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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
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102C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>,
103C<sin>, C<sqrt>, C<srand>
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104
105=item Functions for real @ARRAYs
106
22fae026 107C<pop>, C<push>, C<shift>, C<splice>, C<unshift>
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108
109=item Functions for list data
110
ab4f32c2 111C<grep>, C<join>, C<map>, C<qw/STRING/>, C<reverse>, C<sort>, C<unpack>
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112
113=item Functions for real %HASHes
114
22fae026 115C<delete>, C<each>, C<exists>, C<keys>, C<values>
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116
117=item Input and output functions
118
22fae026
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119C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
120C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
121C<readdir>, C<rewinddir>, C<seek>, C<seekdir>, C<select>, C<syscall>,
122C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>,
123C<warn>, C<write>
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124
125=item Functions for fixed length data or records
126
22fae026 127C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec>
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128
129=item Functions for filehandles, files, or directories
130
22fae026 131C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>,
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132C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>,
133C<readlink>, C<rename>, C<rmdir>, C<stat>, C<symlink>, C<umask>,
134C<unlink>, C<utime>
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135
136=item Keywords related to the control flow of your perl program
137
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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: $!\n"
a0d0e21e
LW
908
909If the value of EXPR does not end in a newline, the current script line
910number and input line number (if any) are also printed, and a newline
7660c0ab
A
911is supplied. Hint: sometimes appending C<", stopped"> to your message
912will cause it to make better sense when the string C<"at foo line 123"> is
a0d0e21e
LW
913appended. Suppose you are running script "canasta".
914
915 die "/etc/games is no good";
916 die "/etc/games is no good, stopped";
917
918produce, respectively
919
920 /etc/games is no good at canasta line 123.
921 /etc/games is no good, stopped at canasta line 123.
922
7660c0ab 923See also C<exit()> and C<warn()>.
a0d0e21e 924
7660c0ab
A
925If LIST is empty and C<$@> already contains a value (typically from a
926previous eval) that value is reused after appending C<"\t...propagated">.
fb73857a
PP
927This is useful for propagating exceptions:
928
929 eval { ... };
930 die unless $@ =~ /Expected exception/;
931
7660c0ab 932If C<$@> is empty then the string C<"Died"> is used.
fb73857a 933
7660c0ab 934You can arrange for a callback to be run just before the C<die()> does
774d564b
PP
935its deed, by setting the C<$SIG{__DIE__}> hook. The associated handler
936will be called with the error text and can change the error message, if
7660c0ab 937it sees fit, by calling C<die()> again. See L<perlvar/$SIG{expr}> for details on
fb73857a
PP
938setting C<%SIG> entries, and L<"eval BLOCK"> for some examples.
939
940Note that the C<$SIG{__DIE__}> hook is called even inside eval()ed
941blocks/strings. If one wants the hook to do nothing in such
942situations, put
943
944 die @_ if $^S;
945
946as the first line of the handler (see L<perlvar/$^S>).
774d564b 947
a0d0e21e
LW
948=item do BLOCK
949
950Not really a function. Returns the value of the last command in the
951sequence of commands indicated by BLOCK. When modified by a loop
98293880
JH
952modifier, executes the BLOCK once before testing the loop condition.
953(On other statements the loop modifiers test the conditional first.)
a0d0e21e 954
4968c1e4 955C<do BLOCK> does I<not> count as a loop, so the loop control statements
98293880 956C<next>, C<last> or C<redo> cannot be used to leave or restart the block.
4968c1e4 957
a0d0e21e
LW
958=item do SUBROUTINE(LIST)
959
960A deprecated form of subroutine call. See L<perlsub>.
961
962=item do EXPR
963
964Uses the value of EXPR as a filename and executes the contents of the
965file as a Perl script. Its primary use is to include subroutines
966from a Perl subroutine library.
967
968 do 'stat.pl';
969
970is just like
971
fb73857a 972 scalar eval `cat stat.pl`;
a0d0e21e 973
5a964f20 974except that it's more efficient and concise, keeps track of the
a0d0e21e
LW
975current filename for error messages, and searches all the B<-I>
976libraries if the file isn't in the current directory (see also the @INC
dc1be6b5
GS
977array in L<perlvar/Predefined Names>). It is also different in how
978code evaluated with C<do FILENAME> doesn't see lexicals in the enclosing
979scope like C<eval STRING> does. It's the same, however, in that it does
54310121 980reparse the file every time you call it, so you probably don't want to
a0d0e21e
LW
981do this inside a loop.
982
8e30cc93
G
983If C<do> cannot read the file, it returns undef and sets C<$!> to the
984error. If C<do> can read the file but cannot compile it, it
985returns undef and sets an error message in C<$@>. If the file is
986successfully compiled, C<do> returns the value of the last expression
987evaluated.
988
a0d0e21e 989Note that inclusion of library modules is better done with the
7660c0ab 990C<use()> and C<require()> operators, which also do automatic error checking
4633a7c4 991and raise an exception if there's a problem.
a0d0e21e 992
5a964f20
TC
993You might like to use C<do> to read in a program configuration
994file. Manual error checking can be done this way:
995
996 # read in config files: system first, then user
f86cebdf
GS
997 for $file ("/share/prog/defaults.rc",
998 "$ENV{HOME}/.someprogrc") {
5a964f20 999 unless ($return = do $file) {
f86cebdf
GS
1000 warn "couldn't parse $file: $@" if $@;
1001 warn "couldn't do $file: $!" unless defined $return;
1002 warn "couldn't run $file" unless $return;
5a964f20
TC
1003 }
1004 }
1005
a0d0e21e
LW
1006=item dump LABEL
1007
1008This causes an immediate core dump. Primarily this is so that you can
1009use the B<undump> program to turn your core dump into an executable binary
1010after having initialized all your variables at the beginning of the
1011program. When the new binary is executed it will begin by executing a
1012C<goto LABEL> (with all the restrictions that C<goto> suffers). Think of
f86cebdf 1013it as a goto with an intervening core dump and reincarnation. If C<LABEL>
5a964f20 1014is omitted, restarts the program from the top. WARNING: Any files
a0d0e21e
LW
1015opened at the time of the dump will NOT be open any more when the
1016program is reincarnated, with possible resulting confusion on the part
1017of Perl. See also B<-u> option in L<perlrun>.
1018
1019Example:
1020
1021 #!/usr/bin/perl
1022 require 'getopt.pl';
1023 require 'stat.pl';
1024 %days = (
1025 'Sun' => 1,
1026 'Mon' => 2,
1027 'Tue' => 3,
1028 'Wed' => 4,
1029 'Thu' => 5,
1030 'Fri' => 6,
1031 'Sat' => 7,
1032 );
1033
1034 dump QUICKSTART if $ARGV[0] eq '-d';
1035
1036 QUICKSTART:
1037 Getopt('f');
1038
5a964f20
TC
1039This operator is largely obsolete, partly because it's very hard to
1040convert a core file into an executable, and because the real perl-to-C
1041compiler has superseded it.
1042
aa689395
PP
1043=item each HASH
1044
5a964f20 1045When called in list context, returns a 2-element list consisting of the
aa689395 1046key and value for the next element of a hash, so that you can iterate over
5a964f20 1047it. When called in scalar context, returns the key for only the "next"
7660c0ab 1048element in the hash. (Note: Keys may be C<"0"> or C<"">, which are logically
2f9daede
TPG
1049false; you may wish to avoid constructs like C<while ($k = each %foo) {}>
1050for this reason.)
1051
ab192400
GS
1052Entries are returned in an apparently random order. The actual random
1053order is subject to change in future versions of perl, but it is guaranteed
1054to be in the same order as either the C<keys()> or C<values()> function
1055would produce on the same (unmodified) hash.
1056
1057When the hash is entirely read, a null array is returned in list context
1058(which when assigned produces a FALSE (C<0>) value), and C<undef> in
7660c0ab
A
1059scalar context. The next call to C<each()> after that will start iterating
1060again. There is a single iterator for each hash, shared by all C<each()>,
1061C<keys()>, and C<values()> function calls in the program; it can be reset by
2f9daede
TPG
1062reading all the elements from the hash, or by evaluating C<keys HASH> or
1063C<values HASH>. If you add or delete elements of a hash while you're
1064iterating over it, you may get entries skipped or duplicated, so don't.
aa689395 1065
f86cebdf 1066The following prints out your environment like the printenv(1) program,
aa689395 1067only in a different order:
a0d0e21e
LW
1068
1069 while (($key,$value) = each %ENV) {
1070 print "$key=$value\n";
1071 }
1072
ab192400 1073See also C<keys()>, C<values()> and C<sort()>.
a0d0e21e
LW
1074
1075=item eof FILEHANDLE
1076
4633a7c4
LW
1077=item eof ()
1078
a0d0e21e
LW
1079=item eof
1080
1081Returns 1 if the next read on FILEHANDLE will return end of file, or if
1082FILEHANDLE is not open. FILEHANDLE may be an expression whose value
5a964f20 1083gives the real filehandle. (Note that this function actually
7660c0ab 1084reads a character and then C<ungetc()>s it, so isn't very useful in an
748a9306
LW
1085interactive context.) Do not read from a terminal file (or call
1086C<eof(FILEHANDLE)> on it) after end-of-file is reached. Filetypes such
1087as terminals may lose the end-of-file condition if you do.
1088
1089An C<eof> without an argument uses the last file read as argument.
5a964f20 1090Using C<eof()> with empty parentheses is very different. It indicates the pseudo file formed of
2f9daede
TPG
1091the files listed on the command line, i.e., C<eof()> is reasonable to
1092use inside a C<while (E<lt>E<gt>)> loop to detect the end of only the
1093last file. Use C<eof(ARGV)> or eof without the parentheses to test
1094I<EACH> file in a while (E<lt>E<gt>) loop. Examples:
a0d0e21e 1095
748a9306
LW
1096 # reset line numbering on each input file
1097 while (<>) {
5a964f20 1098 next if /^\s*#/; # skip comments
748a9306 1099 print "$.\t$_";
5a964f20
TC
1100 } continue {
1101 close ARGV if eof; # Not eof()!
748a9306
LW
1102 }
1103
a0d0e21e
LW
1104 # insert dashes just before last line of last file
1105 while (<>) {
5a964f20 1106 if (eof()) { # check for end of current file
a0d0e21e 1107 print "--------------\n";
748a9306
LW
1108 close(ARGV); # close or break; is needed if we
1109 # are reading from the terminal
a0d0e21e
LW
1110 }
1111 print;
1112 }
1113
a0d0e21e 1114Practical hint: you almost never need to use C<eof> in Perl, because the
3b02c43c
GS
1115input operators return false values when they run out of data, or if there
1116was an error.
a0d0e21e
LW
1117
1118=item eval EXPR
1119
1120=item eval BLOCK
1121
c7cc6f1c
GS
1122In the first form, the return value of EXPR is parsed and executed as if it
1123were a little Perl program. The value of the expression (which is itself
5a964f20 1124determined within scalar context) is first parsed, and if there weren't any
c7cc6f1c 1125errors, executed in the context of the current Perl program, so that any
5f05dabc 1126variable settings or subroutine and format definitions remain afterwards.
c7cc6f1c
GS
1127Note that the value is parsed every time the eval executes. If EXPR is
1128omitted, evaluates C<$_>. This form is typically used to delay parsing
1129and subsequent execution of the text of EXPR until run time.
1130
1131In the second form, the code within the BLOCK is parsed only once--at the
1132same time the code surrounding the eval itself was parsed--and executed
1133within the context of the current Perl program. This form is typically
1134used to trap exceptions more efficiently than the first (see below), while
1135also providing the benefit of checking the code within BLOCK at compile
1136time.
1137
1138The final semicolon, if any, may be omitted from the value of EXPR or within
1139the BLOCK.
1140
1141In both forms, the value returned is the value of the last expression
5a964f20 1142evaluated inside the mini-program; a return statement may be also used, just
c7cc6f1c 1143as with subroutines. The expression providing the return value is evaluated
5a964f20 1144in void, scalar, or list context, depending on the context of the eval itself.
c7cc6f1c 1145See L</wantarray> for more on how the evaluation context can be determined.
a0d0e21e 1146
7660c0ab
A
1147If there is a syntax error or runtime error, or a C<die()> statement is
1148executed, an undefined value is returned by C<eval()>, and C<$@> is set to the
a0d0e21e 1149error message. If there was no error, C<$@> is guaranteed to be a null
7660c0ab 1150string. Beware that using C<eval()> neither silences perl from printing
c7cc6f1c
GS
1151warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1152To do either of those, you have to use the C<$SIG{__WARN__}> facility. See
1153L</warn> and L<perlvar>.
a0d0e21e 1154
7660c0ab
A
1155Note that, because C<eval()> traps otherwise-fatal errors, it is useful for
1156determining whether a particular feature (such as C<socket()> or C<symlink()>)
a0d0e21e
LW
1157is implemented. It is also Perl's exception trapping mechanism, where
1158the die operator is used to raise exceptions.
1159
1160If the code to be executed doesn't vary, you may use the eval-BLOCK
1161form to trap run-time errors without incurring the penalty of
1162recompiling each time. The error, if any, is still returned in C<$@>.
1163Examples:
1164
54310121 1165 # make divide-by-zero nonfatal
a0d0e21e
LW
1166 eval { $answer = $a / $b; }; warn $@ if $@;
1167
1168 # same thing, but less efficient
1169 eval '$answer = $a / $b'; warn $@ if $@;
1170
1171 # a compile-time error
5a964f20 1172 eval { $answer = }; # WRONG
a0d0e21e
LW
1173
1174 # a run-time error
1175 eval '$answer ='; # sets $@
1176
7660c0ab 1177When using the C<eval{}> form as an exception trap in libraries, you may
774d564b
PP
1178wish not to trigger any C<__DIE__> hooks that user code may have
1179installed. You can use the C<local $SIG{__DIE__}> construct for this
1180purpose, as shown in this example:
1181
1182 # a very private exception trap for divide-by-zero
f86cebdf
GS
1183 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1184 warn $@ if $@;
774d564b
PP
1185
1186This is especially significant, given that C<__DIE__> hooks can call
7660c0ab 1187C<die()> again, which has the effect of changing their error messages:
774d564b
PP
1188
1189 # __DIE__ hooks may modify error messages
1190 {
f86cebdf
GS
1191 local $SIG{'__DIE__'} =
1192 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
c7cc6f1c
GS
1193 eval { die "foo lives here" };
1194 print $@ if $@; # prints "bar lives here"
774d564b
PP
1195 }
1196
7660c0ab 1197With an C<eval()>, you should be especially careful to remember what's
a0d0e21e
LW
1198being looked at when:
1199
1200 eval $x; # CASE 1
1201 eval "$x"; # CASE 2
1202
1203 eval '$x'; # CASE 3
1204 eval { $x }; # CASE 4
1205
5a964f20 1206 eval "\$$x++"; # CASE 5
a0d0e21e
LW
1207 $$x++; # CASE 6
1208
2f9daede 1209Cases 1 and 2 above behave identically: they run the code contained in
7660c0ab 1210the variable C<$x>. (Although case 2 has misleading double quotes making
2f9daede 1211the reader wonder what else might be happening (nothing is).) Cases 3
7660c0ab 1212and 4 likewise behave in the same way: they run the code C<'$x'>, which
2f9daede
TPG
1213does nothing but return the value of C<$x>. (Case 4 is preferred for
1214purely visual reasons, but it also has the advantage of compiling at
1215compile-time instead of at run-time.) Case 5 is a place where
54310121 1216normally you I<WOULD> like to use double quotes, except that in this
2f9daede
TPG
1217particular situation, you can just use symbolic references instead, as
1218in case 6.
a0d0e21e 1219
4968c1e4
G
1220C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1221C<next>, C<last> or C<redo> cannot be used to leave or restart the block.
1222
1223
a0d0e21e
LW
1224=item exec LIST
1225
8bf3b016
GS
1226=item exec PROGRAM LIST
1227
7660c0ab
A
1228The C<exec()> function executes a system command I<AND NEVER RETURNS> -
1229use C<system()> instead of C<exec()> if you want it to return. It fails and
fb73857a
PP
1230returns FALSE only if the command does not exist I<and> it is executed
1231directly instead of via your system's command shell (see below).
a0d0e21e 1232
7660c0ab
A
1233Since it's a common mistake to use C<exec()> instead of C<system()>, Perl
1234warns you if there is a following statement which isn't C<die()>, C<warn()>,
1235or C<exit()> (if C<-w> is set - but you always do that). If you
1236I<really> want to follow an C<exec()> with some other statement, you
55d729e4
GS
1237can use one of these styles to avoid the warning:
1238
5a964f20
TC
1239 exec ('foo') or print STDERR "couldn't exec foo: $!";
1240 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
55d729e4 1241
5a964f20 1242If there is more than one argument in LIST, or if LIST is an array
f86cebdf 1243with more than one value, calls execvp(3) with the arguments in LIST.
5a964f20
TC
1244If there is only one scalar argument or an array with one element in it,
1245the argument is checked for shell metacharacters, and if there are any,
1246the entire argument is passed to the system's command shell for parsing
1247(this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1248If there are no shell metacharacters in the argument, it is split into
7660c0ab
A
1249words and passed directly to C<execvp()>, which is more efficient. Note:
1250C<exec()> and C<system()> do not flush your output buffer, so you may need to
5a964f20 1251set C<$|> to avoid lost output. Examples:
a0d0e21e
LW
1252
1253 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1254 exec "sort $outfile | uniq";
1255
1256If you don't really want to execute the first argument, but want to lie
1257to the program you are executing about its own name, you can specify
1258the program you actually want to run as an "indirect object" (without a
1259comma) in front of the LIST. (This always forces interpretation of the
54310121 1260LIST as a multivalued list, even if there is only a single scalar in
a0d0e21e
LW
1261the list.) Example:
1262
1263 $shell = '/bin/csh';
1264 exec $shell '-sh'; # pretend it's a login shell
1265
1266or, more directly,
1267
1268 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1269
bb32b41a
GS
1270When the arguments get executed via the system shell, results will
1271be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1272for details.
1273
ab4f32c2 1274Using an indirect object with C<exec()> or C<system()> is also more secure.
5a964f20
TC
1275This usage forces interpretation of the arguments as a multivalued list,
1276even if the list had just one argument. That way you're safe from the
1277shell expanding wildcards or splitting up words with whitespace in them.
1278
1279 @args = ( "echo surprise" );
1280
f86cebdf
GS
1281 system @args; # subject to shell escapes
1282 # if @args == 1
5a964f20
TC
1283 system { $args[0] } @args; # safe even with one-arg list
1284
1285The first version, the one without the indirect object, ran the I<echo>
1286program, passing it C<"surprise"> an argument. The second version
1287didn't--it tried to run a program literally called I<"echo surprise">,
1288didn't find it, and set C<$?> to a non-zero value indicating failure.
1289
ab4f32c2 1290Note that C<exec()> will not call your C<END> blocks, nor will it call
7660c0ab
A
1291any C<DESTROY> methods in your objects.
1292
a0d0e21e
LW
1293=item exists EXPR
1294
1295Returns TRUE if the specified hash key exists in its hash array, even
1296if the corresponding value is undefined.
1297
1298 print "Exists\n" if exists $array{$key};
1299 print "Defined\n" if defined $array{$key};
1300 print "True\n" if $array{$key};
1301
5f05dabc 1302A hash element can be TRUE only if it's defined, and defined if
a0d0e21e
LW
1303it exists, but the reverse doesn't necessarily hold true.
1304
1305Note that the EXPR can be arbitrarily complicated as long as the final
1306operation is a hash key lookup:
1307
5a964f20
TC
1308 if (exists $ref->{"A"}{"B"}{$key}) { ... }
1309
1310Although the last element will not spring into existence just because its
1311existence was tested, intervening ones will. Thus C<$ref-E<gt>{"A"}>
1312C<$ref-E<gt>{"B"}> will spring into existence due to the existence
1313test for a $key element. This autovivification may be fixed in a later
1314release.
a0d0e21e
LW
1315
1316=item exit EXPR
1317
1318Evaluates EXPR and exits immediately with that value. (Actually, it
1319calls any defined C<END> routines first, but the C<END> routines may not
1320abort the exit. Likewise any object destructors that need to be called
1321are called before exit.) Example:
1322
1323 $ans = <STDIN>;
1324 exit 0 if $ans =~ /^[Xx]/;
1325
7660c0ab
A
1326See also C<die()>. If EXPR is omitted, exits with C<0> status. The only
1327universally portable values for EXPR are C<0> for success and C<1> for error;
f86702cc
PP
1328all other values are subject to unpredictable interpretation depending
1329on the environment in which the Perl program is running.
a0d0e21e 1330
7660c0ab
A
1331You shouldn't use C<exit()> to abort a subroutine if there's any chance that
1332someone might want to trap whatever error happened. Use C<die()> instead,
1333which can be trapped by an C<eval()>.
28757baa 1334
5a964f20
TC
1335All C<END{}> blocks are run at exit time. See L<perlsub> for details.
1336
a0d0e21e
LW
1337=item exp EXPR
1338
54310121 1339=item exp
bbce6d69 1340
54310121 1341Returns I<e> (the natural logarithm base) to the power of EXPR.
a0d0e21e
LW
1342If EXPR is omitted, gives C<exp($_)>.
1343
1344=item fcntl FILEHANDLE,FUNCTION,SCALAR
1345
f86cebdf 1346Implements the fcntl(2) function. You'll probably have to say
a0d0e21e
LW
1347
1348 use Fcntl;
1349
0ade1984 1350first to get the correct constant definitions. Argument processing and
7660c0ab 1351value return works just like C<ioctl()> below.
a0d0e21e
LW
1352For example:
1353
1354 use Fcntl;
5a964f20
TC
1355 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1356 or die "can't fcntl F_GETFL: $!";
1357
ab4f32c2
A
1358You don't have to check for C<defined()> on the return from
1359C<fnctl()>. Like C<ioctl()>, it maps a C<0> return from the system
7660c0ab
A
1360call into "C<0> but true" in Perl. This string is true in
1361boolean context and C<0> in numeric context. It is also
5a964f20
TC
1362exempt from the normal B<-w> warnings on improper numeric
1363conversions.
1364
7660c0ab 1365Note that C<fcntl()> will produce a fatal error if used on a machine that
f86cebdf 1366doesn't implement fcntl(2).
a0d0e21e
LW
1367
1368=item fileno FILEHANDLE
1369
1370Returns the file descriptor for a filehandle. This is useful for
7660c0ab 1371constructing bitmaps for C<select()> and low-level POSIX tty-handling
5a964f20
TC
1372operations. If FILEHANDLE is an expression, the value is taken as
1373an indirect filehandle, generally its name.
1374
1375You can use this to find out whether two handles refer to the
1376same underlying descriptor:
1377
1378 if (fileno(THIS) == fileno(THAT)) {
1379 print "THIS and THAT are dups\n";
1380 }
a0d0e21e
LW
1381
1382=item flock FILEHANDLE,OPERATION
1383
f86cebdf 1384Calls flock(2), or an emulation of it, on FILEHANDLE. Returns TRUE for
68dc0745 1385success, FALSE on failure. Produces a fatal error if used on a machine
f86cebdf 1386that doesn't implement flock(2), fcntl(2) locking, or lockf(3). C<flock()>
68dc0745
PP
1387is Perl's portable file locking interface, although it locks only entire
1388files, not records.
8ebc5c01 1389
a3cb178b 1390On many platforms (including most versions or clones of Unix), locks
7660c0ab 1391established by C<flock()> are B<merely advisory>. Such discretionary locks
5a964f20 1392are more flexible, but offer fewer guarantees. This means that files
7660c0ab
A
1393locked with C<flock()> may be modified by programs that do not also use
1394C<flock()>. Windows NT and OS/2 are among the platforms which
5a964f20 1395enforce mandatory locking. See your local documentation for details.
a3cb178b 1396
8ebc5c01
PP
1397OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1398LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
68dc0745
PP
1399you can use the symbolic names if import them from the Fcntl module,
1400either individually, or as a group using the ':flock' tag. LOCK_SH
1401requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1402releases a previously requested lock. If LOCK_NB is added to LOCK_SH or
7660c0ab 1403LOCK_EX then C<flock()> will return immediately rather than blocking
68dc0745
PP
1404waiting for the lock (check the return status to see if you got it).
1405
1406To avoid the possibility of mis-coordination, Perl flushes FILEHANDLE
1407before (un)locking it.
8ebc5c01 1408
f86cebdf 1409Note that the emulation built with lockf(3) doesn't provide shared
8ebc5c01 1410locks, and it requires that FILEHANDLE be open with write intent. These
f86cebdf
GS
1411are the semantics that lockf(3) implements. Most (all?) systems
1412implement lockf(3) in terms of fcntl(2) locking, though, so the
8ebc5c01
PP
1413differing semantics shouldn't bite too many people.
1414
7660c0ab
A
1415Note also that some versions of C<flock()> cannot lock things over the
1416network; you would need to use the more system-specific C<fcntl()> for
f86cebdf
GS
1417that. If you like you can force Perl to ignore your system's flock(2)
1418function, and so provide its own fcntl(2)-based emulation, by passing
8ebc5c01
PP
1419the switch C<-Ud_flock> to the F<Configure> program when you configure
1420perl.
4633a7c4
LW
1421
1422Here's a mailbox appender for BSD systems.
a0d0e21e 1423
7e1af8bc 1424 use Fcntl ':flock'; # import LOCK_* constants
a0d0e21e
LW
1425
1426 sub lock {
7e1af8bc 1427 flock(MBOX,LOCK_EX);
a0d0e21e
LW
1428 # and, in case someone appended
1429 # while we were waiting...
1430 seek(MBOX, 0, 2);
1431 }
1432
1433 sub unlock {
7e1af8bc 1434 flock(MBOX,LOCK_UN);
a0d0e21e
LW
1435 }
1436
1437 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1438 or die "Can't open mailbox: $!";
1439
1440 lock();
1441 print MBOX $msg,"\n\n";
1442 unlock();
1443
cb1a09d0 1444See also L<DB_File> for other flock() examples.
a0d0e21e
LW
1445
1446=item fork
1447
f86cebdf 1448Does a fork(2) system call. Returns the child pid to the parent process,
7660c0ab 1449C<0> to the child process, or C<undef> if the fork is unsuccessful.
5a964f20 1450
a0d0e21e 1451Note: unflushed buffers remain unflushed in both processes, which means
7660c0ab
A
1452you may need to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()>
1453method of C<IO::Handle> to avoid duplicate output.
a0d0e21e 1454
7660c0ab 1455If you C<fork()> without ever waiting on your children, you will accumulate
a0d0e21e
LW
1456zombies:
1457
4633a7c4 1458 $SIG{CHLD} = sub { wait };
a0d0e21e 1459
54310121 1460There's also the double-fork trick (error checking on
7660c0ab 1461C<fork()> returns omitted);
a0d0e21e
LW
1462
1463 unless ($pid = fork) {
1464 unless (fork) {
1465 exec "what you really wanna do";
1466 die "no exec";
1467 # ... or ...
4633a7c4 1468 ## (some_perl_code_here)
a0d0e21e
LW
1469 exit 0;
1470 }
1471 exit 0;
1472 }
1473 waitpid($pid,0);
1474
cb1a09d0
AD
1475See also L<perlipc> for more examples of forking and reaping
1476moribund children.
1477
28757baa
PP
1478Note that if your forked child inherits system file descriptors like
1479STDIN and STDOUT that are actually connected by a pipe or socket, even
5a964f20 1480if you exit, then the remote server (such as, say, httpd or rsh) won't think
7660c0ab 1481you're done. You should reopen those to F</dev/null> if it's any issue.
28757baa 1482
cb1a09d0
AD
1483=item format
1484
7660c0ab 1485Declare a picture format for use by the C<write()> function. For
cb1a09d0
AD
1486example:
1487
54310121 1488 format Something =
cb1a09d0
AD
1489 Test: @<<<<<<<< @||||| @>>>>>
1490 $str, $%, '$' . int($num)
1491 .
1492
1493 $str = "widget";
184e9718 1494 $num = $cost/$quantity;
cb1a09d0
AD
1495 $~ = 'Something';
1496 write;
1497
1498See L<perlform> for many details and examples.
1499
8903cb82 1500=item formline PICTURE,LIST
a0d0e21e 1501
5a964f20 1502This is an internal function used by C<format>s, though you may call it,
a0d0e21e
LW
1503too. It formats (see L<perlform>) a list of values according to the
1504contents of PICTURE, placing the output into the format output
7660c0ab
A
1505accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
1506Eventually, when a C<write()> is done, the contents of
a0d0e21e 1507C<$^A> are written to some filehandle, but you could also read C<$^A>
7660c0ab
A
1508yourself and then set C<$^A> back to C<"">. Note that a format typically
1509does one C<formline()> per line of form, but the C<formline()> function itself
748a9306 1510doesn't care how many newlines are embedded in the PICTURE. This means
4633a7c4 1511that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
748a9306
LW
1512You may therefore need to use multiple formlines to implement a single
1513record format, just like the format compiler.
1514
5f05dabc 1515Be careful if you put double quotes around the picture, because an "C<@>"
748a9306 1516character may be taken to mean the beginning of an array name.
7660c0ab 1517C<formline()> always returns TRUE. See L<perlform> for other examples.
a0d0e21e
LW
1518
1519=item getc FILEHANDLE
1520
1521=item getc
1522
1523Returns the next character from the input file attached to FILEHANDLE,
9bc64814 1524or the undefined value at end of file, or if there was an error. If
3b02c43c
GS
1525FILEHANDLE is omitted, reads from STDIN. This is not particularly
1526efficient. It cannot be used to get unbuffered single-characters,
1527however. For that, try something more like:
4633a7c4
LW
1528
1529 if ($BSD_STYLE) {
1530 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1531 }
1532 else {
54310121 1533 system "stty", '-icanon', 'eol', "\001";
4633a7c4
LW
1534 }
1535
1536 $key = getc(STDIN);
1537
1538 if ($BSD_STYLE) {
1539 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1540 }
1541 else {
5f05dabc 1542 system "stty", 'icanon', 'eol', '^@'; # ASCII null
4633a7c4
LW
1543 }
1544 print "\n";
1545
54310121
PP
1546Determination of whether $BSD_STYLE should be set
1547is left as an exercise to the reader.
cb1a09d0 1548
7660c0ab 1549The C<POSIX::getattr()> function can do this more portably on systems
5a964f20 1550purporting POSIX compliance.
cb1a09d0 1551See also the C<Term::ReadKey> module from your nearest CPAN site;
b687b08b 1552details on CPAN can be found on L<perlmodlib/CPAN>.
a0d0e21e
LW
1553
1554=item getlogin
1555
5a964f20
TC
1556Implements the C library function of the same name, which on most
1557systems returns the current login from F</etc/utmp>, if any. If null,
7660c0ab 1558use C<getpwuid()>.
a0d0e21e 1559
f86702cc 1560 $login = getlogin || getpwuid($<) || "Kilroy";
a0d0e21e 1561
7660c0ab
A
1562Do not consider C<getlogin()> for authentication: it is not as
1563secure as C<getpwuid()>.
4633a7c4 1564
a0d0e21e
LW
1565=item getpeername SOCKET
1566
1567Returns the packed sockaddr address of other end of the SOCKET connection.
1568
4633a7c4
LW
1569 use Socket;
1570 $hersockaddr = getpeername(SOCK);
1571 ($port, $iaddr) = unpack_sockaddr_in($hersockaddr);
1572 $herhostname = gethostbyaddr($iaddr, AF_INET);
1573 $herstraddr = inet_ntoa($iaddr);
a0d0e21e
LW
1574
1575=item getpgrp PID
1576
47e29363 1577Returns the current process group for the specified PID. Use
7660c0ab 1578a PID of C<0> to get the current process group for the
4633a7c4 1579current process. Will raise an exception if used on a machine that
f86cebdf 1580doesn't implement getpgrp(2). If PID is omitted, returns process
7660c0ab
A
1581group of current process. Note that the POSIX version of C<getpgrp()>
1582does not accept a PID argument, so only C<PID==0> is truly portable.
a0d0e21e
LW
1583
1584=item getppid
1585
1586Returns the process id of the parent process.
1587
1588=item getpriority WHICH,WHO
1589
4633a7c4
LW
1590Returns the current priority for a process, a process group, or a user.
1591(See L<getpriority(2)>.) Will raise a fatal exception if used on a
f86cebdf 1592machine that doesn't implement getpriority(2).
a0d0e21e
LW
1593
1594=item getpwnam NAME
1595
1596=item getgrnam NAME
1597
1598=item gethostbyname NAME
1599
1600=item getnetbyname NAME
1601
1602=item getprotobyname NAME
1603
1604=item getpwuid UID
1605
1606=item getgrgid GID
1607
1608=item getservbyname NAME,PROTO
1609
1610=item gethostbyaddr ADDR,ADDRTYPE
1611
1612=item getnetbyaddr ADDR,ADDRTYPE
1613
1614=item getprotobynumber NUMBER
1615
1616=item getservbyport PORT,PROTO
1617
1618=item getpwent
1619
1620=item getgrent
1621
1622=item gethostent
1623
1624=item getnetent
1625
1626=item getprotoent
1627
1628=item getservent
1629
1630=item setpwent
1631
1632=item setgrent
1633
1634=item sethostent STAYOPEN
1635
1636=item setnetent STAYOPEN
1637
1638=item setprotoent STAYOPEN
1639
1640=item setservent STAYOPEN
1641
1642=item endpwent
1643
1644=item endgrent
1645
1646=item endhostent
1647
1648=item endnetent
1649
1650=item endprotoent
1651
1652=item endservent
1653
1654These routines perform the same functions as their counterparts in the
5a964f20 1655system library. In list context, the return values from the
a0d0e21e
LW
1656various get routines are as follows:
1657
1658 ($name,$passwd,$uid,$gid,
6ee623d5 1659 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
a0d0e21e
LW
1660 ($name,$passwd,$gid,$members) = getgr*
1661 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1662 ($name,$aliases,$addrtype,$net) = getnet*
1663 ($name,$aliases,$proto) = getproto*
1664 ($name,$aliases,$port,$proto) = getserv*
1665
1666(If the entry doesn't exist you get a null list.)
1667
5a964f20 1668In scalar context, you get the name, unless the function was a
a0d0e21e
LW
1669lookup by name, in which case you get the other thing, whatever it is.
1670(If the entry doesn't exist you get the undefined value.) For example:
1671
5a964f20
TC
1672 $uid = getpwnam($name);
1673 $name = getpwuid($num);
1674 $name = getpwent();
1675 $gid = getgrnam($name);
1676 $name = getgrgid($num;
1677 $name = getgrent();
1678 #etc.
a0d0e21e 1679
7660c0ab 1680In I<getpw*()> the fields C<$quota>, C<$comment>, and C<$expire> are special
6ee623d5 1681cases in the sense that in many systems they are unsupported. If the
7660c0ab
A
1682C<$quota> is unsupported, it is an empty scalar. If it is supported, it
1683usually encodes the disk quota. If the C<$comment> field is unsupported,
6ee623d5
GS
1684it is an empty scalar. If it is supported it usually encodes some
1685administrative comment about the user. In some systems the $quota
7660c0ab
A
1686field may be C<$change> or C<$age>, fields that have to do with password
1687aging. In some systems the C<$comment> field may be C<$class>. The C<$expire>
6ee623d5
GS
1688field, if present, encodes the expiration period of the account or the
1689password. For the availability and the exact meaning of these fields
f86cebdf 1690in your system, please consult your getpwnam(3) documentation and your
7660c0ab
A
1691F<pwd.h> file. You can also find out from within Perl which meaning
1692your C<$quota> and C<$comment> fields have and whether you have the C<$expire>
1693field by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
1694C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>.
6ee623d5 1695
7660c0ab 1696The C<$members> value returned by I<getgr*()> is a space separated list of
a0d0e21e
LW
1697the login names of the members of the group.
1698
1699For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1700C, it will be returned to you via C<$?> if the function call fails. The
7660c0ab 1701C<@addrs> value returned by a successful call is a list of the raw
a0d0e21e
LW
1702addresses returned by the corresponding system library call. In the
1703Internet domain, each address is four bytes long and you can unpack it
1704by saying something like:
1705
1706 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1707
5a964f20
TC
1708If you get tired of remembering which element of the return list contains
1709which return value, by-name interfaces are also provided in modules:
7660c0ab
A
1710C<File::stat>, C<Net::hostent>, C<Net::netent>, C<Net::protoent>, C<Net::servent>,
1711C<Time::gmtime>, C<Time::localtime>, and C<User::grent>. These override the
5a964f20
TC
1712normal built-in, replacing them with versions that return objects with
1713the appropriate names for each field. For example:
1714
1715 use File::stat;
1716 use User::pwent;
1717 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
1718
1719Even though it looks like they're the same method calls (uid),
7660c0ab 1720they aren't, because a C<File::stat> object is different from a C<User::pwent> object.
5a964f20 1721
a0d0e21e
LW
1722=item getsockname SOCKET
1723
1724Returns the packed sockaddr address of this end of the SOCKET connection.
1725
4633a7c4
LW
1726 use Socket;
1727 $mysockaddr = getsockname(SOCK);
1728 ($port, $myaddr) = unpack_sockaddr_in($mysockaddr);
a0d0e21e
LW
1729
1730=item getsockopt SOCKET,LEVEL,OPTNAME
1731
5a964f20 1732Returns the socket option requested, or undef if there is an error.
a0d0e21e
LW
1733
1734=item glob EXPR
1735
0a753a76
PP
1736=item glob
1737
7660c0ab 1738Returns the value of EXPR with filename expansions such as the standard Unix shell F</bin/sh> would
68dc0745 1739do. This is the internal function implementing the C<E<lt>*.cE<gt>>
7660c0ab 1740operator, but you can use it directly. If EXPR is omitted, C<$_> is used.
68dc0745
PP
1741The C<E<lt>*.cE<gt>> operator is discussed in more detail in
1742L<perlop/"I/O Operators">.
a0d0e21e
LW
1743
1744=item gmtime EXPR
1745
1746Converts a time as returned by the time function to a 9-element array
54310121 1747with the time localized for the standard Greenwich time zone.
4633a7c4 1748Typically used as follows:
a0d0e21e 1749
54310121 1750 # 0 1 2 3 4 5 6 7 8
a0d0e21e
LW
1751 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1752 gmtime(time);
1753
1754All array elements are numeric, and come straight out of a struct tm.
7660c0ab
A
1755In particular this means that C<$mon> has the range C<0..11> and C<$wday> has
1756the range C<0..6> with sunday as day C<0>. Also, C<$year> is the number of
1757years since 1900, that is, C<$year> is C<123> in year 2023, I<not> simply the last two digits of the year.
2f9daede
TPG
1758
1759If EXPR is omitted, does C<gmtime(time())>.
a0d0e21e 1760
f86cebdf 1761In scalar context, returns the ctime(3) value:
0a753a76
PP
1762
1763 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
1764
7660c0ab 1765Also see the C<timegm()> function provided by the C<Time::Local> module,
f86cebdf 1766and the strftime(3) function available via the POSIX module.
7660c0ab
A
1767
1768This scalar value is B<not> locale dependent, see L<perllocale>, but
1769instead a Perl builtin. Also see the C<Time::Local> module, and the
f86cebdf 1770strftime(3) and mktime(3) function available via the POSIX module. To
7660c0ab
A
1771get somewhat similar but locale dependent date strings, set up your
1772locale environment variables appropriately (please see L<perllocale>)
1773and try for example:
1774
1775 use POSIX qw(strftime);
1776 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
1777
1778Note that the C<%a> and C<%b>, the short forms of the day of the week
1779and the month of the year, may not necessarily be three characters wide.
0a753a76 1780
a0d0e21e
LW
1781=item goto LABEL
1782
748a9306
LW
1783=item goto EXPR
1784
a0d0e21e
LW
1785=item goto &NAME
1786
7660c0ab 1787The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
a0d0e21e 1788execution there. It may not be used to go into any construct that
7660c0ab 1789requires initialization, such as a subroutine or a C<foreach> loop. It
0a753a76 1790also can't be used to go into a construct that is optimized away,
7660c0ab 1791or to get out of a block or subroutine given to C<sort()>.
0a753a76 1792It can be used to go almost anywhere else within the dynamic scope,
a0d0e21e 1793including out of subroutines, but it's usually better to use some other
ab4f32c2 1794construct such as C<last> or C<die()>. The author of Perl has never felt the
7660c0ab 1795need to use this form of C<goto> (in Perl, that is--C is another matter).
a0d0e21e 1796
7660c0ab
A
1797The C<goto-EXPR> form expects a label name, whose scope will be resolved
1798dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
748a9306
LW
1799necessarily recommended if you're optimizing for maintainability:
1800
1801 goto ("FOO", "BAR", "GLARCH")[$i];
1802
7660c0ab 1803The C<goto-&NAME> form is highly magical, and substitutes a call to the
a0d0e21e 1804named subroutine for the currently running subroutine. This is used by
7660c0ab 1805C<AUTOLOAD> subroutines that wish to load another subroutine and then
a0d0e21e 1806pretend that the other subroutine had been called in the first place
7660c0ab
A
1807(except that any modifications to C<@_> in the current subroutine are
1808propagated to the other subroutine.) After the C<goto>, not even C<caller()>
a0d0e21e
LW
1809will be able to tell that this routine was called first.
1810
1811=item grep BLOCK LIST
1812
1813=item grep EXPR,LIST
1814
f86cebdf 1815This is similar in spirit to, but not the same as, grep(1)
2f9daede
TPG
1816and its relatives. In particular, it is not limited to using
1817regular expressions.
1818
a0d0e21e 1819Evaluates the BLOCK or EXPR for each element of LIST (locally setting
7660c0ab 1820C<$_> to each element) and returns the list value consisting of those
a0d0e21e
LW
1821elements for which the expression evaluated to TRUE. In a scalar
1822context, returns the number of times the expression was TRUE.
1823
1824 @foo = grep(!/^#/, @bar); # weed out comments
1825
1826or equivalently,
1827
1828 @foo = grep {!/^#/} @bar; # weed out comments
1829
7660c0ab 1830Note that, because C<$_> is a reference into the list value, it can be used
a0d0e21e
LW
1831to modify the elements of the array. While this is useful and
1832supported, it can cause bizarre results if the LIST is not a named
2f9daede 1833array. Similarly, grep returns aliases into the original list,
f86cebdf 1834much like the way that a for loop's index variable aliases the list
2f9daede 1835elements. That is, modifying an element of a list returned by grep
ab4f32c2 1836(for example, in a C<foreach>, C<map()> or another C<grep()>)
2f9daede 1837actually modifies the element in the original list.
a0d0e21e 1838
fb73857a 1839See also L</map> for an array composed of the results of the BLOCK or EXPR.
38325410 1840
a0d0e21e
LW
1841=item hex EXPR
1842
54310121 1843=item hex
bbce6d69 1844
54310121 1845Interprets EXPR as a hex string and returns the corresponding
2f9daede 1846value. (To convert strings that might start with either 0 or 0x
7660c0ab 1847see L</oct>.) If EXPR is omitted, uses C<$_>.
2f9daede
TPG
1848
1849 print hex '0xAf'; # prints '175'
1850 print hex 'aF'; # same
a0d0e21e
LW
1851
1852=item import
1853
7660c0ab 1854There is no builtin C<import()> function. It is just an ordinary
4633a7c4 1855method (subroutine) defined (or inherited) by modules that wish to export
7660c0ab 1856names to another module. The C<use()> function calls the C<import()> method
54310121 1857for the package used. See also L</use()>, L<perlmod>, and L<Exporter>.
a0d0e21e
LW
1858
1859=item index STR,SUBSTR,POSITION
1860
1861=item index STR,SUBSTR
1862
4633a7c4
LW
1863Returns the position of the first occurrence of SUBSTR in STR at or after
1864POSITION. If POSITION is omitted, starts searching from the beginning of
7660c0ab 1865the string. The return value is based at C<0> (or whatever you've set the C<$[>
4633a7c4 1866variable to--but don't do that). If the substring is not found, returns
7660c0ab 1867one less than the base, ordinarily C<-1>.
a0d0e21e
LW
1868
1869=item int EXPR
1870
54310121 1871=item int
bbce6d69 1872
7660c0ab 1873Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
5a964f20 1874You should not use this for rounding, because it truncates
7660c0ab
A
1875towards C<0>, and because machine representations of floating point
1876numbers can sometimes produce counterintuitive results. Usually C<sprintf()> or C<printf()>,
1877or the C<POSIX::floor> or C<POSIX::ceil> functions, would serve you better.
a0d0e21e
LW
1878
1879=item ioctl FILEHANDLE,FUNCTION,SCALAR
1880
f86cebdf 1881Implements the ioctl(2) function. You'll probably have to say
a0d0e21e 1882
4633a7c4 1883 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
a0d0e21e 1884
4633a7c4 1885first to get the correct function definitions. If F<ioctl.ph> doesn't
a0d0e21e 1886exist or doesn't have the correct definitions you'll have to roll your
4633a7c4 1887own, based on your C header files such as F<E<lt>sys/ioctl.hE<gt>>.
5a964f20 1888(There is a Perl script called B<h2ph> that comes with the Perl kit that
54310121 1889may help you in this, but it's nontrivial.) SCALAR will be read and/or
4633a7c4 1890written depending on the FUNCTION--a pointer to the string value of SCALAR
ab4f32c2 1891will be passed as the third argument of the actual C<ioctl()> call. (If SCALAR
4633a7c4
LW
1892has no string value but does have a numeric value, that value will be
1893passed rather than a pointer to the string value. To guarantee this to be
7660c0ab 1894TRUE, add a C<0> to the scalar before using it.) The C<pack()> and C<unpack()>
4633a7c4 1895functions are useful for manipulating the values of structures used by
7660c0ab 1896C<ioctl()>. The following example sets the erase character to DEL.
a0d0e21e
LW
1897
1898 require 'ioctl.ph';
4633a7c4
LW
1899 $getp = &TIOCGETP;
1900 die "NO TIOCGETP" if $@ || !$getp;
a0d0e21e 1901 $sgttyb_t = "ccccs"; # 4 chars and a short
4633a7c4 1902 if (ioctl(STDIN,$getp,$sgttyb)) {
a0d0e21e
LW
1903 @ary = unpack($sgttyb_t,$sgttyb);
1904 $ary[2] = 127;
1905 $sgttyb = pack($sgttyb_t,@ary);
4633a7c4 1906 ioctl(STDIN,&TIOCSETP,$sgttyb)
a0d0e21e
LW
1907 || die "Can't ioctl: $!";
1908 }
1909
ab4f32c2 1910The return value of C<ioctl()> (and C<fcntl()>) is as follows:
a0d0e21e
LW
1911
1912 if OS returns: then Perl returns:
1913 -1 undefined value
1914 0 string "0 but true"
1915 anything else that number
1916
1917Thus Perl returns TRUE on success and FALSE on failure, yet you can
1918still easily determine the actual value returned by the operating
1919system:
1920
1921 ($retval = ioctl(...)) || ($retval = -1);
1922 printf "System returned %d\n", $retval;
1923
7660c0ab 1924The special string "C<0> but true" is excempt from B<-w> complaints
5a964f20
TC
1925about improper numeric conversions.
1926
a0d0e21e
LW
1927=item join EXPR,LIST
1928
54310121 1929Joins the separate strings of LIST into a single string with
a0d0e21e
LW
1930fields separated by the value of EXPR, and returns the string.
1931Example:
1932
1933 $_ = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
1934
7660c0ab 1935See L</split>.
a0d0e21e 1936
aa689395
PP
1937=item keys HASH
1938
1d2dff63
GS
1939Returns a list consisting of all the keys of the named hash. (In a
1940scalar context, returns the number of keys.) The keys are returned in
ab192400
GS
1941an apparently random order. The actual random order is subject to
1942change in future versions of perl, but it is guaranteed to be the same
1943order as either the C<values()> or C<each()> function produces (given
1944that the hash has not been modified). As a side effect, it resets
1945HASH's iterator.
a0d0e21e 1946
aa689395 1947Here is yet another way to print your environment:
a0d0e21e
LW
1948
1949 @keys = keys %ENV;
1950 @values = values %ENV;
1951 while ($#keys >= 0) {
1952 print pop(@keys), '=', pop(@values), "\n";
1953 }
1954
1955or how about sorted by key:
1956
1957 foreach $key (sort(keys %ENV)) {
1958 print $key, '=', $ENV{$key}, "\n";
1959 }
1960
aca803df 1961To sort a hash by value, you'll need to use a C<sort()> function.
aa689395 1962Here's a descending numeric sort of a hash by its values:
4633a7c4 1963
5a964f20 1964 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
4633a7c4
LW
1965 printf "%4d %s\n", $hash{$key}, $key;
1966 }
1967
ab4f32c2 1968As an lvalue C<keys()> allows you to increase the number of hash buckets
aa689395
PP
1969allocated for the given hash. This can gain you a measure of efficiency if
1970you know the hash is going to get big. (This is similar to pre-extending
1971an array by assigning a larger number to $#array.) If you say
55497cff
PP
1972
1973 keys %hash = 200;
1974
ab192400
GS
1975then C<%hash> will have at least 200 buckets allocated for it--256 of them,
1976in fact, since it rounds up to the next power of two. These
55497cff
PP
1977buckets will be retained even if you do C<%hash = ()>, use C<undef
1978%hash> if you want to free the storage while C<%hash> is still in scope.
1979You can't shrink the number of buckets allocated for the hash using
ab4f32c2 1980C<keys()> in this way (but you needn't worry about doing this by accident,
55497cff
PP
1981as trying has no effect).
1982
ab192400
GS
1983See also C<each()>, C<values()> and C<sort()>.
1984
a0d0e21e
LW
1985=item kill LIST
1986
54310121
PP
1987Sends a signal to a list of processes. The first element of
1988the list must be the signal to send. Returns the number of
4633a7c4 1989processes successfully signaled.
a0d0e21e
LW
1990
1991 $cnt = kill 1, $child1, $child2;
1992 kill 9, @goners;
1993
4633a7c4
LW
1994Unlike in the shell, in Perl if the I<SIGNAL> is negative, it kills
1995process groups instead of processes. (On System V, a negative I<PROCESS>
1996number will also kill process groups, but that's not portable.) That
1997means you usually want to use positive not negative signals. You may also
da0045b7 1998use a signal name in quotes. See L<perlipc/"Signals"> for details.
a0d0e21e
LW
1999
2000=item last LABEL
2001
2002=item last
2003
2004The C<last> command is like the C<break> statement in C (as used in
2005loops); it immediately exits the loop in question. If the LABEL is
2006omitted, the command refers to the innermost enclosing loop. The
2007C<continue> block, if any, is not executed:
2008
4633a7c4
LW
2009 LINE: while (<STDIN>) {
2010 last LINE if /^$/; # exit when done with header
5a964f20 2011 #...
a0d0e21e
LW
2012 }
2013
4968c1e4
G
2014C<last> cannot be used to exit a block which returns a value such as
2015C<eval {}>, C<sub {}> or C<do {}>.
2016
98293880
JH
2017See also L</continue> for an illustration of how C<last>, C<next>, and
2018C<redo> work.
1d2dff63 2019
a0d0e21e
LW
2020=item lc EXPR
2021
54310121 2022=item lc
bbce6d69 2023
a0d0e21e 2024Returns an lowercased version of EXPR. This is the internal function
7660c0ab 2025implementing the C<\L> escape in double-quoted strings.
a0ed51b3 2026Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
a0d0e21e 2027
7660c0ab 2028If EXPR is omitted, uses C<$_>.
bbce6d69 2029
a0d0e21e
LW
2030=item lcfirst EXPR
2031
54310121 2032=item lcfirst
bbce6d69 2033
a0d0e21e 2034Returns the value of EXPR with the first character lowercased. This is
7660c0ab 2035the internal function implementing the C<\l> escape in double-quoted strings.
a0ed51b3 2036Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
a0d0e21e 2037
7660c0ab 2038If EXPR is omitted, uses C<$_>.
bbce6d69 2039
a0d0e21e
LW
2040=item length EXPR
2041
54310121 2042=item length
bbce6d69 2043
a0ed51b3 2044Returns the length in characters of the value of EXPR. If EXPR is
7660c0ab 2045omitted, returns length of C<$_>.
a0d0e21e
LW
2046
2047=item link OLDFILE,NEWFILE
2048
5a964f20
TC
2049Creates a new filename linked to the old filename. Returns TRUE for
2050success, FALSE otherwise.
a0d0e21e
LW
2051
2052=item listen SOCKET,QUEUESIZE
2053
2054Does the same thing that the listen system call does. Returns TRUE if
4633a7c4 2055it succeeded, FALSE otherwise. See example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
2056
2057=item local EXPR
2058
5a964f20
TC
2059A local modifies the listed variables to be local to the enclosing
2060block, file, or eval. If more than one value is listed, the list must
2061be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2062for details, including issues with tied arrays and hashes.
a0d0e21e 2063
7660c0ab 2064You really probably want to be using C<my()> instead, because C<local()> isn't
7b8d334a 2065what most people think of as "local". See L<perlsub/"Private Variables
cb1a09d0 2066via my()"> for details.
a0d0e21e
LW
2067
2068=item localtime EXPR
2069
2070Converts a time as returned by the time function to a 9-element array
5f05dabc 2071with the time analyzed for the local time zone. Typically used as
a0d0e21e
LW
2072follows:
2073
54310121 2074 # 0 1 2 3 4 5 6 7 8
a0d0e21e
LW
2075 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2076 localtime(time);
2077
2078All array elements are numeric, and come straight out of a struct tm.
7660c0ab
A
2079In particular this means that C<$mon> has the range C<0..11> and C<$wday> has
2080the range C<0..6> with sunday as day C<0>. Also, C<$year> is the number of
2081years 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
2082
2083If EXPR is omitted, uses the current time (C<localtime(time)>).
a0d0e21e 2084
f86cebdf 2085In scalar context, returns the ctime(3) value:
a0d0e21e 2086
5f05dabc 2087 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
a0d0e21e 2088
a3cb178b 2089This scalar value is B<not> locale dependent, see L<perllocale>, but
7660c0ab 2090instead a Perl builtin. Also see the C<Time::Local> module, and the
f86cebdf 2091strftime(3) and mktime(3) function available via the POSIX module. To
a3cb178b
GS
2092get somewhat similar but locale dependent date strings, set up your
2093locale environment variables appropriately (please see L<perllocale>)
5a964f20 2094and try for example:
a3cb178b 2095
5a964f20 2096 use POSIX qw(strftime);
a3cb178b
GS
2097 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
2098
2099Note that the C<%a> and C<%b>, the short forms of the day of the week
2100and the month of the year, may not necessarily be three characters wide.
a0d0e21e
LW
2101
2102=item log EXPR
2103
54310121 2104=item log
bbce6d69 2105
5a964f20 2106Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted, returns log
7660c0ab 2107of C<$_>.
a0d0e21e
LW
2108
2109=item lstat FILEHANDLE
2110
2111=item lstat EXPR
2112
54310121 2113=item lstat
bbce6d69 2114
7660c0ab 2115Does the same thing as the C<stat()> function (including setting the
5a964f20
TC
2116special C<_> filehandle) but stats a symbolic link instead of the file
2117the symbolic link points to. If symbolic links are unimplemented on
7660c0ab 2118your system, a normal C<stat()> is done.
a0d0e21e 2119
7660c0ab 2120If EXPR is omitted, stats C<$_>.
bbce6d69 2121
a0d0e21e
LW
2122=item m//
2123
2124The match operator. See L<perlop>.
2125
2126=item map BLOCK LIST
2127
2128=item map EXPR,LIST
2129
7660c0ab 2130Evaluates the BLOCK or EXPR for each element of LIST (locally setting C<$_> to each
a0d0e21e
LW
2131element) and returns the list value composed of the results of each such
2132evaluation. Evaluates BLOCK or EXPR in a list context, so each element of LIST
2133may produce zero, one, or more elements in the returned value.
2134
2135 @chars = map(chr, @nums);
2136
2137translates a list of numbers to the corresponding characters. And
2138
4633a7c4 2139 %hash = map { getkey($_) => $_ } @array;
a0d0e21e
LW
2140
2141is just a funny way to write
2142
2143 %hash = ();
2144 foreach $_ (@array) {
4633a7c4 2145 $hash{getkey($_)} = $_;
a0d0e21e
LW
2146 }
2147
7660c0ab 2148Note that, because C<$_> is a reference into the list value, it can be used
fb73857a
PP
2149to modify the elements of the array. While this is useful and
2150supported, it can cause bizarre results if the LIST is not a named
2151array. See also L</grep> for an array composed of those items of the
2152original list for which the BLOCK or EXPR evaluates to true.
2153
a0d0e21e
LW
2154=item mkdir FILENAME,MODE
2155
0591cd52
NT
2156Creates the directory specified by FILENAME, with permissions
2157specified by MODE (as modified by C<umask>). If it succeeds it
2158returns TRUE, otherwise it returns FALSE and sets C<$!> (errno).
2159
2160In general, it is better to create directories with permissive MODEs,
2161and let the user modify that with their C<umask>, than it is to supply
2162a restrictive MODE and give the user no way to be more permissive.
2163The exceptions to this rule are when the file or directory should be
2164kept private (mail files, for instance). The perlfunc(1) entry on
2165C<umask> discusses the choice of MODE in more detail.
a0d0e21e
LW
2166
2167=item msgctl ID,CMD,ARG
2168
f86cebdf 2169Calls the System V IPC function msgctl(2). You'll probably have to say
0ade1984
JH
2170
2171 use IPC::SysV;
2172
7660c0ab
A
2173first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2174then ARG must be a variable which will hold the returned C<msqid_ds>
ab4f32c2 2175structure. Returns like C<ioctl()>: the undefined value for error, "C<0> but
0ade1984 2176true" for zero, or the actual return value otherwise. See also
7660c0ab 2177C<IPC::SysV> and C<IPC::Semaphore::Msg> documentation.
a0d0e21e
LW
2178
2179=item msgget KEY,FLAGS
2180
f86cebdf 2181Calls the System V IPC function msgget(2). Returns the message queue
7660c0ab
A
2182id, or the undefined value if there is an error. See also C<IPC::SysV>
2183and C<IPC::SysV::Msg> documentation.
a0d0e21e
LW
2184
2185=item msgsnd ID,MSG,FLAGS
2186
2187Calls the System V IPC function msgsnd to send the message MSG to the
2188message queue ID. MSG must begin with the long integer message type,
c07a80fd 2189which may be created with C<pack("l", $type)>. Returns TRUE if
7660c0ab
A
2190successful, or FALSE if there is an error. See also C<IPC::SysV>
2191and C<IPC::SysV::Msg> documentation.
a0d0e21e
LW
2192
2193=item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2194
2195Calls the System V IPC function msgrcv to receive a message from
2196message queue ID into variable VAR with a maximum message size of
0ade1984
JH
2197SIZE. Note that if a message is received, the message type will be
2198the first thing in VAR, and the maximum length of VAR is SIZE plus the
2199size of the message type. Returns TRUE if successful, or FALSE if
7660c0ab 2200there is an error. See also C<IPC::SysV> and C<IPC::SysV::Msg> documentation.
a0d0e21e
LW
2201
2202=item my EXPR
2203
ab4f32c2
A
2204A C<my()> declares the listed variables to be local (lexically) to the
2205enclosing block, file, or C<eval()>. If
5f05dabc 2206more than one value is listed, the list must be placed in parentheses. See
cb1a09d0 2207L<perlsub/"Private Variables via my()"> for details.
4633a7c4 2208
a0d0e21e
LW
2209=item next LABEL
2210
2211=item next
2212
2213The C<next> command is like the C<continue> statement in C; it starts
2214the next iteration of the loop:
2215
4633a7c4
LW
2216 LINE: while (<STDIN>) {
2217 next LINE if /^#/; # discard comments
5a964f20 2218 #...
a0d0e21e
LW
2219 }
2220
2221Note that if there were a C<continue> block on the above, it would get
2222executed even on discarded lines. If the LABEL is omitted, the command
2223refers to the innermost enclosing loop.
2224
4968c1e4
G
2225C<next> cannot be used to exit a block which returns a value such as
2226C<eval {}>, C<sub {}> or C<do {}>.
2227
98293880
JH
2228See also L</continue> for an illustration of how C<last>, C<next>, and
2229C<redo> work.
1d2dff63 2230
a0d0e21e
LW
2231=item no Module LIST
2232
7660c0ab 2233See the L</use> function, which C<no> is the opposite of.
a0d0e21e
LW
2234
2235=item oct EXPR
2236
54310121 2237=item oct
bbce6d69 2238
4633a7c4 2239Interprets EXPR as an octal string and returns the corresponding
4f19785b
WSI
2240value. (If EXPR happens to start off with C<0x>, interprets it as a
2241hex string. If EXPR starts off with C<0b>, it is interpreted as a
2242binary string.) The following will handle decimal, binary, octal, and
4633a7c4 2243hex in the standard Perl or C notation:
a0d0e21e
LW
2244
2245 $val = oct($val) if $val =~ /^0/;
2246
7660c0ab
A
2247If EXPR is omitted, uses C<$_>. This function is commonly used when
2248a string such as C<644> needs to be converted into a file mode, for
2f9daede
TPG
2249example. (Although perl will automatically convert strings into
2250numbers as needed, this automatic conversion assumes base 10.)
a0d0e21e
LW
2251
2252=item open FILEHANDLE,EXPR
2253
2254=item open FILEHANDLE
2255
2256Opens the file whose filename is given by EXPR, and associates it with
5f05dabc
PP
2257FILEHANDLE. If FILEHANDLE is an expression, its value is used as the
2258name of the real filehandle wanted. If EXPR is omitted, the scalar
2259variable of the same name as the FILEHANDLE contains the filename.
ab4f32c2
A
2260(Note that lexical variables--those declared with C<my()>--will not work
2261for this purpose; so if you're using C<my()>, specify EXPR in your call
5f05dabc
PP
2262to open.)
2263
7660c0ab
A
2264If the filename begins with C<'E<lt>'> or nothing, the file is opened for input.
2265If the filename begins with C<'E<gt>'>, the file is truncated and opened for
2266output, being created if necessary. If the filename begins with C<'E<gt>E<gt>'>,
fbb426e4 2267the file is opened for appending, again being created if necessary.
7660c0ab
A
2268You can put a C<'+'> in front of the C<'E<gt>'> or C<'E<lt>'> to indicate that
2269you want both read and write access to the file; thus C<'+E<lt>'> is almost
2270always preferred for read/write updates--the C<'+E<gt>'> mode would clobber the
5a964f20
TC
2271file first. You can't usually use either read-write mode for updating
2272textfiles, since they have variable length records. See the B<-i>
0591cd52
NT
2273switch in L<perlrun> for a better approach. The file is created with
2274permissions of C<0666> modified by the process' C<umask> value.
5a964f20
TC
2275
2276The prefix and the filename may be separated with spaces.
f86cebdf 2277These various prefixes correspond to the fopen(3) modes of C<'r'>, C<'r+'>, C<'w'>,
7660c0ab 2278C<'w+'>, C<'a'>, and C<'a+'>.
5f05dabc 2279
7660c0ab 2280If the filename begins with C<'|'>, the filename is interpreted as a
5a964f20 2281command to which output is to be piped, and if the filename ends with a
f244e06d
GS
2282C<'|'>, the filename is interpreted as a command which pipes output to
2283us. See L<perlipc/"Using open() for IPC">
7660c0ab 2284for more examples of this. (You are not allowed to C<open()> to a command
5a964f20
TC
2285that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2286and L<perlipc/"Bidirectional Communication"> for alternatives.)
cb1a09d0 2287
7660c0ab 2288Opening C<'-'> opens STDIN and opening C<'E<gt>-'> opens STDOUT. Open returns
ab4f32c2 2289nonzero upon success, the undefined value otherwise. If the C<open()>
4633a7c4 2290involved a pipe, the return value happens to be the pid of the
54310121 2291subprocess.
cb1a09d0
AD
2292
2293If you're unfortunate enough to be running Perl on a system that
2294distinguishes between text files and binary files (modern operating
2295systems don't care), then you should check out L</binmode> for tips for
ab4f32c2 2296dealing with this. The key distinction between systems that need C<binmode()>
5a964f20
TC
2297and those that don't is their text file formats. Systems like Unix, MacOS, and
2298Plan9, which delimit lines with a single character, and which encode that
ab4f32c2 2299character in C as C<"\n">, do not need C<binmode()>. The rest need it.
cb1a09d0 2300
fb73857a 2301When opening a file, it's usually a bad idea to continue normal execution
ab4f32c2
A
2302if the request failed, so C<open()> is frequently used in connection with
2303C<die()>. Even if C<die()> won't do what you want (say, in a CGI script,
fb73857a 2304where you want to make a nicely formatted error message (but there are
5a964f20 2305modules that can help with that problem)) you should always check
fb73857a
PP
2306the return value from opening a file. The infrequent exception is when
2307working with an unopened filehandle is actually what you want to do.
2308
cb1a09d0 2309Examples:
a0d0e21e
LW
2310
2311 $ARTICLE = 100;
2312 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2313 while (<ARTICLE>) {...
2314
2315 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
fb73857a 2316 # if the open fails, output is discarded
a0d0e21e 2317
fb73857a
PP
2318 open(DBASE, '+<dbase.mine') # open for update
2319 or die "Can't open 'dbase.mine' for update: $!";
cb1a09d0 2320
fb73857a
PP
2321 open(ARTICLE, "caesar <$article |") # decrypt article
2322 or die "Can't start caesar: $!";
a0d0e21e 2323
fb73857a
PP
2324 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
2325 or die "Can't start sort: $!";
a0d0e21e
LW
2326
2327 # process argument list of files along with any includes
2328
2329 foreach $file (@ARGV) {
2330 process($file, 'fh00');
2331 }
2332
2333 sub process {
5a964f20 2334 my($filename, $input) = @_;
a0d0e21e
LW
2335 $input++; # this is a string increment
2336 unless (open($input, $filename)) {
2337 print STDERR "Can't open $filename: $!\n";
2338 return;
2339 }
2340
5a964f20 2341 local $_;
a0d0e21e
LW
2342 while (<$input>) { # note use of indirection
2343 if (/^#include "(.*)"/) {
2344 process($1, $input);
2345 next;
2346 }
5a964f20 2347 #... # whatever
a0d0e21e
LW
2348 }
2349 }
2350
2351You may also, in the Bourne shell tradition, specify an EXPR beginning
7660c0ab 2352with C<'E<gt>&'>, in which case the rest of the string is interpreted as the
5a964f20 2353name of a filehandle (or file descriptor, if numeric) to be
7660c0ab
A
2354duped and opened. You may use C<&> after C<E<gt>>, C<E<gt>E<gt>>, C<E<lt>>, C<+E<gt>>,
2355C<+E<gt>E<gt>>, and C<+E<lt>>. The
a0d0e21e 2356mode you specify should match the mode of the original filehandle.
184e9718 2357(Duping a filehandle does not take into account any existing contents of
cb1a09d0 2358stdio buffers.)
a0d0e21e
LW
2359Here is a script that saves, redirects, and restores STDOUT and
2360STDERR:
2361
2362 #!/usr/bin/perl
5a964f20
TC
2363 open(OLDOUT, ">&STDOUT");
2364 open(OLDERR, ">&STDERR");
a0d0e21e
LW
2365
2366 open(STDOUT, ">foo.out") || die "Can't redirect stdout";
2367 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
2368
2369 select(STDERR); $| = 1; # make unbuffered
2370 select(STDOUT); $| = 1; # make unbuffered
2371
2372 print STDOUT "stdout 1\n"; # this works for
2373 print STDERR "stderr 1\n"; # subprocesses too
2374
2375 close(STDOUT);
2376 close(STDERR);
2377
5a964f20
TC
2378 open(STDOUT, ">&OLDOUT");
2379 open(STDERR, ">&OLDERR");
a0d0e21e
LW
2380
2381 print STDOUT "stdout 2\n";
2382 print STDERR "stderr 2\n";
2383
2384
7660c0ab
A
2385If you specify C<'E<lt>&=N'>, where C<N> is a number, then Perl will do an
2386equivalent of C's C<fdopen()> of that file descriptor; this is more
4633a7c4 2387parsimonious of file descriptors. For example:
a0d0e21e
LW
2388
2389 open(FILEHANDLE, "<&=$fd")
2390
7660c0ab 2391If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>, then
a0d0e21e 2392there is an implicit fork done, and the return value of open is the pid
7660c0ab 2393of the child within the parent process, and C<0> within the child
184e9718 2394process. (Use C<defined($pid)> to determine whether the open was successful.)
a0d0e21e
LW
2395The filehandle behaves normally for the parent, but i/o to that
2396filehandle is piped from/to the STDOUT/STDIN of the child process.
2397In the child process the filehandle isn't opened--i/o happens from/to
2398the new STDOUT or STDIN. Typically this is used like the normal
2399piped open when you want to exercise more control over just how the
2400pipe command gets executed, such as when you are running setuid, and
54310121 2401don't want to have to scan shell commands for metacharacters.
4633a7c4 2402The following pairs are more or less equivalent:
a0d0e21e
LW
2403
2404 open(FOO, "|tr '[a-z]' '[A-Z]'");
2405 open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
2406
2407 open(FOO, "cat -n '$file'|");
2408 open(FOO, "-|") || exec 'cat', '-n', $file;
2409
4633a7c4
LW
2410See L<perlipc/"Safe Pipe Opens"> for more examples of this.
2411
5a964f20 2412NOTE: On any operation that may do a fork, any unflushed buffers remain
184e9718 2413unflushed in both processes, which means you may need to set C<$|> to
4771b018
GS
2414avoid duplicate output. On systems that support a close-on-exec flag on
2415files, the flag will be set for the newly opened file descriptor as
2416determined by the value of $^F. See L<perlvar/$^F>.
a0d0e21e 2417
0dccf244
CS
2418Closing any piped filehandle causes the parent process to wait for the
2419child to finish, and returns the status value in C<$?>.
2420
5a964f20 2421The filename passed to open will have leading and trailing
f86cebdf 2422whitespace deleted, and the normal redirection characters
5a964f20
TC
2423honored. This property, known as "magic open",
2424can often be used to good effect. A user could specify a filename of
7660c0ab 2425F<"rsh cat file |">, or you could change certain filenames as needed:
5a964f20
TC
2426
2427 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
2428 open(FH, $filename) or die "Can't open $filename: $!";
2429
2430However, to open a file with arbitrary weird characters in it, it's
2431necessary to protect any leading and trailing whitespace:
2432
2433 $file =~ s#^(\s)#./$1#;
2434 open(FOO, "< $file\0");
2435
7660c0ab
A
2436If you want a "real" C C<open()> (see L<open(2)> on your system), then you
2437should use the C<sysopen()> function, which involves no such magic. This is
5a964f20
TC
2438another way to protect your filenames from interpretation. For example:
2439
2440 use IO::Handle;
2441 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
2442 or die "sysopen $path: $!";
2443 $oldfh = select(HANDLE); $| = 1; select($oldfh);
2444 print HANDLE "stuff $$\n");
2445 seek(HANDLE, 0, 0);
2446 print "File contains: ", <HANDLE>;
2447
7660c0ab
A
2448Using the constructor from the C<IO::Handle> package (or one of its
2449subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
5a964f20
TC
2450filehandles that have the scope of whatever variables hold references to
2451them, and automatically close whenever and however you leave that scope:
c07a80fd 2452
5f05dabc 2453 use IO::File;
5a964f20 2454 #...
c07a80fd
PP
2455 sub read_myfile_munged {
2456 my $ALL = shift;
5f05dabc 2457 my $handle = new IO::File;
c07a80fd
PP
2458 open($handle, "myfile") or die "myfile: $!";
2459 $first = <$handle>
2460 or return (); # Automatically closed here.
2461 mung $first or die "mung failed"; # Or here.
2462 return $first, <$handle> if $ALL; # Or here.
2463 $first; # Or here.
2464 }
2465
b687b08b 2466See L</seek> for some details about mixing reading and writing.
a0d0e21e
LW
2467
2468=item opendir DIRHANDLE,EXPR
2469
7660c0ab
A
2470Opens a directory named EXPR for processing by C<readdir()>, C<telldir()>,
2471C<seekdir()>, C<rewinddir()>, and C<closedir()>. Returns TRUE if successful.
a0d0e21e
LW
2472DIRHANDLEs have their own namespace separate from FILEHANDLEs.
2473
2474=item ord EXPR
2475
54310121 2476=item ord
bbce6d69 2477
a0ed51b3 2478Returns the numeric (ASCII or Unicode) value of the first character of EXPR. If
7660c0ab 2479EXPR is omitted, uses C<$_>. For the reverse, see L</chr>.
a0d0e21e
LW
2480
2481=item pack TEMPLATE,LIST
2482
2483Takes an array or list of values and packs it into a binary structure,
2484returning the string containing the structure. The TEMPLATE is a
2485sequence of characters that give the order and type of values, as
2486follows:
2487
2488 A An ascii string, will be space padded.
2489 a An ascii string, will be null padded.
2490 b A bit string (ascending bit order, like vec()).
2491 B A bit string (descending bit order).
2492 h A hex string (low nybble first).
2493 H A hex string (high nybble first).
2494
2495 c A signed char value.
a0ed51b3 2496 C An unsigned char value. Only does bytes. See U for Unicode.
96e4d5b1 2497
a0d0e21e
LW
2498 s A signed short value.
2499 S An unsigned short value.
96e4d5b1
PP
2500 (This 'short' is _exactly_ 16 bits, which may differ from
2501 what a local C compiler calls 'short'.)
2502
a0d0e21e
LW
2503 i A signed integer value.
2504 I An unsigned integer value.
f86cebdf
GS
2505 (This 'integer' is _at_least_ 32 bits wide. Its exact
2506 size depends on what a local C compiler calls 'int',
2507 and may even be larger than the 'long' described in
2508 the next item.)
96e4d5b1 2509
a0d0e21e
LW
2510 l A signed long value.
2511 L An unsigned long value.
96e4d5b1
PP
2512 (This 'long' is _exactly_ 32 bits, which may differ from
2513 what a local C compiler calls 'long'.)
a0d0e21e 2514
96e4d5b1
PP
2515 n A short in "network" (big-endian) order.
2516 N A long in "network" (big-endian) order.
a0d0e21e
LW
2517 v A short in "VAX" (little-endian) order.
2518 V A long in "VAX" (little-endian) order.
96e4d5b1
PP
2519 (These 'shorts' and 'longs' are _exactly_ 16 bits and
2520 _exactly_ 32 bits, respectively.)
a0d0e21e 2521
dae0da7a
JH
2522 q A signed quad (64-bit) value.
2523 Q An unsigned quad value.
2524 (Available only if your system supports 64-bit integer values
2525 _and_ if Perl has been compiled to support those.
2526 Causes a fatal error otherwise.)
2527
a0d0e21e
LW
2528 f A single-precision float in the native format.
2529 d A double-precision float in the native format.
2530
2531 p A pointer to a null-terminated string.
2532 P A pointer to a structure (fixed-length string).
2533
2534 u A uuencoded string.
a0ed51b3
LW
2535 U A Unicode character number. Encodes to UTF-8 internally.
2536 Works even if C<use utf8> is not in effect.
a0d0e21e 2537
96e4d5b1 2538 w A BER compressed integer. Its bytes represent an unsigned
f86cebdf
GS
2539 integer in base 128, most significant digit first, with as
2540 few digits as possible. Bit eight (the high bit) is set
2541 on each byte except the last.
def98dd4 2542
a0d0e21e
LW
2543 x A null byte.
2544 X Back up a byte.
2545 @ Null fill to absolute position.
2546
5a964f20 2547Each letter may optionally be followed by a number giving a repeat
7660c0ab
A
2548count. With all types except C<"a">, C<"A">, C<"b">, C<"B">, C<"h">, C<"H">, and C<"P"> the
2549pack function will gobble up that many values from the LIST. A C<*> for the
2550repeat count means to use however many items are left. The C<"a"> and C<"A">
a0d0e21e 2551types gobble just one value, but pack it as a string of length count,
7660c0ab
A
2552padding with nulls or spaces as necessary. (When unpacking, C<"A"> strips
2553trailing spaces and nulls, but C<"a"> does not.) Likewise, the C<"b"> and C<"B">
2554fields pack a string that many bits long. The C<"h"> and C<"H"> fields pack a
2555string that many nybbles long. The C<"p"> type packs a pointer to a null-
84902520
TB
2556terminated string. You are responsible for ensuring the string is not a
2557temporary value (which can potentially get deallocated before you get
7660c0ab 2558around to using the packed result). The C<"P"> packs a pointer to a structure
61167c6f 2559of the size indicated by the length. A NULL pointer is created if the
7660c0ab 2560corresponding value for C<"p"> or C<"P"> is C<undef>.
61167c6f 2561Real numbers (floats and doubles) are
a0d0e21e
LW
2562in the native machine format only; due to the multiplicity of floating
2563formats around, and the lack of a standard "network" representation, no
2564facility for interchange has been made. This means that packed floating
2565point data written on one machine may not be readable on another - even if
2566both use IEEE floating point arithmetic (as the endian-ness of the memory
2567representation is not part of the IEEE spec). Note that Perl uses doubles
2568internally for all numeric calculation, and converting from double into
5f05dabc 2569float and thence back to double again will lose precision (i.e.,
7660c0ab 2570C<unpack("f", pack("f", $foo)>) will not in general equal C<$foo>).
a0d0e21e
LW
2571
2572Examples:
2573
a0ed51b3 2574 $foo = pack("CCCC",65,66,67,68);
a0d0e21e 2575 # foo eq "ABCD"
a0ed51b3 2576 $foo = pack("C4",65,66,67,68);
a0d0e21e 2577 # same thing
a0ed51b3
LW
2578 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
2579 # same thing with Unicode circled letters
a0d0e21e
LW
2580
2581 $foo = pack("ccxxcc",65,66,67,68);
2582 # foo eq "AB\0\0CD"
2583
2584 $foo = pack("s2",1,2);
2585 # "\1\0\2\0" on little-endian
2586 # "\0\1\0\2" on big-endian
2587
2588 $foo = pack("a4","abcd","x","y","z");
2589 # "abcd"
2590
2591 $foo = pack("aaaa","abcd","x","y","z");
2592 # "axyz"
2593
2594 $foo = pack("a14","abcdefg");
2595 # "abcdefg\0\0\0\0\0\0\0"
2596
2597 $foo = pack("i9pl", gmtime);
2598 # a real struct tm (on my system anyway)
2599
2600 sub bintodec {
2601 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
2602 }
2603
2604The same template may generally also be used in the unpack function.
2605
5a964f20
TC
2606=item package
2607
cb1a09d0
AD
2608=item package NAMESPACE
2609
2610Declares the compilation unit as being in the given namespace. The scope
2611of the package declaration is from the declaration itself through the end of
7660c0ab 2612the enclosing block (the same scope as the C<local()> operator). All further
cb1a09d0 2613unqualified dynamic identifiers will be in this namespace. A package
5f05dabc 2614statement affects only dynamic variables--including those you've used
7660c0ab 2615C<local()> on--but I<not> lexical variables created with C<my()>. Typically it
cb1a09d0
AD
2616would be the first declaration in a file to be included by the C<require>
2617or C<use> operator. You can switch into a package in more than one place;
5a964f20 2618it merely influences which symbol table is used by the compiler for the
cb1a09d0
AD
2619rest of that block. You can refer to variables and filehandles in other
2620packages by prefixing the identifier with the package name and a double
2621colon: C<$Package::Variable>. If the package name is null, the C<main>
2622package as assumed. That is, C<$::sail> is equivalent to C<$main::sail>.
2623
5a964f20
TC
2624If NAMESPACE is omitted, then there is no current package, and all
2625identifiers must be fully qualified or lexicals. This is stricter
2626than C<use strict>, since it also extends to function names.
2627
cb1a09d0
AD
2628See L<perlmod/"Packages"> for more information about packages, modules,
2629and classes. See L<perlsub> for other scoping issues.
2630
a0d0e21e
LW
2631=item pipe READHANDLE,WRITEHANDLE
2632
2633Opens a pair of connected pipes like the corresponding system call.
2634Note that if you set up a loop of piped processes, deadlock can occur
2635unless you are very careful. In addition, note that Perl's pipes use
184e9718 2636stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE
a0d0e21e
LW
2637after each command, depending on the application.
2638
7e1af8bc 2639See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
4633a7c4
LW
2640for examples of such things.
2641
4771b018
GS
2642On systems that support a close-on-exec flag on files, the flag will be set
2643for the newly opened file descriptors as determined by the value of $^F.
2644See L<perlvar/$^F>.
2645
a0d0e21e
LW
2646=item pop ARRAY
2647
54310121 2648=item pop
28757baa 2649
a0d0e21e
LW
2650Pops and returns the last value of the array, shortening the array by
26511. Has a similar effect to
2652
2653 $tmp = $ARRAY[$#ARRAY--];
2654
2655If there are no elements in the array, returns the undefined value.
cb1a09d0 2656If ARRAY is omitted, pops the
7660c0ab
A
2657C<@ARGV> array in the main program, and the C<@_> array in subroutines, just
2658like C<shift()>.
a0d0e21e
LW
2659
2660=item pos SCALAR
2661
54310121 2662=item pos
bbce6d69 2663
4633a7c4 2664Returns the offset of where the last C<m//g> search left off for the variable
7660c0ab 2665is in question (C<$_> is used when the variable is not specified). May be
44a8e56a
PP
2666modified to change that offset. Such modification will also influence
2667the C<\G> zero-width assertion in regular expressions. See L<perlre> and
2668L<perlop>.
a0d0e21e
LW
2669
2670=item print FILEHANDLE LIST
2671
2672=item print LIST
2673
2674=item print
2675
cb1a09d0 2676Prints a string or a comma-separated list of strings. Returns TRUE
a0d0e21e 2677if successful. FILEHANDLE may be a scalar variable name, in which case
cb1a09d0 2678the variable contains the name of or a reference to the filehandle, thus introducing one
a0d0e21e
LW
2679level of indirection. (NOTE: If FILEHANDLE is a variable and the next
2680token is a term, it may be misinterpreted as an operator unless you
7660c0ab 2681interpose a C<+> or put parentheses around the arguments.) If FILEHANDLE is
a0d0e21e 2682omitted, prints by default to standard output (or to the last selected
7660c0ab 2683output channel--see L</select>). If LIST is also omitted, prints C<$_> to
5a964f20 2684the currently selected output channel. To set the default output channel to something other than
a0d0e21e 2685STDOUT use the select operation. Note that, because print takes a
5a964f20 2686LIST, anything in the LIST is evaluated in list context, and any
a0d0e21e 2687subroutine that you call will have one or more of its expressions
5a964f20 2688evaluated in list context. Also be careful not to follow the print
a0d0e21e 2689keyword with a left parenthesis unless you want the corresponding right
7660c0ab 2690parenthesis to terminate the arguments to the print--interpose a C<+> or
5f05dabc 2691put parentheses around all the arguments.
a0d0e21e 2692
4633a7c4 2693Note that if you're storing FILEHANDLES in an array or other expression,
da0045b7 2694you will have to use a block returning its value instead:
4633a7c4
LW
2695
2696 print { $files[$i] } "stuff\n";
2697 print { $OK ? STDOUT : STDERR } "stuff\n";
2698
5f05dabc 2699=item printf FILEHANDLE FORMAT, LIST
a0d0e21e 2700
5f05dabc 2701=item printf FORMAT, LIST
a0d0e21e 2702
7660c0ab 2703Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
a3cb178b 2704(the output record separator) is not appended. The first argument
ab4f32c2 2705of the list will be interpreted as the C<printf()> format. If C<use locale> is
a034a98d
DD
2706in effect, the character used for the decimal point in formatted real numbers
2707is affected by the LC_NUMERIC locale. See L<perllocale>.
a0d0e21e 2708
7660c0ab
A
2709Don't fall into the trap of using a C<printf()> when a simple
2710C<print()> would do. The C<print()> is more efficient and less
28757baa
PP
2711error prone.
2712
da0045b7
PP
2713=item prototype FUNCTION
2714
2715Returns the prototype of a function as a string (or C<undef> if the
5f05dabc
PP
2716function has no prototype). FUNCTION is a reference to, or the name of,
2717the function whose prototype you want to retrieve.
da0045b7 2718
b6c543e3
IZ
2719If FUNCTION is a string starting with C<CORE::>, the rest is taken as
2720a name for Perl builtin. If builtin is not I<overridable> (such as
ab4f32c2
A
2721C<qw//>) or its arguments cannot be expressed by a prototype (such as
2722C<system()>) - in other words, the builtin does not behave like a Perl
b6c543e3
IZ
2723function - returns C<undef>. Otherwise, the string describing the
2724equivalent prototype is returned.
2725
a0d0e21e
LW
2726=item push ARRAY,LIST
2727
2728Treats ARRAY as a stack, and pushes the values of LIST
2729onto the end of ARRAY. The length of ARRAY increases by the length of
2730LIST. Has the same effect as
2731
2732 for $value (LIST) {
2733 $ARRAY[++$#ARRAY] = $value;
2734 }
2735
2736but is more efficient. Returns the new number of elements in the array.
2737
2738=item q/STRING/
2739
2740=item qq/STRING/
2741
8782bef2
GB
2742=item qr/STRING/
2743
a0d0e21e
LW
2744=item qx/STRING/
2745
2746=item qw/STRING/
2747
4b6a7270 2748Generalized quotes. See L<perlop/"Regexp Quote-Like Operators">.
a0d0e21e
LW
2749
2750=item quotemeta EXPR
2751
54310121 2752=item quotemeta
bbce6d69 2753
68dc0745 2754Returns the value of EXPR with all non-alphanumeric
a034a98d
DD
2755characters backslashed. (That is, all characters not matching
2756C</[A-Za-z_0-9]/> will be preceded by a backslash in the
2757returned string, regardless of any locale settings.)
2758This is the internal function implementing
7660c0ab 2759the C<\Q> escape in double-quoted strings.
a0d0e21e 2760
7660c0ab 2761If EXPR is omitted, uses C<$_>.
bbce6d69 2762
a0d0e21e
LW
2763=item rand EXPR
2764
2765=item rand
2766
7660c0ab 2767Returns a random fractional number greater than or equal to C<0> and less
3e3baf6d 2768than the value of EXPR. (EXPR should be positive.) If EXPR is
7660c0ab
A
2769omitted, the value C<1> is used. Automatically calls C<srand()> unless
2770C<srand()> has already been called. See also C<srand()>.
a0d0e21e 2771
2f9daede 2772(Note: If your rand function consistently returns numbers that are too
a0d0e21e 2773large or too small, then your version of Perl was probably compiled
2f9daede 2774with the wrong number of RANDBITS.)
a0d0e21e
LW
2775
2776=item read FILEHANDLE,SCALAR,LENGTH,OFFSET
2777
2778=item read FILEHANDLE,SCALAR,LENGTH
2779
2780Attempts to read LENGTH bytes of data into variable SCALAR from the
3b02c43c
GS
2781specified FILEHANDLE. Returns the number of bytes actually read,
2782C<0> at end of file, or undef if there was an error. SCALAR will be grown
2783or shrunk to the length actually read. An OFFSET may be specified to
2784place the read data at some other place than the beginning of the
f86cebdf
GS
2785string. This call is actually implemented in terms of stdio's fread(3)
2786call. To get a true read(2) system call, see C<sysread()>.
a0d0e21e
LW
2787
2788=item readdir DIRHANDLE
2789
7660c0ab 2790Returns the next directory entry for a directory opened by C<opendir()>.
5a964f20 2791If used in list context, returns all the rest of the entries in the
a0d0e21e 2792directory. If there are no more entries, returns an undefined value in
5a964f20 2793scalar context or a null list in list context.
a0d0e21e 2794
7660c0ab 2795If you're planning to filetest the return values out of a C<readdir()>, you'd
5f05dabc 2796better prepend the directory in question. Otherwise, because we didn't
7660c0ab 2797C<chdir()> there, it would have been testing the wrong file.
cb1a09d0
AD
2798
2799 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
2800 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
2801 closedir DIR;
2802
84902520
TB
2803=item readline EXPR
2804
5a964f20 2805Reads from the filehandle whose typeglob is contained in EXPR. In scalar context, a single line
84902520
TB
2806is read and returned. In list context, reads until end-of-file is
2807reached and returns a list of lines (however you've defined lines
7660c0ab 2808with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
84902520
TB
2809This is the internal function implementing the C<E<lt>EXPRE<gt>>
2810operator, but you can use it directly. The C<E<lt>EXPRE<gt>>
2811operator is discussed in more detail in L<perlop/"I/O Operators">.
2812
5a964f20
TC
2813 $line = <STDIN>;
2814 $line = readline(*STDIN); # same thing
2815
a0d0e21e
LW
2816=item readlink EXPR
2817
54310121 2818=item readlink
bbce6d69 2819
a0d0e21e
LW
2820Returns the value of a symbolic link, if symbolic links are
2821implemented. If not, gives a fatal error. If there is some system
184e9718 2822error, returns the undefined value and sets C<$!> (errno). If EXPR is
7660c0ab 2823omitted, uses C<$_>.
a0d0e21e 2824
84902520
TB
2825=item readpipe EXPR
2826
5a964f20 2827EXPR is executed as a system command.
84902520
TB
2828The collected standard output of the command is returned.
2829In scalar context, it comes back as a single (potentially
2830multi-line) string. In list context, returns a list of lines
7660c0ab 2831(however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
84902520
TB
2832This is the internal function implementing the C<qx/EXPR/>
2833operator, but you can use it directly. The C<qx/EXPR/>
2834operator is discussed in more detail in L<perlop/"I/O Operators">.
2835
a0d0e21e
LW
2836=item recv SOCKET,SCALAR,LEN,FLAGS
2837
2838Receives a message on a socket. Attempts to receive LENGTH bytes of
2839data into variable SCALAR from the specified SOCKET filehandle.
7660c0ab 2840Actually does a C C<recvfrom()>, so that it can return the address of the
a0d0e21e
LW
2841sender. Returns the undefined value if there's an error. SCALAR will
2842be grown or shrunk to the length actually read. Takes the same flags
54310121 2843as the system call of the same name.
4633a7c4 2844See L<perlipc/"UDP: Message Passing"> for examples.
a0d0e21e
LW
2845
2846=item redo LABEL
2847
2848=item redo
2849
2850The C<redo> command restarts the loop block without evaluating the
98293880 2851conditional again. The C<continue> block, if any, is not executed. If
a0d0e21e
LW
2852the LABEL is omitted, the command refers to the innermost enclosing
2853loop. This command is normally used by programs that want to lie to
2854themselves about what was just input:
2855
2856 # a simpleminded Pascal comment stripper
2857 # (warning: assumes no { or } in strings)
4633a7c4 2858 LINE: while (<STDIN>) {
a0d0e21e
LW
2859 while (s|({.*}.*){.*}|$1 |) {}
2860 s|{.*}| |;
2861 if (s|{.*| |) {
2862 $front = $_;
2863 while (<STDIN>) {
2864 if (/}/) { # end of comment?
5a964f20 2865 s|^|$front\{|;
4633a7c4 2866 redo LINE;
a0d0e21e
LW
2867 }
2868 }
2869 }
2870 print;
2871 }
2872
4968c1e4
G
2873C<redo> cannot be used to retry a block which returns a value such as
2874C<eval {}>, C<sub {}> or C<do {}>.
2875
98293880 2876See also L</continue> for an illustration of how C<last>, C<next>, and
1d2dff63
GS
2877C<redo> work.
2878
a0d0e21e
LW
2879=item ref EXPR
2880
54310121 2881=item ref
bbce6d69 2882
2f9daede 2883Returns a TRUE value if EXPR is a reference, FALSE otherwise. If EXPR
7660c0ab 2884is not specified, C<$_> will be used. The value returned depends on the
bbce6d69 2885type of thing the reference is a reference to.
a0d0e21e
LW
2886Builtin types include:
2887
2888 REF
2889 SCALAR
2890 ARRAY
2891 HASH
2892 CODE
2893 GLOB
2894
54310121 2895If the referenced object has been blessed into a package, then that package
7660c0ab 2896name is returned instead. You can think of C<ref()> as a C<typeof()> operator.
a0d0e21e
LW
2897
2898 if (ref($r) eq "HASH") {
aa689395 2899 print "r is a reference to a hash.\n";
54310121 2900 }
5a964f20 2901 if (!ref($r)) {
a0d0e21e 2902 print "r is not a reference at all.\n";
54310121 2903 }
a0d0e21e
LW
2904
2905See also L<perlref>.
2906
2907=item rename OLDNAME,NEWNAME
2908
7660c0ab 2909Changes the name of a file. Returns C<1> for success, C<0> otherwise. Will
5f05dabc 2910not work across file system boundaries.
a0d0e21e
LW
2911
2912=item require EXPR
2913
2914=item require
2915
7660c0ab 2916Demands some semantics specified by EXPR, or by C<$_> if EXPR is not
a0d0e21e 2917supplied. If EXPR is numeric, demands that the current version of Perl
184e9718 2918(C<$]> or $PERL_VERSION) be equal or greater than EXPR.
a0d0e21e
LW
2919
2920Otherwise, demands that a library file be included if it hasn't already
2921been included. The file is included via the do-FILE mechanism, which is
7660c0ab 2922essentially just a variety of C<eval()>. Has semantics similar to the following
a0d0e21e
LW
2923subroutine:
2924
2925 sub require {
5a964f20 2926 my($filename) = @_;
a0d0e21e 2927 return 1 if $INC{$filename};
5a964f20 2928 my($realfilename,$result);
a0d0e21e
LW
2929 ITER: {
2930 foreach $prefix (@INC) {
2931 $realfilename = "$prefix/$filename";
2932 if (-f $realfilename) {
2933 $result = do $realfilename;
2934 last ITER;
2935 }
2936 }
2937 die "Can't find $filename in \@INC";
2938 }
2939 die $@ if $@;
2940 die "$filename did not return true value" unless $result;
2941 $INC{$filename} = $realfilename;
5a964f20 2942 return $result;
a0d0e21e
LW
2943 }
2944
2945Note that the file will not be included twice under the same specified
2946name. The file must return TRUE as the last statement to indicate
2947successful execution of any initialization code, so it's customary to
7660c0ab 2948end such a file with "C<1;>" unless you're sure it'll return TRUE
a0d0e21e
LW
2949otherwise. But it's better just to put the "C<1;>", in case you add more
2950statements.
2951
54310121 2952If EXPR is a bareword, the require assumes a "F<.pm>" extension and
da0045b7 2953replaces "F<::>" with "F</>" in the filename for you,
54310121 2954to make it easy to load standard modules. This form of loading of
a0d0e21e
LW
2955modules does not risk altering your namespace.
2956
ee580363
GS
2957In other words, if you try this:
2958
f86cebdf 2959 require Foo::Bar; # a splendid bareword
ee580363 2960
7660c0ab
A
2961The require function will actually look for the "F<Foo/Bar.pm>" file in the
2962directories specified in the C<@INC> array.
ee580363 2963
5a964f20 2964But if you try this:
ee580363
GS
2965
2966 $class = 'Foo::Bar';
f86cebdf 2967 require $class; # $class is not a bareword
5a964f20 2968 #or
f86cebdf 2969 require "Foo::Bar"; # not a bareword because of the ""
ee580363 2970
7660c0ab
A
2971The require function will look for the "F<Foo::Bar>" file in the @INC array and
2972will complain about not finding "F<Foo::Bar>" there. In this case you can do:
ee580363
GS
2973
2974 eval "require $class";
2975
2976For a yet-more-powerful import facility, see L</use> and L<perlmod>.
a0d0e21e
LW
2977
2978=item reset EXPR
2979
2980=item reset
2981
2982Generally used in a C<continue> block at the end of a loop to clear
7660c0ab 2983variables and reset C<??> searches so that they work again. The
a0d0e21e
LW
2984expression is interpreted as a list of single characters (hyphens
2985allowed for ranges). All variables and arrays beginning with one of
2986those letters are reset to their pristine state. If the expression is
7660c0ab 2987omitted, one-match searches (C<?pattern?>) are reset to match again. Resets
5f05dabc 2988only variables or searches in the current package. Always returns
a0d0e21e
LW
29891. Examples:
2990
2991 reset 'X'; # reset all X variables
2992 reset 'a-z'; # reset lower case variables
2993 reset; # just reset ?? searches
2994
7660c0ab
A
2995Resetting C<"A-Z"> is not recommended because you'll wipe out your
2996C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package variables--lexical variables
a0d0e21e 2997are unaffected, but they clean themselves up on scope exit anyway,
da0045b7 2998so you'll probably want to use them instead. See L</my>.
a0d0e21e 2999
54310121
PP
3000=item return EXPR
3001
3002=item return
3003
7660c0ab 3004Returns from a subroutine, C<eval()>, or C<do FILE> with the value
5a964f20 3005given in EXPR. Evaluation of EXPR may be in list, scalar, or void
54310121 3006context, depending on how the return value will be used, and the context
7660c0ab 3007may vary from one execution to the next (see C<wantarray()>). If no EXPR
5a964f20
TC
3008is given, returns an empty list in list context, an undefined value in
3009scalar context, or nothing in a void context.
a0d0e21e 3010
68dc0745
PP
3011(Note that in the absence of a return, a subroutine, eval, or do FILE
3012will automatically return the value of the last expression evaluated.)
a0d0e21e
LW
3013
3014=item reverse LIST
3015
5a964f20
TC
3016In list context, returns a list value consisting of the elements
3017of LIST in the opposite order. In scalar context, concatenates the
a0ed51b3
LW
3018elements of LIST, and returns a string value with all the characters
3019in the opposite order.
4633a7c4 3020
2f9daede 3021 print reverse <>; # line tac, last line first
4633a7c4 3022
2f9daede 3023 undef $/; # for efficiency of <>
a0ed51b3 3024 print scalar reverse <>; # character tac, last line tsrif
2f9daede
TPG
3025
3026This operator is also handy for inverting a hash, although there are some
3027caveats. If a value is duplicated in the original hash, only one of those
3028can be represented as a key in the inverted hash. Also, this has to
3029unwind one hash and build a whole new one, which may take some time
3030on a large hash.
3031
3032 %by_name = reverse %by_address; # Invert the hash
a0d0e21e
LW
3033
3034=item rewinddir DIRHANDLE
3035
3036Sets the current position to the beginning of the directory for the
7660c0ab 3037C<readdir()> routine on DIRHANDLE.
a0d0e21e
LW
3038
3039=item rindex STR,SUBSTR,POSITION
3040
3041=item rindex STR,SUBSTR
3042
3043Works just like index except that it returns the position of the LAST
3044occurrence of SUBSTR in STR. If POSITION is specified, returns the
3045last occurrence at or before that position.
3046
3047=item rmdir FILENAME
3048
54310121 3049=item rmdir
bbce6d69 3050
5a964f20
TC
3051Deletes the directory specified by FILENAME if that directory is empty. If it
3052succeeds it returns TRUE, otherwise it returns FALSE and sets C<$!> (errno). If
7660c0ab 3053FILENAME is omitted, uses C<$_>.
a0d0e21e
LW
3054
3055=item s///
3056
3057The substitution operator. See L<perlop>.
3058
3059=item scalar EXPR
3060
5a964f20 3061Forces EXPR to be interpreted in scalar context and returns the value
54310121 3062of EXPR.
cb1a09d0
AD
3063
3064 @counts = ( scalar @a, scalar @b, scalar @c );
3065
54310121 3066There is no equivalent operator to force an expression to
5a964f20 3067be interpolated in list context because it's in practice never
cb1a09d0
AD
3068needed. If you really wanted to do so, however, you could use
3069the construction C<@{[ (some expression) ]}>, but usually a simple
3070C<(some expression)> suffices.
a0d0e21e 3071
62c18ce2
GS
3072Though C<scalar> can be considered in general to be a unary operator,
3073EXPR is also allowed to be a parenthesized list. The list in fact
3074behaves as a scalar comma expression, evaluating all but the last
3075element in void context and returning the final element evaluated in
3076a scalar context.
3077
3078The following single statement:
3079
3080 print uc(scalar(&foo,$bar)),$baz;
3081
3082is the moral equivalent of these two:
3083
3084 &foo;
3085 print(uc($bar),$baz);
3086
3087See L<perlop> for more details on unary operators and the comma operator.
3088
a0d0e21e
LW
3089=item seek FILEHANDLE,POSITION,WHENCE
3090
ab4f32c2 3091Sets FILEHANDLE's position, just like the C<fseek()> call of C<stdio()>.
8903cb82 3092FILEHANDLE may be an expression whose value gives the name of the
7660c0ab
A
3093filehandle. The values for WHENCE are C<0> to set the new position to
3094POSITION, C<1> to set it to the current position plus POSITION, and C<2> to
8903cb82 3095set it to EOF plus POSITION (typically negative). For WHENCE you may
7660c0ab
A
3096use the constants C<SEEK_SET>, C<SEEK_CUR>, and C<SEEK_END> from either the
3097C<IO::Seekable> or the POSIX module. Returns C<1> upon success, C<0> otherwise.
8903cb82 3098
7660c0ab
A
3099If you want to position file for C<sysread()> or C<syswrite()>, don't use
3100C<seek()> -- buffering makes its effect on the file's system position
3101unpredictable and non-portable. Use C<sysseek()> instead.
a0d0e21e 3102
cb1a09d0
AD
3103On some systems you have to do a seek whenever you switch between reading
3104and writing. Amongst other things, this may have the effect of calling
f86cebdf 3105stdio's clearerr(3). A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving
8903cb82 3106the file position:
cb1a09d0
AD
3107
3108 seek(TEST,0,1);
3109
3110This is also useful for applications emulating C<tail -f>. Once you hit
3111EOF on your read, and then sleep for a while, you might have to stick in a
7660c0ab 3112seek() to reset things. The C<seek()> doesn't change the current position,
8903cb82
PP
3113but it I<does> clear the end-of-file condition on the handle, so that the
3114next C<E<lt>FILEE<gt>> makes Perl try again to read something. We hope.
cb1a09d0
AD
3115
3116If that doesn't work (some stdios are particularly cantankerous), then
3117you may need something more like this:
3118
3119 for (;;) {
f86cebdf
GS
3120 for ($curpos = tell(FILE); $_ = <FILE>;
3121 $curpos = tell(FILE)) {
cb1a09d0
AD
3122 # search for some stuff and put it into files
3123 }
3124 sleep($for_a_while);
3125 seek(FILE, $curpos, 0);
3126 }
3127
a0d0e21e
LW
3128=item seekdir DIRHANDLE,POS
3129
7660c0ab
A
3130Sets the current position for the C<readdir()> routine on DIRHANDLE. POS
3131must be a value returned by C<telldir()>. Has the same caveats about
a0d0e21e
LW
3132possible directory compaction as the corresponding system library
3133routine.
3134
3135=item select FILEHANDLE
3136
3137=item select
3138
3139Returns the currently selected filehandle. Sets the current default
3140filehandle for output, if FILEHANDLE is supplied. This has two
ab4f32c2 3141effects: first, a C<write()> or a C<print()> without a filehandle will
a0d0e21e
LW
3142default to this FILEHANDLE. Second, references to variables related to
3143output will refer to this output channel. For example, if you have to
3144set the top of form format for more than one output channel, you might
3145do the following:
3146
3147 select(REPORT1);
3148 $^ = 'report1_top';
3149 select(REPORT2);
3150 $^ = 'report2_top';
3151
3152FILEHANDLE may be an expression whose value gives the name of the
3153actual filehandle. Thus:
3154
3155 $oldfh = select(STDERR); $| = 1; select($oldfh);
3156
4633a7c4
LW
3157Some programmers may prefer to think of filehandles as objects with
3158methods, preferring to write the last example as:
a0d0e21e 3159
28757baa 3160 use IO::Handle;
a0d0e21e
LW
3161 STDERR->autoflush(1);
3162
3163=item select RBITS,WBITS,EBITS,TIMEOUT
3164
f86cebdf 3165This calls the select(2) system call with the bit masks specified, which
7660c0ab 3166can be constructed using C<fileno()> and C<vec()>, along these lines:
a0d0e21e
LW
3167
3168 $rin = $win = $ein = '';
3169 vec($rin,fileno(STDIN),1) = 1;
3170 vec($win,fileno(STDOUT),1) = 1;
3171 $ein = $rin | $win;
3172
3173If you want to select on many filehandles you might wish to write a
3174subroutine:
3175
3176 sub fhbits {
5a964f20
TC
3177 my(@fhlist) = split(' ',$_[0]);
3178 my($bits);
a0d0e21e
LW
3179 for (@fhlist) {
3180 vec($bits,fileno($_),1) = 1;
3181 }
3182 $bits;
3183 }
4633a7c4 3184 $rin = fhbits('STDIN TTY SOCK');
a0d0e21e
LW
3185
3186The usual idiom is:
3187
3188 ($nfound,$timeleft) =
3189 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
3190
54310121 3191or to block until something becomes ready just do this
a0d0e21e
LW
3192
3193 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
3194
7660c0ab
A
3195Most systems do not bother to return anything useful in C<$timeleft>, so
3196calling select() in scalar context just returns C<$nfound>.
c07a80fd 3197
5f05dabc 3198Any of the bit masks can also be undef. The timeout, if specified, is
a0d0e21e 3199in seconds, which may be fractional. Note: not all implementations are
7660c0ab
A
3200capable of returning theC<$timeleft>. If not, they always return
3201C<$timeleft> equal to the supplied C<$timeout>.
a0d0e21e 3202
ff68c719 3203You can effect a sleep of 250 milliseconds this way:
a0d0e21e
LW
3204
3205 select(undef, undef, undef, 0.25);
3206
7660c0ab
A
3207B<WARNING>: One should not attempt to mix buffered I/O (like C<read()>
3208or E<lt>FHE<gt>) with C<select()>, except as permitted by POSIX, and even
3209then only on POSIX systems. You have to use C<sysread()> instead.
a0d0e21e
LW
3210
3211=item semctl ID,SEMNUM,CMD,ARG
3212
ab4f32c2 3213Calls the System V IPC function C<semctl()>. You'll probably have to say
0ade1984
JH
3214
3215 use IPC::SysV;
3216
3217first to get the correct constant definitions. If CMD is IPC_STAT or
3218GETALL, then ARG must be a variable which will hold the returned
ab4f32c2 3219semid_ds structure or semaphore value array. Returns like C<ioctl()>: the
7660c0ab
A
3220undefined value for error, "C<0> but true" for zero, or the actual return
3221value otherwise. See also C<IPC::SysV> and C<IPC::Semaphore> documentation.
a0d0e21e
LW
3222
3223=item semget KEY,NSEMS,FLAGS
3224
3225Calls the System V IPC function semget. Returns the semaphore id, or
7660c0ab
A
3226the undefined value if there is an error. See also C<IPC::SysV> and
3227C<IPC::SysV::Semaphore> documentation.
a0d0e21e
LW
3228
3229=item semop KEY,OPSTRING
3230
3231Calls the System V IPC function semop to perform semaphore operations
3232such as signaling and waiting. OPSTRING must be a packed array of
3233semop structures. Each semop structure can be generated with
3234C<pack("sss", $semnum, $semop, $semflag)>. The number of semaphore
3235operations is implied by the length of OPSTRING. Returns TRUE if
3236successful, or FALSE if there is an error. As an example, the
7660c0ab 3237following code waits on semaphore C<$semnum> of semaphore id C<$semid>:
a0d0e21e
LW
3238
3239 $semop = pack("sss", $semnum, -1, 0);
3240 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
3241
7660c0ab
A
3242To signal the semaphore, replace C<-1> with C<1>. See also C<IPC::SysV>
3243and C<IPC::SysV::Semaphore> documentation.
a0d0e21e
LW
3244
3245=item send SOCKET,MSG,FLAGS,TO
3246
3247=item send SOCKET,MSG,FLAGS
3248
3249Sends a message on a socket. Takes the same flags as the system call
3250of the same name. On unconnected sockets you must specify a
7660c0ab 3251destination to send TO, in which case it does a C C<sendto()>. Returns
a0d0e21e
LW
3252the number of characters sent, or the undefined value if there is an
3253error.
4633a7c4 3254See L<perlipc/"UDP: Message Passing"> for examples.
a0d0e21e
LW
3255
3256=item setpgrp PID,PGRP
3257
7660c0ab 3258Sets the current process group for the specified PID, C<0> for the current
a0d0e21e 3259process. Will produce a fatal error if used on a machine that doesn't
f86cebdf 3260implement setpgrp(2). If the arguments are omitted, it defaults to
7660c0ab
A
3261C<0,0>. Note that the POSIX version of C<setpgrp()> does not accept any
3262arguments, so only setpgrp C<0,0> is portable.
a0d0e21e
LW
3263
3264=item setpriority WHICH,WHO,PRIORITY
3265
3266Sets the current priority for a process, a process group, or a user.
f86cebdf
GS
3267(See setpriority(2).) Will produce a fatal error if used on a machine
3268that doesn't implement setpriority(2).
a0d0e21e
LW
3269
3270=item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
3271
3272Sets the socket option requested. Returns undefined if there is an
7660c0ab 3273error. OPTVAL may be specified as C<undef> if you don't want to pass an
a0d0e21e
LW
3274argument.
3275
3276=item shift ARRAY
3277
3278=item shift
3279
3280Shifts the first value of the array off and returns it, shortening the
3281array by 1 and moving everything down. If there are no elements in the
3282array, returns the undefined value. If ARRAY is omitted, shifts the
7660c0ab
A
3283C<@_> array within the lexical scope of subroutines and formats, and the
3284C<@ARGV> array at file scopes or within the lexical scopes established by
977336f5 3285the C<eval ''>, C<BEGIN {}>, C<END {}>, and C<INIT {}> constructs.
7660c0ab
A
3286See also C<unshift()>, C<push()>, and C<pop()>. C<Shift()> and C<unshift()> do the
3287same thing to the left end of an array that C<pop()> and C<push()> do to the
977336f5 3288right end.
a0d0e21e
LW
3289