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avoid spurious "Useless use of variable" warning on C<our $foo;>
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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> and C<sleep> calls.
400(C<sleep> may be internally implemented in your system with C<alarm>)
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
951ba7fe 518Here $subroutine may be C<(eval)> if the frame is not a subroutine
19799a22 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, $evaltext contains the text of the
dc848c6f 522C<eval EXPR> statement. In particular, for a C<eval BLOCK> statement,
951ba7fe 523$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
951ba7fe 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<$1> is defined, despite the fact that it
a5f75d66 916matched "nothing". But it didn't really match nothing--rather, it
2b5ab1e7 917matched something that happened to be zero characters long. This is all
a5f75d66 918very above-board and honest. When a function returns an undefined value,
2f9daede 919it's an admission that it couldn't give you an honest answer. So you
19799a22 920should use C<defined> only when you're questioning the integrity of what
7660c0ab 921you're trying to do. At other times, a simple comparison to C<0> or C<""> is
2f9daede
TPG
922what you want.
923
dc848c6f 924See also L</undef>, L</exists>, L</ref>.
2f9daede 925
a0d0e21e
LW
926=item delete EXPR
927
01020589
GS
928Given an expression that specifies a hash element, array element, hash slice,
929or array slice, deletes the specified element(s) from the hash or array.
930If the array elements happen to be at the end of the array, the size
931of the array will shrink by that number of elements.
a0d0e21e 932
01020589
GS
933Returns each element so deleted or the undefined value if there was no such
934element. Deleting from C<$ENV{}> modifies the environment. Deleting from
935a hash tied to a DBM file deletes the entry from the DBM file. Deleting
936from a C<tie>d hash or array may not necessarily return anything.
937
8ea97a1e
GS
938Deleting an array element effectively returns that position of the array
939to its initial, uninitialized state. Subsequently testing for the same
940element with exists() will return false. See L</exists>.
941
01020589 942The following (inefficiently) deletes all the values of %HASH and @ARRAY:
a0d0e21e 943
5f05dabc
PP
944 foreach $key (keys %HASH) {
945 delete $HASH{$key};
a0d0e21e
LW
946 }
947
01020589
GS
948 foreach $index (0 .. $#ARRAY) {
949 delete $ARRAY[$index];
950 }
951
952And so do these:
5f05dabc 953
01020589
GS
954 delete @HASH{keys %HASH};
955
9740c838 956 delete @ARRAY[0 .. $#ARRAY];
5f05dabc 957
2b5ab1e7 958But both of these are slower than just assigning the empty list
01020589
GS
959or undefining %HASH or @ARRAY:
960
961 %HASH = (); # completely empty %HASH
962 undef %HASH; # forget %HASH ever existed
2b5ab1e7 963
01020589
GS
964 @ARRAY = (); # completely empty @ARRAY
965 undef @ARRAY; # forget @ARRAY ever existed
2b5ab1e7
TC
966
967Note that the EXPR can be arbitrarily complicated as long as the final
01020589
GS
968operation is a hash element, array element, hash slice, or array slice
969lookup:
a0d0e21e
LW
970
971 delete $ref->[$x][$y]{$key};
5f05dabc 972 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
a0d0e21e 973
01020589
GS
974 delete $ref->[$x][$y][$index];
975 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
976
a0d0e21e
LW
977=item die LIST
978
19799a22
GS
979Outside an C<eval>, prints the value of LIST to C<STDERR> and
980exits with the current value of C<$!> (errno). If C<$!> is C<0>,
981exits with the value of C<($? E<gt>E<gt> 8)> (backtick `command`
982status). If C<($? E<gt>E<gt> 8)> is C<0>, exits with C<255>. Inside
983an C<eval(),> the error message is stuffed into C<$@> and the
984C<eval> is terminated with the undefined value. This makes
985C<die> the way to raise an exception.
a0d0e21e
LW
986
987Equivalent examples:
988
989 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
54310121 990 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
a0d0e21e
LW
991
992If the value of EXPR does not end in a newline, the current script line
993number and input line number (if any) are also printed, and a newline
883faa13
GS
994is supplied. Note that the "input line number" (also known as "chunk")
995is subject to whatever notion of "line" happens to be currently in
996effect, and is also available as the special variable C<$.>.
997See L<perlvar/"$/"> and L<perlvar/"$.">.
998
999Hint: sometimes appending C<", stopped"> to your message
7660c0ab 1000will cause it to make better sense when the string C<"at foo line 123"> is
a0d0e21e
LW
1001appended. Suppose you are running script "canasta".
1002
1003 die "/etc/games is no good";
1004 die "/etc/games is no good, stopped";
1005
1006produce, respectively
1007
1008 /etc/games is no good at canasta line 123.
1009 /etc/games is no good, stopped at canasta line 123.
1010
2b5ab1e7 1011See also exit(), warn(), and the Carp module.
a0d0e21e 1012
7660c0ab
A
1013If LIST is empty and C<$@> already contains a value (typically from a
1014previous eval) that value is reused after appending C<"\t...propagated">.
fb73857a
PP
1015This is useful for propagating exceptions:
1016
1017 eval { ... };
1018 die unless $@ =~ /Expected exception/;
1019
7660c0ab 1020If C<$@> is empty then the string C<"Died"> is used.
fb73857a 1021
52531d10
GS
1022die() can also be called with a reference argument. If this happens to be
1023trapped within an eval(), $@ contains the reference. This behavior permits
1024a more elaborate exception handling implementation using objects that
1025maintain arbitary state about the nature of the exception. Such a scheme
1026is sometimes preferable to matching particular string values of $@ using
1027regular expressions. Here's an example:
1028
1029 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1030 if ($@) {
1031 if (ref($@) && UNIVERSAL::isa($@,"Some::Module::Exception")) {
1032 # handle Some::Module::Exception
1033 }
1034 else {
1035 # handle all other possible exceptions
1036 }
1037 }
1038
19799a22 1039Because perl will stringify uncaught exception messages before displaying
52531d10
GS
1040them, you may want to overload stringification operations on such custom
1041exception objects. See L<overload> for details about that.
1042
19799a22
GS
1043You can arrange for a callback to be run just before the C<die>
1044does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1045handler will be called with the error text and can change the error
1046message, if it sees fit, by calling C<die> again. See
1047L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
1048L<"eval BLOCK"> for some examples. Although this feature was meant
1049to be run only right before your program was to exit, this is not
1050currently the case--the C<$SIG{__DIE__}> hook is currently called
1051even inside eval()ed blocks/strings! If one wants the hook to do
1052nothing in such situations, put
fb73857a
PP
1053
1054 die @_ if $^S;
1055
19799a22
GS
1056as the first line of the handler (see L<perlvar/$^S>). Because
1057this promotes strange action at a distance, this counterintuitive
1058behavior may be fixed in a future release.
774d564b 1059
a0d0e21e
LW
1060=item do BLOCK
1061
1062Not really a function. Returns the value of the last command in the
1063sequence of commands indicated by BLOCK. When modified by a loop
98293880
JH
1064modifier, executes the BLOCK once before testing the loop condition.
1065(On other statements the loop modifiers test the conditional first.)
a0d0e21e 1066
4968c1e4 1067C<do BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7
TC
1068C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1069See L<perlsyn> for alternative strategies.
4968c1e4 1070
a0d0e21e
LW
1071=item do SUBROUTINE(LIST)
1072
1073A deprecated form of subroutine call. See L<perlsub>.
1074
1075=item do EXPR
1076
1077Uses the value of EXPR as a filename and executes the contents of the
1078file as a Perl script. Its primary use is to include subroutines
1079from a Perl subroutine library.
1080
1081 do 'stat.pl';
1082
1083is just like
1084
fb73857a 1085 scalar eval `cat stat.pl`;
a0d0e21e 1086
2b5ab1e7
TC
1087except that it's more efficient and concise, keeps track of the current
1088filename for error messages, searches the @INC libraries, and updates
1089C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1090variables. It also differs in that code evaluated with C<do FILENAME>
1091cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1092same, however, in that it does reparse the file every time you call it,
1093so you probably don't want to do this inside a loop.
a0d0e21e 1094
8e30cc93 1095If C<do> cannot read the file, it returns undef and sets C<$!> to the
2b5ab1e7 1096error. If C<do> can read the file but cannot compile it, it
8e30cc93
G
1097returns undef and sets an error message in C<$@>. If the file is
1098successfully compiled, C<do> returns the value of the last expression
1099evaluated.
1100
a0d0e21e 1101Note that inclusion of library modules is better done with the
19799a22 1102C<use> and C<require> operators, which also do automatic error checking
4633a7c4 1103and raise an exception if there's a problem.
a0d0e21e 1104
5a964f20
TC
1105You might like to use C<do> to read in a program configuration
1106file. Manual error checking can be done this way:
1107
1108 # read in config files: system first, then user
f86cebdf 1109 for $file ("/share/prog/defaults.rc",
2b5ab1e7
TC
1110 "$ENV{HOME}/.someprogrc")
1111 {
5a964f20 1112 unless ($return = do $file) {
f86cebdf
GS
1113 warn "couldn't parse $file: $@" if $@;
1114 warn "couldn't do $file: $!" unless defined $return;
1115 warn "couldn't run $file" unless $return;
5a964f20
TC
1116 }
1117 }
1118
a0d0e21e
LW
1119=item dump LABEL
1120
1614b0e3
JD
1121=item dump
1122
19799a22
GS
1123This function causes an immediate core dump. See also the B<-u>
1124command-line switch in L<perlrun>, which does the same thing.
1125Primarily this is so that you can use the B<undump> program (not
1126supplied) to turn your core dump into an executable binary after
1127having initialized all your variables at the beginning of the
1128program. When the new binary is executed it will begin by executing
1129a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1130Think of it as a goto with an intervening core dump and reincarnation.
1131If C<LABEL> is omitted, restarts the program from the top.
1132
1133B<WARNING>: Any files opened at the time of the dump will I<not>
1134be open any more when the program is reincarnated, with possible
1135resulting confusion on the part of Perl.
1136
1137This function is now largely obsolete, partly because it's very
1138hard to convert a core file into an executable, and because the
1139real compiler backends for generating portable bytecode and compilable
1140C code have superseded it.
1141
1142If you're looking to use L<dump> to speed up your program, consider
1143generating bytecode or native C code as described in L<perlcc>. If
1144you're just trying to accelerate a CGI script, consider using the
1145C<mod_perl> extension to B<Apache>, or the CPAN module, Fast::CGI.
1146You might also consider autoloading or selfloading, which at least
1147make your program I<appear> to run faster.
5a964f20 1148
aa689395
PP
1149=item each HASH
1150
5a964f20 1151When called in list context, returns a 2-element list consisting of the
aa689395 1152key and value for the next element of a hash, so that you can iterate over
5a964f20 1153it. When called in scalar context, returns the key for only the "next"
7660c0ab 1154element in the hash. (Note: Keys may be C<"0"> or C<"">, which are logically
2f9daede
TPG
1155false; you may wish to avoid constructs like C<while ($k = each %foo) {}>
1156for this reason.)
1157
ab192400
GS
1158Entries are returned in an apparently random order. The actual random
1159order is subject to change in future versions of perl, but it is guaranteed
19799a22 1160to be in the same order as either the C<keys> or C<values> function
ab192400
GS
1161would produce on the same (unmodified) hash.
1162
1163When the hash is entirely read, a null array is returned in list context
19799a22
GS
1164(which when assigned produces a false (C<0>) value), and C<undef> in
1165scalar context. The next call to C<each> after that will start iterating
1166again. There is a single iterator for each hash, shared by all C<each>,
1167C<keys>, and C<values> function calls in the program; it can be reset by
2f9daede
TPG
1168reading all the elements from the hash, or by evaluating C<keys HASH> or
1169C<values HASH>. If you add or delete elements of a hash while you're
1170iterating over it, you may get entries skipped or duplicated, so don't.
aa689395 1171
f86cebdf 1172The following prints out your environment like the printenv(1) program,
aa689395 1173only in a different order:
a0d0e21e
LW
1174
1175 while (($key,$value) = each %ENV) {
1176 print "$key=$value\n";
1177 }
1178
19799a22 1179See also C<keys>, C<values> and C<sort>.
a0d0e21e
LW
1180
1181=item eof FILEHANDLE
1182
4633a7c4
LW
1183=item eof ()
1184
a0d0e21e
LW
1185=item eof
1186
1187Returns 1 if the next read on FILEHANDLE will return end of file, or if
1188FILEHANDLE is not open. FILEHANDLE may be an expression whose value
5a964f20 1189gives the real filehandle. (Note that this function actually
19799a22 1190reads a character and then C<ungetc>s it, so isn't very useful in an
748a9306 1191interactive context.) Do not read from a terminal file (or call
19799a22 1192C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
748a9306
LW
1193as terminals may lose the end-of-file condition if you do.
1194
820475bd
GS
1195An C<eof> without an argument uses the last file read. Using C<eof()>
1196with empty parentheses is very different. It refers to the pseudo file
1197formed from the files listed on the command line and accessed via the
1198C<E<lt>E<gt>> operator. Since C<E<lt>E<gt>> isn't explicitly opened,
1199as a normal filehandle is, an C<eof()> before C<E<lt>E<gt>> has been
1200used will cause C<@ARGV> to be examined to determine if input is
1201available.
1202
1203In a C<while (E<lt>E<gt>)> loop, C<eof> or C<eof(ARGV)> can be used to
1204detect the end of each file, C<eof()> will only detect the end of the
1205last file. Examples:
a0d0e21e 1206
748a9306
LW
1207 # reset line numbering on each input file
1208 while (<>) {
5a964f20 1209 next if /^\s*#/; # skip comments
748a9306 1210 print "$.\t$_";
5a964f20
TC
1211 } continue {
1212 close ARGV if eof; # Not eof()!
748a9306
LW
1213 }
1214
a0d0e21e
LW
1215 # insert dashes just before last line of last file
1216 while (<>) {
5a964f20 1217 if (eof()) { # check for end of current file
a0d0e21e 1218 print "--------------\n";
2b5ab1e7 1219 close(ARGV); # close or last; is needed if we
748a9306 1220 # are reading from the terminal
a0d0e21e
LW
1221 }
1222 print;
1223 }
1224
a0d0e21e 1225Practical hint: you almost never need to use C<eof> in Perl, because the
3ce0d271
GS
1226input operators typically return C<undef> when they run out of data, or if
1227there was an error.
a0d0e21e
LW
1228
1229=item eval EXPR
1230
1231=item eval BLOCK
1232
c7cc6f1c
GS
1233In the first form, the return value of EXPR is parsed and executed as if it
1234were a little Perl program. The value of the expression (which is itself
5a964f20 1235determined within scalar context) is first parsed, and if there weren't any
c7cc6f1c 1236errors, executed in the context of the current Perl program, so that any
5f05dabc 1237variable settings or subroutine and format definitions remain afterwards.
c7cc6f1c
GS
1238Note that the value is parsed every time the eval executes. If EXPR is
1239omitted, evaluates C<$_>. This form is typically used to delay parsing
1240and subsequent execution of the text of EXPR until run time.
1241
1242In the second form, the code within the BLOCK is parsed only once--at the
1243same time the code surrounding the eval itself was parsed--and executed
1244within the context of the current Perl program. This form is typically
1245used to trap exceptions more efficiently than the first (see below), while
1246also providing the benefit of checking the code within BLOCK at compile
1247time.
1248
1249The final semicolon, if any, may be omitted from the value of EXPR or within
1250the BLOCK.
1251
1252In both forms, the value returned is the value of the last expression
5a964f20 1253evaluated inside the mini-program; a return statement may be also used, just
c7cc6f1c 1254as with subroutines. The expression providing the return value is evaluated
5a964f20 1255in void, scalar, or list context, depending on the context of the eval itself.
c7cc6f1c 1256See L</wantarray> for more on how the evaluation context can be determined.
a0d0e21e 1257
19799a22
GS
1258If there is a syntax error or runtime error, or a C<die> statement is
1259executed, an undefined value is returned by C<eval>, and C<$@> is set to the
a0d0e21e 1260error message. If there was no error, C<$@> is guaranteed to be a null
19799a22 1261string. Beware that using C<eval> neither silences perl from printing
c7cc6f1c
GS
1262warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1263To do either of those, you have to use the C<$SIG{__WARN__}> facility. See
1264L</warn> and L<perlvar>.
a0d0e21e 1265
19799a22
GS
1266Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1267determining whether a particular feature (such as C<socket> or C<symlink>)
a0d0e21e
LW
1268is implemented. It is also Perl's exception trapping mechanism, where
1269the die operator is used to raise exceptions.
1270
1271If the code to be executed doesn't vary, you may use the eval-BLOCK
1272form to trap run-time errors without incurring the penalty of
1273recompiling each time. The error, if any, is still returned in C<$@>.
1274Examples:
1275
54310121 1276 # make divide-by-zero nonfatal
a0d0e21e
LW
1277 eval { $answer = $a / $b; }; warn $@ if $@;
1278
1279 # same thing, but less efficient
1280 eval '$answer = $a / $b'; warn $@ if $@;
1281
1282 # a compile-time error
5a964f20 1283 eval { $answer = }; # WRONG
a0d0e21e
LW
1284
1285 # a run-time error
1286 eval '$answer ='; # sets $@
1287
2b5ab1e7
TC
1288Due to the current arguably broken state of C<__DIE__> hooks, when using
1289the C<eval{}> form as an exception trap in libraries, you may wish not
1290to trigger any C<__DIE__> hooks that user code may have installed.
1291You can use the C<local $SIG{__DIE__}> construct for this purpose,
1292as shown in this example:
774d564b
PP
1293
1294 # a very private exception trap for divide-by-zero
f86cebdf
GS
1295 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1296 warn $@ if $@;
774d564b
PP
1297
1298This is especially significant, given that C<__DIE__> hooks can call
19799a22 1299C<die> again, which has the effect of changing their error messages:
774d564b
PP
1300
1301 # __DIE__ hooks may modify error messages
1302 {
f86cebdf
GS
1303 local $SIG{'__DIE__'} =
1304 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
c7cc6f1c
GS
1305 eval { die "foo lives here" };
1306 print $@ if $@; # prints "bar lives here"
774d564b
PP
1307 }
1308
19799a22 1309Because this promotes action at a distance, this counterintuitive behavior
2b5ab1e7
TC
1310may be fixed in a future release.
1311
19799a22 1312With an C<eval>, you should be especially careful to remember what's
a0d0e21e
LW
1313being looked at when:
1314
1315 eval $x; # CASE 1
1316 eval "$x"; # CASE 2
1317
1318 eval '$x'; # CASE 3
1319 eval { $x }; # CASE 4
1320
5a964f20 1321 eval "\$$x++"; # CASE 5
a0d0e21e
LW
1322 $$x++; # CASE 6
1323
2f9daede 1324Cases 1 and 2 above behave identically: they run the code contained in
19799a22 1325the variable $x. (Although case 2 has misleading double quotes making
2f9daede 1326the reader wonder what else might be happening (nothing is).) Cases 3
7660c0ab 1327and 4 likewise behave in the same way: they run the code C<'$x'>, which
19799a22 1328does nothing but return the value of $x. (Case 4 is preferred for
2f9daede
TPG
1329purely visual reasons, but it also has the advantage of compiling at
1330compile-time instead of at run-time.) Case 5 is a place where
19799a22 1331normally you I<would> like to use double quotes, except that in this
2f9daede
TPG
1332particular situation, you can just use symbolic references instead, as
1333in case 6.
a0d0e21e 1334
4968c1e4 1335C<eval BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7 1336C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
4968c1e4 1337
a0d0e21e
LW
1338=item exec LIST
1339
8bf3b016
GS
1340=item exec PROGRAM LIST
1341
19799a22
GS
1342The C<exec> function executes a system command I<and never returns>--
1343use C<system> instead of C<exec> if you want it to return. It fails and
1344returns false only if the command does not exist I<and> it is executed
fb73857a 1345directly instead of via your system's command shell (see below).
a0d0e21e 1346
19799a22
GS
1347Since it's a common mistake to use C<exec> instead of C<system>, Perl
1348warns you if there is a following statement which isn't C<die>, C<warn>,
1349or C<exit> (if C<-w> is set - but you always do that). If you
1350I<really> want to follow an C<exec> with some other statement, you
55d729e4
GS
1351can use one of these styles to avoid the warning:
1352
5a964f20
TC
1353 exec ('foo') or print STDERR "couldn't exec foo: $!";
1354 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
55d729e4 1355
5a964f20 1356If there is more than one argument in LIST, or if LIST is an array
f86cebdf 1357with more than one value, calls execvp(3) with the arguments in LIST.
5a964f20
TC
1358If there is only one scalar argument or an array with one element in it,
1359the argument is checked for shell metacharacters, and if there are any,
1360the entire argument is passed to the system's command shell for parsing
1361(this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1362If there are no shell metacharacters in the argument, it is split into
19799a22
GS
1363words and passed directly to C<execvp>, which is more efficient.
1364Examples:
a0d0e21e 1365
19799a22
GS
1366 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1367 exec "sort $outfile | uniq";
a0d0e21e
LW
1368
1369If you don't really want to execute the first argument, but want to lie
1370to the program you are executing about its own name, you can specify
1371the program you actually want to run as an "indirect object" (without a
1372comma) in front of the LIST. (This always forces interpretation of the
54310121 1373LIST as a multivalued list, even if there is only a single scalar in
a0d0e21e
LW
1374the list.) Example:
1375
1376 $shell = '/bin/csh';
1377 exec $shell '-sh'; # pretend it's a login shell
1378
1379or, more directly,
1380
1381 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1382
bb32b41a
GS
1383When the arguments get executed via the system shell, results will
1384be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1385for details.
1386
19799a22
GS
1387Using an indirect object with C<exec> or C<system> is also more
1388secure. This usage (which also works fine with system()) forces
1389interpretation of the arguments as a multivalued list, even if the
1390list had just one argument. That way you're safe from the shell
1391expanding wildcards or splitting up words with whitespace in them.
5a964f20
TC
1392
1393 @args = ( "echo surprise" );
1394
2b5ab1e7 1395 exec @args; # subject to shell escapes
f86cebdf 1396 # if @args == 1
2b5ab1e7 1397 exec { $args[0] } @args; # safe even with one-arg list
5a964f20
TC
1398
1399The first version, the one without the indirect object, ran the I<echo>
1400program, passing it C<"surprise"> an argument. The second version
1401didn't--it tried to run a program literally called I<"echo surprise">,
1402didn't find it, and set C<$?> to a non-zero value indicating failure.
1403
19799a22 1404Note that C<exec> will not call your C<END> blocks, nor will it call
7660c0ab
A
1405any C<DESTROY> methods in your objects.
1406
a0d0e21e
LW
1407=item exists EXPR
1408
01020589 1409Given an expression that specifies a hash element or array element,
8ea97a1e
GS
1410returns true if the specified element in the hash or array has ever
1411been initialized, even if the corresponding value is undefined. The
1412element is not autovivified if it doesn't exist.
a0d0e21e 1413
01020589
GS
1414 print "Exists\n" if exists $hash{$key};
1415 print "Defined\n" if defined $hash{$key};
1416 print "True\n" if $hash{$key};
1417
1418 print "Exists\n" if exists $array[$index];
1419 print "Defined\n" if defined $array[$index];
1420 print "True\n" if $array[$index];
a0d0e21e 1421
8ea97a1e 1422A hash or array element can be true only if it's defined, and defined if
a0d0e21e
LW
1423it exists, but the reverse doesn't necessarily hold true.
1424
1425Note that the EXPR can be arbitrarily complicated as long as the final
01020589 1426operation is a hash or array key lookup:
a0d0e21e 1427
2b5ab1e7
TC
1428 if (exists $ref->{A}->{B}->{$key}) { }
1429 if (exists $hash{A}{B}{$key}) { }
1430
01020589
GS
1431 if (exists $ref->{A}->{B}->[$ix]) { }
1432 if (exists $hash{A}{B}[$ix]) { }
1433
1434Although the deepest nested array or hash will not spring into existence
1435just because its existence was tested, any intervening ones will.
1436Thus C<$ref-E<gt>{"A"}> and C<$ref-E<gt>{"A"}-E<gt>{"B"}> will spring
1437into existence due to the existence test for the $key element above.
1438This happens anywhere the arrow operator is used, including even:
5a964f20 1439
2b5ab1e7
TC
1440 undef $ref;
1441 if (exists $ref->{"Some key"}) { }
1442 print $ref; # prints HASH(0x80d3d5c)
1443
1444This surprising autovivification in what does not at first--or even
1445second--glance appear to be an lvalue context may be fixed in a future
5a964f20 1446release.
a0d0e21e 1447
e0478e5a
MS
1448See L<perlref/"Pseudo-hashes"> for specifics on how exists() acts when
1449used on a pseudo-hash.
1450
a0d0e21e
LW
1451=item exit EXPR
1452
2b5ab1e7 1453Evaluates EXPR and exits immediately with that value. Example:
a0d0e21e
LW
1454
1455 $ans = <STDIN>;
1456 exit 0 if $ans =~ /^[Xx]/;
1457
19799a22 1458See also C<die>. If EXPR is omitted, exits with C<0> status. The only
2b5ab1e7
TC
1459universally recognized values for EXPR are C<0> for success and C<1>
1460for error; other values are subject to interpretation depending on the
1461environment in which the Perl program is running. For example, exiting
146269 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1463the mailer to return the item undelivered, but that's not true everywhere.
a0d0e21e 1464
19799a22
GS
1465Don't use C<exit> to abort a subroutine if there's any chance that
1466someone might want to trap whatever error happened. Use C<die> instead,
1467which can be trapped by an C<eval>.
28757baa 1468
19799a22 1469The exit() function does not always exit immediately. It calls any
2b5ab1e7 1470defined C<END> routines first, but these C<END> routines may not
19799a22 1471themselves abort the exit. Likewise any object destructors that need to
2b5ab1e7
TC
1472be called are called before the real exit. If this is a problem, you
1473can call C<POSIX:_exit($status)> to avoid END and destructor processing.
87275199 1474See L<perlmod> for details.
5a964f20 1475
a0d0e21e
LW
1476=item exp EXPR
1477
54310121 1478=item exp
bbce6d69 1479
2b5ab1e7 1480Returns I<e> (the natural logarithm base) to the power of EXPR.
a0d0e21e
LW
1481If EXPR is omitted, gives C<exp($_)>.
1482
1483=item fcntl FILEHANDLE,FUNCTION,SCALAR
1484
f86cebdf 1485Implements the fcntl(2) function. You'll probably have to say
a0d0e21e
LW
1486
1487 use Fcntl;
1488
0ade1984 1489first to get the correct constant definitions. Argument processing and
19799a22 1490value return works just like C<ioctl> below.
a0d0e21e
LW
1491For example:
1492
1493 use Fcntl;
5a964f20
TC
1494 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1495 or die "can't fcntl F_GETFL: $!";
1496
19799a22 1497You don't have to check for C<defined> on the return from C<fnctl>.
951ba7fe
GS
1498Like C<ioctl>, it maps a C<0> return from the system call into
1499C<"0 but true"> in Perl. This string is true in boolean context and C<0>
2b5ab1e7
TC
1500in numeric context. It is also exempt from the normal B<-w> warnings
1501on improper numeric conversions.
5a964f20 1502
19799a22 1503Note that C<fcntl> will produce a fatal error if used on a machine that
2b5ab1e7
TC
1504doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1505manpage to learn what functions are available on your system.
a0d0e21e
LW
1506
1507=item fileno FILEHANDLE
1508
2b5ab1e7
TC
1509Returns the file descriptor for a filehandle, or undefined if the
1510filehandle is not open. This is mainly useful for constructing
19799a22 1511bitmaps for C<select> and low-level POSIX tty-handling operations.
2b5ab1e7
TC
1512If FILEHANDLE is an expression, the value is taken as an indirect
1513filehandle, generally its name.
5a964f20
TC
1514
1515You can use this to find out whether two handles refer to the
1516same underlying descriptor:
1517
1518 if (fileno(THIS) == fileno(THAT)) {
1519 print "THIS and THAT are dups\n";
1520 }
a0d0e21e
LW
1521
1522=item flock FILEHANDLE,OPERATION
1523
19799a22
GS
1524Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1525for success, false on failure. Produces a fatal error if used on a
2b5ab1e7 1526machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
19799a22 1527C<flock> is Perl's portable file locking interface, although it locks
2b5ab1e7
TC
1528only entire files, not records.
1529
1530Two potentially non-obvious but traditional C<flock> semantics are
1531that it waits indefinitely until the lock is granted, and that its locks
1532B<merely advisory>. Such discretionary locks are more flexible, but offer
19799a22
GS
1533fewer guarantees. This means that files locked with C<flock> may be
1534modified by programs that do not also use C<flock>. See L<perlport>,
2b5ab1e7
TC
1535your port's specific documentation, or your system-specific local manpages
1536for details. It's best to assume traditional behavior if you're writing
1537portable programs. (But if you're not, you should as always feel perfectly
1538free to write for your own system's idiosyncrasies (sometimes called
1539"features"). Slavish adherence to portability concerns shouldn't get
1540in the way of your getting your job done.)
a3cb178b 1541
8ebc5c01
PP
1542OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1543LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
68dc0745
PP
1544you can use the symbolic names if import them from the Fcntl module,
1545either individually, or as a group using the ':flock' tag. LOCK_SH
1546requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1547releases a previously requested lock. If LOCK_NB is added to LOCK_SH or
19799a22 1548LOCK_EX then C<flock> will return immediately rather than blocking
68dc0745
PP
1549waiting for the lock (check the return status to see if you got it).
1550
2b5ab1e7
TC
1551To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1552before locking or unlocking it.
8ebc5c01 1553
f86cebdf 1554Note that the emulation built with lockf(3) doesn't provide shared
8ebc5c01 1555locks, and it requires that FILEHANDLE be open with write intent. These
2b5ab1e7 1556are the semantics that lockf(3) implements. Most if not all systems
f86cebdf 1557implement lockf(3) in terms of fcntl(2) locking, though, so the
8ebc5c01
PP
1558differing semantics shouldn't bite too many people.
1559
19799a22
GS
1560Note also that some versions of C<flock> cannot lock things over the
1561network; you would need to use the more system-specific C<fcntl> for
f86cebdf
GS
1562that. If you like you can force Perl to ignore your system's flock(2)
1563function, and so provide its own fcntl(2)-based emulation, by passing
8ebc5c01
PP
1564the switch C<-Ud_flock> to the F<Configure> program when you configure
1565perl.
4633a7c4
LW
1566
1567Here's a mailbox appender for BSD systems.
a0d0e21e 1568
7e1af8bc 1569 use Fcntl ':flock'; # import LOCK_* constants
a0d0e21e
LW
1570
1571 sub lock {
7e1af8bc 1572 flock(MBOX,LOCK_EX);
a0d0e21e
LW
1573 # and, in case someone appended
1574 # while we were waiting...
1575 seek(MBOX, 0, 2);
1576 }
1577
1578 sub unlock {
7e1af8bc 1579 flock(MBOX,LOCK_UN);
a0d0e21e
LW
1580 }
1581
1582 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1583 or die "Can't open mailbox: $!";
1584
1585 lock();
1586 print MBOX $msg,"\n\n";
1587 unlock();
1588
2b5ab1e7
TC
1589On systems that support a real flock(), locks are inherited across fork()
1590calls, whereas those that must resort to the more capricious fcntl()
1591function lose the locks, making it harder to write servers.
1592
cb1a09d0 1593See also L<DB_File> for other flock() examples.
a0d0e21e
LW
1594
1595=item fork
1596
2b5ab1e7
TC
1597Does a fork(2) system call to create a new process running the
1598same program at the same point. It returns the child pid to the
1599parent process, C<0> to the child process, or C<undef> if the fork is
1600unsuccessful. File descriptors (and sometimes locks on those descriptors)
1601are shared, while everything else is copied. On most systems supporting
1602fork(), great care has gone into making it extremely efficient (for
1603example, using copy-on-write technology on data pages), making it the
1604dominant paradigm for multitasking over the last few decades.
5a964f20 1605
45bc9206 1606All files opened for output are flushed before forking the child process.
a0d0e21e 1607
19799a22 1608If you C<fork> without ever waiting on your children, you will
2b5ab1e7
TC
1609accumulate zombies. On some systems, you can avoid this by setting
1610C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1611forking and reaping moribund children.
cb1a09d0 1612
28757baa
PP
1613Note that if your forked child inherits system file descriptors like
1614STDIN and STDOUT that are actually connected by a pipe or socket, even
2b5ab1e7 1615if you exit, then the remote server (such as, say, a CGI script or a
19799a22 1616backgrounded job launched from a remote shell) won't think you're done.
2b5ab1e7 1617You should reopen those to F</dev/null> if it's any issue.
28757baa 1618
cb1a09d0
AD
1619=item format
1620
19799a22 1621Declare a picture format for use by the C<write> function. For
cb1a09d0
AD
1622example:
1623
54310121 1624 format Something =
cb1a09d0
AD
1625 Test: @<<<<<<<< @||||| @>>>>>
1626 $str, $%, '$' . int($num)
1627 .
1628
1629 $str = "widget";
184e9718 1630 $num = $cost/$quantity;
cb1a09d0
AD
1631 $~ = 'Something';
1632 write;
1633
1634See L<perlform> for many details and examples.
1635
8903cb82 1636=item formline PICTURE,LIST
a0d0e21e 1637
5a964f20 1638This is an internal function used by C<format>s, though you may call it,
a0d0e21e
LW
1639too. It formats (see L<perlform>) a list of values according to the
1640contents of PICTURE, placing the output into the format output
7660c0ab 1641accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
19799a22 1642Eventually, when a C<write> is done, the contents of
a0d0e21e 1643C<$^A> are written to some filehandle, but you could also read C<$^A>
7660c0ab 1644yourself and then set C<$^A> back to C<"">. Note that a format typically
19799a22 1645does one C<formline> per line of form, but the C<formline> function itself
748a9306 1646doesn't care how many newlines are embedded in the PICTURE. This means
4633a7c4 1647that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
748a9306
LW
1648You may therefore need to use multiple formlines to implement a single
1649record format, just like the format compiler.
1650
19799a22 1651Be careful if you put double quotes around the picture, because an C<@>
748a9306 1652character may be taken to mean the beginning of an array name.
19799a22 1653C<formline> always returns true. See L<perlform> for other examples.
a0d0e21e
LW
1654
1655=item getc FILEHANDLE
1656
1657=item getc
1658
1659Returns the next character from the input file attached to FILEHANDLE,
2b5ab1e7
TC
1660or the undefined value at end of file, or if there was an error.
1661If FILEHANDLE is omitted, reads from STDIN. This is not particularly
1662efficient. However, it cannot be used by itself to fetch single
1663characters without waiting for the user to hit enter. For that, try
1664something more like:
4633a7c4
LW
1665
1666 if ($BSD_STYLE) {
1667 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1668 }
1669 else {
54310121 1670 system "stty", '-icanon', 'eol', "\001";
4633a7c4
LW
1671 }
1672
1673 $key = getc(STDIN);
1674
1675 if ($BSD_STYLE) {
1676 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1677 }
1678 else {
5f05dabc 1679 system "stty", 'icanon', 'eol', '^@'; # ASCII null
4633a7c4
LW
1680 }
1681 print "\n";
1682
54310121
PP
1683Determination of whether $BSD_STYLE should be set
1684is left as an exercise to the reader.
cb1a09d0 1685
19799a22 1686The C<POSIX::getattr> function can do this more portably on
2b5ab1e7
TC
1687systems purporting POSIX compliance. See also the C<Term::ReadKey>
1688module from your nearest CPAN site; details on CPAN can be found on
1689L<perlmodlib/CPAN>.
a0d0e21e
LW
1690
1691=item getlogin
1692
5a964f20
TC
1693Implements the C library function of the same name, which on most
1694systems returns the current login from F</etc/utmp>, if any. If null,
19799a22 1695use C<getpwuid>.
a0d0e21e 1696
f86702cc 1697 $login = getlogin || getpwuid($<) || "Kilroy";
a0d0e21e 1698
19799a22
GS
1699Do not consider C<getlogin> for authentication: it is not as
1700secure as C<getpwuid>.
4633a7c4 1701
a0d0e21e
LW
1702=item getpeername SOCKET
1703
1704Returns the packed sockaddr address of other end of the SOCKET connection.
1705
4633a7c4
LW
1706 use Socket;
1707 $hersockaddr = getpeername(SOCK);
19799a22 1708 ($port, $iaddr) = sockaddr_in($hersockaddr);
4633a7c4
LW
1709 $herhostname = gethostbyaddr($iaddr, AF_INET);
1710 $herstraddr = inet_ntoa($iaddr);
a0d0e21e
LW
1711
1712=item getpgrp PID
1713
47e29363 1714Returns the current process group for the specified PID. Use
7660c0ab 1715a PID of C<0> to get the current process group for the
4633a7c4 1716current process. Will raise an exception if used on a machine that
f86cebdf 1717doesn't implement getpgrp(2). If PID is omitted, returns process
19799a22 1718group of current process. Note that the POSIX version of C<getpgrp>
7660c0ab 1719does not accept a PID argument, so only C<PID==0> is truly portable.
a0d0e21e
LW
1720
1721=item getppid
1722
1723Returns the process id of the parent process.
1724
1725=item getpriority WHICH,WHO
1726
4633a7c4
LW
1727Returns the current priority for a process, a process group, or a user.
1728(See L<getpriority(2)>.) Will raise a fatal exception if used on a
f86cebdf 1729machine that doesn't implement getpriority(2).
a0d0e21e
LW
1730
1731=item getpwnam NAME
1732
1733=item getgrnam NAME
1734
1735=item gethostbyname NAME
1736
1737=item getnetbyname NAME
1738
1739=item getprotobyname NAME
1740
1741=item getpwuid UID
1742
1743=item getgrgid GID
1744
1745=item getservbyname NAME,PROTO
1746
1747=item gethostbyaddr ADDR,ADDRTYPE
1748
1749=item getnetbyaddr ADDR,ADDRTYPE
1750
1751=item getprotobynumber NUMBER
1752
1753=item getservbyport PORT,PROTO
1754
1755=item getpwent
1756
1757=item getgrent
1758
1759=item gethostent
1760
1761=item getnetent
1762
1763=item getprotoent
1764
1765=item getservent
1766
1767=item setpwent
1768
1769=item setgrent
1770
1771=item sethostent STAYOPEN
1772
1773=item setnetent STAYOPEN
1774
1775=item setprotoent STAYOPEN
1776
1777=item setservent STAYOPEN
1778
1779=item endpwent
1780
1781=item endgrent
1782
1783=item endhostent
1784
1785=item endnetent
1786
1787=item endprotoent
1788
1789=item endservent
1790
1791These routines perform the same functions as their counterparts in the
5a964f20 1792system library. In list context, the return values from the
a0d0e21e
LW
1793various get routines are as follows:
1794
1795 ($name,$passwd,$uid,$gid,
6ee623d5 1796 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
a0d0e21e
LW
1797 ($name,$passwd,$gid,$members) = getgr*
1798 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1799 ($name,$aliases,$addrtype,$net) = getnet*
1800 ($name,$aliases,$proto) = getproto*
1801 ($name,$aliases,$port,$proto) = getserv*
1802
1803(If the entry doesn't exist you get a null list.)
1804
5a964f20 1805In scalar context, you get the name, unless the function was a
a0d0e21e
LW
1806lookup by name, in which case you get the other thing, whatever it is.
1807(If the entry doesn't exist you get the undefined value.) For example:
1808
5a964f20
TC
1809 $uid = getpwnam($name);
1810 $name = getpwuid($num);
1811 $name = getpwent();
1812 $gid = getgrnam($name);
1813 $name = getgrgid($num;
1814 $name = getgrent();
1815 #etc.
a0d0e21e 1816
19799a22 1817In I<getpw*()> the fields $quota, $comment, and $expire are
2b5ab1e7 1818special cases in the sense that in many systems they are unsupported.
19799a22
GS
1819If the $quota is unsupported, it is an empty scalar. If it is
1820supported, it usually encodes the disk quota. If the $comment
2b5ab1e7
TC
1821field is unsupported, it is an empty scalar. If it is supported it
1822usually encodes some administrative comment about the user. In some
19799a22
GS
1823systems the $quota field may be $change or $age, fields that have
1824to do with password aging. In some systems the $comment field may
1825be $class. The $expire field, if present, encodes the expiration
2b5ab1e7
TC
1826period of the account or the password. For the availability and the
1827exact meaning of these fields in your system, please consult your
1828getpwnam(3) documentation and your F<pwd.h> file. You can also find
19799a22
GS
1829out from within Perl what your $quota and $comment fields mean
1830and whether you have the $expire field by using the C<Config> module
2b5ab1e7
TC
1831and the values C<d_pwquota>, C<d_pwage>, C<d_pwchange>, C<d_pwcomment>,
1832and C<d_pwexpire>. Shadow password files are only supported if your
1833vendor has implemented them in the intuitive fashion that calling the
1834regular C library routines gets the shadow versions if you're running
1835under privilege. Those that incorrectly implement a separate library
1836call are not supported.
6ee623d5 1837
19799a22 1838The $members value returned by I<getgr*()> is a space separated list of
a0d0e21e
LW
1839the login names of the members of the group.
1840
1841For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1842C, it will be returned to you via C<$?> if the function call fails. The
7660c0ab 1843C<@addrs> value returned by a successful call is a list of the raw
a0d0e21e
LW
1844addresses returned by the corresponding system library call. In the
1845Internet domain, each address is four bytes long and you can unpack it
1846by saying something like:
1847
1848 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1849
2b5ab1e7
TC
1850The Socket library makes this slightly easier:
1851
1852 use Socket;
1853 $iaddr = inet_aton("127.1"); # or whatever address
1854 $name = gethostbyaddr($iaddr, AF_INET);
1855
1856 # or going the other way
19799a22 1857 $straddr = inet_ntoa($iaddr);
2b5ab1e7 1858
19799a22
GS
1859If you get tired of remembering which element of the return list
1860contains which return value, by-name interfaces are provided
1861in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
1862C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
1863and C<User::grent>. These override the normal built-ins, supplying
1864versions that return objects with the appropriate names
1865for each field. For example:
5a964f20
TC
1866
1867 use File::stat;
1868 use User::pwent;
1869 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
1870
1871Even though it looks like they're the same method calls (uid),
19799a22
GS
1872they aren't, because a C<File::stat> object is different from
1873a C<User::pwent> object.
5a964f20 1874
a0d0e21e
LW
1875=item getsockname SOCKET
1876
19799a22
GS
1877Returns the packed sockaddr address of this end of the SOCKET connection,
1878in case you don't know the address because you have several different
1879IPs that the connection might have come in on.
a0d0e21e 1880
4633a7c4
LW
1881 use Socket;
1882 $mysockaddr = getsockname(SOCK);
19799a22
GS
1883 ($port, $myaddr) = sockaddr_in($mysockaddr);
1884 printf "Connect to %s [%s]\n",
1885 scalar gethostbyaddr($myaddr, AF_INET),
1886 inet_ntoa($myaddr);
a0d0e21e
LW
1887
1888=item getsockopt SOCKET,LEVEL,OPTNAME
1889
5a964f20 1890Returns the socket option requested, or undef if there is an error.
a0d0e21e
LW
1891
1892=item glob EXPR
1893
0a753a76
PP
1894=item glob
1895
2b5ab1e7
TC
1896Returns the value of EXPR with filename expansions such as the
1897standard Unix shell F</bin/csh> would do. This is the internal function
1898implementing the C<E<lt>*.cE<gt>> operator, but you can use it directly.
1899If EXPR is omitted, C<$_> is used. The C<E<lt>*.cE<gt>> operator is
1900discussed in more detail in L<perlop/"I/O Operators">.
a0d0e21e
LW
1901
1902=item gmtime EXPR
1903
19799a22 1904Converts a time as returned by the time function to a 9-element list
54310121 1905with the time localized for the standard Greenwich time zone.
4633a7c4 1906Typically used as follows:
a0d0e21e 1907
54310121 1908 # 0 1 2 3 4 5 6 7 8
a0d0e21e
LW
1909 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1910 gmtime(time);
1911
19799a22
GS
1912All list elements are numeric, and come straight out of a struct tm.
1913In particular this means that $mon has the range C<0..11> and $wday
1914has the range C<0..6> with sunday as day C<0>. Also, $year is the
1915number of years since 1900, that is, $year is C<123> in year 2023,
2b5ab1e7
TC
1916I<not> simply the last two digits of the year. If you assume it is,
1917then you create non-Y2K-compliant programs--and you wouldn't want to do
1918that, would you?
2f9daede 1919
abd75f24
GS
1920The proper way to get a complete 4-digit year is simply:
1921
1922 $year += 1900;
1923
1924And to get the last two digits of the year (e.g., '01' in 2001) do:
1925
1926 $year = sprintf("%02d", $year % 100);
1927
2f9daede 1928If EXPR is omitted, does C<gmtime(time())>.
a0d0e21e 1929
f86cebdf 1930In scalar context, returns the ctime(3) value:
0a753a76
PP
1931
1932 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
1933
19799a22 1934Also see the C<timegm> function provided by the C<Time::Local> module,
f86cebdf 1935and the strftime(3) function available via the POSIX module.
7660c0ab 1936
2b5ab1e7
TC
1937This scalar value is B<not> locale dependent (see L<perllocale>), but
1938is instead a Perl builtin. Also see the C<Time::Local> module, and the
1939strftime(3) and mktime(3) functions available via the POSIX module. To
7660c0ab
A
1940get somewhat similar but locale dependent date strings, set up your
1941locale environment variables appropriately (please see L<perllocale>)
1942and try for example:
1943
1944 use POSIX qw(strftime);
2b5ab1e7 1945 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
7660c0ab 1946
2b5ab1e7
TC
1947Note that the C<%a> and C<%b> escapes, which represent the short forms
1948of the day of the week and the month of the year, may not necessarily
1949be three characters wide in all locales.
0a753a76 1950
a0d0e21e
LW
1951=item goto LABEL
1952
748a9306
LW
1953=item goto EXPR
1954
a0d0e21e
LW
1955=item goto &NAME
1956
7660c0ab 1957The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
a0d0e21e 1958execution there. It may not be used to go into any construct that
7660c0ab 1959requires initialization, such as a subroutine or a C<foreach> loop. It
0a753a76 1960also can't be used to go into a construct that is optimized away,
19799a22 1961or to get out of a block or subroutine given to C<sort>.
0a753a76 1962It can be used to go almost anywhere else within the dynamic scope,
a0d0e21e 1963including out of subroutines, but it's usually better to use some other
19799a22 1964construct such as C<last> or C<die>. The author of Perl has never felt the
7660c0ab 1965need to use this form of C<goto> (in Perl, that is--C is another matter).
a0d0e21e 1966
7660c0ab
A
1967The C<goto-EXPR> form expects a label name, whose scope will be resolved
1968dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
748a9306
LW
1969necessarily recommended if you're optimizing for maintainability:
1970
1971 goto ("FOO", "BAR", "GLARCH")[$i];
1972
6cb9131c
GS
1973The C<goto-&NAME> form is quite different from the other forms of C<goto>.
1974In fact, it isn't a goto in the normal sense at all, and doesn't have
1975the stigma associated with other gotos. Instead, it
1976substitutes a call to the named subroutine for the currently running
1977subroutine. This is used by C<AUTOLOAD> subroutines that wish to load
1978another subroutine and then pretend that the other subroutine had been
1979called in the first place (except that any modifications to C<@_>
1980in the current subroutine are propagated to the other subroutine.)
1981After the C<goto>, not even C<caller> will be able to tell that this
1982routine was called first.
1983
1984NAME needn't be the name of a subroutine; it can be a scalar variable
1985containing a code reference, or a block which evaluates to a code
1986reference.
a0d0e21e
LW
1987
1988=item grep BLOCK LIST
1989
1990=item grep EXPR,LIST
1991
2b5ab1e7
TC
1992This is similar in spirit to, but not the same as, grep(1) and its
1993relatives. In particular, it is not limited to using regular expressions.
2f9daede 1994
a0d0e21e 1995Evaluates the BLOCK or EXPR for each element of LIST (locally setting
7660c0ab 1996C<$_> to each element) and returns the list value consisting of those
19799a22
GS
1997elements for which the expression evaluated to true. In scalar
1998context, returns the number of times the expression was true.
a0d0e21e
LW
1999
2000 @foo = grep(!/^#/, @bar); # weed out comments
2001
2002or equivalently,
2003
2004 @foo = grep {!/^#/} @bar; # weed out comments
2005
2b5ab1e7
TC
2006Note that, because C<$_> is a reference into the list value, it can
2007be used to modify the elements of the array. While this is useful and
2008supported, it can cause bizarre results if the LIST is not a named array.
2009Similarly, grep returns aliases into the original list, much as a for
2010loop's index variable aliases the list elements. That is, modifying an
19799a22
GS
2011element of a list returned by grep (for example, in a C<foreach>, C<map>
2012or another C<grep>) actually modifies the element in the original list.
2b5ab1e7 2013This is usually something to be avoided when writing clear code.
a0d0e21e 2014
19799a22 2015See also L</map> for a list composed of the results of the BLOCK or EXPR.
38325410 2016
a0d0e21e
LW
2017=item hex EXPR
2018
54310121 2019=item hex
bbce6d69 2020
2b5ab1e7
TC
2021Interprets EXPR as a hex string and returns the corresponding value.
2022(To convert strings that might start with either 0, 0x, or 0b, see
2023L</oct>.) If EXPR is omitted, uses C<$_>.
2f9daede
TPG
2024
2025 print hex '0xAf'; # prints '175'
2026 print hex 'aF'; # same
a0d0e21e 2027
19799a22 2028Hex strings may only represent integers. Strings that would cause
c6edd1b7 2029integer overflow trigger a warning.
19799a22 2030
a0d0e21e
LW
2031=item import
2032
19799a22 2033There is no builtin C<import> function. It is just an ordinary
4633a7c4 2034method (subroutine) defined (or inherited) by modules that wish to export
19799a22 2035names to another module. The C<use> function calls the C<import> method
54310121 2036for the package used. See also L</use()>, L<perlmod>, and L<Exporter>.
a0d0e21e
LW
2037
2038=item index STR,SUBSTR,POSITION
2039
2040=item index STR,SUBSTR
2041
2b5ab1e7
TC
2042The index function searches for one string within another, but without
2043the wildcard-like behavior of a full regular-expression pattern match.
2044It returns the position of the first occurrence of SUBSTR in STR at
2045or after POSITION. If POSITION is omitted, starts searching from the
2046beginning of the string. The return value is based at C<0> (or whatever
2047you've set the C<$[> variable to--but don't do that). If the substring
2048is not found, returns one less than the base, ordinarily C<-1>.
a0d0e21e
LW
2049
2050=item int EXPR
2051
54310121 2052=item int
bbce6d69 2053
7660c0ab 2054Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2b5ab1e7
TC
2055You should not use this function for rounding: one because it truncates
2056towards C<0>, and two because machine representations of floating point
2057numbers can sometimes produce counterintuitive results. For example,
2058C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2059because it's really more like -268.99999999999994315658 instead. Usually,
19799a22 2060the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2b5ab1e7 2061functions will serve you better than will int().
a0d0e21e
LW
2062
2063=item ioctl FILEHANDLE,FUNCTION,SCALAR
2064
2b5ab1e7 2065Implements the ioctl(2) function. You'll probably first have to say
a0d0e21e 2066
4633a7c4 2067 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
a0d0e21e 2068
2b5ab1e7 2069to get the correct function definitions. If F<ioctl.ph> doesn't
a0d0e21e 2070exist or doesn't have the correct definitions you'll have to roll your
4633a7c4 2071own, based on your C header files such as F<E<lt>sys/ioctl.hE<gt>>.
5a964f20 2072(There is a Perl script called B<h2ph> that comes with the Perl kit that
54310121 2073may help you in this, but it's nontrivial.) SCALAR will be read and/or
4633a7c4 2074written depending on the FUNCTION--a pointer to the string value of SCALAR
19799a22 2075will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
4633a7c4
LW
2076has no string value but does have a numeric value, that value will be
2077passed rather than a pointer to the string value. To guarantee this to be
19799a22
GS
2078true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2079functions may be needed to manipulate the values of structures used by
2080C<ioctl>.
a0d0e21e 2081
19799a22 2082The return value of C<ioctl> (and C<fcntl>) is as follows:
a0d0e21e
LW
2083
2084 if OS returns: then Perl returns:
2085 -1 undefined value
2086 0 string "0 but true"
2087 anything else that number
2088
19799a22 2089Thus Perl returns true on success and false on failure, yet you can
a0d0e21e
LW
2090still easily determine the actual value returned by the operating
2091system:
2092
2b5ab1e7 2093 $retval = ioctl(...) || -1;
a0d0e21e
LW
2094 printf "System returned %d\n", $retval;
2095
c2611fb3 2096The special string "C<0> but true" is exempt from B<-w> complaints
5a964f20
TC
2097about improper numeric conversions.
2098
19799a22
GS
2099Here's an example of setting a filehandle named C<REMOTE> to be
2100non-blocking at the system level. You'll have to negotiate C<$|>
2101on your own, though.
2102
2103 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
2104
2105 $flags = fcntl(REMOTE, F_GETFL, 0)
2106 or die "Can't get flags for the socket: $!\n";
2107
2108 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
2109 or die "Can't set flags for the socket: $!\n";
2110
a0d0e21e
LW
2111=item join EXPR,LIST
2112
2b5ab1e7
TC
2113Joins the separate strings of LIST into a single string with fields
2114separated by the value of EXPR, and returns that new string. Example:
a0d0e21e 2115
2b5ab1e7 2116 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
a0d0e21e 2117
eb6e2d6f
GS
2118Beware that unlike C<split>, C<join> doesn't take a pattern as its
2119first argument. Compare L</split>.
a0d0e21e 2120
aa689395
PP
2121=item keys HASH
2122
19799a22 2123Returns a list consisting of all the keys of the named hash. (In
1d2dff63 2124scalar context, returns the number of keys.) The keys are returned in
ab192400
GS
2125an apparently random order. The actual random order is subject to
2126change in future versions of perl, but it is guaranteed to be the same
19799a22 2127order as either the C<values> or C<each> function produces (given
ab192400
GS
2128that the hash has not been modified). As a side effect, it resets
2129HASH's iterator.
a0d0e21e 2130
aa689395 2131Here is yet another way to print your environment:
a0d0e21e
LW
2132
2133 @keys = keys %ENV;
2134 @values = values %ENV;
19799a22 2135 while (@keys) {
a0d0e21e
LW
2136 print pop(@keys), '=', pop(@values), "\n";
2137 }
2138
2139or how about sorted by key:
2140
2141 foreach $key (sort(keys %ENV)) {
2142 print $key, '=', $ENV{$key}, "\n";
2143 }
2144
19799a22 2145To sort a hash by value, you'll need to use a C<sort> function.
aa689395 2146Here's a descending numeric sort of a hash by its values:
4633a7c4 2147
5a964f20 2148 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
4633a7c4
LW
2149 printf "%4d %s\n", $hash{$key}, $key;
2150 }
2151
19799a22 2152As an lvalue C<keys> allows you to increase the number of hash buckets
aa689395
PP
2153allocated for the given hash. This can gain you a measure of efficiency if
2154you know the hash is going to get big. (This is similar to pre-extending
2155an array by assigning a larger number to $#array.) If you say
55497cff
PP
2156
2157 keys %hash = 200;
2158
ab192400
GS
2159then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2160in fact, since it rounds up to the next power of two. These
55497cff
PP
2161buckets will be retained even if you do C<%hash = ()>, use C<undef
2162%hash> if you want to free the storage while C<%hash> is still in scope.
2163You can't shrink the number of buckets allocated for the hash using
19799a22 2164C<keys> in this way (but you needn't worry about doing this by accident,
55497cff
PP
2165as trying has no effect).
2166
19799a22 2167See also C<each>, C<values> and C<sort>.
ab192400 2168
b350dd2f 2169=item kill SIGNAL, LIST
a0d0e21e 2170
b350dd2f 2171Sends a signal to a list of processes. Returns the number of
517db077
GS
2172processes successfully signaled (which is not necessarily the
2173same as the number actually killed).
a0d0e21e
LW
2174
2175 $cnt = kill 1, $child1, $child2;
2176 kill 9, @goners;
2177
b350dd2f
GS
2178If SIGNAL is zero, no signal is sent to the process. This is a
2179useful way to check that the process is alive and hasn't changed
2180its UID. See L<perlport> for notes on the portability of this
2181construct.
2182
2183Unlike in the shell, if SIGNAL is negative, it kills
4633a7c4
LW
2184process groups instead of processes. (On System V, a negative I<PROCESS>
2185number will also kill process groups, but that's not portable.) That
2186means you usually want to use positive not negative signals. You may also
da0045b7 2187use a signal name in quotes. See L<perlipc/"Signals"> for details.
a0d0e21e
LW
2188
2189=item last LABEL
2190
2191=item last
2192
2193The C<last> command is like the C<break> statement in C (as used in
2194loops); it immediately exits the loop in question. If the LABEL is
2195omitted, the command refers to the innermost enclosing loop. The
2196C<continue> block, if any, is not executed:
2197
4633a7c4
LW
2198 LINE: while (<STDIN>) {
2199 last LINE if /^$/; # exit when done with header
5a964f20 2200 #...
a0d0e21e
LW
2201 }
2202
4968c1e4 2203C<last> cannot be used to exit a block which returns a value such as
2b5ab1e7
TC
2204C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2205a grep() or map() operation.
4968c1e4 2206
6c1372ed
GS
2207Note that a block by itself is semantically identical to a loop
2208that executes once. Thus C<last> can be used to effect an early
2209exit out of such a block.
2210
98293880
JH
2211See also L</continue> for an illustration of how C<last>, C<next>, and
2212C<redo> work.
1d2dff63 2213
a0d0e21e
LW
2214=item lc EXPR
2215
54310121 2216=item lc
bbce6d69 2217
a0d0e21e 2218Returns an lowercased version of EXPR. This is the internal function
7660c0ab 2219implementing the C<\L> escape in double-quoted strings.
19799a22
GS
2220Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
2221and L<utf8>.
a0d0e21e 2222
7660c0ab 2223If EXPR is omitted, uses C<$_>.
bbce6d69 2224
a0d0e21e
LW
2225=item lcfirst EXPR
2226
54310121 2227=item lcfirst
bbce6d69 2228
a0d0e21e 2229Returns the value of EXPR with the first character lowercased. This is
7660c0ab 2230the internal function implementing the C<\l> escape in double-quoted strings.
a0ed51b3 2231Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
a0d0e21e 2232
7660c0ab 2233If EXPR is omitted, uses C<$_>.
bbce6d69 2234
a0d0e21e
LW
2235=item length EXPR
2236
54310121 2237=item length
bbce6d69 2238
a0ed51b3 2239Returns the length in characters of the value of EXPR. If EXPR is
2b5ab1e7
TC
2240omitted, returns length of C<$_>. Note that this cannot be used on
2241an entire array or hash to find out how many elements these have.
2242For that, use C<scalar @array> and C<scalar keys %hash> respectively.
a0d0e21e
LW
2243
2244=item link OLDFILE,NEWFILE
2245
19799a22
GS
2246Creates a new filename linked to the old filename. Returns true for
2247success, false otherwise.
a0d0e21e
LW
2248
2249=item listen SOCKET,QUEUESIZE
2250
19799a22
GS
2251Does the same thing that the listen system call does. Returns true if
2252it succeeded, false otherwise. See the example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
2253
2254=item local EXPR
2255
19799a22 2256You really probably want to be using C<my> instead, because C<local> isn't
2b5ab1e7
TC
2257what most people think of as "local". See L<perlsub/"Private Variables
2258via my()"> for details.
2259
5a964f20
TC
2260A local modifies the listed variables to be local to the enclosing
2261block, file, or eval. If more than one value is listed, the list must
2262be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2263for details, including issues with tied arrays and hashes.
a0d0e21e 2264
a0d0e21e
LW
2265=item localtime EXPR
2266
19799a22 2267Converts a time as returned by the time function to a 9-element list
5f05dabc 2268with the time analyzed for the local time zone. Typically used as
a0d0e21e
LW
2269follows:
2270
54310121 2271 # 0 1 2 3 4 5 6 7 8
a0d0e21e
LW
2272 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2273 localtime(time);
2274
19799a22
GS
2275All list elements are numeric, and come straight out of a struct tm.
2276In particular this means that $mon has the range C<0..11> and $wday
2277has the range C<0..6> with sunday as day C<0>. Also, $year is the
2278number of years since 1900, that is, $year is C<123> in year 2023,
2b5ab1e7
TC
2279and I<not> simply the last two digits of the year. If you assume it is,
2280then you create non-Y2K-compliant programs--and you wouldn't want to do
2281that, would you?
54310121 2282
abd75f24
GS
2283The proper way to get a complete 4-digit year is simply:
2284
2285 $year += 1900;
2286
2287And to get the last two digits of the year (e.g., '01' in 2001) do:
2288
2289 $year = sprintf("%02d", $year % 100);
2290
54310121 2291If EXPR is omitted, uses the current time (C<localtime(time)>).
a0d0e21e 2292
f86cebdf 2293In scalar context, returns the ctime(3) value:
a0d0e21e 2294
5f05dabc 2295 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
a0d0e21e 2296
a3cb178b 2297This scalar value is B<not> locale dependent, see L<perllocale>, but
68f8bed4
JH
2298instead a Perl builtin. Also see the C<Time::Local> module
2299(to convert the second, minutes, hours, ... back to seconds since the
2300stroke of midnight the 1st of January 1970, the value returned by
2301time()), and the strftime(3) and mktime(3) function available via the
2302POSIX module. To get somewhat similar but locale dependent date
2303strings, set up your locale environment variables appropriately
2304(please see L<perllocale>) and try for example:
a3cb178b 2305
5a964f20 2306 use POSIX qw(strftime);
2b5ab1e7 2307 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
a3cb178b
GS
2308
2309Note that the C<%a> and C<%b>, the short forms of the day of the week
2310and the month of the year, may not necessarily be three characters wide.
a0d0e21e 2311
19799a22
GS
2312=item lock
2313
2314 lock I<THING>
2315
2316This function places an advisory lock on a variable, subroutine,
2317or referenced object contained in I<THING> until the lock goes out
2318of scope. This is a built-in function only if your version of Perl
2319was built with threading enabled, and if you've said C<use Threads>.
2320Otherwise a user-defined function by this name will be called. See
2321L<Thread>.
2322
a0d0e21e
LW
2323=item log EXPR
2324
54310121 2325=item log
bbce6d69 2326
2b5ab1e7
TC
2327Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2328returns log of C<$_>. To get the log of another base, use basic algebra:
19799a22 2329The base-N log of a number is equal to the natural log of that number
2b5ab1e7
TC
2330divided by the natural log of N. For example:
2331
2332 sub log10 {
2333 my $n = shift;
2334 return log($n)/log(10);
2335 }
2336
2337See also L</exp> for the inverse operation.
a0d0e21e
LW
2338
2339=item lstat FILEHANDLE
2340
2341=item lstat EXPR
2342
54310121 2343=item lstat
bbce6d69 2344
19799a22 2345Does the same thing as the C<stat> function (including setting the
5a964f20
TC
2346special C<_> filehandle) but stats a symbolic link instead of the file
2347the symbolic link points to. If symbolic links are unimplemented on
19799a22 2348your system, a normal C<stat> is done.
a0d0e21e 2349
7660c0ab 2350If EXPR is omitted, stats C<$_>.
bbce6d69 2351
a0d0e21e
LW
2352=item m//
2353
2354The match operator. See L<perlop>.
2355
2356=item map BLOCK LIST
2357
2358=item map EXPR,LIST
2359
19799a22
GS
2360Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2361C<$_> to each element) and returns the list value composed of the
2362results of each such evaluation. In scalar context, returns the
2363total number of elements so generated. Evaluates BLOCK or EXPR in
2364list context, so each element of LIST may produce zero, one, or
2365more elements in the returned value.
dd99ebda 2366
a0d0e21e
LW
2367 @chars = map(chr, @nums);
2368
2369translates a list of numbers to the corresponding characters. And
2370
4633a7c4 2371 %hash = map { getkey($_) => $_ } @array;
a0d0e21e
LW
2372
2373is just a funny way to write
2374
2375 %hash = ();
2376 foreach $_ (@array) {
4633a7c4 2377 $hash{getkey($_)} = $_;
a0d0e21e
LW
2378 }
2379
2b5ab1e7
TC
2380Note that, because C<$_> is a reference into the list value, it can
2381be used to modify the elements of the array. While this is useful and
2382supported, it can cause bizarre results if the LIST is not a named array.
2383Using a regular C<foreach> loop for this purpose would be clearer in
2384most cases. See also L</grep> for an array composed of those items of
2385the original list for which the BLOCK or EXPR evaluates to true.
fb73857a 2386
19799a22 2387=item mkdir FILENAME,MASK
a0d0e21e 2388
0591cd52 2389Creates the directory specified by FILENAME, with permissions
19799a22
GS
2390specified by MASK (as modified by C<umask>). If it succeeds it
2391returns true, otherwise it returns false and sets C<$!> (errno).
0591cd52 2392
19799a22 2393In general, it is better to create directories with permissive MASK,
0591cd52 2394and let the user modify that with their C<umask>, than it is to supply
19799a22 2395a restrictive MASK and give the user no way to be more permissive.
0591cd52
NT
2396The exceptions to this rule are when the file or directory should be
2397kept private (mail files, for instance). The perlfunc(1) entry on
19799a22 2398C<umask> discusses the choice of MASK in more detail.
a0d0e21e
LW
2399
2400=item msgctl ID,CMD,ARG
2401
f86cebdf 2402Calls the System V IPC function msgctl(2). You'll probably have to say
0ade1984
JH
2403
2404 use IPC::SysV;
2405
7660c0ab
A
2406first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2407then ARG must be a variable which will hold the returned C<msqid_ds>
951ba7fe
GS
2408structure. Returns like C<ioctl>: the undefined value for error,
2409C<"0 but true"> for zero, or the actual return value otherwise. See also
19799a22 2410C<IPC::SysV> and C<IPC::Semaphore> documentation.
a0d0e21e
LW
2411
2412=item msgget KEY,FLAGS
2413
f86cebdf 2414Calls the System V IPC function msgget(2). Returns the message queue
7660c0ab 2415id, or the undefined value if there is an error. See also C<IPC::SysV>
19799a22 2416and C<IPC::Msg> documentation.
a0d0e21e
LW
2417
2418=item msgsnd ID,MSG,FLAGS
2419
2420Calls the System V IPC function msgsnd to send the message MSG to the
2421message queue ID. MSG must begin with the long integer message type,
19799a22
GS
2422which may be created with C<pack("l", $type)>. Returns true if
2423successful, or false if there is an error. See also C<IPC::SysV>
7660c0ab 2424and C<IPC::SysV::Msg> documentation.
a0d0e21e
LW
2425
2426=item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2427
2428Calls the System V IPC function msgrcv to receive a message from
2429message queue ID into variable VAR with a maximum message size of
0ade1984
JH
2430SIZE. Note that if a message is received, the message type will be
2431the first thing in VAR, and the maximum length of VAR is SIZE plus the
19799a22 2432size of the message type. Returns true if successful, or false if
7660c0ab 2433there is an error. See also C<IPC::SysV> and C<IPC::SysV::Msg> documentation.
a0d0e21e
LW
2434
2435=item my EXPR
2436
09bef843
SB
2437=item my EXPR : ATTRIBUTES
2438
19799a22
GS
2439A C<my> declares the listed variables to be local (lexically) to the
2440enclosing block, file, or C<eval>. If
5f05dabc 2441more than one value is listed, the list must be placed in parentheses. See
cb1a09d0 2442L<perlsub/"Private Variables via my()"> for details.
4633a7c4 2443
a0d0e21e
LW
2444=item next LABEL
2445
2446=item next
2447
2448The C<next> command is like the C<continue> statement in C; it starts
2449the next iteration of the loop:
2450
4633a7c4
LW
2451 LINE: while (<STDIN>) {
2452 next LINE if /^#/; # discard comments
5a964f20 2453 #...
a0d0e21e
LW
2454 }
2455
2456Note that if there were a C<continue> block on the above, it would get
2457executed even on discarded lines. If the LABEL is omitted, the command
2458refers to the innermost enclosing loop.
2459
4968c1e4 2460C<next> cannot be used to exit a block which returns a value such as
2b5ab1e7
TC
2461C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2462a grep() or map() operation.
4968c1e4 2463
6c1372ed
GS
2464Note that a block by itself is semantically identical to a loop
2465that executes once. Thus C<next> will exit such a block early.
2466
98293880
JH
2467See also L</continue> for an illustration of how C<last>, C<next>, and
2468C<redo> work.
1d2dff63 2469
a0d0e21e
LW
2470=item no Module LIST
2471
7660c0ab 2472See the L</use> function, which C<no> is the opposite of.
a0d0e21e
LW
2473
2474=item oct EXPR
2475
54310121 2476=item oct
bbce6d69 2477
4633a7c4 2478Interprets EXPR as an octal string and returns the corresponding
4f19785b
WSI
2479value. (If EXPR happens to start off with C<0x>, interprets it as a
2480hex string. If EXPR starts off with C<0b>, it is interpreted as a
2481binary string.) The following will handle decimal, binary, octal, and
4633a7c4 2482hex in the standard Perl or C notation:
a0d0e21e
LW
2483
2484 $val = oct($val) if $val =~ /^0/;
2485
19799a22
GS
2486If EXPR is omitted, uses C<$_>. To go the other way (produce a number
2487in octal), use sprintf() or printf():
2488
2489 $perms = (stat("filename"))[2] & 07777;
2490 $oct_perms = sprintf "%lo", $perms;
2491
2492The oct() function is commonly used when a string such as C<644> needs
2493to be converted into a file mode, for example. (Although perl will
2494automatically convert strings into numbers as needed, this automatic
2495conversion assumes base 10.)
a0d0e21e 2496
6170680b
IZ
2497=item open FILEHANDLE,MODE,EXPR
2498
a0d0e21e
LW
2499=item open FILEHANDLE,EXPR
2500
2501=item open FILEHANDLE
2502
2503Opens the file whose filename is given by EXPR, and associates it with
5f05dabc
PP
2504FILEHANDLE. If FILEHANDLE is an expression, its value is used as the
2505name of the real filehandle wanted. If EXPR is omitted, the scalar
2506variable of the same name as the FILEHANDLE contains the filename.
19799a22
GS
2507(Note that lexical variables--those declared with C<my>--will not work
2508for this purpose; so if you're using C<my>, specify EXPR in your call
2b5ab1e7
TC
2509to open.) See L<perlopentut> for a kinder, gentler explanation of opening
2510files.
5f05dabc 2511
6170680b
IZ
2512If MODE is C<'E<lt>'> or nothing, the file is opened for input.
2513If MODE is C<'E<gt>'>, the file is truncated and opened for
2514output, being created if necessary. If MODE is C<'E<gt>E<gt>'>,
fbb426e4 2515the file is opened for appending, again being created if necessary.
7660c0ab
A
2516You can put a C<'+'> in front of the C<'E<gt>'> or C<'E<lt>'> to indicate that
2517you want both read and write access to the file; thus C<'+E<lt>'> is almost
2518always preferred for read/write updates--the C<'+E<gt>'> mode would clobber the
5a964f20
TC
2519file first. You can't usually use either read-write mode for updating
2520textfiles, since they have variable length records. See the B<-i>
0591cd52
NT
2521switch in L<perlrun> for a better approach. The file is created with
2522permissions of C<0666> modified by the process' C<umask> value.
5a964f20 2523
f86cebdf 2524These various prefixes correspond to the fopen(3) modes of C<'r'>, C<'r+'>, C<'w'>,
7660c0ab 2525C<'w+'>, C<'a'>, and C<'a+'>.
5f05dabc 2526
6170680b
IZ
2527In the 2-arguments (and 1-argument) form of the call the mode and
2528filename should be concatenated (in this order), possibly separated by
2529spaces. It is possible to omit the mode if the mode is C<'E<lt>'>.
2530
7660c0ab 2531If the filename begins with C<'|'>, the filename is interpreted as a
5a964f20 2532command to which output is to be piped, and if the filename ends with a
f244e06d
GS
2533C<'|'>, the filename is interpreted as a command which pipes output to
2534us. See L<perlipc/"Using open() for IPC">
19799a22 2535for more examples of this. (You are not allowed to C<open> to a command
5a964f20
TC
2536that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2537and L<perlipc/"Bidirectional Communication"> for alternatives.)
cb1a09d0 2538
6170680b
IZ
2539If MODE is C<'|-'>, the filename is interpreted as a
2540command to which output is to be piped, and if MODE is
2541C<'-|'>, the filename is interpreted as a command which pipes output to
2542us. In the 2-arguments (and 1-argument) form one should replace dash
2543(C<'-'>) with the command. See L<perlipc/"Using open() for IPC">
2544for more examples of this. (You are not allowed to C<open> to a command
2545that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2546and L<perlipc/"Bidirectional Communication"> for alternatives.)
2547
2548In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
2549and opening C<'E<gt>-'> opens STDOUT.
2550
2551Open returns
19799a22 2552nonzero upon success, the undefined value otherwise. If the C<open>
4633a7c4 2553involved a pipe, the return value happens to be the pid of the
54310121 2554subprocess.
cb1a09d0
AD
2555
2556If you're unfortunate enough to be running Perl on a system that
2557distinguishes between text files and binary files (modern operating
2558systems don't care), then you should check out L</binmode> for tips for
19799a22 2559dealing with this. The key distinction between systems that need C<binmode>
5a964f20
TC
2560and those that don't is their text file formats. Systems like Unix, MacOS, and
2561Plan9, which delimit lines with a single character, and which encode that
19799a22 2562character in C as C<"\n">, do not need C<binmode>. The rest need it.
cb1a09d0 2563
fb73857a 2564When opening a file, it's usually a bad idea to continue normal execution
19799a22
GS
2565if the request failed, so C<open> is frequently used in connection with
2566C<die>. Even if C<die> won't do what you want (say, in a CGI script,
fb73857a 2567where you want to make a nicely formatted error message (but there are
5a964f20 2568modules that can help with that problem)) you should always check
19799a22 2569the return value from opening a file. The infrequent exception is when
fb73857a
PP
2570working with an unopened filehandle is actually what you want to do.
2571
cb1a09d0 2572Examples:
a0d0e21e
LW
2573
2574 $ARTICLE = 100;
2575 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2576 while (<ARTICLE>) {...
2577
6170680b 2578 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
fb73857a 2579 # if the open fails, output is discarded
a0d0e21e 2580
6170680b 2581 open(DBASE, '+<', 'dbase.mine') # open for update
fb73857a 2582 or die "Can't open 'dbase.mine' for update: $!";
cb1a09d0 2583
6170680b
IZ
2584 open(DBASE, '+<dbase.mine') # ditto
2585 or die "Can't open 'dbase.mine' for update: $!";
2586
2587 open(ARTICLE, '-|', "caesar <$article") # decrypt article
fb73857a 2588 or die "Can't start caesar: $!";
a0d0e21e 2589
6170680b
IZ
2590 open(ARTICLE, "caesar <$article |") # ditto
2591 or die "Can't start caesar: $!";
2592
2593 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
fb73857a 2594 or die "Can't start sort: $!";
a0d0e21e
LW
2595
2596 # process argument list of files along with any includes
2597
2598 foreach $file (@ARGV) {
2599 process($file, 'fh00');
2600 }
2601
2602 sub process {
5a964f20 2603 my($filename, $input) = @_;
a0d0e21e
LW
2604 $input++; # this is a string increment
2605 unless (open($input, $filename)) {
2606 print STDERR "Can't open $filename: $!\n";
2607 return;
2608 }
2609
5a964f20 2610 local $_;
a0d0e21e
LW
2611 while (<$input>) { # note use of indirection
2612 if (/^#include "(.*)"/) {
2613 process($1, $input);
2614 next;
2615 }
5a964f20 2616 #... # whatever
a0d0e21e
LW
2617 }
2618 }
2619
2620You may also, in the Bourne shell tradition, specify an EXPR beginning
7660c0ab 2621with C<'E<gt>&'>, in which case the rest of the string is interpreted as the
5a964f20 2622name of a filehandle (or file descriptor, if numeric) to be
6170680b
IZ
2623duped and opened. You may use C<&> after C<E<gt>>, C<E<gt>E<gt>>,
2624C<E<lt>>, C<+E<gt>>, C<+E<gt>E<gt>>, and C<+E<lt>>. The
a0d0e21e 2625mode you specify should match the mode of the original filehandle.
184e9718 2626(Duping a filehandle does not take into account any existing contents of
6170680b
IZ
2627stdio buffers.) Duping file handles is not yet supported for 3-argument
2628open().
2629
a0d0e21e
LW
2630Here is a script that saves, redirects, and restores STDOUT and
2631STDERR:
2632
2633 #!/usr/bin/perl
5a964f20
TC
2634 open(OLDOUT, ">&STDOUT");
2635 open(OLDERR, ">&STDERR");
a0d0e21e 2636
6170680b
IZ
2637 open(STDOUT, '>', "foo.out") || die "Can't redirect stdout";
2638 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
a0d0e21e
LW
2639
2640 select(STDERR); $| = 1; # make unbuffered
2641 select(STDOUT); $| = 1; # make unbuffered
2642
2643 print STDOUT "stdout 1\n"; # this works for
2644 print STDERR "stderr 1\n"; # subprocesses too
2645
2646 close(STDOUT);
2647 close(STDERR);
2648
5a964f20
TC
2649 open(STDOUT, ">&OLDOUT");
2650 open(STDERR, ">&OLDERR");
a0d0e21e
LW
2651
2652 print STDOUT "stdout 2\n";
2653 print STDERR "stderr 2\n";
2654
7660c0ab 2655If you specify C<'E<lt>&=N'>, where C<N> is a number, then Perl will do an
19799a22 2656equivalent of C's C<fdopen> of that file descriptor; this is more
4633a7c4 2657parsimonious of file descriptors. For example:
a0d0e21e
LW
2658
2659 open(FILEHANDLE, "<&=$fd")
2660
4af147f6
CS
2661Note that this feature depends on the fdopen() C library function.
2662On many UNIX systems, fdopen() is known to fail when file descriptors
2663exceed a certain value, typically 255. If you need more file
2664descriptors than that, consider rebuilding Perl to use the C<sfio>
2665library.
2666
6170680b
IZ
2667If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
2668with 2-arguments (or 1-argument) form of open(), then
a0d0e21e 2669there is an implicit fork done, and the return value of open is the pid
7660c0ab 2670of the child within the parent process, and C<0> within the child
184e9718 2671process. (Use C<defined($pid)> to determine whether the open was successful.)
a0d0e21e
LW
2672The filehandle behaves normally for the parent, but i/o to that
2673filehandle is piped from/to the STDOUT/STDIN of the child process.
2674In the child process the filehandle isn't opened--i/o happens from/to
2675the new STDOUT or STDIN. Typically this is used like the normal
2676piped open when you want to exercise more control over just how the
2677pipe command gets executed, such as when you are running setuid, and
54310121 2678don't want to have to scan shell commands for metacharacters.
6170680b 2679The following triples are more or less equivalent:
a0d0e21e
LW
2680
2681 open(FOO, "|tr '[a-z]' '[A-Z]'");
6170680b
IZ
2682 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
2683 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
a0d0e21e
LW
2684
2685 open(FOO, "cat -n '$file'|");
6170680b
IZ
2686 open(FOO, '-|', "cat -n '$file'");
2687 open(FOO, '-|') || exec 'cat', '-n', $file;
a0d0e21e 2688
4633a7c4
LW
2689See L<perlipc/"Safe Pipe Opens"> for more examples of this.
2690
45bc9206
GS
2691NOTE: On any operation that may do a fork, all files opened for output
2692are flushed before the fork is attempted. On systems that support a
2693close-on-exec flag on files, the flag will be set for the newly opened
2694file descriptor as determined by the value of $^F. See L<perlvar/$^F>.
a0d0e21e 2695
0dccf244
CS
2696Closing any piped filehandle causes the parent process to wait for the
2697child to finish, and returns the status value in C<$?>.
2698
6170680b
IZ
2699The filename passed to 2-argument (or 1-argument) form of open()
2700will have leading and trailing
f86cebdf 2701whitespace deleted, and the normal redirection characters
5a964f20
TC
2702honored. This property, known as "magic open",
2703can often be used to good effect. A user could specify a filename of
7660c0ab 2704F<"rsh cat file |">, or you could change certain filenames as needed:
5a964f20
TC
2705
2706 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
2707 open(FH, $filename) or die "Can't open $filename: $!";
2708
6170680b
IZ
2709Use 3-argument form to open a file with arbitrary weird characters in it,
2710
2711 open(FOO, '<', $file);
2712
2713otherwise it's necessary to protect any leading and trailing whitespace:
5a964f20
TC
2714
2715 $file =~ s#^(\s)#./$1#;
2716 open(FOO, "< $file\0");
2717
6170680b
IZ
2718(this may not work on some bizzare filesystems). One should
2719conscientiously choose between the the I<magic> and 3-arguments form
2720of open():
2721
2722 open IN, $ARGV[0];
2723
2724will allow the user to specify an argument of the form C<"rsh cat file |">,
2725but will not work on a filename which happens to have a trailing space, while
2726
2727 open IN, '<', $ARGV[0];
2728
2729will have exactly the opposite restrictions.
2730
19799a22 2731If you want a "real" C C<open> (see L<open(2)> on your system), then you
6170680b
IZ
2732should use the C<sysopen> function, which involves no such magic (but
2733may use subtly different filemodes than Perl open(), which is mapped
2734to C fopen()). This is
5a964f20
TC
2735another way to protect your filenames from interpretation. For example:
2736
2737 use IO::Handle;
2738 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
2739 or die "sysopen $path: $!";
2740 $oldfh = select(HANDLE); $| = 1; select($oldfh);
2741 print HANDLE "stuff $$\n");
2742 seek(HANDLE, 0, 0);
2743 print "File contains: ", <HANDLE>;
2744
7660c0ab
A
2745Using the constructor from the C<IO::Handle> package (or one of its
2746subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
5a964f20
TC
2747filehandles that have the scope of whatever variables hold references to
2748them, and automatically close whenever and however you leave that scope:
c07a80fd 2749
5f05dabc 2750 use IO::File;
5a964f20 2751 #...
c07a80fd
PP
2752 sub read_myfile_munged {
2753 my $ALL = shift;
5f05dabc 2754 my $handle = new IO::File;
c07a80fd
PP
2755 open($handle, "myfile") or die "myfile: $!";
2756 $first = <$handle>
2757 or return (); # Automatically closed here.
2758 mung $first or die "mung failed"; # Or here.
2759 return $first, <$handle> if $ALL; # Or here.
2760 $first; # Or here.
2761 }
2762
b687b08b 2763See L</seek> for some details about mixing reading and writing.
a0d0e21e
LW
2764
2765=item opendir DIRHANDLE,EXPR
2766
19799a22
GS
2767Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
2768C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
a0d0e21e
LW
2769DIRHANDLEs have their own namespace separate from FILEHANDLEs.
2770
2771=item ord EXPR
2772
54310121 2773=item ord
bbce6d69 2774
a0ed51b3 2775Returns the numeric (ASCII or Unicode) value of the first character of EXPR. If
7660c0ab 2776EXPR is omitted, uses C<$_>. For the reverse, see L</chr>.
2b5ab1e7 2777See L<utf8> for more about Unicode.
a0d0e21e 2778
77ca0c92
LW
2779=item our EXPR
2780
2781An C<our> declares the listed variables to be valid globals within
2782the enclosing block, file, or C<eval>. That is, it has the same
2783scoping rules as a "my" declaration, but does not create a local
2784variable. If more than one value is listed, the list must be placed
2785in parentheses. The C<our> declaration has no semantic effect unless
2786"use strict vars" is in effect, in which case it lets you use the
2787declared global variable without qualifying it with a package name.
2788(But only within the lexical scope of the C<our> declaration. In this
2789it differs from "use vars", which is package scoped.)
2790
a0d0e21e
LW
2791=item pack TEMPLATE,LIST
2792
2b6c5635
GS
2793Takes a LIST of values and converts it into a string using the rules
2794given by the TEMPLATE. The resulting string is the concatenation of
2795the converted values. Typically, each converted value looks
2796like its machine-level representation. For example, on 32-bit machines
2797a converted integer may be represented by a sequence of 4 bytes.
2798
2799The TEMPLATE is a
a0d0e21e
LW
2800sequence of characters that give the order and type of values, as
2801follows:
2802
5a929a98 2803 a A string with arbitrary binary data, will be null padded.
a0d0e21e 2804 A An ascii string, will be space padded.
5a929a98
VU
2805 Z A null terminated (asciz) string, will be null padded.
2806
2b6c5635
GS
2807 b A bit string (ascending bit order inside each byte, like vec()).
2808 B A bit string (descending bit order inside each byte).
a0d0e21e
LW
2809 h A hex string (low nybble first).
2810 H A hex string (high nybble first).
2811
2812 c A signed char value.
a0ed51b3 2813 C An unsigned char value. Only does bytes. See U for Unicode.
96e4d5b1 2814
a0d0e21e
LW
2815 s A signed short value.
2816 S An unsigned short value.
96e4d5b1 2817 (This 'short' is _exactly_ 16 bits, which may differ from
851646ae
JH
2818 what a local C compiler calls 'short'. If you want
2819 native-length shorts, use the '!' suffix.)
96e4d5b1 2820
a0d0e21e
LW
2821 i A signed integer value.
2822 I An unsigned integer value.
19799a22 2823 (This 'integer' is _at_least_ 32 bits wide. Its exact
f86cebdf
GS
2824 size depends on what a local C compiler calls 'int',
2825 and may even be larger than the 'long' described in
2826 the next item.)
96e4d5b1 2827
a0d0e21e
LW
2828 l A signed long value.
2829 L An unsigned long value.
96e4d5b1 2830 (This 'long' is _exactly_ 32 bits, which may differ from
851646ae
JH
2831 what a local C compiler calls 'long'. If you want
2832 native-length longs, use the '!' suffix.)
a0d0e21e 2833
5d11dd56
G
2834 n An unsigned short in "network" (big-endian) order.
2835 N An unsigned long in "network" (big-endian) order.
2836 v An unsigned short in "VAX" (little-endian) order.
2837 V An unsigned long in "VAX" (little-endian) order.
96e4d5b1
PP
2838 (These 'shorts' and 'longs' are _exactly_ 16 bits and
2839 _exactly_ 32 bits, respectively.)
a0d0e21e 2840
dae0da7a
JH
2841 q A signed quad (64-bit) value.
2842 Q An unsigned quad value.
851646ae
JH
2843 (Quads are available only if your system supports 64-bit
2844 integer values _and_ if Perl has been compiled to support those.
dae0da7a
JH
2845 Causes a fatal error otherwise.)
2846
a0d0e21e
LW
2847 f A single-precision float in the native format.
2848 d A double-precision float in the native format.
2849
2850 p A pointer to a null-terminated string.
2851 P A pointer to a structure (fixed-length string).
2852
2853 u A uuencoded string.
a0ed51b3
LW
2854 U A Unicode character number. Encodes to UTF-8 internally.
2855 Works even if C<use utf8> is not in effect.
a0d0e21e 2856
96e4d5b1 2857 w A BER compressed integer. Its bytes represent an unsigned
f86cebdf
GS
2858 integer in base 128, most significant digit first, with as
2859 few digits as possible. Bit eight (the high bit) is set
2860 on each byte except the last.
def98dd4 2861
a0d0e21e
LW
2862 x A null byte.
2863 X Back up a byte.
2864 @ Null fill to absolute position.
2865
5a929a98
VU
2866The following rules apply:
2867
2868=over 8
2869
2870=item *
2871
5a964f20 2872Each letter may optionally be followed by a number giving a repeat
951ba7fe
GS
2873count. With all types except C<a>, C<A>, C<Z>, C<b>, C<B>, C<h>,
2874C<H>, and C<P> the pack function will gobble up that many values from
5a929a98 2875the LIST. A C<*> for the repeat count means to use however many items are
951ba7fe
GS
2876left, except for C<@>, C<x>, C<X>, where it is equivalent
2877to C<0>, and C<u>, where it is equivalent to 1 (or 45, what is the
2b6c5635
GS
2878same).
2879
951ba7fe 2880When used with C<Z>, C<*> results in the addition of a trailing null
2b6c5635
GS
2881byte (so the packed result will be one longer than the byte C<length>
2882of the item).
2883
951ba7fe 2884The repeat count for C<u> is interpreted as the maximal number of bytes
2b6c5635 2885to encode per line of output, with 0 and 1 replaced by 45.
5a929a98
VU
2886
2887=item *
2888
951ba7fe 2889The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
5a929a98 2890string of length count, padding with nulls or spaces as necessary. When
951ba7fe
GS
2891unpacking, C<A> strips trailing spaces and nulls, C<Z> strips everything
2892after the first null, and C<a> returns data verbatim. When packing,
2893C<a>, and C<Z> are equivalent.
2b6c5635
GS
2894
2895If the value-to-pack is too long, it is truncated. If too long and an
951ba7fe
GS
2896explicit count is provided, C<Z> packs only C<$count-1> bytes, followed
2897by a null byte. Thus C<Z> always packs a trailing null byte under
2b6c5635 2898all circumstances.
5a929a98
VU
2899
2900=item *
2901
951ba7fe 2902Likewise, the C<b> and C<B> fields pack a string that many bits long.
c73032f5
IZ
2903Each byte of the input field of pack() generates 1 bit of the result.
2904Each result bit is based on the least-significant bit of the corresponding
2905input byte, i.e., on C<ord($byte)%2>. In particular, bytes C<"0"> and
2906C<"1"> generate bits 0 and 1, as do bytes C<"\0"> and C<"\1">.
2907
2908Starting from the beginning of the input string of pack(), each 8-tuple
951ba7fe 2909of bytes is converted to 1 byte of output. With format C<b>
c73032f5 2910the first byte of the 8-tuple determines the least-significant bit of a
951ba7fe 2911byte, and with format C<B> it determines the most-significant bit of
c73032f5
IZ
2912a byte.
2913
2914If the length of the input string is not exactly divisible by 8, the
2915remainder is packed as if the input string were padded by null bytes
2916at the end. Similarly, during unpack()ing the "extra" bits are ignored.
2917
2918If the input string of pack() is longer than needed, extra bytes are ignored.
2b6c5635
GS
2919A C<*> for the repeat count of pack() means to use all the bytes of
2920the input field. On unpack()ing the bits are converted to a string
2921of C<"0">s and C<"1">s.
5a929a98
VU
2922
2923=item *
2924
951ba7fe 2925The C<h> and C<H> fields pack a string that many nybbles (4-bit groups,
851646ae 2926representable as hexadecimal digits, 0-9a-f) long.
5a929a98 2927
c73032f5
IZ
2928Each byte of the input field of pack() generates 4 bits of the result.
2929For non-alphabetical bytes the result is based on the 4 least-significant
2930bits of the input byte, i.e., on C<ord($byte)%16>. In particular,
2931bytes C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
2932C<"\0"> and C<"\1">. For bytes C<"a".."f"> and C<"A".."F"> the result
2933is compatible with the usual hexadecimal digits, so that C<"a"> and
2934C<"A"> both generate the nybble C<0xa==10>. The result for bytes
2935C<"g".."z"> and C<"G".."Z"> is not well-defined.
2936
2937Starting from the beginning of the input string of pack(), each pair
951ba7fe 2938of bytes is converted to 1 byte of output. With format C<h> the
c73032f5 2939first byte of the pair determines the least-significant nybble of the
951ba7fe 2940output byte, and with format C<H> it determines the most-significant
c73032f5
IZ
2941nybble.
2942
2943If the length of the input string is not even, it behaves as if padded
2944by a null byte at the end. Similarly, during unpack()ing the "extra"
2945nybbles are ignored.
2946
2947If the input string of pack() is longer than needed, extra bytes are ignored.
2948A C<*> for the repeat count of pack() means to use all the bytes of
2949the input field. On unpack()ing the bits are converted to a string
2950of hexadecimal digits.
2951
5a929a98
VU
2952=item *
2953
951ba7fe 2954The C<p> type packs a pointer to a null-terminated string. You are
5a929a98
VU
2955responsible for ensuring the string is not a temporary value (which can
2956potentially get deallocated before you get around to using the packed result).
951ba7fe
GS
2957The C<P> type packs a pointer to a structure of the size indicated by the
2958length. A NULL pointer is created if the corresponding value for C<p> or
2959C<P> is C<undef>, similarly for unpack().
5a929a98
VU
2960
2961=item *
2962
951ba7fe
GS
2963The C</> template character allows packing and unpacking of strings where
2964the packed structure contains a byte count followed by the string itself.
17f4a12d 2965You write I<length-item>C</>I<string-item>.
43192e07
IP
2966
2967The I<length-item> can be any C<pack> template letter,
2968and describes how the length value is packed.
2969The ones likely to be of most use are integer-packing ones like
951ba7fe
GS
2970C<n> (for Java strings), C<w> (for ASN.1 or SNMP)
2971and C<N> (for Sun XDR).
43192e07
IP
2972
2973The I<string-item> must, at present, be C<"A*">, C<"a*"> or C<"Z*">.
2974For C<unpack> the length of the string is obtained from the I<length-item>,
2975but if you put in the '*' it will be ignored.
2976
17f4a12d
IZ
2977 unpack 'C/a', "\04Gurusamy"; gives 'Guru'
2978 unpack 'a3/A* A*', '007 Bond J '; gives (' Bond','J')
2979 pack 'n/a* w/a*','hello,','world'; gives "\000\006hello,\005world"
43192e07
IP
2980
2981The I<length-item> is not returned explicitly from C<unpack>.
2982
951ba7fe
GS
2983Adding a count to the I<length-item> letter is unlikely to do anything
2984useful, unless that letter is C<A>, C<a> or C<Z>. Packing with a
2985I<length-item> of C<a> or C<Z> may introduce C<"\000"> characters,
43192e07
IP
2986which Perl does not regard as legal in numeric strings.
2987
2988=item *
2989
951ba7fe
GS
2990The integer types C<s>, C<S>, C<l>, and C<L> may be
2991immediately followed by a C<!> suffix to signify native shorts or
2992longs--as you can see from above for example a bare C<l> does mean
851646ae
JH
2993exactly 32 bits, the native C<long> (as seen by the local C compiler)
2994may be larger. This is an issue mainly in 64-bit platforms. You can
951ba7fe 2995see whether using C<!> makes any difference by
726ea183 2996
4d0c1c44
GS
2997 print length(pack("s")), " ", length(pack("s!")), "\n";
2998 print length(pack("l")), " ", length(pack("l!")), "\n";
ef54e1a4 2999
951ba7fe
GS
3000C<i!> and C<I!> also work but only because of completeness;
3001they are identical to C<i> and C<I>.
ef54e1a4 3002
19799a22
GS
3003The actual sizes (in bytes) of native shorts, ints, longs, and long
3004longs on the platform where Perl was built are also available via
3005L<Config>:
3006
3007 use Config;
3008 print $Config{shortsize}, "\n";
3009 print $Config{intsize}, "\n";
3010 print $Config{longsize}, "\n";
3011 print $Config{longlongsize}, "\n";
ef54e1a4 3012
5074e145 3013(The C<$Config{longlongsize}> will be undefine if your system does
851646ae
JH
3014not support long longs.)
3015
ef54e1a4
JH
3016=item *
3017
951ba7fe 3018The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, and C<L>
ef54e1a4
JH
3019are inherently non-portable between processors and operating systems
3020because they obey the native byteorder and endianness. For example a
140cb37e 30214-byte integer 0x12345678 (305419896 decimal) be ordered natively
ef54e1a4
JH
3022(arranged in and handled by the CPU registers) into bytes as
3023
719a3cf5
JH
3024 0x12 0x34 0x56 0x78 # little-endian
3025 0x78 0x56 0x34 0x12 # big-endian
ef54e1a4 3026
5d11dd56 3027Basically, the Intel, Alpha, and VAX CPUs are little-endian, while
719a3cf5
JH
3028everybody else, for example Motorola m68k/88k, PPC, Sparc, HP PA,
3029Power, and Cray are big-endian. MIPS can be either: Digital used it
19799a22 3030in little-endian mode; SGI uses it in big-endian mode.
719a3cf5 3031
19799a22 3032The names `big-endian' and `little-endian' are comic references to
ef54e1a4
JH
3033the classic "Gulliver's Travels" (via the paper "On Holy Wars and a
3034Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and
19799a22 3035the egg-eating habits of the Lilliputians.
ef54e1a4 3036
140cb37e 3037Some systems may have even weirder byte orders such as
ef54e1a4
JH
3038
3039 0x56 0x78 0x12 0x34
3040 0x34 0x12 0x78 0x56
3041
3042You can see your system's preference with
3043
3044 print join(" ", map { sprintf "%#02x", $_ }
3045 unpack("C*",pack("L",0x12345678))), "\n";
3046
d99ad34e 3047The byteorder on the platform where Perl was built is also available
726ea183 3048via L<Config>:
ef54e1a4
JH
3049
3050 use Config;
3051 print $Config{byteorder}, "\n";
3052
d99ad34e
JH
3053Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'>
3054and C<'87654321'> are big-endian.
719a3cf5 3055
951ba7fe
GS
3056If you want portable packed integers use the formats C<n>, C<N>,
3057C<v>, and C<V>, their byte endianness and size is known.
851646ae 3058See also L<perlport>.
ef54e1a4
JH
3059
3060=item *
3061
5a929a98
VU
3062Real numbers (floats and doubles) are in the native machine format only;
3063due to the multiplicity of floating formats around, and the lack of a
3064standard "network" representation, no facility for interchange has been
3065made. This means that packed floating point data written on one machine
3066may not be readable on another - even if both use IEEE floating point
3067arithmetic (as the endian-ness of the memory representation is not part
851646ae 3068of the IEEE spec). See also L<perlport>.
5a929a98
VU
3069
3070Note that Perl uses doubles internally for all numeric calculation, and
3071converting from double into float and thence back to double again will
3072lose precision (i.e., C<unpack("f", pack("f", $foo)>) will not in general
19799a22 3073equal $foo).
5a929a98 3074
851646ae
JH
3075=item *
3076
3077You must yourself do any alignment or padding by inserting for example
9ccd05c0
JH
3078enough C<'x'>es while packing. There is no way to pack() and unpack()
3079could know where the bytes are going to or coming from. Therefore
3080C<pack> (and C<unpack>) handle their output and input as flat
3081sequences of bytes.
851646ae 3082
17f4a12d
IZ
3083=item *
3084
3085A comment in a TEMPLATE starts with C<#> and goes to the end of line.
3086
2b6c5635
GS
3087=item *
3088
3089If TEMPLATE requires more arguments to pack() than actually given, pack()
3090assumes additional C<""> arguments. If TEMPLATE requires less arguments
3091to pack() than actually given, extra arguments are ignored.
3092
5a929a98 3093=back
a0d0e21e
LW
3094
3095Examples:
3096
a0ed51b3 3097 $foo = pack("CCCC",65,66,67,68);
a0d0e21e 3098 # foo eq "ABCD"
a0ed51b3 3099 $foo = pack("C4",65,66,67,68);
a0d0e21e 3100 # same thing
a0ed51b3
LW
3101 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
3102 # same thing with Unicode circled letters
a0d0e21e
LW
3103
3104 $foo = pack("ccxxcc",65,66,67,68);
3105 # foo eq "AB\0\0CD"
3106
9ccd05c0
JH
3107 # note: the above examples featuring "C" and "c" are true
3108 # only on ASCII and ASCII-derived systems such as ISO Latin 1
3109 # and UTF-8. In EBCDIC the first example would be
3110 # $foo = pack("CCCC",193,194,195,196);
3111
a0d0e21e
LW
3112 $foo = pack("s2",1,2);
3113 # "\1\0\2\0" on little-endian
3114 # "\0\1\0\2" on big-endian
3115
3116 $foo = pack("a4","abcd","x","y","z");
3117 # "abcd"
3118
3119 $foo = pack("aaaa","abcd","x","y","z");
3120 # "axyz"
3121
3122 $foo = pack("a14","abcdefg");
3123 # "abcdefg\0\0\0\0\0\0\0"
3124
3125 $foo = pack("i9pl", gmtime);
3126 # a real struct tm (on my system anyway)
3127
5a929a98
VU
3128 $utmp_template = "Z8 Z8 Z16 L";
3129 $utmp = pack($utmp_template, @utmp1);
3130 # a struct utmp (BSDish)
3131
3132 @utmp2 = unpack($utmp_template, $utmp);
3133 # "@utmp1" eq "@utmp2"
3134
a0d0e21e
LW
3135 sub bintodec {
3136 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
3137 }
3138
851646ae
JH
3139 $foo = pack('sx2l', 12, 34);
3140 # short 12, two zero bytes padding, long 34
3141 $bar = pack('s@4l', 12, 34);
3142 # short 12, zero fill to position 4, long 34
3143 # $foo eq $bar
3144
5a929a98 3145The same template may generally also be used in unpack().
a0d0e21e 3146
5a964f20
TC
3147=item package
3148
cb1a09d0
AD
3149=item package NAMESPACE
3150
3151Declares the compilation unit as being in the given namespace. The scope
2b5ab1e7 3152of the package declaration is from the declaration itself through the end
19799a22 3153of the enclosing block, file, or eval (the same as the C<my> operator).
2b5ab1e7
TC
3154All further unqualified dynamic identifiers will be in this namespace.
3155A package statement affects only dynamic variables--including those
19799a22
GS
3156you've used C<local> on--but I<not> lexical variables, which are created
3157with C<my>. Typically it would be the first declaration in a file to
2b5ab1e7
TC
3158be included by the C<require> or C<use> operator. You can switch into a
3159package in more than one place; it merely influences which symbol table
3160is used by the compiler for the rest of that block. You can refer to
3161variables and filehandles in other packages by prefixing the identifier
3162with the package name and a double colon: C<$Package::Variable>.
3163If the package name is null, the C<main> package as assumed. That is,
3164C<$::sail> is equivalent to C<$main::sail> (as well as to C<$main'sail>,
3165still seen in older code).
cb1a09d0 3166
5a964f20
TC
3167If NAMESPACE is omitted, then there is no current package, and all
3168identifiers must be fully qualified or lexicals. This is stricter
3169than C<use strict>, since it also extends to function names.
3170
cb1a09d0
AD
3171See L<perlmod/"Packages"> for more information about packages, modules,
3172and classes. See L<perlsub> for other scoping issues.
3173
a0d0e21e
LW
3174=item pipe READHANDLE,WRITEHANDLE
3175
3176Opens a pair of connected pipes like the corresponding system call.
3177Note that if you set up a loop of piped processes, deadlock can occur
3178unless you are very careful. In addition, note that Perl's pipes use
184e9718 3179stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE
a0d0e21e
LW
3180after each command, depending on the application.
3181
7e1af8bc 3182See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
4633a7c4
LW
3183for examples of such things.
3184
4771b018
GS
3185On systems that support a close-on-exec flag on files, the flag will be set
3186for the newly opened file descriptors as determined by the value of $^F.
3187See L<perlvar/$^F>.
3188
a0d0e21e
LW
3189=item pop ARRAY
3190
54310121 3191=item pop
28757baa 3192
a0d0e21e 3193Pops and returns the last value of the array, shortening the array by
19799a22 3194one element. Has an effect similar to
a0d0e21e 3195
19799a22 3196 $ARRAY[$#ARRAY--]
a0d0e21e 3197
19799a22
GS
3198If there are no elements in the array, returns the undefined value
3199(although this may happen at other times as well). If ARRAY is
3200omitted, pops the C<@ARGV> array in the main program, and the C<@_>
3201array in subroutines, just like C<shift>.
a0d0e21e
LW
3202
3203=item pos SCALAR
3204
54310121 3205=item pos
bbce6d69 3206
4633a7c4 3207Returns the offset of where the last C<m//g> search left off for the variable
7660c0ab 3208is in question (C<$_> is used when the variable is not specified). May be
44a8e56a
PP
3209modified to change that offset. Such modification will also influence
3210the C<\G> zero-width assertion in regular expressions. See L<perlre> and
3211L<perlop>.
a0d0e21e
LW
3212
3213=item print FILEHANDLE LIST
3214
3215=item print LIST
3216
3217=item print
3218
19799a22
GS
3219Prints a string or a list of strings. Returns true if successful.
3220FILEHANDLE may be a scalar variable name, in which case the variable
3221contains the name of or a reference to the filehandle, thus introducing
3222one level of indirection. (NOTE: If FILEHANDLE is a variable and
3223the next token is a term, it may be misinterpreted as an operator
2b5ab1e7 3224unless you interpose a C<+> or put parentheses around the arguments.)
19799a22
GS
3225If FILEHANDLE is omitted, prints by default to standard output (or
3226to the last selected output channel--see L</select>). If LIST is
3227also omitted, prints C<$_> to the currently selected output channel.
3228To set the default output channel to something other than STDOUT
3229use the select operation. The current value of C<$,> (if any) is
3230printed between each LIST item. The current value of C<$\> (if
3231any) is printed after the entire LIST has been printed. Because
3232print takes a LIST, anything in the LIST is evaluated in list
3233context, and any subroutine that you call will have one or more of
3234its expressions evaluated in list context. Also be careful not to
3235follow the print keyword with a left parenthesis unless you want
3236the corresponding right parenthesis to terminate the arguments to
3237the print--interpose a C<+> or put parentheses around all the
3238arguments.
a0d0e21e 3239
4633a7c4 3240Note that if you're storing FILEHANDLES in an array or other expression,
da0045b7 3241you will have to use a block returning its value instead:
4633a7c4
LW
3242
3243 print { $files[$i] } "stuff\n";
3244 print { $OK ? STDOUT : STDERR } "stuff\n";
3245
5f05dabc 3246=item printf FILEHANDLE FORMAT, LIST
a0d0e21e 3247
5f05dabc 3248=item printf FORMAT, LIST
a0d0e21e 3249
7660c0ab 3250Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
a3cb178b 3251(the output record separator) is not appended. The first argument
19799a22 3252of the list will be interpreted as the C<printf> format. If C<use locale> is
a034a98d
DD
3253in effect, the character used for the decimal point in formatted real numbers
3254is affected by the LC_NUMERIC locale. See L<perllocale>.
a0d0e21e 3255
19799a22
GS
3256Don't fall into the trap of using a C<printf> when a simple
3257C<print> would do. The C<print> is more efficient and less
28757baa
PP
3258error prone.
3259
da0045b7
PP
3260=item prototype FUNCTION
3261
3262Returns the prototype of a function as a string (or C<undef> if the
5f05dabc
PP
3263function has no prototype). FUNCTION is a reference to, or the name of,
3264the function whose prototype you want to retrieve.
da0045b7 3265
2b5ab1e7
TC
3266If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
3267name for Perl builtin. If the builtin is not I<overridable> (such as
ab4f32c2 3268C<qw//>) or its arguments cannot be expressed by a prototype (such as
19799a22 3269C<system>) returns C<undef> because the builtin does not really behave
2b5ab1e7
TC
3270like a Perl function. Otherwise, the string describing the equivalent
3271prototype is returned.
b6c543e3 3272
a0d0e21e
LW
3273=item push ARRAY,LIST
3274
3275Treats ARRAY as a stack, and pushes the values of LIST
3276onto the end of ARRAY. The length of ARRAY increases by the length of
3277LIST. Has the same effect as
3278
3279 for $value (LIST) {
3280 $ARRAY[++$#ARRAY] = $value;
3281 }
3282
3283but is more efficient. Returns the new number of elements in the array.
3284
3285=item q/STRING/
3286
3287=item qq/STRING/
3288
8782bef2
GB
3289=item qr/STRING/
3290
a0d0e21e
LW
3291=item qx/STRING/
3292
3293=item qw/STRING/
3294
4b6a7270 3295Generalized quotes. See L<perlop/"Regexp Quote-Like Operators">.
a0d0e21e
LW
3296
3297=item quotemeta EXPR
3298
54310121 3299=item quotemeta
bbce6d69 3300