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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
13a2d996 94=over 4
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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
4375e838 149C<caller>, C<import>, C<local>, C<my>, C<our>, C<package>, C<use>
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150
151=item Miscellaneous functions
152
4375e838 153C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<our>, C<reset>,
22fae026 154C<scalar>, C<undef>, C<wantarray>
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155
156=item Functions for processes and process groups
157
22fae026 158C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
ab4f32c2 159C<pipe>, C<qx/STRING/>, C<setpgrp>, C<setpriority>, C<sleep>, C<system>,
22fae026 160C<times>, C<wait>, C<waitpid>
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161
162=item Keywords related to perl modules
163
22fae026 164C<do>, C<import>, C<no>, C<package>, C<require>, C<use>
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165
166=item Keywords related to classes and object-orientedness
167
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168C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
169C<untie>, C<use>
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170
171=item Low-level socket functions
172
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173C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
174C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
175C<socket>, C<socketpair>
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176
177=item System V interprocess communication functions
178
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179C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
180C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>
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181
182=item Fetching user and group info
183
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184C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
185C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
186C<getpwuid>, C<setgrent>, C<setpwent>
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187
188=item Fetching network info
189
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190C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
191C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
192C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
193C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
194C<setnetent>, C<setprotoent>, C<setservent>
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195
196=item Time-related functions
197
22fae026 198C<gmtime>, C<localtime>, C<time>, C<times>
cb1a09d0 199
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200=item Functions new in perl5
201
22fae026 202C<abs>, C<bless>, C<chomp>, C<chr>, C<exists>, C<formline>, C<glob>,
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203C<import>, C<lc>, C<lcfirst>, C<map>, C<my>, C<no>, C<our>, C<prototype>,
204C<qx>, C<qw>, C<readline>, C<readpipe>, C<ref>, C<sub*>, C<sysopen>, C<tie>,
22fae026 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
4375e838 491data contains C<\cZ>, the I/O subsystem will regard it as the end of
30168b04
GS
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, $evaltext contains the text of the
277ddfaf 542C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
951ba7fe 543$filename is C<(eval)>, but $evaltext is undefined. (Note also that
dc848c6f 544each C<use> statement creates a C<require> frame inside an C<eval EXPR>)
277ddfaf 545frame. C<$hasargs> is true if a new instance of C<@_> was set up for the
e476b1b5
GS
546frame. C<$hints> and C<$bitmask> contain pragmatic hints that the caller
547was compiled with. The C<$hints> and C<$bitmask> values are subject to
548change between versions of Perl, and are not meant for external use.
748a9306
LW
549
550Furthermore, when called from within the DB package, caller returns more
7660c0ab 551detailed information: it sets the list variable C<@DB::args> to be the
54310121 552arguments with which the subroutine was invoked.
748a9306 553
7660c0ab 554Be aware that the optimizer might have optimized call frames away before
19799a22 555C<caller> had a chance to get the information. That means that C<caller(N)>
7660c0ab 556might not return information about the call frame you expect it do, for
61eff3bc 557C<< N > 1 >>. In particular, C<@DB::args> might have information from the
19799a22 558previous time C<caller> was called.
7660c0ab 559
a0d0e21e
LW
560=item chdir EXPR
561
2b5ab1e7 562Changes the working directory to EXPR, if possible. If EXPR is omitted,
0bfc1ec4
GS
563changes to the directory specified by C<$ENV{HOME}>, if set; if not,
564changes to the directory specified by C<$ENV{LOGDIR}>. If neither is
565set, C<chdir> does nothing. It returns true upon success, false
566otherwise. See the example under C<die>.
a0d0e21e
LW
567
568=item chmod LIST
569
570Changes the permissions of a list of files. The first element of the
4633a7c4 571list must be the numerical mode, which should probably be an octal
2f9daede
TPG
572number, and which definitely should I<not> a string of octal digits:
573C<0644> is okay, C<'0644'> is not. Returns the number of files
dc848c6f 574successfully changed. See also L</oct>, if all you have is a string.
a0d0e21e
LW
575
576 $cnt = chmod 0755, 'foo', 'bar';
577 chmod 0755, @executables;
f86cebdf
GS
578 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
579 # --w----r-T
2f9daede
TPG
580 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
581 $mode = 0644; chmod $mode, 'foo'; # this is best
a0d0e21e 582
ca6e1c26
JH
583You can also import the symbolic C<S_I*> constants from the Fcntl
584module:
585
586 use Fcntl ':mode';
587
588 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
589 # This is identical to the chmod 0755 of the above example.
590
a0d0e21e
LW
591=item chomp VARIABLE
592
593=item chomp LIST
594
595=item chomp
596
2b5ab1e7
TC
597This safer version of L</chop> removes any trailing string
598that corresponds to the current value of C<$/> (also known as
28757baa
PP
599$INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
600number of characters removed from all its arguments. It's often used to
601remove the newline from the end of an input record when you're worried
2b5ab1e7
TC
602that the final record may be missing its newline. When in paragraph
603mode (C<$/ = "">), it removes all trailing newlines from the string.
4c5a6083
GS
604When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
605a reference to an integer or the like, see L<perlvar>) chomp() won't
19799a22
GS
606remove anything.
607If VARIABLE is omitted, it chomps C<$_>. Example:
a0d0e21e
LW
608
609 while (<>) {
610 chomp; # avoid \n on last field
611 @array = split(/:/);
5a964f20 612 # ...
a0d0e21e
LW
613 }
614
4bf21a6d
RD
615If VARIABLE is a hash, it chomps the hash's values, but not its keys.
616
a0d0e21e
LW
617You can actually chomp anything that's an lvalue, including an assignment:
618
619 chomp($cwd = `pwd`);
620 chomp($answer = <STDIN>);
621
622If you chomp a list, each element is chomped, and the total number of
623characters removed is returned.
624
625=item chop VARIABLE
626
627=item chop LIST
628
629=item chop
630
631Chops off the last character of a string and returns the character
632chopped. It's used primarily to remove the newline from the end of an
633input record, but is much more efficient than C<s/\n//> because it neither
7660c0ab 634scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
a0d0e21e
LW
635Example:
636
637 while (<>) {
638 chop; # avoid \n on last field
639 @array = split(/:/);
5a964f20 640 #...
a0d0e21e
LW
641 }
642
4bf21a6d
RD
643If VARIABLE is a hash, it chops the hash's values, but not its keys.
644
a0d0e21e
LW
645You can actually chop anything that's an lvalue, including an assignment:
646
647 chop($cwd = `pwd`);
648 chop($answer = <STDIN>);
649
650If you chop a list, each element is chopped. Only the value of the
19799a22 651last C<chop> is returned.
a0d0e21e 652
19799a22 653Note that C<chop> returns the last character. To return all but the last
748a9306
LW
654character, use C<substr($string, 0, -1)>.
655
a0d0e21e
LW
656=item chown LIST
657
658Changes the owner (and group) of a list of files. The first two
19799a22
GS
659elements of the list must be the I<numeric> uid and gid, in that
660order. A value of -1 in either position is interpreted by most
661systems to leave that value unchanged. Returns the number of files
662successfully changed.
a0d0e21e
LW
663
664 $cnt = chown $uid, $gid, 'foo', 'bar';
665 chown $uid, $gid, @filenames;
666
54310121 667Here's an example that looks up nonnumeric uids in the passwd file:
a0d0e21e
LW
668
669 print "User: ";
19799a22 670 chomp($user = <STDIN>);
5a964f20 671 print "Files: ";
19799a22 672 chomp($pattern = <STDIN>);
a0d0e21e
LW
673
674 ($login,$pass,$uid,$gid) = getpwnam($user)
675 or die "$user not in passwd file";
676
5a964f20 677 @ary = glob($pattern); # expand filenames
a0d0e21e
LW
678 chown $uid, $gid, @ary;
679
54310121 680On most systems, you are not allowed to change the ownership of the
4633a7c4
LW
681file unless you're the superuser, although you should be able to change
682the group to any of your secondary groups. On insecure systems, these
683restrictions may be relaxed, but this is not a portable assumption.
19799a22
GS
684On POSIX systems, you can detect this condition this way:
685
686 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
687 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
4633a7c4 688
a0d0e21e
LW
689=item chr NUMBER
690
54310121 691=item chr
bbce6d69 692
a0d0e21e 693Returns the character represented by that NUMBER in the character set.
a0ed51b3 694For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
aaa68c4a
SC
695chr(0x263a) is a Unicode smiley face. Within the scope of C<use utf8>,
696characters higher than 127 are encoded in Unicode; if you don't want
697this, temporarily C<use bytes> or use C<pack("C*",...)>
698
699For the reverse, use L</ord>.
2b5ab1e7 700See L<utf8> for more about Unicode.
a0d0e21e 701
7660c0ab 702If NUMBER is omitted, uses C<$_>.
bbce6d69 703
a0d0e21e
LW
704=item chroot FILENAME
705
54310121 706=item chroot
bbce6d69 707
5a964f20 708This function works like the system call by the same name: it makes the
4633a7c4 709named directory the new root directory for all further pathnames that
951ba7fe 710begin with a C</> by your process and all its children. (It doesn't
28757baa 711change your current working directory, which is unaffected.) For security
4633a7c4 712reasons, this call is restricted to the superuser. If FILENAME is
19799a22 713omitted, does a C<chroot> to C<$_>.
a0d0e21e
LW
714
715=item close FILEHANDLE
716
6a518fbc
TP
717=item close
718
19799a22 719Closes the file or pipe associated with the file handle, returning true
a0d0e21e 720only if stdio successfully flushes buffers and closes the system file
19799a22 721descriptor. Closes the currently selected filehandle if the argument
6a518fbc 722is omitted.
fb73857a
PP
723
724You don't have to close FILEHANDLE if you are immediately going to do
19799a22
GS
725another C<open> on it, because C<open> will close it for you. (See
726C<open>.) However, an explicit C<close> on an input file resets the line
727counter (C<$.>), while the implicit close done by C<open> does not.
fb73857a 728
19799a22
GS
729If the file handle came from a piped open C<close> will additionally
730return false if one of the other system calls involved fails or if the
fb73857a 731program exits with non-zero status. (If the only problem was that the
2b5ab1e7
TC
732program exited non-zero C<$!> will be set to C<0>.) Closing a pipe
733also waits for the process executing on the pipe to complete, in case you
734want to look at the output of the pipe afterwards, and
735implicitly puts the exit status value of that command into C<$?>.
5a964f20 736
73689b13
GS
737Prematurely closing the read end of a pipe (i.e. before the process
738writing to it at the other end has closed it) will result in a
739SIGPIPE being delivered to the writer. If the other end can't
740handle that, be sure to read all the data before closing the pipe.
741
fb73857a 742Example:
a0d0e21e 743
fb73857a
PP
744 open(OUTPUT, '|sort >foo') # pipe to sort
745 or die "Can't start sort: $!";
5a964f20 746 #... # print stuff to output
fb73857a
PP
747 close OUTPUT # wait for sort to finish
748 or warn $! ? "Error closing sort pipe: $!"
749 : "Exit status $? from sort";
750 open(INPUT, 'foo') # get sort's results
751 or die "Can't open 'foo' for input: $!";
a0d0e21e 752
5a964f20
TC
753FILEHANDLE may be an expression whose value can be used as an indirect
754filehandle, usually the real filehandle name.
a0d0e21e
LW
755
756=item closedir DIRHANDLE
757
19799a22 758Closes a directory opened by C<opendir> and returns the success of that
5a964f20
TC
759system call.
760
761DIRHANDLE may be an expression whose value can be used as an indirect
762dirhandle, usually the real dirhandle name.
a0d0e21e
LW
763
764=item connect SOCKET,NAME
765
766Attempts to connect to a remote socket, just as the connect system call
19799a22 767does. Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
768packed address of the appropriate type for the socket. See the examples in
769L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 770
cb1a09d0
AD
771=item continue BLOCK
772
773Actually a flow control statement rather than a function. If there is a
98293880
JH
774C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
775C<foreach>), it is always executed just before the conditional is about to
776be evaluated again, just like the third part of a C<for> loop in C. Thus
cb1a09d0
AD
777it can be used to increment a loop variable, even when the loop has been
778continued via the C<next> statement (which is similar to the C C<continue>
779statement).
780
98293880 781C<last>, C<next>, or C<redo> may appear within a C<continue>
19799a22
GS
782block. C<last> and C<redo> will behave as if they had been executed within
783the main block. So will C<next>, but since it will execute a C<continue>
1d2dff63
GS
784block, it may be more entertaining.
785
786 while (EXPR) {
787 ### redo always comes here
788 do_something;
789 } continue {
790 ### next always comes here
791 do_something_else;
792 # then back the top to re-check EXPR
793 }
794 ### last always comes here
795
796Omitting the C<continue> section is semantically equivalent to using an
19799a22 797empty one, logically enough. In that case, C<next> goes directly back
1d2dff63
GS
798to check the condition at the top of the loop.
799
a0d0e21e
LW
800=item cos EXPR
801
d6217f1e
GS
802=item cos
803
5a964f20 804Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
7660c0ab 805takes cosine of C<$_>.
a0d0e21e 806
ca6e1c26 807For the inverse cosine operation, you may use the C<Math::Trig::acos()>
28757baa
PP
808function, or use this relation:
809
810 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
811
a0d0e21e
LW
812=item crypt PLAINTEXT,SALT
813
f86cebdf 814Encrypts a string exactly like the crypt(3) function in the C library
4633a7c4
LW
815(assuming that you actually have a version there that has not been
816extirpated as a potential munition). This can prove useful for checking
817the password file for lousy passwords, amongst other things. Only the
818guys wearing white hats should do this.
a0d0e21e 819
19799a22 820Note that C<crypt> is intended to be a one-way function, much like breaking
11155c91
CS
821eggs to make an omelette. There is no (known) corresponding decrypt
822function. As a result, this function isn't all that useful for
823cryptography. (For that, see your nearby CPAN mirror.)
2f9daede 824
e71965be
RS
825When verifying an existing encrypted string you should use the encrypted
826text as the salt (like C<crypt($plain, $crypted) eq $crypted>). This
19799a22 827allows your code to work with the standard C<crypt> and with more
e71965be
RS
828exotic implementations. When choosing a new salt create a random two
829character string whose characters come from the set C<[./0-9A-Za-z]>
830(like C<join '', ('.', '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>).
831
a0d0e21e
LW
832Here's an example that makes sure that whoever runs this program knows
833their own password:
834
835 $pwd = (getpwuid($<))[1];
a0d0e21e
LW
836
837 system "stty -echo";
838 print "Password: ";
e71965be 839 chomp($word = <STDIN>);
a0d0e21e
LW
840 print "\n";
841 system "stty echo";
842
e71965be 843 if (crypt($word, $pwd) ne $pwd) {
a0d0e21e
LW
844 die "Sorry...\n";
845 } else {
846 print "ok\n";
54310121 847 }
a0d0e21e 848
9f8f0c9d 849Of course, typing in your own password to whoever asks you
748a9306 850for it is unwise.
a0d0e21e 851
19799a22
GS
852The L<crypt> function is unsuitable for encrypting large quantities
853of data, not least of all because you can't get the information
854back. Look at the F<by-module/Crypt> and F<by-module/PGP> directories
855on your favorite CPAN mirror for a slew of potentially useful
856modules.
857
aa689395 858=item dbmclose HASH
a0d0e21e 859
19799a22 860[This function has been largely superseded by the C<untie> function.]
a0d0e21e 861
aa689395 862Breaks the binding between a DBM file and a hash.
a0d0e21e 863
19799a22 864=item dbmopen HASH,DBNAME,MASK
a0d0e21e 865
19799a22 866[This function has been largely superseded by the C<tie> function.]
a0d0e21e 867
7b8d334a 868This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
19799a22
GS
869hash. HASH is the name of the hash. (Unlike normal C<open>, the first
870argument is I<not> a filehandle, even though it looks like one). DBNAME
aa689395
PP
871is the name of the database (without the F<.dir> or F<.pag> extension if
872any). If the database does not exist, it is created with protection
19799a22
GS
873specified by MASK (as modified by the C<umask>). If your system supports
874only the older DBM functions, you may perform only one C<dbmopen> in your
aa689395 875program. In older versions of Perl, if your system had neither DBM nor
19799a22 876ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
aa689395
PP
877sdbm(3).
878
879If you don't have write access to the DBM file, you can only read hash
880variables, not set them. If you want to test whether you can write,
19799a22 881either use file tests or try setting a dummy hash entry inside an C<eval>,
aa689395 882which will trap the error.
a0d0e21e 883
19799a22
GS
884Note that functions such as C<keys> and C<values> may return huge lists
885when used on large DBM files. You may prefer to use the C<each>
a0d0e21e
LW
886function to iterate over large DBM files. Example:
887
888 # print out history file offsets
889 dbmopen(%HIST,'/usr/lib/news/history',0666);
890 while (($key,$val) = each %HIST) {
891 print $key, ' = ', unpack('L',$val), "\n";
892 }
893 dbmclose(%HIST);
894
cb1a09d0 895See also L<AnyDBM_File> for a more general description of the pros and
184e9718 896cons of the various dbm approaches, as well as L<DB_File> for a particularly
cb1a09d0 897rich implementation.
4633a7c4 898
2b5ab1e7
TC
899You can control which DBM library you use by loading that library
900before you call dbmopen():
901
902 use DB_File;
903 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
904 or die "Can't open netscape history file: $!";
905
a0d0e21e
LW
906=item defined EXPR
907
54310121 908=item defined
bbce6d69 909
2f9daede
TPG
910Returns a Boolean value telling whether EXPR has a value other than
911the undefined value C<undef>. If EXPR is not present, C<$_> will be
912checked.
913
914Many operations return C<undef> to indicate failure, end of file,
915system error, uninitialized variable, and other exceptional
916conditions. This function allows you to distinguish C<undef> from
917other values. (A simple Boolean test will not distinguish among
7660c0ab 918C<undef>, zero, the empty string, and C<"0">, which are all equally
2f9daede 919false.) Note that since C<undef> is a valid scalar, its presence
19799a22 920doesn't I<necessarily> indicate an exceptional condition: C<pop>
2f9daede
TPG
921returns C<undef> when its argument is an empty array, I<or> when the
922element to return happens to be C<undef>.
923
f10b0346
GS
924You may also use C<defined(&func)> to check whether subroutine C<&func>
925has ever been defined. The return value is unaffected by any forward
926declarations of C<&foo>.
927
928Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
929used to report whether memory for that aggregate has ever been
930allocated. This behavior may disappear in future versions of Perl.
931You should instead use a simple test for size:
932
933 if (@an_array) { print "has array elements\n" }
934 if (%a_hash) { print "has hash members\n" }
2f9daede
TPG
935
936When used on a hash element, it tells you whether the value is defined,
dc848c6f 937not whether the key exists in the hash. Use L</exists> for the latter
2f9daede 938purpose.
a0d0e21e
LW
939
940Examples:
941
942 print if defined $switch{'D'};
943 print "$val\n" while defined($val = pop(@ary));
944 die "Can't readlink $sym: $!"
945 unless defined($value = readlink $sym);
a0d0e21e 946 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
2f9daede 947 $debugging = 0 unless defined $debugging;
a0d0e21e 948
19799a22 949Note: Many folks tend to overuse C<defined>, and then are surprised to
7660c0ab 950discover that the number C<0> and C<""> (the zero-length string) are, in fact,
2f9daede 951defined values. For example, if you say
a5f75d66
AD
952
953 "ab" =~ /a(.*)b/;
954
7660c0ab 955The pattern match succeeds, and C<$1> is defined, despite the fact that it
a5f75d66 956matched "nothing". But it didn't really match nothing--rather, it
2b5ab1e7 957matched something that happened to be zero characters long. This is all
a5f75d66 958very above-board and honest. When a function returns an undefined value,
2f9daede 959it's an admission that it couldn't give you an honest answer. So you
19799a22 960should use C<defined> only when you're questioning the integrity of what
7660c0ab 961you're trying to do. At other times, a simple comparison to C<0> or C<""> is
2f9daede
TPG
962what you want.
963
dc848c6f 964See also L</undef>, L</exists>, L</ref>.
2f9daede 965
a0d0e21e
LW
966=item delete EXPR
967
01020589
GS
968Given an expression that specifies a hash element, array element, hash slice,
969or array slice, deletes the specified element(s) from the hash or array.
8216c1fd
GS
970In the case of an array, if the array elements happen to be at the end,
971the size of the array will shrink to the highest element that tests
972true for exists() (or 0 if no such element exists).
a0d0e21e 973
01020589
GS
974Returns each element so deleted or the undefined value if there was no such
975element. Deleting from C<$ENV{}> modifies the environment. Deleting from
976a hash tied to a DBM file deletes the entry from the DBM file. Deleting
977from a C<tie>d hash or array may not necessarily return anything.
978
8ea97a1e
GS
979Deleting an array element effectively returns that position of the array
980to its initial, uninitialized state. Subsequently testing for the same
8216c1fd
GS
981element with exists() will return false. Note that deleting array
982elements in the middle of an array will not shift the index of the ones
983after them down--use splice() for that. See L</exists>.
8ea97a1e 984
01020589 985The following (inefficiently) deletes all the values of %HASH and @ARRAY:
a0d0e21e 986
5f05dabc
PP
987 foreach $key (keys %HASH) {
988 delete $HASH{$key};
a0d0e21e
LW
989 }
990
01020589
GS
991 foreach $index (0 .. $#ARRAY) {
992 delete $ARRAY[$index];
993 }
994
995And so do these:
5f05dabc 996
01020589
GS
997 delete @HASH{keys %HASH};
998
9740c838 999 delete @ARRAY[0 .. $#ARRAY];
5f05dabc 1000
2b5ab1e7 1001But both of these are slower than just assigning the empty list
01020589
GS
1002or undefining %HASH or @ARRAY:
1003
1004 %HASH = (); # completely empty %HASH
1005 undef %HASH; # forget %HASH ever existed
2b5ab1e7 1006
01020589
GS
1007 @ARRAY = (); # completely empty @ARRAY
1008 undef @ARRAY; # forget @ARRAY ever existed
2b5ab1e7
TC
1009
1010Note that the EXPR can be arbitrarily complicated as long as the final
01020589
GS
1011operation is a hash element, array element, hash slice, or array slice
1012lookup:
a0d0e21e
LW
1013
1014 delete $ref->[$x][$y]{$key};
5f05dabc 1015 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
a0d0e21e 1016
01020589
GS
1017 delete $ref->[$x][$y][$index];
1018 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1019
a0d0e21e
LW
1020=item die LIST
1021
19799a22
GS
1022Outside an C<eval>, prints the value of LIST to C<STDERR> and
1023exits with the current value of C<$!> (errno). If C<$!> is C<0>,
61eff3bc
JH
1024exits with the value of C<<< ($? >> 8) >>> (backtick `command`
1025status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside
19799a22
GS
1026an C<eval(),> the error message is stuffed into C<$@> and the
1027C<eval> is terminated with the undefined value. This makes
1028C<die> the way to raise an exception.
a0d0e21e
LW
1029
1030Equivalent examples:
1031
1032 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
54310121 1033 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
a0d0e21e
LW
1034
1035If the value of EXPR does not end in a newline, the current script line
1036number and input line number (if any) are also printed, and a newline
883faa13
GS
1037is supplied. Note that the "input line number" (also known as "chunk")
1038is subject to whatever notion of "line" happens to be currently in
1039effect, and is also available as the special variable C<$.>.
1040See L<perlvar/"$/"> and L<perlvar/"$.">.
1041
1042Hint: sometimes appending C<", stopped"> to your message
7660c0ab 1043will cause it to make better sense when the string C<"at foo line 123"> is
a0d0e21e
LW
1044appended. Suppose you are running script "canasta".
1045
1046 die "/etc/games is no good";
1047 die "/etc/games is no good, stopped";
1048
1049produce, respectively
1050
1051 /etc/games is no good at canasta line 123.
1052 /etc/games is no good, stopped at canasta line 123.
1053
2b5ab1e7 1054See also exit(), warn(), and the Carp module.
a0d0e21e 1055
7660c0ab
A
1056If LIST is empty and C<$@> already contains a value (typically from a
1057previous eval) that value is reused after appending C<"\t...propagated">.
fb73857a
PP
1058This is useful for propagating exceptions:
1059
1060 eval { ... };
1061 die unless $@ =~ /Expected exception/;
1062
7660c0ab 1063If C<$@> is empty then the string C<"Died"> is used.
fb73857a 1064
52531d10
GS
1065die() can also be called with a reference argument. If this happens to be
1066trapped within an eval(), $@ contains the reference. This behavior permits
1067a more elaborate exception handling implementation using objects that
4375e838 1068maintain arbitrary state about the nature of the exception. Such a scheme
52531d10
GS
1069is sometimes preferable to matching particular string values of $@ using
1070regular expressions. Here's an example:
1071
1072 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1073 if ($@) {
1074 if (ref($@) && UNIVERSAL::isa($@,"Some::Module::Exception")) {
1075 # handle Some::Module::Exception
1076 }
1077 else {
1078 # handle all other possible exceptions
1079 }
1080 }
1081
19799a22 1082Because perl will stringify uncaught exception messages before displaying
52531d10
GS
1083them, you may want to overload stringification operations on such custom
1084exception objects. See L<overload> for details about that.
1085
19799a22
GS
1086You can arrange for a callback to be run just before the C<die>
1087does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1088handler will be called with the error text and can change the error
1089message, if it sees fit, by calling C<die> again. See
1090L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
1091L<"eval BLOCK"> for some examples. Although this feature was meant
1092to be run only right before your program was to exit, this is not
1093currently the case--the C<$SIG{__DIE__}> hook is currently called
1094even inside eval()ed blocks/strings! If one wants the hook to do
1095nothing in such situations, put
fb73857a
PP
1096
1097 die @_ if $^S;
1098
19799a22
GS
1099as the first line of the handler (see L<perlvar/$^S>). Because
1100this promotes strange action at a distance, this counterintuitive
1101behavior may be fixed in a future release.
774d564b 1102
a0d0e21e
LW
1103=item do BLOCK
1104
1105Not really a function. Returns the value of the last command in the
1106sequence of commands indicated by BLOCK. When modified by a loop
98293880
JH
1107modifier, executes the BLOCK once before testing the loop condition.
1108(On other statements the loop modifiers test the conditional first.)
a0d0e21e 1109
4968c1e4 1110C<do BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7
TC
1111C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1112See L<perlsyn> for alternative strategies.
4968c1e4 1113
a0d0e21e
LW
1114=item do SUBROUTINE(LIST)
1115
1116A deprecated form of subroutine call. See L<perlsub>.
1117
1118=item do EXPR
1119
1120Uses the value of EXPR as a filename and executes the contents of the
1121file as a Perl script. Its primary use is to include subroutines
1122from a Perl subroutine library.
1123
1124 do 'stat.pl';
1125
1126is just like
1127
fb73857a 1128 scalar eval `cat stat.pl`;
a0d0e21e 1129
2b5ab1e7
TC
1130except that it's more efficient and concise, keeps track of the current
1131filename for error messages, searches the @INC libraries, and updates
1132C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1133variables. It also differs in that code evaluated with C<do FILENAME>
1134cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1135same, however, in that it does reparse the file every time you call it,
1136so you probably don't want to do this inside a loop.
a0d0e21e 1137
8e30cc93 1138If C<do> cannot read the file, it returns undef and sets C<$!> to the
2b5ab1e7 1139error. If C<do> can read the file but cannot compile it, it
8e30cc93
G
1140returns undef and sets an error message in C<$@>. If the file is
1141successfully compiled, C<do> returns the value of the last expression
1142evaluated.
1143
a0d0e21e 1144Note that inclusion of library modules is better done with the
19799a22 1145C<use> and C<require> operators, which also do automatic error checking
4633a7c4 1146and raise an exception if there's a problem.
a0d0e21e 1147
5a964f20
TC
1148You might like to use C<do> to read in a program configuration
1149file. Manual error checking can be done this way:
1150
1151 # read in config files: system first, then user
f86cebdf 1152 for $file ("/share/prog/defaults.rc",
2b5ab1e7
TC
1153 "$ENV{HOME}/.someprogrc")
1154 {
5a964f20 1155 unless ($return = do $file) {
f86cebdf
GS
1156 warn "couldn't parse $file: $@" if $@;
1157 warn "couldn't do $file: $!" unless defined $return;
1158 warn "couldn't run $file" unless $return;
5a964f20
TC
1159 }
1160 }
1161
a0d0e21e
LW
1162=item dump LABEL
1163
1614b0e3
JD
1164=item dump
1165
19799a22
GS
1166This function causes an immediate core dump. See also the B<-u>
1167command-line switch in L<perlrun>, which does the same thing.
1168Primarily this is so that you can use the B<undump> program (not
1169supplied) to turn your core dump into an executable binary after
1170having initialized all your variables at the beginning of the
1171program. When the new binary is executed it will begin by executing
1172a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1173Think of it as a goto with an intervening core dump and reincarnation.
1174If C<LABEL> is omitted, restarts the program from the top.
1175
1176B<WARNING>: Any files opened at the time of the dump will I<not>
1177be open any more when the program is reincarnated, with possible
1178resulting confusion on the part of Perl.
1179
1180This function is now largely obsolete, partly because it's very
1181hard to convert a core file into an executable, and because the
1182real compiler backends for generating portable bytecode and compilable
1183C code have superseded it.
1184
1185If you're looking to use L<dump> to speed up your program, consider
1186generating bytecode or native C code as described in L<perlcc>. If
1187you're just trying to accelerate a CGI script, consider using the
1188C<mod_perl> extension to B<Apache>, or the CPAN module, Fast::CGI.
1189You might also consider autoloading or selfloading, which at least
1190make your program I<appear> to run faster.
5a964f20 1191
aa689395
PP
1192=item each HASH
1193
5a964f20 1194When called in list context, returns a 2-element list consisting of the
aa689395 1195key and value for the next element of a hash, so that you can iterate over
74fc8b5f 1196it. When called in scalar context, returns only the key for the next
e902a979 1197element in the hash.
2f9daede 1198
ab192400
GS
1199Entries are returned in an apparently random order. The actual random
1200order is subject to change in future versions of perl, but it is guaranteed
19799a22 1201to be in the same order as either the C<keys> or C<values> function
ab192400
GS
1202would produce on the same (unmodified) hash.
1203
1204When the hash is entirely read, a null array is returned in list context
19799a22
GS
1205(which when assigned produces a false (C<0>) value), and C<undef> in
1206scalar context. The next call to C<each> after that will start iterating
1207again. There is a single iterator for each hash, shared by all C<each>,
1208C<keys>, and C<values> function calls in the program; it can be reset by
2f9daede
TPG
1209reading all the elements from the hash, or by evaluating C<keys HASH> or
1210C<values HASH>. If you add or delete elements of a hash while you're
74fc8b5f
MJD
1211iterating over it, you may get entries skipped or duplicated, so
1212don't. Exception: It is always safe to delete the item most recently
1213returned by C<each()>, which means that the following code will work:
1214
1215 while (($key, $value) = each %hash) {
1216 print $key, "\n";
1217 delete $hash{$key}; # This is safe
1218 }
aa689395 1219
f86cebdf 1220The following prints out your environment like the printenv(1) program,
aa689395 1221only in a different order:
a0d0e21e
LW
1222
1223 while (($key,$value) = each %ENV) {
1224 print "$key=$value\n";
1225 }
1226
19799a22 1227See also C<keys>, C<values> and C<sort>.
a0d0e21e
LW
1228
1229=item eof FILEHANDLE
1230
4633a7c4
LW
1231=item eof ()
1232
a0d0e21e
LW
1233=item eof
1234
1235Returns 1 if the next read on FILEHANDLE will return end of file, or if
1236FILEHANDLE is not open. FILEHANDLE may be an expression whose value
5a964f20 1237gives the real filehandle. (Note that this function actually
19799a22 1238reads a character and then C<ungetc>s it, so isn't very useful in an
748a9306 1239interactive context.) Do not read from a terminal file (or call
19799a22 1240C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
748a9306
LW
1241as terminals may lose the end-of-file condition if you do.
1242
820475bd
GS
1243An C<eof> without an argument uses the last file read. Using C<eof()>
1244with empty parentheses is very different. It refers to the pseudo file
1245formed from the files listed on the command line and accessed via the
61eff3bc
JH
1246C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1247as a normal filehandle is, an C<eof()> before C<< <> >> has been
820475bd
GS
1248used will cause C<@ARGV> to be examined to determine if input is
1249available.
1250
61eff3bc 1251In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
820475bd
GS
1252detect the end of each file, C<eof()> will only detect the end of the
1253last file. Examples:
a0d0e21e 1254
748a9306
LW
1255 # reset line numbering on each input file
1256 while (<>) {
5a964f20 1257 next if /^\s*#/; # skip comments
748a9306 1258 print "$.\t$_";
5a964f20
TC
1259 } continue {
1260 close ARGV if eof; # Not eof()!
748a9306
LW
1261 }
1262
a0d0e21e
LW
1263 # insert dashes just before last line of last file
1264 while (<>) {
5a964f20 1265 if (eof()) { # check for end of current file
a0d0e21e 1266 print "--------------\n";
2b5ab1e7 1267 close(ARGV); # close or last; is needed if we
748a9306 1268 # are reading from the terminal
a0d0e21e
LW
1269 }
1270 print;
1271 }
1272
a0d0e21e 1273Practical hint: you almost never need to use C<eof> in Perl, because the
3ce0d271
GS
1274input operators typically return C<undef> when they run out of data, or if
1275there was an error.
a0d0e21e
LW
1276
1277=item eval EXPR
1278
1279=item eval BLOCK
1280
c7cc6f1c
GS
1281In the first form, the return value of EXPR is parsed and executed as if it
1282were a little Perl program. The value of the expression (which is itself
5a964f20 1283determined within scalar context) is first parsed, and if there weren't any
be3174d2
GS
1284errors, executed in the lexical context of the current Perl program, so
1285that any variable settings or subroutine and format definitions remain
1286afterwards. Note that the value is parsed every time the eval executes.
1287If EXPR is omitted, evaluates C<$_>. This form is typically used to
1288delay parsing and subsequent execution of the text of EXPR until run time.
c7cc6f1c
GS
1289
1290In the second form, the code within the BLOCK is parsed only once--at the
1291same time the code surrounding the eval itself was parsed--and executed
1292within the context of the current Perl program. This form is typically
1293used to trap exceptions more efficiently than the first (see below), while
1294also providing the benefit of checking the code within BLOCK at compile
1295time.
1296
1297The final semicolon, if any, may be omitted from the value of EXPR or within
1298the BLOCK.
1299
1300In both forms, the value returned is the value of the last expression
5a964f20 1301evaluated inside the mini-program; a return statement may be also used, just
c7cc6f1c 1302as with subroutines. The expression providing the return value is evaluated
5a964f20 1303in void, scalar, or list context, depending on the context of the eval itself.
c7cc6f1c 1304See L</wantarray> for more on how the evaluation context can be determined.
a0d0e21e 1305
19799a22
GS
1306If there is a syntax error or runtime error, or a C<die> statement is
1307executed, an undefined value is returned by C<eval>, and C<$@> is set to the
a0d0e21e 1308error message. If there was no error, C<$@> is guaranteed to be a null
19799a22 1309string. Beware that using C<eval> neither silences perl from printing
c7cc6f1c
GS
1310warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1311To do either of those, you have to use the C<$SIG{__WARN__}> facility. See
1312L</warn> and L<perlvar>.
a0d0e21e 1313
19799a22
GS
1314Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1315determining whether a particular feature (such as C<socket> or C<symlink>)
a0d0e21e
LW
1316is implemented. It is also Perl's exception trapping mechanism, where
1317the die operator is used to raise exceptions.
1318
1319If the code to be executed doesn't vary, you may use the eval-BLOCK
1320form to trap run-time errors without incurring the penalty of
1321recompiling each time. The error, if any, is still returned in C<$@>.
1322Examples:
1323
54310121 1324 # make divide-by-zero nonfatal
a0d0e21e
LW
1325 eval { $answer = $a / $b; }; warn $@ if $@;
1326
1327 # same thing, but less efficient
1328 eval '$answer = $a / $b'; warn $@ if $@;
1329
1330 # a compile-time error
5a964f20 1331 eval { $answer = }; # WRONG
a0d0e21e
LW
1332
1333 # a run-time error
1334 eval '$answer ='; # sets $@
1335
2b5ab1e7
TC
1336Due to the current arguably broken state of C<__DIE__> hooks, when using
1337the C<eval{}> form as an exception trap in libraries, you may wish not
1338to trigger any C<__DIE__> hooks that user code may have installed.
1339You can use the C<local $SIG{__DIE__}> construct for this purpose,
1340as shown in this example:
774d564b
PP
1341
1342 # a very private exception trap for divide-by-zero
f86cebdf
GS
1343 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1344 warn $@ if $@;
774d564b
PP
1345
1346This is especially significant, given that C<__DIE__> hooks can call
19799a22 1347C<die> again, which has the effect of changing their error messages:
774d564b
PP
1348
1349 # __DIE__ hooks may modify error messages
1350 {
f86cebdf
GS
1351 local $SIG{'__DIE__'} =
1352 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
c7cc6f1c
GS
1353 eval { die "foo lives here" };
1354 print $@ if $@; # prints "bar lives here"
774d564b
PP
1355 }
1356
19799a22 1357Because this promotes action at a distance, this counterintuitive behavior
2b5ab1e7
TC
1358may be fixed in a future release.
1359
19799a22 1360With an C<eval>, you should be especially careful to remember what's
a0d0e21e
LW
1361being looked at when:
1362
1363 eval $x; # CASE 1
1364 eval "$x"; # CASE 2
1365
1366 eval '$x'; # CASE 3
1367 eval { $x }; # CASE 4
1368
5a964f20 1369 eval "\$$x++"; # CASE 5
a0d0e21e
LW
1370 $$x++; # CASE 6
1371
2f9daede 1372Cases 1 and 2 above behave identically: they run the code contained in
19799a22 1373the variable $x. (Although case 2 has misleading double quotes making
2f9daede 1374the reader wonder what else might be happening (nothing is).) Cases 3
7660c0ab 1375and 4 likewise behave in the same way: they run the code C<'$x'>, which
19799a22 1376does nothing but return the value of $x. (Case 4 is preferred for
2f9daede
TPG
1377purely visual reasons, but it also has the advantage of compiling at
1378compile-time instead of at run-time.) Case 5 is a place where
19799a22 1379normally you I<would> like to use double quotes, except that in this
2f9daede
TPG
1380particular situation, you can just use symbolic references instead, as
1381in case 6.
a0d0e21e 1382
4968c1e4 1383C<eval BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7 1384C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
4968c1e4 1385
a0d0e21e
LW
1386=item exec LIST
1387
8bf3b016
GS
1388=item exec PROGRAM LIST
1389
19799a22
GS
1390The C<exec> function executes a system command I<and never returns>--
1391use C<system> instead of C<exec> if you want it to return. It fails and
1392returns false only if the command does not exist I<and> it is executed
fb73857a 1393directly instead of via your system's command shell (see below).
a0d0e21e 1394
19799a22
GS
1395Since it's a common mistake to use C<exec> instead of C<system>, Perl
1396warns you if there is a following statement which isn't C<die>, C<warn>,
1397or C<exit> (if C<-w> is set - but you always do that). If you
1398I<really> want to follow an C<exec> with some other statement, you
55d729e4
GS
1399can use one of these styles to avoid the warning:
1400
5a964f20
TC
1401 exec ('foo') or print STDERR "couldn't exec foo: $!";
1402 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
55d729e4 1403
5a964f20 1404If there is more than one argument in LIST, or if LIST is an array
f86cebdf 1405with more than one value, calls execvp(3) with the arguments in LIST.
5a964f20
TC
1406If there is only one scalar argument or an array with one element in it,
1407the argument is checked for shell metacharacters, and if there are any,
1408the entire argument is passed to the system's command shell for parsing
1409(this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1410If there are no shell metacharacters in the argument, it is split into
19799a22
GS
1411words and passed directly to C<execvp>, which is more efficient.
1412Examples:
a0d0e21e 1413
19799a22
GS
1414 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1415 exec "sort $outfile | uniq";
a0d0e21e
LW
1416
1417If you don't really want to execute the first argument, but want to lie
1418to the program you are executing about its own name, you can specify
1419the program you actually want to run as an "indirect object" (without a
1420comma) in front of the LIST. (This always forces interpretation of the
54310121 1421LIST as a multivalued list, even if there is only a single scalar in
a0d0e21e
LW
1422the list.) Example:
1423
1424 $shell = '/bin/csh';
1425 exec $shell '-sh'; # pretend it's a login shell
1426
1427or, more directly,
1428
1429 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1430
bb32b41a
GS
1431When the arguments get executed via the system shell, results will
1432be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1433for details.
1434
19799a22
GS
1435Using an indirect object with C<exec> or C<system> is also more
1436secure. This usage (which also works fine with system()) forces
1437interpretation of the arguments as a multivalued list, even if the
1438list had just one argument. That way you're safe from the shell
1439expanding wildcards or splitting up words with whitespace in them.
5a964f20
TC
1440
1441 @args = ( "echo surprise" );
1442
2b5ab1e7 1443 exec @args; # subject to shell escapes
f86cebdf 1444 # if @args == 1
2b5ab1e7 1445 exec { $args[0] } @args; # safe even with one-arg list
5a964f20
TC
1446
1447The first version, the one without the indirect object, ran the I<echo>
1448program, passing it C<"surprise"> an argument. The second version
1449didn't--it tried to run a program literally called I<"echo surprise">,
1450didn't find it, and set C<$?> to a non-zero value indicating failure.
1451
0f897271
GS
1452Beginning with v5.6.0, Perl will attempt to flush all files opened for
1453output before the exec, but this may not be supported on some platforms
1454(see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1455in English) or call the C<autoflush()> method of C<IO::Handle> on any
1456open handles in order to avoid lost output.
1457
19799a22 1458Note that C<exec> will not call your C<END> blocks, nor will it call
7660c0ab
A
1459any C<DESTROY> methods in your objects.
1460
a0d0e21e
LW
1461=item exists EXPR
1462
01020589 1463Given an expression that specifies a hash element or array element,
8ea97a1e
GS
1464returns true if the specified element in the hash or array has ever
1465been initialized, even if the corresponding value is undefined. The
1466element is not autovivified if it doesn't exist.
a0d0e21e 1467
01020589
GS
1468 print "Exists\n" if exists $hash{$key};
1469 print "Defined\n" if defined $hash{$key};
1470 print "True\n" if $hash{$key};
1471
1472 print "Exists\n" if exists $array[$index];
1473 print "Defined\n" if defined $array[$index];
1474 print "True\n" if $array[$index];
a0d0e21e 1475
8ea97a1e 1476A hash or array element can be true only if it's defined, and defined if
a0d0e21e
LW
1477it exists, but the reverse doesn't necessarily hold true.
1478
afebc493
GS
1479Given an expression that specifies the name of a subroutine,
1480returns true if the specified subroutine has ever been declared, even
1481if it is undefined. Mentioning a subroutine name for exists or defined
1482does not count as declaring it.
1483
1484 print "Exists\n" if exists &subroutine;
1485 print "Defined\n" if defined &subroutine;
1486
a0d0e21e 1487Note that the EXPR can be arbitrarily complicated as long as the final
afebc493 1488operation is a hash or array key lookup or subroutine name:
a0d0e21e 1489
2b5ab1e7
TC
1490 if (exists $ref->{A}->{B}->{$key}) { }
1491 if (exists $hash{A}{B}{$key}) { }
1492
01020589
GS
1493 if (exists $ref->{A}->{B}->[$ix]) { }
1494 if (exists $hash{A}{B}[$ix]) { }
1495
afebc493
GS
1496 if (exists &{$ref->{A}{B}{$key}}) { }
1497
01020589
GS
1498Although the deepest nested array or hash will not spring into existence
1499just because its existence was tested, any intervening ones will.
61eff3bc 1500Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
01020589
GS
1501into existence due to the existence test for the $key element above.
1502This happens anywhere the arrow operator is used, including even:
5a964f20 1503
2b5ab1e7
TC
1504 undef $ref;
1505 if (exists $ref->{"Some key"}) { }
1506 print $ref; # prints HASH(0x80d3d5c)
1507
1508This surprising autovivification in what does not at first--or even
1509second--glance appear to be an lvalue context may be fixed in a future
5a964f20 1510release.
a0d0e21e 1511
479ba383
GS
1512See L<perlref/"Pseudo-hashes: Using an array as a hash"> for specifics
1513on how exists() acts when used on a pseudo-hash.
e0478e5a 1514
afebc493
GS
1515Use of a subroutine call, rather than a subroutine name, as an argument
1516to exists() is an error.
1517
1518 exists &sub; # OK
1519 exists &sub(); # Error
1520
a0d0e21e
LW
1521=item exit EXPR
1522
2b5ab1e7 1523Evaluates EXPR and exits immediately with that value. Example:
a0d0e21e
LW
1524
1525 $ans = <STDIN>;
1526 exit 0 if $ans =~ /^[Xx]/;
1527
19799a22 1528See also C<die>. If EXPR is omitted, exits with C<0> status. The only
2b5ab1e7
TC
1529universally recognized values for EXPR are C<0> for success and C<1>
1530for error; other values are subject to interpretation depending on the
1531environment in which the Perl program is running. For example, exiting
153269 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1533the mailer to return the item undelivered, but that's not true everywhere.
a0d0e21e 1534
19799a22
GS
1535Don't use C<exit> to abort a subroutine if there's any chance that
1536someone might want to trap whatever error happened. Use C<die> instead,
1537which can be trapped by an C<eval>.
28757baa 1538
19799a22 1539The exit() function does not always exit immediately. It calls any
2b5ab1e7 1540defined C<END> routines first, but these C<END> routines may not
19799a22 1541themselves abort the exit. Likewise any object destructors that need to
2b5ab1e7
TC
1542be called are called before the real exit. If this is a problem, you
1543can call C<POSIX:_exit($status)> to avoid END and destructor processing.
87275199 1544See L<perlmod> for details.
5a964f20 1545
a0d0e21e
LW
1546=item exp EXPR
1547
54310121 1548=item exp
bbce6d69 1549
2b5ab1e7 1550Returns I<e> (the natural logarithm base) to the power of EXPR.
a0d0e21e
LW
1551If EXPR is omitted, gives C<exp($_)>.
1552
1553=item fcntl FILEHANDLE,FUNCTION,SCALAR
1554
f86cebdf 1555Implements the fcntl(2) function. You'll probably have to say
a0d0e21e
LW
1556
1557 use Fcntl;
1558
0ade1984 1559first to get the correct constant definitions. Argument processing and
19799a22 1560value return works just like C<ioctl> below.
a0d0e21e
LW
1561For example:
1562
1563 use Fcntl;
5a964f20
TC
1564 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1565 or die "can't fcntl F_GETFL: $!";
1566
19799a22 1567You don't have to check for C<defined> on the return from C<fnctl>.
951ba7fe
GS
1568Like C<ioctl>, it maps a C<0> return from the system call into
1569C<"0 but true"> in Perl. This string is true in boolean context and C<0>
2b5ab1e7
TC
1570in numeric context. It is also exempt from the normal B<-w> warnings
1571on improper numeric conversions.
5a964f20 1572
19799a22 1573Note that C<fcntl> will produce a fatal error if used on a machine that
2b5ab1e7
TC
1574doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1575manpage to learn what functions are available on your system.
a0d0e21e
LW
1576
1577=item fileno FILEHANDLE
1578
2b5ab1e7
TC
1579Returns the file descriptor for a filehandle, or undefined if the
1580filehandle is not open. This is mainly useful for constructing
19799a22 1581bitmaps for C<select> and low-level POSIX tty-handling operations.
2b5ab1e7
TC
1582If FILEHANDLE is an expression, the value is taken as an indirect
1583filehandle, generally its name.
5a964f20
TC
1584
1585You can use this to find out whether two handles refer to the
1586same underlying descriptor:
1587
1588 if (fileno(THIS) == fileno(THAT)) {
1589 print "THIS and THAT are dups\n";
1590 }
a0d0e21e
LW
1591
1592=item flock FILEHANDLE,OPERATION
1593
19799a22
GS
1594Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1595for success, false on failure. Produces a fatal error if used on a
2b5ab1e7 1596machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
19799a22 1597C<flock> is Perl's portable file locking interface, although it locks
2b5ab1e7
TC
1598only entire files, not records.
1599
1600Two potentially non-obvious but traditional C<flock> semantics are
1601that it waits indefinitely until the lock is granted, and that its locks
1602B<merely advisory>. Such discretionary locks are more flexible, but offer
19799a22
GS
1603fewer guarantees. This means that files locked with C<flock> may be
1604modified by programs that do not also use C<flock>. See L<perlport>,
2b5ab1e7
TC
1605your port's specific documentation, or your system-specific local manpages
1606for details. It's best to assume traditional behavior if you're writing
1607portable programs. (But if you're not, you should as always feel perfectly
1608free to write for your own system's idiosyncrasies (sometimes called
1609"features"). Slavish adherence to portability concerns shouldn't get
1610in the way of your getting your job done.)
a3cb178b 1611
8ebc5c01
PP
1612OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1613LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
ea3105be 1614you can use the symbolic names if you import them from the Fcntl module,
68dc0745
PP
1615either individually, or as a group using the ':flock' tag. LOCK_SH
1616requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
ea3105be
GS
1617releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1618LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
68dc0745
PP
1619waiting for the lock (check the return status to see if you got it).
1620
2b5ab1e7
TC
1621To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1622before locking or unlocking it.
8ebc5c01 1623
f86cebdf 1624Note that the emulation built with lockf(3) doesn't provide shared
8ebc5c01 1625locks, and it requires that FILEHANDLE be open with write intent. These
2b5ab1e7 1626are the semantics that lockf(3) implements. Most if not all systems
f86cebdf 1627implement lockf(3) in terms of fcntl(2) locking, though, so the
8ebc5c01
PP
1628differing semantics shouldn't bite too many people.
1629
19799a22
GS
1630Note also that some versions of C<flock> cannot lock things over the
1631network; you would need to use the more system-specific C<fcntl> for
f86cebdf
GS
1632that. If you like you can force Perl to ignore your system's flock(2)
1633function, and so provide its own fcntl(2)-based emulation, by passing
8ebc5c01
PP
1634the switch C<-Ud_flock> to the F<Configure> program when you configure
1635perl.
4633a7c4
LW
1636
1637Here's a mailbox appender for BSD systems.
a0d0e21e 1638
7e1af8bc 1639 use Fcntl ':flock'; # import LOCK_* constants
a0d0e21e
LW
1640
1641 sub lock {
7e1af8bc 1642 flock(MBOX,LOCK_EX);
a0d0e21e
LW
1643 # and, in case someone appended
1644 # while we were waiting...
1645 seek(MBOX, 0, 2);
1646 }
1647
1648 sub unlock {
7e1af8bc 1649 flock(MBOX,LOCK_UN);
a0d0e21e
LW
1650 }
1651
1652 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1653 or die "Can't open mailbox: $!";
1654
1655 lock();
1656 print MBOX $msg,"\n\n";
1657 unlock();
1658
2b5ab1e7
TC
1659On systems that support a real flock(), locks are inherited across fork()
1660calls, whereas those that must resort to the more capricious fcntl()
1661function lose the locks, making it harder to write servers.
1662
cb1a09d0 1663See also L<DB_File> for other flock() examples.
a0d0e21e
LW
1664
1665=item fork
1666
2b5ab1e7
TC
1667Does a fork(2) system call to create a new process running the
1668same program at the same point. It returns the child pid to the
1669parent process, C<0> to the child process, or C<undef> if the fork is
1670unsuccessful. File descriptors (and sometimes locks on those descriptors)
1671are shared, while everything else is copied. On most systems supporting
1672fork(), great care has gone into making it extremely efficient (for
1673example, using copy-on-write technology on data pages), making it the
1674dominant paradigm for multitasking over the last few decades.
5a964f20 1675
0f897271
GS
1676Beginning with v5.6.0, Perl will attempt to flush all files opened for
1677output before forking the child process, but this may not be supported
1678on some platforms (see L<perlport>). To be safe, you may need to set
1679C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1680C<IO::Handle> on any open handles in order to avoid duplicate output.
a0d0e21e 1681
19799a22 1682If you C<fork> without ever waiting on your children, you will
2b5ab1e7
TC
1683accumulate zombies. On some systems, you can avoid this by setting
1684C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1685forking and reaping moribund children.
cb1a09d0 1686
28757baa
PP
1687Note that if your forked child inherits system file descriptors like
1688STDIN and STDOUT that are actually connected by a pipe or socket, even
2b5ab1e7 1689if you exit, then the remote server (such as, say, a CGI script or a
19799a22 1690backgrounded job launched from a remote shell) won't think you're done.
2b5ab1e7 1691You should reopen those to F</dev/null> if it's any issue.
28757baa 1692
cb1a09d0
AD
1693=item format
1694
19799a22 1695Declare a picture format for use by the C<write> function. For
cb1a09d0
AD
1696example:
1697
54310121 1698 format Something =
cb1a09d0
AD
1699 Test: @<<<<<<<< @||||| @>>>>>
1700 $str, $%, '$' . int($num)
1701 .
1702
1703 $str = "widget";
184e9718 1704 $num = $cost/$quantity;
cb1a09d0
AD
1705 $~ = 'Something';
1706 write;
1707
1708See L<perlform> for many details and examples.
1709
8903cb82 1710=item formline PICTURE,LIST
a0d0e21e 1711
5a964f20 1712This is an internal function used by C<format>s, though you may call it,
a0d0e21e
LW
1713too. It formats (see L<perlform>) a list of values according to the
1714contents of PICTURE, placing the output into the format output
7660c0ab 1715accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
19799a22 1716Eventually, when a C<write> is done, the contents of
a0d0e21e 1717C<$^A> are written to some filehandle, but you could also read C<$^A>
7660c0ab 1718yourself and then set C<$^A> back to C<"">. Note that a format typically
19799a22 1719does one C<formline> per line of form, but the C<formline> function itself
748a9306 1720doesn't care how many newlines are embedded in the PICTURE. This means
4633a7c4 1721that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
748a9306
LW
1722You may therefore need to use multiple formlines to implement a single
1723record format, just like the format compiler.
1724
19799a22 1725Be careful if you put double quotes around the picture, because an C<@>
748a9306 1726character may be taken to mean the beginning of an array name.
19799a22 1727C<formline> always returns true. See L<perlform> for other examples.
a0d0e21e
LW
1728
1729=item getc FILEHANDLE
1730
1731=item getc
1732
1733Returns the next character from the input file attached to FILEHANDLE,
2b5ab1e7
TC
1734or the undefined value at end of file, or if there was an error.
1735If FILEHANDLE is omitted, reads from STDIN. This is not particularly
1736efficient. However, it cannot be used by itself to fetch single
1737characters without waiting for the user to hit enter. For that, try
1738something more like:
4633a7c4
LW
1739
1740 if ($BSD_STYLE) {
1741 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1742 }
1743 else {
54310121 1744 system "stty", '-icanon', 'eol', "\001";
4633a7c4
LW
1745 }
1746
1747 $key = getc(STDIN);
1748
1749 if ($BSD_STYLE) {
1750 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1751 }
1752 else {
5f05dabc 1753 system "stty", 'icanon', 'eol', '^@'; # ASCII null
4633a7c4
LW
1754 }
1755 print "\n";
1756
54310121
PP
1757Determination of whether $BSD_STYLE should be set
1758is left as an exercise to the reader.
cb1a09d0 1759
19799a22 1760The C<POSIX::getattr> function can do this more portably on
2b5ab1e7
TC
1761systems purporting POSIX compliance. See also the C<Term::ReadKey>
1762module from your nearest CPAN site; details on CPAN can be found on
1763L<perlmodlib/CPAN>.
a0d0e21e
LW
1764
1765=item getlogin
1766
5a964f20
TC
1767Implements the C library function of the same name, which on most
1768systems returns the current login from F</etc/utmp>, if any. If null,
19799a22 1769use C<getpwuid>.
a0d0e21e 1770
f86702cc 1771 $login = getlogin || getpwuid($<) || "Kilroy";
a0d0e21e 1772
19799a22
GS
1773Do not consider C<getlogin> for authentication: it is not as
1774secure as C<getpwuid>.
4633a7c4 1775
a0d0e21e
LW
1776=item getpeername SOCKET
1777
1778Returns the packed sockaddr address of other end of the SOCKET connection.
1779
4633a7c4
LW
1780 use Socket;
1781 $hersockaddr = getpeername(SOCK);
19799a22 1782 ($port, $iaddr) = sockaddr_in($hersockaddr);
4633a7c4
LW
1783 $herhostname = gethostbyaddr($iaddr, AF_INET);
1784 $herstraddr = inet_ntoa($iaddr);
a0d0e21e
LW
1785
1786=item getpgrp PID
1787
47e29363 1788Returns the current process group for the specified PID. Use
7660c0ab 1789a PID of C<0> to get the current process group for the
4633a7c4 1790current process. Will raise an exception if used on a machine that
f86cebdf 1791doesn't implement getpgrp(2). If PID is omitted, returns process
19799a22 1792group of current process. Note that the POSIX version of C<getpgrp>
7660c0ab 1793does not accept a PID argument, so only C<PID==0> is truly portable.
a0d0e21e
LW
1794
1795=item getppid
1796
1797Returns the process id of the parent process.
1798
1799=item getpriority WHICH,WHO
1800
4633a7c4
LW
1801Returns the current priority for a process, a process group, or a user.
1802(See L<getpriority(2)>.) Will raise a fatal exception if used on a
f86cebdf 1803machine that doesn't implement getpriority(2).
a0d0e21e
LW
1804
1805=item getpwnam NAME
1806
1807=item getgrnam NAME
1808
1809=item gethostbyname NAME
1810
1811=item getnetbyname NAME
1812
1813=item getprotobyname NAME
1814
1815=item getpwuid UID
1816
1817=item getgrgid GID
1818
1819=item getservbyname NAME,PROTO
1820
1821=item gethostbyaddr ADDR,ADDRTYPE
1822
1823=item getnetbyaddr ADDR,ADDRTYPE
1824
1825=item getprotobynumber NUMBER
1826
1827=item getservbyport PORT,PROTO
1828
1829=item getpwent
1830
1831=item getgrent
1832
1833=item gethostent
1834
1835=item getnetent
1836
1837=item getprotoent
1838
1839=item getservent
1840
1841=item setpwent
1842
1843=item setgrent
1844
1845=item sethostent STAYOPEN
1846
1847=item setnetent STAYOPEN
1848
1849=item setprotoent STAYOPEN
1850
1851=item setservent STAYOPEN
1852
1853=item endpwent
1854
1855=item endgrent
1856
1857=item endhostent
1858
1859=item endnetent
1860
1861=item endprotoent
1862
1863=item endservent
1864
1865These routines perform the same functions as their counterparts in the
5a964f20 1866system library. In list context, the return values from the
a0d0e21e
LW
1867various get routines are as follows:
1868
1869 ($name,$passwd,$uid,$gid,
6ee623d5 1870 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
a0d0e21e
LW
1871 ($name,$passwd,$gid,$members) = getgr*
1872 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1873 ($name,$aliases,$addrtype,$net) = getnet*
1874 ($name,$aliases,$proto) = getproto*
1875 ($name,$aliases,$port,$proto) = getserv*
1876
1877(If the entry doesn't exist you get a null list.)
1878
4602f195
JH
1879The exact meaning of the $gcos field varies but it usually contains
1880the real name of the user (as opposed to the login name) and other
1881information pertaining to the user. Beware, however, that in many
1882system users are able to change this information and therefore it
106325ad 1883cannot be trusted and therefore the $gcos is tainted (see
2959b6e3
JH
1884L<perlsec>). The $passwd and $shell, user's encrypted password and
1885login shell, are also tainted, because of the same reason.
4602f195 1886
5a964f20 1887In scalar context, you get the name, unless the function was a
a0d0e21e
LW
1888lookup by name, in which case you get the other thing, whatever it is.
1889(If the entry doesn't exist you get the undefined value.) For example:
1890
5a964f20
TC
1891 $uid = getpwnam($name);
1892 $name = getpwuid($num);
1893 $name = getpwent();
1894 $gid = getgrnam($name);
1895 $name = getgrgid($num;
1896 $name = getgrent();
1897 #etc.
a0d0e21e 1898
4602f195
JH
1899In I<getpw*()> the fields $quota, $comment, and $expire are special
1900cases in the sense that in many systems they are unsupported. If the
1901$quota is unsupported, it is an empty scalar. If it is supported, it
1902usually encodes the disk quota. If the $comment field is unsupported,
1903it is an empty scalar. If it is supported it usually encodes some
1904administrative comment about the user. In some systems the $quota
1905field may be $change or $age, fields that have to do with password
1906aging. In some systems the $comment field may be $class. The $expire
1907field, if present, encodes the expiration period of the account or the
1908password. For the availability and the exact meaning of these fields
1909in your system, please consult your getpwnam(3) documentation and your
1910F<pwd.h> file. You can also find out from within Perl what your
1911$quota and $comment fields mean and whether you have the $expire field
1912by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
1913C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
1914files are only supported if your vendor has implemented them in the
1915intuitive fashion that calling the regular C library routines gets the
5d3a0a3b
GS
1916shadow versions if you're running under privilege or if there exists
1917the shadow(3) functions as found in System V ( this includes Solaris
1918and Linux.) Those systems which implement a proprietary shadow password
1919facility are unlikely to be supported.
6ee623d5 1920
19799a22 1921The $members value returned by I<getgr*()> is a space separated list of
a0d0e21e
LW
1922the login names of the members of the group.
1923
1924For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1925C, it will be returned to you via C<$?> if the function call fails. The
7660c0ab 1926C<@addrs> value returned by a successful call is a list of the raw
a0d0e21e
LW
1927addresses returned by the corresponding system library call. In the
1928Internet domain, each address is four bytes long and you can unpack it
1929by saying something like:
1930
1931 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1932
2b5ab1e7
TC
1933The Socket library makes this slightly easier:
1934
1935 use Socket;
1936 $iaddr = inet_aton("127.1"); # or whatever address
1937 $name = gethostbyaddr($iaddr, AF_INET);
1938
1939 # or going the other way
19799a22 1940 $straddr = inet_ntoa($iaddr);
2b5ab1e7 1941
19799a22
GS
1942If you get tired of remembering which element of the return list
1943contains which return value, by-name interfaces are provided
1944in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
1945C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
1946and C<User::grent>. These override the normal built-ins, supplying
1947versions that return objects with the appropriate names
1948for each field. For example:
5a964f20
TC
1949
1950 use File::stat;
1951 use User::pwent;
1952 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
1953
1954Even though it looks like they're the same method calls (uid),
19799a22
GS
1955they aren't, because a C<File::stat> object is different from
1956a C<User::pwent> object.
5a964f20 1957
a0d0e21e
LW
1958=item getsockname SOCKET
1959
19799a22
GS
1960Returns the packed sockaddr address of this end of the SOCKET connection,
1961in case you don't know the address because you have several different
1962IPs that the connection might have come in on.
a0d0e21e 1963
4633a7c4
LW
1964 use Socket;
1965 $mysockaddr = getsockname(SOCK);
19799a22
GS
1966 ($port, $myaddr) = sockaddr_in($mysockaddr);
1967 printf "Connect to %s [%s]\n",
1968 scalar gethostbyaddr($myaddr, AF_INET),
1969 inet_ntoa($myaddr);
a0d0e21e
LW
1970
1971=item getsockopt SOCKET,LEVEL,OPTNAME
1972
5a964f20 1973Returns the socket option requested, or undef if there is an error.
a0d0e21e
LW
1974
1975=item glob EXPR
1976
0a753a76
PP
1977=item glob
1978
2b5ab1e7
TC
1979Returns the value of EXPR with filename expansions such as the
1980standard Unix shell F</bin/csh> would do. This is the internal function
61eff3bc
JH
1981implementing the C<< <*.c> >> operator, but you can use it directly.
1982If EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is
2b5ab1e7 1983discussed in more detail in L<perlop/"I/O Operators">.
a0d0e21e 1984
3a4b19e4
GS
1985Beginning with v5.6.0, this operator is implemented using the standard
1986C<File::Glob> extension. See L<File::Glob> for details.
1987
a0d0e21e
LW
1988=item gmtime EXPR
1989
48a26b3a 1990Converts a time as returned by the time function to a 8-element list
54310121 1991with the time localized for the standard Greenwich time zone.
4633a7c4 1992Typically used as follows:
a0d0e21e 1993
48a26b3a
GS
1994 # 0 1 2 3 4 5 6 7
1995 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday) =
a0d0e21e
LW
1996 gmtime(time);
1997
48a26b3a
GS
1998All list elements are numeric, and come straight out of the C `struct
1999tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2000specified time. $mday is the day of the month, and $mon is the month
2001itself, in the range C<0..11> with 0 indicating January and 11
2002indicating December. $year is the number of years since 1900. That
2003is, $year is C<123> in year 2023. $wday is the day of the week, with
20040 indicating Sunday and 3 indicating Wednesday. $yday is the day of
874b1813 2005the year, in the range C<0..364> (or C<0..365> in leap years.)
48a26b3a
GS
2006
2007Note that the $year element is I<not> simply the last two digits of
2008the year. If you assume it is, then you create non-Y2K-compliant
2009programs--and you wouldn't want to do that, would you?
2f9daede 2010
abd75f24
GS
2011The proper way to get a complete 4-digit year is simply:
2012
2013 $year += 1900;
2014
2015And to get the last two digits of the year (e.g., '01' in 2001) do:
2016
2017 $year = sprintf("%02d", $year % 100);
2018
48a26b3a 2019If EXPR is omitted, C<gmtime()> uses the current time (C<gmtime(time)>).
a0d0e21e 2020
48a26b3a 2021In scalar context, C<gmtime()> returns the ctime(3) value:
0a753a76
PP
2022
2023 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
2024
19799a22 2025Also see the C<timegm> function provided by the C<Time::Local> module,
f86cebdf 2026and the strftime(3) function available via the POSIX module.
7660c0ab 2027
2b5ab1e7
TC
2028This scalar value is B<not> locale dependent (see L<perllocale>), but
2029is instead a Perl builtin. Also see the C<Time::Local> module, and the
2030strftime(3) and mktime(3) functions available via the POSIX module. To
7660c0ab
A
2031get somewhat similar but locale dependent date strings, set up your
2032locale environment variables appropriately (please see L<perllocale>)
2033and try for example:
2034
2035 use POSIX qw(strftime);
2b5ab1e7 2036 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
7660c0ab 2037
2b5ab1e7
TC
2038Note that the C<%a> and C<%b> escapes, which represent the short forms
2039of the day of the week and the month of the year, may not necessarily
2040be three characters wide in all locales.
0a753a76 2041
a0d0e21e
LW
2042=item goto LABEL
2043
748a9306
LW
2044=item goto EXPR
2045
a0d0e21e
LW
2046=item goto &NAME
2047
7660c0ab 2048The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
a0d0e21e 2049execution there. It may not be used to go into any construct that
7660c0ab 2050requires initialization, such as a subroutine or a C<foreach> loop. It
0a753a76 2051also can't be used to go into a construct that is optimized away,
19799a22 2052or to get out of a block or subroutine given to C<sort>.
0a753a76 2053It can be used to go almost anywhere else within the dynamic scope,
a0d0e21e 2054including out of subroutines, but it's usually better to use some other
19799a22 2055construct such as C<last> or C<die>. The author of Perl has never felt the
7660c0ab 2056need to use this form of C<goto> (in Perl, that is--C is another matter).
a0d0e21e 2057
7660c0ab
A
2058The C<goto-EXPR> form expects a label name, whose scope will be resolved
2059dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
748a9306
LW
2060necessarily recommended if you're optimizing for maintainability:
2061
2062 goto ("FOO", "BAR", "GLARCH")[$i];
2063
6cb9131c
GS
2064The C<goto-&NAME> form is quite different from the other forms of C<goto>.
2065In fact, it isn't a goto in the normal sense at all, and doesn't have
2066the stigma associated with other gotos. Instead, it
2067substitutes a call to the named subroutine for the currently running
2068subroutine. This is used by C<AUTOLOAD> subroutines that wish to load
2069another subroutine and then pretend that the other subroutine had been
2070called in the first place (except that any modifications to C<@_>
2071in the current subroutine are propagated to the other subroutine.)
2072After the C<goto>, not even C<caller> will be able to tell that this
2073routine was called first.
2074
2075NAME needn't be the name of a subroutine; it can be a scalar variable
2076containing a code reference, or a block which evaluates to a code
2077reference.
a0d0e21e
LW
2078
2079=item grep BLOCK LIST
2080
2081=item grep EXPR,LIST
2082
2b5ab1e7
TC
2083This is similar in spirit to, but not the same as, grep(1) and its
2084relatives. In particular, it is not limited to using regular expressions.
2f9daede 2085
a0d0e21e 2086Evaluates the BLOCK or EXPR for each element of LIST (locally setting
7660c0ab 2087C<$_> to each element) and returns the list value consisting of those
19799a22
GS
2088elements for which the expression evaluated to true. In scalar
2089context, returns the number of times the expression was true.
a0d0e21e
LW
2090
2091 @foo = grep(!/^#/, @bar); # weed out comments
2092
2093or equivalently,
2094
2095 @foo = grep {!/^#/} @bar; # weed out comments
2096
be3174d2
GS
2097Note that C<$_> is an alias to the list value, so it can be used to
2098modify the elements of the LIST. While this is useful and supported,
2099it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
2100Similarly, grep returns aliases into the original list, much as a for
2101loop's index variable aliases the list elements. That is, modifying an
19799a22
GS
2102element of a list returned by grep (for example, in a C<foreach>, C<map>
2103or another C<grep>) actually modifies the element in the original list.
2b5ab1e7 2104This is usually something to be avoided when writing clear code.
a0d0e21e 2105
19799a22 2106See also L</map> for a list composed of the results of the BLOCK or EXPR.
38325410 2107
a0d0e21e
LW
2108=item hex EXPR
2109
54310121 2110=item hex
bbce6d69 2111
2b5ab1e7
TC
2112Interprets EXPR as a hex string and returns the corresponding value.
2113(To convert strings that might start with either 0, 0x, or 0b, see
2114L</oct>.) If EXPR is omitted, uses C<$_>.
2f9daede
TPG
2115
2116 print hex '0xAf'; # prints '175'
2117 print hex 'aF'; # same
a0d0e21e 2118
19799a22 2119Hex strings may only represent integers. Strings that would cause
c6edd1b7 2120integer overflow trigger a warning.
19799a22 2121
a0d0e21e
LW
2122=item import
2123
19799a22 2124There is no builtin C<import> function. It is just an ordinary
4633a7c4 2125method (subroutine) defined (or inherited) by modules that wish to export
19799a22 2126names to another module. The C<use> function calls the C<import> method
54310121 2127for the package used. See also L</use()>, L<perlmod>, and L<Exporter>.
a0d0e21e
LW
2128
2129=item index STR,SUBSTR,POSITION
2130
2131=item index STR,SUBSTR
2132
2b5ab1e7
TC
2133The index function searches for one string within another, but without
2134the wildcard-like behavior of a full regular-expression pattern match.
2135It returns the position of the first occurrence of SUBSTR in STR at
2136or after POSITION. If POSITION is omitted, starts searching from the
2137beginning of the string. The return value is based at C<0> (or whatever
2138you've set the C<$[> variable to--but don't do that). If the substring
2139is not found, returns one less than the base, ordinarily C<-1>.
a0d0e21e
LW
2140
2141=item int EXPR
2142
54310121 2143=item int
bbce6d69 2144
7660c0ab 2145Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2b5ab1e7
TC
2146You should not use this function for rounding: one because it truncates
2147towards C<0>, and two because machine representations of floating point
2148numbers can sometimes produce counterintuitive results. For example,
2149C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2150because it's really more like -268.99999999999994315658 instead. Usually,
19799a22 2151the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2b5ab1e7 2152functions will serve you better than will int().
a0d0e21e
LW
2153
2154=item ioctl FILEHANDLE,FUNCTION,SCALAR
2155
2b5ab1e7 2156Implements the ioctl(2) function. You'll probably first have to say
a0d0e21e 2157
4633a7c4 2158 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
a0d0e21e 2159
2b5ab1e7 2160to get the correct function definitions. If F<ioctl.ph> doesn't
a0d0e21e 2161exist or doesn't have the correct definitions you'll have to roll your
61eff3bc 2162own, based on your C header files such as F<< <sys/ioctl.h> >>.
5a964f20 2163(There is a Perl script called B<h2ph> that comes with the Perl kit that
54310121 2164may help you in this, but it's nontrivial.) SCALAR will be read and/or
4633a7c4 2165written depending on the FUNCTION--a pointer to the string value of SCALAR
19799a22 2166will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
4633a7c4
LW
2167has no string value but does have a numeric value, that value will be
2168passed rather than a pointer to the string value. To guarantee this to be
19799a22
GS
2169true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2170functions may be needed to manipulate the values of structures used by
2171C<ioctl>.
a0d0e21e 2172
19799a22 2173The return value of C<ioctl> (and C<fcntl>) is as follows:
a0d0e21e
LW
2174
2175 if OS returns: then Perl returns:
2176 -1 undefined value
2177 0 string "0 but true"
2178 anything else that number
2179
19799a22 2180Thus Perl returns true on success and false on failure, yet you can
a0d0e21e
LW
2181still easily determine the actual value returned by the operating
2182system:
2183
2b5ab1e7 2184 $retval = ioctl(...) || -1;
a0d0e21e
LW
2185 printf "System returned %d\n", $retval;
2186
c2611fb3 2187The special string "C<0> but true" is exempt from B<-w> complaints
5a964f20
TC
2188about improper numeric conversions.
2189
19799a22
GS
2190Here's an example of setting a filehandle named C<REMOTE> to be
2191non-blocking at the system level. You'll have to negotiate C<$|>
2192on your own, though.
2193
2194 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
2195
2196 $flags = fcntl(REMOTE, F_GETFL, 0)
2197 or die "Can't get flags for the socket: $!\n";
2198
2199 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
2200 or die "Can't set flags for the socket: $!\n";
2201
a0d0e21e
LW
2202=item join EXPR,LIST
2203
2b5ab1e7
TC
2204Joins the separate strings of LIST into a single string with fields
2205separated by the value of EXPR, and returns that new string. Example:
a0d0e21e 2206
2b5ab1e7 2207 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
a0d0e21e 2208
eb6e2d6f
GS
2209Beware that unlike C<split>, C<join> doesn't take a pattern as its
2210first argument. Compare L</split>.
a0d0e21e 2211
aa689395
PP
2212=item keys HASH
2213
19799a22 2214Returns a list consisting of all the keys of the named hash. (In
1d2dff63 2215scalar context, returns the number of keys.) The keys are returned in
ab192400
GS
2216an apparently random order. The actual random order is subject to
2217change in future versions of perl, but it is guaranteed to be the same
19799a22 2218order as either the C<values> or C<each> function produces (given
ab192400
GS
2219that the hash has not been modified). As a side effect, it resets
2220HASH's iterator.
a0d0e21e 2221
aa689395 2222Here is yet another way to print your environment:
a0d0e21e
LW
2223
2224 @keys = keys %ENV;
2225 @values = values %ENV;
19799a22 2226 while (@keys) {
a0d0e21e
LW
2227 print pop(@keys), '=', pop(@values), "\n";
2228 }
2229
2230or how about sorted by key:
2231
2232 foreach $key (sort(keys %ENV)) {
2233 print $key, '=', $ENV{$key}, "\n";
2234 }
2235
8ea1e5d4
GS
2236The returned values are copies of the original keys in the hash, so
2237modifying them will not affect the original hash. Compare L</values>.
2238
19799a22 2239To sort a hash by value, you'll need to use a C<sort> function.
aa689395 2240Here's a descending numeric sort of a hash by its values:
4633a7c4 2241
5a964f20 2242 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
4633a7c4
LW
2243 printf "%4d %s\n", $hash{$key}, $key;
2244 }
2245
19799a22 2246As an lvalue C<keys> allows you to increase the number of hash buckets
aa689395
PP
2247allocated for the given hash. This can gain you a measure of efficiency if
2248you know the hash is going to get big. (This is similar to pre-extending
2249an array by assigning a larger number to $#array.) If you say
55497cff
PP
2250
2251 keys %hash = 200;
2252
ab192400
GS
2253then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2254in fact, since it rounds up to the next power of two. These
55497cff
PP
2255buckets will be retained even if you do C<%hash = ()>, use C<undef
2256%hash> if you want to free the storage while C<%hash> is still in scope.
2257You can't shrink the number of buckets allocated for the hash using
19799a22 2258C<keys> in this way (but you needn't worry about doing this by accident,
55497cff
PP
2259as trying has no effect).
2260
19799a22 2261See also C<each>, C<values> and C<sort>.
ab192400 2262
b350dd2f 2263=item kill SIGNAL, LIST
a0d0e21e 2264
b350dd2f 2265Sends a signal to a list of processes. Returns the number of
517db077
GS
2266processes successfully signaled (which is not necessarily the
2267same as the number actually killed).
a0d0e21e
LW
2268
2269 $cnt = kill 1, $child1, $child2;
2270 kill 9, @goners;
2271
b350dd2f
GS
2272If SIGNAL is zero, no signal is sent to the process. This is a
2273useful way to check that the process is alive and hasn't changed
2274its UID. See L<perlport> for notes on the portability of this
2275construct.
2276
2277Unlike in the shell, if SIGNAL is negative, it kills
4633a7c4
LW
2278process groups instead of processes. (On System V, a negative I<PROCESS>
2279number will also kill process groups, but that's not portable.) That
2280means you usually want to use positive not negative signals. You may also
da0045b7 2281use a signal name in quotes. See L<perlipc/"Signals"> for details.
a0d0e21e
LW
2282
2283=item last LABEL
2284
2285=item last
2286
2287The C<last> command is like the C<break> statement in C (as used in
2288loops); it immediately exits the loop in question. If the LABEL is
2289omitted, the command refers to the innermost enclosing loop. The
2290C<continue> block, if any, is not executed:
2291
4633a7c4
LW
2292 LINE: while (<STDIN>) {
2293 last LINE if /^$/; # exit when done with header
5a964f20 2294 #...
a0d0e21e
LW
2295 }
2296
4968c1e4 2297C<last> cannot be used to exit a block which returns a value such as
2b5ab1e7
TC
2298C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2299a grep() or map() operation.
4968c1e4 2300
6c1372ed
GS
2301Note that a block by itself is semantically identical to a loop
2302that executes once. Thus C<last> can be used to effect an early
2303exit out of such a block.
2304
98293880
JH
2305See also L</continue> for an illustration of how C<last>, C<next>, and
2306C<redo> work.
1d2dff63 2307
a0d0e21e
LW
2308=item lc EXPR
2309
54310121 2310=item lc
bbce6d69 2311
a0d0e21e 2312Returns an lowercased version of EXPR. This is the internal function
7660c0ab 2313implementing the C<\L> escape in double-quoted strings.
19799a22
GS
2314Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
2315and L<utf8>.
a0d0e21e 2316
7660c0ab 2317If EXPR is omitted, uses C<$_>.
bbce6d69 2318
a0d0e21e
LW
2319=item lcfirst EXPR
2320
54310121 2321=item lcfirst
bbce6d69 2322
a0d0e21e 2323Returns the value of EXPR with the first character lowercased. This is
7660c0ab 2324the internal function implementing the C<\l> escape in double-quoted strings.
a0ed51b3 2325Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
a0d0e21e 2326
7660c0ab 2327If EXPR is omitted, uses C<$_>.
bbce6d69 2328
a0d0e21e
LW
2329=item length EXPR
2330
54310121 2331=item length
bbce6d69 2332
a0ed51b3 2333Returns the length in characters of the value of EXPR. If EXPR is
2b5ab1e7
TC
2334omitted, returns length of C<$_>. Note that this cannot be used on
2335an entire array or hash to find out how many elements these have.
2336For that, use C<scalar @array> and C<scalar keys %hash> respectively.
a0d0e21e
LW
2337
2338=item link OLDFILE,NEWFILE
2339
19799a22
GS
2340Creates a new filename linked to the old filename. Returns true for
2341success, false otherwise.
a0d0e21e
LW
2342
2343=item listen SOCKET,QUEUESIZE
2344
19799a22
GS
2345Does the same thing that the listen system call does. Returns true if
2346it succeeded, false otherwise. See the example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
2347
2348=item local EXPR
2349
19799a22 2350You really probably want to be using C<my> instead, because C<local> isn't
13a2d996
SP
2351what most people think of as "local". See
2352L<perlsub/"Private Variables via my()"> for details.
2b5ab1e7 2353
5a964f20
TC
2354A local modifies the listed variables to be local to the enclosing
2355block, file, or eval. If more than one value is listed, the list must
2356be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2357for details, including issues with tied arrays and hashes.
a0d0e21e 2358
a0d0e21e
LW
2359=item localtime EXPR
2360
19799a22 2361Converts a time as returned by the time function to a 9-element list
5f05dabc 2362with the time analyzed for the local time zone. Typically used as
a0d0e21e
LW
2363follows:
2364
54310121 2365 # 0 1 2 3 4 5 6 7 8
a0d0e21e
LW
2366 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2367 localtime(time);
2368
48a26b3a
GS
2369All list elements are numeric, and come straight out of the C `struct
2370tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2371specified time. $mday is the day of the month, and $mon is the month
2372itself, in the range C<0..11> with 0 indicating January and 11
2373indicating December. $year is the number of years since 1900. That
2374is, $year is C<123> in year 2023. $wday is the day of the week, with
23750 indicating Sunday and 3 indicating Wednesday. $yday is the day of
874b1813 2376the year, in the range C<0..364> (or C<0..365> in leap years.) $isdst
48a26b3a
GS
2377is true if the specified time occurs during daylight savings time,
2378false otherwise.
2379
2380Note that the $year element is I<not> simply the last two digits of
2381the year. If you assume it is, then you create non-Y2K-compliant
2382programs--and you wouldn't want to do that, would you?
54310121 2383
abd75f24
GS
2384The proper way to get a complete 4-digit year is simply:
2385
2386 $year += 1900;
2387
2388And to get the last two digits of the year (e.g., '01' in 2001) do:
2389
2390 $year = sprintf("%02d", $year % 100);
2391
48a26b3a 2392If EXPR is omitted, C<localtime()> uses the current time (C<localtime(time)>).
a0d0e21e 2393
48a26b3a 2394In scalar context, C<localtime()> returns the ctime(3) value:
a0d0e21e 2395
5f05dabc 2396 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
a0d0e21e 2397
a3cb178b 2398This scalar value is B<not> locale dependent, see L<perllocale>, but
68f8bed4
JH
2399instead a Perl builtin. Also see the C<Time::Local> module
2400(to convert the second, minutes, hours, ... back to seconds since the
2401stroke of midnight the 1st of January 1970, the value returned by
ca6e1c26 2402time()), and the strftime(3) and mktime(3) functions available via the
68f8bed4
JH
2403POSIX module. To get somewhat similar but locale dependent date
2404strings, set up your locale environment variables appropriately
2405(please see L<perllocale>) and try for example:
a3cb178b 2406
5a964f20 2407 use POSIX qw(strftime);
2b5ab1e7 2408 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
a3cb178b
GS
2409
2410Note that the C<%a> and C<%b>, the short forms of the day of the week
2411and the month of the year, may not necessarily be three characters wide.
a0d0e21e 2412
19799a22
GS
2413=item lock
2414
2415 lock I<THING>
2416
2417This function places an advisory lock on a variable, subroutine,
2418or referenced object contained in I<THING> until the lock goes out
2419of scope. This is a built-in function only if your version of Perl
2420was built with threading enabled, and if you've said C<use Threads>.
2421Otherwise a user-defined function by this name will be called. See
2422L<Thread>.
2423
a0d0e21e
LW
2424=item log EXPR
2425
54310121 2426=item log
bbce6d69 2427
2b5ab1e7
TC
2428Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2429returns log of C<$_>. To get the log of another base, use basic algebra:
19799a22 2430The base-N log of a number is equal to the natural log of that number
2b5ab1e7
TC
2431divided by the natural log of N. For example:
2432
2433 sub log10 {
2434 my $n = shift;
2435 return log($n)/log(10);
2436 }
2437
2438See also L</exp> for the inverse operation.
a0d0e21e 2439
a0d0e21e
LW
2440=item lstat EXPR
2441
54310121 2442=item lstat
bbce6d69 2443
19799a22 2444Does the same thing as the C<stat> function (including setting the
5a964f20
TC
2445special C<_> filehandle) but stats a symbolic link instead of the file
2446the symbolic link points to. If symbolic links are unimplemented on
19799a22 2447your system, a normal C<stat> is done.
a0d0e21e 2448
7660c0ab 2449If EXPR is omitted, stats C<$_>.
bbce6d69 2450
a0d0e21e
LW
2451=item m//
2452
2453The match operator. See L<perlop>.
2454
2455=item map BLOCK LIST
2456
2457=item map EXPR,LIST
2458
19799a22
GS
2459Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2460C<$_> to each element) and returns the list value composed of the
2461results of each such evaluation. In scalar context, returns the
2462total number of elements so generated. Evaluates BLOCK or EXPR in
2463list context, so each element of LIST may produce zero, one, or
2464more elements in the returned value.
dd99ebda 2465
a0d0e21e
LW
2466 @chars = map(chr, @nums);
2467
2468translates a list of numbers to the corresponding characters. And
2469
4633a7c4 2470 %hash = map { getkey($_) => $_ } @array;
a0d0e21e
LW
2471
2472is just a funny way to write
2473
2474 %hash = ();
2475 foreach $_ (@array) {
4633a7c4 2476 $hash{getkey($_)} = $_;
a0d0e21e
LW
2477 }
2478
be3174d2
GS
2479Note that C<$_> is an alias to the list value, so it can be used to
2480modify the elements of the LIST. While this is useful and supported,
2481it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
2482Using a regular C<foreach> loop for this purpose would be clearer in
2483most cases. See also L</grep> for an array composed of those items of
2484the original list for which the BLOCK or EXPR evaluates to true.
fb73857a 2485
19799a22 2486=item mkdir FILENAME,MASK
a0d0e21e 2487
5a211162
GS
2488=item mkdir FILENAME
2489
0591cd52 2490Creates the directory specified by FILENAME, with permissions
19799a22
GS
2491specified by MASK (as modified by C<umask>). If it succeeds it
2492returns true, otherwise it returns false and sets C<$!> (errno).
5a211162 2493If omitted, MASK defaults to 0777.
0591cd52 2494
19799a22 2495In general, it is better to create directories with permissive MASK,
0591cd52 2496and let the user modify that with their C<umask>, than it is to supply
19799a22 2497a restrictive MASK and give the user no way to be more permissive.
0591cd52
NT
2498The exceptions to this rule are when the file or directory should be
2499kept private (mail files, for instance). The perlfunc(1) entry on
19799a22 2500C<umask> discusses the choice of MASK in more detail.
a0d0e21e
LW
2501
2502=item msgctl ID,CMD,ARG
2503
f86cebdf 2504Calls the System V IPC function msgctl(2). You'll probably have to say
0ade1984
JH
2505
2506 use IPC::SysV;
2507
7660c0ab
A
2508first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2509then ARG must be a variable which will hold the returned C<msqid_ds>
951ba7fe
GS
2510structure. Returns like C<ioctl>: the undefined value for error,
2511C<"0 but true"> for zero, or the actual return value otherwise. See also
4755096e 2512L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation.
a0d0e21e
LW
2513
2514=item msgget KEY,FLAGS
2515
f86cebdf 2516Calls the System V IPC function msgget(2). Returns the message queue
4755096e
GS
2517id, or the undefined value if there is an error. See also
2518L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation.
a0d0e21e 2519
a0d0e21e
LW
2520=item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2521
2522Calls the System V IPC function msgrcv to receive a message from
2523message queue ID into variable VAR with a maximum message size of
41d6edb2
JH
2524SIZE. Note that when a message is received, the message type as a
2525native long integer will be the first thing in VAR, followed by the
2526actual message. This packing may be opened with C<unpack("l! a*")>.
2527Taints the variable. Returns true if successful, or false if there is
4755096e
GS
2528an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and
2529C<IPC::SysV::Msg> documentation.
41d6edb2
JH
2530
2531=item msgsnd ID,MSG,FLAGS
2532
2533Calls the System V IPC function msgsnd to send the message MSG to the
2534message queue ID. MSG must begin with the native long integer message
2535type, and be followed by the length of the actual message, and finally
2536the message itself. This kind of packing can be achieved with
2537C<pack("l! a*", $type, $message)>. Returns true if successful,
2538or false if there is an error. See also C<IPC::SysV>
2539and C<IPC::SysV::Msg> documentation.
a0d0e21e
LW
2540
2541=item my EXPR
2542
09bef843
SB
2543=item my EXPR : ATTRIBUTES
2544
19799a22
GS
2545A C<my> declares the listed variables to be local (lexically) to the
2546enclosing block, file, or C<eval>. If
5f05dabc 2547more than one value is listed, the list must be placed in parentheses. See
cb1a09d0 2548L<perlsub/"Private Variables via my()"> for details.
4633a7c4 2549
a0d0e21e
LW
2550=item next LABEL
2551
2552=item next
2553
2554The C<next> command is like the C<continue> statement in C; it starts
2555the next iteration of the loop:
2556
4633a7c4
LW
2557 LINE: while (<STDIN>) {
2558 next LINE if /^#/; # discard comments
5a964f20 2559 #...
a0d0e21e
LW
2560 }
2561
2562Note that if there were a C<continue> block on the above, it would get
2563executed even on discarded lines. If the LABEL is omitted, the command
2564refers to the innermost enclosing loop.
2565
4968c1e4 2566C<next> cannot be used to exit a block which returns a value such as
2b5ab1e7
TC
2567C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2568a grep() or map() operation.
4968c1e4 2569
6c1372ed
GS
2570Note that a block by itself is semantically identical to a loop
2571that executes once. Thus C<next> will exit such a block early.
2572
98293880
JH
2573See also L</continue> for an illustration of how C<last>, C<next>, and
2574C<redo> work.
1d2dff63 2575
a0d0e21e
LW
2576=item no Module LIST
2577
7660c0ab 2578See the L</use> function, which C<no> is the opposite of.
a0d0e21e
LW
2579
2580=item oct EXPR
2581
54310121 2582=item oct
bbce6d69 2583
4633a7c4 2584Interprets EXPR as an octal string and returns the corresponding
4f19785b
WSI
2585value. (If EXPR happens to start off with C<0x>, interprets it as a
2586hex string. If EXPR starts off with C<0b>, it is interpreted as a
2587binary string.) The following will handle decimal, binary, octal, and
4633a7c4 2588hex in the standard Perl or C notation:
a0d0e21e
LW
2589
2590 $val = oct($val) if $val =~ /^0/;
2591
19799a22
GS
2592If EXPR is omitted, uses C<$_>. To go the other way (produce a number
2593in octal), use sprintf() or printf():
2594
2595 $perms = (stat("filename"))[2] & 07777;
2596 $oct_perms = sprintf "%lo", $perms;
2597
2598The oct() function is commonly used when a string such as C<644> needs
2599to be converted into a file mode, for example. (Although perl will
2600automatically convert strings into numbers as needed, this automatic
2601conversion assumes base 10.)
a0d0e21e 2602
1c1fc3ea 2603=item open FILEHANDLE,MODE,LIST
6170680b 2604
a0d0e21e
LW
2605=item open FILEHANDLE,EXPR
2606
2607=item open FILEHANDLE
2608
2609Opens the file whose filename is given by EXPR, and associates it with
5f05dabc 2610FILEHANDLE. If FILEHANDLE is an expression, its value is used as the
d6fd2b02
GS
2611name of the real filehandle wanted. (This is considered a symbolic
2612reference, so C<use strict 'refs'> should I<not> be in effect.)
2613
2614If EXPR is omitted, the scalar
5f05dabc 2615variable of the same name as the FILEHANDLE contains the filename.
19799a22
GS
2616(Note that lexical variables--those declared with C<my>--will not work
2617for this purpose; so if you're using C<my>, specify EXPR in your call
2b5ab1e7
TC
2618to open.) See L<perlopentut> for a kinder, gentler explanation of opening
2619files.
5f05dabc 2620
61eff3bc
JH
2621If MODE is C<< '<' >> or nothing, the file is opened for input.
2622If MODE is C<< '>' >>, the file is truncated and opened for
2623output, being created if necessary. If MODE is C<<< '>>' >>>,
fbb426e4 2624the file is opened for appending, again being created if necessary.
61eff3bc
JH
2625You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to indicate that
2626you want both read and write access to the file; thus C<< '+<' >> is almost
2627always preferred for read/write updates--the C<< '+>' >> mode would clobber the
5a964f20
TC
2628file first. You can't usually use either read-write mode for updating
2629textfiles, since they have variable length records. See the B<-i>
0591cd52
NT
2630switch in L<perlrun> for a better approach. The file is created with
2631permissions of C<0666> modified by the process' C<umask> value.
5a964f20 2632
61eff3bc
JH
2633These various prefixes correspond to the fopen(3) modes of C<'r'>, C<'r+'>,
2634C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
5f05dabc 2635
6170680b
IZ
2636In the 2-arguments (and 1-argument) form of the call the mode and
2637filename should be concatenated (in this order), possibly separated by
61eff3bc 2638spaces. It is possible to omit the mode if the mode is C<< '<' >>.
6170680b 2639
7660c0ab 2640If the filename begins with C<'|'>, the filename is interpreted as a
5a964f20 2641command to which output is to be piped, and if the filename ends with a
f244e06d
GS
2642C<'|'>, the filename is interpreted as a command which pipes output to
2643us. See L<perlipc/"Using open() for IPC">
19799a22 2644for more examples of this. (You are not allowed to C<open> to a command
5a964f20 2645that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
4a4eefd0
GS
2646and L<perlipc/"Bidirectional Communication with Another Process">
2647for alternatives.)
cb1a09d0 2648
6170680b
IZ
2649If MODE is C<'|-'>, the filename is interpreted as a
2650command to which output is to be piped, and if MODE is
2651C<'-|'>, the filename is interpreted as a command which pipes output to
2652us. In the 2-arguments (and 1-argument) form one should replace dash
2653(C<'-'>) with the command. See L<perlipc/"Using open() for IPC">
2654for more examples of this. (You are not allowed to C<open> to a command
2655that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2656and L<perlipc/"Bidirectional Communication"> for alternatives.)
2657
2658In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
61eff3bc 2659and opening C<< '>-' >> opens STDOUT.
6170680b
IZ
2660
2661Open returns
19799a22 2662nonzero upon success, the undefined value otherwise. If the C<open>
4633a7c4 2663involved a pipe, the return value happens to be the pid of the
54310121 2664subprocess.
cb1a09d0
AD
2665
2666If you're unfortunate enough to be running Perl on a system that
2667distinguishes between text files and binary files (modern operating
2668systems don't care), then you should check out L</binmode> for tips for
19799a22 2669dealing with this. The key distinction between systems that need C<binmode>
5a964f20
TC
2670and those that don't is their text file formats. Systems like Unix, MacOS, and
2671Plan9, which delimit lines with a single character, and which encode that
19799a22 2672character in C as C<"\n">, do not need C<binmode>. The rest need it.
cb1a09d0 2673
fb73857a 2674When opening a file, it's usually a bad idea to continue normal execution
19799a22
GS
2675if the request failed, so C<open> is frequently used in connection with
2676C<die>. Even if C<die> won't do what you want (say, in a CGI script,
fb73857a 2677where you want to make a nicely formatted error message (but there are
5a964f20 2678modules that can help with that problem)) you should always check
19799a22 2679the return value from opening a file. The infrequent exception is when
fb73857a
PP
2680working with an unopened filehandle is actually what you want to do.
2681
cb1a09d0 2682Examples:
a0d0e21e
LW
2683
2684 $ARTICLE = 100;
2685 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2686 while (<ARTICLE>) {...
2687
6170680b 2688 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
fb73857a 2689 # if the open fails, output is discarded
a0d0e21e 2690
6170680b 2691 open(DBASE, '+<', 'dbase.mine') # open for update
fb73857a 2692 or die "Can't open 'dbase.mine' for update: $!";
cb1a09d0 2693
6170680b
IZ
2694 open(DBASE, '+<dbase.mine') # ditto
2695 or die "Can't open 'dbase.mine' for update: $!";
2696
2697 open(ARTICLE, '-|', "caesar <$article") # decrypt article
fb73857a 2698 or die "Can't start caesar: $!";
a0d0e21e 2699
6170680b
IZ
2700 open(ARTICLE, "caesar <$article |") # ditto
2701 or die "Can't start caesar: $!";
2702
2703 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
fb73857a 2704 or die "Can't start sort: $!";
a0d0e21e
LW
2705
2706 # process argument list of files along with any includes
2707
2708 foreach $file (@ARGV) {
2709 process($file, 'fh00');
2710 }
2711
2712 sub process {
5a964f20 2713 my($filename, $input) = @_;
a0d0e21e
LW
2714 $input++; # this is a string increment
2715 unless (open($input, $filename)) {
2716 print STDERR "Can't open $filename: $!\n";
2717 return;
2718 }
2719
5a964f20 2720 local $_;
a0d0e21e
LW
2721 while (<$input>) { # note use of indirection
2722 if (/^#include "(.*)"/) {
2723 process($1, $input);
2724 next;
2725 }
5a964f20 2726 #... # whatever
a0d0e21e
LW
2727 }
2728 }
2729
2730You may also, in the Bourne shell tradition, specify an EXPR beginning
61eff3bc 2731with C<< '>&' >>, in which case the rest of the string is interpreted as the
5a964f20 2732name of a filehandle (or file descriptor, if numeric) to be
61eff3bc
JH
2733duped and opened. You may use C<&> after C<< > >>, C<<< >> >>>,
2734C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>. The
a0d0e21e 2735mode you specify should match the mode of the original filehandle.
184e9718 2736(Duping a filehandle does not take into account any existing contents of
6170680b
IZ
2737stdio buffers.) Duping file handles is not yet supported for 3-argument
2738open().
2739
a0d0e21e
LW
2740Here is a script that saves, redirects, and restores STDOUT and
2741STDERR:
2742
2743 #!/usr/bin/perl
5a964f20
TC
2744 open(OLDOUT, ">&STDOUT");
2745 open(OLDERR, ">&STDERR");
a0d0e21e 2746
6170680b
IZ
2747 open(STDOUT, '>', "foo.out") || die "Can't redirect stdout";
2748 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
a0d0e21e
LW
2749
2750 select(STDERR); $| = 1; # make unbuffered
2751 select(STDOUT); $| = 1; # make unbuffered
2752
2753 print STDOUT "stdout 1\n"; # this works for
2754 print STDERR "stderr 1\n"; # subprocesses too
2755
2756 close(STDOUT);
2757 close(STDERR);
2758
5a964f20
TC
2759 open(STDOUT, ">&OLDOUT");
2760 open(STDERR, ">&OLDERR");
a0d0e21e
LW
2761
2762 print STDOUT "stdout 2\n";
2763 print STDERR "stderr 2\n";
2764
61eff3bc 2765If you specify C<< '<&=N' >>, where C<N> is a number, then Perl will do an
19799a22 2766equivalent of C's C<fdopen> of that file descriptor; this is more
4633a7c4 2767parsimonious of file descriptors. For example:
a0d0e21e
LW
2768
2769 open(FILEHANDLE, "<&=$fd")
2770
4af147f6
CS
2771Note that this feature depends on the fdopen() C library function.
2772On many UNIX systems, fdopen() is known to fail when file descriptors
2773exceed a certain value, typically 255. If you need more file
2774descriptors than that, consider rebuilding Perl to use the C<sfio>
2775library.
2776
6170680b
IZ
2777If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
2778with 2-arguments (or 1-argument) form of open(), then
a0d0e21e 2779there is an implicit fork done, and the return value of open is the pid
7660c0ab 2780of the child within the parent process, and C<0> within the child
184e9718 2781process. (Use C<defined($pid)> to determine whether the open was successful.)
a0d0e21e
LW
2782The filehandle behaves normally for the parent, but i/o to that
2783filehandle is piped from/to the STDOUT/STDIN of the child process.
2784In the child process the filehandle isn't opened--i/o happens from/to
2785the new STDOUT or STDIN. Typically this is used like the normal
2786piped open when you want to exercise more control over just how the
2787pipe command gets executed, such as when you are running setuid, and
54310121 2788don't want to have to scan shell commands for metacharacters.
6170680b 2789The following triples are more or less equivalent:
a0d0e21e
LW
2790
2791 open(FOO, "|tr '[a-z]' '[A-Z]'");
6170680b
IZ
2792 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
2793 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
a0d0e21e
LW
2794
2795 open(FOO, "cat -n '$file'|");
6170680b
IZ
2796 open(FOO, '-|', "cat -n '$file'");
2797 open(FOO, '-|') || exec 'cat', '-n', $file;
a0d0e21e 2798
4633a7c4
LW
2799See L<perlipc/"Safe Pipe Opens"> for more examples of this.
2800
0f897271
GS
2801Beginning with v5.6.0, Perl will attempt to flush all files opened for
2802output before any operation that may do a fork, but this may not be
2803supported on some platforms (see L<perlport>). To be safe, you may need
2804to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
2805of C<IO::Handle> on any open handles.
2806
2807On systems that support a
45bc9206
GS
2808close-on-exec flag on files, the flag will be set for the newly opened
2809file descriptor as determined by the value of $^F. See L<perlvar/$^F>.
a0d0e21e 2810
0dccf244
CS
2811Closing any piped filehandle causes the parent process to wait for the
2812child to finish, and returns the status value in C<$?>.
2813
6170680b
IZ
2814The filename passed to 2-argument (or 1-argument) form of open()
2815will have leading and trailing
f86cebdf 2816whitespace deleted, and the normal redirection characters
5a964f20
TC
2817honored. This property, known as "magic open",
2818can often be used to good effect. A user could specify a filename of
7660c0ab 2819F<"rsh cat file |">, or you could change certain filenames as needed:
5a964f20
TC
2820
2821 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
2822 open(FH, $filename) or die "Can't open $filename: $!";
2823
6170680b
IZ
2824Use 3-argument form to open a file with arbitrary weird characters in it,
2825
2826 open(FOO, '<', $file);
2827
2828otherwise it's necessary to protect any leading and trailing whitespace:
5a964f20
TC
2829
2830 $file =~ s#^(\s)#./$1#;
2831 open(FOO, "< $file\0");
2832
a31a806a 2833(this may not work on some bizarre filesystems). One should
106325ad 2834conscientiously choose between the I<magic> and 3-arguments form
6170680b
IZ
2835of open():
2836
2837 open IN, $ARGV[0];
2838
2839will allow the user to specify an argument of the form C<"rsh cat file |">,
2840but will not work on a filename which happens to have a trailing space, while
2841
2842 open IN, '<', $ARGV[0];
2843
2844will have exactly the opposite restrictions.
2845
19799a22 2846If you want a "real" C C<open> (see L<open(2)> on your system), then you
6170680b
IZ
2847should use the C<sysopen> function, which involves no such magic (but
2848may use subtly different filemodes than Perl open(), which is mapped
2849to C fopen()). This is
5a964f20
TC
2850another way to protect your filenames from interpretation. For example:
2851
2852 use IO::Handle;
2853 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
2854 or die "sysopen $path: $!";
2855 $oldfh = select(HANDLE); $| = 1; select($oldfh);
2856 print HANDLE "stuff $$\n");
2857 seek(HANDLE, 0, 0);
2858 print "File contains: ", <HANDLE>;
2859
7660c0ab
A
2860Using the constructor from the C<IO::Handle> package (or one of its
2861subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
5a964f20
TC
2862filehandles that have the scope of whatever variables hold references to
2863them, and automatically close whenever and however you leave that scope:
c07a80fd 2864
5f05dabc 2865 use IO::File;
5a964f20 2866 #...
c07a80fd
PP
2867 sub read_myfile_munged {
2868 my $ALL = shift;
5f05dabc 2869 my $handle = new IO::File;
c07a80fd
PP
2870 open($handle, "myfile") or die "myfile: $!";
2871 $first = <$handle>
2872 or return (); # Automatically closed here.
2873 mung $first or die "mung failed"; # Or here.
2874 return $first, <$handle> if $ALL; # Or here.
2875 $first; # Or here.
2876 }
2877
b687b08b 2878See L</seek> for some details about mixing reading and writing.
a0d0e21e
LW
2879
2880=item opendir DIRHANDLE,EXPR
2881
19799a22
GS
2882Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
2883C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
a0d0e21e
LW
2884DIRHANDLEs have their own namespace separate from FILEHANDLEs.
2885
2886=item ord EXPR
2887
54310121 2888=item ord
bbce6d69 2889
a0ed51b3 2890Returns the numeric (ASCII or Unicode) value of the first character of EXPR. If
7660c0ab 2891EXPR is omitted, uses C<$_>. For the reverse, see L</chr>.
2b5ab1e7 2892See L<utf8> for more about Unicode.
a0d0e21e 2893
77ca0c92
LW
2894=item our EXPR
2895
2896An C<our> declares the listed variables to be valid globals within
2897the enclosing block, file, or C<eval>. That is, it has the same
2898scoping rules as a "my" declaration, but does not create a local
2899variable. If more than one value is listed, the list must be placed
2900in parentheses. The C<our> declaration has no semantic effect unless
2901"use strict vars" is in effect, in which case it lets you use the
2902declared global variable without qualifying it with a package name.
2903(But only within the lexical scope of the C<our> declaration. In this
2904it differs from "use vars", which is package scoped.)
2905
f472eb5c
GS
2906An C<our> declaration declares a global variable that will be visible
2907across its entire lexical scope, even across package boundaries. The
2908package in which the variable is entered is determined at the point
2909of the declaration, not at the point of use. This means the following
2910behavior holds:
2911
2912 package Foo;
2913 our $bar; # declares $Foo::bar for rest of lexical scope
2914 $bar = 20;
2915
2916 package Bar;
2917 print $bar; # prints 20
2918
2919Multiple C<our> declarations in the same lexical scope are allowed
2920if they are in different packages. If they happened to be in the same
2921package, Perl will emit warnings if you have asked for them.
2922
2923 use warnings;
2924 package Foo;
2925 our $bar; # declares $Foo::bar for rest of lexical scope
2926 $bar = 20;
2927
2928 package Bar;
2929 our $bar = 30; # declares $Bar::bar for rest of lexical scope
2930 print $bar; # prints 30
2931
2932 our $bar; # emits warning
2933
a0d0e21e
LW
2934=item pack TEMPLATE,LIST
2935
2b6c5635
GS
2936Takes a LIST of values and converts it into a string using the rules
2937given by the TEMPLATE. The resulting string is the concatenation of
2938the converted values. Typically, each converted value looks
2939like its machine-level representation. For example, on 32-bit machines
2940a converted integer may be represented by a sequence of 4 bytes.
2941
2942The TEMPLATE is a
a0d0e21e
LW
2943sequence of characters that give the order and type of values, as
2944follows:
2945
5a929a98 2946 a A string with arbitrary binary data, will be null padded.
4375e838 2947 A An ASCII string, will be space padded.
5a929a98
VU
2948 Z A null terminated (asciz) string, will be null padded.
2949
2b6c5635
GS
2950 b A bit string (ascending bit order inside each byte, like vec()).
2951 B A bit string (descending bit order inside each byte).
a0d0e21e
LW
2952 h A hex string (low nybble first).
2953 H A hex string (high nybble first).
2954
2955 c A signed char value.
a0ed51b3 2956 C An unsigned char value. Only does bytes. See U for Unicode.
96e4d5b1 2957
a0d0e21e
LW
2958 s A signed short value.
2959 S An unsigned short value.
96e4d5b1 2960 (This 'short' is _exactly_ 16 bits, which may differ from
851646ae
JH
2961 what a local C compiler calls 'short'. If you want
2962 native-length shorts, use the '!' suffix.)
96e4d5b1 2963
a0d0e21e
LW
2964 i A signed integer value.
2965 I An unsigned integer value.
19799a22 2966 (This 'integer' is _at_least_ 32 bits wide. Its exact
f86cebdf
GS
2967 size depends on what a local C compiler calls 'int',
2968 and may even be larger than the 'long' described in
2969 the next item.)
96e4d5b1 2970
a0d0e21e
LW
2971 l A signed long value.
2972 L An unsigned long value.
96e4d5b1 2973 (This 'long' is _exactly_ 32 bits, which may differ from
851646ae
JH
2974 what a local C compiler calls 'long'. If you want
2975 native-length longs, use the '!' suffix.)
a0d0e21e 2976
5d11dd56
G
2977 n An unsigned short in "network" (big-endian) order.
2978 N An unsigned long in "network" (big-endian) order.
2979 v An unsigned short in "VAX" (little-endian) order.
2980 V An unsigned long in "VAX" (little-endian) order.
96e4d5b1
PP
2981 (These 'shorts' and 'longs' are _exactly_ 16 bits and
2982 _exactly_ 32 bits, respectively.)
a0d0e21e 2983
dae0da7a
JH
2984 q A signed quad (64-bit) value.
2985 Q An unsigned quad value.
851646ae
JH
2986 (Quads are available only if your system supports 64-bit
2987 integer values _and_ if Perl has been compiled to support those.
dae0da7a
JH
2988 Causes a fatal error otherwise.)
2989
a0d0e21e
LW
2990 f A single-precision float in the native format.
2991 d A double-precision float in the native format.
2992
2993 p A pointer to a null-terminated string.
2994 P A pointer to a structure (fixed-length string).
2995
2996 u A uuencoded string.
a0ed51b3
LW
2997 U A Unicode character number. Encodes to UTF-8 internally.
2998 Works even if C<use utf8> is not in effect.
a0d0e21e 2999
96e4d5b1 3000 w A BER compressed integer. Its bytes represent an unsigned
f86cebdf
GS
3001 integer in base 128, most significant digit first, with as
3002 few digits as possible. Bit eight (the high bit) is set
3003 on each byte except the last.
def98dd4 3004
a0d0e21e
LW
3005 x A null byte.
3006 X Back up a byte.
3007 @ Null fill to absolute position.
3008
5a929a98
VU
3009The following rules apply:
3010
3011=over 8
3012
3013=item *
3014
5a964f20 3015Each letter may optionally be followed by a number giving a repeat
951ba7fe
GS
3016count. With all types except C<a>, C<A>, C<Z>, C<b>, C<B>, C<h>,
3017C<H>, and C<P> the pack function will gobble up that many values from
5a929a98 3018the LIST. A C<*> for the repeat count means to use however many items are
951ba7fe
GS
3019left, except for C<@>, C<x>, C<X>, where it is equivalent
3020to C<0>, and C<u>, where it is equivalent to 1 (or 45, what is the
2b6c5635
GS
3021same).
3022
951ba7fe 3023When used with C<Z>, C<*> results in the addition of a trailing null
2b6c5635
GS
3024byte (so the packed result will be one longer than the byte C<length>
3025of the item).
3026
951ba7fe 3027The repeat count for C<u> is interpreted as the maximal number of bytes
2b6c5635 3028to encode per line of output, with 0 and 1 replaced by 45.
5a929a98
VU
3029
3030=item *
3031
951ba7fe 3032The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
5a929a98 3033string of length count, padding with nulls or spaces as necessary. When
951ba7fe
GS
3034unpacking, C<A> strips trailing spaces and nulls, C<Z> strips everything
3035after the first null, and C<a> returns data verbatim. When packing,
3036C<a>, and C<Z> are equivalent.
2b6c5635
GS
3037
3038If the value-to-pack is too long, it is truncated. If too long and an
951ba7fe
GS
3039explicit count is provided, C<Z> packs only C<$count-1> bytes, followed
3040by a null byte. Thus C<Z> always packs a trailing null byte under
2b6c5635 3041all circumstances.
5a929a98
VU
3042
3043=item *
3044
951ba7fe 3045Likewise, the C<b> and C<B> fields pack a string that many bits long.
c73032f5
IZ
3046Each byte of the input field of pack() generates 1 bit of the result.
3047Each result bit is based on the least-significant bit of the corresponding
3048input byte, i.e., on C<ord($byte)%2>. In particular, bytes C<"0"> and
3049C<"1"> generate bits 0 and 1, as do bytes C<"\0"> and C<"\1">.
3050
3051Starting from the beginning of the input string of pack(), each 8-tuple
951ba7fe 3052of bytes is converted to 1 byte of output. With format C<b>
c73032f5 3053the first byte of the 8-tuple determines the least-significant bit of a
951ba7fe 3054byte, and with format C<B> it determines the most-significant bit of
c73032f5
IZ
3055a byte.
3056
3057If the length of the input string is not exactly divisible by 8, the
3058remainder is packed as if the input string were padded by null bytes
3059at the end. Similarly, during unpack()ing the "extra" bits are ignored.
3060
3061If the input string of pack() is longer than needed, extra bytes are ignored.
2b6c5635
GS
3062A C<*> for the repeat count of pack() means to use all the bytes of
3063the input field. On unpack()ing the bits are converted to a string
3064of C<"0">s and C<"1">s.
5a929a98
VU
3065
3066=item *
3067
951ba7fe 3068The C<h> and C<H> fields pack a string that many nybbles (4-bit groups,
851646ae 3069representable as hexadecimal digits, 0-9a-f) long.
5a929a98 3070
c73032f5
IZ
3071Each byte of the input field of pack() generates 4 bits of the result.
3072For non-alphabetical bytes the result is based on the 4 least-significant
3073bits of the input byte, i.e., on C<ord($byte)%16>. In particular,
3074bytes C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
3075C<"\0"> and C<"\1">. For bytes C<"a".."f"> and C<"A".."F"> the result
3076is compatible with the usual hexadecimal digits, so that C<"a"> and
3077C<"A"> both generate the nybble C<0xa==10>. The result for bytes
3078C<"g".."z"> and C<"G".."Z"> is not well-defined.
3079
3080Starting from the beginning of the input string of pack(), each pair
951ba7fe 3081of bytes is converted to 1 byte of output. With format C<h> the
c73032f5 3082first byte of the pair determines the least-significant nybble of the
951ba7fe 3083output byte, and with format C<H> it determines the most-significant
c73032f5
IZ
3084nybble.
3085
3086If the length of the input string is not even, it behaves as if padded
3087by a null byte at the end. Similarly, during unpack()ing the "extra"
3088nybbles are ignored.
3089
3090If the input string of pack() is longer than needed, extra bytes are ignored.
3091A C<*> for the repeat count of pack() means to use all the bytes of
3092the input field. On unpack()ing the bits are converted to a string
3093of hexadecimal digits.
3094
5a929a98
VU
3095=item *
3096
951ba7fe 3097The C<p> type packs a pointer to a null-terminated string. You are
5a929a98
VU
3098responsible for ensuring the string is not a temporary value (which can
3099potentially get deallocated before you get around to using the packed result).
951ba7fe
GS
3100The C<P> type packs a pointer to a structure of the size indicated by the
3101length. A NULL pointer is created if the corresponding value for C<p> or
3102C<P> is C<undef>, similarly for unpack().
5a929a98
VU
3103
3104=item *
3105
951ba7fe
GS
3106The C</> template character allows packing and unpacking of strings where
3107the packed structure contains a byte count followed by the string itself.
17f4a12d 3108You write I<length-item>C</>I<string-item>.
43192e07
IP
3109
3110The I<length-item> can be any C<pack> template letter,
3111and describes how the length value is packed.
3112The ones likely to be of most use are integer-packing ones like
951ba7fe
GS
3113C<n> (for Java strings), C<w> (for ASN.1 or SNMP)
3114and C<N> (for Sun XDR).
43192e07
IP
3115
3116The I<string-item> must, at present, be C<"A*">, C<"a*"> or C<"Z*">.
3117For C<unpack> the length of the string is obtained from the I<length-item>,
3118but if you put in the '*' it will be ignored.
3119
17f4a12d
IZ
3120 unpack 'C/a', "\04Gurusamy"; gives 'Guru'
3121 unpack 'a3/A* A*', '007 Bond J '; gives (' Bond','J')
3122 pack 'n/a* w/a*','hello,','world'; gives "\000\006hello,\005world"
43192e07
IP
3123
3124The I<length-item> is not returned explicitly from C<unpack>.
3125
951ba7fe
GS
3126Adding a count to the I<length-item> letter is unlikely to do anything
3127useful, unless that letter is C<A>, C<a> or C<Z>. Packing with a
3128I<length-item> of C<a> or C<Z> may introduce C<"\000"> characters,
43192e07
IP
3129which Perl does not regard as legal in numeric strings.
3130
3131=item *
3132
951ba7fe
GS
3133The integer types C<s>, C<S>, C<l>, and C<L> may be
3134immediately followed by a C<!> suffix to signify native shorts or
3135longs--as you can see from above for example a bare C<l> does mean
851646ae
JH
3136exactly 32 bits, the native C<long> (as seen by the local C compiler)
3137may be larger. This is an issue mainly in 64-bit platforms. You can
951ba7fe 3138see whether using C<!> makes any difference by
726ea183 3139
4d0c1c44
GS
3140 print length(pack("s")), " ", length(pack("s!")), "\n";
3141 print length(pack("l")), " ", length(pack("l!")), "\n";
ef54e1a4 3142
951ba7fe
GS
3143C<i!> and C<I!> also work but only because of completeness;
3144they are identical to C<i> and C<I>.
ef54e1a4 3145
19799a22
GS
3146The actual sizes (in bytes) of native shorts, ints, longs, and long
3147longs on the platform where Perl was built are also available via
3148L<Config>:
3149
3150 use Config;
3151 print $Config{shortsize}, "\n";
3152 print $Config{intsize}, "\n";
3153 print $Config{longsize}, "\n";
3154 print $Config{longlongsize}, "\n";
ef54e1a4 3155
5074e145 3156(The C<$Config{longlongsize}> will be undefine if your system does
851646ae
JH
3157not support long longs.)
3158
ef54e1a4
JH
3159=item *
3160
951ba7fe 3161The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, and C<L>
ef54e1a4
JH
3162are inherently non-portable between processors and operating systems
3163because they obey the native byteorder and endianness. For example a
140cb37e 31644-byte integer 0x12345678 (305419896 decimal) be ordered natively
ef54e1a4 3165(arranged in and handled by the CPU registers) into bytes as
61eff3bc 3166
b35e152f
JJ
3167 0x12 0x34 0x56 0x78 # big-endian
3168 0x78 0x56 0x34 0x12 # little-endian
61eff3bc 3169
b84d4f81
JH
3170Basically, the Intel and VAX CPUs are little-endian, while everybody
3171else, for example Motorola m68k/88k, PPC, Sparc, HP PA, Power, and
3172Cray are big-endian. Alpha and MIPS can be either: Digital/Compaq
3173used/uses them in little-endian mode; SGI/Cray uses them in big-endian mode.
719a3cf5 3174
19799a22 3175The names `big-endian' and `little-endian' are comic references to
ef54e1a4
JH
3176the classic "Gulliver's Travels" (via the paper "On Holy Wars and a
3177Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and
19799a22 3178the egg-eating habits of the Lilliputians.
61eff3bc 3179
140cb37e 3180Some systems may have even weirder byte orders such as
61eff3bc 3181
ef54e1a4
JH
3182 0x56 0x78 0x12 0x34
3183 0x34 0x12 0x78 0x56
61eff3bc 3184
ef54e1a4
JH
3185You can see your system's preference with
3186
3187 print join(" ", map { sprintf "%#02x", $_ }
3188 unpack("C*",pack("L",0x12345678))), "\n";
3189
d99ad34e 3190The byteorder on the platform where Perl was built is also available
726ea183 3191via L<Config>:
ef54e1a4
JH
3192
3193 use Config;
3194 print $Config{byteorder}, "\n";
3195
d99ad34e
JH
3196Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'>
3197and C<'87654321'> are big-endian.
719a3cf5 3198
951ba7fe
GS
3199If you want portable packed integers use the formats C<n>, C<N>,
3200C<v>, and C<V>, their byte endianness and size is known.
851646ae 3201See also L<perlport>.
ef54e1a4
JH
3202
3203=item *
3204
5a929a98
VU
3205Real numbers (floats and doubles) are in the native machine format only;
3206due to the multiplicity of floating formats around, and the lack of a
3207standard "network" representation, no facility for interchange has been
3208made. This means that packed floating point data written on one machine
3209may not be readable on another - even if both use IEEE floating point
3210arithmetic (as the endian-ness of the memory representation is not part
851646ae 3211of the IEEE spec). See also L<perlport>.
5a929a98
VU
3212
3213Note that Perl uses doubles internally for all numeric calculation, and
3214converting from double into float and thence back to double again will
3215lose precision (i.e., C<unpack("f", pack("f", $foo)>) will not in general
19799a22 3216equal $foo).
5a929a98 3217
851646ae
JH
3218=item *
3219
036b4402
GS
3220If the pattern begins with a C<U>, the resulting string will be treated
3221as Unicode-encoded. You can force UTF8 encoding on in a string with an
3222initial C<U0>, and the bytes that follow will be interpreted as Unicode
3223characters. If you don't want this to happen, you can begin your pattern
3224with C<C0> (or anything else) to force Perl not to UTF8 encode your
3225string, and then follow this with a C<U*> somewhere in your pattern.
3226
3227=item *
3228
851646ae 3229You must yourself do any alignment or padding by inserting for example
9ccd05c0
JH
3230enough C<'x'>es while packing. There is no way to pack() and unpack()
3231could know where the bytes are going to or coming from. Therefore
3232C<pack> (and C<unpack>) handle their output and input as flat
3233sequences of bytes.
851646ae 3234
17f4a12d
IZ
3235=item *
3236
3237A comment in a TEMPLATE starts with C<#> and goes to the end of line.
3238
2b6c5635
GS
3239=item *
3240
3241If TEMPLATE requires more arguments to pack() than actually given, pack()
3242assumes additional C<""> arguments. If TEMPLATE requires less arguments
3243to pack() than actually given, extra arguments are ignored.
3244
5a929a98 3245=back
a0d0e21e
LW
3246
3247Examples:
3248
a0ed51b3 3249 $foo = pack("CCCC",65,66,67,68);
a0d0e21e 3250 # foo eq "ABCD"
a0ed51b3 3251 $foo = pack("C4",65,66,67,68);
a0d0e21e 3252 # same thing
a0ed51b3
LW
3253 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
3254 # same thing with Unicode circled letters
a0d0e21e
LW
3255
3256 $foo = pack("ccxxcc",65,66,67,68);
3257 # foo eq "AB\0\0CD"
3258
9ccd05c0
JH
3259 # note: the above examples featuring "C" and "c" are true
3260 # only on ASCII and ASCII-derived systems such as ISO Latin 1
3261 # and UTF-8. In EBCDIC the first example would be
3262 # $foo = pack("CCCC",193,194,195,196);
3263
a0d0e21e
LW
3264 $foo = pack("s2",1,2);
3265 # "\1\0\2\0" on little-endian
3266 # "\0\1\0\2" on big-endian
3267
3268 $foo = pack("a4","abcd","x","y","z");
3269 # "abcd"
3270
3271 $foo = pack("aaaa","abcd","x","y","z");
3272 # "axyz"
3273
3274 $foo = pack("a14","abcdefg");
3275 # "abcdefg\0\0\0\0\0\0\0"
3276
3277 $foo = pack("i9pl", gmtime);
3278 # a real struct tm (on my system anyway)
3279
5a929a98
VU
3280 $utmp_template = "Z8 Z8 Z16 L";
3281 $utmp = pack($utmp_template, @utmp1);
3282 # a struct utmp (BSDish)
3283
3284 @utmp2 = unpack($utmp_template, $utmp);
3285 # "@utmp1" eq "@utmp2"
3286
a0d0e21e
LW
3287 sub bintodec {
3288 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
3289 }
3290