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1=head1 NAME
2
3perlfunc - Perl builtin functions
4
5=head1 DESCRIPTION
6
7The functions in this section can serve as terms in an expression.
8They fall into two major categories: list operators and named unary
9operators. These differ in their precedence relationship with a
10following comma. (See the precedence table in L<perlop>.) List
11operators take more than one argument, while unary operators can never
12take more than one argument. Thus, a comma terminates the argument of
13a unary operator, but merely separates the arguments of a list
14operator. A unary operator generally provides a scalar context to its
15argument, while a list operator may provide either scalar and list
16contexts for its arguments. If it does both, the scalar arguments will
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17be first, and the list argument will follow. (Note that there can ever
18be only one list argument.) For instance, splice() has three scalar
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19arguments followed by a list.
20
21In the syntax descriptions that follow, list operators that expect a
22list (and provide list context for the elements of the list) are shown
23with LIST as an argument. Such a list may consist of any combination
24of scalar arguments or list values; the list values will be included
25in the list as if each individual element were interpolated at that
26point in the list, forming a longer single-dimensional list value.
27Elements of the LIST should be separated by commas.
28
29Any function in the list below may be used either with or without
30parentheses around its arguments. (The syntax descriptions omit the
5f05dabc 31parentheses.) If you use the parentheses, the simple (but occasionally
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32surprising) rule is this: It I<LOOKS> like a function, therefore it I<IS> a
33function, and precedence doesn't matter. Otherwise it's a list
34operator or unary operator, and precedence does matter. And whitespace
35between the function and left parenthesis doesn't count--so you need to
36be careful sometimes:
37
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38 print 1+2+4; # Prints 7.
39 print(1+2) + 4; # Prints 3.
40 print (1+2)+4; # Also prints 3!
41 print +(1+2)+4; # Prints 7.
42 print ((1+2)+4); # Prints 7.
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43
44If you run Perl with the B<-w> switch it can warn you about this. For
45example, the third line above produces:
46
47 print (...) interpreted as function at - line 1.
48 Useless use of integer addition in void context at - line 1.
49
50For functions that can be used in either a scalar or list context,
54310121 51nonabortive failure is generally indicated in a scalar context by
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52returning the undefined value, and in a list context by returning the
53null list.
54
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55Remember the following important rule: There is B<no rule> that relates
56the behavior of an expression in list context to its behavior in scalar
57context, or vice versa. It might do two totally different things.
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58Each operator and function decides which sort of value it would be most
59appropriate to return in a scalar context. Some operators return the
5a964f20 60length of the list that would have been returned in list context. Some
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61operators return the first value in the list. Some operators return the
62last value in the list. Some operators return a count of successful
63operations. In general, they do what you want, unless you want
64consistency.
65
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66An named array in scalar context is quite different from what would at
67first glance appear to be a list in scalar context. You can't get a list
68like C<(1,2,3)> into being in scalar context, because the compiler knows
69the context at compile time. It would generate the scalar comma operator
70there, not the list construction version of the comma. That means it
71was never a list to start with.
72
73In general, functions in Perl that serve as wrappers for system calls
74of the same name (like chown(2), fork(2), closedir(2), etc.) all return
75true when they succeed and C<undef> otherwise, as is usually mentioned
76in the descriptions below. This is different from the C interfaces,
77which return -1 on failure. Exceptions to this rule are wait(),
78waitpid(), and syscall(). System calls also set the special C<$!>
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
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92C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>,
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
22fae026 98C<m>//, C<pos>, C<quotemeta>, C<s>///, C<split>, C<study>
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99
100=item Numeric functions
101
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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
22fae026 111C<grep>, C<join>, C<map>, C<qw>/STRING/, C<reverse>, C<sort>, C<unpack>
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112
113=item Functions for real %HASHes
114
22fae026 115C<delete>, C<each>, C<exists>, C<keys>, C<values>
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116
117=item Input and output functions
118
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119C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
120C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
121C<readdir>, C<rewinddir>, C<seek>, C<seekdir>, C<select>, C<syscall>,
122C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>,
123C<warn>, C<write>
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124
125=item Functions for fixed length data or records
126
22fae026 127C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec>
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128
129=item Functions for filehandles, files, or directories
130
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131C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>,
132C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>, C<readlink>,
133C<rename>, C<rmdir>, C<stat>, C<symlink>, C<umask>, C<unlink>, C<utime>
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134
135=item Keywords related to the control flow of your perl program
136
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137C<caller>, C<continue>, C<die>, C<do>, C<dump>, C<eval>, C<exit>,
138C<goto>, C<last>, C<next>, C<redo>, C<return>, C<sub>, C<wantarray>
cb1a09d0 139
54310121 140=item Keywords related to scoping
cb1a09d0 141
22fae026 142C<caller>, C<import>, C<local>, C<my>, C<package>, C<use>
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143
144=item Miscellaneous functions
145
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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
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151C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
152C<pipe>, C<qx>/STRING/, C<setpgrp>, C<setpriority>, C<sleep>, C<system>,
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
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193=item Functions new in perl5
194
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195C<abs>, C<bless>, C<chomp>, C<chr>, C<exists>, C<formline>, C<glob>,
196C<import>, C<lc>, C<lcfirst>, C<map>, C<my>, C<no>, C<prototype>, C<qx>,
197C<qw>, C<readline>, C<readpipe>, C<ref>, C<sub*>, C<sysopen>, C<tie>,
198C<tied>, C<uc>, C<ucfirst>, C<untie>, C<use>
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199
200* - C<sub> was a keyword in perl4, but in perl5 it is an
5a964f20 201operator, which can be used in expressions.
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202
203=item Functions obsoleted in perl5
204
22fae026 205C<dbmclose>, C<dbmopen>
37798a01 206
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207=back
208
209=head2 Alphabetical Listing of Perl Functions
210
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211=over 8
212
22fae026 213=item I<-X> FILEHANDLE
a0d0e21e 214
22fae026 215=item I<-X> EXPR
a0d0e21e 216
22fae026 217=item I<-X>
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218
219A file test, where X is one of the letters listed below. This unary
220operator takes one argument, either a filename or a filehandle, and
221tests the associated file to see if something is true about it. If the
222argument is omitted, tests $_, except for C<-t>, which tests STDIN.
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.
246 -p File is a named pipe (FIFO).
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,
5f05dabc 267C<-r>, C<-R>, C<-w>, and C<-W> always return 1, and C<-x> and C<-X> return
a0d0e21e 2681 if any execute bit is set in the mode. Scripts run by the superuser may
5f05dabc 269thus need to do a 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
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295If any of the file tests (or either the stat() or lstat() operators) are given
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.
bbce6d69 319If VALUE is omitted, uses $_.
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320
321=item accept NEWSOCKET,GENERICSOCKET
322
323Accepts an incoming socket connect, just as the accept(2) system call
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
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332specified number of seconds have elapsed. If SECONDS is not specified,
333the value stored in $_ is used. (On some machines,
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334unfortunately, the elapsed time may be up to one second less than you
335specified because of how seconds are counted.) Only one timer may be
336counting at once. Each call disables the previous timer, and an
337argument of 0 may be supplied to cancel the previous timer without
338starting a new one. The returned value is the amount of time remaining
339on the previous timer.
340
4633a7c4 341For delays of finer granularity than one second, you may use Perl's
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342syscall() interface to access setitimer(2) if your system supports it,
343or else see L</select()>. It is usually a mistake to intermix alarm()
4633a7c4 344and sleep() calls.
a0d0e21e 345
ff68c719 346If you want to use alarm() to time out a system call you need to use an
2f9daede 347eval/die pair. You can't rely on the alarm causing the system call to
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348fail with C<$!> set to EINTR because Perl sets up signal handlers to
349restart system calls on some systems. Using eval/die always works,
350modulo the caveats given in L<perlipc/"Signals">.
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351
352 eval {
5a964f20 353 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
36477c24 354 alarm $timeout;
ff68c719 355 $nread = sysread SOCKET, $buffer, $size;
36477c24 356 alarm 0;
ff68c719 357 };
ff68c719 358 if ($@) {
5a964f20 359 die unless $@ eq "alarm\n"; # propagate unexpected errors
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360 # timed out
361 }
362 else {
363 # didn't
364 }
365
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366=item atan2 Y,X
367
368Returns the arctangent of Y/X in the range -PI to PI.
369
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370For the tangent operation, you may use the POSIX::tan()
371function, or use the familiar relation:
372
373 sub tan { sin($_[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
cb1a09d0 390systems that need binmode and those that don't is their text file
5a964f20 391formats. Systems like Unix, MacOS, and Plan9 that delimit lines with a single
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392character, and that encode that character in C as '\n', do not need
393C<binmode>. The rest need it. If FILEHANDLE is an expression, the value
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
5f05dabc 403convenience, because a 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,
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413returns the caller's package name if there is a caller, that is, if
414we're in a subroutine or eval() or 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
426Here $subroutine may be C<"(eval)"> if the frame is not a subroutine
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427call, but an C<eval>. In such a case additional elements $evaltext and
428$is_require are set: $is_require is true if the frame is created by a
429C<require> or C<use> statement, $evaltext contains the text of the
430C<eval EXPR> statement. In particular, for a C<eval BLOCK> statement,
431$filename is C<"(eval)">, but $evaltext is undefined. (Note also that
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
4633a7c4 436detailed information: it sets the list variable @DB::args to be the
54310121 437arguments with which the subroutine was invoked.
748a9306 438
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439=item chdir EXPR
440
441Changes the working directory to EXPR, if possible. If EXPR is
442omitted, changes to home directory. Returns TRUE upon success, FALSE
443otherwise. See example under die().
444
445=item chmod LIST
446
447Changes the permissions of a list of files. The first element of the
4633a7c4 448list must be the numerical mode, which should probably be an octal
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449number, and which definitely should I<not> a string of octal digits:
450C<0644> is okay, C<'0644'> is not. Returns the number of files
dc848c6f 451successfully changed. See also L</oct>, if all you have is a string.
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452
453 $cnt = chmod 0755, 'foo', 'bar';
454 chmod 0755, @executables;
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455 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to --w----r-T
456 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
457 $mode = 0644; chmod $mode, 'foo'; # this is best
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458
459=item chomp VARIABLE
460
461=item chomp LIST
462
463=item chomp
464
3e3baf6d 465This is a slightly safer version of L</chop>. It removes any
a0d0e21e 466line ending that corresponds to the current value of C<$/> (also known as
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467$INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
468number of characters removed from all its arguments. It's often used to
469remove the newline from the end of an input record when you're worried
470that the final record may be missing its newline. When in paragraph mode
471(C<$/ = "">), it removes all trailing newlines from the string. If
472VARIABLE is omitted, it chomps $_. Example:
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473
474 while (<>) {
475 chomp; # avoid \n on last field
476 @array = split(/:/);
5a964f20 477 # ...
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478 }
479
480You can actually chomp anything that's an lvalue, including an assignment:
481
482 chomp($cwd = `pwd`);
483 chomp($answer = <STDIN>);
484
485If you chomp a list, each element is chomped, and the total number of
486characters removed is returned.
487
488=item chop VARIABLE
489
490=item chop LIST
491
492=item chop
493
494Chops off the last character of a string and returns the character
495chopped. It's used primarily to remove the newline from the end of an
496input record, but is much more efficient than C<s/\n//> because it neither
497scans nor copies the string. If VARIABLE is omitted, chops $_.
498Example:
499
500 while (<>) {
501 chop; # avoid \n on last field
502 @array = split(/:/);
5a964f20 503 #...
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504 }
505
506You can actually chop anything that's an lvalue, including an assignment:
507
508 chop($cwd = `pwd`);
509 chop($answer = <STDIN>);
510
511If you chop a list, each element is chopped. Only the value of the
512last chop is returned.
513
748a9306
LW
514Note that chop returns the last character. To return all but the last
515character, use C<substr($string, 0, -1)>.
516
a0d0e21e
LW
517=item chown LIST
518
519Changes the owner (and group) of a list of files. The first two
520elements of the list must be the I<NUMERICAL> uid and gid, in that order.
521Returns the number of files successfully changed.
522
523 $cnt = chown $uid, $gid, 'foo', 'bar';
524 chown $uid, $gid, @filenames;
525
54310121 526Here's an example that looks up nonnumeric uids in the passwd file:
a0d0e21e
LW
527
528 print "User: ";
529 chop($user = <STDIN>);
5a964f20 530 print "Files: ";
a0d0e21e
LW
531 chop($pattern = <STDIN>);
532
533 ($login,$pass,$uid,$gid) = getpwnam($user)
534 or die "$user not in passwd file";
535
5a964f20 536 @ary = glob($pattern); # expand filenames
a0d0e21e
LW
537 chown $uid, $gid, @ary;
538
54310121 539On most systems, you are not allowed to change the ownership of the
4633a7c4
LW
540file unless you're the superuser, although you should be able to change
541the group to any of your secondary groups. On insecure systems, these
542restrictions may be relaxed, but this is not a portable assumption.
543
a0d0e21e
LW
544=item chr NUMBER
545
54310121 546=item chr
bbce6d69 547
a0d0e21e 548Returns the character represented by that NUMBER in the character set.
dc848c6f 549For example, C<chr(65)> is "A" in ASCII. For the reverse, use L</ord>.
a0d0e21e 550
bbce6d69
PP
551If NUMBER is omitted, uses $_.
552
a0d0e21e
LW
553=item chroot FILENAME
554
54310121 555=item chroot
bbce6d69 556
5a964f20 557This function works like the system call by the same name: it makes the
4633a7c4 558named directory the new root directory for all further pathnames that
5a964f20 559begin with a "/" by your process and all its children. (It doesn't
28757baa 560change your current working directory, which is unaffected.) For security
4633a7c4 561reasons, this call is restricted to the superuser. If FILENAME is
5a964f20 562omitted, does a chroot to $_.
a0d0e21e
LW
563
564=item close FILEHANDLE
565
6a518fbc
TP
566=item close
567
a0d0e21e
LW
568Closes the file or pipe associated with the file handle, returning TRUE
569only if stdio successfully flushes buffers and closes the system file
6a518fbc
TP
570descriptor. Closes the currently selected filehandle if the argument
571is omitted.
fb73857a
PP
572
573You don't have to close FILEHANDLE if you are immediately going to do
574another open() on it, because open() will close it for you. (See
a0d0e21e 575open().) However, an explicit close on an input file resets the line
fb73857a
PP
576counter ($.), while the implicit close done by open() does not.
577
578If the file handle came from a piped open C<close> will additionally
579return FALSE if one of the other system calls involved fails or if the
580program exits with non-zero status. (If the only problem was that the
5a964f20
TC
581program exited non-zero $! will be set to 0.) Also, closing a pipe
582waits for the process executing on the pipe to complete, in case you
fb73857a
PP
583want to look at the output of the pipe afterwards. Closing a pipe
584explicitly also puts the exit status value of the command into C<$?>.
5a964f20 585
fb73857a 586Example:
a0d0e21e 587
fb73857a
PP
588 open(OUTPUT, '|sort >foo') # pipe to sort
589 or die "Can't start sort: $!";
5a964f20 590 #... # print stuff to output
fb73857a
PP
591 close OUTPUT # wait for sort to finish
592 or warn $! ? "Error closing sort pipe: $!"
593 : "Exit status $? from sort";
594 open(INPUT, 'foo') # get sort's results
595 or die "Can't open 'foo' for input: $!";
a0d0e21e 596
5a964f20
TC
597FILEHANDLE may be an expression whose value can be used as an indirect
598filehandle, usually the real filehandle name.
a0d0e21e
LW
599
600=item closedir DIRHANDLE
601
5a964f20
TC
602Closes a directory opened by opendir() and returns the success of that
603system call.
604
605DIRHANDLE may be an expression whose value can be used as an indirect
606dirhandle, usually the real dirhandle name.
a0d0e21e
LW
607
608=item connect SOCKET,NAME
609
610Attempts to connect to a remote socket, just as the connect system call
611does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
4633a7c4
LW
612packed address of the appropriate type for the socket. See the examples in
613L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 614
cb1a09d0
AD
615=item continue BLOCK
616
617Actually a flow control statement rather than a function. If there is a
618C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
619C<foreach>), it is always executed just before the conditional is about to
620be evaluated again, just like the third part of a C<for> loop in C. Thus
621it can be used to increment a loop variable, even when the loop has been
622continued via the C<next> statement (which is similar to the C C<continue>
623statement).
624
1d2dff63
GS
625C<last>, C<next>, or C<redo> may appear within a C<continue>
626block. C<last> and C<redo> will behave as if they had been executed within
627the main block. So will C<next>, but since it will execute a C<continue>
628block, it may be more entertaining.
629
630 while (EXPR) {
631 ### redo always comes here
632 do_something;
633 } continue {
634 ### next always comes here
635 do_something_else;
636 # then back the top to re-check EXPR
637 }
638 ### last always comes here
639
640Omitting the C<continue> section is semantically equivalent to using an
641empty one, logically enough. In that case, C<next> goes directly back
642to check the condition at the top of the loop.
643
a0d0e21e
LW
644=item cos EXPR
645
5a964f20 646Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
a0d0e21e
LW
647takes cosine of $_.
648
28757baa
PP
649For the inverse cosine operation, you may use the POSIX::acos()
650function, or use this relation:
651
652 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
653
a0d0e21e
LW
654=item crypt PLAINTEXT,SALT
655
4633a7c4
LW
656Encrypts a string exactly like the crypt(3) function in the C library
657(assuming that you actually have a version there that has not been
658extirpated as a potential munition). This can prove useful for checking
659the password file for lousy passwords, amongst other things. Only the
660guys wearing white hats should do this.
a0d0e21e 661
11155c91
CS
662Note that crypt is intended to be a one-way function, much like breaking
663eggs to make an omelette. There is no (known) corresponding decrypt
664function. As a result, this function isn't all that useful for
665cryptography. (For that, see your nearby CPAN mirror.)
2f9daede 666
a0d0e21e
LW
667Here's an example that makes sure that whoever runs this program knows
668their own password:
669
670 $pwd = (getpwuid($<))[1];
671 $salt = substr($pwd, 0, 2);
672
673 system "stty -echo";
674 print "Password: ";
675 chop($word = <STDIN>);
676 print "\n";
677 system "stty echo";
678
679 if (crypt($word, $salt) ne $pwd) {
680 die "Sorry...\n";
681 } else {
682 print "ok\n";
54310121 683 }
a0d0e21e 684
9f8f0c9d 685Of course, typing in your own password to whoever asks you
748a9306 686for it is unwise.
a0d0e21e 687
aa689395 688=item dbmclose HASH
a0d0e21e
LW
689
690[This function has been superseded by the untie() function.]
691
aa689395 692Breaks the binding between a DBM file and a hash.
a0d0e21e 693
aa689395 694=item dbmopen HASH,DBNAME,MODE
a0d0e21e
LW
695
696[This function has been superseded by the tie() function.]
697
7b8d334a 698This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
aa689395
PP
699hash. HASH is the name of the hash. (Unlike normal open, the first
700argument is I<NOT> a filehandle, even though it looks like one). DBNAME
701is the name of the database (without the F<.dir> or F<.pag> extension if
702any). If the database does not exist, it is created with protection
703specified by MODE (as modified by the umask()). If your system supports
704only the older DBM functions, you may perform only one dbmopen() in your
705program. In older versions of Perl, if your system had neither DBM nor
706ndbm, calling dbmopen() produced a fatal error; it now falls back to
707sdbm(3).
708
709If you don't have write access to the DBM file, you can only read hash
710variables, not set them. If you want to test whether you can write,
711either use file tests or try setting a dummy hash entry inside an eval(),
712which will trap the error.
a0d0e21e 713
1d2dff63
GS
714Note that functions such as keys() and values() may return huge lists
715when used on large DBM files. You may prefer to use the each()
a0d0e21e
LW
716function to iterate over large DBM files. Example:
717
718 # print out history file offsets
719 dbmopen(%HIST,'/usr/lib/news/history',0666);
720 while (($key,$val) = each %HIST) {
721 print $key, ' = ', unpack('L',$val), "\n";
722 }
723 dbmclose(%HIST);
724
cb1a09d0 725See also L<AnyDBM_File> for a more general description of the pros and
184e9718 726cons of the various dbm approaches, as well as L<DB_File> for a particularly
cb1a09d0 727rich implementation.
4633a7c4 728
a0d0e21e
LW
729=item defined EXPR
730
54310121 731=item defined
bbce6d69 732
2f9daede
TPG
733Returns a Boolean value telling whether EXPR has a value other than
734the undefined value C<undef>. If EXPR is not present, C<$_> will be
735checked.
736
737Many operations return C<undef> to indicate failure, end of file,
738system error, uninitialized variable, and other exceptional
739conditions. This function allows you to distinguish C<undef> from
740other values. (A simple Boolean test will not distinguish among
741C<undef>, zero, the empty string, and "0", which are all equally
742false.) Note that since C<undef> is a valid scalar, its presence
743doesn't I<necessarily> indicate an exceptional condition: pop()
744returns C<undef> when its argument is an empty array, I<or> when the
745element to return happens to be C<undef>.
746
5a964f20
TC
747You may also use defined() to check whether a subroutine exists, by
748saying C<defined &func> without parentheses. On the other hand, use
749of defined() upon aggregates (hashes and arrays) is not guaranteed to
750produce intuitive results, and should probably be avoided.
2f9daede
TPG
751
752When used on a hash element, it tells you whether the value is defined,
dc848c6f 753not whether the key exists in the hash. Use L</exists> for the latter
2f9daede 754purpose.
a0d0e21e
LW
755
756Examples:
757
758 print if defined $switch{'D'};
759 print "$val\n" while defined($val = pop(@ary));
760 die "Can't readlink $sym: $!"
761 unless defined($value = readlink $sym);
a0d0e21e 762 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
2f9daede 763 $debugging = 0 unless defined $debugging;
a0d0e21e 764
2f9daede
TPG
765Note: Many folks tend to overuse defined(), and then are surprised to
766discover that the number 0 and "" (the zero-length string) are, in fact,
767defined values. For example, if you say
a5f75d66
AD
768
769 "ab" =~ /a(.*)b/;
770
5a964f20 771The pattern match succeeds, and $1 is defined, despite the fact that it
a5f75d66
AD
772matched "nothing". But it didn't really match nothing--rather, it
773matched something that happened to be 0 characters long. This is all
774very above-board and honest. When a function returns an undefined value,
2f9daede
TPG
775it's an admission that it couldn't give you an honest answer. So you
776should use defined() only when you're questioning the integrity of what
777you're trying to do. At other times, a simple comparison to 0 or "" is
778what you want.
779
780Currently, using defined() on an entire array or hash reports whether
781memory for that aggregate has ever been allocated. So an array you set
782to the empty list appears undefined initially, and one that once was full
783and that you then set to the empty list still appears defined. You
784should instead use a simple test for size:
28757baa
PP
785
786 if (@an_array) { print "has array elements\n" }
787 if (%a_hash) { print "has hash members\n" }
788
789Using undef() on these, however, does clear their memory and then report
5a964f20 790them as not defined anymore, but you shouldn't do that unless you don't
28757baa 791plan to use them again, because it saves time when you load them up
5a964f20
TC
792again to have memory already ready to be filled. The normal way to
793free up space used by an aggregate is to assign the empty list.
28757baa 794
5a964f20 795This counterintuitive behavior of defined() on aggregates may be
28757baa
PP
796changed, fixed, or broken in a future release of Perl.
797
dc848c6f 798See also L</undef>, L</exists>, L</ref>.
2f9daede 799
a0d0e21e
LW
800=item delete EXPR
801
aa689395
PP
802Deletes the specified key(s) and their associated values from a hash.
803For each key, returns the deleted value associated with that key, or
804the undefined value if there was no such key. Deleting from C<$ENV{}>
805modifies the environment. Deleting from a hash tied to a DBM file
5f05dabc
PP
806deletes the entry from the DBM file. (But deleting from a tie()d hash
807doesn't necessarily return anything.)
a0d0e21e 808
aa689395 809The following deletes all the values of a hash:
a0d0e21e 810
5f05dabc
PP
811 foreach $key (keys %HASH) {
812 delete $HASH{$key};
a0d0e21e
LW
813 }
814
5f05dabc
PP
815And so does this:
816
817 delete @HASH{keys %HASH}
818
5a964f20
TC
819(But both of these are slower than just assigning the empty list, or
820using undef().) Note that the EXPR can be arbitrarily complicated as
821long as the final operation is a hash element lookup or hash slice:
a0d0e21e
LW
822
823 delete $ref->[$x][$y]{$key};
5f05dabc 824 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
a0d0e21e
LW
825
826=item die LIST
827
5a964f20 828Outside an eval(), prints the value of LIST to C<STDERR> and exits with
184e9718 829the current value of C<$!> (errno). If C<$!> is 0, exits with the value of
54310121 830C<($? E<gt>E<gt> 8)> (backtick `command` status). If C<($? E<gt>E<gt> 8)>
28757baa 831is 0, exits with 255. Inside an eval(), the error message is stuffed into
5a964f20 832C<$@> and the eval() is terminated with the undefined value. This makes
28757baa 833die() the way to raise an exception.
a0d0e21e
LW
834
835Equivalent examples:
836
837 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
54310121 838 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
a0d0e21e
LW
839
840If the value of EXPR does not end in a newline, the current script line
841number and input line number (if any) are also printed, and a newline
842is supplied. Hint: sometimes appending ", stopped" to your message
843will cause it to make better sense when the string "at foo line 123" is
844appended. Suppose you are running script "canasta".
845
846 die "/etc/games is no good";
847 die "/etc/games is no good, stopped";
848
849produce, respectively
850
851 /etc/games is no good at canasta line 123.
852 /etc/games is no good, stopped at canasta line 123.
853
854See also exit() and warn().
855
fb73857a
PP
856If LIST is empty and $@ already contains a value (typically from a
857previous eval) that value is reused after appending "\t...propagated".
858This is useful for propagating exceptions:
859
860 eval { ... };
861 die unless $@ =~ /Expected exception/;
862
863If $@ is empty then the string "Died" is used.
864
5a964f20 865You can arrange for a callback to be run just before the die() does
774d564b
PP
866its deed, by setting the C<$SIG{__DIE__}> hook. The associated handler
867will be called with the error text and can change the error message, if
fb73857a
PP
868it sees fit, by calling die() again. See L<perlvar/$SIG{expr}> for details on
869setting C<%SIG> entries, and L<"eval BLOCK"> for some examples.
870
871Note that the C<$SIG{__DIE__}> hook is called even inside eval()ed
872blocks/strings. If one wants the hook to do nothing in such
873situations, put
874
875 die @_ if $^S;
876
877as the first line of the handler (see L<perlvar/$^S>).
774d564b 878
a0d0e21e
LW
879=item do BLOCK
880
881Not really a function. Returns the value of the last command in the
882sequence of commands indicated by BLOCK. When modified by a loop
883modifier, executes the BLOCK once before testing the loop condition.
884(On other statements the loop modifiers test the conditional first.)
885
886=item do SUBROUTINE(LIST)
887
888A deprecated form of subroutine call. See L<perlsub>.
889
890=item do EXPR
891
892Uses the value of EXPR as a filename and executes the contents of the
893file as a Perl script. Its primary use is to include subroutines
894from a Perl subroutine library.
895
896 do 'stat.pl';
897
898is just like
899
fb73857a 900 scalar eval `cat stat.pl`;
a0d0e21e 901
5a964f20 902except that it's more efficient and concise, keeps track of the
a0d0e21e
LW
903current filename for error messages, and searches all the B<-I>
904libraries if the file isn't in the current directory (see also the @INC
dc1be6b5
GS
905array in L<perlvar/Predefined Names>). It is also different in how
906code evaluated with C<do FILENAME> doesn't see lexicals in the enclosing
907scope like C<eval STRING> does. It's the same, however, in that it does
54310121 908reparse the file every time you call it, so you probably don't want to
a0d0e21e
LW
909do this inside a loop.
910
911Note that inclusion of library modules is better done with the
5a964f20 912use() and require() operators, which also do automatic error checking
4633a7c4 913and raise an exception if there's a problem.
a0d0e21e 914
5a964f20
TC
915You might like to use C<do> to read in a program configuration
916file. Manual error checking can be done this way:
917
918 # read in config files: system first, then user
919 for $file ('/share/prog/defaults.rc", "$ENV{HOME}/.someprogrc") {
920 unless ($return = do $file) {
921 warn "couldn't parse $file: $@" if $@;
922 warn "couldn't do $file: $!" unless defined $return;
923 warn "couldn't run $file" unless $return;
924 }
925 }
926
a0d0e21e
LW
927=item dump LABEL
928
929This causes an immediate core dump. Primarily this is so that you can
930use the B<undump> program to turn your core dump into an executable binary
931after having initialized all your variables at the beginning of the
932program. When the new binary is executed it will begin by executing a
933C<goto LABEL> (with all the restrictions that C<goto> suffers). Think of
934it as a goto with an intervening core dump and reincarnation. If LABEL
5a964f20 935is omitted, restarts the program from the top. WARNING: Any files
a0d0e21e
LW
936opened at the time of the dump will NOT be open any more when the
937program is reincarnated, with possible resulting confusion on the part
938of Perl. See also B<-u> option in L<perlrun>.
939
940Example:
941
942 #!/usr/bin/perl
943 require 'getopt.pl';
944 require 'stat.pl';
945 %days = (
946 'Sun' => 1,
947 'Mon' => 2,
948 'Tue' => 3,
949 'Wed' => 4,
950 'Thu' => 5,
951 'Fri' => 6,
952 'Sat' => 7,
953 );
954
955 dump QUICKSTART if $ARGV[0] eq '-d';
956
957 QUICKSTART:
958 Getopt('f');
959
5a964f20
TC
960This operator is largely obsolete, partly because it's very hard to
961convert a core file into an executable, and because the real perl-to-C
962compiler has superseded it.
963
aa689395
PP
964=item each HASH
965
5a964f20 966When called in list context, returns a 2-element list consisting of the
aa689395 967key and value for the next element of a hash, so that you can iterate over
5a964f20 968it. When called in scalar context, returns the key for only the "next"
2f9daede
TPG
969element in the hash. (Note: Keys may be "0" or "", which are logically
970false; you may wish to avoid constructs like C<while ($k = each %foo) {}>
971for this reason.)
972
973Entries are returned in an apparently random order. When the hash is
974entirely read, a null array is returned in list context (which when
5a964f20 975assigned produces a FALSE (0) value), and C<undef> in
2f9daede
TPG
976scalar context. The next call to each() after that will start iterating
977again. There is a single iterator for each hash, shared by all each(),
978keys(), and values() function calls in the program; it can be reset by
979reading all the elements from the hash, or by evaluating C<keys HASH> or
980C<values HASH>. If you add or delete elements of a hash while you're
981iterating over it, you may get entries skipped or duplicated, so don't.
aa689395
PP
982
983The following prints out your environment like the printenv(1) program,
984only in a different order:
a0d0e21e
LW
985
986 while (($key,$value) = each %ENV) {
987 print "$key=$value\n";
988 }
989
990See also keys() and values().
991
992=item eof FILEHANDLE
993
4633a7c4
LW
994=item eof ()
995
a0d0e21e
LW
996=item eof
997
998Returns 1 if the next read on FILEHANDLE will return end of file, or if
999FILEHANDLE is not open. FILEHANDLE may be an expression whose value
5a964f20
TC
1000gives the real filehandle. (Note that this function actually
1001reads a character and then ungetc()s it, so isn't very useful in an
748a9306
LW
1002interactive context.) Do not read from a terminal file (or call
1003C<eof(FILEHANDLE)> on it) after end-of-file is reached. Filetypes such
1004as terminals may lose the end-of-file condition if you do.
1005
1006An C<eof> without an argument uses the last file read as argument.
5a964f20 1007Using C<eof()> with empty parentheses is very different. It indicates the pseudo file formed of
2f9daede
TPG
1008the files listed on the command line, i.e., C<eof()> is reasonable to
1009use inside a C<while (E<lt>E<gt>)> loop to detect the end of only the
1010last file. Use C<eof(ARGV)> or eof without the parentheses to test
1011I<EACH> file in a while (E<lt>E<gt>) loop. Examples:
a0d0e21e 1012
748a9306
LW
1013 # reset line numbering on each input file
1014 while (<>) {
5a964f20 1015 next if /^\s*#/; # skip comments
748a9306 1016 print "$.\t$_";
5a964f20
TC
1017 } continue {
1018 close ARGV if eof; # Not eof()!
748a9306
LW
1019 }
1020
a0d0e21e
LW
1021 # insert dashes just before last line of last file
1022 while (<>) {
5a964f20 1023 if (eof()) { # check for end of current file
a0d0e21e 1024 print "--------------\n";
748a9306
LW
1025 close(ARGV); # close or break; is needed if we
1026 # are reading from the terminal
a0d0e21e
LW
1027 }
1028 print;
1029 }
1030
a0d0e21e 1031Practical hint: you almost never need to use C<eof> in Perl, because the
3b02c43c
GS
1032input operators return false values when they run out of data, or if there
1033was an error.
a0d0e21e
LW
1034
1035=item eval EXPR
1036
1037=item eval BLOCK
1038
c7cc6f1c
GS
1039In the first form, the return value of EXPR is parsed and executed as if it
1040were a little Perl program. The value of the expression (which is itself
5a964f20 1041determined within scalar context) is first parsed, and if there weren't any
c7cc6f1c 1042errors, executed in the context of the current Perl program, so that any
5f05dabc 1043variable settings or subroutine and format definitions remain afterwards.
c7cc6f1c
GS
1044Note that the value is parsed every time the eval executes. If EXPR is
1045omitted, evaluates C<$_>. This form is typically used to delay parsing
1046and subsequent execution of the text of EXPR until run time.
1047
1048In the second form, the code within the BLOCK is parsed only once--at the
1049same time the code surrounding the eval itself was parsed--and executed
1050within the context of the current Perl program. This form is typically
1051used to trap exceptions more efficiently than the first (see below), while
1052also providing the benefit of checking the code within BLOCK at compile
1053time.
1054
1055The final semicolon, if any, may be omitted from the value of EXPR or within
1056the BLOCK.
1057
1058In both forms, the value returned is the value of the last expression
5a964f20 1059evaluated inside the mini-program; a return statement may be also used, just
c7cc6f1c 1060as with subroutines. The expression providing the return value is evaluated
5a964f20 1061in void, scalar, or list context, depending on the context of the eval itself.
c7cc6f1c 1062See L</wantarray> for more on how the evaluation context can be determined.
a0d0e21e
LW
1063
1064If there is a syntax error or runtime error, or a die() statement is
1065executed, an undefined value is returned by eval(), and C<$@> is set to the
1066error message. If there was no error, C<$@> is guaranteed to be a null
c7cc6f1c
GS
1067string. Beware that using eval() neither silences perl from printing
1068warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1069To do either of those, you have to use the C<$SIG{__WARN__}> facility. See
1070L</warn> and L<perlvar>.
a0d0e21e 1071
5f05dabc 1072Note that, because eval() traps otherwise-fatal errors, it is useful for
4633a7c4 1073determining whether a particular feature (such as socket() or symlink())
a0d0e21e
LW
1074is implemented. It is also Perl's exception trapping mechanism, where
1075the die operator is used to raise exceptions.
1076
1077If the code to be executed doesn't vary, you may use the eval-BLOCK
1078form to trap run-time errors without incurring the penalty of
1079recompiling each time. The error, if any, is still returned in C<$@>.
1080Examples:
1081
54310121 1082 # make divide-by-zero nonfatal
a0d0e21e
LW
1083 eval { $answer = $a / $b; }; warn $@ if $@;
1084
1085 # same thing, but less efficient
1086 eval '$answer = $a / $b'; warn $@ if $@;
1087
1088 # a compile-time error
5a964f20 1089 eval { $answer = }; # WRONG
a0d0e21e
LW
1090
1091 # a run-time error
1092 eval '$answer ='; # sets $@
1093
774d564b
PP
1094When using the eval{} form as an exception trap in libraries, you may
1095wish not to trigger any C<__DIE__> hooks that user code may have
1096installed. You can use the C<local $SIG{__DIE__}> construct for this
1097purpose, as shown in this example:
1098
1099 # a very private exception trap for divide-by-zero
1100 eval { local $SIG{'__DIE__'}; $answer = $a / $b; }; warn $@ if $@;
1101
1102This is especially significant, given that C<__DIE__> hooks can call
1103die() again, which has the effect of changing their error messages:
1104
1105 # __DIE__ hooks may modify error messages
1106 {
1107 local $SIG{'__DIE__'} = sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
c7cc6f1c
GS
1108 eval { die "foo lives here" };
1109 print $@ if $@; # prints "bar lives here"
774d564b
PP
1110 }
1111
54310121 1112With an eval(), you should be especially careful to remember what's
a0d0e21e
LW
1113being looked at when:
1114
1115 eval $x; # CASE 1
1116 eval "$x"; # CASE 2
1117
1118 eval '$x'; # CASE 3
1119 eval { $x }; # CASE 4
1120
5a964f20 1121 eval "\$$x++"; # CASE 5
a0d0e21e
LW
1122 $$x++; # CASE 6
1123
2f9daede
TPG
1124Cases 1 and 2 above behave identically: they run the code contained in
1125the variable $x. (Although case 2 has misleading double quotes making
1126the reader wonder what else might be happening (nothing is).) Cases 3
1127and 4 likewise behave in the same way: they run the code '$x', which
1128does nothing but return the value of C<$x>. (Case 4 is preferred for
1129purely visual reasons, but it also has the advantage of compiling at
1130compile-time instead of at run-time.) Case 5 is a place where
54310121 1131normally you I<WOULD> like to use double quotes, except that in this
2f9daede
TPG
1132particular situation, you can just use symbolic references instead, as
1133in case 6.
a0d0e21e
LW
1134
1135=item exec LIST
1136
8bf3b016
GS
1137=item exec PROGRAM LIST
1138
fb73857a
PP
1139The exec() function executes a system command I<AND NEVER RETURNS> -
1140use system() instead of exec() if you want it to return. It fails and
1141returns FALSE only if the command does not exist I<and> it is executed
1142directly instead of via your system's command shell (see below).
a0d0e21e 1143
55d729e4 1144Since it's a common mistake to use system() instead of exec(), Perl
5a964f20 1145warns you if there is a following statement which isn't die(), warn(),
55d729e4
GS
1146or exit() (if C<-w> is set - but you always do that). If you
1147I<really> want to follow an exec() with some other statement, you
1148can use one of these styles to avoid the warning:
1149
5a964f20
TC
1150 exec ('foo') or print STDERR "couldn't exec foo: $!";
1151 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
55d729e4 1152
5a964f20
TC
1153If there is more than one argument in LIST, or if LIST is an array
1154with more than one value, calls execvp(3) with the arguments in LIST.
1155If there is only one scalar argument or an array with one element in it,
1156the argument is checked for shell metacharacters, and if there are any,
1157the entire argument is passed to the system's command shell for parsing
1158(this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1159If there are no shell metacharacters in the argument, it is split into
1160words and passed directly to execvp(), which is more efficient. Note:
1161exec() and system() do not flush your output buffer, so you may need to
1162set C<$|> to avoid lost output. Examples:
a0d0e21e
LW
1163
1164 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1165 exec "sort $outfile | uniq";
1166
1167If you don't really want to execute the first argument, but want to lie
1168to the program you are executing about its own name, you can specify
1169the program you actually want to run as an "indirect object" (without a
1170comma) in front of the LIST. (This always forces interpretation of the
54310121 1171LIST as a multivalued list, even if there is only a single scalar in
a0d0e21e
LW
1172the list.) Example:
1173
1174 $shell = '/bin/csh';
1175 exec $shell '-sh'; # pretend it's a login shell
1176
1177or, more directly,
1178
1179 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1180
bb32b41a
GS
1181When the arguments get executed via the system shell, results will
1182be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1183for details.
1184
5a964f20
TC
1185Using an indirect object with C<exec> or C<system> is also more secure.
1186This usage forces interpretation of the arguments as a multivalued list,
1187even if the list had just one argument. That way you're safe from the
1188shell expanding wildcards or splitting up words with whitespace in them.
1189
1190 @args = ( "echo surprise" );
1191
1192 system @args; # subject to shell escapes if @args == 1
1193 system { $args[0] } @args; # safe even with one-arg list
1194
1195The first version, the one without the indirect object, ran the I<echo>
1196program, passing it C<"surprise"> an argument. The second version
1197didn't--it tried to run a program literally called I<"echo surprise">,
1198didn't find it, and set C<$?> to a non-zero value indicating failure.
1199
a0d0e21e
LW
1200=item exists EXPR
1201
1202Returns TRUE if the specified hash key exists in its hash array, even
1203if the corresponding value is undefined.
1204
1205 print "Exists\n" if exists $array{$key};
1206 print "Defined\n" if defined $array{$key};
1207 print "True\n" if $array{$key};
1208
5f05dabc 1209A hash element can be TRUE only if it's defined, and defined if
a0d0e21e
LW
1210it exists, but the reverse doesn't necessarily hold true.
1211
1212Note that the EXPR can be arbitrarily complicated as long as the final
1213operation is a hash key lookup:
1214
5a964f20
TC
1215 if (exists $ref->{"A"}{"B"}{$key}) { ... }
1216
1217Although the last element will not spring into existence just because its
1218existence was tested, intervening ones will. Thus C<$ref-E<gt>{"A"}>
1219C<$ref-E<gt>{"B"}> will spring into existence due to the existence
1220test for a $key element. This autovivification may be fixed in a later
1221release.
a0d0e21e
LW
1222
1223=item exit EXPR
1224
1225Evaluates EXPR and exits immediately with that value. (Actually, it
1226calls any defined C<END> routines first, but the C<END> routines may not
1227abort the exit. Likewise any object destructors that need to be called
1228are called before exit.) Example:
1229
1230 $ans = <STDIN>;
1231 exit 0 if $ans =~ /^[Xx]/;
1232
f86702cc 1233See also die(). If EXPR is omitted, exits with 0 status. The only
54310121 1234universally portable values for EXPR are 0 for success and 1 for error;
f86702cc
PP
1235all other values are subject to unpredictable interpretation depending
1236on the environment in which the Perl program is running.
a0d0e21e 1237
28757baa
PP
1238You shouldn't use exit() to abort a subroutine if there's any chance that
1239someone might want to trap whatever error happened. Use die() instead,
1240which can be trapped by an eval().
1241
5a964f20
TC
1242All C<END{}> blocks are run at exit time. See L<perlsub> for details.
1243
a0d0e21e
LW
1244=item exp EXPR
1245
54310121 1246=item exp
bbce6d69 1247
54310121 1248Returns I<e> (the natural logarithm base) to the power of EXPR.
a0d0e21e
LW
1249If EXPR is omitted, gives C<exp($_)>.
1250
1251=item fcntl FILEHANDLE,FUNCTION,SCALAR
1252
1253Implements the fcntl(2) function. You'll probably have to say
1254
1255 use Fcntl;
1256
0ade1984 1257first to get the correct constant definitions. Argument processing and
5a964f20 1258value return works just like ioctl() below.
a0d0e21e
LW
1259For example:
1260
1261 use Fcntl;
5a964f20
TC
1262 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1263 or die "can't fcntl F_GETFL: $!";
1264
1265You don't have to check for C<defined> on the return from
1266fnctl. Like ioctl, it maps a 0 return from the system
1267call into "0 but true" in Perl. This string is true in
1268boolean context and 0 in numeric context. It is also
1269exempt from the normal B<-w> warnings on improper numeric
1270conversions.
1271
1272Note that fcntl() will produce a fatal error if used on a machine that
1273doesn't implement fcntl(2).
a0d0e21e
LW
1274
1275=item fileno FILEHANDLE
1276
1277Returns the file descriptor for a filehandle. This is useful for
5a964f20
TC
1278constructing bitmaps for select() and low-level POSIX tty-handling
1279operations. If FILEHANDLE is an expression, the value is taken as
1280an indirect filehandle, generally its name.
1281
1282You can use this to find out whether two handles refer to the
1283same underlying descriptor:
1284
1285 if (fileno(THIS) == fileno(THAT)) {
1286 print "THIS and THAT are dups\n";
1287 }
a0d0e21e
LW
1288
1289=item flock FILEHANDLE,OPERATION
1290
8ebc5c01 1291Calls flock(2), or an emulation of it, on FILEHANDLE. Returns TRUE for
68dc0745
PP
1292success, FALSE on failure. Produces a fatal error if used on a machine
1293that doesn't implement flock(2), fcntl(2) locking, or lockf(3). flock()
1294is Perl's portable file locking interface, although it locks only entire
1295files, not records.
8ebc5c01 1296
a3cb178b 1297On many platforms (including most versions or clones of Unix), locks
5a964f20
TC
1298established by flock() are B<merely advisory>. Such discretionary locks
1299are more flexible, but offer fewer guarantees. This means that files
1300locked with flock() may be modified by programs that do not also use
1301flock(). Windows NT and OS/2 are among the platforms which
1302enforce mandatory locking. See your local documentation for details.
a3cb178b 1303
8ebc5c01
PP
1304OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1305LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
68dc0745
PP
1306you can use the symbolic names if import them from the Fcntl module,
1307either individually, or as a group using the ':flock' tag. LOCK_SH
1308requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1309releases a previously requested lock. If LOCK_NB is added to LOCK_SH or
1310LOCK_EX then flock() will return immediately rather than blocking
1311waiting for the lock (check the return status to see if you got it).
1312
1313To avoid the possibility of mis-coordination, Perl flushes FILEHANDLE
1314before (un)locking it.
8ebc5c01
PP
1315
1316Note that the emulation built with lockf(3) doesn't provide shared
1317locks, and it requires that FILEHANDLE be open with write intent. These
1318are the semantics that lockf(3) implements. Most (all?) systems
1319implement lockf(3) in terms of fcntl(2) locking, though, so the
1320differing semantics shouldn't bite too many people.
1321
1322Note also that some versions of flock() cannot lock things over the
1323network; you would need to use the more system-specific fcntl() for
1324that. If you like you can force Perl to ignore your system's flock(2)
1325function, and so provide its own fcntl(2)-based emulation, by passing
1326the switch C<-Ud_flock> to the F<Configure> program when you configure
1327perl.
4633a7c4
LW
1328
1329Here's a mailbox appender for BSD systems.
a0d0e21e 1330
7e1af8bc 1331 use Fcntl ':flock'; # import LOCK_* constants
a0d0e21e
LW
1332
1333 sub lock {
7e1af8bc 1334 flock(MBOX,LOCK_EX);
a0d0e21e
LW
1335 # and, in case someone appended
1336 # while we were waiting...
1337 seek(MBOX, 0, 2);
1338 }
1339
1340 sub unlock {
7e1af8bc 1341 flock(MBOX,LOCK_UN);
a0d0e21e
LW
1342 }
1343
1344 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1345 or die "Can't open mailbox: $!";
1346
1347 lock();
1348 print MBOX $msg,"\n\n";
1349 unlock();
1350
cb1a09d0 1351See also L<DB_File> for other flock() examples.
a0d0e21e
LW
1352
1353=item fork
1354
5a964f20
TC
1355Does a fork(2) system call. Returns the child pid to the parent process,
13560 to the child process, or C<undef> if the fork is unsuccessful.
1357
a0d0e21e 1358Note: unflushed buffers remain unflushed in both processes, which means
28757baa
PP
1359you may need to set C<$|> ($AUTOFLUSH in English) or call the autoflush()
1360method of IO::Handle to avoid duplicate output.
a0d0e21e
LW
1361
1362If you fork() without ever waiting on your children, you will accumulate
1363zombies:
1364
4633a7c4 1365 $SIG{CHLD} = sub { wait };
a0d0e21e 1366
54310121 1367There's also the double-fork trick (error checking on
a0d0e21e
LW
1368fork() returns omitted);
1369
1370 unless ($pid = fork) {
1371 unless (fork) {
1372 exec "what you really wanna do";
1373 die "no exec";
1374 # ... or ...
4633a7c4 1375 ## (some_perl_code_here)
a0d0e21e
LW
1376 exit 0;
1377 }
1378 exit 0;
1379 }
1380 waitpid($pid,0);
1381
cb1a09d0
AD
1382See also L<perlipc> for more examples of forking and reaping
1383moribund children.
1384
28757baa
PP
1385Note that if your forked child inherits system file descriptors like
1386STDIN and STDOUT that are actually connected by a pipe or socket, even
5a964f20 1387if you exit, then the remote server (such as, say, httpd or rsh) won't think
28757baa
PP
1388you're done. You should reopen those to /dev/null if it's any issue.
1389
cb1a09d0
AD
1390=item format
1391
7b8d334a 1392Declare a picture format for use by the write() function. For
cb1a09d0
AD
1393example:
1394
54310121 1395 format Something =
cb1a09d0
AD
1396 Test: @<<<<<<<< @||||| @>>>>>
1397 $str, $%, '$' . int($num)
1398 .
1399
1400 $str = "widget";
184e9718 1401 $num = $cost/$quantity;
cb1a09d0
AD
1402 $~ = 'Something';
1403 write;
1404
1405See L<perlform> for many details and examples.
1406
8903cb82 1407=item formline PICTURE,LIST
a0d0e21e 1408
5a964f20 1409This is an internal function used by C<format>s, though you may call it,
a0d0e21e
LW
1410too. It formats (see L<perlform>) a list of values according to the
1411contents of PICTURE, placing the output into the format output
4633a7c4
LW
1412accumulator, C<$^A> (or $ACCUMULATOR in English).
1413Eventually, when a write() is done, the contents of
a0d0e21e
LW
1414C<$^A> are written to some filehandle, but you could also read C<$^A>
1415yourself and then set C<$^A> back to "". Note that a format typically
1416does one formline() per line of form, but the formline() function itself
748a9306 1417doesn't care how many newlines are embedded in the PICTURE. This means
4633a7c4 1418that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
748a9306
LW
1419You may therefore need to use multiple formlines to implement a single
1420record format, just like the format compiler.
1421
5f05dabc 1422Be careful if you put double quotes around the picture, because an "C<@>"
748a9306 1423character may be taken to mean the beginning of an array name.
4633a7c4 1424formline() always returns TRUE. See L<perlform> for other examples.
a0d0e21e
LW
1425
1426=item getc FILEHANDLE
1427
1428=item getc
1429
1430Returns the next character from the input file attached to FILEHANDLE,
9bc64814 1431or the undefined value at end of file, or if there was an error. If
3b02c43c
GS
1432FILEHANDLE is omitted, reads from STDIN. This is not particularly
1433efficient. It cannot be used to get unbuffered single-characters,
1434however. For that, try something more like:
4633a7c4
LW
1435
1436 if ($BSD_STYLE) {
1437 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1438 }
1439 else {
54310121 1440 system "stty", '-icanon', 'eol', "\001";
4633a7c4
LW
1441 }
1442
1443 $key = getc(STDIN);
1444
1445 if ($BSD_STYLE) {
1446 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1447 }
1448 else {
5f05dabc 1449 system "stty", 'icanon', 'eol', '^@'; # ASCII null
4633a7c4
LW
1450 }
1451 print "\n";
1452
54310121
PP
1453Determination of whether $BSD_STYLE should be set
1454is left as an exercise to the reader.
cb1a09d0 1455
28757baa 1456The POSIX::getattr() function can do this more portably on systems
5a964f20 1457purporting POSIX compliance.
cb1a09d0 1458See also the C<Term::ReadKey> module from your nearest CPAN site;
54310121 1459details on CPAN can be found on L<perlmod/CPAN>.
a0d0e21e
LW
1460
1461=item getlogin
1462
5a964f20
TC
1463Implements the C library function of the same name, which on most
1464systems returns the current login from F</etc/utmp>, if any. If null,
1465use getpwuid().
a0d0e21e 1466
f86702cc 1467 $login = getlogin || getpwuid($<) || "Kilroy";
a0d0e21e 1468
da0045b7 1469Do not consider getlogin() for authentication: it is not as
4633a7c4
LW
1470secure as getpwuid().
1471
a0d0e21e
LW
1472=item getpeername SOCKET
1473
1474Returns the packed sockaddr address of other end of the SOCKET connection.
1475
4633a7c4
LW
1476 use Socket;
1477 $hersockaddr = getpeername(SOCK);
1478 ($port, $iaddr) = unpack_sockaddr_in($hersockaddr);
1479 $herhostname = gethostbyaddr($iaddr, AF_INET);
1480 $herstraddr = inet_ntoa($iaddr);
a0d0e21e
LW
1481
1482=item getpgrp PID
1483
47e29363
PP
1484Returns the current process group for the specified PID. Use
1485a PID of 0 to get the current process group for the
4633a7c4 1486current process. Will raise an exception if used on a machine that
a0d0e21e 1487doesn't implement getpgrp(2). If PID is omitted, returns process
47e29363
PP
1488group of current process. Note that the POSIX version of getpgrp()
1489does not accept a PID argument, so only PID==0 is truly portable.
a0d0e21e
LW
1490
1491=item getppid
1492
1493Returns the process id of the parent process.
1494
1495=item getpriority WHICH,WHO
1496
4633a7c4
LW
1497Returns the current priority for a process, a process group, or a user.
1498(See L<getpriority(2)>.) Will raise a fatal exception if used on a
a0d0e21e
LW
1499machine that doesn't implement getpriority(2).
1500
1501=item getpwnam NAME
1502
1503=item getgrnam NAME
1504
1505=item gethostbyname NAME
1506
1507=item getnetbyname NAME
1508
1509=item getprotobyname NAME
1510
1511=item getpwuid UID
1512
1513=item getgrgid GID
1514
1515=item getservbyname NAME,PROTO
1516
1517=item gethostbyaddr ADDR,ADDRTYPE
1518
1519=item getnetbyaddr ADDR,ADDRTYPE
1520
1521=item getprotobynumber NUMBER
1522
1523=item getservbyport PORT,PROTO
1524
1525=item getpwent
1526
1527=item getgrent
1528
1529=item gethostent
1530
1531=item getnetent
1532
1533=item getprotoent
1534
1535=item getservent
1536
1537=item setpwent
1538
1539=item setgrent
1540
1541=item sethostent STAYOPEN
1542
1543=item setnetent STAYOPEN
1544
1545=item setprotoent STAYOPEN
1546
1547=item setservent STAYOPEN
1548
1549=item endpwent
1550
1551=item endgrent
1552
1553=item endhostent
1554
1555=item endnetent
1556
1557=item endprotoent
1558
1559=item endservent
1560
1561These routines perform the same functions as their counterparts in the
5a964f20 1562system library. In list context, the return values from the
a0d0e21e
LW
1563various get routines are as follows:
1564
1565 ($name,$passwd,$uid,$gid,
6ee623d5 1566 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
a0d0e21e
LW
1567 ($name,$passwd,$gid,$members) = getgr*
1568 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1569 ($name,$aliases,$addrtype,$net) = getnet*
1570 ($name,$aliases,$proto) = getproto*
1571 ($name,$aliases,$port,$proto) = getserv*
1572
1573(If the entry doesn't exist you get a null list.)
1574
5a964f20 1575In scalar context, you get the name, unless the function was a
a0d0e21e
LW
1576lookup by name, in which case you get the other thing, whatever it is.
1577(If the entry doesn't exist you get the undefined value.) For example:
1578
5a964f20
TC
1579 $uid = getpwnam($name);
1580 $name = getpwuid($num);
1581 $name = getpwent();
1582 $gid = getgrnam($name);
1583 $name = getgrgid($num;
1584 $name = getgrent();
1585 #etc.
a0d0e21e 1586
6ee623d5
GS
1587In I<getpw*()> the fields $quota, $comment, and $expire are special
1588cases in the sense that in many systems they are unsupported. If the
1589$quota is unsupported, it is an empty scalar. If it is supported, it
1590usually encodes the disk quota. If the $comment field is unsupported,
1591it is an empty scalar. If it is supported it usually encodes some
1592administrative comment about the user. In some systems the $quota
1593field may be $change or $age, fields that have to do with password
1594aging. In some systems the $comment field may be $class. The $expire
1595field, if present, encodes the expiration period of the account or the
1596password. For the availability and the exact meaning of these fields
1597in your system, please consult your getpwnam(3) documentation and your
1598<pwd.h> file. You can also find out from within Perl which meaning
1599your $quota and $comment fields have and whether you have the $expire
1600field by using the Config module and the values d_pwquota, d_pwage,
1601d_pwchange, d_pwcomment, and d_pwexpire.
1602
a0d0e21e
LW
1603The $members value returned by I<getgr*()> is a space separated list of
1604the login names of the members of the group.
1605
1606For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1607C, it will be returned to you via C<$?> if the function call fails. The
1608@addrs value returned by a successful call is a list of the raw
1609addresses returned by the corresponding system library call. In the
1610Internet domain, each address is four bytes long and you can unpack it
1611by saying something like:
1612
1613 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1614
5a964f20
TC
1615If you get tired of remembering which element of the return list contains
1616which return value, by-name interfaces are also provided in modules:
1617File::stat, Net::hostent, Net::netent, Net::protoent, Net::servent,
1618Time::gmtime, Time::localtime, and User::grent. These override the
1619normal built-in, replacing them with versions that return objects with
1620the appropriate names for each field. For example:
1621
1622 use File::stat;
1623 use User::pwent;
1624 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
1625
1626Even though it looks like they're the same method calls (uid),
1627they aren't, because a File::stat object is different from a User::pwent object.
1628
a0d0e21e
LW
1629=item getsockname SOCKET
1630
1631Returns the packed sockaddr address of this end of the SOCKET connection.
1632
4633a7c4
LW
1633 use Socket;
1634 $mysockaddr = getsockname(SOCK);
1635 ($port, $myaddr) = unpack_sockaddr_in($mysockaddr);
a0d0e21e
LW
1636
1637=item getsockopt SOCKET,LEVEL,OPTNAME
1638
5a964f20 1639Returns the socket option requested, or undef if there is an error.
a0d0e21e
LW
1640
1641=item glob EXPR
1642
0a753a76
PP
1643=item glob
1644
5a964f20 1645Returns the value of EXPR with filename expansions such as the standard Unix shell /bin/sh would
68dc0745
PP
1646do. This is the internal function implementing the C<E<lt>*.cE<gt>>
1647operator, but you can use it directly. If EXPR is omitted, $_ is used.
1648The C<E<lt>*.cE<gt>> operator is discussed in more detail in
1649L<perlop/"I/O Operators">.
a0d0e21e
LW
1650
1651=item gmtime EXPR
1652
1653Converts a time as returned by the time function to a 9-element array
54310121 1654with the time localized for the standard Greenwich time zone.
4633a7c4 1655Typically used as follows:
a0d0e21e 1656
54310121 1657 # 0 1 2 3 4 5 6 7 8
a0d0e21e
LW
1658 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1659 gmtime(time);
1660
1661All array elements are numeric, and come straight out of a struct tm.
1662In particular this means that $mon has the range 0..11 and $wday has
54310121
PP
1663the range 0..6 with sunday as day 0. Also, $year is the number of
1664years since 1900, I<not> simply the last two digits of the year.
2f9daede
TPG
1665
1666If EXPR is omitted, does C<gmtime(time())>.
a0d0e21e 1667
5a964f20 1668In scalar context, returns the ctime(3) value:
0a753a76
PP
1669
1670 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
1671
54310121
PP
1672Also see the timegm() function provided by the Time::Local module,
1673and the strftime(3) function available via the POSIX module.
0a753a76 1674
a0d0e21e
LW
1675=item goto LABEL
1676
748a9306
LW
1677=item goto EXPR
1678
a0d0e21e
LW
1679=item goto &NAME
1680
1681The goto-LABEL form finds the statement labeled with LABEL and resumes
1682execution there. It may not be used to go into any construct that
1683requires initialization, such as a subroutine or a foreach loop. It
0a753a76
PP
1684also can't be used to go into a construct that is optimized away,
1685or to get out of a block or subroutine given to sort().
1686It can be used to go almost anywhere else within the dynamic scope,
a0d0e21e
LW
1687including out of subroutines, but it's usually better to use some other
1688construct such as last or die. The author of Perl has never felt the
1689need to use this form of goto (in Perl, that is--C is another matter).
1690
748a9306
LW
1691The goto-EXPR form expects a label name, whose scope will be resolved
1692dynamically. This allows for computed gotos per FORTRAN, but isn't
1693necessarily recommended if you're optimizing for maintainability:
1694
1695 goto ("FOO", "BAR", "GLARCH")[$i];
1696
a0d0e21e
LW
1697The goto-&NAME form is highly magical, and substitutes a call to the
1698named subroutine for the currently running subroutine. This is used by
1699AUTOLOAD subroutines that wish to load another subroutine and then
1700pretend that the other subroutine had been called in the first place
1701(except that any modifications to @_ in the current subroutine are
1702propagated to the other subroutine.) After the goto, not even caller()
1703will be able to tell that this routine was called first.
1704
1705=item grep BLOCK LIST
1706
1707=item grep EXPR,LIST
1708
54310121 1709This is similar in spirit to, but not the same as, grep(1)
2f9daede
TPG
1710and its relatives. In particular, it is not limited to using
1711regular expressions.
1712
a0d0e21e
LW
1713Evaluates the BLOCK or EXPR for each element of LIST (locally setting
1714$_ to each element) and returns the list value consisting of those
1715elements for which the expression evaluated to TRUE. In a scalar
1716context, returns the number of times the expression was TRUE.
1717
1718 @foo = grep(!/^#/, @bar); # weed out comments
1719
1720or equivalently,
1721
1722 @foo = grep {!/^#/} @bar; # weed out comments
1723
5f05dabc 1724Note that, because $_ is a reference into the list value, it can be used
a0d0e21e
LW
1725to modify the elements of the array. While this is useful and
1726supported, it can cause bizarre results if the LIST is not a named
2f9daede 1727array. Similarly, grep returns aliases into the original list,
7b8d334a 1728much like the way that a for loops's index variable aliases the list
2f9daede 1729elements. That is, modifying an element of a list returned by grep
fb73857a 1730(for example, in a C<foreach>, C<map> or another C<grep>)
2f9daede 1731actually modifies the element in the original list.
a0d0e21e 1732
fb73857a 1733See also L</map> for an array composed of the results of the BLOCK or EXPR.
38325410 1734
a0d0e21e
LW
1735=item hex EXPR
1736
54310121 1737=item hex
bbce6d69 1738
54310121 1739Interprets EXPR as a hex string and returns the corresponding
2f9daede 1740value. (To convert strings that might start with either 0 or 0x
dc848c6f 1741see L</oct>.) If EXPR is omitted, uses $_.
2f9daede
TPG
1742
1743 print hex '0xAf'; # prints '175'
1744 print hex 'aF'; # same
a0d0e21e
LW
1745
1746=item import
1747
5a964f20 1748There is no builtin import() function. It is just an ordinary
4633a7c4 1749method (subroutine) defined (or inherited) by modules that wish to export
a0d0e21e 1750names to another module. The use() function calls the import() method
54310121 1751for the package used. See also L</use()>, L<perlmod>, and L<Exporter>.
a0d0e21e
LW
1752
1753=item index STR,SUBSTR,POSITION
1754
1755=item index STR,SUBSTR
1756
4633a7c4
LW
1757Returns the position of the first occurrence of SUBSTR in STR at or after
1758POSITION. If POSITION is omitted, starts searching from the beginning of
184e9718 1759the string. The return value is based at 0 (or whatever you've set the C<$[>
4633a7c4 1760variable to--but don't do that). If the substring is not found, returns
a0d0e21e
LW
1761one less than the base, ordinarily -1.
1762
1763=item int EXPR
1764
54310121 1765=item int
bbce6d69 1766
a0d0e21e 1767Returns the integer portion of EXPR. If EXPR is omitted, uses $_.
5a964f20
TC
1768You should not use this for rounding, because it truncates
1769towards 0, and because machine representations of floating point
1770numbers can sometimes produce counterintuitive results. Usually sprintf() or printf(),
1771or the POSIX::floor or POSIX::ceil functions, would serve you better.
a0d0e21e
LW
1772
1773=item ioctl FILEHANDLE,FUNCTION,SCALAR
1774
1775Implements the ioctl(2) function. You'll probably have to say
1776
4633a7c4 1777 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
a0d0e21e 1778
4633a7c4 1779first to get the correct function definitions. If F<ioctl.ph> doesn't
a0d0e21e 1780exist or doesn't have the correct definitions you'll have to roll your
4633a7c4 1781own, based on your C header files such as F<E<lt>sys/ioctl.hE<gt>>.
5a964f20 1782(There is a Perl script called B<h2ph> that comes with the Perl kit that
54310121 1783may help you in this, but it's nontrivial.) SCALAR will be read and/or
4633a7c4
LW
1784written depending on the FUNCTION--a pointer to the string value of SCALAR
1785will be passed as the third argument of the actual ioctl call. (If SCALAR
1786has no string value but does have a numeric value, that value will be
1787passed rather than a pointer to the string value. To guarantee this to be
1788TRUE, add a 0 to the scalar before using it.) The pack() and unpack()
1789functions are useful for manipulating the values of structures used by
1790ioctl(). The following example sets the erase character to DEL.
a0d0e21e
LW
1791
1792 require 'ioctl.ph';
4633a7c4
LW
1793 $getp = &TIOCGETP;
1794 die "NO TIOCGETP" if $@ || !$getp;
a0d0e21e 1795 $sgttyb_t = "ccccs"; # 4 chars and a short
4633a7c4 1796 if (ioctl(STDIN,$getp,$sgttyb)) {
a0d0e21e
LW
1797 @ary = unpack($sgttyb_t,$sgttyb);
1798 $ary[2] = 127;
1799 $sgttyb = pack($sgttyb_t,@ary);
4633a7c4 1800 ioctl(STDIN,&TIOCSETP,$sgttyb)
a0d0e21e
LW
1801 || die "Can't ioctl: $!";
1802 }
1803
1804The return value of ioctl (and fcntl) is as follows:
1805
1806 if OS returns: then Perl returns:
1807 -1 undefined value
1808 0 string "0 but true"
1809 anything else that number
1810
1811Thus Perl returns TRUE on success and FALSE on failure, yet you can
1812still easily determine the actual value returned by the operating
1813system:
1814
1815 ($retval = ioctl(...)) || ($retval = -1);
1816 printf "System returned %d\n", $retval;
1817
5a964f20
TC
1818The special string "0 but true" is excempt from B<-w> complaints
1819about improper numeric conversions.
1820
a0d0e21e
LW
1821=item join EXPR,LIST
1822
54310121 1823Joins the separate strings of LIST into a single string with
a0d0e21e
LW
1824fields separated by the value of EXPR, and returns the string.
1825Example:
1826
1827 $_ = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
1828
1829See L<perlfunc/split>.
1830
aa689395
PP
1831=item keys HASH
1832
1d2dff63
GS
1833Returns a list consisting of all the keys of the named hash. (In a
1834scalar context, returns the number of keys.) The keys are returned in
aa689395
PP
1835an apparently random order, but it is the same order as either the
1836values() or each() function produces (given that the hash has not been
1837modified). As a side effect, it resets HASH's iterator.
a0d0e21e 1838
aa689395 1839Here is yet another way to print your environment:
a0d0e21e
LW
1840
1841 @keys = keys %ENV;
1842 @values = values %ENV;
1843 while ($#keys >= 0) {
1844 print pop(@keys), '=', pop(@values), "\n";
1845 }
1846
1847or how about sorted by key:
1848
1849 foreach $key (sort(keys %ENV)) {
1850 print $key, '=', $ENV{$key}, "\n";
1851 }
1852
54310121 1853To sort an array by value, you'll need to use a C<sort> function.
aa689395 1854Here's a descending numeric sort of a hash by its values:
4633a7c4 1855
5a964f20 1856 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
4633a7c4
LW
1857 printf "%4d %s\n", $hash{$key}, $key;
1858 }
1859
55497cff 1860As an lvalue C<keys> allows you to increase the number of hash buckets
aa689395
PP
1861allocated for the given hash. This can gain you a measure of efficiency if
1862you know the hash is going to get big. (This is similar to pre-extending
1863an array by assigning a larger number to $#array.) If you say
55497cff
PP
1864
1865 keys %hash = 200;
1866
5a964f20
TC
1867then C<%hash> will have at least 200 buckets allocated for it--256 of them, in fact, since
1868it rounds up to the next power of two. These
55497cff
PP
1869buckets will be retained even if you do C<%hash = ()>, use C<undef
1870%hash> if you want to free the storage while C<%hash> is still in scope.
1871You can't shrink the number of buckets allocated for the hash using
1872C<keys> in this way (but you needn't worry about doing this by accident,
1873as trying has no effect).
1874
a0d0e21e
LW
1875=item kill LIST
1876
54310121
PP
1877Sends a signal to a list of processes. The first element of
1878the list must be the signal to send. Returns the number of
4633a7c4 1879processes successfully signaled.
a0d0e21e
LW
1880
1881 $cnt = kill 1, $child1, $child2;
1882 kill 9, @goners;
1883
4633a7c4
LW
1884Unlike in the shell, in Perl if the I<SIGNAL> is negative, it kills
1885process groups instead of processes. (On System V, a negative I<PROCESS>
1886number will also kill process groups, but that's not portable.) That
1887means you usually want to use positive not negative signals. You may also
da0045b7 1888use a signal name in quotes. See L<perlipc/"Signals"> for details.
a0d0e21e
LW
1889
1890=item last LABEL
1891
1892=item last
1893
1894The C<last> command is like the C<break> statement in C (as used in
1895loops); it immediately exits the loop in question. If the LABEL is
1896omitted, the command refers to the innermost enclosing loop. The
1897C<continue> block, if any, is not executed:
1898
4633a7c4
LW
1899 LINE: while (<STDIN>) {
1900 last LINE if /^$/; # exit when done with header
5a964f20 1901 #...
a0d0e21e
LW
1902 }
1903
1d2dff63
GS
1904See also L</continue> for an illustration of how C<last>, C<next>, and
1905C<redo> work.
1906
a0d0e21e
LW
1907=item lc EXPR
1908
54310121 1909=item lc
bbce6d69 1910
a0d0e21e 1911Returns an lowercased version of EXPR. This is the internal function
54310121 1912implementing the \L escape in double-quoted strings.
a034a98d 1913Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
a0d0e21e 1914
bbce6d69
PP
1915If EXPR is omitted, uses $_.
1916
a0d0e21e
LW
1917=item lcfirst EXPR
1918
54310121 1919=item lcfirst
bbce6d69 1920
a0d0e21e
LW
1921Returns the value of EXPR with the first character lowercased. This is
1922the internal function implementing the \l escape in double-quoted strings.
a034a98d 1923Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
a0d0e21e 1924
bbce6d69
PP
1925If EXPR is omitted, uses $_.
1926
a0d0e21e
LW
1927=item length EXPR
1928
54310121 1929=item length
bbce6d69 1930
5a964f20 1931Returns the length in bytes of the value of EXPR. If EXPR is
a0d0e21e
LW
1932omitted, returns length of $_.
1933
1934=item link OLDFILE,NEWFILE
1935
5a964f20
TC
1936Creates a new filename linked to the old filename. Returns TRUE for
1937success, FALSE otherwise.
a0d0e21e
LW
1938
1939=item listen SOCKET,QUEUESIZE
1940
1941Does the same thing that the listen system call does. Returns TRUE if
4633a7c4 1942it succeeded, FALSE otherwise. See example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
1943
1944=item local EXPR
1945
5a964f20
TC
1946A local modifies the listed variables to be local to the enclosing
1947block, file, or eval. If more than one value is listed, the list must
1948be placed in parentheses. See L<perlsub/"Temporary Values via local()">
1949for details, including issues with tied arrays and hashes.
a0d0e21e 1950
7b8d334a
GS
1951You really probably want to be using my() instead, because local() isn't
1952what most people think of as "local". See L<perlsub/"Private Variables
cb1a09d0 1953via my()"> for details.
a0d0e21e
LW
1954
1955=item localtime EXPR
1956
1957Converts a time as returned by the time function to a 9-element array
5f05dabc 1958with the time analyzed for the local time zone. Typically used as
a0d0e21e
LW
1959follows:
1960
54310121 1961 # 0 1 2 3 4 5 6 7 8
a0d0e21e
LW
1962 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1963 localtime(time);
1964
1965All array elements are numeric, and come straight out of a struct tm.
1966In particular this means that $mon has the range 0..11 and $wday has
54310121
PP
1967the range 0..6 with sunday as day 0. Also, $year is the number of
1968years since 1900, that is, $year is 123 in year 2023.
1969
1970If EXPR is omitted, uses the current time (C<localtime(time)>).
a0d0e21e 1971
5a964f20 1972In scalar context, returns the ctime(3) value:
a0d0e21e 1973
5f05dabc 1974 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
a0d0e21e 1975
a3cb178b
GS
1976This scalar value is B<not> locale dependent, see L<perllocale>, but
1977instead a Perl builtin. Also see the Time::Local module, and the
1978strftime(3) and mktime(3) function available via the POSIX module. To
1979get somewhat similar but locale dependent date strings, set up your
1980locale environment variables appropriately (please see L<perllocale>)
5a964f20 1981and try for example:
a3cb178b 1982
5a964f20 1983 use POSIX qw(strftime);
a3cb178b
GS
1984 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
1985
1986Note that the C<%a> and C<%b>, the short forms of the day of the week
1987and the month of the year, may not necessarily be three characters wide.
a0d0e21e
LW
1988
1989=item log EXPR
1990
54310121 1991=item log
bbce6d69 1992
5a964f20 1993Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted, returns log
a0d0e21e
LW
1994of $_.
1995
1996=item lstat FILEHANDLE
1997
1998=item lstat EXPR
1999
54310121 2000=item lstat
bbce6d69 2001
5a964f20
TC
2002Does the same thing as the stat() function (including setting the
2003special C<_> filehandle) but stats a symbolic link instead of the file
2004the symbolic link points to. If symbolic links are unimplemented on
2005your system, a normal stat() is done.
a0d0e21e 2006
bbce6d69
PP
2007If EXPR is omitted, stats $_.
2008
a0d0e21e
LW
2009=item m//
2010
2011The match operator. See L<perlop>.
2012
2013=item map BLOCK LIST
2014
2015=item map EXPR,LIST
2016
2017Evaluates the BLOCK or EXPR for each element of LIST (locally setting $_ to each
2018element) and returns the list value composed of the results of each such
2019evaluation. Evaluates BLOCK or EXPR in a list context, so each element of LIST
2020may produce zero, one, or more elements in the returned value.
2021
2022 @chars = map(chr, @nums);
2023
2024translates a list of numbers to the corresponding characters. And
2025
4633a7c4 2026 %hash = map { getkey($_) => $_ } @array;
a0d0e21e
LW
2027
2028is just a funny way to write
2029
2030 %hash = ();
2031 foreach $_ (@array) {
4633a7c4 2032 $hash{getkey($_)} = $_;
a0d0e21e
LW
2033 }
2034
fb73857a
PP
2035Note that, because $_ is a reference into the list value, it can be used
2036to modify the elements of the array. While this is useful and
2037supported, it can cause bizarre results if the LIST is not a named
2038array. See also L</grep> for an array composed of those items of the
2039original list for which the BLOCK or EXPR evaluates to true.
2040
a0d0e21e
LW
2041=item mkdir FILENAME,MODE
2042
2043Creates the directory specified by FILENAME, with permissions specified
5a964f20
TC
2044by MODE (as modified by umask). If it succeeds it returns TRUE, otherwise
2045it returns FALSE and sets C<$!> (errno).
a0d0e21e
LW
2046
2047=item msgctl ID,CMD,ARG
2048
0ade1984
JH
2049Calls the System V IPC function msgctl(2). You'll probably have to say
2050
2051 use IPC::SysV;
2052
2053first to get the correct constant definitions. If CMD is IPC_STAT,
2054then ARG must be a variable which will hold the returned msqid_ds
2055structure. Returns like ioctl: the undefined value for error, "0 but
2056true" for zero, or the actual return value otherwise. See also
2057IPC::SysV and IPC::Semaphore::Msg documentation.
a0d0e21e
LW
2058
2059=item msgget KEY,FLAGS
2060
0ade1984
JH
2061Calls the System V IPC function msgget(2). Returns the message queue
2062id, or the undefined value if there is an error. See also IPC::SysV
2063and IPC::SysV::Msg documentation.
a0d0e21e
LW
2064
2065=item msgsnd ID,MSG,FLAGS
2066
2067Calls the System V IPC function msgsnd to send the message MSG to the
2068message queue ID. MSG must begin with the long integer message type,
c07a80fd 2069which may be created with C<pack("l", $type)>. Returns TRUE if
0ade1984
JH
2070successful, or FALSE if there is an error. See also IPC::SysV
2071and IPC::SysV::Msg documentation.
a0d0e21e
LW
2072
2073=item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2074
2075Calls the System V IPC function msgrcv to receive a message from
2076message queue ID into variable VAR with a maximum message size of
0ade1984
JH
2077SIZE. Note that if a message is received, the message type will be
2078the first thing in VAR, and the maximum length of VAR is SIZE plus the
2079size of the message type. Returns TRUE if successful, or FALSE if
2080there is an error. See also IPC::SysV and IPC::SysV::Msg documentation.
a0d0e21e
LW
2081
2082=item my EXPR
2083
2084A "my" declares the listed variables to be local (lexically) to the
5a964f20 2085enclosing block, file, or C<eval>. If
5f05dabc 2086more than one value is listed, the list must be placed in parentheses. See
cb1a09d0 2087L<perlsub/"Private Variables via my()"> for details.
4633a7c4 2088
a0d0e21e
LW
2089=item next LABEL
2090
2091=item next
2092
2093The C<next> command is like the C<continue> statement in C; it starts
2094the next iteration of the loop:
2095
4633a7c4
LW
2096 LINE: while (<STDIN>) {
2097 next LINE if /^#/; # discard comments
5a964f20 2098 #...
a0d0e21e
LW
2099 }
2100
2101Note that if there were a C<continue> block on the above, it would get
2102executed even on discarded lines. If the LABEL is omitted, the command
2103refers to the innermost enclosing loop.
2104
1d2dff63
GS
2105See also L</continue> for an illustration of how C<last>, C<next>, and
2106C<redo> work.
2107
a0d0e21e
LW
2108=item no Module LIST
2109
2110See the "use" function, which "no" is the opposite of.
2111
2112=item oct EXPR
2113
54310121 2114=item oct
bbce6d69 2115
4633a7c4 2116Interprets EXPR as an octal string and returns the corresponding
2f9daede 2117value. (If EXPR happens to start off with 0x, interprets it as
4633a7c4
LW
2118a hex string instead.) The following will handle decimal, octal, and
2119hex in the standard Perl or C notation:
a0d0e21e
LW
2120
2121 $val = oct($val) if $val =~ /^0/;
2122
2f9daede
TPG
2123If EXPR is omitted, uses $_. This function is commonly used when
2124a string such as "644" needs to be converted into a file mode, for
2125example. (Although perl will automatically convert strings into
2126numbers as needed, this automatic conversion assumes base 10.)
a0d0e21e
LW
2127
2128=item open FILEHANDLE,EXPR
2129
2130=item open FILEHANDLE
2131
2132Opens the file whose filename is given by EXPR, and associates it with
5f05dabc
PP
2133FILEHANDLE. If FILEHANDLE is an expression, its value is used as the
2134name of the real filehandle wanted. If EXPR is omitted, the scalar
2135variable of the same name as the FILEHANDLE contains the filename.
2136(Note that lexical variables--those declared with C<my>--will not work
2137for this purpose; so if you're using C<my>, specify EXPR in your call
2138to open.)
2139
2140If the filename begins with '<' or nothing, the file is opened for input.
2141If the filename begins with '>', the file is truncated and opened for
fbb426e4
TP
2142output, being created if necessary. If the filename begins with '>>',
2143the file is opened for appending, again being created if necessary.
2144You can put a '+' in front of the '>' or '<' to indicate that
5f05dabc
PP
2145you want both read and write access to the file; thus '+<' is almost
2146always preferred for read/write updates--the '+>' mode would clobber the
5a964f20
TC
2147file first. You can't usually use either read-write mode for updating
2148textfiles, since they have variable length records. See the B<-i>
2149switch in L<perlrun> for a better approach.
2150
2151The prefix and the filename may be separated with spaces.
5f05dabc
PP
2152These various prefixes correspond to the fopen(3) modes of 'r', 'r+', 'w',
2153'w+', 'a', and 'a+'.
2154
5a964f20
TC
2155If the filename begins with "|", the filename is interpreted as a
2156command to which output is to be piped, and if the filename ends with a
2157"|", the filename is interpreted See L<perlipc/"Using open() for IPC">
2158for more examples of this. (You are not allowed to open() to a command
2159that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2160and L<perlipc/"Bidirectional Communication"> for alternatives.)
cb1a09d0 2161
184e9718 2162Opening '-' opens STDIN and opening 'E<gt>-' opens STDOUT. Open returns
54310121 2163nonzero upon success, the undefined value otherwise. If the open
4633a7c4 2164involved a pipe, the return value happens to be the pid of the
54310121 2165subprocess.
cb1a09d0
AD
2166
2167If you're unfortunate enough to be running Perl on a system that
2168distinguishes between text files and binary files (modern operating
2169systems don't care), then you should check out L</binmode> for tips for
2170dealing with this. The key distinction between systems that need binmode
5a964f20
TC
2171and those that don't is their text file formats. Systems like Unix, MacOS, and
2172Plan9, which delimit lines with a single character, and which encode that
cb1a09d0
AD
2173character in C as '\n', do not need C<binmode>. The rest need it.
2174
fb73857a
PP
2175When opening a file, it's usually a bad idea to continue normal execution
2176if the request failed, so C<open> is frequently used in connection with
2177C<die>. Even if C<die> won't do what you want (say, in a CGI script,
2178where you want to make a nicely formatted error message (but there are
5a964f20 2179modules that can help with that problem)) you should always check
fb73857a
PP
2180the return value from opening a file. The infrequent exception is when
2181working with an unopened filehandle is actually what you want to do.
2182
cb1a09d0 2183Examples:
a0d0e21e
LW
2184
2185 $ARTICLE = 100;
2186 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2187 while (<ARTICLE>) {...
2188
2189 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
fb73857a 2190 # if the open fails, output is discarded
a0d0e21e 2191
fb73857a
PP
2192 open(DBASE, '+<dbase.mine') # open for update
2193 or die "Can't open 'dbase.mine' for update: $!";
cb1a09d0 2194
fb73857a
PP
2195 open(ARTICLE, "caesar <$article |") # decrypt article
2196 or die "Can't start caesar: $!";
a0d0e21e 2197
fb73857a
PP
2198 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
2199 or die "Can't start sort: $!";
a0d0e21e
LW
2200
2201 # process argument list of files along with any includes
2202
2203 foreach $file (@ARGV) {
2204 process($file, 'fh00');
2205 }
2206
2207 sub process {
5a964f20 2208 my($filename, $input) = @_;
a0d0e21e
LW
2209 $input++; # this is a string increment
2210 unless (open($input, $filename)) {
2211 print STDERR "Can't open $filename: $!\n";
2212 return;
2213 }
2214
5a964f20 2215 local $_;
a0d0e21e
LW
2216 while (<$input>) { # note use of indirection
2217 if (/^#include "(.*)"/) {
2218 process($1, $input);
2219 next;
2220 }
5a964f20 2221 #... # whatever
a0d0e21e
LW
2222 }
2223 }
2224
2225You may also, in the Bourne shell tradition, specify an EXPR beginning
184e9718 2226with "E<gt>&", in which case the rest of the string is interpreted as the
5a964f20 2227name of a filehandle (or file descriptor, if numeric) to be
184e9718 2228duped and opened. You may use & after E<gt>, E<gt>E<gt>, E<lt>, +E<gt>,
5f05dabc 2229+E<gt>E<gt>, and +E<lt>. The
a0d0e21e 2230mode you specify should match the mode of the original filehandle.
184e9718 2231(Duping a filehandle does not take into account any existing contents of
cb1a09d0 2232stdio buffers.)
a0d0e21e
LW
2233Here is a script that saves, redirects, and restores STDOUT and
2234STDERR:
2235
2236 #!/usr/bin/perl
5a964f20
TC
2237 open(OLDOUT, ">&STDOUT");
2238 open(OLDERR, ">&STDERR");
a0d0e21e
LW
2239
2240 open(STDOUT, ">foo.out") || die "Can't redirect stdout";
2241 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
2242
2243 select(STDERR); $| = 1; # make unbuffered
2244 select(STDOUT); $| = 1; # make unbuffered
2245
2246 print STDOUT "stdout 1\n"; # this works for
2247 print STDERR "stderr 1\n"; # subprocesses too
2248
2249 close(STDOUT);
2250 close(STDERR);
2251
5a964f20
TC
2252 open(STDOUT, ">&OLDOUT");
2253 open(STDERR, ">&OLDERR");
a0d0e21e
LW
2254
2255 print STDOUT "stdout 2\n";
2256 print STDERR "stderr 2\n";
2257
2258
184e9718 2259If you specify "E<lt>&=N", where N is a number, then Perl will do an
4633a7c4
LW
2260equivalent of C's fdopen() of that file descriptor; this is more
2261parsimonious of file descriptors. For example:
a0d0e21e
LW
2262
2263 open(FILEHANDLE, "<&=$fd")
2264
5f05dabc 2265If you open a pipe on the command "-", i.e., either "|-" or "-|", then
a0d0e21e
LW
2266there is an implicit fork done, and the return value of open is the pid
2267of the child within the parent process, and 0 within the child
184e9718 2268process. (Use C<defined($pid)> to determine whether the open was successful.)
a0d0e21e
LW
2269The filehandle behaves normally for the parent, but i/o to that
2270filehandle is piped from/to the STDOUT/STDIN of the child process.
2271In the child process the filehandle isn't opened--i/o happens from/to
2272the new STDOUT or STDIN. Typically this is used like the normal
2273piped open when you want to exercise more control over just how the
2274pipe command gets executed, such as when you are running setuid, and
54310121 2275don't want to have to scan shell commands for metacharacters.
4633a7c4 2276The following pairs are more or less equivalent:
a0d0e21e
LW
2277
2278 open(FOO, "|tr '[a-z]' '[A-Z]'");
2279 open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
2280
2281 open(FOO, "cat -n '$file'|");
2282 open(FOO, "-|") || exec 'cat', '-n', $file;
2283
4633a7c4
LW
2284See L<perlipc/"Safe Pipe Opens"> for more examples of this.
2285
5a964f20 2286NOTE: On any operation that may do a fork, any unflushed buffers remain
184e9718 2287unflushed in both processes, which means you may need to set C<$|> to
a0d0e21e
LW
2288avoid duplicate output.
2289
0dccf244
CS
2290Closing any piped filehandle causes the parent process to wait for the
2291child to finish, and returns the status value in C<$?>.
2292
5a964f20
TC
2293The filename passed to open will have leading and trailing
2294whitespace deleted, and the normal redirection chararacters
2295honored. This property, known as "magic open",
2296can often be used to good effect. A user could specify a filename of
2297"rsh cat file |", or you could change certain filenames as needed:
2298
2299 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
2300 open(FH, $filename) or die "Can't open $filename: $!";
2301
2302However, to open a file with arbitrary weird characters in it, it's
2303necessary to protect any leading and trailing whitespace:
2304
2305 $file =~ s#^(\s)#./$1#;
2306 open(FOO, "< $file\0");
2307
2308If you want a "real" C open() (see L<open(2)> on your system), then you
2309should use the sysopen() function, which involves no such magic. This is
2310another way to protect your filenames from interpretation. For example:
2311
2312 use IO::Handle;
2313 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
2314 or die "sysopen $path: $!";
2315 $oldfh = select(HANDLE); $| = 1; select($oldfh);
2316 print HANDLE "stuff $$\n");
2317 seek(HANDLE, 0, 0);
2318 print "File contains: ", <HANDLE>;
2319
5f05dabc 2320Using the constructor from the IO::Handle package (or one of its
5a964f20
TC
2321subclasses, such as IO::File or IO::Socket), you can generate anonymous
2322filehandles that have the scope of whatever variables hold references to
2323them, and automatically close whenever and however you leave that scope:
c07a80fd 2324
5f05dabc 2325 use IO::File;
5a964f20 2326 #...
c07a80fd
PP
2327 sub read_myfile_munged {
2328 my $ALL = shift;
5f05dabc 2329 my $handle = new IO::File;
c07a80fd
PP
2330 open($handle, "myfile") or die "myfile: $!";
2331 $first = <$handle>
2332 or return (); # Automatically closed here.
2333 mung $first or die "mung failed"; # Or here.
2334 return $first, <$handle> if $ALL; # Or here.
2335 $first; # Or here.
2336 }
2337
cb1a09d0 2338See L</seek()> for some details about mixing reading and writing.
a0d0e21e
LW
2339
2340=item opendir DIRHANDLE,EXPR
2341
2342Opens a directory named EXPR for processing by readdir(), telldir(),
5f05dabc 2343seekdir(), rewinddir(), and closedir(). Returns TRUE if successful.
a0d0e21e
LW
2344DIRHANDLEs have their own namespace separate from FILEHANDLEs.
2345
2346=item ord EXPR
2347
54310121 2348=item ord
bbce6d69 2349
a0d0e21e 2350Returns the numeric ascii value of the first character of EXPR. If
dc848c6f 2351EXPR is omitted, uses $_. For the reverse, see L</chr>.
a0d0e21e
LW
2352
2353=item pack TEMPLATE,LIST
2354
2355Takes an array or list of values and packs it into a binary structure,
2356returning the string containing the structure. The TEMPLATE is a
2357sequence of characters that give the order and type of values, as
2358follows:
2359
2360 A An ascii string, will be space padded.
2361 a An ascii string, will be null padded.
2362 b A bit string (ascending bit order, like vec()).
2363 B A bit string (descending bit order).
2364 h A hex string (low nybble first).
2365 H A hex string (high nybble first).
2366
2367 c A signed char value.
2368 C An unsigned char value.
96e4d5b1 2369
a0d0e21e
LW
2370 s A signed short value.
2371 S An unsigned short value.
96e4d5b1
PP
2372 (This 'short' is _exactly_ 16 bits, which may differ from
2373 what a local C compiler calls 'short'.)
2374
a0d0e21e
LW
2375 i A signed integer value.
2376 I An unsigned integer value.
96e4d5b1
PP
2377 (This 'integer' is _at_least_ 32 bits wide. Its exact size
2378 depends on what a local C compiler calls 'int', and may
2379 even be larger than the 'long' described in the next item.)
2380
a0d0e21e
LW
2381 l A signed long value.
2382 L An unsigned long value.
96e4d5b1
PP
2383 (This 'long' is _exactly_ 32 bits, which may differ from
2384 what a local C compiler calls 'long'.)
a0d0e21e 2385
96e4d5b1
PP
2386 n A short in "network" (big-endian) order.
2387 N A long in "network" (big-endian) order.
a0d0e21e
LW
2388 v A short in "VAX" (little-endian) order.
2389 V A long in "VAX" (little-endian) order.
96e4d5b1
PP
2390 (These 'shorts' and 'longs' are _exactly_ 16 bits and
2391 _exactly_ 32 bits, respectively.)
a0d0e21e
LW
2392
2393 f A single-precision float in the native format.
2394 d A double-precision float in the native format.
2395
2396 p A pointer to a null-terminated string.
2397 P A pointer to a structure (fixed-length string).
2398
2399 u A uuencoded string.
2400
96e4d5b1
PP
2401 w A BER compressed integer. Its bytes represent an unsigned
2402 integer in base 128, most significant digit first, with as few
2403 digits as possible. Bit eight (the high bit) is set on each
2404 byte except the last.
def98dd4 2405
a0d0e21e
LW
2406 x A null byte.
2407 X Back up a byte.
2408 @ Null fill to absolute position.
2409
5a964f20 2410Each letter may optionally be followed by a number giving a repeat
5f05dabc 2411count. With all types except "a", "A", "b", "B", "h", "H", and "P" the
a0d0e21e
LW
2412pack function will gobble up that many values from the LIST. A * for the
2413repeat count means to use however many items are left. The "a" and "A"
2414types gobble just one value, but pack it as a string of length count,
2415padding with nulls or spaces as necessary. (When unpacking, "A" strips
2416trailing spaces and nulls, but "a" does not.) Likewise, the "b" and "B"
2417fields pack a string that many bits long. The "h" and "H" fields pack a
84902520
TB
2418string that many nybbles long. The "p" type packs a pointer to a null-
2419terminated string. You are responsible for ensuring the string is not a
2420temporary value (which can potentially get deallocated before you get
2421around to using the packed result). The "P" packs a pointer to a structure
61167c6f
PM
2422of the size indicated by the length. A NULL pointer is created if the
2423corresponding value for "p" or "P" is C<undef>.
2424Real numbers (floats and doubles) are
a0d0e21e
LW
2425in the native machine format only; due to the multiplicity of floating
2426formats around, and the lack of a standard "network" representation, no
2427facility for interchange has been made. This means that packed floating
2428point data written on one machine may not be readable on another - even if
2429both use IEEE floating point arithmetic (as the endian-ness of the memory
2430representation is not part of the IEEE spec). Note that Perl uses doubles
2431internally for all numeric calculation, and converting from double into
5f05dabc 2432float and thence back to double again will lose precision (i.e.,
a0d0e21e
LW
2433C<unpack("f", pack("f", $foo)>) will not in general equal $foo).
2434
2435Examples:
2436
2437 $foo = pack("cccc",65,66,67,68);
2438 # foo eq "ABCD"
2439 $foo = pack("c4",65,66,67,68);
2440 # same thing
2441
2442 $foo = pack("ccxxcc",65,66,67,68);
2443 # foo eq "AB\0\0CD"
2444
2445 $foo = pack("s2",1,2);
2446 # "\1\0\2\0" on little-endian
2447 # "\0\1\0\2" on big-endian
2448
2449 $foo = pack("a4","abcd","x","y","z");
2450 # "abcd"
2451
2452 $foo = pack("aaaa","abcd","x","y","z");
2453 # "axyz"
2454
2455 $foo = pack("a14","abcdefg");
2456 # "abcdefg\0\0\0\0\0\0\0"
2457
2458 $foo = pack("i9pl", gmtime);
2459 # a real struct tm (on my system anyway)
2460
2461 sub bintodec {
2462 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
2463 }
2464
2465The same template may generally also be used in the unpack function.
2466
5a964f20
TC
2467=item package
2468
cb1a09d0
AD
2469=item package NAMESPACE
2470
2471Declares the compilation unit as being in the given namespace. The scope
2472of the package declaration is from the declaration itself through the end of
2473the enclosing block (the same scope as the local() operator). All further
2474unqualified dynamic identifiers will be in this namespace. A package
5f05dabc 2475statement affects only dynamic variables--including those you've used
cb1a09d0
AD
2476local() on--but I<not> lexical variables created with my(). Typically it
2477would be the first declaration in a file to be included by the C<require>
2478or C<use> operator. You can switch into a package in more than one place;
5a964f20 2479it merely influences which symbol table is used by the compiler for the
cb1a09d0
AD
2480rest of that block. You can refer to variables and filehandles in other
2481packages by prefixing the identifier with the package name and a double
2482colon: C<$Package::Variable>. If the package name is null, the C<main>
2483package as assumed. That is, C<$::sail> is equivalent to C<$main::sail>.
2484
5a964f20
TC
2485If NAMESPACE is omitted, then there is no current package, and all
2486identifiers must be fully qualified or lexicals. This is stricter
2487than C<use strict>, since it also extends to function names.
2488
cb1a09d0
AD
2489See L<perlmod/"Packages"> for more information about packages, modules,
2490and classes. See L<perlsub> for other scoping issues.
2491
a0d0e21e
LW
2492=item pipe READHANDLE,WRITEHANDLE
2493
2494Opens a pair of connected pipes like the corresponding system call.
2495Note that if you set up a loop of piped processes, deadlock can occur
2496unless you are very careful. In addition, note that Perl's pipes use
184e9718 2497stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE
a0d0e21e
LW
2498after each command, depending on the application.
2499
7e1af8bc 2500See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
4633a7c4
LW
2501for examples of such things.
2502
a0d0e21e
LW
2503=item pop ARRAY
2504
54310121 2505=item pop
28757baa 2506
a0d0e21e
LW
2507Pops and returns the last value of the array, shortening the array by
25081. Has a similar effect to
2509
2510 $tmp = $ARRAY[$#ARRAY--];
2511
2512If there are no elements in the array, returns the undefined value.
cb1a09d0
AD
2513If ARRAY is omitted, pops the
2514@ARGV array in the main program, and the @_ array in subroutines, just
2515like shift().
a0d0e21e
LW
2516
2517=item pos SCALAR
2518
54310121 2519=item pos
bbce6d69 2520
4633a7c4 2521Returns the offset of where the last C<m//g> search left off for the variable
2f9daede 2522is in question ($_ is used when the variable is not specified). May be
44a8e56a
PP
2523modified to change that offset. Such modification will also influence
2524the C<\G> zero-width assertion in regular expressions. See L<perlre> and
2525L<perlop>.
a0d0e21e
LW
2526
2527=item print FILEHANDLE LIST
2528
2529=item print LIST
2530
2531=item print
2532
cb1a09d0 2533Prints a string or a comma-separated list of strings. Returns TRUE
a0d0e21e 2534if successful. FILEHANDLE may be a scalar variable name, in which case
cb1a09d0 2535the variable contains the name of or a reference to the filehandle, thus introducing one
a0d0e21e
LW
2536level of indirection. (NOTE: If FILEHANDLE is a variable and the next
2537token is a term, it may be misinterpreted as an operator unless you
5f05dabc 2538interpose a + or put parentheses around the arguments.) If FILEHANDLE is
a0d0e21e 2539omitted, prints by default to standard output (or to the last selected
da0045b7 2540output channel--see L</select>). If LIST is also omitted, prints $_ to
5a964f20 2541the currently selected output channel. To set the default output channel to something other than
a0d0e21e 2542STDOUT use the select operation. Note that, because print takes a
5a964f20 2543LIST, anything in the LIST is evaluated in list context, and any
a0d0e21e 2544subroutine that you call will have one or more of its expressions
5a964f20 2545evaluated in list context. Also be careful not to follow the print
a0d0e21e
LW
2546keyword with a left parenthesis unless you want the corresponding right
2547parenthesis to terminate the arguments to the print--interpose a + or
5f05dabc 2548put parentheses around all the arguments.
a0d0e21e 2549
4633a7c4 2550Note that if you're storing FILEHANDLES in an array or other expression,
da0045b7 2551you will have to use a block returning its value instead:
4633a7c4
LW
2552
2553 print { $files[$i] } "stuff\n";
2554 print { $OK ? STDOUT : STDERR } "stuff\n";
2555
5f05dabc 2556=item printf FILEHANDLE FORMAT, LIST
a0d0e21e 2557
5f05dabc 2558=item printf FORMAT, LIST
a0d0e21e 2559
a3cb178b
GS
2560Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that $\
2561(the output record separator) is not appended. The first argument
a034a98d
DD
2562of the list will be interpreted as the printf format. If C<use locale> is
2563in effect, the character used for the decimal point in formatted real numbers
2564is affected by the LC_NUMERIC locale. See L<perllocale>.
a0d0e21e 2565
28757baa 2566Don't fall into the trap of using a printf() when a simple
5a964f20 2567print() would do. The print() is more efficient and less
28757baa
PP
2568error prone.
2569
da0045b7
PP
2570=item prototype FUNCTION
2571
2572Returns the prototype of a function as a string (or C<undef> if the
5f05dabc
PP
2573function has no prototype). FUNCTION is a reference to, or the name of,
2574the function whose prototype you want to retrieve.
da0045b7 2575
b6c543e3
IZ
2576If FUNCTION is a string starting with C<CORE::>, the rest is taken as
2577a name for Perl builtin. If builtin is not I<overridable> (such as
2578C<qw>) or its arguments cannot be expressed by a prototype (such as
2579C<system>) - in other words, the builtin does not behave like a Perl
2580function - returns C<undef>. Otherwise, the string describing the
2581equivalent prototype is returned.
2582
a0d0e21e
LW
2583=item push ARRAY,LIST
2584
2585Treats ARRAY as a stack, and pushes the values of LIST
2586onto the end of ARRAY. The length of ARRAY increases by the length of
2587LIST. Has the same effect as
2588
2589 for $value (LIST) {
2590 $ARRAY[++$#ARRAY] = $value;
2591 }
2592
2593but is more efficient. Returns the new number of elements in the array.
2594
2595=item q/STRING/
2596
2597=item qq/STRING/
2598
2599=item qx/STRING/
2600
2601=item qw/STRING/
2602
2603Generalized quotes. See L<perlop>.
2604
2605=item quotemeta EXPR
2606
54310121 2607=item quotemeta
bbce6d69 2608
68dc0745 2609Returns the value of EXPR with all non-alphanumeric
a034a98d
DD
2610characters backslashed. (That is, all characters not matching
2611C</[A-Za-z_0-9]/> will be preceded by a backslash in the
2612returned string, regardless of any locale settings.)
2613This is the internal function implementing
a0d0e21e
LW
2614the \Q escape in double-quoted strings.
2615
bbce6d69
PP
2616If EXPR is omitted, uses $_.
2617
a0d0e21e
LW
2618=item rand EXPR
2619
2620=item rand
2621
3e3baf6d
TB
2622Returns a random fractional number greater than or equal to 0 and less
2623than the value of EXPR. (EXPR should be positive.) If EXPR is
2624omitted, the value 1 is used. Automatically calls srand() unless
2625srand() has already been called. See also srand().
a0d0e21e 2626
2f9daede 2627(Note: If your rand function consistently returns numbers that are too
a0d0e21e 2628large or too small, then your version of Perl was probably compiled
2f9daede 2629with the wrong number of RANDBITS.)
a0d0e21e
LW
2630
2631=item read FILEHANDLE,SCALAR,LENGTH,OFFSET
2632
2633=item read FILEHANDLE,SCALAR,LENGTH
2634
2635Attempts to read LENGTH bytes of data into variable SCALAR from the
3b02c43c
GS
2636specified FILEHANDLE. Returns the number of bytes actually read,
2637C<0> at end of file, or undef if there was an error. SCALAR will be grown
2638or shrunk to the length actually read. An OFFSET may be specified to
2639place the read data at some other place than the beginning of the
2640string. This call is actually implemented in terms of stdio's fread(3)
2641call. To get a true read(2) system call, see sysread().
a0d0e21e
LW
2642
2643=item readdir DIRHANDLE
2644
2645Returns the next directory entry for a directory opened by opendir().
5a964f20 2646If used in list context, returns all the rest of the entries in the
a0d0e21e 2647directory. If there are no more entries, returns an undefined value in
5a964f20 2648scalar context or a null list in list context.
a0d0e21e 2649
cb1a09d0 2650If you're planning to filetest the return values out of a readdir(), you'd
5f05dabc 2651better prepend the directory in question. Otherwise, because we didn't
cb1a09d0
AD
2652chdir() there, it would have been testing the wrong file.
2653
2654 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
2655 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
2656 closedir DIR;
2657
84902520
TB
2658=item readline EXPR
2659
5a964f20 2660Reads from the filehandle whose typeglob is contained in EXPR. In scalar context, a single line
84902520
TB
2661is read and returned. In list context, reads until end-of-file is
2662reached and returns a list of lines (however you've defined lines
2663with $/ or $INPUT_RECORD_SEPARATOR).
2664This is the internal function implementing the C<E<lt>EXPRE<gt>>
2665operator, but you can use it directly. The C<E<lt>EXPRE<gt>>
2666operator is discussed in more detail in L<perlop/"I/O Operators">.
2667
5a964f20
TC
2668 $line = <STDIN>;
2669 $line = readline(*STDIN); # same thing
2670
a0d0e21e
LW
2671=item readlink EXPR
2672
54310121 2673=item readlink
bbce6d69 2674
a0d0e21e
LW
2675Returns the value of a symbolic link, if symbolic links are
2676implemented. If not, gives a fatal error. If there is some system
184e9718 2677error, returns the undefined value and sets C<$!> (errno). If EXPR is
a0d0e21e
LW
2678omitted, uses $_.
2679
84902520
TB
2680=item readpipe EXPR
2681
5a964f20 2682EXPR is executed as a system command.
84902520
TB
2683The collected standard output of the command is returned.
2684In scalar context, it comes back as a single (potentially
2685multi-line) string. In list context, returns a list of lines
2686(however you've defined lines with $/ or $INPUT_RECORD_SEPARATOR).
2687This is the internal function implementing the C<qx/EXPR/>
2688operator, but you can use it directly. The C<qx/EXPR/>
2689operator is discussed in more detail in L<perlop/"I/O Operators">.
2690
a0d0e21e
LW
2691=item recv SOCKET,SCALAR,LEN,FLAGS
2692
2693Receives a message on a socket. Attempts to receive LENGTH bytes of
2694data into variable SCALAR from the specified SOCKET filehandle.
a3cb178b 2695Actually does a C recvfrom(), so that it can return the address of the
a0d0e21e
LW
2696sender. Returns the undefined value if there's an error. SCALAR will
2697be grown or shrunk to the length actually read. Takes the same flags
54310121 2698as the system call of the same name.
4633a7c4 2699See L<perlipc/"UDP: Message Passing"> for examples.
a0d0e21e
LW
2700
2701=item redo LABEL
2702
2703=item redo
2704
2705The C<redo> command restarts the loop block without evaluating the
2706conditional again. The C<continue> block, if any, is not executed. If
2707the LABEL is omitted, the command refers to the innermost enclosing
2708loop. This command is normally used by programs that want to lie to
2709themselves about what was just input:
2710
2711 # a simpleminded Pascal comment stripper
2712 # (warning: assumes no { or } in strings)
4633a7c4 2713 LINE: while (<STDIN>) {
a0d0e21e
LW
2714 while (s|({.*}.*){.*}|$1 |) {}
2715 s|{.*}| |;
2716 if (s|{.*| |) {
2717 $front = $_;
2718 while (<STDIN>) {
2719 if (/}/) { # end of comment?
5a964f20 2720 s|^|$front\{|;
4633a7c4 2721 redo LINE;
a0d0e21e
LW
2722 }
2723 }
2724 }
2725 print;
2726 }
2727
1d2dff63
GS
2728See also L</continue> for an illustration of how C<last>, C<next>, and
2729C<redo> work.
2730
a0d0e21e
LW
2731=item ref EXPR
2732
54310121 2733=item ref
bbce6d69 2734
2f9daede
TPG
2735Returns a TRUE value if EXPR is a reference, FALSE otherwise. If EXPR
2736is not specified, $_ will be used. The value returned depends on the
bbce6d69 2737type of thing the reference is a reference to.
a0d0e21e
LW
2738Builtin types include:
2739
2740 REF
2741 SCALAR
2742 ARRAY
2743 HASH
2744 CODE
2745 GLOB
2746
54310121 2747If the referenced object has been blessed into a package, then that package
a0d0e21e
LW
2748name is returned instead. You can think of ref() as a typeof() operator.
2749
2750 if (ref($r) eq "HASH") {
aa689395 2751 print "r is a reference to a hash.\n";
54310121 2752 }
5a964f20 2753 if (!ref($r)) {
a0d0e21e 2754 print "r is not a reference at all.\n";
54310121 2755 }
a0d0e21e
LW
2756
2757See also L<perlref>.
2758
2759=item rename OLDNAME,NEWNAME
2760
2761Changes the name of a file. Returns 1 for success, 0 otherwise. Will
5f05dabc 2762not work across file system boundaries.
a0d0e21e
LW
2763
2764=item require EXPR
2765
2766=item require
2767
2768Demands some semantics specified by EXPR, or by $_ if EXPR is not
2769supplied. If EXPR is numeric, demands that the current version of Perl
184e9718 2770(C<$]> or $PERL_VERSION) be equal or greater than EXPR.
a0d0e21e
LW
2771
2772Otherwise, demands that a library file be included if it hasn't already
2773been included. The file is included via the do-FILE mechanism, which is
2774essentially just a variety of eval(). Has semantics similar to the following
2775subroutine:
2776
2777 sub require {
5a964f20 2778 my($filename) = @_;
a0d0e21e 2779 return 1 if $INC{$filename};
5a964f20 2780 my($realfilename,$result);
a0d0e21e
LW
2781 ITER: {
2782 foreach $prefix (@INC) {
2783 $realfilename = "$prefix/$filename";
2784 if (-f $realfilename) {
2785 $result = do $realfilename;
2786 last ITER;
2787 }
2788 }
2789 die "Can't find $filename in \@INC";
2790 }
2791 die $@ if $@;
2792 die "$filename did not return true value" unless $result;
2793 $INC{$filename} = $realfilename;
5a964f20 2794 return $result;
a0d0e21e
LW
2795 }
2796
2797Note that the file will not be included twice under the same specified
2798name. The file must return TRUE as the last statement to indicate
2799successful execution of any initialization code, so it's customary to
2800end such a file with "1;" unless you're sure it'll return TRUE
2801otherwise. But it's better just to put the "C<1;>", in case you add more
2802statements.
2803
54310121 2804If EXPR is a bareword, the require assumes a "F<.pm>" extension and
da0045b7 2805replaces "F<::>" with "F</>" in the filename for you,
54310121 2806to make it easy to load standard modules. This form of loading of
a0d0e21e
LW
2807modules does not risk altering your namespace.
2808
ee580363
GS
2809In other words, if you try this:
2810
5a964f20 2811 require Foo::Bar; # a splendid bareword
ee580363
GS
2812
2813The require function will actually look for the "Foo/Bar.pm" file in the
2814directories specified in the @INC array.
2815
5a964f20 2816But if you try this:
ee580363
GS
2817
2818 $class = 'Foo::Bar';
5a964f20
TC
2819 require $class; # $class is not a bareword
2820 #or
2821 require "Foo::Bar"; # not a bareword because of the ""
ee580363
GS
2822
2823The require function will look for the "Foo::Bar" file in the @INC array and
5a964f20 2824will complain about not finding "Foo::Bar" there. In this case you can do:
ee580363
GS
2825
2826 eval "require $class";
2827
2828For a yet-more-powerful import facility, see L</use> and L<perlmod>.
a0d0e21e
LW
2829
2830=item reset EXPR
2831
2832=item reset
2833
2834Generally used in a C<continue> block at the end of a loop to clear
2835variables and reset ?? searches so that they work again. The
2836expression is interpreted as a list of single characters (hyphens
2837allowed for ranges). All variables and arrays beginning with one of
2838those letters are reset to their pristine state. If the expression is
5f05dabc
PP
2839omitted, one-match searches (?pattern?) are reset to match again. Resets
2840only variables or searches in the current package. Always returns
a0d0e21e
LW
28411. Examples:
2842
2843 reset 'X'; # reset all X variables
2844 reset 'a-z'; # reset lower case variables
2845 reset; # just reset ?? searches
2846
5f05dabc
PP
2847Resetting "A-Z" is not recommended because you'll wipe out your
2848ARGV and ENV arrays. Resets only package variables--lexical variables
a0d0e21e 2849are unaffected, but they clean themselves up on scope exit anyway,
da0045b7 2850so you'll probably want to use them instead. See L</my>.
a0d0e21e 2851
54310121
PP
2852=item return EXPR
2853
2854=item return
2855
5a964f20
TC
2856Returns from a subroutine, eval(), or C<do FILE> with the value
2857given in EXPR. Evaluation of EXPR may be in list, scalar, or void
54310121
PP
2858context, depending on how the return value will be used, and the context
2859may vary from one execution to the next (see wantarray()). If no EXPR
5a964f20
TC
2860is given, returns an empty list in list context, an undefined value in
2861scalar context, or nothing in a void context.
a0d0e21e 2862
68dc0745
PP
2863(Note that in the absence of a return, a subroutine, eval, or do FILE
2864will automatically return the value of the last expression evaluated.)
a0d0e21e
LW
2865
2866=item reverse LIST
2867
5a964f20
TC
2868In list context, returns a list value consisting of the elements
2869of LIST in the opposite order. In scalar context, concatenates the
2f9daede
TPG
2870elements of LIST, and returns a string value consisting of those bytes,
2871but in the opposite order.
4633a7c4 2872
2f9daede 2873 print reverse <>; # line tac, last line first
4633a7c4 2874
2f9daede
TPG
2875 undef $/; # for efficiency of <>
2876 print scalar reverse <>; # byte tac, last line tsrif
2877
2878This operator is also handy for inverting a hash, although there are some
2879caveats. If a value is duplicated in the original hash, only one of those
2880can be represented as a key in the inverted hash. Also, this has to
2881unwind one hash and build a whole new one, which may take some time
2882on a large hash.
2883
2884 %by_name = reverse %by_address; # Invert the hash
a0d0e21e
LW
2885
2886=item rewinddir DIRHANDLE
2887
2888Sets the current position to the beginning of the directory for the
2889readdir() routine on DIRHANDLE.
2890
2891=item rindex STR,SUBSTR,POSITION
2892
2893=item rindex STR,SUBSTR
2894
2895Works just like index except that it returns the position of the LAST
2896occurrence of SUBSTR in STR. If POSITION is specified, returns the
2897last occurrence at or before that position.
2898
2899=item rmdir FILENAME
2900
54310121 2901=item rmdir
bbce6d69 2902
5a964f20
TC
2903Deletes the directory specified by FILENAME if that directory is empty. If it
2904succeeds it returns TRUE, otherwise it returns FALSE and sets C<$!> (errno). If
a0d0e21e
LW
2905FILENAME is omitted, uses $_.
2906
2907=item s///
2908
2909The substitution operator. See L<perlop>.
2910
2911=item scalar EXPR
2912
5a964f20 2913Forces EXPR to be interpreted in scalar context and returns the value
54310121 2914of EXPR.
cb1a09d0
AD
2915
2916 @counts = ( scalar @a, scalar @b, scalar @c );
2917
54310121 2918There is no equivalent operator to force an expression to
5a964f20 2919be interpolated in list context because it's in practice never
cb1a09d0
AD
2920needed. If you really wanted to do so, however, you could use
2921the construction C<@{[ (some expression) ]}>, but usually a simple
2922C<(some expression)> suffices.
a0d0e21e
LW
2923
2924=item seek FILEHANDLE,POSITION,WHENCE
2925
8903cb82
PP
2926Sets FILEHANDLE's position, just like the fseek() call of stdio.
2927FILEHANDLE may be an expression whose value gives the name of the
2928filehandle. The values for WHENCE are 0 to set the new position to
2929POSITION, 1 to set it to the current position plus POSITION, and 2 to
2930set it to EOF plus POSITION (typically negative). For WHENCE you may
2931use the constants SEEK_SET, SEEK_CUR, and SEEK_END from either the
2932IO::Seekable or the POSIX module. Returns 1 upon success, 0 otherwise.
2933
2934If you want to position file for sysread() or syswrite(), don't use
2935seek() -- buffering makes its effect on the file's system position
137443ea 2936unpredictable and non-portable. Use sysseek() instead.
a0d0e21e 2937
cb1a09d0
AD
2938On some systems you have to do a seek whenever you switch between reading
2939and writing. Amongst other things, this may have the effect of calling
8903cb82
PP
2940stdio's clearerr(3). A WHENCE of 1 (SEEK_CUR) is useful for not moving
2941the file position:
cb1a09d0
AD
2942
2943 seek(TEST,0,1);
2944
2945This is also useful for applications emulating C<tail -f>. Once you hit
2946EOF on your read, and then sleep for a while, you might have to stick in a
8903cb82
PP
2947seek() to reset things. The seek() doesn't change the current position,
2948but it I<does> clear the end-of-file condition on the handle, so that the
2949next C<E<lt>FILEE<gt>> makes Perl try again to read something. We hope.
cb1a09d0
AD
2950
2951If that doesn't work (some stdios are particularly cantankerous), then
2952you may need something more like this:
2953
2954 for (;;) {
2955 for ($curpos = tell(FILE); $_ = <FILE>; $curpos = tell(FILE)) {
2956 # search for some stuff and put it into files
2957 }
2958 sleep($for_a_while);
2959 seek(FILE, $curpos, 0);
2960 }
2961
a0d0e21e
LW
2962=item seekdir DIRHANDLE,POS
2963
2964Sets the current position for the readdir() routine on DIRHANDLE. POS
2965must be a value returned by telldir(). Has the same caveats about
2966possible directory compaction as the corresponding system library
2967routine.
2968
2969=item select FILEHANDLE
2970
2971=item select
2972
2973Returns the currently selected filehandle. Sets the current default
2974filehandle for output, if FILEHANDLE is supplied. This has two
2975effects: first, a C<write> or a C<print> without a filehandle will
2976default to this FILEHANDLE. Second, references to variables related to
2977output will refer to this output channel. For example, if you have to
2978set the top of form format for more than one output channel, you might
2979do the following:
2980
2981 select(REPORT1);
2982 $^ = 'report1_top';
2983 select(REPORT2);
2984 $^ = 'report2_top';
2985
2986FILEHANDLE may be an expression whose value gives the name of the
2987actual filehandle. Thus:
2988
2989 $oldfh = select(STDERR); $| = 1; select($oldfh);
2990
4633a7c4
LW
2991Some programmers may prefer to think of filehandles as objects with
2992methods, preferring to write the last example as:
a0d0e21e 2993
28757baa 2994 use IO::Handle;
a0d0e21e
LW
2995 STDERR->autoflush(1);
2996
2997=item select RBITS,WBITS,EBITS,TIMEOUT
2998
5f05dabc 2999This calls the select(2) system call with the bit masks specified, which
a0d0e21e
LW
3000can be constructed using fileno() and vec(), along these lines:
3001
3002 $rin = $win = $ein = '';
3003 vec($rin,fileno(STDIN),1) = 1;
3004 vec($win,fileno(STDOUT),1) = 1;
3005 $ein = $rin | $win;
3006
3007If you want to select on many filehandles you might wish to write a
3008subroutine:
3009
3010 sub fhbits {
5a964f20
TC
3011 my(@fhlist) = split(' ',$_[0]);
3012 my($bits);
a0d0e21e
LW
3013 for (@fhlist) {
3014 vec($bits,fileno($_),1) = 1;
3015 }
3016 $bits;
3017 }
4633a7c4 3018 $rin = fhbits('STDIN TTY SOCK');
a0d0e21e
LW
3019
3020The usual idiom is:
3021
3022 ($nfound,$timeleft) =
3023 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
3024
54310121 3025or to block until something becomes ready just do this
a0d0e21e
LW
3026
3027 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
3028
5f05dabc 3029Most systems do not bother to return anything useful in $timeleft, so
5a964f20 3030calling select() in scalar context just returns $nfound.
c07a80fd 3031
5f05dabc 3032Any of the bit masks can also be undef. The timeout, if specified, is
a0d0e21e
LW
3033in seconds, which may be fractional. Note: not all implementations are
3034capable of returning the $timeleft. If not, they always return
3035$timeleft equal to the supplied $timeout.
3036
ff68c719 3037You can effect a sleep of 250 milliseconds this way:
a0d0e21e
LW
3038
3039 select(undef, undef, undef, 0.25);
3040
5a964f20
TC
3041B<WARNING>: One should not attempt to mix buffered I/O (like read()
3042or E<lt>FHE<gt>) with select(), except as permitted by POSIX, and even
3043then only on POSIX systems. You have to use sysread() instead.
a0d0e21e
LW
3044
3045=item semctl ID,SEMNUM,CMD,ARG
3046
0ade1984
JH
3047Calls the System V IPC function semctl. You'll probably have to say
3048
3049 use IPC::SysV;
3050
3051first to get the correct constant definitions. If CMD is IPC_STAT or
3052GETALL, then ARG must be a variable which will hold the returned
a0d0e21e
LW
3053semid_ds structure or semaphore value array. Returns like ioctl: the
3054undefined value for error, "0 but true" for zero, or the actual return
0ade1984 3055value otherwise. See also IPC::SysV and IPC::Semaphore documentation.
a0d0e21e
LW
3056
3057=item semget KEY,NSEMS,FLAGS
3058
3059Calls the System V IPC function semget. Returns the semaphore id, or
0ade1984
JH
3060the undefined value if there is an error. See also IPC::SysV and
3061IPC::SysV::Semaphore documentation.
a0d0e21e
LW
3062
3063=item semop KEY,OPSTRING
3064
3065Calls the System V IPC function semop to perform semaphore operations
3066such as signaling and waiting. OPSTRING must be a packed array of
3067semop structures. Each semop structure can be generated with
3068C<pack("sss", $semnum, $semop, $semflag)>. The number of semaphore
3069operations is implied by the length of OPSTRING. Returns TRUE if
3070successful, or FALSE if there is an error. As an example, the
3071following code waits on semaphore $semnum of semaphore id $semid:
3072
3073 $semop = pack("sss", $semnum, -1, 0);
3074 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
3075
0ade1984
JH
3076To signal the semaphore, replace "-1" with "1". See also IPC::SysV
3077and IPC::SysV::Semaphore documentation.
a0d0e21e
LW
3078
3079=item send SOCKET,MSG,FLAGS,TO
3080
3081=item send SOCKET,MSG,FLAGS
3082
3083Sends a message on a socket. Takes the same flags as the system call
3084of the same name. On unconnected sockets you must specify a
3085destination to send TO, in which case it does a C sendto(). Returns
3086the number of characters sent, or the undefined value if there is an
3087error.
4633a7c4 3088See L<perlipc/"UDP: Message Passing"> for examples.
a0d0e21e
LW
3089
3090=item setpgrp PID,PGRP
3091
3092Sets the current process group for the specified PID, 0 for the current
3093process. Will produce a fatal error if used on a machine that doesn't
5f05dabc 3094implement setpgrp(2). If the arguments are omitted, it defaults to
47e29363
PP
30950,0. Note that the POSIX version of setpgrp() does not accept any
3096arguments, so only setpgrp 0,0 is portable.
a0d0e21e
LW
3097
3098=item setpriority WHICH,WHO,PRIORITY
3099
3100Sets the current priority for a process, a process group, or a user.
748a9306 3101(See setpriority(2).) Will produce a fatal error if used on a machine
a0d0e21e
LW
3102that doesn't implement setpriority(2).
3103
3104=item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
3105
3106Sets the socket option requested. Returns undefined if there is an
3107error. OPTVAL may be specified as undef if you don't want to pass an
3108argument.
3109
3110=item shift ARRAY
3111
3112=item shift
3113
3114Shifts the first value of the array off and returns it, shortening the
3115array by 1 and moving everything down. If there are no elements in the
3116array, returns the undefined value. If ARRAY is omitted, shifts the
977336f5
GS
3117@_ array within the lexical scope of subroutines and formats, and the
3118@ARGV array at file scopes or within the lexical scopes established by
3119the C<eval ''>, C<BEGIN {}>, C<END {}>, and C<INIT {}> constructs.
3120See also unshift(), push(), and pop(). Shift() and unshift() do the
3121same thing to the left end of an array that pop() and push() do to the
3122right end.
a0d0e21e
LW
3123
3124=item shmctl ID,CMD,ARG
3125
0ade1984
JH
3126Calls the System V IPC function shmctl. You'll probably have to say
3127
3128 use IPC::SysV;
3129
3130first to get the correct constant definitions. If CMD is IPC_STAT,
3131then ARG must be a variable which will hold the returned shmid_ds
3132structure. Returns like ioctl: the undefined value for error, "0 but
3133true" for zero, or the actual return value otherwise.
3134See also IPC::SysV documentation.
a0d0e21e
LW
3135
3136=item shmget KEY,SIZE,FLAGS
3137
3138Calls the System V IPC function shmget. Returns the shared memory
3139segment id, or the undefined value if there is an error.
0ade1984 3140See also IPC::SysV documentation.
a0d0e21e
LW
3141
3142=item shmread ID,VAR,POS,SIZE
3143
3144=item shmwrite ID,STRING,POS,SIZE
3145
3146Reads or writes the System V shared memory segment ID starting at
3147position POS for size SIZE by attaching to it, copying in/out, and
5a964f20 3148detaching from it. When reading, VAR must be a variable that will
a0d0e21e
LW
3149hold the data read. When writing, if STRING is too long, only SIZE
3150bytes are used; if STRING is too short, nulls are written to fill out
3151SIZE bytes. Return TRUE if successful, or FALSE if there is an error.
0ade1984 3152See also IPC::SysV documentation.
a0d0e21e
LW
3153
3154=item shutdown SOCKET,HOW
3155
3156Shuts down a socket connection in the manner indicated by HOW, which
3157has the same interpretation as in the system call of the same name.
3158
5a964f20
TC
3159 shutdown(SOCKET, 0); # I/we have stopped reading data
3160 shutdown(SOCKET, 1); # I/we have stopped writing data
3161 shutdown(SOCKET, 2); # I/we have stopped using this socket
3162
3163This is useful with sockets when you want to tell the other
3164side you're done writing but not done reading, or vice versa.
3165It's also a more insistent form of close because it also
3166disables the filedescriptor in any forked copies in other
3167processes.
3168
a0d0e21e
LW
3169=item sin EXPR
3170
54310121 3171=item sin
bbce6d69 3172
a0d0e21e
LW
3173Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
3174returns sine of $_.
3175
54310121 3176For the inverse sine operation, you may use the POSIX::asin()
28757baa
PP
3177function, or use this relation:
3178
3179 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
3180
a0d0e21e
LW
3181=item sleep EXPR
3182
3183=item sleep
3184
3185Causes the script to sleep for EXPR seconds, or forever if no EXPR.
1d3434b8
GS
3186May be interrupted if the process receives a signal such as SIGALRM.
3187Returns the number of seconds actually slept. You probably cannot
3188mix alarm() and sleep() calls, because sleep() is often implemented
3189using alarm().
a0d0e21e
LW
3190
3191On some older systems, it may sleep up to a full second less than what
3192you requested, depending on how it counts seconds. Most modern systems
5a964f20
TC
3193always sleep the full amount. They may appear to sleep longer than that,
3194however, because your process might not be scheduled right away in a
3195busy multitasking system.
a0d0e21e 3196
cb1a09d0 3197For delays of finer granularity than one second, you may use Perl's
54310121 3198syscall() interface to access setitimer(2) if your system supports it,
7b8d334a 3199or else see L</select()> above.
cb1a09d0 3200
5f05dabc
PP
3201See also the POSIX module's sigpause() function.
3202
a0d0e21e
LW
3203=item socket SOCKET,DOMAIN,TYPE,PROTOCOL
3204
3205Opens a socket of the specified kind and attaches it to filehandle
5f05dabc 3206SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for the
a0d0e21e 3207system call of the same name. You should "use Socket;" first to get
4633a7c4 3208the proper definitions imported. See the example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
3209
3210=item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
3211
3212Creates an unnamed pair of sockets in the specified domain, of the
5f05dabc 3213specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
a0d0e21e
LW
3214for the system call of the same name. If unimplemented, yields a fatal
3215error. Returns TRUE if successful.
3216
5a964f20
TC
3217Some systems defined pipe() in terms of socketpair, in which a call
3218to C<pipe(Rdr, Wtr)> is essentially:
3219
3220 use Socket;
3221 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
3222 shutdown(Rdr, 1); # no more writing for reader
3223 shutdown(Wtr, 0); # no more reading for writer
3224
3225See L<perlipc> for an example of socketpair use.
3226
a0d0e21e
LW
3227=item sort SUBNAME LIST
3228
3229=item sort BLOCK LIST
3230
3231=item sort LIST
3232
2f9daede
TPG
3233Sorts the LIST and returns the sorted list value. If SUBNAME or BLOCK
3234is omitted, sorts in standard string comparison order. If SUBNAME is
3235specified, it gives the name of a subroutine that returns an integer
3236less than, equal to, or greater than 0, depending on how the elements
3237of the array are to be ordered. (The C<E<lt>=E<gt>> and C<cmp>
3238operators are extremely useful in such routines.) SUBNAME may be a
1d3434b8
GS
3239scalar variable name (unsubscripted), in which case the value provides
3240the name of (or a reference to) the actual subroutine to use. In place
3241of a SUBNAME, you can provide a BLOCK as an anonymous, in-line sort
3242subroutine.
a0d0e21e 3243
cb1a09d0
AD
3244In the interests of efficiency the normal calling code for subroutines is
3245bypassed, with the following effects: the subroutine may not be a
3246recursive subroutine, and the two elements to be compared are passed into
3247the subroutine not via @_ but as the package global variables $a and
3248$b (see example below). They are passed by reference, so don't
3249modify $a and $b. And don't try to declare them as lexicals either.
a0d0e21e 3250
0a753a76
PP
3251You also cannot exit out of the sort block or subroutine using any of the
3252loop control operators described in L<perlsyn> or with goto().
3253
a034a98d
DD
3254When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
3255current collation locale. See L<perllocale>.
3256
a0d0e21e
LW
3257Examples:
3258
3259 # sort lexically
3260 @articles = sort @files;
3261
3262 # same thing, but with explicit sort routine
3263 @articles = sort {$a cmp $b} @files;
3264
cb1a09d0 3265 # now case-insensitively
54310121 3266 @articles = sort {uc($a) cmp uc($b)} @files;
cb1a09d0 3267
a0d0e21e
LW
3268 # same thing in reversed order
3269 @articles = sort {$b cmp $a} @files;
3270
3271 # sort numerically ascending
3272 @articles = sort {$a <=> $b} @files;
3273
3274 # sort numerically descending
3275 @articles = sort {$b <=> $a} @files;
3276
3277 # sort using explicit subroutine name
3278 sub byage {
2f9daede 3279 $age{$a} <=> $age{$b}; # presuming numeric
a0d0e21e
LW
3280 }
3281 @sortedclass = sort byage @class;
3282
aa689395
PP
3283 # this sorts the %age hash by value instead of key
3284 # using an in-line function
c07a80fd
PP
3285 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
3286
a0d0e21e
LW
3287 sub backwards { $b cmp $a; }
3288 @harry = ('dog','cat','x','Cain','Abel');
3289 @george = ('gone','chased','yz','Punished','Axed');
3290 print sort @harry;
3291 # prints AbelCaincatdogx
3292 print sort backwards @harry;
3293 # prints xdogcatCainAbel
3294 print sort @george, 'to', @harry;
3295 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
3296
54310121
PP
3297 # inefficiently sort by descending numeric compare using
3298 # the first integer after the first = sign, or the
cb1a09d0
AD
3299 # whole record case-insensitively otherwise
3300
3301 @new = sort {
3302 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
3303 ||
3304 uc($a) cmp uc($b)
3305 } @old;
3306
3307 # same thing, but much more efficiently;
3308 # we'll build auxiliary indices instead
3309 # for speed
3310 @nums = @caps = ();
54310121 3311 for (@old) {
cb1a09d0
AD
3312 push @nums, /=(\d+)/;
3313 push @caps, uc($_);
54310121 3314 }
cb1a09d0
AD
3315
3316 @new = @old[ sort {
3317 $nums[$b] <=> $nums[$a]
3318 ||
3319 $caps[$a] cmp $caps[$b]
3320 } 0..$#old
3321 ];
3322
3323 # same thing using a Schwartzian Transform (no temps)
3324 @new = map { $_->[0] }
3325 sort { $b->[1] <=> $a->[1]
3326 ||
3327 $a->[2] cmp $b->[2]
3328 } map { [$_, /=(\d+)/, uc($_)] } @old;
3329
184e9718 3330If you're using strict, you I<MUST NOT> declare $a
cb1a09d0
AD
3331and $b as lexicals. They are package globals. That means
3332if you're in the C<main> package, it's
3333
3334 @articles = sort {$main::b <=> $main::a} @files;
3335
3336or just
3337
3338 @articles = sort {$::b <=> $::a} @files;
3339
3340but if you're in the C<FooPack> package, it's
3341
3342 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
3343
55497cff
PP
3344The comparison function is required to behave. If it returns
3345inconsistent results (sometimes saying $x[1] is less than $x[2] and
3346sometimes saying the opposite, for example) the Perl interpreter will
3347probably crash and dump core. This is entirely due to and dependent
3348upon your system's qsort(3) library routine; this routine often avoids
3349sanity checks in the interest of speed.
3350
a0d0e21e
LW
3351=item splice ARRAY,OFFSET,LENGTH,LIST
3352
3353=item splice ARRAY,OFFSET,LENGTH
3354
3355=item splice ARRAY,OFFSET
3356
3357Removes the elements designated by OFFSET and LENGTH from an array, and
5a964f20
TC
3358replaces them with the elements of LIST, if any. In list context,
3359returns the elements removed from the array. In scalar context,
43051805 3360returns the last element removed, or C<undef> if no elements are
48cdf507
GA
3361removed. The array grows or shrinks as necessary.
3362If OFFSET is negative then it start that far from the end of the array.
3363If LENGTH is omitted, removes everything from OFFSET onward.
3364If LENGTH is negative, leave that many elements off the end of the array.
3365The following equivalences hold (assuming C<$[ == 0>):
a0d0e21e 3366
48cdf507 3367 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
a0d0e21e
LW
3368 pop(@a) splice(@a,-1)
3369 shift(@a) splice(@a,0,1)
3370 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
5a964f20 3371 $a[$x] = $y splice(@a,$x,1,$y)
a0d0e21e
LW
3372
3373Example, assuming array lengths are passed before arrays:
3374
3375 sub aeq { # compare two list values
5a964f20
TC
3376 my(@a) = splice(@_,0,shift);
3377 my(@b) = splice(@_,0,shift);
a0d0e21e
LW
3378 return 0 unless @a == @b; # same len?
3379 while (@a) {
3380 return 0 if pop(@a) ne pop(@b);
3381 }
3382 return 1;
3383 }
3384 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
3385
3386=item split /PATTERN/,EXPR,LIMIT
3387
3388=item split /PATTERN/,EXPR
3389
3390=item split /PATTERN/
3391
3392=item split
3393
5a964f20
TC
3394Splits a string into an array of strings, and returns it. By default,
3395empty leading fields are preserved, and empty trailing ones are deleted.
a0d0e21e 3396
5a964f20
TC
3397If not in list context, returns the number of fields found and splits into
3398the @_ array. (In list context, you can force the split into @_ by
1d2dff63 3399using C<??> as the pattern delimiters, but it still returns the list
5a964f20
TC
3400value.) The use of implicit split to @_ is deprecated, however, because
3401it clobbers your subroutine arguments.
a0d0e21e
LW
3402
3403If EXPR is omitted, splits the $_ string. If PATTERN is also omitted,
4633a7c4
LW
3404splits on whitespace (after skipping any leading whitespace). Anything
3405matching PATTERN is taken to be a delimiter separating the fields. (Note
fb73857a
PP
3406that the delimiter may be longer than one character.)
3407
5a964f20 3408If LIMIT is specified and positive, splits into no more than that
7b8d334a
GS
3409many fields (though it may split into fewer). If LIMIT is unspecified
3410or zero, trailing null fields are stripped (which potential users
fb73857a
PP
3411of pop() would do well to remember). If LIMIT is negative, it is
3412treated as if an arbitrarily large LIMIT had been specified.
a0d0e21e
LW
3413
3414A pattern matching the null string (not to be confused with
748a9306 3415a null pattern C<//>, which is just one member of the set of patterns
a0d0e21e
LW
3416matching a null string) will split the value of EXPR into separate
3417characters at each point it matches that way. For example:
3418
3419 print join(':', split(/ */, 'hi there'));
3420
3421produces the output 'h:i:t:h:e:r:e'.
3422
5f05dabc 3423The LIMIT parameter can be used to split a line partially
a0d0e21e
LW
3424
3425 ($login, $passwd, $remainder) = split(/:/, $_, 3);
3426
3427When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT
3428one larger than the number of variables in the list, to avoid
3429unnecessary work. For the list above LIMIT would have been 4 by
3430default. In time critical applications it behooves you not to split
3431into more fields than you really need.
3432
3433If the PATTERN contains parentheses, additional array elements are
3434created from each matching substring in the delimiter.
3435
da0045b7 3436 split(/([,-])/, "1-10,20", 3);
a0d0e21e
LW
3437
3438produces the list value
3439
3440 (1, '-', 10, ',', 20)
3441
54310121 3442If you had the entire header of a normal Unix email message in $header,
4633a7c4
LW
3443you could split it up into fields and their values this way:
3444
3445 $header =~ s/\n\s+/ /g; # fix continuation lines
fb73857a 3446 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
4633a7c4 3447
a0d0e21e
LW
3448The pattern C</PATTERN/> may be replaced with an expression to specify
3449patterns that vary at runtime. (To do runtime compilation only once,
748a9306
LW
3450use C</$variable/o>.)
3451
3452As a special case, specifying a PATTERN of space (C<' '>) will split on
3453white space just as split with no arguments does. Thus, split(' ') can
3454be used to emulate B<awk>'s default behavior, whereas C<split(/ /)>
3455will give you as many null initial fields as there are leading spaces.
3456A split on /\s+/ is like a split(' ') except that any leading
3457whitespace produces a null first field. A split with no arguments
3458really does a C<split(' ', $_)> internally.
a0d0e21e
LW
3459
3460Example:
3461
5a964f20
TC
3462 open(PASSWD, '/etc/passwd');
3463 while (<PASSWD>) {
3464 ($login, $passwd, $uid, $gid, $gcos,$home, $shell) = split(/:/);
3465 #...
a0d0e21e
LW
3466 }
3467
54310121 3468(Note that $shell above will still have a newline on it. See L</chop>,
a0d0e21e
LW
3469L</chomp>, and L</join>.)
3470
5f05dabc 3471=item sprintf FORMAT, LIST
a0d0e21e 3472
74a77017
CS
3473Returns a string formatted by the usual printf conventions of the
3474C library function sprintf(). See L<sprintf(3)> or L<printf(3)>
3475on your system for an explanation of the general principles.
3476
5a964f20 3477Perl does its own sprintf() formatting -- it emulates the C
74a77017
CS
3478function sprintf(), but it doesn't use it (except for floating-point
3479numbers, and even then only the standard modifiers are allowed). As a
3480result, any non-standard extensions in your local sprintf() are not
3481available from Perl.
3482
3483Perl's sprintf() permits the following universally-known conversions:
3484
3485 %% a percent sign
3486 %c a character with the given number
3487 %s a string
3488 %d a signed integer, in decimal
3489 %u an unsigned integer, in decimal
3490 %o an unsigned integer, in octal
3491 %x an unsigned integer, in hexadecimal
3492 %e a floating-point number, in scientific notation
3493 %f a floating-point number, in fixed decimal notation
3494 %g a floating-point number, in %e or %f notation
3495
1b3f7d21 3496In addition, Perl permits the following widely-supported conversions:
74a77017 3497
74a77017
CS
3498 %X like %x, but using upper-case letters
3499 %E like %e, but using an upper-case "E"
3500 %G like %g, but with an upper-case "E" (if applicable)
3501 %p a pointer (outputs the Perl value's address in hexadecimal)
1b3f7d21
CS
3502 %n special: *stores* the numbe