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1=head1 NAME
2
3perlfunc - Perl builtin functions
4
5=head1 DESCRIPTION
6
7The functions in this section can serve as terms in an expression.
8They fall into two major categories: list operators and named unary
9operators. These differ in their precedence relationship with a
10following comma. (See the precedence table in L<perlop>.) List
11operators take more than one argument, while unary operators can never
12take more than one argument. Thus, a comma terminates the argument of
13a unary operator, but merely separates the arguments of a list
14operator. A unary operator generally provides a scalar context to its
2b5ab1e7 15argument, while a list operator may provide either scalar or list
a0d0e21e 16contexts for its arguments. If it does both, the scalar arguments will
5f05dabc 17be first, and the list argument will follow. (Note that there can ever
0f31cffe 18be only one such list argument.) For instance, splice() has three scalar
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19arguments followed by a list, whereas gethostbyname() has four scalar
20arguments.
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21
22In the syntax descriptions that follow, list operators that expect a
23list (and provide list context for the elements of the list) are shown
24with LIST as an argument. Such a list may consist of any combination
25of scalar arguments or list values; the list values will be included
26in the list as if each individual element were interpolated at that
27point in the list, forming a longer single-dimensional list value.
28Elements of the LIST should be separated by commas.
29
30Any function in the list below may be used either with or without
31parentheses around its arguments. (The syntax descriptions omit the
5f05dabc 32parentheses.) If you use the parentheses, the simple (but occasionally
19799a22 33surprising) rule is this: It I<looks> like a function, therefore it I<is> a
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34function, and precedence doesn't matter. Otherwise it's a list
35operator or unary operator, and precedence does matter. And whitespace
36between the function and left parenthesis doesn't count--so you need to
37be careful sometimes:
38
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39 print 1+2+4; # Prints 7.
40 print(1+2) + 4; # Prints 3.
41 print (1+2)+4; # Also prints 3!
42 print +(1+2)+4; # Prints 7.
43 print ((1+2)+4); # Prints 7.
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44
45If you run Perl with the B<-w> switch it can warn you about this. For
46example, the third line above produces:
47
48 print (...) interpreted as function at - line 1.
49 Useless use of integer addition in void context at - line 1.
50
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51A few functions take no arguments at all, and therefore work as neither
52unary nor list operators. These include such functions as C<time>
53and C<endpwent>. For example, C<time+86_400> always means
54C<time() + 86_400>.
55
a0d0e21e 56For functions that can be used in either a scalar or list context,
54310121 57nonabortive failure is generally indicated in a scalar context by
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58returning the undefined value, and in a list context by returning the
59null list.
60
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61Remember the following important rule: There is B<no rule> that relates
62the behavior of an expression in list context to its behavior in scalar
63context, or vice versa. It might do two totally different things.
a0d0e21e 64Each operator and function decides which sort of value it would be most
2b5ab1e7 65appropriate to return in scalar context. Some operators return the
5a964f20 66length of the list that would have been returned in list context. Some
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67operators return the first value in the list. Some operators return the
68last value in the list. Some operators return a count of successful
69operations. In general, they do what you want, unless you want
70consistency.
71
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
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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
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179C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
180C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>
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181
182=item Fetching user and group info
183
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184C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
185C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
186C<getpwuid>, C<setgrent>, C<setpwent>
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187
188=item Fetching network info
189
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190C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
191C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
192C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
193C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
194C<setnetent>, C<setprotoent>, C<setservent>
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195
196=item Time-related functions
197
22fae026 198C<gmtime>, C<localtime>, C<time>, C<times>
cb1a09d0 199
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200=item Functions new in perl5
201
22fae026 202C<abs>, C<bless>, C<chomp>, C<chr>, C<exists>, C<formline>, C<glob>,
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>
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206
207* - C<sub> was a keyword in perl4, but in perl5 it is an
5a964f20 208operator, which can be used in expressions.
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209
210=item Functions obsoleted in perl5
211
22fae026 212C<dbmclose>, C<dbmopen>
37798a01 213
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214=back
215
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216=head2 Portability
217
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218Perl was born in Unix and can therefore access all common Unix
219system calls. In non-Unix environments, the functionality of some
220Unix system calls may not be available, or details of the available
221functionality may differ slightly. The Perl functions affected
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222by this are:
223
224C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>,
225C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>,
226C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>,
227C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostent>,
228C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
229C<getppid>, C<getprgp>, C<getpriority>, C<getprotobynumber>,
230C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>,
231C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>,
232C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>,
2b5ab1e7 233C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<rename>, C<select>, C<semctl>,
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234C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>,
235C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>,
236C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>,
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">.
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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 $@ eq "alarm\n"; # propagate unexpected errors
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426 # timed out
427 }
428 else {
429 # didn't
430 }
431
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432=item atan2 Y,X
433
434Returns the arctangent of Y/X in the range -PI to PI.
435
ca6e1c26 436For the tangent operation, you may use the C<Math::Trig::tan>
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437function, or use the familiar relation:
438
439 sub tan { sin($_[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
460UTF-8, use C<:utf8>, and to mark the as bytes, use C<:bytes>.
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 $evaltext is undefined. (Note also that
dc848c6f 557each C<use> statement creates a C<require> frame inside an C<eval EXPR>)
277ddfaf 558frame. C<$hasargs> is true if a new instance of C<@_> was set up for the
e476b1b5
GS
559frame. C<$hints> and C<$bitmask> contain pragmatic hints that the caller
560was compiled with. The C<$hints> and C<$bitmask> values are subject to
561change between versions of Perl, and are not meant for external use.
748a9306
LW
562
563Furthermore, when called from within the DB package, caller returns more
7660c0ab 564detailed information: it sets the list variable C<@DB::args> to be the
54310121 565arguments with which the subroutine was invoked.
748a9306 566
7660c0ab 567Be aware that the optimizer might have optimized call frames away before
19799a22 568C<caller> had a chance to get the information. That means that C<caller(N)>
7660c0ab 569might not return information about the call frame you expect it do, for
b76cc8ba 570C<< N > 1 >>. In particular, C<@DB::args> might have information from the
19799a22 571previous time C<caller> was called.
7660c0ab 572
a0d0e21e
LW
573=item chdir EXPR
574
ffce7b87 575Changes the working directory to EXPR, if possible. If EXPR is omitted,
0bfc1ec4 576changes to the directory specified by C<$ENV{HOME}>, if set; if not,
ffce7b87 577changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
b4ad75f0
AMS
578variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
579neither is set, C<chdir> does nothing. It returns true upon success,
580false otherwise. See the example under C<die>.
a0d0e21e
LW
581
582=item chmod LIST
583
584Changes the permissions of a list of files. The first element of the
4633a7c4 585list must be the numerical mode, which should probably be an octal
2f9daede
TPG
586number, and which definitely should I<not> a string of octal digits:
587C<0644> is okay, C<'0644'> is not. Returns the number of files
dc848c6f 588successfully changed. See also L</oct>, if all you have is a string.
a0d0e21e
LW
589
590 $cnt = chmod 0755, 'foo', 'bar';
591 chmod 0755, @executables;
f86cebdf
GS
592 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
593 # --w----r-T
2f9daede
TPG
594 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
595 $mode = 0644; chmod $mode, 'foo'; # this is best
a0d0e21e 596
ca6e1c26
JH
597You can also import the symbolic C<S_I*> constants from the Fcntl
598module:
599
600 use Fcntl ':mode';
601
602 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
603 # This is identical to the chmod 0755 of the above example.
604
a0d0e21e
LW
605=item chomp VARIABLE
606
313c9f5c 607=item chomp( LIST )
a0d0e21e
LW
608
609=item chomp
610
2b5ab1e7
TC
611This safer version of L</chop> removes any trailing string
612that corresponds to the current value of C<$/> (also known as
28757baa
PP
613$INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
614number of characters removed from all its arguments. It's often used to
615remove the newline from the end of an input record when you're worried
2b5ab1e7
TC
616that the final record may be missing its newline. When in paragraph
617mode (C<$/ = "">), it removes all trailing newlines from the string.
4c5a6083
GS
618When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
619a reference to an integer or the like, see L<perlvar>) chomp() won't
b76cc8ba 620remove anything.
19799a22 621If VARIABLE is omitted, it chomps C<$_>. Example:
a0d0e21e
LW
622
623 while (<>) {
624 chomp; # avoid \n on last field
625 @array = split(/:/);
5a964f20 626 # ...
a0d0e21e
LW
627 }
628
4bf21a6d
RD
629If VARIABLE is a hash, it chomps the hash's values, but not its keys.
630
a0d0e21e
LW
631You can actually chomp anything that's an lvalue, including an assignment:
632
633 chomp($cwd = `pwd`);
634 chomp($answer = <STDIN>);
635
636If you chomp a list, each element is chomped, and the total number of
637characters removed is returned.
638
639=item chop VARIABLE
640
313c9f5c 641=item chop( LIST )
a0d0e21e
LW
642
643=item chop
644
645Chops off the last character of a string and returns the character
5b3eff12 646chopped. It is much more efficient than C<s/.$//s> because it neither
7660c0ab 647scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
4bf21a6d
RD
648If VARIABLE is a hash, it chops the hash's values, but not its keys.
649
5b3eff12 650You can actually chop anything that's an lvalue, including an assignment.
a0d0e21e
LW
651
652If you chop a list, each element is chopped. Only the value of the
19799a22 653last C<chop> is returned.
a0d0e21e 654
19799a22 655Note that C<chop> returns the last character. To return all but the last
748a9306
LW
656character, use C<substr($string, 0, -1)>.
657
a0d0e21e
LW
658=item chown LIST
659
660Changes the owner (and group) of a list of files. The first two
19799a22
GS
661elements of the list must be the I<numeric> uid and gid, in that
662order. A value of -1 in either position is interpreted by most
663systems to leave that value unchanged. Returns the number of files
664successfully changed.
a0d0e21e
LW
665
666 $cnt = chown $uid, $gid, 'foo', 'bar';
667 chown $uid, $gid, @filenames;
668
54310121 669Here's an example that looks up nonnumeric uids in the passwd file:
a0d0e21e
LW
670
671 print "User: ";
19799a22 672 chomp($user = <STDIN>);
5a964f20 673 print "Files: ";
19799a22 674 chomp($pattern = <STDIN>);
a0d0e21e
LW
675
676 ($login,$pass,$uid,$gid) = getpwnam($user)
677 or die "$user not in passwd file";
678
5a964f20 679 @ary = glob($pattern); # expand filenames
a0d0e21e
LW
680 chown $uid, $gid, @ary;
681
54310121 682On most systems, you are not allowed to change the ownership of the
4633a7c4
LW
683file unless you're the superuser, although you should be able to change
684the group to any of your secondary groups. On insecure systems, these
685restrictions may be relaxed, but this is not a portable assumption.
19799a22
GS
686On POSIX systems, you can detect this condition this way:
687
688 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
689 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
4633a7c4 690
a0d0e21e
LW
691=item chr NUMBER
692
54310121 693=item chr
bbce6d69 694
a0d0e21e 695Returns the character represented by that NUMBER in the character set.
a0ed51b3 696For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
121910a4
JH
697chr(0x263a) is a Unicode smiley face. Note that characters from 127
698to 255 (inclusive) are by default not encoded in Unicode for backward
699compatibility reasons (but see L<encoding>).
aaa68c4a 700
b76cc8ba 701For the reverse, use L</ord>.
121910a4 702See L<perlunicode> and L<encoding> for more about Unicode.
a0d0e21e 703
7660c0ab 704If NUMBER is omitted, uses C<$_>.
bbce6d69 705
a0d0e21e
LW
706=item chroot FILENAME
707
54310121 708=item chroot
bbce6d69 709
5a964f20 710This function works like the system call by the same name: it makes the
4633a7c4 711named directory the new root directory for all further pathnames that
951ba7fe 712begin with a C</> by your process and all its children. (It doesn't
28757baa 713change your current working directory, which is unaffected.) For security
4633a7c4 714reasons, this call is restricted to the superuser. If FILENAME is
19799a22 715omitted, does a C<chroot> to C<$_>.
a0d0e21e
LW
716
717=item close FILEHANDLE
718
6a518fbc
TP
719=item close
720
9124316e
JH
721Closes the file or pipe associated with the file handle, returning
722true only if IO buffers are successfully flushed and closes the system
723file descriptor. Closes the currently selected filehandle if the
724argument is omitted.
fb73857a
PP
725
726You don't have to close FILEHANDLE if you are immediately going to do
19799a22
GS
727another C<open> on it, because C<open> will close it for you. (See
728C<open>.) However, an explicit C<close> on an input file resets the line
729counter (C<$.>), while the implicit close done by C<open> does not.
fb73857a 730
19799a22
GS
731If the file handle came from a piped open C<close> will additionally
732return false if one of the other system calls involved fails or if the
fb73857a 733program exits with non-zero status. (If the only problem was that the
b76cc8ba 734program exited non-zero C<$!> will be set to C<0>.) Closing a pipe
2b5ab1e7 735also waits for the process executing on the pipe to complete, in case you
b76cc8ba 736want to look at the output of the pipe afterwards, and
2b5ab1e7 737implicitly puts the exit status value of that command into C<$?>.
5a964f20 738
73689b13
GS
739Prematurely closing the read end of a pipe (i.e. before the process
740writing to it at the other end has closed it) will result in a
741SIGPIPE being delivered to the writer. If the other end can't
742handle that, be sure to read all the data before closing the pipe.
743
fb73857a 744Example:
a0d0e21e 745
fb73857a
PP
746 open(OUTPUT, '|sort >foo') # pipe to sort
747 or die "Can't start sort: $!";
5a964f20 748 #... # print stuff to output
fb73857a
PP
749 close OUTPUT # wait for sort to finish
750 or warn $! ? "Error closing sort pipe: $!"
751 : "Exit status $? from sort";
752 open(INPUT, 'foo') # get sort's results
753 or die "Can't open 'foo' for input: $!";
a0d0e21e 754
5a964f20
TC
755FILEHANDLE may be an expression whose value can be used as an indirect
756filehandle, usually the real filehandle name.
a0d0e21e
LW
757
758=item closedir DIRHANDLE
759
19799a22 760Closes a directory opened by C<opendir> and returns the success of that
5a964f20
TC
761system call.
762
763DIRHANDLE may be an expression whose value can be used as an indirect
764dirhandle, usually the real dirhandle name.
a0d0e21e
LW
765
766=item connect SOCKET,NAME
767
768Attempts to connect to a remote socket, just as the connect system call
19799a22 769does. Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
770packed address of the appropriate type for the socket. See the examples in
771L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 772
cb1a09d0
AD
773=item continue BLOCK
774
775Actually a flow control statement rather than a function. If there is a
98293880
JH
776C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
777C<foreach>), it is always executed just before the conditional is about to
778be evaluated again, just like the third part of a C<for> loop in C. Thus
cb1a09d0
AD
779it can be used to increment a loop variable, even when the loop has been
780continued via the C<next> statement (which is similar to the C C<continue>
781statement).
782
98293880 783C<last>, C<next>, or C<redo> may appear within a C<continue>
19799a22
GS
784block. C<last> and C<redo> will behave as if they had been executed within
785the main block. So will C<next>, but since it will execute a C<continue>
1d2dff63
GS
786block, it may be more entertaining.
787
788 while (EXPR) {
789 ### redo always comes here
790 do_something;
791 } continue {
792 ### next always comes here
793 do_something_else;
794 # then back the top to re-check EXPR
795 }
796 ### last always comes here
797
798Omitting the C<continue> section is semantically equivalent to using an
19799a22 799empty one, logically enough. In that case, C<next> goes directly back
1d2dff63
GS
800to check the condition at the top of the loop.
801
a0d0e21e
LW
802=item cos EXPR
803
d6217f1e
GS
804=item cos
805
5a964f20 806Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
7660c0ab 807takes cosine of C<$_>.
a0d0e21e 808
ca6e1c26 809For the inverse cosine operation, you may use the C<Math::Trig::acos()>
28757baa
PP
810function, or use this relation:
811
812 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
813
a0d0e21e
LW
814=item crypt PLAINTEXT,SALT
815
f86cebdf 816Encrypts a string exactly like the crypt(3) function in the C library
4633a7c4
LW
817(assuming that you actually have a version there that has not been
818extirpated as a potential munition). This can prove useful for checking
819the password file for lousy passwords, amongst other things. Only the
820guys wearing white hats should do this.
a0d0e21e 821
85c16d83
JH
822Note that C<crypt> is intended to be a one-way function, much like
823breaking eggs to make an omelette. There is no (known) corresponding
824decrypt function (in other words, the crypt() is a one-way hash
825function). As a result, this function isn't all that useful for
11155c91 826cryptography. (For that, see your nearby CPAN mirror.)
2f9daede 827
85c16d83
JH
828When verifying an existing encrypted string you should use the
829encrypted text as the salt (like C<crypt($plain, $crypted) eq
830$crypted>). This allows your code to work with the standard C<crypt>
831and with more exotic implementations. In other words, do not assume
832anything about the returned string itself, or how many bytes in
833the encrypted string matter.
834
835Traditionally the result is a string of 13 bytes: two first bytes of
836the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
837the first eight bytes of the encrypted string mattered, but
838alternative hashing schemes (like MD5), higher level security schemes
839(like C2), and implementations on non-UNIX platforms may produce
840different strings.
841
842When choosing a new salt create a random two character string whose
843characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
844'/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>).
e71965be 845
a0d0e21e
LW
846Here's an example that makes sure that whoever runs this program knows
847their own password:
848
849 $pwd = (getpwuid($<))[1];
a0d0e21e
LW
850
851 system "stty -echo";
852 print "Password: ";
e71965be 853 chomp($word = <STDIN>);
a0d0e21e
LW
854 print "\n";
855 system "stty echo";
856
e71965be 857 if (crypt($word, $pwd) ne $pwd) {
a0d0e21e
LW
858 die "Sorry...\n";
859 } else {
860 print "ok\n";
54310121 861 }
a0d0e21e 862
9f8f0c9d 863Of course, typing in your own password to whoever asks you
748a9306 864for it is unwise.
a0d0e21e 865
19799a22
GS
866The L<crypt> function is unsuitable for encrypting large quantities
867of data, not least of all because you can't get the information
868back. Look at the F<by-module/Crypt> and F<by-module/PGP> directories
869on your favorite CPAN mirror for a slew of potentially useful
870modules.
871
f2791508
JH
872If using crypt() on a Unicode string (which I<potentially> has
873characters with codepoints above 255), Perl tries to make sense
874of the situation by trying to downgrade (a copy of the string)
875the string back to an eight-bit byte string before calling crypt()
876(on that copy). If that works, good. If not, crypt() dies with
877C<Wide character in crypt>.
85c16d83 878
aa689395 879=item dbmclose HASH
a0d0e21e 880
19799a22 881[This function has been largely superseded by the C<untie> function.]
a0d0e21e 882
aa689395 883Breaks the binding between a DBM file and a hash.
a0d0e21e 884
19799a22 885=item dbmopen HASH,DBNAME,MASK
a0d0e21e 886
19799a22 887[This function has been largely superseded by the C<tie> function.]
a0d0e21e 888
7b8d334a 889This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
19799a22
GS
890hash. HASH is the name of the hash. (Unlike normal C<open>, the first
891argument is I<not> a filehandle, even though it looks like one). DBNAME
aa689395
PP
892is the name of the database (without the F<.dir> or F<.pag> extension if
893any). If the database does not exist, it is created with protection
19799a22
GS
894specified by MASK (as modified by the C<umask>). If your system supports
895only the older DBM functions, you may perform only one C<dbmopen> in your
aa689395 896program. In older versions of Perl, if your system had neither DBM nor
19799a22 897ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
aa689395
PP
898sdbm(3).
899
900If you don't have write access to the DBM file, you can only read hash
901variables, not set them. If you want to test whether you can write,
19799a22 902either use file tests or try setting a dummy hash entry inside an C<eval>,
aa689395 903which will trap the error.
a0d0e21e 904
19799a22
GS
905Note that functions such as C<keys> and C<values> may return huge lists
906when used on large DBM files. You may prefer to use the C<each>
a0d0e21e
LW
907function to iterate over large DBM files. Example:
908
909 # print out history file offsets
910 dbmopen(%HIST,'/usr/lib/news/history',0666);
911 while (($key,$val) = each %HIST) {
912 print $key, ' = ', unpack('L',$val), "\n";
913 }
914 dbmclose(%HIST);
915
cb1a09d0 916See also L<AnyDBM_File> for a more general description of the pros and
184e9718 917cons of the various dbm approaches, as well as L<DB_File> for a particularly
cb1a09d0 918rich implementation.
4633a7c4 919
2b5ab1e7
TC
920You can control which DBM library you use by loading that library
921before you call dbmopen():
922
923 use DB_File;
924 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
925 or die "Can't open netscape history file: $!";
926
a0d0e21e
LW
927=item defined EXPR
928
54310121 929=item defined
bbce6d69 930
2f9daede
TPG
931Returns a Boolean value telling whether EXPR has a value other than
932the undefined value C<undef>. If EXPR is not present, C<$_> will be
933checked.
934
935Many operations return C<undef> to indicate failure, end of file,
936system error, uninitialized variable, and other exceptional
937conditions. This function allows you to distinguish C<undef> from
938other values. (A simple Boolean test will not distinguish among
7660c0ab 939C<undef>, zero, the empty string, and C<"0">, which are all equally
2f9daede 940false.) Note that since C<undef> is a valid scalar, its presence
19799a22 941doesn't I<necessarily> indicate an exceptional condition: C<pop>
2f9daede
TPG
942returns C<undef> when its argument is an empty array, I<or> when the
943element to return happens to be C<undef>.
944
f10b0346
GS
945You may also use C<defined(&func)> to check whether subroutine C<&func>
946has ever been defined. The return value is unaffected by any forward
847c7ebe
DD
947declarations of C<&foo>. Note that a subroutine which is not defined
948may still be callable: its package may have an C<AUTOLOAD> method that
949makes it spring into existence the first time that it is called -- see
950L<perlsub>.
f10b0346
GS
951
952Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
953used to report whether memory for that aggregate has ever been
954allocated. This behavior may disappear in future versions of Perl.
955You should instead use a simple test for size:
956
957 if (@an_array) { print "has array elements\n" }
958 if (%a_hash) { print "has hash members\n" }
2f9daede
TPG
959
960When used on a hash element, it tells you whether the value is defined,
dc848c6f 961not whether the key exists in the hash. Use L</exists> for the latter
2f9daede 962purpose.
a0d0e21e
LW
963
964Examples:
965
966 print if defined $switch{'D'};
967 print "$val\n" while defined($val = pop(@ary));
968 die "Can't readlink $sym: $!"
969 unless defined($value = readlink $sym);
a0d0e21e 970 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
2f9daede 971 $debugging = 0 unless defined $debugging;
a0d0e21e 972
19799a22 973Note: Many folks tend to overuse C<defined>, and then are surprised to
7660c0ab 974discover that the number C<0> and C<""> (the zero-length string) are, in fact,
2f9daede 975defined values. For example, if you say
a5f75d66
AD
976
977 "ab" =~ /a(.*)b/;
978
7660c0ab 979The pattern match succeeds, and C<$1> is defined, despite the fact that it
a5f75d66 980matched "nothing". But it didn't really match nothing--rather, it
2b5ab1e7 981matched something that happened to be zero characters long. This is all
a5f75d66 982very above-board and honest. When a function returns an undefined value,
2f9daede 983it's an admission that it couldn't give you an honest answer. So you
19799a22 984should use C<defined> only when you're questioning the integrity of what
7660c0ab 985you're trying to do. At other times, a simple comparison to C<0> or C<""> is
2f9daede
TPG
986what you want.
987
dc848c6f 988See also L</undef>, L</exists>, L</ref>.
2f9daede 989
a0d0e21e
LW
990=item delete EXPR
991
01020589
GS
992Given an expression that specifies a hash element, array element, hash slice,
993or array slice, deletes the specified element(s) from the hash or array.
8216c1fd 994In the case of an array, if the array elements happen to be at the end,
b76cc8ba 995the size of the array will shrink to the highest element that tests
8216c1fd 996true for exists() (or 0 if no such element exists).
a0d0e21e 997
01020589
GS
998Returns each element so deleted or the undefined value if there was no such
999element. Deleting from C<$ENV{}> modifies the environment. Deleting from
1000a hash tied to a DBM file deletes the entry from the DBM file. Deleting
1001from a C<tie>d hash or array may not necessarily return anything.
1002
8ea97a1e
GS
1003Deleting an array element effectively returns that position of the array
1004to its initial, uninitialized state. Subsequently testing for the same
8216c1fd
GS
1005element with exists() will return false. Note that deleting array
1006elements in the middle of an array will not shift the index of the ones
1007after them down--use splice() for that. See L</exists>.
8ea97a1e 1008
01020589 1009The following (inefficiently) deletes all the values of %HASH and @ARRAY:
a0d0e21e 1010
5f05dabc
PP
1011 foreach $key (keys %HASH) {
1012 delete $HASH{$key};
a0d0e21e
LW
1013 }
1014
01020589
GS
1015 foreach $index (0 .. $#ARRAY) {
1016 delete $ARRAY[$index];
1017 }
1018
1019And so do these:
5f05dabc 1020
01020589
GS
1021 delete @HASH{keys %HASH};
1022
9740c838 1023 delete @ARRAY[0 .. $#ARRAY];
5f05dabc 1024
2b5ab1e7 1025But both of these are slower than just assigning the empty list
01020589
GS
1026or undefining %HASH or @ARRAY:
1027
1028 %HASH = (); # completely empty %HASH
1029 undef %HASH; # forget %HASH ever existed
2b5ab1e7 1030
01020589
GS
1031 @ARRAY = (); # completely empty @ARRAY
1032 undef @ARRAY; # forget @ARRAY ever existed
2b5ab1e7
TC
1033
1034Note that the EXPR can be arbitrarily complicated as long as the final
01020589
GS
1035operation is a hash element, array element, hash slice, or array slice
1036lookup:
a0d0e21e
LW
1037
1038 delete $ref->[$x][$y]{$key};
5f05dabc 1039 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
a0d0e21e 1040
01020589
GS
1041 delete $ref->[$x][$y][$index];
1042 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1043
a0d0e21e
LW
1044=item die LIST
1045
19799a22
GS
1046Outside an C<eval>, prints the value of LIST to C<STDERR> and
1047exits with the current value of C<$!> (errno). If C<$!> is C<0>,
61eff3bc
JH
1048exits with the value of C<<< ($? >> 8) >>> (backtick `command`
1049status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside
19799a22
GS
1050an C<eval(),> the error message is stuffed into C<$@> and the
1051C<eval> is terminated with the undefined value. This makes
1052C<die> the way to raise an exception.
a0d0e21e
LW
1053
1054Equivalent examples:
1055
1056 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
54310121 1057 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
a0d0e21e 1058
ccac6780 1059If the last element of LIST does not end in a newline, the current
df37ec69
WW
1060script line number and input line number (if any) are also printed,
1061and a newline is supplied. Note that the "input line number" (also
1062known as "chunk") is subject to whatever notion of "line" happens to
1063be currently in effect, and is also available as the special variable
1064C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1065
1066Hint: sometimes appending C<", stopped"> to your message will cause it
1067to make better sense when the string C<"at foo line 123"> is appended.
1068Suppose you are running script "canasta".
a0d0e21e
LW
1069
1070 die "/etc/games is no good";
1071 die "/etc/games is no good, stopped";
1072
1073produce, respectively
1074
1075 /etc/games is no good at canasta line 123.
1076 /etc/games is no good, stopped at canasta line 123.
1077
2b5ab1e7 1078See also exit(), warn(), and the Carp module.
a0d0e21e 1079
7660c0ab
A
1080If LIST is empty and C<$@> already contains a value (typically from a
1081previous eval) that value is reused after appending C<"\t...propagated">.
fb73857a
PP
1082This is useful for propagating exceptions:
1083
1084 eval { ... };
1085 die unless $@ =~ /Expected exception/;
1086
ad216e65
JH
1087If LIST is empty and C<$@> contains an object reference that has a
1088C<PROPAGATE> method, that method will be called with additional file
1089and line number parameters. The return value replaces the value in
1090C<$@>. ie. as if C<<$@ = eval { $@->PROPAGATE(__FILE__, __LINE__) };>>
1091were called.
1092
7660c0ab 1093If C<$@> is empty then the string C<"Died"> is used.
fb73857a 1094
52531d10
GS
1095die() can also be called with a reference argument. If this happens to be
1096trapped within an eval(), $@ contains the reference. This behavior permits
1097a more elaborate exception handling implementation using objects that
4375e838 1098maintain arbitrary state about the nature of the exception. Such a scheme
52531d10
GS
1099is sometimes preferable to matching particular string values of $@ using
1100regular expressions. Here's an example:
1101
1102 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1103 if ($@) {
1104 if (ref($@) && UNIVERSAL::isa($@,"Some::Module::Exception")) {
1105 # handle Some::Module::Exception
1106 }
1107 else {
1108 # handle all other possible exceptions
1109 }
1110 }
1111
19799a22 1112Because perl will stringify uncaught exception messages before displaying
52531d10
GS
1113them, you may want to overload stringification operations on such custom
1114exception objects. See L<overload> for details about that.
1115
19799a22
GS
1116You can arrange for a callback to be run just before the C<die>
1117does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1118handler will be called with the error text and can change the error
1119message, if it sees fit, by calling C<die> again. See
1120L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
1121L<"eval BLOCK"> for some examples. Although this feature was meant
1122to be run only right before your program was to exit, this is not
1123currently the case--the C<$SIG{__DIE__}> hook is currently called
1124even inside eval()ed blocks/strings! If one wants the hook to do
1125nothing in such situations, put
fb73857a
PP
1126
1127 die @_ if $^S;
1128
19799a22
GS
1129as the first line of the handler (see L<perlvar/$^S>). Because
1130this promotes strange action at a distance, this counterintuitive
b76cc8ba 1131behavior may be fixed in a future release.
774d564b 1132
a0d0e21e
LW
1133=item do BLOCK
1134
1135Not really a function. Returns the value of the last command in the
1136sequence of commands indicated by BLOCK. When modified by a loop
98293880
JH
1137modifier, executes the BLOCK once before testing the loop condition.
1138(On other statements the loop modifiers test the conditional first.)
a0d0e21e 1139
4968c1e4 1140C<do BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7
TC
1141C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1142See L<perlsyn> for alternative strategies.
4968c1e4 1143
a0d0e21e
LW
1144=item do SUBROUTINE(LIST)
1145
1146A deprecated form of subroutine call. See L<perlsub>.
1147
1148=item do EXPR
1149
1150Uses the value of EXPR as a filename and executes the contents of the
1151file as a Perl script. Its primary use is to include subroutines
1152from a Perl subroutine library.
1153
1154 do 'stat.pl';
1155
1156is just like
1157
986b19de 1158 eval `cat stat.pl`;
a0d0e21e 1159
2b5ab1e7
TC
1160except that it's more efficient and concise, keeps track of the current
1161filename for error messages, searches the @INC libraries, and updates
1162C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1163variables. It also differs in that code evaluated with C<do FILENAME>
1164cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1165same, however, in that it does reparse the file every time you call it,
1166so you probably don't want to do this inside a loop.
a0d0e21e 1167
8e30cc93 1168If C<do> cannot read the file, it returns undef and sets C<$!> to the
2b5ab1e7 1169error. If C<do> can read the file but cannot compile it, it
8e30cc93
G
1170returns undef and sets an error message in C<$@>. If the file is
1171successfully compiled, C<do> returns the value of the last expression
1172evaluated.
1173
a0d0e21e 1174Note that inclusion of library modules is better done with the
19799a22 1175C<use> and C<require> operators, which also do automatic error checking
4633a7c4 1176and raise an exception if there's a problem.
a0d0e21e 1177
5a964f20
TC
1178You might like to use C<do> to read in a program configuration
1179file. Manual error checking can be done this way:
1180
b76cc8ba 1181 # read in config files: system first, then user
f86cebdf 1182 for $file ("/share/prog/defaults.rc",
b76cc8ba 1183 "$ENV{HOME}/.someprogrc")
2b5ab1e7 1184 {
5a964f20 1185 unless ($return = do $file) {
f86cebdf
GS
1186 warn "couldn't parse $file: $@" if $@;
1187 warn "couldn't do $file: $!" unless defined $return;
1188 warn "couldn't run $file" unless $return;
5a964f20
TC
1189 }
1190 }
1191
a0d0e21e
LW
1192=item dump LABEL
1193
1614b0e3
JD
1194=item dump
1195
19799a22
GS
1196This function causes an immediate core dump. See also the B<-u>
1197command-line switch in L<perlrun>, which does the same thing.
1198Primarily this is so that you can use the B<undump> program (not
1199supplied) to turn your core dump into an executable binary after
1200having initialized all your variables at the beginning of the
1201program. When the new binary is executed it will begin by executing
1202a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1203Think of it as a goto with an intervening core dump and reincarnation.
1204If C<LABEL> is omitted, restarts the program from the top.
1205
1206B<WARNING>: Any files opened at the time of the dump will I<not>
1207be open any more when the program is reincarnated, with possible
b76cc8ba 1208resulting confusion on the part of Perl.
19799a22
GS
1209
1210This function is now largely obsolete, partly because it's very
1211hard to convert a core file into an executable, and because the
1212real compiler backends for generating portable bytecode and compilable
ac206dc8
RGS
1213C code have superseded it. That's why you should now invoke it as
1214C<CORE::dump()>, if you don't want to be warned against a possible
1215typo.
19799a22
GS
1216
1217If you're looking to use L<dump> to speed up your program, consider
1218generating bytecode or native C code as described in L<perlcc>. If
1219you're just trying to accelerate a CGI script, consider using the
210b36aa 1220C<mod_perl> extension to B<Apache>, or the CPAN module, CGI::Fast.
19799a22 1221You might also consider autoloading or selfloading, which at least
b76cc8ba 1222make your program I<appear> to run faster.
5a964f20 1223
aa689395
PP
1224=item each HASH
1225
5a964f20 1226When called in list context, returns a 2-element list consisting of the
aa689395 1227key and value for the next element of a hash, so that you can iterate over
74fc8b5f 1228it. When called in scalar context, returns only the key for the next
e902a979 1229element in the hash.
2f9daede 1230
ab192400
GS
1231Entries are returned in an apparently random order. The actual random
1232order is subject to change in future versions of perl, but it is guaranteed
19799a22 1233to be in the same order as either the C<keys> or C<values> function
ab192400
GS
1234would produce on the same (unmodified) hash.
1235
1236When the hash is entirely read, a null array is returned in list context
19799a22
GS
1237(which when assigned produces a false (C<0>) value), and C<undef> in
1238scalar context. The next call to C<each> after that will start iterating
1239again. There is a single iterator for each hash, shared by all C<each>,
1240C<keys>, and C<values> function calls in the program; it can be reset by
2f9daede
TPG
1241reading all the elements from the hash, or by evaluating C<keys HASH> or
1242C<values HASH>. If you add or delete elements of a hash while you're
74fc8b5f
MJD
1243iterating over it, you may get entries skipped or duplicated, so
1244don't. Exception: It is always safe to delete the item most recently
1245returned by C<each()>, which means that the following code will work:
1246
1247 while (($key, $value) = each %hash) {
1248 print $key, "\n";
1249 delete $hash{$key}; # This is safe
1250 }
aa689395 1251
f86cebdf 1252The following prints out your environment like the printenv(1) program,
aa689395 1253only in a different order:
a0d0e21e
LW
1254
1255 while (($key,$value) = each %ENV) {
1256 print "$key=$value\n";
1257 }
1258
19799a22 1259See also C<keys>, C<values> and C<sort>.
a0d0e21e
LW
1260
1261=item eof FILEHANDLE
1262
4633a7c4
LW
1263=item eof ()
1264
a0d0e21e
LW
1265=item eof
1266
1267Returns 1 if the next read on FILEHANDLE will return end of file, or if
1268FILEHANDLE is not open. FILEHANDLE may be an expression whose value
5a964f20 1269gives the real filehandle. (Note that this function actually
19799a22 1270reads a character and then C<ungetc>s it, so isn't very useful in an
748a9306 1271interactive context.) Do not read from a terminal file (or call
19799a22 1272C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
748a9306
LW
1273as terminals may lose the end-of-file condition if you do.
1274
820475bd
GS
1275An C<eof> without an argument uses the last file read. Using C<eof()>
1276with empty parentheses is very different. It refers to the pseudo file
1277formed from the files listed on the command line and accessed via the
61eff3bc
JH
1278C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1279as a normal filehandle is, an C<eof()> before C<< <> >> has been
820475bd 1280used will cause C<@ARGV> to be examined to determine if input is
efdd0218
RB
1281available. Similarly, an C<eof()> after C<< <> >> has returned
1282end-of-file will assume you are processing another C<@ARGV> list,
1283and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1284see L<perlop/"I/O Operators">.
820475bd 1285
61eff3bc 1286In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
820475bd
GS
1287detect the end of each file, C<eof()> will only detect the end of the
1288last file. Examples:
a0d0e21e 1289
748a9306
LW
1290 # reset line numbering on each input file
1291 while (<>) {
b76cc8ba 1292 next if /^\s*#/; # skip comments
748a9306 1293 print "$.\t$_";
5a964f20
TC
1294 } continue {
1295 close ARGV if eof; # Not eof()!
748a9306
LW
1296 }
1297
a0d0e21e
LW
1298 # insert dashes just before last line of last file
1299 while (<>) {
5a964f20 1300 if (eof()) { # check for end of current file
a0d0e21e 1301 print "--------------\n";
2b5ab1e7 1302 close(ARGV); # close or last; is needed if we
748a9306 1303 # are reading from the terminal
a0d0e21e
LW
1304 }
1305 print;
1306 }
1307
a0d0e21e 1308Practical hint: you almost never need to use C<eof> in Perl, because the
3ce0d271
GS
1309input operators typically return C<undef> when they run out of data, or if
1310there was an error.
a0d0e21e
LW
1311
1312=item eval EXPR
1313
1314=item eval BLOCK
1315
c7cc6f1c
GS
1316In the first form, the return value of EXPR is parsed and executed as if it
1317were a little Perl program. The value of the expression (which is itself
5a964f20 1318determined within scalar context) is first parsed, and if there weren't any
be3174d2
GS
1319errors, executed in the lexical context of the current Perl program, so
1320that any variable settings or subroutine and format definitions remain
1321afterwards. Note that the value is parsed every time the eval executes.
1322If EXPR is omitted, evaluates C<$_>. This form is typically used to
1323delay parsing and subsequent execution of the text of EXPR until run time.
c7cc6f1c
GS
1324
1325In the second form, the code within the BLOCK is parsed only once--at the
1326same time the code surrounding the eval itself was parsed--and executed
1327within the context of the current Perl program. This form is typically
1328used to trap exceptions more efficiently than the first (see below), while
1329also providing the benefit of checking the code within BLOCK at compile
1330time.
1331
1332The final semicolon, if any, may be omitted from the value of EXPR or within
1333the BLOCK.
1334
1335In both forms, the value returned is the value of the last expression
5a964f20 1336evaluated inside the mini-program; a return statement may be also used, just
c7cc6f1c 1337as with subroutines. The expression providing the return value is evaluated
5a964f20 1338in void, scalar, or list context, depending on the context of the eval itself.
c7cc6f1c 1339See L</wantarray> for more on how the evaluation context can be determined.
a0d0e21e 1340
19799a22
GS
1341If there is a syntax error or runtime error, or a C<die> statement is
1342executed, an undefined value is returned by C<eval>, and C<$@> is set to the
a0d0e21e 1343error message. If there was no error, C<$@> is guaranteed to be a null
19799a22 1344string. Beware that using C<eval> neither silences perl from printing
c7cc6f1c 1345warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
d9984052
A
1346To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1347turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1348See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
a0d0e21e 1349
19799a22
GS
1350Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1351determining whether a particular feature (such as C<socket> or C<symlink>)
a0d0e21e
LW
1352is implemented. It is also Perl's exception trapping mechanism, where
1353the die operator is used to raise exceptions.
1354
1355If the code to be executed doesn't vary, you may use the eval-BLOCK
1356form to trap run-time errors without incurring the penalty of
1357recompiling each time. The error, if any, is still returned in C<$@>.
1358Examples:
1359
54310121 1360 # make divide-by-zero nonfatal
a0d0e21e
LW
1361 eval { $answer = $a / $b; }; warn $@ if $@;
1362
1363 # same thing, but less efficient
1364 eval '$answer = $a / $b'; warn $@ if $@;
1365
1366 # a compile-time error
5a964f20 1367 eval { $answer = }; # WRONG
a0d0e21e
LW
1368
1369 # a run-time error
1370 eval '$answer ='; # sets $@
1371
2b5ab1e7
TC
1372Due to the current arguably broken state of C<__DIE__> hooks, when using
1373the C<eval{}> form as an exception trap in libraries, you may wish not
1374to trigger any C<__DIE__> hooks that user code may have installed.
1375You can use the C<local $SIG{__DIE__}> construct for this purpose,
1376as shown in this example:
774d564b
PP
1377
1378 # a very private exception trap for divide-by-zero
f86cebdf
GS
1379 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1380 warn $@ if $@;
774d564b
PP
1381
1382This is especially significant, given that C<__DIE__> hooks can call
19799a22 1383C<die> again, which has the effect of changing their error messages:
774d564b
PP
1384
1385 # __DIE__ hooks may modify error messages
1386 {
f86cebdf
GS
1387 local $SIG{'__DIE__'} =
1388 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
c7cc6f1c
GS
1389 eval { die "foo lives here" };
1390 print $@ if $@; # prints "bar lives here"
774d564b
PP
1391 }
1392
19799a22 1393Because this promotes action at a distance, this counterintuitive behavior
2b5ab1e7
TC
1394may be fixed in a future release.
1395
19799a22 1396With an C<eval>, you should be especially careful to remember what's
a0d0e21e
LW
1397being looked at when:
1398
1399 eval $x; # CASE 1
1400 eval "$x"; # CASE 2
1401
1402 eval '$x'; # CASE 3
1403 eval { $x }; # CASE 4
1404
5a964f20 1405 eval "\$$x++"; # CASE 5
a0d0e21e
LW
1406 $$x++; # CASE 6
1407
2f9daede 1408Cases 1 and 2 above behave identically: they run the code contained in
19799a22 1409the variable $x. (Although case 2 has misleading double quotes making
2f9daede 1410the reader wonder what else might be happening (nothing is).) Cases 3
7660c0ab 1411and 4 likewise behave in the same way: they run the code C<'$x'>, which
19799a22 1412does nothing but return the value of $x. (Case 4 is preferred for
2f9daede
TPG
1413purely visual reasons, but it also has the advantage of compiling at
1414compile-time instead of at run-time.) Case 5 is a place where
19799a22 1415normally you I<would> like to use double quotes, except that in this
2f9daede
TPG
1416particular situation, you can just use symbolic references instead, as
1417in case 6.
a0d0e21e 1418
4968c1e4 1419C<eval BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7 1420C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
4968c1e4 1421
a0d0e21e
LW
1422=item exec LIST
1423
8bf3b016
GS
1424=item exec PROGRAM LIST
1425
19799a22
GS
1426The C<exec> function executes a system command I<and never returns>--
1427use C<system> instead of C<exec> if you want it to return. It fails and
1428returns false only if the command does not exist I<and> it is executed
fb73857a 1429directly instead of via your system's command shell (see below).
a0d0e21e 1430
19799a22
GS
1431Since it's a common mistake to use C<exec> instead of C<system>, Perl
1432warns you if there is a following statement which isn't C<die>, C<warn>,
1433or C<exit> (if C<-w> is set - but you always do that). If you
1434I<really> want to follow an C<exec> with some other statement, you
55d729e4
GS
1435can use one of these styles to avoid the warning:
1436
5a964f20
TC
1437 exec ('foo') or print STDERR "couldn't exec foo: $!";
1438 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
55d729e4 1439
5a964f20 1440If there is more than one argument in LIST, or if LIST is an array
f86cebdf 1441with more than one value, calls execvp(3) with the arguments in LIST.
5a964f20
TC
1442If there is only one scalar argument or an array with one element in it,
1443the argument is checked for shell metacharacters, and if there are any,
1444the entire argument is passed to the system's command shell for parsing
1445(this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1446If there are no shell metacharacters in the argument, it is split into
b76cc8ba 1447words and passed directly to C<execvp>, which is more efficient.
19799a22 1448Examples:
a0d0e21e 1449
19799a22
GS
1450 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1451 exec "sort $outfile | uniq";
a0d0e21e
LW
1452
1453If you don't really want to execute the first argument, but want to lie
1454to the program you are executing about its own name, you can specify
1455the program you actually want to run as an "indirect object" (without a
1456comma) in front of the LIST. (This always forces interpretation of the
54310121 1457LIST as a multivalued list, even if there is only a single scalar in
a0d0e21e
LW
1458the list.) Example:
1459
1460 $shell = '/bin/csh';
1461 exec $shell '-sh'; # pretend it's a login shell
1462
1463or, more directly,
1464
1465 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1466
bb32b41a
GS
1467When the arguments get executed via the system shell, results will
1468be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1469for details.
1470
19799a22
GS
1471Using an indirect object with C<exec> or C<system> is also more
1472secure. This usage (which also works fine with system()) forces
1473interpretation of the arguments as a multivalued list, even if the
1474list had just one argument. That way you're safe from the shell
1475expanding wildcards or splitting up words with whitespace in them.
5a964f20
TC
1476
1477 @args = ( "echo surprise" );
1478
2b5ab1e7 1479 exec @args; # subject to shell escapes
f86cebdf 1480 # if @args == 1
2b5ab1e7 1481 exec { $args[0] } @args; # safe even with one-arg list
5a964f20
TC
1482
1483The first version, the one without the indirect object, ran the I<echo>
1484program, passing it C<"surprise"> an argument. The second version
1485didn't--it tried to run a program literally called I<"echo surprise">,
1486didn't find it, and set C<$?> to a non-zero value indicating failure.
1487
0f897271
GS
1488Beginning with v5.6.0, Perl will attempt to flush all files opened for
1489output before the exec, but this may not be supported on some platforms
1490(see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1491in English) or call the C<autoflush()> method of C<IO::Handle> on any
1492open handles in order to avoid lost output.
1493
19799a22 1494Note that C<exec> will not call your C<END> blocks, nor will it call
7660c0ab
A
1495any C<DESTROY> methods in your objects.
1496
a0d0e21e
LW
1497=item exists EXPR
1498
01020589 1499Given an expression that specifies a hash element or array element,
8ea97a1e
GS
1500returns true if the specified element in the hash or array has ever
1501been initialized, even if the corresponding value is undefined. The
1502element is not autovivified if it doesn't exist.
a0d0e21e 1503
01020589
GS
1504 print "Exists\n" if exists $hash{$key};
1505 print "Defined\n" if defined $hash{$key};
1506 print "True\n" if $hash{$key};
1507
1508 print "Exists\n" if exists $array[$index];
1509 print "Defined\n" if defined $array[$index];
1510 print "True\n" if $array[$index];
a0d0e21e 1511
8ea97a1e 1512A hash or array element can be true only if it's defined, and defined if
a0d0e21e
LW
1513it exists, but the reverse doesn't necessarily hold true.
1514
afebc493
GS
1515Given an expression that specifies the name of a subroutine,
1516returns true if the specified subroutine has ever been declared, even
1517if it is undefined. Mentioning a subroutine name for exists or defined
847c7ebe
DD
1518does not count as declaring it. Note that a subroutine which does not
1519exist may still be callable: its package may have an C<AUTOLOAD>
1520method that makes it spring into existence the first time that it is
1521called -- see L<perlsub>.
afebc493
GS
1522
1523 print "Exists\n" if exists &subroutine;
1524 print "Defined\n" if defined &subroutine;
1525
a0d0e21e 1526Note that the EXPR can be arbitrarily complicated as long as the final
afebc493 1527operation is a hash or array key lookup or subroutine name:
a0d0e21e 1528
2b5ab1e7
TC
1529 if (exists $ref->{A}->{B}->{$key}) { }
1530 if (exists $hash{A}{B}{$key}) { }
1531
01020589
GS
1532 if (exists $ref->{A}->{B}->[$ix]) { }
1533 if (exists $hash{A}{B}[$ix]) { }
1534
afebc493
GS
1535 if (exists &{$ref->{A}{B}{$key}}) { }
1536
01020589
GS
1537Although the deepest nested array or hash will not spring into existence
1538just because its existence was tested, any intervening ones will.
61eff3bc 1539Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
01020589
GS
1540into existence due to the existence test for the $key element above.
1541This happens anywhere the arrow operator is used, including even:
5a964f20 1542
2b5ab1e7
TC
1543 undef $ref;
1544 if (exists $ref->{"Some key"}) { }
1545 print $ref; # prints HASH(0x80d3d5c)
1546
1547This surprising autovivification in what does not at first--or even
1548second--glance appear to be an lvalue context may be fixed in a future
5a964f20 1549release.
a0d0e21e 1550
479ba383
GS
1551See L<perlref/"Pseudo-hashes: Using an array as a hash"> for specifics
1552on how exists() acts when used on a pseudo-hash.
e0478e5a 1553
afebc493
GS
1554Use of a subroutine call, rather than a subroutine name, as an argument
1555to exists() is an error.
1556
1557 exists &sub; # OK
1558 exists &sub(); # Error
1559
a0d0e21e
LW
1560=item exit EXPR
1561
2b5ab1e7 1562Evaluates EXPR and exits immediately with that value. Example:
a0d0e21e
LW
1563
1564 $ans = <STDIN>;
1565 exit 0 if $ans =~ /^[Xx]/;
1566
19799a22 1567See also C<die>. If EXPR is omitted, exits with C<0> status. The only
2b5ab1e7
TC
1568universally recognized values for EXPR are C<0> for success and C<1>
1569for error; other values are subject to interpretation depending on the
1570environment in which the Perl program is running. For example, exiting
157169 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1572the mailer to return the item undelivered, but that's not true everywhere.
a0d0e21e 1573
19799a22
GS
1574Don't use C<exit> to abort a subroutine if there's any chance that
1575someone might want to trap whatever error happened. Use C<die> instead,
1576which can be trapped by an C<eval>.
28757baa 1577
19799a22 1578The exit() function does not always exit immediately. It calls any
2b5ab1e7 1579defined C<END> routines first, but these C<END> routines may not
19799a22 1580themselves abort the exit. Likewise any object destructors that need to
2b5ab1e7
TC
1581be called are called before the real exit. If this is a problem, you
1582can call C<POSIX:_exit($status)> to avoid END and destructor processing.
87275199 1583See L<perlmod> for details.
5a964f20 1584
a0d0e21e
LW
1585=item exp EXPR
1586
54310121 1587=item exp
bbce6d69 1588
b76cc8ba 1589Returns I<e> (the natural logarithm base) to the power of EXPR.
a0d0e21e
LW
1590If EXPR is omitted, gives C<exp($_)>.
1591
1592=item fcntl FILEHANDLE,FUNCTION,SCALAR
1593
f86cebdf 1594Implements the fcntl(2) function. You'll probably have to say
a0d0e21e
LW
1595
1596 use Fcntl;
1597
0ade1984 1598first to get the correct constant definitions. Argument processing and
b76cc8ba 1599value return works just like C<ioctl> below.
a0d0e21e
LW
1600For example:
1601
1602 use Fcntl;
5a964f20
TC
1603 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1604 or die "can't fcntl F_GETFL: $!";
1605
19799a22 1606You don't have to check for C<defined> on the return from C<fnctl>.
951ba7fe
GS
1607Like C<ioctl>, it maps a C<0> return from the system call into
1608C<"0 but true"> in Perl. This string is true in boolean context and C<0>
2b5ab1e7
TC
1609in numeric context. It is also exempt from the normal B<-w> warnings
1610on improper numeric conversions.
5a964f20 1611
19799a22 1612Note that C<fcntl> will produce a fatal error if used on a machine that
2b5ab1e7
TC
1613doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1614manpage to learn what functions are available on your system.
a0d0e21e
LW
1615
1616=item fileno FILEHANDLE
1617
2b5ab1e7
TC
1618Returns the file descriptor for a filehandle, or undefined if the
1619filehandle is not open. This is mainly useful for constructing
19799a22 1620bitmaps for C<select> and low-level POSIX tty-handling operations.
2b5ab1e7
TC
1621If FILEHANDLE is an expression, the value is taken as an indirect
1622filehandle, generally its name.
5a964f20 1623
b76cc8ba 1624You can use this to find out whether two handles refer to the
5a964f20
TC
1625same underlying descriptor:
1626
1627 if (fileno(THIS) == fileno(THAT)) {
1628 print "THIS and THAT are dups\n";
b76cc8ba
NIS
1629 }
1630
1631(Filehandles connected to memory objects via new features of C<open> may
1632return undefined even though they are open.)
1633
a0d0e21e
LW
1634
1635=item flock FILEHANDLE,OPERATION
1636
19799a22
GS
1637Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1638for success, false on failure. Produces a fatal error if used on a
2b5ab1e7 1639machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
19799a22 1640C<flock> is Perl's portable file locking interface, although it locks
2b5ab1e7
TC
1641only entire files, not records.
1642
1643Two potentially non-obvious but traditional C<flock> semantics are
1644that it waits indefinitely until the lock is granted, and that its locks
1645B<merely advisory>. Such discretionary locks are more flexible, but offer
19799a22
GS
1646fewer guarantees. This means that files locked with C<flock> may be
1647modified by programs that do not also use C<flock>. See L<perlport>,
2b5ab1e7
TC
1648your port's specific documentation, or your system-specific local manpages
1649for details. It's best to assume traditional behavior if you're writing
1650portable programs. (But if you're not, you should as always feel perfectly
1651free to write for your own system's idiosyncrasies (sometimes called
1652"features"). Slavish adherence to portability concerns shouldn't get
1653in the way of your getting your job done.)
a3cb178b 1654
8ebc5c01
PP
1655OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1656LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
ea3105be 1657you can use the symbolic names if you import them from the Fcntl module,
68dc0745
PP
1658either individually, or as a group using the ':flock' tag. LOCK_SH
1659requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
ea3105be
GS
1660releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1661LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
68dc0745
PP
1662waiting for the lock (check the return status to see if you got it).
1663
2b5ab1e7
TC
1664To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1665before locking or unlocking it.
8ebc5c01 1666
f86cebdf 1667Note that the emulation built with lockf(3) doesn't provide shared
8ebc5c01 1668locks, and it requires that FILEHANDLE be open with write intent. These
2b5ab1e7 1669are the semantics that lockf(3) implements. Most if not all systems
f86cebdf 1670implement lockf(3) in terms of fcntl(2) locking, though, so the
8ebc5c01
PP
1671differing semantics shouldn't bite too many people.
1672
becacb53
TM
1673Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
1674be open with read intent to use LOCK_SH and requires that it be open
1675with write intent to use LOCK_EX.
1676
19799a22
GS
1677Note also that some versions of C<flock> cannot lock things over the
1678network; you would need to use the more system-specific C<fcntl> for
f86cebdf
GS
1679that. If you like you can force Perl to ignore your system's flock(2)
1680function, and so provide its own fcntl(2)-based emulation, by passing
8ebc5c01
PP
1681the switch C<-Ud_flock> to the F<Configure> program when you configure
1682perl.
4633a7c4
LW
1683
1684Here's a mailbox appender for BSD systems.
a0d0e21e 1685
7e1af8bc 1686 use Fcntl ':flock'; # import LOCK_* constants
a0d0e21e
LW
1687
1688 sub lock {
7e1af8bc 1689 flock(MBOX,LOCK_EX);
a0d0e21e
LW
1690 # and, in case someone appended
1691 # while we were waiting...
1692 seek(MBOX, 0, 2);
1693 }
1694
1695 sub unlock {
7e1af8bc 1696 flock(MBOX,LOCK_UN);
a0d0e21e
LW
1697 }
1698
1699 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1700 or die "Can't open mailbox: $!";
1701
1702 lock();
1703 print MBOX $msg,"\n\n";
1704 unlock();
1705
2b5ab1e7
TC
1706On systems that support a real flock(), locks are inherited across fork()
1707calls, whereas those that must resort to the more capricious fcntl()
1708function lose the locks, making it harder to write servers.
1709
cb1a09d0 1710See also L<DB_File> for other flock() examples.
a0d0e21e
LW
1711
1712=item fork
1713
2b5ab1e7
TC
1714Does a fork(2) system call to create a new process running the
1715same program at the same point. It returns the child pid to the
1716parent process, C<0> to the child process, or C<undef> if the fork is
1717unsuccessful. File descriptors (and sometimes locks on those descriptors)
1718are shared, while everything else is copied. On most systems supporting
1719fork(), great care has gone into making it extremely efficient (for
1720example, using copy-on-write technology on data pages), making it the
1721dominant paradigm for multitasking over the last few decades.
5a964f20 1722
0f897271
GS
1723Beginning with v5.6.0, Perl will attempt to flush all files opened for
1724output before forking the child process, but this may not be supported
1725on some platforms (see L<perlport>). To be safe, you may need to set
1726C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1727C<IO::Handle> on any open handles in order to avoid duplicate output.
a0d0e21e 1728
19799a22 1729If you C<fork> without ever waiting on your children, you will
2b5ab1e7
TC
1730accumulate zombies. On some systems, you can avoid this by setting
1731C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1732forking and reaping moribund children.
cb1a09d0 1733
28757baa
PP
1734Note that if your forked child inherits system file descriptors like
1735STDIN and STDOUT that are actually connected by a pipe or socket, even
2b5ab1e7 1736if you exit, then the remote server (such as, say, a CGI script or a
19799a22 1737backgrounded job launched from a remote shell) won't think you're done.
2b5ab1e7 1738You should reopen those to F</dev/null> if it's any issue.
28757baa 1739
cb1a09d0
AD
1740=item format
1741
19799a22 1742Declare a picture format for use by the C<write> function. For
cb1a09d0
AD
1743example:
1744
54310121 1745 format Something =
cb1a09d0
AD
1746 Test: @<<<<<<<< @||||| @>>>>>
1747 $str, $%, '$' . int($num)
1748 .
1749
1750 $str = "widget";
184e9718 1751 $num = $cost/$quantity;
cb1a09d0
AD
1752 $~ = 'Something';
1753 write;
1754
1755See L<perlform> for many details and examples.
1756
8903cb82 1757=item formline PICTURE,LIST
a0d0e21e 1758
5a964f20 1759This is an internal function used by C<format>s, though you may call it,
a0d0e21e
LW
1760too. It formats (see L<perlform>) a list of values according to the
1761contents of PICTURE, placing the output into the format output
7660c0ab 1762accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
19799a22 1763Eventually, when a C<write> is done, the contents of
a0d0e21e 1764C<$^A> are written to some filehandle, but you could also read C<$^A>
7660c0ab 1765yourself and then set C<$^A> back to C<"">. Note that a format typically
19799a22 1766does one C<formline> per line of form, but the C<formline> function itself
748a9306 1767doesn't care how many newlines are embedded in the PICTURE. This means
4633a7c4 1768that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
748a9306
LW
1769You may therefore need to use multiple formlines to implement a single
1770record format, just like the format compiler.
1771
19799a22 1772Be careful if you put double quotes around the picture, because an C<@>
748a9306 1773character may be taken to mean the beginning of an array name.
19799a22 1774C<formline> always returns true. See L<perlform> for other examples.
a0d0e21e
LW
1775
1776=item getc FILEHANDLE
1777
1778=item getc
1779
1780Returns the next character from the input file attached to FILEHANDLE,
2b5ab1e7
TC
1781or the undefined value at end of file, or if there was an error.
1782If FILEHANDLE is omitted, reads from STDIN. This is not particularly
1783efficient. However, it cannot be used by itself to fetch single
1784characters without waiting for the user to hit enter. For that, try
1785something more like:
4633a7c4
LW
1786
1787 if ($BSD_STYLE) {
1788 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1789 }
1790 else {
54310121 1791 system "stty", '-icanon', 'eol', "\001";
4633a7c4
LW
1792 }
1793
1794 $key = getc(STDIN);
1795
1796 if ($BSD_STYLE) {
1797 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1798 }
1799 else {
5f05dabc 1800 system "stty", 'icanon', 'eol', '^@'; # ASCII null
4633a7c4
LW
1801 }
1802 print "\n";
1803
54310121
PP
1804Determination of whether $BSD_STYLE should be set
1805is left as an exercise to the reader.
cb1a09d0 1806
19799a22 1807The C<POSIX::getattr> function can do this more portably on
2b5ab1e7
TC
1808systems purporting POSIX compliance. See also the C<Term::ReadKey>
1809module from your nearest CPAN site; details on CPAN can be found on
1810L<perlmodlib/CPAN>.
a0d0e21e
LW
1811
1812=item getlogin
1813
5a964f20
TC
1814Implements the C library function of the same name, which on most
1815systems returns the current login from F</etc/utmp>, if any. If null,
19799a22 1816use C<getpwuid>.
a0d0e21e 1817
f86702cc 1818 $login = getlogin || getpwuid($<) || "Kilroy";
a0d0e21e 1819
19799a22
GS
1820Do not consider C<getlogin> for authentication: it is not as
1821secure as C<getpwuid>.
4633a7c4 1822
a0d0e21e
LW
1823=item getpeername SOCKET
1824
1825Returns the packed sockaddr address of other end of the SOCKET connection.
1826
4633a7c4
LW
1827 use Socket;
1828 $hersockaddr = getpeername(SOCK);
19799a22 1829 ($port, $iaddr) = sockaddr_in($hersockaddr);
4633a7c4
LW
1830 $herhostname = gethostbyaddr($iaddr, AF_INET);
1831 $herstraddr = inet_ntoa($iaddr);
a0d0e21e
LW
1832
1833=item getpgrp PID
1834
47e29363 1835Returns the current process group for the specified PID. Use
7660c0ab 1836a PID of C<0> to get the current process group for the
4633a7c4 1837current process. Will raise an exception if used on a machine that
f86cebdf 1838doesn't implement getpgrp(2). If PID is omitted, returns process
19799a22 1839group of current process. Note that the POSIX version of C<getpgrp>
7660c0ab 1840does not accept a PID argument, so only C<PID==0> is truly portable.
a0d0e21e
LW
1841
1842=item getppid
1843
1844Returns the process id of the parent process.
1845
1846=item getpriority WHICH,WHO
1847
4633a7c4
LW
1848Returns the current priority for a process, a process group, or a user.
1849(See L<getpriority(2)>.) Will raise a fatal exception if used on a
f86cebdf 1850machine that doesn't implement getpriority(2).
a0d0e21e
LW
1851
1852=item getpwnam NAME
1853
1854=item getgrnam NAME
1855
1856=item gethostbyname NAME
1857
1858=item getnetbyname NAME
1859
1860=item getprotobyname NAME
1861
1862=item getpwuid UID
1863
1864=item getgrgid GID
1865
1866=item getservbyname NAME,PROTO
1867
1868=item gethostbyaddr ADDR,ADDRTYPE
1869
1870=item getnetbyaddr ADDR,ADDRTYPE
1871
1872=item getprotobynumber NUMBER
1873
1874=item getservbyport PORT,PROTO
1875
1876=item getpwent
1877
1878=item getgrent
1879
1880=item gethostent
1881
1882=item getnetent
1883
1884=item getprotoent
1885
1886=item getservent
1887
1888=item setpwent
1889
1890=item setgrent
1891
1892=item sethostent STAYOPEN
1893
1894=item setnetent STAYOPEN
1895
1896=item setprotoent STAYOPEN
1897
1898=item setservent STAYOPEN
1899
1900=item endpwent
1901
1902=item endgrent
1903
1904=item endhostent
1905
1906=item endnetent
1907
1908=item endprotoent
1909
1910=item endservent
1911
1912These routines perform the same functions as their counterparts in the
5a964f20 1913system library. In list context, the return values from the
a0d0e21e
LW
1914various get routines are as follows:
1915
1916 ($name,$passwd,$uid,$gid,
6ee623d5 1917 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
a0d0e21e
LW
1918 ($name,$passwd,$gid,$members) = getgr*
1919 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1920 ($name,$aliases,$addrtype,$net) = getnet*
1921 ($name,$aliases,$proto) = getproto*
1922 ($name,$aliases,$port,$proto) = getserv*
1923
1924(If the entry doesn't exist you get a null list.)
1925
4602f195
JH
1926The exact meaning of the $gcos field varies but it usually contains
1927the real name of the user (as opposed to the login name) and other
1928information pertaining to the user. Beware, however, that in many
1929system users are able to change this information and therefore it
106325ad 1930cannot be trusted and therefore the $gcos is tainted (see
2959b6e3
JH
1931L<perlsec>). The $passwd and $shell, user's encrypted password and
1932login shell, are also tainted, because of the same reason.
4602f195 1933
5a964f20 1934In scalar context, you get the name, unless the function was a
a0d0e21e
LW
1935lookup by name, in which case you get the other thing, whatever it is.
1936(If the entry doesn't exist you get the undefined value.) For example:
1937
5a964f20
TC
1938 $uid = getpwnam($name);
1939 $name = getpwuid($num);
1940 $name = getpwent();
1941 $gid = getgrnam($name);
1942 $name = getgrgid($num;
1943 $name = getgrent();
1944 #etc.
a0d0e21e 1945
4602f195
JH
1946In I<getpw*()> the fields $quota, $comment, and $expire are special
1947cases in the sense that in many systems they are unsupported. If the
1948$quota is unsupported, it is an empty scalar. If it is supported, it
1949usually encodes the disk quota. If the $comment field is unsupported,
1950it is an empty scalar. If it is supported it usually encodes some
1951administrative comment about the user. In some systems the $quota
1952field may be $change or $age, fields that have to do with password
1953aging. In some systems the $comment field may be $class. The $expire
1954field, if present, encodes the expiration period of the account or the
1955password. For the availability and the exact meaning of these fields
1956in your system, please consult your getpwnam(3) documentation and your
1957F<pwd.h> file. You can also find out from within Perl what your
1958$quota and $comment fields mean and whether you have the $expire field
1959by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
1960C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
1961files are only supported if your vendor has implemented them in the
1962intuitive fashion that calling the regular C library routines gets the
5d3a0a3b
GS
1963shadow versions if you're running under privilege or if there exists
1964the shadow(3) functions as found in System V ( this includes Solaris
1965and Linux.) Those systems which implement a proprietary shadow password
1966facility are unlikely to be supported.
6ee623d5 1967
19799a22 1968The $members value returned by I<getgr*()> is a space separated list of
a0d0e21e
LW
1969the login names of the members of the group.
1970
1971For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1972C, it will be returned to you via C<$?> if the function call fails. The
7660c0ab 1973C<@addrs> value returned by a successful call is a list of the raw
a0d0e21e
LW
1974addresses returned by the corresponding system library call. In the
1975Internet domain, each address is four bytes long and you can unpack it
1976by saying something like:
1977
1978 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1979
2b5ab1e7
TC
1980The Socket library makes this slightly easier:
1981
1982 use Socket;
1983 $iaddr = inet_aton("127.1"); # or whatever address
1984 $name = gethostbyaddr($iaddr, AF_INET);
1985
1986 # or going the other way
19799a22 1987 $straddr = inet_ntoa($iaddr);
2b5ab1e7 1988
19799a22
GS
1989If you get tired of remembering which element of the return list
1990contains which return value, by-name interfaces are provided
1991in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
1992C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
1993and C<User::grent>. These override the normal built-ins, supplying
1994versions that return objects with the appropriate names
1995for each field. For example:
5a964f20
TC
1996
1997 use File::stat;
1998 use User::pwent;
1999 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2000
b76cc8ba
NIS
2001Even though it looks like they're the same method calls (uid),
2002they aren't, because a C<File::stat> object is different from
19799a22 2003a C<User::pwent> object.
5a964f20 2004
a0d0e21e
LW
2005=item getsockname SOCKET
2006
19799a22
GS
2007Returns the packed sockaddr address of this end of the SOCKET connection,
2008in case you don't know the address because you have several different
2009IPs that the connection might have come in on.
a0d0e21e 2010
4633a7c4
LW
2011 use Socket;
2012 $mysockaddr = getsockname(SOCK);
19799a22 2013 ($port, $myaddr) = sockaddr_in($mysockaddr);
b76cc8ba 2014 printf "Connect to %s [%s]\n",
19799a22
GS
2015 scalar gethostbyaddr($myaddr, AF_INET),
2016 inet_ntoa($myaddr);
a0d0e21e
LW
2017
2018=item getsockopt SOCKET,LEVEL,OPTNAME
2019
5a964f20 2020Returns the socket option requested, or undef if there is an error.
a0d0e21e
LW
2021
2022=item glob EXPR
2023
0a753a76
PP
2024=item glob
2025
2b5ab1e7
TC
2026Returns the value of EXPR with filename expansions such as the
2027standard Unix shell F</bin/csh> would do. This is the internal function
61eff3bc
JH
2028implementing the C<< <*.c> >> operator, but you can use it directly.
2029If EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is
2b5ab1e7 2030discussed in more detail in L<perlop/"I/O Operators">.
a0d0e21e 2031
3a4b19e4
GS
2032Beginning with v5.6.0, this operator is implemented using the standard
2033C<File::Glob> extension. See L<File::Glob> for details.
2034
a0d0e21e
LW
2035=item gmtime EXPR
2036
d1be9408 2037Converts a time as returned by the time function to an 8-element list
54310121 2038with the time localized for the standard Greenwich time zone.
4633a7c4 2039Typically used as follows:
a0d0e21e 2040
b76cc8ba 2041 # 0 1 2 3 4 5 6 7
48a26b3a 2042 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday) =
a0d0e21e
LW
2043 gmtime(time);
2044
48a26b3a
GS
2045All list elements are numeric, and come straight out of the C `struct
2046tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2047specified time. $mday is the day of the month, and $mon is the month
2048itself, in the range C<0..11> with 0 indicating January and 11
2049indicating December. $year is the number of years since 1900. That
2050is, $year is C<123> in year 2023. $wday is the day of the week, with
20510 indicating Sunday and 3 indicating Wednesday. $yday is the day of
b76cc8ba 2052the year, in the range C<0..364> (or C<0..365> in leap years.)
48a26b3a
GS
2053
2054Note that the $year element is I<not> simply the last two digits of
2055the year. If you assume it is, then you create non-Y2K-compliant
2056programs--and you wouldn't want to do that, would you?
2f9daede 2057
abd75f24
GS
2058The proper way to get a complete 4-digit year is simply:
2059
2060 $year += 1900;
2061
2062And to get the last two digits of the year (e.g., '01' in 2001) do:
2063
2064 $year = sprintf("%02d", $year % 100);
2065
48a26b3a 2066If EXPR is omitted, C<gmtime()> uses the current time (C<gmtime(time)>).
a0d0e21e 2067
48a26b3a 2068In scalar context, C<gmtime()> returns the ctime(3) value:
0a753a76
PP
2069
2070 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
2071
19799a22 2072Also see the C<timegm> function provided by the C<Time::Local> module,
f86cebdf 2073and the strftime(3) function available via the POSIX module.
7660c0ab 2074
2b5ab1e7
TC
2075This scalar value is B<not> locale dependent (see L<perllocale>), but
2076is instead a Perl builtin. Also see the C<Time::Local> module, and the
2077strftime(3) and mktime(3) functions available via the POSIX module. To
7660c0ab
A
2078get somewhat similar but locale dependent date strings, set up your
2079locale environment variables appropriately (please see L<perllocale>)
2080and try for example:
2081
2082 use POSIX qw(strftime);
2b5ab1e7 2083 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
7660c0ab 2084
2b5ab1e7
TC
2085Note that the C<%a> and C<%b> escapes, which represent the short forms
2086of the day of the week and the month of the year, may not necessarily
2087be three characters wide in all locales.
0a753a76 2088
a0d0e21e
LW
2089=item goto LABEL
2090
748a9306
LW
2091=item goto EXPR
2092
a0d0e21e
LW
2093=item goto &NAME
2094
7660c0ab 2095The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
a0d0e21e 2096execution there. It may not be used to go into any construct that
7660c0ab 2097requires initialization, such as a subroutine or a C<foreach> loop. It
0a753a76 2098also can't be used to go into a construct that is optimized away,
19799a22 2099or to get out of a block or subroutine given to C<sort>.
0a753a76 2100It can be used to go almost anywhere else within the dynamic scope,
a0d0e21e 2101including out of subroutines, but it's usually better to use some other
19799a22 2102construct such as C<last> or C<die>. The author of Perl has never felt the
7660c0ab 2103need to use this form of C<goto> (in Perl, that is--C is another matter).
1b6921cb
BT
2104(The difference being that C does not offer named loops combined with
2105loop control. Perl does, and this replaces most structured uses of C<goto>
2106in other languages.)
a0d0e21e 2107
7660c0ab
A
2108The C<goto-EXPR> form expects a label name, whose scope will be resolved
2109dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
748a9306
LW
2110necessarily recommended if you're optimizing for maintainability:
2111
2112 goto ("FOO", "BAR", "GLARCH")[$i];
2113
1b6921cb
BT
2114The C<goto-&NAME> form is quite different from the other forms of
2115C<goto>. In fact, it isn't a goto in the normal sense at all, and
2116doesn't have the stigma associated with other gotos. Instead, it
2117exits the current subroutine (losing any changes set by local()) and
2118immediately calls in its place the named subroutine using the current
2119value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2120load another subroutine and then pretend that the other subroutine had
2121been called in the first place (except that any modifications to C<@_>
6cb9131c
GS
2122in the current subroutine are propagated to the other subroutine.)
2123After the C<goto>, not even C<caller> will be able to tell that this
2124routine was called first.
2125
2126NAME needn't be the name of a subroutine; it can be a scalar variable
2127containing a code reference, or a block which evaluates to a code
2128reference.
a0d0e21e
LW
2129
2130=item grep BLOCK LIST
2131
2132=item grep EXPR,LIST
2133
2b5ab1e7
TC
2134This is similar in spirit to, but not the same as, grep(1) and its
2135relatives. In particular, it is not limited to using regular expressions.
2f9daede 2136
a0d0e21e 2137Evaluates the BLOCK or EXPR for each element of LIST (locally setting
7660c0ab 2138C<$_> to each element) and returns the list value consisting of those
19799a22
GS
2139elements for which the expression evaluated to true. In scalar
2140context, returns the number of times the expression was true.
a0d0e21e
LW
2141
2142 @foo = grep(!/^#/, @bar); # weed out comments
2143
2144or equivalently,
2145
2146 @foo = grep {!/^#/} @bar; # weed out comments
2147
be3174d2
GS
2148Note that C<$_> is an alias to the list value, so it can be used to
2149modify the elements of the LIST. While this is useful and supported,
2150it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
2151Similarly, grep returns aliases into the original list, much as a for
2152loop's index variable aliases the list elements. That is, modifying an
19799a22
GS
2153element of a list returned by grep (for example, in a C<foreach>, C<map>
2154or another C<grep>) actually modifies the element in the original list.
2b5ab1e7 2155This is usually something to be avoided when writing clear code.
a0d0e21e 2156
19799a22 2157See also L</map> for a list composed of the results of the BLOCK or EXPR.
38325410 2158
a0d0e21e
LW
2159=item hex EXPR
2160
54310121 2161=item hex
bbce6d69 2162
2b5ab1e7
TC
2163Interprets EXPR as a hex string and returns the corresponding value.
2164(To convert strings that might start with either 0, 0x, or 0b, see
2165L</oct>.) If EXPR is omitted, uses C<$_>.
2f9daede
TPG
2166
2167 print hex '0xAf'; # prints '175'
2168 print hex 'aF'; # same
a0d0e21e 2169
19799a22 2170Hex strings may only represent integers. Strings that would cause
53305cf1
NC
2171integer overflow trigger a warning. Leading whitespace is not stripped,
2172unlike oct().
19799a22 2173
a0d0e21e
LW
2174=item import
2175
19799a22 2176There is no builtin C<import> function. It is just an ordinary
4633a7c4 2177method (subroutine) defined (or inherited) by modules that wish to export
19799a22 2178names to another module. The C<use> function calls the C<import> method
cea6626f 2179for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
a0d0e21e
LW
2180
2181=item index STR,SUBSTR,POSITION
2182
2183=item index STR,SUBSTR
2184
2b5ab1e7
TC
2185The index function searches for one string within another, but without
2186the wildcard-like behavior of a full regular-expression pattern match.
2187It returns the position of the first occurrence of SUBSTR in STR at
2188or after POSITION. If POSITION is omitted, starts searching from the
2189beginning of the string. The return value is based at C<0> (or whatever
2190you've set the C<$[> variable to--but don't do that). If the substring
2191is not found, returns one less than the base, ordinarily C<-1>.
a0d0e21e
LW
2192
2193=item int EXPR
2194
54310121 2195=item int
bbce6d69 2196
7660c0ab 2197Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2b5ab1e7
TC
2198You should not use this function for rounding: one because it truncates
2199towards C<0>, and two because machine representations of floating point
2200numbers can sometimes produce counterintuitive results. For example,
2201C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2202because it's really more like -268.99999999999994315658 instead. Usually,
19799a22 2203the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2b5ab1e7 2204functions will serve you better than will int().
a0d0e21e
LW
2205
2206=item ioctl FILEHANDLE,FUNCTION,SCALAR
2207
2b5ab1e7 2208Implements the ioctl(2) function. You'll probably first have to say
a0d0e21e 2209
4633a7c4 2210 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
a0d0e21e 2211
2b5ab1e7 2212to get the correct function definitions. If F<ioctl.ph> doesn't
a0d0e21e 2213exist or doesn't have the correct definitions you'll have to roll your
61eff3bc 2214own, based on your C header files such as F<< <sys/ioctl.h> >>.
5a964f20 2215(There is a Perl script called B<h2ph> that comes with the Perl kit that
54310121 2216may help you in this, but it's nontrivial.) SCALAR will be read and/or
4633a7c4 2217written depending on the FUNCTION--a pointer to the string value of SCALAR
19799a22 2218will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
4633a7c4
LW
2219has no string value but does have a numeric value, that value will be
2220passed rather than a pointer to the string value. To guarantee this to be
19799a22
GS
2221true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2222functions may be needed to manipulate the values of structures used by
b76cc8ba 2223C<ioctl>.
a0d0e21e 2224
19799a22 2225The return value of C<ioctl> (and C<fcntl>) is as follows:
a0d0e21e
LW
2226
2227 if OS returns: then Perl returns:
2228 -1 undefined value
2229 0 string "0 but true"
2230 anything else that number
2231
19799a22 2232Thus Perl returns true on success and false on failure, yet you can
a0d0e21e
LW
2233still easily determine the actual value returned by the operating
2234system:
2235
2b5ab1e7 2236 $retval = ioctl(...) || -1;
a0d0e21e
LW
2237 printf "System returned %d\n", $retval;
2238
c2611fb3 2239The special string "C<0> but true" is exempt from B<-w> complaints
5a964f20
TC
2240about improper numeric conversions.
2241
19799a22
GS
2242Here's an example of setting a filehandle named C<REMOTE> to be
2243non-blocking at the system level. You'll have to negotiate C<$|>
2244on your own, though.
2245
2246 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
2247
2248 $flags = fcntl(REMOTE, F_GETFL, 0)
2249 or die "Can't get flags for the socket: $!\n";
2250
2251 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
2252 or die "Can't set flags for the socket: $!\n";
2253
a0d0e21e
LW
2254=item join EXPR,LIST
2255
2b5ab1e7
TC
2256Joins the separate strings of LIST into a single string with fields
2257separated by the value of EXPR, and returns that new string. Example:
a0d0e21e 2258
2b5ab1e7 2259 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
a0d0e21e 2260
eb6e2d6f
GS
2261Beware that unlike C<split>, C<join> doesn't take a pattern as its
2262first argument. Compare L</split>.
a0d0e21e 2263
aa689395
PP
2264=item keys HASH
2265
19799a22 2266Returns a list consisting of all the keys of the named hash. (In
1d2dff63 2267scalar context, returns the number of keys.) The keys are returned in
ab192400
GS
2268an apparently random order. The actual random order is subject to
2269change in future versions of perl, but it is guaranteed to be the same
19799a22 2270order as either the C<values> or C<each> function produces (given
ab192400
GS
2271that the hash has not been modified). As a side effect, it resets
2272HASH's iterator.
a0d0e21e 2273
aa689395 2274Here is yet another way to print your environment:
a0d0e21e
LW
2275
2276 @keys = keys %ENV;
2277 @values = values %ENV;
b76cc8ba 2278 while (@keys) {
a0d0e21e
LW
2279 print pop(@keys), '=', pop(@values), "\n";
2280 }
2281
2282or how about sorted by key:
2283
2284 foreach $key (sort(keys %ENV)) {
2285 print $key, '=', $ENV{$key}, "\n";
2286 }
2287
8ea1e5d4
GS
2288The returned values are copies of the original keys in the hash, so
2289modifying them will not affect the original hash. Compare L</values>.
2290
19799a22 2291To sort a hash by value, you'll need to use a C<sort> function.
aa689395 2292Here's a descending numeric sort of a hash by its values:
4633a7c4 2293
5a964f20 2294 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
4633a7c4
LW
2295 printf "%4d %s\n", $hash{$key}, $key;
2296 }
2297
19799a22 2298As an lvalue C<keys> allows you to increase the number of hash buckets
aa689395
PP
2299allocated for the given hash. This can gain you a measure of efficiency if
2300you know the hash is going to get big. (This is similar to pre-extending
2301an array by assigning a larger number to $#array.) If you say
55497cff
PP
2302
2303 keys %hash = 200;
2304
ab192400
GS
2305then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2306in fact, since it rounds up to the next power of two. These
55497cff
PP
2307buckets will be retained even if you do C<%hash = ()>, use C<undef
2308%hash> if you want to free the storage while C<%hash> is still in scope.
2309You can't shrink the number of buckets allocated for the hash using
19799a22 2310C<keys> in this way (but you needn't worry about doing this by accident,
55497cff
PP
2311as trying has no effect).
2312
19799a22 2313See also C<each>, C<values> and C<sort>.
ab192400 2314
b350dd2f 2315=item kill SIGNAL, LIST
a0d0e21e 2316
b350dd2f 2317Sends a signal to a list of processes. Returns the number of
517db077
GS
2318processes successfully signaled (which is not necessarily the
2319same as the number actually killed).
a0d0e21e
LW
2320
2321 $cnt = kill 1, $child1, $child2;
2322 kill 9, @goners;
2323
b350dd2f
GS
2324If SIGNAL is zero, no signal is sent to the process. This is a
2325useful way to check that the process is alive and hasn't changed
2326its UID. See L<perlport> for notes on the portability of this
2327construct.
2328
2329Unlike in the shell, if SIGNAL is negative, it kills
4633a7c4
LW
2330process groups instead of processes. (On System V, a negative I<PROCESS>
2331number will also kill process groups, but that's not portable.) That
2332means you usually want to use positive not negative signals. You may also
da0045b7 2333use a signal name in quotes. See L<perlipc/"Signals"> for details.
a0d0e21e
LW
2334
2335=item last LABEL
2336
2337=item last
2338
2339The C<last> command is like the C<break> statement in C (as used in
2340loops); it immediately exits the loop in question. If the LABEL is
2341omitted, the command refers to the innermost enclosing loop. The
2342C<continue> block, if any, is not executed:
2343
4633a7c4
LW
2344 LINE: while (<STDIN>) {
2345 last LINE if /^$/; # exit when done with header
5a964f20 2346 #...
a0d0e21e
LW
2347 }
2348
4968c1e4 2349C<last> cannot be used to exit a block which returns a value such as
2b5ab1e7
TC
2350C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2351a grep() or map() operation.
4968c1e4 2352
6c1372ed
GS
2353Note that a block by itself is semantically identical to a loop
2354that executes once. Thus C<last> can be used to effect an early
2355exit out of such a block.
2356
98293880
JH
2357See also L</continue> for an illustration of how C<last>, C<next>, and
2358C<redo> work.
1d2dff63 2359
a0d0e21e
LW
2360=item lc EXPR
2361
54310121 2362=item lc
bbce6d69 2363
d1be9408 2364Returns a lowercased version of EXPR. This is the internal function
ad0029c4
JH
2365implementing the C<\L> escape in double-quoted strings. Respects
2366current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
983ffd37 2367and L<perlunicode> for more details about locale and Unicode support.
a0d0e21e 2368
7660c0ab 2369If EXPR is omitted, uses C<$_>.
bbce6d69 2370
a0d0e21e
LW
2371=item lcfirst EXPR
2372
54310121 2373=item lcfirst
bbce6d69 2374
ad0029c4
JH
2375Returns the value of EXPR with the first character lowercased. This
2376is the internal function implementing the C<\l> escape in
2377double-quoted strings. Respects current LC_CTYPE locale if C<use
983ffd37
JH
2378locale> in force. See L<perllocale> and L<perlunicode> for more
2379details about locale and Unicode support.
a0d0e21e 2380
7660c0ab 2381If EXPR is omitted, uses C<$_>.
bbce6d69 2382
a0d0e21e
LW
2383=item length EXPR
2384
54310121 2385=item length
bbce6d69 2386
a0ed51b3 2387Returns the length in characters of the value of EXPR. If EXPR is
b76cc8ba 2388omitted, returns length of C<$_>. Note that this cannot be used on
2b5ab1e7
TC
2389an entire array or hash to find out how many elements these have.
2390For that, use C<scalar @array> and C<scalar keys %hash> respectively.
a0d0e21e
LW
2391
2392=item link OLDFILE,NEWFILE
2393
19799a22 2394Creates a new filename linked to the old filename. Returns true for
b76cc8ba 2395success, false otherwise.
a0d0e21e
LW
2396
2397=item listen SOCKET,QUEUESIZE
2398
19799a22 2399Does the same thing that the listen system call does. Returns true if
b76cc8ba 2400it succeeded, false otherwise. See the example in
cea6626f 2401L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
2402
2403=item local EXPR
2404
19799a22 2405You really probably want to be using C<my> instead, because C<local> isn't
b76cc8ba 2406what most people think of as "local". See
13a2d996 2407L<perlsub/"Private Variables via my()"> for details.
2b5ab1e7 2408
5a964f20
TC
2409A local modifies the listed variables to be local to the enclosing
2410block, file, or eval. If more than one value is listed, the list must
2411be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2412for details, including issues with tied arrays and hashes.
a0d0e21e 2413
a0d0e21e
LW
2414=item localtime EXPR
2415
19799a22 2416Converts a time as returned by the time function to a 9-element list
5f05dabc 2417with the time analyzed for the local time zone. Typically used as
a0d0e21e
LW
2418follows:
2419
54310121 2420 # 0 1 2 3 4 5 6 7 8
a0d0e21e
LW
2421 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2422 localtime(time);
2423
48a26b3a
GS
2424All list elements are numeric, and come straight out of the C `struct
2425tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2426specified time. $mday is the day of the month, and $mon is the month
2427itself, in the range C<0..11> with 0 indicating January and 11
2428indicating December. $year is the number of years since 1900. That
2429is, $year is C<123> in year 2023. $wday is the day of the week, with
24300 indicating Sunday and 3 indicating Wednesday. $yday is the day of
874b1813 2431the year, in the range C<0..364> (or C<0..365> in leap years.) $isdst
48a26b3a
GS
2432is true if the specified time occurs during daylight savings time,
2433false otherwise.
2434
2435Note that the $year element is I<not> simply the last two digits of
2436the year. If you assume it is, then you create non-Y2K-compliant
2437programs--and you wouldn't want to do that, would you?
54310121 2438
abd75f24
GS
2439The proper way to get a complete 4-digit year is simply:
2440
2441 $year += 1900;
2442
2443And to get the last two digits of the year (e.g., '01' in 2001) do:
2444
2445 $year = sprintf("%02d", $year % 100);
2446
48a26b3a 2447If EXPR is omitted, C<localtime()> uses the current time (C<localtime(time)>).
a0d0e21e 2448
48a26b3a 2449In scalar context, C<localtime()> returns the ctime(3) value:
a0d0e21e 2450
5f05dabc 2451 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
a0d0e21e 2452
a3cb178b 2453This scalar value is B<not> locale dependent, see L<perllocale>, but
68f8bed4
JH
2454instead a Perl builtin. Also see the C<Time::Local> module
2455(to convert the second, minutes, hours, ... back to seconds since the
2456stroke of midnight the 1st of January 1970, the value returned by
ca6e1c26 2457time()), and the strftime(3) and mktime(3) functions available via the
68f8bed4
JH
2458POSIX module. To get somewhat similar but locale dependent date
2459strings, set up your locale environment variables appropriately
2460(please see L<perllocale>) and try for example:
a3cb178b 2461
5a964f20 2462 use POSIX qw(strftime);
2b5ab1e7 2463 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
a3cb178b
GS
2464
2465Note that the C<%a> and C<%b>, the short forms of the day of the week
2466and the month of the year, may not necessarily be three characters wide.
a0d0e21e 2467
07698885 2468=item lock THING
19799a22
GS
2469
2470This function places an advisory lock on a variable, subroutine,
2471or referenced object contained in I<THING> until the lock goes out
2472of scope. This is a built-in function only if your version of Perl
43d3ddbe
JH
2473was built with threading enabled, and if you've said C<use Thread>.
2474Otherwise a user-defined function by this name will be called.
2475See L<Thread>.
19799a22 2476
a0d0e21e
LW
2477=item log EXPR
2478
54310121 2479=item log
bbce6d69 2480
2b5ab1e7
TC
2481Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2482returns log of C<$_>. To get the log of another base, use basic algebra:
19799a22 2483The base-N log of a number is equal to the natural log of that number
2b5ab1e7
TC
2484divided by the natural log of N. For example:
2485
2486 sub log10 {
2487 my $n = shift;
2488 return log($n)/log(10);
b76cc8ba 2489 }
2b5ab1e7
TC
2490
2491See also L</exp> for the inverse operation.
a0d0e21e 2492
a0d0e21e
LW
2493=item lstat EXPR
2494
54310121 2495=item lstat
bbce6d69 2496
19799a22 2497Does the same thing as the C<stat> function (including setting the
5a964f20
TC
2498special C<_> filehandle) but stats a symbolic link instead of the file
2499the symbolic link points to. If symbolic links are unimplemented on
19799a22 2500your system, a normal C<stat> is done.
a0d0e21e 2501
7660c0ab 2502If EXPR is omitted, stats C<$_>.
bbce6d69 2503
a0d0e21e
LW
2504=item m//
2505
2506The match operator. See L<perlop>.
2507
2508=item map BLOCK LIST
2509
2510=item map EXPR,LIST
2511
19799a22
GS
2512Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2513C<$_> to each element) and returns the list value composed of the
2514results of each such evaluation. In scalar context, returns the
2515total number of elements so generated. Evaluates BLOCK or EXPR in
2516list context, so each element of LIST may produce zero, one, or
2517more elements in the returned value.
dd99ebda 2518
a0d0e21e
LW
2519 @chars = map(chr, @nums);
2520
2521translates a list of numbers to the corresponding characters. And
2522
4633a7c4 2523 %hash = map { getkey($_) => $_ } @array;
a0d0e21e
LW
2524
2525is just a funny way to write
2526
2527 %hash = ();
2528 foreach $_ (@array) {
4633a7c4 2529 $hash{getkey($_)} = $_;
a0d0e21e
LW
2530 }
2531
be3174d2
GS
2532Note that C<$_> is an alias to the list value, so it can be used to
2533modify the elements of the LIST. While this is useful and supported,
2534it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
2535Using a regular C<foreach> loop for this purpose would be clearer in
2536most cases. See also L</grep> for an array composed of those items of
2537the original list for which the BLOCK or EXPR evaluates to true.
fb73857a 2538
205fdb4d
NC
2539C<{> starts both hash references and blocks, so C<map { ...> could be either
2540the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look
2541ahead for the closing C<}> it has to take a guess at which its dealing with
2542based what it finds just after the C<{>. Usually it gets it right, but if it
2543doesn't it won't realize something is wrong until it gets to the C<}> and
2544encounters the missing (or unexpected) comma. The syntax error will be
2545reported close to the C<}> but you'll need to change something near the C<{>
2546such as using a unary C<+> to give perl some help:
2547
2548 %hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong
2549 %hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right
2550 %hash = map { ("\L$_", 1) } @array # this also works
2551 %hash = map { lc($_), 1 } @array # as does this.
2552 %hash = map +( lc($_), 1 ), @array # this is EXPR and works!
cea6626f 2553
205fdb4d
NC
2554 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
2555
2556or to force an anon hash constructor use C<+{>
2557
2558 @hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end
2559
2560and you get list of anonymous hashes each with only 1 entry.
2561
19799a22 2562=item mkdir FILENAME,MASK
a0d0e21e 2563
5a211162
GS
2564=item mkdir FILENAME
2565
0591cd52 2566Creates the directory specified by FILENAME, with permissions
19799a22
GS
2567specified by MASK (as modified by C<umask>). If it succeeds it
2568returns true, otherwise it returns false and sets C<$!> (errno).
5a211162 2569If omitted, MASK defaults to 0777.
0591cd52 2570
19799a22 2571In general, it is better to create directories with permissive MASK,
0591cd52 2572and let the user modify that with their C<umask>, than it is to supply
19799a22 2573a restrictive MASK and give the user no way to be more permissive.
0591cd52
NT
2574The exceptions to this rule are when the file or directory should be
2575kept private (mail files, for instance). The perlfunc(1) entry on
19799a22 2576C<umask> discusses the choice of MASK in more detail.
a0d0e21e 2577
cc1852e8
JH
2578Note that according to the POSIX 1003.1-1996 the FILENAME may have any
2579number of trailing slashes. Some operating and filesystems do not get
2580this right, so Perl automatically removes all trailing slashes to keep
2581everyone happy.
2582
a0d0e21e
LW
2583=item msgctl ID,CMD,ARG
2584
f86cebdf 2585Calls the System V IPC function msgctl(2). You'll probably have to say
0ade1984
JH
2586
2587 use IPC::SysV;
2588
7660c0ab
A
2589first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2590then ARG must be a variable which will hold the returned C<msqid_ds>
951ba7fe
GS
2591structure. Returns like C<ioctl>: the undefined value for error,
2592C<"0 but true"> for zero, or the actual return value otherwise. See also
4755096e 2593L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation.
a0d0e21e
LW
2594
2595=item msgget KEY,FLAGS
2596
f86cebdf 2597Calls the System V IPC function msgget(2). Returns the message queue
4755096e
GS
2598id, or the undefined value if there is an error. See also
2599L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation.
a0d0e21e 2600
a0d0e21e
LW
2601=item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2602
2603Calls the System V IPC function msgrcv to receive a message from
2604message queue ID into variable VAR with a maximum message size of
41d6edb2
JH
2605SIZE. Note that when a message is received, the message type as a
2606native long integer will be the first thing in VAR, followed by the
2607actual message. This packing may be opened with C<unpack("l! a*")>.
2608Taints the variable. Returns true if successful, or false if there is
4755096e
GS
2609an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and
2610C<IPC::SysV::Msg> documentation.
41d6edb2
JH
2611
2612=item msgsnd ID,MSG,FLAGS
2613
2614Calls the System V IPC function msgsnd to send the message MSG to the
2615message queue ID. MSG must begin with the native long integer message
2616type, and be followed by the length of the actual message, and finally
2617the message itself. This kind of packing can be achieved with
2618C<pack("l! a*", $type, $message)>. Returns true if successful,
2619or false if there is an error. See also C<IPC::SysV>
2620and C<IPC::SysV::Msg> documentation.
a0d0e21e
LW
2621
2622=item my EXPR
2623
09bef843
SB
2624=item my EXPR : ATTRIBUTES
2625
19799a22
GS
2626A C<my> declares the listed variables to be local (lexically) to the
2627enclosing block, file, or C<eval>. If
5f05dabc 2628more than one value is listed, the list must be placed in parentheses. See
cb1a09d0 2629L<perlsub/"Private Variables via my()"> for details.
4633a7c4 2630
a0d0e21e
LW
2631=item next LABEL
2632
2633=item next
2634
2635The C<next> command is like the C<continue> statement in C; it starts
2636the next iteration of the loop:
2637
4633a7c4
LW
2638 LINE: while (<STDIN>) {
2639 next LINE if /^#/; # discard comments
5a964f20 2640 #...
a0d0e21e
LW
2641 }
2642
2643Note that if there were a C<continue> block on the above, it would get
2644executed even on discarded lines. If the LABEL is omitted, the command
2645refers to the innermost enclosing loop.
2646
4968c1e4 2647C<next> cannot be used to exit a block which returns a value such as
2b5ab1e7
TC
2648C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2649a grep() or map() operation.
4968c1e4 2650
6c1372ed
GS
2651Note that a block by itself is semantically identical to a loop
2652that executes once. Thus C<next> will exit such a block early.
2653
98293880
JH
2654See also L</continue> for an illustration of how C<last>, C<next>, and
2655C<redo> work.
1d2dff63 2656
4a66ea5a
RGS
2657=item no Module VERSION LIST
2658
2659=item no Module VERSION
2660
a0d0e21e
LW
2661=item no Module LIST
2662
4a66ea5a
RGS
2663=item no Module
2664
7660c0ab 2665See the L</use> function, which C<no> is the opposite of.
a0d0e21e
LW
2666
2667=item oct EXPR
2668
54310121 2669=item oct
bbce6d69 2670
4633a7c4 2671Interprets EXPR as an octal string and returns the corresponding
4f19785b
WSI
2672value. (If EXPR happens to start off with C<0x>, interprets it as a
2673hex string. If EXPR starts off with C<0b>, it is interpreted as a
53305cf1
NC
2674binary string. Leading whitespace is ignored in all three cases.)
2675The following will handle decimal, binary, octal, and hex in the standard
2676Perl or C notation:
a0d0e21e
LW
2677
2678 $val = oct($val) if $val =~ /^0/;
2679
19799a22
GS
2680If EXPR is omitted, uses C<$_>. To go the other way (produce a number
2681in octal), use sprintf() or printf():
2682
2683 $perms = (stat("filename"))[2] & 07777;
2684 $oct_perms = sprintf "%lo", $perms;
2685
2686The oct() function is commonly used when a string such as C<644> needs
2687to be converted into a file mode, for example. (Although perl will
2688automatically convert strings into numbers as needed, this automatic
2689conversion assumes base 10.)
a0d0e21e
LW
2690
2691=item open FILEHANDLE,EXPR
2692
68bd7414
NIS
2693=item open FILEHANDLE,MODE,EXPR
2694
2695=item open FILEHANDLE,MODE,EXPR,LIST
2696
ba964c95
T
2697=item open FILEHANDLE,MODE,REFERENCE
2698
a0d0e21e
LW
2699=item open FILEHANDLE
2700
2701Opens the file whose filename is given by EXPR, and associates it with
ed53a2bb
JH
2702FILEHANDLE.
2703
2704(The following is a comprehensive reference to open(): for a gentler
2705introduction you may consider L<perlopentut>.)
2706
2707If FILEHANDLE is an undefined lexical (C<my>) variable the variable is
2708assigned a reference to a new anonymous filehandle, otherwise if
2709FILEHANDLE is an expression, its value is used as the name of the real
2710filehandle wanted. (This is considered a symbolic reference, so C<use
2711strict 'refs'> should I<not> be in effect.)
2712
2713If EXPR is omitted, the scalar variable of the same name as the
2714FILEHANDLE contains the filename. (Note that lexical variables--those
2715declared with C<my>--will not work for this purpose; so if you're
2716using C<my>, specify EXPR in your call to open.)
2717
2718If three or more arguments are specified then the mode of opening and
2719the file name are separate. If MODE is C<< '<' >> or nothing, the file
2720is opened for input. If MODE is C<< '>' >>, the file is truncated and
2721opened for output, being created if necessary. If MODE is C<<< '>>' >>>,
b76cc8ba 2722the file is opened for appending, again being created if necessary.
5a964f20 2723
ed53a2bb
JH
2724You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to
2725indicate that you want both read and write access to the file; thus
2726C<< '+<' >> is almost always preferred for read/write updates--the C<<
2727'+>' >> mode would clobber the file first. You can't usually use
2728either read-write mode for updating textfiles, since they have
2729variable length records. See the B<-i> switch in L<perlrun> for a
2730better approach. The file is created with permissions of C<0666>
2731modified by the process' C<umask> value.
2732
2733These various prefixes correspond to the fopen(3) modes of C<'r'>,
2734C<'r+'>, C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
5f05dabc 2735
6170680b
IZ
2736In the 2-arguments (and 1-argument) form of the call the mode and
2737filename should be concatenated (in this order), possibly separated by
68bd7414
NIS
2738spaces. It is possible to omit the mode in these forms if the mode is
2739C<< '<' >>.
6170680b 2740
7660c0ab 2741If the filename begins with C<'|'>, the filename is interpreted as a
5a964f20 2742command to which output is to be piped, and if the filename ends with a
f244e06d
GS
2743C<'|'>, the filename is interpreted as a command which pipes output to
2744us. See L<perlipc/"Using open() for IPC">
19799a22 2745for more examples of this. (You are not allowed to C<open> to a command
5a964f20 2746that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
4a4eefd0
GS
2747and L<perlipc/"Bidirectional Communication with Another Process">
2748for alternatives.)
cb1a09d0 2749
ed53a2bb
JH
2750For three or more arguments if MODE is C<'|-'>, the filename is
2751interpreted as a command to which output is to be piped, and if MODE
2752is C<'-|'>, the filename is interpreted as a command which pipes
2753output to us. In the 2-arguments (and 1-argument) form one should
2754replace dash (C<'-'>) with the command.
2755See L<perlipc/"Using open() for IPC"> for more examples of this.
2756(You are not allowed to C<open> to a command that pipes both in I<and>
2757out, but see L<IPC::Open2>, L<IPC::Open3>, and
2758L<perlipc/"Bidirectional Communication"> for alternatives.)
2759
2760In the three-or-more argument form of pipe opens, if LIST is specified
2761(extra arguments after the command name) then LIST becomes arguments
2762to the command invoked if the platform supports it. The meaning of
2763C<open> with more than three arguments for non-pipe modes is not yet
2764specified. Experimental "layers" may give extra LIST arguments
2765meaning.
6170680b
IZ
2766
2767In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
b76cc8ba 2768and opening C<< '>-' >> opens STDOUT.
6170680b 2769
ed53a2bb
JH
2770You may use the three-argument form of open to specify
2771I<I/O disciplines> that affect how the input and output
9124316e 2772are processed: see L</binmode> and L<open>. For example
7207e29d 2773
9124316e
JH
2774 open(FH, "<:utf8", "file")
2775
2776will open the UTF-8 encoded file containing Unicode characters,
2777see L<perluniintro>.
ed53a2bb
JH
2778
2779Open returns nonzero upon success, the undefined value otherwise. If
2780the C<open> involved a pipe, the return value happens to be the pid of
2781the subprocess.
cb1a09d0 2782
ed53a2bb
JH
2783If you're running Perl on a system that distinguishes between text
2784files and binary files, then you should check out L</binmode> for tips
2785for dealing with this. The key distinction between systems that need
2786C<binmode> and those that don't is their text file formats. Systems
2787like Unix, MacOS, and Plan9, which delimit lines with a single
2788character, and which encode that character in C as C<"\n">, do not
2789need C<binmode>. The rest need it.
cb1a09d0 2790
ed53a2bb
JH
2791In the three argument form MODE may also contain a list of IO "layers"
2792(see L<open> and L<PerlIO> for more details) to be applied to the
2793handle. This can be used to achieve the effect of C<binmode> as well
2794as more complex behaviours.
68bd7414 2795
fb73857a 2796When opening a file, it's usually a bad idea to continue normal execution
19799a22
GS
2797if the request failed, so C<open> is frequently used in connection with
2798C<die>. Even if C<die> won't do what you want (say, in a CGI script,
fb73857a 2799where you want to make a nicely formatted error message (but there are
5a964f20 2800modules that can help with that problem)) you should always check
19799a22 2801the return value from opening a file. The infrequent exception is when
fb73857a
PP
2802working with an unopened filehandle is actually what you want to do.
2803
ed53a2bb
JH
2804As a special case the 3 arg form with a read/write mode and the third
2805argument being C<undef>:
b76cc8ba
NIS
2806
2807 open(TMP, "+>", undef) or die ...
2808
2809opens a filehandle to an anonymous temporary file.
2810
ba964c95
T
2811File handles can be opened to "in memory" files held in Perl scalars via:
2812
b996200f
SB
2813 open($fh, '>', \$variable) || ..
2814
2815Though if you try to re-open C<STDOUT> or C<STDERR> as an "in memory"
2816file, you have to close it first:
2817
2818 close STDOUT;
2819 open STDOUT, '>', \$variable or die "Can't open STDOUT: $!";
ba964c95 2820
cb1a09d0 2821Examples:
a0d0e21e
LW
2822
2823 $ARTICLE = 100;
2824 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2825 while (<ARTICLE>) {...
2826
6170680b 2827 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
fb73857a 2828 # if the open fails, output is discarded
a0d0e21e 2829
6170680b 2830 open(DBASE, '+<', 'dbase.mine') # open for update
fb73857a 2831 or die "Can't open 'dbase.mine' for update: $!";
cb1a09d0 2832
6170680b
IZ
2833 open(DBASE, '+<dbase.mine') # ditto
2834 or die "Can't open 'dbase.mine' for update: $!";
2835
2836 open(ARTICLE, '-|', "caesar <$article") # decrypt article
fb73857a 2837 or die "Can't start caesar: $!";
a0d0e21e 2838
6170680b
IZ
2839 open(ARTICLE, "caesar <$article |") # ditto
2840 or die "Can't start caesar: $!";
2841
2842 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
fb73857a 2843 or die "Can't start sort: $!";
a0d0e21e 2844
ba964c95
T
2845 # in memory files
2846 open(MEMORY,'>', \$var)
2847 or die "Can't open memory file: $!";
2848 print MEMORY "foo!\n"; # output will end up in $var
2849
a0d0e21e
LW
2850 # process argument list of files along with any includes
2851
2852 foreach $file (@ARGV) {
2853 process($file, 'fh00');
2854 }
2855
2856 sub process {
5a964f20 2857 my($filename, $input) = @_;
a0d0e21e
LW
2858 $input++; # this is a string increment
2859 unless (open($input, $filename)) {
2860 print STDERR "Can't open $filename: $!\n";
2861 return;
2862 }
2863
5a964f20 2864 local $_;
a0d0e21e
LW
2865 while (<$input>) { # note use of indirection
2866 if (/^#include "(.*)"/) {
2867 process($1, $input);
2868 next;
2869 }
5a964f20 2870 #... # whatever
a0d0e21e
LW
2871 }
2872 }
2873
2874You may also, in the Bourne shell tradition, specify an EXPR beginning
61eff3bc 2875with C<< '>&' >>, in which case the rest of the string is interpreted as the
5a964f20 2876name of a filehandle (or file descriptor, if numeric) to be
61eff3bc
JH
2877duped and opened. You may use C<&> after C<< > >>, C<<< >> >>>,
2878C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>. The
a0d0e21e 2879mode you specify should match the mode of the original filehandle.
184e9718 2880(Duping a filehandle does not take into account any existing contents of
9124316e 2881IO buffers.) If you use the 3 arg form then you can pass either a number,
b76cc8ba 2882the name of a filehandle or the normal "reference to a glob".
6170680b 2883
eae1b76b
SB
2884Here is a script that saves, redirects, and restores C<STDOUT> and
2885C<STDERR> using various methods:
a0d0e21e
LW
2886
2887 #!/usr/bin/perl
eae1b76b
SB
2888 open my $oldout, ">&STDOUT" or die "Can't dup STDOUT: $!";
2889 open OLDERR, ">&", \*STDERR or die "Can't dup STDERR: $!";
2890
2891 open STDOUT, '>', "foo.out" or die "Can't redirect STDOUT: $!";
2892 open STDERR, ">&STDOUT" or die "Can't dup STDOUT: $!";
a0d0e21e 2893
eae1b76b
SB
2894 select STDERR; $| = 1; # make unbuffered
2895 select STDOUT; $| = 1; # make unbuffered
a0d0e21e
LW
2896
2897 print STDOUT "stdout 1\n"; # this works for
2898 print STDERR "stderr 1\n"; # subprocesses too
2899
eae1b76b
SB
2900 close STDOUT;
2901 close STDERR;
a0d0e21e 2902
eae1b76b
SB
2903 open STDOUT, ">&", $oldout or die "Can't dup \$oldout: $!";
2904 open STDERR, ">&OLDERR" or die "Can't dup OLDERR: $!";
a0d0e21e
LW
2905
2906 print STDOUT "stdout 2\n";
2907 print STDERR "stderr 2\n";
2908
df632fdf
JH
2909If you specify C<< '<&=N' >>, where C<N> is a number, then Perl will
2910do an equivalent of C's C<fdopen> of that file descriptor; this is
2911more parsimonious of file descriptors. For example:
a0d0e21e
LW
2912
2913 open(FILEHANDLE, "<&=$fd")
df632fdf 2914
b76cc8ba 2915or
df632fdf 2916
b76cc8ba 2917 open(FILEHANDLE, "<&=", $fd)
a0d0e21e 2918
df632fdf
JH
2919Note that if Perl is using the standard C libraries' fdopen() then on
2920many UNIX systems, fdopen() is known to fail when file descriptors
4af147f6 2921exceed a certain value, typically 255. If you need more file
b76cc8ba 2922descriptors than that, consider rebuilding Perl to use the C<PerlIO>.
4af147f6 2923
df632fdf
JH
2924You can see whether Perl has been compiled with PerlIO or not by
2925running C<perl -V> and looking for C<useperlio=> line. If C<useperlio>
2926is C<define>, you have PerlIO, otherwise you don't.
2927
6170680b
IZ
2928If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
2929with 2-arguments (or 1-argument) form of open(), then
a0d0e21e 2930there is an implicit fork done, and the return value of open is the pid
7660c0ab 2931of the child within the parent process, and C<0> within the child
184e9718 2932process. (Use C<defined($pid)> to determine whether the open was successful.)
a0d0e21e
LW
2933The filehandle behaves normally for the parent, but i/o to that
2934filehandle is piped from/to the STDOUT/STDIN of the child process.
2935In the child process the filehandle isn't opened--i/o happens from/to
2936the new STDOUT or STDIN. Typically this is used like the normal
2937piped open when you want to exercise more control over just how the
2938pipe command gets executed, such as when you are running setuid, and
54310121 2939don't want to have to scan shell commands for metacharacters.
6170680b 2940The following triples are more or less equivalent:
a0d0e21e
LW
2941
2942 open(FOO, "|tr '[a-z]' '[A-Z]'");
6170680b
IZ
2943 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
2944 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
b76cc8ba 2945 open(FOO, '|-', "tr", '[a-z]', '[A-Z]');
a0d0e21e
LW
2946
2947 open(FOO, "cat -n '$file'|");
6170680b
IZ
2948 open(FOO, '-|', "cat -n '$file'");
2949 open(FOO, '-|') || exec 'cat', '-n', $file;
b76cc8ba
NIS
2950 open(FOO, '-|', "cat", '-n', $file);
2951
2952The last example in each block shows the pipe as "list form", which is
2953not yet supported on all platforms.
a0d0e21e 2954
4633a7c4
LW
2955See L<perlipc/"Safe Pipe Opens"> for more examples of this.
2956
0f897271
GS
2957Beginning with v5.6.0, Perl will attempt to flush all files opened for
2958output before any operation that may do a fork, but this may not be
2959supported on some platforms (see L<perlport>). To be safe, you may need
2960to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
2961of C<IO::Handle> on any open handles.
2962
ed53a2bb
JH
2963On systems that support a close-on-exec flag on files, the flag will
2964be set for the newly opened file descriptor as determined by the value
2965of $^F. See L<perlvar/$^F>.
a0d0e21e 2966
0dccf244
CS
2967Closing any piped filehandle causes the parent process to wait for the
2968child to finish, and returns the status value in C<$?>.
2969
ed53a2bb
JH
2970The filename passed to 2-argument (or 1-argument) form of open() will
2971have leading and trailing whitespace deleted, and the normal
2972redirection characters honored. This property, known as "magic open",
5a964f20 2973can often be used to good effect. A user could specify a filename of
7660c0ab 2974F<"rsh cat file |">, or you could change certain filenames as needed:
5a964f20
TC
2975
2976 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
2977 open(FH, $filename) or die "Can't open $filename: $!";
2978
6170680b
IZ
2979Use 3-argument form to open a file with arbitrary weird characters in it,
2980
2981 open(FOO, '<', $file);
2982
2983otherwise it's necessary to protect any leading and trailing whitespace:
5a964f20
TC
2984
2985 $file =~ s#^(\s)#./$1#;
2986 open(FOO, "< $file\0");
2987
a31a806a 2988(this may not work on some bizarre filesystems). One should
106325ad 2989conscientiously choose between the I<magic> and 3-arguments form
6170680b
IZ
2990of open():
2991
2992 open IN, $ARGV[0];
2993
2994will allow the user to specify an argument of the form C<"rsh cat file |">,
2995but will not work on a filename which happens to have a trailing space, while
2996
2997 open IN, '<', $ARGV[0];
2998
2999will have exactly the opposite restrictions.
3000
19799a22 3001If you want a "real" C C<open> (see L<open(2)> on your system), then you
6170680b
IZ
3002should use the C<sysopen> function, which involves no such magic (but
3003may use subtly different filemodes than Perl open(), which is mapped
3004to C fopen()). This is
5a964f20
TC
3005another way to protect your filenames from interpretation. For example:
3006
3007 use IO::Handle;
3008 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
3009 or die "sysopen $path: $!";
3010 $oldfh = select(HANDLE); $| = 1; select($oldfh);
38762f02 3011 print HANDLE "stuff $$\n";
5a964f20
TC
3012 seek(HANDLE, 0, 0);
3013 print "File contains: ", <HANDLE>;
3014
7660c0ab
A
3015Using the constructor from the C<IO::Handle> package (or one of its
3016subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
5a964f20
TC
3017filehandles that have the scope of whatever variables hold references to
3018them, and automatically close whenever and however you leave that scope:
c07a80fd 3019
5f05dabc 3020 use IO::File;
5a964f20 3021 #...
c07a80fd
PP
3022 sub read_myfile_munged {
3023 my $ALL = shift;
5f05dabc 3024 my $handle = new IO::File;
c07a80fd
PP
3025 open($handle, "myfile") or die "myfile: $!";
3026 $first = <$handle>
3027 or return (); # Automatically closed here.
3028 mung $first or die "mung failed"; # Or here.
3029 return $first, <$handle> if $ALL; # Or here.
3030 $first; # Or here.
3031 }
3032
b687b08b 3033See L</seek> for some details about mixing reading and writing.
a0d0e21e
LW
3034
3035=item opendir DIRHANDLE,EXPR
3036
19799a22
GS
3037Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
3038C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
a0d0e21e
LW
3039DIRHANDLEs have their own namespace separate from FILEHANDLEs.
3040
3041=item ord EXPR
3042
54310121 3043=item ord
bbce6d69 3044
121910a4
JH
3045Returns the numeric (the native 8-bit encoding, like ASCII or EBCDIC,
3046or Unicode) value of the first character of EXPR. If EXPR is omitted,
3047uses C<$_>.
3048
3049For the reverse, see L</chr>.
3050See L<perlunicode> and L<encoding> for more about Unicode.
a0d0e21e 3051
77ca0c92
LW
3052=item our EXPR
3053
9969eac4
BS
3054=item our EXPR : ATTRIBUTES
3055
77ca0c92
LW
3056An C<our> declares the listed variables to be valid globals within
3057the enclosing block, file, or C<eval>. That is, it has the same
3058scoping rules as a "my" declaration, but does not create a local
3059variable. If more than one value is listed, the list must be placed
3060in parentheses. The C<our> declaration has no semantic effect unless
3061"use strict vars" is in effect, in which case it lets you use the
3062declared global variable without qualifying it with a package name.
3063(But only within the lexical scope of the C<our> declaration. In this
3064it differs from "use vars", which is package scoped.)
3065
f472eb5c
GS
3066An C<our> declaration declares a global variable that will be visible
3067across its entire lexical scope, even across package boundaries. The
3068package in which the variable is entered is determined at the point
3069of the declaration, not at the point of use. This means the following
3070behavior holds:
3071
3072 package Foo;
3073 our $bar; # declares $Foo::bar for rest of lexical scope
3074 $bar = 20;
3075
3076 package Bar;
3077 print $bar; # prints 20
3078
3079Multiple C<our> declarations in the same lexical scope are allowed
3080if they are in different packages. If they happened to be in the same
3081package, Perl will emit warnings if you have asked for them.
3082
3083 use warnings;
3084 package Foo;
3085 our $bar; # declares $Foo::bar for rest of lexical scope
3086 $bar = 20;
3087
3088 package Bar;
3089 our $bar = 30; # declares $Bar::bar for rest of lexical scope
3090 print $bar; # prints 30
3091
3092 our $bar; # emits warning
3093
9969eac4
BS
3094An C<our> declaration may also have a list of attributes associated
3095with it. B<WARNING>: This is an experimental feature that may be
3096changed or removed in future releases of Perl. It should not be
3097relied upon.
3098
51d2bbcc 3099The only currently recognized attribute is C<unique> which indicates
9969eac4
BS
3100that a single copy of the global is to be used by all interpreters
3101should the program happen to be running in a multi-interpreter
3102environment. (The default behaviour would be for each interpreter to
3103have its own copy of the global.) In such an environment, this
3104attribute also has the effect of making the global readonly.
3105Examples:
3106
51d2bbcc
JH
3107 our @EXPORT : unique = qw(foo);
3108 our %EXPORT_TAGS : unique = (bar => [qw(aa bb cc)]);
3109 our $VERSION : unique = "1.00";
9969eac4
BS
3110
3111Multi-interpreter environments can come to being either through the
3112fork() emulation on Windows platforms, or by embedding perl in a
51d2bbcc 3113multi-threaded application. The C<unique> attribute does nothing in
9969eac4
BS
3114all other environments.
3115
a0d0e21e
LW
3116=item pack TEMPLATE,LIST
3117
2b6c5635
GS
3118Takes a LIST of values and converts it into a string using the rules
3119given by the TEMPLATE. The resulting string is the concatenation of
3120the converted values. Typically, each converted value looks
3121like its machine-level representation. For example, on 32-bit machines
3122a converted integer may be represented by a sequence of 4 bytes.
3123
18529408
IZ
3124The TEMPLATE is a sequence of characters that give the order and type
3125of values, as follows:
a0d0e21e 3126
5a929a98 3127 a A string with arbitrary binary data, will be null padded.
121910a4
JH
3128 A A text (ASCII) string, will be space padded.
3129 Z A null terminated (ASCIZ) string, will be null padded.
5a929a98 3130
2b6c5635
GS
3131 b A bit string (ascending bit order inside each byte, like vec()).
3132 B A bit string (descending bit order inside each byte).
a0d0e21e
LW
3133 h A hex string (low nybble first).
3134 H A hex string (high nybble first).
3135
3136 c A signed char value.
a0ed51b3 3137 C An unsigned char value. Only does bytes. See U for Unicode.
96e4d5b1 3138
a0d0e21e
LW
3139 s A signed short value.
3140 S An unsigned short value.
96e4d5b1 3141 (This 'short' is _exactly_ 16 bits, which may differ from
851646ae
JH
3142 what a local C compiler calls 'short'. If you want
3143 native-length shorts, use the '!' suffix.)
96e4d5b1 3144
a0d0e21e
LW
3145 i A signed integer value.
3146 I An unsigned integer value.
19799a22 3147 (This 'integer' is _at_least_ 32 bits wide. Its exact
f86cebdf
GS
3148 size depends on what a local C compiler calls 'int',
3149 and may even be larger than the 'long' described in
3150 the next item.)
96e4d5b1 3151
a0d0e21e
LW
3152 l A signed long value.
3153 L An unsigned long value.
96e4d5b1 3154 (This 'long' is _exactly_ 32 bits, which may differ from
851646ae
JH
3155 what a local C compiler calls 'long'. If you want
3156 native-length longs, use the '!' suffix.)
a0d0e21e 3157
5d11dd56
G
3158 n An unsigned short in "network" (big-endian) order.
3159 N An unsigned long in "network" (big-endian) order.
3160 v An unsigned short in "VAX" (little-endian) order.
3161 V An unsigned long in "VAX" (little-endian) order.
96e4d5b1
PP
3162 (These 'shorts' and 'longs' are _exactly_ 16 bits and
3163 _exactly_ 32 bits, respectively.)
a0d0e21e 3164
dae0da7a
JH
3165 q A signed quad (64-bit) value.
3166 Q An unsigned quad value.
851646ae
JH
3167 (Quads are available only if your system supports 64-bit
3168 integer values _and_ if Perl has been compiled to support those.
dae0da7a
JH
3169 Causes a fatal error otherwise.)
3170
92d41999
JH
3171 j A signed integer value (a Perl internal integer, IV).
3172 J An unsigned integer value (a Perl internal unsigned integer, UV).
3173
a0d0e21e
LW
3174 f A single-precision float in the native format.
3175 d A double-precision float in the native format.
3176
92d41999
JH
3177 F A floating point value in the native native format
3178 (a Perl internal floating point value, NV).
3179 D A long double-precision float in the native format.
3180 (Long doubles are available only if your system supports long
3181 double values _and_ if Perl has been compiled to support those.
3182 Causes a fatal error otherwise.)
3183
a0d0e21e
LW
3184 p A pointer to a null-terminated string.
3185 P A pointer to a structure (fixed-length string).
3186
3187 u A uuencoded string.
ad0029c4
JH
3188 U A Unicode character number. Encodes to UTF-8 internally
3189 (or UTF-EBCDIC in EBCDIC platforms).
a0d0e21e 3190
96e4d5b1 3191 w A BER compressed integer. Its bytes represent an unsigned
f86cebdf
GS
3192 integer in base 128, most significant digit first, with as
3193 few digits as possible. Bit eight (the high bit) is set
3194 on each byte except the last.
def98dd4 3195
a0d0e21e
LW
3196 x A null byte.
3197 X Back up a byte.
3198 @ Null fill to absolute position.
206947d2 3199 ( Start of a ()-group.
a0d0e21e 3200
5a929a98
VU
3201The following rules apply:
3202
3203=over 8
3204
3205=item *
3206
5a964f20 3207Each letter may optionally be followed by a number giving a repeat
951ba7fe 3208count. With all types except C<a>, C<A>, C<Z>, C<b>, C<B>, C<h>,
206947d2
IZ
3209C<H>, C<@>, C<x>, C<X> and C<P> the pack function will gobble up that
3210many values from the LIST. A C<*> for the repeat count means to use
3211however many items are left, except for C<@>, C<x>, C<X>, where it is
3212equivalent to C<0>, and C<u>, where it is equivalent to 1 (or 45, what
3213is the same). A numeric repeat count may optionally be enclosed in
3214brackets, as in C<pack 'C[80]', @arr>.
3215
3216One can replace the numeric repeat count by a template enclosed in brackets;
3217then the packed length of this template in bytes is used as a count.
62f95557
IZ
3218For example, C<x[L]> skips a long (it skips the number of bytes in a long);
3219the template C<$t X[$t] $t> unpack()s twice what $t unpacks.
3220If the template in brackets contains alignment commands (such as C<x![d]>),
3221its packed length is calculated as if the start of the template has the maximal
3222possible alignment.
2b6c5635 3223
951ba7fe 3224When used with C<Z>, C<*> results in the addition of a trailing null
2b6c5635
GS
3225byte (so the packed result will be one longer than the byte C<length>
3226of the item).
3227
951ba7fe 3228The repeat count for C<u> is interpreted as the maximal number of bytes
2b6c5635 3229to encode per line of output, with 0 and 1 replaced by 45.
5a929a98
VU
3230
3231=item *
3232
951ba7fe 3233The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
5a929a98 3234string of length count, padding with nulls or spaces as necessary. When
951ba7fe
GS
3235unpacking, C<A> strips trailing spaces and nulls, C<Z> strips everything
3236after the first null, and C<a> returns data verbatim. When packing,
3237C<a>, and C<Z> are equivalent.
2b6c5635
GS
3238
3239If the value-to-pack is too long, it is truncated. If too long and an
951ba7fe
GS
3240explicit count is provided, C<Z> packs only C<$count-1> bytes, followed
3241by a null byte. Thus C<Z> always packs a trailing null byte under
2b6c5635 3242all circumstances.
5a929a98
VU
3243
3244=item *
3245
951ba7fe 3246Likewise, the C<b> and C<B> fields pack a string that many bits long.
c73032f5
IZ
3247Each byte of the input field of pack() generates 1 bit of the result.
3248Each result bit is based on the least-significant bit of the corresponding
3249input byte, i.e., on C<ord($byte)%2>. In particular, bytes C<"0"> and
3250C<"1"> generate bits 0 and 1, as do bytes C<"\0"> and C<"\1">.
3251
3252Starting from the beginning of the input string of pack(), each 8-tuple
951ba7fe 3253of bytes is converted to 1 byte of output. With format C<b>
c73032f5 3254the first byte of the 8-tuple determines the least-significant bit of a
951ba7fe 3255byte, and with format C<B> it determines the most-significant bit of
c73032f5
IZ
3256a byte.
3257
3258If the length of the input string is not exactly divisible by 8, the
3259remainder is packed as if the input string were padded by null bytes
3260at the end. Similarly, during unpack()ing the "extra" bits are ignored.
3261
3262If the input string of pack() is longer than needed, extra bytes are ignored.
2b6c5635
GS
3263A C<*> for the repeat count of pack() means to use all the bytes of
3264the input field. On unpack()ing the bits are converted to a string
3265of C<"0">s and C<"1">s.
5a929a98
VU
3266
3267=item *
3268
951ba7fe 3269The C<h> and C<H> fields pack a string that many nybbles (4-bit groups,
851646ae 3270representable as hexadecimal digits, 0-9a-f) long.
5a929a98 3271
c73032f5
IZ