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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
4ad40acf 617number, and which definitely should I<not> be a string of octal digits:
2f9daede 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 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
4633a7c4 734
a0d0e21e
LW
735=item chr NUMBER
736
54310121 737=item chr
bbce6d69 738
a0d0e21e 739Returns the character represented by that NUMBER in the character set.
a0ed51b3 740For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
1e54db1a
JH
741chr(0x263a) is a Unicode smiley face. Note that characters from 128
742to 255 (inclusive) are by default not encoded in UTF-8 Unicode for
743backward compatibility reasons (but see L<encoding>).
aaa68c4a 744
8a064bd6
JH
745Negative values give the Unicode replacement character (chr(0xfffd)),
746except under the L</bytes> pragma, where low eight bits of the value
747(truncated to an integer) are used.
748
974da8e5
JH
749If NUMBER is omitted, uses C<$_>.
750
b76cc8ba 751For the reverse, use L</ord>.
a0d0e21e 752
974da8e5
JH
753Note that under the C<bytes> pragma the NUMBER is masked to
754the low eight bits.
755
756See L<perlunicode> and L<encoding> for more about Unicode.
bbce6d69 757
a0d0e21e
LW
758=item chroot FILENAME
759
54310121 760=item chroot
bbce6d69 761
5a964f20 762This function works like the system call by the same name: it makes the
4633a7c4 763named directory the new root directory for all further pathnames that
951ba7fe 764begin with a C</> by your process and all its children. (It doesn't
28757baa 765change your current working directory, which is unaffected.) For security
4633a7c4 766reasons, this call is restricted to the superuser. If FILENAME is
19799a22 767omitted, does a C<chroot> to C<$_>.
a0d0e21e
LW
768
769=item close FILEHANDLE
770
6a518fbc
TP
771=item close
772
9124316e
JH
773Closes the file or pipe associated with the file handle, returning
774true only if IO buffers are successfully flushed and closes the system
775file descriptor. Closes the currently selected filehandle if the
776argument is omitted.
fb73857a
PP
777
778You don't have to close FILEHANDLE if you are immediately going to do
19799a22
GS
779another C<open> on it, because C<open> will close it for you. (See
780C<open>.) However, an explicit C<close> on an input file resets the line
781counter (C<$.>), while the implicit close done by C<open> does not.
fb73857a 782
dede8123
RGS
783If the file handle came from a piped open, C<close> will additionally
784return false if one of the other system calls involved fails, or if the
fb73857a 785program exits with non-zero status. (If the only problem was that the
dede8123 786program exited non-zero, C<$!> will be set to C<0>.) Closing a pipe
2b5ab1e7 787also waits for the process executing on the pipe to complete, in case you
b76cc8ba 788want to look at the output of the pipe afterwards, and
e5218da5
GA
789implicitly puts the exit status value of that command into C<$?> and
790C<${^CHILD_ERROR_NATIVE}>.
5a964f20 791
73689b13
GS
792Prematurely closing the read end of a pipe (i.e. before the process
793writing to it at the other end has closed it) will result in a
794SIGPIPE being delivered to the writer. If the other end can't
795handle that, be sure to read all the data before closing the pipe.
796
fb73857a 797Example:
a0d0e21e 798
fb73857a
PP
799 open(OUTPUT, '|sort >foo') # pipe to sort
800 or die "Can't start sort: $!";
5a964f20 801 #... # print stuff to output
fb73857a
PP
802 close OUTPUT # wait for sort to finish
803 or warn $! ? "Error closing sort pipe: $!"
804 : "Exit status $? from sort";
805 open(INPUT, 'foo') # get sort's results
806 or die "Can't open 'foo' for input: $!";
a0d0e21e 807
5a964f20
TC
808FILEHANDLE may be an expression whose value can be used as an indirect
809filehandle, usually the real filehandle name.
a0d0e21e
LW
810
811=item closedir DIRHANDLE
812
19799a22 813Closes a directory opened by C<opendir> and returns the success of that
5a964f20
TC
814system call.
815
a0d0e21e
LW
816=item connect SOCKET,NAME
817
818Attempts to connect to a remote socket, just as the connect system call
19799a22 819does. Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
820packed address of the appropriate type for the socket. See the examples in
821L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 822
cb1a09d0
AD
823=item continue BLOCK
824
825Actually a flow control statement rather than a function. If there is a
98293880
JH
826C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
827C<foreach>), it is always executed just before the conditional is about to
828be evaluated again, just like the third part of a C<for> loop in C. Thus
cb1a09d0
AD
829it can be used to increment a loop variable, even when the loop has been
830continued via the C<next> statement (which is similar to the C C<continue>
831statement).
832
98293880 833C<last>, C<next>, or C<redo> may appear within a C<continue>
19799a22
GS
834block. C<last> and C<redo> will behave as if they had been executed within
835the main block. So will C<next>, but since it will execute a C<continue>
1d2dff63
GS
836block, it may be more entertaining.
837
838 while (EXPR) {
839 ### redo always comes here
840 do_something;
841 } continue {
842 ### next always comes here
843 do_something_else;
844 # then back the top to re-check EXPR
845 }
846 ### last always comes here
847
848Omitting the C<continue> section is semantically equivalent to using an
19799a22 849empty one, logically enough. In that case, C<next> goes directly back
1d2dff63
GS
850to check the condition at the top of the loop.
851
a0d0e21e
LW
852=item cos EXPR
853
d6217f1e
GS
854=item cos
855
5a964f20 856Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
7660c0ab 857takes cosine of C<$_>.
a0d0e21e 858
ca6e1c26 859For the inverse cosine operation, you may use the C<Math::Trig::acos()>
28757baa
PP
860function, or use this relation:
861
862 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
863
a0d0e21e
LW
864=item crypt PLAINTEXT,SALT
865
f86cebdf 866Encrypts a string exactly like the crypt(3) function in the C library
4633a7c4
LW
867(assuming that you actually have a version there that has not been
868extirpated as a potential munition). This can prove useful for checking
869the password file for lousy passwords, amongst other things. Only the
870guys wearing white hats should do this.
a0d0e21e 871
a6d05634 872Note that L<crypt|/crypt> is intended to be a one-way function, much like
85c16d83
JH
873breaking eggs to make an omelette. There is no (known) corresponding
874decrypt function (in other words, the crypt() is a one-way hash
875function). As a result, this function isn't all that useful for
11155c91 876cryptography. (For that, see your nearby CPAN mirror.)
2f9daede 877
85c16d83
JH
878When verifying an existing encrypted string you should use the
879encrypted text as the salt (like C<crypt($plain, $crypted) eq
8e2ffcbe 880$crypted>). This allows your code to work with the standard L<crypt|/crypt>
85c16d83
JH
881and with more exotic implementations. In other words, do not assume
882anything about the returned string itself, or how many bytes in
883the encrypted string matter.
884
885Traditionally the result is a string of 13 bytes: two first bytes of
886the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
887the first eight bytes of the encrypted string mattered, but
888alternative hashing schemes (like MD5), higher level security schemes
889(like C2), and implementations on non-UNIX platforms may produce
890different strings.
891
892When choosing a new salt create a random two character string whose
893characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
d3989d75
CW
894'/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of
895characters is just a recommendation; the characters allowed in
896the salt depend solely on your system's crypt library, and Perl can't
897restrict what salts C<crypt()> accepts.
e71965be 898
a0d0e21e
LW
899Here's an example that makes sure that whoever runs this program knows
900their own password:
901
902 $pwd = (getpwuid($<))[1];
a0d0e21e
LW
903
904 system "stty -echo";
905 print "Password: ";
e71965be 906 chomp($word = <STDIN>);
a0d0e21e
LW
907 print "\n";
908 system "stty echo";
909
e71965be 910 if (crypt($word, $pwd) ne $pwd) {
a0d0e21e
LW
911 die "Sorry...\n";
912 } else {
913 print "ok\n";
54310121 914 }
a0d0e21e 915
9f8f0c9d 916Of course, typing in your own password to whoever asks you
748a9306 917for it is unwise.
a0d0e21e 918
8e2ffcbe 919The L<crypt|/crypt> function is unsuitable for encrypting large quantities
19799a22
GS
920of data, not least of all because you can't get the information
921back. Look at the F<by-module/Crypt> and F<by-module/PGP> directories
922on your favorite CPAN mirror for a slew of potentially useful
923modules.
924
f2791508
JH
925If using crypt() on a Unicode string (which I<potentially> has
926characters with codepoints above 255), Perl tries to make sense
927of the situation by trying to downgrade (a copy of the string)
928the string back to an eight-bit byte string before calling crypt()
929(on that copy). If that works, good. If not, crypt() dies with
930C<Wide character in crypt>.
85c16d83 931
aa689395 932=item dbmclose HASH
a0d0e21e 933
19799a22 934[This function has been largely superseded by the C<untie> function.]
a0d0e21e 935
aa689395 936Breaks the binding between a DBM file and a hash.
a0d0e21e 937
19799a22 938=item dbmopen HASH,DBNAME,MASK
a0d0e21e 939
19799a22 940[This function has been largely superseded by the C<tie> function.]
a0d0e21e 941
7b8d334a 942This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
19799a22
GS
943hash. HASH is the name of the hash. (Unlike normal C<open>, the first
944argument is I<not> a filehandle, even though it looks like one). DBNAME
aa689395
PP
945is the name of the database (without the F<.dir> or F<.pag> extension if
946any). If the database does not exist, it is created with protection
19799a22
GS
947specified by MASK (as modified by the C<umask>). If your system supports
948only the older DBM functions, you may perform only one C<dbmopen> in your
aa689395 949program. In older versions of Perl, if your system had neither DBM nor
19799a22 950ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
aa689395
PP
951sdbm(3).
952
953If you don't have write access to the DBM file, you can only read hash
954variables, not set them. If you want to test whether you can write,
19799a22 955either use file tests or try setting a dummy hash entry inside an C<eval>,
aa689395 956which will trap the error.
a0d0e21e 957
19799a22
GS
958Note that functions such as C<keys> and C<values> may return huge lists
959when used on large DBM files. You may prefer to use the C<each>
a0d0e21e
LW
960function to iterate over large DBM files. Example:
961
962 # print out history file offsets
963 dbmopen(%HIST,'/usr/lib/news/history',0666);
964 while (($key,$val) = each %HIST) {
965 print $key, ' = ', unpack('L',$val), "\n";
966 }
967 dbmclose(%HIST);
968
cb1a09d0 969See also L<AnyDBM_File> for a more general description of the pros and
184e9718 970cons of the various dbm approaches, as well as L<DB_File> for a particularly
cb1a09d0 971rich implementation.
4633a7c4 972
2b5ab1e7
TC
973You can control which DBM library you use by loading that library
974before you call dbmopen():
975
976 use DB_File;
977 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
978 or die "Can't open netscape history file: $!";
979
a0d0e21e
LW
980=item defined EXPR
981
54310121 982=item defined
bbce6d69 983
2f9daede
TPG
984Returns a Boolean value telling whether EXPR has a value other than
985the undefined value C<undef>. If EXPR is not present, C<$_> will be
986checked.
987
988Many operations return C<undef> to indicate failure, end of file,
989system error, uninitialized variable, and other exceptional
990conditions. This function allows you to distinguish C<undef> from
991other values. (A simple Boolean test will not distinguish among
7660c0ab 992C<undef>, zero, the empty string, and C<"0">, which are all equally
2f9daede 993false.) Note that since C<undef> is a valid scalar, its presence
19799a22 994doesn't I<necessarily> indicate an exceptional condition: C<pop>
2f9daede
TPG
995returns C<undef> when its argument is an empty array, I<or> when the
996element to return happens to be C<undef>.
997
f10b0346
GS
998You may also use C<defined(&func)> to check whether subroutine C<&func>
999has ever been defined. The return value is unaffected by any forward
04891299 1000declarations of C<&func>. Note that a subroutine which is not defined
847c7ebe
DD
1001may still be callable: its package may have an C<AUTOLOAD> method that
1002makes it spring into existence the first time that it is called -- see
1003L<perlsub>.
f10b0346
GS
1004
1005Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
1006used to report whether memory for that aggregate has ever been
1007allocated. This behavior may disappear in future versions of Perl.
1008You should instead use a simple test for size:
1009
1010 if (@an_array) { print "has array elements\n" }
1011 if (%a_hash) { print "has hash members\n" }
2f9daede
TPG
1012
1013When used on a hash element, it tells you whether the value is defined,
dc848c6f 1014not whether the key exists in the hash. Use L</exists> for the latter
2f9daede 1015purpose.
a0d0e21e
LW
1016
1017Examples:
1018
1019 print if defined $switch{'D'};
1020 print "$val\n" while defined($val = pop(@ary));
1021 die "Can't readlink $sym: $!"
1022 unless defined($value = readlink $sym);
a0d0e21e 1023 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
2f9daede 1024 $debugging = 0 unless defined $debugging;
a0d0e21e 1025
19799a22 1026Note: Many folks tend to overuse C<defined>, and then are surprised to
7660c0ab 1027discover that the number C<0> and C<""> (the zero-length string) are, in fact,
2f9daede 1028defined values. For example, if you say
a5f75d66
AD
1029
1030 "ab" =~ /a(.*)b/;
1031
7660c0ab 1032The pattern match succeeds, and C<$1> is defined, despite the fact that it
a5f75d66 1033matched "nothing". But it didn't really match nothing--rather, it
2b5ab1e7 1034matched something that happened to be zero characters long. This is all
a5f75d66 1035very above-board and honest. When a function returns an undefined value,
2f9daede 1036it's an admission that it couldn't give you an honest answer. So you
19799a22 1037should use C<defined> only when you're questioning the integrity of what
7660c0ab 1038you're trying to do. At other times, a simple comparison to C<0> or C<""> is
2f9daede
TPG
1039what you want.
1040
dc848c6f 1041See also L</undef>, L</exists>, L</ref>.
2f9daede 1042
a0d0e21e
LW
1043=item delete EXPR
1044
01020589
GS
1045Given an expression that specifies a hash element, array element, hash slice,
1046or array slice, deletes the specified element(s) from the hash or array.
8216c1fd 1047In the case of an array, if the array elements happen to be at the end,
b76cc8ba 1048the size of the array will shrink to the highest element that tests
8216c1fd 1049true for exists() (or 0 if no such element exists).
a0d0e21e 1050
eba0920a
EM
1051Returns a list with the same number of elements as the number of elements
1052for which deletion was attempted. Each element of that list consists of
1053either the value of the element deleted, or the undefined value. In scalar
1054context, this means that you get the value of the last element deleted (or
1055the undefined value if that element did not exist).
1056
1057 %hash = (foo => 11, bar => 22, baz => 33);
1058 $scalar = delete $hash{foo}; # $scalar is 11
1059 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
1060 @array = delete @hash{qw(foo bar baz)}; # @array is (undef,undef,33)
1061
1062Deleting from C<%ENV> modifies the environment. Deleting from
01020589
GS
1063a hash tied to a DBM file deletes the entry from the DBM file. Deleting
1064from a C<tie>d hash or array may not necessarily return anything.
1065
8ea97a1e
GS
1066Deleting an array element effectively returns that position of the array
1067to its initial, uninitialized state. Subsequently testing for the same
8216c1fd
GS
1068element with exists() will return false. Note that deleting array
1069elements in the middle of an array will not shift the index of the ones
1070after them down--use splice() for that. See L</exists>.
8ea97a1e 1071
01020589 1072The following (inefficiently) deletes all the values of %HASH and @ARRAY:
a0d0e21e 1073
5f05dabc
PP
1074 foreach $key (keys %HASH) {
1075 delete $HASH{$key};
a0d0e21e
LW
1076 }
1077
01020589
GS
1078 foreach $index (0 .. $#ARRAY) {
1079 delete $ARRAY[$index];
1080 }
1081
1082And so do these:
5f05dabc 1083
01020589
GS
1084 delete @HASH{keys %HASH};
1085
9740c838 1086 delete @ARRAY[0 .. $#ARRAY];
5f05dabc 1087
2b5ab1e7 1088But both of these are slower than just assigning the empty list
01020589
GS
1089or undefining %HASH or @ARRAY:
1090
1091 %HASH = (); # completely empty %HASH
1092 undef %HASH; # forget %HASH ever existed
2b5ab1e7 1093
01020589
GS
1094 @ARRAY = (); # completely empty @ARRAY
1095 undef @ARRAY; # forget @ARRAY ever existed
2b5ab1e7
TC
1096
1097Note that the EXPR can be arbitrarily complicated as long as the final
01020589
GS
1098operation is a hash element, array element, hash slice, or array slice
1099lookup:
a0d0e21e
LW
1100
1101 delete $ref->[$x][$y]{$key};
5f05dabc 1102 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
a0d0e21e 1103
01020589
GS
1104 delete $ref->[$x][$y][$index];
1105 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1106
a0d0e21e
LW
1107=item die LIST
1108
19799a22
GS
1109Outside an C<eval>, prints the value of LIST to C<STDERR> and
1110exits with the current value of C<$!> (errno). If C<$!> is C<0>,
61eff3bc
JH
1111exits with the value of C<<< ($? >> 8) >>> (backtick `command`
1112status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside
19799a22
GS
1113an C<eval(),> the error message is stuffed into C<$@> and the
1114C<eval> is terminated with the undefined value. This makes
1115C<die> the way to raise an exception.
a0d0e21e
LW
1116
1117Equivalent examples:
1118
1119 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
54310121 1120 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
a0d0e21e 1121
ccac6780 1122If the last element of LIST does not end in a newline, the current
df37ec69
WW
1123script line number and input line number (if any) are also printed,
1124and a newline is supplied. Note that the "input line number" (also
1125known as "chunk") is subject to whatever notion of "line" happens to
1126be currently in effect, and is also available as the special variable
1127C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1128
1129Hint: sometimes appending C<", stopped"> to your message will cause it
1130to make better sense when the string C<"at foo line 123"> is appended.
1131Suppose you are running script "canasta".
a0d0e21e
LW
1132
1133 die "/etc/games is no good";
1134 die "/etc/games is no good, stopped";
1135
1136produce, respectively
1137
1138 /etc/games is no good at canasta line 123.
1139 /etc/games is no good, stopped at canasta line 123.
1140
2b5ab1e7 1141See also exit(), warn(), and the Carp module.
a0d0e21e 1142
7660c0ab
A
1143If LIST is empty and C<$@> already contains a value (typically from a
1144previous eval) that value is reused after appending C<"\t...propagated">.
fb73857a
PP
1145This is useful for propagating exceptions:
1146
1147 eval { ... };
1148 die unless $@ =~ /Expected exception/;
1149
ad216e65
JH
1150If LIST is empty and C<$@> contains an object reference that has a
1151C<PROPAGATE> method, that method will be called with additional file
1152and line number parameters. The return value replaces the value in
16869676 1153C<$@>. ie. as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >>
ad216e65
JH
1154were called.
1155
7660c0ab 1156If C<$@> is empty then the string C<"Died"> is used.
fb73857a 1157
52531d10
GS
1158die() can also be called with a reference argument. If this happens to be
1159trapped within an eval(), $@ contains the reference. This behavior permits
1160a more elaborate exception handling implementation using objects that
4375e838 1161maintain arbitrary state about the nature of the exception. Such a scheme
52531d10
GS
1162is sometimes preferable to matching particular string values of $@ using
1163regular expressions. Here's an example:
1164
da279afe 1165 use Scalar::Util 'blessed';
1166
52531d10
GS
1167 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1168 if ($@) {
da279afe 1169 if (blessed($@) && $@->isa("Some::Module::Exception")) {
52531d10
GS
1170 # handle Some::Module::Exception
1171 }
1172 else {
1173 # handle all other possible exceptions
1174 }
1175 }
1176
19799a22 1177Because perl will stringify uncaught exception messages before displaying
52531d10
GS
1178them, you may want to overload stringification operations on such custom
1179exception objects. See L<overload> for details about that.
1180
19799a22
GS
1181You can arrange for a callback to be run just before the C<die>
1182does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1183handler will be called with the error text and can change the error
1184message, if it sees fit, by calling C<die> again. See
1185L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
1186L<"eval BLOCK"> for some examples. Although this feature was meant
1187to be run only right before your program was to exit, this is not
1188currently the case--the C<$SIG{__DIE__}> hook is currently called
1189even inside eval()ed blocks/strings! If one wants the hook to do
1190nothing in such situations, put
fb73857a
PP
1191
1192 die @_ if $^S;
1193
19799a22
GS
1194as the first line of the handler (see L<perlvar/$^S>). Because
1195this promotes strange action at a distance, this counterintuitive
b76cc8ba 1196behavior may be fixed in a future release.
774d564b 1197
a0d0e21e
LW
1198=item do BLOCK
1199
1200Not really a function. Returns the value of the last command in the
1201sequence of commands indicated by BLOCK. When modified by a loop
98293880
JH
1202modifier, executes the BLOCK once before testing the loop condition.
1203(On other statements the loop modifiers test the conditional first.)
a0d0e21e 1204
4968c1e4 1205C<do BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7
TC
1206C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1207See L<perlsyn> for alternative strategies.
4968c1e4 1208
a0d0e21e
LW
1209=item do SUBROUTINE(LIST)
1210
1211A deprecated form of subroutine call. See L<perlsub>.
1212
1213=item do EXPR
1214
1215Uses the value of EXPR as a filename and executes the contents of the
ea63ef19 1216file as a Perl script.
a0d0e21e
LW
1217
1218 do 'stat.pl';
1219
1220is just like
1221
986b19de 1222 eval `cat stat.pl`;
a0d0e21e 1223
2b5ab1e7 1224except that it's more efficient and concise, keeps track of the current
ea63ef19 1225filename for error messages, searches the @INC directories, and updates
2b5ab1e7
TC
1226C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1227variables. It also differs in that code evaluated with C<do FILENAME>
1228cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1229same, however, in that it does reparse the file every time you call it,
1230so you probably don't want to do this inside a loop.
a0d0e21e 1231
8e30cc93 1232If C<do> cannot read the file, it returns undef and sets C<$!> to the
2b5ab1e7 1233error. If C<do> can read the file but cannot compile it, it
8e30cc93
G
1234returns undef and sets an error message in C<$@>. If the file is
1235successfully compiled, C<do> returns the value of the last expression
1236evaluated.
1237
a0d0e21e 1238Note that inclusion of library modules is better done with the
19799a22 1239C<use> and C<require> operators, which also do automatic error checking
4633a7c4 1240and raise an exception if there's a problem.
a0d0e21e 1241
5a964f20
TC
1242You might like to use C<do> to read in a program configuration
1243file. Manual error checking can be done this way:
1244
b76cc8ba 1245 # read in config files: system first, then user
f86cebdf 1246 for $file ("/share/prog/defaults.rc",
b76cc8ba 1247 "$ENV{HOME}/.someprogrc")
2b5ab1e7 1248 {
5a964f20 1249 unless ($return = do $file) {
f86cebdf
GS
1250 warn "couldn't parse $file: $@" if $@;
1251 warn "couldn't do $file: $!" unless defined $return;
1252 warn "couldn't run $file" unless $return;
5a964f20
TC
1253 }
1254 }
1255
a0d0e21e
LW
1256=item dump LABEL
1257
1614b0e3
JD
1258=item dump
1259
19799a22
GS
1260This function causes an immediate core dump. See also the B<-u>
1261command-line switch in L<perlrun>, which does the same thing.
1262Primarily this is so that you can use the B<undump> program (not
1263supplied) to turn your core dump into an executable binary after
1264having initialized all your variables at the beginning of the
1265program. When the new binary is executed it will begin by executing
1266a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1267Think of it as a goto with an intervening core dump and reincarnation.
1268If C<LABEL> is omitted, restarts the program from the top.
1269
1270B<WARNING>: Any files opened at the time of the dump will I<not>
1271be open any more when the program is reincarnated, with possible
b76cc8ba 1272resulting confusion on the part of Perl.
19799a22
GS
1273
1274This function is now largely obsolete, partly because it's very
1275hard to convert a core file into an executable, and because the
1276real compiler backends for generating portable bytecode and compilable
ac206dc8
RGS
1277C code have superseded it. That's why you should now invoke it as
1278C<CORE::dump()>, if you don't want to be warned against a possible
1279typo.
19799a22
GS
1280
1281If you're looking to use L<dump> to speed up your program, consider
1282generating bytecode or native C code as described in L<perlcc>. If
1283you're just trying to accelerate a CGI script, consider using the
210b36aa 1284C<mod_perl> extension to B<Apache>, or the CPAN module, CGI::Fast.
19799a22 1285You might also consider autoloading or selfloading, which at least
b76cc8ba 1286make your program I<appear> to run faster.
5a964f20 1287
aa689395
PP
1288=item each HASH
1289
5a964f20 1290When called in list context, returns a 2-element list consisting of the
aa689395 1291key and value for the next element of a hash, so that you can iterate over
74fc8b5f 1292it. When called in scalar context, returns only the key for the next
e902a979 1293element in the hash.
2f9daede 1294
ab192400 1295Entries are returned in an apparently random order. The actual random
504f80c1
JH
1296order is subject to change in future versions of perl, but it is
1297guaranteed to be in the same order as either the C<keys> or C<values>
4546b9e6
JH
1298function would produce on the same (unmodified) hash. Since Perl
12995.8.1 the ordering is different even between different runs of Perl
1300for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
ab192400
GS
1301
1302When the hash is entirely read, a null array is returned in list context
19799a22
GS
1303(which when assigned produces a false (C<0>) value), and C<undef> in
1304scalar context. The next call to C<each> after that will start iterating
1305again. There is a single iterator for each hash, shared by all C<each>,
1306C<keys>, and C<values> function calls in the program; it can be reset by
2f9daede
TPG
1307reading all the elements from the hash, or by evaluating C<keys HASH> or
1308C<values HASH>. If you add or delete elements of a hash while you're
74fc8b5f
MJD
1309iterating over it, you may get entries skipped or duplicated, so
1310don't. Exception: It is always safe to delete the item most recently
1311returned by C<each()>, which means that the following code will work:
1312
1313 while (($key, $value) = each %hash) {
1314 print $key, "\n";
1315 delete $hash{$key}; # This is safe
1316 }
aa689395 1317
f86cebdf 1318The following prints out your environment like the printenv(1) program,
aa689395 1319only in a different order:
a0d0e21e
LW
1320
1321 while (($key,$value) = each %ENV) {
1322 print "$key=$value\n";
1323 }
1324
19799a22 1325See also C<keys>, C<values> and C<sort>.
a0d0e21e
LW
1326
1327=item eof FILEHANDLE
1328
4633a7c4
LW
1329=item eof ()
1330
a0d0e21e
LW
1331=item eof
1332
1333Returns 1 if the next read on FILEHANDLE will return end of file, or if
1334FILEHANDLE is not open. FILEHANDLE may be an expression whose value
5a964f20 1335gives the real filehandle. (Note that this function actually
19799a22 1336reads a character and then C<ungetc>s it, so isn't very useful in an
748a9306 1337interactive context.) Do not read from a terminal file (or call
19799a22 1338C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
748a9306
LW
1339as terminals may lose the end-of-file condition if you do.
1340
820475bd
GS
1341An C<eof> without an argument uses the last file read. Using C<eof()>
1342with empty parentheses is very different. It refers to the pseudo file
1343formed from the files listed on the command line and accessed via the
61eff3bc
JH
1344C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1345as a normal filehandle is, an C<eof()> before C<< <> >> has been
820475bd 1346used will cause C<@ARGV> to be examined to determine if input is
67408cae 1347available. Similarly, an C<eof()> after C<< <> >> has returned
efdd0218
RB
1348end-of-file will assume you are processing another C<@ARGV> list,
1349and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1350see L<perlop/"I/O Operators">.
820475bd 1351
61eff3bc 1352In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
820475bd
GS
1353detect the end of each file, C<eof()> will only detect the end of the
1354last file. Examples:
a0d0e21e 1355
748a9306
LW
1356 # reset line numbering on each input file
1357 while (<>) {
b76cc8ba 1358 next if /^\s*#/; # skip comments
748a9306 1359 print "$.\t$_";
5a964f20
TC
1360 } continue {
1361 close ARGV if eof; # Not eof()!
748a9306
LW
1362 }
1363
a0d0e21e
LW
1364 # insert dashes just before last line of last file
1365 while (<>) {
6ac88b13 1366 if (eof()) { # check for end of last file
a0d0e21e
LW
1367 print "--------------\n";
1368 }
1369 print;
6ac88b13 1370 last if eof(); # needed if we're reading from a terminal
a0d0e21e
LW
1371 }
1372
a0d0e21e 1373Practical hint: you almost never need to use C<eof> in Perl, because the
3ce0d271
GS
1374input operators typically return C<undef> when they run out of data, or if
1375there was an error.
a0d0e21e
LW
1376
1377=item eval EXPR
1378
1379=item eval BLOCK
1380
c7cc6f1c
GS
1381In the first form, the return value of EXPR is parsed and executed as if it
1382were a little Perl program. The value of the expression (which is itself
5a964f20 1383determined within scalar context) is first parsed, and if there weren't any
be3174d2
GS
1384errors, executed in the lexical context of the current Perl program, so
1385that any variable settings or subroutine and format definitions remain
1386afterwards. Note that the value is parsed every time the eval executes.
1387If EXPR is omitted, evaluates C<$_>. This form is typically used to
1388delay parsing and subsequent execution of the text of EXPR until run time.
c7cc6f1c
GS
1389
1390In the second form, the code within the BLOCK is parsed only once--at the
1391same time the code surrounding the eval itself was parsed--and executed
1392within the context of the current Perl program. This form is typically
1393used to trap exceptions more efficiently than the first (see below), while
1394also providing the benefit of checking the code within BLOCK at compile
1395time.
1396
1397The final semicolon, if any, may be omitted from the value of EXPR or within
1398the BLOCK.
1399
1400In both forms, the value returned is the value of the last expression
5a964f20 1401evaluated inside the mini-program; a return statement may be also used, just
c7cc6f1c 1402as with subroutines. The expression providing the return value is evaluated
5a964f20 1403in void, scalar, or list context, depending on the context of the eval itself.
c7cc6f1c 1404See L</wantarray> for more on how the evaluation context can be determined.
a0d0e21e 1405
19799a22
GS
1406If there is a syntax error or runtime error, or a C<die> statement is
1407executed, an undefined value is returned by C<eval>, and C<$@> is set to the
a0d0e21e 1408error message. If there was no error, C<$@> is guaranteed to be a null
19799a22 1409string. Beware that using C<eval> neither silences perl from printing
c7cc6f1c 1410warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
d9984052
A
1411To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1412turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1413See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
a0d0e21e 1414
19799a22
GS
1415Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1416determining whether a particular feature (such as C<socket> or C<symlink>)
a0d0e21e
LW
1417is implemented. It is also Perl's exception trapping mechanism, where
1418the die operator is used to raise exceptions.
1419
1420If the code to be executed doesn't vary, you may use the eval-BLOCK
1421form to trap run-time errors without incurring the penalty of
1422recompiling each time. The error, if any, is still returned in C<$@>.
1423Examples:
1424
54310121 1425 # make divide-by-zero nonfatal
a0d0e21e
LW
1426 eval { $answer = $a / $b; }; warn $@ if $@;
1427
1428 # same thing, but less efficient
1429 eval '$answer = $a / $b'; warn $@ if $@;
1430
1431 # a compile-time error
5a964f20 1432 eval { $answer = }; # WRONG
a0d0e21e
LW
1433
1434 # a run-time error
1435 eval '$answer ='; # sets $@
1436
2b5ab1e7
TC
1437Due to the current arguably broken state of C<__DIE__> hooks, when using
1438the C<eval{}> form as an exception trap in libraries, you may wish not
1439to trigger any C<__DIE__> hooks that user code may have installed.
1440You can use the C<local $SIG{__DIE__}> construct for this purpose,
1441as shown in this example:
774d564b
PP
1442
1443 # a very private exception trap for divide-by-zero
f86cebdf
GS
1444 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1445 warn $@ if $@;
774d564b
PP
1446
1447This is especially significant, given that C<__DIE__> hooks can call
19799a22 1448C<die> again, which has the effect of changing their error messages:
774d564b
PP
1449
1450 # __DIE__ hooks may modify error messages
1451 {
f86cebdf
GS
1452 local $SIG{'__DIE__'} =
1453 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
c7cc6f1c
GS
1454 eval { die "foo lives here" };
1455 print $@ if $@; # prints "bar lives here"
774d564b
PP
1456 }
1457
19799a22 1458Because this promotes action at a distance, this counterintuitive behavior
2b5ab1e7
TC
1459may be fixed in a future release.
1460
19799a22 1461With an C<eval>, you should be especially careful to remember what's
a0d0e21e
LW
1462being looked at when:
1463
1464 eval $x; # CASE 1
1465 eval "$x"; # CASE 2
1466
1467 eval '$x'; # CASE 3
1468 eval { $x }; # CASE 4
1469
5a964f20 1470 eval "\$$x++"; # CASE 5
a0d0e21e
LW
1471 $$x++; # CASE 6
1472
2f9daede 1473Cases 1 and 2 above behave identically: they run the code contained in
19799a22 1474the variable $x. (Although case 2 has misleading double quotes making
2f9daede 1475the reader wonder what else might be happening (nothing is).) Cases 3
7660c0ab 1476and 4 likewise behave in the same way: they run the code C<'$x'>, which
19799a22 1477does nothing but return the value of $x. (Case 4 is preferred for
2f9daede
TPG
1478purely visual reasons, but it also has the advantage of compiling at
1479compile-time instead of at run-time.) Case 5 is a place where
19799a22 1480normally you I<would> like to use double quotes, except that in this
2f9daede
TPG
1481particular situation, you can just use symbolic references instead, as
1482in case 6.
a0d0e21e 1483
4968c1e4 1484C<eval BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7 1485C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
4968c1e4 1486
d819b83a
DM
1487Note that as a very special case, an C<eval ''> executed within the C<DB>
1488package doesn't see the usual surrounding lexical scope, but rather the
1489scope of the first non-DB piece of code that called it. You don't normally
1490need to worry about this unless you are writing a Perl debugger.
1491
a0d0e21e
LW
1492=item exec LIST
1493
8bf3b016
GS
1494=item exec PROGRAM LIST
1495
19799a22
GS
1496The C<exec> function executes a system command I<and never returns>--
1497use C<system> instead of C<exec> if you want it to return. It fails and
1498returns false only if the command does not exist I<and> it is executed
fb73857a 1499directly instead of via your system's command shell (see below).
a0d0e21e 1500
19799a22
GS
1501Since it's a common mistake to use C<exec> instead of C<system>, Perl
1502warns you if there is a following statement which isn't C<die>, C<warn>,
1503or C<exit> (if C<-w> is set - but you always do that). If you
1504I<really> want to follow an C<exec> with some other statement, you
55d729e4
GS
1505can use one of these styles to avoid the warning:
1506
5a964f20
TC
1507 exec ('foo') or print STDERR "couldn't exec foo: $!";
1508 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
55d729e4 1509
5a964f20 1510If there is more than one argument in LIST, or if LIST is an array
f86cebdf 1511with more than one value, calls execvp(3) with the arguments in LIST.
5a964f20
TC
1512If there is only one scalar argument or an array with one element in it,
1513the argument is checked for shell metacharacters, and if there are any,
1514the entire argument is passed to the system's command shell for parsing
1515(this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1516If there are no shell metacharacters in the argument, it is split into
b76cc8ba 1517words and passed directly to C<execvp>, which is more efficient.
19799a22 1518Examples:
a0d0e21e 1519
19799a22
GS
1520 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1521 exec "sort $outfile | uniq";
a0d0e21e
LW
1522
1523If you don't really want to execute the first argument, but want to lie
1524to the program you are executing about its own name, you can specify
1525the program you actually want to run as an "indirect object" (without a
1526comma) in front of the LIST. (This always forces interpretation of the
54310121 1527LIST as a multivalued list, even if there is only a single scalar in
a0d0e21e
LW
1528the list.) Example:
1529
1530 $shell = '/bin/csh';
1531 exec $shell '-sh'; # pretend it's a login shell
1532
1533or, more directly,
1534
1535 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1536
bb32b41a
GS
1537When the arguments get executed via the system shell, results will
1538be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1539for details.
1540
19799a22
GS
1541Using an indirect object with C<exec> or C<system> is also more
1542secure. This usage (which also works fine with system()) forces
1543interpretation of the arguments as a multivalued list, even if the
1544list had just one argument. That way you're safe from the shell
1545expanding wildcards or splitting up words with whitespace in them.
5a964f20
TC
1546
1547 @args = ( "echo surprise" );
1548
2b5ab1e7 1549 exec @args; # subject to shell escapes
f86cebdf 1550 # if @args == 1
2b5ab1e7 1551 exec { $args[0] } @args; # safe even with one-arg list
5a964f20
TC
1552
1553The first version, the one without the indirect object, ran the I<echo>
1554program, passing it C<"surprise"> an argument. The second version
1555didn't--it tried to run a program literally called I<"echo surprise">,
1556didn't find it, and set C<$?> to a non-zero value indicating failure.
1557
0f897271
GS
1558Beginning with v5.6.0, Perl will attempt to flush all files opened for
1559output before the exec, but this may not be supported on some platforms
1560(see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1561in English) or call the C<autoflush()> method of C<IO::Handle> on any
1562open handles in order to avoid lost output.
1563
19799a22 1564Note that C<exec> will not call your C<END> blocks, nor will it call
7660c0ab
A
1565any C<DESTROY> methods in your objects.
1566
a0d0e21e
LW
1567=item exists EXPR
1568
01020589 1569Given an expression that specifies a hash element or array element,
8ea97a1e
GS
1570returns true if the specified element in the hash or array has ever
1571been initialized, even if the corresponding value is undefined. The
1572element is not autovivified if it doesn't exist.
a0d0e21e 1573
01020589
GS
1574 print "Exists\n" if exists $hash{$key};
1575 print "Defined\n" if defined $hash{$key};
1576 print "True\n" if $hash{$key};
1577
1578 print "Exists\n" if exists $array[$index];
1579 print "Defined\n" if defined $array[$index];
1580 print "True\n" if $array[$index];
a0d0e21e 1581
8ea97a1e 1582A hash or array element can be true only if it's defined, and defined if
a0d0e21e
LW
1583it exists, but the reverse doesn't necessarily hold true.
1584
afebc493
GS
1585Given an expression that specifies the name of a subroutine,
1586returns true if the specified subroutine has ever been declared, even
1587if it is undefined. Mentioning a subroutine name for exists or defined
847c7ebe
DD
1588does not count as declaring it. Note that a subroutine which does not
1589exist may still be callable: its package may have an C<AUTOLOAD>
1590method that makes it spring into existence the first time that it is
1591called -- see L<perlsub>.
afebc493
GS
1592
1593 print "Exists\n" if exists &subroutine;
1594 print "Defined\n" if defined &subroutine;
1595
a0d0e21e 1596Note that the EXPR can be arbitrarily complicated as long as the final
afebc493 1597operation is a hash or array key lookup or subroutine name:
a0d0e21e 1598
2b5ab1e7
TC
1599 if (exists $ref->{A}->{B}->{$key}) { }
1600 if (exists $hash{A}{B}{$key}) { }
1601
01020589
GS
1602 if (exists $ref->{A}->{B}->[$ix]) { }
1603 if (exists $hash{A}{B}[$ix]) { }
1604
afebc493
GS
1605 if (exists &{$ref->{A}{B}{$key}}) { }
1606
01020589
GS
1607Although the deepest nested array or hash will not spring into existence
1608just because its existence was tested, any intervening ones will.
61eff3bc 1609Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
01020589
GS
1610into existence due to the existence test for the $key element above.
1611This happens anywhere the arrow operator is used, including even:
5a964f20 1612
2b5ab1e7
TC
1613 undef $ref;
1614 if (exists $ref->{"Some key"}) { }
1615 print $ref; # prints HASH(0x80d3d5c)
1616
1617This surprising autovivification in what does not at first--or even
1618second--glance appear to be an lvalue context may be fixed in a future
5a964f20 1619release.
a0d0e21e 1620
afebc493
GS
1621Use of a subroutine call, rather than a subroutine name, as an argument
1622to exists() is an error.
1623
1624 exists &sub; # OK
1625 exists &sub(); # Error
1626
a0d0e21e
LW
1627=item exit EXPR
1628
2b5ab1e7 1629Evaluates EXPR and exits immediately with that value. Example:
a0d0e21e
LW
1630
1631 $ans = <STDIN>;
1632 exit 0 if $ans =~ /^[Xx]/;
1633
19799a22 1634See also C<die>. If EXPR is omitted, exits with C<0> status. The only
2b5ab1e7
TC
1635universally recognized values for EXPR are C<0> for success and C<1>
1636for error; other values are subject to interpretation depending on the
1637environment in which the Perl program is running. For example, exiting
163869 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1639the mailer to return the item undelivered, but that's not true everywhere.
a0d0e21e 1640
19799a22
GS
1641Don't use C<exit> to abort a subroutine if there's any chance that
1642someone might want to trap whatever error happened. Use C<die> instead,
1643which can be trapped by an C<eval>.
28757baa 1644
19799a22 1645The exit() function does not always exit immediately. It calls any
2b5ab1e7 1646defined C<END> routines first, but these C<END> routines may not
19799a22 1647themselves abort the exit. Likewise any object destructors that need to
2b5ab1e7
TC
1648be called are called before the real exit. If this is a problem, you
1649can call C<POSIX:_exit($status)> to avoid END and destructor processing.
87275199 1650See L<perlmod> for details.
5a964f20 1651
a0d0e21e
LW
1652=item exp EXPR
1653
54310121 1654=item exp
bbce6d69 1655
b76cc8ba 1656Returns I<e> (the natural logarithm base) to the power of EXPR.
a0d0e21e
LW
1657If EXPR is omitted, gives C<exp($_)>.
1658
1659=item fcntl FILEHANDLE,FUNCTION,SCALAR
1660
f86cebdf 1661Implements the fcntl(2) function. You'll probably have to say
a0d0e21e
LW
1662
1663 use Fcntl;
1664
0ade1984 1665first to get the correct constant definitions. Argument processing and
b76cc8ba 1666value return works just like C<ioctl> below.
a0d0e21e
LW
1667For example:
1668
1669 use Fcntl;
5a964f20
TC
1670 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1671 or die "can't fcntl F_GETFL: $!";
1672
554ad1fc 1673You don't have to check for C<defined> on the return from C<fcntl>.
951ba7fe
GS
1674Like C<ioctl>, it maps a C<0> return from the system call into
1675C<"0 but true"> in Perl. This string is true in boolean context and C<0>
2b5ab1e7
TC
1676in numeric context. It is also exempt from the normal B<-w> warnings
1677on improper numeric conversions.
5a964f20 1678
19799a22 1679Note that C<fcntl> will produce a fatal error if used on a machine that
2b5ab1e7
TC
1680doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1681manpage to learn what functions are available on your system.
a0d0e21e 1682
be2f7487 1683Here's an example of setting a filehandle named C<REMOTE> to be
1684non-blocking at the system level. You'll have to negotiate C<$|>
1685on your own, though.
1686
1687 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
1688
1689 $flags = fcntl(REMOTE, F_GETFL, 0)
1690 or die "Can't get flags for the socket: $!\n";
1691
1692 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
1693 or die "Can't set flags for the socket: $!\n";
1694
a0d0e21e
LW
1695=item fileno FILEHANDLE
1696
2b5ab1e7
TC
1697Returns the file descriptor for a filehandle, or undefined if the
1698filehandle is not open. This is mainly useful for constructing
19799a22 1699bitmaps for C<select> and low-level POSIX tty-handling operations.
2b5ab1e7
TC
1700If FILEHANDLE is an expression, the value is taken as an indirect
1701filehandle, generally its name.
5a964f20 1702
b76cc8ba 1703You can use this to find out whether two handles refer to the
5a964f20
TC
1704same underlying descriptor:
1705
1706 if (fileno(THIS) == fileno(THAT)) {
1707 print "THIS and THAT are dups\n";
b76cc8ba
NIS
1708 }
1709
1710(Filehandles connected to memory objects via new features of C<open> may
1711return undefined even though they are open.)
1712
a0d0e21e
LW
1713
1714=item flock FILEHANDLE,OPERATION
1715
19799a22
GS
1716Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1717for success, false on failure. Produces a fatal error if used on a
2b5ab1e7 1718machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
19799a22 1719C<flock> is Perl's portable file locking interface, although it locks
2b5ab1e7
TC
1720only entire files, not records.
1721
1722Two potentially non-obvious but traditional C<flock> semantics are
1723that it waits indefinitely until the lock is granted, and that its locks
1724B<merely advisory>. Such discretionary locks are more flexible, but offer
19799a22
GS
1725fewer guarantees. This means that files locked with C<flock> may be
1726modified by programs that do not also use C<flock>. See L<perlport>,
2b5ab1e7
TC
1727your port's specific documentation, or your system-specific local manpages
1728for details. It's best to assume traditional behavior if you're writing
1729portable programs. (But if you're not, you should as always feel perfectly
1730free to write for your own system's idiosyncrasies (sometimes called
1731"features"). Slavish adherence to portability concerns shouldn't get
1732in the way of your getting your job done.)
a3cb178b 1733
8ebc5c01
PP
1734OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1735LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
ea3105be 1736you can use the symbolic names if you import them from the Fcntl module,
68dc0745
PP
1737either individually, or as a group using the ':flock' tag. LOCK_SH
1738requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
ea3105be
GS
1739releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1740LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
68dc0745
PP
1741waiting for the lock (check the return status to see if you got it).
1742
2b5ab1e7
TC
1743To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1744before locking or unlocking it.
8ebc5c01 1745
f86cebdf 1746Note that the emulation built with lockf(3) doesn't provide shared
8ebc5c01 1747locks, and it requires that FILEHANDLE be open with write intent. These
2b5ab1e7 1748are the semantics that lockf(3) implements. Most if not all systems
f86cebdf 1749implement lockf(3) in terms of fcntl(2) locking, though, so the
8ebc5c01
PP
1750differing semantics shouldn't bite too many people.
1751
becacb53
TM
1752Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
1753be open with read intent to use LOCK_SH and requires that it be open
1754with write intent to use LOCK_EX.
1755
19799a22
GS
1756Note also that some versions of C<flock> cannot lock things over the
1757network; you would need to use the more system-specific C<fcntl> for
f86cebdf
GS
1758that. If you like you can force Perl to ignore your system's flock(2)
1759function, and so provide its own fcntl(2)-based emulation, by passing
8ebc5c01
PP
1760the switch C<-Ud_flock> to the F<Configure> program when you configure
1761perl.
4633a7c4
LW
1762
1763Here's a mailbox appender for BSD systems.
a0d0e21e 1764
7e1af8bc 1765 use Fcntl ':flock'; # import LOCK_* constants
a0d0e21e
LW
1766
1767 sub lock {
7e1af8bc 1768 flock(MBOX,LOCK_EX);
a0d0e21e
LW
1769 # and, in case someone appended
1770 # while we were waiting...
1771 seek(MBOX, 0, 2);
1772 }
1773
1774 sub unlock {
7e1af8bc 1775 flock(MBOX,LOCK_UN);
a0d0e21e
LW
1776 }
1777
1778 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1779 or die "Can't open mailbox: $!";
1780
1781 lock();
1782 print MBOX $msg,"\n\n";
1783 unlock();
1784
2b5ab1e7
TC
1785On systems that support a real flock(), locks are inherited across fork()
1786calls, whereas those that must resort to the more capricious fcntl()
1787function lose the locks, making it harder to write servers.
1788
cb1a09d0 1789See also L<DB_File> for other flock() examples.
a0d0e21e
LW
1790
1791=item fork
1792
2b5ab1e7
TC
1793Does a fork(2) system call to create a new process running the
1794same program at the same point. It returns the child pid to the
1795parent process, C<0> to the child process, or C<undef> if the fork is
1796unsuccessful. File descriptors (and sometimes locks on those descriptors)
1797are shared, while everything else is copied. On most systems supporting
1798fork(), great care has gone into making it extremely efficient (for
1799example, using copy-on-write technology on data pages), making it the
1800dominant paradigm for multitasking over the last few decades.
5a964f20 1801
0f897271
GS
1802Beginning with v5.6.0, Perl will attempt to flush all files opened for
1803output before forking the child process, but this may not be supported
1804on some platforms (see L<perlport>). To be safe, you may need to set
1805C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1806C<IO::Handle> on any open handles in order to avoid duplicate output.
a0d0e21e 1807
19799a22 1808If you C<fork> without ever waiting on your children, you will
2b5ab1e7
TC
1809accumulate zombies. On some systems, you can avoid this by setting
1810C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1811forking and reaping moribund children.
cb1a09d0 1812
28757baa
PP
1813Note that if your forked child inherits system file descriptors like
1814STDIN and STDOUT that are actually connected by a pipe or socket, even
2b5ab1e7 1815if you exit, then the remote server (such as, say, a CGI script or a
19799a22 1816backgrounded job launched from a remote shell) won't think you're done.
2b5ab1e7 1817You should reopen those to F</dev/null> if it's any issue.
28757baa 1818
cb1a09d0
AD
1819=item format
1820
19799a22 1821Declare a picture format for use by the C<write> function. For
cb1a09d0
AD
1822example:
1823
54310121 1824 format Something =
cb1a09d0
AD
1825 Test: @<<<<<<<< @||||| @>>>>>
1826 $str, $%, '$' . int($num)
1827 .
1828
1829 $str = "widget";
184e9718 1830 $num = $cost/$quantity;
cb1a09d0
AD
1831 $~ = 'Something';
1832 write;
1833
1834See L<perlform> for many details and examples.
1835
8903cb82 1836=item formline PICTURE,LIST
a0d0e21e 1837
5a964f20 1838This is an internal function used by C<format>s, though you may call it,
a0d0e21e
LW
1839too. It formats (see L<perlform>) a list of values according to the
1840contents of PICTURE, placing the output into the format output
7660c0ab 1841accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
19799a22 1842Eventually, when a C<write> is done, the contents of
a0d0e21e 1843C<$^A> are written to some filehandle, but you could also read C<$^A>
7660c0ab 1844yourself and then set C<$^A> back to C<"">. Note that a format typically
19799a22 1845does one C<formline> per line of form, but the C<formline> function itself
748a9306 1846doesn't care how many newlines are embedded in the PICTURE. This means
4633a7c4 1847that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
748a9306
LW
1848You may therefore need to use multiple formlines to implement a single
1849record format, just like the format compiler.
1850
19799a22 1851Be careful if you put double quotes around the picture, because an C<@>
748a9306 1852character may be taken to mean the beginning of an array name.
19799a22 1853C<formline> always returns true. See L<perlform> for other examples.
a0d0e21e
LW
1854
1855=item getc FILEHANDLE
1856
1857=item getc
1858
1859Returns the next character from the input file attached to FILEHANDLE,
b5fe5ca2
SR
1860or the undefined value at end of file, or if there was an error (in
1861the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
1862STDIN. This is not particularly efficient. However, it cannot be
1863used by itself to fetch single characters without waiting for the user
1864to hit enter. For that, try something more like:
4633a7c4
LW
1865
1866 if ($BSD_STYLE) {
1867 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1868 }
1869 else {
54310121 1870 system "stty", '-icanon', 'eol', "\001";
4633a7c4
LW
1871 }
1872
1873 $key = getc(STDIN);
1874
1875 if ($BSD_STYLE) {
1876 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1877 }
1878 else {
5f05dabc 1879 system "stty", 'icanon', 'eol', '^@'; # ASCII null
4633a7c4
LW
1880 }
1881 print "\n";
1882
54310121
PP
1883Determination of whether $BSD_STYLE should be set
1884is left as an exercise to the reader.
cb1a09d0 1885
19799a22 1886The C<POSIX::getattr> function can do this more portably on
2b5ab1e7
TC
1887systems purporting POSIX compliance. See also the C<Term::ReadKey>
1888module from your nearest CPAN site; details on CPAN can be found on
1889L<perlmodlib/CPAN>.
a0d0e21e
LW
1890
1891=item getlogin
1892
5a964f20
TC
1893Implements the C library function of the same name, which on most
1894systems returns the current login from F</etc/utmp>, if any. If null,
19799a22 1895use C<getpwuid>.
a0d0e21e 1896
f86702cc 1897 $login = getlogin || getpwuid($<) || "Kilroy";
a0d0e21e 1898
19799a22
GS
1899Do not consider C<getlogin> for authentication: it is not as
1900secure as C<getpwuid>.
4633a7c4 1901
a0d0e21e
LW
1902=item getpeername SOCKET
1903
1904Returns the packed sockaddr address of other end of the SOCKET connection.
1905
4633a7c4
LW
1906 use Socket;
1907 $hersockaddr = getpeername(SOCK);
19799a22 1908 ($port, $iaddr) = sockaddr_in($hersockaddr);
4633a7c4
LW
1909 $herhostname = gethostbyaddr($iaddr, AF_INET);
1910 $herstraddr = inet_ntoa($iaddr);
a0d0e21e
LW
1911
1912=item getpgrp PID
1913
47e29363 1914Returns the current process group for the specified PID. Use
7660c0ab 1915a PID of C<0> to get the current process group for the
4633a7c4 1916current process. Will raise an exception if used on a machine that
f86cebdf 1917doesn't implement getpgrp(2). If PID is omitted, returns process
19799a22 1918group of current process. Note that the POSIX version of C<getpgrp>
7660c0ab 1919does not accept a PID argument, so only C<PID==0> is truly portable.
a0d0e21e
LW
1920
1921=item getppid
1922
1923Returns the process id of the parent process.
1924
4d76a344
RGS
1925Note for Linux users: on Linux, the C functions C<getpid()> and
1926C<getppid()> return different values from different threads. In order to
1927be portable, this behavior is not reflected by the perl-level function
1928C<getppid()>, that returns a consistent value across threads. If you want
e3256f86
RGS
1929to call the underlying C<getppid()>, you may use the CPAN module
1930C<Linux::Pid>.
4d76a344 1931
a0d0e21e
LW
1932=item getpriority WHICH,WHO
1933
4633a7c4
LW
1934Returns the current priority for a process, a process group, or a user.
1935(See L<getpriority(2)>.) Will raise a fatal exception if used on a
f86cebdf 1936machine that doesn't implement getpriority(2).
a0d0e21e
LW
1937
1938=item getpwnam NAME
1939
1940=item getgrnam NAME
1941
1942=item gethostbyname NAME
1943
1944=item getnetbyname NAME
1945
1946=item getprotobyname NAME
1947
1948=item getpwuid UID
1949
1950=item getgrgid GID
1951
1952=item getservbyname NAME,PROTO
1953
1954=item gethostbyaddr ADDR,ADDRTYPE
1955
1956=item getnetbyaddr ADDR,ADDRTYPE
1957
1958=item getprotobynumber NUMBER
1959
1960=item getservbyport PORT,PROTO
1961
1962=item getpwent
1963
1964=item getgrent
1965
1966=item gethostent
1967
1968=item getnetent
1969
1970=item getprotoent
1971
1972=item getservent
1973
1974=item setpwent
1975
1976=item setgrent
1977
1978=item sethostent STAYOPEN
1979
1980=item setnetent STAYOPEN
1981
1982=item setprotoent STAYOPEN
1983
1984=item setservent STAYOPEN
1985
1986=item endpwent
1987
1988=item endgrent
1989
1990=item endhostent
1991
1992=item endnetent
1993
1994=item endprotoent
1995
1996=item endservent
1997
1998These routines perform the same functions as their counterparts in the
5a964f20 1999system library. In list context, the return values from the
a0d0e21e
LW
2000various get routines are as follows:
2001
2002 ($name,$passwd,$uid,$gid,
6ee623d5 2003 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
a0d0e21e
LW
2004 ($name,$passwd,$gid,$members) = getgr*
2005 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
2006 ($name,$aliases,$addrtype,$net) = getnet*
2007 ($name,$aliases,$proto) = getproto*
2008 ($name,$aliases,$port,$proto) = getserv*
2009
2010(If the entry doesn't exist you get a null list.)
2011
4602f195
JH
2012The exact meaning of the $gcos field varies but it usually contains
2013the real name of the user (as opposed to the login name) and other
2014information pertaining to the user. Beware, however, that in many
2015system users are able to change this information and therefore it
106325ad 2016cannot be trusted and therefore the $gcos is tainted (see
2959b6e3
JH
2017L<perlsec>). The $passwd and $shell, user's encrypted password and
2018login shell, are also tainted, because of the same reason.
4602f195 2019
5a964f20 2020In scalar context, you get the name, unless the function was a
a0d0e21e
LW
2021lookup by name, in which case you get the other thing, whatever it is.
2022(If the entry doesn't exist you get the undefined value.) For example:
2023
5a964f20
TC
2024 $uid = getpwnam($name);
2025 $name = getpwuid($num);
2026 $name = getpwent();
2027 $gid = getgrnam($name);
08a33e13 2028 $name = getgrgid($num);
5a964f20
TC
2029 $name = getgrent();
2030 #etc.
a0d0e21e 2031
4602f195
JH
2032In I<getpw*()> the fields $quota, $comment, and $expire are special
2033cases in the sense that in many systems they are unsupported. If the
2034$quota is unsupported, it is an empty scalar. If it is supported, it
2035usually encodes the disk quota. If the $comment field is unsupported,
2036it is an empty scalar. If it is supported it usually encodes some
2037administrative comment about the user. In some systems the $quota
2038field may be $change or $age, fields that have to do with password
2039aging. In some systems the $comment field may be $class. The $expire
2040field, if present, encodes the expiration period of the account or the
2041password. For the availability and the exact meaning of these fields
2042in your system, please consult your getpwnam(3) documentation and your
2043F<pwd.h> file. You can also find out from within Perl what your
2044$quota and $comment fields mean and whether you have the $expire field
2045by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2046C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
2047files are only supported if your vendor has implemented them in the
2048intuitive fashion that calling the regular C library routines gets the
5d3a0a3b
GS
2049shadow versions if you're running under privilege or if there exists
2050the shadow(3) functions as found in System V ( this includes Solaris
2051and Linux.) Those systems which implement a proprietary shadow password
2052facility are unlikely to be supported.
6ee623d5 2053
19799a22 2054The $members value returned by I<getgr*()> is a space separated list of
a0d0e21e
LW
2055the login names of the members of the group.
2056
2057For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2058C, it will be returned to you via C<$?> if the function call fails. The
7660c0ab 2059C<@addrs> value returned by a successful call is a list of the raw
a0d0e21e
LW
2060addresses returned by the corresponding system library call. In the
2061Internet domain, each address is four bytes long and you can unpack it
2062by saying something like:
2063
f337b084 2064 ($a,$b,$c,$d) = unpack('W4',$addr[0]);
a0d0e21e 2065
2b5ab1e7
TC
2066The Socket library makes this slightly easier:
2067
2068 use Socket;
2069 $iaddr = inet_aton("127.1"); # or whatever address
2070 $name = gethostbyaddr($iaddr, AF_INET);
2071
2072 # or going the other way
19799a22 2073 $straddr = inet_ntoa($iaddr);
2b5ab1e7 2074
19799a22
GS
2075If you get tired of remembering which element of the return list
2076contains which return value, by-name interfaces are provided
2077in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2078C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2079and C<User::grent>. These override the normal built-ins, supplying
2080versions that return objects with the appropriate names
2081for each field. For example:
5a964f20
TC
2082
2083 use File::stat;
2084 use User::pwent;
2085 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2086
b76cc8ba
NIS
2087Even though it looks like they're the same method calls (uid),
2088they aren't, because a C<File::stat> object is different from
19799a22 2089a C<User::pwent> object.
5a964f20 2090
a0d0e21e
LW
2091=item getsockname SOCKET
2092
19799a22
GS
2093Returns the packed sockaddr address of this end of the SOCKET connection,
2094in case you don't know the address because you have several different
2095IPs that the connection might have come in on.
a0d0e21e 2096
4633a7c4
LW
2097 use Socket;
2098 $mysockaddr = getsockname(SOCK);
19799a22 2099 ($port, $myaddr) = sockaddr_in($mysockaddr);
b76cc8ba 2100 printf "Connect to %s [%s]\n",
19799a22
GS
2101 scalar gethostbyaddr($myaddr, AF_INET),
2102 inet_ntoa($myaddr);
a0d0e21e
LW
2103
2104=item getsockopt SOCKET,LEVEL,OPTNAME
2105
636e6b1f
TH
2106Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
2107Options may exist at multiple protocol levels depending on the socket
2108type, but at least the uppermost socket level SOL_SOCKET (defined in the
2109C<Socket> module) will exist. To query options at another level the
2110protocol number of the appropriate protocol controlling the option
2111should be supplied. For example, to indicate that an option is to be
2112interpreted by the TCP protocol, LEVEL should be set to the protocol
2113number of TCP, which you can get using getprotobyname.
2114
2115The call returns a packed string representing the requested socket option,
2116or C<undef> if there is an error (the error reason will be in $!). What
2117exactly is in the packed string depends in the LEVEL and OPTNAME, consult
2118your system documentation for details. A very common case however is that
2119the option is an integer, in which case the result will be an packed
2120integer which you can decode using unpack with the C<i> (or C<I>) format.
2121
2122An example testing if Nagle's algorithm is turned on on a socket:
2123
4852725b 2124 use Socket qw(:all);
636e6b1f
TH
2125
2126 defined(my $tcp = getprotobyname("tcp"))
2127 or die "Could not determine the protocol number for tcp";
4852725b
DD
2128 # my $tcp = IPPROTO_TCP; # Alternative
2129 my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
2130 or die "Could not query TCP_NODELAY socket option: $!";
636e6b1f
TH
2131 my $nodelay = unpack("I", $packed);
2132 print "Nagle's algorithm is turned ", $nodelay ? "off\n" : "on\n";
2133
a0d0e21e
LW
2134
2135=item glob EXPR
2136
0a753a76
PP
2137=item glob
2138
d9a9d457
JL
2139In list context, returns a (possibly empty) list of filename expansions on
2140the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
2141scalar context, glob iterates through such filename expansions, returning
2142undef when the list is exhausted. This is the internal function
2143implementing the C<< <*.c> >> operator, but you can use it directly. If
2144EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2145more detail in L<perlop/"I/O Operators">.
a0d0e21e 2146
3a4b19e4
GS
2147Beginning with v5.6.0, this operator is implemented using the standard
2148C<File::Glob> extension. See L<File::Glob> for details.
2149
a0d0e21e
LW
2150=item gmtime EXPR
2151
d1be9408 2152Converts a time as returned by the time function to an 8-element list
54310121 2153with the time localized for the standard Greenwich time zone.
4633a7c4 2154Typically used as follows:
a0d0e21e 2155
b76cc8ba 2156 # 0 1 2 3 4 5 6 7
48a26b3a 2157 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday) =
a0d0e21e
LW
2158 gmtime(time);
2159
48a26b3a
GS
2160All list elements are numeric, and come straight out of the C `struct
2161tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2162specified time. $mday is the day of the month, and $mon is the month
2163itself, in the range C<0..11> with 0 indicating January and 11
2164indicating December. $year is the number of years since 1900. That
2165is, $year is C<123> in year 2023. $wday is the day of the week, with
21660 indicating Sunday and 3 indicating Wednesday. $yday is the day of
b76cc8ba 2167the year, in the range C<0..364> (or C<0..365> in leap years.)
48a26b3a
GS
2168
2169Note that the $year element is I<not> simply the last two digits of
2170the year. If you assume it is, then you create non-Y2K-compliant
2171programs--and you wouldn't want to do that, would you?
2f9daede 2172
abd75f24
GS
2173The proper way to get a complete 4-digit year is simply:
2174
2175 $year += 1900;
2176
2177And to get the last two digits of the year (e.g., '01' in 2001) do:
2178
2179 $year = sprintf("%02d", $year % 100);
2180
48a26b3a 2181If EXPR is omitted, C<gmtime()> uses the current time (C<gmtime(time)>).
a0d0e21e 2182
48a26b3a 2183In scalar context, C<gmtime()> returns the ctime(3) value:
0a753a76
PP
2184
2185 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
2186
fe86afc2
NC
2187If you need local time instead of GMT use the L</localtime> builtin.
2188See also the C<timegm> function provided by the C<Time::Local> module,
2189and the strftime(3) and mktime(3) functions available via the L<POSIX> module.
7660c0ab 2190
fe86afc2
NC
2191This scalar value is B<not> locale dependent (see L<perllocale>), but is
2192instead a Perl builtin. To get somewhat similar but locale dependent date
2193strings, see the example in L</localtime>.
0a753a76 2194
62aa5637
MS
2195See L<perlport/gmtime> for portability concerns.
2196
a0d0e21e
LW
2197=item goto LABEL
2198
748a9306
LW
2199=item goto EXPR
2200
a0d0e21e
LW
2201=item goto &NAME
2202
7660c0ab 2203The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
a0d0e21e 2204execution there. It may not be used to go into any construct that
7660c0ab 2205requires initialization, such as a subroutine or a C<foreach> loop. It
0a753a76 2206also can't be used to go into a construct that is optimized away,
19799a22 2207or to get out of a block or subroutine given to C<sort>.
0a753a76 2208It can be used to go almost anywhere else within the dynamic scope,
a0d0e21e 2209including out of subroutines, but it's usually better to use some other
19799a22 2210construct such as C<last> or C<die>. The author of Perl has never felt the
7660c0ab 2211need to use this form of C<goto> (in Perl, that is--C is another matter).
1b6921cb
BT
2212(The difference being that C does not offer named loops combined with
2213loop control. Perl does, and this replaces most structured uses of C<goto>
2214in other languages.)
a0d0e21e 2215
7660c0ab
A
2216The C<goto-EXPR> form expects a label name, whose scope will be resolved
2217dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
748a9306
LW
2218necessarily recommended if you're optimizing for maintainability:
2219
2220 goto ("FOO", "BAR", "GLARCH")[$i];
2221
1b6921cb
BT
2222The C<goto-&NAME> form is quite different from the other forms of
2223C<goto>. In fact, it isn't a goto in the normal sense at all, and
2224doesn't have the stigma associated with other gotos. Instead, it
2225exits the current subroutine (losing any changes set by local()) and
2226immediately calls in its place the named subroutine using the current
2227value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2228load another subroutine and then pretend that the other subroutine had
2229been called in the first place (except that any modifications to C<@_>
6cb9131c
GS
2230in the current subroutine are propagated to the other subroutine.)
2231After the C<goto>, not even C<caller> will be able to tell that this
2232routine was called first.
2233
2234NAME needn't be the name of a subroutine; it can be a scalar variable
2235containing a code reference, or a block which evaluates to a code
2236reference.
a0d0e21e
LW
2237
2238=item grep BLOCK LIST
2239
2240=item grep EXPR,LIST
2241
2b5ab1e7
TC
2242This is similar in spirit to, but not the same as, grep(1) and its
2243relatives. In particular, it is not limited to using regular expressions.
2f9daede 2244
a0d0e21e 2245Evaluates the BLOCK or EXPR for each element of LIST (locally setting
7660c0ab 2246C<$_> to each element) and returns the list value consisting of those
19799a22
GS
2247elements for which the expression evaluated to true. In scalar
2248context, returns the number of times the expression was true.
a0d0e21e
LW
2249
2250 @foo = grep(!/^#/, @bar); # weed out comments
2251
2252or equivalently,
2253
2254 @foo = grep {!/^#/} @bar; # weed out comments
2255
be3174d2
GS
2256Note that C<$_> is an alias to the list value, so it can be used to
2257modify the elements of the LIST. While this is useful and supported,
2258it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
2259Similarly, grep returns aliases into the original list, much as a for
2260loop's index variable aliases the list elements. That is, modifying an
19799a22
GS
2261element of a list returned by grep (for example, in a C<foreach>, C<map>
2262or another C<grep>) actually modifies the element in the original list.
2b5ab1e7 2263This is usually something to be avoided when writing clear code.
a0d0e21e 2264
a4fb8298
RGS
2265If C<$_> is lexical in the scope where the C<grep> appears (because it has
2266been declared with C<my $_>) then, in addition the be locally aliased to
2267the list elements, C<$_> keeps being lexical inside the block; i.e. it
2268can't be seen from the outside, avoiding any potential side-effects.
2269
19799a22 2270See also L</map> for a list composed of the results of the BLOCK or EXPR.
38325410 2271
a0d0e21e
LW
2272=item hex EXPR
2273
54310121 2274=item hex
bbce6d69 2275
2b5ab1e7 2276Interprets EXPR as a hex string and returns the corresponding value.
38366c11 2277(To convert strings that might start with either C<0>, C<0x>, or C<0b>, see
2b5ab1e7 2278L</oct>.) If EXPR is omitted, uses C<$_>.
2f9daede
TPG
2279
2280 print hex '0xAf'; # prints '175'
2281 print hex 'aF'; # same
a0d0e21e 2282
19799a22 2283Hex strings may only represent integers. Strings that would cause
53305cf1 2284integer overflow trigger a warning. Leading whitespace is not stripped,
38366c11
DN
2285unlike oct(). To present something as hex, look into L</printf>,
2286L</sprintf>, or L</unpack>.
19799a22 2287
a0d0e21e
LW
2288=item import
2289
19799a22 2290There is no builtin C<import> function. It is just an ordinary
4633a7c4 2291method (subroutine) defined (or inherited) by modules that wish to export
19799a22 2292names to another module. The C<use> function calls the C<import> method
cea6626f 2293for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
a0d0e21e
LW
2294
2295=item index STR,SUBSTR,POSITION
2296
2297=item index STR,SUBSTR
2298
2b5ab1e7
TC
2299The index function searches for one string within another, but without
2300the wildcard-like behavior of a full regular-expression pattern match.
2301It returns the position of the first occurrence of SUBSTR in STR at
2302or after POSITION. If POSITION is omitted, starts searching from the
2303beginning of the string. The return value is based at C<0> (or whatever
2304you've set the C<$[> variable to--but don't do that). If the substring
2305is not found, returns one less than the base, ordinarily C<-1>.
a0d0e21e
LW
2306
2307=item int EXPR
2308
54310121 2309=item int
bbce6d69 2310
7660c0ab 2311Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2b5ab1e7
TC
2312You should not use this function for rounding: one because it truncates
2313towards C<0>, and two because machine representations of floating point
2314numbers can sometimes produce counterintuitive results. For example,
2315C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2316because it's really more like -268.99999999999994315658 instead. Usually,
19799a22 2317the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2b5ab1e7 2318functions will serve you better than will int().
a0d0e21e
LW
2319
2320=item ioctl FILEHANDLE,FUNCTION,SCALAR
2321
2b5ab1e7 2322Implements the ioctl(2) function. You'll probably first have to say
a0d0e21e 2323
a11c483f 2324 require "sys/ioctl.ph"; # probably in $Config{archlib}/ioctl.ph
a0d0e21e 2325
a11c483f 2326to get the correct function definitions. If F<sys/ioctl.ph> doesn't
a0d0e21e 2327exist or doesn't have the correct definitions you'll have to roll your
61eff3bc 2328own, based on your C header files such as F<< <sys/ioctl.h> >>.
5a964f20 2329(There is a Perl script called B<h2ph> that comes with the Perl kit that
54310121 2330may help you in this, but it's nontrivial.) SCALAR will be read and/or
4633a7c4 2331written depending on the FUNCTION--a pointer to the string value of SCALAR
19799a22 2332will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
4633a7c4
LW
2333has no string value but does have a numeric value, that value will be
2334passed rather than a pointer to the string value. To guarantee this to be
19799a22
GS
2335true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2336functions may be needed to manipulate the values of structures used by
b76cc8ba 2337C<ioctl>.
a0d0e21e 2338
19799a22 2339The return value of C<ioctl> (and C<fcntl>) is as follows:
a0d0e21e
LW
2340
2341 if OS returns: then Perl returns:
2342 -1 undefined value
2343 0 string "0 but true"
2344 anything else that number
2345
19799a22 2346Thus Perl returns true on success and false on failure, yet you can
a0d0e21e
LW
2347still easily determine the actual value returned by the operating
2348system:
2349
2b5ab1e7 2350 $retval = ioctl(...) || -1;
a0d0e21e
LW
2351 printf "System returned %d\n", $retval;
2352
be2f7487 2353The special string C<"0 but true"> is exempt from B<-w> complaints
5a964f20
TC
2354about improper numeric conversions.
2355
a0d0e21e
LW
2356=item join EXPR,LIST
2357
2b5ab1e7
TC
2358Joins the separate strings of LIST into a single string with fields
2359separated by the value of EXPR, and returns that new string. Example:
a0d0e21e 2360
2b5ab1e7 2361 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
a0d0e21e 2362
eb6e2d6f
GS
2363Beware that unlike C<split>, C<join> doesn't take a pattern as its
2364first argument. Compare L</split>.
a0d0e21e 2365
aa689395
PP
2366=item keys HASH
2367
504f80c1
JH
2368Returns a list consisting of all the keys of the named hash.
2369(In scalar context, returns the number of keys.)
2370
2371The keys are returned in an apparently random order. The actual
2372random order is subject to change in future versions of perl, but it
2373is guaranteed to be the same order as either the C<values> or C<each>
4546b9e6
JH
2374function produces (given that the hash has not been modified). Since
2375Perl 5.8.1 the ordering is different even between different runs of
2376Perl for security reasons (see L<perlsec/"Algorithmic Complexity
d6df3700 2377Attacks">).
504f80c1
JH
2378
2379As a side effect, calling keys() resets the HASH's internal iterator,
2f65b2f0
RGS
2380see L</each>. (In particular, calling keys() in void context resets
2381the iterator with no other overhead.)
a0d0e21e 2382
aa689395 2383Here is yet another way to print your environment:
a0d0e21e
LW
2384
2385 @keys = keys %ENV;
2386 @values = values %ENV;
b76cc8ba 2387 while (@keys) {
a0d0e21e
LW
2388 print pop(@keys), '=', pop(@values), "\n";
2389 }
2390
2391or how about sorted by key:
2392
2393 foreach $key (sort(keys %ENV)) {
2394 print $key, '=', $ENV{$key}, "\n";
2395 }
2396
8ea1e5d4
GS
2397The returned values are copies of the original keys in the hash, so
2398modifying them will not affect the original hash. Compare L</values>.
2399
19799a22 2400To sort a hash by value, you'll need to use a C<sort> function.
aa689395 2401Here's a descending numeric sort of a hash by its values:
4633a7c4 2402
5a964f20 2403 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
4633a7c4
LW
2404 printf "%4d %s\n", $hash{$key}, $key;
2405 }
2406
19799a22 2407As an lvalue C<keys> allows you to increase the number of hash buckets
aa689395
PP
2408allocated for the given hash. This can gain you a measure of efficiency if
2409you know the hash is going to get big. (This is similar to pre-extending
2410an array by assigning a larger number to $#array.) If you say
55497cff
PP
2411
2412 keys %hash = 200;
2413
ab192400
GS
2414then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2415in fact, since it rounds up to the next power of two. These
55497cff
PP
2416buckets will be retained even if you do C<%hash = ()>, use C<undef
2417%hash> if you want to free the storage while C<%hash> is still in scope.
2418You can't shrink the number of buckets allocated for the hash using
19799a22 2419C<keys> in this way (but you needn't worry about doing this by accident,
55497cff
PP
2420as trying has no effect).
2421
19799a22 2422See also C<each>, C<values> and C<sort>.
ab192400 2423
b350dd2f 2424=item kill SIGNAL, LIST
a0d0e21e 2425
b350dd2f 2426Sends a signal to a list of processes. Returns the number of
517db077
GS
2427processes successfully signaled (which is not necessarily the
2428same as the number actually killed).
a0d0e21e
LW
2429
2430 $cnt = kill 1, $child1, $child2;
2431 kill 9, @goners;
2432
b350dd2f 2433If SIGNAL is zero, no signal is sent to the process. This is a
1e9c1022 2434useful way to check that a child process is alive and hasn't changed
b350dd2f
GS
2435its UID. See L<perlport> for notes on the portability of this
2436construct.
2437
2438Unlike in the shell, if SIGNAL is negative, it kills
4633a7c4
LW
2439process groups instead of processes. (On System V, a negative I<PROCESS>
2440number will also kill process groups, but that's not portable.) That
2441means you usually want to use positive not negative signals. You may also
1e9c1022
JL
2442use a signal name in quotes.
2443
2444See L<perlipc/"Signals"> for more details.
a0d0e21e
LW
2445
2446=item last LABEL
2447
2448=item last
2449
2450The C<last> command is like the C<break> statement in C (as used in
2451loops); it immediately exits the loop in question. If the LABEL is
2452omitted, the command refers to the innermost enclosing loop. The
2453C<continue> block, if any, is not executed:
2454
4633a7c4
LW
2455 LINE: while (<STDIN>) {
2456 last LINE if /^$/; # exit when done with header
5a964f20 2457 #...
a0d0e21e
LW
2458 }
2459
4968c1e4 2460C<last> cannot be used to exit a block which returns a value such as
2b5ab1e7
TC
2461C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2462a grep() or map() operation.
4968c1e4 2463
6c1372ed
GS
2464Note that a block by itself is semantically identical to a loop
2465that executes once. Thus C<last> can be used to effect an early
2466exit out of such a block.
2467
98293880
JH
2468See also L</continue> for an illustration of how C<last>, C<next>, and
2469C<redo> work.
1d2dff63 2470
a0d0e21e
LW
2471=item lc EXPR
2472
54310121 2473=item lc
bbce6d69 2474
d1be9408 2475Returns a lowercased version of EXPR. This is the internal function
ad0029c4
JH
2476implementing the C<\L> escape in double-quoted strings. Respects
2477current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
983ffd37 2478and L<perlunicode> for more details about locale and Unicode support.
a0d0e21e 2479
7660c0ab 2480If EXPR is omitted, uses C<$_>.
bbce6d69 2481
a0d0e21e
LW
2482=item lcfirst EXPR
2483
54310121 2484=item lcfirst
bbce6d69 2485
ad0029c4
JH
2486Returns the value of EXPR with the first character lowercased. This
2487is the internal function implementing the C<\l> escape in
2488double-quoted strings. Respects current LC_CTYPE locale if C<use
983ffd37
JH
2489locale> in force. See L<perllocale> and L<perlunicode> for more
2490details about locale and Unicode support.
a0d0e21e 2491
7660c0ab 2492If EXPR is omitted, uses C<$_>.
bbce6d69 2493
a0d0e21e
LW
2494=item length EXPR
2495
54310121 2496=item length
bbce6d69 2497
974da8e5 2498Returns the length in I<characters> of the value of EXPR. If EXPR is
b76cc8ba 2499omitted, returns length of C<$_>. Note that this cannot be used on
2b5ab1e7
TC
2500an entire array or hash to find out how many elements these have.
2501For that, use C<scalar @array> and C<scalar keys %hash> respectively.
a0d0e21e 2502
974da8e5
JH
2503Note the I<characters>: if the EXPR is in Unicode, you will get the
2504number of characters, not the number of bytes. To get the length
2505in bytes, use C<do { use bytes; length(EXPR) }>, see L<bytes>.
2506
a0d0e21e
LW
2507=item link OLDFILE,NEWFILE
2508
19799a22 2509Creates a new filename linked to the old filename. Returns true for
b76cc8ba 2510success, false otherwise.
a0d0e21e
LW
2511
2512=item listen SOCKET,QUEUESIZE
2513
19799a22 2514Does the same thing that the listen system call does. Returns true if
b76cc8ba 2515it succeeded, false otherwise. See the example in
cea6626f 2516L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
2517
2518=item local EXPR
2519
19799a22 2520You really probably want to be using C<my> instead, because C<local> isn't
b76cc8ba 2521what most people think of as "local". See
13a2d996 2522L<perlsub/"Private Variables via my()"> for details.
2b5ab1e7 2523
5a964f20
TC
2524A local modifies the listed variables to be local to the enclosing
2525block, file, or eval. If more than one value is listed, the list must
2526be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2527for details, including issues with tied arrays and hashes.
a0d0e21e 2528
a0d0e21e
LW
2529=item localtime EXPR
2530
ba053783
AL
2531=item localtime
2532
19799a22 2533Converts a time as returned by the time function to a 9-element list
5f05dabc 2534with the time analyzed for the local time zone. Typically used as
a0d0e21e
LW
2535follows:
2536
54310121 2537 # 0 1 2 3 4 5 6 7 8
a0d0e21e 2538 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
ba053783 2539 localtime(time);
a0d0e21e 2540
48a26b3a 2541All list elements are numeric, and come straight out of the C `struct
ba053783
AL
2542tm'. C<$sec>, C<$min>, and C<$hour> are the seconds, minutes, and hours
2543of the specified time.
48a26b3a 2544
ba053783
AL
2545C<$mday> is the day of the month, and C<$mon> is the month itself, in
2546the range C<0..11> with 0 indicating January and 11 indicating December.
2547This makes it easy to get a month name from a list:
54310121 2548
ba053783
AL
2549 my @abbr = qw( Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec );
2550 print "$abbr[$mon] $mday";
2551 # $mon=9, $mday=18 gives "Oct 18"
abd75f24 2552
ba053783
AL
2553C<$year> is the number of years since 1900, not just the last two digits
2554of the year. That is, C<$year> is C<123> in year 2023. The proper way
2555to get a complete 4-digit year is simply:
abd75f24 2556
ba053783 2557 $year += 1900;
abd75f24 2558
ba053783
AL
2559To get the last two digits of the year (e.g., '01' in 2001) do:
2560
2561 $year = sprintf("%02d", $year % 100);
2562
2563C<$wday> is the day of the week, with 0 indicating Sunday and 3 indicating
2564Wednesday. C<$yday> is the day of the year, in the range C<0..364>
2565(or C<0..365> in leap years.)
2566
2567C<$isdst> is true if the specified time occurs during Daylight Saving
2568Time, false otherwise.
abd75f24 2569
48a26b3a 2570If EXPR is omitted, C<localtime()> uses the current time (C<localtime(time)>).
a0d0e21e 2571
48a26b3a 2572In scalar context, C<localtime()> returns the ctime(3) value:
a0d0e21e 2573
5f05dabc 2574 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
a0d0e21e 2575
fe86afc2
NC
2576This scalar value is B<not> locale dependent but is a Perl builtin. For GMT
2577instead of local time use the L</gmtime> builtin. See also the
2578C<Time::Local> module (to convert the second, minutes, hours, ... back to
2579the integer value returned by time()), and the L<POSIX> module's strftime(3)
2580and mktime(3) functions.
2581
2582To get somewhat similar but locale dependent date strings, set up your
2583locale environment variables appropriately (please see L<perllocale>) and
2584try for example:
a3cb178b 2585
5a964f20 2586 use POSIX qw(strftime);
2b5ab1e7 2587 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
fe86afc2
NC
2588 # or for GMT formatted appropriately for your locale:
2589 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
a3cb178b
GS
2590
2591Note that the C<%a> and C<%b>, the short forms of the day of the week
2592and the month of the year, may not necessarily be three characters wide.
a0d0e21e 2593
62aa5637
MS
2594See L<perlport/localtime> for portability concerns.
2595
07698885 2596=item lock THING
19799a22 2597
01e6739c 2598This function places an advisory lock on a shared variable, or referenced
03730085 2599object contained in I<THING> until the lock goes out of scope.
a6d5524e 2600
f3a23afb 2601lock() is a "weak keyword" : this means that if you've defined a function
67408cae 2602by this name (before any calls to it), that function will be called
03730085
AB
2603instead. (However, if you've said C<use threads>, lock() is always a
2604keyword.) See L<threads>.
19799a22 2605
a0d0e21e
LW
2606=item log EXPR
2607
54310121 2608=item log
bbce6d69 2609
2b5ab1e7
TC
2610Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2611returns log of C<$_>. To get the log of another base, use basic algebra:
19799a22 2612The base-N log of a number is equal to the natural log of that number
2b5ab1e7
TC
2613divided by the natural log of N. For example:
2614
2615 sub log10 {
2616 my $n = shift;
2617 return log($n)/log(10);
b76cc8ba 2618 }
2b5ab1e7
TC
2619
2620See also L</exp> for the inverse operation.
a0d0e21e 2621
a0d0e21e
LW
2622=item lstat EXPR
2623
54310121 2624=item lstat
bbce6d69 2625
19799a22 2626Does the same thing as the C<stat> function (including setting the
5a964f20
TC
2627special C<_> filehandle) but stats a symbolic link instead of the file
2628the symbolic link points to. If symbolic links are unimplemented on
c837d5b4
DP
2629your system, a normal C<stat> is done. For much more detailed
2630information, please see the documentation for C<stat>.
a0d0e21e 2631
7660c0ab 2632If EXPR is omitted, stats C<$_>.
bbce6d69 2633
a0d0e21e
LW
2634=item m//
2635
2636The match operator. See L<perlop>.
2637
2638=item map BLOCK LIST
2639
2640=item map EXPR,LIST
2641
19799a22
GS
2642Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2643C<$_> to each element) and returns the list value composed of the
2644results of each such evaluation. In scalar context, returns the
2645total number of elements so generated. Evaluates BLOCK or EXPR in
2646list context, so each element of LIST may produce zero, one, or
2647more elements in the returned value.
dd99ebda 2648
a0d0e21e
LW
2649 @chars = map(chr, @nums);
2650
2651translates a list of numbers to the corresponding characters. And
2652
4633a7c4 2653 %hash = map { getkey($_) => $_ } @array;
a0d0e21e
LW
2654
2655is just a funny way to write
2656
2657 %hash = ();
2658 foreach $_ (@array) {
4633a7c4 2659 $hash{getkey($_)} = $_;
a0d0e21e
LW
2660 }
2661
be3174d2
GS
2662Note that C<$_> is an alias to the list value, so it can be used to
2663modify the elements of the LIST. While this is useful and supported,
2664it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
2665Using a regular C<foreach> loop for this purpose would be clearer in
2666most cases. See also L</grep> for an array composed of those items of
2667the original list for which the BLOCK or EXPR evaluates to true.
fb73857a 2668
a4fb8298
RGS
2669If C<$_> is lexical in the scope where the C<map> appears (because it has
2670been declared with C<my $_>) then, in addition the be locally aliased to
2671the list elements, C<$_> keeps being lexical inside the block; i.e. it
2672can't be seen from the outside, avoiding any potential side-effects.
2673
205fdb4d
NC
2674C<{> starts both hash references and blocks, so C<map { ...> could be either
2675the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look
2676ahead for the closing C<}> it has to take a guess at which its dealing with
2677based what it finds just after the C<{>. Usually it gets it right, but if it
2678doesn't it won't realize something is wrong until it gets to the C<}> and
2679encounters the missing (or unexpected) comma. The syntax error will be
2680reported close to the C<}> but you'll need to change something near the C<{>
2681such as using a unary C<+> to give perl some help:
2682
2683 %hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong
2684 %hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right
2685 %hash = map { ("\L$_", 1) } @array # this also works
2686 %hash = map { lc($_), 1 } @array # as does this.
2687 %hash = map +( lc($_), 1 ), @array # this is EXPR and works!
cea6626f 2688
205fdb4d
NC
2689 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
2690
2691or to force an anon hash constructor use C<+{>
2692
2693 @hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end
2694
2695and you get list of anonymous hashes each with only 1 entry.
2696
19799a22 2697=item mkdir FILENAME,MASK
a0d0e21e 2698
5a211162
GS
2699=item mkdir FILENAME
2700
491873e5
RGS
2701=item mkdir
2702
0591cd52 2703Creates the directory specified by FILENAME, with permissions
19799a22
GS
2704specified by MASK (as modified by C<umask>). If it succeeds it
2705returns true, otherwise it returns false and sets C<$!> (errno).
491873e5
RGS
2706If omitted, MASK defaults to 0777. If omitted, FILENAME defaults
2707to C<$_>.
0591cd52 2708
19799a22 2709In general, it is better to create directories with permissive MASK,
0591cd52 2710and let the user modify that with their C<umask>, than it is to supply
19799a22 2711a restrictive MASK and give the user no way to be more permissive.
0591cd52
NT
2712The exceptions to this rule are when the file or directory should be
2713kept private (mail files, for instance). The perlfunc(1) entry on
19799a22 2714C<umask> discusses the choice of MASK in more detail.
a0d0e21e 2715
cc1852e8
JH
2716Note that according to the POSIX 1003.1-1996 the FILENAME may have any
2717number of trailing slashes. Some operating and filesystems do not get
2718this right, so Perl automatically removes all trailing slashes to keep
2719everyone happy.
2720
a0d0e21e
LW
2721=item msgctl ID,CMD,ARG
2722
f86cebdf 2723Calls the System V IPC function msgctl(2). You'll probably have to say
0ade1984
JH
2724
2725 use IPC::SysV;
2726
7660c0ab
A
2727first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2728then ARG must be a variable which will hold the returned C<msqid_ds>
951ba7fe
GS
2729structure. Returns like C<ioctl>: the undefined value for error,
2730C<"0 but true"> for zero, or the actual return value otherwise. See also
4755096e 2731L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation.
a0d0e21e
LW
2732
2733=item msgget KEY,FLAGS
2734
f86cebdf 2735Calls the System V IPC function msgget(2). Returns the message queue
4755096e
GS
2736id, or the undefined value if there is an error. See also
2737L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation.
a0d0e21e 2738
a0d0e21e
LW
2739=item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2740
2741Calls the System V IPC function msgrcv to receive a message from
2742message queue ID into variable VAR with a maximum message size of
41d6edb2
JH
2743SIZE. Note that when a message is received, the message type as a
2744native long integer will be the first thing in VAR, followed by the
2745actual message. This packing may be opened with C<unpack("l! a*")>.
2746Taints the variable. Returns true if successful, or false if there is
4755096e
GS
2747an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and
2748C<IPC::SysV::Msg> documentation.
41d6edb2
JH
2749
2750=item msgsnd ID,MSG,FLAGS
2751
2752Calls the System V IPC function msgsnd to send the message MSG to the
2753message queue ID. MSG must begin with the native long integer message
2754type, and be followed by the length of the actual message, and finally
2755the message itself. This kind of packing can be achieved with
2756C<pack("l! a*", $type, $message)>. Returns true if successful,
2757or false if there is an error. See also C<IPC::SysV>
2758and C<IPC::SysV::Msg> documentation.
a0d0e21e
LW
2759
2760=item my EXPR
2761
307ea6df
JH
2762=item my TYPE EXPR
2763
1d2de774 2764=item my EXPR : ATTRS
09bef843 2765
1d2de774 2766=item my TYPE EXPR : ATTRS
307ea6df 2767
19799a22 2768A C<my> declares the listed variables to be local (lexically) to the
1d2de774
JH
2769enclosing block, file, or C<eval>. If more than one value is listed,
2770the list must be placed in parentheses.
307ea6df 2771
1d2de774
JH
2772The exact semantics and interface of TYPE and ATTRS are still
2773evolving. TYPE is currently bound to the use of C<fields> pragma,
307ea6df
JH
2774and attributes are handled using the C<attributes> pragma, or starting
2775from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
2776L<perlsub/"Private Variables via my()"> for details, and L<fields>,
2777L<attributes>, and L<Attribute::Handlers>.
4633a7c4 2778
a0d0e21e
LW
2779=item next LABEL
2780
2781=item next
2782
2783The C<next> command is like the C<continue> statement in C; it starts
2784the next iteration of the loop:
2785
4633a7c4
LW
2786 LINE: while (<STDIN>) {
2787 next LINE if /^#/; # discard comments
5a964f20 2788 #...
a0d0e21e
LW
2789 }
2790
2791Note that if there were a C<continue> block on the above, it would get
2792executed even on discarded lines. If the LABEL is omitted, the command
2793refers to the innermost enclosing loop.
2794
4968c1e4 2795C<next> cannot be used to exit a block which returns a value such as
2b5ab1e7
TC
2796C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2797a grep() or map() operation.
4968c1e4 2798
6c1372ed
GS
2799Note that a block by itself is semantically identical to a loop
2800that executes once. Thus C<next> will exit such a block early.
2801
98293880
JH
2802See also L</continue> for an illustration of how C<last>, C<next>, and
2803C<redo> work.
1d2dff63 2804
4a66ea5a
RGS
2805=item no Module VERSION LIST
2806
2807=item no Module VERSION
2808
a0d0e21e
LW
2809=item no Module LIST
2810
4a66ea5a
RGS
2811=item no Module
2812
593b9c14 2813See the C<use> function, of which C<no> is the opposite.
a0d0e21e
LW
2814
2815=item oct EXPR
2816
54310121 2817=item oct
bbce6d69 2818
4633a7c4 2819Interprets EXPR as an octal string and returns the corresponding
4f19785b
WSI
2820value. (If EXPR happens to start off with C<0x>, interprets it as a
2821hex string. If EXPR starts off with C<0b>, it is interpreted as a
53305cf1
NC
2822binary string. Leading whitespace is ignored in all three cases.)
2823The following will handle decimal, binary, octal, and hex in the standard
2824Perl or C notation:
a0d0e21e
LW
2825
2826 $val = oct($val) if $val =~ /^0/;
2827
19799a22
GS
2828If EXPR is omitted, uses C<$_>. To go the other way (produce a number
2829in octal), use sprintf() or printf():
2830
2831 $perms = (stat("filename"))[2] & 07777;
2832 $oct_perms = sprintf "%lo", $perms;
2833
2834The oct() function is commonly used when a string such as C<644> needs
2835to be converted into a file mode, for example. (Although perl will
2836automatically convert strings into numbers as needed, this automatic
2837conversion assumes base 10.)
a0d0e21e
LW
2838
2839=item open FILEHANDLE,EXPR
2840
68bd7414
NIS
2841=item open FILEHANDLE,MODE,EXPR
2842
2843=item open FILEHANDLE,MODE,EXPR,LIST
2844
ba964c95
T
2845=item open FILEHANDLE,MODE,REFERENCE
2846
a0d0e21e
LW
2847=item open FILEHANDLE
2848
2849Opens the file whose filename is given by EXPR, and associates it with
ed53a2bb
JH
2850FILEHANDLE.
2851
2852(The following is a comprehensive reference to open(): for a gentler
2853introduction you may consider L<perlopentut>.)
2854
a28cd5c9
NT
2855If FILEHANDLE is an undefined scalar variable (or array or hash element)
2856the variable is assigned a reference to a new anonymous filehandle,
2857otherwise if FILEHANDLE is an expression, its value is used as the name of
2858the real filehandle wanted. (This is considered a symbolic reference, so
2859C<use strict 'refs'> should I<not> be in effect.)
ed53a2bb
JH
2860
2861If EXPR is omitted, the scalar variable of the same name as the
2862FILEHANDLE contains the filename. (Note that lexical variables--those
2863declared with C<my>--will not work for this purpose; so if you're
67408cae 2864using C<my>, specify EXPR in your call to open.)
ed53a2bb
JH
2865
2866If three or more arguments are specified then the mode of opening and
2867the file name are separate. If MODE is C<< '<' >> or nothing, the file
2868is opened for input. If MODE is C<< '>' >>, the file is truncated and
2869opened for output, being created if necessary. If MODE is C<<< '>>' >>>,
b76cc8ba 2870the file is opened for appending, again being created if necessary.
5a964f20 2871
ed53a2bb
JH
2872You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to
2873indicate that you want both read and write access to the file; thus
2874C<< '+<' >> is almost always preferred for read/write updates--the C<<
2875'+>' >> mode would clobber the file first. You can't usually use
2876either read-write mode for updating textfiles, since they have
2877variable length records. See the B<-i> switch in L<perlrun> for a
2878better approach. The file is created with permissions of C<0666>
2879modified by the process' C<umask> value.
2880
2881These various prefixes correspond to the fopen(3) modes of C<'r'>,
2882C<'r+'>, C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
5f05dabc 2883
6170680b
IZ
2884In the 2-arguments (and 1-argument) form of the call the mode and
2885filename should be concatenated (in this order), possibly separated by
68bd7414
NIS
2886spaces. It is possible to omit the mode in these forms if the mode is
2887C<< '<' >>.
6170680b 2888
7660c0ab 2889If the filename begins with C<'|'>, the filename is interpreted as a
5a964f20 2890command to which output is to be piped, and if the filename ends with a
f244e06d
GS
2891C<'|'>, the filename is interpreted as a command which pipes output to
2892us. See L<perlipc/"Using open() for IPC">
19799a22 2893for more examples of this. (You are not allowed to C<open> to a command
5a964f20 2894that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
4a4eefd0
GS
2895and L<perlipc/"Bidirectional Communication with Another Process">
2896for alternatives.)
cb1a09d0 2897
ed53a2bb
JH
2898For three or more arguments if MODE is C<'|-'>, the filename is
2899interpreted as a command to which output is to be piped, and if MODE
2900is C<'-|'>, the filename is interpreted as a command which pipes
2901output to us. In the 2-arguments (and 1-argument) form one should
2902replace dash (C<'-'>) with the command.
2903See L<perlipc/"Using open() for IPC"> for more examples of this.
2904(You are not allowed to C<open> to a command that pipes both in I<and>
2905out, but see L<IPC::Open2>, L<IPC::Open3>, and
2906L<perlipc/"Bidirectional Communication"> for alternatives.)
2907
2908In the three-or-more argument form of pipe opens, if LIST is specified
2909(extra arguments after the command name) then LIST becomes arguments
2910to the command invoked if the platform supports it. The meaning of
2911C<open> with more than three arguments for non-pipe modes is not yet
2912specified. Experimental "layers" may give extra LIST arguments
2913meaning.
6170680b
IZ
2914
2915In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
b76cc8ba 2916and opening C<< '>-' >> opens STDOUT.
6170680b 2917
fae2c0fb
RGS
2918You may use the three-argument form of open to specify IO "layers"
2919(sometimes also referred to as "disciplines") to be applied to the handle
2920that affect how the input and output are processed (see L<open> and
2921L<PerlIO> for more details). For example
7207e29d 2922
9124316e
JH
2923 open(FH, "<:utf8", "file")
2924
2925will open the UTF-8 encoded file containing Unicode characters,
fae2c0fb
RGS
2926see L<perluniintro>. (Note that if layers are specified in the
2927three-arg form then default layers set by the C<open> pragma are
01e6739c 2928ignored.)
ed53a2bb
JH
2929
2930Open returns nonzero upon success, the undefined value otherwise. If
2931the C<open> involved a pipe, the return value happens to be the pid of
2932the subprocess.
cb1a09d0 2933
ed53a2bb
JH
2934If you're running Perl on a system that distinguishes between text
2935files and binary files, then you should check out L</binmode> for tips
2936for dealing with this. The key distinction between systems that need
2937C<binmode> and those that don't is their text file formats. Systems
8939ba94 2938like Unix, Mac OS, and Plan 9, which delimit lines with a single
ed53a2bb
JH
2939character, and which encode that character in C as C<"\n">, do not
2940need C<binmode>. The rest need it.
cb1a09d0 2941
fb73857a 2942When opening a file, it's usually a bad idea to continue normal execution
19799a22
GS
2943if the request failed, so C<open> is frequently used in connection with
2944C<die>. Even if C<die> won't do what you want (say, in a CGI script,
fb73857a 2945where you want to make a nicely formatted error message (but there are
5a964f20 2946modules that can help with that problem)) you should always check
19799a22 2947the return value from opening a file. The infrequent exception is when
fb73857a
PP
2948working with an unopened filehandle is actually what you want to do.
2949
ed53a2bb
JH
2950As a special case the 3 arg form with a read/write mode and the third
2951argument being C<undef>:
b76cc8ba
NIS
2952
2953 open(TMP, "+>", undef) or die ...
2954
f253e835
JH
2955opens a filehandle to an anonymous temporary file. Also using "+<"
2956works for symmetry, but you really should consider writing something
2957to the temporary file first. You will need to seek() to do the
2958reading.
b76cc8ba 2959
2ce64696
JC
2960Since v5.8.0, perl has built using PerlIO by default. Unless you've
2961changed this (ie Configure -Uuseperlio), you can open file handles to
2962"in memory" files held in Perl scalars via:
ba964c95 2963
b996200f
SB
2964 open($fh, '>', \$variable) || ..
2965
2966Though if you try to re-open C<STDOUT> or C<STDERR> as an "in memory"
2967file, you have to close it first:
2968
2969 close STDOUT;
2970 open STDOUT, '>', \$variable or die "Can't open STDOUT: $!";
ba964c95 2971
cb1a09d0 2972Examples:
a0d0e21e
LW
2973
2974 $ARTICLE = 100;
2975 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2976 while (<ARTICLE>) {...
2977
6170680b 2978 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
fb73857a 2979 # if the open fails, output is discarded
a0d0e21e 2980
6170680b 2981 open(DBASE, '+<', 'dbase.mine') # open for update
fb73857a 2982 or die "Can't open 'dbase.mine' for update: $!";
cb1a09d0 2983
6170680b
IZ
2984 open(DBASE, '+<dbase.mine') # ditto
2985 or die "Can't open 'dbase.mine' for update: $!";
2986
2987 open(ARTICLE, '-|', "caesar <$article") # decrypt article
fb73857a 2988 or die "Can't start caesar: $!";
a0d0e21e 2989
6170680b
IZ
2990 open(ARTICLE, "caesar <$article |") # ditto
2991 or die "Can't start caesar: $!";
2992
2359510d 2993 open(EXTRACT, "|sort >Tmp$$") # $$ is our process id
fb73857a 2994 or die "Can't start sort: $!";
a0d0e21e 2995
ba964c95
T
2996 # in memory files
2997 open(MEMORY,'>', \$var)
2998 or die "Can't open memory file: $!";
2999 print MEMORY "foo!\n"; # output will end up in $var
3000
a0d0e21e
LW
3001 # process argument list of files along with any includes
3002
3003 foreach $file (@ARGV) {
3004 process($file, 'fh00');
3005 }
3006
3007 sub process {
5a964f20 3008 my($filename, $input) = @_;
a0d0e21e
LW
3009 $input++; # this is a string increment
3010 unless (open($input, $filename)) {
3011 print STDERR "Can't open $filename: $!\n";
3012 return;
3013 }
3014
5a964f20 3015 local $_;
a0d0e21e
LW
3016 while (<$input>) { # note use of indirection
3017 if (/^#include "(.*)"/) {
3018 process($1, $input);
3019 next;
3020 }
5a964f20 3021 #... # whatever
a0d0e21e
LW
3022 }
3023 }
3024
ae4c5402 3025See L<perliol> for detailed info on PerlIO.
2ce64696 3026
a0d0e21e 3027You may also, in the Bourne shell tradition, specify an EXPR beginning
00cafafa
JH
3028with C<< '>&' >>, in which case the rest of the string is interpreted
3029as the name of a filehandle (or file descriptor, if numeric) to be
3030duped (as L<dup(2)>) and opened. You may use C<&> after C<< > >>,
3031C<<< >> >>>, C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>.
3032The mode you specify should match the mode of the original filehandle.
3033(Duping a filehandle does not take into account any existing contents
3034of IO buffers.) If you use the 3 arg form then you can pass either a
3035number, the name of a filehandle or the normal "reference to a glob".
6170680b 3036
eae1b76b
SB
3037Here is a script that saves, redirects, and restores C<STDOUT> and
3038C<STDERR> using various methods:
a0d0e21e
LW
3039
3040 #!/usr/bin/perl
eae1b76b
SB
3041 open my $oldout, ">&STDOUT" or die "Can't dup STDOUT: $!";
3042 open OLDERR, ">&", \*STDERR or die "Can't dup STDERR: $!";
818c4caa 3043
eae1b76b
SB
3044 open STDOUT, '>', "foo.out" or die "Can't redirect STDOUT: $!";
3045 open STDERR, ">&STDOUT" or die "Can't dup STDOUT: $!";
a0d0e21e 3046
eae1b76b
SB
3047 select STDERR; $| = 1; # make unbuffered
3048 select STDOUT; $| = 1; # make unbuffered
a0d0e21e
LW
3049
3050 print STDOUT "stdout 1\n"; # this works for
3051 print STDERR "stderr 1\n"; # subprocesses too
3052
eae1b76b
SB
3053 open STDOUT, ">&", $oldout or die "Can't dup \$oldout: $!";
3054 open STDERR, ">&OLDERR" or die "Can't dup OLDERR: $!";
a0d0e21e
LW
3055
3056 print STDOUT "stdout 2\n";
3057 print STDERR "stderr 2\n";
3058
ef8b303f
JH
3059If you specify C<< '<&=X' >>, where C<X> is a file descriptor number
3060or a filehandle, then Perl will do an equivalent of C's C<fdopen> of
3061that file descriptor (and not call L<dup(2)>); this is more
3062parsimonious of file descriptors. For example:
a0d0e21e 3063
00cafafa 3064 # open for input, reusing the fileno of $fd
a0d0e21e 3065 open(FILEHANDLE, "<&=$fd")
df632fdf 3066
b76cc8ba 3067or
df632fdf 3068
b76cc8ba 3069 open(FILEHANDLE, "<&=", $fd)
a0d0e21e 3070
00cafafa
JH
3071or
3072
3073 # open for append, using the fileno of OLDFH
3074 open(FH, ">>&=", OLDFH)
3075
3076or
3077
3078 open(FH, ">>&=OLDFH")
3079
ef8b303f
JH
3080Being parsimonious on filehandles is also useful (besides being
3081parsimonious) for example when something is dependent on file
3082descriptors, like for example locking using flock(). If you do just
3083C<< open(A, '>>&B') >>, the filehandle A will not have the same file
3084descriptor as B, and therefore flock(A) will not flock(B), and vice
3085versa. But with C<< open(A, '>>&=B') >> the filehandles will share
3086the same file descriptor.
3087
3088Note that if you are using Perls older than 5.8.0, Perl will be using
3089the standard C libraries' fdopen() to implement the "=" functionality.
3090On many UNIX systems fdopen() fails when file descriptors exceed a
3091certain value, typically 255. For Perls 5.8.0 and later, PerlIO is
3092most often the default.
4af147f6 3093
df632fdf
JH
3094You can see whether Perl has been compiled with PerlIO or not by
3095running C<perl -V> and looking for C<useperlio=> line. If C<useperlio>
3096is C<define>, you have PerlIO, otherwise you don't.
3097
6170680b
IZ
3098If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
3099with 2-arguments (or 1-argument) form of open(), then
a0d0e21e 3100there is an implicit fork done, and the return value of open is the pid
7660c0ab 3101of the child within the parent process, and C<0> within the child
184e9718 3102process. (Use C<defined($pid)> to determine whether the open was successful.)
a0d0e21e
LW
3103The filehandle behaves normally for the parent, but i/o to that
3104filehandle is piped from/to the STDOUT/STDIN of the child process.
3105In the child process the filehandle isn't opened--i/o happens from/to
3106the new STDOUT or STDIN. Typically this is used like the normal
3107piped open when you want to exercise more control over just how the
3108pipe command gets executed, such as when you are running setuid, and
54310121 3109don't want to have to scan shell commands for metacharacters.
6170680b 3110The following triples are more or less equivalent:
a0d0e21e
LW
3111
3112 open(FOO, "|tr '[a-z]' '[A-Z]'");
6170680b
IZ
3113 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
3114 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
b76cc8ba 3115 open(FOO, '|-', "tr", '[a-z]', '[A-Z]');
a0d0e21e
LW
3116
3117 open(FOO, "cat -n '$file'|");
6170680b
IZ
3118 open(FOO, '-|', "cat -n '$file'");
3119 open(FOO, '-|') || exec 'cat', '-n', $file;
b76cc8ba
NIS
3120 open(FOO, '-|', "cat", '-n', $file);
3121
3122The last example in each block shows the pipe as "list form", which is
64da03b2
JH
3123not yet supported on all platforms. A good rule of thumb is that if
3124your platform has true C<fork()> (in other words, if your platform is
3125UNIX) you can use the list form.
a0d0e21e 3126
4633a7c4
LW
3127See L<perlipc/"Safe Pipe Opens"> for more examples of this.
3128
0f897271
GS
3129Beginning with v5.6.0, Perl will attempt to flush all files opened for
3130output before any operation that may do a fork, but this may not be
3131supported on some platforms (see L<perlport>). To be safe, you may need
3132to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
3133of C<IO::Handle> on any open handles.
3134
ed53a2bb
JH
3135On systems that support a close-on-exec flag on files, the flag will
3136be set for the newly opened file descriptor as determined by the value
3137of $^F. See L<perlvar/$^F>.
a0d0e21e 3138
0dccf244 3139Closing any piped filehandle causes the parent process to wait for the
e5218da5
GA
3140child to finish, and returns the status value in C<$?> and
3141C<${^CHILD_ERROR_NATIVE}>.
0dccf244 3142
ed53a2bb
JH
3143The filename passed to 2-argument (or 1-argument) form of open() will
3144have leading and trailing whitespace deleted, and the normal
3145redirection characters honored. This property, known as "magic open",
5a964f20 3146can often be used to good effect. A user could specify a filename of
7660c0ab 3147F<"rsh cat file |">, or you could change certain filenames as needed:
5a964f20
TC
3148
3149 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
3150 open(FH, $filename) or die "Can't open $filename: $!";
3151
6170680b
IZ
3152Use 3-argument form to open a file with arbitrary weird characters in it,
3153
3154 open(FOO, '<', $file);
3155
3156otherwise it's necessary to protect any leading and trailing whitespace:
5a964f20
TC
3157
3158 $file =~ s#^(\s)#./$1#;
3159 open(FOO, "< $file\0");
3160
a31a806a 3161(this may not work on some bizarre filesystems). One should
106325ad 3162conscientiously choose between the I<magic> and 3-arguments form
6170680b
IZ
3163of open():
3164
3165 open IN, $ARGV[0];
3166
3167will allow the user to specify an argument of the form C<"rsh cat file |">,
3168but will not work on a filename which happens to have a trailing space, while
3169
3170 open IN, '<', $ARGV[0];
3171
3172will have exactly the opposite restrictions.
3173
19799a22 3174If you want a "real" C C<open> (see L<open(2)> on your system), then you
6170680b
IZ
3175should use the C<sysopen> function, which involves no such magic (but
3176may use subtly different filemodes than Perl open(), which is mapped
3177to C fopen()). This is
5a964f20
TC
3178another way to protect your filenames from interpretation. For example:
3179
3180 use IO::Handle;
3181 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
3182 or die "sysopen $path: $!";
3183 $oldfh = select(HANDLE); $| = 1; select($oldfh);
38762f02 3184 print HANDLE "stuff $$\n";
5a964f20
TC
3185 seek(HANDLE, 0, 0);
3186 print "File contains: ", <HANDLE>;
3187
7660c0ab
A
3188Using the constructor from the C<IO::Handle> package (or one of its
3189subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
5a964f20
TC
3190filehandles that have the scope of whatever variables hold references to
3191them, and automatically close whenever and however you leave that scope:
c07a80fd 3192
5f05dabc 3193 use IO::File;
5a964f20 3194 #...
c07a80fd
PP
3195 sub read_myfile_munged {
3196 my $ALL = shift;
5f05dabc 3197 my $handle = new IO::File;
c07a80fd
PP
3198 open($handle, "myfile") or die "myfile: $!";
3199 $first = <$handle>
3200 or return (); # Automatically closed here.
3201 mung $first or die "mung failed"; # Or here.
3202 return $first, <$handle> if $ALL; # Or here.
3203 $first; # Or here.
3204 }
3205
b687b08b 3206See L</seek> for some details about mixing reading and writing.
a0d0e21e
LW
3207
3208=item opendir DIRHANDLE,EXPR
3209
19799a22
GS
3210Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
3211C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
a28cd5c9
NT
3212DIRHANDLE may be an expression whose value can be used as an indirect
3213dirhandle, usually the real dirhandle name. If DIRHANDLE is an undefined
3214scalar variable (or array or hash element), the variable is assigned a
3215reference to a new anonymous dirhandle.
a0d0e21e
LW
3216DIRHANDLEs have their own namespace separate from FILEHANDLEs.
3217
3218=item ord EXPR
3219
54310121 3220=item ord
bbce6d69 3221
121910a4
JH
3222Returns the numeric (the native 8-bit encoding, like ASCII or EBCDIC,
3223or Unicode) value of the first character of EXPR. If EXPR is omitted,
3224uses C<$_>.
3225
3226For the reverse, see L</chr>.
3227See L<perlunicode> and L<encoding> for more about Unicode.
a0d0e21e 3228
77ca0c92
LW
3229=item our EXPR
3230
307ea6df
JH
3231=item our EXPR TYPE
3232
1d2de774 3233=item our EXPR : ATTRS
9969eac4 3234
1d2de774 3235=item our TYPE EXPR : ATTRS
307ea6df 3236
77ca0c92
LW
3237An C<our> declares the listed variables to be valid globals within
3238the enclosing block, file, or C<eval>. That is, it has the same
3239scoping rules as a "my" declaration, but does not create a local
3240variable. If more than one value is listed, the list must be placed
3241in parentheses. The C<our> declaration has no semantic effect unless
3242"use strict vars" is in effect, in which case it lets you use the
3243declared global variable without qualifying it with a package name.
3244(But only within the lexical scope of the C<our> declaration. In this
3245it differs from "use vars", which is package scoped.)
3246
f472eb5c
GS
3247An C<our> declaration declares a global variable that will be visible
3248across its entire lexical scope, even across package boundaries. The
3249package in which the variable is entered is determined at the point
3250of the declaration, not at the point of use. This means the following
3251behavior holds:
3252
3253 package Foo;
3254 our $bar; # declares $Foo::bar for rest of lexical scope
3255 $bar = 20;