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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
2b5ab1e7 15argument, while a list operator may provide either scalar or list
a0d0e21e 16contexts for its arguments. If it does both, the scalar arguments will
5f05dabc 17be first, and the list argument will follow. (Note that there can ever
0f31cffe 18be only one such list argument.) For instance, splice() has three scalar
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19arguments followed by a list, whereas gethostbyname() has four scalar
20arguments.
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21
22In the syntax descriptions that follow, list operators that expect a
23list (and provide list context for the elements of the list) are shown
24with LIST as an argument. Such a list may consist of any combination
25of scalar arguments or list values; the list values will be included
26in the list as if each individual element were interpolated at that
27point in the list, forming a longer single-dimensional list value.
28Elements of the LIST should be separated by commas.
29
30Any function in the list below may be used either with or without
31parentheses around its arguments. (The syntax descriptions omit the
5f05dabc 32parentheses.) If you use the parentheses, the simple (but occasionally
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33surprising) rule is this: It I<LOOKS> like a function, therefore it I<IS> a
34function, and precedence doesn't matter. Otherwise it's a list
35operator or unary operator, and precedence does matter. And whitespace
36between the function and left parenthesis doesn't count--so you need to
37be careful sometimes:
38
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39 print 1+2+4; # Prints 7.
40 print(1+2) + 4; # Prints 3.
41 print (1+2)+4; # Also prints 3!
42 print +(1+2)+4; # Prints 7.
43 print ((1+2)+4); # Prints 7.
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44
45If you run Perl with the B<-w> switch it can warn you about this. For
46example, the third line above produces:
47
48 print (...) interpreted as function at - line 1.
49 Useless use of integer addition in void context at - line 1.
50
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51A few functions take no arguments at all, and therefore work as neither
52unary nor list operators. These include such functions as C<time>
53and C<endpwent>. For example, C<time+86_400> always means
54C<time() + 86_400>.
55
a0d0e21e 56For functions that can be used in either a scalar or list context,
54310121 57nonabortive failure is generally indicated in a scalar context by
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58returning the undefined value, and in a list context by returning the
59null list.
60
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61Remember the following important rule: There is B<no rule> that relates
62the behavior of an expression in list context to its behavior in scalar
63context, or vice versa. It might do two totally different things.
a0d0e21e 64Each operator and function decides which sort of value it would be most
2b5ab1e7 65appropriate to return in scalar context. Some operators return the
5a964f20 66length of the list that would have been returned in list context. Some
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67operators return the first value in the list. Some operators return the
68last value in the list. Some operators return a count of successful
69operations. In general, they do what you want, unless you want
70consistency.
71
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72An named array in scalar context is quite different from what would at
73first glance appear to be a list in scalar context. You can't get a list
74like C<(1,2,3)> into being in scalar context, because the compiler knows
75the context at compile time. It would generate the scalar comma operator
76there, not the list construction version of the comma. That means it
77was never a list to start with.
78
79In general, functions in Perl that serve as wrappers for system calls
f86cebdf 80of the same name (like chown(2), fork(2), closedir(2), etc.) all return
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81true when they succeed and C<undef> otherwise, as is usually mentioned
82in the descriptions below. This is different from the C interfaces,
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83which return C<-1> on failure. Exceptions to this rule are C<wait()>,
84C<waitpid()>, and C<syscall()>. System calls also set the special C<$!>
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85variable on failure. Other functions do not, except accidentally.
86
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87=head2 Perl Functions by Category
88
89Here are Perl's functions (including things that look like
5a964f20 90functions, like some keywords and named operators)
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91arranged by category. Some functions appear in more
92than one place.
93
94=over
95
96=item Functions for SCALARs or strings
97
22fae026 98C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>,
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99C<length>, C<oct>, C<ord>, C<pack>, C<q/STRING/>, C<qq/STRING/>, C<reverse>,
100C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///>
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101
102=item Regular expressions and pattern matching
103
ab4f32c2 104C<m//>, C<pos>, C<quotemeta>, C<s///>, C<split>, C<study>, C<qr//>
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105
106=item Numeric functions
107
22fae026
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108C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>,
109C<sin>, C<sqrt>, C<srand>
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110
111=item Functions for real @ARRAYs
112
22fae026 113C<pop>, C<push>, C<shift>, C<splice>, C<unshift>
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114
115=item Functions for list data
116
ab4f32c2 117C<grep>, C<join>, C<map>, C<qw/STRING/>, C<reverse>, C<sort>, C<unpack>
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118
119=item Functions for real %HASHes
120
22fae026 121C<delete>, C<each>, C<exists>, C<keys>, C<values>
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122
123=item Input and output functions
124
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125C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
126C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
127C<readdir>, C<rewinddir>, C<seek>, C<seekdir>, C<select>, C<syscall>,
128C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>,
129C<warn>, C<write>
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130
131=item Functions for fixed length data or records
132
22fae026 133C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec>
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134
135=item Functions for filehandles, files, or directories
136
22fae026 137C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>,
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138C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>,
139C<readlink>, C<rename>, C<rmdir>, C<stat>, C<symlink>, C<umask>,
140C<unlink>, C<utime>
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141
142=item Keywords related to the control flow of your perl program
143
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144C<caller>, C<continue>, C<die>, C<do>, C<dump>, C<eval>, C<exit>,
145C<goto>, C<last>, C<next>, C<redo>, C<return>, C<sub>, C<wantarray>
cb1a09d0 146
54310121 147=item Keywords related to scoping
cb1a09d0 148
22fae026 149C<caller>, C<import>, C<local>, C<my>, C<package>, C<use>
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150
151=item Miscellaneous functions
152
22fae026
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153C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<reset>,
154C<scalar>, C<undef>, C<wantarray>
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155
156=item Functions for processes and process groups
157
22fae026 158C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
ab4f32c2 159C<pipe>, C<qx/STRING/>, C<setpgrp>, C<setpriority>, C<sleep>, C<system>,
22fae026 160C<times>, C<wait>, C<waitpid>
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161
162=item Keywords related to perl modules
163
22fae026 164C<do>, C<import>, C<no>, C<package>, C<require>, C<use>
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165
166=item Keywords related to classes and object-orientedness
167
22fae026
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168C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
169C<untie>, C<use>
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170
171=item Low-level socket functions
172
22fae026
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173C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
174C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
175C<socket>, C<socketpair>
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176
177=item System V interprocess communication functions
178
22fae026
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179C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
180C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>
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181
182=item Fetching user and group info
183
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184C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
185C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
186C<getpwuid>, C<setgrent>, C<setpwent>
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187
188=item Fetching network info
189
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190C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
191C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
192C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
193C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
194C<setnetent>, C<setprotoent>, C<setservent>
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195
196=item Time-related functions
197
22fae026 198C<gmtime>, C<localtime>, C<time>, C<times>
cb1a09d0 199
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200=item Functions new in perl5
201
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202C<abs>, C<bless>, C<chomp>, C<chr>, C<exists>, C<formline>, C<glob>,
203C<import>, C<lc>, C<lcfirst>, C<map>, C<my>, C<no>, C<prototype>, C<qx>,
204C<qw>, C<readline>, C<readpipe>, C<ref>, C<sub*>, C<sysopen>, C<tie>,
205C<tied>, C<uc>, C<ucfirst>, C<untie>, C<use>
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206
207* - C<sub> was a keyword in perl4, but in perl5 it is an
5a964f20 208operator, which can be used in expressions.
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209
210=item Functions obsoleted in perl5
211
22fae026 212C<dbmclose>, C<dbmopen>
37798a01 213
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214=back
215
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216=head2 Portability
217
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218Perl was born in Unix and can therefore access all common Unix
219system calls. In non-Unix environments, the functionality of some
220Unix system calls may not be available, or details of the available
221functionality may differ slightly. The Perl functions affected
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222by this are:
223
224C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>,
225C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>,
226C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>,
227C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostent>,
228C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
229C<getppid>, C<getprgp>, C<getpriority>, C<getprotobynumber>,
230C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>,
231C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>,
232C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>,
2b5ab1e7 233C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<rename>, C<select>, C<semctl>,
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234C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>,
235C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>,
236C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>,
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237C<shmwrite>, C<socket>, C<socketpair>, C<stat>, C<symlink>, C<syscall>,
238C<sysopen>, C<system>, C<times>, C<truncate>, C<umask>, C<unlink>,
239C<utime>, C<wait>, C<waitpid>
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240
241For more information about the portability of these functions, see
242L<perlport> and other available platform-specific documentation.
243
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244=head2 Alphabetical Listing of Perl Functions
245
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246=over 8
247
22fae026 248=item I<-X> FILEHANDLE
a0d0e21e 249
22fae026 250=item I<-X> EXPR
a0d0e21e 251
22fae026 252=item I<-X>
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253
254A file test, where X is one of the letters listed below. This unary
255operator takes one argument, either a filename or a filehandle, and
256tests the associated file to see if something is true about it. If the
7660c0ab 257argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
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258Unless otherwise documented, it returns C<1> for TRUE and C<''> for FALSE, or
259the undefined value if the file doesn't exist. Despite the funny
260names, precedence is the same as any other named unary operator, and
261the argument may be parenthesized like any other unary operator. The
262operator may be any of:
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263X<-r>X<-w>X<-x>X<-o>X<-R>X<-W>X<-X>X<-O>X<-e>X<-z>X<-s>X<-f>X<-d>X<-l>X<-p>
264X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
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265
266 -r File is readable by effective uid/gid.
267 -w File is writable by effective uid/gid.
268 -x File is executable by effective uid/gid.
269 -o File is owned by effective uid.
270
271 -R File is readable by real uid/gid.
272 -W File is writable by real uid/gid.
273 -X File is executable by real uid/gid.
274 -O File is owned by real uid.
275
276 -e File exists.
277 -z File has zero size.
54310121 278 -s File has nonzero size (returns size).
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279
280 -f File is a plain file.
281 -d File is a directory.
282 -l File is a symbolic link.
9c4d0f16 283 -p File is a named pipe (FIFO), or Filehandle is a pipe.
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284 -S File is a socket.
285 -b File is a block special file.
286 -c File is a character special file.
287 -t Filehandle is opened to a tty.
288
289 -u File has setuid bit set.
290 -g File has setgid bit set.
291 -k File has sticky bit set.
292
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293 -T File is an ASCII text file.
294 -B File is a "binary" file (opposite of -T).
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295
296 -M Age of file in days when script started.
297 -A Same for access time.
298 -C Same for inode change time.
299
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300Example:
301
302 while (<>) {
303 chop;
304 next unless -f $_; # ignore specials
5a964f20 305 #...
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306 }
307
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308The interpretation of the file permission operators C<-r>, C<-R>,
309C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
310of the file and the uids and gids of the user. There may be other
311reasons you can't actually read, write, or execute the file. Such
312reasons may be for example network filesystem access controls, ACLs
313(access control lists), read-only filesystems, and unrecognized
314executable formats.
315
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316Also note that, for the superuser on the local filesystems, the C<-r>,
317C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
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318if any execute bit is set in the mode. Scripts run by the superuser
319may thus need to do a stat() to determine the actual mode of the file,
2b5ab1e7 320or temporarily set their effective uid to something else.
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321
322If you are using ACLs, there is a pragma called C<filetest> that may
323produce more accurate results than the bare stat() mode bits.
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324When under the C<use filetest 'access'> the above-mentioned filetests
325will test whether the permission can (not) be granted using the
468541a8 326access() family of system calls. Also note that the C<-x> and C<-X> may
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327under this pragma return true even if there are no execute permission
328bits set (nor any extra execute permission ACLs). This strangeness is
329due to the underlying system calls' definitions. Read the
330documentation for the C<filetest> pragma for more information.
331
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332Note that C<-s/a/b/> does not do a negated substitution. Saying
333C<-exp($foo)> still works as expected, however--only single letters
334following a minus are interpreted as file tests.
335
336The C<-T> and C<-B> switches work as follows. The first block or so of the
337file is examined for odd characters such as strange control codes or
5a964f20 338characters with the high bit set. If too many strange characters (E<gt>30%)
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339are found, it's a C<-B> file, otherwise it's a C<-T> file. Also, any file
340containing null in the first block is considered a binary file. If C<-T>
341or C<-B> is used on a filehandle, the current stdio buffer is examined
342rather than the first block. Both C<-T> and C<-B> return TRUE on a null
54310121 343file, or a file at EOF when testing a filehandle. Because you have to
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344read a file to do the C<-T> test, on most occasions you want to use a C<-f>
345against the file first, as in C<next unless -f $file && -T $file>.
a0d0e21e 346
7660c0ab 347If any of the file tests (or either the C<stat()> or C<lstat()> operators) are given
28757baa 348the special filehandle consisting of a solitary underline, then the stat
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349structure of the previous file test (or stat operator) is used, saving
350a system call. (This doesn't work with C<-t>, and you need to remember
351that lstat() and C<-l> will leave values in the stat structure for the
352symbolic link, not the real file.) Example:
353
354 print "Can do.\n" if -r $a || -w _ || -x _;
355
356 stat($filename);
357 print "Readable\n" if -r _;
358 print "Writable\n" if -w _;
359 print "Executable\n" if -x _;
360 print "Setuid\n" if -u _;
361 print "Setgid\n" if -g _;
362 print "Sticky\n" if -k _;
363 print "Text\n" if -T _;
364 print "Binary\n" if -B _;
365
366=item abs VALUE
367
54310121 368=item abs
bbce6d69 369
a0d0e21e 370Returns the absolute value of its argument.
7660c0ab 371If VALUE is omitted, uses C<$_>.
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372
373=item accept NEWSOCKET,GENERICSOCKET
374
f86cebdf 375Accepts an incoming socket connect, just as the accept(2) system call
a0d0e21e 376does. Returns the packed address if it succeeded, FALSE otherwise.
2b5ab1e7 377See the example in L<perlipc/"Sockets: Client/Server Communication">.
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378
379=item alarm SECONDS
380
54310121 381=item alarm
bbce6d69 382
a0d0e21e 383Arranges to have a SIGALRM delivered to this process after the
bbce6d69 384specified number of seconds have elapsed. If SECONDS is not specified,
7660c0ab 385the value stored in C<$_> is used. (On some machines,
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386unfortunately, the elapsed time may be up to one second less than you
387specified because of how seconds are counted.) Only one timer may be
388counting at once. Each call disables the previous timer, and an
7660c0ab 389argument of C<0> may be supplied to cancel the previous timer without
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390starting a new one. The returned value is the amount of time remaining
391on the previous timer.
392
4633a7c4 393For delays of finer granularity than one second, you may use Perl's
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394four-arugment version of select() leaving the first three arguments
395undefined, or you might be able to use the C<syscall()> interface to
396access setitimer(2) if your system supports it. The Time::HiRes module
397from CPAN may also prove useful.
398
399It is usually a mistake to intermix C<alarm()>
7660c0ab 400and C<sleep()> calls.
a0d0e21e 401
7660c0ab 402If you want to use C<alarm()> to time out a system call you need to use an
ab4f32c2 403C<eval()>/C<die()> pair. You can't rely on the alarm causing the system call to
f86cebdf 404fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
ab4f32c2 405restart system calls on some systems. Using C<eval()>/C<die()> always works,
5a964f20 406modulo the caveats given in L<perlipc/"Signals">.
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407
408 eval {
f86cebdf 409 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
36477c24 410 alarm $timeout;
ff68c719 411 $nread = sysread SOCKET, $buffer, $size;
36477c24 412 alarm 0;
ff68c719 413 };
ff68c719 414 if ($@) {
f86cebdf 415 die unless $@ eq "alarm\n"; # propagate unexpected errors
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416 # timed out
417 }
418 else {
419 # didn't
420 }
421
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422=item atan2 Y,X
423
424Returns the arctangent of Y/X in the range -PI to PI.
425
7660c0ab 426For the tangent operation, you may use the C<POSIX::tan()>
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427function, or use the familiar relation:
428
429 sub tan { sin($_[0]) / cos($_[0]) }
430
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431=item bind SOCKET,NAME
432
433Binds a network address to a socket, just as the bind system call
434does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
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435packed address of the appropriate type for the socket. See the examples in
436L<perlipc/"Sockets: Client/Server Communication">.
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437
438=item binmode FILEHANDLE
439
cb1a09d0 440Arranges for the file to be read or written in "binary" mode in operating
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441systems that distinguish between binary and text files. Files that
442are not in binary mode have CR LF sequences translated to LF on input
443and LF translated to CR LF on output. Binmode has no effect under
444many sytems, but in MS-DOS and similarly archaic systems, it may be
445imperative--otherwise your MS-DOS-damaged C library may mangle your file.
446The key distinction between systems that need C<binmode()> and those
447that don't is their text file formats. Systems like Unix, MacOS, and
448Plan9 that delimit lines with a single character, and that encode that
449character in C as C<"\n">, do not need C<binmode()>. The rest may need it.
450If FILEHANDLE is an expression, the value is taken as the name of the
451filehandle.
452
453If the system does care about it, using it when you shouldn't is just as
454perilous as failing to use it when you should. Fortunately for most of
455us, you can't go wrong using binmode() on systems that don't care about
456it, though.
a0d0e21e 457
4633a7c4 458=item bless REF,CLASSNAME
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459
460=item bless REF
461
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462This function tells the thingy referenced by REF that it is now an object
463in the CLASSNAME package. If CLASSNAME is omitted, the current package
464is used. Because a C<bless()> is often the last thing in a constructor.
465it returns the reference for convenience. Always use the two-argument
466version if the function doing the blessing might be inherited by a
467derived class. See L<perltoot> and L<perlobj> for more about the blessing
468(and blessings) of objects.
a0d0e21e 469
57668c4d 470Consider always blessing objects in CLASSNAMEs that are mixed case.
2b5ab1e7
TC
471Namespaces with all lowercase names are considered reserved for
472Perl pragmata. Builtin types have all uppercase names, so to prevent
473confusion, you may wish to avoid such package names as well. Make sure
474that CLASSNAME is a true value.
60ad88b8
GS
475
476See L<perlmod/"Perl Modules">.
477
a0d0e21e
LW
478=item caller EXPR
479
480=item caller
481
5a964f20 482Returns the context of the current subroutine call. In scalar context,
28757baa 483returns the caller's package name if there is a caller, that is, if
7660c0ab 484we're in a subroutine or C<eval()> or C<require()>, and the undefined value
5a964f20 485otherwise. In list context, returns
a0d0e21e 486
748a9306 487 ($package, $filename, $line) = caller;
a0d0e21e
LW
488
489With EXPR, it returns some extra information that the debugger uses to
490print a stack trace. The value of EXPR indicates how many call frames
491to go back before the current one.
492
54310121 493 ($package, $filename, $line, $subroutine,
e7ea3e70
IZ
494 $hasargs, $wantarray, $evaltext, $is_require) = caller($i);
495
7660c0ab 496Here C<$subroutine> may be C<"(eval)"> if the frame is not a subroutine
ab4f32c2 497call, but an C<eval()>. In such a case additional elements C<$evaltext> and
7660c0ab
A
498C<$is_require> are set: C<$is_require> is true if the frame is created by a
499C<require> or C<use> statement, C<$evaltext> contains the text of the
dc848c6f 500C<eval EXPR> statement. In particular, for a C<eval BLOCK> statement,
7660c0ab 501C<$filename> is C<"(eval)">, but C<$evaltext> is undefined. (Note also that
dc848c6f
PP
502each C<use> statement creates a C<require> frame inside an C<eval EXPR>)
503frame.
748a9306
LW
504
505Furthermore, when called from within the DB package, caller returns more
7660c0ab 506detailed information: it sets the list variable C<@DB::args> to be the
54310121 507arguments with which the subroutine was invoked.
748a9306 508
7660c0ab 509Be aware that the optimizer might have optimized call frames away before
ab4f32c2 510C<caller()> had a chance to get the information. That means that C<caller(N)>
7660c0ab
A
511might not return information about the call frame you expect it do, for
512C<N E<gt> 1>. In particular, C<@DB::args> might have information from the
513previous time C<caller()> was called.
514
a0d0e21e
LW
515=item chdir EXPR
516
2b5ab1e7
TC
517Changes the working directory to EXPR, if possible. If EXPR is omitted,
518changes to the user's home directory. Returns TRUE upon success,
519FALSE otherwise. See the example under C<die()>.
a0d0e21e
LW
520
521=item chmod LIST
522
523Changes the permissions of a list of files. The first element of the
4633a7c4 524list must be the numerical mode, which should probably be an octal
2f9daede
TPG
525number, and which definitely should I<not> a string of octal digits:
526C<0644> is okay, C<'0644'> is not. Returns the number of files
dc848c6f 527successfully changed. See also L</oct>, if all you have is a string.
a0d0e21e
LW
528
529 $cnt = chmod 0755, 'foo', 'bar';
530 chmod 0755, @executables;
f86cebdf
GS
531 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
532 # --w----r-T
2f9daede
TPG
533 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
534 $mode = 0644; chmod $mode, 'foo'; # this is best
a0d0e21e
LW
535
536=item chomp VARIABLE
537
538=item chomp LIST
539
540=item chomp
541
2b5ab1e7
TC
542This safer version of L</chop> removes any trailing string
543that corresponds to the current value of C<$/> (also known as
28757baa
PP
544$INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
545number of characters removed from all its arguments. It's often used to
546remove the newline from the end of an input record when you're worried
2b5ab1e7
TC
547that the final record may be missing its newline. When in paragraph
548mode (C<$/ = "">), it removes all trailing newlines from the string.
4c5a6083
GS
549When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
550a reference to an integer or the like, see L<perlvar>) chomp() won't
551remove anything. If VARIABLE is omitted, it chomps C<$_>. Example:
a0d0e21e
LW
552
553 while (<>) {
554 chomp; # avoid \n on last field
555 @array = split(/:/);
5a964f20 556 # ...
a0d0e21e
LW
557 }
558
559You can actually chomp anything that's an lvalue, including an assignment:
560
561 chomp($cwd = `pwd`);
562 chomp($answer = <STDIN>);
563
564If you chomp a list, each element is chomped, and the total number of
565characters removed is returned.
566
567=item chop VARIABLE
568
569=item chop LIST
570
571=item chop
572
573Chops off the last character of a string and returns the character
574chopped. It's used primarily to remove the newline from the end of an
575input record, but is much more efficient than C<s/\n//> because it neither
7660c0ab 576scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
a0d0e21e
LW
577Example:
578
579 while (<>) {
580 chop; # avoid \n on last field
581 @array = split(/:/);
5a964f20 582 #...
a0d0e21e
LW
583 }
584
585You can actually chop anything that's an lvalue, including an assignment:
586
587 chop($cwd = `pwd`);
588 chop($answer = <STDIN>);
589
590If you chop a list, each element is chopped. Only the value of the
ab4f32c2 591last C<chop()> is returned.
a0d0e21e 592
ab4f32c2 593Note that C<chop()> returns the last character. To return all but the last
748a9306
LW
594character, use C<substr($string, 0, -1)>.
595
a0d0e21e
LW
596=item chown LIST
597
598Changes the owner (and group) of a list of files. The first two
599elements of the list must be the I<NUMERICAL> uid and gid, in that order.
600Returns the number of files successfully changed.
601
602 $cnt = chown $uid, $gid, 'foo', 'bar';
603 chown $uid, $gid, @filenames;
604
54310121 605Here's an example that looks up nonnumeric uids in the passwd file:
a0d0e21e
LW
606
607 print "User: ";
608 chop($user = <STDIN>);
5a964f20 609 print "Files: ";
a0d0e21e
LW
610 chop($pattern = <STDIN>);
611
612 ($login,$pass,$uid,$gid) = getpwnam($user)
613 or die "$user not in passwd file";
614
5a964f20 615 @ary = glob($pattern); # expand filenames
a0d0e21e
LW
616 chown $uid, $gid, @ary;
617
54310121 618On most systems, you are not allowed to change the ownership of the
4633a7c4
LW
619file unless you're the superuser, although you should be able to change
620the group to any of your secondary groups. On insecure systems, these
621restrictions may be relaxed, but this is not a portable assumption.
622
a0d0e21e
LW
623=item chr NUMBER
624
54310121 625=item chr
bbce6d69 626
a0d0e21e 627Returns the character represented by that NUMBER in the character set.
a0ed51b3 628For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
2b5ab1e7
TC
629chr(0x263a) is a Unicode smiley face (but only within the scope of
630a C<use utf8>). For the reverse, use L</ord>.
631See L<utf8> for more about Unicode.
a0d0e21e 632
7660c0ab 633If NUMBER is omitted, uses C<$_>.
bbce6d69 634
a0d0e21e
LW
635=item chroot FILENAME
636
54310121 637=item chroot
bbce6d69 638
5a964f20 639This function works like the system call by the same name: it makes the
4633a7c4 640named directory the new root directory for all further pathnames that
7660c0ab 641begin with a C<"/"> by your process and all its children. (It doesn't
28757baa 642change your current working directory, which is unaffected.) For security
4633a7c4 643reasons, this call is restricted to the superuser. If FILENAME is
ab4f32c2 644omitted, does a C<chroot()> to C<$_>.
a0d0e21e
LW
645
646=item close FILEHANDLE
647
6a518fbc
TP
648=item close
649
a0d0e21e
LW
650Closes the file or pipe associated with the file handle, returning TRUE
651only if stdio successfully flushes buffers and closes the system file
6a518fbc
TP
652descriptor. Closes the currently selected filehandle if the argument
653is omitted.
fb73857a
PP
654
655You don't have to close FILEHANDLE if you are immediately going to do
7660c0ab 656another C<open()> on it, because C<open()> will close it for you. (See
ab4f32c2 657C<open()>.) However, an explicit C<close()> on an input file resets the line
7660c0ab 658counter (C<$.>), while the implicit close done by C<open()> does not.
fb73857a 659
ab4f32c2 660If the file handle came from a piped open C<close()> will additionally
fb73857a
PP
661return FALSE if one of the other system calls involved fails or if the
662program exits with non-zero status. (If the only problem was that the
2b5ab1e7
TC
663program exited non-zero C<$!> will be set to C<0>.) Closing a pipe
664also waits for the process executing on the pipe to complete, in case you
665want to look at the output of the pipe afterwards, and
666implicitly puts the exit status value of that command into C<$?>.
5a964f20 667
fb73857a 668Example:
a0d0e21e 669
fb73857a
PP
670 open(OUTPUT, '|sort >foo') # pipe to sort
671 or die "Can't start sort: $!";
5a964f20 672 #... # print stuff to output
fb73857a
PP
673 close OUTPUT # wait for sort to finish
674 or warn $! ? "Error closing sort pipe: $!"
675 : "Exit status $? from sort";
676 open(INPUT, 'foo') # get sort's results
677 or die "Can't open 'foo' for input: $!";
a0d0e21e 678
5a964f20
TC
679FILEHANDLE may be an expression whose value can be used as an indirect
680filehandle, usually the real filehandle name.
a0d0e21e
LW
681
682=item closedir DIRHANDLE
683
7660c0ab 684Closes a directory opened by C<opendir()> and returns the success of that
5a964f20
TC
685system call.
686
687DIRHANDLE may be an expression whose value can be used as an indirect
688dirhandle, usually the real dirhandle name.
a0d0e21e
LW
689
690=item connect SOCKET,NAME
691
692Attempts to connect to a remote socket, just as the connect system call
693does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
4633a7c4
LW
694packed address of the appropriate type for the socket. See the examples in
695L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 696
cb1a09d0
AD
697=item continue BLOCK
698
699Actually a flow control statement rather than a function. If there is a
98293880
JH
700C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
701C<foreach>), it is always executed just before the conditional is about to
702be evaluated again, just like the third part of a C<for> loop in C. Thus
cb1a09d0
AD
703it can be used to increment a loop variable, even when the loop has been
704continued via the C<next> statement (which is similar to the C C<continue>
705statement).
706
98293880 707C<last>, C<next>, or C<redo> may appear within a C<continue>
1d2dff63
GS
708block. C<last> and C<redo> will behave as if they had been executed within
709the main block. So will C<next>, but since it will execute a C<continue>
710block, it may be more entertaining.
711
712 while (EXPR) {
713 ### redo always comes here
714 do_something;
715 } continue {
716 ### next always comes here
717 do_something_else;
718 # then back the top to re-check EXPR
719 }
720 ### last always comes here
721
722Omitting the C<continue> section is semantically equivalent to using an
723empty one, logically enough. In that case, C<next> goes directly back
724to check the condition at the top of the loop.
725
a0d0e21e
LW
726=item cos EXPR
727
5a964f20 728Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
7660c0ab 729takes cosine of C<$_>.
a0d0e21e 730
7660c0ab 731For the inverse cosine operation, you may use the C<POSIX::acos()>
28757baa
PP
732function, or use this relation:
733
734 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
735
a0d0e21e
LW
736=item crypt PLAINTEXT,SALT
737
f86cebdf 738Encrypts a string exactly like the crypt(3) function in the C library
4633a7c4
LW
739(assuming that you actually have a version there that has not been
740extirpated as a potential munition). This can prove useful for checking
741the password file for lousy passwords, amongst other things. Only the
742guys wearing white hats should do this.
a0d0e21e 743
7660c0ab 744Note that C<crypt()> is intended to be a one-way function, much like breaking
11155c91
CS
745eggs to make an omelette. There is no (known) corresponding decrypt
746function. As a result, this function isn't all that useful for
747cryptography. (For that, see your nearby CPAN mirror.)
2f9daede 748
e71965be
RS
749When verifying an existing encrypted string you should use the encrypted
750text as the salt (like C<crypt($plain, $crypted) eq $crypted>). This
751allows your code to work with the standard C<crypt()> and with more
752exotic implementations. When choosing a new salt create a random two
753character string whose characters come from the set C<[./0-9A-Za-z]>
754(like C<join '', ('.', '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>).
755
a0d0e21e
LW
756Here's an example that makes sure that whoever runs this program knows
757their own password:
758
759 $pwd = (getpwuid($<))[1];
a0d0e21e
LW
760
761 system "stty -echo";
762 print "Password: ";
e71965be 763 chomp($word = <STDIN>);
a0d0e21e
LW
764 print "\n";
765 system "stty echo";
766
e71965be 767 if (crypt($word, $pwd) ne $pwd) {
a0d0e21e
LW
768 die "Sorry...\n";
769 } else {
770 print "ok\n";
54310121 771 }
a0d0e21e 772
9f8f0c9d 773Of course, typing in your own password to whoever asks you
748a9306 774for it is unwise.
a0d0e21e 775
aa689395 776=item dbmclose HASH
a0d0e21e 777
2b5ab1e7 778[This function has been largely superseded by the C<untie()> function.]
a0d0e21e 779
aa689395 780Breaks the binding between a DBM file and a hash.
a0d0e21e 781
aa689395 782=item dbmopen HASH,DBNAME,MODE
a0d0e21e 783
2b5ab1e7 784[This function has been largely superseded by the C<tie()> function.]
a0d0e21e 785
7b8d334a 786This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
ab4f32c2 787hash. HASH is the name of the hash. (Unlike normal C<open()>, the first
aa689395
PP
788argument is I<NOT> a filehandle, even though it looks like one). DBNAME
789is the name of the database (without the F<.dir> or F<.pag> extension if
790any). If the database does not exist, it is created with protection
7660c0ab
A
791specified by MODE (as modified by the C<umask()>). If your system supports
792only the older DBM functions, you may perform only one C<dbmopen()> in your
aa689395 793program. In older versions of Perl, if your system had neither DBM nor
7660c0ab 794ndbm, calling C<dbmopen()> produced a fatal error; it now falls back to
aa689395
PP
795sdbm(3).
796
797If you don't have write access to the DBM file, you can only read hash
798variables, not set them. If you want to test whether you can write,
7660c0ab 799either use file tests or try setting a dummy hash entry inside an C<eval()>,
aa689395 800which will trap the error.
a0d0e21e 801
7660c0ab
A
802Note that functions such as C<keys()> and C<values()> may return huge lists
803when used on large DBM files. You may prefer to use the C<each()>
a0d0e21e
LW
804function to iterate over large DBM files. Example:
805
806 # print out history file offsets
807 dbmopen(%HIST,'/usr/lib/news/history',0666);
808 while (($key,$val) = each %HIST) {
809 print $key, ' = ', unpack('L',$val), "\n";
810 }
811 dbmclose(%HIST);
812
cb1a09d0 813See also L<AnyDBM_File> for a more general description of the pros and
184e9718 814cons of the various dbm approaches, as well as L<DB_File> for a particularly
cb1a09d0 815rich implementation.
4633a7c4 816
2b5ab1e7
TC
817You can control which DBM library you use by loading that library
818before you call dbmopen():
819
820 use DB_File;
821 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
822 or die "Can't open netscape history file: $!";
823
a0d0e21e
LW
824=item defined EXPR
825
54310121 826=item defined
bbce6d69 827
2f9daede
TPG
828Returns a Boolean value telling whether EXPR has a value other than
829the undefined value C<undef>. If EXPR is not present, C<$_> will be
830checked.
831
832Many operations return C<undef> to indicate failure, end of file,
833system error, uninitialized variable, and other exceptional
834conditions. This function allows you to distinguish C<undef> from
835other values. (A simple Boolean test will not distinguish among
7660c0ab 836C<undef>, zero, the empty string, and C<"0">, which are all equally
2f9daede 837false.) Note that since C<undef> is a valid scalar, its presence
7660c0ab 838doesn't I<necessarily> indicate an exceptional condition: C<pop()>
2f9daede
TPG
839returns C<undef> when its argument is an empty array, I<or> when the
840element to return happens to be C<undef>.
841
7660c0ab 842You may also use C<defined()> to check whether a subroutine exists, by
5a964f20 843saying C<defined &func> without parentheses. On the other hand, use
7660c0ab 844of C<defined()> upon aggregates (hashes and arrays) is not guaranteed to
5a964f20 845produce intuitive results, and should probably be avoided.
2f9daede
TPG
846
847When used on a hash element, it tells you whether the value is defined,
dc848c6f 848not whether the key exists in the hash. Use L</exists> for the latter
2f9daede 849purpose.
a0d0e21e
LW
850
851Examples:
852
853 print if defined $switch{'D'};
854 print "$val\n" while defined($val = pop(@ary));
855 die "Can't readlink $sym: $!"
856 unless defined($value = readlink $sym);
a0d0e21e 857 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
2f9daede 858 $debugging = 0 unless defined $debugging;
a0d0e21e 859
7660c0ab
A
860Note: Many folks tend to overuse C<defined()>, and then are surprised to
861discover that the number C<0> and C<""> (the zero-length string) are, in fact,
2f9daede 862defined values. For example, if you say
a5f75d66
AD
863
864 "ab" =~ /a(.*)b/;
865
7660c0ab 866The pattern match succeeds, and C<$1> is defined, despite the fact that it
a5f75d66 867matched "nothing". But it didn't really match nothing--rather, it
2b5ab1e7 868matched something that happened to be zero characters long. This is all
a5f75d66 869very above-board and honest. When a function returns an undefined value,
2f9daede 870it's an admission that it couldn't give you an honest answer. So you
7660c0ab
A
871should use C<defined()> only when you're questioning the integrity of what
872you're trying to do. At other times, a simple comparison to C<0> or C<""> is
2f9daede
TPG
873what you want.
874
7660c0ab 875Currently, using C<defined()> on an entire array or hash reports whether
2f9daede
TPG
876memory for that aggregate has ever been allocated. So an array you set
877to the empty list appears undefined initially, and one that once was full
878and that you then set to the empty list still appears defined. You
879should instead use a simple test for size:
28757baa
PP
880
881 if (@an_array) { print "has array elements\n" }
882 if (%a_hash) { print "has hash members\n" }
883
7660c0ab 884Using C<undef()> on these, however, does clear their memory and then report
5a964f20 885them as not defined anymore, but you shouldn't do that unless you don't
28757baa 886plan to use them again, because it saves time when you load them up
5a964f20
TC
887again to have memory already ready to be filled. The normal way to
888free up space used by an aggregate is to assign the empty list.
28757baa 889
7660c0ab 890This counterintuitive behavior of C<defined()> on aggregates may be
28757baa
PP
891changed, fixed, or broken in a future release of Perl.
892
dc848c6f 893See also L</undef>, L</exists>, L</ref>.
2f9daede 894
a0d0e21e
LW
895=item delete EXPR
896
aa689395
PP
897Deletes the specified key(s) and their associated values from a hash.
898For each key, returns the deleted value associated with that key, or
899the undefined value if there was no such key. Deleting from C<$ENV{}>
900modifies the environment. Deleting from a hash tied to a DBM file
7660c0ab 901deletes the entry from the DBM file. (But deleting from a C<tie()>d hash
5f05dabc 902doesn't necessarily return anything.)
a0d0e21e 903
aa689395 904The following deletes all the values of a hash:
a0d0e21e 905
5f05dabc
PP
906 foreach $key (keys %HASH) {
907 delete $HASH{$key};
a0d0e21e
LW
908 }
909
5f05dabc
PP
910And so does this:
911
912 delete @HASH{keys %HASH}
913
2b5ab1e7
TC
914But both of these are slower than just assigning the empty list
915or undefining it:
916
917 %hash = (); # completely empty %hash
918 undef %hash; # forget %hash every existed
919
920Note that the EXPR can be arbitrarily complicated as long as the final
921operation is a hash element lookup or hash slice:
a0d0e21e
LW
922
923 delete $ref->[$x][$y]{$key};
5f05dabc 924 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
a0d0e21e
LW
925
926=item die LIST
927
7660c0ab
A
928Outside an C<eval()>, prints the value of LIST to C<STDERR> and exits with
929the current value of C<$!> (errno). If C<$!> is C<0>, exits with the value of
54310121 930C<($? E<gt>E<gt> 8)> (backtick `command` status). If C<($? E<gt>E<gt> 8)>
7660c0ab
A
931is C<0>, exits with C<255>. Inside an C<eval(),> the error message is stuffed into
932C<$@> and the C<eval()> is terminated with the undefined value. This makes
933C<die()> the way to raise an exception.
a0d0e21e
LW
934
935Equivalent examples:
936
937 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
54310121 938 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
a0d0e21e
LW
939
940If the value of EXPR does not end in a newline, the current script line
941number and input line number (if any) are also printed, and a newline
883faa13
GS
942is supplied. Note that the "input line number" (also known as "chunk")
943is subject to whatever notion of "line" happens to be currently in
944effect, and is also available as the special variable C<$.>.
945See L<perlvar/"$/"> and L<perlvar/"$.">.
946
947Hint: sometimes appending C<", stopped"> to your message
7660c0ab 948will cause it to make better sense when the string C<"at foo line 123"> is
a0d0e21e
LW
949appended. Suppose you are running script "canasta".
950
951 die "/etc/games is no good";
952 die "/etc/games is no good, stopped";
953
954produce, respectively
955
956 /etc/games is no good at canasta line 123.
957 /etc/games is no good, stopped at canasta line 123.
958
2b5ab1e7 959See also exit(), warn(), and the Carp module.
a0d0e21e 960
7660c0ab
A
961If LIST is empty and C<$@> already contains a value (typically from a
962previous eval) that value is reused after appending C<"\t...propagated">.
fb73857a
PP
963This is useful for propagating exceptions:
964
965 eval { ... };
966 die unless $@ =~ /Expected exception/;
967
7660c0ab 968If C<$@> is empty then the string C<"Died"> is used.
fb73857a 969
52531d10
GS
970die() can also be called with a reference argument. If this happens to be
971trapped within an eval(), $@ contains the reference. This behavior permits
972a more elaborate exception handling implementation using objects that
973maintain arbitary state about the nature of the exception. Such a scheme
974is sometimes preferable to matching particular string values of $@ using
975regular expressions. Here's an example:
976
977 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
978 if ($@) {
979 if (ref($@) && UNIVERSAL::isa($@,"Some::Module::Exception")) {
980 # handle Some::Module::Exception
981 }
982 else {
983 # handle all other possible exceptions
984 }
985 }
986
987Since perl will stringify uncaught exception messages before displaying
988them, you may want to overload stringification operations on such custom
989exception objects. See L<overload> for details about that.
990
7660c0ab 991You can arrange for a callback to be run just before the C<die()> does
774d564b
PP
992its deed, by setting the C<$SIG{__DIE__}> hook. The associated handler
993will be called with the error text and can change the error message, if
7660c0ab 994it sees fit, by calling C<die()> again. See L<perlvar/$SIG{expr}> for details on
fb73857a
PP
995setting C<%SIG> entries, and L<"eval BLOCK"> for some examples.
996
2b5ab1e7
TC
997Note that the C<$SIG{__DIE__}> hook is currently called even inside
998eval()ed blocks/strings! If one wants the hook to do nothing in such
fb73857a
PP
999situations, put
1000
1001 die @_ if $^S;
1002
2b5ab1e7
TC
1003as the first line of the handler (see L<perlvar/$^S>). Because this
1004promotes action at a distance, this counterintuitive behavior may be fixed
1005in a future release.
774d564b 1006
a0d0e21e
LW
1007=item do BLOCK
1008
1009Not really a function. Returns the value of the last command in the
1010sequence of commands indicated by BLOCK. When modified by a loop
98293880
JH
1011modifier, executes the BLOCK once before testing the loop condition.
1012(On other statements the loop modifiers test the conditional first.)
a0d0e21e 1013
4968c1e4 1014C<do BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7
TC
1015C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1016See L<perlsyn> for alternative strategies.
4968c1e4 1017
a0d0e21e
LW
1018=item do SUBROUTINE(LIST)
1019
1020A deprecated form of subroutine call. See L<perlsub>.
1021
1022=item do EXPR
1023
1024Uses the value of EXPR as a filename and executes the contents of the
1025file as a Perl script. Its primary use is to include subroutines
1026from a Perl subroutine library.
1027
1028 do 'stat.pl';
1029
1030is just like
1031
fb73857a 1032 scalar eval `cat stat.pl`;
a0d0e21e 1033
2b5ab1e7
TC
1034except that it's more efficient and concise, keeps track of the current
1035filename for error messages, searches the @INC libraries, and updates
1036C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1037variables. It also differs in that code evaluated with C<do FILENAME>
1038cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1039same, however, in that it does reparse the file every time you call it,
1040so you probably don't want to do this inside a loop.
a0d0e21e 1041
8e30cc93 1042If C<do> cannot read the file, it returns undef and sets C<$!> to the
2b5ab1e7 1043error. If C<do> can read the file but cannot compile it, it
8e30cc93
G
1044returns undef and sets an error message in C<$@>. If the file is
1045successfully compiled, C<do> returns the value of the last expression
1046evaluated.
1047
a0d0e21e 1048Note that inclusion of library modules is better done with the
7660c0ab 1049C<use()> and C<require()> operators, which also do automatic error checking
4633a7c4 1050and raise an exception if there's a problem.
a0d0e21e 1051
5a964f20
TC
1052You might like to use C<do> to read in a program configuration
1053file. Manual error checking can be done this way:
1054
1055 # read in config files: system first, then user
f86cebdf 1056 for $file ("/share/prog/defaults.rc",
2b5ab1e7
TC
1057 "$ENV{HOME}/.someprogrc")
1058 {
5a964f20 1059 unless ($return = do $file) {
f86cebdf
GS
1060 warn "couldn't parse $file: $@" if $@;
1061 warn "couldn't do $file: $!" unless defined $return;
1062 warn "couldn't run $file" unless $return;
5a964f20
TC
1063 }
1064 }
1065
a0d0e21e
LW
1066=item dump LABEL
1067
1614b0e3
JD
1068=item dump
1069
a0d0e21e
LW
1070This causes an immediate core dump. Primarily this is so that you can
1071use the B<undump> program to turn your core dump into an executable binary
1072after having initialized all your variables at the beginning of the
1073program. When the new binary is executed it will begin by executing a
1074C<goto LABEL> (with all the restrictions that C<goto> suffers). Think of
f86cebdf 1075it as a goto with an intervening core dump and reincarnation. If C<LABEL>
5a964f20 1076is omitted, restarts the program from the top. WARNING: Any files
a0d0e21e
LW
1077opened at the time of the dump will NOT be open any more when the
1078program is reincarnated, with possible resulting confusion on the part
1079of Perl. See also B<-u> option in L<perlrun>.
1080
1081Example:
1082
1083 #!/usr/bin/perl
1084 require 'getopt.pl';
1085 require 'stat.pl';
1086 %days = (
1087 'Sun' => 1,
1088 'Mon' => 2,
1089 'Tue' => 3,
1090 'Wed' => 4,
1091 'Thu' => 5,
1092 'Fri' => 6,
1093 'Sat' => 7,
1094 );
1095
1096 dump QUICKSTART if $ARGV[0] eq '-d';
1097
1098 QUICKSTART:
1099 Getopt('f');
1100
5a964f20
TC
1101This operator is largely obsolete, partly because it's very hard to
1102convert a core file into an executable, and because the real perl-to-C
1103compiler has superseded it.
1104
aa689395
PP
1105=item each HASH
1106
5a964f20 1107When called in list context, returns a 2-element list consisting of the
aa689395 1108key and value for the next element of a hash, so that you can iterate over
5a964f20 1109it. When called in scalar context, returns the key for only the "next"
7660c0ab 1110element in the hash. (Note: Keys may be C<"0"> or C<"">, which are logically
2f9daede
TPG
1111false; you may wish to avoid constructs like C<while ($k = each %foo) {}>
1112for this reason.)
1113
ab192400
GS
1114Entries are returned in an apparently random order. The actual random
1115order is subject to change in future versions of perl, but it is guaranteed
1116to be in the same order as either the C<keys()> or C<values()> function
1117would produce on the same (unmodified) hash.
1118
1119When the hash is entirely read, a null array is returned in list context
1120(which when assigned produces a FALSE (C<0>) value), and C<undef> in
7660c0ab
A
1121scalar context. The next call to C<each()> after that will start iterating
1122again. There is a single iterator for each hash, shared by all C<each()>,
1123C<keys()>, and C<values()> function calls in the program; it can be reset by
2f9daede
TPG
1124reading all the elements from the hash, or by evaluating C<keys HASH> or
1125C<values HASH>. If you add or delete elements of a hash while you're
1126iterating over it, you may get entries skipped or duplicated, so don't.
aa689395 1127
f86cebdf 1128The following prints out your environment like the printenv(1) program,
aa689395 1129only in a different order:
a0d0e21e
LW
1130
1131 while (($key,$value) = each %ENV) {
1132 print "$key=$value\n";
1133 }
1134
ab192400 1135See also C<keys()>, C<values()> and C<sort()>.
a0d0e21e
LW
1136
1137=item eof FILEHANDLE
1138
4633a7c4
LW
1139=item eof ()
1140
a0d0e21e
LW
1141=item eof
1142
1143Returns 1 if the next read on FILEHANDLE will return end of file, or if
1144FILEHANDLE is not open. FILEHANDLE may be an expression whose value
5a964f20 1145gives the real filehandle. (Note that this function actually
7660c0ab 1146reads a character and then C<ungetc()>s it, so isn't very useful in an
748a9306
LW
1147interactive context.) Do not read from a terminal file (or call
1148C<eof(FILEHANDLE)> on it) after end-of-file is reached. Filetypes such
1149as terminals may lose the end-of-file condition if you do.
1150
1151An C<eof> without an argument uses the last file read as argument.
2b5ab1e7
TC
1152Using C<eof()> with empty parentheses is very different. It indicates
1153the pseudo file formed of the files listed on the command line, i.e.,
1154C<eof()> is reasonable to use inside a C<while (E<lt>E<gt>)> loop to
1155detect the end of only the last file. Use C<eof(ARGV)> or eof without the
1156parentheses to test I<EACH> file in a while (E<lt>E<gt>) loop. Examples:
a0d0e21e 1157
748a9306
LW
1158 # reset line numbering on each input file
1159 while (<>) {
5a964f20 1160 next if /^\s*#/; # skip comments
748a9306 1161 print "$.\t$_";
5a964f20
TC
1162 } continue {
1163 close ARGV if eof; # Not eof()!
748a9306
LW
1164 }
1165
a0d0e21e
LW
1166 # insert dashes just before last line of last file
1167 while (<>) {
5a964f20 1168 if (eof()) { # check for end of current file
a0d0e21e 1169 print "--------------\n";
2b5ab1e7 1170 close(ARGV); # close or last; is needed if we
748a9306 1171 # are reading from the terminal
a0d0e21e
LW
1172 }
1173 print;
1174 }
1175
a0d0e21e 1176Practical hint: you almost never need to use C<eof> in Perl, because the
3b02c43c
GS
1177input operators return false values when they run out of data, or if there
1178was an error.
a0d0e21e
LW
1179
1180=item eval EXPR
1181
1182=item eval BLOCK
1183
c7cc6f1c
GS
1184In the first form, the return value of EXPR is parsed and executed as if it
1185were a little Perl program. The value of the expression (which is itself
5a964f20 1186determined within scalar context) is first parsed, and if there weren't any
c7cc6f1c 1187errors, executed in the context of the current Perl program, so that any
5f05dabc 1188variable settings or subroutine and format definitions remain afterwards.
c7cc6f1c
GS
1189Note that the value is parsed every time the eval executes. If EXPR is
1190omitted, evaluates C<$_>. This form is typically used to delay parsing
1191and subsequent execution of the text of EXPR until run time.
1192
1193In the second form, the code within the BLOCK is parsed only once--at the
1194same time the code surrounding the eval itself was parsed--and executed
1195within the context of the current Perl program. This form is typically
1196used to trap exceptions more efficiently than the first (see below), while
1197also providing the benefit of checking the code within BLOCK at compile
1198time.
1199
1200The final semicolon, if any, may be omitted from the value of EXPR or within
1201the BLOCK.
1202
1203In both forms, the value returned is the value of the last expression
5a964f20 1204evaluated inside the mini-program; a return statement may be also used, just
c7cc6f1c 1205as with subroutines. The expression providing the return value is evaluated
5a964f20 1206in void, scalar, or list context, depending on the context of the eval itself.
c7cc6f1c 1207See L</wantarray> for more on how the evaluation context can be determined.
a0d0e21e 1208
7660c0ab
A
1209If there is a syntax error or runtime error, or a C<die()> statement is
1210executed, an undefined value is returned by C<eval()>, and C<$@> is set to the
a0d0e21e 1211error message. If there was no error, C<$@> is guaranteed to be a null
7660c0ab 1212string. Beware that using C<eval()> neither silences perl from printing
c7cc6f1c
GS
1213warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1214To do either of those, you have to use the C<$SIG{__WARN__}> facility. See
1215L</warn> and L<perlvar>.
a0d0e21e 1216
7660c0ab
A
1217Note that, because C<eval()> traps otherwise-fatal errors, it is useful for
1218determining whether a particular feature (such as C<socket()> or C<symlink()>)
a0d0e21e
LW
1219is implemented. It is also Perl's exception trapping mechanism, where
1220the die operator is used to raise exceptions.
1221
1222If the code to be executed doesn't vary, you may use the eval-BLOCK
1223form to trap run-time errors without incurring the penalty of
1224recompiling each time. The error, if any, is still returned in C<$@>.
1225Examples:
1226
54310121 1227 # make divide-by-zero nonfatal
a0d0e21e
LW
1228 eval { $answer = $a / $b; }; warn $@ if $@;
1229
1230 # same thing, but less efficient
1231 eval '$answer = $a / $b'; warn $@ if $@;
1232
1233 # a compile-time error
5a964f20 1234 eval { $answer = }; # WRONG
a0d0e21e
LW
1235
1236 # a run-time error
1237 eval '$answer ='; # sets $@
1238
2b5ab1e7
TC
1239Due to the current arguably broken state of C<__DIE__> hooks, when using
1240the C<eval{}> form as an exception trap in libraries, you may wish not
1241to trigger any C<__DIE__> hooks that user code may have installed.
1242You can use the C<local $SIG{__DIE__}> construct for this purpose,
1243as shown in this example:
774d564b
PP
1244
1245 # a very private exception trap for divide-by-zero
f86cebdf
GS
1246 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1247 warn $@ if $@;
774d564b
PP
1248
1249This is especially significant, given that C<__DIE__> hooks can call
7660c0ab 1250C<die()> again, which has the effect of changing their error messages:
774d564b
PP
1251
1252 # __DIE__ hooks may modify error messages
1253 {
f86cebdf
GS
1254 local $SIG{'__DIE__'} =
1255 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
c7cc6f1c
GS
1256 eval { die "foo lives here" };
1257 print $@ if $@; # prints "bar lives here"
774d564b
PP
1258 }
1259
2b5ab1e7
TC
1260Because this promotes action at a distance, this counterintuive behavior
1261may be fixed in a future release.
1262
7660c0ab 1263With an C<eval()>, you should be especially careful to remember what's
a0d0e21e
LW
1264being looked at when:
1265
1266 eval $x; # CASE 1
1267 eval "$x"; # CASE 2
1268
1269 eval '$x'; # CASE 3
1270 eval { $x }; # CASE 4
1271
5a964f20 1272 eval "\$$x++"; # CASE 5
a0d0e21e
LW
1273 $$x++; # CASE 6
1274
2f9daede 1275Cases 1 and 2 above behave identically: they run the code contained in
7660c0ab 1276the variable C<$x>. (Although case 2 has misleading double quotes making
2f9daede 1277the reader wonder what else might be happening (nothing is).) Cases 3
7660c0ab 1278and 4 likewise behave in the same way: they run the code C<'$x'>, which
2f9daede
TPG
1279does nothing but return the value of C<$x>. (Case 4 is preferred for
1280purely visual reasons, but it also has the advantage of compiling at
1281compile-time instead of at run-time.) Case 5 is a place where
54310121 1282normally you I<WOULD> like to use double quotes, except that in this
2f9daede
TPG
1283particular situation, you can just use symbolic references instead, as
1284in case 6.
a0d0e21e 1285
4968c1e4 1286C<eval BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7 1287C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
4968c1e4 1288
a0d0e21e
LW
1289=item exec LIST
1290
8bf3b016
GS
1291=item exec PROGRAM LIST
1292
7660c0ab
A
1293The C<exec()> function executes a system command I<AND NEVER RETURNS> -
1294use C<system()> instead of C<exec()> if you want it to return. It fails and
fb73857a
PP
1295returns FALSE only if the command does not exist I<and> it is executed
1296directly instead of via your system's command shell (see below).
a0d0e21e 1297
7660c0ab
A
1298Since it's a common mistake to use C<exec()> instead of C<system()>, Perl
1299warns you if there is a following statement which isn't C<die()>, C<warn()>,
1300or C<exit()> (if C<-w> is set - but you always do that). If you
1301I<really> want to follow an C<exec()> with some other statement, you
55d729e4
GS
1302can use one of these styles to avoid the warning:
1303
5a964f20
TC
1304 exec ('foo') or print STDERR "couldn't exec foo: $!";
1305 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
55d729e4 1306
5a964f20 1307If there is more than one argument in LIST, or if LIST is an array
f86cebdf 1308with more than one value, calls execvp(3) with the arguments in LIST.
5a964f20
TC
1309If there is only one scalar argument or an array with one element in it,
1310the argument is checked for shell metacharacters, and if there are any,
1311the entire argument is passed to the system's command shell for parsing
1312(this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1313If there are no shell metacharacters in the argument, it is split into
45bc9206 1314words and passed directly to C<execvp()>, which is more efficient.
a0d0e21e 1315
45bc9206 1316All files opened for output are flushed before attempting the exec().
a0d0e21e
LW
1317
1318If you don't really want to execute the first argument, but want to lie
1319to the program you are executing about its own name, you can specify
1320the program you actually want to run as an "indirect object" (without a
1321comma) in front of the LIST. (This always forces interpretation of the
54310121 1322LIST as a multivalued list, even if there is only a single scalar in
a0d0e21e
LW
1323the list.) Example:
1324
1325 $shell = '/bin/csh';
1326 exec $shell '-sh'; # pretend it's a login shell
1327
1328or, more directly,
1329
1330 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1331
bb32b41a
GS
1332When the arguments get executed via the system shell, results will
1333be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1334for details.
1335
ab4f32c2 1336Using an indirect object with C<exec()> or C<system()> is also more secure.
5a964f20
TC
1337This usage forces interpretation of the arguments as a multivalued list,
1338even if the list had just one argument. That way you're safe from the
1339shell expanding wildcards or splitting up words with whitespace in them.
1340
1341 @args = ( "echo surprise" );
1342
2b5ab1e7 1343 exec @args; # subject to shell escapes
f86cebdf 1344 # if @args == 1
2b5ab1e7 1345 exec { $args[0] } @args; # safe even with one-arg list
5a964f20
TC
1346
1347The first version, the one without the indirect object, ran the I<echo>
1348program, passing it C<"surprise"> an argument. The second version
1349didn't--it tried to run a program literally called I<"echo surprise">,
1350didn't find it, and set C<$?> to a non-zero value indicating failure.
1351
ab4f32c2 1352Note that C<exec()> will not call your C<END> blocks, nor will it call
7660c0ab
A
1353any C<DESTROY> methods in your objects.
1354
a0d0e21e
LW
1355=item exists EXPR
1356
1357Returns TRUE if the specified hash key exists in its hash array, even
1358if the corresponding value is undefined.
1359
2b5ab1e7
TC
1360 print "Exists\n" if exists $array{$key};
1361 print "Defined\n" if defined $array{$key};
1362 print "True\n" if $array{$key};
a0d0e21e 1363
5f05dabc 1364A hash element can be TRUE only if it's defined, and defined if
a0d0e21e
LW
1365it exists, but the reverse doesn't necessarily hold true.
1366
1367Note that the EXPR can be arbitrarily complicated as long as the final
1368operation is a hash key lookup:
1369
2b5ab1e7
TC
1370 if (exists $ref->{A}->{B}->{$key}) { }
1371 if (exists $hash{A}{B}{$key}) { }
1372
1373Although the last element will not spring into existence just because
1374its existence was tested, intervening ones will. Thus C<$ref-E<gt>{"A"}>
1375and C<$ref-E<gt>{"A"}-E<gt>{"B"}> will spring into existence due to the
1376existence test for a $key element. This happens anywhere the arrow
1377operator is used, including even
5a964f20 1378
2b5ab1e7
TC
1379 undef $ref;
1380 if (exists $ref->{"Some key"}) { }
1381 print $ref; # prints HASH(0x80d3d5c)
1382
1383This surprising autovivification in what does not at first--or even
1384second--glance appear to be an lvalue context may be fixed in a future
5a964f20 1385release.
a0d0e21e
LW
1386
1387=item exit EXPR
1388
2b5ab1e7 1389Evaluates EXPR and exits immediately with that value. Example:
a0d0e21e
LW
1390
1391 $ans = <STDIN>;
1392 exit 0 if $ans =~ /^[Xx]/;
1393
7660c0ab 1394See also C<die()>. If EXPR is omitted, exits with C<0> status. The only
2b5ab1e7
TC
1395universally recognized values for EXPR are C<0> for success and C<1>
1396for error; other values are subject to interpretation depending on the
1397environment in which the Perl program is running. For example, exiting
139869 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1399the mailer to return the item undelivered, but that's not true everywhere.
a0d0e21e 1400
2b5ab1e7 1401Don't use C<exit()> to abort a subroutine if there's any chance that
7660c0ab
A
1402someone might want to trap whatever error happened. Use C<die()> instead,
1403which can be trapped by an C<eval()>.
28757baa 1404
2b5ab1e7
TC
1405The exit() function does not always exit immediately. It calls any
1406defined C<END> routines first, but these C<END> routines may not
1407themselves abort the exit. Likewise any object destructors that need to
1408be called are called before the real exit. If this is a problem, you
1409can call C<POSIX:_exit($status)> to avoid END and destructor processing.
1410See L<perlsub> for details.
5a964f20 1411
a0d0e21e
LW
1412=item exp EXPR
1413
54310121 1414=item exp
bbce6d69 1415
2b5ab1e7 1416Returns I<e> (the natural logarithm base) to the power of EXPR.
a0d0e21e
LW
1417If EXPR is omitted, gives C<exp($_)>.
1418
1419=item fcntl FILEHANDLE,FUNCTION,SCALAR
1420
f86cebdf 1421Implements the fcntl(2) function. You'll probably have to say
a0d0e21e
LW
1422
1423 use Fcntl;
1424
0ade1984 1425first to get the correct constant definitions. Argument processing and
7660c0ab 1426value return works just like C<ioctl()> below.
a0d0e21e
LW
1427For example:
1428
1429 use Fcntl;
5a964f20
TC
1430 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1431 or die "can't fcntl F_GETFL: $!";
1432
2b5ab1e7
TC
1433You don't have to check for C<defined()> on the return from C<fnctl()>.
1434Like C<ioctl()>, it maps a C<0> return from the system call into "C<0>
1435but true" in Perl. This string is true in boolean context and C<0>
1436in numeric context. It is also exempt from the normal B<-w> warnings
1437on improper numeric conversions.
5a964f20 1438
7660c0ab 1439Note that C<fcntl()> will produce a fatal error if used on a machine that
2b5ab1e7
TC
1440doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1441manpage to learn what functions are available on your system.
a0d0e21e
LW
1442
1443=item fileno FILEHANDLE
1444
2b5ab1e7
TC
1445Returns the file descriptor for a filehandle, or undefined if the
1446filehandle is not open. This is mainly useful for constructing
1447bitmaps for C<select()> and low-level POSIX tty-handling operations.
1448If FILEHANDLE is an expression, the value is taken as an indirect
1449filehandle, generally its name.
5a964f20
TC
1450
1451You can use this to find out whether two handles refer to the
1452same underlying descriptor:
1453
1454 if (fileno(THIS) == fileno(THAT)) {
1455 print "THIS and THAT are dups\n";
1456 }
a0d0e21e
LW
1457
1458=item flock FILEHANDLE,OPERATION
1459
2b5ab1e7
TC
1460Calls flock(2), or an emulation of it, on FILEHANDLE. Returns TRUE
1461for success, FALSE on failure. Produces a fatal error if used on a
1462machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
1463C<flock()> is Perl's portable file locking interface, although it locks
1464only entire files, not records.
1465
1466Two potentially non-obvious but traditional C<flock> semantics are
1467that it waits indefinitely until the lock is granted, and that its locks
1468B<merely advisory>. Such discretionary locks are more flexible, but offer
1469fewer guarantees. This means that files locked with C<flock()> may be
1470modified by programs that do not also use C<flock()>. See L<perlport>,
1471your port's specific documentation, or your system-specific local manpages
1472for details. It's best to assume traditional behavior if you're writing
1473portable programs. (But if you're not, you should as always feel perfectly
1474free to write for your own system's idiosyncrasies (sometimes called
1475"features"). Slavish adherence to portability concerns shouldn't get
1476in the way of your getting your job done.)
a3cb178b 1477
8ebc5c01
PP
1478OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1479LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
68dc0745
PP
1480you can use the symbolic names if import them from the Fcntl module,
1481either individually, or as a group using the ':flock' tag. LOCK_SH
1482requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1483releases a previously requested lock. If LOCK_NB is added to LOCK_SH or
7660c0ab 1484LOCK_EX then C<flock()> will return immediately rather than blocking
68dc0745
PP
1485waiting for the lock (check the return status to see if you got it).
1486
2b5ab1e7
TC
1487To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1488before locking or unlocking it.
8ebc5c01 1489
f86cebdf 1490Note that the emulation built with lockf(3) doesn't provide shared
8ebc5c01 1491locks, and it requires that FILEHANDLE be open with write intent. These
2b5ab1e7 1492are the semantics that lockf(3) implements. Most if not all systems
f86cebdf 1493implement lockf(3) in terms of fcntl(2) locking, though, so the
8ebc5c01
PP
1494differing semantics shouldn't bite too many people.
1495
7660c0ab
A
1496Note also that some versions of C<flock()> cannot lock things over the
1497network; you would need to use the more system-specific C<fcntl()> for
f86cebdf
GS
1498that. If you like you can force Perl to ignore your system's flock(2)
1499function, and so provide its own fcntl(2)-based emulation, by passing
8ebc5c01
PP
1500the switch C<-Ud_flock> to the F<Configure> program when you configure
1501perl.
4633a7c4
LW
1502
1503Here's a mailbox appender for BSD systems.
a0d0e21e 1504
7e1af8bc 1505 use Fcntl ':flock'; # import LOCK_* constants
a0d0e21e
LW
1506
1507 sub lock {
7e1af8bc 1508 flock(MBOX,LOCK_EX);
a0d0e21e
LW
1509 # and, in case someone appended
1510 # while we were waiting...
1511 seek(MBOX, 0, 2);
1512 }
1513
1514 sub unlock {
7e1af8bc 1515 flock(MBOX,LOCK_UN);
a0d0e21e
LW
1516 }
1517
1518 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1519 or die "Can't open mailbox: $!";
1520
1521 lock();
1522 print MBOX $msg,"\n\n";
1523 unlock();
1524
2b5ab1e7
TC
1525On systems that support a real flock(), locks are inherited across fork()
1526calls, whereas those that must resort to the more capricious fcntl()
1527function lose the locks, making it harder to write servers.
1528
cb1a09d0 1529See also L<DB_File> for other flock() examples.
a0d0e21e
LW
1530
1531=item fork
1532
2b5ab1e7
TC
1533Does a fork(2) system call to create a new process running the
1534same program at the same point. It returns the child pid to the
1535parent process, C<0> to the child process, or C<undef> if the fork is
1536unsuccessful. File descriptors (and sometimes locks on those descriptors)
1537are shared, while everything else is copied. On most systems supporting
1538fork(), great care has gone into making it extremely efficient (for
1539example, using copy-on-write technology on data pages), making it the
1540dominant paradigm for multitasking over the last few decades.
5a964f20 1541
45bc9206 1542All files opened for output are flushed before forking the child process.
a0d0e21e 1543
2b5ab1e7
TC
1544If you C<fork()> without ever waiting on your children, you will
1545accumulate zombies. On some systems, you can avoid this by setting
1546C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1547forking and reaping moribund children.
cb1a09d0 1548
28757baa
PP
1549Note that if your forked child inherits system file descriptors like
1550STDIN and STDOUT that are actually connected by a pipe or socket, even
2b5ab1e7
TC
1551if you exit, then the remote server (such as, say, a CGI script or a
1552backgrounded job launced from a remote shell) won't think you're done.
1553You should reopen those to F</dev/null> if it's any issue.
28757baa 1554
cb1a09d0
AD
1555=item format
1556
7660c0ab 1557Declare a picture format for use by the C<write()> function. For
cb1a09d0
AD
1558example:
1559
54310121 1560 format Something =
cb1a09d0
AD
1561 Test: @<<<<<<<< @||||| @>>>>>
1562 $str, $%, '$' . int($num)
1563 .
1564
1565 $str = "widget";
184e9718 1566 $num = $cost/$quantity;
cb1a09d0
AD
1567 $~ = 'Something';
1568 write;
1569
1570See L<perlform> for many details and examples.
1571
8903cb82 1572=item formline PICTURE,LIST
a0d0e21e 1573
5a964f20 1574This is an internal function used by C<format>s, though you may call it,
a0d0e21e
LW
1575too. It formats (see L<perlform>) a list of values according to the
1576contents of PICTURE, placing the output into the format output
7660c0ab
A
1577accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
1578Eventually, when a C<write()> is done, the contents of
a0d0e21e 1579C<$^A> are written to some filehandle, but you could also read C<$^A>
7660c0ab
A
1580yourself and then set C<$^A> back to C<"">. Note that a format typically
1581does one C<formline()> per line of form, but the C<formline()> function itself
748a9306 1582doesn't care how many newlines are embedded in the PICTURE. This means
4633a7c4 1583that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
748a9306
LW
1584You may therefore need to use multiple formlines to implement a single
1585record format, just like the format compiler.
1586
5f05dabc 1587Be careful if you put double quotes around the picture, because an "C<@>"
748a9306 1588character may be taken to mean the beginning of an array name.
7660c0ab 1589C<formline()> always returns TRUE. See L<perlform> for other examples.
a0d0e21e
LW
1590
1591=item getc FILEHANDLE
1592
1593=item getc
1594
1595Returns the next character from the input file attached to FILEHANDLE,
2b5ab1e7
TC
1596or the undefined value at end of file, or if there was an error.
1597If FILEHANDLE is omitted, reads from STDIN. This is not particularly
1598efficient. However, it cannot be used by itself to fetch single
1599characters without waiting for the user to hit enter. For that, try
1600something more like:
4633a7c4
LW
1601
1602 if ($BSD_STYLE) {
1603 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1604 }
1605 else {
54310121 1606 system "stty", '-icanon', 'eol', "\001";
4633a7c4
LW
1607 }
1608
1609 $key = getc(STDIN);
1610
1611 if ($BSD_STYLE) {
1612 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1613 }
1614 else {
5f05dabc 1615 system "stty", 'icanon', 'eol', '^@'; # ASCII null
4633a7c4
LW
1616 }
1617 print "\n";
1618
54310121
PP
1619Determination of whether $BSD_STYLE should be set
1620is left as an exercise to the reader.
cb1a09d0 1621
2b5ab1e7
TC
1622The C<POSIX::getattr()> function can do this more portably on
1623systems purporting POSIX compliance. See also the C<Term::ReadKey>
1624module from your nearest CPAN site; details on CPAN can be found on
1625L<perlmodlib/CPAN>.
a0d0e21e
LW
1626
1627=item getlogin
1628
5a964f20
TC
1629Implements the C library function of the same name, which on most
1630systems returns the current login from F</etc/utmp>, if any. If null,
7660c0ab 1631use C<getpwuid()>.
a0d0e21e 1632
f86702cc 1633 $login = getlogin || getpwuid($<) || "Kilroy";
a0d0e21e 1634
7660c0ab
A
1635Do not consider C<getlogin()> for authentication: it is not as
1636secure as C<getpwuid()>.
4633a7c4 1637
a0d0e21e
LW
1638=item getpeername SOCKET
1639
1640Returns the packed sockaddr address of other end of the SOCKET connection.
1641
4633a7c4
LW
1642 use Socket;
1643 $hersockaddr = getpeername(SOCK);
1644 ($port, $iaddr) = unpack_sockaddr_in($hersockaddr);
1645 $herhostname = gethostbyaddr($iaddr, AF_INET);
1646 $herstraddr = inet_ntoa($iaddr);
a0d0e21e
LW
1647
1648=item getpgrp PID
1649
47e29363 1650Returns the current process group for the specified PID. Use
7660c0ab 1651a PID of C<0> to get the current process group for the
4633a7c4 1652current process. Will raise an exception if used on a machine that
f86cebdf 1653doesn't implement getpgrp(2). If PID is omitted, returns process
7660c0ab
A
1654group of current process. Note that the POSIX version of C<getpgrp()>
1655does not accept a PID argument, so only C<PID==0> is truly portable.
a0d0e21e
LW
1656
1657=item getppid
1658
1659Returns the process id of the parent process.
1660
1661=item getpriority WHICH,WHO
1662
4633a7c4
LW
1663Returns the current priority for a process, a process group, or a user.
1664(See L<getpriority(2)>.) Will raise a fatal exception if used on a
f86cebdf 1665machine that doesn't implement getpriority(2).
a0d0e21e
LW
1666
1667=item getpwnam NAME
1668
1669=item getgrnam NAME
1670
1671=item gethostbyname NAME
1672
1673=item getnetbyname NAME
1674
1675=item getprotobyname NAME
1676
1677=item getpwuid UID
1678
1679=item getgrgid GID
1680
1681=item getservbyname NAME,PROTO
1682
1683=item gethostbyaddr ADDR,ADDRTYPE
1684
1685=item getnetbyaddr ADDR,ADDRTYPE
1686
1687=item getprotobynumber NUMBER
1688
1689=item getservbyport PORT,PROTO
1690
1691=item getpwent
1692
1693=item getgrent
1694
1695=item gethostent
1696
1697=item getnetent
1698
1699=item getprotoent
1700
1701=item getservent
1702
1703=item setpwent
1704
1705=item setgrent
1706
1707=item sethostent STAYOPEN
1708
1709=item setnetent STAYOPEN
1710
1711=item setprotoent STAYOPEN
1712
1713=item setservent STAYOPEN
1714
1715=item endpwent
1716
1717=item endgrent
1718
1719=item endhostent
1720
1721=item endnetent
1722
1723=item endprotoent
1724
1725=item endservent
1726
1727These routines perform the same functions as their counterparts in the
5a964f20 1728system library. In list context, the return values from the
a0d0e21e
LW
1729various get routines are as follows:
1730
1731 ($name,$passwd,$uid,$gid,
6ee623d5 1732 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
a0d0e21e
LW
1733 ($name,$passwd,$gid,$members) = getgr*
1734 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1735 ($name,$aliases,$addrtype,$net) = getnet*
1736 ($name,$aliases,$proto) = getproto*
1737 ($name,$aliases,$port,$proto) = getserv*
1738
1739(If the entry doesn't exist you get a null list.)
1740
5a964f20 1741In scalar context, you get the name, unless the function was a
a0d0e21e
LW
1742lookup by name, in which case you get the other thing, whatever it is.
1743(If the entry doesn't exist you get the undefined value.) For example:
1744
5a964f20
TC
1745 $uid = getpwnam($name);
1746 $name = getpwuid($num);
1747 $name = getpwent();
1748 $gid = getgrnam($name);
1749 $name = getgrgid($num;
1750 $name = getgrent();
1751 #etc.
a0d0e21e 1752
2b5ab1e7
TC
1753In I<getpw*()> the fields C<$quota>, C<$comment>, and C<$expire> are
1754special cases in the sense that in many systems they are unsupported.
1755If the C<$quota> is unsupported, it is an empty scalar. If it is
1756supported, it usually encodes the disk quota. If the C<$comment>
1757field is unsupported, it is an empty scalar. If it is supported it
1758usually encodes some administrative comment about the user. In some
1759systems the $quota field may be C<$change> or C<$age>, fields that have
1760to do with password aging. In some systems the C<$comment> field may
1761be C<$class>. The C<$expire> field, if present, encodes the expiration
1762period of the account or the password. For the availability and the
1763exact meaning of these fields in your system, please consult your
1764getpwnam(3) documentation and your F<pwd.h> file. You can also find
1765out from within Perl what your C<$quota> and C<$comment> fields mean
1766and whether you have the C<$expire> field by using the C<Config> module
1767and the values C<d_pwquota>, C<d_pwage>, C<d_pwchange>, C<d_pwcomment>,
1768and C<d_pwexpire>. Shadow password files are only supported if your
1769vendor has implemented them in the intuitive fashion that calling the
1770regular C library routines gets the shadow versions if you're running
1771under privilege. Those that incorrectly implement a separate library
1772call are not supported.
6ee623d5 1773
7660c0ab 1774The C<$members> value returned by I<getgr*()> is a space separated list of
a0d0e21e
LW
1775the login names of the members of the group.
1776
1777For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1778C, it will be returned to you via C<$?> if the function call fails. The
7660c0ab 1779C<@addrs> value returned by a successful call is a list of the raw
a0d0e21e
LW
1780addresses returned by the corresponding system library call. In the
1781Internet domain, each address is four bytes long and you can unpack it
1782by saying something like:
1783
1784 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1785
2b5ab1e7
TC
1786The Socket library makes this slightly easier:
1787
1788 use Socket;
1789 $iaddr = inet_aton("127.1"); # or whatever address
1790 $name = gethostbyaddr($iaddr, AF_INET);
1791
1792 # or going the other way
1793 $straddr = inet_ntoa($iaddr");
1794
5a964f20
TC
1795If you get tired of remembering which element of the return list contains
1796which return value, by-name interfaces are also provided in modules:
7660c0ab
A
1797C<File::stat>, C<Net::hostent>, C<Net::netent>, C<Net::protoent>, C<Net::servent>,
1798C<Time::gmtime>, C<Time::localtime>, and C<User::grent>. These override the
5a964f20
TC
1799normal built-in, replacing them with versions that return objects with
1800the appropriate names for each field. For example:
1801
1802 use File::stat;
1803 use User::pwent;
1804 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
1805
1806Even though it looks like they're the same method calls (uid),
7660c0ab 1807they aren't, because a C<File::stat> object is different from a C<User::pwent> object.
5a964f20 1808
a0d0e21e
LW
1809=item getsockname SOCKET
1810
1811Returns the packed sockaddr address of this end of the SOCKET connection.
1812
4633a7c4
LW
1813 use Socket;
1814 $mysockaddr = getsockname(SOCK);
1815 ($port, $myaddr) = unpack_sockaddr_in($mysockaddr);
a0d0e21e
LW
1816
1817=item getsockopt SOCKET,LEVEL,OPTNAME
1818
5a964f20 1819Returns the socket option requested, or undef if there is an error.
a0d0e21e
LW
1820
1821=item glob EXPR
1822
0a753a76
PP
1823=item glob
1824
2b5ab1e7
TC
1825Returns the value of EXPR with filename expansions such as the
1826standard Unix shell F</bin/csh> would do. This is the internal function
1827implementing the C<E<lt>*.cE<gt>> operator, but you can use it directly.
1828If EXPR is omitted, C<$_> is used. The C<E<lt>*.cE<gt>> operator is
1829discussed in more detail in L<perlop/"I/O Operators">.
a0d0e21e
LW
1830
1831=item gmtime EXPR
1832
1833Converts a time as returned by the time function to a 9-element array
54310121 1834with the time localized for the standard Greenwich time zone.
4633a7c4 1835Typically used as follows:
a0d0e21e 1836
54310121 1837 # 0 1 2 3 4 5 6 7 8
a0d0e21e
LW
1838 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1839 gmtime(time);
1840
1841All array elements are numeric, and come straight out of a struct tm.
2b5ab1e7
TC
1842In particular this means that C<$mon> has the range C<0..11> and C<$wday>
1843has the range C<0..6> with sunday as day C<0>. Also, C<$year> is the
1844number of years since 1900, that is, C<$year> is C<123> in year 2023,
1845I<not> simply the last two digits of the year. If you assume it is,
1846then you create non-Y2K-compliant programs--and you wouldn't want to do
1847that, would you?
2f9daede
TPG
1848
1849If EXPR is omitted, does C<gmtime(time())>.
a0d0e21e 1850
f86cebdf 1851In scalar context, returns the ctime(3) value:
0a753a76
PP
1852
1853 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
1854
7660c0ab 1855Also see the C<timegm()> function provided by the C<Time::Local> module,
f86cebdf 1856and the strftime(3) function available via the POSIX module.
7660c0ab 1857
2b5ab1e7
TC
1858This scalar value is B<not> locale dependent (see L<perllocale>), but
1859is instead a Perl builtin. Also see the C<Time::Local> module, and the
1860strftime(3) and mktime(3) functions available via the POSIX module. To
7660c0ab
A
1861get somewhat similar but locale dependent date strings, set up your
1862locale environment variables appropriately (please see L<perllocale>)
1863and try for example:
1864
1865 use POSIX qw(strftime);
2b5ab1e7 1866 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
7660c0ab 1867
2b5ab1e7
TC
1868Note that the C<%a> and C<%b> escapes, which represent the short forms
1869of the day of the week and the month of the year, may not necessarily
1870be three characters wide in all locales.
0a753a76 1871
a0d0e21e
LW
1872=item goto LABEL
1873
748a9306
LW
1874=item goto EXPR
1875
a0d0e21e
LW
1876=item goto &NAME
1877
7660c0ab 1878The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
a0d0e21e 1879execution there. It may not be used to go into any construct that
7660c0ab 1880requires initialization, such as a subroutine or a C<foreach> loop. It
0a753a76 1881also can't be used to go into a construct that is optimized away,
7660c0ab 1882or to get out of a block or subroutine given to C<sort()>.
0a753a76 1883It can be used to go almost anywhere else within the dynamic scope,
a0d0e21e 1884including out of subroutines, but it's usually better to use some other
ab4f32c2 1885construct such as C<last> or C<die()>. The author of Perl has never felt the
7660c0ab 1886need to use this form of C<goto> (in Perl, that is--C is another matter).
a0d0e21e 1887
7660c0ab
A
1888The C<goto-EXPR> form expects a label name, whose scope will be resolved
1889dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
748a9306
LW
1890necessarily recommended if you're optimizing for maintainability:
1891
1892 goto ("FOO", "BAR", "GLARCH")[$i];
1893
7660c0ab 1894The C<goto-&NAME> form is highly magical, and substitutes a call to the
a0d0e21e 1895named subroutine for the currently running subroutine. This is used by
7660c0ab 1896C<AUTOLOAD> subroutines that wish to load another subroutine and then
a0d0e21e 1897pretend that the other subroutine had been called in the first place
7660c0ab
A
1898(except that any modifications to C<@_> in the current subroutine are
1899propagated to the other subroutine.) After the C<goto>, not even C<caller()>
a0d0e21e
LW
1900will be able to tell that this routine was called first.
1901
1902=item grep BLOCK LIST
1903
1904=item grep EXPR,LIST
1905
2b5ab1e7
TC
1906This is similar in spirit to, but not the same as, grep(1) and its
1907relatives. In particular, it is not limited to using regular expressions.
2f9daede 1908
a0d0e21e 1909Evaluates the BLOCK or EXPR for each element of LIST (locally setting
7660c0ab 1910C<$_> to each element) and returns the list value consisting of those
2b5ab1e7 1911elements for which the expression evaluated to TRUE. In scalar
a0d0e21e
LW
1912context, returns the number of times the expression was TRUE.
1913
1914 @foo = grep(!/^#/, @bar); # weed out comments
1915
1916or equivalently,
1917
1918 @foo = grep {!/^#/} @bar; # weed out comments
1919
2b5ab1e7
TC
1920Note that, because C<$_> is a reference into the list value, it can
1921be used to modify the elements of the array. While this is useful and
1922supported, it can cause bizarre results if the LIST is not a named array.
1923Similarly, grep returns aliases into the original list, much as a for
1924loop's index variable aliases the list elements. That is, modifying an
1925element of a list returned by grep (for example, in a C<foreach>, C<map()>
1926or another C<grep()>) actually modifies the element in the original list.
1927This is usually something to be avoided when writing clear code.
a0d0e21e 1928
fb73857a 1929See also L</map> for an array composed of the results of the BLOCK or EXPR.
38325410 1930
a0d0e21e
LW
1931=item hex EXPR
1932
54310121 1933=item hex
bbce6d69 1934
2b5ab1e7
TC
1935Interprets EXPR as a hex string and returns the corresponding value.
1936(To convert strings that might start with either 0, 0x, or 0b, see
1937L</oct>.) If EXPR is omitted, uses C<$_>.
2f9daede
TPG
1938
1939 print hex '0xAf'; # prints '175'
1940 print hex 'aF'; # same
a0d0e21e
LW
1941
1942=item import
1943
7660c0ab 1944There is no builtin C<import()> function. It is just an ordinary
4633a7c4 1945method (subroutine) defined (or inherited) by modules that wish to export
7660c0ab 1946names to another module. The C<use()> function calls the C<import()> method
54310121 1947for the package used. See also L</use()>, L<perlmod>, and L<Exporter>.
a0d0e21e
LW
1948
1949=item index STR,SUBSTR,POSITION
1950
1951=item index STR,SUBSTR
1952
2b5ab1e7
TC
1953The index function searches for one string within another, but without
1954the wildcard-like behavior of a full regular-expression pattern match.
1955It returns the position of the first occurrence of SUBSTR in STR at
1956or after POSITION. If POSITION is omitted, starts searching from the
1957beginning of the string. The return value is based at C<0> (or whatever
1958you've set the C<$[> variable to--but don't do that). If the substring
1959is not found, returns one less than the base, ordinarily C<-1>.
a0d0e21e
LW
1960
1961=item int EXPR
1962
54310121 1963=item int
bbce6d69 1964
7660c0ab 1965Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2b5ab1e7
TC
1966You should not use this function for rounding: one because it truncates
1967towards C<0>, and two because machine representations of floating point
1968numbers can sometimes produce counterintuitive results. For example,
1969C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
1970because it's really more like -268.99999999999994315658 instead. Usually,
1971the C<sprintf()>, C<printf()>, or the C<POSIX::floor> and C<POSIX::ceil>
1972functions will serve you better than will int().
a0d0e21e
LW
1973
1974=item ioctl FILEHANDLE,FUNCTION,SCALAR
1975
2b5ab1e7 1976Implements the ioctl(2) function. You'll probably first have to say
a0d0e21e 1977
4633a7c4 1978 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
a0d0e21e 1979
2b5ab1e7 1980to get the correct function definitions. If F<ioctl.ph> doesn't
a0d0e21e 1981exist or doesn't have the correct definitions you'll have to roll your
4633a7c4 1982own, based on your C header files such as F<E<lt>sys/ioctl.hE<gt>>.
5a964f20 1983(There is a Perl script called B<h2ph> that comes with the Perl kit that
54310121 1984may help you in this, but it's nontrivial.) SCALAR will be read and/or
4633a7c4 1985written depending on the FUNCTION--a pointer to the string value of SCALAR
ab4f32c2 1986will be passed as the third argument of the actual C<ioctl()> call. (If SCALAR
4633a7c4
LW
1987has no string value but does have a numeric value, that value will be
1988passed rather than a pointer to the string value. To guarantee this to be
7660c0ab 1989TRUE, add a C<0> to the scalar before using it.) The C<pack()> and C<unpack()>
4633a7c4 1990functions are useful for manipulating the values of structures used by
7660c0ab 1991C<ioctl()>. The following example sets the erase character to DEL.
a0d0e21e
LW
1992
1993 require 'ioctl.ph';
4633a7c4
LW
1994 $getp = &TIOCGETP;
1995 die "NO TIOCGETP" if $@ || !$getp;
a0d0e21e 1996 $sgttyb_t = "ccccs"; # 4 chars and a short
4633a7c4 1997 if (ioctl(STDIN,$getp,$sgttyb)) {
a0d0e21e
LW
1998 @ary = unpack($sgttyb_t,$sgttyb);
1999 $ary[2] = 127;
2000 $sgttyb = pack($sgttyb_t,@ary);
4633a7c4 2001 ioctl(STDIN,&TIOCSETP,$sgttyb)
a0d0e21e
LW
2002 || die "Can't ioctl: $!";
2003 }
2004
ab4f32c2 2005The return value of C<ioctl()> (and C<fcntl()>) is as follows:
a0d0e21e
LW
2006
2007 if OS returns: then Perl returns:
2008 -1 undefined value
2009 0 string "0 but true"
2010 anything else that number
2011
2012Thus Perl returns TRUE on success and FALSE on failure, yet you can
2013still easily determine the actual value returned by the operating
2014system:
2015
2b5ab1e7 2016 $retval = ioctl(...) || -1;
a0d0e21e
LW
2017 printf "System returned %d\n", $retval;
2018
c2611fb3 2019The special string "C<0> but true" is exempt from B<-w> complaints
5a964f20
TC
2020about improper numeric conversions.
2021
a0d0e21e
LW
2022=item join EXPR,LIST
2023
2b5ab1e7
TC
2024Joins the separate strings of LIST into a single string with fields
2025separated by the value of EXPR, and returns that new string. Example:
a0d0e21e 2026
2b5ab1e7 2027 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
a0d0e21e 2028
7660c0ab 2029See L</split>.
a0d0e21e 2030
aa689395
PP
2031=item keys HASH
2032
1d2dff63
GS
2033Returns a list consisting of all the keys of the named hash. (In a
2034scalar context, returns the number of keys.) The keys are returned in
ab192400
GS
2035an apparently random order. The actual random order is subject to
2036change in future versions of perl, but it is guaranteed to be the same
2037order as either the C<values()> or C<each()> function produces (given
2038that the hash has not been modified). As a side effect, it resets
2039HASH's iterator.
a0d0e21e 2040
aa689395 2041Here is yet another way to print your environment:
a0d0e21e
LW
2042
2043 @keys = keys %ENV;
2044 @values = values %ENV;
2045 while ($#keys >= 0) {
2046 print pop(@keys), '=', pop(@values), "\n";
2047 }
2048
2049or how about sorted by key:
2050
2051 foreach $key (sort(keys %ENV)) {
2052 print $key, '=', $ENV{$key}, "\n";
2053 }
2054
aca803df 2055To sort a hash by value, you'll need to use a C<sort()> function.
aa689395 2056Here's a descending numeric sort of a hash by its values:
4633a7c4 2057
5a964f20 2058 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
4633a7c4
LW
2059 printf "%4d %s\n", $hash{$key}, $key;
2060 }
2061
ab4f32c2 2062As an lvalue C<keys()> allows you to increase the number of hash buckets
aa689395
PP
2063allocated for the given hash. This can gain you a measure of efficiency if
2064you know the hash is going to get big. (This is similar to pre-extending
2065an array by assigning a larger number to $#array.) If you say
55497cff
PP
2066
2067 keys %hash = 200;
2068
ab192400
GS
2069then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2070in fact, since it rounds up to the next power of two. These
55497cff
PP
2071buckets will be retained even if you do C<%hash = ()>, use C<undef
2072%hash> if you want to free the storage while C<%hash> is still in scope.
2073You can't shrink the number of buckets allocated for the hash using
ab4f32c2 2074C<keys()> in this way (but you needn't worry about doing this by accident,
55497cff
PP
2075as trying has no effect).
2076
ab192400
GS
2077See also C<each()>, C<values()> and C<sort()>.
2078
a0d0e21e
LW
2079=item kill LIST
2080
54310121
PP
2081Sends a signal to a list of processes. The first element of
2082the list must be the signal to send. Returns the number of
4633a7c4 2083processes successfully signaled.
a0d0e21e
LW
2084
2085 $cnt = kill 1, $child1, $child2;
2086 kill 9, @goners;
2087
4633a7c4
LW
2088Unlike in the shell, in Perl if the I<SIGNAL> is negative, it kills
2089process groups instead of processes. (On System V, a negative I<PROCESS>
2090number will also kill process groups, but that's not portable.) That
2091means you usually want to use positive not negative signals. You may also
da0045b7 2092use a signal name in quotes. See L<perlipc/"Signals"> for details.
a0d0e21e
LW
2093
2094=item last LABEL
2095
2096=item last
2097
2098The C<last> command is like the C<break> statement in C (as used in
2099loops); it immediately exits the loop in question. If the LABEL is
2100omitted, the command refers to the innermost enclosing loop. The
2101C<continue> block, if any, is not executed:
2102
4633a7c4
LW
2103 LINE: while (<STDIN>) {
2104 last LINE if /^$/; # exit when done with header
5a964f20 2105 #...
a0d0e21e
LW
2106 }
2107
4968c1e4 2108C<last> cannot be used to exit a block which returns a value such as
2b5ab1e7
TC
2109C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2110a grep() or map() operation.
4968c1e4 2111
98293880
JH
2112See also L</continue> for an illustration of how C<last>, C<next>, and
2113C<redo> work.
1d2dff63 2114
a0d0e21e
LW
2115=item lc EXPR
2116
54310121 2117=item lc
bbce6d69 2118
a0d0e21e 2119Returns an lowercased version of EXPR. This is the internal function
7660c0ab 2120implementing the C<\L> escape in double-quoted strings.
a0ed51b3 2121Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
a0d0e21e 2122
7660c0ab 2123If EXPR is omitted, uses C<$_>.
bbce6d69 2124
a0d0e21e
LW
2125=item lcfirst EXPR
2126
54310121 2127=item lcfirst
bbce6d69 2128
a0d0e21e 2129Returns the value of EXPR with the first character lowercased. This is
7660c0ab 2130the internal function implementing the C<\l> escape in double-quoted strings.
a0ed51b3 2131Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
a0d0e21e 2132
7660c0ab 2133If EXPR is omitted, uses C<$_>.
bbce6d69 2134
a0d0e21e
LW
2135=item length EXPR
2136
54310121 2137=item length
bbce6d69 2138
a0ed51b3 2139Returns the length in characters of the value of EXPR. If EXPR is
2b5ab1e7
TC
2140omitted, returns length of C<$_>. Note that this cannot be used on
2141an entire array or hash to find out how many elements these have.
2142For that, use C<scalar @array> and C<scalar keys %hash> respectively.
a0d0e21e
LW
2143
2144=item link OLDFILE,NEWFILE
2145
5a964f20 2146Creates a new filename linked to the old filename. Returns TRUE for
2b5ab1e7 2147success, FALSE otherwise.
a0d0e21e
LW
2148
2149=item listen SOCKET,QUEUESIZE
2150
2151Does the same thing that the listen system call does. Returns TRUE if
2b5ab1e7 2152it succeeded, FALSE otherwise. See the example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
2153
2154=item local EXPR
2155
2b5ab1e7
TC
2156You really probably want to be using C<my()> instead, because C<local()> isn't
2157what most people think of as "local". See L<perlsub/"Private Variables
2158via my()"> for details.
2159
5a964f20
TC
2160A local modifies the listed variables to be local to the enclosing
2161block, file, or eval. If more than one value is listed, the list must
2162be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2163for details, including issues with tied arrays and hashes.
a0d0e21e 2164
a0d0e21e
LW
2165=item localtime EXPR
2166
2167Converts a time as returned by the time function to a 9-element array
5f05dabc 2168with the time analyzed for the local time zone. Typically used as
a0d0e21e
LW
2169follows:
2170
54310121 2171 # 0 1 2 3 4 5 6 7 8
a0d0e21e
LW
2172 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2173 localtime(time);
2174
2175All array elements are numeric, and come straight out of a struct tm.
2b5ab1e7
TC
2176In particular this means that C<$mon> has the range C<0..11> and C<$wday>
2177has the range C<0..6> with sunday as day C<0>. Also, C<$year> is the
2178number of years since 1900, that is, C<$year> is C<123> in year 2023,
2179and I<not> simply the last two digits of the year. If you assume it is,
2180then you create non-Y2K-compliant programs--and you wouldn't want to do
2181that, would you?
54310121
PP
2182
2183If EXPR is omitted, uses the current time (C<localtime(time)>).
a0d0e21e 2184
f86cebdf 2185In scalar context, returns the ctime(3) value:
a0d0e21e 2186
5f05dabc 2187 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
a0d0e21e 2188
a3cb178b 2189This scalar value is B<not> locale dependent, see L<perllocale>, but
7660c0ab 2190instead a Perl builtin. Also see the C<Time::Local> module, and the
f86cebdf 2191strftime(3) and mktime(3) function available via the POSIX module. To
a3cb178b
GS
2192get somewhat similar but locale dependent date strings, set up your
2193locale environment variables appropriately (please see L<perllocale>)
5a964f20 2194and try for example:
a3cb178b 2195
5a964f20 2196 use POSIX qw(strftime);
2b5ab1e7 2197 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
a3cb178b
GS
2198
2199Note that the C<%a> and C<%b>, the short forms of the day of the week
2200and the month of the year, may not necessarily be three characters wide.
a0d0e21e
LW
2201
2202=item log EXPR
2203
54310121 2204=item log
bbce6d69 2205
2b5ab1e7
TC
2206Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2207returns log of C<$_>. To get the log of another base, use basic algebra:
2208The base-N log of a number is is equal to the natural log of that number
2209divided by the natural log of N. For example:
2210
2211 sub log10 {
2212 my $n = shift;
2213 return log($n)/log(10);
2214 }
2215
2216See also L</exp> for the inverse operation.
a0d0e21e
LW
2217
2218=item lstat FILEHANDLE
2219
2220=item lstat EXPR
2221
54310121 2222=item lstat
bbce6d69 2223
7660c0ab 2224Does the same thing as the C<stat()> function (including setting the
5a964f20
TC
2225special C<_> filehandle) but stats a symbolic link instead of the file
2226the symbolic link points to. If symbolic links are unimplemented on
7660c0ab 2227your system, a normal C<stat()> is done.
a0d0e21e 2228
7660c0ab 2229If EXPR is omitted, stats C<$_>.
bbce6d69 2230
a0d0e21e
LW
2231=item m//
2232
2233The match operator. See L<perlop>.
2234
2235=item map BLOCK LIST
2236
2237=item map EXPR,LIST
2238
7660c0ab 2239Evaluates the BLOCK or EXPR for each element of LIST (locally setting C<$_> to each
a0d0e21e
LW
2240element) and returns the list value composed of the results of each such
2241evaluation. Evaluates BLOCK or EXPR in a list context, so each element of LIST
2242may produce zero, one, or more elements in the returned value.
2243
dd99ebda
GS
2244In scalar context, returns the total number of elements so generated.
2245
a0d0e21e
LW
2246 @chars = map(chr, @nums);
2247
2248translates a list of numbers to the corresponding characters. And
2249
4633a7c4 2250 %hash = map { getkey($_) => $_ } @array;
a0d0e21e
LW
2251
2252is just a funny way to write
2253
2254 %hash = ();
2255 foreach $_ (@array) {
4633a7c4 2256 $hash{getkey($_)} = $_;
a0d0e21e
LW
2257 }
2258
2b5ab1e7
TC
2259Note that, because C<$_> is a reference into the list value, it can
2260be used to modify the elements of the array. While this is useful and
2261supported, it can cause bizarre results if the LIST is not a named array.
2262Using a regular C<foreach> loop for this purpose would be clearer in
2263most cases. See also L</grep> for an array composed of those items of
2264the original list for which the BLOCK or EXPR evaluates to true.
fb73857a 2265
a0d0e21e
LW
2266=item mkdir FILENAME,MODE
2267
0591cd52
NT
2268Creates the directory specified by FILENAME, with permissions
2269specified by MODE (as modified by C<umask>). If it succeeds it
2270returns TRUE, otherwise it returns FALSE and sets C<$!> (errno).
2271
2272In general, it is better to create directories with permissive MODEs,
2273and let the user modify that with their C<umask>, than it is to supply
2274a restrictive MODE and give the user no way to be more permissive.
2275The exceptions to this rule are when the file or directory should be
2276kept private (mail files, for instance). The perlfunc(1) entry on
2277C<umask> discusses the choice of MODE in more detail.
a0d0e21e
LW
2278
2279=item msgctl ID,CMD,ARG
2280
f86cebdf 2281Calls the System V IPC function msgctl(2). You'll probably have to say
0ade1984
JH
2282
2283 use IPC::SysV;
2284
7660c0ab
A
2285first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2286then ARG must be a variable which will hold the returned C<msqid_ds>
ab4f32c2 2287structure. Returns like C<ioctl()>: the undefined value for error, "C<0> but
0ade1984 2288true" for zero, or the actual return value otherwise. See also
7660c0ab 2289C<IPC::SysV> and C<IPC::Semaphore::Msg> documentation.
a0d0e21e
LW
2290
2291=item msgget KEY,FLAGS
2292
f86cebdf 2293Calls the System V IPC function msgget(2). Returns the message queue
7660c0ab
A
2294id, or the undefined value if there is an error. See also C<IPC::SysV>
2295and C<IPC::SysV::Msg> documentation.
a0d0e21e
LW
2296
2297=item msgsnd ID,MSG,FLAGS
2298
2299Calls the System V IPC function msgsnd to send the message MSG to the
2300message queue ID. MSG must begin with the long integer message type,
c07a80fd 2301which may be created with C<pack("l", $type)>. Returns TRUE if
7660c0ab
A
2302successful, or FALSE if there is an error. See also C<IPC::SysV>
2303and C<IPC::SysV::Msg> documentation.
a0d0e21e
LW
2304
2305=item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2306
2307Calls the System V IPC function msgrcv to receive a message from
2308message queue ID into variable VAR with a maximum message size of
0ade1984
JH
2309SIZE. Note that if a message is received, the message type will be
2310the first thing in VAR, and the maximum length of VAR is SIZE plus the
2311size of the message type. Returns TRUE if successful, or FALSE if
7660c0ab 2312there is an error. See also C<IPC::SysV> and C<IPC::SysV::Msg> documentation.
a0d0e21e
LW
2313
2314=item my EXPR
2315
ab4f32c2
A
2316A C<my()> declares the listed variables to be local (lexically) to the
2317enclosing block, file, or C<eval()>. If
5f05dabc 2318more than one value is listed, the list must be placed in parentheses. See
cb1a09d0 2319L<perlsub/"Private Variables via my()"> for details.
4633a7c4 2320
a0d0e21e
LW
2321=item next LABEL
2322
2323=item next
2324
2325The C<next> command is like the C<continue> statement in C; it starts
2326the next iteration of the loop:
2327
4633a7c4
LW
2328 LINE: while (<STDIN>) {
2329 next LINE if /^#/; # discard comments
5a964f20 2330 #...
a0d0e21e
LW
2331 }
2332
2333Note that if there were a C<continue> block on the above, it would get
2334executed even on discarded lines. If the LABEL is omitted, the command
2335refers to the innermost enclosing loop.
2336
4968c1e4 2337C<next> cannot be used to exit a block which returns a value such as
2b5ab1e7
TC
2338C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2339a grep() or map() operation.
4968c1e4 2340
98293880
JH
2341See also L</continue> for an illustration of how C<last>, C<next>, and
2342C<redo> work.
1d2dff63 2343
a0d0e21e
LW
2344=item no Module LIST
2345
7660c0ab 2346See the L</use> function, which C<no> is the opposite of.
a0d0e21e
LW
2347
2348=item oct EXPR
2349
54310121 2350=item oct
bbce6d69 2351
4633a7c4 2352Interprets EXPR as an octal string and returns the corresponding
4f19785b
WSI
2353value. (If EXPR happens to start off with C<0x>, interprets it as a
2354hex string. If EXPR starts off with C<0b>, it is interpreted as a
2355binary string.) The following will handle decimal, binary, octal, and
4633a7c4 2356hex in the standard Perl or C notation:
a0d0e21e
LW
2357
2358 $val = oct($val) if $val =~ /^0/;
2359
7660c0ab
A
2360If EXPR is omitted, uses C<$_>. This function is commonly used when
2361a string such as C<644> needs to be converted into a file mode, for
2f9daede
TPG
2362example. (Although perl will automatically convert strings into
2363numbers as needed, this automatic conversion assumes base 10.)
a0d0e21e
LW
2364
2365=item open FILEHANDLE,EXPR
2366
2367=item open FILEHANDLE
2368
2369Opens the file whose filename is given by EXPR, and associates it with
5f05dabc
PP
2370FILEHANDLE. If FILEHANDLE is an expression, its value is used as the
2371name of the real filehandle wanted. If EXPR is omitted, the scalar
2372variable of the same name as the FILEHANDLE contains the filename.
ab4f32c2
A
2373(Note that lexical variables--those declared with C<my()>--will not work
2374for this purpose; so if you're using C<my()>, specify EXPR in your call
2b5ab1e7
TC
2375to open.) See L<perlopentut> for a kinder, gentler explanation of opening
2376files.
5f05dabc 2377
7660c0ab
A
2378If the filename begins with C<'E<lt>'> or nothing, the file is opened for input.
2379If the filename begins with C<'E<gt>'>, the file is truncated and opened for
2380output, being created if necessary. If the filename begins with C<'E<gt>E<gt>'>,
fbb426e4 2381the file is opened for appending, again being created if necessary.
7660c0ab
A
2382You can put a C<'+'> in front of the C<'E<gt>'> or C<'E<lt>'> to indicate that
2383you want both read and write access to the file; thus C<'+E<lt>'> is almost
2384always preferred for read/write updates--the C<'+E<gt>'> mode would clobber the
5a964f20
TC
2385file first. You can't usually use either read-write mode for updating
2386textfiles, since they have variable length records. See the B<-i>
0591cd52
NT
2387switch in L<perlrun> for a better approach. The file is created with
2388permissions of C<0666> modified by the process' C<umask> value.
5a964f20
TC
2389
2390The prefix and the filename may be separated with spaces.
f86cebdf 2391These various prefixes correspond to the fopen(3) modes of C<'r'>, C<'r+'>, C<'w'>,
7660c0ab 2392C<'w+'>, C<'a'>, and C<'a+'>.
5f05dabc 2393
7660c0ab 2394If the filename begins with C<'|'>, the filename is interpreted as a
5a964f20 2395command to which output is to be piped, and if the filename ends with a
f244e06d
GS
2396C<'|'>, the filename is interpreted as a command which pipes output to
2397us. See L<perlipc/"Using open() for IPC">
7660c0ab 2398for more examples of this. (You are not allowed to C<open()> to a command
5a964f20
TC
2399that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2400and L<perlipc/"Bidirectional Communication"> for alternatives.)
cb1a09d0 2401
7660c0ab 2402Opening C<'-'> opens STDIN and opening C<'E<gt>-'> opens STDOUT. Open returns
ab4f32c2 2403nonzero upon success, the undefined value otherwise. If the C<open()>
4633a7c4 2404involved a pipe, the return value happens to be the pid of the
54310121 2405subprocess.
cb1a09d0
AD
2406
2407If you're unfortunate enough to be running Perl on a system that
2408distinguishes between text files and binary files (modern operating
2409systems don't care), then you should check out L</binmode> for tips for
ab4f32c2 2410dealing with this. The key distinction between systems that need C<binmode()>
5a964f20
TC
2411and those that don't is their text file formats. Systems like Unix, MacOS, and
2412Plan9, which delimit lines with a single character, and which encode that
ab4f32c2 2413character in C as C<"\n">, do not need C<binmode()>. The rest need it.
cb1a09d0 2414
fb73857a 2415When opening a file, it's usually a bad idea to continue normal execution
ab4f32c2
A
2416if the request failed, so C<open()> is frequently used in connection with
2417C<die()>. Even if C<die()> won't do what you want (say, in a CGI script,
fb73857a 2418where you want to make a nicely formatted error message (but there are
5a964f20 2419modules that can help with that problem)) you should always check
fb73857a
PP
2420the return value from opening a file. The infrequent exception is when
2421working with an unopened filehandle is actually what you want to do.
2422
cb1a09d0 2423Examples:
a0d0e21e
LW
2424
2425 $ARTICLE = 100;
2426 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2427 while (<ARTICLE>) {...
2428
2429 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
fb73857a 2430 # if the open fails, output is discarded
a0d0e21e 2431
fb73857a
PP
2432 open(DBASE, '+<dbase.mine') # open for update
2433 or die "Can't open 'dbase.mine' for update: $!";
cb1a09d0 2434
fb73857a
PP
2435 open(ARTICLE, "caesar <$article |") # decrypt article
2436 or die "Can't start caesar: $!";
a0d0e21e 2437
fb73857a
PP
2438 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
2439 or die "Can't start sort: $!";
a0d0e21e
LW
2440
2441 # process argument list of files along with any includes
2442
2443 foreach $file (@ARGV) {
2444 process($file, 'fh00');
2445 }
2446
2447 sub process {
5a964f20 2448 my($filename, $input) = @_;
a0d0e21e
LW
2449 $input++; # this is a string increment
2450 unless (open($input, $filename)) {
2451 print STDERR "Can't open $filename: $!\n";
2452 return;
2453 }
2454
5a964f20 2455 local $_;
a0d0e21e
LW
2456 while (<$input>) { # note use of indirection
2457 if (/^#include "(.*)"/) {
2458 process($1, $input);
2459 next;
2460 }
5a964f20 2461 #... # whatever
a0d0e21e
LW
2462 }
2463 }
2464
2465You may also, in the Bourne shell tradition, specify an EXPR beginning
7660c0ab 2466with C<'E<gt>&'>, in which case the rest of the string is interpreted as the
5a964f20 2467name of a filehandle (or file descriptor, if numeric) to be
7660c0ab
A
2468duped and opened. You may use C<&> after C<E<gt>>, C<E<gt>E<gt>>, C<E<lt>>, C<+E<gt>>,
2469C<+E<gt>E<gt>>, and C<+E<lt>>. The
a0d0e21e 2470mode you specify should match the mode of the original filehandle.
184e9718 2471(Duping a filehandle does not take into account any existing contents of
cb1a09d0 2472stdio buffers.)
a0d0e21e
LW
2473Here is a script that saves, redirects, and restores STDOUT and
2474STDERR:
2475
2476 #!/usr/bin/perl
5a964f20
TC
2477 open(OLDOUT, ">&STDOUT");
2478 open(OLDERR, ">&STDERR");
a0d0e21e
LW
2479
2480 open(STDOUT, ">foo.out") || die "Can't redirect stdout";
2481 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
2482
2483 select(STDERR); $| = 1; # make unbuffered
2484 select(STDOUT); $| = 1; # make unbuffered
2485
2486 print STDOUT "stdout 1\n"; # this works for
2487 print STDERR "stderr 1\n"; # subprocesses too
2488
2489 close(STDOUT);
2490 close(STDERR);
2491
5a964f20
TC
2492 open(STDOUT, ">&OLDOUT");
2493 open(STDERR, ">&OLDERR");
a0d0e21e
LW
2494
2495 print STDOUT "stdout 2\n";
2496 print STDERR "stderr 2\n";
2497
7660c0ab
A
2498If you specify C<'E<lt>&=N'>, where C<N> is a number, then Perl will do an
2499equivalent of C's C<fdopen()> of that file descriptor; this is more
4633a7c4 2500parsimonious of file descriptors. For example:
a0d0e21e
LW
2501
2502 open(FILEHANDLE, "<&=$fd")
2503
7660c0ab 2504If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>, then
a0d0e21e 2505there is an implicit fork done, and the return value of open is the pid
7660c0ab 2506of the child within the parent process, and C<0> within the child
184e9718 2507process. (Use C<defined($pid)> to determine whether the open was successful.)
a0d0e21e
LW
2508The filehandle behaves normally for the parent, but i/o to that
2509filehandle is piped from/to the STDOUT/STDIN of the child process.
2510In the child process the filehandle isn't opened--i/o happens from/to
2511the new STDOUT or STDIN. Typically this is used like the normal
2512piped open when you want to exercise more control over just how the
2513pipe command gets executed, such as when you are running setuid, and
54310121 2514don't want to have to scan shell commands for metacharacters.
4633a7c4 2515The following pairs are more or less equivalent:
a0d0e21e
LW
2516
2517 open(FOO, "|tr '[a-z]' '[A-Z]'");
2518 open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
2519
2520 open(FOO, "cat -n '$file'|");
2521 open(FOO, "-|") || exec 'cat', '-n', $file;
2522
4633a7c4
LW
2523See L<perlipc/"Safe Pipe Opens"> for more examples of this.
2524
45bc9206
GS
2525NOTE: On any operation that may do a fork, all files opened for output
2526are flushed before the fork is attempted. On systems that support a
2527close-on-exec flag on files, the flag will be set for the newly opened
2528file descriptor as determined by the value of $^F. See L<perlvar/$^F>.
a0d0e21e 2529
0dccf244
CS
2530Closing any piped filehandle causes the parent process to wait for the
2531child to finish, and returns the status value in C<$?>.
2532
5a964f20 2533The filename passed to open will have leading and trailing
f86cebdf 2534whitespace deleted, and the normal redirection characters
5a964f20
TC
2535honored. This property, known as "magic open",
2536can often be used to good effect. A user could specify a filename of
7660c0ab 2537F<"rsh cat file |">, or you could change certain filenames as needed:
5a964f20
TC
2538
2539 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
2540 open(FH, $filename) or die "Can't open $filename: $!";
2541
2542However, to open a file with arbitrary weird characters in it, it's
2543necessary to protect any leading and trailing whitespace:
2544
2545 $file =~ s#^(\s)#./$1#;
2546 open(FOO, "< $file\0");
2547
7660c0ab
A
2548If you want a "real" C C<open()> (see L<open(2)> on your system), then you
2549should use the C<sysopen()> function, which involves no such magic. This is
5a964f20
TC
2550another way to protect your filenames from interpretation. For example:
2551
2552 use IO::Handle;
2553 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
2554 or die "sysopen $path: $!";
2555 $oldfh = select(HANDLE); $| = 1; select($oldfh);
2556 print HANDLE "stuff $$\n");
2557 seek(HANDLE, 0, 0);
2558 print "File contains: ", <HANDLE>;
2559
7660c0ab
A
2560Using the constructor from the C<IO::Handle> package (or one of its
2561subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
5a964f20
TC
2562filehandles that have the scope of whatever variables hold references to
2563them, and automatically close whenever and however you leave that scope:
c07a80fd 2564
5f05dabc 2565 use IO::File;
5a964f20 2566 #...
c07a80fd
PP
2567 sub read_myfile_munged {
2568 my $ALL = shift;
5f05dabc 2569 my $handle = new IO::File;
c07a80fd
PP
2570 open($handle, "myfile") or die "myfile: $!";
2571 $first = <$handle>
2572 or return (); # Automatically closed here.
2573 mung $first or die "mung failed"; # Or here.
2574 return $first, <$handle> if $ALL; # Or here.
2575 $first; # Or here.
2576 }
2577
b687b08b 2578See L</seek> for some details about mixing reading and writing.
a0d0e21e
LW
2579
2580=item opendir DIRHANDLE,EXPR
2581
7660c0ab
A
2582Opens a directory named EXPR for processing by C<readdir()>, C<telldir()>,
2583C<seekdir()>, C<rewinddir()>, and C<closedir()>. Returns TRUE if successful.
a0d0e21e
LW
2584DIRHANDLEs have their own namespace separate from FILEHANDLEs.
2585
2586=item ord EXPR
2587
54310121 2588=item ord
bbce6d69 2589
a0ed51b3 2590Returns the numeric (ASCII or Unicode) value of the first character of EXPR. If
7660c0ab 2591EXPR is omitted, uses C<$_>. For the reverse, see L</chr>.
2b5ab1e7 2592See L<utf8> for more about Unicode.
a0d0e21e
LW
2593
2594=item pack TEMPLATE,LIST
2595
2596Takes an array or list of values and packs it into a binary structure,
2597returning the string containing the structure. The TEMPLATE is a
2598sequence of characters that give the order and type of values, as
2599follows:
2600
5a929a98 2601 a A string with arbitrary binary data, will be null padded.
a0d0e21e 2602 A An ascii string, will be space padded.
5a929a98
VU
2603 Z A null terminated (asciz) string, will be null padded.
2604
a0d0e21e
LW
2605 b A bit string (ascending bit order, like vec()).
2606 B A bit string (descending bit order).
2607 h A hex string (low nybble first).
2608 H A hex string (high nybble first).
2609
2610 c A signed char value.
a0ed51b3 2611 C An unsigned char value. Only does bytes. See U for Unicode.
96e4d5b1 2612
a0d0e21e
LW
2613 s A signed short value.
2614 S An unsigned short value.
96e4d5b1
PP
2615 (This 'short' is _exactly_ 16 bits, which may differ from
2616 what a local C compiler calls 'short'.)
2617
a0d0e21e
LW
2618 i A signed integer value.
2619 I An unsigned integer value.
4d0c1c44 2620 (This 'integer' is _at least_ 32 bits wide. Its exact
f86cebdf
GS
2621 size depends on what a local C compiler calls 'int',
2622 and may even be larger than the 'long' described in
2623 the next item.)
96e4d5b1 2624
a0d0e21e
LW
2625 l A signed long value.
2626 L An unsigned long value.
96e4d5b1
PP
2627 (This 'long' is _exactly_ 32 bits, which may differ from
2628 what a local C compiler calls 'long'.)
a0d0e21e 2629
96e4d5b1
PP
2630 n A short in "network" (big-endian) order.
2631 N A long in "network" (big-endian) order.
a0d0e21e
LW
2632 v A short in "VAX" (little-endian) order.
2633 V A long in "VAX" (little-endian) order.
96e4d5b1
PP
2634 (These 'shorts' and 'longs' are _exactly_ 16 bits and
2635 _exactly_ 32 bits, respectively.)
a0d0e21e 2636
dae0da7a
JH
2637 q A signed quad (64-bit) value.
2638 Q An unsigned quad value.
2639 (Available only if your system supports 64-bit integer values
2640 _and_ if Perl has been compiled to support those.
2641 Causes a fatal error otherwise.)
2642
a0d0e21e
LW
2643 f A single-precision float in the native format.
2644 d A double-precision float in the native format.
2645
2646 p A pointer to a null-terminated string.
2647 P A pointer to a structure (fixed-length string).
2648
2649 u A uuencoded string.
a0ed51b3
LW
2650 U A Unicode character number. Encodes to UTF-8 internally.
2651 Works even if C<use utf8> is not in effect.
a0d0e21e 2652
96e4d5b1 2653 w A BER compressed integer. Its bytes represent an unsigned
f86cebdf
GS
2654 integer in base 128, most significant digit first, with as
2655 few digits as possible. Bit eight (the high bit) is set
2656 on each byte except the last.
def98dd4 2657
a0d0e21e
LW
2658 x A null byte.
2659 X Back up a byte.
2660 @ Null fill to absolute position.
2661
5a929a98
VU
2662The following rules apply:
2663
2664=over 8
2665
2666=item *
2667
5a964f20 2668Each letter may optionally be followed by a number giving a repeat
5a929a98
VU
2669count. With all types except C<"a">, C<"A">, C<"Z">, C<"b">, C<"B">, C<"h">,
2670C<"H">, and C<"P"> the pack function will gobble up that many values from
2671the LIST. A C<*> for the repeat count means to use however many items are
2672left.
2673
2674=item *
2675
2b5ab1e7 2676The C<"a">, C<"A">, and C<"Z"> types gobble just one value, but pack it as a
5a929a98
VU
2677string of length count, padding with nulls or spaces as necessary. When
2678unpacking, C<"A"> strips trailing spaces and nulls, C<"Z"> strips everything
2679after the first null, and C<"a"> returns data verbatim.
2680
2681=item *
2682
2683Likewise, the C<"b"> and C<"B"> fields pack a string that many bits long.
2684
2685=item *
2686
2687The C<"h"> and C<"H"> fields pack a string that many nybbles long.
2688
2689=item *
2690
2691The C<"p"> type packs a pointer to a null-terminated string. You are
2692responsible for ensuring the string is not a temporary value (which can
2693potentially get deallocated before you get around to using the packed result).
2694The C<"P"> type packs a pointer to a structure of the size indicated by the
2695length. A NULL pointer is created if the corresponding value for C<"p"> or
2696C<"P"> is C<undef>.
2697
2698=item *
2699
ef54e1a4 2700The integer types C<"s">, C<"S">, C<"l">, and C<"L"> may be
4d0c1c44 2701immediately followed by a C<"!"> to signify native shorts or longs--as
ef54e1a4
JH
2702you can see from above for example a bare C<"l"> does mean exactly 32
2703bits, the native C<long> (as seen by the local C compiler) may be
726ea183 2704larger. This is an issue mainly in 64-bit platforms. You can see
4d0c1c44 2705whether using C<"!"> makes any difference by
726ea183 2706
4d0c1c44
GS
2707 print length(pack("s")), " ", length(pack("s!")), "\n";
2708 print length(pack("l")), " ", length(pack("l!")), "\n";
ef54e1a4 2709
4d0c1c44 2710C<"i!"> and C<"I!"> also work but only because of completeness;
ef54e1a4
JH
2711they are identical to C<"i"> and C<"I">.
2712
726ea183 2713The actual sizes (in bytes) of native shorts, ints, and longs on
d99ad34e 2714the platform where Perl was built are also available via L<Config>:
ef54e1a4
JH
2715
2716 use Config;
2717 print $Config{shortsize}, "\n";
726ea183
JH
2718 print $Config{intsize}, "\n";
2719 print $Config{longsize}, "\n";
ef54e1a4
JH
2720
2721=item *
2722
2723The integer formats C<"s">, C<"S">, C<"i">, C<"I">, C<"l">, and C<"L">
2724are inherently non-portable between processors and operating systems
2725because they obey the native byteorder and endianness. For example a
719a3cf5 27264-byte integer 0x87654321 (2271560481 decimal) be ordered natively
ef54e1a4
JH
2727(arranged in and handled by the CPU registers) into bytes as
2728
719a3cf5
JH
2729 0x12 0x34 0x56 0x78 # little-endian
2730 0x78 0x56 0x34 0x12 # big-endian
ef54e1a4 2731
719a3cf5
JH
2732Basically, the Intel, Alpha, and VAX CPUs and little-endian, while
2733everybody else, for example Motorola m68k/88k, PPC, Sparc, HP PA,
2734Power, and Cray are big-endian. MIPS can be either: Digital used it
2735in little-endian mode, SGI uses it in big-endian mode.
2736
ef54e1a4
JH
2737The names `big-endian' and `little-endian' are joking references to
2738the classic "Gulliver's Travels" (via the paper "On Holy Wars and a
2739Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and
2740the egg-eating habits of the lilliputs.
2741
2742Some systems may even have weird byte orders such as
2743
2744 0x56 0x78 0x12 0x34
2745 0x34 0x12 0x78 0x56
2746
2747You can see your system's preference with
2748
2749 print join(" ", map { sprintf "%#02x", $_ }
2750 unpack("C*",pack("L",0x12345678))), "\n";
2751
d99ad34e 2752The byteorder on the platform where Perl was built is also available
726ea183 2753via L<Config>:
ef54e1a4
JH
2754
2755 use Config;
2756 print $Config{byteorder}, "\n";
2757
d99ad34e
JH
2758Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'>
2759and C<'87654321'> are big-endian.
719a3cf5 2760
d99ad34e
JH
2761If you want portable packed integers use the formats C<"n">, C<"N">,
2762C<"v">, and C<"V">, their byte endianness and size is known.
ef54e1a4
JH
2763
2764=item *
2765
5a929a98
VU
2766Real numbers (floats and doubles) are in the native machine format only;
2767due to the multiplicity of floating formats around, and the lack of a
2768standard "network" representation, no facility for interchange has been
2769made. This means that packed floating point data written on one machine
2770may not be readable on another - even if both use IEEE floating point
2771arithmetic (as the endian-ness of the memory representation is not part
2772of the IEEE spec).
2773
2774Note that Perl uses doubles internally for all numeric calculation, and
2775converting from double into float and thence back to double again will
2776lose precision (i.e., C<unpack("f", pack("f", $foo)>) will not in general
2777equal C<$foo>).
2778
2779=back
a0d0e21e
LW
2780
2781Examples:
2782
a0ed51b3 2783 $foo = pack("CCCC",65,66,67,68);
a0d0e21e 2784 # foo eq "ABCD"
a0ed51b3 2785 $foo = pack("C4",65,66,67,68);
a0d0e21e 2786 # same thing
a0ed51b3
LW
2787 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
2788 # same thing with Unicode circled letters
a0d0e21e
LW
2789
2790 $foo = pack("ccxxcc",65,66,67,68);
2791 # foo eq "AB\0\0CD"
2792
2793 $foo = pack("s2",1,2);
2794 # "\1\0\2\0" on little-endian
2795 # "\0\1\0\2" on big-endian
2796
2797 $foo = pack("a4","abcd","x","y","z");
2798 # "abcd"
2799
2800 $foo = pack("aaaa","abcd","x","y","z");
2801 # "axyz"
2802
2803 $foo = pack("a14","abcdefg");
2804 # "abcdefg\0\0\0\0\0\0\0"
2805
2806 $foo = pack("i9pl", gmtime);
2807 # a real struct tm (on my system anyway)
2808
5a929a98
VU
2809 $utmp_template = "Z8 Z8 Z16 L";
2810 $utmp = pack($utmp_template, @utmp1);
2811 # a struct utmp (BSDish)
2812
2813 @utmp2 = unpack($utmp_template, $utmp);
2814 # "@utmp1" eq "@utmp2"
2815
a0d0e21e
LW
2816 sub bintodec {
2817 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
2818 }
2819
5a929a98 2820The same template may generally also be used in unpack().
a0d0e21e 2821
5a964f20
TC
2822=item package
2823
cb1a09d0
AD
2824=item package NAMESPACE
2825
2826Declares the compilation unit as being in the given namespace. The scope
2b5ab1e7
TC
2827of the package declaration is from the declaration itself through the end
2828of the enclosing block, file, or eval (the same as the C<my()> operator).
2829All further unqualified dynamic identifiers will be in this namespace.
2830A package statement affects only dynamic variables--including those
2831you've used C<local()> on--but I<not> lexical variables, which are created
2832with C<my()>. Typically it would be the first declaration in a file to
2833be included by the C<require> or C<use> operator. You can switch into a
2834package in more than one place; it merely influences which symbol table
2835is used by the compiler for the rest of that block. You can refer to
2836variables and filehandles in other packages by prefixing the identifier
2837with the package name and a double colon: C<$Package::Variable>.
2838If the package name is null, the C<main> package as assumed. That is,
2839C<$::sail> is equivalent to C<$main::sail> (as well as to C<$main'sail>,
2840still seen in older code).
cb1a09d0 2841
5a964f20
TC
2842If NAMESPACE is omitted, then there is no current package, and all
2843identifiers must be fully qualified or lexicals. This is stricter
2844than C<use strict>, since it also extends to function names.
2845
cb1a09d0
AD
2846See L<perlmod/"Packages"> for more information about packages, modules,
2847and classes. See L<perlsub> for other scoping issues.
2848
a0d0e21e
LW
2849=item pipe READHANDLE,WRITEHANDLE
2850
2851Opens a pair of connected pipes like the corresponding system call.
2852Note that if you set up a loop of piped processes, deadlock can occur
2853unless you are very careful. In addition, note that Perl's pipes use
184e9718 2854stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE
a0d0e21e
LW
2855after each command, depending on the application.
2856
7e1af8bc 2857See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
4633a7c4
LW
2858for examples of such things.
2859
4771b018
GS
2860On systems that support a close-on-exec flag on files, the flag will be set
2861for the newly opened file descriptors as determined by the value of $^F.
2862See L<perlvar/$^F>.
2863
a0d0e21e
LW
2864=item pop ARRAY
2865
54310121 2866=item pop
28757baa 2867
a0d0e21e 2868Pops and returns the last value of the array, shortening the array by
2b5ab1e7 2869one element. Has a similar effect to
a0d0e21e
LW
2870
2871 $tmp = $ARRAY[$#ARRAY--];
2872
2873If there are no elements in the array, returns the undefined value.
2b5ab1e7
TC
2874If ARRAY is omitted, pops the C<@ARGV> array in the main program, and
2875the C<@_> array in subroutines, just like C<shift()>.
a0d0e21e
LW
2876
2877=item pos SCALAR
2878
54310121 2879=item pos
bbce6d69 2880
4633a7c4 2881Returns the offset of where the last C<m//g> search left off for the variable
7660c0ab 2882is in question (C<$_> is used when the variable is not specified). May be
44a8e56a
PP
2883modified to change that offset. Such modification will also influence
2884the C<\G> zero-width assertion in regular expressions. See L<perlre> and
2885L<perlop>.
a0d0e21e
LW
2886
2887=item print FILEHANDLE LIST
2888
2889=item print LIST
2890
2891=item print
2892
cb1a09d0 2893Prints a string or a comma-separated list of strings. Returns TRUE
a0d0e21e 2894if successful. FILEHANDLE may be a scalar variable name, in which case
2b5ab1e7
TC
2895the variable contains the name of or a reference to the filehandle, thus
2896introducing one level of indirection. (NOTE: If FILEHANDLE is a variable
2897and the next token is a term, it may be misinterpreted as an operator
2898unless you interpose a C<+> or put parentheses around the arguments.)
2899If FILEHANDLE is omitted, prints by default to standard output (or to the
2900last selected output channel--see L</select>). If LIST is also omitted,
2901prints C<$_> to the currently selected output channel. To set the default
2902output channel to something other than STDOUT use the select operation.
2903Note that, because print takes a LIST, anything in the LIST is evaluated
2904in list context, and any subroutine that you call will have one or
2905more of its expressions evaluated in list context. Also be careful
2906not to follow the print keyword with a left parenthesis unless you want
2907the corresponding right parenthesis to terminate the arguments to the
2908print--interpose a C<+> or put parentheses around all the arguments.
a0d0e21e 2909
4633a7c4 2910Note that if you're storing FILEHANDLES in an array or other expression,
da0045b7 2911you will have to use a block returning its value instead:
4633a7c4
LW
2912
2913 print { $files[$i] } "stuff\n";
2914 print { $OK ? STDOUT : STDERR } "stuff\n";
2915
5f05dabc 2916=item printf FILEHANDLE FORMAT, LIST
a0d0e21e 2917
5f05dabc 2918=item printf FORMAT, LIST
a0d0e21e 2919
7660c0ab 2920Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
a3cb178b 2921(the output record separator) is not appended. The first argument
ab4f32c2 2922of the list will be interpreted as the C<printf()> format. If C<use locale> is
a034a98d
DD
2923in effect, the character used for the decimal point in formatted real numbers
2924is affected by the LC_NUMERIC locale. See L<perllocale>.
a0d0e21e 2925
7660c0ab
A
2926Don't fall into the trap of using a C<printf()> when a simple
2927C<print()> would do. The C<print()> is more efficient and less
28757baa
PP
2928error prone.
2929
da0045b7
PP
2930=item prototype FUNCTION
2931
2932Returns the prototype of a function as a string (or C<undef> if the
5f05dabc
PP
2933function has no prototype). FUNCTION is a reference to, or the name of,
2934the function whose prototype you want to retrieve.
da0045b7 2935
2b5ab1e7
TC
2936If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
2937name for Perl builtin. If the builtin is not I<overridable> (such as
ab4f32c2 2938C<qw//>) or its arguments cannot be expressed by a prototype (such as
2b5ab1e7
TC
2939C<system()>) returns C<undef> because the builtin does not really behave
2940like a Perl function. Otherwise, the string describing the equivalent
2941prototype is returned.
b6c543e3 2942
a0d0e21e
LW
2943=item push ARRAY,LIST
2944
2945Treats ARRAY as a stack, and pushes the values of LIST
2946onto the end of ARRAY. The length of ARRAY increases by the length of
2947LIST. Has the same effect as
2948
2949 for $value (LIST) {
2950 $ARRAY[++$#ARRAY] = $value;
2951 }
2952
2953but is more efficient. Returns the new number of elements in the array.
2954
2955=item q/STRING/
2956
2957=item qq/STRING/
2958
8782bef2
GB
2959=item qr/STRING/
2960
a0d0e21e
LW
2961=item qx/STRING/
2962
2963=item qw/STRING/
2964
4b6a7270 2965Generalized quotes. See L<perlop/"Regexp Quote-Like Operators">.
a0d0e21e
LW
2966
2967=item quotemeta EXPR
2968
54310121 2969=item quotemeta
bbce6d69 2970
68dc0745 2971Returns the value of EXPR with all non-alphanumeric
a034a98d
DD
2972characters backslashed. (That is, all characters not matching
2973C</[A-Za-z_0-9]/> will be preceded by a backslash in the
2974returned string, regardless of any locale settings.)
2975This is the internal function implementing
7660c0ab 2976the C<\Q> escape in double-quoted strings.
a0d0e21e 2977
7660c0ab 2978If EXPR is omitted, uses C<$_>.
bbce6d69 2979
a0d0e21e
LW
2980=item rand EXPR
2981
2982=item rand
2983
7660c0ab 2984Returns a random fractional number greater than or equal to C<0> and less
3e3baf6d 2985than the value of EXPR. (EXPR should be positive.) If EXPR is
7660c0ab
A
2986omitted, the value C<1> is used. Automatically calls C<srand()> unless
2987C<srand()> has already been called. See also C<srand()>.
a0d0e21e 2988
2f9daede 2989(Note: If your rand function consistently returns numbers that are too
a0d0e21e 2990large or too small, then your version of Perl was probably compiled
2f9daede 2991with the wrong number of RANDBITS.)
a0d0e21e
LW
2992
2993=item read FILEHANDLE,SCALAR,LENGTH,OFFSET
2994
2995=item read FILEHANDLE,SCALAR,LENGTH
2996
2997Attempts to read LENGTH bytes of data into variable SCALAR from the
3b02c43c
GS
2998specified FILEHANDLE. Returns the number of bytes actually read,
2999C<0> at end of file, or undef if there was an error. SCALAR will be grown
3000or shrunk to the length actually read. An OFFSET may be specified to
3001place the read data at some other place than the beginning of the
f86cebdf
GS
3002string. This call is actually implemented in terms of stdio's fread(3)
3003call. To get a true read(2) system call, see C<sysread()>.
a0d0e21e
LW
3004
3005=item readdir DIRHANDLE
3006
7660c0ab 3007Returns the next directory entry for a directory opened by C<opendir()>.
5a964f20 3008If used in list context, returns all the rest of the entries in the
a0d0e21e 3009directory. If there are no more entries, returns an undefined value in
5a964f20 3010scalar context or a null list in list context.
a0d0e21e 3011
7660c0ab 3012If you're planning to filetest the return values out of a C<readdir()>, you'd
5f05dabc 3013better prepend the directory in question. Otherwise, because we didn't
7660c0ab 3014C<chdir()> there, it would have been testing the wrong file.
cb1a09d0
AD
3015
3016 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
3017 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
3018 closedir DIR;
3019
84902520
TB
3020=item readline EXPR
3021
fbad3eb5
GS
3022Reads from the filehandle whose typeglob is contained in EXPR. In scalar
3023context, each call reads and returns the next line, until end-of-file is
3024reached, whereupon the subsequent call returns undef. In list context,
3025reads until end-of-file is reached and returns a list of lines. Note that
3026the notion of "line" used here is however you may have defined it
3027with C<$/> or C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">.
3028
2b5ab1e7 3029When C<$/> is set to C<undef>, when readline() is in scalar
449bc448
GS
3030context (i.e. file slurp mode), and when an empty file is read, it
3031returns C<''> the first time, followed by C<undef> subsequently.
fbad3eb5 3032
84902520
TB
3033This is the internal function implementing the C<E<lt>EXPRE<gt>>
3034operator, but you can use it directly. The C<E<lt>EXPRE<gt>>
3035operator is discussed in more detail in L<perlop/"I/O Operators">.
3036
5a964f20
TC
3037 $line = <STDIN>;
3038 $line = readline(*STDIN); # same thing
3039
a0d0e21e
LW
3040=item readlink EXPR
3041
54310121 3042=item readlink
bbce6d69 3043
a0d0e21e
LW
3044Returns the value of a symbolic link, if symbolic links are
3045implemented. If not, gives a fatal error. If there is some system
184e9718 3046error, returns the undefined value and sets C<$!> (errno). If EXPR is
7660c0ab 3047omitted, uses C<$_>.
a0d0e21e 3048
84902520
TB
3049=item readpipe EXPR
3050
5a964f20 3051EXPR is executed as a system command.
84902520
TB
3052The collected standard output of the command is returned.
3053In scalar context, it comes back as a single (potentially
3054multi-line) string. In list context, returns a list of lines
7660c0ab 3055(however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
84902520
TB
3056This is the internal function implementing the C<qx/EXPR/>
3057operator, but you can use it directly. The C<qx/EXPR/>
3058operator is discussed in more detail in L<perlop/"I/O Operators">.
3059
399388f4 3060=item recv SOCKET,SCALAR,LENGTH,FLAGS
a0d0e21e
LW
3061
3062Receives a message on a socket. Attempts to receive LENGTH bytes of
3063data into variable SCALAR from the specified SOCKET filehandle.
7660c0ab 3064Actually does a C C<recvfrom()>, so that it can return the address of the
a0d0e21e
LW
3065sender. Returns the undefined value if there's an error. SCALAR will
3066be grown or shrunk to the length actually read. Takes the same flags
54310121 3067as the system call of the same name.
4633a7c4 3068See L<perlipc/"UDP: Message Passing"> for examples.
a0d0e21e
LW
3069
3070=item redo LABEL
3071
3072=item redo
3073
3074The C<redo> command restarts the loop block without evaluating the
98293880 3075conditional again. The C<continue> block, if any, is not executed. If
a0d0e21e
LW
3076the LABEL is omitted, the command refers to the innermost enclosing
3077loop. This command is normally used by programs that want to lie to
3078themselves about what was just input:
3079
3080 # a simpleminded Pascal comment stripper
3081 # (warning: assumes no { or } in strings)
4633a7c4 3082 LINE: while (<STDIN>) {
a0d0e21e
LW
3083 while (s|({.*}.*){.*}|$1 |) {}
3084 s|{.*}| |;
3085 if (s|{.*| |) {
3086 $front = $_;
3087 while (<STDIN>) {
3088 if (/}/) { # end of comment?
5a964f20 3089 s|^|$front\{|;
4633a7c4 3090 redo LINE;
a0d0e21e
LW
3091 }
3092 }
3093 }
3094 print;
3095 }
3096
4968c1e4 3097C<redo> cannot be used to retry a block which returns a value such as
2b5ab1e7
TC
3098C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
3099a grep() or map() operation.
4968c1e4 3100
98293880 3101See also L</continue> for an illustration of how C<last>, C<next>, and
1d2dff63
GS
3102C<redo> work.
3103
a0d0e21e
LW
3104=item ref EXPR
3105
54310121 3106=item ref
bbce6d69 3107
2f9daede 3108Returns a TRUE value if EXPR is a reference, FALSE otherwise. If EXPR
7660c0ab 3109is not specified, C<$_> will be used. The value returned depends on the
bbce6d69 3110type of thing the reference is a reference to.
a0d0e21e
LW
3111Builtin types include:
3112
3113 REF
3114 SCALAR
3115 ARRAY
3116 HASH
3117 CODE
3118 GLOB
3119
54310121 3120If the referenced object has been blessed into a package, then that package
7660c0ab 3121name is returned instead. You can think of C<ref()> as a C<typeof()> operator.
a0d0e21e
LW
3122
3123 if (ref($r) eq "HASH") {
aa689395 3124 print "r is a reference to a hash.\n";
54310121 3125 }
2b5ab1e7 3126 unless (ref($r)) {
a0d0e21e 3127 print "r is not a reference at all.\n";
54310121 3128 }
2b5ab1e7
TC
3129 if (UNIVERSAL::isa($r, "HASH")) { # for subclassing
3130 print "r is a reference to something that isa hash.\n";
3131 }
a0d0e21e
LW
3132
3133See also L<perlref>.
3134
3135=item rename OLDNAME,NEWNAME
3136
2b5ab1e7
TC
3137Changes the name of a file. Returns C<1> for success, C<0> otherwise.
3138Behavior of this function varies wildly depending on your system
3139implementation. For example, it will usually not work across file system
3140boundaries, even though the system I<mv> command sometimes compensates
3141for this. Other restrictions include whether it works on directories,
3142open files, or pre-existing files. Check L<perlport> and either the
3143rename(2) manpage or equivalent system documentation for details.
a0d0e21e
LW
3144
3145=item require EXPR
3146
3147=item require
3148
7660c0ab 3149Demands some semantics specified by EXPR, or by C<$_> if EXPR is not
a0d0e21e 3150supplied. If EXPR is numeric, demands that the current version of Perl
184e9718 3151(C<$]> or $PERL_VERSION) be equal or greater than EXPR.
a0d0e21e
LW
3152
3153Otherwise, demands that a library file be included if it hasn't already
3154been included. The file is included via the do-FILE mechanism, which is
7660c0ab 3155essentially just a variety of C<eval()>. Has semantics similar to the following
a0d0e21e
LW
3156subroutine:
3157
3158 sub require {
5a964f20 3159 my($filename) = @_;
a0d0e21e 3160 return 1 if $INC{$filename};
5a964f20 3161 my($realfilename,$result);
a0d0e21e
LW
3162 ITER: {
3163 foreach $prefix (@INC) {
3164 $realfilename = "$prefix/$filename";
3165 if (-f $realfilename) {
3166 $result = do $realfilename;
3167 last ITER;
3168 }
3169 }
3170 die "Can't find $filename in \@INC";
3171 }
3172 die $@ if $@;
3173 die "$filename did not return true value" unless $result;
3174 $INC{$filename} = $realfilename;
5a964f20 3175 return $result;
a0d0e21e
LW
3176 }
3177
3178Note that the file will not be included twice under the same specified
3179name. The file must return TRUE as the last statement to indicate
3180successful execution of any initialization code, so it's customary to
7660c0ab 3181end such a file with "C<1;>" unless you're sure it'll return TRUE
a0d0e21e
LW
3182otherwise. But it's better just to put the "C<1;>", in case you add more
3183statements.
3184
54310121 3185If EXPR is a bareword, the require assumes a "F<.pm>" extension and
da0045b7 3186replaces "F<::>" with "F</>" in the filename for you,
54310121 3187to make it easy to load standard modules. This form of loading of
a0d0e21e
LW
3188modules does not risk altering your namespace.
3189
ee580363
GS
3190In other words, if you try this:
3191
f86cebdf 3192 require Foo::Bar; # a splendid bareword
ee580363 3193
7660c0ab
A
3194The require function will actually look for the "F<Foo/Bar.pm>" file in the
3195directories specified in the C<@INC> array.
ee580363 3196
5a964f20 3197But if you try this:
ee580363
GS
3198
3199 $class = 'Foo::Bar';
f86cebdf 3200 require $class; # $class is not a bareword
5a964f20 3201 #or
f86cebdf 3202 require "Foo::Bar"; # not a bareword because of the ""
ee580363 3203
7660c0ab
A
3204The require function will look for the "F<Foo::Bar>" file in the @INC array and
3205will complain about not finding "F<Foo::Bar>" there. In this case you can do:
ee580363
GS
3206
3207 eval "require $class";
3208
3209For a yet-more-powerful import facility, see L</use> and L<perlmod>.
a0d0e21e
LW
3210
3211=item reset EXPR
3212
3213=item reset
3214
3215Generally used in a C<continue> block at the end of a loop to clear
7660c0ab 3216variables and reset C<??> searches so that they work again. The
a0d0e21e
LW
3217expression is interpreted as a list of single characters (hyphens
3218allowed for ranges). All variables and arrays beginning with one of
3219those letters are reset to their pristine state. If the expression is
7660c0ab 3220omitted, one-match searches (C<?pattern?>) are reset to match again. Resets
5f05dabc 3221only variables or searches in the current package. Always returns
a0d0e21e
LW
32221. Examples:
3223
3224 reset 'X'; # reset all X variables
3225 reset 'a-z'; # reset lower case variables
2b5ab1e7 3226 reset; # just reset ?one-time? searches
a0d0e21e 3227
7660c0ab 3228Resetting C<"A-Z"> is not recommended because you'll wipe out your
2b5ab1e7
TC
3229C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package
3230variables--lexical variables are unaffected, but they clean themselves
3231up on scope exit anyway, so you'll probably want to use them instead.
3232See L</my>.
a0d0e21e 3233
54310121
PP
3234=item return EXPR
3235
3236=item return
3237
7660c0ab 3238Returns from a subroutine, C<eval()>, or C<do FILE> with the value
5a964f20 3239given in EXPR. Evaluation of EXPR may be in list, scalar, or void
54310121 3240context, depending on how the return value will be used, and the context
7660c0ab 3241may vary from one execution to the next (see C<wantarray()>). If no EXPR
2b5ab1e7
TC
3242is given, returns an empty list in list context, the undefined value in
3243scalar context, and (of course) nothing at all in a void context.
a0d0e21e 3244
2b5ab1e7
TC
3245(Note that in the absence of a explicit C<return>, a subroutine, eval,
3246or do FILE will automatically return the value of the last expression
3247evaluated.)
a0d0e21e
LW
3248
3249=item reverse LIST
3250
5a964f20
TC
3251In list context, returns a list value consisting of the elements
3252of LIST in the opposite order. In scalar context, concatenates the
2b5ab1e7 3253elements of LIST and returns a string value with all characters
a0ed51b3 3254in the opposite order.
4633a7c4 3255
2f9daede 3256 print reverse <>; # line tac, last line first
4633a7c4 3257
2f9daede 3258 undef $/; # for efficiency of <>
a0ed51b3 3259 print scalar reverse <>; # character tac, last line tsrif
2f9daede
TPG
3260
3261This operator is also handy for inverting a hash, although there are some
3262caveats. If a value is duplicated in the original hash, only one of those
3263can be represented as a key in the inverted hash. Also, this has to
3264unwind one hash and build a whole new one, which may take some time
2b5ab1e7 3265on a large hash, such as from a DBM file.
2f9daede
TPG
3266
3267 %by_name = reverse %by_address; # Invert the hash
a0d0e21e
LW
3268
3269=item rewinddir DIRHANDLE
3270
3271Sets the current position to the beginning of the directory for the
7660c0ab 3272C<readdir()> routine on DIRHANDLE.
a0d0e21e
LW
3273
3274=item rindex STR,SUBSTR,POSITION
3275
3276=item rindex STR,SUBSTR
3277
2b5ab1e7 3278Works just like index() except that it returns the position of the LAST
a0d0e21e
LW
3279occurrence of SUBSTR in STR. If POSITION is specified, returns the
3280last occurrence at or before that position.
3281
3282=item rmdir FILENAME
3283
54310121 3284=item rmdir
bbce6d69 3285
5a964f20
TC
3286Deletes the directory specified by FILENAME if that directory is empty. If it
3287succeeds it returns TRUE, otherwise it returns FALSE and sets C<$!> (errno). If
7660c0ab 3288FILENAME is omitted, uses C<$_>.
a0d0e21e
LW
3289
3290=item s///
3291
3292The substitution operator. See L<perlop>.
3293
3294=item scalar EXPR
3295
5a964f20 3296Forces EXPR to be interpreted in scalar context and returns the value
54310121 3297of EXPR.
cb1a09d0
AD
3298
3299 @counts = ( scalar @a, scalar @b, scalar @c );
3300
54310121 3301There is no equivalent operator to force an expression to
2b5ab1e7 3302be interpolated in list context because in practice, this is never
cb1a09d0
AD
3303needed. If you really wanted to do so, however, you could use
3304the construction C<@{[ (some expression) ]}>, but usually a simple
3305C<(some expression)> suffices.
a0d0e21e 3306
2b5ab1e7
TC
3307Wince C<scalar> is unary operator, if you accidentally use for EXPR a
3308parenthesized list, this behaves as a scalar comma expression, evaluating
3309all but the last element in void context and returning the final element
3310evaluated in scalar context. This is seldom what you want.
62c18ce2
GS
3311
3312The following single statement:
3313
3314 print uc(scalar(&foo,$bar)),$baz;
3315
3316is the moral equivalent of these two:
3317
3318 &foo;
3319 print(uc($bar),$baz);
3320
3321See L<perlop> for more details on unary operators and the comma operator.
3322
a0d0e21e
LW
3323=item seek FILEHANDLE,POSITION,WHENCE