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1=head1 NAME
2
3perlfunc - Perl builtin functions
4
5=head1 DESCRIPTION
6
7The functions in this section can serve as terms in an expression.
8They fall into two major categories: list operators and named unary
9operators. These differ in their precedence relationship with a
10following comma. (See the precedence table in L<perlop>.) List
11operators take more than one argument, while unary operators can never
12take more than one argument. Thus, a comma terminates the argument of
13a unary operator, but merely separates the arguments of a list
14operator. A unary operator generally provides a scalar context to its
15argument, while a list operator may provide either scalar and list
16contexts for its arguments. If it does both, the scalar arguments will
5f05dabc 17be first, and the list argument will follow. (Note that there can ever
18be only one list argument.) For instance, splice() has three scalar
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19arguments followed by a list.
20
21In the syntax descriptions that follow, list operators that expect a
22list (and provide list context for the elements of the list) are shown
23with LIST as an argument. Such a list may consist of any combination
24of scalar arguments or list values; the list values will be included
25in the list as if each individual element were interpolated at that
26point in the list, forming a longer single-dimensional list value.
27Elements of the LIST should be separated by commas.
28
29Any function in the list below may be used either with or without
30parentheses around its arguments. (The syntax descriptions omit the
5f05dabc 31parentheses.) If you use the parentheses, the simple (but occasionally
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32surprising) rule is this: It I<LOOKS> like a function, therefore it I<IS> a
33function, and precedence doesn't matter. Otherwise it's a list
34operator or unary operator, and precedence does matter. And whitespace
35between the function and left parenthesis doesn't count--so you need to
36be careful sometimes:
37
68dc0745 38 print 1+2+4; # Prints 7.
39 print(1+2) + 4; # Prints 3.
40 print (1+2)+4; # Also prints 3!
41 print +(1+2)+4; # Prints 7.
42 print ((1+2)+4); # Prints 7.
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43
44If you run Perl with the B<-w> switch it can warn you about this. For
45example, the third line above produces:
46
47 print (...) interpreted as function at - line 1.
48 Useless use of integer addition in void context at - line 1.
49
50For functions that can be used in either a scalar or list context,
54310121 51nonabortive failure is generally indicated in a scalar context by
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52returning the undefined value, and in a list context by returning the
53null list.
54
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55Remember the following important rule: There is B<no rule> that relates
56the behavior of an expression in list context to its behavior in scalar
57context, or vice versa. It might do two totally different things.
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58Each operator and function decides which sort of value it would be most
59appropriate to return in a scalar context. Some operators return the
5a964f20 60length of the list that would have been returned in list context. Some
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61operators return the first value in the list. Some operators return the
62last value in the list. Some operators return a count of successful
63operations. In general, they do what you want, unless you want
64consistency.
65
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66An named array in scalar context is quite different from what would at
67first glance appear to be a list in scalar context. You can't get a list
68like C<(1,2,3)> into being in scalar context, because the compiler knows
69the context at compile time. It would generate the scalar comma operator
70there, not the list construction version of the comma. That means it
71was never a list to start with.
72
73In general, functions in Perl that serve as wrappers for system calls
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
37798a01 193=item Functions new in perl5
194
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195C<abs>, C<bless>, C<chomp>, C<chr>, C<exists>, C<formline>, C<glob>,
196C<import>, C<lc>, C<lcfirst>, C<map>, C<my>, C<no>, C<prototype>, C<qx>,
197C<qw>, C<readline>, C<readpipe>, C<ref>, C<sub*>, C<sysopen>, C<tie>,
198C<tied>, C<uc>, C<ucfirst>, C<untie>, C<use>
37798a01 199
200* - C<sub> was a keyword in perl4, but in perl5 it is an
5a964f20 201operator, which can be used in expressions.
37798a01 202
203=item Functions obsoleted in perl5
204
22fae026 205C<dbmclose>, C<dbmopen>
37798a01 206
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207=back
208
209=head2 Alphabetical Listing of Perl Functions
210
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211=over 8
212
22fae026 213=item I<-X> FILEHANDLE
a0d0e21e 214
22fae026 215=item I<-X> EXPR
a0d0e21e 216
22fae026 217=item I<-X>
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218
219A file test, where X is one of the letters listed below. This unary
220operator takes one argument, either a filename or a filehandle, and
221tests the associated file to see if something is true about it. If the
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
28757baa 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
bbce6d69 332specified number of seconds have elapsed. If SECONDS is not specified,
333the value stored in $_ is used. (On some machines,
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334unfortunately, the elapsed time may be up to one second less than you
335specified because of how seconds are counted.) Only one timer may be
336counting at once. Each call disables the previous timer, and an
337argument of 0 may be supplied to cancel the previous timer without
338starting a new one. The returned value is the amount of time remaining
339on the previous timer.
340
4633a7c4 341For delays of finer granularity than one second, you may use Perl's
54310121 342syscall() interface to access setitimer(2) if your system supports it,
343or else see L</select()>. It is usually a mistake to intermix alarm()
4633a7c4 344and sleep() calls.
a0d0e21e 345
ff68c719 346If you want to use alarm() to time out a system call you need to use an
2f9daede 347eval/die pair. You can't rely on the alarm causing the system call to
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348fail with C<$!> 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">.
ff68c719 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 $@ eq "alarm\n"; # propagate unexpected errors
ff68c719 360 # timed out
361 }
362 else {
363 # didn't
364 }
365
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366=item atan2 Y,X
367
368Returns the arctangent of Y/X in the range -PI to PI.
369
28757baa 370For the tangent operation, you may use the POSIX::tan()
371function, or use the familiar relation:
372
373 sub tan { sin($_[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,
28757baa 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
dc848c6f 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
28757baa 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 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 $_.
a0d0e21e
LW
563
564=item close FILEHANDLE
565
566Closes the file or pipe associated with the file handle, returning TRUE
567only if stdio successfully flushes buffers and closes the system file
fb73857a 568descriptor.
569
570You don't have to close FILEHANDLE if you are immediately going to do
571another open() on it, because open() will close it for you. (See
a0d0e21e 572open().) However, an explicit close on an input file resets the line
fb73857a 573counter ($.), while the implicit close done by open() does not.
574
575If the file handle came from a piped open C<close> will additionally
576return FALSE if one of the other system calls involved fails or if the
577program exits with non-zero status. (If the only problem was that the
5a964f20
TC
578program exited non-zero $! will be set to 0.) Also, closing a pipe
579waits for the process executing on the pipe to complete, in case you
fb73857a 580want to look at the output of the pipe afterwards. Closing a pipe
581explicitly also puts the exit status value of the command into C<$?>.
5a964f20 582
fb73857a 583Example:
a0d0e21e 584
fb73857a 585 open(OUTPUT, '|sort >foo') # pipe to sort
586 or die "Can't start sort: $!";
5a964f20 587 #... # print stuff to output
fb73857a 588 close OUTPUT # wait for sort to finish
589 or warn $! ? "Error closing sort pipe: $!"
590 : "Exit status $? from sort";
591 open(INPUT, 'foo') # get sort's results
592 or die "Can't open 'foo' for input: $!";
a0d0e21e 593
5a964f20
TC
594FILEHANDLE may be an expression whose value can be used as an indirect
595filehandle, usually the real filehandle name.
a0d0e21e
LW
596
597=item closedir DIRHANDLE
598
5a964f20
TC
599Closes a directory opened by opendir() and returns the success of that
600system call.
601
602DIRHANDLE may be an expression whose value can be used as an indirect
603dirhandle, usually the real dirhandle name.
a0d0e21e
LW
604
605=item connect SOCKET,NAME
606
607Attempts to connect to a remote socket, just as the connect system call
608does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
4633a7c4
LW
609packed address of the appropriate type for the socket. See the examples in
610L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 611
cb1a09d0
AD
612=item continue BLOCK
613
614Actually a flow control statement rather than a function. If there is a
615C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
616C<foreach>), it is always executed just before the conditional is about to
617be evaluated again, just like the third part of a C<for> loop in C. Thus
618it can be used to increment a loop variable, even when the loop has been
619continued via the C<next> statement (which is similar to the C C<continue>
620statement).
621
1d2dff63
GS
622C<last>, C<next>, or C<redo> may appear within a C<continue>
623block. C<last> and C<redo> will behave as if they had been executed within
624the main block. So will C<next>, but since it will execute a C<continue>
625block, it may be more entertaining.
626
627 while (EXPR) {
628 ### redo always comes here
629 do_something;
630 } continue {
631 ### next always comes here
632 do_something_else;
633 # then back the top to re-check EXPR
634 }
635 ### last always comes here
636
637Omitting the C<continue> section is semantically equivalent to using an
638empty one, logically enough. In that case, C<next> goes directly back
639to check the condition at the top of the loop.
640
a0d0e21e
LW
641=item cos EXPR
642
5a964f20 643Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
a0d0e21e
LW
644takes cosine of $_.
645
28757baa 646For the inverse cosine operation, you may use the POSIX::acos()
647function, or use this relation:
648
649 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
650
a0d0e21e
LW
651=item crypt PLAINTEXT,SALT
652
4633a7c4
LW
653Encrypts a string exactly like the crypt(3) function in the C library
654(assuming that you actually have a version there that has not been
655extirpated as a potential munition). This can prove useful for checking
656the password file for lousy passwords, amongst other things. Only the
657guys wearing white hats should do this.
a0d0e21e 658
11155c91
CS
659Note that crypt is intended to be a one-way function, much like breaking
660eggs to make an omelette. There is no (known) corresponding decrypt
661function. As a result, this function isn't all that useful for
662cryptography. (For that, see your nearby CPAN mirror.)
2f9daede 663
a0d0e21e
LW
664Here's an example that makes sure that whoever runs this program knows
665their own password:
666
667 $pwd = (getpwuid($<))[1];
668 $salt = substr($pwd, 0, 2);
669
670 system "stty -echo";
671 print "Password: ";
672 chop($word = <STDIN>);
673 print "\n";
674 system "stty echo";
675
676 if (crypt($word, $salt) ne $pwd) {
677 die "Sorry...\n";
678 } else {
679 print "ok\n";
54310121 680 }
a0d0e21e 681
9f8f0c9d 682Of course, typing in your own password to whoever asks you
748a9306 683for it is unwise.
a0d0e21e 684
aa689395 685=item dbmclose HASH
a0d0e21e
LW
686
687[This function has been superseded by the untie() function.]
688
aa689395 689Breaks the binding between a DBM file and a hash.
a0d0e21e 690
aa689395 691=item dbmopen HASH,DBNAME,MODE
a0d0e21e
LW
692
693[This function has been superseded by the tie() function.]
694
7b8d334a 695This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
aa689395 696hash. HASH is the name of the hash. (Unlike normal open, the first
697argument is I<NOT> a filehandle, even though it looks like one). DBNAME
698is the name of the database (without the F<.dir> or F<.pag> extension if
699any). If the database does not exist, it is created with protection
700specified by MODE (as modified by the umask()). If your system supports
701only the older DBM functions, you may perform only one dbmopen() in your
702program. In older versions of Perl, if your system had neither DBM nor
703ndbm, calling dbmopen() produced a fatal error; it now falls back to
704sdbm(3).
705
706If you don't have write access to the DBM file, you can only read hash
707variables, not set them. If you want to test whether you can write,
708either use file tests or try setting a dummy hash entry inside an eval(),
709which will trap the error.
a0d0e21e 710
1d2dff63
GS
711Note that functions such as keys() and values() may return huge lists
712when used on large DBM files. You may prefer to use the each()
a0d0e21e
LW
713function to iterate over large DBM files. Example:
714
715 # print out history file offsets
716 dbmopen(%HIST,'/usr/lib/news/history',0666);
717 while (($key,$val) = each %HIST) {
718 print $key, ' = ', unpack('L',$val), "\n";
719 }
720 dbmclose(%HIST);
721
cb1a09d0 722See also L<AnyDBM_File> for a more general description of the pros and
184e9718 723cons of the various dbm approaches, as well as L<DB_File> for a particularly
cb1a09d0 724rich implementation.
4633a7c4 725
a0d0e21e
LW
726=item defined EXPR
727
54310121 728=item defined
bbce6d69 729
2f9daede
TP
730Returns a Boolean value telling whether EXPR has a value other than
731the undefined value C<undef>. If EXPR is not present, C<$_> will be
732checked.
733
734Many operations return C<undef> to indicate failure, end of file,
735system error, uninitialized variable, and other exceptional
736conditions. This function allows you to distinguish C<undef> from
737other values. (A simple Boolean test will not distinguish among
738C<undef>, zero, the empty string, and "0", which are all equally
739false.) Note that since C<undef> is a valid scalar, its presence
740doesn't I<necessarily> indicate an exceptional condition: pop()
741returns C<undef> when its argument is an empty array, I<or> when the
742element to return happens to be C<undef>.
743
5a964f20
TC
744You may also use defined() to check whether a subroutine exists, by
745saying C<defined &func> without parentheses. On the other hand, use
746of defined() upon aggregates (hashes and arrays) is not guaranteed to
747produce intuitive results, and should probably be avoided.
2f9daede
TP
748
749When used on a hash element, it tells you whether the value is defined,
dc848c6f 750not whether the key exists in the hash. Use L</exists> for the latter
2f9daede 751purpose.
a0d0e21e
LW
752
753Examples:
754
755 print if defined $switch{'D'};
756 print "$val\n" while defined($val = pop(@ary));
757 die "Can't readlink $sym: $!"
758 unless defined($value = readlink $sym);
a0d0e21e 759 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
2f9daede 760 $debugging = 0 unless defined $debugging;
a0d0e21e 761
2f9daede
TP
762Note: Many folks tend to overuse defined(), and then are surprised to
763discover that the number 0 and "" (the zero-length string) are, in fact,
764defined values. For example, if you say
a5f75d66
AD
765
766 "ab" =~ /a(.*)b/;
767
5a964f20 768The pattern match succeeds, and $1 is defined, despite the fact that it
a5f75d66
AD
769matched "nothing". But it didn't really match nothing--rather, it
770matched something that happened to be 0 characters long. This is all
771very above-board and honest. When a function returns an undefined value,
2f9daede
TP
772it's an admission that it couldn't give you an honest answer. So you
773should use defined() only when you're questioning the integrity of what
774you're trying to do. At other times, a simple comparison to 0 or "" is
775what you want.
776
777Currently, using defined() on an entire array or hash reports whether
778memory for that aggregate has ever been allocated. So an array you set
779to the empty list appears undefined initially, and one that once was full
780and that you then set to the empty list still appears defined. You
781should instead use a simple test for size:
28757baa 782
783 if (@an_array) { print "has array elements\n" }
784 if (%a_hash) { print "has hash members\n" }
785
786Using undef() on these, however, does clear their memory and then report
5a964f20 787them as not defined anymore, but you shouldn't do that unless you don't
28757baa 788plan to use them again, because it saves time when you load them up
5a964f20
TC
789again to have memory already ready to be filled. The normal way to
790free up space used by an aggregate is to assign the empty list.
28757baa 791
5a964f20 792This counterintuitive behavior of defined() on aggregates may be
28757baa 793changed, fixed, or broken in a future release of Perl.
794
dc848c6f 795See also L</undef>, L</exists>, L</ref>.
2f9daede 796
a0d0e21e
LW
797=item delete EXPR
798
aa689395 799Deletes the specified key(s) and their associated values from a hash.
800For each key, returns the deleted value associated with that key, or
801the undefined value if there was no such key. Deleting from C<$ENV{}>
802modifies the environment. Deleting from a hash tied to a DBM file
5f05dabc 803deletes the entry from the DBM file. (But deleting from a tie()d hash
804doesn't necessarily return anything.)
a0d0e21e 805
aa689395 806The following deletes all the values of a hash:
a0d0e21e 807
5f05dabc 808 foreach $key (keys %HASH) {
809 delete $HASH{$key};
a0d0e21e
LW
810 }
811
5f05dabc 812And so does this:
813
814 delete @HASH{keys %HASH}
815
5a964f20
TC
816(But both of these are slower than just assigning the empty list, or
817using undef().) Note that the EXPR can be arbitrarily complicated as
818long as the final operation is a hash element lookup or hash slice:
a0d0e21e
LW
819
820 delete $ref->[$x][$y]{$key};
5f05dabc 821 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
a0d0e21e
LW
822
823=item die LIST
824
5a964f20 825Outside an eval(), prints the value of LIST to C<STDERR> and exits with
184e9718 826the current value of C<$!> (errno). If C<$!> is 0, exits with the value of
54310121 827C<($? E<gt>E<gt> 8)> (backtick `command` status). If C<($? E<gt>E<gt> 8)>
28757baa 828is 0, exits with 255. Inside an eval(), the error message is stuffed into
5a964f20 829C<$@> and the eval() is terminated with the undefined value. This makes
28757baa 830die() the way to raise an exception.
a0d0e21e
LW
831
832Equivalent examples:
833
834 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
54310121 835 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
a0d0e21e
LW
836
837If the value of EXPR does not end in a newline, the current script line
838number and input line number (if any) are also printed, and a newline
839is supplied. Hint: sometimes appending ", stopped" to your message
840will cause it to make better sense when the string "at foo line 123" is
841appended. Suppose you are running script "canasta".
842
843 die "/etc/games is no good";
844 die "/etc/games is no good, stopped";
845
846produce, respectively
847
848 /etc/games is no good at canasta line 123.
849 /etc/games is no good, stopped at canasta line 123.
850
851See also exit() and warn().
852
fb73857a 853If LIST is empty and $@ already contains a value (typically from a
854previous eval) that value is reused after appending "\t...propagated".
855This is useful for propagating exceptions:
856
857 eval { ... };
858 die unless $@ =~ /Expected exception/;
859
860If $@ is empty then the string "Died" is used.
861
5a964f20 862You can arrange for a callback to be run just before the die() does
774d564b 863its deed, by setting the C<$SIG{__DIE__}> hook. The associated handler
864will be called with the error text and can change the error message, if
fb73857a 865it sees fit, by calling die() again. See L<perlvar/$SIG{expr}> for details on
866setting C<%SIG> entries, and L<"eval BLOCK"> for some examples.
867
868Note that the C<$SIG{__DIE__}> hook is called even inside eval()ed
869blocks/strings. If one wants the hook to do nothing in such
870situations, put
871
872 die @_ if $^S;
873
874as the first line of the handler (see L<perlvar/$^S>).
774d564b 875
a0d0e21e
LW
876=item do BLOCK
877
878Not really a function. Returns the value of the last command in the
879sequence of commands indicated by BLOCK. When modified by a loop
880modifier, executes the BLOCK once before testing the loop condition.
881(On other statements the loop modifiers test the conditional first.)
882
883=item do SUBROUTINE(LIST)
884
885A deprecated form of subroutine call. See L<perlsub>.
886
887=item do EXPR
888
889Uses the value of EXPR as a filename and executes the contents of the
890file as a Perl script. Its primary use is to include subroutines
891from a Perl subroutine library.
892
893 do 'stat.pl';
894
895is just like
896
fb73857a 897 scalar eval `cat stat.pl`;
a0d0e21e 898
5a964f20 899except that it's more efficient and concise, keeps track of the
a0d0e21e
LW
900current filename for error messages, and searches all the B<-I>
901libraries if the file isn't in the current directory (see also the @INC
dc1be6b5
GS
902array in L<perlvar/Predefined Names>). It is also different in how
903code evaluated with C<do FILENAME> doesn't see lexicals in the enclosing
904scope like C<eval STRING> does. It's the same, however, in that it does
54310121 905reparse the file every time you call it, so you probably don't want to
a0d0e21e
LW
906do this inside a loop.
907
908Note that inclusion of library modules is better done with the
5a964f20 909use() and require() operators, which also do automatic error checking
4633a7c4 910and raise an exception if there's a problem.
a0d0e21e 911
5a964f20
TC
912You might like to use C<do> to read in a program configuration
913file. Manual error checking can be done this way:
914
915 # read in config files: system first, then user
916 for $file ('/share/prog/defaults.rc", "$ENV{HOME}/.someprogrc") {
917 unless ($return = do $file) {
918 warn "couldn't parse $file: $@" if $@;
919 warn "couldn't do $file: $!" unless defined $return;
920 warn "couldn't run $file" unless $return;
921 }
922 }
923
a0d0e21e
LW
924=item dump LABEL
925
926This causes an immediate core dump. Primarily this is so that you can
927use the B<undump> program to turn your core dump into an executable binary
928after having initialized all your variables at the beginning of the
929program. When the new binary is executed it will begin by executing a
930C<goto LABEL> (with all the restrictions that C<goto> suffers). Think of
931it as a goto with an intervening core dump and reincarnation. If LABEL
5a964f20 932is omitted, restarts the program from the top. WARNING: Any files
a0d0e21e
LW
933opened at the time of the dump will NOT be open any more when the
934program is reincarnated, with possible resulting confusion on the part
935of Perl. See also B<-u> option in L<perlrun>.
936
937Example:
938
939 #!/usr/bin/perl
940 require 'getopt.pl';
941 require 'stat.pl';
942 %days = (
943 'Sun' => 1,
944 'Mon' => 2,
945 'Tue' => 3,
946 'Wed' => 4,
947 'Thu' => 5,
948 'Fri' => 6,
949 'Sat' => 7,
950 );
951
952 dump QUICKSTART if $ARGV[0] eq '-d';
953
954 QUICKSTART:
955 Getopt('f');
956
5a964f20
TC
957This operator is largely obsolete, partly because it's very hard to
958convert a core file into an executable, and because the real perl-to-C
959compiler has superseded it.
960
aa689395 961=item each HASH
962
5a964f20 963When called in list context, returns a 2-element list consisting of the
aa689395 964key and value for the next element of a hash, so that you can iterate over
5a964f20 965it. When called in scalar context, returns the key for only the "next"
2f9daede
TP
966element in the hash. (Note: Keys may be "0" or "", which are logically
967false; you may wish to avoid constructs like C<while ($k = each %foo) {}>
968for this reason.)
969
970Entries are returned in an apparently random order. When the hash is
971entirely read, a null array is returned in list context (which when
5a964f20 972assigned produces a FALSE (0) value), and C<undef> in
2f9daede
TP
973scalar context. The next call to each() after that will start iterating
974again. There is a single iterator for each hash, shared by all each(),
975keys(), and values() function calls in the program; it can be reset by
976reading all the elements from the hash, or by evaluating C<keys HASH> or
977C<values HASH>. If you add or delete elements of a hash while you're
978iterating over it, you may get entries skipped or duplicated, so don't.
aa689395 979
980The following prints out your environment like the printenv(1) program,
981only in a different order:
a0d0e21e
LW
982
983 while (($key,$value) = each %ENV) {
984 print "$key=$value\n";
985 }
986
987See also keys() and values().
988
989=item eof FILEHANDLE
990
4633a7c4
LW
991=item eof ()
992
a0d0e21e
LW
993=item eof
994
995Returns 1 if the next read on FILEHANDLE will return end of file, or if
996FILEHANDLE is not open. FILEHANDLE may be an expression whose value
5a964f20
TC
997gives the real filehandle. (Note that this function actually
998reads a character and then ungetc()s it, so isn't very useful in an
748a9306
LW
999interactive context.) Do not read from a terminal file (or call
1000C<eof(FILEHANDLE)> on it) after end-of-file is reached. Filetypes such
1001as terminals may lose the end-of-file condition if you do.
1002
1003An C<eof> without an argument uses the last file read as argument.
5a964f20 1004Using C<eof()> with empty parentheses is very different. It indicates the pseudo file formed of
2f9daede
TP
1005the files listed on the command line, i.e., C<eof()> is reasonable to
1006use inside a C<while (E<lt>E<gt>)> loop to detect the end of only the
1007last file. Use C<eof(ARGV)> or eof without the parentheses to test
1008I<EACH> file in a while (E<lt>E<gt>) loop. Examples:
a0d0e21e 1009
748a9306
LW
1010 # reset line numbering on each input file
1011 while (<>) {
5a964f20 1012 next if /^\s*#/; # skip comments
748a9306 1013 print "$.\t$_";
5a964f20
TC
1014 } continue {
1015 close ARGV if eof; # Not eof()!
748a9306
LW
1016 }
1017
a0d0e21e
LW
1018 # insert dashes just before last line of last file
1019 while (<>) {
5a964f20 1020 if (eof()) { # check for end of current file
a0d0e21e 1021 print "--------------\n";
748a9306
LW
1022 close(ARGV); # close or break; is needed if we
1023 # are reading from the terminal
a0d0e21e
LW
1024 }
1025 print;
1026 }
1027
a0d0e21e 1028Practical hint: you almost never need to use C<eof> in Perl, because the
5a964f20 1029input operators return C<undef> when they run out of data.
a0d0e21e
LW
1030
1031=item eval EXPR
1032
1033=item eval BLOCK
1034
c7cc6f1c
GS
1035In the first form, the return value of EXPR is parsed and executed as if it
1036were a little Perl program. The value of the expression (which is itself
5a964f20 1037determined within scalar context) is first parsed, and if there weren't any
c7cc6f1c 1038errors, executed in the context of the current Perl program, so that any
5f05dabc 1039variable settings or subroutine and format definitions remain afterwards.
c7cc6f1c
GS
1040Note that the value is parsed every time the eval executes. If EXPR is
1041omitted, evaluates C<$_>. This form is typically used to delay parsing
1042and subsequent execution of the text of EXPR until run time.
1043
1044In the second form, the code within the BLOCK is parsed only once--at the
1045same time the code surrounding the eval itself was parsed--and executed
1046within the context of the current Perl program. This form is typically
1047used to trap exceptions more efficiently than the first (see below), while
1048also providing the benefit of checking the code within BLOCK at compile
1049time.
1050
1051The final semicolon, if any, may be omitted from the value of EXPR or within
1052the BLOCK.
1053
1054In both forms, the value returned is the value of the last expression
5a964f20 1055evaluated inside the mini-program; a return statement may be also used, just
c7cc6f1c 1056as with subroutines. The expression providing the return value is evaluated
5a964f20 1057in void, scalar, or list context, depending on the context of the eval itself.
c7cc6f1c 1058See L</wantarray> for more on how the evaluation context can be determined.
a0d0e21e
LW
1059
1060If there is a syntax error or runtime error, or a die() statement is
1061executed, an undefined value is returned by eval(), and C<$@> is set to the
1062error message. If there was no error, C<$@> is guaranteed to be a null
c7cc6f1c
GS
1063string. Beware that using eval() neither silences perl from printing
1064warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1065To do either of those, you have to use the C<$SIG{__WARN__}> facility. See
1066L</warn> and L<perlvar>.
a0d0e21e 1067
5f05dabc 1068Note that, because eval() traps otherwise-fatal errors, it is useful for
4633a7c4 1069determining whether a particular feature (such as socket() or symlink())
a0d0e21e
LW
1070is implemented. It is also Perl's exception trapping mechanism, where
1071the die operator is used to raise exceptions.
1072
1073If the code to be executed doesn't vary, you may use the eval-BLOCK
1074form to trap run-time errors without incurring the penalty of
1075recompiling each time. The error, if any, is still returned in C<$@>.
1076Examples:
1077
54310121 1078 # make divide-by-zero nonfatal
a0d0e21e
LW
1079 eval { $answer = $a / $b; }; warn $@ if $@;
1080
1081 # same thing, but less efficient
1082 eval '$answer = $a / $b'; warn $@ if $@;
1083
1084 # a compile-time error
5a964f20 1085 eval { $answer = }; # WRONG
a0d0e21e
LW
1086
1087 # a run-time error
1088 eval '$answer ='; # sets $@
1089
774d564b 1090When using the eval{} form as an exception trap in libraries, you may
1091wish not to trigger any C<__DIE__> hooks that user code may have
1092installed. You can use the C<local $SIG{__DIE__}> construct for this
1093purpose, as shown in this example:
1094
1095 # a very private exception trap for divide-by-zero
1096 eval { local $SIG{'__DIE__'}; $answer = $a / $b; }; warn $@ if $@;
1097
1098This is especially significant, given that C<__DIE__> hooks can call
1099die() again, which has the effect of changing their error messages:
1100
1101 # __DIE__ hooks may modify error messages
1102 {
1103 local $SIG{'__DIE__'} = sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
c7cc6f1c
GS
1104 eval { die "foo lives here" };
1105 print $@ if $@; # prints "bar lives here"
774d564b 1106 }
1107
54310121 1108With an eval(), you should be especially careful to remember what's
a0d0e21e
LW
1109being looked at when:
1110
1111 eval $x; # CASE 1
1112 eval "$x"; # CASE 2
1113
1114 eval '$x'; # CASE 3
1115 eval { $x }; # CASE 4
1116
5a964f20 1117 eval "\$$x++"; # CASE 5
a0d0e21e
LW
1118 $$x++; # CASE 6
1119
2f9daede
TP
1120Cases 1 and 2 above behave identically: they run the code contained in
1121the variable $x. (Although case 2 has misleading double quotes making
1122the reader wonder what else might be happening (nothing is).) Cases 3
1123and 4 likewise behave in the same way: they run the code '$x', which
1124does nothing but return the value of C<$x>. (Case 4 is preferred for
1125purely visual reasons, but it also has the advantage of compiling at
1126compile-time instead of at run-time.) Case 5 is a place where
54310121 1127normally you I<WOULD> like to use double quotes, except that in this
2f9daede
TP
1128particular situation, you can just use symbolic references instead, as
1129in case 6.
a0d0e21e
LW
1130
1131=item exec LIST
1132
8bf3b016
GS
1133=item exec PROGRAM LIST
1134
fb73857a 1135The exec() function executes a system command I<AND NEVER RETURNS> -
1136use system() instead of exec() if you want it to return. It fails and
1137returns FALSE only if the command does not exist I<and> it is executed
1138directly instead of via your system's command shell (see below).
a0d0e21e 1139
55d729e4 1140Since it's a common mistake to use system() instead of exec(), Perl
5a964f20 1141warns you if there is a following statement which isn't die(), warn(),
55d729e4
GS
1142or exit() (if C<-w> is set - but you always do that). If you
1143I<really> want to follow an exec() with some other statement, you
1144can use one of these styles to avoid the warning:
1145
5a964f20
TC
1146 exec ('foo') or print STDERR "couldn't exec foo: $!";
1147 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
55d729e4 1148
5a964f20
TC
1149If there is more than one argument in LIST, or if LIST is an array
1150with more than one value, calls execvp(3) with the arguments in LIST.
1151If there is only one scalar argument or an array with one element in it,
1152the argument is checked for shell metacharacters, and if there are any,
1153the entire argument is passed to the system's command shell for parsing
1154(this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1155If there are no shell metacharacters in the argument, it is split into
1156words and passed directly to execvp(), which is more efficient. Note:
1157exec() and system() do not flush your output buffer, so you may need to
1158set C<$|> to avoid lost output. Examples:
a0d0e21e
LW
1159
1160 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1161 exec "sort $outfile | uniq";
1162
1163If you don't really want to execute the first argument, but want to lie
1164to the program you are executing about its own name, you can specify
1165the program you actually want to run as an "indirect object" (without a
1166comma) in front of the LIST. (This always forces interpretation of the
54310121 1167LIST as a multivalued list, even if there is only a single scalar in
a0d0e21e
LW
1168the list.) Example:
1169
1170 $shell = '/bin/csh';
1171 exec $shell '-sh'; # pretend it's a login shell
1172
1173or, more directly,
1174
1175 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1176
bb32b41a
GS
1177When the arguments get executed via the system shell, results will
1178be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1179for details.
1180
5a964f20
TC
1181Using an indirect object with C<exec> or C<system> is also more secure.
1182This usage forces interpretation of the arguments as a multivalued list,
1183even if the list had just one argument. That way you're safe from the
1184shell expanding wildcards or splitting up words with whitespace in them.
1185
1186 @args = ( "echo surprise" );
1187
1188 system @args; # subject to shell escapes if @args == 1
1189 system { $args[0] } @args; # safe even with one-arg list
1190
1191The first version, the one without the indirect object, ran the I<echo>
1192program, passing it C<"surprise"> an argument. The second version
1193didn't--it tried to run a program literally called I<"echo surprise">,
1194didn't find it, and set C<$?> to a non-zero value indicating failure.
1195
a0d0e21e
LW
1196=item exists EXPR
1197
1198Returns TRUE if the specified hash key exists in its hash array, even
1199if the corresponding value is undefined.
1200
1201 print "Exists\n" if exists $array{$key};
1202 print "Defined\n" if defined $array{$key};
1203 print "True\n" if $array{$key};
1204
5f05dabc 1205A hash element can be TRUE only if it's defined, and defined if
a0d0e21e
LW
1206it exists, but the reverse doesn't necessarily hold true.
1207
1208Note that the EXPR can be arbitrarily complicated as long as the final
1209operation is a hash key lookup:
1210
5a964f20
TC
1211 if (exists $ref->{"A"}{"B"}{$key}) { ... }
1212
1213Although the last element will not spring into existence just because its
1214existence was tested, intervening ones will. Thus C<$ref-E<gt>{"A"}>
1215C<$ref-E<gt>{"B"}> will spring into existence due to the existence
1216test for a $key element. This autovivification may be fixed in a later
1217release.
a0d0e21e
LW
1218
1219=item exit EXPR
1220
1221Evaluates EXPR and exits immediately with that value. (Actually, it
1222calls any defined C<END> routines first, but the C<END> routines may not
1223abort the exit. Likewise any object destructors that need to be called
1224are called before exit.) Example:
1225
1226 $ans = <STDIN>;
1227 exit 0 if $ans =~ /^[Xx]/;
1228
f86702cc 1229See also die(). If EXPR is omitted, exits with 0 status. The only
54310121 1230universally portable values for EXPR are 0 for success and 1 for error;
f86702cc 1231all other values are subject to unpredictable interpretation depending
1232on the environment in which the Perl program is running.
a0d0e21e 1233
28757baa 1234You shouldn't use exit() to abort a subroutine if there's any chance that
1235someone might want to trap whatever error happened. Use die() instead,
1236which can be trapped by an eval().
1237
5a964f20
TC
1238All C<END{}> blocks are run at exit time. See L<perlsub> for details.
1239
a0d0e21e
LW
1240=item exp EXPR
1241
54310121 1242=item exp
bbce6d69 1243
54310121 1244Returns I<e> (the natural logarithm base) to the power of EXPR.
a0d0e21e
LW
1245If EXPR is omitted, gives C<exp($_)>.
1246
1247=item fcntl FILEHANDLE,FUNCTION,SCALAR
1248
1249Implements the fcntl(2) function. You'll probably have to say
1250
1251 use Fcntl;
1252
1253first to get the correct function definitions. Argument processing and
5a964f20 1254value return works just like ioctl() below.
a0d0e21e
LW
1255For example:
1256
1257 use Fcntl;
5a964f20
TC
1258 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1259 or die "can't fcntl F_GETFL: $!";
1260
1261You don't have to check for C<defined> on the return from
1262fnctl. Like ioctl, it maps a 0 return from the system
1263call into "0 but true" in Perl. This string is true in
1264boolean context and 0 in numeric context. It is also
1265exempt from the normal B<-w> warnings on improper numeric
1266conversions.
1267
1268Note that fcntl() will produce a fatal error if used on a machine that
1269doesn't implement fcntl(2).
a0d0e21e
LW
1270
1271=item fileno FILEHANDLE
1272
1273Returns the file descriptor for a filehandle. This is useful for
5a964f20
TC
1274constructing bitmaps for select() and low-level POSIX tty-handling
1275operations. If FILEHANDLE is an expression, the value is taken as
1276an indirect filehandle, generally its name.
1277
1278You can use this to find out whether two handles refer to the
1279same underlying descriptor:
1280
1281 if (fileno(THIS) == fileno(THAT)) {
1282 print "THIS and THAT are dups\n";
1283 }
a0d0e21e
LW
1284
1285=item flock FILEHANDLE,OPERATION
1286
8ebc5c01 1287Calls flock(2), or an emulation of it, on FILEHANDLE. Returns TRUE for
68dc0745 1288success, FALSE on failure. Produces a fatal error if used on a machine
1289that doesn't implement flock(2), fcntl(2) locking, or lockf(3). flock()
1290is Perl's portable file locking interface, although it locks only entire
1291files, not records.
8ebc5c01 1292
a3cb178b 1293On many platforms (including most versions or clones of Unix), locks
5a964f20
TC
1294established by flock() are B<merely advisory>. Such discretionary locks
1295are more flexible, but offer fewer guarantees. This means that files
1296locked with flock() may be modified by programs that do not also use
1297flock(). Windows NT and OS/2 are among the platforms which
1298enforce mandatory locking. See your local documentation for details.
a3cb178b 1299
8ebc5c01 1300OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1301LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
68dc0745 1302you can use the symbolic names if import them from the Fcntl module,
1303either individually, or as a group using the ':flock' tag. LOCK_SH
1304requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1305releases a previously requested lock. If LOCK_NB is added to LOCK_SH or
1306LOCK_EX then flock() will return immediately rather than blocking
1307waiting for the lock (check the return status to see if you got it).
1308
1309To avoid the possibility of mis-coordination, Perl flushes FILEHANDLE
1310before (un)locking it.
8ebc5c01 1311
1312Note that the emulation built with lockf(3) doesn't provide shared
1313locks, and it requires that FILEHANDLE be open with write intent. These
1314are the semantics that lockf(3) implements. Most (all?) systems
1315implement lockf(3) in terms of fcntl(2) locking, though, so the
1316differing semantics shouldn't bite too many people.
1317
1318Note also that some versions of flock() cannot lock things over the
1319network; you would need to use the more system-specific fcntl() for
1320that. If you like you can force Perl to ignore your system's flock(2)
1321function, and so provide its own fcntl(2)-based emulation, by passing
1322the switch C<-Ud_flock> to the F<Configure> program when you configure
1323perl.
4633a7c4
LW
1324
1325Here's a mailbox appender for BSD systems.
a0d0e21e 1326
7e1af8bc 1327 use Fcntl ':flock'; # import LOCK_* constants
a0d0e21e
LW
1328
1329 sub lock {
7e1af8bc 1330 flock(MBOX,LOCK_EX);
a0d0e21e
LW
1331 # and, in case someone appended
1332 # while we were waiting...
1333 seek(MBOX, 0, 2);
1334 }
1335
1336 sub unlock {
7e1af8bc 1337 flock(MBOX,LOCK_UN);
a0d0e21e
LW
1338 }
1339
1340 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1341 or die "Can't open mailbox: $!";
1342
1343 lock();
1344 print MBOX $msg,"\n\n";
1345 unlock();
1346
cb1a09d0 1347See also L<DB_File> for other flock() examples.
a0d0e21e
LW
1348
1349=item fork
1350
5a964f20
TC
1351Does a fork(2) system call. Returns the child pid to the parent process,
13520 to the child process, or C<undef> if the fork is unsuccessful.
1353
a0d0e21e 1354Note: unflushed buffers remain unflushed in both processes, which means
28757baa 1355you may need to set C<$|> ($AUTOFLUSH in English) or call the autoflush()
1356method of IO::Handle to avoid duplicate output.
a0d0e21e
LW
1357
1358If you fork() without ever waiting on your children, you will accumulate
1359zombies:
1360
4633a7c4 1361 $SIG{CHLD} = sub { wait };
a0d0e21e 1362
54310121 1363There's also the double-fork trick (error checking on
a0d0e21e
LW
1364fork() returns omitted);
1365
1366 unless ($pid = fork) {
1367 unless (fork) {
1368 exec "what you really wanna do";
1369 die "no exec";
1370 # ... or ...
4633a7c4 1371 ## (some_perl_code_here)
a0d0e21e
LW
1372 exit 0;
1373 }
1374 exit 0;
1375 }
1376 waitpid($pid,0);
1377
cb1a09d0
AD
1378See also L<perlipc> for more examples of forking and reaping
1379moribund children.
1380
28757baa 1381Note that if your forked child inherits system file descriptors like
1382STDIN and STDOUT that are actually connected by a pipe or socket, even
5a964f20 1383if you exit, then the remote server (such as, say, httpd or rsh) won't think
28757baa 1384you're done. You should reopen those to /dev/null if it's any issue.
1385
cb1a09d0
AD
1386=item format
1387
7b8d334a 1388Declare a picture format for use by the write() function. For
cb1a09d0
AD
1389example:
1390
54310121 1391 format Something =
cb1a09d0
AD
1392 Test: @<<<<<<<< @||||| @>>>>>
1393 $str, $%, '$' . int($num)
1394 .
1395
1396 $str = "widget";
184e9718 1397 $num = $cost/$quantity;
cb1a09d0
AD
1398 $~ = 'Something';
1399 write;
1400
1401See L<perlform> for many details and examples.
1402
8903cb82 1403=item formline PICTURE,LIST
a0d0e21e 1404
5a964f20 1405This is an internal function used by C<format>s, though you may call it,
a0d0e21e
LW
1406too. It formats (see L<perlform>) a list of values according to the
1407contents of PICTURE, placing the output into the format output
4633a7c4
LW
1408accumulator, C<$^A> (or $ACCUMULATOR in English).
1409Eventually, when a write() is done, the contents of
a0d0e21e
LW
1410C<$^A> are written to some filehandle, but you could also read C<$^A>
1411yourself and then set C<$^A> back to "". Note that a format typically
1412does one formline() per line of form, but the formline() function itself
748a9306 1413doesn't care how many newlines are embedded in the PICTURE. This means
4633a7c4 1414that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
748a9306
LW
1415You may therefore need to use multiple formlines to implement a single
1416record format, just like the format compiler.
1417
5f05dabc 1418Be careful if you put double quotes around the picture, because an "C<@>"
748a9306 1419character may be taken to mean the beginning of an array name.
4633a7c4 1420formline() always returns TRUE. See L<perlform> for other examples.
a0d0e21e
LW
1421
1422=item getc FILEHANDLE
1423
1424=item getc
1425
1426Returns the next character from the input file attached to FILEHANDLE,
1427or a null string at end of file. If FILEHANDLE is omitted, reads from STDIN.
4633a7c4 1428This is not particularly efficient. It cannot be used to get unbuffered
cb1a09d0 1429single-characters, however. For that, try something more like:
4633a7c4
LW
1430
1431 if ($BSD_STYLE) {
1432 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1433 }
1434 else {
54310121 1435 system "stty", '-icanon', 'eol', "\001";
4633a7c4
LW
1436 }
1437
1438 $key = getc(STDIN);
1439
1440 if ($BSD_STYLE) {
1441 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1442 }
1443 else {
5f05dabc 1444 system "stty", 'icanon', 'eol', '^@'; # ASCII null
4633a7c4
LW
1445 }
1446 print "\n";
1447
54310121 1448Determination of whether $BSD_STYLE should be set
1449is left as an exercise to the reader.
cb1a09d0 1450
28757baa 1451The POSIX::getattr() function can do this more portably on systems
5a964f20 1452purporting POSIX compliance.
cb1a09d0 1453See also the C<Term::ReadKey> module from your nearest CPAN site;
54310121 1454details on CPAN can be found on L<perlmod/CPAN>.
a0d0e21e
LW
1455
1456=item getlogin
1457
5a964f20
TC
1458Implements the C library function of the same name, which on most
1459systems returns the current login from F</etc/utmp>, if any. If null,
1460use getpwuid().
a0d0e21e 1461
f86702cc 1462 $login = getlogin || getpwuid($<) || "Kilroy";
a0d0e21e 1463
da0045b7 1464Do not consider getlogin() for authentication: it is not as
4633a7c4
LW
1465secure as getpwuid().
1466
a0d0e21e
LW
1467=item getpeername SOCKET
1468
1469Returns the packed sockaddr address of other end of the SOCKET connection.
1470
4633a7c4
LW
1471 use Socket;
1472 $hersockaddr = getpeername(SOCK);
1473 ($port, $iaddr) = unpack_sockaddr_in($hersockaddr);
1474 $herhostname = gethostbyaddr($iaddr, AF_INET);
1475 $herstraddr = inet_ntoa($iaddr);
a0d0e21e
LW
1476
1477=item getpgrp PID
1478
47e29363 1479Returns the current process group for the specified PID. Use
1480a PID of 0 to get the current process group for the
4633a7c4 1481current process. Will raise an exception if used on a machine that
a0d0e21e 1482doesn't implement getpgrp(2). If PID is omitted, returns process
47e29363 1483group of current process. Note that the POSIX version of getpgrp()
1484does not accept a PID argument, so only PID==0 is truly portable.
a0d0e21e
LW
1485
1486=item getppid
1487
1488Returns the process id of the parent process.
1489
1490=item getpriority WHICH,WHO
1491
4633a7c4
LW
1492Returns the current priority for a process, a process group, or a user.
1493(See L<getpriority(2)>.) Will raise a fatal exception if used on a
a0d0e21e
LW
1494machine that doesn't implement getpriority(2).
1495
1496=item getpwnam NAME
1497
1498=item getgrnam NAME
1499
1500=item gethostbyname NAME
1501
1502=item getnetbyname NAME
1503
1504=item getprotobyname NAME
1505
1506=item getpwuid UID
1507
1508=item getgrgid GID
1509
1510=item getservbyname NAME,PROTO
1511
1512=item gethostbyaddr ADDR,ADDRTYPE
1513
1514=item getnetbyaddr ADDR,ADDRTYPE
1515
1516=item getprotobynumber NUMBER
1517
1518=item getservbyport PORT,PROTO
1519
1520=item getpwent
1521
1522=item getgrent
1523
1524=item gethostent
1525
1526=item getnetent
1527
1528=item getprotoent
1529
1530=item getservent
1531
1532=item setpwent
1533
1534=item setgrent
1535
1536=item sethostent STAYOPEN
1537
1538=item setnetent STAYOPEN
1539
1540=item setprotoent STAYOPEN
1541
1542=item setservent STAYOPEN
1543
1544=item endpwent
1545
1546=item endgrent
1547
1548=item endhostent
1549
1550=item endnetent
1551
1552=item endprotoent
1553
1554=item endservent
1555
1556These routines perform the same functions as their counterparts in the
5a964f20 1557system library. In list context, the return values from the
a0d0e21e
LW
1558various get routines are as follows:
1559
1560 ($name,$passwd,$uid,$gid,
6ee623d5 1561 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
a0d0e21e
LW
1562 ($name,$passwd,$gid,$members) = getgr*
1563 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1564 ($name,$aliases,$addrtype,$net) = getnet*
1565 ($name,$aliases,$proto) = getproto*
1566 ($name,$aliases,$port,$proto) = getserv*
1567
1568(If the entry doesn't exist you get a null list.)
1569
5a964f20 1570In scalar context, you get the name, unless the function was a
a0d0e21e
LW
1571lookup by name, in which case you get the other thing, whatever it is.
1572(If the entry doesn't exist you get the undefined value.) For example:
1573
5a964f20
TC
1574 $uid = getpwnam($name);
1575 $name = getpwuid($num);
1576 $name = getpwent();
1577 $gid = getgrnam($name);
1578 $name = getgrgid($num;
1579 $name = getgrent();
1580 #etc.
a0d0e21e 1581
6ee623d5
GS
1582In I<getpw*()> the fields $quota, $comment, and $expire are special
1583cases in the sense that in many systems they are unsupported. If the
1584$quota is unsupported, it is an empty scalar. If it is supported, it
1585usually encodes the disk quota. If the $comment field is unsupported,
1586it is an empty scalar. If it is supported it usually encodes some
1587administrative comment about the user. In some systems the $quota
1588field may be $change or $age, fields that have to do with password
1589aging. In some systems the $comment field may be $class. The $expire
1590field, if present, encodes the expiration period of the account or the
1591password. For the availability and the exact meaning of these fields
1592in your system, please consult your getpwnam(3) documentation and your
1593<pwd.h> file. You can also find out from within Perl which meaning
1594your $quota and $comment fields have and whether you have the $expire
1595field by using the Config module and the values d_pwquota, d_pwage,
1596d_pwchange, d_pwcomment, and d_pwexpire.
1597
a0d0e21e
LW
1598The $members value returned by I<getgr*()> is a space separated list of
1599the login names of the members of the group.
1600
1601For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1602C, it will be returned to you via C<$?> if the function call fails. The
1603@addrs value returned by a successful call is a list of the raw
1604addresses returned by the corresponding system library call. In the
1605Internet domain, each address is four bytes long and you can unpack it
1606by saying something like:
1607
1608 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1609
5a964f20
TC
1610If you get tired of remembering which element of the return list contains
1611which return value, by-name interfaces are also provided in modules:
1612File::stat, Net::hostent, Net::netent, Net::protoent, Net::servent,
1613Time::gmtime, Time::localtime, and User::grent. These override the
1614normal built-in, replacing them with versions that return objects with
1615the appropriate names for each field. For example:
1616
1617 use File::stat;
1618 use User::pwent;
1619 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
1620
1621Even though it looks like they're the same method calls (uid),
1622they aren't, because a File::stat object is different from a User::pwent object.
1623
a0d0e21e
LW
1624=item getsockname SOCKET
1625
1626Returns the packed sockaddr address of this end of the SOCKET connection.
1627
4633a7c4
LW
1628 use Socket;
1629 $mysockaddr = getsockname(SOCK);
1630 ($port, $myaddr) = unpack_sockaddr_in($mysockaddr);
a0d0e21e
LW
1631
1632=item getsockopt SOCKET,LEVEL,OPTNAME
1633
5a964f20 1634Returns the socket option requested, or undef if there is an error.
a0d0e21e
LW
1635
1636=item glob EXPR
1637
0a753a76 1638=item glob
1639
5a964f20 1640Returns the value of EXPR with filename expansions such as the standard Unix shell /bin/sh would
68dc0745 1641do. This is the internal function implementing the C<E<lt>*.cE<gt>>
1642operator, but you can use it directly. If EXPR is omitted, $_ is used.
1643The C<E<lt>*.cE<gt>> operator is discussed in more detail in
1644L<perlop/"I/O Operators">.
a0d0e21e
LW
1645
1646=item gmtime EXPR
1647
1648Converts a time as returned by the time function to a 9-element array
54310121 1649with the time localized for the standard Greenwich time zone.
4633a7c4 1650Typically used as follows:
a0d0e21e 1651
54310121 1652 # 0 1 2 3 4 5 6 7 8
a0d0e21e
LW
1653 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1654 gmtime(time);
1655
1656All array elements are numeric, and come straight out of a struct tm.
1657In particular this means that $mon has the range 0..11 and $wday has
54310121 1658the range 0..6 with sunday as day 0. Also, $year is the number of
1659years since 1900, I<not> simply the last two digits of the year.
2f9daede
TP
1660
1661If EXPR is omitted, does C<gmtime(time())>.
a0d0e21e 1662
5a964f20 1663In scalar context, returns the ctime(3) value:
0a753a76 1664
1665 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
1666
54310121 1667Also see the timegm() function provided by the Time::Local module,
1668and the strftime(3) function available via the POSIX module.
0a753a76 1669
a0d0e21e
LW
1670=item goto LABEL
1671
748a9306
LW
1672=item goto EXPR
1673
a0d0e21e
LW
1674=item goto &NAME
1675
1676The goto-LABEL form finds the statement labeled with LABEL and resumes
1677execution there. It may not be used to go into any construct that
1678requires initialization, such as a subroutine or a foreach loop. It
0a753a76 1679also can't be used to go into a construct that is optimized away,
1680or to get out of a block or subroutine given to sort().
1681It can be used to go almost anywhere else within the dynamic scope,
a0d0e21e
LW
1682including out of subroutines, but it's usually better to use some other
1683construct such as last or die. The author of Perl has never felt the
1684need to use this form of goto (in Perl, that is--C is another matter).
1685
748a9306
LW
1686The goto-EXPR form expects a label name, whose scope will be resolved
1687dynamically. This allows for computed gotos per FORTRAN, but isn't
1688necessarily recommended if you're optimizing for maintainability:
1689
1690 goto ("FOO", "BAR", "GLARCH")[$i];
1691
a0d0e21e
LW
1692The goto-&NAME form is highly magical, and substitutes a call to the
1693named subroutine for the currently running subroutine. This is used by
1694AUTOLOAD subroutines that wish to load another subroutine and then
1695pretend that the other subroutine had been called in the first place
1696(except that any modifications to @_ in the current subroutine are
1697propagated to the other subroutine.) After the goto, not even caller()
1698will be able to tell that this routine was called first.
1699
1700=item grep BLOCK LIST
1701
1702=item grep EXPR,LIST
1703
54310121 1704This is similar in spirit to, but not the same as, grep(1)
2f9daede
TP
1705and its relatives. In particular, it is not limited to using
1706regular expressions.
1707
a0d0e21e
LW
1708Evaluates the BLOCK or EXPR for each element of LIST (locally setting
1709$_ to each element) and returns the list value consisting of those
1710elements for which the expression evaluated to TRUE. In a scalar
1711context, returns the number of times the expression was TRUE.
1712
1713 @foo = grep(!/^#/, @bar); # weed out comments
1714
1715or equivalently,
1716
1717 @foo = grep {!/^#/} @bar; # weed out comments
1718
5f05dabc 1719Note that, because $_ is a reference into the list value, it can be used
a0d0e21e
LW
1720to modify the elements of the array. While this is useful and
1721supported, it can cause bizarre results if the LIST is not a named
2f9daede 1722array. Similarly, grep returns aliases into the original list,
7b8d334a 1723much like the way that a for loops's index variable aliases the list
2f9daede 1724elements. That is, modifying an element of a list returned by grep
fb73857a 1725(for example, in a C<foreach>, C<map> or another C<grep>)
2f9daede 1726actually modifies the element in the original list.
a0d0e21e 1727
fb73857a 1728See also L</map> for an array composed of the results of the BLOCK or EXPR.
38325410 1729
a0d0e21e
LW
1730=item hex EXPR
1731
54310121 1732=item hex
bbce6d69 1733
54310121 1734Interprets EXPR as a hex string and returns the corresponding
2f9daede 1735value. (To convert strings that might start with either 0 or 0x
dc848c6f 1736see L</oct>.) If EXPR is omitted, uses $_.
2f9daede
TP
1737
1738 print hex '0xAf'; # prints '175'
1739 print hex 'aF'; # same
a0d0e21e
LW
1740
1741=item import
1742
5a964f20 1743There is no builtin import() function. It is just an ordinary
4633a7c4 1744method (subroutine) defined (or inherited) by modules that wish to export
a0d0e21e 1745names to another module. The use() function calls the import() method
54310121 1746for the package used. See also L</use()>, L<perlmod>, and L<Exporter>.
a0d0e21e
LW
1747
1748=item index STR,SUBSTR,POSITION
1749
1750=item index STR,SUBSTR
1751
4633a7c4
LW
1752Returns the position of the first occurrence of SUBSTR in STR at or after
1753POSITION. If POSITION is omitted, starts searching from the beginning of
184e9718 1754the string. The return value is based at 0 (or whatever you've set the C<$[>
4633a7c4 1755variable to--but don't do that). If the substring is not found, returns
a0d0e21e
LW
1756one less than the base, ordinarily -1.
1757
1758=item int EXPR
1759
54310121 1760=item int
bbce6d69 1761
a0d0e21e 1762Returns the integer portion of EXPR. If EXPR is omitted, uses $_.
5a964f20
TC
1763You should not use this for rounding, because it truncates
1764towards 0, and because machine representations of floating point
1765numbers can sometimes produce counterintuitive results. Usually sprintf() or printf(),
1766or the POSIX::floor or POSIX::ceil functions, would serve you better.
a0d0e21e
LW
1767
1768=item ioctl FILEHANDLE,FUNCTION,SCALAR
1769
1770Implements the ioctl(2) function. You'll probably have to say
1771
4633a7c4 1772 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
a0d0e21e 1773
4633a7c4 1774first to get the correct function definitions. If F<ioctl.ph> doesn't
a0d0e21e 1775exist or doesn't have the correct definitions you'll have to roll your
4633a7c4 1776own, based on your C header files such as F<E<lt>sys/ioctl.hE<gt>>.
5a964f20 1777(There is a Perl script called B<h2ph> that comes with the Perl kit that
54310121 1778may help you in this, but it's nontrivial.) SCALAR will be read and/or
4633a7c4
LW
1779written depending on the FUNCTION--a pointer to the string value of SCALAR
1780will be passed as the third argument of the actual ioctl call. (If SCALAR
1781has no string value but does have a numeric value, that value will be
1782passed rather than a pointer to the string value. To guarantee this to be
1783TRUE, add a 0 to the scalar before using it.) The pack() and unpack()
1784functions are useful for manipulating the values of structures used by
1785ioctl(). The following example sets the erase character to DEL.
a0d0e21e
LW
1786
1787 require 'ioctl.ph';
4633a7c4
LW
1788 $getp = &TIOCGETP;
1789 die "NO TIOCGETP" if $@ || !$getp;
a0d0e21e 1790 $sgttyb_t = "ccccs"; # 4 chars and a short
4633a7c4 1791 if (ioctl(STDIN,$getp,$sgttyb)) {
a0d0e21e
LW
1792 @ary = unpack($sgttyb_t,$sgttyb);
1793 $ary[2] = 127;
1794 $sgttyb = pack($sgttyb_t,@ary);
4633a7c4 1795 ioctl(STDIN,&TIOCSETP,$sgttyb)
a0d0e21e
LW
1796 || die "Can't ioctl: $!";
1797 }
1798
1799The return value of ioctl (and fcntl) is as follows:
1800
1801 if OS returns: then Perl returns:
1802 -1 undefined value
1803 0 string "0 but true"
1804 anything else that number
1805
1806Thus Perl returns TRUE on success and FALSE on failure, yet you can
1807still easily determine the actual value returned by the operating
1808system:
1809
1810 ($retval = ioctl(...)) || ($retval = -1);
1811 printf "System returned %d\n", $retval;
1812
5a964f20
TC
1813The special string "0 but true" is excempt from B<-w> complaints
1814about improper numeric conversions.
1815
a0d0e21e
LW
1816=item join EXPR,LIST
1817
54310121 1818Joins the separate strings of LIST into a single string with
a0d0e21e
LW
1819fields separated by the value of EXPR, and returns the string.
1820Example:
1821
1822 $_ = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
1823
1824See L<perlfunc/split>.
1825
aa689395 1826=item keys HASH
1827
1d2dff63
GS
1828Returns a list consisting of all the keys of the named hash. (In a
1829scalar context, returns the number of keys.) The keys are returned in
aa689395 1830an apparently random order, but it is the same order as either the
1831values() or each() function produces (given that the hash has not been
1832modified). As a side effect, it resets HASH's iterator.
a0d0e21e 1833
aa689395 1834Here is yet another way to print your environment:
a0d0e21e
LW
1835
1836 @keys = keys %ENV;
1837 @values = values %ENV;
1838 while ($#keys >= 0) {
1839 print pop(@keys), '=', pop(@values), "\n";
1840 }
1841
1842or how about sorted by key:
1843
1844 foreach $key (sort(keys %ENV)) {
1845 print $key, '=', $ENV{$key}, "\n";
1846 }
1847
54310121 1848To sort an array by value, you'll need to use a C<sort> function.
aa689395 1849Here's a descending numeric sort of a hash by its values:
4633a7c4 1850
5a964f20 1851 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
4633a7c4
LW
1852 printf "%4d %s\n", $hash{$key}, $key;
1853 }
1854
55497cff 1855As an lvalue C<keys> allows you to increase the number of hash buckets
aa689395 1856allocated for the given hash. This can gain you a measure of efficiency if
1857you know the hash is going to get big. (This is similar to pre-extending
1858an array by assigning a larger number to $#array.) If you say
55497cff 1859
1860 keys %hash = 200;
1861
5a964f20
TC
1862then C<%hash> will have at least 200 buckets allocated for it--256 of them, in fact, since
1863it rounds up to the next power of two. These
55497cff 1864buckets will be retained even if you do C<%hash = ()>, use C<undef
1865%hash> if you want to free the storage while C<%hash> is still in scope.
1866You can't shrink the number of buckets allocated for the hash using
1867C<keys> in this way (but you needn't worry about doing this by accident,
1868as trying has no effect).
1869
a0d0e21e
LW
1870=item kill LIST
1871
54310121 1872Sends a signal to a list of processes. The first element of
1873the list must be the signal to send. Returns the number of
4633a7c4 1874processes successfully signaled.
a0d0e21e
LW
1875
1876 $cnt = kill 1, $child1, $child2;
1877 kill 9, @goners;
1878
4633a7c4
LW
1879Unlike in the shell, in Perl if the I<SIGNAL> is negative, it kills
1880process groups instead of processes. (On System V, a negative I<PROCESS>
1881number will also kill process groups, but that's not portable.) That
1882means you usually want to use positive not negative signals. You may also
da0045b7 1883use a signal name in quotes. See L<perlipc/"Signals"> for details.
a0d0e21e
LW
1884
1885=item last LABEL
1886
1887=item last
1888
1889The C<last> command is like the C<break> statement in C (as used in
1890loops); it immediately exits the loop in question. If the LABEL is
1891omitted, the command refers to the innermost enclosing loop. The
1892C<continue> block, if any, is not executed:
1893
4633a7c4
LW
1894 LINE: while (<STDIN>) {
1895 last LINE if /^$/; # exit when done with header
5a964f20 1896 #...
a0d0e21e
LW
1897 }
1898
1d2dff63
GS
1899See also L</continue> for an illustration of how C<last>, C<next>, and
1900C<redo> work.
1901
a0d0e21e
LW
1902=item lc EXPR
1903
54310121 1904=item lc
bbce6d69 1905
a0d0e21e 1906Returns an lowercased version of EXPR. This is the internal function
54310121 1907implementing the \L escape in double-quoted strings.
a034a98d 1908Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
a0d0e21e 1909
bbce6d69 1910If EXPR is omitted, uses $_.
1911
a0d0e21e
LW
1912=item lcfirst EXPR
1913
54310121 1914=item lcfirst
bbce6d69 1915
a0d0e21e
LW
1916Returns the value of EXPR with the first character lowercased. This is
1917the internal function implementing the \l escape in double-quoted strings.
a034a98d 1918Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
a0d0e21e 1919
bbce6d69 1920If EXPR is omitted, uses $_.
1921
a0d0e21e
LW
1922=item length EXPR
1923
54310121 1924=item length
bbce6d69 1925
5a964f20 1926Returns the length in bytes of the value of EXPR. If EXPR is
a0d0e21e
LW
1927omitted, returns length of $_.
1928
1929=item link OLDFILE,NEWFILE
1930
5a964f20
TC
1931Creates a new filename linked to the old filename. Returns TRUE for
1932success, FALSE otherwise.
a0d0e21e
LW
1933
1934=item listen SOCKET,QUEUESIZE
1935
1936Does the same thing that the listen system call does. Returns TRUE if
4633a7c4 1937it succeeded, FALSE otherwise. See example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
1938
1939=item local EXPR
1940
5a964f20
TC
1941A local modifies the listed variables to be local to the enclosing
1942block, file, or eval. If more than one value is listed, the list must
1943be placed in parentheses. See L<perlsub/"Temporary Values via local()">
1944for details, including issues with tied arrays and hashes.
a0d0e21e 1945
7b8d334a
GS
1946You really probably want to be using my() instead, because local() isn't
1947what most people think of as "local". See L<perlsub/"Private Variables
cb1a09d0 1948via my()"> for details.
a0d0e21e
LW
1949
1950=item localtime EXPR
1951
1952Converts a time as returned by the time function to a 9-element array
5f05dabc 1953with the time analyzed for the local time zone. Typically used as
a0d0e21e
LW
1954follows:
1955
54310121 1956 # 0 1 2 3 4 5 6 7 8
a0d0e21e
LW
1957 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1958 localtime(time);
1959
1960All array elements are numeric, and come straight out of a struct tm.
1961In particular this means that $mon has the range 0..11 and $wday has
54310121 1962the range 0..6 with sunday as day 0. Also, $year is the number of
1963years since 1900, that is, $year is 123 in year 2023.
1964
1965If EXPR is omitted, uses the current time (C<localtime(time)>).
a0d0e21e 1966
5a964f20 1967In scalar context, returns the ctime(3) value:
a0d0e21e 1968
5f05dabc 1969 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
a0d0e21e 1970
a3cb178b
GS
1971This scalar value is B<not> locale dependent, see L<perllocale>, but
1972instead a Perl builtin. Also see the Time::Local module, and the
1973strftime(3) and mktime(3) function available via the POSIX module. To
1974get somewhat similar but locale dependent date strings, set up your
1975locale environment variables appropriately (please see L<perllocale>)
5a964f20 1976and try for example:
a3cb178b 1977
5a964f20 1978 use POSIX qw(strftime);
a3cb178b
GS
1979 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
1980
1981Note that the C<%a> and C<%b>, the short forms of the day of the week
1982and the month of the year, may not necessarily be three characters wide.
a0d0e21e
LW
1983
1984=item log EXPR
1985
54310121 1986=item log
bbce6d69 1987
5a964f20 1988Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted, returns log
a0d0e21e
LW
1989of $_.
1990
1991=item lstat FILEHANDLE
1992
1993=item lstat EXPR
1994
54310121 1995=item lstat
bbce6d69 1996
5a964f20
TC
1997Does the same thing as the stat() function (including setting the
1998special C<_> filehandle) but stats a symbolic link instead of the file
1999the symbolic link points to. If symbolic links are unimplemented on
2000your system, a normal stat() is done.
a0d0e21e 2001
bbce6d69 2002If EXPR is omitted, stats $_.
2003
a0d0e21e
LW
2004=item m//
2005
2006The match operator. See L<perlop>.
2007
2008=item map BLOCK LIST
2009
2010=item map EXPR,LIST
2011
2012Evaluates the BLOCK or EXPR for each element of LIST (locally setting $_ to each
2013element) and returns the list value composed of the results of each such
2014evaluation. Evaluates BLOCK or EXPR in a list context, so each element of LIST
2015may produce zero, one, or more elements in the returned value.
2016
2017 @chars = map(chr, @nums);
2018
2019translates a list of numbers to the corresponding characters. And
2020
4633a7c4 2021 %hash = map { getkey($_) => $_ } @array;
a0d0e21e
LW
2022
2023is just a funny way to write
2024
2025 %hash = ();
2026 foreach $_ (@array) {
4633a7c4 2027 $hash{getkey($_)} = $_;
a0d0e21e
LW
2028 }
2029
fb73857a 2030Note that, because $_ is a reference into the list value, it can be used
2031to modify the elements of the array. While this is useful and
2032supported, it can cause bizarre results if the LIST is not a named
2033array. See also L</grep> for an array composed of those items of the
2034original list for which the BLOCK or EXPR evaluates to true.
2035
a0d0e21e
LW
2036=item mkdir FILENAME,MODE
2037
2038Creates the directory specified by FILENAME, with permissions specified
5a964f20
TC
2039by MODE (as modified by umask). If it succeeds it returns TRUE, otherwise
2040it returns FALSE and sets C<$!> (errno).
a0d0e21e
LW
2041
2042=item msgctl ID,CMD,ARG
2043
4633a7c4 2044Calls the System V IPC function msgctl(2). If CMD is &IPC_STAT, then ARG
5a964f20 2045must be a variable that will hold the returned msqid_ds structure.
a0d0e21e
LW
2046Returns like ioctl: the undefined value for error, "0 but true" for
2047zero, or the actual return value otherwise.
2048
2049=item msgget KEY,FLAGS
2050
4633a7c4 2051Calls the System V IPC function msgget(2). Returns the message queue id,
a0d0e21e
LW
2052or the undefined value if there is an error.
2053
2054=item msgsnd ID,MSG,FLAGS
2055
2056Calls the System V IPC function msgsnd to send the message MSG to the
2057message queue ID. MSG must begin with the long integer message type,
c07a80fd 2058which may be created with C<pack("l", $type)>. Returns TRUE if
a0d0e21e
LW
2059successful, or FALSE if there is an error.
2060
2061=item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2062
2063Calls the System V IPC function msgrcv to receive a message from
2064message queue ID into variable VAR with a maximum message size of
2065SIZE. Note that if a message is received, the message type will be the
2066first thing in VAR, and the maximum length of VAR is SIZE plus the size
2067of the message type. Returns TRUE if successful, or FALSE if there is
2068an error.
2069
2070=item my EXPR
2071
2072A "my" declares the listed variables to be local (lexically) to the
5a964f20 2073enclosing block, file, or C<eval>. If
5f05dabc 2074more than one value is listed, the list must be placed in parentheses. See
cb1a09d0 2075L<perlsub/"Private Variables via my()"> for details.
4633a7c4 2076
a0d0e21e
LW
2077=item next LABEL
2078
2079=item next
2080
2081The C<next> command is like the C<continue> statement in C; it starts
2082the next iteration of the loop:
2083
4633a7c4
LW
2084 LINE: while (<STDIN>) {
2085 next LINE if /^#/; # discard comments
5a964f20 2086 #...
a0d0e21e
LW
2087 }
2088
2089Note that if there were a C<continue> block on the above, it would get
2090executed even on discarded lines. If the LABEL is omitted, the command
2091refers to the innermost enclosing loop.
2092
1d2dff63
GS
2093See also L</continue> for an illustration of how C<last>, C<next>, and
2094C<redo> work.
2095
a0d0e21e
LW
2096=item no Module LIST
2097
2098See the "use" function, which "no" is the opposite of.
2099
2100=item oct EXPR
2101
54310121 2102=item oct
bbce6d69 2103
4633a7c4 2104Interprets EXPR as an octal string and returns the corresponding
2f9daede 2105value. (If EXPR happens to start off with 0x, interprets it as
4633a7c4
LW
2106a hex string instead.) The following will handle decimal, octal, and
2107hex in the standard Perl or C notation:
a0d0e21e
LW
2108
2109 $val = oct($val) if $val =~ /^0/;
2110
2f9daede
TP
2111If EXPR is omitted, uses $_. This function is commonly used when
2112a string such as "644" needs to be converted into a file mode, for
2113example. (Although perl will automatically convert strings into
2114numbers as needed, this automatic conversion assumes base 10.)
a0d0e21e
LW
2115
2116=item open FILEHANDLE,EXPR
2117
2118=item open FILEHANDLE
2119
2120Opens the file whose filename is given by EXPR, and associates it with
5f05dabc 2121FILEHANDLE. If FILEHANDLE is an expression, its value is used as the
2122name of the real filehandle wanted. If EXPR is omitted, the scalar
2123variable of the same name as the FILEHANDLE contains the filename.
2124(Note that lexical variables--those declared with C<my>--will not work
2125for this purpose; so if you're using C<my>, specify EXPR in your call
2126to open.)
2127
2128If the filename begins with '<' or nothing, the file is opened for input.
2129If the filename begins with '>', the file is truncated and opened for
fbb426e4
TP
2130output, being created if necessary. If the filename begins with '>>',
2131the file is opened for appending, again being created if necessary.
2132You can put a '+' in front of the '>' or '<' to indicate that
5f05dabc 2133you want both read and write access to the file; thus '+<' is almost
2134always preferred for read/write updates--the '+>' mode would clobber the
5a964f20
TC
2135file first. You can't usually use either read-write mode for updating
2136textfiles, since they have variable length records. See the B<-i>
2137switch in L<perlrun> for a better approach.
2138
2139The prefix and the filename may be separated with spaces.
5f05dabc 2140These various prefixes correspond to the fopen(3) modes of 'r', 'r+', 'w',
2141'w+', 'a', and 'a+'.
2142
5a964f20
TC
2143If the filename begins with "|", the filename is interpreted as a
2144command to which output is to be piped, and if the filename ends with a
2145"|", the filename is interpreted See L<perlipc/"Using open() for IPC">
2146for more examples of this. (You are not allowed to open() to a command
2147that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2148and L<perlipc/"Bidirectional Communication"> for alternatives.)
cb1a09d0 2149
184e9718 2150Opening '-' opens STDIN and opening 'E<gt>-' opens STDOUT. Open returns
54310121 2151nonzero upon success, the undefined value otherwise. If the open
4633a7c4 2152involved a pipe, the return value happens to be the pid of the
54310121 2153subprocess.
cb1a09d0
AD
2154
2155If you're unfortunate enough to be running Perl on a system that
2156distinguishes between text files and binary files (modern operating
2157systems don't care), then you should check out L</binmode> for tips for
2158dealing with this. The key distinction between systems that need binmode
5a964f20
TC
2159and those that don't is their text file formats. Systems like Unix, MacOS, and
2160Plan9, which delimit lines with a single character, and which encode that
cb1a09d0
AD
2161character in C as '\n', do not need C<binmode>. The rest need it.
2162
fb73857a 2163When opening a file, it's usually a bad idea to continue normal execution
2164if the request failed, so C<open> is frequently used in connection with
2165C<die>. Even if C<die> won't do what you want (say, in a CGI script,
2166where you want to make a nicely formatted error message (but there are
5a964f20 2167modules that can help with that problem)) you should always check
fb73857a 2168the return value from opening a file. The infrequent exception is when
2169working with an unopened filehandle is actually what you want to do.
2170
cb1a09d0 2171Examples:
a0d0e21e
LW
2172
2173 $ARTICLE = 100;
2174 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2175 while (<ARTICLE>) {...
2176
2177 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
fb73857a 2178 # if the open fails, output is discarded
a0d0e21e 2179
fb73857a 2180 open(DBASE, '+<dbase.mine') # open for update
2181 or die "Can't open 'dbase.mine' for update: $!";
cb1a09d0 2182
fb73857a 2183 open(ARTICLE, "caesar <$article |") # decrypt article
2184 or die "Can't start caesar: $!";
a0d0e21e 2185
fb73857a 2186 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
2187 or die "Can't start sort: $!";
a0d0e21e
LW
2188
2189 # process argument list of files along with any includes
2190
2191 foreach $file (@ARGV) {
2192 process($file, 'fh00');
2193 }
2194
2195 sub process {
5a964f20 2196 my($filename, $input) = @_;
a0d0e21e
LW
2197 $input++; # this is a string increment
2198 unless (open($input, $filename)) {
2199 print STDERR "Can't open $filename: $!\n";
2200 return;
2201 }
2202
5a964f20 2203 local $_;
a0d0e21e
LW
2204 while (<$input>) { # note use of indirection
2205 if (/^#include "(.*)"/) {
2206 process($1, $input);
2207 next;
2208 }
5a964f20 2209 #... # whatever
a0d0e21e
LW
2210 }
2211 }
2212
2213You may also, in the Bourne shell tradition, specify an EXPR beginning
184e9718 2214with "E<gt>&", in which case the rest of the string is interpreted as the
5a964f20 2215name of a filehandle (or file descriptor, if numeric) to be
184e9718 2216duped and opened. You may use & after E<gt>, E<gt>E<gt>, E<lt>, +E<gt>,
5f05dabc 2217+E<gt>E<gt>, and +E<lt>. The
a0d0e21e 2218mode you specify should match the mode of the original filehandle.
184e9718 2219(Duping a filehandle does not take into account any existing contents of
cb1a09d0 2220stdio buffers.)
a0d0e21e
LW
2221Here is a script that saves, redirects, and restores STDOUT and
2222STDERR:
2223
2224 #!/usr/bin/perl
5a964f20
TC
2225 open(OLDOUT, ">&STDOUT");
2226 open(OLDERR, ">&STDERR");
a0d0e21e
LW
2227
2228 open(STDOUT, ">foo.out") || die "Can't redirect stdout";
2229 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
2230
2231 select(STDERR); $| = 1; # make unbuffered
2232 select(STDOUT); $| = 1; # make unbuffered
2233
2234 print STDOUT "stdout 1\n"; # this works for
2235 print STDERR "stderr 1\n"; # subprocesses too
2236
2237 close(STDOUT);
2238 close(STDERR);
2239
5a964f20
TC
2240 open(STDOUT, ">&OLDOUT");
2241 open(STDERR, ">&OLDERR");
a0d0e21e
LW
2242
2243 print STDOUT "stdout 2\n";
2244 print STDERR "stderr 2\n";
2245
2246
184e9718 2247If you specify "E<lt>&=N", where N is a number, then Perl will do an
4633a7c4
LW
2248equivalent of C's fdopen() of that file descriptor; this is more
2249parsimonious of file descriptors. For example:
a0d0e21e
LW
2250
2251 open(FILEHANDLE, "<&=$fd")
2252
5f05dabc 2253If you open a pipe on the command "-", i.e., either "|-" or "-|", then
a0d0e21e
LW
2254there is an implicit fork done, and the return value of open is the pid
2255of the child within the parent process, and 0 within the child
184e9718 2256process. (Use C<defined($pid)> to determine whether the open was successful.)
a0d0e21e
LW
2257The filehandle behaves normally for the parent, but i/o to that
2258filehandle is piped from/to the STDOUT/STDIN of the child process.
2259In the child process the filehandle isn't opened--i/o happens from/to
2260the new STDOUT or STDIN. Typically this is used like the normal
2261piped open when you want to exercise more control over just how the
2262pipe command gets executed, such as when you are running setuid, and
54310121 2263don't want to have to scan shell commands for metacharacters.
4633a7c4 2264The following pairs are more or less equivalent:
a0d0e21e
LW
2265
2266 open(FOO, "|tr '[a-z]' '[A-Z]'");
2267 open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
2268
2269 open(FOO, "cat -n '$file'|");
2270 open(FOO, "-|") || exec 'cat', '-n', $file;
2271
4633a7c4
LW
2272See L<perlipc/"Safe Pipe Opens"> for more examples of this.
2273
5a964f20 2274NOTE: On any operation that may do a fork, any unflushed buffers remain
184e9718 2275unflushed in both processes, which means you may need to set C<$|> to
a0d0e21e
LW
2276avoid duplicate output.
2277
0dccf244
CS
2278Closing any piped filehandle causes the parent process to wait for the
2279child to finish, and returns the status value in C<$?>.
2280
5a964f20
TC
2281The filename passed to open will have leading and trailing
2282whitespace deleted, and the normal redirection chararacters
2283honored. This property, known as "magic open",
2284can often be used to good effect. A user could specify a filename of
2285"rsh cat file |", or you could change certain filenames as needed:
2286
2287 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
2288 open(FH, $filename) or die "Can't open $filename: $!";
2289
2290However, to open a file with arbitrary weird characters in it, it's
2291necessary to protect any leading and trailing whitespace:
2292
2293 $file =~ s#^(\s)#./$1#;
2294 open(FOO, "< $file\0");
2295
2296If you want a "real" C open() (see L<open(2)> on your system), then you
2297should use the sysopen() function, which involves no such magic. This is
2298another way to protect your filenames from interpretation. For example:
2299
2300 use IO::Handle;
2301 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
2302 or die "sysopen $path: $!";
2303 $oldfh = select(HANDLE); $| = 1; select($oldfh);
2304 print HANDLE "stuff $$\n");
2305 seek(HANDLE, 0, 0);
2306 print "File contains: ", <HANDLE>;
2307
5f05dabc 2308Using the constructor from the IO::Handle package (or one of its
5a964f20
TC
2309subclasses, such as IO::File or IO::Socket), you can generate anonymous
2310filehandles that have the scope of whatever variables hold references to
2311them, and automatically close whenever and however you leave that scope:
c07a80fd 2312
5f05dabc 2313 use IO::File;
5a964f20 2314 #...
c07a80fd 2315 sub read_myfile_munged {
2316 my $ALL = shift;
5f05dabc 2317 my $handle = new IO::File;
c07a80fd 2318 open($handle, "myfile") or die "myfile: $!";
2319 $first = <$handle>
2320 or return (); # Automatically closed here.
2321 mung $first or die "mung failed"; # Or here.
2322 return $first, <$handle> if $ALL; # Or here.
2323 $first; # Or here.
2324 }
2325
cb1a09d0 2326See L</seek()> for some details about mixing reading and writing.
a0d0e21e
LW
2327
2328=item opendir DIRHANDLE,EXPR
2329
2330Opens a directory named EXPR for processing by readdir(), telldir(),
5f05dabc 2331seekdir(), rewinddir(), and closedir(). Returns TRUE if successful.
a0d0e21e
LW
2332DIRHANDLEs have their own namespace separate from FILEHANDLEs.
2333
2334=item ord EXPR
2335
54310121 2336=item ord
bbce6d69 2337
a0d0e21e 2338Returns the numeric ascii value of the first character of EXPR. If
dc848c6f 2339EXPR is omitted, uses $_. For the reverse, see L</chr>.
a0d0e21e
LW
2340
2341=item pack TEMPLATE,LIST
2342
2343Takes an array or list of values and packs it into a binary structure,
2344returning the string containing the structure. The TEMPLATE is a
2345sequence of characters that give the order and type of values, as
2346follows:
2347
2348 A An ascii string, will be space padded.
2349 a An ascii string, will be null padded.
2350 b A bit string (ascending bit order, like vec()).
2351 B A bit string (descending bit order).
2352 h A hex string (low nybble first).
2353 H A hex string (high nybble first).
2354
2355 c A signed char value.
2356 C An unsigned char value.
96e4d5b1 2357
a0d0e21e
LW
2358 s A signed short value.
2359 S An unsigned short value.
96e4d5b1 2360 (This 'short' is _exactly_ 16 bits, which may differ from
2361 what a local C compiler calls 'short'.)
2362
a0d0e21e
LW
2363 i A signed integer value.
2364 I An unsigned integer value.
96e4d5b1 2365 (This 'integer' is _at_least_ 32 bits wide. Its exact size
2366 depends on what a local C compiler calls 'int', and may
2367 even be larger than the 'long' described in the next item.)
2368
a0d0e21e
LW
2369 l A signed long value.
2370 L An unsigned long value.
96e4d5b1 2371 (This 'long' is _exactly_ 32 bits, which may differ from
2372 what a local C compiler calls 'long'.)
a0d0e21e 2373
96e4d5b1 2374 n A short in "network" (big-endian) order.
2375 N A long in "network" (big-endian) order.
a0d0e21e
LW
2376 v A short in "VAX" (little-endian) order.
2377 V A long in "VAX" (little-endian) order.
96e4d5b1 2378 (These 'shorts' and 'longs' are _exactly_ 16 bits and
2379 _exactly_ 32 bits, respectively.)
a0d0e21e
LW
2380
2381 f A single-precision float in the native format.
2382 d A double-precision float in the native format.
2383
2384 p A pointer to a null-terminated string.
2385 P A pointer to a structure (fixed-length string).
2386
2387 u A uuencoded string.
2388
96e4d5b1 2389 w A BER compressed integer. Its bytes represent an unsigned
2390 integer in base 128, most significant digit first, with as few
2391 digits as possible. Bit eight (the high bit) is set on each
2392 byte except the last.
def98dd4 2393
a0d0e21e
LW
2394 x A null byte.
2395 X Back up a byte.
2396 @ Null fill to absolute position.
2397
5a964f20 2398Each letter may optionally be followed by a number giving a repeat
5f05dabc 2399count. With all types except "a", "A", "b", "B", "h", "H", and "P" the
a0d0e21e
LW
2400pack function will gobble up that many values from the LIST. A * for the
2401repeat count means to use however many items are left. The "a" and "A"
2402types gobble just one value, but pack it as a string of length count,
2403padding with nulls or spaces as necessary. (When unpacking, "A" strips
2404trailing spaces and nulls, but "a" does not.) Likewise, the "b" and "B"
2405fields pack a string that many bits long. The "h" and "H" fields pack a
84902520
TB
2406string that many nybbles long. The "p" type packs a pointer to a null-
2407terminated string. You are responsible for ensuring the string is not a
2408temporary value (which can potentially get deallocated before you get
2409around to using the packed result). The "P" packs a pointer to a structure
61167c6f
PM
2410of the size indicated by the length. A NULL pointer is created if the
2411corresponding value for "p" or "P" is C<undef>.
2412Real numbers (floats and doubles) are
a0d0e21e
LW
2413in the native machine format only; due to the multiplicity of floating
2414formats around, and the lack of a standard "network" representation, no
2415facility for interchange has been made. This means that packed floating
2416point data written on one machine may not be readable on another - even if
2417both use IEEE floating point arithmetic (as the endian-ness of the memory
2418representation is not part of the IEEE spec). Note that Perl uses doubles
2419internally for all numeric calculation, and converting from double into
5f05dabc 2420float and thence back to double again will lose precision (i.e.,
a0d0e21e
LW
2421C<unpack("f", pack("f", $foo)>) will not in general equal $foo).
2422
2423Examples:
2424
2425 $foo = pack("cccc",65,66,67,68);
2426 # foo eq "ABCD"
2427 $foo = pack("c4",65,66,67,68);
2428 # same thing
2429
2430 $foo = pack("ccxxcc",65,66,67,68);
2431 # foo eq "AB\0\0CD"
2432
2433 $foo = pack("s2",1,2);
2434 # "\1\0\2\0" on little-endian
2435 # "\0\1\0\2" on big-endian
2436
2437 $foo = pack("a4","abcd","x","y","z");
2438 # "abcd"
2439
2440 $foo = pack("aaaa","abcd","x","y","z");
2441 # "axyz"
2442
2443 $foo = pack("a14","abcdefg");
2444 # "abcdefg\0\0\0\0\0\0\0"
2445
2446 $foo = pack("i9pl", gmtime);
2447 # a real struct tm (on my system anyway)
2448
2449 sub bintodec {
2450 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
2451 }
2452
2453The same template may generally also be used in the unpack function.
2454
5a964f20
TC
2455=item package
2456
cb1a09d0
AD
2457=item package NAMESPACE
2458
2459Declares the compilation unit as being in the given namespace. The scope
2460of the package declaration is from the declaration itself through the end of
2461the enclosing block (the same scope as the local() operator). All further
2462unqualified dynamic identifiers will be in this namespace. A package
5f05dabc 2463statement affects only dynamic variables--including those you've used
cb1a09d0
AD
2464local() on--but I<not> lexical variables created with my(). Typically it
2465would be the first declaration in a file to be included by the C<require>
2466or C<use> operator. You can switch into a package in more than one place;
5a964f20 2467it merely influences which symbol table is used by the compiler for the
cb1a09d0
AD
2468rest of that block. You can refer to variables and filehandles in other
2469packages by prefixing the identifier with the package name and a double
2470colon: C<$Package::Variable>. If the package name is null, the C<main>
2471package as assumed. That is, C<$::sail> is equivalent to C<$main::sail>.
2472
5a964f20
TC
2473If NAMESPACE is omitted, then there is no current package, and all
2474identifiers must be fully qualified or lexicals. This is stricter
2475than C<use strict>, since it also extends to function names.
2476
cb1a09d0
AD
2477See L<perlmod/"Packages"> for more information about packages, modules,
2478and classes. See L<perlsub> for other scoping issues.
2479
a0d0e21e
LW
2480=item pipe READHANDLE,WRITEHANDLE
2481
2482Opens a pair of connected pipes like the corresponding system call.
2483Note that if you set up a loop of piped processes, deadlock can occur
2484unless you are very careful. In addition, note that Perl's pipes use
184e9718 2485stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE
a0d0e21e
LW
2486after each command, depending on the application.
2487
7e1af8bc 2488See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
4633a7c4
LW
2489for examples of such things.
2490
a0d0e21e
LW
2491=item pop ARRAY
2492
54310121 2493=item pop
28757baa 2494
a0d0e21e
LW
2495Pops and returns the last value of the array, shortening the array by
24961. Has a similar effect to
2497
2498 $tmp = $ARRAY[$#ARRAY--];
2499
2500If there are no elements in the array, returns the undefined value.
cb1a09d0
AD
2501If ARRAY is omitted, pops the
2502@ARGV array in the main program, and the @_ array in subroutines, just
2503like shift().
a0d0e21e
LW
2504
2505=item pos SCALAR
2506
54310121 2507=item pos
bbce6d69 2508
4633a7c4 2509Returns the offset of where the last C<m//g> search left off for the variable
2f9daede 2510is in question ($_ is used when the variable is not specified). May be
44a8e56a 2511modified to change that offset. Such modification will also influence
2512the C<\G> zero-width assertion in regular expressions. See L<perlre> and
2513L<perlop>.
a0d0e21e
LW
2514
2515=item print FILEHANDLE LIST
2516
2517=item print LIST
2518
2519=item print
2520
cb1a09d0 2521Prints a string or a comma-separated list of strings. Returns TRUE
a0d0e21e 2522if successful. FILEHANDLE may be a scalar variable name, in which case
cb1a09d0 2523the variable contains the name of or a reference to the filehandle, thus introducing one
a0d0e21e
LW
2524level of indirection. (NOTE: If FILEHANDLE is a variable and the next
2525token is a term, it may be misinterpreted as an operator unless you
5f05dabc 2526interpose a + or put parentheses around the arguments.) If FILEHANDLE is
a0d0e21e 2527omitted, prints by default to standard output (or to the last selected
da0045b7 2528output channel--see L</select>). If LIST is also omitted, prints $_ to
5a964f20 2529the currently selected output channel. To set the default output channel to something other than
a0d0e21e 2530STDOUT use the select operation. Note that, because print takes a
5a964f20 2531LIST, anything in the LIST is evaluated in list context, and any
a0d0e21e 2532subroutine that you call will have one or more of its expressions
5a964f20 2533evaluated in list context. Also be careful not to follow the print
a0d0e21e
LW
2534keyword with a left parenthesis unless you want the corresponding right
2535parenthesis to terminate the arguments to the print--interpose a + or
5f05dabc 2536put parentheses around all the arguments.
a0d0e21e 2537
4633a7c4 2538Note that if you're storing FILEHANDLES in an array or other expression,
da0045b7 2539you will have to use a block returning its value instead:
4633a7c4
LW
2540
2541 print { $files[$i] } "stuff\n";
2542 print { $OK ? STDOUT : STDERR } "stuff\n";
2543
5f05dabc 2544=item printf FILEHANDLE FORMAT, LIST
a0d0e21e 2545
5f05dabc 2546=item printf FORMAT, LIST
a0d0e21e 2547
a3cb178b
GS
2548Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that $\
2549(the output record separator) is not appended. The first argument
a034a98d
DD
2550of the list will be interpreted as the printf format. If C<use locale> is
2551in effect, the character used for the decimal point in formatted real numbers
2552is affected by the LC_NUMERIC locale. See L<perllocale>.
a0d0e21e 2553
28757baa 2554Don't fall into the trap of using a printf() when a simple
5a964f20 2555print() would do. The print() is more efficient and less
28757baa 2556error prone.
2557
da0045b7 2558=item prototype FUNCTION
2559
2560Returns the prototype of a function as a string (or C<undef> if the
5f05dabc 2561function has no prototype). FUNCTION is a reference to, or the name of,
2562the function whose prototype you want to retrieve.
da0045b7 2563
b6c543e3
IZ
2564If FUNCTION is a string starting with C<CORE::>, the rest is taken as
2565a name for Perl builtin. If builtin is not I<overridable> (such as
2566C<qw>) or its arguments cannot be expressed by a prototype (such as
2567C<system>) - in other words, the builtin does not behave like a Perl
2568function - returns C<undef>. Otherwise, the string describing the
2569equivalent prototype is returned.
2570
a0d0e21e
LW
2571=item push ARRAY,LIST
2572
2573Treats ARRAY as a stack, and pushes the values of LIST
2574onto the end of ARRAY. The length of ARRAY increases by the length of
2575LIST. Has the same effect as
2576
2577 for $value (LIST) {
2578 $ARRAY[++$#ARRAY] = $value;
2579 }
2580
2581but is more efficient. Returns the new number of elements in the array.
2582
2583=item q/STRING/
2584
2585=item qq/STRING/
2586
2587=item qx/STRING/
2588
2589=item qw/STRING/
2590
2591Generalized quotes. See L<perlop>.
2592
2593=item quotemeta EXPR
2594
54310121 2595=item quotemeta
bbce6d69 2596
68dc0745 2597Returns the value of EXPR with all non-alphanumeric
a034a98d
DD
2598characters backslashed. (That is, all characters not matching
2599C</[A-Za-z_0-9]/> will be preceded by a backslash in the
2600returned string, regardless of any locale settings.)
2601This is the internal function implementing
a0d0e21e
LW
2602the \Q escape in double-quoted strings.
2603
bbce6d69 2604If EXPR is omitted, uses $_.
2605
a0d0e21e
LW
2606=item rand EXPR
2607
2608=item rand
2609
3e3baf6d
TB
2610Returns a random fractional number greater than or equal to 0 and less
2611than the value of EXPR. (EXPR should be positive.) If EXPR is
2612omitted, the value 1 is used. Automatically calls srand() unless
2613srand() has already been called. See also srand().
a0d0e21e 2614
2f9daede 2615(Note: If your rand function consistently returns numbers that are too
a0d0e21e 2616large or too small, then your version of Perl was probably compiled
2f9daede 2617with the wrong number of RANDBITS.)
a0d0e21e
LW
2618
2619=item read FILEHANDLE,SCALAR,LENGTH,OFFSET
2620
2621=item read FILEHANDLE,SCALAR,LENGTH
2622
2623Attempts to read LENGTH bytes of data into variable SCALAR from the
2624specified FILEHANDLE. Returns the number of bytes actually read, or
2625undef if there was an error. SCALAR will be grown or shrunk to the
2626length actually read. An OFFSET may be specified to place the read
2627data at some other place than the beginning of the string. This call
5a964f20
TC
2628is actually implemented in terms of stdio's fread(3) call. To get a true
2629read(2) system call, see sysread().
a0d0e21e
LW
2630
2631=item readdir DIRHANDLE
2632
2633Returns the next directory entry for a directory opened by opendir().
5a964f20 2634If used in list context, returns all the rest of the entries in the
a0d0e21e 2635directory. If there are no more entries, returns an undefined value in
5a964f20 2636scalar context or a null list in list context.
a0d0e21e 2637
cb1a09d0 2638If you're planning to filetest the return values out of a readdir(), you'd
5f05dabc 2639better prepend the directory in question. Otherwise, because we didn't
cb1a09d0
AD
2640chdir() there, it would have been testing the wrong file.
2641
2642 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
2643 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
2644 closedir DIR;
2645
84902520
TB
2646=item readline EXPR
2647
5a964f20 2648Reads from the filehandle whose typeglob is contained in EXPR. In scalar context, a single line
84902520
TB
2649is read and returned. In list context, reads until end-of-file is
2650reached and returns a list of lines (however you've defined lines
2651with $/ or $INPUT_RECORD_SEPARATOR).
2652This is the internal function implementing the C<E<lt>EXPRE<gt>>
2653operator, but you can use it directly. The C<E<lt>EXPRE<gt>>
2654operator is discussed in more detail in L<perlop/"I/O Operators">.
2655
5a964f20
TC
2656 $line = <STDIN>;
2657 $line = readline(*STDIN); # same thing
2658
a0d0e21e
LW
2659=item readlink EXPR
2660
54310121 2661=item readlink
bbce6d69 2662
a0d0e21e
LW
2663Returns the value of a symbolic link, if symbolic links are
2664implemented. If not, gives a fatal error. If there is some system
184e9718 2665error, returns the undefined value and sets C<$!> (errno). If EXPR is
a0d0e21e
LW
2666omitted, uses $_.
2667
84902520
TB
2668=item readpipe EXPR
2669
5a964f20 2670EXPR is executed as a system command.
84902520
TB
2671The collected standard output of the command is returned.
2672In scalar context, it comes back as a single (potentially
2673multi-line) string. In list context, returns a list of lines
2674(however you've defined lines with $/ or $INPUT_RECORD_SEPARATOR).
2675This is the internal function implementing the C<qx/EXPR/>
2676operator, but you can use it directly. The C<qx/EXPR/>
2677operator is discussed in more detail in L<perlop/"I/O Operators">.
2678
a0d0e21e
LW
2679=item recv SOCKET,SCALAR,LEN,FLAGS
2680
2681Receives a message on a socket. Attempts to receive LENGTH bytes of
2682data into variable SCALAR from the specified SOCKET filehandle.
a3cb178b 2683Actually does a C recvfrom(), so that it can return the address of the
a0d0e21e
LW
2684sender. Returns the undefined value if there's an error. SCALAR will
2685be grown or shrunk to the length actually read. Takes the same flags
54310121 2686as the system call of the same name.
4633a7c4 2687See L<perlipc/"UDP: Message Passing"> for examples.
a0d0e21e
LW
2688
2689=item redo LABEL
2690
2691=item redo
2692
2693The C<redo> command restarts the loop block without evaluating the
2694conditional again. The C<continue> block, if any, is not executed. If
2695the LABEL is omitted, the command refers to the innermost enclosing
2696loop. This command is normally used by programs that want to lie to
2697themselves about what was just input:
2698
2699 # a simpleminded Pascal comment stripper
2700 # (warning: assumes no { or } in strings)
4633a7c4 2701 LINE: while (<STDIN>) {
a0d0e21e
LW
2702 while (s|({.*}.*){.*}|$1 |) {}
2703 s|{.*}| |;
2704 if (s|{.*| |) {
2705 $front = $_;
2706 while (<STDIN>) {
2707 if (/}/) { # end of comment?
5a964f20 2708 s|^|$front\{|;
4633a7c4 2709 redo LINE;
a0d0e21e
LW
2710 }
2711 }
2712 }
2713 print;
2714 }
2715
1d2dff63
GS
2716See also L</continue> for an illustration of how C<last>, C<next>, and
2717C<redo> work.
2718
a0d0e21e
LW
2719=item ref EXPR
2720
54310121 2721=item ref
bbce6d69 2722
2f9daede
TP
2723Returns a TRUE value if EXPR is a reference, FALSE otherwise. If EXPR
2724is not specified, $_ will be used. The value returned depends on the
bbce6d69 2725type of thing the reference is a reference to.
a0d0e21e
LW
2726Builtin types include:
2727
2728 REF
2729 SCALAR
2730 ARRAY
2731 HASH
2732 CODE
2733 GLOB
2734
54310121 2735If the referenced object has been blessed into a package, then that package
a0d0e21e
LW
2736name is returned instead. You can think of ref() as a typeof() operator.
2737
2738 if (ref($r) eq "HASH") {
aa689395 2739 print "r is a reference to a hash.\n";
54310121 2740 }
5a964f20 2741 if (!ref($r)) {
a0d0e21e 2742 print "r is not a reference at all.\n";
54310121 2743 }
a0d0e21e
LW
2744
2745See also L<perlref>.
2746
2747=item rename OLDNAME,NEWNAME
2748
2749Changes the name of a file. Returns 1 for success, 0 otherwise. Will
5f05dabc 2750not work across file system boundaries.
a0d0e21e
LW
2751
2752=item require EXPR
2753
2754=item require
2755
2756Demands some semantics specified by EXPR, or by $_ if EXPR is not
2757supplied. If EXPR is numeric, demands that the current version of Perl
184e9718 2758(C<$]> or $PERL_VERSION) be equal or greater than EXPR.
a0d0e21e
LW
2759
2760Otherwise, demands that a library file be included if it hasn't already
2761been included. The file is included via the do-FILE mechanism, which is
2762essentially just a variety of eval(). Has semantics similar to the following
2763subroutine:
2764
2765 sub require {
5a964f20 2766 my($filename) = @_;
a0d0e21e 2767 return 1 if $INC{$filename};
5a964f20 2768 my($realfilename,$result);
a0d0e21e
LW
2769 ITER: {
2770 foreach $prefix (@INC) {
2771 $realfilename = "$prefix/$filename";
2772 if (-f $realfilename) {
2773 $result = do $realfilename;
2774 last ITER;
2775 }
2776 }
2777 die "Can't find $filename in \@INC";
2778 }
2779 die $@ if $@;
2780 die "$filename did not return true value" unless $result;
2781 $INC{$filename} = $realfilename;
5a964f20 2782 return $result;
a0d0e21e
LW
2783 }
2784
2785Note that the file will not be included twice under the same specified
2786name. The file must return TRUE as the last statement to indicate
2787successful execution of any initialization code, so it's customary to
2788end such a file with "1;" unless you're sure it'll return TRUE
2789otherwise. But it's better just to put the "C<1;>", in case you add more
2790statements.
2791
54310121 2792If EXPR is a bareword, the require assumes a "F<.pm>" extension and
da0045b7 2793replaces "F<::>" with "F</>" in the filename for you,
54310121 2794to make it easy to load standard modules. This form of loading of
a0d0e21e
LW
2795modules does not risk altering your namespace.
2796
ee580363
GS
2797In other words, if you try this:
2798
5a964f20 2799 require Foo::Bar; # a splendid bareword
ee580363
GS
2800
2801The require function will actually look for the "Foo/Bar.pm" file in the
2802directories specified in the @INC array.
2803
5a964f20 2804But if you try this:
ee580363
GS
2805
2806 $class = 'Foo::Bar';
5a964f20
TC
2807 require $class; # $class is not a bareword
2808 #or
2809 require "Foo::Bar"; # not a bareword because of the ""
ee580363
GS
2810
2811The require function will look for the "Foo::Bar" file in the @INC array and
5a964f20 2812will complain about not finding "Foo::Bar" there. In this case you can do:
ee580363
GS
2813
2814 eval "require $class";
2815
2816For a yet-more-powerful import facility, see L</use> and L<perlmod>.
a0d0e21e
LW
2817
2818=item reset EXPR
2819
2820=item reset
2821
2822Generally used in a C<continue> block at the end of a loop to clear
2823variables and reset ?? searches so that they work again. The
2824expression is interpreted as a list of single characters (hyphens
2825allowed for ranges). All variables and arrays beginning with one of
2826those letters are reset to their pristine state. If the expression is
5f05dabc 2827omitted, one-match searches (?pattern?) are reset to match again. Resets
2828only variables or searches in the current package. Always returns
a0d0e21e
LW
28291. Examples:
2830
2831 reset 'X'; # reset all X variables
2832 reset 'a-z'; # reset lower case variables
2833 reset; # just reset ?? searches
2834
5f05dabc 2835Resetting "A-Z" is not recommended because you'll wipe out your
2836ARGV and ENV arrays. Resets only package variables--lexical variables
a0d0e21e 2837are unaffected, but they clean themselves up on scope exit anyway,
da0045b7 2838so you'll probably want to use them instead. See L</my>.
a0d0e21e 2839
54310121 2840=item return EXPR
2841
2842=item return
2843
5a964f20
TC
2844Returns from a subroutine, eval(), or C<do FILE> with the value
2845given in EXPR. Evaluation of EXPR may be in list, scalar, or void
54310121 2846context, depending on how the return value will be used, and the context
2847may vary from one execution to the next (see wantarray()). If no EXPR
5a964f20
TC
2848is given, returns an empty list in list context, an undefined value in
2849scalar context, or nothing in a void context.
a0d0e21e 2850
68dc0745 2851(Note that in the absence of a return, a subroutine, eval, or do FILE
2852will automatically return the value of the last expression evaluated.)
a0d0e21e
LW
2853
2854=item reverse LIST
2855
5a964f20
TC
2856In list context, returns a list value consisting of the elements
2857of LIST in the opposite order. In scalar context, concatenates the
2f9daede
TP
2858elements of LIST, and returns a string value consisting of those bytes,
2859but in the opposite order.
4633a7c4 2860
2f9daede 2861 print reverse <>; # line tac, last line first
4633a7c4 2862
2f9daede
TP
2863 undef $/; # for efficiency of <>
2864 print scalar reverse <>; # byte tac, last line tsrif
2865
2866This operator is also handy for inverting a hash, although there are some
2867caveats. If a value is duplicated in the original hash, only one of those
2868can be represented as a key in the inverted hash. Also, this has to
2869unwind one hash and build a whole new one, which may take some time
2870on a large hash.
2871
2872 %by_name = reverse %by_address; # Invert the hash
a0d0e21e
LW
2873
2874=item rewinddir DIRHANDLE
2875
2876Sets the current position to the beginning of the directory for the
2877readdir() routine on DIRHANDLE.
2878
2879=item rindex STR,SUBSTR,POSITION
2880
2881=item rindex STR,SUBSTR
2882
2883Works just like index except that it returns the position of the LAST
2884occurrence of SUBSTR in STR. If POSITION is specified, returns the
2885last occurrence at or before that position.
2886
2887=item rmdir FILENAME
2888
54310121 2889=item rmdir
bbce6d69 2890
5a964f20
TC
2891Deletes the directory specified by FILENAME if that directory is empty. If it
2892succeeds it returns TRUE, otherwise it returns FALSE and sets C<$!> (errno). If
a0d0e21e
LW
2893FILENAME is omitted, uses $_.
2894
2895=item s///
2896
2897The substitution operator. See L<perlop>.
2898
2899=item scalar EXPR
2900
5a964f20 2901Forces EXPR to be interpreted in scalar context and returns the value
54310121 2902of EXPR.
cb1a09d0
AD
2903
2904 @counts = ( scalar @a, scalar @b, scalar @c );
2905
54310121 2906There is no equivalent operator to force an expression to
5a964f20 2907be interpolated in list context because it's in practice never
cb1a09d0
AD
2908needed. If you really wanted to do so, however, you could use
2909the construction C<@{[ (some expression) ]}>, but usually a simple
2910C<(some expression)> suffices.
a0d0e21e
LW
2911
2912=item seek FILEHANDLE,POSITION,WHENCE
2913
8903cb82 2914Sets FILEHANDLE's position, just like the fseek() call of stdio.
2915FILEHANDLE may be an expression whose value gives the name of the
2916filehandle. The values for WHENCE are 0 to set the new position to
2917POSITION, 1 to set it to the current position plus POSITION, and 2 to
2918set it to EOF plus POSITION (typically negative). For WHENCE you may
2919use the constants SEEK_SET, SEEK_CUR, and SEEK_END from either the
2920IO::Seekable or the POSIX module. Returns 1 upon success, 0 otherwise.
2921
2922If you want to position file for sysread() or syswrite(), don't use
2923seek() -- buffering makes its effect on the file's system position
137443ea 2924unpredictable and non-portable. Use sysseek() instead.
a0d0e21e 2925
cb1a09d0
AD
2926On some systems you have to do a seek whenever you switch between reading
2927and writing. Amongst other things, this may have the effect of calling
8903cb82 2928stdio's clearerr(3). A WHENCE of 1 (SEEK_CUR) is useful for not moving
2929the file position:
cb1a09d0
AD
2930
2931 seek(TEST,0,1);
2932
2933This is also useful for applications emulating C<tail -f>. Once you hit
2934EOF on your read, and then sleep for a while, you might have to stick in a
8903cb82 2935seek() to reset things. The seek() doesn't change the current position,
2936but it I<does> clear the end-of-file condition on the handle, so that the
2937next C<E<lt>FILEE<gt>> makes Perl try again to read something. We hope.
cb1a09d0
AD
2938
2939If that doesn't work (some stdios are particularly cantankerous), then
2940you may need something more like this:
2941
2942 for (;;) {
2943 for ($curpos = tell(FILE); $_ = <FILE>; $curpos = tell(FILE)) {
2944 # search for some stuff and put it into files
2945 }
2946 sleep($for_a_while);
2947 seek(FILE, $curpos, 0);
2948 }
2949
a0d0e21e
LW
2950=item seekdir DIRHANDLE,POS
2951
2952Sets the current position for the readdir() routine on DIRHANDLE. POS
2953must be a value returned by telldir(). Has the same caveats about
2954possible directory compaction as the corresponding system library
2955routine.
2956
2957=item select FILEHANDLE
2958
2959=item select
2960
2961Returns the currently selected filehandle. Sets the current default
2962filehandle for output, if FILEHANDLE is supplied. This has two
2963effects: first, a C<write> or a C<print> without a filehandle will
2964default to this FILEHANDLE. Second, references to variables related to
2965output will refer to this output channel. For example, if you have to
2966set the top of form format for more than one output channel, you might
2967do the following:
2968
2969 select(REPORT1);
2970 $^ = 'report1_top';
2971 select(REPORT2);
2972 $^ = 'report2_top';
2973
2974FILEHANDLE may be an expression whose value gives the name of the
2975actual filehandle. Thus:
2976
2977 $oldfh = select(STDERR); $| = 1; select($oldfh);
2978
4633a7c4
LW
2979Some programmers may prefer to think of filehandles as objects with
2980methods, preferring to write the last example as:
a0d0e21e 2981
28757baa 2982 use IO::Handle;
a0d0e21e
LW
2983 STDERR->autoflush(1);
2984
2985=item select RBITS,WBITS,EBITS,TIMEOUT
2986
5f05dabc 2987This calls the select(2) system call with the bit masks specified, which
a0d0e21e
LW
2988can be constructed using fileno() and vec(), along these lines:
2989
2990 $rin = $win = $ein = '';
2991 vec($rin,fileno(STDIN),1) = 1;
2992 vec($win,fileno(STDOUT),1) = 1;
2993 $ein = $rin | $win;
2994
2995If you want to select on many filehandles you might wish to write a
2996subroutine:
2997
2998 sub fhbits {
5a964f20
TC
2999 my(@fhlist) = split(' ',$_[0]);
3000 my($bits);
a0d0e21e
LW
3001 for (@fhlist) {
3002 vec($bits,fileno($_),1) = 1;
3003 }
3004 $bits;
3005 }
4633a7c4 3006 $rin = fhbits('STDIN TTY SOCK');
a0d0e21e
LW
3007
3008The usual idiom is:
3009
3010 ($nfound,$timeleft) =
3011 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
3012
54310121 3013or to block until something becomes ready just do this
a0d0e21e
LW
3014
3015 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
3016
5f05dabc 3017Most systems do not bother to return anything useful in $timeleft, so
5a964f20 3018calling select() in scalar context just returns $nfound.
c07a80fd 3019
5f05dabc 3020Any of the bit masks can also be undef. The timeout, if specified, is
a0d0e21e
LW
3021in seconds, which may be fractional. Note: not all implementations are
3022capable of returning the $timeleft. If not, they always return
3023$timeleft equal to the supplied $timeout.
3024
ff68c719 3025You can effect a sleep of 250 milliseconds this way:
a0d0e21e
LW
3026
3027 select(undef, undef, undef, 0.25);
3028
5a964f20
TC
3029B<WARNING>: One should not attempt to mix buffered I/O (like read()
3030or E<lt>FHE<gt>) with select(), except as permitted by POSIX, and even
3031then only on POSIX systems. You have to use sysread() instead.
a0d0e21e
LW
3032
3033=item semctl ID,SEMNUM,CMD,ARG
3034
3035Calls the System V IPC function semctl. If CMD is &IPC_STAT or
5a964f20 3036&GETALL, then ARG must be a variable that will hold the returned
a0d0e21e
LW
3037semid_ds structure or semaphore value array. Returns like ioctl: the
3038undefined value for error, "0 but true" for zero, or the actual return
3039value otherwise.
3040
3041=item semget KEY,NSEMS,FLAGS
3042
3043Calls the System V IPC function semget. Returns the semaphore id, or
3044the undefined value if there is an error.
3045
3046=item semop KEY,OPSTRING
3047
3048Calls the System V IPC function semop to perform semaphore operations
3049such as signaling and waiting. OPSTRING must be a packed array of
3050semop structures. Each semop structure can be generated with
3051C<pack("sss", $semnum, $semop, $semflag)>. The number of semaphore
3052operations is implied by the length of OPSTRING. Returns TRUE if
3053successful, or FALSE if there is an error. As an example, the
3054following code waits on semaphore $semnum of semaphore id $semid:
3055
3056 $semop = pack("sss", $semnum, -1, 0);
3057 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
3058
3059To signal the semaphore, replace "-1" with "1".
3060
3061=item send SOCKET,MSG,FLAGS,TO
3062
3063=item send SOCKET,MSG,FLAGS
3064
3065Sends a message on a socket. Takes the same flags as the system call
3066of the same name. On unconnected sockets you must specify a
3067destination to send TO, in which case it does a C sendto(). Returns
3068the number of characters sent, or the undefined value if there is an
3069error.
4633a7c4 3070See L<perlipc/"UDP: Message Passing"> for examples.
a0d0e21e
LW
3071
3072=item setpgrp PID,PGRP
3073
3074Sets the current process group for the specified PID, 0 for the current
3075process. Will produce a fatal error if used on a machine that doesn't
5f05dabc 3076implement setpgrp(2). If the arguments are omitted, it defaults to
47e29363 30770,0. Note that the POSIX version of setpgrp() does not accept any
3078arguments, so only setpgrp 0,0 is portable.
a0d0e21e
LW
3079
3080=item setpriority WHICH,WHO,PRIORITY
3081
3082Sets the current priority for a process, a process group, or a user.
748a9306 3083(See setpriority(2).) Will produce a fatal error if used on a machine
a0d0e21e
LW
3084that doesn't implement setpriority(2).
3085
3086=item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
3087
3088Sets the socket option requested. Returns undefined if there is an
3089error. OPTVAL may be specified as undef if you don't want to pass an
3090argument.
3091
3092=item shift ARRAY
3093
3094=item shift
3095
3096Shifts the first value of the array off and returns it, shortening the
3097array by 1 and moving everything down. If there are no elements in the
3098array, returns the undefined value. If ARRAY is omitted, shifts the
977336f5
GS
3099@_ array within the lexical scope of subroutines and formats, and the
3100@ARGV array at file scopes or within the lexical scopes established by
3101the C<eval ''>, C<BEGIN {}>, C<END {}>, and C<INIT {}> constructs.
3102See also unshift(), push(), and pop(). Shift() and unshift() do the
3103same thing to the left end of an array that pop() and push() do to the
3104right end.
a0d0e21e
LW
3105
3106=item shmctl ID,CMD,ARG
3107
3108Calls the System V IPC function shmctl. If CMD is &IPC_STAT, then ARG
5a964f20 3109must be a variable that will hold the returned shmid_ds structure.
a0d0e21e
LW
3110Returns like ioctl: the undefined value for error, "0 but true" for
3111zero, or the actual return value otherwise.
3112
3113=item shmget KEY,SIZE,FLAGS
3114
3115Calls the System V IPC function shmget. Returns the shared memory
3116segment id, or the undefined value if there is an error.
3117
3118=item shmread ID,VAR,POS,SIZE
3119
3120=item shmwrite ID,STRING,POS,SIZE
3121
3122Reads or writes the System V shared memory segment ID starting at
3123position POS for size SIZE by attaching to it, copying in/out, and
5a964f20 3124detaching from it. When reading, VAR must be a variable that will
a0d0e21e
LW
3125hold the data read. When writing, if STRING is too long, only SIZE
3126bytes are used; if STRING is too short, nulls are written to fill out
3127SIZE bytes. Return TRUE if successful, or FALSE if there is an error.
3128
3129=item shutdown SOCKET,HOW
3130
3131Shuts down a socket connection in the manner indicated by HOW, which
3132has the same interpretation as in the system call of the same name.
3133
5a964f20
TC
3134 shutdown(SOCKET, 0); # I/we have stopped reading data
3135 shutdown(SOCKET, 1); # I/we have stopped writing data
3136 shutdown(SOCKET, 2); # I/we have stopped using this socket
3137
3138This is useful with sockets when you want to tell the other
3139side you're done writing but not done reading, or vice versa.
3140It's also a more insistent form of close because it also
3141disables the filedescriptor in any forked copies in other
3142processes.
3143
a0d0e21e
LW
3144=item sin EXPR
3145
54310121 3146=item sin
bbce6d69 3147
a0d0e21e
LW
3148Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
3149returns sine of $_.
3150
54310121 3151For the inverse sine operation, you may use the POSIX::asin()
28757baa 3152function, or use this relation:
3153
3154 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
3155
a0d0e21e
LW
3156=item sleep EXPR
3157
3158=item sleep
3159
3160Causes the script to sleep for EXPR seconds, or forever if no EXPR.
1d3434b8
GS
3161May be interrupted if the process receives a signal such as SIGALRM.
3162Returns the number of seconds actually slept. You probably cannot
3163mix alarm() and sleep() calls, because sleep() is often implemented
3164using alarm().
a0d0e21e
LW
3165
3166On some older systems, it may sleep up to a full second less than what
3167you requested, depending on how it counts seconds. Most modern systems
5a964f20
TC
3168always sleep the full amount. They may appear to sleep longer than that,
3169however, because your process might not be scheduled right away in a
3170busy multitasking system.
a0d0e21e 3171
cb1a09d0 3172For delays of finer granularity than one second, you may use Perl's
54310121 3173syscall() interface to access setitimer(2) if your system supports it,
7b8d334a 3174or else see L</select()> above.
cb1a09d0 3175
5f05dabc 3176See also the POSIX module's sigpause() function.
3177
a0d0e21e
LW
3178=item socket SOCKET,DOMAIN,TYPE,PROTOCOL
3179
3180Opens a socket of the specified kind and attaches it to filehandle
5f05dabc 3181SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for the
a0d0e21e 3182system call of the same name. You should "use Socket;" first to get
4633a7c4 3183the proper definitions imported. See the example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
3184
3185=item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
3186
3187Creates an unnamed pair of sockets in the specified domain, of the
5f05dabc 3188specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
a0d0e21e
LW
3189for the system call of the same name. If unimplemented, yields a fatal
3190error. Returns TRUE if successful.
3191
5a964f20
TC
3192Some systems defined pipe() in terms of socketpair, in which a call
3193to C<pipe(Rdr, Wtr)> is essentially:
3194
3195 use Socket;
3196 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
3197 shutdown(Rdr, 1); # no more writing for reader
3198 shutdown(Wtr, 0); # no more reading for writer
3199
3200See L<perlipc> for an example of socketpair use.
3201
a0d0e21e
LW
3202=item sort SUBNAME LIST
3203
3204=item sort BLOCK LIST
3205
3206=item sort LIST
3207
2f9daede
TP
3208Sorts the LIST and returns the sorted list value. If SUBNAME or BLOCK
3209is omitted, sorts in standard string comparison order. If SUBNAME is
3210specified, it gives the name of a subroutine that returns an integer
3211less than, equal to, or greater than 0, depending on how the elements
3212of the array are to be ordered. (The C<E<lt>=E<gt>> and C<cmp>
3213operators are extremely useful in such routines.) SUBNAME may be a
1d3434b8
GS
3214scalar variable name (unsubscripted), in which case the value provides
3215the name of (or a reference to) the actual subroutine to use. In place
3216of a SUBNAME, you can provide a BLOCK as an anonymous, in-line sort
3217subroutine.
a0d0e21e 3218
cb1a09d0
AD
3219In the interests of efficiency the normal calling code for subroutines is
3220bypassed, with the following effects: the subroutine may not be a
3221recursive subroutine, and the two elements to be compared are passed into
3222the subroutine not via @_ but as the package global variables $a and
3223$b (see example below). They are passed by reference, so don't
3224modify $a and $b. And don't try to declare them as lexicals either.
a0d0e21e 3225
0a753a76 3226You also cannot exit out of the sort block or subroutine using any of the
3227loop control operators described in L<perlsyn> or with goto().
3228
a034a98d
DD
3229When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
3230current collation locale. See L<perllocale>.
3231
a0d0e21e
LW
3232Examples:
3233
3234 # sort lexically
3235 @articles = sort @files;
3236
3237 # same thing, but with explicit sort routine
3238 @articles = sort {$a cmp $b} @files;
3239
cb1a09d0 3240 # now case-insensitively
54310121 3241 @articles = sort {uc($a) cmp uc($b)} @files;
cb1a09d0 3242
a0d0e21e
LW
3243 # same thing in reversed order
3244 @articles = sort {$b cmp $a} @files;
3245
3246 # sort numerically ascending
3247 @articles = sort {$a <=> $b} @files;
3248
3249 # sort numerically descending
3250 @articles = sort {$b <=> $a} @files;
3251
3252 # sort using explicit subroutine name
3253 sub byage {
2f9daede 3254 $age{$a} <=> $age{$b}; # presuming numeric
a0d0e21e
LW
3255 }
3256 @sortedclass = sort byage @class;
3257
aa689395 3258 # this sorts the %age hash by value instead of key
3259 # using an in-line function
c07a80fd 3260 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
3261
a0d0e21e
LW
3262 sub backwards { $b cmp $a; }
3263 @harry = ('dog','cat','x','Cain','Abel');
3264 @george = ('gone','chased','yz','Punished','Axed');
3265 print sort @harry;
3266 # prints AbelCaincatdogx
3267 print sort backwards @harry;
3268 # prints xdogcatCainAbel
3269 print sort @george, 'to', @harry;
3270 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
3271
54310121 3272 # inefficiently sort by descending numeric compare using
3273 # the first integer after the first = sign, or the
cb1a09d0
AD
3274 # whole record case-insensitively otherwise
3275
3276 @new = sort {
3277 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
3278 ||
3279 uc($a) cmp uc($b)
3280 } @old;
3281
3282 # same thing, but much more efficiently;
3283 # we'll build auxiliary indices instead
3284 # for speed
3285 @nums = @caps = ();
54310121 3286 for (@old) {
cb1a09d0
AD
3287 push @nums, /=(\d+)/;
3288 push @caps, uc($_);
54310121 3289 }
cb1a09d0
AD
3290
3291 @new = @old[ sort {
3292 $nums[$b] <=> $nums[$a]
3293 ||
3294 $caps[$a] cmp $caps[$b]
3295 } 0..$#old
3296 ];
3297
3298 # same thing using a Schwartzian Transform (no temps)
3299 @new = map { $_->[0] }
3300 sort { $b->[1] <=> $a->[1]
3301 ||
3302 $a->[2] cmp $b->[2]
3303 } map { [$_, /=(\d+)/, uc($_)] } @old;
3304
184e9718 3305If you're using strict, you I<MUST NOT> declare $a
cb1a09d0
AD
3306and $b as lexicals. They are package globals. That means
3307if you're in the C<main> package, it's
3308
3309 @articles = sort {$main::b <=> $main::a} @files;
3310
3311or just
3312
3313 @articles = sort {$::b <=> $::a} @files;
3314
3315but if you're in the C<FooPack> package, it's
3316
3317 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
3318
55497cff 3319The comparison function is required to behave. If it returns
3320inconsistent results (sometimes saying $x[1] is less than $x[2] and
3321sometimes saying the opposite, for example) the Perl interpreter will
3322probably crash and dump core. This is entirely due to and dependent
3323upon your system's qsort(3) library routine; this routine often avoids
3324sanity checks in the interest of speed.
3325
a0d0e21e
LW
3326=item splice ARRAY,OFFSET,LENGTH,LIST
3327
3328=item splice ARRAY,OFFSET,LENGTH
3329
3330=item splice ARRAY,OFFSET
3331
3332Removes the elements designated by OFFSET and LENGTH from an array, and
5a964f20
TC
3333replaces them with the elements of LIST, if any. In list context,
3334returns the elements removed from the array. In scalar context,
43051805
GS
3335returns the last element removed, or C<undef> if no elements are
3336removed. The array grows or shrinks as necessary. If LENGTH is
3337omitted, removes everything from OFFSET onward. The following
3338equivalences hold (assuming C<$[ == 0>):
a0d0e21e
LW
3339
3340 push(@a,$x,$y) splice(@a,$#a+1,0,$x,$y)
3341 pop(@a) splice(@a,-1)
3342 shift(@a) splice(@a,0,1)
3343 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
5a964f20 3344 $a[$x] = $y splice(@a,$x,1,$y)
a0d0e21e
LW
3345
3346Example, assuming array lengths are passed before arrays:
3347
3348 sub aeq { # compare two list values
5a964f20
TC
3349 my(@a) = splice(@_,0,shift);
3350 my(@b) = splice(@_,0,shift);
a0d0e21e
LW
3351 return 0 unless @a == @b; # same len?
3352 while (@a) {
3353 return 0 if pop(@a) ne pop(@b);
3354 }
3355 return 1;
3356 }
3357 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
3358
3359=item split /PATTERN/,EXPR,LIMIT
3360
3361=item split /PATTERN/,EXPR
3362
3363=item split /PATTERN/
3364
3365=item split
3366
5a964f20
TC
3367Splits a string into an array of strings, and returns it. By default,
3368empty leading fields are preserved, and empty trailing ones are deleted.
a0d0e21e 3369
5a964f20
TC
3370If not in list context, returns the number of fields found and splits into
3371the @_ array. (In list context, you can force the split into @_ by
1d2dff63 3372using C<??> as the pattern delimiters, but it still returns the list
5a964f20
TC
3373value.) The use of implicit split to @_ is deprecated, however, because
3374it clobbers your subroutine arguments.
a0d0e21e
LW
3375
3376If EXPR is omitted, splits the $_ string. If PATTERN is also omitted,
4633a7c4
LW
3377splits on whitespace (after skipping any leading whitespace). Anything
3378matching PATTERN is taken to be a delimiter separating the fields. (Note
fb73857a 3379that the delimiter may be longer than one character.)
3380
5a964f20 3381If LIMIT is specified and positive, splits into no more than that
7b8d334a
GS
3382many fields (though it may split into fewer). If LIMIT is unspecified
3383or zero, trailing null fields are stripped (which potential users
fb73857a 3384of pop() would do well to remember). If LIMIT is negative, it is
3385treated as if an arbitrarily large LIMIT had been specified.
a0d0e21e
LW
3386
3387A pattern matching the null string (not to be confused with
748a9306 3388a null pattern C<//>, which is just one member of the set of patterns
a0d0e21e
LW
3389matching a null string) will split the value of EXPR into separate
3390characters at each point it matches that way. For example:
3391
3392 print join(':', split(/ */, 'hi there'));
3393
3394produces the output 'h:i:t:h:e:r:e'.
3395
5f05dabc 3396The LIMIT parameter can be used to split a line partially
a0d0e21e
LW
3397
3398 ($login, $passwd, $remainder) = split(/:/, $_, 3);
3399
3400When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT
3401one larger than the number of variables in the list, to avoid
3402unnecessary work. For the list above LIMIT would have been 4 by
3403default. In time critical applications it behooves you not to split
3404into more fields than you really need.
3405
3406If the PATTERN contains parentheses, additional array elements are
3407created from each matching substring in the delimiter.
3408
da0045b7 3409 split(/([,-])/, "1-10,20", 3);
a0d0e21e
LW
3410
3411produces the list value
3412
3413 (1, '-', 10, ',', 20)
3414
54310121 3415If you had the entire header of a normal Unix email message in $header,
4633a7c4
LW
3416you could split it up into fields and their values this way:
3417
3418 $header =~ s/\n\s+/ /g; # fix continuation lines
fb73857a 3419 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
4633a7c4 3420
a0d0e21e
LW
3421The pattern C</PATTERN/> may be replaced with an expression to specify
3422patterns that vary at runtime. (To do runtime compilation only once,
748a9306
LW
3423use C</$variable/o>.)
3424
3425As a special case, specifying a PATTERN of space (C<' '>) will split on
3426white space just as split with no arguments does. Thus, split(' ') can
3427be used to emulate B<awk>'s default behavior, whereas C<split(/ /)>
3428will give you as many null initial fields as there are leading spaces.
3429A split on /\s+/ is like a split(' ') except that any leading
3430whitespace produces a null first field. A split with no arguments
3431really does a C<split(' ', $_)> internally.
a0d0e21e
LW
3432
3433Example:
3434
5a964f20
TC
3435 open(PASSWD, '/etc/passwd');
3436 while (<PASSWD>) {
3437 ($login, $passwd, $uid, $gid, $gcos,$home, $shell) = split(/:/);
3438 #...
a0d0e21e
LW
3439 }
3440
54310121 3441(Note that $shell above will still have a newline on it. See L</chop>,
a0d0e21e
LW
3442L</chomp>, and L</join>.)
3443
5f05dabc 3444=item sprintf FORMAT, LIST
a0d0e21e 3445
74a77017
CS
3446Returns a string formatted by the usual printf conventions of the
3447C library function sprintf(). See L<sprintf(3)> or L<printf(3)>
3448on your system for an explanation of the general principles.
3449
5a964f20 3450Perl does its own sprintf() formatting -- it emulates the C
74a77017
CS
3451function sprintf(), but it doesn't use it (except for floating-point
3452numbers, and even then only the standard modifiers are allowed). As a
3453result, any non-standard extensions in your local sprintf() are not
3454available from Perl.
3455
3456Perl's sprintf() permits the following universally-known conversions:
3457
3458 %% a percent sign
3459 %c a character with the given number
3460 %s a string
3461 %d a signed integer, in decimal
3462 %u an unsigned integer, in decimal
3463 %o an unsigned integer, in octal
3464 %x an unsigned integer, in hexadecimal
3465 %e a floating-point number, in scientific notation
3466 %f a floating-point number, in fixed decimal notation
3467 %g a floating-point number, in %e or %f notation
3468
1b3f7d21 3469In addition, Perl permits the following widely-supported conversions:
74a77017 3470
74a77017
CS
3471 %X like %x, but using upper-case letters
3472 %E like %e, but using an upper-case "E"
3473 %G like %g, but with an upper-case "E" (if applicable)
3474 %p a pointer (outputs the Perl value's address in hexadecimal)
1b3f7d21
CS
3475 %n special: *stores* the number of characters output so far
3476 into the next variable in the parameter list
74a77017 3477
1b3f7d21
CS
3478Finally, for backward (and we do mean "backward") compatibility, Perl
3479permits these unnecessary but widely-supported conversions:
74a77017 3480
1b3f7d21 3481 %i a synonym for %d
74a77017
CS
3482 %D a synonym for %ld
3483 %U a synonym for %lu
3484 %O a synonym for %lo
3485 %F a synonym for %f
3486
3487Perl permits the following universally-known flags between the C<%>
3488and the conversion letter:
3489
3490 space prefix positive number with a space
3491 + prefix positive number with a plus sign
3492 - left-justify within the field
3493 0 use zeros, not spaces, to right-justify
a3cb178b 3494 # prefix non-zero octal with "0", non-zero hex with "0x"
74a77017
CS
3495 number minimum field width
3496 .number "precision": digits after decimal point for floating-point,
3497 max length for string, minimum length for integer
3498 l interpret integer as C type "long" or "unsigned long"
74a77017
CS
3499 h interpret integer as C type "short" or "unsigned short"
3500
1b3f7d21 3501There is also one Perl-specific flag:
74a77017
CS
3502
3503 V interpret integer as Perl's standard integer type
3504
3505Where a number would appear in the flags, an asterisk ("*") may be
3506used instead, in which case Perl uses the next item in the parameter
3507list as the given number (that is, as the field width or precision).
3508If a field width obtained through "*" is negative, it has the same
3509effect as the '-' flag: left-justification.
3510
3511If C<use locale> is in effect, the character used for the decimal
3512point in formatted real numbers is affected by the LC_NUMERIC locale.
3513See L<perllocale>.
a0d0e21e
LW
3514
3515=item sqrt EXPR
3516
54310121 3517=item sqrt
bbce6d69 3518
a0d0e21e
LW
3519Return the square root of EXPR. If EXPR is omitted, returns square
3520root of $_.
3521
3522=item srand EXPR