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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
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108C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>,
109C<sin>, C<sqrt>, C<srand>
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110
111=item Functions for real @ARRAYs
112
22fae026 113C<pop>, C<push>, C<shift>, C<splice>, C<unshift>
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114
115=item Functions for list data
116
ab4f32c2 117C<grep>, C<join>, C<map>, C<qw/STRING/>, C<reverse>, C<sort>, C<unpack>
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118
119=item Functions for real %HASHes
120
22fae026 121C<delete>, C<each>, C<exists>, C<keys>, C<values>
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122
123=item Input and output functions
124
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125C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
126C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
127C<readdir>, C<rewinddir>, C<seek>, C<seekdir>, C<select>, C<syscall>,
128C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>,
129C<warn>, C<write>
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130
131=item Functions for fixed length data or records
132
22fae026 133C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec>
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134
135=item Functions for filehandles, files, or directories
136
22fae026 137C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>,
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138C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>,
139C<readlink>, C<rename>, C<rmdir>, C<stat>, C<symlink>, C<umask>,
140C<unlink>, C<utime>
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141
142=item Keywords related to the control flow of your perl program
143
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144C<caller>, C<continue>, C<die>, C<do>, C<dump>, C<eval>, C<exit>,
145C<goto>, C<last>, C<next>, C<redo>, C<return>, C<sub>, C<wantarray>
cb1a09d0 146
54310121 147=item Keywords related to scoping
cb1a09d0 148
22fae026 149C<caller>, C<import>, C<local>, C<my>, C<package>, C<use>
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150
151=item Miscellaneous functions
152
22fae026
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153C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<reset>,
154C<scalar>, C<undef>, C<wantarray>
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155
156=item Functions for processes and process groups
157
22fae026 158C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
ab4f32c2 159C<pipe>, C<qx/STRING/>, C<setpgrp>, C<setpriority>, C<sleep>, C<system>,
22fae026 160C<times>, C<wait>, C<waitpid>
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161
162=item Keywords related to perl modules
163
22fae026 164C<do>, C<import>, C<no>, C<package>, C<require>, C<use>
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165
166=item Keywords related to classes and object-orientedness
167
22fae026
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168C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
169C<untie>, C<use>
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170
171=item Low-level socket functions
172
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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
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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
afebc493
GS
1425Given an expression that specifies the name of a subroutine,
1426returns true if the specified subroutine has ever been declared, even
1427if it is undefined. Mentioning a subroutine name for exists or defined
1428does not count as declaring it.
1429
1430 print "Exists\n" if exists &subroutine;
1431 print "Defined\n" if defined &subroutine;
1432
a0d0e21e 1433Note that the EXPR can be arbitrarily complicated as long as the final
afebc493 1434operation is a hash or array key lookup or subroutine name:
a0d0e21e 1435
2b5ab1e7
TC
1436 if (exists $ref->{A}->{B}->{$key}) { }
1437 if (exists $hash{A}{B}{$key}) { }
1438
01020589
GS
1439 if (exists $ref->{A}->{B}->[$ix]) { }
1440 if (exists $hash{A}{B}[$ix]) { }
1441
afebc493
GS
1442 if (exists &{$ref->{A}{B}{$key}}) { }
1443
01020589
GS
1444Although the deepest nested array or hash will not spring into existence
1445just because its existence was tested, any intervening ones will.
1446Thus C<$ref-E<gt>{"A"}> and C<$ref-E<gt>{"A"}-E<gt>{"B"}> will spring
1447into existence due to the existence test for the $key element above.
1448This happens anywhere the arrow operator is used, including even:
5a964f20 1449
2b5ab1e7
TC
1450 undef $ref;
1451 if (exists $ref->{"Some key"}) { }
1452 print $ref; # prints HASH(0x80d3d5c)
1453
1454This surprising autovivification in what does not at first--or even
1455second--glance appear to be an lvalue context may be fixed in a future
5a964f20 1456release.
a0d0e21e 1457
e0478e5a
MS
1458See L<perlref/"Pseudo-hashes"> for specifics on how exists() acts when
1459used on a pseudo-hash.
1460
afebc493
GS
1461Use of a subroutine call, rather than a subroutine name, as an argument
1462to exists() is an error.
1463
1464 exists &sub; # OK
1465 exists &sub(); # Error
1466
a0d0e21e
LW
1467=item exit EXPR
1468
2b5ab1e7 1469Evaluates EXPR and exits immediately with that value. Example:
a0d0e21e
LW
1470
1471 $ans = <STDIN>;
1472 exit 0 if $ans =~ /^[Xx]/;
1473
19799a22 1474See also C<die>. If EXPR is omitted, exits with C<0> status. The only
2b5ab1e7
TC
1475universally recognized values for EXPR are C<0> for success and C<1>
1476for error; other values are subject to interpretation depending on the
1477environment in which the Perl program is running. For example, exiting
147869 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1479the mailer to return the item undelivered, but that's not true everywhere.
a0d0e21e 1480
19799a22
GS
1481Don't use C<exit> to abort a subroutine if there's any chance that
1482someone might want to trap whatever error happened. Use C<die> instead,
1483which can be trapped by an C<eval>.
28757baa 1484
19799a22 1485The exit() function does not always exit immediately. It calls any
2b5ab1e7 1486defined C<END> routines first, but these C<END> routines may not
19799a22 1487themselves abort the exit. Likewise any object destructors that need to
2b5ab1e7
TC
1488be called are called before the real exit. If this is a problem, you
1489can call C<POSIX:_exit($status)> to avoid END and destructor processing.
87275199 1490See L<perlmod> for details.
5a964f20 1491
a0d0e21e
LW
1492=item exp EXPR
1493
54310121 1494=item exp
bbce6d69 1495
2b5ab1e7 1496Returns I<e> (the natural logarithm base) to the power of EXPR.
a0d0e21e
LW
1497If EXPR is omitted, gives C<exp($_)>.
1498
1499=item fcntl FILEHANDLE,FUNCTION,SCALAR
1500
f86cebdf 1501Implements the fcntl(2) function. You'll probably have to say
a0d0e21e
LW
1502
1503 use Fcntl;
1504
0ade1984 1505first to get the correct constant definitions. Argument processing and
19799a22 1506value return works just like C<ioctl> below.
a0d0e21e
LW
1507For example:
1508
1509 use Fcntl;
5a964f20
TC
1510 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1511 or die "can't fcntl F_GETFL: $!";
1512
19799a22 1513You don't have to check for C<defined> on the return from C<fnctl>.
951ba7fe
GS
1514Like C<ioctl>, it maps a C<0> return from the system call into
1515C<"0 but true"> in Perl. This string is true in boolean context and C<0>
2b5ab1e7
TC
1516in numeric context. It is also exempt from the normal B<-w> warnings
1517on improper numeric conversions.
5a964f20 1518
19799a22 1519Note that C<fcntl> will produce a fatal error if used on a machine that
2b5ab1e7
TC
1520doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1521manpage to learn what functions are available on your system.
a0d0e21e
LW
1522
1523=item fileno FILEHANDLE
1524
2b5ab1e7
TC
1525Returns the file descriptor for a filehandle, or undefined if the
1526filehandle is not open. This is mainly useful for constructing
19799a22 1527bitmaps for C<select> and low-level POSIX tty-handling operations.
2b5ab1e7
TC
1528If FILEHANDLE is an expression, the value is taken as an indirect
1529filehandle, generally its name.
5a964f20
TC
1530
1531You can use this to find out whether two handles refer to the
1532same underlying descriptor:
1533
1534 if (fileno(THIS) == fileno(THAT)) {
1535 print "THIS and THAT are dups\n";
1536 }
a0d0e21e
LW
1537
1538=item flock FILEHANDLE,OPERATION
1539
19799a22
GS
1540Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1541for success, false on failure. Produces a fatal error if used on a
2b5ab1e7 1542machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
19799a22 1543C<flock> is Perl's portable file locking interface, although it locks
2b5ab1e7
TC
1544only entire files, not records.
1545
1546Two potentially non-obvious but traditional C<flock> semantics are
1547that it waits indefinitely until the lock is granted, and that its locks
1548B<merely advisory>. Such discretionary locks are more flexible, but offer
19799a22
GS
1549fewer guarantees. This means that files locked with C<flock> may be
1550modified by programs that do not also use C<flock>. See L<perlport>,
2b5ab1e7
TC
1551your port's specific documentation, or your system-specific local manpages
1552for details. It's best to assume traditional behavior if you're writing
1553portable programs. (But if you're not, you should as always feel perfectly
1554free to write for your own system's idiosyncrasies (sometimes called
1555"features"). Slavish adherence to portability concerns shouldn't get
1556in the way of your getting your job done.)
a3cb178b 1557
8ebc5c01
PP
1558OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1559LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
68dc0745
PP
1560you can use the symbolic names if import them from the Fcntl module,
1561either individually, or as a group using the ':flock' tag. LOCK_SH
1562requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1563releases a previously requested lock. If LOCK_NB is added to LOCK_SH or
19799a22 1564LOCK_EX then C<flock> will return immediately rather than blocking
68dc0745
PP
1565waiting for the lock (check the return status to see if you got it).
1566
2b5ab1e7
TC
1567To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1568before locking or unlocking it.
8ebc5c01 1569
f86cebdf 1570Note that the emulation built with lockf(3) doesn't provide shared
8ebc5c01 1571locks, and it requires that FILEHANDLE be open with write intent. These
2b5ab1e7 1572are the semantics that lockf(3) implements. Most if not all systems
f86cebdf 1573implement lockf(3) in terms of fcntl(2) locking, though, so the
8ebc5c01
PP
1574differing semantics shouldn't bite too many people.
1575
19799a22
GS
1576Note also that some versions of C<flock> cannot lock things over the
1577network; you would need to use the more system-specific C<fcntl> for
f86cebdf
GS
1578that. If you like you can force Perl to ignore your system's flock(2)
1579function, and so provide its own fcntl(2)-based emulation, by passing
8ebc5c01
PP
1580the switch C<-Ud_flock> to the F<Configure> program when you configure
1581perl.
4633a7c4
LW
1582
1583Here's a mailbox appender for BSD systems.
a0d0e21e 1584
7e1af8bc 1585 use Fcntl ':flock'; # import LOCK_* constants
a0d0e21e
LW
1586
1587 sub lock {
7e1af8bc 1588 flock(MBOX,LOCK_EX);
a0d0e21e
LW
1589 # and, in case someone appended
1590 # while we were waiting...
1591 seek(MBOX, 0, 2);
1592 }
1593
1594 sub unlock {
7e1af8bc 1595 flock(MBOX,LOCK_UN);
a0d0e21e
LW
1596 }
1597
1598 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1599 or die "Can't open mailbox: $!";
1600
1601 lock();
1602 print MBOX $msg,"\n\n";
1603 unlock();
1604
2b5ab1e7
TC
1605On systems that support a real flock(), locks are inherited across fork()
1606calls, whereas those that must resort to the more capricious fcntl()
1607function lose the locks, making it harder to write servers.
1608
cb1a09d0 1609See also L<DB_File> for other flock() examples.
a0d0e21e
LW
1610
1611=item fork
1612
2b5ab1e7
TC
1613Does a fork(2) system call to create a new process running the
1614same program at the same point. It returns the child pid to the
1615parent process, C<0> to the child process, or C<undef> if the fork is
1616unsuccessful. File descriptors (and sometimes locks on those descriptors)
1617are shared, while everything else is copied. On most systems supporting
1618fork(), great care has gone into making it extremely efficient (for
1619example, using copy-on-write technology on data pages), making it the
1620dominant paradigm for multitasking over the last few decades.
5a964f20 1621
45bc9206 1622All files opened for output are flushed before forking the child process.
a0d0e21e 1623
19799a22 1624If you C<fork> without ever waiting on your children, you will
2b5ab1e7
TC
1625accumulate zombies. On some systems, you can avoid this by setting
1626C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1627forking and reaping moribund children.
cb1a09d0 1628
28757baa
PP
1629Note that if your forked child inherits system file descriptors like
1630STDIN and STDOUT that are actually connected by a pipe or socket, even
2b5ab1e7 1631if you exit, then the remote server (such as, say, a CGI script or a
19799a22 1632backgrounded job launched from a remote shell) won't think you're done.
2b5ab1e7 1633You should reopen those to F</dev/null> if it's any issue.
28757baa 1634
cb1a09d0
AD
1635=item format
1636
19799a22 1637Declare a picture format for use by the C<write> function. For
cb1a09d0
AD
1638example:
1639
54310121 1640 format Something =
cb1a09d0
AD
1641 Test: @<<<<<<<< @||||| @>>>>>
1642 $str, $%, '$' . int($num)
1643 .
1644
1645 $str = "widget";
184e9718 1646 $num = $cost/$quantity;
cb1a09d0
AD
1647 $~ = 'Something';
1648 write;
1649
1650See L<perlform> for many details and examples.
1651
8903cb82 1652=item formline PICTURE,LIST
a0d0e21e 1653
5a964f20 1654This is an internal function used by C<format>s, though you may call it,
a0d0e21e
LW
1655too. It formats (see L<perlform>) a list of values according to the
1656contents of PICTURE, placing the output into the format output
7660c0ab 1657accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
19799a22 1658Eventually, when a C<write> is done, the contents of
a0d0e21e 1659C<$^A> are written to some filehandle, but you could also read C<$^A>
7660c0ab 1660yourself and then set C<$^A> back to C<"">. Note that a format typically
19799a22 1661does one C<formline> per line of form, but the C<formline> function itself
748a9306 1662doesn't care how many newlines are embedded in the PICTURE. This means
4633a7c4 1663that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
748a9306
LW
1664You may therefore need to use multiple formlines to implement a single
1665record format, just like the format compiler.
1666
19799a22 1667Be careful if you put double quotes around the picture, because an C<@>
748a9306 1668character may be taken to mean the beginning of an array name.
19799a22 1669C<formline> always returns true. See L<perlform> for other examples.
a0d0e21e
LW
1670
1671=item getc FILEHANDLE
1672
1673=item getc
1674
1675Returns the next character from the input file attached to FILEHANDLE,
2b5ab1e7
TC
1676or the undefined value at end of file, or if there was an error.
1677If FILEHANDLE is omitted, reads from STDIN. This is not particularly
1678efficient. However, it cannot be used by itself to fetch single
1679characters without waiting for the user to hit enter. For that, try
1680something more like:
4633a7c4
LW
1681
1682 if ($BSD_STYLE) {
1683 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1684 }
1685 else {
54310121 1686 system "stty", '-icanon', 'eol', "\001";
4633a7c4
LW
1687 }
1688
1689 $key = getc(STDIN);
1690
1691 if ($BSD_STYLE) {
1692 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1693 }
1694 else {
5f05dabc 1695 system "stty", 'icanon', 'eol', '^@'; # ASCII null
4633a7c4
LW
1696 }
1697 print "\n";
1698
54310121
PP
1699Determination of whether $BSD_STYLE should be set
1700is left as an exercise to the reader.
cb1a09d0 1701
19799a22 1702The C<POSIX::getattr> function can do this more portably on
2b5ab1e7
TC
1703systems purporting POSIX compliance. See also the C<Term::ReadKey>
1704module from your nearest CPAN site; details on CPAN can be found on
1705L<perlmodlib/CPAN>.
a0d0e21e
LW
1706
1707=item getlogin
1708
5a964f20
TC
1709Implements the C library function of the same name, which on most
1710systems returns the current login from F</etc/utmp>, if any. If null,
19799a22 1711use C<getpwuid>.
a0d0e21e 1712
f86702cc 1713 $login = getlogin || getpwuid($<) || "Kilroy";
a0d0e21e 1714
19799a22
GS
1715Do not consider C<getlogin> for authentication: it is not as
1716secure as C<getpwuid>.
4633a7c4 1717
a0d0e21e
LW
1718=item getpeername SOCKET
1719
1720Returns the packed sockaddr address of other end of the SOCKET connection.
1721
4633a7c4
LW
1722 use Socket;
1723 $hersockaddr = getpeername(SOCK);
19799a22 1724 ($port, $iaddr) = sockaddr_in($hersockaddr);
4633a7c4
LW
1725 $herhostname = gethostbyaddr($iaddr, AF_INET);
1726 $herstraddr = inet_ntoa($iaddr);
a0d0e21e
LW
1727
1728=item getpgrp PID
1729
47e29363 1730Returns the current process group for the specified PID. Use
7660c0ab 1731a PID of C<0> to get the current process group for the
4633a7c4 1732current process. Will raise an exception if used on a machine that
f86cebdf 1733doesn't implement getpgrp(2). If PID is omitted, returns process
19799a22 1734group of current process. Note that the POSIX version of C<getpgrp>
7660c0ab 1735does not accept a PID argument, so only C<PID==0> is truly portable.
a0d0e21e
LW
1736
1737=item getppid
1738
1739Returns the process id of the parent process.
1740
1741=item getpriority WHICH,WHO
1742
4633a7c4
LW
1743Returns the current priority for a process, a process group, or a user.
1744(See L<getpriority(2)>.) Will raise a fatal exception if used on a
f86cebdf 1745machine that doesn't implement getpriority(2).
a0d0e21e
LW
1746
1747=item getpwnam NAME
1748
1749=item getgrnam NAME
1750
1751=item gethostbyname NAME
1752
1753=item getnetbyname NAME
1754
1755=item getprotobyname NAME
1756
1757=item getpwuid UID
1758
1759=item getgrgid GID
1760
1761=item getservbyname NAME,PROTO
1762
1763=item gethostbyaddr ADDR,ADDRTYPE
1764
1765=item getnetbyaddr ADDR,ADDRTYPE
1766
1767=item getprotobynumber NUMBER
1768
1769=item getservbyport PORT,PROTO
1770
1771=item getpwent
1772
1773=item getgrent
1774
1775=item gethostent
1776
1777=item getnetent
1778
1779=item getprotoent
1780
1781=item getservent
1782
1783=item setpwent
1784
1785=item setgrent
1786
1787=item sethostent STAYOPEN
1788
1789=item setnetent STAYOPEN
1790
1791=item setprotoent STAYOPEN
1792
1793=item setservent STAYOPEN
1794
1795=item endpwent
1796
1797=item endgrent
1798
1799=item endhostent
1800
1801=item endnetent
1802
1803=item endprotoent
1804
1805=item endservent
1806
1807These routines perform the same functions as their counterparts in the
5a964f20 1808system library. In list context, the return values from the
a0d0e21e
LW
1809various get routines are as follows:
1810
1811 ($name,$passwd,$uid,$gid,
6ee623d5 1812 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
a0d0e21e
LW
1813 ($name,$passwd,$gid,$members) = getgr*
1814 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1815 ($name,$aliases,$addrtype,$net) = getnet*
1816 ($name,$aliases,$proto) = getproto*
1817 ($name,$aliases,$port,$proto) = getserv*
1818
1819(If the entry doesn't exist you get a null list.)
1820
5a964f20 1821In scalar context, you get the name, unless the function was a
a0d0e21e
LW
1822lookup by name, in which case you get the other thing, whatever it is.
1823(If the entry doesn't exist you get the undefined value.) For example:
1824
5a964f20
TC
1825 $uid = getpwnam($name);
1826 $name = getpwuid($num);
1827 $name = getpwent();
1828 $gid = getgrnam($name);
1829 $name = getgrgid($num;
1830 $name = getgrent();
1831 #etc.
a0d0e21e 1832
19799a22 1833In I<getpw*()> the fields $quota, $comment, and $expire are
2b5ab1e7 1834special cases in the sense that in many systems they are unsupported.
19799a22
GS
1835If the $quota is unsupported, it is an empty scalar. If it is
1836supported, it usually encodes the disk quota. If the $comment
2b5ab1e7
TC
1837field is unsupported, it is an empty scalar. If it is supported it
1838usually encodes some administrative comment about the user. In some
19799a22
GS
1839systems the $quota field may be $change or $age, fields that have
1840to do with password aging. In some systems the $comment field may
1841be $class. The $expire field, if present, encodes the expiration
2b5ab1e7
TC
1842period of the account or the password. For the availability and the
1843exact meaning of these fields in your system, please consult your
1844getpwnam(3) documentation and your F<pwd.h> file. You can also find
19799a22
GS
1845out from within Perl what your $quota and $comment fields mean
1846and whether you have the $expire field by using the C<Config> module
2b5ab1e7
TC
1847and the values C<d_pwquota>, C<d_pwage>, C<d_pwchange>, C<d_pwcomment>,
1848and C<d_pwexpire>. Shadow password files are only supported if your
1849vendor has implemented them in the intuitive fashion that calling the
1850regular C library routines gets the shadow versions if you're running
1851under privilege. Those that incorrectly implement a separate library
1852call are not supported.
6ee623d5 1853
19799a22 1854The $members value returned by I<getgr*()> is a space separated list of
a0d0e21e
LW
1855the login names of the members of the group.
1856
1857For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1858C, it will be returned to you via C<$?> if the function call fails. The
7660c0ab 1859C<@addrs> value returned by a successful call is a list of the raw
a0d0e21e
LW
1860addresses returned by the corresponding system library call. In the
1861Internet domain, each address is four bytes long and you can unpack it
1862by saying something like:
1863
1864 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1865
2b5ab1e7
TC
1866The Socket library makes this slightly easier:
1867
1868 use Socket;
1869 $iaddr = inet_aton("127.1"); # or whatever address
1870 $name = gethostbyaddr($iaddr, AF_INET);
1871
1872 # or going the other way
19799a22 1873 $straddr = inet_ntoa($iaddr);
2b5ab1e7 1874
19799a22
GS
1875If you get tired of remembering which element of the return list
1876contains which return value, by-name interfaces are provided
1877in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
1878C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
1879and C<User::grent>. These override the normal built-ins, supplying
1880versions that return objects with the appropriate names
1881for each field. For example:
5a964f20
TC
1882
1883 use File::stat;
1884 use User::pwent;
1885 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
1886
1887Even though it looks like they're the same method calls (uid),
19799a22
GS
1888they aren't, because a C<File::stat> object is different from
1889a C<User::pwent> object.
5a964f20 1890
a0d0e21e
LW
1891=item getsockname SOCKET
1892
19799a22
GS
1893Returns the packed sockaddr address of this end of the SOCKET connection,
1894in case you don't know the address because you have several different
1895IPs that the connection might have come in on.
a0d0e21e 1896
4633a7c4
LW
1897 use Socket;
1898 $mysockaddr = getsockname(SOCK);
19799a22
GS
1899 ($port, $myaddr) = sockaddr_in($mysockaddr);
1900 printf "Connect to %s [%s]\n",
1901 scalar gethostbyaddr($myaddr, AF_INET),
1902 inet_ntoa($myaddr);
a0d0e21e
LW
1903
1904=item getsockopt SOCKET,LEVEL,OPTNAME
1905
5a964f20 1906Returns the socket option requested, or undef if there is an error.
a0d0e21e
LW
1907
1908=item glob EXPR
1909
0a753a76
PP
1910=item glob
1911
2b5ab1e7
TC
1912Returns the value of EXPR with filename expansions such as the
1913standard Unix shell F</bin/csh> would do. This is the internal function
1914implementing the C<E<lt>*.cE<gt>> operator, but you can use it directly.
1915If EXPR is omitted, C<$_> is used. The C<E<lt>*.cE<gt>> operator is
1916discussed in more detail in L<perlop/"I/O Operators">.
a0d0e21e
LW
1917
1918=item gmtime EXPR
1919
19799a22 1920Converts a time as returned by the time function to a 9-element list
54310121 1921with the time localized for the standard Greenwich time zone.
4633a7c4 1922Typically used as follows:
a0d0e21e 1923
54310121 1924 # 0 1 2 3 4 5 6 7 8
a0d0e21e
LW
1925 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1926 gmtime(time);
1927
19799a22
GS
1928All list elements are numeric, and come straight out of a struct tm.
1929In particular this means that $mon has the range C<0..11> and $wday
1930has the range C<0..6> with sunday as day C<0>. Also, $year is the
1931number of years since 1900, that is, $year is C<123> in year 2023,
2b5ab1e7
TC
1932I<not> simply the last two digits of the year. If you assume it is,
1933then you create non-Y2K-compliant programs--and you wouldn't want to do
1934that, would you?
2f9daede 1935
abd75f24
GS
1936The proper way to get a complete 4-digit year is simply:
1937
1938 $year += 1900;
1939
1940And to get the last two digits of the year (e.g., '01' in 2001) do:
1941
1942 $year = sprintf("%02d", $year % 100);
1943
2f9daede 1944If EXPR is omitted, does C<gmtime(time())>.
a0d0e21e 1945
f86cebdf 1946In scalar context, returns the ctime(3) value:
0a753a76
PP
1947
1948 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
1949
19799a22 1950Also see the C<timegm> function provided by the C<Time::Local> module,
f86cebdf 1951and the strftime(3) function available via the POSIX module.
7660c0ab 1952
2b5ab1e7
TC
1953This scalar value is B<not> locale dependent (see L<perllocale>), but
1954is instead a Perl builtin. Also see the C<Time::Local> module, and the
1955strftime(3) and mktime(3) functions available via the POSIX module. To
7660c0ab
A
1956get somewhat similar but locale dependent date strings, set up your
1957locale environment variables appropriately (please see L<perllocale>)
1958and try for example:
1959
1960 use POSIX qw(strftime);
2b5ab1e7 1961 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
7660c0ab 1962
2b5ab1e7
TC
1963Note that the C<%a> and C<%b> escapes, which represent the short forms
1964of the day of the week and the month of the year, may not necessarily
1965be three characters wide in all locales.
0a753a76 1966
a0d0e21e
LW
1967=item goto LABEL
1968
748a9306
LW
1969=item goto EXPR
1970
a0d0e21e
LW
1971=item goto &NAME
1972
7660c0ab 1973The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
a0d0e21e 1974execution there. It may not be used to go into any construct that
7660c0ab 1975requires initialization, such as a subroutine or a C<foreach> loop. It
0a753a76 1976also can't be used to go into a construct that is optimized away,
19799a22 1977or to get out of a block or subroutine given to C<sort>.
0a753a76 1978It can be used to go almost anywhere else within the dynamic scope,
a0d0e21e 1979including out of subroutines, but it's usually better to use some other
19799a22 1980construct such as C<last> or C<die>. The author of Perl has never felt the
7660c0ab 1981need to use this form of C<goto> (in Perl, that is--C is another matter).
a0d0e21e 1982
7660c0ab
A
1983The C<goto-EXPR> form expects a label name, whose scope will be resolved
1984dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
748a9306
LW
1985necessarily recommended if you're optimizing for maintainability:
1986
1987 goto ("FOO", "BAR", "GLARCH")[$i];
1988
6cb9131c
GS
1989The C<goto-&NAME> form is quite different from the other forms of C<goto>.
1990In fact, it isn't a goto in the normal sense at all, and doesn't have
1991the stigma associated with other gotos. Instead, it
1992substitutes a call to the named subroutine for the currently running
1993subroutine. This is used by C<AUTOLOAD> subroutines that wish to load
1994another subroutine and then pretend that the other subroutine had been
1995called in the first place (except that any modifications to C<@_>
1996in the current subroutine are propagated to the other subroutine.)
1997After the C<goto>, not even C<caller> will be able to tell that this
1998routine was called first.
1999
2000NAME needn't be the name of a subroutine; it can be a scalar variable
2001containing a code reference, or a block which evaluates to a code
2002reference.
a0d0e21e
LW
2003
2004=item grep BLOCK LIST
2005
2006=item grep EXPR,LIST
2007
2b5ab1e7
TC
2008This is similar in spirit to, but not the same as, grep(1) and its
2009relatives. In particular, it is not limited to using regular expressions.
2f9daede 2010
a0d0e21e 2011Evaluates the BLOCK or EXPR for each element of LIST (locally setting
7660c0ab 2012C<$_> to each element) and returns the list value consisting of those
19799a22
GS
2013elements for which the expression evaluated to true. In scalar
2014context, returns the number of times the expression was true.
a0d0e21e
LW
2015
2016 @foo = grep(!/^#/, @bar); # weed out comments
2017
2018or equivalently,
2019
2020 @foo = grep {!/^#/} @bar; # weed out comments
2021
2b5ab1e7
TC
2022Note that, because C<$_> is a reference into the list value, it can
2023be used to modify the elements of the array. While this is useful and
2024supported, it can cause bizarre results if the LIST is not a named array.
2025Similarly, grep returns aliases into the original list, much as a for
2026loop's index variable aliases the list elements. That is, modifying an
19799a22
GS
2027element of a list returned by grep (for example, in a C<foreach>, C<map>
2028or another C<grep>) actually modifies the element in the original list.
2b5ab1e7 2029This is usually something to be avoided when writing clear code.
a0d0e21e 2030
19799a22 2031See also L</map> for a list composed of the results of the BLOCK or EXPR.
38325410 2032
a0d0e21e
LW
2033=item hex EXPR
2034
54310121 2035=item hex
bbce6d69 2036
2b5ab1e7
TC
2037Interprets EXPR as a hex string and returns the corresponding value.
2038(To convert strings that might start with either 0, 0x, or 0b, see
2039L</oct>.) If EXPR is omitted, uses C<$_>.
2f9daede
TPG
2040
2041 print hex '0xAf'; # prints '175'
2042 print hex 'aF'; # same
a0d0e21e 2043
19799a22 2044Hex strings may only represent integers. Strings that would cause
c6edd1b7 2045integer overflow trigger a warning.
19799a22 2046
a0d0e21e
LW
2047=item import
2048
19799a22 2049There is no builtin C<import> function. It is just an ordinary
4633a7c4 2050method (subroutine) defined (or inherited) by modules that wish to export
19799a22 2051names to another module. The C<use> function calls the C<import> method
54310121 2052for the package used. See also L</use()>, L<perlmod>, and L<Exporter>.
a0d0e21e
LW
2053
2054=item index STR,SUBSTR,POSITION
2055
2056=item index STR,SUBSTR
2057
2b5ab1e7
TC
2058The index function searches for one string within another, but without
2059the wildcard-like behavior of a full regular-expression pattern match.
2060It returns the position of the first occurrence of SUBSTR in STR at
2061or after POSITION. If POSITION is omitted, starts searching from the
2062beginning of the string. The return value is based at C<0> (or whatever
2063you've set the C<$[> variable to--but don't do that). If the substring
2064is not found, returns one less than the base, ordinarily C<-1>.
a0d0e21e
LW
2065
2066=item int EXPR
2067
54310121 2068=item int
bbce6d69 2069
7660c0ab 2070Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2b5ab1e7
TC
2071You should not use this function for rounding: one because it truncates
2072towards C<0>, and two because machine representations of floating point
2073numbers can sometimes produce counterintuitive results. For example,
2074C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2075because it's really more like -268.99999999999994315658 instead. Usually,
19799a22 2076the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2b5ab1e7 2077functions will serve you better than will int().
a0d0e21e
LW
2078
2079=item ioctl FILEHANDLE,FUNCTION,SCALAR
2080
2b5ab1e7 2081Implements the ioctl(2) function. You'll probably first have to say
a0d0e21e 2082
4633a7c4 2083 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
a0d0e21e 2084
2b5ab1e7 2085to get the correct function definitions. If F<ioctl.ph> doesn't
a0d0e21e 2086exist or doesn't have the correct definitions you'll have to roll your
4633a7c4 2087own, based on your C header files such as F<E<lt>sys/ioctl.hE<gt>>.
5a964f20 2088(There is a Perl script called B<h2ph> that comes with the Perl kit that
54310121 2089may help you in this, but it's nontrivial.) SCALAR will be read and/or
4633a7c4 2090written depending on the FUNCTION--a pointer to the string value of SCALAR
19799a22 2091will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
4633a7c4
LW
2092has no string value but does have a numeric value, that value will be
2093passed rather than a pointer to the string value. To guarantee this to be
19799a22
GS
2094true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2095functions may be needed to manipulate the values of structures used by
2096C<ioctl>.
a0d0e21e 2097
19799a22 2098The return value of C<ioctl> (and C<fcntl>) is as follows:
a0d0e21e
LW
2099
2100 if OS returns: then Perl returns:
2101 -1 undefined value
2102 0 string "0 but true"
2103 anything else that number
2104
19799a22 2105Thus Perl returns true on success and false on failure, yet you can
a0d0e21e
LW
2106still easily determine the actual value returned by the operating
2107system:
2108
2b5ab1e7 2109 $retval = ioctl(...) || -1;
a0d0e21e
LW
2110 printf "System returned %d\n", $retval;
2111
c2611fb3 2112The special string "C<0> but true" is exempt from B<-w> complaints
5a964f20
TC
2113about improper numeric conversions.
2114
19799a22
GS
2115Here's an example of setting a filehandle named C<REMOTE> to be
2116non-blocking at the system level. You'll have to negotiate C<$|>
2117on your own, though.
2118
2119 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
2120
2121 $flags = fcntl(REMOTE, F_GETFL, 0)
2122 or die "Can't get flags for the socket: $!\n";
2123
2124 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
2125 or die "Can't set flags for the socket: $!\n";
2126
a0d0e21e
LW
2127=item join EXPR,LIST
2128
2b5ab1e7
TC
2129Joins the separate strings of LIST into a single string with fields
2130separated by the value of EXPR, and returns that new string. Example:
a0d0e21e 2131
2b5ab1e7 2132 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
a0d0e21e 2133
eb6e2d6f
GS
2134Beware that unlike C<split>, C<join> doesn't take a pattern as its
2135first argument. Compare L</split>.
a0d0e21e 2136
aa689395
PP
2137=item keys HASH
2138
19799a22 2139Returns a list consisting of all the keys of the named hash. (In
1d2dff63 2140scalar context, returns the number of keys.) The keys are returned in
ab192400
GS
2141an apparently random order. The actual random order is subject to
2142change in future versions of perl, but it is guaranteed to be the same
19799a22 2143order as either the C<values> or C<each> function produces (given
ab192400
GS
2144that the hash has not been modified). As a side effect, it resets
2145HASH's iterator.
a0d0e21e 2146
aa689395 2147Here is yet another way to print your environment:
a0d0e21e
LW
2148
2149 @keys = keys %ENV;
2150 @values = values %ENV;
19799a22 2151 while (@keys) {
a0d0e21e
LW
2152 print pop(@keys), '=', pop(@values), "\n";
2153 }
2154
2155or how about sorted by key:
2156
2157 foreach $key (sort(keys %ENV)) {
2158 print $key, '=', $ENV{$key}, "\n";
2159 }
2160
19799a22 2161To sort a hash by value, you'll need to use a C<sort> function.
aa689395 2162Here's a descending numeric sort of a hash by its values:
4633a7c4 2163
5a964f20 2164 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
4633a7c4
LW
2165 printf "%4d %s\n", $hash{$key}, $key;
2166 }
2167
19799a22 2168As an lvalue C<keys> allows you to increase the number of hash buckets
aa689395
PP
2169allocated for the given hash. This can gain you a measure of efficiency if
2170you know the hash is going to get big. (This is similar to pre-extending
2171an array by assigning a larger number to $#array.) If you say
55497cff
PP
2172
2173 keys %hash = 200;
2174
ab192400
GS
2175then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2176in fact, since it rounds up to the next power of two. These
55497cff
PP
2177buckets will be retained even if you do C<%hash = ()>, use C<undef
2178%hash> if you want to free the storage while C<%hash> is still in scope.
2179You can't shrink the number of buckets allocated for the hash using
19799a22 2180C<keys> in this way (but you needn't worry about doing this by accident,
55497cff
PP
2181as trying has no effect).
2182
19799a22 2183See also C<each>, C<values> and C<sort>.
ab192400 2184
b350dd2f 2185=item kill SIGNAL, LIST
a0d0e21e 2186
b350dd2f 2187Sends a signal to a list of processes. Returns the number of
517db077
GS
2188processes successfully signaled (which is not necessarily the
2189same as the number actually killed).
a0d0e21e
LW
2190
2191 $cnt = kill 1, $child1, $child2;
2192 kill 9, @goners;
2193
b350dd2f
GS
2194If SIGNAL is zero, no signal is sent to the process. This is a
2195useful way to check that the process is alive and hasn't changed
2196its UID. See L<perlport> for notes on the portability of this
2197construct.
2198
2199Unlike in the shell, if SIGNAL is negative, it kills
4633a7c4
LW
2200process groups instead of processes. (On System V, a negative I<PROCESS>
2201number will also kill process groups, but that's not portable.) That
2202means you usually want to use positive not negative signals. You may also
da0045b7 2203use a signal name in quotes. See L<perlipc/"Signals"> for details.
a0d0e21e
LW
2204
2205=item last LABEL
2206
2207=item last
2208
2209The C<last> command is like the C<break> statement in C (as used in
2210loops); it immediately exits the loop in question. If the LABEL is
2211omitted, the command refers to the innermost enclosing loop. The
2212C<continue> block, if any, is not executed:
2213
4633a7c4
LW
2214 LINE: while (<STDIN>) {
2215 last LINE if /^$/; # exit when done with header
5a964f20 2216 #...
a0d0e21e
LW
2217 }
2218
4968c1e4 2219C<last> cannot be used to exit a block which returns a value such as
2b5ab1e7
TC
2220C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2221a grep() or map() operation.
4968c1e4 2222
6c1372ed
GS
2223Note that a block by itself is semantically identical to a loop
2224that executes once. Thus C<last> can be used to effect an early
2225exit out of such a block.
2226
98293880
JH
2227See also L</continue> for an illustration of how C<last>, C<next>, and
2228C<redo> work.
1d2dff63 2229
a0d0e21e
LW
2230=item lc EXPR
2231
54310121 2232=item lc
bbce6d69 2233
a0d0e21e 2234Returns an lowercased version of EXPR. This is the internal function
7660c0ab 2235implementing the C<\L> escape in double-quoted strings.
19799a22
GS
2236Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
2237and L<utf8>.
a0d0e21e 2238
7660c0ab 2239If EXPR is omitted, uses C<$_>.
bbce6d69 2240
a0d0e21e
LW
2241=item lcfirst EXPR
2242
54310121 2243=item lcfirst
bbce6d69 2244
a0d0e21e 2245Returns the value of EXPR with the first character lowercased. This is
7660c0ab 2246the internal function implementing the C<\l> escape in double-quoted strings.
a0ed51b3 2247Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
a0d0e21e 2248
7660c0ab 2249If EXPR is omitted, uses C<$_>.
bbce6d69 2250
a0d0e21e
LW
2251=item length EXPR
2252
54310121 2253=item length
bbce6d69 2254
a0ed51b3 2255Returns the length in characters of the value of EXPR. If EXPR is
2b5ab1e7
TC
2256omitted, returns length of C<$_>. Note that this cannot be used on
2257an entire array or hash to find out how many elements these have.
2258For that, use C<scalar @array> and C<scalar keys %hash> respectively.
a0d0e21e
LW
2259
2260=item link OLDFILE,NEWFILE
2261
19799a22
GS
2262Creates a new filename linked to the old filename. Returns true for
2263success, false otherwise.
a0d0e21e
LW
2264
2265=item listen SOCKET,QUEUESIZE
2266
19799a22
GS
2267Does the same thing that the listen system call does. Returns true if
2268it succeeded, false otherwise. See the example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
2269
2270=item local EXPR
2271
19799a22 2272You really probably want to be using C<my> instead, because C<local> isn't
2b5ab1e7
TC
2273what most people think of as "local". See L<perlsub/"Private Variables
2274via my()"> for details.
2275
5a964f20
TC
2276A local modifies the listed variables to be local to the enclosing
2277block, file, or eval. If more than one value is listed, the list must
2278be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2279for details, including issues with tied arrays and hashes.
a0d0e21e 2280
a0d0e21e
LW
2281=item localtime EXPR
2282
19799a22 2283Converts a time as returned by the time function to a 9-element list
5f05dabc 2284with the time analyzed for the local time zone. Typically used as
a0d0e21e
LW
2285follows:
2286
54310121 2287 # 0 1 2 3 4 5 6 7 8
a0d0e21e
LW
2288 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2289 localtime(time);
2290
19799a22
GS
2291All list elements are numeric, and come straight out of a struct tm.
2292In particular this means that $mon has the range C<0..11> and $wday
2293has the range C<0..6> with sunday as day C<0>. Also, $year is the
2294number of years since 1900, that is, $year is C<123> in year 2023,
2b5ab1e7
TC
2295and I<not> simply the last two digits of the year. If you assume it is,
2296then you create non-Y2K-compliant programs--and you wouldn't want to do
2297that, would you?
54310121 2298
abd75f24
GS
2299The proper way to get a complete 4-digit year is simply:
2300
2301 $year += 1900;
2302
2303And to get the last two digits of the year (e.g., '01' in 2001) do:
2304
2305 $year = sprintf("%02d", $year % 100);
2306
54310121 2307If EXPR is omitted, uses the current time (C<localtime(time)>).
a0d0e21e 2308
f86cebdf 2309In scalar context, returns the ctime(3) value:
a0d0e21e 2310
5f05dabc 2311 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
a0d0e21e 2312
a3cb178b 2313This scalar value is B<not> locale dependent, see L<perllocale>, but
68f8bed4
JH
2314instead a Perl builtin. Also see the C<Time::Local> module
2315(to convert the second, minutes, hours, ... back to seconds since the
2316stroke of midnight the 1st of January 1970, the value returned by
2317time()), and the strftime(3) and mktime(3) function available via the
2318POSIX module. To get somewhat similar but locale dependent date
2319strings, set up your locale environment variables appropriately
2320(please see L<perllocale>) and try for example:
a3cb178b 2321
5a964f20 2322 use POSIX qw(strftime);
2b5ab1e7 2323 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
a3cb178b
GS
2324
2325Note that the C<%a> and C<%b>, the short forms of the day of the week
2326and the month of the year, may not necessarily be three characters wide.
a0d0e21e 2327
19799a22
GS
2328=item lock
2329
2330 lock I<THING>
2331
2332This function places an advisory lock on a variable, subroutine,
2333or referenced object contained in I<THING> until the lock goes out
2334of scope. This is a built-in function only if your version of Perl
2335was built with threading enabled, and if you've said C<use Threads>.
2336Otherwise a user-defined function by this name will be called. See
2337L<Thread>.
2338
a0d0e21e
LW
2339=item log EXPR
2340
54310121 2341=item log
bbce6d69 2342
2b5ab1e7
TC
2343Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2344returns log of C<$_>. To get the log of another base, use basic algebra:
19799a22 2345The base-N log of a number is equal to the natural log of that number
2b5ab1e7
TC
2346divided by the natural log of N. For example:
2347
2348 sub log10 {
2349 my $n = shift;
2350 return log($n)/log(10);
2351 }
2352
2353See also L</exp> for the inverse operation.
a0d0e21e
LW
2354
2355=item lstat FILEHANDLE
2356
2357=item lstat EXPR
2358
54310121 2359=item lstat
bbce6d69 2360
19799a22 2361Does the same thing as the C<stat> function (including setting the
5a964f20
TC
2362special C<_> filehandle) but stats a symbolic link instead of the file
2363the symbolic link points to. If symbolic links are unimplemented on
19799a22 2364your system, a normal C<stat> is done.
a0d0e21e 2365
7660c0ab 2366If EXPR is omitted, stats C<$_>.
bbce6d69 2367
a0d0e21e
LW
2368=item m//
2369
2370The match operator. See L<perlop>.
2371
2372=item map BLOCK LIST
2373
2374=item map EXPR,LIST
2375
19799a22
GS
2376Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2377C<$_> to each element) and returns the list value composed of the
2378results of each such evaluation. In scalar context, returns the
2379total number of elements so generated. Evaluates BLOCK or EXPR in
2380list context, so each element of LIST may produce zero, one, or
2381more elements in the returned value.
dd99ebda 2382
a0d0e21e
LW
2383 @chars = map(chr, @nums);
2384
2385translates a list of numbers to the corresponding characters. And
2386
4633a7c4 2387 %hash = map { getkey($_) => $_ } @array;
a0d0e21e
LW
2388
2389is just a funny way to write
2390
2391 %hash = ();
2392 foreach $_ (@array) {
4633a7c4 2393 $hash{getkey($_)} = $_;
a0d0e21e
LW
2394 }
2395
2b5ab1e7
TC
2396Note that, because C<$_> is a reference into the list value, it can
2397be used to modify the elements of the array. While this is useful and
2398supported, it can cause bizarre results if the LIST is not a named array.
2399Using a regular C<foreach> loop for this purpose would be clearer in
2400most cases. See also L</grep> for an array composed of those items of
2401the original list for which the BLOCK or EXPR evaluates to true.
fb73857a 2402
19799a22 2403=item mkdir FILENAME,MASK
a0d0e21e 2404
0591cd52 2405Creates the directory specified by FILENAME, with permissions
19799a22
GS
2406specified by MASK (as modified by C<umask>). If it succeeds it
2407returns true, otherwise it returns false and sets C<$!> (errno).
0591cd52 2408
19799a22 2409In general, it is better to create directories with permissive MASK,
0591cd52 2410and let the user modify that with their C<umask>, than it is to supply
19799a22 2411a restrictive MASK and give the user no way to be more permissive.
0591cd52
NT
2412The exceptions to this rule are when the file or directory should be
2413kept private (mail files, for instance). The perlfunc(1) entry on
19799a22 2414C<umask> discusses the choice of MASK in more detail.
a0d0e21e
LW
2415
2416=item msgctl ID,CMD,ARG
2417
f86cebdf 2418Calls the System V IPC function msgctl(2). You'll probably have to say
0ade1984
JH
2419
2420 use IPC::SysV;
2421
7660c0ab
A
2422first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2423then ARG must be a variable which will hold the returned C<msqid_ds>
951ba7fe
GS
2424structure. Returns like C<ioctl>: the undefined value for error,
2425C<"0 but true"> for zero, or the actual return value otherwise. See also
19799a22 2426C<IPC::SysV> and C<IPC::Semaphore> documentation.
a0d0e21e
LW
2427
2428=item msgget KEY,FLAGS
2429
f86cebdf 2430Calls the System V IPC function msgget(2). Returns the message queue
7660c0ab 2431id, or the undefined value if there is an error. See also C<IPC::SysV>
19799a22 2432and C<IPC::Msg> documentation.
a0d0e21e
LW
2433
2434=item msgsnd ID,MSG,FLAGS
2435
2436Calls the System V IPC function msgsnd to send the message MSG to the
2437message queue ID. MSG must begin with the long integer message type,
19799a22
GS
2438which may be created with C<pack("l", $type)>. Returns true if
2439successful, or false if there is an error. See also C<IPC::SysV>
7660c0ab 2440and C<IPC::SysV::Msg> documentation.
a0d0e21e
LW
2441
2442=item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2443
2444Calls the System V IPC function msgrcv to receive a message from
2445message queue ID into variable VAR with a maximum message size of
0ade1984
JH
2446SIZE. Note that if a message is received, the message type will be
2447the first thing in VAR, and the maximum length of VAR is SIZE plus the
19799a22 2448size of the message type. Returns true if successful, or false if
7660c0ab 2449there is an error. See also C<IPC::SysV> and C<IPC::SysV::Msg> documentation.
a0d0e21e
LW
2450
2451=item my EXPR
2452
09bef843
SB
2453=item my EXPR : ATTRIBUTES
2454
19799a22
GS
2455A C<my> declares the listed variables to be local (lexically) to the
2456enclosing block, file, or C<eval>. If
5f05dabc 2457more than one value is listed, the list must be placed in parentheses. See
cb1a09d0 2458L<perlsub/"Private Variables via my()"> for details.
4633a7c4 2459
a0d0e21e
LW
2460=item next LABEL
2461
2462=item next
2463
2464The C<next> command is like the C<continue> statement in C; it starts
2465the next iteration of the loop:
2466
4633a7c4
LW
2467 LINE: while (<STDIN>) {
2468 next LINE if /^#/; # discard comments
5a964f20 2469 #...
a0d0e21e
LW
2470 }
2471
2472Note that if there were a C<continue> block on the above, it would get
2473executed even on discarded lines. If the LABEL is omitted, the command
2474refers to the innermost enclosing loop.
2475
4968c1e4 2476C<next> cannot be used to exit a block which returns a value such as
2b5ab1e7
TC
2477C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2478a grep() or map() operation.
4968c1e4 2479
6c1372ed
GS
2480Note that a block by itself is semantically identical to a loop
2481that executes once. Thus C<next> will exit such a block early.
2482
98293880
JH
2483See also L</continue> for an illustration of how C<last>, C<next>, and
2484C<redo> work.
1d2dff63 2485
a0d0e21e
LW
2486=item no Module LIST
2487
7660c0ab 2488See the L</use> function, which C<no> is the opposite of.
a0d0e21e
LW
2489
2490=item oct EXPR
2491
54310121 2492=item oct
bbce6d69 2493
4633a7c4 2494Interprets EXPR as an octal string and returns the corresponding
4f19785b
WSI
2495value. (If EXPR happens to start off with C<0x>, interprets it as a
2496hex string. If EXPR starts off with C<0b>, it is interpreted as a
2497binary string.) The following will handle decimal, binary, octal, and
4633a7c4 2498hex in the standard Perl or C notation:
a0d0e21e
LW
2499
2500 $val = oct($val) if $val =~ /^0/;
2501
19799a22
GS
2502If EXPR is omitted, uses C<$_>. To go the other way (produce a number
2503in octal), use sprintf() or printf():
2504
2505 $perms = (stat("filename"))[2] & 07777;
2506 $oct_perms = sprintf "%lo", $perms;
2507
2508The oct() function is commonly used when a string such as C<644> needs
2509to be converted into a file mode, for example. (Although perl will
2510automatically convert strings into numbers as needed, this automatic
2511conversion assumes base 10.)
a0d0e21e 2512
6170680b
IZ
2513=item open FILEHANDLE,MODE,EXPR
2514
a0d0e21e
LW
2515=item open FILEHANDLE,EXPR
2516
2517=item open FILEHANDLE
2518
2519Opens the file whose filename is given by EXPR, and associates it with
5f05dabc
PP
2520FILEHANDLE. If FILEHANDLE is an expression, its value is used as the
2521name of the real filehandle wanted. If EXPR is omitted, the scalar
2522variable of the same name as the FILEHANDLE contains the filename.
19799a22
GS
2523(Note that lexical variables--those declared with C<my>--will not work
2524for this purpose; so if you're using C<my>, specify EXPR in your call
2b5ab1e7
TC
2525to open.) See L<perlopentut> for a kinder, gentler explanation of opening
2526files.
5f05dabc 2527
6170680b
IZ
2528If MODE is C<'E<lt>'> or nothing, the file is opened for input.
2529If MODE is C<'E<gt>'>, the file is truncated and opened for
2530output, being created if necessary. If MODE is C<'E<gt>E<gt>'>,
fbb426e4 2531the file is opened for appending, again being created if necessary.
7660c0ab
A
2532You can put a C<'+'> in front of the C<'E<gt>'> or C<'E<lt>'> to indicate that
2533you want both read and write access to the file; thus C<'+E<lt>'> is almost
2534always preferred for read/write updates--the C<'+E<gt>'> mode would clobber the
5a964f20
TC
2535file first. You can't usually use either read-write mode for updating
2536textfiles, since they have variable length records. See the B<-i>
0591cd52
NT
2537switch in L<perlrun> for a better approach. The file is created with
2538permissions of C<0666> modified by the process' C<umask> value.
5a964f20 2539
f86cebdf 2540These various prefixes correspond to the fopen(3) modes of C<'r'>, C<'r+'>, C<'w'>,
7660c0ab 2541C<'w+'>, C<'a'>, and C<'a+'>.
5f05dabc 2542
6170680b
IZ
2543In the 2-arguments (and 1-argument) form of the call the mode and
2544filename should be concatenated (in this order), possibly separated by
2545spaces. It is possible to omit the mode if the mode is C<'E<lt>'>.
2546
7660c0ab 2547If the filename begins with C<'|'>, the filename is interpreted as a
5a964f20 2548command to which output is to be piped, and if the filename ends with a
f244e06d
GS
2549C<'|'>, the filename is interpreted as a command which pipes output to
2550us. See L<perlipc/"Using open() for IPC">
19799a22 2551for more examples of this. (You are not allowed to C<open> to a command
5a964f20
TC
2552that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2553and L<perlipc/"Bidirectional Communication"> for alternatives.)
cb1a09d0 2554
6170680b
IZ
2555If MODE is C<'|-'>, the filename is interpreted as a
2556command to which output is to be piped, and if MODE is
2557C<'-|'>, the filename is interpreted as a command which pipes output to
2558us. In the 2-arguments (and 1-argument) form one should replace dash
2559(C<'-'>) with the command. See L<perlipc/"Using open() for IPC">
2560for more examples of this. (You are not allowed to C<open> to a command
2561that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2562and L<perlipc/"Bidirectional Communication"> for alternatives.)
2563
2564In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
2565and opening C<'E<gt>-'> opens STDOUT.
2566
2567Open returns
19799a22 2568nonzero upon success, the undefined value otherwise. If the C<open>
4633a7c4 2569involved a pipe, the return value happens to be the pid of the
54310121 2570subprocess.
cb1a09d0
AD
2571
2572If you're unfortunate enough to be running Perl on a system that
2573distinguishes between text files and binary files (modern operating
2574systems don't care), then you should check out L</binmode> for tips for
19799a22 2575dealing with this. The key distinction between systems that need C<binmode>
5a964f20
TC
2576and those that don't is their text file formats. Systems like Unix, MacOS, and
2577Plan9, which delimit lines with a single character, and which encode that
19799a22 2578character in C as C<"\n">, do not need C<binmode>. The rest need it.
cb1a09d0 2579
fb73857a 2580When opening a file, it's usually a bad idea to continue normal execution
19799a22
GS
2581if the request failed, so C<open> is frequently used in connection with
2582C<die>. Even if C<die> won't do what you want (say, in a CGI script,
fb73857a 2583where you want to make a nicely formatted error message (but there are
5a964f20 2584modules that can help with that problem)) you should always check
19799a22 2585the return value from opening a file. The infrequent exception is when
fb73857a
PP
2586working with an unopened filehandle is actually what you want to do.
2587
cb1a09d0 2588Examples:
a0d0e21e
LW
2589
2590 $ARTICLE = 100;
2591 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2592 while (<ARTICLE>) {...
2593
6170680b 2594 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
fb73857a 2595 # if the open fails, output is discarded
a0d0e21e 2596
6170680b 2597 open(DBASE, '+<', 'dbase.mine') # open for update
fb73857a 2598 or die "Can't open 'dbase.mine' for update: $!";
cb1a09d0 2599
6170680b
IZ
2600 open(DBASE, '+<dbase.mine') # ditto
2601 or die "Can't open 'dbase.mine' for update: $!";
2602
2603 open(ARTICLE, '-|', "caesar <$article") # decrypt article
fb73857a 2604 or die "Can't start caesar: $!";
a0d0e21e 2605
6170680b
IZ
2606 open(ARTICLE, "caesar <$article |") # ditto
2607 or die "Can't start caesar: $!";
2608
2609 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
fb73857a 2610 or die "Can't start sort: $!";
a0d0e21e
LW
2611
2612 # process argument list of files along with any includes
2613
2614 foreach $file (@ARGV) {
2615 process($file, 'fh00');
2616 }
2617
2618 sub process {
5a964f20 2619 my($filename, $input) = @_;
a0d0e21e
LW
2620 $input++; # this is a string increment
2621 unless (open($input, $filename)) {
2622 print STDERR "Can't open $filename: $!\n";
2623 return;
2624 }
2625
5a964f20 2626 local $_;
a0d0e21e
LW
2627 while (<$input>) { # note use of indirection
2628 if (/^#include "(.*)"/) {
2629 process($1, $input);
2630 next;
2631 }
5a964f20 2632 #... # whatever
a0d0e21e
LW
2633 }
2634 }
2635
2636You may also, in the Bourne shell tradition, specify an EXPR beginning
7660c0ab 2637with C<'E<gt>&'>, in which case the rest of the string is interpreted as the
5a964f20 2638name of a filehandle (or file descriptor, if numeric) to be
6170680b
IZ
2639duped and opened. You may use C<&> after C<E<gt>>, C<E<gt>E<gt>>,
2640C<E<lt>>, C<+E<gt>>, C<+E<gt>E<gt>>, and C<+E<lt>>. The
a0d0e21e 2641mode you specify should match the mode of the original filehandle.
184e9718 2642(Duping a filehandle does not take into account any existing contents of
6170680b
IZ
2643stdio buffers.) Duping file handles is not yet supported for 3-argument
2644open().
2645
a0d0e21e
LW
2646Here is a script that saves, redirects, and restores STDOUT and
2647STDERR:
2648
2649 #!/usr/bin/perl
5a964f20
TC
2650 open(OLDOUT, ">&STDOUT");
2651 open(OLDERR, ">&STDERR");
a0d0e21e 2652
6170680b
IZ
2653 open(STDOUT, '>', "foo.out") || die "Can't redirect stdout";
2654 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
a0d0e21e
LW
2655
2656 select(STDERR); $| = 1; # make unbuffered
2657 select(STDOUT); $| = 1; # make unbuffered
2658
2659 print STDOUT "stdout 1\n"; # this works for
2660 print STDERR "stderr 1\n"; # subprocesses too
2661
2662 close(STDOUT);
2663 close(STDERR);
2664
5a964f20
TC
2665 open(STDOUT, ">&OLDOUT");
2666 open(STDERR, ">&OLDERR");
a0d0e21e
LW
2667
2668 print STDOUT "stdout 2\n";
2669 print STDERR "stderr 2\n";
2670
7660c0ab 2671If you specify C<'E<lt>&=N'>, where C<N> is a number, then Perl will do an
19799a22 2672equivalent of C's C<fdopen> of that file descriptor; this is more
4633a7c4 2673parsimonious of file descriptors. For example:
a0d0e21e
LW
2674
2675 open(FILEHANDLE, "<&=$fd")
2676
4af147f6
CS
2677Note that this feature depends on the fdopen() C library function.
2678On many UNIX systems, fdopen() is known to fail when file descriptors
2679exceed a certain value, typically 255. If you need more file
2680descriptors than that, consider rebuilding Perl to use the C<sfio>
2681library.
2682
6170680b
IZ
2683If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
2684with 2-arguments (or 1-argument) form of open(), then
a0d0e21e 2685there is an implicit fork done, and the return value of open is the pid
7660c0ab 2686of the child within the parent process, and C<0> within the child
184e9718 2687process. (Use C<defined($pid)> to determine whether the open was successful.)
a0d0e21e
LW
2688The filehandle behaves normally for the parent, but i/o to that
2689filehandle is piped from/to the STDOUT/STDIN of the child process.
2690In the child process the filehandle isn't opened--i/o happens from/to
2691the new STDOUT or STDIN. Typically this is used like the normal
2692piped open when you want to exercise more control over just how the
2693pipe command gets executed, such as when you are running setuid, and
54310121 2694don't want to have to scan shell commands for metacharacters.
6170680b 2695The following triples are more or less equivalent:
a0d0e21e
LW
2696
2697 open(FOO, "|tr '[a-z]' '[A-Z]'");
6170680b
IZ
2698 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
2699 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
a0d0e21e
LW
2700
2701 open(FOO, "cat -n '$file'|");
6170680b
IZ
2702 open(FOO, '-|', "cat -n '$file'");
2703 open(FOO, '-|') || exec 'cat', '-n', $file;
a0d0e21e 2704
4633a7c4
LW
2705See L<perlipc/"Safe Pipe Opens"> for more examples of this.
2706
45bc9206
GS
2707NOTE: On any operation that may do a fork, all files opened for output
2708are flushed before the fork is attempted. On systems that support a
2709close-on-exec flag on files, the flag will be set for the newly opened
2710file descriptor as determined by the value of $^F. See L<perlvar/$^F>.
a0d0e21e 2711
0dccf244
CS
2712Closing any piped filehandle causes the parent process to wait for the
2713child to finish, and returns the status value in C<$?>.
2714
6170680b
IZ
2715The filename passed to 2-argument (or 1-argument) form of open()
2716will have leading and trailing
f86cebdf 2717whitespace deleted, and the normal redirection characters
5a964f20
TC
2718honored. This property, known as "magic open",
2719can often be used to good effect. A user could specify a filename of
7660c0ab 2720F<"rsh cat file |">, or you could change certain filenames as needed:
5a964f20
TC
2721
2722 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
2723 open(FH, $filename) or die "Can't open $filename: $!";
2724
6170680b
IZ
2725Use 3-argument form to open a file with arbitrary weird characters in it,
2726
2727 open(FOO, '<', $file);
2728
2729otherwise it's necessary to protect any leading and trailing whitespace:
5a964f20
TC
2730
2731 $file =~ s#^(\s)#./$1#;
2732 open(FOO, "< $file\0");
2733
6170680b
IZ
2734(this may not work on some bizzare filesystems). One should
2735conscientiously choose between the the I<magic> and 3-arguments form
2736of open():
2737
2738 open IN, $ARGV[0];
2739
2740will allow the user to specify an argument of the form C<"rsh cat file |">,
2741but will not work on a filename which happens to have a trailing space, while
2742
2743 open IN, '<', $ARGV[0];
2744
2745will have exactly the opposite restrictions.
2746
19799a22 2747If you want a "real" C C<open> (see L<open(2)> on your system), then you
6170680b
IZ
2748should use the C<sysopen> function, which involves no such magic (but
2749may use subtly different filemodes than Perl open(), which is mapped
2750to C fopen()). This is
5a964f20
TC
2751another way to protect your filenames from interpretation. For example:
2752
2753 use IO::Handle;
2754 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
2755 or die "sysopen $path: $!";
2756 $oldfh = select(HANDLE); $| = 1; select($oldfh);
2757 print HANDLE "stuff $$\n");
2758 seek(HANDLE, 0, 0);
2759 print "File contains: ", <HANDLE>;
2760
7660c0ab
A
2761Using the constructor from the C<IO::Handle> package (or one of its
2762subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
5a964f20
TC
2763filehandles that have the scope of whatever variables hold references to
2764them, and automatically close whenever and however you leave that scope:
c07a80fd 2765
5f05dabc 2766 use IO::File;
5a964f20 2767 #...
c07a80fd
PP
2768 sub read_myfile_munged {
2769 my $ALL = shift;
5f05dabc 2770 my $handle = new IO::File;
c07a80fd
PP
2771 open($handle, "myfile") or die "myfile: $!";
2772 $first = <$handle>
2773 or return (); # Automatically closed here.
2774 mung $first or die "mung failed"; # Or here.
2775 return $first, <$handle> if $ALL; # Or here.
2776 $first; # Or here.
2777 }
2778
b687b08b 2779See L</seek> for some details about mixing reading and writing.
a0d0e21e
LW
2780
2781=item opendir DIRHANDLE,EXPR
2782
19799a22
GS
2783Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
2784C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
a0d0e21e
LW
2785DIRHANDLEs have their own namespace separate from FILEHANDLEs.
2786
2787=item ord EXPR
2788
54310121 2789=item ord
bbce6d69 2790
a0ed51b3 2791Returns the numeric (ASCII or Unicode) value of the first character of EXPR. If
7660c0ab 2792EXPR is omitted, uses C<$_>. For the reverse, see L</chr>.
2b5ab1e7 2793See L<utf8> for more about Unicode.
a0d0e21e 2794
77ca0c92
LW
2795=item our EXPR
2796
2797An C<our> declares the listed variables to be valid globals within
2798the enclosing block, file, or C<eval>. That is, it has the same
2799scoping rules as a "my" declaration, but does not create a local
2800variable. If more than one value is listed, the list must be placed
2801in parentheses. The C<our> declaration has no semantic effect unless
2802"use strict vars" is in effect, in which case it lets you use the
2803declared global variable without qualifying it with a package name.
2804(But only within the lexical scope of the C<our> declaration. In this
2805it differs from "use vars", which is package scoped.)
2806
f472eb5c
GS
2807An C<our> declaration declares a global variable that will be visible
2808across its entire lexical scope, even across package boundaries. The
2809package in which the variable is entered is determined at the point
2810of the declaration, not at the point of use. This means the following
2811behavior holds:
2812
2813 package Foo;
2814 our $bar; # declares $Foo::bar for rest of lexical scope
2815 $bar = 20;
2816
2817 package Bar;
2818 print $bar; # prints 20
2819
2820Multiple C<our> declarations in the same lexical scope are allowed
2821if they are in different packages. If they happened to be in the same
2822package, Perl will emit warnings if you have asked for them.
2823
2824 use warnings;
2825 package Foo;
2826 our $bar; # declares $Foo::bar for rest of lexical scope
2827 $bar = 20;
2828
2829 package Bar;
2830 our $bar = 30; # declares $Bar::bar for rest of lexical scope
2831 print $bar; # prints 30
2832
2833 our $bar; # emits warning
2834
a0d0e21e
LW
2835=item pack TEMPLATE,LIST
2836
2b6c5635
GS
2837Takes a LIST of values and converts it into a string using the rules
2838given by the TEMPLATE. The resulting string is the concatenation of
2839the converted values. Typically, each converted value looks
2840like its machine-level representation. For example, on 32-bit machines
2841a converted integer may be represented by a sequence of 4 bytes.
2842
2843The TEMPLATE is a
a0d0e21e
LW
2844sequence of characters that give the order and type of values, as
2845follows:
2846
5a929a98 2847 a A string with arbitrary binary data, will be null padded.
a0d0e21e 2848 A An ascii string, will be space padded.
5a929a98
VU
2849 Z A null terminated (asciz) string, will be null padded.
2850
2b6c5635
GS
2851 b A bit string (ascending bit order inside each byte, like vec()).
2852 B A bit string (descending bit order inside each byte).
a0d0e21e
LW
2853 h A hex string (low nybble first).
2854 H A hex string (high nybble first).
2855
2856 c A signed char value.
a0ed51b3 2857 C An unsigned char value. Only does bytes. See U for Unicode.
96e4d5b1 2858
a0d0e21e
LW
2859 s A signed short value.
2860 S An unsigned short value.
96e4d5b1 2861 (This 'short' is _exactly_ 16 bits, which may differ from
851646ae
JH
2862 what a local C compiler calls 'short'. If you want
2863 native-length shorts, use the '!' suffix.)
96e4d5b1 2864
a0d0e21e
LW
2865 i A signed integer value.
2866 I An unsigned integer value.
19799a22 2867 (This 'integer' is _at_least_ 32 bits wide. Its exact
f86cebdf
GS
2868 size depends on what a local C compiler calls 'int',
2869 and may even be larger than the 'long' described in
2870 the next item.)
96e4d5b1 2871
a0d0e21e
LW
2872 l A signed long value.
2873 L An unsigned long value.
96e4d5b1 2874 (This 'long' is _exactly_ 32 bits, which may differ from
851646ae
JH
2875 what a local C compiler calls 'long'. If you want
2876 native-length longs, use the '!' suffix.)
a0d0e21e 2877
5d11dd56
G
2878 n An unsigned short in "network" (big-endian) order.
2879 N An unsigned long in "network" (big-endian) order.
2880 v An unsigned short in "VAX" (little-endian) order.
2881 V An unsigned long in "VAX" (little-endian) order.
96e4d5b1
PP
2882 (These 'shorts' and 'longs' are _exactly_ 16 bits and
2883 _exactly_ 32 bits, respectively.)
a0d0e21e 2884
dae0da7a
JH
2885 q A signed quad (64-bit) value.
2886 Q An unsigned quad value.
851646ae
JH
2887 (Quads are available only if your system supports 64-bit
2888 integer values _and_ if Perl has been compiled to support those.
dae0da7a
JH
2889 Causes a fatal error otherwise.)
2890
a0d0e21e
LW
2891 f A single-precision float in the native format.
2892 d A double-precision float in the native format.
2893
2894 p A pointer to a null-terminated string.
2895 P A pointer to a structure (fixed-length string).
2896
2897 u A uuencoded string.
a0ed51b3
LW
2898 U A Unicode character number. Encodes to UTF-8 internally.
2899 Works even if C<use utf8> is not in effect.
a0d0e21e 2900
96e4d5b1 2901 w A BER compressed integer. Its bytes represent an unsigned
f86cebdf
GS
2902 integer in base 128, most significant digit first, with as
2903 few digits as possible. Bit eight (the high bit) is set
2904 on each byte except the last.
def98dd4 2905
a0d0e21e
LW
2906 x A null byte.
2907 X Back up a byte.
2908 @ Null fill to absolute position.
2909
5a929a98
VU
2910The following rules apply:
2911
2912=over 8
2913
2914=item *
2915
5a964f20 2916Each letter may optionally be followed by a number giving a repeat
951ba7fe
GS
2917count. With all types except C<a>, C<A>, C<Z>, C<b>, C<B>, C<h>,
2918C<H>, and C<P> the pack function will gobble up that many values from
5a929a98 2919the LIST. A C<*> for the repeat count means to use however many items are
951ba7fe
GS
2920left, except for C<@>, C<x>, C<X>, where it is equivalent
2921to C<0>, and C<u>, where it is equivalent to 1 (or 45, what is the
2b6c5635
GS
2922same).
2923
951ba7fe 2924When used with C<Z>, C<*> results in the addition of a trailing null
2b6c5635
GS
2925byte (so the packed result will be one longer than the byte C<length>
2926of the item).
2927
951ba7fe 2928The repeat count for C<u> is interpreted as the maximal number of bytes
2b6c5635 2929to encode per line of output, with 0 and 1 replaced by 45.
5a929a98
VU
2930
2931=item *
2932
951ba7fe 2933The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
5a929a98 2934string of length count, padding with nulls or spaces as necessary. When
951ba7fe
GS
2935unpacking, C<A> strips trailing spaces and nulls, C<Z> strips everything
2936after the first null, and C<a> returns data verbatim. When packing,
2937C<a>, and C<Z> are equivalent.
2b6c5635
GS
2938
2939If the value-to-pack is too long, it is truncated. If too long and an
951ba7fe
GS
2940explicit count is provided, C<Z> packs only C<$count-1> bytes, followed
2941by a null byte. Thus C<Z> always packs a trailing null byte under
2b6c5635 2942all circumstances.
5a929a98
VU
2943
2944=item *
2945
951ba7fe 2946Likewise, the C<b> and C<B> fields pack a string that many bits long.
c73032f5
IZ
2947Each byte of the input field of pack() generates 1 bit of the result.
2948Each result bit is based on the least-significant bit of the corresponding
2949input byte, i.e., on C<ord($byte)%2>. In particular, bytes C<"0"> and
2950C<"1"> generate bits 0 and 1, as do bytes C<"\0"> and C<"\1">.
2951
2952Starting from the beginning of the input string of pack(), each 8-tuple
951ba7fe 2953of bytes is converted to 1 byte of output. With format C<b>
c73032f5 2954the first byte of the 8-tuple determines the least-significant bit of a
951ba7fe 2955byte, and with format C<B> it determines the most-significant bit of
c73032f5
IZ
2956a byte.
2957
2958If the length of the input string is not exactly divisible by 8, the
2959remainder is packed as if the input string were padded by null bytes
2960at the end. Similarly, during unpack()ing the "extra" bits are ignored.
2961
2962If the input string of pack() is longer than needed, extra bytes are ignored.
2b6c5635
GS
2963A C<*> for the repeat count of pack() means to use all the bytes of
2964the input field. On unpack()ing the bits are converted to a string
2965of C<"0">s and C<"1">s.
5a929a98
VU
2966
2967=item *
2968
951ba7fe 2969The C<h> and C<H> fields pack a string that many nybbles (4-bit groups,
851646ae 2970representable as hexadecimal digits, 0-9a-f) long.
5a929a98 2971
c73032f5
IZ
2972Each byte of the input field of pack() generates 4 bits of the result.
2973For non-alphabetical bytes the result is based on the 4 least-significant
2974bits of the input byte, i.e., on C<ord($byte)%16>. In particular,
2975bytes C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
2976C<"\0"> and C<"\1">. For bytes C<"a".."f"> and C<"A".."F"> the result
2977is compatible with the usual hexadecimal digits, so that C<"a"> and
2978C<"A"> both generate the nybble C<0xa==10>. The result for bytes
2979C<"g".."z"> and C<"G".."Z"> is not well-defined.
2980
2981Starting from the beginning of the input string of pack(), each pair
951ba7fe 2982of bytes is converted to 1 byte of output. With format C<h> the
c73032f5 2983first byte of the pair determines the least-significant nybble of the
951ba7fe 2984output byte, and with format C<H> it determines the most-significant
c73032f5
IZ
2985nybble.
2986
2987If the length of the input string is not even, it behaves as if padded
2988by a null byte at the end. Similarly, during unpack()ing the "extra"
2989nybbles are ignored.
2990
2991If the input string of pack() is longer than needed, extra bytes are ignored.
2992A C<*> for the repeat count of pack() means to use all the bytes of
2993the input field. On unpack()ing the bits are converted to a string
2994of hexadecimal digits.
2995
5a929a98
VU
2996=item *
2997
951ba7fe 2998The C<p> type packs a pointer to a null-terminated string. You are
5a929a98
VU
2999responsible for ensuring the string is not a temporary value (which can
3000potentially get deallocated before you get around to using the packed result).
951ba7fe
GS
3001The C<P> type packs a pointer to a structure of the size indicated by the
3002length. A NULL pointer is created if the corresponding value for C<p> or
3003C<P> is C<undef>, similarly for unpack().
5a929a98
VU
3004
3005=item *
3006
951ba7fe
GS
3007The C</> template character allows packing and unpacking of strings where
3008the packed structure contains a byte count followed by the string itself.
17f4a12d 3009You write I<length-item>C</>I<string-item>.
43192e07
IP
3010
3011The I<length-item> can be any C<pack> template letter,
3012and describes how the length value is packed.
3013The ones likely to be of most use are integer-packing ones like
951ba7fe
GS
3014C<n> (for Java strings), C<w> (for ASN.1 or SNMP)
3015and C<N> (for Sun XDR).
43192e07
IP
3016
3017The I<string-item> must, at present, be C<"A*">, C<"a*"> or C<"Z*">.
3018For C<unpack> the length of the string is obtained from the I<length-item>,
3019but if you put in the '*' it will be ignored.
3020
17f4a12d
IZ
3021 unpack 'C/a', "\04Gurusamy"; gives 'Guru'
3022 unpack 'a3/A* A*', '007 Bond J '; gives (' Bond','J')
3023 pack 'n/a* w/a*','hello,','world'; gives "\000\006hello,\005world"
43192e07
IP
3024
3025The I<length-item> is not returned explicitly from C<unpack>.
3026
951ba7fe
GS
3027Adding a count to the I<length-item> letter is unlikely to do anything
3028useful, unless that letter is C<A>, C<a> or C<Z>. Packing with a
3029I<length-item> of C<a> or C<Z> may introduce C<"\000"> characters,
43192e07
IP
3030which Perl does not regard as legal in numeric strings.
3031
3032=item *
3033
951ba7fe
GS
3034The integer types C<s>, C<S>, C<l>, and C<L> may be
3035immediately followed by a C<!> suffix to signify native shorts or
3036longs--as you can see from above for example a bare C<l> does mean
851646ae
JH
3037exactly 32 bits, the native C<long> (as seen by the local C compiler)
3038may be larger. This is an issue mainly in 64-bit platforms. You can
951ba7fe 3039see whether using C<!> makes any difference by
726ea183 3040
4d0c1c44
GS
3041 print length(pack("s")), " ", length(pack("s!")), "\n";
3042 print length(pack("l")), " ", length(pack("l!")), "\n";
ef54e1a4 3043
951ba7fe
GS
3044C<i!> and C<I!> also work but only because of completeness;
3045they are identical to C<i> and C<I>.
ef54e1a4 3046
19799a22
GS
3047The actual sizes (in bytes) of native shorts, ints, longs, and long
3048longs on the platform where Perl was built are also available via
3049L<Config>:
3050
3051 use Config;
3052 print $Config{shortsize}, "\n";
3053 print $Config{intsize}, "\n";
3054 print $Config{longsize}, "\n";
3055 print $Config{longlongsize}, "\n";
ef54e1a4 3056
5074e145 3057(The C<$Config{longlongsize}> will be undefine if your system does
851646ae
JH
3058not support long longs.)
3059
ef54e1a4
JH
3060=item *
3061
951ba7fe 3062The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, and C<L>
ef54e1a4
JH
3063are inherently non-portable between processors and operating systems
3064because they obey the native byteorder and endianness. For example a
140cb37e 30654-byte integer 0x12345678 (305419896 decimal) be ordered natively
ef54e1a4
JH
3066(arranged in and handled by the CPU registers) into bytes as
3067
719a3cf5
JH
3068 0x12 0x34 0x56 0x78 # little-endian
3069 0x78 0x56 0x34 0x12 # big-endian
ef54e1a4 3070
5d11dd56 3071Basically, the Intel, Alpha, and VAX CPUs are little-endian, while
719a3cf5
JH
3072everybody else, for example Motorola m68k/88k, PPC, Sparc, HP PA,
3073Power, and Cray are big-endian. MIPS can be either: Digital used it
19799a22 3074in little-endian mode; SGI uses it in big-endian mode.
719a3cf5 3075
19799a22 3076The names `big-endian' and `little-endian' are comic references to
ef54e1a4
JH
3077the classic "Gulliver's Travels" (via the paper "On Holy Wars and a
3078Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and
19799a22 3079the egg-eating habits of the Lilliputians.
ef54e1a4 3080
140cb37e 3081Some systems may have even weirder byte orders such as
ef54e1a4
JH
3082
3083 0x56 0x78 0x12 0x34
3084 0x34 0x12 0x78 0x56
3085
3086You can see your system's preference with
3087
3088 print join(" ", map { sprintf "%#02x", $_ }
3089 unpack("C*",pack("L",0x12345678))), "\n";
3090
d99ad34e 3091The byteorder on the platform where Perl was built is also available
726ea183 3092via L<Config>:
ef54e1a4
JH
3093
3094 use Config;
3095 print $Config{byteorder}, "\n";
3096
d99ad34e
JH
3097Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'>
3098and C<'87654321'> are big-endian.
719a3cf5 3099
951ba7fe
GS
3100If you want portable packed integers use the formats C<n>, C<N>,
3101C<v>, and C<V>, their byte endianness and size is known.
851646ae 3102See also L<perlport>.
ef54e1a4
JH
3103
3104=item *
3105
5a929a98
VU
3106Real numbers (floats and doubles) are in the native machine format only;
3107due to the multiplicity of floating formats around, and the lack of a
3108standard "network" representation, no facility for interchange has been
3109made. This means that packed floating point data written on one machine
3110may not be readable on another - even if both use IEEE floating point
3111arithmetic (as the endian-ness of the memory representation is not part
851646ae 3112of the IEEE spec). See also L<perlport>.
5a929a98
VU
3113
3114Note that Perl uses doubles internally for all numeric calculation, and
3115converting from double into float and thence back to double again will
3116lose precision (i.e., C<unpack("f", pack("f", $foo)>) will not in general
19799a22 3117equal $foo).
5a929a98 3118
851646ae
JH
3119=item *
3120
3121You must yourself do any alignment or padding by inserting for example
9ccd05c0
JH
3122enough C<'x'>es while packing. There is no way to pack() and unpack()
3123could know where the bytes are going to or coming from. Therefore
3124C<pack> (and C<unpack>) handle their output and input as flat
3125sequences of bytes.
851646ae 3126
17f4a12d
IZ
3127=item *
3128
3129A comment in a TEMPLATE starts with C<#> and goes to the end of line.
3130
2b6c5635
GS
3131=item *
3132
3133If TEMPLATE requires more arguments to pack() than actually given, pack()
3134assumes additional C<""> arguments. If TEMPLATE requires less arguments
3135to pack() than actually given, extra arguments are ignored.
3136
5a929a98 3137=back
a0d0e21e
LW
3138
3139Examples:
3140
a0ed51b3 3141 $foo = pack("CCCC",65,66,67,68);
a0d0e21e 3142 # foo eq "ABCD"
a0ed51b3 3143 $foo = pack("C4",65,66,67,68);
a0d0e21e 3144 # same thing
a0ed51b3
LW
3145 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
3146 # same thing with Unicode circled letters
a0d0e21e
LW
3147
3148 $foo = pack("ccxxcc",65,66,67,68);
3149 # foo eq "AB\0\0CD"
3150
9ccd05c0
JH
3151 # note: the above examples featuring "C" and "c" are true
3152 # only on ASCII and ASCII-derived systems such as ISO Latin 1
3153 # and UTF-8. In EBCDIC the first example would be
3154 # $foo = pack("CCCC",193,194,195,196);
3155
a0d0e21e
LW
3156 $foo = pack("s2",1,2);
3157 # "\1\0\2\0" on little-endian
3158 # "\0\1\0\2" on big-endian
3159
3160 $foo = pack("a4","abcd","x","y","z");
3161 # "abcd"
3162
3163 $foo = pack("aaaa","abcd","x","y","z");
3164 # "axyz"
3165
3166 $foo = pack("a14","abcdefg");
3167 # "abcdefg\0\0\0\0\0\0\0"
3168
3169 $foo = pack("i9pl", gmtime);
3170 # a real struct tm (on my system anyway)
3171
5a929a98
VU
3172 $utmp_template = "Z8 Z8 Z16 L";
3173 $utmp = pack($utmp_template, @utmp1);
3174 # a struct utmp (BSDish)
3175
3176 @utmp2 = unpack($utmp_template, $utmp);
3177 # "@utmp1" eq "@utmp2"
3178
a0d0e21e
LW
3179 sub bintodec {
3180 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
3181 }
3182
851646ae
JH
3183 $foo = pack('sx2l', 12, 34);
3184 # short 12, two zero bytes padding, long 34
3185 $bar = pack('s@4l', 12, 34);
3186 # short 12, zero fill to position 4, long 34
3187 # $foo eq $bar
3188
5a929a98 3189The same template may generally also be used in unpack().
a0d0e21e 3190
5a964f20
TC
3191=item package
3192
cb1a09d0
AD
3193=item package NAMESPACE
3194
3195Declares the compilation unit as being in the given namespace. The scope
2b5ab1e7 3196of the package declaration is from the declaration itself through the end
19799a22 3197of the enclosing block, file, or eval (the same as the C<my> operator).
2b5ab1e7
TC
3198All further unqualified dynamic identifiers will be in this namespace.
3199A package statement affects only dynamic variables--including those
19799a22
GS
3200you've used C<local> on--but I<not> lexical variables, which are created
3201with C<my>. Typically it would be the first declaration in a file to
2b5ab1e7
TC
3202be included by the C<require> or C<use> operator. You can switch into a
3203package in more than one place; it merely influences which symbol table
3204is used by the compiler for the rest of that block. You can refer to
3205variables and filehandles in other packages by prefixing the identifier
3206with the package name and a double colon: C<$Package::Variable>.
3207If the package name is null, the C<main> package as assumed. That is,
3208C<$::sail> is equivalent to C<$main::sail> (as well as to C<$main'sail>,
3209still seen in older code).
cb1a09d0 3210
5a964f20
TC
3211If NAMESPACE is omitted, then there is no current package, and all
3212identifiers must be fully qualified or lexicals. This is stricter
3213than C<use strict>, since it also extends to function names.
3214
cb1a09d0
AD
3215See L<perlmod/"Packages"> for more information about packages, modules,
3216and classes. See L<perlsub> for other scoping issues.
3217
a0d0e21e
LW
3218=item pipe READHANDLE,WRITEHANDLE
3219
3220Opens a pair of connected pipes like the corresponding system call.
3221Note that if you set up a loop of piped processes, deadlock can occur
3222unless you are very careful. In addition, note that Perl's pipes use
184e9718 3223stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE
a0d0e21e
LW
3224after each command, depending on the application.
3225
7e1af8bc 3226See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
4633a7c4
LW
3227for examples of such things.
3228
4771b018
GS
3229On systems that support a close-on-exec flag on files, the flag will be set
3230for the newly opened file descriptors as determined by the value of $^F.
3231See L<perlvar/$^F>.
3232
a0d0e21e
LW
3233=item pop ARRAY
3234
54310121 3235=item pop
28757baa 3236
a0d0e21e 3237Pops and returns the last value of the array, shortening the array by
19799a22 3238one element. Has an effect similar to
a0d0e21e 3239
19799a22 3240 $ARRAY[$#ARRAY--]
a0d0e21e 3241
19799a22
GS
3242If there are no elements in the array, returns the undefined value
3243(although this may happen at other times as well). If ARRAY is
3244omitted, pops the C<@ARGV> array in the main program, and the C<@_>
3245array in subroutines, just like C<shift>.
a0d0e21e
LW
3246
3247=item pos SCALAR
3248
54310121 3249=item pos
bbce6d69 3250
4633a7c4 3251Returns the offset of where the last C<m//g> search left off for the variable
7660c0ab 3252is in question (C<$_> is used when the variable is not specified). May be
44a8e56a
PP
3253modified to change that offset. Such modification will also influence
3254the C<\G> zero-width assertion in regular expressions. See L<perlre> and
3255L<perlop>.
a0d0e21e
LW
3256
3257=item print FILEHANDLE LIST
3258
3259=item print LIST
3260
3261=item print
3262
19799a22
GS
3263Prints a string or a list of strings. Returns true if successful.
3264FILEHANDLE may be a scalar variable name, in which case the variable
3265contains the name of or a reference to the filehandle, thus introducing
3266one level of indirection. (NOTE: If FILEHANDLE is a variable and
3267the next token is a term, it may be misinterpreted as an operator
2b5ab1e7 3268unless you interpose a C<+> or put parentheses around the arguments.)
19799a22
GS
3269If FILEHANDLE is omitted, prints by default to standard output (or
3270to the last selected output channel--see L</select>). If LIST is
3271also omitted, prints C<$_> to the currently selected output channel.
3272To set the default output channel to something other than STDOUT
3273use the select operation. The current value of C<$,> (if any) is
3274printed between each LIST item. The current value of C<$\> (if
3275any) is printed after the entire LIST has been printed. Because
3276print takes a LIST, anything in the LIST is evaluated in list
3277context, and any subroutine that you call will have one or more of
3278its expressions evaluated in list context. Also be careful not to
3279follow the print keyword with a left parenthesis unless you want
3280the corresponding right parenthesis to terminate the arguments to
3281the print--interpose a C<+> or put parentheses around all the
3282arguments.
a0d0e21e 3283
4633a7c4 3284Note that if you're storing FILEHANDLES in an array or other expression,
da0045b7 3285you will have to use a block returning its value instead:
4633a7c4
LW
3286
3287 print { $files[$i] } "stuff\n";
3288 print { $OK ? STDOUT : STDERR } "stuff\n";
3289
5f05dabc 3290=item printf FILEHANDLE FORMAT, LIST
a0d0e21e 3291
5f05dabc 3292=item printf FORMAT, LIST
a0d0e21e 3293
7660c0ab 3294Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
a3cb178b 3295(the output record separator) is not appended. The first argument
19799a22 3296of the list will be interpreted as the C<printf> format. If C<use locale> is
a034a98d
DD
3297in effect, the character used for the decimal point in formatted real numbers
3298is affected by the LC_NUMERIC locale. See L<perllocale>.
a0d0e21e 3299
19799a22
GS
3300Don't fall into the trap of using a C<printf> when a simple
3301C<print> would do. The C<print> is more efficient and less
28757baa
PP
3302error prone.
3303
da0045b7
PP
3304=item prototype FUNCTION
3305
3306Returns the prototype of a function as a string (or C<undef&g