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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
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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
61eff3bc 338characters with the high bit set. If too many strange characters (>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">.
a0d0e21e 378
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379On systems that support a close-on-exec flag on files, the flag will
380be set for the newly opened file descriptor, as determined by the
381value of $^F. See L<perlvar/$^F>.
382
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383=item alarm SECONDS
384
54310121 385=item alarm
bbce6d69 386
a0d0e21e 387Arranges to have a SIGALRM delivered to this process after the
bbce6d69 388specified number of seconds have elapsed. If SECONDS is not specified,
7660c0ab 389the value stored in C<$_> is used. (On some machines,
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390unfortunately, the elapsed time may be up to one second less than you
391specified because of how seconds are counted.) Only one timer may be
392counting at once. Each call disables the previous timer, and an
7660c0ab 393argument of C<0> may be supplied to cancel the previous timer without
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394starting a new one. The returned value is the amount of time remaining
395on the previous timer.
396
4633a7c4 397For delays of finer granularity than one second, you may use Perl's
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398four-argument version of select() leaving the first three arguments
399undefined, or you might be able to use the C<syscall> interface to
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400access setitimer(2) if your system supports it. The Time::HiRes module
401from CPAN may also prove useful.
402
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403It is usually a mistake to intermix C<alarm> and C<sleep> calls.
404(C<sleep> may be internally implemented in your system with C<alarm>)
a0d0e21e 405
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406If you want to use C<alarm> to time out a system call you need to use an
407C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
f86cebdf 408fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
19799a22 409restart system calls on some systems. Using C<eval>/C<die> always works,
5a964f20 410modulo the caveats given in L<perlipc/"Signals">.
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411
412 eval {
f86cebdf 413 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
36477c24 414 alarm $timeout;
ff68c719 415 $nread = sysread SOCKET, $buffer, $size;
36477c24 416 alarm 0;
ff68c719 417 };
ff68c719 418 if ($@) {
f86cebdf 419 die unless $@ eq "alarm\n"; # propagate unexpected errors
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420 # timed out
421 }
422 else {
423 # didn't
424 }
425
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426=item atan2 Y,X
427
428Returns the arctangent of Y/X in the range -PI to PI.
429
ca6e1c26 430For the tangent operation, you may use the C<Math::Trig::tan>
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431function, or use the familiar relation:
432
433 sub tan { sin($_[0]) / cos($_[0]) }
434
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435=item bind SOCKET,NAME
436
437Binds a network address to a socket, just as the bind system call
19799a22 438does. Returns true if it succeeded, false otherwise. NAME should be a
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439packed address of the appropriate type for the socket. See the examples in
440L<perlipc/"Sockets: Client/Server Communication">.
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441
442=item binmode FILEHANDLE
443
19799a22 444Arranges for FILEHANDLE to be read or written in "binary" mode on
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445systems where the run-time libraries distinguish between binary and
446text files. If FILEHANDLE is an expression, the value is taken as the
447name of the filehandle. binmode() should be called after open() but
448before any I/O is done on the filehandle. The only way to reset
449binary mode on a filehandle is to reopen the file.
450
451On many systems binmode() has no effect, and on some systems it is
452necessary when you're not working with a text file. For the sake of
453portability it is a good idea to always use it when appropriate, and
454to never use it when it isn't appropriate.
455
456In other words: Regardless of platform, use binmode() on binary
457files, and do not use binmode() on text files.
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458
459The operating system, device drivers, C libraries, and Perl run-time
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460system all work together to let the programmer treat a single
461character (C<\n>) as the line terminator, irrespective of the external
462representation. On many operating systems, the native text file
463representation matches the internal representation, but on some
464platforms the external representation of C<\n> is made up of more than
465one character.
466
467Mac OS and all variants of Unix use a single character to end each line
468in the external representation of text (even though that single
469character is not necessarily the same across these platforms).
470Consequently binmode() has no effect on these operating systems. In
471other systems like VMS, MS-DOS and the various flavors of MS-Windows
472your program sees a C<\n> as a simple C<\cJ>, but what's stored in text
473files are the two characters C<\cM\cJ>. That means that, if you don't
474use binmode() on these systems, C<\cM\cJ> sequences on disk will be
475converted to C<\n> on input, and any C<\n> in your program will be
476converted back to C<\cM\cJ> on output. This is what you want for text
477files, but it can be disastrous for binary files.
478
479Another consequence of using binmode() (on some systems) is that
480special end-of-file markers will be seen as part of the data stream.
481For systems from the Microsoft family this means that if your binary
482data contains C<\cZ>, the I/O subsystem will ragard it as the end of
483the file, unless you use binmode().
484
485binmode() is not only important for readline() and print() operations,
486but also when using read(), seek(), sysread(), syswrite() and tell()
487(see L<perlport> for more details). See the C<$/> and C<$\> variables
488in L<perlvar> for how to manually set your input and output
489line-termination sequences.
a0d0e21e 490
4633a7c4 491=item bless REF,CLASSNAME
a0d0e21e
LW
492
493=item bless REF
494
2b5ab1e7
TC
495This function tells the thingy referenced by REF that it is now an object
496in the CLASSNAME package. If CLASSNAME is omitted, the current package
19799a22 497is used. Because a C<bless> is often the last thing in a constructor,
2b5ab1e7
TC
498it returns the reference for convenience. Always use the two-argument
499version if the function doing the blessing might be inherited by a
500derived class. See L<perltoot> and L<perlobj> for more about the blessing
501(and blessings) of objects.
a0d0e21e 502
57668c4d 503Consider always blessing objects in CLASSNAMEs that are mixed case.
2b5ab1e7
TC
504Namespaces with all lowercase names are considered reserved for
505Perl pragmata. Builtin types have all uppercase names, so to prevent
506confusion, you may wish to avoid such package names as well. Make sure
507that CLASSNAME is a true value.
60ad88b8
GS
508
509See L<perlmod/"Perl Modules">.
510
a0d0e21e
LW
511=item caller EXPR
512
513=item caller
514
5a964f20 515Returns the context of the current subroutine call. In scalar context,
28757baa 516returns the caller's package name if there is a caller, that is, if
19799a22 517we're in a subroutine or C<eval> or C<require>, and the undefined value
5a964f20 518otherwise. In list context, returns
a0d0e21e 519
748a9306 520 ($package, $filename, $line) = caller;
a0d0e21e
LW
521
522With EXPR, it returns some extra information that the debugger uses to
523print a stack trace. The value of EXPR indicates how many call frames
524to go back before the current one.
525
f3aa04c2 526 ($package, $filename, $line, $subroutine, $hasargs,
e476b1b5 527 $wantarray, $evaltext, $is_require, $hints, $bitmask) = caller($i);
e7ea3e70 528
951ba7fe 529Here $subroutine may be C<(eval)> if the frame is not a subroutine
19799a22 530call, but an C<eval>. In such a case additional elements $evaltext and
7660c0ab 531C<$is_require> are set: C<$is_require> is true if the frame is created by a
19799a22 532C<require> or C<use> statement, $evaltext contains the text of the
dc848c6f 533C<eval EXPR> statement. In particular, for a C<eval BLOCK> statement,
951ba7fe 534$filename is C<(eval)>, but $evaltext is undefined. (Note also that
dc848c6f 535each C<use> statement creates a C<require> frame inside an C<eval EXPR>)
e476b1b5
GS
536frame. C<$hints> and C<$bitmask> contain pragmatic hints that the caller
537was compiled with. The C<$hints> and C<$bitmask> values are subject to
538change between versions of Perl, and are not meant for external use.
748a9306
LW
539
540Furthermore, when called from within the DB package, caller returns more
7660c0ab 541detailed information: it sets the list variable C<@DB::args> to be the
54310121 542arguments with which the subroutine was invoked.
748a9306 543
7660c0ab 544Be aware that the optimizer might have optimized call frames away before
19799a22 545C<caller> had a chance to get the information. That means that C<caller(N)>
7660c0ab 546might not return information about the call frame you expect it do, for
61eff3bc 547C<< N > 1 >>. In particular, C<@DB::args> might have information from the
19799a22 548previous time C<caller> was called.
7660c0ab 549
a0d0e21e
LW
550=item chdir EXPR
551
2b5ab1e7 552Changes the working directory to EXPR, if possible. If EXPR is omitted,
0bfc1ec4
GS
553changes to the directory specified by C<$ENV{HOME}>, if set; if not,
554changes to the directory specified by C<$ENV{LOGDIR}>. If neither is
555set, C<chdir> does nothing. It returns true upon success, false
556otherwise. See the example under C<die>.
a0d0e21e
LW
557
558=item chmod LIST
559
560Changes the permissions of a list of files. The first element of the
4633a7c4 561list must be the numerical mode, which should probably be an octal
2f9daede
TPG
562number, and which definitely should I<not> a string of octal digits:
563C<0644> is okay, C<'0644'> is not. Returns the number of files
dc848c6f 564successfully changed. See also L</oct>, if all you have is a string.
a0d0e21e
LW
565
566 $cnt = chmod 0755, 'foo', 'bar';
567 chmod 0755, @executables;
f86cebdf
GS
568 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
569 # --w----r-T
2f9daede
TPG
570 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
571 $mode = 0644; chmod $mode, 'foo'; # this is best
a0d0e21e 572
ca6e1c26
JH
573You can also import the symbolic C<S_I*> constants from the Fcntl
574module:
575
576 use Fcntl ':mode';
577
578 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
579 # This is identical to the chmod 0755 of the above example.
580
a0d0e21e
LW
581=item chomp VARIABLE
582
583=item chomp LIST
584
585=item chomp
586
2b5ab1e7
TC
587This safer version of L</chop> removes any trailing string
588that corresponds to the current value of C<$/> (also known as
28757baa
PP
589$INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
590number of characters removed from all its arguments. It's often used to
591remove the newline from the end of an input record when you're worried
2b5ab1e7
TC
592that the final record may be missing its newline. When in paragraph
593mode (C<$/ = "">), it removes all trailing newlines from the string.
4c5a6083
GS
594When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
595a reference to an integer or the like, see L<perlvar>) chomp() won't
19799a22
GS
596remove anything.
597If VARIABLE is omitted, it chomps C<$_>. Example:
a0d0e21e
LW
598
599 while (<>) {
600 chomp; # avoid \n on last field
601 @array = split(/:/);
5a964f20 602 # ...
a0d0e21e
LW
603 }
604
605You can actually chomp anything that's an lvalue, including an assignment:
606
607 chomp($cwd = `pwd`);
608 chomp($answer = <STDIN>);
609
610If you chomp a list, each element is chomped, and the total number of
611characters removed is returned.
612
613=item chop VARIABLE
614
615=item chop LIST
616
617=item chop
618
619Chops off the last character of a string and returns the character
620chopped. It's used primarily to remove the newline from the end of an
621input record, but is much more efficient than C<s/\n//> because it neither
7660c0ab 622scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
a0d0e21e
LW
623Example:
624
625 while (<>) {
626 chop; # avoid \n on last field
627 @array = split(/:/);
5a964f20 628 #...
a0d0e21e
LW
629 }
630
631You can actually chop anything that's an lvalue, including an assignment:
632
633 chop($cwd = `pwd`);
634 chop($answer = <STDIN>);
635
636If you chop a list, each element is chopped. Only the value of the
19799a22 637last C<chop> is returned.
a0d0e21e 638
19799a22 639Note that C<chop> returns the last character. To return all but the last
748a9306
LW
640character, use C<substr($string, 0, -1)>.
641
a0d0e21e
LW
642=item chown LIST
643
644Changes the owner (and group) of a list of files. The first two
19799a22
GS
645elements of the list must be the I<numeric> uid and gid, in that
646order. A value of -1 in either position is interpreted by most
647systems to leave that value unchanged. Returns the number of files
648successfully changed.
a0d0e21e
LW
649
650 $cnt = chown $uid, $gid, 'foo', 'bar';
651 chown $uid, $gid, @filenames;
652
54310121 653Here's an example that looks up nonnumeric uids in the passwd file:
a0d0e21e
LW
654
655 print "User: ";
19799a22 656 chomp($user = <STDIN>);
5a964f20 657 print "Files: ";
19799a22 658 chomp($pattern = <STDIN>);
a0d0e21e
LW
659
660 ($login,$pass,$uid,$gid) = getpwnam($user)
661 or die "$user not in passwd file";
662
5a964f20 663 @ary = glob($pattern); # expand filenames
a0d0e21e
LW
664 chown $uid, $gid, @ary;
665
54310121 666On most systems, you are not allowed to change the ownership of the
4633a7c4
LW
667file unless you're the superuser, although you should be able to change
668the group to any of your secondary groups. On insecure systems, these
669restrictions may be relaxed, but this is not a portable assumption.
19799a22
GS
670On POSIX systems, you can detect this condition this way:
671
672 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
673 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
4633a7c4 674
a0d0e21e
LW
675=item chr NUMBER
676
54310121 677=item chr
bbce6d69 678
a0d0e21e 679Returns the character represented by that NUMBER in the character set.
a0ed51b3 680For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
2b5ab1e7
TC
681chr(0x263a) is a Unicode smiley face (but only within the scope of
682a C<use utf8>). For the reverse, use L</ord>.
683See L<utf8> for more about Unicode.
a0d0e21e 684
7660c0ab 685If NUMBER is omitted, uses C<$_>.
bbce6d69 686
a0d0e21e
LW
687=item chroot FILENAME
688
54310121 689=item chroot
bbce6d69 690
5a964f20 691This function works like the system call by the same name: it makes the
4633a7c4 692named directory the new root directory for all further pathnames that
951ba7fe 693begin with a C</> by your process and all its children. (It doesn't
28757baa 694change your current working directory, which is unaffected.) For security
4633a7c4 695reasons, this call is restricted to the superuser. If FILENAME is
19799a22 696omitted, does a C<chroot> to C<$_>.
a0d0e21e
LW
697
698=item close FILEHANDLE
699
6a518fbc
TP
700=item close
701
19799a22 702Closes the file or pipe associated with the file handle, returning true
a0d0e21e 703only if stdio successfully flushes buffers and closes the system file
19799a22 704descriptor. Closes the currently selected filehandle if the argument
6a518fbc 705is omitted.
fb73857a
PP
706
707You don't have to close FILEHANDLE if you are immediately going to do
19799a22
GS
708another C<open> on it, because C<open> will close it for you. (See
709C<open>.) However, an explicit C<close> on an input file resets the line
710counter (C<$.>), while the implicit close done by C<open> does not.
fb73857a 711
19799a22
GS
712If the file handle came from a piped open C<close> will additionally
713return false if one of the other system calls involved fails or if the
fb73857a 714program exits with non-zero status. (If the only problem was that the
2b5ab1e7
TC
715program exited non-zero C<$!> will be set to C<0>.) Closing a pipe
716also waits for the process executing on the pipe to complete, in case you
717want to look at the output of the pipe afterwards, and
718implicitly puts the exit status value of that command into C<$?>.
5a964f20 719
73689b13
GS
720Prematurely closing the read end of a pipe (i.e. before the process
721writing to it at the other end has closed it) will result in a
722SIGPIPE being delivered to the writer. If the other end can't
723handle that, be sure to read all the data before closing the pipe.
724
fb73857a 725Example:
a0d0e21e 726
fb73857a
PP
727 open(OUTPUT, '|sort >foo') # pipe to sort
728 or die "Can't start sort: $!";
5a964f20 729 #... # print stuff to output
fb73857a
PP
730 close OUTPUT # wait for sort to finish
731 or warn $! ? "Error closing sort pipe: $!"
732 : "Exit status $? from sort";
733 open(INPUT, 'foo') # get sort's results
734 or die "Can't open 'foo' for input: $!";
a0d0e21e 735
5a964f20
TC
736FILEHANDLE may be an expression whose value can be used as an indirect
737filehandle, usually the real filehandle name.
a0d0e21e
LW
738
739=item closedir DIRHANDLE
740
19799a22 741Closes a directory opened by C<opendir> and returns the success of that
5a964f20
TC
742system call.
743
744DIRHANDLE may be an expression whose value can be used as an indirect
745dirhandle, usually the real dirhandle name.
a0d0e21e
LW
746
747=item connect SOCKET,NAME
748
749Attempts to connect to a remote socket, just as the connect system call
19799a22 750does. Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
751packed address of the appropriate type for the socket. See the examples in
752L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 753
cb1a09d0
AD
754=item continue BLOCK
755
756Actually a flow control statement rather than a function. If there is a
98293880
JH
757C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
758C<foreach>), it is always executed just before the conditional is about to
759be evaluated again, just like the third part of a C<for> loop in C. Thus
cb1a09d0
AD
760it can be used to increment a loop variable, even when the loop has been
761continued via the C<next> statement (which is similar to the C C<continue>
762statement).
763
98293880 764C<last>, C<next>, or C<redo> may appear within a C<continue>
19799a22
GS
765block. C<last> and C<redo> will behave as if they had been executed within
766the main block. So will C<next>, but since it will execute a C<continue>
1d2dff63
GS
767block, it may be more entertaining.
768
769 while (EXPR) {
770 ### redo always comes here
771 do_something;
772 } continue {
773 ### next always comes here
774 do_something_else;
775 # then back the top to re-check EXPR
776 }
777 ### last always comes here
778
779Omitting the C<continue> section is semantically equivalent to using an
19799a22 780empty one, logically enough. In that case, C<next> goes directly back
1d2dff63
GS
781to check the condition at the top of the loop.
782
a0d0e21e
LW
783=item cos EXPR
784
5a964f20 785Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
7660c0ab 786takes cosine of C<$_>.
a0d0e21e 787
ca6e1c26 788For the inverse cosine operation, you may use the C<Math::Trig::acos()>
28757baa
PP
789function, or use this relation:
790
791 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
792
a0d0e21e
LW
793=item crypt PLAINTEXT,SALT
794
f86cebdf 795Encrypts a string exactly like the crypt(3) function in the C library
4633a7c4
LW
796(assuming that you actually have a version there that has not been
797extirpated as a potential munition). This can prove useful for checking
798the password file for lousy passwords, amongst other things. Only the
799guys wearing white hats should do this.
a0d0e21e 800
19799a22 801Note that C<crypt> is intended to be a one-way function, much like breaking
11155c91
CS
802eggs to make an omelette. There is no (known) corresponding decrypt
803function. As a result, this function isn't all that useful for
804cryptography. (For that, see your nearby CPAN mirror.)
2f9daede 805
e71965be
RS
806When verifying an existing encrypted string you should use the encrypted
807text as the salt (like C<crypt($plain, $crypted) eq $crypted>). This
19799a22 808allows your code to work with the standard C<crypt> and with more
e71965be
RS
809exotic implementations. When choosing a new salt create a random two
810character string whose characters come from the set C<[./0-9A-Za-z]>
811(like C<join '', ('.', '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>).
812
a0d0e21e
LW
813Here's an example that makes sure that whoever runs this program knows
814their own password:
815
816 $pwd = (getpwuid($<))[1];
a0d0e21e
LW
817
818 system "stty -echo";
819 print "Password: ";
e71965be 820 chomp($word = <STDIN>);
a0d0e21e
LW
821 print "\n";
822 system "stty echo";
823
e71965be 824 if (crypt($word, $pwd) ne $pwd) {
a0d0e21e
LW
825 die "Sorry...\n";
826 } else {
827 print "ok\n";
54310121 828 }
a0d0e21e 829
9f8f0c9d 830Of course, typing in your own password to whoever asks you
748a9306 831for it is unwise.
a0d0e21e 832
19799a22
GS
833The L<crypt> function is unsuitable for encrypting large quantities
834of data, not least of all because you can't get the information
835back. Look at the F<by-module/Crypt> and F<by-module/PGP> directories
836on your favorite CPAN mirror for a slew of potentially useful
837modules.
838
aa689395 839=item dbmclose HASH
a0d0e21e 840
19799a22 841[This function has been largely superseded by the C<untie> function.]
a0d0e21e 842
aa689395 843Breaks the binding between a DBM file and a hash.
a0d0e21e 844
19799a22 845=item dbmopen HASH,DBNAME,MASK
a0d0e21e 846
19799a22 847[This function has been largely superseded by the C<tie> function.]
a0d0e21e 848
7b8d334a 849This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
19799a22
GS
850hash. HASH is the name of the hash. (Unlike normal C<open>, the first
851argument is I<not> a filehandle, even though it looks like one). DBNAME
aa689395
PP
852is the name of the database (without the F<.dir> or F<.pag> extension if
853any). If the database does not exist, it is created with protection
19799a22
GS
854specified by MASK (as modified by the C<umask>). If your system supports
855only the older DBM functions, you may perform only one C<dbmopen> in your
aa689395 856program. In older versions of Perl, if your system had neither DBM nor
19799a22 857ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
aa689395
PP
858sdbm(3).
859
860If you don't have write access to the DBM file, you can only read hash
861variables, not set them. If you want to test whether you can write,
19799a22 862either use file tests or try setting a dummy hash entry inside an C<eval>,
aa689395 863which will trap the error.
a0d0e21e 864
19799a22
GS
865Note that functions such as C<keys> and C<values> may return huge lists
866when used on large DBM files. You may prefer to use the C<each>
a0d0e21e
LW
867function to iterate over large DBM files. Example:
868
869 # print out history file offsets
870 dbmopen(%HIST,'/usr/lib/news/history',0666);
871 while (($key,$val) = each %HIST) {
872 print $key, ' = ', unpack('L',$val), "\n";
873 }
874 dbmclose(%HIST);
875
cb1a09d0 876See also L<AnyDBM_File> for a more general description of the pros and
184e9718 877cons of the various dbm approaches, as well as L<DB_File> for a particularly
cb1a09d0 878rich implementation.
4633a7c4 879
2b5ab1e7
TC
880You can control which DBM library you use by loading that library
881before you call dbmopen():
882
883 use DB_File;
884 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
885 or die "Can't open netscape history file: $!";
886
a0d0e21e
LW
887=item defined EXPR
888
54310121 889=item defined
bbce6d69 890
2f9daede
TPG
891Returns a Boolean value telling whether EXPR has a value other than
892the undefined value C<undef>. If EXPR is not present, C<$_> will be
893checked.
894
895Many operations return C<undef> to indicate failure, end of file,
896system error, uninitialized variable, and other exceptional
897conditions. This function allows you to distinguish C<undef> from
898other values. (A simple Boolean test will not distinguish among
7660c0ab 899C<undef>, zero, the empty string, and C<"0">, which are all equally
2f9daede 900false.) Note that since C<undef> is a valid scalar, its presence
19799a22 901doesn't I<necessarily> indicate an exceptional condition: C<pop>
2f9daede
TPG
902returns C<undef> when its argument is an empty array, I<or> when the
903element to return happens to be C<undef>.
904
f10b0346
GS
905You may also use C<defined(&func)> to check whether subroutine C<&func>
906has ever been defined. The return value is unaffected by any forward
907declarations of C<&foo>.
908
909Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
910used to report whether memory for that aggregate has ever been
911allocated. This behavior may disappear in future versions of Perl.
912You should instead use a simple test for size:
913
914 if (@an_array) { print "has array elements\n" }
915 if (%a_hash) { print "has hash members\n" }
2f9daede
TPG
916
917When used on a hash element, it tells you whether the value is defined,
dc848c6f 918not whether the key exists in the hash. Use L</exists> for the latter
2f9daede 919purpose.
a0d0e21e
LW
920
921Examples:
922
923 print if defined $switch{'D'};
924 print "$val\n" while defined($val = pop(@ary));
925 die "Can't readlink $sym: $!"
926 unless defined($value = readlink $sym);
a0d0e21e 927 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
2f9daede 928 $debugging = 0 unless defined $debugging;
a0d0e21e 929
19799a22 930Note: Many folks tend to overuse C<defined>, and then are surprised to
7660c0ab 931discover that the number C<0> and C<""> (the zero-length string) are, in fact,
2f9daede 932defined values. For example, if you say
a5f75d66
AD
933
934 "ab" =~ /a(.*)b/;
935
7660c0ab 936The pattern match succeeds, and C<$1> is defined, despite the fact that it
a5f75d66 937matched "nothing". But it didn't really match nothing--rather, it
2b5ab1e7 938matched something that happened to be zero characters long. This is all
a5f75d66 939very above-board and honest. When a function returns an undefined value,
2f9daede 940it's an admission that it couldn't give you an honest answer. So you
19799a22 941should use C<defined> only when you're questioning the integrity of what
7660c0ab 942you're trying to do. At other times, a simple comparison to C<0> or C<""> is
2f9daede
TPG
943what you want.
944
dc848c6f 945See also L</undef>, L</exists>, L</ref>.
2f9daede 946
a0d0e21e
LW
947=item delete EXPR
948
01020589
GS
949Given an expression that specifies a hash element, array element, hash slice,
950or array slice, deletes the specified element(s) from the hash or array.
8216c1fd
GS
951In the case of an array, if the array elements happen to be at the end,
952the size of the array will shrink to the highest element that tests
953true for exists() (or 0 if no such element exists).
a0d0e21e 954
01020589
GS
955Returns each element so deleted or the undefined value if there was no such
956element. Deleting from C<$ENV{}> modifies the environment. Deleting from
957a hash tied to a DBM file deletes the entry from the DBM file. Deleting
958from a C<tie>d hash or array may not necessarily return anything.
959
8ea97a1e
GS
960Deleting an array element effectively returns that position of the array
961to its initial, uninitialized state. Subsequently testing for the same
8216c1fd
GS
962element with exists() will return false. Note that deleting array
963elements in the middle of an array will not shift the index of the ones
964after them down--use splice() for that. See L</exists>.
8ea97a1e 965
01020589 966The following (inefficiently) deletes all the values of %HASH and @ARRAY:
a0d0e21e 967
5f05dabc
PP
968 foreach $key (keys %HASH) {
969 delete $HASH{$key};
a0d0e21e
LW
970 }
971
01020589
GS
972 foreach $index (0 .. $#ARRAY) {
973 delete $ARRAY[$index];
974 }
975
976And so do these:
5f05dabc 977
01020589
GS
978 delete @HASH{keys %HASH};
979
9740c838 980 delete @ARRAY[0 .. $#ARRAY];
5f05dabc 981
2b5ab1e7 982But both of these are slower than just assigning the empty list
01020589
GS
983or undefining %HASH or @ARRAY:
984
985 %HASH = (); # completely empty %HASH
986 undef %HASH; # forget %HASH ever existed
2b5ab1e7 987
01020589
GS
988 @ARRAY = (); # completely empty @ARRAY
989 undef @ARRAY; # forget @ARRAY ever existed
2b5ab1e7
TC
990
991Note that the EXPR can be arbitrarily complicated as long as the final
01020589
GS
992operation is a hash element, array element, hash slice, or array slice
993lookup:
a0d0e21e
LW
994
995 delete $ref->[$x][$y]{$key};
5f05dabc 996 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
a0d0e21e 997
01020589
GS
998 delete $ref->[$x][$y][$index];
999 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1000
a0d0e21e
LW
1001=item die LIST
1002
19799a22
GS
1003Outside an C<eval>, prints the value of LIST to C<STDERR> and
1004exits with the current value of C<$!> (errno). If C<$!> is C<0>,
61eff3bc
JH
1005exits with the value of C<<< ($? >> 8) >>> (backtick `command`
1006status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside
19799a22
GS
1007an C<eval(),> the error message is stuffed into C<$@> and the
1008C<eval> is terminated with the undefined value. This makes
1009C<die> the way to raise an exception.
a0d0e21e
LW
1010
1011Equivalent examples:
1012
1013 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
54310121 1014 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
a0d0e21e
LW
1015
1016If the value of EXPR does not end in a newline, the current script line
1017number and input line number (if any) are also printed, and a newline
883faa13
GS
1018is supplied. Note that the "input line number" (also known as "chunk")
1019is subject to whatever notion of "line" happens to be currently in
1020effect, and is also available as the special variable C<$.>.
1021See L<perlvar/"$/"> and L<perlvar/"$.">.
1022
1023Hint: sometimes appending C<", stopped"> to your message
7660c0ab 1024will cause it to make better sense when the string C<"at foo line 123"> is
a0d0e21e
LW
1025appended. Suppose you are running script "canasta".
1026
1027 die "/etc/games is no good";
1028 die "/etc/games is no good, stopped";
1029
1030produce, respectively
1031
1032 /etc/games is no good at canasta line 123.
1033 /etc/games is no good, stopped at canasta line 123.
1034
2b5ab1e7 1035See also exit(), warn(), and the Carp module.
a0d0e21e 1036
7660c0ab
A
1037If LIST is empty and C<$@> already contains a value (typically from a
1038previous eval) that value is reused after appending C<"\t...propagated">.
fb73857a
PP
1039This is useful for propagating exceptions:
1040
1041 eval { ... };
1042 die unless $@ =~ /Expected exception/;
1043
7660c0ab 1044If C<$@> is empty then the string C<"Died"> is used.
fb73857a 1045
52531d10
GS
1046die() can also be called with a reference argument. If this happens to be
1047trapped within an eval(), $@ contains the reference. This behavior permits
1048a more elaborate exception handling implementation using objects that
1049maintain arbitary state about the nature of the exception. Such a scheme
1050is sometimes preferable to matching particular string values of $@ using
1051regular expressions. Here's an example:
1052
1053 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1054 if ($@) {
1055 if (ref($@) && UNIVERSAL::isa($@,"Some::Module::Exception")) {
1056 # handle Some::Module::Exception
1057 }
1058 else {
1059 # handle all other possible exceptions
1060 }
1061 }
1062
19799a22 1063Because perl will stringify uncaught exception messages before displaying
52531d10
GS
1064them, you may want to overload stringification operations on such custom
1065exception objects. See L<overload> for details about that.
1066
19799a22
GS
1067You can arrange for a callback to be run just before the C<die>
1068does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1069handler will be called with the error text and can change the error
1070message, if it sees fit, by calling C<die> again. See
1071L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
1072L<"eval BLOCK"> for some examples. Although this feature was meant
1073to be run only right before your program was to exit, this is not
1074currently the case--the C<$SIG{__DIE__}> hook is currently called
1075even inside eval()ed blocks/strings! If one wants the hook to do
1076nothing in such situations, put
fb73857a
PP
1077
1078 die @_ if $^S;
1079
19799a22
GS
1080as the first line of the handler (see L<perlvar/$^S>). Because
1081this promotes strange action at a distance, this counterintuitive
1082behavior may be fixed in a future release.
774d564b 1083
a0d0e21e
LW
1084=item do BLOCK
1085
1086Not really a function. Returns the value of the last command in the
1087sequence of commands indicated by BLOCK. When modified by a loop
98293880
JH
1088modifier, executes the BLOCK once before testing the loop condition.
1089(On other statements the loop modifiers test the conditional first.)
a0d0e21e 1090
4968c1e4 1091C<do BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7
TC
1092C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1093See L<perlsyn> for alternative strategies.
4968c1e4 1094
a0d0e21e
LW
1095=item do SUBROUTINE(LIST)
1096
1097A deprecated form of subroutine call. See L<perlsub>.
1098
1099=item do EXPR
1100
1101Uses the value of EXPR as a filename and executes the contents of the
1102file as a Perl script. Its primary use is to include subroutines
1103from a Perl subroutine library.
1104
1105 do 'stat.pl';
1106
1107is just like
1108
fb73857a 1109 scalar eval `cat stat.pl`;
a0d0e21e 1110
2b5ab1e7
TC
1111except that it's more efficient and concise, keeps track of the current
1112filename for error messages, searches the @INC libraries, and updates
1113C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1114variables. It also differs in that code evaluated with C<do FILENAME>
1115cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1116same, however, in that it does reparse the file every time you call it,
1117so you probably don't want to do this inside a loop.
a0d0e21e 1118
8e30cc93 1119If C<do> cannot read the file, it returns undef and sets C<$!> to the
2b5ab1e7 1120error. If C<do> can read the file but cannot compile it, it
8e30cc93
G
1121returns undef and sets an error message in C<$@>. If the file is
1122successfully compiled, C<do> returns the value of the last expression
1123evaluated.
1124
a0d0e21e 1125Note that inclusion of library modules is better done with the
19799a22 1126C<use> and C<require> operators, which also do automatic error checking
4633a7c4 1127and raise an exception if there's a problem.
a0d0e21e 1128
5a964f20
TC
1129You might like to use C<do> to read in a program configuration
1130file. Manual error checking can be done this way:
1131
1132 # read in config files: system first, then user
f86cebdf 1133 for $file ("/share/prog/defaults.rc",
2b5ab1e7
TC
1134 "$ENV{HOME}/.someprogrc")
1135 {
5a964f20 1136 unless ($return = do $file) {
f86cebdf
GS
1137 warn "couldn't parse $file: $@" if $@;
1138 warn "couldn't do $file: $!" unless defined $return;
1139 warn "couldn't run $file" unless $return;
5a964f20
TC
1140 }
1141 }
1142
a0d0e21e
LW
1143=item dump LABEL
1144
1614b0e3
JD
1145=item dump
1146
19799a22
GS
1147This function causes an immediate core dump. See also the B<-u>
1148command-line switch in L<perlrun>, which does the same thing.
1149Primarily this is so that you can use the B<undump> program (not
1150supplied) to turn your core dump into an executable binary after
1151having initialized all your variables at the beginning of the
1152program. When the new binary is executed it will begin by executing
1153a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1154Think of it as a goto with an intervening core dump and reincarnation.
1155If C<LABEL> is omitted, restarts the program from the top.
1156
1157B<WARNING>: Any files opened at the time of the dump will I<not>
1158be open any more when the program is reincarnated, with possible
1159resulting confusion on the part of Perl.
1160
1161This function is now largely obsolete, partly because it's very
1162hard to convert a core file into an executable, and because the
1163real compiler backends for generating portable bytecode and compilable
1164C code have superseded it.
1165
1166If you're looking to use L<dump> to speed up your program, consider
1167generating bytecode or native C code as described in L<perlcc>. If
1168you're just trying to accelerate a CGI script, consider using the
1169C<mod_perl> extension to B<Apache>, or the CPAN module, Fast::CGI.
1170You might also consider autoloading or selfloading, which at least
1171make your program I<appear> to run faster.
5a964f20 1172
aa689395
PP
1173=item each HASH
1174
5a964f20 1175When called in list context, returns a 2-element list consisting of the
aa689395 1176key and value for the next element of a hash, so that you can iterate over
5a964f20 1177it. When called in scalar context, returns the key for only the "next"
e902a979 1178element in the hash.
2f9daede 1179
ab192400
GS
1180Entries are returned in an apparently random order. The actual random
1181order is subject to change in future versions of perl, but it is guaranteed
19799a22 1182to be in the same order as either the C<keys> or C<values> function
ab192400
GS
1183would produce on the same (unmodified) hash.
1184
1185When the hash is entirely read, a null array is returned in list context
19799a22
GS
1186(which when assigned produces a false (C<0>) value), and C<undef> in
1187scalar context. The next call to C<each> after that will start iterating
1188again. There is a single iterator for each hash, shared by all C<each>,
1189C<keys>, and C<values> function calls in the program; it can be reset by
2f9daede
TPG
1190reading all the elements from the hash, or by evaluating C<keys HASH> or
1191C<values HASH>. If you add or delete elements of a hash while you're
1192iterating over it, you may get entries skipped or duplicated, so don't.
aa689395 1193
f86cebdf 1194The following prints out your environment like the printenv(1) program,
aa689395 1195only in a different order:
a0d0e21e
LW
1196
1197 while (($key,$value) = each %ENV) {
1198 print "$key=$value\n";
1199 }
1200
19799a22 1201See also C<keys>, C<values> and C<sort>.
a0d0e21e
LW
1202
1203=item eof FILEHANDLE
1204
4633a7c4
LW
1205=item eof ()
1206
a0d0e21e
LW
1207=item eof
1208
1209Returns 1 if the next read on FILEHANDLE will return end of file, or if
1210FILEHANDLE is not open. FILEHANDLE may be an expression whose value
5a964f20 1211gives the real filehandle. (Note that this function actually
19799a22 1212reads a character and then C<ungetc>s it, so isn't very useful in an
748a9306 1213interactive context.) Do not read from a terminal file (or call
19799a22 1214C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
748a9306
LW
1215as terminals may lose the end-of-file condition if you do.
1216
820475bd
GS
1217An C<eof> without an argument uses the last file read. Using C<eof()>
1218with empty parentheses is very different. It refers to the pseudo file
1219formed from the files listed on the command line and accessed via the
61eff3bc
JH
1220C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1221as a normal filehandle is, an C<eof()> before C<< <> >> has been
820475bd
GS
1222used will cause C<@ARGV> to be examined to determine if input is
1223available.
1224
61eff3bc 1225In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
820475bd
GS
1226detect the end of each file, C<eof()> will only detect the end of the
1227last file. Examples:
a0d0e21e 1228
748a9306
LW
1229 # reset line numbering on each input file
1230 while (<>) {
5a964f20 1231 next if /^\s*#/; # skip comments
748a9306 1232 print "$.\t$_";
5a964f20
TC
1233 } continue {
1234 close ARGV if eof; # Not eof()!
748a9306
LW
1235 }
1236
a0d0e21e
LW
1237 # insert dashes just before last line of last file
1238 while (<>) {
5a964f20 1239 if (eof()) { # check for end of current file
a0d0e21e 1240 print "--------------\n";
2b5ab1e7 1241 close(ARGV); # close or last; is needed if we
748a9306 1242 # are reading from the terminal
a0d0e21e
LW
1243 }
1244 print;
1245 }
1246
a0d0e21e 1247Practical hint: you almost never need to use C<eof> in Perl, because the
3ce0d271
GS
1248input operators typically return C<undef> when they run out of data, or if
1249there was an error.
a0d0e21e
LW
1250
1251=item eval EXPR
1252
1253=item eval BLOCK
1254
c7cc6f1c
GS
1255In the first form, the return value of EXPR is parsed and executed as if it
1256were a little Perl program. The value of the expression (which is itself
5a964f20 1257determined within scalar context) is first parsed, and if there weren't any
c7cc6f1c 1258errors, executed in the context of the current Perl program, so that any
5f05dabc 1259variable settings or subroutine and format definitions remain afterwards.
c7cc6f1c
GS
1260Note that the value is parsed every time the eval executes. If EXPR is
1261omitted, evaluates C<$_>. This form is typically used to delay parsing
1262and subsequent execution of the text of EXPR until run time.
1263
1264In the second form, the code within the BLOCK is parsed only once--at the
1265same time the code surrounding the eval itself was parsed--and executed
1266within the context of the current Perl program. This form is typically
1267used to trap exceptions more efficiently than the first (see below), while
1268also providing the benefit of checking the code within BLOCK at compile
1269time.
1270
1271The final semicolon, if any, may be omitted from the value of EXPR or within
1272the BLOCK.
1273
1274In both forms, the value returned is the value of the last expression
5a964f20 1275evaluated inside the mini-program; a return statement may be also used, just
c7cc6f1c 1276as with subroutines. The expression providing the return value is evaluated
5a964f20 1277in void, scalar, or list context, depending on the context of the eval itself.
c7cc6f1c 1278See L</wantarray> for more on how the evaluation context can be determined.
a0d0e21e 1279
19799a22
GS
1280If there is a syntax error or runtime error, or a C<die> statement is
1281executed, an undefined value is returned by C<eval>, and C<$@> is set to the
a0d0e21e 1282error message. If there was no error, C<$@> is guaranteed to be a null
19799a22 1283string. Beware that using C<eval> neither silences perl from printing
c7cc6f1c
GS
1284warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1285To do either of those, you have to use the C<$SIG{__WARN__}> facility. See
1286L</warn> and L<perlvar>.
a0d0e21e 1287
19799a22
GS
1288Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1289determining whether a particular feature (such as C<socket> or C<symlink>)
a0d0e21e
LW
1290is implemented. It is also Perl's exception trapping mechanism, where
1291the die operator is used to raise exceptions.
1292
1293If the code to be executed doesn't vary, you may use the eval-BLOCK
1294form to trap run-time errors without incurring the penalty of
1295recompiling each time. The error, if any, is still returned in C<$@>.
1296Examples:
1297
54310121 1298 # make divide-by-zero nonfatal
a0d0e21e
LW
1299 eval { $answer = $a / $b; }; warn $@ if $@;
1300
1301 # same thing, but less efficient
1302 eval '$answer = $a / $b'; warn $@ if $@;
1303
1304 # a compile-time error
5a964f20 1305 eval { $answer = }; # WRONG
a0d0e21e
LW
1306
1307 # a run-time error
1308 eval '$answer ='; # sets $@
1309
2b5ab1e7
TC
1310Due to the current arguably broken state of C<__DIE__> hooks, when using
1311the C<eval{}> form as an exception trap in libraries, you may wish not
1312to trigger any C<__DIE__> hooks that user code may have installed.
1313You can use the C<local $SIG{__DIE__}> construct for this purpose,
1314as shown in this example:
774d564b
PP
1315
1316 # a very private exception trap for divide-by-zero
f86cebdf
GS
1317 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1318 warn $@ if $@;
774d564b
PP
1319
1320This is especially significant, given that C<__DIE__> hooks can call
19799a22 1321C<die> again, which has the effect of changing their error messages:
774d564b
PP
1322
1323 # __DIE__ hooks may modify error messages
1324 {
f86cebdf
GS
1325 local $SIG{'__DIE__'} =
1326 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
c7cc6f1c
GS
1327 eval { die "foo lives here" };
1328 print $@ if $@; # prints "bar lives here"
774d564b
PP
1329 }
1330
19799a22 1331Because this promotes action at a distance, this counterintuitive behavior
2b5ab1e7
TC
1332may be fixed in a future release.
1333
19799a22 1334With an C<eval>, you should be especially careful to remember what's
a0d0e21e
LW
1335being looked at when:
1336
1337 eval $x; # CASE 1
1338 eval "$x"; # CASE 2
1339
1340 eval '$x'; # CASE 3
1341 eval { $x }; # CASE 4
1342
5a964f20 1343 eval "\$$x++"; # CASE 5
a0d0e21e
LW
1344 $$x++; # CASE 6
1345
2f9daede 1346Cases 1 and 2 above behave identically: they run the code contained in
19799a22 1347the variable $x. (Although case 2 has misleading double quotes making
2f9daede 1348the reader wonder what else might be happening (nothing is).) Cases 3
7660c0ab 1349and 4 likewise behave in the same way: they run the code C<'$x'>, which
19799a22 1350does nothing but return the value of $x. (Case 4 is preferred for
2f9daede
TPG
1351purely visual reasons, but it also has the advantage of compiling at
1352compile-time instead of at run-time.) Case 5 is a place where
19799a22 1353normally you I<would> like to use double quotes, except that in this
2f9daede
TPG
1354particular situation, you can just use symbolic references instead, as
1355in case 6.
a0d0e21e 1356
4968c1e4 1357C<eval BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7 1358C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
4968c1e4 1359
a0d0e21e
LW
1360=item exec LIST
1361
8bf3b016
GS
1362=item exec PROGRAM LIST
1363
19799a22
GS
1364The C<exec> function executes a system command I<and never returns>--
1365use C<system> instead of C<exec> if you want it to return. It fails and
1366returns false only if the command does not exist I<and> it is executed
fb73857a 1367directly instead of via your system's command shell (see below).
a0d0e21e 1368
19799a22
GS
1369Since it's a common mistake to use C<exec> instead of C<system>, Perl
1370warns you if there is a following statement which isn't C<die>, C<warn>,
1371or C<exit> (if C<-w> is set - but you always do that). If you
1372I<really> want to follow an C<exec> with some other statement, you
55d729e4
GS
1373can use one of these styles to avoid the warning:
1374
5a964f20
TC
1375 exec ('foo') or print STDERR "couldn't exec foo: $!";
1376 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
55d729e4 1377
5a964f20 1378If there is more than one argument in LIST, or if LIST is an array
f86cebdf 1379with more than one value, calls execvp(3) with the arguments in LIST.
5a964f20
TC
1380If there is only one scalar argument or an array with one element in it,
1381the argument is checked for shell metacharacters, and if there are any,
1382the entire argument is passed to the system's command shell for parsing
1383(this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1384If there are no shell metacharacters in the argument, it is split into
19799a22
GS
1385words and passed directly to C<execvp>, which is more efficient.
1386Examples:
a0d0e21e 1387
19799a22
GS
1388 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1389 exec "sort $outfile | uniq";
a0d0e21e
LW
1390
1391If you don't really want to execute the first argument, but want to lie
1392to the program you are executing about its own name, you can specify
1393the program you actually want to run as an "indirect object" (without a
1394comma) in front of the LIST. (This always forces interpretation of the
54310121 1395LIST as a multivalued list, even if there is only a single scalar in
a0d0e21e
LW
1396the list.) Example:
1397
1398 $shell = '/bin/csh';
1399 exec $shell '-sh'; # pretend it's a login shell
1400
1401or, more directly,
1402
1403 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1404
bb32b41a
GS
1405When the arguments get executed via the system shell, results will
1406be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1407for details.
1408
19799a22
GS
1409Using an indirect object with C<exec> or C<system> is also more
1410secure. This usage (which also works fine with system()) forces
1411interpretation of the arguments as a multivalued list, even if the
1412list had just one argument. That way you're safe from the shell
1413expanding wildcards or splitting up words with whitespace in them.
5a964f20
TC
1414
1415 @args = ( "echo surprise" );
1416
2b5ab1e7 1417 exec @args; # subject to shell escapes
f86cebdf 1418 # if @args == 1
2b5ab1e7 1419 exec { $args[0] } @args; # safe even with one-arg list
5a964f20
TC
1420
1421The first version, the one without the indirect object, ran the I<echo>
1422program, passing it C<"surprise"> an argument. The second version
1423didn't--it tried to run a program literally called I<"echo surprise">,
1424didn't find it, and set C<$?> to a non-zero value indicating failure.
1425
19799a22 1426Note that C<exec> will not call your C<END> blocks, nor will it call
7660c0ab
A
1427any C<DESTROY> methods in your objects.
1428
a0d0e21e
LW
1429=item exists EXPR
1430
01020589 1431Given an expression that specifies a hash element or array element,
8ea97a1e
GS
1432returns true if the specified element in the hash or array has ever
1433been initialized, even if the corresponding value is undefined. The
1434element is not autovivified if it doesn't exist.
a0d0e21e 1435
01020589
GS
1436 print "Exists\n" if exists $hash{$key};
1437 print "Defined\n" if defined $hash{$key};
1438 print "True\n" if $hash{$key};
1439
1440 print "Exists\n" if exists $array[$index];
1441 print "Defined\n" if defined $array[$index];
1442 print "True\n" if $array[$index];
a0d0e21e 1443
8ea97a1e 1444A hash or array element can be true only if it's defined, and defined if
a0d0e21e
LW
1445it exists, but the reverse doesn't necessarily hold true.
1446
afebc493
GS
1447Given an expression that specifies the name of a subroutine,
1448returns true if the specified subroutine has ever been declared, even
1449if it is undefined. Mentioning a subroutine name for exists or defined
1450does not count as declaring it.
1451
1452 print "Exists\n" if exists &subroutine;
1453 print "Defined\n" if defined &subroutine;
1454
a0d0e21e 1455Note that the EXPR can be arbitrarily complicated as long as the final
afebc493 1456operation is a hash or array key lookup or subroutine name:
a0d0e21e 1457
2b5ab1e7
TC
1458 if (exists $ref->{A}->{B}->{$key}) { }
1459 if (exists $hash{A}{B}{$key}) { }
1460
01020589
GS
1461 if (exists $ref->{A}->{B}->[$ix]) { }
1462 if (exists $hash{A}{B}[$ix]) { }
1463
afebc493
GS
1464 if (exists &{$ref->{A}{B}{$key}}) { }
1465
01020589
GS
1466Although the deepest nested array or hash will not spring into existence
1467just because its existence was tested, any intervening ones will.
61eff3bc 1468Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
01020589
GS
1469into existence due to the existence test for the $key element above.
1470This happens anywhere the arrow operator is used, including even:
5a964f20 1471
2b5ab1e7
TC
1472 undef $ref;
1473 if (exists $ref->{"Some key"}) { }
1474 print $ref; # prints HASH(0x80d3d5c)
1475
1476This surprising autovivification in what does not at first--or even
1477second--glance appear to be an lvalue context may be fixed in a future
5a964f20 1478release.
a0d0e21e 1479
479ba383
GS
1480See L<perlref/"Pseudo-hashes: Using an array as a hash"> for specifics
1481on how exists() acts when used on a pseudo-hash.
e0478e5a 1482
afebc493
GS
1483Use of a subroutine call, rather than a subroutine name, as an argument
1484to exists() is an error.
1485
1486 exists &sub; # OK
1487 exists &sub(); # Error
1488
a0d0e21e
LW
1489=item exit EXPR
1490
2b5ab1e7 1491Evaluates EXPR and exits immediately with that value. Example:
a0d0e21e
LW
1492
1493 $ans = <STDIN>;
1494 exit 0 if $ans =~ /^[Xx]/;
1495
19799a22 1496See also C<die>. If EXPR is omitted, exits with C<0> status. The only
2b5ab1e7
TC
1497universally recognized values for EXPR are C<0> for success and C<1>
1498for error; other values are subject to interpretation depending on the
1499environment in which the Perl program is running. For example, exiting
150069 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1501the mailer to return the item undelivered, but that's not true everywhere.
a0d0e21e 1502
19799a22
GS
1503Don't use C<exit> to abort a subroutine if there's any chance that
1504someone might want to trap whatever error happened. Use C<die> instead,
1505which can be trapped by an C<eval>.
28757baa 1506
19799a22 1507The exit() function does not always exit immediately. It calls any
2b5ab1e7 1508defined C<END> routines first, but these C<END> routines may not
19799a22 1509themselves abort the exit. Likewise any object destructors that need to
2b5ab1e7
TC
1510be called are called before the real exit. If this is a problem, you
1511can call C<POSIX:_exit($status)> to avoid END and destructor processing.
87275199 1512See L<perlmod> for details.
5a964f20 1513
a0d0e21e
LW
1514=item exp EXPR
1515
54310121 1516=item exp
bbce6d69 1517
2b5ab1e7 1518Returns I<e> (the natural logarithm base) to the power of EXPR.
a0d0e21e
LW
1519If EXPR is omitted, gives C<exp($_)>.
1520
1521=item fcntl FILEHANDLE,FUNCTION,SCALAR
1522
f86cebdf 1523Implements the fcntl(2) function. You'll probably have to say
a0d0e21e
LW
1524
1525 use Fcntl;
1526
0ade1984 1527first to get the correct constant definitions. Argument processing and
19799a22 1528value return works just like C<ioctl> below.
a0d0e21e
LW
1529For example:
1530
1531 use Fcntl;
5a964f20
TC
1532 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1533 or die "can't fcntl F_GETFL: $!";
1534
19799a22 1535You don't have to check for C<defined> on the return from C<fnctl>.
951ba7fe
GS
1536Like C<ioctl>, it maps a C<0> return from the system call into
1537C<"0 but true"> in Perl. This string is true in boolean context and C<0>
2b5ab1e7
TC
1538in numeric context. It is also exempt from the normal B<-w> warnings
1539on improper numeric conversions.
5a964f20 1540
19799a22 1541Note that C<fcntl> will produce a fatal error if used on a machine that
2b5ab1e7
TC
1542doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1543manpage to learn what functions are available on your system.
a0d0e21e
LW
1544
1545=item fileno FILEHANDLE
1546
2b5ab1e7
TC
1547Returns the file descriptor for a filehandle, or undefined if the
1548filehandle is not open. This is mainly useful for constructing
19799a22 1549bitmaps for C<select> and low-level POSIX tty-handling operations.
2b5ab1e7
TC
1550If FILEHANDLE is an expression, the value is taken as an indirect
1551filehandle, generally its name.
5a964f20
TC
1552
1553You can use this to find out whether two handles refer to the
1554same underlying descriptor:
1555
1556 if (fileno(THIS) == fileno(THAT)) {
1557 print "THIS and THAT are dups\n";
1558 }
a0d0e21e
LW
1559
1560=item flock FILEHANDLE,OPERATION
1561
19799a22
GS
1562Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1563for success, false on failure. Produces a fatal error if used on a
2b5ab1e7 1564machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
19799a22 1565C<flock> is Perl's portable file locking interface, although it locks
2b5ab1e7
TC
1566only entire files, not records.
1567
1568Two potentially non-obvious but traditional C<flock> semantics are
1569that it waits indefinitely until the lock is granted, and that its locks
1570B<merely advisory>. Such discretionary locks are more flexible, but offer
19799a22
GS
1571fewer guarantees. This means that files locked with C<flock> may be
1572modified by programs that do not also use C<flock>. See L<perlport>,
2b5ab1e7
TC
1573your port's specific documentation, or your system-specific local manpages
1574for details. It's best to assume traditional behavior if you're writing
1575portable programs. (But if you're not, you should as always feel perfectly
1576free to write for your own system's idiosyncrasies (sometimes called
1577"features"). Slavish adherence to portability concerns shouldn't get
1578in the way of your getting your job done.)
a3cb178b 1579
8ebc5c01
PP
1580OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1581LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
ea3105be 1582you can use the symbolic names if you import them from the Fcntl module,
68dc0745
PP
1583either individually, or as a group using the ':flock' tag. LOCK_SH
1584requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
ea3105be
GS
1585releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1586LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
68dc0745
PP
1587waiting for the lock (check the return status to see if you got it).
1588
2b5ab1e7
TC
1589To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1590before locking or unlocking it.
8ebc5c01 1591
f86cebdf 1592Note that the emulation built with lockf(3) doesn't provide shared
8ebc5c01 1593locks, and it requires that FILEHANDLE be open with write intent. These
2b5ab1e7 1594are the semantics that lockf(3) implements. Most if not all systems
f86cebdf 1595implement lockf(3) in terms of fcntl(2) locking, though, so the
8ebc5c01
PP
1596differing semantics shouldn't bite too many people.
1597
19799a22
GS
1598Note also that some versions of C<flock> cannot lock things over the
1599network; you would need to use the more system-specific C<fcntl> for
f86cebdf
GS
1600that. If you like you can force Perl to ignore your system's flock(2)
1601function, and so provide its own fcntl(2)-based emulation, by passing
8ebc5c01
PP
1602the switch C<-Ud_flock> to the F<Configure> program when you configure
1603perl.
4633a7c4
LW
1604
1605Here's a mailbox appender for BSD systems.
a0d0e21e 1606
7e1af8bc 1607 use Fcntl ':flock'; # import LOCK_* constants
a0d0e21e
LW
1608
1609 sub lock {
7e1af8bc 1610 flock(MBOX,LOCK_EX);
a0d0e21e
LW
1611 # and, in case someone appended
1612 # while we were waiting...
1613 seek(MBOX, 0, 2);
1614 }
1615
1616 sub unlock {
7e1af8bc 1617 flock(MBOX,LOCK_UN);
a0d0e21e
LW
1618 }
1619
1620 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1621 or die "Can't open mailbox: $!";
1622
1623 lock();
1624 print MBOX $msg,"\n\n";
1625 unlock();
1626
2b5ab1e7
TC
1627On systems that support a real flock(), locks are inherited across fork()
1628calls, whereas those that must resort to the more capricious fcntl()
1629function lose the locks, making it harder to write servers.
1630
cb1a09d0 1631See also L<DB_File> for other flock() examples.
a0d0e21e
LW
1632
1633=item fork
1634
2b5ab1e7
TC
1635Does a fork(2) system call to create a new process running the
1636same program at the same point. It returns the child pid to the
1637parent process, C<0> to the child process, or C<undef> if the fork is
1638unsuccessful. File descriptors (and sometimes locks on those descriptors)
1639are shared, while everything else is copied. On most systems supporting
1640fork(), great care has gone into making it extremely efficient (for
1641example, using copy-on-write technology on data pages), making it the
1642dominant paradigm for multitasking over the last few decades.
5a964f20 1643
45bc9206 1644All files opened for output are flushed before forking the child process.
a0d0e21e 1645
19799a22 1646If you C<fork> without ever waiting on your children, you will
2b5ab1e7
TC
1647accumulate zombies. On some systems, you can avoid this by setting
1648C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1649forking and reaping moribund children.
cb1a09d0 1650
28757baa
PP
1651Note that if your forked child inherits system file descriptors like
1652STDIN and STDOUT that are actually connected by a pipe or socket, even
2b5ab1e7 1653if you exit, then the remote server (such as, say, a CGI script or a
19799a22 1654backgrounded job launched from a remote shell) won't think you're done.
2b5ab1e7 1655You should reopen those to F</dev/null> if it's any issue.
28757baa 1656
cb1a09d0
AD
1657=item format
1658
19799a22 1659Declare a picture format for use by the C<write> function. For
cb1a09d0
AD
1660example:
1661
54310121 1662 format Something =
cb1a09d0
AD
1663 Test: @<<<<<<<< @||||| @>>>>>
1664 $str, $%, '$' . int($num)
1665 .
1666
1667 $str = "widget";
184e9718 1668 $num = $cost/$quantity;
cb1a09d0
AD
1669 $~ = 'Something';
1670 write;
1671
1672See L<perlform> for many details and examples.
1673
8903cb82 1674=item formline PICTURE,LIST
a0d0e21e 1675
5a964f20 1676This is an internal function used by C<format>s, though you may call it,
a0d0e21e
LW
1677too. It formats (see L<perlform>) a list of values according to the
1678contents of PICTURE, placing the output into the format output
7660c0ab 1679accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
19799a22 1680Eventually, when a C<write> is done, the contents of
a0d0e21e 1681C<$^A> are written to some filehandle, but you could also read C<$^A>
7660c0ab 1682yourself and then set C<$^A> back to C<"">. Note that a format typically
19799a22 1683does one C<formline> per line of form, but the C<formline> function itself
748a9306 1684doesn't care how many newlines are embedded in the PICTURE. This means
4633a7c4 1685that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
748a9306
LW
1686You may therefore need to use multiple formlines to implement a single
1687record format, just like the format compiler.
1688
19799a22 1689Be careful if you put double quotes around the picture, because an C<@>
748a9306 1690character may be taken to mean the beginning of an array name.
19799a22 1691C<formline> always returns true. See L<perlform> for other examples.
a0d0e21e
LW
1692
1693=item getc FILEHANDLE
1694
1695=item getc
1696
1697Returns the next character from the input file attached to FILEHANDLE,
2b5ab1e7
TC
1698or the undefined value at end of file, or if there was an error.
1699If FILEHANDLE is omitted, reads from STDIN. This is not particularly
1700efficient. However, it cannot be used by itself to fetch single
1701characters without waiting for the user to hit enter. For that, try
1702something more like:
4633a7c4
LW
1703
1704 if ($BSD_STYLE) {
1705 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1706 }
1707 else {
54310121 1708 system "stty", '-icanon', 'eol', "\001";
4633a7c4
LW
1709 }
1710
1711 $key = getc(STDIN);
1712
1713 if ($BSD_STYLE) {
1714 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1715 }
1716 else {
5f05dabc 1717 system "stty", 'icanon', 'eol', '^@'; # ASCII null
4633a7c4
LW
1718 }
1719 print "\n";
1720
54310121
PP
1721Determination of whether $BSD_STYLE should be set
1722is left as an exercise to the reader.
cb1a09d0 1723
19799a22 1724The C<POSIX::getattr> function can do this more portably on
2b5ab1e7
TC
1725systems purporting POSIX compliance. See also the C<Term::ReadKey>
1726module from your nearest CPAN site; details on CPAN can be found on
1727L<perlmodlib/CPAN>.
a0d0e21e
LW
1728
1729=item getlogin
1730
5a964f20
TC
1731Implements the C library function of the same name, which on most
1732systems returns the current login from F</etc/utmp>, if any. If null,
19799a22 1733use C<getpwuid>.
a0d0e21e 1734
f86702cc 1735 $login = getlogin || getpwuid($<) || "Kilroy";
a0d0e21e 1736
19799a22
GS
1737Do not consider C<getlogin> for authentication: it is not as
1738secure as C<getpwuid>.
4633a7c4 1739
a0d0e21e
LW
1740=item getpeername SOCKET
1741
1742Returns the packed sockaddr address of other end of the SOCKET connection.
1743
4633a7c4
LW
1744 use Socket;
1745 $hersockaddr = getpeername(SOCK);
19799a22 1746 ($port, $iaddr) = sockaddr_in($hersockaddr);
4633a7c4
LW
1747 $herhostname = gethostbyaddr($iaddr, AF_INET);
1748 $herstraddr = inet_ntoa($iaddr);
a0d0e21e
LW
1749
1750=item getpgrp PID
1751
47e29363 1752Returns the current process group for the specified PID. Use
7660c0ab 1753a PID of C<0> to get the current process group for the
4633a7c4 1754current process. Will raise an exception if used on a machine that
f86cebdf 1755doesn't implement getpgrp(2). If PID is omitted, returns process
19799a22 1756group of current process. Note that the POSIX version of C<getpgrp>
7660c0ab 1757does not accept a PID argument, so only C<PID==0> is truly portable.
a0d0e21e
LW
1758
1759=item getppid
1760
1761Returns the process id of the parent process.
1762
1763=item getpriority WHICH,WHO
1764
4633a7c4
LW
1765Returns the current priority for a process, a process group, or a user.
1766(See L<getpriority(2)>.) Will raise a fatal exception if used on a
f86cebdf 1767machine that doesn't implement getpriority(2).
a0d0e21e
LW
1768
1769=item getpwnam NAME
1770
1771=item getgrnam NAME
1772
1773=item gethostbyname NAME
1774
1775=item getnetbyname NAME
1776
1777=item getprotobyname NAME
1778
1779=item getpwuid UID
1780
1781=item getgrgid GID
1782
1783=item getservbyname NAME,PROTO
1784
1785=item gethostbyaddr ADDR,ADDRTYPE
1786
1787=item getnetbyaddr ADDR,ADDRTYPE
1788
1789=item getprotobynumber NUMBER
1790
1791=item getservbyport PORT,PROTO
1792
1793=item getpwent
1794
1795=item getgrent
1796
1797=item gethostent
1798
1799=item getnetent
1800
1801=item getprotoent
1802
1803=item getservent
1804
1805=item setpwent
1806
1807=item setgrent
1808
1809=item sethostent STAYOPEN
1810
1811=item setnetent STAYOPEN
1812
1813=item setprotoent STAYOPEN
1814
1815=item setservent STAYOPEN
1816
1817=item endpwent
1818
1819=item endgrent
1820
1821=item endhostent
1822
1823=item endnetent
1824
1825=item endprotoent
1826
1827=item endservent
1828
1829These routines perform the same functions as their counterparts in the
5a964f20 1830system library. In list context, the return values from the
a0d0e21e
LW
1831various get routines are as follows:
1832
1833 ($name,$passwd,$uid,$gid,
6ee623d5 1834 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
a0d0e21e
LW
1835 ($name,$passwd,$gid,$members) = getgr*
1836 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1837 ($name,$aliases,$addrtype,$net) = getnet*
1838 ($name,$aliases,$proto) = getproto*
1839 ($name,$aliases,$port,$proto) = getserv*
1840
1841(If the entry doesn't exist you get a null list.)
1842
5a964f20 1843In scalar context, you get the name, unless the function was a
a0d0e21e
LW
1844lookup by name, in which case you get the other thing, whatever it is.
1845(If the entry doesn't exist you get the undefined value.) For example:
1846
5a964f20
TC
1847 $uid = getpwnam($name);
1848 $name = getpwuid($num);
1849 $name = getpwent();
1850 $gid = getgrnam($name);
1851 $name = getgrgid($num;
1852 $name = getgrent();
1853 #etc.
a0d0e21e 1854
19799a22 1855In I<getpw*()> the fields $quota, $comment, and $expire are
2b5ab1e7 1856special cases in the sense that in many systems they are unsupported.
19799a22
GS
1857If the $quota is unsupported, it is an empty scalar. If it is
1858supported, it usually encodes the disk quota. If the $comment
2b5ab1e7
TC
1859field is unsupported, it is an empty scalar. If it is supported it
1860usually encodes some administrative comment about the user. In some
19799a22
GS
1861systems the $quota field may be $change or $age, fields that have
1862to do with password aging. In some systems the $comment field may
1863be $class. The $expire field, if present, encodes the expiration
2b5ab1e7
TC
1864period of the account or the password. For the availability and the
1865exact meaning of these fields in your system, please consult your
1866getpwnam(3) documentation and your F<pwd.h> file. You can also find
19799a22
GS
1867out from within Perl what your $quota and $comment fields mean
1868and whether you have the $expire field by using the C<Config> module
2b5ab1e7
TC
1869and the values C<d_pwquota>, C<d_pwage>, C<d_pwchange>, C<d_pwcomment>,
1870and C<d_pwexpire>. Shadow password files are only supported if your
1871vendor has implemented them in the intuitive fashion that calling the
1872regular C library routines gets the shadow versions if you're running
1873under privilege. Those that incorrectly implement a separate library
1874call are not supported.
6ee623d5 1875
19799a22 1876The $members value returned by I<getgr*()> is a space separated list of
a0d0e21e
LW
1877the login names of the members of the group.
1878
1879For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1880C, it will be returned to you via C<$?> if the function call fails. The
7660c0ab 1881C<@addrs> value returned by a successful call is a list of the raw
a0d0e21e
LW
1882addresses returned by the corresponding system library call. In the
1883Internet domain, each address is four bytes long and you can unpack it
1884by saying something like:
1885
1886 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1887
2b5ab1e7
TC
1888The Socket library makes this slightly easier:
1889
1890 use Socket;
1891 $iaddr = inet_aton("127.1"); # or whatever address
1892 $name = gethostbyaddr($iaddr, AF_INET);
1893
1894 # or going the other way
19799a22 1895 $straddr = inet_ntoa($iaddr);
2b5ab1e7 1896
19799a22
GS
1897If you get tired of remembering which element of the return list
1898contains which return value, by-name interfaces are provided
1899in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
1900C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
1901and C<User::grent>. These override the normal built-ins, supplying
1902versions that return objects with the appropriate names
1903for each field. For example:
5a964f20
TC
1904
1905 use File::stat;
1906 use User::pwent;
1907 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
1908
1909Even though it looks like they're the same method calls (uid),
19799a22
GS
1910they aren't, because a C<File::stat> object is different from
1911a C<User::pwent> object.
5a964f20 1912
a0d0e21e
LW
1913=item getsockname SOCKET
1914
19799a22
GS
1915Returns the packed sockaddr address of this end of the SOCKET connection,
1916in case you don't know the address because you have several different
1917IPs that the connection might have come in on.
a0d0e21e 1918
4633a7c4
LW
1919 use Socket;
1920 $mysockaddr = getsockname(SOCK);
19799a22
GS
1921 ($port, $myaddr) = sockaddr_in($mysockaddr);
1922 printf "Connect to %s [%s]\n",
1923 scalar gethostbyaddr($myaddr, AF_INET),
1924 inet_ntoa($myaddr);
a0d0e21e
LW
1925
1926=item getsockopt SOCKET,LEVEL,OPTNAME
1927
5a964f20 1928Returns the socket option requested, or undef if there is an error.
a0d0e21e
LW
1929
1930=item glob EXPR
1931
0a753a76
PP
1932=item glob
1933
2b5ab1e7
TC
1934Returns the value of EXPR with filename expansions such as the
1935standard Unix shell F</bin/csh> would do. This is the internal function
61eff3bc
JH
1936implementing the C<< <*.c> >> operator, but you can use it directly.
1937If EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is
2b5ab1e7 1938discussed in more detail in L<perlop/"I/O Operators">.
a0d0e21e 1939
3a4b19e4
GS
1940Beginning with v5.6.0, this operator is implemented using the standard
1941C<File::Glob> extension. See L<File::Glob> for details.
1942
a0d0e21e
LW
1943=item gmtime EXPR
1944
19799a22 1945Converts a time as returned by the time function to a 9-element list
54310121 1946with the time localized for the standard Greenwich time zone.
4633a7c4 1947Typically used as follows:
a0d0e21e 1948
54310121 1949 # 0 1 2 3 4 5 6 7 8
a0d0e21e
LW
1950 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1951 gmtime(time);
1952
19799a22
GS
1953All list elements are numeric, and come straight out of a struct tm.
1954In particular this means that $mon has the range C<0..11> and $wday
1955has the range C<0..6> with sunday as day C<0>. Also, $year is the
1956number of years since 1900, that is, $year is C<123> in year 2023,
2b5ab1e7
TC
1957I<not> simply the last two digits of the year. If you assume it is,
1958then you create non-Y2K-compliant programs--and you wouldn't want to do
1959that, would you?
2f9daede 1960
abd75f24
GS
1961The proper way to get a complete 4-digit year is simply:
1962
1963 $year += 1900;
1964
1965And to get the last two digits of the year (e.g., '01' in 2001) do:
1966
1967 $year = sprintf("%02d", $year % 100);
1968
2f9daede 1969If EXPR is omitted, does C<gmtime(time())>.
a0d0e21e 1970
f86cebdf 1971In scalar context, returns the ctime(3) value:
0a753a76
PP
1972
1973 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
1974
19799a22 1975Also see the C<timegm> function provided by the C<Time::Local> module,
f86cebdf 1976and the strftime(3) function available via the POSIX module.
7660c0ab 1977
2b5ab1e7
TC
1978This scalar value is B<not> locale dependent (see L<perllocale>), but
1979is instead a Perl builtin. Also see the C<Time::Local> module, and the
1980strftime(3) and mktime(3) functions available via the POSIX module. To
7660c0ab
A
1981get somewhat similar but locale dependent date strings, set up your
1982locale environment variables appropriately (please see L<perllocale>)
1983and try for example:
1984
1985 use POSIX qw(strftime);
2b5ab1e7 1986 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
7660c0ab 1987
2b5ab1e7
TC
1988Note that the C<%a> and C<%b> escapes, which represent the short forms
1989of the day of the week and the month of the year, may not necessarily
1990be three characters wide in all locales.
0a753a76 1991
a0d0e21e
LW
1992=item goto LABEL
1993
748a9306
LW
1994=item goto EXPR
1995
a0d0e21e
LW
1996=item goto &NAME
1997
7660c0ab 1998The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
a0d0e21e 1999execution there. It may not be used to go into any construct that
7660c0ab 2000requires initialization, such as a subroutine or a C<foreach> loop. It
0a753a76 2001also can't be used to go into a construct that is optimized away,
19799a22 2002or to get out of a block or subroutine given to C<sort>.
0a753a76 2003It can be used to go almost anywhere else within the dynamic scope,
a0d0e21e 2004including out of subroutines, but it's usually better to use some other
19799a22 2005construct such as C<last> or C<die>. The author of Perl has never felt the
7660c0ab 2006need to use this form of C<goto> (in Perl, that is--C is another matter).
a0d0e21e 2007
7660c0ab
A
2008The C<goto-EXPR> form expects a label name, whose scope will be resolved
2009dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
748a9306
LW
2010necessarily recommended if you're optimizing for maintainability:
2011
2012 goto ("FOO", "BAR", "GLARCH")[$i];
2013
6cb9131c
GS
2014The C<goto-&NAME> form is quite different from the other forms of C<goto>.
2015In fact, it isn't a goto in the normal sense at all, and doesn't have
2016the stigma associated with other gotos. Instead, it
2017substitutes a call to the named subroutine for the currently running
2018subroutine. This is used by C<AUTOLOAD> subroutines that wish to load
2019another subroutine and then pretend that the other subroutine had been
2020called in the first place (except that any modifications to C<@_>
2021in the current subroutine are propagated to the other subroutine.)
2022After the C<goto>, not even C<caller> will be able to tell that this
2023routine was called first.
2024
2025NAME needn't be the name of a subroutine; it can be a scalar variable
2026containing a code reference, or a block which evaluates to a code
2027reference.
a0d0e21e
LW
2028
2029=item grep BLOCK LIST
2030
2031=item grep EXPR,LIST
2032
2b5ab1e7
TC
2033This is similar in spirit to, but not the same as, grep(1) and its
2034relatives. In particular, it is not limited to using regular expressions.
2f9daede 2035
a0d0e21e 2036Evaluates the BLOCK or EXPR for each element of LIST (locally setting
7660c0ab 2037C<$_> to each element) and returns the list value consisting of those
19799a22
GS
2038elements for which the expression evaluated to true. In scalar
2039context, returns the number of times the expression was true.
a0d0e21e
LW
2040
2041 @foo = grep(!/^#/, @bar); # weed out comments
2042
2043or equivalently,
2044
2045 @foo = grep {!/^#/} @bar; # weed out comments
2046
2b5ab1e7
TC
2047Note that, because C<$_> is a reference into the list value, it can
2048be used to modify the elements of the array. While this is useful and
2049supported, it can cause bizarre results if the LIST is not a named array.
2050Similarly, grep returns aliases into the original list, much as a for
2051loop's index variable aliases the list elements. That is, modifying an
19799a22
GS
2052element of a list returned by grep (for example, in a C<foreach>, C<map>
2053or another C<grep>) actually modifies the element in the original list.
2b5ab1e7 2054This is usually something to be avoided when writing clear code.
a0d0e21e 2055
19799a22 2056See also L</map> for a list composed of the results of the BLOCK or EXPR.
38325410 2057
a0d0e21e
LW
2058=item hex EXPR
2059
54310121 2060=item hex
bbce6d69 2061
2b5ab1e7
TC
2062Interprets EXPR as a hex string and returns the corresponding value.
2063(To convert strings that might start with either 0, 0x, or 0b, see
2064L</oct>.) If EXPR is omitted, uses C<$_>.
2f9daede
TPG
2065
2066 print hex '0xAf'; # prints '175'
2067 print hex 'aF'; # same
a0d0e21e 2068
19799a22 2069Hex strings may only represent integers. Strings that would cause
c6edd1b7 2070integer overflow trigger a warning.
19799a22 2071
a0d0e21e
LW
2072=item import
2073
19799a22 2074There is no builtin C<import> function. It is just an ordinary
4633a7c4 2075method (subroutine) defined (or inherited) by modules that wish to export
19799a22 2076names to another module. The C<use> function calls the C<import> method
54310121 2077for the package used. See also L</use()>, L<perlmod>, and L<Exporter>.
a0d0e21e
LW
2078
2079=item index STR,SUBSTR,POSITION
2080
2081=item index STR,SUBSTR
2082
2b5ab1e7
TC
2083The index function searches for one string within another, but without
2084the wildcard-like behavior of a full regular-expression pattern match.
2085It returns the position of the first occurrence of SUBSTR in STR at
2086or after POSITION. If POSITION is omitted, starts searching from the
2087beginning of the string. The return value is based at C<0> (or whatever
2088you've set the C<$[> variable to--but don't do that). If the substring
2089is not found, returns one less than the base, ordinarily C<-1>.
a0d0e21e
LW
2090
2091=item int EXPR
2092
54310121 2093=item int
bbce6d69 2094
7660c0ab 2095Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2b5ab1e7
TC
2096You should not use this function for rounding: one because it truncates
2097towards C<0>, and two because machine representations of floating point
2098numbers can sometimes produce counterintuitive results. For example,
2099C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2100because it's really more like -268.99999999999994315658 instead. Usually,
19799a22 2101the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2b5ab1e7 2102functions will serve you better than will int().
a0d0e21e
LW
2103
2104=item ioctl FILEHANDLE,FUNCTION,SCALAR
2105
2b5ab1e7 2106Implements the ioctl(2) function. You'll probably first have to say
a0d0e21e 2107
4633a7c4 2108 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
a0d0e21e 2109
2b5ab1e7 2110to get the correct function definitions. If F<ioctl.ph> doesn't
a0d0e21e 2111exist or doesn't have the correct definitions you'll have to roll your
61eff3bc 2112own, based on your C header files such as F<< <sys/ioctl.h> >>.
5a964f20 2113(There is a Perl script called B<h2ph> that comes with the Perl kit that
54310121 2114may help you in this, but it's nontrivial.) SCALAR will be read and/or
4633a7c4 2115written depending on the FUNCTION--a pointer to the string value of SCALAR
19799a22 2116will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
4633a7c4
LW
2117has no string value but does have a numeric value, that value will be
2118passed rather than a pointer to the string value. To guarantee this to be
19799a22
GS
2119true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2120functions may be needed to manipulate the values of structures used by
2121C<ioctl>.
a0d0e21e 2122
19799a22 2123The return value of C<ioctl> (and C<fcntl>) is as follows:
a0d0e21e
LW
2124
2125 if OS returns: then Perl returns:
2126 -1 undefined value
2127 0 string "0 but true"
2128 anything else that number
2129
19799a22 2130Thus Perl returns true on success and false on failure, yet you can
a0d0e21e
LW
2131still easily determine the actual value returned by the operating
2132system:
2133
2b5ab1e7 2134 $retval = ioctl(...) || -1;
a0d0e21e
LW
2135 printf "System returned %d\n", $retval;
2136
c2611fb3 2137The special string "C<0> but true" is exempt from B<-w> complaints
5a964f20
TC
2138about improper numeric conversions.
2139
19799a22
GS
2140Here's an example of setting a filehandle named C<REMOTE> to be
2141non-blocking at the system level. You'll have to negotiate C<$|>
2142on your own, though.
2143
2144 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
2145
2146 $flags = fcntl(REMOTE, F_GETFL, 0)
2147 or die "Can't get flags for the socket: $!\n";
2148
2149 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
2150 or die "Can't set flags for the socket: $!\n";
2151
a0d0e21e
LW
2152=item join EXPR,LIST
2153
2b5ab1e7
TC
2154Joins the separate strings of LIST into a single string with fields
2155separated by the value of EXPR, and returns that new string. Example:
a0d0e21e 2156
2b5ab1e7 2157 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
a0d0e21e 2158
eb6e2d6f
GS
2159Beware that unlike C<split>, C<join> doesn't take a pattern as its
2160first argument. Compare L</split>.
a0d0e21e 2161
aa689395
PP
2162=item keys HASH
2163
19799a22 2164Returns a list consisting of all the keys of the named hash. (In
1d2dff63 2165scalar context, returns the number of keys.) The keys are returned in
ab192400
GS
2166an apparently random order. The actual random order is subject to
2167change in future versions of perl, but it is guaranteed to be the same
19799a22 2168order as either the C<values> or C<each> function produces (given
ab192400
GS
2169that the hash has not been modified). As a side effect, it resets
2170HASH's iterator.
a0d0e21e 2171
aa689395 2172Here is yet another way to print your environment:
a0d0e21e
LW
2173
2174 @keys = keys %ENV;
2175 @values = values %ENV;
19799a22 2176 while (@keys) {
a0d0e21e
LW
2177 print pop(@keys), '=', pop(@values), "\n";
2178 }
2179
2180or how about sorted by key:
2181
2182 foreach $key (sort(keys %ENV)) {
2183 print $key, '=', $ENV{$key}, "\n";
2184 }
2185
19799a22 2186To sort a hash by value, you'll need to use a C<sort> function.
aa689395 2187Here's a descending numeric sort of a hash by its values:
4633a7c4 2188
5a964f20 2189 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
4633a7c4
LW
2190 printf "%4d %s\n", $hash{$key}, $key;
2191 }
2192
19799a22 2193As an lvalue C<keys> allows you to increase the number of hash buckets
aa689395
PP
2194allocated for the given hash. This can gain you a measure of efficiency if
2195you know the hash is going to get big. (This is similar to pre-extending
2196an array by assigning a larger number to $#array.) If you say
55497cff
PP
2197
2198 keys %hash = 200;
2199
ab192400
GS
2200then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2201in fact, since it rounds up to the next power of two. These
55497cff
PP
2202buckets will be retained even if you do C<%hash = ()>, use C<undef
2203%hash> if you want to free the storage while C<%hash> is still in scope.
2204You can't shrink the number of buckets allocated for the hash using
19799a22 2205C<keys> in this way (but you needn't worry about doing this by accident,
55497cff
PP
2206as trying has no effect).
2207
19799a22 2208See also C<each>, C<values> and C<sort>.
ab192400 2209
b350dd2f 2210=item kill SIGNAL, LIST
a0d0e21e 2211
b350dd2f 2212Sends a signal to a list of processes. Returns the number of
517db077
GS
2213processes successfully signaled (which is not necessarily the
2214same as the number actually killed).
a0d0e21e
LW
2215
2216 $cnt = kill 1, $child1, $child2;
2217 kill 9, @goners;
2218
b350dd2f
GS
2219If SIGNAL is zero, no signal is sent to the process. This is a
2220useful way to check that the process is alive and hasn't changed
2221its UID. See L<perlport> for notes on the portability of this
2222construct.
2223
2224Unlike in the shell, if SIGNAL is negative, it kills
4633a7c4
LW
2225process groups instead of processes. (On System V, a negative I<PROCESS>
2226number will also kill process groups, but that's not portable.) That
2227means you usually want to use positive not negative signals. You may also
da0045b7 2228use a signal name in quotes. See L<perlipc/"Signals"> for details.
a0d0e21e
LW
2229
2230=item last LABEL
2231
2232=item last
2233
2234The C<last> command is like the C<break> statement in C (as used in
2235loops); it immediately exits the loop in question. If the LABEL is
2236omitted, the command refers to the innermost enclosing loop. The
2237C<continue> block, if any, is not executed:
2238
4633a7c4
LW
2239 LINE: while (<STDIN>) {
2240 last LINE if /^$/; # exit when done with header
5a964f20 2241 #...
a0d0e21e
LW
2242 }
2243
4968c1e4 2244C<last> cannot be used to exit a block which returns a value such as
2b5ab1e7
TC
2245C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2246a grep() or map() operation.
4968c1e4 2247
6c1372ed
GS
2248Note that a block by itself is semantically identical to a loop
2249that executes once. Thus C<last> can be used to effect an early
2250exit out of such a block.
2251
98293880
JH
2252See also L</continue> for an illustration of how C<last>, C<next>, and
2253C<redo> work.
1d2dff63 2254
a0d0e21e
LW
2255=item lc EXPR
2256
54310121 2257=item lc
bbce6d69 2258
a0d0e21e 2259Returns an lowercased version of EXPR. This is the internal function
7660c0ab 2260implementing the C<\L> escape in double-quoted strings.
19799a22
GS
2261Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
2262and L<utf8>.
a0d0e21e 2263
7660c0ab 2264If EXPR is omitted, uses C<$_>.
bbce6d69 2265
a0d0e21e
LW
2266=item lcfirst EXPR
2267
54310121 2268=item lcfirst
bbce6d69 2269
a0d0e21e 2270Returns the value of EXPR with the first character lowercased. This is
7660c0ab 2271the internal function implementing the C<\l> escape in double-quoted strings.
a0ed51b3 2272Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
a0d0e21e 2273
7660c0ab 2274If EXPR is omitted, uses C<$_>.
bbce6d69 2275
a0d0e21e
LW
2276=item length EXPR
2277
54310121 2278=item length
bbce6d69 2279
a0ed51b3 2280Returns the length in characters of the value of EXPR. If EXPR is
2b5ab1e7
TC
2281omitted, returns length of C<$_>. Note that this cannot be used on
2282an entire array or hash to find out how many elements these have.
2283For that, use C<scalar @array> and C<scalar keys %hash> respectively.
a0d0e21e
LW
2284
2285=item link OLDFILE,NEWFILE
2286
19799a22
GS
2287Creates a new filename linked to the old filename. Returns true for
2288success, false otherwise.
a0d0e21e
LW
2289
2290=item listen SOCKET,QUEUESIZE
2291
19799a22
GS
2292Does the same thing that the listen system call does. Returns true if
2293it succeeded, false otherwise. See the example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
2294
2295=item local EXPR
2296
19799a22 2297You really probably want to be using C<my> instead, because C<local> isn't
2b5ab1e7
TC
2298what most people think of as "local". See L<perlsub/"Private Variables
2299via my()"> for details.
2300
5a964f20
TC
2301A local modifies the listed variables to be local to the enclosing
2302block, file, or eval. If more than one value is listed, the list must
2303be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2304for details, including issues with tied arrays and hashes.
a0d0e21e 2305
a0d0e21e
LW
2306=item localtime EXPR
2307
19799a22 2308Converts a time as returned by the time function to a 9-element list
5f05dabc 2309with the time analyzed for the local time zone. Typically used as
a0d0e21e
LW
2310follows:
2311
54310121 2312 # 0 1 2 3 4 5 6 7 8
a0d0e21e
LW
2313 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2314 localtime(time);
2315
19799a22
GS
2316All list elements are numeric, and come straight out of a struct tm.
2317In particular this means that $mon has the range C<0..11> and $wday
2318has the range C<0..6> with sunday as day C<0>. Also, $year is the
2319number of years since 1900, that is, $year is C<123> in year 2023,
2b5ab1e7
TC
2320and I<not> simply the last two digits of the year. If you assume it is,
2321then you create non-Y2K-compliant programs--and you wouldn't want to do
2322that, would you?
54310121 2323
abd75f24
GS
2324The proper way to get a complete 4-digit year is simply:
2325
2326 $year += 1900;
2327
2328And to get the last two digits of the year (e.g., '01' in 2001) do:
2329
2330 $year = sprintf("%02d", $year % 100);
2331
54310121 2332If EXPR is omitted, uses the current time (C<localtime(time)>).
a0d0e21e 2333
f86cebdf 2334In scalar context, returns the ctime(3) value:
a0d0e21e 2335
5f05dabc 2336 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
a0d0e21e 2337
a3cb178b 2338This scalar value is B<not> locale dependent, see L<perllocale>, but
68f8bed4
JH
2339instead a Perl builtin. Also see the C<Time::Local> module
2340(to convert the second, minutes, hours, ... back to seconds since the
2341stroke of midnight the 1st of January 1970, the value returned by
ca6e1c26 2342time()), and the strftime(3) and mktime(3) functions available via the
68f8bed4
JH
2343POSIX module. To get somewhat similar but locale dependent date
2344strings, set up your locale environment variables appropriately
2345(please see L<perllocale>) and try for example:
a3cb178b 2346
5a964f20 2347 use POSIX qw(strftime);
2b5ab1e7 2348 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
a3cb178b
GS
2349
2350Note that the C<%a> and C<%b>, the short forms of the day of the week
2351and the month of the year, may not necessarily be three characters wide.
a0d0e21e 2352
19799a22
GS
2353=item lock
2354
2355 lock I<THING>
2356
2357This function places an advisory lock on a variable, subroutine,
2358or referenced object contained in I<THING> until the lock goes out
2359of scope. This is a built-in function only if your version of Perl
2360was built with threading enabled, and if you've said C<use Threads>.
2361Otherwise a user-defined function by this name will be called. See
2362L<Thread>.
2363
a0d0e21e
LW
2364=item log EXPR
2365
54310121 2366=item log
bbce6d69 2367
2b5ab1e7
TC
2368Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2369returns log of C<$_>. To get the log of another base, use basic algebra:
19799a22 2370The base-N log of a number is equal to the natural log of that number
2b5ab1e7
TC
2371divided by the natural log of N. For example:
2372
2373 sub log10 {
2374 my $n = shift;
2375 return log($n)/log(10);
2376 }
2377
2378See also L</exp> for the inverse operation.
a0d0e21e
LW
2379
2380=item lstat FILEHANDLE
2381
2382=item lstat EXPR
2383
54310121 2384=item lstat
bbce6d69 2385
19799a22 2386Does the same thing as the C<stat> function (including setting the
5a964f20
TC
2387special C<_> filehandle) but stats a symbolic link instead of the file
2388the symbolic link points to. If symbolic links are unimplemented on
19799a22 2389your system, a normal C<stat> is done.
a0d0e21e 2390
7660c0ab 2391If EXPR is omitted, stats C<$_>.
bbce6d69 2392
a0d0e21e
LW
2393=item m//
2394
2395The match operator. See L<perlop>.
2396
2397=item map BLOCK LIST
2398
2399=item map EXPR,LIST
2400
19799a22
GS
2401Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2402C<$_> to each element) and returns the list value composed of the
2403results of each such evaluation. In scalar context, returns the
2404total number of elements so generated. Evaluates BLOCK or EXPR in
2405list context, so each element of LIST may produce zero, one, or
2406more elements in the returned value.
dd99ebda 2407
a0d0e21e
LW
2408 @chars = map(chr, @nums);
2409
2410translates a list of numbers to the corresponding characters. And
2411
4633a7c4 2412 %hash = map { getkey($_) => $_ } @array;
a0d0e21e
LW
2413
2414is just a funny way to write
2415
2416 %hash = ();
2417 foreach $_ (@array) {
4633a7c4 2418 $hash{getkey($_)} = $_;
a0d0e21e
LW
2419 }
2420
2b5ab1e7
TC
2421Note that, because C<$_> is a reference into the list value, it can
2422be used to modify the elements of the array. While this is useful and
2423supported, it can cause bizarre results if the LIST is not a named array.
2424Using a regular C<foreach> loop for this purpose would be clearer in
2425most cases. See also L</grep> for an array composed of those items of
2426the original list for which the BLOCK or EXPR evaluates to true.
fb73857a 2427
19799a22 2428=item mkdir FILENAME,MASK
a0d0e21e 2429
5a211162
GS
2430=item mkdir FILENAME
2431
0591cd52 2432Creates the directory specified by FILENAME, with permissions
19799a22
GS
2433specified by MASK (as modified by C<umask>). If it succeeds it
2434returns true, otherwise it returns false and sets C<$!> (errno).
5a211162 2435If omitted, MASK defaults to 0777.
0591cd52 2436
19799a22 2437In general, it is better to create directories with permissive MASK,
0591cd52 2438and let the user modify that with their C<umask>, than it is to supply
19799a22 2439a restrictive MASK and give the user no way to be more permissive.
0591cd52
NT
2440The exceptions to this rule are when the file or directory should be
2441kept private (mail files, for instance). The perlfunc(1) entry on
19799a22 2442C<umask> discusses the choice of MASK in more detail.
a0d0e21e
LW
2443
2444=item msgctl ID,CMD,ARG
2445
f86cebdf 2446Calls the System V IPC function msgctl(2). You'll probably have to say
0ade1984
JH
2447
2448 use IPC::SysV;
2449
7660c0ab
A
2450first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2451then ARG must be a variable which will hold the returned C<msqid_ds>
951ba7fe
GS
2452structure. Returns like C<ioctl>: the undefined value for error,
2453C<"0 but true"> for zero, or the actual return value otherwise. See also
19799a22 2454C<IPC::SysV> and C<IPC::Semaphore> documentation.
a0d0e21e
LW
2455
2456=item msgget KEY,FLAGS
2457
f86cebdf 2458Calls the System V IPC function msgget(2). Returns the message queue
7660c0ab 2459id, or the undefined value if there is an error. See also C<IPC::SysV>
19799a22 2460and C<IPC::Msg> documentation.
a0d0e21e
LW
2461
2462=item msgsnd ID,MSG,FLAGS
2463
2464Calls the System V IPC function msgsnd to send the message MSG to the
e4038a1f
MS
2465message queue ID. MSG must begin with the native long integer message
2466type, which may be created with C<pack("l!", $type)>. Returns true if
2467successful, or false if there is an error. See also C<IPC::SysV> and
2468C<IPC::SysV::Msg> documentation.
a0d0e21e
LW
2469
2470=item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2471
2472Calls the System V IPC function msgrcv to receive a message from
2473message queue ID into variable VAR with a maximum message size of
0ade1984
JH
2474SIZE. Note that if a message is received, the message type will be
2475the first thing in VAR, and the maximum length of VAR is SIZE plus the
19799a22 2476size of the message type. Returns true if successful, or false if
7660c0ab 2477there is an error. See also C<IPC::SysV> and C<IPC::SysV::Msg> documentation.
a0d0e21e
LW
2478
2479=item my EXPR
2480
09bef843
SB
2481=item my EXPR : ATTRIBUTES
2482
19799a22
GS
2483A C<my> declares the listed variables to be local (lexically) to the
2484enclosing block, file, or C<eval>. If
5f05dabc 2485more than one value is listed, the list must be placed in parentheses. See
cb1a09d0 2486L<perlsub/"Private Variables via my()"> for details.
4633a7c4 2487
a0d0e21e
LW
2488=item next LABEL
2489
2490=item next
2491
2492The C<next> command is like the C<continue> statement in C; it starts
2493the next iteration of the loop:
2494
4633a7c4
LW
2495 LINE: while (<STDIN>) {
2496 next LINE if /^#/; # discard comments
5a964f20 2497 #...
a0d0e21e
LW
2498 }
2499
2500Note that if there were a C<continue> block on the above, it would get
2501executed even on discarded lines. If the LABEL is omitted, the command
2502refers to the innermost enclosing loop.
2503
4968c1e4 2504C<next> cannot be used to exit a block which returns a value such as
2b5ab1e7
TC
2505C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2506a grep() or map() operation.
4968c1e4 2507
6c1372ed
GS
2508Note that a block by itself is semantically identical to a loop
2509that executes once. Thus C<next> will exit such a block early.
2510
98293880
JH
2511See also L</continue> for an illustration of how C<last>, C<next>, and
2512C<redo> work.
1d2dff63 2513
a0d0e21e
LW
2514=item no Module LIST
2515
7660c0ab 2516See the L</use> function, which C<no> is the opposite of.
a0d0e21e
LW
2517
2518=item oct EXPR
2519
54310121 2520=item oct
bbce6d69 2521
4633a7c4 2522Interprets EXPR as an octal string and returns the corresponding
4f19785b
WSI
2523value. (If EXPR happens to start off with C<0x>, interprets it as a
2524hex string. If EXPR starts off with C<0b>, it is interpreted as a
2525binary string.) The following will handle decimal, binary, octal, and
4633a7c4 2526hex in the standard Perl or C notation:
a0d0e21e
LW
2527
2528 $val = oct($val) if $val =~ /^0/;
2529
19799a22
GS
2530If EXPR is omitted, uses C<$_>. To go the other way (produce a number
2531in octal), use sprintf() or printf():
2532
2533 $perms = (stat("filename"))[2] & 07777;
2534 $oct_perms = sprintf "%lo", $perms;
2535
2536The oct() function is commonly used when a string such as C<644> needs
2537to be converted into a file mode, for example. (Although perl will
2538automatically convert strings into numbers as needed, this automatic
2539conversion assumes base 10.)
a0d0e21e 2540
6170680b
IZ
2541=item open FILEHANDLE,MODE,EXPR
2542
a0d0e21e
LW
2543=item open FILEHANDLE,EXPR
2544
2545=item open FILEHANDLE
2546
2547Opens the file whose filename is given by EXPR, and associates it with
5f05dabc
PP
2548FILEHANDLE. If FILEHANDLE is an expression, its value is used as the
2549name of the real filehandle wanted. If EXPR is omitted, the scalar
2550variable of the same name as the FILEHANDLE contains the filename.
19799a22
GS
2551(Note that lexical variables--those declared with C<my>--will not work
2552for this purpose; so if you're using C<my>, specify EXPR in your call
2b5ab1e7
TC
2553to open.) See L<perlopentut> for a kinder, gentler explanation of opening
2554files.
5f05dabc 2555
61eff3bc
JH
2556If MODE is C<< '<' >> or nothing, the file is opened for input.
2557If MODE is C<< '>' >>, the file is truncated and opened for
2558output, being created if necessary. If MODE is C<<< '>>' >>>,
fbb426e4 2559the file is opened for appending, again being created if necessary.
61eff3bc
JH
2560You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to indicate that
2561you want both read and write access to the file; thus C<< '+<' >> is almost
2562always preferred for read/write updates--the C<< '+>' >> mode would clobber the
5a964f20
TC
2563file first. You can't usually use either read-write mode for updating
2564textfiles, since they have variable length records. See the B<-i>
0591cd52
NT
2565switch in L<perlrun> for a better approach. The file is created with
2566permissions of C<0666> modified by the process' C<umask> value.
5a964f20 2567
61eff3bc
JH
2568These various prefixes correspond to the fopen(3) modes of C<'r'>, C<'r+'>,
2569C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
5f05dabc 2570
6170680b
IZ
2571In the 2-arguments (and 1-argument) form of the call the mode and
2572filename should be concatenated (in this order), possibly separated by
61eff3bc 2573spaces. It is possible to omit the mode if the mode is C<< '<' >>.
6170680b 2574
7660c0ab 2575If the filename begins with C<'|'>, the filename is interpreted as a
5a964f20 2576command to which output is to be piped, and if the filename ends with a
f244e06d
GS
2577C<'|'>, the filename is interpreted as a command which pipes output to
2578us. See L<perlipc/"Using open() for IPC">
19799a22 2579for more examples of this. (You are not allowed to C<open> to a command
5a964f20
TC
2580that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2581and L<perlipc/"Bidirectional Communication"> for alternatives.)
cb1a09d0 2582
6170680b
IZ
2583If MODE is C<'|-'>, the filename is interpreted as a
2584command to which output is to be piped, and if MODE is
2585C<'-|'>, the filename is interpreted as a command which pipes output to
2586us. In the 2-arguments (and 1-argument) form one should replace dash
2587(C<'-'>) with the command. See L<perlipc/"Using open() for IPC">
2588for more examples of this. (You are not allowed to C<open> to a command
2589that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2590and L<perlipc/"Bidirectional Communication"> for alternatives.)
2591
2592In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
61eff3bc 2593and opening C<< '>-' >> opens STDOUT.
6170680b
IZ
2594
2595Open returns
19799a22 2596nonzero upon success, the undefined value otherwise. If the C<open>
4633a7c4 2597involved a pipe, the return value happens to be the pid of the
54310121 2598subprocess.
cb1a09d0
AD
2599
2600If you're unfortunate enough to be running Perl on a system that
2601distinguishes between text files and binary files (modern operating
2602systems don't care), then you should check out L</binmode> for tips for
19799a22 2603dealing with this. The key distinction between systems that need C<binmode>
5a964f20
TC
2604and those that don't is their text file formats. Systems like Unix, MacOS, and
2605Plan9, which delimit lines with a single character, and which encode that
19799a22 2606character in C as C<"\n">, do not need C<binmode>. The rest need it.
cb1a09d0 2607
fb73857a 2608When opening a file, it's usually a bad idea to continue normal execution
19799a22
GS
2609if the request failed, so C<open> is frequently used in connection with
2610C<die>. Even if C<die> won't do what you want (say, in a CGI script,
fb73857a 2611where you want to make a nicely formatted error message (but there are
5a964f20 2612modules that can help with that problem)) you should always check
19799a22 2613the return value from opening a file. The infrequent exception is when
fb73857a
PP
2614working with an unopened filehandle is actually what you want to do.
2615
cb1a09d0 2616Examples:
a0d0e21e
LW
2617
2618 $ARTICLE = 100;
2619 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2620 while (<ARTICLE>) {...
2621
6170680b 2622 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
fb73857a 2623 # if the open fails, output is discarded
a0d0e21e 2624
6170680b 2625 open(DBASE, '+<', 'dbase.mine') # open for update
fb73857a 2626 or die "Can't open 'dbase.mine' for update: $!";
cb1a09d0 2627
6170680b
IZ
2628 open(DBASE, '+<dbase.mine') # ditto
2629 or die "Can't open 'dbase.mine' for update: $!";
2630
2631 open(ARTICLE, '-|', "caesar <$article") # decrypt article
fb73857a 2632 or die "Can't start caesar: $!";
a0d0e21e 2633
6170680b
IZ
2634 open(ARTICLE, "caesar <$article |") # ditto
2635 or die "Can't start caesar: $!";
2636
2637 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
fb73857a 2638 or die "Can't start sort: $!";
a0d0e21e
LW
2639
2640 # process argument list of files along with any includes
2641
2642 foreach $file (@ARGV) {
2643 process($file, 'fh00');
2644 }
2645
2646 sub process {
5a964f20 2647 my($filename, $input) = @_;
a0d0e21e
LW
2648 $input++; # this is a string increment
2649 unless (open($input, $filename)) {
2650 print STDERR "Can't open $filename: $!\n";
2651 return;
2652 }
2653
5a964f20 2654 local $_;
a0d0e21e
LW
2655 while (<$input>) { # note use of indirection
2656 if (/^#include "(.*)"/) {
2657 process($1, $input);
2658 next;
2659 }
5a964f20 2660 #... # whatever
a0d0e21e
LW
2661 }
2662 }
2663
2664You may also, in the Bourne shell tradition, specify an EXPR beginning
61eff3bc 2665with C<< '>&' >>, in which case the rest of the string is interpreted as the
5a964f20 2666name of a filehandle (or file descriptor, if numeric) to be
61eff3bc
JH
2667duped and opened. You may use C<&> after C<< > >>, C<<< >> >>>,
2668C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>. The
a0d0e21e 2669mode you specify should match the mode of the original filehandle.
184e9718 2670(Duping a filehandle does not take into account any existing contents of
6170680b
IZ
2671stdio buffers.) Duping file handles is not yet supported for 3-argument
2672open().
2673
a0d0e21e
LW
2674Here is a script that saves, redirects, and restores STDOUT and
2675STDERR:
2676
2677 #!/usr/bin/perl
5a964f20
TC
2678 open(OLDOUT, ">&STDOUT");
2679 open(OLDERR, ">&STDERR");
a0d0e21e 2680
6170680b
IZ
2681 open(STDOUT, '>', "foo.out") || die "Can't redirect stdout";
2682 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
a0d0e21e
LW
2683
2684 select(STDERR); $| = 1; # make unbuffered
2685 select(STDOUT); $| = 1; # make unbuffered
2686
2687 print STDOUT "stdout 1\n"; # this works for
2688 print STDERR "stderr 1\n"; # subprocesses too
2689
2690 close(STDOUT);
2691 close(STDERR);
2692
5a964f20
TC
2693 open(STDOUT, ">&OLDOUT");
2694 open(STDERR, ">&OLDERR");
a0d0e21e
LW
2695
2696 print STDOUT "stdout 2\n";
2697 print STDERR "stderr 2\n";
2698
61eff3bc 2699If you specify C<< '<&=N' >>, where C<N> is a number, then Perl will do an
19799a22 2700equivalent of C's C<fdopen> of that file descriptor; this is more
4633a7c4 2701parsimonious of file descriptors. For example:
a0d0e21e
LW
2702
2703 open(FILEHANDLE, "<&=$fd")
2704
4af147f6
CS
2705Note that this feature depends on the fdopen() C library function.
2706On many UNIX systems, fdopen() is known to fail when file descriptors
2707exceed a certain value, typically 255. If you need more file
2708descriptors than that, consider rebuilding Perl to use the C<sfio>
2709library.
2710
6170680b
IZ
2711If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
2712with 2-arguments (or 1-argument) form of open(), then
a0d0e21e 2713there is an implicit fork done, and the return value of open is the pid
7660c0ab 2714of the child within the parent process, and C<0> within the child
184e9718 2715process. (Use C<defined($pid)> to determine whether the open was successful.)
a0d0e21e
LW
2716The filehandle behaves normally for the parent, but i/o to that
2717filehandle is piped from/to the STDOUT/STDIN of the child process.
2718In the child process the filehandle isn't opened--i/o happens from/to
2719the new STDOUT or STDIN. Typically this is used like the normal
2720piped open when you want to exercise more control over just how the
2721pipe command gets executed, such as when you are running setuid, and
54310121 2722don't want to have to scan shell commands for metacharacters.
6170680b 2723The following triples are more or less equivalent:
a0d0e21e
LW
2724
2725 open(FOO, "|tr '[a-z]' '[A-Z]'");
6170680b
IZ
2726 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
2727 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
a0d0e21e
LW
2728
2729 open(FOO, "cat -n '$file'|");
6170680b
IZ
2730 open(FOO, '-|', "cat -n '$file'");
2731 open(FOO, '-|') || exec 'cat', '-n', $file;
a0d0e21e 2732
4633a7c4
LW
2733See L<perlipc/"Safe Pipe Opens"> for more examples of this.
2734
45bc9206
GS
2735NOTE: On any operation that may do a fork, all files opened for output
2736are flushed before the fork is attempted. On systems that support a
2737close-on-exec flag on files, the flag will be set for the newly opened
2738file descriptor as determined by the value of $^F. See L<perlvar/$^F>.
a0d0e21e 2739
0dccf244
CS
2740Closing any piped filehandle causes the parent process to wait for the
2741child to finish, and returns the status value in C<$?>.
2742
6170680b
IZ
2743The filename passed to 2-argument (or 1-argument) form of open()
2744will have leading and trailing
f86cebdf 2745whitespace deleted, and the normal redirection characters
5a964f20
TC
2746honored. This property, known as "magic open",
2747can often be used to good effect. A user could specify a filename of
7660c0ab 2748F<"rsh cat file |">, or you could change certain filenames as needed:
5a964f20
TC
2749
2750 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
2751 open(FH, $filename) or die "Can't open $filename: $!";
2752
6170680b
IZ
2753Use 3-argument form to open a file with arbitrary weird characters in it,
2754
2755 open(FOO, '<', $file);
2756
2757otherwise it's necessary to protect any leading and trailing whitespace:
5a964f20
TC
2758
2759 $file =~ s#^(\s)#./$1#;
2760 open(FOO, "< $file\0");
2761
6170680b
IZ
2762(this may not work on some bizzare filesystems). One should
2763conscientiously choose between the the I<magic> and 3-arguments form
2764of open():
2765
2766 open IN, $ARGV[0];
2767
2768will allow the user to specify an argument of the form C<"rsh cat file |">,
2769but will not work on a filename which happens to have a trailing space, while
2770
2771 open IN, '<', $ARGV[0];
2772
2773will have exactly the opposite restrictions.
2774
19799a22 2775If you want a "real" C C<open> (see L<open(2)> on your system), then you
6170680b
IZ
2776should use the C<sysopen> function, which involves no such magic (but
2777may use subtly different filemodes than Perl open(), which is mapped
2778to C fopen()). This is
5a964f20
TC
2779another way to protect your filenames from interpretation. For example:
2780
2781 use IO::Handle;
2782 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
2783 or die "sysopen $path: $!";
2784 $oldfh = select(HANDLE); $| = 1; select($oldfh);
2785 print HANDLE "stuff $$\n");
2786 seek(HANDLE, 0, 0);
2787 print "File contains: ", <HANDLE>;
2788
7660c0ab
A
2789Using the constructor from the C<IO::Handle> package (or one of its
2790subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
5a964f20
TC
2791filehandles that have the scope of whatever variables hold references to
2792them, and automatically close whenever and however you leave that scope:
c07a80fd 2793
5f05dabc 2794 use IO::File;
5a964f20 2795 #...
c07a80fd
PP
2796 sub read_myfile_munged {
2797 my $ALL = shift;
5f05dabc 2798 my $handle = new IO::File;
c07a80fd
PP
2799 open($handle, "myfile") or die "myfile: $!";
2800 $first = <$handle>
2801 or return (); # Automatically closed here.
2802 mung $first or die "mung failed"; # Or here.
2803 return $first, <$handle> if $ALL; # Or here.
2804 $first; # Or here.
2805 }
2806
b687b08b 2807See L</seek> for some details about mixing reading and writing.
a0d0e21e
LW
2808
2809=item opendir DIRHANDLE,EXPR
2810
19799a22
GS
2811Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
2812C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
a0d0e21e
LW
2813DIRHANDLEs have their own namespace separate from FILEHANDLEs.
2814
2815=item ord EXPR
2816
54310121 2817=item ord
bbce6d69 2818
a0ed51b3 2819Returns the numeric (ASCII or Unicode) value of the first character of EXPR. If
7660c0ab 2820EXPR is omitted, uses C<$_>. For the reverse, see L</chr>.
2b5ab1e7 2821See L<utf8> for more about Unicode.
a0d0e21e 2822
77ca0c92
LW
2823=item our EXPR
2824
2825An C<our> declares the listed variables to be valid globals within
2826the enclosing block, file, or C<eval>. That is, it has the same
2827scoping rules as a "my" declaration, but does not create a local
2828variable. If more than one value is listed, the list must be placed
2829in parentheses. The C<our> declaration has no semantic effect unless
2830"use strict vars" is in effect, in which case it lets you use the
2831declared global variable without qualifying it with a package name.
2832(But only within the lexical scope of the C<our> declaration. In this
2833it differs from "use vars", which is package scoped.)
2834
f472eb5c
GS
2835An C<our> declaration declares a global variable that will be visible
2836across its entire lexical scope, even across package boundaries. The
2837package in which the variable is entered is determined at the point
2838of the declaration, not at the point of use. This means the following
2839behavior holds:
2840
2841 package Foo;
2842 our $bar; # declares $Foo::bar for rest of lexical scope
2843 $bar = 20;
2844
2845 package Bar;
2846 print $bar; # prints 20
2847
2848Multiple C<our> declarations in the same lexical scope are allowed
2849if they are in different packages. If they happened to be in the same
2850package, Perl will emit warnings if you have asked for them.
2851
2852 use warnings;
2853 package Foo;
2854 our $bar; # declares $Foo::bar for rest of lexical scope
2855 $bar = 20;
2856
2857 package Bar;
2858 our $bar = 30; # declares $Bar::bar for rest of lexical scope
2859 print $bar; # prints 30
2860
2861 our $bar; # emits warning
2862
a0d0e21e
LW
2863=item pack TEMPLATE,LIST
2864
2b6c5635
GS
2865Takes a LIST of values and converts it into a string using the rules
2866given by the TEMPLATE. The resulting string is the concatenation of
2867the converted values. Typically, each converted value looks
2868like its machine-level representation. For example, on 32-bit machines
2869a converted integer may be represented by a sequence of 4 bytes.
2870
2871The TEMPLATE is a
a0d0e21e
LW
2872sequence of characters that give the order and type of values, as
2873follows:
2874
5a929a98 2875 a A string with arbitrary binary data, will be null padded.
a0d0e21e 2876 A An ascii string, will be space padded.
5a929a98
VU
2877 Z A null terminated (asciz) string, will be null padded.
2878
2b6c5635
GS
2879 b A bit string (ascending bit order inside each byte, like vec()).
2880 B A bit string (descending bit order inside each byte).
a0d0e21e
LW
2881 h A hex string (low nybble first).
2882 H A hex string (high nybble first).
2883
2884 c A signed char value.
a0ed51b3 2885 C An unsigned char value. Only does bytes. See U for Unicode.
96e4d5b1 2886
a0d0e21e
LW
2887 s A signed short value.
2888 S An unsigned short value.
96e4d5b1 2889 (This 'short' is _exactly_ 16 bits, which may differ from
851646ae
JH
2890 what a local C compiler calls 'short'. If you want
2891 native-length shorts, use the '!' suffix.)
96e4d5b1 2892
a0d0e21e
LW
2893 i A signed integer value.
2894 I An unsigned integer value.
19799a22 2895 (This 'integer' is _at_least_ 32 bits wide. Its exact
f86cebdf
GS
2896 size depends on what a local C compiler calls 'int',
2897 and may even be larger than the 'long' described in
2898 the next item.)
96e4d5b1 2899
a0d0e21e
LW
2900 l A signed long value.
2901 L An unsigned long value.
96e4d5b1 2902 (This 'long' is _exactly_ 32 bits, which may differ from
851646ae
JH
2903 what a local C compiler calls 'long'. If you want
2904 native-length longs, use the '!' suffix.)
a0d0e21e 2905
5d11dd56
G
2906 n An unsigned short in "network" (big-endian) order.
2907 N An unsigned long in "network" (big-endian) order.
2908 v An unsigned short in "VAX" (little-endian) order.
2909 V An unsigned long in "VAX" (little-endian) order.
96e4d5b1
PP
2910 (These 'shorts' and 'longs' are _exactly_ 16 bits and
2911 _exactly_ 32 bits, respectively.)
a0d0e21e 2912
dae0da7a
JH
2913 q A signed quad (64-bit) value.
2914 Q An unsigned quad value.
851646ae
JH
2915 (Quads are available only if your system supports 64-bit
2916 integer values _and_ if Perl has been compiled to support those.
dae0da7a
JH
2917 Causes a fatal error otherwise.)
2918
a0d0e21e
LW
2919 f A single-precision float in the native format.
2920 d A double-precision float in the native format.
2921
2922 p A pointer to a null-terminated string.
2923 P A pointer to a structure (fixed-length string).
2924
2925 u A uuencoded string.
a0ed51b3
LW
2926 U A Unicode character number. Encodes to UTF-8 internally.
2927 Works even if C<use utf8> is not in effect.
a0d0e21e 2928
96e4d5b1 2929 w A BER compressed integer. Its bytes represent an unsigned
f86cebdf
GS
2930 integer in base 128, most significant digit first, with as
2931 few digits as possible. Bit eight (the high bit) is set
2932 on each byte except the last.
def98dd4 2933
a0d0e21e
LW
2934 x A null byte.
2935 X Back up a byte.
2936 @ Null fill to absolute position.
2937
5a929a98
VU
2938The following rules apply:
2939
2940=over 8
2941
2942=item *
2943
5a964f20 2944Each letter may optionally be followed by a number giving a repeat
951ba7fe
GS
2945count. With all types except C<a>, C<A>, C<Z>, C<b>, C<B>, C<h>,
2946C<H>, and C<P> the pack function will gobble up that many values from
5a929a98 2947the LIST. A C<*> for the repeat count means to use however many items are
951ba7fe
GS
2948left, except for C<@>, C<x>, C<X>, where it is equivalent
2949to C<0>, and C<u>, where it is equivalent to 1 (or 45, what is the
2b6c5635
GS
2950same).
2951
951ba7fe 2952When used with C<Z>, C<*> results in the addition of a trailing null
2b6c5635
GS
2953byte (so the packed result will be one longer than the byte C<length>
2954of the item).
2955
951ba7fe 2956The repeat count for C<u> is interpreted as the maximal number of bytes
2b6c5635 2957to encode per line of output, with 0 and 1 replaced by 45.
5a929a98
VU
2958
2959=item *
2960
951ba7fe 2961The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
5a929a98 2962string of length count, padding with nulls or spaces as necessary. When
951ba7fe
GS
2963unpacking, C<A> strips trailing spaces and nulls, C<Z> strips everything
2964after the first null, and C<a> returns data verbatim. When packing,
2965C<a>, and C<Z> are equivalent.
2b6c5635
GS
2966
2967If the value-to-pack is too long, it is truncated. If too long and an
951ba7fe
GS
2968explicit count is provided, C<Z> packs only C<$count-1> bytes, followed
2969by a null byte. Thus C<Z> always packs a trailing null byte under
2b6c5635 2970all circumstances.
5a929a98
VU
2971
2972=item *
2973
951ba7fe 2974Likewise, the C<b> and C<B> fields pack a string that many bits long.
c73032f5
IZ
2975Each byte of the input field of pack() generates 1 bit of the result.
2976Each result bit is based on the least-significant bit of the corresponding
2977input byte, i.e., on C<ord($byte)%2>. In particular, bytes C<"0"> and
2978C<"1"> generate bits 0 and 1, as do bytes C<"\0"> and C<"\1">.
2979
2980Starting from the beginning of the input string of pack(), each 8-tuple
951ba7fe 2981of bytes is converted to 1 byte of output. With format C<b>
c73032f5 2982the first byte of the 8-tuple determines the least-significant bit of a
951ba7fe 2983byte, and with format C<B> it determines the most-significant bit of
c73032f5
IZ
2984a byte.
2985
2986If the length of the input string is not exactly divisible by 8, the
2987remainder is packed as if the input string were padded by null bytes
2988at the end. Similarly, during unpack()ing the "extra" bits are ignored.
2989
2990If the input string of pack() is longer than needed, extra bytes are ignored.
2b6c5635
GS
2991A C<*> for the repeat count of pack() means to use all the bytes of
2992the input field. On unpack()ing the bits are converted to a string
2993of C<"0">s and C<"1">s.
5a929a98
VU
2994
2995=item *
2996
951ba7fe 2997The C<h> and C<H> fields pack a string that many nybbles (4-bit groups,
851646ae 2998representable as hexadecimal digits, 0-9a-f) long.
5a929a98 2999
c73032f5
IZ
3000Each byte of the input field of pack() generates 4 bits of the result.
3001For non-alphabetical bytes the result is based on the 4 least-significant
3002bits of the input byte, i.e., on C<ord($byte)%16>. In particular,
3003bytes C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
3004C<"\0"> and C<"\1">. For bytes C<"a".."f"> and C<"A".."F"> the result
3005is compatible with the usual hexadecimal digits, so that C<"a"> and
3006C<"A"> both generate the nybble C<0xa==10>. The result for bytes
3007C<"g".."z"> and C<"G".."Z"> is not well-defined.
3008
3009Starting from the beginning of the input string of pack(), each pair
951ba7fe 3010of bytes is converted to 1 byte of output. With format C<h> the
c73032f5 3011first byte of the pair determines the least-significant nybble of the
951ba7fe 3012output byte, and with format C<H> it determines the most-significant
c73032f5
IZ
3013nybble.
3014
3015If the length of the input string is not even, it behaves as if padded
3016by a null byte at the end. Similarly, during unpack()ing the "extra"
3017nybbles are ignored.
3018
3019If the input string of pack() is longer than needed, extra bytes are ignored.
3020A C<*> for the repeat count of pack() means to use all the bytes of
3021the input field. On unpack()ing the bits are converted to a string
3022of hexadecimal digits.
3023
5a929a98
VU
3024=item *
3025
951ba7fe 3026The C<p> type packs a pointer to a null-terminated string. You are
5a929a98
VU
3027responsible for ensuring the string is not a temporary value (which can
3028potentially get deallocated before you get around to using the packed result).
951ba7fe
GS
3029The C<P> type packs a pointer to a structure of the size indicated by the
3030length. A NULL pointer is created if the corresponding value for C<p> or
3031C<P> is C<undef>, similarly for unpack().
5a929a98
VU
3032
3033=item *
3034
951ba7fe
GS
3035The C</> template character allows packing and unpacking of strings where
3036the packed structure contains a byte count followed by the string itself.
17f4a12d 3037You write I<length-item>C</>I<string-item>.
43192e07
IP
3038
3039The I<length-item> can be any C<pack> template letter,
3040and describes how the length value is packed.
3041The ones likely to be of most use are integer-packing ones like
951ba7fe
GS
3042C<n> (for Java strings), C<w> (for ASN.1 or SNMP)
3043and C<N> (for Sun XDR).
43192e07
IP
3044
3045The I<string-item> must, at present, be C<"A*">, C<"a*"> or C<"Z*">.
3046For C<unpack> the length of the string is obtained from the I<length-item>,
3047but if you put in the '*' it will be ignored.
3048
17f4a12d
IZ
3049 unpack 'C/a', "\04Gurusamy"; gives 'Guru'
3050 unpack 'a3/A* A*', '007 Bond J '; gives (' Bond','J')
3051 pack 'n/a* w/a*','hello,','world'; gives "\000\006hello,\005world"
43192e07
IP
3052
3053The I<length-item> is not returned explicitly from C<unpack>.
3054
951ba7fe
GS
3055Adding a count to the I<length-item> letter is unlikely to do anything
3056useful, unless that letter is C<A>, C<a> or C<Z>. Packing with a
3057I<length-item> of C<a> or C<Z> may introduce C<"\000"> characters,
43192e07
IP
3058which Perl does not regard as legal in numeric strings.
3059
3060=item *
3061
951ba7fe
GS
3062The integer types C<s>, C<S>, C<l>, and C<L> may be
3063immediately followed by a C<!> suffix to signify native shorts or
3064longs--as you can see from above for example a bare C<l> does mean
851646ae
JH
3065exactly 32 bits, the native C<long> (as seen by the local C compiler)
3066may be larger. This is an issue mainly in 64-bit platforms. You can
951ba7fe 3067see whether using C<!> makes any difference by
726ea183 3068
4d0c1c44
GS
3069 print length(pack("s")), " ", length(pack("s!")), "\n";
3070 print length(pack("l")), " ", length(pack("l!")), "\n";
ef54e1a4 3071
951ba7fe
GS
3072C<i!> and C<I!> also work but only because of completeness;
3073they are identical to C<i> and C<I>.
ef54e1a4 3074
19799a22
GS
3075The actual sizes (in bytes) of native shorts, ints, longs, and long
3076longs on the platform where Perl was built are also available via
3077L<Config>:
3078
3079 use Config;
3080 print $Config{shortsize}, "\n";
3081 print $Config{intsize}, "\n";
3082 print $Config{longsize}, "\n";
3083 print $Config{longlongsize}, "\n";
ef54e1a4 3084
5074e145 3085(The C<$Config{longlongsize}> will be undefine if your system does
851646ae
JH
3086not support long longs.)
3087
ef54e1a4
JH
3088=item *
3089
951ba7fe 3090The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, and C<L>
ef54e1a4
JH
3091are inherently non-portable between processors and operating systems
3092because they obey the native byteorder and endianness. For example a
140cb37e 30934-byte integer 0x12345678 (305419896 decimal) be ordered natively
ef54e1a4 3094(arranged in and handled by the CPU registers) into bytes as
61eff3bc 3095
719a3cf5
JH
3096 0x12 0x34 0x56 0x78 # little-endian
3097 0x78 0x56 0x34 0x12 # big-endian
61eff3bc 3098
5d11dd56 3099Basically, the Intel, Alpha, and VAX CPUs are little-endian, while
719a3cf5
JH
3100everybody else, for example Motorola m68k/88k, PPC, Sparc, HP PA,
3101Power, and Cray are big-endian. MIPS can be either: Digital used it
19799a22 3102in little-endian mode; SGI uses it in big-endian mode.
719a3cf5 3103
19799a22 3104The names `big-endian' and `little-endian' are comic references to
ef54e1a4
JH
3105the classic "Gulliver's Travels" (via the paper "On Holy Wars and a
3106Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and
19799a22 3107the egg-eating habits of the Lilliputians.
61eff3bc 3108
140cb37e 3109Some systems may have even weirder byte orders such as
61eff3bc 3110
ef54e1a4
JH
3111 0x56 0x78 0x12 0x34
3112 0x34 0x12 0x78 0x56
61eff3bc 3113
ef54e1a4
JH
3114You can see your system's preference with
3115
3116 print join(" ", map { sprintf "%#02x", $_ }
3117 unpack("C*",pack("L",0x12345678))), "\n";
3118
d99ad34e 3119The byteorder on the platform where Perl was built is also available
726ea183 3120via L<Config>:
ef54e1a4
JH
3121
3122 use Config;
3123 print $Config{byteorder}, "\n";
3124
d99ad34e
JH
3125Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'>
3126and C<'87654321'> are big-endian.
719a3cf5 3127
951ba7fe
GS
3128If you want portable packed integers use the formats C<n>, C<N>,
3129C<v>, and C<V>, their byte endianness and size is known.
851646ae 3130See also L<perlport>.
ef54e1a4
JH
3131
3132=item *
3133
5a929a98
VU
3134Real numbers (floats and doubles) are in the native machine format only;
3135due to the multiplicity of floating formats around, and the lack of a
3136standard "network" representation, no facility for interchange has been
3137made. This means that packed floating point data written on one machine
3138may not be readable on another - even if both use IEEE floating point
3139arithmetic (as the endian-ness of the memory representation is not part
851646ae 3140of the IEEE spec). See also L<perlport>.
5a929a98
VU
3141
3142Note that Perl uses doubles internally for all numeric calculation, and
3143converting from double into float and thence back to double again will
3144lose precision (i.e., C<unpack("f", pack("f", $foo)>) will not in general
19799a22 3145equal $foo).
5a929a98 3146
851646ae
JH
3147=item *
3148
3149You must yourself do any alignment or padding by inserting for example
9ccd05c0
JH
3150enough C<'x'>es while packing. There is no way to pack() and unpack()
3151could know where the bytes are going to or coming from. Therefore
3152C<pack> (and C<unpack>) handle their output and input as flat
3153sequences of bytes.
851646ae 3154
17f4a12d
IZ
3155=item *
3156
3157A comment in a TEMPLATE starts with C<#> and goes to the end of line.
3158
2b6c5635
GS
3159=item *
3160
3161If TEMPLATE requires more arguments to pack() than actually given, pack()
3162assumes additional C<""> arguments. If TEMPLATE requires less arguments
3163to pack() than actually given, extra arguments are ignored.
3164
5a929a98 3165=back
a0d0e21e
LW
3166
3167Examples:
3168
a0ed51b3 3169 $foo = pack("CCCC",65,66,67,68);
a0d0e21e 3170 # foo eq "ABCD"
a0ed51b3 3171 $foo = pack("C4",65,66,67,68);
a0d0e21e 3172 # same thing
a0ed51b3
LW
3173 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
3174 # same thing with Unicode circled letters
a0d0e21e
LW
3175
3176 $foo = pack("ccxxcc",65,66,67,68);
3177 # foo eq "AB\0\0CD"
3178
9ccd05c0
JH
3179 # note: the above examples featuring "C" and "c" are true
3180 # only on ASCII and ASCII-derived systems such as ISO Latin 1
3181 # and UTF-8. In EBCDIC the first example would be
3182 # $foo = pack("CCCC",193,194,195,196);
3183
a0d0e21e
LW
3184 $foo = pack("s2",1,2);
3185 # "\1\0\2\0" on little-endian
3186 # "\0\1\0\2" on big-endian
3187
3188 $foo = pack("a4","abcd","x","y","z");
3189 # "abcd"
3190
3191 $foo = pack("aaaa","abcd","x","y","z");
3192 # "axyz"
3193
3194 $foo = pack("a14","abcdefg");
3195 # "abcdefg\0\0\0\0\0\0\0"
3196
3197 $foo = pack("i9pl", gmtime);
3198 # a real struct tm (on my system anyway)
3199
5a929a98
VU
3200 $utmp_template = "Z8 Z8 Z16 L";
3201 $utmp = pack($utmp_template, @utmp1);
3202 # a struct utmp (BSDish)
3203
3204 @utmp2 = unpack($utmp_template, $utmp);
3205 # "@utmp1" eq "@utmp2"
3206
a0d0e21e
LW
3207 sub bintodec {
3208 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
3209 }
3210
851646ae
JH
3211 $foo = pack('sx2l', 12, 34);
3212 # short 12, two zero bytes padding, long 34
3213 $bar = pack('s@4l', 12, 34);
3214 # short 12, zero fill to position 4, long 34
3215 # $foo eq $bar
3216
5a929a98 3217The same template may generally also be used in unpack().
a0d0e21e 3218
5a964f20
TC
3219=item package
3220
cb1a09d0
AD
3221=item package NAMESPACE
3222
3223Declares the compilation unit as being in the given namespace. The scope
2b5ab1e7 3224of the package declaration is from the declaration itself through the end
19799a22 3225of the enclosing block, file, or eval (the same as the C<my> operator).
2b5ab1e7
TC
3226All further unqualified dynamic identifiers will be in this namespace.
3227A package statement affects only dynamic variables--including those
19799a22
GS
3228you've used C<local> on--but I<not> lexical variables, which are created
3229with C<my>. Typically it would be the first declaration in a file to
2b5ab1e7
TC
3230be included by the C<require> or C<use> operator. You can switch into a
3231package in more than one place; it merely influences which symbol table
3232is used by the compiler for the rest of that block. You can refer to
3233variables and filehandles in other packages by prefixing the identifier
3234with the package name and a double colon: C<$Package::Variable>.
3235If the package name is null, the C<main> package as assumed. That is,
3236C<$::sail> is equivalent to C<$main::sail> (as well as to C<$main'sail>,
3237still seen in older code).
cb1a09d0 3238
5a964f20
TC
3239If NAMESPACE is omitted, then there is no current package, and all
3240identifiers must be fully qualified or lexicals. This is stricter
3241than C<use strict>, since it also extends to function names.
3242
cb1a09d0
AD
3243See L<perlmod/"Packages"> for more information about packages, modules,
3244and classes. See L<perlsub> for other scoping issues.
3245
a0d0e21e
LW
3246=item pipe READHANDLE,WRITEHANDLE
3247
3248Opens a pair of connected pipes like the corresponding system call.
3249Note that if you set up a loop of piped processes, deadlock can occur
3250unless you are very careful. In addition, note that Perl's pipes use
184e9718 3251stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE
a0d0e21e
LW
3252after each command, depending on the application.
3253
7e1af8bc 3254See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
4633a7c4
LW
3255for examples of such things.
3256
4771b018
GS
3257On systems that support a close-on-exec flag on files, the flag will be set
3258for the newly opened file descriptors as determined by the value of $^F.
3259See L<perlvar/$^F>.
3260
a0d0e21e
LW
3261=item pop ARRAY
3262
54310121 3263=item pop
28757baa 3264
a0d0e21e 3265Pops and returns the last value of the array, shortening the array by
19799a22 3266one element. Has an effect similar to
a0d0e21e 3267
19799a22 3268 $ARRAY[$#ARRAY--]
a0d0e21e 3269
19799a22
GS
3270If there are no elements in the array, returns the undefined value
3271(although this may happen at other times as well). If ARRAY is
3272omitted, pops the C<@ARGV> array in the main program, and the C<@_>
3273array in subroutines, just like C<shift>.
a0d0e21e
LW
3274
3275=item pos SCALAR
3276
54310121 3277=item pos
bbce6d69 3278
4633a7c4 3279Returns the offset of where the last C<m//g> search left off for the variable
7660c0ab 3280is in question (C<$_> is used when the variable is not specified). May be
44a8e56a
PP
3281modified to change that offset. Such modification will also influence
3282the C<\G> zero-width assertion in regular expressions. See L<perlre> and
3283L<perlop>.
a0d0e21e
LW
3284
3285=item print FILEHANDLE LIST
3286
3287=item print LIST
3288
3289=item print
3290
19799a22
GS
3291Prints a string or a list of strings. Returns true if successful.
3292FILEHANDLE may be a scalar variable name, in which case the variable
3293