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1=head1 NAME
2
3perlfunc - Perl builtin functions
4
5=head1 DESCRIPTION
6
7The functions in this section can serve as terms in an expression.
8They fall into two major categories: list operators and named unary
9operators. These differ in their precedence relationship with a
10following comma. (See the precedence table in L<perlop>.) List
11operators take more than one argument, while unary operators can never
12take more than one argument. Thus, a comma terminates the argument of
13a unary operator, but merely separates the arguments of a list
14operator. A unary operator generally provides a scalar context to its
2b5ab1e7 15argument, while a list operator may provide either scalar or list
a0d0e21e 16contexts for its arguments. If it does both, the scalar arguments will
5f05dabc 17be first, and the list argument will follow. (Note that there can ever
0f31cffe 18be only one such list argument.) For instance, splice() has three scalar
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19arguments followed by a list, whereas gethostbyname() has four scalar
20arguments.
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21
22In the syntax descriptions that follow, list operators that expect a
23list (and provide list context for the elements of the list) are shown
24with LIST as an argument. Such a list may consist of any combination
25of scalar arguments or list values; the list values will be included
26in the list as if each individual element were interpolated at that
27point in the list, forming a longer single-dimensional list value.
28Elements of the LIST should be separated by commas.
29
30Any function in the list below may be used either with or without
31parentheses around its arguments. (The syntax descriptions omit the
5f05dabc 32parentheses.) If you use the parentheses, the simple (but occasionally
19799a22 33surprising) rule is this: It I<looks> like a function, therefore it I<is> a
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34function, and precedence doesn't matter. Otherwise it's a list
35operator or unary operator, and precedence does matter. And whitespace
36between the function and left parenthesis doesn't count--so you need to
37be careful sometimes:
38
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39 print 1+2+4; # Prints 7.
40 print(1+2) + 4; # Prints 3.
41 print (1+2)+4; # Also prints 3!
42 print +(1+2)+4; # Prints 7.
43 print ((1+2)+4); # Prints 7.
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44
45If you run Perl with the B<-w> switch it can warn you about this. For
46example, the third line above produces:
47
48 print (...) interpreted as function at - line 1.
49 Useless use of integer addition in void context at - line 1.
50
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51A few functions take no arguments at all, and therefore work as neither
52unary nor list operators. These include such functions as C<time>
53and C<endpwent>. For example, C<time+86_400> always means
54C<time() + 86_400>.
55
a0d0e21e 56For functions that can be used in either a scalar or list context,
54310121 57nonabortive failure is generally indicated in a scalar context by
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58returning the undefined value, and in a list context by returning the
59null list.
60
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61Remember the following important rule: There is B<no rule> that relates
62the behavior of an expression in list context to its behavior in scalar
63context, or vice versa. It might do two totally different things.
a0d0e21e 64Each operator and function decides which sort of value it would be most
2b5ab1e7 65appropriate to return in scalar context. Some operators return the
5a964f20 66length of the list that would have been returned in list context. Some
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67operators return the first value in the list. Some operators return the
68last value in the list. Some operators return a count of successful
69operations. In general, they do what you want, unless you want
70consistency.
71
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72An named array in scalar context is quite different from what would at
73first glance appear to be a list in scalar context. You can't get a list
74like C<(1,2,3)> into being in scalar context, because the compiler knows
75the context at compile time. It would generate the scalar comma operator
76there, not the list construction version of the comma. That means it
77was never a list to start with.
78
79In general, functions in Perl that serve as wrappers for system calls
f86cebdf 80of the same name (like chown(2), fork(2), closedir(2), etc.) all return
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81true when they succeed and C<undef> otherwise, as is usually mentioned
82in the descriptions below. This is different from the C interfaces,
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83which return C<-1> on failure. Exceptions to this rule are C<wait>,
84C<waitpid>, and C<syscall>. System calls also set the special C<$!>
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85variable on failure. Other functions do not, except accidentally.
86
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87=head2 Perl Functions by Category
88
89Here are Perl's functions (including things that look like
5a964f20 90functions, like some keywords and named operators)
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91arranged by category. Some functions appear in more
92than one place.
93
94=over
95
96=item Functions for SCALARs or strings
97
22fae026 98C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>,
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99C<length>, C<oct>, C<ord>, C<pack>, C<q/STRING/>, C<qq/STRING/>, C<reverse>,
100C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///>
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101
102=item Regular expressions and pattern matching
103
ab4f32c2 104C<m//>, C<pos>, C<quotemeta>, C<s///>, C<split>, C<study>, C<qr//>
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105
106=item Numeric functions
107
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108C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>,
109C<sin>, C<sqrt>, C<srand>
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110
111=item Functions for real @ARRAYs
112
22fae026 113C<pop>, C<push>, C<shift>, C<splice>, C<unshift>
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114
115=item Functions for list data
116
ab4f32c2 117C<grep>, C<join>, C<map>, C<qw/STRING/>, C<reverse>, C<sort>, C<unpack>
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118
119=item Functions for real %HASHes
120
22fae026 121C<delete>, C<each>, C<exists>, C<keys>, C<values>
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122
123=item Input and output functions
124
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125C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
126C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
127C<readdir>, C<rewinddir>, C<seek>, C<seekdir>, C<select>, C<syscall>,
128C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>,
129C<warn>, C<write>
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130
131=item Functions for fixed length data or records
132
22fae026 133C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec>
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134
135=item Functions for filehandles, files, or directories
136
22fae026 137C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>,
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138C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>,
139C<readlink>, C<rename>, C<rmdir>, C<stat>, C<symlink>, C<umask>,
140C<unlink>, C<utime>
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141
142=item Keywords related to the control flow of your perl program
143
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144C<caller>, C<continue>, C<die>, C<do>, C<dump>, C<eval>, C<exit>,
145C<goto>, C<last>, C<next>, C<redo>, C<return>, C<sub>, C<wantarray>
cb1a09d0 146
54310121 147=item Keywords related to scoping
cb1a09d0 148
22fae026 149C<caller>, C<import>, C<local>, C<my>, C<package>, C<use>
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150
151=item Miscellaneous functions
152
22fae026
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153C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<reset>,
154C<scalar>, C<undef>, C<wantarray>
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155
156=item Functions for processes and process groups
157
22fae026 158C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
ab4f32c2 159C<pipe>, C<qx/STRING/>, C<setpgrp>, C<setpriority>, C<sleep>, C<system>,
22fae026 160C<times>, C<wait>, C<waitpid>
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161
162=item Keywords related to perl modules
163
22fae026 164C<do>, C<import>, C<no>, C<package>, C<require>, C<use>
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165
166=item Keywords related to classes and object-orientedness
167
22fae026
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168C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
169C<untie>, C<use>
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170
171=item Low-level socket functions
172
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173C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
174C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
175C<socket>, C<socketpair>
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176
177=item System V interprocess communication functions
178
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179C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
180C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>
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181
182=item Fetching user and group info
183
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184C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
185C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
186C<getpwuid>, C<setgrent>, C<setpwent>
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187
188=item Fetching network info
189
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190C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
191C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
192C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
193C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
194C<setnetent>, C<setprotoent>, C<setservent>
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195
196=item Time-related functions
197
22fae026 198C<gmtime>, C<localtime>, C<time>, C<times>
cb1a09d0 199
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200=item Functions new in perl5
201
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202C<abs>, C<bless>, C<chomp>, C<chr>, C<exists>, C<formline>, C<glob>,
203C<import>, C<lc>, C<lcfirst>, C<map>, C<my>, C<no>, C<prototype>, C<qx>,
204C<qw>, C<readline>, C<readpipe>, C<ref>, C<sub*>, C<sysopen>, C<tie>,
205C<tied>, C<uc>, C<ucfirst>, C<untie>, C<use>
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206
207* - C<sub> was a keyword in perl4, but in perl5 it is an
5a964f20 208operator, which can be used in expressions.
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209
210=item Functions obsoleted in perl5
211
22fae026 212C<dbmclose>, C<dbmopen>
37798a01 213
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214=back
215
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216=head2 Portability
217
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218Perl was born in Unix and can therefore access all common Unix
219system calls. In non-Unix environments, the functionality of some
220Unix system calls may not be available, or details of the available
221functionality may differ slightly. The Perl functions affected
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222by this are:
223
224C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>,
225C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>,
226C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>,
227C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostent>,
228C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
229C<getppid>, C<getprgp>, C<getpriority>, C<getprotobynumber>,
230C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>,
231C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>,
232C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>,
2b5ab1e7 233C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<rename>, C<select>, C<semctl>,
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234C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>,
235C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>,
236C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>,
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237C<shmwrite>, C<socket>, C<socketpair>, C<stat>, C<symlink>, C<syscall>,
238C<sysopen>, C<system>, C<times>, C<truncate>, C<umask>, C<unlink>,
239C<utime>, C<wait>, C<waitpid>
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240
241For more information about the portability of these functions, see
242L<perlport> and other available platform-specific documentation.
243
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244=head2 Alphabetical Listing of Perl Functions
245
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246=over 8
247
22fae026 248=item I<-X> FILEHANDLE
a0d0e21e 249
22fae026 250=item I<-X> EXPR
a0d0e21e 251
22fae026 252=item I<-X>
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253
254A file test, where X is one of the letters listed below. This unary
255operator takes one argument, either a filename or a filehandle, and
256tests the associated file to see if something is true about it. If the
7660c0ab 257argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
19799a22 258Unless otherwise documented, it returns C<1> for true and C<''> for false, or
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259the undefined value if the file doesn't exist. Despite the funny
260names, precedence is the same as any other named unary operator, and
261the argument may be parenthesized like any other unary operator. The
262operator may be any of:
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263X<-r>X<-w>X<-x>X<-o>X<-R>X<-W>X<-X>X<-O>X<-e>X<-z>X<-s>X<-f>X<-d>X<-l>X<-p>
264X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
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265
266 -r File is readable by effective uid/gid.
267 -w File is writable by effective uid/gid.
268 -x File is executable by effective uid/gid.
269 -o File is owned by effective uid.
270
271 -R File is readable by real uid/gid.
272 -W File is writable by real uid/gid.
273 -X File is executable by real uid/gid.
274 -O File is owned by real uid.
275
276 -e File exists.
277 -z File has zero size.
54310121 278 -s File has nonzero size (returns size).
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279
280 -f File is a plain file.
281 -d File is a directory.
282 -l File is a symbolic link.
9c4d0f16 283 -p File is a named pipe (FIFO), or Filehandle is a pipe.
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284 -S File is a socket.
285 -b File is a block special file.
286 -c File is a character special file.
287 -t Filehandle is opened to a tty.
288
289 -u File has setuid bit set.
290 -g File has setgid bit set.
291 -k File has sticky bit set.
292
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293 -T File is an ASCII text file.
294 -B File is a "binary" file (opposite of -T).
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295
296 -M Age of file in days when script started.
297 -A Same for access time.
298 -C Same for inode change time.
299
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300Example:
301
302 while (<>) {
303 chop;
304 next unless -f $_; # ignore specials
5a964f20 305 #...
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306 }
307
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308The interpretation of the file permission operators C<-r>, C<-R>,
309C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
310of the file and the uids and gids of the user. There may be other
311reasons you can't actually read, write, or execute the file. Such
312reasons may be for example network filesystem access controls, ACLs
313(access control lists), read-only filesystems, and unrecognized
314executable formats.
315
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316Also note that, for the superuser on the local filesystems, the C<-r>,
317C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
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318if any execute bit is set in the mode. Scripts run by the superuser
319may thus need to do a stat() to determine the actual mode of the file,
2b5ab1e7 320or temporarily set their effective uid to something else.
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321
322If you are using ACLs, there is a pragma called C<filetest> that may
323produce more accurate results than the bare stat() mode bits.
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324When under the C<use filetest 'access'> the above-mentioned filetests
325will test whether the permission can (not) be granted using the
468541a8 326access() family of system calls. Also note that the C<-x> and C<-X> may
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327under this pragma return true even if there are no execute permission
328bits set (nor any extra execute permission ACLs). This strangeness is
329due to the underlying system calls' definitions. Read the
330documentation for the C<filetest> pragma for more information.
331
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332Note that C<-s/a/b/> does not do a negated substitution. Saying
333C<-exp($foo)> still works as expected, however--only single letters
334following a minus are interpreted as file tests.
335
336The C<-T> and C<-B> switches work as follows. The first block or so of the
337file is examined for odd characters such as strange control codes or
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">.
a0d0e21e 441
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442=item binmode FILEHANDLE, DISCIPLINE
443
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444=item binmode FILEHANDLE
445
19799a22 446Arranges for FILEHANDLE to be read or written in "binary" mode on
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447systems where the run-time libraries distinguish between binary and
448text files. If FILEHANDLE is an expression, the value is taken as the
449name of the filehandle. binmode() should be called after open() but
450before any I/O is done on the filehandle. The only way to reset
451binary mode on a filehandle is to reopen the file.
452
453On many systems binmode() has no effect, and on some systems it is
454necessary when you're not working with a text file. For the sake of
455portability it is a good idea to always use it when appropriate, and
456to never use it when it isn't appropriate.
457
458In other words: Regardless of platform, use binmode() on binary
459files, and do not use binmode() on text files.
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GS
460
461The operating system, device drivers, C libraries, and Perl run-time
30168b04
GS
462system all work together to let the programmer treat a single
463character (C<\n>) as the line terminator, irrespective of the external
464representation. On many operating systems, the native text file
465representation matches the internal representation, but on some
466platforms the external representation of C<\n> is made up of more than
467one character.
468
469Mac OS and all variants of Unix use a single character to end each line
470in the external representation of text (even though that single
471character is not necessarily the same across these platforms).
472Consequently binmode() has no effect on these operating systems. In
473other systems like VMS, MS-DOS and the various flavors of MS-Windows
474your program sees a C<\n> as a simple C<\cJ>, but what's stored in text
475files are the two characters C<\cM\cJ>. That means that, if you don't
476use binmode() on these systems, C<\cM\cJ> sequences on disk will be
477converted to C<\n> on input, and any C<\n> in your program will be
478converted back to C<\cM\cJ> on output. This is what you want for text
479files, but it can be disastrous for binary files.
480
481Another consequence of using binmode() (on some systems) is that
482special end-of-file markers will be seen as part of the data stream.
483For systems from the Microsoft family this means that if your binary
484data contains C<\cZ>, the I/O subsystem will ragard it as the end of
485the file, unless you use binmode().
486
487binmode() is not only important for readline() and print() operations,
488but also when using read(), seek(), sysread(), syswrite() and tell()
489(see L<perlport> for more details). See the C<$/> and C<$\> variables
490in L<perlvar> for how to manually set your input and output
491line-termination sequences.
a0d0e21e 492
4633a7c4 493=item bless REF,CLASSNAME
a0d0e21e
LW
494
495=item bless REF
496
2b5ab1e7
TC
497This function tells the thingy referenced by REF that it is now an object
498in the CLASSNAME package. If CLASSNAME is omitted, the current package
19799a22 499is used. Because a C<bless> is often the last thing in a constructor,
2b5ab1e7
TC
500it returns the reference for convenience. Always use the two-argument
501version if the function doing the blessing might be inherited by a
502derived class. See L<perltoot> and L<perlobj> for more about the blessing
503(and blessings) of objects.
a0d0e21e 504
57668c4d 505Consider always blessing objects in CLASSNAMEs that are mixed case.
2b5ab1e7
TC
506Namespaces with all lowercase names are considered reserved for
507Perl pragmata. Builtin types have all uppercase names, so to prevent
508confusion, you may wish to avoid such package names as well. Make sure
509that CLASSNAME is a true value.
60ad88b8
GS
510
511See L<perlmod/"Perl Modules">.
512
a0d0e21e
LW
513=item caller EXPR
514
515=item caller
516
5a964f20 517Returns the context of the current subroutine call. In scalar context,
28757baa 518returns the caller's package name if there is a caller, that is, if
19799a22 519we're in a subroutine or C<eval> or C<require>, and the undefined value
5a964f20 520otherwise. In list context, returns
a0d0e21e 521
748a9306 522 ($package, $filename, $line) = caller;
a0d0e21e
LW
523
524With EXPR, it returns some extra information that the debugger uses to
525print a stack trace. The value of EXPR indicates how many call frames
526to go back before the current one.
527
f3aa04c2 528 ($package, $filename, $line, $subroutine, $hasargs,
e476b1b5 529 $wantarray, $evaltext, $is_require, $hints, $bitmask) = caller($i);
e7ea3e70 530
951ba7fe 531Here $subroutine may be C<(eval)> if the frame is not a subroutine
19799a22 532call, but an C<eval>. In such a case additional elements $evaltext and
7660c0ab 533C<$is_require> are set: C<$is_require> is true if the frame is created by a
19799a22 534C<require> or C<use> statement, $evaltext contains the text of the
dc848c6f 535C<eval EXPR> statement. In particular, for a C<eval BLOCK> statement,
951ba7fe 536$filename is C<(eval)>, but $evaltext is undefined. (Note also that
dc848c6f 537each C<use> statement creates a C<require> frame inside an C<eval EXPR>)
e476b1b5
GS
538frame. C<$hints> and C<$bitmask> contain pragmatic hints that the caller
539was compiled with. The C<$hints> and C<$bitmask> values are subject to
540change between versions of Perl, and are not meant for external use.
748a9306
LW
541
542Furthermore, when called from within the DB package, caller returns more
7660c0ab 543detailed information: it sets the list variable C<@DB::args> to be the
54310121 544arguments with which the subroutine was invoked.
748a9306 545
7660c0ab 546Be aware that the optimizer might have optimized call frames away before
19799a22 547C<caller> had a chance to get the information. That means that C<caller(N)>
7660c0ab 548might not return information about the call frame you expect it do, for
61eff3bc 549C<< N > 1 >>. In particular, C<@DB::args> might have information from the
19799a22 550previous time C<caller> was called.
7660c0ab 551
a0d0e21e
LW
552=item chdir EXPR
553
2b5ab1e7 554Changes the working directory to EXPR, if possible. If EXPR is omitted,
0bfc1ec4
GS
555changes to the directory specified by C<$ENV{HOME}>, if set; if not,
556changes to the directory specified by C<$ENV{LOGDIR}>. If neither is
557set, C<chdir> does nothing. It returns true upon success, false
558otherwise. See the example under C<die>.
a0d0e21e
LW
559
560=item chmod LIST
561
562Changes the permissions of a list of files. The first element of the
4633a7c4 563list must be the numerical mode, which should probably be an octal
2f9daede
TPG
564number, and which definitely should I<not> a string of octal digits:
565C<0644> is okay, C<'0644'> is not. Returns the number of files
dc848c6f 566successfully changed. See also L</oct>, if all you have is a string.
a0d0e21e
LW
567
568 $cnt = chmod 0755, 'foo', 'bar';
569 chmod 0755, @executables;
f86cebdf
GS
570 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
571 # --w----r-T
2f9daede
TPG
572 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
573 $mode = 0644; chmod $mode, 'foo'; # this is best
a0d0e21e 574
ca6e1c26
JH
575You can also import the symbolic C<S_I*> constants from the Fcntl
576module:
577
578 use Fcntl ':mode';
579
580 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
581 # This is identical to the chmod 0755 of the above example.
582
a0d0e21e
LW
583=item chomp VARIABLE
584
585=item chomp LIST
586
587=item chomp
588
2b5ab1e7
TC
589This safer version of L</chop> removes any trailing string
590that corresponds to the current value of C<$/> (also known as
28757baa
PP
591$INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
592number of characters removed from all its arguments. It's often used to
593remove the newline from the end of an input record when you're worried
2b5ab1e7
TC
594that the final record may be missing its newline. When in paragraph
595mode (C<$/ = "">), it removes all trailing newlines from the string.
4c5a6083
GS
596When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
597a reference to an integer or the like, see L<perlvar>) chomp() won't
19799a22
GS
598remove anything.
599If VARIABLE is omitted, it chomps C<$_>. Example:
a0d0e21e
LW
600
601 while (<>) {
602 chomp; # avoid \n on last field
603 @array = split(/:/);
5a964f20 604 # ...
a0d0e21e
LW
605 }
606
607You can actually chomp anything that's an lvalue, including an assignment:
608
609 chomp($cwd = `pwd`);
610 chomp($answer = <STDIN>);
611
612If you chomp a list, each element is chomped, and the total number of
613characters removed is returned.
614
615=item chop VARIABLE
616
617=item chop LIST
618
619=item chop
620
621Chops off the last character of a string and returns the character
622chopped. It's used primarily to remove the newline from the end of an
623input record, but is much more efficient than C<s/\n//> because it neither
7660c0ab 624scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
a0d0e21e
LW
625Example:
626
627 while (<>) {
628 chop; # avoid \n on last field
629 @array = split(/:/);
5a964f20 630 #...
a0d0e21e
LW
631 }
632
633You can actually chop anything that's an lvalue, including an assignment:
634
635 chop($cwd = `pwd`);
636 chop($answer = <STDIN>);
637
638If you chop a list, each element is chopped. Only the value of the
19799a22 639last C<chop> is returned.
a0d0e21e 640
19799a22 641Note that C<chop> returns the last character. To return all but the last
748a9306
LW
642character, use C<substr($string, 0, -1)>.
643
a0d0e21e
LW
644=item chown LIST
645
646Changes the owner (and group) of a list of files. The first two
19799a22
GS
647elements of the list must be the I<numeric> uid and gid, in that
648order. A value of -1 in either position is interpreted by most
649systems to leave that value unchanged. Returns the number of files
650successfully changed.
a0d0e21e
LW
651
652 $cnt = chown $uid, $gid, 'foo', 'bar';
653 chown $uid, $gid, @filenames;
654
54310121 655Here's an example that looks up nonnumeric uids in the passwd file:
a0d0e21e
LW
656
657 print "User: ";
19799a22 658 chomp($user = <STDIN>);
5a964f20 659 print "Files: ";
19799a22 660 chomp($pattern = <STDIN>);
a0d0e21e
LW
661
662 ($login,$pass,$uid,$gid) = getpwnam($user)
663 or die "$user not in passwd file";
664
5a964f20 665 @ary = glob($pattern); # expand filenames
a0d0e21e
LW
666 chown $uid, $gid, @ary;
667
54310121 668On most systems, you are not allowed to change the ownership of the
4633a7c4
LW
669file unless you're the superuser, although you should be able to change
670the group to any of your secondary groups. On insecure systems, these
671restrictions may be relaxed, but this is not a portable assumption.
19799a22
GS
672On POSIX systems, you can detect this condition this way:
673
674 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
675 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
4633a7c4 676
a0d0e21e
LW
677=item chr NUMBER
678
54310121 679=item chr
bbce6d69 680
a0d0e21e 681Returns the character represented by that NUMBER in the character set.
a0ed51b3 682For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
2b5ab1e7
TC
683chr(0x263a) is a Unicode smiley face (but only within the scope of
684a C<use utf8>). For the reverse, use L</ord>.
685See L<utf8> for more about Unicode.
a0d0e21e 686
7660c0ab 687If NUMBER is omitted, uses C<$_>.
bbce6d69 688
a0d0e21e
LW
689=item chroot FILENAME
690
54310121 691=item chroot
bbce6d69 692
5a964f20 693This function works like the system call by the same name: it makes the
4633a7c4 694named directory the new root directory for all further pathnames that
951ba7fe 695begin with a C</> by your process and all its children. (It doesn't
28757baa 696change your current working directory, which is unaffected.) For security
4633a7c4 697reasons, this call is restricted to the superuser. If FILENAME is
19799a22 698omitted, does a C<chroot> to C<$_>.
a0d0e21e
LW
699
700=item close FILEHANDLE
701
6a518fbc
TP
702=item close
703
19799a22 704Closes the file or pipe associated with the file handle, returning true
a0d0e21e 705only if stdio successfully flushes buffers and closes the system file
19799a22 706descriptor. Closes the currently selected filehandle if the argument
6a518fbc 707is omitted.
fb73857a
PP
708
709You don't have to close FILEHANDLE if you are immediately going to do
19799a22
GS
710another C<open> on it, because C<open> will close it for you. (See
711C<open>.) However, an explicit C<close> on an input file resets the line
712counter (C<$.>), while the implicit close done by C<open> does not.
fb73857a 713
19799a22
GS
714If the file handle came from a piped open C<close> will additionally
715return false if one of the other system calls involved fails or if the
fb73857a 716program exits with non-zero status. (If the only problem was that the
2b5ab1e7
TC
717program exited non-zero C<$!> will be set to C<0>.) Closing a pipe
718also waits for the process executing on the pipe to complete, in case you
719want to look at the output of the pipe afterwards, and
720implicitly puts the exit status value of that command into C<$?>.
5a964f20 721
73689b13
GS
722Prematurely closing the read end of a pipe (i.e. before the process
723writing to it at the other end has closed it) will result in a
724SIGPIPE being delivered to the writer. If the other end can't
725handle that, be sure to read all the data before closing the pipe.
726
fb73857a 727Example:
a0d0e21e 728
fb73857a
PP
729 open(OUTPUT, '|sort >foo') # pipe to sort
730 or die "Can't start sort: $!";
5a964f20 731 #... # print stuff to output
fb73857a
PP
732 close OUTPUT # wait for sort to finish
733 or warn $! ? "Error closing sort pipe: $!"
734 : "Exit status $? from sort";
735 open(INPUT, 'foo') # get sort's results
736 or die "Can't open 'foo' for input: $!";
a0d0e21e 737
5a964f20
TC
738FILEHANDLE may be an expression whose value can be used as an indirect
739filehandle, usually the real filehandle name.
a0d0e21e
LW
740
741=item closedir DIRHANDLE
742
19799a22 743Closes a directory opened by C<opendir> and returns the success of that
5a964f20
TC
744system call.
745
746DIRHANDLE may be an expression whose value can be used as an indirect
747dirhandle, usually the real dirhandle name.
a0d0e21e
LW
748
749=item connect SOCKET,NAME
750
751Attempts to connect to a remote socket, just as the connect system call
19799a22 752does. Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
753packed address of the appropriate type for the socket. See the examples in
754L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 755
cb1a09d0
AD
756=item continue BLOCK
757
758Actually a flow control statement rather than a function. If there is a
98293880
JH
759C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
760C<foreach>), it is always executed just before the conditional is about to
761be evaluated again, just like the third part of a C<for> loop in C. Thus
cb1a09d0
AD
762it can be used to increment a loop variable, even when the loop has been
763continued via the C<next> statement (which is similar to the C C<continue>
764statement).
765
98293880 766C<last>, C<next>, or C<redo> may appear within a C<continue>
19799a22
GS
767block. C<last> and C<redo> will behave as if they had been executed within
768the main block. So will C<next>, but since it will execute a C<continue>
1d2dff63
GS
769block, it may be more entertaining.
770
771 while (EXPR) {
772 ### redo always comes here
773 do_something;
774 } continue {
775 ### next always comes here
776 do_something_else;
777 # then back the top to re-check EXPR
778 }
779 ### last always comes here
780
781Omitting the C<continue> section is semantically equivalent to using an
19799a22 782empty one, logically enough. In that case, C<next> goes directly back
1d2dff63
GS
783to check the condition at the top of the loop.
784
a0d0e21e
LW
785=item cos EXPR
786
5a964f20 787Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
7660c0ab 788takes cosine of C<$_>.
a0d0e21e 789
ca6e1c26 790For the inverse cosine operation, you may use the C<Math::Trig::acos()>
28757baa
PP
791function, or use this relation:
792
793 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
794
a0d0e21e
LW
795=item crypt PLAINTEXT,SALT
796
f86cebdf 797Encrypts a string exactly like the crypt(3) function in the C library
4633a7c4
LW
798(assuming that you actually have a version there that has not been
799extirpated as a potential munition). This can prove useful for checking
800the password file for lousy passwords, amongst other things. Only the
801guys wearing white hats should do this.
a0d0e21e 802
19799a22 803Note that C<crypt> is intended to be a one-way function, much like breaking
11155c91
CS
804eggs to make an omelette. There is no (known) corresponding decrypt
805function. As a result, this function isn't all that useful for
806cryptography. (For that, see your nearby CPAN mirror.)
2f9daede 807
e71965be
RS
808When verifying an existing encrypted string you should use the encrypted
809text as the salt (like C<crypt($plain, $crypted) eq $crypted>). This
19799a22 810allows your code to work with the standard C<crypt> and with more
e71965be
RS
811exotic implementations. When choosing a new salt create a random two
812character string whose characters come from the set C<[./0-9A-Za-z]>
813(like C<join '', ('.', '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>).
814
a0d0e21e
LW
815Here's an example that makes sure that whoever runs this program knows
816their own password:
817
818 $pwd = (getpwuid($<))[1];
a0d0e21e
LW
819
820 system "stty -echo";
821 print "Password: ";
e71965be 822 chomp($word = <STDIN>);
a0d0e21e
LW
823 print "\n";
824 system "stty echo";
825
e71965be 826 if (crypt($word, $pwd) ne $pwd) {
a0d0e21e
LW
827 die "Sorry...\n";
828 } else {
829 print "ok\n";
54310121 830 }
a0d0e21e 831
9f8f0c9d 832Of course, typing in your own password to whoever asks you
748a9306 833for it is unwise.
a0d0e21e 834
19799a22
GS
835The L<crypt> function is unsuitable for encrypting large quantities
836of data, not least of all because you can't get the information
837back. Look at the F<by-module/Crypt> and F<by-module/PGP> directories
838on your favorite CPAN mirror for a slew of potentially useful
839modules.
840
aa689395 841=item dbmclose HASH
a0d0e21e 842
19799a22 843[This function has been largely superseded by the C<untie> function.]
a0d0e21e 844
aa689395 845Breaks the binding between a DBM file and a hash.
a0d0e21e 846
19799a22 847=item dbmopen HASH,DBNAME,MASK
a0d0e21e 848
19799a22 849[This function has been largely superseded by the C<tie> function.]
a0d0e21e 850
7b8d334a 851This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
19799a22
GS
852hash. HASH is the name of the hash. (Unlike normal C<open>, the first
853argument is I<not> a filehandle, even though it looks like one). DBNAME
aa689395
PP
854is the name of the database (without the F<.dir> or F<.pag> extension if
855any). If the database does not exist, it is created with protection
19799a22
GS
856specified by MASK (as modified by the C<umask>). If your system supports
857only the older DBM functions, you may perform only one C<dbmopen> in your
aa689395 858program. In older versions of Perl, if your system had neither DBM nor
19799a22 859ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
aa689395
PP
860sdbm(3).
861
862If you don't have write access to the DBM file, you can only read hash
863variables, not set them. If you want to test whether you can write,
19799a22 864either use file tests or try setting a dummy hash entry inside an C<eval>,
aa689395 865which will trap the error.
a0d0e21e 866
19799a22
GS
867Note that functions such as C<keys> and C<values> may return huge lists
868when used on large DBM files. You may prefer to use the C<each>
a0d0e21e
LW
869function to iterate over large DBM files. Example:
870
871 # print out history file offsets
872 dbmopen(%HIST,'/usr/lib/news/history',0666);
873 while (($key,$val) = each %HIST) {
874 print $key, ' = ', unpack('L',$val), "\n";
875 }
876 dbmclose(%HIST);
877
cb1a09d0 878See also L<AnyDBM_File> for a more general description of the pros and
184e9718 879cons of the various dbm approaches, as well as L<DB_File> for a particularly
cb1a09d0 880rich implementation.
4633a7c4 881
2b5ab1e7
TC
882You can control which DBM library you use by loading that library
883before you call dbmopen():
884
885 use DB_File;
886 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
887 or die "Can't open netscape history file: $!";
888
a0d0e21e
LW
889=item defined EXPR
890
54310121 891=item defined
bbce6d69 892
2f9daede
TPG
893Returns a Boolean value telling whether EXPR has a value other than
894the undefined value C<undef>. If EXPR is not present, C<$_> will be
895checked.
896
897Many operations return C<undef> to indicate failure, end of file,
898system error, uninitialized variable, and other exceptional
899conditions. This function allows you to distinguish C<undef> from
900other values. (A simple Boolean test will not distinguish among
7660c0ab 901C<undef>, zero, the empty string, and C<"0">, which are all equally
2f9daede 902false.) Note that since C<undef> is a valid scalar, its presence
19799a22 903doesn't I<necessarily> indicate an exceptional condition: C<pop>
2f9daede
TPG
904returns C<undef> when its argument is an empty array, I<or> when the
905element to return happens to be C<undef>.
906
f10b0346
GS
907You may also use C<defined(&func)> to check whether subroutine C<&func>
908has ever been defined. The return value is unaffected by any forward
909declarations of C<&foo>.
910
911Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
912used to report whether memory for that aggregate has ever been
913allocated. This behavior may disappear in future versions of Perl.
914You should instead use a simple test for size:
915
916 if (@an_array) { print "has array elements\n" }
917 if (%a_hash) { print "has hash members\n" }
2f9daede
TPG
918
919When used on a hash element, it tells you whether the value is defined,
dc848c6f 920not whether the key exists in the hash. Use L</exists> for the latter
2f9daede 921purpose.
a0d0e21e
LW
922
923Examples:
924
925 print if defined $switch{'D'};
926 print "$val\n" while defined($val = pop(@ary));
927 die "Can't readlink $sym: $!"
928 unless defined($value = readlink $sym);
a0d0e21e 929 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
2f9daede 930 $debugging = 0 unless defined $debugging;
a0d0e21e 931
19799a22 932Note: Many folks tend to overuse C<defined>, and then are surprised to
7660c0ab 933discover that the number C<0> and C<""> (the zero-length string) are, in fact,
2f9daede 934defined values. For example, if you say
a5f75d66
AD
935
936 "ab" =~ /a(.*)b/;
937
7660c0ab 938The pattern match succeeds, and C<$1> is defined, despite the fact that it
a5f75d66 939matched "nothing". But it didn't really match nothing--rather, it
2b5ab1e7 940matched something that happened to be zero characters long. This is all
a5f75d66 941very above-board and honest. When a function returns an undefined value,
2f9daede 942it's an admission that it couldn't give you an honest answer. So you
19799a22 943should use C<defined> only when you're questioning the integrity of what
7660c0ab 944you're trying to do. At other times, a simple comparison to C<0> or C<""> is
2f9daede
TPG
945what you want.
946
dc848c6f 947See also L</undef>, L</exists>, L</ref>.
2f9daede 948
a0d0e21e
LW
949=item delete EXPR
950
01020589
GS
951Given an expression that specifies a hash element, array element, hash slice,
952or array slice, deletes the specified element(s) from the hash or array.
8216c1fd
GS
953In the case of an array, if the array elements happen to be at the end,
954the size of the array will shrink to the highest element that tests
955true for exists() (or 0 if no such element exists).
a0d0e21e 956
01020589
GS
957Returns each element so deleted or the undefined value if there was no such
958element. Deleting from C<$ENV{}> modifies the environment. Deleting from
959a hash tied to a DBM file deletes the entry from the DBM file. Deleting
960from a C<tie>d hash or array may not necessarily return anything.
961
8ea97a1e
GS
962Deleting an array element effectively returns that position of the array
963to its initial, uninitialized state. Subsequently testing for the same
8216c1fd
GS
964element with exists() will return false. Note that deleting array
965elements in the middle of an array will not shift the index of the ones
966after them down--use splice() for that. See L</exists>.
8ea97a1e 967
01020589 968The following (inefficiently) deletes all the values of %HASH and @ARRAY:
a0d0e21e 969
5f05dabc
PP
970 foreach $key (keys %HASH) {
971 delete $HASH{$key};
a0d0e21e
LW
972 }
973
01020589
GS
974 foreach $index (0 .. $#ARRAY) {
975 delete $ARRAY[$index];
976 }
977
978And so do these:
5f05dabc 979
01020589
GS
980 delete @HASH{keys %HASH};
981
9740c838 982 delete @ARRAY[0 .. $#ARRAY];
5f05dabc 983
2b5ab1e7 984But both of these are slower than just assigning the empty list
01020589
GS
985or undefining %HASH or @ARRAY:
986
987 %HASH = (); # completely empty %HASH
988 undef %HASH; # forget %HASH ever existed
2b5ab1e7 989
01020589
GS
990 @ARRAY = (); # completely empty @ARRAY
991 undef @ARRAY; # forget @ARRAY ever existed
2b5ab1e7
TC
992
993Note that the EXPR can be arbitrarily complicated as long as the final
01020589
GS
994operation is a hash element, array element, hash slice, or array slice
995lookup:
a0d0e21e
LW
996
997 delete $ref->[$x][$y]{$key};
5f05dabc 998 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
a0d0e21e 999
01020589
GS
1000 delete $ref->[$x][$y][$index];
1001 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1002
a0d0e21e
LW
1003=item die LIST
1004
19799a22
GS
1005Outside an C<eval>, prints the value of LIST to C<STDERR> and
1006exits with the current value of C<$!> (errno). If C<$!> is C<0>,
61eff3bc
JH
1007exits with the value of C<<< ($? >> 8) >>> (backtick `command`
1008status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside
19799a22
GS
1009an C<eval(),> the error message is stuffed into C<$@> and the
1010C<eval> is terminated with the undefined value. This makes
1011C<die> the way to raise an exception.
a0d0e21e
LW
1012
1013Equivalent examples:
1014
1015 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
54310121 1016 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
a0d0e21e
LW
1017
1018If the value of EXPR does not end in a newline, the current script line
1019number and input line number (if any) are also printed, and a newline
883faa13
GS
1020is supplied. Note that the "input line number" (also known as "chunk")
1021is subject to whatever notion of "line" happens to be currently in
1022effect, and is also available as the special variable C<$.>.
1023See L<perlvar/"$/"> and L<perlvar/"$.">.
1024
1025Hint: sometimes appending C<", stopped"> to your message
7660c0ab 1026will cause it to make better sense when the string C<"at foo line 123"> is
a0d0e21e
LW
1027appended. Suppose you are running script "canasta".
1028
1029 die "/etc/games is no good";
1030 die "/etc/games is no good, stopped";
1031
1032produce, respectively
1033
1034 /etc/games is no good at canasta line 123.
1035 /etc/games is no good, stopped at canasta line 123.
1036
2b5ab1e7 1037See also exit(), warn(), and the Carp module.
a0d0e21e 1038
7660c0ab
A
1039If LIST is empty and C<$@> already contains a value (typically from a
1040previous eval) that value is reused after appending C<"\t...propagated">.
fb73857a
PP
1041This is useful for propagating exceptions:
1042
1043 eval { ... };
1044 die unless $@ =~ /Expected exception/;
1045
7660c0ab 1046If C<$@> is empty then the string C<"Died"> is used.
fb73857a 1047
52531d10
GS
1048die() can also be called with a reference argument. If this happens to be
1049trapped within an eval(), $@ contains the reference. This behavior permits
1050a more elaborate exception handling implementation using objects that
1051maintain arbitary state about the nature of the exception. Such a scheme
1052is sometimes preferable to matching particular string values of $@ using
1053regular expressions. Here's an example:
1054
1055 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1056 if ($@) {
1057 if (ref($@) && UNIVERSAL::isa($@,"Some::Module::Exception")) {
1058 # handle Some::Module::Exception
1059 }
1060 else {
1061 # handle all other possible exceptions
1062 }
1063 }
1064
19799a22 1065Because perl will stringify uncaught exception messages before displaying
52531d10
GS
1066them, you may want to overload stringification operations on such custom
1067exception objects. See L<overload> for details about that.
1068
19799a22
GS
1069You can arrange for a callback to be run just before the C<die>
1070does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1071handler will be called with the error text and can change the error
1072message, if it sees fit, by calling C<die> again. See
1073L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
1074L<"eval BLOCK"> for some examples. Although this feature was meant
1075to be run only right before your program was to exit, this is not
1076currently the case--the C<$SIG{__DIE__}> hook is currently called
1077even inside eval()ed blocks/strings! If one wants the hook to do
1078nothing in such situations, put
fb73857a
PP
1079
1080 die @_ if $^S;
1081
19799a22
GS
1082as the first line of the handler (see L<perlvar/$^S>). Because
1083this promotes strange action at a distance, this counterintuitive
1084behavior may be fixed in a future release.
774d564b 1085
a0d0e21e
LW
1086=item do BLOCK
1087
1088Not really a function. Returns the value of the last command in the
1089sequence of commands indicated by BLOCK. When modified by a loop
98293880
JH
1090modifier, executes the BLOCK once before testing the loop condition.
1091(On other statements the loop modifiers test the conditional first.)
a0d0e21e 1092
4968c1e4 1093C<do BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7
TC
1094C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1095See L<perlsyn> for alternative strategies.
4968c1e4 1096
a0d0e21e
LW
1097=item do SUBROUTINE(LIST)
1098
1099A deprecated form of subroutine call. See L<perlsub>.
1100
1101=item do EXPR
1102
1103Uses the value of EXPR as a filename and executes the contents of the
1104file as a Perl script. Its primary use is to include subroutines
1105from a Perl subroutine library.
1106
1107 do 'stat.pl';
1108
1109is just like
1110
fb73857a 1111 scalar eval `cat stat.pl`;
a0d0e21e 1112
2b5ab1e7
TC
1113except that it's more efficient and concise, keeps track of the current
1114filename for error messages, searches the @INC libraries, and updates
1115C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1116variables. It also differs in that code evaluated with C<do FILENAME>
1117cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1118same, however, in that it does reparse the file every time you call it,
1119so you probably don't want to do this inside a loop.
a0d0e21e 1120
8e30cc93 1121If C<do> cannot read the file, it returns undef and sets C<$!> to the
2b5ab1e7 1122error. If C<do> can read the file but cannot compile it, it
8e30cc93
G
1123returns undef and sets an error message in C<$@>. If the file is
1124successfully compiled, C<do> returns the value of the last expression
1125evaluated.
1126
a0d0e21e 1127Note that inclusion of library modules is better done with the
19799a22 1128C<use> and C<require> operators, which also do automatic error checking
4633a7c4 1129and raise an exception if there's a problem.
a0d0e21e 1130
5a964f20
TC
1131You might like to use C<do> to read in a program configuration
1132file. Manual error checking can be done this way:
1133
1134 # read in config files: system first, then user
f86cebdf 1135 for $file ("/share/prog/defaults.rc",
2b5ab1e7
TC
1136 "$ENV{HOME}/.someprogrc")
1137 {
5a964f20 1138 unless ($return = do $file) {
f86cebdf
GS
1139 warn "couldn't parse $file: $@" if $@;
1140 warn "couldn't do $file: $!" unless defined $return;
1141 warn "couldn't run $file" unless $return;
5a964f20
TC
1142 }
1143 }
1144
a0d0e21e
LW
1145=item dump LABEL
1146
1614b0e3
JD
1147=item dump
1148
19799a22
GS
1149This function causes an immediate core dump. See also the B<-u>
1150command-line switch in L<perlrun>, which does the same thing.
1151Primarily this is so that you can use the B<undump> program (not
1152supplied) to turn your core dump into an executable binary after
1153having initialized all your variables at the beginning of the
1154program. When the new binary is executed it will begin by executing
1155a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1156Think of it as a goto with an intervening core dump and reincarnation.
1157If C<LABEL> is omitted, restarts the program from the top.
1158
1159B<WARNING>: Any files opened at the time of the dump will I<not>
1160be open any more when the program is reincarnated, with possible
1161resulting confusion on the part of Perl.
1162
1163This function is now largely obsolete, partly because it's very
1164hard to convert a core file into an executable, and because the
1165real compiler backends for generating portable bytecode and compilable
1166C code have superseded it.
1167
1168If you're looking to use L<dump> to speed up your program, consider
1169generating bytecode or native C code as described in L<perlcc>. If
1170you're just trying to accelerate a CGI script, consider using the
1171C<mod_perl> extension to B<Apache>, or the CPAN module, Fast::CGI.
1172You might also consider autoloading or selfloading, which at least
1173make your program I<appear> to run faster.
5a964f20 1174
aa689395
PP
1175=item each HASH
1176
5a964f20 1177When called in list context, returns a 2-element list consisting of the
aa689395 1178key and value for the next element of a hash, so that you can iterate over
5a964f20 1179it. When called in scalar context, returns the key for only the "next"
e902a979 1180element in the hash.
2f9daede 1181
ab192400
GS
1182Entries are returned in an apparently random order. The actual random
1183order is subject to change in future versions of perl, but it is guaranteed
19799a22 1184to be in the same order as either the C<keys> or C<values> function
ab192400
GS
1185would produce on the same (unmodified) hash.
1186
1187When the hash is entirely read, a null array is returned in list context
19799a22
GS
1188(which when assigned produces a false (C<0>) value), and C<undef> in
1189scalar context. The next call to C<each> after that will start iterating
1190again. There is a single iterator for each hash, shared by all C<each>,
1191C<keys>, and C<values> function calls in the program; it can be reset by
2f9daede
TPG
1192reading all the elements from the hash, or by evaluating C<keys HASH> or
1193C<values HASH>. If you add or delete elements of a hash while you're
1194iterating over it, you may get entries skipped or duplicated, so don't.
aa689395 1195
f86cebdf 1196The following prints out your environment like the printenv(1) program,
aa689395 1197only in a different order:
a0d0e21e
LW
1198
1199 while (($key,$value) = each %ENV) {
1200 print "$key=$value\n";
1201 }
1202
19799a22 1203See also C<keys>, C<values> and C<sort>.
a0d0e21e
LW
1204
1205=item eof FILEHANDLE
1206
4633a7c4
LW
1207=item eof ()
1208
a0d0e21e
LW
1209=item eof
1210
1211Returns 1 if the next read on FILEHANDLE will return end of file, or if
1212FILEHANDLE is not open. FILEHANDLE may be an expression whose value
5a964f20 1213gives the real filehandle. (Note that this function actually
19799a22 1214reads a character and then C<ungetc>s it, so isn't very useful in an
748a9306 1215interactive context.) Do not read from a terminal file (or call
19799a22 1216C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
748a9306
LW
1217as terminals may lose the end-of-file condition if you do.
1218
820475bd
GS
1219An C<eof> without an argument uses the last file read. Using C<eof()>
1220with empty parentheses is very different. It refers to the pseudo file
1221formed from the files listed on the command line and accessed via the
61eff3bc
JH
1222C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1223as a normal filehandle is, an C<eof()> before C<< <> >> has been
820475bd
GS
1224used will cause C<@ARGV> to be examined to determine if input is
1225available.
1226
61eff3bc 1227In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
820475bd
GS
1228detect the end of each file, C<eof()> will only detect the end of the
1229last file. Examples:
a0d0e21e 1230
748a9306
LW
1231 # reset line numbering on each input file
1232 while (<>) {
5a964f20 1233 next if /^\s*#/; # skip comments
748a9306 1234 print "$.\t$_";
5a964f20
TC
1235 } continue {
1236 close ARGV if eof; # Not eof()!
748a9306
LW
1237 }
1238
a0d0e21e
LW
1239 # insert dashes just before last line of last file
1240 while (<>) {
5a964f20 1241 if (eof()) { # check for end of current file
a0d0e21e 1242 print "--------------\n";
2b5ab1e7 1243 close(ARGV); # close or last; is needed if we
748a9306 1244 # are reading from the terminal
a0d0e21e
LW
1245 }
1246 print;
1247 }
1248
a0d0e21e 1249Practical hint: you almost never need to use C<eof> in Perl, because the
3ce0d271
GS
1250input operators typically return C<undef> when they run out of data, or if
1251there was an error.
a0d0e21e
LW
1252
1253=item eval EXPR
1254
1255=item eval BLOCK
1256
c7cc6f1c
GS
1257In the first form, the return value of EXPR is parsed and executed as if it
1258were a little Perl program. The value of the expression (which is itself
5a964f20 1259determined within scalar context) is first parsed, and if there weren't any
c7cc6f1c 1260errors, executed in the context of the current Perl program, so that any
5f05dabc 1261variable settings or subroutine and format definitions remain afterwards.
c7cc6f1c
GS
1262Note that the value is parsed every time the eval executes. If EXPR is
1263omitted, evaluates C<$_>. This form is typically used to delay parsing
1264and subsequent execution of the text of EXPR until run time.
1265
1266In the second form, the code within the BLOCK is parsed only once--at the
1267same time the code surrounding the eval itself was parsed--and executed
1268within the context of the current Perl program. This form is typically
1269used to trap exceptions more efficiently than the first (see below), while
1270also providing the benefit of checking the code within BLOCK at compile
1271time.
1272
1273The final semicolon, if any, may be omitted from the value of EXPR or within
1274the BLOCK.
1275
1276In both forms, the value returned is the value of the last expression
5a964f20 1277evaluated inside the mini-program; a return statement may be also used, just
c7cc6f1c 1278as with subroutines. The expression providing the return value is evaluated
5a964f20 1279in void, scalar, or list context, depending on the context of the eval itself.
c7cc6f1c 1280See L</wantarray> for more on how the evaluation context can be determined.
a0d0e21e 1281
19799a22
GS
1282If there is a syntax error or runtime error, or a C<die> statement is
1283executed, an undefined value is returned by C<eval>, and C<$@> is set to the
a0d0e21e 1284error message. If there was no error, C<$@> is guaranteed to be a null
19799a22 1285string. Beware that using C<eval> neither silences perl from printing
c7cc6f1c
GS
1286warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1287To do either of those, you have to use the C<$SIG{__WARN__}> facility. See
1288L</warn> and L<perlvar>.
a0d0e21e 1289
19799a22
GS
1290Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1291determining whether a particular feature (such as C<socket> or C<symlink>)
a0d0e21e
LW
1292is implemented. It is also Perl's exception trapping mechanism, where
1293the die operator is used to raise exceptions.
1294
1295If the code to be executed doesn't vary, you may use the eval-BLOCK
1296form to trap run-time errors without incurring the penalty of
1297recompiling each time. The error, if any, is still returned in C<$@>.
1298Examples:
1299
54310121 1300 # make divide-by-zero nonfatal
a0d0e21e
LW
1301 eval { $answer = $a / $b; }; warn $@ if $@;
1302
1303 # same thing, but less efficient
1304 eval '$answer = $a / $b'; warn $@ if $@;
1305
1306 # a compile-time error
5a964f20 1307 eval { $answer = }; # WRONG
a0d0e21e
LW
1308
1309 # a run-time error
1310 eval '$answer ='; # sets $@
1311
2b5ab1e7
TC
1312Due to the current arguably broken state of C<__DIE__> hooks, when using
1313the C<eval{}> form as an exception trap in libraries, you may wish not
1314to trigger any C<__DIE__> hooks that user code may have installed.
1315You can use the C<local $SIG{__DIE__}> construct for this purpose,
1316as shown in this example:
774d564b
PP
1317
1318 # a very private exception trap for divide-by-zero
f86cebdf
GS
1319 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1320 warn $@ if $@;
774d564b
PP
1321
1322This is especially significant, given that C<__DIE__> hooks can call
19799a22 1323C<die> again, which has the effect of changing their error messages:
774d564b
PP
1324
1325 # __DIE__ hooks may modify error messages
1326 {
f86cebdf
GS
1327 local $SIG{'__DIE__'} =
1328 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
c7cc6f1c
GS
1329 eval { die "foo lives here" };
1330 print $@ if $@; # prints "bar lives here"
774d564b
PP
1331 }
1332
19799a22 1333Because this promotes action at a distance, this counterintuitive behavior
2b5ab1e7
TC
1334may be fixed in a future release.
1335
19799a22 1336With an C<eval>, you should be especially careful to remember what's
a0d0e21e
LW
1337being looked at when:
1338
1339 eval $x; # CASE 1
1340 eval "$x"; # CASE 2
1341
1342 eval '$x'; # CASE 3
1343 eval { $x }; # CASE 4
1344
5a964f20 1345 eval "\$$x++"; # CASE 5
a0d0e21e
LW
1346 $$x++; # CASE 6
1347
2f9daede 1348Cases 1 and 2 above behave identically: they run the code contained in
19799a22 1349the variable $x. (Although case 2 has misleading double quotes making
2f9daede 1350the reader wonder what else might be happening (nothing is).) Cases 3
7660c0ab 1351and 4 likewise behave in the same way: they run the code C<'$x'>, which
19799a22 1352does nothing but return the value of $x. (Case 4 is preferred for
2f9daede
TPG
1353purely visual reasons, but it also has the advantage of compiling at
1354compile-time instead of at run-time.) Case 5 is a place where
19799a22 1355normally you I<would> like to use double quotes, except that in this
2f9daede
TPG
1356particular situation, you can just use symbolic references instead, as
1357in case 6.
a0d0e21e 1358
4968c1e4 1359C<eval BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7 1360C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
4968c1e4 1361
a0d0e21e
LW
1362=item exec LIST
1363
8bf3b016
GS
1364=item exec PROGRAM LIST
1365
19799a22
GS
1366The C<exec> function executes a system command I<and never returns>--
1367use C<system> instead of C<exec> if you want it to return. It fails and
1368returns false only if the command does not exist I<and> it is executed
fb73857a 1369directly instead of via your system's command shell (see below).
a0d0e21e 1370
19799a22
GS
1371Since it's a common mistake to use C<exec> instead of C<system>, Perl
1372warns you if there is a following statement which isn't C<die>, C<warn>,
1373or C<exit> (if C<-w> is set - but you always do that). If you
1374I<really> want to follow an C<exec> with some other statement, you
55d729e4
GS
1375can use one of these styles to avoid the warning:
1376
5a964f20
TC
1377 exec ('foo') or print STDERR "couldn't exec foo: $!";
1378 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
55d729e4 1379
5a964f20 1380If there is more than one argument in LIST, or if LIST is an array
f86cebdf 1381with more than one value, calls execvp(3) with the arguments in LIST.
5a964f20
TC
1382If there is only one scalar argument or an array with one element in it,
1383the argument is checked for shell metacharacters, and if there are any,
1384the entire argument is passed to the system's command shell for parsing
1385(this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1386If there are no shell metacharacters in the argument, it is split into
19799a22
GS
1387words and passed directly to C<execvp>, which is more efficient.
1388Examples:
a0d0e21e 1389
19799a22
GS
1390 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1391 exec "sort $outfile | uniq";
a0d0e21e
LW
1392
1393If you don't really want to execute the first argument, but want to lie
1394to the program you are executing about its own name, you can specify
1395the program you actually want to run as an "indirect object" (without a
1396comma) in front of the LIST. (This always forces interpretation of the
54310121 1397LIST as a multivalued list, even if there is only a single scalar in
a0d0e21e
LW
1398the list.) Example:
1399
1400 $shell = '/bin/csh';
1401 exec $shell '-sh'; # pretend it's a login shell
1402
1403or, more directly,
1404
1405 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1406
bb32b41a
GS
1407When the arguments get executed via the system shell, results will
1408be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1409for details.
1410
19799a22
GS
1411Using an indirect object with C<exec> or C<system> is also more
1412secure. This usage (which also works fine with system()) forces
1413interpretation of the arguments as a multivalued list, even if the
1414list had just one argument. That way you're safe from the shell
1415expanding wildcards or splitting up words with whitespace in them.
5a964f20
TC
1416
1417 @args = ( "echo surprise" );
1418
2b5ab1e7 1419 exec @args; # subject to shell escapes
f86cebdf 1420 # if @args == 1
2b5ab1e7 1421 exec { $args[0] } @args; # safe even with one-arg list
5a964f20
TC
1422
1423The first version, the one without the indirect object, ran the I<echo>
1424program, passing it C<"surprise"> an argument. The second version
1425didn't--it tried to run a program literally called I<"echo surprise">,
1426didn't find it, and set C<$?> to a non-zero value indicating failure.
1427
19799a22 1428Note that C<exec> will not call your C<END> blocks, nor will it call
7660c0ab
A
1429any C<DESTROY> methods in your objects.
1430
a0d0e21e
LW
1431=item exists EXPR
1432
01020589 1433Given an expression that specifies a hash element or array element,
8ea97a1e
GS
1434returns true if the specified element in the hash or array has ever
1435been initialized, even if the corresponding value is undefined. The
1436element is not autovivified if it doesn't exist.
a0d0e21e 1437
01020589
GS
1438 print "Exists\n" if exists $hash{$key};
1439 print "Defined\n" if defined $hash{$key};
1440 print "True\n" if $hash{$key};
1441
1442 print "Exists\n" if exists $array[$index];
1443 print "Defined\n" if defined $array[$index];
1444 print "True\n" if $array[$index];
a0d0e21e 1445
8ea97a1e 1446A hash or array element can be true only if it's defined, and defined if
a0d0e21e
LW
1447it exists, but the reverse doesn't necessarily hold true.
1448
afebc493
GS
1449Given an expression that specifies the name of a subroutine,
1450returns true if the specified subroutine has ever been declared, even
1451if it is undefined. Mentioning a subroutine name for exists or defined
1452does not count as declaring it.
1453
1454 print "Exists\n" if exists &subroutine;
1455 print "Defined\n" if defined &subroutine;
1456
a0d0e21e 1457Note that the EXPR can be arbitrarily complicated as long as the final
afebc493 1458operation is a hash or array key lookup or subroutine name:
a0d0e21e 1459
2b5ab1e7
TC
1460 if (exists $ref->{A}->{B}->{$key}) { }
1461 if (exists $hash{A}{B}{$key}) { }
1462
01020589
GS
1463 if (exists $ref->{A}->{B}->[$ix]) { }
1464 if (exists $hash{A}{B}[$ix]) { }
1465
afebc493
GS
1466 if (exists &{$ref->{A}{B}{$key}}) { }
1467
01020589
GS
1468Although the deepest nested array or hash will not spring into existence
1469just because its existence was tested, any intervening ones will.
61eff3bc 1470Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
01020589
GS
1471into existence due to the existence test for the $key element above.
1472This happens anywhere the arrow operator is used, including even:
5a964f20 1473
2b5ab1e7
TC
1474 undef $ref;
1475 if (exists $ref->{"Some key"}) { }
1476 print $ref; # prints HASH(0x80d3d5c)
1477
1478This surprising autovivification in what does not at first--or even
1479second--glance appear to be an lvalue context may be fixed in a future
5a964f20 1480release.
a0d0e21e 1481
479ba383
GS
1482See L<perlref/"Pseudo-hashes: Using an array as a hash"> for specifics
1483on how exists() acts when used on a pseudo-hash.
e0478e5a 1484
afebc493
GS
1485Use of a subroutine call, rather than a subroutine name, as an argument
1486to exists() is an error.
1487
1488 exists &sub; # OK
1489 exists &sub(); # Error
1490
a0d0e21e
LW
1491=item exit EXPR
1492
2b5ab1e7 1493Evaluates EXPR and exits immediately with that value. Example:
a0d0e21e
LW
1494
1495 $ans = <STDIN>;
1496 exit 0 if $ans =~ /^[Xx]/;
1497
19799a22 1498See also C<die>. If EXPR is omitted, exits with C<0> status. The only
2b5ab1e7
TC
1499universally recognized values for EXPR are C<0> for success and C<1>
1500for error; other values are subject to interpretation depending on the
1501environment in which the Perl program is running. For example, exiting
150269 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1503the mailer to return the item undelivered, but that's not true everywhere.
a0d0e21e 1504
19799a22
GS
1505Don't use C<exit> to abort a subroutine if there's any chance that
1506someone might want to trap whatever error happened. Use C<die> instead,
1507which can be trapped by an C<eval>.
28757baa 1508
19799a22 1509The exit() function does not always exit immediately. It calls any
2b5ab1e7 1510defined C<END> routines first, but these C<END> routines may not
19799a22 1511themselves abort the exit. Likewise any object destructors that need to
2b5ab1e7
TC
1512be called are called before the real exit. If this is a problem, you
1513can call C<POSIX:_exit($status)> to avoid END and destructor processing.
87275199 1514See L<perlmod> for details.
5a964f20 1515
a0d0e21e
LW
1516=item exp EXPR
1517
54310121 1518=item exp
bbce6d69 1519
2b5ab1e7 1520Returns I<e> (the natural logarithm base) to the power of EXPR.
a0d0e21e
LW
1521If EXPR is omitted, gives C<exp($_)>.
1522
1523=item fcntl FILEHANDLE,FUNCTION,SCALAR
1524
f86cebdf 1525Implements the fcntl(2) function. You'll probably have to say
a0d0e21e
LW
1526
1527 use Fcntl;
1528
0ade1984 1529first to get the correct constant definitions. Argument processing and
19799a22 1530value return works just like C<ioctl> below.
a0d0e21e
LW
1531For example:
1532
1533 use Fcntl;
5a964f20
TC
1534 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1535 or die "can't fcntl F_GETFL: $!";
1536
19799a22 1537You don't have to check for C<defined> on the return from C<fnctl>.
951ba7fe
GS
1538Like C<ioctl>, it maps a C<0> return from the system call into
1539C<"0 but true"> in Perl. This string is true in boolean context and C<0>
2b5ab1e7
TC
1540in numeric context. It is also exempt from the normal B<-w> warnings
1541on improper numeric conversions.
5a964f20 1542
19799a22 1543Note that C<fcntl> will produce a fatal error if used on a machine that
2b5ab1e7
TC
1544doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1545manpage to learn what functions are available on your system.
a0d0e21e
LW
1546
1547=item fileno FILEHANDLE
1548
2b5ab1e7
TC
1549Returns the file descriptor for a filehandle, or undefined if the
1550filehandle is not open. This is mainly useful for constructing
19799a22 1551bitmaps for C<select> and low-level POSIX tty-handling operations.
2b5ab1e7
TC
1552If FILEHANDLE is an expression, the value is taken as an indirect
1553filehandle, generally its name.
5a964f20
TC
1554
1555You can use this to find out whether two handles refer to the
1556same underlying descriptor:
1557
1558 if (fileno(THIS) == fileno(THAT)) {
1559 print "THIS and THAT are dups\n";
1560 }
a0d0e21e
LW
1561
1562=item flock FILEHANDLE,OPERATION
1563
19799a22
GS
1564Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1565for success, false on failure. Produces a fatal error if used on a
2b5ab1e7 1566machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
19799a22 1567C<flock> is Perl's portable file locking interface, although it locks
2b5ab1e7
TC
1568only entire files, not records.
1569
1570Two potentially non-obvious but traditional C<flock> semantics are
1571that it waits indefinitely until the lock is granted, and that its locks
1572B<merely advisory>. Such discretionary locks are more flexible, but offer
19799a22
GS
1573fewer guarantees. This means that files locked with C<flock> may be
1574modified by programs that do not also use C<flock>. See L<perlport>,
2b5ab1e7
TC
1575your port's specific documentation, or your system-specific local manpages
1576for details. It's best to assume traditional behavior if you're writing
1577portable programs. (But if you're not, you should as always feel perfectly
1578free to write for your own system's idiosyncrasies (sometimes called
1579"features"). Slavish adherence to portability concerns shouldn't get
1580in the way of your getting your job done.)
a3cb178b 1581
8ebc5c01
PP
1582OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1583LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
ea3105be 1584you can use the symbolic names if you import them from the Fcntl module,
68dc0745
PP
1585either individually, or as a group using the ':flock' tag. LOCK_SH
1586requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
ea3105be
GS
1587releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1588LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
68dc0745
PP
1589waiting for the lock (check the return status to see if you got it).
1590
2b5ab1e7
TC
1591To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1592before locking or unlocking it.
8ebc5c01 1593
f86cebdf 1594Note that the emulation built with lockf(3) doesn't provide shared
8ebc5c01 1595locks, and it requires that FILEHANDLE be open with write intent. These
2b5ab1e7 1596are the semantics that lockf(3) implements. Most if not all systems
f86cebdf 1597implement lockf(3) in terms of fcntl(2) locking, though, so the
8ebc5c01
PP
1598differing semantics shouldn't bite too many people.
1599
19799a22
GS
1600Note also that some versions of C<flock> cannot lock things over the
1601network; you would need to use the more system-specific C<fcntl> for
f86cebdf
GS
1602that. If you like you can force Perl to ignore your system's flock(2)
1603function, and so provide its own fcntl(2)-based emulation, by passing
8ebc5c01
PP
1604the switch C<-Ud_flock> to the F<Configure> program when you configure
1605perl.
4633a7c4
LW
1606
1607Here's a mailbox appender for BSD systems.
a0d0e21e 1608
7e1af8bc 1609 use Fcntl ':flock'; # import LOCK_* constants
a0d0e21e
LW
1610
1611 sub lock {
7e1af8bc 1612 flock(MBOX,LOCK_EX);
a0d0e21e
LW
1613 # and, in case someone appended
1614 # while we were waiting...
1615 seek(MBOX, 0, 2);
1616 }
1617
1618 sub unlock {
7e1af8bc 1619 flock(MBOX,LOCK_UN);
a0d0e21e
LW
1620 }
1621
1622 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1623 or die "Can't open mailbox: $!";
1624
1625 lock();
1626 print MBOX $msg,"\n\n";
1627 unlock();
1628
2b5ab1e7
TC
1629On systems that support a real flock(), locks are inherited across fork()
1630calls, whereas those that must resort to the more capricious fcntl()
1631function lose the locks, making it harder to write servers.
1632
cb1a09d0 1633See also L<DB_File> for other flock() examples.
a0d0e21e
LW
1634
1635=item fork
1636
2b5ab1e7
TC
1637Does a fork(2) system call to create a new process running the
1638same program at the same point. It returns the child pid to the
1639parent process, C<0> to the child process, or C<undef> if the fork is
1640unsuccessful. File descriptors (and sometimes locks on those descriptors)
1641are shared, while everything else is copied. On most systems supporting
1642fork(), great care has gone into making it extremely efficient (for
1643example, using copy-on-write technology on data pages), making it the
1644dominant paradigm for multitasking over the last few decades.
5a964f20 1645
45bc9206 1646All files opened for output are flushed before forking the child process.
a0d0e21e 1647
19799a22 1648If you C<fork> without ever waiting on your children, you will
2b5ab1e7
TC
1649accumulate zombies. On some systems, you can avoid this by setting
1650C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1651forking and reaping moribund children.
cb1a09d0 1652
28757baa
PP
1653Note that if your forked child inherits system file descriptors like
1654STDIN and STDOUT that are actually connected by a pipe or socket, even
2b5ab1e7 1655if you exit, then the remote server (such as, say, a CGI script or a
19799a22 1656backgrounded job launched from a remote shell) won't think you're done.
2b5ab1e7 1657You should reopen those to F</dev/null> if it's any issue.
28757baa 1658
cb1a09d0
AD
1659=item format
1660
19799a22 1661Declare a picture format for use by the C<write> function. For
cb1a09d0
AD
1662example:
1663
54310121 1664 format Something =
cb1a09d0
AD
1665 Test: @<<<<<<<< @||||| @>>>>>
1666 $str, $%, '$' . int($num)
1667 .
1668
1669 $str = "widget";
184e9718 1670 $num = $cost/$quantity;
cb1a09d0
AD
1671 $~ = 'Something';
1672 write;
1673
1674See L<perlform> for many details and examples.
1675
8903cb82 1676=item formline PICTURE,LIST
a0d0e21e 1677
5a964f20 1678This is an internal function used by C<format>s, though you may call it,
a0d0e21e
LW
1679too. It formats (see L<perlform>) a list of values according to the
1680contents of PICTURE, placing the output into the format output
7660c0ab 1681accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
19799a22 1682Eventually, when a C<write> is done, the contents of
a0d0e21e 1683C<$^A> are written to some filehandle, but you could also read C<$^A>
7660c0ab 1684yourself and then set C<$^A> back to C<"">. Note that a format typically
19799a22 1685does one C<formline> per line of form, but the C<formline> function itself
748a9306 1686doesn't care how many newlines are embedded in the PICTURE. This means
4633a7c4 1687that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
748a9306
LW
1688You may therefore need to use multiple formlines to implement a single
1689record format, just like the format compiler.
1690
19799a22 1691Be careful if you put double quotes around the picture, because an C<@>
748a9306 1692character may be taken to mean the beginning of an array name.
19799a22 1693C<formline> always returns true. See L<perlform> for other examples.
a0d0e21e
LW
1694
1695=item getc FILEHANDLE
1696
1697=item getc
1698
1699Returns the next character from the input file attached to FILEHANDLE,
2b5ab1e7
TC
1700or the undefined value at end of file, or if there was an error.
1701If FILEHANDLE is omitted, reads from STDIN. This is not particularly
1702efficient. However, it cannot be used by itself to fetch single
1703characters without waiting for the user to hit enter. For that, try
1704something more like:
4633a7c4
LW
1705
1706 if ($BSD_STYLE) {
1707 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1708 }
1709 else {
54310121 1710 system "stty", '-icanon', 'eol', "\001";
4633a7c4
LW
1711 }
1712
1713 $key = getc(STDIN);
1714
1715 if ($BSD_STYLE) {
1716 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1717 }
1718 else {
5f05dabc 1719 system "stty", 'icanon', 'eol', '^@'; # ASCII null
4633a7c4
LW
1720 }
1721 print "\n";
1722
54310121
PP
1723Determination of whether $BSD_STYLE should be set
1724is left as an exercise to the reader.
cb1a09d0 1725
19799a22 1726The C<POSIX::getattr> function can do this more portably on
2b5ab1e7
TC
1727systems purporting POSIX compliance. See also the C<Term::ReadKey>
1728module from your nearest CPAN site; details on CPAN can be found on
1729L<perlmodlib/CPAN>.
a0d0e21e
LW
1730
1731=item getlogin
1732
5a964f20
TC
1733Implements the C library function of the same name, which on most
1734systems returns the current login from F</etc/utmp>, if any. If null,
19799a22 1735use C<getpwuid>.
a0d0e21e 1736
f86702cc 1737 $login = getlogin || getpwuid($<) || "Kilroy";
a0d0e21e 1738
19799a22
GS
1739Do not consider C<getlogin> for authentication: it is not as
1740secure as C<getpwuid>.
4633a7c4 1741
a0d0e21e
LW
1742=item getpeername SOCKET
1743
1744Returns the packed sockaddr address of other end of the SOCKET connection.
1745
4633a7c4
LW
1746 use Socket;
1747 $hersockaddr = getpeername(SOCK);
19799a22 1748 ($port, $iaddr) = sockaddr_in($hersockaddr);
4633a7c4
LW
1749 $herhostname = gethostbyaddr($iaddr, AF_INET);
1750 $herstraddr = inet_ntoa($iaddr);
a0d0e21e
LW
1751
1752=item getpgrp PID
1753
47e29363 1754Returns the current process group for the specified PID. Use
7660c0ab 1755a PID of C<0> to get the current process group for the
4633a7c4 1756current process. Will raise an exception if used on a machine that
f86cebdf 1757doesn't implement getpgrp(2). If PID is omitted, returns process
19799a22 1758group of current process. Note that the POSIX version of C<getpgrp>
7660c0ab 1759does not accept a PID argument, so only C<PID==0> is truly portable.
a0d0e21e
LW
1760
1761=item getppid
1762
1763Returns the process id of the parent process.
1764
1765=item getpriority WHICH,WHO
1766
4633a7c4
LW
1767Returns the current priority for a process, a process group, or a user.
1768(See L<getpriority(2)>.) Will raise a fatal exception if used on a
f86cebdf 1769machine that doesn't implement getpriority(2).
a0d0e21e
LW
1770
1771=item getpwnam NAME
1772
1773=item getgrnam NAME
1774
1775=item gethostbyname NAME
1776
1777=item getnetbyname NAME
1778
1779=item getprotobyname NAME
1780
1781=item getpwuid UID
1782
1783=item getgrgid GID
1784
1785=item getservbyname NAME,PROTO
1786
1787=item gethostbyaddr ADDR,ADDRTYPE
1788
1789=item getnetbyaddr ADDR,ADDRTYPE
1790
1791=item getprotobynumber NUMBER
1792
1793=item getservbyport PORT,PROTO
1794
1795=item getpwent
1796
1797=item getgrent
1798
1799=item gethostent
1800
1801=item getnetent
1802
1803=item getprotoent
1804
1805=item getservent
1806
1807=item setpwent
1808
1809=item setgrent
1810
1811=item sethostent STAYOPEN
1812
1813=item setnetent STAYOPEN
1814
1815=item setprotoent STAYOPEN
1816
1817=item setservent STAYOPEN
1818
1819=item endpwent
1820
1821=item endgrent
1822
1823=item endhostent
1824
1825=item endnetent
1826
1827=item endprotoent
1828
1829=item endservent
1830
1831These routines perform the same functions as their counterparts in the
5a964f20 1832system library. In list context, the return values from the
a0d0e21e
LW
1833various get routines are as follows:
1834
1835 ($name,$passwd,$uid,$gid,
6ee623d5 1836 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
a0d0e21e
LW
1837 ($name,$passwd,$gid,$members) = getgr*
1838 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1839 ($name,$aliases,$addrtype,$net) = getnet*
1840 ($name,$aliases,$proto) = getproto*
1841 ($name,$aliases,$port,$proto) = getserv*
1842
1843(If the entry doesn't exist you get a null list.)
1844
5a964f20 1845In scalar context, you get the name, unless the function was a
a0d0e21e
LW
1846lookup by name, in which case you get the other thing, whatever it is.
1847(If the entry doesn't exist you get the undefined value.) For example:
1848
5a964f20
TC
1849 $uid = getpwnam($name);
1850 $name = getpwuid($num);
1851 $name = getpwent();
1852 $gid = getgrnam($name);
1853 $name = getgrgid($num;
1854 $name = getgrent();
1855 #etc.
a0d0e21e 1856
19799a22 1857In I<getpw*()> the fields $quota, $comment, and $expire are
2b5ab1e7 1858special cases in the sense that in many systems they are unsupported.
19799a22
GS
1859If the $quota is unsupported, it is an empty scalar. If it is
1860supported, it usually encodes the disk quota. If the $comment
2b5ab1e7
TC
1861field is unsupported, it is an empty scalar. If it is supported it
1862usually encodes some administrative comment about the user. In some
19799a22
GS
1863systems the $quota field may be $change or $age, fields that have
1864to do with password aging. In some systems the $comment field may
1865be $class. The $expire field, if present, encodes the expiration
2b5ab1e7
TC
1866period of the account or the password. For the availability and the
1867exact meaning of these fields in your system, please consult your
1868getpwnam(3) documentation and your F<pwd.h> file. You can also find
19799a22
GS
1869out from within Perl what your $quota and $comment fields mean
1870and whether you have the $expire field by using the C<Config> module
2b5ab1e7
TC
1871and the values C<d_pwquota>, C<d_pwage>, C<d_pwchange>, C<d_pwcomment>,
1872and C<d_pwexpire>. Shadow password files are only supported if your
1873vendor has implemented them in the intuitive fashion that calling the
1874regular C library routines gets the shadow versions if you're running
1875under privilege. Those that incorrectly implement a separate library
1876call are not supported.
6ee623d5 1877
19799a22 1878The $members value returned by I<getgr*()> is a space separated list of
a0d0e21e
LW
1879the login names of the members of the group.
1880
1881For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1882C, it will be returned to you via C<$?> if the function call fails. The
7660c0ab 1883C<@addrs> value returned by a successful call is a list of the raw
a0d0e21e
LW
1884addresses returned by the corresponding system library call. In the
1885Internet domain, each address is four bytes long and you can unpack it
1886by saying something like:
1887
1888 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1889
2b5ab1e7
TC
1890The Socket library makes this slightly easier:
1891
1892 use Socket;
1893 $iaddr = inet_aton("127.1"); # or whatever address
1894 $name = gethostbyaddr($iaddr, AF_INET);
1895
1896 # or going the other way
19799a22 1897 $straddr = inet_ntoa($iaddr);
2b5ab1e7 1898
19799a22
GS
1899If you get tired of remembering which element of the return list
1900contains which return value, by-name interfaces are provided
1901in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
1902C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
1903and C<User::grent>. These override the normal built-ins, supplying
1904versions that return objects with the appropriate names
1905for each field. For example:
5a964f20
TC
1906
1907 use File::stat;
1908 use User::pwent;
1909 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
1910
1911Even though it looks like they're the same method calls (uid),
19799a22
GS
1912they aren't, because a C<File::stat> object is different from
1913a C<User::pwent> object.
5a964f20 1914
a0d0e21e
LW
1915=item getsockname SOCKET
1916
19799a22
GS
1917Returns the packed sockaddr address of this end of the SOCKET connection,
1918in case you don't know the address because you have several different
1919IPs that the connection might have come in on.
a0d0e21e 1920
4633a7c4
LW
1921 use Socket;
1922 $mysockaddr = getsockname(SOCK);
19799a22
GS
1923 ($port, $myaddr) = sockaddr_in($mysockaddr);
1924 printf "Connect to %s [%s]\n",
1925 scalar gethostbyaddr($myaddr, AF_INET),
1926 inet_ntoa($myaddr);
a0d0e21e
LW
1927
1928=item getsockopt SOCKET,LEVEL,OPTNAME
1929
5a964f20 1930Returns the socket option requested, or undef if there is an error.
a0d0e21e
LW
1931
1932=item glob EXPR
1933
0a753a76
PP
1934=item glob
1935
2b5ab1e7
TC
1936Returns the value of EXPR with filename expansions such as the
1937standard Unix shell F</bin/csh> would do. This is the internal function
61eff3bc
JH
1938implementing the C<< <*.c> >> operator, but you can use it directly.
1939If EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is
2b5ab1e7 1940discussed in more detail in L<perlop/"I/O Operators">.
a0d0e21e 1941
3a4b19e4
GS
1942Beginning with v5.6.0, this operator is implemented using the standard
1943C<File::Glob> extension. See L<File::Glob> for details.
1944
a0d0e21e
LW
1945=item gmtime EXPR
1946
19799a22 1947Converts a time as returned by the time function to a 9-element list
54310121 1948with the time localized for the standard Greenwich time zone.
4633a7c4 1949Typically used as follows:
a0d0e21e 1950
54310121 1951 # 0 1 2 3 4 5 6 7 8
a0d0e21e
LW
1952 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1953 gmtime(time);
1954
19799a22
GS
1955All list elements are numeric, and come straight out of a struct tm.
1956In particular this means that $mon has the range C<0..11> and $wday
1957has the range C<0..6> with sunday as day C<0>. Also, $year is the
1958number of years since 1900, that is, $year is C<123> in year 2023,
2b5ab1e7
TC
1959I<not> simply the last two digits of the year. If you assume it is,
1960then you create non-Y2K-compliant programs--and you wouldn't want to do
1961that, would you?
2f9daede 1962
abd75f24
GS
1963The proper way to get a complete 4-digit year is simply:
1964
1965 $year += 1900;
1966
1967And to get the last two digits of the year (e.g., '01' in 2001) do:
1968
1969 $year = sprintf("%02d", $year % 100);
1970
2f9daede 1971If EXPR is omitted, does C<gmtime(time())>.
a0d0e21e 1972
f86cebdf 1973In scalar context, returns the ctime(3) value:
0a753a76
PP
1974
1975 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
1976
19799a22 1977Also see the C<timegm> function provided by the C<Time::Local> module,
f86cebdf 1978and the strftime(3) function available via the POSIX module.
7660c0ab 1979
2b5ab1e7
TC
1980This scalar value is B<not> locale dependent (see L<perllocale>), but
1981is instead a Perl builtin. Also see the C<Time::Local> module, and the
1982strftime(3) and mktime(3) functions available via the POSIX module. To
7660c0ab
A
1983get somewhat similar but locale dependent date strings, set up your
1984locale environment variables appropriately (please see L<perllocale>)
1985and try for example:
1986
1987 use POSIX qw(strftime);
2b5ab1e7 1988 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
7660c0ab 1989
2b5ab1e7
TC
1990Note that the C<%a> and C<%b> escapes, which represent the short forms
1991of the day of the week and the month of the year, may not necessarily
1992be three characters wide in all locales.
0a753a76 1993
a0d0e21e
LW
1994=item goto LABEL
1995
748a9306
LW
1996=item goto EXPR
1997
a0d0e21e
LW
1998=item goto &NAME
1999
7660c0ab 2000The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
a0d0e21e 2001execution there. It may not be used to go into any construct that
7660c0ab 2002requires initialization, such as a subroutine or a C<foreach> loop. It
0a753a76 2003also can't be used to go into a construct that is optimized away,
19799a22 2004or to get out of a block or subroutine given to C<sort>.
0a753a76 2005It can be used to go almost anywhere else within the dynamic scope,
a0d0e21e 2006including out of subroutines, but it's usually better to use some other
19799a22 2007construct such as C<last> or C<die>. The author of Perl has never felt the
7660c0ab 2008need to use this form of C<goto> (in Perl, that is--C is another matter).
a0d0e21e 2009
7660c0ab
A
2010The C<goto-EXPR> form expects a label name, whose scope will be resolved
2011dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
748a9306
LW
2012necessarily recommended if you're optimizing for maintainability:
2013
2014 goto ("FOO", "BAR", "GLARCH")[$i];
2015
6cb9131c
GS
2016The C<goto-&NAME> form is quite different from the other forms of C<goto>.
2017In fact, it isn't a goto in the normal sense at all, and doesn't have
2018the stigma associated with other gotos. Instead, it
2019substitutes a call to the named subroutine for the currently running
2020subroutine. This is used by C<AUTOLOAD> subroutines that wish to load
2021another subroutine and then pretend that the other subroutine had been
2022called in the first place (except that any modifications to C<@_>
2023in the current subroutine are propagated to the other subroutine.)
2024After the C<goto>, not even C<caller> will be able to tell that this
2025routine was called first.
2026
2027NAME needn't be the name of a subroutine; it can be a scalar variable
2028containing a code reference, or a block which evaluates to a code
2029reference.
a0d0e21e
LW
2030
2031=item grep BLOCK LIST
2032
2033=item grep EXPR,LIST
2034
2b5ab1e7
TC
2035This is similar in spirit to, but not the same as, grep(1) and its
2036relatives. In particular, it is not limited to using regular expressions.
2f9daede 2037
a0d0e21e 2038Evaluates the BLOCK or EXPR for each element of LIST (locally setting
7660c0ab 2039C<$_> to each element) and returns the list value consisting of those
19799a22
GS
2040elements for which the expression evaluated to true. In scalar
2041context, returns the number of times the expression was true.
a0d0e21e
LW
2042
2043 @foo = grep(!/^#/, @bar); # weed out comments
2044
2045or equivalently,
2046
2047 @foo = grep {!/^#/} @bar; # weed out comments
2048
2b5ab1e7
TC
2049Note that, because C<$_> is a reference into the list value, it can
2050be used to modify the elements of the array. While this is useful and
2051supported, it can cause bizarre results if the LIST is not a named array.
2052Similarly, grep returns aliases into the original list, much as a for
2053loop's index variable aliases the list elements. That is, modifying an
19799a22
GS
2054element of a list returned by grep (for example, in a C<foreach>, C<map>
2055or another C<grep>) actually modifies the element in the original list.
2b5ab1e7 2056This is usually something to be avoided when writing clear code.
a0d0e21e 2057
19799a22 2058See also L</map> for a list composed of the results of the BLOCK or EXPR.
38325410 2059
a0d0e21e
LW
2060=item hex EXPR
2061
54310121 2062=item hex
bbce6d69 2063
2b5ab1e7
TC
2064Interprets EXPR as a hex string and returns the corresponding value.
2065(To convert strings that might start with either 0, 0x, or 0b, see
2066L</oct>.) If EXPR is omitted, uses C<$_>.
2f9daede
TPG
2067
2068 print hex '0xAf'; # prints '175'
2069 print hex 'aF'; # same
a0d0e21e 2070
19799a22 2071Hex strings may only represent integers. Strings that would cause
c6edd1b7 2072integer overflow trigger a warning.
19799a22 2073
a0d0e21e
LW
2074=item import
2075
19799a22 2076There is no builtin C<import> function. It is just an ordinary
4633a7c4 2077method (subroutine) defined (or inherited) by modules that wish to export
19799a22 2078names to another module. The C<use> function calls the C<import> method
54310121 2079for the package used. See also L</use()>, L<perlmod>, and L<Exporter>.
a0d0e21e
LW
2080
2081=item index STR,SUBSTR,POSITION
2082
2083=item index STR,SUBSTR
2084
2b5ab1e7
TC
2085The index function searches for one string within another, but without
2086the wildcard-like behavior of a full regular-expression pattern match.
2087It returns the position of the first occurrence of SUBSTR in STR at
2088or after POSITION. If POSITION is omitted, starts searching from the
2089beginning of the string. The return value is based at C<0> (or whatever
2090you've set the C<$[> variable to--but don't do that). If the substring
2091is not found, returns one less than the base, ordinarily C<-1>.
a0d0e21e
LW
2092
2093=item int EXPR
2094
54310121 2095=item int
bbce6d69 2096
7660c0ab 2097Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2b5ab1e7
TC
2098You should not use this function for rounding: one because it truncates
2099towards C<0>, and two because machine representations of floating point
2100numbers can sometimes produce counterintuitive results. For example,
2101C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2102because it's really more like -268.99999999999994315658 instead. Usually,
19799a22 2103the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2b5ab1e7 2104functions will serve you better than will int().
a0d0e21e
LW
2105
2106=item ioctl FILEHANDLE,FUNCTION,SCALAR
2107
2b5ab1e7 2108Implements the ioctl(2) function. You'll probably first have to say
a0d0e21e 2109
4633a7c4 2110 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
a0d0e21e 2111
2b5ab1e7 2112to get the correct function definitions. If F<ioctl.ph> doesn't
a0d0e21e 2113exist or doesn't have the correct definitions you'll have to roll your
61eff3bc 2114own, based on your C header files such as F<< <sys/ioctl.h> >>.
5a964f20 2115(There is a Perl script called B<h2ph> that comes with the Perl kit that
54310121 2116may help you in this, but it's nontrivial.) SCALAR will be read and/or
4633a7c4 2117written depending on the FUNCTION--a pointer to the string value of SCALAR
19799a22 2118will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
4633a7c4
LW
2119has no string value but does have a numeric value, that value will be
2120passed rather than a pointer to the string value. To guarantee this to be
19799a22
GS
2121true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2122functions may be needed to manipulate the values of structures used by
2123C<ioctl>.
a0d0e21e 2124
19799a22 2125The return value of C<ioctl> (and C<fcntl>) is as follows:
a0d0e21e
LW
2126
2127 if OS returns: then Perl returns:
2128 -1 undefined value
2129 0 string "0 but true"
2130 anything else that number
2131
19799a22 2132Thus Perl returns true on success and false on failure, yet you can
a0d0e21e
LW
2133still easily determine the actual value returned by the operating
2134system:
2135
2b5ab1e7 2136 $retval = ioctl(...) || -1;
a0d0e21e
LW
2137 printf "System returned %d\n", $retval;
2138
c2611fb3 2139The special string "C<0> but true" is exempt from B<-w> complaints
5a964f20
TC
2140about improper numeric conversions.
2141
19799a22
GS
2142Here's an example of setting a filehandle named C<REMOTE> to be
2143non-blocking at the system level. You'll have to negotiate C<$|>
2144on your own, though.
2145
2146 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
2147
2148 $flags = fcntl(REMOTE, F_GETFL, 0)
2149 or die "Can't get flags for the socket: $!\n";
2150
2151 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
2152 or die "Can't set flags for the socket: $!\n";
2153
a0d0e21e
LW
2154=item join EXPR,LIST
2155
2b5ab1e7
TC
2156Joins the separate strings of LIST into a single string with fields
2157separated by the value of EXPR, and returns that new string. Example:
a0d0e21e 2158
2b5ab1e7 2159 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
a0d0e21e 2160
eb6e2d6f
GS
2161Beware that unlike C<split>, C<join> doesn't take a pattern as its
2162first argument. Compare L</split>.
a0d0e21e 2163
aa689395
PP
2164=item keys HASH
2165
19799a22 2166Returns a list consisting of all the keys of the named hash. (In
1d2dff63 2167scalar context, returns the number of keys.) The keys are returned in
ab192400
GS
2168an apparently random order. The actual random order is subject to
2169change in future versions of perl, but it is guaranteed to be the same
19799a22 2170order as either the C<values> or C<each> function produces (given
ab192400
GS
2171that the hash has not been modified). As a side effect, it resets
2172HASH's iterator.
a0d0e21e 2173
aa689395 2174Here is yet another way to print your environment:
a0d0e21e
LW
2175
2176 @keys = keys %ENV;
2177 @values = values %ENV;
19799a22 2178 while (@keys) {
a0d0e21e
LW
2179 print pop(@keys), '=', pop(@values), "\n";
2180 }
2181
2182or how about sorted by key:
2183
2184 foreach $key (sort(keys %ENV)) {
2185 print $key, '=', $ENV{$key}, "\n";
2186 }
2187
19799a22 2188To sort a hash by value, you'll need to use a C<sort> function.
aa689395 2189Here's a descending numeric sort of a hash by its values:
4633a7c4 2190
5a964f20 2191 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
4633a7c4
LW
2192 printf "%4d %s\n", $hash{$key}, $key;
2193 }
2194
19799a22 2195As an lvalue C<keys> allows you to increase the number of hash buckets
aa689395
PP
2196allocated for the given hash. This can gain you a measure of efficiency if
2197you know the hash is going to get big. (This is similar to pre-extending
2198an array by assigning a larger number to $#array.) If you say
55497cff
PP
2199
2200 keys %hash = 200;
2201
ab192400
GS
2202then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2203in fact, since it rounds up to the next power of two. These
55497cff
PP
2204buckets will be retained even if you do C<%hash = ()>, use C<undef
2205%hash> if you want to free the storage while C<%hash> is still in scope.
2206You can't shrink the number of buckets allocated for the hash using
19799a22 2207C<keys> in this way (but you needn't worry about doing this by accident,
55497cff
PP
2208as trying has no effect).
2209
19799a22 2210See also C<each>, C<values> and C<sort>.
ab192400 2211
b350dd2f 2212=item kill SIGNAL, LIST
a0d0e21e 2213
b350dd2f 2214Sends a signal to a list of processes. Returns the number of
517db077
GS
2215processes successfully signaled (which is not necessarily the
2216same as the number actually killed).
a0d0e21e
LW
2217
2218 $cnt = kill 1, $child1, $child2;
2219 kill 9, @goners;
2220
b350dd2f
GS
2221If SIGNAL is zero, no signal is sent to the process. This is a
2222useful way to check that the process is alive and hasn't changed
2223its UID. See L<perlport> for notes on the portability of this
2224construct.
2225
2226Unlike in the shell, if SIGNAL is negative, it kills
4633a7c4
LW
2227process groups instead of processes. (On System V, a negative I<PROCESS>
2228number will also kill process groups, but that's not portable.) That
2229means you usually want to use positive not negative signals. You may also
da0045b7 2230use a signal name in quotes. See L<perlipc/"Signals"> for details.
a0d0e21e
LW
2231
2232=item last LABEL
2233
2234=item last
2235
2236The C<last> command is like the C<break> statement in C (as used in
2237loops); it immediately exits the loop in question. If the LABEL is
2238omitted, the command refers to the innermost enclosing loop. The
2239C<continue> block, if any, is not executed:
2240
4633a7c4
LW
2241 LINE: while (<STDIN>) {
2242 last LINE if /^$/; # exit when done with header
5a964f20 2243 #...
a0d0e21e
LW
2244 }
2245
4968c1e4 2246C<last> cannot be used to exit a block which returns a value such as
2b5ab1e7
TC
2247C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2248a grep() or map() operation.
4968c1e4 2249
6c1372ed
GS
2250Note that a block by itself is semantically identical to a loop
2251that executes once. Thus C<last> can be used to effect an early
2252exit out of such a block.
2253
98293880
JH
2254See also L</continue> for an illustration of how C<last>, C<next>, and
2255C<redo> work.
1d2dff63 2256
a0d0e21e
LW
2257=item lc EXPR
2258
54310121 2259=item lc
bbce6d69 2260
a0d0e21e 2261Returns an lowercased version of EXPR. This is the internal function
7660c0ab 2262implementing the C<\L> escape in double-quoted strings.
19799a22
GS
2263Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
2264and L<utf8>.
a0d0e21e 2265
7660c0ab 2266If EXPR is omitted, uses C<$_>.
bbce6d69 2267
a0d0e21e
LW
2268=item lcfirst EXPR
2269
54310121 2270=item lcfirst
bbce6d69 2271
a0d0e21e 2272Returns the value of EXPR with the first character lowercased. This is
7660c0ab 2273the internal function implementing the C<\l> escape in double-quoted strings.
a0ed51b3 2274Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
a0d0e21e 2275
7660c0ab 2276If EXPR is omitted, uses C<$_>.
bbce6d69 2277
a0d0e21e
LW
2278=item length EXPR
2279
54310121 2280=item length
bbce6d69 2281
a0ed51b3 2282Returns the length in characters of the value of EXPR. If EXPR is
2b5ab1e7
TC
2283omitted, returns length of C<$_>. Note that this cannot be used on
2284an entire array or hash to find out how many elements these have.
2285For that, use C<scalar @array> and C<scalar keys %hash> respectively.
a0d0e21e
LW
2286
2287=item link OLDFILE,NEWFILE
2288
19799a22
GS
2289Creates a new filename linked to the old filename. Returns true for
2290success, false otherwise.
a0d0e21e
LW
2291
2292=item listen SOCKET,QUEUESIZE
2293
19799a22
GS
2294Does the same thing that the listen system call does. Returns true if
2295it succeeded, false otherwise. See the example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
2296
2297=item local EXPR
2298
19799a22 2299You really probably want to be using C<my> instead, because C<local> isn't
2b5ab1e7
TC
2300what most people think of as "local". See L<perlsub/"Private Variables
2301via my()"> for details.
2302
5a964f20
TC
2303A local modifies the listed variables to be local to the enclosing
2304block, file, or eval. If more than one value is listed, the list must
2305be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2306for details, including issues with tied arrays and hashes.
a0d0e21e 2307
a0d0e21e
LW
2308=item localtime EXPR
2309
19799a22 2310Converts a time as returned by the time function to a 9-element list
5f05dabc 2311with the time analyzed for the local time zone. Typically used as
a0d0e21e
LW
2312follows:
2313
54310121 2314 # 0 1 2 3 4 5 6 7 8
a0d0e21e
LW
2315 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2316 localtime(time);
2317
19799a22
GS
2318All list elements are numeric, and come straight out of a struct tm.
2319In particular this means that $mon has the range C<0..11> and $wday
2320has the range C<0..6> with sunday as day C<0>. Also, $year is the
2321number of years since 1900, that is, $year is C<123> in year 2023,
2b5ab1e7
TC
2322and I<not> simply the last two digits of the year. If you assume it is,
2323then you create non-Y2K-compliant programs--and you wouldn't want to do
2324that, would you?
54310121 2325
abd75f24
GS
2326The proper way to get a complete 4-digit year is simply:
2327
2328 $year += 1900;
2329
2330And to get the last two digits of the year (e.g., '01' in 2001) do:
2331
2332 $year = sprintf("%02d", $year % 100);
2333
54310121 2334If EXPR is omitted, uses the current time (C<localtime(time)>).
a0d0e21e 2335
f86cebdf 2336In scalar context, returns the ctime(3) value:
a0d0e21e 2337
5f05dabc 2338 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
a0d0e21e 2339
a3cb178b 2340This scalar value is B<not> locale dependent, see L<perllocale>, but
68f8bed4
JH
2341instead a Perl builtin. Also see the C<Time::Local> module
2342(to convert the second, minutes, hours, ... back to seconds since the
2343stroke of midnight the 1st of January 1970, the value returned by
ca6e1c26 2344time()), and the strftime(3) and mktime(3) functions available via the
68f8bed4
JH
2345POSIX module. To get somewhat similar but locale dependent date
2346strings, set up your locale environment variables appropriately
2347(please see L<perllocale>) and try for example:
a3cb178b 2348
5a964f20 2349 use POSIX qw(strftime);
2b5ab1e7 2350 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
a3cb178b
GS
2351
2352Note that the C<%a> and C<%b>, the short forms of the day of the week
2353and the month of the year, may not necessarily be three characters wide.
a0d0e21e 2354
19799a22
GS
2355=item lock
2356
2357 lock I<THING>
2358
2359This function places an advisory lock on a variable, subroutine,
2360or referenced object contained in I<THING> until the lock goes out
2361of scope. This is a built-in function only if your version of Perl
2362was built with threading enabled, and if you've said C<use Threads>.
2363Otherwise a user-defined function by this name will be called. See
2364L<Thread>.
2365
a0d0e21e
LW
2366=item log EXPR
2367
54310121 2368=item log
bbce6d69 2369
2b5ab1e7
TC
2370Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2371returns log of C<$_>. To get the log of another base, use basic algebra:
19799a22 2372The base-N log of a number is equal to the natural log of that number
2b5ab1e7
TC
2373divided by the natural log of N. For example:
2374
2375 sub log10 {
2376 my $n = shift;
2377 return log($n)/log(10);
2378 }
2379
2380See also L</exp> for the inverse operation.
a0d0e21e
LW
2381
2382=item lstat FILEHANDLE
2383
2384=item lstat EXPR
2385
54310121 2386=item lstat
bbce6d69 2387
19799a22 2388Does the same thing as the C<stat> function (including setting the
5a964f20
TC
2389special C<_> filehandle) but stats a symbolic link instead of the file
2390the symbolic link points to. If symbolic links are unimplemented on
19799a22 2391your system, a normal C<stat> is done.
a0d0e21e 2392
7660c0ab 2393If EXPR is omitted, stats C<$_>.
bbce6d69 2394
a0d0e21e
LW
2395=item m//
2396
2397The match operator. See L<perlop>.
2398
2399=item map BLOCK LIST
2400
2401=item map EXPR,LIST
2402
19799a22
GS
2403Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2404C<$_> to each element) and returns the list value composed of the
2405results of each such evaluation. In scalar context, returns the
2406total number of elements so generated. Evaluates BLOCK or EXPR in
2407list context, so each element of LIST may produce zero, one, or
2408more elements in the returned value.
dd99ebda 2409
a0d0e21e
LW
2410 @chars = map(chr, @nums);
2411
2412translates a list of numbers to the corresponding characters. And
2413
4633a7c4 2414 %hash = map { getkey($_) => $_ } @array;
a0d0e21e
LW
2415
2416is just a funny way to write
2417
2418 %hash = ();
2419 foreach $_ (@array) {
4633a7c4 2420 $hash{getkey($_)} = $_;
a0d0e21e
LW
2421 }
2422
2b5ab1e7
TC
2423Note that, because C<$_> is a reference into the list value, it can
2424be used to modify the elements of the array. While this is useful and
2425supported, it can cause bizarre results if the LIST is not a named array.
2426Using a regular C<foreach> loop for this purpose would be clearer in
2427most cases. See also L</grep> for an array composed of those items of
2428the original list for which the BLOCK or EXPR evaluates to true.
fb73857a 2429
19799a22 2430=item mkdir FILENAME,MASK
a0d0e21e 2431
5a211162
GS
2432=item mkdir FILENAME
2433
0591cd52 2434Creates the directory specified by FILENAME, with permissions
19799a22
GS
2435specified by MASK (as modified by C<umask>). If it succeeds it
2436returns true, otherwise it returns false and sets C<$!> (errno).
5a211162 2437If omitted, MASK defaults to 0777.
0591cd52 2438
19799a22 2439In general, it is better to create directories with permissive MASK,
0591cd52 2440and let the user modify that with their C<umask>, than it is to supply
19799a22 2441a restrictive MASK and give the user no way to be more permissive.
0591cd52
NT
2442The exceptions to this rule are when the file or directory should be
2443kept private (mail files, for instance). The perlfunc(1) entry on
19799a22 2444C<umask> discusses the choice of MASK in more detail.
a0d0e21e
LW
2445
2446=item msgctl ID,CMD,ARG
2447
f86cebdf 2448Calls the System V IPC function msgctl(2). You'll probably have to say
0ade1984
JH
2449
2450 use IPC::SysV;
2451
7660c0ab
A
2452first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2453then ARG must be a variable which will hold the returned C<msqid_ds>
951ba7fe
GS
2454structure. Returns like C<ioctl>: the undefined value for error,
2455C<"0 but true"> for zero, or the actual return value otherwise. See also
19799a22 2456C<IPC::SysV> and C<IPC::Semaphore> documentation.
a0d0e21e
LW
2457
2458=item msgget KEY,FLAGS
2459
f86cebdf 2460Calls the System V IPC function msgget(2). Returns the message queue
7660c0ab 2461id, or the undefined value if there is an error. See also C<IPC::SysV>
19799a22 2462and C<IPC::Msg> documentation.
a0d0e21e
LW
2463
2464=item msgsnd ID,MSG,FLAGS
2465
2466Calls the System V IPC function msgsnd to send the message MSG to the
e4038a1f
MS
2467message queue ID. MSG must begin with the native long integer message
2468type, which may be created with C<pack("l!", $type)>. Returns true if
2469successful, or false if there is an error. See also C<IPC::SysV> and
2470C<IPC::SysV::Msg> documentation.
a0d0e21e
LW
2471
2472=item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2473
2474Calls the System V IPC function msgrcv to receive a message from
2475message queue ID into variable VAR with a maximum message size of
0ade1984
JH
2476SIZE. Note that if a message is received, the message type will be
2477the first thing in VAR, and the maximum length of VAR is SIZE plus the
19799a22 2478size of the message type. Returns true if successful, or false if
7660c0ab 2479there is an error. See also C<IPC::SysV> and C<IPC::SysV::Msg> documentation.
a0d0e21e
LW
2480
2481=item my EXPR
2482
09bef843
SB
2483=item my EXPR : ATTRIBUTES
2484
19799a22
GS
2485A C<my> declares the listed variables to be local (lexically) to the
2486enclosing block, file, or C<eval>. If
5f05dabc 2487more than one value is listed, the list must be placed in parentheses. See
cb1a09d0 2488L<perlsub/"Private Variables via my()"> for details.
4633a7c4 2489
a0d0e21e
LW
2490=item next LABEL
2491
2492=item next
2493
2494The C<next> command is like the C<continue> statement in C; it starts
2495the next iteration of the loop:
2496
4633a7c4
LW
2497 LINE: while (<STDIN>) {
2498 next LINE if /^#/; # discard comments
5a964f20 2499 #...
a0d0e21e
LW
2500 }
2501
2502Note that if there were a C<continue> block on the above, it would get
2503executed even on discarded lines. If the LABEL is omitted, the command
2504refers to the innermost enclosing loop.
2505
4968c1e4 2506C<next> cannot be used to exit a block which returns a value such as
2b5ab1e7
TC
2507C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2508a grep() or map() operation.
4968c1e4 2509
6c1372ed
GS
2510Note that a block by itself is semantically identical to a loop
2511that executes once. Thus C<next> will exit such a block early.
2512
98293880
JH
2513See also L</continue> for an illustration of how C<last>, C<next>, and
2514C<redo> work.
1d2dff63 2515
a0d0e21e
LW
2516=item no Module LIST
2517
7660c0ab 2518See the L</use> function, which C<no> is the opposite of.
a0d0e21e
LW
2519
2520=item oct EXPR
2521
54310121 2522=item oct
bbce6d69 2523
4633a7c4 2524Interprets EXPR as an octal string and returns the corresponding
4f19785b
WSI
2525value. (If EXPR happens to start off with C<0x>, interprets it as a
2526hex string. If EXPR starts off with C<0b>, it is interpreted as a
2527binary string.) The following will handle decimal, binary, octal, and
4633a7c4 2528hex in the standard Perl or C notation:
a0d0e21e
LW
2529
2530 $val = oct($val) if $val =~ /^0/;
2531
19799a22
GS
2532If EXPR is omitted, uses C<$_>. To go the other way (produce a number
2533in octal), use sprintf() or printf():
2534
2535 $perms = (stat("filename"))[2] & 07777;
2536 $oct_perms = sprintf "%lo", $perms;
2537
2538The oct() function is commonly used when a string such as C<644> needs
2539to be converted into a file mode, for example. (Although perl will
2540automatically convert strings into numbers as needed, this automatic
2541conversion assumes base 10.)
a0d0e21e 2542
1c1fc3ea 2543=item open FILEHANDLE,MODE,LIST
6170680b 2544
a0d0e21e
LW
2545=item open FILEHANDLE,EXPR
2546
2547=item open FILEHANDLE
2548
2549Opens the file whose filename is given by EXPR, and associates it with
5f05dabc
PP
2550FILEHANDLE. If FILEHANDLE is an expression, its value is used as the
2551name of the real filehandle wanted. If EXPR is omitted, the scalar
2552variable of the same name as the FILEHANDLE contains the filename.
19799a22
GS
2553(Note that lexical variables--those declared with C<my>--will not work
2554for this purpose; so if you're using C<my>, specify EXPR in your call
2b5ab1e7
TC
2555to open.) See L<perlopentut> for a kinder, gentler explanation of opening
2556files.
5f05dabc 2557
61eff3bc
JH
2558If MODE is C<< '<' >> or nothing, the file is opened for input.
2559If MODE is C<< '>' >>, the file is truncated and opened for
2560output, being created if necessary. If MODE is C<<< '>>' >>>,
fbb426e4 2561the file is opened for appending, again being created if necessary.
61eff3bc
JH
2562You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to indicate that
2563you want both read and write access to the file; thus C<< '+<' >> is almost
2564always preferred for read/write updates--the C<< '+>' >> mode would clobber the
5a964f20
TC
2565file first. You can't usually use either read-write mode for updating
2566textfiles, since they have variable length records. See the B<-i>
0591cd52
NT
2567switch in L<perlrun> for a better approach. The file is created with
2568permissions of C<0666> modified by the process' C<umask> value.
5a964f20 2569
61eff3bc
JH
2570These various prefixes correspond to the fopen(3) modes of C<'r'>, C<'r+'>,
2571C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
5f05dabc 2572
6170680b
IZ
2573In the 2-arguments (and 1-argument) form of the call the mode and
2574filename should be concatenated (in this order), possibly separated by
61eff3bc 2575spaces. It is possible to omit the mode if the mode is C<< '<' >>.
6170680b 2576
7660c0ab 2577If the filename begins with C<'|'>, the filename is interpreted as a
5a964f20 2578command to which output is to be piped, and if the filename ends with a
f244e06d
GS
2579C<'|'>, the filename is interpreted as a command which pipes output to
2580us. See L<perlipc/"Using open() for IPC">
19799a22 2581for more examples of this. (You are not allowed to C<open> to a command
5a964f20
TC
2582that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2583and L<perlipc/"Bidirectional Communication"> for alternatives.)
cb1a09d0 2584
6170680b
IZ
2585If MODE is C<'|-'>, the filename is interpreted as a
2586command to which output is to be piped, and if MODE is
2587C<'-|'>, the filename is interpreted as a command which pipes output to
2588us. In the 2-arguments (and 1-argument) form one should replace dash
2589(C<'-'>) with the command. See L<perlipc/"Using open() for IPC">
2590for more examples of this. (You are not allowed to C<open> to a command
2591that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2592and L<perlipc/"Bidirectional Communication"> for alternatives.)
2593
2594In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
61eff3bc 2595and opening C<< '>-' >> opens STDOUT.
6170680b
IZ
2596
2597Open returns
19799a22 2598nonzero upon success, the undefined value otherwise. If the C<open>
4633a7c4 2599involved a pipe, the return value happens to be the pid of the
54310121 2600subprocess.
cb1a09d0
AD
2601
2602If you're unfortunate enough to be running Perl on a system that
2603distinguishes between text files and binary files (modern operating
2604systems don't care), then you should check out L</binmode> for tips for
19799a22 2605dealing with this. The key distinction between systems that need C<binmode>
5a964f20
TC
2606and those that don't is their text file formats. Systems like Unix, MacOS, and
2607Plan9, which delimit lines with a single character, and which encode that
19799a22 2608character in C as C<"\n">, do not need C<binmode>. The rest need it.
cb1a09d0 2609
fb73857a 2610When opening a file, it's usually a bad idea to continue normal execution
19799a22
GS
2611if the request failed, so C<open> is frequently used in connection with
2612C<die>. Even if C<die> won't do what you want (say, in a CGI script,
fb73857a 2613where you want to make a nicely formatted error message (but there are
5a964f20 2614modules that can help with that problem)) you should always check
19799a22 2615the return value from opening a file. The infrequent exception is when
fb73857a
PP
2616working with an unopened filehandle is actually what you want to do.
2617
cb1a09d0 2618Examples:
a0d0e21e
LW
2619
2620 $ARTICLE = 100;
2621 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2622 while (<ARTICLE>) {...
2623
6170680b 2624 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
fb73857a 2625 # if the open fails, output is discarded
a0d0e21e 2626
6170680b 2627 open(DBASE, '+<', 'dbase.mine') # open for update
fb73857a 2628 or die "Can't open 'dbase.mine' for update: $!";
cb1a09d0 2629
6170680b
IZ
2630 open(DBASE, '+<dbase.mine') # ditto
2631 or die "Can't open 'dbase.mine' for update: $!";
2632
2633 open(ARTICLE, '-|', "caesar <$article") # decrypt article
fb73857a 2634 or die "Can't start caesar: $!";
a0d0e21e 2635
6170680b
IZ
2636 open(ARTICLE, "caesar <$article |") # ditto
2637 or die "Can't start caesar: $!";
2638
2639 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
fb73857a 2640 or die "Can't start sort: $!";
a0d0e21e
LW
2641
2642 # process argument list of files along with any includes
2643
2644 foreach $file (@ARGV) {
2645 process($file, 'fh00');
2646 }
2647
2648 sub process {
5a964f20 2649 my($filename, $input) = @_;
a0d0e21e
LW
2650 $input++; # this is a string increment
2651 unless (open($input, $filename)) {
2652 print STDERR "Can't open $filename: $!\n";
2653 return;
2654 }
2655
5a964f20 2656 local $_;
a0d0e21e
LW
2657 while (<$input>) { # note use of indirection
2658 if (/^#include "(.*)"/) {
2659 process($1, $input);
2660 next;
2661 }
5a964f20 2662 #... # whatever
a0d0e21e
LW
2663 }
2664 }
2665
2666You may also, in the Bourne shell tradition, specify an EXPR beginning
61eff3bc 2667with C<< '>&' >>, in which case the rest of the string is interpreted as the
5a964f20 2668name of a filehandle (or file descriptor, if numeric) to be
61eff3bc
JH
2669duped and opened. You may use C<&> after C<< > >>, C<<< >> >>>,
2670C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>. The
a0d0e21e 2671mode you specify should match the mode of the original filehandle.
184e9718 2672(Duping a filehandle does not take into account any existing contents of
6170680b
IZ
2673stdio buffers.) Duping file handles is not yet supported for 3-argument
2674open().
2675
a0d0e21e
LW
2676Here is a script that saves, redirects, and restores STDOUT and
2677STDERR:
2678
2679 #!/usr/bin/perl
5a964f20
TC
2680 open(OLDOUT, ">&STDOUT");
2681 open(OLDERR, ">&STDERR");
a0d0e21e 2682
6170680b
IZ
2683 open(STDOUT, '>', "foo.out") || die "Can't redirect stdout";
2684 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
a0d0e21e
LW
2685
2686 select(STDERR); $| = 1; # make unbuffered
2687 select(STDOUT); $| = 1; # make unbuffered
2688
2689 print STDOUT "stdout 1\n"; # this works for
2690 print STDERR "stderr 1\n"; # subprocesses too
2691
2692 close(STDOUT);
2693 close(STDERR);
2694
5a964f20
TC
2695 open(STDOUT, ">&OLDOUT");
2696 open(STDERR, ">&OLDERR");
a0d0e21e
LW
2697
2698 print STDOUT "stdout 2\n";
2699 print STDERR "stderr 2\n";
2700
61eff3bc 2701If you specify C<< '<&=N' >>, where C<N> is a number, then Perl will do an
19799a22 2702equivalent of C's C<fdopen> of that file descriptor; this is more
4633a7c4 2703parsimonious of file descriptors. For example:
a0d0e21e
LW
2704
2705 open(FILEHANDLE, "<&=$fd")
2706
4af147f6
CS
2707Note that this feature depends on the fdopen() C library function.
2708On many UNIX systems, fdopen() is known to fail when file descriptors
2709exceed a certain value, typically 255. If you need more file
2710descriptors than that, consider rebuilding Perl to use the C<sfio>
2711library.
2712
6170680b
IZ
2713If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
2714with 2-arguments (or 1-argument) form of open(), then
a0d0e21e 2715there is an implicit fork done, and the return value of open is the pid
7660c0ab 2716of the child within the parent process, and C<0> within the child
184e9718 2717process. (Use C<defined($pid)> to determine whether the open was successful.)
a0d0e21e
LW
2718The filehandle behaves normally for the parent, but i/o to that
2719filehandle is piped from/to the STDOUT/STDIN of the child process.
2720In the child process the filehandle isn't opened--i/o happens from/to
2721the new STDOUT or STDIN. Typically this is used like the normal
2722piped open when you want to exercise more control over just how the
2723pipe command gets executed, such as when you are running setuid, and
54310121 2724don't want to have to scan shell commands for metacharacters.
6170680b 2725The following triples are more or less equivalent:
a0d0e21e
LW
2726
2727 open(FOO, "|tr '[a-z]' '[A-Z]'");
6170680b
IZ
2728 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
2729 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
a0d0e21e
LW
2730
2731 open(FOO, "cat -n '$file'|");
6170680b
IZ
2732 open(FOO, '-|', "cat -n '$file'");
2733 open(FOO, '-|') || exec 'cat', '-n', $file;
a0d0e21e 2734
4633a7c4
LW
2735See L<perlipc/"Safe Pipe Opens"> for more examples of this.
2736
45bc9206
GS
2737NOTE: On any operation that may do a fork, all files opened for output
2738are flushed before the fork is attempted. On systems that support a
2739close-on-exec flag on files, the flag will be set for the newly opened
2740file descriptor as determined by the value of $^F. See L<perlvar/$^F>.
a0d0e21e 2741
0dccf244
CS
2742Closing any piped filehandle causes the parent process to wait for the
2743child to finish, and returns the status value in C<$?>.
2744
6170680b
IZ
2745The filename passed to 2-argument (or 1-argument) form of open()
2746will have leading and trailing
f86cebdf 2747whitespace deleted, and the normal redirection characters
5a964f20
TC
2748honored. This property, known as "magic open",
2749can often be used to good effect. A user could specify a filename of
7660c0ab 2750F<"rsh cat file |">, or you could change certain filenames as needed:
5a964f20
TC
2751
2752 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
2753 open(FH, $filename) or die "Can't open $filename: $!";
2754
6170680b
IZ
2755Use 3-argument form to open a file with arbitrary weird characters in it,
2756
2757 open(FOO, '<', $file);
2758
2759otherwise it's necessary to protect any leading and trailing whitespace:
5a964f20
TC
2760
2761 $file =~ s#^(\s)#./$1#;
2762 open(FOO, "< $file\0");
2763
6170680b
IZ
2764(this may not work on some bizzare filesystems). One should
2765conscientiously choose between the the I<magic> and 3-arguments form
2766of open():
2767
2768 open IN, $ARGV[0];
2769
2770will allow the user to specify an argument of the form C<"rsh cat file |">,
2771but will not work on a filename which happens to have a trailing space, while
2772
2773 open IN, '<', $ARGV[0];
2774
2775will have exactly the opposite restrictions.
2776
19799a22 2777If you want a "real" C C<open> (see L<open(2)> on your system), then you
6170680b
IZ
2778should use the C<sysopen> function, which involves no such magic (but
2779may use subtly different filemodes than Perl open(), which is mapped
2780to C fopen()). This is
5a964f20
TC
2781another way to protect your filenames from interpretation. For example:
2782
2783 use IO::Handle;
2784 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
2785 or die "sysopen $path: $!";
2786 $oldfh = select(HANDLE); $| = 1; select($oldfh);
2787 print HANDLE "stuff $$\n");
2788 seek(HANDLE, 0, 0);
2789 print "File contains: ", <HANDLE>;
2790
7660c0ab
A
2791Using the constructor from the C<IO::Handle> package (or one of its
2792subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
5a964f20
TC
2793filehandles that have the scope of whatever variables hold references to
2794them, and automatically close whenever and however you leave that scope:
c07a80fd 2795
5f05dabc 2796 use IO::File;
5a964f20 2797 #...
c07a80fd
PP
2798 sub read_myfile_munged {
2799 my $ALL = shift;
5f05dabc 2800 my $handle = new IO::File;
c07a80fd
PP
2801 open($handle, "myfile") or die "myfile: $!";
2802 $first = <$handle>
2803 or return (); # Automatically closed here.
2804 mung $first or die "mung failed"; # Or here.
2805 return $first, <$handle> if $ALL; # Or here.
2806 $first; # Or here.
2807 }
2808
b687b08b 2809See L</seek> for some details about mixing reading and writing.
a0d0e21e
LW
2810
2811=item opendir DIRHANDLE,EXPR
2812
19799a22
GS
2813Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
2814C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
a0d0e21e
LW
2815DIRHANDLEs have their own namespace separate from FILEHANDLEs.
2816
2817=item ord EXPR
2818
54310121 2819=item ord
bbce6d69 2820
a0ed51b3 2821Returns the numeric (ASCII or Unicode) value of the first character of EXPR. If
7660c0ab 2822EXPR is omitted, uses C<$_>. For the reverse, see L</chr>.
2b5ab1e7 2823See L<utf8> for more about Unicode.
a0d0e21e 2824
77ca0c92
LW
2825=item our EXPR
2826
2827An C<our> declares the listed variables to be valid globals within
2828the enclosing block, file, or C<eval>. That is, it has the same
2829scoping rules as a "my" declaration, but does not create a local
2830variable. If more than one value is listed, the list must be placed
2831in parentheses. The C<our> declaration has no semantic effect unless
2832"use strict vars" is in effect, in which case it lets you use the
2833declared global variable without qualifying it with a package name.
2834(But only within the lexical scope of the C<our> declaration. In this
2835it differs from "use vars", which is package scoped.)
2836
f472eb5c
GS
2837An C<our> declaration declares a global variable that will be visible
2838across its entire lexical scope, even across package boundaries. The
2839package in which the variable is entered is determined at the point
2840of the declaration, not at the point of use. This means the following
2841behavior holds:
2842
2843 package Foo;
2844 our $bar; # declares $Foo::bar for rest of lexical scope
2845 $bar = 20;
2846
2847 package Bar;
2848 print $bar; # prints 20
2849
2850Multiple C<our> declarations in the same lexical scope are allowed
2851if they are in different packages. If they happened to be in the same
2852package, Perl will emit warnings if you have asked for them.
2853
2854 use warnings;
2855 package Foo;
2856 our $bar; # declares $Foo::bar for rest of lexical scope
2857 $bar = 20;
2858
2859 package Bar;
2860 our $bar = 30; # declares $Bar::bar for rest of lexical scope
2861 print $bar; # prints 30
2862
2863 our $bar; # emits warning
2864
a0d0e21e
LW
2865=item pack TEMPLATE,LIST
2866
2b6c5635
GS
2867Takes a LIST of values and converts it into a string using the rules
2868given by the TEMPLATE. The resulting string is the concatenation of
2869the converted values. Typically, each converted value looks
2870like its machine-level representation. For example, on 32-bit machines
2871a converted integer may be represented by a sequence of 4 bytes.
2872
2873The TEMPLATE is a
a0d0e21e
LW
2874sequence of characters that give the order and type of values, as
2875follows:
2876
5a929a98 2877 a A string with arbitrary binary data, will be null padded.
a0d0e21e 2878 A An ascii string, will be space padded.
5a929a98
VU
2879 Z A null terminated (asciz) string, will be null padded.
2880
2b6c5635
GS
2881 b A bit string (ascending bit order inside each byte, like vec()).
2882 B A bit string (descending bit order inside each byte).
a0d0e21e
LW
2883 h A hex string (low nybble first).
2884 H A hex string (high nybble first).
2885
2886 c A signed char value.
a0ed51b3 2887 C An unsigned char value. Only does bytes. See U for Unicode.
96e4d5b1 2888
a0d0e21e
LW
2889 s A signed short value.
2890 S An unsigned short value.
96e4d5b1 2891 (This 'short' is _exactly_ 16 bits, which may differ from
851646ae
JH
2892 what a local C compiler calls 'short'. If you want
2893 native-length shorts, use the '!' suffix.)
96e4d5b1 2894
a0d0e21e
LW
2895 i A signed integer value.
2896 I An unsigned integer value.
19799a22 2897 (This 'integer' is _at_least_ 32 bits wide. Its exact
f86cebdf
GS
2898 size depends on what a local C compiler calls 'int',
2899 and may even be larger than the 'long' described in
2900 the next item.)
96e4d5b1 2901
a0d0e21e
LW
2902 l A signed long value.
2903 L An unsigned long value.
96e4d5b1 2904 (This 'long' is _exactly_ 32 bits, which may differ from
851646ae
JH
2905 what a local C compiler calls 'long'. If you want
2906 native-length longs, use the '!' suffix.)
a0d0e21e 2907
5d11dd56
G
2908 n An unsigned short in "network" (big-endian) order.
2909 N An unsigned long in "network" (big-endian) order.
2910 v An unsigned short in "VAX" (little-endian) order.
2911 V An unsigned long in "VAX" (little-endian) order.
96e4d5b1
PP
2912 (These 'shorts' and 'longs' are _exactly_ 16 bits and
2913 _exactly_ 32 bits, respectively.)
a0d0e21e 2914
dae0da7a
JH
2915 q A signed quad (64-bit) value.
2916 Q An unsigned quad value.
851646ae
JH
2917 (Quads are available only if your system supports 64-bit
2918 integer values _and_ if Perl has been compiled to support those.
dae0da7a
JH
2919 Causes a fatal error otherwise.)
2920
a0d0e21e
LW
2921 f A single-precision float in the native format.
2922 d A double-precision float in the native format.
2923
2924 p A pointer to a null-terminated string.
2925 P A pointer to a structure (fixed-length string).
2926
2927 u A uuencoded string.
a0ed51b3
LW
2928 U A Unicode character number. Encodes to UTF-8 internally.
2929 Works even if C<use utf8> is not in effect.
a0d0e21e 2930
96e4d5b1 2931 w A BER compressed integer. Its bytes represent an unsigned
f86cebdf
GS
2932 integer in base 128, most significant digit first, with as
2933 few digits as possible. Bit eight (the high bit) is set
2934 on each byte except the last.
def98dd4 2935
a0d0e21e
LW
2936 x A null byte.
2937 X Back up a byte.
2938 @ Null fill to absolute position.
2939
5a929a98
VU
2940The following rules apply:
2941
2942=over 8
2943
2944=item *
2945
5a964f20 2946Each letter may optionally be followed by a number giving a repeat
951ba7fe
GS
2947count. With all types except C<a>, C<A>, C<Z>, C<b>, C<B>, C<h>,
2948C<H>, and C<P> the pack function will gobble up that many values from
5a929a98 2949the LIST. A C<*> for the repeat count means to use however many items are
951ba7fe
GS
2950left, except for C<@>, C<x>, C<X>, where it is equivalent
2951to C<0>, and C<u>, where it is equivalent to 1 (or 45, what is the
2b6c5635
GS
2952same).
2953
951ba7fe 2954When used with C<Z>, C<*> results in the addition of a trailing null
2b6c5635
GS
2955byte (so the packed result will be one longer than the byte C<length>
2956of the item).
2957
951ba7fe 2958The repeat count for C<u> is interpreted as the maximal number of bytes
2b6c5635 2959to encode per line of output, with 0 and 1 replaced by 45.
5a929a98
VU
2960
2961=item *
2962
951ba7fe 2963The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
5a929a98 2964string of length count, padding with nulls or spaces as necessary. When
951ba7fe
GS
2965unpacking, C<A> strips trailing spaces and nulls, C<Z> strips everything
2966after the first null, and C<a> returns data verbatim. When packing,
2967C<a>, and C<Z> are equivalent.
2b6c5635
GS
2968
2969If the value-to-pack is too long, it is truncated. If too long and an
951ba7fe
GS
2970explicit count is provided, C<Z> packs only C<$count-1> bytes, followed
2971by a null byte. Thus C<Z> always packs a trailing null byte under
2b6c5635 2972all circumstances.
5a929a98
VU
2973
2974=item *
2975
951ba7fe 2976Likewise, the C<b> and C<B> fields pack a string that many bits long.
c73032f5
IZ
2977Each byte of the input field of pack() generates 1 bit of the result.
2978Each result bit is based on the least-significant bit of the corresponding
2979input byte, i.e., on C<ord($byte)%2>. In particular, bytes C<"0"> and
2980C<"1"> generate bits 0 and 1, as do bytes C<"\0"> and C<"\1">.
2981
2982Starting from the beginning of the input string of pack(), each 8-tuple
951ba7fe 2983of bytes is converted to 1 byte of output. With format C<b>
c73032f5 2984the first byte of the 8-tuple determines the least-significant bit of a
951ba7fe 2985byte, and with format C<B> it determines the most-significant bit of
c73032f5
IZ
2986a byte.
2987
2988If the length of the input string is not exactly divisible by 8, the
2989remainder is packed as if the input string were padded by null bytes
2990at the end. Similarly, during unpack()ing the "extra" bits are ignored.
2991
2992If the input string of pack() is longer than needed, extra bytes are ignored.
2b6c5635
GS
2993A C<*> for the repeat count of pack() means to use all the bytes of
2994the input field. On unpack()ing the bits are converted to a string
2995of C<"0">s and C<"1">s.
5a929a98
VU
2996
2997=item *
2998
951ba7fe 2999The C<h> and C<H> fields pack a string that many nybbles (4-bit groups,
851646ae 3000representable as hexadecimal digits, 0-9a-f) long.
5a929a98 3001
c73032f5
IZ
3002Each byte of the input field of pack() generates 4 bits of the result.
3003For non-alphabetical bytes the result is based on the 4 least-significant
3004bits of the input byte, i.e., on C<ord($byte)%16>. In particular,
3005bytes C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
3006C<"\0"> and C<"\1">. For bytes C<"a".."f"> and C<"A".."F"> the result
3007is compatible with the usual hexadecimal digits, so that C<"a"> and
3008C<"A"> both generate the nybble C<0xa==10>. The result for bytes
3009C<"g".."z"> and C<"G".."Z"> is not well-defined.
3010
3011Starting from the beginning of the input string of pack(), each pair
951ba7fe 3012of bytes is converted to 1 byte of output. With format C<h> the
c73032f5 3013first byte of the pair determines the least-significant nybble of the
951ba7fe 3014output byte, and with format C<H> it determines the most-significant
c73032f5
IZ
3015nybble.
3016
3017If the length of the input string is not even, it behaves as if padded
3018by a null byte at the end. Similarly, during unpack()ing the "extra"
3019nybbles are ignored.
3020
3021If the input string of pack() is longer than needed, extra bytes are ignored.
3022A C<*> for the repeat count of pack() means to use all the bytes of
3023the input field. On unpack()ing the bits are converted to a string
3024of hexadecimal digits.
3025
5a929a98
VU
3026=item *
3027
951ba7fe 3028The C<p> type packs a pointer to a null-terminated string. You are
5a929a98
VU
3029responsible for ensuring the string is not a temporary value (which can
3030potentially get deallocated before you get around to using the packed result).
951ba7fe
GS
3031The C<P> type packs a pointer to a structure of the size indicated by the
3032length. A NULL pointer is created if the corresponding value for C<p> or
3033C<P> is C<undef>, similarly for unpack().
5a929a98
VU
3034
3035=item *
3036
951ba7fe
GS
3037The C</> template character allows packing and unpacking of strings where
3038the packed structure contains a byte count followed by the string itself.
17f4a12d 3039You write I<length-item>C</>I<string-item>.
43192e07
IP
3040
3041The I<length-item> can be any C<pack> template letter,
3042and describes how the length value is packed.
3043The ones likely to be of most use are integer-packing ones like
951ba7fe
GS
3044C<n> (for Java strings), C<w> (for ASN.1 or SNMP)
3045and C<N> (for Sun XDR).
43192e07
IP
3046
3047The I<string-item> must, at present, be C<"A*">, C<"a*"> or C<"Z*">.
3048For C<unpack> the length of the string is obtained from the I<length-item>,
3049but if you put in the '*' it will be ignored.
3050
17f4a12d
IZ
3051 unpack 'C/a', "\04Gurusamy"; gives 'Guru'
3052 unpack 'a3/A* A*', '007 Bond J '; gives (' Bond','J')
3053 pack 'n/a* w/a*','hello,','world'; gives "\000\006hello,\005world"
43192e07
IP
3054
3055The I<length-item> is not returned explicitly from C<unpack>.
3056
951ba7fe
GS
3057Adding a count to the I<length-item> letter is unlikely to do anything
3058useful, unless that letter is C<A>, C<a> or C<Z>. Packing with a
3059I<length-item> of C<a> or C<Z> may introduce C<"\000"> characters,
43192e07
IP
3060which Perl does not regard as legal in numeric strings.
3061
3062=item *
3063
951ba7fe
GS
3064The integer types C<s>, C<S>, C<l>, and C<L> may be
3065immediately followed by a C<!> suffix to signify native shorts or
3066longs--as you can see from above for example a bare C<l> does mean
851646ae
JH
3067exactly 32 bits, the native C<long> (as seen by the local C compiler)
3068may be larger. This is an issue mainly in 64-bit platforms. You can
951ba7fe 3069see whether using C<!> makes any difference by
726ea183 3070
4d0c1c44
GS
3071 print length(pack("s")), " ", length(pack("s!")), "\n";
3072 print length(pack("l")), " ", length(pack("l!")), "\n";
ef54e1a4 3073
951ba7fe
GS
3074C<i!> and C<I!> also work but only because of completeness;
3075they are identical to C<i> and C<I>.
ef54e1a4 3076
19799a22
GS
3077The actual sizes (in bytes) of native shorts, ints, longs, and long
3078longs on the platform where Perl was built are also available via
3079L<Config>:
3080
3081 use Config;
3082 print $Config{shortsize}, "\n";
3083 print $Config{intsize}, "\n";
3084 print $Config{longsize}, "\n";
3085 print $Config{longlongsize}, "\n";
ef54e1a4 3086
5074e145 3087(The C<$Config{longlongsize}> will be undefine if your system does
851646ae
JH
3088not support long longs.)
3089
ef54e1a4
JH
3090=item *
3091
951ba7fe 3092The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, and C<L>
ef54e1a4
JH
3093are inherently non-portable between processors and operating systems
3094because they obey the native byteorder and endianness. For example a
140cb37e 30954-byte integer 0x12345678 (305419896 decimal) be ordered natively
ef54e1a4 3096(arranged in and handled by the CPU registers) into bytes as
61eff3bc 3097
719a3cf5
JH
3098 0x12 0x34 0x56 0x78 # little-endian
3099 0x78 0x56 0x34 0x12 # big-endian
61eff3bc 3100
5d11dd56 3101Basically, the Intel, Alpha, and VAX CPUs are little-endian, while
719a3cf5
JH
3102everybody else, for example Motorola m68k/88k, PPC, Sparc, HP PA,
3103Power, and Cray are big-endian. MIPS can be either: Digital used it
19799a22 3104in little-endian mode; SGI uses it in big-endian mode.
719a3cf5 3105
19799a22 3106The names `big-endian' and `little-endian' are comic references to
ef54e1a4
JH
3107the classic "Gulliver's Travels" (via the paper "On Holy Wars and a
3108Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and
19799a22 3109the egg-eating habits of the Lilliputians.
61eff3bc 3110
140cb37e 3111Some systems may have even weirder byte orders such as
61eff3bc 3112
ef54e1a4
JH
3113 0x56 0x78 0x12 0x34
3114 0x34 0x12 0x78 0x56
61eff3bc 3115
ef54e1a4
JH
3116You can see your system's preference with
3117
3118 print join(" ", map { sprintf "%#02x", $_ }
3119 unpack("C*",pack("L",0x12345678))), "\n";
3120
d99ad34e 3121The byteorder on the platform where Perl was built is also available
726ea183 3122via L<Config>:
ef54e1a4
JH
3123
3124 use Config;
3125 print $Config{byteorder}, "\n";
3126
d99ad34e
JH
3127Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'>
3128and C<'87654321'> are big-endian.
719a3cf5 3129
951ba7fe
GS
3130If you want portable packed integers use the formats C<n>, C<N>,
3131C<v>, and C<V>, their byte endianness and size is known.
851646ae 3132See also L<perlport>.
ef54e1a4
JH
3133
3134=item *
3135
5a929a98
VU
3136Real numbers (floats and doubles) are in the native machine format only;
3137due to the multiplicity of floating formats around, and the lack of a
3138standard "network" representation, no facility for interchange has been
3139made. This means that packed floating point data written on one machine
3140may not be readable on another - even if both use IEEE floating point
3141arithmetic (as the endian-ness of the memory representation is not part
851646ae 3142of the IEEE spec). See also L<perlport>.
5a929a98
VU
3143
3144Note that Perl uses doubles internally for all numeric calculation, and
3145converting from double into float and thence back to double again will
3146lose precision (i.e., C<unpack("f", pack("f", $foo)>) will not in general
19799a22 3147equal $foo).
5a929a98 3148
851646ae
JH
3149=item *
3150
3151You must yourself do any alignment or padding by inserting for example
9ccd05c0
JH
3152enough C<'x'>es while packing. There is no way to pack() and unpack()
3153could know where the bytes are going to or coming from. Therefore
3154C<pack> (and C<unpack>) handle their output and input as flat
3155sequences of bytes.
851646ae 3156
17f4a12d
IZ
3157=item *
3158
3159A comment in a TEMPLATE starts with C<#> and goes to the end of line.
3160
2b6c5635
GS
3161=item *
3162
3163If TEMPLATE requires more arguments to pack() than actually given, pack()
3164assumes additional C<""> arguments. If TEMPLATE requires less arguments
3165to pack() than actually given, extra arguments are ignored.
3166
5a929a98 3167=back
a0d0e21e
LW
3168
3169Examples:
3170
a0ed51b3 3171 $foo = pack("CCCC",65,66,67,68);
a0d0e21e 3172 # foo eq "ABCD"
a0ed51b3 3173 $foo = pack("C4",65,66,67,68);
a0d0e21e 3174 # same thing
a0ed51b3
LW
3175 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
3176 # same thing with Unicode circled letters
a0d0e21e
LW
3177
3178 $foo = pack("ccxxcc",65,66,67,68);
3179 # foo eq "AB\0\0CD"
3180
9ccd05c0
JH
3181 # note: the above examples featuring "C" and "c" are true
3182 # only on ASCII and ASCII-derived systems such as ISO Latin 1
3183 # and UTF-8. In EBCDIC the first example would be
3184 # $foo = pack("CCCC",193,194,195,196);
3185
a0d0e21e
LW
3186 $foo = pack("s2",1,2);
3187 # "\1\0\2\0" on little-endian
3188 # "\0\1\0\2" on big-endian
3189
3190 $foo = pack("a4","abcd","x","y","z");
3191 # "abcd"
3192
3193 $foo = pack("aaaa","abcd","x","y","z");
3194 # "axyz"
3195
3196 $foo = pack("a14","abcdefg");
3197 # "abcdefg\0\0\0\0\0\0\0"
3198
3199 $foo = pack("i9pl", gmtime);
3200 # a real struct tm (on my system anyway)
3201
5a929a98
VU
3202 $utmp_template = "Z8 Z8 Z16 L";
3203 $utmp = pack($utmp_template, @utmp1);
3204 # a struct utmp (BSDish)
3205
3206 @utmp2 = unpack($utmp_template, $utmp);
3207 # "@utmp1" eq "@utmp2"
3208
a0d0e21e
LW
3209 sub bintodec {
3210 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
3211 }
3212
851646ae
JH
3213 $foo = pack('sx2l', 12, 34);
3214 # short 12, two zero bytes padding, long 34
3215 $bar = pack('s@4l', 12, 34);
3216 # short 12, zero fill to position 4, long 34
3217 # $foo eq $bar
3218
5a929a98 3219The same template may generally also be used in unpack().
a0d0e21e 3220
5a964f20
TC
3221=item package
3222
cb1a09d0
AD
3223=item package NAMESPACE
3224
3225Declares the compilation unit as being in the given namespace. The scope
2b5ab1e7 3226of the package declaration is from the declaration itself through the end
19799a22 3227of the enclosing block, file, or eval (the same as the C<my> operator).
2b5ab1e7
TC
3228All further unqualified dynamic identifiers will be in this namespace.
3229A package statement affects only dynamic variables--including those
19799a22
GS
3230you've used C<local> on--but I<not> lexical variables, which are created
3231with C<my>. Typically it would be the first declaration in a file to
2b5ab1e7
TC
3232be included by the C<require> or C<use> operator. You can switch into a
3233package in more than one place; it merely influences which symbol table
3234is used by the compiler for the rest of that block. You can refer to
3235variables and filehandles in other packages by prefixing the identifier
3236with the package name and a double colon: C<$Package::Variable>.
3237If the package name is null, the C<main> package as assumed. That is,
3238C<$::sail> is equivalent to C<$main::sail> (as well as to C<$main'sail>,
3239still seen in older code).
cb1a09d0 3240
5a964f20
TC
3241If NAMESPACE is omitted, then there is no current package, and all
3242identifiers must be fully qualified or lexicals. This is stricter
3243than C<use strict>, since it also extends to function names.
3244
cb1a09d0
AD
3245See L<perlmod/"Packages"> for more information about packages, modules,
3246and classes. See L<perlsub> for other scoping issues.
3247
a0d0e21e
LW
3248=item pipe READHANDLE,WRITEHANDLE
3249
3250Opens a pair of connected pipes like the corresponding system call.
3251Note that if you set up a loop of piped processes, deadlock can occur
3252unless you are very careful. In addition, note that Perl's pipes use
184e9718 3253stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE
a0d0e21e
LW
3254after each command, depending on the application.
3255
7e1af8bc 3256See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
4633a7c4
LW
3257for examples of such things.
3258
4771b018
GS
3259On systems that support a close-on-exec flag on files, the flag will be set
3260for the newly opened file descriptors as determined by the value of $^F.
3261See L<perlvar/$^F>.
3262
a0d0e21e
LW
3263=item pop ARRAY
3264
54310121 3265=item pop
28757baa 3266
a0d0e21e 3267Pops and returns the last value of the array, shortening the array by
19799a22 3268one element. Has an effect similar to
a0d0e21e 3269
19799a22 3270 $ARRAY[$#ARRAY--]
a0d0e21e 3271
19799a22
GS
3272If there are no elements in the array, returns the undefined value
3273(although this may happen at other times as well). If ARRAY is
3274omitted, pops the C<@ARGV> array in the main program, and the C<@_>
3275array in subroutines, just like C<shift>.
a0d0e21e
LW
3276
3277=item pos SCALAR
3278
54310121 3279=item pos
bbce6d69 3280
4633a7c4 3281Returns the offset of where the last C<m//g> search left off for the variable
7660c0ab 3282is in question (C<$_> is used when the variable is not specified). May be
44a8e56a
PP
3283modified to change that offset. Such modification will also influence
3284the C<\G> zero-width assertion in regular expressions. See L<perlre> and
3285L<perlop>.
a0d0e21e
LW
3286
3287=item print FILEHANDLE LIST
3288
3289=item print LIST
3290
3291=item print
3