e21e
LW
2740=item rand EXPR
2741
2742=item rand
2743
7660c0ab 2744Returns a random fractional number greater than or equal to C<0> and less
3e3baf6d 2745than the value of EXPR. (EXPR should be positive.) If EXPR is
7660c0ab
A
2746omitted, the value C<1> is used. Automatically calls C<srand()> unless
2747C<srand()> has already been called. See also C<srand()>.
a0d0e21e 2748
2f9daede 2749(Note: If your rand function consistently returns numbers that are too
a0d0e21e 2750large or too small, then your version of Perl was probably compiled
2f9daede 2751with the wrong number of RANDBITS.)
a0d0e21e
LW
2752
2753=item read FILEHANDLE,SCALAR,LENGTH,OFFSET
2754
2755=item read FILEHANDLE,SCALAR,LENGTH
2756
2757Attempts to read LENGTH bytes of data into variable SCALAR from the
3b02c43c
GS
2758specified FILEHANDLE. Returns the number of bytes actually read,
2759C<0> at end of file, or undef if there was an error. SCALAR will be grown
2760or shrunk to the length actually read. An OFFSET may be specified to
2761place the read data at some other place than the beginning of the
f86cebdf
GS
2762string. This call is actually implemented in terms of stdio's fread(3)
2763call. To get a true read(2) system call, see C<sysread()>.
a0d0e21e
LW
2764
2765=item readdir DIRHANDLE
2766
7660c0ab 2767Returns the next directory entry for a directory opened by C<opendir()>.
5a964f20 2768If used in list context, returns all the rest of the entries in the
a0d0e21e 2769directory. If there are no more entries, returns an undefined value in
5a964f20 2770scalar context or a null list in list context.
a0d0e21e 2771
7660c0ab 2772If you're planning to filetest the return values out of a C<readdir()>, you'd
5f05dabc 2773better prepend the directory in question. Otherwise, because we didn't
7660c0ab 2774C<chdir()> there, it would have been testing the wrong file.
cb1a09d0
AD
2775
2776 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
2777 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
2778 closedir DIR;
2779
84902520
TB
2780=item readline EXPR
2781
5a964f20 2782Reads from the filehandle whose typeglob is contained in EXPR. In scalar context, a single line
84902520
TB
2783is read and returned. In list context, reads until end-of-file is
2784reached and returns a list of lines (however you've defined lines
7660c0ab 2785with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
84902520
TB
2786This is the internal function implementing the C<E<lt>EXPRE<gt>>
2787operator, but you can use it directly. The C<E<lt>EXPRE<gt>>
2788operator is discussed in more detail in L<perlop/"I/O Operators">.
2789
5a964f20
TC
2790 $line = <STDIN>;
2791 $line = readline(*STDIN); # same thing
2792
a0d0e21e
LW
2793=item readlink EXPR
2794
54310121 2795=item readlink
bbce6d69 2796
a0d0e21e
LW
2797Returns the value of a symbolic link, if symbolic links are
2798implemented. If not, gives a fatal error. If there is some system
184e9718 2799error, returns the undefined value and sets C<$!> (errno). If EXPR is
7660c0ab 2800omitted, uses C<$_>.
a0d0e21e 2801
84902520
TB
2802=item readpipe EXPR
2803
5a964f20 2804EXPR is executed as a system command.
84902520
TB
2805The collected standard output of the command is returned.
2806In scalar context, it comes back as a single (potentially
2807multi-line) string. In list context, returns a list of lines
7660c0ab 2808(however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
84902520
TB
2809This is the internal function implementing the C<qx/EXPR/>
2810operator, but you can use it directly. The C<qx/EXPR/>
2811operator is discussed in more detail in L<perlop/"I/O Operators">.
2812
a0d0e21e
LW
2813=item recv SOCKET,SCALAR,LEN,FLAGS
2814
2815Receives a message on a socket. Attempts to receive LENGTH bytes of
2816data into variable SCALAR from the specified SOCKET filehandle.
7660c0ab 2817Actually does a C C<recvfrom()>, so that it can return the address of the
a0d0e21e
LW
2818sender. Returns the undefined value if there's an error. SCALAR will
2819be grown or shrunk to the length actually read. Takes the same flags
54310121 2820as the system call of the same name.
4633a7c4 2821See L<perlipc/"UDP: Message Passing"> for examples.
a0d0e21e
LW
2822
2823=item redo LABEL
2824
2825=item redo
2826
2827The C<redo> command restarts the loop block without evaluating the
2828conditional again. The C<continue> block, if any, is not executed. If
2829the LABEL is omitted, the command refers to the innermost enclosing
2830loop. This command is normally used by programs that want to lie to
2831themselves about what was just input:
2832
2833 # a simpleminded Pascal comment stripper
2834 # (warning: assumes no { or } in strings)
4633a7c4 2835 LINE: while (<STDIN>) {
a0d0e21e
LW
2836 while (s|({.*}.*){.*}|$1 |) {}
2837 s|{.*}| |;
2838 if (s|{.*| |) {
2839 $front = $_;
2840 while (<STDIN>) {
2841 if (/}/) { # end of comment?
5a964f20 2842 s|^|$front\{|;
4633a7c4 2843 redo LINE;
a0d0e21e
LW
2844 }
2845 }
2846 }
2847 print;
2848 }
2849
4968c1e4
G
2850C<redo> cannot be used to retry a block which returns a value such as
2851C<eval {}>, C<sub {}> or C<do {}>.
2852
1d2dff63
GS
2853See also L</continue> for an illustration of how C<last>, C<next>, and
2854C<redo> work.
2855
a0d0e21e
LW
2856=item ref EXPR
2857
54310121 2858=item ref
bbce6d69 2859
2f9daede 2860Returns a TRUE value if EXPR is a reference, FALSE otherwise. If EXPR
7660c0ab 2861is not specified, C<$_> will be used. The value returned depends on the
bbce6d69 2862type of thing the reference is a reference to.
a0d0e21e
LW
2863Builtin types include:
2864
2865 REF
2866 SCALAR
2867 ARRAY
2868 HASH
2869 CODE
2870 GLOB
2871
54310121 2872If the referenced object has been blessed into a package, then that package
7660c0ab 2873name is returned instead. You can think of C<ref()> as a C<typeof()> operator.
a0d0e21e
LW
2874
2875 if (ref($r) eq "HASH") {
aa689395 2876 print "r is a reference to a hash.\n";
54310121 2877 }
5a964f20 2878 if (!ref($r)) {
a0d0e21e 2879 print "r is not a reference at all.\n";
54310121 2880 }
a0d0e21e
LW
2881
2882See also L<perlref>.
2883
2884=item rename OLDNAME,NEWNAME
2885
7660c0ab 2886Changes the name of a file. Returns C<1> for success, C<0> otherwise. Will
5f05dabc 2887not work across file system boundaries.
a0d0e21e
LW
2888
2889=item require EXPR
2890
2891=item require
2892
7660c0ab 2893Demands some semantics specified by EXPR, or by C<$_> if EXPR is not
a0d0e21e 2894supplied. If EXPR is numeric, demands that the current version of Perl
184e9718 2895(C<$]> or $PERL_VERSION) be equal or greater than EXPR.
a0d0e21e
LW
2896
2897Otherwise, demands that a library file be included if it hasn't already
2898been included. The file is included via the do-FILE mechanism, which is
7660c0ab 2899essentially just a variety of C<eval()>. Has semantics similar to the following
a0d0e21e
LW
2900subroutine:
2901
2902 sub require {
5a964f20 2903 my($filename) = @_;
a0d0e21e 2904 return 1 if $INC{$filename};
5a964f20 2905 my($realfilename,$result);
a0d0e21e
LW
2906 ITER: {
2907 foreach $prefix (@INC) {
2908 $realfilename = "$prefix/$filename";
2909 if (-f $realfilename) {
2910 $result = do $realfilename;
2911 last ITER;
2912 }
2913 }
2914 die "Can't find $filename in \@INC";
2915 }
2916 die $@ if $@;
2917 die "$filename did not return true value" unless $result;
2918 $INC{$filename} = $realfilename;
5a964f20 2919 return $result;
a0d0e21e
LW
2920 }
2921
2922Note that the file will not be included twice under the same specified
2923name. The file must return TRUE as the last statement to indicate
2924successful execution of any initialization code, so it's customary to
7660c0ab 2925end such a file with "C<1;>" unless you're sure it'll return TRUE
a0d0e21e
LW
2926otherwise. But it's better just to put the "C<1;>", in case you add more
2927statements.
2928
54310121 2929If EXPR is a bareword, the require assumes a "F<.pm>" extension and
da0045b7 2930replaces "F<::>" with "F</>" in the filename for you,
54310121 2931to make it easy to load standard modules. This form of loading of
a0d0e21e
LW
2932modules does not risk altering your namespace.
2933
ee580363
GS
2934In other words, if you try this:
2935
f86cebdf 2936 require Foo::Bar; # a splendid bareword
ee580363 2937
7660c0ab
A
2938The require function will actually look for the "F<Foo/Bar.pm>" file in the
2939directories specified in the C<@INC> array.
ee580363 2940
5a964f20 2941But if you try this:
ee580363
GS
2942
2943 $class = 'Foo::Bar';
f86cebdf 2944 require $class; # $class is not a bareword
5a964f20 2945 #or
f86cebdf 2946 require "Foo::Bar"; # not a bareword because of the ""
ee580363 2947
7660c0ab
A
2948The require function will look for the "F<Foo::Bar>" file in the @INC array and
2949will complain about not finding "F<Foo::Bar>" there. In this case you can do:
ee580363
GS
2950
2951 eval "require $class";
2952
2953For a yet-more-powerful import facility, see L</use> and L<perlmod>.
a0d0e21e
LW
2954
2955=item reset EXPR
2956
2957=item reset
2958
2959Generally used in a C<continue> block at the end of a loop to clear
7660c0ab 2960variables and reset C<??> searches so that they work again. The
a0d0e21e
LW
2961expression is interpreted as a list of single characters (hyphens
2962allowed for ranges). All variables and arrays beginning with one of
2963those letters are reset to their pristine state. If the expression is
7660c0ab 2964omitted, one-match searches (C<?pattern?>) are reset to match again. Resets
5f05dabc 2965only variables or searches in the current package. Always returns
a0d0e21e
LW
29661. Examples:
2967
2968 reset 'X'; # reset all X variables
2969 reset 'a-z'; # reset lower case variables
2970 reset; # just reset ?? searches
2971
7660c0ab
A
2972Resetting C<"A-Z"> is not recommended because you'll wipe out your
2973C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package variables--lexical variables
a0d0e21e 2974are unaffected, but they clean themselves up on scope exit anyway,
da0045b7 2975so you'll probably want to use them instead. See L</my>.
a0d0e21e 2976
54310121
PP
2977=item return EXPR
2978
2979=item return
2980
7660c0ab 2981Returns from a subroutine, C<eval()>, or C<do FILE> with the value
5a964f20 2982given in EXPR. Evaluation of EXPR may be in list, scalar, or void
54310121 2983context, depending on how the return value will be used, and the context
7660c0ab 2984may vary from one execution to the next (see C<wantarray()>). If no EXPR
5a964f20
TC
2985is given, returns an empty list in list context, an undefined value in
2986scalar context, or nothing in a void context.
a0d0e21e 2987
68dc0745
PP
2988(Note that in the absence of a return, a subroutine, eval, or do FILE
2989will automatically return the value of the last expression evaluated.)
a0d0e21e
LW
2990
2991=item reverse LIST
2992
5a964f20
TC
2993In list context, returns a list value consisting of the elements
2994of LIST in the opposite order. In scalar context, concatenates the
a0ed51b3
LW
2995elements of LIST, and returns a string value with all the characters
2996in the opposite order.
4633a7c4 2997
2f9daede 2998 print reverse <>; # line tac, last line first
4633a7c4 2999
2f9daede 3000 undef $/; # for efficiency of <>
a0ed51b3 3001 print scalar reverse <>; # character tac, last line tsrif
2f9daede
TPG
3002
3003This operator is also handy for inverting a hash, although there are some
3004caveats. If a value is duplicated in the original hash, only one of those
3005can be represented as a key in the inverted hash. Also, this has to
3006unwind one hash and build a whole new one, which may take some time
3007on a large hash.
3008
3009 %by_name = reverse %by_address; # Invert the hash
a0d0e21e
LW
3010
3011=item rewinddir DIRHANDLE
3012
3013Sets the current position to the beginning of the directory for the
7660c0ab 3014C<readdir()> routine on DIRHANDLE.
a0d0e21e
LW
3015
3016=item rindex STR,SUBSTR,POSITION
3017
3018=item rindex STR,SUBSTR
3019
3020Works just like index except that it returns the position of the LAST
3021occurrence of SUBSTR in STR. If POSITION is specified, returns the
3022last occurrence at or before that position.
3023
3024=item rmdir FILENAME
3025
54310121 3026=item rmdir
bbce6d69 3027
5a964f20
TC
3028Deletes the directory specified by FILENAME if that directory is empty. If it
3029succeeds it returns TRUE, otherwise it returns FALSE and sets C<$!> (errno). If
7660c0ab 3030FILENAME is omitted, uses C<$_>.
a0d0e21e
LW
3031
3032=item s///
3033
3034The substitution operator. See L<perlop>.
3035
3036=item scalar EXPR
3037
5a964f20 3038Forces EXPR to be interpreted in scalar context and returns the value
54310121 3039of EXPR.
cb1a09d0
AD
3040
3041 @counts = ( scalar @a, scalar @b, scalar @c );
3042
54310121 3043There is no equivalent operator to force an expression to
5a964f20 3044be interpolated in list context because it's in practice never
cb1a09d0
AD
3045needed. If you really wanted to do so, however, you could use
3046the construction C<@{[ (some expression) ]}>, but usually a simple
3047C<(some expression)> suffices.
a0d0e21e
LW
3048
3049=item seek FILEHANDLE,POSITION,WHENCE
3050
ab4f32c2 3051Sets FILEHANDLE's position, just like the C<fseek()> call of C<stdio()>.
8903cb82 3052FILEHANDLE may be an expression whose value gives the name of the
7660c0ab
A
3053filehandle. The values for WHENCE are C<0> to set the new position to
3054POSITION, C<1> to set it to the current position plus POSITION, and C<2> to
8903cb82 3055set it to EOF plus POSITION (typically negative). For WHENCE you may
7660c0ab
A
3056use the constants C<SEEK_SET>, C<SEEK_CUR>, and C<SEEK_END> from either the
3057C<IO::Seekable> or the POSIX module. Returns C<1> upon success, C<0> otherwise.
8903cb82 3058
7660c0ab
A
3059If you want to position file for C<sysread()> or C<syswrite()>, don't use
3060C<seek()> -- buffering makes its effect on the file's system position
3061unpredictable and non-portable. Use C<sysseek()> instead.
a0d0e21e 3062
cb1a09d0
AD
3063On some systems you have to do a seek whenever you switch between reading
3064and writing. Amongst other things, this may have the effect of calling
f86cebdf 3065stdio's clearerr(3). A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving
8903cb82 3066the file position:
cb1a09d0
AD
3067
3068 seek(TEST,0,1);
3069
3070This is also useful for applications emulating C<tail -f>. Once you hit
3071EOF on your read, and then sleep for a while, you might have to stick in a
7660c0ab 3072seek() to reset things. The C<seek()> doesn't change the current position,
8903cb82
PP
3073but it I<does> clear the end-of-file condition on the handle, so that the
3074next C<E<lt>FILEE<gt>> makes Perl try again to read something. We hope.
cb1a09d0
AD
3075
3076If that doesn't work (some stdios are particularly cantankerous), then
3077you may need something more like this:
3078
3079 for (;;) {
f86cebdf
GS
3080 for ($curpos = tell(FILE); $_ = <FILE>;
3081 $curpos = tell(FILE)) {
cb1a09d0
AD
3082 # search for some stuff and put it into files
3083 }
3084 sleep($for_a_while);
3085 seek(FILE, $curpos, 0);
3086 }
3087
a0d0e21e
LW
3088=item seekdir DIRHANDLE,POS
3089
7660c0ab
A
3090Sets the current position for the C<readdir()> routine on DIRHANDLE. POS
3091must be a value returned by C<telldir()>. Has the same caveats about
a0d0e21e
LW
3092possible directory compaction as the corresponding system library
3093routine.
3094
3095=item select FILEHANDLE
3096
3097=item select
3098
3099Returns the currently selected filehandle. Sets the current default
3100filehandle for output, if FILEHANDLE is supplied. This has two
ab4f32c2 3101effects: first, a C<write()> or a C<print()> without a filehandle will
a0d0e21e
LW
3102default to this FILEHANDLE. Second, references to variables related to
3103output will refer to this output channel. For example, if you have to
3104set the top of form format for more than one output channel, you might
3105do the following:
3106
3107 select(REPORT1);
3108 $^ = 'report1_top';
3109 select(REPORT2);
3110 $^ = 'report2_top';
3111
3112FILEHANDLE may be an expression whose value gives the name of the
3113actual filehandle. Thus:
3114
3115 $oldfh = select(STDERR); $| = 1; select($oldfh);
3116
4633a7c4
LW
3117Some programmers may prefer to think of filehandles as objects with
3118methods, preferring to write the last example as:
a0d0e21e 3119
28757baa 3120 use IO::Handle;
a0d0e21e
LW
3121 STDERR->autoflush(1);
3122
3123=item select RBITS,WBITS,EBITS,TIMEOUT
3124
f86cebdf 3125This calls the select(2) system call with the bit masks specified, which
7660c0ab 3126can be constructed using C<fileno()> and C<vec()>, along these lines:
a0d0e21e
LW
3127
3128 $rin = $win = $ein = '';
3129 vec($rin,fileno(STDIN),1) = 1;
3130 vec($win,fileno(STDOUT),1) = 1;
3131 $ein = $rin | $win;
3132
3133If you want to select on many filehandles you might wish to write a
3134subroutine:
3135
3136 sub fhbits {
5a964f20
TC
3137 my(@fhlist) = split(' ',$_[0]);
3138 my($bits);
a0d0e21e
LW
3139 for (@fhlist) {
3140 vec($bits,fileno($_),1) = 1;
3141 }
3142 $bits;
3143 }
4633a7c4 3144 $rin = fhbits('STDIN TTY SOCK');
a0d0e21e
LW
3145
3146The usual idiom is:
3147
3148 ($nfound,$timeleft) =
3149 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
3150
54310121 3151or to block until something becomes ready just do this
a0d0e21e
LW
3152
3153 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
3154
7660c0ab
A
3155Most systems do not bother to return anything useful in C<$timeleft>, so
3156calling select() in scalar context just returns C<$nfound>.
c07a80fd 3157
5f05dabc 3158Any of the bit masks can also be undef. The timeout, if specified, is
a0d0e21e 3159in seconds, which may be fractional. Note: not all implementations are
7660c0ab
A
3160capable of returning theC<$timeleft>. If not, they always return
3161C<$timeleft> equal to the supplied C<$timeout>.
a0d0e21e 3162
ff68c719 3163You can effect a sleep of 250 milliseconds this way:
a0d0e21e
LW
3164
3165 select(undef, undef, undef, 0.25);
3166
7660c0ab
A
3167B<WARNING>: One should not attempt to mix buffered I/O (like C<read()>
3168or E<lt>FHE<gt>) with C<select()>, except as permitted by POSIX, and even
3169then only on POSIX systems. You have to use C<sysread()> instead.
a0d0e21e
LW
3170
3171=item semctl ID,SEMNUM,CMD,ARG
3172
ab4f32c2 3173Calls the System V IPC function C<semctl()>. You'll probably have to say
0ade1984
JH
3174
3175 use IPC::SysV;
3176
3177first to get the correct constant definitions. If CMD is IPC_STAT or
3178GETALL, then ARG must be a variable which will hold the returned
ab4f32c2 3179semid_ds structure or semaphore value array. Returns like C<ioctl()>: the
7660c0ab
A
3180undefined value for error, "C<0> but true" for zero, or the actual return
3181value otherwise. See also C<IPC::SysV> and C<IPC::Semaphore> documentation.
a0d0e21e
LW
3182
3183=item semget KEY,NSEMS,FLAGS
3184
3185Calls the System V IPC function semget. Returns the semaphore id, or
7660c0ab
A
3186the undefined value if there is an error. See also C<IPC::SysV> and
3187C<IPC::SysV::Semaphore> documentation.
a0d0e21e
LW
3188
3189=item semop KEY,OPSTRING
3190
3191Calls the System V IPC function semop to perform semaphore operations
3192such as signaling and waiting. OPSTRING must be a packed array of
3193semop structures. Each semop structure can be generated with
3194C<pack("sss", $semnum, $semop, $semflag)>. The number of semaphore
3195operations is implied by the length of OPSTRING. Returns TRUE if
3196successful, or FALSE if there is an error. As an example, the
7660c0ab 3197following code waits on semaphore C<$semnum> of semaphore id C<$semid>:
a0d0e21e
LW
3198
3199 $semop = pack("sss", $semnum, -1, 0);
3200 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
3201
7660c0ab
A
3202To signal the semaphore, replace C<-1> with C<1>. See also C<IPC::SysV>
3203and C<IPC::SysV::Semaphore> documentation.
a0d0e21e
LW
3204
3205=item send SOCKET,MSG,FLAGS,TO
3206
3207=item send SOCKET,MSG,FLAGS
3208
3209Sends a message on a socket. Takes the same flags as the system call
3210of the same name. On unconnected sockets you must specify a
7660c0ab 3211destination to send TO, in which case it does a C C<sendto()>. Returns
a0d0e21e
LW
3212the number of characters sent, or the undefined value if there is an
3213error.
4633a7c4 3214See L<perlipc/"UDP: Message Passing"> for examples.
a0d0e21e
LW
3215
3216=item setpgrp PID,PGRP
3217
7660c0ab 3218Sets the current process group for the specified PID, C<0> for the current
a0d0e21e 3219process. Will produce a fatal error if used on a machine that doesn't
f86cebdf 3220implement setpgrp(2). If the arguments are omitted, it defaults to
7660c0ab
A
3221C<0,0>. Note that the POSIX version of C<setpgrp()> does not accept any
3222arguments, so only setpgrp C<0,0> is portable.
a0d0e21e
LW
3223
3224=item setpriority WHICH,WHO,PRIORITY
3225
3226Sets the current priority for a process, a process group, or a user.
f86cebdf
GS
3227(See setpriority(2).) Will produce a fatal error if used on a machine
3228that doesn't implement setpriority(2).
a0d0e21e
LW
3229
3230=item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
3231
3232Sets the socket option requested. Returns undefined if there is an
7660c0ab 3233error. OPTVAL may be specified as C<undef> if you don't want to pass an
a0d0e21e
LW
3234argument.
3235
3236=item shift ARRAY
3237
3238=item shift
3239
3240Shifts the first value of the array off and returns it, shortening the
3241array by 1 and moving everything down. If there are no elements in the
3242array, returns the undefined value. If ARRAY is omitted, shifts the
7660c0ab
A
3243C<@_> array within the lexical scope of subroutines and formats, and the
3244C<@ARGV> array at file scopes or within the lexical scopes established by
977336f5 3245the C<eval ''>, C<BEGIN {}>, C<END {}>, and C<INIT {}> constructs.
7660c0ab
A
3246See also C<unshift()>, C<push()>, and C<pop()>. C<Shift()> and C<unshift()> do the
3247same thing to the left end of an array that C<pop()> and C<push()> do to the
977336f5 3248right end.
a0d0e21e
LW
3249
3250=item shmctl ID,CMD,ARG
3251
0ade1984
JH
3252Calls the System V IPC function shmctl. You'll probably have to say
3253
3254 use IPC::SysV;
3255
7660c0ab
A
3256first to get the correct constant definitions. If CMD is C<IPC_STAT>,
3257then ARG must be a variable which will hold the returned C<shmid_ds>
3258structure. Returns like ioctl: the undefined value for error, "C<0> but
0ade1984 3259true" for zero, or the actual return value otherwise.
7660c0ab 3260See also C<IPC::SysV> documentation.
a0d0e21e
LW
3261
3262=item shmget KEY,SIZE,FLAGS
3263
3264Calls the System V IPC function shmget. Returns the shared memory
3265segment id, or the undefined value if there is an error.
7660c0ab 3266See also C<IPC::SysV> documentation.
a0d0e21e
LW
3267
3268=item shmread ID,VAR,POS,SIZE
3269
3270=item shmwrite ID,STRING,POS,SIZE
3271
3272Reads or writes the System V shared memory segment ID starting at
3273position POS for size SIZE by attaching to it, copying in/out, and
5a964f20 3274detaching from it. When reading, VAR must be a variable that will
a0d0e21e
LW
3275hold the data read. When writing, if STRING is too long, only SIZE
3276bytes are used; if STRING is too short, nulls are written to fill out
3277SIZE bytes. Return TRUE if successful, or FALSE if there is an error.
7660c0ab 3278See also C<IPC::SysV> documentation.
a0d0e21e
LW
3279
3280=item shutdown SOCKET,HOW
3281
3282Shuts down a socket connection in the manner indicated by HOW, which
3283has the same interpretation as in the system call of the same name.
3284
f86cebdf
GS
3285 shutdown(SOCKET, 0); # I/we have stopped reading data
3286 shutdown(SOCKET, 1); # I/we have stopped writing data
3287 shutdown(SOCKET, 2); # I/we have stopped using this socket
5a964f20
TC
3288
3289This is useful with sockets when you want to tell the other
3290side you're done writing but not done reading, or vice versa.
3291It's also a more insistent form of close because it also
3292disables the filedescriptor in any forked copies in other
3293processes.
3294
a0d0e21e
LW
3295=item sin EXPR
3296
54310121