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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
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446Arranges for FILEHANDLE to be read or written in "binary" or "text" mode
447on systems where the run-time libraries distinguish between binary and
30168b04 448text files. If FILEHANDLE is an expression, the value is taken as the
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449name of the filehandle. DISCIPLINE can be either of C<":raw"> for
450binary mode or C<":crlf"> for "text" mode. If the DISCIPLINE is
451omitted, it defaults to C<":raw">.
30168b04 452
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453binmode() should be called after open() but before any I/O is done on
454the filehandle.
455
456On many systems binmode() currently has no effect, but in future, it
457will be extended to support user-defined input and output disciplines.
458On some systems binmode() is necessary when you're not working with a
459text file. For the sake of portability it is a good idea to always use
460it when appropriate, and to never use it when it isn't appropriate.
30168b04
GS
461
462In other words: Regardless of platform, use binmode() on binary
463files, and do not use binmode() on text files.
19799a22 464
16fe6d59
GS
465The C<open> pragma can be used to establish default disciplines.
466See L<open>.
467
19799a22 468The operating system, device drivers, C libraries, and Perl run-time
30168b04
GS
469system all work together to let the programmer treat a single
470character (C<\n>) as the line terminator, irrespective of the external
471representation. On many operating systems, the native text file
472representation matches the internal representation, but on some
473platforms the external representation of C<\n> is made up of more than
474one character.
475
476Mac OS and all variants of Unix use a single character to end each line
477in the external representation of text (even though that single
478character is not necessarily the same across these platforms).
479Consequently binmode() has no effect on these operating systems. In
480other systems like VMS, MS-DOS and the various flavors of MS-Windows
481your program sees a C<\n> as a simple C<\cJ>, but what's stored in text
482files are the two characters C<\cM\cJ>. That means that, if you don't
483use binmode() on these systems, C<\cM\cJ> sequences on disk will be
484converted to C<\n> on input, and any C<\n> in your program will be
485converted back to C<\cM\cJ> on output. This is what you want for text
486files, but it can be disastrous for binary files.
487
488Another consequence of using binmode() (on some systems) is that
489special end-of-file markers will be seen as part of the data stream.
490For systems from the Microsoft family this means that if your binary
491data contains C<\cZ>, the I/O subsystem will ragard it as the end of
492the file, unless you use binmode().
493
494binmode() is not only important for readline() and print() operations,
495but also when using read(), seek(), sysread(), syswrite() and tell()
496(see L<perlport> for more details). See the C<$/> and C<$\> variables
497in L<perlvar> for how to manually set your input and output
498line-termination sequences.
a0d0e21e 499
4633a7c4 500=item bless REF,CLASSNAME
a0d0e21e
LW
501
502=item bless REF
503
2b5ab1e7
TC
504This function tells the thingy referenced by REF that it is now an object
505in the CLASSNAME package. If CLASSNAME is omitted, the current package
19799a22 506is used. Because a C<bless> is often the last thing in a constructor,
2b5ab1e7
TC
507it returns the reference for convenience. Always use the two-argument
508version if the function doing the blessing might be inherited by a
509derived class. See L<perltoot> and L<perlobj> for more about the blessing
510(and blessings) of objects.
a0d0e21e 511
57668c4d 512Consider always blessing objects in CLASSNAMEs that are mixed case.
2b5ab1e7
TC
513Namespaces with all lowercase names are considered reserved for
514Perl pragmata. Builtin types have all uppercase names, so to prevent
515confusion, you may wish to avoid such package names as well. Make sure
516that CLASSNAME is a true value.
60ad88b8
GS
517
518See L<perlmod/"Perl Modules">.
519
a0d0e21e
LW
520=item caller EXPR
521
522=item caller
523
5a964f20 524Returns the context of the current subroutine call. In scalar context,
28757baa 525returns the caller's package name if there is a caller, that is, if
19799a22 526we're in a subroutine or C<eval> or C<require>, and the undefined value
5a964f20 527otherwise. In list context, returns
a0d0e21e 528
748a9306 529 ($package, $filename, $line) = caller;
a0d0e21e
LW
530
531With EXPR, it returns some extra information that the debugger uses to
532print a stack trace. The value of EXPR indicates how many call frames
533to go back before the current one.
534
f3aa04c2 535 ($package, $filename, $line, $subroutine, $hasargs,
e476b1b5 536 $wantarray, $evaltext, $is_require, $hints, $bitmask) = caller($i);
e7ea3e70 537
951ba7fe 538Here $subroutine may be C<(eval)> if the frame is not a subroutine
19799a22 539call, but an C<eval>. In such a case additional elements $evaltext and
7660c0ab 540C<$is_require> are set: C<$is_require> is true if the frame is created by a
19799a22 541C<require> or C<use> statement, $evaltext contains the text of the
dc848c6f 542C<eval EXPR> statement. In particular, for a C<eval BLOCK> statement,
951ba7fe 543$filename is C<(eval)>, but $evaltext is undefined. (Note also that
dc848c6f 544each C<use> statement creates a C<require> frame inside an C<eval EXPR>)
e476b1b5
GS
545frame. C<$hints> and C<$bitmask> contain pragmatic hints that the caller
546was compiled with. The C<$hints> and C<$bitmask> values are subject to
547change between versions of Perl, and are not meant for external use.
748a9306
LW
548
549Furthermore, when called from within the DB package, caller returns more
7660c0ab 550detailed information: it sets the list variable C<@DB::args> to be the
54310121 551arguments with which the subroutine was invoked.
748a9306 552
7660c0ab 553Be aware that the optimizer might have optimized call frames away before
19799a22 554C<caller> had a chance to get the information. That means that C<caller(N)>
7660c0ab 555might not return information about the call frame you expect it do, for
61eff3bc 556C<< N > 1 >>. In particular, C<@DB::args> might have information from the
19799a22 557previous time C<caller> was called.
7660c0ab 558
a0d0e21e
LW
559=item chdir EXPR
560
2b5ab1e7 561Changes the working directory to EXPR, if possible. If EXPR is omitted,
0bfc1ec4
GS
562changes to the directory specified by C<$ENV{HOME}>, if set; if not,
563changes to the directory specified by C<$ENV{LOGDIR}>. If neither is
564set, C<chdir> does nothing. It returns true upon success, false
565otherwise. See the example under C<die>.
a0d0e21e
LW
566
567=item chmod LIST
568
569Changes the permissions of a list of files. The first element of the
4633a7c4 570list must be the numerical mode, which should probably be an octal
2f9daede
TPG
571number, and which definitely should I<not> a string of octal digits:
572C<0644> is okay, C<'0644'> is not. Returns the number of files
dc848c6f 573successfully changed. See also L</oct>, if all you have is a string.
a0d0e21e
LW
574
575 $cnt = chmod 0755, 'foo', 'bar';
576 chmod 0755, @executables;
f86cebdf
GS
577 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
578 # --w----r-T
2f9daede
TPG
579 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
580 $mode = 0644; chmod $mode, 'foo'; # this is best
a0d0e21e 581
ca6e1c26
JH
582You can also import the symbolic C<S_I*> constants from the Fcntl
583module:
584
585 use Fcntl ':mode';
586
587 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
588 # This is identical to the chmod 0755 of the above example.
589
a0d0e21e
LW
590=item chomp VARIABLE
591
592=item chomp LIST
593
594=item chomp
595
2b5ab1e7
TC
596This safer version of L</chop> removes any trailing string
597that corresponds to the current value of C<$/> (also known as
28757baa
PP
598$INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
599number of characters removed from all its arguments. It's often used to
600remove the newline from the end of an input record when you're worried
2b5ab1e7
TC
601that the final record may be missing its newline. When in paragraph
602mode (C<$/ = "">), it removes all trailing newlines from the string.
4c5a6083
GS
603When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
604a reference to an integer or the like, see L<perlvar>) chomp() won't
19799a22
GS
605remove anything.
606If VARIABLE is omitted, it chomps C<$_>. Example:
a0d0e21e
LW
607
608 while (<>) {
609 chomp; # avoid \n on last field
610 @array = split(/:/);
5a964f20 611 # ...
a0d0e21e
LW
612 }
613
614You can actually chomp anything that's an lvalue, including an assignment:
615
616 chomp($cwd = `pwd`);
617 chomp($answer = <STDIN>);
618
619If you chomp a list, each element is chomped, and the total number of
620characters removed is returned.
621
622=item chop VARIABLE
623
624=item chop LIST
625
626=item chop
627
628Chops off the last character of a string and returns the character
629chopped. It's used primarily to remove the newline from the end of an
630input record, but is much more efficient than C<s/\n//> because it neither
7660c0ab 631scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
a0d0e21e
LW
632Example:
633
634 while (<>) {
635 chop; # avoid \n on last field
636 @array = split(/:/);
5a964f20 637 #...
a0d0e21e
LW
638 }
639
640You can actually chop anything that's an lvalue, including an assignment:
641
642 chop($cwd = `pwd`);
643 chop($answer = <STDIN>);
644
645If you chop a list, each element is chopped. Only the value of the
19799a22 646last C<chop> is returned.
a0d0e21e 647
19799a22 648Note that C<chop> returns the last character. To return all but the last
748a9306
LW
649character, use C<substr($string, 0, -1)>.
650
a0d0e21e
LW
651=item chown LIST
652
653Changes the owner (and group) of a list of files. The first two
19799a22
GS
654elements of the list must be the I<numeric> uid and gid, in that
655order. A value of -1 in either position is interpreted by most
656systems to leave that value unchanged. Returns the number of files
657successfully changed.
a0d0e21e
LW
658
659 $cnt = chown $uid, $gid, 'foo', 'bar';
660 chown $uid, $gid, @filenames;
661
54310121 662Here's an example that looks up nonnumeric uids in the passwd file:
a0d0e21e
LW
663
664 print "User: ";
19799a22 665 chomp($user = <STDIN>);
5a964f20 666 print "Files: ";
19799a22 667 chomp($pattern = <STDIN>);
a0d0e21e
LW
668
669 ($login,$pass,$uid,$gid) = getpwnam($user)
670 or die "$user not in passwd file";
671
5a964f20 672 @ary = glob($pattern); # expand filenames
a0d0e21e
LW
673 chown $uid, $gid, @ary;
674
54310121 675On most systems, you are not allowed to change the ownership of the
4633a7c4
LW
676file unless you're the superuser, although you should be able to change
677the group to any of your secondary groups. On insecure systems, these
678restrictions may be relaxed, but this is not a portable assumption.
19799a22
GS
679On POSIX systems, you can detect this condition this way:
680
681 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
682 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
4633a7c4 683
a0d0e21e
LW
684=item chr NUMBER
685
54310121 686=item chr
bbce6d69 687
a0d0e21e 688Returns the character represented by that NUMBER in the character set.
a0ed51b3 689For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
2b5ab1e7
TC
690chr(0x263a) is a Unicode smiley face (but only within the scope of
691a C<use utf8>). For the reverse, use L</ord>.
692See L<utf8> for more about Unicode.
a0d0e21e 693
7660c0ab 694If NUMBER is omitted, uses C<$_>.
bbce6d69 695
a0d0e21e
LW
696=item chroot FILENAME
697
54310121 698=item chroot
bbce6d69 699
5a964f20 700This function works like the system call by the same name: it makes the
4633a7c4 701named directory the new root directory for all further pathnames that
951ba7fe 702begin with a C</> by your process and all its children. (It doesn't
28757baa 703change your current working directory, which is unaffected.) For security
4633a7c4 704reasons, this call is restricted to the superuser. If FILENAME is
19799a22 705omitted, does a C<chroot> to C<$_>.
a0d0e21e
LW
706
707=item close FILEHANDLE
708
6a518fbc
TP
709=item close
710
19799a22 711Closes the file or pipe associated with the file handle, returning true
a0d0e21e 712only if stdio successfully flushes buffers and closes the system file
19799a22 713descriptor. Closes the currently selected filehandle if the argument
6a518fbc 714is omitted.
fb73857a
PP
715
716You don't have to close FILEHANDLE if you are immediately going to do
19799a22
GS
717another C<open> on it, because C<open> will close it for you. (See
718C<open>.) However, an explicit C<close> on an input file resets the line
719counter (C<$.>), while the implicit close done by C<open> does not.
fb73857a 720
19799a22
GS
721If the file handle came from a piped open C<close> will additionally
722return false if one of the other system calls involved fails or if the
fb73857a 723program exits with non-zero status. (If the only problem was that the
2b5ab1e7
TC
724program exited non-zero C<$!> will be set to C<0>.) Closing a pipe
725also waits for the process executing on the pipe to complete, in case you
726want to look at the output of the pipe afterwards, and
727implicitly puts the exit status value of that command into C<$?>.
5a964f20 728
73689b13
GS
729Prematurely closing the read end of a pipe (i.e. before the process
730writing to it at the other end has closed it) will result in a
731SIGPIPE being delivered to the writer. If the other end can't
732handle that, be sure to read all the data before closing the pipe.
733
fb73857a 734Example:
a0d0e21e 735
fb73857a
PP
736 open(OUTPUT, '|sort >foo') # pipe to sort
737 or die "Can't start sort: $!";
5a964f20 738 #... # print stuff to output
fb73857a
PP
739 close OUTPUT # wait for sort to finish
740 or warn $! ? "Error closing sort pipe: $!"
741 : "Exit status $? from sort";
742 open(INPUT, 'foo') # get sort's results
743 or die "Can't open 'foo' for input: $!";
a0d0e21e 744
5a964f20
TC
745FILEHANDLE may be an expression whose value can be used as an indirect
746filehandle, usually the real filehandle name.
a0d0e21e
LW
747
748=item closedir DIRHANDLE
749
19799a22 750Closes a directory opened by C<opendir> and returns the success of that
5a964f20
TC
751system call.
752
753DIRHANDLE may be an expression whose value can be used as an indirect
754dirhandle, usually the real dirhandle name.
a0d0e21e
LW
755
756=item connect SOCKET,NAME
757
758Attempts to connect to a remote socket, just as the connect system call
19799a22 759does. Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
760packed address of the appropriate type for the socket. See the examples in
761L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 762
cb1a09d0
AD
763=item continue BLOCK
764
765Actually a flow control statement rather than a function. If there is a
98293880
JH
766C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
767C<foreach>), it is always executed just before the conditional is about to
768be evaluated again, just like the third part of a C<for> loop in C. Thus
cb1a09d0
AD
769it can be used to increment a loop variable, even when the loop has been
770continued via the C<next> statement (which is similar to the C C<continue>
771statement).
772
98293880 773C<last>, C<next>, or C<redo> may appear within a C<continue>
19799a22
GS
774block. C<last> and C<redo> will behave as if they had been executed within
775the main block. So will C<next>, but since it will execute a C<continue>
1d2dff63
GS
776block, it may be more entertaining.
777
778 while (EXPR) {
779 ### redo always comes here
780 do_something;
781 } continue {
782 ### next always comes here
783 do_something_else;
784 # then back the top to re-check EXPR
785 }
786 ### last always comes here
787
788Omitting the C<continue> section is semantically equivalent to using an
19799a22 789empty one, logically enough. In that case, C<next> goes directly back
1d2dff63
GS
790to check the condition at the top of the loop.
791
a0d0e21e
LW
792=item cos EXPR
793
5a964f20 794Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
7660c0ab 795takes cosine of C<$_>.
a0d0e21e 796
ca6e1c26 797For the inverse cosine operation, you may use the C<Math::Trig::acos()>
28757baa
PP
798function, or use this relation:
799
800 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
801
a0d0e21e
LW
802=item crypt PLAINTEXT,SALT
803
f86cebdf 804Encrypts a string exactly like the crypt(3) function in the C library
4633a7c4
LW
805(assuming that you actually have a version there that has not been
806extirpated as a potential munition). This can prove useful for checking
807the password file for lousy passwords, amongst other things. Only the
808guys wearing white hats should do this.
a0d0e21e 809
19799a22 810Note that C<crypt> is intended to be a one-way function, much like breaking
11155c91
CS
811eggs to make an omelette. There is no (known) corresponding decrypt
812function. As a result, this function isn't all that useful for
813cryptography. (For that, see your nearby CPAN mirror.)
2f9daede 814
e71965be
RS
815When verifying an existing encrypted string you should use the encrypted
816text as the salt (like C<crypt($plain, $crypted) eq $crypted>). This
19799a22 817allows your code to work with the standard C<crypt> and with more
e71965be
RS
818exotic implementations. When choosing a new salt create a random two
819character string whose characters come from the set C<[./0-9A-Za-z]>
820(like C<join '', ('.', '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>).
821
a0d0e21e
LW
822Here's an example that makes sure that whoever runs this program knows
823their own password:
824
825 $pwd = (getpwuid($<))[1];
a0d0e21e
LW
826
827 system "stty -echo";
828 print "Password: ";
e71965be 829 chomp($word = <STDIN>);
a0d0e21e
LW
830 print "\n";
831 system "stty echo";
832
e71965be 833 if (crypt($word, $pwd) ne $pwd) {
a0d0e21e
LW
834 die "Sorry...\n";
835 } else {
836 print "ok\n";
54310121 837 }
a0d0e21e 838
9f8f0c9d 839Of course, typing in your own password to whoever asks you
748a9306 840for it is unwise.
a0d0e21e 841
19799a22
GS
842The L<crypt> function is unsuitable for encrypting large quantities
843of data, not least of all because you can't get the information
844back. Look at the F<by-module/Crypt> and F<by-module/PGP> directories
845on your favorite CPAN mirror for a slew of potentially useful
846modules.
847
aa689395 848=item dbmclose HASH
a0d0e21e 849
19799a22 850[This function has been largely superseded by the C<untie> function.]
a0d0e21e 851
aa689395 852Breaks the binding between a DBM file and a hash.
a0d0e21e 853
19799a22 854=item dbmopen HASH,DBNAME,MASK
a0d0e21e 855
19799a22 856[This function has been largely superseded by the C<tie> function.]
a0d0e21e 857
7b8d334a 858This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
19799a22
GS
859hash. HASH is the name of the hash. (Unlike normal C<open>, the first
860argument is I<not> a filehandle, even though it looks like one). DBNAME
aa689395
PP
861is the name of the database (without the F<.dir> or F<.pag> extension if
862any). If the database does not exist, it is created with protection
19799a22
GS
863specified by MASK (as modified by the C<umask>). If your system supports
864only the older DBM functions, you may perform only one C<dbmopen> in your
aa689395 865program. In older versions of Perl, if your system had neither DBM nor
19799a22 866ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
aa689395
PP
867sdbm(3).
868
869If you don't have write access to the DBM file, you can only read hash
870variables, not set them. If you want to test whether you can write,
19799a22 871either use file tests or try setting a dummy hash entry inside an C<eval>,
aa689395 872which will trap the error.
a0d0e21e 873
19799a22
GS
874Note that functions such as C<keys> and C<values> may return huge lists
875when used on large DBM files. You may prefer to use the C<each>
a0d0e21e
LW
876function to iterate over large DBM files. Example:
877
878 # print out history file offsets
879 dbmopen(%HIST,'/usr/lib/news/history',0666);
880 while (($key,$val) = each %HIST) {
881 print $key, ' = ', unpack('L',$val), "\n";
882 }
883 dbmclose(%HIST);
884
cb1a09d0 885See also L<AnyDBM_File> for a more general description of the pros and
184e9718 886cons of the various dbm approaches, as well as L<DB_File> for a particularly
cb1a09d0 887rich implementation.
4633a7c4 888
2b5ab1e7
TC
889You can control which DBM library you use by loading that library
890before you call dbmopen():
891
892 use DB_File;
893 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
894 or die "Can't open netscape history file: $!";
895
a0d0e21e
LW
896=item defined EXPR
897
54310121 898=item defined
bbce6d69 899
2f9daede
TPG
900Returns a Boolean value telling whether EXPR has a value other than
901the undefined value C<undef>. If EXPR is not present, C<$_> will be
902checked.
903
904Many operations return C<undef> to indicate failure, end of file,
905system error, uninitialized variable, and other exceptional
906conditions. This function allows you to distinguish C<undef> from
907other values. (A simple Boolean test will not distinguish among
7660c0ab 908C<undef>, zero, the empty string, and C<"0">, which are all equally
2f9daede 909false.) Note that since C<undef> is a valid scalar, its presence
19799a22 910doesn't I<necessarily> indicate an exceptional condition: C<pop>
2f9daede
TPG
911returns C<undef> when its argument is an empty array, I<or> when the
912element to return happens to be C<undef>.
913
f10b0346
GS
914You may also use C<defined(&func)> to check whether subroutine C<&func>
915has ever been defined. The return value is unaffected by any forward
916declarations of C<&foo>.
917
918Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
919used to report whether memory for that aggregate has ever been
920allocated. This behavior may disappear in future versions of Perl.
921You should instead use a simple test for size:
922
923 if (@an_array) { print "has array elements\n" }
924 if (%a_hash) { print "has hash members\n" }
2f9daede
TPG
925
926When used on a hash element, it tells you whether the value is defined,
dc848c6f 927not whether the key exists in the hash. Use L</exists> for the latter
2f9daede 928purpose.
a0d0e21e
LW
929
930Examples:
931
932 print if defined $switch{'D'};
933 print "$val\n" while defined($val = pop(@ary));
934 die "Can't readlink $sym: $!"
935 unless defined($value = readlink $sym);
a0d0e21e 936 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
2f9daede 937 $debugging = 0 unless defined $debugging;
a0d0e21e 938
19799a22 939Note: Many folks tend to overuse C<defined>, and then are surprised to
7660c0ab 940discover that the number C<0> and C<""> (the zero-length string) are, in fact,
2f9daede 941defined values. For example, if you say
a5f75d66
AD
942
943 "ab" =~ /a(.*)b/;
944
7660c0ab 945The pattern match succeeds, and C<$1> is defined, despite the fact that it
a5f75d66 946matched "nothing". But it didn't really match nothing--rather, it
2b5ab1e7 947matched something that happened to be zero characters long. This is all
a5f75d66 948very above-board and honest. When a function returns an undefined value,
2f9daede 949it's an admission that it couldn't give you an honest answer. So you
19799a22 950should use C<defined> only when you're questioning the integrity of what
7660c0ab 951you're trying to do. At other times, a simple comparison to C<0> or C<""> is
2f9daede
TPG
952what you want.
953
dc848c6f 954See also L</undef>, L</exists>, L</ref>.
2f9daede 955
a0d0e21e
LW
956=item delete EXPR
957
01020589
GS
958Given an expression that specifies a hash element, array element, hash slice,
959or array slice, deletes the specified element(s) from the hash or array.
8216c1fd
GS
960In the case of an array, if the array elements happen to be at the end,
961the size of the array will shrink to the highest element that tests
962true for exists() (or 0 if no such element exists).
a0d0e21e 963
01020589
GS
964Returns each element so deleted or the undefined value if there was no such
965element. Deleting from C<$ENV{}> modifies the environment. Deleting from
966a hash tied to a DBM file deletes the entry from the DBM file. Deleting
967from a C<tie>d hash or array may not necessarily return anything.
968
8ea97a1e
GS
969Deleting an array element effectively returns that position of the array
970to its initial, uninitialized state. Subsequently testing for the same
8216c1fd
GS
971element with exists() will return false. Note that deleting array
972elements in the middle of an array will not shift the index of the ones
973after them down--use splice() for that. See L</exists>.
8ea97a1e 974
01020589 975The following (inefficiently) deletes all the values of %HASH and @ARRAY:
a0d0e21e 976
5f05dabc
PP
977 foreach $key (keys %HASH) {
978 delete $HASH{$key};
a0d0e21e
LW
979 }
980
01020589
GS
981 foreach $index (0 .. $#ARRAY) {
982 delete $ARRAY[$index];
983 }
984
985And so do these:
5f05dabc 986
01020589
GS
987 delete @HASH{keys %HASH};
988
9740c838 989 delete @ARRAY[0 .. $#ARRAY];
5f05dabc 990
2b5ab1e7 991But both of these are slower than just assigning the empty list
01020589
GS
992or undefining %HASH or @ARRAY:
993
994 %HASH = (); # completely empty %HASH
995 undef %HASH; # forget %HASH ever existed
2b5ab1e7 996
01020589
GS
997 @ARRAY = (); # completely empty @ARRAY
998 undef @ARRAY; # forget @ARRAY ever existed
2b5ab1e7
TC
999
1000Note that the EXPR can be arbitrarily complicated as long as the final
01020589
GS
1001operation is a hash element, array element, hash slice, or array slice
1002lookup:
a0d0e21e
LW
1003
1004 delete $ref->[$x][$y]{$key};
5f05dabc 1005 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
a0d0e21e 1006
01020589
GS
1007 delete $ref->[$x][$y][$index];
1008 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1009
a0d0e21e
LW
1010=item die LIST
1011
19799a22
GS
1012Outside an C<eval>, prints the value of LIST to C<STDERR> and
1013exits with the current value of C<$!> (errno). If C<$!> is C<0>,
61eff3bc
JH
1014exits with the value of C<<< ($? >> 8) >>> (backtick `command`
1015status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside
19799a22
GS
1016an C<eval(),> the error message is stuffed into C<$@> and the
1017C<eval> is terminated with the undefined value. This makes
1018C<die> the way to raise an exception.
a0d0e21e
LW
1019
1020Equivalent examples:
1021
1022 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
54310121 1023 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
a0d0e21e
LW
1024
1025If the value of EXPR does not end in a newline, the current script line
1026number and input line number (if any) are also printed, and a newline
883faa13
GS
1027is supplied. Note that the "input line number" (also known as "chunk")
1028is subject to whatever notion of "line" happens to be currently in
1029effect, and is also available as the special variable C<$.>.
1030See L<perlvar/"$/"> and L<perlvar/"$.">.
1031
1032Hint: sometimes appending C<", stopped"> to your message
7660c0ab 1033will cause it to make better sense when the string C<"at foo line 123"> is
a0d0e21e
LW
1034appended. Suppose you are running script "canasta".
1035
1036 die "/etc/games is no good";
1037 die "/etc/games is no good, stopped";
1038
1039produce, respectively
1040
1041 /etc/games is no good at canasta line 123.
1042 /etc/games is no good, stopped at canasta line 123.
1043
2b5ab1e7 1044See also exit(), warn(), and the Carp module.
a0d0e21e 1045
7660c0ab
A
1046If LIST is empty and C<$@> already contains a value (typically from a
1047previous eval) that value is reused after appending C<"\t...propagated">.
fb73857a
PP
1048This is useful for propagating exceptions:
1049
1050 eval { ... };
1051 die unless $@ =~ /Expected exception/;
1052
7660c0ab 1053If C<$@> is empty then the string C<"Died"> is used.
fb73857a 1054
52531d10
GS
1055die() can also be called with a reference argument. If this happens to be
1056trapped within an eval(), $@ contains the reference. This behavior permits
1057a more elaborate exception handling implementation using objects that
1058maintain arbitary state about the nature of the exception. Such a scheme
1059is sometimes preferable to matching particular string values of $@ using
1060regular expressions. Here's an example:
1061
1062 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1063 if ($@) {
1064 if (ref($@) && UNIVERSAL::isa($@,"Some::Module::Exception")) {
1065 # handle Some::Module::Exception
1066 }
1067 else {
1068 # handle all other possible exceptions
1069 }
1070 }
1071
19799a22 1072Because perl will stringify uncaught exception messages before displaying
52531d10
GS
1073them, you may want to overload stringification operations on such custom
1074exception objects. See L<overload> for details about that.
1075
19799a22
GS
1076You can arrange for a callback to be run just before the C<die>
1077does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1078handler will be called with the error text and can change the error
1079message, if it sees fit, by calling C<die> again. See
1080L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
1081L<"eval BLOCK"> for some examples. Although this feature was meant
1082to be run only right before your program was to exit, this is not
1083currently the case--the C<$SIG{__DIE__}> hook is currently called
1084even inside eval()ed blocks/strings! If one wants the hook to do
1085nothing in such situations, put
fb73857a
PP
1086
1087 die @_ if $^S;
1088
19799a22
GS
1089as the first line of the handler (see L<perlvar/$^S>). Because
1090this promotes strange action at a distance, this counterintuitive
1091behavior may be fixed in a future release.
774d564b 1092
a0d0e21e
LW
1093=item do BLOCK
1094
1095Not really a function. Returns the value of the last command in the
1096sequence of commands indicated by BLOCK. When modified by a loop
98293880
JH
1097modifier, executes the BLOCK once before testing the loop condition.
1098(On other statements the loop modifiers test the conditional first.)
a0d0e21e 1099
4968c1e4 1100C<do BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7
TC
1101C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1102See L<perlsyn> for alternative strategies.
4968c1e4 1103
a0d0e21e
LW
1104=item do SUBROUTINE(LIST)
1105
1106A deprecated form of subroutine call. See L<perlsub>.
1107
1108=item do EXPR
1109
1110Uses the value of EXPR as a filename and executes the contents of the
1111file as a Perl script. Its primary use is to include subroutines
1112from a Perl subroutine library.
1113
1114 do 'stat.pl';
1115
1116is just like
1117
fb73857a 1118 scalar eval `cat stat.pl`;
a0d0e21e 1119
2b5ab1e7
TC
1120except that it's more efficient and concise, keeps track of the current
1121filename for error messages, searches the @INC libraries, and updates
1122C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1123variables. It also differs in that code evaluated with C<do FILENAME>
1124cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1125same, however, in that it does reparse the file every time you call it,
1126so you probably don't want to do this inside a loop.
a0d0e21e 1127
8e30cc93 1128If C<do> cannot read the file, it returns undef and sets C<$!> to the
2b5ab1e7 1129error. If C<do> can read the file but cannot compile it, it
8e30cc93
G
1130returns undef and sets an error message in C<$@>. If the file is
1131successfully compiled, C<do> returns the value of the last expression
1132evaluated.
1133
a0d0e21e 1134Note that inclusion of library modules is better done with the
19799a22 1135C<use> and C<require> operators, which also do automatic error checking
4633a7c4 1136and raise an exception if there's a problem.
a0d0e21e 1137
5a964f20
TC
1138You might like to use C<do> to read in a program configuration
1139file. Manual error checking can be done this way:
1140
1141 # read in config files: system first, then user
f86cebdf 1142 for $file ("/share/prog/defaults.rc",
2b5ab1e7
TC
1143 "$ENV{HOME}/.someprogrc")
1144 {
5a964f20 1145 unless ($return = do $file) {
f86cebdf
GS
1146 warn "couldn't parse $file: $@" if $@;
1147 warn "couldn't do $file: $!" unless defined $return;
1148 warn "couldn't run $file" unless $return;
5a964f20
TC
1149 }
1150 }
1151
a0d0e21e
LW
1152=item dump LABEL
1153
1614b0e3
JD
1154=item dump
1155
19799a22
GS
1156This function causes an immediate core dump. See also the B<-u>
1157command-line switch in L<perlrun>, which does the same thing.
1158Primarily this is so that you can use the B<undump> program (not
1159supplied) to turn your core dump into an executable binary after
1160having initialized all your variables at the beginning of the
1161program. When the new binary is executed it will begin by executing
1162a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1163Think of it as a goto with an intervening core dump and reincarnation.
1164If C<LABEL> is omitted, restarts the program from the top.
1165
1166B<WARNING>: Any files opened at the time of the dump will I<not>
1167be open any more when the program is reincarnated, with possible
1168resulting confusion on the part of Perl.
1169
1170This function is now largely obsolete, partly because it's very
1171hard to convert a core file into an executable, and because the
1172real compiler backends for generating portable bytecode and compilable
1173C code have superseded it.
1174
1175If you're looking to use L<dump> to speed up your program, consider
1176generating bytecode or native C code as described in L<perlcc>. If
1177you're just trying to accelerate a CGI script, consider using the
1178C<mod_perl> extension to B<Apache>, or the CPAN module, Fast::CGI.
1179You might also consider autoloading or selfloading, which at least
1180make your program I<appear> to run faster.
5a964f20 1181
aa689395
PP
1182=item each HASH
1183
5a964f20 1184When called in list context, returns a 2-element list consisting of the
aa689395 1185key and value for the next element of a hash, so that you can iterate over
5a964f20 1186it. When called in scalar context, returns the key for only the "next"
e902a979 1187element in the hash.
2f9daede 1188
ab192400
GS
1189Entries are returned in an apparently random order. The actual random
1190order is subject to change in future versions of perl, but it is guaranteed
19799a22 1191to be in the same order as either the C<keys> or C<values> function
ab192400
GS
1192would produce on the same (unmodified) hash.
1193
1194When the hash is entirely read, a null array is returned in list context
19799a22
GS
1195(which when assigned produces a false (C<0>) value), and C<undef> in
1196scalar context. The next call to C<each> after that will start iterating
1197again. There is a single iterator for each hash, shared by all C<each>,
1198C<keys>, and C<values> function calls in the program; it can be reset by
2f9daede
TPG
1199reading all the elements from the hash, or by evaluating C<keys HASH> or
1200C<values HASH>. If you add or delete elements of a hash while you're
1201iterating over it, you may get entries skipped or duplicated, so don't.
aa689395 1202
f86cebdf 1203The following prints out your environment like the printenv(1) program,
aa689395 1204only in a different order:
a0d0e21e
LW
1205
1206 while (($key,$value) = each %ENV) {
1207 print "$key=$value\n";
1208 }
1209
19799a22 1210See also C<keys>, C<values> and C<sort>.
a0d0e21e
LW
1211
1212=item eof FILEHANDLE
1213
4633a7c4
LW
1214=item eof ()
1215
a0d0e21e
LW
1216=item eof
1217
1218Returns 1 if the next read on FILEHANDLE will return end of file, or if
1219FILEHANDLE is not open. FILEHANDLE may be an expression whose value
5a964f20 1220gives the real filehandle. (Note that this function actually
19799a22 1221reads a character and then C<ungetc>s it, so isn't very useful in an
748a9306 1222interactive context.) Do not read from a terminal file (or call
19799a22 1223C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
748a9306
LW
1224as terminals may lose the end-of-file condition if you do.
1225
820475bd
GS
1226An C<eof> without an argument uses the last file read. Using C<eof()>
1227with empty parentheses is very different. It refers to the pseudo file
1228formed from the files listed on the command line and accessed via the
61eff3bc
JH
1229C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1230as a normal filehandle is, an C<eof()> before C<< <> >> has been
820475bd
GS
1231used will cause C<@ARGV> to be examined to determine if input is
1232available.
1233
61eff3bc 1234In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
820475bd
GS
1235detect the end of each file, C<eof()> will only detect the end of the
1236last file. Examples:
a0d0e21e 1237
748a9306
LW
1238 # reset line numbering on each input file
1239 while (<>) {
5a964f20 1240 next if /^\s*#/; # skip comments
748a9306 1241 print "$.\t$_";
5a964f20
TC
1242 } continue {
1243 close ARGV if eof; # Not eof()!
748a9306
LW
1244 }
1245
a0d0e21e
LW
1246 # insert dashes just before last line of last file
1247 while (<>) {
5a964f20 1248 if (eof()) { # check for end of current file
a0d0e21e 1249 print "--------------\n";
2b5ab1e7 1250 close(ARGV); # close or last; is needed if we
748a9306 1251 # are reading from the terminal
a0d0e21e
LW
1252 }
1253 print;
1254 }
1255
a0d0e21e 1256Practical hint: you almost never need to use C<eof> in Perl, because the
3ce0d271
GS
1257input operators typically return C<undef> when they run out of data, or if
1258there was an error.
a0d0e21e
LW
1259
1260=item eval EXPR
1261
1262=item eval BLOCK
1263
c7cc6f1c
GS
1264In the first form, the return value of EXPR is parsed and executed as if it
1265were a little Perl program. The value of the expression (which is itself
5a964f20 1266determined within scalar context) is first parsed, and if there weren't any
c7cc6f1c 1267errors, executed in the context of the current Perl program, so that any
5f05dabc 1268variable settings or subroutine and format definitions remain afterwards.
c7cc6f1c
GS
1269Note that the value is parsed every time the eval executes. If EXPR is
1270omitted, evaluates C<$_>. This form is typically used to delay parsing
1271and subsequent execution of the text of EXPR until run time.
1272
1273In the second form, the code within the BLOCK is parsed only once--at the
1274same time the code surrounding the eval itself was parsed--and executed
1275within the context of the current Perl program. This form is typically
1276used to trap exceptions more efficiently than the first (see below), while
1277also providing the benefit of checking the code within BLOCK at compile
1278time.
1279
1280The final semicolon, if any, may be omitted from the value of EXPR or within
1281the BLOCK.
1282
1283In both forms, the value returned is the value of the last expression
5a964f20 1284evaluated inside the mini-program; a return statement may be also used, just
c7cc6f1c 1285as with subroutines. The expression providing the return value is evaluated
5a964f20 1286in void, scalar, or list context, depending on the context of the eval itself.
c7cc6f1c 1287See L</wantarray> for more on how the evaluation context can be determined.
a0d0e21e 1288
19799a22
GS
1289If there is a syntax error or runtime error, or a C<die> statement is
1290executed, an undefined value is returned by C<eval>, and C<$@> is set to the
a0d0e21e 1291error message. If there was no error, C<$@> is guaranteed to be a null
19799a22 1292string. Beware that using C<eval> neither silences perl from printing
c7cc6f1c
GS
1293warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1294To do either of those, you have to use the C<$SIG{__WARN__}> facility. See
1295L</warn> and L<perlvar>.
a0d0e21e 1296
19799a22
GS
1297Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1298determining whether a particular feature (such as C<socket> or C<symlink>)
a0d0e21e
LW
1299is implemented. It is also Perl's exception trapping mechanism, where
1300the die operator is used to raise exceptions.
1301
1302If the code to be executed doesn't vary, you may use the eval-BLOCK
1303form to trap run-time errors without incurring the penalty of
1304recompiling each time. The error, if any, is still returned in C<$@>.
1305Examples:
1306
54310121 1307 # make divide-by-zero nonfatal
a0d0e21e
LW
1308 eval { $answer = $a / $b; }; warn $@ if $@;
1309
1310 # same thing, but less efficient
1311 eval '$answer = $a / $b'; warn $@ if $@;
1312
1313 # a compile-time error
5a964f20 1314 eval { $answer = }; # WRONG
a0d0e21e
LW
1315
1316 # a run-time error
1317 eval '$answer ='; # sets $@
1318
2b5ab1e7
TC
1319Due to the current arguably broken state of C<__DIE__> hooks, when using
1320the C<eval{}> form as an exception trap in libraries, you may wish not
1321to trigger any C<__DIE__> hooks that user code may have installed.
1322You can use the C<local $SIG{__DIE__}> construct for this purpose,
1323as shown in this example:
774d564b
PP
1324
1325 # a very private exception trap for divide-by-zero
f86cebdf
GS
1326 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1327 warn $@ if $@;
774d564b
PP
1328
1329This is especially significant, given that C<__DIE__> hooks can call
19799a22 1330C<die> again, which has the effect of changing their error messages:
774d564b
PP
1331
1332 # __DIE__ hooks may modify error messages
1333 {
f86cebdf
GS
1334 local $SIG{'__DIE__'} =
1335 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
c7cc6f1c
GS
1336 eval { die "foo lives here" };
1337 print $@ if $@; # prints "bar lives here"
774d564b
PP
1338 }
1339
19799a22 1340Because this promotes action at a distance, this counterintuitive behavior
2b5ab1e7
TC
1341may be fixed in a future release.
1342
19799a22 1343With an C<eval>, you should be especially careful to remember what's
a0d0e21e
LW
1344being looked at when:
1345
1346 eval $x; # CASE 1
1347 eval "$x"; # CASE 2
1348
1349 eval '$x'; # CASE 3
1350 eval { $x }; # CASE 4
1351
5a964f20 1352 eval "\$$x++"; # CASE 5
a0d0e21e
LW
1353 $$x++; # CASE 6
1354
2f9daede 1355Cases 1 and 2 above behave identically: they run the code contained in
19799a22 1356the variable $x. (Although case 2 has misleading double quotes making
2f9daede 1357the reader wonder what else might be happening (nothing is).) Cases 3
7660c0ab 1358and 4 likewise behave in the same way: they run the code C<'$x'>, which
19799a22 1359does nothing but return the value of $x. (Case 4 is preferred for
2f9daede
TPG
1360purely visual reasons, but it also has the advantage of compiling at
1361compile-time instead of at run-time.) Case 5 is a place where
19799a22 1362normally you I<would> like to use double quotes, except that in this
2f9daede
TPG
1363particular situation, you can just use symbolic references instead, as
1364in case 6.
a0d0e21e 1365
4968c1e4 1366C<eval BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7 1367C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
4968c1e4 1368
a0d0e21e
LW
1369=item exec LIST
1370
8bf3b016
GS
1371=item exec PROGRAM LIST
1372
19799a22
GS
1373The C<exec> function executes a system command I<and never returns>--
1374use C<system> instead of C<exec> if you want it to return. It fails and
1375returns false only if the command does not exist I<and> it is executed
fb73857a 1376directly instead of via your system's command shell (see below).
a0d0e21e 1377
19799a22
GS
1378Since it's a common mistake to use C<exec> instead of C<system>, Perl
1379warns you if there is a following statement which isn't C<die>, C<warn>,
1380or C<exit> (if C<-w> is set - but you always do that). If you
1381I<really> want to follow an C<exec> with some other statement, you
55d729e4
GS
1382can use one of these styles to avoid the warning:
1383
5a964f20
TC
1384 exec ('foo') or print STDERR "couldn't exec foo: $!";
1385 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
55d729e4 1386
5a964f20 1387If there is more than one argument in LIST, or if LIST is an array
f86cebdf 1388with more than one value, calls execvp(3) with the arguments in LIST.
5a964f20
TC
1389If there is only one scalar argument or an array with one element in it,
1390the argument is checked for shell metacharacters, and if there are any,
1391the entire argument is passed to the system's command shell for parsing
1392(this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1393If there are no shell metacharacters in the argument, it is split into
19799a22
GS
1394words and passed directly to C<execvp>, which is more efficient.
1395Examples:
a0d0e21e 1396
19799a22
GS
1397 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1398 exec "sort $outfile | uniq";
a0d0e21e
LW
1399
1400If you don't really want to execute the first argument, but want to lie
1401to the program you are executing about its own name, you can specify
1402the program you actually want to run as an "indirect object" (without a
1403comma) in front of the LIST. (This always forces interpretation of the
54310121 1404LIST as a multivalued list, even if there is only a single scalar in
a0d0e21e
LW
1405the list.) Example:
1406
1407 $shell = '/bin/csh';
1408 exec $shell '-sh'; # pretend it's a login shell
1409
1410or, more directly,
1411
1412 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1413
bb32b41a
GS
1414When the arguments get executed via the system shell, results will
1415be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1416for details.
1417
19799a22
GS
1418Using an indirect object with C<exec> or C<system> is also more
1419secure. This usage (which also works fine with system()) forces
1420interpretation of the arguments as a multivalued list, even if the
1421list had just one argument. That way you're safe from the shell
1422expanding wildcards or splitting up words with whitespace in them.
5a964f20
TC
1423
1424 @args = ( "echo surprise" );
1425
2b5ab1e7 1426 exec @args; # subject to shell escapes
f86cebdf 1427 # if @args == 1
2b5ab1e7 1428 exec { $args[0] } @args; # safe even with one-arg list
5a964f20
TC
1429
1430The first version, the one without the indirect object, ran the I<echo>
1431program, passing it C<"surprise"> an argument. The second version
1432didn't--it tried to run a program literally called I<"echo surprise">,
1433didn't find it, and set C<$?> to a non-zero value indicating failure.
1434
19799a22 1435Note that C<exec> will not call your C<END> blocks, nor will it call
7660c0ab
A
1436any C<DESTROY> methods in your objects.
1437
a0d0e21e
LW
1438=item exists EXPR
1439
01020589 1440Given an expression that specifies a hash element or array element,
8ea97a1e
GS
1441returns true if the specified element in the hash or array has ever
1442been initialized, even if the corresponding value is undefined. The
1443element is not autovivified if it doesn't exist.
a0d0e21e 1444
01020589
GS
1445 print "Exists\n" if exists $hash{$key};
1446 print "Defined\n" if defined $hash{$key};
1447 print "True\n" if $hash{$key};
1448
1449 print "Exists\n" if exists $array[$index];
1450 print "Defined\n" if defined $array[$index];
1451 print "True\n" if $array[$index];
a0d0e21e 1452
8ea97a1e 1453A hash or array element can be true only if it's defined, and defined if
a0d0e21e
LW
1454it exists, but the reverse doesn't necessarily hold true.
1455
afebc493
GS
1456Given an expression that specifies the name of a subroutine,
1457returns true if the specified subroutine has ever been declared, even
1458if it is undefined. Mentioning a subroutine name for exists or defined
1459does not count as declaring it.
1460
1461 print "Exists\n" if exists &subroutine;
1462 print "Defined\n" if defined &subroutine;
1463
a0d0e21e 1464Note that the EXPR can be arbitrarily complicated as long as the final
afebc493 1465operation is a hash or array key lookup or subroutine name:
a0d0e21e 1466
2b5ab1e7
TC
1467 if (exists $ref->{A}->{B}->{$key}) { }
1468 if (exists $hash{A}{B}{$key}) { }
1469
01020589
GS
1470 if (exists $ref->{A}->{B}->[$ix]) { }
1471 if (exists $hash{A}{B}[$ix]) { }
1472
afebc493
GS
1473 if (exists &{$ref->{A}{B}{$key}}) { }
1474
01020589
GS
1475Although the deepest nested array or hash will not spring into existence
1476just because its existence was tested, any intervening ones will.
61eff3bc 1477Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
01020589
GS
1478into existence due to the existence test for the $key element above.
1479This happens anywhere the arrow operator is used, including even:
5a964f20 1480
2b5ab1e7
TC
1481 undef $ref;
1482 if (exists $ref->{"Some key"}) { }
1483 print $ref; # prints HASH(0x80d3d5c)
1484
1485This surprising autovivification in what does not at first--or even
1486second--glance appear to be an lvalue context may be fixed in a future
5a964f20 1487release.
a0d0e21e 1488
479ba383
GS
1489See L<perlref/"Pseudo-hashes: Using an array as a hash"> for specifics
1490on how exists() acts when used on a pseudo-hash.
e0478e5a 1491
afebc493
GS
1492Use of a subroutine call, rather than a subroutine name, as an argument
1493to exists() is an error.
1494
1495 exists &sub; # OK
1496 exists &sub(); # Error
1497
a0d0e21e
LW
1498=item exit EXPR
1499
2b5ab1e7 1500Evaluates EXPR and exits immediately with that value. Example:
a0d0e21e
LW
1501
1502 $ans = <STDIN>;
1503 exit 0 if $ans =~ /^[Xx]/;
1504
19799a22 1505See also C<die>. If EXPR is omitted, exits with C<0> status. The only
2b5ab1e7
TC
1506universally recognized values for EXPR are C<0> for success and C<1>
1507for error; other values are subject to interpretation depending on the
1508environment in which the Perl program is running. For example, exiting
150969 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1510the mailer to return the item undelivered, but that's not true everywhere.
a0d0e21e 1511
19799a22
GS
1512Don't use C<exit> to abort a subroutine if there's any chance that
1513someone might want to trap whatever error happened. Use C<die> instead,
1514which can be trapped by an C<eval>.
28757baa 1515
19799a22 1516The exit() function does not always exit immediately. It calls any
2b5ab1e7 1517defined C<END> routines first, but these C<END> routines may not
19799a22 1518themselves abort the exit. Likewise any object destructors that need to
2b5ab1e7
TC
1519be called are called before the real exit. If this is a problem, you
1520can call C<POSIX:_exit($status)> to avoid END and destructor processing.
87275199 1521See L<perlmod> for details.
5a964f20 1522
a0d0e21e
LW
1523=item exp EXPR
1524
54310121 1525=item exp
bbce6d69 1526
2b5ab1e7 1527Returns I<e> (the natural logarithm base) to the power of EXPR.
a0d0e21e
LW
1528If EXPR is omitted, gives C<exp($_)>.
1529
1530=item fcntl FILEHANDLE,FUNCTION,SCALAR
1531
f86cebdf 1532Implements the fcntl(2) function. You'll probably have to say
a0d0e21e
LW
1533
1534 use Fcntl;
1535
0ade1984 1536first to get the correct constant definitions. Argument processing and
19799a22 1537value return works just like C<ioctl> below.
a0d0e21e
LW
1538For example:
1539
1540 use Fcntl;
5a964f20
TC
1541 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1542 or die "can't fcntl F_GETFL: $!";
1543
19799a22 1544You don't have to check for C<defined> on the return from C<fnctl>.
951ba7fe
GS
1545Like C<ioctl>, it maps a C<0> return from the system call into
1546C<"0 but true"> in Perl. This string is true in boolean context and C<0>
2b5ab1e7
TC
1547in numeric context. It is also exempt from the normal B<-w> warnings
1548on improper numeric conversions.
5a964f20 1549
19799a22 1550Note that C<fcntl> will produce a fatal error if used on a machine that
2b5ab1e7
TC
1551doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1552manpage to learn what functions are available on your system.
a0d0e21e
LW
1553
1554=item fileno FILEHANDLE
1555
2b5ab1e7
TC
1556Returns the file descriptor for a filehandle, or undefined if the
1557filehandle is not open. This is mainly useful for constructing
19799a22 1558bitmaps for C<select> and low-level POSIX tty-handling operations.
2b5ab1e7
TC
1559If FILEHANDLE is an expression, the value is taken as an indirect
1560filehandle, generally its name.
5a964f20
TC
1561
1562You can use this to find out whether two handles refer to the
1563same underlying descriptor:
1564
1565 if (fileno(THIS) == fileno(THAT)) {
1566 print "THIS and THAT are dups\n";
1567 }
a0d0e21e
LW
1568
1569=item flock FILEHANDLE,OPERATION
1570
19799a22
GS
1571Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1572for success, false on failure. Produces a fatal error if used on a
2b5ab1e7 1573machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
19799a22 1574C<flock> is Perl's portable file locking interface, although it locks
2b5ab1e7
TC
1575only entire files, not records.
1576
1577Two potentially non-obvious but traditional C<flock> semantics are
1578that it waits indefinitely until the lock is granted, and that its locks
1579B<merely advisory>. Such discretionary locks are more flexible, but offer
19799a22
GS
1580fewer guarantees. This means that files locked with C<flock> may be
1581modified by programs that do not also use C<flock>. See L<perlport>,
2b5ab1e7
TC
1582your port's specific documentation, or your system-specific local manpages
1583for details. It's best to assume traditional behavior if you're writing
1584portable programs. (But if you're not, you should as always feel perfectly
1585free to write for your own system's idiosyncrasies (sometimes called
1586"features"). Slavish adherence to portability concerns shouldn't get
1587in the way of your getting your job done.)
a3cb178b 1588
8ebc5c01
PP
1589OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1590LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
ea3105be 1591you can use the symbolic names if you import them from the Fcntl module,
68dc0745
PP
1592either individually, or as a group using the ':flock' tag. LOCK_SH
1593requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
ea3105be
GS
1594releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1595LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
68dc0745
PP
1596waiting for the lock (check the return status to see if you got it).
1597
2b5ab1e7
TC
1598To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1599before locking or unlocking it.
8ebc5c01 1600
f86cebdf 1601Note that the emulation built with lockf(3) doesn't provide shared
8ebc5c01 1602locks, and it requires that FILEHANDLE be open with write intent. These
2b5ab1e7 1603are the semantics that lockf(3) implements. Most if not all systems
f86cebdf 1604implement lockf(3) in terms of fcntl(2) locking, though, so the
8ebc5c01
PP
1605differing semantics shouldn't bite too many people.
1606
19799a22
GS
1607Note also that some versions of C<flock> cannot lock things over the
1608network; you would need to use the more system-specific C<fcntl> for
f86cebdf
GS
1609that. If you like you can force Perl to ignore your system's flock(2)
1610function, and so provide its own fcntl(2)-based emulation, by passing
8ebc5c01
PP
1611the switch C<-Ud_flock> to the F<Configure> program when you configure
1612perl.
4633a7c4
LW
1613
1614Here's a mailbox appender for BSD systems.
a0d0e21e 1615
7e1af8bc 1616 use Fcntl ':flock'; # import LOCK_* constants
a0d0e21e
LW
1617
1618 sub lock {
7e1af8bc 1619 flock(MBOX,LOCK_EX);
a0d0e21e
LW
1620 # and, in case someone appended
1621 # while we were waiting...
1622 seek(MBOX, 0, 2);
1623 }
1624
1625 sub unlock {
7e1af8bc 1626 flock(MBOX,LOCK_UN);
a0d0e21e
LW
1627 }
1628
1629 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1630 or die "Can't open mailbox: $!";
1631
1632 lock();
1633 print MBOX $msg,"\n\n";
1634 unlock();
1635
2b5ab1e7
TC
1636On systems that support a real flock(), locks are inherited across fork()
1637calls, whereas those that must resort to the more capricious fcntl()
1638function lose the locks, making it harder to write servers.
1639
cb1a09d0 1640See also L<DB_File> for other flock() examples.
a0d0e21e
LW
1641
1642=item fork
1643
2b5ab1e7
TC
1644Does a fork(2) system call to create a new process running the
1645same program at the same point. It returns the child pid to the
1646parent process, C<0> to the child process, or C<undef> if the fork is
1647unsuccessful. File descriptors (and sometimes locks on those descriptors)
1648are shared, while everything else is copied. On most systems supporting
1649fork(), great care has gone into making it extremely efficient (for
1650example, using copy-on-write technology on data pages), making it the
1651dominant paradigm for multitasking over the last few decades.
5a964f20 1652
45bc9206 1653All files opened for output are flushed before forking the child process.
a0d0e21e 1654
19799a22 1655If you C<fork> without ever waiting on your children, you will
2b5ab1e7
TC
1656accumulate zombies. On some systems, you can avoid this by setting
1657C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1658forking and reaping moribund children.
cb1a09d0 1659
28757baa
PP
1660Note that if your forked child inherits system file descriptors like
1661STDIN and STDOUT that are actually connected by a pipe or socket, even
2b5ab1e7 1662if you exit, then the remote server (such as, say, a CGI script or a
19799a22 1663backgrounded job launched from a remote shell) won't think you're done.
2b5ab1e7 1664You should reopen those to F</dev/null> if it's any issue.
28757baa 1665
cb1a09d0
AD
1666=item format
1667
19799a22 1668Declare a picture format for use by the C<write> function. For
cb1a09d0
AD
1669example:
1670
54310121 1671 format Something =
cb1a09d0
AD
1672 Test: @<<<<<<<< @||||| @>>>>>
1673 $str, $%, '$' . int($num)
1674 .
1675
1676 $str = "widget";
184e9718 1677 $num = $cost/$quantity;
cb1a09d0
AD
1678 $~ = 'Something';
1679 write;
1680
1681See L<perlform> for many details and examples.
1682
8903cb82 1683=item formline PICTURE,LIST
a0d0e21e 1684
5a964f20 1685This is an internal function used by C<format>s, though you may call it,
a0d0e21e
LW
1686too. It formats (see L<perlform>) a list of values according to the
1687contents of PICTURE, placing the output into the format output
7660c0ab 1688accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
19799a22 1689Eventually, when a C<write> is done, the contents of
a0d0e21e 1690C<$^A> are written to some filehandle, but you could also read C<$^A>
7660c0ab 1691yourself and then set C<$^A> back to C<"">. Note that a format typically
19799a22 1692does one C<formline> per line of form, but the C<formline> function itself
748a9306 1693doesn't care how many newlines are embedded in the PICTURE. This means
4633a7c4 1694that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
748a9306
LW
1695You may therefore need to use multiple formlines to implement a single
1696record format, just like the format compiler.
1697
19799a22 1698Be careful if you put double quotes around the picture, because an C<@>
748a9306 1699character may be taken to mean the beginning of an array name.
19799a22 1700C<formline> always returns true. See L<perlform> for other examples.
a0d0e21e
LW
1701
1702=item getc FILEHANDLE
1703
1704=item getc
1705
1706Returns the next character from the input file attached to FILEHANDLE,
2b5ab1e7
TC
1707or the undefined value at end of file, or if there was an error.
1708If FILEHANDLE is omitted, reads from STDIN. This is not particularly
1709efficient. However, it cannot be used by itself to fetch single
1710characters without waiting for the user to hit enter. For that, try
1711something more like:
4633a7c4
LW
1712
1713 if ($BSD_STYLE) {
1714 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1715 }
1716 else {
54310121 1717 system "stty", '-icanon', 'eol', "\001";
4633a7c4
LW
1718 }
1719
1720 $key = getc(STDIN);
1721
1722 if ($BSD_STYLE) {
1723 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1724 }
1725 else {
5f05dabc 1726 system "stty", 'icanon', 'eol', '^@'; # ASCII null
4633a7c4
LW
1727 }
1728 print "\n";
1729
54310121
PP
1730Determination of whether $BSD_STYLE should be set
1731is left as an exercise to the reader.
cb1a09d0 1732
19799a22 1733The C<POSIX::getattr> function can do this more portably on
2b5ab1e7
TC
1734systems purporting POSIX compliance. See also the C<Term::ReadKey>
1735module from your nearest CPAN site; details on CPAN can be found on
1736L<perlmodlib/CPAN>.
a0d0e21e
LW
1737
1738=item getlogin
1739
5a964f20
TC
1740Implements the C library function of the same name, which on most
1741systems returns the current login from F</etc/utmp>, if any. If null,
19799a22 1742use C<getpwuid>.
a0d0e21e 1743
f86702cc 1744 $login = getlogin || getpwuid($<) || "Kilroy";
a0d0e21e 1745
19799a22
GS
1746Do not consider C<getlogin> for authentication: it is not as
1747secure as C<getpwuid>.
4633a7c4 1748
a0d0e21e
LW
1749=item getpeername SOCKET
1750
1751Returns the packed sockaddr address of other end of the SOCKET connection.
1752
4633a7c4
LW
1753 use Socket;
1754 $hersockaddr = getpeername(SOCK);
19799a22 1755 ($port, $iaddr) = sockaddr_in($hersockaddr);
4633a7c4
LW
1756 $herhostname = gethostbyaddr($iaddr, AF_INET);
1757 $herstraddr = inet_ntoa($iaddr);
a0d0e21e
LW
1758
1759=item getpgrp PID
1760
47e29363 1761Returns the current process group for the specified PID. Use
7660c0ab 1762a PID of C<0> to get the current process group for the
4633a7c4 1763current process. Will raise an exception if used on a machine that
f86cebdf 1764doesn't implement getpgrp(2). If PID is omitted, returns process
19799a22 1765group of current process. Note that the POSIX version of C<getpgrp>
7660c0ab 1766does not accept a PID argument, so only C<PID==0> is truly portable.
a0d0e21e
LW
1767
1768=item getppid
1769
1770Returns the process id of the parent process.
1771
1772=item getpriority WHICH,WHO
1773
4633a7c4
LW
1774Returns the current priority for a process, a process group, or a user.
1775(See L<getpriority(2)>.) Will raise a fatal exception if used on a
f86cebdf 1776machine that doesn't implement getpriority(2).
a0d0e21e
LW
1777
1778=item getpwnam NAME
1779
1780=item getgrnam NAME
1781
1782=item gethostbyname NAME
1783
1784=item getnetbyname NAME
1785
1786=item getprotobyname NAME
1787
1788=item getpwuid UID
1789
1790=item getgrgid GID
1791
1792=item getservbyname NAME,PROTO
1793
1794=item gethostbyaddr ADDR,ADDRTYPE
1795
1796=item getnetbyaddr ADDR,ADDRTYPE
1797
1798=item getprotobynumber NUMBER
1799
1800=item getservbyport PORT,PROTO
1801
1802=item getpwent
1803
1804=item getgrent
1805
1806=item gethostent
1807
1808=item getnetent
1809
1810=item getprotoent
1811
1812=item getservent
1813
1814=item setpwent
1815
1816=item setgrent
1817
1818=item sethostent STAYOPEN
1819
1820=item setnetent STAYOPEN
1821
1822=item setprotoent STAYOPEN
1823
1824=item setservent STAYOPEN
1825
1826=item endpwent
1827
1828=item endgrent
1829
1830=item endhostent
1831
1832=item endnetent
1833
1834=item endprotoent
1835
1836=item endservent
1837
1838These routines perform the same functions as their counterparts in the
5a964f20 1839system library. In list context, the return values from the
a0d0e21e
LW
1840various get routines are as follows:
1841
1842 ($name,$passwd,$uid,$gid,
6ee623d5 1843 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
a0d0e21e
LW
1844 ($name,$passwd,$gid,$members) = getgr*
1845 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1846 ($name,$aliases,$addrtype,$net) = getnet*
1847 ($name,$aliases,$proto) = getproto*
1848 ($name,$aliases,$port,$proto) = getserv*
1849
1850(If the entry doesn't exist you get a null list.)
1851
5a964f20 1852In scalar context, you get the name, unless the function was a
a0d0e21e
LW
1853lookup by name, in which case you get the other thing, whatever it is.
1854(If the entry doesn't exist you get the undefined value.) For example:
1855
5a964f20
TC
1856 $uid = getpwnam($name);
1857 $name = getpwuid($num);
1858 $name = getpwent();
1859 $gid = getgrnam($name);
1860 $name = getgrgid($num;
1861 $name = getgrent();
1862 #etc.
a0d0e21e 1863
19799a22 1864In I<getpw*()> the fields $quota, $comment, and $expire are
2b5ab1e7 1865special cases in the sense that in many systems they are unsupported.
19799a22
GS
1866If the $quota is unsupported, it is an empty scalar. If it is
1867supported, it usually encodes the disk quota. If the $comment
2b5ab1e7
TC
1868field is unsupported, it is an empty scalar. If it is supported it
1869usually encodes some administrative comment about the user. In some
19799a22
GS
1870systems the $quota field may be $change or $age, fields that have
1871to do with password aging. In some systems the $comment field may
1872be $class. The $expire field, if present, encodes the expiration
2b5ab1e7
TC
1873period of the account or the password. For the availability and the
1874exact meaning of these fields in your system, please consult your
1875getpwnam(3) documentation and your F<pwd.h> file. You can also find
19799a22
GS
1876out from within Perl what your $quota and $comment fields mean
1877and whether you have the $expire field by using the C<Config> module
2b5ab1e7
TC
1878and the values C<d_pwquota>, C<d_pwage>, C<d_pwchange>, C<d_pwcomment>,
1879and C<d_pwexpire>. Shadow password files are only supported if your
1880vendor has implemented them in the intuitive fashion that calling the
1881regular C library routines gets the shadow versions if you're running
1882under privilege. Those that incorrectly implement a separate library
1883call are not supported.
6ee623d5 1884
19799a22 1885The $members value returned by I<getgr*()> is a space separated list of
a0d0e21e
LW
1886the login names of the members of the group.
1887
1888For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1889C, it will be returned to you via C<$?> if the function call fails. The
7660c0ab 1890C<@addrs> value returned by a successful call is a list of the raw
a0d0e21e
LW
1891addresses returned by the corresponding system library call. In the
1892Internet domain, each address is four bytes long and you can unpack it
1893by saying something like:
1894
1895 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1896
2b5ab1e7
TC
1897The Socket library makes this slightly easier:
1898
1899 use Socket;
1900 $iaddr = inet_aton("127.1"); # or whatever address
1901 $name = gethostbyaddr($iaddr, AF_INET);
1902
1903 # or going the other way
19799a22 1904 $straddr = inet_ntoa($iaddr);
2b5ab1e7 1905
19799a22
GS
1906If you get tired of remembering which element of the return list
1907contains which return value, by-name interfaces are provided
1908in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
1909C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
1910and C<User::grent>. These override the normal built-ins, supplying
1911versions that return objects with the appropriate names
1912for each field. For example:
5a964f20
TC
1913
1914 use File::stat;
1915 use User::pwent;
1916 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
1917
1918Even though it looks like they're the same method calls (uid),
19799a22
GS
1919they aren't, because a C<File::stat> object is different from
1920a C<User::pwent> object.
5a964f20 1921
a0d0e21e
LW
1922=item getsockname SOCKET
1923
19799a22
GS
1924Returns the packed sockaddr address of this end of the SOCKET connection,
1925in case you don't know the address because you have several different
1926IPs that the connection might have come in on.
a0d0e21e 1927
4633a7c4
LW
1928 use Socket;
1929 $mysockaddr = getsockname(SOCK);
19799a22
GS
1930 ($port, $myaddr) = sockaddr_in($mysockaddr);
1931 printf "Connect to %s [%s]\n",
1932 scalar gethostbyaddr($myaddr, AF_INET),
1933 inet_ntoa($myaddr);
a0d0e21e
LW
1934
1935=item getsockopt SOCKET,LEVEL,OPTNAME
1936
5a964f20 1937Returns the socket option requested, or undef if there is an error.
a0d0e21e
LW
1938
1939=item glob EXPR
1940
0a753a76
PP
1941=item glob
1942
2b5ab1e7
TC
1943Returns the value of EXPR with filename expansions such as the
1944standard Unix shell F</bin/csh> would do. This is the internal function
61eff3bc
JH
1945implementing the C<< <*.c> >> operator, but you can use it directly.
1946If EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is
2b5ab1e7 1947discussed in more detail in L<perlop/"I/O Operators">.
a0d0e21e 1948
3a4b19e4
GS
1949Beginning with v5.6.0, this operator is implemented using the standard
1950C<File::Glob> extension. See L<File::Glob> for details.
1951
a0d0e21e
LW
1952=item gmtime EXPR
1953
19799a22 1954Converts a time as returned by the time function to a 9-element list
54310121 1955with the time localized for the standard Greenwich time zone.
4633a7c4 1956Typically used as follows:
a0d0e21e 1957
54310121 1958 # 0 1 2 3 4 5 6 7 8
a0d0e21e
LW
1959 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1960 gmtime(time);
1961
19799a22
GS
1962All list elements are numeric, and come straight out of a struct tm.
1963In particular this means that $mon has the range C<0..11> and $wday
1964has the range C<0..6> with sunday as day C<0>. Also, $year is the
1965number of years since 1900, that is, $year is C<123> in year 2023,
2b5ab1e7
TC
1966I<not> simply the last two digits of the year. If you assume it is,
1967then you create non-Y2K-compliant programs--and you wouldn't want to do
1968that, would you?
2f9daede 1969
abd75f24
GS
1970The proper way to get a complete 4-digit year is simply:
1971
1972 $year += 1900;
1973
1974And to get the last two digits of the year (e.g., '01' in 2001) do:
1975
1976 $year = sprintf("%02d", $year % 100);
1977
2f9daede 1978If EXPR is omitted, does C<gmtime(time())>.
a0d0e21e 1979
f86cebdf 1980In scalar context, returns the ctime(3) value:
0a753a76
PP
1981
1982 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
1983
19799a22 1984Also see the C<timegm> function provided by the C<Time::Local> module,
f86cebdf 1985and the strftime(3) function available via the POSIX module.
7660c0ab 1986
2b5ab1e7
TC
1987This scalar value is B<not> locale dependent (see L<perllocale>), but
1988is instead a Perl builtin. Also see the C<Time::Local> module, and the
1989strftime(3) and mktime(3) functions available via the POSIX module. To
7660c0ab
A
1990get somewhat similar but locale dependent date strings, set up your
1991locale environment variables appropriately (please see L<perllocale>)
1992and try for example:
1993
1994 use POSIX qw(strftime);
2b5ab1e7 1995 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
7660c0ab 1996
2b5ab1e7
TC
1997Note that the C<%a> and C<%b> escapes, which represent the short forms
1998of the day of the week and the month of the year, may not necessarily
1999be three characters wide in all locales.
0a753a76 2000
a0d0e21e
LW
2001=item goto LABEL
2002
748a9306
LW
2003=item goto EXPR
2004
a0d0e21e
LW
2005=item goto &NAME
2006
7660c0ab 2007The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
a0d0e21e 2008execution there. It may not be used to go into any construct that
7660c0ab 2009requires initialization, such as a subroutine or a C<foreach> loop. It
0a753a76 2010also can't be used to go into a construct that is optimized away,
19799a22 2011or to get out of a block or subroutine given to C<sort>.
0a753a76 2012It can be used to go almost anywhere else within the dynamic scope,
a0d0e21e 2013including out of subroutines, but it's usually better to use some other
19799a22 2014construct such as C<last> or C<die>. The author of Perl has never felt the
7660c0ab 2015need to use this form of C<goto> (in Perl, that is--C is another matter).
a0d0e21e 2016
7660c0ab
A
2017The C<goto-EXPR> form expects a label name, whose scope will be resolved
2018dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
748a9306
LW
2019necessarily recommended if you're optimizing for maintainability:
2020
2021 goto ("FOO", "BAR", "GLARCH")[$i];
2022
6cb9131c
GS
2023The C<goto-&NAME> form is quite different from the other forms of C<goto>.
2024In fact, it isn't a goto in the normal sense at all, and doesn't have
2025the stigma associated with other gotos. Instead, it
2026substitutes a call to the named subroutine for the currently running
2027subroutine. This is used by C<AUTOLOAD> subroutines that wish to load
2028another subroutine and then pretend that the other subroutine had been
2029called in the first place (except that any modifications to C<@_>
2030in the current subroutine are propagated to the other subroutine.)
2031After the C<goto>, not even C<caller> will be able to tell that this
2032routine was called first.
2033
2034NAME needn't be the name of a subroutine; it can be a scalar variable
2035containing a code reference, or a block which evaluates to a code
2036reference.
a0d0e21e
LW
2037
2038=item grep BLOCK LIST
2039
2040=item grep EXPR,LIST
2041
2b5ab1e7
TC
2042This is similar in spirit to, but not the same as, grep(1) and its
2043relatives. In particular, it is not limited to using regular expressions.
2f9daede 2044
a0d0e21e 2045Evaluates the BLOCK or EXPR for each element of LIST (locally setting
7660c0ab 2046C<$_> to each element) and returns the list value consisting of those
19799a22
GS
2047elements for which the expression evaluated to true. In scalar
2048context, returns the number of times the expression was true.
a0d0e21e
LW
2049
2050 @foo = grep(!/^#/, @bar); # weed out comments
2051
2052or equivalently,
2053
2054 @foo = grep {!/^#/} @bar; # weed out comments
2055
2b5ab1e7
TC
2056Note that, because C<$_> is a reference into the list value, it can
2057be used to modify the elements of the array. While this is useful and
2058supported, it can cause bizarre results if the LIST is not a named array.
2059Similarly, grep returns aliases into the original list, much as a for
2060loop's index variable aliases the list elements. That is, modifying an
19799a22
GS
2061element of a list returned by grep (for example, in a C<foreach>, C<map>
2062or another C<grep>) actually modifies the element in the original list.
2b5ab1e7 2063This is usually something to be avoided when writing clear code.
a0d0e21e 2064
19799a22 2065See also L</map> for a list composed of the results of the BLOCK or EXPR.
38325410 2066
a0d0e21e
LW
2067=item hex EXPR
2068
54310121 2069=item hex
bbce6d69 2070
2b5ab1e7
TC
2071Interprets EXPR as a hex string and returns the corresponding value.
2072(To convert strings that might start with either 0, 0x, or 0b, see
2073L</oct>.) If EXPR is omitted, uses C<$_>.
2f9daede
TPG
2074
2075 print hex '0xAf'; # prints '175'
2076 print hex 'aF'; # same
a0d0e21e 2077
19799a22 2078Hex strings may only represent integers. Strings that would cause
c6edd1b7 2079integer overflow trigger a warning.
19799a22 2080
a0d0e21e
LW
2081=item import
2082
19799a22 2083There is no builtin C<import> function. It is just an ordinary
4633a7c4 2084method (subroutine) defined (or inherited) by modules that wish to export
19799a22 2085names to another module. The C<use> function calls the C<import> method
54310121 2086for the package used. See also L</use()>, L<perlmod>, and L<Exporter>.
a0d0e21e
LW
2087
2088=item index STR,SUBSTR,POSITION
2089
2090=item index STR,SUBSTR
2091
2b5ab1e7
TC
2092The index function searches for one string within another, but without
2093the wildcard-like behavior of a full regular-expression pattern match.
2094It returns the position of the first occurrence of SUBSTR in STR at
2095or after POSITION. If POSITION is omitted, starts searching from the
2096beginning of the string. The return value is based at C<0> (or whatever
2097you've set the C<$[> variable to--but don't do that). If the substring
2098is not found, returns one less than the base, ordinarily C<-1>.
a0d0e21e
LW
2099
2100=item int EXPR
2101
54310121 2102=item int
bbce6d69 2103
7660c0ab 2104Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2b5ab1e7
TC
2105You should not use this function for rounding: one because it truncates
2106towards C<0>, and two because machine representations of floating point
2107numbers can sometimes produce counterintuitive results. For example,
2108C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2109because it's really more like -268.99999999999994315658 instead. Usually,
19799a22 2110the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2b5ab1e7 2111functions will serve you better than will int().
a0d0e21e
LW
2112
2113=item ioctl FILEHANDLE,FUNCTION,SCALAR
2114
2b5ab1e7 2115Implements the ioctl(2) function. You'll probably first have to say
a0d0e21e 2116
4633a7c4 2117 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
a0d0e21e 2118
2b5ab1e7 2119to get the correct function definitions. If F<ioctl.ph> doesn't
a0d0e21e 2120exist or doesn't have the correct definitions you'll have to roll your
61eff3bc 2121own, based on your C header files such as F<< <sys/ioctl.h> >>.
5a964f20 2122(There is a Perl script called B<h2ph> that comes with the Perl kit that
54310121 2123may help you in this, but it's nontrivial.) SCALAR will be read and/or
4633a7c4 2124written depending on the FUNCTION--a pointer to the string value of SCALAR
19799a22 2125will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
4633a7c4
LW
2126has no string value but does have a numeric value, that value will be
2127passed rather than a pointer to the string value. To guarantee this to be
19799a22
GS
2128true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2129functions may be needed to manipulate the values of structures used by
2130C<ioctl>.
a0d0e21e 2131
19799a22 2132The return value of C<ioctl> (and C<fcntl>) is as follows:
a0d0e21e
LW
2133
2134 if OS returns: then Perl returns:
2135 -1 undefined value
2136 0 string "0 but true"
2137 anything else that number
2138
19799a22 2139Thus Perl returns true on success and false on failure, yet you can
a0d0e21e
LW
2140still easily determine the actual value returned by the operating
2141system:
2142
2b5ab1e7 2143 $retval = ioctl(...) || -1;
a0d0e21e
LW
2144 printf "System returned %d\n", $retval;
2145
c2611fb3 2146The special string "C<0> but true" is exempt from B<-w> complaints
5a964f20
TC
2147about improper numeric conversions.
2148
19799a22
GS
2149Here's an example of setting a filehandle named C<REMOTE> to be
2150non-blocking at the system level. You'll have to negotiate C<$|>
2151on your own, though.
2152
2153 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
2154
2155 $flags = fcntl(REMOTE, F_GETFL, 0)
2156 or die "Can't get flags for the socket: $!\n";
2157
2158 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
2159 or die "Can't set flags for the socket: $!\n";
2160
a0d0e21e
LW
2161=item join EXPR,LIST
2162
2b5ab1e7
TC
2163Joins the separate strings of LIST into a single string with fields
2164separated by the value of EXPR, and returns that new string. Example:
a0d0e21e 2165
2b5ab1e7 2166 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
a0d0e21e 2167
eb6e2d6f
GS
2168Beware that unlike C<split>, C<join> doesn't take a pattern as its
2169first argument. Compare L</split>.
a0d0e21e 2170
aa689395
PP
2171=item keys HASH
2172
19799a22 2173Returns a list consisting of all the keys of the named hash. (In
1d2dff63 2174scalar context, returns the number of keys.) The keys are returned in
ab192400
GS
2175an apparently random order. The actual random order is subject to
2176change in future versions of perl, but it is guaranteed to be the same
19799a22 2177order as either the C<values> or C<each> function produces (given
ab192400
GS
2178that the hash has not been modified). As a side effect, it resets
2179HASH's iterator.
a0d0e21e 2180
aa689395 2181Here is yet another way to print your environment:
a0d0e21e
LW
2182
2183 @keys = keys %ENV;
2184 @values = values %ENV;
19799a22 2185 while (@keys) {
a0d0e21e
LW
2186 print pop(@keys), '=', pop(@values), "\n";
2187 }
2188
2189or how about sorted by key:
2190
2191 foreach $key (sort(keys %ENV)) {
2192 print $key, '=', $ENV{$key}, "\n";
2193 }
2194
19799a22 2195To sort a hash by value, you'll need to use a C<sort> function.
aa689395 2196Here's a descending numeric sort of a hash by its values:
4633a7c4 2197
5a964f20 2198 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
4633a7c4
LW
2199 printf "%4d %s\n", $hash{$key}, $key;
2200 }
2201
19799a22 2202As an lvalue C<keys> allows you to increase the number of hash buckets
aa689395
PP
2203allocated for the given hash. This can gain you a measure of efficiency if
2204you know the hash is going to get big. (This is similar to pre-extending
2205an array by assigning a larger number to $#array.) If you say
55497cff
PP
2206
2207 keys %hash = 200;
2208
ab192400
GS
2209then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2210in fact, since it rounds up to the next power of two. These
55497cff
PP
2211buckets will be retained even if you do C<%hash = ()>, use C<undef
2212%hash> if you want to free the storage while C<%hash> is still in scope.
2213You can't shrink the number of buckets allocated for the hash using
19799a22 2214C<keys> in this way (but you needn't worry about doing this by accident,
55497cff
PP
2215as trying has no effect).
2216
19799a22 2217See also C<each>, C<values> and C<sort>.
ab192400 2218
b350dd2f 2219=item kill SIGNAL, LIST
a0d0e21e 2220
b350dd2f 2221Sends a signal to a list of processes. Returns the number of
517db077
GS
2222processes successfully signaled (which is not necessarily the
2223same as the number actually killed).
a0d0e21e
LW
2224
2225 $cnt = kill 1, $child1, $child2;
2226 kill 9, @goners;
2227
b350dd2f
GS
2228If SIGNAL is zero, no signal is sent to the process. This is a
2229useful way to check that the process is alive and hasn't changed
2230its UID. See L<perlport> for notes on the portability of this
2231construct.
2232
2233Unlike in the shell, if SIGNAL is negative, it kills
4633a7c4
LW
2234process groups instead of processes. (On System V, a negative I<PROCESS>
2235number will also kill process groups, but that's not portable.) That
2236means you usually want to use positive not negative signals. You may also
da0045b7 2237use a signal name in quotes. See L<perlipc/"Signals"> for details.
a0d0e21e
LW
2238
2239=item last LABEL
2240
2241=item last
2242
2243The C<last> command is like the C<break> statement in C (as used in
2244loops); it immediately exits the loop in question. If the LABEL is
2245omitted, the command refers to the innermost enclosing loop. The
2246C<continue> block, if any, is not executed:
2247
4633a7c4
LW
2248 LINE: while (<STDIN>) {
2249 last LINE if /^$/; # exit when done with header
5a964f20 2250 #...
a0d0e21e
LW
2251 }
2252
4968c1e4 2253C<last> cannot be used to exit a block which returns a value such as
2b5ab1e7
TC
2254C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2255a grep() or map() operation.
4968c1e4 2256
6c1372ed
GS
2257Note that a block by itself is semantically identical to a loop
2258that executes once. Thus C<last> can be used to effect an early
2259exit out of such a block.
2260
98293880
JH
2261See also L</continue> for an illustration of how C<last>, C<next>, and
2262C<redo> work.
1d2dff63 2263
a0d0e21e
LW
2264=item lc EXPR
2265
54310121 2266=item lc
bbce6d69 2267
a0d0e21e 2268Returns an lowercased version of EXPR. This is the internal function
7660c0ab 2269implementing the C<\L> escape in double-quoted strings.
19799a22
GS
2270Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
2271and L<utf8>.
a0d0e21e 2272
7660c0ab 2273If EXPR is omitted, uses C<$_>.
bbce6d69 2274
a0d0e21e
LW
2275=item lcfirst EXPR
2276
54310121 2277=item lcfirst
bbce6d69 2278
a0d0e21e 2279Returns the value of EXPR with the first character lowercased. This is
7660c0ab 2280the internal function implementing the C<\l> escape in double-quoted strings.
a0ed51b3 2281Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
a0d0e21e 2282
7660c0ab 2283If EXPR is omitted, uses C<$_>.
bbce6d69 2284
a0d0e21e
LW
2285=item length EXPR
2286
54310121 2287=item length
bbce6d69 2288
a0ed51b3 2289Returns the length in characters of the value of EXPR. If EXPR is
2b5ab1e7
TC
2290omitted, returns length of C<$_>. Note that this cannot be used on
2291an entire array or hash to find out how many elements these have.
2292For that, use C<scalar @array> and C<scalar keys %hash> respectively.
a0d0e21e
LW
2293
2294=item link OLDFILE,NEWFILE
2295
19799a22
GS
2296Creates a new filename linked to the old filename. Returns true for
2297success, false otherwise.
a0d0e21e
LW
2298
2299=item listen SOCKET,QUEUESIZE
2300
19799a22
GS
2301Does the same thing that the listen system call does. Returns true if
2302it succeeded, false otherwise. See the example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
2303
2304=item local EXPR
2305
19799a22 2306You really probably want to be using C<my> instead, because C<local> isn't
2b5ab1e7
TC
2307what most people think of as "local". See L<perlsub/"Private Variables
2308via my()"> for details.
2309
5a964f20
TC
2310A local modifies the listed variables to be local to the enclosing
2311block, file, or eval. If more than one value is listed, the list must
2312be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2313for details, including issues with tied arrays and hashes.
a0d0e21e 2314
a0d0e21e
LW
2315=item localtime EXPR
2316
19799a22 2317Converts a time as returned by the time function to a 9-element list
5f05dabc 2318with the time analyzed for the local time zone. Typically used as
a0d0e21e
LW
2319follows:
2320
54310121 2321 # 0 1 2 3 4 5 6 7 8
a0d0e21e
LW
2322 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2323 localtime(time);
2324
19799a22
GS
2325All list elements are numeric, and come straight out of a struct tm.
2326In particular this means that $mon has the range C<0..11> and $wday
2327has the range C<0..6> with sunday as day C<0>. Also, $year is the
2328number of years since 1900, that is, $year is C<123> in year 2023,
2b5ab1e7
TC
2329and I<not> simply the last two digits of the year. If you assume it is,
2330then you create non-Y2K-compliant programs--and you wouldn't want to do
2331that, would you?
54310121 2332
abd75f24
GS
2333The proper way to get a complete 4-digit year is simply:
2334
2335 $year += 1900;
2336
2337And to get the last two digits of the year (e.g., '01' in 2001) do:
2338
2339 $year = sprintf("%02d", $year % 100);
2340
54310121 2341If EXPR is omitted, uses the current time (C<localtime(time)>).
a0d0e21e 2342
f86cebdf 2343In scalar context, returns the ctime(3) value:
a0d0e21e 2344
5f05dabc 2345 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
a0d0e21e 2346
a3cb178b 2347This scalar value is B<not> locale dependent, see L<perllocale>, but
68f8bed4
JH
2348instead a Perl builtin. Also see the C<Time::Local> module
2349(to convert the second, minutes, hours, ... back to seconds since the
2350stroke of midnight the 1st of January 1970, the value returned by
ca6e1c26 2351time()), and the strftime(3) and mktime(3) functions available via the
68f8bed4
JH
2352POSIX module. To get somewhat similar but locale dependent date
2353strings, set up your locale environment variables appropriately
2354(please see L<perllocale>) and try for example:
a3cb178b 2355
5a964f20 2356 use POSIX qw(strftime);
2b5ab1e7 2357 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
a3cb178b
GS
2358
2359Note that the C<%a> and C<%b>, the short forms of the day of the week
2360and the month of the year, may not necessarily be three characters wide.
a0d0e21e 2361
19799a22
GS
2362=item lock
2363
2364 lock I<THING>
2365
2366This function places an advisory lock on a variable, subroutine,
2367or referenced object contained in I<THING> until the lock goes out
2368of scope. This is a built-in function only if your version of Perl
2369was built with threading enabled, and if you've said C<use Threads>.
2370Otherwise a user-defined function by this name will be called. See
2371L<Thread>.
2372
a0d0e21e
LW
2373=item log EXPR
2374
54310121 2375=item log
bbce6d69 2376
2b5ab1e7
TC
2377Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2378returns log of C<$_>. To get the log of another base, use basic algebra:
19799a22 2379The base-N log of a number is equal to the natural log of that number
2b5ab1e7
TC
2380divided by the natural log of N. For example:
2381
2382 sub log10 {
2383 my $n = shift;
2384 return log($n)/log(10);
2385 }
2386
2387See also L</exp> for the inverse operation.
a0d0e21e
LW
2388
2389=item lstat FILEHANDLE
2390
2391=item lstat EXPR
2392
54310121 2393=item lstat
bbce6d69 2394
19799a22 2395Does the same thing as the C<stat> function (including setting the
5a964f20
TC
2396special C<_> filehandle) but stats a symbolic link instead of the file
2397the symbolic link points to. If symbolic links are unimplemented on
19799a22 2398your system, a normal C<stat> is done.
a0d0e21e 2399
7660c0ab 2400If EXPR is omitted, stats C<$_>.
bbce6d69 2401
a0d0e21e
LW
2402=item m//
2403
2404The match operator. See L<perlop>.
2405
2406=item map BLOCK LIST
2407
2408=item map EXPR,LIST
2409
19799a22
GS
2410Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2411C<$_> to each element) and returns the list value composed of the
2412results of each such evaluation. In scalar context, returns the
2413total number of elements so generated. Evaluates BLOCK or EXPR in
2414list context, so each element of LIST may produce zero, one, or
2415more elements in the returned value.
dd99ebda 2416
a0d0e21e
LW
2417 @chars = map(chr, @nums);
2418
2419translates a list of numbers to the corresponding characters. And
2420
4633a7c4 2421 %hash = map { getkey($_) => $_ } @array;
a0d0e21e
LW
2422
2423is just a funny way to write
2424
2425 %hash = ();
2426 foreach $_ (@array) {
4633a7c4 2427 $hash{getkey($_)} = $_;
a0d0e21e
LW
2428 }
2429
2b5ab1e7
TC
2430Note that, because C<$_> is a reference into the list value, it can
2431be used to modify the elements of the array. While this is useful and
2432supported, it can cause bizarre results if the LIST is not a named array.
2433Using a regular C<foreach> loop for this purpose would be clearer in
2434most cases. See also L</grep> for an array composed of those items of
2435the original list for which the BLOCK or EXPR evaluates to true.
fb73857a 2436
19799a22 2437=item mkdir FILENAME,MASK
a0d0e21e 2438
5a211162
GS
2439=item mkdir FILENAME
2440
0591cd52 2441Creates the directory specified by FILENAME, with permissions
19799a22
GS
2442specified by MASK (as modified by C<umask>). If it succeeds it
2443returns true, otherwise it returns false and sets C<$!> (errno).
5a211162 2444If omitted, MASK defaults to 0777.
0591cd52 2445
19799a22 2446In general, it is better to create directories with permissive MASK,
0591cd52 2447and let the user modify that with their C<umask>, than it is to supply
19799a22 2448a restrictive MASK and give the user no way to be more permissive.
0591cd52
NT
2449The exceptions to this rule are when the file or directory should be
2450kept private (mail files, for instance). The perlfunc(1) entry on
19799a22 2451C<umask> discusses the choice of MASK in more detail.
a0d0e21e
LW
2452
2453=item msgctl ID,CMD,ARG
2454
f86cebdf 2455Calls the System V IPC function msgctl(2). You'll probably have to say
0ade1984
JH
2456
2457 use IPC::SysV;
2458
7660c0ab
A
2459first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2460then ARG must be a variable which will hold the returned C<msqid_ds>
951ba7fe
GS
2461structure. Returns like C<ioctl>: the undefined value for error,
2462C<"0 but true"> for zero, or the actual return value otherwise. See also
19799a22 2463C<IPC::SysV> and C<IPC::Semaphore> documentation.
a0d0e21e
LW
2464
2465=item msgget KEY,FLAGS
2466
f86cebdf 2467Calls the System V IPC function msgget(2). Returns the message queue
7660c0ab 2468id, or the undefined value if there is an error. See also C<IPC::SysV>
19799a22 2469and C<IPC::Msg> documentation.
a0d0e21e
LW
2470
2471=item msgsnd ID,MSG,FLAGS
2472
2473Calls the System V IPC function msgsnd to send the message MSG to the
e4038a1f
MS
2474message queue ID. MSG must begin with the native long integer message
2475type, which may be created with C<pack("l!", $type)>. Returns true if
2476successful, or false if there is an error. See also C<IPC::SysV> and
2477C<IPC::SysV::Msg> documentation.
a0d0e21e
LW
2478
2479=item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2480
2481Calls the System V IPC function msgrcv to receive a message from
2482message queue ID into variable VAR with a maximum message size of
0ade1984
JH
2483SIZE. Note that if a message is received, the message type will be
2484the first thing in VAR, and the maximum length of VAR is SIZE plus the
19799a22 2485size of the message type. Returns true if successful, or false if
7660c0ab 2486there is an error. See also C<IPC::SysV> and C<IPC::SysV::Msg> documentation.
a0d0e21e
LW
2487
2488=item my EXPR
2489
09bef843
SB
2490=item my EXPR : ATTRIBUTES
2491
19799a22
GS
2492A C<my> declares the listed variables to be local (lexically) to the
2493enclosing block, file, or C<eval>. If
5f05dabc 2494more than one value is listed, the list must be placed in parentheses. See
cb1a09d0 2495L<perlsub/"Private Variables via my()"> for details.
4633a7c4 2496
a0d0e21e
LW
2497=item next LABEL
2498
2499=item next
2500
2501The C<next> command is like the C<continue> statement in C; it starts
2502the next iteration of the loop:
2503
4633a7c4
LW
2504 LINE: while (<STDIN>) {
2505 next LINE if /^#/; # discard comments
5a964f20 2506 #...
a0d0e21e
LW
2507 }
2508
2509Note that if there were a C<continue> block on the above, it would get
2510executed even on discarded lines. If the LABEL is omitted, the command
2511refers to the innermost enclosing loop.
2512
4968c1e4 2513C<next> cannot be used to exit a block which returns a value such as
2b5ab1e7
TC
2514C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2515a grep() or map() operation.
4968c1e4 2516
6c1372ed
GS
2517Note that a block by itself is semantically identical to a loop
2518that executes once. Thus C<next> will exit such a block early.
2519
98293880
JH
2520See also L</continue> for an illustration of how C<last>, C<next>, and
2521C<redo> work.
1d2dff63 2522
a0d0e21e
LW
2523=item no Module LIST
2524
7660c0ab 2525See the L</use> function, which C<no> is the opposite of.
a0d0e21e
LW
2526
2527=item oct EXPR
2528
54310121 2529=item oct
bbce6d69 2530
4633a7c4 2531Interprets EXPR as an octal string and returns the corresponding
4f19785b
WSI
2532value. (If EXPR happens to start off with C<0x>, interprets it as a
2533hex string. If EXPR starts off with C<0b>, it is interpreted as a
2534binary string.) The following will handle decimal, binary, octal, and
4633a7c4 2535hex in the standard Perl or C notation:
a0d0e21e
LW
2536
2537 $val = oct($val) if $val =~ /^0/;
2538
19799a22
GS
2539If EXPR is omitted, uses C<$_>. To go the other way (produce a number
2540in octal), use sprintf() or printf():
2541
2542 $perms = (stat("filename"))[2] & 07777;
2543 $oct_perms = sprintf "%lo", $perms;
2544
2545The oct() function is commonly used when a string such as C<644> needs
2546to be converted into a file mode, for example. (Although perl will
2547automatically convert strings into numbers as needed, this automatic
2548conversion assumes base 10.)
a0d0e21e 2549
1c1fc3ea 2550=item open FILEHANDLE,MODE,LIST
6170680b 2551
a0d0e21e
LW
2552=item open FILEHANDLE,EXPR
2553
2554=item open FILEHANDLE
2555
2556Opens the file whose filename is given by EXPR, and associates it with
5f05dabc
PP
2557FILEHANDLE. If FILEHANDLE is an expression, its value is used as the
2558name of the real filehandle wanted. If EXPR is omitted, the scalar
2559variable of the same name as the FILEHANDLE contains the filename.
19799a22
GS
2560(Note that lexical variables--those declared with C<my>--will not work
2561for this purpose; so if you're using C<my>, specify EXPR in your call
2b5ab1e7
TC
2562to open.) See L<perlopentut> for a kinder, gentler explanation of opening
2563files.
5f05dabc 2564
61eff3bc
JH
2565If MODE is C<< '<' >> or nothing, the file is opened for input.
2566If MODE is C<< '>' >>, the file is truncated and opened for
2567output, being created if necessary. If MODE is C<<< '>>' >>>,
fbb426e4 2568the file is opened for appending, again being created if necessary.
61eff3bc
JH
2569You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to indicate that
2570you want both read and write access to the file; thus C<< '+<' >> is almost
2571always preferred for read/write updates--the C<< '+>' >> mode would clobber the
5a964f20
TC
2572file first. You can't usually use either read-write mode for updating
2573textfiles, since they have variable length records. See the B<-i>
0591cd52
NT
2574switch in L<perlrun> for a better approach. The file is created with
2575permissions of C<0666> modified by the process' C<umask> value.
5a964f20 2576
61eff3bc
JH
2577These various prefixes correspond to the fopen(3) modes of C<'r'>, C<'r+'>,
2578C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
5f05dabc 2579
6170680b
IZ
2580In the 2-arguments (and 1-argument) form of the call the mode and
2581filename should be concatenated (in this order), possibly separated by
61eff3bc 2582spaces. It is possible to omit the mode if the mode is C<< '<' >>.
6170680b 2583
7660c0ab 2584If the filename begins with C<'|'>, the filename is interpreted as a
5a964f20 2585command to which output is to be piped, and if the filename ends with a
f244e06d
GS
2586C<'|'>, the filename is interpreted as a command which pipes output to
2587us. See L<perlipc/"Using open() for IPC">
19799a22 2588for more examples of this. (You are not allowed to C<open> to a command
5a964f20
TC
2589that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2590and L<perlipc/"Bidirectional Communication"> for alternatives.)
cb1a09d0 2591
6170680b
IZ
2592If MODE is C<'|-'>, the filename is interpreted as a
2593command to which output is to be piped, and if MODE is
2594C<'-|'>, the filename is interpreted as a command which pipes output to
2595us. In the 2-arguments (and 1-argument) form one should replace dash
2596(C<'-'>) with the command. See L<perlipc/"Using open() for IPC">
2597for more examples of this. (You are not allowed to C<open> to a command
2598that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2599and L<perlipc/"Bidirectional Communication"> for alternatives.)
2600
2601In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
61eff3bc 2602and opening C<< '>-' >> opens STDOUT.
6170680b
IZ
2603
2604Open returns
19799a22 2605nonzero upon success, the undefined value otherwise. If the C<open>
4633a7c4 2606involved a pipe, the return value happens to be the pid of the
54310121 2607subprocess.
cb1a09d0
AD
2608
2609If you're unfortunate enough to be running Perl on a system that
2610distinguishes between text files and binary files (modern operating
2611systems don't care), then you should check out L</binmode> for tips for
19799a22 2612dealing with this. The key distinction between systems that need C<binmode>
5a964f20
TC
2613and those that don't is their text file formats. Systems like Unix, MacOS, and
2614Plan9, which delimit lines with a single character, and which encode that
19799a22 2615character in C as C<"\n">, do not need C<binmode>. The rest need it.
cb1a09d0 2616
fb73857a 2617When opening a file, it's usually a bad idea to continue normal execution
19799a22
GS
2618if the request failed, so C<open> is frequently used in connection with
2619C<die>. Even if C<die> won't do what you want (say, in a CGI script,
fb73857a 2620where you want to make a nicely formatted error message (but there are
5a964f20 2621modules that can help with that problem)) you should always check
19799a22 2622the return value from opening a file. The infrequent exception is when
fb73857a
PP
2623working with an unopened filehandle is actually what you want to do.
2624
cb1a09d0 2625Examples:
a0d0e21e
LW
2626
2627 $ARTICLE = 100;
2628 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2629 while (<ARTICLE>) {...
2630
6170680b 2631 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
fb73857a 2632 # if the open fails, output is discarded
a0d0e21e 2633
6170680b 2634 open(DBASE, '+<', 'dbase.mine') # open for update
fb73857a 2635 or die "Can't open 'dbase.mine' for update: $!";
cb1a09d0 2636
6170680b
IZ
2637 open(DBASE, '+<dbase.mine') # ditto
2638 or die "Can't open 'dbase.mine' for update: $!";
2639
2640 open(ARTICLE, '-|', "caesar <$article") # decrypt article
fb73857a 2641 or die "Can't start caesar: $!";
a0d0e21e 2642
6170680b
IZ
2643 open(ARTICLE, "caesar <$article |") # ditto
2644 or die "Can't start caesar: $!";
2645
2646 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
fb73857a 2647 or die "Can't start sort: $!";
a0d0e21e
LW
2648
2649 # process argument list of files along with any includes
2650
2651 foreach $file (@ARGV) {
2652 process($file, 'fh00');
2653 }
2654
2655 sub process {
5a964f20 2656 my($filename, $input) = @_;
a0d0e21e
LW
2657 $input++; # this is a string increment
2658 unless (open($input, $filename)) {
2659 print STDERR "Can't open $filename: $!\n";
2660 return;
2661 }
2662
5a964f20 2663 local $_;
a0d0e21e
LW
2664 while (<$input>) { # note use of indirection
2665 if (/^#include "(.*)"/) {
2666 process($1, $input);
2667 next;
2668 }
5a964f20 2669 #... # whatever
a0d0e21e
LW
2670 }
2671 }
2672
2673You may also, in the Bourne shell tradition, specify an EXPR beginning
61eff3bc 2674with C<< '>&' >>, in which case the rest of the string is interpreted as the
5a964f20 2675name of a filehandle (or file descriptor, if numeric) to be
61eff3bc
JH
2676duped and opened. You may use C<&> after C<< > >>, C<<< >> >>>,
2677C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>. The
a0d0e21e 2678mode you specify should match the mode of the original filehandle.
184e9718 2679(Duping a filehandle does not take into account any existing contents of
6170680b
IZ
2680stdio buffers.) Duping file handles is not yet supported for 3-argument
2681open().
2682
a0d0e21e
LW
2683Here is a script that saves, redirects, and restores STDOUT and
2684STDERR:
2685
2686 #!/usr/bin/perl
5a964f20
TC
2687 open(OLDOUT, ">&STDOUT");
2688 open(OLDERR, ">&STDERR");
a0d0e21e 2689
6170680b
IZ
2690 open(STDOUT, '>', "foo.out") || die "Can't redirect stdout";
2691 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
a0d0e21e
LW
2692
2693 select(STDERR); $| = 1; # make unbuffered
2694 select(STDOUT); $| = 1; # make unbuffered
2695
2696 print STDOUT "stdout 1\n"; # this works for
2697 print STDERR "stderr 1\n"; # subprocesses too
2698
2699 close(STDOUT);
2700 close(STDERR);
2701
5a964f20
TC
2702 open(STDOUT, ">&OLDOUT");
2703 open(STDERR, ">&OLDERR");
a0d0e21e
LW
2704
2705 print STDOUT "stdout 2\n";
2706 print STDERR "stderr 2\n";
2707
61eff3bc 2708If you specify C<< '<&=N' >>, where C<N> is a number, then Perl will do an
19799a22 2709equivalent of C's C<fdopen> of that file descriptor; this is more
4633a7c4 2710parsimonious of file descriptors. For example:
a0d0e21e
LW
2711
2712 open(FILEHANDLE, "<&=$fd")
2713
4af147f6
CS
2714Note that this feature depends on the fdopen() C library function.
2715On many UNIX systems, fdopen() is known to fail when file descriptors
2716exceed a certain value, typically 255. If you need more file
2717descriptors than that, consider rebuilding Perl to use the C<sfio>
2718library.
2719
6170680b
IZ
2720If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
2721with 2-arguments (or 1-argument) form of open(), then
a0d0e21e 2722there is an implicit fork done, and the return value of open is the pid
7660c0ab 2723of the child within the parent process, and C<0> within the child
184e9718 2724process. (Use C<defined($pid)> to determine whether the open was successful.)
a0d0e21e
LW
2725The filehandle behaves normally for the parent, but i/o to that
2726filehandle is piped from/to the STDOUT/STDIN of the child process.
2727In the child process the filehandle isn't opened--i/o happens from/to
2728the new STDOUT or STDIN. Typically this is used like the normal
2729piped open when you want to exercise more control over just how the
2730pipe command gets executed, such as when you are running setuid, and
54310121 2731don't want to have to scan shell commands for metacharacters.
6170680b 2732The following triples are more or less equivalent:
a0d0e21e
LW
2733
2734 open(FOO, "|tr '[a-z]' '[A-Z]'");
6170680b
IZ
2735 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
2736 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
a0d0e21e
LW
2737
2738 open(FOO, "cat -n '$file'|");
6170680b
IZ
2739 open(FOO, '-|', "cat -n '$file'");
2740 open(FOO, '-|') || exec 'cat', '-n', $file;
a0d0e21e 2741
4633a7c4
LW
2742See L<perlipc/"Safe Pipe Opens"> for more examples of this.
2743
45bc9206
GS
2744NOTE: On any operation that may do a fork, all files opened for output
2745are flushed before the fork is attempted. On systems that support a
2746close-on-exec flag on files, the flag will be set for the newly opened
2747file descriptor as determined by the value of $^F. See L<perlvar/$^F>.
a0d0e21e 2748
0dccf244
CS
2749Closing any piped filehandle causes the parent process to wait for the
2750child to finish, and returns the status value in C<$?>.
2751
6170680b
IZ
2752The filename passed to 2-argument (or 1-argument) form of open()
2753will have leading and trailing
f86cebdf 2754whitespace deleted, and the normal redirection characters
5a964f20
TC
2755honored. This property, known as "magic open",
2756can often be used to good effect. A user could specify a filename of
7660c0ab 2757F<"rsh cat file |">, or you could change certain filenames as needed:
5a964f20
TC
2758
2759 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
2760 open(FH, $filename) or die "Can't open $filename: $!";
2761
6170680b
IZ
2762Use 3-argument form to open a file with arbitrary weird characters in it,
2763
2764 open(FOO, '<', $file);
2765
2766otherwise it's necessary to protect any leading and trailing whitespace:
5a964f20
TC
2767
2768 $file =~ s#^(\s)#./$1#;
2769 open(FOO, "< $file\0");
2770
6170680b
IZ
2771(this may not work on some bizzare filesystems). One should
2772conscientiously choose between the the I<magic> and 3-arguments form
2773of open():
2774
2775 open IN, $ARGV[0];
2776
2777will allow the user to specify an argument of the form C<"rsh cat file |">,
2778but will not work on a filename which happens to have a trailing space, while
2779
2780 open IN, '<', $ARGV[0];
2781
2782will have exactly the opposite restrictions.
2783
19799a22 2784If you want a "real" C C<open> (see L<open(2)> on your system), then you
6170680b
IZ
2785should use the C<sysopen> function, which involves no such magic (but
2786may use subtly different filemodes than Perl open(), which is mapped
2787to C fopen()). This is
5a964f20
TC
2788another way to protect your filenames from interpretation. For example:
2789
2790 use IO::Handle;
2791 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
2792 or die "sysopen $path: $!";
2793 $oldfh = select(HANDLE); $| = 1; select($oldfh);
2794 print HANDLE "stuff $$\n");
2795 seek(HANDLE, 0, 0);
2796 print "File contains: ", <HANDLE>;
2797
7660c0ab
A
2798Using the constructor from the C<IO::Handle> package (or one of its
2799subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
5a964f20
TC
2800filehandles that have the scope of whatever variables hold references to
2801them, and automatically close whenever and however you leave that scope:
c07a80fd 2802
5f05dabc 2803 use IO::File;
5a964f20 2804 #...
c07a80fd
PP
2805 sub read_myfile_munged {
2806 my $ALL = shift;
5f05dabc 2807 my $handle = new IO::File;
c07a80fd
PP
2808 open($handle, "myfile") or die "myfile: $!";
2809 $first = <$handle>
2810 or return (); # Automatically closed here.
2811 mung $first or die "mung failed"; # Or here.
2812 return $first, <$handle> if $ALL; # Or here.
2813 $first; # Or here.
2814 }
2815
b687b08b 2816See L</seek> for some details about mixing reading and writing.
a0d0e21e
LW
2817
2818=item opendir DIRHANDLE,EXPR
2819
19799a22
GS
2820Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
2821C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
a0d0e21e
LW
2822DIRHANDLEs have their own namespace separate from FILEHANDLEs.
2823
2824=item ord EXPR
2825
54310121 2826=item ord
bbce6d69 2827
a0ed51b3 2828Returns the numeric (ASCII or Unicode) value of the first character of EXPR. If
7660c0ab 2829EXPR is omitted, uses C<$_>. For the reverse, see L</chr>.
2b5ab1e7 2830See L<utf8> for more about Unicode.
a0d0e21e 2831
77ca0c92
LW
2832=item our EXPR
2833
2834An C<our> declares the listed variables to be valid globals within
2835the enclosing block, file, or C<eval>. That is, it has the same
2836scoping rules as a "my" declaration, but does not create a local
2837variable. If more than one value is listed, the list must be placed
2838in parentheses. The C<our> declaration has no semantic effect unless
2839"use strict vars" is in effect, in which case it lets you use the
2840declared global variable without qualifying it with a package name.
2841(But only within the lexical scope of the C<our> declaration. In this
2842it differs from "use vars", which is package scoped.)
2843
f472eb5c
GS
2844An C<our> declaration declares a global variable that will be visible
2845across its entire lexical scope, even across package boundaries. The
2846package in which the variable is entered is determined at the point
2847of the declaration, not at the point of use. This means the following
2848behavior holds:
2849
2850 package Foo;
2851 our $bar; # declares $Foo::bar for rest of lexical scope
2852 $bar = 20;
2853
2854 package Bar;
2855 print $bar; # prints 20
2856
2857Multiple C<our> declarations in the same lexical scope are allowed
2858if they are in different packages. If they happened to be in the same
2859package, Perl will emit warnings if you have asked for them.
2860
2861 use warnings;
2862 package Foo;
2863 our $bar; # declares $Foo::bar for rest of lexical scope
2864 $bar = 20;
2865
2866 package Bar;
2867 our $bar = 30; # declares $Bar::bar for rest of lexical scope
2868 print $bar; # prints 30
2869
2870 our $bar; # emits warning
2871
a0d0e21e
LW
2872=item pack TEMPLATE,LIST
2873
2b6c5635
GS
2874Takes a LIST of values and converts it into a string using the rules
2875given by the TEMPLATE. The resulting string is the concatenation of
2876the converted values. Typically, each converted value looks
2877like its machine-level representation. For example, on 32-bit machines
2878a converted integer may be represented by a sequence of 4 bytes.
2879
2880The TEMPLATE is a
a0d0e21e
LW
2881sequence of characters that give the order and type of values, as
2882follows:
2883
5a929a98 2884 a A string with arbitrary binary data, will be null padded.
a0d0e21e 2885 A An ascii string, will be space padded.
5a929a98
VU
2886 Z A null terminated (asciz) string, will be null padded.
2887
2b6c5635
GS
2888 b A bit string (ascending bit order inside each byte, like vec()).
2889 B A bit string (descending bit order inside each byte).
a0d0e21e
LW
2890 h A hex string (low nybble first).
2891 H A hex string (high nybble first).
2892
2893 c A signed char value.
a0ed51b3 2894 C An unsigned char value. Only does bytes. See U for Unicode.
96e4d5b1 2895
a0d0e21e
LW
2896 s A signed short value.
2897 S An unsigned short value.
96e4d5b1 2898 (This 'short' is _exactly_ 16 bits, which may differ from
851646ae
JH
2899 what a local C compiler calls 'short'. If you want
2900 native-length shorts, use the '!' suffix.)
96e4d5b1 2901
a0d0e21e
LW
2902 i A signed integer value.
2903 I An unsigned integer value.
19799a22 2904 (This 'integer' is _at_least_ 32 bits wide. Its exact
f86cebdf
GS
2905 size depends on what a local C compiler calls 'int',
2906 and may even be larger than the 'long' described in
2907 the next item.)
96e4d5b1 2908
a0d0e21e
LW
2909 l A signed long value.
2910 L An unsigned long value.
96e4d5b1 2911 (This 'long' is _exactly_ 32 bits, which may differ from
851646ae
JH
2912 what a local C compiler calls 'long'. If you want
2913 native-length longs, use the '!' suffix.)
a0d0e21e 2914
5d11dd56
G
2915 n An unsigned short in "network" (big-endian) order.
2916 N An unsigned long in "network" (big-endian) order.
2917 v An unsigned short in "VAX" (little-endian) order.
2918 V An unsigned long in "VAX" (little-endian) order.
96e4d5b1
PP
2919 (These 'shorts' and 'longs' are _exactly_ 16 bits and
2920 _exactly_ 32 bits, respectively.)
a0d0e21e 2921
dae0da7a
JH
2922 q A signed quad (64-bit) value.
2923 Q An unsigned quad value.
851646ae
JH
2924 (Quads are available only if your system supports 64-bit
2925 integer values _and_ if Perl has been compiled to support those.
dae0da7a
JH
2926 Causes a fatal error otherwise.)
2927
a0d0e21e
LW
2928 f A single-precision float in the native format.
2929 d A double-precision float in the native format.
2930
2931 p A pointer to a null-terminated string.
2932 P A pointer to a structure (fixed-length string).
2933
2934 u A uuencoded string.
a0ed51b3
LW
2935 U A Unicode character number. Encodes to UTF-8 internally.
2936 Works even if C<use utf8> is not in effect.
a0d0e21e 2937
96e4d5b1 2938 w A BER compressed integer. Its bytes represent an unsigned
f86cebdf
GS
2939 integer in base 128, most significant digit first, with as
2940 few digits as possible. Bit eight (the high bit) is set
2941 on each byte except the last.
def98dd4 2942
a0d0e21e
LW
2943 x A null byte.
2944 X Back up a byte.
2945 @ Null fill to absolute position.
2946
5a929a98
VU
2947The following rules apply:
2948
2949=over 8
2950
2951=item *
2952
5a964f20 2953Each letter may optionally be followed by a number giving a repeat
951ba7fe
GS
2954count. With all types except C<a>, C<A>, C<Z>, C<b>, C<B>, C<h>,
2955C<H>, and C<P> the pack function will gobble up that many values from
5a929a98 2956the LIST. A C<*> for the repeat count means to use however many items are
951ba7fe
GS
2957left, except for C<@>, C<x>, C<X>, where it is equivalent
2958to C<0>, and C<u>, where it is equivalent to 1 (or 45, what is the
2b6c5635
GS
2959same).
2960
951ba7fe 2961When used with C<Z>, C<*> results in the addition of a trailing null
2b6c5635
GS
2962byte (so the packed result will be one longer than the byte C<length>
2963of the item).
2964
951ba7fe 2965The repeat count for C<u> is interpreted as the maximal number of bytes
2b6c5635 2966to encode per line of output, with 0 and 1 replaced by 45.
5a929a98
VU
2967
2968=item *
2969
951ba7fe 2970The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
5a929a98 2971string of length count, padding with nulls or spaces as necessary. When
951ba7fe
GS
2972unpacking, C<A> strips trailing spaces and nulls, C<Z> strips everything
2973after the first null, and C<a> returns data verbatim. When packing,
2974C<a>, and C<Z> are equivalent.
2b6c5635
GS
2975
2976If the value-to-pack is too long, it is truncated. If too long and an
951ba7fe
GS
2977explicit count is provided, C<Z> packs only C<$count-1> bytes, followed
2978by a null byte. Thus C<Z> always packs a trailing null byte under
2b6c5635 2979all circumstances.
5a929a98
VU
2980
2981=item *
2982
951ba7fe 2983Likewise, the C<b> and C<B> fields pack a string that many bits long.
c73032f5
IZ
2984Each byte of the input field of pack() generates 1 bit of the result.
2985Each result bit is based on the least-significant bit of the corresponding
2986input byte, i.e., on C<ord($byte)%2>. In particular, bytes C<"0"> and
2987C<"1"> generate bits 0 and 1, as do bytes C<"\0"> and C<"\1">.
2988
2989Starting from the beginning of the input string of pack(), each 8-tuple
951ba7fe 2990of bytes is converted to 1 byte of output. With format C<b>
c73032f5 2991the first byte of the 8-tuple determines the least-significant bit of a
951ba7fe 2992byte, and with format C<B> it determines the most-significant bit of
c73032f5
IZ
2993a byte.
2994
2995If the length of the input string is not exactly divisible by 8, the
2996remainder is packed as if the input string were padded by null bytes
2997at the end. Similarly, during unpack()ing the "extra" bits are ignored.
2998
2999If the input string of pack() is longer than needed, extra bytes are ignored.
2b6c5635
GS
3000A C<*> for the repeat count of pack() means to use all the bytes of
3001the input field. On unpack()ing the bits are converted to a string
3002of C<"0">s and C<"1">s.
5a929a98
VU
3003
3004=item *
3005
951ba7fe 3006The C<h> and C<H> fields pack a string that many nybbles (4-bit groups,
851646ae 3007representable as hexadecimal digits, 0-9a-f) long.
5a929a98 3008
c73032f5
IZ
3009Each byte of the input field of pack() generates 4 bits of the result.
3010For non-alphabetical bytes the result is based on the 4 least-significant
3011bits of the input byte, i.e., on C<ord($byte)%16>. In particular,
3012bytes C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
3013C<"\0"> and C<"\1">. For bytes C<"a".."f"> and C<"A".."F"> the result
3014is compatible with the usual hexadecimal digits, so that C<"a"> and
3015C<"A"> both generate the nybble C<0xa==10>. The result for bytes
3016C<"g".."z"> and C<"G".."Z"> is not well-defined.
3017
3018Starting from the beginning of the input string of pack(), each pair
951ba7fe 3019of bytes is converted to 1 byte of output. With format C<h> the
c73032f5 3020first byte of the pair determines the least-significant nybble of the
951ba7fe 3021output byte, and with format C<H> it determines the most-significant
c73032f5
IZ
3022nybble.
3023
3024If the length of the input string is not even, it behaves as if padded
3025by a null byte at the end. Similarly, during unpack()ing the "extra"
3026nybbles are ignored.
3027
3028If the input string of pack() is longer than needed, extra bytes are ignored.
3029A C<*> for the repeat count of pack() means to use all the bytes of
3030the input field. On unpack()ing the bits are converted to a string
3031of hexadecimal digits.
3032
5a929a98
VU
3033=item *
3034
951ba7fe 3035The C<p> type packs a pointer to a null-terminated string. You are
5a929a98
VU
3036responsible for ensuring the string is not a temporary value (which can
3037potentially get deallocated before you get around to using the packed result).
951ba7fe
GS
3038The C<P> type packs a pointer to a structure of the size indicated by the
3039length. A NULL pointer is created if the corresponding value for C<p> or
3040C<P> is C<undef>, similarly for unpack().
5a929a98
VU
3041
3042=item *
3043
951ba7fe
GS
3044The C</> template character allows packing and unpacking of strings where
3045the packed structure contains a byte count followed by the string itself.
17f4a12d 3046You write I<length-item>C</>I<string-item>.
43192e07
IP
3047
3048The I<length-item> can be any C<pack> template letter,
3049and describes how the length value is packed.
3050The ones likely to be of most use are integer-packing ones like
951ba7fe
GS
3051C<n> (for Java strings), C<w> (for ASN.1 or SNMP)
3052and C<N> (for Sun XDR).
43192e07
IP
3053
3054The I<string-item> must, at present, be C<"A*">, C<"a*"> or C<"Z*">.
3055For C<unpack> the length of the string is obtained from the I<length-item>,
3056but if you put in the '*' it will be ignored.
3057
17f4a12d
IZ
3058 unpack 'C/a', "\04Gurusamy"; gives 'Guru'
3059 unpack 'a3/A* A*', '007 Bond J '; gives (' Bond','J')
3060 pack 'n/a* w/a*','hello,','world'; gives "\000\006hello,\005world"
43192e07
IP
3061
3062The I<length-item> is not returned explicitly from C<unpack>.
3063
951ba7fe
GS
3064Adding a count to the I<length-item> letter is unlikely to do anything
3065useful, unless that letter is C<A>, C<a> or C<Z>. Packing with a
3066I<length-item> of C<a> or C<Z> may introduce C<"\000"> characters,
43192e07
IP
3067which Perl does not regard as legal in numeric strings.
3068
3069=item *
3070
951ba7fe
GS
3071The integer types C<s>, C<S>, C<l>, and C<L> may be
3072immediately followed by a C<!> suffix to signify native shorts or
3073longs--as you can see from above for example a bare C<l> does mean
851646ae
JH
3074exactly 32 bits, the native C<long> (as seen by the local C compiler)
3075may be larger. This is an issue mainly in 64-bit platforms. You can
951ba7fe 3076see whether using C<!> makes any difference by
726ea183 3077
4d0c1c44
GS
3078 print length(pack("s")), " ", length(pack("s!")), "\n";
3079 print length(pack("l")), " ", length(pack("l!")), "\n";
ef54e1a4 3080
951ba7fe
GS
3081C<i!> and C<I!> also work but only because of completeness;
3082they are identical to C<i> and C<I>.
ef54e1a4 3083
19799a22
GS
3084The actual sizes (in bytes) of native shorts, ints, longs, and long
3085longs on the platform where Perl was built are also available via
3086L<Config>:
3087
3088 use Config;
3089 print $Config{shortsize}, "\n";
3090 print $Config{intsize}, "\n";
3091 print $Config{longsize}, "\n";
3092 print $Config{longlongsize}, "\n";
ef54e1a4 3093
5074e145 3094(The C<$Config{longlongsize}> will be undefine if your system does
851646ae
JH
3095not support long longs.)
3096
ef54e1a4
JH
3097=item *
3098
951ba7fe 3099The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, and C<L>
ef54e1a4
JH
3100are inherently non-portable between processors and operating systems
3101because they obey the native byteorder and endianness. For example a
140cb37e 31024-byte integer 0x12345678 (305419896 decimal) be ordered natively
ef54e1a4 3103(arranged in and handled by the CPU registers) into bytes as
61eff3bc 3104
719a3cf5
JH
3105 0x12 0x34 0x56 0x78 # little-endian
3106 0x78 0x56 0x34 0x12 # big-endian
61eff3bc 3107
5d11dd56 3108Basically, the Intel, Alpha, and VAX CPUs are little-endian, while
719a3cf5
JH
3109everybody else, for example Motorola m68k/88k, PPC, Sparc, HP PA,
3110Power, and Cray are big-endian. MIPS can be either: Digital used it
19799a22 3111in little-endian mode; SGI uses it in big-endian mode.
719a3cf5 3112
19799a22 3113The names `big-endian' and `little-endian' are comic references to
ef54e1a4
JH
3114the classic "Gulliver's Travels" (via the paper "On Holy Wars and a
3115Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and
19799a22 3116the egg-eating habits of the Lilliputians.
61eff3bc 3117
140cb37e 3118Some systems may have even weirder byte orders such as
61eff3bc 3119
ef54e1a4
JH
3120 0x56 0x78 0x12 0x34
3121 0x34 0x12 0x78 0x56
61eff3bc 3122
ef54e1a4
JH
3123You can see your system's preference with
3124
3125 print join(" ", map { sprintf "%#02x", $_ }
3126 unpack("C*",pack("L",0x12345678))), "\n";
3127
d99ad34e 3128The byteorder on the platform where Perl was built is also available
726ea183 3129via L<Config>:
ef54e1a4
JH
3130
3131 use Config;
3132 print $Config{byteorder}, "\n";
3133
d99ad34e
JH
3134Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'>
3135and C<'87654321'> are big-endian.
719a3cf5 3136
951ba7fe
GS
3137If you want portable packed integers use the formats C<n>, C<N>,
3138C<v>, and C<V>, their byte endianness and size is known.
851646ae 3139See also L<perlport>.
ef54e1a4
JH
3140
3141=item *
3142
5a929a98
VU
3143Real numbers (floats and doubles) are in the native machine format only;
3144due to the multiplicity of floating formats around, and the lack of a
3145standard "network" representation, no facility for interchange has been
3146made. This means that packed floating point data written on one machine
3147may not be readable on another - even if both use IEEE floating point
3148arithmetic (as the endian-ness of the memory representation is not part
851646ae 3149of the IEEE spec). See also L<perlport>.
5a929a98
VU
3150
3151Note that Perl uses doubles internally for all numeric calculation, and
3152converting from double into float and thence back to double again will
3153lose precision (i.e., C<unpack("f", pack("f", $foo)>) will not in general
19799a22 3154equal $foo).
5a929a98 3155
851646ae
JH
3156=item *
3157
3158You must yourself do any alignment or padding by inserting for example
9ccd05c0
JH
3159enough C<'x'>es while packing. There is no way to pack() and unpack()
3160could know where the bytes are going to or coming from. Therefore
3161C<pack> (and C<unpack>) handle their output and input as flat
3162sequences of bytes.
851646ae 3163
17f4a12d
IZ
3164=item *
3165
3166A comment in a TEMPLATE starts with C<#> and goes to the end of line.
3167
2b6c5635
GS
3168=item *
3169
3170If TEMPLATE requires more arguments to pack() than actually given, pack()
3171assumes additional C<""> arguments. If TEMPLATE requires less arguments
3172to pack() than actually given, extra arguments are ignored.
3173
5a929a98 3174=back
a0d0e21e
LW
3175
3176Examples:
3177
a0ed51b3 3178 $foo = pack("CCCC",65,66,67,68);
a0d0e21e 3179 # foo eq "ABCD"
a0ed51b3 3180 $foo = pack("C4",65,66,67,68);
a0d0e21e 3181 # same thing
a0ed51b3
LW
3182 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
3183 # same thing with Unicode circled letters
a0d0e21e
LW
3184
3185 $foo = pack("ccxxcc",65,66,67,68);
3186 # foo eq "AB\0\0CD"
3187
9ccd05c0
JH
3188 # note: the above examples featuring "C" and "c" are true
3189 # only on ASCII and ASCII-derived systems such as ISO Latin 1
3190 # and UTF-8. In EBCDIC the first example would be
3191 # $foo = pack("CCCC",193,194,195,196);
3192
a0d0e21e
LW
3193 $foo = pack("s2",1,2);
3194 # "\1\0\2\0" on little-endian
3195 # "\0\1\0\2" on big-endian
3196
3197 $foo = pack("a4","abcd","x","y","z");
3198 # "abcd"
3199
3200 $foo = pack("aaaa","abcd","x","y","z");
3201 # "axyz"
3202
3203 $foo = pack("a14","abcdefg");
3204 # "abcdefg\0\0\0\0\0\0\0"
3205
3206 $foo = pack("i9pl", gmtime);
3207 # a real struct tm (on my system anyway)
3208
5a929a98
VU
3209 $utmp_template = "Z8 Z8 Z16 L";
3210 $utmp = pack($utmp_template, @utmp1);
3211 # a struct utmp (BSDish)
3212
3213 @utmp2 = unpack($utmp_template, $utmp);
3214 # "@utmp1" eq "@utmp2"
3215
a0d0e21e
LW
3216 sub bintodec {
3217 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
3218 }
3219
851646ae
JH
3220 $foo = pack('sx2l', 12, 34);
3221 # short 12, two zero bytes padding, long 34
3222 $bar = pack('s@4l', 12, 34);
3223 # short 12, zero fill to position 4, long 34
3224 # $foo eq $bar
3225
5a929a98 3226The same template may generally also be used in unpack().
a0d0e21e 3227
5a964f20
TC
3228=item package
3229
cb1a09d0
AD
3230=item package NAMESPACE
3231
3232Declares the compilation unit as being in the given namespace. The scope
2b5ab1e7 3233of the package declaration is from the declaration itself through the end
19799a22 3234of the enclosing block, file, or eval (the same as the C<my> operator).
2b5ab1e7
TC
3235All further unqualified dynamic identifiers will be in this namespace.
3236A package statement affects only dynamic variables--including those
19799a22
GS
3237you've used C<local> on--but I<not> lexical variables, which are created
3238with C<my>. Typically it would be the first declaration in a file to
2b5ab1e7
TC
3239be included by the C<require> or C<use> operator. You can switch into a
3240package in more than one place; it merely influences which symbol table
3241is used by the compiler for the rest of that block. You can refer to
3242variables and filehandles in other packages by prefixing the identifier
3243with the package name and a double colon: C<$Package::Variable>.
3244If the package name is null, the C<main> package as assumed. That is,
3245C<$::sail> is equivalent to C<$main::sail> (as well as to C<$main'sail>,
3246still seen in older code).
cb1a09d0 3247
5a964f20
TC
3248If NAMESPACE is omitted, then there is no current package, and all
3249identifiers must be fully qualified or lexicals. This is stricter
3250than C<use strict>, since it also extends to function names.
3251
cb1a09d0
AD
3252See L<perlmod/"Packages"> for more information about packages, modules,
3253and classes. See L<perlsub> for other scoping issues.
3254
a0d0e21e
LW
3255=item pipe READHANDLE,WRITEHANDLE
3256
3257Opens a pair of connected pipes like the corresponding system call.
3258Note that if you set up a loop of piped processes, deadlock can occur
3259unless you are very careful. In addition, note that Perl's pipes use
184e9718 3260stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE
a0d0e21e
LW
3261after each command, depending on the application.
3262
7e1af8bc 3263See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
4633a7c4
LW
3264for examples of such things.
3265
4771b018
GS
3266On systems that support a close-on-exec flag on files, the flag will be set
3267for the newly opened file descriptors as determined by the value of $^F.
3268See L<perlvar/$^F>.
3269
a0d0e21e
LW
3270=item pop ARRAY
3271
54310121 3272=item pop
28757baa 3273
a0d0e21e 3274Pops and returns the last value of the array, shortening the array by
19799a22 3275one element. Has an effect similar to
a0d0e21e 3276
19799a22 3277 $ARRAY[$#ARRAY--]
a0d0e21e 3278
19799a22
GS
3279If there are no elements in the array, returns the undefined value
3280(although this may happen at other times as well). If ARRAY is
3281omitted, pops the C<@ARGV> array in the main program, and the C<@_>
3282array in subroutines, just like C<shift>.
a0d0e21e
LW
3283
3284=item pos SCALAR
3285
54310121 3286=item pos
bbce6d69 3287
4633a7c4 3288Returns the offset of where the last C<m//g> search left off for the variable
7660c0ab