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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 $evaltext is undefined. (Note also that
dc848c6f 524each C<use> statement creates a C<require> frame inside an C<eval EXPR>)
f3aa04c2 525frame. C<$hints> contains pragmatic hints that the caller was
8004f2ac
GS
526compiled with. The C<$hints> value is subject to change between versions
527of Perl, and is not meant for external use.
748a9306
LW
528
529Furthermore, when called from within the DB package, caller returns more
7660c0ab 530detailed information: it sets the list variable C<@DB::args> to be the
54310121 531arguments with which the subroutine was invoked.
748a9306 532
7660c0ab 533Be aware that the optimizer might have optimized call frames away before
19799a22 534C<caller> had a chance to get the information. That means that C<caller(N)>
7660c0ab 535might not return information about the call frame you expect it do, for
19799a22
GS
536C<N E<gt> 1>. In particular, C<@DB::args> might have information from the
537previous time C<caller> was called.
7660c0ab 538
a0d0e21e
LW
539=item chdir EXPR
540
2b5ab1e7 541Changes the working directory to EXPR, if possible. If EXPR is omitted,
0bfc1ec4
GS
542changes to the directory specified by C<$ENV{HOME}>, if set; if not,
543changes to the directory specified by C<$ENV{LOGDIR}>. If neither is
544set, C<chdir> does nothing. It returns true upon success, false
545otherwise. See the example under C<die>.
a0d0e21e
LW
546
547=item chmod LIST
548
549Changes the permissions of a list of files. The first element of the
4633a7c4 550list must be the numerical mode, which should probably be an octal
2f9daede
TPG
551number, and which definitely should I<not> a string of octal digits:
552C<0644> is okay, C<'0644'> is not. Returns the number of files
dc848c6f 553successfully changed. See also L</oct>, if all you have is a string.
a0d0e21e
LW
554
555 $cnt = chmod 0755, 'foo', 'bar';
556 chmod 0755, @executables;
f86cebdf
GS
557 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
558 # --w----r-T
2f9daede
TPG
559 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
560 $mode = 0644; chmod $mode, 'foo'; # this is best
a0d0e21e
LW
561
562=item chomp VARIABLE
563
564=item chomp LIST
565
566=item chomp
567
2b5ab1e7
TC
568This safer version of L</chop> removes any trailing string
569that corresponds to the current value of C<$/> (also known as
28757baa
PP
570$INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
571number of characters removed from all its arguments. It's often used to
572remove the newline from the end of an input record when you're worried
2b5ab1e7
TC
573that the final record may be missing its newline. When in paragraph
574mode (C<$/ = "">), it removes all trailing newlines from the string.
4c5a6083
GS
575When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
576a reference to an integer or the like, see L<perlvar>) chomp() won't
19799a22
GS
577remove anything.
578If VARIABLE is omitted, it chomps C<$_>. Example:
a0d0e21e
LW
579
580 while (<>) {
581 chomp; # avoid \n on last field
582 @array = split(/:/);
5a964f20 583 # ...
a0d0e21e
LW
584 }
585
586You can actually chomp anything that's an lvalue, including an assignment:
587
588 chomp($cwd = `pwd`);
589 chomp($answer = <STDIN>);
590
591If you chomp a list, each element is chomped, and the total number of
592characters removed is returned.
593
594=item chop VARIABLE
595
596=item chop LIST
597
598=item chop
599
600Chops off the last character of a string and returns the character
601chopped. It's used primarily to remove the newline from the end of an
602input record, but is much more efficient than C<s/\n//> because it neither
7660c0ab 603scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
a0d0e21e
LW
604Example:
605
606 while (<>) {
607 chop; # avoid \n on last field
608 @array = split(/:/);
5a964f20 609 #...
a0d0e21e
LW
610 }
611
612You can actually chop anything that's an lvalue, including an assignment:
613
614 chop($cwd = `pwd`);
615 chop($answer = <STDIN>);
616
617If you chop a list, each element is chopped. Only the value of the
19799a22 618last C<chop> is returned.
a0d0e21e 619
19799a22 620Note that C<chop> returns the last character. To return all but the last
748a9306
LW
621character, use C<substr($string, 0, -1)>.
622
a0d0e21e
LW
623=item chown LIST
624
625Changes the owner (and group) of a list of files. The first two
19799a22
GS
626elements of the list must be the I<numeric> uid and gid, in that
627order. A value of -1 in either position is interpreted by most
628systems to leave that value unchanged. Returns the number of files
629successfully changed.
a0d0e21e
LW
630
631 $cnt = chown $uid, $gid, 'foo', 'bar';
632 chown $uid, $gid, @filenames;
633
54310121 634Here's an example that looks up nonnumeric uids in the passwd file:
a0d0e21e
LW
635
636 print "User: ";
19799a22 637 chomp($user = <STDIN>);
5a964f20 638 print "Files: ";
19799a22 639 chomp($pattern = <STDIN>);
a0d0e21e
LW
640
641 ($login,$pass,$uid,$gid) = getpwnam($user)
642 or die "$user not in passwd file";
643
5a964f20 644 @ary = glob($pattern); # expand filenames
a0d0e21e
LW
645 chown $uid, $gid, @ary;
646
54310121 647On most systems, you are not allowed to change the ownership of the
4633a7c4
LW
648file unless you're the superuser, although you should be able to change
649the group to any of your secondary groups. On insecure systems, these
650restrictions may be relaxed, but this is not a portable assumption.
19799a22
GS
651On POSIX systems, you can detect this condition this way:
652
653 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
654 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
4633a7c4 655
a0d0e21e
LW
656=item chr NUMBER
657
54310121 658=item chr
bbce6d69 659
a0d0e21e 660Returns the character represented by that NUMBER in the character set.
a0ed51b3 661For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
2b5ab1e7
TC
662chr(0x263a) is a Unicode smiley face (but only within the scope of
663a C<use utf8>). For the reverse, use L</ord>.
664See L<utf8> for more about Unicode.
a0d0e21e 665
7660c0ab 666If NUMBER is omitted, uses C<$_>.
bbce6d69 667
a0d0e21e
LW
668=item chroot FILENAME
669
54310121 670=item chroot
bbce6d69 671
5a964f20 672This function works like the system call by the same name: it makes the
4633a7c4 673named directory the new root directory for all further pathnames that
951ba7fe 674begin with a C</> by your process and all its children. (It doesn't
28757baa 675change your current working directory, which is unaffected.) For security
4633a7c4 676reasons, this call is restricted to the superuser. If FILENAME is
19799a22 677omitted, does a C<chroot> to C<$_>.
a0d0e21e
LW
678
679=item close FILEHANDLE
680
6a518fbc
TP
681=item close
682
19799a22 683Closes the file or pipe associated with the file handle, returning true
a0d0e21e 684only if stdio successfully flushes buffers and closes the system file
19799a22 685descriptor. Closes the currently selected filehandle if the argument
6a518fbc 686is omitted.
fb73857a
PP
687
688You don't have to close FILEHANDLE if you are immediately going to do
19799a22
GS
689another C<open> on it, because C<open> will close it for you. (See
690C<open>.) However, an explicit C<close> on an input file resets the line
691counter (C<$.>), while the implicit close done by C<open> does not.
fb73857a 692
19799a22
GS
693If the file handle came from a piped open C<close> will additionally
694return false if one of the other system calls involved fails or if the
fb73857a 695program exits with non-zero status. (If the only problem was that the
2b5ab1e7
TC
696program exited non-zero C<$!> will be set to C<0>.) Closing a pipe
697also waits for the process executing on the pipe to complete, in case you
698want to look at the output of the pipe afterwards, and
699implicitly puts the exit status value of that command into C<$?>.
5a964f20 700
73689b13
GS
701Prematurely closing the read end of a pipe (i.e. before the process
702writing to it at the other end has closed it) will result in a
703SIGPIPE being delivered to the writer. If the other end can't
704handle that, be sure to read all the data before closing the pipe.
705
fb73857a 706Example:
a0d0e21e 707
fb73857a
PP
708 open(OUTPUT, '|sort >foo') # pipe to sort
709 or die "Can't start sort: $!";
5a964f20 710 #... # print stuff to output
fb73857a
PP
711 close OUTPUT # wait for sort to finish
712 or warn $! ? "Error closing sort pipe: $!"
713 : "Exit status $? from sort";
714 open(INPUT, 'foo') # get sort's results
715 or die "Can't open 'foo' for input: $!";
a0d0e21e 716
5a964f20
TC
717FILEHANDLE may be an expression whose value can be used as an indirect
718filehandle, usually the real filehandle name.
a0d0e21e
LW
719
720=item closedir DIRHANDLE
721
19799a22 722Closes a directory opened by C<opendir> and returns the success of that
5a964f20
TC
723system call.
724
725DIRHANDLE may be an expression whose value can be used as an indirect
726dirhandle, usually the real dirhandle name.
a0d0e21e
LW
727
728=item connect SOCKET,NAME
729
730Attempts to connect to a remote socket, just as the connect system call
19799a22 731does. Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
732packed address of the appropriate type for the socket. See the examples in
733L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 734
cb1a09d0
AD
735=item continue BLOCK
736
737Actually a flow control statement rather than a function. If there is a
98293880
JH
738C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
739C<foreach>), it is always executed just before the conditional is about to
740be evaluated again, just like the third part of a C<for> loop in C. Thus
cb1a09d0
AD
741it can be used to increment a loop variable, even when the loop has been
742continued via the C<next> statement (which is similar to the C C<continue>
743statement).
744
98293880 745C<last>, C<next>, or C<redo> may appear within a C<continue>
19799a22
GS
746block. C<last> and C<redo> will behave as if they had been executed within
747the main block. So will C<next>, but since it will execute a C<continue>
1d2dff63
GS
748block, it may be more entertaining.
749
750 while (EXPR) {
751 ### redo always comes here
752 do_something;
753 } continue {
754 ### next always comes here
755 do_something_else;
756 # then back the top to re-check EXPR
757 }
758 ### last always comes here
759
760Omitting the C<continue> section is semantically equivalent to using an
19799a22 761empty one, logically enough. In that case, C<next> goes directly back
1d2dff63
GS
762to check the condition at the top of the loop.
763
a0d0e21e
LW
764=item cos EXPR
765
5a964f20 766Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
7660c0ab 767takes cosine of C<$_>.
a0d0e21e 768
7660c0ab 769For the inverse cosine operation, you may use the C<POSIX::acos()>
28757baa
PP
770function, or use this relation:
771
772 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
773
a0d0e21e
LW
774=item crypt PLAINTEXT,SALT
775
f86cebdf 776Encrypts a string exactly like the crypt(3) function in the C library
4633a7c4
LW
777(assuming that you actually have a version there that has not been
778extirpated as a potential munition). This can prove useful for checking
779the password file for lousy passwords, amongst other things. Only the
780guys wearing white hats should do this.
a0d0e21e 781
19799a22 782Note that C<crypt> is intended to be a one-way function, much like breaking
11155c91
CS
783eggs to make an omelette. There is no (known) corresponding decrypt
784function. As a result, this function isn't all that useful for
785cryptography. (For that, see your nearby CPAN mirror.)
2f9daede 786
e71965be
RS
787When verifying an existing encrypted string you should use the encrypted
788text as the salt (like C<crypt($plain, $crypted) eq $crypted>). This
19799a22 789allows your code to work with the standard C<crypt> and with more
e71965be
RS
790exotic implementations. When choosing a new salt create a random two
791character string whose characters come from the set C<[./0-9A-Za-z]>
792(like C<join '', ('.', '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>).
793
a0d0e21e
LW
794Here's an example that makes sure that whoever runs this program knows
795their own password:
796
797 $pwd = (getpwuid($<))[1];
a0d0e21e
LW
798
799 system "stty -echo";
800 print "Password: ";
e71965be 801 chomp($word = <STDIN>);
a0d0e21e
LW
802 print "\n";
803 system "stty echo";
804
e71965be 805 if (crypt($word, $pwd) ne $pwd) {
a0d0e21e
LW
806 die "Sorry...\n";
807 } else {
808 print "ok\n";
54310121 809 }
a0d0e21e 810
9f8f0c9d 811Of course, typing in your own password to whoever asks you
748a9306 812for it is unwise.
a0d0e21e 813
19799a22
GS
814The L<crypt> function is unsuitable for encrypting large quantities
815of data, not least of all because you can't get the information
816back. Look at the F<by-module/Crypt> and F<by-module/PGP> directories
817on your favorite CPAN mirror for a slew of potentially useful
818modules.
819
aa689395 820=item dbmclose HASH
a0d0e21e 821
19799a22 822[This function has been largely superseded by the C<untie> function.]
a0d0e21e 823
aa689395 824Breaks the binding between a DBM file and a hash.
a0d0e21e 825
19799a22 826=item dbmopen HASH,DBNAME,MASK
a0d0e21e 827
19799a22 828[This function has been largely superseded by the C<tie> function.]
a0d0e21e 829
7b8d334a 830This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
19799a22
GS
831hash. HASH is the name of the hash. (Unlike normal C<open>, the first
832argument is I<not> a filehandle, even though it looks like one). DBNAME
aa689395
PP
833is the name of the database (without the F<.dir> or F<.pag> extension if
834any). If the database does not exist, it is created with protection
19799a22
GS
835specified by MASK (as modified by the C<umask>). If your system supports
836only the older DBM functions, you may perform only one C<dbmopen> in your
aa689395 837program. In older versions of Perl, if your system had neither DBM nor
19799a22 838ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
aa689395
PP
839sdbm(3).
840
841If you don't have write access to the DBM file, you can only read hash
842variables, not set them. If you want to test whether you can write,
19799a22 843either use file tests or try setting a dummy hash entry inside an C<eval>,
aa689395 844which will trap the error.
a0d0e21e 845
19799a22
GS
846Note that functions such as C<keys> and C<values> may return huge lists
847when used on large DBM files. You may prefer to use the C<each>
a0d0e21e
LW
848function to iterate over large DBM files. Example:
849
850 # print out history file offsets
851 dbmopen(%HIST,'/usr/lib/news/history',0666);
852 while (($key,$val) = each %HIST) {
853 print $key, ' = ', unpack('L',$val), "\n";
854 }
855 dbmclose(%HIST);
856
cb1a09d0 857See also L<AnyDBM_File> for a more general description of the pros and
184e9718 858cons of the various dbm approaches, as well as L<DB_File> for a particularly
cb1a09d0 859rich implementation.
4633a7c4 860
2b5ab1e7
TC
861You can control which DBM library you use by loading that library
862before you call dbmopen():
863
864 use DB_File;
865 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
866 or die "Can't open netscape history file: $!";
867
a0d0e21e
LW
868=item defined EXPR
869
54310121 870=item defined
bbce6d69 871
2f9daede
TPG
872Returns a Boolean value telling whether EXPR has a value other than
873the undefined value C<undef>. If EXPR is not present, C<$_> will be
874checked.
875
876Many operations return C<undef> to indicate failure, end of file,
877system error, uninitialized variable, and other exceptional
878conditions. This function allows you to distinguish C<undef> from
879other values. (A simple Boolean test will not distinguish among
7660c0ab 880C<undef>, zero, the empty string, and C<"0">, which are all equally
2f9daede 881false.) Note that since C<undef> is a valid scalar, its presence
19799a22 882doesn't I<necessarily> indicate an exceptional condition: C<pop>
2f9daede
TPG
883returns C<undef> when its argument is an empty array, I<or> when the
884element to return happens to be C<undef>.
885
f10b0346
GS
886You may also use C<defined(&func)> to check whether subroutine C<&func>
887has ever been defined. The return value is unaffected by any forward
888declarations of C<&foo>.
889
890Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
891used to report whether memory for that aggregate has ever been
892allocated. This behavior may disappear in future versions of Perl.
893You should instead use a simple test for size:
894
895 if (@an_array) { print "has array elements\n" }
896 if (%a_hash) { print "has hash members\n" }
2f9daede
TPG
897
898When used on a hash element, it tells you whether the value is defined,
dc848c6f 899not whether the key exists in the hash. Use L</exists> for the latter
2f9daede 900purpose.
a0d0e21e
LW
901
902Examples:
903
904 print if defined $switch{'D'};
905 print "$val\n" while defined($val = pop(@ary));
906 die "Can't readlink $sym: $!"
907 unless defined($value = readlink $sym);
a0d0e21e 908 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
2f9daede 909 $debugging = 0 unless defined $debugging;
a0d0e21e 910
19799a22 911Note: Many folks tend to overuse C<defined>, and then are surprised to
7660c0ab 912discover that the number C<0> and C<""> (the zero-length string) are, in fact,
2f9daede 913defined values. For example, if you say
a5f75d66
AD
914
915 "ab" =~ /a(.*)b/;
916
7660c0ab 917The pattern match succeeds, and C<$1> is defined, despite the fact that it
a5f75d66 918matched "nothing". But it didn't really match nothing--rather, it
2b5ab1e7 919matched something that happened to be zero characters long. This is all
a5f75d66 920very above-board and honest. When a function returns an undefined value,
2f9daede 921it's an admission that it couldn't give you an honest answer. So you
19799a22 922should use C<defined> only when you're questioning the integrity of what
7660c0ab 923you're trying to do. At other times, a simple comparison to C<0> or C<""> is
2f9daede
TPG
924what you want.
925
dc848c6f 926See also L</undef>, L</exists>, L</ref>.
2f9daede 927
a0d0e21e
LW
928=item delete EXPR
929
01020589
GS
930Given an expression that specifies a hash element, array element, hash slice,
931or array slice, deletes the specified element(s) from the hash or array.
932If the array elements happen to be at the end of the array, the size
933of the array will shrink by that number of elements.
a0d0e21e 934
01020589
GS
935Returns each element so deleted or the undefined value if there was no such
936element. Deleting from C<$ENV{}> modifies the environment. Deleting from
937a hash tied to a DBM file deletes the entry from the DBM file. Deleting
938from a C<tie>d hash or array may not necessarily return anything.
939
8ea97a1e
GS
940Deleting an array element effectively returns that position of the array
941to its initial, uninitialized state. Subsequently testing for the same
942element with exists() will return false. See L</exists>.
943
01020589 944The following (inefficiently) deletes all the values of %HASH and @ARRAY:
a0d0e21e 945
5f05dabc
PP
946 foreach $key (keys %HASH) {
947 delete $HASH{$key};
a0d0e21e
LW
948 }
949
01020589
GS
950 foreach $index (0 .. $#ARRAY) {
951 delete $ARRAY[$index];
952 }
953
954And so do these:
5f05dabc 955
01020589
GS
956 delete @HASH{keys %HASH};
957
9740c838 958 delete @ARRAY[0 .. $#ARRAY];
5f05dabc 959
2b5ab1e7 960But both of these are slower than just assigning the empty list
01020589
GS
961or undefining %HASH or @ARRAY:
962
963 %HASH = (); # completely empty %HASH
964 undef %HASH; # forget %HASH ever existed
2b5ab1e7 965
01020589
GS
966 @ARRAY = (); # completely empty @ARRAY
967 undef @ARRAY; # forget @ARRAY ever existed
2b5ab1e7
TC
968
969Note that the EXPR can be arbitrarily complicated as long as the final
01020589
GS
970operation is a hash element, array element, hash slice, or array slice
971lookup:
a0d0e21e
LW
972
973 delete $ref->[$x][$y]{$key};
5f05dabc 974 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
a0d0e21e 975
01020589
GS
976 delete $ref->[$x][$y][$index];
977 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
978
a0d0e21e
LW
979=item die LIST
980
19799a22
GS
981Outside an C<eval>, prints the value of LIST to C<STDERR> and
982exits with the current value of C<$!> (errno). If C<$!> is C<0>,
983exits with the value of C<($? E<gt>E<gt> 8)> (backtick `command`
984status). If C<($? E<gt>E<gt> 8)> is C<0>, exits with C<255>. Inside
985an C<eval(),> the error message is stuffed into C<$@> and the
986C<eval> is terminated with the undefined value. This makes
987C<die> the way to raise an exception.
a0d0e21e
LW
988
989Equivalent examples:
990
991 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
54310121 992 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
a0d0e21e
LW
993
994If the value of EXPR does not end in a newline, the current script line
995number and input line number (if any) are also printed, and a newline
883faa13
GS
996is supplied. Note that the "input line number" (also known as "chunk")
997is subject to whatever notion of "line" happens to be currently in
998effect, and is also available as the special variable C<$.>.
999See L<perlvar/"$/"> and L<perlvar/"$.">.
1000
1001Hint: sometimes appending C<", stopped"> to your message
7660c0ab 1002will cause it to make better sense when the string C<"at foo line 123"> is
a0d0e21e
LW
1003appended. Suppose you are running script "canasta".
1004
1005 die "/etc/games is no good";
1006 die "/etc/games is no good, stopped";
1007
1008produce, respectively
1009
1010 /etc/games is no good at canasta line 123.
1011 /etc/games is no good, stopped at canasta line 123.
1012
2b5ab1e7 1013See also exit(), warn(), and the Carp module.
a0d0e21e 1014
7660c0ab
A
1015If LIST is empty and C<$@> already contains a value (typically from a
1016previous eval) that value is reused after appending C<"\t...propagated">.
fb73857a
PP
1017This is useful for propagating exceptions:
1018
1019 eval { ... };
1020 die unless $@ =~ /Expected exception/;
1021
7660c0ab 1022If C<$@> is empty then the string C<"Died"> is used.
fb73857a 1023
52531d10
GS
1024die() can also be called with a reference argument. If this happens to be
1025trapped within an eval(), $@ contains the reference. This behavior permits
1026a more elaborate exception handling implementation using objects that
1027maintain arbitary state about the nature of the exception. Such a scheme
1028is sometimes preferable to matching particular string values of $@ using
1029regular expressions. Here's an example:
1030
1031 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1032 if ($@) {
1033 if (ref($@) && UNIVERSAL::isa($@,"Some::Module::Exception")) {
1034 # handle Some::Module::Exception
1035 }
1036 else {
1037 # handle all other possible exceptions
1038 }
1039 }
1040
19799a22 1041Because perl will stringify uncaught exception messages before displaying
52531d10
GS
1042them, you may want to overload stringification operations on such custom
1043exception objects. See L<overload> for details about that.
1044
19799a22
GS
1045You can arrange for a callback to be run just before the C<die>
1046does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1047handler will be called with the error text and can change the error
1048message, if it sees fit, by calling C<die> again. See
1049L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
1050L<"eval BLOCK"> for some examples. Although this feature was meant
1051to be run only right before your program was to exit, this is not
1052currently the case--the C<$SIG{__DIE__}> hook is currently called
1053even inside eval()ed blocks/strings! If one wants the hook to do
1054nothing in such situations, put
fb73857a
PP
1055
1056 die @_ if $^S;
1057
19799a22
GS
1058as the first line of the handler (see L<perlvar/$^S>). Because
1059this promotes strange action at a distance, this counterintuitive
1060behavior may be fixed in a future release.
774d564b 1061
a0d0e21e
LW
1062=item do BLOCK
1063
1064Not really a function. Returns the value of the last command in the
1065sequence of commands indicated by BLOCK. When modified by a loop
98293880
JH
1066modifier, executes the BLOCK once before testing the loop condition.
1067(On other statements the loop modifiers test the conditional first.)
a0d0e21e 1068
4968c1e4 1069C<do BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7
TC
1070C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1071See L<perlsyn> for alternative strategies.
4968c1e4 1072
a0d0e21e
LW
1073=item do SUBROUTINE(LIST)
1074
1075A deprecated form of subroutine call. See L<perlsub>.
1076
1077=item do EXPR
1078
1079Uses the value of EXPR as a filename and executes the contents of the
1080file as a Perl script. Its primary use is to include subroutines
1081from a Perl subroutine library.
1082
1083 do 'stat.pl';
1084
1085is just like
1086
fb73857a 1087 scalar eval `cat stat.pl`;
a0d0e21e 1088
2b5ab1e7
TC
1089except that it's more efficient and concise, keeps track of the current
1090filename for error messages, searches the @INC libraries, and updates
1091C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1092variables. It also differs in that code evaluated with C<do FILENAME>
1093cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1094same, however, in that it does reparse the file every time you call it,
1095so you probably don't want to do this inside a loop.
a0d0e21e 1096
8e30cc93 1097If C<do> cannot read the file, it returns undef and sets C<$!> to the
2b5ab1e7 1098error. If C<do> can read the file but cannot compile it, it
8e30cc93
G
1099returns undef and sets an error message in C<$@>. If the file is
1100successfully compiled, C<do> returns the value of the last expression
1101evaluated.
1102
a0d0e21e 1103Note that inclusion of library modules is better done with the
19799a22 1104C<use> and C<require> operators, which also do automatic error checking
4633a7c4 1105and raise an exception if there's a problem.
a0d0e21e 1106
5a964f20
TC
1107You might like to use C<do> to read in a program configuration
1108file. Manual error checking can be done this way:
1109
1110 # read in config files: system first, then user
f86cebdf 1111 for $file ("/share/prog/defaults.rc",
2b5ab1e7
TC
1112 "$ENV{HOME}/.someprogrc")
1113 {
5a964f20 1114 unless ($return = do $file) {
f86cebdf
GS
1115 warn "couldn't parse $file: $@" if $@;
1116 warn "couldn't do $file: $!" unless defined $return;
1117 warn "couldn't run $file" unless $return;
5a964f20
TC
1118 }
1119 }
1120
a0d0e21e
LW
1121=item dump LABEL
1122
1614b0e3
JD
1123=item dump
1124
19799a22
GS
1125This function causes an immediate core dump. See also the B<-u>
1126command-line switch in L<perlrun>, which does the same thing.
1127Primarily this is so that you can use the B<undump> program (not
1128supplied) to turn your core dump into an executable binary after
1129having initialized all your variables at the beginning of the
1130program. When the new binary is executed it will begin by executing
1131a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1132Think of it as a goto with an intervening core dump and reincarnation.
1133If C<LABEL> is omitted, restarts the program from the top.
1134
1135B<WARNING>: Any files opened at the time of the dump will I<not>
1136be open any more when the program is reincarnated, with possible
1137resulting confusion on the part of Perl.
1138
1139This function is now largely obsolete, partly because it's very
1140hard to convert a core file into an executable, and because the
1141real compiler backends for generating portable bytecode and compilable
1142C code have superseded it.
1143
1144If you're looking to use L<dump> to speed up your program, consider
1145generating bytecode or native C code as described in L<perlcc>. If
1146you're just trying to accelerate a CGI script, consider using the
1147C<mod_perl> extension to B<Apache>, or the CPAN module, Fast::CGI.
1148You might also consider autoloading or selfloading, which at least
1149make your program I<appear> to run faster.
5a964f20 1150
aa689395
PP
1151=item each HASH
1152
5a964f20 1153When called in list context, returns a 2-element list consisting of the
aa689395 1154key and value for the next element of a hash, so that you can iterate over
5a964f20 1155it. When called in scalar context, returns the key for only the "next"
7660c0ab 1156element in the hash. (Note: Keys may be C<"0"> or C<"">, which are logically
2f9daede
TPG
1157false; you may wish to avoid constructs like C<while ($k = each %foo) {}>
1158for this reason.)
1159
ab192400
GS
1160Entries are returned in an apparently random order. The actual random
1161order is subject to change in future versions of perl, but it is guaranteed
19799a22 1162to be in the same order as either the C<keys> or C<values> function
ab192400
GS
1163would produce on the same (unmodified) hash.
1164
1165When the hash is entirely read, a null array is returned in list context
19799a22
GS
1166(which when assigned produces a false (C<0>) value), and C<undef> in
1167scalar context. The next call to C<each> after that will start iterating
1168again. There is a single iterator for each hash, shared by all C<each>,
1169C<keys>, and C<values> function calls in the program; it can be reset by
2f9daede
TPG
1170reading all the elements from the hash, or by evaluating C<keys HASH> or
1171C<values HASH>. If you add or delete elements of a hash while you're
1172iterating over it, you may get entries skipped or duplicated, so don't.
aa689395 1173
f86cebdf 1174The following prints out your environment like the printenv(1) program,
aa689395 1175only in a different order:
a0d0e21e
LW
1176
1177 while (($key,$value) = each %ENV) {
1178 print "$key=$value\n";
1179 }
1180
19799a22 1181See also C<keys>, C<values> and C<sort>.
a0d0e21e
LW
1182
1183=item eof FILEHANDLE
1184
4633a7c4
LW
1185=item eof ()
1186
a0d0e21e
LW
1187=item eof
1188
1189Returns 1 if the next read on FILEHANDLE will return end of file, or if
1190FILEHANDLE is not open. FILEHANDLE may be an expression whose value
5a964f20 1191gives the real filehandle. (Note that this function actually
19799a22 1192reads a character and then C<ungetc>s it, so isn't very useful in an
748a9306 1193interactive context.) Do not read from a terminal file (or call
19799a22 1194C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
748a9306
LW
1195as terminals may lose the end-of-file condition if you do.
1196
820475bd
GS
1197An C<eof> without an argument uses the last file read. Using C<eof()>
1198with empty parentheses is very different. It refers to the pseudo file
1199formed from the files listed on the command line and accessed via the
1200C<E<lt>E<gt>> operator. Since C<E<lt>E<gt>> isn't explicitly opened,
1201as a normal filehandle is, an C<eof()> before C<E<lt>E<gt>> has been
1202used will cause C<@ARGV> to be examined to determine if input is
1203available.
1204
1205In a C<while (E<lt>E<gt>)> loop, C<eof> or C<eof(ARGV)> can be used to
1206detect the end of each file, C<eof()> will only detect the end of the
1207last file. Examples:
a0d0e21e 1208
748a9306
LW
1209 # reset line numbering on each input file
1210 while (<>) {
5a964f20 1211 next if /^\s*#/; # skip comments
748a9306 1212 print "$.\t$_";
5a964f20
TC
1213 } continue {
1214 close ARGV if eof; # Not eof()!
748a9306
LW
1215 }
1216
a0d0e21e
LW
1217 # insert dashes just before last line of last file
1218 while (<>) {
5a964f20 1219 if (eof()) { # check for end of current file
a0d0e21e 1220 print "--------------\n";
2b5ab1e7 1221 close(ARGV); # close or last; is needed if we
748a9306 1222 # are reading from the terminal
a0d0e21e
LW
1223 }
1224 print;
1225 }
1226
a0d0e21e 1227Practical hint: you almost never need to use C<eof> in Perl, because the
3ce0d271
GS
1228input operators typically return C<undef> when they run out of data, or if
1229there was an error.
a0d0e21e
LW
1230
1231=item eval EXPR
1232
1233=item eval BLOCK
1234
c7cc6f1c
GS
1235In the first form, the return value of EXPR is parsed and executed as if it
1236were a little Perl program. The value of the expression (which is itself
5a964f20 1237determined within scalar context) is first parsed, and if there weren't any
c7cc6f1c 1238errors, executed in the context of the current Perl program, so that any
5f05dabc 1239variable settings or subroutine and format definitions remain afterwards.
c7cc6f1c
GS
1240Note that the value is parsed every time the eval executes. If EXPR is
1241omitted, evaluates C<$_>. This form is typically used to delay parsing
1242and subsequent execution of the text of EXPR until run time.
1243
1244In the second form, the code within the BLOCK is parsed only once--at the
1245same time the code surrounding the eval itself was parsed--and executed
1246within the context of the current Perl program. This form is typically
1247used to trap exceptions more efficiently than the first (see below), while
1248also providing the benefit of checking the code within BLOCK at compile
1249time.
1250
1251The final semicolon, if any, may be omitted from the value of EXPR or within
1252the BLOCK.
1253
1254In both forms, the value returned is the value of the last expression
5a964f20 1255evaluated inside the mini-program; a return statement may be also used, just
c7cc6f1c 1256as with subroutines. The expression providing the return value is evaluated
5a964f20 1257in void, scalar, or list context, depending on the context of the eval itself.
c7cc6f1c 1258See L</wantarray> for more on how the evaluation context can be determined.
a0d0e21e 1259
19799a22
GS
1260If there is a syntax error or runtime error, or a C<die> statement is
1261executed, an undefined value is returned by C<eval>, and C<$@> is set to the
a0d0e21e 1262error message. If there was no error, C<$@> is guaranteed to be a null
19799a22 1263string. Beware that using C<eval> neither silences perl from printing
c7cc6f1c
GS
1264warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1265To do either of those, you have to use the C<$SIG{__WARN__}> facility. See
1266L</warn> and L<perlvar>.
a0d0e21e 1267
19799a22
GS
1268Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1269determining whether a particular feature (such as C<socket> or C<symlink>)
a0d0e21e
LW
1270is implemented. It is also Perl's exception trapping mechanism, where
1271the die operator is used to raise exceptions.
1272
1273If the code to be executed doesn't vary, you may use the eval-BLOCK
1274form to trap run-time errors without incurring the penalty of
1275recompiling each time. The error, if any, is still returned in C<$@>.
1276Examples:
1277
54310121 1278 # make divide-by-zero nonfatal
a0d0e21e
LW
1279 eval { $answer = $a / $b; }; warn $@ if $@;
1280
1281 # same thing, but less efficient
1282 eval '$answer = $a / $b'; warn $@ if $@;
1283
1284 # a compile-time error
5a964f20 1285 eval { $answer = }; # WRONG
a0d0e21e
LW
1286
1287 # a run-time error
1288 eval '$answer ='; # sets $@
1289
2b5ab1e7
TC
1290Due to the current arguably broken state of C<__DIE__> hooks, when using
1291the C<eval{}> form as an exception trap in libraries, you may wish not
1292to trigger any C<__DIE__> hooks that user code may have installed.
1293You can use the C<local $SIG{__DIE__}> construct for this purpose,
1294as shown in this example:
774d564b
PP
1295
1296 # a very private exception trap for divide-by-zero
f86cebdf
GS
1297 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1298 warn $@ if $@;
774d564b
PP
1299
1300This is especially significant, given that C<__DIE__> hooks can call
19799a22 1301C<die> again, which has the effect of changing their error messages:
774d564b
PP
1302
1303 # __DIE__ hooks may modify error messages
1304 {
f86cebdf
GS
1305 local $SIG{'__DIE__'} =
1306 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
c7cc6f1c
GS
1307 eval { die "foo lives here" };
1308 print $@ if $@; # prints "bar lives here"
774d564b
PP
1309 }
1310
19799a22 1311Because this promotes action at a distance, this counterintuitive behavior
2b5ab1e7
TC
1312may be fixed in a future release.
1313
19799a22 1314With an C<eval>, you should be especially careful to remember what's
a0d0e21e
LW
1315being looked at when:
1316
1317 eval $x; # CASE 1
1318 eval "$x"; # CASE 2
1319
1320 eval '$x'; # CASE 3
1321 eval { $x }; # CASE 4
1322
5a964f20 1323 eval "\$$x++"; # CASE 5
a0d0e21e
LW
1324 $$x++; # CASE 6
1325
2f9daede 1326Cases 1 and 2 above behave identically: they run the code contained in
19799a22 1327the variable $x. (Although case 2 has misleading double quotes making
2f9daede 1328the reader wonder what else might be happening (nothing is).) Cases 3
7660c0ab 1329and 4 likewise behave in the same way: they run the code C<'$x'>, which
19799a22 1330does nothing but return the value of $x. (Case 4 is preferred for
2f9daede
TPG
1331purely visual reasons, but it also has the advantage of compiling at
1332compile-time instead of at run-time.) Case 5 is a place where
19799a22 1333normally you I<would> like to use double quotes, except that in this
2f9daede
TPG
1334particular situation, you can just use symbolic references instead, as
1335in case 6.
a0d0e21e 1336
4968c1e4 1337C<eval BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7 1338C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
4968c1e4 1339
a0d0e21e
LW
1340=item exec LIST
1341
8bf3b016
GS
1342=item exec PROGRAM LIST
1343
19799a22
GS
1344The C<exec> function executes a system command I<and never returns>--
1345use C<system> instead of C<exec> if you want it to return. It fails and
1346returns false only if the command does not exist I<and> it is executed
fb73857a 1347directly instead of via your system's command shell (see below).
a0d0e21e 1348
19799a22
GS
1349Since it's a common mistake to use C<exec> instead of C<system>, Perl
1350warns you if there is a following statement which isn't C<die>, C<warn>,
1351or C<exit> (if C<-w> is set - but you always do that). If you
1352I<really> want to follow an C<exec> with some other statement, you
55d729e4
GS
1353can use one of these styles to avoid the warning:
1354
5a964f20
TC
1355 exec ('foo') or print STDERR "couldn't exec foo: $!";
1356 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
55d729e4 1357
5a964f20 1358If there is more than one argument in LIST, or if LIST is an array
f86cebdf 1359with more than one value, calls execvp(3) with the arguments in LIST.
5a964f20
TC
1360If there is only one scalar argument or an array with one element in it,
1361the argument is checked for shell metacharacters, and if there are any,
1362the entire argument is passed to the system's command shell for parsing
1363(this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1364If there are no shell metacharacters in the argument, it is split into
19799a22
GS
1365words and passed directly to C<execvp>, which is more efficient.
1366Examples:
a0d0e21e 1367
19799a22
GS
1368 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1369 exec "sort $outfile | uniq";
a0d0e21e
LW
1370
1371If you don't really want to execute the first argument, but want to lie
1372to the program you are executing about its own name, you can specify
1373the program you actually want to run as an "indirect object" (without a
1374comma) in front of the LIST. (This always forces interpretation of the
54310121 1375LIST as a multivalued list, even if there is only a single scalar in
a0d0e21e
LW
1376the list.) Example:
1377
1378 $shell = '/bin/csh';
1379 exec $shell '-sh'; # pretend it's a login shell
1380
1381or, more directly,
1382
1383 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1384
bb32b41a
GS
1385When the arguments get executed via the system shell, results will
1386be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1387for details.
1388
19799a22
GS
1389Using an indirect object with C<exec> or C<system> is also more
1390secure. This usage (which also works fine with system()) forces
1391interpretation of the arguments as a multivalued list, even if the
1392list had just one argument. That way you're safe from the shell
1393expanding wildcards or splitting up words with whitespace in them.
5a964f20
TC
1394
1395 @args = ( "echo surprise" );
1396
2b5ab1e7 1397 exec @args; # subject to shell escapes
f86cebdf 1398 # if @args == 1
2b5ab1e7 1399 exec { $args[0] } @args; # safe even with one-arg list
5a964f20
TC
1400
1401The first version, the one without the indirect object, ran the I<echo>
1402program, passing it C<"surprise"> an argument. The second version
1403didn't--it tried to run a program literally called I<"echo surprise">,
1404didn't find it, and set C<$?> to a non-zero value indicating failure.
1405
19799a22 1406Note that C<exec> will not call your C<END> blocks, nor will it call
7660c0ab
A
1407any C<DESTROY> methods in your objects.
1408
a0d0e21e
LW
1409=item exists EXPR
1410
01020589 1411Given an expression that specifies a hash element or array element,
8ea97a1e
GS
1412returns true if the specified element in the hash or array has ever
1413been initialized, even if the corresponding value is undefined. The
1414element is not autovivified if it doesn't exist.
a0d0e21e 1415
01020589
GS
1416 print "Exists\n" if exists $hash{$key};
1417 print "Defined\n" if defined $hash{$key};
1418 print "True\n" if $hash{$key};
1419
1420 print "Exists\n" if exists $array[$index];
1421 print "Defined\n" if defined $array[$index];
1422 print "True\n" if $array[$index];
a0d0e21e 1423
8ea97a1e 1424A hash or array element can be true only if it's defined, and defined if
a0d0e21e
LW
1425it exists, but the reverse doesn't necessarily hold true.
1426
afebc493
GS
1427Given an expression that specifies the name of a subroutine,
1428returns true if the specified subroutine has ever been declared, even
1429if it is undefined. Mentioning a subroutine name for exists or defined
1430does not count as declaring it.
1431
1432 print "Exists\n" if exists &subroutine;
1433 print "Defined\n" if defined &subroutine;
1434
a0d0e21e 1435Note that the EXPR can be arbitrarily complicated as long as the final
afebc493 1436operation is a hash or array key lookup or subroutine name:
a0d0e21e 1437
2b5ab1e7
TC
1438 if (exists $ref->{A}->{B}->{$key}) { }
1439 if (exists $hash{A}{B}{$key}) { }
1440
01020589
GS
1441 if (exists $ref->{A}->{B}->[$ix]) { }
1442 if (exists $hash{A}{B}[$ix]) { }
1443
afebc493
GS
1444 if (exists &{$ref->{A}{B}{$key}}) { }
1445
01020589
GS
1446Although the deepest nested array or hash will not spring into existence
1447just because its existence was tested, any intervening ones will.
1448Thus C<$ref-E<gt>{"A"}> and C<$ref-E<gt>{"A"}-E<gt>{"B"}> will spring
1449into existence due to the existence test for the $key element above.
1450This happens anywhere the arrow operator is used, including even:
5a964f20 1451
2b5ab1e7
TC
1452 undef $ref;
1453 if (exists $ref->{"Some key"}) { }
1454 print $ref; # prints HASH(0x80d3d5c)
1455
1456This surprising autovivification in what does not at first--or even
1457second--glance appear to be an lvalue context may be fixed in a future
5a964f20 1458release.
a0d0e21e 1459
e0478e5a
MS
1460See L<perlref/"Pseudo-hashes"> for specifics on how exists() acts when
1461used on a pseudo-hash.
1462
afebc493
GS
1463Use of a subroutine call, rather than a subroutine name, as an argument
1464to exists() is an error.
1465
1466 exists &sub; # OK
1467 exists &sub(); # Error
1468
a0d0e21e
LW
1469=item exit EXPR
1470
2b5ab1e7 1471Evaluates EXPR and exits immediately with that value. Example:
a0d0e21e
LW
1472
1473 $ans = <STDIN>;
1474 exit 0 if $ans =~ /^[Xx]/;
1475
19799a22 1476See also C<die>. If EXPR is omitted, exits with C<0> status. The only
2b5ab1e7
TC
1477universally recognized values for EXPR are C<0> for success and C<1>
1478for error; other values are subject to interpretation depending on the
1479environment in which the Perl program is running. For example, exiting
148069 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1481the mailer to return the item undelivered, but that's not true everywhere.
a0d0e21e 1482
19799a22
GS
1483Don't use C<exit> to abort a subroutine if there's any chance that
1484someone might want to trap whatever error happened. Use C<die> instead,
1485which can be trapped by an C<eval>.
28757baa 1486
19799a22 1487The exit() function does not always exit immediately. It calls any
2b5ab1e7 1488defined C<END> routines first, but these C<END> routines may not
19799a22 1489themselves abort the exit. Likewise any object destructors that need to
2b5ab1e7
TC
1490be called are called before the real exit. If this is a problem, you
1491can call C<POSIX:_exit($status)> to avoid END and destructor processing.
87275199 1492See L<perlmod> for details.
5a964f20 1493
a0d0e21e
LW
1494=item exp EXPR
1495
54310121 1496=item exp
bbce6d69 1497
2b5ab1e7 1498Returns I<e> (the natural logarithm base) to the power of EXPR.
a0d0e21e
LW
1499If EXPR is omitted, gives C<exp($_)>.
1500
1501=item fcntl FILEHANDLE,FUNCTION,SCALAR
1502
f86cebdf 1503Implements the fcntl(2) function. You'll probably have to say
a0d0e21e
LW
1504
1505 use Fcntl;
1506
0ade1984 1507first to get the correct constant definitions. Argument processing and
19799a22 1508value return works just like C<ioctl> below.
a0d0e21e
LW
1509For example:
1510
1511 use Fcntl;
5a964f20
TC
1512 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1513 or die "can't fcntl F_GETFL: $!";
1514
19799a22 1515You don't have to check for C<defined> on the return from C<fnctl>.
951ba7fe
GS
1516Like C<ioctl>, it maps a C<0> return from the system call into
1517C<"0 but true"> in Perl. This string is true in boolean context and C<0>
2b5ab1e7
TC
1518in numeric context. It is also exempt from the normal B<-w> warnings
1519on improper numeric conversions.
5a964f20 1520
19799a22 1521Note that C<fcntl> will produce a fatal error if used on a machine that
2b5ab1e7
TC
1522doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1523manpage to learn what functions are available on your system.
a0d0e21e
LW
1524
1525=item fileno FILEHANDLE
1526
2b5ab1e7
TC
1527Returns the file descriptor for a filehandle, or undefined if the
1528filehandle is not open. This is mainly useful for constructing
19799a22 1529bitmaps for C<select> and low-level POSIX tty-handling operations.
2b5ab1e7
TC
1530If FILEHANDLE is an expression, the value is taken as an indirect
1531filehandle, generally its name.
5a964f20
TC
1532
1533You can use this to find out whether two handles refer to the
1534same underlying descriptor:
1535
1536 if (fileno(THIS) == fileno(THAT)) {
1537 print "THIS and THAT are dups\n";
1538 }
a0d0e21e
LW
1539
1540=item flock FILEHANDLE,OPERATION
1541
19799a22
GS
1542Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1543for success, false on failure. Produces a fatal error if used on a
2b5ab1e7 1544machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
19799a22 1545C<flock> is Perl's portable file locking interface, although it locks
2b5ab1e7
TC
1546only entire files, not records.
1547
1548Two potentially non-obvious but traditional C<flock> semantics are
1549that it waits indefinitely until the lock is granted, and that its locks
1550B<merely advisory>. Such discretionary locks are more flexible, but offer
19799a22
GS
1551fewer guarantees. This means that files locked with C<flock> may be
1552modified by programs that do not also use C<flock>. See L<perlport>,
2b5ab1e7
TC
1553your port's specific documentation, or your system-specific local manpages
1554for details. It's best to assume traditional behavior if you're writing
1555portable programs. (But if you're not, you should as always feel perfectly
1556free to write for your own system's idiosyncrasies (sometimes called
1557"features"). Slavish adherence to portability concerns shouldn't get
1558in the way of your getting your job done.)
a3cb178b 1559
8ebc5c01
PP
1560OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1561LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
ea3105be 1562you can use the symbolic names if you import them from the Fcntl module,
68dc0745
PP
1563either individually, or as a group using the ':flock' tag. LOCK_SH
1564requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
ea3105be
GS
1565releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1566LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
68dc0745
PP
1567waiting for the lock (check the return status to see if you got it).
1568
2b5ab1e7
TC
1569To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1570before locking or unlocking it.
8ebc5c01 1571
f86cebdf 1572Note that the emulation built with lockf(3) doesn't provide shared
8ebc5c01 1573locks, and it requires that FILEHANDLE be open with write intent. These
2b5ab1e7 1574are the semantics that lockf(3) implements. Most if not all systems
f86cebdf 1575implement lockf(3) in terms of fcntl(2) locking, though, so the
8ebc5c01
PP
1576differing semantics shouldn't bite too many people.
1577
19799a22
GS
1578Note also that some versions of C<flock> cannot lock things over the
1579network; you would need to use the more system-specific C<fcntl> for
f86cebdf
GS
1580that. If you like you can force Perl to ignore your system's flock(2)
1581function, and so provide its own fcntl(2)-based emulation, by passing
8ebc5c01
PP
1582the switch C<-Ud_flock> to the F<Configure> program when you configure
1583perl.
4633a7c4
LW
1584
1585Here's a mailbox appender for BSD systems.
a0d0e21e 1586
7e1af8bc 1587 use Fcntl ':flock'; # import LOCK_* constants
a0d0e21e
LW
1588
1589 sub lock {
7e1af8bc 1590 flock(MBOX,LOCK_EX);
a0d0e21e
LW
1591 # and, in case someone appended
1592 # while we were waiting...
1593 seek(MBOX, 0, 2);
1594 }
1595
1596 sub unlock {
7e1af8bc 1597 flock(MBOX,LOCK_UN);
a0d0e21e
LW
1598 }
1599
1600 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1601 or die "Can't open mailbox: $!";
1602
1603 lock();
1604 print MBOX $msg,"\n\n";
1605 unlock();
1606
2b5ab1e7
TC
1607On systems that support a real flock(), locks are inherited across fork()
1608calls, whereas those that must resort to the more capricious fcntl()
1609function lose the locks, making it harder to write servers.
1610
cb1a09d0 1611See also L<DB_File> for other flock() examples.
a0d0e21e
LW
1612
1613=item fork
1614
2b5ab1e7
TC
1615Does a fork(2) system call to create a new process running the
1616same program at the same point. It returns the child pid to the
1617parent process, C<0> to the child process, or C<undef> if the fork is
1618unsuccessful. File descriptors (and sometimes locks on those descriptors)
1619are shared, while everything else is copied. On most systems supporting
1620fork(), great care has gone into making it extremely efficient (for
1621example, using copy-on-write technology on data pages), making it the
1622dominant paradigm for multitasking over the last few decades.
5a964f20 1623
45bc9206 1624All files opened for output are flushed before forking the child process.
a0d0e21e 1625
19799a22 1626If you C<fork> without ever waiting on your children, you will
2b5ab1e7
TC
1627accumulate zombies. On some systems, you can avoid this by setting
1628C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1629forking and reaping moribund children.
cb1a09d0 1630
28757baa
PP
1631Note that if your forked child inherits system file descriptors like
1632STDIN and STDOUT that are actually connected by a pipe or socket, even
2b5ab1e7 1633if you exit, then the remote server (such as, say, a CGI script or a
19799a22 1634backgrounded job launched from a remote shell) won't think you're done.
2b5ab1e7 1635You should reopen those to F</dev/null> if it's any issue.
28757baa 1636
cb1a09d0
AD
1637=item format
1638
19799a22 1639Declare a picture format for use by the C<write> function. For
cb1a09d0
AD
1640example:
1641
54310121 1642 format Something =
cb1a09d0
AD
1643 Test: @<<<<<<<< @||||| @>>>>>
1644 $str, $%, '$' . int($num)
1645 .
1646
1647 $str = "widget";
184e9718 1648 $num = $cost/$quantity;
cb1a09d0
AD
1649 $~ = 'Something';
1650 write;
1651
1652See L<perlform> for many details and examples.
1653
8903cb82 1654=item formline PICTURE,LIST
a0d0e21e 1655
5a964f20 1656This is an internal function used by C<format>s, though you may call it,
a0d0e21e
LW
1657too. It formats (see L<perlform>) a list of values according to the
1658contents of PICTURE, placing the output into the format output
7660c0ab 1659accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
19799a22 1660Eventually, when a C<write> is done, the contents of
a0d0e21e 1661C<$^A> are written to some filehandle, but you could also read C<$^A>
7660c0ab 1662yourself and then set C<$^A> back to C<"">. Note that a format typically
19799a22 1663does one C<formline> per line of form, but the C<formline> function itself
748a9306 1664doesn't care how many newlines are embedded in the PICTURE. This means
4633a7c4 1665that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
748a9306
LW
1666You may therefore need to use multiple formlines to implement a single
1667record format, just like the format compiler.
1668
19799a22 1669Be careful if you put double quotes around the picture, because an C<@>
748a9306 1670character may be taken to mean the beginning of an array name.
19799a22 1671C<formline> always returns true. See L<perlform> for other examples.
a0d0e21e
LW
1672
1673=item getc FILEHANDLE
1674
1675=item getc
1676
1677Returns the next character from the input file attached to FILEHANDLE,
2b5ab1e7
TC
1678or the undefined value at end of file, or if there was an error.
1679If FILEHANDLE is omitted, reads from STDIN. This is not particularly
1680efficient. However, it cannot be used by itself to fetch single
1681characters without waiting for the user to hit enter. For that, try
1682something more like:
4633a7c4
LW
1683
1684 if ($BSD_STYLE) {
1685 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1686 }
1687 else {
54310121 1688 system "stty", '-icanon', 'eol', "\001";
4633a7c4
LW
1689 }
1690
1691 $key = getc(STDIN);
1692
1693 if ($BSD_STYLE) {
1694 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1695 }
1696 else {
5f05dabc 1697 system "stty", 'icanon', 'eol', '^@'; # ASCII null
4633a7c4
LW
1698 }
1699 print "\n";
1700
54310121
PP
1701Determination of whether $BSD_STYLE should be set
1702is left as an exercise to the reader.
cb1a09d0 1703
19799a22 1704The C<POSIX::getattr> function can do this more portably on
2b5ab1e7
TC
1705systems purporting POSIX compliance. See also the C<Term::ReadKey>
1706module from your nearest CPAN site; details on CPAN can be found on
1707L<perlmodlib/CPAN>.
a0d0e21e
LW
1708
1709=item getlogin
1710
5a964f20
TC
1711Implements the C library function of the same name, which on most
1712systems returns the current login from F</etc/utmp>, if any. If null,
19799a22 1713use C<getpwuid>.
a0d0e21e 1714
f86702cc 1715 $login = getlogin || getpwuid($<) || "Kilroy";
a0d0e21e 1716
19799a22
GS
1717Do not consider C<getlogin> for authentication: it is not as
1718secure as C<getpwuid>.
4633a7c4 1719
a0d0e21e
LW
1720=item getpeername SOCKET
1721
1722Returns the packed sockaddr address of other end of the SOCKET connection.
1723
4633a7c4
LW
1724 use Socket;
1725 $hersockaddr = getpeername(SOCK);
19799a22 1726 ($port, $iaddr) = sockaddr_in($hersockaddr);
4633a7c4
LW
1727 $herhostname = gethostbyaddr($iaddr, AF_INET);
1728 $herstraddr = inet_ntoa($iaddr);
a0d0e21e
LW
1729
1730=item getpgrp PID
1731
47e29363 1732Returns the current process group for the specified PID. Use
7660c0ab 1733a PID of C<0> to get the current process group for the
4633a7c4 1734current process. Will raise an exception if used on a machine that
f86cebdf 1735doesn't implement getpgrp(2). If PID is omitted, returns process
19799a22 1736group of current process. Note that the POSIX version of C<getpgrp>
7660c0ab 1737does not accept a PID argument, so only C<PID==0> is truly portable.
a0d0e21e
LW
1738
1739=item getppid
1740
1741Returns the process id of the parent process.
1742
1743=item getpriority WHICH,WHO
1744
4633a7c4
LW
1745Returns the current priority for a process, a process group, or a user.
1746(See L<getpriority(2)>.) Will raise a fatal exception if used on a
f86cebdf 1747machine that doesn't implement getpriority(2).
a0d0e21e
LW
1748
1749=item getpwnam NAME
1750
1751=item getgrnam NAME
1752
1753=item gethostbyname NAME
1754
1755=item getnetbyname NAME
1756
1757=item getprotobyname NAME
1758
1759=item getpwuid UID
1760
1761=item getgrgid GID
1762
1763=item getservbyname NAME,PROTO
1764
1765=item gethostbyaddr ADDR,ADDRTYPE
1766
1767=item getnetbyaddr ADDR,ADDRTYPE
1768
1769=item getprotobynumber NUMBER
1770
1771=item getservbyport PORT,PROTO
1772
1773=item getpwent
1774
1775=item getgrent
1776
1777=item gethostent
1778
1779=item getnetent
1780
1781=item getprotoent
1782
1783=item getservent
1784
1785=item setpwent
1786
1787=item setgrent
1788
1789=item sethostent STAYOPEN
1790
1791=item setnetent STAYOPEN
1792
1793=item setprotoent STAYOPEN
1794
1795=item setservent STAYOPEN
1796
1797=item endpwent
1798
1799=item endgrent
1800
1801=item endhostent
1802
1803=item endnetent
1804
1805=item endprotoent
1806
1807=item endservent
1808
1809These routines perform the same functions as their counterparts in the
5a964f20 1810system library. In list context, the return values from the
a0d0e21e
LW
1811various get routines are as follows:
1812
1813 ($name,$passwd,$uid,$gid,
6ee623d5 1814 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
a0d0e21e
LW
1815 ($name,$passwd,$gid,$members) = getgr*
1816 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1817 ($name,$aliases,$addrtype,$net) = getnet*
1818 ($name,$aliases,$proto) = getproto*
1819 ($name,$aliases,$port,$proto) = getserv*
1820
1821(If the entry doesn't exist you get a null list.)
1822
5a964f20 1823In scalar context, you get the name, unless the function was a
a0d0e21e
LW
1824lookup by name, in which case you get the other thing, whatever it is.
1825(If the entry doesn't exist you get the undefined value.) For example:
1826
5a964f20
TC
1827 $uid = getpwnam($name);
1828 $name = getpwuid($num);
1829 $name = getpwent();
1830 $gid = getgrnam($name);
1831 $name = getgrgid($num;
1832 $name = getgrent();
1833 #etc.
a0d0e21e 1834
19799a22 1835In I<getpw*()> the fields $quota, $comment, and $expire are
2b5ab1e7 1836special cases in the sense that in many systems they are unsupported.
19799a22
GS
1837If the $quota is unsupported, it is an empty scalar. If it is
1838supported, it usually encodes the disk quota. If the $comment
2b5ab1e7
TC
1839field is unsupported, it is an empty scalar. If it is supported it
1840usually encodes some administrative comment about the user. In some
19799a22
GS
1841systems the $quota field may be $change or $age, fields that have
1842to do with password aging. In some systems the $comment field may
1843be $class. The $expire field, if present, encodes the expiration
2b5ab1e7
TC
1844period of the account or the password. For the availability and the
1845exact meaning of these fields in your system, please consult your
1846getpwnam(3) documentation and your F<pwd.h> file. You can also find
19799a22
GS
1847out from within Perl what your $quota and $comment fields mean
1848and whether you have the $expire field by using the C<Config> module
2b5ab1e7
TC
1849and the values C<d_pwquota>, C<d_pwage>, C<d_pwchange>, C<d_pwcomment>,
1850and C<d_pwexpire>. Shadow password files are only supported if your
1851vendor has implemented them in the intuitive fashion that calling the
1852regular C library routines gets the shadow versions if you're running
1853under privilege. Those that incorrectly implement a separate library
1854call are not supported.
6ee623d5 1855
19799a22 1856The $members value returned by I<getgr*()> is a space separated list of
a0d0e21e
LW
1857the login names of the members of the group.
1858
1859For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1860C, it will be returned to you via C<$?> if the function call fails. The
7660c0ab 1861C<@addrs> value returned by a successful call is a list of the raw
a0d0e21e
LW
1862addresses returned by the corresponding system library call. In the
1863Internet domain, each address is four bytes long and you can unpack it
1864by saying something like:
1865
1866 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1867
2b5ab1e7
TC
1868The Socket library makes this slightly easier:
1869
1870 use Socket;
1871 $iaddr = inet_aton("127.1"); # or whatever address
1872 $name = gethostbyaddr($iaddr, AF_INET);
1873
1874 # or going the other way
19799a22 1875 $straddr = inet_ntoa($iaddr);
2b5ab1e7 1876
19799a22
GS
1877If you get tired of remembering which element of the return list
1878contains which return value, by-name interfaces are provided
1879in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
1880C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
1881and C<User::grent>. These override the normal built-ins, supplying
1882versions that return objects with the appropriate names
1883for each field. For example:
5a964f20
TC
1884
1885 use File::stat;
1886 use User::pwent;
1887 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
1888
1889Even though it looks like they're the same method calls (uid),
19799a22
GS
1890they aren't, because a C<File::stat> object is different from
1891a C<User::pwent> object.
5a964f20 1892
a0d0e21e
LW
1893=item getsockname SOCKET
1894
19799a22
GS
1895Returns the packed sockaddr address of this end of the SOCKET connection,
1896in case you don't know the address because you have several different
1897IPs that the connection might have come in on.
a0d0e21e 1898
4633a7c4
LW
1899 use Socket;
1900 $mysockaddr = getsockname(SOCK);
19799a22
GS
1901 ($port, $myaddr) = sockaddr_in($mysockaddr);
1902 printf "Connect to %s [%s]\n",
1903 scalar gethostbyaddr($myaddr, AF_INET),
1904 inet_ntoa($myaddr);
a0d0e21e
LW
1905
1906=item getsockopt SOCKET,LEVEL,OPTNAME
1907
5a964f20 1908Returns the socket option requested, or undef if there is an error.
a0d0e21e
LW
1909
1910=item glob EXPR
1911
0a753a76
PP
1912=item glob
1913
2b5ab1e7
TC
1914Returns the value of EXPR with filename expansions such as the
1915standard Unix shell F</bin/csh> would do. This is the internal function
1916implementing the C<E<lt>*.cE<gt>> operator, but you can use it directly.
1917If EXPR is omitted, C<$_> is used. The C<E<lt>*.cE<gt>> operator is
1918discussed in more detail in L<perlop/"I/O Operators">.
a0d0e21e
LW
1919
1920=item gmtime EXPR
1921
19799a22 1922Converts a time as returned by the time function to a 9-element list
54310121 1923with the time localized for the standard Greenwich time zone.
4633a7c4 1924Typically used as follows:
a0d0e21e 1925
54310121 1926 # 0 1 2 3 4 5 6 7 8
a0d0e21e
LW
1927 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1928 gmtime(time);
1929
19799a22
GS
1930All list elements are numeric, and come straight out of a struct tm.
1931In particular this means that $mon has the range C<0..11> and $wday
1932has the range C<0..6> with sunday as day C<0>. Also, $year is the
1933number of years since 1900, that is, $year is C<123> in year 2023,
2b5ab1e7
TC
1934I<not> simply the last two digits of the year. If you assume it is,
1935then you create non-Y2K-compliant programs--and you wouldn't want to do
1936that, would you?
2f9daede 1937
abd75f24
GS
1938The proper way to get a complete 4-digit year is simply:
1939
1940 $year += 1900;
1941
1942And to get the last two digits of the year (e.g., '01' in 2001) do:
1943
1944 $year = sprintf("%02d", $year % 100);
1945
2f9daede 1946If EXPR is omitted, does C<gmtime(time())>.
a0d0e21e 1947
f86cebdf 1948In scalar context, returns the ctime(3) value:
0a753a76
PP
1949
1950 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
1951
19799a22 1952Also see the C<timegm> function provided by the C<Time::Local> module,
f86cebdf 1953and the strftime(3) function available via the POSIX module.
7660c0ab 1954
2b5ab1e7
TC
1955This scalar value is B<not> locale dependent (see L<perllocale>), but
1956is instead a Perl builtin. Also see the C<Time::Local> module, and the
1957strftime(3) and mktime(3) functions available via the POSIX module. To
7660c0ab
A
1958get somewhat similar but locale dependent date strings, set up your
1959locale environment variables appropriately (please see L<perllocale>)
1960and try for example:
1961
1962 use POSIX qw(strftime);
2b5ab1e7 1963 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
7660c0ab 1964
2b5ab1e7
TC
1965Note that the C<%a> and C<%b> escapes, which represent the short forms
1966of the day of the week and the month of the year, may not necessarily
1967be three characters wide in all locales.
0a753a76 1968
a0d0e21e
LW
1969=item goto LABEL
1970
748a9306
LW
1971=item goto EXPR
1972
a0d0e21e
LW
1973=item goto &NAME
1974
7660c0ab 1975The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
a0d0e21e 1976execution there. It may not be used to go into any construct that
7660c0ab 1977requires initialization, such as a subroutine or a C<foreach> loop. It
0a753a76 1978also can't be used to go into a construct that is optimized away,
19799a22 1979or to get out of a block or subroutine given to C<sort>.
0a753a76 1980It can be used to go almost anywhere else within the dynamic scope,
a0d0e21e 1981including out of subroutines, but it's usually better to use some other
19799a22 1982construct such as C<last> or C<die>. The author of Perl has never felt the
7660c0ab 1983need to use this form of C<goto> (in Perl, that is--C is another matter).
a0d0e21e 1984
7660c0ab
A
1985The C<goto-EXPR> form expects a label name, whose scope will be resolved
1986dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
748a9306
LW
1987necessarily recommended if you're optimizing for maintainability:
1988
1989 goto ("FOO", "BAR", "GLARCH")[$i];
1990
6cb9131c
GS
1991The C<goto-&NAME> form is quite different from the other forms of C<goto>.
1992In fact, it isn't a goto in the normal sense at all, and doesn't have
1993the stigma associated with other gotos. Instead, it
1994substitutes a call to the named subroutine for the currently running
1995subroutine. This is used by C<AUTOLOAD> subroutines that wish to load
1996another subroutine and then pretend that the other subroutine had been
1997called in the first place (except that any modifications to C<@_>
1998in the current subroutine are propagated to the other subroutine.)
1999After the C<goto>, not even C<caller> will be able to tell that this
2000routine was called first.
2001
2002NAME needn't be the name of a subroutine; it can be a scalar variable
2003containing a code reference, or a block which evaluates to a code
2004reference.
a0d0e21e
LW
2005
2006=item grep BLOCK LIST
2007
2008=item grep EXPR,LIST
2009
2b5ab1e7
TC
2010This is similar in spirit to, but not the same as, grep(1) and its
2011relatives. In particular, it is not limited to using regular expressions.
2f9daede 2012
a0d0e21e 2013Evaluates the BLOCK or EXPR for each element of LIST (locally setting
7660c0ab 2014C<$_> to each element) and returns the list value consisting of those
19799a22
GS
2015elements for which the expression evaluated to true. In scalar
2016context, returns the number of times the expression was true.
a0d0e21e
LW
2017
2018 @foo = grep(!/^#/, @bar); # weed out comments
2019
2020or equivalently,
2021
2022 @foo = grep {!/^#/} @bar; # weed out comments
2023
2b5ab1e7
TC
2024Note that, because C<$_> is a reference into the list value, it can
2025be used to modify the elements of the array. While this is useful and
2026supported, it can cause bizarre results if the LIST is not a named array.
2027Similarly, grep returns aliases into the original list, much as a for
2028loop's index variable aliases the list elements. That is, modifying an
19799a22
GS
2029element of a list returned by grep (for example, in a C<foreach>, C<map>
2030or another C<grep>) actually modifies the element in the original list.
2b5ab1e7 2031This is usually something to be avoided when writing clear code.
a0d0e21e 2032
19799a22 2033See also L</map> for a list composed of the results of the BLOCK or EXPR.
38325410 2034
a0d0e21e
LW
2035=item hex EXPR
2036
54310121 2037=item hex
bbce6d69 2038
2b5ab1e7
TC
2039Interprets EXPR as a hex string and returns the corresponding value.
2040(To convert strings that might start with either 0, 0x, or 0b, see
2041L</oct>.) If EXPR is omitted, uses C<$_>.
2f9daede
TPG
2042
2043 print hex '0xAf'; # prints '175'
2044 print hex 'aF'; # same
a0d0e21e 2045
19799a22 2046Hex strings may only represent integers. Strings that would cause
c6edd1b7 2047integer overflow trigger a warning.
19799a22 2048
a0d0e21e
LW
2049=item import
2050
19799a22 2051There is no builtin C<import> function. It is just an ordinary
4633a7c4 2052method (subroutine) defined (or inherited) by modules that wish to export
19799a22 2053names to another module. The C<use> function calls the C<import> method
54310121 2054for the package used. See also L</use()>, L<perlmod>, and L<Exporter>.
a0d0e21e
LW
2055
2056=item index STR,SUBSTR,POSITION
2057
2058=item index STR,SUBSTR
2059
2b5ab1e7
TC
2060The index function searches for one string within another, but without
2061the wildcard-like behavior of a full regular-expression pattern match.
2062It returns the position of the first occurrence of SUBSTR in STR at
2063or after POSITION. If POSITION is omitted, starts searching from the
2064beginning of the string. The return value is based at C<0> (or whatever
2065you've set the C<$[> variable to--but don't do that). If the substring
2066is not found, returns one less than the base, ordinarily C<-1>.
a0d0e21e
LW
2067
2068=item int EXPR
2069
54310121 2070=item int
bbce6d69 2071
7660c0ab 2072Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2b5ab1e7
TC
2073You should not use this function for rounding: one because it truncates
2074towards C<0>, and two because machine representations of floating point
2075numbers can sometimes produce counterintuitive results. For example,
2076C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2077because it's really more like -268.99999999999994315658 instead. Usually,
19799a22 2078the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2b5ab1e7 2079functions will serve you better than will int().
a0d0e21e
LW
2080
2081=item ioctl FILEHANDLE,FUNCTION,SCALAR
2082
2b5ab1e7 2083Implements the ioctl(2) function. You'll probably first have to say
a0d0e21e 2084
4633a7c4 2085 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
a0d0e21e 2086
2b5ab1e7 2087to get the correct function definitions. If F<ioctl.ph> doesn't
a0d0e21e 2088exist or doesn't have the correct definitions you'll have to roll your
4633a7c4 2089own, based on your C header files such as F<E<lt>sys/ioctl.hE<gt>>.
5a964f20 2090(There is a Perl script called B<h2ph> that comes with the Perl kit that
54310121 2091may help you in this, but it's nontrivial.) SCALAR will be read and/or
4633a7c4 2092written depending on the FUNCTION--a pointer to the string value of SCALAR
19799a22 2093will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
4633a7c4
LW
2094has no string value but does have a numeric value, that value will be
2095passed rather than a pointer to the string value. To guarantee this to be
19799a22
GS
2096true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2097functions may be needed to manipulate the values of structures used by
2098C<ioctl>.
a0d0e21e 2099
19799a22 2100The return value of C<ioctl> (and C<fcntl>) is as follows:
a0d0e21e
LW
2101
2102 if OS returns: then Perl returns:
2103 -1 undefined value
2104 0 string "0 but true"
2105 anything else that number
2106
19799a22 2107Thus Perl returns true on success and false on failure, yet you can
a0d0e21e
LW
2108still easily determine the actual value returned by the operating
2109system:
2110
2b5ab1e7 2111 $retval = ioctl(...) || -1;
a0d0e21e
LW
2112 printf "System returned %d\n", $retval;
2113
c2611fb3 2114The special string "C<0> but true" is exempt from B<-w> complaints
5a964f20
TC
2115about improper numeric conversions.
2116
19799a22
GS
2117Here's an example of setting a filehandle named C<REMOTE> to be
2118non-blocking at the system level. You'll have to negotiate C<$|>
2119on your own, though.
2120
2121 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
2122
2123 $flags = fcntl(REMOTE, F_GETFL, 0)
2124 or die "Can't get flags for the socket: $!\n";
2125
2126 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
2127 or die "Can't set flags for the socket: $!\n";
2128
a0d0e21e
LW
2129=item join EXPR,LIST
2130
2b5ab1e7
TC
2131Joins the separate strings of LIST into a single string with fields
2132separated by the value of EXPR, and returns that new string. Example:
a0d0e21e 2133
2b5ab1e7 2134 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
a0d0e21e 2135
eb6e2d6f
GS
2136Beware that unlike C<split>, C<join> doesn't take a pattern as its
2137first argument. Compare L</split>.
a0d0e21e 2138
aa689395
PP
2139=item keys HASH
2140
19799a22 2141Returns a list consisting of all the keys of the named hash. (In
1d2dff63 2142scalar context, returns the number of keys.) The keys are returned in
ab192400
GS
2143an apparently random order. The actual random order is subject to
2144change in future versions of perl, but it is guaranteed to be the same
19799a22 2145order as either the C<values> or C<each> function produces (given
ab192400
GS
2146that the hash has not been modified). As a side effect, it resets
2147HASH's iterator.
a0d0e21e 2148
aa689395 2149Here is yet another way to print your environment:
a0d0e21e
LW
2150
2151 @keys = keys %ENV;
2152 @values = values %ENV;
19799a22 2153 while (@keys) {
a0d0e21e
LW
2154 print pop(@keys), '=', pop(@values), "\n";
2155 }
2156
2157or how about sorted by key:
2158
2159 foreach $key (sort(keys %ENV)) {
2160 print $key, '=', $ENV{$key}, "\n";
2161 }
2162
19799a22 2163To sort a hash by value, you'll need to use a C<sort> function.
aa689395 2164Here's a descending numeric sort of a hash by its values:
4633a7c4 2165
5a964f20 2166 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
4633a7c4
LW
2167 printf "%4d %s\n", $hash{$key}, $key;
2168 }
2169
19799a22 2170As an lvalue C<keys> allows you to increase the number of hash buckets
aa689395
PP
2171allocated for the given hash. This can gain you a measure of efficiency if
2172you know the hash is going to get big. (This is similar to pre-extending
2173an array by assigning a larger number to $#array.) If you say
55497cff
PP
2174
2175 keys %hash = 200;
2176
ab192400
GS
2177then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2178in fact, since it rounds up to the next power of two. These
55497cff
PP
2179buckets will be retained even if you do C<%hash = ()>, use C<undef
2180%hash> if you want to free the storage while C<%hash> is still in scope.
2181You can't shrink the number of buckets allocated for the hash using
19799a22 2182C<keys> in this way (but you needn't worry about doing this by accident,
55497cff
PP
2183as trying has no effect).
2184
19799a22 2185See also C<each>, C<values> and C<sort>.
ab192400 2186
b350dd2f 2187=item kill SIGNAL, LIST
a0d0e21e 2188
b350dd2f 2189Sends a signal to a list of processes. Returns the number of
517db077
GS
2190processes successfully signaled (which is not necessarily the
2191same as the number actually killed).
a0d0e21e
LW
2192
2193 $cnt = kill 1, $child1, $child2;
2194 kill 9, @goners;
2195
b350dd2f
GS
2196If SIGNAL is zero, no signal is sent to the process. This is a
2197useful way to check that the process is alive and hasn't changed
2198its UID. See L<perlport> for notes on the portability of this
2199construct.
2200
2201Unlike in the shell, if SIGNAL is negative, it kills
4633a7c4
LW
2202process groups instead of processes. (On System V, a negative I<PROCESS>
2203number will also kill process groups, but that's not portable.) That
2204means you usually want to use positive not negative signals. You may also
da0045b7 2205use a signal name in quotes. See L<perlipc/"Signals"> for details.
a0d0e21e
LW
2206
2207=item last LABEL
2208
2209=item last
2210
2211The C<last> command is like the C<break> statement in C (as used in
2212loops); it immediately exits the loop in question. If the LABEL is
2213omitted, the command refers to the innermost enclosing loop. The
2214C<continue> block, if any, is not executed:
2215
4633a7c4
LW
2216 LINE: while (<STDIN>) {
2217 last LINE if /^$/; # exit when done with header
5a964f20 2218 #...
a0d0e21e
LW
2219 }
2220
4968c1e4 2221C<last> cannot be used to exit a block which returns a value such as
2b5ab1e7
TC
2222C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2223a grep() or map() operation.
4968c1e4 2224
6c1372ed
GS
2225Note that a block by itself is semantically identical to a loop
2226that executes once. Thus C<last> can be used to effect an early
2227exit out of such a block.
2228
98293880
JH
2229See also L</continue> for an illustration of how C<last>, C<next>, and
2230C<redo> work.
1d2dff63 2231
a0d0e21e
LW
2232=item lc EXPR
2233
54310121 2234=item lc
bbce6d69 2235
a0d0e21e 2236Returns an lowercased version of EXPR. This is the internal function
7660c0ab 2237implementing the C<\L> escape in double-quoted strings.
19799a22
GS
2238Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
2239and L<utf8>.
a0d0e21e 2240
7660c0ab 2241If EXPR is omitted, uses C<$_>.
bbce6d69 2242
a0d0e21e
LW
2243=item lcfirst EXPR
2244
54310121 2245=item lcfirst
bbce6d69 2246
a0d0e21e 2247Returns the value of EXPR with the first character lowercased. This is
7660c0ab 2248the internal function implementing the C<\l> escape in double-quoted strings.
a0ed51b3 2249Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
a0d0e21e 2250
7660c0ab 2251If EXPR is omitted, uses C<$_>.
bbce6d69 2252
a0d0e21e
LW
2253=item length EXPR
2254
54310121 2255=item length
bbce6d69 2256
a0ed51b3 2257Returns the length in characters of the value of EXPR. If EXPR is
2b5ab1e7
TC
2258omitted, returns length of C<$_>. Note that this cannot be used on
2259an entire array or hash to find out how many elements these have.
2260For that, use C<scalar @array> and C<scalar keys %hash> respectively.
a0d0e21e
LW
2261
2262=item link OLDFILE,NEWFILE
2263
19799a22
GS
2264Creates a new filename linked to the old filename. Returns true for
2265success, false otherwise.
a0d0e21e
LW
2266
2267=item listen SOCKET,QUEUESIZE
2268
19799a22
GS
2269Does the same thing that the listen system call does. Returns true if
2270it succeeded, false otherwise. See the example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
2271
2272=item local EXPR
2273
19799a22 2274You really probably want to be using C<my> instead, because C<local> isn't
2b5ab1e7
TC
2275what most people think of as "local". See L<perlsub/"Private Variables
2276via my()"> for details.
2277
5a964f20
TC
2278A local modifies the listed variables to be local to the enclosing
2279block, file, or eval. If more than one value is listed, the list must
2280be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2281for details, including issues with tied arrays and hashes.
a0d0e21e 2282
a0d0e21e
LW
2283=item localtime EXPR
2284
19799a22 2285Converts a time as returned by the time function to a 9-element list
5f05dabc 2286with the time analyzed for the local time zone. Typically used as
a0d0e21e
LW
2287follows:
2288
54310121 2289 # 0 1 2 3 4 5 6 7 8
a0d0e21e
LW
2290 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2291 localtime(time);
2292
19799a22
GS
2293All list elements are numeric, and come straight out of a struct tm.
2294In particular this means that $mon has the range C<0..11> and $wday
2295has the range C<0..6> with sunday as day C<0>. Also, $year is the
2296number of years since 1900, that is, $year is C<123> in year 2023,
2b5ab1e7
TC
2297and I<not> simply the last two digits of the year. If you assume it is,
2298then you create non-Y2K-compliant programs--and you wouldn't want to do
2299that, would you?
54310121 2300
abd75f24
GS
2301The proper way to get a complete 4-digit year is simply:
2302
2303 $year += 1900;
2304
2305And to get the last two digits of the year (e.g., '01' in 2001) do:
2306
2307 $year = sprintf("%02d", $year % 100);
2308
54310121 2309If EXPR is omitted, uses the current time (C<localtime(time)>).
a0d0e21e 2310
f86cebdf 2311In scalar context, returns the ctime(3) value:
a0d0e21e 2312
5f05dabc 2313 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
a0d0e21e 2314
a3cb178b 2315This scalar value is B<not> locale dependent, see L<perllocale>, but
68f8bed4
JH
2316instead a Perl builtin. Also see the C<Time::Local> module
2317(to convert the second, minutes, hours, ... back to seconds since the
2318stroke of midnight the 1st of January 1970, the value returned by
2319time()), and the strftime(3) and mktime(3) function available via the
2320POSIX module. To get somewhat similar but locale dependent date
2321strings, set up your locale environment variables appropriately
2322(please see L<perllocale>) and try for example:
a3cb178b 2323
5a964f20 2324 use POSIX qw(strftime);
2b5ab1e7 2325 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
a3cb178b
GS
2326
2327Note that the C<%a> and C<%b>, the short forms of the day of the week
2328and the month of the year, may not necessarily be three characters wide.
a0d0e21e 2329
19799a22
GS
2330=item lock
2331
2332 lock I<THING>
2333
2334This function places an advisory lock on a variable, subroutine,
2335or referenced object contained in I<THING> until the lock goes out
2336of scope. This is a built-in function only if your version of Perl
2337was built with threading enabled, and if you've said C<use Threads>.
2338Otherwise a user-defined function by this name will be called. See
2339L<Thread>.
2340
a0d0e21e
LW
2341=item log EXPR
2342
54310121 2343=item log
bbce6d69 2344
2b5ab1e7
TC
2345Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2346returns log of C<$_>. To get the log of another base, use basic algebra:
19799a22 2347The base-N log of a number is equal to the natural log of that number
2b5ab1e7
TC
2348divided by the natural log of N. For example:
2349
2350 sub log10 {
2351 my $n = shift;
2352 return log($n)/log(10);
2353 }
2354
2355See also L</exp> for the inverse operation.
a0d0e21e
LW
2356
2357=item lstat FILEHANDLE
2358
2359=item lstat EXPR
2360
54310121 2361=item lstat
bbce6d69 2362
19799a22 2363Does the same thing as the C<stat> function (including setting the
5a964f20
TC
2364special C<_> filehandle) but stats a symbolic link instead of the file
2365the symbolic link points to. If symbolic links are unimplemented on
19799a22 2366your system, a normal C<stat> is done.
a0d0e21e 2367
7660c0ab 2368If EXPR is omitted, stats C<$_>.
bbce6d69 2369
a0d0e21e
LW
2370=item m//
2371
2372The match operator. See L<perlop>.
2373
2374=item map BLOCK LIST
2375
2376=item map EXPR,LIST
2377
19799a22
GS
2378Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2379C<$_> to each element) and returns the list value composed of the
2380results of each such evaluation. In scalar context, returns the
2381total number of elements so generated. Evaluates BLOCK or EXPR in
2382list context, so each element of LIST may produce zero, one, or
2383more elements in the returned value.
dd99ebda 2384
a0d0e21e
LW
2385 @chars = map(chr, @nums);
2386
2387translates a list of numbers to the corresponding characters. And
2388
4633a7c4 2389 %hash = map { getkey($_) => $_ } @array;
a0d0e21e
LW
2390
2391is just a funny way to write
2392
2393 %hash = ();
2394 foreach $_ (@array) {
4633a7c4 2395 $hash{getkey($_)} = $_;
a0d0e21e
LW
2396 }
2397
2b5ab1e7
TC
2398Note that, because C<$_> is a reference into the list value, it can
2399be used to modify the elements of the array. While this is useful and
2400supported, it can cause bizarre results if the LIST is not a named array.
2401Using a regular C<foreach> loop for this purpose would be clearer in
2402most cases. See also L</grep> for an array composed of those items of
2403the original list for which the BLOCK or EXPR evaluates to true.
fb73857a 2404
19799a22 2405=item mkdir FILENAME,MASK
a0d0e21e 2406
0591cd52 2407Creates the directory specified by FILENAME, with permissions
19799a22
GS
2408specified by MASK (as modified by C<umask>). If it succeeds it
2409returns true, otherwise it returns false and sets C<$!> (errno).
0591cd52 2410
19799a22 2411In general, it is better to create directories with permissive MASK,
0591cd52 2412and let the user modify that with their C<umask>, than it is to supply
19799a22 2413a restrictive MASK and give the user no way to be more permissive.
0591cd52
NT
2414The exceptions to this rule are when the file or directory should be
2415kept private (mail files, for instance). The perlfunc(1) entry on
19799a22 2416C<umask> discusses the choice of MASK in more detail.
a0d0e21e
LW
2417
2418=item msgctl ID,CMD,ARG
2419
f86cebdf 2420Calls the System V IPC function msgctl(2). You'll probably have to say
0ade1984
JH
2421
2422 use IPC::SysV;
2423
7660c0ab
A
2424first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2425then ARG must be a variable which will hold the returned C<msqid_ds>
951ba7fe
GS
2426structure. Returns like C<ioctl>: the undefined value for error,
2427C<"0 but true"> for zero, or the actual return value otherwise. See also
19799a22 2428C<IPC::SysV> and C<IPC::Semaphore> documentation.
a0d0e21e
LW
2429
2430=item msgget KEY,FLAGS
2431
f86cebdf 2432Calls the System V IPC function msgget(2). Returns the message queue
7660c0ab 2433id, or the undefined value if there is an error. See also C<IPC::SysV>
19799a22 2434and C<IPC::Msg> documentation.
a0d0e21e
LW
2435
2436=item msgsnd ID,MSG,FLAGS
2437
2438Calls the System V IPC function msgsnd to send the message MSG to the
2439message queue ID. MSG must begin with the long integer message type,
19799a22
GS
2440which may be created with C<pack("l", $type)>. Returns true if
2441successful, or false if there is an error. See also C<IPC::SysV>
7660c0ab 2442and C<IPC::SysV::Msg> documentation.
a0d0e21e
LW
2443
2444=item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2445
2446Calls the System V IPC function msgrcv to receive a message from
2447message queue ID into variable VAR with a maximum message size of
0ade1984
JH
2448SIZE. Note that if a message is received, the message type will be
2449the first thing in VAR, and the maximum length of VAR is SIZE plus the
19799a22 2450size of the message type. Returns true if successful, or false if
7660c0ab 2451there is an error. See also C<IPC::SysV> and C<IPC::SysV::Msg> documentation.
a0d0e21e
LW
2452
2453=item my EXPR
2454
09bef843
SB
2455=item my EXPR : ATTRIBUTES
2456
19799a22
GS
2457A C<my> declares the listed variables to be local (lexically) to the
2458enclosing block, file, or C<eval>. If
5f05dabc 2459more than one value is listed, the list must be placed in parentheses. See
cb1a09d0 2460L<perlsub/"Private Variables via my()"> for details.
4633a7c4 2461
a0d0e21e
LW
2462=item next LABEL
2463
2464=item next
2465
2466The C<next> command is like the C<continue> statement in C; it starts
2467the next iteration of the loop:
2468
4633a7c4
LW
2469 LINE: while (<STDIN>) {
2470 next LINE if /^#/; # discard comments
5a964f20 2471 #...
a0d0e21e
LW
2472 }
2473
2474Note that if there were a C<continue> block on the above, it would get
2475executed even on discarded lines. If the LABEL is omitted, the command
2476refers to the innermost enclosing loop.
2477
4968c1e4 2478C<next> cannot be used to exit a block which returns a value such as
2b5ab1e7
TC
2479C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2480a grep() or map() operation.
4968c1e4 2481
6c1372ed
GS
2482Note that a block by itself is semantically identical to a loop
2483that executes once. Thus C<next> will exit such a block early.
2484
98293880
JH
2485See also L</continue> for an illustration of how C<last>, C<next>, and
2486C<redo> work.
1d2dff63 2487
a0d0e21e
LW
2488=item no Module LIST
2489
7660c0ab 2490See the L</use> function, which C<no> is the opposite of.
a0d0e21e
LW
2491
2492=item oct EXPR
2493
54310121 2494=item oct
bbce6d69 2495
4633a7c4 2496Interprets EXPR as an octal string and returns the corresponding
4f19785b
WSI
2497value. (If EXPR happens to start off with C<0x>, interprets it as a
2498hex string. If EXPR starts off with C<0b>, it is interpreted as a
2499binary string.) The following will handle decimal, binary, octal, and
4633a7c4 2500hex in the standard Perl or C notation:
a0d0e21e
LW
2501
2502 $val = oct($val) if $val =~ /^0/;
2503
19799a22
GS
2504If EXPR is omitted, uses C<$_>. To go the other way (produce a number
2505in octal), use sprintf() or printf():
2506
2507 $perms = (stat("filename"))[2] & 07777;
2508 $oct_perms = sprintf "%lo", $perms;
2509
2510The oct() function is commonly used when a string such as C<644> needs
2511to be converted into a file mode, for example. (Although perl will
2512automatically convert strings into numbers as needed, this automatic
2513conversion assumes base 10.)
a0d0e21e 2514
6170680b
IZ
2515=item open FILEHANDLE,MODE,EXPR
2516
a0d0e21e
LW
2517=item open FILEHANDLE,EXPR
2518
2519=item open FILEHANDLE
2520
2521Opens the file whose filename is given by EXPR, and associates it with
5f05dabc
PP
2522FILEHANDLE. If FILEHANDLE is an expression, its value is used as the
2523name of the real filehandle wanted. If EXPR is omitted, the scalar
2524variable of the same name as the FILEHANDLE contains the filename.
19799a22
GS
2525(Note that lexical variables--those declared with C<my>--will not work
2526for this purpose; so if you're using C<my>, specify EXPR in your call
2b5ab1e7
TC
2527to open.) See L<perlopentut> for a kinder, gentler explanation of opening
2528files.
5f05dabc 2529
6170680b
IZ
2530If MODE is C<'E<lt>'> or nothing, the file is opened for input.
2531If MODE is C<'E<gt>'>, the file is truncated and opened for
2532output, being created if necessary. If MODE is C<'E<gt>E<gt>'>,
fbb426e4 2533the file is opened for appending, again being created if necessary.
7660c0ab
A
2534You can put a C<'+'> in front of the C<'E<gt>'> or C<'E<lt>'> to indicate that
2535you want both read and write access to the file; thus C<'+E<lt>'> is almost
2536always preferred for read/write updates--the C<'+E<gt>'> mode would clobber the
5a964f20
TC
2537file first. You can't usually use either read-write mode for updating
2538textfiles, since they have variable length records. See the B<-i>
0591cd52
NT
2539switch in L<perlrun> for a better approach. The file is created with
2540permissions of C<0666> modified by the process' C<umask> value.
5a964f20 2541
f86cebdf 2542These various prefixes correspond to the fopen(3) modes of C<'r'>, C<'r+'>, C<'w'>,
7660c0ab 2543C<'w+'>, C<'a'>, and C<'a+'>.
5f05dabc 2544
6170680b
IZ
2545In the 2-arguments (and 1-argument) form of the call the mode and
2546filename should be concatenated (in this order), possibly separated by
2547spaces. It is possible to omit the mode if the mode is C<'E<lt>'>.
2548
7660c0ab 2549If the filename begins with C<'|'>, the filename is interpreted as a
5a964f20 2550command to which output is to be piped, and if the filename ends with a
f244e06d
GS
2551C<'|'>, the filename is interpreted as a command which pipes output to
2552us. See L<perlipc/"Using open() for IPC">
19799a22 2553for more examples of this. (You are not allowed to C<open> to a command
5a964f20
TC
2554that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2555and L<perlipc/"Bidirectional Communication"> for alternatives.)
cb1a09d0 2556
6170680b
IZ
2557If MODE is C<'|-'>, the filename is interpreted as a
2558command to which output is to be piped, and if MODE is
2559C<'-|'>, the filename is interpreted as a command which pipes output to
2560us. In the 2-arguments (and 1-argument) form one should replace dash
2561(C<'-'>) with the command. See L<perlipc/"Using open() for IPC">
2562for more examples of this. (You are not allowed to C<open> to a command
2563that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2564and L<perlipc/"Bidirectional Communication"> for alternatives.)
2565
2566In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
2567and opening C<'E<gt>-'> opens STDOUT.
2568
2569Open returns
19799a22 2570nonzero upon success, the undefined value otherwise. If the C<open>
4633a7c4 2571involved a pipe, the return value happens to be the pid of the
54310121 2572subprocess.
cb1a09d0
AD
2573
2574If you're unfortunate enough to be running Perl on a system that
2575distinguishes between text files and binary files (modern operating
2576systems don't care), then you should check out L</binmode> for tips for
19799a22 2577dealing with this. The key distinction between systems that need C<binmode>
5a964f20
TC
2578and those that don't is their text file formats. Systems like Unix, MacOS, and
2579Plan9, which delimit lines with a single character, and which encode that
19799a22 2580character in C as C<"\n">, do not need C<binmode>. The rest need it.
cb1a09d0 2581
fb73857a 2582When opening a file, it's usually a bad idea to continue normal execution
19799a22
GS
2583if the request failed, so C<open> is frequently used in connection with
2584C<die>. Even if C<die> won't do what you want (say, in a CGI script,
fb73857a 2585where you want to make a nicely formatted error message (but there are
5a964f20 2586modules that can help with that problem)) you should always check
19799a22 2587the return value from opening a file. The infrequent exception is when
fb73857a
PP
2588working with an unopened filehandle is actually what you want to do.
2589
cb1a09d0 2590Examples:
a0d0e21e
LW
2591
2592 $ARTICLE = 100;
2593 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2594 while (<ARTICLE>) {...
2595
6170680b 2596 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
fb73857a 2597 # if the open fails, output is discarded
a0d0e21e 2598
6170680b 2599 open(DBASE, '+<', 'dbase.mine') # open for update
fb73857a 2600 or die "Can't open 'dbase.mine' for update: $!";
cb1a09d0 2601
6170680b
IZ
2602 open(DBASE, '+<dbase.mine') # ditto
2603 or die "Can't open 'dbase.mine' for update: $!";
2604
2605 open(ARTICLE, '-|', "caesar <$article") # decrypt article
fb73857a 2606 or die "Can't start caesar: $!";
a0d0e21e 2607
6170680b
IZ
2608 open(ARTICLE, "caesar <$article |") # ditto
2609 or die "Can't start caesar: $!";
2610
2611 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
fb73857a 2612 or die "Can't start sort: $!";
a0d0e21e
LW
2613
2614 # process argument list of files along with any includes
2615
2616 foreach $file (@ARGV) {
2617 process($file, 'fh00');
2618 }
2619
2620 sub process {
5a964f20 2621 my($filename, $input) = @_;
a0d0e21e
LW
2622 $input++; # this is a string increment
2623 unless (open($input, $filename)) {
2624 print STDERR "Can't open $filename: $!\n";
2625 return;
2626 }
2627
5a964f20 2628 local $_;
a0d0e21e
LW
2629 while (<$input>) { # note use of indirection
2630 if (/^#include "(.*)"/) {
2631 process($1, $input);
2632 next;
2633 }
5a964f20 2634 #... # whatever
a0d0e21e
LW
2635 }
2636 }
2637
2638You may also, in the Bourne shell tradition, specify an EXPR beginning
7660c0ab 2639with C<'E<gt>&'>, in which case the rest of the string is interpreted as the
5a964f20 2640name of a filehandle (or file descriptor, if numeric) to be
6170680b
IZ
2641duped and opened. You may use C<&> after C<E<gt>>, C<E<gt>E<gt>>,
2642C<E<lt>>, C<+E<gt>>, C<+E<gt>E<gt>>, and C<+E<lt>>. The
a0d0e21e 2643mode you specify should match the mode of the original filehandle.
184e9718 2644(Duping a filehandle does not take into account any existing contents of
6170680b
IZ
2645stdio buffers.) Duping file handles is not yet supported for 3-argument
2646open().
2647
a0d0e21e
LW
2648Here is a script that saves, redirects, and restores STDOUT and
2649STDERR:
2650
2651 #!/usr/bin/perl
5a964f20
TC
2652 open(OLDOUT, ">&STDOUT");
2653 open(OLDERR, ">&STDERR");
a0d0e21e 2654
6170680b
IZ
2655 open(STDOUT, '>', "foo.out") || die "Can't redirect stdout";
2656 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
a0d0e21e
LW
2657
2658 select(STDERR); $| = 1; # make unbuffered
2659 select(STDOUT); $| = 1; # make unbuffered
2660
2661 print STDOUT "stdout 1\n"; # this works for
2662 print STDERR "stderr 1\n"; # subprocesses too
2663
2664 close(STDOUT);
2665 close(STDERR);
2666
5a964f20
TC
2667 open(STDOUT, ">&OLDOUT");
2668 open(STDERR, ">&OLDERR");
a0d0e21e
LW
2669
2670 print STDOUT "stdout 2\n";
2671 print STDERR "stderr 2\n";
2672
7660c0ab 2673If you specify C<'E<lt>&=N'>, where C<N> is a number, then Perl will do an
19799a22 2674equivalent of C's C<fdopen> of that file descriptor; this is more
4633a7c4 2675parsimonious of file descriptors. For example:
a0d0e21e
LW
2676
2677 open(FILEHANDLE, "<&=$fd")
2678
4af147f6
CS
2679Note that this feature depends on the fdopen() C library function.
2680On many UNIX systems, fdopen() is known to fail when file descriptors
2681exceed a certain value, typically 255. If you need more file
2682descriptors than that, consider rebuilding Perl to use the C<sfio>
2683library.
2684
6170680b
IZ
2685If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
2686with 2-arguments (or 1-argument) form of open(), then
a0d0e21e 2687there is an implicit fork done, and the return value of open is the pid
7660c0ab 2688of the child within the parent process, and C<0> within the child
184e9718 2689process. (Use C<defined($pid)> to determine whether the open was successful.)
a0d0e21e
LW
2690The filehandle behaves normally for the parent, but i/o to that
2691filehandle is piped from/to the STDOUT/STDIN of the child process.
2692In the child process the filehandle isn't opened--i/o happens from/to
2693the new STDOUT or STDIN. Typically this is used like the normal
2694piped open when you want to exercise more control over just how the
2695pipe command gets executed, such as when you are running setuid, and
54310121 2696don't want to have to scan shell commands for metacharacters.
6170680b 2697The following triples are more or less equivalent:
a0d0e21e
LW
2698
2699 open(FOO, "|tr '[a-z]' '[A-Z]'");
6170680b
IZ
2700 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
2701 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
a0d0e21e
LW
2702
2703 open(FOO, "cat -n '$file'|");
6170680b
IZ
2704 open(FOO, '-|', "cat -n '$file'");
2705 open(FOO, '-|') || exec 'cat', '-n', $file;
a0d0e21e 2706
4633a7c4
LW
2707See L<perlipc/"Safe Pipe Opens"> for more examples of this.
2708
45bc9206
GS
2709NOTE: On any operation that may do a fork, all files opened for output
2710are flushed before the fork is attempted. On systems that support a
2711close-on-exec flag on files, the flag will be set for the newly opened
2712file descriptor as determined by the value of $^F. See L<perlvar/$^F>.
a0d0e21e 2713
0dccf244
CS
2714Closing any piped filehandle causes the parent process to wait for the
2715child to finish, and returns the status value in C<$?>.
2716
6170680b
IZ
2717The filename passed to 2-argument (or 1-argument) form of open()
2718will have leading and trailing
f86cebdf 2719whitespace deleted, and the normal redirection characters
5a964f20
TC
2720honored. This property, known as "magic open",
2721can often be used to good effect. A user could specify a filename of
7660c0ab 2722F<"rsh cat file |">, or you could change certain filenames as needed:
5a964f20
TC
2723
2724 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
2725 open(FH, $filename) or die "Can't open $filename: $!";
2726
6170680b
IZ
2727Use 3-argument form to open a file with arbitrary weird characters in it,
2728
2729 open(FOO, '<', $file);
2730
2731otherwise it's necessary to protect any leading and trailing whitespace:
5a964f20
TC
2732
2733 $file =~ s#^(\s)#./$1#;
2734 open(FOO, "< $file\0");
2735
6170680b
IZ
2736(this may not work on some bizzare filesystems). One should
2737conscientiously choose between the the I<magic> and 3-arguments form
2738of open():
2739
2740 open IN, $ARGV[0];
2741
2742will allow the user to specify an argument of the form C<"rsh cat file |">,
2743but will not work on a filename which happens to have a trailing space, while
2744
2745 open IN, '<', $ARGV[0];
2746
2747will have exactly the opposite restrictions.
2748
19799a22 2749If you want a "real" C C<open> (see L<open(2)> on your system), then you
6170680b
IZ
2750should use the C<sysopen> function, which involves no such magic (but
2751may use subtly different filemodes than Perl open(), which is mapped
2752to C fopen()). This is
5a964f20
TC
2753another way to protect your filenames from interpretation. For example:
2754
2755 use IO::Handle;
2756 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
2757 or die "sysopen $path: $!";
2758 $oldfh = select(HANDLE); $| = 1; select($oldfh);
2759 print HANDLE "stuff $$\n");
2760 seek(HANDLE, 0, 0);
2761 print "File contains: ", <HANDLE>;
2762
7660c0ab
A
2763Using the constructor from the C<IO::Handle> package (or one of its
2764subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
5a964f20
TC
2765filehandles that have the scope of whatever variables hold references to
2766them, and automatically close whenever and however you leave that scope:
c07a80fd 2767
5f05dabc 2768 use IO::File;
5a964f20 2769 #...
c07a80fd
PP
2770 sub read_myfile_munged {
2771 my $ALL = shift;
5f05dabc 2772 my $handle = new IO::File;
c07a80fd
PP
2773 open($handle, "myfile") or die "myfile: $!";
2774 $first = <$handle>
2775 or return (); # Automatically closed here.
2776 mung $first or die "mung failed"; # Or here.
2777 return $first, <$handle> if $ALL; # Or here.
2778 $first; # Or here.
2779 }
2780
b687b08b 2781See L</seek> for some details about mixing reading and writing.
a0d0e21e
LW
2782
2783=item opendir DIRHANDLE,EXPR
2784
19799a22
GS
2785Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
2786C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
a0d0e21e
LW
2787DIRHANDLEs have their own namespace separate from FILEHANDLEs.
2788
2789=item ord EXPR
2790
54310121 2791=item ord
bbce6d69 2792
a0ed51b3 2793Returns the numeric (ASCII or Unicode) value of the first character of EXPR. If
7660c0ab 2794EXPR is omitted, uses C<$_>. For the reverse, see L</chr>.
2b5ab1e7 2795See L<utf8> for more about Unicode.
a0d0e21e 2796
77ca0c92
LW
2797=item our EXPR
2798
2799An C<our> declares the listed variables to be valid globals within
2800the enclosing block, file, or C<eval>. That is, it has the same
2801scoping rules as a "my" declaration, but does not create a local
2802variable. If more than one value is listed, the list must be placed
2803in parentheses. The C<our> declaration has no semantic effect unless
2804"use strict vars" is in effect, in which case it lets you use the
2805declared global variable without qualifying it with a package name.
2806(But only within the lexical scope of the C<our> declaration. In this
2807it differs from "use vars", which is package scoped.)
2808
f472eb5c
GS
2809An C<our> declaration declares a global variable that will be visible
2810across its entire lexical scope, even across package boundaries. The
2811package in which the variable is entered is determined at the point
2812of the declaration, not at the point of use. This means the following
2813behavior holds:
2814
2815 package Foo;
2816 our $bar; # declares $Foo::bar for rest of lexical scope
2817 $bar = 20;
2818
2819 package Bar;
2820 print $bar; # prints 20
2821
2822Multiple C<our> declarations in the same lexical scope are allowed
2823if they are in different packages. If they happened to be in the same
2824package, Perl will emit warnings if you have asked for them.
2825
2826 use warnings;
2827 package Foo;
2828 our $bar; # declares $Foo::bar for rest of lexical scope
2829 $bar = 20;
2830
2831 package Bar;
2832 our $bar = 30; # declares $Bar::bar for rest of lexical scope
2833 print $bar; # prints 30
2834
2835 our $bar; # emits warning
2836
a0d0e21e
LW
2837=item pack TEMPLATE,LIST
2838
2b6c5635
GS
2839Takes a LIST of values and converts it into a string using the rules
2840given by the TEMPLATE. The resulting string is the concatenation of
2841the converted values. Typically, each converted value looks
2842like its machine-level representation. For example, on 32-bit machines
2843a converted integer may be represented by a sequence of 4 bytes.
2844
2845The TEMPLATE is a
a0d0e21e
LW
2846sequence of characters that give the order and type of values, as
2847follows:
2848
5a929a98 2849 a A string with arbitrary binary data, will be null padded.
a0d0e21e 2850 A An ascii string, will be space padded.
5a929a98
VU
2851 Z A null terminated (asciz) string, will be null padded.
2852
2b6c5635
GS
2853 b A bit string (ascending bit order inside each byte, like vec()).
2854 B A bit string (descending bit order inside each byte).
a0d0e21e
LW
2855 h A hex string (low nybble first).
2856 H A hex string (high nybble first).
2857
2858 c A signed char value.
a0ed51b3 2859 C An unsigned char value. Only does bytes. See U for Unicode.
96e4d5b1 2860
a0d0e21e
LW
2861 s A signed short value.
2862 S An unsigned short value.
96e4d5b1 2863 (This 'short' is _exactly_ 16 bits, which may differ from
851646ae
JH
2864 what a local C compiler calls 'short'. If you want
2865 native-length shorts, use the '!' suffix.)
96e4d5b1 2866
a0d0e21e
LW
2867 i A signed integer value.
2868 I An unsigned integer value.
19799a22 2869 (This 'integer' is _at_least_ 32 bits wide. Its exact
f86cebdf
GS
2870 size depends on what a local C compiler calls 'int',
2871 and may even be larger than the 'long' described in
2872 the next item.)
96e4d5b1 2873
a0d0e21e
LW
2874 l A signed long value.
2875 L An unsigned long value.
96e4d5b1 2876 (This 'long' is _exactly_ 32 bits, which may differ from
851646ae
JH
2877 what a local C compiler calls 'long'. If you want
2878 native-length longs, use the '!' suffix.)
a0d0e21e 2879
5d11dd56
G
2880 n An unsigned short in "network" (big-endian) order.
2881 N An unsigned long in "network" (big-endian) order.
2882 v An unsigned short in "VAX" (little-endian) order.
2883 V An unsigned long in "VAX" (little-endian) order.
96e4d5b1
PP
2884 (These 'shorts' and 'longs' are _exactly_ 16 bits and
2885 _exactly_ 32 bits, respectively.)
a0d0e21e 2886
dae0da7a
JH
2887 q A signed quad (64-bit) value.
2888 Q An unsigned quad value.
851646ae
JH
2889 (Quads are available only if your system supports 64-bit
2890 integer values _and_ if Perl has been compiled to support those.
dae0da7a
JH
2891 Causes a fatal error otherwise.)
2892
a0d0e21e
LW
2893 f A single-precision float in the native format.
2894 d A double-precision float in the native format.
2895
2896 p A pointer to a null-terminated string.
2897 P A pointer to a structure (fixed-length string).
2898
2899 u A uuencoded string.
a0ed51b3
LW
2900 U A Unicode character number. Encodes to UTF-8 internally.
2901 Works even if C<use utf8> is not in effect.
a0d0e21e 2902
96e4d5b1 2903 w A BER compressed integer. Its bytes represent an unsigned
f86cebdf
GS
2904 integer in base 128, most significant digit first, with as
2905 few digits as possible. Bit eight (the high bit) is set
2906 on each byte except the last.
def98dd4 2907
a0d0e21e
LW
2908 x A null byte.
2909 X Back up a byte.
2910 @ Null fill to absolute position.
2911
5a929a98
VU
2912The following rules apply:
2913
2914=over 8
2915
2916=item *
2917
5a964f20 2918Each letter may optionally be followed by a number giving a repeat
951ba7fe
GS
2919count. With all types except C<a>, C<A>, C<Z>, C<b>, C<B>, C<h>,
2920C<H>, and C<P> the pack function will gobble up that many values from
5a929a98 2921the LIST. A C<*> for the repeat count means to use however many items are
951ba7fe
GS
2922left, except for C<@>, C<x>, C<X>, where it is equivalent
2923to C<0>, and C<u>, where it is equivalent to 1 (or 45, what is the
2b6c5635
GS
2924same).
2925
951ba7fe 2926When used with C<Z>, C<*> results in the addition of a trailing null
2b6c5635
GS
2927byte (so the packed result will be one longer than the byte C<length>
2928of the item).
2929
951ba7fe 2930The repeat count for C<u> is interpreted as the maximal number of bytes
2b6c5635 2931to encode per line of output, with 0 and 1 replaced by 45.
5a929a98
VU
2932
2933=item *
2934
951ba7fe 2935The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
5a929a98 2936string of length count, padding with nulls or spaces as necessary. When
951ba7fe
GS
2937unpacking, C<A> strips trailing spaces and nulls, C<Z> strips everything
2938after the first null, and C<a> returns data verbatim. When packing,
2939C<a>, and C<Z> are equivalent.
2b6c5635
GS
2940
2941If the value-to-pack is too long, it is truncated. If too long and an
951ba7fe
GS
2942explicit count is provided, C<Z> packs only C<$count-1> bytes, followed
2943by a null byte. Thus C<Z> always packs a trailing null byte under
2b6c5635 2944all circumstances.
5a929a98
VU
2945
2946=item *
2947
951ba7fe 2948Likewise, the C<b> and C<B> fields pack a string that many bits long.
c73032f5
IZ
2949Each byte of the input field of pack() generates 1 bit of the result.
2950Each result bit is based on the least-significant bit of the corresponding
2951input byte, i.e., on C<ord($byte)%2>. In particular, bytes C<"0"> and
2952C<"1"> generate bits 0 and 1, as do bytes C<"\0"> and C<"\1">.
2953
2954Starting from the beginning of the input string of pack(), each 8-tuple
951ba7fe 2955of bytes is converted to 1 byte of output. With format C<b>
c73032f5 2956the first byte of the 8-tuple determines the least-significant bit of a
951ba7fe 2957byte, and with format C<B> it determines the most-significant bit of
c73032f5
IZ
2958a byte.
2959
2960If the length of the input string is not exactly divisible by 8, the
2961remainder is packed as if the input string were padded by null bytes
2962at the end. Similarly, during unpack()ing the "extra" bits are ignored.
2963
2964If the input string of pack() is longer than needed, extra bytes are ignored.
2b6c5635
GS
2965A C<*> for the repeat count of pack() means to use all the bytes of
2966the input field. On unpack()ing the bits are converted to a string
2967of C<"0">s and C<"1">s.
5a929a98
VU
2968
2969=item *
2970
951ba7fe 2971The C<h> and C<H> fields pack a string that many nybbles (4-bit groups,
851646ae 2972representable as hexadecimal digits, 0-9a-f) long.
5a929a98 2973
c73032f5
IZ
2974Each byte of the input field of pack() generates 4 bits of the result.
2975For non-alphabetical bytes the result is based on the 4 least-significant
2976bits of the input byte, i.e., on C<ord($byte)%16>. In particular,
2977bytes C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
2978C<"\0"> and C<"\1">. For bytes C<"a".."f"> and C<"A".."F"> the result
2979is compatible with the usual hexadecimal digits, so that C<"a"> and
2980C<"A"> both generate the nybble C<0xa==10>. The result for bytes
2981C<"g".."z"> and C<"G".."Z"> is not well-defined.
2982
2983Starting from the beginning of the input string of pack(), each pair
951ba7fe 2984of bytes is converted to 1 byte of output. With format C<h> the
c73032f5 2985first byte of the pair determines the least-significant nybble of the
951ba7fe 2986output byte, and with format C<H> it determines the most-significant
c73032f5
IZ
2987nybble.
2988
2989If the length of the input string is not even, it behaves as if padded
2990by a null byte at the end. Similarly, during unpack()ing the "extra"
2991nybbles are ignored.
2992
2993If the input string of pack() is longer than needed, extra bytes are ignored.
2994A C<*> for the repeat count of pack() means to use all the bytes of
2995the input field. On unpack()ing the bits are converted to a string
2996of hexadecimal digits.
2997
5a929a98
VU
2998=item *
2999
951ba7fe 3000The C<p> type packs a pointer to a null-terminated string. You are
5a929a98
VU
3001responsible for ensuring the string is not a temporary value (which can
3002potentially get deallocated before you get around to using the packed result).
951ba7fe
GS
3003The C<P> type packs a pointer to a structure of the size indicated by the
3004length. A NULL pointer is created if the corresponding value for C<p> or
3005C<P> is C<undef>, similarly for unpack().
5a929a98
VU
3006
3007=item *
3008
951ba7fe
GS
3009The C</> template character allows packing and unpacking of strings where
3010the packed structure contains a byte count followed by the string itself.
17f4a12d 3011You write I<length-item>C</>I<string-item>.
43192e07
IP
3012
3013The I<length-item> can be any C<pack> template letter,
3014and describes how the length value is packed.
3015The ones likely to be of most use are integer-packing ones like
951ba7fe
GS
3016C<n> (for Java strings), C<w> (for ASN.1 or SNMP)
3017and C<N> (for Sun XDR).
43192e07
IP
3018
3019The I<string-item> must, at present, be C<"A*">, C<"a*"> or C<"Z*">.
3020For C<unpack> the length of the string is obtained from the I<length-item>,
3021but if you put in the '*' it will be ignored.
3022
17f4a12d
IZ
3023 unpack 'C/a', "\04Gurusamy"; gives 'Guru'
3024 unpack 'a3/A* A*', '007 Bond J '; gives (' Bond','J')
3025 pack 'n/a* w/a*','hello,','world'; gives "\000\006hello,\005world"
43192e07
IP
3026
3027The I<length-item> is not returned explicitly from C<unpack>.
3028
951ba7fe
GS
3029Adding a count to the I<length-item> letter is unlikely to do anything
3030useful, unless that letter is C<A>, C<a> or C<Z>. Packing with a
3031I<length-item> of C<a> or C<Z> may introduce C<"\000"> characters,
43192e07
IP
3032which Perl does not regard as legal in numeric strings.
3033
3034=item *
3035
951ba7fe
GS
3036The integer types C<s>, C<S>, C<l>, and C<L> may be
3037immediately followed by a C<!> suffix to signify native shorts or
3038longs--as you can see from above for example a bare C<l> does mean
851646ae
JH
3039exactly 32 bits, the native C<long> (as seen by the local C compiler)
3040may be larger. This is an issue mainly in 64-bit platforms. You can
951ba7fe 3041see whether using C<!> makes any difference by
726ea183 3042
4d0c1c44
GS
3043 print length(pack("s")), " ", length(pack("s!")), "\n";
3044 print length(pack("l")), " ", length(pack("l!")), "\n";
ef54e1a4 3045
951ba7fe
GS
3046C<i!> and C<I!> also work but only because of completeness;
3047they are identical to C<i> and C<I>.
ef54e1a4 3048
19799a22
GS
3049The actual sizes (in bytes) of native shorts, ints, longs, and long
3050longs on the platform where Perl was built are also available via
3051L<Config>:
3052
3053 use Config;
3054 print $Config{shortsize}, "\n";
3055 print $Config{intsize}, "\n";
3056 print $Config{longsize}, "\n";
3057 print $Config{longlongsize}, "\n";
ef54e1a4 3058
5074e145 3059(The C<$Config{longlongsize}> will be undefine if your system does
851646ae
JH
3060not support long longs.)
3061
ef54e1a4
JH
3062=item *
3063
951ba7fe 3064The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, and C<L>
ef54e1a4
JH
3065are inherently non-portable between processors and operating systems
3066because they obey the native byteorder and endianness. For example a
140cb37e 30674-byte integer 0x12345678 (305419896 decimal) be ordered natively
ef54e1a4
JH
3068(arranged in and handled by the CPU registers) into bytes as
3069
719a3cf5
JH
3070 0x12 0x34 0x56 0x78 # little-endian
3071 0x78 0x56 0x34 0x12 # big-endian
ef54e1a4 3072
5d11dd56 3073Basically, the Intel, Alpha, and VAX CPUs are little-endian, while
719a3cf5
JH
3074everybody else, for example Motorola m68k/88k, PPC, Sparc, HP PA,
3075Power, and Cray are big-endian. MIPS can be either: Digital used it
19799a22 3076in little-endian mode; SGI uses it in big-endian mode.
719a3cf5 3077
19799a22 3078The names `big-endian' and `little-endian' are comic references to
ef54e1a4
JH
3079the classic "Gulliver's Travels" (via the paper "On Holy Wars and a
3080Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and
19799a22 3081the egg-eating habits of the Lilliputians.
ef54e1a4 3082
140cb37e 3083Some systems may have even weirder byte orders such as
ef54e1a4
JH
3084
3085 0x56 0x78 0x12 0x34
3086 0x34 0x12 0x78 0x56
3087
3088You can see your system's preference with
3089
3090 print join(" ", map { sprintf "%#02x", $_ }
3091 unpack("C*",pack("L",0x12345678))), "\n";
3092
d99ad34e 3093The byteorder on the platform where Perl was built is also available
726ea183 3094via L<Config>:
ef54e1a4
JH
3095
3096 use Config;
3097 print $Config{byteorder}, "\n";
3098
d99ad34e
JH
3099Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'>
3100and C<'87654321'> are big-endian.
719a3cf5 3101
951ba7fe
GS
3102If you want portable packed integers use the formats C<n>, C<N>,
3103C<v>, and C<V>, their byte endianness and size is known.
851646ae 3104See also L<perlport>.
ef54e1a4
JH
3105
3106=item *
3107
5a929a98
VU
3108Real numbers (floats and doubles) are in the native machine format only;
3109due to the multiplicity of floating formats around, and the lack of a
3110standard "network" representation, no facility for interchange has been
3111made. This means that packed floating point data written on one machine
3112may not be readable on another - even if both use IEEE floating point
3113arithmetic (as the endian-ness of the memory representation is not part
851646ae 3114of the IEEE spec). See also L<perlport>.
5a929a98
VU
3115
3116Note that Perl uses doubles internally for all numeric calculation, and
3117converting from double into float and thence back to double again will
3118lose precision (i.e., C<unpack("f", pack("f", $foo)>) will not in general
19799a22 3119equal $foo).
5a929a98 3120
851646ae
JH
3121=item *
3122
3123You must yourself do any alignment or padding by inserting for example
9ccd05c0
JH
3124enough C<'x'>es while packing. There is no way to pack() and unpack()
3125could know where the bytes are going to or coming from. Therefore
3126C<pack> (and C<unpack>) handle their output and input as flat
3127sequences of bytes.
851646ae 3128
17f4a12d
IZ
3129=item *
3130
3131A comment in a TEMPLATE starts with C<#> and goes to the end of line.
3132
2b6c5635
GS
3133=item *
3134
3135If TEMPLATE requires more arguments to pack() than actually given, pack()
3136assumes additional C<""> arguments. If TEMPLATE requires less arguments
3137to pack() than actually given, extra arguments are ignored.
3138
5a929a98 3139=back
a0d0e21e
LW
3140
3141Examples:
3142
a0ed51b3 3143 $foo = pack("CCCC",65,66,67,68);
a0d0e21e 3144 # foo eq "ABCD"
a0ed51b3 3145 $foo = pack("C4",65,66,67,68);
a0d0e21e 3146 # same thing
a0ed51b3
LW
3147 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
3148 # same thing with Unicode circled letters
a0d0e21e
LW
3149
3150 $foo = pack("ccxxcc",65,66,67,68);
3151 # foo eq "AB\0\0CD"
3152
9ccd05c0
JH
3153 # note: the above examples featuring "C" and "c" are true
3154 # only on ASCII and ASCII-derived systems such as ISO Latin 1
3155 # and UTF-8. In EBCDIC the first example would be
3156 # $foo = pack("CCCC",193,194,195,196);
3157
a0d0e21e
LW
3158 $foo = pack("s2",1,2);
3159 # "\1\0\2\0" on little-endian
3160 # "\0\1\0\2" on big-endian
3161
3162 $foo = pack("a4","abcd","x","y","z");
3163 # "abcd"
3164
3165 $foo = pack("aaaa","abcd","x","y","z");
3166 # "axyz"
3167
3168 $foo = pack("a14","abcdefg");
3169 # "abcdefg\0\0\0\0\0\0\0"
3170
3171 $foo = pack("i9pl", gmtime);
3172 # a real struct tm (on my system anyway)
3173
5a929a98
VU
3174 $utmp_template = "Z8 Z8 Z16 L";
3175 $utmp = pack($utmp_template, @utmp1);
3176 # a struct utmp (BSDish)
3177
3178 @utmp2 = unpack($utmp_template, $utmp);
3179 # "@utmp1" eq "@utmp2"
3180
a0d0e21e
LW
3181 sub bintodec {
3182 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
3183 }
3184
851646ae
JH
3185 $foo = pack('sx2l', 12, 34);
3186 # short 12, two zero bytes padding, long 34
3187 $bar = pack('s@4l', 12, 34);
3188 # short 12, zero fill to position 4, long 34
3189 # $foo eq $bar
3190
5a929a98 3191The same template may generally also be used in unpack().
a0d0e21e 3192
5a964f20
TC
3193=item package
3194
cb1a09d0
AD
3195=item package NAMESPACE
3196
3197Declares the compilation unit as being in the given namespace. The scope
2b5ab1e7 3198of the package declaration is from the declaration itself through the end
19799a22 3199of the enclosing block, file, or eval (the same as the C<my> operator).
2b5ab1e7
TC
3200All further unqualified dynamic identifiers will be in this namespace.
3201A package statement affects only dynamic variables--including those
19799a22
GS
3202you've used C<local> on--but I<not> lexical variables, which are created
3203with C<my>. Typically it would be the first declaration in a file to
2b5ab1e7
TC
3204be included by the C<require> or C<use> operator. You can switch into a
3205package in more than one place; it merely influences which symbol table
3206is used by the compiler for the rest of that block. You can refer to
3207variables and filehandles in other packages by prefixing the identifier
3208with the package name and a double colon: C<$Package::Variable>.
3209If the package name is null, the C<main> package as assumed. That is,
3210C<$::sail> is equivalent to C<$main::sail> (as well as to C<$main'sail>,
3211still seen in older code).
cb1a09d0 3212
5a964f20
TC
3213If NAMESPACE is omitted, then there is no current package, and all
3214identifiers must be fully qualified or lexicals. This is stricter
3215than C<use strict>, since it also extends to function names.
3216
cb1a09d0
AD
3217See L<perlmod/"Packages"> for more information about packages, modules,
3218and classes. See L<perlsub> for other scoping issues.
3219
a0d0e21e
LW
3220=item pipe READHANDLE,WRITEHANDLE
3221
3222Opens a pair of connected pipes like the corresponding system call.
3223Note that if you set up a loop of piped processes, deadlock can occur
3224unless you are very careful. In addition, note that Perl's pipes use
184e9718 3225stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE
a0d0e21e
LW
3226after each command, depending on the application.
3227
7e1af8bc 3228See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
4633a7c4
LW
3229for examples of such things.
3230
4771b018
GS
3231On systems that support a close-on-exec flag on files, the flag will be set
3232for the newly opened file descriptors as determined by the value of $^F.
3233See L<perlvar/$^F>.
3234
a0d0e21e
LW
3235=item pop ARRAY
3236
54310121 3237=item pop
28757baa 3238
a0d0e21e 3239Pops and returns the last value of the array, shortening the array by
19799a22 3240one element. Has an effect similar to
a0d0e21e 3241
19799a22 3242 $ARRAY[$#ARRAY--]
a0d0e21e 3243
19799a22
GS
3244If there are no elements in the array, returns the undefined value
3245(although this may happen at other times as well). If ARRAY is
3246omitted, pops the C<@ARGV> array in the main program, and the C<@_>
3247array in subroutines, just like C<shift>.
a0d0e21e
LW
3248
3249=item pos SCALAR
3250
54310121 3251=item pos
bbce6d69 3252
4633a7c4 3253Returns the offset of where the last C<m//g> search left off for the variable
7660c0ab 3254is in question (C<$_> is used when the variable is not specified). May be
44a8e56a
PP
3255modified to change that offset. Such modification will also influence
3256the C<\G> zero-width assertion in regular expressions. See L<perlre> and
3257L<perlop>.
a0d0e21e
LW
3258
3259=item print FILEHANDLE LIST
3260
3261=item print LIST
3262
3263=item print
3264
19799a22
GS
3265Prints a string or a list of strings. Returns true if successful.
3266FILEHANDLE may be a scalar variable name, in which case the variable
3267contains the name of or a reference to the filehandle, thus introducing
3268one level of indirection. (NOTE: If FILEHANDLE is a variable and
3269the next token is a term, it may be misinterpreted as an operator
2b5ab1e7 3270unless you interpose a C<+> or put parentheses around the arguments.)
19799a22
GS
3271If FILEHANDLE is omitted, prints by default to standard output (or
3272to the last selected output channel--see L</select>). If LIST is
3273also omitted, prints C<$_> to the currently selected output channel.
3274To set the default output channel to something other than STDOUT
3275use the select operation. The current value of C<$,> (if any) is
3276printed between each LIST item. The current value of C<$\> (if
3277any) is printed after the entire LIST has been printed. Because
3278print takes a LIST, anything in the LIST is evaluated in list
3279context, and any subroutine that you call will have one or more of
3280its expressions evaluated in list context. Also be careful not to
3281follow the print keyword with a left parenthesis unless you want
3282the corresponding right parenthesis to terminate the arguments to
3283the print--interpose a C<+> or put parentheses around all the
3284arguments.
a0d0e21e 3285
4633a7c4 3286Note that if you're storing FILEHANDLES in an array or other expression,
da0045b7 3287you will have to use a block returning its value instead:
4633a7c4
LW
3288
3289 print { $files[$i] } "stuff\n";
3290 print { $OK ? STDOUT : STDERR } "stuff\n";
3291
5f05dabc 3292=item printf FILEHANDLE FORMAT, LIST
a0d0e21e 3293
5f05dabc 3294=item printf FORMAT, LIST
a0d0e21e 3295
7660c0ab 3296Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
a3cb178b 3297(the output record separator) is not appended. The first argument
19799a22 3298of the list will be interpreted as the C<printf> format. If C<use locale> is
a034a98d
DD
3299in effect, the character used for the decimal point in formatted real numbers
3300is affected by the LC_NUMERIC locale. See L<perllocale>.
a0d0e21e 3301
19799a22
GS
3302Don't fall into the trap of using a C<printf> when a simple
3303C<print> would do. The C<print> is more efficient and less
28757baa
PP
3304error prone.