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1=head1 NAME
2
3perlfunc - Perl builtin functions
4
5=head1 DESCRIPTION
6
7The functions in this section can serve as terms in an expression.
8They fall into two major categories: list operators and named unary
9operators. These differ in their precedence relationship with a
10following comma. (See the precedence table in L<perlop>.) List
11operators take more than one argument, while unary operators can never
12take more than one argument. Thus, a comma terminates the argument of
13a unary operator, but merely separates the arguments of a list
14operator. A unary operator generally provides a scalar context to its
2b5ab1e7 15argument, while a list operator may provide either scalar or list
a0d0e21e 16contexts for its arguments. If it does both, the scalar arguments will
5f05dabc 17be first, and the list argument will follow. (Note that there can ever
0f31cffe 18be only one such list argument.) For instance, splice() has three scalar
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19arguments followed by a list, whereas gethostbyname() has four scalar
20arguments.
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21
22In the syntax descriptions that follow, list operators that expect a
23list (and provide list context for the elements of the list) are shown
24with LIST as an argument. Such a list may consist of any combination
25of scalar arguments or list values; the list values will be included
26in the list as if each individual element were interpolated at that
27point in the list, forming a longer single-dimensional list value.
28Elements of the LIST should be separated by commas.
29
30Any function in the list below may be used either with or without
31parentheses around its arguments. (The syntax descriptions omit the
5f05dabc 32parentheses.) If you use the parentheses, the simple (but occasionally
19799a22 33surprising) rule is this: It I<looks> like a function, therefore it I<is> a
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34function, and precedence doesn't matter. Otherwise it's a list
35operator or unary operator, and precedence does matter. And whitespace
36between the function and left parenthesis doesn't count--so you need to
37be careful sometimes:
38
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39 print 1+2+4; # Prints 7.
40 print(1+2) + 4; # Prints 3.
41 print (1+2)+4; # Also prints 3!
42 print +(1+2)+4; # Prints 7.
43 print ((1+2)+4); # Prints 7.
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44
45If you run Perl with the B<-w> switch it can warn you about this. For
46example, the third line above produces:
47
48 print (...) interpreted as function at - line 1.
49 Useless use of integer addition in void context at - line 1.
50
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51A few functions take no arguments at all, and therefore work as neither
52unary nor list operators. These include such functions as C<time>
53and C<endpwent>. For example, C<time+86_400> always means
54C<time() + 86_400>.
55
a0d0e21e 56For functions that can be used in either a scalar or list context,
54310121 57nonabortive failure is generally indicated in a scalar context by
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58returning the undefined value, and in a list context by returning the
59null list.
60
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61Remember the following important rule: There is B<no rule> that relates
62the behavior of an expression in list context to its behavior in scalar
63context, or vice versa. It might do two totally different things.
a0d0e21e 64Each operator and function decides which sort of value it would be most
2b5ab1e7 65appropriate to return in scalar context. Some operators return the
5a964f20 66length of the list that would have been returned in list context. Some
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67operators return the first value in the list. Some operators return the
68last value in the list. Some operators return a count of successful
69operations. In general, they do what you want, unless you want
70consistency.
71
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72An named array in scalar context is quite different from what would at
73first glance appear to be a list in scalar context. You can't get a list
74like C<(1,2,3)> into being in scalar context, because the compiler knows
75the context at compile time. It would generate the scalar comma operator
76there, not the list construction version of the comma. That means it
77was never a list to start with.
78
79In general, functions in Perl that serve as wrappers for system calls
f86cebdf 80of the same name (like chown(2), fork(2), closedir(2), etc.) all return
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81true when they succeed and C<undef> otherwise, as is usually mentioned
82in the descriptions below. This is different from the C interfaces,
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83which return C<-1> on failure. Exceptions to this rule are C<wait>,
84C<waitpid>, and C<syscall>. System calls also set the special C<$!>
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85variable on failure. Other functions do not, except accidentally.
86
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87=head2 Perl Functions by Category
88
89Here are Perl's functions (including things that look like
5a964f20 90functions, like some keywords and named operators)
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91arranged by category. Some functions appear in more
92than one place.
93
94=over
95
96=item Functions for SCALARs or strings
97
22fae026 98C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>,
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99C<length>, C<oct>, C<ord>, C<pack>, C<q/STRING/>, C<qq/STRING/>, C<reverse>,
100C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///>
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101
102=item Regular expressions and pattern matching
103
ab4f32c2 104C<m//>, C<pos>, C<quotemeta>, C<s///>, C<split>, C<study>, C<qr//>
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105
106=item Numeric functions
107
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108C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>,
109C<sin>, C<sqrt>, C<srand>
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110
111=item Functions for real @ARRAYs
112
22fae026 113C<pop>, C<push>, C<shift>, C<splice>, C<unshift>
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114
115=item Functions for list data
116
ab4f32c2 117C<grep>, C<join>, C<map>, C<qw/STRING/>, C<reverse>, C<sort>, C<unpack>
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118
119=item Functions for real %HASHes
120
22fae026 121C<delete>, C<each>, C<exists>, C<keys>, C<values>
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122
123=item Input and output functions
124
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125C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
126C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
127C<readdir>, C<rewinddir>, C<seek>, C<seekdir>, C<select>, C<syscall>,
128C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>,
129C<warn>, C<write>
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130
131=item Functions for fixed length data or records
132
22fae026 133C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec>
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134
135=item Functions for filehandles, files, or directories
136
22fae026 137C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>,
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138C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>,
139C<readlink>, C<rename>, C<rmdir>, C<stat>, C<symlink>, C<umask>,
140C<unlink>, C<utime>
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141
142=item Keywords related to the control flow of your perl program
143
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144C<caller>, C<continue>, C<die>, C<do>, C<dump>, C<eval>, C<exit>,
145C<goto>, C<last>, C<next>, C<redo>, C<return>, C<sub>, C<wantarray>
cb1a09d0 146
54310121 147=item Keywords related to scoping
cb1a09d0 148
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
22fae026
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168C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
169C<untie>, C<use>
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170
171=item Low-level socket functions
172
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173C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
174C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
175C<socket>, C<socketpair>
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176
177=item System V interprocess communication functions
178
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179C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
180C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>
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181
182=item Fetching user and group info
183
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184C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
185C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
186C<getpwuid>, C<setgrent>, C<setpwent>
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187
188=item Fetching network info
189
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190C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
191C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
192C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
193C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
194C<setnetent>, C<setprotoent>, C<setservent>
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195
196=item Time-related functions
197
22fae026 198C<gmtime>, C<localtime>, C<time>, C<times>
cb1a09d0 199
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200=item Functions new in perl5
201
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, $evaltext contains the text of the
dc848c6f 542C<eval EXPR> statement. In particular, for a C<eval BLOCK> statement,
951ba7fe 543$filename is C<(eval)>, but $evaltext is undefined. (Note also that
dc848c6f 544each C<use> statement creates a C<require> frame inside an C<eval EXPR>)
e476b1b5
GS
545frame. C<$hints> and C<$bitmask> contain pragmatic hints that the caller
546was compiled with. The C<$hints> and C<$bitmask> values are subject to
547change between versions of Perl, and are not meant for external use.
748a9306
LW
548
549Furthermore, when called from within the DB package, caller returns more
7660c0ab 550detailed information: it sets the list variable C<@DB::args> to be the
54310121 551arguments with which the subroutine was invoked.
748a9306 552
7660c0ab 553Be aware that the optimizer might have optimized call frames away before
19799a22 554C<caller> had a chance to get the information. That means that C<caller(N)>
7660c0ab 555might not return information about the call frame you expect it do, for
61eff3bc 556C<< N > 1 >>. In particular, C<@DB::args> might have information from the
19799a22 557previous time C<caller> was called.
7660c0ab 558
a0d0e21e
LW
559=item chdir EXPR
560
2b5ab1e7 561Changes the working directory to EXPR, if possible. If EXPR is omitted,
0bfc1ec4
GS
562changes to the directory specified by C<$ENV{HOME}>, if set; if not,
563changes to the directory specified by C<$ENV{LOGDIR}>. If neither is
564set, C<chdir> does nothing. It returns true upon success, false
565otherwise. See the example under C<die>.
a0d0e21e
LW
566
567=item chmod LIST
568
569Changes the permissions of a list of files. The first element of the
4633a7c4 570list must be the numerical mode, which should probably be an octal
2f9daede
TPG
571number, and which definitely should I<not> a string of octal digits:
572C<0644> is okay, C<'0644'> is not. Returns the number of files
dc848c6f 573successfully changed. See also L</oct>, if all you have is a string.
a0d0e21e
LW
574
575 $cnt = chmod 0755, 'foo', 'bar';
576 chmod 0755, @executables;
f86cebdf
GS
577 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
578 # --w----r-T
2f9daede
TPG
579 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
580 $mode = 0644; chmod $mode, 'foo'; # this is best
a0d0e21e 581
ca6e1c26
JH
582You can also import the symbolic C<S_I*> constants from the Fcntl
583module:
584
585 use Fcntl ':mode';
586
587 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
588 # This is identical to the chmod 0755 of the above example.
589
a0d0e21e
LW
590=item chomp VARIABLE
591
592=item chomp LIST
593
594=item chomp
595
2b5ab1e7
TC
596This safer version of L</chop> removes any trailing string
597that corresponds to the current value of C<$/> (also known as
28757baa
PP
598$INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
599number of characters removed from all its arguments. It's often used to
600remove the newline from the end of an input record when you're worried
2b5ab1e7
TC
601that the final record may be missing its newline. When in paragraph
602mode (C<$/ = "">), it removes all trailing newlines from the string.
4c5a6083
GS
603When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
604a reference to an integer or the like, see L<perlvar>) chomp() won't
19799a22
GS
605remove anything.
606If VARIABLE is omitted, it chomps C<$_>. Example:
a0d0e21e
LW
607
608 while (<>) {
609 chomp; # avoid \n on last field
610 @array = split(/:/);
5a964f20 611 # ...
a0d0e21e
LW
612 }
613
614You can actually chomp anything that's an lvalue, including an assignment:
615
616 chomp($cwd = `pwd`);
617 chomp($answer = <STDIN>);
618
619If you chomp a list, each element is chomped, and the total number of
620characters removed is returned.
621
622=item chop VARIABLE
623
624=item chop LIST
625
626=item chop
627
628Chops off the last character of a string and returns the character
629chopped. It's used primarily to remove the newline from the end of an
630input record, but is much more efficient than C<s/\n//> because it neither
7660c0ab 631scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
a0d0e21e
LW
632Example:
633
634 while (<>) {
635 chop; # avoid \n on last field
636 @array = split(/:/);
5a964f20 637 #...
a0d0e21e
LW
638 }
639
640You can actually chop anything that's an lvalue, including an assignment:
641
642 chop($cwd = `pwd`);
643 chop($answer = <STDIN>);
644
645If you chop a list, each element is chopped. Only the value of the
19799a22 646last C<chop> is returned.
a0d0e21e 647
19799a22 648Note that C<chop> returns the last character. To return all but the last
748a9306
LW
649character, use C<substr($string, 0, -1)>.
650
a0d0e21e
LW
651=item chown LIST
652
653Changes the owner (and group) of a list of files. The first two
19799a22
GS
654elements of the list must be the I<numeric> uid and gid, in that
655order. A value of -1 in either position is interpreted by most
656systems to leave that value unchanged. Returns the number of files
657successfully changed.
a0d0e21e
LW
658
659 $cnt = chown $uid, $gid, 'foo', 'bar';
660 chown $uid, $gid, @filenames;
661
54310121 662Here's an example that looks up nonnumeric uids in the passwd file:
a0d0e21e
LW
663
664 print "User: ";
19799a22 665 chomp($user = <STDIN>);
5a964f20 666 print "Files: ";
19799a22 667 chomp($pattern = <STDIN>);
a0d0e21e
LW
668
669 ($login,$pass,$uid,$gid) = getpwnam($user)
670 or die "$user not in passwd file";
671
5a964f20 672 @ary = glob($pattern); # expand filenames
a0d0e21e
LW
673 chown $uid, $gid, @ary;
674
54310121 675On most systems, you are not allowed to change the ownership of the
4633a7c4
LW
676file unless you're the superuser, although you should be able to change
677the group to any of your secondary groups. On insecure systems, these
678restrictions may be relaxed, but this is not a portable assumption.
19799a22
GS
679On POSIX systems, you can detect this condition this way:
680
681 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
682 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
4633a7c4 683
a0d0e21e
LW
684=item chr NUMBER
685
54310121 686=item chr
bbce6d69 687
a0d0e21e 688Returns the character represented by that NUMBER in the character set.
a0ed51b3 689For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
2b5ab1e7
TC
690chr(0x263a) is a Unicode smiley face (but only within the scope of
691a C<use utf8>). For the reverse, use L</ord>.
692See L<utf8> for more about Unicode.
a0d0e21e 693
7660c0ab 694If NUMBER is omitted, uses C<$_>.
bbce6d69 695
a0d0e21e
LW
696=item chroot FILENAME
697
54310121 698=item chroot
bbce6d69 699
5a964f20 700This function works like the system call by the same name: it makes the
4633a7c4 701named directory the new root directory for all further pathnames that
951ba7fe 702begin with a C</> by your process and all its children. (It doesn't
28757baa 703change your current working directory, which is unaffected.) For security
4633a7c4 704reasons, this call is restricted to the superuser. If FILENAME is
19799a22 705omitted, does a C<chroot> to C<$_>.
a0d0e21e
LW
706
707=item close FILEHANDLE
708
6a518fbc
TP
709=item close
710
19799a22 711Closes the file or pipe associated with the file handle, returning true
a0d0e21e 712only if stdio successfully flushes buffers and closes the system file
19799a22 713descriptor. Closes the currently selected filehandle if the argument
6a518fbc 714is omitted.
fb73857a
PP
715
716You don't have to close FILEHANDLE if you are immediately going to do
19799a22
GS
717another C<open> on it, because C<open> will close it for you. (See
718C<open>.) However, an explicit C<close> on an input file resets the line
719counter (C<$.>), while the implicit close done by C<open> does not.
fb73857a 720
19799a22
GS
721If the file handle came from a piped open C<close> will additionally
722return false if one of the other system calls involved fails or if the
fb73857a 723program exits with non-zero status. (If the only problem was that the
2b5ab1e7
TC
724program exited non-zero C<$!> will be set to C<0>.) Closing a pipe
725also waits for the process executing on the pipe to complete, in case you
726want to look at the output of the pipe afterwards, and
727implicitly puts the exit status value of that command into C<$?>.
5a964f20 728
73689b13
GS
729Prematurely closing the read end of a pipe (i.e. before the process
730writing to it at the other end has closed it) will result in a
731SIGPIPE being delivered to the writer. If the other end can't
732handle that, be sure to read all the data before closing the pipe.
733
fb73857a 734Example:
a0d0e21e 735
fb73857a
PP
736 open(OUTPUT, '|sort >foo') # pipe to sort
737 or die "Can't start sort: $!";
5a964f20 738 #... # print stuff to output
fb73857a
PP
739 close OUTPUT # wait for sort to finish
740 or warn $! ? "Error closing sort pipe: $!"
741 : "Exit status $? from sort";
742 open(INPUT, 'foo') # get sort's results
743 or die "Can't open 'foo' for input: $!";
a0d0e21e 744
5a964f20
TC
745FILEHANDLE may be an expression whose value can be used as an indirect
746filehandle, usually the real filehandle name.
a0d0e21e
LW
747
748=item closedir DIRHANDLE
749
19799a22 750Closes a directory opened by C<opendir> and returns the success of that
5a964f20
TC
751system call.
752
753DIRHANDLE may be an expression whose value can be used as an indirect
754dirhandle, usually the real dirhandle name.
a0d0e21e
LW
755
756=item connect SOCKET,NAME
757
758Attempts to connect to a remote socket, just as the connect system call
19799a22 759does. Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
760packed address of the appropriate type for the socket. See the examples in
761L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 762
cb1a09d0
AD
763=item continue BLOCK
764
765Actually a flow control statement rather than a function. If there is a
98293880
JH
766C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
767C<foreach>), it is always executed just before the conditional is about to
768be evaluated again, just like the third part of a C<for> loop in C. Thus
cb1a09d0
AD
769it can be used to increment a loop variable, even when the loop has been
770continued via the C<next> statement (which is similar to the C C<continue>
771statement).
772
98293880 773C<last>, C<next>, or C<redo> may appear within a C<continue>
19799a22
GS
774block. C<last> and C<redo> will behave as if they had been executed within
775the main block. So will C<next>, but since it will execute a C<continue>
1d2dff63
GS
776block, it may be more entertaining.
777
778 while (EXPR) {
779 ### redo always comes here
780 do_something;
781 } continue {
782 ### next always comes here
783 do_something_else;
784 # then back the top to re-check EXPR
785 }
786 ### last always comes here
787
788Omitting the C<continue> section is semantically equivalent to using an
19799a22 789empty one, logically enough. In that case, C<next> goes directly back
1d2dff63
GS
790to check the condition at the top of the loop.
791
a0d0e21e
LW
792=item cos EXPR
793
5a964f20 794Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
7660c0ab 795takes cosine of C<$_>.
a0d0e21e 796
ca6e1c26 797For the inverse cosine operation, you may use the C<Math::Trig::acos()>
28757baa
PP
798function, or use this relation:
799
800 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
801
a0d0e21e
LW
802=item crypt PLAINTEXT,SALT
803
f86cebdf 804Encrypts a string exactly like the crypt(3) function in the C library
4633a7c4
LW
805(assuming that you actually have a version there that has not been
806extirpated as a potential munition). This can prove useful for checking
807the password file for lousy passwords, amongst other things. Only the
808guys wearing white hats should do this.
a0d0e21e 809
19799a22 810Note that C<crypt> is intended to be a one-way function, much like breaking
11155c91
CS
811eggs to make an omelette. There is no (known) corresponding decrypt
812function. As a result, this function isn't all that useful for
813cryptography. (For that, see your nearby CPAN mirror.)
2f9daede 814
e71965be
RS
815When verifying an existing encrypted string you should use the encrypted
816text as the salt (like C<crypt($plain, $crypted) eq $crypted>). This
19799a22 817allows your code to work with the standard C<crypt> and with more
e71965be
RS
818exotic implementations. When choosing a new salt create a random two
819character string whose characters come from the set C<[./0-9A-Za-z]>
820(like C<join '', ('.', '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>).
821
a0d0e21e
LW
822Here's an example that makes sure that whoever runs this program knows
823their own password:
824
825 $pwd = (getpwuid($<))[1];
a0d0e21e
LW
826
827 system "stty -echo";
828 print "Password: ";
e71965be 829 chomp($word = <STDIN>);
a0d0e21e
LW
830 print "\n";
831 system "stty echo";
832
e71965be 833 if (crypt($word, $pwd) ne $pwd) {
a0d0e21e
LW
834 die "Sorry...\n";
835 } else {
836 print "ok\n";
54310121 837 }
a0d0e21e 838
9f8f0c9d 839Of course, typing in your own password to whoever asks you
748a9306 840for it is unwise.
a0d0e21e 841
19799a22
GS
842The L<crypt> function is unsuitable for encrypting large quantities
843of data, not least of all because you can't get the information
844back. Look at the F<by-module/Crypt> and F<by-module/PGP> directories
845on your favorite CPAN mirror for a slew of potentially useful
846modules.
847
aa689395 848=item dbmclose HASH
a0d0e21e 849
19799a22 850[This function has been largely superseded by the C<untie> function.]
a0d0e21e 851
aa689395 852Breaks the binding between a DBM file and a hash.
a0d0e21e 853
19799a22 854=item dbmopen HASH,DBNAME,MASK
a0d0e21e 855
19799a22 856[This function has been largely superseded by the C<tie> function.]
a0d0e21e 857
7b8d334a 858This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
19799a22
GS
859hash. HASH is the name of the hash. (Unlike normal C<open>, the first
860argument is I<not> a filehandle, even though it looks like one). DBNAME
aa689395
PP
861is the name of the database (without the F<.dir> or F<.pag> extension if
862any). If the database does not exist, it is created with protection
19799a22
GS
863specified by MASK (as modified by the C<umask>). If your system supports
864only the older DBM functions, you may perform only one C<dbmopen> in your
aa689395 865program. In older versions of Perl, if your system had neither DBM nor
19799a22 866ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
aa689395
PP
867sdbm(3).
868
869If you don't have write access to the DBM file, you can only read hash
870variables, not set them. If you want to test whether you can write,
19799a22 871either use file tests or try setting a dummy hash entry inside an C<eval>,
aa689395 872which will trap the error.
a0d0e21e 873
19799a22
GS
874Note that functions such as C<keys> and C<values> may return huge lists
875when used on large DBM files. You may prefer to use the C<each>
a0d0e21e
LW
876function to iterate over large DBM files. Example:
877
878 # print out history file offsets
879 dbmopen(%HIST,'/usr/lib/news/history',0666);
880 while (($key,$val) = each %HIST) {
881 print $key, ' = ', unpack('L',$val), "\n";
882 }
883 dbmclose(%HIST);
884
cb1a09d0 885See also L<AnyDBM_File> for a more general description of the pros and
184e9718 886cons of the various dbm approaches, as well as L<DB_File> for a particularly
cb1a09d0 887rich implementation.
4633a7c4 888
2b5ab1e7
TC
889You can control which DBM library you use by loading that library
890before you call dbmopen():
891
892 use DB_File;
893 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
894 or die "Can't open netscape history file: $!";
895
a0d0e21e
LW
896=item defined EXPR
897
54310121 898=item defined
bbce6d69 899
2f9daede
TPG
900Returns a Boolean value telling whether EXPR has a value other than
901the undefined value C<undef>. If EXPR is not present, C<$_> will be
902checked.
903
904Many operations return C<undef> to indicate failure, end of file,
905system error, uninitialized variable, and other exceptional
906conditions. This function allows you to distinguish C<undef> from
907other values. (A simple Boolean test will not distinguish among
7660c0ab 908C<undef>, zero, the empty string, and C<"0">, which are all equally
2f9daede 909false.) Note that since C<undef> is a valid scalar, its presence
19799a22 910doesn't I<necessarily> indicate an exceptional condition: C<pop>
2f9daede
TPG
911returns C<undef> when its argument is an empty array, I<or> when the
912element to return happens to be C<undef>.
913
f10b0346
GS
914You may also use C<defined(&func)> to check whether subroutine C<&func>
915has ever been defined. The return value is unaffected by any forward
916declarations of C<&foo>.
917
918Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
919used to report whether memory for that aggregate has ever been
920allocated. This behavior may disappear in future versions of Perl.
921You should instead use a simple test for size:
922
923 if (@an_array) { print "has array elements\n" }
924 if (%a_hash) { print "has hash members\n" }
2f9daede
TPG
925
926When used on a hash element, it tells you whether the value is defined,
dc848c6f 927not whether the key exists in the hash. Use L</exists> for the latter
2f9daede 928purpose.
a0d0e21e
LW
929
930Examples:
931
932 print if defined $switch{'D'};
933 print "$val\n" while defined($val = pop(@ary));
934 die "Can't readlink $sym: $!"
935 unless defined($value = readlink $sym);
a0d0e21e 936 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
2f9daede 937 $debugging = 0 unless defined $debugging;
a0d0e21e 938
19799a22 939Note: Many folks tend to overuse C<defined>, and then are surprised to
7660c0ab 940discover that the number C<0> and C<""> (the zero-length string) are, in fact,
2f9daede 941defined values. For example, if you say
a5f75d66
AD
942
943 "ab" =~ /a(.*)b/;
944
7660c0ab 945The pattern match succeeds, and C<$1> is defined, despite the fact that it
a5f75d66 946matched "nothing". But it didn't really match nothing--rather, it
2b5ab1e7 947matched something that happened to be zero characters long. This is all
a5f75d66 948very above-board and honest. When a function returns an undefined value,
2f9daede 949it's an admission that it couldn't give you an honest answer. So you
19799a22 950should use C<defined> only when you're questioning the integrity of what
7660c0ab 951you're trying to do. At other times, a simple comparison to C<0> or C<""> is
2f9daede
TPG
952what you want.
953
dc848c6f 954See also L</undef>, L</exists>, L</ref>.
2f9daede 955
a0d0e21e
LW
956=item delete EXPR
957
01020589
GS
958Given an expression that specifies a hash element, array element, hash slice,
959or array slice, deletes the specified element(s) from the hash or array.
8216c1fd
GS
960In the case of an array, if the array elements happen to be at the end,
961the size of the array will shrink to the highest element that tests
962true for exists() (or 0 if no such element exists).
a0d0e21e 963
01020589
GS
964Returns each element so deleted or the undefined value if there was no such
965element. Deleting from C<$ENV{}> modifies the environment. Deleting from
966a hash tied to a DBM file deletes the entry from the DBM file. Deleting
967from a C<tie>d hash or array may not necessarily return anything.
968
8ea97a1e
GS
969Deleting an array element effectively returns that position of the array
970to its initial, uninitialized state. Subsequently testing for the same
8216c1fd
GS
971element with exists() will return false. Note that deleting array
972elements in the middle of an array will not shift the index of the ones
973after them down--use splice() for that. See L</exists>.
8ea97a1e 974
01020589 975The following (inefficiently) deletes all the values of %HASH and @ARRAY:
a0d0e21e 976
5f05dabc
PP
977 foreach $key (keys %HASH) {
978 delete $HASH{$key};
a0d0e21e
LW
979 }
980
01020589
GS
981 foreach $index (0 .. $#ARRAY) {
982 delete $ARRAY[$index];
983 }
984
985And so do these:
5f05dabc 986
01020589
GS
987 delete @HASH{keys %HASH};
988
9740c838 989 delete @ARRAY[0 .. $#ARRAY];
5f05dabc 990
2b5ab1e7 991But both of these are slower than just assigning the empty list
01020589
GS
992or undefining %HASH or @ARRAY:
993
994 %HASH = (); # completely empty %HASH
995 undef %HASH; # forget %HASH ever existed
2b5ab1e7 996
01020589
GS
997 @ARRAY = (); # completely empty @ARRAY
998 undef @ARRAY; # forget @ARRAY ever existed
2b5ab1e7
TC
999
1000Note that the EXPR can be arbitrarily complicated as long as the final
01020589
GS
1001operation is a hash element, array element, hash slice, or array slice
1002lookup:
a0d0e21e
LW
1003
1004 delete $ref->[$x][$y]{$key};
5f05dabc 1005 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
a0d0e21e 1006
01020589
GS
1007 delete $ref->[$x][$y][$index];
1008 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1009
a0d0e21e
LW
1010=item die LIST
1011
19799a22
GS
1012Outside an C<eval>, prints the value of LIST to C<STDERR> and
1013exits with the current value of C<$!> (errno). If C<$!> is C<0>,
61eff3bc
JH
1014exits with the value of C<<< ($? >> 8) >>> (backtick `command`
1015status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside
19799a22
GS
1016an C<eval(),> the error message is stuffed into C<$@> and the
1017C<eval> is terminated with the undefined value. This makes
1018C<die> the way to raise an exception.
a0d0e21e
LW
1019
1020Equivalent examples:
1021
1022 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
54310121 1023 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
a0d0e21e
LW
1024
1025If the value of EXPR does not end in a newline, the current script line
1026number and input line number (if any) are also printed, and a newline
883faa13
GS
1027is supplied. Note that the "input line number" (also known as "chunk")
1028is subject to whatever notion of "line" happens to be currently in
1029effect, and is also available as the special variable C<$.>.
1030See L<perlvar/"$/"> and L<perlvar/"$.">.
1031
1032Hint: sometimes appending C<", stopped"> to your message
7660c0ab 1033will cause it to make better sense when the string C<"at foo line 123"> is
a0d0e21e
LW
1034appended. Suppose you are running script "canasta".
1035
1036 die "/etc/games is no good";
1037 die "/etc/games is no good, stopped";
1038
1039produce, respectively
1040
1041 /etc/games is no good at canasta line 123.
1042 /etc/games is no good, stopped at canasta line 123.
1043
2b5ab1e7 1044See also exit(), warn(), and the Carp module.
a0d0e21e 1045
7660c0ab
A
1046If LIST is empty and C<$@> already contains a value (typically from a
1047previous eval) that value is reused after appending C<"\t...propagated">.
fb73857a
PP
1048This is useful for propagating exceptions:
1049
1050 eval { ... };
1051 die unless $@ =~ /Expected exception/;
1052
7660c0ab 1053If C<$@> is empty then the string C<"Died"> is used.
fb73857a 1054
52531d10
GS
1055die() can also be called with a reference argument. If this happens to be
1056trapped within an eval(), $@ contains the reference. This behavior permits
1057a more elaborate exception handling implementation using objects that
4375e838 1058maintain arbitrary state about the nature of the exception. Such a scheme
52531d10
GS
1059is sometimes preferable to matching particular string values of $@ using
1060regular expressions. Here's an example:
1061
1062 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1063 if ($@) {
1064 if (ref($@) && UNIVERSAL::isa($@,"Some::Module::Exception")) {
1065 # handle Some::Module::Exception
1066 }
1067 else {
1068 # handle all other possible exceptions
1069 }
1070 }
1071
19799a22 1072Because perl will stringify uncaught exception messages before displaying
52531d10
GS
1073them, you may want to overload stringification operations on such custom
1074exception objects. See L<overload> for details about that.
1075
19799a22
GS
1076You can arrange for a callback to be run just before the C<die>
1077does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1078handler will be called with the error text and can change the error
1079message, if it sees fit, by calling C<die> again. See
1080L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
1081L<"eval BLOCK"> for some examples. Although this feature was meant
1082to be run only right before your program was to exit, this is not
1083currently the case--the C<$SIG{__DIE__}> hook is currently called
1084even inside eval()ed blocks/strings! If one wants the hook to do
1085nothing in such situations, put
fb73857a
PP
1086
1087 die @_ if $^S;
1088
19799a22
GS
1089as the first line of the handler (see L<perlvar/$^S>). Because
1090this promotes strange action at a distance, this counterintuitive
1091behavior may be fixed in a future release.
774d564b 1092
a0d0e21e
LW
1093=item do BLOCK
1094
1095Not really a function. Returns the value of the last command in the
1096sequence of commands indicated by BLOCK. When modified by a loop
98293880
JH
1097modifier, executes the BLOCK once before testing the loop condition.
1098(On other statements the loop modifiers test the conditional first.)
a0d0e21e 1099
4968c1e4 1100C<do BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7
TC
1101C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1102See L<perlsyn> for alternative strategies.
4968c1e4 1103
a0d0e21e
LW
1104=item do SUBROUTINE(LIST)
1105
1106A deprecated form of subroutine call. See L<perlsub>.
1107
1108=item do EXPR
1109
1110Uses the value of EXPR as a filename and executes the contents of the
1111file as a Perl script. Its primary use is to include subroutines
1112from a Perl subroutine library.
1113
1114 do 'stat.pl';
1115
1116is just like
1117
fb73857a 1118 scalar eval `cat stat.pl`;
a0d0e21e 1119
2b5ab1e7
TC
1120except that it's more efficient and concise, keeps track of the current
1121filename for error messages, searches the @INC libraries, and updates
1122C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1123variables. It also differs in that code evaluated with C<do FILENAME>
1124cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1125same, however, in that it does reparse the file every time you call it,
1126so you probably don't want to do this inside a loop.
a0d0e21e 1127
8e30cc93 1128If C<do> cannot read the file, it returns undef and sets C<$!> to the
2b5ab1e7 1129error. If C<do> can read the file but cannot compile it, it
8e30cc93
G
1130returns undef and sets an error message in C<$@>. If the file is
1131successfully compiled, C<do> returns the value of the last expression
1132evaluated.
1133
a0d0e21e 1134Note that inclusion of library modules is better done with the
19799a22 1135C<use> and C<require> operators, which also do automatic error checking
4633a7c4 1136and raise an exception if there's a problem.
a0d0e21e 1137
5a964f20
TC
1138You might like to use C<do> to read in a program configuration
1139file. Manual error checking can be done this way:
1140
1141 # read in config files: system first, then user
f86cebdf 1142 for $file ("/share/prog/defaults.rc",
2b5ab1e7
TC
1143 "$ENV{HOME}/.someprogrc")
1144 {
5a964f20 1145 unless ($return = do $file) {
f86cebdf
GS
1146 warn "couldn't parse $file: $@" if $@;
1147 warn "couldn't do $file: $!" unless defined $return;
1148 warn "couldn't run $file" unless $return;
5a964f20
TC
1149 }
1150 }
1151
a0d0e21e
LW
1152=item dump LABEL
1153
1614b0e3
JD
1154=item dump
1155
19799a22
GS
1156This function causes an immediate core dump. See also the B<-u>
1157command-line switch in L<perlrun>, which does the same thing.
1158Primarily this is so that you can use the B<undump> program (not
1159supplied) to turn your core dump into an executable binary after
1160having initialized all your variables at the beginning of the
1161program. When the new binary is executed it will begin by executing
1162a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1163Think of it as a goto with an intervening core dump and reincarnation.
1164If C<LABEL> is omitted, restarts the program from the top.
1165
1166B<WARNING>: Any files opened at the time of the dump will I<not>
1167be open any more when the program is reincarnated, with possible
1168resulting confusion on the part of Perl.
1169
1170This function is now largely obsolete, partly because it's very
1171hard to convert a core file into an executable, and because the
1172real compiler backends for generating portable bytecode and compilable
1173C code have superseded it.
1174
1175If you're looking to use L<dump> to speed up your program, consider
1176generating bytecode or native C code as described in L<perlcc>. If
1177you're just trying to accelerate a CGI script, consider using the
1178C<mod_perl> extension to B<Apache>, or the CPAN module, Fast::CGI.
1179You might also consider autoloading or selfloading, which at least
1180make your program I<appear> to run faster.
5a964f20 1181
aa689395
PP
1182=item each HASH
1183
5a964f20 1184When called in list context, returns a 2-element list consisting of the
aa689395 1185key and value for the next element of a hash, so that you can iterate over
5a964f20 1186it. When called in scalar context, returns the key for only the "next"
e902a979 1187element in the hash.
2f9daede 1188
ab192400
GS
1189Entries are returned in an apparently random order. The actual random
1190order is subject to change in future versions of perl, but it is guaranteed
19799a22 1191to be in the same order as either the C<keys> or C<values> function
ab192400
GS
1192would produce on the same (unmodified) hash.
1193
1194When the hash is entirely read, a null array is returned in list context
19799a22
GS
1195(which when assigned produces a false (C<0>) value), and C<undef> in
1196scalar context. The next call to C<each> after that will start iterating
1197again. There is a single iterator for each hash, shared by all C<each>,
1198C<keys>, and C<values> function calls in the program; it can be reset by
2f9daede
TPG
1199reading all the elements from the hash, or by evaluating C<keys HASH> or
1200C<values HASH>. If you add or delete elements of a hash while you're
1201iterating over it, you may get entries skipped or duplicated, so don't.
aa689395 1202
f86cebdf 1203The following prints out your environment like the printenv(1) program,
aa689395 1204only in a different order:
a0d0e21e
LW
1205
1206 while (($key,$value) = each %ENV) {
1207 print "$key=$value\n";
1208 }
1209
19799a22 1210See also C<keys>, C<values> and C<sort>.
a0d0e21e
LW
1211
1212=item eof FILEHANDLE
1213
4633a7c4
LW
1214=item eof ()
1215
a0d0e21e
LW
1216=item eof
1217
1218Returns 1 if the next read on FILEHANDLE will return end of file, or if
1219FILEHANDLE is not open. FILEHANDLE may be an expression whose value
5a964f20 1220gives the real filehandle. (Note that this function actually
19799a22 1221reads a character and then C<ungetc>s it, so isn't very useful in an
748a9306 1222interactive context.) Do not read from a terminal file (or call
19799a22 1223C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
748a9306
LW
1224as terminals may lose the end-of-file condition if you do.
1225
820475bd
GS
1226An C<eof> without an argument uses the last file read. Using C<eof()>
1227with empty parentheses is very different. It refers to the pseudo file
1228formed from the files listed on the command line and accessed via the
61eff3bc
JH
1229C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1230as a normal filehandle is, an C<eof()> before C<< <> >> has been
820475bd
GS
1231used will cause C<@ARGV> to be examined to determine if input is
1232available.
1233
61eff3bc 1234In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
820475bd
GS
1235detect the end of each file, C<eof()> will only detect the end of the
1236last file. Examples:
a0d0e21e 1237
748a9306
LW
1238 # reset line numbering on each input file
1239 while (<>) {
5a964f20 1240 next if /^\s*#/; # skip comments
748a9306 1241 print "$.\t$_";
5a964f20
TC
1242 } continue {
1243 close ARGV if eof; # Not eof()!
748a9306
LW
1244 }
1245
a0d0e21e
LW
1246 # insert dashes just before last line of last file
1247 while (<>) {
5a964f20 1248 if (eof()) { # check for end of current file
a0d0e21e 1249 print "--------------\n";
2b5ab1e7 1250 close(ARGV); # close or last; is needed if we
748a9306 1251 # are reading from the terminal
a0d0e21e
LW
1252 }
1253 print;
1254 }
1255
a0d0e21e 1256Practical hint: you almost never need to use C<eof> in Perl, because the
3ce0d271
GS
1257input operators typically return C<undef> when they run out of data, or if
1258there was an error.
a0d0e21e
LW
1259
1260=item eval EXPR
1261
1262=item eval BLOCK
1263
c7cc6f1c
GS
1264In the first form, the return value of EXPR is parsed and executed as if it
1265were a little Perl program. The value of the expression (which is itself
5a964f20 1266determined within scalar context) is first parsed, and if there weren't any
c7cc6f1c 1267errors, executed in the context of the current Perl program, so that any
5f05dabc 1268variable settings or subroutine and format definitions remain afterwards.
c7cc6f1c
GS
1269Note that the value is parsed every time the eval executes. If EXPR is
1270omitted, evaluates C<$_>. This form is typically used to delay parsing
1271and subsequent execution of the text of EXPR until run time.
1272
1273In the second form, the code within the BLOCK is parsed only once--at the
1274same time the code surrounding the eval itself was parsed--and executed
1275within the context of the current Perl program. This form is typically
1276used to trap exceptions more efficiently than the first (see below), while
1277also providing the benefit of checking the code within BLOCK at compile
1278time.
1279
1280The final semicolon, if any, may be omitted from the value of EXPR or within
1281the BLOCK.
1282
1283In both forms, the value returned is the value of the last expression
5a964f20 1284evaluated inside the mini-program; a return statement may be also used, just
c7cc6f1c 1285as with subroutines. The expression providing the return value is evaluated
5a964f20 1286in void, scalar, or list context, depending on the context of the eval itself.
c7cc6f1c 1287See L</wantarray> for more on how the evaluation context can be determined.
a0d0e21e 1288
19799a22
GS
1289If there is a syntax error or runtime error, or a C<die> statement is
1290executed, an undefined value is returned by C<eval>, and C<$@> is set to the
a0d0e21e 1291error message. If there was no error, C<$@> is guaranteed to be a null
19799a22 1292string. Beware that using C<eval> neither silences perl from printing
c7cc6f1c
GS
1293warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1294To do either of those, you have to use the C<$SIG{__WARN__}> facility. See
1295L</warn> and L<perlvar>.
a0d0e21e 1296
19799a22
GS
1297Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1298determining whether a particular feature (such as C<socket> or C<symlink>)
a0d0e21e
LW
1299is implemented. It is also Perl's exception trapping mechanism, where
1300the die operator is used to raise exceptions.
1301
1302If the code to be executed doesn't vary, you may use the eval-BLOCK
1303form to trap run-time errors without incurring the penalty of
1304recompiling each time. The error, if any, is still returned in C<$@>.
1305Examples:
1306
54310121 1307 # make divide-by-zero nonfatal
a0d0e21e
LW
1308 eval { $answer = $a / $b; }; warn $@ if $@;
1309
1310 # same thing, but less efficient
1311 eval '$answer = $a / $b'; warn $@ if $@;
1312
1313 # a compile-time error
5a964f20 1314 eval { $answer = }; # WRONG
a0d0e21e
LW
1315
1316 # a run-time error
1317 eval '$answer ='; # sets $@
1318
2b5ab1e7
TC
1319Due to the current arguably broken state of C<__DIE__> hooks, when using
1320the C<eval{}> form as an exception trap in libraries, you may wish not
1321to trigger any C<__DIE__> hooks that user code may have installed.
1322You can use the C<local $SIG{__DIE__}> construct for this purpose,
1323as shown in this example:
774d564b
PP
1324
1325 # a very private exception trap for divide-by-zero
f86cebdf
GS
1326 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1327 warn $@ if $@;
774d564b
PP
1328
1329This is especially significant, given that C<__DIE__> hooks can call
19799a22 1330C<die> again, which has the effect of changing their error messages:
774d564b
PP
1331
1332 # __DIE__ hooks may modify error messages
1333 {
f86cebdf
GS
1334 local $SIG{'__DIE__'} =
1335 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
c7cc6f1c
GS
1336 eval { die "foo lives here" };
1337 print $@ if $@; # prints "bar lives here"
774d564b
PP
1338 }
1339
19799a22 1340Because this promotes action at a distance, this counterintuitive behavior
2b5ab1e7
TC
1341may be fixed in a future release.
1342
19799a22 1343With an C<eval>, you should be especially careful to remember what's
a0d0e21e
LW
1344being looked at when:
1345
1346 eval $x; # CASE 1
1347 eval "$x"; # CASE 2
1348
1349 eval '$x'; # CASE 3
1350 eval { $x }; # CASE 4
1351
5a964f20 1352 eval "\$$x++"; # CASE 5
a0d0e21e
LW
1353 $$x++; # CASE 6
1354
2f9daede 1355Cases 1 and 2 above behave identically: they run the code contained in
19799a22 1356the variable $x. (Although case 2 has misleading double quotes making
2f9daede 1357the reader wonder what else might be happening (nothing is).) Cases 3
7660c0ab 1358and 4 likewise behave in the same way: they run the code C<'$x'>, which
19799a22 1359does nothing but return the value of $x. (Case 4 is preferred for
2f9daede
TPG
1360purely visual reasons, but it also has the advantage of compiling at
1361compile-time instead of at run-time.) Case 5 is a place where
19799a22 1362normally you I<would> like to use double quotes, except that in this
2f9daede
TPG
1363particular situation, you can just use symbolic references instead, as
1364in case 6.
a0d0e21e 1365
4968c1e4 1366C<eval BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7 1367C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
4968c1e4 1368
a0d0e21e
LW
1369=item exec LIST
1370
8bf3b016
GS
1371=item exec PROGRAM LIST
1372
19799a22
GS
1373The C<exec> function executes a system command I<and never returns>--
1374use C<system> instead of C<exec> if you want it to return. It fails and
1375returns false only if the command does not exist I<and> it is executed
fb73857a 1376directly instead of via your system's command shell (see below).
a0d0e21e 1377
19799a22
GS
1378Since it's a common mistake to use C<exec> instead of C<system>, Perl
1379warns you if there is a following statement which isn't C<die>, C<warn>,
1380or C<exit> (if C<-w> is set - but you always do that). If you
1381I<really> want to follow an C<exec> with some other statement, you
55d729e4
GS
1382can use one of these styles to avoid the warning:
1383
5a964f20
TC
1384 exec ('foo') or print STDERR "couldn't exec foo: $!";
1385 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
55d729e4 1386
5a964f20 1387If there is more than one argument in LIST, or if LIST is an array
f86cebdf 1388with more than one value, calls execvp(3) with the arguments in LIST.
5a964f20
TC
1389If there is only one scalar argument or an array with one element in it,
1390the argument is checked for shell metacharacters, and if there are any,
1391the entire argument is passed to the system's command shell for parsing
1392(this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1393If there are no shell metacharacters in the argument, it is split into
19799a22
GS
1394words and passed directly to C<execvp>, which is more efficient.
1395Examples:
a0d0e21e 1396
19799a22
GS
1397 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1398 exec "sort $outfile | uniq";
a0d0e21e
LW
1399
1400If you don't really want to execute the first argument, but want to lie
1401to the program you are executing about its own name, you can specify
1402the program you actually want to run as an "indirect object" (without a
1403comma) in front of the LIST. (This always forces interpretation of the
54310121 1404LIST as a multivalued list, even if there is only a single scalar in
a0d0e21e
LW
1405the list.) Example:
1406
1407 $shell = '/bin/csh';
1408 exec $shell '-sh'; # pretend it's a login shell
1409
1410or, more directly,
1411
1412 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1413
bb32b41a
GS
1414When the arguments get executed via the system shell, results will
1415be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1416for details.
1417
19799a22
GS
1418Using an indirect object with C<exec> or C<system> is also more
1419secure. This usage (which also works fine with system()) forces
1420interpretation of the arguments as a multivalued list, even if the
1421list had just one argument. That way you're safe from the shell
1422expanding wildcards or splitting up words with whitespace in them.
5a964f20
TC
1423
1424 @args = ( "echo surprise" );
1425
2b5ab1e7 1426 exec @args; # subject to shell escapes
f86cebdf 1427 # if @args == 1
2b5ab1e7 1428 exec { $args[0] } @args; # safe even with one-arg list
5a964f20
TC
1429
1430The first version, the one without the indirect object, ran the I<echo>
1431program, passing it C<"surprise"> an argument. The second version
1432didn't--it tried to run a program literally called I<"echo surprise">,
1433didn't find it, and set C<$?> to a non-zero value indicating failure.
1434
0f897271
GS
1435Beginning with v5.6.0, Perl will attempt to flush all files opened for
1436output before the exec, but this may not be supported on some platforms
1437(see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1438in English) or call the C<autoflush()> method of C<IO::Handle> on any
1439open handles in order to avoid lost output.
1440
19799a22 1441Note that C<exec> will not call your C<END> blocks, nor will it call
7660c0ab
A
1442any C<DESTROY> methods in your objects.
1443
a0d0e21e
LW
1444=item exists EXPR
1445
01020589 1446Given an expression that specifies a hash element or array element,
8ea97a1e
GS
1447returns true if the specified element in the hash or array has ever
1448been initialized, even if the corresponding value is undefined. The
1449element is not autovivified if it doesn't exist.
a0d0e21e 1450
01020589
GS
1451 print "Exists\n" if exists $hash{$key};
1452 print "Defined\n" if defined $hash{$key};
1453 print "True\n" if $hash{$key};
1454
1455 print "Exists\n" if exists $array[$index];
1456 print "Defined\n" if defined $array[$index];
1457 print "True\n" if $array[$index];
a0d0e21e 1458
8ea97a1e 1459A hash or array element can be true only if it's defined, and defined if
a0d0e21e
LW
1460it exists, but the reverse doesn't necessarily hold true.
1461
afebc493
GS
1462Given an expression that specifies the name of a subroutine,
1463returns true if the specified subroutine has ever been declared, even
1464if it is undefined. Mentioning a subroutine name for exists or defined
1465does not count as declaring it.
1466
1467 print "Exists\n" if exists &subroutine;
1468 print "Defined\n" if defined &subroutine;
1469
a0d0e21e 1470Note that the EXPR can be arbitrarily complicated as long as the final
afebc493 1471operation is a hash or array key lookup or subroutine name:
a0d0e21e 1472
2b5ab1e7
TC
1473 if (exists $ref->{A}->{B}->{$key}) { }
1474 if (exists $hash{A}{B}{$key}) { }
1475
01020589
GS
1476 if (exists $ref->{A}->{B}->[$ix]) { }
1477 if (exists $hash{A}{B}[$ix]) { }
1478
afebc493
GS
1479 if (exists &{$ref->{A}{B}{$key}}) { }
1480
01020589
GS
1481Although the deepest nested array or hash will not spring into existence
1482just because its existence was tested, any intervening ones will.
61eff3bc 1483Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
01020589
GS
1484into existence due to the existence test for the $key element above.
1485This happens anywhere the arrow operator is used, including even:
5a964f20 1486
2b5ab1e7
TC
1487 undef $ref;
1488 if (exists $ref->{"Some key"}) { }
1489 print $ref; # prints HASH(0x80d3d5c)
1490
1491This surprising autovivification in what does not at first--or even
1492second--glance appear to be an lvalue context may be fixed in a future
5a964f20 1493release.
a0d0e21e 1494
479ba383
GS
1495See L<perlref/"Pseudo-hashes: Using an array as a hash"> for specifics
1496on how exists() acts when used on a pseudo-hash.
e0478e5a 1497
afebc493
GS
1498Use of a subroutine call, rather than a subroutine name, as an argument
1499to exists() is an error.
1500
1501 exists &sub; # OK
1502 exists &sub(); # Error
1503
a0d0e21e
LW
1504=item exit EXPR
1505
2b5ab1e7 1506Evaluates EXPR and exits immediately with that value. Example:
a0d0e21e
LW
1507
1508 $ans = <STDIN>;
1509 exit 0 if $ans =~ /^[Xx]/;
1510
19799a22 1511See also C<die>. If EXPR is omitted, exits with C<0> status. The only
2b5ab1e7
TC
1512universally recognized values for EXPR are C<0> for success and C<1>
1513for error; other values are subject to interpretation depending on the
1514environment in which the Perl program is running. For example, exiting
151569 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1516the mailer to return the item undelivered, but that's not true everywhere.
a0d0e21e 1517
19799a22
GS
1518Don't use C<exit> to abort a subroutine if there's any chance that
1519someone might want to trap whatever error happened. Use C<die> instead,
1520which can be trapped by an C<eval>.
28757baa 1521
19799a22 1522The exit() function does not always exit immediately. It calls any
2b5ab1e7 1523defined C<END> routines first, but these C<END> routines may not
19799a22 1524themselves abort the exit. Likewise any object destructors that need to
2b5ab1e7
TC
1525be called are called before the real exit. If this is a problem, you
1526can call C<POSIX:_exit($status)> to avoid END and destructor processing.
87275199 1527See L<perlmod> for details.
5a964f20 1528
a0d0e21e
LW
1529=item exp EXPR
1530
54310121 1531=item exp
bbce6d69 1532
2b5ab1e7 1533Returns I<e> (the natural logarithm base) to the power of EXPR.
a0d0e21e
LW
1534If EXPR is omitted, gives C<exp($_)>.
1535
1536=item fcntl FILEHANDLE,FUNCTION,SCALAR
1537
f86cebdf 1538Implements the fcntl(2) function. You'll probably have to say
a0d0e21e
LW
1539
1540 use Fcntl;
1541
0ade1984 1542first to get the correct constant definitions. Argument processing and
19799a22 1543value return works just like C<ioctl> below.
a0d0e21e
LW
1544For example:
1545
1546 use Fcntl;
5a964f20
TC
1547 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1548 or die "can't fcntl F_GETFL: $!";
1549
19799a22 1550You don't have to check for C<defined> on the return from C<fnctl>.
951ba7fe
GS
1551Like C<ioctl>, it maps a C<0> return from the system call into
1552C<"0 but true"> in Perl. This string is true in boolean context and C<0>
2b5ab1e7
TC
1553in numeric context. It is also exempt from the normal B<-w> warnings
1554on improper numeric conversions.
5a964f20 1555
19799a22 1556Note that C<fcntl> will produce a fatal error if used on a machine that
2b5ab1e7
TC
1557doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1558manpage to learn what functions are available on your system.
a0d0e21e
LW
1559
1560=item fileno FILEHANDLE
1561
2b5ab1e7
TC
1562Returns the file descriptor for a filehandle, or undefined if the
1563filehandle is not open. This is mainly useful for constructing
19799a22 1564bitmaps for C<select> and low-level POSIX tty-handling operations.
2b5ab1e7
TC
1565If FILEHANDLE is an expression, the value is taken as an indirect
1566filehandle, generally its name.
5a964f20
TC
1567
1568You can use this to find out whether two handles refer to the
1569same underlying descriptor:
1570
1571 if (fileno(THIS) == fileno(THAT)) {
1572 print "THIS and THAT are dups\n";
1573 }
a0d0e21e
LW
1574
1575=item flock FILEHANDLE,OPERATION
1576
19799a22
GS
1577Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1578for success, false on failure. Produces a fatal error if used on a
2b5ab1e7 1579machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
19799a22 1580C<flock> is Perl's portable file locking interface, although it locks
2b5ab1e7
TC
1581only entire files, not records.
1582
1583Two potentially non-obvious but traditional C<flock> semantics are
1584that it waits indefinitely until the lock is granted, and that its locks
1585B<merely advisory>. Such discretionary locks are more flexible, but offer
19799a22
GS
1586fewer guarantees. This means that files locked with C<flock> may be
1587modified by programs that do not also use C<flock>. See L<perlport>,
2b5ab1e7
TC
1588your port's specific documentation, or your system-specific local manpages
1589for details. It's best to assume traditional behavior if you're writing
1590portable programs. (But if you're not, you should as always feel perfectly
1591free to write for your own system's idiosyncrasies (sometimes called
1592"features"). Slavish adherence to portability concerns shouldn't get
1593in the way of your getting your job done.)
a3cb178b 1594
8ebc5c01
PP
1595OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1596LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
ea3105be 1597you can use the symbolic names if you import them from the Fcntl module,
68dc0745
PP
1598either individually, or as a group using the ':flock' tag. LOCK_SH
1599requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
ea3105be
GS
1600releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1601LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
68dc0745
PP
1602waiting for the lock (check the return status to see if you got it).
1603
2b5ab1e7
TC
1604To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1605before locking or unlocking it.
8ebc5c01 1606
f86cebdf 1607Note that the emulation built with lockf(3) doesn't provide shared
8ebc5c01 1608locks, and it requires that FILEHANDLE be open with write intent. These
2b5ab1e7 1609are the semantics that lockf(3) implements. Most if not all systems
f86cebdf 1610implement lockf(3) in terms of fcntl(2) locking, though, so the
8ebc5c01
PP
1611differing semantics shouldn't bite too many people.
1612
19799a22
GS
1613Note also that some versions of C<flock> cannot lock things over the
1614network; you would need to use the more system-specific C<fcntl> for
f86cebdf
GS
1615that. If you like you can force Perl to ignore your system's flock(2)
1616function, and so provide its own fcntl(2)-based emulation, by passing
8ebc5c01
PP
1617the switch C<-Ud_flock> to the F<Configure> program when you configure
1618perl.
4633a7c4
LW
1619
1620Here's a mailbox appender for BSD systems.
a0d0e21e 1621
7e1af8bc 1622 use Fcntl ':flock'; # import LOCK_* constants
a0d0e21e
LW
1623
1624 sub lock {
7e1af8bc 1625 flock(MBOX,LOCK_EX);
a0d0e21e
LW
1626 # and, in case someone appended
1627 # while we were waiting...
1628 seek(MBOX, 0, 2);
1629 }
1630
1631 sub unlock {
7e1af8bc 1632 flock(MBOX,LOCK_UN);
a0d0e21e
LW
1633 }
1634
1635 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1636 or die "Can't open mailbox: $!";
1637
1638 lock();
1639 print MBOX $msg,"\n\n";
1640 unlock();
1641
2b5ab1e7
TC
1642On systems that support a real flock(), locks are inherited across fork()
1643calls, whereas those that must resort to the more capricious fcntl()
1644function lose the locks, making it harder to write servers.
1645
cb1a09d0 1646See also L<DB_File> for other flock() examples.
a0d0e21e
LW
1647
1648=item fork
1649
2b5ab1e7
TC
1650Does a fork(2) system call to create a new process running the
1651same program at the same point. It returns the child pid to the
1652parent process, C<0> to the child process, or C<undef> if the fork is
1653unsuccessful. File descriptors (and sometimes locks on those descriptors)
1654are shared, while everything else is copied. On most systems supporting
1655fork(), great care has gone into making it extremely efficient (for
1656example, using copy-on-write technology on data pages), making it the
1657dominant paradigm for multitasking over the last few decades.
5a964f20 1658
0f897271
GS
1659Beginning with v5.6.0, Perl will attempt to flush all files opened for
1660output before forking the child process, but this may not be supported
1661on some platforms (see L<perlport>). To be safe, you may need to set
1662C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1663C<IO::Handle> on any open handles in order to avoid duplicate output.
a0d0e21e 1664
19799a22 1665If you C<fork> without ever waiting on your children, you will
2b5ab1e7
TC
1666accumulate zombies. On some systems, you can avoid this by setting
1667C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1668forking and reaping moribund children.
cb1a09d0 1669
28757baa
PP
1670Note that if your forked child inherits system file descriptors like
1671STDIN and STDOUT that are actually connected by a pipe or socket, even
2b5ab1e7 1672if you exit, then the remote server (such as, say, a CGI script or a
19799a22 1673backgrounded job launched from a remote shell) won't think you're done.
2b5ab1e7 1674You should reopen those to F</dev/null> if it's any issue.
28757baa 1675
cb1a09d0
AD
1676=item format
1677
19799a22 1678Declare a picture format for use by the C<write> function. For
cb1a09d0
AD
1679example:
1680
54310121 1681 format Something =
cb1a09d0
AD
1682 Test: @<<<<<<<< @||||| @>>>>>
1683 $str, $%, '$' . int($num)
1684 .
1685
1686 $str = "widget";
184e9718 1687 $num = $cost/$quantity;
cb1a09d0
AD
1688 $~ = 'Something';
1689 write;
1690
1691See L<perlform> for many details and examples.
1692
8903cb82 1693=item formline PICTURE,LIST
a0d0e21e 1694
5a964f20 1695This is an internal function used by C<format>s, though you may call it,
a0d0e21e
LW
1696too. It formats (see L<perlform>) a list of values according to the
1697contents of PICTURE, placing the output into the format output
7660c0ab 1698accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
19799a22 1699Eventually, when a C<write> is done, the contents of
a0d0e21e 1700C<$^A> are written to some filehandle, but you could also read C<$^A>
7660c0ab 1701yourself and then set C<$^A> back to C<"">. Note that a format typically
19799a22 1702does one C<formline> per line of form, but the C<formline> function itself
748a9306 1703doesn't care how many newlines are embedded in the PICTURE. This means
4633a7c4 1704that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
748a9306
LW
1705You may therefore need to use multiple formlines to implement a single
1706record format, just like the format compiler.
1707
19799a22 1708Be careful if you put double quotes around the picture, because an C<@>
748a9306 1709character may be taken to mean the beginning of an array name.
19799a22 1710C<formline> always returns true. See L<perlform> for other examples.
a0d0e21e
LW
1711
1712=item getc FILEHANDLE
1713
1714=item getc
1715
1716Returns the next character from the input file attached to FILEHANDLE,
2b5ab1e7
TC
1717or the undefined value at end of file, or if there was an error.
1718If FILEHANDLE is omitted, reads from STDIN. This is not particularly
1719efficient. However, it cannot be used by itself to fetch single
1720characters without waiting for the user to hit enter. For that, try
1721something more like:
4633a7c4
LW
1722
1723 if ($BSD_STYLE) {
1724 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1725 }
1726 else {
54310121 1727 system "stty", '-icanon', 'eol', "\001";
4633a7c4
LW
1728 }
1729
1730 $key = getc(STDIN);
1731
1732 if ($BSD_STYLE) {
1733 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1734 }
1735 else {
5f05dabc 1736 system "stty", 'icanon', 'eol', '^@'; # ASCII null
4633a7c4
LW
1737 }
1738 print "\n";
1739
54310121
PP
1740Determination of whether $BSD_STYLE should be set
1741is left as an exercise to the reader.
cb1a09d0 1742
19799a22 1743The C<POSIX::getattr> function can do this more portably on
2b5ab1e7
TC
1744systems purporting POSIX compliance. See also the C<Term::ReadKey>
1745module from your nearest CPAN site; details on CPAN can be found on
1746L<perlmodlib/CPAN>.
a0d0e21e
LW
1747
1748=item getlogin
1749
5a964f20
TC
1750Implements the C library function of the same name, which on most
1751systems returns the current login from F</etc/utmp>, if any. If null,
19799a22 1752use C<getpwuid>.
a0d0e21e 1753
f86702cc 1754 $login = getlogin || getpwuid($<) || "Kilroy";
a0d0e21e 1755
19799a22
GS
1756Do not consider C<getlogin> for authentication: it is not as
1757secure as C<getpwuid>.
4633a7c4 1758
a0d0e21e
LW
1759=item getpeername SOCKET
1760
1761Returns the packed sockaddr address of other end of the SOCKET connection.
1762
4633a7c4
LW
1763 use Socket;
1764 $hersockaddr = getpeername(SOCK);
19799a22 1765 ($port, $iaddr) = sockaddr_in($hersockaddr);
4633a7c4
LW
1766 $herhostname = gethostbyaddr($iaddr, AF_INET);
1767 $herstraddr = inet_ntoa($iaddr);
a0d0e21e
LW
1768
1769=item getpgrp PID
1770
47e29363 1771Returns the current process group for the specified PID. Use
7660c0ab 1772a PID of C<0> to get the current process group for the
4633a7c4 1773current process. Will raise an exception if used on a machine that
f86cebdf 1774doesn't implement getpgrp(2). If PID is omitted, returns process
19799a22 1775group of current process. Note that the POSIX version of C<getpgrp>
7660c0ab 1776does not accept a PID argument, so only C<PID==0> is truly portable.
a0d0e21e
LW
1777
1778=item getppid
1779
1780Returns the process id of the parent process.
1781
1782=item getpriority WHICH,WHO
1783
4633a7c4
LW
1784Returns the current priority for a process, a process group, or a user.
1785(See L<getpriority(2)>.) Will raise a fatal exception if used on a
f86cebdf 1786machine that doesn't implement getpriority(2).
a0d0e21e
LW
1787
1788=item getpwnam NAME
1789
1790=item getgrnam NAME
1791
1792=item gethostbyname NAME
1793
1794=item getnetbyname NAME
1795
1796=item getprotobyname NAME
1797
1798=item getpwuid UID
1799
1800=item getgrgid GID
1801
1802=item getservbyname NAME,PROTO
1803
1804=item gethostbyaddr ADDR,ADDRTYPE
1805
1806=item getnetbyaddr ADDR,ADDRTYPE
1807
1808=item getprotobynumber NUMBER
1809
1810=item getservbyport PORT,PROTO
1811
1812=item getpwent
1813
1814=item getgrent
1815
1816=item gethostent
1817
1818=item getnetent
1819
1820=item getprotoent
1821
1822=item getservent
1823
1824=item setpwent
1825
1826=item setgrent
1827
1828=item sethostent STAYOPEN
1829
1830=item setnetent STAYOPEN
1831
1832=item setprotoent STAYOPEN
1833
1834=item setservent STAYOPEN
1835
1836=item endpwent
1837
1838=item endgrent
1839
1840=item endhostent
1841
1842=item endnetent
1843
1844=item endprotoent
1845
1846=item endservent
1847
1848These routines perform the same functions as their counterparts in the
5a964f20 1849system library. In list context, the return values from the
a0d0e21e
LW
1850various get routines are as follows:
1851
1852 ($name,$passwd,$uid,$gid,
6ee623d5 1853 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
a0d0e21e
LW
1854 ($name,$passwd,$gid,$members) = getgr*
1855 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1856 ($name,$aliases,$addrtype,$net) = getnet*
1857 ($name,$aliases,$proto) = getproto*
1858 ($name,$aliases,$port,$proto) = getserv*
1859
1860(If the entry doesn't exist you get a null list.)
1861
4602f195
JH
1862The exact meaning of the $gcos field varies but it usually contains
1863the real name of the user (as opposed to the login name) and other
1864information pertaining to the user. Beware, however, that in many
1865system users are able to change this information and therefore it
106325ad 1866cannot be trusted and therefore the $gcos is tainted (see
2959b6e3
JH
1867L<perlsec>). The $passwd and $shell, user's encrypted password and
1868login shell, are also tainted, because of the same reason.
4602f195 1869
5a964f20 1870In scalar context, you get the name, unless the function was a
a0d0e21e
LW
1871lookup by name, in which case you get the other thing, whatever it is.
1872(If the entry doesn't exist you get the undefined value.) For example:
1873
5a964f20
TC
1874 $uid = getpwnam($name);
1875 $name = getpwuid($num);
1876 $name = getpwent();
1877 $gid = getgrnam($name);
1878 $name = getgrgid($num;
1879 $name = getgrent();
1880 #etc.
a0d0e21e 1881
4602f195
JH
1882In I<getpw*()> the fields $quota, $comment, and $expire are special
1883cases in the sense that in many systems they are unsupported. If the
1884$quota is unsupported, it is an empty scalar. If it is supported, it
1885usually encodes the disk quota. If the $comment field is unsupported,
1886it is an empty scalar. If it is supported it usually encodes some
1887administrative comment about the user. In some systems the $quota
1888field may be $change or $age, fields that have to do with password
1889aging. In some systems the $comment field may be $class. The $expire
1890field, if present, encodes the expiration period of the account or the
1891password. For the availability and the exact meaning of these fields
1892in your system, please consult your getpwnam(3) documentation and your
1893F<pwd.h> file. You can also find out from within Perl what your
1894$quota and $comment fields mean and whether you have the $expire field
1895by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
1896C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
1897files are only supported if your vendor has implemented them in the
1898intuitive fashion that calling the regular C library routines gets the
5d3a0a3b
GS
1899shadow versions if you're running under privilege or if there exists
1900the shadow(3) functions as found in System V ( this includes Solaris
1901and Linux.) Those systems which implement a proprietary shadow password
1902facility are unlikely to be supported.
6ee623d5 1903
19799a22 1904The $members value returned by I<getgr*()> is a space separated list of
a0d0e21e
LW
1905the login names of the members of the group.
1906
1907For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1908C, it will be returned to you via C<$?> if the function call fails. The
7660c0ab 1909C<@addrs> value returned by a successful call is a list of the raw
a0d0e21e
LW
1910addresses returned by the corresponding system library call. In the
1911Internet domain, each address is four bytes long and you can unpack it
1912by saying something like:
1913
1914 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1915
2b5ab1e7
TC
1916The Socket library makes this slightly easier:
1917
1918 use Socket;
1919 $iaddr = inet_aton("127.1"); # or whatever address
1920 $name = gethostbyaddr($iaddr, AF_INET);
1921
1922 # or going the other way
19799a22 1923 $straddr = inet_ntoa($iaddr);
2b5ab1e7 1924
19799a22
GS
1925If you get tired of remembering which element of the return list
1926contains which return value, by-name interfaces are provided
1927in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
1928C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
1929and C<User::grent>. These override the normal built-ins, supplying
1930versions that return objects with the appropriate names
1931for each field. For example:
5a964f20
TC
1932
1933 use File::stat;
1934 use User::pwent;
1935 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
1936
1937Even though it looks like they're the same method calls (uid),
19799a22
GS
1938they aren't, because a C<File::stat> object is different from
1939a C<User::pwent> object.
5a964f20 1940
a0d0e21e
LW
1941=item getsockname SOCKET
1942
19799a22
GS
1943Returns the packed sockaddr address of this end of the SOCKET connection,
1944in case you don't know the address because you have several different
1945IPs that the connection might have come in on.
a0d0e21e 1946
4633a7c4
LW
1947 use Socket;
1948 $mysockaddr = getsockname(SOCK);
19799a22
GS
1949 ($port, $myaddr) = sockaddr_in($mysockaddr);
1950 printf "Connect to %s [%s]\n",
1951 scalar gethostbyaddr($myaddr, AF_INET),
1952 inet_ntoa($myaddr);
a0d0e21e
LW
1953
1954=item getsockopt SOCKET,LEVEL,OPTNAME
1955
5a964f20 1956Returns the socket option requested, or undef if there is an error.
a0d0e21e
LW
1957
1958=item glob EXPR
1959
0a753a76
PP
1960=item glob
1961
2b5ab1e7
TC
1962Returns the value of EXPR with filename expansions such as the
1963standard Unix shell F</bin/csh> would do. This is the internal function
61eff3bc
JH
1964implementing the C<< <*.c> >> operator, but you can use it directly.
1965If EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is
2b5ab1e7 1966discussed in more detail in L<perlop/"I/O Operators">.
a0d0e21e 1967
3a4b19e4
GS
1968Beginning with v5.6.0, this operator is implemented using the standard
1969C<File::Glob> extension. See L<File::Glob> for details.
1970
a0d0e21e
LW
1971=item gmtime EXPR
1972
48a26b3a 1973Converts a time as returned by the time function to a 8-element list
54310121 1974with the time localized for the standard Greenwich time zone.
4633a7c4 1975Typically used as follows:
a0d0e21e 1976
48a26b3a
GS
1977 # 0 1 2 3 4 5 6 7
1978 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday) =
a0d0e21e
LW
1979 gmtime(time);
1980
48a26b3a
GS
1981All list elements are numeric, and come straight out of the C `struct
1982tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
1983specified time. $mday is the day of the month, and $mon is the month
1984itself, in the range C<0..11> with 0 indicating January and 11
1985indicating December. $year is the number of years since 1900. That
1986is, $year is C<123> in year 2023. $wday is the day of the week, with
19870 indicating Sunday and 3 indicating Wednesday. $yday is the day of
874b1813 1988the year, in the range C<0..364> (or C<0..365> in leap years.)
48a26b3a
GS
1989
1990Note that the $year element is I<not> simply the last two digits of
1991the year. If you assume it is, then you create non-Y2K-compliant
1992programs--and you wouldn't want to do that, would you?
2f9daede 1993
abd75f24
GS
1994The proper way to get a complete 4-digit year is simply:
1995
1996 $year += 1900;
1997
1998And to get the last two digits of the year (e.g., '01' in 2001) do:
1999
2000 $year = sprintf("%02d", $year % 100);
2001
48a26b3a 2002If EXPR is omitted, C<gmtime()> uses the current time (C<gmtime(time)>).
a0d0e21e 2003
48a26b3a 2004In scalar context, C<gmtime()> returns the ctime(3) value:
0a753a76
PP
2005
2006 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
2007
19799a22 2008Also see the C<timegm> function provided by the C<Time::Local> module,
f86cebdf 2009and the strftime(3) function available via the POSIX module.
7660c0ab 2010
2b5ab1e7
TC
2011This scalar value is B<not> locale dependent (see L<perllocale>), but
2012is instead a Perl builtin. Also see the C<Time::Local> module, and the
2013strftime(3) and mktime(3) functions available via the POSIX module. To
7660c0ab
A
2014get somewhat similar but locale dependent date strings, set up your
2015locale environment variables appropriately (please see L<perllocale>)
2016and try for example:
2017
2018 use POSIX qw(strftime);
2b5ab1e7 2019 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
7660c0ab 2020
2b5ab1e7
TC
2021Note that the C<%a> and C<%b> escapes, which represent the short forms
2022of the day of the week and the month of the year, may not necessarily
2023be three characters wide in all locales.
0a753a76 2024
a0d0e21e
LW
2025=item goto LABEL
2026
748a9306
LW
2027=item goto EXPR
2028
a0d0e21e
LW
2029=item goto &NAME
2030
7660c0ab 2031The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
a0d0e21e 2032execution there. It may not be used to go into any construct that
7660c0ab 2033requires initialization, such as a subroutine or a C<foreach> loop. It
0a753a76 2034also can't be used to go into a construct that is optimized away,
19799a22 2035or to get out of a block or subroutine given to C<sort>.
0a753a76 2036It can be used to go almost anywhere else within the dynamic scope,
a0d0e21e 2037including out of subroutines, but it's usually better to use some other
19799a22 2038construct such as C<last> or C<die>. The author of Perl has never felt the
7660c0ab 2039need to use this form of C<goto> (in Perl, that is--C is another matter).
a0d0e21e 2040
7660c0ab
A
2041The C<goto-EXPR> form expects a label name, whose scope will be resolved
2042dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
748a9306
LW
2043necessarily recommended if you're optimizing for maintainability:
2044
2045 goto ("FOO", "BAR", "GLARCH")[$i];
2046
6cb9131c
GS
2047The C<goto-&NAME> form is quite different from the other forms of C<goto>.
2048In fact, it isn't a goto in the normal sense at all, and doesn't have
2049the stigma associated with other gotos. Instead, it
2050substitutes a call to the named subroutine for the currently running
2051subroutine. This is used by C<AUTOLOAD> subroutines that wish to load
2052another subroutine and then pretend that the other subroutine had been
2053called in the first place (except that any modifications to C<@_>
2054in the current subroutine are propagated to the other subroutine.)
2055After the C<goto>, not even C<caller> will be able to tell that this
2056routine was called first.
2057
2058NAME needn't be the name of a subroutine; it can be a scalar variable
2059containing a code reference, or a block which evaluates to a code
2060reference.
a0d0e21e
LW
2061
2062=item grep BLOCK LIST
2063
2064=item grep EXPR,LIST
2065
2b5ab1e7
TC
2066This is similar in spirit to, but not the same as, grep(1) and its
2067relatives. In particular, it is not limited to using regular expressions.
2f9daede 2068
a0d0e21e 2069Evaluates the BLOCK or EXPR for each element of LIST (locally setting
7660c0ab 2070C<$_> to each element) and returns the list value consisting of those
19799a22
GS
2071elements for which the expression evaluated to true. In scalar
2072context, returns the number of times the expression was true.
a0d0e21e
LW
2073
2074 @foo = grep(!/^#/, @bar); # weed out comments
2075
2076or equivalently,
2077
2078 @foo = grep {!/^#/} @bar; # weed out comments
2079
2b5ab1e7
TC
2080Note that, because C<$_> is a reference into the list value, it can
2081be used to modify the elements of the array. While this is useful and
2082supported, it can cause bizarre results if the LIST is not a named array.
2083Similarly, grep returns aliases into the original list, much as a for
2084loop's index variable aliases the list elements. That is, modifying an
19799a22
GS
2085element of a list returned by grep (for example, in a C<foreach>, C<map>
2086or another C<grep>) actually modifies the element in the original list.
2b5ab1e7 2087This is usually something to be avoided when writing clear code.
a0d0e21e 2088
19799a22 2089See also L</map> for a list composed of the results of the BLOCK or EXPR.
38325410 2090
a0d0e21e
LW
2091=item hex EXPR
2092
54310121 2093=item hex
bbce6d69 2094
2b5ab1e7
TC
2095Interprets EXPR as a hex string and returns the corresponding value.
2096(To convert strings that might start with either 0, 0x, or 0b, see
2097L</oct>.) If EXPR is omitted, uses C<$_>.
2f9daede
TPG
2098
2099 print hex '0xAf'; # prints '175'
2100 print hex 'aF'; # same
a0d0e21e 2101
19799a22 2102Hex strings may only represent integers. Strings that would cause
c6edd1b7 2103integer overflow trigger a warning.
19799a22 2104
a0d0e21e
LW
2105=item import
2106
19799a22 2107There is no builtin C<import> function. It is just an ordinary
4633a7c4 2108method (subroutine) defined (or inherited) by modules that wish to export
19799a22 2109names to another module. The C<use> function calls the C<import> method
54310121 2110for the package used. See also L</use()>, L<perlmod>, and L<Exporter>.
a0d0e21e
LW
2111
2112=item index STR,SUBSTR,POSITION
2113
2114=item index STR,SUBSTR
2115
2b5ab1e7
TC
2116The index function searches for one string within another, but without
2117the wildcard-like behavior of a full regular-expression pattern match.
2118It returns the position of the first occurrence of SUBSTR in STR at
2119or after POSITION. If POSITION is omitted, starts searching from the
2120beginning of the string. The return value is based at C<0> (or whatever
2121you've set the C<$[> variable to--but don't do that). If the substring
2122is not found, returns one less than the base, ordinarily C<-1>.
a0d0e21e
LW
2123
2124=item int EXPR
2125
54310121 2126=item int
bbce6d69 2127
7660c0ab 2128Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2b5ab1e7
TC
2129You should not use this function for rounding: one because it truncates
2130towards C<0>, and two because machine representations of floating point
2131numbers can sometimes produce counterintuitive results. For example,
2132C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2133because it's really more like -268.99999999999994315658 instead. Usually,
19799a22 2134the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2b5ab1e7 2135functions will serve you better than will int().
a0d0e21e
LW
2136
2137=item ioctl FILEHANDLE,FUNCTION,SCALAR
2138
2b5ab1e7 2139Implements the ioctl(2) function. You'll probably first have to say
a0d0e21e 2140
4633a7c4 2141 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
a0d0e21e 2142
2b5ab1e7 2143to get the correct function definitions. If F<ioctl.ph> doesn't
a0d0e21e 2144exist or doesn't have the correct definitions you'll have to roll your
61eff3bc 2145own, based on your C header files such as F<< <sys/ioctl.h> >>.
5a964f20 2146(There is a Perl script called B<h2ph> that comes with the Perl kit that
54310121 2147may help you in this, but it's nontrivial.) SCALAR will be read and/or
4633a7c4 2148written depending on the FUNCTION--a pointer to the string value of SCALAR
19799a22 2149will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
4633a7c4
LW
2150has no string value but does have a numeric value, that value will be
2151passed rather than a pointer to the string value. To guarantee this to be
19799a22
GS
2152true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2153functions may be needed to manipulate the values of structures used by
2154C<ioctl>.
a0d0e21e 2155
19799a22 2156The return value of C<ioctl> (and C<fcntl>) is as follows:
a0d0e21e
LW
2157
2158 if OS returns: then Perl returns:
2159 -1 undefined value
2160 0 string "0 but true"
2161 anything else that number
2162
19799a22 2163Thus Perl returns true on success and false on failure, yet you can
a0d0e21e
LW
2164still easily determine the actual value returned by the operating
2165system:
2166
2b5ab1e7 2167 $retval = ioctl(...) || -1;
a0d0e21e
LW
2168 printf "System returned %d\n", $retval;
2169
c2611fb3 2170The special string "C<0> but true" is exempt from B<-w> complaints
5a964f20
TC
2171about improper numeric conversions.
2172
19799a22
GS
2173Here's an example of setting a filehandle named C<REMOTE> to be
2174non-blocking at the system level. You'll have to negotiate C<$|>
2175on your own, though.
2176
2177 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
2178
2179 $flags = fcntl(REMOTE, F_GETFL, 0)
2180 or die "Can't get flags for the socket: $!\n";
2181
2182 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
2183 or die "Can't set flags for the socket: $!\n";
2184
a0d0e21e
LW
2185=item join EXPR,LIST
2186
2b5ab1e7
TC
2187Joins the separate strings of LIST into a single string with fields
2188separated by the value of EXPR, and returns that new string. Example:
a0d0e21e 2189
2b5ab1e7 2190 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
a0d0e21e 2191
eb6e2d6f
GS
2192Beware that unlike C<split>, C<join> doesn't take a pattern as its
2193first argument. Compare L</split>.
a0d0e21e 2194
aa689395
PP
2195=item keys HASH
2196
19799a22 2197Returns a list consisting of all the keys of the named hash. (In
1d2dff63 2198scalar context, returns the number of keys.) The keys are returned in
ab192400
GS
2199an apparently random order. The actual random order is subject to
2200change in future versions of perl, but it is guaranteed to be the same
19799a22 2201order as either the C<values> or C<each> function produces (given
ab192400
GS
2202that the hash has not been modified). As a side effect, it resets
2203HASH's iterator.
a0d0e21e 2204
aa689395 2205Here is yet another way to print your environment:
a0d0e21e
LW
2206
2207 @keys = keys %ENV;
2208 @values = values %ENV;
19799a22 2209 while (@keys) {
a0d0e21e
LW
2210 print pop(@keys), '=', pop(@values), "\n";
2211 }
2212
2213or how about sorted by key:
2214
2215 foreach $key (sort(keys %ENV)) {
2216 print $key, '=', $ENV{$key}, "\n";
2217 }
2218
19799a22 2219To sort a hash by value, you'll need to use a C<sort> function.
aa689395 2220Here's a descending numeric sort of a hash by its values:
4633a7c4 2221
5a964f20 2222 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
4633a7c4
LW
2223 printf "%4d %s\n", $hash{$key}, $key;
2224 }
2225
19799a22 2226As an lvalue C<keys> allows you to increase the number of hash buckets
aa689395
PP
2227allocated for the given hash. This can gain you a measure of efficiency if
2228you know the hash is going to get big. (This is similar to pre-extending
2229an array by assigning a larger number to $#array.) If you say
55497cff
PP
2230
2231 keys %hash = 200;
2232
ab192400
GS
2233then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2234in fact, since it rounds up to the next power of two. These
55497cff
PP
2235buckets will be retained even if you do C<%hash = ()>, use C<undef
2236%hash> if you want to free the storage while C<%hash> is still in scope.
2237You can't shrink the number of buckets allocated for the hash using
19799a22 2238C<keys> in this way (but you needn't worry about doing this by accident,
55497cff
PP
2239as trying has no effect).
2240
19799a22 2241See also C<each>, C<values> and C<sort>.
ab192400 2242
b350dd2f 2243=item kill SIGNAL, LIST
a0d0e21e 2244
b350dd2f 2245Sends a signal to a list of processes. Returns the number of
517db077
GS
2246processes successfully signaled (which is not necessarily the
2247same as the number actually killed).
a0d0e21e
LW
2248
2249 $cnt = kill 1, $child1, $child2;
2250 kill 9, @goners;
2251
b350dd2f
GS
2252If SIGNAL is zero, no signal is sent to the process. This is a
2253useful way to check that the process is alive and hasn't changed
2254its UID. See L<perlport> for notes on the portability of this
2255construct.
2256
2257Unlike in the shell, if SIGNAL is negative, it kills
4633a7c4
LW
2258process groups instead of processes. (On System V, a negative I<PROCESS>
2259number will also kill process groups, but that's not portable.) That
2260means you usually want to use positive not negative signals. You may also
da0045b7 2261use a signal name in quotes. See L<perlipc/"Signals"> for details.
a0d0e21e
LW
2262
2263=item last LABEL
2264
2265=item last
2266
2267The C<last> command is like the C<break> statement in C (as used in
2268loops); it immediately exits the loop in question. If the LABEL is
2269omitted, the command refers to the innermost enclosing loop. The
2270C<continue> block, if any, is not executed:
2271
4633a7c4
LW
2272 LINE: while (<STDIN>) {
2273 last LINE if /^$/; # exit when done with header
5a964f20 2274 #...
a0d0e21e
LW
2275 }
2276
4968c1e4 2277C<last> cannot be used to exit a block which returns a value such as
2b5ab1e7
TC
2278C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2279a grep() or map() operation.
4968c1e4 2280
6c1372ed
GS
2281Note that a block by itself is semantically identical to a loop
2282that executes once. Thus C<last> can be used to effect an early
2283exit out of such a block.
2284
98293880
JH
2285See also L</continue> for an illustration of how C<last>, C<next>, and
2286C<redo> work.
1d2dff63 2287
a0d0e21e
LW
2288=item lc EXPR
2289
54310121 2290=item lc
bbce6d69 2291
a0d0e21e 2292Returns an lowercased version of EXPR. This is the internal function
7660c0ab 2293implementing the C<\L> escape in double-quoted strings.
19799a22
GS
2294Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
2295and L<utf8>.
a0d0e21e 2296
7660c0ab 2297If EXPR is omitted, uses C<$_>.
bbce6d69 2298
a0d0e21e
LW
2299=item lcfirst EXPR
2300
54310121 2301=item lcfirst
bbce6d69 2302
a0d0e21e 2303Returns the value of EXPR with the first character lowercased. This is
7660c0ab 2304the internal function implementing the C<\l> escape in double-quoted strings.
a0ed51b3 2305Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
a0d0e21e 2306
7660c0ab 2307If EXPR is omitted, uses C<$_>.
bbce6d69 2308
a0d0e21e
LW
2309=item length EXPR
2310
54310121 2311=item length
bbce6d69 2312
a0ed51b3 2313Returns the length in characters of the value of EXPR. If EXPR is
2b5ab1e7
TC
2314omitted, returns length of C<$_>. Note that this cannot be used on
2315an entire array or hash to find out how many elements these have.
2316For that, use C<scalar @array> and C<scalar keys %hash> respectively.
a0d0e21e
LW
2317
2318=item link OLDFILE,NEWFILE
2319
19799a22
GS
2320Creates a new filename linked to the old filename. Returns true for
2321success, false otherwise.
a0d0e21e
LW
2322
2323=item listen SOCKET,QUEUESIZE
2324
19799a22
GS
2325Does the same thing that the listen system call does. Returns true if
2326it succeeded, false otherwise. See the example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
2327
2328=item local EXPR
2329
19799a22 2330You really probably want to be using C<my> instead, because C<local> isn't
2b5ab1e7
TC
2331what most people think of as "local". See L<perlsub/"Private Variables
2332via my()"> for details.
2333
5a964f20
TC
2334A local modifies the listed variables to be local to the enclosing
2335block, file, or eval. If more than one value is listed, the list must
2336be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2337for details, including issues with tied arrays and hashes.
a0d0e21e 2338
a0d0e21e
LW
2339=item localtime EXPR
2340
19799a22 2341Converts a time as returned by the time function to a 9-element list
5f05dabc 2342with the time analyzed for the local time zone. Typically used as
a0d0e21e
LW
2343follows:
2344
54310121 2345 # 0 1 2 3 4 5 6 7 8
a0d0e21e
LW
2346 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2347 localtime(time);
2348
48a26b3a
GS
2349All list elements are numeric, and come straight out of the C `struct
2350tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2351specified time. $mday is the day of the month, and $mon is the month
2352itself, in the range C<0..11> with 0 indicating January and 11
2353indicating December. $year is the number of years since 1900. That
2354is, $year is C<123> in year 2023. $wday is the day of the week, with
23550 indicating Sunday and 3 indicating Wednesday. $yday is the day of
874b1813 2356the year, in the range C<0..364> (or C<0..365> in leap years.) $isdst
48a26b3a
GS
2357is true if the specified time occurs during daylight savings time,
2358false otherwise.
2359
2360Note that the $year element is I<not> simply the last two digits of
2361the year. If you assume it is, then you create non-Y2K-compliant
2362programs--and you wouldn't want to do that, would you?
54310121 2363
abd75f24
GS
2364The proper way to get a complete 4-digit year is simply:
2365
2366 $year += 1900;
2367
2368And to get the last two digits of the year (e.g., '01' in 2001) do:
2369
2370 $year = sprintf("%02d", $year % 100);
2371
48a26b3a 2372If EXPR is omitted, C<localtime()> uses the current time (C<localtime(time)>).
a0d0e21e 2373
48a26b3a 2374In scalar context, C<localtime()> returns the ctime(3) value:
a0d0e21e 2375
5f05dabc 2376 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
a0d0e21e 2377
a3cb178b 2378This scalar value is B<not> locale dependent, see L<perllocale>, but
68f8bed4
JH
2379instead a Perl builtin. Also see the C<Time::Local> module
2380(to convert the second, minutes, hours, ... back to seconds since the
2381stroke of midnight the 1st of January 1970, the value returned by
ca6e1c26 2382time()), and the strftime(3) and mktime(3) functions available via the
68f8bed4
JH
2383POSIX module. To get somewhat similar but locale dependent date
2384strings, set up your locale environment variables appropriately
2385(please see L<perllocale>) and try for example:
a3cb178b 2386
5a964f20 2387 use POSIX qw(strftime);
2b5ab1e7 2388 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
a3cb178b
GS
2389
2390Note that the C<%a> and C<%b>, the short forms of the day of the week
2391and the month of the year, may not necessarily be three characters wide.
a0d0e21e 2392
19799a22
GS
2393=item lock
2394
2395 lock I<THING>
2396
2397This function places an advisory lock on a variable, subroutine,
2398or referenced object contained in I<THING> until the lock goes out
2399of scope. This is a built-in function only if your version of Perl
2400was built with threading enabled, and if you've said C<use Threads>.
2401Otherwise a user-defined function by this name will be called. See
2402L<Thread>.
2403
a0d0e21e
LW
2404=item log EXPR
2405
54310121 2406=item log
bbce6d69 2407
2b5ab1e7
TC
2408Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2409returns log of C<$_>. To get the log of another base, use basic algebra:
19799a22 2410The base-N log of a number is equal to the natural log of that number
2b5ab1e7
TC
2411divided by the natural log of N. For example:
2412
2413 sub log10 {
2414 my $n = shift;
2415 return log($n)/log(10);
2416 }
2417
2418See also L</exp> for the inverse operation.
a0d0e21e
LW
2419
2420=item lstat FILEHANDLE
2421
2422=item lstat EXPR
2423
54310121 2424=item lstat
bbce6d69 2425
19799a22 2426Does the same thing as the C<stat> function (including setting the
5a964f20
TC
2427special C<_> filehandle) but stats a symbolic link instead of the file
2428the symbolic link points to. If symbolic links are unimplemented on
19799a22 2429your system, a normal C<stat> is done.
a0d0e21e 2430
7660c0ab 2431If EXPR is omitted, stats C<$_>.
bbce6d69 2432
a0d0e21e
LW
2433=item m//
2434
2435The match operator. See L<perlop>.
2436
2437=item map BLOCK LIST
2438
2439=item map EXPR,LIST
2440
19799a22
GS
2441Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2442C<$_> to each element) and returns the list value composed of the
2443results of each such evaluation. In scalar context, returns the
2444total number of elements so generated. Evaluates BLOCK or EXPR in
2445list context, so each element of LIST may produce zero, one, or
2446more elements in the returned value.
dd99ebda 2447
a0d0e21e
LW
2448 @chars = map(chr, @nums);
2449
2450translates a list of numbers to the corresponding characters. And
2451
4633a7c4 2452 %hash = map { getkey($_) => $_ } @array;
a0d0e21e
LW
2453
2454is just a funny way to write
2455
2456 %hash = ();
2457 foreach $_ (@array) {
4633a7c4 2458 $hash{getkey($_)} = $_;
a0d0e21e
LW
2459 }
2460
2b5ab1e7
TC
2461Note that, because C<$_> is a reference into the list value, it can
2462be used to modify the elements of the array. While this is useful and
2463supported, it can cause bizarre results if the LIST is not a named array.
2464Using a regular C<foreach> loop for this purpose would be clearer in
2465most cases. See also L</grep> for an array composed of those items of
2466the original list for which the BLOCK or EXPR evaluates to true.
fb73857a 2467
19799a22 2468=item mkdir FILENAME,MASK
a0d0e21e 2469
5a211162
GS
2470=item mkdir FILENAME
2471
0591cd52 2472Creates the directory specified by FILENAME, with permissions
19799a22
GS
2473specified by MASK (as modified by C<umask>). If it succeeds it
2474returns true, otherwise it returns false and sets C<$!> (errno).
5a211162 2475If omitted, MASK defaults to 0777.
0591cd52 2476
19799a22 2477In general, it is better to create directories with permissive MASK,
0591cd52 2478and let the user modify that with their C<umask>, than it is to supply
19799a22 2479a restrictive MASK and give the user no way to be more permissive.
0591cd52
NT
2480The exceptions to this rule are when the file or directory should be
2481kept private (mail files, for instance). The perlfunc(1) entry on
19799a22 2482C<umask> discusses the choice of MASK in more detail.
a0d0e21e
LW
2483
2484=item msgctl ID,CMD,ARG
2485
f86cebdf 2486Calls the System V IPC function msgctl(2). You'll probably have to say
0ade1984
JH
2487
2488 use IPC::SysV;
2489
7660c0ab
A
2490first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2491then ARG must be a variable which will hold the returned C<msqid_ds>
951ba7fe
GS
2492structure. Returns like C<ioctl>: the undefined value for error,
2493C<"0 but true"> for zero, or the actual return value otherwise. See also
19799a22 2494C<IPC::SysV> and C<IPC::Semaphore> documentation.
a0d0e21e
LW
2495
2496=item msgget KEY,FLAGS
2497
f86cebdf 2498Calls the System V IPC function msgget(2). Returns the message queue
7660c0ab 2499id, or the undefined value if there is an error. See also C<IPC::SysV>
19799a22 2500and C<IPC::Msg> documentation.
a0d0e21e 2501
a0d0e21e
LW
2502=item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2503
2504Calls the System V IPC function msgrcv to receive a message from
2505message queue ID into variable VAR with a maximum message size of
41d6edb2
JH
2506SIZE. Note that when a message is received, the message type as a
2507native long integer will be the first thing in VAR, followed by the
2508actual message. This packing may be opened with C<unpack("l! a*")>.
2509Taints the variable. Returns true if successful, or false if there is
2510an error. See also C<IPC::SysV> and C<IPC::SysV::Msg> documentation.
2511
2512=item msgsnd ID,MSG,FLAGS
2513
2514Calls the System V IPC function msgsnd to send the message MSG to the
2515message queue ID. MSG must begin with the native long integer message
2516type, and be followed by the length of the actual message, and finally
2517the message itself. This kind of packing can be achieved with
2518C<pack("l! a*", $type, $message)>. Returns true if successful,
2519or false if there is an error. See also C<IPC::SysV>
2520and C<IPC::SysV::Msg> documentation.
a0d0e21e
LW
2521
2522=item my EXPR
2523
09bef843
SB
2524=item my EXPR : ATTRIBUTES
2525
19799a22
GS
2526A C<my> declares the listed variables to be local (lexically) to the
2527enclosing block, file, or C<eval>. If
5f05dabc 2528more than one value is listed, the list must be placed in parentheses. See
cb1a09d0 2529L<perlsub/"Private Variables via my()"> for details.
4633a7c4 2530
a0d0e21e
LW
2531=item next LABEL
2532
2533=item next
2534
2535The C<next> command is like the C<continue> statement in C; it starts
2536the next iteration of the loop:
2537
4633a7c4
LW
2538 LINE: while (<STDIN>) {
2539 next LINE if /^#/; # discard comments
5a964f20 2540 #...
a0d0e21e
LW
2541 }
2542
2543Note that if there were a C<continue> block on the above, it would get
2544executed even on discarded lines. If the LABEL is omitted, the command
2545refers to the innermost enclosing loop.
2546
4968c1e4 2547C<next> cannot be used to exit a block which returns a value such as
2b5ab1e7
TC
2548C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2549a grep() or map() operation.
4968c1e4 2550
6c1372ed
GS
2551Note that a block by itself is semantically identical to a loop
2552that executes once. Thus C<next> will exit such a block early.
2553
98293880
JH
2554See also L</continue> for an illustration of how C<last>, C<next>, and
2555C<redo> work.
1d2dff63 2556
a0d0e21e
LW
2557=item no Module LIST
2558
7660c0ab 2559See the L</use> function, which C<no> is the opposite of.
a0d0e21e
LW
2560
2561=item oct EXPR
2562
54310121 2563=item oct
bbce6d69 2564
4633a7c4 2565Interprets EXPR as an octal string and returns the corresponding
4f19785b
WSI
2566value. (If EXPR happens to start off with C<0x>, interprets it as a
2567hex string. If EXPR starts off with C<0b>, it is interpreted as a
2568binary string.) The following will handle decimal, binary, octal, and
4633a7c4 2569hex in the standard Perl or C notation:
a0d0e21e
LW
2570
2571 $val = oct($val) if $val =~ /^0/;
2572
19799a22
GS
2573If EXPR is omitted, uses C<$_>. To go the other way (produce a number
2574in octal), use sprintf() or printf():
2575
2576 $perms = (stat("filename"))[2] & 07777;
2577 $oct_perms = sprintf "%lo", $perms;
2578
2579The oct() function is commonly used when a string such as C<644> needs
2580to be converted into a file mode, for example. (Although perl will
2581automatically convert strings into numbers as needed, this automatic
2582conversion assumes base 10.)
a0d0e21e 2583
1c1fc3ea 2584=item open FILEHANDLE,MODE,LIST
6170680b 2585
a0d0e21e
LW
2586=item open FILEHANDLE,EXPR
2587
2588=item open FILEHANDLE
2589
2590Opens the file whose filename is given by EXPR, and associates it with
5f05dabc 2591FILEHANDLE. If FILEHANDLE is an expression, its value is used as the
d6fd2b02
GS
2592name of the real filehandle wanted. (This is considered a symbolic
2593reference, so C<use strict 'refs'> should I<not> be in effect.)
2594
2595If EXPR is omitted, the scalar
5f05dabc 2596variable of the same name as the FILEHANDLE contains the filename.
19799a22
GS
2597(Note that lexical variables--those declared with C<my>--will not work
2598for this purpose; so if you're using C<my>, specify EXPR in your call
2b5ab1e7
TC
2599to open.) See L<perlopentut> for a kinder, gentler explanation of opening
2600files.
5f05dabc 2601
61eff3bc
JH
2602If MODE is C<< '<' >> or nothing, the file is opened for input.
2603If MODE is C<< '>' >>, the file is truncated and opened for
2604output, being created if necessary. If MODE is C<<< '>>' >>>,
fbb426e4 2605the file is opened for appending, again being created if necessary.
61eff3bc
JH
2606You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to indicate that
2607you want both read and write access to the file; thus C<< '+<' >> is almost
2608always preferred for read/write updates--the C<< '+>' >> mode would clobber the
5a964f20
TC
2609file first. You can't usually use either read-write mode for updating
2610textfiles, since they have variable length records. See the B<-i>
0591cd52
NT
2611switch in L<perlrun> for a better approach. The file is created with
2612permissions of C<0666> modified by the process' C<umask> value.
5a964f20 2613
61eff3bc
JH
2614These various prefixes correspond to the fopen(3) modes of C<'r'>, C<'r+'>,
2615C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
5f05dabc 2616
6170680b
IZ
2617In the 2-arguments (and 1-argument) form of the call the mode and
2618filename should be concatenated (in this order), possibly separated by
61eff3bc 2619spaces. It is possible to omit the mode if the mode is C<< '<' >>.
6170680b 2620
7660c0ab 2621If the filename begins with C<'|'>, the filename is interpreted as a
5a964f20 2622command to which output is to be piped, and if the filename ends with a
f244e06d
GS
2623C<'|'>, the filename is interpreted as a command which pipes output to
2624us. See L<perlipc/"Using open() for IPC">
19799a22 2625for more examples of this. (You are not allowed to C<open> to a command
5a964f20 2626that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
4a4eefd0
GS
2627and L<perlipc/"Bidirectional Communication with Another Process">
2628for alternatives.)
cb1a09d0 2629
6170680b
IZ
2630If MODE is C<'|-'>, the filename is interpreted as a
2631command to which output is to be piped, and if MODE is
2632C<'-|'>, the filename is interpreted as a command which pipes output to
2633us. In the 2-arguments (and 1-argument) form one should replace dash
2634(C<'-'>) with the command. See L<perlipc/"Using open() for IPC">
2635for more examples of this. (You are not allowed to C<open> to a command
2636that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2637and L<perlipc/"Bidirectional Communication"> for alternatives.)
2638
2639In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
61eff3bc 2640and opening C<< '>-' >> opens STDOUT.
6170680b
IZ
2641
2642Open returns
19799a22 2643nonzero upon success, the undefined value otherwise. If the C<open>
4633a7c4 2644involved a pipe, the return value happens to be the pid of the
54310121 2645subprocess.
cb1a09d0
AD
2646
2647If you're unfortunate enough to be running Perl on a system that
2648distinguishes between text files and binary files (modern operating
2649systems don't care), then you should check out L</binmode> for tips for
19799a22 2650dealing with this. The key distinction between systems that need C<binmode>
5a964f20
TC
2651and those that don't is their text file formats. Systems like Unix, MacOS, and
2652Plan9, which delimit lines with a single character, and which encode that
19799a22 2653character in C as C<"\n">, do not need C<binmode>. The rest need it.
cb1a09d0 2654
fb73857a 2655When opening a file, it's usually a bad idea to continue normal execution
19799a22
GS
2656if the request failed, so C<open> is frequently used in connection with
2657C<die>. Even if C<die> won't do what you want (say, in a CGI script,
fb73857a 2658where you want to make a nicely formatted error message (but there are
5a964f20 2659modules that can help with that problem)) you should always check
19799a22 2660the return value from opening a file. The infrequent exception is when
fb73857a
PP
2661working with an unopened filehandle is actually what you want to do.
2662
cb1a09d0 2663Examples:
a0d0e21e
LW
2664
2665 $ARTICLE = 100;
2666 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2667 while (<ARTICLE>) {...
2668
6170680b 2669 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
fb73857a 2670 # if the open fails, output is discarded
a0d0e21e 2671
6170680b 2672 open(DBASE, '+<', 'dbase.mine') # open for update
fb73857a 2673 or die "Can't open 'dbase.mine' for update: $!";
cb1a09d0 2674
6170680b
IZ
2675 open(DBASE, '+<dbase.mine') # ditto
2676 or die "Can't open 'dbase.mine' for update: $!";
2677
2678 open(ARTICLE, '-|', "caesar <$article") # decrypt article
fb73857a 2679 or die "Can't start caesar: $!";
a0d0e21e 2680
6170680b
IZ
2681 open(ARTICLE, "caesar <$article |") # ditto
2682 or die "Can't start caesar: $!";
2683
2684 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
fb73857a 2685 or die "Can't start sort: $!";
a0d0e21e
LW
2686
2687 # process argument list of files along with any includes
2688
2689 foreach $file (@ARGV) {
2690 process($file, 'fh00');
2691 }
2692
2693 sub process {
5a964f20 2694 my($filename, $input) = @_;
a0d0e21e
LW
2695 $input++; # this is a string increment
2696 unless (open($input, $filename)) {
2697 print STDERR "Can't open $filename: $!\n";
2698 return;
2699 }
2700
5a964f20 2701 local $_;
a0d0e21e
LW
2702 while (<$input>) { # note use of indirection
2703 if (/^#include "(.*)"/) {
2704 process($1, $input);
2705 next;
2706 }
5a964f20 2707 #... # whatever
a0d0e21e
LW
2708 }
2709 }
2710
2711You may also, in the Bourne shell tradition, specify an EXPR beginning
61eff3bc 2712with C<< '>&' >>, in which case the rest of the string is interpreted as the
5a964f20 2713name of a filehandle (or file descriptor, if numeric) to be
61eff3bc
JH
2714duped and opened. You may use C<&> after C<< > >>, C<<< >> >>>,
2715C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>. The
a0d0e21e 2716mode you specify should match the mode of the original filehandle.
184e9718 2717(Duping a filehandle does not take into account any existing contents of
6170680b
IZ
2718stdio buffers.) Duping file handles is not yet supported for 3-argument
2719open().
2720
a0d0e21e
LW
2721Here is a script that saves, redirects, and restores STDOUT and
2722STDERR:
2723
2724 #!/usr/bin/perl
5a964f20
TC
2725 open(OLDOUT, ">&STDOUT");
2726 open(OLDERR, ">&STDERR");
a0d0e21e 2727
6170680b
IZ
2728 open(STDOUT, '>', "foo.out") || die "Can't redirect stdout";
2729 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
a0d0e21e
LW
2730
2731 select(STDERR); $| = 1; # make unbuffered
2732 select(STDOUT); $| = 1; # make unbuffered
2733
2734 print STDOUT "stdout 1\n"; # this works for
2735 print STDERR "stderr 1\n"; # subprocesses too
2736
2737 close(STDOUT);
2738 close(STDERR);
2739
5a964f20
TC
2740 open(STDOUT, ">&OLDOUT");
2741 open(STDERR, ">&OLDERR");
a0d0e21e
LW
2742
2743 print STDOUT "stdout 2\n";
2744 print STDERR "stderr 2\n";
2745
61eff3bc 2746If you specify C<< '<&=N' >>, where C<N> is a number, then Perl will do an
19799a22 2747equivalent of C's C<fdopen> of that file descriptor; this is more
4633a7c4 2748parsimonious of file descriptors. For example:
a0d0e21e
LW
2749
2750 open(FILEHANDLE, "<&=$fd")
2751
4af147f6
CS
2752Note that this feature depends on the fdopen() C library function.
2753On many UNIX systems, fdopen() is known to fail when file descriptors
2754exceed a certain value, typically 255. If you need more file
2755descriptors than that, consider rebuilding Perl to use the C<sfio>
2756library.
2757
6170680b
IZ
2758If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
2759with 2-arguments (or 1-argument) form of open(), then
a0d0e21e 2760there is an implicit fork done, and the return value of open is the pid
7660c0ab 2761of the child within the parent process, and C<0> within the child
184e9718 2762process. (Use C<defined($pid)> to determine whether the open was successful.)
a0d0e21e
LW
2763The filehandle behaves normally for the parent, but i/o to that
2764filehandle is piped from/to the STDOUT/STDIN of the child process.
2765In the child process the filehandle isn't opened--i/o happens from/to
2766the new STDOUT or STDIN. Typically this is used like the normal
2767piped open when you want to exercise more control over just how the
2768pipe command gets executed, such as when you are running setuid, and
54310121 2769don't want to have to scan shell commands for metacharacters.
6170680b 2770The following triples are more or less equivalent:
a0d0e21e
LW
2771
2772 open(FOO, "|tr '[a-z]' '[A-Z]'");
6170680b
IZ
2773 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
2774 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
a0d0e21e
LW
2775
2776 open(FOO, "cat -n '$file'|");
6170680b
IZ
2777 open(FOO, '-|', "cat -n '$file'");
2778 open(FOO, '-|') || exec 'cat', '-n', $file;
a0d0e21e 2779
4633a7c4
LW
2780See L<perlipc/"Safe Pipe Opens"> for more examples of this.
2781
0f897271
GS
2782Beginning with v5.6.0, Perl will attempt to flush all files opened for
2783output before any operation that may do a fork, but this may not be
2784supported on some platforms (see L<perlport>). To be safe, you may need
2785to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
2786of C<IO::Handle> on any open handles.
2787
2788On systems that support a
45bc9206
GS
2789close-on-exec flag on files, the flag will be set for the newly opened
2790file descriptor as determined by the value of $^F. See L<perlvar/$^F>.
a0d0e21e 2791
0dccf244
CS
2792Closing any piped filehandle causes the parent process to wait for the
2793child to finish, and returns the status value in C<$?>.
2794
6170680b
IZ
2795The filename passed to 2-argument (or 1-argument) form of open()
2796will have leading and trailing
f86cebdf 2797whitespace deleted, and the normal redirection characters
5a964f20
TC
2798honored. This property, known as "magic open",
2799can often be used to good effect. A user could specify a filename of
7660c0ab 2800F<"rsh cat file |">, or you could change certain filenames as needed:
5a964f20
TC
2801
2802 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
2803 open(FH, $filename) or die "Can't open $filename: $!";
2804
6170680b
IZ
2805Use 3-argument form to open a file with arbitrary weird characters in it,
2806
2807 open(FOO, '<', $file);
2808
2809otherwise it's necessary to protect any leading and trailing whitespace:
5a964f20
TC
2810
2811 $file =~ s#^(\s)#./$1#;
2812 open(FOO, "< $file\0");
2813
6170680b 2814(this may not work on some bizzare filesystems). One should
106325ad 2815conscientiously choose between the I<magic> and 3-arguments form
6170680b
IZ
2816of open():
2817
2818 open IN, $ARGV[0];
2819
2820will allow the user to specify an argument of the form C<"rsh cat file |">,
2821but will not work on a filename which happens to have a trailing space, while
2822
2823 open IN, '<', $ARGV[0];
2824
2825will have exactly the opposite restrictions.
2826
19799a22 2827If you want a "real" C C<open> (see L<open(2)> on your system), then you
6170680b
IZ
2828should use the C<sysopen> function, which involves no such magic (but
2829may use subtly different filemodes than Perl open(), which is mapped
2830to C fopen()). This is
5a964f20
TC
2831another way to protect your filenames from interpretation. For example:
2832
2833 use IO::Handle;
2834 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
2835 or die "sysopen $path: $!";
2836 $oldfh = select(HANDLE); $| = 1; select($oldfh);
2837 print HANDLE "stuff $$\n");
2838 seek(HANDLE, 0, 0);
2839 print "File contains: ", <HANDLE>;
2840
7660c0ab
A
2841Using the constructor from the C<IO::Handle> package (or one of its
2842subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
5a964f20
TC
2843filehandles that have the scope of whatever variables hold references to
2844them, and automatically close whenever and however you leave that scope:
c07a80fd 2845
5f05dabc 2846 use IO::File;
5a964f20 2847 #...
c07a80fd
PP
2848 sub read_myfile_munged {
2849 my $ALL = shift;
5f05dabc 2850 my $handle = new IO::File;
c07a80fd
PP
2851 open($handle, "myfile") or die "myfile: $!";
2852 $first = <$handle>
2853 or return (); # Automatically closed here.
2854 mung $first or die "mung failed"; # Or here.
2855 return $first, <$handle> if $ALL; # Or here.
2856 $first; # Or here.
2857 }
2858
b687b08b 2859See L</seek> for some details about mixing reading and writing.
a0d0e21e
LW
2860
2861=item opendir DIRHANDLE,EXPR
2862
19799a22
GS
2863Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
2864C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
a0d0e21e
LW
2865DIRHANDLEs have their own namespace separate from FILEHANDLEs.
2866
2867=item ord EXPR
2868
54310121 2869=item ord
bbce6d69 2870
a0ed51b3 2871Returns the numeric (ASCII or Unicode) value of the first character of EXPR. If
7660c0ab 2872EXPR is omitted, uses C<$_>. For the reverse, see L</chr>.
2b5ab1e7 2873See L<utf8> for more about Unicode.
a0d0e21e 2874
77ca0c92
LW
2875=item our EXPR
2876
2877An C<our> declares the listed variables to be valid globals within
2878the enclosing block, file, or C<eval>. That is, it has the same
2879scoping rules as a "my" declaration, but does not create a local
2880variable. If more than one value is listed, the list must be placed
2881in parentheses. The C<our> declaration has no semantic effect unless
2882"use strict vars" is in effect, in which case it lets you use the
2883declared global variable without qualifying it with a package name.
2884(But only within the lexical scope of the C<our> declaration. In this
2885it differs from "use vars", which is package scoped.)
2886
f472eb5c
GS
2887An C<our> declaration declares a global variable that will be visible
2888across its entire lexical scope, even across package boundaries. The
2889package in which the variable is entered is determined at the point
2890of the declaration, not at the point of use. This means the following
2891behavior holds:
2892
2893 package Foo;
2894 our $bar; # declares $Foo::bar for rest of lexical scope
2895 $bar = 20;
2896
2897 package Bar;
2898 print $bar; # prints 20
2899
2900Multiple C<our> declarations in the same lexical scope are allowed
2901if they are in different packages. If they happened to be in the same
2902package, Perl will emit warnings if you have asked for them.
2903
2904 use warnings;
2905 package Foo;
2906 our $bar; # declares $Foo::bar for rest of lexical scope
2907 $bar = 20;
2908
2909 package Bar;
2910 our $bar = 30; # declares $Bar::bar for rest of lexical scope
2911 print $bar; # prints 30
2912
2913 our $bar; # emits warning
2914
a0d0e21e
LW
2915=item pack TEMPLATE,LIST
2916
2b6c5635
GS
2917Takes a LIST of values and converts it into a string using the rules
2918given by the TEMPLATE. The resulting string is the concatenation of
2919the converted values. Typically, each converted value looks
2920like its machine-level representation. For example, on 32-bit machines
2921a converted integer may be represented by a sequence of 4 bytes.
2922
2923The TEMPLATE is a
a0d0e21e
LW
2924sequence of characters that give the order and type of values, as
2925follows:
2926
5a929a98 2927 a A string with arbitrary binary data, will be null padded.
4375e838 2928 A An ASCII string, will be space padded.
5a929a98
VU
2929 Z A null terminated (asciz) string, will be null padded.
2930
2b6c5635
GS
2931 b A bit string (ascending bit order inside each byte, like vec()).
2932 B A bit string (descending bit order inside each byte).
a0d0e21e
LW
2933 h A hex string (low nybble first).
2934 H A hex string (high nybble first).
2935
2936 c A signed char value.
a0ed51b3 2937 C An unsigned char value. Only does bytes. See U for Unicode.
96e4d5b1 2938
a0d0e21e
LW
2939 s A signed short value.
2940 S An unsigned short value.
96e4d5b1 2941 (This 'short' is _exactly_ 16 bits, which may differ from
851646ae
JH
2942 what a local C compiler calls 'short'. If you want
2943 native-length shorts, use the '!' suffix.)
96e4d5b1 2944
a0d0e21e
LW
2945 i A signed integer value.
2946 I An unsigned integer value.
19799a22 2947 (This 'integer' is _at_least_ 32 bits wide. Its exact
f86cebdf
GS
2948 size depends on what a local C compiler calls 'int',
2949 and may even be larger than the 'long' described in
2950 the next item.)
96e4d5b1 2951
a0d0e21e
LW
2952 l A signed long value.
2953 L An unsigned long value.
96e4d5b1 2954 (This 'long' is _exactly_ 32 bits, which may differ from
851646ae
JH
2955 what a local C compiler calls 'long'. If you want
2956 native-length longs, use the '!' suffix.)
a0d0e21e 2957
5d11dd56
G
2958 n An unsigned short in "network" (big-endian) order.
2959 N An unsigned long in "network" (big-endian) order.
2960 v An unsigned short in "VAX" (little-endian) order.
2961 V An unsigned long in "VAX" (little-endian) order.
96e4d5b1
PP
2962 (These 'shorts' and 'longs' are _exactly_ 16 bits and
2963 _exactly_ 32 bits, respectively.)
a0d0e21e 2964
dae0da7a
JH
2965 q A signed quad (64-bit) value.
2966 Q An unsigned quad value.
851646ae
JH
2967 (Quads are available only if your system supports 64-bit
2968 integer values _and_ if Perl has been compiled to support those.
dae0da7a
JH
2969 Causes a fatal error otherwise.)
2970
a0d0e21e
LW
2971 f A single-precision float in the native format.
2972 d A double-precision float in the native format.
2973
2974 p A pointer to a null-terminated string.
2975 P A pointer to a structure (fixed-length string).
2976
2977 u A uuencoded string.
a0ed51b3
LW
2978 U A Unicode character number. Encodes to UTF-8 internally.
2979 Works even if C<use utf8> is not in effect.
a0d0e21e 2980
96e4d5b1 2981 w A BER compressed integer. Its bytes represent an unsigned
f86cebdf
GS
2982 integer in base 128, most significant digit first, with as
2983 few digits as possible. Bit eight (the high bit) is set
2984 on each byte except the last.
def98dd4 2985
a0d0e21e
LW
2986 x A null byte.
2987 X Back up a byte.
2988 @ Null fill to absolute position.
2989
5a929a98
VU
2990The following rules apply:
2991
2992=over 8
2993
2994=item *
2995
5a964f20 2996Each letter may optionally be followed by a number giving a repeat
951ba7fe
GS
2997count. With all types except C<a>, C<A>, C<Z>, C<b>, C<B>, C<h>,
2998C<H>, and C<P> the pack function will gobble up that many values from
5a929a98 2999the LIST. A C<*> for the repeat count means to use however many items are
951ba7fe
GS
3000left, except for C<@>, C<x>, C<X>, where it is equivalent
3001to C<0>, and C<u>, where it is equivalent to 1 (or 45, what is the
2b6c5635
GS
3002same).
3003
951ba7fe 3004When used with C<Z>, C<*> results in the addition of a trailing null
2b6c5635
GS
3005byte (so the packed result will be one longer than the byte C<length>
3006of the item).
3007
951ba7fe 3008The repeat count for C<u> is interpreted as the maximal number of bytes
2b6c5635 3009to encode per line of output, with 0 and 1 replaced by 45.
5a929a98
VU
3010
3011=item *
3012
951ba7fe 3013The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
5a929a98 3014string of length count, padding with nulls or spaces as necessary. When
951ba7fe
GS
3015unpacking, C<A> strips trailing spaces and nulls, C<Z> strips everything
3016after the first null, and C<a> returns data verbatim. When packing,
3017C<a>, and C<Z> are equivalent.
2b6c5635
GS
3018
3019If the value-to-pack is too long, it is truncated. If too long and an
951ba7fe
GS
3020explicit count is provided, C<Z> packs only C<$count-1> bytes, followed
3021by a null byte. Thus C<Z> always packs a trailing null byte under
2b6c5635 3022all circumstances.
5a929a98
VU
3023
3024=item *
3025
951ba7fe 3026Likewise, the C<b> and C<B> fields pack a string that many bits long.
c73032f5
IZ
3027Each byte of the input field of pack() generates 1 bit of the result.
3028Each result bit is based on the least-significant bit of the corresponding
3029input byte, i.e., on C<ord($byte)%2>. In particular, bytes C<"0"> and
3030C<"1"> generate bits 0 and 1, as do bytes C<"\0"> and C<"\1">.
3031
3032Starting from the beginning of the input string of pack(), each 8-tuple
951ba7fe 3033of bytes is converted to 1 byte of output. With format C<b>
c73032f5 3034the first byte of the 8-tuple determines the least-significant bit of a
951ba7fe 3035byte, and with format C<B> it determines the most-significant bit of
c73032f5
IZ
3036a byte.
3037
3038If the length of the input string is not exactly divisible by 8, the
3039remainder is packed as if the input string were padded by null bytes
3040at the end. Similarly, during unpack()ing the "extra" bits are ignored.
3041
3042If the input string of pack() is longer than needed, extra bytes are ignored.
2b6c5635
GS
3043A C<*> for the repeat count of pack() means to use all the bytes of
3044the input field. On unpack()ing the bits are converted to a string
3045of C<"0">s and C<"1">s.
5a929a98
VU
3046
3047=item *
3048
951ba7fe 3049The C<h> and C<H> fields pack a string that many nybbles (4-bit groups,
851646ae 3050representable as hexadecimal digits, 0-9a-f) long.
5a929a98 3051
c73032f5
IZ
3052Each byte of the input field of pack() generates 4 bits of the result.
3053For non-alphabetical bytes the result is based on the 4 least-significant
3054bits of the input byte, i.e., on C<ord($byte)%16>. In particular,
3055bytes C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
3056C<"\0"> and C<"\1">. For bytes C<"a".."f"> and C<"A".."F"> the result
3057is compatible with the usual hexadecimal digits, so that C<"a"> and
3058C<"A"> both generate the nybble C<0xa==10>. The result for bytes
3059C<"g".."z"> and C<"G".."Z"> is not well-defined.
3060
3061Starting from the beginning of the input string of pack(), each pair
951ba7fe 3062of bytes is converted to 1 byte of output. With format C<h> the
c73032f5 3063first byte of the pair determines the least-significant nybble of the
951ba7fe 3064output byte, and with format C<H> it determines the most-significant
c73032f5
IZ
3065nybble.
3066
3067If the length of the input string is not even, it behaves as if padded
3068by a null byte at the end. Similarly, during unpack()ing the "extra"
3069nybbles are ignored.
3070
3071If the input string of pack() is longer than needed, extra bytes are ignored.
3072A C<*> for the repeat count of pack() means to use all the bytes of
3073the input field. On unpack()ing the bits are converted to a string
3074of hexadecimal digits.
3075
5a929a98
VU
3076=item *
3077
951ba7fe 3078The C<p> type packs a pointer to a null-terminated string. You are
5a929a98
VU
3079responsible for ensuring the string is not a temporary value (which can
3080potentially get deallocated before you get around to using the packed result).
951ba7fe
GS
3081The C<P> type packs a pointer to a structure of the size indicated by the
3082length. A NULL pointer is created if the corresponding value for C<p> or
3083C<P> is C<undef>, similarly for unpack().
5a929a98
VU
3084
3085=item *
3086
951ba7fe
GS
3087The C</> template character allows packing and unpacking of strings where
3088the packed structure contains a byte count followed by the string itself.
17f4a12d 3089You write I<length-item>C</>I<string-item>.
43192e07
IP
3090
3091The I<length-item> can be any C<pack> template letter,
3092and describes how the length value is packed.
3093The ones likely to be of most use are integer-packing ones like
951ba7fe
GS
3094C<n> (for Java strings), C<w> (for ASN.1 or SNMP)
3095and C<N> (for Sun XDR).
43192e07
IP
3096
3097The I<string-item> must, at present, be C<"A*">, C<"a*"> or C<"Z*">.
3098For C<unpack> the length of the string is obtained from the I<length-item>,
3099but if you put in the '*' it will be ignored.
3100
17f4a12d
IZ
3101 unpack 'C/a', "\04Gurusamy"; gives 'Guru'
3102 unpack 'a3/A* A*', '007 Bond J '; gives (' Bond','J')
3103 pack 'n/a* w/a*','hello,','world'; gives "\000\006hello,\005world"
43192e07
IP
3104
3105The I<length-item> is not returned explicitly from C<unpack>.
3106
951ba7fe
GS
3107Adding a count to the I<length-item> letter is unlikely to do anything
3108useful, unless that letter is C<A>, C<a> or C<Z>. Packing with a
3109I<length-item> of C<a> or C<Z> may introduce C<"\000"> characters,
43192e07
IP
3110which Perl does not regard as legal in numeric strings.
3111
3112=item *
3113
951ba7fe
GS
3114The integer types C<s>, C<S>, C<l>, and C<L> may be
3115immediately followed by a C<!> suffix to signify native shorts or
3116longs--as you can see from above for example a bare C<l> does mean
851646ae
JH
3117exactly 32 bits, the native C<long> (as seen by the local C compiler)
3118may be larger. This is an issue mainly in 64-bit platforms. You can
951ba7fe 3119see whether using C<!> makes any difference by
726ea183 3120
4d0c1c44
GS
3121 print length(pack("s")), " ", length(pack("s!")), "\n";
3122 print length(pack("l")), " ", length(pack("l!")), "\n";
ef54e1a4 3123
951ba7fe
GS
3124C<i!> and C<I!> also work but only because of completeness;
3125they are identical to C<i> and C<I>.
ef54e1a4 3126
19799a22
GS
3127The actual sizes (in bytes) of native shorts, ints, longs, and long
3128longs on the platform where Perl was built are also available via
3129L<Config>:
3130
3131 use Config;
3132 print $Config{shortsize}, "\n";
3133 print $Config{intsize}, "\n";
3134 print $Config{longsize}, "\n";
3135 print $Config{longlongsize}, "\n";
ef54e1a4 3136
5074e145 3137(The C<$Config{longlongsize}> will be undefine if your system does
851646ae
JH
3138not support long longs.)
3139
ef54e1a4
JH
3140=item *
3141
951ba7fe 3142The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, and C<L>
ef54e1a4
JH
3143are inherently non-portable between processors and operating systems
3144because they obey the native byteorder and endianness. For example a
140cb37e 31454-byte integer 0x12345678 (305419896 decimal) be ordered natively
ef54e1a4 3146(arranged in and handled by the CPU registers) into bytes as
61eff3bc 3147
719a3cf5
JH
3148 0x12 0x34 0x56 0x78 # little-endian
3149 0x78 0x56 0x34 0x12 # big-endian
61eff3bc 3150
5d11dd56 3151Basically, the Intel, Alpha, and VAX CPUs are little-endian, while
719a3cf5
JH
3152everybody else, for example Motorola m68k/88k, PPC, Sparc, HP PA,
3153Power, and Cray are big-endian. MIPS can be either: Digital used it
19799a22 3154in little-endian mode; SGI uses it in big-endian mode.
719a3cf5 3155
19799a22 3156The names `big-endian' and `little-endian' are comic references to
ef54e1a4
JH
3157the classic "Gulliver's Travels" (via the paper "On Holy Wars and a
3158Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and
19799a22 3159the egg-eating habits of the Lilliputians.
61eff3bc 3160
140cb37e 3161Some systems may have even weirder byte orders such as
61eff3bc 3162
ef54e1a4
JH
3163 0x56 0x78 0x12 0x34
3164 0x34 0x12 0x78 0x56
61eff3bc 3165
ef54e1a4
JH
3166You can see your system's preference with
3167
3168 print join(" ", map { sprintf "%#02x", $_ }
3169 unpack("C*",pack("L",0x12345678))), "\n";
3170
d99ad34e 3171The byteorder on the platform where Perl was built is also available
726ea183 3172via L<Config>:
ef54e1a4
JH
3173
3174 use Config;
3175 print $Config{byteorder}, "\n";
3176
d99ad34e
JH
3177Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'>
3178and C<'87654321'> are big-endian.
719a3cf5 3179
951ba7fe
GS
3180If you want portable packed integers use the formats C<n>, C<N>,
3181C<v>, and C<V>, their byte endianness and size is known.
851646ae 3182See also L<perlport>.
ef54e1a4
JH
3183
3184=item *
3185
5a929a98
VU
3186Real numbers (floats and doubles) are in the native machine format only;
3187due to the multiplicity of floating formats around, and the lack of a
3188standard "network" representation, no facility for interchange has been
3189made. This means that packed floating point data written on one machine
3190may not be readable on another - even if both use IEEE floating point
3191arithmetic (as the endian-ness of the memory representation is not part
851646ae 3192of the IEEE spec). See also L<perlport>.
5a929a98
VU
3193
3194Note that Perl uses doubles internally for all numeric calculation, and
3195converting from double into float and thence back to double again will
3196lose precision (i.e., C<unpack("f", pack("f", $foo)>) will not in general
19799a22 3197equal $foo).
5a929a98 3198
851646ae
JH
3199=item *
3200
3201You must yourself do any alignment or padding by inserting for example
9ccd05c0
JH
3202enough C<'x'>es while packing. There is no way to pack() and unpack()
3203could know where the bytes are going to or coming from. Therefore
3204C<pack> (and C<unpack>) handle their output and input as flat
3205sequences of bytes.
851646ae 3206
17f4a12d
IZ
3207=item *
3208
3209A comment in a TEMPLATE starts with C<#> and goes to the end of line.
3210
2b6c5635
GS
3211=item *
3212
3213If TEMPLATE requires more arguments to pack() than actually given, pack()
3214assumes additional C<""> arguments. If TEMPLATE requires less arguments
3215to pack() than actually given, extra arguments are ignored.
3216
5a929a98 3217=back
a0d0e21e
LW
3218
3219Examples:
3220
a0ed51b3 3221 $foo = pack("CCCC",65,66,67,68);
a0d0e21e 3222 # foo eq "ABCD"
a0ed51b3 3223 $foo = pack("C4",65,66,67,68);
a0d0e21e 3224 # same thing
a0ed51b3
LW
3225 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
3226 # same thing with Unicode circled letters
a0d0e21e
LW
3227
3228 $foo = pack("ccxxcc",65,66,67,68);
3229 # foo eq "AB\0\0CD"
3230
9ccd05c0
JH
3231 # note: the above examples featuring "C" and "c" are true
3232 # only on ASCII and ASCII-derived systems such as ISO Latin 1
3233 # and UTF-8. In EBCDIC the first example would be
3234 # $foo = pack("CCCC",193,194,195,196);
3235
a0d0e21e
LW
3236 $foo = pack("s2",1,2);
3237 # "\1\0\2\0" on little-endian
3238 # "\0\1\0\2" on big-endian
3239
3240 $foo = pack("a4","abcd","x","y","z");
3241 # "abcd"
3242
3243 $foo = pack("aaaa","abcd","x","y","z");
3244 # "axyz"
3245
3246 $foo = pack("a14","abcdefg");
3247 # "abcdefg\0\0\0\0\0\0\0"
3248
3249 $foo = pack("i9pl", gmtime);
3250 # a real struct tm (on my system anyway)
3251
5a929a98
VU
3252 $utmp_template = "Z8 Z8 Z16 L";
3253 $utmp = pack($utmp_template, @utmp1);
3254 # a struct utmp (BSDish)
3255
3256 @utmp2 = unpack($utmp_template, $utmp);
3257 # "@utmp1" eq "@utmp2"
3258
a0d0e21e
LW
3259 sub bintodec {
3260 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
3261 }
3262
851646ae
JH
3263 $foo = pack('sx2l', 12, 34);
3264 # short 12, two zero bytes padding, long 34
3265 $bar = pack('s@4l', 12, 34);
3266 # short 12, zero fill to position 4, long 34
3267 # $foo eq $bar
3268
5a929a98 3269The same template may generally also be used in unpack().
a0d0e21e 3270
5a964f20
TC
3271=item package
3272
cb1a09d0
AD
3273=item package NAMESPACE
3274
3275Declares the compilation unit as being in the given namespace. The scope
2b5ab1e7 3276of the package declaration is from the declaration itself through the end
19799a22 3277of the enclosing block, file, or eval (the same as the C<my> operator).
2b5ab1e7
TC
3278All further unqualified dynamic identifiers will be in this namespace.
3279A package statement affects only dynamic variables--including those
19799a22
GS
3280you've used C<local> on--but I<not> lexical variables, which are created
3281with C<my>. Typically it would be the first declaration in a file to
2b5ab1e7
TC
3282be included by the C<require> or C<use> operator. You can switch into a
3283package in more than one place; it merely influences which symbol table
3284is used by the compiler for the rest of that block. You can refer to
3285variables and filehandles in other packages by prefixing the identifier
3286with the package name and a double colon: C<$Package::Variable>.
3287If the package name is null, the C<main> package as assumed. That is,
3288C<$::sail> is equivalent to C<$main::sail> (as well as to C<$main'sail>,
3289still seen in older code).
cb1a09d0 3290
5a964f20
TC
3291If NAMESPACE is omitted, then there is no current package, and all
3292identifiers must be fully qualified or lexicals. This is stricter
329