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1=head1 NAME
2
3perlfunc - Perl builtin functions
4
5=head1 DESCRIPTION
6
7The functions in this section can serve as terms in an expression.
8They fall into two major categories: list operators and named unary
9operators. These differ in their precedence relationship with a
10following comma. (See the precedence table in L<perlop>.) List
11operators take more than one argument, while unary operators can never
12take more than one argument. Thus, a comma terminates the argument of
13a unary operator, but merely separates the arguments of a list
14operator. A unary operator generally provides a scalar context to its
15argument, while a list operator may provide either scalar and list
16contexts for its arguments. If it does both, the scalar arguments will
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17be first, and the list argument will follow. (Note that there can ever
18be only one list argument.) For instance, splice() has three scalar
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19arguments followed by a list.
20
21In the syntax descriptions that follow, list operators that expect a
22list (and provide list context for the elements of the list) are shown
23with LIST as an argument. Such a list may consist of any combination
24of scalar arguments or list values; the list values will be included
25in the list as if each individual element were interpolated at that
26point in the list, forming a longer single-dimensional list value.
27Elements of the LIST should be separated by commas.
28
29Any function in the list below may be used either with or without
30parentheses around its arguments. (The syntax descriptions omit the
5f05dabc 31parentheses.) If you use the parentheses, the simple (but occasionally
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32surprising) rule is this: It I<LOOKS> like a function, therefore it I<IS> a
33function, and precedence doesn't matter. Otherwise it's a list
34operator or unary operator, and precedence does matter. And whitespace
35between the function and left parenthesis doesn't count--so you need to
36be careful sometimes:
37
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38 print 1+2+4; # Prints 7.
39 print(1+2) + 4; # Prints 3.
40 print (1+2)+4; # Also prints 3!
41 print +(1+2)+4; # Prints 7.
42 print ((1+2)+4); # Prints 7.
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43
44If you run Perl with the B<-w> switch it can warn you about this. For
45example, the third line above produces:
46
47 print (...) interpreted as function at - line 1.
48 Useless use of integer addition in void context at - line 1.
49
50For functions that can be used in either a scalar or list context,
54310121 51nonabortive failure is generally indicated in a scalar context by
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52returning the undefined value, and in a list context by returning the
53null list.
54
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55Remember the following important rule: There is B<no rule> that relates
56the behavior of an expression in list context to its behavior in scalar
57context, or vice versa. It might do two totally different things.
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58Each operator and function decides which sort of value it would be most
59appropriate to return in a scalar context. Some operators return the
5a964f20 60length of the list that would have been returned in list context. Some
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61operators return the first value in the list. Some operators return the
62last value in the list. Some operators return a count of successful
63operations. In general, they do what you want, unless you want
64consistency.
65
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66An named array in scalar context is quite different from what would at
67first glance appear to be a list in scalar context. You can't get a list
68like C<(1,2,3)> into being in scalar context, because the compiler knows
69the context at compile time. It would generate the scalar comma operator
70there, not the list construction version of the comma. That means it
71was never a list to start with.
72
73In general, functions in Perl that serve as wrappers for system calls
f86cebdf 74of the same name (like chown(2), fork(2), closedir(2), etc.) all return
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75true when they succeed and C<undef> otherwise, as is usually mentioned
76in the descriptions below. This is different from the C interfaces,
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77which return C<-1> on failure. Exceptions to this rule are C<wait()>,
78C<waitpid()>, and C<syscall()>. System calls also set the special C<$!>
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79variable on failure. Other functions do not, except accidentally.
80
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81=head2 Perl Functions by Category
82
83Here are Perl's functions (including things that look like
5a964f20 84functions, like some keywords and named operators)
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85arranged by category. Some functions appear in more
86than one place.
87
88=over
89
90=item Functions for SCALARs or strings
91
22fae026 92C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>,
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93C<length>, C<oct>, C<ord>, C<pack>, C<q/STRING/>, C<qq/STRING/>, C<reverse>,
94C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///>
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95
96=item Regular expressions and pattern matching
97
ab4f32c2 98C<m//>, C<pos>, C<quotemeta>, C<s///>, C<split>, C<study>, C<qr//>
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99
100=item Numeric functions
101
22fae026
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102C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>,
103C<sin>, C<sqrt>, C<srand>
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104
105=item Functions for real @ARRAYs
106
22fae026 107C<pop>, C<push>, C<shift>, C<splice>, C<unshift>
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108
109=item Functions for list data
110
ab4f32c2 111C<grep>, C<join>, C<map>, C<qw/STRING/>, C<reverse>, C<sort>, C<unpack>
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112
113=item Functions for real %HASHes
114
22fae026 115C<delete>, C<each>, C<exists>, C<keys>, C<values>
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116
117=item Input and output functions
118
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119C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
120C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
121C<readdir>, C<rewinddir>, C<seek>, C<seekdir>, C<select>, C<syscall>,
122C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>,
123C<warn>, C<write>
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124
125=item Functions for fixed length data or records
126
22fae026 127C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec>
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128
129=item Functions for filehandles, files, or directories
130
22fae026 131C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>,
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132C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>,
133C<readlink>, C<rename>, C<rmdir>, C<stat>, C<symlink>, C<umask>,
134C<unlink>, C<utime>
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135
136=item Keywords related to the control flow of your perl program
137
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138C<caller>, C<continue>, C<die>, C<do>, C<dump>, C<else>, C<elsif>,
139C<eval>, C<exit>, C<for>, C<foreach>, C<goto>, C<if>, C<last>,
140C<next>, C<redo>, C<return>, C<sub>, C<unless>, C<wantarray>,
141C<while>, C<until>
cb1a09d0 142
54310121 143=item Keywords related to scoping
cb1a09d0 144
22fae026 145C<caller>, C<import>, C<local>, C<my>, C<package>, C<use>
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146
147=item Miscellaneous functions
148
22fae026
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149C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<reset>,
150C<scalar>, C<undef>, C<wantarray>
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151
152=item Functions for processes and process groups
153
22fae026 154C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
ab4f32c2 155C<pipe>, C<qx/STRING/>, C<setpgrp>, C<setpriority>, C<sleep>, C<system>,
22fae026 156C<times>, C<wait>, C<waitpid>
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157
158=item Keywords related to perl modules
159
22fae026 160C<do>, C<import>, C<no>, C<package>, C<require>, C<use>
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161
162=item Keywords related to classes and object-orientedness
163
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164C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
165C<untie>, C<use>
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166
167=item Low-level socket functions
168
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169C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
170C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
171C<socket>, C<socketpair>
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172
173=item System V interprocess communication functions
174
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175C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
176C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>
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177
178=item Fetching user and group info
179
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180C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
181C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
182C<getpwuid>, C<setgrent>, C<setpwent>
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183
184=item Fetching network info
185
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186C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
187C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
188C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
189C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
190C<setnetent>, C<setprotoent>, C<setservent>
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191
192=item Time-related functions
193
22fae026 194C<gmtime>, C<localtime>, C<time>, C<times>
cb1a09d0 195
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196=item Functions new in perl5
197
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198C<abs>, C<bless>, C<chomp>, C<chr>, C<exists>, C<formline>, C<glob>,
199C<import>, C<lc>, C<lcfirst>, C<map>, C<my>, C<no>, C<prototype>, C<qx>,
200C<qw>, C<readline>, C<readpipe>, C<ref>, C<sub*>, C<sysopen>, C<tie>,
201C<tied>, C<uc>, C<ucfirst>, C<untie>, C<use>
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202
203* - C<sub> was a keyword in perl4, but in perl5 it is an
5a964f20 204operator, which can be used in expressions.
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205
206=item Functions obsoleted in perl5
207
22fae026 208C<dbmclose>, C<dbmopen>
37798a01 209
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210=back
211
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212=head2 Portability
213
214Perl was born in UNIX and therefore it can access all the common UNIX
215system calls. In non-UNIX environments the functionality of many
216UNIX system calls may not be available or the details of the available
217functionality may be slightly different. The Perl functions affected
218by this are:
219
220C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>,
221C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>,
222C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>,
223C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostent>,
224C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
225C<getppid>, C<getprgp>, C<getpriority>, C<getprotobynumber>,
226C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>,
227C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>,
228C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>,
229C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<select>, C<semctl>,
230C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>,
231C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>,
232C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>,
233C<shmwrite>, C<socketpair>, C<stat>, C<symlink>, C<syscall>,
234C<sysopen>, C<system>, C<times>, C<truncate>, C<umask>, C<utime>,
235C<wait>, C<waitpid>
236
237For more information about the portability of these functions, see
238L<perlport> and other available platform-specific documentation.
239
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240=head2 Alphabetical Listing of Perl Functions
241
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242=over 8
243
22fae026 244=item I<-X> FILEHANDLE
a0d0e21e 245
22fae026 246=item I<-X> EXPR
a0d0e21e 247
22fae026 248=item I<-X>
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249
250A file test, where X is one of the letters listed below. This unary
251operator takes one argument, either a filename or a filehandle, and
252tests the associated file to see if something is true about it. If the
7660c0ab 253argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
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254Unless otherwise documented, it returns C<1> for TRUE and C<''> for FALSE, or
255the undefined value if the file doesn't exist. Despite the funny
256names, precedence is the same as any other named unary operator, and
257the argument may be parenthesized like any other unary operator. The
258operator may be any of:
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259X<-r>X<-w>X<-x>X<-o>X<-R>X<-W>X<-X>X<-O>X<-e>X<-z>X<-s>X<-f>X<-d>X<-l>X<-p>
260X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
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261
262 -r File is readable by effective uid/gid.
263 -w File is writable by effective uid/gid.
264 -x File is executable by effective uid/gid.
265 -o File is owned by effective uid.
266
267 -R File is readable by real uid/gid.
268 -W File is writable by real uid/gid.
269 -X File is executable by real uid/gid.
270 -O File is owned by real uid.
271
272 -e File exists.
273 -z File has zero size.
54310121 274 -s File has nonzero size (returns size).
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275
276 -f File is a plain file.
277 -d File is a directory.
278 -l File is a symbolic link.
9c4d0f16 279 -p File is a named pipe (FIFO), or Filehandle is a pipe.
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280 -S File is a socket.
281 -b File is a block special file.
282 -c File is a character special file.
283 -t Filehandle is opened to a tty.
284
285 -u File has setuid bit set.
286 -g File has setgid bit set.
287 -k File has sticky bit set.
288
289 -T File is a text file.
290 -B File is a binary file (opposite of -T).
291
292 -M Age of file in days when script started.
293 -A Same for access time.
294 -C Same for inode change time.
295
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296Example:
297
298 while (<>) {
299 chop;
300 next unless -f $_; # ignore specials
5a964f20 301 #...
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302 }
303
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304The interpretation of the file permission operators C<-r>, C<-R>,
305C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
306of the file and the uids and gids of the user. There may be other
307reasons you can't actually read, write, or execute the file. Such
308reasons may be for example network filesystem access controls, ACLs
309(access control lists), read-only filesystems, and unrecognized
310executable formats.
311
312Also note that, for the superuser on the local filesystems, C<-r>,
313C<-R>, C<-w>, and C<-W> always return 1, and C<-x> and C<-X> return 1
314if any execute bit is set in the mode. Scripts run by the superuser
315may thus need to do a stat() to determine the actual mode of the file,
316or temporarily set the uid to something else.
317
318If you are using ACLs, there is a pragma called C<filetest> that may
319produce more accurate results than the bare stat() mode bits.
320
321When under the C<use filetest 'access'> the above-mentioned filetests
322will test whether the permission can (not) be granted using the
468541a8 323access() family of system calls. Also note that the C<-x> and C<-X> may
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324under this pragma return true even if there are no execute permission
325bits set (nor any extra execute permission ACLs). This strangeness is
326due to the underlying system calls' definitions. Read the
327documentation for the C<filetest> pragma for more information.
328
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329Note that C<-s/a/b/> does not do a negated substitution. Saying
330C<-exp($foo)> still works as expected, however--only single letters
331following a minus are interpreted as file tests.
332
333The C<-T> and C<-B> switches work as follows. The first block or so of the
334file is examined for odd characters such as strange control codes or
5a964f20 335characters with the high bit set. If too many strange characters (E<gt>30%)
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336are found, it's a C<-B> file, otherwise it's a C<-T> file. Also, any file
337containing null in the first block is considered a binary file. If C<-T>
338or C<-B> is used on a filehandle, the current stdio buffer is examined
339rather than the first block. Both C<-T> and C<-B> return TRUE on a null
54310121 340file, or a file at EOF when testing a filehandle. Because you have to
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341read a file to do the C<-T> test, on most occasions you want to use a C<-f>
342against the file first, as in C<next unless -f $file && -T $file>.
a0d0e21e 343
7660c0ab 344If any of the file tests (or either the C<stat()> or C<lstat()> operators) are given
28757baa 345the special filehandle consisting of a solitary underline, then the stat
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346structure of the previous file test (or stat operator) is used, saving
347a system call. (This doesn't work with C<-t>, and you need to remember
348that lstat() and C<-l> will leave values in the stat structure for the
349symbolic link, not the real file.) Example:
350
351 print "Can do.\n" if -r $a || -w _ || -x _;
352
353 stat($filename);
354 print "Readable\n" if -r _;
355 print "Writable\n" if -w _;
356 print "Executable\n" if -x _;
357 print "Setuid\n" if -u _;
358 print "Setgid\n" if -g _;
359 print "Sticky\n" if -k _;
360 print "Text\n" if -T _;
361 print "Binary\n" if -B _;
362
363=item abs VALUE
364
54310121 365=item abs
bbce6d69 366
a0d0e21e 367Returns the absolute value of its argument.
7660c0ab 368If VALUE is omitted, uses C<$_>.
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369
370=item accept NEWSOCKET,GENERICSOCKET
371
f86cebdf 372Accepts an incoming socket connect, just as the accept(2) system call
a0d0e21e 373does. Returns the packed address if it succeeded, FALSE otherwise.
4633a7c4 374See example in L<perlipc/"Sockets: Client/Server Communication">.
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375
376=item alarm SECONDS
377
54310121 378=item alarm
bbce6d69 379
a0d0e21e 380Arranges to have a SIGALRM delivered to this process after the
bbce6d69 381specified number of seconds have elapsed. If SECONDS is not specified,
7660c0ab 382the value stored in C<$_> is used. (On some machines,
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383unfortunately, the elapsed time may be up to one second less than you
384specified because of how seconds are counted.) Only one timer may be
385counting at once. Each call disables the previous timer, and an
7660c0ab 386argument of C<0> may be supplied to cancel the previous timer without
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387starting a new one. The returned value is the amount of time remaining
388on the previous timer.
389
4633a7c4 390For delays of finer granularity than one second, you may use Perl's
f86cebdf 391C<syscall()> interface to access setitimer(2) if your system supports it,
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392or else see L</select()>. It is usually a mistake to intermix C<alarm()>
393and C<sleep()> calls.
a0d0e21e 394
7660c0ab 395If you want to use C<alarm()> to time out a system call you need to use an
ab4f32c2 396C<eval()>/C<die()> pair. You can't rely on the alarm causing the system call to
f86cebdf 397fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
ab4f32c2 398restart system calls on some systems. Using C<eval()>/C<die()> always works,
5a964f20 399modulo the caveats given in L<perlipc/"Signals">.
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400
401 eval {
f86cebdf 402 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
36477c24 403 alarm $timeout;
ff68c719 404 $nread = sysread SOCKET, $buffer, $size;
36477c24 405 alarm 0;
ff68c719 406 };
ff68c719 407 if ($@) {
f86cebdf 408 die unless $@ eq "alarm\n"; # propagate unexpected errors
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409 # timed out
410 }
411 else {
412 # didn't
413 }
414
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415=item atan2 Y,X
416
417Returns the arctangent of Y/X in the range -PI to PI.
418
7660c0ab 419For the tangent operation, you may use the C<POSIX::tan()>
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420function, or use the familiar relation:
421
422 sub tan { sin($_[0]) / cos($_[0]) }
423
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424=item bind SOCKET,NAME
425
426Binds a network address to a socket, just as the bind system call
427does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
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428packed address of the appropriate type for the socket. See the examples in
429L<perlipc/"Sockets: Client/Server Communication">.
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430
431=item binmode FILEHANDLE
432
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433Arranges for the file to be read or written in "binary" mode in operating
434systems that distinguish between binary and text files. Files that are
435not in binary mode have CR LF sequences translated to LF on input and LF
54310121 436translated to CR LF on output. Binmode has no effect under Unix; in MS-DOS
cb1a09d0 437and similarly archaic systems, it may be imperative--otherwise your
54310121 438MS-DOS-damaged C library may mangle your file. The key distinction between
ab4f32c2 439systems that need C<binmode()> and those that don't is their text file
5a964f20 440formats. Systems like Unix, MacOS, and Plan9 that delimit lines with a single
7660c0ab 441character, and that encode that character in C as C<"\n">, do not need
ab4f32c2 442C<binmode()>. The rest need it. If FILEHANDLE is an expression, the value
cb1a09d0 443is taken as the name of the filehandle.
a0d0e21e 444
4633a7c4 445=item bless REF,CLASSNAME
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446
447=item bless REF
448
28757baa 449This function tells the thingy referenced by REF that it is now
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450an object in the CLASSNAME package--or the current package if no CLASSNAME
451is specified, which is often the case. It returns the reference for
7660c0ab 452convenience, because a C<bless()> is often the last thing in a constructor.
4633a7c4 453Always use the two-argument version if the function doing the blessing
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454might be inherited by a derived class. See L<perltoot> and L<perlobj>
455for more about the blessing (and blessings) of objects.
a0d0e21e 456
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457Creating objects in lowercased CLASSNAMEs should be avoided. Such
458namespaces should be considered reserved for Perl pragmata and objects
459that may be created to implement internal operations.
460
461See L<perlmod/"Perl Modules">.
462
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463=item caller EXPR
464
465=item caller
466
5a964f20 467Returns the context of the current subroutine call. In scalar context,
28757baa 468returns the caller's package name if there is a caller, that is, if
7660c0ab 469we're in a subroutine or C<eval()> or C<require()>, and the undefined value
5a964f20 470otherwise. In list context, returns
a0d0e21e 471
748a9306 472 ($package, $filename, $line) = caller;
a0d0e21e
LW
473
474With EXPR, it returns some extra information that the debugger uses to
475print a stack trace. The value of EXPR indicates how many call frames
476to go back before the current one.
477
54310121 478 ($package, $filename, $line, $subroutine,
e7ea3e70
IZ
479 $hasargs, $wantarray, $evaltext, $is_require) = caller($i);
480
7660c0ab 481Here C<$subroutine> may be C<"(eval)"> if the frame is not a subroutine
ab4f32c2 482call, but an C<eval()>. In such a case additional elements C<$evaltext> and
7660c0ab
A
483C<$is_require> are set: C<$is_require> is true if the frame is created by a
484C<require> or C<use> statement, C<$evaltext> contains the text of the
dc848c6f 485C<eval EXPR> statement. In particular, for a C<eval BLOCK> statement,
7660c0ab 486C<$filename> is C<"(eval)">, but C<$evaltext> is undefined. (Note also that
dc848c6f
PP
487each C<use> statement creates a C<require> frame inside an C<eval EXPR>)
488frame.
748a9306
LW
489
490Furthermore, when called from within the DB package, caller returns more
7660c0ab 491detailed information: it sets the list variable C<@DB::args> to be the
54310121 492arguments with which the subroutine was invoked.
748a9306 493
7660c0ab 494Be aware that the optimizer might have optimized call frames away before
ab4f32c2 495C<caller()> had a chance to get the information. That means that C<caller(N)>
7660c0ab
A
496might not return information about the call frame you expect it do, for
497C<N E<gt> 1>. In particular, C<@DB::args> might have information from the
498previous time C<caller()> was called.
499
a0d0e21e
LW
500=item chdir EXPR
501
502Changes the working directory to EXPR, if possible. If EXPR is
503omitted, changes to home directory. Returns TRUE upon success, FALSE
7660c0ab 504otherwise. See example under C<die()>.
a0d0e21e
LW
505
506=item chmod LIST
507
508Changes the permissions of a list of files. The first element of the
4633a7c4 509list must be the numerical mode, which should probably be an octal
2f9daede
TPG
510number, and which definitely should I<not> a string of octal digits:
511C<0644> is okay, C<'0644'> is not. Returns the number of files
dc848c6f 512successfully changed. See also L</oct>, if all you have is a string.
a0d0e21e
LW
513
514 $cnt = chmod 0755, 'foo', 'bar';
515 chmod 0755, @executables;
f86cebdf
GS
516 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
517 # --w----r-T
2f9daede
TPG
518 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
519 $mode = 0644; chmod $mode, 'foo'; # this is best
a0d0e21e
LW
520
521=item chomp VARIABLE
522
523=item chomp LIST
524
525=item chomp
526
3e3baf6d 527This is a slightly safer version of L</chop>. It removes any
a0d0e21e 528line ending that corresponds to the current value of C<$/> (also known as
28757baa
PP
529$INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
530number of characters removed from all its arguments. It's often used to
531remove the newline from the end of an input record when you're worried
532that the final record may be missing its newline. When in paragraph mode
533(C<$/ = "">), it removes all trailing newlines from the string. If
7660c0ab 534VARIABLE is omitted, it chomps C<$_>. Example:
a0d0e21e
LW
535
536 while (<>) {
537 chomp; # avoid \n on last field
538 @array = split(/:/);
5a964f20 539 # ...
a0d0e21e
LW
540 }
541
542You can actually chomp anything that's an lvalue, including an assignment:
543
544 chomp($cwd = `pwd`);
545 chomp($answer = <STDIN>);
546
547If you chomp a list, each element is chomped, and the total number of
548characters removed is returned.
549
550=item chop VARIABLE
551
552=item chop LIST
553
554=item chop
555
556Chops off the last character of a string and returns the character
557chopped. It's used primarily to remove the newline from the end of an
558input record, but is much more efficient than C<s/\n//> because it neither
7660c0ab 559scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
a0d0e21e
LW
560Example:
561
562 while (<>) {
563 chop; # avoid \n on last field
564 @array = split(/:/);
5a964f20 565 #...
a0d0e21e
LW
566 }
567
568You can actually chop anything that's an lvalue, including an assignment:
569
570 chop($cwd = `pwd`);
571 chop($answer = <STDIN>);
572
573If you chop a list, each element is chopped. Only the value of the
ab4f32c2 574last C<chop()> is returned.
a0d0e21e 575
ab4f32c2 576Note that C<chop()> returns the last character. To return all but the last
748a9306
LW
577character, use C<substr($string, 0, -1)>.
578
a0d0e21e
LW
579=item chown LIST
580
581Changes the owner (and group) of a list of files. The first two
582elements of the list must be the I<NUMERICAL> uid and gid, in that order.
583Returns the number of files successfully changed.
584
585 $cnt = chown $uid, $gid, 'foo', 'bar';
586 chown $uid, $gid, @filenames;
587
54310121 588Here's an example that looks up nonnumeric uids in the passwd file:
a0d0e21e
LW
589
590 print "User: ";
591 chop($user = <STDIN>);
5a964f20 592 print "Files: ";
a0d0e21e
LW
593 chop($pattern = <STDIN>);
594
595 ($login,$pass,$uid,$gid) = getpwnam($user)
596 or die "$user not in passwd file";
597
5a964f20 598 @ary = glob($pattern); # expand filenames
a0d0e21e
LW
599 chown $uid, $gid, @ary;
600
54310121 601On most systems, you are not allowed to change the ownership of the
4633a7c4
LW
602file unless you're the superuser, although you should be able to change
603the group to any of your secondary groups. On insecure systems, these
604restrictions may be relaxed, but this is not a portable assumption.
605
a0d0e21e
LW
606=item chr NUMBER
607
54310121 608=item chr
bbce6d69 609
a0d0e21e 610Returns the character represented by that NUMBER in the character set.
a0ed51b3
LW
611For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
612chr(0x263a) is a Unicode smiley face (but only within the scope of a
613C<use utf8>). For the reverse, use L</ord>.
a0d0e21e 614
7660c0ab 615If NUMBER is omitted, uses C<$_>.
bbce6d69 616
a0d0e21e
LW
617=item chroot FILENAME
618
54310121 619=item chroot
bbce6d69 620
5a964f20 621This function works like the system call by the same name: it makes the
4633a7c4 622named directory the new root directory for all further pathnames that
7660c0ab 623begin with a C<"/"> by your process and all its children. (It doesn't
28757baa 624change your current working directory, which is unaffected.) For security
4633a7c4 625reasons, this call is restricted to the superuser. If FILENAME is
ab4f32c2 626omitted, does a C<chroot()> to C<$_>.
a0d0e21e
LW
627
628=item close FILEHANDLE
629
6a518fbc
TP
630=item close
631
a0d0e21e
LW
632Closes the file or pipe associated with the file handle, returning TRUE
633only if stdio successfully flushes buffers and closes the system file
6a518fbc
TP
634descriptor. Closes the currently selected filehandle if the argument
635is omitted.
fb73857a
PP
636
637You don't have to close FILEHANDLE if you are immediately going to do
7660c0ab 638another C<open()> on it, because C<open()> will close it for you. (See
ab4f32c2 639C<open()>.) However, an explicit C<close()> on an input file resets the line
7660c0ab 640counter (C<$.>), while the implicit close done by C<open()> does not.
fb73857a 641
ab4f32c2 642If the file handle came from a piped open C<close()> will additionally
fb73857a
PP
643return FALSE if one of the other system calls involved fails or if the
644program exits with non-zero status. (If the only problem was that the
7660c0ab 645program exited non-zero C<$!> will be set to C<0>.) Also, closing a pipe
5a964f20 646waits for the process executing on the pipe to complete, in case you
fb73857a
PP
647want to look at the output of the pipe afterwards. Closing a pipe
648explicitly also puts the exit status value of the command into C<$?>.
5a964f20 649
fb73857a 650Example:
a0d0e21e 651
fb73857a
PP
652 open(OUTPUT, '|sort >foo') # pipe to sort
653 or die "Can't start sort: $!";
5a964f20 654 #... # print stuff to output
fb73857a
PP
655 close OUTPUT # wait for sort to finish
656 or warn $! ? "Error closing sort pipe: $!"
657 : "Exit status $? from sort";
658 open(INPUT, 'foo') # get sort's results
659 or die "Can't open 'foo' for input: $!";
a0d0e21e 660
5a964f20
TC
661FILEHANDLE may be an expression whose value can be used as an indirect
662filehandle, usually the real filehandle name.
a0d0e21e
LW
663
664=item closedir DIRHANDLE
665
7660c0ab 666Closes a directory opened by C<opendir()> and returns the success of that
5a964f20
TC
667system call.
668
669DIRHANDLE may be an expression whose value can be used as an indirect
670dirhandle, usually the real dirhandle name.
a0d0e21e
LW
671
672=item connect SOCKET,NAME
673
674Attempts to connect to a remote socket, just as the connect system call
675does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
4633a7c4
LW
676packed address of the appropriate type for the socket. See the examples in
677L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 678
cb1a09d0
AD
679=item continue BLOCK
680
681Actually a flow control statement rather than a function. If there is a
dc9e4912 682C<continue> BLOCK attached to a BLOCK (typically in a L</while> or
a189bff9
JH
683L</foreach>), it is always executed just before the conditional is about to
684be evaluated again, just like the third part of a L</for> loop in C. Thus
cb1a09d0
AD
685it can be used to increment a loop variable, even when the loop has been
686continued via the C<next> statement (which is similar to the C C<continue>
687statement).
688
a189bff9 689L</last>, L</next>, or L</redo> may appear within a C<continue>
1d2dff63
GS
690block. C<last> and C<redo> will behave as if they had been executed within
691the main block. So will C<next>, but since it will execute a C<continue>
692block, it may be more entertaining.
693
694 while (EXPR) {
695 ### redo always comes here
696 do_something;
697 } continue {
698 ### next always comes here
699 do_something_else;
700 # then back the top to re-check EXPR
701 }
702 ### last always comes here
703
704Omitting the C<continue> section is semantically equivalent to using an
705empty one, logically enough. In that case, C<next> goes directly back
706to check the condition at the top of the loop.
707
a189bff9
JH
708See also L<perlsyn>.
709
a0d0e21e
LW
710=item cos EXPR
711
5a964f20 712Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
7660c0ab 713takes cosine of C<$_>.
a0d0e21e 714
7660c0ab 715For the inverse cosine operation, you may use the C<POSIX::acos()>
28757baa
PP
716function, or use this relation:
717
718 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
719
a0d0e21e
LW
720=item crypt PLAINTEXT,SALT
721
f86cebdf 722Encrypts a string exactly like the crypt(3) function in the C library
4633a7c4
LW
723(assuming that you actually have a version there that has not been
724extirpated as a potential munition). This can prove useful for checking
725the password file for lousy passwords, amongst other things. Only the
726guys wearing white hats should do this.
a0d0e21e 727
7660c0ab 728Note that C<crypt()> is intended to be a one-way function, much like breaking
11155c91
CS
729eggs to make an omelette. There is no (known) corresponding decrypt
730function. As a result, this function isn't all that useful for
731cryptography. (For that, see your nearby CPAN mirror.)
2f9daede 732
e71965be
RS
733When verifying an existing encrypted string you should use the encrypted
734text as the salt (like C<crypt($plain, $crypted) eq $crypted>). This
735allows your code to work with the standard C<crypt()> and with more
736exotic implementations. When choosing a new salt create a random two
737character string whose characters come from the set C<[./0-9A-Za-z]>
738(like C<join '', ('.', '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>).
739
a0d0e21e
LW
740Here's an example that makes sure that whoever runs this program knows
741their own password:
742
743 $pwd = (getpwuid($<))[1];
a0d0e21e
LW
744
745 system "stty -echo";
746 print "Password: ";
e71965be 747 chomp($word = <STDIN>);
a0d0e21e
LW
748 print "\n";
749 system "stty echo";
750
e71965be 751 if (crypt($word, $pwd) ne $pwd) {
a0d0e21e
LW
752 die "Sorry...\n";
753 } else {
754 print "ok\n";
54310121 755 }
a0d0e21e 756
9f8f0c9d 757Of course, typing in your own password to whoever asks you
748a9306 758for it is unwise.
a0d0e21e 759
aa689395 760=item dbmclose HASH
a0d0e21e 761
7660c0ab 762[This function has been superseded by the C<untie()> function.]
a0d0e21e 763
aa689395 764Breaks the binding between a DBM file and a hash.
a0d0e21e 765
aa689395 766=item dbmopen HASH,DBNAME,MODE
a0d0e21e 767
7660c0ab 768[This function has been superseded by the C<tie()> function.]
a0d0e21e 769
7b8d334a 770This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
ab4f32c2 771hash. HASH is the name of the hash. (Unlike normal C<open()>, the first
aa689395
PP
772argument is I<NOT> a filehandle, even though it looks like one). DBNAME
773is the name of the database (without the F<.dir> or F<.pag> extension if
774any). If the database does not exist, it is created with protection
7660c0ab
A
775specified by MODE (as modified by the C<umask()>). If your system supports
776only the older DBM functions, you may perform only one C<dbmopen()> in your
aa689395 777program. In older versions of Perl, if your system had neither DBM nor
7660c0ab 778ndbm, calling C<dbmopen()> produced a fatal error; it now falls back to
aa689395
PP
779sdbm(3).
780
781If you don't have write access to the DBM file, you can only read hash
782variables, not set them. If you want to test whether you can write,
7660c0ab 783either use file tests or try setting a dummy hash entry inside an C<eval()>,
aa689395 784which will trap the error.
a0d0e21e 785
7660c0ab
A
786Note that functions such as C<keys()> and C<values()> may return huge lists
787when used on large DBM files. You may prefer to use the C<each()>
a0d0e21e
LW
788function to iterate over large DBM files. Example:
789
790 # print out history file offsets
791 dbmopen(%HIST,'/usr/lib/news/history',0666);
792 while (($key,$val) = each %HIST) {
793 print $key, ' = ', unpack('L',$val), "\n";
794 }
795 dbmclose(%HIST);
796
cb1a09d0 797See also L<AnyDBM_File> for a more general description of the pros and
184e9718 798cons of the various dbm approaches, as well as L<DB_File> for a particularly
cb1a09d0 799rich implementation.
4633a7c4 800
a0d0e21e
LW
801=item defined EXPR
802
54310121 803=item defined
bbce6d69 804
2f9daede
TPG
805Returns a Boolean value telling whether EXPR has a value other than
806the undefined value C<undef>. If EXPR is not present, C<$_> will be
807checked.
808
809Many operations return C<undef> to indicate failure, end of file,
810system error, uninitialized variable, and other exceptional
811conditions. This function allows you to distinguish C<undef> from
812other values. (A simple Boolean test will not distinguish among
7660c0ab 813C<undef>, zero, the empty string, and C<"0">, which are all equally
2f9daede 814false.) Note that since C<undef> is a valid scalar, its presence
7660c0ab 815doesn't I<necessarily> indicate an exceptional condition: C<pop()>
2f9daede
TPG
816returns C<undef> when its argument is an empty array, I<or> when the
817element to return happens to be C<undef>.
818
7660c0ab 819You may also use C<defined()> to check whether a subroutine exists, by
5a964f20 820saying C<defined &func> without parentheses. On the other hand, use
7660c0ab 821of C<defined()> upon aggregates (hashes and arrays) is not guaranteed to
5a964f20 822produce intuitive results, and should probably be avoided.
2f9daede
TPG
823
824When used on a hash element, it tells you whether the value is defined,
dc848c6f 825not whether the key exists in the hash. Use L</exists> for the latter
2f9daede 826purpose.
a0d0e21e
LW
827
828Examples:
829
830 print if defined $switch{'D'};
831 print "$val\n" while defined($val = pop(@ary));
832 die "Can't readlink $sym: $!"
833 unless defined($value = readlink $sym);
a0d0e21e 834 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
2f9daede 835 $debugging = 0 unless defined $debugging;
a0d0e21e 836
7660c0ab
A
837Note: Many folks tend to overuse C<defined()>, and then are surprised to
838discover that the number C<0> and C<""> (the zero-length string) are, in fact,
2f9daede 839defined values. For example, if you say
a5f75d66
AD
840
841 "ab" =~ /a(.*)b/;
842
7660c0ab 843The pattern match succeeds, and C<$1> is defined, despite the fact that it
a5f75d66 844matched "nothing". But it didn't really match nothing--rather, it
7660c0ab 845matched something that happened to be C<0> characters long. This is all
a5f75d66 846very above-board and honest. When a function returns an undefined value,
2f9daede 847it's an admission that it couldn't give you an honest answer. So you
7660c0ab
A
848should use C<defined()> only when you're questioning the integrity of what
849you're trying to do. At other times, a simple comparison to C<0> or C<""> is
2f9daede
TPG
850what you want.
851
7660c0ab 852Currently, using C<defined()> on an entire array or hash reports whether
2f9daede
TPG
853memory for that aggregate has ever been allocated. So an array you set
854to the empty list appears undefined initially, and one that once was full
855and that you then set to the empty list still appears defined. You
856should instead use a simple test for size:
28757baa
PP
857
858 if (@an_array) { print "has array elements\n" }
859 if (%a_hash) { print "has hash members\n" }
860
7660c0ab 861Using C<undef()> on these, however, does clear their memory and then report
5a964f20 862them as not defined anymore, but you shouldn't do that unless you don't
28757baa 863plan to use them again, because it saves time when you load them up
5a964f20
TC
864again to have memory already ready to be filled. The normal way to
865free up space used by an aggregate is to assign the empty list.
28757baa 866
7660c0ab 867This counterintuitive behavior of C<defined()> on aggregates may be
28757baa
PP
868changed, fixed, or broken in a future release of Perl.
869
dc848c6f 870See also L</undef>, L</exists>, L</ref>.
2f9daede 871
a0d0e21e
LW
872=item delete EXPR
873
aa689395
PP
874Deletes the specified key(s) and their associated values from a hash.
875For each key, returns the deleted value associated with that key, or
876the undefined value if there was no such key. Deleting from C<$ENV{}>
877modifies the environment. Deleting from a hash tied to a DBM file
7660c0ab 878deletes the entry from the DBM file. (But deleting from a C<tie()>d hash
5f05dabc 879doesn't necessarily return anything.)
a0d0e21e 880
aa689395 881The following deletes all the values of a hash:
a0d0e21e 882
5f05dabc
PP
883 foreach $key (keys %HASH) {
884 delete $HASH{$key};
a0d0e21e
LW
885 }
886
5f05dabc
PP
887And so does this:
888
889 delete @HASH{keys %HASH}
890
5a964f20 891(But both of these are slower than just assigning the empty list, or
7660c0ab 892using C<undef()>.) Note that the EXPR can be arbitrarily complicated as
5a964f20 893long as the final operation is a hash element lookup or hash slice:
a0d0e21e
LW
894
895 delete $ref->[$x][$y]{$key};
5f05dabc 896 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
a0d0e21e
LW
897
898=item die LIST
899
7660c0ab
A
900Outside an C<eval()>, prints the value of LIST to C<STDERR> and exits with
901the current value of C<$!> (errno). If C<$!> is C<0>, exits with the value of
54310121 902C<($? E<gt>E<gt> 8)> (backtick `command` status). If C<($? E<gt>E<gt> 8)>
7660c0ab
A
903is C<0>, exits with C<255>. Inside an C<eval(),> the error message is stuffed into
904C<$@> and the C<eval()> is terminated with the undefined value. This makes
905C<die()> the way to raise an exception.
a0d0e21e
LW
906
907Equivalent examples:
908
909 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
54310121 910 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
a0d0e21e
LW
911
912If the value of EXPR does not end in a newline, the current script line
913number and input line number (if any) are also printed, and a newline
7660c0ab
A
914is supplied. Hint: sometimes appending C<", stopped"> to your message
915will cause it to make better sense when the string C<"at foo line 123"> is
a0d0e21e
LW
916appended. Suppose you are running script "canasta".
917
918 die "/etc/games is no good";
919 die "/etc/games is no good, stopped";
920
921produce, respectively
922
923 /etc/games is no good at canasta line 123.
924 /etc/games is no good, stopped at canasta line 123.
925
7660c0ab 926See also C<exit()> and C<warn()>.
a0d0e21e 927
7660c0ab
A
928If LIST is empty and C<$@> already contains a value (typically from a
929previous eval) that value is reused after appending C<"\t...propagated">.
fb73857a
PP
930This is useful for propagating exceptions:
931
932 eval { ... };
933 die unless $@ =~ /Expected exception/;
934
7660c0ab 935If C<$@> is empty then the string C<"Died"> is used.
fb73857a 936
7660c0ab 937You can arrange for a callback to be run just before the C<die()> does
774d564b
PP
938its deed, by setting the C<$SIG{__DIE__}> hook. The associated handler
939will be called with the error text and can change the error message, if
7660c0ab 940it sees fit, by calling C<die()> again. See L<perlvar/$SIG{expr}> for details on
fb73857a
PP
941setting C<%SIG> entries, and L<"eval BLOCK"> for some examples.
942
943Note that the C<$SIG{__DIE__}> hook is called even inside eval()ed
944blocks/strings. If one wants the hook to do nothing in such
945situations, put
946
947 die @_ if $^S;
948
949as the first line of the handler (see L<perlvar/$^S>).
774d564b 950
a0d0e21e
LW
951=item do BLOCK
952
953Not really a function. Returns the value of the last command in the
954sequence of commands indicated by BLOCK. When modified by a loop
a189bff9
JH
955modifier such as L</while> or L</until>, executes the BLOCK once
956before testing the loop condition. (On other statements the loop
957modifiers test the conditional first.)
a0d0e21e 958
4968c1e4 959C<do BLOCK> does I<not> count as a loop, so the loop control statements
a189bff9 960L</next>, L</last> or L</redo> cannot be used to leave or restart the block.
4968c1e4 961
a0d0e21e
LW
962=item do SUBROUTINE(LIST)
963
964A deprecated form of subroutine call. See L<perlsub>.
965
966=item do EXPR
967
968Uses the value of EXPR as a filename and executes the contents of the
969file as a Perl script. Its primary use is to include subroutines
970from a Perl subroutine library.
971
972 do 'stat.pl';
973
974is just like
975
fb73857a 976 scalar eval `cat stat.pl`;
a0d0e21e 977
5a964f20 978except that it's more efficient and concise, keeps track of the
a0d0e21e
LW
979current filename for error messages, and searches all the B<-I>
980libraries if the file isn't in the current directory (see also the @INC
dc1be6b5
GS
981array in L<perlvar/Predefined Names>). It is also different in how
982code evaluated with C<do FILENAME> doesn't see lexicals in the enclosing
983scope like C<eval STRING> does. It's the same, however, in that it does
54310121 984reparse the file every time you call it, so you probably don't want to
a0d0e21e
LW
985do this inside a loop.
986
8e30cc93
G
987If C<do> cannot read the file, it returns undef and sets C<$!> to the
988error. If C<do> can read the file but cannot compile it, it
989returns undef and sets an error message in C<$@>. If the file is
990successfully compiled, C<do> returns the value of the last expression
991evaluated.
992
a0d0e21e 993Note that inclusion of library modules is better done with the
7660c0ab 994C<use()> and C<require()> operators, which also do automatic error checking
4633a7c4 995and raise an exception if there's a problem.
a0d0e21e 996
5a964f20
TC
997You might like to use C<do> to read in a program configuration
998file. Manual error checking can be done this way:
999
1000 # read in config files: system first, then user
f86cebdf
GS
1001 for $file ("/share/prog/defaults.rc",
1002 "$ENV{HOME}/.someprogrc") {
5a964f20 1003 unless ($return = do $file) {
f86cebdf
GS
1004 warn "couldn't parse $file: $@" if $@;
1005 warn "couldn't do $file: $!" unless defined $return;
1006 warn "couldn't run $file" unless $return;
5a964f20
TC
1007 }
1008 }
1009
a0d0e21e
LW
1010=item dump LABEL
1011
1012This causes an immediate core dump. Primarily this is so that you can
1013use the B<undump> program to turn your core dump into an executable binary
1014after having initialized all your variables at the beginning of the
1015program. When the new binary is executed it will begin by executing a
1016C<goto LABEL> (with all the restrictions that C<goto> suffers). Think of
f86cebdf 1017it as a goto with an intervening core dump and reincarnation. If C<LABEL>
5a964f20 1018is omitted, restarts the program from the top. WARNING: Any files
a0d0e21e
LW
1019opened at the time of the dump will NOT be open any more when the
1020program is reincarnated, with possible resulting confusion on the part
1021of Perl. See also B<-u> option in L<perlrun>.
1022
1023Example:
1024
1025 #!/usr/bin/perl
1026 require 'getopt.pl';
1027 require 'stat.pl';
1028 %days = (
1029 'Sun' => 1,
1030 'Mon' => 2,
1031 'Tue' => 3,
1032 'Wed' => 4,
1033 'Thu' => 5,
1034 'Fri' => 6,
1035 'Sat' => 7,
1036 );
1037
1038 dump QUICKSTART if $ARGV[0] eq '-d';
1039
1040 QUICKSTART:
1041 Getopt('f');
1042
5a964f20
TC
1043This operator is largely obsolete, partly because it's very hard to
1044convert a core file into an executable, and because the real perl-to-C
1045compiler has superseded it.
1046
aa689395
PP
1047=item each HASH
1048
5a964f20 1049When called in list context, returns a 2-element list consisting of the
aa689395 1050key and value for the next element of a hash, so that you can iterate over
5a964f20 1051it. When called in scalar context, returns the key for only the "next"
7660c0ab 1052element in the hash. (Note: Keys may be C<"0"> or C<"">, which are logically
2f9daede
TPG
1053false; you may wish to avoid constructs like C<while ($k = each %foo) {}>
1054for this reason.)
1055
ab192400
GS
1056Entries are returned in an apparently random order. The actual random
1057order is subject to change in future versions of perl, but it is guaranteed
1058to be in the same order as either the C<keys()> or C<values()> function
1059would produce on the same (unmodified) hash.
1060
1061When the hash is entirely read, a null array is returned in list context
1062(which when assigned produces a FALSE (C<0>) value), and C<undef> in
7660c0ab
A
1063scalar context. The next call to C<each()> after that will start iterating
1064again. There is a single iterator for each hash, shared by all C<each()>,
1065C<keys()>, and C<values()> function calls in the program; it can be reset by
2f9daede
TPG
1066reading all the elements from the hash, or by evaluating C<keys HASH> or
1067C<values HASH>. If you add or delete elements of a hash while you're
1068iterating over it, you may get entries skipped or duplicated, so don't.
aa689395 1069
f86cebdf 1070The following prints out your environment like the printenv(1) program,
aa689395 1071only in a different order:
a0d0e21e
LW
1072
1073 while (($key,$value) = each %ENV) {
1074 print "$key=$value\n";
1075 }
1076
ab192400 1077See also C<keys()>, C<values()> and C<sort()>.
a0d0e21e 1078
a189bff9
JH
1079=item else BLOCK
1080
1081=item elsif (EXPR) BLOCK
1082
1083See L</if>.
1084
a0d0e21e
LW
1085=item eof FILEHANDLE
1086
4633a7c4
LW
1087=item eof ()
1088
a0d0e21e
LW
1089=item eof
1090
1091Returns 1 if the next read on FILEHANDLE will return end of file, or if
1092FILEHANDLE is not open. FILEHANDLE may be an expression whose value
5a964f20 1093gives the real filehandle. (Note that this function actually
7660c0ab 1094reads a character and then C<ungetc()>s it, so isn't very useful in an
748a9306
LW
1095interactive context.) Do not read from a terminal file (or call
1096C<eof(FILEHANDLE)> on it) after end-of-file is reached. Filetypes such
1097as terminals may lose the end-of-file condition if you do.
1098
1099An C<eof> without an argument uses the last file read as argument.
5a964f20 1100Using C<eof()> with empty parentheses is very different. It indicates the pseudo file formed of
2f9daede
TPG
1101the files listed on the command line, i.e., C<eof()> is reasonable to
1102use inside a C<while (E<lt>E<gt>)> loop to detect the end of only the
1103last file. Use C<eof(ARGV)> or eof without the parentheses to test
1104I<EACH> file in a while (E<lt>E<gt>) loop. Examples:
a0d0e21e 1105
748a9306
LW
1106 # reset line numbering on each input file
1107 while (<>) {
5a964f20 1108 next if /^\s*#/; # skip comments
748a9306 1109 print "$.\t$_";
5a964f20
TC
1110 } continue {
1111 close ARGV if eof; # Not eof()!
748a9306
LW
1112 }
1113
a0d0e21e
LW
1114 # insert dashes just before last line of last file
1115 while (<>) {
5a964f20 1116 if (eof()) { # check for end of current file
a0d0e21e 1117 print "--------------\n";
748a9306
LW
1118 close(ARGV); # close or break; is needed if we
1119 # are reading from the terminal
a0d0e21e
LW
1120 }
1121 print;
1122 }
1123
a0d0e21e 1124Practical hint: you almost never need to use C<eof> in Perl, because the
3b02c43c
GS
1125input operators return false values when they run out of data, or if there
1126was an error.
a0d0e21e
LW
1127
1128=item eval EXPR
1129
1130=item eval BLOCK
1131
c7cc6f1c
GS
1132In the first form, the return value of EXPR is parsed and executed as if it
1133were a little Perl program. The value of the expression (which is itself
5a964f20 1134determined within scalar context) is first parsed, and if there weren't any
c7cc6f1c 1135errors, executed in the context of the current Perl program, so that any
5f05dabc 1136variable settings or subroutine and format definitions remain afterwards.
c7cc6f1c
GS
1137Note that the value is parsed every time the eval executes. If EXPR is
1138omitted, evaluates C<$_>. This form is typically used to delay parsing
1139and subsequent execution of the text of EXPR until run time.
1140
1141In the second form, the code within the BLOCK is parsed only once--at the
1142same time the code surrounding the eval itself was parsed--and executed
1143within the context of the current Perl program. This form is typically
1144used to trap exceptions more efficiently than the first (see below), while
1145also providing the benefit of checking the code within BLOCK at compile
1146time.
1147
1148The final semicolon, if any, may be omitted from the value of EXPR or within
1149the BLOCK.
1150
1151In both forms, the value returned is the value of the last expression
5a964f20 1152evaluated inside the mini-program; a return statement may be also used, just
c7cc6f1c 1153as with subroutines. The expression providing the return value is evaluated
5a964f20 1154in void, scalar, or list context, depending on the context of the eval itself.
c7cc6f1c 1155See L</wantarray> for more on how the evaluation context can be determined.
a0d0e21e 1156
7660c0ab
A
1157If there is a syntax error or runtime error, or a C<die()> statement is
1158executed, an undefined value is returned by C<eval()>, and C<$@> is set to the
a0d0e21e 1159error message. If there was no error, C<$@> is guaranteed to be a null
7660c0ab 1160string. Beware that using C<eval()> neither silences perl from printing
c7cc6f1c
GS
1161warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1162To do either of those, you have to use the C<$SIG{__WARN__}> facility. See
1163L</warn> and L<perlvar>.
a0d0e21e 1164
7660c0ab
A
1165Note that, because C<eval()> traps otherwise-fatal errors, it is useful for
1166determining whether a particular feature (such as C<socket()> or C<symlink()>)
a0d0e21e
LW
1167is implemented. It is also Perl's exception trapping mechanism, where
1168the die operator is used to raise exceptions.
1169
1170If the code to be executed doesn't vary, you may use the eval-BLOCK
1171form to trap run-time errors without incurring the penalty of
1172recompiling each time. The error, if any, is still returned in C<$@>.
1173Examples:
1174
54310121 1175 # make divide-by-zero nonfatal
a0d0e21e
LW
1176 eval { $answer = $a / $b; }; warn $@ if $@;
1177
1178 # same thing, but less efficient
1179 eval '$answer = $a / $b'; warn $@ if $@;
1180
1181 # a compile-time error
5a964f20 1182 eval { $answer = }; # WRONG
a0d0e21e
LW
1183
1184 # a run-time error
1185 eval '$answer ='; # sets $@
1186
7660c0ab 1187When using the C<eval{}> form as an exception trap in libraries, you may
774d564b
PP
1188wish not to trigger any C<__DIE__> hooks that user code may have
1189installed. You can use the C<local $SIG{__DIE__}> construct for this
1190purpose, as shown in this example:
1191
1192 # a very private exception trap for divide-by-zero
f86cebdf
GS
1193 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1194 warn $@ if $@;
774d564b
PP
1195
1196This is especially significant, given that C<__DIE__> hooks can call
7660c0ab 1197C<die()> again, which has the effect of changing their error messages:
774d564b
PP
1198
1199 # __DIE__ hooks may modify error messages
1200 {
f86cebdf
GS
1201 local $SIG{'__DIE__'} =
1202 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
c7cc6f1c
GS
1203 eval { die "foo lives here" };
1204 print $@ if $@; # prints "bar lives here"
774d564b
PP
1205 }
1206
7660c0ab 1207With an C<eval()>, you should be especially careful to remember what's
a0d0e21e
LW
1208being looked at when:
1209
1210 eval $x; # CASE 1
1211 eval "$x"; # CASE 2
1212
1213 eval '$x'; # CASE 3
1214 eval { $x }; # CASE 4
1215
5a964f20 1216 eval "\$$x++"; # CASE 5
a0d0e21e
LW
1217 $$x++; # CASE 6
1218
2f9daede 1219Cases 1 and 2 above behave identically: they run the code contained in
7660c0ab 1220the variable C<$x>. (Although case 2 has misleading double quotes making
2f9daede 1221the reader wonder what else might be happening (nothing is).) Cases 3
7660c0ab 1222and 4 likewise behave in the same way: they run the code C<'$x'>, which
2f9daede
TPG
1223does nothing but return the value of C<$x>. (Case 4 is preferred for
1224purely visual reasons, but it also has the advantage of compiling at
1225compile-time instead of at run-time.) Case 5 is a place where
54310121 1226normally you I<WOULD> like to use double quotes, except that in this
2f9daede
TPG
1227particular situation, you can just use symbolic references instead, as
1228in case 6.
a0d0e21e 1229
4968c1e4
G
1230C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1231C<next>, C<last> or C<redo> cannot be used to leave or restart the block.
1232
1233
a0d0e21e
LW
1234=item exec LIST
1235
8bf3b016
GS
1236=item exec PROGRAM LIST
1237
7660c0ab
A
1238The C<exec()> function executes a system command I<AND NEVER RETURNS> -
1239use C<system()> instead of C<exec()> if you want it to return. It fails and
fb73857a
PP
1240returns FALSE only if the command does not exist I<and> it is executed
1241directly instead of via your system's command shell (see below).
a0d0e21e 1242
7660c0ab
A
1243Since it's a common mistake to use C<exec()> instead of C<system()>, Perl
1244warns you if there is a following statement which isn't C<die()>, C<warn()>,
1245or C<exit()> (if C<-w> is set - but you always do that). If you
1246I<really> want to follow an C<exec()> with some other statement, you
55d729e4
GS
1247can use one of these styles to avoid the warning:
1248
5a964f20
TC
1249 exec ('foo') or print STDERR "couldn't exec foo: $!";
1250 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
55d729e4 1251
5a964f20 1252If there is more than one argument in LIST, or if LIST is an array
f86cebdf 1253with more than one value, calls execvp(3) with the arguments in LIST.
5a964f20
TC
1254If there is only one scalar argument or an array with one element in it,
1255the argument is checked for shell metacharacters, and if there are any,
1256the entire argument is passed to the system's command shell for parsing
1257(this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1258If there are no shell metacharacters in the argument, it is split into
7660c0ab
A
1259words and passed directly to C<execvp()>, which is more efficient. Note:
1260C<exec()> and C<system()> do not flush your output buffer, so you may need to
5a964f20 1261set C<$|> to avoid lost output. Examples:
a0d0e21e
LW
1262
1263 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1264 exec "sort $outfile | uniq";
1265
1266If you don't really want to execute the first argument, but want to lie
1267to the program you are executing about its own name, you can specify
1268the program you actually want to run as an "indirect object" (without a
1269comma) in front of the LIST. (This always forces interpretation of the
54310121 1270LIST as a multivalued list, even if there is only a single scalar in
a0d0e21e
LW
1271the list.) Example:
1272
1273 $shell = '/bin/csh';
1274 exec $shell '-sh'; # pretend it's a login shell
1275
1276or, more directly,
1277
1278 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1279
bb32b41a
GS
1280When the arguments get executed via the system shell, results will
1281be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1282for details.
1283
ab4f32c2 1284Using an indirect object with C<exec()> or C<system()> is also more secure.
5a964f20
TC
1285This usage forces interpretation of the arguments as a multivalued list,
1286even if the list had just one argument. That way you're safe from the
1287shell expanding wildcards or splitting up words with whitespace in them.
1288
1289 @args = ( "echo surprise" );
1290
f86cebdf
GS
1291 system @args; # subject to shell escapes
1292 # if @args == 1
5a964f20
TC
1293 system { $args[0] } @args; # safe even with one-arg list
1294
1295The first version, the one without the indirect object, ran the I<echo>
1296program, passing it C<"surprise"> an argument. The second version
1297didn't--it tried to run a program literally called I<"echo surprise">,
1298didn't find it, and set C<$?> to a non-zero value indicating failure.
1299
ab4f32c2 1300Note that C<exec()> will not call your C<END> blocks, nor will it call
7660c0ab
A
1301any C<DESTROY> methods in your objects.
1302
a0d0e21e
LW
1303=item exists EXPR
1304
1305Returns TRUE if the specified hash key exists in its hash array, even
1306if the corresponding value is undefined.
1307
1308 print "Exists\n" if exists $array{$key};
1309 print "Defined\n" if defined $array{$key};
1310 print "True\n" if $array{$key};
1311
5f05dabc 1312A hash element can be TRUE only if it's defined, and defined if
a0d0e21e
LW
1313it exists, but the reverse doesn't necessarily hold true.
1314
1315Note that the EXPR can be arbitrarily complicated as long as the final
1316operation is a hash key lookup:
1317
5a964f20
TC
1318 if (exists $ref->{"A"}{"B"}{$key}) { ... }
1319
1320Although the last element will not spring into existence just because its
1321existence was tested, intervening ones will. Thus C<$ref-E<gt>{"A"}>
1322C<$ref-E<gt>{"B"}> will spring into existence due to the existence
1323test for a $key element. This autovivification may be fixed in a later
1324release.
a0d0e21e
LW
1325
1326=item exit EXPR
1327
1328Evaluates EXPR and exits immediately with that value. (Actually, it
1329calls any defined C<END> routines first, but the C<END> routines may not
1330abort the exit. Likewise any object destructors that need to be called
1331are called before exit.) Example:
1332
1333 $ans = <STDIN>;
1334 exit 0 if $ans =~ /^[Xx]/;
1335
7660c0ab
A
1336See also C<die()>. If EXPR is omitted, exits with C<0> status. The only
1337universally portable values for EXPR are C<0> for success and C<1> for error;
f86702cc
PP
1338all other values are subject to unpredictable interpretation depending
1339on the environment in which the Perl program is running.
a0d0e21e 1340
7660c0ab
A
1341You shouldn't use C<exit()> to abort a subroutine if there's any chance that
1342someone might want to trap whatever error happened. Use C<die()> instead,
1343which can be trapped by an C<eval()>.
28757baa 1344
5a964f20
TC
1345All C<END{}> blocks are run at exit time. See L<perlsub> for details.
1346
a0d0e21e
LW
1347=item exp EXPR
1348
54310121 1349=item exp
bbce6d69 1350
54310121 1351Returns I<e> (the natural logarithm base) to the power of EXPR.
a0d0e21e
LW
1352If EXPR is omitted, gives C<exp($_)>.
1353
1354=item fcntl FILEHANDLE,FUNCTION,SCALAR
1355
f86cebdf 1356Implements the fcntl(2) function. You'll probably have to say
a0d0e21e
LW
1357
1358 use Fcntl;
1359
0ade1984 1360first to get the correct constant definitions. Argument processing and
7660c0ab 1361value return works just like C<ioctl()> below.
a0d0e21e
LW
1362For example:
1363
1364 use Fcntl;
5a964f20
TC
1365 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1366 or die "can't fcntl F_GETFL: $!";
1367
ab4f32c2
A
1368You don't have to check for C<defined()> on the return from
1369C<fnctl()>. Like C<ioctl()>, it maps a C<0> return from the system
7660c0ab
A
1370call into "C<0> but true" in Perl. This string is true in
1371boolean context and C<0> in numeric context. It is also
5a964f20
TC
1372exempt from the normal B<-w> warnings on improper numeric
1373conversions.
1374
7660c0ab 1375Note that C<fcntl()> will produce a fatal error if used on a machine that
f86cebdf 1376doesn't implement fcntl(2).
a0d0e21e
LW
1377
1378=item fileno FILEHANDLE
1379
1380Returns the file descriptor for a filehandle. This is useful for
7660c0ab 1381constructing bitmaps for C<select()> and low-level POSIX tty-handling
5a964f20
TC
1382operations. If FILEHANDLE is an expression, the value is taken as
1383an indirect filehandle, generally its name.
1384
1385You can use this to find out whether two handles refer to the
1386same underlying descriptor:
1387
1388 if (fileno(THIS) == fileno(THAT)) {
1389 print "THIS and THAT are dups\n";
1390 }
a0d0e21e
LW
1391
1392=item flock FILEHANDLE,OPERATION
1393
f86cebdf 1394Calls flock(2), or an emulation of it, on FILEHANDLE. Returns TRUE for
68dc0745 1395success, FALSE on failure. Produces a fatal error if used on a machine
f86cebdf 1396that doesn't implement flock(2), fcntl(2) locking, or lockf(3). C<flock()>
68dc0745
PP
1397is Perl's portable file locking interface, although it locks only entire
1398files, not records.
8ebc5c01 1399
a3cb178b 1400On many platforms (including most versions or clones of Unix), locks
7660c0ab 1401established by C<flock()> are B<merely advisory>. Such discretionary locks
5a964f20 1402are more flexible, but offer fewer guarantees. This means that files
7660c0ab
A
1403locked with C<flock()> may be modified by programs that do not also use
1404C<flock()>. Windows NT and OS/2 are among the platforms which
5a964f20 1405enforce mandatory locking. See your local documentation for details.
a3cb178b 1406
8ebc5c01
PP
1407OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1408LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
68dc0745
PP
1409you can use the symbolic names if import them from the Fcntl module,
1410either individually, or as a group using the ':flock' tag. LOCK_SH
1411requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1412releases a previously requested lock. If LOCK_NB is added to LOCK_SH or
7660c0ab 1413LOCK_EX then C<flock()> will return immediately rather than blocking
68dc0745
PP
1414waiting for the lock (check the return status to see if you got it).
1415
1416To avoid the possibility of mis-coordination, Perl flushes FILEHANDLE
1417before (un)locking it.
8ebc5c01 1418
f86cebdf 1419Note that the emulation built with lockf(3) doesn't provide shared
8ebc5c01 1420locks, and it requires that FILEHANDLE be open with write intent. These
f86cebdf
GS
1421are the semantics that lockf(3) implements. Most (all?) systems
1422implement lockf(3) in terms of fcntl(2) locking, though, so the
8ebc5c01
PP
1423differing semantics shouldn't bite too many people.
1424
7660c0ab
A
1425Note also that some versions of C<flock()> cannot lock things over the
1426network; you would need to use the more system-specific C<fcntl()> for
f86cebdf
GS
1427that. If you like you can force Perl to ignore your system's flock(2)
1428function, and so provide its own fcntl(2)-based emulation, by passing
8ebc5c01
PP
1429the switch C<-Ud_flock> to the F<Configure> program when you configure
1430perl.
4633a7c4
LW
1431
1432Here's a mailbox appender for BSD systems.
a0d0e21e 1433
7e1af8bc 1434 use Fcntl ':flock'; # import LOCK_* constants
a0d0e21e
LW
1435
1436 sub lock {
7e1af8bc 1437 flock(MBOX,LOCK_EX);
a0d0e21e
LW
1438 # and, in case someone appended
1439 # while we were waiting...
1440 seek(MBOX, 0, 2);
1441 }
1442
1443 sub unlock {
7e1af8bc 1444 flock(MBOX,LOCK_UN);
a0d0e21e
LW
1445 }
1446
1447 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1448 or die "Can't open mailbox: $!";
1449
1450 lock();
1451 print MBOX $msg,"\n\n";
1452 unlock();
1453
cb1a09d0 1454See also L<DB_File> for other flock() examples.
a0d0e21e 1455
a189bff9
JH
1456=item for (INITIAL; WHILE; EACH) BLOCK
1457
1458Do INITIAL, enter BLOCK while EXPR is true, at the end of each round
1459do EACH. For example:
1460
1461 for ($i = 0, $j = 0; $i < 10; $i++) {
1462 if ($i % 3 == 0) { $j++ }
1463 print "i = $i, j = $j\n";
1464 }
1465
1466See L<perlsyn> for more details. See also L</foreach>, a twin of
1467C<for>, L</while> and L</until>, close cousins of L<for>, and
1468L</last>, L</next>, and L</redo> for additional control flow.
1469
1470=item foreach LOOPVAR (LIST) BLOCK
1471
1472Enter BLOCK as LOOPVAR set in turn to each element of LIST.
1473For example:
1474
1475 foreach $rolling (@stones) { print "rolling $stone\n" }
1476
1477 foreach my $file (@files) { print "file $file\n" }
1478
1479The LOOPVAR is optional and defaults to C<$_>. If the elements are
1480modifiable (as opposed to constants or tied variables) you can modify them.
1481
1482 foreach (@words) { tr/abc/xyz/ }
1483
1484See L<perlsyn> for more details. See also L</for>, a twin of
1485C<foreach>, L</while> and L</until>, close cousins of L<for>, and
1486L</last>, L</next>, and L</redo> for additional control flow.
1487
a0d0e21e
LW
1488=item fork
1489
f86cebdf 1490Does a fork(2) system call. Returns the child pid to the parent process,
7660c0ab 1491C<0> to the child process, or C<undef> if the fork is unsuccessful.
5a964f20 1492
a0d0e21e 1493Note: unflushed buffers remain unflushed in both processes, which means
7660c0ab
A
1494you may need to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()>
1495method of C<IO::Handle> to avoid duplicate output.
a0d0e21e 1496
7660c0ab 1497If you C<fork()> without ever waiting on your children, you will accumulate
a0d0e21e
LW
1498zombies:
1499
4633a7c4 1500 $SIG{CHLD} = sub { wait };
a0d0e21e 1501
54310121 1502There's also the double-fork trick (error checking on
7660c0ab 1503C<fork()> returns omitted);
a0d0e21e
LW
1504
1505 unless ($pid = fork) {
1506 unless (fork) {
1507 exec "what you really wanna do";
1508 die "no exec";
1509 # ... or ...
4633a7c4 1510 ## (some_perl_code_here)
a0d0e21e
LW
1511 exit 0;
1512 }
1513 exit 0;
1514 }
1515 waitpid($pid,0);
1516
cb1a09d0
AD
1517See also L<perlipc> for more examples of forking and reaping
1518moribund children.
1519
28757baa
PP
1520Note that if your forked child inherits system file descriptors like
1521STDIN and STDOUT that are actually connected by a pipe or socket, even
5a964f20 1522if you exit, then the remote server (such as, say, httpd or rsh) won't think
7660c0ab 1523you're done. You should reopen those to F</dev/null> if it's any issue.
28757baa 1524
cb1a09d0
AD
1525=item format
1526
7660c0ab 1527Declare a picture format for use by the C<write()> function. For
cb1a09d0
AD
1528example:
1529
54310121 1530 format Something =
cb1a09d0
AD
1531 Test: @<<<<<<<< @||||| @>>>>>
1532 $str, $%, '$' . int($num)
1533 .
1534
1535 $str = "widget";
184e9718 1536 $num = $cost/$quantity;
cb1a09d0
AD
1537 $~ = 'Something';
1538 write;
1539
1540See L<perlform> for many details and examples.
1541
8903cb82 1542=item formline PICTURE,LIST
a0d0e21e 1543
5a964f20 1544This is an internal function used by C<format>s, though you may call it,
a0d0e21e
LW
1545too. It formats (see L<perlform>) a list of values according to the
1546contents of PICTURE, placing the output into the format output
7660c0ab
A
1547accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
1548Eventually, when a C<write()> is done, the contents of
a0d0e21e 1549C<$^A> are written to some filehandle, but you could also read C<$^A>
7660c0ab
A
1550yourself and then set C<$^A> back to C<"">. Note that a format typically
1551does one C<formline()> per line of form, but the C<formline()> function itself
748a9306 1552doesn't care how many newlines are embedded in the PICTURE. This means
4633a7c4 1553that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
748a9306
LW
1554You may therefore need to use multiple formlines to implement a single
1555record format, just like the format compiler.
1556
5f05dabc 1557Be careful if you put double quotes around the picture, because an "C<@>"
748a9306 1558character may be taken to mean the beginning of an array name.
7660c0ab 1559C<formline()> always returns TRUE. See L<perlform> for other examples.
a0d0e21e
LW
1560
1561=item getc FILEHANDLE
1562
1563=item getc
1564
1565Returns the next character from the input file attached to FILEHANDLE,
9bc64814 1566or the undefined value at end of file, or if there was an error. If
3b02c43c
GS
1567FILEHANDLE is omitted, reads from STDIN. This is not particularly
1568efficient. It cannot be used to get unbuffered single-characters,
1569however. For that, try something more like:
4633a7c4
LW
1570
1571 if ($BSD_STYLE) {
1572 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1573 }
1574 else {
54310121 1575 system "stty", '-icanon', 'eol', "\001";
4633a7c4
LW
1576 }
1577
1578 $key = getc(STDIN);
1579
1580 if ($BSD_STYLE) {
1581 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1582 }
1583 else {
5f05dabc 1584 system "stty", 'icanon', 'eol', '^@'; # ASCII null
4633a7c4
LW
1585 }
1586 print "\n";
1587
54310121
PP
1588Determination of whether $BSD_STYLE should be set
1589is left as an exercise to the reader.
cb1a09d0 1590
7660c0ab 1591The C<POSIX::getattr()> function can do this more portably on systems
5a964f20 1592purporting POSIX compliance.
cb1a09d0 1593See also the C<Term::ReadKey> module from your nearest CPAN site;
54310121 1594details on CPAN can be found on L<perlmod/CPAN>.
a0d0e21e
LW
1595
1596=item getlogin
1597
5a964f20
TC
1598Implements the C library function of the same name, which on most
1599systems returns the current login from F</etc/utmp>, if any. If null,
7660c0ab 1600use C<getpwuid()>.
a0d0e21e 1601
f86702cc 1602 $login = getlogin || getpwuid($<) || "Kilroy";
a0d0e21e 1603
7660c0ab
A
1604Do not consider C<getlogin()> for authentication: it is not as
1605secure as C<getpwuid()>.
4633a7c4 1606
a0d0e21e
LW
1607=item getpeername SOCKET
1608
1609Returns the packed sockaddr address of other end of the SOCKET connection.
1610
4633a7c4
LW
1611 use Socket;
1612 $hersockaddr = getpeername(SOCK);
1613 ($port, $iaddr) = unpack_sockaddr_in($hersockaddr);
1614 $herhostname = gethostbyaddr($iaddr, AF_INET);
1615 $herstraddr = inet_ntoa($iaddr);
a0d0e21e
LW
1616
1617=item getpgrp PID
1618
47e29363 1619Returns the current process group for the specified PID. Use
7660c0ab 1620a PID of C<0> to get the current process group for the
4633a7c4 1621current process. Will raise an exception if used on a machine that
f86cebdf 1622doesn't implement getpgrp(2). If PID is omitted, returns process
7660c0ab
A
1623group of current process. Note that the POSIX version of C<getpgrp()>
1624does not accept a PID argument, so only C<PID==0> is truly portable.
a0d0e21e
LW
1625
1626=item getppid
1627
1628Returns the process id of the parent process.
1629
1630=item getpriority WHICH,WHO
1631
4633a7c4
LW
1632Returns the current priority for a process, a process group, or a user.
1633(See L<getpriority(2)>.) Will raise a fatal exception if used on a
f86cebdf 1634machine that doesn't implement getpriority(2).
a0d0e21e
LW
1635
1636=item getpwnam NAME
1637
1638=item getgrnam NAME
1639
1640=item gethostbyname NAME
1641
1642=item getnetbyname NAME
1643
1644=item getprotobyname NAME
1645
1646=item getpwuid UID
1647
1648=item getgrgid GID
1649
1650=item getservbyname NAME,PROTO
1651
1652=item gethostbyaddr ADDR,ADDRTYPE
1653
1654=item getnetbyaddr ADDR,ADDRTYPE
1655
1656=item getprotobynumber NUMBER
1657
1658=item getservbyport PORT,PROTO
1659
1660=item getpwent
1661
1662=item getgrent
1663
1664=item gethostent
1665
1666=item getnetent
1667
1668=item getprotoent
1669
1670=item getservent
1671
1672=item setpwent
1673
1674=item setgrent
1675
1676=item sethostent STAYOPEN
1677
1678=item setnetent STAYOPEN
1679
1680=item setprotoent STAYOPEN
1681
1682=item setservent STAYOPEN
1683
1684=item endpwent
1685
1686=item endgrent
1687
1688=item endhostent
1689
1690=item endnetent
1691
1692=item endprotoent
1693
1694=item endservent
1695
1696These routines perform the same functions as their counterparts in the
5a964f20 1697system library. In list context, the return values from the
a0d0e21e
LW
1698various get routines are as follows:
1699
1700 ($name,$passwd,$uid,$gid,
6ee623d5 1701 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
a0d0e21e
LW
1702 ($name,$passwd,$gid,$members) = getgr*
1703 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1704 ($name,$aliases,$addrtype,$net) = getnet*
1705 ($name,$aliases,$proto) = getproto*
1706 ($name,$aliases,$port,$proto) = getserv*
1707
1708(If the entry doesn't exist you get a null list.)
1709
5a964f20 1710In scalar context, you get the name, unless the function was a
a0d0e21e
LW
1711lookup by name, in which case you get the other thing, whatever it is.
1712(If the entry doesn't exist you get the undefined value.) For example:
1713
5a964f20
TC
1714 $uid = getpwnam($name);
1715 $name = getpwuid($num);
1716 $name = getpwent();
1717 $gid = getgrnam($name);
1718 $name = getgrgid($num;
1719 $name = getgrent();
1720 #etc.
a0d0e21e 1721
7660c0ab 1722In I<getpw*()> the fields C<$quota>, C<$comment>, and C<$expire> are special
6ee623d5 1723cases in the sense that in many systems they are unsupported. If the
7660c0ab
A
1724C<$quota> is unsupported, it is an empty scalar. If it is supported, it
1725usually encodes the disk quota. If the C<$comment> field is unsupported,
6ee623d5
GS
1726it is an empty scalar. If it is supported it usually encodes some
1727administrative comment about the user. In some systems the $quota
7660c0ab
A
1728field may be C<$change> or C<$age>, fields that have to do with password
1729aging. In some systems the C<$comment> field may be C<$class>. The C<$expire>
6ee623d5
GS
1730field, if present, encodes the expiration period of the account or the
1731password. For the availability and the exact meaning of these fields
f86cebdf 1732in your system, please consult your getpwnam(3) documentation and your
7660c0ab
A
1733F<pwd.h> file. You can also find out from within Perl which meaning
1734your C<$quota> and C<$comment> fields have and whether you have the C<$expire>
1735field by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
1736C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>.
6ee623d5 1737
7660c0ab 1738The C<$members> value returned by I<getgr*()> is a space separated list of
a0d0e21e
LW
1739the login names of the members of the group.
1740
1741For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1742C, it will be returned to you via C<$?> if the function call fails. The
7660c0ab 1743C<@addrs> value returned by a successful call is a list of the raw
a0d0e21e
LW
1744addresses returned by the corresponding system library call. In the
1745Internet domain, each address is four bytes long and you can unpack it
1746by saying something like:
1747
1748 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1749
5a964f20
TC
1750If you get tired of remembering which element of the return list contains
1751which return value, by-name interfaces are also provided in modules:
7660c0ab
A
1752C<File::stat>, C<Net::hostent>, C<Net::netent>, C<Net::protoent>, C<Net::servent>,
1753C<Time::gmtime>, C<Time::localtime>, and C<User::grent>. These override the
5a964f20
TC
1754normal built-in, replacing them with versions that return objects with
1755the appropriate names for each field. For example:
1756
1757 use File::stat;
1758 use User::pwent;
1759 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
1760
1761Even though it looks like they're the same method calls (uid),
7660c0ab 1762they aren't, because a C<File::stat> object is different from a C<User::pwent> object.
5a964f20 1763
a0d0e21e
LW
1764=item getsockname SOCKET
1765
1766Returns the packed sockaddr address of this end of the SOCKET connection.
1767
4633a7c4
LW
1768 use Socket;
1769 $mysockaddr = getsockname(SOCK);
1770 ($port, $myaddr) = unpack_sockaddr_in($mysockaddr);
a0d0e21e
LW
1771
1772=item getsockopt SOCKET,LEVEL,OPTNAME
1773
5a964f20 1774Returns the socket option requested, or undef if there is an error.
a0d0e21e
LW
1775
1776=item glob EXPR
1777
0a753a76
PP
1778=item glob
1779
7660c0ab 1780Returns the value of EXPR with filename expansions such as the standard Unix shell F</bin/sh> would
68dc0745 1781do. This is the internal function implementing the C<E<lt>*.cE<gt>>
7660c0ab 1782operator, but you can use it directly. If EXPR is omitted, C<$_> is used.
68dc0745
PP
1783The C<E<lt>*.cE<gt>> operator is discussed in more detail in
1784L<perlop/"I/O Operators">.
a0d0e21e
LW
1785
1786=item gmtime EXPR
1787
1788Converts a time as returned by the time function to a 9-element array
54310121 1789with the time localized for the standard Greenwich time zone.
4633a7c4 1790Typically used as follows:
a0d0e21e 1791
54310121 1792 # 0 1 2 3 4 5 6 7 8
a0d0e21e
LW
1793 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1794 gmtime(time);
1795
1796All array elements are numeric, and come straight out of a struct tm.
7660c0ab
A
1797In particular this means that C<$mon> has the range C<0..11> and C<$wday> has
1798the range C<0..6> with sunday as day C<0>. Also, C<$year> is the number of
1799years since 1900, that is, C<$year> is C<123> in year 2023, I<not> simply the last two digits of the year.
2f9daede
TPG
1800
1801If EXPR is omitted, does C<gmtime(time())>.
a0d0e21e 1802
f86cebdf 1803In scalar context, returns the ctime(3) value:
0a753a76
PP
1804
1805 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
1806
7660c0ab 1807Also see the C<timegm()> function provided by the C<Time::Local> module,
f86cebdf 1808and the strftime(3) function available via the POSIX module.
7660c0ab
A
1809
1810This scalar value is B<not> locale dependent, see L<perllocale>, but
1811instead a Perl builtin. Also see the C<Time::Local> module, and the
f86cebdf 1812strftime(3) and mktime(3) function available via the POSIX module. To
7660c0ab
A
1813get somewhat similar but locale dependent date strings, set up your
1814locale environment variables appropriately (please see L<perllocale>)
1815and try for example:
1816
1817 use POSIX qw(strftime);
1818 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
1819
1820Note that the C<%a> and C<%b>, the short forms of the day of the week
1821and the month of the year, may not necessarily be three characters wide.
0a753a76 1822
a0d0e21e
LW
1823=item goto LABEL
1824
748a9306
LW
1825=item goto EXPR
1826
a0d0e21e
LW
1827=item goto &NAME
1828
7660c0ab 1829The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
a0d0e21e 1830execution there. It may not be used to go into any construct that
7660c0ab 1831requires initialization, such as a subroutine or a C<foreach> loop. It
0a753a76 1832also can't be used to go into a construct that is optimized away,
7660c0ab 1833or to get out of a block or subroutine given to C<sort()>.
0a753a76 1834It can be used to go almost anywhere else within the dynamic scope,
a0d0e21e 1835including out of subroutines, but it's usually better to use some other
ab4f32c2 1836construct such as C<last> or C<die()>. The author of Perl has never felt the
7660c0ab 1837need to use this form of C<goto> (in Perl, that is--C is another matter).
a0d0e21e 1838
7660c0ab
A
1839The C<goto-EXPR> form expects a label name, whose scope will be resolved
1840dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
748a9306
LW
1841necessarily recommended if you're optimizing for maintainability:
1842
1843 goto ("FOO", "BAR", "GLARCH")[$i];
1844
7660c0ab 1845The C<goto-&NAME> form is highly magical, and substitutes a call to the
a0d0e21e 1846named subroutine for the currently running subroutine. This is used by
7660c0ab 1847C<AUTOLOAD> subroutines that wish to load another subroutine and then
a0d0e21e 1848pretend that the other subroutine had been called in the first place
7660c0ab
A
1849(except that any modifications to C<@_> in the current subroutine are
1850propagated to the other subroutine.) After the C<goto>, not even C<caller()>
a0d0e21e
LW
1851will be able to tell that this routine was called first.
1852
1853=item grep BLOCK LIST
1854
1855=item grep EXPR,LIST
1856
f86cebdf 1857This is similar in spirit to, but not the same as, grep(1)
2f9daede
TPG
1858and its relatives. In particular, it is not limited to using
1859regular expressions.
1860
a0d0e21e 1861Evaluates the BLOCK or EXPR for each element of LIST (locally setting
7660c0ab 1862C<$_> to each element) and returns the list value consisting of those
a0d0e21e
LW
1863elements for which the expression evaluated to TRUE. In a scalar
1864context, returns the number of times the expression was TRUE.
1865
1866 @foo = grep(!/^#/, @bar); # weed out comments
1867
1868or equivalently,
1869
1870 @foo = grep {!/^#/} @bar; # weed out comments
1871
7660c0ab 1872Note that, because C<$_> is a reference into the list value, it can be used
a0d0e21e
LW
1873to modify the elements of the array. While this is useful and
1874supported, it can cause bizarre results if the LIST is not a named
2f9daede 1875array. Similarly, grep returns aliases into the original list,
f86cebdf 1876much like the way that a for loop's index variable aliases the list
2f9daede 1877elements. That is, modifying an element of a list returned by grep
ab4f32c2 1878(for example, in a C<foreach>, C<map()> or another C<grep()>)
2f9daede 1879actually modifies the element in the original list.
a0d0e21e 1880
fb73857a 1881See also L</map> for an array composed of the results of the BLOCK or EXPR.
38325410 1882
a0d0e21e
LW
1883=item hex EXPR
1884
54310121 1885=item hex
bbce6d69 1886
54310121 1887Interprets EXPR as a hex string and returns the corresponding
2f9daede 1888value. (To convert strings that might start with either 0 or 0x
7660c0ab 1889see L</oct>.) If EXPR is omitted, uses C<$_>.
2f9daede
TPG
1890
1891 print hex '0xAf'; # prints '175'
1892 print hex 'aF'; # same
a0d0e21e 1893
a189bff9
JH
1894=item if (EXPR) BLOCK
1895
1896=item if (EXPR) BLOCK else BLOCK2
1897
1898=item if (EXPR) BLOCK elsif (EXPR2) BLOCK2
1899
1900Enter BLOCKs conditionally. The first EXPR to return true
1901causes the corresponding BLOCK to be entered, or, in the case
1902of C<else>, the fall-through default BLOCK.
1903
a91c35b0
JH
1904Note 1: Perl wants BLOCKS, expressions won't do (like they do
1905e.g. in C, C++, Java, Pascal).
1906
1907Note 2: It's C<elsif>, not C<elseif>. You can have as many
1908C<elsif>s as you want.
a189bff9
JH
1909
1910See L<perlsyn> for more details. See also C<unless>.
1911
a0d0e21e
LW
1912=item import
1913
7660c0ab 1914There is no builtin C<import()> function. It is just an ordinary
4633a7c4 1915method (subroutine) defined (or inherited) by modules that wish to export
7660c0ab 1916names to another module. The C<use()> function calls the C<import()> method
54310121 1917for the package used. See also L</use()>, L<perlmod>, and L<Exporter>.
a0d0e21e
LW
1918
1919=item index STR,SUBSTR,POSITION
1920
1921=item index STR,SUBSTR
1922
4633a7c4
LW
1923Returns the position of the first occurrence of SUBSTR in STR at or after
1924POSITION. If POSITION is omitted, starts searching from the beginning of
7660c0ab 1925the string. The return value is based at C<0> (or whatever you've set the C<$[>
4633a7c4 1926variable to--but don't do that). If the substring is not found, returns
7660c0ab 1927one less than the base, ordinarily C<-1>.
a0d0e21e
LW
1928
1929=item int EXPR
1930
54310121 1931=item int
bbce6d69 1932
7660c0ab 1933Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
5a964f20 1934You should not use this for rounding, because it truncates
7660c0ab
A
1935towards C<0>, and because machine representations of floating point
1936numbers can sometimes produce counterintuitive results. Usually C<sprintf()> or C<printf()>,
1937or the C<POSIX::floor> or C<POSIX::ceil> functions, would serve you better.
a0d0e21e
LW
1938
1939=item ioctl FILEHANDLE,FUNCTION,SCALAR
1940
f86cebdf 1941Implements the ioctl(2) function. You'll probably have to say
a0d0e21e 1942
4633a7c4 1943 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
a0d0e21e 1944
4633a7c4 1945first to get the correct function definitions. If F<ioctl.ph> doesn't
a0d0e21e 1946exist or doesn't have the correct definitions you'll have to roll your
4633a7c4 1947own, based on your C header files such as F<E<lt>sys/ioctl.hE<gt>>.
5a964f20 1948(There is a Perl script called B<h2ph> that comes with the Perl kit that
54310121 1949may help you in this, but it's nontrivial.) SCALAR will be read and/or
4633a7c4 1950written depending on the FUNCTION--a pointer to the string value of SCALAR
ab4f32c2 1951will be passed as the third argument of the actual C<ioctl()> call. (If SCALAR
4633a7c4
LW
1952has no string value but does have a numeric value, that value will be
1953passed rather than a pointer to the string value. To guarantee this to be
7660c0ab 1954TRUE, add a C<0> to the scalar before using it.) The C<pack()> and C<unpack()>
4633a7c4 1955functions are useful for manipulating the values of structures used by
7660c0ab 1956C<ioctl()>. The following example sets the erase character to DEL.
a0d0e21e
LW
1957
1958 require 'ioctl.ph';
4633a7c4
LW
1959 $getp = &TIOCGETP;
1960 die "NO TIOCGETP" if $@ || !$getp;
a0d0e21e 1961 $sgttyb_t = "ccccs"; # 4 chars and a short
4633a7c4 1962 if (ioctl(STDIN,$getp,$sgttyb)) {
a0d0e21e
LW
1963 @ary = unpack($sgttyb_t,$sgttyb);
1964 $ary[2] = 127;
1965 $sgttyb = pack($sgttyb_t,@ary);
4633a7c4 1966 ioctl(STDIN,&TIOCSETP,$sgttyb)
a0d0e21e
LW
1967 || die "Can't ioctl: $!";
1968 }
1969
ab4f32c2 1970The return value of C<ioctl()> (and C<fcntl()>) is as follows:
a0d0e21e
LW
1971
1972 if OS returns: then Perl returns:
1973 -1 undefined value
1974 0 string "0 but true"
1975 anything else that number
1976
1977Thus Perl returns TRUE on success and FALSE on failure, yet you can
1978still easily determine the actual value returned by the operating
1979system:
1980
1981 ($retval = ioctl(...)) || ($retval = -1);
1982 printf "System returned %d\n", $retval;
1983
7660c0ab 1984The special string "C<0> but true" is excempt from B<-w> complaints
5a964f20
TC
1985about improper numeric conversions.
1986
a0d0e21e
LW
1987=item join EXPR,LIST
1988
54310121 1989Joins the separate strings of LIST into a single string with
a0d0e21e
LW
1990fields separated by the value of EXPR, and returns the string.
1991Example:
1992
1993 $_ = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
1994
7660c0ab 1995See L</split>.
a0d0e21e 1996
aa689395
PP
1997=item keys HASH
1998
1d2dff63
GS
1999Returns a list consisting of all the keys of the named hash. (In a
2000scalar context, returns the number of keys.) The keys are returned in
ab192400
GS
2001an apparently random order. The actual random order is subject to
2002change in future versions of perl, but it is guaranteed to be the same
2003order as either the C<values()> or C<each()> function produces (given
2004that the hash has not been modified). As a side effect, it resets
2005HASH's iterator.
a0d0e21e 2006
aa689395 2007Here is yet another way to print your environment:
a0d0e21e
LW
2008
2009 @keys = keys %ENV;
2010 @values = values %ENV;
2011 while ($#keys >= 0) {
2012 print pop(@keys), '=', pop(@values), "\n";
2013 }
2014
2015or how about sorted by key:
2016
2017 foreach $key (sort(keys %ENV)) {
2018 print $key, '=', $ENV{$key}, "\n";
2019 }
2020
aca803df 2021To sort a hash by value, you'll need to use a C<sort()> function.
aa689395 2022Here's a descending numeric sort of a hash by its values:
4633a7c4 2023
5a964f20 2024 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
4633a7c4
LW
2025 printf "%4d %s\n", $hash{$key}, $key;
2026 }
2027
ab4f32c2 2028As an lvalue C<keys()> allows you to increase the number of hash buckets
aa689395
PP
2029allocated for the given hash. This can gain you a measure of efficiency if
2030you know the hash is going to get big. (This is similar to pre-extending
2031an array by assigning a larger number to $#array.) If you say
55497cff
PP
2032
2033 keys %hash = 200;
2034
ab192400
GS
2035then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2036in fact, since it rounds up to the next power of two. These
55497cff
PP
2037buckets will be retained even if you do C<%hash = ()>, use C<undef
2038%hash> if you want to free the storage while C<%hash> is still in scope.
2039You can't shrink the number of buckets allocated for the hash using
ab4f32c2 2040C<keys()> in this way (but you needn't worry about doing this by accident,
55497cff
PP
2041as trying has no effect).
2042
ab192400
GS
2043See also C<each()>, C<values()> and C<sort()>.
2044
a0d0e21e
LW
2045=item kill LIST
2046
54310121
PP
2047Sends a signal to a list of processes. The first element of
2048the list must be the signal to send. Returns the number of
4633a7c4 2049processes successfully signaled.
a0d0e21e
LW
2050
2051 $cnt = kill 1, $child1, $child2;
2052 kill 9, @goners;
2053
4633a7c4
LW
2054Unlike in the shell, in Perl if the I<SIGNAL> is negative, it kills
2055process groups instead of processes. (On System V, a negative I<PROCESS>
2056number will also kill process groups, but that's not portable.) That
2057means you usually want to use positive not negative signals. You may also
da0045b7 2058use a signal name in quotes. See L<perlipc/"Signals"> for details.
a0d0e21e
LW
2059
2060=item last LABEL
2061
2062=item last
2063
2064The C<last> command is like the C<break> statement in C (as used in
2065loops); it immediately exits the loop in question. If the LABEL is
2066omitted, the command refers to the innermost enclosing loop. The
2067C<continue> block, if any, is not executed:
2068
4633a7c4
LW
2069 LINE: while (<STDIN>) {
2070 last LINE if /^$/; # exit when done with header
5a964f20 2071 #...
a0d0e21e
LW
2072 }
2073
4968c1e4
G
2074C<last> cannot be used to exit a block which returns a value such as
2075C<eval {}>, C<sub {}> or C<do {}>.
2076
a189bff9
JH
2077See also L</continue> for an illustration of how C<last>, L</next>, and
2078L</redo> work.
2079
2080See also L<perlsyn>.
1d2dff63 2081
a0d0e21e
LW
2082=item lc EXPR
2083
54310121 2084=item lc
bbce6d69 2085
a0d0e21e 2086Returns an lowercased version of EXPR. This is the internal function
7660c0ab 2087implementing the C<\L> escape in double-quoted strings.
a0ed51b3 2088Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
a0d0e21e 2089
7660c0ab 2090If EXPR is omitted, uses C<$_>.
bbce6d69 2091
a0d0e21e
LW
2092=item lcfirst EXPR
2093
54310121 2094=item lcfirst
bbce6d69 2095
a0d0e21e 2096Returns the value of EXPR with the first character lowercased. This is
7660c0ab 2097the internal function implementing the C<\l> escape in double-quoted strings.
a0ed51b3 2098Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
a0d0e21e 2099
7660c0ab 2100If EXPR is omitted, uses C<$_>.
bbce6d69 2101
a0d0e21e
LW
2102=item length EXPR
2103
54310121 2104=item length
bbce6d69 2105
a0ed51b3 2106Returns the length in characters of the value of EXPR. If EXPR is
7660c0ab 2107omitted, returns length of C<$_>.
a0d0e21e
LW
2108
2109=item link OLDFILE,NEWFILE
2110
5a964f20
TC
2111Creates a new filename linked to the old filename. Returns TRUE for
2112success, FALSE otherwise.
a0d0e21e
LW
2113
2114=item listen SOCKET,QUEUESIZE
2115
2116Does the same thing that the listen system call does. Returns TRUE if
4633a7c4 2117it succeeded, FALSE otherwise. See example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
2118
2119=item local EXPR
2120
5a964f20
TC
2121A local modifies the listed variables to be local to the enclosing
2122block, file, or eval. If more than one value is listed, the list must
2123be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2124for details, including issues with tied arrays and hashes.
a0d0e21e 2125
7660c0ab 2126You really probably want to be using C<my()> instead, because C<local()> isn't
7b8d334a 2127what most people think of as "local". See L<perlsub/"Private Variables
cb1a09d0 2128via my()"> for details.
a0d0e21e
LW
2129
2130=item localtime EXPR
2131
2132Converts a time as returned by the time function to a 9-element array
5f05dabc 2133with the time analyzed for the local time zone. Typically used as
a0d0e21e
LW
2134follows:
2135
54310121 2136 # 0 1 2 3 4 5 6 7 8
a0d0e21e
LW
2137 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2138 localtime(time);
2139
2140All array elements are numeric, and come straight out of a struct tm.
7660c0ab
A
2141In particular this means that C<$mon> has the range C<0..11> and C<$wday> has
2142the range C<0..6> with sunday as day C<0>. Also, C<$year> is the number of
2143years 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
2144
2145If EXPR is omitted, uses the current time (C<localtime(time)>).
a0d0e21e 2146
f86cebdf 2147In scalar context, returns the ctime(3) value:
a0d0e21e 2148
5f05dabc 2149 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
a0d0e21e 2150
a3cb178b 2151This scalar value is B<not> locale dependent, see L<perllocale>, but
7660c0ab 2152instead a Perl builtin. Also see the C<Time::Local> module, and the
f86cebdf 2153strftime(3) and mktime(3) function available via the POSIX module. To
a3cb178b
GS
2154get somewhat similar but locale dependent date strings, set up your
2155locale environment variables appropriately (please see L<perllocale>)
5a964f20 2156and try for example:
a3cb178b 2157
5a964f20 2158 use POSIX qw(strftime);
a3cb178b
GS
2159 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
2160
2161Note that the C<%a> and C<%b>, the short forms of the day of the week
2162and the month of the year, may not necessarily be three characters wide.
a0d0e21e
LW
2163
2164=item log EXPR
2165
54310121 2166=item log
bbce6d69 2167
5a964f20 2168Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted, returns log
7660c0ab 2169of C<$_>.
a0d0e21e
LW
2170
2171=item lstat FILEHANDLE
2172
2173=item lstat EXPR
2174
54310121 2175=item lstat
bbce6d69 2176
7660c0ab 2177Does the same thing as the C<stat()> function (including setting the
5a964f20
TC
2178special C<_> filehandle) but stats a symbolic link instead of the file
2179the symbolic link points to. If symbolic links are unimplemented on
7660c0ab 2180your system, a normal C<stat()> is done.
a0d0e21e 2181
7660c0ab 2182If EXPR is omitted, stats C<$_>.
bbce6d69 2183
a0d0e21e
LW
2184=item m//
2185
2186The match operator. See L<perlop>.
2187
2188=item map BLOCK LIST
2189
2190=item map EXPR,LIST
2191
7660c0ab 2192Evaluates the BLOCK or EXPR for each element of LIST (locally setting C<$_> to each
a0d0e21e
LW
2193element) and returns the list value composed of the results of each such
2194evaluation. Evaluates BLOCK or EXPR in a list context, so each element of LIST
2195may produce zero, one, or more elements in the returned value.
2196
2197 @chars = map(chr, @nums);
2198
2199translates a list of numbers to the corresponding characters. And
2200
4633a7c4 2201 %hash = map { getkey($_) => $_ } @array;
a0d0e21e
LW
2202
2203is just a funny way to write
2204
2205 %hash = ();
2206 foreach $_ (@array) {
4633a7c4 2207 $hash{getkey($_)} = $_;
a0d0e21e
LW
2208 }
2209
7660c0ab 2210Note that, because C<$_> is a reference into the list value, it can be used
fb73857a
PP
2211to modify the elements of the array. While this is useful and
2212supported, it can cause bizarre results if the LIST is not a named
2213array. See also L</grep> for an array composed of those items of the
2214original list for which the BLOCK or EXPR evaluates to true.
2215
a0d0e21e
LW
2216=item mkdir FILENAME,MODE
2217
0591cd52
NT
2218Creates the directory specified by FILENAME, with permissions
2219specified by MODE (as modified by C<umask>). If it succeeds it
2220returns TRUE, otherwise it returns FALSE and sets C<$!> (errno).
2221
2222In general, it is better to create directories with permissive MODEs,
2223and let the user modify that with their C<umask>, than it is to supply
2224a restrictive MODE and give the user no way to be more permissive.
2225The exceptions to this rule are when the file or directory should be
2226kept private (mail files, for instance). The perlfunc(1) entry on
2227C<umask> discusses the choice of MODE in more detail.
a0d0e21e
LW
2228
2229=item msgctl ID,CMD,ARG
2230
f86cebdf 2231Calls the System V IPC function msgctl(2). You'll probably have to say
0ade1984
JH
2232
2233 use IPC::SysV;
2234
7660c0ab
A
2235first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2236then ARG must be a variable which will hold the returned C<msqid_ds>
ab4f32c2 2237structure. Returns like C<ioctl()>: the undefined value for error, "C<0> but
0ade1984 2238true" for zero, or the actual return value otherwise. See also
7660c0ab 2239C<IPC::SysV> and C<IPC::Semaphore::Msg> documentation.
a0d0e21e
LW
2240
2241=item msgget KEY,FLAGS
2242
f86cebdf 2243Calls the System V IPC function msgget(2). Returns the message queue
7660c0ab
A
2244id, or the undefined value if there is an error. See also C<IPC::SysV>
2245and C<IPC::SysV::Msg> documentation.
a0d0e21e
LW
2246
2247=item msgsnd ID,MSG,FLAGS
2248
2249Calls the System V IPC function msgsnd to send the message MSG to the
2250message queue ID. MSG must begin with the long integer message type,
c07a80fd 2251which may be created with C<pack("l", $type)>. Returns TRUE if
7660c0ab
A
2252successful, or FALSE if there is an error. See also C<IPC::SysV>
2253and C<IPC::SysV::Msg> documentation.
a0d0e21e
LW
2254
2255=item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2256
2257Calls the System V IPC function msgrcv to receive a message from
2258message queue ID into variable VAR with a maximum message size of
0ade1984
JH
2259SIZE. Note that if a message is received, the message type will be
2260the first thing in VAR, and the maximum length of VAR is SIZE plus the
2261size of the message type. Returns TRUE if successful, or FALSE if
7660c0ab 2262there is an error. See also C<IPC::SysV> and C<IPC::SysV::Msg> documentation.
a0d0e21e
LW
2263
2264=item my EXPR
2265
ab4f32c2
A
2266A C<my()> declares the listed variables to be local (lexically) to the
2267enclosing block, file, or C<eval()>. If
5f05dabc 2268more than one value is listed, the list must be placed in parentheses. See
cb1a09d0 2269L<perlsub/"Private Variables via my()"> for details.
4633a7c4 2270
a0d0e21e
LW
2271=item next LABEL
2272
2273=item next
2274
2275The C<next> command is like the C<continue> statement in C; it starts
2276the next iteration of the loop:
2277
4633a7c4
LW
2278 LINE: while (<STDIN>) {
2279 next LINE if /^#/; # discard comments
5a964f20 2280 #...
a0d0e21e
LW
2281 }
2282
2283Note that if there were a C<continue> block on the above, it would get
2284executed even on discarded lines. If the LABEL is omitted, the command
2285refers to the innermost enclosing loop.
2286
4968c1e4
G
2287C<next> cannot be used to exit a block which returns a value such as
2288C<eval {}>, C<sub {}> or C<do {}>.
2289
a189bff9
JH
2290See also L</continue> for an illustration of how L</last>, C<next>, and
2291L</redo> work.
2292
2293See also L<perlsyn>.
1d2dff63 2294
a0d0e21e
LW
2295=item no Module LIST
2296
7660c0ab 2297See the L</use> function, which C<no> is the opposite of.
a0d0e21e
LW
2298
2299=item oct EXPR
2300
54310121 2301=item oct
bbce6d69 2302
4633a7c4 2303Interprets EXPR as an octal string and returns the corresponding
7660c0ab 2304value. (If EXPR happens to start off with C<0x>, interprets it as
4633a7c4
LW
2305a hex string instead.) The following will handle decimal, octal, and
2306hex in the standard Perl or C notation:
a0d0e21e
LW
2307
2308 $val = oct($val) if $val =~ /^0/;
2309
7660c0ab
A
2310If EXPR is omitted, uses C<$_>. This function is commonly used when
2311a string such as C<644> needs to be converted into a file mode, for
2f9daede
TPG
2312example. (Although perl will automatically convert strings into
2313numbers as needed, this automatic conversion assumes base 10.)
a0d0e21e
LW
2314
2315=item open FILEHANDLE,EXPR
2316
2317=item open FILEHANDLE
2318
2319Opens the file whose filename is given by EXPR, and associates it with
5f05dabc
PP
2320FILEHANDLE. If FILEHANDLE is an expression, its value is used as the
2321name of the real filehandle wanted. If EXPR is omitted, the scalar
2322variable of the same name as the FILEHANDLE contains the filename.
ab4f32c2
A
2323(Note that lexical variables--those declared with C<my()>--will not work
2324for this purpose; so if you're using C<my()>, specify EXPR in your call
5f05dabc
PP
2325to open.)
2326
7660c0ab
A
2327If the filename begins with C<'E<lt>'> or nothing, the file is opened for input.
2328If the filename begins with C<'E<gt>'>, the file is truncated and opened for
2329output, being created if necessary. If the filename begins with C<'E<gt>E<gt>'>,
fbb426e4 2330the file is opened for appending, again being created if necessary.
7660c0ab
A
2331You can put a C<'+'> in front of the C<'E<gt>'> or C<'E<lt>'> to indicate that
2332you want both read and write access to the file; thus C<'+E<lt>'> is almost
2333always preferred for read/write updates--the C<'+E<gt>'> mode would clobber the
5a964f20
TC
2334file first. You can't usually use either read-write mode for updating
2335textfiles, since they have variable length records. See the B<-i>
0591cd52
NT
2336switch in L<perlrun> for a better approach. The file is created with
2337permissions of C<0666> modified by the process' C<umask> value.
5a964f20
TC
2338
2339The prefix and the filename may be separated with spaces.
f86cebdf 2340These various prefixes correspond to the fopen(3) modes of C<'r'>, C<'r+'>, C<'w'>,
7660c0ab 2341C<'w+'>, C<'a'>, and C<'a+'>.
5f05dabc 2342
7660c0ab 2343If the filename begins with C<'|'>, the filename is interpreted as a
5a964f20 2344command to which output is to be piped, and if the filename ends with a
7660c0ab
A
2345C<'|'>, the filename is interpreted See L<perlipc/"Using open() for IPC">
2346for more examples of this. (You are not allowed to C<open()> to a command
5a964f20
TC
2347that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2348and L<perlipc/"Bidirectional Communication"> for alternatives.)
cb1a09d0 2349
7660c0ab 2350Opening C<'-'> opens STDIN and opening C<'E<gt>-'> opens STDOUT. Open returns
ab4f32c2 2351nonzero upon success, the undefined value otherwise. If the C<open()>
4633a7c4 2352involved a pipe, the return value happens to be the pid of the
54310121 2353subprocess.
cb1a09d0
AD
2354
2355If you're unfortunate enough to be running Perl on a system that
2356distinguishes between text files and binary files (modern operating
2357systems don't care), then you should check out L</binmode> for tips for
ab4f32c2 2358dealing with this. The key distinction between systems that need C<binmode()>
5a964f20
TC
2359and those that don't is their text file formats. Systems like Unix, MacOS, and
2360Plan9, which delimit lines with a single character, and which encode that
ab4f32c2 2361character in C as C<"\n">, do not need C<binmode()>. The rest need it.
cb1a09d0 2362
fb73857a 2363When opening a file, it's usually a bad idea to continue normal execution
ab4f32c2
A
2364if the request failed, so C<open()> is frequently used in connection with
2365C<die()>. Even if C<die()> won't do what you want (say, in a CGI script,
fb73857a 2366where you want to make a nicely formatted error message (but there are
5a964f20 2367modules that can help with that problem)) you should always check
fb73857a
PP
2368the return value from opening a file. The infrequent exception is when
2369working with an unopened filehandle is actually what you want to do.
2370
cb1a09d0 2371Examples:
a0d0e21e
LW
2372
2373 $ARTICLE = 100;
2374 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2375 while (<ARTICLE>) {...
2376
2377 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
fb73857a 2378 # if the open fails, output is discarded
a0d0e21e 2379
fb73857a
PP
2380 open(DBASE, '+<dbase.mine') # open for update
2381 or die "Can't open 'dbase.mine' for update: $!";
cb1a09d0 2382
fb73857a
PP
2383 open(ARTICLE, "caesar <$article |") # decrypt article
2384 or die "Can't start caesar: $!";
a0d0e21e 2385
fb73857a
PP
2386 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
2387 or die "Can't start sort: $!";
a0d0e21e
LW
2388
2389 # process argument list of files along with any includes
2390
2391 foreach $file (@ARGV) {
2392 process($file, 'fh00');
2393 }
2394
2395 sub process {
5a964f20 2396 my($filename, $input) = @_;
a0d0e21e
LW
2397 $input++; # this is a string increment
2398 unless (open($input, $filename)) {
2399 print STDERR "Can't open $filename: $!\n";
2400 return;
2401 }
2402
5a964f20 2403 local $_;
a0d0e21e
LW
2404 while (<$input>) { # note use of indirection
2405 if (/^#include "(.*)"/) {
2406 process($1, $input);
2407 next;
2408 }
5a964f20 2409 #... # whatever
a0d0e21e
LW
2410 }
2411 }
2412
2413You may also, in the Bourne shell tradition, specify an EXPR beginning
7660c0ab 2414with C<'E<gt>&'>, in which case the rest of the string is interpreted as the
5a964f20 2415name of a filehandle (or file descriptor, if numeric) to be
7660c0ab
A
2416duped and opened. You may use C<&> after C<E<gt>>, C<E<gt>E<gt>>, C<E<lt>>, C<+E<gt>>,
2417C<+E<gt>E<gt>>, and C<+E<lt>>. The
a0d0e21e 2418mode you specify should match the mode of the original filehandle.
184e9718 2419(Duping a filehandle does not take into account any existing contents of
cb1a09d0 2420stdio buffers.)
a0d0e21e
LW
2421Here is a script that saves, redirects, and restores STDOUT and
2422STDERR:
2423
2424 #!/usr/bin/perl
5a964f20
TC
2425 open(OLDOUT, ">&STDOUT");
2426 open(OLDERR, ">&STDERR");
a0d0e21e
LW
2427
2428 open(STDOUT, ">foo.out") || die "Can't redirect stdout";
2429 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
2430
2431 select(STDERR); $| = 1; # make unbuffered
2432 select(STDOUT); $| = 1; # make unbuffered
2433
2434 print STDOUT "stdout 1\n"; # this works for
2435 print STDERR "stderr 1\n"; # subprocesses too
2436
2437 close(STDOUT);
2438 close(STDERR);
2439
5a964f20
TC
2440 open(STDOUT, ">&OLDOUT");
2441 open(STDERR, ">&OLDERR");
a0d0e21e
LW
2442
2443 print STDOUT "stdout 2\n";
2444 print STDERR "stderr 2\n";
2445
2446
7660c0ab
A
2447If you specify C<'E<lt>&=N'>, where C<N> is a number, then Perl will do an
2448equivalent of C's C<fdopen()> of that file descriptor; this is more
4633a7c4 2449parsimonious of file descriptors. For example:
a0d0e21e
LW
2450
2451 open(FILEHANDLE, "<&=$fd")
2452
7660c0ab 2453If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>, then
a0d0e21e 2454there is an implicit fork done, and the return value of open is the pid
7660c0ab 2455of the child within the parent process, and C<0> within the child
184e9718 2456process. (Use C<defined($pid)> to determine whether the open was successful.)
a0d0e21e
LW
2457The filehandle behaves normally for the parent, but i/o to that
2458filehandle is piped from/to the STDOUT/STDIN of the child process.
2459In the child process the filehandle isn't opened--i/o happens from/to
2460the new STDOUT or STDIN. Typically this is used like the normal
2461piped open when you want to exercise more control over just how the
2462pipe command gets executed, such as when you are running setuid, and
54310121 2463don't want to have to scan shell commands for metacharacters.
4633a7c4 2464The following pairs are more or less equivalent:
a0d0e21e
LW
2465
2466 open(FOO, "|tr '[a-z]' '[A-Z]'");
2467 open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
2468
2469 open(FOO, "cat -n '$file'|");
2470 open(FOO, "-|") || exec 'cat', '-n', $file;
2471
4633a7c4
LW
2472See L<perlipc/"Safe Pipe Opens"> for more examples of this.
2473
5a964f20 2474NOTE: On any operation that may do a fork, any unflushed buffers remain
184e9718 2475unflushed in both processes, which means you may need to set C<$|> to
4771b018
GS
2476avoid duplicate output. On systems that support a close-on-exec flag on
2477files, the flag will be set for the newly opened file descriptor as
2478determined by the value of $^F. See L<perlvar/$^F>.
a0d0e21e 2479
0dccf244
CS
2480Closing any piped filehandle causes the parent process to wait for the
2481child to finish, and returns the status value in C<$?>.
2482
5a964f20 2483The filename passed to open will have leading and trailing
f86cebdf 2484whitespace deleted, and the normal redirection characters
5a964f20
TC
2485honored. This property, known as "magic open",
2486can often be used to good effect. A user could specify a filename of
7660c0ab 2487F<"rsh cat file |">, or you could change certain filenames as needed:
5a964f20
TC
2488
2489 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
2490 open(FH, $filename) or die "Can't open $filename: $!";
2491
2492However, to open a file with arbitrary weird characters in it, it's
2493necessary to protect any leading and trailing whitespace:
2494
2495 $file =~ s#^(\s)#./$1#;
2496 open(FOO, "< $file\0");
2497
7660c0ab
A
2498If you want a "real" C C<open()> (see L<open(2)> on your system), then you
2499should use the C<sysopen()> function, which involves no such magic. This is
5a964f20
TC
2500another way to protect your filenames from interpretation. For example:
2501
2502 use IO::Handle;
2503 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
2504 or die "sysopen $path: $!";
2505 $oldfh = select(HANDLE); $| = 1; select($oldfh);
2506 print HANDLE "stuff $$\n");
2507 seek(HANDLE, 0, 0);
2508 print "File contains: ", <HANDLE>;
2509
7660c0ab
A
2510Using the constructor from the C<IO::Handle> package (or one of its
2511subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
5a964f20
TC
2512filehandles that have the scope of whatever variables hold references to
2513them, and automatically close whenever and however you leave that scope:
c07a80fd 2514
5f05dabc 2515 use IO::File;
5a964f20 2516 #...
c07a80fd
PP
2517 sub read_myfile_munged {
2518 my $ALL = shift;
5f05dabc 2519 my $handle = new IO::File;
c07a80fd
PP
2520 open($handle, "myfile") or die "myfile: $!";
2521 $first = <$handle>
2522 or return (); # Automatically closed here.
2523 mung $first or die "mung failed"; # Or here.
2524 return $first, <$handle> if $ALL; # Or here.
2525 $first; # Or here.
2526 }
2527
cb1a09d0 2528See L</seek()> for some details about mixing reading and writing.
a0d0e21e
LW
2529
2530=item opendir DIRHANDLE,EXPR
2531
7660c0ab
A
2532Opens a directory named EXPR for processing by C<readdir()>, C<telldir()>,
2533C<seekdir()>, C<rewinddir()>, and C<closedir()>. Returns TRUE if successful.
a0d0e21e
LW
2534DIRHANDLEs have their own namespace separate from FILEHANDLEs.
2535
2536=item ord EXPR
2537
54310121 2538=item ord
bbce6d69 2539
a0ed51b3 2540Returns the numeric (ASCII or Unicode) value of the first character of EXPR. If
7660c0ab 2541EXPR is omitted, uses C<$_>. For the reverse, see L</chr>.
a0d0e21e
LW
2542
2543=item pack TEMPLATE,LIST
2544
2545Takes an array or list of values and packs it into a binary structure,
2546returning the string containing the structure. The TEMPLATE is a
2547sequence of characters that give the order and type of values, as
2548follows:
2549
2550 A An ascii string, will be space padded.
2551 a An ascii string, will be null padded.
2552 b A bit string (ascending bit order, like vec()).
2553 B A bit string (descending bit order).
2554 h A hex string (low nybble first).
2555 H A hex string (high nybble first).
2556
2557 c A signed char value.
a0ed51b3 2558 C An unsigned char value. Only does bytes. See U for Unicode.
96e4d5b1 2559
a0d0e21e
LW
2560 s A signed short value.
2561 S An unsigned short value.
96e4d5b1
PP
2562 (This 'short' is _exactly_ 16 bits, which may differ from
2563 what a local C compiler calls 'short'.)
2564
a0d0e21e
LW
2565 i A signed integer value.
2566 I An unsigned integer value.
f86cebdf
GS
2567 (This 'integer' is _at_least_ 32 bits wide. Its exact
2568 size depends on what a local C compiler calls 'int',
2569 and may even be larger than the 'long' described in
2570 the next item.)
96e4d5b1 2571
a0d0e21e
LW
2572 l A signed long value.
2573 L An unsigned long value.
96e4d5b1
PP
2574 (This 'long' is _exactly_ 32 bits, which may differ from
2575 what a local C compiler calls 'long'.)
a0d0e21e 2576
96e4d5b1
PP
2577 n A short in "network" (big-endian) order.
2578 N A long in "network" (big-endian) order.
a0d0e21e
LW
2579 v A short in "VAX" (little-endian) order.
2580 V A long in "VAX" (little-endian) order.
96e4d5b1
PP
2581 (These 'shorts' and 'longs' are _exactly_ 16 bits and
2582 _exactly_ 32 bits, respectively.)
a0d0e21e 2583
dae0da7a
JH
2584 q A signed quad (64-bit) value.
2585 Q An unsigned quad value.
2586 (Available only if your system supports 64-bit integer values
2587 _and_ if Perl has been compiled to support those.
2588 Causes a fatal error otherwise.)
2589
a0d0e21e
LW
2590 f A single-precision float in the native format.
2591 d A double-precision float in the native format.
2592
2593 p A pointer to a null-terminated string.
2594 P A pointer to a structure (fixed-length string).
2595
2596 u A uuencoded string.
a0ed51b3
LW
2597 U A Unicode character number. Encodes to UTF-8 internally.
2598 Works even if C<use utf8> is not in effect.
a0d0e21e 2599
96e4d5b1 2600 w A BER compressed integer. Its bytes represent an unsigned
f86cebdf
GS
2601 integer in base 128, most significant digit first, with as
2602 few digits as possible. Bit eight (the high bit) is set
2603 on each byte except the last.
def98dd4 2604
a0d0e21e
LW
2605 x A null byte.
2606 X Back up a byte.
2607 @ Null fill to absolute position.
2608
5a964f20 2609Each letter may optionally be followed by a number giving a repeat
7660c0ab
A
2610count. With all types except C<"a">, C<"A">, C<"b">, C<"B">, C<"h">, C<"H">, and C<"P"> the
2611pack function will gobble up that many values from the LIST. A C<*> for the
2612repeat count means to use however many items are left. The C<"a"> and C<"A">
a0d0e21e 2613types gobble just one value, but pack it as a string of length count,
7660c0ab
A
2614padding with nulls or spaces as necessary. (When unpacking, C<"A"> strips
2615trailing spaces and nulls, but C<"a"> does not.) Likewise, the C<"b"> and C<"B">
2616fields pack a string that many bits long. The C<"h"> and C<"H"> fields pack a
2617string that many nybbles long. The C<"p"> type packs a pointer to a null-
84902520
TB
2618terminated string. You are responsible for ensuring the string is not a
2619temporary value (which can potentially get deallocated before you get
7660c0ab 2620around to using the packed result). The C<"P"> packs a pointer to a structure
61167c6f 2621of the size indicated by the length. A NULL pointer is created if the
7660c0ab 2622corresponding value for C<"p"> or C<"P"> is C<undef>.
61167c6f 2623Real numbers (floats and doubles) are
a0d0e21e
LW
2624in the native machine format only; due to the multiplicity of floating
2625formats around, and the lack of a standard "network" representation, no
2626facility for interchange has been made. This means that packed floating
2627point data written on one machine may not be readable on another - even if
2628both use IEEE floating point arithmetic (as the endian-ness of the memory
2629representation is not part of the IEEE spec). Note that Perl uses doubles
2630internally for all numeric calculation, and converting from double into
5f05dabc 2631float and thence back to double again will lose precision (i.e.,
7660c0ab 2632C<unpack("f", pack("f", $foo)>) will not in general equal C<$foo>).
a0d0e21e
LW
2633
2634Examples:
2635
a0ed51b3 2636 $foo = pack("CCCC",65,66,67,68);
a0d0e21e 2637 # foo eq "ABCD"
a0ed51b3 2638 $foo = pack("C4",65,66,67,68);
a0d0e21e 2639 # same thing
a0ed51b3
LW
2640 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
2641 # same thing with Unicode circled letters
a0d0e21e
LW
2642
2643 $foo = pack("ccxxcc",65,66,67,68);
2644 # foo eq "AB\0\0CD"
2645
2646 $foo = pack("s2",1,2);
2647 # "\1\0\2\0" on little-endian
2648 # "\0\1\0\2" on big-endian
2649
2650 $foo = pack("a4","abcd","x","y","z");
2651 # "abcd"
2652
2653 $foo = pack("aaaa","abcd","x","y","z");
2654 # "axyz"
2655
2656 $foo = pack("a14","abcdefg");
2657 # "abcdefg\0\0\0\0\0\0\0"
2658
2659 $foo = pack("i9pl", gmtime);
2660 # a real struct tm (on my system anyway)
2661
2662 sub bintodec {
2663 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
2664 }
2665
2666The same template may generally also be used in the unpack function.
2667
5a964f20
TC
2668=item package
2669
cb1a09d0
AD
2670=item package NAMESPACE
2671
2672Declares the compilation unit as being in the given namespace. The scope
2673of the package declaration is from the declaration itself through the end of
7660c0ab 2674the enclosing block (the same scope as the C<local()> operator). All further
cb1a09d0 2675unqualified dynamic identifiers will be in this namespace. A package
5f05dabc 2676statement affects only dynamic variables--including those you've used
7660c0ab 2677C<local()> on--but I<not> lexical variables created with C<my()>. Typically it
cb1a09d0
AD
2678would be the first declaration in a file to be included by the C<require>
2679or C<use> operator. You can switch into a package in more than one place;
5a964f20 2680it merely influences which symbol table is used by the compiler for the
cb1a09d0
AD
2681rest of that block. You can refer to variables and filehandles in other
2682packages by prefixing the identifier with the package name and a double
2683colon: C<$Package::Variable>. If the package name is null, the C<main>
2684package as assumed. That is, C<$::sail> is equivalent to C<$main::sail>.
2685
5a964f20
TC
2686If NAMESPACE is omitted, then there is no current package, and all
2687identifiers must be fully qualified or lexicals. This is stricter
2688than C<use strict>, since it also extends to function names.
2689
cb1a09d0
AD
2690See L<perlmod/"Packages"> for more information about packages, modules,
2691and classes. See L<perlsub> for other scoping issues.
2692
a0d0e21e
LW
2693=item pipe READHANDLE,WRITEHANDLE
2694
2695Opens a pair of connected pipes like the corresponding system call.
2696Note that if you set up a loop of piped processes, deadlock can occur
2697unless you are very careful. In addition, note that Perl's pipes use
184e9718 2698stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE
a0d0e21e
LW
2699after each command, depending on the application.
2700
7e1af8bc 2701See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
4633a7c4
LW
2702for examples of such things.
2703
4771b018
GS
2704On systems that support a close-on-exec flag on files, the flag will be set
2705for the newly opened file descriptors as determined by the value of $^F.
2706See L<perlvar/$^F>.
2707
a0d0e21e
LW
2708=item pop ARRAY
2709
54310121 2710=item pop
28757baa 2711
a0d0e21e
LW
2712Pops and returns the last value of the array, shortening the array by
27131. Has a similar effect to
2714
2715 $tmp = $ARRAY[$#ARRAY--];
2716
2717If there are no elements in the array, returns the undefined value.
cb1a09d0 2718If ARRAY is omitted, pops the
7660c0ab
A
2719C<@ARGV> array in the main program, and the C<@_> array in subroutines, just
2720like C<shift()>.
a0d0e21e
LW
2721
2722=item pos SCALAR
2723
54310121 2724=item pos
bbce6d69 2725
4633a7c4 2726Returns the offset of where the last C<m//g> search left off for the variable
7660c0ab 2727is in question (C<$_> is used when the variable is not specified). May be
44a8e56a
PP
2728modified to change that offset. Such modification will also influence
2729the C<\G> zero-width assertion in regular expressions. See L<perlre> and
2730L<perlop>.
a0d0e21e
LW
2731
2732=item print FILEHANDLE LIST
2733
2734=item print LIST
2735
2736=item print
2737
cb1a09d0 2738Prints a string or a comma-separated list of strings. Returns TRUE
a0d0e21e 2739if successful. FILEHANDLE may be a scalar variable name, in which case
cb1a09d0 2740the variable contains the name of or a reference to the filehandle, thus introducing one
a0d0e21e
LW
2741level of indirection. (NOTE: If FILEHANDLE is a variable and the next
2742token is a term, it may be misinterpreted as an operator unless you
7660c0ab 2743interpose a C<+> or put parentheses around the arguments.) If FILEHANDLE is
a0d0e21e 2744omitted, prints by default to standard output (or to the last selected
7660c0ab 2745output channel--see L</select>). If LIST is also omitted, prints C<$_> to
5a964f20 2746the currently selected output channel. To set the default output channel to something other than
a0d0e21e 2747STDOUT use the select operation. Note that, because print takes a
5a964f20 2748LIST, anything in the LIST is evaluated in list context, and any
a0d0e21e 2749subroutine that you call will have one or more of its expressions
5a964f20 2750evaluated in list context. Also be careful not to follow the print
a0d0e21e 2751keyword with a left parenthesis unless you want the corresponding right
7660c0ab 2752parenthesis to terminate the arguments to the print--interpose a C<+> or
5f05dabc 2753put parentheses around all the arguments.
a0d0e21e 2754
4633a7c4 2755Note that if you're storing FILEHANDLES in an array or other expression,
da0045b7 2756you will have to use a block returning its value instead:
4633a7c4
LW
2757
2758 print { $files[$i] } "stuff\n";
2759 print { $OK ? STDOUT : STDERR } "stuff\n";
2760
5f05dabc 2761=item printf FILEHANDLE FORMAT, LIST
a0d0e21e 2762
5f05dabc 2763=item printf FORMAT, LIST
a0d0e21e 2764
7660c0ab 2765Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
a3cb178b 2766(the output record separator) is not appended. The first argument
ab4f32c2 2767of the list will be interpreted as the C<printf()> format. If C<use locale> is
a034a98d
DD
2768in effect, the character used for the decimal point in formatted real numbers
2769is affected by the LC_NUMERIC locale. See L<perllocale>.
a0d0e21e 2770
7660c0ab
A
2771Don't fall into the trap of using a C<printf()> when a simple
2772C<print()> would do. The C<print()> is more efficient and less
28757baa
PP
2773error prone.
2774
da0045b7
PP
2775=item prototype FUNCTION
2776
2777Returns the prototype of a function as a string (or C<undef> if the
5f05dabc
PP
2778function has no prototype). FUNCTION is a reference to, or the name of,
2779the function whose prototype you want to retrieve.
da0045b7 2780
b6c543e3
IZ
2781If FUNCTION is a string starting with C<CORE::>, the rest is taken as
2782a name for Perl builtin. If builtin is not I<overridable> (such as
ab4f32c2
A
2783C<qw//>) or its arguments cannot be expressed by a prototype (such as
2784C<system()>) - in other words, the builtin does not behave like a Perl
b6c543e3
IZ
2785function - returns C<undef>. Otherwise, the string describing the
2786equivalent prototype is returned.
2787
a0d0e21e
LW
2788=item push ARRAY,LIST
2789
2790Treats ARRAY as a stack, and pushes the values of LIST
2791onto the end of ARRAY. The length of ARRAY increases by the length of
2792LIST. Has the same effect as
2793
2794 for $value (LIST) {
2795 $ARRAY[++$#ARRAY] = $value;
2796 }
2797
2798but is more efficient. Returns the new number of elements in the array.
2799
2800=item q/STRING/
2801
2802=item qq/STRING/
2803
8782bef2
GB
2804=item qr/STRING/
2805
a0d0e21e
LW
2806=item qx/STRING/
2807
2808=item qw/STRING/
2809
4b6a7270 2810Generalized quotes. See L<perlop/"Regexp Quote-Like Operators">.
a0d0e21e
LW
2811
2812=item quotemeta EXPR
2813
54310121 2814=item quotemeta
bbce6d69 2815
68dc0745 2816Returns the value of EXPR with all non-alphanumeric
a034a98d
DD
2817characters backslashed. (That is, all characters not matching
2818C</[A-Za-z_0-9]/> will be preceded by a backslash in the
2819returned string, regardless of any locale settings.)
2820This is the internal function implementing
7660c0ab 2821the C<\Q> escape in double-quoted strings.
a0d0e21e 2822
7660c0ab 2823If EXPR is omitted, uses C<$_>.
bbce6d69 2824
a0d0e21e
LW
2825=item rand EXPR
2826
2827=item rand
2828
7660c0ab 2829Returns a random fractional number greater than or equal to C<0> and less
3e3baf6d 2830than the value of EXPR. (EXPR should be positive.) If EXPR is
7660c0ab
A
2831omitted, the value C<1> is used. Automatically calls C<srand()> unless
2832C<srand()> has already been called. See also C<srand()>.
a0d0e21e 2833
2f9daede 2834(Note: If your rand function consistently returns numbers that are too
a0d0e21e 2835large or too small, then your version of Perl was probably compiled
2f9daede 2836with the wrong number of RANDBITS.)
a0d0e21e
LW
2837
2838=item read FILEHANDLE,SCALAR,LENGTH,OFFSET
2839
2840=item read FILEHANDLE,SCALAR,LENGTH
2841
2842Attempts to read LENGTH bytes of data into variable SCALAR from the
3b02c43c
GS
2843specified FILEHANDLE. Returns the number of bytes actually read,
2844C<0> at end of file, or undef if there was an error. SCALAR will be grown
2845or shrunk to the length actually read. An OFFSET may be specified to
2846place the read data at some other place than the beginning of the
f86cebdf
GS
2847string. This call is actually implemented in terms of stdio's fread(3)
2848call. To get a true read(2) system call, see C<sysread()>.
a0d0e21e
LW
2849
2850=item readdir DIRHANDLE
2851
7660c0ab 2852Returns the next directory entry for a directory opened by C<opendir()>.
5a964f20 2853If used in list context, returns all the rest of the entries in the
a0d0e21e 2854directory. If there are no more entries, returns an undefined value in
5a964f20 2855scalar context or a null list in list context.
a0d0e21e 2856
7660c0ab 2857If you're planning to filetest the return values out of a C<readdir()>, you'd
5f05dabc 2858better prepend the directory in question. Otherwise, because we didn't
7660c0ab 2859C<chdir()> there, it would have been testing the wrong file.
cb1a09d0
AD
2860
2861 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
2862 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
2863 closedir DIR;
2864
84902520
TB
2865=item readline EXPR
2866
5a964f20 2867Reads from the filehandle whose typeglob is contained in EXPR. In scalar context, a single line
84902520
TB
2868is read and returned. In list context, reads until end-of-file is
2869reached and returns a list of lines (however you've defined lines
7660c0ab 2870with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
84902520
TB
2871This is the internal function implementing the C<E<lt>EXPRE<gt>>
2872operator, but you can use it directly. The C<E<lt>EXPRE<gt>>
2873operator is discussed in more detail in L<perlop/"I/O Operators">.
2874
5a964f20
TC
2875 $line = <STDIN>;
2876 $line = readline(*STDIN); # same thing
2877
a0d0e21e
LW
2878=item readlink EXPR
2879
54310121 2880=item readlink
bbce6d69 2881
a0d0e21e
LW
2882Returns the value of a symbolic link, if symbolic links are
2883implemented. If not, gives a fatal error. If there is some system
184e9718 2884error, returns the undefined value and sets C<$!> (errno). If EXPR is
7660c0ab 2885omitted, uses C<$_>.
a0d0e21e 2886
84902520
TB
2887=item readpipe EXPR
2888
5a964f20 2889EXPR is executed as a system command.
84902520
TB
2890The collected standard output of the command is returned.
2891In scalar context, it comes back as a single (potentially
2892multi-line) string. In list context, returns a list of lines
7660c0ab 2893(however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
84902520
TB
2894This is the internal function implementing the C<qx/EXPR/>
2895operator, but you can use it directly. The C<qx/EXPR/>
2896operator is discussed in more detail in L<perlop/"I/O Operators">.
2897
a0d0e21e
LW
2898=item recv SOCKET,SCALAR,LEN,FLAGS
2899
2900Receives a message on a socket. Attempts to receive LENGTH bytes of
2901data into variable SCALAR from the specified SOCKET filehandle.
7660c0ab 2902Actually does a C C<recvfrom()>, so that it can return the address of the
a0d0e21e
LW
2903sender. Returns the undefined value if there's an error. SCALAR will
2904be grown or shrunk to the length actually read. Takes the same flags
54310121 2905as the system call of the same name.
4633a7c4 2906See L<perlipc/"UDP: Message Passing"> for examples.
a0d0e21e
LW
2907
2908=item redo LABEL
2909
2910=item redo
2911
2912The C<redo> command restarts the loop block without evaluating the
a189bff9 2913conditional again. The L</continue> block, if any, is not executed. If
a0d0e21e
LW
2914the LABEL is omitted, the command refers to the innermost enclosing
2915loop. This command is normally used by programs that want to lie to
2916themselves about what was just input:
2917
2918 # a simpleminded Pascal comment stripper
2919 # (warning: assumes no { or } in strings)
4633a7c4 2920 LINE: while (<STDIN>) {
a0d0e21e
LW
2921 while (s|({.*}.*){.*}|$1 |) {}
2922 s|{.*}| |;
2923 if (s|{.*| |) {
2924 $front = $_;
2925 while (<STDIN>) {
2926 if (/}/) { # end of comment?
5a964f20 2927 s|^|$front\{|;
4633a7c4 2928 redo LINE;
a0d0e21e
LW
2929 }
2930 }
2931 }
2932 print;
2933 }
2934
4968c1e4
G
2935C<redo> cannot be used to retry a block which returns a value such as
2936C<eval {}>, C<sub {}> or C<do {}>.
2937
a189bff9 2938See also L</continue> for an illustration of how L</last>, L</next>, and
1d2dff63
GS
2939C<redo> work.
2940
a189bff9
JH
2941See also L<perlsyn>.
2942
a0d0e21e
LW
2943=item ref EXPR
2944
54310121 2945=item ref
bbce6d69 2946
2f9daede 2947Returns a TRUE value if EXPR is a reference, FALSE otherwise. If EXPR
7660c0ab 2948is not specified, C<$_> will be used. The value returned depends on the
bbce6d69 2949type of thing the reference is a reference to.
a0d0e21e
LW
2950Builtin types include:
2951
2952 REF
2953 SCALAR
2954 ARRAY
2955 HASH
2956 CODE
2957 GLOB
2958
54310121 2959If the referenced object has been blessed into a package, then that package
7660c0ab 2960name is returned instead. You can think of C<ref()> as a C<typeof()> operator.
a0d0e21e
LW
2961
2962 if (ref($r) eq "HASH") {
aa689395 2963 print "r is a reference to a hash.\n";
54310121 2964 }
5a964f20 2965 if (!ref($r)) {
a0d0e21e 2966 print "r is not a reference at all.\n";
54310121 2967 }
a0d0e21e
LW
2968
2969See also L<perlref>.
2970
2971=item rename OLDNAME,NEWNAME
2972
7660c0ab 2973Changes the name of a file. Returns C<1> for success, C<0> otherwise. Will
5f05dabc 2974not work across file system boundaries.
a0d0e21e
LW
2975
2976=item require EXPR
2977
2978=item require
2979
7660c0ab 2980Demands some semantics specified by EXPR, or by C<$_> if EXPR is not
a0d0e21e 2981supplied. If EXPR is numeric, demands that the current version of Perl
184e9718 2982(C<$]> or $PERL_VERSION) be equal or greater than EXPR.
a0d0e21e
LW
2983
2984Otherwise, demands that a library file be included if it hasn't already
2985been included. The file is included via the do-FILE mechanism, which is
7660c0ab 2986essentially just a variety of C<eval()>. Has semantics similar to the following
a0d0e21e
LW
2987subroutine:
2988
2989 sub require {
5a964f20 2990 my($filename) = @_;
a0d0e21e 2991 return 1 if $INC{$filename};
5a964f20 2992 my($realfilename,$result);
a0d0e21e
LW
2993 ITER: {
2994 foreach $prefix (@INC) {
2995 $realfilename = "$prefix/$filename";
2996 if (-f $realfilename) {
2997 $result = do $realfilename;
2998 last ITER;
2999 }
3000 }
3001 die "Can't find $filename in \@INC";
3002 }
3003 die $@ if $@;
3004 die "$filename did not return true value" unless $result;
3005 $INC{$filename} = $realfilename;
5a964f20 3006 return $result;
a0d0e21e
LW
3007 }
3008
3009Note that the file will not be included twice under the same specified
3010name. The file must return TRUE as the last statement to indicate
3011successful execution of any initialization code, so it's customary to
7660c0ab 3012end such a file with "C<1;>" unless you're sure it'll return TRUE
a0d0e21e
LW
3013otherwise. But it's better just to put the "C<1;>", in case you add more
3014statements.
3015
54310121 3016If EXPR is a bareword, the require assumes a "F<.pm>" extension and
da0045b7 3017replaces "F<::>" with "F</>" in the filename for you,
54310121 3018to make it easy to load standard modules. This form of loading of
a0d0e21e
LW
3019modules does not risk altering your namespace.
3020
ee580363
GS
3021In other words, if you try this:
3022
f86cebdf 3023 require Foo::Bar; # a splendid bareword
ee580363 3024
7660c0ab
A
3025The require function will actually look for the "F<Foo/Bar.pm>" file in the
3026directories specified in the C<@INC> array.
ee580363 3027
5a964f20 3028But if you try this:
ee580363
GS
3029
3030 $class = 'Foo::Bar';
f86cebdf 3031 require $class; # $class is not a bareword
5a964f20 3032 #or
f86cebdf 3033 require "Foo::Bar"; # not a bareword because of the ""
ee580363 3034
7660c0ab
A
3035The require function will look for the "F<Foo::Bar>" file in the @INC array and
3036will complain about not finding "F<Foo::Bar>" there. In this case you can do:
ee580363
GS
3037
3038 eval "require $class";
3039
3040For a yet-more-powerful import facility, see L</use> and L<perlmod>.
a0d0e21e
LW
3041
3042=item reset EXPR
3043
3044=item reset
3045
3046Generally used in a C<continue> block at the end of a loop to clear
7660c0ab 3047variables and reset C<??> searches so that they work again. The
a0d0e21e
LW
3048expression is interpreted as a list of single characters (hyphens
3049allowed for ranges). All variables and arrays beginning with one of
3050those letters are reset to their pristine state. If the expression is
7660c0ab 3051omitted, one-match searches (C<?pattern?>) are reset to match again. Resets
5f05dabc 3052only variables or searches in the current package. Always returns
a0d0e21e
LW
30531. Examples:
3054
3055 reset 'X'; # reset all X variables
3056 reset 'a-z'; # reset lower case variables
3057 reset; # just reset ?? searches
3058
7660c0ab
A
3059Resetting C<"A-Z"> is not recommended because you'll wipe out your
3060C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package variables--lexical variables
a0d0e21e 3061are unaffected, but they clean themselves up on scope exit anyway,
da0045b7 3062so you'll probably want to use them instead. See L</my>.
a0d0e21e 3063
54310121
PP
3064=item return EXPR
3065
3066=item return
3067
7660c0ab 3068Returns from a subroutine, C<eval()>, or C<do FILE> with the value
5a964f20 3069given in EXPR. Evaluation of EXPR may be in list, scalar, or void
54310121 3070context, depending on how the return value will be used, and the context
7660c0ab 3071may vary from one execution to the next (see C<wantarray()>). If no EXPR
5a964f20
TC
3072is given, returns an empty list in list context, an undefined value in
3073scalar context, or nothing in a void context.
a0d0e21e 3074
68dc0745
PP
3075(Note that in the absence of a return, a subroutine, eval, or do FILE
3076will automatically return the value of the last expression evaluated.)
a0d0e21e
LW
3077
3078=item reverse LIST
3079
5a964f20
TC
3080In list context, returns a list value consisting of the elements
3081of LIST in the opposite order. In scalar context, concatenates the
a0ed51b3
LW
3082elements of LIST, and returns a string value with all the characters
3083in the opposite order.
4633a7c4 3084
2f9daede 3085 print reverse <>; # line tac, last line first
4633a7c4 3086
2f9daede 3087 undef $/; # for efficiency of <>
a0ed51b3 3088 print scalar reverse <>; # character tac, last line tsrif
2f9daede
TPG
3089
3090This operator is also handy for inverting a hash, although there are some
3091caveats. If a value is duplicated in the original hash, only one of those
3092can be represented as a key in the inverted hash. Also, this has to
3093unwind one hash and build a whole new one, which may take some time
3094on a large hash.
3095
3096 %by_name = reverse %by_address; # Invert the hash
a0d0e21e
LW
3097
3098=item rewinddir DIRHANDLE
3099
3100Sets the current position to the beginning of the directory for the
7660c0ab 3101C<readdir()> routine on DIRHANDLE.
a0d0e21e
LW
3102
3103=item rindex STR,SUBSTR,POSITION
3104
3105=item rindex STR,SUBSTR
3106
3107Works just like index except that it returns the position of the LAST
3108occurrence of SUBSTR in STR. If POSITION is specified, returns the
3109last occurrence at or before that position.
3110
3111=item rmdir FILENAME
3112
54310121 3113=item rmdir
bbce6d69 3114
5a964f20
TC
3115Deletes the directory specified by FILENAME if that directory is empty. If it
3116succeeds it returns TRUE, otherwise it returns FALSE and sets C<$!> (errno). If
7660c0ab 3117FILENAME is omitted, uses C<$_>.
a0d0e21e
LW
3118
3119=item s///
3120
3121The substitution operator. See L<perlop>.
3122
3123=item scalar EXPR
3124
5a964f20 3125Forces EXPR to be interpreted in scalar context and returns the value
54310121 3126of EXPR.
cb1a09d0
AD
3127
3128 @counts = ( scalar @a, scalar @b, scalar @c );
3129
54310121 3130There is no equivalent operator to force an expression to
5a964f20 3131be interpolated in list context because it's in practice never
cb1a09d0
AD
3132needed. If you really wanted to do so, however, you could use
3133the construction C<@{[ (some expression) ]}>, but usually a simple
3134C<(some expression)> suffices.
a0d0e21e
LW
3135
3136=item seek FILEHANDLE,POSITION,WHENCE
3137
ab4f32c2 3138Sets FILEHANDLE's position, just like the C<fseek()> call of C<stdio()>.
8903cb82 3139FILEHANDLE may be an expression whose value gives the name of the
7660c0ab
A
3140filehandle. The values for WHENCE are C<0> to set the new position to
3141POSITION, C<1> to set it to the current position plus POSITION, and C<2> to
8903cb82 3142set it to EOF plus POSITION (typically negative). For WHENCE you may
7660c0ab
A
3143use the constants C<SEEK_SET>, C<SEEK_CUR>, and C<SEEK_END> from either the
3144C<IO::Seekable> or the POSIX module. Returns C<1> upon success, C<0> otherwise.
8903cb82 3145
7660c0ab
A
3146If you want to position file for C<sysread()> or C<syswrite()>, don't use
3147C<seek()> -- buffering makes its effect on the file's system position
3148unpredictable and non-portable. Use C<sysseek()> instead.
a0d0e21e 3149
cb1a09d0
AD
3150On some systems you have to do a seek whenever you switch between reading
3151and writing. Amongst other things, this may have the effect of calling
f86cebdf 3152stdio's clearerr(3). A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving
8903cb82 3153the file position:
cb1a09d0
AD
3154
3155 seek(TEST,0,1);
3156
3157This is also useful for applications emulating C<tail -f>. Once you hit
3158EOF on your read, and then sleep for a while, you might have to stick in a
7660c0ab 3159seek() to reset things. The C<seek()> doesn't change the current position,
8903cb82
PP
3160but it I<does> clear the end-of-file condition on the handle, so that the
3161next C<E<lt>FILEE<gt>> makes Perl try again to read something. We hope.
cb1a09d0
AD
3162
3163If that doesn't work (some stdios are particularly cantankerous), then
3164you may need something more like this:
3165
3166 for (;;) {
f86cebdf
GS
3167 for ($curpos = tell(FILE); $_ = <FILE>;
3168 $curpos = tell(FILE)) {
cb1a09d0
AD
3169 # search for some stuff and put it into files
3170 }
3171 sleep($for_a_while);
3172 seek(FILE, $curpos, 0);
3173 }
3174
a0d0e21e
LW
3175=item seekdir DIRHANDLE,POS
3176
7660c0ab
A
3177Sets the current position for the C<readdir()> routine on DIRHANDLE. POS
3178must be a value returned by C<telldir()>. Has the same caveats about
a0d0e21e
LW
3179possible directory compaction as the corresponding system library
3180routine.
3181
3182=item select FILEHANDLE
3183
3184=item select
3185
3186Returns the currently selected filehandle. Sets the current default
3187filehandle for output, if FILEHANDLE is supplied. This has two
ab4f32c2 3188effects: first, a C<write()> or a C<print()> without a filehandle will
a0d0e21e
LW
3189default to this FILEHANDLE. Second, references to variables related to
3190output will refer to this output channel. For example, if you have to
3191set the top of form format for more than one output channel, you might
3192do the following:
3193
3194 select(REPORT1);
3195 $^ = 'report1_top';
3196 select(REPORT2);
3197 $^ = 'report2_top';
3198
3199FILEHANDLE may be an expression whose value gives the name of the
3200actual filehandle. Thus:
3201
3202 $oldfh = select(STDERR); $| = 1; select($oldfh);
3203
4633a7c4
LW
3204Some programmers may prefer to think of filehandles as objects with
3205methods, preferring to write the last example as:
a0d0e21e 3206
28757baa 3207 use IO::Handle;
a0d0e21e
LW
3208 STDERR->autoflush(1);
3209
3210=item select RBITS,WBITS,EBITS,TIMEOUT
3211
f86cebdf 3212This calls the select(2) system call with the bit masks specified, which
7660c0ab 3213can be constructed using C<fileno()> and C<vec()>, along these lines:
a0d0e21e
LW
3214
3215 $rin = $win = $ein = '';
3216 vec($rin,fileno(STDIN),1) = 1;
3217 vec($win,fileno(STDOUT),1) = 1;
3218 $ein = $rin | $win;
3219
3220If you want to select on many filehandles you might wish to write a
3221subroutine:
3222
3223 sub fhbits {
5a964f20
TC
3224 my(@fhlist) = split(' ',$_[0]);
3225 my($bits);
a0d0e21e
LW
3226 for (@fhlist) {
3227 vec($bits,fileno($_),1) = 1;
3228 }
3229 $bits;
3230 }
4633a7c4 3231 $rin = fhbits('STDIN TTY SOCK');
a0d0e21e
LW
3232
3233The usual idiom is:
3234
3235 ($nfound,$timeleft) =
3236 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
3237
54310121 3238or to block until something becomes ready just do this
a0d0e21e
LW
3239
3240 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
3241
7660c0ab
A
3242Most systems do not bother to return anything useful in C<$timeleft>, so
3243calling select() in scalar context just returns C<$nfound>.
c07a80fd 3244
5f05dabc 3245Any of the bit masks can also be undef. The timeout, if specified, is
a0d0e21e 3246in seconds, which may be fractional. Note: not all implementations are
7660c0ab
A
3247capable of returning theC<$timeleft>. If not, they always return
3248C<$timeleft> equal to the supplied C<$timeout>.
a0d0e21e 3249
ff68c719 3250You can effect a sleep of 250 milliseconds this way:
a0d0e21e
LW
3251
3252 select(undef, undef, undef, 0.25);
3253
7660c0ab
A
3254B<WARNING>: One should not attempt to mix buffered I/O (like C<read()>
3255or E<lt>FHE<gt>) with C<select()>, except as permitted by POSIX, and even
3256then only on POSIX systems. You have to use C<sysread()> instead.
a0d0e21e
LW
3257
3258=item semctl ID,SEMNUM,CMD,ARG
3259
ab4f32c2 3260Calls the System V IPC function C<semctl()>. You'll probably have to say
0ade1984
JH
3261
3262 use IPC::SysV;
3263
3264first to get the correct constant definitions. If CMD is IPC_STAT or
3265GETALL, then ARG must be a variable which will hold the returned
ab4f32c2 3266semid_ds structure or semaphore value array. Returns like C<ioctl()>: the
7660c0ab
A
3267undefined value for error, "C<0> but true" for zero, or the actual return
3268value otherwise. See also C<IPC::SysV> and C<IPC::Semaphore> documentation.
a0d0e21e
LW
3269
3270=item semget KEY,NSEMS,FLAGS
3271
3272Calls the System V IPC function semget. Returns the semaphore id, or
7660c0ab
A
3273the undefined value if there is an error. See also C<IPC::SysV> and
3274C<IPC::SysV::Semaphore> documentation.
a0d0e21e
LW
3275
3276=item semop KEY,OPSTRING
3277
3278Calls the System V IPC function semop to perform semaphore operations
3279such as signaling and waiting. OPSTRING must be a packed array of
3280semop structures. Each semop structure can be generated with
3281C<pack("sss", $semnum, $semop, $semflag)>. The number of semaphore
3282operations is implied by the length of OPSTRING. Returns TRUE if
3283successful, or FALSE if there is an error. As an example, the
7660c0ab 3284following code waits on semaphore C<$semnum> of semaphore id C<$semid>:
a0d0e21e
LW
3285
3286 $semop = pack("sss", $semnum, -1, 0);
3287 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
3288
7660c0ab
A
3289To signal the semaphore, replace C<-1> with C<1>. See also C<IPC::SysV>
3290and C<IPC::SysV::Semaphore> documentation.
a0d0e21e
LW
3291
3292=item send SOCKET,MSG,FLAGS,TO
3293
3294=item send SOCKET,MSG,FLAGS
3295
3296Sends a message on a socket. Takes the same flags as the system call
3297of the same name. On unconnected sockets you must specify a
7660c0ab 3298destination to send TO, in which case it does a C C<sendto()>. Returns
a0d0e21e
LW
3299the number of characters sent, or the undefined value if there is an
3300error.