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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
19799a22 33surprising) rule is this: It I<looks> like a function, therefore it I<is> a
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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
22fae026
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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.
19799a22 258Unless otherwise documented, it returns C<1> for true and C<''> for false, or
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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
19799a22 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
19799a22 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<$_>.
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372
373=item accept NEWSOCKET,GENERICSOCKET
374
f86cebdf 375Accepts an incoming socket connect, just as the accept(2) system call
19799a22 376does. Returns the packed address if it succeeded, false otherwise.
2b5ab1e7 377See the example in L<perlipc/"Sockets: Client/Server Communication">.
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378
379=item alarm SECONDS
380
54310121 381=item alarm
bbce6d69 382
a0d0e21e 383Arranges to have a SIGALRM delivered to this process after the
bbce6d69 384specified number of seconds have elapsed. If SECONDS is not specified,
7660c0ab 385the value stored in C<$_> 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-argument version of select() leaving the first three arguments
395undefined, or you might be able to use the C<syscall> interface to
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396access setitimer(2) if your system supports it. The Time::HiRes module
397from CPAN may also prove useful.
398
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399It is usually a mistake to intermix C<alarm>
400and C<sleep> calls.
a0d0e21e 401
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402If you want to use C<alarm> to time out a system call you need to use an
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
19799a22 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
19799a22 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
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440Arranges for FILEHANDLE to be read or written in "binary" mode on
441systems whose run-time libraries force the programmer to guess
442between binary and text files. If FILEHANDLE is an expression, the
443value is taken as the name of the filehandle. binmode() should be
444called after the C<open> but before any I/O is done on the filehandle.
445The only way to reset binary mode on a filehandle is to reopen the
446file.
447
448The operating system, device drivers, C libraries, and Perl run-time
449system all conspire to let the programmer conveniently treat a
450simple, one-byte C<\n> as the line terminator, irrespective of its
451external representation. On Unix and its brethren, the native file
452representation exactly matches the internal representation, making
453everyone's lives unbelievably simpler. Consequently, L<binmode>
454has no effect under Unix, Plan9, or Mac OS, all of which use C<\n>
455to end each line. (Unix and Plan9 think C<\n> means C<\cJ> and
456C<\r> means C<\cM>, whereas the Mac goes the other way--it uses
457C<\cM> for c<\n> and C<\cJ> to mean C<\r>. But that's ok, because
458it's only one byte, and the internal and external representations
459match.)
460
461In legacy systems like MS-DOS and its embellishments, your program
462sees a C<\n> as a simple C<\cJ> (just as in Unix), but oddly enough,
463that's not what's physically stored on disk. What's worse, these
464systems refuse to help you with this; it's up to you to remember
465what to do. And you mustn't go applying binmode() with wild abandon,
466either, because if your system does care about binmode(), then using
467it when you shouldn't is just as perilous as failing to use it when
468you should.
469
470That means that on any version of Microsoft WinXX that you might
471care to name (or not), binmode() causes C<\cM\cJ> sequences on disk
472to be converted to C<\n> when read into your program, and causes
473any C<\n> in your program to be converted back to C<\cM\cJ> on
474output to disk. This sad discrepancy leads to no end of
475problems in not just the readline operator, but also when using
476seek(), tell(), and read() calls. See L<perlport> for other painful
477details. See the C<$/> and C<$\> variables in L<perlvar> for how
478to manually set your input and output line-termination sequences.
a0d0e21e 479
4633a7c4 480=item bless REF,CLASSNAME
a0d0e21e
LW
481
482=item bless REF
483
2b5ab1e7
TC
484This function tells the thingy referenced by REF that it is now an object
485in the CLASSNAME package. If CLASSNAME is omitted, the current package
19799a22 486is used. Because a C<bless> is often the last thing in a constructor,
2b5ab1e7
TC
487it returns the reference for convenience. Always use the two-argument
488version if the function doing the blessing might be inherited by a
489derived class. See L<perltoot> and L<perlobj> for more about the blessing
490(and blessings) of objects.
a0d0e21e 491
57668c4d 492Consider always blessing objects in CLASSNAMEs that are mixed case.
2b5ab1e7
TC
493Namespaces with all lowercase names are considered reserved for
494Perl pragmata. Builtin types have all uppercase names, so to prevent
495confusion, you may wish to avoid such package names as well. Make sure
496that CLASSNAME is a true value.
60ad88b8
GS
497
498See L<perlmod/"Perl Modules">.
499
a0d0e21e
LW
500=item caller EXPR
501
502=item caller
503
5a964f20 504Returns the context of the current subroutine call. In scalar context,
28757baa 505returns the caller's package name if there is a caller, that is, if
19799a22 506we're in a subroutine or C<eval> or C<require>, and the undefined value
5a964f20 507otherwise. In list context, returns
a0d0e21e 508
748a9306 509 ($package, $filename, $line) = caller;
a0d0e21e
LW
510
511With EXPR, it returns some extra information that the debugger uses to
512print a stack trace. The value of EXPR indicates how many call frames
513to go back before the current one.
514
f3aa04c2
GS
515 ($package, $filename, $line, $subroutine, $hasargs,
516 $wantarray, $evaltext, $is_require, $hints) = caller($i);
e7ea3e70 517
19799a22
GS
518Here $subroutine may be C<"(eval)"> if the frame is not a subroutine
519call, but an C<eval>. In such a case additional elements $evaltext and
7660c0ab 520C<$is_require> are set: C<$is_require> is true if the frame is created by a
19799a22 521C<require> or C<use> statement, $evaltext contains the text of the
dc848c6f 522C<eval EXPR> statement. In particular, for a C<eval BLOCK> statement,
19799a22 523$filename is C<"(eval)">, but $evaltext is undefined. (Note also that
dc848c6f 524each C<use> statement creates a C<require> frame inside an C<eval EXPR>)
f3aa04c2
GS
525frame. C<$hints> contains pragmatic hints that the caller was
526compiled with. It currently only reflects the hint corresponding to
527C<use utf8>.
748a9306
LW
528
529Furthermore, when called from within the DB package, caller returns more
7660c0ab 530detailed information: it sets the list variable C<@DB::args> to be the
54310121 531arguments with which the subroutine was invoked.
748a9306 532
7660c0ab 533Be aware that the optimizer might have optimized call frames away before
19799a22 534C<caller> had a chance to get the information. That means that C<caller(N)>
7660c0ab 535might not return information about the call frame you expect it do, for
19799a22
GS
536C<N E<gt> 1>. In particular, C<@DB::args> might have information from the
537previous time C<caller> was called.
7660c0ab 538
a0d0e21e
LW
539=item chdir EXPR
540
2b5ab1e7 541Changes the working directory to EXPR, if possible. If EXPR is omitted,
19799a22
GS
542changes to the user's home directory. Returns true upon success,
543false otherwise. See the example under C<die>.
a0d0e21e
LW
544
545=item chmod LIST
546
547Changes the permissions of a list of files. The first element of the
4633a7c4 548list must be the numerical mode, which should probably be an octal
2f9daede
TPG
549number, and which definitely should I<not> a string of octal digits:
550C<0644> is okay, C<'0644'> is not. Returns the number of files
dc848c6f 551successfully changed. See also L</oct>, if all you have is a string.
a0d0e21e
LW
552
553 $cnt = chmod 0755, 'foo', 'bar';
554 chmod 0755, @executables;
f86cebdf
GS
555 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
556 # --w----r-T
2f9daede
TPG
557 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
558 $mode = 0644; chmod $mode, 'foo'; # this is best
a0d0e21e
LW
559
560=item chomp VARIABLE
561
562=item chomp LIST
563
564=item chomp
565
2b5ab1e7
TC
566This safer version of L</chop> removes any trailing string
567that corresponds to the current value of C<$/> (also known as
28757baa
PP
568$INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
569number of characters removed from all its arguments. It's often used to
570remove the newline from the end of an input record when you're worried
2b5ab1e7
TC
571that the final record may be missing its newline. When in paragraph
572mode (C<$/ = "">), it removes all trailing newlines from the string.
4c5a6083
GS
573When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
574a reference to an integer or the like, see L<perlvar>) chomp() won't
19799a22
GS
575remove anything.
576If VARIABLE is omitted, it chomps C<$_>. Example:
a0d0e21e
LW
577
578 while (<>) {
579 chomp; # avoid \n on last field
580 @array = split(/:/);
5a964f20 581 # ...
a0d0e21e
LW
582 }
583
584You can actually chomp anything that's an lvalue, including an assignment:
585
586 chomp($cwd = `pwd`);
587 chomp($answer = <STDIN>);
588
589If you chomp a list, each element is chomped, and the total number of
590characters removed is returned.
591
592=item chop VARIABLE
593
594=item chop LIST
595
596=item chop
597
598Chops off the last character of a string and returns the character
599chopped. It's used primarily to remove the newline from the end of an
600input record, but is much more efficient than C<s/\n//> because it neither
7660c0ab 601scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
a0d0e21e
LW
602Example:
603
604 while (<>) {
605 chop; # avoid \n on last field
606 @array = split(/:/);
5a964f20 607 #...
a0d0e21e
LW
608 }
609
610You can actually chop anything that's an lvalue, including an assignment:
611
612 chop($cwd = `pwd`);
613 chop($answer = <STDIN>);
614
615If you chop a list, each element is chopped. Only the value of the
19799a22 616last C<chop> is returned.
a0d0e21e 617
19799a22 618Note that C<chop> returns the last character. To return all but the last
748a9306
LW
619character, use C<substr($string, 0, -1)>.
620
a0d0e21e
LW
621=item chown LIST
622
623Changes the owner (and group) of a list of files. The first two
19799a22
GS
624elements of the list must be the I<numeric> uid and gid, in that
625order. A value of -1 in either position is interpreted by most
626systems to leave that value unchanged. Returns the number of files
627successfully changed.
a0d0e21e
LW
628
629 $cnt = chown $uid, $gid, 'foo', 'bar';
630 chown $uid, $gid, @filenames;
631
54310121 632Here's an example that looks up nonnumeric uids in the passwd file:
a0d0e21e
LW
633
634 print "User: ";
19799a22 635 chomp($user = <STDIN>);
5a964f20 636 print "Files: ";
19799a22 637 chomp($pattern = <STDIN>);
a0d0e21e
LW
638
639 ($login,$pass,$uid,$gid) = getpwnam($user)
640 or die "$user not in passwd file";
641
5a964f20 642 @ary = glob($pattern); # expand filenames
a0d0e21e
LW
643 chown $uid, $gid, @ary;
644
54310121 645On most systems, you are not allowed to change the ownership of the
4633a7c4
LW
646file unless you're the superuser, although you should be able to change
647the group to any of your secondary groups. On insecure systems, these
648restrictions may be relaxed, but this is not a portable assumption.
19799a22
GS
649On POSIX systems, you can detect this condition this way:
650
651 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
652 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
4633a7c4 653
a0d0e21e
LW
654=item chr NUMBER
655
54310121 656=item chr
bbce6d69 657
a0d0e21e 658Returns the character represented by that NUMBER in the character set.
a0ed51b3 659For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
2b5ab1e7
TC
660chr(0x263a) is a Unicode smiley face (but only within the scope of
661a C<use utf8>). For the reverse, use L</ord>.
662See L<utf8> for more about Unicode.
a0d0e21e 663
7660c0ab 664If NUMBER is omitted, uses C<$_>.
bbce6d69 665
a0d0e21e
LW
666=item chroot FILENAME
667
54310121 668=item chroot
bbce6d69 669
5a964f20 670This function works like the system call by the same name: it makes the
4633a7c4 671named directory the new root directory for all further pathnames that
7660c0ab 672begin with a C<"/"> by your process and all its children. (It doesn't
28757baa 673change your current working directory, which is unaffected.) For security
4633a7c4 674reasons, this call is restricted to the superuser. If FILENAME is
19799a22 675omitted, does a C<chroot> to C<$_>.
a0d0e21e
LW
676
677=item close FILEHANDLE
678
6a518fbc
TP
679=item close
680
19799a22 681Closes the file or pipe associated with the file handle, returning true
a0d0e21e 682only if stdio successfully flushes buffers and closes the system file
19799a22 683descriptor. Closes the currently selected filehandle if the argument
6a518fbc 684is omitted.
fb73857a
PP
685
686You don't have to close FILEHANDLE if you are immediately going to do
19799a22
GS
687another C<open> on it, because C<open> will close it for you. (See
688C<open>.) However, an explicit C<close> on an input file resets the line
689counter (C<$.>), while the implicit close done by C<open> does not.
fb73857a 690
19799a22
GS
691If the file handle came from a piped open C<close> will additionally
692return false if one of the other system calls involved fails or if the
fb73857a 693program exits with non-zero status. (If the only problem was that the
2b5ab1e7
TC
694program exited non-zero C<$!> will be set to C<0>.) Closing a pipe
695also waits for the process executing on the pipe to complete, in case you
696want to look at the output of the pipe afterwards, and
697implicitly puts the exit status value of that command into C<$?>.
5a964f20 698
73689b13
GS
699Prematurely closing the read end of a pipe (i.e. before the process
700writing to it at the other end has closed it) will result in a
701SIGPIPE being delivered to the writer. If the other end can't
702handle that, be sure to read all the data before closing the pipe.
703
fb73857a 704Example:
a0d0e21e 705
fb73857a
PP
706 open(OUTPUT, '|sort >foo') # pipe to sort
707 or die "Can't start sort: $!";
5a964f20 708 #... # print stuff to output
fb73857a
PP
709 close OUTPUT # wait for sort to finish
710 or warn $! ? "Error closing sort pipe: $!"
711 : "Exit status $? from sort";
712 open(INPUT, 'foo') # get sort's results
713 or die "Can't open 'foo' for input: $!";
a0d0e21e 714
5a964f20
TC
715FILEHANDLE may be an expression whose value can be used as an indirect
716filehandle, usually the real filehandle name.
a0d0e21e
LW
717
718=item closedir DIRHANDLE
719
19799a22 720Closes a directory opened by C<opendir> and returns the success of that
5a964f20
TC
721system call.
722
723DIRHANDLE may be an expression whose value can be used as an indirect
724dirhandle, usually the real dirhandle name.
a0d0e21e
LW
725
726=item connect SOCKET,NAME
727
728Attempts to connect to a remote socket, just as the connect system call
19799a22 729does. Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
730packed address of the appropriate type for the socket. See the examples in
731L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 732
cb1a09d0
AD
733=item continue BLOCK
734
735Actually a flow control statement rather than a function. If there is a
98293880
JH
736C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
737C<foreach>), it is always executed just before the conditional is about to
738be evaluated again, just like the third part of a C<for> loop in C. Thus
cb1a09d0
AD
739it can be used to increment a loop variable, even when the loop has been
740continued via the C<next> statement (which is similar to the C C<continue>
741statement).
742
98293880 743C<last>, C<next>, or C<redo> may appear within a C<continue>
19799a22
GS
744block. C<last> and C<redo> will behave as if they had been executed within
745the main block. So will C<next>, but since it will execute a C<continue>
1d2dff63
GS
746block, it may be more entertaining.
747
748 while (EXPR) {
749 ### redo always comes here
750 do_something;
751 } continue {
752 ### next always comes here
753 do_something_else;
754 # then back the top to re-check EXPR
755 }
756 ### last always comes here
757
758Omitting the C<continue> section is semantically equivalent to using an
19799a22 759empty one, logically enough. In that case, C<next> goes directly back
1d2dff63
GS
760to check the condition at the top of the loop.
761
a0d0e21e
LW
762=item cos EXPR
763
5a964f20 764Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
7660c0ab 765takes cosine of C<$_>.
a0d0e21e 766
7660c0ab 767For the inverse cosine operation, you may use the C<POSIX::acos()>
28757baa
PP
768function, or use this relation:
769
770 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
771
a0d0e21e
LW
772=item crypt PLAINTEXT,SALT
773
f86cebdf 774Encrypts a string exactly like the crypt(3) function in the C library
4633a7c4
LW
775(assuming that you actually have a version there that has not been
776extirpated as a potential munition). This can prove useful for checking
777the password file for lousy passwords, amongst other things. Only the
778guys wearing white hats should do this.
a0d0e21e 779
19799a22 780Note that C<crypt> is intended to be a one-way function, much like breaking
11155c91
CS
781eggs to make an omelette. There is no (known) corresponding decrypt
782function. As a result, this function isn't all that useful for
783cryptography. (For that, see your nearby CPAN mirror.)
2f9daede 784
e71965be
RS
785When verifying an existing encrypted string you should use the encrypted
786text as the salt (like C<crypt($plain, $crypted) eq $crypted>). This
19799a22 787allows your code to work with the standard C<crypt> and with more
e71965be
RS
788exotic implementations. When choosing a new salt create a random two
789character string whose characters come from the set C<[./0-9A-Za-z]>
790(like C<join '', ('.', '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>).
791
a0d0e21e
LW
792Here's an example that makes sure that whoever runs this program knows
793their own password:
794
795 $pwd = (getpwuid($<))[1];
a0d0e21e
LW
796
797 system "stty -echo";
798 print "Password: ";
e71965be 799 chomp($word = <STDIN>);
a0d0e21e
LW
800 print "\n";
801 system "stty echo";
802
e71965be 803 if (crypt($word, $pwd) ne $pwd) {
a0d0e21e
LW
804 die "Sorry...\n";
805 } else {
806 print "ok\n";
54310121 807 }
a0d0e21e 808
9f8f0c9d 809Of course, typing in your own password to whoever asks you
748a9306 810for it is unwise.
a0d0e21e 811
19799a22
GS
812The L<crypt> function is unsuitable for encrypting large quantities
813of data, not least of all because you can't get the information
814back. Look at the F<by-module/Crypt> and F<by-module/PGP> directories
815on your favorite CPAN mirror for a slew of potentially useful
816modules.
817
aa689395 818=item dbmclose HASH
a0d0e21e 819
19799a22 820[This function has been largely superseded by the C<untie> function.]
a0d0e21e 821
aa689395 822Breaks the binding between a DBM file and a hash.
a0d0e21e 823
19799a22 824=item dbmopen HASH,DBNAME,MASK
a0d0e21e 825
19799a22 826[This function has been largely superseded by the C<tie> function.]
a0d0e21e 827
7b8d334a 828This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
19799a22
GS
829hash. HASH is the name of the hash. (Unlike normal C<open>, the first
830argument is I<not> a filehandle, even though it looks like one). DBNAME
aa689395
PP
831is the name of the database (without the F<.dir> or F<.pag> extension if
832any). If the database does not exist, it is created with protection
19799a22
GS
833specified by MASK (as modified by the C<umask>). If your system supports
834only the older DBM functions, you may perform only one C<dbmopen> in your
aa689395 835program. In older versions of Perl, if your system had neither DBM nor
19799a22 836ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
aa689395
PP
837sdbm(3).
838
839If you don't have write access to the DBM file, you can only read hash
840variables, not set them. If you want to test whether you can write,
19799a22 841either use file tests or try setting a dummy hash entry inside an C<eval>,
aa689395 842which will trap the error.
a0d0e21e 843
19799a22
GS
844Note that functions such as C<keys> and C<values> may return huge lists
845when used on large DBM files. You may prefer to use the C<each>
a0d0e21e
LW
846function to iterate over large DBM files. Example:
847
848 # print out history file offsets
849 dbmopen(%HIST,'/usr/lib/news/history',0666);
850 while (($key,$val) = each %HIST) {
851 print $key, ' = ', unpack('L',$val), "\n";
852 }
853 dbmclose(%HIST);
854
cb1a09d0 855See also L<AnyDBM_File> for a more general description of the pros and
184e9718 856cons of the various dbm approaches, as well as L<DB_File> for a particularly
cb1a09d0 857rich implementation.
4633a7c4 858
2b5ab1e7
TC
859You can control which DBM library you use by loading that library
860before you call dbmopen():
861
862 use DB_File;
863 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
864 or die "Can't open netscape history file: $!";
865
a0d0e21e
LW
866=item defined EXPR
867
54310121 868=item defined
bbce6d69 869
2f9daede
TPG
870Returns a Boolean value telling whether EXPR has a value other than
871the undefined value C<undef>. If EXPR is not present, C<$_> will be
872checked.
873
874Many operations return C<undef> to indicate failure, end of file,
875system error, uninitialized variable, and other exceptional
876conditions. This function allows you to distinguish C<undef> from
877other values. (A simple Boolean test will not distinguish among
7660c0ab 878C<undef>, zero, the empty string, and C<"0">, which are all equally
2f9daede 879false.) Note that since C<undef> is a valid scalar, its presence
19799a22 880doesn't I<necessarily> indicate an exceptional condition: C<pop>
2f9daede
TPG
881returns C<undef> when its argument is an empty array, I<or> when the
882element to return happens to be C<undef>.
883
f10b0346
GS
884You may also use C<defined(&func)> to check whether subroutine C<&func>
885has ever been defined. The return value is unaffected by any forward
886declarations of C<&foo>.
887
888Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
889used to report whether memory for that aggregate has ever been
890allocated. This behavior may disappear in future versions of Perl.
891You should instead use a simple test for size:
892
893 if (@an_array) { print "has array elements\n" }
894 if (%a_hash) { print "has hash members\n" }
2f9daede
TPG
895
896When used on a hash element, it tells you whether the value is defined,
dc848c6f 897not whether the key exists in the hash. Use L</exists> for the latter
2f9daede 898purpose.
a0d0e21e
LW
899
900Examples:
901
902 print if defined $switch{'D'};
903 print "$val\n" while defined($val = pop(@ary));
904 die "Can't readlink $sym: $!"
905 unless defined($value = readlink $sym);
a0d0e21e 906 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
2f9daede 907 $debugging = 0 unless defined $debugging;
a0d0e21e 908
19799a22 909Note: Many folks tend to overuse C<defined>, and then are surprised to
7660c0ab 910discover that the number C<0> and C<""> (the zero-length string) are, in fact,
2f9daede 911defined values. For example, if you say
a5f75d66
AD
912
913 "ab" =~ /a(.*)b/;
914
7660c0ab 915The pattern match succeeds, and C<$1> is defined, despite the fact that it
a5f75d66 916matched "nothing". But it didn't really match nothing--rather, it
2b5ab1e7 917matched something that happened to be zero characters long. This is all
a5f75d66 918very above-board and honest. When a function returns an undefined value,
2f9daede 919it's an admission that it couldn't give you an honest answer. So you
19799a22 920should use C<defined> only when you're questioning the integrity of what
7660c0ab 921you're trying to do. At other times, a simple comparison to C<0> or C<""> is
2f9daede
TPG
922what you want.
923
dc848c6f 924See also L</undef>, L</exists>, L</ref>.
2f9daede 925
a0d0e21e
LW
926=item delete EXPR
927
aa689395
PP
928Deletes the specified key(s) and their associated values from a hash.
929For each key, returns the deleted value associated with that key, or
930the undefined value if there was no such key. Deleting from C<$ENV{}>
931modifies the environment. Deleting from a hash tied to a DBM file
19799a22 932deletes the entry from the DBM file. (But deleting from a C<tie>d hash
5f05dabc 933doesn't necessarily return anything.)
a0d0e21e 934
aa689395 935The following deletes all the values of a hash:
a0d0e21e 936
5f05dabc
PP
937 foreach $key (keys %HASH) {
938 delete $HASH{$key};
a0d0e21e
LW
939 }
940
5f05dabc
PP
941And so does this:
942
943 delete @HASH{keys %HASH}
944
2b5ab1e7
TC
945But both of these are slower than just assigning the empty list
946or undefining it:
947
948 %hash = (); # completely empty %hash
949 undef %hash; # forget %hash every existed
950
951Note that the EXPR can be arbitrarily complicated as long as the final
952operation is a hash element lookup or hash slice:
a0d0e21e
LW
953
954 delete $ref->[$x][$y]{$key};
5f05dabc 955 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
a0d0e21e
LW
956
957=item die LIST
958
19799a22
GS
959Outside an C<eval>, prints the value of LIST to C<STDERR> and
960exits with the current value of C<$!> (errno). If C<$!> is C<0>,
961exits with the value of C<($? E<gt>E<gt> 8)> (backtick `command`
962status). If C<($? E<gt>E<gt> 8)> is C<0>, exits with C<255>. Inside
963an C<eval(),> the error message is stuffed into C<$@> and the
964C<eval> is terminated with the undefined value. This makes
965C<die> the way to raise an exception.
a0d0e21e
LW
966
967Equivalent examples:
968
969 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
54310121 970 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
a0d0e21e
LW
971
972If the value of EXPR does not end in a newline, the current script line
973number and input line number (if any) are also printed, and a newline
883faa13
GS
974is supplied. Note that the "input line number" (also known as "chunk")
975is subject to whatever notion of "line" happens to be currently in
976effect, and is also available as the special variable C<$.>.
977See L<perlvar/"$/"> and L<perlvar/"$.">.
978
979Hint: sometimes appending C<", stopped"> to your message
7660c0ab 980will cause it to make better sense when the string C<"at foo line 123"> is
a0d0e21e
LW
981appended. Suppose you are running script "canasta".
982
983 die "/etc/games is no good";
984 die "/etc/games is no good, stopped";
985
986produce, respectively
987
988 /etc/games is no good at canasta line 123.
989 /etc/games is no good, stopped at canasta line 123.
990
2b5ab1e7 991See also exit(), warn(), and the Carp module.
a0d0e21e 992
7660c0ab
A
993If LIST is empty and C<$@> already contains a value (typically from a
994previous eval) that value is reused after appending C<"\t...propagated">.
fb73857a
PP
995This is useful for propagating exceptions:
996
997 eval { ... };
998 die unless $@ =~ /Expected exception/;
999
7660c0ab 1000If C<$@> is empty then the string C<"Died"> is used.
fb73857a 1001
52531d10
GS
1002die() can also be called with a reference argument. If this happens to be
1003trapped within an eval(), $@ contains the reference. This behavior permits
1004a more elaborate exception handling implementation using objects that
1005maintain arbitary state about the nature of the exception. Such a scheme
1006is sometimes preferable to matching particular string values of $@ using
1007regular expressions. Here's an example:
1008
1009 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1010 if ($@) {
1011 if (ref($@) && UNIVERSAL::isa($@,"Some::Module::Exception")) {
1012 # handle Some::Module::Exception
1013 }
1014 else {
1015 # handle all other possible exceptions
1016 }
1017 }
1018
19799a22 1019Because perl will stringify uncaught exception messages before displaying
52531d10
GS
1020them, you may want to overload stringification operations on such custom
1021exception objects. See L<overload> for details about that.
1022
19799a22
GS
1023You can arrange for a callback to be run just before the C<die>
1024does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1025handler will be called with the error text and can change the error
1026message, if it sees fit, by calling C<die> again. See
1027L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
1028L<"eval BLOCK"> for some examples. Although this feature was meant
1029to be run only right before your program was to exit, this is not
1030currently the case--the C<$SIG{__DIE__}> hook is currently called
1031even inside eval()ed blocks/strings! If one wants the hook to do
1032nothing in such situations, put
fb73857a
PP
1033
1034 die @_ if $^S;
1035
19799a22
GS
1036as the first line of the handler (see L<perlvar/$^S>). Because
1037this promotes strange action at a distance, this counterintuitive
1038behavior may be fixed in a future release.
774d564b 1039
a0d0e21e
LW
1040=item do BLOCK
1041
1042Not really a function. Returns the value of the last command in the
1043sequence of commands indicated by BLOCK. When modified by a loop
98293880
JH
1044modifier, executes the BLOCK once before testing the loop condition.
1045(On other statements the loop modifiers test the conditional first.)
a0d0e21e 1046
4968c1e4 1047C<do BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7
TC
1048C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1049See L<perlsyn> for alternative strategies.
4968c1e4 1050
a0d0e21e
LW
1051=item do SUBROUTINE(LIST)
1052
1053A deprecated form of subroutine call. See L<perlsub>.
1054
1055=item do EXPR
1056
1057Uses the value of EXPR as a filename and executes the contents of the
1058file as a Perl script. Its primary use is to include subroutines
1059from a Perl subroutine library.
1060
1061 do 'stat.pl';
1062
1063is just like
1064
fb73857a 1065 scalar eval `cat stat.pl`;
a0d0e21e 1066
2b5ab1e7
TC
1067except that it's more efficient and concise, keeps track of the current
1068filename for error messages, searches the @INC libraries, and updates
1069C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1070variables. It also differs in that code evaluated with C<do FILENAME>
1071cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1072same, however, in that it does reparse the file every time you call it,
1073so you probably don't want to do this inside a loop.
a0d0e21e 1074
8e30cc93 1075If C<do> cannot read the file, it returns undef and sets C<$!> to the
2b5ab1e7 1076error. If C<do> can read the file but cannot compile it, it
8e30cc93
G
1077returns undef and sets an error message in C<$@>. If the file is
1078successfully compiled, C<do> returns the value of the last expression
1079evaluated.
1080
a0d0e21e 1081Note that inclusion of library modules is better done with the
19799a22 1082C<use> and C<require> operators, which also do automatic error checking
4633a7c4 1083and raise an exception if there's a problem.
a0d0e21e 1084
5a964f20
TC
1085You might like to use C<do> to read in a program configuration
1086file. Manual error checking can be done this way:
1087
1088 # read in config files: system first, then user
f86cebdf 1089 for $file ("/share/prog/defaults.rc",
2b5ab1e7
TC
1090 "$ENV{HOME}/.someprogrc")
1091 {
5a964f20 1092 unless ($return = do $file) {
f86cebdf
GS
1093 warn "couldn't parse $file: $@" if $@;
1094 warn "couldn't do $file: $!" unless defined $return;
1095 warn "couldn't run $file" unless $return;
5a964f20
TC
1096 }
1097 }
1098
a0d0e21e
LW
1099=item dump LABEL
1100
1614b0e3
JD
1101=item dump
1102
19799a22
GS
1103This function causes an immediate core dump. See also the B<-u>
1104command-line switch in L<perlrun>, which does the same thing.
1105Primarily this is so that you can use the B<undump> program (not
1106supplied) to turn your core dump into an executable binary after
1107having initialized all your variables at the beginning of the
1108program. When the new binary is executed it will begin by executing
1109a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1110Think of it as a goto with an intervening core dump and reincarnation.
1111If C<LABEL> is omitted, restarts the program from the top.
1112
1113B<WARNING>: Any files opened at the time of the dump will I<not>
1114be open any more when the program is reincarnated, with possible
1115resulting confusion on the part of Perl.
1116
1117This function is now largely obsolete, partly because it's very
1118hard to convert a core file into an executable, and because the
1119real compiler backends for generating portable bytecode and compilable
1120C code have superseded it.
1121
1122If you're looking to use L<dump> to speed up your program, consider
1123generating bytecode or native C code as described in L<perlcc>. If
1124you're just trying to accelerate a CGI script, consider using the
1125C<mod_perl> extension to B<Apache>, or the CPAN module, Fast::CGI.
1126You might also consider autoloading or selfloading, which at least
1127make your program I<appear> to run faster.
5a964f20 1128
aa689395
PP
1129=item each HASH
1130
5a964f20 1131When called in list context, returns a 2-element list consisting of the
aa689395 1132key and value for the next element of a hash, so that you can iterate over
5a964f20 1133it. When called in scalar context, returns the key for only the "next"
7660c0ab 1134element in the hash. (Note: Keys may be C<"0"> or C<"">, which are logically
2f9daede
TPG
1135false; you may wish to avoid constructs like C<while ($k = each %foo) {}>
1136for this reason.)
1137
ab192400
GS
1138Entries are returned in an apparently random order. The actual random
1139order is subject to change in future versions of perl, but it is guaranteed
19799a22 1140to be in the same order as either the C<keys> or C<values> function
ab192400
GS
1141would produce on the same (unmodified) hash.
1142
1143When the hash is entirely read, a null array is returned in list context
19799a22
GS
1144(which when assigned produces a false (C<0>) value), and C<undef> in
1145scalar context. The next call to C<each> after that will start iterating
1146again. There is a single iterator for each hash, shared by all C<each>,
1147C<keys>, and C<values> function calls in the program; it can be reset by
2f9daede
TPG
1148reading all the elements from the hash, or by evaluating C<keys HASH> or
1149C<values HASH>. If you add or delete elements of a hash while you're
1150iterating over it, you may get entries skipped or duplicated, so don't.
aa689395 1151
f86cebdf 1152The following prints out your environment like the printenv(1) program,
aa689395 1153only in a different order:
a0d0e21e
LW
1154
1155 while (($key,$value) = each %ENV) {
1156 print "$key=$value\n";
1157 }
1158
19799a22 1159See also C<keys>, C<values> and C<sort>.
a0d0e21e
LW
1160
1161=item eof FILEHANDLE
1162
4633a7c4
LW
1163=item eof ()
1164
a0d0e21e
LW
1165=item eof
1166
1167Returns 1 if the next read on FILEHANDLE will return end of file, or if
1168FILEHANDLE is not open. FILEHANDLE may be an expression whose value
5a964f20 1169gives the real filehandle. (Note that this function actually
19799a22 1170reads a character and then C<ungetc>s it, so isn't very useful in an
748a9306 1171interactive context.) Do not read from a terminal file (or call
19799a22 1172C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
748a9306
LW
1173as terminals may lose the end-of-file condition if you do.
1174
1175An C<eof> without an argument uses the last file read as argument.
2b5ab1e7 1176Using C<eof()> with empty parentheses is very different. It indicates
19799a22
GS
1177the pseudo file formed of the files listed on the command line,
1178i.e., C<eof()> is reasonable to use inside a C<while (E<lt>E<gt>)>
1179loop to detect the end of only the last file. Use C<eof(ARGV)> or
1180C<eof> without the parentheses to test I<each> file in a while
1181(E<lt>E<gt>) loop. Examples:
a0d0e21e 1182
748a9306
LW
1183 # reset line numbering on each input file
1184 while (<>) {
5a964f20 1185 next if /^\s*#/; # skip comments
748a9306 1186 print "$.\t$_";
5a964f20
TC
1187 } continue {
1188 close ARGV if eof; # Not eof()!
748a9306
LW
1189 }
1190
a0d0e21e
LW
1191 # insert dashes just before last line of last file
1192 while (<>) {
5a964f20 1193 if (eof()) { # check for end of current file
a0d0e21e 1194 print "--------------\n";
2b5ab1e7 1195 close(ARGV); # close or last; is needed if we
748a9306 1196 # are reading from the terminal
a0d0e21e
LW
1197 }
1198 print;
1199 }
1200
a0d0e21e 1201Practical hint: you almost never need to use C<eof> in Perl, because the
3b02c43c
GS
1202input operators return false values when they run out of data, or if there
1203was an error.
a0d0e21e
LW
1204
1205=item eval EXPR
1206
1207=item eval BLOCK
1208
c7cc6f1c
GS
1209In the first form, the return value of EXPR is parsed and executed as if it
1210were a little Perl program. The value of the expression (which is itself
5a964f20 1211determined within scalar context) is first parsed, and if there weren't any
c7cc6f1c 1212errors, executed in the context of the current Perl program, so that any
5f05dabc 1213variable settings or subroutine and format definitions remain afterwards.
c7cc6f1c
GS
1214Note that the value is parsed every time the eval executes. If EXPR is
1215omitted, evaluates C<$_>. This form is typically used to delay parsing
1216and subsequent execution of the text of EXPR until run time.
1217
1218In the second form, the code within the BLOCK is parsed only once--at the
1219same time the code surrounding the eval itself was parsed--and executed
1220within the context of the current Perl program. This form is typically
1221used to trap exceptions more efficiently than the first (see below), while
1222also providing the benefit of checking the code within BLOCK at compile
1223time.
1224
1225The final semicolon, if any, may be omitted from the value of EXPR or within
1226the BLOCK.
1227
1228In both forms, the value returned is the value of the last expression
5a964f20 1229evaluated inside the mini-program; a return statement may be also used, just
c7cc6f1c 1230as with subroutines. The expression providing the return value is evaluated
5a964f20 1231in void, scalar, or list context, depending on the context of the eval itself.
c7cc6f1c 1232See L</wantarray> for more on how the evaluation context can be determined.
a0d0e21e 1233
19799a22
GS
1234If there is a syntax error or runtime error, or a C<die> statement is
1235executed, an undefined value is returned by C<eval>, and C<$@> is set to the
a0d0e21e 1236error message. If there was no error, C<$@> is guaranteed to be a null
19799a22 1237string. Beware that using C<eval> neither silences perl from printing
c7cc6f1c
GS
1238warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1239To do either of those, you have to use the C<$SIG{__WARN__}> facility. See
1240L</warn> and L<perlvar>.
a0d0e21e 1241
19799a22
GS
1242Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1243determining whether a particular feature (such as C<socket> or C<symlink>)
a0d0e21e
LW
1244is implemented. It is also Perl's exception trapping mechanism, where
1245the die operator is used to raise exceptions.
1246
1247If the code to be executed doesn't vary, you may use the eval-BLOCK
1248form to trap run-time errors without incurring the penalty of
1249recompiling each time. The error, if any, is still returned in C<$@>.
1250Examples:
1251
54310121 1252 # make divide-by-zero nonfatal
a0d0e21e
LW
1253 eval { $answer = $a / $b; }; warn $@ if $@;
1254
1255 # same thing, but less efficient
1256 eval '$answer = $a / $b'; warn $@ if $@;
1257
1258 # a compile-time error
5a964f20 1259 eval { $answer = }; # WRONG
a0d0e21e
LW
1260
1261 # a run-time error
1262 eval '$answer ='; # sets $@
1263
2b5ab1e7
TC
1264Due to the current arguably broken state of C<__DIE__> hooks, when using
1265the C<eval{}> form as an exception trap in libraries, you may wish not
1266to trigger any C<__DIE__> hooks that user code may have installed.
1267You can use the C<local $SIG{__DIE__}> construct for this purpose,
1268as shown in this example:
774d564b
PP
1269
1270 # a very private exception trap for divide-by-zero
f86cebdf
GS
1271 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1272 warn $@ if $@;
774d564b
PP
1273
1274This is especially significant, given that C<__DIE__> hooks can call
19799a22 1275C<die> again, which has the effect of changing their error messages:
774d564b
PP
1276
1277 # __DIE__ hooks may modify error messages
1278 {
f86cebdf
GS
1279 local $SIG{'__DIE__'} =
1280 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
c7cc6f1c
GS
1281 eval { die "foo lives here" };
1282 print $@ if $@; # prints "bar lives here"
774d564b
PP
1283 }
1284
19799a22 1285Because this promotes action at a distance, this counterintuitive behavior
2b5ab1e7
TC
1286may be fixed in a future release.
1287
19799a22 1288With an C<eval>, you should be especially careful to remember what's
a0d0e21e
LW
1289being looked at when:
1290
1291 eval $x; # CASE 1
1292 eval "$x"; # CASE 2
1293
1294 eval '$x'; # CASE 3
1295 eval { $x }; # CASE 4
1296
5a964f20 1297 eval "\$$x++"; # CASE 5
a0d0e21e
LW
1298 $$x++; # CASE 6
1299
2f9daede 1300Cases 1 and 2 above behave identically: they run the code contained in
19799a22 1301the variable $x. (Although case 2 has misleading double quotes making
2f9daede 1302the reader wonder what else might be happening (nothing is).) Cases 3
7660c0ab 1303and 4 likewise behave in the same way: they run the code C<'$x'>, which
19799a22 1304does nothing but return the value of $x. (Case 4 is preferred for
2f9daede
TPG
1305purely visual reasons, but it also has the advantage of compiling at
1306compile-time instead of at run-time.) Case 5 is a place where
19799a22 1307normally you I<would> like to use double quotes, except that in this
2f9daede
TPG
1308particular situation, you can just use symbolic references instead, as
1309in case 6.
a0d0e21e 1310
4968c1e4 1311C<eval BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7 1312C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
4968c1e4 1313
a0d0e21e
LW
1314=item exec LIST
1315
8bf3b016
GS
1316=item exec PROGRAM LIST
1317
19799a22
GS
1318The C<exec> function executes a system command I<and never returns>--
1319use C<system> instead of C<exec> if you want it to return. It fails and
1320returns false only if the command does not exist I<and> it is executed
fb73857a 1321directly instead of via your system's command shell (see below).
a0d0e21e 1322
19799a22
GS
1323Since it's a common mistake to use C<exec> instead of C<system>, Perl
1324warns you if there is a following statement which isn't C<die>, C<warn>,
1325or C<exit> (if C<-w> is set - but you always do that). If you
1326I<really> want to follow an C<exec> with some other statement, you
55d729e4
GS
1327can use one of these styles to avoid the warning:
1328
5a964f20
TC
1329 exec ('foo') or print STDERR "couldn't exec foo: $!";
1330 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
55d729e4 1331
5a964f20 1332If there is more than one argument in LIST, or if LIST is an array
f86cebdf 1333with more than one value, calls execvp(3) with the arguments in LIST.
5a964f20
TC
1334If there is only one scalar argument or an array with one element in it,
1335the argument is checked for shell metacharacters, and if there are any,
1336the entire argument is passed to the system's command shell for parsing
1337(this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1338If there are no shell metacharacters in the argument, it is split into
19799a22
GS
1339words and passed directly to C<execvp>, which is more efficient.
1340Examples:
a0d0e21e 1341
19799a22
GS
1342 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1343 exec "sort $outfile | uniq";
a0d0e21e
LW
1344
1345If you don't really want to execute the first argument, but want to lie
1346to the program you are executing about its own name, you can specify
1347the program you actually want to run as an "indirect object" (without a
1348comma) in front of the LIST. (This always forces interpretation of the
54310121 1349LIST as a multivalued list, even if there is only a single scalar in
a0d0e21e
LW
1350the list.) Example:
1351
1352 $shell = '/bin/csh';
1353 exec $shell '-sh'; # pretend it's a login shell
1354
1355or, more directly,
1356
1357 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1358
bb32b41a
GS
1359When the arguments get executed via the system shell, results will
1360be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1361for details.
1362
19799a22
GS
1363Using an indirect object with C<exec> or C<system> is also more
1364secure. This usage (which also works fine with system()) forces
1365interpretation of the arguments as a multivalued list, even if the
1366list had just one argument. That way you're safe from the shell
1367expanding wildcards or splitting up words with whitespace in them.
5a964f20
TC
1368
1369 @args = ( "echo surprise" );
1370
2b5ab1e7 1371 exec @args; # subject to shell escapes
f86cebdf 1372 # if @args == 1
2b5ab1e7 1373 exec { $args[0] } @args; # safe even with one-arg list
5a964f20
TC
1374
1375The first version, the one without the indirect object, ran the I<echo>
1376program, passing it C<"surprise"> an argument. The second version
1377didn't--it tried to run a program literally called I<"echo surprise">,
1378didn't find it, and set C<$?> to a non-zero value indicating failure.
1379
19799a22 1380Note that C<exec> will not call your C<END> blocks, nor will it call
7660c0ab
A
1381any C<DESTROY> methods in your objects.
1382
a0d0e21e
LW
1383=item exists EXPR
1384
19799a22 1385Returns true if the specified hash key exists in its hash, even
a0d0e21e
LW
1386if the corresponding value is undefined.
1387
2b5ab1e7
TC
1388 print "Exists\n" if exists $array{$key};
1389 print "Defined\n" if defined $array{$key};
1390 print "True\n" if $array{$key};
a0d0e21e 1391
19799a22 1392A hash element can be true only if it's defined, and defined if
a0d0e21e
LW
1393it exists, but the reverse doesn't necessarily hold true.
1394
1395Note that the EXPR can be arbitrarily complicated as long as the final
1396operation is a hash key lookup:
1397
2b5ab1e7
TC
1398 if (exists $ref->{A}->{B}->{$key}) { }
1399 if (exists $hash{A}{B}{$key}) { }
1400
1401Although the last element will not spring into existence just because
1402its existence was tested, intervening ones will. Thus C<$ref-E<gt>{"A"}>
1403and C<$ref-E<gt>{"A"}-E<gt>{"B"}> will spring into existence due to the
1404existence test for a $key element. This happens anywhere the arrow
1405operator is used, including even
5a964f20 1406
2b5ab1e7
TC
1407 undef $ref;
1408 if (exists $ref->{"Some key"}) { }
1409 print $ref; # prints HASH(0x80d3d5c)
1410
1411This surprising autovivification in what does not at first--or even
1412second--glance appear to be an lvalue context may be fixed in a future
5a964f20 1413release.
a0d0e21e
LW
1414
1415=item exit EXPR
1416
2b5ab1e7 1417Evaluates EXPR and exits immediately with that value. Example:
a0d0e21e
LW
1418
1419 $ans = <STDIN>;
1420 exit 0 if $ans =~ /^[Xx]/;
1421
19799a22 1422See also C<die>. If EXPR is omitted, exits with C<0> status. The only
2b5ab1e7
TC
1423universally recognized values for EXPR are C<0> for success and C<1>
1424for error; other values are subject to interpretation depending on the
1425environment in which the Perl program is running. For example, exiting
142669 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1427the mailer to return the item undelivered, but that's not true everywhere.
a0d0e21e 1428
19799a22
GS
1429Don't use C<exit> to abort a subroutine if there's any chance that
1430someone might want to trap whatever error happened. Use C<die> instead,
1431which can be trapped by an C<eval>.
28757baa 1432
19799a22 1433The exit() function does not always exit immediately. It calls any
2b5ab1e7 1434defined C<END> routines first, but these C<END> routines may not
19799a22 1435themselves abort the exit. Likewise any object destructors that need to
2b5ab1e7
TC
1436be called are called before the real exit. If this is a problem, you
1437can call C<POSIX:_exit($status)> to avoid END and destructor processing.
87275199 1438See L<perlmod> for details.
5a964f20 1439
a0d0e21e
LW
1440=item exp EXPR
1441
54310121 1442=item exp
bbce6d69 1443
2b5ab1e7 1444Returns I<e> (the natural logarithm base) to the power of EXPR.
a0d0e21e
LW
1445If EXPR is omitted, gives C<exp($_)>.
1446
1447=item fcntl FILEHANDLE,FUNCTION,SCALAR
1448
f86cebdf 1449Implements the fcntl(2) function. You'll probably have to say
a0d0e21e
LW
1450
1451 use Fcntl;
1452
0ade1984 1453first to get the correct constant definitions. Argument processing and
19799a22 1454value return works just like C<ioctl> below.
a0d0e21e
LW
1455For example:
1456
1457 use Fcntl;
5a964f20
TC
1458 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1459 or die "can't fcntl F_GETFL: $!";
1460
19799a22
GS
1461You don't have to check for C<defined> on the return from C<fnctl>.
1462Like C<ioctl>, it maps a C<0> return from the system call into C<"0
1463but true"> in Perl. This string is true in boolean context and C<0>
2b5ab1e7
TC
1464in numeric context. It is also exempt from the normal B<-w> warnings
1465on improper numeric conversions.
5a964f20 1466
19799a22 1467Note that C<fcntl> will produce a fatal error if used on a machine that
2b5ab1e7
TC
1468doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1469manpage to learn what functions are available on your system.
a0d0e21e
LW
1470
1471=item fileno FILEHANDLE
1472
2b5ab1e7
TC
1473Returns the file descriptor for a filehandle, or undefined if the
1474filehandle is not open. This is mainly useful for constructing
19799a22 1475bitmaps for C<select> and low-level POSIX tty-handling operations.
2b5ab1e7
TC
1476If FILEHANDLE is an expression, the value is taken as an indirect
1477filehandle, generally its name.
5a964f20
TC
1478
1479You can use this to find out whether two handles refer to the
1480same underlying descriptor:
1481
1482 if (fileno(THIS) == fileno(THAT)) {
1483 print "THIS and THAT are dups\n";
1484 }
a0d0e21e
LW
1485
1486=item flock FILEHANDLE,OPERATION
1487
19799a22
GS
1488Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1489for success, false on failure. Produces a fatal error if used on a
2b5ab1e7 1490machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
19799a22 1491C<flock> is Perl's portable file locking interface, although it locks
2b5ab1e7
TC
1492only entire files, not records.
1493
1494Two potentially non-obvious but traditional C<flock> semantics are
1495that it waits indefinitely until the lock is granted, and that its locks
1496B<merely advisory>. Such discretionary locks are more flexible, but offer
19799a22
GS
1497fewer guarantees. This means that files locked with C<flock> may be
1498modified by programs that do not also use C<flock>. See L<perlport>,
2b5ab1e7
TC
1499your port's specific documentation, or your system-specific local manpages
1500for details. It's best to assume traditional behavior if you're writing
1501portable programs. (But if you're not, you should as always feel perfectly
1502free to write for your own system's idiosyncrasies (sometimes called
1503"features"). Slavish adherence to portability concerns shouldn't get
1504in the way of your getting your job done.)
a3cb178b 1505
8ebc5c01
PP
1506OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1507LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
68dc0745
PP
1508you can use the symbolic names if import them from the Fcntl module,
1509either individually, or as a group using the ':flock' tag. LOCK_SH
1510requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1511releases a previously requested lock. If LOCK_NB is added to LOCK_SH or
19799a22 1512LOCK_EX then C<flock> will return immediately rather than blocking
68dc0745
PP
1513waiting for the lock (check the return status to see if you got it).
1514
2b5ab1e7
TC
1515To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1516before locking or unlocking it.
8ebc5c01 1517
f86cebdf 1518Note that the emulation built with lockf(3) doesn't provide shared
8ebc5c01 1519locks, and it requires that FILEHANDLE be open with write intent. These
2b5ab1e7 1520are the semantics that lockf(3) implements. Most if not all systems
f86cebdf 1521implement lockf(3) in terms of fcntl(2) locking, though, so the
8ebc5c01
PP
1522differing semantics shouldn't bite too many people.
1523
19799a22
GS
1524Note also that some versions of C<flock> cannot lock things over the
1525network; you would need to use the more system-specific C<fcntl> for
f86cebdf
GS
1526that. If you like you can force Perl to ignore your system's flock(2)
1527function, and so provide its own fcntl(2)-based emulation, by passing
8ebc5c01
PP
1528the switch C<-Ud_flock> to the F<Configure> program when you configure
1529perl.
4633a7c4
LW
1530
1531Here's a mailbox appender for BSD systems.
a0d0e21e 1532
7e1af8bc 1533 use Fcntl ':flock'; # import LOCK_* constants
a0d0e21e
LW
1534
1535 sub lock {
7e1af8bc 1536 flock(MBOX,LOCK_EX);
a0d0e21e
LW
1537 # and, in case someone appended
1538 # while we were waiting...
1539 seek(MBOX, 0, 2);
1540 }
1541
1542 sub unlock {
7e1af8bc 1543 flock(MBOX,LOCK_UN);
a0d0e21e
LW
1544 }
1545
1546 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1547 or die "Can't open mailbox: $!";
1548
1549 lock();
1550 print MBOX $msg,"\n\n";
1551 unlock();
1552
2b5ab1e7
TC
1553On systems that support a real flock(), locks are inherited across fork()
1554calls, whereas those that must resort to the more capricious fcntl()
1555function lose the locks, making it harder to write servers.
1556
cb1a09d0 1557See also L<DB_File> for other flock() examples.
a0d0e21e
LW
1558
1559=item fork
1560
2b5ab1e7
TC
1561Does a fork(2) system call to create a new process running the
1562same program at the same point. It returns the child pid to the
1563parent process, C<0> to the child process, or C<undef> if the fork is
1564unsuccessful. File descriptors (and sometimes locks on those descriptors)
1565are shared, while everything else is copied. On most systems supporting
1566fork(), great care has gone into making it extremely efficient (for
1567example, using copy-on-write technology on data pages), making it the
1568dominant paradigm for multitasking over the last few decades.
5a964f20 1569
45bc9206 1570All files opened for output are flushed before forking the child process.
a0d0e21e 1571
19799a22 1572If you C<fork> without ever waiting on your children, you will
2b5ab1e7
TC
1573accumulate zombies. On some systems, you can avoid this by setting
1574C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1575forking and reaping moribund children.
cb1a09d0 1576
28757baa
PP
1577Note that if your forked child inherits system file descriptors like
1578STDIN and STDOUT that are actually connected by a pipe or socket, even
2b5ab1e7 1579if you exit, then the remote server (such as, say, a CGI script or a
19799a22 1580backgrounded job launched from a remote shell) won't think you're done.
2b5ab1e7 1581You should reopen those to F</dev/null> if it's any issue.
28757baa 1582
cb1a09d0
AD
1583=item format
1584
19799a22 1585Declare a picture format for use by the C<write> function. For
cb1a09d0
AD
1586example:
1587
54310121 1588 format Something =
cb1a09d0
AD
1589 Test: @<<<<<<<< @||||| @>>>>>
1590 $str, $%, '$' . int($num)
1591 .
1592
1593 $str = "widget";
184e9718 1594 $num = $cost/$quantity;
cb1a09d0
AD
1595 $~ = 'Something';
1596 write;
1597
1598See L<perlform> for many details and examples.
1599
8903cb82 1600=item formline PICTURE,LIST
a0d0e21e 1601
5a964f20 1602This is an internal function used by C<format>s, though you may call it,
a0d0e21e
LW
1603too. It formats (see L<perlform>) a list of values according to the
1604contents of PICTURE, placing the output into the format output
7660c0ab 1605accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
19799a22 1606Eventually, when a C<write> is done, the contents of
a0d0e21e 1607C<$^A> are written to some filehandle, but you could also read C<$^A>
7660c0ab 1608yourself and then set C<$^A> back to C<"">. Note that a format typically
19799a22 1609does one C<formline> per line of form, but the C<formline> function itself
748a9306 1610doesn't care how many newlines are embedded in the PICTURE. This means
4633a7c4 1611that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
748a9306
LW
1612You may therefore need to use multiple formlines to implement a single
1613record format, just like the format compiler.
1614
19799a22 1615Be careful if you put double quotes around the picture, because an C<@>
748a9306 1616character may be taken to mean the beginning of an array name.
19799a22 1617C<formline> always returns true. See L<perlform> for other examples.
a0d0e21e
LW
1618
1619=item getc FILEHANDLE
1620
1621=item getc
1622
1623Returns the next character from the input file attached to FILEHANDLE,
2b5ab1e7
TC
1624or the undefined value at end of file, or if there was an error.
1625If FILEHANDLE is omitted, reads from STDIN. This is not particularly
1626efficient. However, it cannot be used by itself to fetch single
1627characters without waiting for the user to hit enter. For that, try
1628something more like:
4633a7c4
LW
1629
1630 if ($BSD_STYLE) {
1631 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1632 }
1633 else {
54310121 1634 system "stty", '-icanon', 'eol', "\001";
4633a7c4
LW
1635 }
1636
1637 $key = getc(STDIN);
1638
1639 if ($BSD_STYLE) {
1640 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1641 }
1642 else {
5f05dabc 1643 system "stty", 'icanon', 'eol', '^@'; # ASCII null
4633a7c4
LW
1644 }
1645 print "\n";
1646
54310121
PP
1647Determination of whether $BSD_STYLE should be set
1648is left as an exercise to the reader.
cb1a09d0 1649
19799a22 1650The C<POSIX::getattr> function can do this more portably on
2b5ab1e7
TC
1651systems purporting POSIX compliance. See also the C<Term::ReadKey>
1652module from your nearest CPAN site; details on CPAN can be found on
1653L<perlmodlib/CPAN>.
a0d0e21e
LW
1654
1655=item getlogin
1656
5a964f20
TC
1657Implements the C library function of the same name, which on most
1658systems returns the current login from F</etc/utmp>, if any. If null,
19799a22 1659use C<getpwuid>.
a0d0e21e 1660
f86702cc 1661 $login = getlogin || getpwuid($<) || "Kilroy";
a0d0e21e 1662
19799a22
GS
1663Do not consider C<getlogin> for authentication: it is not as
1664secure as C<getpwuid>.
4633a7c4 1665
a0d0e21e
LW
1666=item getpeername SOCKET
1667
1668Returns the packed sockaddr address of other end of the SOCKET connection.
1669
4633a7c4
LW
1670 use Socket;
1671 $hersockaddr = getpeername(SOCK);
19799a22 1672 ($port, $iaddr) = sockaddr_in($hersockaddr);
4633a7c4
LW
1673 $herhostname = gethostbyaddr($iaddr, AF_INET);
1674 $herstraddr = inet_ntoa($iaddr);
a0d0e21e
LW
1675
1676=item getpgrp PID
1677
47e29363 1678Returns the current process group for the specified PID. Use
7660c0ab 1679a PID of C<0> to get the current process group for the
4633a7c4 1680current process. Will raise an exception if used on a machine that
f86cebdf 1681doesn't implement getpgrp(2). If PID is omitted, returns process
19799a22 1682group of current process. Note that the POSIX version of C<getpgrp>
7660c0ab 1683does not accept a PID argument, so only C<PID==0> is truly portable.
a0d0e21e
LW
1684
1685=item getppid
1686
1687Returns the process id of the parent process.
1688
1689=item getpriority WHICH,WHO
1690
4633a7c4
LW
1691Returns the current priority for a process, a process group, or a user.
1692(See L<getpriority(2)>.) Will raise a fatal exception if used on a
f86cebdf 1693machine that doesn't implement getpriority(2).
a0d0e21e
LW
1694
1695=item getpwnam NAME
1696
1697=item getgrnam NAME
1698
1699=item gethostbyname NAME
1700
1701=item getnetbyname NAME
1702
1703=item getprotobyname NAME
1704
1705=item getpwuid UID
1706
1707=item getgrgid GID
1708
1709=item getservbyname NAME,PROTO
1710
1711=item gethostbyaddr ADDR,ADDRTYPE
1712
1713=item getnetbyaddr ADDR,ADDRTYPE
1714
1715=item getprotobynumber NUMBER
1716
1717=item getservbyport PORT,PROTO
1718
1719=item getpwent
1720
1721=item getgrent
1722
1723=item gethostent
1724
1725=item getnetent
1726
1727=item getprotoent
1728
1729=item getservent
1730
1731=item setpwent
1732
1733=item setgrent
1734
1735=item sethostent STAYOPEN
1736
1737=item setnetent STAYOPEN
1738
1739=item setprotoent STAYOPEN
1740
1741=item setservent STAYOPEN
1742
1743=item endpwent
1744
1745=item endgrent
1746
1747=item endhostent
1748
1749=item endnetent
1750
1751=item endprotoent
1752
1753=item endservent
1754
1755These routines perform the same functions as their counterparts in the
5a964f20 1756system library. In list context, the return values from the
a0d0e21e
LW
1757various get routines are as follows:
1758
1759 ($name,$passwd,$uid,$gid,
6ee623d5 1760 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
a0d0e21e
LW
1761 ($name,$passwd,$gid,$members) = getgr*
1762 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1763 ($name,$aliases,$addrtype,$net) = getnet*
1764 ($name,$aliases,$proto) = getproto*
1765 ($name,$aliases,$port,$proto) = getserv*
1766
1767(If the entry doesn't exist you get a null list.)
1768
5a964f20 1769In scalar context, you get the name, unless the function was a
a0d0e21e
LW
1770lookup by name, in which case you get the other thing, whatever it is.
1771(If the entry doesn't exist you get the undefined value.) For example:
1772
5a964f20
TC
1773 $uid = getpwnam($name);
1774 $name = getpwuid($num);
1775 $name = getpwent();
1776 $gid = getgrnam($name);
1777 $name = getgrgid($num;
1778 $name = getgrent();
1779 #etc.
a0d0e21e 1780
19799a22 1781In I<getpw*()> the fields $quota, $comment, and $expire are
2b5ab1e7 1782special cases in the sense that in many systems they are unsupported.
19799a22
GS
1783If the $quota is unsupported, it is an empty scalar. If it is
1784supported, it usually encodes the disk quota. If the $comment
2b5ab1e7
TC
1785field is unsupported, it is an empty scalar. If it is supported it
1786usually encodes some administrative comment about the user. In some
19799a22
GS
1787systems the $quota field may be $change or $age, fields that have
1788to do with password aging. In some systems the $comment field may
1789be $class. The $expire field, if present, encodes the expiration
2b5ab1e7
TC
1790period of the account or the password. For the availability and the
1791exact meaning of these fields in your system, please consult your
1792getpwnam(3) documentation and your F<pwd.h> file. You can also find
19799a22
GS
1793out from within Perl what your $quota and $comment fields mean
1794and whether you have the $expire field by using the C<Config> module
2b5ab1e7
TC
1795and the values C<d_pwquota>, C<d_pwage>, C<d_pwchange>, C<d_pwcomment>,
1796and C<d_pwexpire>. Shadow password files are only supported if your
1797vendor has implemented them in the intuitive fashion that calling the
1798regular C library routines gets the shadow versions if you're running
1799under privilege. Those that incorrectly implement a separate library
1800call are not supported.
6ee623d5 1801
19799a22 1802The $members value returned by I<getgr*()> is a space separated list of
a0d0e21e
LW
1803the login names of the members of the group.
1804
1805For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1806C, it will be returned to you via C<$?> if the function call fails. The
7660c0ab 1807C<@addrs> value returned by a successful call is a list of the raw
a0d0e21e
LW
1808addresses returned by the corresponding system library call. In the
1809Internet domain, each address is four bytes long and you can unpack it
1810by saying something like:
1811
1812 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1813
2b5ab1e7
TC
1814The Socket library makes this slightly easier:
1815
1816 use Socket;
1817 $iaddr = inet_aton("127.1"); # or whatever address
1818 $name = gethostbyaddr($iaddr, AF_INET);
1819
1820 # or going the other way
19799a22 1821 $straddr = inet_ntoa($iaddr);
2b5ab1e7 1822
19799a22
GS
1823If you get tired of remembering which element of the return list
1824contains which return value, by-name interfaces are provided
1825in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
1826C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
1827and C<User::grent>. These override the normal built-ins, supplying
1828versions that return objects with the appropriate names
1829for each field. For example:
5a964f20
TC
1830
1831 use File::stat;
1832 use User::pwent;
1833 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
1834
1835Even though it looks like they're the same method calls (uid),
19799a22
GS
1836they aren't, because a C<File::stat> object is different from
1837a C<User::pwent> object.
5a964f20 1838
a0d0e21e
LW
1839=item getsockname SOCKET
1840
19799a22
GS
1841Returns the packed sockaddr address of this end of the SOCKET connection,
1842in case you don't know the address because you have several different
1843IPs that the connection might have come in on.
a0d0e21e 1844
4633a7c4
LW
1845 use Socket;
1846 $mysockaddr = getsockname(SOCK);
19799a22
GS
1847 ($port, $myaddr) = sockaddr_in($mysockaddr);
1848 printf "Connect to %s [%s]\n",
1849 scalar gethostbyaddr($myaddr, AF_INET),
1850 inet_ntoa($myaddr);
a0d0e21e
LW
1851
1852=item getsockopt SOCKET,LEVEL,OPTNAME
1853
5a964f20 1854Returns the socket option requested, or undef if there is an error.
a0d0e21e
LW
1855
1856=item glob EXPR
1857
0a753a76
PP
1858=item glob
1859
2b5ab1e7
TC
1860Returns the value of EXPR with filename expansions such as the
1861standard Unix shell F</bin/csh> would do. This is the internal function
1862implementing the C<E<lt>*.cE<gt>> operator, but you can use it directly.
1863If EXPR is omitted, C<$_> is used. The C<E<lt>*.cE<gt>> operator is
1864discussed in more detail in L<perlop/"I/O Operators">.
a0d0e21e
LW
1865
1866=item gmtime EXPR
1867
19799a22 1868Converts a time as returned by the time function to a 9-element list
54310121 1869with the time localized for the standard Greenwich time zone.
4633a7c4 1870Typically used as follows:
a0d0e21e 1871
54310121 1872 # 0 1 2 3 4 5 6 7 8
a0d0e21e
LW
1873 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1874 gmtime(time);
1875
19799a22
GS
1876All list elements are numeric, and come straight out of a struct tm.
1877In particular this means that $mon has the range C<0..11> and $wday
1878has the range C<0..6> with sunday as day C<0>. Also, $year is the
1879number of years since 1900, that is, $year is C<123> in year 2023,
2b5ab1e7
TC
1880I<not> simply the last two digits of the year. If you assume it is,
1881then you create non-Y2K-compliant programs--and you wouldn't want to do
1882that, would you?
2f9daede
TPG
1883
1884If EXPR is omitted, does C<gmtime(time())>.
a0d0e21e 1885
f86cebdf 1886In scalar context, returns the ctime(3) value:
0a753a76
PP
1887
1888 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
1889
19799a22 1890Also see the C<timegm> function provided by the C<Time::Local> module,
f86cebdf 1891and the strftime(3) function available via the POSIX module.
7660c0ab 1892
2b5ab1e7
TC
1893This scalar value is B<not> locale dependent (see L<perllocale>), but
1894is instead a Perl builtin. Also see the C<Time::Local> module, and the
1895strftime(3) and mktime(3) functions available via the POSIX module. To
7660c0ab
A
1896get somewhat similar but locale dependent date strings, set up your
1897locale environment variables appropriately (please see L<perllocale>)
1898and try for example:
1899
1900 use POSIX qw(strftime);
2b5ab1e7 1901 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
7660c0ab 1902
2b5ab1e7
TC
1903Note that the C<%a> and C<%b> escapes, which represent the short forms
1904of the day of the week and the month of the year, may not necessarily
1905be three characters wide in all locales.
0a753a76 1906
a0d0e21e
LW
1907=item goto LABEL
1908
748a9306
LW
1909=item goto EXPR
1910
a0d0e21e
LW
1911=item goto &NAME
1912
7660c0ab 1913The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
a0d0e21e 1914execution there. It may not be used to go into any construct that
7660c0ab 1915requires initialization, such as a subroutine or a C<foreach> loop. It
0a753a76 1916also can't be used to go into a construct that is optimized away,
19799a22 1917or to get out of a block or subroutine given to C<sort>.
0a753a76 1918It can be used to go almost anywhere else within the dynamic scope,
a0d0e21e 1919including out of subroutines, but it's usually better to use some other
19799a22 1920construct such as C<last> or C<die>. The author of Perl has never felt the
7660c0ab 1921need to use this form of C<goto> (in Perl, that is--C is another matter).
a0d0e21e 1922
7660c0ab
A
1923The C<goto-EXPR> form expects a label name, whose scope will be resolved
1924dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
748a9306
LW
1925necessarily recommended if you're optimizing for maintainability:
1926
1927 goto ("FOO", "BAR", "GLARCH")[$i];
1928
7660c0ab 1929The C<goto-&NAME> form is highly magical, and substitutes a call to the
a0d0e21e 1930named subroutine for the currently running subroutine. This is used by
7660c0ab 1931C<AUTOLOAD> subroutines that wish to load another subroutine and then
a0d0e21e 1932pretend that the other subroutine had been called in the first place
7660c0ab 1933(except that any modifications to C<@_> in the current subroutine are
19799a22 1934propagated to the other subroutine.) After the C<goto>, not even C<caller>
a0d0e21e
LW
1935will be able to tell that this routine was called first.
1936
1937=item grep BLOCK LIST
1938
1939=item grep EXPR,LIST
1940
2b5ab1e7
TC
1941This is similar in spirit to, but not the same as, grep(1) and its
1942relatives. In particular, it is not limited to using regular expressions.
2f9daede 1943
a0d0e21e 1944Evaluates the BLOCK or EXPR for each element of LIST (locally setting
7660c0ab 1945C<$_> to each element) and returns the list value consisting of those
19799a22
GS
1946elements for which the expression evaluated to true. In scalar
1947context, returns the number of times the expression was true.
a0d0e21e
LW
1948
1949 @foo = grep(!/^#/, @bar); # weed out comments
1950
1951or equivalently,
1952
1953 @foo = grep {!/^#/} @bar; # weed out comments
1954
2b5ab1e7
TC
1955Note that, because C<$_> is a reference into the list value, it can
1956be used to modify the elements of the array. While this is useful and
1957supported, it can cause bizarre results if the LIST is not a named array.
1958Similarly, grep returns aliases into the original list, much as a for
1959loop's index variable aliases the list elements. That is, modifying an
19799a22
GS
1960element of a list returned by grep (for example, in a C<foreach>, C<map>
1961or another C<grep>) actually modifies the element in the original list.
2b5ab1e7 1962This is usually something to be avoided when writing clear code.
a0d0e21e 1963
19799a22 1964See also L</map> for a list composed of the results of the BLOCK or EXPR.
38325410 1965
a0d0e21e
LW
1966=item hex EXPR
1967
54310121 1968=item hex
bbce6d69 1969
2b5ab1e7
TC
1970Interprets EXPR as a hex string and returns the corresponding value.
1971(To convert strings that might start with either 0, 0x, or 0b, see
1972L</oct>.) If EXPR is omitted, uses C<$_>.
2f9daede
TPG
1973
1974 print hex '0xAf'; # prints '175'
1975 print hex 'aF'; # same
a0d0e21e 1976
19799a22
GS
1977Hex strings may only represent integers. Strings that would cause
1978integer overflow trigger a mandatory error message.
1979
a0d0e21e
LW
1980=item import
1981
19799a22 1982There is no builtin C<import> function. It is just an ordinary
4633a7c4 1983method (subroutine) defined (or inherited) by modules that wish to export
19799a22 1984names to another module. The C<use> function calls the C<import> method
54310121 1985for the package used. See also L</use()>, L<perlmod>, and L<Exporter>.
a0d0e21e
LW
1986
1987=item index STR,SUBSTR,POSITION
1988
1989=item index STR,SUBSTR
1990
2b5ab1e7
TC
1991The index function searches for one string within another, but without
1992the wildcard-like behavior of a full regular-expression pattern match.
1993It returns the position of the first occurrence of SUBSTR in STR at
1994or after POSITION. If POSITION is omitted, starts searching from the
1995beginning of the string. The return value is based at C<0> (or whatever
1996you've set the C<$[> variable to--but don't do that). If the substring
1997is not found, returns one less than the base, ordinarily C<-1>.
a0d0e21e
LW
1998
1999=item int EXPR
2000
54310121 2001=item int
bbce6d69 2002
7660c0ab 2003Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2b5ab1e7
TC
2004You should not use this function for rounding: one because it truncates
2005towards C<0>, and two because machine representations of floating point
2006numbers can sometimes produce counterintuitive results. For example,
2007C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2008because it's really more like -268.99999999999994315658 instead. Usually,
19799a22 2009the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2b5ab1e7 2010functions will serve you better than will int().
a0d0e21e
LW
2011
2012=item ioctl FILEHANDLE,FUNCTION,SCALAR
2013
2b5ab1e7 2014Implements the ioctl(2) function. You'll probably first have to say
a0d0e21e 2015
4633a7c4 2016 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
a0d0e21e 2017
2b5ab1e7 2018to get the correct function definitions. If F<ioctl.ph> doesn't
a0d0e21e 2019exist or doesn't have the correct definitions you'll have to roll your
4633a7c4 2020own, based on your C header files such as F<E<lt>sys/ioctl.hE<gt>>.
5a964f20 2021(There is a Perl script called B<h2ph> that comes with the Perl kit that
54310121 2022may help you in this, but it's nontrivial.) SCALAR will be read and/or
4633a7c4 2023written depending on the FUNCTION--a pointer to the string value of SCALAR
19799a22 2024will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
4633a7c4
LW
2025has no string value but does have a numeric value, that value will be
2026passed rather than a pointer to the string value. To guarantee this to be
19799a22
GS
2027true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2028functions may be needed to manipulate the values of structures used by
2029C<ioctl>.
a0d0e21e 2030
19799a22 2031The return value of C<ioctl> (and C<fcntl>) is as follows:
a0d0e21e
LW
2032
2033 if OS returns: then Perl returns:
2034 -1 undefined value
2035 0 string "0 but true"
2036 anything else that number
2037
19799a22 2038Thus Perl returns true on success and false on failure, yet you can
a0d0e21e
LW
2039still easily determine the actual value returned by the operating
2040system:
2041
2b5ab1e7 2042 $retval = ioctl(...) || -1;
a0d0e21e
LW
2043 printf "System returned %d\n", $retval;
2044
c2611fb3 2045The special string "C<0> but true" is exempt from B<-w> complaints
5a964f20
TC
2046about improper numeric conversions.
2047
19799a22
GS
2048Here's an example of setting a filehandle named C<REMOTE> to be
2049non-blocking at the system level. You'll have to negotiate C<$|>
2050on your own, though.
2051
2052 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
2053
2054 $flags = fcntl(REMOTE, F_GETFL, 0)
2055 or die "Can't get flags for the socket: $!\n";
2056
2057 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
2058 or die "Can't set flags for the socket: $!\n";
2059
a0d0e21e
LW
2060=item join EXPR,LIST
2061
2b5ab1e7
TC
2062Joins the separate strings of LIST into a single string with fields
2063separated by the value of EXPR, and returns that new string. Example:
a0d0e21e 2064
2b5ab1e7 2065 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
a0d0e21e 2066
7660c0ab 2067See L</split>.
a0d0e21e 2068
aa689395
PP
2069=item keys HASH
2070
19799a22 2071Returns a list consisting of all the keys of the named hash. (In
1d2dff63 2072scalar context, returns the number of keys.) The keys are returned in
ab192400
GS
2073an apparently random order. The actual random order is subject to
2074change in future versions of perl, but it is guaranteed to be the same
19799a22 2075order as either the C<values> or C<each> function produces (given
ab192400
GS
2076that the hash has not been modified). As a side effect, it resets
2077HASH's iterator.
a0d0e21e 2078
aa689395 2079Here is yet another way to print your environment:
a0d0e21e
LW
2080
2081 @keys = keys %ENV;
2082 @values = values %ENV;
19799a22 2083 while (@keys) {
a0d0e21e
LW
2084 print pop(@keys), '=', pop(@values), "\n";
2085 }
2086
2087or how about sorted by key:
2088
2089 foreach $key (sort(keys %ENV)) {
2090 print $key, '=', $ENV{$key}, "\n";
2091 }
2092
19799a22 2093To sort a hash by value, you'll need to use a C<sort> function.
aa689395 2094Here's a descending numeric sort of a hash by its values:
4633a7c4 2095
5a964f20 2096 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
4633a7c4
LW
2097 printf "%4d %s\n", $hash{$key}, $key;
2098 }
2099
19799a22 2100As an lvalue C<keys> allows you to increase the number of hash buckets
aa689395
PP
2101allocated for the given hash. This can gain you a measure of efficiency if
2102you know the hash is going to get big. (This is similar to pre-extending
2103an array by assigning a larger number to $#array.) If you say
55497cff
PP
2104
2105 keys %hash = 200;
2106
ab192400
GS
2107then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2108in fact, since it rounds up to the next power of two. These
55497cff
PP
2109buckets will be retained even if you do C<%hash = ()>, use C<undef
2110%hash> if you want to free the storage while C<%hash> is still in scope.
2111You can't shrink the number of buckets allocated for the hash using
19799a22 2112C<keys> in this way (but you needn't worry about doing this by accident,
55497cff
PP
2113as trying has no effect).
2114
19799a22 2115See also C<each>, C<values> and C<sort>.
ab192400 2116
a0d0e21e
LW
2117=item kill LIST
2118
54310121
PP
2119Sends a signal to a list of processes. The first element of
2120the list must be the signal to send. Returns the number of
517db077
GS
2121processes successfully signaled (which is not necessarily the
2122same as the number actually killed).
a0d0e21e
LW
2123
2124 $cnt = kill 1, $child1, $child2;
2125 kill 9, @goners;
2126
4633a7c4
LW
2127Unlike in the shell, in Perl if the I<SIGNAL> is negative, it kills
2128process groups instead of processes. (On System V, a negative I<PROCESS>
2129number will also kill process groups, but that's not portable.) That
2130means you usually want to use positive not negative signals. You may also
da0045b7 2131use a signal name in quotes. See L<perlipc/"Signals"> for details.
a0d0e21e
LW
2132
2133=item last LABEL
2134
2135=item last
2136
2137The C<last> command is like the C<break> statement in C (as used in
2138loops); it immediately exits the loop in question. If the LABEL is
2139omitted, the command refers to the innermost enclosing loop. The
2140C<continue> block, if any, is not executed:
2141
4633a7c4
LW
2142 LINE: while (<STDIN>) {
2143 last LINE if /^$/; # exit when done with header
5a964f20 2144 #...
a0d0e21e
LW
2145 }
2146
4968c1e4 2147C<last> cannot be used to exit a block which returns a value such as
2b5ab1e7
TC
2148C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2149a grep() or map() operation.
4968c1e4 2150
98293880
JH
2151See also L</continue> for an illustration of how C<last>, C<next>, and
2152C<redo> work.
1d2dff63 2153
a0d0e21e
LW
2154=item lc EXPR
2155
54310121 2156=item lc
bbce6d69 2157
a0d0e21e 2158Returns an lowercased version of EXPR. This is the internal function
7660c0ab 2159implementing the C<\L> escape in double-quoted strings.
19799a22
GS
2160Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
2161and L<utf8>.
a0d0e21e 2162
7660c0ab 2163If EXPR is omitted, uses C<$_>.
bbce6d69 2164
a0d0e21e
LW
2165=item lcfirst EXPR
2166
54310121 2167=item lcfirst
bbce6d69 2168
a0d0e21e 2169Returns the value of EXPR with the first character lowercased. This is
7660c0ab 2170the internal function implementing the C<\l> escape in double-quoted strings.
a0ed51b3 2171Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
a0d0e21e 2172
7660c0ab 2173If EXPR is omitted, uses C<$_>.
bbce6d69 2174
a0d0e21e
LW
2175=item length EXPR
2176
54310121 2177=item length
bbce6d69 2178
a0ed51b3 2179Returns the length in characters of the value of EXPR. If EXPR is
2b5ab1e7
TC
2180omitted, returns length of C<$_>. Note that this cannot be used on
2181an entire array or hash to find out how many elements these have.
2182For that, use C<scalar @array> and C<scalar keys %hash> respectively.
a0d0e21e
LW
2183
2184=item link OLDFILE,NEWFILE
2185
19799a22
GS
2186Creates a new filename linked to the old filename. Returns true for
2187success, false otherwise.
a0d0e21e
LW
2188
2189=item listen SOCKET,QUEUESIZE
2190
19799a22
GS
2191Does the same thing that the listen system call does. Returns true if
2192it succeeded, false otherwise. See the example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
2193
2194=item local EXPR
2195
19799a22 2196You really probably want to be using C<my> instead, because C<local> isn't
2b5ab1e7
TC
2197what most people think of as "local". See L<perlsub/"Private Variables
2198via my()"> for details.
2199
5a964f20
TC
2200A local modifies the listed variables to be local to the enclosing
2201block, file, or eval. If more than one value is listed, the list must
2202be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2203for details, including issues with tied arrays and hashes.
a0d0e21e 2204
a0d0e21e
LW
2205=item localtime EXPR
2206
19799a22 2207Converts a time as returned by the time function to a 9-element list
5f05dabc 2208with the time analyzed for the local time zone. Typically used as
a0d0e21e
LW
2209follows:
2210
54310121 2211 # 0 1 2 3 4 5 6 7 8
a0d0e21e
LW
2212 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2213 localtime(time);
2214
19799a22
GS
2215All list elements are numeric, and come straight out of a struct tm.
2216In particular this means that $mon has the range C<0..11> and $wday
2217has the range C<0..6> with sunday as day C<0>. Also, $year is the
2218number of years since 1900, that is, $year is C<123> in year 2023,
2b5ab1e7
TC
2219and I<not> simply the last two digits of the year. If you assume it is,
2220then you create non-Y2K-compliant programs--and you wouldn't want to do
2221that, would you?
54310121
PP
2222
2223If EXPR is omitted, uses the current time (C<localtime(time)>).
a0d0e21e 2224
f86cebdf 2225In scalar context, returns the ctime(3) value:
a0d0e21e 2226
5f05dabc 2227 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
a0d0e21e 2228
a3cb178b 2229This scalar value is B<not> locale dependent, see L<perllocale>, but
7660c0ab 2230instead a Perl builtin. Also see the C<Time::Local> module, and the
f86cebdf 2231strftime(3) and mktime(3) function available via the POSIX module. To
a3cb178b
GS
2232get somewhat similar but locale dependent date strings, set up your
2233locale environment variables appropriately (please see L<perllocale>)
5a964f20 2234and try for example:
a3cb178b 2235
5a964f20 2236 use POSIX qw(strftime);
2b5ab1e7 2237 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
a3cb178b
GS
2238
2239Note that the C<%a> and C<%b>, the short forms of the day of the week
2240and the month of the year, may not necessarily be three characters wide.
a0d0e21e 2241
19799a22
GS
2242=item lock
2243
2244 lock I<THING>
2245
2246This function places an advisory lock on a variable, subroutine,
2247or referenced object contained in I<THING> until the lock goes out
2248of scope. This is a built-in function only if your version of Perl
2249was built with threading enabled, and if you've said C<use Threads>.
2250Otherwise a user-defined function by this name will be called. See
2251L<Thread>.
2252
a0d0e21e
LW
2253=item log EXPR
2254
54310121 2255=item log
bbce6d69 2256
2b5ab1e7
TC
2257Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2258returns log of C<$_>. To get the log of another base, use basic algebra:
19799a22 2259The base-N log of a number is equal to the natural log of that number
2b5ab1e7
TC
2260divided by the natural log of N. For example:
2261
2262 sub log10 {
2263 my $n = shift;
2264 return log($n)/log(10);
2265 }
2266
2267See also L</exp> for the inverse operation.
a0d0e21e
LW
2268
2269=item lstat FILEHANDLE
2270
2271=item lstat EXPR
2272
54310121 2273=item lstat
bbce6d69 2274
19799a22 2275Does the same thing as the C<stat> function (including setting the
5a964f20
TC
2276special C<_> filehandle) but stats a symbolic link instead of the file
2277the symbolic link points to. If symbolic links are unimplemented on
19799a22 2278your system, a normal C<stat> is done.
a0d0e21e 2279
7660c0ab 2280If EXPR is omitted, stats C<$_>.
bbce6d69 2281
a0d0e21e
LW
2282=item m//
2283
2284The match operator. See L<perlop>.
2285
2286=item map BLOCK LIST
2287
2288=item map EXPR,LIST
2289
19799a22
GS
2290Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2291C<$_> to each element) and returns the list value composed of the
2292results of each such evaluation. In scalar context, returns the
2293total number of elements so generated. Evaluates BLOCK or EXPR in
2294list context, so each element of LIST may produce zero, one, or
2295more elements in the returned value.
dd99ebda 2296
a0d0e21e
LW
2297 @chars = map(chr, @nums);
2298
2299translates a list of numbers to the corresponding characters. And
2300
4633a7c4 2301 %hash = map { getkey($_) => $_ } @array;
a0d0e21e
LW
2302
2303is just a funny way to write
2304
2305 %hash = ();
2306 foreach $_ (@array) {
4633a7c4 2307 $hash{getkey($_)} = $_;
a0d0e21e
LW
2308 }
2309
2b5ab1e7
TC
2310Note that, because C<$_> is a reference into the list value, it can
2311be used to modify the elements of the array. While this is useful and
2312supported, it can cause bizarre results if the LIST is not a named array.
2313Using a regular C<foreach> loop for this purpose would be clearer in
2314most cases. See also L</grep> for an array composed of those items of
2315the original list for which the BLOCK or EXPR evaluates to true.
fb73857a 2316
19799a22 2317=item mkdir FILENAME,MASK
a0d0e21e 2318
0591cd52 2319Creates the directory specified by FILENAME, with permissions
19799a22
GS
2320specified by MASK (as modified by C<umask>). If it succeeds it
2321returns true, otherwise it returns false and sets C<$!> (errno).
0591cd52 2322
19799a22 2323In general, it is better to create directories with permissive MASK,
0591cd52 2324and let the user modify that with their C<umask>, than it is to supply
19799a22 2325a restrictive MASK and give the user no way to be more permissive.
0591cd52
NT
2326The exceptions to this rule are when the file or directory should be
2327kept private (mail files, for instance). The perlfunc(1) entry on
19799a22 2328C<umask> discusses the choice of MASK in more detail.
a0d0e21e
LW
2329
2330=item msgctl ID,CMD,ARG
2331
f86cebdf 2332Calls the System V IPC function msgctl(2). You'll probably have to say
0ade1984
JH
2333
2334 use IPC::SysV;
2335
7660c0ab
A
2336first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2337then ARG must be a variable which will hold the returned C<msqid_ds>
19799a22
GS
2338structure. Returns like C<ioctl>: the undefined value for error, C<"0 but
2339true"> for zero, or the actual return value otherwise. See also
2340C<IPC::SysV> and C<IPC::Semaphore> documentation.
a0d0e21e
LW
2341
2342=item msgget KEY,FLAGS
2343
f86cebdf 2344Calls the System V IPC function msgget(2). Returns the message queue
7660c0ab 2345id, or the undefined value if there is an error. See also C<IPC::SysV>
19799a22 2346and C<IPC::Msg> documentation.
a0d0e21e
LW
2347
2348=item msgsnd ID,MSG,FLAGS
2349
2350Calls the System V IPC function msgsnd to send the message MSG to the
2351message queue ID. MSG must begin with the long integer message type,
19799a22
GS
2352which may be created with C<pack("l", $type)>. Returns true if
2353successful, or false if there is an error. See also C<IPC::SysV>
7660c0ab 2354and C<IPC::SysV::Msg> documentation.
a0d0e21e
LW
2355
2356=item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2357
2358Calls the System V IPC function msgrcv to receive a message from
2359message queue ID into variable VAR with a maximum message size of
0ade1984
JH
2360SIZE. Note that if a message is received, the message type will be
2361the first thing in VAR, and the maximum length of VAR is SIZE plus the
19799a22 2362size of the message type. Returns true if successful, or false if
7660c0ab 2363there is an error. See also C<IPC::SysV> and C<IPC::SysV::Msg> documentation.
a0d0e21e
LW
2364
2365=item my EXPR
2366
19799a22
GS
2367A C<my> declares the listed variables to be local (lexically) to the
2368enclosing block, file, or C<eval>. If
5f05dabc 2369more than one value is listed, the list must be placed in parentheses. See
cb1a09d0 2370L<perlsub/"Private Variables via my()"> for details.
4633a7c4 2371
a0d0e21e
LW
2372=item next LABEL
2373
2374=item next
2375
2376The C<next> command is like the C<continue> statement in C; it starts
2377the next iteration of the loop:
2378
4633a7c4
LW
2379 LINE: while (<STDIN>) {
2380 next LINE if /^#/; # discard comments
5a964f20 2381 #...
a0d0e21e
LW
2382 }
2383
2384Note that if there were a C<continue> block on the above, it would get
2385executed even on discarded lines. If the LABEL is omitted, the command
2386refers to the innermost enclosing loop.
2387
4968c1e4 2388C<next> cannot be used to exit a block which returns a value such as
2b5ab1e7
TC
2389C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2390a grep() or map() operation.
4968c1e4 2391
98293880
JH
2392See also L</continue> for an illustration of how C<last>, C<next>, and
2393C<redo> work.
1d2dff63 2394
a0d0e21e
LW
2395=item no Module LIST
2396
7660c0ab 2397See the L</use> function, which C<no> is the opposite of.
a0d0e21e
LW
2398
2399=item oct EXPR
2400
54310121 2401=item oct
bbce6d69 2402
4633a7c4 2403Interprets EXPR as an octal string and returns the corresponding
4f19785b
WSI
2404value. (If EXPR happens to start off with C<0x>, interprets it as a
2405hex string. If EXPR starts off with C<0b>, it is interpreted as a
2406binary string.) The following will handle decimal, binary, octal, and
4633a7c4 2407hex in the standard Perl or C notation:
a0d0e21e
LW
2408
2409 $val = oct($val) if $val =~ /^0/;
2410
19799a22
GS
2411If EXPR is omitted, uses C<$_>. To go the other way (produce a number
2412in octal), use sprintf() or printf():
2413
2414 $perms = (stat("filename"))[2] & 07777;
2415 $oct_perms = sprintf "%lo", $perms;
2416
2417The oct() function is commonly used when a string such as C<644> needs
2418to be converted into a file mode, for example. (Although perl will
2419automatically convert strings into numbers as needed, this automatic
2420conversion assumes base 10.)
a0d0e21e
LW
2421
2422=item open FILEHANDLE,EXPR
2423
2424=item open FILEHANDLE
2425
2426Opens the file whose filename is given by EXPR, and associates it with
5f05dabc
PP
2427FILEHANDLE. If FILEHANDLE is an expression, its value is used as the
2428name of the real filehandle wanted. If EXPR is omitted, the scalar
2429variable of the same name as the FILEHANDLE contains the filename.
19799a22
GS
2430(Note that lexical variables--those declared with C<my>--will not work
2431for this purpose; so if you're using C<my>, specify EXPR in your call
2b5ab1e7
TC
2432to open.) See L<perlopentut> for a kinder, gentler explanation of opening
2433files.
5f05dabc 2434
7660c0ab
A
2435If the filename begins with C<'E<lt>'> or nothing, the file is opened for input.
2436If the filename begins with C<'E<gt>'>, the file is truncated and opened for
19799a22 2437output, being created if necessary. If the filename begins with C<'E<gt>E<gt>'>,
fbb426e4 2438the file is opened for appending, again being created if necessary.
7660c0ab
A
2439You can put a C<'+'> in front of the C<'E<gt>'> or C<'E<lt>'> to indicate that
2440you want both read and write access to the file; thus C<'+E<lt>'> is almost
2441always preferred for read/write updates--the C<'+E<gt>'> mode would clobber the
5a964f20
TC
2442file first. You can't usually use either read-write mode for updating
2443textfiles, since they have variable length records. See the B<-i>
0591cd52
NT
2444switch in L<perlrun> for a better approach. The file is created with
2445permissions of C<0666> modified by the process' C<umask> value.
5a964f20
TC
2446
2447The prefix and the filename may be separated with spaces.
f86cebdf 2448These various prefixes correspond to the fopen(3) modes of C<'r'>, C<'r+'>, C<'w'>,
7660c0ab 2449C<'w+'>, C<'a'>, and C<'a+'>.
5f05dabc 2450
7660c0ab 2451If the filename begins with C<'|'>, the filename is interpreted as a
5a964f20 2452command to which output is to be piped, and if the filename ends with a
f244e06d
GS
2453C<'|'>, the filename is interpreted as a command which pipes output to
2454us. See L<perlipc/"Using open() for IPC">
19799a22 2455for more examples of this. (You are not allowed to C<open> to a command
5a964f20
TC
2456that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2457and L<perlipc/"Bidirectional Communication"> for alternatives.)
cb1a09d0 2458
7660c0ab 2459Opening C<'-'> opens STDIN and opening C<'E<gt>-'> opens STDOUT. Open returns
19799a22 2460nonzero upon success, the undefined value otherwise. If the C<open>
4633a7c4 2461involved a pipe, the return value happens to be the pid of the
54310121 2462subprocess.
cb1a09d0
AD
2463
2464If you're unfortunate enough to be running Perl on a system that
2465distinguishes between text files and binary files (modern operating
2466systems don't care), then you should check out L</binmode> for tips for
19799a22 2467dealing with this. The key distinction between systems that need C<binmode>
5a964f20
TC
2468and those that don't is their text file formats. Systems like Unix, MacOS, and
2469Plan9, which delimit lines with a single character, and which encode that
19799a22 2470character in C as C<"\n">, do not need C<binmode>. The rest need it.
cb1a09d0 2471
fb73857a 2472When opening a file, it's usually a bad idea to continue normal execution
19799a22
GS
2473if the request failed, so C<open> is frequently used in connection with
2474C<die>. Even if C<die> won't do what you want (say, in a CGI script,
fb73857a 2475where you want to make a nicely formatted error message (but there are
5a964f20 2476modules that can help with that problem)) you should always check
19799a22 2477the return value from opening a file. The infrequent exception is when
fb73857a
PP
2478working with an unopened filehandle is actually what you want to do.
2479
cb1a09d0 2480Examples:
a0d0e21e
LW
2481
2482 $ARTICLE = 100;
2483 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2484 while (<ARTICLE>) {...
2485
2486 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
fb73857a 2487 # if the open fails, output is discarded
a0d0e21e 2488
fb73857a
PP
2489 open(DBASE, '+<dbase.mine') # open for update
2490 or die "Can't open 'dbase.mine' for update: $!";
cb1a09d0 2491
fb73857a
PP
2492 open(ARTICLE, "caesar <$article |") # decrypt article
2493 or die "Can't start caesar: $!";
a0d0e21e 2494
fb73857a
PP
2495 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
2496 or die "Can't start sort: $!";
a0d0e21e
LW
2497
2498 # process argument list of files along with any includes
2499
2500 foreach $file (@ARGV) {
2501 process($file, 'fh00');
2502 }
2503
2504 sub process {
5a964f20 2505 my($filename, $input) = @_;
a0d0e21e
LW
2506 $input++; # this is a string increment
2507 unless (open($input, $filename)) {
2508 print STDERR "Can't open $filename: $!\n";
2509 return;
2510 }
2511
5a964f20 2512 local $_;
a0d0e21e
LW
2513 while (<$input>) { # note use of indirection
2514 if (/^#include "(.*)"/) {
2515 process($1, $input);
2516 next;
2517 }
5a964f20 2518 #... # whatever
a0d0e21e
LW
2519 }
2520 }
2521
2522You may also, in the Bourne shell tradition, specify an EXPR beginning
7660c0ab 2523with C<'E<gt>&'>, in which case the rest of the string is interpreted as the
5a964f20 2524name of a filehandle (or file descriptor, if numeric) to be
7660c0ab
A
2525duped and opened. You may use C<&> after C<E<gt>>, C<E<gt>E<gt>>, C<E<lt>>, C<+E<gt>>,
2526C<+E<gt>E<gt>>, and C<+E<lt>>. The
a0d0e21e 2527mode you specify should match the mode of the original filehandle.
184e9718 2528(Duping a filehandle does not take into account any existing contents of
cb1a09d0 2529stdio buffers.)
a0d0e21e
LW
2530Here is a script that saves, redirects, and restores STDOUT and
2531STDERR:
2532
2533 #!/usr/bin/perl
5a964f20
TC
2534 open(OLDOUT, ">&STDOUT");
2535 open(OLDERR, ">&STDERR");
a0d0e21e
LW
2536
2537 open(STDOUT, ">foo.out") || die "Can't redirect stdout";
2538 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
2539
2540 select(STDERR); $| = 1; # make unbuffered
2541 select(STDOUT); $| = 1; # make unbuffered
2542
2543 print STDOUT "stdout 1\n"; # this works for
2544 print STDERR "stderr 1\n"; # subprocesses too
2545
2546 close(STDOUT);
2547 close(STDERR);
2548
5a964f20
TC
2549 open(STDOUT, ">&OLDOUT");
2550 open(STDERR, ">&OLDERR");
a0d0e21e
LW
2551
2552 print STDOUT "stdout 2\n";
2553 print STDERR "stderr 2\n";
2554
7660c0ab 2555If you specify C<'E<lt>&=N'>, where C<N> is a number, then Perl will do an
19799a22 2556equivalent of C's C<fdopen> of that file descriptor; this is more
4633a7c4 2557parsimonious of file descriptors. For example:
a0d0e21e
LW
2558
2559 open(FILEHANDLE, "<&=$fd")
2560
7660c0ab 2561If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>, then
a0d0e21e 2562there is an implicit fork done, and the return value of open is the pid
7660c0ab 2563of the child within the parent process, and C<0> within the child
184e9718 2564process. (Use C<defined($pid)> to determine whether the open was successful.)
a0d0e21e
LW
2565The filehandle behaves normally for the parent, but i/o to that
2566filehandle is piped from/to the STDOUT/STDIN of the child process.
2567In the child process the filehandle isn't opened--i/o happens from/to
2568the new STDOUT or STDIN. Typically this is used like the normal
2569piped open when you want to exercise more control over just how the
2570pipe command gets executed, such as when you are running setuid, and
54310121 2571don't want to have to scan shell commands for metacharacters.
4633a7c4 2572The following pairs are more or less equivalent:
a0d0e21e
LW
2573
2574 open(FOO, "|tr '[a-z]' '[A-Z]'");
2575 open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
2576
2577 open(FOO, "cat -n '$file'|");
2578 open(FOO, "-|") || exec 'cat', '-n', $file;
2579
4633a7c4
LW
2580See L<perlipc/"Safe Pipe Opens"> for more examples of this.
2581
45bc9206
GS
2582NOTE: On any operation that may do a fork, all files opened for output
2583are flushed before the fork is attempted. On systems that support a
2584close-on-exec flag on files, the flag will be set for the newly opened
2585file descriptor as determined by the value of $^F. See L<perlvar/$^F>.
a0d0e21e 2586
0dccf244
CS
2587Closing any piped filehandle causes the parent process to wait for the
2588child to finish, and returns the status value in C<$?>.
2589
5a964f20 2590The filename passed to open will have leading and trailing
f86cebdf 2591whitespace deleted, and the normal redirection characters
5a964f20
TC
2592honored. This property, known as "magic open",
2593can often be used to good effect. A user could specify a filename of
7660c0ab 2594F<"rsh cat file |">, or you could change certain filenames as needed:
5a964f20
TC
2595
2596 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
2597 open(FH, $filename) or die "Can't open $filename: $!";
2598
2599However, to open a file with arbitrary weird characters in it, it's
2600necessary to protect any leading and trailing whitespace:
2601
2602 $file =~ s#^(\s)#./$1#;
2603 open(FOO, "< $file\0");
2604
19799a22
GS
2605If you want a "real" C C<open> (see L<open(2)> on your system), then you
2606should use the C<sysopen> function, which involves no such magic. This is
5a964f20
TC
2607another way to protect your filenames from interpretation. For example:
2608
2609 use IO::Handle;
2610 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
2611 or die "sysopen $path: $!";
2612 $oldfh = select(HANDLE); $| = 1; select($oldfh);
2613 print HANDLE "stuff $$\n");
2614 seek(HANDLE, 0, 0);
2615 print "File contains: ", <HANDLE>;
2616
7660c0ab
A
2617Using the constructor from the C<IO::Handle> package (or one of its
2618subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
5a964f20
TC
2619filehandles that have the scope of whatever variables hold references to
2620them, and automatically close whenever and however you leave that scope:
c07a80fd 2621
5f05dabc 2622 use IO::File;
5a964f20 2623 #...
c07a80fd
PP
2624 sub read_myfile_munged {
2625 my $ALL = shift;
5f05dabc 2626 my $handle = new IO::File;
c07a80fd
PP
2627 open($handle, "myfile") or die "myfile: $!";
2628 $first = <$handle>
2629 or return (); # Automatically closed here.
2630 mung $first or die "mung failed"; # Or here.
2631 return $first, <$handle> if $ALL; # Or here.
2632 $first; # Or here.
2633 }
2634
b687b08b 2635See L</seek> for some details about mixing reading and writing.
a0d0e21e
LW
2636
2637=item opendir DIRHANDLE,EXPR
2638
19799a22
GS
2639Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
2640C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
a0d0e21e
LW
2641DIRHANDLEs have their own namespace separate from FILEHANDLEs.
2642
2643=item ord EXPR
2644
54310121 2645=item ord
bbce6d69 2646
a0ed51b3 2647Returns the numeric (ASCII or Unicode) value of the first character of EXPR. If
7660c0ab 2648EXPR is omitted, uses C<$_>. For the reverse, see L</chr>.
2b5ab1e7 2649See L<utf8> for more about Unicode.
a0d0e21e
LW
2650
2651=item pack TEMPLATE,LIST
2652
19799a22 2653Takes a list of values and packs it into a binary structure,
a0d0e21e
LW
2654returning the string containing the structure. The TEMPLATE is a
2655sequence of characters that give the order and type of values, as
2656follows:
2657
5a929a98 2658 a A string with arbitrary binary data, will be null padded.
a0d0e21e 2659 A An ascii string, will be space padded.
5a929a98
VU
2660 Z A null terminated (asciz) string, will be null padded.
2661
a0d0e21e
LW
2662 b A bit string (ascending bit order, like vec()).
2663 B A bit string (descending bit order).
2664 h A hex string (low nybble first).
2665 H A hex string (high nybble first).
2666
2667 c A signed char value.
a0ed51b3 2668 C An unsigned char value. Only does bytes. See U for Unicode.
96e4d5b1 2669
a0d0e21e
LW
2670 s A signed short value.
2671 S An unsigned short value.
96e4d5b1
PP
2672 (This 'short' is _exactly_ 16 bits, which may differ from
2673 what a local C compiler calls 'short'.)
2674
a0d0e21e
LW
2675 i A signed integer value.
2676 I An unsigned integer value.
19799a22 2677 (This 'integer' is _at_least_ 32 bits wide. Its exact
f86cebdf
GS
2678 size depends on what a local C compiler calls 'int',
2679 and may even be larger than the 'long' described in
2680 the next item.)
96e4d5b1 2681
a0d0e21e
LW
2682 l A signed long value.
2683 L An unsigned long value.
96e4d5b1
PP
2684 (This 'long' is _exactly_ 32 bits, which may differ from
2685 what a local C compiler calls 'long'.)
a0d0e21e 2686
96e4d5b1
PP
2687 n A short in "network" (big-endian) order.
2688 N A long in "network" (big-endian) order.
a0d0e21e
LW
2689 v A short in "VAX" (little-endian) order.
2690 V A long in "VAX" (little-endian) order.
96e4d5b1
PP
2691 (These 'shorts' and 'longs' are _exactly_ 16 bits and
2692 _exactly_ 32 bits, respectively.)
a0d0e21e 2693
dae0da7a
JH
2694 q A signed quad (64-bit) value.
2695 Q An unsigned quad value.
2696 (Available only if your system supports 64-bit integer values
2697 _and_ if Perl has been compiled to support those.
2698 Causes a fatal error otherwise.)
2699
a0d0e21e
LW
2700 f A single-precision float in the native format.
2701 d A double-precision float in the native format.
2702
2703 p A pointer to a null-terminated string.
2704 P A pointer to a structure (fixed-length string).
2705
2706 u A uuencoded string.
a0ed51b3
LW
2707 U A Unicode character number. Encodes to UTF-8 internally.
2708 Works even if C<use utf8> is not in effect.
a0d0e21e 2709
96e4d5b1 2710 w A BER compressed integer. Its bytes represent an unsigned
f86cebdf
GS
2711 integer in base 128, most significant digit first, with as
2712 few digits as possible. Bit eight (the high bit) is set
2713 on each byte except the last.
def98dd4 2714
a0d0e21e
LW
2715 x A null byte.
2716 X Back up a byte.
2717 @ Null fill to absolute position.
2718
5a929a98
VU
2719The following rules apply:
2720
2721=over 8
2722
2723=item *
2724
5a964f20 2725Each letter may optionally be followed by a number giving a repeat
5a929a98
VU
2726count. With all types except C<"a">, C<"A">, C<"Z">, C<"b">, C<"B">, C<"h">,
2727C<"H">, and C<"P"> the pack function will gobble up that many values from
2728the LIST. A C<*> for the repeat count means to use however many items are
2729left.
2730
2731=item *
2732
2b5ab1e7 2733The C<"a">, C<"A">, and C<"Z"> types gobble just one value, but pack it as a
5a929a98
VU
2734string of length count, padding with nulls or spaces as necessary. When
2735unpacking, C<"A"> strips trailing spaces and nulls, C<"Z"> strips everything
2736after the first null, and C<"a"> returns data verbatim.
2737
2738=item *
2739
2740Likewise, the C<"b"> and C<"B"> fields pack a string that many bits long.
2741
2742=item *
2743
2744The C<"h"> and C<"H"> fields pack a string that many nybbles long.
2745
2746=item *
2747
2748The C<"p"> type packs a pointer to a null-terminated string. You are
2749responsible for ensuring the string is not a temporary value (which can
2750potentially get deallocated before you get around to using the packed result).
2751The C<"P"> type packs a pointer to a structure of the size indicated by the
19799a22 2752length. A NULL pointer is created if the corresponding value for C<"p"> or
5a929a98
VU
2753C<"P"> is C<undef>.
2754
2755=item *
2756
43192e07
IP
2757The C<"#"> character allows packing and unpacking of strings where the
2758packed structure contains a byte count followed by the string itself.
2759You write I<length-item>C<#>I<string-item>.
2760
2761The I<length-item> can be any C<pack> template letter,
2762and describes how the length value is packed.
2763The ones likely to be of most use are integer-packing ones like
2764C<"n"> (for Java strings), C<"w"> (for ASN.1 or SNMP)
2765and C<"N"> (for Sun XDR).
2766
2767The I<string-item> must, at present, be C<"A*">, C<"a*"> or C<"Z*">.
2768For C<unpack> the length of the string is obtained from the I<length-item>,
2769but if you put in the '*' it will be ignored.
2770
2771 unpack 'C#a', "\04Gurusamy"; gives 'Guru'
2772 unpack 'a3#A* A*', '007 Bond J '; gives (' Bond','J')
2773 pack 'n#a* w#a*','hello,','world'; gives "\000\006hello,\005world"
2774
2775The I<length-item> is not returned explicitly from C<unpack>.
2776
2777Adding a count to the I<length-item> letter
2778is unlikely to do anything useful,
2779unless that letter is C<"A">, C<"a"> or C<"Z">.
2780Packing with a I<length-item> of C<"a"> or C<"Z">
2781may introduce C<"\000"> characters,
2782which Perl does not regard as legal in numeric strings.
2783
2784=item *
2785
ef54e1a4 2786The integer types C<"s">, C<"S">, C<"l">, and C<"L"> may be
4d0c1c44 2787immediately followed by a C<"!"> to signify native shorts or longs--as
ef54e1a4
JH
2788you can see from above for example a bare C<"l"> does mean exactly 32
2789bits, the native C<long> (as seen by the local C compiler) may be
726ea183 2790larger. This is an issue mainly in 64-bit platforms. You can see
4d0c1c44 2791whether using C<"!"> makes any difference by
726ea183 2792
4d0c1c44
GS
2793 print length(pack("s")), " ", length(pack("s!")), "\n";
2794 print length(pack("l")), " ", length(pack("l!")), "\n";
ef54e1a4 2795
4d0c1c44 2796C<"i!"> and C<"I!"> also work but only because of completeness;
ef54e1a4
JH
2797they are identical to C<"i"> and C<"I">.
2798
726ea183 2799The actual sizes (in bytes) of native shorts, ints, and longs on
d99ad34e 2800the platform where Perl was built are also available via L<Config>:
ef54e1a4 2801
19799a22
GS
2802The actual sizes (in bytes) of native shorts, ints, longs, and long
2803longs on the platform where Perl was built are also available via
2804L<Config>:
2805
2806 use Config;
2807 print $Config{shortsize}, "\n";
2808 print $Config{intsize}, "\n";
2809 print $Config{longsize}, "\n";
2810 print $Config{longlongsize}, "\n";
ef54e1a4
JH
2811
2812=item *
2813
2814The integer formats C<"s">, C<"S">, C<"i">, C<"I">, C<"l">, and C<"L">
2815are inherently non-portable between processors and operating systems
2816because they obey the native byteorder and endianness. For example a
719a3cf5 28174-byte integer 0x87654321 (2271560481 decimal) be ordered natively
ef54e1a4
JH
2818(arranged in and handled by the CPU registers) into bytes as
2819
719a3cf5
JH
2820 0x12 0x34 0x56 0x78 # little-endian
2821 0x78 0x56 0x34 0x12 # big-endian
ef54e1a4 2822
719a3cf5
JH
2823Basically, the Intel, Alpha, and VAX CPUs and little-endian, while
2824everybody else, for example Motorola m68k/88k, PPC, Sparc, HP PA,
2825Power, and Cray are big-endian. MIPS can be either: Digital used it
19799a22 2826in little-endian mode; SGI uses it in big-endian mode.
719a3cf5 2827
19799a22 2828The names `big-endian' and `little-endian' are comic references to
ef54e1a4
JH
2829the classic "Gulliver's Travels" (via the paper "On Holy Wars and a
2830Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and
19799a22 2831the egg-eating habits of the Lilliputians.
ef54e1a4
JH
2832
2833Some systems may even have weird byte orders such as
2834
2835 0x56 0x78 0x12 0x34
2836 0x34 0x12 0x78 0x56
2837
2838You can see your system's preference with
2839
2840 print join(" ", map { sprintf "%#02x", $_ }
2841 unpack("C*",pack("L",0x12345678))), "\n";
2842
d99ad34e 2843The byteorder on the platform where Perl was built is also available
726ea183 2844via L<Config>:
ef54e1a4
JH
2845
2846 use Config;
2847 print $Config{byteorder}, "\n";
2848
d99ad34e
JH
2849Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'>
2850and C<'87654321'> are big-endian.
719a3cf5 2851
d99ad34e
JH
2852If you want portable packed integers use the formats C<"n">, C<"N">,
2853C<"v">, and C<"V">, their byte endianness and size is known.
ef54e1a4
JH
2854
2855=item *
2856
5a929a98
VU
2857Real numbers (floats and doubles) are in the native machine format only;
2858due to the multiplicity of floating formats around, and the lack of a
2859standard "network" representation, no facility for interchange has been
2860made. This means that packed floating point data written on one machine
2861may not be readable on another - even if both use IEEE floating point
2862arithmetic (as the endian-ness of the memory representation is not part
2863of the IEEE spec).
2864
2865Note that Perl uses doubles internally for all numeric calculation, and
2866converting from double into float and thence back to double again will
2867lose precision (i.e., C<unpack("f", pack("f", $foo)>) will not in general
19799a22 2868equal $foo).
5a929a98
VU
2869
2870=back
a0d0e21e
LW
2871
2872Examples:
2873
a0ed51b3 2874 $foo = pack("CCCC",65,66,67,68);
a0d0e21e 2875 # foo eq "ABCD"
a0ed51b3 2876 $foo = pack("C4",65,66,67,68);
a0d0e21e 2877 # same thing
a0ed51b3
LW
2878 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
2879 # same thing with Unicode circled letters
a0d0e21e
LW
2880
2881 $foo = pack("ccxxcc",65,66,67,68);
2882 # foo eq "AB\0\0CD"
2883
2884 $foo = pack("s2",1,2);
2885 # "\1\0\2\0" on little-endian
2886 # "\0\1\0\2" on big-endian
2887
2888 $foo = pack("a4","abcd","x","y","z");
2889 # "abcd"
2890
2891 $foo = pack("aaaa","abcd","x","y","z");
2892 # "axyz"
2893
2894 $foo = pack("a14","abcdefg");
2895 # "abcdefg\0\0\0\0\0\0\0"
2896
2897 $foo = pack("i9pl", gmtime);
2898 # a real struct tm (on my system anyway)
2899
5a929a98
VU
2900 $utmp_template = "Z8 Z8 Z16 L";
2901 $utmp = pack($utmp_template, @utmp1);
2902 # a struct utmp (BSDish)
2903
2904 @utmp2 = unpack($utmp_template, $utmp);
2905 # "@utmp1" eq "@utmp2"
2906
a0d0e21e
LW
2907 sub bintodec {
2908 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
2909 }
2910
5a929a98 2911The same template may generally also be used in unpack().
a0d0e21e 2912
5a964f20
TC
2913=item package
2914
cb1a09d0
AD
2915=item package NAMESPACE
2916
2917Declares the compilation unit as being in the given namespace. The scope
2b5ab1e7 2918of the package declaration is from the declaration itself through the end
19799a22 2919of the enclosing block, file, or eval (the same as the C<my> operator).
2b5ab1e7
TC
2920All further unqualified dynamic identifiers will be in this namespace.
2921A package statement affects only dynamic variables--including those
19799a22
GS
2922you've used C<local> on--but I<not> lexical variables, which are created
2923with C<my>. Typically it would be the first declaration in a file to
2b5ab1e7
TC
2924be included by the C<require> or C<use> operator. You can switch into a
2925package in more than one place; it merely influences which symbol table
2926is used by the compiler for the rest of that block. You can refer to
2927variables and filehandles in other packages by prefixing the identifier
2928with the package name and a double colon: C<$Package::Variable>.
2929If the package name is null, the C<main> package as assumed. That is,
2930C<$::sail> is equivalent to C<$main::sail> (as well as to C<$main'sail>,
2931still seen in older code).
cb1a09d0 2932
5a964f20
TC
2933If NAMESPACE is omitted, then there is no current package, and all
2934identifiers must be fully qualified or lexicals. This is stricter
2935than C<use strict>, since it also extends to function names.
2936
cb1a09d0
AD
2937See L<perlmod/"Packages"> for more information about packages, modules,
2938and classes. See L<perlsub> for other scoping issues.
2939
a0d0e21e
LW
2940=item pipe READHANDLE,WRITEHANDLE
2941
2942Opens a pair of connected pipes like the corresponding system call.
2943Note that if you set up a loop of piped processes, deadlock can occur
2944unless you are very careful. In addition, note that Perl's pipes use
184e9718 2945stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE
a0d0e21e
LW
2946after each command, depending on the application.
2947
7e1af8bc 2948See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
4633a7c4
LW
2949for examples of such things.
2950
4771b018
GS
2951On systems that support a close-on-exec flag on files, the flag will be set
2952for the newly opened file descriptors as determined by the value of $^F.
2953See L<perlvar/$^F>.
2954
a0d0e21e
LW
2955=item pop ARRAY
2956
54310121 2957=item pop
28757baa 2958
a0d0e21e 2959Pops and returns the last value of the array, shortening the array by
19799a22 2960one element. Has an effect similar to
a0d0e21e 2961
19799a22 2962 $ARRAY[$#ARRAY--]
a0d0e21e 2963
19799a22
GS
2964If there are no elements in the array, returns the undefined value
2965(although this may happen at other times as well). If ARRAY is
2966omitted, pops the C<@ARGV> array in the main program, and the C<@_>
2967array in subroutines, just like C<shift>.
a0d0e21e
LW
2968
2969=item pos SCALAR
2970
54310121 2971=item pos
bbce6d69 2972
4633a7c4 2973Returns the offset of where the last C<m//g> search left off for the variable
7660c0ab 2974is in question (C<$_> is used when the variable is not specified). May be
44a8e56a
PP
2975modified to change that offset. Such modification will also influence
2976the C<\G> zero-width assertion in regular expressions. See L<perlre> and
2977L<perlop>.
a0d0e21e
LW
2978
2979=item print FILEHANDLE LIST
2980
2981=item print LIST
2982
2983=item print
2984
19799a22
GS
2985Prints a string or a list of strings. Returns true if successful.
2986FILEHANDLE may be a scalar variable name, in which case the variable
2987contains the name of or a reference to the filehandle, thus introducing
2988one level of indirection. (NOTE: If FILEHANDLE is a variable and
2989the next token is a term, it may be misinterpreted as an operator
2b5ab1e7 2990unless you interpose a C<+> or put parentheses around the arguments.)
19799a22
GS
2991If FILEHANDLE is omitted, prints by default to standard output (or
2992to the last selected output channel--see L</select>). If LIST is
2993also omitted, prints C<$_> to the currently selected output channel.
2994To set the default output channel to something other than STDOUT
2995use the select operation. The current value of C<$,> (if any) is
2996printed between each LIST item. The current value of C<$\> (if
2997any) is printed after the entire LIST has been printed. Because
2998print takes a LIST, anything in the LIST is evaluated in list
2999context, and any subroutine that you call will have one or more of
3000its expressions evaluated in list context. Also be careful not to
3001follow the print keyword with a left parenthesis unless you want
3002the corresponding right parenthesis to terminate the arguments to
3003the print--interpose a C<+> or put parentheses around all the
3004arguments.
a0d0e21e 3005
4633a7c4 3006Note that if you're storing FILEHANDLES in an array or other expression,
da0045b7 3007you will have to use a block returning its value instead:
4633a7c4
LW
3008
3009 print { $files[$i] } "stuff\n";
3010 print { $OK ? STDOUT : STDERR } "stuff\n";
3011
5f05dabc 3012=item printf FILEHANDLE FORMAT, LIST
a0d0e21e 3013
5f05dabc 3014=item printf FORMAT, LIST
a0d0e21e 3015
7660c0ab 3016Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
a3cb178b 3017(the output record separator) is not appended. The first argument
19799a22 3018of the list will be interpreted as the C<printf> format. If C<use locale> is
a034a98d
DD
3019in effect, the character used for the decimal point in formatted real numbers
3020is affected by the LC_NUMERIC locale. See L<perllocale>.
a0d0e21e 3021
19799a22
GS
3022Don't fall into the trap of using a C<printf> when a simple
3023C<print> would do. The C<print> is more efficient and less
28757baa
PP
3024error prone.
3025
da0045b7
PP
3026=item prototype FUNCTION
3027
3028Returns the prototype of a function as a string (or C<undef> if the
5f05dabc
PP
3029function has no prototype). FUNCTION is a reference to, or the name of,
3030the function whose prototype you want to retrieve.
da0045b7 3031
2b5ab1e7
TC
3032If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
3033name for Perl builtin. If the builtin is not I<overridable> (such as
ab4f32c2 3034C<qw//>) or its arguments cannot be expressed by a prototype (such as
19799a22 3035C<system>) returns C<undef> because the builtin does not really behave
2b5ab1e7
TC
3036like a Perl function. Otherwise, the string describing the equivalent
3037prototype is returned.
b6c543e3 3038
a0d0e21e
LW
3039=item push ARRAY,LIST
3040
3041Treats ARRAY as a stack, and pushes the values of LIST
3042onto the end of ARRAY. The length of ARRAY increases by the length of
3043LIST. Has the same effect as
3044
3045 for $value (LIST) {
3046 $ARRAY[++$#ARRAY] = $value;
3047 }
3048
3049but is more efficient. Returns the new number of elements in the array.
3050
3051=item q/STRING/
3052
3053=item qq/STRING/
3054
8782bef2
GB
3055=item qr/STRING/
3056
a0d0e21e
LW
3057=item qx/STRING/
3058
3059=item qw/STRING/
3060
4b6a7270 3061Generalized quotes. See L<perlop/"Regexp Quote-Like Operators">.
a0d0e21e
LW
3062
3063=item quotemeta EXPR
3064
54310121 3065=item quotemeta
bbce6d69 3066
68dc0745 3067Returns the value of EXPR with all non-alphanumeric
a034a98d
DD
3068characters backslashed. (That is, all characters not matching
3069C</[A-Za-z_0-9]/> will be preceded by a backslash in the
3070returned string, regardless of any locale settings.)
3071This is the internal function implementing
7660c0ab 3072the C<\Q> escape in double-quoted strings.
a0d0e21e 3073
7660c0ab 3074If EXPR is omitted, uses C<$_>.
bbce6d69 3075
a0d0e21e
LW
3076=item rand EXPR
3077
3078=item rand
3079
7660c0ab 3080Returns a random fractional number greater than or equal to C<0> and less
3e3baf6d 3081than the value of EXPR. (EXPR should be positive.) If EXPR is
19799a22
GS
3082omitted, the value C<1> is used. Automatically calls C<srand> unless
3083C<srand> has already been called. See also C<srand>.
a0d0e21e 3084
2f9daede 3085(Note: If your rand function consistently returns numbers that are too
a0d0e21e 3086large or too small, then your version of Perl was probably compiled
2f9daede 3087with the wrong number of RANDBITS.)
a0d0e21e
LW
3088
3089=item read FILEHANDLE,SCALAR,LENGTH,OFFSET
3090
3091=item read FILEHANDLE,SCALAR,LENGTH
3092
3093Attempts to read LENGTH bytes of data into variable SCALAR from the
3b02c43c
GS
3094specified FILEHANDLE. Returns the number of bytes actually read,
3095C<0> at end of file, or undef if there was an error. SCALAR will be grown
3096or shrunk to the length actually read. An OFFSET may be specified to
3097place the read data at some other place than the beginning of the
f86cebdf 3098string. This call is actually implemented in terms of stdio's fread(3)
19799a22 3099call. To get a true read(2) system call, see C<sysread>.
a0d0e21e
LW
3100
3101=item readdir DIRHANDLE
3102
19799a22 3103Returns the next directory entry for a directory opened by C<opendir>.
5a964f20 3104If used in list context, returns all the rest of the entries in the
a0d0e21e 3105directory. If there are no more entries, returns an undefined value in
5a964f20 3106scalar context or a null list in list context.
a0d0e21e 3107
19799a22 3108If you're planning to filetest the return values out of a C<readdir>, you'd
5f05dabc 3109better prepend the directory in question. Otherwise, because we didn't
19799a22 3110C<chdir> there, it would have been testing the wrong file.
cb1a09d0
AD
3111
3112 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
3113 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
3114 closedir DIR;
3115
84902520
TB
3116=item readline EXPR
3117
fbad3eb5
GS
3118Reads from the filehandle whose typeglob is contained in EXPR. In scalar
3119context, each call reads and returns the next line, until end-of-file is
3120reached, whereupon the subsequent call returns undef. In list context,
3121reads until end-of-file is reached and returns a list of lines. Note that
3122the notion of "line" used here is however you may have defined it
3123with C<$/> or C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">.
3124
2b5ab1e7 3125When C<$/> is set to C<undef>, when readline() is in scalar
449bc448
GS
3126context (i.e. file slurp mode), and when an empty file is read, it
3127returns C<''> the first time, followed by C<undef> subsequently.
fbad3eb5 3128
84902520
TB
3129This is the internal function implementing the C<E<lt>EXPRE<gt>>
3130operator, but you can use it directly. The C<E<lt>EXPRE<gt>>
3131operator is discussed in more detail in L<perlop/"I/O Operators">.
3132
5a964f20
TC
3133 $line = <STDIN>;
3134 $line = readline(*STDIN); # same thing
3135
a0d0e21e
LW
3136=item readlink EXPR
3137
54310121 3138=item readlink
bbce6d69 3139
a0d0e21e
LW
3140Returns the value of a symbolic link, if symbolic links are
3141implemented. If not, gives a fatal error. If there is some system
184e9718 3142error, returns the undefined value and sets C<$!> (errno). If EXPR is
7660c0ab 3143omitted, uses C<$_>.
a0d0e21e 3144
84902520
TB
3145=item readpipe EXPR
3146
5a964f20 3147EXPR is executed as a system command.
84902520
TB
3148The collected standard output of the command is returned.
3149In scalar context, it comes back as a single (potentially
3150multi-line) string. In list context, returns a list of lines
7660c0ab 3151(however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
84902520
TB
3152This is the internal function implementing the C<qx/EXPR/>
3153operator, but you can use it directly. The C<qx/EXPR/>
3154operator is discussed in more detail in L<perlop/"I/O Operators">.
3155
399388f4 3156=item recv SOCKET,SCALAR,LENGTH,FLAGS
a0d0e21e
LW
3157
3158Receives a message on a socket. Attempts to receive LENGTH bytes of
3159data into variable SCALAR from the specified SOCKET filehandle.
19799a22 3160Actually does a C C<recvfrom>, so that it can return the address of the
a0d0e21e
LW
3161sender. Returns the undefined value if there's an error. SCALAR will
3162be grown or shrunk to the length actually read. Takes the same flags
54310121 3163as the system call of the same name.
4633a7c4 3164See L<perlipc/"UDP: Message Passing"> for examples.
a0d0e21e
LW
3165
3166=item redo LABEL
3167
3168=item redo
3169
3170The C<redo> command restarts the loop block without evaluating the
98293880 3171conditional again. The C<continue> block, if any, is not executed. If
a0d0e21e
LW
3172the LABEL is omitted, the command refers to the innermost enclosing
3173loop. This command is normally used by programs that want to lie to
3174themselves about what was just input:
3175
3176 # a simpleminded Pascal comment stripper
3177 # (warning: assumes no { or } in strings)
4633a7c4 3178 LINE: while (<STDIN>) {
a0d0e21e
LW
3179 while (s|({.*}.*){.*}|$1 |) {}
3180 s|{.*}| |;
3181 if (s|{.*| |) {
3182 $front = $_;
3183 while (<STDIN>) {
3184 if (/}/) { # end of comment?
5a964f20 3185 s|^|$front\{|;
4633a7c4 3186 redo LINE;
a0d0e21e
LW
3187 }
3188 }
3189 }
3190 print;
3191 }
3192
4968c1e4 3193C<redo> cannot be used to retry a block which returns a value such as
2b5ab1e7
TC
3194C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
3195a grep() or map() operation.
4968c1e4 3196
98293880 3197See also L</continue> for an illustration of how C<last>, C<next>, and
1d2dff63
GS
3198C<redo> work.
3199
a0d0e21e
LW
3200=item ref EXPR
3201
54310121 3202=item ref
bbce6d69 3203
19799a22 3204Returns a true value if EXPR is a reference, false otherwise. If EXPR
7660c0ab 3205is not specified, C<$_> will be used. The value returned depends on the
bbce6d69 3206type of thing the reference is a reference to.
a0d0e21e
LW
3207Builtin types include:
3208
a0d0e21e
LW
3209 SCALAR
3210 ARRAY
3211 HASH
3212 CODE
19799a22 3213 REF
a0d0e21e 3214 GLOB
19799a22 3215 LVALUE
a0d0e21e 3216
54310121 3217If the referenced object has been blessed into a package, then that package
19799a22 3218name is returned instead. You can think of C<ref> as a C<typeof> operator.
a0d0e21e
LW
3219
3220 if (ref($r) eq "HASH") {
aa689395 3221 print "r is a reference to a hash.\n";
54310121 3222 }
2b5ab1e7 3223 unless (ref($r)) {
a0d0e21e 3224 print "r is not a reference at all.\n";
54310121 3225 }
2b5ab1e7
TC
3226 if (UNIVERSAL::isa($r, "HASH")) { # for subclassing
3227 print "r is a reference to something that isa hash.\n";
3228 }
a0d0e21e
LW
3229
3230See also L<perlref>.
3231
3232=item rename OLDNAME,NEWNAME
3233
19799a22
GS
3234Changes the name of a file; an existing file NEWNAME will be
3235clobbered. Returns true for success, false otherwise.
3236
2b5ab1e7
TC
3237Behavior of this function varies wildly depending on your system
3238implementation. For example, it will usually not work across file system
3239boundaries, even though the system I<mv> command sometimes compensates
3240for this. Other restrictions include whether it works on directories,
3241open files, or pre-existing files. Check L<perlport> and either the
3242rename(2) manpage or equivalent system documentation for details.
a0d0e21e
LW
3243
3244=item require EXPR
3245
3246=item require
3247
7660c0ab 3248Demands some semantics specified by EXPR, or by C<$_> if EXPR is not
a0d0e21e 3249supplied. If EXPR is numeric, demands that the current version of Perl
184e9718 3250(C<$]> or $PERL_VERSION) be equal or greater than EXPR.
a0d0e21e
LW
3251
3252Otherwise, demands that a library file be included if it hasn't already
3253been included. The file is included via the do-FILE mechanism, which is
19799a22 3254essentially just a variety of C<eval>. Has semantics similar to the following
a0d0e21e
LW
3255subroutine:
3256
3257 sub require {
5a964f20 3258 my($filename) = @_;
a0d0e21e 3259 return 1 if $INC{$filename};
5a964f20 3260 my($realfilename,$result);
a0d0e21e
LW
3261 ITER: {
3262 foreach $prefix (@INC) {
3263 $realfilename = "$prefix/$filename";
3264 if (-f $realfilename) {
3265 $result = do $realfilename;
3266 last ITER;
3267 }
3268 }
3269 die "Can't find $filename in \@INC";
3270 }
3271 die $@ if $@;
3272 die "$filename did not return true value" unless $result;
3273 $INC{$filename} = $realfilename;
5a964f20 3274 return $result;
a0d0e21e
LW
3275 }
3276
3277Note that the file will not be included twice under the same specified
19799a22 3278name. The file must return true as the last statement to indicate
a0d0e21e 3279successful execution of any initialization code, so it's customary to
19799a22
GS
3280end such a file with C<1;> unless you're sure it'll return true
3281otherwise. But it's better just to put the C<1;>, in case you add more
a0d0e21e
LW
3282statements.
3283
54310121 3284If EXPR is a bareword, the require assumes a "F<.pm>" extension and
da0045b7 3285replaces "F<::>" with "F</>" in the filename for you,
54310121 3286to make it easy to load standard modules. This form of loading of
a0d0e21e
LW
3287modules does not risk altering your namespace.
3288
ee580363
GS
3289In other words, if you try this:
3290
f86cebdf 3291 require Foo::Bar; # a splendid bareword
ee580363 3292