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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
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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
22fae026
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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.
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456
457=item caller EXPR
458
459=item caller
460
5a964f20 461Returns the context of the current subroutine call. In scalar context,
28757baa 462returns the caller's package name if there is a caller, that is, if
7660c0ab 463we're in a subroutine or C<eval()> or C<require()>, and the undefined value
5a964f20 464otherwise. In list context, returns
a0d0e21e 465
748a9306 466 ($package, $filename, $line) = caller;
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467
468With EXPR, it returns some extra information that the debugger uses to
469print a stack trace. The value of EXPR indicates how many call frames
470to go back before the current one.
471
54310121 472 ($package, $filename, $line, $subroutine,
e7ea3e70
IZ
473 $hasargs, $wantarray, $evaltext, $is_require) = caller($i);
474
7660c0ab 475Here C<$subroutine> may be C<"(eval)"> if the frame is not a subroutine
ab4f32c2 476call, but an C<eval()>. In such a case additional elements C<$evaltext> and
7660c0ab
A
477C<$is_require> are set: C<$is_require> is true if the frame is created by a
478C<require> or C<use> statement, C<$evaltext> contains the text of the
dc848c6f 479C<eval EXPR> statement. In particular, for a C<eval BLOCK> statement,
7660c0ab 480C<$filename> is C<"(eval)">, but C<$evaltext> is undefined. (Note also that
dc848c6f
PP
481each C<use> statement creates a C<require> frame inside an C<eval EXPR>)
482frame.
748a9306
LW
483
484Furthermore, when called from within the DB package, caller returns more
7660c0ab 485detailed information: it sets the list variable C<@DB::args> to be the
54310121 486arguments with which the subroutine was invoked.
748a9306 487
7660c0ab 488Be aware that the optimizer might have optimized call frames away before
ab4f32c2 489C<caller()> had a chance to get the information. That means that C<caller(N)>
7660c0ab
A
490might not return information about the call frame you expect it do, for
491C<N E<gt> 1>. In particular, C<@DB::args> might have information from the
492previous time C<caller()> was called.
493
a0d0e21e
LW
494=item chdir EXPR
495
496Changes the working directory to EXPR, if possible. If EXPR is
497omitted, changes to home directory. Returns TRUE upon success, FALSE
7660c0ab 498otherwise. See example under C<die()>.
a0d0e21e
LW
499
500=item chmod LIST
501
502Changes the permissions of a list of files. The first element of the
4633a7c4 503list must be the numerical mode, which should probably be an octal
2f9daede
TPG
504number, and which definitely should I<not> a string of octal digits:
505C<0644> is okay, C<'0644'> is not. Returns the number of files
dc848c6f 506successfully changed. See also L</oct>, if all you have is a string.
a0d0e21e
LW
507
508 $cnt = chmod 0755, 'foo', 'bar';
509 chmod 0755, @executables;
f86cebdf
GS
510 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
511 # --w----r-T
2f9daede
TPG
512 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
513 $mode = 0644; chmod $mode, 'foo'; # this is best
a0d0e21e
LW
514
515=item chomp VARIABLE
516
517=item chomp LIST
518
519=item chomp
520
3e3baf6d 521This is a slightly safer version of L</chop>. It removes any
a0d0e21e 522line ending that corresponds to the current value of C<$/> (also known as
28757baa
PP
523$INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
524number of characters removed from all its arguments. It's often used to
525remove the newline from the end of an input record when you're worried
526that the final record may be missing its newline. When in paragraph mode
527(C<$/ = "">), it removes all trailing newlines from the string. If
7660c0ab 528VARIABLE is omitted, it chomps C<$_>. Example:
a0d0e21e
LW
529
530 while (<>) {
531 chomp; # avoid \n on last field
532 @array = split(/:/);
5a964f20 533 # ...
a0d0e21e
LW
534 }
535
536You can actually chomp anything that's an lvalue, including an assignment:
537
538 chomp($cwd = `pwd`);
539 chomp($answer = <STDIN>);
540
541If you chomp a list, each element is chomped, and the total number of
542characters removed is returned.
543
544=item chop VARIABLE
545
546=item chop LIST
547
548=item chop
549
550Chops off the last character of a string and returns the character
551chopped. It's used primarily to remove the newline from the end of an
552input record, but is much more efficient than C<s/\n//> because it neither
7660c0ab 553scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
a0d0e21e
LW
554Example:
555
556 while (<>) {
557 chop; # avoid \n on last field
558 @array = split(/:/);
5a964f20 559 #...
a0d0e21e
LW
560 }
561
562You can actually chop anything that's an lvalue, including an assignment:
563
564 chop($cwd = `pwd`);
565 chop($answer = <STDIN>);
566
567If you chop a list, each element is chopped. Only the value of the
ab4f32c2 568last C<chop()> is returned.
a0d0e21e 569
ab4f32c2 570Note that C<chop()> returns the last character. To return all but the last
748a9306
LW
571character, use C<substr($string, 0, -1)>.
572
a0d0e21e
LW
573=item chown LIST
574
575Changes the owner (and group) of a list of files. The first two
576elements of the list must be the I<NUMERICAL> uid and gid, in that order.
577Returns the number of files successfully changed.
578
579 $cnt = chown $uid, $gid, 'foo', 'bar';
580 chown $uid, $gid, @filenames;
581
54310121 582Here's an example that looks up nonnumeric uids in the passwd file:
a0d0e21e
LW
583
584 print "User: ";
585 chop($user = <STDIN>);
5a964f20 586 print "Files: ";
a0d0e21e
LW
587 chop($pattern = <STDIN>);
588
589 ($login,$pass,$uid,$gid) = getpwnam($user)
590 or die "$user not in passwd file";
591
5a964f20 592 @ary = glob($pattern); # expand filenames
a0d0e21e
LW
593 chown $uid, $gid, @ary;
594
54310121 595On most systems, you are not allowed to change the ownership of the
4633a7c4
LW
596file unless you're the superuser, although you should be able to change
597the group to any of your secondary groups. On insecure systems, these
598restrictions may be relaxed, but this is not a portable assumption.
599
a0d0e21e
LW
600=item chr NUMBER
601
54310121 602=item chr
bbce6d69 603
a0d0e21e 604Returns the character represented by that NUMBER in the character set.
a0ed51b3
LW
605For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
606chr(0x263a) is a Unicode smiley face (but only within the scope of a
607C<use utf8>). For the reverse, use L</ord>.
a0d0e21e 608
7660c0ab 609If NUMBER is omitted, uses C<$_>.
bbce6d69 610
a0d0e21e
LW
611=item chroot FILENAME
612
54310121 613=item chroot
bbce6d69 614
5a964f20 615This function works like the system call by the same name: it makes the
4633a7c4 616named directory the new root directory for all further pathnames that
7660c0ab 617begin with a C<"/"> by your process and all its children. (It doesn't
28757baa 618change your current working directory, which is unaffected.) For security
4633a7c4 619reasons, this call is restricted to the superuser. If FILENAME is
ab4f32c2 620omitted, does a C<chroot()> to C<$_>.
a0d0e21e
LW
621
622=item close FILEHANDLE
623
6a518fbc
TP
624=item close
625
a0d0e21e
LW
626Closes the file or pipe associated with the file handle, returning TRUE
627only if stdio successfully flushes buffers and closes the system file
6a518fbc
TP
628descriptor. Closes the currently selected filehandle if the argument
629is omitted.
fb73857a
PP
630
631You don't have to close FILEHANDLE if you are immediately going to do
7660c0ab 632another C<open()> on it, because C<open()> will close it for you. (See
ab4f32c2 633C<open()>.) However, an explicit C<close()> on an input file resets the line
7660c0ab 634counter (C<$.>), while the implicit close done by C<open()> does not.
fb73857a 635
ab4f32c2 636If the file handle came from a piped open C<close()> will additionally
fb73857a
PP
637return FALSE if one of the other system calls involved fails or if the
638program exits with non-zero status. (If the only problem was that the
7660c0ab 639program exited non-zero C<$!> will be set to C<0>.) Also, closing a pipe
5a964f20 640waits for the process executing on the pipe to complete, in case you
fb73857a
PP
641want to look at the output of the pipe afterwards. Closing a pipe
642explicitly also puts the exit status value of the command into C<$?>.
5a964f20 643
fb73857a 644Example:
a0d0e21e 645
fb73857a
PP
646 open(OUTPUT, '|sort >foo') # pipe to sort
647 or die "Can't start sort: $!";
5a964f20 648 #... # print stuff to output
fb73857a
PP
649 close OUTPUT # wait for sort to finish
650 or warn $! ? "Error closing sort pipe: $!"
651 : "Exit status $? from sort";
652 open(INPUT, 'foo') # get sort's results
653 or die "Can't open 'foo' for input: $!";
a0d0e21e 654
5a964f20
TC
655FILEHANDLE may be an expression whose value can be used as an indirect
656filehandle, usually the real filehandle name.
a0d0e21e
LW
657
658=item closedir DIRHANDLE
659
7660c0ab 660Closes a directory opened by C<opendir()> and returns the success of that
5a964f20
TC
661system call.
662
663DIRHANDLE may be an expression whose value can be used as an indirect
664dirhandle, usually the real dirhandle name.
a0d0e21e
LW
665
666=item connect SOCKET,NAME
667
668Attempts to connect to a remote socket, just as the connect system call
669does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
4633a7c4
LW
670packed address of the appropriate type for the socket. See the examples in
671L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 672
cb1a09d0
AD
673=item continue BLOCK
674
675Actually a flow control statement rather than a function. If there is a
dc9e4912 676C<continue> BLOCK attached to a BLOCK (typically in a L</while> or
a189bff9
JH
677L</foreach>), it is always executed just before the conditional is about to
678be evaluated again, just like the third part of a L</for> loop in C. Thus
cb1a09d0
AD
679it can be used to increment a loop variable, even when the loop has been
680continued via the C<next> statement (which is similar to the C C<continue>
681statement).
682
a189bff9 683L</last>, L</next>, or L</redo> may appear within a C<continue>
1d2dff63
GS
684block. C<last> and C<redo> will behave as if they had been executed within
685the main block. So will C<next>, but since it will execute a C<continue>
686block, it may be more entertaining.
687
688 while (EXPR) {
689 ### redo always comes here
690 do_something;
691 } continue {
692 ### next always comes here
693 do_something_else;
694 # then back the top to re-check EXPR
695 }
696 ### last always comes here
697
698Omitting the C<continue> section is semantically equivalent to using an
699empty one, logically enough. In that case, C<next> goes directly back
700to check the condition at the top of the loop.
701
a189bff9
JH
702See also L<perlsyn>.
703
a0d0e21e
LW
704=item cos EXPR
705
5a964f20 706Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
7660c0ab 707takes cosine of C<$_>.
a0d0e21e 708
7660c0ab 709For the inverse cosine operation, you may use the C<POSIX::acos()>
28757baa
PP
710function, or use this relation:
711
712 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
713
a0d0e21e
LW
714=item crypt PLAINTEXT,SALT
715
f86cebdf 716Encrypts a string exactly like the crypt(3) function in the C library
4633a7c4
LW
717(assuming that you actually have a version there that has not been
718extirpated as a potential munition). This can prove useful for checking
719the password file for lousy passwords, amongst other things. Only the
720guys wearing white hats should do this.
a0d0e21e 721
7660c0ab 722Note that C<crypt()> is intended to be a one-way function, much like breaking
11155c91
CS
723eggs to make an omelette. There is no (known) corresponding decrypt
724function. As a result, this function isn't all that useful for
725cryptography. (For that, see your nearby CPAN mirror.)
2f9daede 726
e71965be
RS
727When verifying an existing encrypted string you should use the encrypted
728text as the salt (like C<crypt($plain, $crypted) eq $crypted>). This
729allows your code to work with the standard C<crypt()> and with more
730exotic implementations. When choosing a new salt create a random two
731character string whose characters come from the set C<[./0-9A-Za-z]>
732(like C<join '', ('.', '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>).
733
a0d0e21e
LW
734Here's an example that makes sure that whoever runs this program knows
735their own password:
736
737 $pwd = (getpwuid($<))[1];
a0d0e21e
LW
738
739 system "stty -echo";
740 print "Password: ";
e71965be 741 chomp($word = <STDIN>);
a0d0e21e
LW
742 print "\n";
743 system "stty echo";
744
e71965be 745 if (crypt($word, $pwd) ne $pwd) {
a0d0e21e
LW
746 die "Sorry...\n";
747 } else {
748 print "ok\n";
54310121 749 }
a0d0e21e 750
9f8f0c9d 751Of course, typing in your own password to whoever asks you
748a9306 752for it is unwise.
a0d0e21e 753
aa689395 754=item dbmclose HASH
a0d0e21e 755
7660c0ab 756[This function has been superseded by the C<untie()> function.]
a0d0e21e 757
aa689395 758Breaks the binding between a DBM file and a hash.
a0d0e21e 759
aa689395 760=item dbmopen HASH,DBNAME,MODE
a0d0e21e 761
7660c0ab 762[This function has been superseded by the C<tie()> function.]
a0d0e21e 763
7b8d334a 764This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
ab4f32c2 765hash. HASH is the name of the hash. (Unlike normal C<open()>, the first
aa689395
PP
766argument is I<NOT> a filehandle, even though it looks like one). DBNAME
767is the name of the database (without the F<.dir> or F<.pag> extension if
768any). If the database does not exist, it is created with protection
7660c0ab
A
769specified by MODE (as modified by the C<umask()>). If your system supports
770only the older DBM functions, you may perform only one C<dbmopen()> in your
aa689395 771program. In older versions of Perl, if your system had neither DBM nor
7660c0ab 772ndbm, calling C<dbmopen()> produced a fatal error; it now falls back to
aa689395
PP
773sdbm(3).
774
775If you don't have write access to the DBM file, you can only read hash
776variables, not set them. If you want to test whether you can write,
7660c0ab 777either use file tests or try setting a dummy hash entry inside an C<eval()>,
aa689395 778which will trap the error.
a0d0e21e 779
7660c0ab
A
780Note that functions such as C<keys()> and C<values()> may return huge lists
781when used on large DBM files. You may prefer to use the C<each()>
a0d0e21e
LW
782function to iterate over large DBM files. Example:
783
784 # print out history file offsets
785 dbmopen(%HIST,'/usr/lib/news/history',0666);
786 while (($key,$val) = each %HIST) {
787 print $key, ' = ', unpack('L',$val), "\n";
788 }
789 dbmclose(%HIST);
790
cb1a09d0 791See also L<AnyDBM_File> for a more general description of the pros and
184e9718 792cons of the various dbm approaches, as well as L<DB_File> for a particularly
cb1a09d0 793rich implementation.
4633a7c4 794
a0d0e21e
LW
795=item defined EXPR
796
54310121 797=item defined
bbce6d69 798
2f9daede
TPG
799Returns a Boolean value telling whether EXPR has a value other than
800the undefined value C<undef>. If EXPR is not present, C<$_> will be
801checked.
802
803Many operations return C<undef> to indicate failure, end of file,
804system error, uninitialized variable, and other exceptional
805conditions. This function allows you to distinguish C<undef> from
806other values. (A simple Boolean test will not distinguish among
7660c0ab 807C<undef>, zero, the empty string, and C<"0">, which are all equally
2f9daede 808false.) Note that since C<undef> is a valid scalar, its presence
7660c0ab 809doesn't I<necessarily> indicate an exceptional condition: C<pop()>
2f9daede
TPG
810returns C<undef> when its argument is an empty array, I<or> when the
811element to return happens to be C<undef>.
812
7660c0ab 813You may also use C<defined()> to check whether a subroutine exists, by
5a964f20 814saying C<defined &func> without parentheses. On the other hand, use
7660c0ab 815of C<defined()> upon aggregates (hashes and arrays) is not guaranteed to
5a964f20 816produce intuitive results, and should probably be avoided.
2f9daede
TPG
817
818When used on a hash element, it tells you whether the value is defined,
dc848c6f 819not whether the key exists in the hash. Use L</exists> for the latter
2f9daede 820purpose.
a0d0e21e
LW
821
822Examples:
823
824 print if defined $switch{'D'};
825 print "$val\n" while defined($val = pop(@ary));
826 die "Can't readlink $sym: $!"
827 unless defined($value = readlink $sym);
a0d0e21e 828 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
2f9daede 829 $debugging = 0 unless defined $debugging;
a0d0e21e 830
7660c0ab
A
831Note: Many folks tend to overuse C<defined()>, and then are surprised to
832discover that the number C<0> and C<""> (the zero-length string) are, in fact,
2f9daede 833defined values. For example, if you say
a5f75d66
AD
834
835 "ab" =~ /a(.*)b/;
836
7660c0ab 837The pattern match succeeds, and C<$1> is defined, despite the fact that it
a5f75d66 838matched "nothing". But it didn't really match nothing--rather, it
7660c0ab 839matched something that happened to be C<0> characters long. This is all
a5f75d66 840very above-board and honest. When a function returns an undefined value,
2f9daede 841it's an admission that it couldn't give you an honest answer. So you
7660c0ab
A
842should use C<defined()> only when you're questioning the integrity of what
843you're trying to do. At other times, a simple comparison to C<0> or C<""> is
2f9daede
TPG
844what you want.
845
7660c0ab 846Currently, using C<defined()> on an entire array or hash reports whether
2f9daede
TPG
847memory for that aggregate has ever been allocated. So an array you set
848to the empty list appears undefined initially, and one that once was full
849and that you then set to the empty list still appears defined. You
850should instead use a simple test for size:
28757baa
PP
851
852 if (@an_array) { print "has array elements\n" }
853 if (%a_hash) { print "has hash members\n" }
854
7660c0ab 855Using C<undef()> on these, however, does clear their memory and then report
5a964f20 856them as not defined anymore, but you shouldn't do that unless you don't
28757baa 857plan to use them again, because it saves time when you load them up
5a964f20
TC
858again to have memory already ready to be filled. The normal way to
859free up space used by an aggregate is to assign the empty list.
28757baa 860
7660c0ab 861This counterintuitive behavior of C<defined()> on aggregates may be
28757baa
PP
862changed, fixed, or broken in a future release of Perl.
863
dc848c6f 864See also L</undef>, L</exists>, L</ref>.
2f9daede 865
a0d0e21e
LW
866=item delete EXPR
867
aa689395
PP
868Deletes the specified key(s) and their associated values from a hash.
869For each key, returns the deleted value associated with that key, or
870the undefined value if there was no such key. Deleting from C<$ENV{}>
871modifies the environment. Deleting from a hash tied to a DBM file
7660c0ab 872deletes the entry from the DBM file. (But deleting from a C<tie()>d hash
5f05dabc 873doesn't necessarily return anything.)
a0d0e21e 874
aa689395 875The following deletes all the values of a hash:
a0d0e21e 876
5f05dabc
PP
877 foreach $key (keys %HASH) {
878 delete $HASH{$key};
a0d0e21e
LW
879 }
880
5f05dabc
PP
881And so does this:
882
883 delete @HASH{keys %HASH}
884
5a964f20 885(But both of these are slower than just assigning the empty list, or
7660c0ab 886using C<undef()>.) Note that the EXPR can be arbitrarily complicated as
5a964f20 887long as the final operation is a hash element lookup or hash slice:
a0d0e21e
LW
888
889 delete $ref->[$x][$y]{$key};
5f05dabc 890 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
a0d0e21e
LW
891
892=item die LIST
893
7660c0ab
A
894Outside an C<eval()>, prints the value of LIST to C<STDERR> and exits with
895the current value of C<$!> (errno). If C<$!> is C<0>, exits with the value of
54310121 896C<($? E<gt>E<gt> 8)> (backtick `command` status). If C<($? E<gt>E<gt> 8)>
7660c0ab
A
897is C<0>, exits with C<255>. Inside an C<eval(),> the error message is stuffed into
898C<$@> and the C<eval()> is terminated with the undefined value. This makes
899C<die()> the way to raise an exception.
a0d0e21e
LW
900
901Equivalent examples:
902
903 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
54310121 904 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
a0d0e21e
LW
905
906If the value of EXPR does not end in a newline, the current script line
907number and input line number (if any) are also printed, and a newline
7660c0ab
A
908is supplied. Hint: sometimes appending C<", stopped"> to your message
909will cause it to make better sense when the string C<"at foo line 123"> is
a0d0e21e
LW
910appended. Suppose you are running script "canasta".
911
912 die "/etc/games is no good";
913 die "/etc/games is no good, stopped";
914
915produce, respectively
916
917 /etc/games is no good at canasta line 123.
918 /etc/games is no good, stopped at canasta line 123.
919
7660c0ab 920See also C<exit()> and C<warn()>.
a0d0e21e 921
7660c0ab
A
922If LIST is empty and C<$@> already contains a value (typically from a
923previous eval) that value is reused after appending C<"\t...propagated">.
fb73857a
PP
924This is useful for propagating exceptions:
925
926 eval { ... };
927 die unless $@ =~ /Expected exception/;
928
7660c0ab 929If C<$@> is empty then the string C<"Died"> is used.
fb73857a 930
7660c0ab 931You can arrange for a callback to be run just before the C<die()> does
774d564b
PP
932its deed, by setting the C<$SIG{__DIE__}> hook. The associated handler
933will be called with the error text and can change the error message, if
7660c0ab 934it sees fit, by calling C<die()> again. See L<perlvar/$SIG{expr}> for details on
fb73857a
PP
935setting C<%SIG> entries, and L<"eval BLOCK"> for some examples.
936
937Note that the C<$SIG{__DIE__}> hook is called even inside eval()ed
938blocks/strings. If one wants the hook to do nothing in such
939situations, put
940
941 die @_ if $^S;
942
943as the first line of the handler (see L<perlvar/$^S>).
774d564b 944
a0d0e21e
LW
945=item do BLOCK
946
947Not really a function. Returns the value of the last command in the
948sequence of commands indicated by BLOCK. When modified by a loop
a189bff9
JH
949modifier such as L</while> or L</until>, executes the BLOCK once
950before testing the loop condition. (On other statements the loop
951modifiers test the conditional first.)
a0d0e21e 952
4968c1e4 953C<do BLOCK> does I<not> count as a loop, so the loop control statements
a189bff9 954L</next>, L</last> or L</redo> cannot be used to leave or restart the block.
4968c1e4 955
a0d0e21e
LW
956=item do SUBROUTINE(LIST)
957
958A deprecated form of subroutine call. See L<perlsub>.
959
960=item do EXPR
961
962Uses the value of EXPR as a filename and executes the contents of the
963file as a Perl script. Its primary use is to include subroutines
964from a Perl subroutine library.
965
966 do 'stat.pl';
967
968is just like
969
fb73857a 970 scalar eval `cat stat.pl`;
a0d0e21e 971
5a964f20 972except that it's more efficient and concise, keeps track of the
a0d0e21e
LW
973current filename for error messages, and searches all the B<-I>
974libraries if the file isn't in the current directory (see also the @INC
dc1be6b5
GS
975array in L<perlvar/Predefined Names>). It is also different in how
976code evaluated with C<do FILENAME> doesn't see lexicals in the enclosing
977scope like C<eval STRING> does. It's the same, however, in that it does
54310121 978reparse the file every time you call it, so you probably don't want to
a0d0e21e
LW
979do this inside a loop.
980
8e30cc93
G
981If C<do> cannot read the file, it returns undef and sets C<$!> to the
982error. If C<do> can read the file but cannot compile it, it
983returns undef and sets an error message in C<$@>. If the file is
984successfully compiled, C<do> returns the value of the last expression
985evaluated.
986
a0d0e21e 987Note that inclusion of library modules is better done with the
7660c0ab 988C<use()> and C<require()> operators, which also do automatic error checking
4633a7c4 989and raise an exception if there's a problem.
a0d0e21e 990
5a964f20
TC
991You might like to use C<do> to read in a program configuration
992file. Manual error checking can be done this way:
993
994 # read in config files: system first, then user
f86cebdf
GS
995 for $file ("/share/prog/defaults.rc",
996 "$ENV{HOME}/.someprogrc") {
5a964f20 997 unless ($return = do $file) {
f86cebdf
GS
998 warn "couldn't parse $file: $@" if $@;
999 warn "couldn't do $file: $!" unless defined $return;
1000 warn "couldn't run $file" unless $return;
5a964f20
TC
1001 }
1002 }
1003
a0d0e21e
LW
1004=item dump LABEL
1005
1006This causes an immediate core dump. Primarily this is so that you can
1007use the B<undump> program to turn your core dump into an executable binary
1008after having initialized all your variables at the beginning of the
1009program. When the new binary is executed it will begin by executing a
1010C<goto LABEL> (with all the restrictions that C<goto> suffers). Think of
f86cebdf 1011it as a goto with an intervening core dump and reincarnation. If C<LABEL>
5a964f20 1012is omitted, restarts the program from the top. WARNING: Any files
a0d0e21e
LW
1013opened at the time of the dump will NOT be open any more when the
1014program is reincarnated, with possible resulting confusion on the part
1015of Perl. See also B<-u> option in L<perlrun>.
1016
1017Example:
1018
1019 #!/usr/bin/perl
1020 require 'getopt.pl';
1021 require 'stat.pl';
1022 %days = (
1023 'Sun' => 1,
1024 'Mon' => 2,
1025 'Tue' => 3,
1026 'Wed' => 4,
1027 'Thu' => 5,
1028 'Fri' => 6,
1029 'Sat' => 7,
1030 );
1031
1032 dump QUICKSTART if $ARGV[0] eq '-d';
1033
1034 QUICKSTART:
1035 Getopt('f');
1036
5a964f20
TC
1037This operator is largely obsolete, partly because it's very hard to
1038convert a core file into an executable, and because the real perl-to-C
1039compiler has superseded it.
1040
aa689395
PP
1041=item each HASH
1042
5a964f20 1043When called in list context, returns a 2-element list consisting of the
aa689395 1044key and value for the next element of a hash, so that you can iterate over
5a964f20 1045it. When called in scalar context, returns the key for only the "next"
7660c0ab 1046element in the hash. (Note: Keys may be C<"0"> or C<"">, which are logically
2f9daede
TPG
1047false; you may wish to avoid constructs like C<while ($k = each %foo) {}>
1048for this reason.)
1049
1050Entries are returned in an apparently random order. When the hash is
1051entirely read, a null array is returned in list context (which when
7660c0ab
A
1052assigned produces a FALSE (C<0>) value), and C<undef> in
1053scalar context. The next call to C<each()> after that will start iterating
1054again. There is a single iterator for each hash, shared by all C<each()>,
1055C<keys()>, and C<values()> function calls in the program; it can be reset by
2f9daede
TPG
1056reading all the elements from the hash, or by evaluating C<keys HASH> or
1057C<values HASH>. If you add or delete elements of a hash while you're
1058iterating over it, you may get entries skipped or duplicated, so don't.
aa689395 1059
f86cebdf 1060The following prints out your environment like the printenv(1) program,
aa689395 1061only in a different order:
a0d0e21e
LW
1062
1063 while (($key,$value) = each %ENV) {
1064 print "$key=$value\n";
1065 }
1066
7660c0ab 1067See also C<keys()> and C<values()>.
a0d0e21e 1068
a189bff9
JH
1069=item else BLOCK
1070
1071=item elsif (EXPR) BLOCK
1072
1073See L</if>.
1074
a0d0e21e
LW
1075=item eof FILEHANDLE
1076
4633a7c4
LW
1077=item eof ()
1078
a0d0e21e
LW
1079=item eof
1080
1081Returns 1 if the next read on FILEHANDLE will return end of file, or if
1082FILEHANDLE is not open. FILEHANDLE may be an expression whose value
5a964f20 1083gives the real filehandle. (Note that this function actually
7660c0ab 1084reads a character and then C<ungetc()>s it, so isn't very useful in an
748a9306
LW
1085interactive context.) Do not read from a terminal file (or call
1086C<eof(FILEHANDLE)> on it) after end-of-file is reached. Filetypes such
1087as terminals may lose the end-of-file condition if you do.
1088
1089An C<eof> without an argument uses the last file read as argument.
5a964f20 1090Using C<eof()> with empty parentheses is very different. It indicates the pseudo file formed of
2f9daede
TPG
1091the files listed on the command line, i.e., C<eof()> is reasonable to
1092use inside a C<while (E<lt>E<gt>)> loop to detect the end of only the
1093last file. Use C<eof(ARGV)> or eof without the parentheses to test
1094I<EACH> file in a while (E<lt>E<gt>) loop. Examples:
a0d0e21e 1095
748a9306
LW
1096 # reset line numbering on each input file
1097 while (<>) {
5a964f20 1098 next if /^\s*#/; # skip comments
748a9306 1099 print "$.\t$_";
5a964f20
TC
1100 } continue {
1101 close ARGV if eof; # Not eof()!
748a9306
LW
1102 }
1103
a0d0e21e
LW
1104 # insert dashes just before last line of last file
1105 while (<>) {
5a964f20 1106 if (eof()) { # check for end of current file
a0d0e21e 1107 print "--------------\n";
748a9306
LW
1108 close(ARGV); # close or break; is needed if we
1109 # are reading from the terminal
a0d0e21e
LW
1110 }
1111 print;
1112 }
1113
a0d0e21e 1114Practical hint: you almost never need to use C<eof> in Perl, because the
3b02c43c
GS
1115input operators return false values when they run out of data, or if there
1116was an error.
a0d0e21e
LW
1117
1118=item eval EXPR
1119
1120=item eval BLOCK
1121
c7cc6f1c
GS
1122In the first form, the return value of EXPR is parsed and executed as if it
1123were a little Perl program. The value of the expression (which is itself
5a964f20 1124determined within scalar context) is first parsed, and if there weren't any
c7cc6f1c 1125errors, executed in the context of the current Perl program, so that any
5f05dabc 1126variable settings or subroutine and format definitions remain afterwards.
c7cc6f1c
GS
1127Note that the value is parsed every time the eval executes. If EXPR is
1128omitted, evaluates C<$_>. This form is typically used to delay parsing
1129and subsequent execution of the text of EXPR until run time.
1130
1131In the second form, the code within the BLOCK is parsed only once--at the
1132same time the code surrounding the eval itself was parsed--and executed
1133within the context of the current Perl program. This form is typically
1134used to trap exceptions more efficiently than the first (see below), while
1135also providing the benefit of checking the code within BLOCK at compile
1136time.
1137
1138The final semicolon, if any, may be omitted from the value of EXPR or within
1139the BLOCK.
1140
1141In both forms, the value returned is the value of the last expression
5a964f20 1142evaluated inside the mini-program; a return statement may be also used, just
c7cc6f1c 1143as with subroutines. The expression providing the return value is evaluated
5a964f20 1144in void, scalar, or list context, depending on the context of the eval itself.
c7cc6f1c 1145See L</wantarray> for more on how the evaluation context can be determined.
a0d0e21e 1146
7660c0ab
A
1147If there is a syntax error or runtime error, or a C<die()> statement is
1148executed, an undefined value is returned by C<eval()>, and C<$@> is set to the
a0d0e21e 1149error message. If there was no error, C<$@> is guaranteed to be a null
7660c0ab 1150string. Beware that using C<eval()> neither silences perl from printing
c7cc6f1c
GS
1151warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1152To do either of those, you have to use the C<$SIG{__WARN__}> facility. See
1153L</warn> and L<perlvar>.
a0d0e21e 1154
7660c0ab
A
1155Note that, because C<eval()> traps otherwise-fatal errors, it is useful for
1156determining whether a particular feature (such as C<socket()> or C<symlink()>)
a0d0e21e
LW
1157is implemented. It is also Perl's exception trapping mechanism, where
1158the die operator is used to raise exceptions.
1159
1160If the code to be executed doesn't vary, you may use the eval-BLOCK
1161form to trap run-time errors without incurring the penalty of
1162recompiling each time. The error, if any, is still returned in C<$@>.
1163Examples:
1164
54310121 1165 # make divide-by-zero nonfatal
a0d0e21e
LW
1166 eval { $answer = $a / $b; }; warn $@ if $@;
1167
1168 # same thing, but less efficient
1169 eval '$answer = $a / $b'; warn $@ if $@;
1170
1171 # a compile-time error
5a964f20 1172 eval { $answer = }; # WRONG
a0d0e21e
LW
1173
1174 # a run-time error
1175 eval '$answer ='; # sets $@
1176
7660c0ab 1177When using the C<eval{}> form as an exception trap in libraries, you may
774d564b
PP
1178wish not to trigger any C<__DIE__> hooks that user code may have
1179installed. You can use the C<local $SIG{__DIE__}> construct for this
1180purpose, as shown in this example:
1181
1182 # a very private exception trap for divide-by-zero
f86cebdf
GS
1183 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1184 warn $@ if $@;
774d564b
PP
1185
1186This is especially significant, given that C<__DIE__> hooks can call
7660c0ab 1187C<die()> again, which has the effect of changing their error messages:
774d564b
PP
1188
1189 # __DIE__ hooks may modify error messages
1190 {
f86cebdf
GS
1191 local $SIG{'__DIE__'} =
1192 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
c7cc6f1c
GS
1193 eval { die "foo lives here" };
1194 print $@ if $@; # prints "bar lives here"
774d564b
PP
1195 }
1196
7660c0ab 1197With an C<eval()>, you should be especially careful to remember what's
a0d0e21e
LW
1198being looked at when:
1199
1200 eval $x; # CASE 1
1201 eval "$x"; # CASE 2
1202
1203 eval '$x'; # CASE 3
1204 eval { $x }; # CASE 4
1205
5a964f20 1206 eval "\$$x++"; # CASE 5
a0d0e21e
LW
1207 $$x++; # CASE 6
1208
2f9daede 1209Cases 1 and 2 above behave identically: they run the code contained in
7660c0ab 1210the variable C<$x>. (Although case 2 has misleading double quotes making
2f9daede 1211the reader wonder what else might be happening (nothing is).) Cases 3
7660c0ab 1212and 4 likewise behave in the same way: they run the code C<'$x'>, which
2f9daede
TPG
1213does nothing but return the value of C<$x>. (Case 4 is preferred for
1214purely visual reasons, but it also has the advantage of compiling at
1215compile-time instead of at run-time.) Case 5 is a place where
54310121 1216normally you I<WOULD> like to use double quotes, except that in this
2f9daede
TPG
1217particular situation, you can just use symbolic references instead, as
1218in case 6.
a0d0e21e 1219
4968c1e4
G
1220C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1221C<next>, C<last> or C<redo> cannot be used to leave or restart the block.
1222
1223
a0d0e21e
LW
1224=item exec LIST
1225
8bf3b016
GS
1226=item exec PROGRAM LIST
1227
7660c0ab
A
1228The C<exec()> function executes a system command I<AND NEVER RETURNS> -
1229use C<system()> instead of C<exec()> if you want it to return. It fails and
fb73857a
PP
1230returns FALSE only if the command does not exist I<and> it is executed
1231directly instead of via your system's command shell (see below).
a0d0e21e 1232
7660c0ab
A
1233Since it's a common mistake to use C<exec()> instead of C<system()>, Perl
1234warns you if there is a following statement which isn't C<die()>, C<warn()>,
1235or C<exit()> (if C<-w> is set - but you always do that). If you
1236I<really> want to follow an C<exec()> with some other statement, you
55d729e4
GS
1237can use one of these styles to avoid the warning:
1238
5a964f20
TC
1239 exec ('foo') or print STDERR "couldn't exec foo: $!";
1240 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
55d729e4 1241
5a964f20 1242If there is more than one argument in LIST, or if LIST is an array
f86cebdf 1243with more than one value, calls execvp(3) with the arguments in LIST.
5a964f20
TC
1244If there is only one scalar argument or an array with one element in it,
1245the argument is checked for shell metacharacters, and if there are any,
1246the entire argument is passed to the system's command shell for parsing
1247(this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1248If there are no shell metacharacters in the argument, it is split into
7660c0ab
A
1249words and passed directly to C<execvp()>, which is more efficient. Note:
1250C<exec()> and C<system()> do not flush your output buffer, so you may need to
5a964f20 1251set C<$|> to avoid lost output. Examples:
a0d0e21e
LW
1252
1253 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1254 exec "sort $outfile | uniq";
1255
1256If you don't really want to execute the first argument, but want to lie
1257to the program you are executing about its own name, you can specify
1258the program you actually want to run as an "indirect object" (without a
1259comma) in front of the LIST. (This always forces interpretation of the
54310121 1260LIST as a multivalued list, even if there is only a single scalar in
a0d0e21e
LW
1261the list.) Example:
1262
1263 $shell = '/bin/csh';
1264 exec $shell '-sh'; # pretend it's a login shell
1265
1266or, more directly,
1267
1268 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1269
bb32b41a
GS
1270When the arguments get executed via the system shell, results will
1271be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1272for details.
1273
ab4f32c2 1274Using an indirect object with C<exec()> or C<system()> is also more secure.
5a964f20
TC
1275This usage forces interpretation of the arguments as a multivalued list,
1276even if the list had just one argument. That way you're safe from the
1277shell expanding wildcards or splitting up words with whitespace in them.
1278
1279 @args = ( "echo surprise" );
1280
f86cebdf
GS
1281 system @args; # subject to shell escapes
1282 # if @args == 1
5a964f20
TC
1283 system { $args[0] } @args; # safe even with one-arg list
1284
1285The first version, the one without the indirect object, ran the I<echo>
1286program, passing it C<"surprise"> an argument. The second version
1287didn't--it tried to run a program literally called I<"echo surprise">,
1288didn't find it, and set C<$?> to a non-zero value indicating failure.
1289
ab4f32c2 1290Note that C<exec()> will not call your C<END> blocks, nor will it call
7660c0ab
A
1291any C<DESTROY> methods in your objects.
1292
a0d0e21e
LW
1293=item exists EXPR
1294
1295Returns TRUE if the specified hash key exists in its hash array, even
1296if the corresponding value is undefined.
1297
1298 print "Exists\n" if exists $array{$key};
1299 print "Defined\n" if defined $array{$key};
1300 print "True\n" if $array{$key};
1301
5f05dabc 1302A hash element can be TRUE only if it's defined, and defined if
a0d0e21e
LW
1303it exists, but the reverse doesn't necessarily hold true.
1304
1305Note that the EXPR can be arbitrarily complicated as long as the final
1306operation is a hash key lookup:
1307
5a964f20
TC
1308 if (exists $ref->{"A"}{"B"}{$key}) { ... }
1309
1310Although the last element will not spring into existence just because its
1311existence was tested, intervening ones will. Thus C<$ref-E<gt>{"A"}>
1312C<$ref-E<gt>{"B"}> will spring into existence due to the existence
1313test for a $key element. This autovivification may be fixed in a later
1314release.
a0d0e21e
LW
1315
1316=item exit EXPR
1317
1318Evaluates EXPR and exits immediately with that value. (Actually, it
1319calls any defined C<END> routines first, but the C<END> routines may not
1320abort the exit. Likewise any object destructors that need to be called
1321are called before exit.) Example:
1322
1323 $ans = <STDIN>;
1324 exit 0 if $ans =~ /^[Xx]/;
1325
7660c0ab
A
1326See also C<die()>. If EXPR is omitted, exits with C<0> status. The only
1327universally portable values for EXPR are C<0> for success and C<1> for error;
f86702cc
PP
1328all other values are subject to unpredictable interpretation depending
1329on the environment in which the Perl program is running.
a0d0e21e 1330
7660c0ab
A
1331You shouldn't use C<exit()> to abort a subroutine if there's any chance that
1332someone might want to trap whatever error happened. Use C<die()> instead,
1333which can be trapped by an C<eval()>.
28757baa 1334
5a964f20
TC
1335All C<END{}> blocks are run at exit time. See L<perlsub> for details.
1336
a0d0e21e
LW
1337=item exp EXPR
1338
54310121 1339=item exp
bbce6d69 1340
54310121 1341Returns I<e> (the natural logarithm base) to the power of EXPR.
a0d0e21e
LW
1342If EXPR is omitted, gives C<exp($_)>.
1343
1344=item fcntl FILEHANDLE,FUNCTION,SCALAR
1345
f86cebdf 1346Implements the fcntl(2) function. You'll probably have to say
a0d0e21e
LW
1347
1348 use Fcntl;
1349
0ade1984 1350first to get the correct constant definitions. Argument processing and
7660c0ab 1351value return works just like C<ioctl()> below.
a0d0e21e
LW
1352For example:
1353
1354 use Fcntl;
5a964f20
TC
1355 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1356 or die "can't fcntl F_GETFL: $!";
1357
ab4f32c2
A
1358You don't have to check for C<defined()> on the return from
1359C<fnctl()>. Like C<ioctl()>, it maps a C<0> return from the system
7660c0ab
A
1360call into "C<0> but true" in Perl. This string is true in
1361boolean context and C<0> in numeric context. It is also
5a964f20
TC
1362exempt from the normal B<-w> warnings on improper numeric
1363conversions.
1364
7660c0ab 1365Note that C<fcntl()> will produce a fatal error if used on a machine that
f86cebdf 1366doesn't implement fcntl(2).
a0d0e21e
LW
1367
1368=item fileno FILEHANDLE
1369
1370Returns the file descriptor for a filehandle. This is useful for
7660c0ab 1371constructing bitmaps for C<select()> and low-level POSIX tty-handling
5a964f20
TC
1372operations. If FILEHANDLE is an expression, the value is taken as
1373an indirect filehandle, generally its name.
1374
1375You can use this to find out whether two handles refer to the
1376same underlying descriptor:
1377
1378 if (fileno(THIS) == fileno(THAT)) {
1379 print "THIS and THAT are dups\n";
1380 }
a0d0e21e
LW
1381
1382=item flock FILEHANDLE,OPERATION
1383
f86cebdf 1384Calls flock(2), or an emulation of it, on FILEHANDLE. Returns TRUE for
68dc0745 1385success, FALSE on failure. Produces a fatal error if used on a machine
f86cebdf 1386that doesn't implement flock(2), fcntl(2) locking, or lockf(3). C<flock()>
68dc0745
PP
1387is Perl's portable file locking interface, although it locks only entire
1388files, not records.
8ebc5c01 1389
a3cb178b 1390On many platforms (including most versions or clones of Unix), locks
7660c0ab 1391established by C<flock()> are B<merely advisory>. Such discretionary locks
5a964f20 1392are more flexible, but offer fewer guarantees. This means that files
7660c0ab
A
1393locked with C<flock()> may be modified by programs that do not also use
1394C<flock()>. Windows NT and OS/2 are among the platforms which
5a964f20 1395enforce mandatory locking. See your local documentation for details.
a3cb178b 1396
8ebc5c01
PP
1397OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1398LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
68dc0745
PP
1399you can use the symbolic names if import them from the Fcntl module,
1400either individually, or as a group using the ':flock' tag. LOCK_SH
1401requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1402releases a previously requested lock. If LOCK_NB is added to LOCK_SH or
7660c0ab 1403LOCK_EX then C<flock()> will return immediately rather than blocking
68dc0745
PP
1404waiting for the lock (check the return status to see if you got it).
1405
1406To avoid the possibility of mis-coordination, Perl flushes FILEHANDLE
1407before (un)locking it.
8ebc5c01 1408
f86cebdf 1409Note that the emulation built with lockf(3) doesn't provide shared
8ebc5c01 1410locks, and it requires that FILEHANDLE be open with write intent. These
f86cebdf
GS
1411are the semantics that lockf(3) implements. Most (all?) systems
1412implement lockf(3) in terms of fcntl(2) locking, though, so the
8ebc5c01
PP
1413differing semantics shouldn't bite too many people.
1414
7660c0ab
A
1415Note also that some versions of C<flock()> cannot lock things over the
1416network; you would need to use the more system-specific C<fcntl()> for
f86cebdf
GS
1417that. If you like you can force Perl to ignore your system's flock(2)
1418function, and so provide its own fcntl(2)-based emulation, by passing
8ebc5c01
PP
1419the switch C<-Ud_flock> to the F<Configure> program when you configure
1420perl.
4633a7c4
LW
1421
1422Here's a mailbox appender for BSD systems.
a0d0e21e 1423
7e1af8bc 1424 use Fcntl ':flock'; # import LOCK_* constants
a0d0e21e
LW
1425
1426 sub lock {
7e1af8bc 1427 flock(MBOX,LOCK_EX);
a0d0e21e
LW
1428 # and, in case someone appended
1429 # while we were waiting...
1430 seek(MBOX, 0, 2);
1431 }
1432
1433 sub unlock {
7e1af8bc 1434 flock(MBOX,LOCK_UN);
a0d0e21e
LW
1435 }
1436
1437 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1438 or die "Can't open mailbox: $!";
1439
1440 lock();
1441 print MBOX $msg,"\n\n";
1442 unlock();
1443
cb1a09d0 1444See also L<DB_File> for other flock() examples.
a0d0e21e 1445
a189bff9
JH
1446=item for (INITIAL; WHILE; EACH) BLOCK
1447
1448Do INITIAL, enter BLOCK while EXPR is true, at the end of each round
1449do EACH. For example:
1450
1451 for ($i = 0, $j = 0; $i < 10; $i++) {
1452 if ($i % 3 == 0) { $j++ }
1453 print "i = $i, j = $j\n";
1454 }
1455
1456See L<perlsyn> for more details. See also L</foreach>, a twin of
1457C<for>, L</while> and L</until>, close cousins of L<for>, and
1458L</last>, L</next>, and L</redo> for additional control flow.
1459
1460=item foreach LOOPVAR (LIST) BLOCK
1461
1462Enter BLOCK as LOOPVAR set in turn to each element of LIST.
1463For example:
1464
1465 foreach $rolling (@stones) { print "rolling $stone\n" }
1466
1467 foreach my $file (@files) { print "file $file\n" }
1468
1469The LOOPVAR is optional and defaults to C<$_>. If the elements are
1470modifiable (as opposed to constants or tied variables) you can modify them.
1471
1472 foreach (@words) { tr/abc/xyz/ }
1473
1474See L<perlsyn> for more details. See also L</for>, a twin of
1475C<foreach>, L</while> and L</until>, close cousins of L<for>, and
1476L</last>, L</next>, and L</redo> for additional control flow.
1477
a0d0e21e
LW
1478=item fork
1479
f86cebdf 1480Does a fork(2) system call. Returns the child pid to the parent process,
7660c0ab 1481C<0> to the child process, or C<undef> if the fork is unsuccessful.
5a964f20 1482
a0d0e21e 1483Note: unflushed buffers remain unflushed in both processes, which means
7660c0ab
A
1484you may need to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()>
1485method of C<IO::Handle> to avoid duplicate output.
a0d0e21e 1486
7660c0ab 1487If you C<fork()> without ever waiting on your children, you will accumulate
a0d0e21e
LW
1488zombies:
1489
4633a7c4 1490 $SIG{CHLD} = sub { wait };
a0d0e21e 1491
54310121 1492There's also the double-fork trick (error checking on
7660c0ab 1493C<fork()> returns omitted);
a0d0e21e
LW
1494
1495 unless ($pid = fork) {
1496 unless (fork) {
1497 exec "what you really wanna do";
1498 die "no exec";
1499 # ... or ...
4633a7c4 1500 ## (some_perl_code_here)
a0d0e21e
LW
1501 exit 0;
1502 }
1503 exit 0;
1504 }
1505 waitpid($pid,0);
1506
cb1a09d0
AD
1507See also L<perlipc> for more examples of forking and reaping
1508moribund children.
1509
28757baa
PP
1510Note that if your forked child inherits system file descriptors like
1511STDIN and STDOUT that are actually connected by a pipe or socket, even
5a964f20 1512if you exit, then the remote server (such as, say, httpd or rsh) won't think
7660c0ab 1513you're done. You should reopen those to F</dev/null> if it's any issue.
28757baa 1514
cb1a09d0
AD
1515=item format
1516
7660c0ab 1517Declare a picture format for use by the C<write()> function. For
cb1a09d0
AD
1518example:
1519
54310121 1520 format Something =
cb1a09d0
AD
1521 Test: @<<<<<<<< @||||| @>>>>>
1522 $str, $%, '$' . int($num)
1523 .
1524
1525 $str = "widget";
184e9718 1526 $num = $cost/$quantity;
cb1a09d0
AD
1527 $~ = 'Something';
1528 write;
1529
1530See L<perlform> for many details and examples.
1531
8903cb82 1532=item formline PICTURE,LIST
a0d0e21e 1533
5a964f20 1534This is an internal function used by C<format>s, though you may call it,
a0d0e21e
LW
1535too. It formats (see L<perlform>) a list of values according to the
1536contents of PICTURE, placing the output into the format output
7660c0ab
A
1537accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
1538Eventually, when a C<write()> is done, the contents of
a0d0e21e 1539C<$^A> are written to some filehandle, but you could also read C<$^A>
7660c0ab
A
1540yourself and then set C<$^A> back to C<"">. Note that a format typically
1541does one C<formline()> per line of form, but the C<formline()> function itself
748a9306 1542doesn't care how many newlines are embedded in the PICTURE. This means
4633a7c4 1543that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
748a9306
LW
1544You may therefore need to use multiple formlines to implement a single
1545record format, just like the format compiler.
1546
5f05dabc 1547Be careful if you put double quotes around the picture, because an "C<@>"
748a9306 1548character may be taken to mean the beginning of an array name.
7660c0ab 1549C<formline()> always returns TRUE. See L<perlform> for other examples.
a0d0e21e
LW
1550
1551=item getc FILEHANDLE
1552
1553=item getc
1554
1555Returns the next character from the input file attached to FILEHANDLE,
9bc64814 1556or the undefined value at end of file, or if there was an error. If
3b02c43c
GS
1557FILEHANDLE is omitted, reads from STDIN. This is not particularly
1558efficient. It cannot be used to get unbuffered single-characters,
1559however. For that, try something more like:
4633a7c4
LW
1560
1561 if ($BSD_STYLE) {
1562 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1563 }
1564 else {
54310121 1565 system "stty", '-icanon', 'eol', "\001";
4633a7c4
LW
1566 }
1567
1568 $key = getc(STDIN);
1569
1570 if ($BSD_STYLE) {
1571 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1572 }
1573 else {
5f05dabc 1574 system "stty", 'icanon', 'eol', '^@'; # ASCII null
4633a7c4
LW
1575 }
1576 print "\n";
1577
54310121
PP
1578Determination of whether $BSD_STYLE should be set
1579is left as an exercise to the reader.
cb1a09d0 1580
7660c0ab 1581The C<POSIX::getattr()> function can do this more portably on systems
5a964f20 1582purporting POSIX compliance.
cb1a09d0 1583See also the C<Term::ReadKey> module from your nearest CPAN site;
54310121 1584details on CPAN can be found on L<perlmod/CPAN>.
a0d0e21e
LW
1585
1586=item getlogin
1587
5a964f20
TC
1588Implements the C library function of the same name, which on most
1589systems returns the current login from F</etc/utmp>, if any. If null,
7660c0ab 1590use C<getpwuid()>.
a0d0e21e 1591
f86702cc 1592 $login = getlogin || getpwuid($<) || "Kilroy";
a0d0e21e 1593
7660c0ab
A
1594Do not consider C<getlogin()> for authentication: it is not as
1595secure as C<getpwuid()>.
4633a7c4 1596
a0d0e21e
LW
1597=item getpeername SOCKET
1598
1599Returns the packed sockaddr address of other end of the SOCKET connection.
1600
4633a7c4
LW
1601 use Socket;
1602 $hersockaddr = getpeername(SOCK);
1603 ($port, $iaddr) = unpack_sockaddr_in($hersockaddr);
1604 $herhostname = gethostbyaddr($iaddr, AF_INET);
1605 $herstraddr = inet_ntoa($iaddr);
a0d0e21e
LW
1606
1607=item getpgrp PID
1608
47e29363 1609Returns the current process group for the specified PID. Use
7660c0ab 1610a PID of C<0> to get the current process group for the
4633a7c4 1611current process. Will raise an exception if used on a machine that
f86cebdf 1612doesn't implement getpgrp(2). If PID is omitted, returns process
7660c0ab
A
1613group of current process. Note that the POSIX version of C<getpgrp()>
1614does not accept a PID argument, so only C<PID==0> is truly portable.
a0d0e21e
LW
1615
1616=item getppid
1617
1618Returns the process id of the parent process.
1619
1620=item getpriority WHICH,WHO
1621
4633a7c4
LW
1622Returns the current priority for a process, a process group, or a user.
1623(See L<getpriority(2)>.) Will raise a fatal exception if used on a
f86cebdf 1624machine that doesn't implement getpriority(2).
a0d0e21e
LW
1625
1626=item getpwnam NAME
1627
1628=item getgrnam NAME
1629
1630=item gethostbyname NAME
1631
1632=item getnetbyname NAME
1633
1634=item getprotobyname NAME
1635
1636=item getpwuid UID
1637
1638=item getgrgid GID
1639
1640=item getservbyname NAME,PROTO
1641
1642=item gethostbyaddr ADDR,ADDRTYPE
1643
1644=item getnetbyaddr ADDR,ADDRTYPE
1645
1646=item getprotobynumber NUMBER
1647
1648=item getservbyport PORT,PROTO
1649
1650=item getpwent
1651
1652=item getgrent
1653
1654=item gethostent
1655
1656=item getnetent
1657
1658=item getprotoent
1659
1660=item getservent
1661
1662=item setpwent
1663
1664=item setgrent
1665
1666=item sethostent STAYOPEN
1667
1668=item setnetent STAYOPEN
1669
1670=item setprotoent STAYOPEN
1671
1672=item setservent STAYOPEN
1673
1674=item endpwent
1675
1676=item endgrent
1677
1678=item endhostent
1679
1680=item endnetent
1681
1682=item endprotoent
1683
1684=item endservent
1685
1686These routines perform the same functions as their counterparts in the
5a964f20 1687system library. In list context, the return values from the
a0d0e21e
LW
1688various get routines are as follows:
1689
1690 ($name,$passwd,$uid,$gid,
6ee623d5 1691 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
a0d0e21e
LW
1692 ($name,$passwd,$gid,$members) = getgr*
1693 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1694 ($name,$aliases,$addrtype,$net) = getnet*
1695 ($name,$aliases,$proto) = getproto*
1696 ($name,$aliases,$port,$proto) = getserv*
1697
1698(If the entry doesn't exist you get a null list.)
1699
5a964f20 1700In scalar context, you get the name, unless the function was a
a0d0e21e
LW
1701lookup by name, in which case you get the other thing, whatever it is.
1702(If the entry doesn't exist you get the undefined value.) For example:
1703
5a964f20
TC
1704 $uid = getpwnam($name);
1705 $name = getpwuid($num);
1706 $name = getpwent();
1707 $gid = getgrnam($name);
1708 $name = getgrgid($num;
1709 $name = getgrent();
1710 #etc.
a0d0e21e 1711
7660c0ab 1712In I<getpw*()> the fields C<$quota>, C<$comment>, and C<$expire> are special
6ee623d5 1713cases in the sense that in many systems they are unsupported. If the
7660c0ab
A
1714C<$quota> is unsupported, it is an empty scalar. If it is supported, it
1715usually encodes the disk quota. If the C<$comment> field is unsupported,
6ee623d5
GS
1716it is an empty scalar. If it is supported it usually encodes some
1717administrative comment about the user. In some systems the $quota
7660c0ab
A
1718field may be C<$change> or C<$age>, fields that have to do with password
1719aging. In some systems the C<$comment> field may be C<$class>. The C<$expire>
6ee623d5
GS
1720field, if present, encodes the expiration period of the account or the
1721password. For the availability and the exact meaning of these fields
f86cebdf 1722in your system, please consult your getpwnam(3) documentation and your
7660c0ab
A
1723F<pwd.h> file. You can also find out from within Perl which meaning
1724your C<$quota> and C<$comment> fields have and whether you have the C<$expire>
1725field by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
1726C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>.
6ee623d5 1727
7660c0ab 1728The C<$members> value returned by I<getgr*()> is a space separated list of
a0d0e21e
LW
1729the login names of the members of the group.
1730
1731For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1732C, it will be returned to you via C<$?> if the function call fails. The
7660c0ab 1733C<@addrs> value returned by a successful call is a list of the raw
a0d0e21e
LW
1734addresses returned by the corresponding system library call. In the
1735Internet domain, each address is four bytes long and you can unpack it
1736by saying something like:
1737
1738 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1739
5a964f20
TC
1740If you get tired of remembering which element of the return list contains
1741which return value, by-name interfaces are also provided in modules:
7660c0ab
A
1742C<File::stat>, C<Net::hostent>, C<Net::netent>, C<Net::protoent>, C<Net::servent>,
1743C<Time::gmtime>, C<Time::localtime>, and C<User::grent>. These override the
5a964f20
TC
1744normal built-in, replacing them with versions that return objects with
1745the appropriate names for each field. For example:
1746
1747 use File::stat;
1748 use User::pwent;
1749 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
1750
1751Even though it looks like they're the same method calls (uid),
7660c0ab 1752they aren't, because a C<File::stat> object is different from a C<User::pwent> object.
5a964f20 1753
a0d0e21e
LW
1754=item getsockname SOCKET
1755
1756Returns the packed sockaddr address of this end of the SOCKET connection.
1757
4633a7c4
LW
1758 use Socket;
1759 $mysockaddr = getsockname(SOCK);
1760 ($port, $myaddr) = unpack_sockaddr_in($mysockaddr);
a0d0e21e
LW
1761
1762=item getsockopt SOCKET,LEVEL,OPTNAME
1763
5a964f20 1764Returns the socket option requested, or undef if there is an error.
a0d0e21e
LW
1765
1766=item glob EXPR
1767
0a753a76
PP
1768=item glob
1769
7660c0ab 1770Returns the value of EXPR with filename expansions such as the standard Unix shell F</bin/sh> would
68dc0745 1771do. This is the internal function implementing the C<E<lt>*.cE<gt>>
7660c0ab 1772operator, but you can use it directly. If EXPR is omitted, C<$_> is used.
68dc0745
PP
1773The C<E<lt>*.cE<gt>> operator is discussed in more detail in
1774L<perlop/"I/O Operators">.
a0d0e21e
LW
1775
1776=item gmtime EXPR
1777
1778Converts a time as returned by the time function to a 9-element array
54310121 1779with the time localized for the standard Greenwich time zone.
4633a7c4 1780Typically used as follows:
a0d0e21e 1781
54310121 1782 # 0 1 2 3 4 5 6 7 8
a0d0e21e
LW
1783 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1784 gmtime(time);
1785
1786All array elements are numeric, and come straight out of a struct tm.
7660c0ab
A
1787In particular this means that C<$mon> has the range C<0..11> and C<$wday> has
1788the range C<0..6> with sunday as day C<0>. Also, C<$year> is the number of
1789years since 1900, that is, C<$year> is C<123> in year 2023, I<not> simply the last two digits of the year.
2f9daede
TPG
1790
1791If EXPR is omitted, does C<gmtime(time())>.
a0d0e21e 1792
f86cebdf 1793In scalar context, returns the ctime(3) value:
0a753a76
PP
1794
1795 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
1796
7660c0ab 1797Also see the C<timegm()> function provided by the C<Time::Local> module,
f86cebdf 1798and the strftime(3) function available via the POSIX module.
7660c0ab
A
1799
1800This scalar value is B<not> locale dependent, see L<perllocale>, but
1801instead a Perl builtin. Also see the C<Time::Local> module, and the
f86cebdf 1802strftime(3) and mktime(3) function available via the POSIX module. To
7660c0ab
A
1803get somewhat similar but locale dependent date strings, set up your
1804locale environment variables appropriately (please see L<perllocale>)
1805and try for example:
1806
1807 use POSIX qw(strftime);
1808 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
1809
1810Note that the C<%a> and C<%b>, the short forms of the day of the week
1811and the month of the year, may not necessarily be three characters wide.
0a753a76 1812
a0d0e21e
LW
1813=item goto LABEL
1814
748a9306
LW
1815=item goto EXPR
1816
a0d0e21e
LW
1817=item goto &NAME
1818
7660c0ab 1819The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
a0d0e21e 1820execution there. It may not be used to go into any construct that
7660c0ab 1821requires initialization, such as a subroutine or a C<foreach> loop. It
0a753a76 1822also can't be used to go into a construct that is optimized away,
7660c0ab 1823or to get out of a block or subroutine given to C<sort()>.
0a753a76 1824It can be used to go almost anywhere else within the dynamic scope,
a0d0e21e 1825including out of subroutines, but it's usually better to use some other
ab4f32c2 1826construct such as C<last> or C<die()>. The author of Perl has never felt the
7660c0ab 1827need to use this form of C<goto> (in Perl, that is--C is another matter).
a0d0e21e 1828
7660c0ab
A
1829The C<goto-EXPR> form expects a label name, whose scope will be resolved
1830dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
748a9306
LW
1831necessarily recommended if you're optimizing for maintainability:
1832
1833 goto ("FOO", "BAR", "GLARCH")[$i];
1834
7660c0ab 1835The C<goto-&NAME> form is highly magical, and substitutes a call to the
a0d0e21e 1836named subroutine for the currently running subroutine. This is used by
7660c0ab 1837C<AUTOLOAD> subroutines that wish to load another subroutine and then
a0d0e21e 1838pretend that the other subroutine had been called in the first place
7660c0ab
A
1839(except that any modifications to C<@_> in the current subroutine are
1840propagated to the other subroutine.) After the C<goto>, not even C<caller()>
a0d0e21e
LW
1841will be able to tell that this routine was called first.
1842
1843=item grep BLOCK LIST
1844
1845=item grep EXPR,LIST
1846
f86cebdf 1847This is similar in spirit to, but not the same as, grep(1)
2f9daede
TPG
1848and its relatives. In particular, it is not limited to using
1849regular expressions.
1850
a0d0e21e 1851Evaluates the BLOCK or EXPR for each element of LIST (locally setting
7660c0ab 1852C<$_> to each element) and returns the list value consisting of those
a0d0e21e
LW
1853elements for which the expression evaluated to TRUE. In a scalar
1854context, returns the number of times the expression was TRUE.
1855
1856 @foo = grep(!/^#/, @bar); # weed out comments
1857
1858or equivalently,
1859
1860 @foo = grep {!/^#/} @bar; # weed out comments
1861
7660c0ab 1862Note that, because C<$_> is a reference into the list value, it can be used
a0d0e21e
LW
1863to modify the elements of the array. While this is useful and
1864supported, it can cause bizarre results if the LIST is not a named
2f9daede 1865array. Similarly, grep returns aliases into the original list,
f86cebdf 1866much like the way that a for loop's index variable aliases the list
2f9daede 1867elements. That is, modifying an element of a list returned by grep
ab4f32c2 1868(for example, in a C<foreach>, C<map()> or another C<grep()>)
2f9daede 1869actually modifies the element in the original list.
a0d0e21e 1870
fb73857a 1871See also L</map> for an array composed of the results of the BLOCK or EXPR.
38325410 1872
a0d0e21e
LW
1873=item hex EXPR
1874
54310121 1875=item hex
bbce6d69 1876
54310121 1877Interprets EXPR as a hex string and returns the corresponding
2f9daede 1878value. (To convert strings that might start with either 0 or 0x
7660c0ab 1879see L</oct>.) If EXPR is omitted, uses C<$_>.
2f9daede
TPG
1880
1881 print hex '0xAf'; # prints '175'
1882 print hex 'aF'; # same
a0d0e21e 1883
a189bff9
JH
1884=item if (EXPR) BLOCK
1885
1886=item if (EXPR) BLOCK else BLOCK2
1887
1888=item if (EXPR) BLOCK elsif (EXPR2) BLOCK2
1889
1890Enter BLOCKs conditionally. The first EXPR to return true
1891causes the corresponding BLOCK to be entered, or, in the case
1892of C<else>, the fall-through default BLOCK.
1893
1894Take notice: Perl wants BLOCKS, expressions (like e.g. in C, C++, or
1895Pascal) won't do.
1896
1897See L<perlsyn> for more details. See also C<unless>.
1898
a0d0e21e
LW
1899=item import
1900
7660c0ab 1901There is no builtin C<import()> function. It is just an ordinary
4633a7c4 1902method (subroutine) defined (or inherited) by modules that wish to export
7660c0ab 1903names to another module. The C<use()> function calls the C<import()> method
54310121 1904for the package used. See also L</use()>, L<perlmod>, and L<Exporter>.
a0d0e21e
LW
1905
1906=item index STR,SUBSTR,POSITION
1907
1908=item index STR,SUBSTR
1909
4633a7c4
LW
1910Returns the position of the first occurrence of SUBSTR in STR at or after
1911POSITION. If POSITION is omitted, starts searching from the beginning of
7660c0ab 1912the string. The return value is based at C<0> (or whatever you've set the C<$[>
4633a7c4 1913variable to--but don't do that). If the substring is not found, returns
7660c0ab 1914one less than the base, ordinarily C<-1>.
a0d0e21e
LW
1915
1916=item int EXPR
1917
54310121 1918=item int
bbce6d69 1919
7660c0ab 1920Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
5a964f20 1921You should not use this for rounding, because it truncates
7660c0ab
A
1922towards C<0>, and because machine representations of floating point
1923numbers can sometimes produce counterintuitive results. Usually C<sprintf()> or C<printf()>,
1924or the C<POSIX::floor> or C<POSIX::ceil> functions, would serve you better.
a0d0e21e
LW
1925
1926=item ioctl FILEHANDLE,FUNCTION,SCALAR
1927
f86cebdf 1928Implements the ioctl(2) function. You'll probably have to say
a0d0e21e 1929
4633a7c4 1930 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
a0d0e21e 1931
4633a7c4 1932first to get the correct function definitions. If F<ioctl.ph> doesn't
a0d0e21e 1933exist or doesn't have the correct definitions you'll have to roll your
4633a7c4 1934own, based on your C header files such as F<E<lt>sys/ioctl.hE<gt>>.
5a964f20 1935(There is a Perl script called B<h2ph> that comes with the Perl kit that
54310121 1936may help you in this, but it's nontrivial.) SCALAR will be read and/or
4633a7c4 1937written depending on the FUNCTION--a pointer to the string value of SCALAR
ab4f32c2 1938will be passed as the third argument of the actual C<ioctl()> call. (If SCALAR
4633a7c4
LW
1939has no string value but does have a numeric value, that value will be
1940passed rather than a pointer to the string value. To guarantee this to be
7660c0ab 1941TRUE, add a C<0> to the scalar before using it.) The C<pack()> and C<unpack()>
4633a7c4 1942functions are useful for manipulating the values of structures used by
7660c0ab 1943C<ioctl()>. The following example sets the erase character to DEL.
a0d0e21e
LW
1944
1945 require 'ioctl.ph';
4633a7c4
LW
1946 $getp = &TIOCGETP;
1947 die "NO TIOCGETP" if $@ || !$getp;
a0d0e21e 1948 $sgttyb_t = "ccccs"; # 4 chars and a short
4633a7c4 1949 if (ioctl(STDIN,$getp,$sgttyb)) {
a0d0e21e
LW
1950 @ary = unpack($sgttyb_t,$sgttyb);
1951 $ary[2] = 127;
1952 $sgttyb = pack($sgttyb_t,@ary);
4633a7c4 1953 ioctl(STDIN,&TIOCSETP,$sgttyb)
a0d0e21e
LW
1954 || die "Can't ioctl: $!";
1955 }
1956
ab4f32c2 1957The return value of C<ioctl()> (and C<fcntl()>) is as follows:
a0d0e21e
LW
1958
1959 if OS returns: then Perl returns:
1960 -1 undefined value
1961 0 string "0 but true"
1962 anything else that number
1963
1964Thus Perl returns TRUE on success and FALSE on failure, yet you can
1965still easily determine the actual value returned by the operating
1966system:
1967
1968 ($retval = ioctl(...)) || ($retval = -1);
1969 printf "System returned %d\n", $retval;
1970
7660c0ab 1971The special string "C<0> but true" is excempt from B<-w> complaints
5a964f20
TC
1972about improper numeric conversions.
1973
a0d0e21e
LW
1974=item join EXPR,LIST
1975
54310121 1976Joins the separate strings of LIST into a single string with
a0d0e21e
LW
1977fields separated by the value of EXPR, and returns the string.
1978Example:
1979
1980 $_ = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
1981
7660c0ab 1982See L</split>.
a0d0e21e 1983
aa689395
PP
1984=item keys HASH
1985
1d2dff63
GS
1986Returns a list consisting of all the keys of the named hash. (In a
1987scalar context, returns the number of keys.) The keys are returned in
aa689395 1988an apparently random order, but it is the same order as either the
7660c0ab 1989C<values()> or C<each()> function produces (given that the hash has not been
aa689395 1990modified). As a side effect, it resets HASH's iterator.
a0d0e21e 1991
aa689395 1992Here is yet another way to print your environment:
a0d0e21e
LW
1993
1994 @keys = keys %ENV;
1995 @values = values %ENV;
1996 while ($#keys >= 0) {
1997 print pop(@keys), '=', pop(@values), "\n";
1998 }
1999
2000or how about sorted by key:
2001
2002 foreach $key (sort(keys %ENV)) {
2003 print $key, '=', $ENV{$key}, "\n";
2004 }
2005
aca803df 2006To sort a hash by value, you'll need to use a C<sort()> function.
aa689395 2007Here's a descending numeric sort of a hash by its values:
4633a7c4 2008
5a964f20 2009 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
4633a7c4
LW
2010 printf "%4d %s\n", $hash{$key}, $key;
2011 }
2012
ab4f32c2 2013As an lvalue C<keys()> allows you to increase the number of hash buckets
aa689395
PP
2014allocated for the given hash. This can gain you a measure of efficiency if
2015you know the hash is going to get big. (This is similar to pre-extending
2016an array by assigning a larger number to $#array.) If you say
55497cff
PP
2017
2018 keys %hash = 200;
2019
5a964f20
TC
2020then C<%hash> will have at least 200 buckets allocated for it--256 of them, in fact, since
2021it rounds up to the next power of two. These
55497cff
PP
2022buckets will be retained even if you do C<%hash = ()>, use C<undef
2023%hash> if you want to free the storage while C<%hash> is still in scope.
2024You can't shrink the number of buckets allocated for the hash using
ab4f32c2 2025C<keys()> in this way (but you needn't worry about doing this by accident,
55497cff
PP
2026as trying has no effect).
2027
a0d0e21e
LW
2028=item kill LIST
2029
54310121
PP
2030Sends a signal to a list of processes. The first element of
2031the list must be the signal to send. Returns the number of
4633a7c4 2032processes successfully signaled.
a0d0e21e
LW
2033
2034 $cnt = kill 1, $child1, $child2;
2035 kill 9, @goners;
2036
4633a7c4
LW
2037Unlike in the shell, in Perl if the I<SIGNAL> is negative, it kills
2038process groups instead of processes. (On System V, a negative I<PROCESS>
2039number will also kill process groups, but that's not portable.) That
2040means you usually want to use positive not negative signals. You may also
da0045b7 2041use a signal name in quotes. See L<perlipc/"Signals"> for details.
a0d0e21e
LW
2042
2043=item last LABEL
2044
2045=item last
2046
2047The C<last> command is like the C<break> statement in C (as used in
2048loops); it immediately exits the loop in question. If the LABEL is
2049omitted, the command refers to the innermost enclosing loop. The
2050C<continue> block, if any, is not executed:
2051
4633a7c4
LW
2052 LINE: while (<STDIN>) {
2053 last LINE if /^$/; # exit when done with header
5a964f20 2054 #...
a0d0e21e
LW
2055 }
2056
4968c1e4
G
2057C<last> cannot be used to exit a block which returns a value such as
2058C<eval {}>, C<sub {}> or C<do {}>.
2059
a189bff9
JH
2060See also L</continue> for an illustration of how C<last>, L</next>, and
2061L</redo> work.
2062
2063See also L<perlsyn>.
1d2dff63 2064
a0d0e21e
LW
2065=item lc EXPR
2066
54310121 2067=item lc
bbce6d69 2068
a0d0e21e 2069Returns an lowercased version of EXPR. This is the internal function
7660c0ab 2070implementing the C<\L> escape in double-quoted strings.
a0ed51b3 2071Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
a0d0e21e 2072
7660c0ab 2073If EXPR is omitted, uses C<$_>.
bbce6d69 2074
a0d0e21e
LW
2075=item lcfirst EXPR
2076
54310121 2077=item lcfirst
bbce6d69 2078
a0d0e21e 2079Returns the value of EXPR with the first character lowercased. This is
7660c0ab 2080the internal function implementing the C<\l> escape in double-quoted strings.
a0ed51b3 2081Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
a0d0e21e 2082
7660c0ab 2083If EXPR is omitted, uses C<$_>.
bbce6d69 2084
a0d0e21e
LW
2085=item length EXPR
2086
54310121 2087=item length
bbce6d69 2088
a0ed51b3 2089Returns the length in characters of the value of EXPR. If EXPR is
7660c0ab 2090omitted, returns length of C<$_>.
a0d0e21e
LW
2091
2092=item link OLDFILE,NEWFILE
2093
5a964f20
TC
2094Creates a new filename linked to the old filename. Returns TRUE for
2095success, FALSE otherwise.
a0d0e21e
LW
2096
2097=item listen SOCKET,QUEUESIZE
2098
2099Does the same thing that the listen system call does. Returns TRUE if
4633a7c4 2100it succeeded, FALSE otherwise. See example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
2101
2102=item local EXPR
2103
5a964f20
TC
2104A local modifies the listed variables to be local to the enclosing
2105block, file, or eval. If more than one value is listed, the list must
2106be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2107for details, including issues with tied arrays and hashes.
a0d0e21e 2108
7660c0ab 2109You really probably want to be using C<my()> instead, because C<local()> isn't
7b8d334a 2110what most people think of as "local". See L<perlsub/"Private Variables
cb1a09d0 2111via my()"> for details.
a0d0e21e
LW
2112
2113=item localtime EXPR
2114
2115Converts a time as returned by the time function to a 9-element array
5f05dabc 2116with the time analyzed for the local time zone. Typically used as
a0d0e21e
LW
2117follows:
2118
54310121 2119 # 0 1 2 3 4 5 6 7 8
a0d0e21e
LW
2120 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2121 localtime(time);
2122
2123All array elements are numeric, and come straight out of a struct tm.
7660c0ab
A
2124In particular this means that C<$mon> has the range C<0..11> and C<$wday> has
2125the range C<0..6> with sunday as day C<0>. Also, C<$year> is the number of
2126years 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
2127
2128If EXPR is omitted, uses the current time (C<localtime(time)>).
a0d0e21e 2129
f86cebdf 2130In scalar context, returns the ctime(3) value:
a0d0e21e 2131
5f05dabc 2132 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
a0d0e21e 2133
a3cb178b 2134This scalar value is B<not> locale dependent, see L<perllocale>, but
7660c0ab 2135instead a Perl builtin. Also see the C<Time::Local> module, and the
f86cebdf 2136strftime(3) and mktime(3) function available via the POSIX module. To
a3cb178b
GS
2137get somewhat similar but locale dependent date strings, set up your
2138locale environment variables appropriately (please see L<perllocale>)
5a964f20 2139and try for example:
a3cb178b 2140
5a964f20 2141 use POSIX qw(strftime);
a3cb178b
GS
2142 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
2143
2144Note that the C<%a> and C<%b>, the short forms of the day of the week
2145and the month of the year, may not necessarily be three characters wide.
a0d0e21e
LW
2146
2147=item log EXPR
2148
54310121 2149=item log
bbce6d69 2150
5a964f20 2151Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted, returns log
7660c0ab 2152of C<$_>.
a0d0e21e
LW
2153
2154=item lstat FILEHANDLE
2155
2156=item lstat EXPR
2157
54310121 2158=item lstat
bbce6d69 2159
7660c0ab 2160Does the same thing as the C<stat()> function (including setting the
5a964f20
TC
2161special C<_> filehandle) but stats a symbolic link instead of the file
2162the symbolic link points to. If symbolic links are unimplemented on
7660c0ab 2163your system, a normal C<stat()> is done.
a0d0e21e 2164
7660c0ab 2165If EXPR is omitted, stats C<$_>.
bbce6d69 2166
a0d0e21e
LW
2167=item m//
2168
2169The match operator. See L<perlop>.
2170
2171=item map BLOCK LIST
2172
2173=item map EXPR,LIST
2174
7660c0ab 2175Evaluates the BLOCK or EXPR for each element of LIST (locally setting C<$_> to each
a0d0e21e
LW
2176element) and returns the list value composed of the results of each such
2177evaluation. Evaluates BLOCK or EXPR in a list context, so each element of LIST
2178may produce zero, one, or more elements in the returned value.
2179
2180 @chars = map(chr, @nums);
2181
2182translates a list of numbers to the corresponding characters. And
2183
4633a7c4 2184 %hash = map { getkey($_) => $_ } @array;
a0d0e21e
LW
2185
2186is just a funny way to write
2187
2188 %hash = ();
2189 foreach $_ (@array) {
4633a7c4 2190 $hash{getkey($_)} = $_;
a0d0e21e
LW
2191 }
2192
7660c0ab 2193Note that, because C<$_> is a reference into the list value, it can be used
fb73857a
PP
2194to modify the elements of the array. While this is useful and
2195supported, it can cause bizarre results if the LIST is not a named
2196array. See also L</grep> for an array composed of those items of the
2197original list for which the BLOCK or EXPR evaluates to true.
2198
a0d0e21e
LW
2199=item mkdir FILENAME,MODE
2200
0591cd52
NT
2201Creates the directory specified by FILENAME, with permissions
2202specified by MODE (as modified by C<umask>). If it succeeds it
2203returns TRUE, otherwise it returns FALSE and sets C<$!> (errno).
2204
2205In general, it is better to create directories with permissive MODEs,
2206and let the user modify that with their C<umask>, than it is to supply
2207a restrictive MODE and give the user no way to be more permissive.
2208The exceptions to this rule are when the file or directory should be
2209kept private (mail files, for instance). The perlfunc(1) entry on
2210C<umask> discusses the choice of MODE in more detail.
a0d0e21e
LW
2211
2212=item msgctl ID,CMD,ARG
2213
f86cebdf 2214Calls the System V IPC function msgctl(2). You'll probably have to say
0ade1984
JH
2215
2216 use IPC::SysV;
2217
7660c0ab
A
2218first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2219then ARG must be a variable which will hold the returned C<msqid_ds>
ab4f32c2 2220structure. Returns like C<ioctl()>: the undefined value for error, "C<0> but
0ade1984 2221true" for zero, or the actual return value otherwise. See also
7660c0ab 2222C<IPC::SysV> and C<IPC::Semaphore::Msg> documentation.
a0d0e21e
LW
2223
2224=item msgget KEY,FLAGS
2225
f86cebdf 2226Calls the System V IPC function msgget(2). Returns the message queue
7660c0ab
A
2227id, or the undefined value if there is an error. See also C<IPC::SysV>
2228and C<IPC::SysV::Msg> documentation.
a0d0e21e
LW
2229
2230=item msgsnd ID,MSG,FLAGS
2231
2232Calls the System V IPC function msgsnd to send the message MSG to the
2233message queue ID. MSG must begin with the long integer message type,
c07a80fd 2234which may be created with C<pack("l", $type)>. Returns TRUE if
7660c0ab
A
2235successful, or FALSE if there is an error. See also C<IPC::SysV>
2236and C<IPC::SysV::Msg> documentation.
a0d0e21e
LW
2237
2238=item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2239
2240Calls the System V IPC function msgrcv to receive a message from
2241message queue ID into variable VAR with a maximum message size of
0ade1984
JH
2242SIZE. Note that if a message is received, the message type will be
2243the first thing in VAR, and the maximum length of VAR is SIZE plus the
2244size of the message type. Returns TRUE if successful, or FALSE if
7660c0ab 2245there is an error. See also C<IPC::SysV> and C<IPC::SysV::Msg> documentation.
a0d0e21e
LW
2246
2247=item my EXPR
2248
ab4f32c2
A
2249A C<my()> declares the listed variables to be local (lexically) to the
2250enclosing block, file, or C<eval()>. If
5f05dabc 2251more than one value is listed, the list must be placed in parentheses. See
cb1a09d0 2252L<perlsub/"Private Variables via my()"> for details.
4633a7c4 2253
a0d0e21e
LW
2254=item next LABEL
2255
2256=item next
2257
2258The C<next> command is like the C<continue> statement in C; it starts
2259the next iteration of the loop:
2260
4633a7c4
LW
2261 LINE: while (<STDIN>) {
2262 next LINE if /^#/; # discard comments
5a964f20 2263 #...
a0d0e21e
LW
2264 }
2265
2266Note that if there were a C<continue> block on the above, it would get
2267executed even on discarded lines. If the LABEL is omitted, the command
2268refers to the innermost enclosing loop.
2269
4968c1e4
G
2270C<next> cannot be used to exit a block which returns a value such as
2271C<eval {}>, C<sub {}> or C<do {}>.
2272
a189bff9
JH
2273See also L</continue> for an illustration of how L</last>, C<next>, and
2274L</redo> work.
2275
2276See also L<perlsyn>.
1d2dff63 2277
a0d0e21e
LW
2278=item no Module LIST
2279
7660c0ab 2280See the L</use> function, which C<no> is the opposite of.
a0d0e21e
LW
2281
2282=item oct EXPR
2283
54310121 2284=item oct
bbce6d69 2285
4633a7c4 2286Interprets EXPR as an octal string and returns the corresponding
7660c0ab 2287value. (If EXPR happens to start off with C<0x>, interprets it as
4633a7c4
LW
2288a hex string instead.) The following will handle decimal, octal, and
2289hex in the standard Perl or C notation:
a0d0e21e
LW
2290
2291 $val = oct($val) if $val =~ /^0/;
2292
7660c0ab
A
2293If EXPR is omitted, uses C<$_>. This function is commonly used when
2294a string such as C<644> needs to be converted into a file mode, for
2f9daede
TPG
2295example. (Although perl will automatically convert strings into
2296numbers as needed, this automatic conversion assumes base 10.)
a0d0e21e
LW
2297
2298=item open FILEHANDLE,EXPR
2299
2300=item open FILEHANDLE
2301
2302Opens the file whose filename is given by EXPR, and associates it with
5f05dabc
PP
2303FILEHANDLE. If FILEHANDLE is an expression, its value is used as the
2304name of the real filehandle wanted. If EXPR is omitted, the scalar
2305variable of the same name as the FILEHANDLE contains the filename.
ab4f32c2
A
2306(Note that lexical variables--those declared with C<my()>--will not work
2307for this purpose; so if you're using C<my()>, specify EXPR in your call
5f05dabc
PP
2308to open.)
2309
7660c0ab
A
2310If the filename begins with C<'E<lt>'> or nothing, the file is opened for input.
2311If the filename begins with C<'E<gt>'>, the file is truncated and opened for
2312output, being created if necessary. If the filename begins with C<'E<gt>E<gt>'>,
fbb426e4 2313the file is opened for appending, again being created if necessary.
7660c0ab
A
2314You can put a C<'+'> in front of the C<'E<gt>'> or C<'E<lt>'> to indicate that
2315you want both read and write access to the file; thus C<'+E<lt>'> is almost
2316always preferred for read/write updates--the C<'+E<gt>'> mode would clobber the
5a964f20
TC
2317file first. You can't usually use either read-write mode for updating
2318textfiles, since they have variable length records. See the B<-i>
0591cd52
NT
2319switch in L<perlrun> for a better approach. The file is created with
2320permissions of C<0666> modified by the process' C<umask> value.
5a964f20
TC
2321
2322The prefix and the filename may be separated with spaces.
f86cebdf 2323These various prefixes correspond to the fopen(3) modes of C<'r'>, C<'r+'>, C<'w'>,
7660c0ab 2324C<'w+'>, C<'a'>, and C<'a+'>.
5f05dabc 2325
7660c0ab 2326If the filename begins with C<'|'>, the filename is interpreted as a
5a964f20 2327command to which output is to be piped, and if the filename ends with a
7660c0ab
A
2328C<'|'>, the filename is interpreted See L<perlipc/"Using open() for IPC">
2329for more examples of this. (You are not allowed to C<open()> to a command
5a964f20
TC
2330that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2331and L<perlipc/"Bidirectional Communication"> for alternatives.)
cb1a09d0 2332
7660c0ab 2333Opening C<'-'> opens STDIN and opening C<'E<gt>-'> opens STDOUT. Open returns
ab4f32c2 2334nonzero upon success, the undefined value otherwise. If the C<open()>
4633a7c4 2335involved a pipe, the return value happens to be the pid of the
54310121 2336subprocess.
cb1a09d0
AD
2337
2338If you're unfortunate enough to be running Perl on a system that
2339distinguishes between text files and binary files (modern operating
2340systems don't care), then you should check out L</binmode> for tips for
ab4f32c2 2341dealing with this. The key distinction between systems that need C<binmode()>
5a964f20
TC
2342and those that don't is their text file formats. Systems like Unix, MacOS, and
2343Plan9, which delimit lines with a single character, and which encode that
ab4f32c2 2344character in C as C<"\n">, do not need C<binmode()>. The rest need it.
cb1a09d0 2345
fb73857a 2346When opening a file, it's usually a bad idea to continue normal execution
ab4f32c2
A
2347if the request failed, so C<open()> is frequently used in connection with
2348C<die()>. Even if C<die()> won't do what you want (say, in a CGI script,
fb73857a 2349where you want to make a nicely formatted error message (but there are
5a964f20 2350modules that can help with that problem)) you should always check
fb73857a
PP
2351the return value from opening a file. The infrequent exception is when
2352working with an unopened filehandle is actually what you want to do.
2353
cb1a09d0 2354Examples:
a0d0e21e
LW
2355
2356 $ARTICLE = 100;
2357 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2358 while (<ARTICLE>) {...
2359
2360 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
fb73857a 2361 # if the open fails, output is discarded
a0d0e21e 2362
fb73857a
PP
2363 open(DBASE, '+<dbase.mine') # open for update
2364 or die "Can't open 'dbase.mine' for update: $!";
cb1a09d0 2365
fb73857a
PP
2366 open(ARTICLE, "caesar <$article |") # decrypt article
2367 or die "Can't start caesar: $!";
a0d0e21e 2368
fb73857a
PP
2369 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
2370 or die "Can't start sort: $!";
a0d0e21e
LW
2371
2372 # process argument list of files along with any includes
2373
2374 foreach $file (@ARGV) {
2375 process($file, 'fh00');
2376 }
2377
2378 sub process {
5a964f20 2379 my($filename, $input) = @_;
a0d0e21e
LW
2380 $input++; # this is a string increment
2381 unless (open($input, $filename)) {
2382 print STDERR "Can't open $filename: $!\n";
2383 return;
2384 }
2385
5a964f20 2386 local $_;
a0d0e21e
LW
2387 while (<$input>) { # note use of indirection
2388 if (/^#include "(.*)"/) {
2389 process($1, $input);
2390 next;
2391 }
5a964f20 2392 #... # whatever
a0d0e21e
LW
2393 }
2394 }
2395
2396You may also, in the Bourne shell tradition, specify an EXPR beginning
7660c0ab 2397with C<'E<gt>&'>, in which case the rest of the string is interpreted as the
5a964f20 2398name of a filehandle (or file descriptor, if numeric) to be
7660c0ab
A
2399duped and opened. You may use C<&> after C<E<gt>>, C<E<gt>E<gt>>, C<E<lt>>, C<+E<gt>>,
2400C<+E<gt>E<gt>>, and C<+E<lt>>. The
a0d0e21e 2401mode you specify should match the mode of the original filehandle.
184e9718 2402(Duping a filehandle does not take into account any existing contents of
cb1a09d0 2403stdio buffers.)
a0d0e21e
LW
2404Here is a script that saves, redirects, and restores STDOUT and
2405STDERR:
2406
2407 #!/usr/bin/perl
5a964f20
TC
2408 open(OLDOUT, ">&STDOUT");
2409 open(OLDERR, ">&STDERR");
a0d0e21e
LW
2410
2411 open(STDOUT, ">foo.out") || die "Can't redirect stdout";
2412 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
2413
2414 select(STDERR); $| = 1; # make unbuffered
2415 select(STDOUT); $| = 1; # make unbuffered
2416
2417 print STDOUT "stdout 1\n"; # this works for
2418 print STDERR "stderr 1\n"; # subprocesses too
2419
2420 close(STDOUT);
2421 close(STDERR);
2422
5a964f20
TC
2423 open(STDOUT, ">&OLDOUT");
2424 open(STDERR, ">&OLDERR");
a0d0e21e
LW
2425
2426 print STDOUT "stdout 2\n";
2427 print STDERR "stderr 2\n";
2428
2429
7660c0ab
A
2430If you specify C<'E<lt>&=N'>, where C<N> is a number, then Perl will do an
2431equivalent of C's C<fdopen()> of that file descriptor; this is more
4633a7c4 2432parsimonious of file descriptors. For example:
a0d0e21e
LW
2433
2434 open(FILEHANDLE, "<&=$fd")
2435
7660c0ab 2436If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>, then
a0d0e21e 2437there is an implicit fork done, and the return value of open is the pid
7660c0ab 2438of the child within the parent process, and C<0> within the child
184e9718 2439process. (Use C<defined($pid)> to determine whether the open was successful.)
a0d0e21e
LW
2440The filehandle behaves normally for the parent, but i/o to that
2441filehandle is piped from/to the STDOUT/STDIN of the child process.
2442In the child process the filehandle isn't opened--i/o happens from/to
2443the new STDOUT or STDIN. Typically this is used like the normal
2444piped open when you want to exercise more control over just how the
2445pipe command gets executed, such as when you are running setuid, and
54310121 2446don't want to have to scan shell commands for metacharacters.
4633a7c4 2447The following pairs are more or less equivalent:
a0d0e21e
LW
2448
2449 open(FOO, "|tr '[a-z]' '[A-Z]'");
2450 open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
2451
2452 open(FOO, "cat -n '$file'|");
2453 open(FOO, "-|") || exec 'cat', '-n', $file;
2454
4633a7c4
LW
2455See L<perlipc/"Safe Pipe Opens"> for more examples of this.
2456
5a964f20 2457NOTE: On any operation that may do a fork, any unflushed buffers remain
184e9718 2458unflushed in both processes, which means you may need to set C<$|> to
4771b018
GS
2459avoid duplicate output. On systems that support a close-on-exec flag on
2460files, the flag will be set for the newly opened file descriptor as
2461determined by the value of $^F. See L<perlvar/$^F>.
a0d0e21e 2462
0dccf244
CS
2463Closing any piped filehandle causes the parent process to wait for the
2464child to finish, and returns the status value in C<$?>.
2465
5a964f20 2466The filename passed to open will have leading and trailing
f86cebdf 2467whitespace deleted, and the normal redirection characters
5a964f20
TC
2468honored. This property, known as "magic open",
2469can often be used to good effect. A user could specify a filename of
7660c0ab 2470F<"rsh cat file |">, or you could change certain filenames as needed:
5a964f20
TC
2471
2472 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
2473 open(FH, $filename) or die "Can't open $filename: $!";
2474
2475However, to open a file with arbitrary weird characters in it, it's
2476necessary to protect any leading and trailing whitespace:
2477
2478 $file =~ s#^(\s)#./$1#;
2479 open(FOO, "< $file\0");
2480
7660c0ab
A
2481If you want a "real" C C<open()> (see L<open(2)> on your system), then you
2482should use the C<sysopen()> function, which involves no such magic. This is
5a964f20
TC
2483another way to protect your filenames from interpretation. For example:
2484
2485 use IO::Handle;
2486 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
2487 or die "sysopen $path: $!";
2488 $oldfh = select(HANDLE); $| = 1; select($oldfh);
2489 print HANDLE "stuff $$\n");
2490 seek(HANDLE, 0, 0);
2491 print "File contains: ", <HANDLE>;
2492
7660c0ab
A
2493Using the constructor from the C<IO::Handle> package (or one of its
2494subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
5a964f20
TC
2495filehandles that have the scope of whatever variables hold references to
2496them, and automatically close whenever and however you leave that scope:
c07a80fd 2497
5f05dabc 2498 use IO::File;
5a964f20 2499 #...
c07a80fd
PP
2500 sub read_myfile_munged {
2501 my $ALL = shift;
5f05dabc 2502 my $handle = new IO::File;
c07a80fd
PP
2503 open($handle, "myfile") or die "myfile: $!";
2504 $first = <$handle>
2505 or return (); # Automatically closed here.
2506 mung $first or die "mung failed"; # Or here.
2507 return $first, <$handle> if $ALL; # Or here.
2508 $first; # Or here.
2509 }
2510
cb1a09d0 2511See L</seek()> for some details about mixing reading and writing.
a0d0e21e
LW
2512
2513=item opendir DIRHANDLE,EXPR
2514
7660c0ab
A
2515Opens a directory named EXPR for processing by C<readdir()>, C<telldir()>,
2516C<seekdir()>, C<rewinddir()>, and C<closedir()>. Returns TRUE if successful.
a0d0e21e
LW
2517DIRHANDLEs have their own namespace separate from FILEHANDLEs.
2518
2519=item ord EXPR
2520
54310121 2521=item ord
bbce6d69 2522
a0ed51b3 2523Returns the numeric (ASCII or Unicode) value of the first character of EXPR. If
7660c0ab 2524EXPR is omitted, uses C<$_>. For the reverse, see L</chr>.
a0d0e21e
LW
2525
2526=item pack TEMPLATE,LIST
2527
2528Takes an array or list of values and packs it into a binary structure,
2529returning the string containing the structure. The TEMPLATE is a
2530sequence of characters that give the order and type of values, as
2531follows:
2532
2533 A An ascii string, will be space padded.
2534 a An ascii string, will be null padded.
2535 b A bit string (ascending bit order, like vec()).
2536 B A bit string (descending bit order).
2537 h A hex string (low nybble first).
2538 H A hex string (high nybble first).
2539
2540 c A signed char value.
a0ed51b3 2541 C An unsigned char value. Only does bytes. See U for Unicode.
96e4d5b1 2542
a0d0e21e
LW
2543 s A signed short value.
2544 S An unsigned short value.
96e4d5b1
PP
2545 (This 'short' is _exactly_ 16 bits, which may differ from
2546 what a local C compiler calls 'short'.)
2547
a0d0e21e
LW
2548 i A signed integer value.
2549 I An unsigned integer value.
f86cebdf
GS
2550 (This 'integer' is _at_least_ 32 bits wide. Its exact
2551 size depends on what a local C compiler calls 'int',
2552 and may even be larger than the 'long' described in
2553 the next item.)
96e4d5b1 2554
a0d0e21e
LW
2555 l A signed long value.
2556 L An unsigned long value.
96e4d5b1
PP
2557 (This 'long' is _exactly_ 32 bits, which may differ from
2558 what a local C compiler calls 'long'.)
a0d0e21e 2559
96e4d5b1
PP
2560 n A short in "network" (big-endian) order.
2561 N A long in "network" (big-endian) order.
a0d0e21e
LW
2562 v A short in "VAX" (little-endian) order.
2563 V A long in "VAX" (little-endian) order.
96e4d5b1
PP
2564 (These 'shorts' and 'longs' are _exactly_ 16 bits and
2565 _exactly_ 32 bits, respectively.)
a0d0e21e 2566
dae0da7a
JH
2567 q A signed quad (64-bit) value.
2568 Q An unsigned quad value.
2569 (Available only if your system supports 64-bit integer values
2570 _and_ if Perl has been compiled to support those.
2571 Causes a fatal error otherwise.)
2572
a0d0e21e
LW
2573 f A single-precision float in the native format.
2574 d A double-precision float in the native format.
2575
2576 p A pointer to a null-terminated string.
2577 P A pointer to a structure (fixed-length string).
2578
2579 u A uuencoded string.
a0ed51b3
LW
2580 U A Unicode character number. Encodes to UTF-8 internally.
2581 Works even if C<use utf8> is not in effect.
a0d0e21e 2582
96e4d5b1 2583 w A BER compressed integer. Its bytes represent an unsigned
f86cebdf
GS
2584 integer in base 128, most significant digit first, with as
2585 few digits as possible. Bit eight (the high bit) is set
2586 on each byte except the last.
def98dd4 2587
a0d0e21e
LW
2588 x A null byte.
2589 X Back up a byte.
2590 @ Null fill to absolute position.
2591
5a964f20 2592Each letter may optionally be followed by a number giving a repeat
7660c0ab
A
2593count. With all types except C<"a">, C<"A">, C<"b">, C<"B">, C<"h">, C<"H">, and C<"P"> the
2594pack function will gobble up that many values from the LIST. A C<*> for the
2595repeat count means to use however many items are left. The C<"a"> and C<"A">
a0d0e21e 2596types gobble just one value, but pack it as a string of length count,
7660c0ab
A
2597padding with nulls or spaces as necessary. (When unpacking, C<"A"> strips
2598trailing spaces and nulls, but C<"a"> does not.) Likewise, the C<"b"> and C<"B">
2599fields pack a string that many bits long. The C<"h"> and C<"H"> fields pack a
2600string that many nybbles long. The C<"p"> type packs a pointer to a null-
84902520
TB
2601terminated string. You are responsible for ensuring the string is not a
2602temporary value (which can potentially get deallocated before you get
7660c0ab 2603around to using the packed result). The C<"P"> packs a pointer to a structure
61167c6f 2604of the size indicated by the length. A NULL pointer is created if the
7660c0ab 2605corresponding value for C<"p"> or C<"P"> is C<undef>.
61167c6f 2606Real numbers (floats and doubles) are
a0d0e21e
LW
2607in the native machine format only; due to the multiplicity of floating
2608formats around, and the lack of a standard "network" representation, no
2609facility for interchange has been made. This means that packed floating
2610point data written on one machine may not be readable on another - even if
2611both use IEEE floating point arithmetic (as the endian-ness of the memory
2612representation is not part of the IEEE spec). Note that Perl uses doubles
2613internally for all numeric calculation, and converting from double into
5f05dabc 2614float and thence back to double again will lose precision (i.e.,
7660c0ab 2615C<unpack("f", pack("f", $foo)>) will not in general equal C<$foo>).
a0d0e21e
LW
2616
2617Examples:
2618
a0ed51b3 2619 $foo = pack("CCCC",65,66,67,68);
a0d0e21e 2620 # foo eq "ABCD"
a0ed51b3 2621 $foo = pack("C4",65,66,67,68);
a0d0e21e 2622 # same thing
a0ed51b3
LW
2623 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
2624 # same thing with Unicode circled letters
a0d0e21e
LW
2625
2626 $foo = pack("ccxxcc",65,66,67,68);
2627 # foo eq "AB\0\0CD"
2628
2629 $foo = pack("s2",1,2);
2630 # "\1\0\2\0" on little-endian
2631 # "\0\1\0\2" on big-endian
2632
2633 $foo = pack("a4","abcd","x","y","z");
2634 # "abcd"
2635
2636 $foo = pack("aaaa","abcd","x","y","z");
2637 # "axyz"
2638
2639 $foo = pack("a14","abcdefg");
2640 # "abcdefg\0\0\0\0\0\0\0"
2641
2642 $foo = pack("i9pl", gmtime);
2643 # a real struct tm (on my system anyway)
2644
2645 sub bintodec {
2646 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
2647 }
2648
2649The same template may generally also be used in the unpack function.
2650
5a964f20
TC
2651=item package
2652
cb1a09d0
AD
2653=item package NAMESPACE
2654
2655Declares the compilation unit as being in the given namespace. The scope
2656of the package declaration is from the declaration itself through the end of
7660c0ab 2657the enclosing block (the same scope as the C<local()> operator). All further
cb1a09d0 2658unqualified dynamic identifiers will be in this namespace. A package
5f05dabc 2659statement affects only dynamic variables--including those you've used
7660c0ab 2660C<local()> on--but I<not> lexical variables created with C<my()>. Typically it
cb1a09d0
AD
2661would be the first declaration in a file to be included by the C<require>
2662or C<use> operator. You can switch into a package in more than one place;
5a964f20 2663it merely influences which symbol table is used by the compiler for the
cb1a09d0
AD
2664rest of that block. You can refer to variables and filehandles in other
2665packages by prefixing the identifier with the package name and a double
2666colon: C<$Package::Variable>. If the package name is null, the C<main>
2667package as assumed. That is, C<$::sail> is equivalent to C<$main::sail>.
2668
5a964f20
TC
2669If NAMESPACE is omitted, then there is no current package, and all
2670identifiers must be fully qualified or lexicals. This is stricter
2671than C<use strict>, since it also extends to function names.
2672
cb1a09d0
AD
2673See L<perlmod/"Packages"> for more information about packages, modules,
2674and classes. See L<perlsub> for other scoping issues.
2675
a0d0e21e
LW
2676=item pipe READHANDLE,WRITEHANDLE
2677
2678Opens a pair of connected pipes like the corresponding system call.
2679Note that if you set up a loop of piped processes, deadlock can occur
2680unless you are very careful. In addition, note that Perl's pipes use
184e9718 2681stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE
a0d0e21e
LW
2682after each command, depending on the application.
2683
7e1af8bc 2684See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
4633a7c4
LW
2685for examples of such things.
2686
4771b018
GS
2687On systems that support a close-on-exec flag on files, the flag will be set
2688for the newly opened file descriptors as determined by the value of $^F.
2689See L<perlvar/$^F>.
2690
a0d0e21e
LW
2691=item pop ARRAY
2692
54310121 2693=item pop
28757baa 2694
a0d0e21e
LW
2695Pops and returns the last value of the array, shortening the array by
26961. Has a similar effect to
2697
2698 $tmp = $ARRAY[$#ARRAY--];
2699
2700If there are no elements in the array, returns the undefined value.
cb1a09d0 2701If ARRAY is omitted, pops the
7660c0ab
A
2702C<@ARGV> array in the main program, and the C<@_> array in subroutines, just
2703like C<shift()>.
a0d0e21e
LW
2704
2705=item pos SCALAR
2706
54310121 2707=item pos
bbce6d69 2708
4633a7c4 2709Returns the offset of where the last C<m//g> search left off for the variable
7660c0ab 2710is in question (C<$_> is used when the variable is not specified). May be
44a8e56a
PP
2711modified to change that offset. Such modification will also influence
2712the C<\G> zero-width assertion in regular expressions. See L<perlre> and
2713L<perlop>.
a0d0e21e
LW
2714
2715=item print FILEHANDLE LIST
2716
2717=item print LIST
2718
2719=item print
2720
cb1a09d0 2721Prints a string or a comma-separated list of strings. Returns TRUE
a0d0e21e 2722if successful. FILEHANDLE may be a scalar variable name, in which case
cb1a09d0 2723the variable contains the name of or a reference to the filehandle, thus introducing one
a0d0e21e
LW
2724level of indirection. (NOTE: If FILEHANDLE is a variable and the next
2725token is a term, it may be misinterpreted as an operator unless you
7660c0ab 2726interpose a C<+> or put parentheses around the arguments.) If FILEHANDLE is
a0d0e21e 2727omitted, prints by default to standard output (or to the last selected
7660c0ab 2728output channel--see L</select>). If LIST is also omitted, prints C<$_> to
5a964f20 2729the currently selected output channel. To set the default output channel to something other than
a0d0e21e 2730STDOUT use the select operation. Note that, because print takes a
5a964f20 2731LIST, anything in the LIST is evaluated in list context, and any
a0d0e21e 2732subroutine that you call will have one or more of its expressions
5a964f20 2733evaluated in list context. Also be careful not to follow the print
a0d0e21e 2734keyword with a left parenthesis unless you want the corresponding right
7660c0ab 2735parenthesis to terminate the arguments to the print--interpose a C<+> or
5f05dabc 2736put parentheses around all the arguments.
a0d0e21e 2737
4633a7c4 2738Note that if you're storing FILEHANDLES in an array or other expression,
da0045b7 2739you will have to use a block returning its value instead:
4633a7c4
LW
2740
2741 print { $files[$i] } "stuff\n";
2742 print { $OK ? STDOUT : STDERR } "stuff\n";
2743
5f05dabc 2744=item printf FILEHANDLE FORMAT, LIST
a0d0e21e 2745
5f05dabc 2746=item printf FORMAT, LIST
a0d0e21e 2747
7660c0ab 2748Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
a3cb178b 2749(the output record separator) is not appended. The first argument
ab4f32c2 2750of the list will be interpreted as the C<printf()> format. If C<use locale> is
a034a98d
DD
2751in effect, the character used for the decimal point in formatted real numbers
2752is affected by the LC_NUMERIC locale. See L<perllocale>.
a0d0e21e 2753
7660c0ab
A
2754Don't fall into the trap of using a C<printf()> when a simple
2755C<print()> would do. The C<print()> is more efficient and less
28757baa
PP
2756error prone.
2757
da0045b7
PP
2758=item prototype FUNCTION
2759
2760Returns the prototype of a function as a string (or C<undef> if the
5f05dabc
PP
2761function has no prototype). FUNCTION is a reference to, or the name of,
2762the function whose prototype you want to retrieve.
da0045b7 2763
b6c543e3
IZ
2764If FUNCTION is a string starting with C<CORE::>, the rest is taken as
2765a name for Perl builtin. If builtin is not I<overridable> (such as
ab4f32c2
A
2766C<qw//>) or its arguments cannot be expressed by a prototype (such as
2767C<system()>) - in other words, the builtin does not behave like a Perl
b6c543e3
IZ
2768function - returns C<undef>. Otherwise, the string describing the
2769equivalent prototype is returned.
2770
a0d0e21e
LW
2771=item push ARRAY,LIST
2772
2773Treats ARRAY as a stack, and pushes the values of LIST
2774onto the end of ARRAY. The length of ARRAY increases by the length of
2775LIST. Has the same effect as
2776
2777 for $value (LIST) {
2778 $ARRAY[++$#ARRAY] = $value;
2779 }
2780
2781but is more efficient. Returns the new number of elements in the array.
2782
2783=item q/STRING/
2784
2785=item qq/STRING/
2786
8782bef2
GB
2787=item qr/STRING/
2788
a0d0e21e
LW
2789=item qx/STRING/
2790
2791=item qw/STRING/
2792
2793Generalized quotes. See L<perlop>.
2794
2795=item quotemeta EXPR
2796
54310121 2797=item quotemeta
bbce6d69 2798
68dc0745 2799Returns the value of EXPR with all non-alphanumeric
a034a98d
DD
2800characters backslashed. (That is, all characters not matching
2801C</[A-Za-z_0-9]/> will be preceded by a backslash in the
2802returned string, regardless of any locale settings.)
2803This is the internal function implementing
7660c0ab 2804the C<\Q> escape in double-quoted strings.
a0d0e21e 2805
7660c0ab 2806If EXPR is omitted, uses C<$_>.
bbce6d69 2807
a0d0e21e
LW
2808=item rand EXPR
2809
2810=item rand
2811
7660c0ab 2812Returns a random fractional number greater than or equal to C<0> and less
3e3baf6d 2813than the value of EXPR. (EXPR should be positive.) If EXPR is
7660c0ab
A
2814omitted, the value C<1> is used. Automatically calls C<srand()> unless
2815C<srand()> has already been called. See also C<srand()>.
a0d0e21e 2816
2f9daede 2817(Note: If your rand function consistently returns numbers that are too
a0d0e21e 2818large or too small, then your version of Perl was probably compiled
2f9daede 2819with the wrong number of RANDBITS.)
a0d0e21e
LW
2820
2821=item read FILEHANDLE,SCALAR,LENGTH,OFFSET
2822
2823=item read FILEHANDLE,SCALAR,LENGTH
2824
2825Attempts to read LENGTH bytes of data into variable SCALAR from the
3b02c43c
GS
2826specified FILEHANDLE. Returns the number of bytes actually read,
2827C<0> at end of file, or undef if there was an error. SCALAR will be grown
2828or shrunk to the length actually read. An OFFSET may be specified to
2829place the read data at some other place than the beginning of the
f86cebdf
GS
2830string. This call is actually implemented in terms of stdio's fread(3)
2831call. To get a true read(2) system call, see C<sysread()>.
a0d0e21e
LW
2832
2833=item readdir DIRHANDLE
2834
7660c0ab 2835Returns the next directory entry for a directory opened by C<opendir()>.
5a964f20 2836If used in list context, returns all the rest of the entries in the
a0d0e21e 2837directory. If there are no more entries, returns an undefined value in
5a964f20 2838scalar context or a null list in list context.
a0d0e21e 2839
7660c0ab 2840If you're planning to filetest the return values out of a C<readdir()>, you'd
5f05dabc 2841better prepend the directory in question. Otherwise, because we didn't
7660c0ab 2842C<chdir()> there, it would have been testing the wrong file.
cb1a09d0
AD
2843
2844 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
2845 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
2846 closedir DIR;
2847
84902520
TB
2848=item readline EXPR
2849
5a964f20 2850Reads from the filehandle whose typeglob is contained in EXPR. In scalar context, a single line
84902520
TB
2851is read and returned. In list context, reads until end-of-file is
2852reached and returns a list of lines (however you've defined lines
7660c0ab 2853with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
84902520
TB
2854This is the internal function implementing the C<E<lt>EXPRE<gt>>
2855operator, but you can use it directly. The C<E<lt>EXPRE<gt>>
2856operator is discussed in more detail in L<perlop/"I/O Operators">.
2857
5a964f20
TC
2858 $line = <STDIN>;
2859 $line = readline(*STDIN); # same thing
2860
a0d0e21e
LW
2861=item readlink EXPR
2862
54310121 2863=item readlink
bbce6d69 2864
a0d0e21e
LW
2865Returns the value of a symbolic link, if symbolic links are
2866implemented. If not, gives a fatal error. If there is some system
184e9718 2867error, returns the undefined value and sets C<$!> (errno). If EXPR is
7660c0ab 2868omitted, uses C<$_>.
a0d0e21e 2869
84902520
TB
2870=item readpipe EXPR
2871
5a964f20 2872EXPR is executed as a system command.
84902520
TB
2873The collected standard output of the command is returned.
2874In scalar context, it comes back as a single (potentially
2875multi-line) string. In list context, returns a list of lines
7660c0ab 2876(however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
84902520
TB
2877This is the internal function implementing the C<qx/EXPR/>
2878operator, but you can use it directly. The C<qx/EXPR/>
2879operator is discussed in more detail in L<perlop/"I/O Operators">.
2880
a0d0e21e
LW
2881=item recv SOCKET,SCALAR,LEN,FLAGS
2882
2883Receives a message on a socket. Attempts to receive LENGTH bytes of
2884data into variable SCALAR from the specified SOCKET filehandle.
7660c0ab 2885Actually does a C C<recvfrom()>, so that it can return the address of the
a0d0e21e
LW
2886sender. Returns the undefined value if there's an error. SCALAR will
2887be grown or shrunk to the length actually read. Takes the same flags
54310121 2888as the system call of the same name.
4633a7c4 2889See L<perlipc/"UDP: Message Passing"> for examples.
a0d0e21e
LW
2890
2891=item redo LABEL
2892
2893=item redo
2894
2895The C<redo> command restarts the loop block without evaluating the
a189bff9 2896conditional again. The L</continue> block, if any, is not executed. If
a0d0e21e
LW
2897the LABEL is omitted, the command refers to the innermost enclosing
2898loop. This command is normally used by programs that want to lie to
2899themselves about what was just input:
2900
2901 # a simpleminded Pascal comment stripper
2902 # (warning: assumes no { or } in strings)
4633a7c4 2903 LINE: while (<STDIN>) {
a0d0e21e
LW
2904 while (s|({.*}.*){.*}|$1 |) {}
2905 s|{.*}| |;
2906 if (s|{.*| |) {
2907 $front = $_;
2908 while (<STDIN>) {
2909 if (/}/) { # end of comment?
5a964f20 2910 s|^|$front\{|;
4633a7c4 2911 redo LINE;
a0d0e21e
LW
2912 }
2913 }
2914 }
2915 print;
2916 }
2917
4968c1e4
G
2918C<redo> cannot be used to retry a block which returns a value such as
2919C<eval {}>, C<sub {}> or C<do {}>.
2920
a189bff9 2921See also L</continue> for an illustration of how L</last>, L</next>, and
1d2dff63
GS
2922C<redo> work.
2923
a189bff9
JH
2924See also L<perlsyn>.
2925
a0d0e21e
LW
2926=item ref EXPR
2927
54310121 2928=item ref
bbce6d69 2929
2f9daede 2930Returns a TRUE value if EXPR is a reference, FALSE otherwise. If EXPR
7660c0ab 2931is not specified, C<$_> will be used. The value returned depends on the
bbce6d69 2932type of thing the reference is a reference to.
a0d0e21e
LW
2933Builtin types include:
2934
2935 REF
2936 SCALAR
2937 ARRAY
2938 HASH
2939 CODE
2940 GLOB
2941
54310121 2942If the referenced object has been blessed into a package, then that package
7660c0ab 2943name is returned instead. You can think of C<ref()> as a C<typeof()> operator.
a0d0e21e
LW
2944
2945 if (ref($r) eq "HASH") {
aa689395 2946 print "r is a reference to a hash.\n";
54310121 2947 }
5a964f20 2948 if (!ref($r)) {
a0d0e21e 2949 print "r is not a reference at all.\n";
54310121 2950 }
a0d0e21e
LW
2951
2952See also L<perlref>.
2953
2954=item rename OLDNAME,NEWNAME
2955
7660c0ab 2956Changes the name of a file. Returns C<1> for success, C<0> otherwise. Will
5f05dabc 2957not work across file system boundaries.
a0d0e21e
LW
2958
2959=item require EXPR
2960
2961=item require
2962
7660c0ab 2963Demands some semantics specified by EXPR, or by C<$_> if EXPR is not
a0d0e21e 2964supplied. If EXPR is numeric, demands that the current version of Perl
184e9718 2965(C<$]> or $PERL_VERSION) be equal or greater than EXPR.
a0d0e21e
LW
2966
2967Otherwise, demands that a library file be included if it hasn't already
2968been included. The file is included via the do-FILE mechanism, which is
7660c0ab 2969essentially just a variety of C<eval()>. Has semantics similar to the following
a0d0e21e
LW
2970subroutine:
2971
2972 sub require {
5a964f20 2973 my($filename) = @_;
a0d0e21e 2974 return 1 if $INC{$filename};
5a964f20 2975 my($realfilename,$result);
a0d0e21e
LW
2976 ITER: {
2977 foreach $prefix (@INC) {
2978 $realfilename = "$prefix/$filename";
2979 if (-f $realfilename) {
2980 $result = do $realfilename;
2981 last ITER;
2982 }
2983 }
2984 die "Can't find $filename in \@INC";
2985 }
2986 die $@ if $@;
2987 die "$filename did not return true value" unless $result;
2988 $INC{$filename} = $realfilename;
5a964f20 2989 return $result;
a0d0e21e
LW
2990 }
2991
2992Note that the file will not be included twice under the same specified
2993name. The file must return TRUE as the last statement to indicate
2994successful execution of any initialization code, so it's customary to
7660c0ab 2995end such a file with "C<1;>" unless you're sure it'll return TRUE
a0d0e21e
LW
2996otherwise. But it's better just to put the "C<1;>", in case you add more
2997statements.
2998
54310121 2999If EXPR is a bareword, the require assumes a "F<.pm>" extension and
da0045b7 3000replaces "F<::>" with "F</>" in the filename for you,
54310121 3001to make it easy to load standard modules. This form of loading of
a0d0e21e
LW
3002modules does not risk altering your namespace.
3003
ee580363
GS
3004In other words, if you try this:
3005
f86cebdf 3006 require Foo::Bar; # a splendid bareword
ee580363 3007
7660c0ab
A
3008The require function will actually look for the "F<Foo/Bar.pm>" file in the
3009directories specified in the C<@INC> array.
ee580363 3010
5a964f20 3011But if you try this:
ee580363
GS
3012
3013 $class = 'Foo::Bar';
f86cebdf 3014 require $class; # $class is not a bareword
5a964f20 3015 #or
f86cebdf 3016 require "Foo::Bar"; # not a bareword because of the ""
ee580363 3017
7660c0ab
A
3018The require function will look for the "F<Foo::Bar>" file in the @INC array and
3019will complain about not finding "F<Foo::Bar>" there. In this case you can do:
ee580363
GS
3020
3021 eval "require $class";
3022
3023For a yet-more-powerful import facility, see L</use> and L<perlmod>.
a0d0e21e
LW
3024
3025=item reset EXPR
3026
3027=item reset
3028
3029Generally used in a C<continue> block at the end of a loop to clear
7660c0ab 3030variables and reset C<??> searches so that they work again. The
a0d0e21e
LW
3031expression is interpreted as a list of single characters (hyphens
3032allowed for ranges). All variables and arrays beginning with one of
3033those letters are reset to their pristine state. If the expression is
7660c0ab 3034omitted, one-match searches (C<?pattern?>) are reset to match again. Resets
5f05dabc 3035only variables or searches in the current package. Always returns
a0d0e21e
LW
30361. Examples:
3037
3038 reset 'X'; # reset all X variables
3039 reset 'a-z'; # reset lower case variables
3040 reset; # just reset ?? searches
3041
7660c0ab
A
3042Resetting C<"A-Z"> is not recommended because you'll wipe out your
3043C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package variables--lexical variables
a0d0e21e 3044are unaffected, but they clean themselves up on scope exit anyway,
da0045b7 3045so you'll probably want to use them instead. See L</my>.
a0d0e21e 3046
54310121
PP
3047=item return EXPR
3048
3049=item return
3050
7660c0ab 3051Returns from a subroutine, C<eval()>, or C<do FILE> with the value
5a964f20 3052given in EXPR. Evaluation of EXPR may be in list, scalar, or void
54310121 3053context, depending on how the return value will be used, and the context
7660c0ab 3054may vary from one execution to the next (see C<wantarray()>). If no EXPR
5a964f20
TC
3055is given, returns an empty list in list context, an undefined value in
3056scalar context, or nothing in a void context.
a0d0e21e 3057
68dc0745
PP
3058(Note that in the absence of a return, a subroutine, eval, or do FILE
3059will automatically return the value of the last expression evaluated.)
a0d0e21e
LW
3060
3061=item reverse LIST
3062
5a964f20
TC
3063In list context, returns a list value consisting of the elements
3064of LIST in the opposite order. In scalar context, concatenates the
a0ed51b3
LW
3065elements of LIST, and returns a string value with all the characters
3066in the opposite order.
4633a7c4 3067
2f9daede 3068 print reverse <>; # line tac, last line first
4633a7c4 3069
2f9daede 3070 undef $/; # for efficiency of <>
a0ed51b3 3071 print scalar reverse <>; # character tac, last line tsrif
2f9daede
TPG
3072
3073This operator is also handy for inverting a hash, although there are some
3074caveats. If a value is duplicated in the original hash, only one of those
3075can be represented as a key in the inverted hash. Also, this has to
3076unwind one hash and build a whole new one, which may take some time
3077on a large hash.
3078
3079 %by_name = reverse %by_address; # Invert the hash
a0d0e21e
LW
3080
3081=item rewinddir DIRHANDLE
3082
3083Sets the current position to the beginning of the directory for the
7660c0ab 3084C<readdir()> routine on DIRHANDLE.
a0d0e21e
LW
3085
3086=item rindex STR,SUBSTR,POSITION
3087
3088=item rindex STR,SUBSTR
3089
3090Works just like index except that it returns the position of the LAST
3091occurrence of SUBSTR in STR. If POSITION is specified, returns the
3092last occurrence at or before that position.
3093
3094=item rmdir FILENAME
3095
54310121 3096=item rmdir
bbce6d69 3097
5a964f20
TC
3098Deletes the directory specified by FILENAME if that directory is empty. If it
3099succeeds it returns TRUE, otherwise it returns FALSE and sets C<$!> (errno). If
7660c0ab 3100FILENAME is omitted, uses C<$_>.
a0d0e21e
LW
3101
3102=item s///
3103
3104The substitution operator. See L<perlop>.
3105
3106=item scalar EXPR
3107
5a964f20 3108Forces EXPR to be interpreted in scalar context and returns the value
54310121 3109of EXPR.
cb1a09d0
AD
3110
3111 @counts = ( scalar @a, scalar @b, scalar @c );
3112
54310121 3113There is no equivalent operator to force an expression to
5a964f20 3114be interpolated in list context because it's in practice never
cb1a09d0
AD
3115needed. If you really wanted to do so, however, you could use
3116the construction C<@{[ (some expression) ]}>, but usually a simple
3117C<(some expression)> suffices.
a0d0e21e
LW
3118
3119=item seek FILEHANDLE,POSITION,WHENCE
3120
ab4f32c2 3121Sets FILEHANDLE's position, just like the C<fseek()> call of C<stdio()>.
8903cb82 3122FILEHANDLE may be an expression whose value gives the name of the
7660c0ab
A
3123filehandle. The values for WHENCE are C<0> to set the new position to
3124POSITION, C<1> to set it to the current position plus POSITION, and C<2> to
8903cb82 3125set it to EOF plus POSITION (typically negative). For WHENCE you may
7660c0ab
A
3126use the constants C<SEEK_SET>, C<SEEK_CUR>, and C<SEEK_END> from either the
3127C<IO::Seekable> or the POSIX module. Returns C<1> upon success, C<0> otherwise.
8903cb82 3128
7660c0ab
A
3129If you want to position file for C<sysread()> or C<syswrite()>, don't use
3130C<seek()> -- buffering makes its effect on the file's system position
3131unpredictable and non-portable. Use C<sysseek()> instead.
a0d0e21e 3132
cb1a09d0
AD
3133On some systems you have to do a seek whenever you switch between reading
3134and writing. Amongst other things, this may have the effect of calling
f86cebdf 3135stdio's clearerr(3). A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving
8903cb82 3136the file position:
cb1a09d0
AD
3137
3138 seek(TEST,0,1);
3139
3140This is also useful for applications emulating C<tail -f>. Once you hit
3141EOF on your read, and then sleep for a while, you might have to stick in a
7660c0ab 3142seek() to reset things. The C<seek()> doesn't change the current position,
8903cb82
PP
3143but it I<does> clear the end-of-file condition on the handle, so that the
3144next C<E<lt>FILEE<gt>> makes Perl try again to read something. We hope.
cb1a09d0
AD
3145
3146If that doesn't work (some stdios are particularly cantankerous), then
3147you may need something more like this:
3148
3149 for (;;) {
f86cebdf
GS
3150 for ($curpos = tell(FILE); $_ = <FILE>;
3151 $curpos = tell(FILE)) {
cb1a09d0
AD
3152 # search for some stuff and put it into files
3153 }
3154 sleep($for_a_while);
3155 seek(FILE, $curpos, 0);
3156 }
3157
a0d0e21e
LW
3158=item seekdir DIRHANDLE,POS
3159
7660c0ab
A
3160Sets the current position for the C<readdir()> routine on DIRHANDLE. POS
3161must be a value returned by C<telldir()>. Has the same caveats about
a0d0e21e
LW
3162possible directory compaction as the corresponding system library
3163routine.
3164
3165=item select FILEHANDLE
3166
3167=item select
3168
3169Returns the currently selected filehandle. Sets the current default
3170filehandle for output, if FILEHANDLE is supplied. This has two
ab4f32c2 3171effects: first, a C<write()> or a C<print()> without a filehandle will
a0d0e21e
LW
3172default to this FILEHANDLE. Second, references to variables related to
3173output will refer to this output channel. For example, if you have to
3174set the top of form format for more than one output channel, you might
3175do the following:
3176
3177 select(REPORT1);
3178 $^ = 'report1_top';
3179 select(REPORT2);
3180 $^ = 'report2_top';
3181
3182FILEHANDLE may be an expression whose value gives the name of the
3183actual filehandle. Thus:
3184
3185 $oldfh = select(STDERR); $| = 1; select($oldfh);
3186
4633a7c4
LW
3187Some programmers may prefer to think of filehandles as objects with
3188methods, preferring to write the last example as:
a0d0e21e 3189
28757baa 3190 use IO::Handle;
a0d0e21e
LW
3191 STDERR->autoflush(1);
3192
3193=item select RBITS,WBITS,EBITS,TIMEOUT
3194
f86cebdf 3195This calls the select(2) system call with the bit masks specified, which
7660c0ab 3196can be constructed using C<fileno()> and C<vec()>, along these lines:
a0d0e21e
LW
3197
3198 $rin = $win = $ein = '';
3199 vec($rin,fileno(STDIN),1) = 1;
3200 vec($win,fileno(STDOUT),1) = 1;
3201 $ein = $rin | $win;
3202
3203If you want to select on many filehandles you might wish to write a
3204subroutine:
3205
3206 sub fhbits {
5a964f20
TC
3207 my(@fhlist) = split(' ',$_[0]);
3208 my($bits);
a0d0e21e
LW
3209 for (@fhlist) {
3210 vec($bits,fileno($_),1) = 1;
3211 }
3212 $bits;
3213 }
4633a7c4 3214 $rin = fhbits('STDIN TTY SOCK');
a0d0e21e
LW
3215
3216The usual idiom is:
3217
3218 ($nfound,$timeleft) =
3219 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
3220
54310121 3221or to block until something becomes ready just do this
a0d0e21e
LW
3222
3223 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
3224
7660c0ab
A
3225Most systems do not bother to return anything useful in C<$timeleft>, so
3226calling select() in scalar context just returns C<$nfound>.
c07a80fd 3227
5f05dabc 3228Any of the bit masks can also be undef. The timeout, if specified, is
a0d0e21e 3229in seconds, which may be fractional. Note: not all implementations are
7660c0ab
A
3230capable of returning theC<$timeleft>. If not, they always return
3231C<$timeleft> equal to the supplied C<$timeout>.
a0d0e21e 3232
ff68c719 3233You can effect a sleep of 250 milliseconds this way:
a0d0e21e
LW
3234
3235 select(undef, undef, undef, 0.25);
3236
7660c0ab
A
3237B<WARNING>: One should not attempt to mix buffered I/O (like C<read()>
3238or E<lt>FHE<gt>) with C<select()>, except as permitted by POSIX, and even
3239then only on POSIX systems. You have to use C<sysread()> instead.
a0d0e21e
LW
3240
3241=item semctl ID,SEMNUM,CMD,ARG
3242
ab4f32c2 3243Calls the System V IPC function C<semctl()>. You'll probably have to say
0ade1984
JH
3244
3245 use IPC::SysV;
3246
3247first to get the correct constant definitions. If CMD is IPC_STAT or
3248GETALL, then ARG must be a variable which will hold the returned
ab4f32c2 3249semid_ds structure or semaphore value array. Returns like C<ioctl()>: the
7660c0ab
A
3250undefined value for error, "C<0> but true" for zero, or the actual return
3251value otherwise. See also C<IPC::SysV> and C<IPC::Semaphore> documentation.
a0d0e21e
LW
3252
3253=item semget KEY,NSEMS,FLAGS
3254
3255Calls the System V IPC function semget. Returns the semaphore id, or
7660c0ab
A
3256the undefined value if there is an error. See also C<IPC::SysV> and
3257C<IPC::SysV::Semaphore> documentation.
a0d0e21e
LW
3258
3259=item semop KEY,OPSTRING
3260
3261Calls the System V IPC function semop to perform semaphore operations
3262such as signaling and waiting. OPSTRING must be a packed array of
3263semop structures. Each semop structure can be generated with
3264C<pack("sss", $semnum, $semop, $semflag)>. The number of semaphore
3265operations is implied by the length of OPSTRING. Returns TRUE if
3266successful, or FALSE if there is an error. As an example, the
7660c0ab 3267following code waits on semaphore C<$semnum> of semaphore id C<$semid>:
a0d0e21e
LW
3268
3269 $semop = pack("sss", $semnum, -1, 0);
3270 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
3271
7660c0ab
A
3272To signal the semaphore, replace C<-1> with C<1>. See also C<IPC::SysV>
3273and C<IPC::SysV::Semaphore> documentation.
a0d0e21e
LW
3274
3275=item send SOCKET,MSG,FLAGS,TO
3276
3277=item send SOCKET,MSG,FLAGS
3278
3279Sends a message on a socket. Takes the same flags as the system call
3280of the same name. On unconnected sockets you must specify a
7660c0ab 3281destination to send TO, in which case it does a C C<sendto()>. Returns
a0d0e21e
LW
3282the number of characters sent, or the undefined value if there is an
3283error.
4633a7c4 3284See L<perlipc/"UDP: Message Passing"> for examples.
a0d0e21e
LW
3285
3286=item setpgrp PID,PGRP
3287
7660c0ab 3288Sets the current process group for the specified PID, C<0> for the current
a0d0e21e 3289process. Will produce a fatal error if used on a machine that doesn't
f86cebdf 3290implement setpgrp(2). If the arguments are omitted, it defaults to
7660c0ab
A
3291C<0,0>. Note that the POSIX version of C<setpgrp()> does not accept any
3292arguments, so only setpgrp C<0,0> is portable.
a0d0e21e
LW
3293
3294=item setpriority WHICH,WHO,PRIORITY
3295
3296Sets the current priority for a process, a process group, or a user.
f86cebdf
GS
3297(See setpriority(2).) Will produce a fatal error if used on a machine
3298that doesn't implement setpriority(2).