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1=head1 NAME
2
3perlfunc - Perl builtin functions
4
5=head1 DESCRIPTION
6
7The functions in this section can serve as terms in an expression.
8They fall into two major categories: list operators and named unary
9operators. These differ in their precedence relationship with a
10following comma. (See the precedence table in L<perlop>.) List
11operators take more than one argument, while unary operators can never
12take more than one argument. Thus, a comma terminates the argument of
13a unary operator, but merely separates the arguments of a list
14operator. A unary operator generally provides a scalar context to its
2b5ab1e7 15argument, while a list operator may provide either scalar or list
a0d0e21e 16contexts for its arguments. If it does both, the scalar arguments will
5f05dabc 17be first, and the list argument will follow. (Note that there can ever
0f31cffe 18be only one such list argument.) For instance, splice() has three scalar
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19arguments followed by a list, whereas gethostbyname() has four scalar
20arguments.
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21
22In the syntax descriptions that follow, list operators that expect a
23list (and provide list context for the elements of the list) are shown
24with LIST as an argument. Such a list may consist of any combination
25of scalar arguments or list values; the list values will be included
26in the list as if each individual element were interpolated at that
27point in the list, forming a longer single-dimensional list value.
28Elements of the LIST should be separated by commas.
29
30Any function in the list below may be used either with or without
31parentheses around its arguments. (The syntax descriptions omit the
5f05dabc 32parentheses.) If you use the parentheses, the simple (but occasionally
19799a22 33surprising) rule is this: It I<looks> like a function, therefore it I<is> a
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34function, and precedence doesn't matter. Otherwise it's a list
35operator or unary operator, and precedence does matter. And whitespace
36between the function and left parenthesis doesn't count--so you need to
37be careful sometimes:
38
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39 print 1+2+4; # Prints 7.
40 print(1+2) + 4; # Prints 3.
41 print (1+2)+4; # Also prints 3!
42 print +(1+2)+4; # Prints 7.
43 print ((1+2)+4); # Prints 7.
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44
45If you run Perl with the B<-w> switch it can warn you about this. For
46example, the third line above produces:
47
48 print (...) interpreted as function at - line 1.
49 Useless use of integer addition in void context at - line 1.
50
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51A few functions take no arguments at all, and therefore work as neither
52unary nor list operators. These include such functions as C<time>
53and C<endpwent>. For example, C<time+86_400> always means
54C<time() + 86_400>.
55
a0d0e21e 56For functions that can be used in either a scalar or list context,
54310121 57nonabortive failure is generally indicated in a scalar context by
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58returning the undefined value, and in a list context by returning the
59null list.
60
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61Remember the following important rule: There is B<no rule> that relates
62the behavior of an expression in list context to its behavior in scalar
63context, or vice versa. It might do two totally different things.
a0d0e21e 64Each operator and function decides which sort of value it would be most
2b5ab1e7 65appropriate to return in scalar context. Some operators return the
5a964f20 66length of the list that would have been returned in list context. Some
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67operators return the first value in the list. Some operators return the
68last value in the list. Some operators return a count of successful
69operations. In general, they do what you want, unless you want
70consistency.
71
d1be9408 72A named array in scalar context is quite different from what would at
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73first glance appear to be a list in scalar context. You can't get a list
74like C<(1,2,3)> into being in scalar context, because the compiler knows
75the context at compile time. It would generate the scalar comma operator
76there, not the list construction version of the comma. That means it
77was never a list to start with.
78
79In general, functions in Perl that serve as wrappers for system calls
f86cebdf 80of the same name (like chown(2), fork(2), closedir(2), etc.) all return
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81true when they succeed and C<undef> otherwise, as is usually mentioned
82in the descriptions below. This is different from the C interfaces,
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83which return C<-1> on failure. Exceptions to this rule are C<wait>,
84C<waitpid>, and C<syscall>. System calls also set the special C<$!>
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85variable on failure. Other functions do not, except accidentally.
86
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87=head2 Perl Functions by Category
88
89Here are Perl's functions (including things that look like
5a964f20 90functions, like some keywords and named operators)
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91arranged by category. Some functions appear in more
92than one place.
93
13a2d996 94=over 4
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95
96=item Functions for SCALARs or strings
97
22fae026 98C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>,
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99C<length>, C<oct>, C<ord>, C<pack>, C<q/STRING/>, C<qq/STRING/>, C<reverse>,
100C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///>
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101
102=item Regular expressions and pattern matching
103
ab4f32c2 104C<m//>, C<pos>, C<quotemeta>, C<s///>, C<split>, C<study>, C<qr//>
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105
106=item Numeric functions
107
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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
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200=item Functions new in perl5
201
22fae026 202C<abs>, C<bless>, C<chomp>, C<chr>, C<exists>, C<formline>, C<glob>,
b76cc8ba 203C<import>, C<lc>, C<lcfirst>, C<map>, C<my>, C<no>, C<our>, C<prototype>,
4375e838 204C<qx>, C<qw>, C<readline>, C<readpipe>, C<ref>, C<sub*>, C<sysopen>, C<tie>,
22fae026 205C<tied>, C<uc>, C<ucfirst>, C<untie>, C<use>
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206
207* - C<sub> was a keyword in perl4, but in perl5 it is an
5a964f20 208operator, which can be used in expressions.
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209
210=item Functions obsoleted in perl5
211
22fae026 212C<dbmclose>, C<dbmopen>
37798a01 213
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214=back
215
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216=head2 Portability
217
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218Perl was born in Unix and can therefore access all common Unix
219system calls. In non-Unix environments, the functionality of some
220Unix system calls may not be available, or details of the available
221functionality may differ slightly. The Perl functions affected
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222by this are:
223
224C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>,
225C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>,
226C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>,
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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
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369As of Perl 5.9.1, as a form of purely syntactic sugar, you can stack file
370test operators, in a way that C<-f -w -x $file> is equivalent to
371C<-x $file && -w _ && -f _>. (This is only syntax fancy : if you use
372the return value of C<-f $file> as an argument to another filetest
373operator, no special magic will happen.)
374
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375=item abs VALUE
376
54310121 377=item abs
bbce6d69 378
a0d0e21e 379Returns the absolute value of its argument.
7660c0ab 380If VALUE is omitted, uses C<$_>.
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381
382=item accept NEWSOCKET,GENERICSOCKET
383
f86cebdf 384Accepts an incoming socket connect, just as the accept(2) system call
19799a22 385does. Returns the packed address if it succeeded, false otherwise.
2b5ab1e7 386See the example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 387
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388On systems that support a close-on-exec flag on files, the flag will
389be set for the newly opened file descriptor, as determined by the
390value of $^F. See L<perlvar/$^F>.
391
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392=item alarm SECONDS
393
54310121 394=item alarm
bbce6d69 395
a0d0e21e 396Arranges to have a SIGALRM delivered to this process after the
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397specified number of wallclock seconds have elapsed. If SECONDS is not
398specified, the value stored in C<$_> is used. (On some machines,
399unfortunately, the elapsed time may be up to one second less or more
400than you specified because of how seconds are counted, and process
401scheduling may delay the delivery of the signal even further.)
402
403Only one timer may be counting at once. Each call disables the
404previous timer, and an argument of C<0> may be supplied to cancel the
405previous timer without starting a new one. The returned value is the
406amount of time remaining on the previous timer.
a0d0e21e 407
4633a7c4 408For delays of finer granularity than one second, you may use Perl's
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409four-argument version of select() leaving the first three arguments
410undefined, or you might be able to use the C<syscall> interface to
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411access setitimer(2) if your system supports it. The Time::HiRes
412module (from CPAN, and starting from Perl 5.8 part of the standard
413distribution) may also prove useful.
2b5ab1e7 414
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415It is usually a mistake to intermix C<alarm> and C<sleep> calls.
416(C<sleep> may be internally implemented in your system with C<alarm>)
a0d0e21e 417
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418If you want to use C<alarm> to time out a system call you need to use an
419C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
f86cebdf 420fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
19799a22 421restart system calls on some systems. Using C<eval>/C<die> always works,
5a964f20 422modulo the caveats given in L<perlipc/"Signals">.
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423
424 eval {
f86cebdf 425 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
36477c24 426 alarm $timeout;
ff68c719 427 $nread = sysread SOCKET, $buffer, $size;
36477c24 428 alarm 0;
ff68c719 429 };
ff68c719 430 if ($@) {
f86cebdf 431 die unless $@ eq "alarm\n"; # propagate unexpected errors
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432 # timed out
433 }
434 else {
435 # didn't
436 }
437
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438For more information see L<perlipc>.
439
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440=item atan2 Y,X
441
442Returns the arctangent of Y/X in the range -PI to PI.
443
ca6e1c26 444For the tangent operation, you may use the C<Math::Trig::tan>
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445function, or use the familiar relation:
446
447 sub tan { sin($_[0]) / cos($_[0]) }
448
a0d0e21e
LW
449=item bind SOCKET,NAME
450
451Binds a network address to a socket, just as the bind system call
19799a22 452does. Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
453packed address of the appropriate type for the socket. See the examples in
454L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 455
fae2c0fb 456=item binmode FILEHANDLE, LAYER
1c1fc3ea 457
a0d0e21e
LW
458=item binmode FILEHANDLE
459
1cbfc93d
NIS
460Arranges for FILEHANDLE to be read or written in "binary" or "text"
461mode on systems where the run-time libraries distinguish between
462binary and text files. If FILEHANDLE is an expression, the value is
463taken as the name of the filehandle. Returns true on success,
b5fe5ca2 464otherwise it returns C<undef> and sets C<$!> (errno).
1cbfc93d 465
d807c6f4
JH
466On some systems (in general, DOS and Windows-based systems) binmode()
467is necessary when you're not working with a text file. For the sake
468of portability it is a good idea to always use it when appropriate,
469and to never use it when it isn't appropriate. Also, people can
470set their I/O to be by default UTF-8 encoded Unicode, not bytes.
471
472In other words: regardless of platform, use binmode() on binary data,
473like for example images.
474
475If LAYER is present it is a single string, but may contain multiple
476directives. The directives alter the behaviour of the file handle.
477When LAYER is present using binmode on text file makes sense.
478
fae2c0fb 479If LAYER is omitted or specified as C<:raw> the filehandle is made
0226bbdb
NIS
480suitable for passing binary data. This includes turning off possible CRLF
481translation and marking it as bytes (as opposed to Unicode characters).
749683d2
YST
482Note that, despite what may be implied in I<"Programming Perl"> (the
483Camel) or elsewhere, C<:raw> is I<not> the simply inverse of C<:crlf>
fae2c0fb 484-- other layers which would affect binary nature of the stream are
0226bbdb
NIS
485I<also> disabled. See L<PerlIO>, L<perlrun> and the discussion about the
486PERLIO environment variable.
01e6739c 487
d807c6f4
JH
488The C<:bytes>, C<:crlf>, and C<:utf8>, and any other directives of the
489form C<:...>, are called I/O I<layers>. The C<open> pragma can be used to
490establish default I/O layers. See L<open>.
491
fae2c0fb
RGS
492I<The LAYER parameter of the binmode() function is described as "DISCIPLINE"
493in "Programming Perl, 3rd Edition". However, since the publishing of this
494book, by many known as "Camel III", the consensus of the naming of this
495functionality has moved from "discipline" to "layer". All documentation
496of this version of Perl therefore refers to "layers" rather than to
497"disciplines". Now back to the regularly scheduled documentation...>
498
01e6739c 499To mark FILEHANDLE as UTF-8, use C<:utf8>.
1cbfc93d 500
ed53a2bb 501In general, binmode() should be called after open() but before any I/O
01e6739c
NIS
502is done on the filehandle. Calling binmode() will normally flush any
503pending buffered output data (and perhaps pending input data) on the
fae2c0fb 504handle. An exception to this is the C<:encoding> layer that
01e6739c 505changes the default character encoding of the handle, see L<open>.
fae2c0fb 506The C<:encoding> layer sometimes needs to be called in
3874323d
JH
507mid-stream, and it doesn't flush the stream. The C<:encoding>
508also implicitly pushes on top of itself the C<:utf8> layer because
509internally Perl will operate on UTF-8 encoded Unicode characters.
16fe6d59 510
19799a22 511The operating system, device drivers, C libraries, and Perl run-time
30168b04
GS
512system all work together to let the programmer treat a single
513character (C<\n>) as the line terminator, irrespective of the external
514representation. On many operating systems, the native text file
515representation matches the internal representation, but on some
516platforms the external representation of C<\n> is made up of more than
517one character.
518
68bd7414
NIS
519Mac OS, all variants of Unix, and Stream_LF files on VMS use a single
520character to end each line in the external representation of text (even
5e12dbfa 521though that single character is CARRIAGE RETURN on Mac OS and LINE FEED
01e6739c
NIS
522on Unix and most VMS files). In other systems like OS/2, DOS and the
523various flavors of MS-Windows your program sees a C<\n> as a simple C<\cJ>,
524but what's stored in text files are the two characters C<\cM\cJ>. That
525means that, if you don't use binmode() on these systems, C<\cM\cJ>
526sequences on disk will be converted to C<\n> on input, and any C<\n> in
527your program will be converted back to C<\cM\cJ> on output. This is what
528you want for text files, but it can be disastrous for binary files.
30168b04
GS
529
530Another consequence of using binmode() (on some systems) is that
531special end-of-file markers will be seen as part of the data stream.
532For systems from the Microsoft family this means that if your binary
4375e838 533data contains C<\cZ>, the I/O subsystem will regard it as the end of
30168b04
GS
534the file, unless you use binmode().
535
536binmode() is not only important for readline() and print() operations,
537but also when using read(), seek(), sysread(), syswrite() and tell()
538(see L<perlport> for more details). See the C<$/> and C<$\> variables
539in L<perlvar> for how to manually set your input and output
540line-termination sequences.
a0d0e21e 541
4633a7c4 542=item bless REF,CLASSNAME
a0d0e21e
LW
543
544=item bless REF
545
2b5ab1e7
TC
546This function tells the thingy referenced by REF that it is now an object
547in the CLASSNAME package. If CLASSNAME is omitted, the current package
19799a22 548is used. Because a C<bless> is often the last thing in a constructor,
2b5ab1e7
TC
549it returns the reference for convenience. Always use the two-argument
550version if the function doing the blessing might be inherited by a
551derived class. See L<perltoot> and L<perlobj> for more about the blessing
552(and blessings) of objects.
a0d0e21e 553
57668c4d 554Consider always blessing objects in CLASSNAMEs that are mixed case.
2b5ab1e7
TC
555Namespaces with all lowercase names are considered reserved for
556Perl pragmata. Builtin types have all uppercase names, so to prevent
557confusion, you may wish to avoid such package names as well. Make sure
558that CLASSNAME is a true value.
60ad88b8
GS
559
560See L<perlmod/"Perl Modules">.
561
a0d0e21e
LW
562=item caller EXPR
563
564=item caller
565
5a964f20 566Returns the context of the current subroutine call. In scalar context,
28757baa 567returns the caller's package name if there is a caller, that is, if
19799a22 568we're in a subroutine or C<eval> or C<require>, and the undefined value
5a964f20 569otherwise. In list context, returns
a0d0e21e 570
748a9306 571 ($package, $filename, $line) = caller;
a0d0e21e
LW
572
573With EXPR, it returns some extra information that the debugger uses to
574print a stack trace. The value of EXPR indicates how many call frames
575to go back before the current one.
576
f3aa04c2 577 ($package, $filename, $line, $subroutine, $hasargs,
e476b1b5 578 $wantarray, $evaltext, $is_require, $hints, $bitmask) = caller($i);
e7ea3e70 579
951ba7fe 580Here $subroutine may be C<(eval)> if the frame is not a subroutine
19799a22 581call, but an C<eval>. In such a case additional elements $evaltext and
7660c0ab 582C<$is_require> are set: C<$is_require> is true if the frame is created by a
19799a22 583C<require> or C<use> statement, $evaltext contains the text of the
277ddfaf 584C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
951ba7fe 585$filename is C<(eval)>, but $evaltext is undefined. (Note also that
0fc9dec4
RGS
586each C<use> statement creates a C<require> frame inside an C<eval EXPR>
587frame.) $subroutine may also be C<(unknown)> if this particular
588subroutine happens to have been deleted from the symbol table.
589C<$hasargs> is true if a new instance of C<@_> was set up for the frame.
590C<$hints> and C<$bitmask> contain pragmatic hints that the caller was
591compiled with. The C<$hints> and C<$bitmask> values are subject to change
592between versions of Perl, and are not meant for external use.
748a9306
LW
593
594Furthermore, when called from within the DB package, caller returns more
7660c0ab 595detailed information: it sets the list variable C<@DB::args> to be the
54310121 596arguments with which the subroutine was invoked.
748a9306 597
7660c0ab 598Be aware that the optimizer might have optimized call frames away before
19799a22 599C<caller> had a chance to get the information. That means that C<caller(N)>
7660c0ab 600might not return information about the call frame you expect it do, for
b76cc8ba 601C<< N > 1 >>. In particular, C<@DB::args> might have information from the
19799a22 602previous time C<caller> was called.
7660c0ab 603
a0d0e21e
LW
604=item chdir EXPR
605
ffce7b87 606Changes the working directory to EXPR, if possible. If EXPR is omitted,
0bfc1ec4 607changes to the directory specified by C<$ENV{HOME}>, if set; if not,
ffce7b87 608changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
b4ad75f0
AMS
609variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
610neither is set, C<chdir> does nothing. It returns true upon success,
611false otherwise. See the example under C<die>.
a0d0e21e
LW
612
613=item chmod LIST
614
615Changes the permissions of a list of files. The first element of the
4633a7c4 616list must be the numerical mode, which should probably be an octal
2f9daede
TPG
617number, and which definitely should I<not> a string of octal digits:
618C<0644> is okay, C<'0644'> is not. Returns the number of files
dc848c6f 619successfully changed. See also L</oct>, if all you have is a string.
a0d0e21e
LW
620
621 $cnt = chmod 0755, 'foo', 'bar';
622 chmod 0755, @executables;
f86cebdf
GS
623 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
624 # --w----r-T
2f9daede
TPG
625 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
626 $mode = 0644; chmod $mode, 'foo'; # this is best
a0d0e21e 627
ca6e1c26
JH
628You can also import the symbolic C<S_I*> constants from the Fcntl
629module:
630
631 use Fcntl ':mode';
632
633 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
634 # This is identical to the chmod 0755 of the above example.
635
a0d0e21e
LW
636=item chomp VARIABLE
637
313c9f5c 638=item chomp( LIST )
a0d0e21e
LW
639
640=item chomp
641
2b5ab1e7
TC
642This safer version of L</chop> removes any trailing string
643that corresponds to the current value of C<$/> (also known as
28757baa
PP
644$INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
645number of characters removed from all its arguments. It's often used to
646remove the newline from the end of an input record when you're worried
2b5ab1e7
TC
647that the final record may be missing its newline. When in paragraph
648mode (C<$/ = "">), it removes all trailing newlines from the string.
4c5a6083
GS
649When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
650a reference to an integer or the like, see L<perlvar>) chomp() won't
b76cc8ba 651remove anything.
19799a22 652If VARIABLE is omitted, it chomps C<$_>. Example:
a0d0e21e
LW
653
654 while (<>) {
655 chomp; # avoid \n on last field
656 @array = split(/:/);
5a964f20 657 # ...
a0d0e21e
LW
658 }
659
4bf21a6d
RD
660If VARIABLE is a hash, it chomps the hash's values, but not its keys.
661
a0d0e21e
LW
662You can actually chomp anything that's an lvalue, including an assignment:
663
664 chomp($cwd = `pwd`);
665 chomp($answer = <STDIN>);
666
667If you chomp a list, each element is chomped, and the total number of
668characters removed is returned.
669
442a8c12
NC
670If the C<encoding> pragma is in scope then the lengths returned are
671calculated from the length of C<$/> in Unicode characters, which is not
672always the same as the length of C<$/> in the native encoding.
673
15e44fd8
RGS
674Note that parentheses are necessary when you're chomping anything
675that is not a simple variable. This is because C<chomp $cwd = `pwd`;>
676is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as
677C<chomp( $cwd = `pwd` )> which you might expect. Similarly,
678C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than
679as C<chomp($a, $b)>.
680
a0d0e21e
LW
681=item chop VARIABLE
682
313c9f5c 683=item chop( LIST )
a0d0e21e
LW
684
685=item chop
686
687Chops off the last character of a string and returns the character
5b3eff12 688chopped. It is much more efficient than C<s/.$//s> because it neither
7660c0ab 689scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
4bf21a6d
RD
690If VARIABLE is a hash, it chops the hash's values, but not its keys.
691
5b3eff12 692You can actually chop anything that's an lvalue, including an assignment.
a0d0e21e
LW
693
694If you chop a list, each element is chopped. Only the value of the
19799a22 695last C<chop> is returned.
a0d0e21e 696
19799a22 697Note that C<chop> returns the last character. To return all but the last
748a9306
LW
698character, use C<substr($string, 0, -1)>.
699
15e44fd8
RGS
700See also L</chomp>.
701
a0d0e21e
LW
702=item chown LIST
703
704Changes the owner (and group) of a list of files. The first two
19799a22
GS
705elements of the list must be the I<numeric> uid and gid, in that
706order. A value of -1 in either position is interpreted by most
707systems to leave that value unchanged. Returns the number of files
708successfully changed.
a0d0e21e
LW
709
710 $cnt = chown $uid, $gid, 'foo', 'bar';
711 chown $uid, $gid, @filenames;
712
54310121 713Here's an example that looks up nonnumeric uids in the passwd file:
a0d0e21e
LW
714
715 print "User: ";
19799a22 716 chomp($user = <STDIN>);
5a964f20 717 print "Files: ";
19799a22 718 chomp($pattern = <STDIN>);
a0d0e21e
LW
719
720 ($login,$pass,$uid,$gid) = getpwnam($user)
721 or die "$user not in passwd file";
722
5a964f20 723 @ary = glob($pattern); # expand filenames
a0d0e21e
LW
724 chown $uid, $gid, @ary;
725
54310121 726On most systems, you are not allowed to change the ownership of the
4633a7c4
LW
727file unless you're the superuser, although you should be able to change
728the group to any of your secondary groups. On insecure systems, these
729restrictions may be relaxed, but this is not a portable assumption.
19799a22
GS
730On POSIX systems, you can detect this condition this way:
731
732 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
733 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
4633a7c4 734
a0d0e21e
LW
735=item chr NUMBER
736
54310121 737=item chr
bbce6d69 738
a0d0e21e 739Returns the character represented by that NUMBER in the character set.
a0ed51b3 740For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
1e54db1a
JH
741chr(0x263a) is a Unicode smiley face. Note that characters from 128
742to 255 (inclusive) are by default not encoded in UTF-8 Unicode for
743backward compatibility reasons (but see L<encoding>).
aaa68c4a 744
974da8e5
JH
745If NUMBER is omitted, uses C<$_>.
746
b76cc8ba 747For the reverse, use L</ord>.
a0d0e21e 748
974da8e5
JH
749Note that under the C<bytes> pragma the NUMBER is masked to
750the low eight bits.
751
752See L<perlunicode> and L<encoding> for more about Unicode.
bbce6d69 753
a0d0e21e
LW
754=item chroot FILENAME
755
54310121 756=item chroot
bbce6d69 757
5a964f20 758This function works like the system call by the same name: it makes the
4633a7c4 759named directory the new root directory for all further pathnames that
951ba7fe 760begin with a C</> by your process and all its children. (It doesn't
28757baa 761change your current working directory, which is unaffected.) For security
4633a7c4 762reasons, this call is restricted to the superuser. If FILENAME is
19799a22 763omitted, does a C<chroot> to C<$_>.
a0d0e21e
LW
764
765=item close FILEHANDLE
766
6a518fbc
TP
767=item close
768
9124316e
JH
769Closes the file or pipe associated with the file handle, returning
770true only if IO buffers are successfully flushed and closes the system
771file descriptor. Closes the currently selected filehandle if the
772argument is omitted.
fb73857a
PP
773
774You don't have to close FILEHANDLE if you are immediately going to do
19799a22
GS
775another C<open> on it, because C<open> will close it for you. (See
776C<open>.) However, an explicit C<close> on an input file resets the line
777counter (C<$.>), while the implicit close done by C<open> does not.
fb73857a 778
dede8123
RGS
779If the file handle came from a piped open, C<close> will additionally
780return false if one of the other system calls involved fails, or if the
fb73857a 781program exits with non-zero status. (If the only problem was that the
dede8123 782program exited non-zero, C<$!> will be set to C<0>.) Closing a pipe
2b5ab1e7 783also waits for the process executing on the pipe to complete, in case you
b76cc8ba 784want to look at the output of the pipe afterwards, and
2b5ab1e7 785implicitly puts the exit status value of that command into C<$?>.
5a964f20 786
73689b13
GS
787Prematurely closing the read end of a pipe (i.e. before the process
788writing to it at the other end has closed it) will result in a
789SIGPIPE being delivered to the writer. If the other end can't
790handle that, be sure to read all the data before closing the pipe.
791
fb73857a 792Example:
a0d0e21e 793
fb73857a
PP
794 open(OUTPUT, '|sort >foo') # pipe to sort
795 or die "Can't start sort: $!";
5a964f20 796 #... # print stuff to output
fb73857a
PP
797 close OUTPUT # wait for sort to finish
798 or warn $! ? "Error closing sort pipe: $!"
799 : "Exit status $? from sort";
800 open(INPUT, 'foo') # get sort's results
801 or die "Can't open 'foo' for input: $!";
a0d0e21e 802
5a964f20
TC
803FILEHANDLE may be an expression whose value can be used as an indirect
804filehandle, usually the real filehandle name.
a0d0e21e
LW
805
806=item closedir DIRHANDLE
807
19799a22 808Closes a directory opened by C<opendir> and returns the success of that
5a964f20
TC
809system call.
810
a0d0e21e
LW
811=item connect SOCKET,NAME
812
813Attempts to connect to a remote socket, just as the connect system call
19799a22 814does. Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
815packed address of the appropriate type for the socket. See the examples in
816L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 817
cb1a09d0
AD
818=item continue BLOCK
819
820Actually a flow control statement rather than a function. If there is a
98293880
JH
821C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
822C<foreach>), it is always executed just before the conditional is about to
823be evaluated again, just like the third part of a C<for> loop in C. Thus
cb1a09d0
AD
824it can be used to increment a loop variable, even when the loop has been
825continued via the C<next> statement (which is similar to the C C<continue>
826statement).
827
98293880 828C<last>, C<next>, or C<redo> may appear within a C<continue>
19799a22
GS
829block. C<last> and C<redo> will behave as if they had been executed within
830the main block. So will C<next>, but since it will execute a C<continue>
1d2dff63
GS
831block, it may be more entertaining.
832
833 while (EXPR) {
834 ### redo always comes here
835 do_something;
836 } continue {
837 ### next always comes here
838 do_something_else;
839 # then back the top to re-check EXPR
840 }
841 ### last always comes here
842
843Omitting the C<continue> section is semantically equivalent to using an
19799a22 844empty one, logically enough. In that case, C<next> goes directly back
1d2dff63
GS
845to check the condition at the top of the loop.
846
a0d0e21e
LW
847=item cos EXPR
848
d6217f1e
GS
849=item cos
850
5a964f20 851Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
7660c0ab 852takes cosine of C<$_>.
a0d0e21e 853
ca6e1c26 854For the inverse cosine operation, you may use the C<Math::Trig::acos()>
28757baa
PP
855function, or use this relation:
856
857 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
858
a0d0e21e
LW
859=item crypt PLAINTEXT,SALT
860
f86cebdf 861Encrypts a string exactly like the crypt(3) function in the C library
4633a7c4
LW
862(assuming that you actually have a version there that has not been
863extirpated as a potential munition). This can prove useful for checking
864the password file for lousy passwords, amongst other things. Only the
865guys wearing white hats should do this.
a0d0e21e 866
a6d05634 867Note that L<crypt|/crypt> is intended to be a one-way function, much like
85c16d83
JH
868breaking eggs to make an omelette. There is no (known) corresponding
869decrypt function (in other words, the crypt() is a one-way hash
870function). As a result, this function isn't all that useful for
11155c91 871cryptography. (For that, see your nearby CPAN mirror.)
2f9daede 872
85c16d83
JH
873When verifying an existing encrypted string you should use the
874encrypted text as the salt (like C<crypt($plain, $crypted) eq
8e2ffcbe 875$crypted>). This allows your code to work with the standard L<crypt|/crypt>
85c16d83
JH
876and with more exotic implementations. In other words, do not assume
877anything about the returned string itself, or how many bytes in
878the encrypted string matter.
879
880Traditionally the result is a string of 13 bytes: two first bytes of
881the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
882the first eight bytes of the encrypted string mattered, but
883alternative hashing schemes (like MD5), higher level security schemes
884(like C2), and implementations on non-UNIX platforms may produce
885different strings.
886
887When choosing a new salt create a random two character string whose
888characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
d3989d75
CW
889'/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of
890characters is just a recommendation; the characters allowed in
891the salt depend solely on your system's crypt library, and Perl can't
892restrict what salts C<crypt()> accepts.
e71965be 893
a0d0e21e
LW
894Here's an example that makes sure that whoever runs this program knows
895their own password:
896
897 $pwd = (getpwuid($<))[1];
a0d0e21e
LW
898
899 system "stty -echo";
900 print "Password: ";
e71965be 901 chomp($word = <STDIN>);
a0d0e21e
LW
902 print "\n";
903 system "stty echo";
904
e71965be 905 if (crypt($word, $pwd) ne $pwd) {
a0d0e21e
LW
906 die "Sorry...\n";
907 } else {
908 print "ok\n";
54310121 909 }
a0d0e21e 910
9f8f0c9d 911Of course, typing in your own password to whoever asks you
748a9306 912for it is unwise.
a0d0e21e 913
8e2ffcbe 914The L<crypt|/crypt> function is unsuitable for encrypting large quantities
19799a22
GS
915of data, not least of all because you can't get the information
916back. Look at the F<by-module/Crypt> and F<by-module/PGP> directories
917on your favorite CPAN mirror for a slew of potentially useful
918modules.
919
f2791508
JH
920If using crypt() on a Unicode string (which I<potentially> has
921characters with codepoints above 255), Perl tries to make sense
922of the situation by trying to downgrade (a copy of the string)
923the string back to an eight-bit byte string before calling crypt()
924(on that copy). If that works, good. If not, crypt() dies with
925C<Wide character in crypt>.
85c16d83 926
aa689395 927=item dbmclose HASH
a0d0e21e 928
19799a22 929[This function has been largely superseded by the C<untie> function.]
a0d0e21e 930
aa689395 931Breaks the binding between a DBM file and a hash.
a0d0e21e 932
19799a22 933=item dbmopen HASH,DBNAME,MASK
a0d0e21e 934
19799a22 935[This function has been largely superseded by the C<tie> function.]
a0d0e21e 936
7b8d334a 937This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
19799a22
GS
938hash. HASH is the name of the hash. (Unlike normal C<open>, the first
939argument is I<not> a filehandle, even though it looks like one). DBNAME
aa689395
PP
940is the name of the database (without the F<.dir> or F<.pag> extension if
941any). If the database does not exist, it is created with protection
19799a22
GS
942specified by MASK (as modified by the C<umask>). If your system supports
943only the older DBM functions, you may perform only one C<dbmopen> in your
aa689395 944program. In older versions of Perl, if your system had neither DBM nor
19799a22 945ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
aa689395
PP
946sdbm(3).
947
948If you don't have write access to the DBM file, you can only read hash
949variables, not set them. If you want to test whether you can write,
19799a22 950either use file tests or try setting a dummy hash entry inside an C<eval>,
aa689395 951which will trap the error.
a0d0e21e 952
19799a22
GS
953Note that functions such as C<keys> and C<values> may return huge lists
954when used on large DBM files. You may prefer to use the C<each>
a0d0e21e
LW
955function to iterate over large DBM files. Example:
956
957 # print out history file offsets
958 dbmopen(%HIST,'/usr/lib/news/history',0666);
959 while (($key,$val) = each %HIST) {
960 print $key, ' = ', unpack('L',$val), "\n";
961 }
962 dbmclose(%HIST);
963
cb1a09d0 964See also L<AnyDBM_File> for a more general description of the pros and
184e9718 965cons of the various dbm approaches, as well as L<DB_File> for a particularly
cb1a09d0 966rich implementation.
4633a7c4 967
2b5ab1e7
TC
968You can control which DBM library you use by loading that library
969before you call dbmopen():
970
971 use DB_File;
972 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
973 or die "Can't open netscape history file: $!";
974
a0d0e21e
LW
975=item defined EXPR
976
54310121 977=item defined
bbce6d69 978
2f9daede
TPG
979Returns a Boolean value telling whether EXPR has a value other than
980the undefined value C<undef>. If EXPR is not present, C<$_> will be
981checked.
982
983Many operations return C<undef> to indicate failure, end of file,
984system error, uninitialized variable, and other exceptional
985conditions. This function allows you to distinguish C<undef> from
986other values. (A simple Boolean test will not distinguish among
7660c0ab 987C<undef>, zero, the empty string, and C<"0">, which are all equally
2f9daede 988false.) Note that since C<undef> is a valid scalar, its presence
19799a22 989doesn't I<necessarily> indicate an exceptional condition: C<pop>
2f9daede
TPG
990returns C<undef> when its argument is an empty array, I<or> when the
991element to return happens to be C<undef>.
992
f10b0346
GS
993You may also use C<defined(&func)> to check whether subroutine C<&func>
994has ever been defined. The return value is unaffected by any forward
04891299 995declarations of C<&func>. Note that a subroutine which is not defined
847c7ebe
DD
996may still be callable: its package may have an C<AUTOLOAD> method that
997makes it spring into existence the first time that it is called -- see
998L<perlsub>.
f10b0346
GS
999
1000Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
1001used to report whether memory for that aggregate has ever been
1002allocated. This behavior may disappear in future versions of Perl.
1003You should instead use a simple test for size:
1004
1005 if (@an_array) { print "has array elements\n" }
1006 if (%a_hash) { print "has hash members\n" }
2f9daede
TPG
1007
1008When used on a hash element, it tells you whether the value is defined,
dc848c6f 1009not whether the key exists in the hash. Use L</exists> for the latter
2f9daede 1010purpose.
a0d0e21e
LW
1011
1012Examples:
1013
1014 print if defined $switch{'D'};
1015 print "$val\n" while defined($val = pop(@ary));
1016 die "Can't readlink $sym: $!"
1017 unless defined($value = readlink $sym);
a0d0e21e 1018 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
2f9daede 1019 $debugging = 0 unless defined $debugging;
a0d0e21e 1020
19799a22 1021Note: Many folks tend to overuse C<defined>, and then are surprised to
7660c0ab 1022discover that the number C<0> and C<""> (the zero-length string) are, in fact,
2f9daede 1023defined values. For example, if you say
a5f75d66
AD
1024
1025 "ab" =~ /a(.*)b/;
1026
7660c0ab 1027The pattern match succeeds, and C<$1> is defined, despite the fact that it
a5f75d66 1028matched "nothing". But it didn't really match nothing--rather, it
2b5ab1e7 1029matched something that happened to be zero characters long. This is all
a5f75d66 1030very above-board and honest. When a function returns an undefined value,
2f9daede 1031it's an admission that it couldn't give you an honest answer. So you
19799a22 1032should use C<defined> only when you're questioning the integrity of what
7660c0ab 1033you're trying to do. At other times, a simple comparison to C<0> or C<""> is
2f9daede
TPG
1034what you want.
1035
dc848c6f 1036See also L</undef>, L</exists>, L</ref>.
2f9daede 1037
a0d0e21e
LW
1038=item delete EXPR
1039
01020589
GS
1040Given an expression that specifies a hash element, array element, hash slice,
1041or array slice, deletes the specified element(s) from the hash or array.
8216c1fd 1042In the case of an array, if the array elements happen to be at the end,
b76cc8ba 1043the size of the array will shrink to the highest element that tests
8216c1fd 1044true for exists() (or 0 if no such element exists).
a0d0e21e 1045
eba0920a
EM
1046Returns a list with the same number of elements as the number of elements
1047for which deletion was attempted. Each element of that list consists of
1048either the value of the element deleted, or the undefined value. In scalar
1049context, this means that you get the value of the last element deleted (or
1050the undefined value if that element did not exist).
1051
1052 %hash = (foo => 11, bar => 22, baz => 33);
1053 $scalar = delete $hash{foo}; # $scalar is 11
1054 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
1055 @array = delete @hash{qw(foo bar baz)}; # @array is (undef,undef,33)
1056
1057Deleting from C<%ENV> modifies the environment. Deleting from
01020589
GS
1058a hash tied to a DBM file deletes the entry from the DBM file. Deleting
1059from a C<tie>d hash or array may not necessarily return anything.
1060
8ea97a1e
GS
1061Deleting an array element effectively returns that position of the array
1062to its initial, uninitialized state. Subsequently testing for the same
8216c1fd
GS
1063element with exists() will return false. Note that deleting array
1064elements in the middle of an array will not shift the index of the ones
1065after them down--use splice() for that. See L</exists>.
8ea97a1e 1066
01020589 1067The following (inefficiently) deletes all the values of %HASH and @ARRAY:
a0d0e21e 1068
5f05dabc
PP
1069 foreach $key (keys %HASH) {
1070 delete $HASH{$key};
a0d0e21e
LW
1071 }
1072
01020589
GS
1073 foreach $index (0 .. $#ARRAY) {
1074 delete $ARRAY[$index];
1075 }
1076
1077And so do these:
5f05dabc 1078
01020589
GS
1079 delete @HASH{keys %HASH};
1080
9740c838 1081 delete @ARRAY[0 .. $#ARRAY];
5f05dabc 1082
2b5ab1e7 1083But both of these are slower than just assigning the empty list
01020589
GS
1084or undefining %HASH or @ARRAY:
1085
1086 %HASH = (); # completely empty %HASH
1087 undef %HASH; # forget %HASH ever existed
2b5ab1e7 1088
01020589
GS
1089 @ARRAY = (); # completely empty @ARRAY
1090 undef @ARRAY; # forget @ARRAY ever existed
2b5ab1e7
TC
1091
1092Note that the EXPR can be arbitrarily complicated as long as the final
01020589
GS
1093operation is a hash element, array element, hash slice, or array slice
1094lookup:
a0d0e21e
LW
1095
1096 delete $ref->[$x][$y]{$key};
5f05dabc 1097 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
a0d0e21e 1098
01020589
GS
1099 delete $ref->[$x][$y][$index];
1100 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1101
a0d0e21e
LW
1102=item die LIST
1103
19799a22
GS
1104Outside an C<eval>, prints the value of LIST to C<STDERR> and
1105exits with the current value of C<$!> (errno). If C<$!> is C<0>,
61eff3bc
JH
1106exits with the value of C<<< ($? >> 8) >>> (backtick `command`
1107status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside
19799a22
GS
1108an C<eval(),> the error message is stuffed into C<$@> and the
1109C<eval> is terminated with the undefined value. This makes
1110C<die> the way to raise an exception.
a0d0e21e
LW
1111
1112Equivalent examples:
1113
1114 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
54310121 1115 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
a0d0e21e 1116
ccac6780 1117If the last element of LIST does not end in a newline, the current
df37ec69
WW
1118script line number and input line number (if any) are also printed,
1119and a newline is supplied. Note that the "input line number" (also
1120known as "chunk") is subject to whatever notion of "line" happens to
1121be currently in effect, and is also available as the special variable
1122C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1123
1124Hint: sometimes appending C<", stopped"> to your message will cause it
1125to make better sense when the string C<"at foo line 123"> is appended.
1126Suppose you are running script "canasta".
a0d0e21e
LW
1127
1128 die "/etc/games is no good";
1129 die "/etc/games is no good, stopped";
1130
1131produce, respectively
1132
1133 /etc/games is no good at canasta line 123.
1134 /etc/games is no good, stopped at canasta line 123.
1135
2b5ab1e7 1136See also exit(), warn(), and the Carp module.
a0d0e21e 1137
7660c0ab
A
1138If LIST is empty and C<$@> already contains a value (typically from a
1139previous eval) that value is reused after appending C<"\t...propagated">.
fb73857a
PP
1140This is useful for propagating exceptions:
1141
1142 eval { ... };
1143 die unless $@ =~ /Expected exception/;
1144
ad216e65
JH
1145If LIST is empty and C<$@> contains an object reference that has a
1146C<PROPAGATE> method, that method will be called with additional file
1147and line number parameters. The return value replaces the value in
16869676 1148C<$@>. ie. as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >>
ad216e65
JH
1149were called.
1150
7660c0ab 1151If C<$@> is empty then the string C<"Died"> is used.
fb73857a 1152
52531d10
GS
1153die() can also be called with a reference argument. If this happens to be
1154trapped within an eval(), $@ contains the reference. This behavior permits
1155a more elaborate exception handling implementation using objects that
4375e838 1156maintain arbitrary state about the nature of the exception. Such a scheme
52531d10
GS
1157is sometimes preferable to matching particular string values of $@ using
1158regular expressions. Here's an example:
1159
1160 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1161 if ($@) {
1162 if (ref($@) && UNIVERSAL::isa($@,"Some::Module::Exception")) {
1163 # handle Some::Module::Exception
1164 }
1165 else {
1166 # handle all other possible exceptions
1167 }
1168 }
1169
19799a22 1170Because perl will stringify uncaught exception messages before displaying
52531d10
GS
1171them, you may want to overload stringification operations on such custom
1172exception objects. See L<overload> for details about that.
1173
19799a22
GS
1174You can arrange for a callback to be run just before the C<die>
1175does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1176handler will be called with the error text and can change the error
1177message, if it sees fit, by calling C<die> again. See
1178L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
1179L<"eval BLOCK"> for some examples. Although this feature was meant
1180to be run only right before your program was to exit, this is not
1181currently the case--the C<$SIG{__DIE__}> hook is currently called
1182even inside eval()ed blocks/strings! If one wants the hook to do
1183nothing in such situations, put
fb73857a
PP
1184
1185 die @_ if $^S;
1186
19799a22
GS
1187as the first line of the handler (see L<perlvar/$^S>). Because
1188this promotes strange action at a distance, this counterintuitive
b76cc8ba 1189behavior may be fixed in a future release.
774d564b 1190
a0d0e21e
LW
1191=item do BLOCK
1192
1193Not really a function. Returns the value of the last command in the
1194sequence of commands indicated by BLOCK. When modified by a loop
98293880
JH
1195modifier, executes the BLOCK once before testing the loop condition.
1196(On other statements the loop modifiers test the conditional first.)
a0d0e21e 1197
4968c1e4 1198C<do BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7
TC
1199C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1200See L<perlsyn> for alternative strategies.
4968c1e4 1201
a0d0e21e
LW
1202=item do SUBROUTINE(LIST)
1203
1204A deprecated form of subroutine call. See L<perlsub>.
1205
1206=item do EXPR
1207
1208Uses the value of EXPR as a filename and executes the contents of the
1209file as a Perl script. Its primary use is to include subroutines
1210from a Perl subroutine library.
1211
1212 do 'stat.pl';
1213
1214is just like
1215
986b19de 1216 eval `cat stat.pl`;
a0d0e21e 1217
2b5ab1e7
TC
1218except that it's more efficient and concise, keeps track of the current
1219filename for error messages, searches the @INC libraries, and updates
1220C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1221variables. It also differs in that code evaluated with C<do FILENAME>
1222cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1223same, however, in that it does reparse the file every time you call it,
1224so you probably don't want to do this inside a loop.
a0d0e21e 1225
8e30cc93 1226If C<do> cannot read the file, it returns undef and sets C<$!> to the
2b5ab1e7 1227error. If C<do> can read the file but cannot compile it, it
8e30cc93
G
1228returns undef and sets an error message in C<$@>. If the file is
1229successfully compiled, C<do> returns the value of the last expression
1230evaluated.
1231
a0d0e21e 1232Note that inclusion of library modules is better done with the
19799a22 1233C<use> and C<require> operators, which also do automatic error checking
4633a7c4 1234and raise an exception if there's a problem.
a0d0e21e 1235
5a964f20
TC
1236You might like to use C<do> to read in a program configuration
1237file. Manual error checking can be done this way:
1238
b76cc8ba 1239 # read in config files: system first, then user
f86cebdf 1240 for $file ("/share/prog/defaults.rc",
b76cc8ba 1241 "$ENV{HOME}/.someprogrc")
2b5ab1e7 1242 {
5a964f20 1243 unless ($return = do $file) {
f86cebdf
GS
1244 warn "couldn't parse $file: $@" if $@;
1245 warn "couldn't do $file: $!" unless defined $return;
1246 warn "couldn't run $file" unless $return;
5a964f20
TC
1247 }
1248 }
1249
a0d0e21e
LW
1250=item dump LABEL
1251
1614b0e3
JD
1252=item dump
1253
19799a22
GS
1254This function causes an immediate core dump. See also the B<-u>
1255command-line switch in L<perlrun>, which does the same thing.
1256Primarily this is so that you can use the B<undump> program (not
1257supplied) to turn your core dump into an executable binary after
1258having initialized all your variables at the beginning of the
1259program. When the new binary is executed it will begin by executing
1260a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1261Think of it as a goto with an intervening core dump and reincarnation.
1262If C<LABEL> is omitted, restarts the program from the top.
1263
1264B<WARNING>: Any files opened at the time of the dump will I<not>
1265be open any more when the program is reincarnated, with possible
b76cc8ba 1266resulting confusion on the part of Perl.
19799a22
GS
1267
1268This function is now largely obsolete, partly because it's very
1269hard to convert a core file into an executable, and because the
1270real compiler backends for generating portable bytecode and compilable
ac206dc8
RGS
1271C code have superseded it. That's why you should now invoke it as
1272C<CORE::dump()>, if you don't want to be warned against a possible
1273typo.
19799a22
GS
1274
1275If you're looking to use L<dump> to speed up your program, consider
1276generating bytecode or native C code as described in L<perlcc>. If
1277you're just trying to accelerate a CGI script, consider using the
210b36aa 1278C<mod_perl> extension to B<Apache>, or the CPAN module, CGI::Fast.
19799a22 1279You might also consider autoloading or selfloading, which at least
b76cc8ba 1280make your program I<appear> to run faster.
5a964f20 1281
aa689395
PP
1282=item each HASH
1283
5a964f20 1284When called in list context, returns a 2-element list consisting of the
aa689395 1285key and value for the next element of a hash, so that you can iterate over
74fc8b5f 1286it. When called in scalar context, returns only the key for the next
e902a979 1287element in the hash.
2f9daede 1288
ab192400 1289Entries are returned in an apparently random order. The actual random
504f80c1
JH
1290order is subject to change in future versions of perl, but it is
1291guaranteed to be in the same order as either the C<keys> or C<values>
4546b9e6
JH
1292function would produce on the same (unmodified) hash. Since Perl
12935.8.1 the ordering is different even between different runs of Perl
1294for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
ab192400
GS
1295
1296When the hash is entirely read, a null array is returned in list context
19799a22
GS
1297(which when assigned produces a false (C<0>) value), and C<undef> in
1298scalar context. The next call to C<each> after that will start iterating
1299again. There is a single iterator for each hash, shared by all C<each>,
1300C<keys>, and C<values> function calls in the program; it can be reset by
2f9daede
TPG
1301reading all the elements from the hash, or by evaluating C<keys HASH> or
1302C<values HASH>. If you add or delete elements of a hash while you're
74fc8b5f
MJD
1303iterating over it, you may get entries skipped or duplicated, so
1304don't. Exception: It is always safe to delete the item most recently
1305returned by C<each()>, which means that the following code will work:
1306
1307 while (($key, $value) = each %hash) {
1308 print $key, "\n";
1309 delete $hash{$key}; # This is safe
1310 }
aa689395 1311
f86cebdf 1312The following prints out your environment like the printenv(1) program,
aa689395 1313only in a different order:
a0d0e21e
LW
1314
1315 while (($key,$value) = each %ENV) {
1316 print "$key=$value\n";
1317 }
1318
19799a22 1319See also C<keys>, C<values> and C<sort>.
a0d0e21e
LW
1320
1321=item eof FILEHANDLE
1322
4633a7c4
LW
1323=item eof ()
1324
a0d0e21e
LW
1325=item eof
1326
1327Returns 1 if the next read on FILEHANDLE will return end of file, or if
1328FILEHANDLE is not open. FILEHANDLE may be an expression whose value
5a964f20 1329gives the real filehandle. (Note that this function actually
19799a22 1330reads a character and then C<ungetc>s it, so isn't very useful in an
748a9306 1331interactive context.) Do not read from a terminal file (or call
19799a22 1332C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
748a9306
LW
1333as terminals may lose the end-of-file condition if you do.
1334
820475bd
GS
1335An C<eof> without an argument uses the last file read. Using C<eof()>
1336with empty parentheses is very different. It refers to the pseudo file
1337formed from the files listed on the command line and accessed via the
61eff3bc
JH
1338C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1339as a normal filehandle is, an C<eof()> before C<< <> >> has been
820475bd 1340used will cause C<@ARGV> to be examined to determine if input is
67408cae 1341available. Similarly, an C<eof()> after C<< <> >> has returned
efdd0218
RB
1342end-of-file will assume you are processing another C<@ARGV> list,
1343and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1344see L<perlop/"I/O Operators">.
820475bd 1345
61eff3bc 1346In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
820475bd
GS
1347detect the end of each file, C<eof()> will only detect the end of the
1348last file. Examples:
a0d0e21e 1349
748a9306
LW
1350 # reset line numbering on each input file
1351 while (<>) {
b76cc8ba 1352 next if /^\s*#/; # skip comments
748a9306 1353 print "$.\t$_";
5a964f20
TC
1354 } continue {
1355 close ARGV if eof; # Not eof()!
748a9306
LW
1356 }
1357
a0d0e21e
LW
1358 # insert dashes just before last line of last file
1359 while (<>) {
6ac88b13 1360 if (eof()) { # check for end of last file
a0d0e21e
LW
1361 print "--------------\n";
1362 }
1363 print;
6ac88b13 1364 last if eof(); # needed if we're reading from a terminal
a0d0e21e
LW
1365 }
1366
a0d0e21e 1367Practical hint: you almost never need to use C<eof> in Perl, because the
3ce0d271
GS
1368input operators typically return C<undef> when they run out of data, or if
1369there was an error.
a0d0e21e
LW
1370
1371=item eval EXPR
1372
1373=item eval BLOCK
1374
c7cc6f1c
GS
1375In the first form, the return value of EXPR is parsed and executed as if it
1376were a little Perl program. The value of the expression (which is itself
5a964f20 1377determined within scalar context) is first parsed, and if there weren't any
be3174d2
GS
1378errors, executed in the lexical context of the current Perl program, so
1379that any variable settings or subroutine and format definitions remain
1380afterwards. Note that the value is parsed every time the eval executes.
1381If EXPR is omitted, evaluates C<$_>. This form is typically used to
1382delay parsing and subsequent execution of the text of EXPR until run time.
c7cc6f1c
GS
1383
1384In the second form, the code within the BLOCK is parsed only once--at the
1385same time the code surrounding the eval itself was parsed--and executed
1386within the context of the current Perl program. This form is typically
1387used to trap exceptions more efficiently than the first (see below), while
1388also providing the benefit of checking the code within BLOCK at compile
1389time.
1390
1391The final semicolon, if any, may be omitted from the value of EXPR or within
1392the BLOCK.
1393
1394In both forms, the value returned is the value of the last expression
5a964f20 1395evaluated inside the mini-program; a return statement may be also used, just
c7cc6f1c 1396as with subroutines. The expression providing the return value is evaluated
5a964f20 1397in void, scalar, or list context, depending on the context of the eval itself.
c7cc6f1c 1398See L</wantarray> for more on how the evaluation context can be determined.
a0d0e21e 1399
19799a22
GS
1400If there is a syntax error or runtime error, or a C<die> statement is
1401executed, an undefined value is returned by C<eval>, and C<$@> is set to the
a0d0e21e 1402error message. If there was no error, C<$@> is guaranteed to be a null
19799a22 1403string. Beware that using C<eval> neither silences perl from printing
c7cc6f1c 1404warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
d9984052
A
1405To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1406turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1407See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
a0d0e21e 1408
19799a22
GS
1409Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1410determining whether a particular feature (such as C<socket> or C<symlink>)
a0d0e21e
LW
1411is implemented. It is also Perl's exception trapping mechanism, where
1412the die operator is used to raise exceptions.
1413
1414If the code to be executed doesn't vary, you may use the eval-BLOCK
1415form to trap run-time errors without incurring the penalty of
1416recompiling each time. The error, if any, is still returned in C<$@>.
1417Examples:
1418
54310121 1419 # make divide-by-zero nonfatal
a0d0e21e
LW
1420 eval { $answer = $a / $b; }; warn $@ if $@;
1421
1422 # same thing, but less efficient
1423 eval '$answer = $a / $b'; warn $@ if $@;
1424
1425 # a compile-time error
5a964f20 1426 eval { $answer = }; # WRONG
a0d0e21e
LW
1427
1428 # a run-time error
1429 eval '$answer ='; # sets $@
1430
2b5ab1e7
TC
1431Due to the current arguably broken state of C<__DIE__> hooks, when using
1432the C<eval{}> form as an exception trap in libraries, you may wish not
1433to trigger any C<__DIE__> hooks that user code may have installed.
1434You can use the C<local $SIG{__DIE__}> construct for this purpose,
1435as shown in this example:
774d564b
PP
1436
1437 # a very private exception trap for divide-by-zero
f86cebdf
GS
1438 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1439 warn $@ if $@;
774d564b
PP
1440
1441This is especially significant, given that C<__DIE__> hooks can call
19799a22 1442C<die> again, which has the effect of changing their error messages:
774d564b
PP
1443
1444 # __DIE__ hooks may modify error messages
1445 {
f86cebdf
GS
1446 local $SIG{'__DIE__'} =
1447 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
c7cc6f1c
GS
1448 eval { die "foo lives here" };
1449 print $@ if $@; # prints "bar lives here"
774d564b
PP
1450 }
1451
19799a22 1452Because this promotes action at a distance, this counterintuitive behavior
2b5ab1e7
TC
1453may be fixed in a future release.
1454
19799a22 1455With an C<eval>, you should be especially careful to remember what's
a0d0e21e
LW
1456being looked at when:
1457
1458 eval $x; # CASE 1
1459 eval "$x"; # CASE 2
1460
1461 eval '$x'; # CASE 3
1462 eval { $x }; # CASE 4
1463
5a964f20 1464 eval "\$$x++"; # CASE 5
a0d0e21e
LW
1465 $$x++; # CASE 6
1466
2f9daede 1467Cases 1 and 2 above behave identically: they run the code contained in
19799a22 1468the variable $x. (Although case 2 has misleading double quotes making
2f9daede 1469the reader wonder what else might be happening (nothing is).) Cases 3
7660c0ab 1470and 4 likewise behave in the same way: they run the code C<'$x'>, which
19799a22 1471does nothing but return the value of $x. (Case 4 is preferred for
2f9daede
TPG
1472purely visual reasons, but it also has the advantage of compiling at
1473compile-time instead of at run-time.) Case 5 is a place where
19799a22 1474normally you I<would> like to use double quotes, except that in this
2f9daede
TPG
1475particular situation, you can just use symbolic references instead, as
1476in case 6.
a0d0e21e 1477
4968c1e4 1478C<eval BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7 1479C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
4968c1e4 1480
d819b83a
DM
1481Note that as a very special case, an C<eval ''> executed within the C<DB>
1482package doesn't see the usual surrounding lexical scope, but rather the
1483scope of the first non-DB piece of code that called it. You don't normally
1484need to worry about this unless you are writing a Perl debugger.
1485
a0d0e21e
LW
1486=item exec LIST
1487
8bf3b016
GS
1488=item exec PROGRAM LIST
1489
19799a22
GS
1490The C<exec> function executes a system command I<and never returns>--
1491use C<system> instead of C<exec> if you want it to return. It fails and
1492returns false only if the command does not exist I<and> it is executed
fb73857a 1493directly instead of via your system's command shell (see below).
a0d0e21e 1494
19799a22
GS
1495Since it's a common mistake to use C<exec> instead of C<system>, Perl
1496warns you if there is a following statement which isn't C<die>, C<warn>,
1497or C<exit> (if C<-w> is set - but you always do that). If you
1498I<really> want to follow an C<exec> with some other statement, you
55d729e4
GS
1499can use one of these styles to avoid the warning:
1500
5a964f20
TC
1501 exec ('foo') or print STDERR "couldn't exec foo: $!";
1502 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
55d729e4 1503
5a964f20 1504If there is more than one argument in LIST, or if LIST is an array
f86cebdf 1505with more than one value, calls execvp(3) with the arguments in LIST.
5a964f20
TC
1506If there is only one scalar argument or an array with one element in it,
1507the argument is checked for shell metacharacters, and if there are any,
1508the entire argument is passed to the system's command shell for parsing
1509(this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1510If there are no shell metacharacters in the argument, it is split into
b76cc8ba 1511words and passed directly to C<execvp>, which is more efficient.
19799a22 1512Examples:
a0d0e21e 1513
19799a22
GS
1514 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1515 exec "sort $outfile | uniq";
a0d0e21e
LW
1516
1517If you don't really want to execute the first argument, but want to lie
1518to the program you are executing about its own name, you can specify
1519the program you actually want to run as an "indirect object" (without a
1520comma) in front of the LIST. (This always forces interpretation of the
54310121 1521LIST as a multivalued list, even if there is only a single scalar in
a0d0e21e
LW
1522the list.) Example:
1523
1524 $shell = '/bin/csh';
1525 exec $shell '-sh'; # pretend it's a login shell
1526
1527or, more directly,
1528
1529 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1530
bb32b41a
GS
1531When the arguments get executed via the system shell, results will
1532be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1533for details.
1534
19799a22
GS
1535Using an indirect object with C<exec> or C<system> is also more
1536secure. This usage (which also works fine with system()) forces
1537interpretation of the arguments as a multivalued list, even if the
1538list had just one argument. That way you're safe from the shell
1539expanding wildcards or splitting up words with whitespace in them.
5a964f20
TC
1540
1541 @args = ( "echo surprise" );
1542
2b5ab1e7 1543 exec @args; # subject to shell escapes
f86cebdf 1544 # if @args == 1
2b5ab1e7 1545 exec { $args[0] } @args; # safe even with one-arg list
5a964f20
TC
1546
1547The first version, the one without the indirect object, ran the I<echo>
1548program, passing it C<"surprise"> an argument. The second version
1549didn't--it tried to run a program literally called I<"echo surprise">,
1550didn't find it, and set C<$?> to a non-zero value indicating failure.
1551
0f897271
GS
1552Beginning with v5.6.0, Perl will attempt to flush all files opened for
1553output before the exec, but this may not be supported on some platforms
1554(see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1555in English) or call the C<autoflush()> method of C<IO::Handle> on any
1556open handles in order to avoid lost output.
1557
19799a22 1558Note that C<exec> will not call your C<END> blocks, nor will it call
7660c0ab
A
1559any C<DESTROY> methods in your objects.
1560
a0d0e21e
LW
1561=item exists EXPR
1562
01020589 1563Given an expression that specifies a hash element or array element,
8ea97a1e
GS
1564returns true if the specified element in the hash or array has ever
1565been initialized, even if the corresponding value is undefined. The
1566element is not autovivified if it doesn't exist.
a0d0e21e 1567
01020589
GS
1568 print "Exists\n" if exists $hash{$key};
1569 print "Defined\n" if defined $hash{$key};
1570 print "True\n" if $hash{$key};
1571
1572 print "Exists\n" if exists $array[$index];
1573 print "Defined\n" if defined $array[$index];
1574 print "True\n" if $array[$index];
a0d0e21e 1575
8ea97a1e 1576A hash or array element can be true only if it's defined, and defined if
a0d0e21e
LW
1577it exists, but the reverse doesn't necessarily hold true.
1578
afebc493
GS
1579Given an expression that specifies the name of a subroutine,
1580returns true if the specified subroutine has ever been declared, even
1581if it is undefined. Mentioning a subroutine name for exists or defined
847c7ebe
DD
1582does not count as declaring it. Note that a subroutine which does not
1583exist may still be callable: its package may have an C<AUTOLOAD>
1584method that makes it spring into existence the first time that it is
1585called -- see L<perlsub>.
afebc493
GS
1586
1587 print "Exists\n" if exists &subroutine;
1588 print "Defined\n" if defined &subroutine;
1589
a0d0e21e 1590Note that the EXPR can be arbitrarily complicated as long as the final
afebc493 1591operation is a hash or array key lookup or subroutine name:
a0d0e21e 1592
2b5ab1e7
TC
1593 if (exists $ref->{A}->{B}->{$key}) { }
1594 if (exists $hash{A}{B}{$key}) { }
1595
01020589
GS
1596 if (exists $ref->{A}->{B}->[$ix]) { }
1597 if (exists $hash{A}{B}[$ix]) { }
1598
afebc493
GS
1599 if (exists &{$ref->{A}{B}{$key}}) { }
1600
01020589
GS
1601Although the deepest nested array or hash will not spring into existence
1602just because its existence was tested, any intervening ones will.
61eff3bc 1603Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
01020589
GS
1604into existence due to the existence test for the $key element above.
1605This happens anywhere the arrow operator is used, including even:
5a964f20 1606
2b5ab1e7
TC
1607 undef $ref;
1608 if (exists $ref->{"Some key"}) { }
1609 print $ref; # prints HASH(0x80d3d5c)
1610
1611This surprising autovivification in what does not at first--or even
1612second--glance appear to be an lvalue context may be fixed in a future
5a964f20 1613release.
a0d0e21e 1614
afebc493
GS
1615Use of a subroutine call, rather than a subroutine name, as an argument
1616to exists() is an error.
1617
1618 exists &sub; # OK
1619 exists &sub(); # Error
1620
a0d0e21e
LW
1621=item exit EXPR
1622
2b5ab1e7 1623Evaluates EXPR and exits immediately with that value. Example:
a0d0e21e
LW
1624
1625 $ans = <STDIN>;
1626 exit 0 if $ans =~ /^[Xx]/;
1627
19799a22 1628See also C<die>. If EXPR is omitted, exits with C<0> status. The only
2b5ab1e7
TC
1629universally recognized values for EXPR are C<0> for success and C<1>
1630for error; other values are subject to interpretation depending on the
1631environment in which the Perl program is running. For example, exiting
163269 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1633the mailer to return the item undelivered, but that's not true everywhere.
a0d0e21e 1634
19799a22
GS
1635Don't use C<exit> to abort a subroutine if there's any chance that
1636someone might want to trap whatever error happened. Use C<die> instead,
1637which can be trapped by an C<eval>.
28757baa 1638
19799a22 1639The exit() function does not always exit immediately. It calls any
2b5ab1e7 1640defined C<END> routines first, but these C<END> routines may not
19799a22 1641themselves abort the exit. Likewise any object destructors that need to
2b5ab1e7
TC
1642be called are called before the real exit. If this is a problem, you
1643can call C<POSIX:_exit($status)> to avoid END and destructor processing.
87275199 1644See L<perlmod> for details.
5a964f20 1645
a0d0e21e
LW
1646=item exp EXPR
1647
54310121 1648=item exp
bbce6d69 1649
b76cc8ba 1650Returns I<e> (the natural logarithm base) to the power of EXPR.
a0d0e21e
LW
1651If EXPR is omitted, gives C<exp($_)>.
1652
1653=item fcntl FILEHANDLE,FUNCTION,SCALAR
1654
f86cebdf 1655Implements the fcntl(2) function. You'll probably have to say
a0d0e21e
LW
1656
1657 use Fcntl;
1658
0ade1984 1659first to get the correct constant definitions. Argument processing and
b76cc8ba 1660value return works just like C<ioctl> below.
a0d0e21e
LW
1661For example:
1662
1663 use Fcntl;
5a964f20
TC
1664 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1665 or die "can't fcntl F_GETFL: $!";
1666
554ad1fc 1667You don't have to check for C<defined> on the return from C<fcntl>.
951ba7fe
GS
1668Like C<ioctl>, it maps a C<0> return from the system call into
1669C<"0 but true"> in Perl. This string is true in boolean context and C<0>
2b5ab1e7
TC
1670in numeric context. It is also exempt from the normal B<-w> warnings
1671on improper numeric conversions.
5a964f20 1672
19799a22 1673Note that C<fcntl> will produce a fatal error if used on a machine that
2b5ab1e7
TC
1674doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1675manpage to learn what functions are available on your system.
a0d0e21e 1676
be2f7487 1677Here's an example of setting a filehandle named C<REMOTE> to be
1678non-blocking at the system level. You'll have to negotiate C<$|>
1679on your own, though.
1680
1681 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
1682
1683 $flags = fcntl(REMOTE, F_GETFL, 0)
1684 or die "Can't get flags for the socket: $!\n";
1685
1686 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
1687 or die "Can't set flags for the socket: $!\n";
1688
a0d0e21e
LW
1689=item fileno FILEHANDLE
1690
2b5ab1e7
TC
1691Returns the file descriptor for a filehandle, or undefined if the
1692filehandle is not open. This is mainly useful for constructing
19799a22 1693bitmaps for C<select> and low-level POSIX tty-handling operations.
2b5ab1e7
TC
1694If FILEHANDLE is an expression, the value is taken as an indirect
1695filehandle, generally its name.
5a964f20 1696
b76cc8ba 1697You can use this to find out whether two handles refer to the
5a964f20
TC
1698same underlying descriptor:
1699
1700 if (fileno(THIS) == fileno(THAT)) {
1701 print "THIS and THAT are dups\n";
b76cc8ba
NIS
1702 }
1703
1704(Filehandles connected to memory objects via new features of C<open> may
1705return undefined even though they are open.)
1706
a0d0e21e
LW
1707
1708=item flock FILEHANDLE,OPERATION
1709
19799a22
GS
1710Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1711for success, false on failure. Produces a fatal error if used on a
2b5ab1e7 1712machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
19799a22 1713C<flock> is Perl's portable file locking interface, although it locks
2b5ab1e7
TC
1714only entire files, not records.
1715
1716Two potentially non-obvious but traditional C<flock> semantics are
1717that it waits indefinitely until the lock is granted, and that its locks
1718B<merely advisory>. Such discretionary locks are more flexible, but offer
19799a22
GS
1719fewer guarantees. This means that files locked with C<flock> may be
1720modified by programs that do not also use C<flock>. See L<perlport>,
2b5ab1e7
TC
1721your port's specific documentation, or your system-specific local manpages
1722for details. It's best to assume traditional behavior if you're writing
1723portable programs. (But if you're not, you should as always feel perfectly
1724free to write for your own system's idiosyncrasies (sometimes called
1725"features"). Slavish adherence to portability concerns shouldn't get
1726in the way of your getting your job done.)
a3cb178b 1727
8ebc5c01
PP
1728OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1729LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
ea3105be 1730you can use the symbolic names if you import them from the Fcntl module,
68dc0745
PP
1731either individually, or as a group using the ':flock' tag. LOCK_SH
1732requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
ea3105be
GS
1733releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1734LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
68dc0745
PP
1735waiting for the lock (check the return status to see if you got it).
1736
2b5ab1e7
TC
1737To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1738before locking or unlocking it.
8ebc5c01 1739
f86cebdf 1740Note that the emulation built with lockf(3) doesn't provide shared
8ebc5c01 1741locks, and it requires that FILEHANDLE be open with write intent. These
2b5ab1e7 1742are the semantics that lockf(3) implements. Most if not all systems
f86cebdf 1743implement lockf(3) in terms of fcntl(2) locking, though, so the
8ebc5c01
PP
1744differing semantics shouldn't bite too many people.
1745
becacb53
TM
1746Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
1747be open with read intent to use LOCK_SH and requires that it be open
1748with write intent to use LOCK_EX.
1749
19799a22
GS
1750Note also that some versions of C<flock> cannot lock things over the
1751network; you would need to use the more system-specific C<fcntl> for
f86cebdf
GS
1752that. If you like you can force Perl to ignore your system's flock(2)
1753function, and so provide its own fcntl(2)-based emulation, by passing
8ebc5c01
PP
1754the switch C<-Ud_flock> to the F<Configure> program when you configure
1755perl.
4633a7c4
LW
1756
1757Here's a mailbox appender for BSD systems.
a0d0e21e 1758
7e1af8bc 1759 use Fcntl ':flock'; # import LOCK_* constants
a0d0e21e
LW
1760
1761 sub lock {
7e1af8bc 1762 flock(MBOX,LOCK_EX);
a0d0e21e
LW
1763 # and, in case someone appended
1764 # while we were waiting...
1765 seek(MBOX, 0, 2);
1766 }
1767
1768 sub unlock {
7e1af8bc 1769 flock(MBOX,LOCK_UN);
a0d0e21e
LW
1770 }
1771
1772 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1773 or die "Can't open mailbox: $!";
1774
1775 lock();
1776 print MBOX $msg,"\n\n";
1777 unlock();
1778
2b5ab1e7
TC
1779On systems that support a real flock(), locks are inherited across fork()
1780calls, whereas those that must resort to the more capricious fcntl()
1781function lose the locks, making it harder to write servers.
1782
cb1a09d0 1783See also L<DB_File> for other flock() examples.
a0d0e21e
LW
1784
1785=item fork
1786
2b5ab1e7
TC
1787Does a fork(2) system call to create a new process running the
1788same program at the same point. It returns the child pid to the
1789parent process, C<0> to the child process, or C<undef> if the fork is
1790unsuccessful. File descriptors (and sometimes locks on those descriptors)
1791are shared, while everything else is copied. On most systems supporting
1792fork(), great care has gone into making it extremely efficient (for
1793example, using copy-on-write technology on data pages), making it the
1794dominant paradigm for multitasking over the last few decades.
5a964f20 1795
0f897271
GS
1796Beginning with v5.6.0, Perl will attempt to flush all files opened for
1797output before forking the child process, but this may not be supported
1798on some platforms (see L<perlport>). To be safe, you may need to set
1799C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1800C<IO::Handle> on any open handles in order to avoid duplicate output.
a0d0e21e 1801
19799a22 1802If you C<fork> without ever waiting on your children, you will
2b5ab1e7
TC
1803accumulate zombies. On some systems, you can avoid this by setting
1804C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1805forking and reaping moribund children.
cb1a09d0 1806
28757baa
PP
1807Note that if your forked child inherits system file descriptors like
1808STDIN and STDOUT that are actually connected by a pipe or socket, even
2b5ab1e7 1809if you exit, then the remote server (such as, say, a CGI script or a
19799a22 1810backgrounded job launched from a remote shell) won't think you're done.
2b5ab1e7 1811You should reopen those to F</dev/null> if it's any issue.
28757baa 1812
cb1a09d0
AD
1813=item format
1814
19799a22 1815Declare a picture format for use by the C<write> function. For
cb1a09d0
AD
1816example:
1817
54310121 1818 format Something =
cb1a09d0
AD
1819 Test: @<<<<<<<< @||||| @>>>>>
1820 $str, $%, '$' . int($num)
1821 .
1822
1823 $str = "widget";
184e9718 1824 $num = $cost/$quantity;
cb1a09d0
AD
1825 $~ = 'Something';
1826 write;
1827
1828See L<perlform> for many details and examples.
1829
8903cb82 1830=item formline PICTURE,LIST
a0d0e21e 1831
5a964f20 1832This is an internal function used by C<format>s, though you may call it,
a0d0e21e
LW
1833too. It formats (see L<perlform>) a list of values according to the
1834contents of PICTURE, placing the output into the format output
7660c0ab 1835accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
19799a22 1836Eventually, when a C<write> is done, the contents of
a0d0e21e 1837C<$^A> are written to some filehandle, but you could also read C<$^A>
7660c0ab 1838yourself and then set C<$^A> back to C<"">. Note that a format typically
19799a22 1839does one C<formline> per line of form, but the C<formline> function itself
748a9306 1840doesn't care how many newlines are embedded in the PICTURE. This means
4633a7c4 1841that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
748a9306
LW
1842You may therefore need to use multiple formlines to implement a single
1843record format, just like the format compiler.
1844
19799a22 1845Be careful if you put double quotes around the picture, because an C<@>
748a9306 1846character may be taken to mean the beginning of an array name.
19799a22 1847C<formline> always returns true. See L<perlform> for other examples.
a0d0e21e
LW
1848
1849=item getc FILEHANDLE
1850
1851=item getc
1852
1853Returns the next character from the input file attached to FILEHANDLE,
b5fe5ca2
SR
1854or the undefined value at end of file, or if there was an error (in
1855the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
1856STDIN. This is not particularly efficient. However, it cannot be
1857used by itself to fetch single characters without waiting for the user
1858to hit enter. For that, try something more like:
4633a7c4
LW
1859
1860 if ($BSD_STYLE) {
1861 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1862 }
1863 else {
54310121 1864 system "stty", '-icanon', 'eol', "\001";
4633a7c4
LW
1865 }
1866
1867 $key = getc(STDIN);
1868
1869 if ($BSD_STYLE) {
1870 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1871 }
1872 else {
5f05dabc 1873 system "stty", 'icanon', 'eol', '^@'; # ASCII null
4633a7c4
LW
1874 }
1875 print "\n";
1876
54310121
PP
1877Determination of whether $BSD_STYLE should be set
1878is left as an exercise to the reader.
cb1a09d0 1879
19799a22 1880The C<POSIX::getattr> function can do this more portably on
2b5ab1e7
TC
1881systems purporting POSIX compliance. See also the C<Term::ReadKey>
1882module from your nearest CPAN site; details on CPAN can be found on
1883L<perlmodlib/CPAN>.
a0d0e21e
LW
1884
1885=item getlogin
1886
5a964f20
TC
1887Implements the C library function of the same name, which on most
1888systems returns the current login from F</etc/utmp>, if any. If null,
19799a22 1889use C<getpwuid>.
a0d0e21e 1890
f86702cc 1891 $login = getlogin || getpwuid($<) || "Kilroy";
a0d0e21e 1892
19799a22
GS
1893Do not consider C<getlogin> for authentication: it is not as
1894secure as C<getpwuid>.
4633a7c4 1895
a0d0e21e
LW
1896=item getpeername SOCKET
1897
1898Returns the packed sockaddr address of other end of the SOCKET connection.
1899
4633a7c4
LW
1900 use Socket;
1901 $hersockaddr = getpeername(SOCK);
19799a22 1902 ($port, $iaddr) = sockaddr_in($hersockaddr);
4633a7c4
LW
1903 $herhostname = gethostbyaddr($iaddr, AF_INET);
1904 $herstraddr = inet_ntoa($iaddr);
a0d0e21e
LW
1905
1906=item getpgrp PID
1907
47e29363 1908Returns the current process group for the specified PID. Use
7660c0ab 1909a PID of C<0> to get the current process group for the
4633a7c4 1910current process. Will raise an exception if used on a machine that
f86cebdf 1911doesn't implement getpgrp(2). If PID is omitted, returns process
19799a22 1912group of current process. Note that the POSIX version of C<getpgrp>
7660c0ab 1913does not accept a PID argument, so only C<PID==0> is truly portable.
a0d0e21e
LW
1914
1915=item getppid
1916
1917Returns the process id of the parent process.
1918
4d76a344
RGS
1919Note for Linux users: on Linux, the C functions C<getpid()> and
1920C<getppid()> return different values from different threads. In order to
1921be portable, this behavior is not reflected by the perl-level function
1922C<getppid()>, that returns a consistent value across threads. If you want
e3256f86
RGS
1923to call the underlying C<getppid()>, you may use the CPAN module
1924C<Linux::Pid>.
4d76a344 1925
a0d0e21e
LW
1926=item getpriority WHICH,WHO
1927
4633a7c4
LW
1928Returns the current priority for a process, a process group, or a user.
1929(See L<getpriority(2)>.) Will raise a fatal exception if used on a
f86cebdf 1930machine that doesn't implement getpriority(2).
a0d0e21e
LW
1931
1932=item getpwnam NAME
1933
1934=item getgrnam NAME
1935
1936=item gethostbyname NAME
1937
1938=item getnetbyname NAME
1939
1940=item getprotobyname NAME
1941
1942=item getpwuid UID
1943
1944=item getgrgid GID
1945
1946=item getservbyname NAME,PROTO
1947
1948=item gethostbyaddr ADDR,ADDRTYPE
1949
1950=item getnetbyaddr ADDR,ADDRTYPE
1951
1952=item getprotobynumber NUMBER
1953
1954=item getservbyport PORT,PROTO
1955
1956=item getpwent
1957
1958=item getgrent
1959
1960=item gethostent
1961
1962=item getnetent
1963
1964=item getprotoent
1965
1966=item getservent
1967
1968=item setpwent
1969
1970=item setgrent
1971
1972=item sethostent STAYOPEN
1973
1974=item setnetent STAYOPEN
1975
1976=item setprotoent STAYOPEN
1977
1978=item setservent STAYOPEN
1979
1980=item endpwent
1981
1982=item endgrent
1983
1984=item endhostent
1985
1986=item endnetent
1987
1988=item endprotoent
1989
1990=item endservent
1991
1992These routines perform the same functions as their counterparts in the
5a964f20 1993system library. In list context, the return values from the
a0d0e21e
LW
1994various get routines are as follows:
1995
1996 ($name,$passwd,$uid,$gid,
6ee623d5 1997 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
a0d0e21e
LW
1998 ($name,$passwd,$gid,$members) = getgr*
1999 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
2000 ($name,$aliases,$addrtype,$net) = getnet*
2001 ($name,$aliases,$proto) = getproto*
2002 ($name,$aliases,$port,$proto) = getserv*
2003
2004(If the entry doesn't exist you get a null list.)
2005
4602f195
JH
2006The exact meaning of the $gcos field varies but it usually contains
2007the real name of the user (as opposed to the login name) and other
2008information pertaining to the user. Beware, however, that in many
2009system users are able to change this information and therefore it
106325ad 2010cannot be trusted and therefore the $gcos is tainted (see
2959b6e3
JH
2011L<perlsec>). The $passwd and $shell, user's encrypted password and
2012login shell, are also tainted, because of the same reason.
4602f195 2013
5a964f20 2014In scalar context, you get the name, unless the function was a
a0d0e21e
LW
2015lookup by name, in which case you get the other thing, whatever it is.
2016(If the entry doesn't exist you get the undefined value.) For example:
2017
5a964f20
TC
2018 $uid = getpwnam($name);
2019 $name = getpwuid($num);
2020 $name = getpwent();
2021 $gid = getgrnam($name);
08a33e13 2022 $name = getgrgid($num);
5a964f20
TC
2023 $name = getgrent();
2024 #etc.
a0d0e21e 2025
4602f195
JH
2026In I<getpw*()> the fields $quota, $comment, and $expire are special
2027cases in the sense that in many systems they are unsupported. If the
2028$quota is unsupported, it is an empty scalar. If it is supported, it
2029usually encodes the disk quota. If the $comment field is unsupported,
2030it is an empty scalar. If it is supported it usually encodes some
2031administrative comment about the user. In some systems the $quota
2032field may be $change or $age, fields that have to do with password
2033aging. In some systems the $comment field may be $class. The $expire
2034field, if present, encodes the expiration period of the account or the
2035password. For the availability and the exact meaning of these fields
2036in your system, please consult your getpwnam(3) documentation and your
2037F<pwd.h> file. You can also find out from within Perl what your
2038$quota and $comment fields mean and whether you have the $expire field
2039by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2040C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
2041files are only supported if your vendor has implemented them in the
2042intuitive fashion that calling the regular C library routines gets the
5d3a0a3b
GS
2043shadow versions if you're running under privilege or if there exists
2044the shadow(3) functions as found in System V ( this includes Solaris
2045and Linux.) Those systems which implement a proprietary shadow password
2046facility are unlikely to be supported.
6ee623d5 2047
19799a22 2048The $members value returned by I<getgr*()> is a space separated list of
a0d0e21e
LW
2049the login names of the members of the group.
2050
2051For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2052C, it will be returned to you via C<$?> if the function call fails. The
7660c0ab 2053C<@addrs> value returned by a successful call is a list of the raw
a0d0e21e
LW
2054addresses returned by the corresponding system library call. In the
2055Internet domain, each address is four bytes long and you can unpack it
2056by saying something like:
2057
2058 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
2059
2b5ab1e7
TC
2060The Socket library makes this slightly easier:
2061
2062 use Socket;
2063 $iaddr = inet_aton("127.1"); # or whatever address
2064 $name = gethostbyaddr($iaddr, AF_INET);
2065
2066 # or going the other way
19799a22 2067 $straddr = inet_ntoa($iaddr);
2b5ab1e7 2068
19799a22
GS
2069If you get tired of remembering which element of the return list
2070contains which return value, by-name interfaces are provided
2071in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2072C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2073and C<User::grent>. These override the normal built-ins, supplying
2074versions that return objects with the appropriate names
2075for each field. For example:
5a964f20
TC
2076
2077 use File::stat;
2078 use User::pwent;
2079 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2080
b76cc8ba
NIS
2081Even though it looks like they're the same method calls (uid),
2082they aren't, because a C<File::stat> object is different from
19799a22 2083a C<User::pwent> object.
5a964f20 2084
a0d0e21e
LW
2085=item getsockname SOCKET
2086
19799a22
GS
2087Returns the packed sockaddr address of this end of the SOCKET connection,
2088in case you don't know the address because you have several different
2089IPs that the connection might have come in on.
a0d0e21e 2090
4633a7c4
LW
2091 use Socket;
2092 $mysockaddr = getsockname(SOCK);
19799a22 2093 ($port, $myaddr) = sockaddr_in($mysockaddr);
b76cc8ba 2094 printf "Connect to %s [%s]\n",
19799a22
GS
2095 scalar gethostbyaddr($myaddr, AF_INET),
2096 inet_ntoa($myaddr);
a0d0e21e
LW
2097
2098=item getsockopt SOCKET,LEVEL,OPTNAME
2099
5a964f20 2100Returns the socket option requested, or undef if there is an error.
a0d0e21e
LW
2101
2102=item glob EXPR
2103
0a753a76
PP
2104=item glob
2105
d9a9d457
JL
2106In list context, returns a (possibly empty) list of filename expansions on
2107the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
2108scalar context, glob iterates through such filename expansions, returning
2109undef when the list is exhausted. This is the internal function
2110implementing the C<< <*.c> >> operator, but you can use it directly. If
2111EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2112more detail in L<perlop/"I/O Operators">.
a0d0e21e 2113
3a4b19e4
GS
2114Beginning with v5.6.0, this operator is implemented using the standard
2115C<File::Glob> extension. See L<File::Glob> for details.
2116
a0d0e21e
LW
2117=item gmtime EXPR
2118
d1be9408 2119Converts a time as returned by the time function to an 8-element list
54310121 2120with the time localized for the standard Greenwich time zone.
4633a7c4 2121Typically used as follows:
a0d0e21e 2122
b76cc8ba 2123 # 0 1 2 3 4 5 6 7
48a26b3a 2124 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday) =
a0d0e21e
LW
2125 gmtime(time);
2126
48a26b3a
GS
2127All list elements are numeric, and come straight out of the C `struct
2128tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2129specified time. $mday is the day of the month, and $mon is the month
2130itself, in the range C<0..11> with 0 indicating January and 11
2131indicating December. $year is the number of years since 1900. That
2132is, $year is C<123> in year 2023. $wday is the day of the week, with
21330 indicating Sunday and 3 indicating Wednesday. $yday is the day of
b76cc8ba 2134the year, in the range C<0..364> (or C<0..365> in leap years.)
48a26b3a
GS
2135
2136Note that the $year element is I<not> simply the last two digits of
2137the year. If you assume it is, then you create non-Y2K-compliant
2138programs--and you wouldn't want to do that, would you?
2f9daede 2139
abd75f24
GS
2140The proper way to get a complete 4-digit year is simply:
2141
2142 $year += 1900;
2143
2144And to get the last two digits of the year (e.g., '01' in 2001) do:
2145
2146 $year = sprintf("%02d", $year % 100);
2147
48a26b3a 2148If EXPR is omitted, C<gmtime()> uses the current time (C<gmtime(time)>).
a0d0e21e 2149
48a26b3a 2150In scalar context, C<gmtime()> returns the ctime(3) value:
0a753a76
PP
2151
2152 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
2153
fe86afc2
NC
2154If you need local time instead of GMT use the L</localtime> builtin.
2155See also the C<timegm> function provided by the C<Time::Local> module,
2156and the strftime(3) and mktime(3) functions available via the L<POSIX> module.
7660c0ab 2157
fe86afc2
NC
2158This scalar value is B<not> locale dependent (see L<perllocale>), but is
2159instead a Perl builtin. To get somewhat similar but locale dependent date
2160strings, see the example in L</localtime>.
0a753a76 2161
a0d0e21e
LW
2162=item goto LABEL
2163
748a9306
LW
2164=item goto EXPR
2165
a0d0e21e
LW
2166=item goto &NAME
2167
7660c0ab 2168The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
a0d0e21e 2169execution there. It may not be used to go into any construct that
7660c0ab 2170requires initialization, such as a subroutine or a C<foreach> loop. It
0a753a76 2171also can't be used to go into a construct that is optimized away,
19799a22 2172or to get out of a block or subroutine given to C<sort>.
0a753a76 2173It can be used to go almost anywhere else within the dynamic scope,
a0d0e21e 2174including out of subroutines, but it's usually better to use some other
19799a22 2175construct such as C<last> or C<die>. The author of Perl has never felt the
7660c0ab 2176need to use this form of C<goto> (in Perl, that is--C is another matter).
1b6921cb
BT
2177(The difference being that C does not offer named loops combined with
2178loop control. Perl does, and this replaces most structured uses of C<goto>
2179in other languages.)
a0d0e21e 2180
7660c0ab
A
2181The C<goto-EXPR> form expects a label name, whose scope will be resolved
2182dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
748a9306
LW
2183necessarily recommended if you're optimizing for maintainability:
2184
2185 goto ("FOO", "BAR", "GLARCH")[$i];
2186
1b6921cb
BT
2187The C<goto-&NAME> form is quite different from the other forms of
2188C<goto>. In fact, it isn't a goto in the normal sense at all, and
2189doesn't have the stigma associated with other gotos. Instead, it
2190exits the current subroutine (losing any changes set by local()) and
2191immediately calls in its place the named subroutine using the current
2192value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2193load another subroutine and then pretend that the other subroutine had
2194been called in the first place (except that any modifications to C<@_>
6cb9131c
GS
2195in the current subroutine are propagated to the other subroutine.)
2196After the C<goto>, not even C<caller> will be able to tell that this
2197routine was called first.
2198
2199NAME needn't be the name of a subroutine; it can be a scalar variable
2200containing a code reference, or a block which evaluates to a code
2201reference.
a0d0e21e
LW
2202
2203=item grep BLOCK LIST
2204
2205=item grep EXPR,LIST
2206
2b5ab1e7
TC
2207This is similar in spirit to, but not the same as, grep(1) and its
2208relatives. In particular, it is not limited to using regular expressions.
2f9daede 2209
a0d0e21e 2210Evaluates the BLOCK or EXPR for each element of LIST (locally setting
7660c0ab 2211C<$_> to each element) and returns the list value consisting of those
19799a22
GS
2212elements for which the expression evaluated to true. In scalar
2213context, returns the number of times the expression was true.
a0d0e21e
LW
2214
2215 @foo = grep(!/^#/, @bar); # weed out comments
2216
2217or equivalently,
2218
2219 @foo = grep {!/^#/} @bar; # weed out comments
2220
be3174d2
GS
2221Note that C<$_> is an alias to the list value, so it can be used to
2222modify the elements of the LIST. While this is useful and supported,
2223it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
2224Similarly, grep returns aliases into the original list, much as a for
2225loop's index variable aliases the list elements. That is, modifying an
19799a22
GS
2226element of a list returned by grep (for example, in a C<foreach>, C<map>
2227or another C<grep>) actually modifies the element in the original list.
2b5ab1e7 2228This is usually something to be avoided when writing clear code.
a0d0e21e 2229
a4fb8298
RGS
2230If C<$_> is lexical in the scope where the C<grep> appears (because it has
2231been declared with C<my $_>) then, in addition the be locally aliased to
2232the list elements, C<$_> keeps being lexical inside the block; i.e. it
2233can't be seen from the outside, avoiding any potential side-effects.
2234
19799a22 2235See also L</map> for a list composed of the results of the BLOCK or EXPR.
38325410 2236
a0d0e21e
LW
2237=item hex EXPR
2238
54310121 2239=item hex
bbce6d69 2240
2b5ab1e7
TC
2241Interprets EXPR as a hex string and returns the corresponding value.
2242(To convert strings that might start with either 0, 0x, or 0b, see
2243L</oct>.) If EXPR is omitted, uses C<$_>.
2f9daede
TPG
2244
2245 print hex '0xAf'; # prints '175'
2246 print hex 'aF'; # same
a0d0e21e 2247
19799a22 2248Hex strings may only represent integers. Strings that would cause
53305cf1
NC
2249integer overflow trigger a warning. Leading whitespace is not stripped,
2250unlike oct().
19799a22 2251
a0d0e21e
LW
2252=item import
2253
19799a22 2254There is no builtin C<import> function. It is just an ordinary
4633a7c4 2255method (subroutine) defined (or inherited) by modules that wish to export
19799a22 2256names to another module. The C<use> function calls the C<import> method
cea6626f 2257for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
a0d0e21e
LW
2258
2259=item index STR,SUBSTR,POSITION
2260
2261=item index STR,SUBSTR
2262
2b5ab1e7
TC
2263The index function searches for one string within another, but without
2264the wildcard-like behavior of a full regular-expression pattern match.
2265It returns the position of the first occurrence of SUBSTR in STR at
2266or after POSITION. If POSITION is omitted, starts searching from the
2267beginning of the string. The return value is based at C<0> (or whatever
2268you've set the C<$[> variable to--but don't do that). If the substring
2269is not found, returns one less than the base, ordinarily C<-1>.
a0d0e21e
LW
2270
2271=item int EXPR
2272
54310121 2273=item int
bbce6d69 2274
7660c0ab 2275Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2b5ab1e7
TC
2276You should not use this function for rounding: one because it truncates
2277towards C<0>, and two because machine representations of floating point
2278numbers can sometimes produce counterintuitive results. For example,
2279C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2280because it's really more like -268.99999999999994315658 instead. Usually,
19799a22 2281the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2b5ab1e7 2282functions will serve you better than will int().
a0d0e21e
LW
2283
2284=item ioctl FILEHANDLE,FUNCTION,SCALAR
2285
2b5ab1e7 2286Implements the ioctl(2) function. You'll probably first have to say
a0d0e21e 2287
4633a7c4 2288 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
a0d0e21e 2289
2b5ab1e7 2290to get the correct function definitions. If F<ioctl.ph> doesn't
a0d0e21e 2291exist or doesn't have the correct definitions you'll have to roll your
61eff3bc 2292own, based on your C header files such as F<< <sys/ioctl.h> >>.
5a964f20 2293(There is a Perl script called B<h2ph> that comes with the Perl kit that
54310121 2294may help you in this, but it's nontrivial.) SCALAR will be read and/or
4633a7c4 2295written depending on the FUNCTION--a pointer to the string value of SCALAR
19799a22 2296will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
4633a7c4
LW
2297has no string value but does have a numeric value, that value will be
2298passed rather than a pointer to the string value. To guarantee this to be
19799a22
GS
2299true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2300functions may be needed to manipulate the values of structures used by
b76cc8ba 2301C<ioctl>.
a0d0e21e 2302
19799a22 2303The return value of C<ioctl> (and C<fcntl>) is as follows:
a0d0e21e
LW
2304
2305 if OS returns: then Perl returns:
2306 -1 undefined value
2307 0 string "0 but true"
2308 anything else that number
2309
19799a22 2310Thus Perl returns true on success and false on failure, yet you can
a0d0e21e
LW
2311still easily determine the actual value returned by the operating
2312system:
2313
2b5ab1e7 2314 $retval = ioctl(...) || -1;
a0d0e21e
LW
2315 printf "System returned %d\n", $retval;
2316
be2f7487 2317The special string C<"0 but true"> is exempt from B<-w> complaints
5a964f20
TC
2318about improper numeric conversions.
2319
a0d0e21e
LW
2320=item join EXPR,LIST
2321
2b5ab1e7
TC
2322Joins the separate strings of LIST into a single string with fields
2323separated by the value of EXPR, and returns that new string. Example:
a0d0e21e 2324
2b5ab1e7 2325 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
a0d0e21e 2326
eb6e2d6f
GS
2327Beware that unlike C<split>, C<join> doesn't take a pattern as its
2328first argument. Compare L</split>.
a0d0e21e 2329
aa689395
PP
2330=item keys HASH
2331
504f80c1
JH
2332Returns a list consisting of all the keys of the named hash.
2333(In scalar context, returns the number of keys.)
2334
2335The keys are returned in an apparently random order. The actual
2336random order is subject to change in future versions of perl, but it
2337is guaranteed to be the same order as either the C<values> or C<each>
4546b9e6
JH
2338function produces (given that the hash has not been modified). Since
2339Perl 5.8.1 the ordering is different even between different runs of
2340Perl for security reasons (see L<perlsec/"Algorithmic Complexity
d6df3700 2341Attacks">).
504f80c1
JH
2342
2343As a side effect, calling keys() resets the HASH's internal iterator,
2f65b2f0
RGS
2344see L</each>. (In particular, calling keys() in void context resets
2345the iterator with no other overhead.)
a0d0e21e 2346
aa689395 2347Here is yet another way to print your environment:
a0d0e21e
LW
2348
2349 @keys = keys %ENV;
2350 @values = values %ENV;
b76cc8ba 2351 while (@keys) {
a0d0e21e
LW
2352 print pop(@keys), '=', pop(@values), "\n";
2353 }
2354
2355or how about sorted by key:
2356
2357 foreach $key (sort(keys %ENV)) {
2358 print $key, '=', $ENV{$key}, "\n";
2359 }
2360
8ea1e5d4
GS
2361The returned values are copies of the original keys in the hash, so
2362modifying them will not affect the original hash. Compare L</values>.
2363
19799a22 2364To sort a hash by value, you'll need to use a C<sort> function.
aa689395 2365Here's a descending numeric sort of a hash by its values:
4633a7c4 2366
5a964f20 2367 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
4633a7c4
LW
2368 printf "%4d %s\n", $hash{$key}, $key;
2369 }
2370
19799a22 2371As an lvalue C<keys> allows you to increase the number of hash buckets
aa689395
PP
2372allocated for the given hash. This can gain you a measure of efficiency if
2373you know the hash is going to get big. (This is similar to pre-extending
2374an array by assigning a larger number to $#array.) If you say
55497cff
PP
2375
2376 keys %hash = 200;
2377
ab192400
GS
2378then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2379in fact, since it rounds up to the next power of two. These
55497cff
PP
2380buckets will be retained even if you do C<%hash = ()>, use C<undef
2381%hash> if you want to free the storage while C<%hash> is still in scope.
2382You can't shrink the number of buckets allocated for the hash using
19799a22 2383C<keys> in this way (but you needn't worry about doing this by accident,
55497cff
PP
2384as trying has no effect).
2385
19799a22 2386See also C<each>, C<values> and C<sort>.
ab192400 2387
b350dd2f 2388=item kill SIGNAL, LIST
a0d0e21e 2389
b350dd2f 2390Sends a signal to a list of processes. Returns the number of
517db077
GS
2391processes successfully signaled (which is not necessarily the
2392same as the number actually killed).
a0d0e21e
LW
2393
2394 $cnt = kill 1, $child1, $child2;
2395 kill 9, @goners;
2396
b350dd2f 2397If SIGNAL is zero, no signal is sent to the process. This is a
1e9c1022 2398useful way to check that a child process is alive and hasn't changed
b350dd2f
GS
2399its UID. See L<perlport> for notes on the portability of this
2400construct.
2401
2402Unlike in the shell, if SIGNAL is negative, it kills
4633a7c4
LW
2403process groups instead of processes. (On System V, a negative I<PROCESS>
2404number will also kill process groups, but that's not portable.) That
2405means you usually want to use positive not negative signals. You may also
1e9c1022
JL
2406use a signal name in quotes.
2407
2408See L<perlipc/"Signals"> for more details.
a0d0e21e
LW
2409
2410=item last LABEL
2411
2412=item last
2413
2414The C<last> command is like the C<break> statement in C (as used in
2415loops); it immediately exits the loop in question. If the LABEL is
2416omitted, the command refers to the innermost enclosing loop. The
2417C<continue> block, if any, is not executed:
2418
4633a7c4
LW
2419 LINE: while (<STDIN>) {
2420 last LINE if /^$/; # exit when done with header
5a964f20 2421 #...
a0d0e21e
LW
2422 }
2423
4968c1e4 2424C<last> cannot be used to exit a block which returns a value such as
2b5ab1e7
TC
2425C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2426a grep() or map() operation.
4968c1e4 2427
6c1372ed
GS
2428Note that a block by itself is semantically identical to a loop
2429that executes once. Thus C<last> can be used to effect an early
2430exit out of such a block.
2431
98293880
JH
2432See also L</continue> for an illustration of how C<last>, C<next>, and
2433C<redo> work.
1d2dff63 2434
a0d0e21e
LW
2435=item lc EXPR
2436
54310121 2437=item lc
bbce6d69 2438
d1be9408 2439Returns a lowercased version of EXPR. This is the internal function
ad0029c4
JH
2440implementing the C<\L> escape in double-quoted strings. Respects
2441current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
983ffd37 2442and L<perlunicode> for more details about locale and Unicode support.
a0d0e21e 2443
7660c0ab 2444If EXPR is omitted, uses C<$_>.
bbce6d69 2445
a0d0e21e
LW
2446=item lcfirst EXPR
2447
54310121 2448=item lcfirst
bbce6d69 2449
ad0029c4
JH
2450Returns the value of EXPR with the first character lowercased. This
2451is the internal function implementing the C<\l> escape in
2452double-quoted strings. Respects current LC_CTYPE locale if C<use
983ffd37
JH
2453locale> in force. See L<perllocale> and L<perlunicode> for more
2454details about locale and Unicode support.
a0d0e21e 2455
7660c0ab 2456If EXPR is omitted, uses C<$_>.
bbce6d69 2457
a0d0e21e
LW
2458=item length EXPR
2459
54310121 2460=item length
bbce6d69 2461
974da8e5 2462Returns the length in I<characters> of the value of EXPR. If EXPR is
b76cc8ba 2463omitted, returns length of C<$_>. Note that this cannot be used on
2b5ab1e7
TC
2464an entire array or hash to find out how many elements these have.
2465For that, use C<scalar @array> and C<scalar keys %hash> respectively.
a0d0e21e 2466
974da8e5
JH
2467Note the I<characters>: if the EXPR is in Unicode, you will get the
2468number of characters, not the number of bytes. To get the length
2469in bytes, use C<do { use bytes; length(EXPR) }>, see L<bytes>.
2470
a0d0e21e
LW
2471=item link OLDFILE,NEWFILE
2472
19799a22 2473Creates a new filename linked to the old filename. Returns true for
b76cc8ba 2474success, false otherwise.
a0d0e21e
LW
2475
2476=item listen SOCKET,QUEUESIZE
2477
19799a22 2478Does the same thing that the listen system call does. Returns true if
b76cc8ba 2479it succeeded, false otherwise. See the example in
cea6626f 2480L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
2481
2482=item local EXPR
2483
19799a22 2484You really probably want to be using C<my> instead, because C<local> isn't
b76cc8ba 2485what most people think of as "local". See
13a2d996 2486L<perlsub/"Private Variables via my()"> for details.
2b5ab1e7 2487
5a964f20
TC
2488A local modifies the listed variables to be local to the enclosing
2489block, file, or eval. If more than one value is listed, the list must
2490be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2491for details, including issues with tied arrays and hashes.
a0d0e21e 2492
a0d0e21e
LW
2493=item localtime EXPR
2494
19799a22 2495Converts a time as returned by the time function to a 9-element list
5f05dabc 2496with the time analyzed for the local time zone. Typically used as
a0d0e21e
LW
2497follows:
2498
54310121 2499 # 0 1 2 3 4 5 6 7 8
a0d0e21e
LW
2500 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2501 localtime(time);
2502
48a26b3a
GS
2503All list elements are numeric, and come straight out of the C `struct
2504tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2505specified time. $mday is the day of the month, and $mon is the month
2506itself, in the range C<0..11> with 0 indicating January and 11
2507indicating December. $year is the number of years since 1900. That
2508is, $year is C<123> in year 2023. $wday is the day of the week, with
25090 indicating Sunday and 3 indicating Wednesday. $yday is the day of
874b1813 2510the year, in the range C<0..364> (or C<0..365> in leap years.) $isdst
48a26b3a
GS
2511is true if the specified time occurs during daylight savings time,
2512false otherwise.
2513
2514Note that the $year element is I<not> simply the last two digits of
2515the year. If you assume it is, then you create non-Y2K-compliant
2516programs--and you wouldn't want to do that, would you?
54310121 2517
abd75f24
GS
2518The proper way to get a complete 4-digit year is simply:
2519
2520 $year += 1900;
2521
2522And to get the last two digits of the year (e.g., '01' in 2001) do:
2523
2524 $year = sprintf("%02d", $year % 100);
2525
48a26b3a 2526If EXPR is omitted, C<localtime()> uses the current time (C<localtime(time)>).
a0d0e21e 2527
48a26b3a 2528In scalar context, C<localtime()> returns the ctime(3) value:
a0d0e21e 2529
5f05dabc 2530 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
a0d0e21e 2531
fe86afc2
NC
2532This scalar value is B<not> locale dependent but is a Perl builtin. For GMT
2533instead of local time use the L</gmtime> builtin. See also the
2534C<Time::Local> module (to convert the second, minutes, hours, ... back to
2535the integer value returned by time()), and the L<POSIX> module's strftime(3)
2536and mktime(3) functions.
2537
2538To get somewhat similar but locale dependent date strings, set up your
2539locale environment variables appropriately (please see L<perllocale>) and
2540try for example:
a3cb178b 2541
5a964f20 2542 use POSIX qw(strftime);
2b5ab1e7 2543 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
fe86afc2
NC
2544 # or for GMT formatted appropriately for your locale:
2545 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
a3cb178b
GS
2546
2547Note that the C<%a> and C<%b>, the short forms of the day of the week
2548and the month of the year, may not necessarily be three characters wide.
a0d0e21e 2549
07698885 2550=item lock THING
19799a22 2551
01e6739c 2552This function places an advisory lock on a shared variable, or referenced
03730085 2553object contained in I<THING> until the lock goes out of scope.
a6d5524e 2554
f3a23afb 2555lock() is a "weak keyword" : this means that if you've defined a function
67408cae 2556by this name (before any calls to it), that function will be called
03730085
AB
2557instead. (However, if you've said C<use threads>, lock() is always a
2558keyword.) See L<threads>.
19799a22 2559
a0d0e21e
LW
2560=item log EXPR
2561
54310121 2562=item log
bbce6d69 2563
2b5ab1e7
TC
2564Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2565returns log of C<$_>. To get the log of another base, use basic algebra:
19799a22 2566The base-N log of a number is equal to the natural log of that number
2b5ab1e7
TC
2567divided by the natural log of N. For example:
2568
2569 sub log10 {
2570 my $n = shift;
2571 return log($n)/log(10);
b76cc8ba 2572 }
2b5ab1e7
TC
2573
2574See also L</exp> for the inverse operation.
a0d0e21e 2575
a0d0e21e
LW
2576=item lstat EXPR
2577
54310121 2578=item lstat
bbce6d69 2579
19799a22 2580Does the same thing as the C<stat> function (including setting the
5a964f20
TC
2581special C<_> filehandle) but stats a symbolic link instead of the file
2582the symbolic link points to. If symbolic links are unimplemented on
c837d5b4
DP
2583your system, a normal C<stat> is done. For much more detailed
2584information, please see the documentation for C<stat>.
a0d0e21e 2585
7660c0ab 2586If EXPR is omitted, stats C<$_>.
bbce6d69 2587
a0d0e21e
LW
2588=item m//
2589
2590The match operator. See L<perlop>.
2591
2592=item map BLOCK LIST
2593
2594=item map EXPR,LIST
2595
19799a22
GS
2596Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2597C<$_> to each element) and returns the list value composed of the
2598results of each such evaluation. In scalar context, returns the
2599total number of elements so generated. Evaluates BLOCK or EXPR in
2600list context, so each element of LIST may produce zero, one, or
2601more elements in the returned value.
dd99ebda 2602
a0d0e21e
LW
2603 @chars = map(chr, @nums);
2604
2605translates a list of numbers to the corresponding characters. And
2606
4633a7c4 2607 %hash = map { getkey($_) => $_ } @array;
a0d0e21e
LW
2608
2609is just a funny way to write
2610
2611 %hash = ();
2612 foreach $_ (@array) {
4633a7c4 2613 $hash{getkey($_)} = $_;
a0d0e21e
LW
2614 }
2615
be3174d2
GS
2616Note that C<$_> is an alias to the list value, so it can be used to
2617modify the elements of the LIST. While this is useful and supported,
2618it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
2619Using a regular C<foreach> loop for this purpose would be clearer in
2620most cases. See also L</grep> for an array composed of those items of
2621the original list for which the BLOCK or EXPR evaluates to true.
fb73857a 2622
a4fb8298
RGS
2623If C<$_> is lexical in the scope where the C<map> appears (because it has
2624been declared with C<my $_>) then, in addition the be locally aliased to
2625the list elements, C<$_> keeps being lexical inside the block; i.e. it
2626can't be seen from the outside, avoiding any potential side-effects.
2627
205fdb4d
NC
2628C<{> starts both hash references and blocks, so C<map { ...> could be either
2629the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look
2630ahead for the closing C<}> it has to take a guess at which its dealing with
2631based what it finds just after the C<{>. Usually it gets it right, but if it
2632doesn't it won't realize something is wrong until it gets to the C<}> and
2633encounters the missing (or unexpected) comma. The syntax error will be
2634reported close to the C<}> but you'll need to change something near the C<{>
2635such as using a unary C<+> to give perl some help:
2636
2637 %hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong
2638 %hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right
2639 %hash = map { ("\L$_", 1) } @array # this also works
2640 %hash = map { lc($_), 1 } @array # as does this.
2641 %hash = map +( lc($_), 1 ), @array # this is EXPR and works!
cea6626f 2642
205fdb4d
NC
2643 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
2644
2645or to force an anon hash constructor use C<+{>
2646
2647 @hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end
2648
2649and you get list of anonymous hashes each with only 1 entry.
2650
19799a22 2651=item mkdir FILENAME,MASK
a0d0e21e 2652
5a211162
GS
2653=item mkdir FILENAME
2654
0591cd52 2655Creates the directory specified by FILENAME, with permissions
19799a22
GS
2656specified by MASK (as modified by C<umask>). If it succeeds it
2657returns true, otherwise it returns false and sets C<$!> (errno).
5a211162 2658If omitted, MASK defaults to 0777.
0591cd52 2659
19799a22 2660In general, it is better to create directories with permissive MASK,
0591cd52 2661and let the user modify that with their C<umask>, than it is to supply
19799a22 2662a restrictive MASK and give the user no way to be more permissive.
0591cd52
NT
2663The exceptions to this rule are when the file or directory should be
2664kept private (mail files, for instance). The perlfunc(1) entry on
19799a22 2665C<umask> discusses the choice of MASK in more detail.
a0d0e21e 2666
cc1852e8
JH
2667Note that according to the POSIX 1003.1-1996 the FILENAME may have any
2668number of trailing slashes. Some operating and filesystems do not get
2669this right, so Perl automatically removes all trailing slashes to keep
2670everyone happy.
2671
a0d0e21e
LW
2672=item msgctl ID,CMD,ARG
2673
f86cebdf 2674Calls the System V IPC function msgctl(2). You'll probably have to say
0ade1984
JH
2675
2676 use IPC::SysV;
2677
7660c0ab
A
2678first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2679then ARG must be a variable which will hold the returned C<msqid_ds>
951ba7fe
GS
2680structure. Returns like C<ioctl>: the undefined value for error,
2681C<"0 but true"> for zero, or the actual return value otherwise. See also
4755096e 2682L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation.
a0d0e21e
LW
2683
2684=item msgget KEY,FLAGS
2685
f86cebdf 2686Calls the System V IPC function msgget(2). Returns the message queue
4755096e
GS
2687id, or the undefined value if there is an error. See also
2688L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation.
a0d0e21e 2689
a0d0e21e
LW
2690=item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2691
2692Calls the System V IPC function msgrcv to receive a message from
2693message queue ID into variable VAR with a maximum message size of
41d6edb2
JH
2694SIZE. Note that when a message is received, the message type as a
2695native long integer will be the first thing in VAR, followed by the
2696actual message. This packing may be opened with C<unpack("l! a*")>.
2697Taints the variable. Returns true if successful, or false if there is
4755096e
GS
2698an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and
2699C<IPC::SysV::Msg> documentation.
41d6edb2
JH
2700
2701=item msgsnd ID,MSG,FLAGS
2702
2703Calls the System V IPC function msgsnd to send the message MSG to the
2704message queue ID. MSG must begin with the native long integer message
2705type, and be followed by the length of the actual message, and finally
2706the message itself. This kind of packing can be achieved with
2707C<pack("l! a*", $type, $message)>. Returns true if successful,
2708or false if there is an error. See also C<IPC::SysV>
2709and C<IPC::SysV::Msg> documentation.
a0d0e21e
LW
2710
2711=item my EXPR
2712
307ea6df
JH
2713=item my TYPE EXPR
2714
1d2de774 2715=item my EXPR : ATTRS
09bef843 2716
1d2de774 2717=item my TYPE EXPR : ATTRS
307ea6df 2718
19799a22 2719A C<my> declares the listed variables to be local (lexically) to the
1d2de774
JH
2720enclosing block, file, or C<eval>. If more than one value is listed,
2721the list must be placed in parentheses.
307ea6df 2722
1d2de774
JH
2723The exact semantics and interface of TYPE and ATTRS are still
2724evolving. TYPE is currently bound to the use of C<fields> pragma,
307ea6df
JH
2725and attributes are handled using the C<attributes> pragma, or starting
2726from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
2727L<perlsub/"Private Variables via my()"> for details, and L<fields>,
2728L<attributes>, and L<Attribute::Handlers>.
4633a7c4 2729
a0d0e21e
LW
2730=item next LABEL
2731
2732=item next
2733
2734The C<next> command is like the C<continue> statement in C; it starts
2735the next iteration of the loop:
2736
4633a7c4
LW
2737 LINE: while (<STDIN>) {
2738 next LINE if /^#/; # discard comments
5a964f20 2739 #...
a0d0e21e
LW
2740 }
2741
2742Note that if there were a C<continue> block on the above, it would get
2743executed even on discarded lines. If the LABEL is omitted, the command
2744refers to the innermost enclosing loop.
2745
4968c1e4 2746C<next> cannot be used to exit a block which returns a value such as
2b5ab1e7
TC
2747C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2748a grep() or map() operation.
4968c1e4 2749
6c1372ed
GS
2750Note that a block by itself is semantically identical to a loop
2751that executes once. Thus C<next> will exit such a block early.
2752
98293880
JH
2753See also L</continue> for an illustration of how C<last>, C<next>, and
2754C<redo> work.
1d2dff63 2755
4a66ea5a
RGS
2756=item no Module VERSION LIST
2757
2758=item no Module VERSION
2759
a0d0e21e
LW
2760=item no Module LIST
2761
4a66ea5a
RGS
2762=item no Module
2763
593b9c14 2764See the C<use> function, of which C<no> is the opposite.
a0d0e21e
LW
2765
2766=item oct EXPR
2767
54310121 2768=item oct
bbce6d69 2769
4633a7c4 2770Interprets EXPR as an octal string and returns the corresponding
4f19785b
WSI
2771value. (If EXPR happens to start off with C<0x>, interprets it as a
2772hex string. If EXPR starts off with C<0b>, it is interpreted as a
53305cf1
NC
2773binary string. Leading whitespace is ignored in all three cases.)
2774The following will handle decimal, binary, octal, and hex in the standard
2775Perl or C notation:
a0d0e21e
LW
2776
2777 $val = oct($val) if $val =~ /^0/;
2778
19799a22
GS
2779If EXPR is omitted, uses C<$_>. To go the other way (produce a number
2780in octal), use sprintf() or printf():
2781
2782 $perms = (stat("filename"))[2] & 07777;
2783 $oct_perms = sprintf "%lo", $perms;
2784
2785The oct() function is commonly used when a string such as C<644> needs
2786to be converted into a file mode, for example. (Although perl will
2787automatically convert strings into numbers as needed, this automatic
2788conversion assumes base 10.)
a0d0e21e
LW
2789
2790=item open FILEHANDLE,EXPR
2791
68bd7414
NIS
2792=item open FILEHANDLE,MODE,EXPR
2793
2794=item open FILEHANDLE,MODE,EXPR,LIST
2795
ba964c95
T
2796=item open FILEHANDLE,MODE,REFERENCE
2797
a0d0e21e
LW
2798=item open FILEHANDLE
2799
2800Opens the file whose filename is given by EXPR, and associates it with
ed53a2bb
JH
2801FILEHANDLE.
2802
2803(The following is a comprehensive reference to open(): for a gentler
2804introduction you may consider L<perlopentut>.)
2805
a28cd5c9
NT
2806If FILEHANDLE is an undefined scalar variable (or array or hash element)
2807the variable is assigned a reference to a new anonymous filehandle,
2808otherwise if FILEHANDLE is an expression, its value is used as the name of
2809the real filehandle wanted. (This is considered a symbolic reference, so
2810C<use strict 'refs'> should I<not> be in effect.)
ed53a2bb
JH
2811
2812If EXPR is omitted, the scalar variable of the same name as the
2813FILEHANDLE contains the filename. (Note that lexical variables--those
2814declared with C<my>--will not work for this purpose; so if you're
67408cae 2815using C<my>, specify EXPR in your call to open.)
ed53a2bb
JH
2816
2817If three or more arguments are specified then the mode of opening and
2818the file name are separate. If MODE is C<< '<' >> or nothing, the file
2819is opened for input. If MODE is C<< '>' >>, the file is truncated and
2820opened for output, being created if necessary. If MODE is C<<< '>>' >>>,
b76cc8ba 2821the file is opened for appending, again being created if necessary.
5a964f20 2822
ed53a2bb
JH
2823You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to
2824indicate that you want both read and write access to the file; thus
2825C<< '+<' >> is almost always preferred for read/write updates--the C<<
2826'+>' >> mode would clobber the file first. You can't usually use
2827either read-write mode for updating textfiles, since they have
2828variable length records. See the B<-i> switch in L<perlrun> for a
2829better approach. The file is created with permissions of C<0666>
2830modified by the process' C<umask> value.
2831
2832These various prefixes correspond to the fopen(3) modes of C<'r'>,
2833C<'r+'>, C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
5f05dabc 2834
6170680b
IZ
2835In the 2-arguments (and 1-argument) form of the call the mode and
2836filename should be concatenated (in this order), possibly separated by
68bd7414
NIS
2837spaces. It is possible to omit the mode in these forms if the mode is
2838C<< '<' >>.
6170680b 2839
7660c0ab 2840If the filename begins with C<'|'>, the filename is interpreted as a
5a964f20 2841command to which output is to be piped, and if the filename ends with a
f244e06d
GS
2842C<'|'>, the filename is interpreted as a command which pipes output to
2843us. See L<perlipc/"Using open() for IPC">
19799a22 2844for more examples of this. (You are not allowed to C<open> to a command
5a964f20 2845that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
4a4eefd0
GS
2846and L<perlipc/"Bidirectional Communication with Another Process">
2847for alternatives.)
cb1a09d0 2848
ed53a2bb
JH
2849For three or more arguments if MODE is C<'|-'>, the filename is
2850interpreted as a command to which output is to be piped, and if MODE
2851is C<'-|'>, the filename is interpreted as a command which pipes
2852output to us. In the 2-arguments (and 1-argument) form one should
2853replace dash (C<'-'>) with the command.
2854See L<perlipc/"Using open() for IPC"> for more examples of this.
2855(You are not allowed to C<open> to a command that pipes both in I<and>
2856out, but see L<IPC::Open2>, L<IPC::Open3>, and
2857L<perlipc/"Bidirectional Communication"> for alternatives.)
2858
2859In the three-or-more argument form of pipe opens, if LIST is specified
2860(extra arguments after the command name) then LIST becomes arguments
2861to the command invoked if the platform supports it. The meaning of
2862C<open> with more than three arguments for non-pipe modes is not yet
2863specified. Experimental "layers" may give extra LIST arguments
2864meaning.
6170680b
IZ
2865
2866In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
b76cc8ba 2867and opening C<< '>-' >> opens STDOUT.
6170680b 2868
fae2c0fb
RGS
2869You may use the three-argument form of open to specify IO "layers"
2870(sometimes also referred to as "disciplines") to be applied to the handle
2871that affect how the input and output are processed (see L<open> and
2872L<PerlIO> for more details). For example
7207e29d 2873
9124316e
JH
2874 open(FH, "<:utf8", "file")
2875
2876will open the UTF-8 encoded file containing Unicode characters,
fae2c0fb
RGS
2877see L<perluniintro>. (Note that if layers are specified in the
2878three-arg form then default layers set by the C<open> pragma are
01e6739c 2879ignored.)
ed53a2bb
JH
2880
2881Open returns nonzero upon success, the undefined value otherwise. If
2882the C<open> involved a pipe, the return value happens to be the pid of
2883the subprocess.
cb1a09d0 2884
ed53a2bb
JH
2885If you're running Perl on a system that distinguishes between text
2886files and binary files, then you should check out L</binmode> for tips
2887for dealing with this. The key distinction between systems that need
2888C<binmode> and those that don't is their text file formats. Systems
8939ba94 2889like Unix, Mac OS, and Plan 9, which delimit lines with a single
ed53a2bb
JH
2890character, and which encode that character in C as C<"\n">, do not
2891need C<binmode>. The rest need it.
cb1a09d0 2892
fb73857a 2893When opening a file, it's usually a bad idea to continue normal execution
19799a22
GS
2894if the request failed, so C<open> is frequently used in connection with
2895C<die>. Even if C<die> won't do what you want (say, in a CGI script,
fb73857a 2896where you want to make a nicely formatted error message (but there are
5a964f20 2897modules that can help with that problem)) you should always check
19799a22 2898the return value from opening a file. The infrequent exception is when
fb73857a
PP
2899working with an unopened filehandle is actually what you want to do.
2900
ed53a2bb
JH
2901As a special case the 3 arg form with a read/write mode and the third
2902argument being C<undef>:
b76cc8ba
NIS
2903
2904 open(TMP, "+>", undef) or die ...
2905
f253e835
JH
2906opens a filehandle to an anonymous temporary file. Also using "+<"
2907works for symmetry, but you really should consider writing something
2908to the temporary file first. You will need to seek() to do the
2909reading.
b76cc8ba 2910
2ce64696
JC
2911Since v5.8.0, perl has built using PerlIO by default. Unless you've
2912changed this (ie Configure -Uuseperlio), you can open file handles to
2913"in memory" files held in Perl scalars via:
ba964c95 2914
b996200f
SB
2915 open($fh, '>', \$variable) || ..
2916
2917Though if you try to re-open C<STDOUT> or C<STDERR> as an "in memory"
2918file, you have to close it first:
2919
2920 close STDOUT;
2921 open STDOUT, '>', \$variable or die "Can't open STDOUT: $!";
ba964c95 2922
cb1a09d0 2923Examples:
a0d0e21e
LW
2924
2925 $ARTICLE = 100;
2926 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2927 while (<ARTICLE>) {...
2928
6170680b 2929 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
fb73857a 2930 # if the open fails, output is discarded
a0d0e21e 2931
6170680b 2932 open(DBASE, '+<', 'dbase.mine') # open for update
fb73857a 2933 or die "Can't open 'dbase.mine' for update: $!";
cb1a09d0 2934
6170680b
IZ
2935 open(DBASE, '+<dbase.mine') # ditto
2936 or die "Can't open 'dbase.mine' for update: $!";
2937
2938 open(ARTICLE, '-|', "caesar <$article") # decrypt article
fb73857a 2939 or die "Can't start caesar: $!";
a0d0e21e 2940
6170680b
IZ
2941 open(ARTICLE, "caesar <$article |") # ditto
2942 or die "Can't start caesar: $!";
2943
2359510d 2944 open(EXTRACT, "|sort >Tmp$$") # $$ is our process id
fb73857a 2945 or die "Can't start sort: $!";
a0d0e21e 2946
ba964c95
T
2947 # in memory files
2948 open(MEMORY,'>', \$var)
2949 or die "Can't open memory file: $!";
2950 print MEMORY "foo!\n"; # output will end up in $var
2951
a0d0e21e
LW
2952 # process argument list of files along with any includes
2953
2954 foreach $file (@ARGV) {
2955 process($file, 'fh00');
2956 }
2957
2958 sub process {
5a964f20 2959 my($filename, $input) = @_;
a0d0e21e
LW
2960 $input++; # this is a string increment
2961 unless (open($input, $filename)) {
2962 print STDERR "Can't open $filename: $!\n";
2963 return;
2964 }
2965
5a964f20 2966 local $_;
a0d0e21e
LW
2967 while (<$input>) { # note use of indirection
2968 if (/^#include "(.*)"/) {
2969 process($1, $input);
2970 next;
2971 }
5a964f20 2972 #... # whatever
a0d0e21e
LW
2973 }
2974 }
2975
2ce64696
JC
2976See L<perliol/> for detailed info on PerlIO.
2977
a0d0e21e 2978You may also, in the Bourne shell tradition, specify an EXPR beginning
00cafafa
JH
2979with C<< '>&' >>, in which case the rest of the string is interpreted
2980as the name of a filehandle (or file descriptor, if numeric) to be
2981duped (as L<dup(2)>) and opened. You may use C<&> after C<< > >>,
2982C<<< >> >>>, C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>.
2983The mode you specify should match the mode of the original filehandle.
2984(Duping a filehandle does not take into account any existing contents
2985of IO buffers.) If you use the 3 arg form then you can pass either a
2986number, the name of a filehandle or the normal "reference to a glob".
6170680b 2987
eae1b76b
SB
2988Here is a script that saves, redirects, and restores C<STDOUT> and
2989C<STDERR> using various methods:
a0d0e21e
LW
2990
2991 #!/usr/bin/perl
eae1b76b
SB
2992 open my $oldout, ">&STDOUT" or die "Can't dup STDOUT: $!";
2993 open OLDERR, ">&", \*STDERR or die "Can't dup STDERR: $!";
818c4caa 2994
eae1b76b
SB
2995 open STDOUT, '>', "foo.out" or die "Can't redirect STDOUT: $!";
2996 open STDERR, ">&STDOUT" or die "Can't dup STDOUT: $!";
a0d0e21e 2997
eae1b76b
SB
2998 select STDERR; $| = 1; # make unbuffered
2999 select STDOUT; $| = 1; # make unbuffered
a0d0e21e
LW
3000
3001 print STDOUT "stdout 1\n"; # this works for
3002 print STDERR "stderr 1\n"; # subprocesses too
3003
eae1b76b
SB
3004 open STDOUT, ">&", $oldout or die "Can't dup \$oldout: $!";
3005 open STDERR, ">&OLDERR" or die "Can't dup OLDERR: $!";
a0d0e21e
LW
3006
3007 print STDOUT "stdout 2\n";
3008 print STDERR "stderr 2\n";
3009
ef8b303f
JH
3010If you specify C<< '<&=X' >>, where C<X> is a file descriptor number
3011or a filehandle, then Perl will do an equivalent of C's C<fdopen> of
3012that file descriptor (and not call L<dup(2)>); this is more
3013parsimonious of file descriptors. For example:
a0d0e21e 3014
00cafafa 3015 # open for input, reusing the fileno of $fd
a0d0e21e 3016 open(FILEHANDLE, "<&=$fd")
df632fdf 3017
b76cc8ba 3018or
df632fdf 3019
b76cc8ba 3020 open(FILEHANDLE, "<&=", $fd)
a0d0e21e 3021
00cafafa
JH
3022or
3023
3024 # open for append, using the fileno of OLDFH
3025 open(FH, ">>&=", OLDFH)
3026
3027or
3028
3029 open(FH, ">>&=OLDFH")
3030
ef8b303f
JH
3031Being parsimonious on filehandles is also useful (besides being
3032parsimonious) for example when something is dependent on file
3033descriptors, like for example locking using flock(). If you do just
3034C<< open(A, '>>&B') >>, the filehandle A will not have the same file
3035descriptor as B, and therefore flock(A) will not flock(B), and vice
3036versa. But with C<< open(A, '>>&=B') >> the filehandles will share
3037the same file descriptor.
3038
3039Note that if you are using Perls older than 5.8.0, Perl will be using
3040the standard C libraries' fdopen() to implement the "=" functionality.
3041On many UNIX systems fdopen() fails when file descriptors exceed a
3042certain value, typically 255. For Perls 5.8.0 and later, PerlIO is
3043most often the default.
4af147f6 3044
df632fdf
JH
3045You can see whether Perl has been compiled with PerlIO or not by
3046running C<perl -V> and looking for C<useperlio=> line. If C<useperlio>
3047is C<define>, you have PerlIO, otherwise you don't.
3048
6170680b
IZ
3049If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
3050with 2-arguments (or 1-argument) form of open(), then
a0d0e21e 3051there is an implicit fork done, and the return value of open is the pid
7660c0ab 3052of the child within the parent process, and C<0> within the child
184e9718 3053process. (Use C<defined($pid)> to determine whether the open was successful.)
a0d0e21e
LW
3054The filehandle behaves normally for the parent, but i/o to that
3055filehandle is piped from/to the STDOUT/STDIN of the child process.
3056In the child process the filehandle isn't opened--i/o happens from/to
3057the new STDOUT or STDIN. Typically this is used like the normal
3058piped open when you want to exercise more control over just how the
3059pipe command gets executed, such as when you are running setuid, and
54310121 3060don't want to have to scan shell commands for metacharacters.
6170680b 3061The following triples are more or less equivalent:
a0d0e21e
LW
3062
3063 open(FOO, "|tr '[a-z]' '[A-Z]'");
6170680b
IZ
3064 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
3065 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
b76cc8ba 3066 open(FOO, '|-', "tr", '[a-z]', '[A-Z]');
a0d0e21e
LW
3067
3068 open(FOO, "cat -n '$file'|");
6170680b
IZ
3069 open(FOO, '-|', "cat -n '$file'");
3070 open(FOO, '-|') || exec 'cat', '-n', $file;
b76cc8ba
NIS
3071 open(FOO, '-|', "cat", '-n', $file);
3072
3073The last example in each block shows the pipe as "list form", which is
64da03b2
JH
3074not yet supported on all platforms. A good rule of thumb is that if
3075your platform has true C<fork()> (in other words, if your platform is
3076UNIX) you can use the list form.
a0d0e21e 3077
4633a7c4
LW
3078See L<perlipc/"Safe Pipe Opens"> for more examples of this.
3079
0f897271
GS
3080Beginning with v5.6.0, Perl will attempt to flush all files opened for
3081output before any operation that may do a fork, but this may not be
3082supported on some platforms (see L<perlport>). To be safe, you may need
3083to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
3084of C<IO::Handle> on any open handles.
3085
ed53a2bb
JH
3086On systems that support a close-on-exec flag on files, the flag will
3087be set for the newly opened file descriptor as determined by the value
3088of $^F. See L<perlvar/$^F>.
a0d0e21e 3089
0dccf244
CS
3090Closing any piped filehandle causes the parent process to wait for the
3091child to finish, and returns the status value in C<$?>.
3092
ed53a2bb
JH
3093The filename passed to 2-argument (or 1-argument) form of open() will
3094have leading and trailing whitespace deleted, and the normal
3095redirection characters honored. This property, known as "magic open",
5a964f20 3096can often be used to good effect. A user could specify a filename of
7660c0ab 3097F<"rsh cat file |">, or you could change certain filenames as needed:
5a964f20
TC
3098
3099 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
3100 open(FH, $filename) or die "Can't open $filename: $!";
3101
6170680b
IZ
3102Use 3-argument form to open a file with arbitrary weird characters in it,
3103
3104 open(FOO, '<', $file);
3105
3106otherwise it's necessary to protect any leading and trailing whitespace:
5a964f20
TC
3107
3108 $file =~ s#^(\s)#./$1#;
3109 open(FOO, "< $file\0");
3110
a31a806a 3111(this may not work on some bizarre filesystems). One should
106325ad 3112conscientiously choose between the I<magic> and 3-arguments form
6170680b
IZ
3113of open():
3114
3115 open IN, $ARGV[0];
3116
3117will allow the user to specify an argument of the form C<"rsh cat file |">,
3118but will not work on a filename which happens to have a trailing space, while
3119
3120 open IN, '<', $ARGV[0];
3121
3122will have exactly the opposite restrictions.
3123
19799a22 3124If you want a "real" C C<open> (see L<open(2)> on your system), then you
6170680b
IZ
3125should use the C<sysopen> function, which involves no such magic (but
3126may use subtly different filemodes than Perl open(), which is mapped
3127to C fopen()). This is
5a964f20
TC
3128another way to protect your filenames from interpretation. For example:
3129
3130 use IO::Handle;
3131 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
3132 or die "sysopen $path: $!";
3133 $oldfh = select(HANDLE); $| = 1; select($oldfh);
38762f02 3134 print HANDLE "stuff $$\n";
5a964f20
TC
3135 seek(HANDLE, 0, 0);
3136 print "File contains: ", <HANDLE>;
3137
7660c0ab
A
3138Using the constructor from the C<IO::Handle> package (or one of its
3139subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
5a964f20
TC
3140filehandles that have the scope of whatever variables hold references to
3141them, and automatically close whenever and however you leave that scope:
c07a80fd 3142
5f05dabc 3143 use IO::File;
5a964f20 3144 #...
c07a80fd
PP
3145 sub read_myfile_munged {
3146 my $ALL = shift;
5f05dabc 3147 my $handle = new IO::File;
c07a80fd
PP
3148 open($handle, "myfile") or die "myfile: $!";
3149 $first = <$handle>
3150 or return (); # Automatically closed here.
3151 mung $first or die "mung failed"; # Or here.
3152 return $first, <$handle> if $ALL; # Or here.
3153 $first; # Or here.
3154 }
3155
b687b08b 3156See L</seek> for some details about mixing reading and writing.
a0d0e21e
LW
3157
3158=item opendir DIRHANDLE,EXPR
3159
19799a22
GS
3160Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
3161C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
a28cd5c9
NT
3162DIRHANDLE may be an expression whose value can be used as an indirect
3163dirhandle, usually the real dirhandle name. If DIRHANDLE is an undefined
3164scalar variable (or array or hash element), the variable is assigned a
3165reference to a new anonymous dirhandle.
a0d0e21e
LW
3166DIRHANDLEs have their own namespace separate from FILEHANDLEs.
3167
3168=item ord EXPR
3169
54310121 3170=item ord
bbce6d69 3171
121910a4
JH
3172Returns the numeric (the native 8-bit encoding, like ASCII or EBCDIC,
3173or Unicode) value of the first character of EXPR. If EXPR is omitted,
3174uses C<$_>.
3175
3176For the reverse, see L</chr>.
3177See L<perlunicode> and L<encoding> for more about Unicode.
a0d0e21e 3178
77ca0c92
LW
3179=item our EXPR
3180
307ea6df
JH
3181=item our EXPR TYPE
3182
1d2de774 3183=item our EXPR : ATTRS
9969eac4 3184
1d2de774 3185=item our TYPE EXPR : ATTRS
307ea6df 3186
77ca0c92
LW
3187An C<our> declares the listed variables to be valid globals within
3188the enclosing block, file, or C<eval>. That is, it has the same
3189scoping rules as a "my" declaration, but does not create a local
3190variable. If more than one value is listed, the list must be placed
3191in parentheses. The C<our> declaration has no semantic effect unless
3192"use strict vars" is in effect, in which case it lets you use the
3193declared global variable without qualifying it with a package name.
3194(But only within the lexical scope of the C<our> declaration. In this
3195it differs from "use vars", which is package scoped.)
3196
f472eb5c
GS
3197An C<our> declaration declares a global variable that will be visible
3198across its entire lexical scope, even across package boundaries. The
3199package in which the variable is entered is determined at the point
3200of the declaration, not at the point of use. This means the following
3201behavior holds:
3202
3203 package Foo;
3204 our $bar; # declares $Foo::bar for rest of lexical scope
3205 $bar = 20;
3206
3207 package Bar;
3208 print $bar; # prints 20
3209
3210Multiple C<our> declarations in the same lexical scope are allowed
3211if they are in different packages. If they happened to be in the same
3212package, Perl will emit warnings if you have asked for them.
3213
3214 use warnings;
3215 package Foo;
3216 our $bar; # declares $Foo::bar for rest of lexical scope
3217 $bar = 20;
3218
3219 package Bar;
3220 our $bar = 30; # declares $Bar::bar for rest of lexical scope
3221 print $bar; # prints 30
3222
3223 our $bar; # emits warning
3224
9969eac4 3225An C<our> declaration may also have a list of attributes associated
307ea6df
JH
3226with it.
3227
1d2de774
JH
3228The exact semantics and interface of TYPE and ATTRS are still
3229evolving. TYPE is currently bound to the use of C<fields> pragma,
307ea6df
JH
3230and attributes are handled using the C<attributes> pragma, or starting
3231from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
3232L<perlsub/"Private Variables via my()"> for details, and L<fields>,
3233L<attributes>, and L<Attribute::Handlers>.
3234
3235The only currently recognized C<our()> attribute is C<unique> which
3236indicates that a single copy of the global is to be used by all
3237interpreters should the program happen to be running in a
3238multi-interpreter environment. (The default behaviour would be for
3239each interpreter to have its own copy of the global.) Examples:
9969eac4 3240
51d2bbcc
JH
3241 our @EXPORT : unique = qw(foo);
3242 our %EXPORT_TAGS : unique = (bar => [qw(aa bb cc)]);
3243 our $VERSION : unique = "1.00";
9969eac4 3244
96fa8c42 3245Note that this attribute also has the effect of making the global
72e53bfb
JH
3246readonly when the first new interpreter is cloned (for example,
3247when the first new thread is created).
96fa8c42 3248
9969eac4
BS
3249Multi-interpreter environments can come to being either through the
3250fork() emulation on Windows platforms, or by embedding perl in a
51d2bbcc 3251multi-threaded application. The C<unique> attribute does nothing in
9969eac4
BS
3252all other environments.
3253
72982a0c 3254Warning: the current implementation of this attribute operates on the
20fe557d
DM
3255typeglob associated with the variable; this means that C<our $x : unique>
3256also has the effect of C<our @x : unique; our %x : unique>. This may be
3257subject to change.
3258
a0d0e21e
LW
3259=item pack TEMPLATE,LIST
3260
2b6c5635
GS
3261Takes a LIST of values and converts it into a string using the rules
3262given by the TEMPLATE. The resulting string is the concatenation of
3263the converted values. Typically, each converted value looks
3264like its machine-level representation. For example, on 32-bit machines
3265a converted integer may be represented by a sequence of 4 bytes.
3266
18529408
IZ
3267The TEMPLATE is a sequence of characters that give the order and type
3268of values, as follows: