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1=head1 NAME
2
3perlfunc - Perl builtin functions
4
5=head1 DESCRIPTION
6
7The functions in this section can serve as terms in an expression.
8They fall into two major categories: list operators and named unary
9operators. These differ in their precedence relationship with a
10following comma. (See the precedence table in L<perlop>.) List
11operators take more than one argument, while unary operators can never
12take more than one argument. Thus, a comma terminates the argument of
13a unary operator, but merely separates the arguments of a list
14operator. A unary operator generally provides a scalar context to its
2b5ab1e7 15argument, while a list operator may provide either scalar or list
a0d0e21e 16contexts for its arguments. If it does both, the scalar arguments will
5f05dabc 17be first, and the list argument will follow. (Note that there can ever
0f31cffe 18be only one such list argument.) For instance, splice() has three scalar
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19arguments followed by a list, whereas gethostbyname() has four scalar
20arguments.
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21
22In the syntax descriptions that follow, list operators that expect a
23list (and provide list context for the elements of the list) are shown
24with LIST as an argument. Such a list may consist of any combination
25of scalar arguments or list values; the list values will be included
26in the list as if each individual element were interpolated at that
27point in the list, forming a longer single-dimensional list value.
28Elements of the LIST should be separated by commas.
29
30Any function in the list below may be used either with or without
31parentheses around its arguments. (The syntax descriptions omit the
5f05dabc 32parentheses.) If you use the parentheses, the simple (but occasionally
19799a22 33surprising) rule is this: It I<looks> like a function, therefore it I<is> a
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34function, and precedence doesn't matter. Otherwise it's a list
35operator or unary operator, and precedence does matter. And whitespace
36between the function and left parenthesis doesn't count--so you need to
37be careful sometimes:
38
68dc0745 39 print 1+2+4; # Prints 7.
40 print(1+2) + 4; # Prints 3.
41 print (1+2)+4; # Also prints 3!
42 print +(1+2)+4; # Prints 7.
43 print ((1+2)+4); # Prints 7.
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44
45If you run Perl with the B<-w> switch it can warn you about this. For
46example, the third line above produces:
47
48 print (...) interpreted as function at - line 1.
49 Useless use of integer addition in void context at - line 1.
50
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51A few functions take no arguments at all, and therefore work as neither
52unary nor list operators. These include such functions as C<time>
53and C<endpwent>. For example, C<time+86_400> always means
54C<time() + 86_400>.
55
a0d0e21e 56For functions that can be used in either a scalar or list context,
54310121 57nonabortive failure is generally indicated in a scalar context by
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58returning the undefined value, and in a list context by returning the
59null list.
60
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61Remember the following important rule: There is B<no rule> that relates
62the behavior of an expression in list context to its behavior in scalar
63context, or vice versa. It might do two totally different things.
a0d0e21e 64Each operator and function decides which sort of value it would be most
2b5ab1e7 65appropriate to return in scalar context. Some operators return the
5a964f20 66length of the list that would have been returned in list context. Some
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67operators return the first value in the list. Some operators return the
68last value in the list. Some operators return a count of successful
69operations. In general, they do what you want, unless you want
70consistency.
71
d1be9408 72A named array in scalar context is quite different from what would at
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73first glance appear to be a list in scalar context. You can't get a list
74like C<(1,2,3)> into being in scalar context, because the compiler knows
75the context at compile time. It would generate the scalar comma operator
76there, not the list construction version of the comma. That means it
77was never a list to start with.
78
79In general, functions in Perl that serve as wrappers for system calls
f86cebdf 80of the same name (like chown(2), fork(2), closedir(2), etc.) all return
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81true when they succeed and C<undef> otherwise, as is usually mentioned
82in the descriptions below. This is different from the C interfaces,
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83which return C<-1> on failure. Exceptions to this rule are C<wait>,
84C<waitpid>, and C<syscall>. System calls also set the special C<$!>
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85variable on failure. Other functions do not, except accidentally.
86
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87=head2 Perl Functions by Category
88
89Here are Perl's functions (including things that look like
5a964f20 90functions, like some keywords and named operators)
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91arranged by category. Some functions appear in more
92than one place.
93
13a2d996 94=over 4
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95
96=item Functions for SCALARs or strings
97
22fae026 98C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>,
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99C<length>, C<oct>, C<ord>, C<pack>, C<q/STRING/>, C<qq/STRING/>, C<reverse>,
100C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///>
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101
102=item Regular expressions and pattern matching
103
ab4f32c2 104C<m//>, C<pos>, C<quotemeta>, C<s///>, C<split>, C<study>, C<qr//>
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105
106=item Numeric functions
107
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108C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>,
109C<sin>, C<sqrt>, C<srand>
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110
111=item Functions for real @ARRAYs
112
22fae026 113C<pop>, C<push>, C<shift>, C<splice>, C<unshift>
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114
115=item Functions for list data
116
ab4f32c2 117C<grep>, C<join>, C<map>, C<qw/STRING/>, C<reverse>, C<sort>, C<unpack>
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118
119=item Functions for real %HASHes
120
22fae026 121C<delete>, C<each>, C<exists>, C<keys>, C<values>
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122
123=item Input and output functions
124
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125C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
126C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
127C<readdir>, C<rewinddir>, C<seek>, C<seekdir>, C<select>, C<syscall>,
128C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>,
129C<warn>, C<write>
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130
131=item Functions for fixed length data or records
132
22fae026 133C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec>
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134
135=item Functions for filehandles, files, or directories
136
22fae026 137C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>,
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
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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
37798a01 200=item Functions new in perl5
201
22fae026 202C<abs>, C<bless>, C<chomp>, C<chr>, C<exists>, C<formline>, C<glob>,
b76cc8ba 203C<import>, C<lc>, C<lcfirst>, C<map>, C<my>, C<no>, C<our>, C<prototype>,
4375e838 204C<qx>, C<qw>, C<readline>, C<readpipe>, C<ref>, C<sub*>, C<sysopen>, C<tie>,
22fae026 205C<tied>, C<uc>, C<ucfirst>, C<untie>, C<use>
37798a01 206
207* - C<sub> was a keyword in perl4, but in perl5 it is an
5a964f20 208operator, which can be used in expressions.
37798a01 209
210=item Functions obsoleted in perl5
211
22fae026 212C<dbmclose>, C<dbmopen>
37798a01 213
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214=back
215
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216=head2 Portability
217
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218Perl was born in Unix and can therefore access all common Unix
219system calls. In non-Unix environments, the functionality of some
220Unix system calls may not be available, or details of the available
221functionality may differ slightly. The Perl functions affected
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222by this are:
223
224C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>,
225C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>,
226C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>,
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227C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostbyname>,
228C<gethostent>, C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
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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
5b3c99c0 249=item -X FILEHANDLE
a0d0e21e 250
5b3c99c0 251=item -X EXPR
a0d0e21e 252
5b3c99c0 253=item -X
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254
255A file test, where X is one of the letters listed below. This unary
256operator takes one argument, either a filename or a filehandle, and
257tests the associated file to see if something is true about it. If the
7660c0ab 258argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
19799a22 259Unless otherwise documented, it returns C<1> for true and C<''> for false, or
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260the undefined value if the file doesn't exist. Despite the funny
261names, precedence is the same as any other named unary operator, and
262the argument may be parenthesized like any other unary operator. The
263operator may be any of:
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264X<-r>X<-w>X<-x>X<-o>X<-R>X<-W>X<-X>X<-O>X<-e>X<-z>X<-s>X<-f>X<-d>X<-l>X<-p>
265X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
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266
267 -r File is readable by effective uid/gid.
268 -w File is writable by effective uid/gid.
269 -x File is executable by effective uid/gid.
270 -o File is owned by effective uid.
271
272 -R File is readable by real uid/gid.
273 -W File is writable by real uid/gid.
274 -X File is executable by real uid/gid.
275 -O File is owned by real uid.
276
277 -e File exists.
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278 -z File has zero size (is empty).
279 -s File has nonzero size (returns size in bytes).
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280
281 -f File is a plain file.
282 -d File is a directory.
283 -l File is a symbolic link.
9c4d0f16 284 -p File is a named pipe (FIFO), or Filehandle is a pipe.
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285 -S File is a socket.
286 -b File is a block special file.
287 -c File is a character special file.
288 -t Filehandle is opened to a tty.
289
290 -u File has setuid bit set.
291 -g File has setgid bit set.
292 -k File has sticky bit set.
293
121910a4 294 -T File is an ASCII text file (heuristic guess).
2cdbc966 295 -B File is a "binary" file (opposite of -T).
a0d0e21e 296
95a3fe12 297 -M Script start time minus file modification time, in days.
a0d0e21e 298 -A Same for access time.
95a3fe12 299 -C Same for inode change time (Unix, may differ for other platforms)
a0d0e21e 300
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301Example:
302
303 while (<>) {
5b3eff12 304 chomp;
a0d0e21e 305 next unless -f $_; # ignore specials
5a964f20 306 #...
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307 }
308
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309The interpretation of the file permission operators C<-r>, C<-R>,
310C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
311of the file and the uids and gids of the user. There may be other
312reasons you can't actually read, write, or execute the file. Such
313reasons may be for example network filesystem access controls, ACLs
314(access control lists), read-only filesystems, and unrecognized
315executable formats.
316
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317Also note that, for the superuser on the local filesystems, the C<-r>,
318C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
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319if any execute bit is set in the mode. Scripts run by the superuser
320may thus need to do a stat() to determine the actual mode of the file,
2b5ab1e7 321or temporarily set their effective uid to something else.
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322
323If you are using ACLs, there is a pragma called C<filetest> that may
324produce more accurate results than the bare stat() mode bits.
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325When under the C<use filetest 'access'> the above-mentioned filetests
326will test whether the permission can (not) be granted using the
468541a8 327access() family of system calls. Also note that the C<-x> and C<-X> may
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328under this pragma return true even if there are no execute permission
329bits set (nor any extra execute permission ACLs). This strangeness is
330due to the underlying system calls' definitions. Read the
331documentation for the C<filetest> pragma for more information.
332
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333Note that C<-s/a/b/> does not do a negated substitution. Saying
334C<-exp($foo)> still works as expected, however--only single letters
335following a minus are interpreted as file tests.
336
337The C<-T> and C<-B> switches work as follows. The first block or so of the
338file is examined for odd characters such as strange control codes or
61eff3bc 339characters with the high bit set. If too many strange characters (>30%)
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340are found, it's a C<-B> file, otherwise it's a C<-T> file. Also, any file
341containing null in the first block is considered a binary file. If C<-T>
9124316e 342or C<-B> is used on a filehandle, the current IO buffer is examined
19799a22 343rather than the first block. Both C<-T> and C<-B> return true on a null
54310121 344file, or a file at EOF when testing a filehandle. Because you have to
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345read a file to do the C<-T> test, on most occasions you want to use a C<-f>
346against the file first, as in C<next unless -f $file && -T $file>.
a0d0e21e 347
19799a22 348If any of the file tests (or either the C<stat> or C<lstat> operators) are given
28757baa 349the special filehandle consisting of a solitary underline, then the stat
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350structure of the previous file test (or stat operator) is used, saving
351a system call. (This doesn't work with C<-t>, and you need to remember
352that lstat() and C<-l> will leave values in the stat structure for the
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353symbolic link, not the real file.) (Also, if the stat buffer was filled by
354a C<lstat> call, C<-T> and C<-B> will reset it with the results of C<stat _>).
355Example:
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356
357 print "Can do.\n" if -r $a || -w _ || -x _;
358
359 stat($filename);
360 print "Readable\n" if -r _;
361 print "Writable\n" if -w _;
362 print "Executable\n" if -x _;
363 print "Setuid\n" if -u _;
364 print "Setgid\n" if -g _;
365 print "Sticky\n" if -k _;
366 print "Text\n" if -T _;
367 print "Binary\n" if -B _;
368
369=item abs VALUE
370
54310121 371=item abs
bbce6d69 372
a0d0e21e 373Returns the absolute value of its argument.
7660c0ab 374If VALUE is omitted, uses C<$_>.
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375
376=item accept NEWSOCKET,GENERICSOCKET
377
f86cebdf 378Accepts an incoming socket connect, just as the accept(2) system call
19799a22 379does. Returns the packed address if it succeeded, false otherwise.
2b5ab1e7 380See the example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 381
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382On systems that support a close-on-exec flag on files, the flag will
383be set for the newly opened file descriptor, as determined by the
384value of $^F. See L<perlvar/$^F>.
385
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386=item alarm SECONDS
387
54310121 388=item alarm
bbce6d69 389
a0d0e21e 390Arranges to have a SIGALRM delivered to this process after the
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391specified number of wallclock seconds have elapsed. If SECONDS is not
392specified, the value stored in C<$_> is used. (On some machines,
393unfortunately, the elapsed time may be up to one second less or more
394than you specified because of how seconds are counted, and process
395scheduling may delay the delivery of the signal even further.)
396
397Only one timer may be counting at once. Each call disables the
398previous timer, and an argument of C<0> may be supplied to cancel the
399previous timer without starting a new one. The returned value is the
400amount of time remaining on the previous timer.
a0d0e21e 401
4633a7c4 402For delays of finer granularity than one second, you may use Perl's
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403four-argument version of select() leaving the first three arguments
404undefined, or you might be able to use the C<syscall> interface to
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405access setitimer(2) if your system supports it. The Time::HiRes
406module (from CPAN, and starting from Perl 5.8 part of the standard
407distribution) may also prove useful.
2b5ab1e7 408
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409It is usually a mistake to intermix C<alarm> and C<sleep> calls.
410(C<sleep> may be internally implemented in your system with C<alarm>)
a0d0e21e 411
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412If you want to use C<alarm> to time out a system call you need to use an
413C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
f86cebdf 414fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
19799a22 415restart system calls on some systems. Using C<eval>/C<die> always works,
5a964f20 416modulo the caveats given in L<perlipc/"Signals">.
ff68c719 417
418 eval {
f86cebdf 419 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
36477c24 420 alarm $timeout;
ff68c719 421 $nread = sysread SOCKET, $buffer, $size;
36477c24 422 alarm 0;
ff68c719 423 };
ff68c719 424 if ($@) {
f86cebdf 425 die unless $@ eq "alarm\n"; # propagate unexpected errors
ff68c719 426 # timed out
427 }
428 else {
429 # didn't
430 }
431
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432For more information see L<perlipc>.
433
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434=item atan2 Y,X
435
436Returns the arctangent of Y/X in the range -PI to PI.
437
ca6e1c26 438For the tangent operation, you may use the C<Math::Trig::tan>
28757baa 439function, or use the familiar relation:
440
441 sub tan { sin($_[0]) / cos($_[0]) }
442
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443=item bind SOCKET,NAME
444
445Binds a network address to a socket, just as the bind system call
19799a22 446does. Returns true if it succeeded, false otherwise. NAME should be a
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447packed address of the appropriate type for the socket. See the examples in
448L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 449
fae2c0fb 450=item binmode FILEHANDLE, LAYER
1c1fc3ea 451
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452=item binmode FILEHANDLE
453
1cbfc93d
NIS
454Arranges for FILEHANDLE to be read or written in "binary" or "text"
455mode on systems where the run-time libraries distinguish between
456binary and text files. If FILEHANDLE is an expression, the value is
457taken as the name of the filehandle. Returns true on success,
b5fe5ca2 458otherwise it returns C<undef> and sets C<$!> (errno).
1cbfc93d 459
d807c6f4
JH
460On some systems (in general, DOS and Windows-based systems) binmode()
461is necessary when you're not working with a text file. For the sake
462of portability it is a good idea to always use it when appropriate,
463and to never use it when it isn't appropriate. Also, people can
464set their I/O to be by default UTF-8 encoded Unicode, not bytes.
465
466In other words: regardless of platform, use binmode() on binary data,
467like for example images.
468
469If LAYER is present it is a single string, but may contain multiple
470directives. The directives alter the behaviour of the file handle.
471When LAYER is present using binmode on text file makes sense.
472
fae2c0fb 473If LAYER is omitted or specified as C<:raw> the filehandle is made
0226bbdb
NIS
474suitable for passing binary data. This includes turning off possible CRLF
475translation and marking it as bytes (as opposed to Unicode characters).
749683d2
YST
476Note that, despite what may be implied in I<"Programming Perl"> (the
477Camel) or elsewhere, C<:raw> is I<not> the simply inverse of C<:crlf>
fae2c0fb 478-- other layers which would affect binary nature of the stream are
0226bbdb
NIS
479I<also> disabled. See L<PerlIO>, L<perlrun> and the discussion about the
480PERLIO environment variable.
01e6739c 481
d807c6f4
JH
482The C<:bytes>, C<:crlf>, and C<:utf8>, and any other directives of the
483form C<:...>, are called I/O I<layers>. The C<open> pragma can be used to
484establish default I/O layers. See L<open>.
485
fae2c0fb
RGS
486I<The LAYER parameter of the binmode() function is described as "DISCIPLINE"
487in "Programming Perl, 3rd Edition". However, since the publishing of this
488book, by many known as "Camel III", the consensus of the naming of this
489functionality has moved from "discipline" to "layer". All documentation
490of this version of Perl therefore refers to "layers" rather than to
491"disciplines". Now back to the regularly scheduled documentation...>
492
01e6739c 493To mark FILEHANDLE as UTF-8, use C<:utf8>.
1cbfc93d 494
ed53a2bb 495In general, binmode() should be called after open() but before any I/O
01e6739c
NIS
496is done on the filehandle. Calling binmode() will normally flush any
497pending buffered output data (and perhaps pending input data) on the
fae2c0fb 498handle. An exception to this is the C<:encoding> layer that
01e6739c 499changes the default character encoding of the handle, see L<open>.
fae2c0fb 500The C<:encoding> layer sometimes needs to be called in
3874323d
JH
501mid-stream, and it doesn't flush the stream. The C<:encoding>
502also implicitly pushes on top of itself the C<:utf8> layer because
503internally Perl will operate on UTF-8 encoded Unicode characters.
16fe6d59 504
19799a22 505The operating system, device drivers, C libraries, and Perl run-time
30168b04
GS
506system all work together to let the programmer treat a single
507character (C<\n>) as the line terminator, irrespective of the external
508representation. On many operating systems, the native text file
509representation matches the internal representation, but on some
510platforms the external representation of C<\n> is made up of more than
511one character.
512
68bd7414
NIS
513Mac OS, all variants of Unix, and Stream_LF files on VMS use a single
514character to end each line in the external representation of text (even
5e12dbfa 515though that single character is CARRIAGE RETURN on Mac OS and LINE FEED
01e6739c
NIS
516on Unix and most VMS files). In other systems like OS/2, DOS and the
517various flavors of MS-Windows your program sees a C<\n> as a simple C<\cJ>,
518but what's stored in text files are the two characters C<\cM\cJ>. That
519means that, if you don't use binmode() on these systems, C<\cM\cJ>
520sequences on disk will be converted to C<\n> on input, and any C<\n> in
521your program will be converted back to C<\cM\cJ> on output. This is what
522you want for text files, but it can be disastrous for binary files.
30168b04
GS
523
524Another consequence of using binmode() (on some systems) is that
525special end-of-file markers will be seen as part of the data stream.
526For systems from the Microsoft family this means that if your binary
4375e838 527data contains C<\cZ>, the I/O subsystem will regard it as the end of
30168b04
GS
528the file, unless you use binmode().
529
530binmode() is not only important for readline() and print() operations,
531but also when using read(), seek(), sysread(), syswrite() and tell()
532(see L<perlport> for more details). See the C<$/> and C<$\> variables
533in L<perlvar> for how to manually set your input and output
534line-termination sequences.
a0d0e21e 535
4633a7c4 536=item bless REF,CLASSNAME
a0d0e21e
LW
537
538=item bless REF
539
2b5ab1e7
TC
540This function tells the thingy referenced by REF that it is now an object
541in the CLASSNAME package. If CLASSNAME is omitted, the current package
19799a22 542is used. Because a C<bless> is often the last thing in a constructor,
2b5ab1e7
TC
543it returns the reference for convenience. Always use the two-argument
544version if the function doing the blessing might be inherited by a
545derived class. See L<perltoot> and L<perlobj> for more about the blessing
546(and blessings) of objects.
a0d0e21e 547
57668c4d 548Consider always blessing objects in CLASSNAMEs that are mixed case.
2b5ab1e7
TC
549Namespaces with all lowercase names are considered reserved for
550Perl pragmata. Builtin types have all uppercase names, so to prevent
551confusion, you may wish to avoid such package names as well. Make sure
552that CLASSNAME is a true value.
60ad88b8
GS
553
554See L<perlmod/"Perl Modules">.
555
a0d0e21e
LW
556=item caller EXPR
557
558=item caller
559
5a964f20 560Returns the context of the current subroutine call. In scalar context,
28757baa 561returns the caller's package name if there is a caller, that is, if
19799a22 562we're in a subroutine or C<eval> or C<require>, and the undefined value
5a964f20 563otherwise. In list context, returns
a0d0e21e 564
748a9306 565 ($package, $filename, $line) = caller;
a0d0e21e
LW
566
567With EXPR, it returns some extra information that the debugger uses to
568print a stack trace. The value of EXPR indicates how many call frames
569to go back before the current one.
570
f3aa04c2 571 ($package, $filename, $line, $subroutine, $hasargs,
e476b1b5 572 $wantarray, $evaltext, $is_require, $hints, $bitmask) = caller($i);
e7ea3e70 573
951ba7fe 574Here $subroutine may be C<(eval)> if the frame is not a subroutine
19799a22 575call, but an C<eval>. In such a case additional elements $evaltext and
7660c0ab 576C<$is_require> are set: C<$is_require> is true if the frame is created by a
19799a22 577C<require> or C<use> statement, $evaltext contains the text of the
277ddfaf 578C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
951ba7fe 579$filename is C<(eval)>, but $evaltext is undefined. (Note also that
0fc9dec4
RGS
580each C<use> statement creates a C<require> frame inside an C<eval EXPR>
581frame.) $subroutine may also be C<(unknown)> if this particular
582subroutine happens to have been deleted from the symbol table.
583C<$hasargs> is true if a new instance of C<@_> was set up for the frame.
584C<$hints> and C<$bitmask> contain pragmatic hints that the caller was
585compiled with. The C<$hints> and C<$bitmask> values are subject to change
586between versions of Perl, and are not meant for external use.
748a9306
LW
587
588Furthermore, when called from within the DB package, caller returns more
7660c0ab 589detailed information: it sets the list variable C<@DB::args> to be the
54310121 590arguments with which the subroutine was invoked.
748a9306 591
7660c0ab 592Be aware that the optimizer might have optimized call frames away before
19799a22 593C<caller> had a chance to get the information. That means that C<caller(N)>
7660c0ab 594might not return information about the call frame you expect it do, for
b76cc8ba 595C<< N > 1 >>. In particular, C<@DB::args> might have information from the
19799a22 596previous time C<caller> was called.
7660c0ab 597
a0d0e21e
LW
598=item chdir EXPR
599
ffce7b87 600Changes the working directory to EXPR, if possible. If EXPR is omitted,
0bfc1ec4 601changes to the directory specified by C<$ENV{HOME}>, if set; if not,
ffce7b87 602changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
b4ad75f0
AMS
603variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
604neither is set, C<chdir> does nothing. It returns true upon success,
605false otherwise. See the example under C<die>.
a0d0e21e
LW
606
607=item chmod LIST
608
609Changes the permissions of a list of files. The first element of the
4633a7c4 610list must be the numerical mode, which should probably be an octal
2f9daede
TP
611number, and which definitely should I<not> a string of octal digits:
612C<0644> is okay, C<'0644'> is not. Returns the number of files
dc848c6f 613successfully changed. See also L</oct>, if all you have is a string.
a0d0e21e
LW
614
615 $cnt = chmod 0755, 'foo', 'bar';
616 chmod 0755, @executables;
f86cebdf
GS
617 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
618 # --w----r-T
2f9daede
TP
619 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
620 $mode = 0644; chmod $mode, 'foo'; # this is best
a0d0e21e 621
ca6e1c26
JH
622You can also import the symbolic C<S_I*> constants from the Fcntl
623module:
624
625 use Fcntl ':mode';
626
627 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
628 # This is identical to the chmod 0755 of the above example.
629
a0d0e21e
LW
630=item chomp VARIABLE
631
313c9f5c 632=item chomp( LIST )
a0d0e21e
LW
633
634=item chomp
635
2b5ab1e7
TC
636This safer version of L</chop> removes any trailing string
637that corresponds to the current value of C<$/> (also known as
28757baa 638$INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
639number of characters removed from all its arguments. It's often used to
640remove the newline from the end of an input record when you're worried
2b5ab1e7
TC
641that the final record may be missing its newline. When in paragraph
642mode (C<$/ = "">), it removes all trailing newlines from the string.
4c5a6083
GS
643When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
644a reference to an integer or the like, see L<perlvar>) chomp() won't
b76cc8ba 645remove anything.
19799a22 646If VARIABLE is omitted, it chomps C<$_>. Example:
a0d0e21e
LW
647
648 while (<>) {
649 chomp; # avoid \n on last field
650 @array = split(/:/);
5a964f20 651 # ...
a0d0e21e
LW
652 }
653
4bf21a6d
RD
654If VARIABLE is a hash, it chomps the hash's values, but not its keys.
655
a0d0e21e
LW
656You can actually chomp anything that's an lvalue, including an assignment:
657
658 chomp($cwd = `pwd`);
659 chomp($answer = <STDIN>);
660
661If you chomp a list, each element is chomped, and the total number of
662characters removed is returned.
663
15e44fd8
RGS
664Note that parentheses are necessary when you're chomping anything
665that is not a simple variable. This is because C<chomp $cwd = `pwd`;>
666is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as
667C<chomp( $cwd = `pwd` )> which you might expect. Similarly,
668C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than
669as C<chomp($a, $b)>.
670
a0d0e21e
LW
671=item chop VARIABLE
672
313c9f5c 673=item chop( LIST )
a0d0e21e
LW
674
675=item chop
676
677Chops off the last character of a string and returns the character
5b3eff12 678chopped. It is much more efficient than C<s/.$//s> because it neither
7660c0ab 679scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
4bf21a6d
RD
680If VARIABLE is a hash, it chops the hash's values, but not its keys.
681
5b3eff12 682You can actually chop anything that's an lvalue, including an assignment.
a0d0e21e
LW
683
684If you chop a list, each element is chopped. Only the value of the
19799a22 685last C<chop> is returned.
a0d0e21e 686
19799a22 687Note that C<chop> returns the last character. To return all but the last
748a9306
LW
688character, use C<substr($string, 0, -1)>.
689
15e44fd8
RGS
690See also L</chomp>.
691
a0d0e21e
LW
692=item chown LIST
693
694Changes the owner (and group) of a list of files. The first two
19799a22
GS
695elements of the list must be the I<numeric> uid and gid, in that
696order. A value of -1 in either position is interpreted by most
697systems to leave that value unchanged. Returns the number of files
698successfully changed.
a0d0e21e
LW
699
700 $cnt = chown $uid, $gid, 'foo', 'bar';
701 chown $uid, $gid, @filenames;
702
54310121 703Here's an example that looks up nonnumeric uids in the passwd file:
a0d0e21e
LW
704
705 print "User: ";
19799a22 706 chomp($user = <STDIN>);
5a964f20 707 print "Files: ";
19799a22 708 chomp($pattern = <STDIN>);
a0d0e21e
LW
709
710 ($login,$pass,$uid,$gid) = getpwnam($user)
711 or die "$user not in passwd file";
712
5a964f20 713 @ary = glob($pattern); # expand filenames
a0d0e21e
LW
714 chown $uid, $gid, @ary;
715
54310121 716On most systems, you are not allowed to change the ownership of the
4633a7c4
LW
717file unless you're the superuser, although you should be able to change
718the group to any of your secondary groups. On insecure systems, these
719restrictions may be relaxed, but this is not a portable assumption.
19799a22
GS
720On POSIX systems, you can detect this condition this way:
721
722 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
723 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
4633a7c4 724
a0d0e21e
LW
725=item chr NUMBER
726
54310121 727=item chr
bbce6d69 728
a0d0e21e 729Returns the character represented by that NUMBER in the character set.
a0ed51b3 730For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
1e54db1a
JH
731chr(0x263a) is a Unicode smiley face. Note that characters from 128
732to 255 (inclusive) are by default not encoded in UTF-8 Unicode for
733backward compatibility reasons (but see L<encoding>).
aaa68c4a 734
974da8e5
JH
735If NUMBER is omitted, uses C<$_>.
736
b76cc8ba 737For the reverse, use L</ord>.
a0d0e21e 738
974da8e5
JH
739Note that under the C<bytes> pragma the NUMBER is masked to
740the low eight bits.
741
742See L<perlunicode> and L<encoding> for more about Unicode.
bbce6d69 743
a0d0e21e
LW
744=item chroot FILENAME
745
54310121 746=item chroot
bbce6d69 747
5a964f20 748This function works like the system call by the same name: it makes the
4633a7c4 749named directory the new root directory for all further pathnames that
951ba7fe 750begin with a C</> by your process and all its children. (It doesn't
28757baa 751change your current working directory, which is unaffected.) For security
4633a7c4 752reasons, this call is restricted to the superuser. If FILENAME is
19799a22 753omitted, does a C<chroot> to C<$_>.
a0d0e21e
LW
754
755=item close FILEHANDLE
756
6a518fbc
TP
757=item close
758
9124316e
JH
759Closes the file or pipe associated with the file handle, returning
760true only if IO buffers are successfully flushed and closes the system
761file descriptor. Closes the currently selected filehandle if the
762argument is omitted.
fb73857a 763
764You don't have to close FILEHANDLE if you are immediately going to do
19799a22
GS
765another C<open> on it, because C<open> will close it for you. (See
766C<open>.) However, an explicit C<close> on an input file resets the line
767counter (C<$.>), while the implicit close done by C<open> does not.
fb73857a 768
19799a22
GS
769If the file handle came from a piped open C<close> will additionally
770return false if one of the other system calls involved fails or if the
fb73857a 771program exits with non-zero status. (If the only problem was that the
b76cc8ba 772program exited non-zero C<$!> will be set to C<0>.) Closing a pipe
2b5ab1e7 773also waits for the process executing on the pipe to complete, in case you
b76cc8ba 774want to look at the output of the pipe afterwards, and
2b5ab1e7 775implicitly puts the exit status value of that command into C<$?>.
5a964f20 776
73689b13
GS
777Prematurely closing the read end of a pipe (i.e. before the process
778writing to it at the other end has closed it) will result in a
779SIGPIPE being delivered to the writer. If the other end can't
780handle that, be sure to read all the data before closing the pipe.
781
fb73857a 782Example:
a0d0e21e 783
fb73857a 784 open(OUTPUT, '|sort >foo') # pipe to sort
785 or die "Can't start sort: $!";
5a964f20 786 #... # print stuff to output
fb73857a 787 close OUTPUT # wait for sort to finish
788 or warn $! ? "Error closing sort pipe: $!"
789 : "Exit status $? from sort";
790 open(INPUT, 'foo') # get sort's results
791 or die "Can't open 'foo' for input: $!";
a0d0e21e 792
5a964f20
TC
793FILEHANDLE may be an expression whose value can be used as an indirect
794filehandle, usually the real filehandle name.
a0d0e21e
LW
795
796=item closedir DIRHANDLE
797
19799a22 798Closes a directory opened by C<opendir> and returns the success of that
5a964f20
TC
799system call.
800
a0d0e21e
LW
801=item connect SOCKET,NAME
802
803Attempts to connect to a remote socket, just as the connect system call
19799a22 804does. Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
805packed address of the appropriate type for the socket. See the examples in
806L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 807
cb1a09d0
AD
808=item continue BLOCK
809
810Actually a flow control statement rather than a function. If there is a
98293880
JH
811C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
812C<foreach>), it is always executed just before the conditional is about to
813be evaluated again, just like the third part of a C<for> loop in C. Thus
cb1a09d0
AD
814it can be used to increment a loop variable, even when the loop has been
815continued via the C<next> statement (which is similar to the C C<continue>
816statement).
817
98293880 818C<last>, C<next>, or C<redo> may appear within a C<continue>
19799a22
GS
819block. C<last> and C<redo> will behave as if they had been executed within
820the main block. So will C<next>, but since it will execute a C<continue>
1d2dff63
GS
821block, it may be more entertaining.
822
823 while (EXPR) {
824 ### redo always comes here
825 do_something;
826 } continue {
827 ### next always comes here
828 do_something_else;
829 # then back the top to re-check EXPR
830 }
831 ### last always comes here
832
833Omitting the C<continue> section is semantically equivalent to using an
19799a22 834empty one, logically enough. In that case, C<next> goes directly back
1d2dff63
GS
835to check the condition at the top of the loop.
836
a0d0e21e
LW
837=item cos EXPR
838
d6217f1e
GS
839=item cos
840
5a964f20 841Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
7660c0ab 842takes cosine of C<$_>.
a0d0e21e 843
ca6e1c26 844For the inverse cosine operation, you may use the C<Math::Trig::acos()>
28757baa 845function, or use this relation:
846
847 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
848
a0d0e21e
LW
849=item crypt PLAINTEXT,SALT
850
f86cebdf 851Encrypts a string exactly like the crypt(3) function in the C library
4633a7c4
LW
852(assuming that you actually have a version there that has not been
853extirpated as a potential munition). This can prove useful for checking
854the password file for lousy passwords, amongst other things. Only the
855guys wearing white hats should do this.
a0d0e21e 856
a6d05634 857Note that L<crypt|/crypt> is intended to be a one-way function, much like
85c16d83
JH
858breaking eggs to make an omelette. There is no (known) corresponding
859decrypt function (in other words, the crypt() is a one-way hash
860function). As a result, this function isn't all that useful for
11155c91 861cryptography. (For that, see your nearby CPAN mirror.)
2f9daede 862
85c16d83
JH
863When verifying an existing encrypted string you should use the
864encrypted text as the salt (like C<crypt($plain, $crypted) eq
8e2ffcbe 865$crypted>). This allows your code to work with the standard L<crypt|/crypt>
85c16d83
JH
866and with more exotic implementations. In other words, do not assume
867anything about the returned string itself, or how many bytes in
868the encrypted string matter.
869
870Traditionally the result is a string of 13 bytes: two first bytes of
871the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
872the first eight bytes of the encrypted string mattered, but
873alternative hashing schemes (like MD5), higher level security schemes
874(like C2), and implementations on non-UNIX platforms may produce
875different strings.
876
877When choosing a new salt create a random two character string whose
878characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
d3989d75
CW
879'/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of
880characters is just a recommendation; the characters allowed in
881the salt depend solely on your system's crypt library, and Perl can't
882restrict what salts C<crypt()> accepts.
e71965be 883
a0d0e21e
LW
884Here's an example that makes sure that whoever runs this program knows
885their own password:
886
887 $pwd = (getpwuid($<))[1];
a0d0e21e
LW
888
889 system "stty -echo";
890 print "Password: ";
e71965be 891 chomp($word = <STDIN>);
a0d0e21e
LW
892 print "\n";
893 system "stty echo";
894
e71965be 895 if (crypt($word, $pwd) ne $pwd) {
a0d0e21e
LW
896 die "Sorry...\n";
897 } else {
898 print "ok\n";
54310121 899 }
a0d0e21e 900
9f8f0c9d 901Of course, typing in your own password to whoever asks you
748a9306 902for it is unwise.
a0d0e21e 903
8e2ffcbe 904The L<crypt|/crypt> function is unsuitable for encrypting large quantities
19799a22
GS
905of data, not least of all because you can't get the information
906back. Look at the F<by-module/Crypt> and F<by-module/PGP> directories
907on your favorite CPAN mirror for a slew of potentially useful
908modules.
909
f2791508
JH
910If using crypt() on a Unicode string (which I<potentially> has
911characters with codepoints above 255), Perl tries to make sense
912of the situation by trying to downgrade (a copy of the string)
913the string back to an eight-bit byte string before calling crypt()
914(on that copy). If that works, good. If not, crypt() dies with
915C<Wide character in crypt>.
85c16d83 916
aa689395 917=item dbmclose HASH
a0d0e21e 918
19799a22 919[This function has been largely superseded by the C<untie> function.]
a0d0e21e 920
aa689395 921Breaks the binding between a DBM file and a hash.
a0d0e21e 922
19799a22 923=item dbmopen HASH,DBNAME,MASK
a0d0e21e 924
19799a22 925[This function has been largely superseded by the C<tie> function.]
a0d0e21e 926
7b8d334a 927This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
19799a22
GS
928hash. HASH is the name of the hash. (Unlike normal C<open>, the first
929argument is I<not> a filehandle, even though it looks like one). DBNAME
aa689395 930is the name of the database (without the F<.dir> or F<.pag> extension if
931any). If the database does not exist, it is created with protection
19799a22
GS
932specified by MASK (as modified by the C<umask>). If your system supports
933only the older DBM functions, you may perform only one C<dbmopen> in your
aa689395 934program. In older versions of Perl, if your system had neither DBM nor
19799a22 935ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
aa689395 936sdbm(3).
937
938If you don't have write access to the DBM file, you can only read hash
939variables, not set them. If you want to test whether you can write,
19799a22 940either use file tests or try setting a dummy hash entry inside an C<eval>,
aa689395 941which will trap the error.
a0d0e21e 942
19799a22
GS
943Note that functions such as C<keys> and C<values> may return huge lists
944when used on large DBM files. You may prefer to use the C<each>
a0d0e21e
LW
945function to iterate over large DBM files. Example:
946
947 # print out history file offsets
948 dbmopen(%HIST,'/usr/lib/news/history',0666);
949 while (($key,$val) = each %HIST) {
950 print $key, ' = ', unpack('L',$val), "\n";
951 }
952 dbmclose(%HIST);
953
cb1a09d0 954See also L<AnyDBM_File> for a more general description of the pros and
184e9718 955cons of the various dbm approaches, as well as L<DB_File> for a particularly
cb1a09d0 956rich implementation.
4633a7c4 957
2b5ab1e7
TC
958You can control which DBM library you use by loading that library
959before you call dbmopen():
960
961 use DB_File;
962 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
963 or die "Can't open netscape history file: $!";
964
a0d0e21e
LW
965=item defined EXPR
966
54310121 967=item defined
bbce6d69 968
2f9daede
TP
969Returns a Boolean value telling whether EXPR has a value other than
970the undefined value C<undef>. If EXPR is not present, C<$_> will be
971checked.
972
973Many operations return C<undef> to indicate failure, end of file,
974system error, uninitialized variable, and other exceptional
975conditions. This function allows you to distinguish C<undef> from
976other values. (A simple Boolean test will not distinguish among
7660c0ab 977C<undef>, zero, the empty string, and C<"0">, which are all equally
2f9daede 978false.) Note that since C<undef> is a valid scalar, its presence
19799a22 979doesn't I<necessarily> indicate an exceptional condition: C<pop>
2f9daede
TP
980returns C<undef> when its argument is an empty array, I<or> when the
981element to return happens to be C<undef>.
982
f10b0346
GS
983You may also use C<defined(&func)> to check whether subroutine C<&func>
984has ever been defined. The return value is unaffected by any forward
04891299 985declarations of C<&func>. Note that a subroutine which is not defined
847c7ebe
DD
986may still be callable: its package may have an C<AUTOLOAD> method that
987makes it spring into existence the first time that it is called -- see
988L<perlsub>.
f10b0346
GS
989
990Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
991used to report whether memory for that aggregate has ever been
992allocated. This behavior may disappear in future versions of Perl.
993You should instead use a simple test for size:
994
995 if (@an_array) { print "has array elements\n" }
996 if (%a_hash) { print "has hash members\n" }
2f9daede
TP
997
998When used on a hash element, it tells you whether the value is defined,
dc848c6f 999not whether the key exists in the hash. Use L</exists> for the latter
2f9daede 1000purpose.
a0d0e21e
LW
1001
1002Examples:
1003
1004 print if defined $switch{'D'};
1005 print "$val\n" while defined($val = pop(@ary));
1006 die "Can't readlink $sym: $!"
1007 unless defined($value = readlink $sym);
a0d0e21e 1008 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
2f9daede 1009 $debugging = 0 unless defined $debugging;
a0d0e21e 1010
19799a22 1011Note: Many folks tend to overuse C<defined>, and then are surprised to
7660c0ab 1012discover that the number C<0> and C<""> (the zero-length string) are, in fact,
2f9daede 1013defined values. For example, if you say
a5f75d66
AD
1014
1015 "ab" =~ /a(.*)b/;
1016
7660c0ab 1017The pattern match succeeds, and C<$1> is defined, despite the fact that it
a5f75d66 1018matched "nothing". But it didn't really match nothing--rather, it
2b5ab1e7 1019matched something that happened to be zero characters long. This is all
a5f75d66 1020very above-board and honest. When a function returns an undefined value,
2f9daede 1021it's an admission that it couldn't give you an honest answer. So you
19799a22 1022should use C<defined> only when you're questioning the integrity of what
7660c0ab 1023you're trying to do. At other times, a simple comparison to C<0> or C<""> is
2f9daede
TP
1024what you want.
1025
dc848c6f 1026See also L</undef>, L</exists>, L</ref>.
2f9daede 1027
a0d0e21e
LW
1028=item delete EXPR
1029
01020589
GS
1030Given an expression that specifies a hash element, array element, hash slice,
1031or array slice, deletes the specified element(s) from the hash or array.
8216c1fd 1032In the case of an array, if the array elements happen to be at the end,
b76cc8ba 1033the size of the array will shrink to the highest element that tests
8216c1fd 1034true for exists() (or 0 if no such element exists).
a0d0e21e 1035
eba0920a
EM
1036Returns a list with the same number of elements as the number of elements
1037for which deletion was attempted. Each element of that list consists of
1038either the value of the element deleted, or the undefined value. In scalar
1039context, this means that you get the value of the last element deleted (or
1040the undefined value if that element did not exist).
1041
1042 %hash = (foo => 11, bar => 22, baz => 33);
1043 $scalar = delete $hash{foo}; # $scalar is 11
1044 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
1045 @array = delete @hash{qw(foo bar baz)}; # @array is (undef,undef,33)
1046
1047Deleting from C<%ENV> modifies the environment. Deleting from
01020589
GS
1048a hash tied to a DBM file deletes the entry from the DBM file. Deleting
1049from a C<tie>d hash or array may not necessarily return anything.
1050
8ea97a1e
GS
1051Deleting an array element effectively returns that position of the array
1052to its initial, uninitialized state. Subsequently testing for the same
8216c1fd
GS
1053element with exists() will return false. Note that deleting array
1054elements in the middle of an array will not shift the index of the ones
1055after them down--use splice() for that. See L</exists>.
8ea97a1e 1056
01020589 1057The following (inefficiently) deletes all the values of %HASH and @ARRAY:
a0d0e21e 1058
5f05dabc 1059 foreach $key (keys %HASH) {
1060 delete $HASH{$key};
a0d0e21e
LW
1061 }
1062
01020589
GS
1063 foreach $index (0 .. $#ARRAY) {
1064 delete $ARRAY[$index];
1065 }
1066
1067And so do these:
5f05dabc 1068
01020589
GS
1069 delete @HASH{keys %HASH};
1070
9740c838 1071 delete @ARRAY[0 .. $#ARRAY];
5f05dabc 1072
2b5ab1e7 1073But both of these are slower than just assigning the empty list
01020589
GS
1074or undefining %HASH or @ARRAY:
1075
1076 %HASH = (); # completely empty %HASH
1077 undef %HASH; # forget %HASH ever existed
2b5ab1e7 1078
01020589
GS
1079 @ARRAY = (); # completely empty @ARRAY
1080 undef @ARRAY; # forget @ARRAY ever existed
2b5ab1e7
TC
1081
1082Note that the EXPR can be arbitrarily complicated as long as the final
01020589
GS
1083operation is a hash element, array element, hash slice, or array slice
1084lookup:
a0d0e21e
LW
1085
1086 delete $ref->[$x][$y]{$key};
5f05dabc 1087 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
a0d0e21e 1088
01020589
GS
1089 delete $ref->[$x][$y][$index];
1090 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1091
a0d0e21e
LW
1092=item die LIST
1093
19799a22
GS
1094Outside an C<eval>, prints the value of LIST to C<STDERR> and
1095exits with the current value of C<$!> (errno). If C<$!> is C<0>,
61eff3bc
JH
1096exits with the value of C<<< ($? >> 8) >>> (backtick `command`
1097status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside
19799a22
GS
1098an C<eval(),> the error message is stuffed into C<$@> and the
1099C<eval> is terminated with the undefined value. This makes
1100C<die> the way to raise an exception.
a0d0e21e
LW
1101
1102Equivalent examples:
1103
1104 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
54310121 1105 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
a0d0e21e 1106
ccac6780 1107If the last element of LIST does not end in a newline, the current
df37ec69
WW
1108script line number and input line number (if any) are also printed,
1109and a newline is supplied. Note that the "input line number" (also
1110known as "chunk") is subject to whatever notion of "line" happens to
1111be currently in effect, and is also available as the special variable
1112C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1113
1114Hint: sometimes appending C<", stopped"> to your message will cause it
1115to make better sense when the string C<"at foo line 123"> is appended.
1116Suppose you are running script "canasta".
a0d0e21e
LW
1117
1118 die "/etc/games is no good";
1119 die "/etc/games is no good, stopped";
1120
1121produce, respectively
1122
1123 /etc/games is no good at canasta line 123.
1124 /etc/games is no good, stopped at canasta line 123.
1125
2b5ab1e7 1126See also exit(), warn(), and the Carp module.
a0d0e21e 1127
7660c0ab
A
1128If LIST is empty and C<$@> already contains a value (typically from a
1129previous eval) that value is reused after appending C<"\t...propagated">.
fb73857a 1130This is useful for propagating exceptions:
1131
1132 eval { ... };
1133 die unless $@ =~ /Expected exception/;
1134
ad216e65
JH
1135If LIST is empty and C<$@> contains an object reference that has a
1136C<PROPAGATE> method, that method will be called with additional file
1137and line number parameters. The return value replaces the value in
16869676 1138C<$@>. ie. as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >>
ad216e65
JH
1139were called.
1140
7660c0ab 1141If C<$@> is empty then the string C<"Died"> is used.
fb73857a 1142
52531d10
GS
1143die() can also be called with a reference argument. If this happens to be
1144trapped within an eval(), $@ contains the reference. This behavior permits
1145a more elaborate exception handling implementation using objects that
4375e838 1146maintain arbitrary state about the nature of the exception. Such a scheme
52531d10
GS
1147is sometimes preferable to matching particular string values of $@ using
1148regular expressions. Here's an example:
1149
1150 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1151 if ($@) {
1152 if (ref($@) && UNIVERSAL::isa($@,"Some::Module::Exception")) {
1153 # handle Some::Module::Exception
1154 }
1155 else {
1156 # handle all other possible exceptions
1157 }
1158 }
1159
19799a22 1160Because perl will stringify uncaught exception messages before displaying
52531d10
GS
1161them, you may want to overload stringification operations on such custom
1162exception objects. See L<overload> for details about that.
1163
19799a22
GS
1164You can arrange for a callback to be run just before the C<die>
1165does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1166handler will be called with the error text and can change the error
1167message, if it sees fit, by calling C<die> again. See
1168L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
1169L<"eval BLOCK"> for some examples. Although this feature was meant
1170to be run only right before your program was to exit, this is not
1171currently the case--the C<$SIG{__DIE__}> hook is currently called
1172even inside eval()ed blocks/strings! If one wants the hook to do
1173nothing in such situations, put
fb73857a 1174
1175 die @_ if $^S;
1176
19799a22
GS
1177as the first line of the handler (see L<perlvar/$^S>). Because
1178this promotes strange action at a distance, this counterintuitive
b76cc8ba 1179behavior may be fixed in a future release.
774d564b 1180
a0d0e21e
LW
1181=item do BLOCK
1182
1183Not really a function. Returns the value of the last command in the
1184sequence of commands indicated by BLOCK. When modified by a loop
98293880
JH
1185modifier, executes the BLOCK once before testing the loop condition.
1186(On other statements the loop modifiers test the conditional first.)
a0d0e21e 1187
4968c1e4 1188C<do BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7
TC
1189C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1190See L<perlsyn> for alternative strategies.
4968c1e4 1191
a0d0e21e
LW
1192=item do SUBROUTINE(LIST)
1193
1194A deprecated form of subroutine call. See L<perlsub>.
1195
1196=item do EXPR
1197
1198Uses the value of EXPR as a filename and executes the contents of the
1199file as a Perl script. Its primary use is to include subroutines
1200from a Perl subroutine library.
1201
1202 do 'stat.pl';
1203
1204is just like
1205
986b19de 1206 eval `cat stat.pl`;
a0d0e21e 1207
2b5ab1e7
TC
1208except that it's more efficient and concise, keeps track of the current
1209filename for error messages, searches the @INC libraries, and updates
1210C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1211variables. It also differs in that code evaluated with C<do FILENAME>
1212cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1213same, however, in that it does reparse the file every time you call it,
1214so you probably don't want to do this inside a loop.
a0d0e21e 1215
8e30cc93 1216If C<do> cannot read the file, it returns undef and sets C<$!> to the
2b5ab1e7 1217error. If C<do> can read the file but cannot compile it, it
8e30cc93
MG
1218returns undef and sets an error message in C<$@>. If the file is
1219successfully compiled, C<do> returns the value of the last expression
1220evaluated.
1221
a0d0e21e 1222Note that inclusion of library modules is better done with the
19799a22 1223C<use> and C<require> operators, which also do automatic error checking
4633a7c4 1224and raise an exception if there's a problem.
a0d0e21e 1225
5a964f20
TC
1226You might like to use C<do> to read in a program configuration
1227file. Manual error checking can be done this way:
1228
b76cc8ba 1229 # read in config files: system first, then user
f86cebdf 1230 for $file ("/share/prog/defaults.rc",
b76cc8ba 1231 "$ENV{HOME}/.someprogrc")
2b5ab1e7 1232 {
5a964f20 1233 unless ($return = do $file) {
f86cebdf
GS
1234 warn "couldn't parse $file: $@" if $@;
1235 warn "couldn't do $file: $!" unless defined $return;
1236 warn "couldn't run $file" unless $return;
5a964f20
TC
1237 }
1238 }
1239
a0d0e21e
LW
1240=item dump LABEL
1241
1614b0e3
JD
1242=item dump
1243
19799a22
GS
1244This function causes an immediate core dump. See also the B<-u>
1245command-line switch in L<perlrun>, which does the same thing.
1246Primarily this is so that you can use the B<undump> program (not
1247supplied) to turn your core dump into an executable binary after
1248having initialized all your variables at the beginning of the
1249program. When the new binary is executed it will begin by executing
1250a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1251Think of it as a goto with an intervening core dump and reincarnation.
1252If C<LABEL> is omitted, restarts the program from the top.
1253
1254B<WARNING>: Any files opened at the time of the dump will I<not>
1255be open any more when the program is reincarnated, with possible
b76cc8ba 1256resulting confusion on the part of Perl.
19799a22
GS
1257
1258This function is now largely obsolete, partly because it's very
1259hard to convert a core file into an executable, and because the
1260real compiler backends for generating portable bytecode and compilable
ac206dc8
RGS
1261C code have superseded it. That's why you should now invoke it as
1262C<CORE::dump()>, if you don't want to be warned against a possible
1263typo.
19799a22
GS
1264
1265If you're looking to use L<dump> to speed up your program, consider
1266generating bytecode or native C code as described in L<perlcc>. If
1267you're just trying to accelerate a CGI script, consider using the
210b36aa 1268C<mod_perl> extension to B<Apache>, or the CPAN module, CGI::Fast.
19799a22 1269You might also consider autoloading or selfloading, which at least
b76cc8ba 1270make your program I<appear> to run faster.
5a964f20 1271
aa689395 1272=item each HASH
1273
5a964f20 1274When called in list context, returns a 2-element list consisting of the
aa689395 1275key and value for the next element of a hash, so that you can iterate over
74fc8b5f 1276it. When called in scalar context, returns only the key for the next
e902a979 1277element in the hash.
2f9daede 1278
ab192400 1279Entries are returned in an apparently random order. The actual random
504f80c1
JH
1280order is subject to change in future versions of perl, but it is
1281guaranteed to be in the same order as either the C<keys> or C<values>
4546b9e6
JH
1282function would produce on the same (unmodified) hash. Since Perl
12835.8.1 the ordering is different even between different runs of Perl
1284for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
ab192400
GS
1285
1286When the hash is entirely read, a null array is returned in list context
19799a22
GS
1287(which when assigned produces a false (C<0>) value), and C<undef> in
1288scalar context. The next call to C<each> after that will start iterating
1289again. There is a single iterator for each hash, shared by all C<each>,
1290C<keys>, and C<values> function calls in the program; it can be reset by
2f9daede
TP
1291reading all the elements from the hash, or by evaluating C<keys HASH> or
1292C<values HASH>. If you add or delete elements of a hash while you're
74fc8b5f
MJD
1293iterating over it, you may get entries skipped or duplicated, so
1294don't. Exception: It is always safe to delete the item most recently
1295returned by C<each()>, which means that the following code will work:
1296
1297 while (($key, $value) = each %hash) {
1298 print $key, "\n";
1299 delete $hash{$key}; # This is safe
1300 }
aa689395 1301
f86cebdf 1302The following prints out your environment like the printenv(1) program,
aa689395 1303only in a different order:
a0d0e21e
LW
1304
1305 while (($key,$value) = each %ENV) {
1306 print "$key=$value\n";
1307 }
1308
19799a22 1309See also C<keys>, C<values> and C<sort>.
a0d0e21e
LW
1310
1311=item eof FILEHANDLE
1312
4633a7c4
LW
1313=item eof ()
1314
a0d0e21e
LW
1315=item eof
1316
1317Returns 1 if the next read on FILEHANDLE will return end of file, or if
1318FILEHANDLE is not open. FILEHANDLE may be an expression whose value
5a964f20 1319gives the real filehandle. (Note that this function actually
19799a22 1320reads a character and then C<ungetc>s it, so isn't very useful in an
748a9306 1321interactive context.) Do not read from a terminal file (or call
19799a22 1322C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
748a9306
LW
1323as terminals may lose the end-of-file condition if you do.
1324
820475bd
GS
1325An C<eof> without an argument uses the last file read. Using C<eof()>
1326with empty parentheses is very different. It refers to the pseudo file
1327formed from the files listed on the command line and accessed via the
61eff3bc
JH
1328C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1329as a normal filehandle is, an C<eof()> before C<< <> >> has been
820475bd 1330used will cause C<@ARGV> to be examined to determine if input is
67408cae 1331available. Similarly, an C<eof()> after C<< <> >> has returned
efdd0218
RB
1332end-of-file will assume you are processing another C<@ARGV> list,
1333and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1334see L<perlop/"I/O Operators">.
820475bd 1335
61eff3bc 1336In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
820475bd
GS
1337detect the end of each file, C<eof()> will only detect the end of the
1338last file. Examples:
a0d0e21e 1339
748a9306
LW
1340 # reset line numbering on each input file
1341 while (<>) {
b76cc8ba 1342 next if /^\s*#/; # skip comments
748a9306 1343 print "$.\t$_";
5a964f20
TC
1344 } continue {
1345 close ARGV if eof; # Not eof()!
748a9306
LW
1346 }
1347
a0d0e21e
LW
1348 # insert dashes just before last line of last file
1349 while (<>) {
6ac88b13 1350 if (eof()) { # check for end of last file
a0d0e21e
LW
1351 print "--------------\n";
1352 }
1353 print;
6ac88b13 1354 last if eof(); # needed if we're reading from a terminal
a0d0e21e
LW
1355 }
1356
a0d0e21e 1357Practical hint: you almost never need to use C<eof> in Perl, because the
3ce0d271
GS
1358input operators typically return C<undef> when they run out of data, or if
1359there was an error.
a0d0e21e
LW
1360
1361=item eval EXPR
1362
1363=item eval BLOCK
1364
c7cc6f1c
GS
1365In the first form, the return value of EXPR is parsed and executed as if it
1366were a little Perl program. The value of the expression (which is itself
5a964f20 1367determined within scalar context) is first parsed, and if there weren't any
be3174d2
GS
1368errors, executed in the lexical context of the current Perl program, so
1369that any variable settings or subroutine and format definitions remain
1370afterwards. Note that the value is parsed every time the eval executes.
1371If EXPR is omitted, evaluates C<$_>. This form is typically used to
1372delay parsing and subsequent execution of the text of EXPR until run time.
c7cc6f1c
GS
1373
1374In the second form, the code within the BLOCK is parsed only once--at the
1375same time the code surrounding the eval itself was parsed--and executed
1376within the context of the current Perl program. This form is typically
1377used to trap exceptions more efficiently than the first (see below), while
1378also providing the benefit of checking the code within BLOCK at compile
1379time.
1380
1381The final semicolon, if any, may be omitted from the value of EXPR or within
1382the BLOCK.
1383
1384In both forms, the value returned is the value of the last expression
5a964f20 1385evaluated inside the mini-program; a return statement may be also used, just
c7cc6f1c 1386as with subroutines. The expression providing the return value is evaluated
5a964f20 1387in void, scalar, or list context, depending on the context of the eval itself.
c7cc6f1c 1388See L</wantarray> for more on how the evaluation context can be determined.
a0d0e21e 1389
19799a22
GS
1390If there is a syntax error or runtime error, or a C<die> statement is
1391executed, an undefined value is returned by C<eval>, and C<$@> is set to the
a0d0e21e 1392error message. If there was no error, C<$@> is guaranteed to be a null
19799a22 1393string. Beware that using C<eval> neither silences perl from printing
c7cc6f1c 1394warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
d9984052
A
1395To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1396turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1397See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
a0d0e21e 1398
19799a22
GS
1399Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1400determining whether a particular feature (such as C<socket> or C<symlink>)
a0d0e21e
LW
1401is implemented. It is also Perl's exception trapping mechanism, where
1402the die operator is used to raise exceptions.
1403
1404If the code to be executed doesn't vary, you may use the eval-BLOCK
1405form to trap run-time errors without incurring the penalty of
1406recompiling each time. The error, if any, is still returned in C<$@>.
1407Examples:
1408
54310121 1409 # make divide-by-zero nonfatal
a0d0e21e
LW
1410 eval { $answer = $a / $b; }; warn $@ if $@;
1411
1412 # same thing, but less efficient
1413 eval '$answer = $a / $b'; warn $@ if $@;
1414
1415 # a compile-time error
5a964f20 1416 eval { $answer = }; # WRONG
a0d0e21e
LW
1417
1418 # a run-time error
1419 eval '$answer ='; # sets $@
1420
2b5ab1e7
TC
1421Due to the current arguably broken state of C<__DIE__> hooks, when using
1422the C<eval{}> form as an exception trap in libraries, you may wish not
1423to trigger any C<__DIE__> hooks that user code may have installed.
1424You can use the C<local $SIG{__DIE__}> construct for this purpose,
1425as shown in this example:
774d564b 1426
1427 # a very private exception trap for divide-by-zero
f86cebdf
GS
1428 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1429 warn $@ if $@;
774d564b 1430
1431This is especially significant, given that C<__DIE__> hooks can call
19799a22 1432C<die> again, which has the effect of changing their error messages:
774d564b 1433
1434 # __DIE__ hooks may modify error messages
1435 {
f86cebdf
GS
1436 local $SIG{'__DIE__'} =
1437 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
c7cc6f1c
GS
1438 eval { die "foo lives here" };
1439 print $@ if $@; # prints "bar lives here"
774d564b 1440 }
1441
19799a22 1442Because this promotes action at a distance, this counterintuitive behavior
2b5ab1e7
TC
1443may be fixed in a future release.
1444
19799a22 1445With an C<eval>, you should be especially careful to remember what's
a0d0e21e
LW
1446being looked at when:
1447
1448 eval $x; # CASE 1
1449 eval "$x"; # CASE 2
1450
1451 eval '$x'; # CASE 3
1452 eval { $x }; # CASE 4
1453
5a964f20 1454 eval "\$$x++"; # CASE 5
a0d0e21e
LW
1455 $$x++; # CASE 6
1456
2f9daede 1457Cases 1 and 2 above behave identically: they run the code contained in
19799a22 1458the variable $x. (Although case 2 has misleading double quotes making
2f9daede 1459the reader wonder what else might be happening (nothing is).) Cases 3
7660c0ab 1460and 4 likewise behave in the same way: they run the code C<'$x'>, which
19799a22 1461does nothing but return the value of $x. (Case 4 is preferred for
2f9daede
TP
1462purely visual reasons, but it also has the advantage of compiling at
1463compile-time instead of at run-time.) Case 5 is a place where
19799a22 1464normally you I<would> like to use double quotes, except that in this
2f9daede
TP
1465particular situation, you can just use symbolic references instead, as
1466in case 6.
a0d0e21e 1467
4968c1e4 1468C<eval BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7 1469C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
4968c1e4 1470
d819b83a
DM
1471Note that as a very special case, an C<eval ''> executed within the C<DB>
1472package doesn't see the usual surrounding lexical scope, but rather the
1473scope of the first non-DB piece of code that called it. You don't normally
1474need to worry about this unless you are writing a Perl debugger.
1475
a0d0e21e
LW
1476=item exec LIST
1477
8bf3b016
GS
1478=item exec PROGRAM LIST
1479
19799a22
GS
1480The C<exec> function executes a system command I<and never returns>--
1481use C<system> instead of C<exec> if you want it to return. It fails and
1482returns false only if the command does not exist I<and> it is executed
fb73857a 1483directly instead of via your system's command shell (see below).
a0d0e21e 1484
19799a22
GS
1485Since it's a common mistake to use C<exec> instead of C<system>, Perl
1486warns you if there is a following statement which isn't C<die>, C<warn>,
1487or C<exit> (if C<-w> is set - but you always do that). If you
1488I<really> want to follow an C<exec> with some other statement, you
55d729e4
GS
1489can use one of these styles to avoid the warning:
1490
5a964f20
TC
1491 exec ('foo') or print STDERR "couldn't exec foo: $!";
1492 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
55d729e4 1493
5a964f20 1494If there is more than one argument in LIST, or if LIST is an array
f86cebdf 1495with more than one value, calls execvp(3) with the arguments in LIST.
5a964f20
TC
1496If there is only one scalar argument or an array with one element in it,
1497the argument is checked for shell metacharacters, and if there are any,
1498the entire argument is passed to the system's command shell for parsing
1499(this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1500If there are no shell metacharacters in the argument, it is split into
b76cc8ba 1501words and passed directly to C<execvp>, which is more efficient.
19799a22 1502Examples:
a0d0e21e 1503
19799a22
GS
1504 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1505 exec "sort $outfile | uniq";
a0d0e21e
LW
1506
1507If you don't really want to execute the first argument, but want to lie
1508to the program you are executing about its own name, you can specify
1509the program you actually want to run as an "indirect object" (without a
1510comma) in front of the LIST. (This always forces interpretation of the
54310121 1511LIST as a multivalued list, even if there is only a single scalar in
a0d0e21e
LW
1512the list.) Example:
1513
1514 $shell = '/bin/csh';
1515 exec $shell '-sh'; # pretend it's a login shell
1516
1517or, more directly,
1518
1519 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1520
bb32b41a
GS
1521When the arguments get executed via the system shell, results will
1522be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1523for details.
1524
19799a22
GS
1525Using an indirect object with C<exec> or C<system> is also more
1526secure. This usage (which also works fine with system()) forces
1527interpretation of the arguments as a multivalued list, even if the
1528list had just one argument. That way you're safe from the shell
1529expanding wildcards or splitting up words with whitespace in them.
5a964f20
TC
1530
1531 @args = ( "echo surprise" );
1532
2b5ab1e7 1533 exec @args; # subject to shell escapes
f86cebdf 1534 # if @args == 1
2b5ab1e7 1535 exec { $args[0] } @args; # safe even with one-arg list
5a964f20
TC
1536
1537The first version, the one without the indirect object, ran the I<echo>
1538program, passing it C<"surprise"> an argument. The second version
1539didn't--it tried to run a program literally called I<"echo surprise">,
1540didn't find it, and set C<$?> to a non-zero value indicating failure.
1541
0f897271
GS
1542Beginning with v5.6.0, Perl will attempt to flush all files opened for
1543output before the exec, but this may not be supported on some platforms
1544(see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1545in English) or call the C<autoflush()> method of C<IO::Handle> on any
1546open handles in order to avoid lost output.
1547
19799a22 1548Note that C<exec> will not call your C<END> blocks, nor will it call
7660c0ab
A
1549any C<DESTROY> methods in your objects.
1550
a0d0e21e
LW
1551=item exists EXPR
1552
01020589 1553Given an expression that specifies a hash element or array element,
8ea97a1e
GS
1554returns true if the specified element in the hash or array has ever
1555been initialized, even if the corresponding value is undefined. The
1556element is not autovivified if it doesn't exist.
a0d0e21e 1557
01020589
GS
1558 print "Exists\n" if exists $hash{$key};
1559 print "Defined\n" if defined $hash{$key};
1560 print "True\n" if $hash{$key};
1561
1562 print "Exists\n" if exists $array[$index];
1563 print "Defined\n" if defined $array[$index];
1564 print "True\n" if $array[$index];
a0d0e21e 1565
8ea97a1e 1566A hash or array element can be true only if it's defined, and defined if
a0d0e21e
LW
1567it exists, but the reverse doesn't necessarily hold true.
1568
afebc493
GS
1569Given an expression that specifies the name of a subroutine,
1570returns true if the specified subroutine has ever been declared, even
1571if it is undefined. Mentioning a subroutine name for exists or defined
847c7ebe
DD
1572does not count as declaring it. Note that a subroutine which does not
1573exist may still be callable: its package may have an C<AUTOLOAD>
1574method that makes it spring into existence the first time that it is
1575called -- see L<perlsub>.
afebc493
GS
1576
1577 print "Exists\n" if exists &subroutine;
1578 print "Defined\n" if defined &subroutine;
1579
a0d0e21e 1580Note that the EXPR can be arbitrarily complicated as long as the final
afebc493 1581operation is a hash or array key lookup or subroutine name:
a0d0e21e 1582
2b5ab1e7
TC
1583 if (exists $ref->{A}->{B}->{$key}) { }
1584 if (exists $hash{A}{B}{$key}) { }
1585
01020589
GS
1586 if (exists $ref->{A}->{B}->[$ix]) { }
1587 if (exists $hash{A}{B}[$ix]) { }
1588
afebc493
GS
1589 if (exists &{$ref->{A}{B}{$key}}) { }
1590
01020589
GS
1591Although the deepest nested array or hash will not spring into existence
1592just because its existence was tested, any intervening ones will.
61eff3bc 1593Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
01020589
GS
1594into existence due to the existence test for the $key element above.
1595This happens anywhere the arrow operator is used, including even:
5a964f20 1596
2b5ab1e7
TC
1597 undef $ref;
1598 if (exists $ref->{"Some key"}) { }
1599 print $ref; # prints HASH(0x80d3d5c)
1600
1601This surprising autovivification in what does not at first--or even
1602second--glance appear to be an lvalue context may be fixed in a future
5a964f20 1603release.
a0d0e21e 1604
afebc493
GS
1605Use of a subroutine call, rather than a subroutine name, as an argument
1606to exists() is an error.
1607
1608 exists &sub; # OK
1609 exists &sub(); # Error
1610
a0d0e21e
LW
1611=item exit EXPR
1612
2b5ab1e7 1613Evaluates EXPR and exits immediately with that value. Example:
a0d0e21e
LW
1614
1615 $ans = <STDIN>;
1616 exit 0 if $ans =~ /^[Xx]/;
1617
19799a22 1618See also C<die>. If EXPR is omitted, exits with C<0> status. The only
2b5ab1e7
TC
1619universally recognized values for EXPR are C<0> for success and C<1>
1620for error; other values are subject to interpretation depending on the
1621environment in which the Perl program is running. For example, exiting
162269 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1623the mailer to return the item undelivered, but that's not true everywhere.
a0d0e21e 1624
19799a22
GS
1625Don't use C<exit> to abort a subroutine if there's any chance that
1626someone might want to trap whatever error happened. Use C<die> instead,
1627which can be trapped by an C<eval>.
28757baa 1628
19799a22 1629The exit() function does not always exit immediately. It calls any
2b5ab1e7 1630defined C<END> routines first, but these C<END> routines may not
19799a22 1631themselves abort the exit. Likewise any object destructors that need to
2b5ab1e7
TC
1632be called are called before the real exit. If this is a problem, you
1633can call C<POSIX:_exit($status)> to avoid END and destructor processing.
87275199 1634See L<perlmod> for details.
5a964f20 1635
a0d0e21e
LW
1636=item exp EXPR
1637
54310121 1638=item exp
bbce6d69 1639
b76cc8ba 1640Returns I<e> (the natural logarithm base) to the power of EXPR.
a0d0e21e
LW
1641If EXPR is omitted, gives C<exp($_)>.
1642
1643=item fcntl FILEHANDLE,FUNCTION,SCALAR
1644
f86cebdf 1645Implements the fcntl(2) function. You'll probably have to say
a0d0e21e
LW
1646
1647 use Fcntl;
1648
0ade1984 1649first to get the correct constant definitions. Argument processing and
b76cc8ba 1650value return works just like C<ioctl> below.
a0d0e21e
LW
1651For example:
1652
1653 use Fcntl;
5a964f20
TC
1654 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1655 or die "can't fcntl F_GETFL: $!";
1656
554ad1fc 1657You don't have to check for C<defined> on the return from C<fcntl>.
951ba7fe
GS
1658Like C<ioctl>, it maps a C<0> return from the system call into
1659C<"0 but true"> in Perl. This string is true in boolean context and C<0>
2b5ab1e7
TC
1660in numeric context. It is also exempt from the normal B<-w> warnings
1661on improper numeric conversions.
5a964f20 1662
19799a22 1663Note that C<fcntl> will produce a fatal error if used on a machine that
2b5ab1e7
TC
1664doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1665manpage to learn what functions are available on your system.
a0d0e21e 1666
be2f7487
TH
1667Here's an example of setting a filehandle named C<REMOTE> to be
1668non-blocking at the system level. You'll have to negotiate C<$|>
1669on your own, though.
1670
1671 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
1672
1673 $flags = fcntl(REMOTE, F_GETFL, 0)
1674 or die "Can't get flags for the socket: $!\n";
1675
1676 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
1677 or die "Can't set flags for the socket: $!\n";
1678
a0d0e21e
LW
1679=item fileno FILEHANDLE
1680
2b5ab1e7
TC
1681Returns the file descriptor for a filehandle, or undefined if the
1682filehandle is not open. This is mainly useful for constructing
19799a22 1683bitmaps for C<select> and low-level POSIX tty-handling operations.
2b5ab1e7
TC
1684If FILEHANDLE is an expression, the value is taken as an indirect
1685filehandle, generally its name.
5a964f20 1686
b76cc8ba 1687You can use this to find out whether two handles refer to the
5a964f20
TC
1688same underlying descriptor:
1689
1690 if (fileno(THIS) == fileno(THAT)) {
1691 print "THIS and THAT are dups\n";
b76cc8ba
NIS
1692 }
1693
1694(Filehandles connected to memory objects via new features of C<open> may
1695return undefined even though they are open.)
1696
a0d0e21e
LW
1697
1698=item flock FILEHANDLE,OPERATION
1699
19799a22
GS
1700Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1701for success, false on failure. Produces a fatal error if used on a
2b5ab1e7 1702machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
19799a22 1703C<flock> is Perl's portable file locking interface, although it locks
2b5ab1e7
TC
1704only entire files, not records.
1705
1706Two potentially non-obvious but traditional C<flock> semantics are
1707that it waits indefinitely until the lock is granted, and that its locks
1708B<merely advisory>. Such discretionary locks are more flexible, but offer
19799a22
GS
1709fewer guarantees. This means that files locked with C<flock> may be
1710modified by programs that do not also use C<flock>. See L<perlport>,
2b5ab1e7
TC
1711your port's specific documentation, or your system-specific local manpages
1712for details. It's best to assume traditional behavior if you're writing
1713portable programs. (But if you're not, you should as always feel perfectly
1714free to write for your own system's idiosyncrasies (sometimes called
1715"features"). Slavish adherence to portability concerns shouldn't get
1716in the way of your getting your job done.)
a3cb178b 1717
8ebc5c01 1718OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1719LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
ea3105be 1720you can use the symbolic names if you import them from the Fcntl module,
68dc0745 1721either individually, or as a group using the ':flock' tag. LOCK_SH
1722requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
ea3105be
GS
1723releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1724LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
68dc0745 1725waiting for the lock (check the return status to see if you got it).
1726
2b5ab1e7
TC
1727To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1728before locking or unlocking it.
8ebc5c01 1729
f86cebdf 1730Note that the emulation built with lockf(3) doesn't provide shared
8ebc5c01 1731locks, and it requires that FILEHANDLE be open with write intent. These
2b5ab1e7 1732are the semantics that lockf(3) implements. Most if not all systems
f86cebdf 1733implement lockf(3) in terms of fcntl(2) locking, though, so the
8ebc5c01 1734differing semantics shouldn't bite too many people.
1735
becacb53
TM
1736Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
1737be open with read intent to use LOCK_SH and requires that it be open
1738with write intent to use LOCK_EX.
1739
19799a22
GS
1740Note also that some versions of C<flock> cannot lock things over the
1741network; you would need to use the more system-specific C<fcntl> for
f86cebdf
GS
1742that. If you like you can force Perl to ignore your system's flock(2)
1743function, and so provide its own fcntl(2)-based emulation, by passing
8ebc5c01 1744the switch C<-Ud_flock> to the F<Configure> program when you configure
1745perl.
4633a7c4
LW
1746
1747Here's a mailbox appender for BSD systems.
a0d0e21e 1748
7e1af8bc 1749 use Fcntl ':flock'; # import LOCK_* constants
a0d0e21e
LW
1750
1751 sub lock {
7e1af8bc 1752 flock(MBOX,LOCK_EX);
a0d0e21e
LW
1753 # and, in case someone appended
1754 # while we were waiting...
1755 seek(MBOX, 0, 2);
1756 }
1757
1758 sub unlock {
7e1af8bc 1759 flock(MBOX,LOCK_UN);
a0d0e21e
LW
1760 }
1761
1762 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1763 or die "Can't open mailbox: $!";
1764
1765 lock();
1766 print MBOX $msg,"\n\n";
1767 unlock();
1768
2b5ab1e7
TC
1769On systems that support a real flock(), locks are inherited across fork()
1770calls, whereas those that must resort to the more capricious fcntl()
1771function lose the locks, making it harder to write servers.
1772
cb1a09d0 1773See also L<DB_File> for other flock() examples.
a0d0e21e
LW
1774
1775=item fork
1776
2b5ab1e7
TC
1777Does a fork(2) system call to create a new process running the
1778same program at the same point. It returns the child pid to the
1779parent process, C<0> to the child process, or C<undef> if the fork is
1780unsuccessful. File descriptors (and sometimes locks on those descriptors)
1781are shared, while everything else is copied. On most systems supporting
1782fork(), great care has gone into making it extremely efficient (for
1783example, using copy-on-write technology on data pages), making it the
1784dominant paradigm for multitasking over the last few decades.
5a964f20 1785
0f897271
GS
1786Beginning with v5.6.0, Perl will attempt to flush all files opened for
1787output before forking the child process, but this may not be supported
1788on some platforms (see L<perlport>). To be safe, you may need to set
1789C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1790C<IO::Handle> on any open handles in order to avoid duplicate output.
a0d0e21e 1791
19799a22 1792If you C<fork> without ever waiting on your children, you will
2b5ab1e7
TC
1793accumulate zombies. On some systems, you can avoid this by setting
1794C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1795forking and reaping moribund children.
cb1a09d0 1796
28757baa 1797Note that if your forked child inherits system file descriptors like
1798STDIN and STDOUT that are actually connected by a pipe or socket, even
2b5ab1e7 1799if you exit, then the remote server (such as, say, a CGI script or a
19799a22 1800backgrounded job launched from a remote shell) won't think you're done.
2b5ab1e7 1801You should reopen those to F</dev/null> if it's any issue.
28757baa 1802
cb1a09d0
AD
1803=item format
1804
19799a22 1805Declare a picture format for use by the C<write> function. For
cb1a09d0
AD
1806example:
1807
54310121 1808 format Something =
cb1a09d0
AD
1809 Test: @<<<<<<<< @||||| @>>>>>
1810 $str, $%, '$' . int($num)
1811 .
1812
1813 $str = "widget";
184e9718 1814 $num = $cost/$quantity;
cb1a09d0
AD
1815 $~ = 'Something';
1816 write;
1817
1818See L<perlform> for many details and examples.
1819
8903cb82 1820=item formline PICTURE,LIST
a0d0e21e 1821
5a964f20 1822This is an internal function used by C<format>s, though you may call it,
a0d0e21e
LW
1823too. It formats (see L<perlform>) a list of values according to the
1824contents of PICTURE, placing the output into the format output
7660c0ab 1825accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
19799a22 1826Eventually, when a C<write> is done, the contents of
a0d0e21e 1827C<$^A> are written to some filehandle, but you could also read C<$^A>
7660c0ab 1828yourself and then set C<$^A> back to C<"">. Note that a format typically
19799a22 1829does one C<formline> per line of form, but the C<formline> function itself
748a9306 1830doesn't care how many newlines are embedded in the PICTURE. This means
4633a7c4 1831that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
748a9306
LW
1832You may therefore need to use multiple formlines to implement a single
1833record format, just like the format compiler.
1834
19799a22 1835Be careful if you put double quotes around the picture, because an C<@>
748a9306 1836character may be taken to mean the beginning of an array name.
19799a22 1837C<formline> always returns true. See L<perlform> for other examples.
a0d0e21e
LW
1838
1839=item getc FILEHANDLE
1840
1841=item getc
1842
1843Returns the next character from the input file attached to FILEHANDLE,
b5fe5ca2
SR
1844or the undefined value at end of file, or if there was an error (in
1845the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
1846STDIN. This is not particularly efficient. However, it cannot be
1847used by itself to fetch single characters without waiting for the user
1848to hit enter. For that, try something more like:
4633a7c4
LW
1849
1850 if ($BSD_STYLE) {
1851 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1852 }
1853 else {
54310121 1854 system "stty", '-icanon', 'eol', "\001";
4633a7c4
LW
1855 }
1856
1857 $key = getc(STDIN);
1858
1859 if ($BSD_STYLE) {
1860 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1861 }
1862 else {
5f05dabc 1863 system "stty", 'icanon', 'eol', '^@'; # ASCII null
4633a7c4
LW
1864 }
1865 print "\n";
1866
54310121 1867Determination of whether $BSD_STYLE should be set
1868is left as an exercise to the reader.
cb1a09d0 1869
19799a22 1870The C<POSIX::getattr> function can do this more portably on
2b5ab1e7
TC
1871systems purporting POSIX compliance. See also the C<Term::ReadKey>
1872module from your nearest CPAN site; details on CPAN can be found on
1873L<perlmodlib/CPAN>.
a0d0e21e
LW
1874
1875=item getlogin
1876
5a964f20
TC
1877Implements the C library function of the same name, which on most
1878systems returns the current login from F</etc/utmp>, if any. If null,
19799a22 1879use C<getpwuid>.
a0d0e21e 1880
f86702cc 1881 $login = getlogin || getpwuid($<) || "Kilroy";
a0d0e21e 1882
19799a22
GS
1883Do not consider C<getlogin> for authentication: it is not as
1884secure as C<getpwuid>.
4633a7c4 1885
a0d0e21e
LW
1886=item getpeername SOCKET
1887
1888Returns the packed sockaddr address of other end of the SOCKET connection.
1889
4633a7c4
LW
1890 use Socket;
1891 $hersockaddr = getpeername(SOCK);
19799a22 1892 ($port, $iaddr) = sockaddr_in($hersockaddr);
4633a7c4
LW
1893 $herhostname = gethostbyaddr($iaddr, AF_INET);
1894 $herstraddr = inet_ntoa($iaddr);
a0d0e21e
LW
1895
1896=item getpgrp PID
1897
47e29363 1898Returns the current process group for the specified PID. Use
7660c0ab 1899a PID of C<0> to get the current process group for the
4633a7c4 1900current process. Will raise an exception if used on a machine that
f86cebdf 1901doesn't implement getpgrp(2). If PID is omitted, returns process
19799a22 1902group of current process. Note that the POSIX version of C<getpgrp>
7660c0ab 1903does not accept a PID argument, so only C<PID==0> is truly portable.
a0d0e21e
LW
1904
1905=item getppid
1906
1907Returns the process id of the parent process.
1908
4d76a344
RGS
1909Note for Linux users: on Linux, the C functions C<getpid()> and
1910C<getppid()> return different values from different threads. In order to
1911be portable, this behavior is not reflected by the perl-level function
1912C<getppid()>, that returns a consistent value across threads. If you want
e3256f86
RGS
1913to call the underlying C<getppid()>, you may use the CPAN module
1914C<Linux::Pid>.
4d76a344 1915
a0d0e21e
LW
1916=item getpriority WHICH,WHO
1917
4633a7c4
LW
1918Returns the current priority for a process, a process group, or a user.
1919(See L<getpriority(2)>.) Will raise a fatal exception if used on a
f86cebdf 1920machine that doesn't implement getpriority(2).
a0d0e21e
LW
1921
1922=item getpwnam NAME
1923
1924=item getgrnam NAME
1925
1926=item gethostbyname NAME
1927
1928=item getnetbyname NAME
1929
1930=item getprotobyname NAME
1931
1932=item getpwuid UID
1933
1934=item getgrgid GID
1935
1936=item getservbyname NAME,PROTO
1937
1938=item gethostbyaddr ADDR,ADDRTYPE
1939
1940=item getnetbyaddr ADDR,ADDRTYPE
1941
1942=item getprotobynumber NUMBER
1943
1944=item getservbyport PORT,PROTO
1945
1946=item getpwent
1947
1948=item getgrent
1949
1950=item gethostent
1951
1952=item getnetent
1953
1954=item getprotoent
1955
1956=item getservent
1957
1958=item setpwent
1959
1960=item setgrent
1961
1962=item sethostent STAYOPEN
1963
1964=item setnetent STAYOPEN
1965
1966=item setprotoent STAYOPEN
1967
1968=item setservent STAYOPEN
1969
1970=item endpwent
1971
1972=item endgrent
1973
1974=item endhostent
1975
1976=item endnetent
1977
1978=item endprotoent
1979
1980=item endservent
1981
1982These routines perform the same functions as their counterparts in the
5a964f20 1983system library. In list context, the return values from the
a0d0e21e
LW
1984various get routines are as follows:
1985
1986 ($name,$passwd,$uid,$gid,
6ee623d5 1987 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
a0d0e21e
LW
1988 ($name,$passwd,$gid,$members) = getgr*
1989 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1990 ($name,$aliases,$addrtype,$net) = getnet*
1991 ($name,$aliases,$proto) = getproto*
1992 ($name,$aliases,$port,$proto) = getserv*
1993
1994(If the entry doesn't exist you get a null list.)
1995
4602f195
JH
1996The exact meaning of the $gcos field varies but it usually contains
1997the real name of the user (as opposed to the login name) and other
1998information pertaining to the user. Beware, however, that in many
1999system users are able to change this information and therefore it
106325ad 2000cannot be trusted and therefore the $gcos is tainted (see
2959b6e3
JH
2001L<perlsec>). The $passwd and $shell, user's encrypted password and
2002login shell, are also tainted, because of the same reason.
4602f195 2003
5a964f20 2004In scalar context, you get the name, unless the function was a
a0d0e21e
LW
2005lookup by name, in which case you get the other thing, whatever it is.
2006(If the entry doesn't exist you get the undefined value.) For example:
2007
5a964f20
TC
2008 $uid = getpwnam($name);
2009 $name = getpwuid($num);
2010 $name = getpwent();
2011 $gid = getgrnam($name);
08a33e13 2012 $name = getgrgid($num);
5a964f20
TC
2013 $name = getgrent();
2014 #etc.
a0d0e21e 2015
4602f195
JH
2016In I<getpw*()> the fields $quota, $comment, and $expire are special
2017cases in the sense that in many systems they are unsupported. If the
2018$quota is unsupported, it is an empty scalar. If it is supported, it
2019usually encodes the disk quota. If the $comment field is unsupported,
2020it is an empty scalar. If it is supported it usually encodes some
2021administrative comment about the user. In some systems the $quota
2022field may be $change or $age, fields that have to do with password
2023aging. In some systems the $comment field may be $class. The $expire
2024field, if present, encodes the expiration period of the account or the
2025password. For the availability and the exact meaning of these fields
2026in your system, please consult your getpwnam(3) documentation and your
2027F<pwd.h> file. You can also find out from within Perl what your
2028$quota and $comment fields mean and whether you have the $expire field
2029by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2030C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
2031files are only supported if your vendor has implemented them in the
2032intuitive fashion that calling the regular C library routines gets the
5d3a0a3b
GS
2033shadow versions if you're running under privilege or if there exists
2034the shadow(3) functions as found in System V ( this includes Solaris
2035and Linux.) Those systems which implement a proprietary shadow password
2036facility are unlikely to be supported.
6ee623d5 2037
19799a22 2038The $members value returned by I<getgr*()> is a space separated list of
a0d0e21e
LW
2039the login names of the members of the group.
2040
2041For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2042C, it will be returned to you via C<$?> if the function call fails. The
7660c0ab 2043C<@addrs> value returned by a successful call is a list of the raw
a0d0e21e
LW
2044addresses returned by the corresponding system library call. In the
2045Internet domain, each address is four bytes long and you can unpack it
2046by saying something like:
2047
2048 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
2049
2b5ab1e7
TC
2050The Socket library makes this slightly easier:
2051
2052 use Socket;
2053 $iaddr = inet_aton("127.1"); # or whatever address
2054 $name = gethostbyaddr($iaddr, AF_INET);
2055
2056 # or going the other way
19799a22 2057 $straddr = inet_ntoa($iaddr);
2b5ab1e7 2058
19799a22
GS
2059If you get tired of remembering which element of the return list
2060contains which return value, by-name interfaces are provided
2061in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2062C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2063and C<User::grent>. These override the normal built-ins, supplying
2064versions that return objects with the appropriate names
2065for each field. For example:
5a964f20
TC
2066
2067 use File::stat;
2068 use User::pwent;
2069 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2070
b76cc8ba
NIS
2071Even though it looks like they're the same method calls (uid),
2072they aren't, because a C<File::stat> object is different from
19799a22 2073a C<User::pwent> object.
5a964f20 2074
a0d0e21e
LW
2075=item getsockname SOCKET
2076
19799a22
GS
2077Returns the packed sockaddr address of this end of the SOCKET connection,
2078in case you don't know the address because you have several different
2079IPs that the connection might have come in on.
a0d0e21e 2080
4633a7c4
LW
2081 use Socket;
2082 $mysockaddr = getsockname(SOCK);
19799a22 2083 ($port, $myaddr) = sockaddr_in($mysockaddr);
b76cc8ba 2084 printf "Connect to %s [%s]\n",
19799a22
GS
2085 scalar gethostbyaddr($myaddr, AF_INET),
2086 inet_ntoa($myaddr);
a0d0e21e
LW
2087
2088=item getsockopt SOCKET,LEVEL,OPTNAME
2089
5a964f20 2090Returns the socket option requested, or undef if there is an error.
a0d0e21e
LW
2091
2092=item glob EXPR
2093
0a753a76 2094=item glob
2095
d9a9d457
JL
2096In list context, returns a (possibly empty) list of filename expansions on
2097the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
2098scalar context, glob iterates through such filename expansions, returning
2099undef when the list is exhausted. This is the internal function
2100implementing the C<< <*.c> >> operator, but you can use it directly. If
2101EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2102more detail in L<perlop/"I/O Operators">.
a0d0e21e 2103
3a4b19e4
GS
2104Beginning with v5.6.0, this operator is implemented using the standard
2105C<File::Glob> extension. See L<File::Glob> for details.
2106
a0d0e21e
LW
2107=item gmtime EXPR
2108
d1be9408 2109Converts a time as returned by the time function to an 8-element list
54310121 2110with the time localized for the standard Greenwich time zone.
4633a7c4 2111Typically used as follows:
a0d0e21e 2112
b76cc8ba 2113 # 0 1 2 3 4 5 6 7
48a26b3a 2114 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday) =
a0d0e21e
LW
2115 gmtime(time);
2116
48a26b3a
GS
2117All list elements are numeric, and come straight out of the C `struct
2118tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2119specified time. $mday is the day of the month, and $mon is the month
2120itself, in the range C<0..11> with 0 indicating January and 11
2121indicating December. $year is the number of years since 1900. That
2122is, $year is C<123> in year 2023. $wday is the day of the week, with
21230 indicating Sunday and 3 indicating Wednesday. $yday is the day of
b76cc8ba 2124the year, in the range C<0..364> (or C<0..365> in leap years.)
48a26b3a
GS
2125
2126Note that the $year element is I<not> simply the last two digits of
2127the year. If you assume it is, then you create non-Y2K-compliant
2128programs--and you wouldn't want to do that, would you?
2f9daede 2129
abd75f24
GS
2130The proper way to get a complete 4-digit year is simply:
2131
2132 $year += 1900;
2133
2134And to get the last two digits of the year (e.g., '01' in 2001) do:
2135
2136 $year = sprintf("%02d", $year % 100);
2137
48a26b3a 2138If EXPR is omitted, C<gmtime()> uses the current time (C<gmtime(time)>).
a0d0e21e 2139
48a26b3a 2140In scalar context, C<gmtime()> returns the ctime(3) value:
0a753a76 2141
2142 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
2143
19799a22 2144Also see the C<timegm> function provided by the C<Time::Local> module,
f86cebdf 2145and the strftime(3) function available via the POSIX module.
7660c0ab 2146
2b5ab1e7
TC
2147This scalar value is B<not> locale dependent (see L<perllocale>), but
2148is instead a Perl builtin. Also see the C<Time::Local> module, and the
2149strftime(3) and mktime(3) functions available via the POSIX module. To
7660c0ab
A
2150get somewhat similar but locale dependent date strings, set up your
2151locale environment variables appropriately (please see L<perllocale>)
2152and try for example:
2153
2154 use POSIX qw(strftime);
2b5ab1e7 2155 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
7660c0ab 2156
2b5ab1e7
TC
2157Note that the C<%a> and C<%b> escapes, which represent the short forms
2158of the day of the week and the month of the year, may not necessarily
2159be three characters wide in all locales.
0a753a76 2160
a0d0e21e
LW
2161=item goto LABEL
2162
748a9306
LW
2163=item goto EXPR
2164
a0d0e21e
LW
2165=item goto &NAME
2166
7660c0ab 2167The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
a0d0e21e 2168execution there. It may not be used to go into any construct that
7660c0ab 2169requires initialization, such as a subroutine or a C<foreach> loop. It
0a753a76 2170also can't be used to go into a construct that is optimized away,
19799a22 2171or to get out of a block or subroutine given to C<sort>.
0a753a76 2172It can be used to go almost anywhere else within the dynamic scope,
a0d0e21e 2173including out of subroutines, but it's usually better to use some other
19799a22 2174construct such as C<last> or C<die>. The author of Perl has never felt the
7660c0ab 2175need to use this form of C<goto> (in Perl, that is--C is another matter).
1b6921cb
BT
2176(The difference being that C does not offer named loops combined with
2177loop control. Perl does, and this replaces most structured uses of C<goto>
2178in other languages.)
a0d0e21e 2179
7660c0ab
A
2180The C<goto-EXPR> form expects a label name, whose scope will be resolved
2181dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
748a9306
LW
2182necessarily recommended if you're optimizing for maintainability:
2183
2184 goto ("FOO", "BAR", "GLARCH")[$i];
2185
1b6921cb
BT
2186The C<goto-&NAME> form is quite different from the other forms of
2187C<goto>. In fact, it isn't a goto in the normal sense at all, and
2188doesn't have the stigma associated with other gotos. Instead, it
2189exits the current subroutine (losing any changes set by local()) and
2190immediately calls in its place the named subroutine using the current
2191value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2192load another subroutine and then pretend that the other subroutine had
2193been called in the first place (except that any modifications to C<@_>
6cb9131c
GS
2194in the current subroutine are propagated to the other subroutine.)
2195After the C<goto>, not even C<caller> will be able to tell that this
2196routine was called first.
2197
2198NAME needn't be the name of a subroutine; it can be a scalar variable
2199containing a code reference, or a block which evaluates to a code
2200reference.
a0d0e21e
LW
2201
2202=item grep BLOCK LIST
2203
2204=item grep EXPR,LIST
2205
2b5ab1e7
TC
2206This is similar in spirit to, but not the same as, grep(1) and its
2207relatives. In particular, it is not limited to using regular expressions.
2f9daede 2208
a0d0e21e 2209Evaluates the BLOCK or EXPR for each element of LIST (locally setting
7660c0ab 2210C<$_> to each element) and returns the list value consisting of those
19799a22
GS
2211elements for which the expression evaluated to true. In scalar
2212context, returns the number of times the expression was true.
a0d0e21e
LW
2213
2214 @foo = grep(!/^#/, @bar); # weed out comments
2215
2216or equivalently,
2217
2218 @foo = grep {!/^#/} @bar; # weed out comments
2219
be3174d2
GS
2220Note that C<$_> is an alias to the list value, so it can be used to
2221modify the elements of the LIST. While this is useful and supported,
2222it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
2223Similarly, grep returns aliases into the original list, much as a for
2224loop's index variable aliases the list elements. That is, modifying an
19799a22
GS
2225element of a list returned by grep (for example, in a C<foreach>, C<map>
2226or another C<grep>) actually modifies the element in the original list.
2b5ab1e7 2227This is usually something to be avoided when writing clear code.
a0d0e21e 2228
19799a22 2229See also L</map> for a list composed of the results of the BLOCK or EXPR.
38325410 2230
a0d0e21e
LW
2231=item hex EXPR
2232
54310121 2233=item hex
bbce6d69 2234
2b5ab1e7
TC
2235Interprets EXPR as a hex string and returns the corresponding value.
2236(To convert strings that might start with either 0, 0x, or 0b, see
2237L</oct>.) If EXPR is omitted, uses C<$_>.
2f9daede
TP
2238
2239 print hex '0xAf'; # prints '175'
2240 print hex 'aF'; # same
a0d0e21e 2241
19799a22 2242Hex strings may only represent integers. Strings that would cause
53305cf1
NC
2243integer overflow trigger a warning. Leading whitespace is not stripped,
2244unlike oct().
19799a22 2245
a0d0e21e
LW
2246=item import
2247
19799a22 2248There is no builtin C<import> function. It is just an ordinary
4633a7c4 2249method (subroutine) defined (or inherited) by modules that wish to export
19799a22 2250names to another module. The C<use> function calls the C<import> method
cea6626f 2251for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
a0d0e21e
LW
2252
2253=item index STR,SUBSTR,POSITION
2254
2255=item index STR,SUBSTR
2256
2b5ab1e7
TC
2257The index function searches for one string within another, but without
2258the wildcard-like behavior of a full regular-expression pattern match.
2259It returns the position of the first occurrence of SUBSTR in STR at
2260or after POSITION. If POSITION is omitted, starts searching from the
2261beginning of the string. The return value is based at C<0> (or whatever
2262you've set the C<$[> variable to--but don't do that). If the substring
2263is not found, returns one less than the base, ordinarily C<-1>.
a0d0e21e
LW
2264
2265=item int EXPR
2266
54310121 2267=item int
bbce6d69 2268
7660c0ab 2269Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2b5ab1e7
TC
2270You should not use this function for rounding: one because it truncates
2271towards C<0>, and two because machine representations of floating point
2272numbers can sometimes produce counterintuitive results. For example,
2273C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2274because it's really more like -268.99999999999994315658 instead. Usually,
19799a22 2275the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2b5ab1e7 2276functions will serve you better than will int().
a0d0e21e
LW
2277
2278=item ioctl FILEHANDLE,FUNCTION,SCALAR
2279
2b5ab1e7 2280Implements the ioctl(2) function. You'll probably first have to say
a0d0e21e 2281
4633a7c4 2282 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
a0d0e21e 2283
2b5ab1e7 2284to get the correct function definitions. If F<ioctl.ph> doesn't
a0d0e21e 2285exist or doesn't have the correct definitions you'll have to roll your
61eff3bc 2286own, based on your C header files such as F<< <sys/ioctl.h> >>.
5a964f20 2287(There is a Perl script called B<h2ph> that comes with the Perl kit that
54310121 2288may help you in this, but it's nontrivial.) SCALAR will be read and/or
4633a7c4 2289written depending on the FUNCTION--a pointer to the string value of SCALAR
19799a22 2290will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
4633a7c4
LW
2291has no string value but does have a numeric value, that value will be
2292passed rather than a pointer to the string value. To guarantee this to be
19799a22
GS
2293true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2294functions may be needed to manipulate the values of structures used by
b76cc8ba 2295C<ioctl>.
a0d0e21e 2296
19799a22 2297The return value of C<ioctl> (and C<fcntl>) is as follows:
a0d0e21e
LW
2298
2299 if OS returns: then Perl returns:
2300 -1 undefined value
2301 0 string "0 but true"
2302 anything else that number
2303
19799a22 2304Thus Perl returns true on success and false on failure, yet you can
a0d0e21e
LW
2305still easily determine the actual value returned by the operating
2306system:
2307
2b5ab1e7 2308 $retval = ioctl(...) || -1;
a0d0e21e
LW
2309 printf "System returned %d\n", $retval;
2310
be2f7487 2311The special string C<"0 but true"> is exempt from B<-w> complaints
5a964f20
TC
2312about improper numeric conversions.
2313
a0d0e21e
LW
2314=item join EXPR,LIST
2315
2b5ab1e7
TC
2316Joins the separate strings of LIST into a single string with fields
2317separated by the value of EXPR, and returns that new string. Example:
a0d0e21e 2318
2b5ab1e7 2319 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
a0d0e21e 2320
eb6e2d6f
GS
2321Beware that unlike C<split>, C<join> doesn't take a pattern as its
2322first argument. Compare L</split>.
a0d0e21e 2323
aa689395 2324=item keys HASH
2325
504f80c1
JH
2326Returns a list consisting of all the keys of the named hash.
2327(In scalar context, returns the number of keys.)
2328
2329The keys are returned in an apparently random order. The actual
2330random order is subject to change in future versions of perl, but it
2331is guaranteed to be the same order as either the C<values> or C<each>
4546b9e6
JH
2332function produces (given that the hash has not been modified). Since
2333Perl 5.8.1 the ordering is different even between different runs of
2334Perl for security reasons (see L<perlsec/"Algorithmic Complexity
d6df3700 2335Attacks">).
504f80c1
JH
2336
2337As a side effect, calling keys() resets the HASH's internal iterator,
2f65b2f0
RGS
2338see L</each>. (In particular, calling keys() in void context resets
2339the iterator with no other overhead.)
a0d0e21e 2340
aa689395 2341Here is yet another way to print your environment:
a0d0e21e
LW
2342
2343 @keys = keys %ENV;
2344 @values = values %ENV;
b76cc8ba 2345 while (@keys) {
a0d0e21e
LW
2346 print pop(@keys), '=', pop(@values), "\n";
2347 }
2348
2349or how about sorted by key:
2350
2351 foreach $key (sort(keys %ENV)) {
2352 print $key, '=', $ENV{$key}, "\n";
2353 }
2354
8ea1e5d4
GS
2355The returned values are copies of the original keys in the hash, so
2356modifying them will not affect the original hash. Compare L</values>.
2357
19799a22 2358To sort a hash by value, you'll need to use a C<sort> function.
aa689395 2359Here's a descending numeric sort of a hash by its values:
4633a7c4 2360
5a964f20 2361 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
4633a7c4
LW
2362 printf "%4d %s\n", $hash{$key}, $key;
2363 }
2364
19799a22 2365As an lvalue C<keys> allows you to increase the number of hash buckets
aa689395 2366allocated for the given hash. This can gain you a measure of efficiency if
2367you know the hash is going to get big. (This is similar to pre-extending
2368an array by assigning a larger number to $#array.) If you say
55497cff 2369
2370 keys %hash = 200;
2371
ab192400
GS
2372then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2373in fact, since it rounds up to the next power of two. These
55497cff 2374buckets will be retained even if you do C<%hash = ()>, use C<undef
2375%hash> if you want to free the storage while C<%hash> is still in scope.
2376You can't shrink the number of buckets allocated for the hash using
19799a22 2377C<keys> in this way (but you needn't worry about doing this by accident,
55497cff 2378as trying has no effect).
2379
19799a22 2380See also C<each>, C<values> and C<sort>.
ab192400 2381
b350dd2f 2382=item kill SIGNAL, LIST
a0d0e21e 2383
b350dd2f 2384Sends a signal to a list of processes. Returns the number of
517db077
GS
2385processes successfully signaled (which is not necessarily the
2386same as the number actually killed).
a0d0e21e
LW
2387
2388 $cnt = kill 1, $child1, $child2;
2389 kill 9, @goners;
2390
b350dd2f 2391If SIGNAL is zero, no signal is sent to the process. This is a
1e9c1022 2392useful way to check that a child process is alive and hasn't changed
b350dd2f
GS
2393its UID. See L<perlport> for notes on the portability of this
2394construct.
2395
2396Unlike in the shell, if SIGNAL is negative, it kills
4633a7c4
LW
2397process groups instead of processes. (On System V, a negative I<PROCESS>
2398number will also kill process groups, but that's not portable.) That
2399means you usually want to use positive not negative signals. You may also
1e9c1022
JL
2400use a signal name in quotes.
2401
2402See L<perlipc/"Signals"> for more details.
a0d0e21e
LW
2403
2404=item last LABEL
2405
2406=item last
2407
2408The C<last> command is like the C<break> statement in C (as used in
2409loops); it immediately exits the loop in question. If the LABEL is
2410omitted, the command refers to the innermost enclosing loop. The
2411C<continue> block, if any, is not executed:
2412
4633a7c4
LW
2413 LINE: while (<STDIN>) {
2414 last LINE if /^$/; # exit when done with header
5a964f20 2415 #...
a0d0e21e
LW
2416 }
2417
4968c1e4 2418C<last> cannot be used to exit a block which returns a value such as
2b5ab1e7
TC
2419C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2420a grep() or map() operation.
4968c1e4 2421
6c1372ed
GS
2422Note that a block by itself is semantically identical to a loop
2423that executes once. Thus C<last> can be used to effect an early
2424exit out of such a block.
2425
98293880
JH
2426See also L</continue> for an illustration of how C<last>, C<next>, and
2427C<redo> work.
1d2dff63 2428
a0d0e21e
LW
2429=item lc EXPR
2430
54310121 2431=item lc
bbce6d69 2432
d1be9408 2433Returns a lowercased version of EXPR. This is the internal function
ad0029c4
JH
2434implementing the C<\L> escape in double-quoted strings. Respects
2435current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
983ffd37 2436and L<perlunicode> for more details about locale and Unicode support.
a0d0e21e 2437
7660c0ab 2438If EXPR is omitted, uses C<$_>.
bbce6d69 2439
a0d0e21e
LW
2440=item lcfirst EXPR
2441
54310121 2442=item lcfirst
bbce6d69 2443
ad0029c4
JH
2444Returns the value of EXPR with the first character lowercased. This
2445is the internal function implementing the C<\l> escape in
2446double-quoted strings. Respects current LC_CTYPE locale if C<use
983ffd37
JH
2447locale> in force. See L<perllocale> and L<perlunicode> for more
2448details about locale and Unicode support.
a0d0e21e 2449
7660c0ab 2450If EXPR is omitted, uses C<$_>.
bbce6d69 2451
a0d0e21e
LW
2452=item length EXPR
2453
54310121 2454=item length
bbce6d69 2455
974da8e5 2456Returns the length in I<characters> of the value of EXPR. If EXPR is
b76cc8ba 2457omitted, returns length of C<$_>. Note that this cannot be used on
2b5ab1e7
TC
2458an entire array or hash to find out how many elements these have.
2459For that, use C<scalar @array> and C<scalar keys %hash> respectively.
a0d0e21e 2460
974da8e5
JH
2461Note the I<characters>: if the EXPR is in Unicode, you will get the
2462number of characters, not the number of bytes. To get the length
2463in bytes, use C<do { use bytes; length(EXPR) }>, see L<bytes>.
2464
a0d0e21e
LW
2465=item link OLDFILE,NEWFILE
2466
19799a22 2467Creates a new filename linked to the old filename. Returns true for
b76cc8ba 2468success, false otherwise.
a0d0e21e
LW
2469
2470=item listen SOCKET,QUEUESIZE
2471
19799a22 2472Does the same thing that the listen system call does. Returns true if
b76cc8ba 2473it succeeded, false otherwise. See the example in
cea6626f 2474L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
2475
2476=item local EXPR
2477
19799a22 2478You really probably want to be using C<my> instead, because C<local> isn't
b76cc8ba 2479what most people think of as "local". See
13a2d996 2480L<perlsub/"Private Variables via my()"> for details.
2b5ab1e7 2481
5a964f20
TC
2482A local modifies the listed variables to be local to the enclosing
2483block, file, or eval. If more than one value is listed, the list must
2484be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2485for details, including issues with tied arrays and hashes.
a0d0e21e 2486
a0d0e21e
LW
2487=item localtime EXPR
2488
19799a22 2489Converts a time as returned by the time function to a 9-element list
5f05dabc 2490with the time analyzed for the local time zone. Typically used as
a0d0e21e
LW
2491follows:
2492
54310121 2493 # 0 1 2 3 4 5 6 7 8
a0d0e21e
LW
2494 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2495 localtime(time);
2496
48a26b3a
GS
2497All list elements are numeric, and come straight out of the C `struct
2498tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2499specified time. $mday is the day of the month, and $mon is the month
2500itself, in the range C<0..11> with 0 indicating January and 11
2501indicating December. $year is the number of years since 1900. That
2502is, $year is C<123> in year 2023. $wday is the day of the week, with
25030 indicating Sunday and 3 indicating Wednesday. $yday is the day of
874b1813 2504the year, in the range C<0..364> (or C<0..365> in leap years.) $isdst
48a26b3a
GS
2505is true if the specified time occurs during daylight savings time,
2506false otherwise.
2507
2508Note that the $year element is I<not> simply the last two digits of
2509the year. If you assume it is, then you create non-Y2K-compliant
2510programs--and you wouldn't want to do that, would you?
54310121 2511
abd75f24
GS
2512The proper way to get a complete 4-digit year is simply:
2513
2514 $year += 1900;
2515
2516And to get the last two digits of the year (e.g., '01' in 2001) do:
2517
2518 $year = sprintf("%02d", $year % 100);
2519
48a26b3a 2520If EXPR is omitted, C<localtime()> uses the current time (C<localtime(time)>).
a0d0e21e 2521
48a26b3a 2522In scalar context, C<localtime()> returns the ctime(3) value:
a0d0e21e 2523
5f05dabc 2524 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
a0d0e21e 2525
a3cb178b 2526This scalar value is B<not> locale dependent, see L<perllocale>, but
68f8bed4
JH
2527instead a Perl builtin. Also see the C<Time::Local> module
2528(to convert the second, minutes, hours, ... back to seconds since the
2529stroke of midnight the 1st of January 1970, the value returned by
ca6e1c26 2530time()), and the strftime(3) and mktime(3) functions available via the
68f8bed4
JH
2531POSIX module. To get somewhat similar but locale dependent date
2532strings, set up your locale environment variables appropriately
2533(please see L<perllocale>) and try for example:
a3cb178b 2534
5a964f20 2535 use POSIX qw(strftime);
2b5ab1e7 2536 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
a3cb178b
GS
2537
2538Note that the C<%a> and C<%b>, the short forms of the day of the week
2539and the month of the year, may not necessarily be three characters wide.
a0d0e21e 2540
07698885 2541=item lock THING
19799a22 2542
01e6739c 2543This function places an advisory lock on a shared variable, or referenced
03730085 2544object contained in I<THING> until the lock goes out of scope.
a6d5524e 2545
f3a23afb 2546lock() is a "weak keyword" : this means that if you've defined a function
67408cae 2547by this name (before any calls to it), that function will be called
03730085
AB
2548instead. (However, if you've said C<use threads>, lock() is always a
2549keyword.) See L<threads>.
19799a22 2550
a0d0e21e
LW
2551=item log EXPR
2552
54310121 2553=item log
bbce6d69 2554
2b5ab1e7
TC
2555Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2556returns log of C<$_>. To get the log of another base, use basic algebra:
19799a22 2557The base-N log of a number is equal to the natural log of that number
2b5ab1e7
TC
2558divided by the natural log of N. For example:
2559
2560 sub log10 {
2561 my $n = shift;
2562 return log($n)/log(10);
b76cc8ba 2563 }
2b5ab1e7
TC
2564
2565See also L</exp> for the inverse operation.
a0d0e21e 2566
a0d0e21e
LW
2567=item lstat EXPR
2568
54310121 2569=item lstat
bbce6d69 2570
19799a22 2571Does the same thing as the C<stat> function (including setting the
5a964f20
TC
2572special C<_> filehandle) but stats a symbolic link instead of the file
2573the symbolic link points to. If symbolic links are unimplemented on
c837d5b4
DP
2574your system, a normal C<stat> is done. For much more detailed
2575information, please see the documentation for C<stat>.
a0d0e21e 2576
7660c0ab 2577If EXPR is omitted, stats C<$_>.
bbce6d69 2578
a0d0e21e
LW
2579=item m//
2580
2581The match operator. See L<perlop>.
2582
2583=item map BLOCK LIST
2584
2585=item map EXPR,LIST
2586
19799a22
GS
2587Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2588C<$_> to each element) and returns the list value composed of the
2589results of each such evaluation. In scalar context, returns the
2590total number of elements so generated. Evaluates BLOCK or EXPR in
2591list context, so each element of LIST may produce zero, one, or
2592more elements in the returned value.
dd99ebda 2593
a0d0e21e
LW
2594 @chars = map(chr, @nums);
2595
2596translates a list of numbers to the corresponding characters. And
2597
4633a7c4 2598 %hash = map { getkey($_) => $_ } @array;
a0d0e21e
LW
2599
2600is just a funny way to write
2601
2602 %hash = ();
2603 foreach $_ (@array) {
4633a7c4 2604 $hash{getkey($_)} = $_;
a0d0e21e
LW
2605 }
2606
be3174d2
GS
2607Note that C<$_> is an alias to the list value, so it can be used to
2608modify the elements of the LIST. While this is useful and supported,
2609it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
2610Using a regular C<foreach> loop for this purpose would be clearer in
2611most cases. See also L</grep> for an array composed of those items of
2612the original list for which the BLOCK or EXPR evaluates to true.
fb73857a 2613
205fdb4d
NC
2614C<{> starts both hash references and blocks, so C<map { ...> could be either
2615the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look
2616ahead for the closing C<}> it has to take a guess at which its dealing with
2617based what it finds just after the C<{>. Usually it gets it right, but if it
2618doesn't it won't realize something is wrong until it gets to the C<}> and
2619encounters the missing (or unexpected) comma. The syntax error will be
2620reported close to the C<}> but you'll need to change something near the C<{>
2621such as using a unary C<+> to give perl some help:
2622
2623 %hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong
2624 %hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right
2625 %hash = map { ("\L$_", 1) } @array # this also works
2626 %hash = map { lc($_), 1 } @array # as does this.
2627 %hash = map +( lc($_), 1 ), @array # this is EXPR and works!
cea6626f 2628
205fdb4d
NC
2629 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
2630
2631or to force an anon hash constructor use C<+{>
2632
2633 @hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end
2634
2635and you get list of anonymous hashes each with only 1 entry.
2636
19799a22 2637=item mkdir FILENAME,MASK
a0d0e21e 2638
5a211162
GS
2639=item mkdir FILENAME
2640
0591cd52 2641Creates the directory specified by FILENAME, with permissions
19799a22
GS
2642specified by MASK (as modified by C<umask>). If it succeeds it
2643returns true, otherwise it returns false and sets C<$!> (errno).
5a211162 2644If omitted, MASK defaults to 0777.
0591cd52 2645
19799a22 2646In general, it is better to create directories with permissive MASK,
0591cd52 2647and let the user modify that with their C<umask>, than it is to supply
19799a22 2648a restrictive MASK and give the user no way to be more permissive.
0591cd52
NT
2649The exceptions to this rule are when the file or directory should be
2650kept private (mail files, for instance). The perlfunc(1) entry on
19799a22 2651C<umask> discusses the choice of MASK in more detail.
a0d0e21e 2652
cc1852e8
JH
2653Note that according to the POSIX 1003.1-1996 the FILENAME may have any
2654number of trailing slashes. Some operating and filesystems do not get
2655this right, so Perl automatically removes all trailing slashes to keep
2656everyone happy.
2657
a0d0e21e
LW
2658=item msgctl ID,CMD,ARG
2659
f86cebdf 2660Calls the System V IPC function msgctl(2). You'll probably have to say
0ade1984
JH
2661
2662 use IPC::SysV;
2663
7660c0ab
A
2664first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2665then ARG must be a variable which will hold the returned C<msqid_ds>
951ba7fe
GS
2666structure. Returns like C<ioctl>: the undefined value for error,
2667C<"0 but true"> for zero, or the actual return value otherwise. See also
4755096e 2668L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation.
a0d0e21e
LW
2669
2670=item msgget KEY,FLAGS
2671
f86cebdf 2672Calls the System V IPC function msgget(2). Returns the message queue
4755096e
GS
2673id, or the undefined value if there is an error. See also
2674L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation.
a0d0e21e 2675
a0d0e21e
LW
2676=item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2677
2678Calls the System V IPC function msgrcv to receive a message from
2679message queue ID into variable VAR with a maximum message size of
41d6edb2
JH
2680SIZE. Note that when a message is received, the message type as a
2681native long integer will be the first thing in VAR, followed by the
2682actual message. This packing may be opened with C<unpack("l! a*")>.
2683Taints the variable. Returns true if successful, or false if there is
4755096e
GS
2684an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and
2685C<IPC::SysV::Msg> documentation.
41d6edb2
JH
2686
2687=item msgsnd ID,MSG,FLAGS
2688
2689Calls the System V IPC function msgsnd to send the message MSG to the
2690message queue ID. MSG must begin with the native long integer message
2691type, and be followed by the length of the actual message, and finally
2692the message itself. This kind of packing can be achieved with
2693C<pack("l! a*", $type, $message)>. Returns true if successful,
2694or false if there is an error. See also C<IPC::SysV>
2695and C<IPC::SysV::Msg> documentation.
a0d0e21e
LW
2696
2697=item my EXPR
2698
307ea6df
JH
2699=item my TYPE EXPR
2700
1d2de774 2701=item my EXPR : ATTRS
09bef843 2702
1d2de774 2703=item my TYPE EXPR : ATTRS
307ea6df 2704
19799a22 2705A C<my> declares the listed variables to be local (lexically) to the
1d2de774
JH
2706enclosing block, file, or C<eval>. If more than one value is listed,
2707the list must be placed in parentheses.
307ea6df 2708
1d2de774
JH
2709The exact semantics and interface of TYPE and ATTRS are still
2710evolving. TYPE is currently bound to the use of C<fields> pragma,
307ea6df
JH
2711and attributes are handled using the C<attributes> pragma, or starting
2712from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
2713L<perlsub/"Private Variables via my()"> for details, and L<fields>,
2714L<attributes>, and L<Attribute::Handlers>.
4633a7c4 2715
a0d0e21e
LW
2716=item next LABEL
2717
2718=item next
2719
2720The C<next> command is like the C<continue> statement in C; it starts
2721the next iteration of the loop:
2722
4633a7c4
LW
2723 LINE: while (<STDIN>) {
2724 next LINE if /^#/; # discard comments
5a964f20 2725 #...
a0d0e21e
LW
2726 }
2727
2728Note that if there were a C<continue> block on the above, it would get
2729executed even on discarded lines. If the LABEL is omitted, the command
2730refers to the innermost enclosing loop.
2731
4968c1e4 2732C<next> cannot be used to exit a block which returns a value such as
2b5ab1e7
TC
2733C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2734a grep() or map() operation.
4968c1e4 2735
6c1372ed
GS
2736Note that a block by itself is semantically identical to a loop
2737that executes once. Thus C<next> will exit such a block early.
2738
98293880
JH
2739See also L</continue> for an illustration of how C<last>, C<next>, and
2740C<redo> work.
1d2dff63 2741
4a66ea5a
RGS
2742=item no Module VERSION LIST
2743
2744=item no Module VERSION
2745
a0d0e21e
LW
2746=item no Module LIST
2747
4a66ea5a
RGS
2748=item no Module
2749
593b9c14 2750See the C<use> function, of which C<no> is the opposite.
a0d0e21e
LW
2751
2752=item oct EXPR
2753
54310121 2754=item oct
bbce6d69 2755
4633a7c4 2756Interprets EXPR as an octal string and returns the corresponding
4f19785b
WSI
2757value. (If EXPR happens to start off with C<0x>, interprets it as a
2758hex string. If EXPR starts off with C<0b>, it is interpreted as a
53305cf1
NC
2759binary string. Leading whitespace is ignored in all three cases.)
2760The following will handle decimal, binary, octal, and hex in the standard
2761Perl or C notation:
a0d0e21e
LW
2762
2763 $val = oct($val) if $val =~ /^0/;
2764
19799a22
GS
2765If EXPR is omitted, uses C<$_>. To go the other way (produce a number
2766in octal), use sprintf() or printf():
2767
2768 $perms = (stat("filename"))[2] & 07777;
2769 $oct_perms = sprintf "%lo", $perms;
2770
2771The oct() function is commonly used when a string such as C<644> needs
2772to be converted into a file mode, for example. (Although perl will
2773automatically convert strings into numbers as needed, this automatic
2774conversion assumes base 10.)
a0d0e21e
LW
2775
2776=item open FILEHANDLE,EXPR
2777
68bd7414
NIS
2778=item open FILEHANDLE,MODE,EXPR
2779
2780=item open FILEHANDLE,MODE,EXPR,LIST
2781
ba964c95
T
2782=item open FILEHANDLE,MODE,REFERENCE
2783
a0d0e21e
LW
2784=item open FILEHANDLE
2785
2786Opens the file whose filename is given by EXPR, and associates it with
ed53a2bb
JH
2787FILEHANDLE.
2788
2789(The following is a comprehensive reference to open(): for a gentler
2790introduction you may consider L<perlopentut>.)
2791
a28cd5c9
NT
2792If FILEHANDLE is an undefined scalar variable (or array or hash element)
2793the variable is assigned a reference to a new anonymous filehandle,
2794otherwise if FILEHANDLE is an expression, its value is used as the name of
2795the real filehandle wanted. (This is considered a symbolic reference, so
2796C<use strict 'refs'> should I<not> be in effect.)
ed53a2bb
JH
2797
2798If EXPR is omitted, the scalar variable of the same name as the
2799FILEHANDLE contains the filename. (Note that lexical variables--those
2800declared with C<my>--will not work for this purpose; so if you're
67408cae 2801using C<my>, specify EXPR in your call to open.)
ed53a2bb
JH
2802
2803If three or more arguments are specified then the mode of opening and
2804the file name are separate. If MODE is C<< '<' >> or nothing, the file
2805is opened for input. If MODE is C<< '>' >>, the file is truncated and
2806opened for output, being created if necessary. If MODE is C<<< '>>' >>>,
b76cc8ba 2807the file is opened for appending, again being created if necessary.
5a964f20 2808
ed53a2bb
JH
2809You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to
2810indicate that you want both read and write access to the file; thus
2811C<< '+<' >> is almost always preferred for read/write updates--the C<<
2812'+>' >> mode would clobber the file first. You can't usually use
2813either read-write mode for updating textfiles, since they have
2814variable length records. See the B<-i> switch in L<perlrun> for a
2815better approach. The file is created with permissions of C<0666>
2816modified by the process' C<umask> value.
2817
2818These various prefixes correspond to the fopen(3) modes of C<'r'>,
2819C<'r+'>, C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
5f05dabc 2820
6170680b
IZ
2821In the 2-arguments (and 1-argument) form of the call the mode and
2822filename should be concatenated (in this order), possibly separated by
68bd7414
NIS
2823spaces. It is possible to omit the mode in these forms if the mode is
2824C<< '<' >>.
6170680b 2825
7660c0ab 2826If the filename begins with C<'|'>, the filename is interpreted as a
5a964f20 2827command to which output is to be piped, and if the filename ends with a
f244e06d
GS
2828C<'|'>, the filename is interpreted as a command which pipes output to
2829us. See L<perlipc/"Using open() for IPC">
19799a22 2830for more examples of this. (You are not allowed to C<open> to a command
5a964f20 2831that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
4a4eefd0
GS
2832and L<perlipc/"Bidirectional Communication with Another Process">
2833for alternatives.)
cb1a09d0 2834
ed53a2bb
JH
2835For three or more arguments if MODE is C<'|-'>, the filename is
2836interpreted as a command to which output is to be piped, and if MODE
2837is C<'-|'>, the filename is interpreted as a command which pipes
2838output to us. In the 2-arguments (and 1-argument) form one should
2839replace dash (C<'-'>) with the command.
2840See L<perlipc/"Using open() for IPC"> for more examples of this.
2841(You are not allowed to C<open> to a command that pipes both in I<and>
2842out, but see L<IPC::Open2>, L<IPC::Open3>, and
2843L<perlipc/"Bidirectional Communication"> for alternatives.)
2844
2845In the three-or-more argument form of pipe opens, if LIST is specified
2846(extra arguments after the command name) then LIST becomes arguments
2847to the command invoked if the platform supports it. The meaning of
2848C<open> with more than three arguments for non-pipe modes is not yet
2849specified. Experimental "layers" may give extra LIST arguments
2850meaning.
6170680b
IZ
2851
2852In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
b76cc8ba 2853and opening C<< '>-' >> opens STDOUT.
6170680b 2854
fae2c0fb
RGS
2855You may use the three-argument form of open to specify IO "layers"
2856(sometimes also referred to as "disciplines") to be applied to the handle
2857that affect how the input and output are processed (see L<open> and
2858L<PerlIO> for more details). For example
7207e29d 2859
9124316e
JH
2860 open(FH, "<:utf8", "file")
2861
2862will open the UTF-8 encoded file containing Unicode characters,
fae2c0fb
RGS
2863see L<perluniintro>. (Note that if layers are specified in the
2864three-arg form then default layers set by the C<open> pragma are
01e6739c 2865ignored.)
ed53a2bb
JH
2866
2867Open returns nonzero upon success, the undefined value otherwise. If
2868the C<open> involved a pipe, the return value happens to be the pid of
2869the subprocess.
cb1a09d0 2870
ed53a2bb
JH
2871If you're running Perl on a system that distinguishes between text
2872files and binary files, then you should check out L</binmode> for tips
2873for dealing with this. The key distinction between systems that need
2874C<binmode> and those that don't is their text file formats. Systems
8939ba94 2875like Unix, Mac OS, and Plan 9, which delimit lines with a single
ed53a2bb
JH
2876character, and which encode that character in C as C<"\n">, do not
2877need C<binmode>. The rest need it.
cb1a09d0 2878
fb73857a 2879When opening a file, it's usually a bad idea to continue normal execution
19799a22
GS
2880if the request failed, so C<open> is frequently used in connection with
2881C<die>. Even if C<die> won't do what you want (say, in a CGI script,
fb73857a 2882where you want to make a nicely formatted error message (but there are
5a964f20 2883modules that can help with that problem)) you should always check
19799a22 2884the return value from opening a file. The infrequent exception is when
fb73857a 2885working with an unopened filehandle is actually what you want to do.
2886
ed53a2bb
JH
2887As a special case the 3 arg form with a read/write mode and the third
2888argument being C<undef>:
b76cc8ba
NIS
2889
2890 open(TMP, "+>", undef) or die ...
2891
f253e835
JH
2892opens a filehandle to an anonymous temporary file. Also using "+<"
2893works for symmetry, but you really should consider writing something
2894to the temporary file first. You will need to seek() to do the
2895reading.
b76cc8ba 2896
ba964c95
T
2897File handles can be opened to "in memory" files held in Perl scalars via:
2898
b996200f
SB
2899 open($fh, '>', \$variable) || ..
2900
2901Though if you try to re-open C<STDOUT> or C<STDERR> as an "in memory"
2902file, you have to close it first:
2903
2904 close STDOUT;
2905 open STDOUT, '>', \$variable or die "Can't open STDOUT: $!";
ba964c95 2906
cb1a09d0 2907Examples:
a0d0e21e
LW
2908
2909 $ARTICLE = 100;
2910 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2911 while (<ARTICLE>) {...
2912
6170680b 2913 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
fb73857a 2914 # if the open fails, output is discarded
a0d0e21e 2915
6170680b 2916 open(DBASE, '+<', 'dbase.mine') # open for update
fb73857a 2917 or die "Can't open 'dbase.mine' for update: $!";
cb1a09d0 2918
6170680b
IZ
2919 open(DBASE, '+<dbase.mine') # ditto
2920 or die "Can't open 'dbase.mine' for update: $!";
2921
2922 open(ARTICLE, '-|', "caesar <$article") # decrypt article
fb73857a 2923 or die "Can't start caesar: $!";
a0d0e21e 2924
6170680b
IZ
2925 open(ARTICLE, "caesar <$article |") # ditto
2926 or die "Can't start caesar: $!";
2927
2928 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
fb73857a 2929 or die "Can't start sort: $!";
a0d0e21e 2930
ba964c95
T
2931 # in memory files
2932 open(MEMORY,'>', \$var)
2933 or die "Can't open memory file: $!";
2934 print MEMORY "foo!\n"; # output will end up in $var
2935
a0d0e21e
LW
2936 # process argument list of files along with any includes
2937
2938 foreach $file (@ARGV) {
2939 process($file, 'fh00');
2940 }
2941
2942 sub process {
5a964f20 2943 my($filename, $input) = @_;
a0d0e21e
LW
2944 $input++; # this is a string increment
2945 unless (open($input, $filename)) {
2946 print STDERR "Can't open $filename: $!\n";
2947 return;
2948 }
2949
5a964f20 2950 local $_;
a0d0e21e
LW
2951 while (<$input>) { # note use of indirection
2952 if (/^#include "(.*)"/) {
2953 process($1, $input);
2954 next;
2955 }
5a964f20 2956 #... # whatever
a0d0e21e
LW
2957 }
2958 }
2959
2960You may also, in the Bourne shell tradition, specify an EXPR beginning
00cafafa
JH
2961with C<< '>&' >>, in which case the rest of the string is interpreted
2962as the name of a filehandle (or file descriptor, if numeric) to be
2963duped (as L<dup(2)>) and opened. You may use C<&> after C<< > >>,
2964C<<< >> >>>, C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>.
2965The mode you specify should match the mode of the original filehandle.
2966(Duping a filehandle does not take into account any existing contents
2967of IO buffers.) If you use the 3 arg form then you can pass either a
2968number, the name of a filehandle or the normal "reference to a glob".
6170680b 2969
eae1b76b
SB
2970Here is a script that saves, redirects, and restores C<STDOUT> and
2971C<STDERR> using various methods:
a0d0e21e
LW
2972
2973 #!/usr/bin/perl
eae1b76b
SB
2974 open my $oldout, ">&STDOUT" or die "Can't dup STDOUT: $!";
2975 open OLDERR, ">&", \*STDERR or die "Can't dup STDERR: $!";
818c4caa 2976
eae1b76b
SB
2977 open STDOUT, '>', "foo.out" or die "Can't redirect STDOUT: $!";
2978 open STDERR, ">&STDOUT" or die "Can't dup STDOUT: $!";
a0d0e21e 2979
eae1b76b
SB
2980 select STDERR; $| = 1; # make unbuffered
2981 select STDOUT; $| = 1; # make unbuffered
a0d0e21e
LW
2982
2983 print STDOUT "stdout 1\n"; # this works for
2984 print STDERR "stderr 1\n"; # subprocesses too
2985
eae1b76b
SB
2986 close STDOUT;
2987 close STDERR;
a0d0e21e 2988
eae1b76b
SB
2989 open STDOUT, ">&", $oldout or die "Can't dup \$oldout: $!";
2990 open STDERR, ">&OLDERR" or die "Can't dup OLDERR: $!";
a0d0e21e
LW
2991
2992 print STDOUT "stdout 2\n";
2993 print STDERR "stderr 2\n";
2994
ef8b303f
JH
2995If you specify C<< '<&=X' >>, where C<X> is a file descriptor number
2996or a filehandle, then Perl will do an equivalent of C's C<fdopen> of
2997that file descriptor (and not call L<dup(2)>); this is more
2998parsimonious of file descriptors. For example:
a0d0e21e 2999
00cafafa 3000 # open for input, reusing the fileno of $fd
a0d0e21e 3001 open(FILEHANDLE, "<&=$fd")
df632fdf 3002
b76cc8ba 3003or
df632fdf 3004
b76cc8ba 3005 open(FILEHANDLE, "<&=", $fd)
a0d0e21e 3006
00cafafa
JH
3007or
3008
3009 # open for append, using the fileno of OLDFH
3010 open(FH, ">>&=", OLDFH)
3011
3012or
3013
3014 open(FH, ">>&=OLDFH")
3015
ef8b303f
JH
3016Being parsimonious on filehandles is also useful (besides being
3017parsimonious) for example when something is dependent on file
3018descriptors, like for example locking using flock(). If you do just
3019C<< open(A, '>>&B') >>, the filehandle A will not have the same file
3020descriptor as B, and therefore flock(A) will not flock(B), and vice
3021versa. But with C<< open(A, '>>&=B') >> the filehandles will share
3022the same file descriptor.
3023
3024Note that if you are using Perls older than 5.8.0, Perl will be using
3025the standard C libraries' fdopen() to implement the "=" functionality.
3026On many UNIX systems fdopen() fails when file descriptors exceed a
3027certain value, typically 255. For Perls 5.8.0 and later, PerlIO is
3028most often the default.
4af147f6 3029
df632fdf
JH
3030You can see whether Perl has been compiled with PerlIO or not by
3031running C<perl -V> and looking for C<useperlio=> line. If C<useperlio>
3032is C<define>, you have PerlIO, otherwise you don't.
3033
6170680b
IZ
3034If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
3035with 2-arguments (or 1-argument) form of open(), then
a0d0e21e 3036there is an implicit fork done, and the return value of open is the pid
7660c0ab 3037of the child within the parent process, and C<0> within the child
184e9718 3038process. (Use C<defined($pid)> to determine whether the open was successful.)
a0d0e21e
LW
3039The filehandle behaves normally for the parent, but i/o to that
3040filehandle is piped from/to the STDOUT/STDIN of the child process.
3041In the child process the filehandle isn't opened--i/o happens from/to
3042the new STDOUT or STDIN. Typically this is used like the normal
3043piped open when you want to exercise more control over just how the
3044pipe command gets executed, such as when you are running setuid, and
54310121 3045don't want to have to scan shell commands for metacharacters.
6170680b 3046The following triples are more or less equivalent:
a0d0e21e
LW
3047
3048 open(FOO, "|tr '[a-z]' '[A-Z]'");
6170680b
IZ
3049 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
3050 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
b76cc8ba 3051 open(FOO, '|-', "tr", '[a-z]', '[A-Z]');
a0d0e21e
LW
3052
3053 open(FOO, "cat -n '$file'|");
6170680b
IZ
3054 open(FOO, '-|', "cat -n '$file'");
3055 open(FOO, '-|') || exec 'cat', '-n', $file;
b76cc8ba
NIS
3056 open(FOO, '-|', "cat", '-n', $file);
3057
3058The last example in each block shows the pipe as "list form", which is
64da03b2
JH
3059not yet supported on all platforms. A good rule of thumb is that if
3060your platform has true C<fork()> (in other words, if your platform is
3061UNIX) you can use the list form.
a0d0e21e 3062
4633a7c4
LW
3063See L<perlipc/"Safe Pipe Opens"> for more examples of this.
3064
0f897271
GS
3065Beginning with v5.6.0, Perl will attempt to flush all files opened for
3066output before any operation that may do a fork, but this may not be
3067supported on some platforms (see L<perlport>). To be safe, you may need
3068to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
3069of C<IO::Handle> on any open handles.
3070
ed53a2bb
JH
3071On systems that support a close-on-exec flag on files, the flag will
3072be set for the newly opened file descriptor as determined by the value
3073of $^F. See L<perlvar/$^F>.
a0d0e21e 3074
0dccf244
CS
3075Closing any piped filehandle causes the parent process to wait for the
3076child to finish, and returns the status value in C<$?>.
3077
ed53a2bb
JH
3078The filename passed to 2-argument (or 1-argument) form of open() will
3079have leading and trailing whitespace deleted, and the normal
3080redirection characters honored. This property, known as "magic open",
5a964f20 3081can often be used to good effect. A user could specify a filename of
7660c0ab 3082F<"rsh cat file |">, or you could change certain filenames as needed:
5a964f20
TC
3083
3084 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
3085 open(FH, $filename) or die "Can't open $filename: $!";
3086
6170680b
IZ
3087Use 3-argument form to open a file with arbitrary weird characters in it,
3088
3089 open(FOO, '<', $file);
3090
3091otherwise it's necessary to protect any leading and trailing whitespace:
5a964f20
TC
3092
3093 $file =~ s#^(\s)#./$1#;
3094 open(FOO, "< $file\0");
3095
a31a806a 3096(this may not work on some bizarre filesystems). One should
106325ad 3097conscientiously choose between the I<magic> and 3-arguments form
6170680b
IZ
3098of open():
3099
3100 open IN, $ARGV[0];
3101
3102will allow the user to specify an argument of the form C<"rsh cat file |">,
3103but will not work on a filename which happens to have a trailing space, while
3104
3105 open IN, '<', $ARGV[0];
3106
3107will have exactly the opposite restrictions.
3108
19799a22 3109If you want a "real" C C<open> (see L<open(2)> on your system), then you
6170680b
IZ
3110should use the C<sysopen> function, which involves no such magic (but
3111may use subtly different filemodes than Perl open(), which is mapped
3112to C fopen()). This is
5a964f20
TC
3113another way to protect your filenames from interpretation. For example:
3114
3115 use IO::Handle;
3116 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
3117 or die "sysopen $path: $!";
3118 $oldfh = select(HANDLE); $| = 1; select($oldfh);
38762f02 3119 print HANDLE "stuff $$\n";
5a964f20
TC
3120 seek(HANDLE, 0, 0);
3121 print "File contains: ", <HANDLE>;
3122
7660c0ab
A
3123Using the constructor from the C<IO::Handle> package (or one of its
3124subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
5a964f20
TC
3125filehandles that have the scope of whatever variables hold references to
3126them, and automatically close whenever and however you leave that scope:
c07a80fd 3127
5f05dabc 3128 use IO::File;
5a964f20 3129 #...
c07a80fd 3130 sub read_myfile_munged {
3131 my $ALL = shift;
5f05dabc 3132 my $handle = new IO::File;
c07a80fd 3133 open($handle, "myfile") or die "myfile: $!";
3134 $first = <$handle>
3135 or return (); # Automatically closed here.
3136 mung $first or die "mung failed"; # Or here.
3137 return $first, <$handle> if $ALL; # Or here.
3138 $first; # Or here.
3139 }
3140
b687b08b 3141See L</seek> for some details about mixing reading and writing.
a0d0e21e
LW
3142
3143=item opendir DIRHANDLE,EXPR
3144
19799a22
GS
3145Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
3146C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
a28cd5c9
NT
3147DIRHANDLE may be an expression whose value can be used as an indirect
3148dirhandle, usually the real dirhandle name. If DIRHANDLE is an undefined
3149scalar variable (or array or hash element), the variable is assigned a
3150reference to a new anonymous dirhandle.
a0d0e21e
LW
3151DIRHANDLEs have their own namespace separate from FILEHANDLEs.
3152
3153=item ord EXPR
3154
54310121 3155=item ord
bbce6d69 3156
121910a4
JH
3157Returns the numeric (the native 8-bit encoding, like ASCII or EBCDIC,
3158or Unicode) value of the first character of EXPR. If EXPR is omitted,
3159uses C<$_>.
3160
3161For the reverse, see L</chr>.
3162See L<perlunicode> and L<encoding> for more about Unicode.
a0d0e21e 3163
77ca0c92
LW
3164=item our EXPR
3165
307ea6df
JH
3166=item our EXPR TYPE
3167
1d2de774 3168=item our EXPR : ATTRS
9969eac4 3169
1d2de774 3170=item our TYPE EXPR : ATTRS
307ea6df 3171
77ca0c92
LW
3172An C<our> declares the listed variables to be valid globals within
3173the enclosing block, file, or C<eval>. That is, it has the same
3174scoping rules as a "my" declaration, but does not create a local
3175variable. If more than one value is listed, the list must be placed
3176in parentheses. The C<our> declaration has no semantic effect unless
3177"use strict vars" is in effect, in which case it lets you use the
3178declared global variable without qualifying it with a package name.
3179(But only within the lexical scope of the C<our> declaration. In this
3180it differs from "use vars", which is package scoped.)
3181
f472eb5c
GS
3182An C<our> declaration declares a global variable that will be visible
3183across its entire lexical scope, even across package boundaries. The
3184package in which the variable is entered is determined at the point
3185of the declaration, not at the point of use. This means the following
3186behavior holds:
3187
3188 package Foo;
3189 our $bar; # declares $Foo::bar for rest of lexical scope
3190 $bar = 20;
3191
3192 package Bar;
3193 print $bar; # prints 20
3194
3195Multiple C<our> declarations in the same lexical scope are allowed
3196if they are in different packages. If they happened to be in the same
3197package, Perl will emit warnings if you have asked for them.
3198
3199 use warnings;
3200 package Foo;
3201 our $bar; # declares $Foo::bar for rest of lexical scope
3202 $bar = 20;
3203
3204 package Bar;
3205 our $bar = 30; # declares $Bar::bar for rest of lexical scope
3206 print $bar; # prints 30
3207
3208 our $bar; # emits warning
3209
9969eac4 3210An C<our> declaration may also have a list of attributes associated
307ea6df
JH
3211with it.
3212
1d2de774
JH
3213The exact semantics and interface of TYPE and ATTRS are still
3214evolving. TYPE is currently bound to the use of C<fields> pragma,
307ea6df
JH
3215and attributes are handled using the C<attributes> pragma, or starting
3216from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
3217L<perlsub/"Private Variables via my()"> for details, and L<fields>,
3218L<attributes>, and L<Attribute::Handlers>.
3219
3220The only currently recognized C<our()> attribute is C<unique> which
3221indicates that a single copy of the global is to be used by all
3222interpreters should the program happen to be running in a
3223multi-interpreter environment. (The default behaviour would be for
3224each interpreter to have its own copy of the global.) Examples:
9969eac4 3225
51d2bbcc
JH
3226 our @EXPORT : unique = qw(foo);
3227 our %EXPORT_TAGS : unique = (bar => [qw(aa bb cc)]);
3228 our $VERSION : unique = "1.00";
9969eac4 3229
96fa8c42 3230Note that this attribute also has the effect of making the global
72e53bfb
JH
3231readonly when the first new interpreter is cloned (for example,
3232when the first new thread is created).
96fa8c42 3233
9969eac4
BS
3234Multi-interpreter environments can come to being either through the
3235fork() emulation on Windows platforms, or by embedding perl in a
51d2bbcc 3236multi-threaded application. The C<unique> attribute does nothing in
9969eac4
BS
3237all other environments.
3238
a0d0e21e
LW
3239=item pack TEMPLATE,LIST
3240
2b6c5635
GS
3241Takes a LIST of values and converts it into a string using the rules
3242given by the TEMPLATE. The resulting string is the concatenation of
3243the converted values. Typically, each converted value looks
3244like its machine-level representation. For example, on 32-bit machines
3245a converted integer may be represented by a sequence of 4 bytes.
3246
18529408
IZ
3247The TEMPLATE is a sequence of characters that give the order and type
3248of values, as follows:
a0d0e21e 3249
5a929a98 3250 a A string with arbitrary binary data, will be null padded.
121910a4
JH
3251 A A text (ASCII) string, will be space padded.
3252 Z A null terminated (ASCIZ) string, will be null padded.
5a929a98 3253
2b6c5635
GS
3254 b A bit string (ascending bit order inside each byte, like vec()).
3255 B A bit string (descending bit order inside each byte).
a0d0e21e
LW
3256 h A hex string (low nybble first).
3257 H A hex string (high nybble first).
3258
3259 c A signed char value.
a0ed51b3 3260 C An unsigned char value. Only does bytes. See U for Unicode.
96e4d5b1 3261
a0d0e21e
LW
3262 s A signed short value.
3263 S An unsigned short value.
96e4d5b1 3264 (This 'short' is _exactly_ 16 bits, which may differ from
851646ae
JH
3265 what a local C compiler calls 'short'. If you want
3266 native-length shorts, use the '!' suffix.)
96e4d5b1 3267
a0d0e21e
LW
3268 i A signed integer value.
3269 I An unsigned integer value.
19799a22 3270 (This 'integer' is _at_least_ 32 bits wide. Its exact