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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
5f05dabc 17be first, and the list argument will follow. (Note that there can ever
0f31cffe 18be only one such list argument.) For instance, splice() has three scalar
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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
TM
102C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>,
103C<sin>, C<sqrt>, C<srand>
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104
105=item Functions for real @ARRAYs
106
22fae026 107C<pop>, C<push>, C<shift>, C<splice>, C<unshift>
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108
109=item Functions for list data
110
ab4f32c2 111C<grep>, C<join>, C<map>, C<qw/STRING/>, C<reverse>, C<sort>, C<unpack>
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112
113=item Functions for real %HASHes
114
22fae026 115C<delete>, C<each>, C<exists>, C<keys>, C<values>
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116
117=item Input and output functions
118
22fae026
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119C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
120C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
121C<readdir>, C<rewinddir>, C<seek>, C<seekdir>, C<select>, C<syscall>,
122C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>,
123C<warn>, C<write>
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124
125=item Functions for fixed length data or records
126
22fae026 127C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec>
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128
129=item Functions for filehandles, files, or directories
130
22fae026 131C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>,
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132C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>,
133C<readlink>, C<rename>, C<rmdir>, C<stat>, C<symlink>, C<umask>,
134C<unlink>, C<utime>
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135
136=item Keywords related to the control flow of your perl program
137
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138C<caller>, C<continue>, C<die>, C<do>, C<dump>, C<eval>, C<exit>,
139C<goto>, C<last>, C<next>, C<redo>, C<return>, C<sub>, C<wantarray>
cb1a09d0 140
54310121 141=item Keywords related to scoping
cb1a09d0 142
22fae026 143C<caller>, C<import>, C<local>, C<my>, C<package>, C<use>
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144
145=item Miscellaneous functions
146
22fae026
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147C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<reset>,
148C<scalar>, C<undef>, C<wantarray>
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149
150=item Functions for processes and process groups
151
22fae026 152C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
ab4f32c2 153C<pipe>, C<qx/STRING/>, C<setpgrp>, C<setpriority>, C<sleep>, C<system>,
22fae026 154C<times>, C<wait>, C<waitpid>
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155
156=item Keywords related to perl modules
157
22fae026 158C<do>, C<import>, C<no>, C<package>, C<require>, C<use>
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159
160=item Keywords related to classes and object-orientedness
161
22fae026
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162C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
163C<untie>, C<use>
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164
165=item Low-level socket functions
166
22fae026
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167C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
168C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
169C<socket>, C<socketpair>
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170
171=item System V interprocess communication functions
172
22fae026
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173C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
174C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>
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175
176=item Fetching user and group info
177
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178C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
179C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
180C<getpwuid>, C<setgrent>, C<setpwent>
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181
182=item Fetching network info
183
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184C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
185C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
186C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
187C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
188C<setnetent>, C<setprotoent>, C<setservent>
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189
190=item Time-related functions
191
22fae026 192C<gmtime>, C<localtime>, C<time>, C<times>
cb1a09d0 193
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194=item Functions new in perl5
195
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196C<abs>, C<bless>, C<chomp>, C<chr>, C<exists>, C<formline>, C<glob>,
197C<import>, C<lc>, C<lcfirst>, C<map>, C<my>, C<no>, C<prototype>, C<qx>,
198C<qw>, C<readline>, C<readpipe>, C<ref>, C<sub*>, C<sysopen>, C<tie>,
199C<tied>, C<uc>, C<ucfirst>, C<untie>, C<use>
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200
201* - C<sub> was a keyword in perl4, but in perl5 it is an
5a964f20 202operator, which can be used in expressions.
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203
204=item Functions obsoleted in perl5
205
22fae026 206C<dbmclose>, C<dbmopen>
37798a01 207
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208=back
209
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210=head2 Portability
211
212Perl was born in UNIX and therefore it can access all the common UNIX
213system calls. In non-UNIX environments the functionality of many
214UNIX system calls may not be available or the details of the available
215functionality may be slightly different. The Perl functions affected
216by this are:
217
218C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>,
219C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>,
220C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>,
221C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostent>,
222C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
223C<getppid>, C<getprgp>, C<getpriority>, C<getprotobynumber>,
224C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>,
225C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>,
226C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>,
227C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<select>, C<semctl>,
228C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>,
229C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>,
230C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>,
231C<shmwrite>, C<socketpair>, C<stat>, C<symlink>, C<syscall>,
232C<sysopen>, C<system>, C<times>, C<truncate>, C<umask>, C<utime>,
233C<wait>, C<waitpid>
234
235For more information about the portability of these functions, see
236L<perlport> and other available platform-specific documentation.
237
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238=head2 Alphabetical Listing of Perl Functions
239
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240=over 8
241
22fae026 242=item I<-X> FILEHANDLE
a0d0e21e 243
22fae026 244=item I<-X> EXPR
a0d0e21e 245
22fae026 246=item I<-X>
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247
248A file test, where X is one of the letters listed below. This unary
249operator takes one argument, either a filename or a filehandle, and
250tests the associated file to see if something is true about it. If the
7660c0ab 251argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
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252Unless otherwise documented, it returns C<1> for TRUE and C<''> for FALSE, or
253the undefined value if the file doesn't exist. Despite the funny
254names, precedence is the same as any other named unary operator, and
255the argument may be parenthesized like any other unary operator. The
256operator may be any of:
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257X<-r>X<-w>X<-x>X<-o>X<-R>X<-W>X<-X>X<-O>X<-e>X<-z>X<-s>X<-f>X<-d>X<-l>X<-p>
258X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
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259
260 -r File is readable by effective uid/gid.
261 -w File is writable by effective uid/gid.
262 -x File is executable by effective uid/gid.
263 -o File is owned by effective uid.
264
265 -R File is readable by real uid/gid.
266 -W File is writable by real uid/gid.
267 -X File is executable by real uid/gid.
268 -O File is owned by real uid.
269
270 -e File exists.
271 -z File has zero size.
54310121 272 -s File has nonzero size (returns size).
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273
274 -f File is a plain file.
275 -d File is a directory.
276 -l File is a symbolic link.
9c4d0f16 277 -p File is a named pipe (FIFO), or Filehandle is a pipe.
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278 -S File is a socket.
279 -b File is a block special file.
280 -c File is a character special file.
281 -t Filehandle is opened to a tty.
282
283 -u File has setuid bit set.
284 -g File has setgid bit set.
285 -k File has sticky bit set.
286
287 -T File is a text file.
288 -B File is a binary file (opposite of -T).
289
290 -M Age of file in days when script started.
291 -A Same for access time.
292 -C Same for inode change time.
293
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294Example:
295
296 while (<>) {
297 chop;
298 next unless -f $_; # ignore specials
5a964f20 299 #...
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300 }
301
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302The interpretation of the file permission operators C<-r>, C<-R>,
303C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
304of the file and the uids and gids of the user. There may be other
305reasons you can't actually read, write, or execute the file. Such
306reasons may be for example network filesystem access controls, ACLs
307(access control lists), read-only filesystems, and unrecognized
308executable formats.
309
310Also note that, for the superuser on the local filesystems, C<-r>,
311C<-R>, C<-w>, and C<-W> always return 1, and C<-x> and C<-X> return 1
312if any execute bit is set in the mode. Scripts run by the superuser
313may thus need to do a stat() to determine the actual mode of the file,
314or temporarily set the uid to something else.
315
316If you are using ACLs, there is a pragma called C<filetest> that may
317produce more accurate results than the bare stat() mode bits.
318
319When under the C<use filetest 'access'> the above-mentioned filetests
320will test whether the permission can (not) be granted using the
468541a8 321access() family of system calls. Also note that the C<-x> and C<-X> may
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322under this pragma return true even if there are no execute permission
323bits set (nor any extra execute permission ACLs). This strangeness is
324due to the underlying system calls' definitions. Read the
325documentation for the C<filetest> pragma for more information.
326
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327Note that C<-s/a/b/> does not do a negated substitution. Saying
328C<-exp($foo)> still works as expected, however--only single letters
329following a minus are interpreted as file tests.
330
331The C<-T> and C<-B> switches work as follows. The first block or so of the
332file is examined for odd characters such as strange control codes or
5a964f20 333characters with the high bit set. If too many strange characters (E<gt>30%)
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334are found, it's a C<-B> file, otherwise it's a C<-T> file. Also, any file
335containing null in the first block is considered a binary file. If C<-T>
336or C<-B> is used on a filehandle, the current stdio buffer is examined
337rather than the first block. Both C<-T> and C<-B> return TRUE on a null
54310121 338file, or a file at EOF when testing a filehandle. Because you have to
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339read a file to do the C<-T> test, on most occasions you want to use a C<-f>
340against the file first, as in C<next unless -f $file && -T $file>.
a0d0e21e 341
7660c0ab 342If any of the file tests (or either the C<stat()> or C<lstat()> operators) are given
28757baa 343the special filehandle consisting of a solitary underline, then the stat
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344structure of the previous file test (or stat operator) is used, saving
345a system call. (This doesn't work with C<-t>, and you need to remember
346that lstat() and C<-l> will leave values in the stat structure for the
347symbolic link, not the real file.) Example:
348
349 print "Can do.\n" if -r $a || -w _ || -x _;
350
351 stat($filename);
352 print "Readable\n" if -r _;
353 print "Writable\n" if -w _;
354 print "Executable\n" if -x _;
355 print "Setuid\n" if -u _;
356 print "Setgid\n" if -g _;
357 print "Sticky\n" if -k _;
358 print "Text\n" if -T _;
359 print "Binary\n" if -B _;
360
361=item abs VALUE
362
54310121 363=item abs
bbce6d69 364
a0d0e21e 365Returns the absolute value of its argument.
7660c0ab 366If VALUE is omitted, uses C<$_>.
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367
368=item accept NEWSOCKET,GENERICSOCKET
369
f86cebdf 370Accepts an incoming socket connect, just as the accept(2) system call
a0d0e21e 371does. Returns the packed address if it succeeded, FALSE otherwise.
4633a7c4 372See example in L<perlipc/"Sockets: Client/Server Communication">.
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373
374=item alarm SECONDS
375
54310121 376=item alarm
bbce6d69 377
a0d0e21e 378Arranges to have a SIGALRM delivered to this process after the
bbce6d69 379specified number of seconds have elapsed. If SECONDS is not specified,
7660c0ab 380the value stored in C<$_> is used. (On some machines,
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381unfortunately, the elapsed time may be up to one second less than you
382specified because of how seconds are counted.) Only one timer may be
383counting at once. Each call disables the previous timer, and an
7660c0ab 384argument of C<0> may be supplied to cancel the previous timer without
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385starting a new one. The returned value is the amount of time remaining
386on the previous timer.
387
4633a7c4 388For delays of finer granularity than one second, you may use Perl's
f86cebdf 389C<syscall()> interface to access setitimer(2) if your system supports it,
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390or else see L</select()>. It is usually a mistake to intermix C<alarm()>
391and C<sleep()> calls.
a0d0e21e 392
7660c0ab 393If you want to use C<alarm()> to time out a system call you need to use an
ab4f32c2 394C<eval()>/C<die()> pair. You can't rely on the alarm causing the system call to
f86cebdf 395fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
ab4f32c2 396restart system calls on some systems. Using C<eval()>/C<die()> always works,
5a964f20 397modulo the caveats given in L<perlipc/"Signals">.
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398
399 eval {
f86cebdf 400 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
36477c24 401 alarm $timeout;
ff68c719 402 $nread = sysread SOCKET, $buffer, $size;
36477c24 403 alarm 0;
ff68c719 404 };
ff68c719 405 if ($@) {
f86cebdf 406 die unless $@ eq "alarm\n"; # propagate unexpected errors
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407 # timed out
408 }
409 else {
410 # didn't
411 }
412
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413=item atan2 Y,X
414
415Returns the arctangent of Y/X in the range -PI to PI.
416
7660c0ab 417For the tangent operation, you may use the C<POSIX::tan()>
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418function, or use the familiar relation:
419
420 sub tan { sin($_[0]) / cos($_[0]) }
421
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422=item bind SOCKET,NAME
423
424Binds a network address to a socket, just as the bind system call
425does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
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426packed address of the appropriate type for the socket. See the examples in
427L<perlipc/"Sockets: Client/Server Communication">.
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428
429=item binmode FILEHANDLE
430
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431Arranges for the file to be read or written in "binary" mode in operating
432systems that distinguish between binary and text files. Files that are
433not in binary mode have CR LF sequences translated to LF on input and LF
54310121 434translated to CR LF on output. Binmode has no effect under Unix; in MS-DOS
cb1a09d0 435and similarly archaic systems, it may be imperative--otherwise your
54310121 436MS-DOS-damaged C library may mangle your file. The key distinction between
ab4f32c2 437systems that need C<binmode()> and those that don't is their text file
5a964f20 438formats. Systems like Unix, MacOS, and Plan9 that delimit lines with a single
7660c0ab 439character, and that encode that character in C as C<"\n">, do not need
ab4f32c2 440C<binmode()>. The rest need it. If FILEHANDLE is an expression, the value
cb1a09d0 441is taken as the name of the filehandle.
a0d0e21e 442
4633a7c4 443=item bless REF,CLASSNAME
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444
445=item bless REF
446
28757baa 447This function tells the thingy referenced by REF that it is now
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448an object in the CLASSNAME package--or the current package if no CLASSNAME
449is specified, which is often the case. It returns the reference for
7660c0ab 450convenience, because a C<bless()> is often the last thing in a constructor.
4633a7c4 451Always use the two-argument version if the function doing the blessing
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452might be inherited by a derived class. See L<perltoot> and L<perlobj>
453for more about the blessing (and blessings) of objects.
a0d0e21e 454
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455Consider always blessing objects in CLASSNAMEs that are mixed case.
456Namespaces with all lowercase names are considered reserved for Perl
457pragmata. Builtin types have all uppercase names, so to prevent confusion,
458it is best to avoid such package names as well.
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459
460See L<perlmod/"Perl Modules">.
461
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462=item caller EXPR
463
464=item caller
465
5a964f20 466Returns the context of the current subroutine call. In scalar context,
28757baa 467returns the caller's package name if there is a caller, that is, if
7660c0ab 468we're in a subroutine or C<eval()> or C<require()>, and the undefined value
5a964f20 469otherwise. In list context, returns
a0d0e21e 470
748a9306 471 ($package, $filename, $line) = caller;
a0d0e21e
LW
472
473With EXPR, it returns some extra information that the debugger uses to
474print a stack trace. The value of EXPR indicates how many call frames
475to go back before the current one.
476
54310121 477 ($package, $filename, $line, $subroutine,
e7ea3e70
IZ
478 $hasargs, $wantarray, $evaltext, $is_require) = caller($i);
479
7660c0ab 480Here C<$subroutine> may be C<"(eval)"> if the frame is not a subroutine
ab4f32c2 481call, but an C<eval()>. In such a case additional elements C<$evaltext> and
7660c0ab
A
482C<$is_require> are set: C<$is_require> is true if the frame is created by a
483C<require> or C<use> statement, C<$evaltext> contains the text of the
dc848c6f 484C<eval EXPR> statement. In particular, for a C<eval BLOCK> statement,
7660c0ab 485C<$filename> is C<"(eval)">, but C<$evaltext> is undefined. (Note also that
dc848c6f
PP
486each C<use> statement creates a C<require> frame inside an C<eval EXPR>)
487frame.
748a9306
LW
488
489Furthermore, when called from within the DB package, caller returns more
7660c0ab 490detailed information: it sets the list variable C<@DB::args> to be the
54310121 491arguments with which the subroutine was invoked.
748a9306 492
7660c0ab 493Be aware that the optimizer might have optimized call frames away before
ab4f32c2 494C<caller()> had a chance to get the information. That means that C<caller(N)>
7660c0ab
A
495might not return information about the call frame you expect it do, for
496C<N E<gt> 1>. In particular, C<@DB::args> might have information from the
497previous time C<caller()> was called.
498
a0d0e21e
LW
499=item chdir EXPR
500
501Changes the working directory to EXPR, if possible. If EXPR is
502omitted, changes to home directory. Returns TRUE upon success, FALSE
7660c0ab 503otherwise. See example under C<die()>.
a0d0e21e
LW
504
505=item chmod LIST
506
507Changes the permissions of a list of files. The first element of the
4633a7c4 508list must be the numerical mode, which should probably be an octal
2f9daede
TPG
509number, and which definitely should I<not> a string of octal digits:
510C<0644> is okay, C<'0644'> is not. Returns the number of files
dc848c6f 511successfully changed. See also L</oct>, if all you have is a string.
a0d0e21e
LW
512
513 $cnt = chmod 0755, 'foo', 'bar';
514 chmod 0755, @executables;
f86cebdf
GS
515 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
516 # --w----r-T
2f9daede
TPG
517 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
518 $mode = 0644; chmod $mode, 'foo'; # this is best
a0d0e21e
LW
519
520=item chomp VARIABLE
521
522=item chomp LIST
523
524=item chomp
525
3e3baf6d 526This is a slightly safer version of L</chop>. It removes any
a0d0e21e 527line ending that corresponds to the current value of C<$/> (also known as
28757baa
PP
528$INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
529number of characters removed from all its arguments. It's often used to
530remove the newline from the end of an input record when you're worried
531that the final record may be missing its newline. When in paragraph mode
532(C<$/ = "">), it removes all trailing newlines from the string. If
7660c0ab 533VARIABLE is omitted, it chomps C<$_>. Example:
a0d0e21e
LW
534
535 while (<>) {
536 chomp; # avoid \n on last field
537 @array = split(/:/);
5a964f20 538 # ...
a0d0e21e
LW
539 }
540
541You can actually chomp anything that's an lvalue, including an assignment:
542
543 chomp($cwd = `pwd`);
544 chomp($answer = <STDIN>);
545
546If you chomp a list, each element is chomped, and the total number of
547characters removed is returned.
548
549=item chop VARIABLE
550
551=item chop LIST
552
553=item chop
554
555Chops off the last character of a string and returns the character
556chopped. It's used primarily to remove the newline from the end of an
557input record, but is much more efficient than C<s/\n//> because it neither
7660c0ab 558scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
a0d0e21e
LW
559Example:
560
561 while (<>) {
562 chop; # avoid \n on last field
563 @array = split(/:/);
5a964f20 564 #...
a0d0e21e
LW
565 }
566
567You can actually chop anything that's an lvalue, including an assignment:
568
569 chop($cwd = `pwd`);
570 chop($answer = <STDIN>);
571
572If you chop a list, each element is chopped. Only the value of the
ab4f32c2 573last C<chop()> is returned.
a0d0e21e 574
ab4f32c2 575Note that C<chop()> returns the last character. To return all but the last
748a9306
LW
576character, use C<substr($string, 0, -1)>.
577
a0d0e21e
LW
578=item chown LIST
579
580Changes the owner (and group) of a list of files. The first two
581elements of the list must be the I<NUMERICAL> uid and gid, in that order.
582Returns the number of files successfully changed.
583
584 $cnt = chown $uid, $gid, 'foo', 'bar';
585 chown $uid, $gid, @filenames;
586
54310121 587Here's an example that looks up nonnumeric uids in the passwd file:
a0d0e21e
LW
588
589 print "User: ";
590 chop($user = <STDIN>);
5a964f20 591 print "Files: ";
a0d0e21e
LW
592 chop($pattern = <STDIN>);
593
594 ($login,$pass,$uid,$gid) = getpwnam($user)
595 or die "$user not in passwd file";
596
5a964f20 597 @ary = glob($pattern); # expand filenames
a0d0e21e
LW
598 chown $uid, $gid, @ary;
599
54310121 600On most systems, you are not allowed to change the ownership of the
4633a7c4
LW
601file unless you're the superuser, although you should be able to change
602the group to any of your secondary groups. On insecure systems, these
603restrictions may be relaxed, but this is not a portable assumption.
604
a0d0e21e
LW
605=item chr NUMBER
606
54310121 607=item chr
bbce6d69 608
a0d0e21e 609Returns the character represented by that NUMBER in the character set.
a0ed51b3
LW
610For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
611chr(0x263a) is a Unicode smiley face (but only within the scope of a
612C<use utf8>). For the reverse, use L</ord>.
a0d0e21e 613
7660c0ab 614If NUMBER is omitted, uses C<$_>.
bbce6d69 615
a0d0e21e
LW
616=item chroot FILENAME
617
54310121 618=item chroot
bbce6d69 619
5a964f20 620This function works like the system call by the same name: it makes the
4633a7c4 621named directory the new root directory for all further pathnames that
7660c0ab 622begin with a C<"/"> by your process and all its children. (It doesn't
28757baa 623change your current working directory, which is unaffected.) For security
4633a7c4 624reasons, this call is restricted to the superuser. If FILENAME is
ab4f32c2 625omitted, does a C<chroot()> to C<$_>.
a0d0e21e
LW
626
627=item close FILEHANDLE
628
6a518fbc
TP
629=item close
630
a0d0e21e
LW
631Closes the file or pipe associated with the file handle, returning TRUE
632only if stdio successfully flushes buffers and closes the system file
6a518fbc
TP
633descriptor. Closes the currently selected filehandle if the argument
634is omitted.
fb73857a
PP
635
636You don't have to close FILEHANDLE if you are immediately going to do
7660c0ab 637another C<open()> on it, because C<open()> will close it for you. (See
ab4f32c2 638C<open()>.) However, an explicit C<close()> on an input file resets the line
7660c0ab 639counter (C<$.>), while the implicit close done by C<open()> does not.
fb73857a 640
ab4f32c2 641If the file handle came from a piped open C<close()> will additionally
fb73857a
PP
642return FALSE if one of the other system calls involved fails or if the
643program exits with non-zero status. (If the only problem was that the
7660c0ab 644program exited non-zero C<$!> will be set to C<0>.) Also, closing a pipe
5a964f20 645waits for the process executing on the pipe to complete, in case you
fb73857a
PP
646want to look at the output of the pipe afterwards. Closing a pipe
647explicitly also puts the exit status value of the command into C<$?>.
5a964f20 648
fb73857a 649Example:
a0d0e21e 650
fb73857a
PP
651 open(OUTPUT, '|sort >foo') # pipe to sort
652 or die "Can't start sort: $!";
5a964f20 653 #... # print stuff to output
fb73857a
PP
654 close OUTPUT # wait for sort to finish
655 or warn $! ? "Error closing sort pipe: $!"
656 : "Exit status $? from sort";
657 open(INPUT, 'foo') # get sort's results
658 or die "Can't open 'foo' for input: $!";
a0d0e21e 659
5a964f20
TC
660FILEHANDLE may be an expression whose value can be used as an indirect
661filehandle, usually the real filehandle name.
a0d0e21e
LW
662
663=item closedir DIRHANDLE
664
7660c0ab 665Closes a directory opened by C<opendir()> and returns the success of that
5a964f20
TC
666system call.
667
668DIRHANDLE may be an expression whose value can be used as an indirect
669dirhandle, usually the real dirhandle name.
a0d0e21e
LW
670
671=item connect SOCKET,NAME
672
673Attempts to connect to a remote socket, just as the connect system call
674does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
4633a7c4
LW
675packed address of the appropriate type for the socket. See the examples in
676L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 677
cb1a09d0
AD
678=item continue BLOCK
679
680Actually a flow control statement rather than a function. If there is a
98293880
JH
681C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
682C<foreach>), it is always executed just before the conditional is about to
683be evaluated again, just like the third part of a C<for> loop in C. Thus
cb1a09d0
AD
684it can be used to increment a loop variable, even when the loop has been
685continued via the C<next> statement (which is similar to the C C<continue>
686statement).
687
98293880 688C<last>, C<next>, or C<redo> may appear within a C<continue>
1d2dff63
GS
689block. C<last> and C<redo> will behave as if they had been executed within
690the main block. So will C<next>, but since it will execute a C<continue>
691block, it may be more entertaining.
692
693 while (EXPR) {
694 ### redo always comes here
695 do_something;
696 } continue {
697 ### next always comes here
698 do_something_else;
699 # then back the top to re-check EXPR
700 }
701 ### last always comes here
702
703Omitting the C<continue> section is semantically equivalent to using an
704empty one, logically enough. In that case, C<next> goes directly back
705to check the condition at the top of the loop.
706
a0d0e21e
LW
707=item cos EXPR
708
5a964f20 709Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
7660c0ab 710takes cosine of C<$_>.
a0d0e21e 711
7660c0ab 712For the inverse cosine operation, you may use the C<POSIX::acos()>
28757baa
PP
713function, or use this relation:
714
715 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
716
a0d0e21e
LW
717=item crypt PLAINTEXT,SALT
718
f86cebdf 719Encrypts a string exactly like the crypt(3) function in the C library
4633a7c4
LW
720(assuming that you actually have a version there that has not been
721extirpated as a potential munition). This can prove useful for checking
722the password file for lousy passwords, amongst other things. Only the
723guys wearing white hats should do this.
a0d0e21e 724
7660c0ab 725Note that C<crypt()> is intended to be a one-way function, much like breaking
11155c91
CS
726eggs to make an omelette. There is no (known) corresponding decrypt
727function. As a result, this function isn't all that useful for
728cryptography. (For that, see your nearby CPAN mirror.)
2f9daede 729
e71965be
RS
730When verifying an existing encrypted string you should use the encrypted
731text as the salt (like C<crypt($plain, $crypted) eq $crypted>). This
732allows your code to work with the standard C<crypt()> and with more
733exotic implementations. When choosing a new salt create a random two
734character string whose characters come from the set C<[./0-9A-Za-z]>
735(like C<join '', ('.', '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>).
736
a0d0e21e
LW
737Here's an example that makes sure that whoever runs this program knows
738their own password:
739
740 $pwd = (getpwuid($<))[1];
a0d0e21e
LW
741
742 system "stty -echo";
743 print "Password: ";
e71965be 744 chomp($word = <STDIN>);
a0d0e21e
LW
745 print "\n";
746 system "stty echo";
747
e71965be 748 if (crypt($word, $pwd) ne $pwd) {
a0d0e21e
LW
749 die "Sorry...\n";
750 } else {
751 print "ok\n";
54310121 752 }
a0d0e21e 753
9f8f0c9d 754Of course, typing in your own password to whoever asks you
748a9306 755for it is unwise.
a0d0e21e 756
aa689395 757=item dbmclose HASH
a0d0e21e 758
7660c0ab 759[This function has been superseded by the C<untie()> function.]
a0d0e21e 760
aa689395 761Breaks the binding between a DBM file and a hash.
a0d0e21e 762
aa689395 763=item dbmopen HASH,DBNAME,MODE
a0d0e21e 764
7660c0ab 765[This function has been superseded by the C<tie()> function.]
a0d0e21e 766
7b8d334a 767This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
ab4f32c2 768hash. HASH is the name of the hash. (Unlike normal C<open()>, the first
aa689395
PP
769argument is I<NOT> a filehandle, even though it looks like one). DBNAME
770is the name of the database (without the F<.dir> or F<.pag> extension if
771any). If the database does not exist, it is created with protection
7660c0ab
A
772specified by MODE (as modified by the C<umask()>). If your system supports
773only the older DBM functions, you may perform only one C<dbmopen()> in your
aa689395 774program. In older versions of Perl, if your system had neither DBM nor
7660c0ab 775ndbm, calling C<dbmopen()> produced a fatal error; it now falls back to
aa689395
PP
776sdbm(3).
777
778If you don't have write access to the DBM file, you can only read hash
779variables, not set them. If you want to test whether you can write,
7660c0ab 780either use file tests or try setting a dummy hash entry inside an C<eval()>,
aa689395 781which will trap the error.
a0d0e21e 782
7660c0ab
A
783Note that functions such as C<keys()> and C<values()> may return huge lists
784when used on large DBM files. You may prefer to use the C<each()>
a0d0e21e
LW
785function to iterate over large DBM files. Example:
786
787 # print out history file offsets
788 dbmopen(%HIST,'/usr/lib/news/history',0666);
789 while (($key,$val) = each %HIST) {
790 print $key, ' = ', unpack('L',$val), "\n";
791 }
792 dbmclose(%HIST);
793
cb1a09d0 794See also L<AnyDBM_File> for a more general description of the pros and
184e9718 795cons of the various dbm approaches, as well as L<DB_File> for a particularly
cb1a09d0 796rich implementation.
4633a7c4 797
a0d0e21e
LW
798=item defined EXPR
799
54310121 800=item defined
bbce6d69 801
2f9daede
TPG
802Returns a Boolean value telling whether EXPR has a value other than
803the undefined value C<undef>. If EXPR is not present, C<$_> will be
804checked.
805
806Many operations return C<undef> to indicate failure, end of file,
807system error, uninitialized variable, and other exceptional
808conditions. This function allows you to distinguish C<undef> from
809other values. (A simple Boolean test will not distinguish among
7660c0ab 810C<undef>, zero, the empty string, and C<"0">, which are all equally
2f9daede 811false.) Note that since C<undef> is a valid scalar, its presence
7660c0ab 812doesn't I<necessarily> indicate an exceptional condition: C<pop()>
2f9daede
TPG
813returns C<undef> when its argument is an empty array, I<or> when the
814element to return happens to be C<undef>.
815
7660c0ab 816You may also use C<defined()> to check whether a subroutine exists, by
5a964f20 817saying C<defined &func> without parentheses. On the other hand, use
7660c0ab 818of C<defined()> upon aggregates (hashes and arrays) is not guaranteed to
5a964f20 819produce intuitive results, and should probably be avoided.
2f9daede
TPG
820
821When used on a hash element, it tells you whether the value is defined,
dc848c6f 822not whether the key exists in the hash. Use L</exists> for the latter
2f9daede 823purpose.
a0d0e21e
LW
824
825Examples:
826
827 print if defined $switch{'D'};
828 print "$val\n" while defined($val = pop(@ary));
829 die "Can't readlink $sym: $!"
830 unless defined($value = readlink $sym);
a0d0e21e 831 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
2f9daede 832 $debugging = 0 unless defined $debugging;
a0d0e21e 833
7660c0ab
A
834Note: Many folks tend to overuse C<defined()>, and then are surprised to
835discover that the number C<0> and C<""> (the zero-length string) are, in fact,
2f9daede 836defined values. For example, if you say
a5f75d66
AD
837
838 "ab" =~ /a(.*)b/;
839
7660c0ab 840The pattern match succeeds, and C<$1> is defined, despite the fact that it
a5f75d66 841matched "nothing". But it didn't really match nothing--rather, it
7660c0ab 842matched something that happened to be C<0> characters long. This is all
a5f75d66 843very above-board and honest. When a function returns an undefined value,
2f9daede 844it's an admission that it couldn't give you an honest answer. So you
7660c0ab
A
845should use C<defined()> only when you're questioning the integrity of what
846you're trying to do. At other times, a simple comparison to C<0> or C<""> is
2f9daede
TPG
847what you want.
848
7660c0ab 849Currently, using C<defined()> on an entire array or hash reports whether
2f9daede
TPG
850memory for that aggregate has ever been allocated. So an array you set
851to the empty list appears undefined initially, and one that once was full
852and that you then set to the empty list still appears defined. You
853should instead use a simple test for size:
28757baa
PP
854
855 if (@an_array) { print "has array elements\n" }
856 if (%a_hash) { print "has hash members\n" }
857
7660c0ab 858Using C<undef()> on these, however, does clear their memory and then report
5a964f20 859them as not defined anymore, but you shouldn't do that unless you don't
28757baa 860plan to use them again, because it saves time when you load them up
5a964f20
TC
861again to have memory already ready to be filled. The normal way to
862free up space used by an aggregate is to assign the empty list.
28757baa 863
7660c0ab 864This counterintuitive behavior of C<defined()> on aggregates may be
28757baa
PP
865changed, fixed, or broken in a future release of Perl.
866
dc848c6f 867See also L</undef>, L</exists>, L</ref>.
2f9daede 868
a0d0e21e
LW
869=item delete EXPR
870
aa689395
PP
871Deletes the specified key(s) and their associated values from a hash.
872For each key, returns the deleted value associated with that key, or
873the undefined value if there was no such key. Deleting from C<$ENV{}>
874modifies the environment. Deleting from a hash tied to a DBM file
7660c0ab 875deletes the entry from the DBM file. (But deleting from a C<tie()>d hash
5f05dabc 876doesn't necessarily return anything.)
a0d0e21e 877
aa689395 878The following deletes all the values of a hash:
a0d0e21e 879
5f05dabc
PP
880 foreach $key (keys %HASH) {
881 delete $HASH{$key};
a0d0e21e
LW
882 }
883
5f05dabc
PP
884And so does this:
885
886 delete @HASH{keys %HASH}
887
5a964f20 888(But both of these are slower than just assigning the empty list, or
7660c0ab 889using C<undef()>.) Note that the EXPR can be arbitrarily complicated as
5a964f20 890long as the final operation is a hash element lookup or hash slice:
a0d0e21e
LW
891
892 delete $ref->[$x][$y]{$key};
5f05dabc 893 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
a0d0e21e
LW
894
895=item die LIST
896
7660c0ab
A
897Outside an C<eval()>, prints the value of LIST to C<STDERR> and exits with
898the current value of C<$!> (errno). If C<$!> is C<0>, exits with the value of
54310121 899C<($? E<gt>E<gt> 8)> (backtick `command` status). If C<($? E<gt>E<gt> 8)>
7660c0ab
A
900is C<0>, exits with C<255>. Inside an C<eval(),> the error message is stuffed into
901C<$@> and the C<eval()> is terminated with the undefined value. This makes
902C<die()> the way to raise an exception.
a0d0e21e
LW
903
904Equivalent examples:
905
906 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
54310121 907 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
a0d0e21e
LW
908
909If the value of EXPR does not end in a newline, the current script line
910number and input line number (if any) are also printed, and a newline
883faa13
GS
911is supplied. Note that the "input line number" (also known as "chunk")
912is subject to whatever notion of "line" happens to be currently in
913effect, and is also available as the special variable C<$.>.
914See L<perlvar/"$/"> and L<perlvar/"$.">.
915
916Hint: sometimes appending C<", stopped"> to your message
7660c0ab 917will cause it to make better sense when the string C<"at foo line 123"> is
a0d0e21e
LW
918appended. Suppose you are running script "canasta".
919
920 die "/etc/games is no good";
921 die "/etc/games is no good, stopped";
922
923produce, respectively
924
925 /etc/games is no good at canasta line 123.
926 /etc/games is no good, stopped at canasta line 123.
927
7660c0ab 928See also C<exit()> and C<warn()>.
a0d0e21e 929
7660c0ab
A
930If LIST is empty and C<$@> already contains a value (typically from a
931previous eval) that value is reused after appending C<"\t...propagated">.
fb73857a
PP
932This is useful for propagating exceptions:
933
934 eval { ... };
935 die unless $@ =~ /Expected exception/;
936
7660c0ab 937If C<$@> is empty then the string C<"Died"> is used.
fb73857a 938
7660c0ab 939You can arrange for a callback to be run just before the C<die()> does
774d564b
PP
940its deed, by setting the C<$SIG{__DIE__}> hook. The associated handler
941will be called with the error text and can change the error message, if
7660c0ab 942it sees fit, by calling C<die()> again. See L<perlvar/$SIG{expr}> for details on
fb73857a
PP
943setting C<%SIG> entries, and L<"eval BLOCK"> for some examples.
944
945Note that the C<$SIG{__DIE__}> hook is called even inside eval()ed
946blocks/strings. If one wants the hook to do nothing in such
947situations, put
948
949 die @_ if $^S;
950
951as the first line of the handler (see L<perlvar/$^S>).
774d564b 952
a0d0e21e
LW
953=item do BLOCK
954
955Not really a function. Returns the value of the last command in the
956sequence of commands indicated by BLOCK. When modified by a loop
98293880
JH
957modifier, executes the BLOCK once before testing the loop condition.
958(On other statements the loop modifiers test the conditional first.)
a0d0e21e 959
4968c1e4 960C<do BLOCK> does I<not> count as a loop, so the loop control statements
98293880 961C<next>, C<last> or C<redo> cannot be used to leave or restart the block.
4968c1e4 962
a0d0e21e
LW
963=item do SUBROUTINE(LIST)
964
965A deprecated form of subroutine call. See L<perlsub>.
966
967=item do EXPR
968
969Uses the value of EXPR as a filename and executes the contents of the
970file as a Perl script. Its primary use is to include subroutines
971from a Perl subroutine library.
972
973 do 'stat.pl';
974
975is just like
976
fb73857a 977 scalar eval `cat stat.pl`;
a0d0e21e 978
5a964f20 979except that it's more efficient and concise, keeps track of the
a0d0e21e
LW
980current filename for error messages, and searches all the B<-I>
981libraries if the file isn't in the current directory (see also the @INC
dc1be6b5
GS
982array in L<perlvar/Predefined Names>). It is also different in how
983code evaluated with C<do FILENAME> doesn't see lexicals in the enclosing
984scope like C<eval STRING> does. It's the same, however, in that it does
54310121 985reparse the file every time you call it, so you probably don't want to
a0d0e21e
LW
986do this inside a loop.
987
8e30cc93
G
988If C<do> cannot read the file, it returns undef and sets C<$!> to the
989error. If C<do> can read the file but cannot compile it, it
990returns undef and sets an error message in C<$@>. If the file is
991successfully compiled, C<do> returns the value of the last expression
992evaluated.
993
a0d0e21e 994Note that inclusion of library modules is better done with the
7660c0ab 995C<use()> and C<require()> operators, which also do automatic error checking
4633a7c4 996and raise an exception if there's a problem.
a0d0e21e 997
5a964f20
TC
998You might like to use C<do> to read in a program configuration
999file. Manual error checking can be done this way:
1000
1001 # read in config files: system first, then user
f86cebdf
GS
1002 for $file ("/share/prog/defaults.rc",
1003 "$ENV{HOME}/.someprogrc") {
5a964f20 1004 unless ($return = do $file) {
f86cebdf
GS
1005 warn "couldn't parse $file: $@" if $@;
1006 warn "couldn't do $file: $!" unless defined $return;
1007 warn "couldn't run $file" unless $return;
5a964f20
TC
1008 }
1009 }
1010
a0d0e21e
LW
1011=item dump LABEL
1012
1614b0e3
JD
1013=item dump
1014
a0d0e21e
LW
1015This causes an immediate core dump. Primarily this is so that you can
1016use the B<undump> program to turn your core dump into an executable binary
1017after having initialized all your variables at the beginning of the
1018program. When the new binary is executed it will begin by executing a
1019C<goto LABEL> (with all the restrictions that C<goto> suffers). Think of
f86cebdf 1020it as a goto with an intervening core dump and reincarnation. If C<LABEL>
5a964f20 1021is omitted, restarts the program from the top. WARNING: Any files
a0d0e21e
LW
1022opened at the time of the dump will NOT be open any more when the
1023program is reincarnated, with possible resulting confusion on the part
1024of Perl. See also B<-u> option in L<perlrun>.
1025
1026Example:
1027
1028 #!/usr/bin/perl
1029 require 'getopt.pl';
1030 require 'stat.pl';
1031 %days = (
1032 'Sun' => 1,
1033 'Mon' => 2,
1034 'Tue' => 3,
1035 'Wed' => 4,
1036 'Thu' => 5,
1037 'Fri' => 6,
1038 'Sat' => 7,
1039 );
1040
1041 dump QUICKSTART if $ARGV[0] eq '-d';
1042
1043 QUICKSTART:
1044 Getopt('f');
1045
5a964f20
TC
1046This operator is largely obsolete, partly because it's very hard to
1047convert a core file into an executable, and because the real perl-to-C
1048compiler has superseded it.
1049
aa689395
PP
1050=item each HASH
1051
5a964f20 1052When called in list context, returns a 2-element list consisting of the
aa689395 1053key and value for the next element of a hash, so that you can iterate over
5a964f20 1054it. When called in scalar context, returns the key for only the "next"
7660c0ab 1055element in the hash. (Note: Keys may be C<"0"> or C<"">, which are logically
2f9daede
TPG
1056false; you may wish to avoid constructs like C<while ($k = each %foo) {}>
1057for this reason.)
1058
ab192400
GS
1059Entries are returned in an apparently random order. The actual random
1060order is subject to change in future versions of perl, but it is guaranteed
1061to be in the same order as either the C<keys()> or C<values()> function
1062would produce on the same (unmodified) hash.
1063
1064When the hash is entirely read, a null array is returned in list context
1065(which when assigned produces a FALSE (C<0>) value), and C<undef> in
7660c0ab
A
1066scalar context. The next call to C<each()> after that will start iterating
1067again. There is a single iterator for each hash, shared by all C<each()>,
1068C<keys()>, and C<values()> function calls in the program; it can be reset by
2f9daede
TPG
1069reading all the elements from the hash, or by evaluating C<keys HASH> or
1070C<values HASH>. If you add or delete elements of a hash while you're
1071iterating over it, you may get entries skipped or duplicated, so don't.
aa689395 1072
f86cebdf 1073The following prints out your environment like the printenv(1) program,
aa689395 1074only in a different order:
a0d0e21e
LW
1075
1076 while (($key,$value) = each %ENV) {
1077 print "$key=$value\n";
1078 }
1079
ab192400 1080See also C<keys()>, C<values()> and C<sort()>.
a0d0e21e
LW
1081
1082=item eof FILEHANDLE
1083
4633a7c4
LW
1084=item eof ()
1085
a0d0e21e
LW
1086=item eof
1087
1088Returns 1 if the next read on FILEHANDLE will return end of file, or if
1089FILEHANDLE is not open. FILEHANDLE may be an expression whose value
5a964f20 1090gives the real filehandle. (Note that this function actually
7660c0ab 1091reads a character and then C<ungetc()>s it, so isn't very useful in an
748a9306
LW
1092interactive context.) Do not read from a terminal file (or call
1093C<eof(FILEHANDLE)> on it) after end-of-file is reached. Filetypes such
1094as terminals may lose the end-of-file condition if you do.
1095
1096An C<eof> without an argument uses the last file read as argument.
5a964f20 1097Using C<eof()> with empty parentheses is very different. It indicates the pseudo file formed of
2f9daede
TPG
1098the files listed on the command line, i.e., C<eof()> is reasonable to
1099use inside a C<while (E<lt>E<gt>)> loop to detect the end of only the
1100last file. Use C<eof(ARGV)> or eof without the parentheses to test
1101I<EACH> file in a while (E<lt>E<gt>) loop. Examples:
a0d0e21e 1102
748a9306
LW
1103 # reset line numbering on each input file
1104 while (<>) {
5a964f20 1105 next if /^\s*#/; # skip comments
748a9306 1106 print "$.\t$_";
5a964f20
TC
1107 } continue {
1108 close ARGV if eof; # Not eof()!
748a9306
LW
1109 }
1110
a0d0e21e
LW
1111 # insert dashes just before last line of last file
1112 while (<>) {
5a964f20 1113 if (eof()) { # check for end of current file
a0d0e21e 1114 print "--------------\n";
748a9306
LW
1115 close(ARGV); # close or break; is needed if we
1116 # are reading from the terminal
a0d0e21e
LW
1117 }
1118 print;
1119 }
1120
a0d0e21e 1121Practical hint: you almost never need to use C<eof> in Perl, because the
3b02c43c
GS
1122input operators return false values when they run out of data, or if there
1123was an error.
a0d0e21e
LW
1124
1125=item eval EXPR
1126
1127=item eval BLOCK
1128
c7cc6f1c
GS
1129In the first form, the return value of EXPR is parsed and executed as if it
1130were a little Perl program. The value of the expression (which is itself
5a964f20 1131determined within scalar context) is first parsed, and if there weren't any
c7cc6f1c 1132errors, executed in the context of the current Perl program, so that any
5f05dabc 1133variable settings or subroutine and format definitions remain afterwards.
c7cc6f1c
GS
1134Note that the value is parsed every time the eval executes. If EXPR is
1135omitted, evaluates C<$_>. This form is typically used to delay parsing
1136and subsequent execution of the text of EXPR until run time.
1137
1138In the second form, the code within the BLOCK is parsed only once--at the
1139same time the code surrounding the eval itself was parsed--and executed
1140within the context of the current Perl program. This form is typically
1141used to trap exceptions more efficiently than the first (see below), while
1142also providing the benefit of checking the code within BLOCK at compile
1143time.
1144
1145The final semicolon, if any, may be omitted from the value of EXPR or within
1146the BLOCK.
1147
1148In both forms, the value returned is the value of the last expression
5a964f20 1149evaluated inside the mini-program; a return statement may be also used, just
c7cc6f1c 1150as with subroutines. The expression providing the return value is evaluated
5a964f20 1151in void, scalar, or list context, depending on the context of the eval itself.
c7cc6f1c 1152See L</wantarray> for more on how the evaluation context can be determined.
a0d0e21e 1153
7660c0ab
A
1154If there is a syntax error or runtime error, or a C<die()> statement is
1155executed, an undefined value is returned by C<eval()>, and C<$@> is set to the
a0d0e21e 1156error message. If there was no error, C<$@> is guaranteed to be a null
7660c0ab 1157string. Beware that using C<eval()> neither silences perl from printing
c7cc6f1c
GS
1158warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1159To do either of those, you have to use the C<$SIG{__WARN__}> facility. See
1160L</warn> and L<perlvar>.
a0d0e21e 1161
7660c0ab
A
1162Note that, because C<eval()> traps otherwise-fatal errors, it is useful for
1163determining whether a particular feature (such as C<socket()> or C<symlink()>)
a0d0e21e
LW
1164is implemented. It is also Perl's exception trapping mechanism, where
1165the die operator is used to raise exceptions.
1166
1167If the code to be executed doesn't vary, you may use the eval-BLOCK
1168form to trap run-time errors without incurring the penalty of
1169recompiling each time. The error, if any, is still returned in C<$@>.
1170Examples:
1171
54310121 1172 # make divide-by-zero nonfatal
a0d0e21e
LW
1173 eval { $answer = $a / $b; }; warn $@ if $@;
1174
1175 # same thing, but less efficient
1176 eval '$answer = $a / $b'; warn $@ if $@;
1177
1178 # a compile-time error
5a964f20 1179 eval { $answer = }; # WRONG
a0d0e21e
LW
1180
1181 # a run-time error
1182 eval '$answer ='; # sets $@
1183
7660c0ab 1184When using the C<eval{}> form as an exception trap in libraries, you may
774d564b
PP
1185wish not to trigger any C<__DIE__> hooks that user code may have
1186installed. You can use the C<local $SIG{__DIE__}> construct for this
1187purpose, as shown in this example:
1188
1189 # a very private exception trap for divide-by-zero
f86cebdf
GS
1190 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1191 warn $@ if $@;
774d564b
PP
1192
1193This is especially significant, given that C<__DIE__> hooks can call
7660c0ab 1194C<die()> again, which has the effect of changing their error messages:
774d564b
PP
1195
1196 # __DIE__ hooks may modify error messages
1197 {
f86cebdf
GS
1198 local $SIG{'__DIE__'} =
1199 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
c7cc6f1c
GS
1200 eval { die "foo lives here" };
1201 print $@ if $@; # prints "bar lives here"
774d564b
PP
1202 }
1203
7660c0ab 1204With an C<eval()>, you should be especially careful to remember what's
a0d0e21e
LW
1205being looked at when:
1206
1207 eval $x; # CASE 1
1208 eval "$x"; # CASE 2
1209
1210 eval '$x'; # CASE 3
1211 eval { $x }; # CASE 4
1212
5a964f20 1213 eval "\$$x++"; # CASE 5
a0d0e21e
LW
1214 $$x++; # CASE 6
1215
2f9daede 1216Cases 1 and 2 above behave identically: they run the code contained in
7660c0ab 1217the variable C<$x>. (Although case 2 has misleading double quotes making
2f9daede 1218the reader wonder what else might be happening (nothing is).) Cases 3
7660c0ab 1219and 4 likewise behave in the same way: they run the code C<'$x'>, which
2f9daede
TPG
1220does nothing but return the value of C<$x>. (Case 4 is preferred for
1221purely visual reasons, but it also has the advantage of compiling at
1222compile-time instead of at run-time.) Case 5 is a place where
54310121 1223normally you I<WOULD> like to use double quotes, except that in this
2f9daede
TPG
1224particular situation, you can just use symbolic references instead, as
1225in case 6.
a0d0e21e 1226
4968c1e4
G
1227C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1228C<next>, C<last> or C<redo> cannot be used to leave or restart the block.
1229
1230
a0d0e21e
LW
1231=item exec LIST
1232
8bf3b016
GS
1233=item exec PROGRAM LIST
1234
7660c0ab
A
1235The C<exec()> function executes a system command I<AND NEVER RETURNS> -
1236use C<system()> instead of C<exec()> if you want it to return. It fails and
fb73857a
PP
1237returns FALSE only if the command does not exist I<and> it is executed
1238directly instead of via your system's command shell (see below).
a0d0e21e 1239
7660c0ab
A
1240Since it's a common mistake to use C<exec()> instead of C<system()>, Perl
1241warns you if there is a following statement which isn't C<die()>, C<warn()>,
1242or C<exit()> (if C<-w> is set - but you always do that). If you
1243I<really> want to follow an C<exec()> with some other statement, you
55d729e4
GS
1244can use one of these styles to avoid the warning:
1245
5a964f20
TC
1246 exec ('foo') or print STDERR "couldn't exec foo: $!";
1247 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
55d729e4 1248
5a964f20 1249If there is more than one argument in LIST, or if LIST is an array
f86cebdf 1250with more than one value, calls execvp(3) with the arguments in LIST.
5a964f20
TC
1251If there is only one scalar argument or an array with one element in it,
1252the argument is checked for shell metacharacters, and if there are any,
1253the entire argument is passed to the system's command shell for parsing
1254(this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1255If there are no shell metacharacters in the argument, it is split into
7660c0ab
A
1256words and passed directly to C<execvp()>, which is more efficient. Note:
1257C<exec()> and C<system()> do not flush your output buffer, so you may need to
5a964f20 1258set C<$|> to avoid lost output. Examples:
a0d0e21e
LW
1259
1260 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1261 exec "sort $outfile | uniq";
1262
1263If you don't really want to execute the first argument, but want to lie
1264to the program you are executing about its own name, you can specify
1265the program you actually want to run as an "indirect object" (without a
1266comma) in front of the LIST. (This always forces interpretation of the
54310121 1267LIST as a multivalued list, even if there is only a single scalar in
a0d0e21e
LW
1268the list.) Example:
1269
1270 $shell = '/bin/csh';
1271 exec $shell '-sh'; # pretend it's a login shell
1272
1273or, more directly,
1274
1275 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1276
bb32b41a
GS
1277When the arguments get executed via the system shell, results will
1278be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1279for details.
1280
ab4f32c2 1281Using an indirect object with C<exec()> or C<system()> is also more secure.
5a964f20
TC
1282This usage forces interpretation of the arguments as a multivalued list,
1283even if the list had just one argument. That way you're safe from the
1284shell expanding wildcards or splitting up words with whitespace in them.
1285
1286 @args = ( "echo surprise" );
1287
f86cebdf
GS
1288 system @args; # subject to shell escapes
1289 # if @args == 1
5a964f20
TC
1290 system { $args[0] } @args; # safe even with one-arg list
1291
1292The first version, the one without the indirect object, ran the I<echo>
1293program, passing it C<"surprise"> an argument. The second version
1294didn't--it tried to run a program literally called I<"echo surprise">,
1295didn't find it, and set C<$?> to a non-zero value indicating failure.
1296
ab4f32c2 1297Note that C<exec()> will not call your C<END> blocks, nor will it call
7660c0ab
A
1298any C<DESTROY> methods in your objects.
1299
a0d0e21e
LW
1300=item exists EXPR
1301
1302Returns TRUE if the specified hash key exists in its hash array, even
1303if the corresponding value is undefined.
1304
1305 print "Exists\n" if exists $array{$key};
1306 print "Defined\n" if defined $array{$key};
1307 print "True\n" if $array{$key};
1308
5f05dabc 1309A hash element can be TRUE only if it's defined, and defined if
a0d0e21e
LW
1310it exists, but the reverse doesn't necessarily hold true.
1311
1312Note that the EXPR can be arbitrarily complicated as long as the final
1313operation is a hash key lookup:
1314
5a964f20
TC
1315 if (exists $ref->{"A"}{"B"}{$key}) { ... }
1316
1317Although the last element will not spring into existence just because its
1318existence was tested, intervening ones will. Thus C<$ref-E<gt>{"A"}>
1319C<$ref-E<gt>{"B"}> will spring into existence due to the existence
1320test for a $key element. This autovivification may be fixed in a later
1321release.
a0d0e21e
LW
1322
1323=item exit EXPR
1324
1325Evaluates EXPR and exits immediately with that value. (Actually, it
1326calls any defined C<END> routines first, but the C<END> routines may not
1327abort the exit. Likewise any object destructors that need to be called
1328are called before exit.) Example:
1329
1330 $ans = <STDIN>;
1331 exit 0 if $ans =~ /^[Xx]/;
1332
7660c0ab
A
1333See also C<die()>. If EXPR is omitted, exits with C<0> status. The only
1334universally portable values for EXPR are C<0> for success and C<1> for error;
f86702cc
PP
1335all other values are subject to unpredictable interpretation depending
1336on the environment in which the Perl program is running.
a0d0e21e 1337
7660c0ab
A
1338You shouldn't use C<exit()> to abort a subroutine if there's any chance that
1339someone might want to trap whatever error happened. Use C<die()> instead,
1340which can be trapped by an C<eval()>.
28757baa 1341
5a964f20
TC
1342All C<END{}> blocks are run at exit time. See L<perlsub> for details.
1343
a0d0e21e
LW
1344=item exp EXPR
1345
54310121 1346=item exp
bbce6d69 1347
54310121 1348Returns I<e> (the natural logarithm base) to the power of EXPR.
a0d0e21e
LW
1349If EXPR is omitted, gives C<exp($_)>.
1350
1351=item fcntl FILEHANDLE,FUNCTION,SCALAR
1352
f86cebdf 1353Implements the fcntl(2) function. You'll probably have to say
a0d0e21e
LW
1354
1355 use Fcntl;
1356
0ade1984 1357first to get the correct constant definitions. Argument processing and
7660c0ab 1358value return works just like C<ioctl()> below.
a0d0e21e
LW
1359For example:
1360
1361 use Fcntl;
5a964f20
TC
1362 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1363 or die "can't fcntl F_GETFL: $!";
1364
ab4f32c2
A
1365You don't have to check for C<defined()> on the return from
1366C<fnctl()>. Like C<ioctl()>, it maps a C<0> return from the system
7660c0ab
A
1367call into "C<0> but true" in Perl. This string is true in
1368boolean context and C<0> in numeric context. It is also
5a964f20
TC
1369exempt from the normal B<-w> warnings on improper numeric
1370conversions.
1371
7660c0ab 1372Note that C<fcntl()> will produce a fatal error if used on a machine that
f86cebdf 1373doesn't implement fcntl(2).
a0d0e21e
LW
1374
1375=item fileno FILEHANDLE
1376
1377Returns the file descriptor for a filehandle. This is useful for
7660c0ab 1378constructing bitmaps for C<select()> and low-level POSIX tty-handling
5a964f20
TC
1379operations. If FILEHANDLE is an expression, the value is taken as
1380an indirect filehandle, generally its name.
1381
1382You can use this to find out whether two handles refer to the
1383same underlying descriptor:
1384
1385 if (fileno(THIS) == fileno(THAT)) {
1386 print "THIS and THAT are dups\n";
1387 }
a0d0e21e
LW
1388
1389=item flock FILEHANDLE,OPERATION
1390
f86cebdf 1391Calls flock(2), or an emulation of it, on FILEHANDLE. Returns TRUE for
68dc0745 1392success, FALSE on failure. Produces a fatal error if used on a machine
f86cebdf 1393that doesn't implement flock(2), fcntl(2) locking, or lockf(3). C<flock()>
68dc0745
PP
1394is Perl's portable file locking interface, although it locks only entire
1395files, not records.
8ebc5c01 1396
a3cb178b 1397On many platforms (including most versions or clones of Unix), locks
7660c0ab 1398established by C<flock()> are B<merely advisory>. Such discretionary locks
5a964f20 1399are more flexible, but offer fewer guarantees. This means that files
7660c0ab
A
1400locked with C<flock()> may be modified by programs that do not also use
1401C<flock()>. Windows NT and OS/2 are among the platforms which
5a964f20 1402enforce mandatory locking. See your local documentation for details.
a3cb178b 1403
8ebc5c01
PP
1404OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1405LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
68dc0745
PP
1406you can use the symbolic names if import them from the Fcntl module,
1407either individually, or as a group using the ':flock' tag. LOCK_SH
1408requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1409releases a previously requested lock. If LOCK_NB is added to LOCK_SH or
7660c0ab 1410LOCK_EX then C<flock()> will return immediately rather than blocking
68dc0745
PP
1411waiting for the lock (check the return status to see if you got it).
1412
1413To avoid the possibility of mis-coordination, Perl flushes FILEHANDLE
1414before (un)locking it.
8ebc5c01 1415
f86cebdf 1416Note that the emulation built with lockf(3) doesn't provide shared
8ebc5c01 1417locks, and it requires that FILEHANDLE be open with write intent. These
f86cebdf
GS
1418are the semantics that lockf(3) implements. Most (all?) systems
1419implement lockf(3) in terms of fcntl(2) locking, though, so the
8ebc5c01
PP
1420differing semantics shouldn't bite too many people.
1421
7660c0ab
A
1422Note also that some versions of C<flock()> cannot lock things over the
1423network; you would need to use the more system-specific C<fcntl()> for
f86cebdf
GS
1424that. If you like you can force Perl to ignore your system's flock(2)
1425function, and so provide its own fcntl(2)-based emulation, by passing
8ebc5c01
PP
1426the switch C<-Ud_flock> to the F<Configure> program when you configure
1427perl.
4633a7c4
LW
1428
1429Here's a mailbox appender for BSD systems.
a0d0e21e 1430
7e1af8bc 1431 use Fcntl ':flock'; # import LOCK_* constants
a0d0e21e
LW
1432
1433 sub lock {
7e1af8bc 1434 flock(MBOX,LOCK_EX);
a0d0e21e
LW
1435 # and, in case someone appended
1436 # while we were waiting...
1437 seek(MBOX, 0, 2);
1438 }
1439
1440 sub unlock {
7e1af8bc 1441 flock(MBOX,LOCK_UN);
a0d0e21e
LW
1442 }
1443
1444 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1445 or die "Can't open mailbox: $!";
1446
1447 lock();
1448 print MBOX $msg,"\n\n";
1449 unlock();
1450
cb1a09d0 1451See also L<DB_File> for other flock() examples.
a0d0e21e
LW
1452
1453=item fork
1454
f86cebdf 1455Does a fork(2) system call. Returns the child pid to the parent process,
7660c0ab 1456C<0> to the child process, or C<undef> if the fork is unsuccessful.
5a964f20 1457
a0d0e21e 1458Note: unflushed buffers remain unflushed in both processes, which means
7660c0ab
A
1459you may need to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()>
1460method of C<IO::Handle> to avoid duplicate output.
a0d0e21e 1461
7660c0ab 1462If you C<fork()> without ever waiting on your children, you will accumulate
a0d0e21e
LW
1463zombies:
1464
4633a7c4 1465 $SIG{CHLD} = sub { wait };
a0d0e21e 1466
54310121 1467There's also the double-fork trick (error checking on
7660c0ab 1468C<fork()> returns omitted);
a0d0e21e
LW
1469
1470 unless ($pid = fork) {
1471 unless (fork) {
1472 exec "what you really wanna do";
1473 die "no exec";
1474 # ... or ...
4633a7c4 1475 ## (some_perl_code_here)
a0d0e21e
LW
1476 exit 0;
1477 }
1478 exit 0;
1479 }
1480 waitpid($pid,0);
1481
cb1a09d0
AD
1482See also L<perlipc> for more examples of forking and reaping
1483moribund children.
1484
28757baa
PP
1485Note that if your forked child inherits system file descriptors like
1486STDIN and STDOUT that are actually connected by a pipe or socket, even
5a964f20 1487if you exit, then the remote server (such as, say, httpd or rsh) won't think
7660c0ab 1488you're done. You should reopen those to F</dev/null> if it's any issue.
28757baa 1489
cb1a09d0
AD
1490=item format
1491
7660c0ab 1492Declare a picture format for use by the C<write()> function. For
cb1a09d0
AD
1493example:
1494
54310121 1495 format Something =
cb1a09d0
AD
1496 Test: @<<<<<<<< @||||| @>>>>>
1497 $str, $%, '$' . int($num)
1498 .
1499
1500 $str = "widget";
184e9718 1501 $num = $cost/$quantity;
cb1a09d0
AD
1502 $~ = 'Something';
1503 write;
1504
1505See L<perlform> for many details and examples.
1506
8903cb82 1507=item formline PICTURE,LIST
a0d0e21e 1508
5a964f20 1509This is an internal function used by C<format>s, though you may call it,
a0d0e21e
LW
1510too. It formats (see L<perlform>) a list of values according to the
1511contents of PICTURE, placing the output into the format output
7660c0ab
A
1512accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
1513Eventually, when a C<write()> is done, the contents of
a0d0e21e 1514C<$^A> are written to some filehandle, but you could also read C<$^A>
7660c0ab
A
1515yourself and then set C<$^A> back to C<"">. Note that a format typically
1516does one C<formline()> per line of form, but the C<formline()> function itself
748a9306 1517doesn't care how many newlines are embedded in the PICTURE. This means
4633a7c4 1518that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
748a9306
LW
1519You may therefore need to use multiple formlines to implement a single
1520record format, just like the format compiler.
1521
5f05dabc 1522Be careful if you put double quotes around the picture, because an "C<@>"
748a9306 1523character may be taken to mean the beginning of an array name.
7660c0ab 1524C<formline()> always returns TRUE. See L<perlform> for other examples.
a0d0e21e
LW
1525
1526=item getc FILEHANDLE
1527
1528=item getc
1529
1530Returns the next character from the input file attached to FILEHANDLE,
9bc64814 1531or the undefined value at end of file, or if there was an error. If
3b02c43c
GS
1532FILEHANDLE is omitted, reads from STDIN. This is not particularly
1533efficient. It cannot be used to get unbuffered single-characters,
1534however. For that, try something more like:
4633a7c4
LW
1535
1536 if ($BSD_STYLE) {
1537 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1538 }
1539 else {
54310121 1540 system "stty", '-icanon', 'eol', "\001";
4633a7c4
LW
1541 }
1542
1543 $key = getc(STDIN);
1544
1545 if ($BSD_STYLE) {
1546 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1547 }
1548 else {
5f05dabc 1549 system "stty", 'icanon', 'eol', '^@'; # ASCII null
4633a7c4
LW
1550 }
1551 print "\n";
1552
54310121
PP
1553Determination of whether $BSD_STYLE should be set
1554is left as an exercise to the reader.
cb1a09d0 1555
7660c0ab 1556The C<POSIX::getattr()> function can do this more portably on systems
5a964f20 1557purporting POSIX compliance.
cb1a09d0 1558See also the C<Term::ReadKey> module from your nearest CPAN site;
b687b08b 1559details on CPAN can be found on L<perlmodlib/CPAN>.
a0d0e21e
LW
1560
1561=item getlogin
1562
5a964f20
TC
1563Implements the C library function of the same name, which on most
1564systems returns the current login from F</etc/utmp>, if any. If null,
7660c0ab 1565use C<getpwuid()>.
a0d0e21e 1566
f86702cc 1567 $login = getlogin || getpwuid($<) || "Kilroy";
a0d0e21e 1568
7660c0ab
A
1569Do not consider C<getlogin()> for authentication: it is not as
1570secure as C<getpwuid()>.
4633a7c4 1571
a0d0e21e
LW
1572=item getpeername SOCKET
1573
1574Returns the packed sockaddr address of other end of the SOCKET connection.
1575
4633a7c4
LW
1576 use Socket;
1577 $hersockaddr = getpeername(SOCK);
1578 ($port, $iaddr) = unpack_sockaddr_in($hersockaddr);
1579 $herhostname = gethostbyaddr($iaddr, AF_INET);
1580 $herstraddr = inet_ntoa($iaddr);
a0d0e21e
LW
1581
1582=item getpgrp PID
1583
47e29363 1584Returns the current process group for the specified PID. Use
7660c0ab 1585a PID of C<0> to get the current process group for the
4633a7c4 1586current process. Will raise an exception if used on a machine that
f86cebdf 1587doesn't implement getpgrp(2). If PID is omitted, returns process
7660c0ab
A
1588group of current process. Note that the POSIX version of C<getpgrp()>
1589does not accept a PID argument, so only C<PID==0> is truly portable.
a0d0e21e
LW
1590
1591=item getppid
1592
1593Returns the process id of the parent process.
1594
1595=item getpriority WHICH,WHO
1596
4633a7c4
LW
1597Returns the current priority for a process, a process group, or a user.
1598(See L<getpriority(2)>.) Will raise a fatal exception if used on a
f86cebdf 1599machine that doesn't implement getpriority(2).
a0d0e21e
LW
1600
1601=item getpwnam NAME
1602
1603=item getgrnam NAME
1604
1605=item gethostbyname NAME
1606
1607=item getnetbyname NAME
1608
1609=item getprotobyname NAME
1610
1611=item getpwuid UID
1612
1613=item getgrgid GID
1614
1615=item getservbyname NAME,PROTO
1616
1617=item gethostbyaddr ADDR,ADDRTYPE
1618
1619=item getnetbyaddr ADDR,ADDRTYPE
1620
1621=item getprotobynumber NUMBER
1622
1623=item getservbyport PORT,PROTO
1624
1625=item getpwent
1626
1627=item getgrent
1628
1629=item gethostent
1630
1631=item getnetent
1632
1633=item getprotoent
1634
1635=item getservent
1636
1637=item setpwent
1638
1639=item setgrent
1640
1641=item sethostent STAYOPEN
1642
1643=item setnetent STAYOPEN
1644
1645=item setprotoent STAYOPEN
1646
1647=item setservent STAYOPEN
1648
1649=item endpwent
1650
1651=item endgrent
1652
1653=item endhostent
1654
1655=item endnetent
1656
1657=item endprotoent
1658
1659=item endservent
1660
1661These routines perform the same functions as their counterparts in the
5a964f20 1662system library. In list context, the return values from the
a0d0e21e
LW
1663various get routines are as follows:
1664
1665 ($name,$passwd,$uid,$gid,
6ee623d5 1666 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
a0d0e21e
LW
1667 ($name,$passwd,$gid,$members) = getgr*
1668 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1669 ($name,$aliases,$addrtype,$net) = getnet*
1670 ($name,$aliases,$proto) = getproto*
1671 ($name,$aliases,$port,$proto) = getserv*
1672
1673(If the entry doesn't exist you get a null list.)
1674
5a964f20 1675In scalar context, you get the name, unless the function was a
a0d0e21e
LW
1676lookup by name, in which case you get the other thing, whatever it is.
1677(If the entry doesn't exist you get the undefined value.) For example:
1678
5a964f20
TC
1679 $uid = getpwnam($name);
1680 $name = getpwuid($num);
1681 $name = getpwent();
1682 $gid = getgrnam($name);
1683 $name = getgrgid($num;
1684 $name = getgrent();
1685 #etc.
a0d0e21e 1686
7660c0ab 1687In I<getpw*()> the fields C<$quota>, C<$comment>, and C<$expire> are special
6ee623d5 1688cases in the sense that in many systems they are unsupported. If the
7660c0ab
A
1689C<$quota> is unsupported, it is an empty scalar. If it is supported, it
1690usually encodes the disk quota. If the C<$comment> field is unsupported,
6ee623d5
GS
1691it is an empty scalar. If it is supported it usually encodes some
1692administrative comment about the user. In some systems the $quota
7660c0ab
A
1693field may be C<$change> or C<$age>, fields that have to do with password
1694aging. In some systems the C<$comment> field may be C<$class>. The C<$expire>
6ee623d5
GS
1695field, if present, encodes the expiration period of the account or the
1696password. For the availability and the exact meaning of these fields
f86cebdf 1697in your system, please consult your getpwnam(3) documentation and your
7660c0ab
A
1698F<pwd.h> file. You can also find out from within Perl which meaning
1699your C<$quota> and C<$comment> fields have and whether you have the C<$expire>
1700field by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
1701C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>.
6ee623d5 1702
7660c0ab 1703The C<$members> value returned by I<getgr*()> is a space separated list of
a0d0e21e
LW
1704the login names of the members of the group.
1705
1706For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1707C, it will be returned to you via C<$?> if the function call fails. The
7660c0ab 1708C<@addrs> value returned by a successful call is a list of the raw
a0d0e21e
LW
1709addresses returned by the corresponding system library call. In the
1710Internet domain, each address is four bytes long and you can unpack it
1711by saying something like:
1712
1713 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1714
5a964f20
TC
1715If you get tired of remembering which element of the return list contains
1716which return value, by-name interfaces are also provided in modules:
7660c0ab
A
1717C<File::stat>, C<Net::hostent>, C<Net::netent>, C<Net::protoent>, C<Net::servent>,
1718C<Time::gmtime>, C<Time::localtime>, and C<User::grent>. These override the
5a964f20
TC
1719normal built-in, replacing them with versions that return objects with
1720the appropriate names for each field. For example:
1721
1722 use File::stat;
1723 use User::pwent;
1724 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
1725
1726Even though it looks like they're the same method calls (uid),
7660c0ab 1727they aren't, because a C<File::stat> object is different from a C<User::pwent> object.
5a964f20 1728
a0d0e21e
LW
1729=item getsockname SOCKET
1730
1731Returns the packed sockaddr address of this end of the SOCKET connection.
1732
4633a7c4
LW
1733 use Socket;
1734 $mysockaddr = getsockname(SOCK);
1735 ($port, $myaddr) = unpack_sockaddr_in($mysockaddr);
a0d0e21e
LW
1736
1737=item getsockopt SOCKET,LEVEL,OPTNAME
1738
5a964f20 1739Returns the socket option requested, or undef if there is an error.
a0d0e21e
LW
1740
1741=item glob EXPR
1742
0a753a76
PP
1743=item glob
1744
7660c0ab 1745Returns the value of EXPR with filename expansions such as the standard Unix shell F</bin/sh> would
68dc0745 1746do. This is the internal function implementing the C<E<lt>*.cE<gt>>
7660c0ab 1747operator, but you can use it directly. If EXPR is omitted, C<$_> is used.
68dc0745
PP
1748The C<E<lt>*.cE<gt>> operator is discussed in more detail in
1749L<perlop/"I/O Operators">.
a0d0e21e
LW
1750
1751=item gmtime EXPR
1752
1753Converts a time as returned by the time function to a 9-element array
54310121 1754with the time localized for the standard Greenwich time zone.
4633a7c4 1755Typically used as follows:
a0d0e21e 1756
54310121 1757 # 0 1 2 3 4 5 6 7 8
a0d0e21e
LW
1758 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1759 gmtime(time);
1760
1761All array elements are numeric, and come straight out of a struct tm.
7660c0ab
A
1762In particular this means that C<$mon> has the range C<0..11> and C<$wday> has
1763the range C<0..6> with sunday as day C<0>. Also, C<$year> is the number of
1764years since 1900, that is, C<$year> is C<123> in year 2023, I<not> simply the last two digits of the year.
2f9daede
TPG
1765
1766If EXPR is omitted, does C<gmtime(time())>.
a0d0e21e 1767
f86cebdf 1768In scalar context, returns the ctime(3) value:
0a753a76
PP
1769
1770 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
1771
7660c0ab 1772Also see the C<timegm()> function provided by the C<Time::Local> module,
f86cebdf 1773and the strftime(3) function available via the POSIX module.
7660c0ab
A
1774
1775This scalar value is B<not> locale dependent, see L<perllocale>, but
1776instead a Perl builtin. Also see the C<Time::Local> module, and the
f86cebdf 1777strftime(3) and mktime(3) function available via the POSIX module. To
7660c0ab
A
1778get somewhat similar but locale dependent date strings, set up your
1779locale environment variables appropriately (please see L<perllocale>)
1780and try for example:
1781
1782 use POSIX qw(strftime);
1783 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
1784
1785Note that the C<%a> and C<%b>, the short forms of the day of the week
1786and the month of the year, may not necessarily be three characters wide.
0a753a76 1787
a0d0e21e
LW
1788=item goto LABEL
1789
748a9306
LW
1790=item goto EXPR
1791
a0d0e21e
LW
1792=item goto &NAME
1793
7660c0ab 1794The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
a0d0e21e 1795execution there. It may not be used to go into any construct that
7660c0ab 1796requires initialization, such as a subroutine or a C<foreach> loop. It
0a753a76 1797also can't be used to go into a construct that is optimized away,
7660c0ab 1798or to get out of a block or subroutine given to C<sort()>.
0a753a76 1799It can be used to go almost anywhere else within the dynamic scope,
a0d0e21e 1800including out of subroutines, but it's usually better to use some other
ab4f32c2 1801construct such as C<last> or C<die()>. The author of Perl has never felt the
7660c0ab 1802need to use this form of C<goto> (in Perl, that is--C is another matter).
a0d0e21e 1803
7660c0ab
A
1804The C<goto-EXPR> form expects a label name, whose scope will be resolved
1805dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
748a9306
LW
1806necessarily recommended if you're optimizing for maintainability:
1807
1808 goto ("FOO", "BAR", "GLARCH")[$i];
1809
7660c0ab 1810The C<goto-&NAME> form is highly magical, and substitutes a call to the
a0d0e21e 1811named subroutine for the currently running subroutine. This is used by
7660c0ab 1812C<AUTOLOAD> subroutines that wish to load another subroutine and then
a0d0e21e 1813pretend that the other subroutine had been called in the first place
7660c0ab
A
1814(except that any modifications to C<@_> in the current subroutine are
1815propagated to the other subroutine.) After the C<goto>, not even C<caller()>
a0d0e21e
LW
1816will be able to tell that this routine was called first.
1817
1818=item grep BLOCK LIST
1819
1820=item grep EXPR,LIST
1821
f86cebdf 1822This is similar in spirit to, but not the same as, grep(1)
2f9daede
TPG
1823and its relatives. In particular, it is not limited to using
1824regular expressions.
1825
a0d0e21e 1826Evaluates the BLOCK or EXPR for each element of LIST (locally setting
7660c0ab 1827C<$_> to each element) and returns the list value consisting of those
a0d0e21e
LW
1828elements for which the expression evaluated to TRUE. In a scalar
1829context, returns the number of times the expression was TRUE.
1830
1831 @foo = grep(!/^#/, @bar); # weed out comments
1832
1833or equivalently,
1834
1835 @foo = grep {!/^#/} @bar; # weed out comments
1836
7660c0ab 1837Note that, because C<$_> is a reference into the list value, it can be used
a0d0e21e
LW
1838to modify the elements of the array. While this is useful and
1839supported, it can cause bizarre results if the LIST is not a named
2f9daede 1840array. Similarly, grep returns aliases into the original list,
f86cebdf 1841much like the way that a for loop's index variable aliases the list
2f9daede 1842elements. That is, modifying an element of a list returned by grep
ab4f32c2 1843(for example, in a C<foreach>, C<map()> or another C<grep()>)
2f9daede 1844actually modifies the element in the original list.
a0d0e21e 1845
fb73857a 1846See also L</map> for an array composed of the results of the BLOCK or EXPR.
38325410 1847
a0d0e21e
LW
1848=item hex EXPR
1849
54310121 1850=item hex
bbce6d69 1851
54310121 1852Interprets EXPR as a hex string and returns the corresponding
2f9daede 1853value. (To convert strings that might start with either 0 or 0x
7660c0ab 1854see L</oct>.) If EXPR is omitted, uses C<$_>.
2f9daede
TPG
1855
1856 print hex '0xAf'; # prints '175'
1857 print hex 'aF'; # same
a0d0e21e
LW
1858
1859=item import
1860
7660c0ab 1861There is no builtin C<import()> function. It is just an ordinary
4633a7c4 1862method (subroutine) defined (or inherited) by modules that wish to export
7660c0ab 1863names to another module. The C<use()> function calls the C<import()> method
54310121 1864for the package used. See also L</use()>, L<perlmod>, and L<Exporter>.
a0d0e21e
LW
1865
1866=item index STR,SUBSTR,POSITION
1867
1868=item index STR,SUBSTR
1869
4633a7c4
LW
1870Returns the position of the first occurrence of SUBSTR in STR at or after
1871POSITION. If POSITION is omitted, starts searching from the beginning of
7660c0ab 1872the string. The return value is based at C<0> (or whatever you've set the C<$[>
4633a7c4 1873variable to--but don't do that). If the substring is not found, returns
7660c0ab 1874one less than the base, ordinarily C<-1>.
a0d0e21e
LW
1875
1876=item int EXPR
1877
54310121 1878=item int
bbce6d69 1879
7660c0ab 1880Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
5a964f20 1881You should not use this for rounding, because it truncates
7660c0ab
A
1882towards C<0>, and because machine representations of floating point
1883numbers can sometimes produce counterintuitive results. Usually C<sprintf()> or C<printf()>,
1884or the C<POSIX::floor> or C<POSIX::ceil> functions, would serve you better.
a0d0e21e
LW
1885
1886=item ioctl FILEHANDLE,FUNCTION,SCALAR
1887
f86cebdf 1888Implements the ioctl(2) function. You'll probably have to say
a0d0e21e 1889
4633a7c4 1890 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
a0d0e21e 1891
4633a7c4 1892first to get the correct function definitions. If F<ioctl.ph> doesn't
a0d0e21e 1893exist or doesn't have the correct definitions you'll have to roll your
4633a7c4 1894own, based on your C header files such as F<E<lt>sys/ioctl.hE<gt>>.
5a964f20 1895(There is a Perl script called B<h2ph> that comes with the Perl kit that
54310121 1896may help you in this, but it's nontrivial.) SCALAR will be read and/or
4633a7c4 1897written depending on the FUNCTION--a pointer to the string value of SCALAR
ab4f32c2 1898will be passed as the third argument of the actual C<ioctl()> call. (If SCALAR
4633a7c4
LW
1899has no string value but does have a numeric value, that value will be
1900passed rather than a pointer to the string value. To guarantee this to be
7660c0ab 1901TRUE, add a C<0> to the scalar before using it.) The C<pack()> and C<unpack()>
4633a7c4 1902functions are useful for manipulating the values of structures used by
7660c0ab 1903C<ioctl()>. The following example sets the erase character to DEL.
a0d0e21e
LW
1904
1905 require 'ioctl.ph';
4633a7c4
LW
1906 $getp = &TIOCGETP;
1907 die "NO TIOCGETP" if $@ || !$getp;
a0d0e21e 1908 $sgttyb_t = "ccccs"; # 4 chars and a short
4633a7c4 1909 if (ioctl(STDIN,$getp,$sgttyb)) {
a0d0e21e
LW
1910 @ary = unpack($sgttyb_t,$sgttyb);
1911 $ary[2] = 127;
1912 $sgttyb = pack($sgttyb_t,@ary);
4633a7c4 1913 ioctl(STDIN,&TIOCSETP,$sgttyb)
a0d0e21e
LW
1914 || die "Can't ioctl: $!";
1915 }
1916
ab4f32c2 1917The return value of C<ioctl()> (and C<fcntl()>) is as follows:
a0d0e21e
LW
1918
1919 if OS returns: then Perl returns:
1920 -1 undefined value
1921 0 string "0 but true"
1922 anything else that number
1923
1924Thus Perl returns TRUE on success and FALSE on failure, yet you can
1925still easily determine the actual value returned by the operating
1926system:
1927
1928 ($retval = ioctl(...)) || ($retval = -1);
1929 printf "System returned %d\n", $retval;
1930
c2611fb3 1931The special string "C<0> but true" is exempt from B<-w> complaints
5a964f20
TC
1932about improper numeric conversions.
1933
a0d0e21e
LW
1934=item join EXPR,LIST
1935
54310121 1936Joins the separate strings of LIST into a single string with
a0d0e21e
LW
1937fields separated by the value of EXPR, and returns the string.
1938Example:
1939
1940 $_ = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
1941
7660c0ab 1942See L</split>.
a0d0e21e 1943
aa689395
PP
1944=item keys HASH
1945
1d2dff63
GS
1946Returns a list consisting of all the keys of the named hash. (In a
1947scalar context, returns the number of keys.) The keys are returned in
ab192400
GS
1948an apparently random order. The actual random order is subject to
1949change in future versions of perl, but it is guaranteed to be the same
1950order as either the C<values()> or C<each()> function produces (given
1951that the hash has not been modified). As a side effect, it resets
1952HASH's iterator.
a0d0e21e 1953
aa689395 1954Here is yet another way to print your environment:
a0d0e21e
LW
1955
1956 @keys = keys %ENV;
1957 @values = values %ENV;
1958 while ($#keys >= 0) {
1959 print pop(@keys), '=', pop(@values), "\n";
1960 }
1961
1962or how about sorted by key:
1963
1964 foreach $key (sort(keys %ENV)) {
1965 print $key, '=', $ENV{$key}, "\n";
1966 }
1967
aca803df 1968To sort a hash by value, you'll need to use a C<sort()> function.
aa689395 1969Here's a descending numeric sort of a hash by its values:
4633a7c4 1970
5a964f20 1971 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
4633a7c4
LW
1972 printf "%4d %s\n", $hash{$key}, $key;
1973 }
1974
ab4f32c2 1975As an lvalue C<keys()> allows you to increase the number of hash buckets
aa689395
PP
1976allocated for the given hash. This can gain you a measure of efficiency if
1977you know the hash is going to get big. (This is similar to pre-extending
1978an array by assigning a larger number to $#array.) If you say
55497cff
PP
1979
1980 keys %hash = 200;
1981
ab192400
GS
1982then C<%hash> will have at least 200 buckets allocated for it--256 of them,
1983in fact, since it rounds up to the next power of two. These
55497cff
PP
1984buckets will be retained even if you do C<%hash = ()>, use C<undef
1985%hash> if you want to free the storage while C<%hash> is still in scope.
1986You can't shrink the number of buckets allocated for the hash using
ab4f32c2 1987C<keys()> in this way (but you needn't worry about doing this by accident,
55497cff
PP
1988as trying has no effect).
1989
ab192400
GS
1990See also C<each()>, C<values()> and C<sort()>.
1991
a0d0e21e
LW
1992=item kill LIST
1993
54310121
PP
1994Sends a signal to a list of processes. The first element of
1995the list must be the signal to send. Returns the number of
4633a7c4 1996processes successfully signaled.
a0d0e21e
LW
1997
1998 $cnt = kill 1, $child1, $child2;
1999 kill 9, @goners;
2000
4633a7c4
LW
2001Unlike in the shell, in Perl if the I<SIGNAL> is negative, it kills
2002process groups instead of processes. (On System V, a negative I<PROCESS>
2003number will also kill process groups, but that's not portable.) That
2004means you usually want to use positive not negative signals. You may also
da0045b7 2005use a signal name in quotes. See L<perlipc/"Signals"> for details.
a0d0e21e
LW
2006
2007=item last LABEL
2008
2009=item last
2010
2011The C<last> command is like the C<break> statement in C (as used in
2012loops); it immediately exits the loop in question. If the LABEL is
2013omitted, the command refers to the innermost enclosing loop. The
2014C<continue> block, if any, is not executed:
2015
4633a7c4
LW
2016 LINE: while (<STDIN>) {
2017 last LINE if /^$/; # exit when done with header
5a964f20 2018 #...
a0d0e21e
LW
2019 }
2020
4968c1e4
G
2021C<last> cannot be used to exit a block which returns a value such as
2022C<eval {}>, C<sub {}> or C<do {}>.
2023
98293880
JH
2024See also L</continue> for an illustration of how C<last>, C<next>, and
2025C<redo> work.
1d2dff63 2026
a0d0e21e
LW
2027=item lc EXPR
2028
54310121 2029=item lc
bbce6d69 2030
a0d0e21e 2031Returns an lowercased version of EXPR. This is the internal function
7660c0ab 2032implementing the C<\L> escape in double-quoted strings.
a0ed51b3 2033Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
a0d0e21e 2034
7660c0ab 2035If EXPR is omitted, uses C<$_>.
bbce6d69 2036
a0d0e21e
LW
2037=item lcfirst EXPR
2038
54310121 2039=item lcfirst
bbce6d69 2040
a0d0e21e 2041Returns the value of EXPR with the first character lowercased. This is
7660c0ab 2042the internal function implementing the C<\l> escape in double-quoted strings.
a0ed51b3 2043Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
a0d0e21e 2044
7660c0ab 2045If EXPR is omitted, uses C<$_>.
bbce6d69 2046
a0d0e21e
LW
2047=item length EXPR
2048
54310121 2049=item length
bbce6d69 2050
a0ed51b3 2051Returns the length in characters of the value of EXPR. If EXPR is
7660c0ab 2052omitted, returns length of C<$_>.
a0d0e21e
LW
2053
2054=item link OLDFILE,NEWFILE
2055
5a964f20
TC
2056Creates a new filename linked to the old filename. Returns TRUE for
2057success, FALSE otherwise.
a0d0e21e
LW
2058
2059=item listen SOCKET,QUEUESIZE
2060
2061Does the same thing that the listen system call does. Returns TRUE if
4633a7c4 2062it succeeded, FALSE otherwise. See example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
2063
2064=item local EXPR
2065
5a964f20
TC
2066A local modifies the listed variables to be local to the enclosing
2067block, file, or eval. If more than one value is listed, the list must
2068be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2069for details, including issues with tied arrays and hashes.
a0d0e21e 2070
7660c0ab 2071You really probably want to be using C<my()> instead, because C<local()> isn't
7b8d334a 2072what most people think of as "local". See L<perlsub/"Private Variables
cb1a09d0 2073via my()"> for details.
a0d0e21e
LW
2074
2075=item localtime EXPR
2076
2077Converts a time as returned by the time function to a 9-element array
5f05dabc 2078with the time analyzed for the local time zone. Typically used as
a0d0e21e
LW
2079follows:
2080
54310121 2081 # 0 1 2 3 4 5 6 7 8
a0d0e21e
LW
2082 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2083 localtime(time);
2084
2085All array elements are numeric, and come straight out of a struct tm.
7660c0ab
A
2086In particular this means that C<$mon> has the range C<0..11> and C<$wday> has
2087the range C<0..6> with sunday as day C<0>. Also, C<$year> is the number of
2088years 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
2089
2090If EXPR is omitted, uses the current time (C<localtime(time)>).
a0d0e21e 2091
f86cebdf 2092In scalar context, returns the ctime(3) value:
a0d0e21e 2093
5f05dabc 2094 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
a0d0e21e 2095
a3cb178b 2096This scalar value is B<not> locale dependent, see L<perllocale>, but
7660c0ab 2097instead a Perl builtin. Also see the C<Time::Local> module, and the
f86cebdf 2098strftime(3) and mktime(3) function available via the POSIX module. To
a3cb178b
GS
2099get somewhat similar but locale dependent date strings, set up your
2100locale environment variables appropriately (please see L<perllocale>)
5a964f20 2101and try for example:
a3cb178b 2102
5a964f20 2103 use POSIX qw(strftime);
a3cb178b
GS
2104 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
2105
2106Note that the C<%a> and C<%b>, the short forms of the day of the week
2107and the month of the year, may not necessarily be three characters wide.
a0d0e21e
LW
2108
2109=item log EXPR
2110
54310121 2111=item log
bbce6d69 2112
5a964f20 2113Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted, returns log
7660c0ab 2114of C<$_>.
a0d0e21e
LW
2115
2116=item lstat FILEHANDLE
2117
2118=item lstat EXPR
2119
54310121 2120=item lstat
bbce6d69 2121
7660c0ab 2122Does the same thing as the C<stat()> function (including setting the
5a964f20
TC
2123special C<_> filehandle) but stats a symbolic link instead of the file
2124the symbolic link points to. If symbolic links are unimplemented on
7660c0ab 2125your system, a normal C<stat()> is done.
a0d0e21e 2126
7660c0ab 2127If EXPR is omitted, stats C<$_>.
bbce6d69 2128
a0d0e21e
LW
2129=item m//
2130
2131The match operator. See L<perlop>.
2132
2133=item map BLOCK LIST
2134
2135=item map EXPR,LIST
2136
7660c0ab 2137Evaluates the BLOCK or EXPR for each element of LIST (locally setting C<$_> to each
a0d0e21e
LW
2138element) and returns the list value composed of the results of each such
2139evaluation. Evaluates BLOCK or EXPR in a list context, so each element of LIST
2140may produce zero, one, or more elements in the returned value.
2141
2142 @chars = map(chr, @nums);
2143
2144translates a list of numbers to the corresponding characters. And
2145
4633a7c4 2146 %hash = map { getkey($_) => $_ } @array;
a0d0e21e
LW
2147
2148is just a funny way to write
2149
2150 %hash = ();
2151 foreach $_ (@array) {
4633a7c4 2152 $hash{getkey($_)} = $_;
a0d0e21e
LW
2153 }
2154
7660c0ab 2155Note that, because C<$_> is a reference into the list value, it can be used
fb73857a
PP
2156to modify the elements of the array. While this is useful and
2157supported, it can cause bizarre results if the LIST is not a named
2158array. See also L</grep> for an array composed of those items of the
2159original list for which the BLOCK or EXPR evaluates to true.
2160
a0d0e21e
LW
2161=item mkdir FILENAME,MODE
2162
0591cd52
NT
2163Creates the directory specified by FILENAME, with permissions
2164specified by MODE (as modified by C<umask>). If it succeeds it
2165returns TRUE, otherwise it returns FALSE and sets C<$!> (errno).
2166
2167In general, it is better to create directories with permissive MODEs,
2168and let the user modify that with their C<umask>, than it is to supply
2169a restrictive MODE and give the user no way to be more permissive.
2170The exceptions to this rule are when the file or directory should be
2171kept private (mail files, for instance). The perlfunc(1) entry on
2172C<umask> discusses the choice of MODE in more detail.
a0d0e21e
LW
2173
2174=item msgctl ID,CMD,ARG
2175
f86cebdf 2176Calls the System V IPC function msgctl(2). You'll probably have to say
0ade1984
JH
2177
2178 use IPC::SysV;
2179
7660c0ab
A
2180first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2181then ARG must be a variable which will hold the returned C<msqid_ds>
ab4f32c2 2182structure. Returns like C<ioctl()>: the undefined value for error, "C<0> but
0ade1984 2183true" for zero, or the actual return value otherwise. See also
7660c0ab 2184C<IPC::SysV> and C<IPC::Semaphore::Msg> documentation.
a0d0e21e
LW
2185
2186=item msgget KEY,FLAGS
2187
f86cebdf 2188Calls the System V IPC function msgget(2). Returns the message queue
7660c0ab
A
2189id, or the undefined value if there is an error. See also C<IPC::SysV>
2190and C<IPC::SysV::Msg> documentation.
a0d0e21e
LW
2191
2192=item msgsnd ID,MSG,FLAGS
2193
2194Calls the System V IPC function msgsnd to send the message MSG to the
2195message queue ID. MSG must begin with the long integer message type,
c07a80fd 2196which may be created with C<pack("l", $type)>. Returns TRUE if
7660c0ab
A
2197successful, or FALSE if there is an error. See also C<IPC::SysV>
2198and C<IPC::SysV::Msg> documentation.
a0d0e21e
LW
2199
2200=item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2201
2202Calls the System V IPC function msgrcv to receive a message from
2203message queue ID into variable VAR with a maximum message size of
0ade1984
JH
2204SIZE. Note that if a message is received, the message type will be
2205the first thing in VAR, and the maximum length of VAR is SIZE plus the
2206size of the message type. Returns TRUE if successful, or FALSE if
7660c0ab 2207there is an error. See also C<IPC::SysV> and C<IPC::SysV::Msg> documentation.
a0d0e21e
LW
2208
2209=item my EXPR
2210
ab4f32c2
A
2211A C<my()> declares the listed variables to be local (lexically) to the
2212enclosing block, file, or C<eval()>. If
5f05dabc 2213more than one value is listed, the list must be placed in parentheses. See
cb1a09d0 2214L<perlsub/"Private Variables via my()"> for details.
4633a7c4 2215
a0d0e21e
LW
2216=item next LABEL
2217
2218=item next
2219
2220The C<next> command is like the C<continue> statement in C; it starts
2221the next iteration of the loop:
2222
4633a7c4
LW
2223 LINE: while (<STDIN>) {
2224 next LINE if /^#/; # discard comments
5a964f20 2225 #...
a0d0e21e
LW
2226 }
2227
2228Note that if there were a C<continue> block on the above, it would get
2229executed even on discarded lines. If the LABEL is omitted, the command
2230refers to the innermost enclosing loop.
2231
4968c1e4
G
2232C<next> cannot be used to exit a block which returns a value such as
2233C<eval {}>, C<sub {}> or C<do {}>.
2234
98293880
JH
2235See also L</continue> for an illustration of how C<last>, C<next>, and
2236C<redo> work.
1d2dff63 2237
a0d0e21e
LW
2238=item no Module LIST
2239
7660c0ab 2240See the L</use> function, which C<no> is the opposite of.
a0d0e21e
LW
2241
2242=item oct EXPR
2243
54310121 2244=item oct
bbce6d69 2245
4633a7c4 2246Interprets EXPR as an octal string and returns the corresponding
4f19785b
WSI
2247value. (If EXPR happens to start off with C<0x>, interprets it as a
2248hex string. If EXPR starts off with C<0b>, it is interpreted as a
2249binary string.) The following will handle decimal, binary, octal, and
4633a7c4 2250hex in the standard Perl or C notation:
a0d0e21e
LW
2251
2252 $val = oct($val) if $val =~ /^0/;
2253
7660c0ab
A
2254If EXPR is omitted, uses C<$_>. This function is commonly used when
2255a string such as C<644> needs to be converted into a file mode, for
2f9daede
TPG
2256example. (Although perl will automatically convert strings into
2257numbers as needed, this automatic conversion assumes base 10.)
a0d0e21e
LW
2258
2259=item open FILEHANDLE,EXPR
2260
2261=item open FILEHANDLE
2262
2263Opens the file whose filename is given by EXPR, and associates it with
5f05dabc
PP
2264FILEHANDLE. If FILEHANDLE is an expression, its value is used as the
2265name of the real filehandle wanted. If EXPR is omitted, the scalar
2266variable of the same name as the FILEHANDLE contains the filename.
ab4f32c2
A
2267(Note that lexical variables--those declared with C<my()>--will not work
2268for this purpose; so if you're using C<my()>, specify EXPR in your call
5f05dabc
PP
2269to open.)
2270
7660c0ab
A
2271If the filename begins with C<'E<lt>'> or nothing, the file is opened for input.
2272If the filename begins with C<'E<gt>'>, the file is truncated and opened for
2273output, being created if necessary. If the filename begins with C<'E<gt>E<gt>'>,
fbb426e4 2274the file is opened for appending, again being created if necessary.
7660c0ab
A
2275You can put a C<'+'> in front of the C<'E<gt>'> or C<'E<lt>'> to indicate that
2276you want both read and write access to the file; thus C<'+E<lt>'> is almost
2277always preferred for read/write updates--the C<'+E<gt>'> mode would clobber the
5a964f20
TC
2278file first. You can't usually use either read-write mode for updating
2279textfiles, since they have variable length records. See the B<-i>
0591cd52
NT
2280switch in L<perlrun> for a better approach. The file is created with
2281permissions of C<0666> modified by the process' C<umask> value.
5a964f20
TC
2282
2283The prefix and the filename may be separated with spaces.
f86cebdf 2284These various prefixes correspond to the fopen(3) modes of C<'r'>, C<'r+'>, C<'w'>,
7660c0ab 2285C<'w+'>, C<'a'>, and C<'a+'>.
5f05dabc 2286
7660c0ab 2287If the filename begins with C<'|'>, the filename is interpreted as a
5a964f20 2288command to which output is to be piped, and if the filename ends with a
f244e06d
GS
2289C<'|'>, the filename is interpreted as a command which pipes output to
2290us. See L<perlipc/"Using open() for IPC">
7660c0ab 2291for more examples of this. (You are not allowed to C<open()> to a command
5a964f20
TC
2292that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2293and L<perlipc/"Bidirectional Communication"> for alternatives.)
cb1a09d0 2294
7660c0ab 2295Opening C<'-'> opens STDIN and opening C<'E<gt>-'> opens STDOUT. Open returns
ab4f32c2 2296nonzero upon success, the undefined value otherwise. If the C<open()>
4633a7c4 2297involved a pipe, the return value happens to be the pid of the
54310121 2298subprocess.
cb1a09d0
AD
2299
2300If you're unfortunate enough to be running Perl on a system that
2301distinguishes between text files and binary files (modern operating
2302systems don't care), then you should check out L</binmode> for tips for
ab4f32c2 2303dealing with this. The key distinction between systems that need C<binmode()>
5a964f20
TC
2304and those that don't is their text file formats. Systems like Unix, MacOS, and
2305Plan9, which delimit lines with a single character, and which encode that
ab4f32c2 2306character in C as C<"\n">, do not need C<binmode()>. The rest need it.
cb1a09d0 2307
fb73857a 2308When opening a file, it's usually a bad idea to continue normal execution
ab4f32c2
A
2309if the request failed, so C<open()> is frequently used in connection with
2310C<die()>. Even if C<die()> won't do what you want (say, in a CGI script,
fb73857a 2311where you want to make a nicely formatted error message (but there are
5a964f20 2312modules that can help with that problem)) you should always check
fb73857a
PP
2313the return value from opening a file. The infrequent exception is when
2314working with an unopened filehandle is actually what you want to do.
2315
cb1a09d0 2316Examples:
a0d0e21e
LW
2317
2318 $ARTICLE = 100;
2319 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2320 while (<ARTICLE>) {...
2321
2322 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
fb73857a 2323 # if the open fails, output is discarded
a0d0e21e 2324
fb73857a
PP
2325 open(DBASE, '+<dbase.mine') # open for update
2326 or die "Can't open 'dbase.mine' for update: $!";
cb1a09d0 2327
fb73857a
PP
2328 open(ARTICLE, "caesar <$article |") # decrypt article
2329 or die "Can't start caesar: $!";
a0d0e21e 2330
fb73857a
PP
2331 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
2332 or die "Can't start sort: $!";
a0d0e21e
LW
2333
2334 # process argument list of files along with any includes
2335
2336 foreach $file (@ARGV) {
2337 process($file, 'fh00');
2338 }
2339
2340 sub process {
5a964f20 2341 my($filename, $input) = @_;
a0d0e21e
LW
2342 $input++; # this is a string increment
2343 unless (open($input, $filename)) {
2344 print STDERR "Can't open $filename: $!\n";
2345 return;
2346 }
2347
5a964f20 2348 local $_;
a0d0e21e
LW
2349 while (<$input>) { # note use of indirection
2350 if (/^#include "(.*)"/) {
2351 process($1, $input);
2352 next;
2353 }
5a964f20 2354 #... # whatever
a0d0e21e
LW
2355 }
2356 }
2357
2358You may also, in the Bourne shell tradition, specify an EXPR beginning
7660c0ab 2359with C<'E<gt>&'>, in which case the rest of the string is interpreted as the
5a964f20 2360name of a filehandle (or file descriptor, if numeric) to be
7660c0ab
A
2361duped and opened. You may use C<&> after C<E<gt>>, C<E<gt>E<gt>>, C<E<lt>>, C<+E<gt>>,
2362C<+E<gt>E<gt>>, and C<+E<lt>>. The
a0d0e21e 2363mode you specify should match the mode of the original filehandle.
184e9718 2364(Duping a filehandle does not take into account any existing contents of
cb1a09d0 2365stdio buffers.)
a0d0e21e
LW
2366Here is a script that saves, redirects, and restores STDOUT and
2367STDERR:
2368
2369 #!/usr/bin/perl
5a964f20
TC
2370 open(OLDOUT, ">&STDOUT");
2371 open(OLDERR, ">&STDERR");
a0d0e21e
LW
2372
2373 open(STDOUT, ">foo.out") || die "Can't redirect stdout";
2374 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
2375
2376 select(STDERR); $| = 1; # make unbuffered
2377 select(STDOUT); $| = 1; # make unbuffered
2378
2379 print STDOUT "stdout 1\n"; # this works for
2380 print STDERR "stderr 1\n"; # subprocesses too
2381
2382 close(STDOUT);
2383 close(STDERR);
2384
5a964f20
TC
2385 open(STDOUT, ">&OLDOUT");
2386 open(STDERR, ">&OLDERR");
a0d0e21e
LW
2387
2388 print STDOUT "stdout 2\n";
2389 print STDERR "stderr 2\n";
2390
2391
7660c0ab
A
2392If you specify C<'E<lt>&=N'>, where C<N> is a number, then Perl will do an
2393equivalent of C's C<fdopen()> of that file descriptor; this is more
4633a7c4 2394parsimonious of file descriptors. For example:
a0d0e21e
LW
2395
2396 open(FILEHANDLE, "<&=$fd")
2397
7660c0ab 2398If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>, then
a0d0e21e 2399there is an implicit fork done, and the return value of open is the pid
7660c0ab 2400of the child within the parent process, and C<0> within the child
184e9718 2401process. (Use C<defined($pid)> to determine whether the open was successful.)
a0d0e21e
LW
2402The filehandle behaves normally for the parent, but i/o to that
2403filehandle is piped from/to the STDOUT/STDIN of the child process.
2404In the child process the filehandle isn't opened--i/o happens from/to
2405the new STDOUT or STDIN. Typically this is used like the normal
2406piped open when you want to exercise more control over just how the
2407pipe command gets executed, such as when you are running setuid, and
54310121 2408don't want to have to scan shell commands for metacharacters.
4633a7c4 2409The following pairs are more or less equivalent:
a0d0e21e
LW
2410
2411 open(FOO, "|tr '[a-z]' '[A-Z]'");
2412 open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
2413
2414 open(FOO, "cat -n '$file'|");
2415 open(FOO, "-|") || exec 'cat', '-n', $file;
2416
4633a7c4
LW
2417See L<perlipc/"Safe Pipe Opens"> for more examples of this.
2418
5a964f20 2419NOTE: On any operation that may do a fork, any unflushed buffers remain
184e9718 2420unflushed in both processes, which means you may need to set C<$|> to
4771b018
GS
2421avoid duplicate output. On systems that support a close-on-exec flag on
2422files, the flag will be set for the newly opened file descriptor as
2423determined by the value of $^F. See L<perlvar/$^F>.
a0d0e21e 2424
0dccf244
CS
2425Closing any piped filehandle causes the parent process to wait for the
2426child to finish, and returns the status value in C<$?>.
2427
5a964f20 2428The filename passed to open will have leading and trailing
f86cebdf 2429whitespace deleted, and the normal redirection characters
5a964f20
TC
2430honored. This property, known as "magic open",
2431can often be used to good effect. A user could specify a filename of
7660c0ab 2432F<"rsh cat file |">, or you could change certain filenames as needed:
5a964f20
TC
2433
2434 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
2435 open(FH, $filename) or die "Can't open $filename: $!";
2436
2437However, to open a file with arbitrary weird characters in it, it's
2438necessary to protect any leading and trailing whitespace:
2439
2440 $file =~ s#^(\s)#./$1#;
2441 open(FOO, "< $file\0");
2442
7660c0ab
A
2443If you want a "real" C C<open()> (see L<open(2)> on your system), then you
2444should use the C<sysopen()> function, which involves no such magic. This is
5a964f20
TC
2445another way to protect your filenames from interpretation. For example:
2446
2447 use IO::Handle;
2448 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
2449 or die "sysopen $path: $!";
2450 $oldfh = select(HANDLE); $| = 1; select($oldfh);
2451 print HANDLE "stuff $$\n");
2452 seek(HANDLE, 0, 0);
2453 print "File contains: ", <HANDLE>;
2454
7660c0ab
A
2455Using the constructor from the C<IO::Handle> package (or one of its
2456subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
5a964f20
TC
2457filehandles that have the scope of whatever variables hold references to
2458them, and automatically close whenever and however you leave that scope:
c07a80fd 2459
5f05dabc 2460 use IO::File;
5a964f20 2461 #...
c07a80fd
PP
2462 sub read_myfile_munged {
2463 my $ALL = shift;
5f05dabc 2464 my $handle = new IO::File;
c07a80fd
PP
2465 open($handle, "myfile") or die "myfile: $!";
2466 $first = <$handle>
2467 or return (); # Automatically closed here.
2468 mung $first or die "mung failed"; # Or here.
2469 return $first, <$handle> if $ALL; # Or here.
2470 $first; # Or here.
2471 }
2472
b687b08b 2473See L</seek> for some details about mixing reading and writing.
a0d0e21e
LW
2474
2475=item opendir DIRHANDLE,EXPR
2476
7660c0ab
A
2477Opens a directory named EXPR for processing by C<readdir()>, C<telldir()>,
2478C<seekdir()>, C<rewinddir()>, and C<closedir()>. Returns TRUE if successful.
a0d0e21e
LW
2479DIRHANDLEs have their own namespace separate from FILEHANDLEs.
2480
2481=item ord EXPR
2482
54310121 2483=item ord
bbce6d69 2484
a0ed51b3 2485Returns the numeric (ASCII or Unicode) value of the first character of EXPR. If
7660c0ab 2486EXPR is omitted, uses C<$_>. For the reverse, see L</chr>.
a0d0e21e
LW
2487
2488=item pack TEMPLATE,LIST
2489
2490Takes an array or list of values and packs it into a binary structure,
2491returning the string containing the structure. The TEMPLATE is a
2492sequence of characters that give the order and type of values, as
2493follows:
2494
5a929a98 2495 a A string with arbitrary binary data, will be null padded.
a0d0e21e 2496 A An ascii string, will be space padded.
5a929a98
VU
2497 Z A null terminated (asciz) string, will be null padded.
2498
a0d0e21e
LW
2499 b A bit string (ascending bit order, like vec()).
2500 B A bit string (descending bit order).
2501 h A hex string (low nybble first).
2502 H A hex string (high nybble first).
2503
2504 c A signed char value.
a0ed51b3 2505 C An unsigned char value. Only does bytes. See U for Unicode.
96e4d5b1 2506
a0d0e21e
LW
2507 s A signed short value.
2508 S An unsigned short value.
96e4d5b1
PP
2509 (This 'short' is _exactly_ 16 bits, which may differ from
2510 what a local C compiler calls 'short'.)
2511
a0d0e21e
LW
2512 i A signed integer value.
2513 I An unsigned integer value.
f86cebdf
GS
2514 (This 'integer' is _at_least_ 32 bits wide. Its exact
2515 size depends on what a local C compiler calls 'int',
2516 and may even be larger than the 'long' described in
2517 the next item.)
96e4d5b1 2518
a0d0e21e
LW
2519 l A signed long value.
2520 L An unsigned long value.
96e4d5b1
PP
2521 (This 'long' is _exactly_ 32 bits, which may differ from
2522 what a local C compiler calls 'long'.)
a0d0e21e 2523
96e4d5b1
PP
2524 n A short in "network" (big-endian) order.
2525 N A long in "network" (big-endian) order.
a0d0e21e
LW
2526 v A short in "VAX" (little-endian) order.
2527 V A long in "VAX" (little-endian) order.
96e4d5b1
PP
2528 (These 'shorts' and 'longs' are _exactly_ 16 bits and
2529 _exactly_ 32 bits, respectively.)
a0d0e21e 2530
dae0da7a
JH
2531 q A signed quad (64-bit) value.
2532 Q An unsigned quad value.
2533 (Available only if your system supports 64-bit integer values
2534 _and_ if Perl has been compiled to support those.
2535 Causes a fatal error otherwise.)
2536
a0d0e21e
LW
2537 f A single-precision float in the native format.
2538 d A double-precision float in the native format.
2539
2540 p A pointer to a null-terminated string.
2541 P A pointer to a structure (fixed-length string).
2542
2543 u A uuencoded string.
a0ed51b3
LW
2544 U A Unicode character number. Encodes to UTF-8 internally.
2545 Works even if C<use utf8> is not in effect.
a0d0e21e 2546
96e4d5b1 2547 w A BER compressed integer. Its bytes represent an unsigned
f86cebdf
GS
2548 integer in base 128, most significant digit first, with as
2549 few digits as possible. Bit eight (the high bit) is set
2550 on each byte except the last.
def98dd4 2551
a0d0e21e
LW
2552 x A null byte.
2553 X Back up a byte.
2554 @ Null fill to absolute position.
2555
5a929a98
VU
2556The following rules apply:
2557
2558=over 8
2559
2560=item *
2561
5a964f20 2562Each letter may optionally be followed by a number giving a repeat
5a929a98
VU
2563count. With all types except C<"a">, C<"A">, C<"Z">, C<"b">, C<"B">, C<"h">,
2564C<"H">, and C<"P"> the pack function will gobble up that many values from
2565the LIST. A C<*> for the repeat count means to use however many items are
2566left.
2567
2568=item *
2569
2570The C<"a">, C<"A"> and C<"Z"> types gobble just one value, but pack it as a
2571string of length count, padding with nulls or spaces as necessary. When
2572unpacking, C<"A"> strips trailing spaces and nulls, C<"Z"> strips everything
2573after the first null, and C<"a"> returns data verbatim.
2574
2575=item *
2576
2577Likewise, the C<"b"> and C<"B"> fields pack a string that many bits long.
2578
2579=item *
2580
2581The C<"h"> and C<"H"> fields pack a string that many nybbles long.
2582
2583=item *
2584
2585The C<"p"> type packs a pointer to a null-terminated string. You are
2586responsible for ensuring the string is not a temporary value (which can
2587potentially get deallocated before you get around to using the packed result).
2588The C<"P"> type packs a pointer to a structure of the size indicated by the
2589length. A NULL pointer is created if the corresponding value for C<"p"> or
2590C<"P"> is C<undef>.
2591
2592=item *
2593
2594Real numbers (floats and doubles) are in the native machine format only;
2595due to the multiplicity of floating formats around, and the lack of a
2596standard "network" representation, no facility for interchange has been
2597made. This means that packed floating point data written on one machine
2598may not be readable on another - even if both use IEEE floating point
2599arithmetic (as the endian-ness of the memory representation is not part
2600of the IEEE spec).
2601
2602Note that Perl uses doubles internally for all numeric calculation, and
2603converting from double into float and thence back to double again will
2604lose precision (i.e., C<unpack("f", pack("f", $foo)>) will not in general
2605equal C<$foo>).
2606
2607=back
a0d0e21e
LW
2608
2609Examples:
2610
a0ed51b3 2611 $foo = pack("CCCC",65,66,67,68);
a0d0e21e 2612 # foo eq "ABCD"
a0ed51b3 2613 $foo = pack("C4",65,66,67,68);
a0d0e21e 2614 # same thing
a0ed51b3
LW
2615 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
2616 # same thing with Unicode circled letters
a0d0e21e
LW
2617
2618 $foo = pack("ccxxcc",65,66,67,68);
2619 # foo eq "AB\0\0CD"
2620
2621 $foo = pack("s2",1,2);
2622 # "\1\0\2\0" on little-endian
2623 # "\0\1\0\2" on big-endian
2624
2625 $foo = pack("a4","abcd","x","y","z");
2626 # "abcd"
2627
2628 $foo = pack("aaaa","abcd","x","y","z");
2629 # "axyz"
2630
2631 $foo = pack("a14","abcdefg");
2632 # "abcdefg\0\0\0\0\0\0\0"
2633
2634 $foo = pack("i9pl", gmtime);
2635 # a real struct tm (on my system anyway)
2636
5a929a98
VU
2637 $utmp_template = "Z8 Z8 Z16 L";
2638 $utmp = pack($utmp_template, @utmp1);
2639 # a struct utmp (BSDish)
2640
2641 @utmp2 = unpack($utmp_template, $utmp);
2642 # "@utmp1" eq "@utmp2"
2643
a0d0e21e
LW
2644 sub bintodec {
2645 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
2646 }
2647
5a929a98 2648The same template may generally also be used in unpack().
a0d0e21e 2649
5a964f20
TC
2650=item package
2651
cb1a09d0
AD
2652=item package NAMESPACE
2653
2654Declares the compilation unit as being in the given namespace. The scope
2655of the package declaration is from the declaration itself through the end of
7660c0ab 2656the enclosing block (the same scope as the C<local()> operator). All further
cb1a09d0 2657unqualified dynamic identifiers will be in this namespace. A package
5f05dabc 2658statement affects only dynamic variables--including those you've used
7660c0ab 2659C<local()> on--but I<not> lexical variables created with C<my()>. Typically it
cb1a09d0
AD
2660would be the first declaration in a file to be included by the C<require>
2661or C<use> operator. You can switch into a package in more than one place;
5a964f20 2662it merely influences which symbol table is used by the compiler for the
cb1a09d0
AD
2663rest of that block. You can refer to variables and filehandles in other
2664packages by prefixing the identifier with the package name and a double
2665colon: C<$Package::Variable>. If the package name is null, the C<main>
2666package as assumed. That is, C<$::sail> is equivalent to C<$main::sail>.
2667
5a964f20
TC
2668If NAMESPACE is omitted, then there is no current package, and all
2669identifiers must be fully qualified or lexicals. This is stricter
2670than C<use strict>, since it also extends to function names.
2671
cb1a09d0
AD
2672See L<perlmod/"Packages"> for more information about packages, modules,
2673and classes. See L<perlsub> for other scoping issues.
2674
a0d0e21e
LW
2675=item pipe READHANDLE,WRITEHANDLE
2676
2677Opens a pair of connected pipes like the corresponding system call.
2678Note that if you set up a loop of piped processes, deadlock can occur
2679unless you are very careful. In addition, note that Perl's pipes use
184e9718 2680stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE
a0d0e21e
LW
2681after each command, depending on the application.
2682
7e1af8bc 2683See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
4633a7c4
LW
2684for examples of such things.
2685
4771b018
GS
2686On systems that support a close-on-exec flag on files, the flag will be set
2687for the newly opened file descriptors as determined by the value of $^F.
2688See L<perlvar/$^F>.
2689
a0d0e21e
LW
2690=item pop ARRAY
2691
54310121 2692=item pop
28757baa 2693
a0d0e21e
LW
2694Pops and returns the last value of the array, shortening the array by
26951. Has a similar effect to
2696
2697 $tmp = $ARRAY[$#ARRAY--];
2698
2699If there are no elements in the array, returns the undefined value.
cb1a09d0 2700If ARRAY is omitted, pops the
7660c0ab
A
2701C<@ARGV> array in the main program, and the C<@_> array in subroutines, just
2702like C<shift()>.
a0d0e21e
LW
2703
2704=item pos SCALAR
2705
54310121 2706=item pos
bbce6d69 2707
4633a7c4 2708Returns the offset of where the last C<m//g> search left off for the variable
7660c0ab 2709is in question (C<$_> is used when the variable is not specified). May be
44a8e56a
PP
2710modified to change that offset. Such modification will also influence
2711the C<\G> zero-width assertion in regular expressions. See L<perlre> and
2712L<perlop>.
a0d0e21e
LW
2713
2714=item print FILEHANDLE LIST
2715
2716=item print LIST
2717
2718=item print
2719
cb1a09d0 2720Prints a string or a comma-separated list of strings. Returns TRUE
a0d0e21e 2721if successful. FILEHANDLE may be a scalar variable name, in which case
cb1a09d0 2722the variable contains the name of or a reference to the filehandle, thus introducing one
a0d0e21e
LW
2723level of indirection. (NOTE: If FILEHANDLE is a variable and the next
2724token is a term, it may be misinterpreted as an operator unless you
7660c0ab 2725interpose a C<+> or put parentheses around the arguments.) If FILEHANDLE is
a0d0e21e 2726omitted, prints by default to standard output (or to the last selected
7660c0ab 2727output channel--see L</select>). If LIST is also omitted, prints C<$_> to
5a964f20 2728the currently selected output channel. To set the default output channel to something other than
a0d0e21e 2729STDOUT use the select operation. Note that, because print takes a
5a964f20 2730LIST, anything in the LIST is evaluated in list context, and any
a0d0e21e 2731subroutine that you call will have one or more of its expressions
5a964f20 2732evaluated in list context. Also be careful not to follow the print
a0d0e21e 2733keyword with a left parenthesis unless you want the corresponding right
7660c0ab 2734parenthesis to terminate the arguments to the print--interpose a C<+> or
5f05dabc 2735put parentheses around all the arguments.
a0d0e21e 2736
4633a7c4 2737Note that if you're storing FILEHANDLES in an array or other expression,
da0045b7 2738you will have to use a block returning its value instead:
4633a7c4
LW
2739
2740 print { $files[$i] } "stuff\n";
2741 print { $OK ? STDOUT : STDERR } "stuff\n";
2742
5f05dabc 2743=item printf FILEHANDLE FORMAT, LIST
a0d0e21e 2744
5f05dabc 2745=item printf FORMAT, LIST
a0d0e21e 2746
7660c0ab 2747Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
a3cb178b 2748(the output record separator) is not appended. The first argument
ab4f32c2 2749of the list will be interpreted as the C<printf()> format. If C<use locale> is
a034a98d
DD
2750in effect, the character used for the decimal point in formatted real numbers
2751is affected by the LC_NUMERIC locale. See L<perllocale>.
a0d0e21e 2752
7660c0ab
A
2753Don't fall into the trap of using a C<printf()> when a simple
2754C<print()> would do. The C<print()> is more efficient and less
28757baa
PP
2755error prone.
2756
da0045b7
PP
2757=item prototype FUNCTION
2758
2759Returns the prototype of a function as a string (or C<undef> if the
5f05dabc
PP
2760function has no prototype). FUNCTION is a reference to, or the name of,
2761the function whose prototype you want to retrieve.
da0045b7 2762
b6c543e3
IZ
2763If FUNCTION is a string starting with C<CORE::>, the rest is taken as
2764a name for Perl builtin. If builtin is not I<overridable> (such as
ab4f32c2
A
2765C<qw//>) or its arguments cannot be expressed by a prototype (such as
2766C<system()>) - in other words, the builtin does not behave like a Perl
b6c543e3
IZ
2767function - returns C<undef>. Otherwise, the string describing the
2768equivalent prototype is returned.
2769
a0d0e21e
LW
2770=item push ARRAY,LIST
2771
2772Treats ARRAY as a stack, and pushes the values of LIST
2773onto the end of ARRAY. The length of ARRAY increases by the length of
2774LIST. Has the same effect as
2775
2776 for $value (LIST) {
2777 $ARRAY[++$#ARRAY] = $value;
2778 }
2779
2780but is more efficient. Returns the new number of elements in the array.
2781
2782=item q/STRING/
2783
2784=item qq/STRING/
2785
8782bef2
GB
2786=item qr/STRING/
2787
a0d0e21e
LW
2788=item qx/STRING/
2789
2790=item qw/STRING/
2791
4b6a7270 2792Generalized quotes. See L<perlop/"Regexp Quote-Like Operators">.
a0d0e21e
LW
2793
2794=item quotemeta EXPR
2795
54310121 2796=item quotemeta
bbce6d69 2797
68dc0745 2798Returns the value of EXPR with all non-alphanumeric
a034a98d
DD
2799characters backslashed. (That is, all characters not matching
2800C</[A-Za-z_0-9]/> will be preceded by a backslash in the
2801returned string, regardless of any locale settings.)
2802This is the internal function implementing
7660c0ab 2803the C<\Q> escape in double-quoted strings.
a0d0e21e 2804
7660c0ab 2805If EXPR is omitted, uses C<$_>.
bbce6d69 2806
a0d0e21e
LW
2807=item rand EXPR
2808
2809=item rand
2810
7660c0ab 2811Returns a random fractional number greater than or equal to C<0> and less
3e3baf6d 2812than the value of EXPR. (EXPR should be positive.) If EXPR is
7660c0ab
A
2813omitted, the value C<1> is used. Automatically calls C<srand()> unless
2814C<srand()> has already been called. See also C<srand()>.
a0d0e21e 2815
2f9daede 2816(Note: If your rand function consistently returns numbers that are too
a0d0e21e 2817large or too small, then your version of Perl was probably compiled
2f9daede 2818with the wrong number of RANDBITS.)
a0d0e21e
LW
2819
2820=item read FILEHANDLE,SCALAR,LENGTH,OFFSET
2821
2822=item read FILEHANDLE,SCALAR,LENGTH
2823
2824Attempts to read LENGTH bytes of data into variable SCALAR from the
3b02c43c
GS
2825specified FILEHANDLE. Returns the number of bytes actually read,
2826C<0> at end of file, or undef if there was an error. SCALAR will be grown
2827or shrunk to the length actually read. An OFFSET may be specified to
2828place the read data at some other place than the beginning of the
f86cebdf
GS
2829string. This call is actually implemented in terms of stdio's fread(3)
2830call. To get a true read(2) system call, see C<sysread()>.
a0d0e21e
LW
2831
2832=item readdir DIRHANDLE
2833
7660c0ab 2834Returns the next directory entry for a directory opened by C<opendir()>.
5a964f20 2835If used in list context, returns all the rest of the entries in the
a0d0e21e 2836directory. If there are no more entries, returns an undefined value in
5a964f20 2837scalar context or a null list in list context.
a0d0e21e 2838
7660c0ab 2839If you're planning to filetest the return values out of a C<readdir()>, you'd
5f05dabc 2840better prepend the directory in question. Otherwise, because we didn't
7660c0ab 2841C<chdir()> there, it would have been testing the wrong file.
cb1a09d0
AD
2842
2843 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
2844 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
2845 closedir DIR;
2846
84902520
TB
2847=item readline EXPR
2848
fbad3eb5
GS
2849Reads from the filehandle whose typeglob is contained in EXPR. In scalar
2850context, each call reads and returns the next line, until end-of-file is
2851reached, whereupon the subsequent call returns undef. In list context,
2852reads until end-of-file is reached and returns a list of lines. Note that
2853the notion of "line" used here is however you may have defined it
2854with C<$/> or C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">.
2855
2856When C<$/> is set to C<undef> and when readline() is in a scalar
2857context (i.e. file slurp mode), it returns C<''> the first time,
2858followed by C<undef> subsequently.
2859
84902520
TB
2860This is the internal function implementing the C<E<lt>EXPRE<gt>>
2861operator, but you can use it directly. The C<E<lt>EXPRE<gt>>
2862operator is discussed in more detail in L<perlop/"I/O Operators">.
2863
5a964f20
TC
2864 $line = <STDIN>;
2865 $line = readline(*STDIN); # same thing
2866
a0d0e21e
LW
2867=item readlink EXPR
2868
54310121 2869=item readlink
bbce6d69 2870
a0d0e21e
LW
2871Returns the value of a symbolic link, if symbolic links are
2872implemented. If not, gives a fatal error. If there is some system
184e9718 2873error, returns the undefined value and sets C<$!> (errno). If EXPR is
7660c0ab 2874omitted, uses C<$_>.
a0d0e21e 2875
84902520
TB
2876=item readpipe EXPR
2877
5a964f20 2878EXPR is executed as a system command.
84902520
TB
2879The collected standard output of the command is returned.
2880In scalar context, it comes back as a single (potentially
2881multi-line) string. In list context, returns a list of lines
7660c0ab 2882(however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
84902520
TB
2883This is the internal function implementing the C<qx/EXPR/>
2884operator, but you can use it directly. The C<qx/EXPR/>
2885operator is discussed in more detail in L<perlop/"I/O Operators">.
2886
a0d0e21e
LW
2887=item recv SOCKET,SCALAR,LEN,FLAGS
2888
2889Receives a message on a socket. Attempts to receive LENGTH bytes of
2890data into variable SCALAR from the specified SOCKET filehandle.
7660c0ab 2891Actually does a C C<recvfrom()>, so that it can return the address of the
a0d0e21e
LW
2892sender. Returns the undefined value if there's an error. SCALAR will
2893be grown or shrunk to the length actually read. Takes the same flags
54310121 2894as the system call of the same name.
4633a7c4 2895See L<perlipc/"UDP: Message Passing"> for examples.
a0d0e21e
LW
2896
2897=item redo LABEL
2898
2899=item redo
2900
2901The C<redo> command restarts the loop block without evaluating the
98293880 2902conditional again. The C<continue> block, if any, is not executed. If
a0d0e21e
LW
2903the LABEL is omitted, the command refers to the innermost enclosing
2904loop. This command is normally used by programs that want to lie to
2905themselves about what was just input:
2906
2907 # a simpleminded Pascal comment stripper
2908 # (warning: assumes no { or } in strings)
4633a7c4 2909 LINE: while (<STDIN>) {
a0d0e21e
LW
2910 while (s|({.*}.*){.*}|$1 |) {}
2911 s|{.*}| |;
2912 if (s|{.*| |) {
2913 $front = $_;
2914 while (<STDIN>) {
2915 if (/}/) { # end of comment?
5a964f20 2916 s|^|$front\{|;
4633a7c4 2917 redo LINE;
a0d0e21e
LW
2918 }
2919 }
2920 }
2921 print;
2922 }
2923
4968c1e4
G
2924C<redo> cannot be used to retry a block which returns a value such as
2925C<eval {}>, C<sub {}> or C<do {}>.
2926
98293880 2927See also L</continue> for an illustration of how C<last>, C<next>, and
1d2dff63
GS
2928C<redo> work.
2929
a0d0e21e
LW
2930=item ref EXPR
2931
54310121 2932=item ref
bbce6d69 2933
2f9daede 2934Returns a TRUE value if EXPR is a reference, FALSE otherwise. If EXPR
7660c0ab 2935is not specified, C<$_> will be used. The value returned depends on the
bbce6d69 2936type of thing the reference is a reference to.
a0d0e21e
LW
2937Builtin types include:
2938
2939 REF
2940 SCALAR
2941 ARRAY
2942 HASH
2943 CODE
2944 GLOB
2945
54310121 2946If the referenced object has been blessed into a package, then that package
7660c0ab 2947name is returned instead. You can think of C<ref()> as a C<typeof()> operator.
a0d0e21e
LW
2948
2949 if (ref($r) eq "HASH") {
aa689395 2950 print "r is a reference to a hash.\n";
54310121 2951 }
5a964f20 2952 if (!ref($r)) {
a0d0e21e 2953 print "r is not a reference at all.\n";
54310121 2954 }
a0d0e21e
LW
2955
2956See also L<perlref>.
2957
2958=item rename OLDNAME,NEWNAME
2959
7660c0ab 2960Changes the name of a file. Returns C<1> for success, C<0> otherwise. Will
5f05dabc 2961not work across file system boundaries.
a0d0e21e
LW
2962
2963=item require EXPR
2964
2965=item require
2966
7660c0ab 2967Demands some semantics specified by EXPR, or by C<$_> if EXPR is not
a0d0e21e 2968supplied. If EXPR is numeric, demands that the current version of Perl
184e9718 2969(C<$]> or $PERL_VERSION) be equal or greater than EXPR.
a0d0e21e
LW
2970
2971Otherwise, demands that a library file be included if it hasn't already
2972been included. The file is included via the do-FILE mechanism, which is
7660c0ab 2973essentially just a variety of C<eval()>. Has semantics similar to the following
a0d0e21e
LW
2974subroutine:
2975
2976 sub require {
5a964f20 2977 my($filename) = @_;
a0d0e21e 2978 return 1 if $INC{$filename};
5a964f20 2979 my($realfilename,$result);
a0d0e21e
LW
2980 ITER: {
2981 foreach $prefix (@INC) {
2982 $realfilename = "$prefix/$filename";
2983 if (-f $realfilename) {
2984 $result = do $realfilename;
2985 last ITER;
2986 }
2987 }
2988 die "Can't find $filename in \@INC";
2989 }
2990 die $@ if $@;
2991 die "$filename did not return true value" unless $result;
2992 $INC{$filename} = $realfilename;
5a964f20 2993 return $result;
a0d0e21e
LW
2994 }
2995
2996Note that the file will not be included twice under the same specified
2997name. The file must return TRUE as the last statement to indicate
2998successful execution of any initialization code, so it's customary to
7660c0ab 2999end such a file with "C<1;>" unless you're sure it'll return TRUE
a0d0e21e
LW
3000otherwise. But it's better just to put the "C<1;>", in case you add more
3001statements.
3002
54310121 3003If EXPR is a bareword, the require assumes a "F<.pm>" extension and
da0045b7 3004replaces "F<::>" with "F</>" in the filename for you,
54310121 3005to make it easy to load standard modules. This form of loading of
a0d0e21e
LW
3006modules does not risk altering your namespace.
3007
ee580363
GS
3008In other words, if you try this:
3009
f86cebdf 3010 require Foo::Bar; # a splendid bareword
ee580363 3011
7660c0ab
A
3012The require function will actually look for the "F<Foo/Bar.pm>" file in the
3013directories specified in the C<@INC> array.
ee580363 3014
5a964f20 3015But if you try this:
ee580363
GS
3016
3017 $class = 'Foo::Bar';
f86cebdf 3018 require $class; # $class is not a bareword
5a964f20 3019 #or
f86cebdf 3020 require "Foo::Bar"; # not a bareword because of the ""
ee580363 3021
7660c0ab
A
3022The require function will look for the "F<Foo::Bar>" file in the @INC array and
3023will complain about not finding "F<Foo::Bar>" there. In this case you can do:
ee580363
GS
3024
3025 eval "require $class";
3026
3027For a yet-more-powerful import facility, see L</use> and L<perlmod>.
a0d0e21e
LW
3028
3029=item reset EXPR
3030
3031=item reset
3032
3033Generally used in a C<continue> block at the end of a loop to clear
7660c0ab 3034variables and reset C<??> searches so that they work again. The
a0d0e21e
LW
3035expression is interpreted as a list of single characters (hyphens
3036allowed for ranges). All variables and arrays beginning with one of
3037those letters are reset to their pristine state. If the expression is
7660c0ab 3038omitted, one-match searches (C<?pattern?>) are reset to match again. Resets
5f05dabc 3039only variables or searches in the current package. Always returns
a0d0e21e
LW
30401. Examples:
3041
3042 reset 'X'; # reset all X variables
3043 reset 'a-z'; # reset lower case variables
3044 reset; # just reset ?? searches
3045
7660c0ab
A
3046Resetting C<"A-Z"> is not recommended because you'll wipe out your
3047C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package variables--lexical variables
a0d0e21e 3048are unaffected, but they clean themselves up on scope exit anyway,
da0045b7 3049so you'll probably want to use them instead. See L</my>.
a0d0e21e 3050
54310121
PP
3051=item return EXPR
3052
3053=item return
3054
7660c0ab 3055Returns from a subroutine, C<eval()>, or C<do FILE> with the value
5a964f20 3056given in EXPR. Evaluation of EXPR may be in list, scalar, or void
54310121 3057context, depending on how the return value will be used, and the context
7660c0ab 3058may vary from one execution to the next (see C<wantarray()>). If no EXPR
5a964f20
TC
3059is given, returns an empty list in list context, an undefined value in
3060scalar context, or nothing in a void context.
a0d0e21e 3061
68dc0745
PP
3062(Note that in the absence of a return, a subroutine, eval, or do FILE
3063will automatically return the value of the last expression evaluated.)
a0d0e21e
LW
3064
3065=item reverse LIST
3066
5a964f20
TC
3067In list context, returns a list value consisting of the elements
3068of LIST in the opposite order. In scalar context, concatenates the
a0ed51b3
LW
3069elements of LIST, and returns a string value with all the characters
3070in the opposite order.
4633a7c4 3071
2f9daede 3072 print reverse <>; # line tac, last line first
4633a7c4 3073
2f9daede 3074 undef $/; # for efficiency of <>
a0ed51b3 3075 print scalar reverse <>; # character tac, last line tsrif
2f9daede
TPG
3076
3077This operator is also handy for inverting a hash, although there are some
3078caveats. If a value is duplicated in the original hash, only one of those
3079can be represented as a key in the inverted hash. Also, this has to
3080unwind one hash and build a whole new one, which may take some time
3081on a large hash.
3082
3083 %by_name = reverse %by_address; # Invert the hash
a0d0e21e
LW
3084
3085=item rewinddir DIRHANDLE
3086
3087Sets the current position to the beginning of the directory for the
7660c0ab 3088C<readdir()> routine on DIRHANDLE.
a0d0e21e
LW
3089
3090=item rindex STR,SUBSTR,POSITION
3091
3092=item rindex STR,SUBSTR
3093
3094Works just like index except that it returns the position of the LAST
3095occurrence of SUBSTR in STR. If POSITION is specified, returns the
3096last occurrence at or before that position.
3097
3098=item rmdir FILENAME
3099
54310121 3100=item rmdir
bbce6d69 3101
5a964f20
TC
3102Deletes the directory specified by FILENAME if that directory is empty. If it
3103succeeds it returns TRUE, otherwise it returns FALSE and sets C<$!> (errno). If
7660c0ab 3104FILENAME is omitted, uses C<$_>.
a0d0e21e
LW
3105
3106=item s///
3107
3108The substitution operator. See L<perlop>.
3109
3110=item scalar EXPR
3111
5a964f20 3112Forces EXPR to be interpreted in scalar context and returns the value
54310121 3113of EXPR.
cb1a09d0
AD
3114
3115 @counts = ( scalar @a, scalar @b, scalar @c );
3116
54310121 3117There is no equivalent operator to force an expression to
5a964f20 3118be interpolated in list context because it's in practice never
cb1a09d0
AD
3119needed. If you really wanted to do so, however, you could use
3120the construction C<@{[ (some expression) ]}>, but usually a simple
3121C<(some expression)> suffices.
a0d0e21e 3122
62c18ce2
GS
3123Though C<scalar> can be considered in general to be a unary operator,
3124EXPR is also allowed to be a parenthesized list. The list in fact
3125behaves as a scalar comma expression, evaluating all but the last
3126element in void context and returning the final element evaluated in
3127a scalar context.
3128
3129The following single statement:
3130
3131 print uc(scalar(&foo,$bar)),$baz;
3132
3133is the moral equivalent of these two:
3134
3135 &foo;
3136 print(uc($bar),$baz);
3137
3138See L<perlop> for more details on unary operators and the comma operator.
3139
a0d0e21e
LW
3140=item seek FILEHANDLE,POSITION,WHENCE
3141
ab4f32c2 3142Sets FILEHANDLE's position, just like the C<fseek()> call of C<stdio()>.
8903cb82 3143FILEHANDLE may be an expression whose value gives the name of the
7660c0ab
A
3144filehandle. The values for WHENCE are C<0> to set the new position to
3145POSITION, C<1> to set it to the current position plus POSITION, and C<2> to
8903cb82 3146set it to EOF plus POSITION (typically negative). For WHENCE you may
7660c0ab
A
3147use the constants C<SEEK_SET>, C<SEEK_CUR>, and C<SEEK_END> from either the
3148C<IO::Seekable> or the POSIX module. Returns C<1> upon success, C<0> otherwise.
8903cb82 3149
7660c0ab
A
3150If you want to position file for C<sysread()> or C<syswrite()>, don't use
3151C<seek()> -- buffering makes its effect on the file's system position
3152unpredictable and non-portable. Use C<sysseek()> instead.
a0d0e21e 3153
cb1a09d0
AD
3154On some systems you have to do a seek whenever you switch between reading
3155and writing. Amongst other things, this may have the effect of calling
f86cebdf 3156stdio's clearerr(3). A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving
8903cb82 3157the file position:
cb1a09d0
AD
3158
3159 seek(TEST,0,1);
3160
3161This is also useful for applications emulating C<tail -f>. Once you hit
3162EOF on your read, and then sleep for a while, you might have to stick in a
7660c0ab 3163seek() to reset things. The C<seek()> doesn't change the current position,
8903cb82
PP
3164but it I<does> clear the end-of-file condition on the handle, so that the
3165next C<E<lt>FILEE<gt>> makes Perl try again to read something. We hope.
cb1a09d0
AD
3166
3167If that doesn't work (some stdios are particularly cantankerous), then
3168you may need something more like this:
3169
3170 for (;;) {
f86cebdf
GS
3171 for ($curpos = tell(FILE); $_ = <FILE>;
3172 $curpos = tell(FILE)) {
cb1a09d0
AD
3173 # search for some stuff and put it into files
3174 }
3175 sleep($for_a_while);
3176 seek(FILE, $curpos, 0);
3177 }
3178
a0d0e21e
LW
3179=item seekdir DIRHANDLE,POS
3180
7660c0ab
A
3181Sets the current position for the C<readdir()> routine on DIRHANDLE. POS
3182must be a value returned by C<telldir()>. Has the same caveats about
a0d0e21e
LW
3183possible directory compaction as the corresponding system library
3184routine.
3185
3186=item select FILEHANDLE
3187
3188=item select
3189
3190Returns the currently selected filehandle. Sets the current default
3191filehandle for output, if FILEHANDLE is supplied. This has two
ab4f32c2 3192effects: first, a C<write()> or a C<print()> without a filehandle will
a0d0e21e
LW
3193default to this FILEHANDLE. Second, references to variables related to
3194output will refer to this output channel. For example, if you have to
3195set the top of form format for more than one output channel, you might
3196do the following:
3197
3198 select(REPORT1);
3199 $^ = 'report1_top';
3200 select(REPORT2);
3201 $^ = 'report2_top';
3202
3203FILEHANDLE may be an expression whose value gives the name of the
3204actual filehandle. Thus:
3205
3206 $oldfh = select(STDERR); $| = 1; select($oldfh);
3207
4633a7c4
LW
3208Some programmers may prefer to think of filehandles as objects with
3209methods, preferring to write the last example as:
a0d0e21e 3210
28757baa 3211 use IO::Handle;
a0d0e21e
LW
3212 STDERR->autoflush(1);
3213
3214=item select RBITS,WBITS,EBITS,TIMEOUT
3215
f86cebdf 3216This calls the select(2) system call with the bit masks specified, which
7660c0ab 3217can be constructed using C<fileno()> and C<vec()>, along these lines:
a0d0e21e
LW
3218
3219 $rin = $win = $ein = '';
3220 vec($rin,fileno(STDIN),1) = 1;
3221 vec($win,fileno(STDOUT),1) = 1;
3222 $ein = $rin | $win;
3223
3224If you want to select on many filehandles you might wish to write a
3225subroutine:
3226
3227 sub fhbits {
5a964f20
TC
3228 my(@fhlist) = split(' ',$_[0]);
3229 my($bits);
a0d0e21e
LW
3230 for (@fhlist) {
3231 vec($bits,fileno($_),1) = 1;
3232 }
3233 $bits;
3234 }
4633a7c4 3235 $rin = fhbits('STDIN TTY SOCK');
a0d0e21e
LW
3236
3237The usual idiom is:
3238
3239 ($nfound,$timeleft) =
3240 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
3241
54310121 3242or to block until something becomes ready just do this
a0d0e21e
LW
3243
3244 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
3245
7660c0ab
A
3246Most systems do not bother to return anything useful in C<$timeleft>, so
3247calling select() in scalar context just returns C<$nfound>.
c07a80fd 3248
5f05dabc 3249Any of the bit masks can also be undef. The timeout, if specified, is
a0d0e21e 3250in seconds, which may be fractional. Note: not all implementations are
7660c0ab
A
3251capable of returning theC<$timeleft>. If not, they always return
3252C<$timeleft> equal to the supplied C<$timeout>.
a0d0e21e 3253
ff68c719 3254You can effect a sleep of 250 milliseconds this way:
a0d0e21e
LW
3255
3256 select(undef, undef, undef, 0.25);
3257
7660c0ab
A
3258B<WARNING>: One should not attempt to mix buffered I/O (like C<read()>
3259or E<lt>FHE<gt>) with C<select()>, except as permitted by POSIX, and even
3260then only on POSIX systems. You have to use C<sysread()> instead.
a0d0e21e
LW
3261
3262=item semctl ID,SEMNUM,CMD,ARG
3263
ab4f32c2 3264Calls the System V IPC function C<semctl()>. You'll probably have to say
0ade1984
JH
3265
3266 use IPC::SysV;
3267
3268first to get the correct constant definitions. If CMD is IPC_STAT or
3269GETALL, then ARG must be a variable which will hold the returned
ab4f32c2 3270semid_ds structure or semaphore value array. Returns like C<ioctl()>: the
7660c0ab
A
3271undefined value for error, "C<0> but true" for zero, or the actual return
3272value otherwise. See also C<IPC::SysV> and C<IPC::Semaphore> documentation.
a0d0e21e
LW
3273
3274=item semget KEY,NSEMS,FLAGS
3275
3276Calls the System V IPC function semget. Returns the semaphore id, or
7660c0ab
A
3277the undefined value if there is an error. See also C<IPC::SysV> and
3278C<IPC::SysV::Semaphore> documentation.
a0d0e21e
LW
3279
3280=item semop KEY,OPSTRING
3281
3282Calls the System V IPC function semop to perform semaphore operations
3283such as signaling and waiting. OPSTRING must be a packed array of
3284semop structures. Each semop structure can be generated with
3285C<pack("sss", $semnum, $semop, $semflag)>. The number of semaphore
3286operations is implied by the length of OPSTRING. Returns TRUE if
3287successful, or FALSE if there is an error. As an example, the
7660c0ab 3288following code waits on semaphore C<$semnum> of semaphore id C<$semid>:
a0d0e21e
LW
3289
3290 $semop = pack("sss", $semnum, -1, 0);
3291 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
3292
7660c0ab
A
3293To signal the semaphore, replace C<-1> with C<1>. See also C<IPC::SysV>
3294and C<IPC::SysV::Semaphore> documentation.
a0d0e21e
LW
3295
3296=item send SOCKET,MSG,FLAGS,TO
3297
3298=item send SOCKET,MSG,FLAGS
3299
3300Sends a message on a socket. Takes the same flags as the system call
3301of the same name. On unconnected sockets you must specify a
7660c0ab 3302destination to send TO, in which case it does a C C<sendto()>. Returns
a0d0e21e
LW
3303the number of characters sent, or the undefined value if there is an
3304error.
4633a7c4 3305See L<perlipc/"UDP: Message Passing"> for examples.