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fix bogus integerization of pop()'s return value
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1=head1 NAME
2
3perlfunc - Perl builtin functions
4
5=head1 DESCRIPTION
6
7The functions in this section can serve as terms in an expression.
8They fall into two major categories: list operators and named unary
9operators. These differ in their precedence relationship with a
10following comma. (See the precedence table in L<perlop>.) List
11operators take more than one argument, while unary operators can never
12take more than one argument. Thus, a comma terminates the argument of
13a unary operator, but merely separates the arguments of a list
14operator. A unary operator generally provides a scalar context to its
15argument, while a list operator may provide either scalar and list
16contexts for its arguments. If it does both, the scalar arguments will
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17be first, and the list argument will follow. (Note that there can ever
18be only one list argument.) For instance, splice() has three scalar
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19arguments followed by a list.
20
21In the syntax descriptions that follow, list operators that expect a
22list (and provide list context for the elements of the list) are shown
23with LIST as an argument. Such a list may consist of any combination
24of scalar arguments or list values; the list values will be included
25in the list as if each individual element were interpolated at that
26point in the list, forming a longer single-dimensional list value.
27Elements of the LIST should be separated by commas.
28
29Any function in the list below may be used either with or without
30parentheses around its arguments. (The syntax descriptions omit the
5f05dabc 31parentheses.) If you use the parentheses, the simple (but occasionally
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32surprising) rule is this: It I<LOOKS> like a function, therefore it I<IS> a
33function, and precedence doesn't matter. Otherwise it's a list
34operator or unary operator, and precedence does matter. And whitespace
35between the function and left parenthesis doesn't count--so you need to
36be careful sometimes:
37
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38 print 1+2+4; # Prints 7.
39 print(1+2) + 4; # Prints 3.
40 print (1+2)+4; # Also prints 3!
41 print +(1+2)+4; # Prints 7.
42 print ((1+2)+4); # Prints 7.
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43
44If you run Perl with the B<-w> switch it can warn you about this. For
45example, the third line above produces:
46
47 print (...) interpreted as function at - line 1.
48 Useless use of integer addition in void context at - line 1.
49
50For functions that can be used in either a scalar or list context,
54310121 51nonabortive failure is generally indicated in a scalar context by
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52returning the undefined value, and in a list context by returning the
53null list.
54
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55Remember the following important rule: There is B<no rule> that relates
56the behavior of an expression in list context to its behavior in scalar
57context, or vice versa. It might do two totally different things.
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58Each operator and function decides which sort of value it would be most
59appropriate to return in a scalar context. Some operators return the
5a964f20 60length of the list that would have been returned in list context. Some
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61operators return the first value in the list. Some operators return the
62last value in the list. Some operators return a count of successful
63operations. In general, they do what you want, unless you want
64consistency.
65
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66An named array in scalar context is quite different from what would at
67first glance appear to be a list in scalar context. You can't get a list
68like C<(1,2,3)> into being in scalar context, because the compiler knows
69the context at compile time. It would generate the scalar comma operator
70there, not the list construction version of the comma. That means it
71was never a list to start with.
72
73In general, functions in Perl that serve as wrappers for system calls
f86cebdf 74of the same name (like chown(2), fork(2), closedir(2), etc.) all return
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75true when they succeed and C<undef> otherwise, as is usually mentioned
76in the descriptions below. This is different from the C interfaces,
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77which return C<-1> on failure. Exceptions to this rule are C<wait()>,
78C<waitpid()>, and C<syscall()>. System calls also set the special C<$!>
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79variable on failure. Other functions do not, except accidentally.
80
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81=head2 Perl Functions by Category
82
83Here are Perl's functions (including things that look like
5a964f20 84functions, like some keywords and named operators)
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85arranged by category. Some functions appear in more
86than one place.
87
88=over
89
90=item Functions for SCALARs or strings
91
22fae026 92C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>,
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93C<length>, C<oct>, C<ord>, C<pack>, C<q/STRING/>, C<qq/STRING/>, C<reverse>,
94C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///>
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95
96=item Regular expressions and pattern matching
97
ab4f32c2 98C<m//>, C<pos>, C<quotemeta>, C<s///>, C<split>, C<study>, C<qr//>
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99
100=item Numeric functions
101
22fae026
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102C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>,
103C<sin>, C<sqrt>, C<srand>
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104
105=item Functions for real @ARRAYs
106
22fae026 107C<pop>, C<push>, C<shift>, C<splice>, C<unshift>
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108
109=item Functions for list data
110
ab4f32c2 111C<grep>, C<join>, C<map>, C<qw/STRING/>, C<reverse>, C<sort>, C<unpack>
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112
113=item Functions for real %HASHes
114
22fae026 115C<delete>, C<each>, C<exists>, C<keys>, C<values>
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116
117=item Input and output functions
118
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119C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
120C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
121C<readdir>, C<rewinddir>, C<seek>, C<seekdir>, C<select>, C<syscall>,
122C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>,
123C<warn>, C<write>
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124
125=item Functions for fixed length data or records
126
22fae026 127C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec>
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128
129=item Functions for filehandles, files, or directories
130
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131C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>,
132C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>, C<readlink>,
133C<rename>, C<rmdir>, C<stat>, C<symlink>, C<umask>, C<unlink>, C<utime>
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134
135=item Keywords related to the control flow of your perl program
136
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137C<caller>, C<continue>, C<die>, C<do>, C<dump>, C<eval>, C<exit>,
138C<goto>, C<last>, C<next>, C<redo>, C<return>, C<sub>, C<wantarray>
cb1a09d0 139
54310121 140=item Keywords related to scoping
cb1a09d0 141
22fae026 142C<caller>, C<import>, C<local>, C<my>, C<package>, C<use>
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143
144=item Miscellaneous functions
145
22fae026
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146C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<reset>,
147C<scalar>, C<undef>, C<wantarray>
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148
149=item Functions for processes and process groups
150
22fae026 151C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
ab4f32c2 152C<pipe>, C<qx/STRING/>, C<setpgrp>, C<setpriority>, C<sleep>, C<system>,
22fae026 153C<times>, C<wait>, C<waitpid>
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154
155=item Keywords related to perl modules
156
22fae026 157C<do>, C<import>, C<no>, C<package>, C<require>, C<use>
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158
159=item Keywords related to classes and object-orientedness
160
22fae026
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161C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
162C<untie>, C<use>
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163
164=item Low-level socket functions
165
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166C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
167C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
168C<socket>, C<socketpair>
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169
170=item System V interprocess communication functions
171
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172C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
173C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>
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174
175=item Fetching user and group info
176
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177C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
178C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
179C<getpwuid>, C<setgrent>, C<setpwent>
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180
181=item Fetching network info
182
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183C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
184C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
185C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
186C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
187C<setnetent>, C<setprotoent>, C<setservent>
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188
189=item Time-related functions
190
22fae026 191C<gmtime>, C<localtime>, C<time>, C<times>
cb1a09d0 192
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193=item Functions new in perl5
194
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195C<abs>, C<bless>, C<chomp>, C<chr>, C<exists>, C<formline>, C<glob>,
196C<import>, C<lc>, C<lcfirst>, C<map>, C<my>, C<no>, C<prototype>, C<qx>,
197C<qw>, C<readline>, C<readpipe>, C<ref>, C<sub*>, C<sysopen>, C<tie>,
198C<tied>, C<uc>, C<ucfirst>, C<untie>, C<use>
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199
200* - C<sub> was a keyword in perl4, but in perl5 it is an
5a964f20 201operator, which can be used in expressions.
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202
203=item Functions obsoleted in perl5
204
22fae026 205C<dbmclose>, C<dbmopen>
37798a01 206
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207=back
208
209=head2 Alphabetical Listing of Perl Functions
210
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211=over 8
212
22fae026 213=item I<-X> FILEHANDLE
a0d0e21e 214
22fae026 215=item I<-X> EXPR
a0d0e21e 216
22fae026 217=item I<-X>
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218
219A file test, where X is one of the letters listed below. This unary
220operator takes one argument, either a filename or a filehandle, and
221tests the associated file to see if something is true about it. If the
7660c0ab 222argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
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223Unless otherwise documented, it returns C<1> for TRUE and C<''> for FALSE, or
224the undefined value if the file doesn't exist. Despite the funny
225names, precedence is the same as any other named unary operator, and
226the argument may be parenthesized like any other unary operator. The
227operator may be any of:
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228X<-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>
229X<-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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230
231 -r File is readable by effective uid/gid.
232 -w File is writable by effective uid/gid.
233 -x File is executable by effective uid/gid.
234 -o File is owned by effective uid.
235
236 -R File is readable by real uid/gid.
237 -W File is writable by real uid/gid.
238 -X File is executable by real uid/gid.
239 -O File is owned by real uid.
240
241 -e File exists.
242 -z File has zero size.
54310121 243 -s File has nonzero size (returns size).
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244
245 -f File is a plain file.
246 -d File is a directory.
247 -l File is a symbolic link.
9c4d0f16 248 -p File is a named pipe (FIFO), or Filehandle is a pipe.
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249 -S File is a socket.
250 -b File is a block special file.
251 -c File is a character special file.
252 -t Filehandle is opened to a tty.
253
254 -u File has setuid bit set.
255 -g File has setgid bit set.
256 -k File has sticky bit set.
257
258 -T File is a text file.
259 -B File is a binary file (opposite of -T).
260
261 -M Age of file in days when script started.
262 -A Same for access time.
263 -C Same for inode change time.
264
265The interpretation of the file permission operators C<-r>, C<-R>, C<-w>,
5f05dabc 266C<-W>, C<-x>, and C<-X> is based solely on the mode of the file and the
a0d0e21e 267uids and gids of the user. There may be other reasons you can't actually
5a964f20 268read, write, or execute the file, such as AFS access control lists. Also note that, for the superuser,
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269C<-r>, C<-R>, C<-w>, and C<-W> always return C<1>, and C<-x> and C<-X> return
270C<1> if any execute bit is set in the mode. Scripts run by the superuser may
271thus need to do a C<stat()> to determine the actual mode of the
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272file, or temporarily set the uid to something else.
273
274Example:
275
276 while (<>) {
277 chop;
278 next unless -f $_; # ignore specials
5a964f20 279 #...
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280 }
281
282Note that C<-s/a/b/> does not do a negated substitution. Saying
283C<-exp($foo)> still works as expected, however--only single letters
284following a minus are interpreted as file tests.
285
286The C<-T> and C<-B> switches work as follows. The first block or so of the
287file is examined for odd characters such as strange control codes or
5a964f20 288characters with the high bit set. If too many strange characters (E<gt>30%)
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289are found, it's a C<-B> file, otherwise it's a C<-T> file. Also, any file
290containing null in the first block is considered a binary file. If C<-T>
291or C<-B> is used on a filehandle, the current stdio buffer is examined
292rather than the first block. Both C<-T> and C<-B> return TRUE on a null
54310121 293file, or a file at EOF when testing a filehandle. Because you have to
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294read a file to do the C<-T> test, on most occasions you want to use a C<-f>
295against the file first, as in C<next unless -f $file && -T $file>.
a0d0e21e 296
7660c0ab 297If any of the file tests (or either the C<stat()> or C<lstat()> operators) are given
28757baa 298the special filehandle consisting of a solitary underline, then the stat
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299structure of the previous file test (or stat operator) is used, saving
300a system call. (This doesn't work with C<-t>, and you need to remember
301that lstat() and C<-l> will leave values in the stat structure for the
302symbolic link, not the real file.) Example:
303
304 print "Can do.\n" if -r $a || -w _ || -x _;
305
306 stat($filename);
307 print "Readable\n" if -r _;
308 print "Writable\n" if -w _;
309 print "Executable\n" if -x _;
310 print "Setuid\n" if -u _;
311 print "Setgid\n" if -g _;
312 print "Sticky\n" if -k _;
313 print "Text\n" if -T _;
314 print "Binary\n" if -B _;
315
316=item abs VALUE
317
54310121 318=item abs
bbce6d69 319
a0d0e21e 320Returns the absolute value of its argument.
7660c0ab 321If VALUE is omitted, uses C<$_>.
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322
323=item accept NEWSOCKET,GENERICSOCKET
324
f86cebdf 325Accepts an incoming socket connect, just as the accept(2) system call
a0d0e21e 326does. Returns the packed address if it succeeded, FALSE otherwise.
4633a7c4 327See example in L<perlipc/"Sockets: Client/Server Communication">.
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328
329=item alarm SECONDS
330
54310121 331=item alarm
bbce6d69 332
a0d0e21e 333Arranges to have a SIGALRM delivered to this process after the
bbce6d69 334specified number of seconds have elapsed. If SECONDS is not specified,
7660c0ab 335the value stored in C<$_> is used. (On some machines,
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336unfortunately, the elapsed time may be up to one second less than you
337specified because of how seconds are counted.) Only one timer may be
338counting at once. Each call disables the previous timer, and an
7660c0ab 339argument of C<0> may be supplied to cancel the previous timer without
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340starting a new one. The returned value is the amount of time remaining
341on the previous timer.
342
4633a7c4 343For delays of finer granularity than one second, you may use Perl's
f86cebdf 344C<syscall()> interface to access setitimer(2) if your system supports it,
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345or else see L</select()>. It is usually a mistake to intermix C<alarm()>
346and C<sleep()> calls.
a0d0e21e 347
7660c0ab 348If you want to use C<alarm()> to time out a system call you need to use an
ab4f32c2 349C<eval()>/C<die()> pair. You can't rely on the alarm causing the system call to
f86cebdf 350fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
ab4f32c2 351restart system calls on some systems. Using C<eval()>/C<die()> always works,
5a964f20 352modulo the caveats given in L<perlipc/"Signals">.
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353
354 eval {
f86cebdf 355 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
36477c24 356 alarm $timeout;
ff68c719 357 $nread = sysread SOCKET, $buffer, $size;
36477c24 358 alarm 0;
ff68c719 359 };
ff68c719 360 if ($@) {
f86cebdf 361 die unless $@ eq "alarm\n"; # propagate unexpected errors
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362 # timed out
363 }
364 else {
365 # didn't
366 }
367
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368=item atan2 Y,X
369
370Returns the arctangent of Y/X in the range -PI to PI.
371
7660c0ab 372For the tangent operation, you may use the C<POSIX::tan()>
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373function, or use the familiar relation:
374
375 sub tan { sin($_[0]) / cos($_[0]) }
376
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377=item bind SOCKET,NAME
378
379Binds a network address to a socket, just as the bind system call
380does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
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381packed address of the appropriate type for the socket. See the examples in
382L<perlipc/"Sockets: Client/Server Communication">.
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383
384=item binmode FILEHANDLE
385
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386Arranges for the file to be read or written in "binary" mode in operating
387systems that distinguish between binary and text files. Files that are
388not in binary mode have CR LF sequences translated to LF on input and LF
54310121 389translated to CR LF on output. Binmode has no effect under Unix; in MS-DOS
cb1a09d0 390and similarly archaic systems, it may be imperative--otherwise your
54310121 391MS-DOS-damaged C library may mangle your file. The key distinction between
ab4f32c2 392systems that need C<binmode()> and those that don't is their text file
5a964f20 393formats. Systems like Unix, MacOS, and Plan9 that delimit lines with a single
7660c0ab 394character, and that encode that character in C as C<"\n">, do not need
ab4f32c2 395C<binmode()>. The rest need it. If FILEHANDLE is an expression, the value
cb1a09d0 396is taken as the name of the filehandle.
a0d0e21e 397
4633a7c4 398=item bless REF,CLASSNAME
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399
400=item bless REF
401
28757baa 402This function tells the thingy referenced by REF that it is now
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403an object in the CLASSNAME package--or the current package if no CLASSNAME
404is specified, which is often the case. It returns the reference for
7660c0ab 405convenience, because a C<bless()> is often the last thing in a constructor.
4633a7c4 406Always use the two-argument version if the function doing the blessing
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407might be inherited by a derived class. See L<perltoot> and L<perlobj>
408for more about the blessing (and blessings) of objects.
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409
410=item caller EXPR
411
412=item caller
413
5a964f20 414Returns the context of the current subroutine call. In scalar context,
28757baa 415returns the caller's package name if there is a caller, that is, if
7660c0ab 416we're in a subroutine or C<eval()> or C<require()>, and the undefined value
5a964f20 417otherwise. In list context, returns
a0d0e21e 418
748a9306 419 ($package, $filename, $line) = caller;
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420
421With EXPR, it returns some extra information that the debugger uses to
422print a stack trace. The value of EXPR indicates how many call frames
423to go back before the current one.
424
54310121 425 ($package, $filename, $line, $subroutine,
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426 $hasargs, $wantarray, $evaltext, $is_require) = caller($i);
427
7660c0ab 428Here C<$subroutine> may be C<"(eval)"> if the frame is not a subroutine
ab4f32c2 429call, but an C<eval()>. In such a case additional elements C<$evaltext> and
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430C<$is_require> are set: C<$is_require> is true if the frame is created by a
431C<require> or C<use> statement, C<$evaltext> contains the text of the
dc848c6f 432C<eval EXPR> statement. In particular, for a C<eval BLOCK> statement,
7660c0ab 433C<$filename> is C<"(eval)">, but C<$evaltext> is undefined. (Note also that
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434each C<use> statement creates a C<require> frame inside an C<eval EXPR>)
435frame.
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436
437Furthermore, when called from within the DB package, caller returns more
7660c0ab 438detailed information: it sets the list variable C<@DB::args> to be the
54310121 439arguments with which the subroutine was invoked.
748a9306 440
7660c0ab 441Be aware that the optimizer might have optimized call frames away before
ab4f32c2 442C<caller()> had a chance to get the information. That means that C<caller(N)>
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443might not return information about the call frame you expect it do, for
444C<N E<gt> 1>. In particular, C<@DB::args> might have information from the
445previous time C<caller()> was called.
446
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447=item chdir EXPR
448
449Changes the working directory to EXPR, if possible. If EXPR is
450omitted, changes to home directory. Returns TRUE upon success, FALSE
7660c0ab 451otherwise. See example under C<die()>.
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452
453=item chmod LIST
454
455Changes the permissions of a list of files. The first element of the
4633a7c4 456list must be the numerical mode, which should probably be an octal
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457number, and which definitely should I<not> a string of octal digits:
458C<0644> is okay, C<'0644'> is not. Returns the number of files
dc848c6f 459successfully changed. See also L</oct>, if all you have is a string.
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460
461 $cnt = chmod 0755, 'foo', 'bar';
462 chmod 0755, @executables;
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463 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
464 # --w----r-T
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TPG
465 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
466 $mode = 0644; chmod $mode, 'foo'; # this is best
a0d0e21e
LW
467
468=item chomp VARIABLE
469
470=item chomp LIST
471
472=item chomp
473
3e3baf6d 474This is a slightly safer version of L</chop>. It removes any
a0d0e21e 475line ending that corresponds to the current value of C<$/> (also known as
28757baa
PP
476$INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
477number of characters removed from all its arguments. It's often used to
478remove the newline from the end of an input record when you're worried
479that the final record may be missing its newline. When in paragraph mode
480(C<$/ = "">), it removes all trailing newlines from the string. If
7660c0ab 481VARIABLE is omitted, it chomps C<$_>. Example:
a0d0e21e
LW
482
483 while (<>) {
484 chomp; # avoid \n on last field
485 @array = split(/:/);
5a964f20 486 # ...
a0d0e21e
LW
487 }
488
489You can actually chomp anything that's an lvalue, including an assignment:
490
491 chomp($cwd = `pwd`);
492 chomp($answer = <STDIN>);
493
494If you chomp a list, each element is chomped, and the total number of
495characters removed is returned.
496
497=item chop VARIABLE
498
499=item chop LIST
500
501=item chop
502
503Chops off the last character of a string and returns the character
504chopped. It's used primarily to remove the newline from the end of an
505input record, but is much more efficient than C<s/\n//> because it neither
7660c0ab 506scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
a0d0e21e
LW
507Example:
508
509 while (<>) {
510 chop; # avoid \n on last field
511 @array = split(/:/);
5a964f20 512 #...
a0d0e21e
LW
513 }
514
515You can actually chop anything that's an lvalue, including an assignment:
516
517 chop($cwd = `pwd`);
518 chop($answer = <STDIN>);
519
520If you chop a list, each element is chopped. Only the value of the
ab4f32c2 521last C<chop()> is returned.
a0d0e21e 522
ab4f32c2 523Note that C<chop()> returns the last character. To return all but the last
748a9306
LW
524character, use C<substr($string, 0, -1)>.
525
a0d0e21e
LW
526=item chown LIST
527
528Changes the owner (and group) of a list of files. The first two
529elements of the list must be the I<NUMERICAL> uid and gid, in that order.
530Returns the number of files successfully changed.
531
532 $cnt = chown $uid, $gid, 'foo', 'bar';
533 chown $uid, $gid, @filenames;
534
54310121 535Here's an example that looks up nonnumeric uids in the passwd file:
a0d0e21e
LW
536
537 print "User: ";
538 chop($user = <STDIN>);
5a964f20 539 print "Files: ";
a0d0e21e
LW
540 chop($pattern = <STDIN>);
541
542 ($login,$pass,$uid,$gid) = getpwnam($user)
543 or die "$user not in passwd file";
544
5a964f20 545 @ary = glob($pattern); # expand filenames
a0d0e21e
LW
546 chown $uid, $gid, @ary;
547
54310121 548On most systems, you are not allowed to change the ownership of the
4633a7c4
LW
549file unless you're the superuser, although you should be able to change
550the group to any of your secondary groups. On insecure systems, these
551restrictions may be relaxed, but this is not a portable assumption.
552
a0d0e21e
LW
553=item chr NUMBER
554
54310121 555=item chr
bbce6d69 556
a0d0e21e 557Returns the character represented by that NUMBER in the character set.
a0ed51b3
LW
558For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
559chr(0x263a) is a Unicode smiley face (but only within the scope of a
560C<use utf8>). For the reverse, use L</ord>.
a0d0e21e 561
7660c0ab 562If NUMBER is omitted, uses C<$_>.
bbce6d69 563
a0d0e21e
LW
564=item chroot FILENAME
565
54310121 566=item chroot
bbce6d69 567
5a964f20 568This function works like the system call by the same name: it makes the
4633a7c4 569named directory the new root directory for all further pathnames that
7660c0ab 570begin with a C<"/"> by your process and all its children. (It doesn't
28757baa 571change your current working directory, which is unaffected.) For security
4633a7c4 572reasons, this call is restricted to the superuser. If FILENAME is
ab4f32c2 573omitted, does a C<chroot()> to C<$_>.
a0d0e21e
LW
574
575=item close FILEHANDLE
576
6a518fbc
TP
577=item close
578
a0d0e21e
LW
579Closes the file or pipe associated with the file handle, returning TRUE
580only if stdio successfully flushes buffers and closes the system file
6a518fbc
TP
581descriptor. Closes the currently selected filehandle if the argument
582is omitted.
fb73857a
PP
583
584You don't have to close FILEHANDLE if you are immediately going to do
7660c0ab 585another C<open()> on it, because C<open()> will close it for you. (See
ab4f32c2 586C<open()>.) However, an explicit C<close()> on an input file resets the line
7660c0ab 587counter (C<$.>), while the implicit close done by C<open()> does not.
fb73857a 588
ab4f32c2 589If the file handle came from a piped open C<close()> will additionally
fb73857a
PP
590return FALSE if one of the other system calls involved fails or if the
591program exits with non-zero status. (If the only problem was that the
7660c0ab 592program exited non-zero C<$!> will be set to C<0>.) Also, closing a pipe
5a964f20 593waits for the process executing on the pipe to complete, in case you
fb73857a
PP
594want to look at the output of the pipe afterwards. Closing a pipe
595explicitly also puts the exit status value of the command into C<$?>.
5a964f20 596
fb73857a 597Example:
a0d0e21e 598
fb73857a
PP
599 open(OUTPUT, '|sort >foo') # pipe to sort
600 or die "Can't start sort: $!";
5a964f20 601 #... # print stuff to output
fb73857a
PP
602 close OUTPUT # wait for sort to finish
603 or warn $! ? "Error closing sort pipe: $!"
604 : "Exit status $? from sort";
605 open(INPUT, 'foo') # get sort's results
606 or die "Can't open 'foo' for input: $!";
a0d0e21e 607
5a964f20
TC
608FILEHANDLE may be an expression whose value can be used as an indirect
609filehandle, usually the real filehandle name.
a0d0e21e
LW
610
611=item closedir DIRHANDLE
612
7660c0ab 613Closes a directory opened by C<opendir()> and returns the success of that
5a964f20
TC
614system call.
615
616DIRHANDLE may be an expression whose value can be used as an indirect
617dirhandle, usually the real dirhandle name.
a0d0e21e
LW
618
619=item connect SOCKET,NAME
620
621Attempts to connect to a remote socket, just as the connect system call
622does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
4633a7c4
LW
623packed address of the appropriate type for the socket. See the examples in
624L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 625
cb1a09d0
AD
626=item continue BLOCK
627
628Actually a flow control statement rather than a function. If there is a
629C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
630C<foreach>), it is always executed just before the conditional is about to
631be evaluated again, just like the third part of a C<for> loop in C. Thus
632it can be used to increment a loop variable, even when the loop has been
633continued via the C<next> statement (which is similar to the C C<continue>
634statement).
635
1d2dff63
GS
636C<last>, C<next>, or C<redo> may appear within a C<continue>
637block. C<last> and C<redo> will behave as if they had been executed within
638the main block. So will C<next>, but since it will execute a C<continue>
639block, it may be more entertaining.
640
641 while (EXPR) {
642 ### redo always comes here
643 do_something;
644 } continue {
645 ### next always comes here
646 do_something_else;
647 # then back the top to re-check EXPR
648 }
649 ### last always comes here
650
651Omitting the C<continue> section is semantically equivalent to using an
652empty one, logically enough. In that case, C<next> goes directly back
653to check the condition at the top of the loop.
654
a0d0e21e
LW
655=item cos EXPR
656
5a964f20 657Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
7660c0ab 658takes cosine of C<$_>.
a0d0e21e 659
7660c0ab 660For the inverse cosine operation, you may use the C<POSIX::acos()>
28757baa
PP
661function, or use this relation:
662
663 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
664
a0d0e21e
LW
665=item crypt PLAINTEXT,SALT
666
f86cebdf 667Encrypts a string exactly like the crypt(3) function in the C library
4633a7c4
LW
668(assuming that you actually have a version there that has not been
669extirpated as a potential munition). This can prove useful for checking
670the password file for lousy passwords, amongst other things. Only the
671guys wearing white hats should do this.
a0d0e21e 672
7660c0ab 673Note that C<crypt()> is intended to be a one-way function, much like breaking
11155c91
CS
674eggs to make an omelette. There is no (known) corresponding decrypt
675function. As a result, this function isn't all that useful for
676cryptography. (For that, see your nearby CPAN mirror.)
2f9daede 677
a0d0e21e
LW
678Here's an example that makes sure that whoever runs this program knows
679their own password:
680
681 $pwd = (getpwuid($<))[1];
682 $salt = substr($pwd, 0, 2);
683
684 system "stty -echo";
685 print "Password: ";
686 chop($word = <STDIN>);
687 print "\n";
688 system "stty echo";
689
690 if (crypt($word, $salt) ne $pwd) {
691 die "Sorry...\n";
692 } else {
693 print "ok\n";
54310121 694 }
a0d0e21e 695
9f8f0c9d 696Of course, typing in your own password to whoever asks you
748a9306 697for it is unwise.
a0d0e21e 698
aa689395 699=item dbmclose HASH
a0d0e21e 700
7660c0ab 701[This function has been superseded by the C<untie()> function.]
a0d0e21e 702
aa689395 703Breaks the binding between a DBM file and a hash.
a0d0e21e 704
aa689395 705=item dbmopen HASH,DBNAME,MODE
a0d0e21e 706
7660c0ab 707[This function has been superseded by the C<tie()> function.]
a0d0e21e 708
7b8d334a 709This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
ab4f32c2 710hash. HASH is the name of the hash. (Unlike normal C<open()>, the first
aa689395
PP
711argument is I<NOT> a filehandle, even though it looks like one). DBNAME
712is the name of the database (without the F<.dir> or F<.pag> extension if
713any). If the database does not exist, it is created with protection
7660c0ab
A
714specified by MODE (as modified by the C<umask()>). If your system supports
715only the older DBM functions, you may perform only one C<dbmopen()> in your
aa689395 716program. In older versions of Perl, if your system had neither DBM nor
7660c0ab 717ndbm, calling C<dbmopen()> produced a fatal error; it now falls back to
aa689395
PP
718sdbm(3).
719
720If you don't have write access to the DBM file, you can only read hash
721variables, not set them. If you want to test whether you can write,
7660c0ab 722either use file tests or try setting a dummy hash entry inside an C<eval()>,
aa689395 723which will trap the error.
a0d0e21e 724
7660c0ab
A
725Note that functions such as C<keys()> and C<values()> may return huge lists
726when used on large DBM files. You may prefer to use the C<each()>
a0d0e21e
LW
727function to iterate over large DBM files. Example:
728
729 # print out history file offsets
730 dbmopen(%HIST,'/usr/lib/news/history',0666);
731 while (($key,$val) = each %HIST) {
732 print $key, ' = ', unpack('L',$val), "\n";
733 }
734 dbmclose(%HIST);
735
cb1a09d0 736See also L<AnyDBM_File> for a more general description of the pros and
184e9718 737cons of the various dbm approaches, as well as L<DB_File> for a particularly
cb1a09d0 738rich implementation.
4633a7c4 739
a0d0e21e
LW
740=item defined EXPR
741
54310121 742=item defined
bbce6d69 743
2f9daede
TPG
744Returns a Boolean value telling whether EXPR has a value other than
745the undefined value C<undef>. If EXPR is not present, C<$_> will be
746checked.
747
748Many operations return C<undef> to indicate failure, end of file,
749system error, uninitialized variable, and other exceptional
750conditions. This function allows you to distinguish C<undef> from
751other values. (A simple Boolean test will not distinguish among
7660c0ab 752C<undef>, zero, the empty string, and C<"0">, which are all equally
2f9daede 753false.) Note that since C<undef> is a valid scalar, its presence
7660c0ab 754doesn't I<necessarily> indicate an exceptional condition: C<pop()>
2f9daede
TPG
755returns C<undef> when its argument is an empty array, I<or> when the
756element to return happens to be C<undef>.
757
7660c0ab 758You may also use C<defined()> to check whether a subroutine exists, by
5a964f20 759saying C<defined &func> without parentheses. On the other hand, use
7660c0ab 760of C<defined()> upon aggregates (hashes and arrays) is not guaranteed to
5a964f20 761produce intuitive results, and should probably be avoided.
2f9daede
TPG
762
763When used on a hash element, it tells you whether the value is defined,
dc848c6f 764not whether the key exists in the hash. Use L</exists> for the latter
2f9daede 765purpose.
a0d0e21e
LW
766
767Examples:
768
769 print if defined $switch{'D'};
770 print "$val\n" while defined($val = pop(@ary));
771 die "Can't readlink $sym: $!"
772 unless defined($value = readlink $sym);
a0d0e21e 773 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
2f9daede 774 $debugging = 0 unless defined $debugging;
a0d0e21e 775
7660c0ab
A
776Note: Many folks tend to overuse C<defined()>, and then are surprised to
777discover that the number C<0> and C<""> (the zero-length string) are, in fact,
2f9daede 778defined values. For example, if you say
a5f75d66
AD
779
780 "ab" =~ /a(.*)b/;
781
7660c0ab 782The pattern match succeeds, and C<$1> is defined, despite the fact that it
a5f75d66 783matched "nothing". But it didn't really match nothing--rather, it
7660c0ab 784matched something that happened to be C<0> characters long. This is all
a5f75d66 785very above-board and honest. When a function returns an undefined value,
2f9daede 786it's an admission that it couldn't give you an honest answer. So you
7660c0ab
A
787should use C<defined()> only when you're questioning the integrity of what
788you're trying to do. At other times, a simple comparison to C<0> or C<""> is
2f9daede
TPG
789what you want.
790
7660c0ab 791Currently, using C<defined()> on an entire array or hash reports whether
2f9daede
TPG
792memory for that aggregate has ever been allocated. So an array you set
793to the empty list appears undefined initially, and one that once was full
794and that you then set to the empty list still appears defined. You
795should instead use a simple test for size:
28757baa
PP
796
797 if (@an_array) { print "has array elements\n" }
798 if (%a_hash) { print "has hash members\n" }
799
7660c0ab 800Using C<undef()> on these, however, does clear their memory and then report
5a964f20 801them as not defined anymore, but you shouldn't do that unless you don't
28757baa 802plan to use them again, because it saves time when you load them up
5a964f20
TC
803again to have memory already ready to be filled. The normal way to
804free up space used by an aggregate is to assign the empty list.
28757baa 805
7660c0ab 806This counterintuitive behavior of C<defined()> on aggregates may be
28757baa
PP
807changed, fixed, or broken in a future release of Perl.
808
dc848c6f 809See also L</undef>, L</exists>, L</ref>.
2f9daede 810
a0d0e21e
LW
811=item delete EXPR
812
aa689395
PP
813Deletes the specified key(s) and their associated values from a hash.
814For each key, returns the deleted value associated with that key, or
815the undefined value if there was no such key. Deleting from C<$ENV{}>
816modifies the environment. Deleting from a hash tied to a DBM file
7660c0ab 817deletes the entry from the DBM file. (But deleting from a C<tie()>d hash
5f05dabc 818doesn't necessarily return anything.)
a0d0e21e 819
aa689395 820The following deletes all the values of a hash:
a0d0e21e 821
5f05dabc
PP
822 foreach $key (keys %HASH) {
823 delete $HASH{$key};
a0d0e21e
LW
824 }
825
5f05dabc
PP
826And so does this:
827
828 delete @HASH{keys %HASH}
829
5a964f20 830(But both of these are slower than just assigning the empty list, or
7660c0ab 831using C<undef()>.) Note that the EXPR can be arbitrarily complicated as
5a964f20 832long as the final operation is a hash element lookup or hash slice:
a0d0e21e
LW
833
834 delete $ref->[$x][$y]{$key};
5f05dabc 835 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
a0d0e21e
LW
836
837=item die LIST
838
7660c0ab
A
839Outside an C<eval()>, prints the value of LIST to C<STDERR> and exits with
840the current value of C<$!> (errno). If C<$!> is C<0>, exits with the value of
54310121 841C<($? E<gt>E<gt> 8)> (backtick `command` status). If C<($? E<gt>E<gt> 8)>
7660c0ab
A
842is C<0>, exits with C<255>. Inside an C<eval(),> the error message is stuffed into
843C<$@> and the C<eval()> is terminated with the undefined value. This makes
844C<die()> the way to raise an exception.
a0d0e21e
LW
845
846Equivalent examples:
847
848 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
54310121 849 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
a0d0e21e
LW
850
851If the value of EXPR does not end in a newline, the current script line
852number and input line number (if any) are also printed, and a newline
7660c0ab
A
853is supplied. Hint: sometimes appending C<", stopped"> to your message
854will cause it to make better sense when the string C<"at foo line 123"> is
a0d0e21e
LW
855appended. Suppose you are running script "canasta".
856
857 die "/etc/games is no good";
858 die "/etc/games is no good, stopped";
859
860produce, respectively
861
862 /etc/games is no good at canasta line 123.
863 /etc/games is no good, stopped at canasta line 123.
864
7660c0ab 865See also C<exit()> and C<warn()>.
a0d0e21e 866
7660c0ab
A
867If LIST is empty and C<$@> already contains a value (typically from a
868previous eval) that value is reused after appending C<"\t...propagated">.
fb73857a
PP
869This is useful for propagating exceptions:
870
871 eval { ... };
872 die unless $@ =~ /Expected exception/;
873
7660c0ab 874If C<$@> is empty then the string C<"Died"> is used.
fb73857a 875
7660c0ab 876You can arrange for a callback to be run just before the C<die()> does
774d564b
PP
877its deed, by setting the C<$SIG{__DIE__}> hook. The associated handler
878will be called with the error text and can change the error message, if
7660c0ab 879it sees fit, by calling C<die()> again. See L<perlvar/$SIG{expr}> for details on
fb73857a
PP
880setting C<%SIG> entries, and L<"eval BLOCK"> for some examples.
881
882Note that the C<$SIG{__DIE__}> hook is called even inside eval()ed
883blocks/strings. If one wants the hook to do nothing in such
884situations, put
885
886 die @_ if $^S;
887
888as the first line of the handler (see L<perlvar/$^S>).
774d564b 889
a0d0e21e
LW
890=item do BLOCK
891
892Not really a function. Returns the value of the last command in the
893sequence of commands indicated by BLOCK. When modified by a loop
894modifier, executes the BLOCK once before testing the loop condition.
895(On other statements the loop modifiers test the conditional first.)
896
897=item do SUBROUTINE(LIST)
898
899A deprecated form of subroutine call. See L<perlsub>.
900
901=item do EXPR
902
903Uses the value of EXPR as a filename and executes the contents of the
904file as a Perl script. Its primary use is to include subroutines
905from a Perl subroutine library.
906
907 do 'stat.pl';
908
909is just like
910
fb73857a 911 scalar eval `cat stat.pl`;
a0d0e21e 912
5a964f20 913except that it's more efficient and concise, keeps track of the
a0d0e21e
LW
914current filename for error messages, and searches all the B<-I>
915libraries if the file isn't in the current directory (see also the @INC
dc1be6b5
GS
916array in L<perlvar/Predefined Names>). It is also different in how
917code evaluated with C<do FILENAME> doesn't see lexicals in the enclosing
918scope like C<eval STRING> does. It's the same, however, in that it does
54310121 919reparse the file every time you call it, so you probably don't want to
a0d0e21e
LW
920do this inside a loop.
921
8e30cc93
G
922If C<do> cannot read the file, it returns undef and sets C<$!> to the
923error. If C<do> can read the file but cannot compile it, it
924returns undef and sets an error message in C<$@>. If the file is
925successfully compiled, C<do> returns the value of the last expression
926evaluated.
927
a0d0e21e 928Note that inclusion of library modules is better done with the
7660c0ab 929C<use()> and C<require()> operators, which also do automatic error checking
4633a7c4 930and raise an exception if there's a problem.
a0d0e21e 931
5a964f20
TC
932You might like to use C<do> to read in a program configuration
933file. Manual error checking can be done this way:
934
935 # read in config files: system first, then user
f86cebdf
GS
936 for $file ("/share/prog/defaults.rc",
937 "$ENV{HOME}/.someprogrc") {
5a964f20 938 unless ($return = do $file) {
f86cebdf
GS
939 warn "couldn't parse $file: $@" if $@;
940 warn "couldn't do $file: $!" unless defined $return;
941 warn "couldn't run $file" unless $return;
5a964f20
TC
942 }
943 }
944
a0d0e21e
LW
945=item dump LABEL
946
947This causes an immediate core dump. Primarily this is so that you can
948use the B<undump> program to turn your core dump into an executable binary
949after having initialized all your variables at the beginning of the
950program. When the new binary is executed it will begin by executing a
951C<goto LABEL> (with all the restrictions that C<goto> suffers). Think of
f86cebdf 952it as a goto with an intervening core dump and reincarnation. If C<LABEL>
5a964f20 953is omitted, restarts the program from the top. WARNING: Any files
a0d0e21e
LW
954opened at the time of the dump will NOT be open any more when the
955program is reincarnated, with possible resulting confusion on the part
956of Perl. See also B<-u> option in L<perlrun>.
957
958Example:
959
960 #!/usr/bin/perl
961 require 'getopt.pl';
962 require 'stat.pl';
963 %days = (
964 'Sun' => 1,
965 'Mon' => 2,
966 'Tue' => 3,
967 'Wed' => 4,
968 'Thu' => 5,
969 'Fri' => 6,
970 'Sat' => 7,
971 );
972
973 dump QUICKSTART if $ARGV[0] eq '-d';
974
975 QUICKSTART:
976 Getopt('f');
977
5a964f20
TC
978This operator is largely obsolete, partly because it's very hard to
979convert a core file into an executable, and because the real perl-to-C
980compiler has superseded it.
981
aa689395
PP
982=item each HASH
983
5a964f20 984When called in list context, returns a 2-element list consisting of the
aa689395 985key and value for the next element of a hash, so that you can iterate over
5a964f20 986it. When called in scalar context, returns the key for only the "next"
7660c0ab 987element in the hash. (Note: Keys may be C<"0"> or C<"">, which are logically
2f9daede
TPG
988false; you may wish to avoid constructs like C<while ($k = each %foo) {}>
989for this reason.)
990
991Entries are returned in an apparently random order. When the hash is
992entirely read, a null array is returned in list context (which when
7660c0ab
A
993assigned produces a FALSE (C<0>) value), and C<undef> in
994scalar context. The next call to C<each()> after that will start iterating
995again. There is a single iterator for each hash, shared by all C<each()>,
996C<keys()>, and C<values()> function calls in the program; it can be reset by
2f9daede
TPG
997reading all the elements from the hash, or by evaluating C<keys HASH> or
998C<values HASH>. If you add or delete elements of a hash while you're
999iterating over it, you may get entries skipped or duplicated, so don't.
aa689395 1000
f86cebdf 1001The following prints out your environment like the printenv(1) program,
aa689395 1002only in a different order:
a0d0e21e
LW
1003
1004 while (($key,$value) = each %ENV) {
1005 print "$key=$value\n";
1006 }
1007
7660c0ab 1008See also C<keys()> and C<values()>.
a0d0e21e
LW
1009
1010=item eof FILEHANDLE
1011
4633a7c4
LW
1012=item eof ()
1013
a0d0e21e
LW
1014=item eof
1015
1016Returns 1 if the next read on FILEHANDLE will return end of file, or if
1017FILEHANDLE is not open. FILEHANDLE may be an expression whose value
5a964f20 1018gives the real filehandle. (Note that this function actually
7660c0ab 1019reads a character and then C<ungetc()>s it, so isn't very useful in an
748a9306
LW
1020interactive context.) Do not read from a terminal file (or call
1021C<eof(FILEHANDLE)> on it) after end-of-file is reached. Filetypes such
1022as terminals may lose the end-of-file condition if you do.
1023
1024An C<eof> without an argument uses the last file read as argument.
5a964f20 1025Using C<eof()> with empty parentheses is very different. It indicates the pseudo file formed of
2f9daede
TPG
1026the files listed on the command line, i.e., C<eof()> is reasonable to
1027use inside a C<while (E<lt>E<gt>)> loop to detect the end of only the
1028last file. Use C<eof(ARGV)> or eof without the parentheses to test
1029I<EACH> file in a while (E<lt>E<gt>) loop. Examples:
a0d0e21e 1030
748a9306
LW
1031 # reset line numbering on each input file
1032 while (<>) {
5a964f20 1033 next if /^\s*#/; # skip comments
748a9306 1034 print "$.\t$_";
5a964f20
TC
1035 } continue {
1036 close ARGV if eof; # Not eof()!
748a9306
LW
1037 }
1038
a0d0e21e
LW
1039 # insert dashes just before last line of last file
1040 while (<>) {
5a964f20 1041 if (eof()) { # check for end of current file
a0d0e21e 1042 print "--------------\n";
748a9306
LW
1043 close(ARGV); # close or break; is needed if we
1044 # are reading from the terminal
a0d0e21e
LW
1045 }
1046 print;
1047 }
1048
a0d0e21e 1049Practical hint: you almost never need to use C<eof> in Perl, because the
3b02c43c
GS
1050input operators return false values when they run out of data, or if there
1051was an error.
a0d0e21e
LW
1052
1053=item eval EXPR
1054
1055=item eval BLOCK
1056
c7cc6f1c
GS
1057In the first form, the return value of EXPR is parsed and executed as if it
1058were a little Perl program. The value of the expression (which is itself
5a964f20 1059determined within scalar context) is first parsed, and if there weren't any
c7cc6f1c 1060errors, executed in the context of the current Perl program, so that any
5f05dabc 1061variable settings or subroutine and format definitions remain afterwards.
c7cc6f1c
GS
1062Note that the value is parsed every time the eval executes. If EXPR is
1063omitted, evaluates C<$_>. This form is typically used to delay parsing
1064and subsequent execution of the text of EXPR until run time.
1065
1066In the second form, the code within the BLOCK is parsed only once--at the
1067same time the code surrounding the eval itself was parsed--and executed
1068within the context of the current Perl program. This form is typically
1069used to trap exceptions more efficiently than the first (see below), while
1070also providing the benefit of checking the code within BLOCK at compile
1071time.
1072
1073The final semicolon, if any, may be omitted from the value of EXPR or within
1074the BLOCK.
1075
1076In both forms, the value returned is the value of the last expression
5a964f20 1077evaluated inside the mini-program; a return statement may be also used, just
c7cc6f1c 1078as with subroutines. The expression providing the return value is evaluated
5a964f20 1079in void, scalar, or list context, depending on the context of the eval itself.
c7cc6f1c 1080See L</wantarray> for more on how the evaluation context can be determined.
a0d0e21e 1081
7660c0ab
A
1082If there is a syntax error or runtime error, or a C<die()> statement is
1083executed, an undefined value is returned by C<eval()>, and C<$@> is set to the
a0d0e21e 1084error message. If there was no error, C<$@> is guaranteed to be a null
7660c0ab 1085string. Beware that using C<eval()> neither silences perl from printing
c7cc6f1c
GS
1086warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1087To do either of those, you have to use the C<$SIG{__WARN__}> facility. See
1088L</warn> and L<perlvar>.
a0d0e21e 1089
7660c0ab
A
1090Note that, because C<eval()> traps otherwise-fatal errors, it is useful for
1091determining whether a particular feature (such as C<socket()> or C<symlink()>)
a0d0e21e
LW
1092is implemented. It is also Perl's exception trapping mechanism, where
1093the die operator is used to raise exceptions.
1094
1095If the code to be executed doesn't vary, you may use the eval-BLOCK
1096form to trap run-time errors without incurring the penalty of
1097recompiling each time. The error, if any, is still returned in C<$@>.
1098Examples:
1099
54310121 1100 # make divide-by-zero nonfatal
a0d0e21e
LW
1101 eval { $answer = $a / $b; }; warn $@ if $@;
1102
1103 # same thing, but less efficient
1104 eval '$answer = $a / $b'; warn $@ if $@;
1105
1106 # a compile-time error
5a964f20 1107 eval { $answer = }; # WRONG
a0d0e21e
LW
1108
1109 # a run-time error
1110 eval '$answer ='; # sets $@
1111
7660c0ab 1112When using the C<eval{}> form as an exception trap in libraries, you may
774d564b
PP
1113wish not to trigger any C<__DIE__> hooks that user code may have
1114installed. You can use the C<local $SIG{__DIE__}> construct for this
1115purpose, as shown in this example:
1116
1117 # a very private exception trap for divide-by-zero
f86cebdf
GS
1118 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1119 warn $@ if $@;
774d564b
PP
1120
1121This is especially significant, given that C<__DIE__> hooks can call
7660c0ab 1122C<die()> again, which has the effect of changing their error messages:
774d564b
PP
1123
1124 # __DIE__ hooks may modify error messages
1125 {
f86cebdf
GS
1126 local $SIG{'__DIE__'} =
1127 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
c7cc6f1c
GS
1128 eval { die "foo lives here" };
1129 print $@ if $@; # prints "bar lives here"
774d564b
PP
1130 }
1131
7660c0ab 1132With an C<eval()>, you should be especially careful to remember what's
a0d0e21e
LW
1133being looked at when:
1134
1135 eval $x; # CASE 1
1136 eval "$x"; # CASE 2
1137
1138 eval '$x'; # CASE 3
1139 eval { $x }; # CASE 4
1140
5a964f20 1141 eval "\$$x++"; # CASE 5
a0d0e21e
LW
1142 $$x++; # CASE 6
1143
2f9daede 1144Cases 1 and 2 above behave identically: they run the code contained in
7660c0ab 1145the variable C<$x>. (Although case 2 has misleading double quotes making
2f9daede 1146the reader wonder what else might be happening (nothing is).) Cases 3
7660c0ab 1147and 4 likewise behave in the same way: they run the code C<'$x'>, which
2f9daede
TPG
1148does nothing but return the value of C<$x>. (Case 4 is preferred for
1149purely visual reasons, but it also has the advantage of compiling at
1150compile-time instead of at run-time.) Case 5 is a place where
54310121 1151normally you I<WOULD> like to use double quotes, except that in this
2f9daede
TPG
1152particular situation, you can just use symbolic references instead, as
1153in case 6.
a0d0e21e
LW
1154
1155=item exec LIST
1156
8bf3b016
GS
1157=item exec PROGRAM LIST
1158
7660c0ab
A
1159The C<exec()> function executes a system command I<AND NEVER RETURNS> -
1160use C<system()> instead of C<exec()> if you want it to return. It fails and
fb73857a
PP
1161returns FALSE only if the command does not exist I<and> it is executed
1162directly instead of via your system's command shell (see below).
a0d0e21e 1163
7660c0ab
A
1164Since it's a common mistake to use C<exec()> instead of C<system()>, Perl
1165warns you if there is a following statement which isn't C<die()>, C<warn()>,
1166or C<exit()> (if C<-w> is set - but you always do that). If you
1167I<really> want to follow an C<exec()> with some other statement, you
55d729e4
GS
1168can use one of these styles to avoid the warning:
1169
5a964f20
TC
1170 exec ('foo') or print STDERR "couldn't exec foo: $!";
1171 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
55d729e4 1172
5a964f20 1173If there is more than one argument in LIST, or if LIST is an array
f86cebdf 1174with more than one value, calls execvp(3) with the arguments in LIST.
5a964f20
TC
1175If there is only one scalar argument or an array with one element in it,
1176the argument is checked for shell metacharacters, and if there are any,
1177the entire argument is passed to the system's command shell for parsing
1178(this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1179If there are no shell metacharacters in the argument, it is split into
7660c0ab
A
1180words and passed directly to C<execvp()>, which is more efficient. Note:
1181C<exec()> and C<system()> do not flush your output buffer, so you may need to
5a964f20 1182set C<$|> to avoid lost output. Examples:
a0d0e21e
LW
1183
1184 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1185 exec "sort $outfile | uniq";
1186
1187If you don't really want to execute the first argument, but want to lie
1188to the program you are executing about its own name, you can specify
1189the program you actually want to run as an "indirect object" (without a
1190comma) in front of the LIST. (This always forces interpretation of the
54310121 1191LIST as a multivalued list, even if there is only a single scalar in
a0d0e21e
LW
1192the list.) Example:
1193
1194 $shell = '/bin/csh';
1195 exec $shell '-sh'; # pretend it's a login shell
1196
1197or, more directly,
1198
1199 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1200
bb32b41a
GS
1201When the arguments get executed via the system shell, results will
1202be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1203for details.
1204
ab4f32c2 1205Using an indirect object with C<exec()> or C<system()> is also more secure.
5a964f20
TC
1206This usage forces interpretation of the arguments as a multivalued list,
1207even if the list had just one argument. That way you're safe from the
1208shell expanding wildcards or splitting up words with whitespace in them.
1209
1210 @args = ( "echo surprise" );
1211
f86cebdf
GS
1212 system @args; # subject to shell escapes
1213 # if @args == 1
5a964f20
TC
1214 system { $args[0] } @args; # safe even with one-arg list
1215
1216The first version, the one without the indirect object, ran the I<echo>
1217program, passing it C<"surprise"> an argument. The second version
1218didn't--it tried to run a program literally called I<"echo surprise">,
1219didn't find it, and set C<$?> to a non-zero value indicating failure.
1220
ab4f32c2 1221Note that C<exec()> will not call your C<END> blocks, nor will it call
7660c0ab
A
1222any C<DESTROY> methods in your objects.
1223
a0d0e21e
LW
1224=item exists EXPR
1225
1226Returns TRUE if the specified hash key exists in its hash array, even
1227if the corresponding value is undefined.
1228
1229 print "Exists\n" if exists $array{$key};
1230 print "Defined\n" if defined $array{$key};
1231 print "True\n" if $array{$key};
1232
5f05dabc 1233A hash element can be TRUE only if it's defined, and defined if
a0d0e21e
LW
1234it exists, but the reverse doesn't necessarily hold true.
1235
1236Note that the EXPR can be arbitrarily complicated as long as the final
1237operation is a hash key lookup:
1238
5a964f20
TC
1239 if (exists $ref->{"A"}{"B"}{$key}) { ... }
1240
1241Although the last element will not spring into existence just because its
1242existence was tested, intervening ones will. Thus C<$ref-E<gt>{"A"}>
1243C<$ref-E<gt>{"B"}> will spring into existence due to the existence
1244test for a $key element. This autovivification may be fixed in a later
1245release.
a0d0e21e
LW
1246
1247=item exit EXPR
1248
1249Evaluates EXPR and exits immediately with that value. (Actually, it
1250calls any defined C<END> routines first, but the C<END> routines may not
1251abort the exit. Likewise any object destructors that need to be called
1252are called before exit.) Example:
1253
1254 $ans = <STDIN>;
1255 exit 0 if $ans =~ /^[Xx]/;
1256
7660c0ab
A
1257See also C<die()>. If EXPR is omitted, exits with C<0> status. The only
1258universally portable values for EXPR are C<0> for success and C<1> for error;
f86702cc
PP
1259all other values are subject to unpredictable interpretation depending
1260on the environment in which the Perl program is running.
a0d0e21e 1261
7660c0ab
A
1262You shouldn't use C<exit()> to abort a subroutine if there's any chance that
1263someone might want to trap whatever error happened. Use C<die()> instead,
1264which can be trapped by an C<eval()>.
28757baa 1265
5a964f20
TC
1266All C<END{}> blocks are run at exit time. See L<perlsub> for details.
1267
a0d0e21e
LW
1268=item exp EXPR
1269
54310121 1270=item exp
bbce6d69 1271
54310121 1272Returns I<e> (the natural logarithm base) to the power of EXPR.
a0d0e21e
LW
1273If EXPR is omitted, gives C<exp($_)>.
1274
1275=item fcntl FILEHANDLE,FUNCTION,SCALAR
1276
f86cebdf 1277Implements the fcntl(2) function. You'll probably have to say
a0d0e21e
LW
1278
1279 use Fcntl;
1280
0ade1984 1281first to get the correct constant definitions. Argument processing and
7660c0ab 1282value return works just like C<ioctl()> below.
a0d0e21e
LW
1283For example:
1284
1285 use Fcntl;
5a964f20
TC
1286 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1287 or die "can't fcntl F_GETFL: $!";
1288
ab4f32c2
A
1289You don't have to check for C<defined()> on the return from
1290C<fnctl()>. Like C<ioctl()>, it maps a C<0> return from the system
7660c0ab
A
1291call into "C<0> but true" in Perl. This string is true in
1292boolean context and C<0> in numeric context. It is also
5a964f20
TC
1293exempt from the normal B<-w> warnings on improper numeric
1294conversions.
1295
7660c0ab 1296Note that C<fcntl()> will produce a fatal error if used on a machine that
f86cebdf 1297doesn't implement fcntl(2).
a0d0e21e
LW
1298
1299=item fileno FILEHANDLE
1300
1301Returns the file descriptor for a filehandle. This is useful for
7660c0ab 1302constructing bitmaps for C<select()> and low-level POSIX tty-handling
5a964f20
TC
1303operations. If FILEHANDLE is an expression, the value is taken as
1304an indirect filehandle, generally its name.
1305
1306You can use this to find out whether two handles refer to the
1307same underlying descriptor:
1308
1309 if (fileno(THIS) == fileno(THAT)) {
1310 print "THIS and THAT are dups\n";
1311 }
a0d0e21e
LW
1312
1313=item flock FILEHANDLE,OPERATION
1314
f86cebdf 1315Calls flock(2), or an emulation of it, on FILEHANDLE. Returns TRUE for
68dc0745 1316success, FALSE on failure. Produces a fatal error if used on a machine
f86cebdf 1317that doesn't implement flock(2), fcntl(2) locking, or lockf(3). C<flock()>
68dc0745
PP
1318is Perl's portable file locking interface, although it locks only entire
1319files, not records.
8ebc5c01 1320
a3cb178b 1321On many platforms (including most versions or clones of Unix), locks
7660c0ab 1322established by C<flock()> are B<merely advisory>. Such discretionary locks
5a964f20 1323are more flexible, but offer fewer guarantees. This means that files
7660c0ab
A
1324locked with C<flock()> may be modified by programs that do not also use
1325C<flock()>. Windows NT and OS/2 are among the platforms which
5a964f20 1326enforce mandatory locking. See your local documentation for details.
a3cb178b 1327
8ebc5c01
PP
1328OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1329LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
68dc0745
PP
1330you can use the symbolic names if import them from the Fcntl module,
1331either individually, or as a group using the ':flock' tag. LOCK_SH
1332requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1333releases a previously requested lock. If LOCK_NB is added to LOCK_SH or
7660c0ab 1334LOCK_EX then C<flock()> will return immediately rather than blocking
68dc0745
PP
1335waiting for the lock (check the return status to see if you got it).
1336
1337To avoid the possibility of mis-coordination, Perl flushes FILEHANDLE
1338before (un)locking it.
8ebc5c01 1339
f86cebdf 1340Note that the emulation built with lockf(3) doesn't provide shared
8ebc5c01 1341locks, and it requires that FILEHANDLE be open with write intent. These
f86cebdf
GS
1342are the semantics that lockf(3) implements. Most (all?) systems
1343implement lockf(3) in terms of fcntl(2) locking, though, so the
8ebc5c01
PP
1344differing semantics shouldn't bite too many people.
1345
7660c0ab
A
1346Note also that some versions of C<flock()> cannot lock things over the
1347network; you would need to use the more system-specific C<fcntl()> for
f86cebdf
GS
1348that. If you like you can force Perl to ignore your system's flock(2)
1349function, and so provide its own fcntl(2)-based emulation, by passing
8ebc5c01
PP
1350the switch C<-Ud_flock> to the F<Configure> program when you configure
1351perl.
4633a7c4
LW
1352
1353Here's a mailbox appender for BSD systems.
a0d0e21e 1354
7e1af8bc 1355 use Fcntl ':flock'; # import LOCK_* constants
a0d0e21e
LW
1356
1357 sub lock {
7e1af8bc 1358 flock(MBOX,LOCK_EX);
a0d0e21e
LW
1359 # and, in case someone appended
1360 # while we were waiting...
1361 seek(MBOX, 0, 2);
1362 }
1363
1364 sub unlock {
7e1af8bc 1365 flock(MBOX,LOCK_UN);
a0d0e21e
LW
1366 }
1367
1368 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1369 or die "Can't open mailbox: $!";
1370
1371 lock();
1372 print MBOX $msg,"\n\n";
1373 unlock();
1374
cb1a09d0 1375See also L<DB_File> for other flock() examples.
a0d0e21e
LW
1376
1377=item fork
1378
f86cebdf 1379Does a fork(2) system call. Returns the child pid to the parent process,
7660c0ab 1380C<0> to the child process, or C<undef> if the fork is unsuccessful.
5a964f20 1381
a0d0e21e 1382Note: unflushed buffers remain unflushed in both processes, which means
7660c0ab
A
1383you may need to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()>
1384method of C<IO::Handle> to avoid duplicate output.
a0d0e21e 1385
7660c0ab 1386If you C<fork()> without ever waiting on your children, you will accumulate
a0d0e21e
LW
1387zombies:
1388
4633a7c4 1389 $SIG{CHLD} = sub { wait };
a0d0e21e 1390
54310121 1391There's also the double-fork trick (error checking on
7660c0ab 1392C<fork()> returns omitted);
a0d0e21e
LW
1393
1394 unless ($pid = fork) {
1395 unless (fork) {
1396 exec "what you really wanna do";
1397 die "no exec";
1398 # ... or ...
4633a7c4 1399 ## (some_perl_code_here)
a0d0e21e
LW
1400 exit 0;
1401 }
1402 exit 0;
1403 }
1404 waitpid($pid,0);
1405
cb1a09d0
AD
1406See also L<perlipc> for more examples of forking and reaping
1407moribund children.
1408
28757baa
PP
1409Note that if your forked child inherits system file descriptors like
1410STDIN and STDOUT that are actually connected by a pipe or socket, even
5a964f20 1411if you exit, then the remote server (such as, say, httpd or rsh) won't think
7660c0ab 1412you're done. You should reopen those to F</dev/null> if it's any issue.
28757baa 1413
cb1a09d0
AD
1414=item format
1415
7660c0ab 1416Declare a picture format for use by the C<write()> function. For
cb1a09d0
AD
1417example:
1418
54310121 1419 format Something =
cb1a09d0
AD
1420 Test: @<<<<<<<< @||||| @>>>>>
1421 $str, $%, '$' . int($num)
1422 .
1423
1424 $str = "widget";
184e9718 1425 $num = $cost/$quantity;
cb1a09d0
AD
1426 $~ = 'Something';
1427 write;
1428
1429See L<perlform> for many details and examples.
1430
8903cb82 1431=item formline PICTURE,LIST
a0d0e21e 1432
5a964f20 1433This is an internal function used by C<format>s, though you may call it,
a0d0e21e
LW
1434too. It formats (see L<perlform>) a list of values according to the
1435contents of PICTURE, placing the output into the format output
7660c0ab
A
1436accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
1437Eventually, when a C<write()> is done, the contents of
a0d0e21e 1438C<$^A> are written to some filehandle, but you could also read C<$^A>
7660c0ab
A
1439yourself and then set C<$^A> back to C<"">. Note that a format typically
1440does one C<formline()> per line of form, but the C<formline()> function itself
748a9306 1441doesn't care how many newlines are embedded in the PICTURE. This means
4633a7c4 1442that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
748a9306
LW
1443You may therefore need to use multiple formlines to implement a single
1444record format, just like the format compiler.
1445
5f05dabc 1446Be careful if you put double quotes around the picture, because an "C<@>"
748a9306 1447character may be taken to mean the beginning of an array name.
7660c0ab 1448C<formline()> always returns TRUE. See L<perlform> for other examples.
a0d0e21e
LW
1449
1450=item getc FILEHANDLE
1451
1452=item getc
1453
1454Returns the next character from the input file attached to FILEHANDLE,
9bc64814 1455or the undefined value at end of file, or if there was an error. If
3b02c43c
GS
1456FILEHANDLE is omitted, reads from STDIN. This is not particularly
1457efficient. It cannot be used to get unbuffered single-characters,
1458however. For that, try something more like:
4633a7c4
LW
1459
1460 if ($BSD_STYLE) {
1461 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1462 }
1463 else {
54310121 1464 system "stty", '-icanon', 'eol', "\001";
4633a7c4
LW
1465 }
1466
1467 $key = getc(STDIN);
1468
1469 if ($BSD_STYLE) {
1470 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1471 }
1472 else {
5f05dabc 1473 system "stty", 'icanon', 'eol', '^@'; # ASCII null
4633a7c4
LW
1474 }
1475 print "\n";
1476
54310121
PP
1477Determination of whether $BSD_STYLE should be set
1478is left as an exercise to the reader.
cb1a09d0 1479
7660c0ab 1480The C<POSIX::getattr()> function can do this more portably on systems
5a964f20 1481purporting POSIX compliance.
cb1a09d0 1482See also the C<Term::ReadKey> module from your nearest CPAN site;
54310121 1483details on CPAN can be found on L<perlmod/CPAN>.
a0d0e21e
LW
1484
1485=item getlogin
1486
5a964f20
TC
1487Implements the C library function of the same name, which on most
1488systems returns the current login from F</etc/utmp>, if any. If null,
7660c0ab 1489use C<getpwuid()>.
a0d0e21e 1490
f86702cc 1491 $login = getlogin || getpwuid($<) || "Kilroy";
a0d0e21e 1492
7660c0ab
A
1493Do not consider C<getlogin()> for authentication: it is not as
1494secure as C<getpwuid()>.
4633a7c4 1495
a0d0e21e
LW
1496=item getpeername SOCKET
1497
1498Returns the packed sockaddr address of other end of the SOCKET connection.
1499
4633a7c4
LW
1500 use Socket;
1501 $hersockaddr = getpeername(SOCK);
1502 ($port, $iaddr) = unpack_sockaddr_in($hersockaddr);
1503 $herhostname = gethostbyaddr($iaddr, AF_INET);
1504 $herstraddr = inet_ntoa($iaddr);
a0d0e21e
LW
1505
1506=item getpgrp PID
1507
47e29363 1508Returns the current process group for the specified PID. Use
7660c0ab 1509a PID of C<0> to get the current process group for the
4633a7c4 1510current process. Will raise an exception if used on a machine that
f86cebdf 1511doesn't implement getpgrp(2). If PID is omitted, returns process
7660c0ab
A
1512group of current process. Note that the POSIX version of C<getpgrp()>
1513does not accept a PID argument, so only C<PID==0> is truly portable.
a0d0e21e
LW
1514
1515=item getppid
1516
1517Returns the process id of the parent process.
1518
1519=item getpriority WHICH,WHO
1520
4633a7c4
LW
1521Returns the current priority for a process, a process group, or a user.
1522(See L<getpriority(2)>.) Will raise a fatal exception if used on a
f86cebdf 1523machine that doesn't implement getpriority(2).
a0d0e21e
LW
1524
1525=item getpwnam NAME
1526
1527=item getgrnam NAME
1528
1529=item gethostbyname NAME
1530
1531=item getnetbyname NAME
1532
1533=item getprotobyname NAME
1534
1535=item getpwuid UID
1536
1537=item getgrgid GID
1538
1539=item getservbyname NAME,PROTO
1540
1541=item gethostbyaddr ADDR,ADDRTYPE
1542
1543=item getnetbyaddr ADDR,ADDRTYPE
1544
1545=item getprotobynumber NUMBER
1546
1547=item getservbyport PORT,PROTO
1548
1549=item getpwent
1550
1551=item getgrent
1552
1553=item gethostent
1554
1555=item getnetent
1556
1557=item getprotoent
1558
1559=item getservent
1560
1561=item setpwent
1562
1563=item setgrent
1564
1565=item sethostent STAYOPEN
1566
1567=item setnetent STAYOPEN
1568
1569=item setprotoent STAYOPEN
1570
1571=item setservent STAYOPEN
1572
1573=item endpwent
1574
1575=item endgrent
1576
1577=item endhostent
1578
1579=item endnetent
1580
1581=item endprotoent
1582
1583=item endservent
1584
1585These routines perform the same functions as their counterparts in the
5a964f20 1586system library. In list context, the return values from the
a0d0e21e
LW
1587various get routines are as follows:
1588
1589 ($name,$passwd,$uid,$gid,
6ee623d5 1590 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
a0d0e21e
LW
1591 ($name,$passwd,$gid,$members) = getgr*
1592 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1593 ($name,$aliases,$addrtype,$net) = getnet*
1594 ($name,$aliases,$proto) = getproto*
1595 ($name,$aliases,$port,$proto) = getserv*
1596
1597(If the entry doesn't exist you get a null list.)
1598
5a964f20 1599In scalar context, you get the name, unless the function was a
a0d0e21e
LW
1600lookup by name, in which case you get the other thing, whatever it is.
1601(If the entry doesn't exist you get the undefined value.) For example:
1602
5a964f20
TC
1603 $uid = getpwnam($name);
1604 $name = getpwuid($num);
1605 $name = getpwent();
1606 $gid = getgrnam($name);
1607 $name = getgrgid($num;
1608 $name = getgrent();
1609 #etc.
a0d0e21e 1610
7660c0ab 1611In I<getpw*()> the fields C<$quota>, C<$comment>, and C<$expire> are special
6ee623d5 1612cases in the sense that in many systems they are unsupported. If the
7660c0ab
A
1613C<$quota> is unsupported, it is an empty scalar. If it is supported, it
1614usually encodes the disk quota. If the C<$comment> field is unsupported,
6ee623d5
GS
1615it is an empty scalar. If it is supported it usually encodes some
1616administrative comment about the user. In some systems the $quota
7660c0ab
A
1617field may be C<$change> or C<$age>, fields that have to do with password
1618aging. In some systems the C<$comment> field may be C<$class>. The C<$expire>
6ee623d5
GS
1619field, if present, encodes the expiration period of the account or the
1620password. For the availability and the exact meaning of these fields
f86cebdf 1621in your system, please consult your getpwnam(3) documentation and your
7660c0ab
A
1622F<pwd.h> file. You can also find out from within Perl which meaning
1623your C<$quota> and C<$comment> fields have and whether you have the C<$expire>
1624field by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
1625C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>.
6ee623d5 1626
7660c0ab 1627The C<$members> value returned by I<getgr*()> is a space separated list of
a0d0e21e
LW
1628the login names of the members of the group.
1629
1630For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1631C, it will be returned to you via C<$?> if the function call fails. The
7660c0ab 1632C<@addrs> value returned by a successful call is a list of the raw
a0d0e21e
LW
1633addresses returned by the corresponding system library call. In the
1634Internet domain, each address is four bytes long and you can unpack it
1635by saying something like:
1636
1637 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1638
5a964f20
TC
1639If you get tired of remembering which element of the return list contains
1640which return value, by-name interfaces are also provided in modules:
7660c0ab
A
1641C<File::stat>, C<Net::hostent>, C<Net::netent>, C<Net::protoent>, C<Net::servent>,
1642C<Time::gmtime>, C<Time::localtime>, and C<User::grent>. These override the
5a964f20
TC
1643normal built-in, replacing them with versions that return objects with
1644the appropriate names for each field. For example:
1645
1646 use File::stat;
1647 use User::pwent;
1648 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
1649
1650Even though it looks like they're the same method calls (uid),
7660c0ab 1651they aren't, because a C<File::stat> object is different from a C<User::pwent> object.
5a964f20 1652
a0d0e21e
LW
1653=item getsockname SOCKET
1654
1655Returns the packed sockaddr address of this end of the SOCKET connection.
1656
4633a7c4
LW
1657 use Socket;
1658 $mysockaddr = getsockname(SOCK);
1659 ($port, $myaddr) = unpack_sockaddr_in($mysockaddr);
a0d0e21e
LW
1660
1661=item getsockopt SOCKET,LEVEL,OPTNAME
1662
5a964f20 1663Returns the socket option requested, or undef if there is an error.
a0d0e21e
LW
1664
1665=item glob EXPR
1666
0a753a76
PP
1667=item glob
1668
7660c0ab 1669Returns the value of EXPR with filename expansions such as the standard Unix shell F</bin/sh> would
68dc0745 1670do. This is the internal function implementing the C<E<lt>*.cE<gt>>
7660c0ab 1671operator, but you can use it directly. If EXPR is omitted, C<$_> is used.
68dc0745
PP
1672The C<E<lt>*.cE<gt>> operator is discussed in more detail in
1673L<perlop/"I/O Operators">.
a0d0e21e
LW
1674
1675=item gmtime EXPR
1676
1677Converts a time as returned by the time function to a 9-element array
54310121 1678with the time localized for the standard Greenwich time zone.
4633a7c4 1679Typically used as follows:
a0d0e21e 1680
54310121 1681 # 0 1 2 3 4 5 6 7 8
a0d0e21e
LW
1682 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1683 gmtime(time);
1684
1685All array elements are numeric, and come straight out of a struct tm.
7660c0ab
A
1686In particular this means that C<$mon> has the range C<0..11> and C<$wday> has
1687the range C<0..6> with sunday as day C<0>. Also, C<$year> is the number of
1688years since 1900, that is, C<$year> is C<123> in year 2023, I<not> simply the last two digits of the year.
2f9daede
TPG
1689
1690If EXPR is omitted, does C<gmtime(time())>.
a0d0e21e 1691
f86cebdf 1692In scalar context, returns the ctime(3) value:
0a753a76
PP
1693
1694 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
1695
7660c0ab 1696Also see the C<timegm()> function provided by the C<Time::Local> module,
f86cebdf 1697and the strftime(3) function available via the POSIX module.
7660c0ab
A
1698
1699This scalar value is B<not> locale dependent, see L<perllocale>, but
1700instead a Perl builtin. Also see the C<Time::Local> module, and the
f86cebdf 1701strftime(3) and mktime(3) function available via the POSIX module. To
7660c0ab
A
1702get somewhat similar but locale dependent date strings, set up your
1703locale environment variables appropriately (please see L<perllocale>)
1704and try for example:
1705
1706 use POSIX qw(strftime);
1707 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
1708
1709Note that the C<%a> and C<%b>, the short forms of the day of the week
1710and the month of the year, may not necessarily be three characters wide.
0a753a76 1711
a0d0e21e
LW
1712=item goto LABEL
1713
748a9306
LW
1714=item goto EXPR
1715
a0d0e21e
LW
1716=item goto &NAME
1717
7660c0ab 1718The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
a0d0e21e 1719execution there. It may not be used to go into any construct that
7660c0ab 1720requires initialization, such as a subroutine or a C<foreach> loop. It
0a753a76 1721also can't be used to go into a construct that is optimized away,
7660c0ab 1722or to get out of a block or subroutine given to C<sort()>.
0a753a76 1723It can be used to go almost anywhere else within the dynamic scope,
a0d0e21e 1724including out of subroutines, but it's usually better to use some other
ab4f32c2 1725construct such as C<last> or C<die()>. The author of Perl has never felt the
7660c0ab 1726need to use this form of C<goto> (in Perl, that is--C is another matter).
a0d0e21e 1727
7660c0ab
A
1728The C<goto-EXPR> form expects a label name, whose scope will be resolved
1729dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
748a9306
LW
1730necessarily recommended if you're optimizing for maintainability:
1731
1732 goto ("FOO", "BAR", "GLARCH")[$i];
1733
7660c0ab 1734The C<goto-&NAME> form is highly magical, and substitutes a call to the
a0d0e21e 1735named subroutine for the currently running subroutine. This is used by
7660c0ab 1736C<AUTOLOAD> subroutines that wish to load another subroutine and then
a0d0e21e 1737pretend that the other subroutine had been called in the first place
7660c0ab
A
1738(except that any modifications to C<@_> in the current subroutine are
1739propagated to the other subroutine.) After the C<goto>, not even C<caller()>
a0d0e21e
LW
1740will be able to tell that this routine was called first.
1741
1742=item grep BLOCK LIST
1743
1744=item grep EXPR,LIST
1745
f86cebdf 1746This is similar in spirit to, but not the same as, grep(1)
2f9daede
TPG
1747and its relatives. In particular, it is not limited to using
1748regular expressions.
1749
a0d0e21e 1750Evaluates the BLOCK or EXPR for each element of LIST (locally setting
7660c0ab 1751C<$_> to each element) and returns the list value consisting of those
a0d0e21e
LW
1752elements for which the expression evaluated to TRUE. In a scalar
1753context, returns the number of times the expression was TRUE.
1754
1755 @foo = grep(!/^#/, @bar); # weed out comments
1756
1757or equivalently,
1758
1759 @foo = grep {!/^#/} @bar; # weed out comments
1760
7660c0ab 1761Note that, because C<$_> is a reference into the list value, it can be used
a0d0e21e
LW
1762to modify the elements of the array. While this is useful and
1763supported, it can cause bizarre results if the LIST is not a named
2f9daede 1764array. Similarly, grep returns aliases into the original list,
f86cebdf 1765much like the way that a for loop's index variable aliases the list
2f9daede 1766elements. That is, modifying an element of a list returned by grep
ab4f32c2 1767(for example, in a C<foreach>, C<map()> or another C<grep()>)
2f9daede 1768actually modifies the element in the original list.
a0d0e21e 1769
fb73857a 1770See also L</map> for an array composed of the results of the BLOCK or EXPR.
38325410 1771
a0d0e21e
LW
1772=item hex EXPR
1773
54310121 1774=item hex
bbce6d69 1775
54310121 1776Interprets EXPR as a hex string and returns the corresponding
2f9daede 1777value. (To convert strings that might start with either 0 or 0x
7660c0ab 1778see L</oct>.) If EXPR is omitted, uses C<$_>.
2f9daede
TPG
1779
1780 print hex '0xAf'; # prints '175'
1781 print hex 'aF'; # same
a0d0e21e
LW
1782
1783=item import
1784
7660c0ab 1785There is no builtin C<import()> function. It is just an ordinary
4633a7c4 1786method (subroutine) defined (or inherited) by modules that wish to export
7660c0ab 1787names to another module. The C<use()> function calls the C<import()> method
54310121 1788for the package used. See also L</use()>, L<perlmod>, and L<Exporter>.
a0d0e21e
LW
1789
1790=item index STR,SUBSTR,POSITION
1791
1792=item index STR,SUBSTR
1793
4633a7c4
LW
1794Returns the position of the first occurrence of SUBSTR in STR at or after
1795POSITION. If POSITION is omitted, starts searching from the beginning of
7660c0ab 1796the string. The return value is based at C<0> (or whatever you've set the C<$[>
4633a7c4 1797variable to--but don't do that). If the substring is not found, returns
7660c0ab 1798one less than the base, ordinarily C<-1>.
a0d0e21e
LW
1799
1800=item int EXPR
1801
54310121 1802=item int
bbce6d69 1803
7660c0ab 1804Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
5a964f20 1805You should not use this for rounding, because it truncates
7660c0ab
A
1806towards C<0>, and because machine representations of floating point
1807numbers can sometimes produce counterintuitive results. Usually C<sprintf()> or C<printf()>,
1808or the C<POSIX::floor> or C<POSIX::ceil> functions, would serve you better.
a0d0e21e
LW
1809
1810=item ioctl FILEHANDLE,FUNCTION,SCALAR
1811
f86cebdf 1812Implements the ioctl(2) function. You'll probably have to say
a0d0e21e 1813
4633a7c4 1814 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
a0d0e21e 1815
4633a7c4 1816first to get the correct function definitions. If F<ioctl.ph> doesn't
a0d0e21e 1817exist or doesn't have the correct definitions you'll have to roll your
4633a7c4 1818own, based on your C header files such as F<E<lt>sys/ioctl.hE<gt>>.
5a964f20 1819(There is a Perl script called B<h2ph> that comes with the Perl kit that
54310121 1820may help you in this, but it's nontrivial.) SCALAR will be read and/or
4633a7c4 1821written depending on the FUNCTION--a pointer to the string value of SCALAR
ab4f32c2 1822will be passed as the third argument of the actual C<ioctl()> call. (If SCALAR
4633a7c4
LW
1823has no string value but does have a numeric value, that value will be
1824passed rather than a pointer to the string value. To guarantee this to be
7660c0ab 1825TRUE, add a C<0> to the scalar before using it.) The C<pack()> and C<unpack()>
4633a7c4 1826functions are useful for manipulating the values of structures used by
7660c0ab 1827C<ioctl()>. The following example sets the erase character to DEL.
a0d0e21e
LW
1828
1829 require 'ioctl.ph';
4633a7c4
LW
1830 $getp = &TIOCGETP;
1831 die "NO TIOCGETP" if $@ || !$getp;
a0d0e21e 1832 $sgttyb_t = "ccccs"; # 4 chars and a short
4633a7c4 1833 if (ioctl(STDIN,$getp,$sgttyb)) {
a0d0e21e
LW
1834 @ary = unpack($sgttyb_t,$sgttyb);
1835 $ary[2] = 127;
1836 $sgttyb = pack($sgttyb_t,@ary);
4633a7c4 1837 ioctl(STDIN,&TIOCSETP,$sgttyb)
a0d0e21e
LW
1838 || die "Can't ioctl: $!";
1839 }
1840
ab4f32c2 1841The return value of C<ioctl()> (and C<fcntl()>) is as follows:
a0d0e21e
LW
1842
1843 if OS returns: then Perl returns:
1844 -1 undefined value
1845 0 string "0 but true"
1846 anything else that number
1847
1848Thus Perl returns TRUE on success and FALSE on failure, yet you can
1849still easily determine the actual value returned by the operating
1850system:
1851
1852 ($retval = ioctl(...)) || ($retval = -1);
1853 printf "System returned %d\n", $retval;
1854
7660c0ab 1855The special string "C<0> but true" is excempt from B<-w> complaints
5a964f20
TC
1856about improper numeric conversions.
1857
a0d0e21e
LW
1858=item join EXPR,LIST
1859
54310121 1860Joins the separate strings of LIST into a single string with
a0d0e21e
LW
1861fields separated by the value of EXPR, and returns the string.
1862Example:
1863
1864 $_ = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
1865
7660c0ab 1866See L</split>.
a0d0e21e 1867
aa689395
PP
1868=item keys HASH
1869
1d2dff63
GS
1870Returns a list consisting of all the keys of the named hash. (In a
1871scalar context, returns the number of keys.) The keys are returned in
aa689395 1872an apparently random order, but it is the same order as either the
7660c0ab 1873C<values()> or C<each()> function produces (given that the hash has not been
aa689395 1874modified). As a side effect, it resets HASH's iterator.
a0d0e21e 1875
aa689395 1876Here is yet another way to print your environment:
a0d0e21e
LW
1877
1878 @keys = keys %ENV;
1879 @values = values %ENV;
1880 while ($#keys >= 0) {
1881 print pop(@keys), '=', pop(@values), "\n";
1882 }
1883
1884or how about sorted by key:
1885
1886 foreach $key (sort(keys %ENV)) {
1887 print $key, '=', $ENV{$key}, "\n";
1888 }
1889
aca803df 1890To sort a hash by value, you'll need to use a C<sort()> function.
aa689395 1891Here's a descending numeric sort of a hash by its values:
4633a7c4 1892
5a964f20 1893 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
4633a7c4
LW
1894 printf "%4d %s\n", $hash{$key}, $key;
1895 }
1896
ab4f32c2 1897As an lvalue C<keys()> allows you to increase the number of hash buckets
aa689395
PP
1898allocated for the given hash. This can gain you a measure of efficiency if
1899you know the hash is going to get big. (This is similar to pre-extending
1900an array by assigning a larger number to $#array.) If you say
55497cff
PP
1901
1902 keys %hash = 200;
1903
5a964f20
TC
1904then C<%hash> will have at least 200 buckets allocated for it--256 of them, in fact, since
1905it rounds up to the next power of two. These
55497cff
PP
1906buckets will be retained even if you do C<%hash = ()>, use C<undef
1907%hash> if you want to free the storage while C<%hash> is still in scope.
1908You can't shrink the number of buckets allocated for the hash using
ab4f32c2 1909C<keys()> in this way (but you needn't worry about doing this by accident,
55497cff
PP
1910as trying has no effect).
1911
a0d0e21e
LW
1912=item kill LIST
1913
54310121
PP
1914Sends a signal to a list of processes. The first element of
1915the list must be the signal to send. Returns the number of
4633a7c4 1916processes successfully signaled.
a0d0e21e
LW
1917
1918 $cnt = kill 1, $child1, $child2;
1919 kill 9, @goners;
1920
4633a7c4
LW
1921Unlike in the shell, in Perl if the I<SIGNAL> is negative, it kills
1922process groups instead of processes. (On System V, a negative I<PROCESS>
1923number will also kill process groups, but that's not portable.) That
1924means you usually want to use positive not negative signals. You may also
da0045b7 1925use a signal name in quotes. See L<perlipc/"Signals"> for details.
a0d0e21e
LW
1926
1927=item last LABEL
1928
1929=item last
1930
1931The C<last> command is like the C<break> statement in C (as used in
1932loops); it immediately exits the loop in question. If the LABEL is
1933omitted, the command refers to the innermost enclosing loop. The
1934C<continue> block, if any, is not executed:
1935
4633a7c4
LW
1936 LINE: while (<STDIN>) {
1937 last LINE if /^$/; # exit when done with header
5a964f20 1938 #...
a0d0e21e
LW
1939 }
1940
1d2dff63
GS
1941See also L</continue> for an illustration of how C<last>, C<next>, and
1942C<redo> work.
1943
a0d0e21e
LW
1944=item lc EXPR
1945
54310121 1946=item lc
bbce6d69 1947
a0d0e21e 1948Returns an lowercased version of EXPR. This is the internal function
7660c0ab 1949implementing the C<\L> escape in double-quoted strings.
a0ed51b3 1950Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
a0d0e21e 1951
7660c0ab 1952If EXPR is omitted, uses C<$_>.
bbce6d69 1953
a0d0e21e
LW
1954=item lcfirst EXPR
1955
54310121 1956=item lcfirst
bbce6d69 1957
a0d0e21e 1958Returns the value of EXPR with the first character lowercased. This is
7660c0ab 1959the internal function implementing the C<\l> escape in double-quoted strings.
a0ed51b3 1960Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
a0d0e21e 1961
7660c0ab 1962If EXPR is omitted, uses C<$_>.
bbce6d69 1963
a0d0e21e
LW
1964=item length EXPR
1965
54310121 1966=item length
bbce6d69 1967
a0ed51b3 1968Returns the length in characters of the value of EXPR. If EXPR is
7660c0ab 1969omitted, returns length of C<$_>.
a0d0e21e
LW
1970
1971=item link OLDFILE,NEWFILE
1972
5a964f20
TC
1973Creates a new filename linked to the old filename. Returns TRUE for
1974success, FALSE otherwise.
a0d0e21e
LW
1975
1976=item listen SOCKET,QUEUESIZE
1977
1978Does the same thing that the listen system call does. Returns TRUE if
4633a7c4 1979it succeeded, FALSE otherwise. See example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
1980
1981=item local EXPR
1982
5a964f20
TC
1983A local modifies the listed variables to be local to the enclosing
1984block, file, or eval. If more than one value is listed, the list must
1985be placed in parentheses. See L<perlsub/"Temporary Values via local()">
1986for details, including issues with tied arrays and hashes.
a0d0e21e 1987
7660c0ab 1988You really probably want to be using C<my()> instead, because C<local()> isn't
7b8d334a 1989what most people think of as "local". See L<perlsub/"Private Variables
cb1a09d0 1990via my()"> for details.
a0d0e21e
LW
1991
1992=item localtime EXPR
1993
1994Converts a time as returned by the time function to a 9-element array
5f05dabc 1995with the time analyzed for the local time zone. Typically used as
a0d0e21e
LW
1996follows:
1997
54310121 1998 # 0 1 2 3 4 5 6 7 8
a0d0e21e
LW
1999 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2000 localtime(time);
2001
2002All array elements are numeric, and come straight out of a struct tm.
7660c0ab
A
2003In particular this means that C<$mon> has the range C<0..11> and C<$wday> has
2004the range C<0..6> with sunday as day C<0>. Also, C<$year> is the number of
2005years 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
2006
2007If EXPR is omitted, uses the current time (C<localtime(time)>).
a0d0e21e 2008
f86cebdf 2009In scalar context, returns the ctime(3) value:
a0d0e21e 2010
5f05dabc 2011 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
a0d0e21e 2012
a3cb178b 2013This scalar value is B<not> locale dependent, see L<perllocale>, but
7660c0ab 2014instead a Perl builtin. Also see the C<Time::Local> module, and the
f86cebdf 2015strftime(3) and mktime(3) function available via the POSIX module. To
a3cb178b
GS
2016get somewhat similar but locale dependent date strings, set up your
2017locale environment variables appropriately (please see L<perllocale>)
5a964f20 2018and try for example:
a3cb178b 2019
5a964f20 2020 use POSIX qw(strftime);
a3cb178b
GS
2021 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
2022
2023Note that the C<%a> and C<%b>, the short forms of the day of the week
2024and the month of the year, may not necessarily be three characters wide.
a0d0e21e
LW
2025
2026=item log EXPR
2027
54310121 2028=item log
bbce6d69 2029
5a964f20 2030Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted, returns log
7660c0ab 2031of C<$_>.
a0d0e21e
LW
2032
2033=item lstat FILEHANDLE
2034
2035=item lstat EXPR
2036
54310121 2037=item lstat
bbce6d69 2038
7660c0ab 2039Does the same thing as the C<stat()> function (including setting the
5a964f20
TC
2040special C<_> filehandle) but stats a symbolic link instead of the file
2041the symbolic link points to. If symbolic links are unimplemented on
7660c0ab 2042your system, a normal C<stat()> is done.
a0d0e21e 2043
7660c0ab 2044If EXPR is omitted, stats C<$_>.
bbce6d69 2045
a0d0e21e
LW
2046=item m//
2047
2048The match operator. See L<perlop>.
2049
2050=item map BLOCK LIST
2051
2052=item map EXPR,LIST
2053
7660c0ab 2054Evaluates the BLOCK or EXPR for each element of LIST (locally setting C<$_> to each
a0d0e21e
LW
2055element) and returns the list value composed of the results of each such
2056evaluation. Evaluates BLOCK or EXPR in a list context, so each element of LIST
2057may produce zero, one, or more elements in the returned value.
2058
2059 @chars = map(chr, @nums);
2060
2061translates a list of numbers to the corresponding characters. And
2062
4633a7c4 2063 %hash = map { getkey($_) => $_ } @array;
a0d0e21e
LW
2064
2065is just a funny way to write
2066
2067 %hash = ();
2068 foreach $_ (@array) {
4633a7c4 2069 $hash{getkey($_)} = $_;
a0d0e21e
LW
2070 }
2071
7660c0ab 2072Note that, because C<$_> is a reference into the list value, it can be used
fb73857a
PP
2073to modify the elements of the array. While this is useful and
2074supported, it can cause bizarre results if the LIST is not a named
2075array. See also L</grep> for an array composed of those items of the
2076original list for which the BLOCK or EXPR evaluates to true.
2077
a0d0e21e
LW
2078=item mkdir FILENAME,MODE
2079
2080Creates the directory specified by FILENAME, with permissions specified
5a964f20
TC
2081by MODE (as modified by umask). If it succeeds it returns TRUE, otherwise
2082it returns FALSE and sets C<$!> (errno).
a0d0e21e
LW
2083
2084=item msgctl ID,CMD,ARG
2085
f86cebdf 2086Calls the System V IPC function msgctl(2). You'll probably have to say
0ade1984
JH
2087
2088 use IPC::SysV;
2089
7660c0ab
A
2090first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2091then ARG must be a variable which will hold the returned C<msqid_ds>
ab4f32c2 2092structure. Returns like C<ioctl()>: the undefined value for error, "C<0> but
0ade1984 2093true" for zero, or the actual return value otherwise. See also
7660c0ab 2094C<IPC::SysV> and C<IPC::Semaphore::Msg> documentation.
a0d0e21e
LW
2095
2096=item msgget KEY,FLAGS
2097
f86cebdf 2098Calls the System V IPC function msgget(2). Returns the message queue
7660c0ab
A
2099id, or the undefined value if there is an error. See also C<IPC::SysV>
2100and C<IPC::SysV::Msg> documentation.
a0d0e21e
LW
2101
2102=item msgsnd ID,MSG,FLAGS
2103
2104Calls the System V IPC function msgsnd to send the message MSG to the
2105message queue ID. MSG must begin with the long integer message type,
c07a80fd 2106which may be created with C<pack("l", $type)>. Returns TRUE if
7660c0ab
A
2107successful, or FALSE if there is an error. See also C<IPC::SysV>
2108and C<IPC::SysV::Msg> documentation.
a0d0e21e
LW
2109
2110=item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2111
2112Calls the System V IPC function msgrcv to receive a message from
2113message queue ID into variable VAR with a maximum message size of
0ade1984
JH
2114SIZE. Note that if a message is received, the message type will be
2115the first thing in VAR, and the maximum length of VAR is SIZE plus the
2116size of the message type. Returns TRUE if successful, or FALSE if
7660c0ab 2117there is an error. See also C<IPC::SysV> and C<IPC::SysV::Msg> documentation.
a0d0e21e
LW
2118
2119=item my EXPR
2120
ab4f32c2
A
2121A C<my()> declares the listed variables to be local (lexically) to the
2122enclosing block, file, or C<eval()>. If
5f05dabc 2123more than one value is listed, the list must be placed in parentheses. See
cb1a09d0 2124L<perlsub/"Private Variables via my()"> for details.
4633a7c4 2125
a0d0e21e
LW
2126=item next LABEL
2127
2128=item next
2129
2130The C<next> command is like the C<continue> statement in C; it starts
2131the next iteration of the loop:
2132
4633a7c4
LW
2133 LINE: while (<STDIN>) {
2134 next LINE if /^#/; # discard comments
5a964f20 2135 #...
a0d0e21e
LW
2136 }
2137
2138Note that if there were a C<continue> block on the above, it would get
2139executed even on discarded lines. If the LABEL is omitted, the command
2140refers to the innermost enclosing loop.
2141
1d2dff63
GS
2142See also L</continue> for an illustration of how C<last>, C<next>, and
2143C<redo> work.
2144
a0d0e21e
LW
2145=item no Module LIST
2146
7660c0ab 2147See the L</use> function, which C<no> is the opposite of.
a0d0e21e
LW
2148
2149=item oct EXPR
2150
54310121 2151=item oct
bbce6d69 2152
4633a7c4 2153Interprets EXPR as an octal string and returns the corresponding
7660c0ab 2154value. (If EXPR happens to start off with C<0x>, interprets it as
4633a7c4
LW
2155a hex string instead.) The following will handle decimal, octal, and
2156hex in the standard Perl or C notation:
a0d0e21e
LW
2157
2158 $val = oct($val) if $val =~ /^0/;
2159
7660c0ab
A
2160If EXPR is omitted, uses C<$_>. This function is commonly used when
2161a string such as C<644> needs to be converted into a file mode, for
2f9daede
TPG
2162example. (Although perl will automatically convert strings into
2163numbers as needed, this automatic conversion assumes base 10.)
a0d0e21e
LW
2164
2165=item open FILEHANDLE,EXPR
2166
2167=item open FILEHANDLE
2168
2169Opens the file whose filename is given by EXPR, and associates it with
5f05dabc
PP
2170FILEHANDLE. If FILEHANDLE is an expression, its value is used as the
2171name of the real filehandle wanted. If EXPR is omitted, the scalar
2172variable of the same name as the FILEHANDLE contains the filename.
ab4f32c2
A
2173(Note that lexical variables--those declared with C<my()>--will not work
2174for this purpose; so if you're using C<my()>, specify EXPR in your call
5f05dabc
PP
2175to open.)
2176
7660c0ab
A
2177If the filename begins with C<'E<lt>'> or nothing, the file is opened for input.
2178If the filename begins with C<'E<gt>'>, the file is truncated and opened for
2179output, being created if necessary. If the filename begins with C<'E<gt>E<gt>'>,
fbb426e4 2180the file is opened for appending, again being created if necessary.
7660c0ab
A
2181You can put a C<'+'> in front of the C<'E<gt>'> or C<'E<lt>'> to indicate that
2182you want both read and write access to the file; thus C<'+E<lt>'> is almost
2183always preferred for read/write updates--the C<'+E<gt>'> mode would clobber the
5a964f20
TC
2184file first. You can't usually use either read-write mode for updating
2185textfiles, since they have variable length records. See the B<-i>
2186switch in L<perlrun> for a better approach.
2187
2188The prefix and the filename may be separated with spaces.
f86cebdf 2189These various prefixes correspond to the fopen(3) modes of C<'r'>, C<'r+'>, C<'w'>,
7660c0ab 2190C<'w+'>, C<'a'>, and C<'a+'>.
5f05dabc 2191
7660c0ab 2192If the filename begins with C<'|'>, the filename is interpreted as a
5a964f20 2193command to which output is to be piped, and if the filename ends with a
7660c0ab
A
2194C<'|'>, the filename is interpreted See L<perlipc/"Using open() for IPC">
2195for more examples of this. (You are not allowed to C<open()> to a command
5a964f20
TC
2196that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2197and L<perlipc/"Bidirectional Communication"> for alternatives.)
cb1a09d0 2198
7660c0ab 2199Opening C<'-'> opens STDIN and opening C<'E<gt>-'> opens STDOUT. Open returns
ab4f32c2 2200nonzero upon success, the undefined value otherwise. If the C<open()>
4633a7c4 2201involved a pipe, the return value happens to be the pid of the
54310121 2202subprocess.
cb1a09d0
AD
2203
2204If you're unfortunate enough to be running Perl on a system that
2205distinguishes between text files and binary files (modern operating
2206systems don't care), then you should check out L</binmode> for tips for
ab4f32c2 2207dealing with this. The key distinction between systems that need C<binmode()>
5a964f20
TC
2208and those that don't is their text file formats. Systems like Unix, MacOS, and
2209Plan9, which delimit lines with a single character, and which encode that
ab4f32c2 2210character in C as C<"\n">, do not need C<binmode()>. The rest need it.
cb1a09d0 2211
fb73857a 2212When opening a file, it's usually a bad idea to continue normal execution
ab4f32c2
A
2213if the request failed, so C<open()> is frequently used in connection with
2214C<die()>. Even if C<die()> won't do what you want (say, in a CGI script,
fb73857a 2215where you want to make a nicely formatted error message (but there are
5a964f20 2216modules that can help with that problem)) you should always check
fb73857a
PP
2217the return value from opening a file. The infrequent exception is when
2218working with an unopened filehandle is actually what you want to do.
2219
cb1a09d0 2220Examples:
a0d0e21e
LW
2221
2222 $ARTICLE = 100;
2223 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2224 while (<ARTICLE>) {...
2225
2226 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
fb73857a 2227 # if the open fails, output is discarded
a0d0e21e 2228
fb73857a
PP
2229 open(DBASE, '+<dbase.mine') # open for update
2230 or die "Can't open 'dbase.mine' for update: $!";
cb1a09d0 2231
fb73857a
PP
2232 open(ARTICLE, "caesar <$article |") # decrypt article
2233 or die "Can't start caesar: $!";
a0d0e21e 2234
fb73857a
PP
2235 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
2236 or die "Can't start sort: $!";
a0d0e21e
LW
2237
2238 # process argument list of files along with any includes
2239
2240 foreach $file (@ARGV) {
2241 process($file, 'fh00');
2242 }
2243
2244 sub process {
5a964f20 2245 my($filename, $input) = @_;
a0d0e21e
LW
2246 $input++; # this is a string increment
2247 unless (open($input, $filename)) {
2248 print STDERR "Can't open $filename: $!\n";
2249 return;
2250 }
2251
5a964f20 2252 local $_;
a0d0e21e
LW
2253 while (<$input>) { # note use of indirection
2254 if (/^#include "(.*)"/) {
2255 process($1, $input);
2256 next;
2257 }
5a964f20 2258 #... # whatever
a0d0e21e
LW
2259 }
2260 }
2261
2262You may also, in the Bourne shell tradition, specify an EXPR beginning
7660c0ab 2263with C<'E<gt>&'>, in which case the rest of the string is interpreted as the
5a964f20 2264name of a filehandle (or file descriptor, if numeric) to be
7660c0ab
A
2265duped and opened. You may use C<&> after C<E<gt>>, C<E<gt>E<gt>>, C<E<lt>>, C<+E<gt>>,
2266C<+E<gt>E<gt>>, and C<+E<lt>>. The
a0d0e21e 2267mode you specify should match the mode of the original filehandle.
184e9718 2268(Duping a filehandle does not take into account any existing contents of
cb1a09d0 2269stdio buffers.)
a0d0e21e
LW
2270Here is a script that saves, redirects, and restores STDOUT and
2271STDERR:
2272
2273 #!/usr/bin/perl
5a964f20
TC
2274 open(OLDOUT, ">&STDOUT");
2275 open(OLDERR, ">&STDERR");
a0d0e21e
LW
2276
2277 open(STDOUT, ">foo.out") || die "Can't redirect stdout";
2278 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
2279
2280 select(STDERR); $| = 1; # make unbuffered
2281 select(STDOUT); $| = 1; # make unbuffered
2282
2283 print STDOUT "stdout 1\n"; # this works for
2284 print STDERR "stderr 1\n"; # subprocesses too
2285
2286 close(STDOUT);
2287 close(STDERR);
2288
5a964f20
TC
2289 open(STDOUT, ">&OLDOUT");
2290 open(STDERR, ">&OLDERR");
a0d0e21e
LW
2291
2292 print STDOUT "stdout 2\n";
2293 print STDERR "stderr 2\n";
2294
2295
7660c0ab
A
2296If you specify C<'E<lt>&=N'>, where C<N> is a number, then Perl will do an
2297equivalent of C's C<fdopen()> of that file descriptor; this is more
4633a7c4 2298parsimonious of file descriptors. For example:
a0d0e21e
LW
2299
2300 open(FILEHANDLE, "<&=$fd")
2301
7660c0ab 2302If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>, then
a0d0e21e 2303there is an implicit fork done, and the return value of open is the pid
7660c0ab 2304of the child within the parent process, and C<0> within the child
184e9718 2305process. (Use C<defined($pid)> to determine whether the open was successful.)
a0d0e21e
LW
2306The filehandle behaves normally for the parent, but i/o to that
2307filehandle is piped from/to the STDOUT/STDIN of the child process.
2308In the child process the filehandle isn't opened--i/o happens from/to
2309the new STDOUT or STDIN. Typically this is used like the normal
2310piped open when you want to exercise more control over just how the
2311pipe command gets executed, such as when you are running setuid, and
54310121 2312don't want to have to scan shell commands for metacharacters.
4633a7c4 2313The following pairs are more or less equivalent:
a0d0e21e
LW
2314
2315 open(FOO, "|tr '[a-z]' '[A-Z]'");
2316 open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
2317
2318 open(FOO, "cat -n '$file'|");
2319 open(FOO, "-|") || exec 'cat', '-n', $file;
2320
4633a7c4
LW
2321See L<perlipc/"Safe Pipe Opens"> for more examples of this.
2322
5a964f20 2323NOTE: On any operation that may do a fork, any unflushed buffers remain
184e9718 2324unflushed in both processes, which means you may need to set C<$|> to
a0d0e21e
LW
2325avoid duplicate output.
2326
0dccf244
CS
2327Closing any piped filehandle causes the parent process to wait for the
2328child to finish, and returns the status value in C<$?>.
2329
5a964f20 2330The filename passed to open will have leading and trailing
f86cebdf 2331whitespace deleted, and the normal redirection characters
5a964f20
TC
2332honored. This property, known as "magic open",
2333can often be used to good effect. A user could specify a filename of
7660c0ab 2334F<"rsh cat file |">, or you could change certain filenames as needed:
5a964f20
TC
2335
2336 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
2337 open(FH, $filename) or die "Can't open $filename: $!";
2338
2339However, to open a file with arbitrary weird characters in it, it's
2340necessary to protect any leading and trailing whitespace:
2341
2342 $file =~ s#^(\s)#./$1#;
2343 open(FOO, "< $file\0");
2344
7660c0ab
A
2345If you want a "real" C C<open()> (see L<open(2)> on your system), then you
2346should use the C<sysopen()> function, which involves no such magic. This is
5a964f20
TC
2347another way to protect your filenames from interpretation. For example:
2348
2349 use IO::Handle;
2350 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
2351 or die "sysopen $path: $!";
2352 $oldfh = select(HANDLE); $| = 1; select($oldfh);
2353 print HANDLE "stuff $$\n");
2354 seek(HANDLE, 0, 0);
2355 print "File contains: ", <HANDLE>;
2356
7660c0ab
A
2357Using the constructor from the C<IO::Handle> package (or one of its
2358subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
5a964f20
TC
2359filehandles that have the scope of whatever variables hold references to
2360them, and automatically close whenever and however you leave that scope:
c07a80fd 2361
5f05dabc 2362 use IO::File;
5a964f20 2363 #...
c07a80fd
PP
2364 sub read_myfile_munged {
2365 my $ALL = shift;
5f05dabc 2366 my $handle = new IO::File;
c07a80fd
PP
2367 open($handle, "myfile") or die "myfile: $!";
2368 $first = <$handle>
2369 or return (); # Automatically closed here.
2370 mung $first or die "mung failed"; # Or here.
2371 return $first, <$handle> if $ALL; # Or here.
2372 $first; # Or here.
2373 }
2374
cb1a09d0 2375See L</seek()> for some details about mixing reading and writing.
a0d0e21e
LW
2376
2377=item opendir DIRHANDLE,EXPR
2378
7660c0ab
A
2379Opens a directory named EXPR for processing by C<readdir()>, C<telldir()>,
2380C<seekdir()>, C<rewinddir()>, and C<closedir()>. Returns TRUE if successful.
a0d0e21e
LW
2381DIRHANDLEs have their own namespace separate from FILEHANDLEs.
2382
2383=item ord EXPR
2384
54310121 2385=item ord
bbce6d69 2386
a0ed51b3 2387Returns the numeric (ASCII or Unicode) value of the first character of EXPR. If
7660c0ab 2388EXPR is omitted, uses C<$_>. For the reverse, see L</chr>.
a0d0e21e
LW
2389
2390=item pack TEMPLATE,LIST
2391
2392Takes an array or list of values and packs it into a binary structure,
2393returning the string containing the structure. The TEMPLATE is a
2394sequence of characters that give the order and type of values, as
2395follows:
2396
2397 A An ascii string, will be space padded.
2398 a An ascii string, will be null padded.
2399 b A bit string (ascending bit order, like vec()).
2400 B A bit string (descending bit order).
2401 h A hex string (low nybble first).
2402 H A hex string (high nybble first).
2403
2404 c A signed char value.
a0ed51b3 2405 C An unsigned char value. Only does bytes. See U for Unicode.
96e4d5b1 2406
a0d0e21e
LW
2407 s A signed short value.
2408 S An unsigned short value.
96e4d5b1
PP
2409 (This 'short' is _exactly_ 16 bits, which may differ from
2410 what a local C compiler calls 'short'.)
2411
a0d0e21e
LW
2412 i A signed integer value.
2413 I An unsigned integer value.
f86cebdf
GS
2414 (This 'integer' is _at_least_ 32 bits wide. Its exact
2415 size depends on what a local C compiler calls 'int',
2416 and may even be larger than the 'long' described in
2417 the next item.)
96e4d5b1 2418
a0d0e21e
LW
2419 l A signed long value.
2420 L An unsigned long value.
96e4d5b1
PP
2421 (This 'long' is _exactly_ 32 bits, which may differ from
2422 what a local C compiler calls 'long'.)
a0d0e21e 2423
96e4d5b1
PP
2424 n A short in "network" (big-endian) order.
2425 N A long in "network" (big-endian) order.
a0d0e21e
LW
2426 v A short in "VAX" (little-endian) order.
2427 V A long in "VAX" (little-endian) order.
96e4d5b1
PP
2428 (These 'shorts' and 'longs' are _exactly_ 16 bits and
2429 _exactly_ 32 bits, respectively.)
a0d0e21e
LW
2430
2431 f A single-precision float in the native format.
2432 d A double-precision float in the native format.
2433
2434 p A pointer to a null-terminated string.
2435 P A pointer to a structure (fixed-length string).
2436
2437 u A uuencoded string.
a0ed51b3
LW
2438 U A Unicode character number. Encodes to UTF-8 internally.
2439 Works even if C<use utf8> is not in effect.
a0d0e21e 2440
96e4d5b1 2441 w A BER compressed integer. Its bytes represent an unsigned
f86cebdf
GS
2442 integer in base 128, most significant digit first, with as
2443 few digits as possible. Bit eight (the high bit) is set
2444 on each byte except the last.
def98dd4 2445
a0d0e21e
LW
2446 x A null byte.
2447 X Back up a byte.
2448 @ Null fill to absolute position.
2449
5a964f20 2450Each letter may optionally be followed by a number giving a repeat
7660c0ab
A
2451count. With all types except C<"a">, C<"A">, C<"b">, C<"B">, C<"h">, C<"H">, and C<"P"> the
2452pack function will gobble up that many values from the LIST. A C<*> for the
2453repeat count means to use however many items are left. The C<"a"> and C<"A">
a0d0e21e 2454types gobble just one value, but pack it as a string of length count,
7660c0ab
A
2455padding with nulls or spaces as necessary. (When unpacking, C<"A"> strips
2456trailing spaces and nulls, but C<"a"> does not.) Likewise, the C<"b"> and C<"B">
2457fields pack a string that many bits long. The C<"h"> and C<"H"> fields pack a
2458string that many nybbles long. The C<"p"> type packs a pointer to a null-
84902520
TB
2459terminated string. You are responsible for ensuring the string is not a
2460temporary value (which can potentially get deallocated before you get
7660c0ab 2461around to using the packed result). The C<"P"> packs a pointer to a structure
61167c6f 2462of the size indicated by the length. A NULL pointer is created if the
7660c0ab 2463corresponding value for C<"p"> or C<"P"> is C<undef>.
61167c6f 2464Real numbers (floats and doubles) are
a0d0e21e
LW
2465in the native machine format only; due to the multiplicity of floating
2466formats around, and the lack of a standard "network" representation, no
2467facility for interchange has been made. This means that packed floating
2468point data written on one machine may not be readable on another - even if
2469both use IEEE floating point arithmetic (as the endian-ness of the memory
2470representation is not part of the IEEE spec). Note that Perl uses doubles
2471internally for all numeric calculation, and converting from double into
5f05dabc 2472float and thence back to double again will lose precision (i.e.,
7660c0ab 2473C<unpack("f", pack("f", $foo)>) will not in general equal C<$foo>).
a0d0e21e
LW
2474
2475Examples:
2476
a0ed51b3 2477 $foo = pack("CCCC",65,66,67,68);
a0d0e21e 2478 # foo eq "ABCD"
a0ed51b3 2479 $foo = pack("C4",65,66,67,68);
a0d0e21e 2480 # same thing
a0ed51b3
LW
2481 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
2482 # same thing with Unicode circled letters
a0d0e21e
LW
2483
2484 $foo = pack("ccxxcc",65,66,67,68);
2485 # foo eq "AB\0\0CD"
2486
2487 $foo = pack("s2",1,2);
2488 # "\1\0\2\0" on little-endian
2489 # "\0\1\0\2" on big-endian
2490
2491 $foo = pack("a4","abcd","x","y","z");
2492 # "abcd"
2493
2494 $foo = pack("aaaa","abcd","x","y","z");
2495 # "axyz"
2496
2497 $foo = pack("a14","abcdefg");
2498 # "abcdefg\0\0\0\0\0\0\0"
2499
2500 $foo = pack("i9pl", gmtime);
2501 # a real struct tm (on my system anyway)
2502
2503 sub bintodec {
2504 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
2505 }
2506
2507The same template may generally also be used in the unpack function.
2508
5a964f20
TC
2509=item package
2510
cb1a09d0
AD
2511=item package NAMESPACE
2512
2513Declares the compilation unit as being in the given namespace. The scope
2514of the package declaration is from the declaration itself through the end of
7660c0ab 2515the enclosing block (the same scope as the C<local()> operator). All further
cb1a09d0 2516unqualified dynamic identifiers will be in this namespace. A package
5f05dabc 2517statement affects only dynamic variables--including those you've used
7660c0ab 2518C<local()> on--but I<not> lexical variables created with C<my()>. Typically it
cb1a09d0
AD
2519would be the first declaration in a file to be included by the C<require>
2520or C<use> operator. You can switch into a package in more than one place;
5a964f20 2521it merely influences which symbol table is used by the compiler for the
cb1a09d0
AD
2522rest of that block. You can refer to variables and filehandles in other
2523packages by prefixing the identifier with the package name and a double
2524colon: C<$Package::Variable>. If the package name is null, the C<main>
2525package as assumed. That is, C<$::sail> is equivalent to C<$main::sail>.
2526
5a964f20
TC
2527If NAMESPACE is omitted, then there is no current package, and all
2528identifiers must be fully qualified or lexicals. This is stricter
2529than C<use strict>, since it also extends to function names.
2530
cb1a09d0
AD
2531See L<perlmod/"Packages"> for more information about packages, modules,
2532and classes. See L<perlsub> for other scoping issues.
2533
a0d0e21e
LW
2534=item pipe READHANDLE,WRITEHANDLE
2535
2536Opens a pair of connected pipes like the corresponding system call.
2537Note that if you set up a loop of piped processes, deadlock can occur
2538unless you are very careful. In addition, note that Perl's pipes use
184e9718 2539stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE
a0d0e21e
LW
2540after each command, depending on the application.
2541
7e1af8bc 2542See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
4633a7c4
LW
2543for examples of such things.
2544
a0d0e21e
LW
2545=item pop ARRAY
2546
54310121 2547=item pop
28757baa 2548
a0d0e21e
LW
2549Pops and returns the last value of the array, shortening the array by
25501. Has a similar effect to
2551
2552 $tmp = $ARRAY[$#ARRAY--];
2553
2554If there are no elements in the array, returns the undefined value.
cb1a09d0 2555If ARRAY is omitted, pops the
7660c0ab
A
2556C<@ARGV> array in the main program, and the C<@_> array in subroutines, just
2557like C<shift()>.
a0d0e21e
LW
2558
2559=item pos SCALAR
2560
54310121 2561=item pos
bbce6d69 2562
4633a7c4 2563Returns the offset of where the last C<m//g> search left off for the variable
7660c0ab 2564is in question (C<$_> is used when the variable is not specified). May be
44a8e56a
PP
2565modified to change that offset. Such modification will also influence
2566the C<\G> zero-width assertion in regular expressions. See L<perlre> and
2567L<perlop>.
a0d0e21e
LW
2568
2569=item print FILEHANDLE LIST
2570
2571=item print LIST
2572
2573=item print
2574
cb1a09d0 2575Prints a string or a comma-separated list of strings. Returns TRUE
a0d0e21e 2576if successful. FILEHANDLE may be a scalar variable name, in which case
cb1a09d0 2577the variable contains the name of or a reference to the filehandle, thus introducing one
a0d0e21e
LW
2578level of indirection. (NOTE: If FILEHANDLE is a variable and the next
2579token is a term, it may be misinterpreted as an operator unless you
7660c0ab 2580interpose a C<+> or put parentheses around the arguments.) If FILEHANDLE is
a0d0e21e 2581omitted, prints by default to standard output (or to the last selected
7660c0ab 2582output channel--see L</select>). If LIST is also omitted, prints C<$_> to
5a964f20 2583the currently selected output channel. To set the default output channel to something other than
a0d0e21e 2584STDOUT use the select operation. Note that, because print takes a
5a964f20 2585LIST, anything in the LIST is evaluated in list context, and any
a0d0e21e 2586subroutine that you call will have one or more of its expressions
5a964f20 2587evaluated in list context. Also be careful not to follow the print
a0d0e21e 2588keyword with a left parenthesis unless you want the corresponding right
7660c0ab 2589parenthesis to terminate the arguments to the print--interpose a C<+> or
5f05dabc 2590put parentheses around all the arguments.
a0d0e21e 2591
4633a7c4 2592Note that if you're storing FILEHANDLES in an array or other expression,
da0045b7 2593you will have to use a block returning its value instead:
4633a7c4
LW
2594
2595 print { $files[$i] } "stuff\n";
2596 print { $OK ? STDOUT : STDERR } "stuff\n";
2597
5f05dabc 2598=item printf FILEHANDLE FORMAT, LIST
a0d0e21e 2599
5f05dabc 2600=item printf FORMAT, LIST
a0d0e21e 2601
7660c0ab 2602Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
a3cb178b 2603(the output record separator) is not appended. The first argument
ab4f32c2 2604of the list will be interpreted as the C<printf()> format. If C<use locale> is
a034a98d
DD
2605in effect, the character used for the decimal point in formatted real numbers
2606is affected by the LC_NUMERIC locale. See L<perllocale>.
a0d0e21e 2607
7660c0ab
A
2608Don't fall into the trap of using a C<printf()> when a simple
2609C<print()> would do. The C<print()> is more efficient and less
28757baa
PP
2610error prone.
2611
da0045b7
PP
2612=item prototype FUNCTION
2613
2614Returns the prototype of a function as a string (or C<undef> if the
5f05dabc
PP
2615function has no prototype). FUNCTION is a reference to, or the name of,
2616the function whose prototype you want to retrieve.
da0045b7 2617
b6c543e3
IZ
2618If FUNCTION is a string starting with C<CORE::>, the rest is taken as
2619a name for Perl builtin. If builtin is not I<overridable> (such as
ab4f32c2
A
2620C<qw//>) or its arguments cannot be expressed by a prototype (such as
2621C<system()>) - in other words, the builtin does not behave like a Perl
b6c543e3
IZ
2622function - returns C<undef>. Otherwise, the string describing the
2623equivalent prototype is returned.
2624
a0d0e21e
LW
2625=item push ARRAY,LIST
2626
2627Treats ARRAY as a stack, and pushes the values of LIST
2628onto the end of ARRAY. The length of ARRAY increases by the length of
2629LIST. Has the same effect as
2630
2631 for $value (LIST) {
2632 $ARRAY[++$#ARRAY] = $value;
2633 }
2634
2635but is more efficient. Returns the new number of elements in the array.
2636
2637=item q/STRING/
2638
2639=item qq/STRING/
2640
8782bef2
GB
2641=item qr/STRING/
2642
a0d0e21e
LW
2643=item qx/STRING/
2644
2645=item qw/STRING/
2646
2647Generalized quotes. See L<perlop>.
2648
2649=item quotemeta EXPR
2650
54310121 2651=item quotemeta
bbce6d69 2652
68dc0745 2653Returns the value of EXPR with all non-alphanumeric
a034a98d
DD
2654characters backslashed. (That is, all characters not matching
2655C</[A-Za-z_0-9]/> will be preceded by a backslash in the
2656returned string, regardless of any locale settings.)
2657This is the internal function implementing
7660c0ab 2658the C<\Q> escape in double-quoted strings.
a0d0e21e 2659
7660c0ab 2660If EXPR is omitted, uses C<$_>.
bbce6d69 2661
a0d0e21e
LW
2662=item rand EXPR
2663
2664=item rand
2665
7660c0ab 2666Returns a random fractional number greater than or equal to C<0> and less
3e3baf6d 2667than the value of EXPR. (EXPR should be positive.) If EXPR is
7660c0ab
A
2668omitted, the value C<1> is used. Automatically calls C<srand()> unless
2669C<srand()> has already been called. See also C<srand()>.
a0d0e21e 2670
2f9daede 2671(Note: If your rand function consistently returns numbers that are too
a0d0e21e 2672large or too small, then your version of Perl was probably compiled
2f9daede 2673with the wrong number of RANDBITS.)
a0d0e21e
LW
2674
2675=item read FILEHANDLE,SCALAR,LENGTH,OFFSET
2676
2677=item read FILEHANDLE,SCALAR,LENGTH
2678
2679Attempts to read LENGTH bytes of data into variable SCALAR from the
3b02c43c
GS
2680specified FILEHANDLE. Returns the number of bytes actually read,
2681C<0> at end of file, or undef if there was an error. SCALAR will be grown
2682or shrunk to the length actually read. An OFFSET may be specified to
2683place the read data at some other place than the beginning of the
f86cebdf
GS
2684string. This call is actually implemented in terms of stdio's fread(3)
2685call. To get a true read(2) system call, see C<sysread()>.
a0d0e21e
LW
2686
2687=item readdir DIRHANDLE
2688
7660c0ab 2689Returns the next directory entry for a directory opened by C<opendir()>.
5a964f20 2690If used in list context, returns all the rest of the entries in the
a0d0e21e 2691directory. If there are no more entries, returns an undefined value in
5a964f20 2692scalar context or a null list in list context.
a0d0e21e 2693
7660c0ab 2694If you're planning to filetest the return values out of a C<readdir()>, you'd
5f05dabc 2695better prepend the directory in question. Otherwise, because we didn't
7660c0ab 2696C<chdir()> there, it would have been testing the wrong file.
cb1a09d0
AD
2697
2698 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
2699 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
2700 closedir DIR;
2701
84902520
TB
2702=item readline EXPR
2703
5a964f20 2704Reads from the filehandle whose typeglob is contained in EXPR. In scalar context, a single line
84902520
TB
2705is read and returned. In list context, reads until end-of-file is
2706reached and returns a list of lines (however you've defined lines
7660c0ab 2707with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
84902520
TB
2708This is the internal function implementing the C<E<lt>EXPRE<gt>>
2709operator, but you can use it directly. The C<E<lt>EXPRE<gt>>
2710operator is discussed in more detail in L<perlop/"I/O Operators">.
2711
5a964f20
TC
2712 $line = <STDIN>;
2713 $line = readline(*STDIN); # same thing
2714
a0d0e21e
LW
2715=item readlink EXPR
2716
54310121 2717=item readlink
bbce6d69 2718
a0d0e21e
LW
2719Returns the value of a symbolic link, if symbolic links are
2720implemented. If not, gives a fatal error. If there is some system
184e9718 2721error, returns the undefined value and sets C<$!> (errno). If EXPR is
7660c0ab 2722omitted, uses C<$_>.
a0d0e21e 2723
84902520
TB
2724=item readpipe EXPR
2725
5a964f20 2726EXPR is executed as a system command.
84902520
TB
2727The collected standard output of the command is returned.
2728In scalar context, it comes back as a single (potentially
2729multi-line) string. In list context, returns a list of lines
7660c0ab 2730(however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
84902520
TB
2731This is the internal function implementing the C<qx/EXPR/>
2732operator, but you can use it directly. The C<qx/EXPR/>
2733operator is discussed in more detail in L<perlop/"I/O Operators">.
2734
a0d0e21e
LW
2735=item recv SOCKET,SCALAR,LEN,FLAGS
2736
2737Receives a message on a socket. Attempts to receive LENGTH bytes of
2738data into variable SCALAR from the specified SOCKET filehandle.
7660c0ab 2739Actually does a C C<recvfrom()>, so that it can return the address of the
a0d0e21e
LW
2740sender. Returns the undefined value if there's an error. SCALAR will
2741be grown or shrunk to the length actually read. Takes the same flags
54310121 2742as the system call of the same name.
4633a7c4 2743See L<perlipc/"UDP: Message Passing"> for examples.
a0d0e21e
LW
2744
2745=item redo LABEL
2746
2747=item redo
2748
2749The C<redo> command restarts the loop block without evaluating the
2750conditional again. The C<continue> block, if any, is not executed. If
2751the LABEL is omitted, the command refers to the innermost enclosing
2752loop. This command is normally used by programs that want to lie to
2753themselves about what was just input:
2754
2755 # a simpleminded Pascal comment stripper
2756 # (warning: assumes no { or } in strings)
4633a7c4 2757 LINE: while (<STDIN>) {
a0d0e21e
LW
2758 while (s|({.*}.*){.*}|$1 |) {}
2759 s|{.*}| |;
2760 if (s|{.*| |) {
2761 $front = $_;
2762 while (<STDIN>) {
2763 if (/}/) { # end of comment?
5a964f20 2764 s|^|$front\{|;
4633a7c4 2765 redo LINE;
a0d0e21e
LW
2766 }
2767 }
2768 }
2769 print;
2770 }
2771
1d2dff63
GS
2772See also L</continue> for an illustration of how C<last>, C<next>, and
2773C<redo> work.
2774
a0d0e21e
LW
2775=item ref EXPR
2776
54310121 2777=item ref
bbce6d69 2778
2f9daede 2779Returns a TRUE value if EXPR is a reference, FALSE otherwise. If EXPR
7660c0ab 2780is not specified, C<$_> will be used. The value returned depends on the
bbce6d69 2781type of thing the reference is a reference to.
a0d0e21e
LW
2782Builtin types include:
2783
2784 REF
2785 SCALAR
2786 ARRAY
2787 HASH
2788 CODE
2789 GLOB
2790
54310121 2791If the referenced object has been blessed into a package, then that package
7660c0ab 2792name is returned instead. You can think of C<ref()> as a C<typeof()> operator.
a0d0e21e
LW
2793
2794 if (ref($r) eq "HASH") {
aa689395 2795 print "r is a reference to a hash.\n";
54310121 2796 }
5a964f20 2797 if (!ref($r)) {
a0d0e21e 2798 print "r is not a reference at all.\n";
54310121 2799 }
a0d0e21e
LW
2800
2801See also L<perlref>.
2802
2803=item rename OLDNAME,NEWNAME
2804
7660c0ab 2805Changes the name of a file. Returns C<1> for success, C<0> otherwise. Will
5f05dabc 2806not work across file system boundaries.
a0d0e21e
LW
2807
2808=item require EXPR
2809
2810=item require
2811
7660c0ab 2812Demands some semantics specified by EXPR, or by C<$_> if EXPR is not
a0d0e21e 2813supplied. If EXPR is numeric, demands that the current version of Perl
184e9718 2814(C<$]> or $PERL_VERSION) be equal or greater than EXPR.
a0d0e21e
LW
2815
2816Otherwise, demands that a library file be included if it hasn't already
2817been included. The file is included via the do-FILE mechanism, which is
7660c0ab 2818essentially just a variety of C<eval()>. Has semantics similar to the following
a0d0e21e
LW
2819subroutine:
2820
2821 sub require {
5a964f20 2822 my($filename) = @_;
a0d0e21e 2823 return 1 if $INC{$filename};
5a964f20 2824 my($realfilename,$result);
a0d0e21e
LW
2825 ITER: {
2826 foreach $prefix (@INC) {
2827 $realfilename = "$prefix/$filename";
2828 if (-f $realfilename) {
2829 $result = do $realfilename;
2830 last ITER;
2831 }
2832 }
2833 die "Can't find $filename in \@INC";
2834 }
2835 die $@ if $@;
2836 die "$filename did not return true value" unless $result;
2837 $INC{$filename} = $realfilename;
5a964f20 2838 return $result;
a0d0e21e
LW
2839 }
2840
2841Note that the file will not be included twice under the same specified
2842name. The file must return TRUE as the last statement to indicate
2843successful execution of any initialization code, so it's customary to
7660c0ab 2844end such a file with "C<1;>" unless you're sure it'll return TRUE
a0d0e21e
LW
2845otherwise. But it's better just to put the "C<1;>", in case you add more
2846statements.
2847
54310121 2848If EXPR is a bareword, the require assumes a "F<.pm>" extension and
da0045b7 2849replaces "F<::>" with "F</>" in the filename for you,
54310121 2850to make it easy to load standard modules. This form of loading of
a0d0e21e
LW
2851modules does not risk altering your namespace.
2852
ee580363
GS
2853In other words, if you try this:
2854
f86cebdf 2855 require Foo::Bar; # a splendid bareword
ee580363 2856
7660c0ab
A
2857The require function will actually look for the "F<Foo/Bar.pm>" file in the
2858directories specified in the C<@INC> array.
ee580363 2859
5a964f20 2860But if you try this:
ee580363
GS
2861
2862 $class = 'Foo::Bar';
f86cebdf 2863 require $class; # $class is not a bareword
5a964f20 2864 #or
f86cebdf 2865 require "Foo::Bar"; # not a bareword because of the ""
ee580363 2866
7660c0ab
A
2867The require function will look for the "F<Foo::Bar>" file in the @INC array and
2868will complain about not finding "F<Foo::Bar>" there. In this case you can do:
ee580363
GS
2869
2870 eval "require $class";
2871
2872For a yet-more-powerful import facility, see L</use> and L<perlmod>.
a0d0e21e
LW
2873
2874=item reset EXPR
2875
2876=item reset
2877
2878Generally used in a C<continue> block at the end of a loop to clear
7660c0ab 2879variables and reset C<??> searches so that they work again. The
a0d0e21e
LW
2880expression is interpreted as a list of single characters (hyphens
2881allowed for ranges). All variables and arrays beginning with one of
2882those letters are reset to their pristine state. If the expression is
7660c0ab 2883omitted, one-match searches (C<?pattern?>) are reset to match again. Resets
5f05dabc 2884only variables or searches in the current package. Always returns
a0d0e21e
LW
28851. Examples:
2886
2887 reset 'X'; # reset all X variables
2888 reset 'a-z'; # reset lower case variables
2889 reset; # just reset ?? searches
2890
7660c0ab
A
2891Resetting C<"A-Z"> is not recommended because you'll wipe out your
2892C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package variables--lexical variables
a0d0e21e 2893are unaffected, but they clean themselves up on scope exit anyway,
da0045b7 2894so you'll probably want to use them instead. See L</my>.
a0d0e21e 2895
54310121
PP
2896=item return EXPR
2897
2898=item return
2899
7660c0ab 2900Returns from a subroutine, C<eval()>, or C<do FILE> with the value
5a964f20 2901given in EXPR. Evaluation of EXPR may be in list, scalar, or void
54310121 2902context, depending on how the return value will be used, and the context
7660c0ab 2903may vary from one execution to the next (see C<wantarray()>). If no EXPR
5a964f20
TC
2904is given, returns an empty list in list context, an undefined value in
2905scalar context, or nothing in a void context.
a0d0e21e 2906
68dc0745
PP
2907(Note that in the absence of a return, a subroutine, eval, or do FILE
2908will automatically return the value of the last expression evaluated.)
a0d0e21e
LW
2909
2910=item reverse LIST
2911
5a964f20
TC
2912In list context, returns a list value consisting of the elements
2913of LIST in the opposite order. In scalar context, concatenates the
a0ed51b3
LW
2914elements of LIST, and returns a string value with all the characters
2915in the opposite order.
4633a7c4 2916
2f9daede 2917 print reverse <>; # line tac, last line first
4633a7c4 2918
2f9daede 2919 undef $/; # for efficiency of <>
a0ed51b3 2920 print scalar reverse <>; # character tac, last line tsrif
2f9daede
TPG
2921
2922This operator is also handy for inverting a hash, although there are some
2923caveats. If a value is duplicated in the original hash, only one of those
2924can be represented as a key in the inverted hash. Also, this has to
2925unwind one hash and build a whole new one, which may take some time
2926on a large hash.
2927
2928 %by_name = reverse %by_address; # Invert the hash
a0d0e21e
LW
2929
2930=item rewinddir DIRHANDLE
2931
2932Sets the current position to the beginning of the directory for the
7660c0ab 2933C<readdir()> routine on DIRHANDLE.
a0d0e21e
LW
2934
2935=item rindex STR,SUBSTR,POSITION
2936
2937=item rindex STR,SUBSTR
2938
2939Works just like index except that it returns the position of the LAST
2940occurrence of SUBSTR in STR. If POSITION is specified, returns the
2941last occurrence at or before that position.
2942
2943=item rmdir FILENAME
2944
54310121 2945=item rmdir
bbce6d69 2946
5a964f20
TC
2947Deletes the directory specified by FILENAME if that directory is empty. If it
2948succeeds it returns TRUE, otherwise it returns FALSE and sets C<$!> (errno). If
7660c0ab 2949FILENAME is omitted, uses C<$_>.
a0d0e21e
LW
2950
2951=item s///
2952
2953The substitution operator. See L<perlop>.
2954
2955=item scalar EXPR
2956
5a964f20 2957Forces EXPR to be interpreted in scalar context and returns the value
54310121 2958of EXPR.
cb1a09d0
AD
2959
2960 @counts = ( scalar @a, scalar @b, scalar @c );
2961
54310121 2962There is no equivalent operator to force an expression to
5a964f20 2963be interpolated in list context because it's in practice never
cb1a09d0
AD
2964needed. If you really wanted to do so, however, you could use
2965the construction C<@{[ (some expression) ]}>, but usually a simple
2966C<(some expression)> suffices.
a0d0e21e
LW
2967
2968=item seek FILEHANDLE,POSITION,WHENCE
2969
ab4f32c2 2970Sets FILEHANDLE's position, just like the C<fseek()> call of C<stdio()>.
8903cb82 2971FILEHANDLE may be an expression whose value gives the name of the
7660c0ab
A
2972filehandle. The values for WHENCE are C<0> to set the new position to
2973POSITION, C<1> to set it to the current position plus POSITION, and C<2> to
8903cb82 2974set it to EOF plus POSITION (typically negative). For WHENCE you may
7660c0ab
A
2975use the constants C<SEEK_SET>, C<SEEK_CUR>, and C<SEEK_END> from either the
2976C<IO::Seekable> or the POSIX module. Returns C<1> upon success, C<0> otherwise.
8903cb82 2977
7660c0ab
A
2978If you want to position file for C<sysread()> or C<syswrite()>, don't use
2979C<seek()> -- buffering makes its effect on the file's system position
2980unpredictable and non-portable. Use C<sysseek()> instead.
a0d0e21e 2981
cb1a09d0
AD
2982On some systems you have to do a seek whenever you switch between reading
2983and writing. Amongst other things, this may have the effect of calling
f86cebdf 2984stdio's clearerr(3). A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving
8903cb82 2985the file position:
cb1a09d0
AD
2986
2987 seek(TEST,0,1);
2988
2989This is also useful for applications emulating C<tail -f>. Once you hit
2990EOF on your read, and then sleep for a while, you might have to stick in a
7660c0ab 2991seek() to reset things. The C<seek()> doesn't change the current position,
8903cb82
PP
2992but it I<does> clear the end-of-file condition on the handle, so that the
2993next C<E<lt>FILEE<gt>> makes Perl try again to read something. We hope.
cb1a09d0
AD
2994
2995If that doesn't work (some stdios are particularly cantankerous), then
2996you may need something more like this:
2997
2998 for (;;) {
f86cebdf
GS
2999 for ($curpos = tell(FILE); $_ = <FILE>;
3000 $curpos = tell(FILE)) {
cb1a09d0
AD
3001 # search for some stuff and put it into files
3002 }
3003 sleep($for_a_while);
3004 seek(FILE, $curpos, 0);
3005 }
3006
a0d0e21e
LW
3007=item seekdir DIRHANDLE,POS
3008
7660c0ab
A
3009Sets the current position for the C<readdir()> routine on DIRHANDLE. POS
3010must be a value returned by C<telldir()>. Has the same caveats about
a0d0e21e
LW
3011possible directory compaction as the corresponding system library
3012routine.
3013
3014=item select FILEHANDLE
3015
3016=item select
3017
3018Returns the currently selected filehandle. Sets the current default
3019filehandle for output, if FILEHANDLE is supplied. This has two
ab4f32c2 3020effects: first, a C<write()> or a C<print()> without a filehandle will
a0d0e21e
LW
3021default to this FILEHANDLE. Second, references to variables related to
3022output will refer to this output channel. For example, if you have to
3023set the top of form format for more than one output channel, you might
3024do the following:
3025
3026 select(REPORT1);
3027 $^ = 'report1_top';
3028 select(REPORT2);
3029 $^ = 'report2_top';
3030
3031FILEHANDLE may be an expression whose value gives the name of the
3032actual filehandle. Thus:
3033
3034 $oldfh = select(STDERR); $| = 1; select($oldfh);
3035
4633a7c4
LW
3036Some programmers may prefer to think of filehandles as objects with
3037methods, preferring to write the last example as:
a0d0e21e 3038
28757baa 3039 use IO::Handle;
a0d0e21e
LW
3040 STDERR->autoflush(1);
3041
3042=item select RBITS,WBITS,EBITS,TIMEOUT
3043
f86cebdf 3044This calls the select(2) system call with the bit masks specified, which
7660c0ab 3045can be constructed using C<fileno()> and C<vec()>, along these lines:
a0d0e21e
LW
3046
3047 $rin = $win = $ein = '';
3048 vec($rin,fileno(STDIN),1) = 1;
3049 vec($win,fileno(STDOUT),1) = 1;
3050 $ein = $rin | $win;
3051
3052If you want to select on many filehandles you might wish to write a
3053subroutine:
3054
3055 sub fhbits {
5a964f20
TC
3056 my(@fhlist) = split(' ',$_[0]);
3057 my($bits);
a0d0e21e
LW
3058 for (@fhlist) {
3059 vec($bits,fileno($_),1) = 1;
3060 }
3061 $bits;
3062 }
4633a7c4 3063 $rin = fhbits('STDIN TTY SOCK');
a0d0e21e
LW
3064
3065The usual idiom is:
3066
3067 ($nfound,$timeleft) =
3068 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
3069
54310121 3070or to block until something becomes ready just do this
a0d0e21e
LW
3071
3072 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
3073
7660c0ab
A
3074Most systems do not bother to return anything useful in C<$timeleft>, so
3075calling select() in scalar context just returns C<$nfound>.
c07a80fd 3076
5f05dabc 3077Any of the bit masks can also be undef. The timeout, if specified, is
a0d0e21e 3078in seconds, which may be fractional. Note: not all implementations are
7660c0ab
A
3079capable of returning theC<$timeleft>. If not, they always return
3080C<$timeleft> equal to the supplied C<$timeout>.
a0d0e21e 3081
ff68c719 3082You can effect a sleep of 250 milliseconds this way:
a0d0e21e
LW
3083
3084 select(undef, undef, undef, 0.25);
3085
7660c0ab
A
3086B<WARNING>: One should not attempt to mix buffered I/O (like C<read()>
3087or E<lt>FHE<gt>) with C<select()>, except as permitted by POSIX, and even
3088then only on POSIX systems. You have to use C<sysread()> instead.
a0d0e21e
LW
3089
3090=item semctl ID,SEMNUM,CMD,ARG
3091
ab4f32c2 3092Calls the System V IPC function C<semctl()>. You'll probably have to say
0ade1984
JH
3093
3094 use IPC::SysV;
3095
3096first to get the correct constant definitions. If CMD is IPC_STAT or
3097GETALL, then ARG must be a variable which will hold the returned
ab4f32c2 3098semid_ds structure or semaphore value array. Returns like C<ioctl()>: the
7660c0ab
A
3099undefined value for error, "C<0> but true" for zero, or the actual return
3100value otherwise. See also C<IPC::SysV> and C<IPC::Semaphore> documentation.
a0d0e21e
LW
3101
3102=item semget KEY,NSEMS,FLAGS
3103
3104Calls the System V IPC function semget. Returns the semaphore id, or
7660c0ab
A
3105the undefined value if there is an error. See also C<IPC::SysV> and
3106C<IPC::SysV::Semaphore> documentation.
a0d0e21e
LW
3107
3108=item semop KEY,OPSTRING
3109
3110Calls the System V IPC function semop to perform semaphore operations
3111such as signaling and waiting. OPSTRING must be a packed array of
3112semop structures. Each semop structure can be generated with
3113C<pack("sss", $semnum, $semop, $semflag)>. The number of semaphore
3114operations is implied by the length of OPSTRING. Returns TRUE if
3115successful, or FALSE if there is an error. As an example, the
7660c0ab 3116following code waits on semaphore C<$semnum> of semaphore id C<$semid>:
a0d0e21e
LW
3117
3118 $semop = pack("sss", $semnum, -1, 0);
3119 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
3120
7660c0ab
A
3121To signal the semaphore, replace C<-1> with C<1>. See also C<IPC::SysV>
3122and C<IPC::SysV::Semaphore> documentation.
a0d0e21e
LW
3123
3124=item send SOCKET,MSG,FLAGS,TO
3125
3126=item send SOCKET,MSG,FLAGS
3127
3128Sends a message on a socket. Takes the same flags as the system call
3129of the same name. On unconnected sockets you must specify a
7660c0ab 3130destination to send TO, in which case it does a C C<sendto()>. Returns
a0d0e21e
LW
3131the number of characters sent, or the undefined value if there is an
3132error.
4633a7c4 3133See L<perlipc/"UDP: Message Passing"> for examples.
a0d0e21e
LW
3134
3135=item setpgrp PID,PGRP
3136
7660c0ab 3137Sets the current process group for the specified PID, C<0> for the current
a0d0e21e 3138process. Will produce a fatal error if used on a machine that doesn't
f86cebdf 3139implement setpgrp(2). If the arguments are omitted, it defaults to
7660c0ab
A
3140C<0,0>. Note that the POSIX version of C<setpgrp()> does not accept any
3141arguments, so only setpgrp C<0,0> is portable.
a0d0e21e
LW
3142
3143=item setpriority WHICH,WHO,PRIORITY
3144
3145Sets the current priority for a process, a process group, or a user.
f86cebdf
GS
3146(See setpriority(2).) Will produce a fatal error if used on a machine
3147that doesn't implement setpriority(2).
a0d0e21e
LW
3148
3149=item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
3150
3151Sets the socket option requested. Returns undefined if there is an
7660c0ab 3152error. OPTVAL may be specified as C<undef> if you don't want to pass an
a0d0e21e
LW
3153argument.
3154
3155=item shift ARRAY
3156
3157=item shift
3158
3159Shifts the first value of the array off and returns it, shortening the
3160array by 1 and moving everything down. If there are no elements in the
3161array, returns the undefined value. If ARRAY is omitted, shifts the
7660c0ab
A
3162C<@_> array within the lexical scope of subroutines and formats, and the
3163C<@ARGV> array at file scopes or within the lexical scopes established by
977336f5 3164the C<eval ''>, C<BEGIN {}>, C<END {}>, and C<INIT {}> constructs.
7660c0ab
A
3165See also C<unshift()>, C<push()>, and C<pop()>. C<Shift()> and C<unshift()> do the
3166same thing to the left end of an array that C<pop()> and C<push()> do to the
977336f5 3167right end.
a0d0e21e
LW
3168
3169=item shmctl ID,CMD,ARG
3170
0ade1984
JH
3171Calls the System V IPC function shmctl. You'll probably have to say
3172
3173 use IPC::SysV;
3174
7660c0ab
A
3175first to get the correct constant definitions. If CMD is C<IPC_STAT>,
3176then ARG must be a variable which will hold the returned C<shmid_ds>
3177structure. Returns like ioctl: the undefined value for error, "C<0> but
0ade1984 3178true" for zero, or the actual return value otherwise.
7660c0ab 3179See also C<IPC::SysV> documentation.
a0d0e21e
LW
3180
3181=item shmget KEY,SIZE,FLAGS
3182
3183Calls the System V IPC function shmget. Returns the shared memory
3184segment id, or the undefined value if there is an error.
7660c0ab 3185See also C<IPC::SysV> documentation.
a0d0e21e
LW
3186
3187=item shmread ID,VAR,POS,SIZE
3188
3189=item shmwrite ID,STRING,POS,SIZE
3190
3191Reads or writes the System V shared memory segment ID starting at
3192position POS for size SIZE by attaching to it, copying in/out, and
5a964f20 3193detaching from it. When reading, VAR must be a variable that will
a0d0e21e
LW
3194hold the data read. When writing, if STRING is too long, only SIZE
3195bytes are used; if STRING is too short, nulls are written to fill out
3196SIZE bytes. Return TRUE if successful, or FALSE if there is an error.
7660c0ab 3197See also C<IPC::SysV> documentation.
a0d0e21e
LW
3198
3199=item shutdown SOCKET,HOW
3200
3201Shuts down a socket connection in the manner indicated by HOW, which
3202has the same interpretation as in the system call of the same name.
3203
f86cebdf
GS
3204 shutdown(SOCKET, 0); # I/we have stopped reading data
3205 shutdown(SOCKET, 1); # I/we have stopped writing data
3206 shutdown(SOCKET, 2); # I/we have stopped using this socket
5a964f20
TC
3207
3208This is useful with sockets when you want to tell the other
3209side you're done writing but not done reading, or vice versa.
3210It's also a more insistent form of close because it also
3211disables the filedescriptor in any forked copies in other
3212processes.
3213
a0d0e21e
LW
3214=item sin EXPR
3215
54310121 3216=item sin
bbce6d69 3217
a0d0e21e 3218Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
7660c0ab 3219returns sine of C<$_>.
a0d0e21e 3220
7660c0ab 3221For the inverse sine operation, you may use the C<POSIX::asin()>
28757baa
PP
3222function, or use this relation:
3223
3224 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
3225
a0d0e21e
LW
3226=item sleep EXPR
3227
3228=item sleep
3229
3230Causes the script to sleep for EXPR seconds, or forever if no EXPR.
7660c0ab 3231May be interrupted if the process receives a signal such as C<SIGALRM>.
1d3434b8 3232Returns the number of seconds actually slept. You probably cannot
7660c0ab
A
3233mix C<alarm()> and C<sleep()> calls, because C<sleep()> is often implemented
3234using C<alarm()>.
a0d0e21e
LW
3235
3236On some older systems, it may sleep up to a full second less than what
3237you requested, depending on how it counts seconds. Most modern systems
5a964f20
TC
3238always sleep the full amount. They may appear to sleep longer than that,
3239however, because your process might not be scheduled right away in a
3240busy multitasking system.
a0d0e21e 3241
cb1a09d0 3242For delays of finer granularity than one second, you may use Perl's
f86cebdf 3243C<syscall()> interface to access setitimer(2) if your system supports it,
7b8d334a 3244or else see L</select()> above.
cb1a09d0 3245
7660c0ab 3246See also the POSIX module's C<sigpause()> function.
5f05dabc 3247
a0d0e21e
LW
3248=item socket SOCKET,DOMAIN,TYPE,PROTOCOL
3249
3250Opens a socket of the specified kind and attaches it to filehandle
5f05dabc 3251SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for the
7660c0ab 3252system call of the same name. You should "C<use Socket;>" first to get
4633a7c4 3253the proper definitions imported. See the example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
3254
3255=item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
3256
3257Creates an unnamed pair of sockets in the specified domain, of the
5f05dabc 3258specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
a0d0e21e
LW
3259for the system call of the same name. If unimplemented, yields a fatal
3260error. Returns TRUE if successful.
3261
ab4f32c2 3262Some systems defined C<pipe()> in terms of C<socketpair()>, in which a call
5a964f20
TC
3263to C<pipe(Rdr, Wtr)> is essentially:
3264
3265 use Socket;
3266 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
3267 shutdown(Rdr, 1); # no more writing for reader
3268 shutdown(Wtr, 0); # no more reading for writer
3269
3270See L<perlipc> for an example of socketpair use.
3271
a0d0e21e
LW
3272=item sort SUBNAME LIST
3273
3274=item sort BLOCK LIST
3275
3276=item sort LIST
3277
2f9daede 3278Sorts the LIST and returns the sorted list value. If SUBNAME or BLOCK
ab4f32c2 3279is omitted, C<sort()>s in standard string comparison order. If SUBNAME is
2f9daede 3280specified, it gives the name of a subroutine that returns an integer
7660c0ab 3281less than, equal to, or greater than C<0>, depending on how the elements
2f9daede
TPG
3282of the array are to be ordered. (The C<E<lt>=E<gt>> and C<cmp>
3283operators are extremely useful in such routines.) SUBNAME may be a
1d3434b8
GS
3284scalar variable name (unsubscripted), in which case the value provides
3285the name of (or a reference to) the actual subroutine to use. In place
3286of a SUBNAME, you can provide a BLOCK as an anonymous, in-line sort