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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
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168C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
169C<untie>, C<use>
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170
171=item Low-level socket functions
172
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173C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
174C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
175C<socket>, C<socketpair>
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176
177=item System V interprocess communication functions
178
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179C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
180C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>
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181
182=item Fetching user and group info
183
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184C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
185C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
186C<getpwuid>, C<setgrent>, C<setpwent>
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187
188=item Fetching network info
189
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190C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
191C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
192C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
193C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
194C<setnetent>, C<setprotoent>, C<setservent>
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195
196=item Time-related functions
197
22fae026 198C<gmtime>, C<localtime>, C<time>, C<times>
cb1a09d0 199
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200=item Functions new in perl5
201
22fae026 202C<abs>, C<bless>, C<chomp>, C<chr>, C<exists>, C<formline>, C<glob>,
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203C<import>, C<lc>, C<lcfirst>, C<map>, C<my>, C<no>, C<our>, C<prototype>,
204C<qx>, C<qw>, C<readline>, C<readpipe>, C<ref>, C<sub*>, C<sysopen>, C<tie>,
22fae026 205C<tied>, C<uc>, C<ucfirst>, C<untie>, C<use>
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206
207* - C<sub> was a keyword in perl4, but in perl5 it is an
5a964f20 208operator, which can be used in expressions.
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209
210=item Functions obsoleted in perl5
211
22fae026 212C<dbmclose>, C<dbmopen>
37798a01 213
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214=back
215
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216=head2 Portability
217
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218Perl was born in Unix and can therefore access all common Unix
219system calls. In non-Unix environments, the functionality of some
220Unix system calls may not be available, or details of the available
221functionality may differ slightly. The Perl functions affected
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222by this are:
223
224C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>,
225C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>,
226C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>,
227C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostent>,
228C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
229C<getppid>, C<getprgp>, C<getpriority>, C<getprotobynumber>,
230C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>,
231C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>,
232C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>,
2b5ab1e7 233C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<rename>, C<select>, C<semctl>,
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234C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>,
235C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>,
236C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>,
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237C<shmwrite>, C<socket>, C<socketpair>, C<stat>, C<symlink>, C<syscall>,
238C<sysopen>, C<system>, C<times>, C<truncate>, C<umask>, C<unlink>,
239C<utime>, C<wait>, C<waitpid>
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240
241For more information about the portability of these functions, see
242L<perlport> and other available platform-specific documentation.
243
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244=head2 Alphabetical Listing of Perl Functions
245
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246=over 8
247
22fae026 248=item I<-X> FILEHANDLE
a0d0e21e 249
22fae026 250=item I<-X> EXPR
a0d0e21e 251
22fae026 252=item I<-X>
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253
254A file test, where X is one of the letters listed below. This unary
255operator takes one argument, either a filename or a filehandle, and
256tests the associated file to see if something is true about it. If the
7660c0ab 257argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
19799a22 258Unless otherwise documented, it returns C<1> for true and C<''> for false, or
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259the undefined value if the file doesn't exist. Despite the funny
260names, precedence is the same as any other named unary operator, and
261the argument may be parenthesized like any other unary operator. The
262operator may be any of:
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263X<-r>X<-w>X<-x>X<-o>X<-R>X<-W>X<-X>X<-O>X<-e>X<-z>X<-s>X<-f>X<-d>X<-l>X<-p>
264X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
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265
266 -r File is readable by effective uid/gid.
267 -w File is writable by effective uid/gid.
268 -x File is executable by effective uid/gid.
269 -o File is owned by effective uid.
270
271 -R File is readable by real uid/gid.
272 -W File is writable by real uid/gid.
273 -X File is executable by real uid/gid.
274 -O File is owned by real uid.
275
276 -e File exists.
277 -z File has zero size.
54310121 278 -s File has nonzero size (returns size).
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279
280 -f File is a plain file.
281 -d File is a directory.
282 -l File is a symbolic link.
9c4d0f16 283 -p File is a named pipe (FIFO), or Filehandle is a pipe.
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284 -S File is a socket.
285 -b File is a block special file.
286 -c File is a character special file.
287 -t Filehandle is opened to a tty.
288
289 -u File has setuid bit set.
290 -g File has setgid bit set.
291 -k File has sticky bit set.
292
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293 -T File is an ASCII text file.
294 -B File is a "binary" file (opposite of -T).
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295
296 -M Age of file in days when script started.
297 -A Same for access time.
298 -C Same for inode change time.
299
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300Example:
301
302 while (<>) {
303 chop;
304 next unless -f $_; # ignore specials
5a964f20 305 #...
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306 }
307
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308The interpretation of the file permission operators C<-r>, C<-R>,
309C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
310of the file and the uids and gids of the user. There may be other
311reasons you can't actually read, write, or execute the file. Such
312reasons may be for example network filesystem access controls, ACLs
313(access control lists), read-only filesystems, and unrecognized
314executable formats.
315
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316Also note that, for the superuser on the local filesystems, the C<-r>,
317C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
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318if any execute bit is set in the mode. Scripts run by the superuser
319may thus need to do a stat() to determine the actual mode of the file,
2b5ab1e7 320or temporarily set their effective uid to something else.
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321
322If you are using ACLs, there is a pragma called C<filetest> that may
323produce more accurate results than the bare stat() mode bits.
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324When under the C<use filetest 'access'> the above-mentioned filetests
325will test whether the permission can (not) be granted using the
468541a8 326access() family of system calls. Also note that the C<-x> and C<-X> may
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327under this pragma return true even if there are no execute permission
328bits set (nor any extra execute permission ACLs). This strangeness is
329due to the underlying system calls' definitions. Read the
330documentation for the C<filetest> pragma for more information.
331
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332Note that C<-s/a/b/> does not do a negated substitution. Saying
333C<-exp($foo)> still works as expected, however--only single letters
334following a minus are interpreted as file tests.
335
336The C<-T> and C<-B> switches work as follows. The first block or so of the
337file is examined for odd characters such as strange control codes or
61eff3bc 338characters with the high bit set. If too many strange characters (>30%)
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339are found, it's a C<-B> file, otherwise it's a C<-T> file. Also, any file
340containing null in the first block is considered a binary file. If C<-T>
341or C<-B> is used on a filehandle, the current stdio buffer is examined
19799a22 342rather than the first block. Both C<-T> and C<-B> return true on a null
54310121 343file, or a file at EOF when testing a filehandle. Because you have to
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344read a file to do the C<-T> test, on most occasions you want to use a C<-f>
345against the file first, as in C<next unless -f $file && -T $file>.
a0d0e21e 346
19799a22 347If any of the file tests (or either the C<stat> or C<lstat> operators) are given
28757baa 348the special filehandle consisting of a solitary underline, then the stat
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349structure of the previous file test (or stat operator) is used, saving
350a system call. (This doesn't work with C<-t>, and you need to remember
351that lstat() and C<-l> will leave values in the stat structure for the
352symbolic link, not the real file.) Example:
353
354 print "Can do.\n" if -r $a || -w _ || -x _;
355
356 stat($filename);
357 print "Readable\n" if -r _;
358 print "Writable\n" if -w _;
359 print "Executable\n" if -x _;
360 print "Setuid\n" if -u _;
361 print "Setgid\n" if -g _;
362 print "Sticky\n" if -k _;
363 print "Text\n" if -T _;
364 print "Binary\n" if -B _;
365
366=item abs VALUE
367
54310121 368=item abs
bbce6d69 369
a0d0e21e 370Returns the absolute value of its argument.
7660c0ab 371If VALUE is omitted, uses C<$_>.
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372
373=item accept NEWSOCKET,GENERICSOCKET
374
f86cebdf 375Accepts an incoming socket connect, just as the accept(2) system call
19799a22 376does. Returns the packed address if it succeeded, false otherwise.
2b5ab1e7 377See the example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 378
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379On systems that support a close-on-exec flag on files, the flag will
380be set for the newly opened file descriptor, as determined by the
381value of $^F. See L<perlvar/$^F>.
382
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383=item alarm SECONDS
384
54310121 385=item alarm
bbce6d69 386
a0d0e21e 387Arranges to have a SIGALRM delivered to this process after the
bbce6d69 388specified number of seconds have elapsed. If SECONDS is not specified,
7660c0ab 389the value stored in C<$_> is used. (On some machines,
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390unfortunately, the elapsed time may be up to one second less than you
391specified because of how seconds are counted.) Only one timer may be
392counting at once. Each call disables the previous timer, and an
7660c0ab 393argument of C<0> may be supplied to cancel the previous timer without
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394starting a new one. The returned value is the amount of time remaining
395on the previous timer.
396
4633a7c4 397For delays of finer granularity than one second, you may use Perl's
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398four-argument version of select() leaving the first three arguments
399undefined, or you might be able to use the C<syscall> interface to
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400access setitimer(2) if your system supports it. The Time::HiRes module
401from CPAN may also prove useful.
402
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403It is usually a mistake to intermix C<alarm> and C<sleep> calls.
404(C<sleep> may be internally implemented in your system with C<alarm>)
a0d0e21e 405
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406If you want to use C<alarm> to time out a system call you need to use an
407C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
f86cebdf 408fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
19799a22 409restart system calls on some systems. Using C<eval>/C<die> always works,
5a964f20 410modulo the caveats given in L<perlipc/"Signals">.
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411
412 eval {
f86cebdf 413 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
36477c24 414 alarm $timeout;
ff68c719 415 $nread = sysread SOCKET, $buffer, $size;
36477c24 416 alarm 0;
ff68c719 417 };
ff68c719 418 if ($@) {
f86cebdf 419 die unless $@ eq "alarm\n"; # propagate unexpected errors
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420 # timed out
421 }
422 else {
423 # didn't
424 }
425
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426=item atan2 Y,X
427
428Returns the arctangent of Y/X in the range -PI to PI.
429
ca6e1c26 430For the tangent operation, you may use the C<Math::Trig::tan>
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431function, or use the familiar relation:
432
433 sub tan { sin($_[0]) / cos($_[0]) }
434
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435=item bind SOCKET,NAME
436
437Binds a network address to a socket, just as the bind system call
19799a22 438does. Returns true if it succeeded, false otherwise. NAME should be a
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439packed address of the appropriate type for the socket. See the examples in
440L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 441
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442=item binmode FILEHANDLE, DISCIPLINE
443
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444=item binmode FILEHANDLE
445
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446Arranges for FILEHANDLE to be read or written in "binary" or "text" mode
447on systems where the run-time libraries distinguish between binary and
30168b04 448text files. If FILEHANDLE is an expression, the value is taken as the
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449name of the filehandle. DISCIPLINE can be either of C<":raw"> for
450binary mode or C<":crlf"> for "text" mode. If the DISCIPLINE is
451omitted, it defaults to C<":raw">.
30168b04 452
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453binmode() should be called after open() but before any I/O is done on
454the filehandle.
455
456On many systems binmode() currently has no effect, but in future, it
457will be extended to support user-defined input and output disciplines.
458On some systems binmode() is necessary when you're not working with a
459text file. For the sake of portability it is a good idea to always use
460it when appropriate, and to never use it when it isn't appropriate.
30168b04
GS
461
462In other words: Regardless of platform, use binmode() on binary
463files, and do not use binmode() on text files.
19799a22 464
16fe6d59
GS
465The C<open> pragma can be used to establish default disciplines.
466See L<open>.
467
19799a22 468The operating system, device drivers, C libraries, and Perl run-time
30168b04
GS
469system all work together to let the programmer treat a single
470character (C<\n>) as the line terminator, irrespective of the external
471representation. On many operating systems, the native text file
472representation matches the internal representation, but on some
473platforms the external representation of C<\n> is made up of more than
474one character.
475
476Mac OS and all variants of Unix use a single character to end each line
477in the external representation of text (even though that single
478character is not necessarily the same across these platforms).
479Consequently binmode() has no effect on these operating systems. In
480other systems like VMS, MS-DOS and the various flavors of MS-Windows
481your program sees a C<\n> as a simple C<\cJ>, but what's stored in text
482files are the two characters C<\cM\cJ>. That means that, if you don't
483use binmode() on these systems, C<\cM\cJ> sequences on disk will be
484converted to C<\n> on input, and any C<\n> in your program will be
485converted back to C<\cM\cJ> on output. This is what you want for text
486files, but it can be disastrous for binary files.
487
488Another consequence of using binmode() (on some systems) is that
489special end-of-file markers will be seen as part of the data stream.
490For systems from the Microsoft family this means that if your binary
4375e838 491data contains C<\cZ>, the I/O subsystem will regard it as the end of
30168b04
GS
492the file, unless you use binmode().
493
494binmode() is not only important for readline() and print() operations,
495but also when using read(), seek(), sysread(), syswrite() and tell()
496(see L<perlport> for more details). See the C<$/> and C<$\> variables
497in L<perlvar> for how to manually set your input and output
498line-termination sequences.
a0d0e21e 499
4633a7c4 500=item bless REF,CLASSNAME
a0d0e21e
LW
501
502=item bless REF
503
2b5ab1e7
TC
504This function tells the thingy referenced by REF that it is now an object
505in the CLASSNAME package. If CLASSNAME is omitted, the current package
19799a22 506is used. Because a C<bless> is often the last thing in a constructor,
2b5ab1e7
TC
507it returns the reference for convenience. Always use the two-argument
508version if the function doing the blessing might be inherited by a
509derived class. See L<perltoot> and L<perlobj> for more about the blessing
510(and blessings) of objects.
a0d0e21e 511
57668c4d 512Consider always blessing objects in CLASSNAMEs that are mixed case.
2b5ab1e7
TC
513Namespaces with all lowercase names are considered reserved for
514Perl pragmata. Builtin types have all uppercase names, so to prevent
515confusion, you may wish to avoid such package names as well. Make sure
516that CLASSNAME is a true value.
60ad88b8
GS
517
518See L<perlmod/"Perl Modules">.
519
a0d0e21e
LW
520=item caller EXPR
521
522=item caller
523
5a964f20 524Returns the context of the current subroutine call. In scalar context,
28757baa 525returns the caller's package name if there is a caller, that is, if
19799a22 526we're in a subroutine or C<eval> or C<require>, and the undefined value
5a964f20 527otherwise. In list context, returns
a0d0e21e 528
748a9306 529 ($package, $filename, $line) = caller;
a0d0e21e
LW
530
531With EXPR, it returns some extra information that the debugger uses to
532print a stack trace. The value of EXPR indicates how many call frames
533to go back before the current one.
534
f3aa04c2 535 ($package, $filename, $line, $subroutine, $hasargs,
e476b1b5 536 $wantarray, $evaltext, $is_require, $hints, $bitmask) = caller($i);
e7ea3e70 537
951ba7fe 538Here $subroutine may be C<(eval)> if the frame is not a subroutine
19799a22 539call, but an C<eval>. In such a case additional elements $evaltext and
7660c0ab 540C<$is_require> are set: C<$is_require> is true if the frame is created by a
19799a22 541C<require> or C<use> statement, $evaltext contains the text of the
277ddfaf 542C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
951ba7fe 543$filename is C<(eval)>, but $evaltext is undefined. (Note also that
dc848c6f 544each C<use> statement creates a C<require> frame inside an C<eval EXPR>)
277ddfaf 545frame. C<$hasargs> is true if a new instance of C<@_> was set up for the
e476b1b5
GS
546frame. C<$hints> and C<$bitmask> contain pragmatic hints that the caller
547was compiled with. The C<$hints> and C<$bitmask> values are subject to
548change between versions of Perl, and are not meant for external use.
748a9306
LW
549
550Furthermore, when called from within the DB package, caller returns more
7660c0ab 551detailed information: it sets the list variable C<@DB::args> to be the
54310121 552arguments with which the subroutine was invoked.
748a9306 553
7660c0ab 554Be aware that the optimizer might have optimized call frames away before
19799a22 555C<caller> had a chance to get the information. That means that C<caller(N)>
7660c0ab 556might not return information about the call frame you expect it do, for
61eff3bc 557C<< N > 1 >>. In particular, C<@DB::args> might have information from the
19799a22 558previous time C<caller> was called.
7660c0ab 559
a0d0e21e
LW
560=item chdir EXPR
561
2b5ab1e7 562Changes the working directory to EXPR, if possible. If EXPR is omitted,
0bfc1ec4
GS
563changes to the directory specified by C<$ENV{HOME}>, if set; if not,
564changes to the directory specified by C<$ENV{LOGDIR}>. If neither is
565set, C<chdir> does nothing. It returns true upon success, false
566otherwise. See the example under C<die>.
a0d0e21e
LW
567
568=item chmod LIST
569
570Changes the permissions of a list of files. The first element of the
4633a7c4 571list must be the numerical mode, which should probably be an octal
2f9daede
TPG
572number, and which definitely should I<not> a string of octal digits:
573C<0644> is okay, C<'0644'> is not. Returns the number of files
dc848c6f 574successfully changed. See also L</oct>, if all you have is a string.
a0d0e21e
LW
575
576 $cnt = chmod 0755, 'foo', 'bar';
577 chmod 0755, @executables;
f86cebdf
GS
578 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
579 # --w----r-T
2f9daede
TPG
580 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
581 $mode = 0644; chmod $mode, 'foo'; # this is best
a0d0e21e 582
ca6e1c26
JH
583You can also import the symbolic C<S_I*> constants from the Fcntl
584module:
585
586 use Fcntl ':mode';
587
588 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
589 # This is identical to the chmod 0755 of the above example.
590
a0d0e21e
LW
591=item chomp VARIABLE
592
593=item chomp LIST
594
595=item chomp
596
2b5ab1e7
TC
597This safer version of L</chop> removes any trailing string
598that corresponds to the current value of C<$/> (also known as
28757baa
PP
599$INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
600number of characters removed from all its arguments. It's often used to
601remove the newline from the end of an input record when you're worried
2b5ab1e7
TC
602that the final record may be missing its newline. When in paragraph
603mode (C<$/ = "">), it removes all trailing newlines from the string.
4c5a6083
GS
604When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
605a reference to an integer or the like, see L<perlvar>) chomp() won't
19799a22
GS
606remove anything.
607If VARIABLE is omitted, it chomps C<$_>. Example:
a0d0e21e
LW
608
609 while (<>) {
610 chomp; # avoid \n on last field
611 @array = split(/:/);
5a964f20 612 # ...
a0d0e21e
LW
613 }
614
615You can actually chomp anything that's an lvalue, including an assignment:
616
617 chomp($cwd = `pwd`);
618 chomp($answer = <STDIN>);
619
620If you chomp a list, each element is chomped, and the total number of
621characters removed is returned.
622
623=item chop VARIABLE
624
625=item chop LIST
626
627=item chop
628
629Chops off the last character of a string and returns the character
630chopped. It's used primarily to remove the newline from the end of an
631input record, but is much more efficient than C<s/\n//> because it neither
7660c0ab 632scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
a0d0e21e
LW
633Example:
634
635 while (<>) {
636 chop; # avoid \n on last field
637 @array = split(/:/);
5a964f20 638 #...
a0d0e21e
LW
639 }
640
641You can actually chop anything that's an lvalue, including an assignment:
642
643 chop($cwd = `pwd`);
644 chop($answer = <STDIN>);
645
646If you chop a list, each element is chopped. Only the value of the
19799a22 647last C<chop> is returned.
a0d0e21e 648
19799a22 649Note that C<chop> returns the last character. To return all but the last
748a9306
LW
650character, use C<substr($string, 0, -1)>.
651
a0d0e21e
LW
652=item chown LIST
653
654Changes the owner (and group) of a list of files. The first two
19799a22
GS
655elements of the list must be the I<numeric> uid and gid, in that
656order. A value of -1 in either position is interpreted by most
657systems to leave that value unchanged. Returns the number of files
658successfully changed.
a0d0e21e
LW
659
660 $cnt = chown $uid, $gid, 'foo', 'bar';
661 chown $uid, $gid, @filenames;
662
54310121 663Here's an example that looks up nonnumeric uids in the passwd file:
a0d0e21e
LW
664
665 print "User: ";
19799a22 666 chomp($user = <STDIN>);
5a964f20 667 print "Files: ";
19799a22 668 chomp($pattern = <STDIN>);
a0d0e21e
LW
669
670 ($login,$pass,$uid,$gid) = getpwnam($user)
671 or die "$user not in passwd file";
672
5a964f20 673 @ary = glob($pattern); # expand filenames
a0d0e21e
LW
674 chown $uid, $gid, @ary;
675
54310121 676On most systems, you are not allowed to change the ownership of the
4633a7c4
LW
677file unless you're the superuser, although you should be able to change
678the group to any of your secondary groups. On insecure systems, these
679restrictions may be relaxed, but this is not a portable assumption.
19799a22
GS
680On POSIX systems, you can detect this condition this way:
681
682 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
683 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
4633a7c4 684
a0d0e21e
LW
685=item chr NUMBER
686
54310121 687=item chr
bbce6d69 688
a0d0e21e 689Returns the character represented by that NUMBER in the character set.
a0ed51b3 690For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
2b5ab1e7
TC
691chr(0x263a) is a Unicode smiley face (but only within the scope of
692a C<use utf8>). For the reverse, use L</ord>.
693See L<utf8> for more about Unicode.
a0d0e21e 694
7660c0ab 695If NUMBER is omitted, uses C<$_>.
bbce6d69 696
a0d0e21e
LW
697=item chroot FILENAME
698
54310121 699=item chroot
bbce6d69 700
5a964f20 701This function works like the system call by the same name: it makes the
4633a7c4 702named directory the new root directory for all further pathnames that
951ba7fe 703begin with a C</> by your process and all its children. (It doesn't
28757baa 704change your current working directory, which is unaffected.) For security
4633a7c4 705reasons, this call is restricted to the superuser. If FILENAME is
19799a22 706omitted, does a C<chroot> to C<$_>.
a0d0e21e
LW
707
708=item close FILEHANDLE
709
6a518fbc
TP
710=item close
711
19799a22 712Closes the file or pipe associated with the file handle, returning true
a0d0e21e 713only if stdio successfully flushes buffers and closes the system file
19799a22 714descriptor. Closes the currently selected filehandle if the argument
6a518fbc 715is omitted.
fb73857a
PP
716
717You don't have to close FILEHANDLE if you are immediately going to do
19799a22
GS
718another C<open> on it, because C<open> will close it for you. (See
719C<open>.) However, an explicit C<close> on an input file resets the line
720counter (C<$.>), while the implicit close done by C<open> does not.
fb73857a 721
19799a22
GS
722If the file handle came from a piped open C<close> will additionally
723return false if one of the other system calls involved fails or if the
fb73857a 724program exits with non-zero status. (If the only problem was that the
2b5ab1e7
TC
725program exited non-zero C<$!> will be set to C<0>.) Closing a pipe
726also waits for the process executing on the pipe to complete, in case you
727want to look at the output of the pipe afterwards, and
728implicitly puts the exit status value of that command into C<$?>.
5a964f20 729
73689b13
GS
730Prematurely closing the read end of a pipe (i.e. before the process
731writing to it at the other end has closed it) will result in a
732SIGPIPE being delivered to the writer. If the other end can't
733handle that, be sure to read all the data before closing the pipe.
734
fb73857a 735Example:
a0d0e21e 736
fb73857a
PP
737 open(OUTPUT, '|sort >foo') # pipe to sort
738 or die "Can't start sort: $!";
5a964f20 739 #... # print stuff to output
fb73857a
PP
740 close OUTPUT # wait for sort to finish
741 or warn $! ? "Error closing sort pipe: $!"
742 : "Exit status $? from sort";
743 open(INPUT, 'foo') # get sort's results
744 or die "Can't open 'foo' for input: $!";
a0d0e21e 745
5a964f20
TC
746FILEHANDLE may be an expression whose value can be used as an indirect
747filehandle, usually the real filehandle name.
a0d0e21e
LW
748
749=item closedir DIRHANDLE
750
19799a22 751Closes a directory opened by C<opendir> and returns the success of that
5a964f20
TC
752system call.
753
754DIRHANDLE may be an expression whose value can be used as an indirect
755dirhandle, usually the real dirhandle name.
a0d0e21e
LW
756
757=item connect SOCKET,NAME
758
759Attempts to connect to a remote socket, just as the connect system call
19799a22 760does. Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
761packed address of the appropriate type for the socket. See the examples in
762L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 763
cb1a09d0
AD
764=item continue BLOCK
765
766Actually a flow control statement rather than a function. If there is a
98293880
JH
767C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
768C<foreach>), it is always executed just before the conditional is about to
769be evaluated again, just like the third part of a C<for> loop in C. Thus
cb1a09d0
AD
770it can be used to increment a loop variable, even when the loop has been
771continued via the C<next> statement (which is similar to the C C<continue>
772statement).
773
98293880 774C<last>, C<next>, or C<redo> may appear within a C<continue>
19799a22
GS
775block. C<last> and C<redo> will behave as if they had been executed within
776the main block. So will C<next>, but since it will execute a C<continue>
1d2dff63
GS
777block, it may be more entertaining.
778
779 while (EXPR) {
780 ### redo always comes here
781 do_something;
782 } continue {
783 ### next always comes here
784 do_something_else;
785 # then back the top to re-check EXPR
786 }
787 ### last always comes here
788
789Omitting the C<continue> section is semantically equivalent to using an
19799a22 790empty one, logically enough. In that case, C<next> goes directly back
1d2dff63
GS
791to check the condition at the top of the loop.
792
a0d0e21e
LW
793=item cos EXPR
794
5a964f20 795Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
7660c0ab 796takes cosine of C<$_>.
a0d0e21e 797
ca6e1c26 798For the inverse cosine operation, you may use the C<Math::Trig::acos()>
28757baa
PP
799function, or use this relation:
800
801 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
802
a0d0e21e
LW
803=item crypt PLAINTEXT,SALT
804
f86cebdf 805Encrypts a string exactly like the crypt(3) function in the C library
4633a7c4
LW
806(assuming that you actually have a version there that has not been
807extirpated as a potential munition). This can prove useful for checking
808the password file for lousy passwords, amongst other things. Only the
809guys wearing white hats should do this.
a0d0e21e 810
19799a22 811Note that C<crypt> is intended to be a one-way function, much like breaking
11155c91
CS
812eggs to make an omelette. There is no (known) corresponding decrypt
813function. As a result, this function isn't all that useful for
814cryptography. (For that, see your nearby CPAN mirror.)
2f9daede 815
e71965be
RS
816When verifying an existing encrypted string you should use the encrypted
817text as the salt (like C<crypt($plain, $crypted) eq $crypted>). This
19799a22 818allows your code to work with the standard C<crypt> and with more
e71965be
RS
819exotic implementations. When choosing a new salt create a random two
820character string whose characters come from the set C<[./0-9A-Za-z]>
821(like C<join '', ('.', '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>).
822
a0d0e21e
LW
823Here's an example that makes sure that whoever runs this program knows
824their own password:
825
826 $pwd = (getpwuid($<))[1];
a0d0e21e
LW
827
828 system "stty -echo";
829 print "Password: ";
e71965be 830 chomp($word = <STDIN>);
a0d0e21e
LW
831 print "\n";
832 system "stty echo";
833
e71965be 834 if (crypt($word, $pwd) ne $pwd) {
a0d0e21e
LW
835 die "Sorry...\n";
836 } else {
837 print "ok\n";
54310121 838 }
a0d0e21e 839
9f8f0c9d 840Of course, typing in your own password to whoever asks you
748a9306 841for it is unwise.
a0d0e21e 842
19799a22
GS
843The L<crypt> function is unsuitable for encrypting large quantities
844of data, not least of all because you can't get the information
845back. Look at the F<by-module/Crypt> and F<by-module/PGP> directories
846on your favorite CPAN mirror for a slew of potentially useful
847modules.
848
aa689395 849=item dbmclose HASH
a0d0e21e 850
19799a22 851[This function has been largely superseded by the C<untie> function.]
a0d0e21e 852
aa689395 853Breaks the binding between a DBM file and a hash.
a0d0e21e 854
19799a22 855=item dbmopen HASH,DBNAME,MASK
a0d0e21e 856
19799a22 857[This function has been largely superseded by the C<tie> function.]
a0d0e21e 858
7b8d334a 859This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
19799a22
GS
860hash. HASH is the name of the hash. (Unlike normal C<open>, the first
861argument is I<not> a filehandle, even though it looks like one). DBNAME
aa689395
PP
862is the name of the database (without the F<.dir> or F<.pag> extension if
863any). If the database does not exist, it is created with protection
19799a22
GS
864specified by MASK (as modified by the C<umask>). If your system supports
865only the older DBM functions, you may perform only one C<dbmopen> in your
aa689395 866program. In older versions of Perl, if your system had neither DBM nor
19799a22 867ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
aa689395
PP
868sdbm(3).
869
870If you don't have write access to the DBM file, you can only read hash
871variables, not set them. If you want to test whether you can write,
19799a22 872either use file tests or try setting a dummy hash entry inside an C<eval>,
aa689395 873which will trap the error.
a0d0e21e 874
19799a22
GS
875Note that functions such as C<keys> and C<values> may return huge lists
876when used on large DBM files. You may prefer to use the C<each>
a0d0e21e
LW
877function to iterate over large DBM files. Example:
878
879 # print out history file offsets
880 dbmopen(%HIST,'/usr/lib/news/history',0666);
881 while (($key,$val) = each %HIST) {
882 print $key, ' = ', unpack('L',$val), "\n";
883 }
884 dbmclose(%HIST);
885
cb1a09d0 886See also L<AnyDBM_File> for a more general description of the pros and
184e9718 887cons of the various dbm approaches, as well as L<DB_File> for a particularly
cb1a09d0 888rich implementation.
4633a7c4 889
2b5ab1e7
TC
890You can control which DBM library you use by loading that library
891before you call dbmopen():
892
893 use DB_File;
894 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
895 or die "Can't open netscape history file: $!";
896
a0d0e21e
LW
897=item defined EXPR
898
54310121 899=item defined
bbce6d69 900
2f9daede
TPG
901Returns a Boolean value telling whether EXPR has a value other than
902the undefined value C<undef>. If EXPR is not present, C<$_> will be
903checked.
904
905Many operations return C<undef> to indicate failure, end of file,
906system error, uninitialized variable, and other exceptional
907conditions. This function allows you to distinguish C<undef> from
908other values. (A simple Boolean test will not distinguish among
7660c0ab 909C<undef>, zero, the empty string, and C<"0">, which are all equally
2f9daede 910false.) Note that since C<undef> is a valid scalar, its presence
19799a22 911doesn't I<necessarily> indicate an exceptional condition: C<pop>
2f9daede
TPG
912returns C<undef> when its argument is an empty array, I<or> when the
913element to return happens to be C<undef>.
914
f10b0346
GS
915You may also use C<defined(&func)> to check whether subroutine C<&func>
916has ever been defined. The return value is unaffected by any forward
917declarations of C<&foo>.
918
919Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
920used to report whether memory for that aggregate has ever been
921allocated. This behavior may disappear in future versions of Perl.
922You should instead use a simple test for size:
923
924 if (@an_array) { print "has array elements\n" }
925 if (%a_hash) { print "has hash members\n" }
2f9daede
TPG
926
927When used on a hash element, it tells you whether the value is defined,
dc848c6f 928not whether the key exists in the hash. Use L</exists> for the latter
2f9daede 929purpose.
a0d0e21e
LW
930
931Examples:
932
933 print if defined $switch{'D'};
934 print "$val\n" while defined($val = pop(@ary));
935 die "Can't readlink $sym: $!"
936 unless defined($value = readlink $sym);
a0d0e21e 937 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
2f9daede 938 $debugging = 0 unless defined $debugging;
a0d0e21e 939
19799a22 940Note: Many folks tend to overuse C<defined>, and then are surprised to
7660c0ab 941discover that the number C<0> and C<""> (the zero-length string) are, in fact,
2f9daede 942defined values. For example, if you say
a5f75d66
AD
943
944 "ab" =~ /a(.*)b/;
945
7660c0ab 946The pattern match succeeds, and C<$1> is defined, despite the fact that it
a5f75d66 947matched "nothing". But it didn't really match nothing--rather, it
2b5ab1e7 948matched something that happened to be zero characters long. This is all
a5f75d66 949very above-board and honest. When a function returns an undefined value,
2f9daede 950it's an admission that it couldn't give you an honest answer. So you
19799a22 951should use C<defined> only when you're questioning the integrity of what
7660c0ab 952you're trying to do. At other times, a simple comparison to C<0> or C<""> is
2f9daede
TPG
953what you want.
954
dc848c6f 955See also L</undef>, L</exists>, L</ref>.
2f9daede 956
a0d0e21e
LW
957=item delete EXPR
958
01020589
GS
959Given an expression that specifies a hash element, array element, hash slice,
960or array slice, deletes the specified element(s) from the hash or array.
8216c1fd
GS
961In the case of an array, if the array elements happen to be at the end,
962the size of the array will shrink to the highest element that tests
963true for exists() (or 0 if no such element exists).
a0d0e21e 964
01020589
GS
965Returns each element so deleted or the undefined value if there was no such
966element. Deleting from C<$ENV{}> modifies the environment. Deleting from
967a hash tied to a DBM file deletes the entry from the DBM file. Deleting
968from a C<tie>d hash or array may not necessarily return anything.
969
8ea97a1e
GS
970Deleting an array element effectively returns that position of the array
971to its initial, uninitialized state. Subsequently testing for the same
8216c1fd
GS
972element with exists() will return false. Note that deleting array
973elements in the middle of an array will not shift the index of the ones
974after them down--use splice() for that. See L</exists>.
8ea97a1e 975
01020589 976The following (inefficiently) deletes all the values of %HASH and @ARRAY:
a0d0e21e 977
5f05dabc
PP
978 foreach $key (keys %HASH) {
979 delete $HASH{$key};
a0d0e21e
LW
980 }
981
01020589
GS
982 foreach $index (0 .. $#ARRAY) {
983 delete $ARRAY[$index];
984 }
985
986And so do these:
5f05dabc 987
01020589
GS
988 delete @HASH{keys %HASH};
989
9740c838 990 delete @ARRAY[0 .. $#ARRAY];
5f05dabc 991
2b5ab1e7 992But both of these are slower than just assigning the empty list
01020589
GS
993or undefining %HASH or @ARRAY:
994
995 %HASH = (); # completely empty %HASH
996 undef %HASH; # forget %HASH ever existed
2b5ab1e7 997
01020589
GS
998 @ARRAY = (); # completely empty @ARRAY
999 undef @ARRAY; # forget @ARRAY ever existed
2b5ab1e7
TC
1000
1001Note that the EXPR can be arbitrarily complicated as long as the final
01020589
GS
1002operation is a hash element, array element, hash slice, or array slice
1003lookup:
a0d0e21e
LW
1004
1005 delete $ref->[$x][$y]{$key};
5f05dabc 1006 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
a0d0e21e 1007
01020589
GS
1008 delete $ref->[$x][$y][$index];
1009 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1010
a0d0e21e
LW
1011=item die LIST
1012
19799a22
GS
1013Outside an C<eval>, prints the value of LIST to C<STDERR> and
1014exits with the current value of C<$!> (errno). If C<$!> is C<0>,
61eff3bc
JH
1015exits with the value of C<<< ($? >> 8) >>> (backtick `command`
1016status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside
19799a22
GS
1017an C<eval(),> the error message is stuffed into C<$@> and the
1018C<eval> is terminated with the undefined value. This makes
1019C<die> the way to raise an exception.
a0d0e21e
LW
1020
1021Equivalent examples:
1022
1023 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
54310121 1024 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
a0d0e21e
LW
1025
1026If the value of EXPR does not end in a newline, the current script line
1027number and input line number (if any) are also printed, and a newline
883faa13
GS
1028is supplied. Note that the "input line number" (also known as "chunk")
1029is subject to whatever notion of "line" happens to be currently in
1030effect, and is also available as the special variable C<$.>.
1031See L<perlvar/"$/"> and L<perlvar/"$.">.
1032
1033Hint: sometimes appending C<", stopped"> to your message
7660c0ab 1034will cause it to make better sense when the string C<"at foo line 123"> is
a0d0e21e
LW
1035appended. Suppose you are running script "canasta".
1036
1037 die "/etc/games is no good";
1038 die "/etc/games is no good, stopped";
1039
1040produce, respectively
1041
1042 /etc/games is no good at canasta line 123.
1043 /etc/games is no good, stopped at canasta line 123.
1044
2b5ab1e7 1045See also exit(), warn(), and the Carp module.
a0d0e21e 1046
7660c0ab
A
1047If LIST is empty and C<$@> already contains a value (typically from a
1048previous eval) that value is reused after appending C<"\t...propagated">.
fb73857a
PP
1049This is useful for propagating exceptions:
1050
1051 eval { ... };
1052 die unless $@ =~ /Expected exception/;
1053
7660c0ab 1054If C<$@> is empty then the string C<"Died"> is used.
fb73857a 1055
52531d10
GS
1056die() can also be called with a reference argument. If this happens to be
1057trapped within an eval(), $@ contains the reference. This behavior permits
1058a more elaborate exception handling implementation using objects that
4375e838 1059maintain arbitrary state about the nature of the exception. Such a scheme
52531d10
GS
1060is sometimes preferable to matching particular string values of $@ using
1061regular expressions. Here's an example:
1062
1063 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1064 if ($@) {
1065 if (ref($@) && UNIVERSAL::isa($@,"Some::Module::Exception")) {
1066 # handle Some::Module::Exception
1067 }
1068 else {
1069 # handle all other possible exceptions
1070 }
1071 }
1072
19799a22 1073Because perl will stringify uncaught exception messages before displaying
52531d10
GS
1074them, you may want to overload stringification operations on such custom
1075exception objects. See L<overload> for details about that.
1076
19799a22
GS
1077You can arrange for a callback to be run just before the C<die>
1078does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1079handler will be called with the error text and can change the error
1080message, if it sees fit, by calling C<die> again. See
1081L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
1082L<"eval BLOCK"> for some examples. Although this feature was meant
1083to be run only right before your program was to exit, this is not
1084currently the case--the C<$SIG{__DIE__}> hook is currently called
1085even inside eval()ed blocks/strings! If one wants the hook to do
1086nothing in such situations, put
fb73857a
PP
1087
1088 die @_ if $^S;
1089
19799a22
GS
1090as the first line of the handler (see L<perlvar/$^S>). Because
1091this promotes strange action at a distance, this counterintuitive
1092behavior may be fixed in a future release.
774d564b 1093
a0d0e21e
LW
1094=item do BLOCK
1095
1096Not really a function. Returns the value of the last command in the
1097sequence of commands indicated by BLOCK. When modified by a loop
98293880
JH
1098modifier, executes the BLOCK once before testing the loop condition.
1099(On other statements the loop modifiers test the conditional first.)
a0d0e21e 1100
4968c1e4 1101C<do BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7
TC
1102C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1103See L<perlsyn> for alternative strategies.
4968c1e4 1104
a0d0e21e
LW
1105=item do SUBROUTINE(LIST)
1106
1107A deprecated form of subroutine call. See L<perlsub>.
1108
1109=item do EXPR
1110
1111Uses the value of EXPR as a filename and executes the contents of the
1112file as a Perl script. Its primary use is to include subroutines
1113from a Perl subroutine library.
1114
1115 do 'stat.pl';
1116
1117is just like
1118
fb73857a 1119 scalar eval `cat stat.pl`;
a0d0e21e 1120
2b5ab1e7
TC
1121except that it's more efficient and concise, keeps track of the current
1122filename for error messages, searches the @INC libraries, and updates
1123C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1124variables. It also differs in that code evaluated with C<do FILENAME>
1125cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1126same, however, in that it does reparse the file every time you call it,
1127so you probably don't want to do this inside a loop.
a0d0e21e 1128
8e30cc93 1129If C<do> cannot read the file, it returns undef and sets C<$!> to the
2b5ab1e7 1130error. If C<do> can read the file but cannot compile it, it
8e30cc93
G
1131returns undef and sets an error message in C<$@>. If the file is
1132successfully compiled, C<do> returns the value of the last expression
1133evaluated.
1134
a0d0e21e 1135Note that inclusion of library modules is better done with the
19799a22 1136C<use> and C<require> operators, which also do automatic error checking
4633a7c4 1137and raise an exception if there's a problem.
a0d0e21e 1138
5a964f20
TC
1139You might like to use C<do> to read in a program configuration
1140file. Manual error checking can be done this way:
1141
1142 # read in config files: system first, then user
f86cebdf 1143 for $file ("/share/prog/defaults.rc",
2b5ab1e7
TC
1144 "$ENV{HOME}/.someprogrc")
1145 {
5a964f20 1146 unless ($return = do $file) {
f86cebdf
GS
1147 warn "couldn't parse $file: $@" if $@;
1148 warn "couldn't do $file: $!" unless defined $return;
1149 warn "couldn't run $file" unless $return;
5a964f20
TC
1150 }
1151 }
1152
a0d0e21e
LW
1153=item dump LABEL
1154
1614b0e3
JD
1155=item dump
1156
19799a22
GS
1157This function causes an immediate core dump. See also the B<-u>
1158command-line switch in L<perlrun>, which does the same thing.
1159Primarily this is so that you can use the B<undump> program (not
1160supplied) to turn your core dump into an executable binary after
1161having initialized all your variables at the beginning of the
1162program. When the new binary is executed it will begin by executing
1163a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1164Think of it as a goto with an intervening core dump and reincarnation.
1165If C<LABEL> is omitted, restarts the program from the top.
1166
1167B<WARNING>: Any files opened at the time of the dump will I<not>
1168be open any more when the program is reincarnated, with possible
1169resulting confusion on the part of Perl.
1170
1171This function is now largely obsolete, partly because it's very
1172hard to convert a core file into an executable, and because the
1173real compiler backends for generating portable bytecode and compilable
1174C code have superseded it.
1175
1176If you're looking to use L<dump> to speed up your program, consider
1177generating bytecode or native C code as described in L<perlcc>. If
1178you're just trying to accelerate a CGI script, consider using the
1179C<mod_perl> extension to B<Apache>, or the CPAN module, Fast::CGI.
1180You might also consider autoloading or selfloading, which at least
1181make your program I<appear> to run faster.
5a964f20 1182
aa689395
PP
1183=item each HASH
1184
5a964f20 1185When called in list context, returns a 2-element list consisting of the
aa689395 1186key and value for the next element of a hash, so that you can iterate over
5a964f20 1187it. When called in scalar context, returns the key for only the "next"
e902a979 1188element in the hash.
2f9daede 1189
ab192400
GS
1190Entries are returned in an apparently random order. The actual random
1191order is subject to change in future versions of perl, but it is guaranteed
19799a22 1192to be in the same order as either the C<keys> or C<values> function
ab192400
GS
1193would produce on the same (unmodified) hash.
1194
1195When the hash is entirely read, a null array is returned in list context
19799a22
GS
1196(which when assigned produces a false (C<0>) value), and C<undef> in
1197scalar context. The next call to C<each> after that will start iterating
1198again. There is a single iterator for each hash, shared by all C<each>,
1199C<keys>, and C<values> function calls in the program; it can be reset by
2f9daede
TPG
1200reading all the elements from the hash, or by evaluating C<keys HASH> or
1201C<values HASH>. If you add or delete elements of a hash while you're
1202iterating over it, you may get entries skipped or duplicated, so don't.
aa689395 1203
f86cebdf 1204The following prints out your environment like the printenv(1) program,
aa689395 1205only in a different order:
a0d0e21e
LW
1206
1207 while (($key,$value) = each %ENV) {
1208 print "$key=$value\n";
1209 }
1210
19799a22 1211See also C<keys>, C<values> and C<sort>.
a0d0e21e
LW
1212
1213=item eof FILEHANDLE
1214
4633a7c4
LW
1215=item eof ()
1216
a0d0e21e
LW
1217=item eof
1218
1219Returns 1 if the next read on FILEHANDLE will return end of file, or if
1220FILEHANDLE is not open. FILEHANDLE may be an expression whose value
5a964f20 1221gives the real filehandle. (Note that this function actually
19799a22 1222reads a character and then C<ungetc>s it, so isn't very useful in an
748a9306 1223interactive context.) Do not read from a terminal file (or call
19799a22 1224C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
748a9306
LW
1225as terminals may lose the end-of-file condition if you do.
1226
820475bd
GS
1227An C<eof> without an argument uses the last file read. Using C<eof()>
1228with empty parentheses is very different. It refers to the pseudo file
1229formed from the files listed on the command line and accessed via the
61eff3bc
JH
1230C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1231as a normal filehandle is, an C<eof()> before C<< <> >> has been
820475bd
GS
1232used will cause C<@ARGV> to be examined to determine if input is
1233available.
1234
61eff3bc 1235In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
820475bd
GS
1236detect the end of each file, C<eof()> will only detect the end of the
1237last file. Examples:
a0d0e21e 1238
748a9306
LW
1239 # reset line numbering on each input file
1240 while (<>) {
5a964f20 1241 next if /^\s*#/; # skip comments
748a9306 1242 print "$.\t$_";
5a964f20
TC
1243 } continue {
1244 close ARGV if eof; # Not eof()!
748a9306
LW
1245 }
1246
a0d0e21e
LW
1247 # insert dashes just before last line of last file
1248 while (<>) {
5a964f20 1249 if (eof()) { # check for end of current file
a0d0e21e 1250 print "--------------\n";
2b5ab1e7 1251 close(ARGV); # close or last; is needed if we
748a9306 1252 # are reading from the terminal
a0d0e21e
LW
1253 }
1254 print;
1255 }
1256
a0d0e21e 1257Practical hint: you almost never need to use C<eof> in Perl, because the
3ce0d271
GS
1258input operators typically return C<undef> when they run out of data, or if
1259there was an error.
a0d0e21e
LW
1260
1261=item eval EXPR
1262
1263=item eval BLOCK
1264
c7cc6f1c
GS
1265In the first form, the return value of EXPR is parsed and executed as if it
1266were a little Perl program. The value of the expression (which is itself
5a964f20 1267determined within scalar context) is first parsed, and if there weren't any
c7cc6f1c 1268errors, executed in the context of the current Perl program, so that any
5f05dabc 1269variable settings or subroutine and format definitions remain afterwards.
c7cc6f1c
GS
1270Note that the value is parsed every time the eval executes. If EXPR is
1271omitted, evaluates C<$_>. This form is typically used to delay parsing
1272and subsequent execution of the text of EXPR until run time.
1273
1274In the second form, the code within the BLOCK is parsed only once--at the
1275same time the code surrounding the eval itself was parsed--and executed
1276within the context of the current Perl program. This form is typically
1277used to trap exceptions more efficiently than the first (see below), while
1278also providing the benefit of checking the code within BLOCK at compile
1279time.
1280
1281The final semicolon, if any, may be omitted from the value of EXPR or within
1282the BLOCK.
1283
1284In both forms, the value returned is the value of the last expression
5a964f20 1285evaluated inside the mini-program; a return statement may be also used, just
c7cc6f1c 1286as with subroutines. The expression providing the return value is evaluated
5a964f20 1287in void, scalar, or list context, depending on the context of the eval itself.
c7cc6f1c 1288See L</wantarray> for more on how the evaluation context can be determined.
a0d0e21e 1289
19799a22
GS
1290If there is a syntax error or runtime error, or a C<die> statement is
1291executed, an undefined value is returned by C<eval>, and C<$@> is set to the
a0d0e21e 1292error message. If there was no error, C<$@> is guaranteed to be a null
19799a22 1293string. Beware that using C<eval> neither silences perl from printing
c7cc6f1c
GS
1294warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1295To do either of those, you have to use the C<$SIG{__WARN__}> facility. See
1296L</warn> and L<perlvar>.
a0d0e21e 1297
19799a22
GS
1298Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1299determining whether a particular feature (such as C<socket> or C<symlink>)
a0d0e21e
LW
1300is implemented. It is also Perl's exception trapping mechanism, where
1301the die operator is used to raise exceptions.
1302
1303If the code to be executed doesn't vary, you may use the eval-BLOCK
1304form to trap run-time errors without incurring the penalty of
1305recompiling each time. The error, if any, is still returned in C<$@>.
1306Examples:
1307
54310121 1308 # make divide-by-zero nonfatal
a0d0e21e
LW
1309 eval { $answer = $a / $b; }; warn $@ if $@;
1310
1311 # same thing, but less efficient
1312 eval '$answer = $a / $b'; warn $@ if $@;
1313
1314 # a compile-time error
5a964f20 1315 eval { $answer = }; # WRONG
a0d0e21e
LW
1316
1317 # a run-time error
1318 eval '$answer ='; # sets $@
1319
2b5ab1e7
TC
1320Due to the current arguably broken state of C<__DIE__> hooks, when using
1321the C<eval{}> form as an exception trap in libraries, you may wish not
1322to trigger any C<__DIE__> hooks that user code may have installed.
1323You can use the C<local $SIG{__DIE__}> construct for this purpose,
1324as shown in this example:
774d564b
PP
1325
1326 # a very private exception trap for divide-by-zero
f86cebdf
GS
1327 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1328 warn $@ if $@;
774d564b
PP
1329
1330This is especially significant, given that C<__DIE__> hooks can call
19799a22 1331C<die> again, which has the effect of changing their error messages:
774d564b
PP
1332
1333 # __DIE__ hooks may modify error messages
1334 {
f86cebdf
GS
1335 local $SIG{'__DIE__'} =
1336 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
c7cc6f1c
GS
1337 eval { die "foo lives here" };
1338 print $@ if $@; # prints "bar lives here"
774d564b
PP
1339 }
1340
19799a22 1341Because this promotes action at a distance, this counterintuitive behavior
2b5ab1e7
TC
1342may be fixed in a future release.
1343
19799a22 1344With an C<eval>, you should be especially careful to remember what's
a0d0e21e
LW
1345being looked at when:
1346
1347 eval $x; # CASE 1
1348 eval "$x"; # CASE 2
1349
1350 eval '$x'; # CASE 3
1351 eval { $x }; # CASE 4
1352
5a964f20 1353 eval "\$$x++"; # CASE 5
a0d0e21e
LW
1354 $$x++; # CASE 6
1355
2f9daede 1356Cases 1 and 2 above behave identically: they run the code contained in
19799a22 1357the variable $x. (Although case 2 has misleading double quotes making
2f9daede 1358the reader wonder what else might be happening (nothing is).) Cases 3
7660c0ab 1359and 4 likewise behave in the same way: they run the code C<'$x'>, which
19799a22 1360does nothing but return the value of $x. (Case 4 is preferred for
2f9daede
TPG
1361purely visual reasons, but it also has the advantage of compiling at
1362compile-time instead of at run-time.) Case 5 is a place where
19799a22 1363normally you I<would> like to use double quotes, except that in this
2f9daede
TPG
1364particular situation, you can just use symbolic references instead, as
1365in case 6.
a0d0e21e 1366
4968c1e4 1367C<eval BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7 1368C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
4968c1e4 1369
a0d0e21e
LW
1370=item exec LIST
1371
8bf3b016
GS
1372=item exec PROGRAM LIST
1373
19799a22
GS
1374The C<exec> function executes a system command I<and never returns>--
1375use C<system> instead of C<exec> if you want it to return. It fails and
1376returns false only if the command does not exist I<and> it is executed
fb73857a 1377directly instead of via your system's command shell (see below).
a0d0e21e 1378
19799a22
GS
1379Since it's a common mistake to use C<exec> instead of C<system>, Perl
1380warns you if there is a following statement which isn't C<die>, C<warn>,
1381or C<exit> (if C<-w> is set - but you always do that). If you
1382I<really> want to follow an C<exec> with some other statement, you
55d729e4
GS
1383can use one of these styles to avoid the warning:
1384
5a964f20
TC
1385 exec ('foo') or print STDERR "couldn't exec foo: $!";
1386 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
55d729e4 1387
5a964f20 1388If there is more than one argument in LIST, or if LIST is an array
f86cebdf 1389with more than one value, calls execvp(3) with the arguments in LIST.
5a964f20
TC
1390If there is only one scalar argument or an array with one element in it,
1391the argument is checked for shell metacharacters, and if there are any,
1392the entire argument is passed to the system's command shell for parsing
1393(this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1394If there are no shell metacharacters in the argument, it is split into
19799a22
GS
1395words and passed directly to C<execvp>, which is more efficient.
1396Examples:
a0d0e21e 1397
19799a22
GS
1398 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1399 exec "sort $outfile | uniq";
a0d0e21e
LW
1400
1401If you don't really want to execute the first argument, but want to lie
1402to the program you are executing about its own name, you can specify
1403the program you actually want to run as an "indirect object" (without a
1404comma) in front of the LIST. (This always forces interpretation of the
54310121 1405LIST as a multivalued list, even if there is only a single scalar in
a0d0e21e
LW
1406the list.) Example:
1407
1408 $shell = '/bin/csh';
1409 exec $shell '-sh'; # pretend it's a login shell
1410
1411or, more directly,
1412
1413 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1414
bb32b41a
GS
1415When the arguments get executed via the system shell, results will
1416be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1417for details.
1418
19799a22
GS
1419Using an indirect object with C<exec> or C<system> is also more
1420secure. This usage (which also works fine with system()) forces
1421interpretation of the arguments as a multivalued list, even if the
1422list had just one argument. That way you're safe from the shell
1423expanding wildcards or splitting up words with whitespace in them.
5a964f20
TC
1424
1425 @args = ( "echo surprise" );
1426
2b5ab1e7 1427 exec @args; # subject to shell escapes
f86cebdf 1428 # if @args == 1
2b5ab1e7 1429 exec { $args[0] } @args; # safe even with one-arg list
5a964f20
TC
1430
1431The first version, the one without the indirect object, ran the I<echo>
1432program, passing it C<"surprise"> an argument. The second version
1433didn't--it tried to run a program literally called I<"echo surprise">,
1434didn't find it, and set C<$?> to a non-zero value indicating failure.
1435
0f897271
GS
1436Beginning with v5.6.0, Perl will attempt to flush all files opened for
1437output before the exec, but this may not be supported on some platforms
1438(see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1439in English) or call the C<autoflush()> method of C<IO::Handle> on any
1440open handles in order to avoid lost output.
1441
19799a22 1442Note that C<exec> will not call your C<END> blocks, nor will it call
7660c0ab
A
1443any C<DESTROY> methods in your objects.
1444
a0d0e21e
LW
1445=item exists EXPR
1446
01020589 1447Given an expression that specifies a hash element or array element,
8ea97a1e
GS
1448returns true if the specified element in the hash or array has ever
1449been initialized, even if the corresponding value is undefined. The
1450element is not autovivified if it doesn't exist.
a0d0e21e 1451
01020589
GS
1452 print "Exists\n" if exists $hash{$key};
1453 print "Defined\n" if defined $hash{$key};
1454 print "True\n" if $hash{$key};
1455
1456 print "Exists\n" if exists $array[$index];
1457 print "Defined\n" if defined $array[$index];
1458 print "True\n" if $array[$index];
a0d0e21e 1459
8ea97a1e 1460A hash or array element can be true only if it's defined, and defined if
a0d0e21e
LW
1461it exists, but the reverse doesn't necessarily hold true.
1462
afebc493
GS
1463Given an expression that specifies the name of a subroutine,
1464returns true if the specified subroutine has ever been declared, even
1465if it is undefined. Mentioning a subroutine name for exists or defined
1466does not count as declaring it.
1467
1468 print "Exists\n" if exists &subroutine;
1469 print "Defined\n" if defined &subroutine;
1470
a0d0e21e 1471Note that the EXPR can be arbitrarily complicated as long as the final
afebc493 1472operation is a hash or array key lookup or subroutine name:
a0d0e21e 1473
2b5ab1e7
TC
1474 if (exists $ref->{A}->{B}->{$key}) { }
1475 if (exists $hash{A}{B}{$key}) { }
1476
01020589
GS
1477 if (exists $ref->{A}->{B}->[$ix]) { }
1478 if (exists $hash{A}{B}[$ix]) { }
1479
afebc493
GS
1480 if (exists &{$ref->{A}{B}{$key}}) { }
1481
01020589
GS
1482Although the deepest nested array or hash will not spring into existence
1483just because its existence was tested, any intervening ones will.
61eff3bc 1484Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
01020589
GS
1485into existence due to the existence test for the $key element above.
1486This happens anywhere the arrow operator is used, including even:
5a964f20 1487
2b5ab1e7
TC
1488 undef $ref;
1489 if (exists $ref->{"Some key"}) { }
1490 print $ref; # prints HASH(0x80d3d5c)
1491
1492This surprising autovivification in what does not at first--or even
1493second--glance appear to be an lvalue context may be fixed in a future
5a964f20 1494release.
a0d0e21e 1495
479ba383
GS
1496See L<perlref/"Pseudo-hashes: Using an array as a hash"> for specifics
1497on how exists() acts when used on a pseudo-hash.
e0478e5a 1498
afebc493
GS
1499Use of a subroutine call, rather than a subroutine name, as an argument
1500to exists() is an error.
1501
1502 exists &sub; # OK
1503 exists &sub(); # Error
1504
a0d0e21e
LW
1505=item exit EXPR
1506
2b5ab1e7 1507Evaluates EXPR and exits immediately with that value. Example:
a0d0e21e
LW
1508
1509 $ans = <STDIN>;
1510 exit 0 if $ans =~ /^[Xx]/;
1511
19799a22 1512See also C<die>. If EXPR is omitted, exits with C<0> status. The only
2b5ab1e7
TC
1513universally recognized values for EXPR are C<0> for success and C<1>
1514for error; other values are subject to interpretation depending on the
1515environment in which the Perl program is running. For example, exiting
151669 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1517the mailer to return the item undelivered, but that's not true everywhere.
a0d0e21e 1518
19799a22
GS
1519Don't use C<exit> to abort a subroutine if there's any chance that
1520someone might want to trap whatever error happened. Use C<die> instead,
1521which can be trapped by an C<eval>.
28757baa 1522
19799a22 1523The exit() function does not always exit immediately. It calls any
2b5ab1e7 1524defined C<END> routines first, but these C<END> routines may not
19799a22 1525themselves abort the exit. Likewise any object destructors that need to
2b5ab1e7
TC
1526be called are called before the real exit. If this is a problem, you
1527can call C<POSIX:_exit($status)> to avoid END and destructor processing.
87275199 1528See L<perlmod> for details.
5a964f20 1529
a0d0e21e
LW
1530=item exp EXPR
1531
54310121 1532=item exp
bbce6d69 1533
2b5ab1e7 1534Returns I<e> (the natural logarithm base) to the power of EXPR.
a0d0e21e
LW
1535If EXPR is omitted, gives C<exp($_)>.
1536
1537=item fcntl FILEHANDLE,FUNCTION,SCALAR
1538
f86cebdf 1539Implements the fcntl(2) function. You'll probably have to say
a0d0e21e
LW
1540
1541 use Fcntl;
1542
0ade1984 1543first to get the correct constant definitions. Argument processing and
19799a22 1544value return works just like C<ioctl> below.
a0d0e21e
LW
1545For example:
1546
1547 use Fcntl;
5a964f20
TC
1548 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1549 or die "can't fcntl F_GETFL: $!";
1550
19799a22 1551You don't have to check for C<defined> on the return from C<fnctl>.
951ba7fe
GS
1552Like C<ioctl>, it maps a C<0> return from the system call into
1553C<"0 but true"> in Perl. This string is true in boolean context and C<0>
2b5ab1e7
TC
1554in numeric context. It is also exempt from the normal B<-w> warnings
1555on improper numeric conversions.
5a964f20 1556
19799a22 1557Note that C<fcntl> will produce a fatal error if used on a machine that
2b5ab1e7
TC
1558doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1559manpage to learn what functions are available on your system.
a0d0e21e
LW
1560
1561=item fileno FILEHANDLE
1562
2b5ab1e7
TC
1563Returns the file descriptor for a filehandle, or undefined if the
1564filehandle is not open. This is mainly useful for constructing
19799a22 1565bitmaps for C<select> and low-level POSIX tty-handling operations.
2b5ab1e7
TC
1566If FILEHANDLE is an expression, the value is taken as an indirect
1567filehandle, generally its name.
5a964f20
TC
1568
1569You can use this to find out whether two handles refer to the
1570same underlying descriptor:
1571
1572 if (fileno(THIS) == fileno(THAT)) {
1573 print "THIS and THAT are dups\n";
1574 }
a0d0e21e
LW
1575
1576=item flock FILEHANDLE,OPERATION
1577
19799a22
GS
1578Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1579for success, false on failure. Produces a fatal error if used on a
2b5ab1e7 1580machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
19799a22 1581C<flock> is Perl's portable file locking interface, although it locks
2b5ab1e7
TC
1582only entire files, not records.
1583
1584Two potentially non-obvious but traditional C<flock> semantics are
1585that it waits indefinitely until the lock is granted, and that its locks
1586B<merely advisory>. Such discretionary locks are more flexible, but offer
19799a22
GS
1587fewer guarantees. This means that files locked with C<flock> may be
1588modified by programs that do not also use C<flock>. See L<perlport>,
2b5ab1e7
TC
1589your port's specific documentation, or your system-specific local manpages
1590for details. It's best to assume traditional behavior if you're writing
1591portable programs. (But if you're not, you should as always feel perfectly
1592free to write for your own system's idiosyncrasies (sometimes called
1593"features"). Slavish adherence to portability concerns shouldn't get
1594in the way of your getting your job done.)
a3cb178b 1595
8ebc5c01
PP
1596OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1597LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
ea3105be 1598you can use the symbolic names if you import them from the Fcntl module,
68dc0745
PP
1599either individually, or as a group using the ':flock' tag. LOCK_SH
1600requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
ea3105be
GS
1601releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1602LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
68dc0745
PP
1603waiting for the lock (check the return status to see if you got it).
1604
2b5ab1e7
TC
1605To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1606before locking or unlocking it.
8ebc5c01 1607
f86cebdf 1608Note that the emulation built with lockf(3) doesn't provide shared
8ebc5c01 1609locks, and it requires that FILEHANDLE be open with write intent. These
2b5ab1e7 1610are the semantics that lockf(3) implements. Most if not all systems
f86cebdf 1611implement lockf(3) in terms of fcntl(2) locking, though, so the
8ebc5c01
PP
1612differing semantics shouldn't bite too many people.
1613
19799a22
GS
1614Note also that some versions of C<flock> cannot lock things over the
1615network; you would need to use the more system-specific C<fcntl> for
f86cebdf
GS
1616that. If you like you can force Perl to ignore your system's flock(2)
1617function, and so provide its own fcntl(2)-based emulation, by passing
8ebc5c01
PP
1618the switch C<-Ud_flock> to the F<Configure> program when you configure
1619perl.
4633a7c4
LW
1620
1621Here's a mailbox appender for BSD systems.
a0d0e21e 1622
7e1af8bc 1623 use Fcntl ':flock'; # import LOCK_* constants
a0d0e21e
LW
1624
1625 sub lock {
7e1af8bc 1626 flock(MBOX,LOCK_EX);
a0d0e21e
LW
1627 # and, in case someone appended
1628 # while we were waiting...
1629 seek(MBOX, 0, 2);
1630 }
1631
1632 sub unlock {
7e1af8bc 1633 flock(MBOX,LOCK_UN);
a0d0e21e
LW
1634 }
1635
1636 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1637 or die "Can't open mailbox: $!";
1638
1639 lock();
1640 print MBOX $msg,"\n\n";
1641 unlock();
1642
2b5ab1e7
TC
1643On systems that support a real flock(), locks are inherited across fork()
1644calls, whereas those that must resort to the more capricious fcntl()
1645function lose the locks, making it harder to write servers.
1646
cb1a09d0 1647See also L<DB_File> for other flock() examples.
a0d0e21e
LW
1648
1649=item fork
1650
2b5ab1e7
TC
1651Does a fork(2) system call to create a new process running the
1652same program at the same point. It returns the child pid to the
1653parent process, C<0> to the child process, or C<undef> if the fork is
1654unsuccessful. File descriptors (and sometimes locks on those descriptors)
1655are shared, while everything else is copied. On most systems supporting
1656fork(), great care has gone into making it extremely efficient (for
1657example, using copy-on-write technology on data pages), making it the
1658dominant paradigm for multitasking over the last few decades.
5a964f20 1659
0f897271
GS
1660Beginning with v5.6.0, Perl will attempt to flush all files opened for
1661output before forking the child process, but this may not be supported
1662on some platforms (see L<perlport>). To be safe, you may need to set
1663C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1664C<IO::Handle> on any open handles in order to avoid duplicate output.
a0d0e21e 1665
19799a22 1666If you C<fork> without ever waiting on your children, you will
2b5ab1e7
TC
1667accumulate zombies. On some systems, you can avoid this by setting
1668C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1669forking and reaping moribund children.
cb1a09d0 1670
28757baa
PP
1671Note that if your forked child inherits system file descriptors like
1672STDIN and STDOUT that are actually connected by a pipe or socket, even
2b5ab1e7 1673if you exit, then the remote server (such as, say, a CGI script or a
19799a22 1674backgrounded job launched from a remote shell) won't think you're done.
2b5ab1e7 1675You should reopen those to F</dev/null> if it's any issue.
28757baa 1676
cb1a09d0
AD
1677=item format
1678
19799a22 1679Declare a picture format for use by the C<write> function. For
cb1a09d0
AD
1680example:
1681
54310121 1682 format Something =
cb1a09d0
AD
1683 Test: @<<<<<<<< @||||| @>>>>>
1684 $str, $%, '$' . int($num)
1685 .
1686
1687 $str = "widget";
184e9718 1688 $num = $cost/$quantity;
cb1a09d0
AD
1689 $~ = 'Something';
1690 write;
1691
1692See L<perlform> for many details and examples.
1693
8903cb82 1694=item formline PICTURE,LIST
a0d0e21e 1695
5a964f20 1696This is an internal function used by C<format>s, though you may call it,
a0d0e21e
LW
1697too. It formats (see L<perlform>) a list of values according to the
1698contents of PICTURE, placing the output into the format output
7660c0ab 1699accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
19799a22 1700Eventually, when a C<write> is done, the contents of
a0d0e21e 1701C<$^A> are written to some filehandle, but you could also read C<$^A>
7660c0ab 1702yourself and then set C<$^A> back to C<"">. Note that a format typically
19799a22 1703does one C<formline> per line of form, but the C<formline> function itself
748a9306 1704doesn't care how many newlines are embedded in the PICTURE. This means
4633a7c4 1705that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
748a9306
LW
1706You may therefore need to use multiple formlines to implement a single
1707record format, just like the format compiler.
1708
19799a22 1709Be careful if you put double quotes around the picture, because an C<@>
748a9306 1710character may be taken to mean the beginning of an array name.
19799a22 1711C<formline> always returns true. See L<perlform> for other examples.
a0d0e21e
LW
1712
1713=item getc FILEHANDLE
1714
1715=item getc
1716
1717Returns the next character from the input file attached to FILEHANDLE,
2b5ab1e7
TC
1718or the undefined value at end of file, or if there was an error.
1719If FILEHANDLE is omitted, reads from STDIN. This is not particularly
1720efficient. However, it cannot be used by itself to fetch single
1721characters without waiting for the user to hit enter. For that, try
1722something more like:
4633a7c4
LW
1723
1724 if ($BSD_STYLE) {
1725 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1726 }
1727 else {
54310121 1728 system "stty", '-icanon', 'eol', "\001";
4633a7c4
LW
1729 }
1730
1731 $key = getc(STDIN);
1732
1733 if ($BSD_STYLE) {
1734 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1735 }
1736 else {
5f05dabc 1737 system "stty", 'icanon', 'eol', '^@'; # ASCII null
4633a7c4
LW
1738 }
1739 print "\n";
1740
54310121
PP
1741Determination of whether $BSD_STYLE should be set
1742is left as an exercise to the reader.
cb1a09d0 1743
19799a22 1744The C<POSIX::getattr> function can do this more portably on
2b5ab1e7
TC
1745systems purporting POSIX compliance. See also the C<Term::ReadKey>
1746module from your nearest CPAN site; details on CPAN can be found on
1747L<perlmodlib/CPAN>.
a0d0e21e
LW
1748
1749=item getlogin
1750
5a964f20
TC
1751Implements the C library function of the same name, which on most
1752systems returns the current login from F</etc/utmp>, if any. If null,
19799a22 1753use C<getpwuid>.
a0d0e21e 1754
f86702cc 1755 $login = getlogin || getpwuid($<) || "Kilroy";
a0d0e21e 1756
19799a22
GS
1757Do not consider C<getlogin> for authentication: it is not as
1758secure as C<getpwuid>.
4633a7c4 1759
a0d0e21e
LW
1760=item getpeername SOCKET
1761
1762Returns the packed sockaddr address of other end of the SOCKET connection.
1763
4633a7c4
LW
1764 use Socket;
1765 $hersockaddr = getpeername(SOCK);
19799a22 1766 ($port, $iaddr) = sockaddr_in($hersockaddr);
4633a7c4
LW
1767 $herhostname = gethostbyaddr($iaddr, AF_INET);
1768 $herstraddr = inet_ntoa($iaddr);
a0d0e21e
LW
1769
1770=item getpgrp PID
1771
47e29363 1772Returns the current process group for the specified PID. Use
7660c0ab 1773a PID of C<0> to get the current process group for the
4633a7c4 1774current process. Will raise an exception if used on a machine that
f86cebdf 1775doesn't implement getpgrp(2). If PID is omitted, returns process
19799a22 1776group of current process. Note that the POSIX version of C<getpgrp>
7660c0ab 1777does not accept a PID argument, so only C<PID==0> is truly portable.
a0d0e21e
LW
1778
1779=item getppid
1780
1781Returns the process id of the parent process.
1782
1783=item getpriority WHICH,WHO
1784
4633a7c4
LW
1785Returns the current priority for a process, a process group, or a user.
1786(See L<getpriority(2)>.) Will raise a fatal exception if used on a
f86cebdf 1787machine that doesn't implement getpriority(2).
a0d0e21e
LW
1788
1789=item getpwnam NAME
1790
1791=item getgrnam NAME
1792
1793=item gethostbyname NAME
1794
1795=item getnetbyname NAME
1796
1797=item getprotobyname NAME
1798
1799=item getpwuid UID
1800
1801=item getgrgid GID
1802
1803=item getservbyname NAME,PROTO
1804
1805=item gethostbyaddr ADDR,ADDRTYPE
1806
1807=item getnetbyaddr ADDR,ADDRTYPE
1808
1809=item getprotobynumber NUMBER
1810
1811=item getservbyport PORT,PROTO
1812
1813=item getpwent
1814
1815=item getgrent
1816
1817=item gethostent
1818
1819=item getnetent
1820
1821=item getprotoent
1822
1823=item getservent
1824
1825=item setpwent
1826
1827=item setgrent
1828
1829=item sethostent STAYOPEN
1830
1831=item setnetent STAYOPEN
1832
1833=item setprotoent STAYOPEN
1834
1835=item setservent STAYOPEN
1836
1837=item endpwent
1838
1839=item endgrent
1840
1841=item endhostent
1842
1843=item endnetent
1844
1845=item endprotoent
1846
1847=item endservent
1848
1849These routines perform the same functions as their counterparts in the
5a964f20 1850system library. In list context, the return values from the
a0d0e21e
LW
1851various get routines are as follows:
1852
1853 ($name,$passwd,$uid,$gid,
6ee623d5 1854 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
a0d0e21e
LW
1855 ($name,$passwd,$gid,$members) = getgr*
1856 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1857 ($name,$aliases,$addrtype,$net) = getnet*
1858 ($name,$aliases,$proto) = getproto*
1859 ($name,$aliases,$port,$proto) = getserv*
1860
1861(If the entry doesn't exist you get a null list.)
1862
4602f195
JH
1863The exact meaning of the $gcos field varies but it usually contains
1864the real name of the user (as opposed to the login name) and other
1865information pertaining to the user. Beware, however, that in many
1866system users are able to change this information and therefore it
106325ad 1867cannot be trusted and therefore the $gcos is tainted (see
2959b6e3
JH
1868L<perlsec>). The $passwd and $shell, user's encrypted password and
1869login shell, are also tainted, because of the same reason.
4602f195 1870
5a964f20 1871In scalar context, you get the name, unless the function was a
a0d0e21e
LW
1872lookup by name, in which case you get the other thing, whatever it is.
1873(If the entry doesn't exist you get the undefined value.) For example:
1874
5a964f20
TC
1875 $uid = getpwnam($name);
1876 $name = getpwuid($num);
1877 $name = getpwent();
1878 $gid = getgrnam($name);
1879 $name = getgrgid($num;
1880 $name = getgrent();
1881 #etc.
a0d0e21e 1882
4602f195
JH
1883In I<getpw*()> the fields $quota, $comment, and $expire are special
1884cases in the sense that in many systems they are unsupported. If the
1885$quota is unsupported, it is an empty scalar. If it is supported, it
1886usually encodes the disk quota. If the $comment field is unsupported,
1887it is an empty scalar. If it is supported it usually encodes some
1888administrative comment about the user. In some systems the $quota
1889field may be $change or $age, fields that have to do with password
1890aging. In some systems the $comment field may be $class. The $expire
1891field, if present, encodes the expiration period of the account or the
1892password. For the availability and the exact meaning of these fields
1893in your system, please consult your getpwnam(3) documentation and your
1894F<pwd.h> file. You can also find out from within Perl what your
1895$quota and $comment fields mean and whether you have the $expire field
1896by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
1897C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
1898files are only supported if your vendor has implemented them in the
1899intuitive fashion that calling the regular C library routines gets the
5d3a0a3b
GS
1900shadow versions if you're running under privilege or if there exists
1901the shadow(3) functions as found in System V ( this includes Solaris
1902and Linux.) Those systems which implement a proprietary shadow password
1903facility are unlikely to be supported.
6ee623d5 1904
19799a22 1905The $members value returned by I<getgr*()> is a space separated list of
a0d0e21e
LW
1906the login names of the members of the group.
1907
1908For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1909C, it will be returned to you via C<$?> if the function call fails. The
7660c0ab 1910C<@addrs> value returned by a successful call is a list of the raw
a0d0e21e
LW
1911addresses returned by the corresponding system library call. In the
1912Internet domain, each address is four bytes long and you can unpack it
1913by saying something like:
1914
1915 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1916
2b5ab1e7
TC
1917The Socket library makes this slightly easier:
1918
1919 use Socket;
1920 $iaddr = inet_aton("127.1"); # or whatever address
1921 $name = gethostbyaddr($iaddr, AF_INET);
1922
1923 # or going the other way
19799a22 1924 $straddr = inet_ntoa($iaddr);
2b5ab1e7 1925
19799a22
GS
1926If you get tired of remembering which element of the return list
1927contains which return value, by-name interfaces are provided
1928in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
1929C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
1930and C<User::grent>. These override the normal built-ins, supplying
1931versions that return objects with the appropriate names
1932for each field. For example:
5a964f20
TC
1933
1934 use File::stat;
1935 use User::pwent;
1936 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
1937
1938Even though it looks like they're the same method calls (uid),
19799a22
GS
1939they aren't, because a C<File::stat> object is different from
1940a C<User::pwent> object.
5a964f20 1941
a0d0e21e
LW
1942=item getsockname SOCKET
1943
19799a22
GS
1944Returns the packed sockaddr address of this end of the SOCKET connection,
1945in case you don't know the address because you have several different
1946IPs that the connection might have come in on.
a0d0e21e 1947
4633a7c4
LW
1948 use Socket;
1949 $mysockaddr = getsockname(SOCK);
19799a22
GS
1950 ($port, $myaddr) = sockaddr_in($mysockaddr);
1951 printf "Connect to %s [%s]\n",
1952 scalar gethostbyaddr($myaddr, AF_INET),
1953 inet_ntoa($myaddr);
a0d0e21e
LW
1954
1955=item getsockopt SOCKET,LEVEL,OPTNAME
1956
5a964f20 1957Returns the socket option requested, or undef if there is an error.
a0d0e21e
LW
1958
1959=item glob EXPR
1960
0a753a76
PP
1961=item glob
1962
2b5ab1e7
TC
1963Returns the value of EXPR with filename expansions such as the
1964standard Unix shell F</bin/csh> would do. This is the internal function
61eff3bc
JH
1965implementing the C<< <*.c> >> operator, but you can use it directly.
1966If EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is
2b5ab1e7 1967discussed in more detail in L<perlop/"I/O Operators">.
a0d0e21e 1968
3a4b19e4
GS
1969Beginning with v5.6.0, this operator is implemented using the standard
1970C<File::Glob> extension. See L<File::Glob> for details.
1971
a0d0e21e
LW
1972=item gmtime EXPR
1973
48a26b3a 1974Converts a time as returned by the time function to a 8-element list
54310121 1975with the time localized for the standard Greenwich time zone.
4633a7c4 1976Typically used as follows:
a0d0e21e 1977
48a26b3a
GS
1978 # 0 1 2 3 4 5 6 7
1979 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday) =
a0d0e21e
LW
1980 gmtime(time);
1981
48a26b3a
GS
1982All list elements are numeric, and come straight out of the C `struct
1983tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
1984specified time. $mday is the day of the month, and $mon is the month
1985itself, in the range C<0..11> with 0 indicating January and 11
1986indicating December. $year is the number of years since 1900. That
1987is, $year is C<123> in year 2023. $wday is the day of the week, with
19880 indicating Sunday and 3 indicating Wednesday. $yday is the day of
874b1813 1989the year, in the range C<0..364> (or C<0..365> in leap years.)
48a26b3a
GS
1990
1991Note that the $year element is I<not> simply the last two digits of
1992the year. If you assume it is, then you create non-Y2K-compliant
1993programs--and you wouldn't want to do that, would you?
2f9daede 1994
abd75f24
GS
1995The proper way to get a complete 4-digit year is simply:
1996
1997 $year += 1900;
1998
1999And to get the last two digits of the year (e.g., '01' in 2001) do:
2000
2001 $year = sprintf("%02d", $year % 100);
2002
48a26b3a 2003If EXPR is omitted, C<gmtime()> uses the current time (C<gmtime(time)>).
a0d0e21e 2004
48a26b3a 2005In scalar context, C<gmtime()> returns the ctime(3) value:
0a753a76
PP
2006
2007 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
2008
19799a22 2009Also see the C<timegm> function provided by the C<Time::Local> module,
f86cebdf 2010and the strftime(3) function available via the POSIX module.
7660c0ab 2011
2b5ab1e7
TC
2012This scalar value is B<not> locale dependent (see L<perllocale>), but
2013is instead a Perl builtin. Also see the C<Time::Local> module, and the
2014strftime(3) and mktime(3) functions available via the POSIX module. To
7660c0ab
A
2015get somewhat similar but locale dependent date strings, set up your
2016locale environment variables appropriately (please see L<perllocale>)
2017and try for example:
2018
2019 use POSIX qw(strftime);
2b5ab1e7 2020 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
7660c0ab 2021
2b5ab1e7
TC
2022Note that the C<%a> and C<%b> escapes, which represent the short forms
2023of the day of the week and the month of the year, may not necessarily
2024be three characters wide in all locales.
0a753a76 2025
a0d0e21e
LW
2026=item goto LABEL
2027
748a9306
LW
2028=item goto EXPR
2029
a0d0e21e
LW
2030=item goto &NAME
2031
7660c0ab 2032The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
a0d0e21e 2033execution there. It may not be used to go into any construct that
7660c0ab 2034requires initialization, such as a subroutine or a C<foreach> loop. It
0a753a76 2035also can't be used to go into a construct that is optimized away,
19799a22 2036or to get out of a block or subroutine given to C<sort>.
0a753a76 2037It can be used to go almost anywhere else within the dynamic scope,
a0d0e21e 2038including out of subroutines, but it's usually better to use some other
19799a22 2039construct such as C<last> or C<die>. The author of Perl has never felt the
7660c0ab 2040need to use this form of C<goto> (in Perl, that is--C is another matter).
a0d0e21e 2041
7660c0ab
A
2042The C<goto-EXPR> form expects a label name, whose scope will be resolved
2043dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
748a9306
LW
2044necessarily recommended if you're optimizing for maintainability:
2045
2046 goto ("FOO", "BAR", "GLARCH")[$i];
2047
6cb9131c
GS
2048The C<goto-&NAME> form is quite different from the other forms of C<goto>.
2049In fact, it isn't a goto in the normal sense at all, and doesn't have
2050the stigma associated with other gotos. Instead, it
2051substitutes a call to the named subroutine for the currently running
2052subroutine. This is used by C<AUTOLOAD> subroutines that wish to load
2053another subroutine and then pretend that the other subroutine had been
2054called in the first place (except that any modifications to C<@_>
2055in the current subroutine are propagated to the other subroutine.)
2056After the C<goto>, not even C<caller> will be able to tell that this
2057routine was called first.
2058
2059NAME needn't be the name of a subroutine; it can be a scalar variable
2060containing a code reference, or a block which evaluates to a code
2061reference.
a0d0e21e
LW
2062
2063=item grep BLOCK LIST
2064
2065=item grep EXPR,LIST
2066
2b5ab1e7
TC
2067This is similar in spirit to, but not the same as, grep(1) and its
2068relatives. In particular, it is not limited to using regular expressions.
2f9daede 2069
a0d0e21e 2070Evaluates the BLOCK or EXPR for each element of LIST (locally setting
7660c0ab 2071C<$_> to each element) and returns the list value consisting of those
19799a22
GS
2072elements for which the expression evaluated to true. In scalar
2073context, returns the number of times the expression was true.
a0d0e21e
LW
2074
2075 @foo = grep(!/^#/, @bar); # weed out comments
2076
2077or equivalently,
2078
2079 @foo = grep {!/^#/} @bar; # weed out comments
2080
2b5ab1e7
TC
2081Note that, because C<$_> is a reference into the list value, it can
2082be used to modify the elements of the array. While this is useful and
2083supported, it can cause bizarre results if the LIST is not a named array.
2084Similarly, grep returns aliases into the original list, much as a for
2085loop's index variable aliases the list elements. That is, modifying an
19799a22
GS
2086element of a list returned by grep (for example, in a C<foreach>, C<map>
2087or another C<grep>) actually modifies the element in the original list.
2b5ab1e7 2088This is usually something to be avoided when writing clear code.
a0d0e21e 2089
19799a22 2090See also L</map> for a list composed of the results of the BLOCK or EXPR.
38325410 2091
a0d0e21e
LW
2092=item hex EXPR
2093
54310121 2094=item hex
bbce6d69 2095
2b5ab1e7
TC
2096Interprets EXPR as a hex string and returns the corresponding value.
2097(To convert strings that might start with either 0, 0x, or 0b, see
2098L</oct>.) If EXPR is omitted, uses C<$_>.
2f9daede
TPG
2099
2100 print hex '0xAf'; # prints '175'
2101 print hex 'aF'; # same
a0d0e21e 2102
19799a22 2103Hex strings may only represent integers. Strings that would cause
c6edd1b7 2104integer overflow trigger a warning.
19799a22 2105
a0d0e21e
LW
2106=item import
2107
19799a22 2108There is no builtin C<import> function. It is just an ordinary
4633a7c4 2109method (subroutine) defined (or inherited) by modules that wish to export
19799a22 2110names to another module. The C<use> function calls the C<import> method
54310121 2111for the package used. See also L</use()>, L<perlmod>, and L<Exporter>.
a0d0e21e
LW
2112
2113=item index STR,SUBSTR,POSITION
2114
2115=item index STR,SUBSTR
2116
2b5ab1e7
TC
2117The index function searches for one string within another, but without
2118the wildcard-like behavior of a full regular-expression pattern match.
2119It returns the position of the first occurrence of SUBSTR in STR at
2120or after POSITION. If POSITION is omitted, starts searching from the
2121beginning of the string. The return value is based at C<0> (or whatever
2122you've set the C<$[> variable to--but don't do that). If the substring
2123is not found, returns one less than the base, ordinarily C<-1>.
a0d0e21e
LW
2124
2125=item int EXPR
2126
54310121 2127=item int
bbce6d69 2128
7660c0ab 2129Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2b5ab1e7
TC
2130You should not use this function for rounding: one because it truncates
2131towards C<0>, and two because machine representations of floating point
2132numbers can sometimes produce counterintuitive results. For example,
2133C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2134because it's really more like -268.99999999999994315658 instead. Usually,
19799a22 2135the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2b5ab1e7 2136functions will serve you better than will int().
a0d0e21e
LW
2137
2138=item ioctl FILEHANDLE,FUNCTION,SCALAR
2139
2b5ab1e7 2140Implements the ioctl(2) function. You'll probably first have to say
a0d0e21e 2141
4633a7c4 2142 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
a0d0e21e 2143
2b5ab1e7 2144to get the correct function definitions. If F<ioctl.ph> doesn't
a0d0e21e 2145exist or doesn't have the correct definitions you'll have to roll your
61eff3bc 2146own, based on your C header files such as F<< <sys/ioctl.h> >>.
5a964f20 2147(There is a Perl script called B<h2ph> that comes with the Perl kit that
54310121 2148may help you in this, but it's nontrivial.) SCALAR will be read and/or
4633a7c4 2149written depending on the FUNCTION--a pointer to the string value of SCALAR
19799a22 2150will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
4633a7c4
LW
2151has no string value but does have a numeric value, that value will be
2152passed rather than a pointer to the string value. To guarantee this to be
19799a22
GS
2153true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2154functions may be needed to manipulate the values of structures used by
2155C<ioctl>.
a0d0e21e 2156
19799a22 2157The return value of C<ioctl> (and C<fcntl>) is as follows:
a0d0e21e
LW
2158
2159 if OS returns: then Perl returns:
2160 -1 undefined value
2161 0 string "0 but true"
2162 anything else that number
2163
19799a22 2164Thus Perl returns true on success and false on failure, yet you can
a0d0e21e
LW
2165still easily determine the actual value returned by the operating
2166system:
2167
2b5ab1e7 2168 $retval = ioctl(...) || -1;
a0d0e21e
LW
2169 printf "System returned %d\n", $retval;
2170
c2611fb3 2171The special string "C<0> but true" is exempt from B<-w> complaints
5a964f20
TC
2172about improper numeric conversions.
2173
19799a22
GS
2174Here's an example of setting a filehandle named C<REMOTE> to be
2175non-blocking at the system level. You'll have to negotiate C<$|>
2176on your own, though.
2177
2178 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
2179
2180 $flags = fcntl(REMOTE, F_GETFL, 0)
2181 or die "Can't get flags for the socket: $!\n";
2182
2183 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
2184 or die "Can't set flags for the socket: $!\n";
2185
a0d0e21e
LW
2186=item join EXPR,LIST
2187
2b5ab1e7
TC
2188Joins the separate strings of LIST into a single string with fields
2189separated by the value of EXPR, and returns that new string. Example:
a0d0e21e 2190
2b5ab1e7 2191 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
a0d0e21e 2192
eb6e2d6f
GS
2193Beware that unlike C<split>, C<join> doesn't take a pattern as its
2194first argument. Compare L</split>.
a0d0e21e 2195
aa689395
PP
2196=item keys HASH
2197
19799a22 2198Returns a list consisting of all the keys of the named hash. (In
1d2dff63 2199scalar context, returns the number of keys.) The keys are returned in
ab192400
GS
2200an apparently random order. The actual random order is subject to
2201change in future versions of perl, but it is guaranteed to be the same
19799a22 2202order as either the C<values> or C<each> function produces (given
ab192400
GS
2203that the hash has not been modified). As a side effect, it resets
2204HASH's iterator.
a0d0e21e 2205
aa689395 2206Here is yet another way to print your environment:
a0d0e21e
LW
2207
2208 @keys = keys %ENV;
2209 @values = values %ENV;
19799a22 2210 while (@keys) {
a0d0e21e
LW
2211 print pop(@keys), '=', pop(@values), "\n";
2212 }
2213
2214or how about sorted by key:
2215
2216 foreach $key (sort(keys %ENV)) {
2217 print $key, '=', $ENV{$key}, "\n";
2218 }
2219
8ea1e5d4
GS
2220The returned values are copies of the original keys in the hash, so
2221modifying them will not affect the original hash. Compare L</values>.
2222
19799a22 2223To sort a hash by value, you'll need to use a C<sort> function.
aa689395 2224Here's a descending numeric sort of a hash by its values:
4633a7c4 2225
5a964f20 2226 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
4633a7c4
LW
2227 printf "%4d %s\n", $hash{$key}, $key;
2228 }
2229
19799a22 2230As an lvalue C<keys> allows you to increase the number of hash buckets
aa689395
PP
2231allocated for the given hash. This can gain you a measure of efficiency if
2232you know the hash is going to get big. (This is similar to pre-extending
2233an array by assigning a larger number to $#array.) If you say
55497cff
PP
2234
2235 keys %hash = 200;
2236
ab192400
GS
2237then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2238in fact, since it rounds up to the next power of two. These
55497cff
PP
2239buckets will be retained even if you do C<%hash = ()>, use C<undef
2240%hash> if you want to free the storage while C<%hash> is still in scope.
2241You can't shrink the number of buckets allocated for the hash using
19799a22 2242C<keys> in this way (but you needn't worry about doing this by accident,
55497cff
PP
2243as trying has no effect).
2244
19799a22 2245See also C<each>, C<values> and C<sort>.
ab192400 2246
b350dd2f 2247=item kill SIGNAL, LIST
a0d0e21e 2248
b350dd2f 2249Sends a signal to a list of processes. Returns the number of
517db077
GS
2250processes successfully signaled (which is not necessarily the
2251same as the number actually killed).
a0d0e21e
LW
2252
2253 $cnt = kill 1, $child1, $child2;
2254 kill 9, @goners;
2255
b350dd2f
GS
2256If SIGNAL is zero, no signal is sent to the process. This is a
2257useful way to check that the process is alive and hasn't changed
2258its UID. See L<perlport> for notes on the portability of this
2259construct.
2260
2261Unlike in the shell, if SIGNAL is negative, it kills
4633a7c4
LW
2262process groups instead of processes. (On System V, a negative I<PROCESS>
2263number will also kill process groups, but that's not portable.) That
2264means you usually want to use positive not negative signals. You may also
da0045b7 2265use a signal name in quotes. See L<perlipc/"Signals"> for details.
a0d0e21e
LW
2266
2267=item last LABEL
2268
2269=item last
2270
2271The C<last> command is like the C<break> statement in C (as used in
2272loops); it immediately exits the loop in question. If the LABEL is
2273omitted, the command refers to the innermost enclosing loop. The
2274C<continue> block, if any, is not executed:
2275
4633a7c4
LW
2276 LINE: while (<STDIN>) {
2277 last LINE if /^$/; # exit when done with header
5a964f20 2278 #...
a0d0e21e
LW
2279 }
2280
4968c1e4 2281C<last> cannot be used to exit a block which returns a value such as
2b5ab1e7
TC
2282C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2283a grep() or map() operation.
4968c1e4 2284
6c1372ed
GS
2285Note that a block by itself is semantically identical to a loop
2286that executes once. Thus C<last> can be used to effect an early
2287exit out of such a block.
2288
98293880
JH
2289See also L</continue> for an illustration of how C<last>, C<next>, and
2290C<redo> work.
1d2dff63 2291
a0d0e21e
LW
2292=item lc EXPR
2293
54310121 2294=item lc
bbce6d69 2295
a0d0e21e 2296Returns an lowercased version of EXPR. This is the internal function
7660c0ab 2297implementing the C<\L> escape in double-quoted strings.
19799a22
GS
2298Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
2299and L<utf8>.
a0d0e21e 2300
7660c0ab 2301If EXPR is omitted, uses C<$_>.
bbce6d69 2302
a0d0e21e
LW
2303=item lcfirst EXPR
2304
54310121 2305=item lcfirst
bbce6d69 2306
a0d0e21e 2307Returns the value of EXPR with the first character lowercased. This is
7660c0ab 2308the internal function implementing the C<\l> escape in double-quoted strings.
a0ed51b3 2309Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
a0d0e21e 2310
7660c0ab 2311If EXPR is omitted, uses C<$_>.
bbce6d69 2312
a0d0e21e
LW
2313=item length EXPR
2314
54310121 2315=item length
bbce6d69 2316
a0ed51b3 2317Returns the length in characters of the value of EXPR. If EXPR is
2b5ab1e7
TC
2318omitted, returns length of C<$_>. Note that this cannot be used on
2319an entire array or hash to find out how many elements these have.
2320For that, use C<scalar @array> and C<scalar keys %hash> respectively.
a0d0e21e
LW
2321
2322=item link OLDFILE,NEWFILE
2323
19799a22
GS
2324Creates a new filename linked to the old filename. Returns true for
2325success, false otherwise.
a0d0e21e
LW
2326
2327=item listen SOCKET,QUEUESIZE
2328
19799a22
GS
2329Does the same thing that the listen system call does. Returns true if
2330it succeeded, false otherwise. See the example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
2331
2332=item local EXPR
2333
19799a22 2334You really probably want to be using C<my> instead, because C<local> isn't
2b5ab1e7
TC
2335what most people think of as "local". See L<perlsub/"Private Variables
2336via my()"> for details.
2337
5a964f20
TC
2338A local modifies the listed variables to be local to the enclosing
2339block, file, or eval. If more than one value is listed, the list must
2340be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2341for details, including issues with tied arrays and hashes.
a0d0e21e 2342
a0d0e21e
LW
2343=item localtime EXPR
2344
19799a22 2345Converts a time as returned by the time function to a 9-element list
5f05dabc 2346with the time analyzed for the local time zone. Typically used as
a0d0e21e
LW
2347follows:
2348
54310121 2349 # 0 1 2 3 4 5 6 7 8
a0d0e21e
LW
2350 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2351 localtime(time);
2352
48a26b3a
GS
2353All list elements are numeric, and come straight out of the C `struct
2354tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2355specified time. $mday is the day of the month, and $mon is the month
2356itself, in the range C<0..11> with 0 indicating January and 11
2357indicating December. $year is the number of years since 1900. That
2358is, $year is C<123> in year 2023. $wday is the day of the week, with
23590 indicating Sunday and 3 indicating Wednesday. $yday is the day of
874b1813 2360the year, in the range C<0..364> (or C<0..365> in leap years.) $isdst
48a26b3a
GS
2361is true if the specified time occurs during daylight savings time,
2362false otherwise.
2363
2364Note that the $year element is I<not> simply the last two digits of
2365the year. If you assume it is, then you create non-Y2K-compliant
2366programs--and you wouldn't want to do that, would you?
54310121 2367
abd75f24
GS
2368The proper way to get a complete 4-digit year is simply:
2369
2370 $year += 1900;
2371
2372And to get the last two digits of the year (e.g., '01' in 2001) do:
2373
2374 $year = sprintf("%02d", $year % 100);
2375
48a26b3a 2376If EXPR is omitted, C<localtime()> uses the current time (C<localtime(time)>).
a0d0e21e 2377
48a26b3a 2378In scalar context, C<localtime()> returns the ctime(3) value:
a0d0e21e 2379
5f05dabc 2380 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
a0d0e21e 2381
a3cb178b 2382This scalar value is B<not> locale dependent, see L<perllocale>, but
68f8bed4
JH
2383instead a Perl builtin. Also see the C<Time::Local> module
2384(to convert the second, minutes, hours, ... back to seconds since the
2385stroke of midnight the 1st of January 1970, the value returned by
ca6e1c26 2386time()), and the strftime(3) and mktime(3) functions available via the
68f8bed4
JH
2387POSIX module. To get somewhat similar but locale dependent date
2388strings, set up your locale environment variables appropriately
2389(please see L<perllocale>) and try for example:
a3cb178b 2390
5a964f20 2391 use POSIX qw(strftime);
2b5ab1e7 2392 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
a3cb178b
GS
2393
2394Note that the C<%a> and C<%b>, the short forms of the day of the week
2395and the month of the year, may not necessarily be three characters wide.
a0d0e21e 2396
19799a22
GS
2397=item lock
2398
2399 lock I<THING>
2400
2401This function places an advisory lock on a variable, subroutine,
2402or referenced object contained in I<THING> until the lock goes out
2403of scope. This is a built-in function only if your version of Perl
2404was built with threading enabled, and if you've said C<use Threads>.
2405Otherwise a user-defined function by this name will be called. See
2406L<Thread>.
2407
a0d0e21e
LW
2408=item log EXPR
2409
54310121 2410=item log
bbce6d69 2411
2b5ab1e7
TC
2412Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2413returns log of C<$_>. To get the log of another base, use basic algebra:
19799a22 2414The base-N log of a number is equal to the natural log of that number
2b5ab1e7
TC
2415divided by the natural log of N. For example:
2416
2417 sub log10 {
2418 my $n = shift;
2419 return log($n)/log(10);
2420 }
2421
2422See also L</exp> for the inverse operation.
a0d0e21e
LW
2423
2424=item lstat FILEHANDLE
2425
2426=item lstat EXPR
2427
54310121 2428=item lstat
bbce6d69 2429
19799a22 2430Does the same thing as the C<stat> function (including setting the
5a964f20
TC
2431special C<_> filehandle) but stats a symbolic link instead of the file
2432the symbolic link points to. If symbolic links are unimplemented on
19799a22 2433your system, a normal C<stat> is done.
a0d0e21e 2434
7660c0ab 2435If EXPR is omitted, stats C<$_>.
bbce6d69 2436
a0d0e21e
LW
2437=item m//
2438
2439The match operator. See L<perlop>.
2440
2441=item map BLOCK LIST
2442
2443=item map EXPR,LIST
2444
19799a22
GS
2445Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2446C<$_> to each element) and returns the list value composed of the
2447results of each such evaluation. In scalar context, returns the
2448total number of elements so generated. Evaluates BLOCK or EXPR in
2449list context, so each element of LIST may produce zero, one, or
2450more elements in the returned value.
dd99ebda 2451
a0d0e21e
LW
2452 @chars = map(chr, @nums);
2453
2454translates a list of numbers to the corresponding characters. And
2455
4633a7c4 2456 %hash = map { getkey($_) => $_ } @array;
a0d0e21e
LW
2457
2458is just a funny way to write
2459
2460 %hash = ();
2461 foreach $_ (@array) {
4633a7c4 2462 $hash{getkey($_)} = $_;
a0d0e21e
LW
2463 }
2464
2b5ab1e7
TC
2465Note that, because C<$_> is a reference into the list value, it can
2466be used to modify the elements of the array. While this is useful and
2467supported, it can cause bizarre results if the LIST is not a named array.
2468Using a regular C<foreach> loop for this purpose would be clearer in
2469most cases. See also L</grep> for an array composed of those items of
2470the original list for which the BLOCK or EXPR evaluates to true.
fb73857a 2471
19799a22 2472=item mkdir FILENAME,MASK
a0d0e21e 2473
5a211162
GS
2474=item mkdir FILENAME
2475
0591cd52 2476Creates the directory specified by FILENAME, with permissions
19799a22
GS
2477specified by MASK (as modified by C<umask>). If it succeeds it
2478returns true, otherwise it returns false and sets C<$!> (errno).
5a211162 2479If omitted, MASK defaults to 0777.
0591cd52 2480
19799a22 2481In general, it is better to create directories with permissive MASK,
0591cd52 2482and let the user modify that with their C<umask>, than it is to supply
19799a22 2483a restrictive MASK and give the user no way to be more permissive.
0591cd52
NT
2484The exceptions to this rule are when the file or directory should be
2485kept private (mail files, for instance). The perlfunc(1) entry on
19799a22 2486C<umask> discusses the choice of MASK in more detail.
a0d0e21e
LW
2487
2488=item msgctl ID,CMD,ARG
2489
f86cebdf 2490Calls the System V IPC function msgctl(2). You'll probably have to say
0ade1984
JH
2491
2492 use IPC::SysV;
2493
7660c0ab
A
2494first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2495then ARG must be a variable which will hold the returned C<msqid_ds>
951ba7fe
GS
2496structure. Returns like C<ioctl>: the undefined value for error,
2497C<"0 but true"> for zero, or the actual return value otherwise. See also
19799a22 2498C<IPC::SysV> and C<IPC::Semaphore> documentation.
a0d0e21e
LW
2499
2500=item msgget KEY,FLAGS
2501
f86cebdf 2502Calls the System V IPC function msgget(2). Returns the message queue
7660c0ab 2503id, or the undefined value if there is an error. See also C<IPC::SysV>
19799a22 2504and C<IPC::Msg> documentation.
a0d0e21e 2505
a0d0e21e
LW
2506=item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2507
2508Calls the System V IPC function msgrcv to receive a message from
2509message queue ID into variable VAR with a maximum message size of
41d6edb2
JH
2510SIZE. Note that when a message is received, the message type as a
2511native long integer will be the first thing in VAR, followed by the
2512actual message. This packing may be opened with C<unpack("l! a*")>.
2513Taints the variable. Returns true if successful, or false if there is
2514an error. See also C<IPC::SysV> and C<IPC::SysV::Msg> documentation.
2515
2516=item msgsnd ID,MSG,FLAGS
2517
2518Calls the System V IPC function msgsnd to send the message MSG to the
2519message queue ID. MSG must begin with the native long integer message
2520type, and be followed by the length of the actual message, and finally
2521the message itself. This kind of packing can be achieved with
2522C<pack("l! a*", $type, $message)>. Returns true if successful,
2523or false if there is an error. See also C<IPC::SysV>
2524and C<IPC::SysV::Msg> documentation.
a0d0e21e
LW
2525
2526=item my EXPR
2527
09bef843
SB
2528=item my EXPR : ATTRIBUTES
2529
19799a22
GS
2530A C<my> declares the listed variables to be local (lexically) to the
2531enclosing block, file, or C<eval>. If
5f05dabc 2532more than one value is listed, the list must be placed in parentheses. See
cb1a09d0 2533L<perlsub/"Private Variables via my()"> for details.
4633a7c4 2534
a0d0e21e
LW
2535=item next LABEL
2536
2537=item next
2538
2539The C<next> command is like the C<continue> statement in C; it starts
2540the next iteration of the loop:
2541
4633a7c4
LW
2542 LINE: while (<STDIN>) {
2543 next LINE if /^#/; # discard comments
5a964f20 2544 #...
a0d0e21e
LW
2545 }
2546
2547Note that if there were a C<continue> block on the above, it would get
2548executed even on discarded lines. If the LABEL is omitted, the command
2549refers to the innermost enclosing loop.
2550
4968c1e4 2551C<next> cannot be used to exit a block which returns a value such as
2b5ab1e7
TC
2552C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2553a grep() or map() operation.
4968c1e4 2554
6c1372ed
GS
2555Note that a block by itself is semantically identical to a loop
2556that executes once. Thus C<next> will exit such a block early.
2557
98293880
JH
2558See also L</continue> for an illustration of how C<last>, C<next>, and
2559C<redo> work.
1d2dff63 2560
a0d0e21e
LW
2561=item no Module LIST
2562
7660c0ab 2563See the L</use> function, which C<no> is the opposite of.
a0d0e21e
LW
2564
2565=item oct EXPR
2566
54310121 2567=item oct
bbce6d69 2568
4633a7c4 2569Interprets EXPR as an octal string and returns the corresponding
4f19785b
WSI
2570value. (If EXPR happens to start off with C<0x>, interprets it as a
2571hex string. If EXPR starts off with C<0b>, it is interpreted as a
2572binary string.) The following will handle decimal, binary, octal, and
4633a7c4 2573hex in the standard Perl or C notation:
a0d0e21e
LW
2574
2575 $val = oct($val) if $val =~ /^0/;
2576
19799a22
GS
2577If EXPR is omitted, uses C<$_>. To go the other way (produce a number
2578in octal), use sprintf() or printf():
2579
2580 $perms = (stat("filename"))[2] & 07777;
2581 $oct_perms = sprintf "%lo", $perms;
2582
2583The oct() function is commonly used when a string such as C<644> needs
2584to be converted into a file mode, for example. (Although perl will
2585automatically convert strings into numbers as needed, this automatic
2586conversion assumes base 10.)
a0d0e21e 2587
1c1fc3ea 2588=item open FILEHANDLE,MODE,LIST
6170680b 2589
a0d0e21e
LW
2590=item open FILEHANDLE,EXPR
2591
2592=item open FILEHANDLE
2593
2594Opens the file whose filename is given by EXPR, and associates it with
5f05dabc 2595FILEHANDLE. If FILEHANDLE is an expression, its value is used as the
d6fd2b02
GS
2596name of the real filehandle wanted. (This is considered a symbolic
2597reference, so C<use strict 'refs'> should I<not> be in effect.)
2598
2599If EXPR is omitted, the scalar
5f05dabc 2600variable of the same name as the FILEHANDLE contains the filename.
19799a22
GS
2601(Note that lexical variables--those declared with C<my>--will not work
2602for this purpose; so if you're using C<my>, specify EXPR in your call
2b5ab1e7
TC
2603to open.) See L<perlopentut> for a kinder, gentler explanation of opening
2604files.
5f05dabc 2605
61eff3bc
JH
2606If MODE is C<< '<' >> or nothing, the file is opened for input.
2607If MODE is C<< '>' >>, the file is truncated and opened for
2608output, being created if necessary. If MODE is C<<< '>>' >>>,
fbb426e4 2609the file is opened for appending, again being created if necessary.
61eff3bc
JH
2610You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to indicate that
2611you want both read and write access to the file; thus C<< '+<' >> is almost
2612always preferred for read/write updates--the C<< '+>' >> mode would clobber the
5a964f20
TC
2613file first. You can't usually use either read-write mode for updating
2614textfiles, since they have variable length records. See the B<-i>
0591cd52
NT
2615switch in L<perlrun> for a better approach. The file is created with
2616permissions of C<0666> modified by the process' C<umask> value.
5a964f20 2617
61eff3bc
JH
2618These various prefixes correspond to the fopen(3) modes of C<'r'>, C<'r+'>,
2619C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
5f05dabc 2620
6170680b
IZ
2621In the 2-arguments (and 1-argument) form of the call the mode and
2622filename should be concatenated (in this order), possibly separated by
61eff3bc 2623spaces. It is possible to omit the mode if the mode is C<< '<' >>.
6170680b 2624
7660c0ab 2625If the filename begins with C<'|'>, the filename is interpreted as a
5a964f20 2626command to which output is to be piped, and if the filename ends with a
f244e06d
GS
2627C<'|'>, the filename is interpreted as a command which pipes output to
2628us. See L<perlipc/"Using open() for IPC">
19799a22 2629for more examples of this. (You are not allowed to C<open> to a command
5a964f20 2630that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
4a4eefd0
GS
2631and L<perlipc/"Bidirectional Communication with Another Process">
2632for alternatives.)
cb1a09d0 2633
6170680b
IZ
2634If MODE is C<'|-'>, the filename is interpreted as a
2635command to which output is to be piped, and if MODE is
2636C<'-|'>, the filename is interpreted as a command which pipes output to
2637us. In the 2-arguments (and 1-argument) form one should replace dash
2638(C<'-'>) with the command. See L<perlipc/"Using open() for IPC">
2639for more examples of this. (You are not allowed to C<open> to a command
2640that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2641and L<perlipc/"Bidirectional Communication"> for alternatives.)
2642
2643In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
61eff3bc 2644and opening C<< '>-' >> opens STDOUT.
6170680b
IZ
2645
2646Open returns
19799a22 2647nonzero upon success, the undefined value otherwise. If the C<open>
4633a7c4 2648involved a pipe, the return value happens to be the pid of the
54310121 2649subprocess.
cb1a09d0
AD
2650
2651If you're unfortunate enough to be running Perl on a system that
2652distinguishes between text files and binary files (modern operating
2653systems don't care), then you should check out L</binmode> for tips for
19799a22 2654dealing with this. The key distinction between systems that need C<binmode>
5a964f20
TC
2655and those that don't is their text file formats. Systems like Unix, MacOS, and
2656Plan9, which delimit lines with a single character, and which encode that
19799a22 2657character in C as C<"\n">, do not need C<binmode>. The rest need it.
cb1a09d0 2658
fb73857a 2659When opening a file, it's usually a bad idea to continue normal execution
19799a22
GS
2660if the request failed, so C<open> is frequently used in connection with
2661C<die>. Even if C<die> won't do what you want (say, in a CGI script,
fb73857a 2662where you want to make a nicely formatted error message (but there are
5a964f20 2663modules that can help with that problem)) you should always check
19799a22 2664the return value from opening a file. The infrequent exception is when
fb73857a
PP
2665working with an unopened filehandle is actually what you want to do.
2666
cb1a09d0 2667Examples:
a0d0e21e
LW
2668
2669 $ARTICLE = 100;
2670 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2671 while (<ARTICLE>) {...
2672
6170680b 2673 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
fb73857a 2674 # if the open fails, output is discarded
a0d0e21e 2675
6170680b 2676 open(DBASE, '+<', 'dbase.mine') # open for update
fb73857a 2677 or die "Can't open 'dbase.mine' for update: $!";
cb1a09d0 2678
6170680b
IZ
2679 open(DBASE, '+<dbase.mine') # ditto
2680 or die "Can't open 'dbase.mine' for update: $!";
2681
2682 open(ARTICLE, '-|', "caesar <$article") # decrypt article
fb73857a 2683 or die "Can't start caesar: $!";
a0d0e21e 2684
6170680b
IZ
2685 open(ARTICLE, "caesar <$article |") # ditto
2686 or die "Can't start caesar: $!";
2687
2688 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
fb73857a 2689 or die "Can't start sort: $!";
a0d0e21e
LW
2690
2691 # process argument list of files along with any includes
2692
2693 foreach $file (@ARGV) {
2694 process($file, 'fh00');
2695 }
2696
2697 sub process {
5a964f20 2698 my($filename, $input) = @_;
a0d0e21e
LW
2699 $input++; # this is a string increment
2700 unless (open($input, $filename)) {
2701 print STDERR "Can't open $filename: $!\n";
2702 return;
2703 }
2704
5a964f20 2705 local $_;
a0d0e21e
LW
2706 while (<$input>) { # note use of indirection
2707 if (/^#include "(.*)"/) {
2708 process($1, $input);
2709 next;
2710 }
5a964f20 2711 #... # whatever
a0d0e21e
LW
2712 }
2713 }
2714
2715You may also, in the Bourne shell tradition, specify an EXPR beginning
61eff3bc 2716with C<< '>&' >>, in which case the rest of the string is interpreted as the
5a964f20 2717name of a filehandle (or file descriptor, if numeric) to be
61eff3bc
JH
2718duped and opened. You may use C<&> after C<< > >>, C<<< >> >>>,
2719C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>. The
a0d0e21e 2720mode you specify should match the mode of the original filehandle.
184e9718 2721(Duping a filehandle does not take into account any existing contents of
6170680b
IZ
2722stdio buffers.) Duping file handles is not yet supported for 3-argument
2723open().
2724
a0d0e21e
LW
2725Here is a script that saves, redirects, and restores STDOUT and
2726STDERR:
2727
2728 #!/usr/bin/perl
5a964f20
TC
2729 open(OLDOUT, ">&STDOUT");
2730 open(OLDERR, ">&STDERR");
a0d0e21e 2731
6170680b
IZ
2732 open(STDOUT, '>', "foo.out") || die "Can't redirect stdout";
2733 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
a0d0e21e
LW
2734
2735 select(STDERR); $| = 1; # make unbuffered
2736 select(STDOUT); $| = 1; # make unbuffered
2737
2738 print STDOUT "stdout 1\n"; # this works for
2739 print STDERR "stderr 1\n"; # subprocesses too
2740
2741 close(STDOUT);
2742 close(STDERR);
2743
5a964f20
TC
2744 open(STDOUT, ">&OLDOUT");
2745 open(STDERR, ">&OLDERR");
a0d0e21e
LW
2746
2747 print STDOUT "stdout 2\n";
2748 print STDERR "stderr 2\n";
2749
61eff3bc 2750If you specify C<< '<&=N' >>, where C<N> is a number, then Perl will do an
19799a22 2751equivalent of C's C<fdopen> of that file descriptor; this is more
4633a7c4 2752parsimonious of file descriptors. For example:
a0d0e21e
LW
2753
2754 open(FILEHANDLE, "<&=$fd")
2755
4af147f6
CS
2756Note that this feature depends on the fdopen() C library function.
2757On many UNIX systems, fdopen() is known to fail when file descriptors
2758exceed a certain value, typically 255. If you need more file
2759descriptors than that, consider rebuilding Perl to use the C<sfio>
2760library.
2761
6170680b
IZ
2762If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
2763with 2-arguments (or 1-argument) form of open(), then
a0d0e21e 2764there is an implicit fork done, and the return value of open is the pid
7660c0ab 2765of the child within the parent process, and C<0> within the child
184e9718 2766process. (Use C<defined($pid)> to determine whether the open was successful.)
a0d0e21e
LW
2767The filehandle behaves normally for the parent, but i/o to that
2768filehandle is piped from/to the STDOUT/STDIN of the child process.
2769In the child process the filehandle isn't opened--i/o happens from/to
2770the new STDOUT or STDIN. Typically this is used like the normal
2771piped open when you want to exercise more control over just how the
2772pipe command gets executed, such as when you are running setuid, and
54310121 2773don't want to have to scan shell commands for metacharacters.
6170680b 2774The following triples are more or less equivalent:
a0d0e21e
LW
2775
2776 open(FOO, "|tr '[a-z]' '[A-Z]'");
6170680b
IZ
2777 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
2778 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
a0d0e21e
LW
2779
2780 open(FOO, "cat -n '$file'|");
6170680b
IZ
2781 open(FOO, '-|', "cat -n '$file'");
2782 open(FOO, '-|') || exec 'cat', '-n', $file;
a0d0e21e 2783
4633a7c4
LW
2784See L<perlipc/"Safe Pipe Opens"> for more examples of this.
2785
0f897271
GS
2786Beginning with v5.6.0, Perl will attempt to flush all files opened for
2787output before any operation that may do a fork, but this may not be
2788supported on some platforms (see L<perlport>). To be safe, you may need
2789to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
2790of C<IO::Handle> on any open handles.
2791
2792On systems that support a
45bc9206
GS
2793close-on-exec flag on files, the flag will be set for the newly opened
2794file descriptor as determined by the value of $^F. See L<perlvar/$^F>.
a0d0e21e 2795
0dccf244
CS
2796Closing any piped filehandle causes the parent process to wait for the
2797child to finish, and returns the status value in C<$?>.
2798
6170680b
IZ
2799The filename passed to 2-argument (or 1-argument) form of open()
2800will have leading and trailing
f86cebdf 2801whitespace deleted, and the normal redirection characters
5a964f20
TC
2802honored. This property, known as "magic open",
2803can often be used to good effect. A user could specify a filename of
7660c0ab 2804F<"rsh cat file |">, or you could change certain filenames as needed:
5a964f20
TC
2805
2806 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
2807 open(FH, $filename) or die "Can't open $filename: $!";
2808
6170680b
IZ
2809Use 3-argument form to open a file with arbitrary weird characters in it,
2810
2811 open(FOO, '<', $file);
2812
2813otherwise it's necessary to protect any leading and trailing whitespace:
5a964f20
TC
2814
2815 $file =~ s#^(\s)#./$1#;
2816 open(FOO, "< $file\0");
2817
6170680b 2818(this may not work on some bizzare filesystems). One should
106325ad 2819conscientiously choose between the I<magic> and 3-arguments form
6170680b
IZ
2820of open():
2821
2822 open IN, $ARGV[0];
2823
2824will allow the user to specify an argument of the form C<"rsh cat file |">,
2825but will not work on a filename which happens to have a trailing space, while
2826
2827 open IN, '<', $ARGV[0];
2828
2829will have exactly the opposite restrictions.
2830
19799a22 2831If you want a "real" C C<open> (see L<open(2)> on your system), then you
6170680b
IZ
2832should use the C<sysopen> function, which involves no such magic (but
2833may use subtly different filemodes than Perl open(), which is mapped
2834to C fopen()). This is
5a964f20
TC
2835another way to protect your filenames from interpretation. For example:
2836
2837 use IO::Handle;
2838 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
2839 or die "sysopen $path: $!";
2840 $oldfh = select(HANDLE); $| = 1; select($oldfh);
2841 print HANDLE "stuff $$\n");
2842 seek(HANDLE, 0, 0);
2843 print "File contains: ", <HANDLE>;
2844
7660c0ab
A
2845Using the constructor from the C<IO::Handle> package (or one of its
2846subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
5a964f20
TC
2847filehandles that have the scope of whatever variables hold references to
2848them, and automatically close whenever and however you leave that scope:
c07a80fd 2849
5f05dabc 2850 use IO::File;
5a964f20 2851 #...
c07a80fd
PP
2852 sub read_myfile_munged {
2853 my $ALL = shift;
5f05dabc 2854 my $handle = new IO::File;
c07a80fd
PP
2855 open($handle, "myfile") or die "myfile: $!";
2856 $first = <$handle>
2857 or return (); # Automatically closed here.
2858 mung $first or die "mung failed"; # Or here.
2859 return $first, <$handle> if $ALL; # Or here.
2860 $first; # Or here.
2861 }
2862
b687b08b 2863See L</seek> for some details about mixing reading and writing.
a0d0e21e
LW
2864
2865=item opendir DIRHANDLE,EXPR
2866
19799a22
GS
2867Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
2868C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
a0d0e21e
LW
2869DIRHANDLEs have their own namespace separate from FILEHANDLEs.
2870
2871=item ord EXPR
2872
54310121 2873=item ord
bbce6d69 2874
a0ed51b3 2875Returns the numeric (ASCII or Unicode) value of the first character of EXPR. If
7660c0ab 2876EXPR is omitted, uses C<$_>. For the reverse, see L</chr>.
2b5ab1e7 2877See L<utf8> for more about Unicode.
a0d0e21e 2878
77ca0c92
LW
2879=item our EXPR
2880
2881An C<our> declares the listed variables to be valid globals within
2882the enclosing block, file, or C<eval>. That is, it has the same
2883scoping rules as a "my" declaration, but does not create a local
2884variable. If more than one value is listed, the list must be placed
2885in parentheses. The C<our> declaration has no semantic effect unless
2886"use strict vars" is in effect, in which case it lets you use the
2887declared global variable without qualifying it with a package name.
2888(But only within the lexical scope of the C<our> declaration. In this
2889it differs from "use vars", which is package scoped.)
2890
f472eb5c
GS
2891An C<our> declaration declares a global variable that will be visible
2892across its entire lexical scope, even across package boundaries. The
2893package in which the variable is entered is determined at the point
2894of the declaration, not at the point of use. This means the following
2895behavior holds:
2896
2897 package Foo;
2898 our $bar; # declares $Foo::bar for rest of lexical scope
2899 $bar = 20;
2900
2901 package Bar;
2902 print $bar; # prints 20
2903
2904Multiple C<our> declarations in the same lexical scope are allowed
2905if they are in different packages. If they happened to be in the same
2906package, Perl will emit warnings if you have asked for them.
2907
2908 use warnings;
2909 package Foo;
2910 our $bar; # declares $Foo::bar for rest of lexical scope
2911 $bar = 20;
2912
2913 package Bar;
2914 our $bar = 30; # declares $Bar::bar for rest of lexical scope
2915 print $bar; # prints 30
2916
2917 our $bar; # emits warning
2918
a0d0e21e
LW
2919=item pack TEMPLATE,LIST
2920
2b6c5635
GS
2921Takes a LIST of values and converts it into a string using the rules
2922given by the TEMPLATE. The resulting string is the concatenation of
2923the converted values. Typically, each converted value looks
2924like its machine-level representation. For example, on 32-bit machines
2925a converted integer may be represented by a sequence of 4 bytes.
2926
2927The TEMPLATE is a
a0d0e21e
LW
2928sequence of characters that give the order and type of values, as
2929follows:
2930
5a929a98 2931 a A string with arbitrary binary data, will be null padded.
4375e838 2932 A An ASCII string, will be space padded.
5a929a98
VU
2933 Z A null terminated (asciz) string, will be null padded.
2934
2b6c5635
GS
2935 b A bit string (ascending bit order inside each byte, like vec()).
2936 B A bit string (descending bit order inside each byte).
a0d0e21e
LW
2937 h A hex string (low nybble first).
2938 H A hex string (high nybble first).
2939
2940 c A signed char value.
a0ed51b3 2941 C An unsigned char value. Only does bytes. See U for Unicode.
96e4d5b1 2942
a0d0e21e
LW
2943 s A signed short value.
2944 S An unsigned short value.
96e4d5b1 2945 (This 'short' is _exactly_ 16 bits, which may differ from
851646ae
JH
2946 what a local C compiler calls 'short'. If you want
2947 native-length shorts, use the '!' suffix.)
96e4d5b1 2948
a0d0e21e
LW
2949 i A signed integer value.
2950 I An unsigned integer value.
19799a22 2951 (This 'integer' is _at_least_ 32 bits wide. Its exact
f86cebdf
GS
2952 size depends on what a local C compiler calls 'int',
2953 and may even be larger than the 'long' described in
2954 the next item.)
96e4d5b1 2955
a0d0e21e
LW
2956 l A signed long value.
2957 L An unsigned long value.
96e4d5b1 2958 (This 'long' is _exactly_ 32 bits, which may differ from
851646ae
JH
2959 what a local C compiler calls 'long'. If you want
2960 native-length longs, use the '!' suffix.)
a0d0e21e 2961
5d11dd56
G
2962 n An unsigned short in "network" (big-endian) order.
2963 N An unsigned long in "network" (big-endian) order.
2964 v An unsigned short in "VAX" (little-endian) order.
2965 V An unsigned long in "VAX" (little-endian) order.
96e4d5b1
PP
2966 (These 'shorts' and 'longs' are _exactly_ 16 bits and
2967 _exactly_ 32 bits, respectively.)
a0d0e21e 2968
dae0da7a
JH
2969 q A signed quad (64-bit) value.
2970 Q An unsigned quad value.
851646ae
JH
2971 (Quads are available only if your system supports 64-bit
2972 integer values _and_ if Perl has been compiled to support those.
dae0da7a
JH
2973 Causes a fatal error otherwise.)
2974
a0d0e21e
LW
2975 f A single-precision float in the native format.
2976 d A double-precision float in the native format.
2977
2978 p A pointer to a null-terminated string.
2979 P A pointer to a structure (fixed-length string).
2980
2981 u A uuencoded string.
a0ed51b3
LW
2982 U A Unicode character number. Encodes to UTF-8 internally.
2983 Works even if C<use utf8> is not in effect.
a0d0e21e 2984
96e4d5b1 2985 w A BER compressed integer. Its bytes represent an unsigned
f86cebdf
GS
2986 integer in base 128, most significant digit first, with as
2987 few digits as possible. Bit eight (the high bit) is set
2988 on each byte except the last.
def98dd4 2989
a0d0e21e
LW
2990 x A null byte.
2991 X Back up a byte.
2992 @ Null fill to absolute position.
2993
5a929a98
VU
2994The following rules apply:
2995
2996=over 8
2997
2998=item *
2999
5a964f20 3000Each letter may optionally be followed by a number giving a repeat
951ba7fe
GS
3001count. With all types except C<a>, C<A>, C<Z>, C<b>, C<B>, C<h>,
3002C<H>, and C<P> the pack function will gobble up that many values from
5a929a98 3003the LIST. A C<*> for the repeat count means to use however many items are
951ba7fe
GS
3004left, except for C<@>, C<x>, C<X>, where it is equivalent
3005to C<0>, and C<u>, where it is equivalent to 1 (or 45, what is the
2b6c5635
GS
3006same).
3007
951ba7fe 3008When used with C<Z>, C<*> results in the addition of a trailing null
2b6c5635
GS
3009byte (so the packed result will be one longer than the byte C<length>
3010of the item).
3011
951ba7fe 3012The repeat count for C<u> is interpreted as the maximal number of bytes
2b6c5635 3013to encode per line of output, with 0 and 1 replaced by 45.
5a929a98
VU
3014
3015=item *
3016
951ba7fe 3017The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
5a929a98 3018string of length count, padding with nulls or spaces as necessary. When
951ba7fe
GS
3019unpacking, C<A> strips trailing spaces and nulls, C<Z> strips everything
3020after the first null, and C<a> returns data verbatim. When packing,
3021C<a>, and C<Z> are equivalent.
2b6c5635
GS
3022
3023If the value-to-pack is too long, it is truncated. If too long and an
951ba7fe
GS
3024explicit count is provided, C<Z> packs only C<$count-1> bytes, followed
3025by a null byte. Thus C<Z> always packs a trailing null byte under
2b6c5635 3026all circumstances.
5a929a98
VU
3027
3028=item *
3029
951ba7fe 3030Likewise, the C<b> and C<B> fields pack a string that many bits long.
c73032f5
IZ
3031Each byte of the input field of pack() generates 1 bit of the result.
3032Each result bit is based on the least-significant bit of the corresponding
3033input byte, i.e., on C<ord($byte)%2>. In particular, bytes C<"0"> and
3034C<"1"> generate bits 0 and 1, as do bytes C<"\0"> and C<"\1">.
3035
3036Starting from the beginning of the input string of pack(), each 8-tuple
951ba7fe 3037of bytes is converted to 1 byte of output. With format C<b>
c73032f5 3038the first byte of the 8-tuple determines the least-significant bit of a
951ba7fe 3039byte, and with format C<B> it determines the most-significant bit of
c73032f5
IZ
3040a byte.
3041
3042If the length of the input string is not exactly divisible by 8, the
3043remainder is packed as if the input string were padded by null bytes
3044at the end. Similarly, during unpack()ing the "extra" bits are ignored.
3045
3046If the input string of pack() is longer than needed, extra bytes are ignored.
2b6c5635
GS
3047A C<*> for the repeat count of pack() means to use all the bytes of
3048the input field. On unpack()ing the bits are converted to a string
3049of C<"0">s and C<"1">s.
5a929a98
VU
3050
3051=item *
3052
951ba7fe 3053The C<h> and C<H> fields pack a string that many nybbles (4-bit groups,
851646ae 3054representable as hexadecimal digits, 0-9a-f) long.
5a929a98 3055
c73032f5
IZ
3056Each byte of the input field of pack() generates 4 bits of the result.
3057For non-alphabetical bytes the result is based on the 4 least-significant
3058bits of the input byte, i.e., on C<ord($byte)%16>. In particular,
3059bytes C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
3060C<"\0"> and C<"\1">. For bytes C<"a".."f"> and C<"A".."F"> the result
3061is compatible with the usual hexadecimal digits, so that C<"a"> and
3062C<"A"> both generate the nybble C<0xa==10>. The result for bytes
3063C<"g".."z"> and C<"G".."Z"> is not well-defined.
3064
3065Starting from the beginning of the input string of pack(), each pair
951ba7fe 3066of bytes is converted to 1 byte of output. With format C<h> the
c73032f5 3067first byte of the pair determines the least-significant nybble of the
951ba7fe 3068output byte, and with format C<H> it determines the most-significant
c73032f5
IZ
3069nybble.
3070
3071If the length of the input string is not even, it behaves as if padded
3072by a null byte at the end. Similarly, during unpack()ing the "extra"
3073nybbles are ignored.
3074
3075If the input string of pack() is longer than needed, extra bytes are ignored.
3076A C<*> for the repeat count of pack() means to use all the bytes of
3077the input field. On unpack()ing the bits are converted to a string
3078of hexadecimal digits.
3079
5a929a98
VU
3080=item *
3081
951ba7fe 3082The C<p> type packs a pointer to a null-terminated string. You are
5a929a98
VU
3083responsible for ensuring the string is not a temporary value (which can
3084potentially get deallocated before you get around to using the packed result).
951ba7fe
GS
3085The C<P> type packs a pointer to a structure of the size indicated by the
3086length. A NULL pointer is created if the corresponding value for C<p> or
3087C<P> is C<undef>, similarly for unpack().
5a929a98
VU
3088
3089=item *
3090
951ba7fe
GS
3091The C</> template character allows packing and unpacking of strings where
3092the packed structure contains a byte count followed by the string itself.
17f4a12d 3093You write I<length-item>C</>I<string-item>.
43192e07
IP
3094
3095The I<length-item> can be any C<pack> template letter,
3096and describes how the length value is packed.
3097The ones likely to be of most use are integer-packing ones like
951ba7fe
GS
3098C<n> (for Java strings), C<w> (for ASN.1 or SNMP)
3099and C<N> (for Sun XDR).
43192e07
IP
3100
3101The I<string-item> must, at present, be C<"A*">, C<"a*"> or C<"Z*">.
3102For C<unpack> the length of the string is obtained from the I<length-item>,
3103but if you put in the '*' it will be ignored.
3104
17f4a12d
IZ
3105 unpack 'C/a', "\04Gurusamy"; gives 'Guru'
3106 unpack 'a3/A* A*', '007 Bond J '; gives (' Bond','J')
3107 pack 'n/a* w/a*','hello,','world'; gives "\000\006hello,\005world"
43192e07
IP
3108
3109The I<length-item> is not returned explicitly from C<unpack>.
3110
951ba7fe
GS
3111Adding a count to the I<length-item> letter is unlikely to do anything
3112useful, unless that letter is C<A>, C<a> or C<Z>. Packing with a
3113I<length-item> of C<a> or C<Z> may introduce C<"\000"> characters,
43192e07
IP
3114which Perl does not regard as legal in numeric strings.
3115
3116=item *
3117
951ba7fe
GS
3118The integer types C<s>, C<S>, C<l>, and C<L> may be
3119immediately followed by a C<!> suffix to signify native shorts or
3120longs--as you can see from above for example a bare C<l> does mean
851646ae
JH
3121exactly 32 bits, the native C<long> (as seen by the local C compiler)
3122may be larger. This is an issue mainly in 64-bit platforms. You can
951ba7fe 3123see whether using C<!> makes any difference by
726ea183 3124
4d0c1c44
GS
3125 print length(pack("s")), " ", length(pack("s!")), "\n";
3126 print length(pack("l")), " ", length(pack("l!")), "\n";
ef54e1a4 3127
951ba7fe
GS
3128C<i!> and C<I!> also work but only because of completeness;
3129they are identical to C<i> and C<I>.
ef54e1a4 3130
19799a22
GS
3131The actual sizes (in bytes) of native shorts, ints, longs, and long
3132longs on the platform where Perl was built are also available via
3133L<Config>:
3134
3135 use Config;
3136 print $Config{shortsize}, "\n";
3137 print $Config{intsize}, "\n";
3138 print $Config{longsize}, "\n";
3139 print $Config{longlongsize}, "\n";
ef54e1a4 3140
5074e145 3141(The C<$Config{longlongsize}> will be undefine if your system does
851646ae
JH
3142not support long longs.)
3143
ef54e1a4
JH
3144=item *
3145
951ba7fe 3146The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, and C<L>
ef54e1a4
JH
3147are inherently non-portable between processors and operating systems
3148because they obey the native byteorder and endianness. For example a
140cb37e 31494-byte integer 0x12345678 (305419896 decimal) be ordered natively
ef54e1a4 3150(arranged in and handled by the CPU registers) into bytes as
61eff3bc 3151
719a3cf5
JH
3152 0x12 0x34 0x56 0x78 # little-endian
3153 0x78 0x56 0x34 0x12 # big-endian
61eff3bc 3154
5d11dd56 3155Basically, the Intel, Alpha, and VAX CPUs are little-endian, while
719a3cf5
JH
3156everybody else, for example Motorola m68k/88k, PPC, Sparc, HP PA,
3157Power, and Cray are big-endian. MIPS can be either: Digital used it
19799a22 3158in little-endian mode; SGI uses it in big-endian mode.
719a3cf5 3159
19799a22 3160The names `big-endian' and `little-endian' are comic references to
ef54e1a4
JH
3161the classic "Gulliver's Travels" (via the paper "On Holy Wars and a
3162Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and
19799a22 3163the egg-eating habits of the Lilliputians.
61eff3bc 3164
140cb37e 3165Some systems may have even weirder byte orders such as
61eff3bc 3166
ef54e1a4
JH
3167 0x56 0x78 0x12 0x34
3168 0x34 0x12 0x78 0x56
61eff3bc 3169
ef54e1a4
JH
3170You can see your system's preference with
3171
3172 print join(" ", map { sprintf "%#02x", $_ }
3173 unpack("C*",pack("L",0x12345678))), "\n";
3174
d99ad34e 3175The byteorder on the platform where Perl was built is also available
726ea183 3176via L<Config>:
ef54e1a4
JH
3177
3178 use Config;
3179 print $Config{byteorder}, "\n";
3180
d99ad34e
JH
3181Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'>
3182and C<'87654321'> are big-endian.
719a3cf5 3183
951ba7fe
GS
3184If you want portable packed integers use the formats C<n>, C<N>,
3185C<v>, and C<V>, their byte endianness and size is known.
851646ae 3186See also L<perlport>.
ef54e1a4
JH
3187
3188=item *
3189
5a929a98
VU
3190Real numbers (floats and doubles) are in the native machine format only;
3191due to the multiplicity of floating formats around, and the lack of a
3192standard "network" representation, no facility for interchange has been
3193made. This means that packed floating point data written on one machine
3194may not be readable on another - even if both use IEEE floating point
3195arithmetic (as the endian-ness of the memory representation is not part
851646ae 3196of the IEEE spec). See also L<perlport>.
5a929a98
VU
3197
3198Note that Perl uses doubles internally for all numeric calculation, and
3199converting from double into float and thence back to double again will
3200lose precision (i.e., C<unpack("f", pack("f", $foo)>) will not in general
19799a22 3201equal $foo).
5a929a98 3202
851646ae
JH
3203=item *
3204
b1fb3988
GS
3205If the pattern begins with a C<U>, the resulting string will be treated
3206as Unicode-encoded. You can force UTF8 encoding on in a string with an
3207initial C<U0>, and the bytes that follow will be interpreted as Unicode
3208characters. If you don't want this to happen, you can begin your pattern
3209with C<C0> (or anything else) to force Perl not to UTF8 encode your
3210string, and then follow this with a C<U*> somewhere in your pattern.
3211
3212=item *
3213
851646ae 3214You must yourself do any alignment or padding by inserting for example
9ccd05c0
JH
3215enough C<'x'>es while packing. There is no way to pack() and unpack()
3216could know where the bytes are going to or coming from. Therefore
3217C<pack> (and C<unpack>) handle their output and input as flat
3218sequences of bytes.
851646ae 3219
17f4a12d
IZ
3220=item *
3221
3222A comment in a TEMPLATE starts with C<#> and goes to the end of line.
3223
2b6c5635
GS
3224=item *
3225
3226If TEMPLATE requires more arguments to pack() than actually given, pack()
3227assumes additional C<""> arguments. If TEMPLATE requires less arguments
3228to pack() than actually given, extra arguments are ignored.
3229
5a929a98 3230=back
a0d0e21e
LW
3231
3232Examples:
3233
a0ed51b3 3234 $foo = pack("CCCC",65,66,67,68);
a0d0e21e 3235 # foo eq "ABCD"
a0ed51b3 3236 $foo = pack("C4",65,66,67,68);
a0d0e21e 3237 # same thing
a0ed51b3
LW
3238 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
3239 # same thing with Unicode circled letters
a0d0e21e
LW
3240
3241 $foo = pack("ccxxcc",65,66,67,68);
3242 # foo eq "AB\0\0CD"
3243
9ccd05c0
JH
3244 # note: the above examples featuring "C" and "c" are true
3245 # only on ASCII and ASCII-derived systems such as ISO Latin 1
3246 # and UTF-8. In EBCDIC the first example would be
3247 # $foo = pack("CCCC",193,194,195,196);
3248
a0d0e21e
LW
3249 $foo = pack("s2",1,2);
3250 # "\1\0\2\0" on little-endian
3251 # "\0\1\0\2" on big-endian
3252
3253 $foo = pack("a4","abcd","x","y","z");
3254 # "abcd"
3255
3256 $foo = pack("aaaa","abcd","x","y","z");
3257 # "axyz"
3258
3259 $foo = pack("a14","abcdefg");
3260 # "abcdefg\0\0\0\0\0\0\0"
3261
3262 $foo = pack("i9pl", gmtime);
3263 # a real struct tm (on my system anyway)
3264
5a929a98
VU
3265 $utmp_template = "Z8 Z8 Z16 L";
3266 $utmp = pack($utmp_template, @utmp1);
3267 # a struct utmp (BSDish)
3268
3269 @utmp2 = unpack($utmp_template, $utmp);
3270 # "@utmp1" eq "@utmp2"
3271
a0d0e21e
LW
3272 sub bintodec {
3273 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
3274 }
3275
851646ae
JH
3276 $foo = pack('sx2l', 12, 34);
3277 # short 12, two zero bytes padding, long 34
3278 $bar = pack('s@4l', 12, 34);
3279 # short 12, zero fill to position 4, long 34
3280 # $foo eq $bar
3281
5a929a98 3282The same template may generally also be used in unpack().
a0d0e21e 3283
5a964f20
TC
3284=item package
3285
cb1a09d0
AD
3286=item package NAMESPACE
3287
3288Declares the compilation unit as being in the given namespace. The scope
2b5ab1e7 3289of the package declaration is from the declaration itself through the end
19799a22 3290of the enclosing block, file, or eval (the same as the C<my> operator).
2b5ab1e7
TC
3291All further unqualified dynamic identifiers will be in this namespace.
3292A package statement affects only dynamic variables--including those
19799a22
GS
3293you've used C<local> on--but I<no