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1=head1 NAME
2
3perlfunc - Perl builtin functions
4
5=head1 DESCRIPTION
6
7The functions in this section can serve as terms in an expression.
8They fall into two major categories: list operators and named unary
9operators. These differ in their precedence relationship with a
10following comma. (See the precedence table in L<perlop>.) List
11operators take more than one argument, while unary operators can never
12take more than one argument. Thus, a comma terminates the argument of
13a unary operator, but merely separates the arguments of a list
14operator. A unary operator generally provides a scalar context to its
15argument, while a list operator may provide either scalar and list
16contexts for its arguments. If it does both, the scalar arguments will
5f05dabc 17be first, and the list argument will follow. (Note that there can ever
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
68dc0745 38 print 1+2+4; # Prints 7.
39 print(1+2) + 4; # Prints 3.
40 print (1+2)+4; # Also prints 3!
41 print +(1+2)+4; # Prints 7.
42 print ((1+2)+4); # Prints 7.
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43
44If you run Perl with the B<-w> switch it can warn you about this. For
45example, the third line above produces:
46
47 print (...) interpreted as function at - line 1.
48 Useless use of integer addition in void context at - line 1.
49
50For functions that can be used in either a scalar or list context,
54310121 51nonabortive failure is generally indicated in a scalar context by
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52returning the undefined value, and in a list context by returning the
53null list.
54
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55Remember the following important rule: There is B<no rule> that relates
56the behavior of an expression in list context to its behavior in scalar
57context, or vice versa. It might do two totally different things.
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58Each operator and function decides which sort of value it would be most
59appropriate to return in a scalar context. Some operators return the
5a964f20 60length of the list that would have been returned in list context. Some
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61operators return the first value in the list. Some operators return the
62last value in the list. Some operators return a count of successful
63operations. In general, they do what you want, unless you want
64consistency.
65
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66An named array in scalar context is quite different from what would at
67first glance appear to be a list in scalar context. You can't get a list
68like C<(1,2,3)> into being in scalar context, because the compiler knows
69the context at compile time. It would generate the scalar comma operator
70there, not the list construction version of the comma. That means it
71was never a list to start with.
72
73In general, functions in Perl that serve as wrappers for system calls
f86cebdf 74of the same name (like chown(2), fork(2), closedir(2), etc.) all return
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75true when they succeed and C<undef> otherwise, as is usually mentioned
76in the descriptions below. This is different from the C interfaces,
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77which return C<-1> on failure. Exceptions to this rule are C<wait()>,
78C<waitpid()>, and C<syscall()>. System calls also set the special C<$!>
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79variable on failure. Other functions do not, except accidentally.
80
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81=head2 Perl Functions by Category
82
83Here are Perl's functions (including things that look like
5a964f20 84functions, like some keywords and named operators)
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85arranged by category. Some functions appear in more
86than one place.
87
88=over
89
90=item Functions for SCALARs or strings
91
22fae026 92C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>,
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93C<length>, C<oct>, C<ord>, C<pack>, C<q/STRING/>, C<qq/STRING/>, C<reverse>,
94C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///>
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95
96=item Regular expressions and pattern matching
97
ab4f32c2 98C<m//>, C<pos>, C<quotemeta>, C<s///>, C<split>, C<study>, C<qr//>
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99
100=item Numeric functions
101
22fae026
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102C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>,
103C<sin>, C<sqrt>, C<srand>
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104
105=item Functions for real @ARRAYs
106
22fae026 107C<pop>, C<push>, C<shift>, C<splice>, C<unshift>
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108
109=item Functions for list data
110
ab4f32c2 111C<grep>, C<join>, C<map>, C<qw/STRING/>, C<reverse>, C<sort>, C<unpack>
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112
113=item Functions for real %HASHes
114
22fae026 115C<delete>, C<each>, C<exists>, C<keys>, C<values>
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116
117=item Input and output functions
118
22fae026
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119C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
120C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
121C<readdir>, C<rewinddir>, C<seek>, C<seekdir>, C<select>, C<syscall>,
122C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>,
123C<warn>, C<write>
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124
125=item Functions for fixed length data or records
126
22fae026 127C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec>
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128
129=item Functions for filehandles, files, or directories
130
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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
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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
37798a01 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>
37798a01 199
200* - C<sub> was a keyword in perl4, but in perl5 it is an
5a964f20 201operator, which can be used in expressions.
37798a01 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:
228
229 -r File is readable by effective uid/gid.
230 -w File is writable by effective uid/gid.
231 -x File is executable by effective uid/gid.
232 -o File is owned by effective uid.
233
234 -R File is readable by real uid/gid.
235 -W File is writable by real uid/gid.
236 -X File is executable by real uid/gid.
237 -O File is owned by real uid.
238
239 -e File exists.
240 -z File has zero size.
54310121 241 -s File has nonzero size (returns size).
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242
243 -f File is a plain file.
244 -d File is a directory.
245 -l File is a symbolic link.
9c4d0f16 246 -p File is a named pipe (FIFO), or Filehandle is a pipe.
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247 -S File is a socket.
248 -b File is a block special file.
249 -c File is a character special file.
250 -t Filehandle is opened to a tty.
251
252 -u File has setuid bit set.
253 -g File has setgid bit set.
254 -k File has sticky bit set.
255
256 -T File is a text file.
257 -B File is a binary file (opposite of -T).
258
259 -M Age of file in days when script started.
260 -A Same for access time.
261 -C Same for inode change time.
262
263The interpretation of the file permission operators C<-r>, C<-R>, C<-w>,
5f05dabc 264C<-W>, C<-x>, and C<-X> is based solely on the mode of the file and the
a0d0e21e 265uids and gids of the user. There may be other reasons you can't actually
5a964f20 266read, write, or execute the file, such as AFS access control lists. Also note that, for the superuser,
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267C<-r>, C<-R>, C<-w>, and C<-W> always return C<1>, and C<-x> and C<-X> return
268C<1> if any execute bit is set in the mode. Scripts run by the superuser may
269thus need to do a C<stat()> to determine the actual mode of the
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270file, or temporarily set the uid to something else.
271
272Example:
273
274 while (<>) {
275 chop;
276 next unless -f $_; # ignore specials
5a964f20 277 #...
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278 }
279
280Note that C<-s/a/b/> does not do a negated substitution. Saying
281C<-exp($foo)> still works as expected, however--only single letters
282following a minus are interpreted as file tests.
283
284The C<-T> and C<-B> switches work as follows. The first block or so of the
285file is examined for odd characters such as strange control codes or
5a964f20 286characters with the high bit set. If too many strange characters (E<gt>30%)
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287are found, it's a C<-B> file, otherwise it's a C<-T> file. Also, any file
288containing null in the first block is considered a binary file. If C<-T>
289or C<-B> is used on a filehandle, the current stdio buffer is examined
290rather than the first block. Both C<-T> and C<-B> return TRUE on a null
54310121 291file, or a file at EOF when testing a filehandle. Because you have to
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292read a file to do the C<-T> test, on most occasions you want to use a C<-f>
293against the file first, as in C<next unless -f $file && -T $file>.
a0d0e21e 294
7660c0ab 295If any of the file tests (or either the C<stat()> or C<lstat()> operators) are given
28757baa 296the special filehandle consisting of a solitary underline, then the stat
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297structure of the previous file test (or stat operator) is used, saving
298a system call. (This doesn't work with C<-t>, and you need to remember
299that lstat() and C<-l> will leave values in the stat structure for the
300symbolic link, not the real file.) Example:
301
302 print "Can do.\n" if -r $a || -w _ || -x _;
303
304 stat($filename);
305 print "Readable\n" if -r _;
306 print "Writable\n" if -w _;
307 print "Executable\n" if -x _;
308 print "Setuid\n" if -u _;
309 print "Setgid\n" if -g _;
310 print "Sticky\n" if -k _;
311 print "Text\n" if -T _;
312 print "Binary\n" if -B _;
313
314=item abs VALUE
315
54310121 316=item abs
bbce6d69 317
a0d0e21e 318Returns the absolute value of its argument.
7660c0ab 319If VALUE is omitted, uses C<$_>.
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320
321=item accept NEWSOCKET,GENERICSOCKET
322
f86cebdf 323Accepts an incoming socket connect, just as the accept(2) system call
a0d0e21e 324does. Returns the packed address if it succeeded, FALSE otherwise.
4633a7c4 325See example in L<perlipc/"Sockets: Client/Server Communication">.
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326
327=item alarm SECONDS
328
54310121 329=item alarm
bbce6d69 330
a0d0e21e 331Arranges to have a SIGALRM delivered to this process after the
bbce6d69 332specified number of seconds have elapsed. If SECONDS is not specified,
7660c0ab 333the value stored in C<$_> is used. (On some machines,
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334unfortunately, the elapsed time may be up to one second less than you
335specified because of how seconds are counted.) Only one timer may be
336counting at once. Each call disables the previous timer, and an
7660c0ab 337argument of C<0> may be supplied to cancel the previous timer without
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338starting a new one. The returned value is the amount of time remaining
339on the previous timer.
340
4633a7c4 341For delays of finer granularity than one second, you may use Perl's
f86cebdf 342C<syscall()> interface to access setitimer(2) if your system supports it,
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343or else see L</select()>. It is usually a mistake to intermix C<alarm()>
344and C<sleep()> calls.
a0d0e21e 345
7660c0ab 346If you want to use C<alarm()> to time out a system call you need to use an
ab4f32c2 347C<eval()>/C<die()> pair. You can't rely on the alarm causing the system call to
f86cebdf 348fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
ab4f32c2 349restart system calls on some systems. Using C<eval()>/C<die()> always works,
5a964f20 350modulo the caveats given in L<perlipc/"Signals">.
ff68c719 351
352 eval {
f86cebdf 353 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
36477c24 354 alarm $timeout;
ff68c719 355 $nread = sysread SOCKET, $buffer, $size;
36477c24 356 alarm 0;
ff68c719 357 };
ff68c719 358 if ($@) {
f86cebdf 359 die unless $@ eq "alarm\n"; # propagate unexpected errors
ff68c719 360 # timed out
361 }
362 else {
363 # didn't
364 }
365
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366=item atan2 Y,X
367
368Returns the arctangent of Y/X in the range -PI to PI.
369
7660c0ab 370For the tangent operation, you may use the C<POSIX::tan()>
28757baa 371function, or use the familiar relation:
372
373 sub tan { sin($_[0]) / cos($_[0]) }
374
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375=item bind SOCKET,NAME
376
377Binds a network address to a socket, just as the bind system call
378does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
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379packed address of the appropriate type for the socket. See the examples in
380L<perlipc/"Sockets: Client/Server Communication">.
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381
382=item binmode FILEHANDLE
383
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384Arranges for the file to be read or written in "binary" mode in operating
385systems that distinguish between binary and text files. Files that are
386not in binary mode have CR LF sequences translated to LF on input and LF
54310121 387translated to CR LF on output. Binmode has no effect under Unix; in MS-DOS
cb1a09d0 388and similarly archaic systems, it may be imperative--otherwise your
54310121 389MS-DOS-damaged C library may mangle your file. The key distinction between
ab4f32c2 390systems that need C<binmode()> and those that don't is their text file
5a964f20 391formats. Systems like Unix, MacOS, and Plan9 that delimit lines with a single
7660c0ab 392character, and that encode that character in C as C<"\n">, do not need
ab4f32c2 393C<binmode()>. The rest need it. If FILEHANDLE is an expression, the value
cb1a09d0 394is taken as the name of the filehandle.
a0d0e21e 395
4633a7c4 396=item bless REF,CLASSNAME
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397
398=item bless REF
399
28757baa 400This function tells the thingy referenced by REF that it is now
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401an object in the CLASSNAME package--or the current package if no CLASSNAME
402is specified, which is often the case. It returns the reference for
7660c0ab 403convenience, because a C<bless()> is often the last thing in a constructor.
4633a7c4 404Always use the two-argument version if the function doing the blessing
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405might be inherited by a derived class. See L<perltoot> and L<perlobj>
406for more about the blessing (and blessings) of objects.
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407
408=item caller EXPR
409
410=item caller
411
5a964f20 412Returns the context of the current subroutine call. In scalar context,
28757baa 413returns the caller's package name if there is a caller, that is, if
7660c0ab 414we're in a subroutine or C<eval()> or C<require()>, and the undefined value
5a964f20 415otherwise. In list context, returns
a0d0e21e 416
748a9306 417 ($package, $filename, $line) = caller;
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418
419With EXPR, it returns some extra information that the debugger uses to
420print a stack trace. The value of EXPR indicates how many call frames
421to go back before the current one.
422
54310121 423 ($package, $filename, $line, $subroutine,
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424 $hasargs, $wantarray, $evaltext, $is_require) = caller($i);
425
7660c0ab 426Here C<$subroutine> may be C<"(eval)"> if the frame is not a subroutine
ab4f32c2 427call, but an C<eval()>. In such a case additional elements C<$evaltext> and
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428C<$is_require> are set: C<$is_require> is true if the frame is created by a
429C<require> or C<use> statement, C<$evaltext> contains the text of the
dc848c6f 430C<eval EXPR> statement. In particular, for a C<eval BLOCK> statement,
7660c0ab 431C<$filename> is C<"(eval)">, but C<$evaltext> is undefined. (Note also that
dc848c6f 432each C<use> statement creates a C<require> frame inside an C<eval EXPR>)
433frame.
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434
435Furthermore, when called from within the DB package, caller returns more
7660c0ab 436detailed information: it sets the list variable C<@DB::args> to be the
54310121 437arguments with which the subroutine was invoked.
748a9306 438
7660c0ab 439Be aware that the optimizer might have optimized call frames away before
ab4f32c2 440C<caller()> had a chance to get the information. That means that C<caller(N)>
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441might not return information about the call frame you expect it do, for
442C<N E<gt> 1>. In particular, C<@DB::args> might have information from the
443previous time C<caller()> was called.
444
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445=item chdir EXPR
446
447Changes the working directory to EXPR, if possible. If EXPR is
448omitted, changes to home directory. Returns TRUE upon success, FALSE
7660c0ab 449otherwise. See example under C<die()>.
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450
451=item chmod LIST
452
453Changes the permissions of a list of files. The first element of the
4633a7c4 454list must be the numerical mode, which should probably be an octal
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455number, and which definitely should I<not> a string of octal digits:
456C<0644> is okay, C<'0644'> is not. Returns the number of files
dc848c6f 457successfully changed. See also L</oct>, if all you have is a string.
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458
459 $cnt = chmod 0755, 'foo', 'bar';
460 chmod 0755, @executables;
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461 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
462 # --w----r-T
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463 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
464 $mode = 0644; chmod $mode, 'foo'; # this is best
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465
466=item chomp VARIABLE
467
468=item chomp LIST
469
470=item chomp
471
3e3baf6d 472This is a slightly safer version of L</chop>. It removes any
a0d0e21e 473line ending that corresponds to the current value of C<$/> (also known as
28757baa 474$INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
475number of characters removed from all its arguments. It's often used to
476remove the newline from the end of an input record when you're worried
477that the final record may be missing its newline. When in paragraph mode
478(C<$/ = "">), it removes all trailing newlines from the string. If
7660c0ab 479VARIABLE is omitted, it chomps C<$_>. Example:
a0d0e21e
LW
480
481 while (<>) {
482 chomp; # avoid \n on last field
483 @array = split(/:/);
5a964f20 484 # ...
a0d0e21e
LW
485 }
486
487You can actually chomp anything that's an lvalue, including an assignment:
488
489 chomp($cwd = `pwd`);
490 chomp($answer = <STDIN>);
491
492If you chomp a list, each element is chomped, and the total number of
493characters removed is returned.
494
495=item chop VARIABLE
496
497=item chop LIST
498
499=item chop
500
501Chops off the last character of a string and returns the character
502chopped. It's used primarily to remove the newline from the end of an
503input record, but is much more efficient than C<s/\n//> because it neither
7660c0ab 504scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
a0d0e21e
LW
505Example:
506
507 while (<>) {
508 chop; # avoid \n on last field
509 @array = split(/:/);
5a964f20 510 #...
a0d0e21e
LW
511 }
512
513You can actually chop anything that's an lvalue, including an assignment:
514
515 chop($cwd = `pwd`);
516 chop($answer = <STDIN>);
517
518If you chop a list, each element is chopped. Only the value of the
ab4f32c2 519last C<chop()> is returned.
a0d0e21e 520
ab4f32c2 521Note that C<chop()> returns the last character. To return all but the last
748a9306
LW
522character, use C<substr($string, 0, -1)>.
523
a0d0e21e
LW
524=item chown LIST
525
526Changes the owner (and group) of a list of files. The first two
527elements of the list must be the I<NUMERICAL> uid and gid, in that order.
528Returns the number of files successfully changed.
529
530 $cnt = chown $uid, $gid, 'foo', 'bar';
531 chown $uid, $gid, @filenames;
532
54310121 533Here's an example that looks up nonnumeric uids in the passwd file:
a0d0e21e
LW
534
535 print "User: ";
536 chop($user = <STDIN>);
5a964f20 537 print "Files: ";
a0d0e21e
LW
538 chop($pattern = <STDIN>);
539
540 ($login,$pass,$uid,$gid) = getpwnam($user)
541 or die "$user not in passwd file";
542
5a964f20 543 @ary = glob($pattern); # expand filenames
a0d0e21e
LW
544 chown $uid, $gid, @ary;
545
54310121 546On most systems, you are not allowed to change the ownership of the
4633a7c4
LW
547file unless you're the superuser, although you should be able to change
548the group to any of your secondary groups. On insecure systems, these
549restrictions may be relaxed, but this is not a portable assumption.
550
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LW
551=item chr NUMBER
552
54310121 553=item chr
bbce6d69 554
a0d0e21e 555Returns the character represented by that NUMBER in the character set.
7660c0ab 556For example, C<chr(65)> is C<"A"> in ASCII. For the reverse, use L</ord>.
a0d0e21e 557
7660c0ab 558If NUMBER is omitted, uses C<$_>.
bbce6d69 559
a0d0e21e
LW
560=item chroot FILENAME
561
54310121 562=item chroot
bbce6d69 563
5a964f20 564This function works like the system call by the same name: it makes the
4633a7c4 565named directory the new root directory for all further pathnames that
7660c0ab 566begin with a C<"/"> by your process and all its children. (It doesn't
28757baa 567change your current working directory, which is unaffected.) For security
4633a7c4 568reasons, this call is restricted to the superuser. If FILENAME is
ab4f32c2 569omitted, does a C<chroot()> to C<$_>.
a0d0e21e
LW
570
571=item close FILEHANDLE
572
6a518fbc
TP
573=item close
574
a0d0e21e
LW
575Closes the file or pipe associated with the file handle, returning TRUE
576only if stdio successfully flushes buffers and closes the system file
6a518fbc
TP
577descriptor. Closes the currently selected filehandle if the argument
578is omitted.
fb73857a 579
580You don't have to close FILEHANDLE if you are immediately going to do
7660c0ab 581another C<open()> on it, because C<open()> will close it for you. (See
ab4f32c2 582C<open()>.) However, an explicit C<close()> on an input file resets the line
7660c0ab 583counter (C<$.>), while the implicit close done by C<open()> does not.
fb73857a 584
ab4f32c2 585If the file handle came from a piped open C<close()> will additionally
fb73857a 586return FALSE if one of the other system calls involved fails or if the
587program exits with non-zero status. (If the only problem was that the
7660c0ab 588program exited non-zero C<$!> will be set to C<0>.) Also, closing a pipe
5a964f20 589waits for the process executing on the pipe to complete, in case you
fb73857a 590want to look at the output of the pipe afterwards. Closing a pipe
591explicitly also puts the exit status value of the command into C<$?>.
5a964f20 592
fb73857a 593Example:
a0d0e21e 594
fb73857a 595 open(OUTPUT, '|sort >foo') # pipe to sort
596 or die "Can't start sort: $!";
5a964f20 597 #... # print stuff to output
fb73857a 598 close OUTPUT # wait for sort to finish
599 or warn $! ? "Error closing sort pipe: $!"
600 : "Exit status $? from sort";
601 open(INPUT, 'foo') # get sort's results
602 or die "Can't open 'foo' for input: $!";
a0d0e21e 603
5a964f20
TC
604FILEHANDLE may be an expression whose value can be used as an indirect
605filehandle, usually the real filehandle name.
a0d0e21e
LW
606
607=item closedir DIRHANDLE
608
7660c0ab 609Closes a directory opened by C<opendir()> and returns the success of that
5a964f20
TC
610system call.
611
612DIRHANDLE may be an expression whose value can be used as an indirect
613dirhandle, usually the real dirhandle name.
a0d0e21e
LW
614
615=item connect SOCKET,NAME
616
617Attempts to connect to a remote socket, just as the connect system call
618does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
4633a7c4
LW
619packed address of the appropriate type for the socket. See the examples in
620L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 621
cb1a09d0
AD
622=item continue BLOCK
623
624Actually a flow control statement rather than a function. If there is a
625C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
626C<foreach>), it is always executed just before the conditional is about to
627be evaluated again, just like the third part of a C<for> loop in C. Thus
628it can be used to increment a loop variable, even when the loop has been
629continued via the C<next> statement (which is similar to the C C<continue>
630statement).
631
1d2dff63
GS
632C<last>, C<next>, or C<redo> may appear within a C<continue>
633block. C<last> and C<redo> will behave as if they had been executed within
634the main block. So will C<next>, but since it will execute a C<continue>
635block, it may be more entertaining.
636
637 while (EXPR) {
638 ### redo always comes here
639 do_something;
640 } continue {
641 ### next always comes here
642 do_something_else;
643 # then back the top to re-check EXPR
644 }
645 ### last always comes here
646
647Omitting the C<continue> section is semantically equivalent to using an
648empty one, logically enough. In that case, C<next> goes directly back
649to check the condition at the top of the loop.
650
a0d0e21e
LW
651=item cos EXPR
652
5a964f20 653Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
7660c0ab 654takes cosine of C<$_>.
a0d0e21e 655
7660c0ab 656For the inverse cosine operation, you may use the C<POSIX::acos()>
28757baa 657function, or use this relation:
658
659 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
660
a0d0e21e
LW
661=item crypt PLAINTEXT,SALT
662
f86cebdf 663Encrypts a string exactly like the crypt(3) function in the C library
4633a7c4
LW
664(assuming that you actually have a version there that has not been
665extirpated as a potential munition). This can prove useful for checking
666the password file for lousy passwords, amongst other things. Only the
667guys wearing white hats should do this.
a0d0e21e 668
7660c0ab 669Note that C<crypt()> is intended to be a one-way function, much like breaking
11155c91
CS
670eggs to make an omelette. There is no (known) corresponding decrypt
671function. As a result, this function isn't all that useful for
672cryptography. (For that, see your nearby CPAN mirror.)
2f9daede 673
a0d0e21e
LW
674Here's an example that makes sure that whoever runs this program knows
675their own password:
676
677 $pwd = (getpwuid($<))[1];
678 $salt = substr($pwd, 0, 2);
679
680 system "stty -echo";
681 print "Password: ";
682 chop($word = <STDIN>);
683 print "\n";
684 system "stty echo";
685
686 if (crypt($word, $salt) ne $pwd) {
687 die "Sorry...\n";
688 } else {
689 print "ok\n";
54310121 690 }
a0d0e21e 691
9f8f0c9d 692Of course, typing in your own password to whoever asks you
748a9306 693for it is unwise.
a0d0e21e 694
aa689395 695=item dbmclose HASH
a0d0e21e 696
7660c0ab 697[This function has been superseded by the C<untie()> function.]
a0d0e21e 698
aa689395 699Breaks the binding between a DBM file and a hash.
a0d0e21e 700
aa689395 701=item dbmopen HASH,DBNAME,MODE
a0d0e21e 702
7660c0ab 703[This function has been superseded by the C<tie()> function.]
a0d0e21e 704
7b8d334a 705This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
ab4f32c2 706hash. HASH is the name of the hash. (Unlike normal C<open()>, the first
aa689395 707argument is I<NOT> a filehandle, even though it looks like one). DBNAME
708is the name of the database (without the F<.dir> or F<.pag> extension if
709any). If the database does not exist, it is created with protection
7660c0ab
A
710specified by MODE (as modified by the C<umask()>). If your system supports
711only the older DBM functions, you may perform only one C<dbmopen()> in your
aa689395 712program. In older versions of Perl, if your system had neither DBM nor
7660c0ab 713ndbm, calling C<dbmopen()> produced a fatal error; it now falls back to
aa689395 714sdbm(3).
715
716If you don't have write access to the DBM file, you can only read hash
717variables, not set them. If you want to test whether you can write,
7660c0ab 718either use file tests or try setting a dummy hash entry inside an C<eval()>,
aa689395 719which will trap the error.
a0d0e21e 720
7660c0ab
A
721Note that functions such as C<keys()> and C<values()> may return huge lists
722when used on large DBM files. You may prefer to use the C<each()>
a0d0e21e
LW
723function to iterate over large DBM files. Example:
724
725 # print out history file offsets
726 dbmopen(%HIST,'/usr/lib/news/history',0666);
727 while (($key,$val) = each %HIST) {
728 print $key, ' = ', unpack('L',$val), "\n";
729 }
730 dbmclose(%HIST);
731
cb1a09d0 732See also L<AnyDBM_File> for a more general description of the pros and
184e9718 733cons of the various dbm approaches, as well as L<DB_File> for a particularly
cb1a09d0 734rich implementation.
4633a7c4 735
a0d0e21e
LW
736=item defined EXPR
737
54310121 738=item defined
bbce6d69 739
2f9daede
TP
740Returns a Boolean value telling whether EXPR has a value other than
741the undefined value C<undef>. If EXPR is not present, C<$_> will be
742checked.
743
744Many operations return C<undef> to indicate failure, end of file,
745system error, uninitialized variable, and other exceptional
746conditions. This function allows you to distinguish C<undef> from
747other values. (A simple Boolean test will not distinguish among
7660c0ab 748C<undef>, zero, the empty string, and C<"0">, which are all equally
2f9daede 749false.) Note that since C<undef> is a valid scalar, its presence
7660c0ab 750doesn't I<necessarily> indicate an exceptional condition: C<pop()>
2f9daede
TP
751returns C<undef> when its argument is an empty array, I<or> when the
752element to return happens to be C<undef>.
753
7660c0ab 754You may also use C<defined()> to check whether a subroutine exists, by
5a964f20 755saying C<defined &func> without parentheses. On the other hand, use
7660c0ab 756of C<defined()> upon aggregates (hashes and arrays) is not guaranteed to
5a964f20 757produce intuitive results, and should probably be avoided.
2f9daede
TP
758
759When used on a hash element, it tells you whether the value is defined,
dc848c6f 760not whether the key exists in the hash. Use L</exists> for the latter
2f9daede 761purpose.
a0d0e21e
LW
762
763Examples:
764
765 print if defined $switch{'D'};
766 print "$val\n" while defined($val = pop(@ary));
767 die "Can't readlink $sym: $!"
768 unless defined($value = readlink $sym);
a0d0e21e 769 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
2f9daede 770 $debugging = 0 unless defined $debugging;
a0d0e21e 771
7660c0ab
A
772Note: Many folks tend to overuse C<defined()>, and then are surprised to
773discover that the number C<0> and C<""> (the zero-length string) are, in fact,
2f9daede 774defined values. For example, if you say
a5f75d66
AD
775
776 "ab" =~ /a(.*)b/;
777
7660c0ab 778The pattern match succeeds, and C<$1> is defined, despite the fact that it
a5f75d66 779matched "nothing". But it didn't really match nothing--rather, it
7660c0ab 780matched something that happened to be C<0> characters long. This is all
a5f75d66 781very above-board and honest. When a function returns an undefined value,
2f9daede 782it's an admission that it couldn't give you an honest answer. So you
7660c0ab
A
783should use C<defined()> only when you're questioning the integrity of what
784you're trying to do. At other times, a simple comparison to C<0> or C<""> is
2f9daede
TP
785what you want.
786
7660c0ab 787Currently, using C<defined()> on an entire array or hash reports whether
2f9daede
TP
788memory for that aggregate has ever been allocated. So an array you set
789to the empty list appears undefined initially, and one that once was full
790and that you then set to the empty list still appears defined. You
791should instead use a simple test for size:
28757baa 792
793 if (@an_array) { print "has array elements\n" }
794 if (%a_hash) { print "has hash members\n" }
795
7660c0ab 796Using C<undef()> on these, however, does clear their memory and then report
5a964f20 797them as not defined anymore, but you shouldn't do that unless you don't
28757baa 798plan to use them again, because it saves time when you load them up
5a964f20
TC
799again to have memory already ready to be filled. The normal way to
800free up space used by an aggregate is to assign the empty list.
28757baa 801
7660c0ab 802This counterintuitive behavior of C<defined()> on aggregates may be
28757baa 803changed, fixed, or broken in a future release of Perl.
804
dc848c6f 805See also L</undef>, L</exists>, L</ref>.
2f9daede 806
a0d0e21e
LW
807=item delete EXPR
808
aa689395 809Deletes the specified key(s) and their associated values from a hash.
810For each key, returns the deleted value associated with that key, or
811the undefined value if there was no such key. Deleting from C<$ENV{}>
812modifies the environment. Deleting from a hash tied to a DBM file
7660c0ab 813deletes the entry from the DBM file. (But deleting from a C<tie()>d hash
5f05dabc 814doesn't necessarily return anything.)
a0d0e21e 815
aa689395 816The following deletes all the values of a hash:
a0d0e21e 817
5f05dabc 818 foreach $key (keys %HASH) {
819 delete $HASH{$key};
a0d0e21e
LW
820 }
821
5f05dabc 822And so does this:
823
824 delete @HASH{keys %HASH}
825
5a964f20 826(But both of these are slower than just assigning the empty list, or
7660c0ab 827using C<undef()>.) Note that the EXPR can be arbitrarily complicated as
5a964f20 828long as the final operation is a hash element lookup or hash slice:
a0d0e21e
LW
829
830 delete $ref->[$x][$y]{$key};
5f05dabc 831 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
a0d0e21e
LW
832
833=item die LIST
834
7660c0ab
A
835Outside an C<eval()>, prints the value of LIST to C<STDERR> and exits with
836the current value of C<$!> (errno). If C<$!> is C<0>, exits with the value of
54310121 837C<($? E<gt>E<gt> 8)> (backtick `command` status). If C<($? E<gt>E<gt> 8)>
7660c0ab
A
838is C<0>, exits with C<255>. Inside an C<eval(),> the error message is stuffed into
839C<$@> and the C<eval()> is terminated with the undefined value. This makes
840C<die()> the way to raise an exception.
a0d0e21e
LW
841
842Equivalent examples:
843
844 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
54310121 845 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
a0d0e21e
LW
846
847If the value of EXPR does not end in a newline, the current script line
848number and input line number (if any) are also printed, and a newline
7660c0ab
A
849is supplied. Hint: sometimes appending C<", stopped"> to your message
850will cause it to make better sense when the string C<"at foo line 123"> is
a0d0e21e
LW
851appended. Suppose you are running script "canasta".
852
853 die "/etc/games is no good";
854 die "/etc/games is no good, stopped";
855
856produce, respectively
857
858 /etc/games is no good at canasta line 123.
859 /etc/games is no good, stopped at canasta line 123.
860
7660c0ab 861See also C<exit()> and C<warn()>.
a0d0e21e 862
7660c0ab
A
863If LIST is empty and C<$@> already contains a value (typically from a
864previous eval) that value is reused after appending C<"\t...propagated">.
fb73857a 865This is useful for propagating exceptions:
866
867 eval { ... };
868 die unless $@ =~ /Expected exception/;
869
7660c0ab 870If C<$@> is empty then the string C<"Died"> is used.
fb73857a 871
7660c0ab 872You can arrange for a callback to be run just before the C<die()> does
774d564b 873its deed, by setting the C<$SIG{__DIE__}> hook. The associated handler
874will be called with the error text and can change the error message, if
7660c0ab 875it sees fit, by calling C<die()> again. See L<perlvar/$SIG{expr}> for details on
fb73857a 876setting C<%SIG> entries, and L<"eval BLOCK"> for some examples.
877
878Note that the C<$SIG{__DIE__}> hook is called even inside eval()ed
879blocks/strings. If one wants the hook to do nothing in such
880situations, put
881
882 die @_ if $^S;
883
884as the first line of the handler (see L<perlvar/$^S>).
774d564b 885
a0d0e21e
LW
886=item do BLOCK
887
888Not really a function. Returns the value of the last command in the
889sequence of commands indicated by BLOCK. When modified by a loop
890modifier, executes the BLOCK once before testing the loop condition.
891(On other statements the loop modifiers test the conditional first.)
892
893=item do SUBROUTINE(LIST)
894
895A deprecated form of subroutine call. See L<perlsub>.
896
897=item do EXPR
898
899Uses the value of EXPR as a filename and executes the contents of the
900file as a Perl script. Its primary use is to include subroutines
901from a Perl subroutine library.
902
903 do 'stat.pl';
904
905is just like
906
fb73857a 907 scalar eval `cat stat.pl`;
a0d0e21e 908
5a964f20 909except that it's more efficient and concise, keeps track of the
a0d0e21e
LW
910current filename for error messages, and searches all the B<-I>
911libraries if the file isn't in the current directory (see also the @INC
dc1be6b5
GS
912array in L<perlvar/Predefined Names>). It is also different in how
913code evaluated with C<do FILENAME> doesn't see lexicals in the enclosing
914scope like C<eval STRING> does. It's the same, however, in that it does
54310121 915reparse the file every time you call it, so you probably don't want to
a0d0e21e
LW
916do this inside a loop.
917
918Note that inclusion of library modules is better done with the
7660c0ab 919C<use()> and C<require()> operators, which also do automatic error checking
4633a7c4 920and raise an exception if there's a problem.
a0d0e21e 921
5a964f20
TC
922You might like to use C<do> to read in a program configuration
923file. Manual error checking can be done this way:
924
925 # read in config files: system first, then user
f86cebdf
GS
926 for $file ("/share/prog/defaults.rc",
927 "$ENV{HOME}/.someprogrc") {
5a964f20 928 unless ($return = do $file) {
f86cebdf
GS
929 warn "couldn't parse $file: $@" if $@;
930 warn "couldn't do $file: $!" unless defined $return;
931 warn "couldn't run $file" unless $return;
5a964f20
TC
932 }
933 }
934
a0d0e21e
LW
935=item dump LABEL
936
937This causes an immediate core dump. Primarily this is so that you can
938use the B<undump> program to turn your core dump into an executable binary
939after having initialized all your variables at the beginning of the
940program. When the new binary is executed it will begin by executing a
941C<goto LABEL> (with all the restrictions that C<goto> suffers). Think of
f86cebdf 942it as a goto with an intervening core dump and reincarnation. If C<LABEL>
5a964f20 943is omitted, restarts the program from the top. WARNING: Any files
a0d0e21e
LW
944opened at the time of the dump will NOT be open any more when the
945program is reincarnated, with possible resulting confusion on the part
946of Perl. See also B<-u> option in L<perlrun>.
947
948Example:
949
950 #!/usr/bin/perl
951 require 'getopt.pl';
952 require 'stat.pl';
953 %days = (
954 'Sun' => 1,
955 'Mon' => 2,
956 'Tue' => 3,
957 'Wed' => 4,
958 'Thu' => 5,
959 'Fri' => 6,
960 'Sat' => 7,
961 );
962
963 dump QUICKSTART if $ARGV[0] eq '-d';
964
965 QUICKSTART:
966 Getopt('f');
967
5a964f20
TC
968This operator is largely obsolete, partly because it's very hard to
969convert a core file into an executable, and because the real perl-to-C
970compiler has superseded it.
971
aa689395 972=item each HASH
973
5a964f20 974When called in list context, returns a 2-element list consisting of the
aa689395 975key and value for the next element of a hash, so that you can iterate over
5a964f20 976it. When called in scalar context, returns the key for only the "next"
7660c0ab 977element in the hash. (Note: Keys may be C<"0"> or C<"">, which are logically
2f9daede
TP
978false; you may wish to avoid constructs like C<while ($k = each %foo) {}>
979for this reason.)
980
981Entries are returned in an apparently random order. When the hash is
982entirely read, a null array is returned in list context (which when
7660c0ab
A
983assigned produces a FALSE (C<0>) value), and C<undef> in
984scalar context. The next call to C<each()> after that will start iterating
985again. There is a single iterator for each hash, shared by all C<each()>,
986C<keys()>, and C<values()> function calls in the program; it can be reset by
2f9daede
TP
987reading all the elements from the hash, or by evaluating C<keys HASH> or
988C<values HASH>. If you add or delete elements of a hash while you're
989iterating over it, you may get entries skipped or duplicated, so don't.
aa689395 990
f86cebdf 991The following prints out your environment like the printenv(1) program,
aa689395 992only in a different order:
a0d0e21e
LW
993
994 while (($key,$value) = each %ENV) {
995 print "$key=$value\n";
996 }
997
7660c0ab 998See also C<keys()> and C<values()>.
a0d0e21e
LW
999
1000=item eof FILEHANDLE
1001
4633a7c4
LW
1002=item eof ()
1003
a0d0e21e
LW
1004=item eof
1005
1006Returns 1 if the next read on FILEHANDLE will return end of file, or if
1007FILEHANDLE is not open. FILEHANDLE may be an expression whose value
5a964f20 1008gives the real filehandle. (Note that this function actually
7660c0ab 1009reads a character and then C<ungetc()>s it, so isn't very useful in an
748a9306
LW
1010interactive context.) Do not read from a terminal file (or call
1011C<eof(FILEHANDLE)> on it) after end-of-file is reached. Filetypes such
1012as terminals may lose the end-of-file condition if you do.
1013
1014An C<eof> without an argument uses the last file read as argument.
5a964f20 1015Using C<eof()> with empty parentheses is very different. It indicates the pseudo file formed of
2f9daede
TP
1016the files listed on the command line, i.e., C<eof()> is reasonable to
1017use inside a C<while (E<lt>E<gt>)> loop to detect the end of only the
1018last file. Use C<eof(ARGV)> or eof without the parentheses to test
1019I<EACH> file in a while (E<lt>E<gt>) loop. Examples:
a0d0e21e 1020
748a9306
LW
1021 # reset line numbering on each input file
1022 while (<>) {
5a964f20 1023 next if /^\s*#/; # skip comments
748a9306 1024 print "$.\t$_";
5a964f20
TC
1025 } continue {
1026 close ARGV if eof; # Not eof()!
748a9306
LW
1027 }
1028
a0d0e21e
LW
1029 # insert dashes just before last line of last file
1030 while (<>) {
5a964f20 1031 if (eof()) { # check for end of current file
a0d0e21e 1032 print "--------------\n";
748a9306
LW
1033 close(ARGV); # close or break; is needed if we
1034 # are reading from the terminal
a0d0e21e
LW
1035 }
1036 print;
1037 }
1038
a0d0e21e 1039Practical hint: you almost never need to use C<eof> in Perl, because the
3b02c43c
GS
1040input operators return false values when they run out of data, or if there
1041was an error.
a0d0e21e
LW
1042
1043=item eval EXPR
1044
1045=item eval BLOCK
1046
c7cc6f1c
GS
1047In the first form, the return value of EXPR is parsed and executed as if it
1048were a little Perl program. The value of the expression (which is itself
5a964f20 1049determined within scalar context) is first parsed, and if there weren't any
c7cc6f1c 1050errors, executed in the context of the current Perl program, so that any
5f05dabc 1051variable settings or subroutine and format definitions remain afterwards.
c7cc6f1c
GS
1052Note that the value is parsed every time the eval executes. If EXPR is
1053omitted, evaluates C<$_>. This form is typically used to delay parsing
1054and subsequent execution of the text of EXPR until run time.
1055
1056In the second form, the code within the BLOCK is parsed only once--at the
1057same time the code surrounding the eval itself was parsed--and executed
1058within the context of the current Perl program. This form is typically
1059used to trap exceptions more efficiently than the first (see below), while
1060also providing the benefit of checking the code within BLOCK at compile
1061time.
1062
1063The final semicolon, if any, may be omitted from the value of EXPR or within
1064the BLOCK.
1065
1066In both forms, the value returned is the value of the last expression
5a964f20 1067evaluated inside the mini-program; a return statement may be also used, just
c7cc6f1c 1068as with subroutines. The expression providing the return value is evaluated
5a964f20 1069in void, scalar, or list context, depending on the context of the eval itself.
c7cc6f1c 1070See L</wantarray> for more on how the evaluation context can be determined.
a0d0e21e 1071
7660c0ab
A
1072If there is a syntax error or runtime error, or a C<die()> statement is
1073executed, an undefined value is returned by C<eval()>, and C<$@> is set to the
a0d0e21e 1074error message. If there was no error, C<$@> is guaranteed to be a null
7660c0ab 1075string. Beware that using C<eval()> neither silences perl from printing
c7cc6f1c
GS
1076warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1077To do either of those, you have to use the C<$SIG{__WARN__}> facility. See
1078L</warn> and L<perlvar>.
a0d0e21e 1079
7660c0ab
A
1080Note that, because C<eval()> traps otherwise-fatal errors, it is useful for
1081determining whether a particular feature (such as C<socket()> or C<symlink()>)
a0d0e21e
LW
1082is implemented. It is also Perl's exception trapping mechanism, where
1083the die operator is used to raise exceptions.
1084
1085If the code to be executed doesn't vary, you may use the eval-BLOCK
1086form to trap run-time errors without incurring the penalty of
1087recompiling each time. The error, if any, is still returned in C<$@>.
1088Examples:
1089
54310121 1090 # make divide-by-zero nonfatal
a0d0e21e
LW
1091 eval { $answer = $a / $b; }; warn $@ if $@;
1092
1093 # same thing, but less efficient
1094 eval '$answer = $a / $b'; warn $@ if $@;
1095
1096 # a compile-time error
5a964f20 1097 eval { $answer = }; # WRONG
a0d0e21e
LW
1098
1099 # a run-time error
1100 eval '$answer ='; # sets $@
1101
7660c0ab 1102When using the C<eval{}> form as an exception trap in libraries, you may
774d564b 1103wish not to trigger any C<__DIE__> hooks that user code may have
1104installed. You can use the C<local $SIG{__DIE__}> construct for this
1105purpose, as shown in this example:
1106
1107 # a very private exception trap for divide-by-zero
f86cebdf
GS
1108 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1109 warn $@ if $@;
774d564b 1110
1111This is especially significant, given that C<__DIE__> hooks can call
7660c0ab 1112C<die()> again, which has the effect of changing their error messages:
774d564b 1113
1114 # __DIE__ hooks may modify error messages
1115 {
f86cebdf
GS
1116 local $SIG{'__DIE__'} =
1117 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
c7cc6f1c
GS
1118 eval { die "foo lives here" };
1119 print $@ if $@; # prints "bar lives here"
774d564b 1120 }
1121
7660c0ab 1122With an C<eval()>, you should be especially careful to remember what's
a0d0e21e
LW
1123being looked at when:
1124
1125 eval $x; # CASE 1
1126 eval "$x"; # CASE 2
1127
1128 eval '$x'; # CASE 3
1129 eval { $x }; # CASE 4
1130
5a964f20 1131 eval "\$$x++"; # CASE 5
a0d0e21e
LW
1132 $$x++; # CASE 6
1133
2f9daede 1134Cases 1 and 2 above behave identically: they run the code contained in
7660c0ab 1135the variable C<$x>. (Although case 2 has misleading double quotes making
2f9daede 1136the reader wonder what else might be happening (nothing is).) Cases 3
7660c0ab 1137and 4 likewise behave in the same way: they run the code C<'$x'>, which
2f9daede
TP
1138does nothing but return the value of C<$x>. (Case 4 is preferred for
1139purely visual reasons, but it also has the advantage of compiling at
1140compile-time instead of at run-time.) Case 5 is a place where
54310121 1141normally you I<WOULD> like to use double quotes, except that in this
2f9daede
TP
1142particular situation, you can just use symbolic references instead, as
1143in case 6.
a0d0e21e
LW
1144
1145=item exec LIST
1146
8bf3b016
GS
1147=item exec PROGRAM LIST
1148
7660c0ab
A
1149The C<exec()> function executes a system command I<AND NEVER RETURNS> -
1150use C<system()> instead of C<exec()> if you want it to return. It fails and
fb73857a 1151returns FALSE only if the command does not exist I<and> it is executed
1152directly instead of via your system's command shell (see below).
a0d0e21e 1153
7660c0ab
A
1154Since it's a common mistake to use C<exec()> instead of C<system()>, Perl
1155warns you if there is a following statement which isn't C<die()>, C<warn()>,
1156or C<exit()> (if C<-w> is set - but you always do that). If you
1157I<really> want to follow an C<exec()> with some other statement, you
55d729e4
GS
1158can use one of these styles to avoid the warning:
1159
5a964f20
TC
1160 exec ('foo') or print STDERR "couldn't exec foo: $!";
1161 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
55d729e4 1162
5a964f20 1163If there is more than one argument in LIST, or if LIST is an array
f86cebdf 1164with more than one value, calls execvp(3) with the arguments in LIST.
5a964f20
TC
1165If there is only one scalar argument or an array with one element in it,
1166the argument is checked for shell metacharacters, and if there are any,
1167the entire argument is passed to the system's command shell for parsing
1168(this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1169If there are no shell metacharacters in the argument, it is split into
7660c0ab
A
1170words and passed directly to C<execvp()>, which is more efficient. Note:
1171C<exec()> and C<system()> do not flush your output buffer, so you may need to
5a964f20 1172set C<$|> to avoid lost output. Examples:
a0d0e21e
LW
1173
1174 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1175 exec "sort $outfile | uniq";
1176
1177If you don't really want to execute the first argument, but want to lie
1178to the program you are executing about its own name, you can specify
1179the program you actually want to run as an "indirect object" (without a
1180comma) in front of the LIST. (This always forces interpretation of the
54310121 1181LIST as a multivalued list, even if there is only a single scalar in
a0d0e21e
LW
1182the list.) Example:
1183
1184 $shell = '/bin/csh';
1185 exec $shell '-sh'; # pretend it's a login shell
1186
1187or, more directly,
1188
1189 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1190
bb32b41a
GS
1191When the arguments get executed via the system shell, results will
1192be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1193for details.
1194
ab4f32c2 1195Using an indirect object with C<exec()> or C<system()> is also more secure.
5a964f20
TC
1196This usage forces interpretation of the arguments as a multivalued list,
1197even if the list had just one argument. That way you're safe from the
1198shell expanding wildcards or splitting up words with whitespace in them.
1199
1200 @args = ( "echo surprise" );
1201
f86cebdf
GS
1202 system @args; # subject to shell escapes
1203 # if @args == 1
5a964f20
TC
1204 system { $args[0] } @args; # safe even with one-arg list
1205
1206The first version, the one without the indirect object, ran the I<echo>
1207program, passing it C<"surprise"> an argument. The second version
1208didn't--it tried to run a program literally called I<"echo surprise">,
1209didn't find it, and set C<$?> to a non-zero value indicating failure.
1210
ab4f32c2 1211Note that C<exec()> will not call your C<END> blocks, nor will it call
7660c0ab
A
1212any C<DESTROY> methods in your objects.
1213
a0d0e21e
LW
1214=item exists EXPR
1215
1216Returns TRUE if the specified hash key exists in its hash array, even
1217if the corresponding value is undefined.
1218
1219 print "Exists\n" if exists $array{$key};
1220 print "Defined\n" if defined $array{$key};
1221 print "True\n" if $array{$key};
1222
5f05dabc 1223A hash element can be TRUE only if it's defined, and defined if
a0d0e21e
LW
1224it exists, but the reverse doesn't necessarily hold true.
1225
1226Note that the EXPR can be arbitrarily complicated as long as the final
1227operation is a hash key lookup:
1228
5a964f20
TC
1229 if (exists $ref->{"A"}{"B"}{$key}) { ... }
1230
1231Although the last element will not spring into existence just because its
1232existence was tested, intervening ones will. Thus C<$ref-E<gt>{"A"}>
1233C<$ref-E<gt>{"B"}> will spring into existence due to the existence
1234test for a $key element. This autovivification may be fixed in a later
1235release.
a0d0e21e
LW
1236
1237=item exit EXPR
1238
1239Evaluates EXPR and exits immediately with that value. (Actually, it
1240calls any defined C<END> routines first, but the C<END> routines may not
1241abort the exit. Likewise any object destructors that need to be called
1242are called before exit.) Example:
1243
1244 $ans = <STDIN>;
1245 exit 0 if $ans =~ /^[Xx]/;
1246
7660c0ab
A
1247See also C<die()>. If EXPR is omitted, exits with C<0> status. The only
1248universally portable values for EXPR are C<0> for success and C<1> for error;
f86702cc 1249all other values are subject to unpredictable interpretation depending
1250on the environment in which the Perl program is running.
a0d0e21e 1251
7660c0ab
A
1252You shouldn't use C<exit()> to abort a subroutine if there's any chance that
1253someone might want to trap whatever error happened. Use C<die()> instead,
1254which can be trapped by an C<eval()>.
28757baa 1255
5a964f20
TC
1256All C<END{}> blocks are run at exit time. See L<perlsub> for details.
1257
a0d0e21e
LW
1258=item exp EXPR
1259
54310121 1260=item exp
bbce6d69 1261
54310121 1262Returns I<e> (the natural logarithm base) to the power of EXPR.
a0d0e21e
LW
1263If EXPR is omitted, gives C<exp($_)>.
1264
1265=item fcntl FILEHANDLE,FUNCTION,SCALAR
1266
f86cebdf 1267Implements the fcntl(2) function. You'll probably have to say
a0d0e21e
LW
1268
1269 use Fcntl;
1270
0ade1984 1271first to get the correct constant definitions. Argument processing and
7660c0ab 1272value return works just like C<ioctl()> below.
a0d0e21e
LW
1273For example:
1274
1275 use Fcntl;
5a964f20
TC
1276 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1277 or die "can't fcntl F_GETFL: $!";
1278
ab4f32c2
A
1279You don't have to check for C<defined()> on the return from
1280C<fnctl()>. Like C<ioctl()>, it maps a C<0> return from the system
7660c0ab
A
1281call into "C<0> but true" in Perl. This string is true in
1282boolean context and C<0> in numeric context. It is also
5a964f20
TC
1283exempt from the normal B<-w> warnings on improper numeric
1284conversions.
1285
7660c0ab 1286Note that C<fcntl()> will produce a fatal error if used on a machine that
f86cebdf 1287doesn't implement fcntl(2).
a0d0e21e
LW
1288
1289=item fileno FILEHANDLE
1290
1291Returns the file descriptor for a filehandle. This is useful for
7660c0ab 1292constructing bitmaps for C<select()> and low-level POSIX tty-handling
5a964f20
TC
1293operations. If FILEHANDLE is an expression, the value is taken as
1294an indirect filehandle, generally its name.
1295
1296You can use this to find out whether two handles refer to the
1297same underlying descriptor:
1298
1299 if (fileno(THIS) == fileno(THAT)) {
1300 print "THIS and THAT are dups\n";
1301 }
a0d0e21e
LW
1302
1303=item flock FILEHANDLE,OPERATION
1304
f86cebdf 1305Calls flock(2), or an emulation of it, on FILEHANDLE. Returns TRUE for
68dc0745 1306success, FALSE on failure. Produces a fatal error if used on a machine
f86cebdf 1307that doesn't implement flock(2), fcntl(2) locking, or lockf(3). C<flock()>
68dc0745 1308is Perl's portable file locking interface, although it locks only entire
1309files, not records.
8ebc5c01 1310
a3cb178b 1311On many platforms (including most versions or clones of Unix), locks
7660c0ab 1312established by C<flock()> are B<merely advisory>. Such discretionary locks
5a964f20 1313are more flexible, but offer fewer guarantees. This means that files
7660c0ab
A
1314locked with C<flock()> may be modified by programs that do not also use
1315C<flock()>. Windows NT and OS/2 are among the platforms which
5a964f20 1316enforce mandatory locking. See your local documentation for details.
a3cb178b 1317
8ebc5c01 1318OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1319LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
68dc0745 1320you can use the symbolic names if import them from the Fcntl module,
1321either individually, or as a group using the ':flock' tag. LOCK_SH
1322requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1323releases a previously requested lock. If LOCK_NB is added to LOCK_SH or
7660c0ab 1324LOCK_EX then C<flock()> will return immediately rather than blocking
68dc0745 1325waiting for the lock (check the return status to see if you got it).
1326
1327To avoid the possibility of mis-coordination, Perl flushes FILEHANDLE
1328before (un)locking it.
8ebc5c01 1329
f86cebdf 1330Note that the emulation built with lockf(3) doesn't provide shared
8ebc5c01 1331locks, and it requires that FILEHANDLE be open with write intent. These
f86cebdf
GS
1332are the semantics that lockf(3) implements. Most (all?) systems
1333implement lockf(3) in terms of fcntl(2) locking, though, so the
8ebc5c01 1334differing semantics shouldn't bite too many people.
1335
7660c0ab
A
1336Note also that some versions of C<flock()> cannot lock things over the
1337network; you would need to use the more system-specific C<fcntl()> for
f86cebdf
GS
1338that. If you like you can force Perl to ignore your system's flock(2)
1339function, and so provide its own fcntl(2)-based emulation, by passing
8ebc5c01 1340the switch C<-Ud_flock> to the F<Configure> program when you configure
1341perl.
4633a7c4
LW
1342
1343Here's a mailbox appender for BSD systems.
a0d0e21e 1344
7e1af8bc 1345 use Fcntl ':flock'; # import LOCK_* constants
a0d0e21e
LW
1346
1347 sub lock {
7e1af8bc 1348 flock(MBOX,LOCK_EX);
a0d0e21e
LW
1349 # and, in case someone appended
1350 # while we were waiting...
1351 seek(MBOX, 0, 2);
1352 }
1353
1354 sub unlock {
7e1af8bc 1355 flock(MBOX,LOCK_UN);
a0d0e21e
LW
1356 }
1357
1358 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1359 or die "Can't open mailbox: $!";
1360
1361 lock();
1362 print MBOX $msg,"\n\n";
1363 unlock();
1364
cb1a09d0 1365See also L<DB_File> for other flock() examples.
a0d0e21e
LW
1366
1367=item fork
1368
f86cebdf 1369Does a fork(2) system call. Returns the child pid to the parent process,
7660c0ab 1370C<0> to the child process, or C<undef> if the fork is unsuccessful.
5a964f20 1371
a0d0e21e 1372Note: unflushed buffers remain unflushed in both processes, which means
7660c0ab
A
1373you may need to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()>
1374method of C<IO::Handle> to avoid duplicate output.
a0d0e21e 1375
7660c0ab 1376If you C<fork()> without ever waiting on your children, you will accumulate
a0d0e21e
LW
1377zombies:
1378
4633a7c4 1379 $SIG{CHLD} = sub { wait };
a0d0e21e 1380
54310121 1381There's also the double-fork trick (error checking on
7660c0ab 1382C<fork()> returns omitted);
a0d0e21e
LW
1383
1384 unless ($pid = fork) {
1385 unless (fork) {
1386 exec "what you really wanna do";
1387 die "no exec";
1388 # ... or ...
4633a7c4 1389 ## (some_perl_code_here)
a0d0e21e
LW
1390 exit 0;
1391 }
1392 exit 0;
1393 }
1394 waitpid($pid,0);
1395
cb1a09d0
AD
1396See also L<perlipc> for more examples of forking and reaping
1397moribund children.
1398
28757baa 1399Note that if your forked child inherits system file descriptors like
1400STDIN and STDOUT that are actually connected by a pipe or socket, even
5a964f20 1401if you exit, then the remote server (such as, say, httpd or rsh) won't think
7660c0ab 1402you're done. You should reopen those to F</dev/null> if it's any issue.
28757baa 1403
cb1a09d0
AD
1404=item format
1405
7660c0ab 1406Declare a picture format for use by the C<write()> function. For
cb1a09d0
AD
1407example:
1408
54310121 1409 format Something =
cb1a09d0
AD
1410 Test: @<<<<<<<< @||||| @>>>>>
1411 $str, $%, '$' . int($num)
1412 .
1413
1414 $str = "widget";
184e9718 1415 $num = $cost/$quantity;
cb1a09d0
AD
1416 $~ = 'Something';
1417 write;
1418
1419See L<perlform> for many details and examples.
1420
8903cb82 1421=item formline PICTURE,LIST
a0d0e21e 1422
5a964f20 1423This is an internal function used by C<format>s, though you may call it,
a0d0e21e
LW
1424too. It formats (see L<perlform>) a list of values according to the
1425contents of PICTURE, placing the output into the format output
7660c0ab
A
1426accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
1427Eventually, when a C<write()> is done, the contents of
a0d0e21e 1428C<$^A> are written to some filehandle, but you could also read C<$^A>
7660c0ab
A
1429yourself and then set C<$^A> back to C<"">. Note that a format typically
1430does one C<formline()> per line of form, but the C<formline()> function itself
748a9306 1431doesn't care how many newlines are embedded in the PICTURE. This means
4633a7c4 1432that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
748a9306
LW
1433You may therefore need to use multiple formlines to implement a single
1434record format, just like the format compiler.
1435
5f05dabc 1436Be careful if you put double quotes around the picture, because an "C<@>"
748a9306 1437character may be taken to mean the beginning of an array name.
7660c0ab 1438C<formline()> always returns TRUE. See L<perlform> for other examples.
a0d0e21e
LW
1439
1440=item getc FILEHANDLE
1441
1442=item getc
1443
1444Returns the next character from the input file attached to FILEHANDLE,
9bc64814 1445or the undefined value at end of file, or if there was an error. If
3b02c43c
GS
1446FILEHANDLE is omitted, reads from STDIN. This is not particularly
1447efficient. It cannot be used to get unbuffered single-characters,
1448however. For that, try something more like:
4633a7c4
LW
1449
1450 if ($BSD_STYLE) {
1451 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1452 }
1453 else {
54310121 1454 system "stty", '-icanon', 'eol', "\001";
4633a7c4
LW
1455 }
1456
1457 $key = getc(STDIN);
1458
1459 if ($BSD_STYLE) {
1460 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1461 }
1462 else {
5f05dabc 1463 system "stty", 'icanon', 'eol', '^@'; # ASCII null
4633a7c4
LW
1464 }
1465 print "\n";
1466
54310121 1467Determination of whether $BSD_STYLE should be set
1468is left as an exercise to the reader.
cb1a09d0 1469
7660c0ab 1470The C<POSIX::getattr()> function can do this more portably on systems
5a964f20 1471purporting POSIX compliance.
cb1a09d0 1472See also the C<Term::ReadKey> module from your nearest CPAN site;
54310121 1473details on CPAN can be found on L<perlmod/CPAN>.
a0d0e21e
LW
1474
1475=item getlogin
1476
5a964f20
TC
1477Implements the C library function of the same name, which on most
1478systems returns the current login from F</etc/utmp>, if any. If null,
7660c0ab 1479use C<getpwuid()>.
a0d0e21e 1480
f86702cc 1481 $login = getlogin || getpwuid($<) || "Kilroy";
a0d0e21e 1482
7660c0ab
A
1483Do not consider C<getlogin()> for authentication: it is not as
1484secure as C<getpwuid()>.
4633a7c4 1485
a0d0e21e
LW
1486=item getpeername SOCKET
1487
1488Returns the packed sockaddr address of other end of the SOCKET connection.
1489
4633a7c4
LW
1490 use Socket;
1491 $hersockaddr = getpeername(SOCK);
1492 ($port, $iaddr) = unpack_sockaddr_in($hersockaddr);
1493 $herhostname = gethostbyaddr($iaddr, AF_INET);
1494 $herstraddr = inet_ntoa($iaddr);
a0d0e21e
LW
1495
1496=item getpgrp PID
1497
47e29363 1498Returns the current process group for the specified PID. Use
7660c0ab 1499a PID of C<0> to get the current process group for the
4633a7c4 1500current process. Will raise an exception if used on a machine that
f86cebdf 1501doesn't implement getpgrp(2). If PID is omitted, returns process
7660c0ab
A
1502group of current process. Note that the POSIX version of C<getpgrp()>
1503does not accept a PID argument, so only C<PID==0> is truly portable.
a0d0e21e
LW
1504
1505=item getppid
1506
1507Returns the process id of the parent process.
1508
1509=item getpriority WHICH,WHO
1510
4633a7c4
LW
1511Returns the current priority for a process, a process group, or a user.
1512(See L<getpriority(2)>.) Will raise a fatal exception if used on a
f86cebdf 1513machine that doesn't implement getpriority(2).
a0d0e21e
LW
1514
1515=item getpwnam NAME
1516
1517=item getgrnam NAME
1518
1519=item gethostbyname NAME
1520
1521=item getnetbyname NAME
1522
1523=item getprotobyname NAME
1524
1525=item getpwuid UID
1526
1527=item getgrgid GID
1528
1529=item getservbyname NAME,PROTO
1530
1531=item gethostbyaddr ADDR,ADDRTYPE
1532
1533=item getnetbyaddr ADDR,ADDRTYPE
1534
1535=item getprotobynumber NUMBER
1536
1537=item getservbyport PORT,PROTO
1538
1539=item getpwent
1540
1541=item getgrent
1542
1543=item gethostent
1544
1545=item getnetent
1546
1547=item getprotoent
1548
1549=item getservent
1550
1551=item setpwent
1552
1553=item setgrent
1554
1555=item sethostent STAYOPEN
1556
1557=item setnetent STAYOPEN
1558
1559=item setprotoent STAYOPEN
1560
1561=item setservent STAYOPEN
1562
1563=item endpwent
1564
1565=item endgrent
1566
1567=item endhostent
1568
1569=item endnetent
1570
1571=item endprotoent
1572
1573=item endservent
1574
1575These routines perform the same functions as their counterparts in the
5a964f20 1576system library. In list context, the return values from the
a0d0e21e
LW
1577various get routines are as follows:
1578
1579 ($name,$passwd,$uid,$gid,
6ee623d5 1580 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
a0d0e21e
LW
1581 ($name,$passwd,$gid,$members) = getgr*
1582 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1583 ($name,$aliases,$addrtype,$net) = getnet*
1584 ($name,$aliases,$proto) = getproto*
1585 ($name,$aliases,$port,$proto) = getserv*
1586
1587(If the entry doesn't exist you get a null list.)
1588
5a964f20 1589In scalar context, you get the name, unless the function was a
a0d0e21e
LW
1590lookup by name, in which case you get the other thing, whatever it is.
1591(If the entry doesn't exist you get the undefined value.) For example:
1592
5a964f20
TC
1593 $uid = getpwnam($name);
1594 $name = getpwuid($num);
1595 $name = getpwent();
1596 $gid = getgrnam($name);
1597 $name = getgrgid($num;
1598 $name = getgrent();
1599 #etc.
a0d0e21e 1600
7660c0ab 1601In I<getpw*()> the fields C<$quota>, C<$comment>, and C<$expire> are special
6ee623d5 1602cases in the sense that in many systems they are unsupported. If the
7660c0ab
A
1603C<$quota> is unsupported, it is an empty scalar. If it is supported, it
1604usually encodes the disk quota. If the C<$comment> field is unsupported,
6ee623d5
GS
1605it is an empty scalar. If it is supported it usually encodes some
1606administrative comment about the user. In some systems the $quota
7660c0ab
A
1607field may be C<$change> or C<$age>, fields that have to do with password
1608aging. In some systems the C<$comment> field may be C<$class>. The C<$expire>
6ee623d5
GS
1609field, if present, encodes the expiration period of the account or the
1610password. For the availability and the exact meaning of these fields
f86cebdf 1611in your system, please consult your getpwnam(3) documentation and your
7660c0ab
A
1612F<pwd.h> file. You can also find out from within Perl which meaning
1613your C<$quota> and C<$comment> fields have and whether you have the C<$expire>
1614field by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
1615C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>.
6ee623d5 1616
7660c0ab 1617The C<$members> value returned by I<getgr*()> is a space separated list of
a0d0e21e
LW
1618the login names of the members of the group.
1619
1620For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1621C, it will be returned to you via C<$?> if the function call fails. The
7660c0ab 1622C<@addrs> value returned by a successful call is a list of the raw
a0d0e21e
LW
1623addresses returned by the corresponding system library call. In the
1624Internet domain, each address is four bytes long and you can unpack it
1625by saying something like:
1626
1627 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1628
5a964f20
TC
1629If you get tired of remembering which element of the return list contains
1630which return value, by-name interfaces are also provided in modules:
7660c0ab
A
1631C<File::stat>, C<Net::hostent>, C<Net::netent>, C<Net::protoent>, C<Net::servent>,
1632C<Time::gmtime>, C<Time::localtime>, and C<User::grent>. These override the
5a964f20
TC
1633normal built-in, replacing them with versions that return objects with
1634the appropriate names for each field. For example:
1635
1636 use File::stat;
1637 use User::pwent;
1638 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
1639
1640Even though it looks like they're the same method calls (uid),
7660c0ab 1641they aren't, because a C<File::stat> object is different from a C<User::pwent> object.
5a964f20 1642
a0d0e21e
LW
1643=item getsockname SOCKET
1644
1645Returns the packed sockaddr address of this end of the SOCKET connection.
1646
4633a7c4
LW
1647 use Socket;
1648 $mysockaddr = getsockname(SOCK);
1649 ($port, $myaddr) = unpack_sockaddr_in($mysockaddr);
a0d0e21e
LW
1650
1651=item getsockopt SOCKET,LEVEL,OPTNAME
1652
5a964f20 1653Returns the socket option requested, or undef if there is an error.
a0d0e21e
LW
1654
1655=item glob EXPR
1656
0a753a76 1657=item glob
1658
7660c0ab 1659Returns the value of EXPR with filename expansions such as the standard Unix shell F</bin/sh> would
68dc0745 1660do. This is the internal function implementing the C<E<lt>*.cE<gt>>
7660c0ab 1661operator, but you can use it directly. If EXPR is omitted, C<$_> is used.
68dc0745 1662The C<E<lt>*.cE<gt>> operator is discussed in more detail in
1663L<perlop/"I/O Operators">.
a0d0e21e
LW
1664
1665=item gmtime EXPR
1666
1667Converts a time as returned by the time function to a 9-element array
54310121 1668with the time localized for the standard Greenwich time zone.
4633a7c4 1669Typically used as follows:
a0d0e21e 1670
54310121 1671 # 0 1 2 3 4 5 6 7 8
a0d0e21e
LW
1672 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1673 gmtime(time);
1674
1675All array elements are numeric, and come straight out of a struct tm.
7660c0ab
A
1676In particular this means that C<$mon> has the range C<0..11> and C<$wday> has
1677the range C<0..6> with sunday as day C<0>. Also, C<$year> is the number of
1678years since 1900, that is, C<$year> is C<123> in year 2023, I<not> simply the last two digits of the year.
2f9daede
TP
1679
1680If EXPR is omitted, does C<gmtime(time())>.
a0d0e21e 1681
f86cebdf 1682In scalar context, returns the ctime(3) value:
0a753a76 1683
1684 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
1685
7660c0ab 1686Also see the C<timegm()> function provided by the C<Time::Local> module,
f86cebdf 1687and the strftime(3) function available via the POSIX module.
7660c0ab
A
1688
1689This scalar value is B<not> locale dependent, see L<perllocale>, but
1690instead a Perl builtin. Also see the C<Time::Local> module, and the
f86cebdf 1691strftime(3) and mktime(3) function available via the POSIX module. To
7660c0ab
A
1692get somewhat similar but locale dependent date strings, set up your
1693locale environment variables appropriately (please see L<perllocale>)
1694and try for example:
1695
1696 use POSIX qw(strftime);
1697 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
1698
1699Note that the C<%a> and C<%b>, the short forms of the day of the week
1700and the month of the year, may not necessarily be three characters wide.
0a753a76 1701
a0d0e21e
LW
1702=item goto LABEL
1703
748a9306
LW
1704=item goto EXPR
1705
a0d0e21e
LW
1706=item goto &NAME
1707
7660c0ab 1708The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
a0d0e21e 1709execution there. It may not be used to go into any construct that
7660c0ab 1710requires initialization, such as a subroutine or a C<foreach> loop. It
0a753a76 1711also can't be used to go into a construct that is optimized away,
7660c0ab 1712or to get out of a block or subroutine given to C<sort()>.
0a753a76 1713It can be used to go almost anywhere else within the dynamic scope,
a0d0e21e 1714including out of subroutines, but it's usually better to use some other
ab4f32c2 1715construct such as C<last> or C<die()>. The author of Perl has never felt the
7660c0ab 1716need to use this form of C<goto> (in Perl, that is--C is another matter).
a0d0e21e 1717
7660c0ab
A
1718The C<goto-EXPR> form expects a label name, whose scope will be resolved
1719dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
748a9306
LW
1720necessarily recommended if you're optimizing for maintainability:
1721
1722 goto ("FOO", "BAR", "GLARCH")[$i];
1723
7660c0ab 1724The C<goto-&NAME> form is highly magical, and substitutes a call to the
a0d0e21e 1725named subroutine for the currently running subroutine. This is used by
7660c0ab 1726C<AUTOLOAD> subroutines that wish to load another subroutine and then
a0d0e21e 1727pretend that the other subroutine had been called in the first place
7660c0ab
A
1728(except that any modifications to C<@_> in the current subroutine are
1729propagated to the other subroutine.) After the C<goto>, not even C<caller()>
a0d0e21e
LW
1730will be able to tell that this routine was called first.
1731
1732=item grep BLOCK LIST
1733
1734=item grep EXPR,LIST
1735
f86cebdf 1736This is similar in spirit to, but not the same as, grep(1)
2f9daede
TP
1737and its relatives. In particular, it is not limited to using
1738regular expressions.
1739
a0d0e21e 1740Evaluates the BLOCK or EXPR for each element of LIST (locally setting
7660c0ab 1741C<$_> to each element) and returns the list value consisting of those
a0d0e21e
LW
1742elements for which the expression evaluated to TRUE. In a scalar
1743context, returns the number of times the expression was TRUE.
1744
1745 @foo = grep(!/^#/, @bar); # weed out comments
1746
1747or equivalently,
1748
1749 @foo = grep {!/^#/} @bar; # weed out comments
1750
7660c0ab 1751Note that, because C<$_> is a reference into the list value, it can be used
a0d0e21e
LW
1752to modify the elements of the array. While this is useful and
1753supported, it can cause bizarre results if the LIST is not a named
2f9daede 1754array. Similarly, grep returns aliases into the original list,
f86cebdf 1755much like the way that a for loop's index variable aliases the list
2f9daede 1756elements. That is, modifying an element of a list returned by grep
ab4f32c2 1757(for example, in a C<foreach>, C<map()> or another C<grep()>)
2f9daede 1758actually modifies the element in the original list.
a0d0e21e 1759
fb73857a 1760See also L</map> for an array composed of the results of the BLOCK or EXPR.
38325410 1761
a0d0e21e
LW
1762=item hex EXPR
1763
54310121 1764=item hex
bbce6d69 1765
54310121 1766Interprets EXPR as a hex string and returns the corresponding
2f9daede 1767value. (To convert strings that might start with either 0 or 0x
7660c0ab 1768see L</oct>.) If EXPR is omitted, uses C<$_>.
2f9daede
TP
1769
1770 print hex '0xAf'; # prints '175'
1771 print hex 'aF'; # same
a0d0e21e
LW
1772
1773=item import
1774
7660c0ab 1775There is no builtin C<import()> function. It is just an ordinary
4633a7c4 1776method (subroutine) defined (or inherited) by modules that wish to export
7660c0ab 1777names to another module. The C<use()> function calls the C<import()> method
54310121 1778for the package used. See also L</use()>, L<perlmod>, and L<Exporter>.
a0d0e21e
LW
1779
1780=item index STR,SUBSTR,POSITION
1781
1782=item index STR,SUBSTR
1783
4633a7c4
LW
1784Returns the position of the first occurrence of SUBSTR in STR at or after
1785POSITION. If POSITION is omitted, starts searching from the beginning of
7660c0ab 1786the string. The return value is based at C<0> (or whatever you've set the C<$[>
4633a7c4 1787variable to--but don't do that). If the substring is not found, returns
7660c0ab 1788one less than the base, ordinarily C<-1>.
a0d0e21e
LW
1789
1790=item int EXPR
1791
54310121 1792=item int
bbce6d69 1793
7660c0ab 1794Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
5a964f20 1795You should not use this for rounding, because it truncates
7660c0ab
A
1796towards C<0>, and because machine representations of floating point
1797numbers can sometimes produce counterintuitive results. Usually C<sprintf()> or C<printf()>,
1798or the C<POSIX::floor> or C<POSIX::ceil> functions, would serve you better.
a0d0e21e
LW
1799
1800=item ioctl FILEHANDLE,FUNCTION,SCALAR
1801
f86cebdf 1802Implements the ioctl(2) function. You'll probably have to say
a0d0e21e 1803
4633a7c4 1804 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
a0d0e21e 1805
4633a7c4 1806first to get the correct function definitions. If F<ioctl.ph> doesn't
a0d0e21e 1807exist or doesn't have the correct definitions you'll have to roll your
4633a7c4 1808own, based on your C header files such as F<E<lt>sys/ioctl.hE<gt>>.
5a964f20 1809(There is a Perl script called B<h2ph> that comes with the Perl kit that
54310121 1810may help you in this, but it's nontrivial.) SCALAR will be read and/or
4633a7c4 1811written depending on the FUNCTION--a pointer to the string value of SCALAR
ab4f32c2 1812will be passed as the third argument of the actual C<ioctl()> call. (If SCALAR
4633a7c4
LW
1813has no string value but does have a numeric value, that value will be
1814passed rather than a pointer to the string value. To guarantee this to be
7660c0ab 1815TRUE, add a C<0> to the scalar before using it.) The C<pack()> and C<unpack()>
4633a7c4 1816functions are useful for manipulating the values of structures used by
7660c0ab 1817C<ioctl()>. The following example sets the erase character to DEL.
a0d0e21e
LW
1818
1819 require 'ioctl.ph';
4633a7c4
LW
1820 $getp = &TIOCGETP;
1821 die "NO TIOCGETP" if $@ || !$getp;
a0d0e21e 1822 $sgttyb_t = "ccccs"; # 4 chars and a short
4633a7c4 1823 if (ioctl(STDIN,$getp,$sgttyb)) {
a0d0e21e
LW
1824 @ary = unpack($sgttyb_t,$sgttyb);
1825 $ary[2] = 127;
1826 $sgttyb = pack($sgttyb_t,@ary);
4633a7c4 1827 ioctl(STDIN,&TIOCSETP,$sgttyb)
a0d0e21e
LW
1828 || die "Can't ioctl: $!";
1829 }
1830
ab4f32c2 1831The return value of C<ioctl()> (and C<fcntl()>) is as follows:
a0d0e21e
LW
1832
1833 if OS returns: then Perl returns:
1834 -1 undefined value
1835 0 string "0 but true"
1836 anything else that number
1837
1838Thus Perl returns TRUE on success and FALSE on failure, yet you can
1839still easily determine the actual value returned by the operating
1840system:
1841
1842 ($retval = ioctl(...)) || ($retval = -1);
1843 printf "System returned %d\n", $retval;
1844
7660c0ab 1845The special string "C<0> but true" is excempt from B<-w> complaints
5a964f20
TC
1846about improper numeric conversions.
1847
a0d0e21e
LW
1848=item join EXPR,LIST
1849
54310121 1850Joins the separate strings of LIST into a single string with
a0d0e21e
LW
1851fields separated by the value of EXPR, and returns the string.
1852Example:
1853
1854 $_ = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
1855
7660c0ab 1856See L</split>.
a0d0e21e 1857
aa689395 1858=item keys HASH
1859
1d2dff63
GS
1860Returns a list consisting of all the keys of the named hash. (In a
1861scalar context, returns the number of keys.) The keys are returned in
aa689395 1862an apparently random order, but it is the same order as either the
7660c0ab 1863C<values()> or C<each()> function produces (given that the hash has not been
aa689395 1864modified). As a side effect, it resets HASH's iterator.
a0d0e21e 1865
aa689395 1866Here is yet another way to print your environment:
a0d0e21e
LW
1867
1868 @keys = keys %ENV;
1869 @values = values %ENV;
1870 while ($#keys >= 0) {
1871 print pop(@keys), '=', pop(@values), "\n";
1872 }
1873
1874or how about sorted by key:
1875
1876 foreach $key (sort(keys %ENV)) {
1877 print $key, '=', $ENV{$key}, "\n";
1878 }
1879
ab4f32c2 1880To sort an array by value, you'll need to use a C<sort()> function.
aa689395 1881Here's a descending numeric sort of a hash by its values:
4633a7c4 1882
5a964f20 1883 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
4633a7c4
LW
1884 printf "%4d %s\n", $hash{$key}, $key;
1885 }
1886
ab4f32c2 1887As an lvalue C<keys()> allows you to increase the number of hash buckets
aa689395 1888allocated for the given hash. This can gain you a measure of efficiency if
1889you know the hash is going to get big. (This is similar to pre-extending
1890an array by assigning a larger number to $#array.) If you say
55497cff 1891
1892 keys %hash = 200;
1893
5a964f20
TC
1894then C<%hash> will have at least 200 buckets allocated for it--256 of them, in fact, since
1895it rounds up to the next power of two. These
55497cff 1896buckets will be retained even if you do C<%hash = ()>, use C<undef
1897%hash> if you want to free the storage while C<%hash> is still in scope.
1898You can't shrink the number of buckets allocated for the hash using
ab4f32c2 1899C<keys()> in this way (but you needn't worry about doing this by accident,
55497cff 1900as trying has no effect).
1901
a0d0e21e
LW
1902=item kill LIST
1903
54310121 1904Sends a signal to a list of processes. The first element of
1905the list must be the signal to send. Returns the number of
4633a7c4 1906processes successfully signaled.
a0d0e21e
LW
1907
1908 $cnt = kill 1, $child1, $child2;
1909 kill 9, @goners;
1910
4633a7c4
LW
1911Unlike in the shell, in Perl if the I<SIGNAL> is negative, it kills
1912process groups instead of processes. (On System V, a negative I<PROCESS>
1913number will also kill process groups, but that's not portable.) That
1914means you usually want to use positive not negative signals. You may also
da0045b7 1915use a signal name in quotes. See L<perlipc/"Signals"> for details.
a0d0e21e
LW
1916
1917=item last LABEL
1918
1919=item last
1920
1921The C<last> command is like the C<break> statement in C (as used in
1922loops); it immediately exits the loop in question. If the LABEL is
1923omitted, the command refers to the innermost enclosing loop. The
1924C<continue> block, if any, is not executed:
1925
4633a7c4
LW
1926 LINE: while (<STDIN>) {
1927 last LINE if /^$/; # exit when done with header
5a964f20 1928 #...
a0d0e21e
LW
1929 }
1930
1d2dff63
GS
1931See also L</continue> for an illustration of how C<last>, C<next>, and
1932C<redo> work.
1933
a0d0e21e
LW
1934=item lc EXPR
1935
54310121 1936=item lc
bbce6d69 1937
a0d0e21e 1938Returns an lowercased version of EXPR. This is the internal function
7660c0ab
A
1939implementing the C<\L> escape in double-quoted strings.
1940Respects current C<LC_CTYPE> locale if C<use locale> in force. See L<perllocale>.
a0d0e21e 1941
7660c0ab 1942If EXPR is omitted, uses C<$_>.
bbce6d69 1943
a0d0e21e
LW
1944=item lcfirst EXPR
1945
54310121 1946=item lcfirst
bbce6d69 1947
a0d0e21e 1948Returns the value of EXPR with the first character lowercased. This is
7660c0ab
A
1949the internal function implementing the C<\l> escape in double-quoted strings.
1950Respects current C<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 length EXPR
1955
54310121 1956=item length
bbce6d69 1957
5a964f20 1958Returns the length in bytes of the value of EXPR. If EXPR is
7660c0ab 1959omitted, returns length of C<$_>.
a0d0e21e
LW
1960
1961=item link OLDFILE,NEWFILE
1962
5a964f20
TC
1963Creates a new filename linked to the old filename. Returns TRUE for
1964success, FALSE otherwise.
a0d0e21e
LW
1965
1966=item listen SOCKET,QUEUESIZE
1967
1968Does the same thing that the listen system call does. Returns TRUE if
4633a7c4 1969it succeeded, FALSE otherwise. See example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
1970
1971=item local EXPR
1972
5a964f20
TC
1973A local modifies the listed variables to be local to the enclosing
1974block, file, or eval. If more than one value is listed, the list must
1975be placed in parentheses. See L<perlsub/"Temporary Values via local()">
1976for details, including issues with tied arrays and hashes.
a0d0e21e 1977
7660c0ab 1978You really probably want to be using C<my()> instead, because C<local()> isn't
7b8d334a 1979what most people think of as "local". See L<perlsub/"Private Variables
cb1a09d0 1980via my()"> for details.
a0d0e21e
LW
1981
1982=item localtime EXPR
1983
1984Converts a time as returned by the time function to a 9-element array
5f05dabc 1985with the time analyzed for the local time zone. Typically used as
a0d0e21e
LW
1986follows:
1987
54310121 1988 # 0 1 2 3 4 5 6 7 8
a0d0e21e
LW
1989 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1990 localtime(time);
1991
1992All array elements are numeric, and come straight out of a struct tm.
7660c0ab
A
1993In particular this means that C<$mon> has the range C<0..11> and C<$wday> has
1994the range C<0..6> with sunday as day C<0>. Also, C<$year> is the number of
1995years since 1900, that is, C<$year> is C<123> in year 2023, and I<not> simply the last two digits of the year.
54310121 1996
1997If EXPR is omitted, uses the current time (C<localtime(time)>).
a0d0e21e 1998
f86cebdf 1999In scalar context, returns the ctime(3) value:
a0d0e21e 2000
5f05dabc 2001 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
a0d0e21e 2002
a3cb178b 2003This scalar value is B<not> locale dependent, see L<perllocale>, but
7660c0ab 2004instead a Perl builtin. Also see the C<Time::Local> module, and the
f86cebdf 2005strftime(3) and mktime(3) function available via the POSIX module. To
a3cb178b
GS
2006get somewhat similar but locale dependent date strings, set up your
2007locale environment variables appropriately (please see L<perllocale>)
5a964f20 2008and try for example:
a3cb178b 2009
5a964f20 2010 use POSIX qw(strftime);
a3cb178b
GS
2011 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
2012
2013Note that the C<%a> and C<%b>, the short forms of the day of the week
2014and the month of the year, may not necessarily be three characters wide.
a0d0e21e
LW
2015
2016=item log EXPR
2017
54310121 2018=item log
bbce6d69 2019
5a964f20 2020Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted, returns log
7660c0ab 2021of C<$_>.
a0d0e21e
LW
2022
2023=item lstat FILEHANDLE
2024
2025=item lstat EXPR
2026
54310121 2027=item lstat
bbce6d69 2028
7660c0ab 2029Does the same thing as the C<stat()> function (including setting the
5a964f20
TC
2030special C<_> filehandle) but stats a symbolic link instead of the file
2031the symbolic link points to. If symbolic links are unimplemented on
7660c0ab 2032your system, a normal C<stat()> is done.
a0d0e21e 2033
7660c0ab 2034If EXPR is omitted, stats C<$_>.
bbce6d69 2035
a0d0e21e
LW
2036=item m//
2037
2038The match operator. See L<perlop>.
2039
2040=item map BLOCK LIST
2041
2042=item map EXPR,LIST
2043
7660c0ab 2044Evaluates the BLOCK or EXPR for each element of LIST (locally setting C<$_> to each
a0d0e21e
LW
2045element) and returns the list value composed of the results of each such
2046evaluation. Evaluates BLOCK or EXPR in a list context, so each element of LIST
2047may produce zero, one, or more elements in the returned value.
2048
2049 @chars = map(chr, @nums);
2050
2051translates a list of numbers to the corresponding characters. And
2052
4633a7c4 2053 %hash = map { getkey($_) => $_ } @array;
a0d0e21e
LW
2054
2055is just a funny way to write
2056
2057 %hash = ();
2058 foreach $_ (@array) {
4633a7c4 2059 $hash{getkey($_)} = $_;
a0d0e21e
LW
2060 }
2061
7660c0ab 2062Note that, because C<$_> is a reference into the list value, it can be used
fb73857a 2063to modify the elements of the array. While this is useful and
2064supported, it can cause bizarre results if the LIST is not a named
2065array. See also L</grep> for an array composed of those items of the
2066original list for which the BLOCK or EXPR evaluates to true.
2067
a0d0e21e
LW
2068=item mkdir FILENAME,MODE
2069
2070Creates the directory specified by FILENAME, with permissions specified
5a964f20
TC
2071by MODE (as modified by umask). If it succeeds it returns TRUE, otherwise
2072it returns FALSE and sets C<$!> (errno).
a0d0e21e
LW
2073
2074=item msgctl ID,CMD,ARG
2075
f86cebdf 2076Calls the System V IPC function msgctl(2). You'll probably have to say
0ade1984
JH
2077
2078 use IPC::SysV;
2079
7660c0ab
A
2080first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2081then ARG must be a variable which will hold the returned C<msqid_ds>
ab4f32c2 2082structure. Returns like C<ioctl()>: the undefined value for error, "C<0> but
0ade1984 2083true" for zero, or the actual return value otherwise. See also
7660c0ab 2084C<IPC::SysV> and C<IPC::Semaphore::Msg> documentation.
a0d0e21e
LW
2085
2086=item msgget KEY,FLAGS
2087
f86cebdf 2088Calls the System V IPC function msgget(2). Returns the message queue
7660c0ab
A
2089id, or the undefined value if there is an error. See also C<IPC::SysV>
2090and C<IPC::SysV::Msg> documentation.
a0d0e21e
LW
2091
2092=item msgsnd ID,MSG,FLAGS
2093
2094Calls the System V IPC function msgsnd to send the message MSG to the
2095message queue ID. MSG must begin with the long integer message type,
c07a80fd 2096which may be created with C<pack("l", $type)>. Returns TRUE if
7660c0ab
A
2097successful, or FALSE if there is an error. See also C<IPC::SysV>
2098and C<IPC::SysV::Msg> documentation.
a0d0e21e
LW
2099
2100=item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2101
2102Calls the System V IPC function msgrcv to receive a message from
2103message queue ID into variable VAR with a maximum message size of
0ade1984
JH
2104SIZE. Note that if a message is received, the message type will be
2105the first thing in VAR, and the maximum length of VAR is SIZE plus the
2106size of the message type. Returns TRUE if successful, or FALSE if
7660c0ab 2107there is an error. See also C<IPC::SysV> and C<IPC::SysV::Msg> documentation.
a0d0e21e
LW
2108
2109=item my EXPR
2110
ab4f32c2
A
2111A C<my()> declares the listed variables to be local (lexically) to the
2112enclosing block, file, or C<eval()>. If
5f05dabc 2113more than one value is listed, the list must be placed in parentheses. See
cb1a09d0 2114L<perlsub/"Private Variables via my()"> for details.
4633a7c4 2115
a0d0e21e
LW
2116=item next LABEL
2117
2118=item next
2119
2120The C<next> command is like the C<continue> statement in C; it starts
2121the next iteration of the loop:
2122
4633a7c4
LW
2123 LINE: while (<STDIN>) {
2124 next LINE if /^#/; # discard comments
5a964f20 2125 #...
a0d0e21e
LW
2126 }
2127
2128Note that if there were a C<continue> block on the above, it would get
2129executed even on discarded lines. If the LABEL is omitted, the command
2130refers to the innermost enclosing loop.
2131
1d2dff63
GS
2132See also L</continue> for an illustration of how C<last>, C<next>, and
2133C<redo> work.
2134
a0d0e21e
LW
2135=item no Module LIST
2136
7660c0ab 2137See the L</use> function, which C<no> is the opposite of.
a0d0e21e
LW
2138
2139=item oct EXPR
2140
54310121 2141=item oct
bbce6d69 2142
4633a7c4 2143Interprets EXPR as an octal string and returns the corresponding
7660c0ab 2144value. (If EXPR happens to start off with C<0x>, interprets it as
4633a7c4
LW
2145a hex string instead.) The following will handle decimal, octal, and
2146hex in the standard Perl or C notation:
a0d0e21e
LW
2147
2148 $val = oct($val) if $val =~ /^0/;
2149
7660c0ab
A
2150If EXPR is omitted, uses C<$_>. This function is commonly used when
2151a string such as C<644> needs to be converted into a file mode, for
2f9daede
TP
2152example. (Although perl will automatically convert strings into
2153numbers as needed, this automatic conversion assumes base 10.)
a0d0e21e
LW
2154
2155=item open FILEHANDLE,EXPR
2156
2157=item open FILEHANDLE
2158
2159Opens the file whose filename is given by EXPR, and associates it with
5f05dabc 2160FILEHANDLE. If FILEHANDLE is an expression, its value is used as the
2161name of the real filehandle wanted. If EXPR is omitted, the scalar
2162variable of the same name as the FILEHANDLE contains the filename.
ab4f32c2
A
2163(Note that lexical variables--those declared with C<my()>--will not work
2164for this purpose; so if you're using C<my()>, specify EXPR in your call
5f05dabc 2165to open.)
2166
7660c0ab
A
2167If the filename begins with C<'E<lt>'> or nothing, the file is opened for input.
2168If the filename begins with C<'E<gt>'>, the file is truncated and opened for
2169output, being created if necessary. If the filename begins with C<'E<gt>E<gt>'>,
fbb426e4 2170the file is opened for appending, again being created if necessary.
7660c0ab
A
2171You can put a C<'+'> in front of the C<'E<gt>'> or C<'E<lt>'> to indicate that
2172you want both read and write access to the file; thus C<'+E<lt>'> is almost
2173always preferred for read/write updates--the C<'+E<gt>'> mode would clobber the
5a964f20
TC
2174file first. You can't usually use either read-write mode for updating
2175textfiles, since they have variable length records. See the B<-i>
2176switch in L<perlrun> for a better approach.
2177
2178The prefix and the filename may be separated with spaces.
f86cebdf 2179These various prefixes correspond to the fopen(3) modes of C<'r'>, C<'r+'>, C<'w'>,
7660c0ab 2180C<'w+'>, C<'a'>, and C<'a+'>.
5f05dabc 2181
7660c0ab 2182If the filename begins with C<'|'>, the filename is interpreted as a
5a964f20 2183command to which output is to be piped, and if the filename ends with a
7660c0ab
A
2184C<'|'>, the filename is interpreted See L<perlipc/"Using open() for IPC">
2185for more examples of this. (You are not allowed to C<open()> to a command
5a964f20
TC
2186that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2187and L<perlipc/"Bidirectional Communication"> for alternatives.)
cb1a09d0 2188
7660c0ab 2189Opening C<'-'> opens STDIN and opening C<'E<gt>-'> opens STDOUT. Open returns
ab4f32c2 2190nonzero upon success, the undefined value otherwise. If the C<open()>
4633a7c4 2191involved a pipe, the return value happens to be the pid of the
54310121 2192subprocess.
cb1a09d0
AD
2193
2194If you're unfortunate enough to be running Perl on a system that
2195distinguishes between text files and binary files (modern operating
2196systems don't care), then you should check out L</binmode> for tips for
ab4f32c2 2197dealing with this. The key distinction between systems that need C<binmode()>
5a964f20
TC
2198and those that don't is their text file formats. Systems like Unix, MacOS, and
2199Plan9, which delimit lines with a single character, and which encode that
ab4f32c2 2200character in C as C<"\n">, do not need C<binmode()>. The rest need it.
cb1a09d0 2201
fb73857a 2202When opening a file, it's usually a bad idea to continue normal execution
ab4f32c2
A
2203if the request failed, so C<open()> is frequently used in connection with
2204C<die()>. Even if C<die()> won't do what you want (say, in a CGI script,
fb73857a 2205where you want to make a nicely formatted error message (but there are
5a964f20 2206modules that can help with that problem)) you should always check
fb73857a 2207the return value from opening a file. The infrequent exception is when
2208working with an unopened filehandle is actually what you want to do.
2209
cb1a09d0 2210Examples:
a0d0e21e
LW
2211
2212 $ARTICLE = 100;
2213 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2214 while (<ARTICLE>) {...
2215
2216 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
fb73857a 2217 # if the open fails, output is discarded
a0d0e21e 2218
fb73857a 2219 open(DBASE, '+<dbase.mine') # open for update
2220 or die "Can't open 'dbase.mine' for update: $!";
cb1a09d0 2221
fb73857a 2222 open(ARTICLE, "caesar <$article |") # decrypt article
2223 or die "Can't start caesar: $!";
a0d0e21e 2224
fb73857a 2225 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
2226 or die "Can't start sort: $!";
a0d0e21e
LW
2227
2228 # process argument list of files along with any includes
2229
2230 foreach $file (@ARGV) {
2231 process($file, 'fh00');
2232 }
2233
2234 sub process {
5a964f20 2235 my($filename, $input) = @_;
a0d0e21e
LW
2236 $input++; # this is a string increment
2237 unless (open($input, $filename)) {
2238 print STDERR "Can't open $filename: $!\n";
2239 return;
2240 }
2241
5a964f20 2242 local $_;
a0d0e21e
LW
2243 while (<$input>) { # note use of indirection
2244 if (/^#include "(.*)"/) {
2245 process($1, $input);
2246 next;
2247 }
5a964f20 2248 #... # whatever
a0d0e21e
LW
2249 }
2250 }
2251
2252You may also, in the Bourne shell tradition, specify an EXPR beginning
7660c0ab 2253with C<'E<gt>&'>, in which case the rest of the string is interpreted as the
5a964f20 2254name of a filehandle (or file descriptor, if numeric) to be
7660c0ab
A
2255duped and opened. You may use C<&> after C<E<gt>>, C<E<gt>E<gt>>, C<E<lt>>, C<+E<gt>>,
2256C<+E<gt>E<gt>>, and C<+E<lt>>. The
a0d0e21e 2257mode you specify should match the mode of the original filehandle.
184e9718 2258(Duping a filehandle does not take into account any existing contents of
cb1a09d0 2259stdio buffers.)
a0d0e21e
LW
2260Here is a script that saves, redirects, and restores STDOUT and
2261STDERR:
2262
2263 #!/usr/bin/perl
5a964f20
TC
2264 open(OLDOUT, ">&STDOUT");
2265 open(OLDERR, ">&STDERR");
a0d0e21e
LW
2266
2267 open(STDOUT, ">foo.out") || die "Can't redirect stdout";
2268 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
2269
2270 select(STDERR); $| = 1; # make unbuffered
2271 select(STDOUT); $| = 1; # make unbuffered
2272
2273 print STDOUT "stdout 1\n"; # this works for
2274 print STDERR "stderr 1\n"; # subprocesses too
2275
2276 close(STDOUT);
2277 close(STDERR);
2278
5a964f20
TC
2279 open(STDOUT, ">&OLDOUT");
2280 open(STDERR, ">&OLDERR");
a0d0e21e
LW
2281
2282 print STDOUT "stdout 2\n";
2283 print STDERR "stderr 2\n";
2284
2285
7660c0ab
A
2286If you specify C<'E<lt>&=N'>, where C<N> is a number, then Perl will do an
2287equivalent of C's C<fdopen()> of that file descriptor; this is more
4633a7c4 2288parsimonious of file descriptors. For example:
a0d0e21e
LW
2289
2290 open(FILEHANDLE, "<&=$fd")
2291
7660c0ab 2292If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>, then
a0d0e21e 2293there is an implicit fork done, and the return value of open is the pid
7660c0ab 2294of the child within the parent process, and C<0> within the child
184e9718 2295process. (Use C<defined($pid)> to determine whether the open was successful.)
a0d0e21e
LW
2296The filehandle behaves normally for the parent, but i/o to that
2297filehandle is piped from/to the STDOUT/STDIN of the child process.
2298In the child process the filehandle isn't opened--i/o happens from/to
2299the new STDOUT or STDIN. Typically this is used like the normal
2300piped open when you want to exercise more control over just how the
2301pipe command gets executed, such as when you are running setuid, and
54310121 2302don't want to have to scan shell commands for metacharacters.
4633a7c4 2303The following pairs are more or less equivalent:
a0d0e21e
LW
2304
2305 open(FOO, "|tr '[a-z]' '[A-Z]'");
2306 open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
2307
2308 open(FOO, "cat -n '$file'|");
2309 open(FOO, "-|") || exec 'cat', '-n', $file;
2310
4633a7c4
LW
2311See L<perlipc/"Safe Pipe Opens"> for more examples of this.
2312
5a964f20 2313NOTE: On any operation that may do a fork, any unflushed buffers remain
184e9718 2314unflushed in both processes, which means you may need to set C<$|> to
a0d0e21e
LW
2315avoid duplicate output.
2316
0dccf244
CS
2317Closing any piped filehandle causes the parent process to wait for the
2318child to finish, and returns the status value in C<$?>.
2319
5a964f20 2320The filename passed to open will have leading and trailing
f86cebdf 2321whitespace deleted, and the normal redirection characters
5a964f20
TC
2322honored. This property, known as "magic open",
2323can often be used to good effect. A user could specify a filename of
7660c0ab 2324F<"rsh cat file |">, or you could change certain filenames as needed:
5a964f20
TC
2325
2326 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
2327 open(FH, $filename) or die "Can't open $filename: $!";
2328
2329However, to open a file with arbitrary weird characters in it, it's
2330necessary to protect any leading and trailing whitespace:
2331
2332 $file =~ s#^(\s)#./$1#;
2333 open(FOO, "< $file\0");
2334
7660c0ab
A
2335If you want a "real" C C<open()> (see L<open(2)> on your system), then you
2336should use the C<sysopen()> function, which involves no such magic. This is
5a964f20
TC
2337another way to protect your filenames from interpretation. For example:
2338
2339 use IO::Handle;
2340 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
2341 or die "sysopen $path: $!";
2342 $oldfh = select(HANDLE); $| = 1; select($oldfh);
2343 print HANDLE "stuff $$\n");
2344 seek(HANDLE, 0, 0);
2345 print "File contains: ", <HANDLE>;
2346
7660c0ab
A
2347Using the constructor from the C<IO::Handle> package (or one of its
2348subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
5a964f20
TC
2349filehandles that have the scope of whatever variables hold references to
2350them, and automatically close whenever and however you leave that scope:
c07a80fd 2351
5f05dabc 2352 use IO::File;
5a964f20 2353 #...
c07a80fd 2354 sub read_myfile_munged {
2355 my $ALL = shift;
5f05dabc 2356 my $handle = new IO::File;
c07a80fd 2357 open($handle, "myfile") or die "myfile: $!";
2358 $first = <$handle>
2359 or return (); # Automatically closed here.
2360 mung $first or die "mung failed"; # Or here.
2361 return $first, <$handle> if $ALL; # Or here.
2362 $first; # Or here.
2363 }
2364
cb1a09d0 2365See L</seek()> for some details about mixing reading and writing.
a0d0e21e
LW
2366
2367=item opendir DIRHANDLE,EXPR
2368
7660c0ab
A
2369Opens a directory named EXPR for processing by C<readdir()>, C<telldir()>,
2370C<seekdir()>, C<rewinddir()>, and C<closedir()>. Returns TRUE if successful.
a0d0e21e
LW
2371DIRHANDLEs have their own namespace separate from FILEHANDLEs.
2372
2373=item ord EXPR
2374
54310121 2375=item ord
bbce6d69 2376
a0d0e21e 2377Returns the numeric ascii value of the first character of EXPR. If
7660c0ab 2378EXPR is omitted, uses C<$_>. For the reverse, see L</chr>.
a0d0e21e
LW
2379
2380=item pack TEMPLATE,LIST
2381
2382Takes an array or list of values and packs it into a binary structure,
2383returning the string containing the structure. The TEMPLATE is a
2384sequence of characters that give the order and type of values, as
2385follows:
2386
2387 A An ascii string, will be space padded.
2388 a An ascii string, will be null padded.
2389 b A bit string (ascending bit order, like vec()).
2390 B A bit string (descending bit order).
2391 h A hex string (low nybble first).
2392 H A hex string (high nybble first).
2393
2394 c A signed char value.
2395 C An unsigned char value.
96e4d5b1 2396
a0d0e21e
LW
2397 s A signed short value.
2398 S An unsigned short value.
96e4d5b1 2399 (This 'short' is _exactly_ 16 bits, which may differ from
2400 what a local C compiler calls 'short'.)
2401
a0d0e21e
LW
2402 i A signed integer value.
2403 I An unsigned integer value.
f86cebdf
GS
2404 (This 'integer' is _at_least_ 32 bits wide. Its exact
2405 size depends on what a local C compiler calls 'int',
2406 and may even be larger than the 'long' described in
2407 the next item.)
96e4d5b1 2408
a0d0e21e
LW
2409 l A signed long value.
2410 L An unsigned long value.
96e4d5b1 2411 (This 'long' is _exactly_ 32 bits, which may differ from
2412 what a local C compiler calls 'long'.)
a0d0e21e 2413
96e4d5b1 2414 n A short in "network" (big-endian) order.
2415 N A long in "network" (big-endian) order.
a0d0e21e
LW
2416 v A short in "VAX" (little-endian) order.
2417 V A long in "VAX" (little-endian) order.
96e4d5b1 2418 (These 'shorts' and 'longs' are _exactly_ 16 bits and
2419 _exactly_ 32 bits, respectively.)
a0d0e21e
LW
2420
2421 f A single-precision float in the native format.
2422 d A double-precision float in the native format.
2423
2424 p A pointer to a null-terminated string.
2425 P A pointer to a structure (fixed-length string).
2426
2427 u A uuencoded string.
2428
96e4d5b1 2429 w A BER compressed integer. Its bytes represent an unsigned
f86cebdf
GS
2430 integer in base 128, most significant digit first, with as
2431 few digits as possible. Bit eight (the high bit) is set
2432 on each byte except the last.
def98dd4 2433
a0d0e21e
LW
2434 x A null byte.
2435 X Back up a byte.
2436 @ Null fill to absolute position.
2437
5a964f20 2438Each letter may optionally be followed by a number giving a repeat
7660c0ab
A
2439count. With all types except C<"a">, C<"A">, C<"b">, C<"B">, C<"h">, C<"H">, and C<"P"> the
2440pack function will gobble up that many values from the LIST. A C<*> for the
2441repeat count means to use however many items are left. The C<"a"> and C<"A">
a0d0e21e 2442types gobble just one value, but pack it as a string of length count,
7660c0ab
A
2443padding with nulls or spaces as necessary. (When unpacking, C<"A"> strips
2444trailing spaces and nulls, but C<"a"> does not.) Likewise, the C<"b"> and C<"B">
2445fields pack a string that many bits long. The C<"h"> and C<"H"> fields pack a
2446string that many nybbles long. The C<"p"> type packs a pointer to a null-
84902520
TB
2447terminated string. You are responsible for ensuring the string is not a
2448temporary value (which can potentially get deallocated before you get
7660c0ab 2449around to using the packed result). The C<"P"> packs a pointer to a structure
61167c6f 2450of the size indicated by the length. A NULL pointer is created if the
7660c0ab 2451corresponding value for C<"p"> or C<"P"> is C<undef>.
61167c6f 2452Real numbers (floats and doubles) are
a0d0e21e
LW
2453in the native machine format only; due to the multiplicity of floating
2454formats around, and the lack of a standard "network" representation, no
2455facility for interchange has been made. This means that packed floating
2456point data written on one machine may not be readable on another - even if
2457both use IEEE floating point arithmetic (as the endian-ness of the memory
2458representation is not part of the IEEE spec). Note that Perl uses doubles
2459internally for all numeric calculation, and converting from double into
5f05dabc 2460float and thence back to double again will lose precision (i.e.,
7660c0ab 2461C<unpack("f", pack("f", $foo)>) will not in general equal C<$foo>).
a0d0e21e
LW
2462
2463Examples:
2464
2465 $foo = pack("cccc",65,66,67,68);
2466 # foo eq "ABCD"
2467 $foo = pack("c4",65,66,67,68);
2468 # same thing
2469
2470 $foo = pack("ccxxcc",65,66,67,68);
2471 # foo eq "AB\0\0CD"
2472
2473 $foo = pack("s2",1,2);
2474 # "\1\0\2\0" on little-endian
2475 # "\0\1\0\2" on big-endian
2476
2477 $foo = pack("a4","abcd","x","y","z");
2478 # "abcd"
2479
2480 $foo = pack("aaaa","abcd","x","y","z");
2481 # "axyz"
2482
2483 $foo = pack("a14","abcdefg");
2484 # "abcdefg\0\0\0\0\0\0\0"
2485
2486 $foo = pack("i9pl", gmtime);
2487 # a real struct tm (on my system anyway)
2488
2489 sub bintodec {
2490 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
2491 }
2492
2493The same template may generally also be used in the unpack function.
2494
5a964f20
TC
2495=item package
2496
cb1a09d0
AD
2497=item package NAMESPACE
2498
2499Declares the compilation unit as being in the given namespace. The scope
2500of the package declaration is from the declaration itself through the end of
7660c0ab 2501the enclosing block (the same scope as the C<local()> operator). All further
cb1a09d0 2502unqualified dynamic identifiers will be in this namespace. A package
5f05dabc 2503statement affects only dynamic variables--including those you've used
7660c0ab 2504C<local()> on--but I<not> lexical variables created with C<my()>. Typically it
cb1a09d0
AD
2505would be the first declaration in a file to be included by the C<require>
2506or C<use> operator. You can switch into a package in more than one place;
5a964f20 2507it merely influences which symbol table is used by the compiler for the
cb1a09d0
AD
2508rest of that block. You can refer to variables and filehandles in other
2509packages by prefixing the identifier with the package name and a double
2510colon: C<$Package::Variable>. If the package name is null, the C<main>
2511package as assumed. That is, C<$::sail> is equivalent to C<$main::sail>.
2512
5a964f20
TC
2513If NAMESPACE is omitted, then there is no current package, and all
2514identifiers must be fully qualified or lexicals. This is stricter
2515than C<use strict>, since it also extends to function names.
2516
cb1a09d0
AD
2517See L<perlmod/"Packages"> for more information about packages, modules,
2518and classes. See L<perlsub> for other scoping issues.
2519
a0d0e21e
LW
2520=item pipe READHANDLE,WRITEHANDLE
2521
2522Opens a pair of connected pipes like the corresponding system call.
2523Note that if you set up a loop of piped processes, deadlock can occur
2524unless you are very careful. In addition, note that Perl's pipes use
184e9718 2525stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE
a0d0e21e
LW
2526after each command, depending on the application.
2527
7e1af8bc 2528See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
4633a7c4
LW
2529for examples of such things.
2530
a0d0e21e
LW
2531=item pop ARRAY
2532
54310121 2533=item pop
28757baa 2534
a0d0e21e
LW
2535Pops and returns the last value of the array, shortening the array by
25361. Has a similar effect to
2537
2538 $tmp = $ARRAY[$#ARRAY--];
2539
2540If there are no elements in the array, returns the undefined value.
cb1a09d0 2541If ARRAY is omitted, pops the
7660c0ab
A
2542C<@ARGV> array in the main program, and the C<@_> array in subroutines, just
2543like C<shift()>.
a0d0e21e
LW
2544
2545=item pos SCALAR
2546
54310121 2547=item pos
bbce6d69 2548
4633a7c4 2549Returns the offset of where the last C<m//g> search left off for the variable
7660c0ab 2550is in question (C<$_> is used when the variable is not specified). May be
44a8e56a 2551modified to change that offset. Such modification will also influence
2552the C<\G> zero-width assertion in regular expressions. See L<perlre> and
2553L<perlop>.
a0d0e21e
LW
2554
2555=item print FILEHANDLE LIST
2556
2557=item print LIST
2558
2559=item print
2560
cb1a09d0 2561Prints a string or a comma-separated list of strings. Returns TRUE
a0d0e21e 2562if successful. FILEHANDLE may be a scalar variable name, in which case
cb1a09d0 2563the variable contains the name of or a reference to the filehandle, thus introducing one
a0d0e21e
LW
2564level of indirection. (NOTE: If FILEHANDLE is a variable and the next
2565token is a term, it may be misinterpreted as an operator unless you
7660c0ab 2566interpose a C<+> or put parentheses around the arguments.) If FILEHANDLE is
a0d0e21e 2567omitted, prints by default to standard output (or to the last selected
7660c0ab 2568output channel--see L</select>). If LIST is also omitted, prints C<$_> to
5a964f20 2569the currently selected output channel. To set the default output channel to something other than
a0d0e21e 2570STDOUT use the select operation. Note that, because print takes a
5a964f20 2571LIST, anything in the LIST is evaluated in list context, and any
a0d0e21e 2572subroutine that you call will have one or more of its expressions
5a964f20 2573evaluated in list context. Also be careful not to follow the print
a0d0e21e 2574keyword with a left parenthesis unless you want the corresponding right
7660c0ab 2575parenthesis to terminate the arguments to the print--interpose a C<+> or
5f05dabc 2576put parentheses around all the arguments.
a0d0e21e 2577
4633a7c4 2578Note that if you're storing FILEHANDLES in an array or other expression,
da0045b7 2579you will have to use a block returning its value instead:
4633a7c4
LW
2580
2581 print { $files[$i] } "stuff\n";
2582 print { $OK ? STDOUT : STDERR } "stuff\n";
2583
5f05dabc 2584=item printf FILEHANDLE FORMAT, LIST
a0d0e21e 2585
5f05dabc 2586=item printf FORMAT, LIST
a0d0e21e 2587
7660c0ab 2588Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
a3cb178b 2589(the output record separator) is not appended. The first argument
ab4f32c2 2590of the list will be interpreted as the C<printf()> format. If C<use locale> is
a034a98d
DD
2591in effect, the character used for the decimal point in formatted real numbers
2592is affected by the LC_NUMERIC locale. See L<perllocale>.
a0d0e21e 2593
7660c0ab
A
2594Don't fall into the trap of using a C<printf()> when a simple
2595C<print()> would do. The C<print()> is more efficient and less
28757baa 2596error prone.
2597
da0045b7 2598=item prototype FUNCTION
2599
2600Returns the prototype of a function as a string (or C<undef> if the
5f05dabc 2601function has no prototype). FUNCTION is a reference to, or the name of,
2602the function whose prototype you want to retrieve.
da0045b7 2603
b6c543e3
IZ
2604If FUNCTION is a string starting with C<CORE::>, the rest is taken as
2605a name for Perl builtin. If builtin is not I<overridable> (such as
ab4f32c2
A
2606C<qw//>) or its arguments cannot be expressed by a prototype (such as
2607C<system()>) - in other words, the builtin does not behave like a Perl
b6c543e3
IZ
2608function - returns C<undef>. Otherwise, the string describing the
2609equivalent prototype is returned.
2610
a0d0e21e
LW
2611=item push ARRAY,LIST
2612
2613Treats ARRAY as a stack, and pushes the values of LIST
2614onto the end of ARRAY. The length of ARRAY increases by the length of
2615LIST. Has the same effect as
2616
2617 for $value (LIST) {
2618 $ARRAY[++$#ARRAY] = $value;
2619 }
2620
2621but is more efficient. Returns the new number of elements in the array.
2622
2623=item q/STRING/
2624
2625=item qq/STRING/
2626
8782bef2
GB
2627=item qr/STRING/
2628
a0d0e21e
LW
2629=item qx/STRING/
2630
2631=item qw/STRING/
2632
2633Generalized quotes. See L<perlop>.
2634
2635=item quotemeta EXPR
2636
54310121 2637=item quotemeta
bbce6d69 2638
68dc0745 2639Returns the value of EXPR with all non-alphanumeric
a034a98d
DD
2640characters backslashed. (That is, all characters not matching
2641C</[A-Za-z_0-9]/> will be preceded by a backslash in the
2642returned string, regardless of any locale settings.)
2643This is the internal function implementing
7660c0ab 2644the C<\Q> escape in double-quoted strings.
a0d0e21e 2645
7660c0ab 2646If EXPR is omitted, uses C<$_>.
bbce6d69 2647
a0d0e21e
LW
2648=item rand EXPR
2649
2650=item rand
2651
7660c0ab 2652Returns a random fractional number greater than or equal to C<0> and less
3e3baf6d 2653than the value of EXPR. (EXPR should be positive.) If EXPR is
7660c0ab
A
2654omitted, the value C<1> is used. Automatically calls C<srand()> unless
2655C<srand()> has already been called. See also C<srand()>.
a0d0e21e 2656
2f9daede 2657(Note: If your rand function consistently returns numbers that are too
a0d0e21e 2658large or too small, then your version of Perl was probably compiled
2f9daede 2659with the wrong number of RANDBITS.)
a0d0e21e
LW
2660
2661=item read FILEHANDLE,SCALAR,LENGTH,OFFSET
2662
2663=item read FILEHANDLE,SCALAR,LENGTH
2664
2665Attempts to read LENGTH bytes of data into variable SCALAR from the
3b02c43c
GS
2666specified FILEHANDLE. Returns the number of bytes actually read,
2667C<0> at end of file, or undef if there was an error. SCALAR will be grown
2668or shrunk to the length actually read. An OFFSET may be specified to
2669place the read data at some other place than the beginning of the
f86cebdf
GS
2670string. This call is actually implemented in terms of stdio's fread(3)
2671call. To get a true read(2) system call, see C<sysread()>.
a0d0e21e
LW
2672
2673=item readdir DIRHANDLE
2674
7660c0ab 2675Returns the next directory entry for a directory opened by C<opendir()>.
5a964f20 2676If used in list context, returns all the rest of the entries in the
a0d0e21e 2677directory. If there are no more entries, returns an undefined value in
5a964f20 2678scalar context or a null list in list context.
a0d0e21e 2679
7660c0ab 2680If you're planning to filetest the return values out of a C<readdir()>, you'd
5f05dabc 2681better prepend the directory in question. Otherwise, because we didn't
7660c0ab 2682C<chdir()> there, it would have been testing the wrong file.
cb1a09d0
AD
2683
2684 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
2685 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
2686 closedir DIR;
2687
84902520
TB
2688=item readline EXPR
2689
5a964f20 2690Reads from the filehandle whose typeglob is contained in EXPR. In scalar context, a single line
84902520
TB
2691is read and returned. In list context, reads until end-of-file is
2692reached and returns a list of lines (however you've defined lines
7660c0ab 2693with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
84902520
TB
2694This is the internal function implementing the C<E<lt>EXPRE<gt>>
2695operator, but you can use it directly. The C<E<lt>EXPRE<gt>>
2696operator is discussed in more detail in L<perlop/"I/O Operators">.
2697
5a964f20
TC
2698 $line = <STDIN>;
2699 $line = readline(*STDIN); # same thing
2700
a0d0e21e
LW
2701=item readlink EXPR
2702
54310121 2703=item readlink
bbce6d69 2704
a0d0e21e
LW
2705Returns the value of a symbolic link, if symbolic links are
2706implemented. If not, gives a fatal error. If there is some system
184e9718 2707error, returns the undefined value and sets C<$!> (errno). If EXPR is
7660c0ab 2708omitted, uses C<$_>.
a0d0e21e 2709
84902520
TB
2710=item readpipe EXPR
2711
5a964f20 2712EXPR is executed as a system command.
84902520
TB
2713The collected standard output of the command is returned.
2714In scalar context, it comes back as a single (potentially
2715multi-line) string. In list context, returns a list of lines
7660c0ab 2716(however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
84902520
TB
2717This is the internal function implementing the C<qx/EXPR/>
2718operator, but you can use it directly. The C<qx/EXPR/>
2719operator is discussed in more detail in L<perlop/"I/O Operators">.
2720
a0d0e21e
LW
2721=item recv SOCKET,SCALAR,LEN,FLAGS
2722
2723Receives a message on a socket. Attempts to receive LENGTH bytes of
2724data into variable SCALAR from the specified SOCKET filehandle.
7660c0ab 2725Actually does a C C<recvfrom()>, so that it can return the address of the
a0d0e21e
LW
2726sender. Returns the undefined value if there's an error. SCALAR will
2727be grown or shrunk to the length actually read. Takes the same flags
54310121 2728as the system call of the same name.
4633a7c4 2729See L<perlipc/"UDP: Message Passing"> for examples.
a0d0e21e
LW
2730
2731=item redo LABEL
2732
2733=item redo
2734
2735The C<redo> command restarts the loop block without evaluating the
2736conditional again. The C<continue> block, if any, is not executed. If
2737the LABEL is omitted, the command refers to the innermost enclosing
2738loop. This command is normally used by programs that want to lie to
2739themselves about what was just input:
2740
2741 # a simpleminded Pascal comment stripper
2742 # (warning: assumes no { or } in strings)
4633a7c4 2743 LINE: while (<STDIN>) {
a0d0e21e
LW
2744 while (s|({.*}.*){.*}|$1 |) {}
2745 s|{.*}| |;
2746 if (s|{.*| |) {
2747 $front = $_;
2748 while (<STDIN>) {
2749 if (/}/) { # end of comment?
5a964f20 2750 s|^|$front\{|;
4633a7c4 2751 redo LINE;
a0d0e21e
LW
2752 }
2753 }
2754 }
2755 print;
2756 }
2757
1d2dff63
GS
2758See also L</continue> for an illustration of how C<last>, C<next>, and
2759C<redo> work.
2760
a0d0e21e
LW
2761=item ref EXPR
2762
54310121 2763=item ref
bbce6d69 2764
2f9daede 2765Returns a TRUE value if EXPR is a reference, FALSE otherwise. If EXPR
7660c0ab 2766is not specified, C<$_> will be used. The value returned depends on the
bbce6d69 2767type of thing the reference is a reference to.
a0d0e21e
LW
2768Builtin types include:
2769
2770 REF
2771 SCALAR
2772 ARRAY
2773 HASH
2774 CODE
2775 GLOB
2776
54310121 2777If the referenced object has been blessed into a package, then that package
7660c0ab 2778name is returned instead. You can think of C<ref()> as a C<typeof()> operator.
a0d0e21e
LW
2779
2780 if (ref($r) eq "HASH") {
aa689395 2781 print "r is a reference to a hash.\n";
54310121 2782 }
5a964f20 2783 if (!ref($r)) {
a0d0e21e 2784 print "r is not a reference at all.\n";
54310121 2785 }
a0d0e21e
LW
2786
2787See also L<perlref>.
2788
2789=item rename OLDNAME,NEWNAME
2790
7660c0ab 2791Changes the name of a file. Returns C<1> for success, C<0> otherwise. Will
5f05dabc 2792not work across file system boundaries.
a0d0e21e
LW
2793
2794=item require EXPR
2795
2796=item require
2797
7660c0ab 2798Demands some semantics specified by EXPR, or by C<$_> if EXPR is not
a0d0e21e 2799supplied. If EXPR is numeric, demands that the current version of Perl
184e9718 2800(C<$]> or $PERL_VERSION) be equal or greater than EXPR.
a0d0e21e
LW
2801
2802Otherwise, demands that a library file be included if it hasn't already
2803been included. The file is included via the do-FILE mechanism, which is
7660c0ab 2804essentially just a variety of C<eval()>. Has semantics similar to the following
a0d0e21e
LW
2805subroutine:
2806
2807 sub require {
5a964f20 2808 my($filename) = @_;
a0d0e21e 2809 return 1 if $INC{$filename};
5a964f20 2810 my($realfilename,$result);
a0d0e21e
LW
2811 ITER: {
2812 foreach $prefix (@INC) {
2813 $realfilename = "$prefix/$filename";
2814 if (-f $realfilename) {
2815 $result = do $realfilename;
2816 last ITER;
2817 }
2818 }
2819 die "Can't find $filename in \@INC";
2820 }
2821 die $@ if $@;
2822 die "$filename did not return true value" unless $result;
2823 $INC{$filename} = $realfilename;
5a964f20 2824 return $result;
a0d0e21e
LW
2825 }
2826
2827Note that the file will not be included twice under the same specified
2828name. The file must return TRUE as the last statement to indicate
2829successful execution of any initialization code, so it's customary to
7660c0ab 2830end such a file with "C<1;>" unless you're sure it'll return TRUE
a0d0e21e
LW
2831otherwise. But it's better just to put the "C<1;>", in case you add more
2832statements.
2833
54310121 2834If EXPR is a bareword, the require assumes a "F<.pm>" extension and
da0045b7 2835replaces "F<::>" with "F</>" in the filename for you,
54310121 2836to make it easy to load standard modules. This form of loading of
a0d0e21e
LW
2837modules does not risk altering your namespace.
2838
ee580363
GS
2839In other words, if you try this:
2840
f86cebdf 2841 require Foo::Bar; # a splendid bareword
ee580363 2842
7660c0ab
A
2843The require function will actually look for the "F<Foo/Bar.pm>" file in the
2844directories specified in the C<@INC> array.
ee580363 2845
5a964f20 2846But if you try this:
ee580363
GS
2847
2848 $class = 'Foo::Bar';
f86cebdf 2849 require $class; # $class is not a bareword
5a964f20 2850 #or
f86cebdf 2851 require "Foo::Bar"; # not a bareword because of the ""
ee580363 2852
7660c0ab
A
2853The require function will look for the "F<Foo::Bar>" file in the @INC array and
2854will complain about not finding "F<Foo::Bar>" there. In this case you can do:
ee580363
GS
2855
2856 eval "require $class";
2857
2858For a yet-more-powerful import facility, see L</use> and L<perlmod>.
a0d0e21e
LW
2859
2860=item reset EXPR
2861
2862=item reset
2863
2864Generally used in a C<continue> block at the end of a loop to clear
7660c0ab 2865variables and reset C<??> searches so that they work again. The
a0d0e21e
LW
2866expression is interpreted as a list of single characters (hyphens
2867allowed for ranges). All variables and arrays beginning with one of
2868those letters are reset to their pristine state. If the expression is
7660c0ab 2869omitted, one-match searches (C<?pattern?>) are reset to match again. Resets
5f05dabc 2870only variables or searches in the current package. Always returns
a0d0e21e
LW
28711. Examples:
2872
2873 reset 'X'; # reset all X variables
2874 reset 'a-z'; # reset lower case variables
2875 reset; # just reset ?? searches
2876
7660c0ab
A
2877Resetting C<"A-Z"> is not recommended because you'll wipe out your
2878C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package variables--lexical variables
a0d0e21e 2879are unaffected, but they clean themselves up on scope exit anyway,
da0045b7 2880so you'll probably want to use them instead. See L</my>.
a0d0e21e 2881
54310121 2882=item return EXPR
2883
2884=item return
2885
7660c0ab 2886Returns from a subroutine, C<eval()>, or C<do FILE> with the value
5a964f20 2887given in EXPR. Evaluation of EXPR may be in list, scalar, or void
54310121 2888context, depending on how the return value will be used, and the context
7660c0ab 2889may vary from one execution to the next (see C<wantarray()>). If no EXPR
5a964f20
TC
2890is given, returns an empty list in list context, an undefined value in
2891scalar context, or nothing in a void context.
a0d0e21e 2892
68dc0745 2893(Note that in the absence of a return, a subroutine, eval, or do FILE
2894will automatically return the value of the last expression evaluated.)
a0d0e21e
LW
2895
2896=item reverse LIST
2897
5a964f20
TC
2898In list context, returns a list value consisting of the elements
2899of LIST in the opposite order. In scalar context, concatenates the
2f9daede
TP
2900elements of LIST, and returns a string value consisting of those bytes,
2901but in the opposite order.
4633a7c4 2902
2f9daede 2903 print reverse <>; # line tac, last line first
4633a7c4 2904
2f9daede
TP
2905 undef $/; # for efficiency of <>
2906 print scalar reverse <>; # byte tac, last line tsrif
2907
2908This operator is also handy for inverting a hash, although there are some
2909caveats. If a value is duplicated in the original hash, only one of those
2910can be represented as a key in the inverted hash. Also, this has to
2911unwind one hash and build a whole new one, which may take some time
2912on a large hash.
2913
2914 %by_name = reverse %by_address; # Invert the hash
a0d0e21e
LW
2915
2916=item rewinddir DIRHANDLE
2917
2918Sets the current position to the beginning of the directory for the
7660c0ab 2919C<readdir()> routine on DIRHANDLE.
a0d0e21e
LW
2920
2921=item rindex STR,SUBSTR,POSITION
2922
2923=item rindex STR,SUBSTR
2924
2925Works just like index except that it returns the position of the LAST
2926occurrence of SUBSTR in STR. If POSITION is specified, returns the
2927last occurrence at or before that position.
2928
2929=item rmdir FILENAME
2930
54310121 2931=item rmdir
bbce6d69 2932
5a964f20
TC
2933Deletes the directory specified by FILENAME if that directory is empty. If it
2934succeeds it returns TRUE, otherwise it returns FALSE and sets C<$!> (errno). If
7660c0ab 2935FILENAME is omitted, uses C<$_>.
a0d0e21e
LW
2936
2937=item s///
2938
2939The substitution operator. See L<perlop>.
2940
2941=item scalar EXPR
2942
5a964f20 2943Forces EXPR to be interpreted in scalar context and returns the value
54310121 2944of EXPR.
cb1a09d0
AD
2945
2946 @counts = ( scalar @a, scalar @b, scalar @c );
2947
54310121 2948There is no equivalent operator to force an expression to
5a964f20 2949be interpolated in list context because it's in practice never
cb1a09d0
AD
2950needed. If you really wanted to do so, however, you could use
2951the construction C<@{[ (some expression) ]}>, but usually a simple
2952C<(some expression)> suffices.
a0d0e21e
LW
2953
2954=item seek FILEHANDLE,POSITION,WHENCE
2955
ab4f32c2 2956Sets FILEHANDLE's position, just like the C<fseek()> call of C<stdio()>.
8903cb82 2957FILEHANDLE may be an expression whose value gives the name of the
7660c0ab
A
2958filehandle. The values for WHENCE are C<0> to set the new position to
2959POSITION, C<1> to set it to the current position plus POSITION, and C<2> to
8903cb82 2960set it to EOF plus POSITION (typically negative). For WHENCE you may
7660c0ab
A
2961use the constants C<SEEK_SET>, C<SEEK_CUR>, and C<SEEK_END> from either the
2962C<IO::Seekable> or the POSIX module. Returns C<1> upon success, C<0> otherwise.
8903cb82 2963
7660c0ab
A
2964If you want to position file for C<sysread()> or C<syswrite()>, don't use
2965C<seek()> -- buffering makes its effect on the file's system position
2966unpredictable and non-portable. Use C<sysseek()> instead.
a0d0e21e 2967
cb1a09d0
AD
2968On some systems you have to do a seek whenever you switch between reading
2969and writing. Amongst other things, this may have the effect of calling
f86cebdf 2970stdio's clearerr(3). A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving
8903cb82 2971the file position:
cb1a09d0
AD
2972
2973 seek(TEST,0,1);
2974
2975This is also useful for applications emulating C<tail -f>. Once you hit
2976EOF on your read, and then sleep for a while, you might have to stick in a
7660c0ab 2977seek() to reset things. The C<seek()> doesn't change the current position,
8903cb82 2978but it I<does> clear the end-of-file condition on the handle, so that the
2979next C<E<lt>FILEE<gt>> makes Perl try again to read something. We hope.
cb1a09d0
AD
2980
2981If that doesn't work (some stdios are particularly cantankerous), then
2982you may need something more like this:
2983
2984 for (;;) {
f86cebdf
GS
2985 for ($curpos = tell(FILE); $_ = <FILE>;
2986 $curpos = tell(FILE)) {
cb1a09d0
AD
2987 # search for some stuff and put it into files
2988 }
2989 sleep($for_a_while);
2990 seek(FILE, $curpos, 0);
2991 }
2992
a0d0e21e
LW
2993=item seekdir DIRHANDLE,POS
2994
7660c0ab
A
2995Sets the current position for the C<readdir()> routine on DIRHANDLE. POS
2996must be a value returned by C<telldir()>. Has the same caveats about
a0d0e21e
LW
2997possible directory compaction as the corresponding system library
2998routine.
2999
3000=item select FILEHANDLE
3001
3002=item select
3003
3004Returns the currently selected filehandle. Sets the current default
3005filehandle for output, if FILEHANDLE is supplied. This has two
ab4f32c2 3006effects: first, a C<write()> or a C<print()> without a filehandle will
a0d0e21e
LW
3007default to this FILEHANDLE. Second, references to variables related to
3008output will refer to this output channel. For example, if you have to
3009set the top of form format for more than one output channel, you might
3010do the following:
3011
3012 select(REPORT1);
3013 $^ = 'report1_top';
3014 select(REPORT2);
3015 $^ = 'report2_top';
3016
3017FILEHANDLE may be an expression whose value gives the name of the
3018actual filehandle. Thus:
3019
3020 $oldfh = select(STDERR); $| = 1; select($oldfh);
3021
4633a7c4
LW
3022Some programmers may prefer to think of filehandles as objects with
3023methods, preferring to write the last example as:
a0d0e21e 3024
28757baa 3025 use IO::Handle;
a0d0e21e
LW
3026 STDERR->autoflush(1);
3027
3028=item select RBITS,WBITS,EBITS,TIMEOUT
3029
f86cebdf 3030This calls the select(2) system call with the bit masks specified, which
7660c0ab 3031can be constructed using C<fileno()> and C<vec()>, along these lines:
a0d0e21e
LW
3032
3033 $rin = $win = $ein = '';
3034 vec($rin,fileno(STDIN),1) = 1;
3035 vec($win,fileno(STDOUT),1) = 1;
3036 $ein = $rin | $win;
3037
3038If you want to select on many filehandles you might wish to write a
3039subroutine:
3040
3041 sub fhbits {
5a964f20
TC
3042 my(@fhlist) = split(' ',$_[0]);
3043 my($bits);
a0d0e21e
LW
3044 for (@fhlist) {
3045 vec($bits,fileno($_),1) = 1;
3046 }
3047 $bits;
3048 }
4633a7c4 3049 $rin = fhbits('STDIN TTY SOCK');
a0d0e21e
LW
3050
3051The usual idiom is:
3052
3053 ($nfound,$timeleft) =
3054 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
3055
54310121 3056or to block until something becomes ready just do this
a0d0e21e
LW
3057
3058 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
3059
7660c0ab
A
3060Most systems do not bother to return anything useful in C<$timeleft>, so
3061calling select() in scalar context just returns C<$nfound>.
c07a80fd 3062
5f05dabc 3063Any of the bit masks can also be undef. The timeout, if specified, is
a0d0e21e 3064in seconds, which may be fractional. Note: not all implementations are
7660c0ab
A
3065capable of returning theC<$timeleft>. If not, they always return
3066C<$timeleft> equal to the supplied C<$timeout>.
a0d0e21e 3067
ff68c719 3068You can effect a sleep of 250 milliseconds this way:
a0d0e21e
LW
3069
3070 select(undef, undef, undef, 0.25);
3071
7660c0ab
A
3072B<WARNING>: One should not attempt to mix buffered I/O (like C<read()>
3073or E<lt>FHE<gt>) with C<select()>, except as permitted by POSIX, and even
3074then only on POSIX systems. You have to use C<sysread()> instead.
a0d0e21e
LW
3075
3076=item semctl ID,SEMNUM,CMD,ARG
3077
ab4f32c2 3078Calls the System V IPC function C<semctl()>. You'll probably have to say
0ade1984
JH
3079
3080 use IPC::SysV;
3081
3082first to get the correct constant definitions. If CMD is IPC_STAT or
3083GETALL, then ARG must be a variable which will hold the returned
ab4f32c2 3084semid_ds structure or semaphore value array. Returns like C<ioctl()>: the
7660c0ab
A
3085undefined value for error, "C<0> but true" for zero, or the actual return
3086value otherwise. See also C<IPC::SysV> and C<IPC::Semaphore> documentation.
a0d0e21e
LW
3087
3088=item semget KEY,NSEMS,FLAGS
3089
3090Calls the System V IPC function semget. Returns the semaphore id, or
7660c0ab
A
3091the undefined value if there is an error. See also C<IPC::SysV> and
3092C<IPC::SysV::Semaphore> documentation.
a0d0e21e
LW
3093
3094=item semop KEY,OPSTRING
3095
3096Calls the System V IPC function semop to perform semaphore operations
3097such as signaling and waiting. OPSTRING must be a packed array of
3098semop structures. Each semop structure can be generated with
3099C<pack("sss", $semnum, $semop, $semflag)>. The number of semaphore
3100operations is implied by the length of OPSTRING. Returns TRUE if
3101successful, or FALSE if there is an error. As an example, the
7660c0ab 3102following code waits on semaphore C<$semnum> of semaphore id C<$semid>:
a0d0e21e
LW
3103
3104 $semop = pack("sss", $semnum, -1, 0);
3105 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
3106
7660c0ab
A
3107To signal the semaphore, replace C<-1> with C<1>. See also C<IPC::SysV>
3108and C<IPC::SysV::Semaphore> documentation.
a0d0e21e
LW
3109
3110=item send SOCKET,MSG,FLAGS,TO
3111
3112=item send SOCKET,MSG,FLAGS
3113
3114Sends a message on a socket. Takes the same flags as the system call
3115of the same name. On unconnected sockets you must specify a
7660c0ab 3116destination to send TO, in which case it does a C C<sendto()>. Returns
a0d0e21e
LW
3117the number of characters sent, or the undefined value if there is an
3118error.
4633a7c4 3119See L<perlipc/"UDP: Message Passing"> for examples.
a0d0e21e
LW
3120
3121=item setpgrp PID,PGRP
3122
7660c0ab 3123Sets the current process group for the specified PID, C<0> for the current
a0d0e21e 3124process. Will produce a fatal error if used on a machine that doesn't
f86cebdf 3125implement setpgrp(2). If the arguments are omitted, it defaults to
7660c0ab
A
3126C<0,0>. Note that the POSIX version of C<setpgrp()> does not accept any
3127arguments, so only setpgrp C<0,0> is portable.
a0d0e21e
LW
3128
3129=item setpriority WHICH,WHO,PRIORITY
3130
3131Sets the current priority for a process, a process group, or a user.
f86cebdf
GS
3132(See setpriority(2).) Will produce a fatal error if used on a machine
3133that doesn't implement setpriority(2).
a0d0e21e
LW
3134
3135=item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
3136
3137Sets the socket option requested. Returns undefined if there is an
7660c0ab 3138error. OPTVAL may be specified as C<undef> if you don't want to pass an
a0d0e21e
LW
3139argument.
3140
3141=item shift ARRAY
3142
3143=item shift
3144
3145Shifts the first value of the array off and returns it, shortening the
3146array by 1 and moving everything down. If there are no elements in the
3147array, returns the undefined value. If ARRAY is omitted, shifts the
7660c0ab
A
3148C<@_> array within the lexical scope of subroutines and formats, and the
3149C<@ARGV> array at file scopes or within the lexical scopes established by
977336f5 3150the C<eval ''>, C<BEGIN {}>, C<END {}>, and C<INIT {}> constructs.
7660c0ab
A
3151See also C<unshift()>, C<push()>, and C<pop()>. C<Shift()> and C<unshift()> do the
3152same thing to the left end of an array that C<pop()> and C<push()> do to the
977336f5 3153right end.
a0d0e21e
LW
3154
3155=item shmctl ID,CMD,ARG
3156
0ade1984
JH
3157Calls the System V IPC function shmctl. You'll probably have to say
3158
3159 use IPC::SysV;
3160
7660c0ab
A
3161first to get the correct constant definitions. If CMD is C<IPC_STAT>,
3162then ARG must be a variable which will hold the returned C<shmid_ds>
3163structure. Returns like ioctl: the undefined value for error, "C<0> but
0ade1984 3164true" for zero, or the actual return value otherwise.
7660c0ab 3165See also C<IPC::SysV> documentation.
a0d0e21e
LW
3166
3167=item shmget KEY,SIZE,FLAGS
3168
3169Calls the System V IPC function shmget. Returns the shared memory
3170segment id, or the undefined value if there is an error.
7660c0ab 3171See also C<IPC::SysV> documentation.
a0d0e21e
LW
3172
3173=item shmread ID,VAR,POS,SIZE
3174
3175=item shmwrite ID,STRING,POS,SIZE
3176
3177Reads or writes the System V shared memory segment ID starting at
3178position POS for size SIZE by attaching to it, copying in/out, and
5a964f20 3179detaching from it. When reading, VAR must be a variable that will
a0d0e21e
LW
3180hold the data read. When writing, if STRING is too long, only SIZE
3181bytes are used; if STRING is too short, nulls are written to fill out
3182SIZE bytes. Return TRUE if successful, or FALSE if there is an error.
7660c0ab 3183See also C<IPC::SysV> documentation.
a0d0e21e
LW
3184
3185=item shutdown SOCKET,HOW
3186
3187Shuts down a socket connection in the manner indicated by HOW, which
3188has the same interpretation as in the system call of the same name.
3189
f86cebdf
GS
3190 shutdown(SOCKET, 0); # I/we have stopped reading data
3191 shutdown(SOCKET, 1); # I/we have stopped writing data
3192 shutdown(SOCKET, 2); # I/we have stopped using this socket
5a964f20
TC
3193
3194This is useful with sockets when you want to tell the other
3195side you're done writing but not done reading, or vice versa.
3196It's also a more insistent form of close because it also
3197disables the filedescriptor in any forked copies in other
3198processes.
3199
a0d0e21e
LW
3200=item sin EXPR
3201
54310121 3202=item sin
bbce6d69 3203
a0d0e21e 3204Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
7660c0ab 3205returns sine of C<$_>.
a0d0e21e 3206
7660c0ab 3207For the inverse sine operation, you may use the C<POSIX::asin()>
28757baa 3208function, or use this relation:
3209
3210 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
3211
a0d0e21e
LW
3212=item sleep EXPR
3213
3214=item sleep
3215
3216Causes the script to sleep for EXPR seconds, or forever if no EXPR.
7660c0ab 3217May be interrupted if the process receives a signal such as C<SIGALRM>.
1d3434b8 3218Returns the number of seconds actually slept. You probably cannot
7660c0ab
A
3219mix C<alarm()> and C<sleep()> calls, because C<sleep()> is often implemented
3220using C<alarm()>.
a0d0e21e
LW
3221
3222On some older systems, it may sleep up to a full second less than what
3223you requested, depending on how it counts seconds. Most modern systems
5a964f20
TC
3224always sleep the full amount. They may appear to sleep longer than that,
3225however, because your process might not be scheduled right away in a
3226busy multitasking system.
a0d0e21e 3227
cb1a09d0 3228For delays of finer granularity than one second, you may use Perl's
f86cebdf 3229C<syscall()> interface to access setitimer(2) if your system supports it,
7b8d334a 3230or else see L</select()> above.
cb1a09d0 3231
7660c0ab 3232See also the POSIX module's C<sigpause()> function.
5f05dabc 3233
a0d0e21e
LW
3234=item socket SOCKET,DOMAIN,TYPE,PROTOCOL
3235
3236Opens a socket of the specified kind and attaches it to filehandle
5f05dabc 3237SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for the
7660c0ab 3238system call of the same name. You should "C<use Socket;>" first to get
4633a7c4 3239the proper definitions imported. See the example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
3240
3241=item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
3242
3243Creates an unnamed pair of sockets in the specified domain, of the
5f05dabc 3244specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
a0d0e21e
LW
3245for the system call of the same name. If unimplemented, yields a fatal
3246error. Returns TRUE if successful.
3247
ab4f32c2 3248Some systems defined C<pipe()> in terms of C<socketpair()>, in which a call
5a964f20
TC
3249to C<pipe(Rdr, Wtr)> is essentially:
3250
3251 use Socket;
3252 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
3253 shutdown(Rdr, 1); # no more writing for reader
3254 shutdown(Wtr, 0); # no more reading for writer
3255
3256See L<perlipc> for an example of socketpair use.
3257
a0d0e21e
LW
3258=item sort SUBNAME LIST
3259
3260=item sort BLOCK LIST
3261
3262=item sort LIST
3263
2f9daede 3264Sorts the LIST and returns the sorted list value. If SUBNAME or BLOCK
ab4f32c2 3265is omitted, C<sort()>s in standard string comparison order. If SUBNAME is
2f9daede 3266specified, it gives the name of a subroutine that returns an integer
7660c0ab 3267less than, equal to, or greater than C<0>, depending on how the elements
2f9daede
TP
3268of the array are to be ordered. (The C<E<lt>=E<gt>> and C<cmp>
3269operators are extremely useful in such routines.) SUBNAME may be a
1d3434b8
GS
3270scalar variable name (unsubscripted), in which case the value provides
3271the name of (or a reference to) the actual subroutine to use. In place
3272of a SUBNAME, you can provide a BLOCK as an anonymous, in-line sort
3273subroutine.
a0d0e21e 3274
cb1a09d0
AD
3275In the interests of efficiency the normal calling code for subroutines is
3276bypassed, with the following effects: the subroutine may not be a
3277recursive subroutine, and the two elements to be compared are passed into
7660c0ab
A
3278the subroutine not via C<@_> but as the package global variables C<$a> and
3279C<$b> (see example below). They are passed by reference, so don't
3280modify C<$a> and C<$b>. And don't try to declare them as lexicals either.
a0d0e21e 3281
0a753a76 3282You also cannot exit out of the sort block or subroutine using any of the
7660c0ab 3283loop control operators described in L<perlsyn> or with C<goto()>.
0a753a76 3284
a034a98d
DD
3285When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
3286current collation locale. See L<perllocale>.
3287
a0d0e21e
LW
3288Examples:
3289
3290 # sort lexically
3291 @articles = sort @files;
3292
3293 # same thing, but with explicit sort routine
3294 @articles = sort {$a cmp $b} @files;
3295
cb1a09d0 3296 # now case-insensitively
54310121 3297 @articles = sort {uc($a) cmp uc($b)} @files;
cb1a09d0 3298
a0d0e21e
LW
3299 # same thing in reversed order
3300 @articles = sort {$b cmp $a} @files;
3301
3302 # sort numerically ascending
3303 @articles = sort {$a <=> $b} @files;
3304
3305 # sort numerically descending
3306<