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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>,
5ff3f7a4 138C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>,
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139C<readlink>, C<rename>, C<rmdir>, C<stat>, C<symlink>, C<sysopen>,
140C<umask>, C<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>,
737dd4b4 175C<socket>, C<socketpair>
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176
177=item System V interprocess communication functions
178
22fae026
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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
22fae026
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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
22fae026
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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>,
737dd4b4 237C<shmwrite>, C<socket>, C<socketpair>,
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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
16537909
JH
455name of the filehandle.
456
457DISCIPLINE can be either of C<:raw> for binary mode or C<:crlf> for
458"text" mode. If the DISCIPLINE is omitted, it defaults to C<:raw>.
459Returns true on success, C<undef> on failure. To mark FILEHANDLE as
e5f9105d 460UTF-8, use C<:utf8>, and to mark it as bytes, use C<:bytes>.
16537909
JH
461
462The C<:raw> are C<:clrf>, and any other directives of the form
463C<:...>, are called I/O I<disciplines>. The C<open> pragma can be
464used to establish default I/O disciplines. See L<open>.
ed53a2bb
JH
465
466In general, binmode() should be called after open() but before any I/O
467is done on the filehandle. Calling binmode() will flush any possibly
468pending buffered input or output data on the handle. The only
469exception to this is the C<:encoding> discipline that changes
470the default character encoding of the handle, see L<open>.
471The C<:encoding> discipline sometimes needs to be called in
472mid-stream, and it doesn't flush the stream.
16fe6d59 473
16fe6d59
GS
474On some systems binmode() is necessary when you're not working with a
475text file. For the sake of portability it is a good idea to always use
476it when appropriate, and to never use it when it isn't appropriate.
30168b04
GS
477
478In other words: Regardless of platform, use binmode() on binary
479files, and do not use binmode() on text files.
19799a22
GS
480
481The operating system, device drivers, C libraries, and Perl run-time
30168b04
GS
482system all work together to let the programmer treat a single
483character (C<\n>) as the line terminator, irrespective of the external
484representation. On many operating systems, the native text file
485representation matches the internal representation, but on some
486platforms the external representation of C<\n> is made up of more than
487one character.
488
68bd7414
NIS
489Mac OS, all variants of Unix, and Stream_LF files on VMS use a single
490character to end each line in the external representation of text (even
5e12dbfa 491though that single character is CARRIAGE RETURN on Mac OS and LINE FEED
68bd7414
NIS
492on Unix and most VMS files). Consequently binmode() has no effect on
493these operating systems. In other systems like OS/2, DOS and the various
494flavors of MS-Windows your program sees a C<\n> as a simple C<\cJ>, but
495what's stored in text files are the two characters C<\cM\cJ>. That means
496that, if you don't use binmode() on these systems, C<\cM\cJ> sequences on
497disk will be converted to C<\n> on input, and any C<\n> in your program
5e12dbfa
PP
498will be converted back to C<\cM\cJ> on output. This is what you want for
499text files, but it can be disastrous for binary files.
30168b04
GS
500
501Another consequence of using binmode() (on some systems) is that
502special end-of-file markers will be seen as part of the data stream.
503For systems from the Microsoft family this means that if your binary
4375e838 504data contains C<\cZ>, the I/O subsystem will regard it as the end of
30168b04
GS
505the file, unless you use binmode().
506
507binmode() is not only important for readline() and print() operations,
508but also when using read(), seek(), sysread(), syswrite() and tell()
509(see L<perlport> for more details). See the C<$/> and C<$\> variables
510in L<perlvar> for how to manually set your input and output
511line-termination sequences.
a0d0e21e 512
4633a7c4 513=item bless REF,CLASSNAME
a0d0e21e
LW
514
515=item bless REF
516
2b5ab1e7
TC
517This function tells the thingy referenced by REF that it is now an object
518in the CLASSNAME package. If CLASSNAME is omitted, the current package
19799a22 519is used. Because a C<bless> is often the last thing in a constructor,
2b5ab1e7
TC
520it returns the reference for convenience. Always use the two-argument
521version if the function doing the blessing might be inherited by a
522derived class. See L<perltoot> and L<perlobj> for more about the blessing
523(and blessings) of objects.
a0d0e21e 524
57668c4d 525Consider always blessing objects in CLASSNAMEs that are mixed case.
2b5ab1e7
TC
526Namespaces with all lowercase names are considered reserved for
527Perl pragmata. Builtin types have all uppercase names, so to prevent
528confusion, you may wish to avoid such package names as well. Make sure
529that CLASSNAME is a true value.
60ad88b8
GS
530
531See L<perlmod/"Perl Modules">.
532
a0d0e21e
LW
533=item caller EXPR
534
535=item caller
536
5a964f20 537Returns the context of the current subroutine call. In scalar context,
28757baa 538returns the caller's package name if there is a caller, that is, if
19799a22 539we're in a subroutine or C<eval> or C<require>, and the undefined value
5a964f20 540otherwise. In list context, returns
a0d0e21e 541
748a9306 542 ($package, $filename, $line) = caller;
a0d0e21e
LW
543
544With EXPR, it returns some extra information that the debugger uses to
545print a stack trace. The value of EXPR indicates how many call frames
546to go back before the current one.
547
f3aa04c2 548 ($package, $filename, $line, $subroutine, $hasargs,
e476b1b5 549 $wantarray, $evaltext, $is_require, $hints, $bitmask) = caller($i);
e7ea3e70 550
951ba7fe 551Here $subroutine may be C<(eval)> if the frame is not a subroutine
19799a22 552call, but an C<eval>. In such a case additional elements $evaltext and
7660c0ab 553C<$is_require> are set: C<$is_require> is true if the frame is created by a
19799a22 554C<require> or C<use> statement, $evaltext contains the text of the
277ddfaf 555C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
951ba7fe 556$filename is C<(eval)>, but $evaltext is undefined. (Note also that
0fc9dec4
RGS
557each C<use> statement creates a C<require> frame inside an C<eval EXPR>
558frame.) $subroutine may also be C<(unknown)> if this particular
559subroutine happens to have been deleted from the symbol table.
560C<$hasargs> is true if a new instance of C<@_> was set up for the frame.
561C<$hints> and C<$bitmask> contain pragmatic hints that the caller was
562compiled with. The C<$hints> and C<$bitmask> values are subject to change
563between versions of Perl, and are not meant for external use.
748a9306
LW
564
565Furthermore, when called from within the DB package, caller returns more
7660c0ab 566detailed information: it sets the list variable C<@DB::args> to be the
54310121 567arguments with which the subroutine was invoked.
748a9306 568
7660c0ab 569Be aware that the optimizer might have optimized call frames away before
19799a22 570C<caller> had a chance to get the information. That means that C<caller(N)>
7660c0ab 571might not return information about the call frame you expect it do, for
b76cc8ba 572C<< N > 1 >>. In particular, C<@DB::args> might have information from the
19799a22 573previous time C<caller> was called.
7660c0ab 574
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LW
575=item chdir EXPR
576
ffce7b87 577Changes the working directory to EXPR, if possible. If EXPR is omitted,
0bfc1ec4 578changes to the directory specified by C<$ENV{HOME}>, if set; if not,
ffce7b87 579changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
b4ad75f0
AMS
580variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
581neither is set, C<chdir> does nothing. It returns true upon success,
582false otherwise. See the example under C<die>.
a0d0e21e
LW
583
584=item chmod LIST
585
586Changes the permissions of a list of files. The first element of the
4633a7c4 587list must be the numerical mode, which should probably be an octal
2f9daede
TP
588number, and which definitely should I<not> a string of octal digits:
589C<0644> is okay, C<'0644'> is not. Returns the number of files
dc848c6f 590successfully changed. See also L</oct>, if all you have is a string.
a0d0e21e
LW
591
592 $cnt = chmod 0755, 'foo', 'bar';
593 chmod 0755, @executables;
f86cebdf
GS
594 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
595 # --w----r-T
2f9daede
TP
596 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
597 $mode = 0644; chmod $mode, 'foo'; # this is best
a0d0e21e 598
ca6e1c26
JH
599You can also import the symbolic C<S_I*> constants from the Fcntl
600module:
601
602 use Fcntl ':mode';
603
604 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
605 # This is identical to the chmod 0755 of the above example.
606
a0d0e21e
LW
607=item chomp VARIABLE
608
313c9f5c 609=item chomp( LIST )
a0d0e21e
LW
610
611=item chomp
612
2b5ab1e7
TC
613This safer version of L</chop> removes any trailing string
614that corresponds to the current value of C<$/> (also known as
28757baa 615$INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
616number of characters removed from all its arguments. It's often used to
617remove the newline from the end of an input record when you're worried
2b5ab1e7
TC
618that the final record may be missing its newline. When in paragraph
619mode (C<$/ = "">), it removes all trailing newlines from the string.
4c5a6083
GS
620When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
621a reference to an integer or the like, see L<perlvar>) chomp() won't
b76cc8ba 622remove anything.
19799a22 623If VARIABLE is omitted, it chomps C<$_>. Example:
a0d0e21e
LW
624
625 while (<>) {
626 chomp; # avoid \n on last field
627 @array = split(/:/);
5a964f20 628 # ...
a0d0e21e
LW
629 }
630
4bf21a6d
RD
631If VARIABLE is a hash, it chomps the hash's values, but not its keys.
632
a0d0e21e
LW
633You can actually chomp anything that's an lvalue, including an assignment:
634
635 chomp($cwd = `pwd`);
636 chomp($answer = <STDIN>);
637
638If you chomp a list, each element is chomped, and the total number of
639characters removed is returned.
640
641=item chop VARIABLE
642
313c9f5c 643=item chop( LIST )
a0d0e21e
LW
644
645=item chop
646
647Chops off the last character of a string and returns the character
5b3eff12 648chopped. It is much more efficient than C<s/.$//s> because it neither
7660c0ab 649scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
4bf21a6d
RD
650If VARIABLE is a hash, it chops the hash's values, but not its keys.
651
5b3eff12 652You can actually chop anything that's an lvalue, including an assignment.
a0d0e21e
LW
653
654If you chop a list, each element is chopped. Only the value of the
19799a22 655last C<chop> is returned.
a0d0e21e 656
19799a22 657Note that C<chop> returns the last character. To return all but the last
748a9306
LW
658character, use C<substr($string, 0, -1)>.
659
a0d0e21e
LW
660=item chown LIST
661
662Changes the owner (and group) of a list of files. The first two
19799a22
GS
663elements of the list must be the I<numeric> uid and gid, in that
664order. A value of -1 in either position is interpreted by most
665systems to leave that value unchanged. Returns the number of files
666successfully changed.
a0d0e21e
LW
667
668 $cnt = chown $uid, $gid, 'foo', 'bar';
669 chown $uid, $gid, @filenames;
670
54310121 671Here's an example that looks up nonnumeric uids in the passwd file:
a0d0e21e
LW
672
673 print "User: ";
19799a22 674 chomp($user = <STDIN>);
5a964f20 675 print "Files: ";
19799a22 676 chomp($pattern = <STDIN>);
a0d0e21e
LW
677
678 ($login,$pass,$uid,$gid) = getpwnam($user)
679 or die "$user not in passwd file";
680
5a964f20 681 @ary = glob($pattern); # expand filenames
a0d0e21e
LW
682 chown $uid, $gid, @ary;
683
54310121 684On most systems, you are not allowed to change the ownership of the
4633a7c4
LW
685file unless you're the superuser, although you should be able to change
686the group to any of your secondary groups. On insecure systems, these
687restrictions may be relaxed, but this is not a portable assumption.
19799a22
GS
688On POSIX systems, you can detect this condition this way:
689
690 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
691 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
4633a7c4 692
a0d0e21e
LW
693=item chr NUMBER
694
54310121 695=item chr
bbce6d69 696
a0d0e21e 697Returns the character represented by that NUMBER in the character set.
a0ed51b3 698For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
121910a4
JH
699chr(0x263a) is a Unicode smiley face. Note that characters from 127
700to 255 (inclusive) are by default not encoded in Unicode for backward
701compatibility reasons (but see L<encoding>).
aaa68c4a 702
b76cc8ba 703For the reverse, use L</ord>.
121910a4 704See L<perlunicode> and L<encoding> for more about Unicode.
a0d0e21e 705
7660c0ab 706If NUMBER is omitted, uses C<$_>.
bbce6d69 707
a0d0e21e
LW
708=item chroot FILENAME
709
54310121 710=item chroot
bbce6d69 711
5a964f20 712This function works like the system call by the same name: it makes the
4633a7c4 713named directory the new root directory for all further pathnames that
951ba7fe 714begin with a C</> by your process and all its children. (It doesn't
28757baa 715change your current working directory, which is unaffected.) For security
4633a7c4 716reasons, this call is restricted to the superuser. If FILENAME is
19799a22 717omitted, does a C<chroot> to C<$_>.
a0d0e21e
LW
718
719=item close FILEHANDLE
720
6a518fbc
TP
721=item close
722
9124316e
JH
723Closes the file or pipe associated with the file handle, returning
724true only if IO buffers are successfully flushed and closes the system
725file descriptor. Closes the currently selected filehandle if the
726argument is omitted.
fb73857a 727
728You don't have to close FILEHANDLE if you are immediately going to do
19799a22
GS
729another C<open> on it, because C<open> will close it for you. (See
730C<open>.) However, an explicit C<close> on an input file resets the line
731counter (C<$.>), while the implicit close done by C<open> does not.
fb73857a 732
19799a22
GS
733If the file handle came from a piped open C<close> will additionally
734return false if one of the other system calls involved fails or if the
fb73857a 735program exits with non-zero status. (If the only problem was that the
b76cc8ba 736program exited non-zero C<$!> will be set to C<0>.) Closing a pipe
2b5ab1e7 737also waits for the process executing on the pipe to complete, in case you
b76cc8ba 738want to look at the output of the pipe afterwards, and
2b5ab1e7 739implicitly puts the exit status value of that command into C<$?>.
5a964f20 740
73689b13
GS
741Prematurely closing the read end of a pipe (i.e. before the process
742writing to it at the other end has closed it) will result in a
743SIGPIPE being delivered to the writer. If the other end can't
744handle that, be sure to read all the data before closing the pipe.
745
fb73857a 746Example:
a0d0e21e 747
fb73857a 748 open(OUTPUT, '|sort >foo') # pipe to sort
749 or die "Can't start sort: $!";
5a964f20 750 #... # print stuff to output
fb73857a 751 close OUTPUT # wait for sort to finish
752 or warn $! ? "Error closing sort pipe: $!"
753 : "Exit status $? from sort";
754 open(INPUT, 'foo') # get sort's results
755 or die "Can't open 'foo' for input: $!";
a0d0e21e 756
5a964f20
TC
757FILEHANDLE may be an expression whose value can be used as an indirect
758filehandle, usually the real filehandle name.
a0d0e21e
LW
759
760=item closedir DIRHANDLE
761
19799a22 762Closes a directory opened by C<opendir> and returns the success of that
5a964f20
TC
763system call.
764
765DIRHANDLE may be an expression whose value can be used as an indirect
766dirhandle, usually the real dirhandle name.
a0d0e21e
LW
767
768=item connect SOCKET,NAME
769
770Attempts to connect to a remote socket, just as the connect system call
19799a22 771does. Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
772packed address of the appropriate type for the socket. See the examples in
773L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 774
cb1a09d0
AD
775=item continue BLOCK
776
777Actually a flow control statement rather than a function. If there is a
98293880
JH
778C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
779C<foreach>), it is always executed just before the conditional is about to
780be evaluated again, just like the third part of a C<for> loop in C. Thus
cb1a09d0
AD
781it can be used to increment a loop variable, even when the loop has been
782continued via the C<next> statement (which is similar to the C C<continue>
783statement).
784
98293880 785C<last>, C<next>, or C<redo> may appear within a C<continue>
19799a22
GS
786block. C<last> and C<redo> will behave as if they had been executed within
787the main block. So will C<next>, but since it will execute a C<continue>
1d2dff63
GS
788block, it may be more entertaining.
789
790 while (EXPR) {
791 ### redo always comes here
792 do_something;
793 } continue {
794 ### next always comes here
795 do_something_else;
796 # then back the top to re-check EXPR
797 }
798 ### last always comes here
799
800Omitting the C<continue> section is semantically equivalent to using an
19799a22 801empty one, logically enough. In that case, C<next> goes directly back
1d2dff63
GS
802to check the condition at the top of the loop.
803
a0d0e21e
LW
804=item cos EXPR
805
d6217f1e
GS
806=item cos
807
5a964f20 808Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
7660c0ab 809takes cosine of C<$_>.
a0d0e21e 810
ca6e1c26 811For the inverse cosine operation, you may use the C<Math::Trig::acos()>
28757baa 812function, or use this relation:
813
814 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
815
a0d0e21e
LW
816=item crypt PLAINTEXT,SALT
817
f86cebdf 818Encrypts a string exactly like the crypt(3) function in the C library
4633a7c4
LW
819(assuming that you actually have a version there that has not been
820extirpated as a potential munition). This can prove useful for checking
821the password file for lousy passwords, amongst other things. Only the
822guys wearing white hats should do this.
a0d0e21e 823
85c16d83
JH
824Note that C<crypt> is intended to be a one-way function, much like
825breaking eggs to make an omelette. There is no (known) corresponding
826decrypt function (in other words, the crypt() is a one-way hash
827function). As a result, this function isn't all that useful for
11155c91 828cryptography. (For that, see your nearby CPAN mirror.)
2f9daede 829
85c16d83
JH
830When verifying an existing encrypted string you should use the
831encrypted text as the salt (like C<crypt($plain, $crypted) eq
832$crypted>). This allows your code to work with the standard C<crypt>
833and with more exotic implementations. In other words, do not assume
834anything about the returned string itself, or how many bytes in
835the encrypted string matter.
836
837Traditionally the result is a string of 13 bytes: two first bytes of
838the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
839the first eight bytes of the encrypted string mattered, but
840alternative hashing schemes (like MD5), higher level security schemes
841(like C2), and implementations on non-UNIX platforms may produce
842different strings.
843
844When choosing a new salt create a random two character string whose
845characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
846'/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>).
e71965be 847
a0d0e21e
LW
848Here's an example that makes sure that whoever runs this program knows
849their own password:
850
851 $pwd = (getpwuid($<))[1];
a0d0e21e
LW
852
853 system "stty -echo";
854 print "Password: ";
e71965be 855 chomp($word = <STDIN>);
a0d0e21e
LW
856 print "\n";
857 system "stty echo";
858
e71965be 859 if (crypt($word, $pwd) ne $pwd) {
a0d0e21e
LW
860 die "Sorry...\n";
861 } else {
862 print "ok\n";
54310121 863 }
a0d0e21e 864
9f8f0c9d 865Of course, typing in your own password to whoever asks you
748a9306 866for it is unwise.
a0d0e21e 867
19799a22
GS
868The L<crypt> function is unsuitable for encrypting large quantities
869of data, not least of all because you can't get the information
870back. Look at the F<by-module/Crypt> and F<by-module/PGP> directories
871on your favorite CPAN mirror for a slew of potentially useful
872modules.
873
f2791508
JH
874If using crypt() on a Unicode string (which I<potentially> has
875characters with codepoints above 255), Perl tries to make sense
876of the situation by trying to downgrade (a copy of the string)
877the string back to an eight-bit byte string before calling crypt()
878(on that copy). If that works, good. If not, crypt() dies with
879C<Wide character in crypt>.
85c16d83 880
aa689395 881=item dbmclose HASH
a0d0e21e 882
19799a22 883[This function has been largely superseded by the C<untie> function.]
a0d0e21e 884
aa689395 885Breaks the binding between a DBM file and a hash.
a0d0e21e 886
19799a22 887=item dbmopen HASH,DBNAME,MASK
a0d0e21e 888
19799a22 889[This function has been largely superseded by the C<tie> function.]
a0d0e21e 890
7b8d334a 891This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
19799a22
GS
892hash. HASH is the name of the hash. (Unlike normal C<open>, the first
893argument is I<not> a filehandle, even though it looks like one). DBNAME
aa689395 894is the name of the database (without the F<.dir> or F<.pag> extension if
895any). If the database does not exist, it is created with protection
19799a22
GS
896specified by MASK (as modified by the C<umask>). If your system supports
897only the older DBM functions, you may perform only one C<dbmopen> in your
aa689395 898program. In older versions of Perl, if your system had neither DBM nor
19799a22 899ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
aa689395 900sdbm(3).
901
902If you don't have write access to the DBM file, you can only read hash
903variables, not set them. If you want to test whether you can write,
19799a22 904either use file tests or try setting a dummy hash entry inside an C<eval>,
aa689395 905which will trap the error.
a0d0e21e 906
19799a22
GS
907Note that functions such as C<keys> and C<values> may return huge lists
908when used on large DBM files. You may prefer to use the C<each>
a0d0e21e
LW
909function to iterate over large DBM files. Example:
910
911 # print out history file offsets
912 dbmopen(%HIST,'/usr/lib/news/history',0666);
913 while (($key,$val) = each %HIST) {
914 print $key, ' = ', unpack('L',$val), "\n";
915 }
916 dbmclose(%HIST);
917
cb1a09d0 918See also L<AnyDBM_File> for a more general description of the pros and
184e9718 919cons of the various dbm approaches, as well as L<DB_File> for a particularly
cb1a09d0 920rich implementation.
4633a7c4 921
2b5ab1e7
TC
922You can control which DBM library you use by loading that library
923before you call dbmopen():
924
925 use DB_File;
926 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
927 or die "Can't open netscape history file: $!";
928
a0d0e21e
LW
929=item defined EXPR
930
54310121 931=item defined
bbce6d69 932
2f9daede
TP
933Returns a Boolean value telling whether EXPR has a value other than
934the undefined value C<undef>. If EXPR is not present, C<$_> will be
935checked.
936
937Many operations return C<undef> to indicate failure, end of file,
938system error, uninitialized variable, and other exceptional
939conditions. This function allows you to distinguish C<undef> from
940other values. (A simple Boolean test will not distinguish among
7660c0ab 941C<undef>, zero, the empty string, and C<"0">, which are all equally
2f9daede 942false.) Note that since C<undef> is a valid scalar, its presence
19799a22 943doesn't I<necessarily> indicate an exceptional condition: C<pop>
2f9daede
TP
944returns C<undef> when its argument is an empty array, I<or> when the
945element to return happens to be C<undef>.
946
f10b0346
GS
947You may also use C<defined(&func)> to check whether subroutine C<&func>
948has ever been defined. The return value is unaffected by any forward
847c7ebe
DD
949declarations of C<&foo>. Note that a subroutine which is not defined
950may still be callable: its package may have an C<AUTOLOAD> method that
951makes it spring into existence the first time that it is called -- see
952L<perlsub>.
f10b0346
GS
953
954Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
955used to report whether memory for that aggregate has ever been
956allocated. This behavior may disappear in future versions of Perl.
957You should instead use a simple test for size:
958
959 if (@an_array) { print "has array elements\n" }
960 if (%a_hash) { print "has hash members\n" }
2f9daede
TP
961
962When used on a hash element, it tells you whether the value is defined,
dc848c6f 963not whether the key exists in the hash. Use L</exists> for the latter
2f9daede 964purpose.
a0d0e21e
LW
965
966Examples:
967
968 print if defined $switch{'D'};
969 print "$val\n" while defined($val = pop(@ary));
970 die "Can't readlink $sym: $!"
971 unless defined($value = readlink $sym);
a0d0e21e 972 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
2f9daede 973 $debugging = 0 unless defined $debugging;
a0d0e21e 974
19799a22 975Note: Many folks tend to overuse C<defined>, and then are surprised to
7660c0ab 976discover that the number C<0> and C<""> (the zero-length string) are, in fact,
2f9daede 977defined values. For example, if you say
a5f75d66
AD
978
979 "ab" =~ /a(.*)b/;
980
7660c0ab 981The pattern match succeeds, and C<$1> is defined, despite the fact that it
a5f75d66 982matched "nothing". But it didn't really match nothing--rather, it
2b5ab1e7 983matched something that happened to be zero characters long. This is all
a5f75d66 984very above-board and honest. When a function returns an undefined value,
2f9daede 985it's an admission that it couldn't give you an honest answer. So you
19799a22 986should use C<defined> only when you're questioning the integrity of what
7660c0ab 987you're trying to do. At other times, a simple comparison to C<0> or C<""> is
2f9daede
TP
988what you want.
989
dc848c6f 990See also L</undef>, L</exists>, L</ref>.
2f9daede 991
a0d0e21e
LW
992=item delete EXPR
993
01020589
GS
994Given an expression that specifies a hash element, array element, hash slice,
995or array slice, deletes the specified element(s) from the hash or array.
8216c1fd 996In the case of an array, if the array elements happen to be at the end,
b76cc8ba 997the size of the array will shrink to the highest element that tests
8216c1fd 998true for exists() (or 0 if no such element exists).
a0d0e21e 999
01020589
GS
1000Returns each element so deleted or the undefined value if there was no such
1001element. Deleting from C<$ENV{}> modifies the environment. Deleting from
1002a hash tied to a DBM file deletes the entry from the DBM file. Deleting
1003from a C<tie>d hash or array may not necessarily return anything.
1004
8ea97a1e
GS
1005Deleting an array element effectively returns that position of the array
1006to its initial, uninitialized state. Subsequently testing for the same
8216c1fd
GS
1007element with exists() will return false. Note that deleting array
1008elements in the middle of an array will not shift the index of the ones
1009after them down--use splice() for that. See L</exists>.
8ea97a1e 1010
01020589 1011The following (inefficiently) deletes all the values of %HASH and @ARRAY:
a0d0e21e 1012
5f05dabc 1013 foreach $key (keys %HASH) {
1014 delete $HASH{$key};
a0d0e21e
LW
1015 }
1016
01020589
GS
1017 foreach $index (0 .. $#ARRAY) {
1018 delete $ARRAY[$index];
1019 }
1020
1021And so do these:
5f05dabc 1022
01020589
GS
1023 delete @HASH{keys %HASH};
1024
9740c838 1025 delete @ARRAY[0 .. $#ARRAY];
5f05dabc 1026
2b5ab1e7 1027But both of these are slower than just assigning the empty list
01020589
GS
1028or undefining %HASH or @ARRAY:
1029
1030 %HASH = (); # completely empty %HASH
1031 undef %HASH; # forget %HASH ever existed
2b5ab1e7 1032
01020589
GS
1033 @ARRAY = (); # completely empty @ARRAY
1034 undef @ARRAY; # forget @ARRAY ever existed
2b5ab1e7
TC
1035
1036Note that the EXPR can be arbitrarily complicated as long as the final
01020589
GS
1037operation is a hash element, array element, hash slice, or array slice
1038lookup:
a0d0e21e
LW
1039
1040 delete $ref->[$x][$y]{$key};
5f05dabc 1041 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
a0d0e21e 1042
01020589
GS
1043 delete $ref->[$x][$y][$index];
1044 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1045
a0d0e21e
LW
1046=item die LIST
1047
19799a22
GS
1048Outside an C<eval>, prints the value of LIST to C<STDERR> and
1049exits with the current value of C<$!> (errno). If C<$!> is C<0>,
61eff3bc
JH
1050exits with the value of C<<< ($? >> 8) >>> (backtick `command`
1051status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside
19799a22
GS
1052an C<eval(),> the error message is stuffed into C<$@> and the
1053C<eval> is terminated with the undefined value. This makes
1054C<die> the way to raise an exception.
a0d0e21e
LW
1055
1056Equivalent examples:
1057
1058 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
54310121 1059 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
a0d0e21e 1060
ccac6780 1061If the last element of LIST does not end in a newline, the current
df37ec69
WW
1062script line number and input line number (if any) are also printed,
1063and a newline is supplied. Note that the "input line number" (also
1064known as "chunk") is subject to whatever notion of "line" happens to
1065be currently in effect, and is also available as the special variable
1066C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1067
1068Hint: sometimes appending C<", stopped"> to your message will cause it
1069to make better sense when the string C<"at foo line 123"> is appended.
1070Suppose you are running script "canasta".
a0d0e21e
LW
1071
1072 die "/etc/games is no good";
1073 die "/etc/games is no good, stopped";
1074
1075produce, respectively
1076
1077 /etc/games is no good at canasta line 123.
1078 /etc/games is no good, stopped at canasta line 123.
1079
2b5ab1e7 1080See also exit(), warn(), and the Carp module.
a0d0e21e 1081
7660c0ab
A
1082If LIST is empty and C<$@> already contains a value (typically from a
1083previous eval) that value is reused after appending C<"\t...propagated">.
fb73857a 1084This is useful for propagating exceptions:
1085
1086 eval { ... };
1087 die unless $@ =~ /Expected exception/;
1088
ad216e65
JH
1089If LIST is empty and C<$@> contains an object reference that has a
1090C<PROPAGATE> method, that method will be called with additional file
1091and line number parameters. The return value replaces the value in
1092C<$@>. ie. as if C<<$@ = eval { $@->PROPAGATE(__FILE__, __LINE__) };>>
1093were called.
1094
7660c0ab 1095If C<$@> is empty then the string C<"Died"> is used.
fb73857a 1096
52531d10
GS
1097die() can also be called with a reference argument. If this happens to be
1098trapped within an eval(), $@ contains the reference. This behavior permits
1099a more elaborate exception handling implementation using objects that
4375e838 1100maintain arbitrary state about the nature of the exception. Such a scheme
52531d10
GS
1101is sometimes preferable to matching particular string values of $@ using
1102regular expressions. Here's an example:
1103
1104 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1105 if ($@) {
1106 if (ref($@) && UNIVERSAL::isa($@,"Some::Module::Exception")) {
1107 # handle Some::Module::Exception
1108 }
1109 else {
1110 # handle all other possible exceptions
1111 }
1112 }
1113
19799a22 1114Because perl will stringify uncaught exception messages before displaying
52531d10
GS
1115them, you may want to overload stringification operations on such custom
1116exception objects. See L<overload> for details about that.
1117
19799a22
GS
1118You can arrange for a callback to be run just before the C<die>
1119does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1120handler will be called with the error text and can change the error
1121message, if it sees fit, by calling C<die> again. See
1122L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
1123L<"eval BLOCK"> for some examples. Although this feature was meant
1124to be run only right before your program was to exit, this is not
1125currently the case--the C<$SIG{__DIE__}> hook is currently called
1126even inside eval()ed blocks/strings! If one wants the hook to do
1127nothing in such situations, put
fb73857a 1128
1129 die @_ if $^S;
1130
19799a22
GS
1131as the first line of the handler (see L<perlvar/$^S>). Because
1132this promotes strange action at a distance, this counterintuitive
b76cc8ba 1133behavior may be fixed in a future release.
774d564b 1134
a0d0e21e
LW
1135=item do BLOCK
1136
1137Not really a function. Returns the value of the last command in the
1138sequence of commands indicated by BLOCK. When modified by a loop
98293880
JH
1139modifier, executes the BLOCK once before testing the loop condition.
1140(On other statements the loop modifiers test the conditional first.)
a0d0e21e 1141
4968c1e4 1142C<do BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7
TC
1143C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1144See L<perlsyn> for alternative strategies.
4968c1e4 1145
a0d0e21e
LW
1146=item do SUBROUTINE(LIST)
1147
1148A deprecated form of subroutine call. See L<perlsub>.
1149
1150=item do EXPR
1151
1152Uses the value of EXPR as a filename and executes the contents of the
1153file as a Perl script. Its primary use is to include subroutines
1154from a Perl subroutine library.
1155
1156 do 'stat.pl';
1157
1158is just like
1159
986b19de 1160 eval `cat stat.pl`;
a0d0e21e 1161
2b5ab1e7
TC
1162except that it's more efficient and concise, keeps track of the current
1163filename for error messages, searches the @INC libraries, and updates
1164C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1165variables. It also differs in that code evaluated with C<do FILENAME>
1166cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1167same, however, in that it does reparse the file every time you call it,
1168so you probably don't want to do this inside a loop.
a0d0e21e 1169
8e30cc93 1170If C<do> cannot read the file, it returns undef and sets C<$!> to the
2b5ab1e7 1171error. If C<do> can read the file but cannot compile it, it
8e30cc93
MG
1172returns undef and sets an error message in C<$@>. If the file is
1173successfully compiled, C<do> returns the value of the last expression
1174evaluated.
1175
a0d0e21e 1176Note that inclusion of library modules is better done with the
19799a22 1177C<use> and C<require> operators, which also do automatic error checking
4633a7c4 1178and raise an exception if there's a problem.
a0d0e21e 1179
5a964f20
TC
1180You might like to use C<do> to read in a program configuration
1181file. Manual error checking can be done this way:
1182
b76cc8ba 1183 # read in config files: system first, then user
f86cebdf 1184 for $file ("/share/prog/defaults.rc",
b76cc8ba 1185 "$ENV{HOME}/.someprogrc")
2b5ab1e7 1186 {
5a964f20 1187 unless ($return = do $file) {
f86cebdf
GS
1188 warn "couldn't parse $file: $@" if $@;
1189 warn "couldn't do $file: $!" unless defined $return;
1190 warn "couldn't run $file" unless $return;
5a964f20
TC
1191 }
1192 }
1193
a0d0e21e
LW
1194=item dump LABEL
1195
1614b0e3
JD
1196=item dump
1197
19799a22
GS
1198This function causes an immediate core dump. See also the B<-u>
1199command-line switch in L<perlrun>, which does the same thing.
1200Primarily this is so that you can use the B<undump> program (not
1201supplied) to turn your core dump into an executable binary after
1202having initialized all your variables at the beginning of the
1203program. When the new binary is executed it will begin by executing
1204a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1205Think of it as a goto with an intervening core dump and reincarnation.
1206If C<LABEL> is omitted, restarts the program from the top.
1207
1208B<WARNING>: Any files opened at the time of the dump will I<not>
1209be open any more when the program is reincarnated, with possible
b76cc8ba 1210resulting confusion on the part of Perl.
19799a22
GS
1211
1212This function is now largely obsolete, partly because it's very
1213hard to convert a core file into an executable, and because the
1214real compiler backends for generating portable bytecode and compilable
ac206dc8
RGS
1215C code have superseded it. That's why you should now invoke it as
1216C<CORE::dump()>, if you don't want to be warned against a possible
1217typo.
19799a22
GS
1218
1219If you're looking to use L<dump> to speed up your program, consider
1220generating bytecode or native C code as described in L<perlcc>. If
1221you're just trying to accelerate a CGI script, consider using the
210b36aa 1222C<mod_perl> extension to B<Apache>, or the CPAN module, CGI::Fast.
19799a22 1223You might also consider autoloading or selfloading, which at least
b76cc8ba 1224make your program I<appear> to run faster.
5a964f20 1225
aa689395 1226=item each HASH
1227
5a964f20 1228When called in list context, returns a 2-element list consisting of the
aa689395 1229key and value for the next element of a hash, so that you can iterate over
74fc8b5f 1230it. When called in scalar context, returns only the key for the next
e902a979 1231element in the hash.
2f9daede 1232
ab192400
GS
1233Entries are returned in an apparently random order. The actual random
1234order is subject to change in future versions of perl, but it is guaranteed
19799a22 1235to be in the same order as either the C<keys> or C<values> function
ab192400
GS
1236would produce on the same (unmodified) hash.
1237
1238When the hash is entirely read, a null array is returned in list context
19799a22
GS
1239(which when assigned produces a false (C<0>) value), and C<undef> in
1240scalar context. The next call to C<each> after that will start iterating
1241again. There is a single iterator for each hash, shared by all C<each>,
1242C<keys>, and C<values> function calls in the program; it can be reset by
2f9daede
TP
1243reading all the elements from the hash, or by evaluating C<keys HASH> or
1244C<values HASH>. If you add or delete elements of a hash while you're
74fc8b5f
MJD
1245iterating over it, you may get entries skipped or duplicated, so
1246don't. Exception: It is always safe to delete the item most recently
1247returned by C<each()>, which means that the following code will work:
1248
1249 while (($key, $value) = each %hash) {
1250 print $key, "\n";
1251 delete $hash{$key}; # This is safe
1252 }
aa689395 1253
f86cebdf 1254The following prints out your environment like the printenv(1) program,
aa689395 1255only in a different order:
a0d0e21e
LW
1256
1257 while (($key,$value) = each %ENV) {
1258 print "$key=$value\n";
1259 }
1260
19799a22 1261See also C<keys>, C<values> and C<sort>.
a0d0e21e
LW
1262
1263=item eof FILEHANDLE
1264
4633a7c4
LW
1265=item eof ()
1266
a0d0e21e
LW
1267=item eof
1268
1269Returns 1 if the next read on FILEHANDLE will return end of file, or if
1270FILEHANDLE is not open. FILEHANDLE may be an expression whose value
5a964f20 1271gives the real filehandle. (Note that this function actually
19799a22 1272reads a character and then C<ungetc>s it, so isn't very useful in an
748a9306 1273interactive context.) Do not read from a terminal file (or call
19799a22 1274C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
748a9306
LW
1275as terminals may lose the end-of-file condition if you do.
1276
820475bd
GS
1277An C<eof> without an argument uses the last file read. Using C<eof()>
1278with empty parentheses is very different. It refers to the pseudo file
1279formed from the files listed on the command line and accessed via the
61eff3bc
JH
1280C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1281as a normal filehandle is, an C<eof()> before C<< <> >> has been
820475bd 1282used will cause C<@ARGV> to be examined to determine if input is
efdd0218
RB
1283available. Similarly, an C<eof()> after C<< <> >> has returned
1284end-of-file will assume you are processing another C<@ARGV> list,
1285and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1286see L<perlop/"I/O Operators">.
820475bd 1287
61eff3bc 1288In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
820475bd
GS
1289detect the end of each file, C<eof()> will only detect the end of the
1290last file. Examples:
a0d0e21e 1291
748a9306
LW
1292 # reset line numbering on each input file
1293 while (<>) {
b76cc8ba 1294 next if /^\s*#/; # skip comments
748a9306 1295 print "$.\t$_";
5a964f20
TC
1296 } continue {
1297 close ARGV if eof; # Not eof()!
748a9306
LW
1298 }
1299
a0d0e21e
LW
1300 # insert dashes just before last line of last file
1301 while (<>) {
5a964f20 1302 if (eof()) { # check for end of current file
a0d0e21e 1303 print "--------------\n";
2b5ab1e7 1304 close(ARGV); # close or last; is needed if we
748a9306 1305 # are reading from the terminal
a0d0e21e
LW
1306 }
1307 print;
1308 }
1309
a0d0e21e 1310Practical hint: you almost never need to use C<eof> in Perl, because the
3ce0d271
GS
1311input operators typically return C<undef> when they run out of data, or if
1312there was an error.
a0d0e21e
LW
1313
1314=item eval EXPR
1315
1316=item eval BLOCK
1317
c7cc6f1c
GS
1318In the first form, the return value of EXPR is parsed and executed as if it
1319were a little Perl program. The value of the expression (which is itself
5a964f20 1320determined within scalar context) is first parsed, and if there weren't any
be3174d2
GS
1321errors, executed in the lexical context of the current Perl program, so
1322that any variable settings or subroutine and format definitions remain
1323afterwards. Note that the value is parsed every time the eval executes.
1324If EXPR is omitted, evaluates C<$_>. This form is typically used to
1325delay parsing and subsequent execution of the text of EXPR until run time.
c7cc6f1c
GS
1326
1327In the second form, the code within the BLOCK is parsed only once--at the
1328same time the code surrounding the eval itself was parsed--and executed
1329within the context of the current Perl program. This form is typically
1330used to trap exceptions more efficiently than the first (see below), while
1331also providing the benefit of checking the code within BLOCK at compile
1332time.
1333
1334The final semicolon, if any, may be omitted from the value of EXPR or within
1335the BLOCK.
1336
1337In both forms, the value returned is the value of the last expression
5a964f20 1338evaluated inside the mini-program; a return statement may be also used, just
c7cc6f1c 1339as with subroutines. The expression providing the return value is evaluated
5a964f20 1340in void, scalar, or list context, depending on the context of the eval itself.
c7cc6f1c 1341See L</wantarray> for more on how the evaluation context can be determined.
a0d0e21e 1342
19799a22
GS
1343If there is a syntax error or runtime error, or a C<die> statement is
1344executed, an undefined value is returned by C<eval>, and C<$@> is set to the
a0d0e21e 1345error message. If there was no error, C<$@> is guaranteed to be a null
19799a22 1346string. Beware that using C<eval> neither silences perl from printing
c7cc6f1c 1347warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
d9984052
A
1348To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1349turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1350See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
a0d0e21e 1351
19799a22
GS
1352Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1353determining whether a particular feature (such as C<socket> or C<symlink>)
a0d0e21e
LW
1354is implemented. It is also Perl's exception trapping mechanism, where
1355the die operator is used to raise exceptions.
1356
1357If the code to be executed doesn't vary, you may use the eval-BLOCK
1358form to trap run-time errors without incurring the penalty of
1359recompiling each time. The error, if any, is still returned in C<$@>.
1360Examples:
1361
54310121 1362 # make divide-by-zero nonfatal
a0d0e21e
LW
1363 eval { $answer = $a / $b; }; warn $@ if $@;
1364
1365 # same thing, but less efficient
1366 eval '$answer = $a / $b'; warn $@ if $@;
1367
1368 # a compile-time error
5a964f20 1369 eval { $answer = }; # WRONG
a0d0e21e
LW
1370
1371 # a run-time error
1372 eval '$answer ='; # sets $@
1373
2b5ab1e7
TC
1374Due to the current arguably broken state of C<__DIE__> hooks, when using
1375the C<eval{}> form as an exception trap in libraries, you may wish not
1376to trigger any C<__DIE__> hooks that user code may have installed.
1377You can use the C<local $SIG{__DIE__}> construct for this purpose,
1378as shown in this example:
774d564b 1379
1380 # a very private exception trap for divide-by-zero
f86cebdf
GS
1381 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1382 warn $@ if $@;
774d564b 1383
1384This is especially significant, given that C<__DIE__> hooks can call
19799a22 1385C<die> again, which has the effect of changing their error messages:
774d564b 1386
1387 # __DIE__ hooks may modify error messages
1388 {
f86cebdf
GS
1389 local $SIG{'__DIE__'} =
1390 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
c7cc6f1c
GS
1391 eval { die "foo lives here" };
1392 print $@ if $@; # prints "bar lives here"
774d564b 1393 }
1394
19799a22 1395Because this promotes action at a distance, this counterintuitive behavior
2b5ab1e7
TC
1396may be fixed in a future release.
1397
19799a22 1398With an C<eval>, you should be especially careful to remember what's
a0d0e21e
LW
1399being looked at when:
1400
1401 eval $x; # CASE 1
1402 eval "$x"; # CASE 2
1403
1404 eval '$x'; # CASE 3
1405 eval { $x }; # CASE 4
1406
5a964f20 1407 eval "\$$x++"; # CASE 5
a0d0e21e
LW
1408 $$x++; # CASE 6
1409
2f9daede 1410Cases 1 and 2 above behave identically: they run the code contained in
19799a22 1411the variable $x. (Although case 2 has misleading double quotes making
2f9daede 1412the reader wonder what else might be happening (nothing is).) Cases 3
7660c0ab 1413and 4 likewise behave in the same way: they run the code C<'$x'>, which
19799a22 1414does nothing but return the value of $x. (Case 4 is preferred for
2f9daede
TP
1415purely visual reasons, but it also has the advantage of compiling at
1416compile-time instead of at run-time.) Case 5 is a place where
19799a22 1417normally you I<would> like to use double quotes, except that in this
2f9daede
TP
1418particular situation, you can just use symbolic references instead, as
1419in case 6.
a0d0e21e 1420
4968c1e4 1421C<eval BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7 1422C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
4968c1e4 1423
a0d0e21e
LW
1424=item exec LIST
1425
8bf3b016
GS
1426=item exec PROGRAM LIST
1427
19799a22
GS
1428The C<exec> function executes a system command I<and never returns>--
1429use C<system> instead of C<exec> if you want it to return. It fails and
1430returns false only if the command does not exist I<and> it is executed
fb73857a 1431directly instead of via your system's command shell (see below).
a0d0e21e 1432
19799a22
GS
1433Since it's a common mistake to use C<exec> instead of C<system>, Perl
1434warns you if there is a following statement which isn't C<die>, C<warn>,
1435or C<exit> (if C<-w> is set - but you always do that). If you
1436I<really> want to follow an C<exec> with some other statement, you
55d729e4
GS
1437can use one of these styles to avoid the warning:
1438
5a964f20
TC
1439 exec ('foo') or print STDERR "couldn't exec foo: $!";
1440 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
55d729e4 1441
5a964f20 1442If there is more than one argument in LIST, or if LIST is an array
f86cebdf 1443with more than one value, calls execvp(3) with the arguments in LIST.
5a964f20
TC
1444If there is only one scalar argument or an array with one element in it,
1445the argument is checked for shell metacharacters, and if there are any,
1446the entire argument is passed to the system's command shell for parsing
1447(this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1448If there are no shell metacharacters in the argument, it is split into
b76cc8ba 1449words and passed directly to C<execvp>, which is more efficient.
19799a22 1450Examples:
a0d0e21e 1451
19799a22
GS
1452 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1453 exec "sort $outfile | uniq";
a0d0e21e
LW
1454
1455If you don't really want to execute the first argument, but want to lie
1456to the program you are executing about its own name, you can specify
1457the program you actually want to run as an "indirect object" (without a
1458comma) in front of the LIST. (This always forces interpretation of the
54310121 1459LIST as a multivalued list, even if there is only a single scalar in
a0d0e21e
LW
1460the list.) Example:
1461
1462 $shell = '/bin/csh';
1463 exec $shell '-sh'; # pretend it's a login shell
1464
1465or, more directly,
1466
1467 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1468
bb32b41a
GS
1469When the arguments get executed via the system shell, results will
1470be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1471for details.
1472
19799a22
GS
1473Using an indirect object with C<exec> or C<system> is also more
1474secure. This usage (which also works fine with system()) forces
1475interpretation of the arguments as a multivalued list, even if the
1476list had just one argument. That way you're safe from the shell
1477expanding wildcards or splitting up words with whitespace in them.
5a964f20
TC
1478
1479 @args = ( "echo surprise" );
1480
2b5ab1e7 1481 exec @args; # subject to shell escapes
f86cebdf 1482 # if @args == 1
2b5ab1e7 1483 exec { $args[0] } @args; # safe even with one-arg list
5a964f20
TC
1484
1485The first version, the one without the indirect object, ran the I<echo>
1486program, passing it C<"surprise"> an argument. The second version
1487didn't--it tried to run a program literally called I<"echo surprise">,
1488didn't find it, and set C<$?> to a non-zero value indicating failure.
1489
0f897271
GS
1490Beginning with v5.6.0, Perl will attempt to flush all files opened for
1491output before the exec, but this may not be supported on some platforms
1492(see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1493in English) or call the C<autoflush()> method of C<IO::Handle> on any
1494open handles in order to avoid lost output.
1495
19799a22 1496Note that C<exec> will not call your C<END> blocks, nor will it call
7660c0ab
A
1497any C<DESTROY> methods in your objects.
1498
a0d0e21e
LW
1499=item exists EXPR
1500
01020589 1501Given an expression that specifies a hash element or array element,
8ea97a1e
GS
1502returns true if the specified element in the hash or array has ever
1503been initialized, even if the corresponding value is undefined. The
1504element is not autovivified if it doesn't exist.
a0d0e21e 1505
01020589
GS
1506 print "Exists\n" if exists $hash{$key};
1507 print "Defined\n" if defined $hash{$key};
1508 print "True\n" if $hash{$key};
1509
1510 print "Exists\n" if exists $array[$index];
1511 print "Defined\n" if defined $array[$index];
1512 print "True\n" if $array[$index];
a0d0e21e 1513
8ea97a1e 1514A hash or array element can be true only if it's defined, and defined if
a0d0e21e
LW
1515it exists, but the reverse doesn't necessarily hold true.
1516
afebc493
GS
1517Given an expression that specifies the name of a subroutine,
1518returns true if the specified subroutine has ever been declared, even
1519if it is undefined. Mentioning a subroutine name for exists or defined
847c7ebe
DD
1520does not count as declaring it. Note that a subroutine which does not
1521exist may still be callable: its package may have an C<AUTOLOAD>
1522method that makes it spring into existence the first time that it is
1523called -- see L<perlsub>.
afebc493
GS
1524
1525 print "Exists\n" if exists &subroutine;
1526 print "Defined\n" if defined &subroutine;
1527
a0d0e21e 1528Note that the EXPR can be arbitrarily complicated as long as the final
afebc493 1529operation is a hash or array key lookup or subroutine name:
a0d0e21e 1530
2b5ab1e7
TC
1531 if (exists $ref->{A}->{B}->{$key}) { }
1532 if (exists $hash{A}{B}{$key}) { }
1533
01020589
GS
1534 if (exists $ref->{A}->{B}->[$ix]) { }
1535 if (exists $hash{A}{B}[$ix]) { }
1536
afebc493
GS
1537 if (exists &{$ref->{A}{B}{$key}}) { }
1538
01020589
GS
1539Although the deepest nested array or hash will not spring into existence
1540just because its existence was tested, any intervening ones will.
61eff3bc 1541Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
01020589
GS
1542into existence due to the existence test for the $key element above.
1543This happens anywhere the arrow operator is used, including even:
5a964f20 1544
2b5ab1e7
TC
1545 undef $ref;
1546 if (exists $ref->{"Some key"}) { }
1547 print $ref; # prints HASH(0x80d3d5c)
1548
1549This surprising autovivification in what does not at first--or even
1550second--glance appear to be an lvalue context may be fixed in a future
5a964f20 1551release.
a0d0e21e 1552
479ba383
GS
1553See L<perlref/"Pseudo-hashes: Using an array as a hash"> for specifics
1554on how exists() acts when used on a pseudo-hash.
e0478e5a 1555
afebc493
GS
1556Use of a subroutine call, rather than a subroutine name, as an argument
1557to exists() is an error.
1558
1559 exists &sub; # OK
1560 exists &sub(); # Error
1561
a0d0e21e
LW
1562=item exit EXPR
1563
2b5ab1e7 1564Evaluates EXPR and exits immediately with that value. Example:
a0d0e21e
LW
1565
1566 $ans = <STDIN>;
1567 exit 0 if $ans =~ /^[Xx]/;
1568
19799a22 1569See also C<die>. If EXPR is omitted, exits with C<0> status. The only
2b5ab1e7
TC
1570universally recognized values for EXPR are C<0> for success and C<1>
1571for error; other values are subject to interpretation depending on the
1572environment in which the Perl program is running. For example, exiting
157369 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1574the mailer to return the item undelivered, but that's not true everywhere.
a0d0e21e 1575
19799a22
GS
1576Don't use C<exit> to abort a subroutine if there's any chance that
1577someone might want to trap whatever error happened. Use C<die> instead,
1578which can be trapped by an C<eval>.
28757baa 1579
19799a22 1580The exit() function does not always exit immediately. It calls any
2b5ab1e7 1581defined C<END> routines first, but these C<END> routines may not
19799a22 1582themselves abort the exit. Likewise any object destructors that need to
2b5ab1e7
TC
1583be called are called before the real exit. If this is a problem, you
1584can call C<POSIX:_exit($status)> to avoid END and destructor processing.
87275199 1585See L<perlmod> for details.
5a964f20 1586
a0d0e21e
LW
1587=item exp EXPR
1588
54310121 1589=item exp
bbce6d69 1590
b76cc8ba 1591Returns I<e> (the natural logarithm base) to the power of EXPR.
a0d0e21e
LW
1592If EXPR is omitted, gives C<exp($_)>.
1593
1594=item fcntl FILEHANDLE,FUNCTION,SCALAR
1595
f86cebdf 1596Implements the fcntl(2) function. You'll probably have to say
a0d0e21e
LW
1597
1598 use Fcntl;
1599
0ade1984 1600first to get the correct constant definitions. Argument processing and
b76cc8ba 1601value return works just like C<ioctl> below.
a0d0e21e
LW
1602For example:
1603
1604 use Fcntl;
5a964f20
TC
1605 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1606 or die "can't fcntl F_GETFL: $!";
1607
19799a22 1608You don't have to check for C<defined> on the return from C<fnctl>.
951ba7fe
GS
1609Like C<ioctl>, it maps a C<0> return from the system call into
1610C<"0 but true"> in Perl. This string is true in boolean context and C<0>
2b5ab1e7
TC
1611in numeric context. It is also exempt from the normal B<-w> warnings
1612on improper numeric conversions.
5a964f20 1613
19799a22 1614Note that C<fcntl> will produce a fatal error if used on a machine that
2b5ab1e7
TC
1615doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1616manpage to learn what functions are available on your system.
a0d0e21e
LW
1617
1618=item fileno FILEHANDLE
1619
2b5ab1e7
TC
1620Returns the file descriptor for a filehandle, or undefined if the
1621filehandle is not open. This is mainly useful for constructing
19799a22 1622bitmaps for C<select> and low-level POSIX tty-handling operations.
2b5ab1e7
TC
1623If FILEHANDLE is an expression, the value is taken as an indirect
1624filehandle, generally its name.
5a964f20 1625
b76cc8ba 1626You can use this to find out whether two handles refer to the
5a964f20
TC
1627same underlying descriptor:
1628
1629 if (fileno(THIS) == fileno(THAT)) {
1630 print "THIS and THAT are dups\n";
b76cc8ba
NIS
1631 }
1632
1633(Filehandles connected to memory objects via new features of C<open> may
1634return undefined even though they are open.)
1635
a0d0e21e
LW
1636
1637=item flock FILEHANDLE,OPERATION
1638
19799a22
GS
1639Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1640for success, false on failure. Produces a fatal error if used on a
2b5ab1e7 1641machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
19799a22 1642C<flock> is Perl's portable file locking interface, although it locks
2b5ab1e7
TC
1643only entire files, not records.
1644
1645Two potentially non-obvious but traditional C<flock> semantics are
1646that it waits indefinitely until the lock is granted, and that its locks
1647B<merely advisory>. Such discretionary locks are more flexible, but offer
19799a22
GS
1648fewer guarantees. This means that files locked with C<flock> may be
1649modified by programs that do not also use C<flock>. See L<perlport>,
2b5ab1e7
TC
1650your port's specific documentation, or your system-specific local manpages
1651for details. It's best to assume traditional behavior if you're writing
1652portable programs. (But if you're not, you should as always feel perfectly
1653free to write for your own system's idiosyncrasies (sometimes called
1654"features"). Slavish adherence to portability concerns shouldn't get
1655in the way of your getting your job done.)
a3cb178b 1656
8ebc5c01 1657OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1658LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
ea3105be 1659you can use the symbolic names if you import them from the Fcntl module,
68dc0745 1660either individually, or as a group using the ':flock' tag. LOCK_SH
1661requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
ea3105be
GS
1662releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1663LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
68dc0745 1664waiting for the lock (check the return status to see if you got it).
1665
2b5ab1e7
TC
1666To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1667before locking or unlocking it.
8ebc5c01 1668
f86cebdf 1669Note that the emulation built with lockf(3) doesn't provide shared
8ebc5c01 1670locks, and it requires that FILEHANDLE be open with write intent. These
2b5ab1e7 1671are the semantics that lockf(3) implements. Most if not all systems
f86cebdf 1672implement lockf(3) in terms of fcntl(2) locking, though, so the
8ebc5c01 1673differing semantics shouldn't bite too many people.
1674
becacb53
TM
1675Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
1676be open with read intent to use LOCK_SH and requires that it be open
1677with write intent to use LOCK_EX.
1678
19799a22
GS
1679Note also that some versions of C<flock> cannot lock things over the
1680network; you would need to use the more system-specific C<fcntl> for
f86cebdf
GS
1681that. If you like you can force Perl to ignore your system's flock(2)
1682function, and so provide its own fcntl(2)-based emulation, by passing
8ebc5c01 1683the switch C<-Ud_flock> to the F<Configure> program when you configure
1684perl.
4633a7c4
LW
1685
1686Here's a mailbox appender for BSD systems.
a0d0e21e 1687
7e1af8bc 1688 use Fcntl ':flock'; # import LOCK_* constants
a0d0e21e
LW
1689
1690 sub lock {
7e1af8bc 1691 flock(MBOX,LOCK_EX);
a0d0e21e
LW
1692 # and, in case someone appended
1693 # while we were waiting...
1694 seek(MBOX, 0, 2);
1695 }
1696
1697 sub unlock {
7e1af8bc 1698 flock(MBOX,LOCK_UN);
a0d0e21e
LW
1699 }
1700
1701 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1702 or die "Can't open mailbox: $!";
1703
1704 lock();
1705 print MBOX $msg,"\n\n";
1706 unlock();
1707
2b5ab1e7
TC
1708On systems that support a real flock(), locks are inherited across fork()
1709calls, whereas those that must resort to the more capricious fcntl()
1710function lose the locks, making it harder to write servers.
1711
cb1a09d0 1712See also L<DB_File> for other flock() examples.
a0d0e21e
LW
1713
1714=item fork
1715
2b5ab1e7
TC
1716Does a fork(2) system call to create a new process running the
1717same program at the same point. It returns the child pid to the
1718parent process, C<0> to the child process, or C<undef> if the fork is
1719unsuccessful. File descriptors (and sometimes locks on those descriptors)
1720are shared, while everything else is copied. On most systems supporting
1721fork(), great care has gone into making it extremely efficient (for
1722example, using copy-on-write technology on data pages), making it the
1723dominant paradigm for multitasking over the last few decades.
5a964f20 1724
0f897271
GS
1725Beginning with v5.6.0, Perl will attempt to flush all files opened for
1726output before forking the child process, but this may not be supported
1727on some platforms (see L<perlport>). To be safe, you may need to set
1728C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1729C<IO::Handle> on any open handles in order to avoid duplicate output.
a0d0e21e 1730
19799a22 1731If you C<fork> without ever waiting on your children, you will
2b5ab1e7
TC
1732accumulate zombies. On some systems, you can avoid this by setting
1733C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1734forking and reaping moribund children.
cb1a09d0 1735
28757baa 1736Note that if your forked child inherits system file descriptors like
1737STDIN and STDOUT that are actually connected by a pipe or socket, even
2b5ab1e7 1738if you exit, then the remote server (such as, say, a CGI script or a
19799a22 1739backgrounded job launched from a remote shell) won't think you're done.
2b5ab1e7 1740You should reopen those to F</dev/null> if it's any issue.
28757baa 1741
cb1a09d0
AD
1742=item format
1743
19799a22 1744Declare a picture format for use by the C<write> function. For
cb1a09d0
AD
1745example:
1746
54310121 1747 format Something =
cb1a09d0
AD
1748 Test: @<<<<<<<< @||||| @>>>>>
1749 $str, $%, '$' . int($num)
1750 .
1751
1752 $str = "widget";
184e9718 1753 $num = $cost/$quantity;
cb1a09d0
AD
1754 $~ = 'Something';
1755 write;
1756
1757See L<perlform> for many details and examples.
1758
8903cb82 1759=item formline PICTURE,LIST
a0d0e21e 1760
5a964f20 1761This is an internal function used by C<format>s, though you may call it,
a0d0e21e
LW
1762too. It formats (see L<perlform>) a list of values according to the
1763contents of PICTURE, placing the output into the format output
7660c0ab 1764accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
19799a22 1765Eventually, when a C<write> is done, the contents of
a0d0e21e 1766C<$^A> are written to some filehandle, but you could also read C<$^A>
7660c0ab 1767yourself and then set C<$^A> back to C<"">. Note that a format typically
19799a22 1768does one C<formline> per line of form, but the C<formline> function itself
748a9306 1769doesn't care how many newlines are embedded in the PICTURE. This means
4633a7c4 1770that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
748a9306
LW
1771You may therefore need to use multiple formlines to implement a single
1772record format, just like the format compiler.
1773
19799a22 1774Be careful if you put double quotes around the picture, because an C<@>
748a9306 1775character may be taken to mean the beginning of an array name.
19799a22 1776C<formline> always returns true. See L<perlform> for other examples.
a0d0e21e
LW
1777
1778=item getc FILEHANDLE
1779
1780=item getc
1781
1782Returns the next character from the input file attached to FILEHANDLE,
2b5ab1e7
TC
1783or the undefined value at end of file, or if there was an error.
1784If FILEHANDLE is omitted, reads from STDIN. This is not particularly
1785efficient. However, it cannot be used by itself to fetch single
1786characters without waiting for the user to hit enter. For that, try
1787something more like:
4633a7c4
LW
1788
1789 if ($BSD_STYLE) {
1790 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1791 }
1792 else {
54310121 1793 system "stty", '-icanon', 'eol', "\001";
4633a7c4
LW
1794 }
1795
1796 $key = getc(STDIN);
1797
1798 if ($BSD_STYLE) {
1799 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1800 }
1801 else {
5f05dabc 1802 system "stty", 'icanon', 'eol', '^@'; # ASCII null
4633a7c4
LW
1803 }
1804 print "\n";
1805
54310121 1806Determination of whether $BSD_STYLE should be set
1807is left as an exercise to the reader.
cb1a09d0 1808
19799a22 1809The C<POSIX::getattr> function can do this more portably on
2b5ab1e7
TC
1810systems purporting POSIX compliance. See also the C<Term::ReadKey>
1811module from your nearest CPAN site; details on CPAN can be found on
1812L<perlmodlib/CPAN>.
a0d0e21e
LW
1813
1814=item getlogin
1815
5a964f20
TC
1816Implements the C library function of the same name, which on most
1817systems returns the current login from F</etc/utmp>, if any. If null,
19799a22 1818use C<getpwuid>.
a0d0e21e 1819
f86702cc 1820 $login = getlogin || getpwuid($<) || "Kilroy";
a0d0e21e 1821
19799a22
GS
1822Do not consider C<getlogin> for authentication: it is not as
1823secure as C<getpwuid>.
4633a7c4 1824
a0d0e21e
LW
1825=item getpeername SOCKET
1826
1827Returns the packed sockaddr address of other end of the SOCKET connection.
1828
4633a7c4
LW
1829 use Socket;
1830 $hersockaddr = getpeername(SOCK);
19799a22 1831 ($port, $iaddr) = sockaddr_in($hersockaddr);
4633a7c4
LW
1832 $herhostname = gethostbyaddr($iaddr, AF_INET);
1833 $herstraddr = inet_ntoa($iaddr);
a0d0e21e
LW
1834
1835=item getpgrp PID
1836
47e29363 1837Returns the current process group for the specified PID. Use
7660c0ab 1838a PID of C<0> to get the current process group for the
4633a7c4 1839current process. Will raise an exception if used on a machine that
f86cebdf 1840doesn't implement getpgrp(2). If PID is omitted, returns process
19799a22 1841group of current process. Note that the POSIX version of C<getpgrp>
7660c0ab 1842does not accept a PID argument, so only C<PID==0> is truly portable.
a0d0e21e
LW
1843
1844=item getppid
1845
1846Returns the process id of the parent process.
1847
1848=item getpriority WHICH,WHO
1849
4633a7c4
LW
1850Returns the current priority for a process, a process group, or a user.
1851(See L<getpriority(2)>.) Will raise a fatal exception if used on a
f86cebdf 1852machine that doesn't implement getpriority(2).
a0d0e21e
LW
1853
1854=item getpwnam NAME
1855
1856=item getgrnam NAME
1857
1858=item gethostbyname NAME
1859
1860=item getnetbyname NAME
1861
1862=item getprotobyname NAME
1863
1864=item getpwuid UID
1865
1866=item getgrgid GID
1867
1868=item getservbyname NAME,PROTO
1869
1870=item gethostbyaddr ADDR,ADDRTYPE
1871
1872=item getnetbyaddr ADDR,ADDRTYPE
1873
1874=item getprotobynumber NUMBER
1875
1876=item getservbyport PORT,PROTO
1877
1878=item getpwent
1879
1880=item getgrent
1881
1882=item gethostent
1883
1884=item getnetent
1885
1886=item getprotoent
1887
1888=item getservent
1889
1890=item setpwent
1891
1892=item setgrent
1893
1894=item sethostent STAYOPEN
1895
1896=item setnetent STAYOPEN
1897
1898=item setprotoent STAYOPEN
1899
1900=item setservent STAYOPEN
1901
1902=item endpwent
1903
1904=item endgrent
1905
1906=item endhostent
1907
1908=item endnetent
1909
1910=item endprotoent
1911
1912=item endservent
1913
1914These routines perform the same functions as their counterparts in the
5a964f20 1915system library. In list context, the return values from the
a0d0e21e
LW
1916various get routines are as follows:
1917
1918 ($name,$passwd,$uid,$gid,
6ee623d5 1919 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
a0d0e21e
LW
1920 ($name,$passwd,$gid,$members) = getgr*
1921 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1922 ($name,$aliases,$addrtype,$net) = getnet*
1923 ($name,$aliases,$proto) = getproto*
1924 ($name,$aliases,$port,$proto) = getserv*
1925
1926(If the entry doesn't exist you get a null list.)
1927
4602f195
JH
1928The exact meaning of the $gcos field varies but it usually contains
1929the real name of the user (as opposed to the login name) and other
1930information pertaining to the user. Beware, however, that in many
1931system users are able to change this information and therefore it
106325ad 1932cannot be trusted and therefore the $gcos is tainted (see
2959b6e3
JH
1933L<perlsec>). The $passwd and $shell, user's encrypted password and
1934login shell, are also tainted, because of the same reason.
4602f195 1935
5a964f20 1936In scalar context, you get the name, unless the function was a
a0d0e21e
LW
1937lookup by name, in which case you get the other thing, whatever it is.
1938(If the entry doesn't exist you get the undefined value.) For example:
1939
5a964f20
TC
1940 $uid = getpwnam($name);
1941 $name = getpwuid($num);
1942 $name = getpwent();
1943 $gid = getgrnam($name);
1944 $name = getgrgid($num;
1945 $name = getgrent();
1946 #etc.
a0d0e21e 1947
4602f195
JH
1948In I<getpw*()> the fields $quota, $comment, and $expire are special
1949cases in the sense that in many systems they are unsupported. If the
1950$quota is unsupported, it is an empty scalar. If it is supported, it
1951usually encodes the disk quota. If the $comment field is unsupported,
1952it is an empty scalar. If it is supported it usually encodes some
1953administrative comment about the user. In some systems the $quota
1954field may be $change or $age, fields that have to do with password
1955aging. In some systems the $comment field may be $class. The $expire
1956field, if present, encodes the expiration period of the account or the
1957password. For the availability and the exact meaning of these fields
1958in your system, please consult your getpwnam(3) documentation and your
1959F<pwd.h> file. You can also find out from within Perl what your
1960$quota and $comment fields mean and whether you have the $expire field
1961by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
1962C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
1963files are only supported if your vendor has implemented them in the
1964intuitive fashion that calling the regular C library routines gets the
5d3a0a3b
GS
1965shadow versions if you're running under privilege or if there exists
1966the shadow(3) functions as found in System V ( this includes Solaris
1967and Linux.) Those systems which implement a proprietary shadow password
1968facility are unlikely to be supported.
6ee623d5 1969
19799a22 1970The $members value returned by I<getgr*()> is a space separated list of
a0d0e21e
LW
1971the login names of the members of the group.
1972
1973For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1974C, it will be returned to you via C<$?> if the function call fails. The
7660c0ab 1975C<@addrs> value returned by a successful call is a list of the raw
a0d0e21e
LW
1976addresses returned by the corresponding system library call. In the
1977Internet domain, each address is four bytes long and you can unpack it
1978by saying something like:
1979
1980 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1981
2b5ab1e7
TC
1982The Socket library makes this slightly easier:
1983
1984 use Socket;
1985 $iaddr = inet_aton("127.1"); # or whatever address
1986 $name = gethostbyaddr($iaddr, AF_INET);
1987
1988 # or going the other way
19799a22 1989 $straddr = inet_ntoa($iaddr);
2b5ab1e7 1990
19799a22
GS
1991If you get tired of remembering which element of the return list
1992contains which return value, by-name interfaces are provided
1993in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
1994C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
1995and C<User::grent>. These override the normal built-ins, supplying
1996versions that return objects with the appropriate names
1997for each field. For example:
5a964f20
TC
1998
1999 use File::stat;
2000 use User::pwent;
2001 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2002
b76cc8ba
NIS
2003Even though it looks like they're the same method calls (uid),
2004they aren't, because a C<File::stat> object is different from
19799a22 2005a C<User::pwent> object.
5a964f20 2006
a0d0e21e
LW
2007=item getsockname SOCKET
2008
19799a22
GS
2009Returns the packed sockaddr address of this end of the SOCKET connection,
2010in case you don't know the address because you have several different
2011IPs that the connection might have come in on.
a0d0e21e 2012
4633a7c4
LW
2013 use Socket;
2014 $mysockaddr = getsockname(SOCK);
19799a22 2015 ($port, $myaddr) = sockaddr_in($mysockaddr);
b76cc8ba 2016 printf "Connect to %s [%s]\n",
19799a22
GS
2017 scalar gethostbyaddr($myaddr, AF_INET),
2018 inet_ntoa($myaddr);
a0d0e21e
LW
2019
2020=item getsockopt SOCKET,LEVEL,OPTNAME
2021
5a964f20 2022Returns the socket option requested, or undef if there is an error.
a0d0e21e
LW
2023
2024=item glob EXPR
2025
0a753a76 2026=item glob
2027
2b5ab1e7
TC
2028Returns the value of EXPR with filename expansions such as the
2029standard Unix shell F</bin/csh> would do. This is the internal function
61eff3bc
JH
2030implementing the C<< <*.c> >> operator, but you can use it directly.
2031If EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is
2b5ab1e7 2032discussed in more detail in L<perlop/"I/O Operators">.
a0d0e21e 2033
3a4b19e4
GS
2034Beginning with v5.6.0, this operator is implemented using the standard
2035C<File::Glob> extension. See L<File::Glob> for details.
2036
a0d0e21e
LW
2037=item gmtime EXPR
2038
d1be9408 2039Converts a time as returned by the time function to an 8-element list
54310121 2040with the time localized for the standard Greenwich time zone.
4633a7c4 2041Typically used as follows:
a0d0e21e 2042
b76cc8ba 2043 # 0 1 2 3 4 5 6 7
48a26b3a 2044 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday) =
a0d0e21e
LW
2045 gmtime(time);
2046
48a26b3a
GS
2047All list elements are numeric, and come straight out of the C `struct
2048tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2049specified time. $mday is the day of the month, and $mon is the month
2050itself, in the range C<0..11> with 0 indicating January and 11
2051indicating December. $year is the number of years since 1900. That
2052is, $year is C<123> in year 2023. $wday is the day of the week, with
20530 indicating Sunday and 3 indicating Wednesday. $yday is the day of
b76cc8ba 2054the year, in the range C<0..364> (or C<0..365> in leap years.)
48a26b3a
GS
2055
2056Note that the $year element is I<not> simply the last two digits of
2057the year. If you assume it is, then you create non-Y2K-compliant
2058programs--and you wouldn't want to do that, would you?
2f9daede 2059
abd75f24
GS
2060The proper way to get a complete 4-digit year is simply:
2061
2062 $year += 1900;
2063
2064And to get the last two digits of the year (e.g., '01' in 2001) do:
2065
2066 $year = sprintf("%02d", $year % 100);
2067
48a26b3a 2068If EXPR is omitted, C<gmtime()> uses the current time (C<gmtime(time)>).
a0d0e21e 2069
48a26b3a 2070In scalar context, C<gmtime()> returns the ctime(3) value:
0a753a76 2071
2072 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
2073
19799a22 2074Also see the C<timegm> function provided by the C<Time::Local> module,
f86cebdf 2075and the strftime(3) function available via the POSIX module.
7660c0ab 2076
2b5ab1e7
TC
2077This scalar value is B<not> locale dependent (see L<perllocale>), but
2078is instead a Perl builtin. Also see the C<Time::Local> module, and the
2079strftime(3) and mktime(3) functions available via the POSIX module. To
7660c0ab
A
2080get somewhat similar but locale dependent date strings, set up your
2081locale environment variables appropriately (please see L<perllocale>)
2082and try for example:
2083
2084 use POSIX qw(strftime);
2b5ab1e7 2085 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
7660c0ab 2086
2b5ab1e7
TC
2087Note that the C<%a> and C<%b> escapes, which represent the short forms
2088of the day of the week and the month of the year, may not necessarily
2089be three characters wide in all locales.
0a753a76 2090
a0d0e21e
LW
2091=item goto LABEL
2092
748a9306
LW
2093=item goto EXPR
2094
a0d0e21e
LW
2095=item goto &NAME
2096
7660c0ab 2097The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
a0d0e21e 2098execution there. It may not be used to go into any construct that
7660c0ab 2099requires initialization, such as a subroutine or a C<foreach> loop. It
0a753a76 2100also can't be used to go into a construct that is optimized away,
19799a22 2101or to get out of a block or subroutine given to C<sort>.
0a753a76 2102It can be used to go almost anywhere else within the dynamic scope,
a0d0e21e 2103including out of subroutines, but it's usually better to use some other
19799a22 2104construct such as C<last> or C<die>. The author of Perl has never felt the
7660c0ab 2105need to use this form of C<goto> (in Perl, that is--C is another matter).
1b6921cb
BT
2106(The difference being that C does not offer named loops combined with
2107loop control. Perl does, and this replaces most structured uses of C<goto>
2108in other languages.)
a0d0e21e 2109
7660c0ab
A
2110The C<goto-EXPR> form expects a label name, whose scope will be resolved
2111dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
748a9306
LW
2112necessarily recommended if you're optimizing for maintainability:
2113
2114 goto ("FOO", "BAR", "GLARCH")[$i];
2115
1b6921cb
BT
2116The C<goto-&NAME> form is quite different from the other forms of
2117C<goto>. In fact, it isn't a goto in the normal sense at all, and
2118doesn't have the stigma associated with other gotos. Instead, it
2119exits the current subroutine (losing any changes set by local()) and
2120immediately calls in its place the named subroutine using the current
2121value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2122load another subroutine and then pretend that the other subroutine had
2123been called in the first place (except that any modifications to C<@_>
6cb9131c
GS
2124in the current subroutine are propagated to the other subroutine.)
2125After the C<goto>, not even C<caller> will be able to tell that this
2126routine was called first.
2127
2128NAME needn't be the name of a subroutine; it can be a scalar variable
2129containing a code reference, or a block which evaluates to a code
2130reference.
a0d0e21e
LW
2131
2132=item grep BLOCK LIST
2133
2134=item grep EXPR,LIST
2135
2b5ab1e7
TC
2136This is similar in spirit to, but not the same as, grep(1) and its
2137relatives. In particular, it is not limited to using regular expressions.
2f9daede 2138
a0d0e21e 2139Evaluates the BLOCK or EXPR for each element of LIST (locally setting
7660c0ab 2140C<$_> to each element) and returns the list value consisting of those
19799a22
GS
2141elements for which the expression evaluated to true. In scalar
2142context, returns the number of times the expression was true.
a0d0e21e
LW
2143
2144 @foo = grep(!/^#/, @bar); # weed out comments
2145
2146or equivalently,
2147
2148 @foo = grep {!/^#/} @bar; # weed out comments
2149
be3174d2
GS
2150Note that C<$_> is an alias to the list value, so it can be used to
2151modify the elements of the LIST. While this is useful and supported,
2152it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
2153Similarly, grep returns aliases into the original list, much as a for
2154loop's index variable aliases the list elements. That is, modifying an
19799a22
GS
2155element of a list returned by grep (for example, in a C<foreach>, C<map>
2156or another C<grep>) actually modifies the element in the original list.
2b5ab1e7 2157This is usually something to be avoided when writing clear code.
a0d0e21e 2158
19799a22 2159See also L</map> for a list composed of the results of the BLOCK or EXPR.
38325410 2160
a0d0e21e
LW
2161=item hex EXPR
2162
54310121 2163=item hex
bbce6d69 2164
2b5ab1e7
TC
2165Interprets EXPR as a hex string and returns the corresponding value.
2166(To convert strings that might start with either 0, 0x, or 0b, see
2167L</oct>.) If EXPR is omitted, uses C<$_>.
2f9daede
TP
2168
2169 print hex '0xAf'; # prints '175'
2170 print hex 'aF'; # same
a0d0e21e 2171
19799a22 2172Hex strings may only represent integers. Strings that would cause
53305cf1
NC
2173integer overflow trigger a warning. Leading whitespace is not stripped,
2174unlike oct().
19799a22 2175
a0d0e21e
LW
2176=item import
2177
19799a22 2178There is no builtin C<import> function. It is just an ordinary
4633a7c4 2179method (subroutine) defined (or inherited) by modules that wish to export
19799a22 2180names to another module. The C<use> function calls the C<import> method
cea6626f 2181for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
a0d0e21e
LW
2182
2183=item index STR,SUBSTR,POSITION
2184
2185=item index STR,SUBSTR
2186
2b5ab1e7
TC
2187The index function searches for one string within another, but without
2188the wildcard-like behavior of a full regular-expression pattern match.
2189It returns the position of the first occurrence of SUBSTR in STR at
2190or after POSITION. If POSITION is omitted, starts searching from the
2191beginning of the string. The return value is based at C<0> (or whatever
2192you've set the C<$[> variable to--but don't do that). If the substring
2193is not found, returns one less than the base, ordinarily C<-1>.
a0d0e21e
LW
2194
2195=item int EXPR
2196
54310121 2197=item int
bbce6d69 2198
7660c0ab 2199Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2b5ab1e7
TC
2200You should not use this function for rounding: one because it truncates
2201towards C<0>, and two because machine representations of floating point
2202numbers can sometimes produce counterintuitive results. For example,
2203C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2204because it's really more like -268.99999999999994315658 instead. Usually,
19799a22 2205the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2b5ab1e7 2206functions will serve you better than will int().
a0d0e21e
LW
2207
2208=item ioctl FILEHANDLE,FUNCTION,SCALAR
2209
2b5ab1e7 2210Implements the ioctl(2) function. You'll probably first have to say
a0d0e21e 2211
4633a7c4 2212 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
a0d0e21e 2213
2b5ab1e7 2214to get the correct function definitions. If F<ioctl.ph> doesn't
a0d0e21e 2215exist or doesn't have the correct definitions you'll have to roll your
61eff3bc 2216own, based on your C header files such as F<< <sys/ioctl.h> >>.
5a964f20 2217(There is a Perl script called B<h2ph> that comes with the Perl kit that
54310121 2218may help you in this, but it's nontrivial.) SCALAR will be read and/or
4633a7c4 2219written depending on the FUNCTION--a pointer to the string value of SCALAR
19799a22 2220will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
4633a7c4
LW
2221has no string value but does have a numeric value, that value will be
2222passed rather than a pointer to the string value. To guarantee this to be
19799a22
GS
2223true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2224functions may be needed to manipulate the values of structures used by
b76cc8ba 2225C<ioctl>.
a0d0e21e 2226
19799a22 2227The return value of C<ioctl> (and C<fcntl>) is as follows:
a0d0e21e
LW
2228
2229 if OS returns: then Perl returns:
2230 -1 undefined value
2231 0 string "0 but true"
2232 anything else that number
2233
19799a22 2234Thus Perl returns true on success and false on failure, yet you can
a0d0e21e
LW
2235still easily determine the actual value returned by the operating
2236system:
2237
2b5ab1e7 2238 $retval = ioctl(...) || -1;
a0d0e21e
LW
2239 printf "System returned %d\n", $retval;
2240
c2611fb3 2241The special string "C<0> but true" is exempt from B<-w> complaints
5a964f20
TC
2242about improper numeric conversions.
2243
19799a22
GS
2244Here's an example of setting a filehandle named C<REMOTE> to be
2245non-blocking at the system level. You'll have to negotiate C<$|>
2246on your own, though.
2247
2248 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
2249
2250 $flags = fcntl(REMOTE, F_GETFL, 0)
2251 or die "Can't get flags for the socket: $!\n";
2252
2253 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
2254 or die "Can't set flags for the socket: $!\n";
2255
a0d0e21e
LW
2256=item join EXPR,LIST
2257
2b5ab1e7
TC
2258Joins the separate strings of LIST into a single string with fields
2259separated by the value of EXPR, and returns that new string. Example:
a0d0e21e 2260
2b5ab1e7 2261 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
a0d0e21e 2262
eb6e2d6f
GS
2263Beware that unlike C<split>, C<join> doesn't take a pattern as its
2264first argument. Compare L</split>.
a0d0e21e 2265
aa689395 2266=item keys HASH
2267
19799a22 2268Returns a list consisting of all the keys of the named hash. (In
1d2dff63 2269scalar context, returns the number of keys.) The keys are returned in
ab192400
GS
2270an apparently random order. The actual random order is subject to
2271change in future versions of perl, but it is guaranteed to be the same
19799a22 2272order as either the C<values> or C<each> function produces (given
ab192400
GS
2273that the hash has not been modified). As a side effect, it resets
2274HASH's iterator.
a0d0e21e 2275
aa689395 2276Here is yet another way to print your environment:
a0d0e21e
LW
2277
2278 @keys = keys %ENV;
2279 @values = values %ENV;
b76cc8ba 2280 while (@keys) {
a0d0e21e
LW
2281 print pop(@keys), '=', pop(@values), "\n";
2282 }
2283
2284or how about sorted by key:
2285
2286 foreach $key (sort(keys %ENV)) {
2287 print $key, '=', $ENV{$key}, "\n";
2288 }
2289
8ea1e5d4
GS
2290The returned values are copies of the original keys in the hash, so
2291modifying them will not affect the original hash. Compare L</values>.
2292
19799a22 2293To sort a hash by value, you'll need to use a C<sort> function.
aa689395 2294Here's a descending numeric sort of a hash by its values:
4633a7c4 2295
5a964f20 2296 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
4633a7c4
LW
2297 printf "%4d %s\n", $hash{$key}, $key;
2298 }
2299
19799a22 2300As an lvalue C<keys> allows you to increase the number of hash buckets
aa689395 2301allocated for the given hash. This can gain you a measure of efficiency if
2302you know the hash is going to get big. (This is similar to pre-extending
2303an array by assigning a larger number to $#array.) If you say
55497cff 2304
2305 keys %hash = 200;
2306
ab192400
GS
2307then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2308in fact, since it rounds up to the next power of two. These
55497cff 2309buckets will be retained even if you do C<%hash = ()>, use C<undef
2310%hash> if you want to free the storage while C<%hash> is still in scope.
2311You can't shrink the number of buckets allocated for the hash using
19799a22 2312C<keys> in this way (but you needn't worry about doing this by accident,
55497cff 2313as trying has no effect).
2314
19799a22 2315See also C<each>, C<values> and C<sort>.
ab192400 2316
b350dd2f 2317=item kill SIGNAL, LIST
a0d0e21e 2318
b350dd2f 2319Sends a signal to a list of processes. Returns the number of
517db077
GS
2320processes successfully signaled (which is not necessarily the
2321same as the number actually killed).
a0d0e21e
LW
2322
2323 $cnt = kill 1, $child1, $child2;
2324 kill 9, @goners;
2325
b350dd2f
GS
2326If SIGNAL is zero, no signal is sent to the process. This is a
2327useful way to check that the process is alive and hasn't changed
2328its UID. See L<perlport> for notes on the portability of this
2329construct.
2330
2331Unlike in the shell, if SIGNAL is negative, it kills
4633a7c4
LW
2332process groups instead of processes. (On System V, a negative I<PROCESS>
2333number will also kill process groups, but that's not portable.) That
2334means you usually want to use positive not negative signals. You may also
da0045b7 2335use a signal name in quotes. See L<perlipc/"Signals"> for details.
a0d0e21e
LW
2336
2337=item last LABEL
2338
2339=item last
2340
2341The C<last> command is like the C<break> statement in C (as used in
2342loops); it immediately exits the loop in question. If the LABEL is
2343omitted, the command refers to the innermost enclosing loop. The
2344C<continue> block, if any, is not executed:
2345
4633a7c4
LW
2346 LINE: while (<STDIN>) {
2347 last LINE if /^$/; # exit when done with header
5a964f20 2348 #...
a0d0e21e
LW
2349 }
2350
4968c1e4 2351C<last> cannot be used to exit a block which returns a value such as
2b5ab1e7
TC
2352C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2353a grep() or map() operation.
4968c1e4 2354
6c1372ed
GS
2355Note that a block by itself is semantically identical to a loop
2356that executes once. Thus C<last> can be used to effect an early
2357exit out of such a block.
2358
98293880
JH
2359See also L</continue> for an illustration of how C<last>, C<next>, and
2360C<redo> work.
1d2dff63 2361
a0d0e21e
LW
2362=item lc EXPR
2363
54310121 2364=item lc
bbce6d69 2365
d1be9408 2366Returns a lowercased version of EXPR. This is the internal function
ad0029c4
JH
2367implementing the C<\L> escape in double-quoted strings. Respects
2368current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
983ffd37 2369and L<perlunicode> for more details about locale and Unicode support.
a0d0e21e 2370
7660c0ab 2371If EXPR is omitted, uses C<$_>.
bbce6d69 2372
a0d0e21e
LW
2373=item lcfirst EXPR
2374
54310121 2375=item lcfirst
bbce6d69 2376
ad0029c4
JH
2377Returns the value of EXPR with the first character lowercased. This
2378is the internal function implementing the C<\l> escape in
2379double-quoted strings. Respects current LC_CTYPE locale if C<use
983ffd37
JH
2380locale> in force. See L<perllocale> and L<perlunicode> for more
2381details about locale and Unicode support.
a0d0e21e 2382
7660c0ab 2383If EXPR is omitted, uses C<$_>.
bbce6d69 2384
a0d0e21e
LW
2385=item length EXPR
2386
54310121 2387=item length
bbce6d69 2388
a0ed51b3 2389Returns the length in characters of the value of EXPR. If EXPR is
b76cc8ba 2390omitted, returns length of C<$_>. Note that this cannot be used on
2b5ab1e7
TC
2391an entire array or hash to find out how many elements these have.
2392For that, use C<scalar @array> and C<scalar keys %hash> respectively.
a0d0e21e
LW
2393
2394=item link OLDFILE,NEWFILE
2395
19799a22 2396Creates a new filename linked to the old filename. Returns true for
b76cc8ba 2397success, false otherwise.
a0d0e21e
LW
2398
2399=item listen SOCKET,QUEUESIZE
2400
19799a22 2401Does the same thing that the listen system call does. Returns true if
b76cc8ba 2402it succeeded, false otherwise. See the example in
cea6626f 2403L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
2404
2405=item local EXPR
2406
19799a22 2407You really probably want to be using C<my> instead, because C<local> isn't
b76cc8ba 2408what most people think of as "local". See
13a2d996 2409L<perlsub/"Private Variables via my()"> for details.
2b5ab1e7 2410
5a964f20
TC
2411A local modifies the listed variables to be local to the enclosing
2412block, file, or eval. If more than one value is listed, the list must
2413be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2414for details, including issues with tied arrays and hashes.
a0d0e21e 2415
a0d0e21e
LW
2416=item localtime EXPR
2417
19799a22 2418Converts a time as returned by the time function to a 9-element list
5f05dabc 2419with the time analyzed for the local time zone. Typically used as
a0d0e21e
LW
2420follows:
2421
54310121 2422 # 0 1 2 3 4 5 6 7 8
a0d0e21e
LW
2423 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2424 localtime(time);
2425
48a26b3a
GS
2426All list elements are numeric, and come straight out of the C `struct
2427tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2428specified time. $mday is the day of the month, and $mon is the month
2429itself, in the range C<0..11> with 0 indicating January and 11
2430indicating December. $year is the number of years since 1900. That
2431is, $year is C<123> in year 2023. $wday is the day of the week, with
24320 indicating Sunday and 3 indicating Wednesday. $yday is the day of
874b1813 2433the year, in the range C<0..364> (or C<0..365> in leap years.) $isdst
48a26b3a
GS
2434is true if the specified time occurs during daylight savings time,
2435false otherwise.
2436
2437Note that the $year element is I<not> simply the last two digits of
2438the year. If you assume it is, then you create non-Y2K-compliant
2439programs--and you wouldn't want to do that, would you?
54310121 2440
abd75f24
GS
2441The proper way to get a complete 4-digit year is simply:
2442
2443 $year += 1900;
2444
2445And to get the last two digits of the year (e.g., '01' in 2001) do:
2446
2447 $year = sprintf("%02d", $year % 100);
2448
48a26b3a 2449If EXPR is omitted, C<localtime()> uses the current time (C<localtime(time)>).
a0d0e21e 2450
48a26b3a 2451In scalar context, C<localtime()> returns the ctime(3) value:
a0d0e21e 2452
5f05dabc 2453 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
a0d0e21e 2454
a3cb178b 2455This scalar value is B<not> locale dependent, see L<perllocale>, but
68f8bed4
JH
2456instead a Perl builtin. Also see the C<Time::Local> module
2457(to convert the second, minutes, hours, ... back to seconds since the
2458stroke of midnight the 1st of January 1970, the value returned by
ca6e1c26 2459time()), and the strftime(3) and mktime(3) functions available via the
68f8bed4
JH
2460POSIX module. To get somewhat similar but locale dependent date
2461strings, set up your locale environment variables appropriately
2462(please see L<perllocale>) and try for example:
a3cb178b 2463
5a964f20 2464 use POSIX qw(strftime);
2b5ab1e7 2465 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
a3cb178b
GS
2466
2467Note that the C<%a> and C<%b>, the short forms of the day of the week
2468and the month of the year, may not necessarily be three characters wide.
a0d0e21e 2469
07698885 2470=item lock THING
19799a22
GS
2471
2472This function places an advisory lock on a variable, subroutine,
2473or referenced object contained in I<THING> until the lock goes out
a6d5524e
RGS
2474of scope.
2475
2476lock() is a "weak keyword" : that means that if you've defined a
2477function by this name, this function will be called instead.
2478(However, if you've said C<use Thread>, lock() is always a keyword.)
43d3ddbe 2479See L<Thread>.
19799a22 2480
a0d0e21e
LW
2481=item log EXPR
2482
54310121 2483=item log
bbce6d69 2484
2b5ab1e7
TC
2485Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2486returns log of C<$_>. To get the log of another base, use basic algebra:
19799a22 2487The base-N log of a number is equal to the natural log of that number
2b5ab1e7
TC
2488divided by the natural log of N. For example:
2489
2490 sub log10 {
2491 my $n = shift;
2492 return log($n)/log(10);
b76cc8ba 2493 }
2b5ab1e7
TC
2494
2495See also L</exp> for the inverse operation.
a0d0e21e 2496
a0d0e21e
LW
2497=item lstat EXPR
2498
54310121 2499=item lstat
bbce6d69 2500
19799a22 2501Does the same thing as the C<stat> function (including setting the
5a964f20
TC
2502special C<_> filehandle) but stats a symbolic link instead of the file
2503the symbolic link points to. If symbolic links are unimplemented on
19799a22 2504your system, a normal C<stat> is done.
a0d0e21e 2505
7660c0ab 2506If EXPR is omitted, stats C<$_>.
bbce6d69 2507
a0d0e21e
LW
2508=item m//
2509
2510The match operator. See L<perlop>.
2511
2512=item map BLOCK LIST
2513
2514=item map EXPR,LIST
2515
19799a22
GS
2516Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2517C<$_> to each element) and returns the list value composed of the
2518results of each such evaluation. In scalar context, returns the
2519total number of elements so generated. Evaluates BLOCK or EXPR in
2520list context, so each element of LIST may produce zero, one, or
2521more elements in the returned value.
dd99ebda 2522
a0d0e21e
LW
2523 @chars = map(chr, @nums);
2524
2525translates a list of numbers to the corresponding characters. And
2526
4633a7c4 2527 %hash = map { getkey($_) => $_ } @array;
a0d0e21e
LW
2528
2529is just a funny way to write
2530
2531 %hash = ();
2532 foreach $_ (@array) {
4633a7c4 2533 $hash{getkey($_)} = $_;
a0d0e21e
LW
2534 }
2535
be3174d2
GS
2536Note that C<$_> is an alias to the list value, so it can be used to
2537modify the elements of the LIST. While this is useful and supported,
2538it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
2539Using a regular C<foreach> loop for this purpose would be clearer in
2540most cases. See also L</grep> for an array composed of those items of
2541the original list for which the BLOCK or EXPR evaluates to true.
fb73857a 2542
205fdb4d
NC
2543C<{> starts both hash references and blocks, so C<map { ...> could be either
2544the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look
2545ahead for the closing C<}> it has to take a guess at which its dealing with
2546based what it finds just after the C<{>. Usually it gets it right, but if it
2547doesn't it won't realize something is wrong until it gets to the C<}> and
2548encounters the missing (or unexpected) comma. The syntax error will be
2549reported close to the C<}> but you'll need to change something near the C<{>
2550such as using a unary C<+> to give perl some help:
2551
2552 %hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong
2553 %hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right
2554 %hash = map { ("\L$_", 1) } @array # this also works
2555 %hash = map { lc($_), 1 } @array # as does this.
2556 %hash = map +( lc($_), 1 ), @array # this is EXPR and works!
cea6626f 2557
205fdb4d
NC
2558 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
2559
2560or to force an anon hash constructor use C<+{>
2561
2562 @hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end
2563
2564and you get list of anonymous hashes each with only 1 entry.
2565
19799a22 2566=item mkdir FILENAME,MASK
a0d0e21e 2567
5a211162
GS
2568=item mkdir FILENAME
2569
0591cd52 2570Creates the directory specified by FILENAME, with permissions
19799a22
GS
2571specified by MASK (as modified by C<umask>). If it succeeds it
2572returns true, otherwise it returns false and sets C<$!> (errno).
5a211162 2573If omitted, MASK defaults to 0777.
0591cd52 2574
19799a22 2575In general, it is better to create directories with permissive MASK,
0591cd52 2576and let the user modify that with their C<umask>, than it is to supply
19799a22 2577a restrictive MASK and give the user no way to be more permissive.
0591cd52
NT
2578The exceptions to this rule are when the file or directory should be
2579kept private (mail files, for instance). The perlfunc(1) entry on
19799a22 2580C<umask> discusses the choice of MASK in more detail.
a0d0e21e 2581
cc1852e8
JH
2582Note that according to the POSIX 1003.1-1996 the FILENAME may have any
2583number of trailing slashes. Some operating and filesystems do not get
2584this right, so Perl automatically removes all trailing slashes to keep
2585everyone happy.
2586
a0d0e21e
LW
2587=item msgctl ID,CMD,ARG
2588
f86cebdf 2589Calls the System V IPC function msgctl(2). You'll probably have to say
0ade1984
JH
2590
2591 use IPC::SysV;
2592
7660c0ab
A
2593first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2594then ARG must be a variable which will hold the returned C<msqid_ds>
951ba7fe
GS
2595structure. Returns like C<ioctl>: the undefined value for error,
2596C<"0 but true"> for zero, or the actual return value otherwise. See also
4755096e 2597L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation.
a0d0e21e
LW
2598
2599=item msgget KEY,FLAGS
2600
f86cebdf 2601Calls the System V IPC function msgget(2). Returns the message queue
4755096e
GS
2602id, or the undefined value if there is an error. See also
2603L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation.
a0d0e21e 2604
a0d0e21e
LW
2605=item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2606
2607Calls the System V IPC function msgrcv to receive a message from
2608message queue ID into variable VAR with a maximum message size of
41d6edb2
JH
2609SIZE. Note that when a message is received, the message type as a
2610native long integer will be the first thing in VAR, followed by the
2611actual message. This packing may be opened with C<unpack("l! a*")>.
2612Taints the variable. Returns true if successful, or false if there is
4755096e
GS
2613an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and
2614C<IPC::SysV::Msg> documentation.
41d6edb2
JH
2615
2616=item msgsnd ID,MSG,FLAGS
2617
2618Calls the System V IPC function msgsnd to send the message MSG to the
2619message queue ID. MSG must begin with the native long integer message
2620type, and be followed by the length of the actual message, and finally
2621the message itself. This kind of packing can be achieved with
2622C<pack("l! a*", $type, $message)>. Returns true if successful,
2623or false if there is an error. See also C<IPC::SysV>
2624and C<IPC::SysV::Msg> documentation.
a0d0e21e
LW
2625
2626=item my EXPR
2627
09bef843
SB
2628=item my EXPR : ATTRIBUTES
2629
19799a22
GS
2630A C<my> declares the listed variables to be local (lexically) to the
2631enclosing block, file, or C<eval>. If
5f05dabc 2632more than one value is listed, the list must be placed in parentheses. See
cb1a09d0 2633L<perlsub/"Private Variables via my()"> for details.
4633a7c4 2634
a0d0e21e
LW
2635=item next LABEL
2636
2637=item next
2638
2639The C<next> command is like the C<continue> statement in C; it starts
2640the next iteration of the loop:
2641
4633a7c4
LW
2642 LINE: while (<STDIN>) {
2643 next LINE if /^#/; # discard comments
5a964f20 2644 #...
a0d0e21e
LW
2645 }
2646
2647Note that if there were a C<continue> block on the above, it would get
2648executed even on discarded lines. If the LABEL is omitted, the command
2649refers to the innermost enclosing loop.
2650
4968c1e4 2651C<next> cannot be used to exit a block which returns a value such as
2b5ab1e7
TC
2652C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2653a grep() or map() operation.
4968c1e4 2654
6c1372ed
GS
2655Note that a block by itself is semantically identical to a loop
2656that executes once. Thus C<next> will exit such a block early.
2657
98293880
JH
2658See also L</continue> for an illustration of how C<last>, C<next>, and
2659C<redo> work.
1d2dff63 2660
4a66ea5a
RGS
2661=item no Module VERSION LIST
2662
2663=item no Module VERSION
2664
a0d0e21e
LW
2665=item no Module LIST
2666
4a66ea5a
RGS
2667=item no Module
2668
7660c0ab 2669See the L</use> function, which C<no> is the opposite of.
a0d0e21e
LW
2670
2671=item oct EXPR
2672
54310121 2673=item oct
bbce6d69 2674
4633a7c4 2675Interprets EXPR as an octal string and returns the corresponding
4f19785b
WSI
2676value. (If EXPR happens to start off with C<0x>, interprets it as a
2677hex string. If EXPR starts off with C<0b>, it is interpreted as a
53305cf1
NC
2678binary string. Leading whitespace is ignored in all three cases.)
2679The following will handle decimal, binary, octal, and hex in the standard
2680Perl or C notation:
a0d0e21e
LW
2681
2682 $val = oct($val) if $val =~ /^0/;
2683
19799a22
GS
2684If EXPR is omitted, uses C<$_>. To go the other way (produce a number
2685in octal), use sprintf() or printf():
2686
2687 $perms = (stat("filename"))[2] & 07777;
2688 $oct_perms = sprintf "%lo", $perms;
2689
2690The oct() function is commonly used when a string such as C<644> needs
2691to be converted into a file mode, for example. (Although perl will
2692automatically convert strings into numbers as needed, this automatic
2693conversion assumes base 10.)
a0d0e21e
LW
2694
2695=item open FILEHANDLE,EXPR
2696
68bd7414
NIS
2697=item open FILEHANDLE,MODE,EXPR
2698
2699=item open FILEHANDLE,MODE,EXPR,LIST
2700
ba964c95
T
2701=item open FILEHANDLE,MODE,REFERENCE
2702
a0d0e21e
LW
2703=item open FILEHANDLE
2704
2705Opens the file whose filename is given by EXPR, and associates it with
ed53a2bb
JH
2706FILEHANDLE.
2707
2708(The following is a comprehensive reference to open(): for a gentler
2709introduction you may consider L<perlopentut>.)
2710
2711If FILEHANDLE is an undefined lexical (C<my>) variable the variable is
2712assigned a reference to a new anonymous filehandle, otherwise if
2713FILEHANDLE is an expression, its value is used as the name of the real
2714filehandle wanted. (This is considered a symbolic reference, so C<use
2715strict 'refs'> should I<not> be in effect.)
2716
2717If EXPR is omitted, the scalar variable of the same name as the
2718FILEHANDLE contains the filename. (Note that lexical variables--those
2719declared with C<my>--will not work for this purpose; so if you're
2720using C<my>, specify EXPR in your call to open.)
2721
2722If three or more arguments are specified then the mode of opening and
2723the file name are separate. If MODE is C<< '<' >> or nothing, the file
2724is opened for input. If MODE is C<< '>' >>, the file is truncated and
2725opened for output, being created if necessary. If MODE is C<<< '>>' >>>,
b76cc8ba 2726the file is opened for appending, again being created if necessary.
5a964f20 2727
ed53a2bb
JH
2728You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to
2729indicate that you want both read and write access to the file; thus
2730C<< '+<' >> is almost always preferred for read/write updates--the C<<
2731'+>' >> mode would clobber the file first. You can't usually use
2732either read-write mode for updating textfiles, since they have
2733variable length records. See the B<-i> switch in L<perlrun> for a
2734better approach. The file is created with permissions of C<0666>
2735modified by the process' C<umask> value.
2736
2737These various prefixes correspond to the fopen(3) modes of C<'r'>,
2738C<'r+'>, C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
5f05dabc 2739
6170680b
IZ
2740In the 2-arguments (and 1-argument) form of the call the mode and
2741filename should be concatenated (in this order), possibly separated by
68bd7414
NIS
2742spaces. It is possible to omit the mode in these forms if the mode is
2743C<< '<' >>.
6170680b 2744
7660c0ab 2745If the filename begins with C<'|'>, the filename is interpreted as a
5a964f20 2746command to which output is to be piped, and if the filename ends with a
f244e06d
GS
2747C<'|'>, the filename is interpreted as a command which pipes output to
2748us. See L<perlipc/"Using open() for IPC">
19799a22 2749for more examples of this. (You are not allowed to C<open> to a command
5a964f20 2750that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
4a4eefd0
GS
2751and L<perlipc/"Bidirectional Communication with Another Process">
2752for alternatives.)
cb1a09d0 2753
ed53a2bb
JH
2754For three or more arguments if MODE is C<'|-'>, the filename is
2755interpreted as a command to which output is to be piped, and if MODE
2756is C<'-|'>, the filename is interpreted as a command which pipes
2757output to us. In the 2-arguments (and 1-argument) form one should
2758replace dash (C<'-'>) with the command.
2759See L<perlipc/"Using open() for IPC"> for more examples of this.
2760(You are not allowed to C<open> to a command that pipes both in I<and>
2761out, but see L<IPC::Open2>, L<IPC::Open3>, and
2762L<perlipc/"Bidirectional Communication"> for alternatives.)
2763
2764In the three-or-more argument form of pipe opens, if LIST is specified
2765(extra arguments after the command name) then LIST becomes arguments
2766to the command invoked if the platform supports it. The meaning of
2767C<open> with more than three arguments for non-pipe modes is not yet
2768specified. Experimental "layers" may give extra LIST arguments
2769meaning.
6170680b
IZ
2770
2771In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
b76cc8ba 2772and opening C<< '>-' >> opens STDOUT.
6170680b 2773
ed53a2bb
JH
2774You may use the three-argument form of open to specify
2775I<I/O disciplines> that affect how the input and output
9124316e 2776are processed: see L</binmode> and L<open>. For example
7207e29d 2777
9124316e
JH
2778 open(FH, "<:utf8", "file")
2779
2780will open the UTF-8 encoded file containing Unicode characters,
2781see L<perluniintro>.
ed53a2bb
JH
2782
2783Open returns nonzero upon success, the undefined value otherwise. If
2784the C<open> involved a pipe, the return value happens to be the pid of
2785the subprocess.
cb1a09d0 2786
ed53a2bb
JH
2787If you're running Perl on a system that distinguishes between text
2788files and binary files, then you should check out L</binmode> for tips
2789for dealing with this. The key distinction between systems that need
2790C<binmode> and those that don't is their text file formats. Systems
2791like Unix, MacOS, and Plan9, which delimit lines with a single
2792character, and which encode that character in C as C<"\n">, do not
2793need C<binmode>. The rest need it.
cb1a09d0 2794
ed53a2bb
JH
2795In the three argument form MODE may also contain a list of IO "layers"
2796(see L<open> and L<PerlIO> for more details) to be applied to the
2797handle. This can be used to achieve the effect of C<binmode> as well
2798as more complex behaviours.
68bd7414 2799
fb73857a 2800When opening a file, it's usually a bad idea to continue normal execution
19799a22
GS
2801if the request failed, so C<open> is frequently used in connection with
2802C<die>. Even if C<die> won't do what you want (say, in a CGI script,
fb73857a 2803where you want to make a nicely formatted error message (but there are
5a964f20 2804modules that can help with that problem)) you should always check
19799a22 2805the return value from opening a file. The infrequent exception is when
fb73857a 2806working with an unopened filehandle is actually what you want to do.
2807
ed53a2bb
JH
2808As a special case the 3 arg form with a read/write mode and the third
2809argument being C<undef>:
b76cc8ba
NIS
2810
2811 open(TMP, "+>", undef) or die ...
2812
2813opens a filehandle to an anonymous temporary file.
2814
ba964c95
T
2815File handles can be opened to "in memory" files held in Perl scalars via:
2816
b996200f
SB
2817 open($fh, '>', \$variable) || ..
2818
2819Though if you try to re-open C<STDOUT> or C<STDERR> as an "in memory"
2820file, you have to close it first:
2821
2822 close STDOUT;
2823 open STDOUT, '>', \$variable or die "Can't open STDOUT: $!";
ba964c95 2824
cb1a09d0 2825Examples:
a0d0e21e
LW
2826
2827 $ARTICLE = 100;
2828 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2829 while (<ARTICLE>) {...
2830
6170680b 2831 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
fb73857a 2832 # if the open fails, output is discarded
a0d0e21e 2833
6170680b 2834 open(DBASE, '+<', 'dbase.mine') # open for update
fb73857a 2835 or die "Can't open 'dbase.mine' for update: $!";
cb1a09d0 2836
6170680b
IZ
2837 open(DBASE, '+<dbase.mine') # ditto
2838 or die "Can't open 'dbase.mine' for update: $!";
2839
2840 open(ARTICLE, '-|', "caesar <$article") # decrypt article
fb73857a 2841 or die "Can't start caesar: $!";
a0d0e21e 2842
6170680b
IZ
2843 open(ARTICLE, "caesar <$article |") # ditto
2844 or die "Can't start caesar: $!";
2845
2846 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
fb73857a 2847 or die "Can't start sort: $!";
a0d0e21e 2848
ba964c95
T
2849 # in memory files
2850 open(MEMORY,'>', \$var)
2851 or die "Can't open memory file: $!";
2852 print MEMORY "foo!\n"; # output will end up in $var
2853
a0d0e21e
LW
2854 # process argument list of files along with any includes
2855
2856 foreach $file (@ARGV) {
2857 process($file, 'fh00');
2858 }
2859
2860 sub process {
5a964f20 2861 my($filename, $input) = @_;
a0d0e21e
LW
2862 $input++; # this is a string increment
2863 unless (open($input, $filename)) {
2864 print STDERR "Can't open $filename: $!\n";
2865 return;
2866 }
2867
5a964f20 2868 local $_;
a0d0e21e
LW
2869 while (<$input>) { # note use of indirection
2870 if (/^#include "(.*)"/) {
2871 process($1, $input);
2872 next;
2873 }
5a964f20 2874 #... # whatever
a0d0e21e
LW
2875 }
2876 }
2877
2878You may also, in the Bourne shell tradition, specify an EXPR beginning
61eff3bc 2879with C<< '>&' >>, in which case the rest of the string is interpreted as the
5a964f20 2880name of a filehandle (or file descriptor, if numeric) to be
61eff3bc
JH
2881duped and opened. You may use C<&> after C<< > >>, C<<< >> >>>,
2882C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>. The
a0d0e21e 2883mode you specify should match the mode of the original filehandle.
184e9718 2884(Duping a filehandle does not take into account any existing contents of
9124316e 2885IO buffers.) If you use the 3 arg form then you can pass either a number,
b76cc8ba 2886the name of a filehandle or the normal "reference to a glob".
6170680b 2887
eae1b76b
SB
2888Here is a script that saves, redirects, and restores C<STDOUT> and
2889C<STDERR> using various methods:
a0d0e21e
LW
2890
2891 #!/usr/bin/perl
eae1b76b
SB
2892 open my $oldout, ">&STDOUT" or die "Can't dup STDOUT: $!";
2893 open OLDERR, ">&", \*STDERR or die "Can't dup STDERR: $!";
818c4caa 2894
eae1b76b
SB
2895 open STDOUT, '>', "foo.out" or die "Can't redirect STDOUT: $!";
2896 open STDERR, ">&STDOUT" or die "Can't dup STDOUT: $!";
a0d0e21e 2897
eae1b76b
SB
2898 select STDERR; $| = 1; # make unbuffered
2899 select STDOUT; $| = 1; # make unbuffered
a0d0e21e
LW
2900
2901 print STDOUT "stdout 1\n"; # this works for
2902 print STDERR "stderr 1\n"; # subprocesses too
2903
eae1b76b
SB
2904 close STDOUT;
2905 close STDERR;
a0d0e21e 2906
eae1b76b
SB
2907 open STDOUT, ">&", $oldout or die "Can't dup \$oldout: $!";
2908 open STDERR, ">&OLDERR" or die "Can't dup OLDERR: $!";
a0d0e21e
LW
2909
2910 print STDOUT "stdout 2\n";
2911 print STDERR "stderr 2\n";
2912
df632fdf
JH
2913If you specify C<< '<&=N' >>, where C<N> is a number, then Perl will
2914do an equivalent of C's C<fdopen> of that file descriptor; this is
2915more parsimonious of file descriptors. For example:
a0d0e21e
LW
2916
2917 open(FILEHANDLE, "<&=$fd")
df632fdf 2918
b76cc8ba 2919or
df632fdf 2920
b76cc8ba 2921 open(FILEHANDLE, "<&=", $fd)
a0d0e21e 2922
df632fdf
JH
2923Note that if Perl is using the standard C libraries' fdopen() then on
2924many UNIX systems, fdopen() is known to fail when file descriptors
4af147f6 2925exceed a certain value, typically 255. If you need more file
b76cc8ba 2926descriptors than that, consider rebuilding Perl to use the C<PerlIO>.
4af147f6 2927
df632fdf
JH
2928You can see whether Perl has been compiled with PerlIO or not by
2929running C<perl -V> and looking for C<useperlio=> line. If C<useperlio>
2930is C<define>, you have PerlIO, otherwise you don't.
2931
6170680b
IZ
2932If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
2933with 2-arguments (or 1-argument) form of open(), then
a0d0e21e 2934there is an implicit fork done, and the return value of open is the pid
7660c0ab 2935of the child within the parent process, and C<0> within the child
184e9718 2936process. (Use C<defined($pid)> to determine whether the open was successful.)
a0d0e21e
LW
2937The filehandle behaves normally for the parent, but i/o to that
2938filehandle is piped from/to the STDOUT/STDIN of the child process.
2939In the child process the filehandle isn't opened--i/o happens from/to
2940the new STDOUT or STDIN. Typically this is used like the normal
2941piped open when you want to exercise more control over just how the
2942pipe command gets executed, such as when you are running setuid, and
54310121 2943don't want to have to scan shell commands for metacharacters.
6170680b 2944The following triples are more or less equivalent:
a0d0e21e
LW
2945
2946 open(FOO, "|tr '[a-z]' '[A-Z]'");
6170680b
IZ
2947 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
2948 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
b76cc8ba 2949 open(FOO, '|-', "tr", '[a-z]', '[A-Z]');
a0d0e21e
LW
2950
2951 open(FOO, "cat -n '$file'|");
6170680b
IZ
2952 open(FOO, '-|', "cat -n '$file'");
2953 open(FOO, '-|') || exec 'cat', '-n', $file;
b76cc8ba
NIS
2954 open(FOO, '-|', "cat", '-n', $file);
2955
2956The last example in each block shows the pipe as "list form", which is
2957not yet supported on all platforms.
a0d0e21e 2958
4633a7c4
LW
2959See L<perlipc/"Safe Pipe Opens"> for more examples of this.
2960
0f897271
GS
2961Beginning with v5.6.0, Perl will attempt to flush all files opened for
2962output before any operation that may do a fork, but this may not be
2963supported on some platforms (see L<perlport>). To be safe, you may need
2964to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
2965of C<IO::Handle> on any open handles.
2966
ed53a2bb
JH
2967On systems that support a close-on-exec flag on files, the flag will
2968be set for the newly opened file descriptor as determined by the value
2969of $^F. See L<perlvar/$^F>.
a0d0e21e 2970
0dccf244
CS
2971Closing any piped filehandle causes the parent process to wait for the
2972child to finish, and returns the status value in C<$?>.
2973
ed53a2bb
JH
2974The filename passed to 2-argument (or 1-argument) form of open() will
2975have leading and trailing whitespace deleted, and the normal
2976redirection characters honored. This property, known as "magic open",
5a964f20 2977can often be used to good effect. A user could specify a filename of
7660c0ab 2978F<"rsh cat file |">, or you could change certain filenames as needed:
5a964f20
TC
2979
2980 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
2981 open(FH, $filename) or die "Can't open $filename: $!";
2982
6170680b
IZ
2983Use 3-argument form to open a file with arbitrary weird characters in it,
2984
2985 open(FOO, '<', $file);
2986
2987otherwise it's necessary to protect any leading and trailing whitespace:
5a964f20
TC
2988
2989 $file =~ s#^(\s)#./$1#;
2990 open(FOO, "< $file\0");
2991
a31a806a 2992(this may not work on some bizarre filesystems). One should
106325ad 2993conscientiously choose between the I<magic> and 3-arguments form
6170680b
IZ
2994of open():
2995
2996 open IN, $ARGV[0];
2997
2998will allow the user to specify an argument of the form C<"rsh cat file |">,
2999but will not work on a filename which happens to have a trailing space, while
3000
3001 open IN, '<', $ARGV[0];
3002
3003will have exactly the opposite restrictions.
3004
19799a22 3005If you want a "real" C C<open> (see L<open(2)> on your system), then you
6170680b
IZ
3006should use the C<sysopen> function, which involves no such magic (but
3007may use subtly different filemodes than Perl open(), which is mapped
3008to C fopen()). This is
5a964f20
TC
3009another way to protect your filenames from interpretation. For example:
3010
3011 use IO::Handle;
3012 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
3013 or die "sysopen $path: $!";
3014 $oldfh = select(HANDLE); $| = 1; select($oldfh);
38762f02 3015 print HANDLE "stuff $$\n";
5a964f20
TC
3016 seek(HANDLE, 0, 0);
3017 print "File contains: ", <HANDLE>;
3018
7660c0ab
A
3019Using the constructor from the C<IO::Handle> package (or one of its
3020subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
5a964f20
TC
3021filehandles that have the scope of whatever variables hold references to
3022them, and automatically close whenever and however you leave that scope:
c07a80fd 3023
5f05dabc 3024 use IO::File;
5a964f20 3025 #...
c07a80fd 3026 sub read_myfile_munged {
3027 my $ALL = shift;
5f05dabc 3028 my $handle = new IO::File;
c07a80fd 3029 open($handle, "myfile") or die "myfile: $!";
3030 $first = <$handle>
3031 or return (); # Automatically closed here.
3032 mung $first or die "mung failed"; # Or here.
3033 return $first, <$handle> if $ALL; # Or here.
3034 $first; # Or here.
3035 }
3036
b687b08b 3037See L</seek> for some details about mixing reading and writing.
a0d0e21e
LW
3038
3039=item opendir DIRHANDLE,EXPR
3040
19799a22
GS
3041Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
3042C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
a0d0e21e
LW
3043DIRHANDLEs have their own namespace separate from FILEHANDLEs.
3044
3045=item ord EXPR
3046
54310121 3047=item ord
bbce6d69 3048
121910a4
JH
3049Returns the numeric (the native 8-bit encoding, like ASCII or EBCDIC,
3050or Unicode) value of the first character of EXPR. If EXPR is omitted,
3051uses C<$_>.
3052
3053For the reverse, see L</chr>.
3054See L<perlunicode> and L<encoding> for more about Unicode.
a0d0e21e 3055
77ca0c92
LW
3056=item our EXPR
3057
9969eac4
BS
3058=item our EXPR : ATTRIBUTES
3059
77ca0c92
LW
3060An C<our> declares the listed variables to be valid globals within
3061the enclosing block, file, or C<eval>. That is, it has the same
3062scoping rules as a "my" declaration, but does not create a local
3063variable. If more than one value is listed, the list must be placed
3064in parentheses. The C<our> declaration has no semantic effect unless
3065"use strict vars" is in effect, in which case it lets you use the
3066declared global variable without qualifying it with a package name.
3067(But only within the lexical scope of the C<our> declaration. In this
3068it differs from "use vars", which is package scoped.)
3069
f472eb5c
GS
3070An C<our> declaration declares a global variable that will be visible
3071across its entire lexical scope, even across package boundaries. The
3072package in which the variable is entered is determined at the point
3073of the declaration, not at the point of use. This means the following
3074behavior holds:
3075
3076 package Foo;
3077 our $bar; # declares $Foo::bar for rest of lexical scope
3078 $bar = 20;
3079
3080 package Bar;
3081 print $bar; # prints 20
3082
3083Multiple C<our> declarations in the same lexical scope are allowed
3084if they are in different packages. If they happened to be in the same
3085package, Perl will emit warnings if you have asked for them.
3086
3087 use warnings;
3088 package Foo;
3089 our $bar; # declares $Foo::bar for rest of lexical scope
3090 $bar = 20;
3091
3092 package Bar;
3093 our $bar = 30; # declares $Bar::bar for rest of lexical scope
3094 print $bar; # prints 30
3095
3096 our $bar; # emits warning
3097
9969eac4
BS
3098An C<our> declaration may also have a list of attributes associated
3099with it. B<WARNING>: This is an experimental feature that may be
3100changed or removed in future releases of Perl. It should not be
3101relied upon.
3102
51d2bbcc 3103The only currently recognized attribute is C<unique> which indicates
9969eac4
BS
3104that a single copy of the global is to be used by all interpreters
3105should the program happen to be running in a multi-interpreter
3106environment. (The default behaviour would be for each interpreter to
3107have its own copy of the global.) In such an environment, this
3108attribute also has the effect of making the global readonly.
3109Examples:
3110
51d2bbcc
JH
3111 our @EXPORT : unique = qw(foo);
3112 our %EXPORT_TAGS : unique = (bar => [qw(aa bb cc)]);
3113 our $VERSION : unique = "1.00";
9969eac4
BS
3114
3115Multi-interpreter environments can come to being either through the
3116fork() emulation on Windows platforms, or by embedding perl in a
51d2bbcc 3117multi-threaded application. The C<unique> attribute does nothing in
9969eac4
BS
3118all other environments.
3119
a0d0e21e
LW
3120=item pack TEMPLATE,LIST
3121
2b6c5635
GS
3122Takes a LIST of values and converts it into a string using the rules
3123given by the TEMPLATE. The resulting string is the concatenation of
3124the converted values. Typically, each converted value looks
3125like its machine-level representation. For example, on 32-bit machines
3126a converted integer may be represented by a sequence of 4 bytes.
3127
18529408
IZ
3128The TEMPLATE is a sequence of characters that give the order and type
3129of values, as follows:
a0d0e21e 3130
5a929a98 3131 a A string with arbitrary binary data, will be null padded.
121910a4
JH
3132 A A text (ASCII) string, will be space padded.
3133 Z A null terminated (ASCIZ) string, will be null padded.
5a929a98 3134
2b6c5635
GS
3135 b A bit string (ascending bit order inside each byte, like vec()).
3136 B A bit string (descending bit order inside each byte).
a0d0e21e
LW
3137 h A hex string (low nybble first).
3138 H A hex string (high nybble first).
3139
3140 c A signed char value.
a0ed51b3 3141 C An unsigned char value. Only does bytes. See U for Unicode.
96e4d5b1 3142
a0d0e21e
LW
3143 s A signed short value.
3144 S An unsigned short value.
96e4d5b1 3145 (This 'short' is _exactly_ 16 bits, which may differ from
851646ae
JH
3146 what a local C compiler calls 'short'. If you want
3147 native-length shorts, use the '!' suffix.)
96e4d5b1 3148
a0d0e21e
LW
3149 i A signed integer value.
3150 I An unsigned integer value.
19799a22 3151 (This 'integer' is _at_least_ 32 bits wide. Its exact
f86cebdf
GS
3152 size depends on what a local C compiler calls 'int',
3153 and may even be larger than the 'long' described in
3154 the next item.)
96e4d5b1 3155
a0d0e21e
LW
3156 l A signed long value.
3157 L An unsigned long value.
96e4d5b1 3158 (This 'long' is _exactly_ 32 bits, which may differ from
851646ae
JH
3159 what a local C compiler calls 'long'. If you want
3160 native-length longs, use the '!' suffix.)
a0d0e21e 3161
5d11dd56
MG
3162 n An unsigned short in "network" (big-endian) order.
3163 N An unsigned long in "network" (big-endian) order.
3164 v An unsigned short in "VAX" (little-endian) order.
3165 V An unsigned long in "VAX" (little-endian) order.
96e4d5b1 3166 (These 'shorts' and 'longs' are _exactly_ 16 bits and
3167 _exactly_ 32 bits, respectively.)
a0d0e21e 3168
dae0da7a
JH
3169 q A signed quad (64-bit) value.
3170 Q An unsigned quad value.
851646ae
JH
3171 (Quads are available only if your system supports 64-bit
3172 integer values _and_ if Perl has been compiled to support those.
dae0da7a
JH
3173 Causes a fatal error otherwise.)
3174
92d41999
JH
3175 j A signed integer value (a Perl internal integer, IV).
3176 J An unsigned integer value (a Perl internal unsigned integer, UV).
3177
a0d0e21e
LW
3178 f A single-precision float in the native format.
3179 d A double-precision float in the native format.
3180
92d41999
JH
3181 F A floating point value in the native native format
3182 (a Perl internal floating point value, NV).
3183 D A long double-precision float in the native format.
3184 (Long doubles are available only if your system supports long
3185 double values _and_ if Perl has been compiled to support those.
3186 Causes a fatal error otherwise.)
3187
a0d0e21e
LW
3188 p A pointer to a null-terminated string.
3189 P A pointer to a structure (fixed-length string).
3190
3191 u A uuencoded string.
ad0029c4
JH
3192 U A Unicode character number. Encodes to UTF-8 internally
3193 (or UTF-EBCDIC in EBCDIC platforms).
a0d0e21e 3194
96e4d5b1 3195 w A BER compressed integer. Its bytes represent an unsigned
f86cebdf
GS
3196 integer in base 128, most significant digit first, with as
3197 few digits as possible. Bit eight (the high bit) is set
3198 on each byte except the last.
def98dd4 3199
a0d0e21e
LW
3200 x A null byte.
3201 X Back up a byte.
3202 @ Null fill to absolute position.
206947d2 3203 ( Start of a ()-group.
a0d0e21e 3204
5a929a98
VU
3205The following rules apply:
3206
3207=over 8
3208
3209=item *
3210
5a964f20 3211Each letter may optionally be followed by a number giving a repeat
951ba7fe 3212count. With all types except C<a>, C<A>, C<Z>, C<b>, C<B>, C<h>,
206947d2
IZ
3213C<H>, C<@>, C<x>, C<X> and C<P> the pack function will gobble up that
3214many values from the LIST. A C<*> for the repeat count means to use
3215however many items are left, except for C<@>, C<x>, C<X>, where it is
3216equivalent to C<0>, and C<u>, where it is equivalent to 1 (or 45, what
3217is the same). A numeric repeat count may optionally be enclosed in
3218brackets, as in C<pack 'C[80]', @arr>.
3219
3220One can replace the numeric repeat count by a template enclosed in brackets;
3221then the packed length of this template in bytes is used as a count.
62f95557
IZ
3222For example, C<x[L]> skips a long (it skips the number of bytes in a long);
3223the template C<$t X[$t] $t> unpack()s twice what $t unpacks.
3224If the template in brackets contains alignment commands (such as C<x![d]>),
3225its packed length is calculated as if the start of the template has the maximal
3226possible alignment.
2b6c5635 3227
951ba7fe 3228When used with C<Z>, C<*> results in the addition of a trailing null
2b6c5635
GS
3229byte (so the packed result will be one longer than the byte C<length>
3230of the item).
3231
951ba7fe 3232The repeat count for C<u> is interpreted as the maximal number of bytes
2b6c5635 3233to encode per line of output, with 0 and 1 replaced by 45.
5a929a98
VU
3234
3235=item *
3236
951ba7fe 3237The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
5a929a98 3238string of length count, padding with nulls or spaces as necessary. When
951ba7fe
GS
3239unpacking, C<A> strips trailing spaces and nulls, C<Z> strips everything
3240after the first null, and C<a> returns data verbatim. When packing,
3241C<a>, and C<Z> are equivalent.
2b6c5635
GS
3242
3243If the value-to-pack is too long, it is truncated. If too long and an
951ba7fe
GS
3244explicit count is provided, C<Z> packs only C<$count-1> bytes, followed
3245by a null byte. Thus C<Z> always packs a trailing null byte under
2b6c5635 3246all circumstances.
5a929a98
VU
3247
3248=item *
3249
951ba7fe 3250Likewise, the C<b> and C<B> fields pack a string that many bits long.
c73032f5
IZ
3251Each byte of the input field of pack() generates 1 bit of the result.
3252Each result bit is based on the least-significant bit of the corresponding
3253input byte, i.e., on C<ord($byte)%2>. In particular, bytes C<"0"> and
3254C<"1"> generate bits 0 and 1, as do bytes C<"\0"> and C<"\1">.
3255
3256Starting from the beginning of the input string of pack(), each 8-tuple
951ba7fe 3257of bytes is converted to 1 byte of output. With format C<b>
c73032f5 3258the first byte of the 8-tuple determines the least-significant bit of a
951ba7fe 3259byte, and with format C<B> it determines the most-significant bit of
c73032f5
IZ
3260a byte.
3261
3262If the length of the input string is not exactly divisible by 8, the
3263remainder is packed as if the input string were padded by null bytes
3264at the end. Similarly, during unpack()ing the "extra" bits are ignored.
3265
3266If the input string of pack() is longer than needed, extra bytes are ignored.
2b6c5635
GS
3267A C<*> for the repeat count of pack() means to use all the bytes of
3268the input field. On unpack()ing the bits are converted to a string
3269of C<"0">s and C<"1">s.
5a929a98
VU
3270
3271=item *
3272
951ba7fe 3273The C<h> and C<H> fields pack a string that many nybbles (4-bit groups,
851646ae 3274representable as hexadecimal digits, 0-9a-f) long.
5a929a98 3275
c73032f5
IZ
3276Each byte of the input field of pack() generates 4 bits of the result.
3277For non-alphabetical bytes the result is based on the 4 least-significant
3278