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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
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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
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138C<caller>, C<continue>, C<die>, C<do>, C<dump>, C<else>, C<elsif>,
139C<eval>, C<exit>, C<for>, C<foreach>, C<goto>, C<if>, C<last>,
140C<next>, C<redo>, C<return>, C<sub>, C<unless>, C<wantarray>,
141C<while>, C<until>
cb1a09d0 142
54310121 143=item Keywords related to scoping
cb1a09d0 144
22fae026 145C<caller>, C<import>, C<local>, C<my>, C<package>, C<use>
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146
147=item Miscellaneous functions
148
22fae026
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149C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<reset>,
150C<scalar>, C<undef>, C<wantarray>
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151
152=item Functions for processes and process groups
153
22fae026 154C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
ab4f32c2 155C<pipe>, C<qx/STRING/>, C<setpgrp>, C<setpriority>, C<sleep>, C<system>,
22fae026 156C<times>, C<wait>, C<waitpid>
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157
158=item Keywords related to perl modules
159
22fae026 160C<do>, C<import>, C<no>, C<package>, C<require>, C<use>
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161
162=item Keywords related to classes and object-orientedness
163
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164C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
165C<untie>, C<use>
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166
167=item Low-level socket functions
168
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169C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
170C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
171C<socket>, C<socketpair>
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172
173=item System V interprocess communication functions
174
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175C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
176C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>
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177
178=item Fetching user and group info
179
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180C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
181C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
182C<getpwuid>, C<setgrent>, C<setpwent>
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183
184=item Fetching network info
185
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186C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
187C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
188C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
189C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
190C<setnetent>, C<setprotoent>, C<setservent>
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191
192=item Time-related functions
193
22fae026 194C<gmtime>, C<localtime>, C<time>, C<times>
cb1a09d0 195
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196=item Functions new in perl5
197
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198C<abs>, C<bless>, C<chomp>, C<chr>, C<exists>, C<formline>, C<glob>,
199C<import>, C<lc>, C<lcfirst>, C<map>, C<my>, C<no>, C<prototype>, C<qx>,
200C<qw>, C<readline>, C<readpipe>, C<ref>, C<sub*>, C<sysopen>, C<tie>,
201C<tied>, C<uc>, C<ucfirst>, C<untie>, C<use>
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202
203* - C<sub> was a keyword in perl4, but in perl5 it is an
5a964f20 204operator, which can be used in expressions.
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205
206=item Functions obsoleted in perl5
207
22fae026 208C<dbmclose>, C<dbmopen>
37798a01 209
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210=back
211
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212=head2 Portability
213
214Perl was born in UNIX and therefore it can access all the common UNIX
215system calls. In non-UNIX environments the functionality of many
216UNIX system calls may not be available or the details of the available
217functionality may be slightly different. The Perl functions affected
218by this are:
219
220C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>,
221C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>,
222C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>,
223C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostent>,
224C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
225C<getppid>, C<getprgp>, C<getpriority>, C<getprotobynumber>,
226C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>,
227C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>,
228C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>,
229C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<select>, C<semctl>,
230C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>,
231C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>,
232C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>,
233C<shmwrite>, C<socketpair>, C<stat>, C<symlink>, C<syscall>,
234C<sysopen>, C<system>, C<times>, C<truncate>, C<umask>, C<utime>,
235C<wait>, C<waitpid>
236
237For more information about the portability of these functions, see
238L<perlport> and other available platform-specific documentation.
239
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240=head2 Alphabetical Listing of Perl Functions
241
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242=over 8
243
22fae026 244=item I<-X> FILEHANDLE
a0d0e21e 245
22fae026 246=item I<-X> EXPR
a0d0e21e 247
22fae026 248=item I<-X>
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249
250A file test, where X is one of the letters listed below. This unary
251operator takes one argument, either a filename or a filehandle, and
252tests the associated file to see if something is true about it. If the
7660c0ab 253argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
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254Unless otherwise documented, it returns C<1> for TRUE and C<''> for FALSE, or
255the undefined value if the file doesn't exist. Despite the funny
256names, precedence is the same as any other named unary operator, and
257the argument may be parenthesized like any other unary operator. The
258operator may be any of:
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259X<-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>
260X<-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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261
262 -r File is readable by effective uid/gid.
263 -w File is writable by effective uid/gid.
264 -x File is executable by effective uid/gid.
265 -o File is owned by effective uid.
266
267 -R File is readable by real uid/gid.
268 -W File is writable by real uid/gid.
269 -X File is executable by real uid/gid.
270 -O File is owned by real uid.
271
272 -e File exists.
273 -z File has zero size.
54310121 274 -s File has nonzero size (returns size).
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275
276 -f File is a plain file.
277 -d File is a directory.
278 -l File is a symbolic link.
9c4d0f16 279 -p File is a named pipe (FIFO), or Filehandle is a pipe.
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280 -S File is a socket.
281 -b File is a block special file.
282 -c File is a character special file.
283 -t Filehandle is opened to a tty.
284
285 -u File has setuid bit set.
286 -g File has setgid bit set.
287 -k File has sticky bit set.
288
289 -T File is a text file.
290 -B File is a binary file (opposite of -T).
291
292 -M Age of file in days when script started.
293 -A Same for access time.
294 -C Same for inode change time.
295
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296Example:
297
298 while (<>) {
299 chop;
300 next unless -f $_; # ignore specials
5a964f20 301 #...
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302 }
303
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304The interpretation of the file permission operators C<-r>, C<-R>,
305C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
306of the file and the uids and gids of the user. There may be other
307reasons you can't actually read, write, or execute the file. Such
308reasons may be for example network filesystem access controls, ACLs
309(access control lists), read-only filesystems, and unrecognized
310executable formats.
311
312Also note that, for the superuser on the local filesystems, C<-r>,
313C<-R>, C<-w>, and C<-W> always return 1, and C<-x> and C<-X> return 1
314if any execute bit is set in the mode. Scripts run by the superuser
315may thus need to do a stat() to determine the actual mode of the file,
316or temporarily set the uid to something else.
317
318If you are using ACLs, there is a pragma called C<filetest> that may
319produce more accurate results than the bare stat() mode bits.
320
321When under the C<use filetest 'access'> the above-mentioned filetests
322will test whether the permission can (not) be granted using the
468541a8 323access() family of system calls. Also note that the C<-x> and C<-X> may
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324under this pragma return true even if there are no execute permission
325bits set (nor any extra execute permission ACLs). This strangeness is
326due to the underlying system calls' definitions. Read the
327documentation for the C<filetest> pragma for more information.
328
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329Note that C<-s/a/b/> does not do a negated substitution. Saying
330C<-exp($foo)> still works as expected, however--only single letters
331following a minus are interpreted as file tests.
332
333The C<-T> and C<-B> switches work as follows. The first block or so of the
334file is examined for odd characters such as strange control codes or
5a964f20 335characters with the high bit set. If too many strange characters (E<gt>30%)
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336are found, it's a C<-B> file, otherwise it's a C<-T> file. Also, any file
337containing null in the first block is considered a binary file. If C<-T>
338or C<-B> is used on a filehandle, the current stdio buffer is examined
339rather than the first block. Both C<-T> and C<-B> return TRUE on a null
54310121 340file, or a file at EOF when testing a filehandle. Because you have to
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341read a file to do the C<-T> test, on most occasions you want to use a C<-f>
342against the file first, as in C<next unless -f $file && -T $file>.
a0d0e21e 343
7660c0ab 344If any of the file tests (or either the C<stat()> or C<lstat()> operators) are given
28757baa 345the special filehandle consisting of a solitary underline, then the stat
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346structure of the previous file test (or stat operator) is used, saving
347a system call. (This doesn't work with C<-t>, and you need to remember
348that lstat() and C<-l> will leave values in the stat structure for the
349symbolic link, not the real file.) Example:
350
351 print "Can do.\n" if -r $a || -w _ || -x _;
352
353 stat($filename);
354 print "Readable\n" if -r _;
355 print "Writable\n" if -w _;
356 print "Executable\n" if -x _;
357 print "Setuid\n" if -u _;
358 print "Setgid\n" if -g _;
359 print "Sticky\n" if -k _;
360 print "Text\n" if -T _;
361 print "Binary\n" if -B _;
362
363=item abs VALUE
364
54310121 365=item abs
bbce6d69 366
a0d0e21e 367Returns the absolute value of its argument.
7660c0ab 368If VALUE is omitted, uses C<$_>.
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369
370=item accept NEWSOCKET,GENERICSOCKET
371
f86cebdf 372Accepts an incoming socket connect, just as the accept(2) system call
a0d0e21e 373does. Returns the packed address if it succeeded, FALSE otherwise.
4633a7c4 374See example in L<perlipc/"Sockets: Client/Server Communication">.
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375
376=item alarm SECONDS
377
54310121 378=item alarm
bbce6d69 379
a0d0e21e 380Arranges to have a SIGALRM delivered to this process after the
bbce6d69 381specified number of seconds have elapsed. If SECONDS is not specified,
7660c0ab 382the value stored in C<$_> is used. (On some machines,
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383unfortunately, the elapsed time may be up to one second less than you
384specified because of how seconds are counted.) Only one timer may be
385counting at once. Each call disables the previous timer, and an
7660c0ab 386argument of C<0> may be supplied to cancel the previous timer without
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387starting a new one. The returned value is the amount of time remaining
388on the previous timer.
389
4633a7c4 390For delays of finer granularity than one second, you may use Perl's
f86cebdf 391C<syscall()> interface to access setitimer(2) if your system supports it,
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392or else see L</select()>. It is usually a mistake to intermix C<alarm()>
393and C<sleep()> calls.
a0d0e21e 394
7660c0ab 395If you want to use C<alarm()> to time out a system call you need to use an
ab4f32c2 396C<eval()>/C<die()> pair. You can't rely on the alarm causing the system call to
f86cebdf 397fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
ab4f32c2 398restart system calls on some systems. Using C<eval()>/C<die()> always works,
5a964f20 399modulo the caveats given in L<perlipc/"Signals">.
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400
401 eval {
f86cebdf 402 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
36477c24 403 alarm $timeout;
ff68c719 404 $nread = sysread SOCKET, $buffer, $size;
36477c24 405 alarm 0;
ff68c719 406 };
ff68c719 407 if ($@) {
f86cebdf 408 die unless $@ eq "alarm\n"; # propagate unexpected errors
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409 # timed out
410 }
411 else {
412 # didn't
413 }
414
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415=item atan2 Y,X
416
417Returns the arctangent of Y/X in the range -PI to PI.
418
7660c0ab 419For the tangent operation, you may use the C<POSIX::tan()>
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420function, or use the familiar relation:
421
422 sub tan { sin($_[0]) / cos($_[0]) }
423
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424=item bind SOCKET,NAME
425
426Binds a network address to a socket, just as the bind system call
427does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
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428packed address of the appropriate type for the socket. See the examples in
429L<perlipc/"Sockets: Client/Server Communication">.
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430
431=item binmode FILEHANDLE
432
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433Arranges for the file to be read or written in "binary" mode in operating
434systems that distinguish between binary and text files. Files that are
435not in binary mode have CR LF sequences translated to LF on input and LF
54310121 436translated to CR LF on output. Binmode has no effect under Unix; in MS-DOS
cb1a09d0 437and similarly archaic systems, it may be imperative--otherwise your
54310121 438MS-DOS-damaged C library may mangle your file. The key distinction between
ab4f32c2 439systems that need C<binmode()> and those that don't is their text file
5a964f20 440formats. Systems like Unix, MacOS, and Plan9 that delimit lines with a single
7660c0ab 441character, and that encode that character in C as C<"\n">, do not need
ab4f32c2 442C<binmode()>. The rest need it. If FILEHANDLE is an expression, the value
cb1a09d0 443is taken as the name of the filehandle.
a0d0e21e 444
4633a7c4 445=item bless REF,CLASSNAME
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446
447=item bless REF
448
28757baa 449This function tells the thingy referenced by REF that it is now
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450an object in the CLASSNAME package--or the current package if no CLASSNAME
451is specified, which is often the case. It returns the reference for
7660c0ab 452convenience, because a C<bless()> is often the last thing in a constructor.
4633a7c4 453Always use the two-argument version if the function doing the blessing
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454might be inherited by a derived class. See L<perltoot> and L<perlobj>
455for more about the blessing (and blessings) of objects.
a0d0e21e 456
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457Consider always blessing objects in CLASSNAMEs that are mixed case.
458Namespaces with all lowercase names are considered reserved for Perl
459pragmata. Builtin types have all uppercase names, so to prevent confusion,
460it is best to avoid such package names as well.
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461
462See L<perlmod/"Perl Modules">.
463
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464=item caller EXPR
465
466=item caller
467
5a964f20 468Returns the context of the current subroutine call. In scalar context,
28757baa 469returns the caller's package name if there is a caller, that is, if
7660c0ab 470we're in a subroutine or C<eval()> or C<require()>, and the undefined value
5a964f20 471otherwise. In list context, returns
a0d0e21e 472
748a9306 473 ($package, $filename, $line) = caller;
a0d0e21e
LW
474
475With EXPR, it returns some extra information that the debugger uses to
476print a stack trace. The value of EXPR indicates how many call frames
477to go back before the current one.
478
54310121 479 ($package, $filename, $line, $subroutine,
e7ea3e70
IZ
480 $hasargs, $wantarray, $evaltext, $is_require) = caller($i);
481
7660c0ab 482Here C<$subroutine> may be C<"(eval)"> if the frame is not a subroutine
ab4f32c2 483call, but an C<eval()>. In such a case additional elements C<$evaltext> and
7660c0ab
A
484C<$is_require> are set: C<$is_require> is true if the frame is created by a
485C<require> or C<use> statement, C<$evaltext> contains the text of the
dc848c6f 486C<eval EXPR> statement. In particular, for a C<eval BLOCK> statement,
7660c0ab 487C<$filename> is C<"(eval)">, but C<$evaltext> is undefined. (Note also that
dc848c6f
PP
488each C<use> statement creates a C<require> frame inside an C<eval EXPR>)
489frame.
748a9306
LW
490
491Furthermore, when called from within the DB package, caller returns more
7660c0ab 492detailed information: it sets the list variable C<@DB::args> to be the
54310121 493arguments with which the subroutine was invoked.
748a9306 494
7660c0ab 495Be aware that the optimizer might have optimized call frames away before
ab4f32c2 496C<caller()> had a chance to get the information. That means that C<caller(N)>
7660c0ab
A
497might not return information about the call frame you expect it do, for
498C<N E<gt> 1>. In particular, C<@DB::args> might have information from the
499previous time C<caller()> was called.
500
a0d0e21e
LW
501=item chdir EXPR
502
503Changes the working directory to EXPR, if possible. If EXPR is
504omitted, changes to home directory. Returns TRUE upon success, FALSE
7660c0ab 505otherwise. See example under C<die()>.
a0d0e21e
LW
506
507=item chmod LIST
508
509Changes the permissions of a list of files. The first element of the
4633a7c4 510list must be the numerical mode, which should probably be an octal
2f9daede
TPG
511number, and which definitely should I<not> a string of octal digits:
512C<0644> is okay, C<'0644'> is not. Returns the number of files
dc848c6f 513successfully changed. See also L</oct>, if all you have is a string.
a0d0e21e
LW
514
515 $cnt = chmod 0755, 'foo', 'bar';
516 chmod 0755, @executables;
f86cebdf
GS
517 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
518 # --w----r-T
2f9daede
TPG
519 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
520 $mode = 0644; chmod $mode, 'foo'; # this is best
a0d0e21e
LW
521
522=item chomp VARIABLE
523
524=item chomp LIST
525
526=item chomp
527
3e3baf6d 528This is a slightly safer version of L</chop>. It removes any
a0d0e21e 529line ending that corresponds to the current value of C<$/> (also known as
28757baa
PP
530$INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
531number of characters removed from all its arguments. It's often used to
532remove the newline from the end of an input record when you're worried
533that the final record may be missing its newline. When in paragraph mode
534(C<$/ = "">), it removes all trailing newlines from the string. If
7660c0ab 535VARIABLE is omitted, it chomps C<$_>. Example:
a0d0e21e
LW
536
537 while (<>) {
538 chomp; # avoid \n on last field
539 @array = split(/:/);
5a964f20 540 # ...
a0d0e21e
LW
541 }
542
543You can actually chomp anything that's an lvalue, including an assignment:
544
545 chomp($cwd = `pwd`);
546 chomp($answer = <STDIN>);
547
548If you chomp a list, each element is chomped, and the total number of
549characters removed is returned.
550
551=item chop VARIABLE
552
553=item chop LIST
554
555=item chop
556
557Chops off the last character of a string and returns the character
558chopped. It's used primarily to remove the newline from the end of an
559input record, but is much more efficient than C<s/\n//> because it neither
7660c0ab 560scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
a0d0e21e
LW
561Example:
562
563 while (<>) {
564 chop; # avoid \n on last field
565 @array = split(/:/);
5a964f20 566 #...
a0d0e21e
LW
567 }
568
569You can actually chop anything that's an lvalue, including an assignment:
570
571 chop($cwd = `pwd`);
572 chop($answer = <STDIN>);
573
574If you chop a list, each element is chopped. Only the value of the
ab4f32c2 575last C<chop()> is returned.
a0d0e21e 576
ab4f32c2 577Note that C<chop()> returns the last character. To return all but the last
748a9306
LW
578character, use C<substr($string, 0, -1)>.
579
a0d0e21e
LW
580=item chown LIST
581
582Changes the owner (and group) of a list of files. The first two
583elements of the list must be the I<NUMERICAL> uid and gid, in that order.
584Returns the number of files successfully changed.
585
586 $cnt = chown $uid, $gid, 'foo', 'bar';
587 chown $uid, $gid, @filenames;
588
54310121 589Here's an example that looks up nonnumeric uids in the passwd file:
a0d0e21e
LW
590
591 print "User: ";
592 chop($user = <STDIN>);
5a964f20 593 print "Files: ";
a0d0e21e
LW
594 chop($pattern = <STDIN>);
595
596 ($login,$pass,$uid,$gid) = getpwnam($user)
597 or die "$user not in passwd file";
598
5a964f20 599 @ary = glob($pattern); # expand filenames
a0d0e21e
LW
600 chown $uid, $gid, @ary;
601
54310121 602On most systems, you are not allowed to change the ownership of the
4633a7c4
LW
603file unless you're the superuser, although you should be able to change
604the group to any of your secondary groups. On insecure systems, these
605restrictions may be relaxed, but this is not a portable assumption.
606
a0d0e21e
LW
607=item chr NUMBER
608
54310121 609=item chr
bbce6d69 610
a0d0e21e 611Returns the character represented by that NUMBER in the character set.
a0ed51b3
LW
612For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
613chr(0x263a) is a Unicode smiley face (but only within the scope of a
614C<use utf8>). For the reverse, use L</ord>.
a0d0e21e 615
7660c0ab 616If NUMBER is omitted, uses C<$_>.
bbce6d69 617
a0d0e21e
LW
618=item chroot FILENAME
619
54310121 620=item chroot
bbce6d69 621
5a964f20 622This function works like the system call by the same name: it makes the
4633a7c4 623named directory the new root directory for all further pathnames that
7660c0ab 624begin with a C<"/"> by your process and all its children. (It doesn't
28757baa 625change your current working directory, which is unaffected.) For security
4633a7c4 626reasons, this call is restricted to the superuser. If FILENAME is
ab4f32c2 627omitted, does a C<chroot()> to C<$_>.
a0d0e21e
LW
628
629=item close FILEHANDLE
630
6a518fbc
TP
631=item close
632
a0d0e21e
LW
633Closes the file or pipe associated with the file handle, returning TRUE
634only if stdio successfully flushes buffers and closes the system file
6a518fbc
TP
635descriptor. Closes the currently selected filehandle if the argument
636is omitted.
fb73857a
PP
637
638You don't have to close FILEHANDLE if you are immediately going to do
7660c0ab 639another C<open()> on it, because C<open()> will close it for you. (See
ab4f32c2 640C<open()>.) However, an explicit C<close()> on an input file resets the line
7660c0ab 641counter (C<$.>), while the implicit close done by C<open()> does not.
fb73857a 642
ab4f32c2 643If the file handle came from a piped open C<close()> will additionally
fb73857a
PP
644return FALSE if one of the other system calls involved fails or if the
645program exits with non-zero status. (If the only problem was that the
7660c0ab 646program exited non-zero C<$!> will be set to C<0>.) Also, closing a pipe
5a964f20 647waits for the process executing on the pipe to complete, in case you
fb73857a
PP
648want to look at the output of the pipe afterwards. Closing a pipe
649explicitly also puts the exit status value of the command into C<$?>.
5a964f20 650
fb73857a 651Example:
a0d0e21e 652
fb73857a
PP
653 open(OUTPUT, '|sort >foo') # pipe to sort
654 or die "Can't start sort: $!";
5a964f20 655 #... # print stuff to output
fb73857a
PP
656 close OUTPUT # wait for sort to finish
657 or warn $! ? "Error closing sort pipe: $!"
658 : "Exit status $? from sort";
659 open(INPUT, 'foo') # get sort's results
660 or die "Can't open 'foo' for input: $!";
a0d0e21e 661
5a964f20
TC
662FILEHANDLE may be an expression whose value can be used as an indirect
663filehandle, usually the real filehandle name.
a0d0e21e
LW
664
665=item closedir DIRHANDLE
666
7660c0ab 667Closes a directory opened by C<opendir()> and returns the success of that
5a964f20
TC
668system call.
669
670DIRHANDLE may be an expression whose value can be used as an indirect
671dirhandle, usually the real dirhandle name.
a0d0e21e
LW
672
673=item connect SOCKET,NAME
674
675Attempts to connect to a remote socket, just as the connect system call
676does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
4633a7c4
LW
677packed address of the appropriate type for the socket. See the examples in
678L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 679
cb1a09d0
AD
680=item continue BLOCK
681
682Actually a flow control statement rather than a function. If there is a
dc9e4912 683C<continue> BLOCK attached to a BLOCK (typically in a L</while> or
a189bff9
JH
684L</foreach>), it is always executed just before the conditional is about to
685be evaluated again, just like the third part of a L</for> loop in C. Thus
cb1a09d0
AD
686it can be used to increment a loop variable, even when the loop has been
687continued via the C<next> statement (which is similar to the C C<continue>
688statement).
689
a189bff9 690L</last>, L</next>, or L</redo> may appear within a C<continue>
1d2dff63
GS
691block. C<last> and C<redo> will behave as if they had been executed within
692the main block. So will C<next>, but since it will execute a C<continue>
693block, it may be more entertaining.
694
695 while (EXPR) {
696 ### redo always comes here
697 do_something;
698 } continue {
699 ### next always comes here
700 do_something_else;
701 # then back the top to re-check EXPR
702 }
703 ### last always comes here
704
705Omitting the C<continue> section is semantically equivalent to using an
706empty one, logically enough. In that case, C<next> goes directly back
707to check the condition at the top of the loop.
708
a189bff9
JH
709See also L<perlsyn>.
710
a0d0e21e
LW
711=item cos EXPR
712
5a964f20 713Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
7660c0ab 714takes cosine of C<$_>.
a0d0e21e 715
7660c0ab 716For the inverse cosine operation, you may use the C<POSIX::acos()>
28757baa
PP
717function, or use this relation:
718
719 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
720
a0d0e21e
LW
721=item crypt PLAINTEXT,SALT
722
f86cebdf 723Encrypts a string exactly like the crypt(3) function in the C library
4633a7c4
LW
724(assuming that you actually have a version there that has not been
725extirpated as a potential munition). This can prove useful for checking
726the password file for lousy passwords, amongst other things. Only the
727guys wearing white hats should do this.
a0d0e21e 728
7660c0ab 729Note that C<crypt()> is intended to be a one-way function, much like breaking
11155c91
CS
730eggs to make an omelette. There is no (known) corresponding decrypt
731function. As a result, this function isn't all that useful for
732cryptography. (For that, see your nearby CPAN mirror.)
2f9daede 733
e71965be
RS
734When verifying an existing encrypted string you should use the encrypted
735text as the salt (like C<crypt($plain, $crypted) eq $crypted>). This
736allows your code to work with the standard C<crypt()> and with more
737exotic implementations. When choosing a new salt create a random two
738character string whose characters come from the set C<[./0-9A-Za-z]>
739(like C<join '', ('.', '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>).
740
a0d0e21e
LW
741Here's an example that makes sure that whoever runs this program knows
742their own password:
743
744 $pwd = (getpwuid($<))[1];
a0d0e21e
LW
745
746 system "stty -echo";
747 print "Password: ";
e71965be 748 chomp($word = <STDIN>);
a0d0e21e
LW
749 print "\n";
750 system "stty echo";
751
e71965be 752 if (crypt($word, $pwd) ne $pwd) {
a0d0e21e
LW
753 die "Sorry...\n";
754 } else {
755 print "ok\n";
54310121 756 }
a0d0e21e 757
9f8f0c9d 758Of course, typing in your own password to whoever asks you
748a9306 759for it is unwise.
a0d0e21e 760
aa689395 761=item dbmclose HASH
a0d0e21e 762
7660c0ab 763[This function has been superseded by the C<untie()> function.]
a0d0e21e 764
aa689395 765Breaks the binding between a DBM file and a hash.
a0d0e21e 766
aa689395 767=item dbmopen HASH,DBNAME,MODE
a0d0e21e 768
7660c0ab 769[This function has been superseded by the C<tie()> function.]
a0d0e21e 770
7b8d334a 771This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
ab4f32c2 772hash. HASH is the name of the hash. (Unlike normal C<open()>, the first
aa689395
PP
773argument is I<NOT> a filehandle, even though it looks like one). DBNAME
774is the name of the database (without the F<.dir> or F<.pag> extension if
775any). If the database does not exist, it is created with protection
7660c0ab
A
776specified by MODE (as modified by the C<umask()>). If your system supports
777only the older DBM functions, you may perform only one C<dbmopen()> in your
aa689395 778program. In older versions of Perl, if your system had neither DBM nor
7660c0ab 779ndbm, calling C<dbmopen()> produced a fatal error; it now falls back to
aa689395
PP
780sdbm(3).
781
782If you don't have write access to the DBM file, you can only read hash
783variables, not set them. If you want to test whether you can write,
7660c0ab 784either use file tests or try setting a dummy hash entry inside an C<eval()>,
aa689395 785which will trap the error.
a0d0e21e 786
7660c0ab
A
787Note that functions such as C<keys()> and C<values()> may return huge lists
788when used on large DBM files. You may prefer to use the C<each()>
a0d0e21e
LW
789function to iterate over large DBM files. Example:
790
791 # print out history file offsets
792 dbmopen(%HIST,'/usr/lib/news/history',0666);
793 while (($key,$val) = each %HIST) {
794 print $key, ' = ', unpack('L',$val), "\n";
795 }
796 dbmclose(%HIST);
797
cb1a09d0 798See also L<AnyDBM_File> for a more general description of the pros and
184e9718 799cons of the various dbm approaches, as well as L<DB_File> for a particularly
cb1a09d0 800rich implementation.
4633a7c4 801
a0d0e21e
LW
802=item defined EXPR
803
54310121 804=item defined
bbce6d69 805
2f9daede
TPG
806Returns a Boolean value telling whether EXPR has a value other than
807the undefined value C<undef>. If EXPR is not present, C<$_> will be
808checked.
809
810Many operations return C<undef> to indicate failure, end of file,
811system error, uninitialized variable, and other exceptional
812conditions. This function allows you to distinguish C<undef> from
813other values. (A simple Boolean test will not distinguish among
7660c0ab 814C<undef>, zero, the empty string, and C<"0">, which are all equally
2f9daede 815false.) Note that since C<undef> is a valid scalar, its presence
7660c0ab 816doesn't I<necessarily> indicate an exceptional condition: C<pop()>
2f9daede
TPG
817returns C<undef> when its argument is an empty array, I<or> when the
818element to return happens to be C<undef>.
819
7660c0ab 820You may also use C<defined()> to check whether a subroutine exists, by
5a964f20 821saying C<defined &func> without parentheses. On the other hand, use
7660c0ab 822of C<defined()> upon aggregates (hashes and arrays) is not guaranteed to
5a964f20 823produce intuitive results, and should probably be avoided.
2f9daede
TPG
824
825When used on a hash element, it tells you whether the value is defined,
dc848c6f 826not whether the key exists in the hash. Use L</exists> for the latter
2f9daede 827purpose.
a0d0e21e
LW
828
829Examples:
830
831 print if defined $switch{'D'};
832 print "$val\n" while defined($val = pop(@ary));
833 die "Can't readlink $sym: $!"
834 unless defined($value = readlink $sym);
a0d0e21e 835 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
2f9daede 836 $debugging = 0 unless defined $debugging;
a0d0e21e 837
7660c0ab
A
838Note: Many folks tend to overuse C<defined()>, and then are surprised to
839discover that the number C<0> and C<""> (the zero-length string) are, in fact,
2f9daede 840defined values. For example, if you say
a5f75d66
AD
841
842 "ab" =~ /a(.*)b/;
843
7660c0ab 844The pattern match succeeds, and C<$1> is defined, despite the fact that it
a5f75d66 845matched "nothing". But it didn't really match nothing--rather, it
7660c0ab 846matched something that happened to be C<0> characters long. This is all
a5f75d66 847very above-board and honest. When a function returns an undefined value,
2f9daede 848it's an admission that it couldn't give you an honest answer. So you
7660c0ab
A
849should use C<defined()> only when you're questioning the integrity of what
850you're trying to do. At other times, a simple comparison to C<0> or C<""> is
2f9daede
TPG
851what you want.
852
7660c0ab 853Currently, using C<defined()> on an entire array or hash reports whether
2f9daede
TPG
854memory for that aggregate has ever been allocated. So an array you set
855to the empty list appears undefined initially, and one that once was full
856and that you then set to the empty list still appears defined. You
857should instead use a simple test for size:
28757baa
PP
858
859 if (@an_array) { print "has array elements\n" }
860 if (%a_hash) { print "has hash members\n" }
861
7660c0ab 862Using C<undef()> on these, however, does clear their memory and then report
5a964f20 863them as not defined anymore, but you shouldn't do that unless you don't
28757baa 864plan to use them again, because it saves time when you load them up
5a964f20
TC
865again to have memory already ready to be filled. The normal way to
866free up space used by an aggregate is to assign the empty list.
28757baa 867
7660c0ab 868This counterintuitive behavior of C<defined()> on aggregates may be
28757baa
PP
869changed, fixed, or broken in a future release of Perl.
870
dc848c6f 871See also L</undef>, L</exists>, L</ref>.
2f9daede 872
a0d0e21e
LW
873=item delete EXPR
874
aa689395
PP
875Deletes the specified key(s) and their associated values from a hash.
876For each key, returns the deleted value associated with that key, or
877the undefined value if there was no such key. Deleting from C<$ENV{}>
878modifies the environment. Deleting from a hash tied to a DBM file
7660c0ab 879deletes the entry from the DBM file. (But deleting from a C<tie()>d hash
5f05dabc 880doesn't necessarily return anything.)
a0d0e21e 881
aa689395 882The following deletes all the values of a hash:
a0d0e21e 883
5f05dabc
PP
884 foreach $key (keys %HASH) {
885 delete $HASH{$key};
a0d0e21e
LW
886 }
887
5f05dabc
PP
888And so does this:
889
890 delete @HASH{keys %HASH}
891
5a964f20 892(But both of these are slower than just assigning the empty list, or
7660c0ab 893using C<undef()>.) Note that the EXPR can be arbitrarily complicated as
5a964f20 894long as the final operation is a hash element lookup or hash slice:
a0d0e21e
LW
895
896 delete $ref->[$x][$y]{$key};
5f05dabc 897 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
a0d0e21e
LW
898
899=item die LIST
900
7660c0ab
A
901Outside an C<eval()>, prints the value of LIST to C<STDERR> and exits with
902the current value of C<$!> (errno). If C<$!> is C<0>, exits with the value of
54310121 903C<($? E<gt>E<gt> 8)> (backtick `command` status). If C<($? E<gt>E<gt> 8)>
7660c0ab
A
904is C<0>, exits with C<255>. Inside an C<eval(),> the error message is stuffed into
905C<$@> and the C<eval()> is terminated with the undefined value. This makes
906C<die()> the way to raise an exception.
a0d0e21e
LW
907
908Equivalent examples:
909
910 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
54310121 911 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
a0d0e21e
LW
912
913If the value of EXPR does not end in a newline, the current script line
914number and input line number (if any) are also printed, and a newline
7660c0ab
A
915is supplied. Hint: sometimes appending C<", stopped"> to your message
916will cause it to make better sense when the string C<"at foo line 123"> is
a0d0e21e
LW
917appended. Suppose you are running script "canasta".
918
919 die "/etc/games is no good";
920 die "/etc/games is no good, stopped";
921
922produce, respectively
923
924 /etc/games is no good at canasta line 123.
925 /etc/games is no good, stopped at canasta line 123.
926
7660c0ab 927See also C<exit()> and C<warn()>.
a0d0e21e 928
7660c0ab
A
929If LIST is empty and C<$@> already contains a value (typically from a
930previous eval) that value is reused after appending C<"\t...propagated">.
fb73857a
PP
931This is useful for propagating exceptions:
932
933 eval { ... };
934 die unless $@ =~ /Expected exception/;
935
7660c0ab 936If C<$@> is empty then the string C<"Died"> is used.
fb73857a 937
7660c0ab 938You can arrange for a callback to be run just before the C<die()> does
774d564b
PP
939its deed, by setting the C<$SIG{__DIE__}> hook. The associated handler
940will be called with the error text and can change the error message, if
7660c0ab 941it sees fit, by calling C<die()> again. See L<perlvar/$SIG{expr}> for details on
fb73857a
PP
942setting C<%SIG> entries, and L<"eval BLOCK"> for some examples.
943
944Note that the C<$SIG{__DIE__}> hook is called even inside eval()ed
945blocks/strings. If one wants the hook to do nothing in such
946situations, put
947
948 die @_ if $^S;
949
950as the first line of the handler (see L<perlvar/$^S>).
774d564b 951
a0d0e21e
LW
952=item do BLOCK
953
954Not really a function. Returns the value of the last command in the
955sequence of commands indicated by BLOCK. When modified by a loop
a189bff9
JH
956modifier such as L</while> or L</until>, executes the BLOCK once
957before testing the loop condition. (On other statements the loop
958modifiers test the conditional first.)
a0d0e21e 959
4968c1e4 960C<do BLOCK> does I<not> count as a loop, so the loop control statements
a189bff9 961L</next>, L</last> or L</redo> cannot be used to leave or restart the block.
4968c1e4 962
a0d0e21e
LW
963=item do SUBROUTINE(LIST)
964
965A deprecated form of subroutine call. See L<perlsub>.
966
967=item do EXPR
968
969Uses the value of EXPR as a filename and executes the contents of the
970file as a Perl script. Its primary use is to include subroutines
971from a Perl subroutine library.
972
973 do 'stat.pl';
974
975is just like
976
fb73857a 977 scalar eval `cat stat.pl`;
a0d0e21e 978
5a964f20 979except that it's more efficient and concise, keeps track of the
a0d0e21e
LW
980current filename for error messages, and searches all the B<-I>
981libraries if the file isn't in the current directory (see also the @INC
dc1be6b5
GS
982array in L<perlvar/Predefined Names>). It is also different in how
983code evaluated with C<do FILENAME> doesn't see lexicals in the enclosing
984scope like C<eval STRING> does. It's the same, however, in that it does
54310121 985reparse the file every time you call it, so you probably don't want to
a0d0e21e
LW
986do this inside a loop.
987
8e30cc93
G
988If C<do> cannot read the file, it returns undef and sets C<$!> to the
989error. If C<do> can read the file but cannot compile it, it
990returns undef and sets an error message in C<$@>. If the file is
991successfully compiled, C<do> returns the value of the last expression
992evaluated.
993
a0d0e21e 994Note that inclusion of library modules is better done with the
7660c0ab 995C<use()> and C<require()> operators, which also do automatic error checking
4633a7c4 996and raise an exception if there's a problem.
a0d0e21e 997
5a964f20
TC
998You might like to use C<do> to read in a program configuration
999file. Manual error checking can be done this way:
1000
1001 # read in config files: system first, then user
f86cebdf
GS
1002 for $file ("/share/prog/defaults.rc",
1003 "$ENV{HOME}/.someprogrc") {
5a964f20 1004 unless ($return = do $file) {
f86cebdf
GS
1005 warn "couldn't parse $file: $@" if $@;
1006 warn "couldn't do $file: $!" unless defined $return;
1007 warn "couldn't run $file" unless $return;
5a964f20
TC
1008 }
1009 }
1010
a0d0e21e
LW
1011=item dump LABEL
1012
1013This causes an immediate core dump. Primarily this is so that you can
1014use the B<undump> program to turn your core dump into an executable binary
1015after having initialized all your variables at the beginning of the
1016program. When the new binary is executed it will begin by executing a
1017C<goto LABEL> (with all the restrictions that C<goto> suffers). Think of
f86cebdf 1018it as a goto with an intervening core dump and reincarnation. If C<LABEL>
5a964f20 1019is omitted, restarts the program from the top. WARNING: Any files
a0d0e21e
LW
1020opened at the time of the dump will NOT be open any more when the
1021program is reincarnated, with possible resulting confusion on the part
1022of Perl. See also B<-u> option in L<perlrun>.
1023
1024Example:
1025
1026 #!/usr/bin/perl
1027 require 'getopt.pl';
1028 require 'stat.pl';
1029 %days = (
1030 'Sun' => 1,
1031 'Mon' => 2,
1032 'Tue' => 3,
1033 'Wed' => 4,
1034 'Thu' => 5,
1035 'Fri' => 6,
1036 'Sat' => 7,
1037 );
1038
1039 dump QUICKSTART if $ARGV[0] eq '-d';
1040
1041 QUICKSTART:
1042 Getopt('f');
1043
5a964f20
TC
1044This operator is largely obsolete, partly because it's very hard to
1045convert a core file into an executable, and because the real perl-to-C
1046compiler has superseded it.
1047
aa689395
PP
1048=item each HASH
1049
5a964f20 1050When called in list context, returns a 2-element list consisting of the
aa689395 1051key and value for the next element of a hash, so that you can iterate over
5a964f20 1052it. When called in scalar context, returns the key for only the "next"
7660c0ab 1053element in the hash. (Note: Keys may be C<"0"> or C<"">, which are logically
2f9daede
TPG
1054false; you may wish to avoid constructs like C<while ($k = each %foo) {}>
1055for this reason.)
1056
ab192400
GS
1057Entries are returned in an apparently random order. The actual random
1058order is subject to change in future versions of perl, but it is guaranteed
1059to be in the same order as either the C<keys()> or C<values()> function
1060would produce on the same (unmodified) hash.
1061
1062When the hash is entirely read, a null array is returned in list context
1063(which when assigned produces a FALSE (C<0>) value), and C<undef> in
7660c0ab
A
1064scalar context. The next call to C<each()> after that will start iterating
1065again. There is a single iterator for each hash, shared by all C<each()>,
1066C<keys()>, and C<values()> function calls in the program; it can be reset by
2f9daede
TPG
1067reading all the elements from the hash, or by evaluating C<keys HASH> or
1068C<values HASH>. If you add or delete elements of a hash while you're
1069iterating over it, you may get entries skipped or duplicated, so don't.
aa689395 1070
f86cebdf 1071The following prints out your environment like the printenv(1) program,
aa689395 1072only in a different order:
a0d0e21e
LW
1073
1074 while (($key,$value) = each %ENV) {
1075 print "$key=$value\n";
1076 }
1077
ab192400 1078See also C<keys()>, C<values()> and C<sort()>.
a0d0e21e 1079
a189bff9
JH
1080=item else BLOCK
1081
1082=item elsif (EXPR) BLOCK
1083
1084See L</if>.
1085
a0d0e21e
LW
1086=item eof FILEHANDLE
1087
4633a7c4
LW
1088=item eof ()
1089
a0d0e21e
LW
1090=item eof
1091
1092Returns 1 if the next read on FILEHANDLE will return end of file, or if
1093FILEHANDLE is not open. FILEHANDLE may be an expression whose value
5a964f20 1094gives the real filehandle. (Note that this function actually
7660c0ab 1095reads a character and then C<ungetc()>s it, so isn't very useful in an
748a9306
LW
1096interactive context.) Do not read from a terminal file (or call
1097C<eof(FILEHANDLE)> on it) after end-of-file is reached. Filetypes such
1098as terminals may lose the end-of-file condition if you do.
1099
1100An C<eof> without an argument uses the last file read as argument.
5a964f20 1101Using C<eof()> with empty parentheses is very different. It indicates the pseudo file formed of
2f9daede
TPG
1102the files listed on the command line, i.e., C<eof()> is reasonable to
1103use inside a C<while (E<lt>E<gt>)> loop to detect the end of only the
1104last file. Use C<eof(ARGV)> or eof without the parentheses to test
1105I<EACH> file in a while (E<lt>E<gt>) loop. Examples:
a0d0e21e 1106
748a9306
LW
1107 # reset line numbering on each input file
1108 while (<>) {
5a964f20 1109 next if /^\s*#/; # skip comments
748a9306 1110 print "$.\t$_";
5a964f20
TC
1111 } continue {
1112 close ARGV if eof; # Not eof()!
748a9306
LW
1113 }
1114
a0d0e21e
LW
1115 # insert dashes just before last line of last file
1116 while (<>) {
5a964f20 1117 if (eof()) { # check for end of current file
a0d0e21e 1118 print "--------------\n";
748a9306
LW
1119 close(ARGV); # close or break; is needed if we
1120 # are reading from the terminal
a0d0e21e
LW
1121 }
1122 print;
1123 }
1124
a0d0e21e 1125Practical hint: you almost never need to use C<eof> in Perl, because the
3b02c43c
GS
1126input operators return false values when they run out of data, or if there
1127was an error.
a0d0e21e
LW
1128
1129=item eval EXPR
1130
1131=item eval BLOCK
1132
c7cc6f1c
GS
1133In the first form, the return value of EXPR is parsed and executed as if it
1134were a little Perl program. The value of the expression (which is itself
5a964f20 1135determined within scalar context) is first parsed, and if there weren't any
c7cc6f1c 1136errors, executed in the context of the current Perl program, so that any
5f05dabc 1137variable settings or subroutine and format definitions remain afterwards.
c7cc6f1c
GS
1138Note that the value is parsed every time the eval executes. If EXPR is
1139omitted, evaluates C<$_>. This form is typically used to delay parsing
1140and subsequent execution of the text of EXPR until run time.
1141
1142In the second form, the code within the BLOCK is parsed only once--at the
1143same time the code surrounding the eval itself was parsed--and executed
1144within the context of the current Perl program. This form is typically
1145used to trap exceptions more efficiently than the first (see below), while
1146also providing the benefit of checking the code within BLOCK at compile
1147time.
1148
1149The final semicolon, if any, may be omitted from the value of EXPR or within
1150the BLOCK.
1151
1152In both forms, the value returned is the value of the last expression
5a964f20 1153evaluated inside the mini-program; a return statement may be also used, just
c7cc6f1c 1154as with subroutines. The expression providing the return value is evaluated
5a964f20 1155in void, scalar, or list context, depending on the context of the eval itself.
c7cc6f1c 1156See L</wantarray> for more on how the evaluation context can be determined.
a0d0e21e 1157
7660c0ab
A
1158If there is a syntax error or runtime error, or a C<die()> statement is
1159executed, an undefined value is returned by C<eval()>, and C<$@> is set to the
a0d0e21e 1160error message. If there was no error, C<$@> is guaranteed to be a null
7660c0ab 1161string. Beware that using C<eval()> neither silences perl from printing
c7cc6f1c
GS
1162warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1163To do either of those, you have to use the C<$SIG{__WARN__}> facility. See
1164L</warn> and L<perlvar>.
a0d0e21e 1165
7660c0ab
A
1166Note that, because C<eval()> traps otherwise-fatal errors, it is useful for
1167determining whether a particular feature (such as C<socket()> or C<symlink()>)
a0d0e21e
LW
1168is implemented. It is also Perl's exception trapping mechanism, where
1169the die operator is used to raise exceptions.
1170
1171If the code to be executed doesn't vary, you may use the eval-BLOCK
1172form to trap run-time errors without incurring the penalty of
1173recompiling each time. The error, if any, is still returned in C<$@>.
1174Examples:
1175
54310121 1176 # make divide-by-zero nonfatal
a0d0e21e
LW
1177 eval { $answer = $a / $b; }; warn $@ if $@;
1178
1179 # same thing, but less efficient
1180 eval '$answer = $a / $b'; warn $@ if $@;
1181
1182 # a compile-time error
5a964f20 1183 eval { $answer = }; # WRONG
a0d0e21e
LW
1184
1185 # a run-time error
1186 eval '$answer ='; # sets $@
1187
7660c0ab 1188When using the C<eval{}> form as an exception trap in libraries, you may
774d564b
PP
1189wish not to trigger any C<__DIE__> hooks that user code may have
1190installed. You can use the C<local $SIG{__DIE__}> construct for this
1191purpose, as shown in this example:
1192
1193 # a very private exception trap for divide-by-zero
f86cebdf
GS
1194 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1195 warn $@ if $@;
774d564b
PP
1196
1197This is especially significant, given that C<__DIE__> hooks can call
7660c0ab 1198C<die()> again, which has the effect of changing their error messages:
774d564b
PP
1199
1200 # __DIE__ hooks may modify error messages
1201 {
f86cebdf
GS
1202 local $SIG{'__DIE__'} =
1203 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
c7cc6f1c
GS
1204 eval { die "foo lives here" };
1205 print $@ if $@; # prints "bar lives here"
774d564b
PP
1206 }
1207
7660c0ab 1208With an C<eval()>, you should be especially careful to remember what's
a0d0e21e
LW
1209being looked at when:
1210
1211 eval $x; # CASE 1
1212 eval "$x"; # CASE 2
1213
1214 eval '$x'; # CASE 3
1215 eval { $x }; # CASE 4
1216
5a964f20 1217 eval "\$$x++"; # CASE 5
a0d0e21e
LW
1218 $$x++; # CASE 6
1219
2f9daede 1220Cases 1 and 2 above behave identically: they run the code contained in
7660c0ab 1221the variable C<$x>. (Although case 2 has misleading double quotes making
2f9daede 1222the reader wonder what else might be happening (nothing is).) Cases 3
7660c0ab 1223and 4 likewise behave in the same way: they run the code C<'$x'>, which
2f9daede
TPG
1224does nothing but return the value of C<$x>. (Case 4 is preferred for
1225purely visual reasons, but it also has the advantage of compiling at
1226compile-time instead of at run-time.) Case 5 is a place where
54310121 1227normally you I<WOULD> like to use double quotes, except that in this
2f9daede
TPG
1228particular situation, you can just use symbolic references instead, as
1229in case 6.
a0d0e21e 1230
4968c1e4
G
1231C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1232C<next>, C<last> or C<redo> cannot be used to leave or restart the block.
1233
1234
a0d0e21e
LW
1235=item exec LIST
1236
8bf3b016
GS
1237=item exec PROGRAM LIST
1238
7660c0ab
A
1239The C<exec()> function executes a system command I<AND NEVER RETURNS> -
1240use C<system()> instead of C<exec()> if you want it to return. It fails and
fb73857a
PP
1241returns FALSE only if the command does not exist I<and> it is executed
1242directly instead of via your system's command shell (see below).
a0d0e21e 1243
7660c0ab
A
1244Since it's a common mistake to use C<exec()> instead of C<system()>, Perl
1245warns you if there is a following statement which isn't C<die()>, C<warn()>,
1246or C<exit()> (if C<-w> is set - but you always do that). If you
1247I<really> want to follow an C<exec()> with some other statement, you
55d729e4
GS
1248can use one of these styles to avoid the warning:
1249
5a964f20
TC
1250 exec ('foo') or print STDERR "couldn't exec foo: $!";
1251 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
55d729e4 1252
5a964f20 1253If there is more than one argument in LIST, or if LIST is an array
f86cebdf 1254with more than one value, calls execvp(3) with the arguments in LIST.
5a964f20
TC
1255If there is only one scalar argument or an array with one element in it,
1256the argument is checked for shell metacharacters, and if there are any,
1257the entire argument is passed to the system's command shell for parsing
1258(this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1259If there are no shell metacharacters in the argument, it is split into
7660c0ab
A
1260words and passed directly to C<execvp()>, which is more efficient. Note:
1261C<exec()> and C<system()> do not flush your output buffer, so you may need to
5a964f20 1262set C<$|> to avoid lost output. Examples:
a0d0e21e
LW
1263
1264 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1265 exec "sort $outfile | uniq";
1266
1267If you don't really want to execute the first argument, but want to lie
1268to the program you are executing about its own name, you can specify
1269the program you actually want to run as an "indirect object" (without a
1270comma) in front of the LIST. (This always forces interpretation of the
54310121 1271LIST as a multivalued list, even if there is only a single scalar in
a0d0e21e
LW
1272the list.) Example:
1273
1274 $shell = '/bin/csh';
1275 exec $shell '-sh'; # pretend it's a login shell
1276
1277or, more directly,
1278
1279 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1280
bb32b41a
GS
1281When the arguments get executed via the system shell, results will
1282be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1283for details.
1284
ab4f32c2 1285Using an indirect object with C<exec()> or C<system()> is also more secure.
5a964f20
TC
1286This usage forces interpretation of the arguments as a multivalued list,
1287even if the list had just one argument. That way you're safe from the
1288shell expanding wildcards or splitting up words with whitespace in them.
1289
1290 @args = ( "echo surprise" );
1291
f86cebdf
GS
1292 system @args; # subject to shell escapes
1293 # if @args == 1
5a964f20
TC
1294 system { $args[0] } @args; # safe even with one-arg list
1295
1296The first version, the one without the indirect object, ran the I<echo>
1297program, passing it C<"surprise"> an argument. The second version
1298didn't--it tried to run a program literally called I<"echo surprise">,
1299didn't find it, and set C<$?> to a non-zero value indicating failure.
1300
ab4f32c2 1301Note that C<exec()> will not call your C<END> blocks, nor will it call
7660c0ab
A
1302any C<DESTROY> methods in your objects.
1303
a0d0e21e
LW
1304=item exists EXPR
1305
1306Returns TRUE if the specified hash key exists in its hash array, even
1307if the corresponding value is undefined.
1308
1309 print "Exists\n" if exists $array{$key};
1310 print "Defined\n" if defined $array{$key};
1311 print "True\n" if $array{$key};
1312
5f05dabc 1313A hash element can be TRUE only if it's defined, and defined if
a0d0e21e
LW
1314it exists, but the reverse doesn't necessarily hold true.
1315
1316Note that the EXPR can be arbitrarily complicated as long as the final
1317operation is a hash key lookup:
1318
5a964f20
TC
1319 if (exists $ref->{"A"}{"B"}{$key}) { ... }
1320
1321Although the last element will not spring into existence just because its
1322existence was tested, intervening ones will. Thus C<$ref-E<gt>{"A"}>
1323C<$ref-E<gt>{"B"}> will spring into existence due to the existence
1324test for a $key element. This autovivification may be fixed in a later
1325release.
a0d0e21e
LW
1326
1327=item exit EXPR
1328
1329Evaluates EXPR and exits immediately with that value. (Actually, it
1330calls any defined C<END> routines first, but the C<END> routines may not
1331abort the exit. Likewise any object destructors that need to be called
1332are called before exit.) Example:
1333
1334 $ans = <STDIN>;
1335 exit 0 if $ans =~ /^[Xx]/;
1336
7660c0ab
A
1337See also C<die()>. If EXPR is omitted, exits with C<0> status. The only
1338universally portable values for EXPR are C<0> for success and C<1> for error;
f86702cc
PP
1339all other values are subject to unpredictable interpretation depending
1340on the environment in which the Perl program is running.
a0d0e21e 1341
7660c0ab
A
1342You shouldn't use C<exit()> to abort a subroutine if there's any chance that
1343someone might want to trap whatever error happened. Use C<die()> instead,
1344which can be trapped by an C<eval()>.
28757baa 1345
5a964f20
TC
1346All C<END{}> blocks are run at exit time. See L<perlsub> for details.
1347
a0d0e21e
LW
1348=item exp EXPR
1349
54310121 1350=item exp
bbce6d69 1351
54310121 1352Returns I<e> (the natural logarithm base) to the power of EXPR.
a0d0e21e
LW
1353If EXPR is omitted, gives C<exp($_)>.
1354
1355=item fcntl FILEHANDLE,FUNCTION,SCALAR
1356
f86cebdf 1357Implements the fcntl(2) function. You'll probably have to say
a0d0e21e
LW
1358
1359 use Fcntl;
1360
0ade1984 1361first to get the correct constant definitions. Argument processing and
7660c0ab 1362value return works just like C<ioctl()> below.
a0d0e21e
LW
1363For example:
1364
1365 use Fcntl;
5a964f20
TC
1366 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1367 or die "can't fcntl F_GETFL: $!";
1368
ab4f32c2
A
1369You don't have to check for C<defined()> on the return from
1370C<fnctl()>. Like C<ioctl()>, it maps a C<0> return from the system
7660c0ab
A
1371call into "C<0> but true" in Perl. This string is true in
1372boolean context and C<0> in numeric context. It is also
5a964f20
TC
1373exempt from the normal B<-w> warnings on improper numeric
1374conversions.
1375
7660c0ab 1376Note that C<fcntl()> will produce a fatal error if used on a machine that
f86cebdf 1377doesn't implement fcntl(2).
a0d0e21e
LW
1378
1379=item fileno FILEHANDLE
1380
1381Returns the file descriptor for a filehandle. This is useful for
7660c0ab 1382constructing bitmaps for C<select()> and low-level POSIX tty-handling
5a964f20
TC
1383operations. If FILEHANDLE is an expression, the value is taken as
1384an indirect filehandle, generally its name.
1385
1386You can use this to find out whether two handles refer to the
1387same underlying descriptor:
1388
1389 if (fileno(THIS) == fileno(THAT)) {
1390 print "THIS and THAT are dups\n";
1391 }
a0d0e21e
LW
1392
1393=item flock FILEHANDLE,OPERATION
1394
f86cebdf 1395Calls flock(2), or an emulation of it, on FILEHANDLE. Returns TRUE for
68dc0745 1396success, FALSE on failure. Produces a fatal error if used on a machine
f86cebdf 1397that doesn't implement flock(2), fcntl(2) locking, or lockf(3). C<flock()>
68dc0745
PP
1398is Perl's portable file locking interface, although it locks only entire
1399files, not records.
8ebc5c01 1400
a3cb178b 1401On many platforms (including most versions or clones of Unix), locks
7660c0ab 1402established by C<flock()> are B<merely advisory>. Such discretionary locks
5a964f20 1403are more flexible, but offer fewer guarantees. This means that files
7660c0ab
A
1404locked with C<flock()> may be modified by programs that do not also use
1405C<flock()>. Windows NT and OS/2 are among the platforms which
5a964f20 1406enforce mandatory locking. See your local documentation for details.
a3cb178b 1407
8ebc5c01
PP
1408OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1409LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
68dc0745
PP
1410you can use the symbolic names if import them from the Fcntl module,
1411either individually, or as a group using the ':flock' tag. LOCK_SH
1412requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1413releases a previously requested lock. If LOCK_NB is added to LOCK_SH or
7660c0ab 1414LOCK_EX then C<flock()> will return immediately rather than blocking
68dc0745
PP
1415waiting for the lock (check the return status to see if you got it).
1416
1417To avoid the possibility of mis-coordination, Perl flushes FILEHANDLE
1418before (un)locking it.
8ebc5c01 1419
f86cebdf 1420Note that the emulation built with lockf(3) doesn't provide shared
8ebc5c01 1421locks, and it requires that FILEHANDLE be open with write intent. These
f86cebdf
GS
1422are the semantics that lockf(3) implements. Most (all?) systems
1423implement lockf(3) in terms of fcntl(2) locking, though, so the
8ebc5c01
PP
1424differing semantics shouldn't bite too many people.
1425
7660c0ab
A
1426Note also that some versions of C<flock()> cannot lock things over the
1427network; you would need to use the more system-specific C<fcntl()> for
f86cebdf
GS
1428that. If you like you can force Perl to ignore your system's flock(2)
1429function, and so provide its own fcntl(2)-based emulation, by passing
8ebc5c01
PP
1430the switch C<-Ud_flock> to the F<Configure> program when you configure
1431perl.
4633a7c4
LW
1432
1433Here's a mailbox appender for BSD systems.
a0d0e21e 1434
7e1af8bc 1435 use Fcntl ':flock'; # import LOCK_* constants
a0d0e21e
LW
1436
1437 sub lock {
7e1af8bc 1438 flock(MBOX,LOCK_EX);
a0d0e21e
LW
1439 # and, in case someone appended
1440 # while we were waiting...
1441 seek(MBOX, 0, 2);
1442 }
1443
1444 sub unlock {
7e1af8bc 1445 flock(MBOX,LOCK_UN);
a0d0e21e
LW
1446 }
1447
1448 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1449 or die "Can't open mailbox: $!";
1450
1451 lock();
1452 print MBOX $msg,"\n\n";
1453 unlock();
1454
cb1a09d0 1455See also L<DB_File> for other flock() examples.
a0d0e21e 1456
a189bff9
JH
1457=item for (INITIAL; WHILE; EACH) BLOCK
1458
1459Do INITIAL, enter BLOCK while EXPR is true, at the end of each round
1460do EACH. For example:
1461
1462 for ($i = 0, $j = 0; $i < 10; $i++) {
1463 if ($i % 3 == 0) { $j++ }
1464 print "i = $i, j = $j\n";
1465 }
1466
1467See L<perlsyn> for more details. See also L</foreach>, a twin of
1468C<for>, L</while> and L</until>, close cousins of L<for>, and
1469L</last>, L</next>, and L</redo> for additional control flow.
1470
1471=item foreach LOOPVAR (LIST) BLOCK
1472
1473Enter BLOCK as LOOPVAR set in turn to each element of LIST.
1474For example:
1475
1476 foreach $rolling (@stones) { print "rolling $stone\n" }
1477
1478 foreach my $file (@files) { print "file $file\n" }
1479
1480The LOOPVAR is optional and defaults to C<$_>. If the elements are
1481modifiable (as opposed to constants or tied variables) you can modify them.
1482
1483 foreach (@words) { tr/abc/xyz/ }
1484
1485See L<perlsyn> for more details. See also L</for>, a twin of
1486C<foreach>, L</while> and L</until>, close cousins of L<for>, and
1487L</last>, L</next>, and L</redo> for additional control flow.
1488
a0d0e21e
LW
1489=item fork
1490
f86cebdf 1491Does a fork(2) system call. Returns the child pid to the parent process,
7660c0ab 1492C<0> to the child process, or C<undef> if the fork is unsuccessful.
5a964f20 1493
a0d0e21e 1494Note: unflushed buffers remain unflushed in both processes, which means
7660c0ab
A
1495you may need to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()>
1496method of C<IO::Handle> to avoid duplicate output.
a0d0e21e 1497
7660c0ab 1498If you C<fork()> without ever waiting on your children, you will accumulate
a0d0e21e
LW
1499zombies:
1500
4633a7c4 1501 $SIG{CHLD} = sub { wait };
a0d0e21e 1502
54310121 1503There's also the double-fork trick (error checking on
7660c0ab 1504C<fork()> returns omitted);
a0d0e21e
LW
1505
1506 unless ($pid = fork) {
1507 unless (fork) {
1508 exec "what you really wanna do";
1509 die "no exec";
1510 # ... or ...
4633a7c4 1511 ## (some_perl_code_here)
a0d0e21e
LW
1512 exit 0;
1513 }
1514 exit 0;
1515 }
1516 waitpid($pid,0);
1517
cb1a09d0
AD
1518See also L<perlipc> for more examples of forking and reaping
1519moribund children.
1520
28757baa
PP
1521Note that if your forked child inherits system file descriptors like
1522STDIN and STDOUT that are actually connected by a pipe or socket, even
5a964f20 1523if you exit, then the remote server (such as, say, httpd or rsh) won't think
7660c0ab 1524you're done. You should reopen those to F</dev/null> if it's any issue.
28757baa 1525
cb1a09d0
AD
1526=item format
1527
7660c0ab 1528Declare a picture format for use by the C<write()> function. For
cb1a09d0
AD
1529example:
1530
54310121 1531 format Something =
cb1a09d0
AD
1532 Test: @<<<<<<<< @||||| @>>>>>
1533 $str, $%, '$' . int($num)
1534 .
1535
1536 $str = "widget";
184e9718 1537 $num = $cost/$quantity;
cb1a09d0
AD
1538 $~ = 'Something';
1539 write;
1540
1541See L<perlform> for many details and examples.
1542
8903cb82 1543=item formline PICTURE,LIST
a0d0e21e 1544
5a964f20 1545This is an internal function used by C<format>s, though you may call it,
a0d0e21e
LW
1546too. It formats (see L<perlform>) a list of values according to the
1547contents of PICTURE, placing the output into the format output
7660c0ab
A
1548accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
1549Eventually, when a C<write()> is done, the contents of
a0d0e21e 1550C<$^A> are written to some filehandle, but you could also read C<$^A>
7660c0ab
A
1551yourself and then set C<$^A> back to C<"">. Note that a format typically
1552does one C<formline()> per line of form, but the C<formline()> function itself
748a9306 1553doesn't care how many newlines are embedded in the PICTURE. This means
4633a7c4 1554that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
748a9306
LW
1555You may therefore need to use multiple formlines to implement a single
1556record format, just like the format compiler.
1557
5f05dabc 1558Be careful if you put double quotes around the picture, because an "C<@>"
748a9306 1559character may be taken to mean the beginning of an array name.
7660c0ab 1560C<formline()> always returns TRUE. See L<perlform> for other examples.
a0d0e21e
LW
1561
1562=item getc FILEHANDLE
1563
1564=item getc
1565
1566Returns the next character from the input file attached to FILEHANDLE,
9bc64814 1567or the undefined value at end of file, or if there was an error. If
3b02c43c
GS
1568FILEHANDLE is omitted, reads from STDIN. This is not particularly
1569efficient. It cannot be used to get unbuffered single-characters,
1570however. For that, try something more like:
4633a7c4
LW
1571
1572 if ($BSD_STYLE) {
1573 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1574 }
1575 else {
54310121 1576 system "stty", '-icanon', 'eol', "\001";
4633a7c4
LW
1577 }
1578
1579 $key = getc(STDIN);
1580
1581 if ($BSD_STYLE) {
1582 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1583 }
1584 else {
5f05dabc 1585 system "stty", 'icanon', 'eol', '^@'; # ASCII null
4633a7c4
LW
1586 }
1587 print "\n";
1588
54310121
PP
1589Determination of whether $BSD_STYLE should be set
1590is left as an exercise to the reader.
cb1a09d0 1591
7660c0ab 1592The C<POSIX::getattr()> function can do this more portably on systems
5a964f20 1593purporting POSIX compliance.
cb1a09d0 1594See also the C<Term::ReadKey> module from your nearest CPAN site;
b687b08b 1595details on CPAN can be found on L<perlmodlib/CPAN>.
a0d0e21e
LW
1596
1597=item getlogin
1598
5a964f20
TC
1599Implements the C library function of the same name, which on most
1600systems returns the current login from F</etc/utmp>, if any. If null,
7660c0ab 1601use C<getpwuid()>.
a0d0e21e 1602
f86702cc 1603 $login = getlogin || getpwuid($<) || "Kilroy";
a0d0e21e 1604
7660c0ab
A
1605Do not consider C<getlogin()> for authentication: it is not as
1606secure as C<getpwuid()>.
4633a7c4 1607
a0d0e21e
LW
1608=item getpeername SOCKET
1609
1610Returns the packed sockaddr address of other end of the SOCKET connection.
1611
4633a7c4
LW
1612 use Socket;
1613 $hersockaddr = getpeername(SOCK);
1614 ($port, $iaddr) = unpack_sockaddr_in($hersockaddr);
1615 $herhostname = gethostbyaddr($iaddr, AF_INET);
1616 $herstraddr = inet_ntoa($iaddr);
a0d0e21e
LW
1617
1618=item getpgrp PID
1619
47e29363 1620Returns the current process group for the specified PID. Use
7660c0ab 1621a PID of C<0> to get the current process group for the
4633a7c4 1622current process. Will raise an exception if used on a machine that
f86cebdf 1623doesn't implement getpgrp(2). If PID is omitted, returns process
7660c0ab
A
1624group of current process. Note that the POSIX version of C<getpgrp()>
1625does not accept a PID argument, so only C<PID==0> is truly portable.
a0d0e21e
LW
1626
1627=item getppid
1628
1629Returns the process id of the parent process.
1630
1631=item getpriority WHICH,WHO
1632
4633a7c4
LW
1633Returns the current priority for a process, a process group, or a user.
1634(See L<getpriority(2)>.) Will raise a fatal exception if used on a
f86cebdf 1635machine that doesn't implement getpriority(2).
a0d0e21e
LW
1636
1637=item getpwnam NAME
1638
1639=item getgrnam NAME
1640
1641=item gethostbyname NAME
1642
1643=item getnetbyname NAME
1644
1645=item getprotobyname NAME
1646
1647=item getpwuid UID
1648
1649=item getgrgid GID
1650
1651=item getservbyname NAME,PROTO
1652
1653=item gethostbyaddr ADDR,ADDRTYPE
1654
1655=item getnetbyaddr ADDR,ADDRTYPE
1656
1657=item getprotobynumber NUMBER
1658
1659=item getservbyport PORT,PROTO
1660
1661=item getpwent
1662
1663=item getgrent
1664
1665=item gethostent
1666
1667=item getnetent
1668
1669=item getprotoent
1670
1671=item getservent
1672
1673=item setpwent
1674
1675=item setgrent
1676
1677=item sethostent STAYOPEN
1678
1679=item setnetent STAYOPEN
1680
1681=item setprotoent STAYOPEN
1682
1683=item setservent STAYOPEN
1684
1685=item endpwent
1686
1687=item endgrent
1688
1689=item endhostent
1690
1691=item endnetent
1692
1693=item endprotoent
1694
1695=item endservent
1696
1697These routines perform the same functions as their counterparts in the
5a964f20 1698system library. In list context, the return values from the
a0d0e21e
LW
1699various get routines are as follows:
1700
1701 ($name,$passwd,$uid,$gid,
6ee623d5 1702 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
a0d0e21e
LW
1703 ($name,$passwd,$gid,$members) = getgr*
1704 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1705 ($name,$aliases,$addrtype,$net) = getnet*
1706 ($name,$aliases,$proto) = getproto*
1707 ($name,$aliases,$port,$proto) = getserv*
1708
1709(If the entry doesn't exist you get a null list.)
1710
5a964f20 1711In scalar context, you get the name, unless the function was a
a0d0e21e
LW
1712lookup by name, in which case you get the other thing, whatever it is.
1713(If the entry doesn't exist you get the undefined value.) For example:
1714
5a964f20
TC
1715 $uid = getpwnam($name);
1716 $name = getpwuid($num);
1717 $name = getpwent();
1718 $gid = getgrnam($name);
1719 $name = getgrgid($num;
1720 $name = getgrent();
1721 #etc.
a0d0e21e 1722
7660c0ab 1723In I<getpw*()> the fields C<$quota>, C<$comment>, and C<$expire> are special
6ee623d5 1724cases in the sense that in many systems they are unsupported. If the
7660c0ab
A
1725C<$quota> is unsupported, it is an empty scalar. If it is supported, it
1726usually encodes the disk quota. If the C<$comment> field is unsupported,
6ee623d5
GS
1727it is an empty scalar. If it is supported it usually encodes some
1728administrative comment about the user. In some systems the $quota
7660c0ab
A
1729field may be C<$change> or C<$age>, fields that have to do with password
1730aging. In some systems the C<$comment> field may be C<$class>. The C<$expire>
6ee623d5
GS
1731field, if present, encodes the expiration period of the account or the
1732password. For the availability and the exact meaning of these fields
f86cebdf 1733in your system, please consult your getpwnam(3) documentation and your
7660c0ab
A
1734F<pwd.h> file. You can also find out from within Perl which meaning
1735your C<$quota> and C<$comment> fields have and whether you have the C<$expire>
1736field by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
1737C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>.
6ee623d5 1738
7660c0ab 1739The C<$members> value returned by I<getgr*()> is a space separated list of
a0d0e21e
LW
1740the login names of the members of the group.
1741
1742For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1743C, it will be returned to you via C<$?> if the function call fails. The
7660c0ab 1744C<@addrs> value returned by a successful call is a list of the raw
a0d0e21e
LW
1745addresses returned by the corresponding system library call. In the
1746Internet domain, each address is four bytes long and you can unpack it
1747by saying something like:
1748
1749 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1750
5a964f20
TC
1751If you get tired of remembering which element of the return list contains
1752which return value, by-name interfaces are also provided in modules:
7660c0ab
A
1753C<File::stat>, C<Net::hostent>, C<Net::netent>, C<Net::protoent>, C<Net::servent>,
1754C<Time::gmtime>, C<Time::localtime>, and C<User::grent>. These override the
5a964f20
TC
1755normal built-in, replacing them with versions that return objects with
1756the appropriate names for each field. For example:
1757
1758 use File::stat;
1759 use User::pwent;
1760 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
1761
1762Even though it looks like they're the same method calls (uid),
7660c0ab 1763they aren't, because a C<File::stat> object is different from a C<User::pwent> object.
5a964f20 1764
a0d0e21e
LW
1765=item getsockname SOCKET
1766
1767Returns the packed sockaddr address of this end of the SOCKET connection.
1768
4633a7c4
LW
1769 use Socket;
1770 $mysockaddr = getsockname(SOCK);
1771 ($port, $myaddr) = unpack_sockaddr_in($mysockaddr);
a0d0e21e
LW
1772
1773=item getsockopt SOCKET,LEVEL,OPTNAME
1774
5a964f20 1775Returns the socket option requested, or undef if there is an error.
a0d0e21e
LW
1776
1777=item glob EXPR
1778
0a753a76
PP
1779=item glob
1780
7660c0ab 1781Returns the value of EXPR with filename expansions such as the standard Unix shell F</bin/sh> would
68dc0745 1782do. This is the internal function implementing the C<E<lt>*.cE<gt>>
7660c0ab 1783operator, but you can use it directly. If EXPR is omitted, C<$_> is used.
68dc0745
PP
1784The C<E<lt>*.cE<gt>> operator is discussed in more detail in
1785L<perlop/"I/O Operators">.
a0d0e21e
LW
1786
1787=item gmtime EXPR
1788
1789Converts a time as returned by the time function to a 9-element array
54310121 1790with the time localized for the standard Greenwich time zone.
4633a7c4 1791Typically used as follows:
a0d0e21e 1792
54310121 1793 # 0 1 2 3 4 5 6 7 8
a0d0e21e
LW
1794 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1795 gmtime(time);
1796
1797All array elements are numeric, and come straight out of a struct tm.
7660c0ab
A
1798In particular this means that C<$mon> has the range C<0..11> and C<$wday> has
1799the range C<0..6> with sunday as day C<0>. Also, C<$year> is the number of
1800years since 1900, that is, C<$year> is C<123> in year 2023, I<not> simply the last two digits of the year.
2f9daede
TPG
1801
1802If EXPR is omitted, does C<gmtime(time())>.
a0d0e21e 1803
f86cebdf 1804In scalar context, returns the ctime(3) value:
0a753a76
PP
1805
1806 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
1807
7660c0ab 1808Also see the C<timegm()> function provided by the C<Time::Local> module,
f86cebdf 1809and the strftime(3) function available via the POSIX module.
7660c0ab
A
1810
1811This scalar value is B<not> locale dependent, see L<perllocale>, but
1812instead a Perl builtin. Also see the C<Time::Local> module, and the
f86cebdf 1813strftime(3) and mktime(3) function available via the POSIX module. To
7660c0ab
A
1814get somewhat similar but locale dependent date strings, set up your
1815locale environment variables appropriately (please see L<perllocale>)
1816and try for example:
1817
1818 use POSIX qw(strftime);
1819 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
1820
1821Note that the C<%a> and C<%b>, the short forms of the day of the week
1822and the month of the year, may not necessarily be three characters wide.
0a753a76 1823
a0d0e21e
LW
1824=item goto LABEL
1825
748a9306
LW
1826=item goto EXPR
1827
a0d0e21e
LW
1828=item goto &NAME
1829
7660c0ab 1830The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
a0d0e21e 1831execution there. It may not be used to go into any construct that
7660c0ab 1832requires initialization, such as a subroutine or a C<foreach> loop. It
0a753a76 1833also can't be used to go into a construct that is optimized away,
7660c0ab 1834or to get out of a block or subroutine given to C<sort()>.
0a753a76 1835It can be used to go almost anywhere else within the dynamic scope,
a0d0e21e 1836including out of subroutines, but it's usually better to use some other
ab4f32c2 1837construct such as C<last> or C<die()>. The author of Perl has never felt the
7660c0ab 1838need to use this form of C<goto> (in Perl, that is--C is another matter).
a0d0e21e 1839
7660c0ab
A
1840The C<goto-EXPR> form expects a label name, whose scope will be resolved
1841dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
748a9306
LW
1842necessarily recommended if you're optimizing for maintainability:
1843
1844 goto ("FOO", "BAR", "GLARCH")[$i];
1845
7660c0ab 1846The C<goto-&NAME> form is highly magical, and substitutes a call to the
a0d0e21e 1847named subroutine for the currently running subroutine. This is used by
7660c0ab 1848C<AUTOLOAD> subroutines that wish to load another subroutine and then
a0d0e21e 1849pretend that the other subroutine had been called in the first place
7660c0ab
A
1850(except that any modifications to C<@_> in the current subroutine are
1851propagated to the other subroutine.) After the C<goto>, not even C<caller()>
a0d0e21e
LW
1852will be able to tell that this routine was called first.
1853
1854=item grep BLOCK LIST
1855
1856=item grep EXPR,LIST
1857
f86cebdf 1858This is similar in spirit to, but not the same as, grep(1)
2f9daede
TPG
1859and its relatives. In particular, it is not limited to using
1860regular expressions.
1861
a0d0e21e 1862Evaluates the BLOCK or EXPR for each element of LIST (locally setting
7660c0ab 1863C<$_> to each element) and returns the list value consisting of those
a0d0e21e
LW
1864elements for which the expression evaluated to TRUE. In a scalar
1865context, returns the number of times the expression was TRUE.
1866
1867 @foo = grep(!/^#/, @bar); # weed out comments
1868
1869or equivalently,
1870
1871 @foo = grep {!/^#/} @bar; # weed out comments
1872
7660c0ab 1873Note that, because C<$_> is a reference into the list value, it can be used
a0d0e21e
LW
1874to modify the elements of the array. While this is useful and
1875supported, it can cause bizarre results if the LIST is not a named
2f9daede 1876array. Similarly, grep returns aliases into the original list,
f86cebdf 1877much like the way that a for loop's index variable aliases the list
2f9daede 1878elements. That is, modifying an element of a list returned by grep
ab4f32c2 1879(for example, in a C<foreach>, C<map()> or another C<grep()>)
2f9daede 1880actually modifies the element in the original list.
a0d0e21e 1881
fb73857a 1882See also L</map> for an array composed of the results of the BLOCK or EXPR.
38325410 1883
a0d0e21e
LW
1884=item hex EXPR
1885
54310121 1886=item hex
bbce6d69 1887
54310121 1888Interprets EXPR as a hex string and returns the corresponding
2f9daede 1889value. (To convert strings that might start with either 0 or 0x
7660c0ab 1890see L</oct>.) If EXPR is omitted, uses C<$_>.
2f9daede
TPG
1891
1892 print hex '0xAf'; # prints '175'
1893 print hex 'aF'; # same
a0d0e21e 1894
a189bff9
JH
1895=item if (EXPR) BLOCK
1896
1897=item if (EXPR) BLOCK else BLOCK2
1898
1899=item if (EXPR) BLOCK elsif (EXPR2) BLOCK2
1900
1901Enter BLOCKs conditionally. The first EXPR to return true
1902causes the corresponding BLOCK to be entered, or, in the case
1903of C<else>, the fall-through default BLOCK.
1904
a91c35b0
JH
1905Note 1: Perl wants BLOCKS, expressions won't do (like they do
1906e.g. in C, C++, Java, Pascal).
1907
1908Note 2: It's C<elsif>, not C<elseif>. You can have as many
1909C<elsif>s as you want.
a189bff9
JH
1910
1911See L<perlsyn> for more details. See also C<unless>.
1912
a0d0e21e
LW
1913=item import
1914
7660c0ab 1915There is no builtin C<import()> function. It is just an ordinary
4633a7c4 1916method (subroutine) defined (or inherited) by modules that wish to export
7660c0ab 1917names to another module. The C<use()> function calls the C<import()> method
54310121 1918for the package used. See also L</use()>, L<perlmod>, and L<Exporter>.
a0d0e21e
LW
1919
1920=item index STR,SUBSTR,POSITION
1921
1922=item index STR,SUBSTR
1923
4633a7c4
LW
1924Returns the position of the first occurrence of SUBSTR in STR at or after
1925POSITION. If POSITION is omitted, starts searching from the beginning of
7660c0ab 1926the string. The return value is based at C<0> (or whatever you've set the C<$[>
4633a7c4 1927variable to--but don't do that). If the substring is not found, returns
7660c0ab 1928one less than the base, ordinarily C<-1>.
a0d0e21e
LW
1929
1930=item int EXPR
1931
54310121 1932=item int
bbce6d69 1933
7660c0ab 1934Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
5a964f20 1935You should not use this for rounding, because it truncates
7660c0ab
A
1936towards C<0>, and because machine representations of floating point
1937numbers can sometimes produce counterintuitive results. Usually C<sprintf()> or C<printf()>,
1938or the C<POSIX::floor> or C<POSIX::ceil> functions, would serve you better.
a0d0e21e
LW
1939
1940=item ioctl FILEHANDLE,FUNCTION,SCALAR
1941
f86cebdf 1942Implements the ioctl(2) function. You'll probably have to say
a0d0e21e 1943
4633a7c4 1944 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
a0d0e21e 1945
4633a7c4 1946first to get the correct function definitions. If F<ioctl.ph> doesn't
a0d0e21e 1947exist or doesn't have the correct definitions you'll have to roll your
4633a7c4 1948own, based on your C header files such as F<E<lt>sys/ioctl.hE<gt>>.
5a964f20 1949(There is a Perl script called B<h2ph> that comes with the Perl kit that
54310121 1950may help you in this, but it's nontrivial.) SCALAR will be read and/or
4633a7c4 1951written depending on the FUNCTION--a pointer to the string value of SCALAR
ab4f32c2 1952will be passed as the third argument of the actual C<ioctl()> call. (If SCALAR
4633a7c4
LW
1953has no string value but does have a numeric value, that value will be
1954passed rather than a pointer to the string value. To guarantee this to be
7660c0ab 1955TRUE, add a C<0> to the scalar before using it.) The C<pack()> and C<unpack()>
4633a7c4 1956functions are useful for manipulating the values of structures used by
7660c0ab 1957C<ioctl()>. The following example sets the erase character to DEL.
a0d0e21e
LW
1958
1959 require 'ioctl.ph';
4633a7c4
LW
1960 $getp = &TIOCGETP;
1961 die "NO TIOCGETP" if $@ || !$getp;
a0d0e21e 1962 $sgttyb_t = "ccccs"; # 4 chars and a short
4633a7c4 1963 if (ioctl(STDIN,$getp,$sgttyb)) {
a0d0e21e
LW
1964 @ary = unpack($sgttyb_t,$sgttyb);
1965 $ary[2] = 127;
1966 $sgttyb = pack($sgttyb_t,@ary);
4633a7c4 1967 ioctl(STDIN,&TIOCSETP,$sgttyb)
a0d0e21e
LW
1968 || die "Can't ioctl: $!";
1969 }
1970
ab4f32c2 1971The return value of C<ioctl()> (and C<fcntl()>) is as follows:
a0d0e21e
LW
1972
1973 if OS returns: then Perl returns:
1974 -1 undefined value
1975 0 string "0 but true"
1976 anything else that number
1977
1978Thus Perl returns TRUE on success and FALSE on failure, yet you can
1979still easily determine the actual value returned by the operating
1980system:
1981
1982 ($retval = ioctl(...)) || ($retval = -1);
1983 printf "System returned %d\n", $retval;
1984
7660c0ab 1985The special string "C<0> but true" is excempt from B<-w> complaints
5a964f20
TC
1986about improper numeric conversions.
1987
a0d0e21e
LW
1988=item join EXPR,LIST
1989
54310121 1990Joins the separate strings of LIST into a single string with
a0d0e21e
LW
1991fields separated by the value of EXPR, and returns the string.
1992Example:
1993
1994 $_ = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
1995
7660c0ab 1996See L</split>.
a0d0e21e 1997
aa689395
PP
1998=item keys HASH
1999
1d2dff63
GS
2000Returns a list consisting of all the keys of the named hash. (In a
2001scalar context, returns the number of keys.) The keys are returned in
ab192400
GS
2002an apparently random order. The actual random order is subject to
2003change in future versions of perl, but it is guaranteed to be the same
2004order as either the C<values()> or C<each()> function produces (given
2005that the hash has not been modified). As a side effect, it resets
2006HASH's iterator.
a0d0e21e 2007
aa689395 2008Here is yet another way to print your environment:
a0d0e21e
LW
2009
2010 @keys = keys %ENV;
2011 @values = values %ENV;
2012 while ($#keys >= 0) {
2013 print pop(@keys), '=', pop(@values), "\n";
2014 }
2015
2016or how about sorted by key:
2017
2018 foreach $key (sort(keys %ENV)) {
2019 print $key, '=', $ENV{$key}, "\n";
2020 }
2021
aca803df 2022To sort a hash by value, you'll need to use a C<sort()> function.
aa689395 2023Here's a descending numeric sort of a hash by its values:
4633a7c4 2024
5a964f20 2025 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
4633a7c4
LW
2026 printf "%4d %s\n", $hash{$key}, $key;
2027 }
2028
ab4f32c2 2029As an lvalue C<keys()> allows you to increase the number of hash buckets
aa689395
PP
2030allocated for the given hash. This can gain you a measure of efficiency if
2031you know the hash is going to get big. (This is similar to pre-extending
2032an array by assigning a larger number to $#array.) If you say
55497cff
PP
2033
2034 keys %hash = 200;
2035
ab192400
GS
2036then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2037in fact, since it rounds up to the next power of two. These
55497cff
PP
2038buckets will be retained even if you do C<%hash = ()>, use C<undef
2039%hash> if you want to free the storage while C<%hash> is still in scope.
2040You can't shrink the number of buckets allocated for the hash using
ab4f32c2 2041C<keys()> in this way (but you needn't worry about doing this by accident,
55497cff
PP
2042as trying has no effect).
2043
ab192400
GS
2044See also C<each()>, C<values()> and C<sort()>.
2045
a0d0e21e
LW
2046=item kill LIST
2047
54310121
PP
2048Sends a signal to a list of processes. The first element of
2049the list must be the signal to send. Returns the number of
4633a7c4 2050processes successfully signaled.
a0d0e21e
LW
2051
2052 $cnt = kill 1, $child1, $child2;
2053 kill 9, @goners;
2054
4633a7c4
LW
2055Unlike in the shell, in Perl if the I<SIGNAL> is negative, it kills
2056process groups instead of processes. (On System V, a negative I<PROCESS>
2057number will also kill process groups, but that's not portable.) That
2058means you usually want to use positive not negative signals. You may also
da0045b7 2059use a signal name in quotes. See L<perlipc/"Signals"> for details.
a0d0e21e
LW
2060
2061=item last LABEL
2062
2063=item last
2064
2065The C<last> command is like the C<break> statement in C (as used in
2066loops); it immediately exits the loop in question. If the LABEL is
2067omitted, the command refers to the innermost enclosing loop. The
2068C<continue> block, if any, is not executed:
2069
4633a7c4
LW
2070 LINE: while (<STDIN>) {
2071 last LINE if /^$/; # exit when done with header
5a964f20 2072 #...
a0d0e21e
LW
2073 }
2074
4968c1e4
G
2075C<last> cannot be used to exit a block which returns a value such as
2076C<eval {}>, C<sub {}> or C<do {}>.
2077
a189bff9
JH
2078See also L</continue> for an illustration of how C<last>, L</next>, and
2079L</redo> work.
2080
2081See also L<perlsyn>.
1d2dff63 2082
a0d0e21e
LW
2083=item lc EXPR
2084
54310121 2085=item lc
bbce6d69 2086
a0d0e21e 2087Returns an lowercased version of EXPR. This is the internal function
7660c0ab 2088implementing the C<\L> escape in double-quoted strings.
a0ed51b3 2089Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
a0d0e21e 2090
7660c0ab 2091If EXPR is omitted, uses C<$_>.
bbce6d69 2092
a0d0e21e
LW
2093=item lcfirst EXPR
2094
54310121 2095=item lcfirst
bbce6d69 2096
a0d0e21e 2097Returns the value of EXPR with the first character lowercased. This is
7660c0ab 2098the internal function implementing the C<\l> escape in double-quoted strings.
a0ed51b3 2099Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
a0d0e21e 2100
7660c0ab 2101If EXPR is omitted, uses C<$_>.
bbce6d69 2102
a0d0e21e
LW
2103=item length EXPR
2104
54310121 2105=item length
bbce6d69 2106
a0ed51b3 2107Returns the length in characters of the value of EXPR. If EXPR is
7660c0ab 2108omitted, returns length of C<$_>.
a0d0e21e
LW
2109
2110=item link OLDFILE,NEWFILE
2111
5a964f20
TC
2112Creates a new filename linked to the old filename. Returns TRUE for
2113success, FALSE otherwise.
a0d0e21e
LW
2114
2115=item listen SOCKET,QUEUESIZE
2116
2117Does the same thing that the listen system call does. Returns TRUE if
4633a7c4 2118it succeeded, FALSE otherwise. See example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
2119
2120=item local EXPR
2121
5a964f20
TC
2122A local modifies the listed variables to be local to the enclosing
2123block, file, or eval. If more than one value is listed, the list must
2124be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2125for details, including issues with tied arrays and hashes.
a0d0e21e 2126
7660c0ab 2127You really probably want to be using C<my()> instead, because C<local()> isn't
7b8d334a 2128what most people think of as "local". See L<perlsub/"Private Variables
cb1a09d0 2129via my()"> for details.
a0d0e21e
LW
2130
2131=item localtime EXPR
2132
2133Converts a time as returned by the time function to a 9-element array
5f05dabc 2134with the time analyzed for the local time zone. Typically used as
a0d0e21e
LW
2135follows:
2136
54310121 2137 # 0 1 2 3 4 5 6 7 8
a0d0e21e
LW
2138 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2139 localtime(time);
2140
2141All array elements are numeric, and come straight out of a struct tm.
7660c0ab
A
2142In particular this means that C<$mon> has the range C<0..11> and C<$wday> has
2143the range C<0..6> with sunday as day C<0>. Also, C<$year> is the number of
2144years 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
2145
2146If EXPR is omitted, uses the current time (C<localtime(time)>).
a0d0e21e 2147
f86cebdf 2148In scalar context, returns the ctime(3) value:
a0d0e21e 2149
5f05dabc 2150 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
a0d0e21e 2151
a3cb178b 2152This scalar value is B<not> locale dependent, see L<perllocale>, but
7660c0ab 2153instead a Perl builtin. Also see the C<Time::Local> module, and the
f86cebdf 2154strftime(3) and mktime(3) function available via the POSIX module. To
a3cb178b
GS
2155get somewhat similar but locale dependent date strings, set up your
2156locale environment variables appropriately (please see L<perllocale>)
5a964f20 2157and try for example:
a3cb178b 2158
5a964f20 2159 use POSIX qw(strftime);
a3cb178b
GS
2160 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
2161
2162Note that the C<%a> and C<%b>, the short forms of the day of the week
2163and the month of the year, may not necessarily be three characters wide.
a0d0e21e
LW
2164
2165=item log EXPR
2166
54310121 2167=item log
bbce6d69 2168
5a964f20 2169Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted, returns log
7660c0ab 2170of C<$_>.
a0d0e21e
LW
2171
2172=item lstat FILEHANDLE
2173
2174=item lstat EXPR
2175
54310121 2176=item lstat
bbce6d69 2177
7660c0ab 2178Does the same thing as the C<stat()> function (including setting the
5a964f20
TC
2179special C<_> filehandle) but stats a symbolic link instead of the file
2180the symbolic link points to. If symbolic links are unimplemented on
7660c0ab 2181your system, a normal C<stat()> is done.
a0d0e21e 2182
7660c0ab 2183If EXPR is omitted, stats C<$_>.
bbce6d69 2184
a0d0e21e
LW
2185=item m//
2186
2187The match operator. See L<perlop>.
2188
2189=item map BLOCK LIST
2190
2191=item map EXPR,LIST
2192
7660c0ab 2193Evaluates the BLOCK or EXPR for each element of LIST (locally setting C<$_> to each
a0d0e21e
LW
2194element) and returns the list value composed of the results of each such
2195evaluation. Evaluates BLOCK or EXPR in a list context, so each element of LIST
2196may produce zero, one, or more elements in the returned value.
2197
2198 @chars = map(chr, @nums);
2199
2200translates a list of numbers to the corresponding characters. And
2201
4633a7c4 2202 %hash = map { getkey($_) => $_ } @array;
a0d0e21e
LW
2203
2204is just a funny way to write
2205
2206 %hash = ();
2207 foreach $_ (@array) {
4633a7c4 2208 $hash{getkey($_)} = $_;
a0d0e21e
LW
2209 }
2210
7660c0ab 2211Note that, because C<$_> is a reference into the list value, it can be used
fb73857a
PP
2212to modify the elements of the array. While this is useful and
2213supported, it can cause bizarre results if the LIST is not a named
2214array. See also L</grep> for an array composed of those items of the
2215original list for which the BLOCK or EXPR evaluates to true.
2216
a0d0e21e
LW
2217=item mkdir FILENAME,MODE
2218
0591cd52
NT
2219Creates the directory specified by FILENAME, with permissions
2220specified by MODE (as modified by C<umask>). If it succeeds it
2221returns TRUE, otherwise it returns FALSE and sets C<$!> (errno).
2222
2223In general, it is better to create directories with permissive MODEs,
2224and let the user modify that with their C<umask>, than it is to supply
2225a restrictive MODE and give the user no way to be more permissive.
2226The exceptions to this rule are when the file or directory should be
2227kept private (mail files, for instance). The perlfunc(1) entry on
2228C<umask> discusses the choice of MODE in more detail.
a0d0e21e
LW
2229
2230=item msgctl ID,CMD,ARG
2231
f86cebdf 2232Calls the System V IPC function msgctl(2). You'll probably have to say
0ade1984
JH
2233
2234 use IPC::SysV;
2235
7660c0ab
A
2236first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2237then ARG must be a variable which will hold the returned C<msqid_ds>
ab4f32c2 2238structure. Returns like C<ioctl()>: the undefined value for error, "C<0> but
0ade1984 2239true" for zero, or the actual return value otherwise. See also
7660c0ab 2240C<IPC::SysV> and C<IPC::Semaphore::Msg> documentation.
a0d0e21e
LW
2241
2242=item msgget KEY,FLAGS
2243
f86cebdf 2244Calls the System V IPC function msgget(2). Returns the message queue
7660c0ab
A
2245id, or the undefined value if there is an error. See also C<IPC::SysV>
2246and C<IPC::SysV::Msg> documentation.
a0d0e21e
LW
2247
2248=item msgsnd ID,MSG,FLAGS
2249
2250Calls the System V IPC function msgsnd to send the message MSG to the
2251message queue ID. MSG must begin with the long integer message type,
c07a80fd 2252which may be created with C<pack("l", $type)>. Returns TRUE if
7660c0ab
A
2253successful, or FALSE if there is an error. See also C<IPC::SysV>
2254and C<IPC::SysV::Msg> documentation.
a0d0e21e
LW
2255
2256=item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2257
2258Calls the System V IPC function msgrcv to receive a message from
2259message queue ID into variable VAR with a maximum message size of
0ade1984
JH
2260SIZE. Note that if a message is received, the message type will be
2261the first thing in VAR, and the maximum length of VAR is SIZE plus the
2262size of the message type. Returns TRUE if successful, or FALSE if
7660c0ab 2263there is an error. See also C<IPC::SysV> and C<IPC::SysV::Msg> documentation.
a0d0e21e
LW
2264
2265=item my EXPR
2266
ab4f32c2
A
2267A C<my()> declares the listed variables to be local (lexically) to the
2268enclosing block, file, or C<eval()>. If
5f05dabc 2269more than one value is listed, the list must be placed in parentheses. See
cb1a09d0 2270L<perlsub/"Private Variables via my()"> for details.
4633a7c4 2271
a0d0e21e
LW
2272=item next LABEL
2273
2274=item next
2275
2276The C<next> command is like the C<continue> statement in C; it starts
2277the next iteration of the loop:
2278
4633a7c4
LW
2279 LINE: while (<STDIN>) {
2280 next LINE if /^#/; # discard comments
5a964f20 2281 #...
a0d0e21e
LW
2282 }
2283
2284Note that if there were a C<continue> block on the above, it would get
2285executed even on discarded lines. If the LABEL is omitted, the command
2286refers to the innermost enclosing loop.
2287
4968c1e4
G
2288C<next> cannot be used to exit a block which returns a value such as
2289C<eval {}>, C<sub {}> or C<do {}>.
2290
a189bff9
JH
2291See also L</continue> for an illustration of how L</last>, C<next>, and
2292L</redo> work.
2293
2294See also L<perlsyn>.
1d2dff63 2295
a0d0e21e
LW
2296=item no Module LIST
2297
7660c0ab 2298See the L</use> function, which C<no> is the opposite of.
a0d0e21e
LW
2299
2300=item oct EXPR
2301
54310121 2302=item oct
bbce6d69 2303
4633a7c4 2304Interprets EXPR as an octal string and returns the corresponding
7660c0ab 2305value. (If EXPR happens to start off with C<0x>, interprets it as
4633a7c4
LW
2306a hex string instead.) The following will handle decimal, octal, and
2307hex in the standard Perl or C notation:
a0d0e21e
LW
2308
2309 $val = oct($val) if $val =~ /^0/;
2310
7660c0ab
A
2311If EXPR is omitted, uses C<$_>. This function is commonly used when
2312a string such as C<644> needs to be converted into a file mode, for
2f9daede
TPG
2313example. (Although perl will automatically convert strings into
2314numbers as needed, this automatic conversion assumes base 10.)
a0d0e21e
LW
2315
2316=item open FILEHANDLE,EXPR
2317
2318=item open FILEHANDLE
2319
2320Opens the file whose filename is given by EXPR, and associates it with
5f05dabc
PP
2321FILEHANDLE. If FILEHANDLE is an expression, its value is used as the
2322name of the real filehandle wanted. If EXPR is omitted, the scalar
2323variable of the same name as the FILEHANDLE contains the filename.
ab4f32c2
A
2324(Note that lexical variables--those declared with C<my()>--will not work
2325for this purpose; so if you're using C<my()>, specify EXPR in your call
5f05dabc
PP
2326to open.)
2327
7660c0ab
A
2328If the filename begins with C<'E<lt>'> or nothing, the file is opened for input.
2329If the filename begins with C<'E<gt>'>, the file is truncated and opened for
2330output, being created if necessary. If the filename begins with C<'E<gt>E<gt>'>,
fbb426e4 2331the file is opened for appending, again being created if necessary.
7660c0ab
A
2332You can put a C<'+'> in front of the C<'E<gt>'> or C<'E<lt>'> to indicate that
2333you want both read and write access to the file; thus C<'+E<lt>'> is almost
2334always preferred for read/write updates--the C<'+E<gt>'> mode would clobber the
5a964f20
TC
2335file first. You can't usually use either read-write mode for updating
2336textfiles, since they have variable length records. See the B<-i>
0591cd52
NT
2337switch in L<perlrun> for a better approach. The file is created with
2338permissions of C<0666> modified by the process' C<umask> value.
5a964f20
TC
2339
2340The prefix and the filename may be separated with spaces.
f86cebdf 2341These various prefixes correspond to the fopen(3) modes of C<'r'>, C<'r+'>, C<'w'>,
7660c0ab 2342C<'w+'>, C<'a'>, and C<'a+'>.
5f05dabc 2343
7660c0ab 2344If the filename begins with C<'|'>, the filename is interpreted as a
5a964f20 2345command to which output is to be piped, and if the filename ends with a
7660c0ab
A
2346C<'|'>, the filename is interpreted See L<perlipc/"Using open() for IPC">
2347for more examples of this. (You are not allowed to C<open()> to a command
5a964f20
TC
2348that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2349and L<perlipc/"Bidirectional Communication"> for alternatives.)
cb1a09d0 2350
7660c0ab 2351Opening C<'-'> opens STDIN and opening C<'E<gt>-'> opens STDOUT. Open returns
ab4f32c2 2352nonzero upon success, the undefined value otherwise. If the C<open()>
4633a7c4 2353involved a pipe, the return value happens to be the pid of the
54310121 2354subprocess.
cb1a09d0
AD
2355
2356If you're unfortunate enough to be running Perl on a system that
2357distinguishes between text files and binary files (modern operating
2358systems don't care), then you should check out L</binmode> for tips for
ab4f32c2 2359dealing with this. The key distinction between systems that need C<binmode()>
5a964f20
TC
2360and those that don't is their text file formats. Systems like Unix, MacOS, and
2361Plan9, which delimit lines with a single character, and which encode that
ab4f32c2 2362character in C as C<"\n">, do not need C<binmode()>. The rest need it.
cb1a09d0 2363
fb73857a 2364When opening a file, it's usually a bad idea to continue normal execution
ab4f32c2
A
2365if the request failed, so C<open()> is frequently used in connection with
2366C<die()>. Even if C<die()> won't do what you want (say, in a CGI script,
fb73857a 2367where you want to make a nicely formatted error message (but there are
5a964f20 2368modules that can help with that problem)) you should always check
fb73857a
PP
2369the return value from opening a file. The infrequent exception is when
2370working with an unopened filehandle is actually what you want to do.
2371
cb1a09d0 2372Examples:
a0d0e21e
LW
2373
2374 $ARTICLE = 100;
2375 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2376 while (<ARTICLE>) {...
2377
2378 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
fb73857a 2379 # if the open fails, output is discarded
a0d0e21e 2380
fb73857a
PP
2381 open(DBASE, '+<dbase.mine') # open for update
2382 or die "Can't open 'dbase.mine' for update: $!";
cb1a09d0 2383
fb73857a
PP
2384 open(ARTICLE, "caesar <$article |") # decrypt article
2385 or die "Can't start caesar: $!";
a0d0e21e 2386
fb73857a
PP
2387 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
2388 or die "Can't start sort: $!";
a0d0e21e
LW
2389
2390 # process argument list of files along with any includes
2391
2392 foreach $file (@ARGV) {
2393 process($file, 'fh00');
2394 }
2395
2396 sub process {
5a964f20 2397 my($filename, $input) = @_;
a0d0e21e
LW
2398 $input++; # this is a string increment
2399 unless (open($input, $filename)) {
2400 print STDERR "Can't open $filename: $!\n";
2401 return;
2402 }
2403
5a964f20 2404 local $_;
a0d0e21e
LW
2405 while (<$input>) { # note use of indirection
2406 if (/^#include "(.*)"/) {
2407 process($1, $input);
2408 next;
2409 }
5a964f20 2410 #... # whatever
a0d0e21e
LW
2411 }
2412 }
2413
2414You may also, in the Bourne shell tradition, specify an EXPR beginning
7660c0ab 2415with C<'E<gt>&'>, in which case the rest of the string is interpreted as the
5a964f20 2416name of a filehandle (or file descriptor, if numeric) to be
7660c0ab
A
2417duped and opened. You may use C<&> after C<E<gt>>, C<E<gt>E<gt>>, C<E<lt>>, C<+E<gt>>,
2418C<+E<gt>E<gt>>, and C<+E<lt>>. The
a0d0e21e 2419mode you specify should match the mode of the original filehandle.
184e9718 2420(Duping a filehandle does not take into account any existing contents of
cb1a09d0 2421stdio buffers.)
a0d0e21e
LW
2422Here is a script that saves, redirects, and restores STDOUT and
2423STDERR:
2424
2425 #!/usr/bin/perl
5a964f20
TC
2426 open(OLDOUT, ">&STDOUT");
2427 open(OLDERR, ">&STDERR");
a0d0e21e
LW
2428
2429 open(STDOUT, ">foo.out") || die "Can't redirect stdout";
2430 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
2431
2432 select(STDERR); $| = 1; # make unbuffered
2433 select(STDOUT); $| = 1; # make unbuffered
2434
2435 print STDOUT "stdout 1\n"; # this works for
2436 print STDERR "stderr 1\n"; # subprocesses too
2437
2438 close(STDOUT);
2439 close(STDERR);
2440
5a964f20
TC
2441 open(STDOUT, ">&OLDOUT");
2442 open(STDERR, ">&OLDERR");
a0d0e21e
LW
2443
2444 print STDOUT "stdout 2\n";
2445 print STDERR "stderr 2\n";
2446
2447
7660c0ab
A
2448If you specify C<'E<lt>&=N'>, where C<N> is a number, then Perl will do an
2449equivalent of C's C<fdopen()> of that file descriptor; this is more
4633a7c4 2450parsimonious of file descriptors. For example:
a0d0e21e
LW
2451
2452 open(FILEHANDLE, "<&=$fd")
2453
7660c0ab 2454If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>, then
a0d0e21e 2455there is an implicit fork done, and the return value of open is the pid
7660c0ab 2456of the child within the parent process, and C<0> within the child
184e9718 2457process. (Use C<defined($pid)> to determine whether the open was successful.)
a0d0e21e
LW
2458The filehandle behaves normally for the parent, but i/o to that
2459filehandle is piped from/to the STDOUT/STDIN of the child process.
2460In the child process the filehandle isn't opened--i/o happens from/to
2461the new STDOUT or STDIN. Typically this is used like the normal
2462piped open when you want to exercise more control over just how the
2463pipe command gets executed, such as when you are running setuid, and
54310121 2464don't want to have to scan shell commands for metacharacters.
4633a7c4 2465The following pairs are more or less equivalent:
a0d0e21e
LW
2466
2467 open(FOO, "|tr '[a-z]' '[A-Z]'");
2468 open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
2469
2470 open(FOO, "cat -n '$file'|");
2471 open(FOO, "-|") || exec 'cat', '-n', $file;
2472
4633a7c4
LW
2473See L<perlipc/"Safe Pipe Opens"> for more examples of this.
2474
5a964f20 2475NOTE: On any operation that may do a fork, any unflushed buffers remain
184e9718 2476unflushed in both processes, which means you may need to set C<$|> to
4771b018
GS
2477avoid duplicate output. On systems that support a close-on-exec flag on
2478files, the flag will be set for the newly opened file descriptor as
2479determined by the value of $^F. See L<perlvar/$^F>.
a0d0e21e 2480
0dccf244
CS
2481Closing any piped filehandle causes the parent process to wait for the
2482child to finish, and returns the status value in C<$?>.
2483
5a964f20 2484The filename passed to open will have leading and trailing
f86cebdf 2485whitespace deleted, and the normal redirection characters
5a964f20
TC
2486honored. This property, known as "magic open",
2487can often be used to good effect. A user could specify a filename of
7660c0ab 2488F<"rsh cat file |">, or you could change certain filenames as needed:
5a964f20
TC
2489
2490 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
2491 open(FH, $filename) or die "Can't open $filename: $!";
2492
2493However, to open a file with arbitrary weird characters in it, it's
2494necessary to protect any leading and trailing whitespace:
2495
2496 $file =~ s#^(\s)#./$1#;
2497 open(FOO, "< $file\0");
2498
7660c0ab
A
2499If you want a "real" C C<open()> (see L<open(2)> on your system), then you
2500should use the C<sysopen()> function, which involves no such magic. This is
5a964f20
TC
2501another way to protect your filenames from interpretation. For example:
2502
2503 use IO::Handle;
2504 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
2505 or die "sysopen $path: $!";
2506 $oldfh = select(HANDLE); $| = 1; select($oldfh);
2507 print HANDLE "stuff $$\n");
2508 seek(HANDLE, 0, 0);
2509 print "File contains: ", <HANDLE>;
2510
7660c0ab
A
2511Using the constructor from the C<IO::Handle> package (or one of its
2512subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
5a964f20
TC
2513filehandles that have the scope of whatever variables hold references to
2514them, and automatically close whenever and however you leave that scope:
c07a80fd 2515
5f05dabc 2516 use IO::File;
5a964f20 2517 #...
c07a80fd
PP
2518 sub read_myfile_munged {
2519 my $ALL = shift;
5f05dabc 2520 my $handle = new IO::File;
c07a80fd
PP
2521 open($handle, "myfile") or die "myfile: $!";
2522 $first = <$handle>
2523 or return (); # Automatically closed here.
2524 mung $first or die "mung failed"; # Or here.
2525 return $first, <$handle> if $ALL; # Or here.
2526 $first; # Or here.
2527 }
2528
b687b08b 2529See L</seek> for some details about mixing reading and writing.
a0d0e21e
LW
2530
2531=item opendir DIRHANDLE,EXPR
2532
7660c0ab
A
2533Opens a directory named EXPR for processing by C<readdir()>, C<telldir()>,
2534C<seekdir()>, C<rewinddir()>, and C<closedir()>. Returns TRUE if successful.
a0d0e21e
LW
2535DIRHANDLEs have their own namespace separate from FILEHANDLEs.
2536
2537=item ord EXPR
2538
54310121 2539=item ord
bbce6d69 2540
a0ed51b3 2541Returns the numeric (ASCII or Unicode) value of the first character of EXPR. If
7660c0ab 2542EXPR is omitted, uses C<$_>. For the reverse, see L</chr>.
a0d0e21e
LW
2543
2544=item pack TEMPLATE,LIST
2545
2546Takes an array or list of values and packs it into a binary structure,
2547returning the string containing the structure. The TEMPLATE is a
2548sequence of characters that give the order and type of values, as
2549follows:
2550
2551 A An ascii string, will be space padded.
2552 a An ascii string, will be null padded.
2553 b A bit string (ascending bit order, like vec()).
2554 B A bit string (descending bit order).
2555 h A hex string (low nybble first).
2556 H A hex string (high nybble first).
2557
2558 c A signed char value.
a0ed51b3 2559 C An unsigned char value. Only does bytes. See U for Unicode.
96e4d5b1 2560
a0d0e21e
LW
2561 s A signed short value.
2562 S An unsigned short value.
96e4d5b1
PP
2563 (This 'short' is _exactly_ 16 bits, which may differ from
2564 what a local C compiler calls 'short'.)
2565
a0d0e21e
LW
2566 i A signed integer value.
2567 I An unsigned integer value.
f86cebdf
GS
2568 (This 'integer' is _at_least_ 32 bits wide. Its exact
2569 size depends on what a local C compiler calls 'int',
2570 and may even be larger than the 'long' described in
2571 the next item.)
96e4d5b1 2572
a0d0e21e
LW
2573 l A signed long value.
2574 L An unsigned long value.
96e4d5b1
PP
2575 (This 'long' is _exactly_ 32 bits, which may differ from
2576 what a local C compiler calls 'long'.)
a0d0e21e 2577
96e4d5b1
PP
2578 n A short in "network" (big-endian) order.
2579 N A long in "network" (big-endian) order.
a0d0e21e
LW
2580 v A short in "VAX" (little-endian) order.
2581 V A long in "VAX" (little-endian) order.
96e4d5b1
PP
2582 (These 'shorts' and 'longs' are _exactly_ 16 bits and
2583 _exactly_ 32 bits, respectively.)
a0d0e21e 2584
dae0da7a
JH
2585 q A signed quad (64-bit) value.
2586 Q An unsigned quad value.
2587 (Available only if your system supports 64-bit integer values
2588 _and_ if Perl has been compiled to support those.
2589 Causes a fatal error otherwise.)
2590
a0d0e21e
LW
2591 f A single-precision float in the native format.
2592 d A double-precision float in the native format.
2593
2594 p A pointer to a null-terminated string.
2595 P A pointer to a structure (fixed-length string).
2596
2597 u A uuencoded string.
a0ed51b3
LW
2598 U A Unicode character number. Encodes to UTF-8 internally.
2599 Works even if C<use utf8> is not in effect.
a0d0e21e 2600
96e4d5b1 2601 w A BER compressed integer. Its bytes represent an unsigned
f86cebdf
GS
2602 integer in base 128, most significant digit first, with as
2603 few digits as possible. Bit eight (the high bit) is set
2604 on each byte except the last.
def98dd4 2605
a0d0e21e
LW
2606 x A null byte.
2607 X Back up a byte.
2608 @ Null fill to absolute position.
2609
5a964f20 2610Each letter may optionally be followed by a number giving a repeat
7660c0ab
A
2611count. With all types except C<"a">, C<"A">, C<"b">, C<"B">, C<"h">, C<"H">, and C<"P"> the
2612pack function will gobble up that many values from the LIST. A C<*> for the
2613repeat count means to use however many items are left. The C<"a"> and C<"A">
a0d0e21e 2614types gobble just one value, but pack it as a string of length count,
7660c0ab
A
2615padding with nulls or spaces as necessary. (When unpacking, C<"A"> strips
2616trailing spaces and nulls, but C<"a"> does not.) Likewise, the C<"b"> and C<"B">
2617fields pack a string that many bits long. The C<"h"> and C<"H"> fields pack a
2618string that many nybbles long. The C<"p"> type packs a pointer to a null-
84902520
TB
2619terminated string. You are responsible for ensuring the string is not a
2620temporary value (which can potentially get deallocated before you get
7660c0ab 2621around to using the packed result). The C<"P"> packs a pointer to a structure
61167c6f 2622of the size indicated by the length. A NULL pointer is created if the
7660c0ab 2623corresponding value for C<"p"> or C<"P"> is C<undef>.
61167c6f 2624Real numbers (floats and doubles) are
a0d0e21e
LW
2625in the native machine format only; due to the multiplicity of floating
2626formats around, and the lack of a standard "network" representation, no
2627facility for interchange has been made. This means that packed floating
2628point data written on one machine may not be readable on another - even if
2629both use IEEE floating point arithmetic (as the endian-ness of the memory
2630representation is not part of the IEEE spec). Note that Perl uses doubles
2631internally for all numeric calculation, and converting from double into
5f05dabc 2632float and thence back to double again will lose precision (i.e.,
7660c0ab 2633C<unpack("f", pack("f", $foo)>) will not in general equal C<$foo>).
a0d0e21e
LW
2634
2635Examples:
2636
a0ed51b3 2637 $foo = pack("CCCC",65,66,67,68);
a0d0e21e 2638 # foo eq "ABCD"
a0ed51b3 2639 $foo = pack("C4",65,66,67,68);
a0d0e21e 2640 # same thing
a0ed51b3
LW
2641 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
2642 # same thing with Unicode circled letters
a0d0e21e
LW
2643
2644 $foo = pack("ccxxcc",65,66,67,68);
2645 # foo eq "AB\0\0CD"
2646
2647 $foo = pack("s2",1,2);
2648 # "\1\0\2\0" on little-endian
2649 # "\0\1\0\2" on big-endian
2650
2651 $foo = pack("a4","abcd","x","y","z");
2652 # "abcd"
2653
2654 $foo = pack("aaaa","abcd","x","y","z");
2655 # "axyz"
2656
2657 $foo = pack("a14","abcdefg");
2658 # "abcdefg\0\0\0\0\0\0\0"
2659
2660 $foo = pack("i9pl", gmtime);
2661 # a real struct tm (on my system anyway)
2662
2663 sub bintodec {
2664 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
2665 }
2666
2667The same template may generally also be used in the unpack function.
2668
5a964f20
TC
2669=item package
2670
cb1a09d0
AD
2671=item package NAMESPACE
2672
2673Declares the compilation unit as being in the given namespace. The scope
2674of the package declaration is from the declaration itself through the end of
7660c0ab 2675the enclosing block (the same scope as the C<local()> operator). All further
cb1a09d0 2676unqualified dynamic identifiers will be in this namespace. A package
5f05dabc 2677statement affects only dynamic variables--including those you've used
7660c0ab 2678C<local()> on--but I<not> lexical variables created with C<my()>. Typically it
cb1a09d0
AD
2679would be the first declaration in a file to be included by the C<require>
2680or C<use> operator. You can switch into a package in more than one place;
5a964f20 2681it merely influences which symbol table is used by the compiler for the
cb1a09d0
AD
2682rest of that block. You can refer to variables and filehandles in other
2683packages by prefixing the identifier with the package name and a double
2684colon: C<$Package::Variable>. If the package name is null, the C<main>
2685package as assumed. That is, C<$::sail> is equivalent to C<$main::sail>.
2686
5a964f20
TC
2687If NAMESPACE is omitted, then there is no current package, and all
2688identifiers must be fully qualified or lexicals. This is stricter
2689than C<use strict>, since it also extends to function names.
2690
cb1a09d0
AD
2691See L<perlmod/"Packages"> for more information about packages, modules,
2692and classes. See L<perlsub> for other scoping issues.
2693
a0d0e21e
LW
2694=item pipe READHANDLE,WRITEHANDLE
2695
2696Opens a pair of connected pipes like the corresponding system call.
2697Note that if you set up a loop of piped processes, deadlock can occur
2698unless you are very careful. In addition, note that Perl's pipes use
184e9718 2699stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE
a0d0e21e
LW
2700after each command, depending on the application.
2701
7e1af8bc 2702See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
4633a7c4
LW
2703for examples of such things.
2704
4771b018
GS
2705On systems that support a close-on-exec flag on files, the flag will be set
2706for the newly opened file descriptors as determined by the value of $^F.
2707See L<perlvar/$^F>.
2708
a0d0e21e
LW
2709=item pop ARRAY
2710
54310121 2711=item pop
28757baa 2712
a0d0e21e
LW
2713Pops and returns the last value of the array, shortening the array by
27141. Has a similar effect to
2715
2716 $tmp = $ARRAY[$#ARRAY--];
2717
2718If there are no elements in the array, returns the undefined value.
cb1a09d0 2719If ARRAY is omitted, pops the
7660c0ab
A
2720C<@ARGV> array in the main program, and the C<@_> array in subroutines, just
2721like C<shift()>.
a0d0e21e
LW
2722
2723=item pos SCALAR
2724
54310121 2725=item pos
bbce6d69 2726
4633a7c4 2727Returns the offset of where the last C<m//g> search left off for the variable
7660c0ab 2728is in question (C<$_> is used when the variable is not specified). May be
44a8e56a
PP
2729modified to change that offset. Such modification will also influence
2730the C<\G> zero-width assertion in regular expressions. See L<perlre> and
2731L<perlop>.
a0d0e21e
LW
2732
2733=item print FILEHANDLE LIST
2734
2735=item print LIST
2736
2737=item print
2738
cb1a09d0 2739Prints a string or a comma-separated list of strings. Returns TRUE
a0d0e21e 2740if successful. FILEHANDLE may be a scalar variable name, in which case
cb1a09d0 2741the variable contains the name of or a reference to the filehandle, thus introducing one
a0d0e21e
LW
2742level of indirection. (NOTE: If FILEHANDLE is a variable and the next
2743token is a term, it may be misinterpreted as an operator unless you
7660c0ab 2744interpose a C<+> or put parentheses around the arguments.) If FILEHANDLE is
a0d0e21e 2745omitted, prints by default to standard output (or to the last selected
7660c0ab 2746output channel--see L</select>). If LIST is also omitted, prints C<$_> to
5a964f20 2747the currently selected output channel. To set the default output channel to something other than
a0d0e21e 2748STDOUT use the select operation. Note that, because print takes a
5a964f20 2749LIST, anything in the LIST is evaluated in list context, and any
a0d0e21e 2750subroutine that you call will have one or more of its expressions
5a964f20 2751evaluated in list context. Also be careful not to follow the print
a0d0e21e 2752keyword with a left parenthesis unless you want the corresponding right
7660c0ab 2753parenthesis to terminate the arguments to the print--interpose a C<+> or
5f05dabc 2754put parentheses around all the arguments.
a0d0e21e 2755
4633a7c4 2756Note that if you're storing FILEHANDLES in an array or other expression,
da0045b7 2757you will have to use a block returning its value instead:
4633a7c4
LW
2758
2759 print { $files[$i] } "stuff\n";
2760 print { $OK ? STDOUT : STDERR } "stuff\n";
2761
5f05dabc 2762=item printf FILEHANDLE FORMAT, LIST
a0d0e21e 2763
5f05dabc 2764=item printf FORMAT, LIST
a0d0e21e 2765
7660c0ab 2766Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
a3cb178b 2767(the output record separator) is not appended. The first argument
ab4f32c2 2768of the list will be interpreted as the C<printf()> format. If C<use locale> is
a034a98d
DD
2769in effect, the character used for the decimal point in formatted real numbers
2770is affected by the LC_NUMERIC locale. See L<perllocale>.
a0d0e21e 2771
7660c0ab
A
2772Don't fall into the trap of using a C<printf()> when a simple
2773C<print()> would do. The C<print()> is more efficient and less
28757baa
PP
2774error prone.
2775
da0045b7
PP
2776=item prototype FUNCTION
2777
2778Returns the prototype of a function as a string (or C<undef> if the
5f05dabc
PP
2779function has no prototype). FUNCTION is a reference to, or the name of,
2780the function whose prototype you want to retrieve.
da0045b7 2781
b6c543e3
IZ
2782If FUNCTION is a string starting with C<CORE::>, the rest is taken as
2783a name for Perl builtin. If builtin is not I<overridable> (such as
ab4f32c2
A
2784C<qw//>) or its arguments cannot be expressed by a prototype (such as
2785C<system()>) - in other words, the builtin does not behave like a Perl
b6c543e3
IZ
2786function - returns C<undef>. Otherwise, the string describing the
2787equivalent prototype is returned.
2788
a0d0e21e
LW
2789=item push ARRAY,LIST
2790
2791Treats ARRAY as a stack, and pushes the values of LIST
2792onto the end of ARRAY. The length of ARRAY increases by the length of
2793LIST. Has the same effect as
2794
2795 for $value (LIST) {
2796 $ARRAY[++$#ARRAY] = $value;
2797 }
2798
2799but is more efficient. Returns the new number of elements in the array.
2800
2801=item q/STRING/
2802
2803=item qq/STRING/
2804
8782bef2
GB
2805=item qr/STRING/
2806
a0d0e21e
LW
2807=item qx/STRING/
2808
2809=item qw/STRING/
2810
4b6a7270 2811Generalized quotes. See L<perlop/"Regexp Quote-Like Operators">.
a0d0e21e
LW
2812
2813=item quotemeta EXPR
2814
54310121 2815=item quotemeta
bbce6d69 2816
68dc0745 2817Returns the value of EXPR with all non-alphanumeric
a034a98d
DD
2818characters backslashed. (That is, all characters not matching
2819C</[A-Za-z_0-9]/> will be preceded by a backslash in the
2820returned string, regardless of any locale settings.)
2821This is the internal function implementing
7660c0ab 2822the C<\Q> escape in double-quoted strings.
a0d0e21e 2823
7660c0ab 2824If EXPR is omitted, uses C<$_>.
bbce6d69 2825
a0d0e21e
LW
2826=item rand EXPR
2827
2828=item rand
2829
7660c0ab 2830Returns a random fractional number greater than or equal to C<0> and less
3e3baf6d 2831than the value of EXPR. (EXPR should be positive.) If EXPR is
7660c0ab
A
2832omitted, the value C<1> is used. Automatically calls C<srand()> unless
2833C<srand()> has already been called. See also C<srand()>.
a0d0e21e 2834
2f9daede 2835(Note: If your rand function consistently returns numbers that are too
a0d0e21e 2836large or too small, then your version of Perl was probably compiled
2f9daede 2837with the wrong number of RANDBITS.)
a0d0e21e
LW
2838
2839=item read FILEHANDLE,SCALAR,LENGTH,OFFSET
2840
2841=item read FILEHANDLE,SCALAR,LENGTH
2842
2843Attempts to read LENGTH bytes of data into variable SCALAR from the
3b02c43c
GS
2844specified FILEHANDLE. Returns the number of bytes actually read,
2845C<0> at end of file, or undef if there was an error. SCALAR will be grown
2846or shrunk to the length actually read. An OFFSET may be specified to
2847place the read data at some other place than the beginning of the
f86cebdf
GS
2848string. This call is actually implemented in terms of stdio's fread(3)
2849call. To get a true read(2) system call, see C<sysread()>.
a0d0e21e
LW
2850
2851=item readdir DIRHANDLE
2852
7660c0ab 2853Returns the next directory entry for a directory opened by C<opendir()>.
5a964f20 2854If used in list context, returns all the rest of the entries in the
a0d0e21e 2855directory. If there are no more entries, returns an undefined value in
5a964f20 2856scalar context or a null list in list context.
a0d0e21e 2857
7660c0ab 2858If you're planning to filetest the return values out of a C<readdir()>, you'd
5f05dabc 2859better prepend the directory in question. Otherwise, because we didn't
7660c0ab 2860C<chdir()> there, it would have been testing the wrong file.
cb1a09d0
AD
2861
2862 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
2863 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
2864 closedir DIR;
2865
84902520
TB
2866=item readline EXPR
2867
5a964f20 2868Reads from the filehandle whose typeglob is contained in EXPR. In scalar context, a single line
84902520
TB
2869is read and returned. In list context, reads until end-of-file is
2870reached and returns a list of lines (however you've defined lines
7660c0ab 2871with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
84902520
TB
2872This is the internal function implementing the C<E<lt>EXPRE<gt>>
2873operator, but you can use it directly. The C<E<lt>EXPRE<gt>>
2874operator is discussed in more detail in L<perlop/"I/O Operators">.
2875
5a964f20
TC
2876 $line = <STDIN>;
2877 $line = readline(*STDIN); # same thing
2878
a0d0e21e
LW
2879=item readlink EXPR
2880
54310121 2881=item readlink
bbce6d69 2882
a0d0e21e
LW
2883Returns the value of a symbolic link, if symbolic links are
2884implemented. If not, gives a fatal error. If there is some system
184e9718 2885error, returns the undefined value and sets C<$!> (errno). If EXPR is
7660c0ab 2886omitted, uses C<$_>.
a0d0e21e 2887
84902520
TB
2888=item readpipe EXPR
2889
5a964f20 2890EXPR is executed as a system command.
84902520
TB
2891The collected standard output of the command is returned.
2892In scalar context, it comes back as a single (potentially
2893multi-line) string. In list context, returns a list of lines
7660c0ab 2894(however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
84902520
TB
2895This is the internal function implementing the C<qx/EXPR/>
2896operator, but you can use it directly. The C<qx/EXPR/>
2897operator is discussed in more detail in L<perlop/"I/O Operators">.
2898
a0d0e21e
LW
2899=item recv SOCKET,SCALAR,LEN,FLAGS
2900
2901Receives a message on a socket. Attempts to receive LENGTH bytes of
2902data into variable SCALAR from the specified SOCKET filehandle.
7660c0ab 2903Actually does a C C<recvfrom()>, so that it can return the address of the
a0d0e21e
LW
2904sender. Returns the undefined value if there's an error. SCALAR will
2905be grown or shrunk to the length actually read. Takes the same flags
54310121 2906as the system call of the same name.
4633a7c4 2907See L<perlipc/"UDP: Message Passing"> for examples.
a0d0e21e
LW
2908
2909=item redo LABEL
2910
2911=item redo
2912
2913The C<redo> command restarts the loop block without evaluating the
a189bff9 2914conditional again. The L</continue> block, if any, is not executed. If
a0d0e21e
LW
2915the LABEL is omitted, the command refers to the innermost enclosing
2916loop. This command is normally used by programs that want to lie to
2917themselves about what was just input:
2918
2919 # a simpleminded Pascal comment stripper
2920 # (warning: assumes no { or } in strings)
4633a7c4 2921 LINE: while (<STDIN>) {
a0d0e21e
LW
2922 while (s|({.*}.*){.*}|$1 |) {}
2923 s|{.*}| |;
2924 if (s|{.*| |) {
2925 $front = $_;
2926 while (<STDIN>) {
2927 if (/}/) { # end of comment?
5a964f20 2928 s|^|$front\{|;
4633a7c4 2929 redo LINE;
a0d0e21e
LW
2930 }
2931 }
2932 }
2933 print;
2934 }
2935
4968c1e4
G
2936C<redo> cannot be used to retry a block which returns a value such as
2937C<eval {}>, C<sub {}> or C<do {}>.
2938
a189bff9 2939See also L</continue> for an illustration of how L</last>, L</next>, and
1d2dff63
GS
2940C<redo> work.
2941
a189bff9
JH
2942See also L<perlsyn>.
2943
a0d0e21e
LW
2944=item ref EXPR
2945
54310121 2946=item ref
bbce6d69 2947
2f9daede 2948Returns a TRUE value if EXPR is a reference, FALSE otherwise. If EXPR
7660c0ab 2949is not specified, C<$_> will be used. The value returned depends on the
bbce6d69 2950type of thing the reference is a reference to.
a0d0e21e
LW
2951Builtin types include:
2952
2953 REF
2954 SCALAR
2955 ARRAY
2956 HASH
2957 CODE
2958 GLOB
2959
54310121 2960If the referenced object has been blessed into a package, then that package
7660c0ab 2961name is returned instead. You can think of C<ref()> as a C<typeof()> operator.
a0d0e21e
LW
2962
2963 if (ref($r) eq "HASH") {
aa689395 2964 print "r is a reference to a hash.\n";
54310121 2965 }
5a964f20 2966 if (!ref($r)) {
a0d0e21e 2967 print "r is not a reference at all.\n";
54310121 2968 }
a0d0e21e
LW
2969
2970See also L<perlref>.
2971
2972=item rename OLDNAME,NEWNAME
2973
7660c0ab 2974Changes the name of a file. Returns C<1> for success, C<0> otherwise. Will
5f05dabc 2975not work across file system boundaries.
a0d0e21e
LW
2976
2977=item require EXPR
2978
2979=item require
2980
7660c0ab 2981Demands some semantics specified by EXPR, or by C<$_> if EXPR is not
a0d0e21e 2982supplied. If EXPR is numeric, demands that the current version of Perl
184e9718 2983(C<$]> or $PERL_VERSION) be equal or greater than EXPR.
a0d0e21e
LW
2984
2985Otherwise, demands that a library file be included if it hasn't already
2986been included. The file is included via the do-FILE mechanism, which is
7660c0ab 2987essentially just a variety of C<eval()>. Has semantics similar to the following
a0d0e21e
LW
2988subroutine:
2989
2990 sub require {
5a964f20 2991 my($filename) = @_;
a0d0e21e 2992 return 1 if $INC{$filename};
5a964f20 2993 my($realfilename,$result);
a0d0e21e
LW
2994 ITER: {
2995 foreach $prefix (@INC) {
2996 $realfilename = "$prefix/$filename";
2997 if (-f $realfilename) {
2998 $result = do $realfilename;
2999 last ITER;
3000 }
3001 }
3002 die "Can't find $filename in \@INC";
3003 }
3004 die $@ if $@;
3005 die "$filename did not return true value" unless $result;
3006 $INC{$filename} = $realfilename;
5a964f20 3007 return $result;
a0d0e21e
LW
3008 }
3009
3010Note that the file will not be included twice under the same specified
3011name. The file must return TRUE as the last statement to indicate
3012successful execution of any initialization code, so it's customary to
7660c0ab 3013end such a file with "C<1;>" unless you're sure it'll return TRUE
a0d0e21e
LW
3014otherwise. But it's better just to put the "C<1;>", in case you add more
3015statements.
3016
54310121 3017If EXPR is a bareword, the require assumes a "F<.pm>" extension and
da0045b7 3018replaces "F<::>" with "F</>" in the filename for you,
54310121 3019to make it easy to load standard modules. This form of loading of
a0d0e21e
LW
3020modules does not risk altering your namespace.
3021
ee580363
GS
3022In other words, if you try this:
3023
f86cebdf 3024 require Foo::Bar; # a splendid bareword
ee580363 3025
7660c0ab
A
3026The require function will actually look for the "F<Foo/Bar.pm>" file in the
3027directories specified in the C<@INC> array.
ee580363 3028
5a964f20 3029But if you try this:
ee580363
GS
3030
3031 $class = 'Foo::Bar';
f86cebdf 3032 require $class; # $class is not a bareword
5a964f20 3033 #or
f86cebdf 3034 require "Foo::Bar"; # not a bareword because of the ""
ee580363 3035
7660c0ab
A
3036The require function will look for the "F<Foo::Bar>" file in the @INC array and
3037will complain about not finding "F<Foo::Bar>" there. In this case you can do:
ee580363
GS
3038
3039 eval "require $class";
3040
3041For a yet-more-powerful import facility, see L</use> and L<perlmod>.
a0d0e21e
LW
3042
3043=item reset EXPR
3044
3045=item reset
3046
3047Generally used in a C<continue> block at the end of a loop to clear
7660c0ab 3048variables and reset C<??> searches so that they work again. The
a0d0e21e
LW
3049expression is interpreted as a list of single characters (hyphens
3050allowed for ranges). All variables and arrays beginning with one of
3051those letters are reset to their pristine state. If the expression is
7660c0ab 3052omitted, one-match searches (C<?pattern?>) are reset to match again. Resets
5f05dabc 3053only variables or searches in the current package. Always returns
a0d0e21e
LW
30541. Examples:
3055
3056 reset 'X'; # reset all X variables
3057 reset 'a-z'; # reset lower case variables
3058 reset; # just reset ?? searches
3059
7660c0ab
A
3060Resetting C<"A-Z"> is not recommended because you'll wipe out your
3061C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package variables--lexical variables
a0d0e21e 3062are unaffected, but they clean themselves up on scope exit anyway,
da0045b7 3063so you'll probably want to use them instead. See L</my>.
a0d0e21e 3064
54310121
PP
3065=item return EXPR
3066
3067=item return
3068
7660c0ab 3069Returns from a subroutine, C<eval()>, or C<do FILE> with the value
5a964f20 3070given in EXPR. Evaluation of EXPR may be in list, scalar, or void
54310121 3071context, depending on how the return value will be used, and the context
7660c0ab 3072may vary from one execution to the next (see C<wantarray()>). If no EXPR
5a964f20
TC
3073is given, returns an empty list in list context, an undefined value in
3074scalar context, or nothing in a void context.
a0d0e21e 3075
68dc0745
PP
3076(Note that in the absence of a return, a subroutine, eval, or do FILE
3077will automatically return the value of the last expression evaluated.)
a0d0e21e
LW
3078
3079=item reverse LIST
3080
5a964f20
TC
3081In list context, returns a list value consisting of the elements
3082of LIST in the opposite order. In scalar context, concatenates the
a0ed51b3
LW
3083elements of LIST, and returns a string value with all the characters
3084in the opposite order.
4633a7c4 3085
2f9daede 3086 print reverse <>; # line tac, last line first
4633a7c4 3087
2f9daede 3088 undef $/; # for efficiency of <>
a0ed51b3 3089 print scalar reverse <>; # character tac, last line tsrif
2f9daede
TPG
3090
3091This operator is also handy for inverting a hash, although there are some
3092caveats. If a value is duplicated in the original hash, only one of those
3093can be represented as a key in the inverted hash. Also, this has to
3094unwind one hash and build a whole new one, which may take some time
3095on a large hash.
3096
3097 %by_name = reverse %by_address; # Invert the hash
a0d0e21e
LW
3098
3099=item rewinddir DIRHANDLE
3100
3101Sets the current position to the beginning of the directory for the
7660c0ab 3102C<readdir()> routine on DIRHANDLE.
a0d0e21e
LW
3103
3104=item rindex STR,SUBSTR,POSITION
3105
3106=item rindex STR,SUBSTR
3107
3108Works just like index except that it returns the position of the LAST
3109occurrence of SUBSTR in STR. If POSITION is specified, returns the
3110last occurrence at or before that position.
3111
3112=item rmdir FILENAME
3113
54310121 3114=item rmdir
bbce6d69 3115
5a964f20
TC
3116Deletes the directory specified by FILENAME if that directory is empty. If it
3117succeeds it returns TRUE, otherwise it returns FALSE and sets C<$!> (errno). If
7660c0ab 3118FILENAME is omitted, uses C<$_>.
a0d0e21e
LW
3119
3120=item s///
3121
3122The substitution operator. See L<perlop>.
3123
3124=item scalar EXPR
3125
5a964f20 3126Forces EXPR to be interpreted in scalar context and returns the value
54310121 3127of EXPR.
cb1a09d0
AD
3128
3129 @counts = ( scalar @a, scalar @b, scalar @c );
3130
54310121 3131There is no equivalent operator to force an expression to
5a964f20 3132be interpolated in list context because it's in practice never
cb1a09d0
AD
3133needed. If you really wanted to do so, however, you could use
3134the construction C<@{[ (some expression) ]}>, but usually a simple
3135C<(some expression)> suffices.
a0d0e21e
LW
3136
3137=item seek FILEHANDLE,POSITION,WHENCE
3138
ab4f32c2 3139Sets FILEHANDLE's position, just like the C<fseek()> call of C<stdio()>.
8903cb82 3140FILEHANDLE may be an expression whose value gives the name of the
7660c0ab
A
3141filehandle. The values for WHENCE are C<0> to set the new position to
3142POSITION, C<1> to set it to the current position plus POSITION, and C<2> to
8903cb82 3143set it to EOF plus POSITION (typically negative). For WHENCE you may
7660c0ab
A
3144use the constants C<SEEK_SET>, C<SEEK_CUR>, and C<SEEK_END> from either the
3145C<IO::Seekable> or the POSIX module. Returns C<1> upon success, C<0> otherwise.
8903cb82 3146
7660c0ab
A
3147If you want to position file for C<sysread()> or C<syswrite()>, don't use
3148C<seek()> -- buffering makes its effect on the file's system position
3149unpredictable and non-portable. Use C<sysseek()> instead.
a0d0e21e 3150
cb1a09d0
AD
3151On some systems you have to do a seek whenever you switch between reading
3152and writing. Amongst other things, this may have the effect of calling
f86cebdf 3153stdio's clearerr(3). A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving
8903cb82 3154the file position:
cb1a09d0
AD
3155
3156 seek(TEST,0,1);
3157
3158This is also useful for applications emulating C<tail -f>. Once you hit
3159EOF on your read, and then sleep for a while, you might have to stick in a
7660c0ab 3160seek() to reset things. The C<seek()> doesn't change the current position,
8903cb82
PP
3161but it I<does> clear the end-of-file condition on the handle, so that the
3162next C<E<lt>FILEE<gt>> makes Perl try again to read something. We hope.
cb1a09d0
AD
3163
3164If that doesn't work (some stdios are particularly cantankerous), then
3165you may need something more like this:
3166
3167 for (;;) {
f86cebdf
GS
3168 for ($curpos = tell(FILE); $_ = <FILE>;
3169 $curpos = tell(FILE)) {
cb1a09d0
AD
3170 # search for some stuff and put it into files
3171 }
3172 sleep($for_a_while);
3173 seek(FILE, $curpos, 0);
3174 }
3175
a0d0e21e
LW
3176=item seekdir DIRHANDLE,POS
3177
7660c0ab
A
3178Sets the current position for the C<readdir()> routine on DIRHANDLE. POS
3179must be a value returned by C<telldir()>. Has the same caveats about
a0d0e21e
LW
3180possible directory compaction as the corresponding system library
3181routine.
3182
3183=item select FILEHANDLE
3184
3185=item select
3186
3187Returns the currently selected filehandle. Sets the current default
3188filehandle for output, if FILEHANDLE is supplied. This has two
ab4f32c2 3189effects: first, a C<write()> or a C<print()> without a filehandle will
a0d0e21e
LW
3190default to this FILEHANDLE. Second, references to variables related to
3191output will refer to this output channel. For example, if you have to
3192set the top of form format for more than one output channel, you might
3193do the following:
3194
3195 select(REPORT1);
3196 $^ = 'report1_top';
3197 select(REPORT2);
3198 $^ = 'report2_top';
3199
3200FILEHANDLE may be an expression whose value gives the name of the
3201actual filehandle. Thus:
3202
3203 $oldfh = select(STDERR); $| = 1; select($oldfh);
3204
4633a7c4
LW
3205Some programmers may prefer to think of filehandles as objects with
3206methods, preferring to write the last example as:
a0d0e21e 3207
28757baa 3208 use IO::Handle;
a0d0e21e
LW
3209 STDERR->autoflush(1);
3210
3211=item select RBITS,WBITS,EBITS,TIMEOUT
3212
f86cebdf 3213This calls the select(2) system call with the bit masks specified, which
7660c0ab 3214can be constructed using C<fileno()> and C<vec()>, along these lines:
a0d0e21e
LW
3215
3216 $rin = $win = $ein = '';
3217 vec($rin,fileno(STDIN),1) = 1;
3218 vec($win,fileno(STDOUT),1) = 1;
3219 $ein = $rin | $win;
3220
3221If you want to select on many filehandles you might wish to write a
3222subroutine:
3223
3224 sub fhbits {
5a964f20
TC
3225 my(@fhlist) = split(' ',$_[0]);
3226 my($bits);
a0d0e21e
LW
3227 for (@fhlist) {
3228 vec($bits,fileno($_),1) = 1;
3229 }
3230 $bits;
3231 }
4633a7c4 3232 $rin = fhbits('STDIN TTY SOCK');
a0d0e21e
LW
3233
3234The usual idiom is:
3235
3236 ($nfound,$timeleft) =
3237 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
3238
54310121 3239or to block until something becomes ready just do this
a0d0e21e
LW
3240
3241 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
3242
7660c0ab
A
3243Most systems do not bother to return anything useful in C<$timeleft>, so
3244calling select() in scalar context just returns C<$nfound>.
c07a80fd 3245
5f05dabc 3246Any of the bit masks can also be undef. The timeout, if specified, is
a0d0e21e 3247in seconds, which may be fractional. Note: not all implementations are
7660c0ab
A
3248capable of returning theC<$timeleft>. If not, they always return
3249C<$timeleft> equal to the supplied C<$timeout>.
a0d0e21e 3250
ff68c719 3251You can effect a sleep of 250 milliseconds this way:
a0d0e21e
LW
3252
3253 select(undef, undef, undef, 0.25);
3254
7660c0ab
A
3255B<WARNING>: One should not attempt to mix buffered I/O (like C<read()>
3256or E<lt>FHE<gt>) with C<select()>, except as permitted by POSIX, and even
3257then only on POSIX systems. You have to use C<sysread()> instead.
a0d0e21e
LW
3258
3259=item semctl ID,SEMNUM,CMD,ARG
3260
ab4f32c2 3261Calls the System V IPC function C<semctl()>. You'll probably have to say
0ade1984
JH
3262
3263 use IPC::SysV;
3264
3265first to get the correct constant definitions. If CMD is IPC_STAT or
3266GETALL, then ARG must be a variable which will hold the returned
ab4f32c2 3267semid_ds structure or semaphore value array. Returns like C<ioctl()>: the
7660c0ab
A
3268undefined value for error, "C<0> but true" for zero, or the actual return
3269value otherwise. See also C<IPC::SysV> and C<IPC::Semaphore> documentation.
a0d0e21e
LW
3270
3271=item semget KEY,NSEMS,FLAGS
3272
3273Calls the System V IPC function semget. Returns the semaphore id, or
7660c0ab
A
3274the undefined value if there is an error. See also C<IPC::SysV> and
3275C<IPC::SysV::Semaphore> documentation.
a0d0e21e
LW
3276
3277=item semop KEY,OPSTRING
3278
3279Calls the System V IPC function semop to perform semaphore operations
3280such as signaling and waiting. OPSTRING must be a packed array of
3281semop structures. Each semop structure can be generated with
3282C<pack("sss", $semnum, $semop, $semflag)>. The number of semaphore
3283operations is implied by the length of OPSTRING. Returns TRUE if
3284successful, or FALSE if there is an error. As an example, the
7660c0ab 3285following code waits on semaphore C<$semnum> of semaphore id C<$semid>:
a0d0e21e
LW
3286
3287 $semop = pack("sss", $semnum, -1, 0);
3288 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
3289
7660c0ab
A
3290To signal the semaphore, replace C<-1> with C<1>. See also C<IPC::SysV>
3291and C<IPC::SysV::Semaphore> documentation.
a0d0e21e
LW
3292
3293=item send SOCKET,MSG,FLAGS,TO
3294
3295=item send SOCKET,MSG,FLAGS
3296
3297Sends a message on a socket. Takes the same flags as the system call
3298of the same name. On unconnected sockets you must specify a
7660c0ab 3299destination to send TO, in which case it does a C C<sendto()>. Returns