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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
68dc0745 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
d1be9408 72A named array in scalar context is quite different from what would at
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73first glance appear to be a list in scalar context. You can't get a list
74like C<(1,2,3)> into being in scalar context, because the compiler knows
75the context at compile time. It would generate the scalar comma operator
76there, not the list construction version of the comma. That means it
77was never a list to start with.
78
79In general, functions in Perl that serve as wrappers for system calls
f86cebdf 80of the same name (like chown(2), fork(2), closedir(2), etc.) all return
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81true when they succeed and C<undef> otherwise, as is usually mentioned
82in the descriptions below. This is different from the C interfaces,
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83which return C<-1> on failure. Exceptions to this rule are C<wait>,
84C<waitpid>, and C<syscall>. System calls also set the special C<$!>
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85variable on failure. Other functions do not, except accidentally.
86
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87=head2 Perl Functions by Category
88
89Here are Perl's functions (including things that look like
5a964f20 90functions, like some keywords and named operators)
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91arranged by category. Some functions appear in more
92than one place.
93
13a2d996 94=over 4
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95
96=item Functions for SCALARs or strings
97
22fae026 98C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>,
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99C<length>, C<oct>, C<ord>, C<pack>, C<q/STRING/>, C<qq/STRING/>, C<reverse>,
100C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///>
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101
102=item Regular expressions and pattern matching
103
ab4f32c2 104C<m//>, C<pos>, C<quotemeta>, C<s///>, C<split>, C<study>, C<qr//>
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105
106=item Numeric functions
107
22fae026
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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
22fae026
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125C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
126C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
127C<readdir>, C<rewinddir>, C<seek>, C<seekdir>, C<select>, C<syscall>,
128C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>,
129C<warn>, C<write>
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130
131=item Functions for fixed length data or records
132
22fae026 133C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec>
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134
135=item Functions for filehandles, files, or directories
136
22fae026 137C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>,
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138C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>,
139C<readlink>, C<rename>, C<rmdir>, C<stat>, C<symlink>, C<umask>,
140C<unlink>, C<utime>
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141
142=item Keywords related to the control flow of your perl program
143
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144C<caller>, C<continue>, C<die>, C<do>, C<dump>, C<eval>, C<exit>,
145C<goto>, C<last>, C<next>, C<redo>, C<return>, C<sub>, C<wantarray>
cb1a09d0 146
54310121 147=item Keywords related to scoping
cb1a09d0 148
4375e838 149C<caller>, C<import>, C<local>, C<my>, C<our>, C<package>, C<use>
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150
151=item Miscellaneous functions
152
4375e838 153C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<our>, C<reset>,
22fae026 154C<scalar>, C<undef>, C<wantarray>
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155
156=item Functions for processes and process groups
157
22fae026 158C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
ab4f32c2 159C<pipe>, C<qx/STRING/>, C<setpgrp>, C<setpriority>, C<sleep>, C<system>,
22fae026 160C<times>, C<wait>, C<waitpid>
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161
162=item Keywords related to perl modules
163
22fae026 164C<do>, C<import>, C<no>, C<package>, C<require>, C<use>
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165
166=item Keywords related to classes and object-orientedness
167
22fae026
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168C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
169C<untie>, C<use>
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170
171=item Low-level socket functions
172
22fae026
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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>,
80cbd5ad 175C<sockatmark>, C<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
37798a01 200=item Functions new in perl5
201
22fae026 202C<abs>, C<bless>, C<chomp>, C<chr>, C<exists>, C<formline>, C<glob>,
b76cc8ba 203C<import>, C<lc>, C<lcfirst>, C<map>, C<my>, C<no>, C<our>, C<prototype>,
4375e838 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>
37798a01 206
207* - C<sub> was a keyword in perl4, but in perl5 it is an
5a964f20 208operator, which can be used in expressions.
37798a01 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<sockatmark>, C<socket>, C<socketpair>,
238C<stat>, C<symlink>, C<syscall>, C<sysopen>, C<system>,
239C<times>, C<truncate>, C<umask>, C<unlink>,
2b5ab1e7 240C<utime>, C<wait>, C<waitpid>
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241
242For more information about the portability of these functions, see
243L<perlport> and other available platform-specific documentation.
244
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245=head2 Alphabetical Listing of Perl Functions
246
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247=over 8
248
22fae026 249=item I<-X> FILEHANDLE
a0d0e21e 250
22fae026 251=item I<-X> EXPR
a0d0e21e 252
22fae026 253=item I<-X>
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254
255A file test, where X is one of the letters listed below. This unary
256operator takes one argument, either a filename or a filehandle, and
257tests the associated file to see if something is true about it. If the
7660c0ab 258argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
19799a22 259Unless otherwise documented, it returns C<1> for true and C<''> for false, or
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260the undefined value if the file doesn't exist. Despite the funny
261names, precedence is the same as any other named unary operator, and
262the argument may be parenthesized like any other unary operator. The
263operator may be any of:
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264X<-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>
265X<-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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266
267 -r File is readable by effective uid/gid.
268 -w File is writable by effective uid/gid.
269 -x File is executable by effective uid/gid.
270 -o File is owned by effective uid.
271
272 -R File is readable by real uid/gid.
273 -W File is writable by real uid/gid.
274 -X File is executable by real uid/gid.
275 -O File is owned by real uid.
276
277 -e File exists.
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278 -z File has zero size (is empty).
279 -s File has nonzero size (returns size in bytes).
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280
281 -f File is a plain file.
282 -d File is a directory.
283 -l File is a symbolic link.
9c4d0f16 284 -p File is a named pipe (FIFO), or Filehandle is a pipe.
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285 -S File is a socket.
286 -b File is a block special file.
287 -c File is a character special file.
288 -t Filehandle is opened to a tty.
289
290 -u File has setuid bit set.
291 -g File has setgid bit set.
292 -k File has sticky bit set.
293
121910a4 294 -T File is an ASCII text file (heuristic guess).
2cdbc966 295 -B File is a "binary" file (opposite of -T).
a0d0e21e 296
95a3fe12 297 -M Script start time minus file modification time, in days.
a0d0e21e 298 -A Same for access time.
95a3fe12 299 -C Same for inode change time (Unix, may differ for other platforms)
a0d0e21e 300
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301Example:
302
303 while (<>) {
5b3eff12 304 chomp;
a0d0e21e 305 next unless -f $_; # ignore specials
5a964f20 306 #...
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307 }
308
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309The interpretation of the file permission operators C<-r>, C<-R>,
310C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
311of the file and the uids and gids of the user. There may be other
312reasons you can't actually read, write, or execute the file. Such
313reasons may be for example network filesystem access controls, ACLs
314(access control lists), read-only filesystems, and unrecognized
315executable formats.
316
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317Also note that, for the superuser on the local filesystems, the C<-r>,
318C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
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319if any execute bit is set in the mode. Scripts run by the superuser
320may thus need to do a stat() to determine the actual mode of the file,
2b5ab1e7 321or temporarily set their effective uid to something else.
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322
323If you are using ACLs, there is a pragma called C<filetest> that may
324produce more accurate results than the bare stat() mode bits.
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325When under the C<use filetest 'access'> the above-mentioned filetests
326will test whether the permission can (not) be granted using the
468541a8 327access() family of system calls. Also note that the C<-x> and C<-X> may
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328under this pragma return true even if there are no execute permission
329bits set (nor any extra execute permission ACLs). This strangeness is
330due to the underlying system calls' definitions. Read the
331documentation for the C<filetest> pragma for more information.
332
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333Note that C<-s/a/b/> does not do a negated substitution. Saying
334C<-exp($foo)> still works as expected, however--only single letters
335following a minus are interpreted as file tests.
336
337The C<-T> and C<-B> switches work as follows. The first block or so of the
338file is examined for odd characters such as strange control codes or
61eff3bc 339characters with the high bit set. If too many strange characters (>30%)
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340are found, it's a C<-B> file, otherwise it's a C<-T> file. Also, any file
341containing null in the first block is considered a binary file. If C<-T>
9124316e 342or C<-B> is used on a filehandle, the current IO buffer is examined
19799a22 343rather than the first block. Both C<-T> and C<-B> return true on a null
54310121 344file, or a file at EOF when testing a filehandle. Because you have to
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345read a file to do the C<-T> test, on most occasions you want to use a C<-f>
346against the file first, as in C<next unless -f $file && -T $file>.
a0d0e21e 347
19799a22 348If any of the file tests (or either the C<stat> or C<lstat> operators) are given
28757baa 349the special filehandle consisting of a solitary underline, then the stat
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350structure of the previous file test (or stat operator) is used, saving
351a system call. (This doesn't work with C<-t>, and you need to remember
352that lstat() and C<-l> will leave values in the stat structure for the
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353symbolic link, not the real file.) (Also, if the stat buffer was filled by
354a C<lstat> call, C<-T> and C<-B> will reset it with the results of C<stat _>).
355Example:
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356
357 print "Can do.\n" if -r $a || -w _ || -x _;
358
359 stat($filename);
360 print "Readable\n" if -r _;
361 print "Writable\n" if -w _;
362 print "Executable\n" if -x _;
363 print "Setuid\n" if -u _;
364 print "Setgid\n" if -g _;
365 print "Sticky\n" if -k _;
366 print "Text\n" if -T _;
367 print "Binary\n" if -B _;
368
369=item abs VALUE
370
54310121 371=item abs
bbce6d69 372
a0d0e21e 373Returns the absolute value of its argument.
7660c0ab 374If VALUE is omitted, uses C<$_>.
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375
376=item accept NEWSOCKET,GENERICSOCKET
377
f86cebdf 378Accepts an incoming socket connect, just as the accept(2) system call
19799a22 379does. Returns the packed address if it succeeded, false otherwise.
2b5ab1e7 380See the example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 381
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382On systems that support a close-on-exec flag on files, the flag will
383be set for the newly opened file descriptor, as determined by the
384value of $^F. See L<perlvar/$^F>.
385
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386=item alarm SECONDS
387
54310121 388=item alarm
bbce6d69 389
a0d0e21e 390Arranges to have a SIGALRM delivered to this process after the
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391specified number of wallclock seconds have elapsed. If SECONDS is not
392specified, the value stored in C<$_> is used. (On some machines,
393unfortunately, the elapsed time may be up to one second less or more
394than you specified because of how seconds are counted, and process
395scheduling may delay the delivery of the signal even further.)
396
397Only one timer may be counting at once. Each call disables the
398previous timer, and an argument of C<0> may be supplied to cancel the
399previous timer without starting a new one. The returned value is the
400amount of time remaining on the previous timer.
a0d0e21e 401
4633a7c4 402For delays of finer granularity than one second, you may use Perl's
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403four-argument version of select() leaving the first three arguments
404undefined, or you might be able to use the C<syscall> interface to
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405access setitimer(2) if your system supports it. The Time::HiRes
406module (from CPAN, and starting from Perl 5.8 part of the standard
407distribution) may also prove useful.
2b5ab1e7 408
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409It is usually a mistake to intermix C<alarm> and C<sleep> calls.
410(C<sleep> may be internally implemented in your system with C<alarm>)
a0d0e21e 411
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412If you want to use C<alarm> to time out a system call you need to use an
413C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
f86cebdf 414fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
19799a22 415restart system calls on some systems. Using C<eval>/C<die> always works,
5a964f20 416modulo the caveats given in L<perlipc/"Signals">.
ff68c719 417
418 eval {
f86cebdf 419 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
36477c24 420 alarm $timeout;
ff68c719 421 $nread = sysread SOCKET, $buffer, $size;
36477c24 422 alarm 0;
ff68c719 423 };
ff68c719 424 if ($@) {
f86cebdf 425 die unless $@ eq "alarm\n"; # propagate unexpected errors
ff68c719 426 # timed out
427 }
428 else {
429 # didn't
430 }
431
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432=item atan2 Y,X
433
434Returns the arctangent of Y/X in the range -PI to PI.
435
ca6e1c26 436For the tangent operation, you may use the C<Math::Trig::tan>
28757baa 437function, or use the familiar relation:
438
439 sub tan { sin($_[0]) / cos($_[0]) }
440
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441=item bind SOCKET,NAME
442
443Binds a network address to a socket, just as the bind system call
19799a22 444does. Returns true if it succeeded, false otherwise. NAME should be a
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445packed address of the appropriate type for the socket. See the examples in
446L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 447
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448=item binmode FILEHANDLE, DISCIPLINE
449
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450=item binmode FILEHANDLE
451
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452Arranges for FILEHANDLE to be read or written in "binary" or "text" mode
453on systems where the run-time libraries distinguish between binary and
30168b04 454text files. If FILEHANDLE is an expression, the value is taken as the
ed53a2bb
JH
455name of the filehandle. DISCIPLINE can be either of C<:raw> for
456binary mode or C<:crlf> for "text" mode. If the DISCIPLINE is
457omitted, it defaults to C<:raw>. Returns true on success, C<undef> on
458failure. The C<:raw> are C<:clrf>, and any other directives of the
459form C<:...>, are called I/O I<disciplines>.
30168b04 460
ed53a2bb
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461The C<open> pragma can be used to establish default I/O disciplines.
462See L<open>.
463
464In general, binmode() should be called after open() but before any I/O
465is done on the filehandle. Calling binmode() will flush any possibly
466pending buffered input or output data on the handle. The only
467exception to this is the C<:encoding> discipline that changes
468the default character encoding of the handle, see L<open>.
469The C<:encoding> discipline sometimes needs to be called in
470mid-stream, and it doesn't flush the stream.
16fe6d59 471
16fe6d59
GS
472On some systems binmode() is necessary when you're not working with a
473text file. For the sake of portability it is a good idea to always use
474it when appropriate, and to never use it when it isn't appropriate.
30168b04
GS
475
476In other words: Regardless of platform, use binmode() on binary
477files, and do not use binmode() on text files.
19799a22
GS
478
479The operating system, device drivers, C libraries, and Perl run-time
30168b04
GS
480system all work together to let the programmer treat a single
481character (C<\n>) as the line terminator, irrespective of the external
482representation. On many operating systems, the native text file
483representation matches the internal representation, but on some
484platforms the external representation of C<\n> is made up of more than
485one character.
486
68bd7414
NIS
487Mac OS, all variants of Unix, and Stream_LF files on VMS use a single
488character to end each line in the external representation of text (even
5e12dbfa 489though that single character is CARRIAGE RETURN on Mac OS and LINE FEED
68bd7414
NIS
490on Unix and most VMS files). Consequently binmode() has no effect on
491these operating systems. In other systems like OS/2, DOS and the various
492flavors of MS-Windows your program sees a C<\n> as a simple C<\cJ>, but
493what's stored in text files are the two characters C<\cM\cJ>. That means
494that, if you don't use binmode() on these systems, C<\cM\cJ> sequences on
495disk will be converted to C<\n> on input, and any C<\n> in your program
5e12dbfa
PP
496will be converted back to C<\cM\cJ> on output. This is what you want for
497text files, but it can be disastrous for binary files.
30168b04
GS
498
499Another consequence of using binmode() (on some systems) is that
500special end-of-file markers will be seen as part of the data stream.
501For systems from the Microsoft family this means that if your binary
4375e838 502data contains C<\cZ>, the I/O subsystem will regard it as the end of
30168b04
GS
503the file, unless you use binmode().
504
505binmode() is not only important for readline() and print() operations,
506but also when using read(), seek(), sysread(), syswrite() and tell()
507(see L<perlport> for more details). See the C<$/> and C<$\> variables
508in L<perlvar> for how to manually set your input and output
509line-termination sequences.
a0d0e21e 510
4633a7c4 511=item bless REF,CLASSNAME
a0d0e21e
LW
512
513=item bless REF
514
2b5ab1e7
TC
515This function tells the thingy referenced by REF that it is now an object
516in the CLASSNAME package. If CLASSNAME is omitted, the current package
19799a22 517is used. Because a C<bless> is often the last thing in a constructor,
2b5ab1e7
TC
518it returns the reference for convenience. Always use the two-argument
519version if the function doing the blessing might be inherited by a
520derived class. See L<perltoot> and L<perlobj> for more about the blessing
521(and blessings) of objects.
a0d0e21e 522
57668c4d 523Consider always blessing objects in CLASSNAMEs that are mixed case.
2b5ab1e7
TC
524Namespaces with all lowercase names are considered reserved for
525Perl pragmata. Builtin types have all uppercase names, so to prevent
526confusion, you may wish to avoid such package names as well. Make sure
527that CLASSNAME is a true value.
60ad88b8
GS
528
529See L<perlmod/"Perl Modules">.
530
a0d0e21e
LW
531=item caller EXPR
532
533=item caller
534
5a964f20 535Returns the context of the current subroutine call. In scalar context,
28757baa 536returns the caller's package name if there is a caller, that is, if
19799a22 537we're in a subroutine or C<eval> or C<require>, and the undefined value
5a964f20 538otherwise. In list context, returns
a0d0e21e 539
748a9306 540 ($package, $filename, $line) = caller;
a0d0e21e
LW
541
542With EXPR, it returns some extra information that the debugger uses to
543print a stack trace. The value of EXPR indicates how many call frames
544to go back before the current one.
545
f3aa04c2 546 ($package, $filename, $line, $subroutine, $hasargs,
e476b1b5 547 $wantarray, $evaltext, $is_require, $hints, $bitmask) = caller($i);
e7ea3e70 548
951ba7fe 549Here $subroutine may be C<(eval)> if the frame is not a subroutine
19799a22 550call, but an C<eval>. In such a case additional elements $evaltext and
7660c0ab 551C<$is_require> are set: C<$is_require> is true if the frame is created by a
19799a22 552C<require> or C<use> statement, $evaltext contains the text of the
277ddfaf 553C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
951ba7fe 554$filename is C<(eval)>, but $evaltext is undefined. (Note also that
dc848c6f 555each C<use> statement creates a C<require> frame inside an C<eval EXPR>)
277ddfaf 556frame. C<$hasargs> is true if a new instance of C<@_> was set up for the
e476b1b5
GS
557frame. C<$hints> and C<$bitmask> contain pragmatic hints that the caller
558was compiled with. The C<$hints> and C<$bitmask> values are subject to
559change between versions of Perl, and are not meant for external use.
748a9306
LW
560
561Furthermore, when called from within the DB package, caller returns more
7660c0ab 562detailed information: it sets the list variable C<@DB::args> to be the
54310121 563arguments with which the subroutine was invoked.
748a9306 564
7660c0ab 565Be aware that the optimizer might have optimized call frames away before
19799a22 566C<caller> had a chance to get the information. That means that C<caller(N)>
7660c0ab 567might not return information about the call frame you expect it do, for
b76cc8ba 568C<< N > 1 >>. In particular, C<@DB::args> might have information from the
19799a22 569previous time C<caller> was called.
7660c0ab 570
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LW
571=item chdir EXPR
572
ffce7b87 573Changes the working directory to EXPR, if possible. If EXPR is omitted,
0bfc1ec4 574changes to the directory specified by C<$ENV{HOME}>, if set; if not,
ffce7b87 575changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
b4ad75f0
AMS
576variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
577neither is set, C<chdir> does nothing. It returns true upon success,
578false otherwise. See the example under C<die>.
a0d0e21e
LW
579
580=item chmod LIST
581
582Changes the permissions of a list of files. The first element of the
4633a7c4 583list must be the numerical mode, which should probably be an octal
2f9daede
TP
584number, and which definitely should I<not> a string of octal digits:
585C<0644> is okay, C<'0644'> is not. Returns the number of files
dc848c6f 586successfully changed. See also L</oct>, if all you have is a string.
a0d0e21e
LW
587
588 $cnt = chmod 0755, 'foo', 'bar';
589 chmod 0755, @executables;
f86cebdf
GS
590 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
591 # --w----r-T
2f9daede
TP
592 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
593 $mode = 0644; chmod $mode, 'foo'; # this is best
a0d0e21e 594
ca6e1c26
JH
595You can also import the symbolic C<S_I*> constants from the Fcntl
596module:
597
598 use Fcntl ':mode';
599
600 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
601 # This is identical to the chmod 0755 of the above example.
602
a0d0e21e
LW
603=item chomp VARIABLE
604
313c9f5c 605=item chomp( LIST )
a0d0e21e
LW
606
607=item chomp
608
2b5ab1e7
TC
609This safer version of L</chop> removes any trailing string
610that corresponds to the current value of C<$/> (also known as
28757baa 611$INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
612number of characters removed from all its arguments. It's often used to
613remove the newline from the end of an input record when you're worried
2b5ab1e7
TC
614that the final record may be missing its newline. When in paragraph
615mode (C<$/ = "">), it removes all trailing newlines from the string.
4c5a6083
GS
616When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
617a reference to an integer or the like, see L<perlvar>) chomp() won't
b76cc8ba 618remove anything.
19799a22 619If VARIABLE is omitted, it chomps C<$_>. Example:
a0d0e21e
LW
620
621 while (<>) {
622 chomp; # avoid \n on last field
623 @array = split(/:/);
5a964f20 624 # ...
a0d0e21e
LW
625 }
626
4bf21a6d
RD
627If VARIABLE is a hash, it chomps the hash's values, but not its keys.
628
a0d0e21e
LW
629You can actually chomp anything that's an lvalue, including an assignment:
630
631 chomp($cwd = `pwd`);
632 chomp($answer = <STDIN>);
633
634If you chomp a list, each element is chomped, and the total number of
635characters removed is returned.
636
637=item chop VARIABLE
638
313c9f5c 639=item chop( LIST )
a0d0e21e
LW
640
641=item chop
642
643Chops off the last character of a string and returns the character
5b3eff12 644chopped. It is much more efficient than C<s/.$//s> because it neither
7660c0ab 645scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
4bf21a6d
RD
646If VARIABLE is a hash, it chops the hash's values, but not its keys.
647
5b3eff12 648You can actually chop anything that's an lvalue, including an assignment.
a0d0e21e
LW
649
650If you chop a list, each element is chopped. Only the value of the
19799a22 651last C<chop> is returned.
a0d0e21e 652
19799a22 653Note that C<chop> returns the last character. To return all but the last
748a9306
LW
654character, use C<substr($string, 0, -1)>.
655
a0d0e21e
LW
656=item chown LIST
657
658Changes the owner (and group) of a list of files. The first two
19799a22
GS
659elements of the list must be the I<numeric> uid and gid, in that
660order. A value of -1 in either position is interpreted by most
661systems to leave that value unchanged. Returns the number of files
662successfully changed.
a0d0e21e
LW
663
664 $cnt = chown $uid, $gid, 'foo', 'bar';
665 chown $uid, $gid, @filenames;
666
54310121 667Here's an example that looks up nonnumeric uids in the passwd file:
a0d0e21e
LW
668
669 print "User: ";
19799a22 670 chomp($user = <STDIN>);
5a964f20 671 print "Files: ";
19799a22 672 chomp($pattern = <STDIN>);
a0d0e21e
LW
673
674 ($login,$pass,$uid,$gid) = getpwnam($user)
675 or die "$user not in passwd file";
676
5a964f20 677 @ary = glob($pattern); # expand filenames
a0d0e21e
LW
678 chown $uid, $gid, @ary;
679
54310121 680On most systems, you are not allowed to change the ownership of the
4633a7c4
LW
681file unless you're the superuser, although you should be able to change
682the group to any of your secondary groups. On insecure systems, these
683restrictions may be relaxed, but this is not a portable assumption.
19799a22
GS
684On POSIX systems, you can detect this condition this way:
685
686 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
687 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
4633a7c4 688
a0d0e21e
LW
689=item chr NUMBER
690
54310121 691=item chr
bbce6d69 692
a0d0e21e 693Returns the character represented by that NUMBER in the character set.
a0ed51b3 694For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
121910a4
JH
695chr(0x263a) is a Unicode smiley face. Note that characters from 127
696to 255 (inclusive) are by default not encoded in Unicode for backward
697compatibility reasons (but see L<encoding>).
aaa68c4a 698
b76cc8ba 699For the reverse, use L</ord>.
121910a4 700See L<perlunicode> and L<encoding> for more about Unicode.
a0d0e21e 701
7660c0ab 702If NUMBER is omitted, uses C<$_>.
bbce6d69 703
a0d0e21e
LW
704=item chroot FILENAME
705
54310121 706=item chroot
bbce6d69 707
5a964f20 708This function works like the system call by the same name: it makes the
4633a7c4 709named directory the new root directory for all further pathnames that
951ba7fe 710begin with a C</> by your process and all its children. (It doesn't
28757baa 711change your current working directory, which is unaffected.) For security
4633a7c4 712reasons, this call is restricted to the superuser. If FILENAME is
19799a22 713omitted, does a C<chroot> to C<$_>.
a0d0e21e
LW
714
715=item close FILEHANDLE
716
6a518fbc
TP
717=item close
718
9124316e
JH
719Closes the file or pipe associated with the file handle, returning
720true only if IO buffers are successfully flushed and closes the system
721file descriptor. Closes the currently selected filehandle if the
722argument is omitted.
fb73857a 723
724You don't have to close FILEHANDLE if you are immediately going to do
19799a22
GS
725another C<open> on it, because C<open> will close it for you. (See
726C<open>.) However, an explicit C<close> on an input file resets the line
727counter (C<$.>), while the implicit close done by C<open> does not.
fb73857a 728
19799a22
GS
729If the file handle came from a piped open C<close> will additionally
730return false if one of the other system calls involved fails or if the
fb73857a 731program exits with non-zero status. (If the only problem was that the
b76cc8ba 732program exited non-zero C<$!> will be set to C<0>.) Closing a pipe
2b5ab1e7 733also waits for the process executing on the pipe to complete, in case you
b76cc8ba 734want to look at the output of the pipe afterwards, and
2b5ab1e7 735implicitly puts the exit status value of that command into C<$?>.
5a964f20 736
73689b13
GS
737Prematurely closing the read end of a pipe (i.e. before the process
738writing to it at the other end has closed it) will result in a
739SIGPIPE being delivered to the writer. If the other end can't
740handle that, be sure to read all the data before closing the pipe.
741
fb73857a 742Example:
a0d0e21e 743
fb73857a 744 open(OUTPUT, '|sort >foo') # pipe to sort
745 or die "Can't start sort: $!";
5a964f20 746 #... # print stuff to output
fb73857a 747 close OUTPUT # wait for sort to finish
748 or warn $! ? "Error closing sort pipe: $!"
749 : "Exit status $? from sort";
750 open(INPUT, 'foo') # get sort's results
751 or die "Can't open 'foo' for input: $!";
a0d0e21e 752
5a964f20
TC
753FILEHANDLE may be an expression whose value can be used as an indirect
754filehandle, usually the real filehandle name.
a0d0e21e
LW
755
756=item closedir DIRHANDLE
757
19799a22 758Closes a directory opened by C<opendir> and returns the success of that
5a964f20
TC
759system call.
760
761DIRHANDLE may be an expression whose value can be used as an indirect
762dirhandle, usually the real dirhandle name.
a0d0e21e
LW
763
764=item connect SOCKET,NAME
765
766Attempts to connect to a remote socket, just as the connect system call
19799a22 767does. Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
768packed address of the appropriate type for the socket. See the examples in
769L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 770
cb1a09d0
AD
771=item continue BLOCK
772
773Actually a flow control statement rather than a function. If there is a
98293880
JH
774C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
775C<foreach>), it is always executed just before the conditional is about to
776be evaluated again, just like the third part of a C<for> loop in C. Thus
cb1a09d0
AD
777it can be used to increment a loop variable, even when the loop has been
778continued via the C<next> statement (which is similar to the C C<continue>
779statement).
780
98293880 781C<last>, C<next>, or C<redo> may appear within a C<continue>
19799a22
GS
782block. C<last> and C<redo> will behave as if they had been executed within
783the main block. So will C<next>, but since it will execute a C<continue>
1d2dff63
GS
784block, it may be more entertaining.
785
786 while (EXPR) {
787 ### redo always comes here
788 do_something;
789 } continue {
790 ### next always comes here
791 do_something_else;
792 # then back the top to re-check EXPR
793 }
794 ### last always comes here
795
796Omitting the C<continue> section is semantically equivalent to using an
19799a22 797empty one, logically enough. In that case, C<next> goes directly back
1d2dff63
GS
798to check the condition at the top of the loop.
799
a0d0e21e
LW
800=item cos EXPR
801
d6217f1e
GS
802=item cos
803
5a964f20 804Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
7660c0ab 805takes cosine of C<$_>.
a0d0e21e 806
ca6e1c26 807For the inverse cosine operation, you may use the C<Math::Trig::acos()>
28757baa 808function, or use this relation:
809
810 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
811
a0d0e21e
LW
812=item crypt PLAINTEXT,SALT
813
f86cebdf 814Encrypts a string exactly like the crypt(3) function in the C library
4633a7c4
LW
815(assuming that you actually have a version there that has not been
816extirpated as a potential munition). This can prove useful for checking
817the password file for lousy passwords, amongst other things. Only the
818guys wearing white hats should do this.
a0d0e21e 819
85c16d83
JH
820Note that C<crypt> is intended to be a one-way function, much like
821breaking eggs to make an omelette. There is no (known) corresponding
822decrypt function (in other words, the crypt() is a one-way hash
823function). As a result, this function isn't all that useful for
11155c91 824cryptography. (For that, see your nearby CPAN mirror.)
2f9daede 825
85c16d83
JH
826When verifying an existing encrypted string you should use the
827encrypted text as the salt (like C<crypt($plain, $crypted) eq
828$crypted>). This allows your code to work with the standard C<crypt>
829and with more exotic implementations. In other words, do not assume
830anything about the returned string itself, or how many bytes in
831the encrypted string matter.
832
833Traditionally the result is a string of 13 bytes: two first bytes of
834the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
835the first eight bytes of the encrypted string mattered, but
836alternative hashing schemes (like MD5), higher level security schemes
837(like C2), and implementations on non-UNIX platforms may produce
838different strings.
839
840When choosing a new salt create a random two character string whose
841characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
842'/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>).
e71965be 843
a0d0e21e
LW
844Here's an example that makes sure that whoever runs this program knows
845their own password:
846
847 $pwd = (getpwuid($<))[1];
a0d0e21e
LW
848
849 system "stty -echo";
850 print "Password: ";
e71965be 851 chomp($word = <STDIN>);
a0d0e21e
LW
852 print "\n";
853 system "stty echo";
854
e71965be 855 if (crypt($word, $pwd) ne $pwd) {
a0d0e21e
LW
856 die "Sorry...\n";
857 } else {
858 print "ok\n";
54310121 859 }
a0d0e21e 860
9f8f0c9d 861Of course, typing in your own password to whoever asks you
748a9306 862for it is unwise.
a0d0e21e 863
19799a22
GS
864The L<crypt> function is unsuitable for encrypting large quantities
865of data, not least of all because you can't get the information
866back. Look at the F<by-module/Crypt> and F<by-module/PGP> directories
867on your favorite CPAN mirror for a slew of potentially useful
868modules.
869
f2791508
JH
870If using crypt() on a Unicode string (which I<potentially> has
871characters with codepoints above 255), Perl tries to make sense
872of the situation by trying to downgrade (a copy of the string)
873the string back to an eight-bit byte string before calling crypt()
874(on that copy). If that works, good. If not, crypt() dies with
875C<Wide character in crypt>.
85c16d83 876
aa689395 877=item dbmclose HASH
a0d0e21e 878
19799a22 879[This function has been largely superseded by the C<untie> function.]
a0d0e21e 880
aa689395 881Breaks the binding between a DBM file and a hash.
a0d0e21e 882
19799a22 883=item dbmopen HASH,DBNAME,MASK
a0d0e21e 884
19799a22 885[This function has been largely superseded by the C<tie> function.]
a0d0e21e 886
7b8d334a 887This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
19799a22
GS
888hash. HASH is the name of the hash. (Unlike normal C<open>, the first
889argument is I<not> a filehandle, even though it looks like one). DBNAME
aa689395 890is the name of the database (without the F<.dir> or F<.pag> extension if
891any). If the database does not exist, it is created with protection
19799a22
GS
892specified by MASK (as modified by the C<umask>). If your system supports
893only the older DBM functions, you may perform only one C<dbmopen> in your
aa689395 894program. In older versions of Perl, if your system had neither DBM nor
19799a22 895ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
aa689395 896sdbm(3).
897
898If you don't have write access to the DBM file, you can only read hash
899variables, not set them. If you want to test whether you can write,
19799a22 900either use file tests or try setting a dummy hash entry inside an C<eval>,
aa689395 901which will trap the error.
a0d0e21e 902
19799a22
GS
903Note that functions such as C<keys> and C<values> may return huge lists
904when used on large DBM files. You may prefer to use the C<each>
a0d0e21e
LW
905function to iterate over large DBM files. Example:
906
907 # print out history file offsets
908 dbmopen(%HIST,'/usr/lib/news/history',0666);
909 while (($key,$val) = each %HIST) {
910 print $key, ' = ', unpack('L',$val), "\n";
911 }
912 dbmclose(%HIST);
913
cb1a09d0 914See also L<AnyDBM_File> for a more general description of the pros and
184e9718 915cons of the various dbm approaches, as well as L<DB_File> for a particularly
cb1a09d0 916rich implementation.
4633a7c4 917
2b5ab1e7
TC
918You can control which DBM library you use by loading that library
919before you call dbmopen():
920
921 use DB_File;
922 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
923 or die "Can't open netscape history file: $!";
924
a0d0e21e
LW
925=item defined EXPR
926
54310121 927=item defined
bbce6d69 928
2f9daede
TP
929Returns a Boolean value telling whether EXPR has a value other than
930the undefined value C<undef>. If EXPR is not present, C<$_> will be
931checked.
932
933Many operations return C<undef> to indicate failure, end of file,
934system error, uninitialized variable, and other exceptional
935conditions. This function allows you to distinguish C<undef> from
936other values. (A simple Boolean test will not distinguish among
7660c0ab 937C<undef>, zero, the empty string, and C<"0">, which are all equally
2f9daede 938false.) Note that since C<undef> is a valid scalar, its presence
19799a22 939doesn't I<necessarily> indicate an exceptional condition: C<pop>
2f9daede
TP
940returns C<undef> when its argument is an empty array, I<or> when the
941element to return happens to be C<undef>.
942
f10b0346
GS
943You may also use C<defined(&func)> to check whether subroutine C<&func>
944has ever been defined. The return value is unaffected by any forward
847c7ebe
DD
945declarations of C<&foo>. Note that a subroutine which is not defined
946may still be callable: its package may have an C<AUTOLOAD> method that
947makes it spring into existence the first time that it is called -- see
948L<perlsub>.
f10b0346
GS
949
950Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
951used to report whether memory for that aggregate has ever been
952allocated. This behavior may disappear in future versions of Perl.
953You should instead use a simple test for size:
954
955 if (@an_array) { print "has array elements\n" }
956 if (%a_hash) { print "has hash members\n" }
2f9daede
TP
957
958When used on a hash element, it tells you whether the value is defined,
dc848c6f 959not whether the key exists in the hash. Use L</exists> for the latter
2f9daede 960purpose.
a0d0e21e
LW
961
962Examples:
963
964 print if defined $switch{'D'};
965 print "$val\n" while defined($val = pop(@ary));
966 die "Can't readlink $sym: $!"
967 unless defined($value = readlink $sym);
a0d0e21e 968 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
2f9daede 969 $debugging = 0 unless defined $debugging;
a0d0e21e 970
19799a22 971Note: Many folks tend to overuse C<defined>, and then are surprised to
7660c0ab 972discover that the number C<0> and C<""> (the zero-length string) are, in fact,
2f9daede 973defined values. For example, if you say
a5f75d66
AD
974
975 "ab" =~ /a(.*)b/;
976
7660c0ab 977The pattern match succeeds, and C<$1> is defined, despite the fact that it
a5f75d66 978matched "nothing". But it didn't really match nothing--rather, it
2b5ab1e7 979matched something that happened to be zero characters long. This is all
a5f75d66 980very above-board and honest. When a function returns an undefined value,
2f9daede 981it's an admission that it couldn't give you an honest answer. So you
19799a22 982should use C<defined> only when you're questioning the integrity of what
7660c0ab 983you're trying to do. At other times, a simple comparison to C<0> or C<""> is
2f9daede
TP
984what you want.
985
dc848c6f 986See also L</undef>, L</exists>, L</ref>.
2f9daede 987
a0d0e21e
LW
988=item delete EXPR
989
01020589
GS
990Given an expression that specifies a hash element, array element, hash slice,
991or array slice, deletes the specified element(s) from the hash or array.
8216c1fd 992In the case of an array, if the array elements happen to be at the end,
b76cc8ba 993the size of the array will shrink to the highest element that tests
8216c1fd 994true for exists() (or 0 if no such element exists).
a0d0e21e 995
01020589
GS
996Returns each element so deleted or the undefined value if there was no such
997element. Deleting from C<$ENV{}> modifies the environment. Deleting from
998a hash tied to a DBM file deletes the entry from the DBM file. Deleting
999from a C<tie>d hash or array may not necessarily return anything.
1000
8ea97a1e
GS
1001Deleting an array element effectively returns that position of the array
1002to its initial, uninitialized state. Subsequently testing for the same
8216c1fd
GS
1003element with exists() will return false. Note that deleting array
1004elements in the middle of an array will not shift the index of the ones
1005after them down--use splice() for that. See L</exists>.
8ea97a1e 1006
01020589 1007The following (inefficiently) deletes all the values of %HASH and @ARRAY:
a0d0e21e 1008
5f05dabc 1009 foreach $key (keys %HASH) {
1010 delete $HASH{$key};
a0d0e21e
LW
1011 }
1012
01020589
GS
1013 foreach $index (0 .. $#ARRAY) {
1014 delete $ARRAY[$index];
1015 }
1016
1017And so do these:
5f05dabc 1018
01020589
GS
1019 delete @HASH{keys %HASH};
1020
9740c838 1021 delete @ARRAY[0 .. $#ARRAY];
5f05dabc 1022
2b5ab1e7 1023But both of these are slower than just assigning the empty list
01020589
GS
1024or undefining %HASH or @ARRAY:
1025
1026 %HASH = (); # completely empty %HASH
1027 undef %HASH; # forget %HASH ever existed
2b5ab1e7 1028
01020589
GS
1029 @ARRAY = (); # completely empty @ARRAY
1030 undef @ARRAY; # forget @ARRAY ever existed
2b5ab1e7
TC
1031
1032Note that the EXPR can be arbitrarily complicated as long as the final
01020589
GS
1033operation is a hash element, array element, hash slice, or array slice
1034lookup:
a0d0e21e
LW
1035
1036 delete $ref->[$x][$y]{$key};
5f05dabc 1037 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
a0d0e21e 1038
01020589
GS
1039 delete $ref->[$x][$y][$index];
1040 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1041
a0d0e21e
LW
1042=item die LIST
1043
19799a22
GS
1044Outside an C<eval>, prints the value of LIST to C<STDERR> and
1045exits with the current value of C<$!> (errno). If C<$!> is C<0>,
61eff3bc
JH
1046exits with the value of C<<< ($? >> 8) >>> (backtick `command`
1047status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside
19799a22
GS
1048an C<eval(),> the error message is stuffed into C<$@> and the
1049C<eval> is terminated with the undefined value. This makes
1050C<die> the way to raise an exception.
a0d0e21e
LW
1051
1052Equivalent examples:
1053
1054 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
54310121 1055 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
a0d0e21e 1056
ccac6780 1057If the last element of LIST does not end in a newline, the current
df37ec69
WW
1058script line number and input line number (if any) are also printed,
1059and a newline is supplied. Note that the "input line number" (also
1060known as "chunk") is subject to whatever notion of "line" happens to
1061be currently in effect, and is also available as the special variable
1062C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1063
1064Hint: sometimes appending C<", stopped"> to your message will cause it
1065to make better sense when the string C<"at foo line 123"> is appended.
1066Suppose you are running script "canasta".
a0d0e21e
LW
1067
1068 die "/etc/games is no good";
1069 die "/etc/games is no good, stopped";
1070
1071produce, respectively
1072
1073 /etc/games is no good at canasta line 123.
1074 /etc/games is no good, stopped at canasta line 123.
1075
2b5ab1e7 1076See also exit(), warn(), and the Carp module.
a0d0e21e 1077
7660c0ab
A
1078If LIST is empty and C<$@> already contains a value (typically from a
1079previous eval) that value is reused after appending C<"\t...propagated">.
fb73857a 1080This is useful for propagating exceptions:
1081
1082 eval { ... };
1083 die unless $@ =~ /Expected exception/;
1084
ad216e65
JH
1085If LIST is empty and C<$@> contains an object reference that has a
1086C<PROPAGATE> method, that method will be called with additional file
1087and line number parameters. The return value replaces the value in
1088C<$@>. ie. as if C<<$@ = eval { $@->PROPAGATE(__FILE__, __LINE__) };>>
1089were called.
1090
7660c0ab 1091If C<$@> is empty then the string C<"Died"> is used.
fb73857a 1092
52531d10
GS
1093die() can also be called with a reference argument. If this happens to be
1094trapped within an eval(), $@ contains the reference. This behavior permits
1095a more elaborate exception handling implementation using objects that
4375e838 1096maintain arbitrary state about the nature of the exception. Such a scheme
52531d10
GS
1097is sometimes preferable to matching particular string values of $@ using
1098regular expressions. Here's an example:
1099
1100 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1101 if ($@) {
1102 if (ref($@) && UNIVERSAL::isa($@,"Some::Module::Exception")) {
1103 # handle Some::Module::Exception
1104 }
1105 else {
1106 # handle all other possible exceptions
1107 }
1108 }
1109
19799a22 1110Because perl will stringify uncaught exception messages before displaying
52531d10
GS
1111them, you may want to overload stringification operations on such custom
1112exception objects. See L<overload> for details about that.
1113
19799a22
GS
1114You can arrange for a callback to be run just before the C<die>
1115does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1116handler will be called with the error text and can change the error
1117message, if it sees fit, by calling C<die> again. See
1118L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
1119L<"eval BLOCK"> for some examples. Although this feature was meant
1120to be run only right before your program was to exit, this is not
1121currently the case--the C<$SIG{__DIE__}> hook is currently called
1122even inside eval()ed blocks/strings! If one wants the hook to do
1123nothing in such situations, put
fb73857a 1124
1125 die @_ if $^S;
1126
19799a22
GS
1127as the first line of the handler (see L<perlvar/$^S>). Because
1128this promotes strange action at a distance, this counterintuitive
b76cc8ba 1129behavior may be fixed in a future release.
774d564b 1130
a0d0e21e
LW
1131=item do BLOCK
1132
1133Not really a function. Returns the value of the last command in the
1134sequence of commands indicated by BLOCK. When modified by a loop
98293880
JH
1135modifier, executes the BLOCK once before testing the loop condition.
1136(On other statements the loop modifiers test the conditional first.)
a0d0e21e 1137
4968c1e4 1138C<do BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7
TC
1139C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1140See L<perlsyn> for alternative strategies.
4968c1e4 1141
a0d0e21e
LW
1142=item do SUBROUTINE(LIST)
1143
1144A deprecated form of subroutine call. See L<perlsub>.
1145
1146=item do EXPR
1147
1148Uses the value of EXPR as a filename and executes the contents of the
1149file as a Perl script. Its primary use is to include subroutines
1150from a Perl subroutine library.
1151
1152 do 'stat.pl';
1153
1154is just like
1155
986b19de 1156 eval `cat stat.pl`;
a0d0e21e 1157
2b5ab1e7
TC
1158except that it's more efficient and concise, keeps track of the current
1159filename for error messages, searches the @INC libraries, and updates
1160C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1161variables. It also differs in that code evaluated with C<do FILENAME>
1162cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1163same, however, in that it does reparse the file every time you call it,
1164so you probably don't want to do this inside a loop.
a0d0e21e 1165
8e30cc93 1166If C<do> cannot read the file, it returns undef and sets C<$!> to the
2b5ab1e7 1167error. If C<do> can read the file but cannot compile it, it
8e30cc93
MG
1168returns undef and sets an error message in C<$@>. If the file is
1169successfully compiled, C<do> returns the value of the last expression
1170evaluated.
1171
a0d0e21e 1172Note that inclusion of library modules is better done with the
19799a22 1173C<use> and C<require> operators, which also do automatic error checking
4633a7c4 1174and raise an exception if there's a problem.
a0d0e21e 1175
5a964f20
TC
1176You might like to use C<do> to read in a program configuration
1177file. Manual error checking can be done this way:
1178
b76cc8ba 1179 # read in config files: system first, then user
f86cebdf 1180 for $file ("/share/prog/defaults.rc",
b76cc8ba 1181 "$ENV{HOME}/.someprogrc")
2b5ab1e7 1182 {
5a964f20 1183 unless ($return = do $file) {
f86cebdf
GS
1184 warn "couldn't parse $file: $@" if $@;
1185 warn "couldn't do $file: $!" unless defined $return;
1186 warn "couldn't run $file" unless $return;
5a964f20
TC
1187 }
1188 }
1189
a0d0e21e
LW
1190=item dump LABEL
1191
1614b0e3
JD
1192=item dump
1193
19799a22
GS
1194This function causes an immediate core dump. See also the B<-u>
1195command-line switch in L<perlrun>, which does the same thing.
1196Primarily this is so that you can use the B<undump> program (not
1197supplied) to turn your core dump into an executable binary after
1198having initialized all your variables at the beginning of the
1199program. When the new binary is executed it will begin by executing
1200a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1201Think of it as a goto with an intervening core dump and reincarnation.
1202If C<LABEL> is omitted, restarts the program from the top.
1203
1204B<WARNING>: Any files opened at the time of the dump will I<not>
1205be open any more when the program is reincarnated, with possible
b76cc8ba 1206resulting confusion on the part of Perl.
19799a22
GS
1207
1208This function is now largely obsolete, partly because it's very
1209hard to convert a core file into an executable, and because the
1210real compiler backends for generating portable bytecode and compilable
ac206dc8
RGS
1211C code have superseded it. That's why you should now invoke it as
1212C<CORE::dump()>, if you don't want to be warned against a possible
1213typo.
19799a22
GS
1214
1215If you're looking to use L<dump> to speed up your program, consider
1216generating bytecode or native C code as described in L<perlcc>. If
1217you're just trying to accelerate a CGI script, consider using the
210b36aa 1218C<mod_perl> extension to B<Apache>, or the CPAN module, CGI::Fast.
19799a22 1219You might also consider autoloading or selfloading, which at least
b76cc8ba 1220make your program I<appear> to run faster.
5a964f20 1221
aa689395 1222=item each HASH
1223
5a964f20 1224When called in list context, returns a 2-element list consisting of the
aa689395 1225key and value for the next element of a hash, so that you can iterate over
74fc8b5f 1226it. When called in scalar context, returns only the key for the next
e902a979 1227element in the hash.
2f9daede 1228
ab192400
GS
1229Entries are returned in an apparently random order. The actual random
1230order is subject to change in future versions of perl, but it is guaranteed
19799a22 1231to be in the same order as either the C<keys> or C<values> function
ab192400
GS
1232would produce on the same (unmodified) hash.
1233
1234When the hash is entirely read, a null array is returned in list context
19799a22
GS
1235(which when assigned produces a false (C<0>) value), and C<undef> in
1236scalar context. The next call to C<each> after that will start iterating
1237again. There is a single iterator for each hash, shared by all C<each>,
1238C<keys>, and C<values> function calls in the program; it can be reset by
2f9daede
TP
1239reading all the elements from the hash, or by evaluating C<keys HASH> or
1240C<values HASH>. If you add or delete elements of a hash while you're
74fc8b5f
MJD
1241iterating over it, you may get entries skipped or duplicated, so
1242don't. Exception: It is always safe to delete the item most recently
1243returned by C<each()>, which means that the following code will work:
1244
1245 while (($key, $value) = each %hash) {
1246 print $key, "\n";
1247 delete $hash{$key}; # This is safe
1248 }
aa689395 1249
f86cebdf 1250The following prints out your environment like the printenv(1) program,
aa689395 1251only in a different order:
a0d0e21e
LW
1252
1253 while (($key,$value) = each %ENV) {
1254 print "$key=$value\n";
1255 }
1256
19799a22 1257See also C<keys>, C<values> and C<sort>.
a0d0e21e
LW
1258
1259=item eof FILEHANDLE
1260
4633a7c4
LW
1261=item eof ()
1262
a0d0e21e
LW
1263=item eof
1264
1265Returns 1 if the next read on FILEHANDLE will return end of file, or if
1266FILEHANDLE is not open. FILEHANDLE may be an expression whose value
5a964f20 1267gives the real filehandle. (Note that this function actually
19799a22 1268reads a character and then C<ungetc>s it, so isn't very useful in an
748a9306 1269interactive context.) Do not read from a terminal file (or call
19799a22 1270C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
748a9306
LW
1271as terminals may lose the end-of-file condition if you do.
1272
820475bd
GS
1273An C<eof> without an argument uses the last file read. Using C<eof()>
1274with empty parentheses is very different. It refers to the pseudo file
1275formed from the files listed on the command line and accessed via the
61eff3bc
JH
1276C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1277as a normal filehandle is, an C<eof()> before C<< <> >> has been
820475bd 1278used will cause C<@ARGV> to be examined to determine if input is
efdd0218
RB
1279available. Similarly, an C<eof()> after C<< <> >> has returned
1280end-of-file will assume you are processing another C<@ARGV> list,
1281and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1282see L<perlop/"I/O Operators">.
820475bd 1283
61eff3bc 1284In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
820475bd
GS
1285detect the end of each file, C<eof()> will only detect the end of the
1286last file. Examples:
a0d0e21e 1287
748a9306
LW
1288 # reset line numbering on each input file
1289 while (<>) {
b76cc8ba 1290 next if /^\s*#/; # skip comments
748a9306 1291 print "$.\t$_";
5a964f20
TC
1292 } continue {
1293 close ARGV if eof; # Not eof()!
748a9306
LW
1294 }
1295
a0d0e21e
LW
1296 # insert dashes just before last line of last file
1297 while (<>) {
5a964f20 1298 if (eof()) { # check for end of current file
a0d0e21e 1299 print "--------------\n";
2b5ab1e7 1300 close(ARGV); # close or last; is needed if we
748a9306 1301 # are reading from the terminal
a0d0e21e
LW
1302 }
1303 print;
1304 }
1305
a0d0e21e 1306Practical hint: you almost never need to use C<eof> in Perl, because the
3ce0d271
GS
1307input operators typically return C<undef> when they run out of data, or if
1308there was an error.
a0d0e21e
LW
1309
1310=item eval EXPR
1311
1312=item eval BLOCK
1313
c7cc6f1c
GS
1314In the first form, the return value of EXPR is parsed and executed as if it
1315were a little Perl program. The value of the expression (which is itself
5a964f20 1316determined within scalar context) is first parsed, and if there weren't any
be3174d2
GS
1317errors, executed in the lexical context of the current Perl program, so
1318that any variable settings or subroutine and format definitions remain
1319afterwards. Note that the value is parsed every time the eval executes.
1320If EXPR is omitted, evaluates C<$_>. This form is typically used to
1321delay parsing and subsequent execution of the text of EXPR until run time.
c7cc6f1c
GS
1322
1323In the second form, the code within the BLOCK is parsed only once--at the
1324same time the code surrounding the eval itself was parsed--and executed
1325within the context of the current Perl program. This form is typically
1326used to trap exceptions more efficiently than the first (see below), while
1327also providing the benefit of checking the code within BLOCK at compile
1328time.
1329
1330The final semicolon, if any, may be omitted from the value of EXPR or within
1331the BLOCK.
1332
1333In both forms, the value returned is the value of the last expression
5a964f20 1334evaluated inside the mini-program; a return statement may be also used, just
c7cc6f1c 1335as with subroutines. The expression providing the return value is evaluated
5a964f20 1336in void, scalar, or list context, depending on the context of the eval itself.
c7cc6f1c 1337See L</wantarray> for more on how the evaluation context can be determined.
a0d0e21e 1338
19799a22
GS
1339If there is a syntax error or runtime error, or a C<die> statement is
1340executed, an undefined value is returned by C<eval>, and C<$@> is set to the
a0d0e21e 1341error message. If there was no error, C<$@> is guaranteed to be a null
19799a22 1342string. Beware that using C<eval> neither silences perl from printing
c7cc6f1c 1343warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
d9984052
A
1344To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1345turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1346See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
a0d0e21e 1347
19799a22
GS
1348Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1349determining whether a particular feature (such as C<socket> or C<symlink>)
a0d0e21e
LW
1350is implemented. It is also Perl's exception trapping mechanism, where
1351the die operator is used to raise exceptions.
1352
1353If the code to be executed doesn't vary, you may use the eval-BLOCK
1354form to trap run-time errors without incurring the penalty of
1355recompiling each time. The error, if any, is still returned in C<$@>.
1356Examples:
1357
54310121 1358 # make divide-by-zero nonfatal
a0d0e21e
LW
1359 eval { $answer = $a / $b; }; warn $@ if $@;
1360
1361 # same thing, but less efficient
1362 eval '$answer = $a / $b'; warn $@ if $@;
1363
1364 # a compile-time error
5a964f20 1365 eval { $answer = }; # WRONG
a0d0e21e
LW
1366
1367 # a run-time error
1368 eval '$answer ='; # sets $@
1369
2b5ab1e7
TC
1370Due to the current arguably broken state of C<__DIE__> hooks, when using
1371the C<eval{}> form as an exception trap in libraries, you may wish not
1372to trigger any C<__DIE__> hooks that user code may have installed.
1373You can use the C<local $SIG{__DIE__}> construct for this purpose,
1374as shown in this example:
774d564b 1375
1376 # a very private exception trap for divide-by-zero
f86cebdf
GS
1377 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1378 warn $@ if $@;
774d564b 1379
1380This is especially significant, given that C<__DIE__> hooks can call
19799a22 1381C<die> again, which has the effect of changing their error messages:
774d564b 1382
1383 # __DIE__ hooks may modify error messages
1384 {
f86cebdf
GS
1385 local $SIG{'__DIE__'} =
1386 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
c7cc6f1c
GS
1387 eval { die "foo lives here" };
1388 print $@ if $@; # prints "bar lives here"
774d564b 1389 }
1390
19799a22 1391Because this promotes action at a distance, this counterintuitive behavior
2b5ab1e7
TC
1392may be fixed in a future release.
1393
19799a22 1394With an C<eval>, you should be especially careful to remember what's
a0d0e21e
LW
1395being looked at when:
1396
1397 eval $x; # CASE 1
1398 eval "$x"; # CASE 2
1399
1400 eval '$x'; # CASE 3
1401 eval { $x }; # CASE 4
1402
5a964f20 1403 eval "\$$x++"; # CASE 5
a0d0e21e
LW
1404 $$x++; # CASE 6
1405
2f9daede 1406Cases 1 and 2 above behave identically: they run the code contained in
19799a22 1407the variable $x. (Although case 2 has misleading double quotes making
2f9daede 1408the reader wonder what else might be happening (nothing is).) Cases 3
7660c0ab 1409and 4 likewise behave in the same way: they run the code C<'$x'>, which
19799a22 1410does nothing but return the value of $x. (Case 4 is preferred for
2f9daede
TP
1411purely visual reasons, but it also has the advantage of compiling at
1412compile-time instead of at run-time.) Case 5 is a place where
19799a22 1413normally you I<would> like to use double quotes, except that in this
2f9daede
TP
1414particular situation, you can just use symbolic references instead, as
1415in case 6.
a0d0e21e 1416
4968c1e4 1417C<eval BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7 1418C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
4968c1e4 1419
a0d0e21e
LW
1420=item exec LIST
1421
8bf3b016
GS
1422=item exec PROGRAM LIST
1423
19799a22
GS
1424The C<exec> function executes a system command I<and never returns>--
1425use C<system> instead of C<exec> if you want it to return. It fails and
1426returns false only if the command does not exist I<and> it is executed
fb73857a 1427directly instead of via your system's command shell (see below).
a0d0e21e 1428
19799a22
GS
1429Since it's a common mistake to use C<exec> instead of C<system>, Perl
1430warns you if there is a following statement which isn't C<die>, C<warn>,
1431or C<exit> (if C<-w> is set - but you always do that). If you
1432I<really> want to follow an C<exec> with some other statement, you
55d729e4
GS
1433can use one of these styles to avoid the warning:
1434
5a964f20
TC
1435 exec ('foo') or print STDERR "couldn't exec foo: $!";
1436 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
55d729e4 1437
5a964f20 1438If there is more than one argument in LIST, or if LIST is an array
f86cebdf 1439with more than one value, calls execvp(3) with the arguments in LIST.
5a964f20
TC
1440If there is only one scalar argument or an array with one element in it,
1441the argument is checked for shell metacharacters, and if there are any,
1442the entire argument is passed to the system's command shell for parsing
1443(this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1444If there are no shell metacharacters in the argument, it is split into
b76cc8ba 1445words and passed directly to C<execvp>, which is more efficient.
19799a22 1446Examples:
a0d0e21e 1447
19799a22
GS
1448 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1449 exec "sort $outfile | uniq";
a0d0e21e
LW
1450
1451If you don't really want to execute the first argument, but want to lie
1452to the program you are executing about its own name, you can specify
1453the program you actually want to run as an "indirect object" (without a
1454comma) in front of the LIST. (This always forces interpretation of the
54310121 1455LIST as a multivalued list, even if there is only a single scalar in
a0d0e21e
LW
1456the list.) Example:
1457
1458 $shell = '/bin/csh';
1459 exec $shell '-sh'; # pretend it's a login shell
1460
1461or, more directly,
1462
1463 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1464
bb32b41a
GS
1465When the arguments get executed via the system shell, results will
1466be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1467for details.
1468
19799a22
GS
1469Using an indirect object with C<exec> or C<system> is also more
1470secure. This usage (which also works fine with system()) forces
1471interpretation of the arguments as a multivalued list, even if the
1472list had just one argument. That way you're safe from the shell
1473expanding wildcards or splitting up words with whitespace in them.
5a964f20
TC
1474
1475 @args = ( "echo surprise" );
1476
2b5ab1e7 1477 exec @args; # subject to shell escapes
f86cebdf 1478 # if @args == 1
2b5ab1e7 1479 exec { $args[0] } @args; # safe even with one-arg list
5a964f20
TC
1480
1481The first version, the one without the indirect object, ran the I<echo>
1482program, passing it C<"surprise"> an argument. The second version
1483didn't--it tried to run a program literally called I<"echo surprise">,
1484didn't find it, and set C<$?> to a non-zero value indicating failure.
1485
0f897271
GS
1486Beginning with v5.6.0, Perl will attempt to flush all files opened for
1487output before the exec, but this may not be supported on some platforms
1488(see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1489in English) or call the C<autoflush()> method of C<IO::Handle> on any
1490open handles in order to avoid lost output.
1491
19799a22 1492Note that C<exec> will not call your C<END> blocks, nor will it call
7660c0ab
A
1493any C<DESTROY> methods in your objects.
1494
a0d0e21e
LW
1495=item exists EXPR
1496
01020589 1497Given an expression that specifies a hash element or array element,
8ea97a1e
GS
1498returns true if the specified element in the hash or array has ever
1499been initialized, even if the corresponding value is undefined. The
1500element is not autovivified if it doesn't exist.
a0d0e21e 1501
01020589
GS
1502 print "Exists\n" if exists $hash{$key};
1503 print "Defined\n" if defined $hash{$key};
1504 print "True\n" if $hash{$key};
1505
1506 print "Exists\n" if exists $array[$index];
1507 print "Defined\n" if defined $array[$index];
1508 print "True\n" if $array[$index];
a0d0e21e 1509
8ea97a1e 1510A hash or array element can be true only if it's defined, and defined if
a0d0e21e
LW
1511it exists, but the reverse doesn't necessarily hold true.
1512
afebc493
GS
1513Given an expression that specifies the name of a subroutine,
1514returns true if the specified subroutine has ever been declared, even
1515if it is undefined. Mentioning a subroutine name for exists or defined
847c7ebe
DD
1516does not count as declaring it. Note that a subroutine which does not
1517exist may still be callable: its package may have an C<AUTOLOAD>
1518method that makes it spring into existence the first time that it is
1519called -- see L<perlsub>.
afebc493
GS
1520
1521 print "Exists\n" if exists &subroutine;
1522 print "Defined\n" if defined &subroutine;
1523
a0d0e21e 1524Note that the EXPR can be arbitrarily complicated as long as the final
afebc493 1525operation is a hash or array key lookup or subroutine name:
a0d0e21e 1526
2b5ab1e7
TC
1527 if (exists $ref->{A}->{B}->{$key}) { }
1528 if (exists $hash{A}{B}{$key}) { }
1529
01020589
GS
1530 if (exists $ref->{A}->{B}->[$ix]) { }
1531 if (exists $hash{A}{B}[$ix]) { }
1532
afebc493
GS
1533 if (exists &{$ref->{A}{B}{$key}}) { }
1534
01020589
GS
1535Although the deepest nested array or hash will not spring into existence
1536just because its existence was tested, any intervening ones will.
61eff3bc 1537Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
01020589
GS
1538into existence due to the existence test for the $key element above.
1539This happens anywhere the arrow operator is used, including even:
5a964f20 1540
2b5ab1e7
TC
1541 undef $ref;
1542 if (exists $ref->{"Some key"}) { }
1543 print $ref; # prints HASH(0x80d3d5c)
1544
1545This surprising autovivification in what does not at first--or even
1546second--glance appear to be an lvalue context may be fixed in a future
5a964f20 1547release.
a0d0e21e 1548
479ba383
GS
1549See L<perlref/"Pseudo-hashes: Using an array as a hash"> for specifics
1550on how exists() acts when used on a pseudo-hash.
e0478e5a 1551
afebc493
GS
1552Use of a subroutine call, rather than a subroutine name, as an argument
1553to exists() is an error.
1554
1555 exists &sub; # OK
1556 exists &sub(); # Error
1557
a0d0e21e
LW
1558=item exit EXPR
1559
2b5ab1e7 1560Evaluates EXPR and exits immediately with that value. Example:
a0d0e21e
LW
1561
1562 $ans = <STDIN>;
1563 exit 0 if $ans =~ /^[Xx]/;
1564
19799a22 1565See also C<die>. If EXPR is omitted, exits with C<0> status. The only
2b5ab1e7
TC
1566universally recognized values for EXPR are C<0> for success and C<1>
1567for error; other values are subject to interpretation depending on the
1568environment in which the Perl program is running. For example, exiting
156969 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1570the mailer to return the item undelivered, but that's not true everywhere.
a0d0e21e 1571
19799a22
GS
1572Don't use C<exit> to abort a subroutine if there's any chance that
1573someone might want to trap whatever error happened. Use C<die> instead,
1574which can be trapped by an C<eval>.
28757baa 1575
19799a22 1576The exit() function does not always exit immediately. It calls any
2b5ab1e7 1577defined C<END> routines first, but these C<END> routines may not
19799a22 1578themselves abort the exit. Likewise any object destructors that need to
2b5ab1e7
TC
1579be called are called before the real exit. If this is a problem, you
1580can call C<POSIX:_exit($status)> to avoid END and destructor processing.
87275199 1581See L<perlmod> for details.
5a964f20 1582
a0d0e21e
LW
1583=item exp EXPR
1584
54310121 1585=item exp
bbce6d69 1586
b76cc8ba 1587Returns I<e> (the natural logarithm base) to the power of EXPR.
a0d0e21e
LW
1588If EXPR is omitted, gives C<exp($_)>.
1589
1590=item fcntl FILEHANDLE,FUNCTION,SCALAR
1591
f86cebdf 1592Implements the fcntl(2) function. You'll probably have to say
a0d0e21e
LW
1593
1594 use Fcntl;
1595
0ade1984 1596first to get the correct constant definitions. Argument processing and
b76cc8ba 1597value return works just like C<ioctl> below.
a0d0e21e
LW
1598For example:
1599
1600 use Fcntl;
5a964f20
TC
1601 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1602 or die "can't fcntl F_GETFL: $!";
1603
19799a22 1604You don't have to check for C<defined> on the return from C<fnctl>.
951ba7fe
GS
1605Like C<ioctl>, it maps a C<0> return from the system call into
1606C<"0 but true"> in Perl. This string is true in boolean context and C<0>
2b5ab1e7
TC
1607in numeric context. It is also exempt from the normal B<-w> warnings
1608on improper numeric conversions.
5a964f20 1609
19799a22 1610Note that C<fcntl> will produce a fatal error if used on a machine that
2b5ab1e7
TC
1611doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1612manpage to learn what functions are available on your system.
a0d0e21e
LW
1613
1614=item fileno FILEHANDLE
1615
2b5ab1e7
TC
1616Returns the file descriptor for a filehandle, or undefined if the
1617filehandle is not open. This is mainly useful for constructing
19799a22 1618bitmaps for C<select> and low-level POSIX tty-handling operations.
2b5ab1e7
TC
1619If FILEHANDLE is an expression, the value is taken as an indirect
1620filehandle, generally its name.
5a964f20 1621
b76cc8ba 1622You can use this to find out whether two handles refer to the
5a964f20
TC
1623same underlying descriptor:
1624
1625 if (fileno(THIS) == fileno(THAT)) {
1626 print "THIS and THAT are dups\n";
b76cc8ba
NIS
1627 }
1628
1629(Filehandles connected to memory objects via new features of C<open> may
1630return undefined even though they are open.)
1631
a0d0e21e
LW
1632
1633=item flock FILEHANDLE,OPERATION
1634
19799a22
GS
1635Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1636for success, false on failure. Produces a fatal error if used on a
2b5ab1e7 1637machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
19799a22 1638C<flock> is Perl's portable file locking interface, although it locks
2b5ab1e7
TC
1639only entire files, not records.
1640
1641Two potentially non-obvious but traditional C<flock> semantics are
1642that it waits indefinitely until the lock is granted, and that its locks
1643B<merely advisory>. Such discretionary locks are more flexible, but offer
19799a22
GS
1644fewer guarantees. This means that files locked with C<flock> may be
1645modified by programs that do not also use C<flock>. See L<perlport>,
2b5ab1e7
TC
1646your port's specific documentation, or your system-specific local manpages
1647for details. It's best to assume traditional behavior if you're writing
1648portable programs. (But if you're not, you should as always feel perfectly
1649free to write for your own system's idiosyncrasies (sometimes called
1650"features"). Slavish adherence to portability concerns shouldn't get
1651in the way of your getting your job done.)
a3cb178b 1652
8ebc5c01 1653OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1654LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
ea3105be 1655you can use the symbolic names if you import them from the Fcntl module,
68dc0745 1656either individually, or as a group using the ':flock' tag. LOCK_SH
1657requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
ea3105be
GS
1658releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1659LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
68dc0745 1660waiting for the lock (check the return status to see if you got it).
1661
2b5ab1e7
TC
1662To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1663before locking or unlocking it.
8ebc5c01 1664
f86cebdf 1665Note that the emulation built with lockf(3) doesn't provide shared
8ebc5c01 1666locks, and it requires that FILEHANDLE be open with write intent. These
2b5ab1e7 1667are the semantics that lockf(3) implements. Most if not all systems
f86cebdf 1668implement lockf(3) in terms of fcntl(2) locking, though, so the
8ebc5c01 1669differing semantics shouldn't bite too many people.
1670
becacb53
TM
1671Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
1672be open with read intent to use LOCK_SH and requires that it be open
1673with write intent to use LOCK_EX.
1674
19799a22
GS
1675Note also that some versions of C<flock> cannot lock things over the
1676network; you would need to use the more system-specific C<fcntl> for
f86cebdf
GS
1677that. If you like you can force Perl to ignore your system's flock(2)
1678function, and so provide its own fcntl(2)-based emulation, by passing
8ebc5c01 1679the switch C<-Ud_flock> to the F<Configure> program when you configure
1680perl.
4633a7c4
LW
1681
1682Here's a mailbox appender for BSD systems.
a0d0e21e 1683
7e1af8bc 1684 use Fcntl ':flock'; # import LOCK_* constants
a0d0e21e
LW
1685
1686 sub lock {
7e1af8bc 1687 flock(MBOX,LOCK_EX);
a0d0e21e
LW
1688 # and, in case someone appended
1689 # while we were waiting...
1690 seek(MBOX, 0, 2);
1691 }
1692
1693 sub unlock {
7e1af8bc 1694 flock(MBOX,LOCK_UN);
a0d0e21e
LW
1695 }
1696
1697 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1698 or die "Can't open mailbox: $!";
1699
1700 lock();
1701 print MBOX $msg,"\n\n";
1702 unlock();
1703
2b5ab1e7
TC
1704On systems that support a real flock(), locks are inherited across fork()
1705calls, whereas those that must resort to the more capricious fcntl()
1706function lose the locks, making it harder to write servers.
1707
cb1a09d0 1708See also L<DB_File> for other flock() examples.
a0d0e21e
LW
1709
1710=item fork
1711
2b5ab1e7
TC
1712Does a fork(2) system call to create a new process running the
1713same program at the same point. It returns the child pid to the
1714parent process, C<0> to the child process, or C<undef> if the fork is
1715unsuccessful. File descriptors (and sometimes locks on those descriptors)
1716are shared, while everything else is copied. On most systems supporting
1717fork(), great care has gone into making it extremely efficient (for
1718example, using copy-on-write technology on data pages), making it the
1719dominant paradigm for multitasking over the last few decades.
5a964f20 1720
0f897271
GS
1721Beginning with v5.6.0, Perl will attempt to flush all files opened for
1722output before forking the child process, but this may not be supported
1723on some platforms (see L<perlport>). To be safe, you may need to set
1724C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1725C<IO::Handle> on any open handles in order to avoid duplicate output.
a0d0e21e 1726
19799a22 1727If you C<fork> without ever waiting on your children, you will
2b5ab1e7
TC
1728accumulate zombies. On some systems, you can avoid this by setting
1729C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1730forking and reaping moribund children.
cb1a09d0 1731
28757baa 1732Note that if your forked child inherits system file descriptors like
1733STDIN and STDOUT that are actually connected by a pipe or socket, even
2b5ab1e7 1734if you exit, then the remote server (such as, say, a CGI script or a
19799a22 1735backgrounded job launched from a remote shell) won't think you're done.
2b5ab1e7 1736You should reopen those to F</dev/null> if it's any issue.
28757baa 1737
cb1a09d0
AD
1738=item format
1739
19799a22 1740Declare a picture format for use by the C<write> function. For
cb1a09d0
AD
1741example:
1742
54310121 1743 format Something =
cb1a09d0
AD
1744 Test: @<<<<<<<< @||||| @>>>>>
1745 $str, $%, '$' . int($num)
1746 .
1747
1748 $str = "widget";
184e9718 1749 $num = $cost/$quantity;
cb1a09d0
AD
1750 $~ = 'Something';
1751 write;
1752
1753See L<perlform> for many details and examples.
1754
8903cb82 1755=item formline PICTURE,LIST
a0d0e21e 1756
5a964f20 1757This is an internal function used by C<format>s, though you may call it,
a0d0e21e
LW
1758too. It formats (see L<perlform>) a list of values according to the
1759contents of PICTURE, placing the output into the format output
7660c0ab 1760accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
19799a22 1761Eventually, when a C<write> is done, the contents of
a0d0e21e 1762C<$^A> are written to some filehandle, but you could also read C<$^A>
7660c0ab 1763yourself and then set C<$^A> back to C<"">. Note that a format typically
19799a22 1764does one C<formline> per line of form, but the C<formline> function itself
748a9306 1765doesn't care how many newlines are embedded in the PICTURE. This means
4633a7c4 1766that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
748a9306
LW
1767You may therefore need to use multiple formlines to implement a single
1768record format, just like the format compiler.
1769
19799a22 1770Be careful if you put double quotes around the picture, because an C<@>
748a9306 1771character may be taken to mean the beginning of an array name.
19799a22 1772C<formline> always returns true. See L<perlform> for other examples.
a0d0e21e
LW
1773
1774=item getc FILEHANDLE
1775
1776=item getc
1777
1778Returns the next character from the input file attached to FILEHANDLE,
2b5ab1e7
TC
1779or the undefined value at end of file, or if there was an error.
1780If FILEHANDLE is omitted, reads from STDIN. This is not particularly
1781efficient. However, it cannot be used by itself to fetch single
1782characters without waiting for the user to hit enter. For that, try
1783something more like:
4633a7c4
LW
1784
1785 if ($BSD_STYLE) {
1786 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1787 }
1788 else {
54310121 1789 system "stty", '-icanon', 'eol', "\001";
4633a7c4
LW
1790 }
1791
1792 $key = getc(STDIN);
1793
1794 if ($BSD_STYLE) {
1795 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1796 }
1797 else {
5f05dabc 1798 system "stty", 'icanon', 'eol', '^@'; # ASCII null
4633a7c4
LW
1799 }
1800 print "\n";
1801
54310121 1802Determination of whether $BSD_STYLE should be set
1803is left as an exercise to the reader.
cb1a09d0 1804
19799a22 1805The C<POSIX::getattr> function can do this more portably on
2b5ab1e7
TC
1806systems purporting POSIX compliance. See also the C<Term::ReadKey>
1807module from your nearest CPAN site; details on CPAN can be found on
1808L<perlmodlib/CPAN>.
a0d0e21e
LW
1809
1810=item getlogin
1811
5a964f20
TC
1812Implements the C library function of the same name, which on most
1813systems returns the current login from F</etc/utmp>, if any. If null,
19799a22 1814use C<getpwuid>.
a0d0e21e 1815
f86702cc 1816 $login = getlogin || getpwuid($<) || "Kilroy";
a0d0e21e 1817
19799a22
GS
1818Do not consider C<getlogin> for authentication: it is not as
1819secure as C<getpwuid>.
4633a7c4 1820
a0d0e21e
LW
1821=item getpeername SOCKET
1822
1823Returns the packed sockaddr address of other end of the SOCKET connection.
1824
4633a7c4
LW
1825 use Socket;
1826 $hersockaddr = getpeername(SOCK);
19799a22 1827 ($port, $iaddr) = sockaddr_in($hersockaddr);
4633a7c4
LW
1828 $herhostname = gethostbyaddr($iaddr, AF_INET);
1829 $herstraddr = inet_ntoa($iaddr);
a0d0e21e
LW
1830
1831=item getpgrp PID
1832
47e29363 1833Returns the current process group for the specified PID. Use
7660c0ab 1834a PID of C<0> to get the current process group for the
4633a7c4 1835current process. Will raise an exception if used on a machine that
f86cebdf 1836doesn't implement getpgrp(2). If PID is omitted, returns process
19799a22 1837group of current process. Note that the POSIX version of C<getpgrp>
7660c0ab 1838does not accept a PID argument, so only C<PID==0> is truly portable.
a0d0e21e
LW
1839
1840=item getppid
1841
1842Returns the process id of the parent process.
1843
1844=item getpriority WHICH,WHO
1845
4633a7c4
LW
1846Returns the current priority for a process, a process group, or a user.
1847(See L<getpriority(2)>.) Will raise a fatal exception if used on a
f86cebdf 1848machine that doesn't implement getpriority(2).
a0d0e21e
LW
1849
1850=item getpwnam NAME
1851
1852=item getgrnam NAME
1853
1854=item gethostbyname NAME
1855
1856=item getnetbyname NAME
1857
1858=item getprotobyname NAME
1859
1860=item getpwuid UID
1861
1862=item getgrgid GID
1863
1864=item getservbyname NAME,PROTO
1865
1866=item gethostbyaddr ADDR,ADDRTYPE
1867
1868=item getnetbyaddr ADDR,ADDRTYPE
1869
1870=item getprotobynumber NUMBER
1871
1872=item getservbyport PORT,PROTO
1873
1874=item getpwent
1875
1876=item getgrent
1877
1878=item gethostent
1879
1880=item getnetent
1881
1882=item getprotoent
1883
1884=item getservent
1885
1886=item setpwent
1887
1888=item setgrent
1889
1890=item sethostent STAYOPEN
1891
1892=item setnetent STAYOPEN
1893
1894=item setprotoent STAYOPEN
1895
1896=item setservent STAYOPEN
1897
1898=item endpwent
1899
1900=item endgrent
1901
1902=item endhostent
1903
1904=item endnetent
1905
1906=item endprotoent
1907
1908=item endservent
1909
1910These routines perform the same functions as their counterparts in the
5a964f20 1911system library. In list context, the return values from the
a0d0e21e
LW
1912various get routines are as follows:
1913
1914 ($name,$passwd,$uid,$gid,
6ee623d5 1915 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
a0d0e21e
LW
1916 ($name,$passwd,$gid,$members) = getgr*
1917 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1918 ($name,$aliases,$addrtype,$net) = getnet*
1919 ($name,$aliases,$proto) = getproto*
1920 ($name,$aliases,$port,$proto) = getserv*
1921
1922(If the entry doesn't exist you get a null list.)
1923
4602f195
JH
1924The exact meaning of the $gcos field varies but it usually contains
1925the real name of the user (as opposed to the login name) and other
1926information pertaining to the user. Beware, however, that in many
1927system users are able to change this information and therefore it
106325ad 1928cannot be trusted and therefore the $gcos is tainted (see
2959b6e3
JH
1929L<perlsec>). The $passwd and $shell, user's encrypted password and
1930login shell, are also tainted, because of the same reason.
4602f195 1931
5a964f20 1932In scalar context, you get the name, unless the function was a
a0d0e21e
LW
1933lookup by name, in which case you get the other thing, whatever it is.
1934(If the entry doesn't exist you get the undefined value.) For example:
1935
5a964f20
TC
1936 $uid = getpwnam($name);
1937 $name = getpwuid($num);
1938 $name = getpwent();
1939 $gid = getgrnam($name);
1940 $name = getgrgid($num;
1941 $name = getgrent();
1942 #etc.
a0d0e21e 1943
4602f195
JH
1944In I<getpw*()> the fields $quota, $comment, and $expire are special
1945cases in the sense that in many systems they are unsupported. If the
1946$quota is unsupported, it is an empty scalar. If it is supported, it
1947usually encodes the disk quota. If the $comment field is unsupported,
1948it is an empty scalar. If it is supported it usually encodes some
1949administrative comment about the user. In some systems the $quota
1950field may be $change or $age, fields that have to do with password
1951aging. In some systems the $comment field may be $class. The $expire
1952field, if present, encodes the expiration period of the account or the
1953password. For the availability and the exact meaning of these fields
1954in your system, please consult your getpwnam(3) documentation and your
1955F<pwd.h> file. You can also find out from within Perl what your
1956$quota and $comment fields mean and whether you have the $expire field
1957by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
1958C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
1959files are only supported if your vendor has implemented them in the
1960intuitive fashion that calling the regular C library routines gets the
5d3a0a3b
GS
1961shadow versions if you're running under privilege or if there exists
1962the shadow(3) functions as found in System V ( this includes Solaris
1963and Linux.) Those systems which implement a proprietary shadow password
1964facility are unlikely to be supported.
6ee623d5 1965
19799a22 1966The $members value returned by I<getgr*()> is a space separated list of
a0d0e21e
LW
1967the login names of the members of the group.
1968
1969For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1970C, it will be returned to you via C<$?> if the function call fails. The
7660c0ab 1971C<@addrs> value returned by a successful call is a list of the raw
a0d0e21e
LW
1972addresses returned by the corresponding system library call. In the
1973Internet domain, each address is four bytes long and you can unpack it
1974by saying something like:
1975
1976 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1977
2b5ab1e7
TC
1978The Socket library makes this slightly easier:
1979
1980 use Socket;
1981 $iaddr = inet_aton("127.1"); # or whatever address
1982 $name = gethostbyaddr($iaddr, AF_INET);
1983
1984 # or going the other way
19799a22 1985 $straddr = inet_ntoa($iaddr);
2b5ab1e7 1986
19799a22
GS
1987If you get tired of remembering which element of the return list
1988contains which return value, by-name interfaces are provided
1989in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
1990C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
1991and C<User::grent>. These override the normal built-ins, supplying
1992versions that return objects with the appropriate names
1993for each field. For example:
5a964f20
TC
1994
1995 use File::stat;
1996 use User::pwent;
1997 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
1998
b76cc8ba
NIS
1999Even though it looks like they're the same method calls (uid),
2000they aren't, because a C<File::stat> object is different from
19799a22 2001a C<User::pwent> object.
5a964f20 2002
a0d0e21e
LW
2003=item getsockname SOCKET
2004
19799a22
GS
2005Returns the packed sockaddr address of this end of the SOCKET connection,
2006in case you don't know the address because you have several different
2007IPs that the connection might have come in on.
a0d0e21e 2008
4633a7c4
LW
2009 use Socket;
2010 $mysockaddr = getsockname(SOCK);
19799a22 2011 ($port, $myaddr) = sockaddr_in($mysockaddr);
b76cc8ba 2012 printf "Connect to %s [%s]\n",
19799a22
GS
2013 scalar gethostbyaddr($myaddr, AF_INET),
2014 inet_ntoa($myaddr);
a0d0e21e
LW
2015
2016=item getsockopt SOCKET,LEVEL,OPTNAME
2017
5a964f20 2018Returns the socket option requested, or undef if there is an error.
a0d0e21e
LW
2019
2020=item glob EXPR
2021
0a753a76 2022=item glob
2023
2b5ab1e7
TC
2024Returns the value of EXPR with filename expansions such as the
2025standard Unix shell F</bin/csh> would do. This is the internal function
61eff3bc
JH
2026implementing the C<< <*.c> >> operator, but you can use it directly.
2027If EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is
2b5ab1e7 2028discussed in more detail in L<perlop/"I/O Operators">.
a0d0e21e 2029
3a4b19e4
GS
2030Beginning with v5.6.0, this operator is implemented using the standard
2031C<File::Glob> extension. See L<File::Glob> for details.
2032
a0d0e21e
LW
2033=item gmtime EXPR
2034
d1be9408 2035Converts a time as returned by the time function to an 8-element list
54310121 2036with the time localized for the standard Greenwich time zone.
4633a7c4 2037Typically used as follows:
a0d0e21e 2038
b76cc8ba 2039 # 0 1 2 3 4 5 6 7
48a26b3a 2040 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday) =
a0d0e21e
LW
2041 gmtime(time);
2042
48a26b3a
GS
2043All list elements are numeric, and come straight out of the C `struct
2044tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2045specified time. $mday is the day of the month, and $mon is the month
2046itself, in the range C<0..11> with 0 indicating January and 11
2047indicating December. $year is the number of years since 1900. That
2048is, $year is C<123> in year 2023. $wday is the day of the week, with
20490 indicating Sunday and 3 indicating Wednesday. $yday is the day of
b76cc8ba 2050the year, in the range C<0..364> (or C<0..365> in leap years.)
48a26b3a
GS
2051
2052Note that the $year element is I<not> simply the last two digits of
2053the year. If you assume it is, then you create non-Y2K-compliant
2054programs--and you wouldn't want to do that, would you?
2f9daede 2055
abd75f24
GS
2056The proper way to get a complete 4-digit year is simply:
2057
2058 $year += 1900;
2059
2060And to get the last two digits of the year (e.g., '01' in 2001) do:
2061
2062 $year = sprintf("%02d", $year % 100);
2063
48a26b3a 2064If EXPR is omitted, C<gmtime()> uses the current time (C<gmtime(time)>).
a0d0e21e 2065
48a26b3a 2066In scalar context, C<gmtime()> returns the ctime(3) value:
0a753a76 2067
2068 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
2069
19799a22 2070Also see the C<timegm> function provided by the C<Time::Local> module,
f86cebdf 2071and the strftime(3) function available via the POSIX module.
7660c0ab 2072
2b5ab1e7
TC
2073This scalar value is B<not> locale dependent (see L<perllocale>), but
2074is instead a Perl builtin. Also see the C<Time::Local> module, and the
2075strftime(3) and mktime(3) functions available via the POSIX module. To
7660c0ab
A
2076get somewhat similar but locale dependent date strings, set up your
2077locale environment variables appropriately (please see L<perllocale>)
2078and try for example:
2079
2080 use POSIX qw(strftime);
2b5ab1e7 2081 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
7660c0ab 2082
2b5ab1e7
TC
2083Note that the C<%a> and C<%b> escapes, which represent the short forms
2084of the day of the week and the month of the year, may not necessarily
2085be three characters wide in all locales.
0a753a76 2086
a0d0e21e
LW
2087=item goto LABEL
2088
748a9306
LW
2089=item goto EXPR
2090
a0d0e21e
LW
2091=item goto &NAME
2092
7660c0ab 2093The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
a0d0e21e 2094execution there. It may not be used to go into any construct that
7660c0ab 2095requires initialization, such as a subroutine or a C<foreach> loop. It
0a753a76 2096also can't be used to go into a construct that is optimized away,
19799a22 2097or to get out of a block or subroutine given to C<sort>.
0a753a76 2098It can be used to go almost anywhere else within the dynamic scope,
a0d0e21e 2099including out of subroutines, but it's usually better to use some other
19799a22 2100construct such as C<last> or C<die>. The author of Perl has never felt the
7660c0ab 2101need to use this form of C<goto> (in Perl, that is--C is another matter).
1b6921cb
BT
2102(The difference being that C does not offer named loops combined with
2103loop control. Perl does, and this replaces most structured uses of C<goto>
2104in other languages.)
a0d0e21e 2105
7660c0ab
A
2106The C<goto-EXPR> form expects a label name, whose scope will be resolved
2107dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
748a9306
LW
2108necessarily recommended if you're optimizing for maintainability:
2109
2110 goto ("FOO", "BAR", "GLARCH")[$i];
2111
1b6921cb
BT
2112The C<goto-&NAME> form is quite different from the other forms of
2113C<goto>. In fact, it isn't a goto in the normal sense at all, and
2114doesn't have the stigma associated with other gotos. Instead, it
2115exits the current subroutine (losing any changes set by local()) and
2116immediately calls in its place the named subroutine using the current
2117value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2118load another subroutine and then pretend that the other subroutine had
2119been called in the first place (except that any modifications to C<@_>
6cb9131c
GS
2120in the current subroutine are propagated to the other subroutine.)
2121After the C<goto>, not even C<caller> will be able to tell that this
2122routine was called first.
2123
2124NAME needn't be the name of a subroutine; it can be a scalar variable
2125containing a code reference, or a block which evaluates to a code
2126reference.
a0d0e21e
LW
2127
2128=item grep BLOCK LIST
2129
2130=item grep EXPR,LIST
2131
2b5ab1e7
TC
2132This is similar in spirit to, but not the same as, grep(1) and its
2133relatives. In particular, it is not limited to using regular expressions.
2f9daede 2134
a0d0e21e 2135Evaluates the BLOCK or EXPR for each element of LIST (locally setting
7660c0ab 2136C<$_> to each element) and returns the list value consisting of those
19799a22
GS
2137elements for which the expression evaluated to true. In scalar
2138context, returns the number of times the expression was true.
a0d0e21e
LW
2139
2140 @foo = grep(!/^#/, @bar); # weed out comments
2141
2142or equivalently,
2143
2144 @foo = grep {!/^#/} @bar; # weed out comments
2145
be3174d2
GS
2146Note that C<$_> is an alias to the list value, so it can be used to
2147modify the elements of the LIST. While this is useful and supported,
2148it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
2149Similarly, grep returns aliases into the original list, much as a for
2150loop's index variable aliases the list elements. That is, modifying an
19799a22
GS
2151element of a list returned by grep (for example, in a C<foreach>, C<map>
2152or another C<grep>) actually modifies the element in the original list.
2b5ab1e7 2153This is usually something to be avoided when writing clear code.
a0d0e21e 2154
19799a22 2155See also L</map> for a list composed of the results of the BLOCK or EXPR.
38325410 2156
a0d0e21e
LW
2157=item hex EXPR
2158
54310121 2159=item hex
bbce6d69 2160
2b5ab1e7
TC
2161Interprets EXPR as a hex string and returns the corresponding value.
2162(To convert strings that might start with either 0, 0x, or 0b, see
2163L</oct>.) If EXPR is omitted, uses C<$_>.
2f9daede
TP
2164
2165 print hex '0xAf'; # prints '175'
2166 print hex 'aF'; # same
a0d0e21e 2167
19799a22 2168Hex strings may only represent integers. Strings that would cause
53305cf1
NC
2169integer overflow trigger a warning. Leading whitespace is not stripped,
2170unlike oct().
19799a22 2171
a0d0e21e
LW
2172=item import
2173
19799a22 2174There is no builtin C<import> function. It is just an ordinary
4633a7c4 2175method (subroutine) defined (or inherited) by modules that wish to export
19799a22 2176names to another module. The C<use> function calls the C<import> method
cea6626f 2177for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
a0d0e21e
LW
2178
2179=item index STR,SUBSTR,POSITION
2180
2181=item index STR,SUBSTR
2182
2b5ab1e7
TC
2183The index function searches for one string within another, but without
2184the wildcard-like behavior of a full regular-expression pattern match.
2185It returns the position of the first occurrence of SUBSTR in STR at
2186or after POSITION. If POSITION is omitted, starts searching from the
2187beginning of the string. The return value is based at C<0> (or whatever
2188you've set the C<$[> variable to--but don't do that). If the substring
2189is not found, returns one less than the base, ordinarily C<-1>.
a0d0e21e
LW
2190
2191=item int EXPR
2192
54310121 2193=item int
bbce6d69 2194
7660c0ab 2195Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2b5ab1e7
TC
2196You should not use this function for rounding: one because it truncates
2197towards C<0>, and two because machine representations of floating point
2198numbers can sometimes produce counterintuitive results. For example,
2199C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2200because it's really more like -268.99999999999994315658 instead. Usually,
19799a22 2201the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2b5ab1e7 2202functions will serve you better than will int().
a0d0e21e
LW
2203
2204=item ioctl FILEHANDLE,FUNCTION,SCALAR
2205
2b5ab1e7 2206Implements the ioctl(2) function. You'll probably first have to say
a0d0e21e 2207
4633a7c4 2208 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
a0d0e21e 2209
2b5ab1e7 2210to get the correct function definitions. If F<ioctl.ph> doesn't
a0d0e21e 2211exist or doesn't have the correct definitions you'll have to roll your
61eff3bc 2212own, based on your C header files such as F<< <sys/ioctl.h> >>.
5a964f20 2213(There is a Perl script called B<h2ph> that comes with the Perl kit that
54310121 2214may help you in this, but it's nontrivial.) SCALAR will be read and/or
4633a7c4 2215written depending on the FUNCTION--a pointer to the string value of SCALAR
19799a22 2216will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
4633a7c4
LW
2217has no string value but does have a numeric value, that value will be
2218passed rather than a pointer to the string value. To guarantee this to be
19799a22
GS
2219true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2220functions may be needed to manipulate the values of structures used by
b76cc8ba 2221C<ioctl>.
a0d0e21e 2222
19799a22 2223The return value of C<ioctl> (and C<fcntl>) is as follows:
a0d0e21e
LW
2224
2225 if OS returns: then Perl returns:
2226 -1 undefined value
2227 0 string "0 but true"
2228 anything else that number
2229
19799a22 2230Thus Perl returns true on success and false on failure, yet you can
a0d0e21e
LW
2231still easily determine the actual value returned by the operating
2232system:
2233
2b5ab1e7 2234 $retval = ioctl(...) || -1;
a0d0e21e
LW
2235 printf "System returned %d\n", $retval;
2236
c2611fb3 2237The special string "C<0> but true" is exempt from B<-w> complaints
5a964f20
TC
2238about improper numeric conversions.
2239
19799a22
GS
2240Here's an example of setting a filehandle named C<REMOTE> to be
2241non-blocking at the system level. You'll have to negotiate C<$|>
2242on your own, though.
2243
2244 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
2245
2246 $flags = fcntl(REMOTE, F_GETFL, 0)
2247 or die "Can't get flags for the socket: $!\n";
2248
2249 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
2250 or die "Can't set flags for the socket: $!\n";
2251
a0d0e21e
LW
2252=item join EXPR,LIST
2253
2b5ab1e7
TC
2254Joins the separate strings of LIST into a single string with fields
2255separated by the value of EXPR, and returns that new string. Example:
a0d0e21e 2256
2b5ab1e7 2257 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
a0d0e21e 2258
eb6e2d6f
GS
2259Beware that unlike C<split>, C<join> doesn't take a pattern as its
2260first argument. Compare L</split>.
a0d0e21e 2261
aa689395 2262=item keys HASH
2263
19799a22 2264Returns a list consisting of all the keys of the named hash. (In
1d2dff63 2265scalar context, returns the number of keys.) The keys are returned in
ab192400
GS
2266an apparently random order. The actual random order is subject to
2267change in future versions of perl, but it is guaranteed to be the same
19799a22 2268order as either the C<values> or C<each> function produces (given
ab192400
GS
2269that the hash has not been modified). As a side effect, it resets
2270HASH's iterator.
a0d0e21e 2271
aa689395 2272Here is yet another way to print your environment:
a0d0e21e
LW
2273
2274 @keys = keys %ENV;
2275 @values = values %ENV;
b76cc8ba 2276 while (@keys) {
a0d0e21e
LW
2277 print pop(@keys), '=', pop(@values), "\n";
2278 }
2279
2280or how about sorted by key:
2281
2282 foreach $key (sort(keys %ENV)) {
2283 print $key, '=', $ENV{$key}, "\n";
2284 }
2285
8ea1e5d4
GS
2286The returned values are copies of the original keys in the hash, so
2287modifying them will not affect the original hash. Compare L</values>.
2288
19799a22 2289To sort a hash by value, you'll need to use a C<sort> function.
aa689395 2290Here's a descending numeric sort of a hash by its values:
4633a7c4 2291
5a964f20 2292 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
4633a7c4
LW
2293 printf "%4d %s\n", $hash{$key}, $key;
2294 }
2295
19799a22 2296As an lvalue C<keys> allows you to increase the number of hash buckets
aa689395 2297allocated for the given hash. This can gain you a measure of efficiency if
2298you know the hash is going to get big. (This is similar to pre-extending
2299an array by assigning a larger number to $#array.) If you say
55497cff 2300
2301 keys %hash = 200;
2302
ab192400
GS
2303then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2304in fact, since it rounds up to the next power of two. These
55497cff 2305buckets will be retained even if you do C<%hash = ()>, use C<undef
2306%hash> if you want to free the storage while C<%hash> is still in scope.
2307You can't shrink the number of buckets allocated for the hash using
19799a22 2308C<keys> in this way (but you needn't worry about doing this by accident,
55497cff 2309as trying has no effect).
2310
19799a22 2311See also C<each>, C<values> and C<sort>.
ab192400 2312
b350dd2f 2313=item kill SIGNAL, LIST
a0d0e21e 2314
b350dd2f 2315Sends a signal to a list of processes. Returns the number of
517db077
GS
2316processes successfully signaled (which is not necessarily the
2317same as the number actually killed).
a0d0e21e
LW
2318
2319 $cnt = kill 1, $child1, $child2;
2320 kill 9, @goners;
2321
b350dd2f
GS
2322If SIGNAL is zero, no signal is sent to the process. This is a
2323useful way to check that the process is alive and hasn't changed
2324its UID. See L<perlport> for notes on the portability of this
2325construct.
2326
2327Unlike in the shell, if SIGNAL is negative, it kills
4633a7c4
LW
2328process groups instead of processes. (On System V, a negative I<PROCESS>
2329number will also kill process groups, but that's not portable.) That
2330means you usually want to use positive not negative signals. You may also
da0045b7 2331use a signal name in quotes. See L<perlipc/"Signals"> for details.
a0d0e21e
LW
2332
2333=item last LABEL
2334
2335=item last
2336
2337The C<last> command is like the C<break> statement in C (as used in
2338loops); it immediately exits the loop in question. If the LABEL is
2339omitted, the command refers to the innermost enclosing loop. The
2340C<continue> block, if any, is not executed:
2341
4633a7c4
LW
2342 LINE: while (<STDIN>) {
2343 last LINE if /^$/; # exit when done with header
5a964f20 2344 #...
a0d0e21e
LW
2345 }
2346
4968c1e4 2347C<last> cannot be used to exit a block which returns a value such as
2b5ab1e7
TC
2348C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2349a grep() or map() operation.
4968c1e4 2350
6c1372ed
GS
2351Note that a block by itself is semantically identical to a loop
2352that executes once. Thus C<last> can be used to effect an early
2353exit out of such a block.
2354
98293880
JH
2355See also L</continue> for an illustration of how C<last>, C<next>, and
2356C<redo> work.
1d2dff63 2357
a0d0e21e
LW
2358=item lc EXPR
2359
54310121 2360=item lc
bbce6d69 2361
d1be9408 2362Returns a lowercased version of EXPR. This is the internal function
ad0029c4
JH
2363implementing the C<\L> escape in double-quoted strings. Respects
2364current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
983ffd37 2365and L<perlunicode> for more details about locale and Unicode support.
a0d0e21e 2366
7660c0ab 2367If EXPR is omitted, uses C<$_>.
bbce6d69 2368
a0d0e21e
LW
2369=item lcfirst EXPR
2370
54310121 2371=item lcfirst
bbce6d69 2372
ad0029c4
JH
2373Returns the value of EXPR with the first character lowercased. This
2374is the internal function implementing the C<\l> escape in
2375double-quoted strings. Respects current LC_CTYPE locale if C<use
983ffd37
JH
2376locale> in force. See L<perllocale> and L<perlunicode> for more
2377details about locale and Unicode support.
a0d0e21e 2378
7660c0ab 2379If EXPR is omitted, uses C<$_>.
bbce6d69 2380
a0d0e21e
LW
2381=item length EXPR
2382
54310121 2383=item length
bbce6d69 2384
a0ed51b3 2385Returns the length in characters of the value of EXPR. If EXPR is
b76cc8ba 2386omitted, returns length of C<$_>. Note that this cannot be used on
2b5ab1e7
TC
2387an entire array or hash to find out how many elements these have.
2388For that, use C<scalar @array> and C<scalar keys %hash> respectively.
a0d0e21e
LW
2389
2390=item link OLDFILE,NEWFILE
2391
19799a22 2392Creates a new filename linked to the old filename. Returns true for
b76cc8ba 2393success, false otherwise.
a0d0e21e
LW
2394
2395=item listen SOCKET,QUEUESIZE
2396
19799a22 2397Does the same thing that the listen system call does. Returns true if
b76cc8ba 2398it succeeded, false otherwise. See the example in
cea6626f 2399L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
2400
2401=item local EXPR
2402
19799a22 2403You really probably want to be using C<my> instead, because C<local> isn't
b76cc8ba 2404what most people think of as "local". See
13a2d996 2405L<perlsub/"Private Variables via my()"> for details.
2b5ab1e7 2406
5a964f20
TC
2407A local modifies the listed variables to be local to the enclosing
2408block, file, or eval. If more than one value is listed, the list must
2409be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2410for details, including issues with tied arrays and hashes.
a0d0e21e 2411
a0d0e21e
LW
2412=item localtime EXPR
2413
19799a22 2414Converts a time as returned by the time function to a 9-element list
5f05dabc 2415with the time analyzed for the local time zone. Typically used as
a0d0e21e
LW
2416follows:
2417
54310121 2418 # 0 1 2 3 4 5 6 7 8
a0d0e21e
LW
2419 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2420 localtime(time);
2421
48a26b3a
GS
2422All list elements are numeric, and come straight out of the C `struct
2423tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2424specified time. $mday is the day of the month, and $mon is the month
2425itself, in the range C<0..11> with 0 indicating January and 11
2426indicating December. $year is the number of years since 1900. That
2427is, $year is C<123> in year 2023. $wday is the day of the week, with
24280 indicating Sunday and 3 indicating Wednesday. $yday is the day of
874b1813 2429the year, in the range C<0..364> (or C<0..365> in leap years.) $isdst
48a26b3a
GS
2430is true if the specified time occurs during daylight savings time,
2431false otherwise.
2432
2433Note that the $year element is I<not> simply the last two digits of
2434the year. If you assume it is, then you create non-Y2K-compliant
2435programs--and you wouldn't want to do that, would you?
54310121 2436
abd75f24
GS
2437The proper way to get a complete 4-digit year is simply:
2438
2439 $year += 1900;
2440
2441And to get the last two digits of the year (e.g., '01' in 2001) do:
2442
2443 $year = sprintf("%02d", $year % 100);
2444
48a26b3a 2445If EXPR is omitted, C<localtime()> uses the current time (C<localtime(time)>).
a0d0e21e 2446
48a26b3a 2447In scalar context, C<localtime()> returns the ctime(3) value:
a0d0e21e 2448
5f05dabc 2449 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
a0d0e21e 2450
a3cb178b 2451This scalar value is B<not> locale dependent, see L<perllocale>, but
68f8bed4
JH
2452instead a Perl builtin. Also see the C<Time::Local> module
2453(to convert the second, minutes, hours, ... back to seconds since the
2454stroke of midnight the 1st of January 1970, the value returned by
ca6e1c26 2455time()), and the strftime(3) and mktime(3) functions available via the
68f8bed4
JH
2456POSIX module. To get somewhat similar but locale dependent date
2457strings, set up your locale environment variables appropriately
2458(please see L<perllocale>) and try for example:
a3cb178b 2459
5a964f20 2460 use POSIX qw(strftime);
2b5ab1e7 2461 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
a3cb178b
GS
2462
2463Note that the C<%a> and C<%b>, the short forms of the day of the week
2464and the month of the year, may not necessarily be three characters wide.
a0d0e21e 2465
07698885 2466=item lock THING
19799a22
GS
2467
2468This function places an advisory lock on a variable, subroutine,
2469or referenced object contained in I<THING> until the lock goes out
2470of scope. This is a built-in function only if your version of Perl
43d3ddbe
JH
2471was built with threading enabled, and if you've said C<use Thread>.
2472Otherwise a user-defined function by this name will be called.
2473See L<Thread>.
19799a22 2474
a0d0e21e
LW
2475=item log EXPR
2476
54310121 2477=item log
bbce6d69 2478
2b5ab1e7
TC
2479Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2480returns log of C<$_>. To get the log of another base, use basic algebra:
19799a22 2481The base-N log of a number is equal to the natural log of that number
2b5ab1e7
TC
2482divided by the natural log of N. For example:
2483
2484 sub log10 {
2485 my $n = shift;
2486 return log($n)/log(10);
b76cc8ba 2487 }
2b5ab1e7
TC
2488
2489See also L</exp> for the inverse operation.
a0d0e21e 2490
a0d0e21e
LW
2491=item lstat EXPR
2492
54310121 2493=item lstat
bbce6d69 2494
19799a22 2495Does the same thing as the C<stat> function (including setting the
5a964f20
TC
2496special C<_> filehandle) but stats a symbolic link instead of the file
2497the symbolic link points to. If symbolic links are unimplemented on
19799a22 2498your system, a normal C<stat> is done.
a0d0e21e 2499
7660c0ab 2500If EXPR is omitted, stats C<$_>.
bbce6d69 2501
a0d0e21e
LW
2502=item m//
2503
2504The match operator. See L<perlop>.
2505
2506=item map BLOCK LIST
2507
2508=item map EXPR,LIST
2509
19799a22
GS
2510Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2511C<$_> to each element) and returns the list value composed of the
2512results of each such evaluation. In scalar context, returns the
2513total number of elements so generated. Evaluates BLOCK or EXPR in
2514list context, so each element of LIST may produce zero, one, or
2515more elements in the returned value.
dd99ebda 2516
a0d0e21e
LW
2517 @chars = map(chr, @nums);
2518
2519translates a list of numbers to the corresponding characters. And
2520
4633a7c4 2521 %hash = map { getkey($_) => $_ } @array;
a0d0e21e
LW
2522
2523is just a funny way to write
2524
2525 %hash = ();
2526 foreach $_ (@array) {
4633a7c4 2527 $hash{getkey($_)} = $_;
a0d0e21e
LW
2528 }
2529
be3174d2
GS
2530Note that C<$_> is an alias to the list value, so it can be used to
2531modify the elements of the LIST. While this is useful and supported,
2532it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
2533Using a regular C<foreach> loop for this purpose would be clearer in
2534most cases. See also L</grep> for an array composed of those items of
2535the original list for which the BLOCK or EXPR evaluates to true.
fb73857a 2536
205fdb4d
NC
2537C<{> starts both hash references and blocks, so C<map { ...> could be either
2538the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look
2539ahead for the closing C<}> it has to take a guess at which its dealing with
2540based what it finds just after the C<{>. Usually it gets it right, but if it
2541doesn't it won't realize something is wrong until it gets to the C<}> and
2542encounters the missing (or unexpected) comma. The syntax error will be
2543reported close to the C<}> but you'll need to change something near the C<{>
2544such as using a unary C<+> to give perl some help:
2545
2546 %hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong
2547 %hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right
2548 %hash = map { ("\L$_", 1) } @array # this also works
2549 %hash = map { lc($_), 1 } @array # as does this.
2550 %hash = map +( lc($_), 1 ), @array # this is EXPR and works!
cea6626f 2551
205fdb4d
NC
2552 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
2553
2554or to force an anon hash constructor use C<+{>
2555
2556 @hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end
2557
2558and you get list of anonymous hashes each with only 1 entry.
2559
19799a22 2560=item mkdir FILENAME,MASK
a0d0e21e 2561
5a211162
GS
2562=item mkdir FILENAME
2563
0591cd52 2564Creates the directory specified by FILENAME, with permissions
19799a22
GS
2565specified by MASK (as modified by C<umask>). If it succeeds it
2566returns true, otherwise it returns false and sets C<$!> (errno).
5a211162 2567If omitted, MASK defaults to 0777.
0591cd52 2568
19799a22 2569In general, it is better to create directories with permissive MASK,
0591cd52 2570and let the user modify that with their C<umask>, than it is to supply
19799a22 2571a restrictive MASK and give the user no way to be more permissive.
0591cd52
NT
2572The exceptions to this rule are when the file or directory should be
2573kept private (mail files, for instance). The perlfunc(1) entry on
19799a22 2574C<umask> discusses the choice of MASK in more detail.
a0d0e21e 2575
cc1852e8
JH
2576Note that according to the POSIX 1003.1-1996 the FILENAME may have any
2577number of trailing slashes. Some operating and filesystems do not get
2578this right, so Perl automatically removes all trailing slashes to keep
2579everyone happy.
2580
a0d0e21e
LW
2581=item msgctl ID,CMD,ARG
2582
f86cebdf 2583Calls the System V IPC function msgctl(2). You'll probably have to say
0ade1984
JH
2584
2585 use IPC::SysV;
2586
7660c0ab
A
2587first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2588then ARG must be a variable which will hold the returned C<msqid_ds>
951ba7fe
GS
2589structure. Returns like C<ioctl>: the undefined value for error,
2590C<"0 but true"> for zero, or the actual return value otherwise. See also
4755096e 2591L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation.
a0d0e21e
LW
2592
2593=item msgget KEY,FLAGS
2594
f86cebdf 2595Calls the System V IPC function msgget(2). Returns the message queue
4755096e
GS
2596id, or the undefined value if there is an error. See also
2597L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation.
a0d0e21e 2598
a0d0e21e
LW
2599=item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2600
2601Calls the System V IPC function msgrcv to receive a message from
2602message queue ID into variable VAR with a maximum message size of
41d6edb2
JH
2603SIZE. Note that when a message is received, the message type as a
2604native long integer will be the first thing in VAR, followed by the
2605actual message. This packing may be opened with C<unpack("l! a*")>.
2606Taints the variable. Returns true if successful, or false if there is
4755096e
GS
2607an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and
2608C<IPC::SysV::Msg> documentation.
41d6edb2
JH
2609
2610=item msgsnd ID,MSG,FLAGS
2611
2612Calls the System V IPC function msgsnd to send the message MSG to the
2613message queue ID. MSG must begin with the native long integer message
2614type, and be followed by the length of the actual message, and finally
2615the message itself. This kind of packing can be achieved with
2616C<pack("l! a*", $type, $message)>. Returns true if successful,
2617or false if there is an error. See also C<IPC::SysV>
2618and C<IPC::SysV::Msg> documentation.
a0d0e21e
LW
2619
2620=item my EXPR
2621
09bef843
SB
2622=item my EXPR : ATTRIBUTES
2623
19799a22
GS
2624A C<my> declares the listed variables to be local (lexically) to the
2625enclosing block, file, or C<eval>. If
5f05dabc 2626more than one value is listed, the list must be placed in parentheses. See
cb1a09d0 2627L<perlsub/"Private Variables via my()"> for details.
4633a7c4 2628
a0d0e21e
LW
2629=item next LABEL
2630
2631=item next
2632
2633The C<next> command is like the C<continue> statement in C; it starts
2634the next iteration of the loop:
2635
4633a7c4
LW
2636 LINE: while (<STDIN>) {
2637 next LINE if /^#/; # discard comments
5a964f20 2638 #...
a0d0e21e
LW
2639 }
2640
2641Note that if there were a C<continue> block on the above, it would get
2642executed even on discarded lines. If the LABEL is omitted, the command
2643refers to the innermost enclosing loop.
2644
4968c1e4 2645C<next> cannot be used to exit a block which returns a value such as
2b5ab1e7
TC
2646C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2647a grep() or map() operation.
4968c1e4 2648
6c1372ed
GS
2649Note that a block by itself is semantically identical to a loop
2650that executes once. Thus C<next> will exit such a block early.
2651
98293880
JH
2652See also L</continue> for an illustration of how C<last>, C<next>, and
2653C<redo> work.
1d2dff63 2654
4a66ea5a
RGS
2655=item no Module VERSION LIST
2656
2657=item no Module VERSION
2658
a0d0e21e
LW
2659=item no Module LIST
2660
4a66ea5a
RGS
2661=item no Module
2662
7660c0ab 2663See the L</use> function, which C<no> is the opposite of.
a0d0e21e
LW
2664
2665=item oct EXPR
2666
54310121 2667=item oct
bbce6d69 2668
4633a7c4 2669Interprets EXPR as an octal string and returns the corresponding
4f19785b
WSI
2670value. (If EXPR happens to start off with C<0x>, interprets it as a
2671hex string. If EXPR starts off with C<0b>, it is interpreted as a
53305cf1
NC
2672binary string. Leading whitespace is ignored in all three cases.)
2673The following will handle decimal, binary, octal, and hex in the standard
2674Perl or C notation:
a0d0e21e
LW
2675
2676 $val = oct($val) if $val =~ /^0/;
2677
19799a22
GS
2678If EXPR is omitted, uses C<$_>. To go the other way (produce a number
2679in octal), use sprintf() or printf():
2680
2681 $perms = (stat("filename"))[2] & 07777;
2682 $oct_perms = sprintf "%lo", $perms;
2683
2684The oct() function is commonly used when a string such as C<644> needs
2685to be converted into a file mode, for example. (Although perl will
2686automatically convert strings into numbers as needed, this automatic
2687conversion assumes base 10.)
a0d0e21e
LW
2688
2689=item open FILEHANDLE,EXPR
2690
68bd7414
NIS
2691=item open FILEHANDLE,MODE,EXPR
2692
2693=item open FILEHANDLE,MODE,EXPR,LIST
2694
ba964c95
T
2695=item open FILEHANDLE,MODE,REFERENCE
2696
a0d0e21e
LW
2697=item open FILEHANDLE
2698
2699Opens the file whose filename is given by EXPR, and associates it with
ed53a2bb
JH
2700FILEHANDLE.
2701
2702(The following is a comprehensive reference to open(): for a gentler
2703introduction you may consider L<perlopentut>.)
2704
2705If FILEHANDLE is an undefined lexical (C<my>) variable the variable is
2706assigned a reference to a new anonymous filehandle, otherwise if
2707FILEHANDLE is an expression, its value is used as the name of the real
2708filehandle wanted. (This is considered a symbolic reference, so C<use
2709strict 'refs'> should I<not> be in effect.)
2710
2711If EXPR is omitted, the scalar variable of the same name as the
2712FILEHANDLE contains the filename. (Note that lexical variables--those
2713declared with C<my>--will not work for this purpose; so if you're
2714using C<my>, specify EXPR in your call to open.)
2715
2716If three or more arguments are specified then the mode of opening and
2717the file name are separate. If MODE is C<< '<' >> or nothing, the file
2718is opened for input. If MODE is C<< '>' >>, the file is truncated and
2719opened for output, being created if necessary. If MODE is C<<< '>>' >>>,
b76cc8ba 2720the file is opened for appending, again being created if necessary.
5a964f20 2721
ed53a2bb
JH
2722You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to
2723indicate that you want both read and write access to the file; thus
2724C<< '+<' >> is almost always preferred for read/write updates--the C<<
2725'+>' >> mode would clobber the file first. You can't usually use
2726either read-write mode for updating textfiles, since they have
2727variable length records. See the B<-i> switch in L<perlrun> for a
2728better approach. The file is created with permissions of C<0666>
2729modified by the process' C<umask> value.
2730
2731These various prefixes correspond to the fopen(3) modes of C<'r'>,
2732C<'r+'>, C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
5f05dabc 2733
6170680b
IZ
2734In the 2-arguments (and 1-argument) form of the call the mode and
2735filename should be concatenated (in this order), possibly separated by
68bd7414
NIS
2736spaces. It is possible to omit the mode in these forms if the mode is
2737C<< '<' >>.
6170680b 2738
7660c0ab 2739If the filename begins with C<'|'>, the filename is interpreted as a
5a964f20 2740command to which output is to be piped, and if the filename ends with a
f244e06d
GS
2741C<'|'>, the filename is interpreted as a command which pipes output to
2742us. See L<perlipc/"Using open() for IPC">
19799a22 2743for more examples of this. (You are not allowed to C<open> to a command
5a964f20 2744that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
4a4eefd0
GS
2745and L<perlipc/"Bidirectional Communication with Another Process">
2746for alternatives.)
cb1a09d0 2747
ed53a2bb
JH
2748For three or more arguments if MODE is C<'|-'>, the filename is
2749interpreted as a command to which output is to be piped, and if MODE
2750is C<'-|'>, the filename is interpreted as a command which pipes
2751output to us. In the 2-arguments (and 1-argument) form one should
2752replace dash (C<'-'>) with the command.
2753See L<perlipc/"Using open() for IPC"> for more examples of this.
2754(You are not allowed to C<open> to a command that pipes both in I<and>
2755out, but see L<IPC::Open2>, L<IPC::Open3>, and
2756L<perlipc/"Bidirectional Communication"> for alternatives.)
2757
2758In the three-or-more argument form of pipe opens, if LIST is specified
2759(extra arguments after the command name) then LIST becomes arguments
2760to the command invoked if the platform supports it. The meaning of
2761C<open> with more than three arguments for non-pipe modes is not yet
2762specified. Experimental "layers" may give extra LIST arguments
2763meaning.
6170680b
IZ
2764
2765In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
b76cc8ba 2766and opening C<< '>-' >> opens STDOUT.
6170680b 2767
ed53a2bb
JH
2768You may use the three-argument form of open to specify
2769I<I/O disciplines> that affect how the input and output
9124316e 2770are processed: see L</binmode> and L<open>. For example
7207e29d 2771
9124316e
JH
2772 open(FH, "<:utf8", "file")
2773
2774will open the UTF-8 encoded file containing Unicode characters,
2775see L<perluniintro>.
ed53a2bb
JH
2776
2777Open returns nonzero upon success, the undefined value otherwise. If
2778the C<open> involved a pipe, the return value happens to be the pid of
2779the subprocess.
cb1a09d0 2780
ed53a2bb
JH
2781If you're running Perl on a system that distinguishes between text
2782files and binary files, then you should check out L</binmode> for tips
2783for dealing with this. The key distinction between systems that need
2784C<binmode> and those that don't is their text file formats. Systems
2785like Unix, MacOS, and Plan9, which delimit lines with a single
2786character, and which encode that character in C as C<"\n">, do not
2787need C<binmode>. The rest need it.
cb1a09d0 2788
ed53a2bb
JH
2789In the three argument form MODE may also contain a list of IO "layers"
2790(see L<open> and L<PerlIO> for more details) to be applied to the
2791handle. This can be used to achieve the effect of C<binmode> as well
2792as more complex behaviours.
68bd7414 2793
fb73857a 2794When opening a file, it's usually a bad idea to continue normal execution
19799a22
GS
2795if the request failed, so C<open> is frequently used in connection with
2796C<die>. Even if C<die> won't do what you want (say, in a CGI script,
fb73857a 2797where you want to make a nicely formatted error message (but there are
5a964f20 2798modules that can help with that problem)) you should always check
19799a22 2799the return value from opening a file. The infrequent exception is when
fb73857a 2800working with an unopened filehandle is actually what you want to do.
2801
ed53a2bb
JH
2802As a special case the 3 arg form with a read/write mode and the third
2803argument being C<undef>:
b76cc8ba
NIS
2804
2805 open(TMP, "+>", undef) or die ...
2806
2807opens a filehandle to an anonymous temporary file.
2808
ba964c95
T
2809File handles can be opened to "in memory" files held in Perl scalars via:
2810
2811 open($fh,'>', \$variable) || ..
2812
cb1a09d0 2813Examples:
a0d0e21e
LW
2814
2815 $ARTICLE = 100;
2816 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2817 while (<ARTICLE>) {...
2818
6170680b 2819 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
fb73857a 2820 # if the open fails, output is discarded
a0d0e21e 2821
6170680b 2822 open(DBASE, '+<', 'dbase.mine') # open for update
fb73857a 2823 or die "Can't open 'dbase.mine' for update: $!";
cb1a09d0 2824
6170680b
IZ
2825 open(DBASE, '+<dbase.mine') # ditto
2826 or die "Can't open 'dbase.mine' for update: $!";
2827
2828 open(ARTICLE, '-|', "caesar <$article") # decrypt article
fb73857a 2829 or die "Can't start caesar: $!";
a0d0e21e 2830
6170680b
IZ
2831 open(ARTICLE, "caesar <$article |") # ditto
2832 or die "Can't start caesar: $!";
2833
2834 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
fb73857a 2835 or die "Can't start sort: $!";
a0d0e21e 2836
ba964c95
T
2837 # in memory files
2838 open(MEMORY,'>', \$var)
2839 or die "Can't open memory file: $!";
2840 print MEMORY "foo!\n"; # output will end up in $var
2841
a0d0e21e
LW
2842 # process argument list of files along with any includes
2843
2844 foreach $file (@ARGV) {
2845 process($file, 'fh00');
2846 }
2847
2848 sub process {
5a964f20 2849 my($filename, $input) = @_;
a0d0e21e
LW
2850 $input++; # this is a string increment
2851 unless (open($input, $filename)) {
2852 print STDERR "Can't open $filename: $!\n";
2853 return;
2854 }
2855
5a964f20 2856 local $_;
a0d0e21e
LW
2857 while (<$input>) { # note use of indirection
2858 if (/^#include "(.*)"/) {
2859 process($1, $input);
2860 next;
2861 }
5a964f20 2862 #... # whatever
a0d0e21e
LW
2863 }
2864 }
2865
2866You may also, in the Bourne shell tradition, specify an EXPR beginning
61eff3bc 2867with C<< '>&' >>, in which case the rest of the string is interpreted as the
5a964f20 2868name of a filehandle (or file descriptor, if numeric) to be
61eff3bc
JH
2869duped and opened. You may use C<&> after C<< > >>, C<<< >> >>>,
2870C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>. The
a0d0e21e 2871mode you specify should match the mode of the original filehandle.
184e9718 2872(Duping a filehandle does not take into account any existing contents of
9124316e 2873IO buffers.) If you use the 3 arg form then you can pass either a number,
b76cc8ba 2874the name of a filehandle or the normal "reference to a glob".
6170680b 2875
eae1b76b
SB
2876Here is a script that saves, redirects, and restores C<STDOUT> and
2877C<STDERR> using various methods:
a0d0e21e
LW
2878
2879 #!/usr/bin/perl
eae1b76b
SB
2880 open my $oldout, ">&STDOUT" or die "Can't dup STDOUT: $!";
2881 open OLDERR, ">&", \*STDERR or die "Can't dup STDERR: $!";
2882
2883 open STDOUT, '>', "foo.out" or die "Can't redirect STDOUT: $!";
2884 open STDERR, ">&STDOUT" or die "Can't dup STDOUT: $!";
a0d0e21e 2885
eae1b76b
SB
2886 select STDERR; $| = 1; # make unbuffered
2887 select STDOUT; $| = 1; # make unbuffered
a0d0e21e
LW
2888
2889 print STDOUT "stdout 1\n"; # this works for
2890 print STDERR "stderr 1\n"; # subprocesses too
2891
eae1b76b
SB
2892 close STDOUT;
2893 close STDERR;
a0d0e21e 2894
eae1b76b
SB
2895 open STDOUT, ">&", $oldout or die "Can't dup \$oldout: $!";
2896 open STDERR, ">&OLDERR" or die "Can't dup OLDERR: $!";
a0d0e21e
LW
2897
2898 print STDOUT "stdout 2\n";
2899 print STDERR "stderr 2\n";
2900
df632fdf
JH
2901If you specify C<< '<&=N' >>, where C<N> is a number, then Perl will
2902do an equivalent of C's C<fdopen> of that file descriptor; this is
2903more parsimonious of file descriptors. For example:
a0d0e21e
LW
2904
2905 open(FILEHANDLE, "<&=$fd")
df632fdf 2906
b76cc8ba 2907or
df632fdf 2908
b76cc8ba 2909 open(FILEHANDLE, "<&=", $fd)
a0d0e21e 2910
df632fdf
JH
2911Note that if Perl is using the standard C libraries' fdopen() then on
2912many UNIX systems, fdopen() is known to fail when file descriptors
4af147f6 2913exceed a certain value, typically 255. If you need more file
b76cc8ba 2914descriptors than that, consider rebuilding Perl to use the C<PerlIO>.
4af147f6 2915
df632fdf
JH
2916You can see whether Perl has been compiled with PerlIO or not by
2917running C<perl -V> and looking for C<useperlio=> line. If C<useperlio>
2918is C<define>, you have PerlIO, otherwise you don't.
2919
6170680b
IZ
2920If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
2921with 2-arguments (or 1-argument) form of open(), then
a0d0e21e 2922there is an implicit fork done, and the return value of open is the pid
7660c0ab 2923of the child within the parent process, and C<0> within the child
184e9718 2924process. (Use C<defined($pid)> to determine whether the open was successful.)
a0d0e21e
LW
2925The filehandle behaves normally for the parent, but i/o to that
2926filehandle is piped from/to the STDOUT/STDIN of the child process.
2927In the child process the filehandle isn't opened--i/o happens from/to
2928the new STDOUT or STDIN. Typically this is used like the normal
2929piped open when you want to exercise more control over just how the
2930pipe command gets executed, such as when you are running setuid, and
54310121 2931don't want to have to scan shell commands for metacharacters.
6170680b 2932The following triples are more or less equivalent:
a0d0e21e
LW
2933
2934 open(FOO, "|tr '[a-z]' '[A-Z]'");
6170680b
IZ
2935 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
2936 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
b76cc8ba 2937 open(FOO, '|-', "tr", '[a-z]', '[A-Z]');
a0d0e21e
LW
2938
2939 open(FOO, "cat -n '$file'|");
6170680b
IZ
2940 open(FOO, '-|', "cat -n '$file'");
2941 open(FOO, '-|') || exec 'cat', '-n', $file;
b76cc8ba
NIS
2942 open(FOO, '-|', "cat", '-n', $file);
2943
2944The last example in each block shows the pipe as "list form", which is
2945not yet supported on all platforms.
a0d0e21e 2946
4633a7c4
LW
2947See L<perlipc/"Safe Pipe Opens"> for more examples of this.
2948
0f897271
GS
2949Beginning with v5.6.0, Perl will attempt to flush all files opened for
2950output before any operation that may do a fork, but this may not be
2951supported on some platforms (see L<perlport>). To be safe, you may need
2952to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
2953of C<IO::Handle> on any open handles.
2954
ed53a2bb
JH
2955On systems that support a close-on-exec flag on files, the flag will
2956be set for the newly opened file descriptor as determined by the value
2957of $^F. See L<perlvar/$^F>.
a0d0e21e 2958
0dccf244
CS
2959Closing any piped filehandle causes the parent process to wait for the
2960child to finish, and returns the status value in C<$?>.
2961
ed53a2bb
JH
2962The filename passed to 2-argument (or 1-argument) form of open() will
2963have leading and trailing whitespace deleted, and the normal
2964redirection characters honored. This property, known as "magic open",
5a964f20 2965can often be used to good effect. A user could specify a filename of
7660c0ab 2966F<"rsh cat file |">, or you could change certain filenames as needed:
5a964f20
TC
2967
2968 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
2969 open(FH, $filename) or die "Can't open $filename: $!";
2970
6170680b
IZ
2971Use 3-argument form to open a file with arbitrary weird characters in it,
2972
2973 open(FOO, '<', $file);
2974
2975otherwise it's necessary to protect any leading and trailing whitespace:
5a964f20
TC
2976
2977 $file =~ s#^(\s)#./$1#;
2978 open(FOO, "< $file\0");
2979
a31a806a 2980(this may not work on some bizarre filesystems). One should
106325ad 2981conscientiously choose between the I<magic> and 3-arguments form
6170680b
IZ
2982of open():
2983
2984 open IN, $ARGV[0];
2985
2986will allow the user to specify an argument of the form C<"rsh cat file |">,
2987but will not work on a filename which happens to have a trailing space, while
2988
2989 open IN, '<', $ARGV[0];
2990
2991will have exactly the opposite restrictions.
2992
19799a22 2993If you want a "real" C C<open> (see L<open(2)> on your system), then you
6170680b
IZ
2994should use the C<sysopen> function, which involves no such magic (but
2995may use subtly different filemodes than Perl open(), which is mapped
2996to C fopen()). This is
5a964f20
TC
2997another way to protect your filenames from interpretation. For example:
2998
2999 use IO::Handle;
3000 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
3001 or die "sysopen $path: $!";
3002 $oldfh = select(HANDLE); $| = 1; select($oldfh);
38762f02 3003 print HANDLE "stuff $$\n";
5a964f20
TC
3004 seek(HANDLE, 0, 0);
3005 print "File contains: ", <HANDLE>;
3006
7660c0ab
A
3007Using the constructor from the C<IO::Handle> package (or one of its
3008subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
5a964f20
TC
3009filehandles that have the scope of whatever variables hold references to
3010them, and automatically close whenever and however you leave that scope:
c07a80fd 3011
5f05dabc 3012 use IO::File;
5a964f20 3013 #...
c07a80fd 3014 sub read_myfile_munged {
3015 my $ALL = shift;
5f05dabc 3016 my $handle = new IO::File;
c07a80fd 3017 open($handle, "myfile") or die "myfile: $!";
3018 $first = <$handle>
3019 or return (); # Automatically closed here.
3020 mung $first or die "mung failed"; # Or here.
3021 return $first, <$handle> if $ALL; # Or here.
3022 $first; # Or here.
3023 }
3024
b687b08b 3025See L</seek> for some details about mixing reading and writing.
a0d0e21e
LW
3026
3027=item opendir DIRHANDLE,EXPR
3028
19799a22
GS
3029Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
3030C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
a0d0e21e
LW
3031DIRHANDLEs have their own namespace separate from FILEHANDLEs.
3032
3033=item ord EXPR
3034
54310121 3035=item ord
bbce6d69 3036
121910a4
JH
3037Returns the numeric (the native 8-bit encoding, like ASCII or EBCDIC,
3038or Unicode) value of the first character of EXPR. If EXPR is omitted,
3039uses C<$_>.
3040
3041For the reverse, see L</chr>.
3042See L<perlunicode> and L<encoding> for more about Unicode.
a0d0e21e 3043
77ca0c92
LW
3044=item our EXPR
3045
9969eac4
BS
3046=item our EXPR : ATTRIBUTES
3047
77ca0c92
LW
3048An C<our> declares the listed variables to be valid globals within
3049the enclosing block, file, or C<eval>. That is, it has the same
3050scoping rules as a "my" declaration, but does not create a local
3051variable. If more than one value is listed, the list must be placed
3052in parentheses. The C<our> declaration has no semantic effect unless
3053"use strict vars" is in effect, in which case it lets you use the
3054declared global variable without qualifying it with a package name.
3055(But only within the lexical scope of the C<our> declaration. In this
3056it differs from "use vars", which is package scoped.)
3057
f472eb5c
GS
3058An C<our> declaration declares a global variable that will be visible
3059across its entire lexical scope, even across package boundaries. The
3060package in which the variable is entered is determined at the point
3061of the declaration, not at the point of use. This means the following
3062behavior holds:
3063
3064 package Foo;
3065 our $bar; # declares $Foo::bar for rest of lexical scope
3066 $bar = 20;
3067
3068 package Bar;
3069 print $bar; # prints 20
3070
3071Multiple C<our> declarations in the same lexical scope are allowed
3072if they are in different packages. If they happened to be in the same
3073package, Perl will emit warnings if you have asked for them.
3074
3075 use warnings;
3076 package Foo;
3077 our $bar; # declares $Foo::bar for rest of lexical scope
3078 $bar = 20;
3079
3080 package Bar;
3081 our $bar = 30; # declares $Bar::bar for rest of lexical scope
3082 print $bar; # prints 30
3083
3084 our $bar; # emits warning
3085
9969eac4
BS
3086An C<our> declaration may also have a list of attributes associated
3087with it. B<WARNING>: This is an experimental feature that may be
3088changed or removed in future releases of Perl. It should not be
3089relied upon.
3090
51d2bbcc 3091The only currently recognized attribute is C<unique> which indicates
9969eac4
BS
3092that a single copy of the global is to be used by all interpreters
3093should the program happen to be running in a multi-interpreter
3094environment. (The default behaviour would be for each interpreter to
3095have its own copy of the global.) In such an environment, this
3096attribute also has the effect of making the global readonly.
3097Examples:
3098
51d2bbcc
JH
3099 our @EXPORT : unique = qw(foo);
3100 our %EXPORT_TAGS : unique = (bar => [qw(aa bb cc)]);
3101 our $VERSION : unique = "1.00";
9969eac4
BS
3102
3103Multi-interpreter environments can come to being either through the
3104fork() emulation on Windows platforms, or by embedding perl in a
51d2bbcc 3105multi-threaded application. The C<unique> attribute does nothing in
9969eac4
BS
3106all other environments.
3107
a0d0e21e
LW
3108=item pack TEMPLATE,LIST
3109
2b6c5635
GS
3110Takes a LIST of values and converts it into a string using the rules
3111given by the TEMPLATE. The resulting string is the concatenation of
3112the converted values. Typically, each converted value looks
3113like its machine-level representation. For example, on 32-bit machines
3114a converted integer may be represented by a sequence of 4 bytes.
3115
18529408
IZ
3116The TEMPLATE is a sequence of characters that give the order and type
3117of values, as follows:
a0d0e21e 3118
5a929a98 3119 a A string with arbitrary binary data, will be null padded.
121910a4
JH
3120 A A text (ASCII) string, will be space padded.
3121 Z A null terminated (ASCIZ) string, will be null padded.
5a929a98 3122
2b6c5635
GS
3123 b A bit string (ascending bit order inside each byte, like vec()).
3124 B A bit string (descending bit order inside each byte).
a0d0e21e
LW
3125 h A hex string (low nybble first).
3126 H A hex string (high nybble first).
3127
3128 c A signed char value.
a0ed51b3 3129 C An unsigned char value. Only does bytes. See U for Unicode.
96e4d5b1 3130
a0d0e21e
LW
3131 s A signed short value.
3132 S An unsigned short value.
96e4d5b1 3133 (This 'short' is _exactly_ 16 bits, which may differ from
851646ae
JH
3134 what a local C compiler calls 'short'. If you want
3135 native-length shorts, use the '!' suffix.)
96e4d5b1 3136
a0d0e21e
LW
3137 i A signed integer value.
3138 I An unsigned integer value.
19799a22 3139 (This 'integer' is _at_least_ 32 bits wide. Its exact
f86cebdf
GS
3140 size depends on what a local C compiler calls 'int',
3141 and may even be larger than the 'long' described in
3142 the next item.)
96e4d5b1 3143
a0d0e21e
LW
3144 l A signed long value.
3145 L An unsigned long value.
96e4d5b1 3146 (This 'long' is _exactly_ 32 bits, which may differ from
851646ae
JH
3147 what a local C compiler calls 'long'. If you want
3148 native-length longs, use the '!' suffix.)
a0d0e21e 3149
5d11dd56
MG
3150 n An unsigned short in "network" (big-endian) order.
3151 N An unsigned long in "network" (big-endian) order.
3152 v An unsigned short in "VAX" (little-endian) order.
3153 V An unsigned long in "VAX" (little-endian) order.
96e4d5b1 3154 (These 'shorts' and 'longs' are _exactly_ 16 bits and
3155 _exactly_ 32 bits, respectively.)
a0d0e21e 3156
dae0da7a
JH
3157 q A signed quad (64-bit) value.
3158 Q An unsigned quad value.
851646ae
JH
3159 (Quads are available only if your system supports 64-bit
3160 integer values _and_ if Perl has been compiled to support those.
dae0da7a
JH
3161 Causes a fatal error otherwise.)
3162
92d41999
JH
3163 j A signed integer value (a Perl internal integer, IV).
3164 J An unsigned integer value (a Perl internal unsigned integer, UV).
3165
a0d0e21e
LW
3166 f A single-precision float in the native format.
3167 d A double-precision float in the native format.
3168
92d41999
JH
3169 F A floating point value in the native native format
3170 (a Perl internal floating point value, NV).
3171 D A long double-precision float in the native format.
3172 (Long doubles are available only if your system supports long
3173 double values _and_ if Perl has been compiled to support those.
3174 Causes a fatal error otherwise.)
3175
a0d0e21e
LW
3176 p A pointer to a null-terminated string.
3177 P A pointer to a structure (fixed-length string).
3178
3179 u A uuencoded string.
ad0029c4
JH
3180 U A Unicode character number. Encodes to UTF-8 internally
3181 (or UTF-EBCDIC in EBCDIC platforms).
a0d0e21e 3182
96e4d5b1 3183 w A BER compressed integer. Its bytes represent an unsigned
f86cebdf
GS
3184 integer in base 128, most significant digit first, with as
3185 few digits as possible. Bit eight (the high bit) is set
3186 on each byte except the last.
def98dd4 3187
a0d0e21e
LW
3188 x A null byte.
3189 X Back up a byte.
3190 @ Null fill to absolute position.
206947d2 3191 ( Start of a ()-group.
a0d0e21e 3192
5a929a98
VU
3193The following rules apply:
3194
3195=over 8
3196
3197=item *
3198
5a964f20 3199Each letter may optionally be followed by a number giving a repeat
951ba7fe 3200count. With all types except C<a>, C<A>, C<Z>, C<b>, C<B>, C<h>,
206947d2
IZ
3201C<H>, C<@>, C<x>, C<X> and C<P> the pack function will gobble up that
3202many values from the LIST. A C<*> for the repeat count means to use
3203however many items are left, except for C<@>, C<x>, C<X>, where it is
3204equivalent to C<0>, and C<u>, where it is equivalent to 1 (or 45, what
3205is the same). A numeric repeat count may optionally be enclosed in
3206brackets, as in C<pack 'C[80]', @arr>.
3207
3208One can replace the numeric repeat count by a template enclosed in brackets;
3209then the packed length of this template in bytes is used as a count.
62f95557
IZ
3210For example, C<x[L]> skips a long (it skips the number of bytes in a long);
3211the template C<$t X[$t] $t> unpack()s twice what $t unpacks.
3212If the template in brackets contains alignment commands (such as C<x![d]>),
3213its packed length is calculated as if the start of the template has the maximal
3214possible alignment.
2b6c5635 3215
951ba7fe 3216When used with C<Z>, C<*> results in the addition of a trailing null
2b6c5635
GS
3217byte (so the packed result will be one longer than the byte C<length>
3218of the item).
3219
951ba7fe 3220The repeat count for C<u> is interpreted as the maximal number of bytes
2b6c5635 3221to encode per line of output, with 0 and 1 replaced by 45.
5a929a98
VU
3222
3223=item *
3224
951ba7fe 3225The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
5a929a98 3226string of length count, padding with nulls or spaces as necessary. When
951ba7fe
GS
3227unpacking, C<A> strips trailing spaces and nulls, C<Z> strips everything
3228after the first null, and C<a> returns data verbatim. When packing,
3229C<a>, and C<Z> are equivalent.
2b6c5635
GS
3230
3231If the value-to-pack is too long, it is truncated. If too long and an
951ba7fe
GS
3232explicit count is provided, C<Z> packs only C<$count-1> bytes, followed
3233by a null byte. Thus C<Z> always packs a trailing null byte under
2b6c5635 3234all circumstances.
5a929a98
VU
3235
3236=item *
3237
951ba7fe 3238Likewise, the C<b> and C<B> fields pack a string that many bits long.
c73032f5
IZ
3239Each byte of the input field of pack() generates 1 bit of the result.
3240Each result bit is based on the least-significant bit of the corresponding
3241input byte, i.e., on C<ord($byte)%2>. In particular, bytes C<"0"> and
3242C<"1"> generate bits 0 and 1, as do bytes C<"\0"> and C<"\1">.
3243
3244Starting from the beginning of the input string of pack(), each 8-tuple
951ba7fe 3245of bytes is converted to 1 byte of output. With format C<b>
c73032f5 3246the first byte of the 8-tuple determines the least-significant bit of a
951ba7fe 3247byte, and with format C<B> it determines the most-significant bit of
c73032f5
IZ
3248a byte.
3249
3250If the length of the input string is not exactly divisible by 8, the
3251remainder is packed as if the input string were padded by null bytes
3252at the end. Similarly, during unpack()ing the "extra" bits are ignored.
3253
3254If the input string of pack() is longer than needed, extra bytes are ignored.
2b6c5635
GS
3255A C<*> for the repeat count of pack() means to use all the bytes of
3256the input field. On unpack()ing the bits are converted to a string
3257of C<"0">s and C<"1">s.
5a929a98
VU
3258
3259=item *
3260
951ba7fe 3261The C<h> and C<H> fields pack a string that many nybbles (4-bit groups,
851646ae 3262representable as hexadecimal digits, 0-9a-f) long.
5a929a98 3263
c73032f5
IZ
3264Each byte of the input field of pack() generates 4 bits of the result.
3265For non-alphabetical bytes the result is based on the 4 least-significant
3266bits of the input byte, i.e., on C<ord($byte)%16>. In particular,
3267bytes C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
3268C<"\0"> and C<"\1">. For bytes C<"a".."f"> and C<"A".."F"> the result
3269is compatible with the usual hexadecimal digits, so that C<"a"> and
3270C<"A"> both generate the nybble C<0xa==10>. The result for bytes
3271C<"g".."z"> and C<"G".."Z"> is not well-defined.
3272
3273Starting from the beginning of the input string of pack(), each pair
951ba7fe 3274of bytes is converted to 1 byte of output. With format C<h> the
c73032f5 3275first byte of the pair determines the least-significant nybble of the
951ba7fe 3276output byte, and with format C<H> it determines the most-significant
c73032f5
IZ
3277nybble.