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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($_[0]) }
448
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LW
449=item bind SOCKET,NAME
450
451Binds a network address to a socket, just as the bind system call
19799a22 452does. Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
453packed address of the appropriate type for the socket. See the examples in
454L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 455
fae2c0fb 456=item binmode FILEHANDLE, LAYER
1c1fc3ea 457
a0d0e21e
LW
458=item binmode FILEHANDLE
459
1cbfc93d
NIS
460Arranges for FILEHANDLE to be read or written in "binary" or "text"
461mode on systems where the run-time libraries distinguish between
462binary and text files. If FILEHANDLE is an expression, the value is
463taken as the name of the filehandle. Returns true on success,
b5fe5ca2 464otherwise it returns C<undef> and sets C<$!> (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
ce2984c3
PF
606=item chdir
607
ffce7b87 608Changes the working directory to EXPR, if possible. If EXPR is omitted,
0bfc1ec4 609changes to the directory specified by C<$ENV{HOME}>, if set; if not,
ffce7b87 610changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
b4ad75f0
AMS
611variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
612neither is set, C<chdir> does nothing. It returns true upon success,
613false otherwise. See the example under C<die>.
a0d0e21e
LW
614
615=item chmod LIST
616
617Changes the permissions of a list of files. The first element of the
4633a7c4 618list must be the numerical mode, which should probably be an octal
4ad40acf 619number, and which definitely should I<not> be a string of octal digits:
2f9daede 620C<0644> is okay, C<'0644'> is not. Returns the number of files
dc848c6f 621successfully changed. See also L</oct>, if all you have is a string.
a0d0e21e
LW
622
623 $cnt = chmod 0755, 'foo', 'bar';
624 chmod 0755, @executables;
f86cebdf
GS
625 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
626 # --w----r-T
2f9daede
TPG
627 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
628 $mode = 0644; chmod $mode, 'foo'; # this is best
a0d0e21e 629
ca6e1c26
JH
630You can also import the symbolic C<S_I*> constants from the Fcntl
631module:
632
633 use Fcntl ':mode';
634
635 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
636 # This is identical to the chmod 0755 of the above example.
637
a0d0e21e
LW
638=item chomp VARIABLE
639
313c9f5c 640=item chomp( LIST )
a0d0e21e
LW
641
642=item chomp
643
2b5ab1e7
TC
644This safer version of L</chop> removes any trailing string
645that corresponds to the current value of C<$/> (also known as
28757baa
PP
646$INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
647number of characters removed from all its arguments. It's often used to
648remove the newline from the end of an input record when you're worried
2b5ab1e7
TC
649that the final record may be missing its newline. When in paragraph
650mode (C<$/ = "">), it removes all trailing newlines from the string.
4c5a6083
GS
651When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
652a reference to an integer or the like, see L<perlvar>) chomp() won't
b76cc8ba 653remove anything.
19799a22 654If VARIABLE is omitted, it chomps C<$_>. Example:
a0d0e21e
LW
655
656 while (<>) {
657 chomp; # avoid \n on last field
658 @array = split(/:/);
5a964f20 659 # ...
a0d0e21e
LW
660 }
661
4bf21a6d
RD
662If VARIABLE is a hash, it chomps the hash's values, but not its keys.
663
a0d0e21e
LW
664You can actually chomp anything that's an lvalue, including an assignment:
665
666 chomp($cwd = `pwd`);
667 chomp($answer = <STDIN>);
668
669If you chomp a list, each element is chomped, and the total number of
670characters removed is returned.
671
442a8c12
NC
672If the C<encoding> pragma is in scope then the lengths returned are
673calculated from the length of C<$/> in Unicode characters, which is not
674always the same as the length of C<$/> in the native encoding.
675
15e44fd8
RGS
676Note that parentheses are necessary when you're chomping anything
677that is not a simple variable. This is because C<chomp $cwd = `pwd`;>
678is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as
679C<chomp( $cwd = `pwd` )> which you might expect. Similarly,
680C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than
681as C<chomp($a, $b)>.
682
a0d0e21e
LW
683=item chop VARIABLE
684
313c9f5c 685=item chop( LIST )
a0d0e21e
LW
686
687=item chop
688
689Chops off the last character of a string and returns the character
5b3eff12 690chopped. It is much more efficient than C<s/.$//s> because it neither
7660c0ab 691scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
4bf21a6d
RD
692If VARIABLE is a hash, it chops the hash's values, but not its keys.
693
5b3eff12 694You can actually chop anything that's an lvalue, including an assignment.
a0d0e21e
LW
695
696If you chop a list, each element is chopped. Only the value of the
19799a22 697last C<chop> is returned.
a0d0e21e 698
19799a22 699Note that C<chop> returns the last character. To return all but the last
748a9306
LW
700character, use C<substr($string, 0, -1)>.
701
15e44fd8
RGS
702See also L</chomp>.
703
a0d0e21e
LW
704=item chown LIST
705
706Changes the owner (and group) of a list of files. The first two
19799a22
GS
707elements of the list must be the I<numeric> uid and gid, in that
708order. A value of -1 in either position is interpreted by most
709systems to leave that value unchanged. Returns the number of files
710successfully changed.
a0d0e21e
LW
711
712 $cnt = chown $uid, $gid, 'foo', 'bar';
713 chown $uid, $gid, @filenames;
714
54310121 715Here's an example that looks up nonnumeric uids in the passwd file:
a0d0e21e
LW
716
717 print "User: ";
19799a22 718 chomp($user = <STDIN>);
5a964f20 719 print "Files: ";
19799a22 720 chomp($pattern = <STDIN>);
a0d0e21e
LW
721
722 ($login,$pass,$uid,$gid) = getpwnam($user)
723 or die "$user not in passwd file";
724
5a964f20 725 @ary = glob($pattern); # expand filenames
a0d0e21e
LW
726 chown $uid, $gid, @ary;
727
54310121 728On most systems, you are not allowed to change the ownership of the
4633a7c4
LW
729file unless you're the superuser, although you should be able to change
730the group to any of your secondary groups. On insecure systems, these
731restrictions may be relaxed, but this is not a portable assumption.
19799a22
GS
732On POSIX systems, you can detect this condition this way:
733
734 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
735 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
4633a7c4 736
a0d0e21e
LW
737=item chr NUMBER
738
54310121 739=item chr
bbce6d69 740
a0d0e21e 741Returns the character represented by that NUMBER in the character set.
a0ed51b3 742For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
1e54db1a
JH
743chr(0x263a) is a Unicode smiley face. Note that characters from 128
744to 255 (inclusive) are by default not encoded in UTF-8 Unicode for
745backward compatibility reasons (but see L<encoding>).
aaa68c4a 746
8a064bd6
JH
747Negative values give the Unicode replacement character (chr(0xfffd)),
748except under the L</bytes> pragma, where low eight bits of the value
749(truncated to an integer) are used.
750
974da8e5
JH
751If NUMBER is omitted, uses C<$_>.
752
b76cc8ba 753For the reverse, use L</ord>.
a0d0e21e 754
974da8e5
JH
755Note that under the C<bytes> pragma the NUMBER is masked to
756the low eight bits.
757
758See L<perlunicode> and L<encoding> for more about Unicode.
bbce6d69 759
a0d0e21e
LW
760=item chroot FILENAME
761
54310121 762=item chroot
bbce6d69 763
5a964f20 764This function works like the system call by the same name: it makes the
4633a7c4 765named directory the new root directory for all further pathnames that
951ba7fe 766begin with a C</> by your process and all its children. (It doesn't
28757baa 767change your current working directory, which is unaffected.) For security
4633a7c4 768reasons, this call is restricted to the superuser. If FILENAME is
19799a22 769omitted, does a C<chroot> to C<$_>.
a0d0e21e
LW
770
771=item close FILEHANDLE
772
6a518fbc
TP
773=item close
774
9124316e
JH
775Closes the file or pipe associated with the file handle, returning
776true only if IO buffers are successfully flushed and closes the system
777file descriptor. Closes the currently selected filehandle if the
778argument is omitted.
fb73857a
PP
779
780You don't have to close FILEHANDLE if you are immediately going to do
19799a22
GS
781another C<open> on it, because C<open> will close it for you. (See
782C<open>.) However, an explicit C<close> on an input file resets the line
783counter (C<$.>), while the implicit close done by C<open> does not.
fb73857a 784
dede8123
RGS
785If the file handle came from a piped open, C<close> will additionally
786return false if one of the other system calls involved fails, or if the
fb73857a 787program exits with non-zero status. (If the only problem was that the
dede8123 788program exited non-zero, C<$!> will be set to C<0>.) Closing a pipe
2b5ab1e7 789also waits for the process executing on the pipe to complete, in case you
b76cc8ba 790want to look at the output of the pipe afterwards, and
e5218da5
GA
791implicitly puts the exit status value of that command into C<$?> and
792C<${^CHILD_ERROR_NATIVE}>.
5a964f20 793
73689b13
GS
794Prematurely closing the read end of a pipe (i.e. before the process
795writing to it at the other end has closed it) will result in a
796SIGPIPE being delivered to the writer. If the other end can't
797handle that, be sure to read all the data before closing the pipe.
798
fb73857a 799Example:
a0d0e21e 800
fb73857a
PP
801 open(OUTPUT, '|sort >foo') # pipe to sort
802 or die "Can't start sort: $!";
5a964f20 803 #... # print stuff to output
fb73857a
PP
804 close OUTPUT # wait for sort to finish
805 or warn $! ? "Error closing sort pipe: $!"
806 : "Exit status $? from sort";
807 open(INPUT, 'foo') # get sort's results
808 or die "Can't open 'foo' for input: $!";
a0d0e21e 809
5a964f20
TC
810FILEHANDLE may be an expression whose value can be used as an indirect
811filehandle, usually the real filehandle name.
a0d0e21e
LW
812
813=item closedir DIRHANDLE
814
19799a22 815Closes a directory opened by C<opendir> and returns the success of that
5a964f20
TC
816system call.
817
a0d0e21e
LW
818=item connect SOCKET,NAME
819
820Attempts to connect to a remote socket, just as the connect system call
19799a22 821does. Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
822packed address of the appropriate type for the socket. See the examples in
823L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 824
cb1a09d0
AD
825=item continue BLOCK
826
827Actually a flow control statement rather than a function. If there is a
98293880
JH
828C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
829C<foreach>), it is always executed just before the conditional is about to
830be evaluated again, just like the third part of a C<for> loop in C. Thus
cb1a09d0
AD
831it can be used to increment a loop variable, even when the loop has been
832continued via the C<next> statement (which is similar to the C C<continue>
833statement).
834
98293880 835C<last>, C<next>, or C<redo> may appear within a C<continue>
19799a22
GS
836block. C<last> and C<redo> will behave as if they had been executed within
837the main block. So will C<next>, but since it will execute a C<continue>
1d2dff63
GS
838block, it may be more entertaining.
839
840 while (EXPR) {
841 ### redo always comes here
842 do_something;
843 } continue {
844 ### next always comes here
845 do_something_else;
846 # then back the top to re-check EXPR
847 }
848 ### last always comes here
849
850Omitting the C<continue> section is semantically equivalent to using an
19799a22 851empty one, logically enough. In that case, C<next> goes directly back
1d2dff63
GS
852to check the condition at the top of the loop.
853
a0d0e21e
LW
854=item cos EXPR
855
d6217f1e
GS
856=item cos
857
5a964f20 858Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
7660c0ab 859takes cosine of C<$_>.
a0d0e21e 860
ca6e1c26 861For the inverse cosine operation, you may use the C<Math::Trig::acos()>
28757baa
PP
862function, or use this relation:
863
864 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
865
a0d0e21e
LW
866=item crypt PLAINTEXT,SALT
867
f86cebdf 868Encrypts a string exactly like the crypt(3) function in the C library
4633a7c4
LW
869(assuming that you actually have a version there that has not been
870extirpated as a potential munition). This can prove useful for checking
871the password file for lousy passwords, amongst other things. Only the
872guys wearing white hats should do this.
a0d0e21e 873
a6d05634 874Note that L<crypt|/crypt> is intended to be a one-way function, much like
85c16d83
JH
875breaking eggs to make an omelette. There is no (known) corresponding
876decrypt function (in other words, the crypt() is a one-way hash
877function). As a result, this function isn't all that useful for
11155c91 878cryptography. (For that, see your nearby CPAN mirror.)
2f9daede 879
85c16d83
JH
880When verifying an existing encrypted string you should use the
881encrypted text as the salt (like C<crypt($plain, $crypted) eq
8e2ffcbe 882$crypted>). This allows your code to work with the standard L<crypt|/crypt>
85c16d83
JH
883and with more exotic implementations. In other words, do not assume
884anything about the returned string itself, or how many bytes in
885the encrypted string matter.
886
887Traditionally the result is a string of 13 bytes: two first bytes of
888the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
889the first eight bytes of the encrypted string mattered, but
890alternative hashing schemes (like MD5), higher level security schemes
891(like C2), and implementations on non-UNIX platforms may produce
892different strings.
893
894When choosing a new salt create a random two character string whose
895characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
d3989d75
CW
896'/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of
897characters is just a recommendation; the characters allowed in
898the salt depend solely on your system's crypt library, and Perl can't
899restrict what salts C<crypt()> accepts.
e71965be 900
a0d0e21e
LW
901Here's an example that makes sure that whoever runs this program knows
902their own password:
903
904 $pwd = (getpwuid($<))[1];
a0d0e21e
LW
905
906 system "stty -echo";
907 print "Password: ";
e71965be 908 chomp($word = <STDIN>);
a0d0e21e
LW
909 print "\n";
910 system "stty echo";
911
e71965be 912 if (crypt($word, $pwd) ne $pwd) {
a0d0e21e
LW
913 die "Sorry...\n";
914 } else {
915 print "ok\n";
54310121 916 }
a0d0e21e 917
9f8f0c9d 918Of course, typing in your own password to whoever asks you
748a9306 919for it is unwise.
a0d0e21e 920
8e2ffcbe 921The L<crypt|/crypt> function is unsuitable for encrypting large quantities
19799a22
GS
922of data, not least of all because you can't get the information
923back. Look at the F<by-module/Crypt> and F<by-module/PGP> directories
924on your favorite CPAN mirror for a slew of potentially useful
925modules.
926
f2791508
JH
927If using crypt() on a Unicode string (which I<potentially> has
928characters with codepoints above 255), Perl tries to make sense
929of the situation by trying to downgrade (a copy of the string)
930the string back to an eight-bit byte string before calling crypt()
931(on that copy). If that works, good. If not, crypt() dies with
932C<Wide character in crypt>.
85c16d83 933
aa689395 934=item dbmclose HASH
a0d0e21e 935
19799a22 936[This function has been largely superseded by the C<untie> function.]
a0d0e21e 937
aa689395 938Breaks the binding between a DBM file and a hash.
a0d0e21e 939
19799a22 940=item dbmopen HASH,DBNAME,MASK
a0d0e21e 941
19799a22 942[This function has been largely superseded by the C<tie> function.]
a0d0e21e 943
7b8d334a 944This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
19799a22
GS
945hash. HASH is the name of the hash. (Unlike normal C<open>, the first
946argument is I<not> a filehandle, even though it looks like one). DBNAME
aa689395
PP
947is the name of the database (without the F<.dir> or F<.pag> extension if
948any). If the database does not exist, it is created with protection
19799a22
GS
949specified by MASK (as modified by the C<umask>). If your system supports
950only the older DBM functions, you may perform only one C<dbmopen> in your
aa689395 951program. In older versions of Perl, if your system had neither DBM nor
19799a22 952ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
aa689395
PP
953sdbm(3).
954
955If you don't have write access to the DBM file, you can only read hash
956variables, not set them. If you want to test whether you can write,
19799a22 957either use file tests or try setting a dummy hash entry inside an C<eval>,
aa689395 958which will trap the error.
a0d0e21e 959
19799a22
GS
960Note that functions such as C<keys> and C<values> may return huge lists
961when used on large DBM files. You may prefer to use the C<each>
a0d0e21e
LW
962function to iterate over large DBM files. Example:
963
964 # print out history file offsets
965 dbmopen(%HIST,'/usr/lib/news/history',0666);
966 while (($key,$val) = each %HIST) {
967 print $key, ' = ', unpack('L',$val), "\n";
968 }
969 dbmclose(%HIST);
970
cb1a09d0 971See also L<AnyDBM_File> for a more general description of the pros and
184e9718 972cons of the various dbm approaches, as well as L<DB_File> for a particularly
cb1a09d0 973rich implementation.
4633a7c4 974
2b5ab1e7
TC
975You can control which DBM library you use by loading that library
976before you call dbmopen():
977
978 use DB_File;
979 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
980 or die "Can't open netscape history file: $!";
981
a0d0e21e
LW
982=item defined EXPR
983
54310121 984=item defined
bbce6d69 985
2f9daede
TPG
986Returns a Boolean value telling whether EXPR has a value other than
987the undefined value C<undef>. If EXPR is not present, C<$_> will be
988checked.
989
990Many operations return C<undef> to indicate failure, end of file,
991system error, uninitialized variable, and other exceptional
992conditions. This function allows you to distinguish C<undef> from
993other values. (A simple Boolean test will not distinguish among
7660c0ab 994C<undef>, zero, the empty string, and C<"0">, which are all equally
2f9daede 995false.) Note that since C<undef> is a valid scalar, its presence
19799a22 996doesn't I<necessarily> indicate an exceptional condition: C<pop>
2f9daede
TPG
997returns C<undef> when its argument is an empty array, I<or> when the
998element to return happens to be C<undef>.
999
f10b0346
GS
1000You may also use C<defined(&func)> to check whether subroutine C<&func>
1001has ever been defined. The return value is unaffected by any forward
04891299 1002declarations of C<&func>. Note that a subroutine which is not defined
847c7ebe
DD
1003may still be callable: its package may have an C<AUTOLOAD> method that
1004makes it spring into existence the first time that it is called -- see
1005L<perlsub>.
f10b0346
GS
1006
1007Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
1008used to report whether memory for that aggregate has ever been
1009allocated. This behavior may disappear in future versions of Perl.
1010You should instead use a simple test for size:
1011
1012 if (@an_array) { print "has array elements\n" }
1013 if (%a_hash) { print "has hash members\n" }
2f9daede
TPG
1014
1015When used on a hash element, it tells you whether the value is defined,
dc848c6f 1016not whether the key exists in the hash. Use L</exists> for the latter
2f9daede 1017purpose.
a0d0e21e
LW
1018
1019Examples:
1020
1021 print if defined $switch{'D'};
1022 print "$val\n" while defined($val = pop(@ary));
1023 die "Can't readlink $sym: $!"
1024 unless defined($value = readlink $sym);
a0d0e21e 1025 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
2f9daede 1026 $debugging = 0 unless defined $debugging;
a0d0e21e 1027
19799a22 1028Note: Many folks tend to overuse C<defined>, and then are surprised to
7660c0ab 1029discover that the number C<0> and C<""> (the zero-length string) are, in fact,
2f9daede 1030defined values. For example, if you say
a5f75d66
AD
1031
1032 "ab" =~ /a(.*)b/;
1033
7660c0ab 1034The pattern match succeeds, and C<$1> is defined, despite the fact that it
a5f75d66 1035matched "nothing". But it didn't really match nothing--rather, it
2b5ab1e7 1036matched something that happened to be zero characters long. This is all
a5f75d66 1037very above-board and honest. When a function returns an undefined value,
2f9daede 1038it's an admission that it couldn't give you an honest answer. So you
19799a22 1039should use C<defined> only when you're questioning the integrity of what
7660c0ab 1040you're trying to do. At other times, a simple comparison to C<0> or C<""> is
2f9daede
TPG
1041what you want.
1042
dc848c6f 1043See also L</undef>, L</exists>, L</ref>.
2f9daede 1044
a0d0e21e
LW
1045=item delete EXPR
1046
01020589
GS
1047Given an expression that specifies a hash element, array element, hash slice,
1048or array slice, deletes the specified element(s) from the hash or array.
8216c1fd 1049In the case of an array, if the array elements happen to be at the end,
b76cc8ba 1050the size of the array will shrink to the highest element that tests
8216c1fd 1051true for exists() (or 0 if no such element exists).
a0d0e21e 1052
eba0920a
EM
1053Returns a list with the same number of elements as the number of elements
1054for which deletion was attempted. Each element of that list consists of
1055either the value of the element deleted, or the undefined value. In scalar
1056context, this means that you get the value of the last element deleted (or
1057the undefined value if that element did not exist).
1058
1059 %hash = (foo => 11, bar => 22, baz => 33);
1060 $scalar = delete $hash{foo}; # $scalar is 11
1061 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
1062 @array = delete @hash{qw(foo bar baz)}; # @array is (undef,undef,33)
1063
1064Deleting from C<%ENV> modifies the environment. Deleting from
01020589
GS
1065a hash tied to a DBM file deletes the entry from the DBM file. Deleting
1066from a C<tie>d hash or array may not necessarily return anything.
1067
8ea97a1e
GS
1068Deleting an array element effectively returns that position of the array
1069to its initial, uninitialized state. Subsequently testing for the same
8216c1fd
GS
1070element with exists() will return false. Note that deleting array
1071elements in the middle of an array will not shift the index of the ones
1072after them down--use splice() for that. See L</exists>.
8ea97a1e 1073
01020589 1074The following (inefficiently) deletes all the values of %HASH and @ARRAY:
a0d0e21e 1075
5f05dabc
PP
1076 foreach $key (keys %HASH) {
1077 delete $HASH{$key};
a0d0e21e
LW
1078 }
1079
01020589
GS
1080 foreach $index (0 .. $#ARRAY) {
1081 delete $ARRAY[$index];
1082 }
1083
1084And so do these:
5f05dabc 1085
01020589
GS
1086 delete @HASH{keys %HASH};
1087
9740c838 1088 delete @ARRAY[0 .. $#ARRAY];
5f05dabc 1089
2b5ab1e7 1090But both of these are slower than just assigning the empty list
01020589
GS
1091or undefining %HASH or @ARRAY:
1092
1093 %HASH = (); # completely empty %HASH
1094 undef %HASH; # forget %HASH ever existed
2b5ab1e7 1095
01020589
GS
1096 @ARRAY = (); # completely empty @ARRAY
1097 undef @ARRAY; # forget @ARRAY ever existed
2b5ab1e7
TC
1098
1099Note that the EXPR can be arbitrarily complicated as long as the final
01020589
GS
1100operation is a hash element, array element, hash slice, or array slice
1101lookup:
a0d0e21e
LW
1102
1103 delete $ref->[$x][$y]{$key};
5f05dabc 1104 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
a0d0e21e 1105
01020589
GS
1106 delete $ref->[$x][$y][$index];
1107 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1108
a0d0e21e
LW
1109=item die LIST
1110
19799a22
GS
1111Outside an C<eval>, prints the value of LIST to C<STDERR> and
1112exits with the current value of C<$!> (errno). If C<$!> is C<0>,
61eff3bc
JH
1113exits with the value of C<<< ($? >> 8) >>> (backtick `command`
1114status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside
19799a22
GS
1115an C<eval(),> the error message is stuffed into C<$@> and the
1116C<eval> is terminated with the undefined value. This makes
1117C<die> the way to raise an exception.
a0d0e21e
LW
1118
1119Equivalent examples:
1120
1121 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
54310121 1122 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
a0d0e21e 1123
ccac6780 1124If the last element of LIST does not end in a newline, the current
df37ec69
WW
1125script line number and input line number (if any) are also printed,
1126and a newline is supplied. Note that the "input line number" (also
1127known as "chunk") is subject to whatever notion of "line" happens to
1128be currently in effect, and is also available as the special variable
1129C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1130
1131Hint: sometimes appending C<", stopped"> to your message will cause it
1132to make better sense when the string C<"at foo line 123"> is appended.
1133Suppose you are running script "canasta".
a0d0e21e
LW
1134
1135 die "/etc/games is no good";
1136 die "/etc/games is no good, stopped";
1137
1138produce, respectively
1139
1140 /etc/games is no good at canasta line 123.
1141 /etc/games is no good, stopped at canasta line 123.
1142
2b5ab1e7 1143See also exit(), warn(), and the Carp module.
a0d0e21e 1144
7660c0ab
A
1145If LIST is empty and C<$@> already contains a value (typically from a
1146previous eval) that value is reused after appending C<"\t...propagated">.
fb73857a
PP
1147This is useful for propagating exceptions:
1148
1149 eval { ... };
1150 die unless $@ =~ /Expected exception/;
1151
ad216e65
JH
1152If LIST is empty and C<$@> contains an object reference that has a
1153C<PROPAGATE> method, that method will be called with additional file
1154and line number parameters. The return value replaces the value in
16869676 1155C<$@>. ie. as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >>
ad216e65
JH
1156were called.
1157
7660c0ab 1158If C<$@> is empty then the string C<"Died"> is used.
fb73857a 1159
52531d10
GS
1160die() can also be called with a reference argument. If this happens to be
1161trapped within an eval(), $@ contains the reference. This behavior permits
1162a more elaborate exception handling implementation using objects that
4375e838 1163maintain arbitrary state about the nature of the exception. Such a scheme
52531d10
GS
1164is sometimes preferable to matching particular string values of $@ using
1165regular expressions. Here's an example:
1166
da279afe 1167 use Scalar::Util 'blessed';
1168
52531d10
GS
1169 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1170 if ($@) {
da279afe 1171 if (blessed($@) && $@->isa("Some::Module::Exception")) {
52531d10
GS
1172 # handle Some::Module::Exception
1173 }
1174 else {
1175 # handle all other possible exceptions
1176 }
1177 }
1178
19799a22 1179Because perl will stringify uncaught exception messages before displaying
52531d10
GS
1180them, you may want to overload stringification operations on such custom
1181exception objects. See L<overload> for details about that.
1182
19799a22
GS
1183You can arrange for a callback to be run just before the C<die>
1184does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1185handler will be called with the error text and can change the error
1186message, if it sees fit, by calling C<die> again. See
1187L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
1188L<"eval BLOCK"> for some examples. Although this feature was meant
1189to be run only right before your program was to exit, this is not
1190currently the case--the C<$SIG{__DIE__}> hook is currently called
1191even inside eval()ed blocks/strings! If one wants the hook to do
1192nothing in such situations, put
fb73857a
PP
1193
1194 die @_ if $^S;
1195
19799a22
GS
1196as the first line of the handler (see L<perlvar/$^S>). Because
1197this promotes strange action at a distance, this counterintuitive
b76cc8ba 1198behavior may be fixed in a future release.
774d564b 1199
a0d0e21e
LW
1200=item do BLOCK
1201
1202Not really a function. Returns the value of the last command in the
1203sequence of commands indicated by BLOCK. When modified by a loop
98293880
JH
1204modifier, executes the BLOCK once before testing the loop condition.
1205(On other statements the loop modifiers test the conditional first.)
a0d0e21e 1206
4968c1e4 1207C<do BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7
TC
1208C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1209See L<perlsyn> for alternative strategies.
4968c1e4 1210
a0d0e21e
LW
1211=item do SUBROUTINE(LIST)
1212
1213A deprecated form of subroutine call. See L<perlsub>.
1214
1215=item do EXPR
1216
1217Uses the value of EXPR as a filename and executes the contents of the
ea63ef19 1218file as a Perl script.
a0d0e21e
LW
1219
1220 do 'stat.pl';
1221
1222is just like
1223
986b19de 1224 eval `cat stat.pl`;
a0d0e21e 1225
2b5ab1e7 1226except that it's more efficient and concise, keeps track of the current
ea63ef19 1227filename for error messages, searches the @INC directories, and updates
2b5ab1e7
TC
1228C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1229variables. It also differs in that code evaluated with C<do FILENAME>
1230cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1231same, however, in that it does reparse the file every time you call it,
1232so you probably don't want to do this inside a loop.
a0d0e21e 1233
8e30cc93 1234If C<do> cannot read the file, it returns undef and sets C<$!> to the
2b5ab1e7 1235error. If C<do> can read the file but cannot compile it, it
8e30cc93
G
1236returns undef and sets an error message in C<$@>. If the file is
1237successfully compiled, C<do> returns the value of the last expression
1238evaluated.
1239
a0d0e21e 1240Note that inclusion of library modules is better done with the
19799a22 1241C<use> and C<require> operators, which also do automatic error checking
4633a7c4 1242and raise an exception if there's a problem.
a0d0e21e 1243
5a964f20
TC
1244You might like to use C<do> to read in a program configuration
1245file. Manual error checking can be done this way:
1246
b76cc8ba 1247 # read in config files: system first, then user
f86cebdf 1248 for $file ("/share/prog/defaults.rc",
b76cc8ba 1249 "$ENV{HOME}/.someprogrc")
2b5ab1e7 1250 {
5a964f20 1251 unless ($return = do $file) {
f86cebdf
GS
1252 warn "couldn't parse $file: $@" if $@;
1253 warn "couldn't do $file: $!" unless defined $return;
1254 warn "couldn't run $file" unless $return;
5a964f20
TC
1255 }
1256 }
1257
a0d0e21e
LW
1258=item dump LABEL
1259
1614b0e3
JD
1260=item dump
1261
19799a22
GS
1262This function causes an immediate core dump. See also the B<-u>
1263command-line switch in L<perlrun>, which does the same thing.
1264Primarily this is so that you can use the B<undump> program (not
1265supplied) to turn your core dump into an executable binary after
1266having initialized all your variables at the beginning of the
1267program. When the new binary is executed it will begin by executing
1268a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1269Think of it as a goto with an intervening core dump and reincarnation.
1270If C<LABEL> is omitted, restarts the program from the top.
1271
1272B<WARNING>: Any files opened at the time of the dump will I<not>
1273be open any more when the program is reincarnated, with possible
b76cc8ba 1274resulting confusion on the part of Perl.
19799a22
GS
1275
1276This function is now largely obsolete, partly because it's very
1277hard to convert a core file into an executable, and because the
1278real compiler backends for generating portable bytecode and compilable
ac206dc8
RGS
1279C code have superseded it. That's why you should now invoke it as
1280C<CORE::dump()>, if you don't want to be warned against a possible
1281typo.
19799a22
GS
1282
1283If you're looking to use L<dump> to speed up your program, consider
1284generating bytecode or native C code as described in L<perlcc>. If
1285you're just trying to accelerate a CGI script, consider using the
210b36aa 1286C<mod_perl> extension to B<Apache>, or the CPAN module, CGI::Fast.
19799a22 1287You might also consider autoloading or selfloading, which at least
b76cc8ba 1288make your program I<appear> to run faster.
5a964f20 1289
aa689395
PP
1290=item each HASH
1291
5a964f20 1292When called in list context, returns a 2-element list consisting of the
aa689395 1293key and value for the next element of a hash, so that you can iterate over
74fc8b5f 1294it. When called in scalar context, returns only the key for the next
e902a979 1295element in the hash.
2f9daede 1296
ab192400 1297Entries are returned in an apparently random order. The actual random
504f80c1
JH
1298order is subject to change in future versions of perl, but it is
1299guaranteed to be in the same order as either the C<keys> or C<values>
4546b9e6
JH
1300function would produce on the same (unmodified) hash. Since Perl
13015.8.1 the ordering is different even between different runs of Perl
1302for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
ab192400
GS
1303
1304When the hash is entirely read, a null array is returned in list context
19799a22
GS
1305(which when assigned produces a false (C<0>) value), and C<undef> in
1306scalar context. The next call to C<each> after that will start iterating
1307again. There is a single iterator for each hash, shared by all C<each>,
1308C<keys>, and C<values> function calls in the program; it can be reset by
2f9daede
TPG
1309reading all the elements from the hash, or by evaluating C<keys HASH> or
1310C<values HASH>. If you add or delete elements of a hash while you're
74fc8b5f
MJD
1311iterating over it, you may get entries skipped or duplicated, so
1312don't. Exception: It is always safe to delete the item most recently
1313returned by C<each()>, which means that the following code will work:
1314
1315 while (($key, $value) = each %hash) {
1316 print $key, "\n";
1317 delete $hash{$key}; # This is safe
1318 }
aa689395 1319
f86cebdf 1320The following prints out your environment like the printenv(1) program,
aa689395 1321only in a different order:
a0d0e21e
LW
1322
1323 while (($key,$value) = each %ENV) {
1324 print "$key=$value\n";
1325 }
1326
19799a22 1327See also C<keys>, C<values> and C<sort>.
a0d0e21e
LW
1328
1329=item eof FILEHANDLE
1330
4633a7c4
LW
1331=item eof ()
1332
a0d0e21e
LW
1333=item eof
1334
1335Returns 1 if the next read on FILEHANDLE will return end of file, or if
1336FILEHANDLE is not open. FILEHANDLE may be an expression whose value
5a964f20 1337gives the real filehandle. (Note that this function actually
19799a22 1338reads a character and then C<ungetc>s it, so isn't very useful in an
748a9306 1339interactive context.) Do not read from a terminal file (or call
19799a22 1340C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
748a9306
LW
1341as terminals may lose the end-of-file condition if you do.
1342
820475bd
GS
1343An C<eof> without an argument uses the last file read. Using C<eof()>
1344with empty parentheses is very different. It refers to the pseudo file
1345formed from the files listed on the command line and accessed via the
61eff3bc
JH
1346C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1347as a normal filehandle is, an C<eof()> before C<< <> >> has been
820475bd 1348used will cause C<@ARGV> to be examined to determine if input is
67408cae 1349available. Similarly, an C<eof()> after C<< <> >> has returned
efdd0218
RB
1350end-of-file will assume you are processing another C<@ARGV> list,
1351and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1352see L<perlop/"I/O Operators">.
820475bd 1353
61eff3bc 1354In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
820475bd
GS
1355detect the end of each file, C<eof()> will only detect the end of the
1356last file. Examples:
a0d0e21e 1357
748a9306
LW
1358 # reset line numbering on each input file
1359 while (<>) {
b76cc8ba 1360 next if /^\s*#/; # skip comments
748a9306 1361 print "$.\t$_";
5a964f20
TC
1362 } continue {
1363 close ARGV if eof; # Not eof()!
748a9306
LW
1364 }
1365
a0d0e21e
LW
1366 # insert dashes just before last line of last file
1367 while (<>) {
6ac88b13 1368 if (eof()) { # check for end of last file
a0d0e21e
LW
1369 print "--------------\n";
1370 }
1371 print;
6ac88b13 1372 last if eof(); # needed if we're reading from a terminal
a0d0e21e
LW
1373 }
1374
a0d0e21e 1375Practical hint: you almost never need to use C<eof> in Perl, because the
3ce0d271
GS
1376input operators typically return C<undef> when they run out of data, or if
1377there was an error.
a0d0e21e
LW
1378
1379=item eval EXPR
1380
1381=item eval BLOCK
1382
ce2984c3
PF
1383=item eval
1384
c7cc6f1c
GS
1385In the first form, the return value of EXPR is parsed and executed as if it
1386were a little Perl program. The value of the expression (which is itself
5a964f20 1387determined within scalar context) is first parsed, and if there weren't any
be3174d2
GS
1388errors, executed in the lexical context of the current Perl program, so
1389that any variable settings or subroutine and format definitions remain
1390afterwards. Note that the value is parsed every time the eval executes.
1391If EXPR is omitted, evaluates C<$_>. This form is typically used to
1392delay parsing and subsequent execution of the text of EXPR until run time.
c7cc6f1c
GS
1393
1394In the second form, the code within the BLOCK is parsed only once--at the
1395same time the code surrounding the eval itself was parsed--and executed
1396within the context of the current Perl program. This form is typically
1397used to trap exceptions more efficiently than the first (see below), while
1398also providing the benefit of checking the code within BLOCK at compile
1399time.
1400
1401The final semicolon, if any, may be omitted from the value of EXPR or within
1402the BLOCK.
1403
1404In both forms, the value returned is the value of the last expression
5a964f20 1405evaluated inside the mini-program; a return statement may be also used, just
c7cc6f1c 1406as with subroutines. The expression providing the return value is evaluated
5a964f20 1407in void, scalar, or list context, depending on the context of the eval itself.
c7cc6f1c 1408See L</wantarray> for more on how the evaluation context can be determined.
a0d0e21e 1409
19799a22
GS
1410If there is a syntax error or runtime error, or a C<die> statement is
1411executed, an undefined value is returned by C<eval>, and C<$@> is set to the
a0d0e21e 1412error message. If there was no error, C<$@> is guaranteed to be a null
19799a22 1413string. Beware that using C<eval> neither silences perl from printing
c7cc6f1c 1414warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
d9984052
A
1415To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1416turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1417See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
a0d0e21e 1418
19799a22
GS
1419Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1420determining whether a particular feature (such as C<socket> or C<symlink>)
a0d0e21e
LW
1421is implemented. It is also Perl's exception trapping mechanism, where
1422the die operator is used to raise exceptions.
1423
1424If the code to be executed doesn't vary, you may use the eval-BLOCK
1425form to trap run-time errors without incurring the penalty of
1426recompiling each time. The error, if any, is still returned in C<$@>.
1427Examples:
1428
54310121 1429 # make divide-by-zero nonfatal
a0d0e21e
LW
1430 eval { $answer = $a / $b; }; warn $@ if $@;
1431
1432 # same thing, but less efficient
1433 eval '$answer = $a / $b'; warn $@ if $@;
1434
1435 # a compile-time error
5a964f20 1436 eval { $answer = }; # WRONG
a0d0e21e
LW
1437
1438 # a run-time error
1439 eval '$answer ='; # sets $@
1440
2b5ab1e7
TC
1441Due to the current arguably broken state of C<__DIE__> hooks, when using
1442the C<eval{}> form as an exception trap in libraries, you may wish not
1443to trigger any C<__DIE__> hooks that user code may have installed.
1444You can use the C<local $SIG{__DIE__}> construct for this purpose,
1445as shown in this example:
774d564b
PP
1446
1447 # a very private exception trap for divide-by-zero
f86cebdf
GS
1448 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1449 warn $@ if $@;
774d564b
PP
1450
1451This is especially significant, given that C<__DIE__> hooks can call
19799a22 1452C<die> again, which has the effect of changing their error messages:
774d564b
PP
1453
1454 # __DIE__ hooks may modify error messages
1455 {
f86cebdf
GS
1456 local $SIG{'__DIE__'} =
1457 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
c7cc6f1c
GS
1458 eval { die "foo lives here" };
1459 print $@ if $@; # prints "bar lives here"
774d564b
PP
1460 }
1461
19799a22 1462Because this promotes action at a distance, this counterintuitive behavior
2b5ab1e7
TC
1463may be fixed in a future release.
1464
19799a22 1465With an C<eval>, you should be especially careful to remember what's
a0d0e21e
LW
1466being looked at when:
1467
1468 eval $x; # CASE 1
1469 eval "$x"; # CASE 2
1470
1471 eval '$x'; # CASE 3
1472 eval { $x }; # CASE 4
1473
5a964f20 1474 eval "\$$x++"; # CASE 5
a0d0e21e
LW
1475 $$x++; # CASE 6
1476
2f9daede 1477Cases 1 and 2 above behave identically: they run the code contained in
19799a22 1478the variable $x. (Although case 2 has misleading double quotes making
2f9daede 1479the reader wonder what else might be happening (nothing is).) Cases 3
7660c0ab 1480and 4 likewise behave in the same way: they run the code C<'$x'>, which
19799a22 1481does nothing but return the value of $x. (Case 4 is preferred for
2f9daede
TPG
1482purely visual reasons, but it also has the advantage of compiling at
1483compile-time instead of at run-time.) Case 5 is a place where
19799a22 1484normally you I<would> like to use double quotes, except that in this
2f9daede
TPG
1485particular situation, you can just use symbolic references instead, as
1486in case 6.
a0d0e21e 1487
4968c1e4 1488C<eval BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7 1489C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
4968c1e4 1490
d819b83a
DM
1491Note that as a very special case, an C<eval ''> executed within the C<DB>
1492package doesn't see the usual surrounding lexical scope, but rather the
1493scope of the first non-DB piece of code that called it. You don't normally
1494need to worry about this unless you are writing a Perl debugger.
1495
a0d0e21e
LW
1496=item exec LIST
1497
8bf3b016
GS
1498=item exec PROGRAM LIST
1499
19799a22
GS
1500The C<exec> function executes a system command I<and never returns>--
1501use C<system> instead of C<exec> if you want it to return. It fails and
1502returns false only if the command does not exist I<and> it is executed
fb73857a 1503directly instead of via your system's command shell (see below).
a0d0e21e 1504
19799a22
GS
1505Since it's a common mistake to use C<exec> instead of C<system>, Perl
1506warns you if there is a following statement which isn't C<die>, C<warn>,
1507or C<exit> (if C<-w> is set - but you always do that). If you
1508I<really> want to follow an C<exec> with some other statement, you
55d729e4
GS
1509can use one of these styles to avoid the warning:
1510
5a964f20
TC
1511 exec ('foo') or print STDERR "couldn't exec foo: $!";
1512 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
55d729e4 1513
5a964f20 1514If there is more than one argument in LIST, or if LIST is an array
f86cebdf 1515with more than one value, calls execvp(3) with the arguments in LIST.
5a964f20
TC
1516If there is only one scalar argument or an array with one element in it,
1517the argument is checked for shell metacharacters, and if there are any,
1518the entire argument is passed to the system's command shell for parsing
1519(this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1520If there are no shell metacharacters in the argument, it is split into
b76cc8ba 1521words and passed directly to C<execvp>, which is more efficient.
19799a22 1522Examples:
a0d0e21e 1523
19799a22
GS
1524 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1525 exec "sort $outfile | uniq";
a0d0e21e
LW
1526
1527If you don't really want to execute the first argument, but want to lie
1528to the program you are executing about its own name, you can specify
1529the program you actually want to run as an "indirect object" (without a
1530comma) in front of the LIST. (This always forces interpretation of the
54310121 1531LIST as a multivalued list, even if there is only a single scalar in
a0d0e21e
LW
1532the list.) Example:
1533
1534 $shell = '/bin/csh';
1535 exec $shell '-sh'; # pretend it's a login shell
1536
1537or, more directly,
1538
1539 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1540
bb32b41a
GS
1541When the arguments get executed via the system shell, results will
1542be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1543for details.
1544
19799a22
GS
1545Using an indirect object with C<exec> or C<system> is also more
1546secure. This usage (which also works fine with system()) forces
1547interpretation of the arguments as a multivalued list, even if the
1548list had just one argument. That way you're safe from the shell
1549expanding wildcards or splitting up words with whitespace in them.
5a964f20
TC
1550
1551 @args = ( "echo surprise" );
1552
2b5ab1e7 1553 exec @args; # subject to shell escapes
f86cebdf 1554 # if @args == 1
2b5ab1e7 1555 exec { $args[0] } @args; # safe even with one-arg list
5a964f20
TC
1556
1557The first version, the one without the indirect object, ran the I<echo>
1558program, passing it C<"surprise"> an argument. The second version
1559didn't--it tried to run a program literally called I<"echo surprise">,
1560didn't find it, and set C<$?> to a non-zero value indicating failure.
1561
0f897271
GS
1562Beginning with v5.6.0, Perl will attempt to flush all files opened for
1563output before the exec, but this may not be supported on some platforms
1564(see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1565in English) or call the C<autoflush()> method of C<IO::Handle> on any
1566open handles in order to avoid lost output.
1567
19799a22 1568Note that C<exec> will not call your C<END> blocks, nor will it call
7660c0ab
A
1569any C<DESTROY> methods in your objects.
1570
a0d0e21e
LW
1571=item exists EXPR
1572
01020589 1573Given an expression that specifies a hash element or array element,
8ea97a1e
GS
1574returns true if the specified element in the hash or array has ever
1575been initialized, even if the corresponding value is undefined. The
1576element is not autovivified if it doesn't exist.
a0d0e21e 1577
01020589
GS
1578 print "Exists\n" if exists $hash{$key};
1579 print "Defined\n" if defined $hash{$key};
1580 print "True\n" if $hash{$key};
1581
1582 print "Exists\n" if exists $array[$index];
1583 print "Defined\n" if defined $array[$index];
1584 print "True\n" if $array[$index];
a0d0e21e 1585
8ea97a1e 1586A hash or array element can be true only if it's defined, and defined if
a0d0e21e
LW
1587it exists, but the reverse doesn't necessarily hold true.
1588
afebc493
GS
1589Given an expression that specifies the name of a subroutine,
1590returns true if the specified subroutine has ever been declared, even
1591if it is undefined. Mentioning a subroutine name for exists or defined
847c7ebe
DD
1592does not count as declaring it. Note that a subroutine which does not
1593exist may still be callable: its package may have an C<AUTOLOAD>
1594method that makes it spring into existence the first time that it is
1595called -- see L<perlsub>.
afebc493
GS
1596
1597 print "Exists\n" if exists &subroutine;
1598 print "Defined\n" if defined &subroutine;
1599
a0d0e21e 1600Note that the EXPR can be arbitrarily complicated as long as the final
afebc493 1601operation is a hash or array key lookup or subroutine name:
a0d0e21e 1602
2b5ab1e7
TC
1603 if (exists $ref->{A}->{B}->{$key}) { }
1604 if (exists $hash{A}{B}{$key}) { }
1605
01020589
GS
1606 if (exists $ref->{A}->{B}->[$ix]) { }
1607 if (exists $hash{A}{B}[$ix]) { }
1608
afebc493
GS
1609 if (exists &{$ref->{A}{B}{$key}}) { }
1610
01020589
GS
1611Although the deepest nested array or hash will not spring into existence
1612just because its existence was tested, any intervening ones will.
61eff3bc 1613Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
01020589
GS
1614into existence due to the existence test for the $key element above.
1615This happens anywhere the arrow operator is used, including even:
5a964f20 1616
2b5ab1e7
TC
1617 undef $ref;
1618 if (exists $ref->{"Some key"}) { }
1619 print $ref; # prints HASH(0x80d3d5c)
1620
1621This surprising autovivification in what does not at first--or even
1622second--glance appear to be an lvalue context may be fixed in a future
5a964f20 1623release.
a0d0e21e 1624
afebc493
GS
1625Use of a subroutine call, rather than a subroutine name, as an argument
1626to exists() is an error.
1627
1628 exists &sub; # OK
1629 exists &sub(); # Error
1630
a0d0e21e
LW
1631=item exit EXPR
1632
ce2984c3
PF
1633=item exit
1634
2b5ab1e7 1635Evaluates EXPR and exits immediately with that value. Example:
a0d0e21e
LW
1636
1637 $ans = <STDIN>;
1638 exit 0 if $ans =~ /^[Xx]/;
1639
19799a22 1640See also C<die>. If EXPR is omitted, exits with C<0> status. The only
2b5ab1e7
TC
1641universally recognized values for EXPR are C<0> for success and C<1>
1642for error; other values are subject to interpretation depending on the
1643environment in which the Perl program is running. For example, exiting
164469 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1645the mailer to return the item undelivered, but that's not true everywhere.
a0d0e21e 1646
19799a22
GS
1647Don't use C<exit> to abort a subroutine if there's any chance that
1648someone might want to trap whatever error happened. Use C<die> instead,
1649which can be trapped by an C<eval>.
28757baa 1650
19799a22 1651The exit() function does not always exit immediately. It calls any
2b5ab1e7 1652defined C<END> routines first, but these C<END> routines may not
19799a22 1653themselves abort the exit. Likewise any object destructors that need to
2b5ab1e7
TC
1654be called are called before the real exit. If this is a problem, you
1655can call C<POSIX:_exit($status)> to avoid END and destructor processing.
87275199 1656See L<perlmod> for details.
5a964f20 1657
a0d0e21e
LW
1658=item exp EXPR
1659
54310121 1660=item exp
bbce6d69 1661
b76cc8ba 1662Returns I<e> (the natural logarithm base) to the power of EXPR.
a0d0e21e
LW
1663If EXPR is omitted, gives C<exp($_)>.
1664
1665=item fcntl FILEHANDLE,FUNCTION,SCALAR
1666
f86cebdf 1667Implements the fcntl(2) function. You'll probably have to say
a0d0e21e
LW
1668
1669 use Fcntl;
1670
0ade1984 1671first to get the correct constant definitions. Argument processing and
b76cc8ba 1672value return works just like C<ioctl> below.
a0d0e21e
LW
1673For example:
1674
1675 use Fcntl;
5a964f20
TC
1676 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1677 or die "can't fcntl F_GETFL: $!";
1678
554ad1fc 1679You don't have to check for C<defined> on the return from C<fcntl>.
951ba7fe
GS
1680Like C<ioctl>, it maps a C<0> return from the system call into
1681C<"0 but true"> in Perl. This string is true in boolean context and C<0>
2b5ab1e7
TC
1682in numeric context. It is also exempt from the normal B<-w> warnings
1683on improper numeric conversions.
5a964f20 1684
19799a22 1685Note that C<fcntl> will produce a fatal error if used on a machine that
2b5ab1e7
TC
1686doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1687manpage to learn what functions are available on your system.
a0d0e21e 1688
be2f7487 1689Here's an example of setting a filehandle named C<REMOTE> to be
1690non-blocking at the system level. You'll have to negotiate C<$|>
1691on your own, though.
1692
1693 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
1694
1695 $flags = fcntl(REMOTE, F_GETFL, 0)
1696 or die "Can't get flags for the socket: $!\n";
1697
1698 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
1699 or die "Can't set flags for the socket: $!\n";
1700
a0d0e21e
LW
1701=item fileno FILEHANDLE
1702
2b5ab1e7
TC
1703Returns the file descriptor for a filehandle, or undefined if the
1704filehandle is not open. This is mainly useful for constructing
19799a22 1705bitmaps for C<select> and low-level POSIX tty-handling operations.
2b5ab1e7
TC
1706If FILEHANDLE is an expression, the value is taken as an indirect
1707filehandle, generally its name.
5a964f20 1708
b76cc8ba 1709You can use this to find out whether two handles refer to the
5a964f20
TC
1710same underlying descriptor:
1711
1712 if (fileno(THIS) == fileno(THAT)) {
1713 print "THIS and THAT are dups\n";
b76cc8ba
NIS
1714 }
1715
1716(Filehandles connected to memory objects via new features of C<open> may
1717return undefined even though they are open.)
1718
a0d0e21e
LW
1719
1720=item flock FILEHANDLE,OPERATION
1721
19799a22
GS
1722Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1723for success, false on failure. Produces a fatal error if used on a
2b5ab1e7 1724machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
19799a22 1725C<flock> is Perl's portable file locking interface, although it locks
2b5ab1e7
TC
1726only entire files, not records.
1727
1728Two potentially non-obvious but traditional C<flock> semantics are
1729that it waits indefinitely until the lock is granted, and that its locks
1730B<merely advisory>. Such discretionary locks are more flexible, but offer
19799a22
GS
1731fewer guarantees. This means that files locked with C<flock> may be
1732modified by programs that do not also use C<flock>. See L<perlport>,
2b5ab1e7
TC
1733your port's specific documentation, or your system-specific local manpages
1734for details. It's best to assume traditional behavior if you're writing
1735portable programs. (But if you're not, you should as always feel perfectly
1736free to write for your own system's idiosyncrasies (sometimes called
1737"features"). Slavish adherence to portability concerns shouldn't get
1738in the way of your getting your job done.)
a3cb178b 1739
8ebc5c01
PP
1740OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1741LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
ea3105be 1742you can use the symbolic names if you import them from the Fcntl module,
68dc0745
PP
1743either individually, or as a group using the ':flock' tag. LOCK_SH
1744requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
ea3105be
GS
1745releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1746LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
68dc0745
PP
1747waiting for the lock (check the return status to see if you got it).
1748
2b5ab1e7
TC
1749To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1750before locking or unlocking it.
8ebc5c01 1751
f86cebdf 1752Note that the emulation built with lockf(3) doesn't provide shared
8ebc5c01 1753locks, and it requires that FILEHANDLE be open with write intent. These
2b5ab1e7 1754are the semantics that lockf(3) implements. Most if not all systems
f86cebdf 1755implement lockf(3) in terms of fcntl(2) locking, though, so the
8ebc5c01
PP
1756differing semantics shouldn't bite too many people.
1757
becacb53
TM
1758Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
1759be open with read intent to use LOCK_SH and requires that it be open
1760with write intent to use LOCK_EX.
1761
19799a22
GS
1762Note also that some versions of C<flock> cannot lock things over the
1763network; you would need to use the more system-specific C<fcntl> for
f86cebdf
GS
1764that. If you like you can force Perl to ignore your system's flock(2)
1765function, and so provide its own fcntl(2)-based emulation, by passing
8ebc5c01
PP
1766the switch C<-Ud_flock> to the F<Configure> program when you configure
1767perl.
4633a7c4
LW
1768
1769Here's a mailbox appender for BSD systems.
a0d0e21e 1770
7e1af8bc 1771 use Fcntl ':flock'; # import LOCK_* constants
a0d0e21e
LW
1772
1773 sub lock {
7e1af8bc 1774 flock(MBOX,LOCK_EX);
a0d0e21e
LW
1775 # and, in case someone appended
1776 # while we were waiting...
1777 seek(MBOX, 0, 2);
1778 }
1779
1780 sub unlock {
7e1af8bc 1781 flock(MBOX,LOCK_UN);
a0d0e21e
LW
1782 }
1783
1784 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1785 or die "Can't open mailbox: $!";
1786
1787 lock();
1788 print MBOX $msg,"\n\n";
1789 unlock();
1790
2b5ab1e7
TC
1791On systems that support a real flock(), locks are inherited across fork()
1792calls, whereas those that must resort to the more capricious fcntl()
1793function lose the locks, making it harder to write servers.
1794
cb1a09d0 1795See also L<DB_File> for other flock() examples.
a0d0e21e
LW
1796
1797=item fork
1798
2b5ab1e7
TC
1799Does a fork(2) system call to create a new process running the
1800same program at the same point. It returns the child pid to the
1801parent process, C<0> to the child process, or C<undef> if the fork is
1802unsuccessful. File descriptors (and sometimes locks on those descriptors)
1803are shared, while everything else is copied. On most systems supporting
1804fork(), great care has gone into making it extremely efficient (for
1805example, using copy-on-write technology on data pages), making it the
1806dominant paradigm for multitasking over the last few decades.
5a964f20 1807
0f897271
GS
1808Beginning with v5.6.0, Perl will attempt to flush all files opened for
1809output before forking the child process, but this may not be supported
1810on some platforms (see L<perlport>). To be safe, you may need to set
1811C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1812C<IO::Handle> on any open handles in order to avoid duplicate output.
a0d0e21e 1813
19799a22 1814If you C<fork> without ever waiting on your children, you will
2b5ab1e7
TC
1815accumulate zombies. On some systems, you can avoid this by setting
1816C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1817forking and reaping moribund children.
cb1a09d0 1818
28757baa
PP
1819Note that if your forked child inherits system file descriptors like
1820STDIN and STDOUT that are actually connected by a pipe or socket, even
2b5ab1e7 1821if you exit, then the remote server (such as, say, a CGI script or a
19799a22 1822backgrounded job launched from a remote shell) won't think you're done.
2b5ab1e7 1823You should reopen those to F</dev/null> if it's any issue.
28757baa 1824
cb1a09d0
AD
1825=item format
1826
19799a22 1827Declare a picture format for use by the C<write> function. For
cb1a09d0
AD
1828example:
1829
54310121 1830 format Something =
cb1a09d0
AD
1831 Test: @<<<<<<<< @||||| @>>>>>
1832 $str, $%, '$' . int($num)
1833 .
1834
1835 $str = "widget";
184e9718 1836 $num = $cost/$quantity;
cb1a09d0
AD
1837 $~ = 'Something';
1838 write;
1839
1840See L<perlform> for many details and examples.
1841
8903cb82 1842=item formline PICTURE,LIST
a0d0e21e 1843
5a964f20 1844This is an internal function used by C<format>s, though you may call it,
a0d0e21e
LW
1845too. It formats (see L<perlform>) a list of values according to the
1846contents of PICTURE, placing the output into the format output
7660c0ab 1847accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
19799a22 1848Eventually, when a C<write> is done, the contents of
a0d0e21e 1849C<$^A> are written to some filehandle, but you could also read C<$^A>
7660c0ab 1850yourself and then set C<$^A> back to C<"">. Note that a format typically
19799a22 1851does one C<formline> per line of form, but the C<formline> function itself
748a9306 1852doesn't care how many newlines are embedded in the PICTURE. This means
4633a7c4 1853that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
748a9306
LW
1854You may therefore need to use multiple formlines to implement a single
1855record format, just like the format compiler.
1856
19799a22 1857Be careful if you put double quotes around the picture, because an C<@>
748a9306 1858character may be taken to mean the beginning of an array name.
19799a22 1859C<formline> always returns true. See L<perlform> for other examples.
a0d0e21e
LW
1860
1861=item getc FILEHANDLE
1862
1863=item getc
1864
1865Returns the next character from the input file attached to FILEHANDLE,
b5fe5ca2
SR
1866or the undefined value at end of file, or if there was an error (in
1867the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
1868STDIN. This is not particularly efficient. However, it cannot be
1869used by itself to fetch single characters without waiting for the user
1870to hit enter. For that, try something more like:
4633a7c4
LW
1871
1872 if ($BSD_STYLE) {
1873 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1874 }
1875 else {
54310121 1876 system "stty", '-icanon', 'eol', "\001";
4633a7c4
LW
1877 }
1878
1879 $key = getc(STDIN);
1880
1881 if ($BSD_STYLE) {
1882 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1883 }
1884 else {
5f05dabc 1885 system "stty", 'icanon', 'eol', '^@'; # ASCII null
4633a7c4
LW
1886 }
1887 print "\n";
1888
54310121
PP
1889Determination of whether $BSD_STYLE should be set
1890is left as an exercise to the reader.
cb1a09d0 1891
19799a22 1892The C<POSIX::getattr> function can do this more portably on
2b5ab1e7
TC
1893systems purporting POSIX compliance. See also the C<Term::ReadKey>
1894module from your nearest CPAN site; details on CPAN can be found on
1895L<perlmodlib/CPAN>.
a0d0e21e
LW
1896
1897=item getlogin
1898
5a964f20
TC
1899Implements the C library function of the same name, which on most
1900systems returns the current login from F</etc/utmp>, if any. If null,
19799a22 1901use C<getpwuid>.
a0d0e21e 1902
f86702cc 1903 $login = getlogin || getpwuid($<) || "Kilroy";
a0d0e21e 1904
19799a22
GS
1905Do not consider C<getlogin> for authentication: it is not as
1906secure as C<getpwuid>.
4633a7c4 1907
a0d0e21e
LW
1908=item getpeername SOCKET
1909
1910Returns the packed sockaddr address of other end of the SOCKET connection.
1911
4633a7c4
LW
1912 use Socket;
1913 $hersockaddr = getpeername(SOCK);
19799a22 1914 ($port, $iaddr) = sockaddr_in($hersockaddr);
4633a7c4
LW
1915 $herhostname = gethostbyaddr($iaddr, AF_INET);
1916 $herstraddr = inet_ntoa($iaddr);
a0d0e21e
LW
1917
1918=item getpgrp PID
1919
47e29363 1920Returns the current process group for the specified PID. Use
7660c0ab 1921a PID of C<0> to get the current process group for the
4633a7c4 1922current process. Will raise an exception if used on a machine that
f86cebdf 1923doesn't implement getpgrp(2). If PID is omitted, returns process
19799a22 1924group of current process. Note that the POSIX version of C<getpgrp>
7660c0ab 1925does not accept a PID argument, so only C<PID==0> is truly portable.
a0d0e21e
LW
1926
1927=item getppid
1928
1929Returns the process id of the parent process.
1930
4d76a344
RGS
1931Note for Linux users: on Linux, the C functions C<getpid()> and
1932C<getppid()> return different values from different threads. In order to
1933be portable, this behavior is not reflected by the perl-level function
1934C<getppid()>, that returns a consistent value across threads. If you want
e3256f86
RGS
1935to call the underlying C<getppid()>, you may use the CPAN module
1936C<Linux::Pid>.
4d76a344 1937
a0d0e21e
LW
1938=item getpriority WHICH,WHO
1939
4633a7c4
LW
1940Returns the current priority for a process, a process group, or a user.
1941(See L<getpriority(2)>.) Will raise a fatal exception if used on a
f86cebdf 1942machine that doesn't implement getpriority(2).
a0d0e21e
LW
1943
1944=item getpwnam NAME
1945
1946=item getgrnam NAME
1947
1948=item gethostbyname NAME
1949
1950=item getnetbyname NAME
1951
1952=item getprotobyname NAME
1953
1954=item getpwuid UID
1955
1956=item getgrgid GID
1957
1958=item getservbyname NAME,PROTO
1959
1960=item gethostbyaddr ADDR,ADDRTYPE
1961
1962=item getnetbyaddr ADDR,ADDRTYPE
1963
1964=item getprotobynumber NUMBER
1965
1966=item getservbyport PORT,PROTO
1967
1968=item getpwent
1969
1970=item getgrent
1971
1972=item gethostent
1973
1974=item getnetent
1975
1976=item getprotoent
1977
1978=item getservent
1979
1980=item setpwent
1981
1982=item setgrent
1983
1984=item sethostent STAYOPEN
1985
1986=item setnetent STAYOPEN
1987
1988=item setprotoent STAYOPEN
1989
1990=item setservent STAYOPEN
1991
1992=item endpwent
1993
1994=item endgrent
1995
1996=item endhostent
1997
1998=item endnetent
1999
2000=item endprotoent
2001
2002=item endservent
2003
2004These routines perform the same functions as their counterparts in the
5a964f20 2005system library. In list context, the return values from the
a0d0e21e
LW
2006various get routines are as follows:
2007
2008 ($name,$passwd,$uid,$gid,
6ee623d5 2009 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
a0d0e21e
LW
2010 ($name,$passwd,$gid,$members) = getgr*
2011 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
2012 ($name,$aliases,$addrtype,$net) = getnet*
2013 ($name,$aliases,$proto) = getproto*
2014 ($name,$aliases,$port,$proto) = getserv*
2015
2016(If the entry doesn't exist you get a null list.)
2017
4602f195
JH
2018The exact meaning of the $gcos field varies but it usually contains
2019the real name of the user (as opposed to the login name) and other
2020information pertaining to the user. Beware, however, that in many
2021system users are able to change this information and therefore it
106325ad 2022cannot be trusted and therefore the $gcos is tainted (see
2959b6e3
JH
2023L<perlsec>). The $passwd and $shell, user's encrypted password and
2024login shell, are also tainted, because of the same reason.
4602f195 2025
5a964f20 2026In scalar context, you get the name, unless the function was a
a0d0e21e
LW
2027lookup by name, in which case you get the other thing, whatever it is.
2028(If the entry doesn't exist you get the undefined value.) For example:
2029
5a964f20
TC
2030 $uid = getpwnam($name);
2031 $name = getpwuid($num);
2032 $name = getpwent();
2033 $gid = getgrnam($name);
08a33e13 2034 $name = getgrgid($num);
5a964f20
TC
2035 $name = getgrent();
2036 #etc.
a0d0e21e 2037
4602f195
JH
2038In I<getpw*()> the fields $quota, $comment, and $expire are special
2039cases in the sense that in many systems they are unsupported. If the
2040$quota is unsupported, it is an empty scalar. If it is supported, it
2041usually encodes the disk quota. If the $comment field is unsupported,
2042it is an empty scalar. If it is supported it usually encodes some
2043administrative comment about the user. In some systems the $quota
2044field may be $change or $age, fields that have to do with password
2045aging. In some systems the $comment field may be $class. The $expire
2046field, if present, encodes the expiration period of the account or the
2047password. For the availability and the exact meaning of these fields
2048in your system, please consult your getpwnam(3) documentation and your
2049F<pwd.h> file. You can also find out from within Perl what your
2050$quota and $comment fields mean and whether you have the $expire field
2051by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2052C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
2053files are only supported if your vendor has implemented them in the
2054intuitive fashion that calling the regular C library routines gets the
5d3a0a3b
GS
2055shadow versions if you're running under privilege or if there exists
2056the shadow(3) functions as found in System V ( this includes Solaris
2057and Linux.) Those systems which implement a proprietary shadow password
2058facility are unlikely to be supported.
6ee623d5 2059
19799a22 2060The $members value returned by I<getgr*()> is a space separated list of
a0d0e21e
LW
2061the login names of the members of the group.
2062
2063For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2064C, it will be returned to you via C<$?> if the function call fails. The
7660c0ab 2065C<@addrs> value returned by a successful call is a list of the raw
a0d0e21e
LW
2066addresses returned by the corresponding system library call. In the
2067Internet domain, each address is four bytes long and you can unpack it
2068by saying something like:
2069
f337b084 2070 ($a,$b,$c,$d) = unpack('W4',$addr[0]);
a0d0e21e 2071
2b5ab1e7
TC
2072The Socket library makes this slightly easier:
2073
2074 use Socket;
2075 $iaddr = inet_aton("127.1"); # or whatever address
2076 $name = gethostbyaddr($iaddr, AF_INET);
2077
2078 # or going the other way
19799a22 2079 $straddr = inet_ntoa($iaddr);
2b5ab1e7 2080
19799a22
GS
2081If you get tired of remembering which element of the return list
2082contains which return value, by-name interfaces are provided
2083in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2084C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2085and C<User::grent>. These override the normal built-ins, supplying
2086versions that return objects with the appropriate names
2087for each field. For example:
5a964f20
TC
2088
2089 use File::stat;
2090 use User::pwent;
2091 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2092
b76cc8ba
NIS
2093Even though it looks like they're the same method calls (uid),
2094they aren't, because a C<File::stat> object is different from
19799a22 2095a C<User::pwent> object.
5a964f20 2096
a0d0e21e
LW
2097=item getsockname SOCKET
2098
19799a22
GS
2099Returns the packed sockaddr address of this end of the SOCKET connection,
2100in case you don't know the address because you have several different
2101IPs that the connection might have come in on.
a0d0e21e 2102
4633a7c4
LW
2103 use Socket;
2104 $mysockaddr = getsockname(SOCK);
19799a22 2105 ($port, $myaddr) = sockaddr_in($mysockaddr);
b76cc8ba 2106 printf "Connect to %s [%s]\n",
19799a22
GS
2107 scalar gethostbyaddr($myaddr, AF_INET),
2108 inet_ntoa($myaddr);
a0d0e21e
LW
2109
2110=item getsockopt SOCKET,LEVEL,OPTNAME
2111
636e6b1f
TH
2112Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
2113Options may exist at multiple protocol levels depending on the socket
2114type, but at least the uppermost socket level SOL_SOCKET (defined in the
2115C<Socket> module) will exist. To query options at another level the
2116protocol number of the appropriate protocol controlling the option
2117should be supplied. For example, to indicate that an option is to be
2118interpreted by the TCP protocol, LEVEL should be set to the protocol
2119number of TCP, which you can get using getprotobyname.
2120
2121The call returns a packed string representing the requested socket option,
2122or C<undef> if there is an error (the error reason will be in $!). What
2123exactly is in the packed string depends in the LEVEL and OPTNAME, consult
2124your system documentation for details. A very common case however is that
2125the option is an integer, in which case the result will be an packed
2126integer which you can decode using unpack with the C<i> (or C<I>) format.
2127
2128An example testing if Nagle's algorithm is turned on on a socket:
2129
4852725b 2130 use Socket qw(:all);
636e6b1f
TH
2131
2132 defined(my $tcp = getprotobyname("tcp"))
2133 or die "Could not determine the protocol number for tcp";
4852725b
DD
2134 # my $tcp = IPPROTO_TCP; # Alternative
2135 my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
2136 or die "Could not query TCP_NODELAY socket option: $!";
636e6b1f
TH
2137 my $nodelay = unpack("I", $packed);
2138 print "Nagle's algorithm is turned ", $nodelay ? "off\n" : "on\n";
2139
a0d0e21e
LW
2140
2141=item glob EXPR
2142
0a753a76
PP
2143=item glob
2144
d9a9d457
JL
2145In list context, returns a (possibly empty) list of filename expansions on
2146the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
2147scalar context, glob iterates through such filename expansions, returning
2148undef when the list is exhausted. This is the internal function
2149implementing the C<< <*.c> >> operator, but you can use it directly. If
2150EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2151more detail in L<perlop/"I/O Operators">.
a0d0e21e 2152
3a4b19e4
GS
2153Beginning with v5.6.0, this operator is implemented using the standard
2154C<File::Glob> extension. See L<File::Glob> for details.
2155
a0d0e21e
LW
2156=item gmtime EXPR
2157
ce2984c3
PF
2158=item gmtime
2159
d1be9408 2160Converts a time as returned by the time function to an 8-element list
54310121 2161with the time localized for the standard Greenwich time zone.
4633a7c4 2162Typically used as follows:
a0d0e21e 2163
b76cc8ba 2164 # 0 1 2 3 4 5 6 7
48a26b3a 2165 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday) =
a0d0e21e
LW
2166 gmtime(time);
2167
48a26b3a
GS
2168All list elements are numeric, and come straight out of the C `struct
2169tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2170specified time. $mday is the day of the month, and $mon is the month
2171itself, in the range C<0..11> with 0 indicating January and 11
2172indicating December. $year is the number of years since 1900. That
2173is, $year is C<123> in year 2023. $wday is the day of the week, with
21740 indicating Sunday and 3 indicating Wednesday. $yday is the day of
b76cc8ba 2175the year, in the range C<0..364> (or C<0..365> in leap years.)
48a26b3a
GS
2176
2177Note that the $year element is I<not> simply the last two digits of
2178the year. If you assume it is, then you create non-Y2K-compliant
2179programs--and you wouldn't want to do that, would you?
2f9daede 2180
abd75f24
GS
2181The proper way to get a complete 4-digit year is simply:
2182
2183 $year += 1900;
2184
2185And to get the last two digits of the year (e.g., '01' in 2001) do:
2186
2187 $year = sprintf("%02d", $year % 100);
2188
48a26b3a 2189If EXPR is omitted, C<gmtime()> uses the current time (C<gmtime(time)>).
a0d0e21e 2190
48a26b3a 2191In scalar context, C<gmtime()> returns the ctime(3) value:
0a753a76
PP
2192
2193 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
2194
fe86afc2
NC
2195If you need local time instead of GMT use the L</localtime> builtin.
2196See also the C<timegm> function provided by the C<Time::Local> module,
2197and the strftime(3) and mktime(3) functions available via the L<POSIX> module.
7660c0ab 2198
fe86afc2
NC
2199This scalar value is B<not> locale dependent (see L<perllocale>), but is
2200instead a Perl builtin. To get somewhat similar but locale dependent date
2201strings, see the example in L</localtime>.
0a753a76 2202
62aa5637
MS
2203See L<perlport/gmtime> for portability concerns.
2204
a0d0e21e
LW
2205=item goto LABEL
2206
748a9306
LW
2207=item goto EXPR
2208
a0d0e21e
LW
2209=item goto &NAME
2210
7660c0ab 2211The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
a0d0e21e 2212execution there. It may not be used to go into any construct that
7660c0ab 2213requires initialization, such as a subroutine or a C<foreach> loop. It
0a753a76 2214also can't be used to go into a construct that is optimized away,
19799a22 2215or to get out of a block or subroutine given to C<sort>.
0a753a76 2216It can be used to go almost anywhere else within the dynamic scope,
a0d0e21e 2217including out of subroutines, but it's usually better to use some other
19799a22 2218construct such as C<last> or C<die>. The author of Perl has never felt the
7660c0ab 2219need to use this form of C<goto> (in Perl, that is--C is another matter).
1b6921cb
BT
2220(The difference being that C does not offer named loops combined with
2221loop control. Perl does, and this replaces most structured uses of C<goto>
2222in other languages.)
a0d0e21e 2223
7660c0ab
A
2224The C<goto-EXPR> form expects a label name, whose scope will be resolved
2225dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
748a9306
LW
2226necessarily recommended if you're optimizing for maintainability:
2227
2228 goto ("FOO", "BAR", "GLARCH")[$i];
2229
1b6921cb
BT
2230The C<goto-&NAME> form is quite different from the other forms of
2231C<goto>. In fact, it isn't a goto in the normal sense at all, and
2232doesn't have the stigma associated with other gotos. Instead, it
2233exits the current subroutine (losing any changes set by local()) and
2234immediately calls in its place the named subroutine using the current
2235value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2236load another subroutine and then pretend that the other subroutine had
2237been called in the first place (except that any modifications to C<@_>
6cb9131c
GS
2238in the current subroutine are propagated to the other subroutine.)
2239After the C<goto>, not even C<caller> will be able to tell that this
2240routine was called first.
2241
2242NAME needn't be the name of a subroutine; it can be a scalar variable
2243containing a code reference, or a block which evaluates to a code
2244reference.
a0d0e21e
LW
2245
2246=item grep BLOCK LIST
2247
2248=item grep EXPR,LIST
2249
2b5ab1e7
TC
2250This is similar in spirit to, but not the same as, grep(1) and its
2251relatives. In particular, it is not limited to using regular expressions.
2f9daede 2252
a0d0e21e 2253Evaluates the BLOCK or EXPR for each element of LIST (locally setting
7660c0ab 2254C<$_> to each element) and returns the list value consisting of those
19799a22
GS
2255elements for which the expression evaluated to true. In scalar
2256context, returns the number of times the expression was true.
a0d0e21e
LW
2257
2258 @foo = grep(!/^#/, @bar); # weed out comments
2259
2260or equivalently,
2261
2262 @foo = grep {!/^#/} @bar; # weed out comments
2263
be3174d2
GS
2264Note that C<$_> is an alias to the list value, so it can be used to
2265modify the elements of the LIST. While this is useful and supported,
2266it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
2267Similarly, grep returns aliases into the original list, much as a for
2268loop's index variable aliases the list elements. That is, modifying an
19799a22
GS
2269element of a list returned by grep (for example, in a C<foreach>, C<map>
2270or another C<grep>) actually modifies the element in the original list.
2b5ab1e7 2271This is usually something to be avoided when writing clear code.
a0d0e21e 2272
a4fb8298
RGS
2273If C<$_> is lexical in the scope where the C<grep> appears (because it has
2274been declared with C<my $_>) then, in addition the be locally aliased to
2275the list elements, C<$_> keeps being lexical inside the block; i.e. it
2276can't be seen from the outside, avoiding any potential side-effects.
2277
19799a22 2278See also L</map> for a list composed of the results of the BLOCK or EXPR.
38325410 2279
a0d0e21e
LW
2280=item hex EXPR
2281
54310121 2282=item hex
bbce6d69 2283
2b5ab1e7 2284Interprets EXPR as a hex string and returns the corresponding value.
38366c11 2285(To convert strings that might start with either C<0>, C<0x>, or C<0b>, see
2b5ab1e7 2286L</oct>.) If EXPR is omitted, uses C<$_>.
2f9daede
TPG
2287
2288 print hex '0xAf'; # prints '175'
2289 print hex 'aF'; # same
a0d0e21e 2290
19799a22 2291Hex strings may only represent integers. Strings that would cause
53305cf1 2292integer overflow trigger a warning. Leading whitespace is not stripped,
38366c11
DN
2293unlike oct(). To present something as hex, look into L</printf>,
2294L</sprintf>, or L</unpack>.
19799a22 2295
ce2984c3 2296=item import LIST
a0d0e21e 2297
19799a22 2298There is no builtin C<import> function. It is just an ordinary
4633a7c4 2299method (subroutine) defined (or inherited) by modules that wish to export
19799a22 2300names to another module. The C<use> function calls the C<import> method
cea6626f 2301for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
a0d0e21e
LW
2302
2303=item index STR,SUBSTR,POSITION
2304
2305=item index STR,SUBSTR
2306
2b5ab1e7
TC
2307The index function searches for one string within another, but without
2308the wildcard-like behavior of a full regular-expression pattern match.
2309It returns the position of the first occurrence of SUBSTR in STR at
2310or after POSITION. If POSITION is omitted, starts searching from the
2311beginning of the string. The return value is based at C<0> (or whatever
2312you've set the C<$[> variable to--but don't do that). If the substring
2313is not found, returns one less than the base, ordinarily C<-1>.
a0d0e21e
LW
2314
2315=item int EXPR
2316
54310121 2317=item int
bbce6d69 2318
7660c0ab 2319Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2b5ab1e7
TC
2320You should not use this function for rounding: one because it truncates
2321towards C<0>, and two because machine representations of floating point
2322numbers can sometimes produce counterintuitive results. For example,
2323C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2324because it's really more like -268.99999999999994315658 instead. Usually,
19799a22 2325the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2b5ab1e7 2326functions will serve you better than will int().
a0d0e21e
LW
2327
2328=item ioctl FILEHANDLE,FUNCTION,SCALAR
2329
2b5ab1e7 2330Implements the ioctl(2) function. You'll probably first have to say
a0d0e21e 2331
a11c483f 2332 require "sys/ioctl.ph"; # probably in $Config{archlib}/ioctl.ph
a0d0e21e 2333
a11c483f 2334to get the correct function definitions. If F<sys/ioctl.ph> doesn't
a0d0e21e 2335exist or doesn't have the correct definitions you'll have to roll your
61eff3bc 2336own, based on your C header files such as F<< <sys/ioctl.h> >>.
5a964f20 2337(There is a Perl script called B<h2ph> that comes with the Perl kit that
54310121 2338may help you in this, but it's nontrivial.) SCALAR will be read and/or
4633a7c4 2339written depending on the FUNCTION--a pointer to the string value of SCALAR
19799a22 2340will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
4633a7c4
LW
2341has no string value but does have a numeric value, that value will be
2342passed rather than a pointer to the string value. To guarantee this to be
19799a22
GS
2343true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2344functions may be needed to manipulate the values of structures used by
b76cc8ba 2345C<ioctl>.
a0d0e21e 2346
19799a22 2347The return value of C<ioctl> (and C<fcntl>) is as follows:
a0d0e21e
LW
2348
2349 if OS returns: then Perl returns:
2350 -1 undefined value
2351 0 string "0 but true"
2352 anything else that number
2353
19799a22 2354Thus Perl returns true on success and false on failure, yet you can
a0d0e21e
LW
2355still easily determine the actual value returned by the operating
2356system:
2357
2b5ab1e7 2358 $retval = ioctl(...) || -1;
a0d0e21e
LW
2359 printf "System returned %d\n", $retval;
2360
be2f7487 2361The special string C<"0 but true"> is exempt from B<-w> complaints
5a964f20
TC
2362about improper numeric conversions.
2363
a0d0e21e
LW
2364=item join EXPR,LIST
2365
2b5ab1e7
TC
2366Joins the separate strings of LIST into a single string with fields
2367separated by the value of EXPR, and returns that new string. Example:
a0d0e21e 2368
2b5ab1e7 2369 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
a0d0e21e 2370
eb6e2d6f
GS
2371Beware that unlike C<split>, C<join> doesn't take a pattern as its
2372first argument. Compare L</split>.
a0d0e21e 2373
aa689395
PP
2374=item keys HASH
2375
504f80c1
JH
2376Returns a list consisting of all the keys of the named hash.
2377(In scalar context, returns the number of keys.)
2378
2379The keys are returned in an apparently random order. The actual
2380random order is subject to change in future versions of perl, but it
2381is guaranteed to be the same order as either the C<values> or C<each>
4546b9e6
JH
2382function produces (given that the hash has not been modified). Since
2383Perl 5.8.1 the ordering is different even between different runs of
2384Perl for security reasons (see L<perlsec/"Algorithmic Complexity
d6df3700 2385Attacks">).
504f80c1
JH
2386
2387As a side effect, calling keys() resets the HASH's internal iterator,
2f65b2f0
RGS
2388see L</each>. (In particular, calling keys() in void context resets
2389the iterator with no other overhead.)
a0d0e21e 2390
aa689395 2391Here is yet another way to print your environment:
a0d0e21e
LW
2392
2393 @keys = keys %ENV;
2394 @values = values %ENV;
b76cc8ba 2395 while (@keys) {
a0d0e21e
LW
2396 print pop(@keys), '=', pop(@values), "\n";
2397 }
2398
2399or how about sorted by key:
2400
2401 foreach $key (sort(keys %ENV)) {
2402 print $key, '=', $ENV{$key}, "\n";
2403 }
2404
8ea1e5d4
GS
2405The returned values are copies of the original keys in the hash, so
2406modifying them will not affect the original hash. Compare L</values>.
2407
19799a22 2408To sort a hash by value, you'll need to use a C<sort> function.
aa689395 2409Here's a descending numeric sort of a hash by its values:
4633a7c4 2410
5a964f20 2411 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
4633a7c4
LW
2412 printf "%4d %s\n", $hash{$key}, $key;
2413 }
2414
19799a22 2415As an lvalue C<keys> allows you to increase the number of hash buckets
aa689395
PP
2416allocated for the given hash. This can gain you a measure of efficiency if
2417you know the hash is going to get big. (This is similar to pre-extending
2418an array by assigning a larger number to $#array.) If you say
55497cff
PP
2419
2420 keys %hash = 200;
2421
ab192400
GS
2422then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2423in fact, since it rounds up to the next power of two. These
55497cff
PP
2424buckets will be retained even if you do C<%hash = ()>, use C<undef
2425%hash> if you want to free the storage while C<%hash> is still in scope.
2426You can't shrink the number of buckets allocated for the hash using
19799a22 2427C<keys> in this way (but you needn't worry about doing this by accident,
55497cff
PP
2428as trying has no effect).
2429
19799a22 2430See also C<each>, C<values> and C<sort>.
ab192400 2431
b350dd2f 2432=item kill SIGNAL, LIST
a0d0e21e 2433
b350dd2f 2434Sends a signal to a list of processes. Returns the number of
517db077
GS
2435processes successfully signaled (which is not necessarily the
2436same as the number actually killed).
a0d0e21e
LW
2437
2438 $cnt = kill 1, $child1, $child2;
2439 kill 9, @goners;
2440
b350dd2f 2441If SIGNAL is zero, no signal is sent to the process. This is a
1e9c1022 2442useful way to check that a child process is alive and hasn't changed
b350dd2f
GS
2443its UID. See L<perlport> for notes on the portability of this
2444construct.
2445
2446Unlike in the shell, if SIGNAL is negative, it kills
4633a7c4
LW
2447process groups instead of processes. (On System V, a negative I<PROCESS>
2448number will also kill process groups, but that's not portable.) That
2449means you usually want to use positive not negative signals. You may also
1e9c1022
JL
2450use a signal name in quotes.
2451
2452See L<perlipc/"Signals"> for more details.
a0d0e21e
LW
2453
2454=item last LABEL
2455
2456=item last
2457
2458The C<last> command is like the C<break> statement in C (as used in
2459loops); it immediately exits the loop in question. If the LABEL is
2460omitted, the command refers to the innermost enclosing loop. The
2461C<continue> block, if any, is not executed:
2462
4633a7c4
LW
2463 LINE: while (<STDIN>) {
2464 last LINE if /^$/; # exit when done with header
5a964f20 2465 #...
a0d0e21e
LW
2466 }
2467
4968c1e4 2468C<last> cannot be used to exit a block which returns a value such as
2b5ab1e7
TC
2469C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2470a grep() or map() operation.
4968c1e4 2471
6c1372ed
GS
2472Note that a block by itself is semantically identical to a loop
2473that executes once. Thus C<last> can be used to effect an early
2474exit out of such a block.
2475
98293880
JH
2476See also L</continue> for an illustration of how C<last>, C<next>, and
2477C<redo> work.
1d2dff63 2478
a0d0e21e
LW
2479=item lc EXPR
2480
54310121 2481=item lc
bbce6d69 2482
d1be9408 2483Returns a lowercased version of EXPR. This is the internal function
ad0029c4
JH
2484implementing the C<\L> escape in double-quoted strings. Respects
2485current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
983ffd37 2486and L<perlunicode> for more details about locale and Unicode support.
a0d0e21e 2487
7660c0ab 2488If EXPR is omitted, uses C<$_>.
bbce6d69 2489
a0d0e21e
LW
2490=item lcfirst EXPR
2491
54310121 2492=item lcfirst
bbce6d69 2493
ad0029c4
JH
2494Returns the value of EXPR with the first character lowercased. This
2495is the internal function implementing the C<\l> escape in
2496double-quoted strings. Respects current LC_CTYPE locale if C<use
983ffd37
JH
2497locale> in force. See L<perllocale> and L<perlunicode> for more
2498details about locale and Unicode support.
a0d0e21e 2499
7660c0ab 2500If EXPR is omitted, uses C<$_>.
bbce6d69 2501
a0d0e21e
LW
2502=item length EXPR
2503
54310121 2504=item length
bbce6d69 2505
974da8e5 2506Returns the length in I<characters> of the value of EXPR. If EXPR is
b76cc8ba 2507omitted, returns length of C<$_>. Note that this cannot be used on
2b5ab1e7
TC
2508an entire array or hash to find out how many elements these have.
2509For that, use C<scalar @array> and C<scalar keys %hash> respectively.
a0d0e21e 2510
974da8e5
JH
2511Note the I<characters>: if the EXPR is in Unicode, you will get the
2512number of characters, not the number of bytes. To get the length
2513in bytes, use C<do { use bytes; length(EXPR) }>, see L<bytes>.
2514
a0d0e21e
LW
2515=item link OLDFILE,NEWFILE
2516
19799a22 2517Creates a new filename linked to the old filename. Returns true for
b76cc8ba 2518success, false otherwise.
a0d0e21e
LW
2519
2520=item listen SOCKET,QUEUESIZE
2521
19799a22 2522Does the same thing that the listen system call does. Returns true if
b76cc8ba 2523it succeeded, false otherwise. See the example in
cea6626f 2524L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
2525
2526=item local EXPR
2527
19799a22 2528You really probably want to be using C<my> instead, because C<local> isn't
b76cc8ba 2529what most people think of as "local". See
13a2d996 2530L<perlsub/"Private Variables via my()"> for details.
2b5ab1e7 2531
5a964f20
TC
2532A local modifies the listed variables to be local to the enclosing
2533block, file, or eval. If more than one value is listed, the list must
2534be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2535for details, including issues with tied arrays and hashes.
a0d0e21e 2536
a0d0e21e
LW
2537=item localtime EXPR
2538
ba053783
AL
2539=item localtime
2540
19799a22 2541Converts a time as returned by the time function to a 9-element list
5f05dabc 2542with the time analyzed for the local time zone. Typically used as
a0d0e21e
LW
2543follows:
2544
54310121 2545 # 0 1 2 3 4 5 6 7 8
a0d0e21e 2546 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
ba053783 2547 localtime(time);
a0d0e21e 2548
48a26b3a 2549All list elements are numeric, and come straight out of the C `struct
ba053783
AL
2550tm'. C<$sec>, C<$min>, and C<$hour> are the seconds, minutes, and hours
2551of the specified time.
48a26b3a 2552
ba053783
AL
2553C<$mday> is the day of the month, and C<$mon> is the month itself, in
2554the range C<0..11> with 0 indicating January and 11 indicating December.
2555This makes it easy to get a month name from a list:
54310121 2556
ba053783
AL
2557 my @abbr = qw( Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec );
2558 print "$abbr[$mon] $mday";
2559 # $mon=9, $mday=18 gives "Oct 18"
abd75f24 2560
ba053783
AL
2561C<$year> is the number of years since 1900, not just the last two digits
2562of the year. That is, C<$year> is C<123> in year 2023. The proper way
2563to get a complete 4-digit year is simply:
abd75f24 2564
ba053783 2565 $year += 1900;
abd75f24 2566
ba053783
AL
2567To get the last two digits of the year (e.g., '01' in 2001) do:
2568
2569 $year = sprintf("%02d", $year % 100);
2570
2571C<$wday> is the day of the week, with 0 indicating Sunday and 3 indicating
2572Wednesday. C<$yday> is the day of the year, in the range C<0..364>
2573(or C<0..365> in leap years.)
2574
2575C<$isdst> is true if the specified time occurs during Daylight Saving
2576Time, false otherwise.
abd75f24 2577
48a26b3a 2578If EXPR is omitted, C<localtime()> uses the current time (C<localtime(time)>).
a0d0e21e 2579
48a26b3a 2580In scalar context, C<localtime()> returns the ctime(3) value:
a0d0e21e 2581
5f05dabc 2582 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
a0d0e21e 2583
fe86afc2
NC
2584This scalar value is B<not> locale dependent but is a Perl builtin. For GMT
2585instead of local time use the L</gmtime> builtin. See also the
2586C<Time::Local> module (to convert the second, minutes, hours, ... back to
2587the integer value returned by time()), and the L<POSIX> module's strftime(3)
2588and mktime(3) functions.
2589
2590To get somewhat similar but locale dependent date strings, set up your
2591locale environment variables appropriately (please see L<perllocale>) and
2592try for example:
a3cb178b 2593
5a964f20 2594 use POSIX qw(strftime);
2b5ab1e7 2595 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
fe86afc2
NC
2596 # or for GMT formatted appropriately for your locale:
2597 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
a3cb178b
GS
2598
2599Note that the C<%a> and C<%b>, the short forms of the day of the week
2600and the month of the year, may not necessarily be three characters wide.
a0d0e21e 2601
62aa5637
MS
2602See L<perlport/localtime> for portability concerns.
2603
07698885 2604=item lock THING
19799a22 2605
01e6739c 2606This function places an advisory lock on a shared variable, or referenced
03730085 2607object contained in I<THING> until the lock goes out of scope.
a6d5524e 2608
f3a23afb 2609lock() is a "weak keyword" : this means that if you've defined a function
67408cae 2610by this name (before any calls to it), that function will be called
03730085
AB
2611instead. (However, if you've said C<use threads>, lock() is always a
2612keyword.) See L<threads>.
19799a22 2613
a0d0e21e
LW
2614=item log EXPR
2615
54310121 2616=item log
bbce6d69 2617
2b5ab1e7
TC
2618Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2619returns log of C<$_>. To get the log of another base, use basic algebra:
19799a22 2620The base-N log of a number is equal to the natural log of that number
2b5ab1e7
TC
2621divided by the natural log of N. For example:
2622
2623 sub log10 {
2624 my $n = shift;
2625 return log($n)/log(10);
b76cc8ba 2626 }
2b5ab1e7
TC
2627
2628See also L</exp> for the inverse operation.
a0d0e21e 2629
a0d0e21e
LW
2630=item lstat EXPR
2631
54310121 2632=item lstat
bbce6d69 2633
19799a22 2634Does the same thing as the C<stat> function (including setting the
5a964f20
TC
2635special C<_> filehandle) but stats a symbolic link instead of the file
2636the symbolic link points to. If symbolic links are unimplemented on
c837d5b4
DP
2637your system, a normal C<stat> is done. For much more detailed
2638information, please see the documentation for C<stat>.
a0d0e21e 2639
7660c0ab 2640If EXPR is omitted, stats C<$_>.
bbce6d69 2641
a0d0e21e
LW
2642=item m//
2643
2644The match operator. See L<perlop>.
2645
2646=item map BLOCK LIST
2647
2648=item map EXPR,LIST
2649
19799a22
GS
2650Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2651C<$_> to each element) and returns the list value composed of the
2652results of each such evaluation. In scalar context, returns the
2653total number of elements so generated. Evaluates BLOCK or EXPR in
2654list context, so each element of LIST may produce zero, one, or
2655more elements in the returned value.
dd99ebda 2656
a0d0e21e
LW
2657 @chars = map(chr, @nums);
2658
2659translates a list of numbers to the corresponding characters. And
2660
4633a7c4 2661 %hash = map { getkey($_) => $_ } @array;
a0d0e21e
LW
2662
2663is just a funny way to write
2664
2665 %hash = ();
2666 foreach $_ (@array) {
4633a7c4 2667 $hash{getkey($_)} = $_;
a0d0e21e
LW
2668 }
2669
be3174d2
GS
2670Note that C<$_> is an alias to the list value, so it can be used to
2671modify the elements of the LIST. While this is useful and supported,
2672it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
2673Using a regular C<foreach> loop for this purpose would be clearer in
2674most cases. See also L</grep> for an array composed of those items of
2675the original list for which the BLOCK or EXPR evaluates to true.
fb73857a 2676
a4fb8298
RGS
2677If C<$_> is lexical in the scope where the C<map> appears (because it has
2678been declared with C<my $_>) then, in addition the be locally aliased to
2679the list elements, C<$_> keeps being lexical inside the block; i.e. it
2680can't be seen from the outside, avoiding any potential side-effects.
2681
205fdb4d
NC
2682C<{> starts both hash references and blocks, so C<map { ...> could be either
2683the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look
2684ahead for the closing C<}> it has to take a guess at which its dealing with
2685based what it finds just after the C<{>. Usually it gets it right, but if it
2686doesn't it won't realize something is wrong until it gets to the C<}> and
2687encounters the missing (or unexpected) comma. The syntax error will be
2688reported close to the C<}> but you'll need to change something near the C<{>
2689such as using a unary C<+> to give perl some help:
2690
2691 %hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong
2692 %hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right
2693 %hash = map { ("\L$_", 1) } @array # this also works
2694 %hash = map { lc($_), 1 } @array # as does this.
2695 %hash = map +( lc($_), 1 ), @array # this is EXPR and works!
cea6626f 2696
205fdb4d
NC
2697 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
2698
2699or to force an anon hash constructor use C<+{>
2700
2701 @hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end
2702
2703and you get list of anonymous hashes each with only 1 entry.
2704
19799a22 2705=item mkdir FILENAME,MASK
a0d0e21e 2706
5a211162
GS
2707=item mkdir FILENAME
2708
491873e5
RGS
2709=item mkdir
2710
0591cd52 2711Creates the directory specified by FILENAME, with permissions
19799a22
GS
2712specified by MASK (as modified by C<umask>). If it succeeds it
2713returns true, otherwise it returns false and sets C<$!> (errno).
491873e5
RGS
2714If omitted, MASK defaults to 0777. If omitted, FILENAME defaults
2715to C<$_>.
0591cd52 2716
19799a22 2717In general, it is better to create directories with permissive MASK,
0591cd52 2718and let the user modify that with their C<umask>, than it is to supply
19799a22 2719a restrictive MASK and give the user no way to be more permissive.
0591cd52
NT
2720The exceptions to this rule are when the file or directory should be
2721kept private (mail files, for instance). The perlfunc(1) entry on
19799a22 2722C<umask> discusses the choice of MASK in more detail.
a0d0e21e 2723
cc1852e8
JH
2724Note that according to the POSIX 1003.1-1996 the FILENAME may have any
2725number of trailing slashes. Some operating and filesystems do not get
2726this right, so Perl automatically removes all trailing slashes to keep
2727everyone happy.
2728
a0d0e21e
LW
2729=item msgctl ID,CMD,ARG
2730
f86cebdf 2731Calls the System V IPC function msgctl(2). You'll probably have to say
0ade1984
JH
2732
2733 use IPC::SysV;
2734
7660c0ab
A
2735first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2736then ARG must be a variable which will hold the returned C<msqid_ds>
951ba7fe
GS
2737structure. Returns like C<ioctl>: the undefined value for error,
2738C<"0 but true"> for zero, or the actual return value otherwise. See also
4755096e 2739L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation.
a0d0e21e
LW
2740
2741=item msgget KEY,FLAGS
2742
f86cebdf 2743Calls the System V IPC function msgget(2). Returns the message queue
4755096e
GS
2744id, or the undefined value if there is an error. See also
2745L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation.
a0d0e21e 2746
a0d0e21e
LW
2747=item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2748
2749Calls the System V IPC function msgrcv to receive a message from
2750message queue ID into variable VAR with a maximum message size of
41d6edb2
JH
2751SIZE. Note that when a message is received, the message type as a
2752native long integer will be the first thing in VAR, followed by the
2753actual message. This packing may be opened with C<unpack("l! a*")>.
2754Taints the variable. Returns true if successful, or false if there is
4755096e
GS
2755an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and
2756C<IPC::SysV::Msg> documentation.
41d6edb2
JH
2757
2758=item msgsnd ID,MSG,FLAGS
2759
2760Calls the System V IPC function msgsnd to send the message MSG to the
2761message queue ID. MSG must begin with the native long integer message
2762type, and be followed by the length of the actual message, and finally
2763the message itself. This kind of packing can be achieved with
2764C<pack("l! a*", $type, $message)>. Returns true if successful,
2765or false if there is an error. See also C<IPC::SysV>
2766and C<IPC::SysV::Msg> documentation.
a0d0e21e
LW
2767
2768=item my EXPR
2769
307ea6df
JH
2770=item my TYPE EXPR
2771
1d2de774 2772=item my EXPR : ATTRS
09bef843 2773
1d2de774 2774=item my TYPE EXPR : ATTRS
307ea6df 2775
19799a22 2776A C<my> declares the listed variables to be local (lexically) to the
1d2de774
JH
2777enclosing block, file, or C<eval>. If more than one value is listed,
2778the list must be placed in parentheses.
307ea6df 2779
1d2de774
JH
2780The exact semantics and interface of TYPE and ATTRS are still
2781evolving. TYPE is currently bound to the use of C<fields> pragma,
307ea6df
JH
2782and attributes are handled using the C<attributes> pragma, or starting
2783from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
2784L<perlsub/"Private Variables via my()"> for details, and L<fields>,
2785L<attributes>, and L<Attribute::Handlers>.
4633a7c4 2786
a0d0e21e
LW
2787=item next LABEL
2788
2789=item next
2790
2791The C<next> command is like the C<continue> statement in C; it starts
2792the next iteration of the loop:
2793
4633a7c4
LW
2794 LINE: while (<STDIN>) {
2795 next LINE if /^#/; # discard comments
5a964f20 2796 #...
a0d0e21e
LW
2797 }
2798
2799Note that if there were a C<continue> block on the above, it would get
2800executed even on discarded lines. If the LABEL is omitted, the command
2801refers to the innermost enclosing loop.
2802
4968c1e4 2803C<next> cannot be used to exit a block which returns a value such as
2b5ab1e7
TC
2804C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2805a grep() or map() operation.
4968c1e4 2806
6c1372ed
GS
2807Note that a block by itself is semantically identical to a loop
2808that executes once. Thus C<next> will exit such a block early.
2809
98293880
JH
2810See also L</continue> for an illustration of how C<last>, C<next>, and
2811C<redo> work.
1d2dff63 2812
4a66ea5a
RGS
2813=item no Module VERSION LIST
2814
2815=item no Module VERSION
2816
a0d0e21e
LW
2817=item no Module LIST
2818
4a66ea5a
RGS
2819=item no Module
2820
593b9c14 2821See the C<use> function, of which C<no> is the opposite.
a0d0e21e
LW
2822
2823=item oct EXPR
2824
54310121 2825=item oct
bbce6d69 2826
4633a7c4 2827Interprets EXPR as an octal string and returns the corresponding
4f19785b
WSI
2828value. (If EXPR happens to start off with C<0x>, interprets it as a
2829hex string. If EXPR starts off with C<0b>, it is interpreted as a
53305cf1
NC
2830binary string. Leading whitespace is ignored in all three cases.)
2831The following will handle decimal, binary, octal, and hex in the standard
2832Perl or C notation:
a0d0e21e
LW
2833
2834 $val = oct($val) if $val =~ /^0/;
2835
19799a22
GS
2836If EXPR is omitted, uses C<$_>. To go the other way (produce a number
2837in octal), use sprintf() or printf():
2838
2839 $perms = (stat("filename"))[2] & 07777;
2840 $oct_perms = sprintf "%lo", $perms;
2841
2842The oct() function is commonly used when a string such as C<644> needs
2843to be converted into a file mode, for example. (Although perl will
2844automatically convert strings into numbers as needed, this automatic
2845conversion assumes base 10.)
a0d0e21e
LW
2846
2847=item open FILEHANDLE,EXPR
2848
68bd7414
NIS
2849=item open FILEHANDLE,MODE,EXPR
2850
2851=item open FILEHANDLE,MODE,EXPR,LIST
2852
ba964c95
T
2853=item open FILEHANDLE,MODE,REFERENCE
2854
a0d0e21e
LW
2855=item open FILEHANDLE
2856
2857Opens the file whose filename is given by EXPR, and associates it with
ed53a2bb
JH
2858FILEHANDLE.
2859
2860(The following is a comprehensive reference to open(): for a gentler
2861introduction you may consider L<perlopentut>.)
2862
a28cd5c9
NT
2863If FILEHANDLE is an undefined scalar variable (or array or hash element)
2864the variable is assigned a reference to a new anonymous filehandle,
2865otherwise if FILEHANDLE is an expression, its value is used as the name of
2866the real filehandle wanted. (This is considered a symbolic reference, so
2867C<use strict 'refs'> should I<not> be in effect.)
ed53a2bb
JH
2868
2869If EXPR is omitted, the scalar variable of the same name as the
2870FILEHANDLE contains the filename. (Note that lexical variables--those
2871declared with C<my>--will not work for this purpose; so if you're
67408cae 2872using C<my>, specify EXPR in your call to open.)
ed53a2bb
JH
2873
2874If three or more arguments are specified then the mode of opening and
2875the file name are separate. If MODE is C<< '<' >> or nothing, the file
2876is opened for input. If MODE is C<< '>' >>, the file is truncated and
2877opened for output, being created if necessary. If MODE is C<<< '>>' >>>,
b76cc8ba 2878the file is opened for appending, again being created if necessary.
5a964f20 2879
ed53a2bb
JH
2880You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to
2881indicate that you want both read and write access to the file; thus
2882C<< '+<' >> is almost always preferred for read/write updates--the C<<
2883'+>' >> mode would clobber the file first. You can't usually use
2884either read-write mode for updating textfiles, since they have
2885variable length records. See the B<-i> switch in L<perlrun> for a
2886better approach. The file is created with permissions of C<0666>
2887modified by the process' C<umask> value.
2888
2889These various prefixes correspond to the fopen(3) modes of C<'r'>,
2890C<'r+'>, C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
5f05dabc 2891
6170680b
IZ
2892In the 2-arguments (and 1-argument) form of the call the mode and
2893filename should be concatenated (in this order), possibly separated by
68bd7414
NIS
2894spaces. It is possible to omit the mode in these forms if the mode is
2895C<< '<' >>.
6170680b 2896
7660c0ab 2897If the filename begins with C<'|'>, the filename is interpreted as a
5a964f20 2898command to which output is to be piped, and if the filename ends with a
f244e06d
GS
2899C<'|'>, the filename is interpreted as a command which pipes output to
2900us. See L<perlipc/"Using open() for IPC">
19799a22 2901for more examples of this. (You are not allowed to C<open> to a command
5a964f20 2902that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
4a4eefd0
GS
2903and L<perlipc/"Bidirectional Communication with Another Process">
2904for alternatives.)
cb1a09d0 2905
ed53a2bb
JH
2906For three or more arguments if MODE is C<'|-'>, the filename is
2907interpreted as a command to which output is to be piped, and if MODE
2908is C<'-|'>, the filename is interpreted as a command which pipes
2909output to us. In the 2-arguments (and 1-argument) form one should
2910replace dash (C<'-'>) with the command.
2911See L<perlipc/"Using open() for IPC"> for more examples of this.
2912(You are not allowed to C<open> to a command that pipes both in I<and>
2913out, but see L<IPC::Open2>, L<IPC::Open3>, and
2914L<perlipc/"Bidirectional Communication"> for alternatives.)
2915
2916In the three-or-more argument form of pipe opens, if LIST is specified
2917(extra arguments after the command name) then LIST becomes arguments
2918to the command invoked if the platform supports it. The meaning of
2919C<open> with more than three arguments for non-pipe modes is not yet
2920specified. Experimental "layers" may give extra LIST arguments
2921meaning.
6170680b
IZ
2922
2923In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
b76cc8ba 2924and opening C<< '>-' >> opens STDOUT.
6170680b 2925
fae2c0fb
RGS
2926You may use the three-argument form of open to specify IO "layers"
2927(sometimes also referred to as "disciplines") to be applied to the handle
2928that affect how the input and output are processed (see L<open> and
2929L<PerlIO> for more details). For example
7207e29d 2930
9124316e
JH
2931 open(FH, "<:utf8", "file")
2932
2933will open the UTF-8 encoded file containing Unicode characters,
fae2c0fb
RGS
2934see L<perluniintro>. (Note that if layers are specified in the
2935three-arg form then default layers set by the C<open> pragma are
01e6739c 2936ignored.)
ed53a2bb
JH
2937
2938Open returns nonzero upon success, the undefined value otherwise. If
2939the C<open> involved a pipe, the return value happens to be the pid of
2940the subprocess.
cb1a09d0 2941
ed53a2bb
JH
2942If you're running Perl on a system that distinguishes between text
2943files and binary files, then you should check out L</binmode> for tips
2944for dealing with this. The key distinction between systems that need
2945C<binmode> and those that don't is their text file formats. Systems
8939ba94 2946like Unix, Mac OS, and Plan 9, which delimit lines with a single
ed53a2bb
JH
2947character, and which encode that character in C as C<"\n">, do not
2948need C<binmode>. The rest need it.
cb1a09d0 2949
fb73857a 2950When opening a file, it's usually a bad idea to continue normal execution
19799a22
GS
2951if the request failed, so C<open> is frequently used in connection with
2952C<die>. Even if C<die> won't do what you want (say, in a CGI script,
fb73857a 2953where you want to make a nicely formatted error message (but there are
5a964f20 2954modules that can help with that problem)) you should always check
19799a22 2955the return value from opening a file. The infrequent exception is when
fb73857a
PP
2956working with an unopened filehandle is actually what you want to do.
2957
ed53a2bb
JH
2958As a special case the 3 arg form with a read/write mode and the third
2959argument being C<undef>:
b76cc8ba
NIS
2960
2961 open(TMP, "+>", undef) or die ...
2962
f253e835
JH
2963opens a filehandle to an anonymous temporary file. Also using "+<"
2964works for symmetry, but you really should consider writing something
2965to the temporary file first. You will need to seek() to do the
2966reading.
b76cc8ba 2967
2ce64696
JC
2968Since v5.8.0, perl has built using PerlIO by default. Unless you've
2969changed this (ie Configure -Uuseperlio), you can open file handles to
2970"in memory" files held in Perl scalars via:
ba964c95 2971
b996200f
SB
2972 open($fh, '>', \$variable) || ..
2973
2974Though if you try to re-open C<STDOUT> or C<STDERR> as an "in memory"
2975file, you have to close it first:
2976
2977 close STDOUT;
2978 open STDOUT, '>', \$variable or die "Can't open STDOUT: $!";
ba964c95 2979
cb1a09d0 2980Examples:
a0d0e21e
LW
2981
2982 $ARTICLE = 100;
2983 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2984 while (<ARTICLE>) {...
2985
6170680b 2986 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
fb73857a 2987 # if the open fails, output is discarded
a0d0e21e 2988
6170680b 2989 open(DBASE, '+<', 'dbase.mine') # open for update
fb73857a 2990 or die "Can't open 'dbase.mine' for update: $!";
cb1a09d0 2991
6170680b
IZ
2992 open(DBASE, '+<dbase.mine') # ditto
2993 or die "Can't open 'dbase.mine' for update: $!";
2994
2995 open(ARTICLE, '-|', "caesar <$article") # decrypt article
fb73857a 2996 or die "Can't start caesar: $!";
a0d0e21e 2997
6170680b
IZ
2998 open(ARTICLE, "caesar <$article |") # ditto
2999 or die "Can't start caesar: $!";
3000
2359510d 3001 open(EXTRACT, "|sort >Tmp$$") # $$ is our process id
fb73857a 3002 or die "Can't start sort: $!";
a0d0e21e 3003
ba964c95
T
3004 # in memory files
3005 open(MEMORY,'>', \$var)
3006 or die "Can't open memory file: $!";
3007 print MEMORY "foo!\n"; # output will end up in $var
3008
a0d0e21e
LW
3009 # process argument list of files along with any includes
3010
3011 foreach $file (@ARGV) {
3012 process($file, 'fh00');
3013 }
3014
3015 sub process {
5a964f20 3016 my($filename, $input) = @_;
a0d0e21e
LW
3017 $input++; # this is a string increment
3018 unless (open($input, $filename)) {
3019 print STDERR "Can't open $filename: $!\n";
3020 return;
3021 }
3022
5a964f20 3023 local $_;
a0d0e21e
LW
3024 while (<$input>) { # note use of indirection
3025 if (/^#include "(.*)"/) {
3026 process($1, $input);
3027 next;
3028 }
5a964f20 3029 #... # whatever
a0d0e21e
LW
3030 }
3031 }
3032
ae4c5402 3033See L<perliol> for detailed info on PerlIO.
2ce64696 3034
a0d0e21e 3035You may also, in the Bourne shell tradition, specify an EXPR beginning
00cafafa
JH
3036with C<< '>&' >>, in which case the rest of the string is interpreted
3037as the name of a filehandle (or file descriptor, if numeric) to be
3038duped (as L<dup(2)>) and opened. You may use C<&> after C<< > >>,
3039C<<< >> >>>, C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>.
3040The mode you specify should match the mode of the original filehandle.
3041(Duping a filehandle does not take into account any existing contents
3042of IO buffers.) If you use the 3 arg form then you can pass either a
3043number, the name of a filehandle or the normal "reference to a glob".
6170680b 3044
eae1b76b
SB
3045Here is a script that saves, redirects, and restores C<STDOUT> and
3046C<STDERR> using various methods:
a0d0e21e
LW
3047
3048 #!/usr/bin/perl
eae1b76b
SB
3049 open my $oldout, ">&STDOUT" or die "Can't dup STDOUT: $!";
3050 open OLDERR, ">&", \*STDERR or die "Can't dup STDERR: $!";
818c4caa 3051
eae1b76b
SB
3052 open STDOUT, '>', "foo.out" or die "Can't redirect STDOUT: $!";
3053 open STDERR, ">&STDOUT" or die "Can't dup STDOUT: $!";
a0d0e21e 3054
eae1b76b
SB
3055 select STDERR; $| = 1; # make unbuffered
3056 select STDOUT; $| = 1; # make unbuffered
a0d0e21e
LW
3057
3058 print STDOUT "stdout 1\n"; # this works for
3059 print STDERR "stderr 1\n"; # subprocesses too
3060
eae1b76b
SB
3061 open STDOUT, ">&", $oldout or die "Can't dup \$oldout: $!";
3062 open STDERR, ">&OLDERR" or die "Can't dup OLDERR: $!";
a0d0e21e
LW
3063
3064 print STDOUT "stdout 2\n";
3065 print STDERR "stderr 2\n";
3066
ef8b303f
JH
3067If you specify C<< '<&=X' >>, where C<X> is a file descriptor number
3068or a filehandle, then Perl will do an equivalent of C's C<fdopen> of
3069that file descriptor (and not call L<dup(2)>); this is more
3070parsimonious of file descriptors. For example:
a0d0e21e 3071
00cafafa 3072 # open for input, reusing the fileno of $fd
a0d0e21e 3073 open(FILEHANDLE, "<&=$fd")
df632fdf 3074
b76cc8ba 3075or
df632fdf 3076
b76cc8ba 3077 open(FILEHANDLE, "<&=", $fd)
a0d0e21e 3078
00cafafa
JH
3079or
3080
3081 # open for append, using the fileno of OLDFH
3082 open(FH, ">>&=", OLDFH)
3083
3084or
3085
3086 open(FH, ">>&=OLDFH")
3087
ef8b303f
JH
3088Being parsimonious on filehandles is also useful (besides being
3089parsimonious) for example when something is dependent on file
3090descriptors, like for example locking using flock(). If you do just
3091C<< open(A, '>>&B') >>, the filehandle A will not have the same file
3092descriptor as B, and therefore flock(A) will not flock(B), and vice
3093versa. But with C<< open(A, '>>&=B') >> the filehandles will share
3094the same file descriptor.
3095
3096Note that if you are using Perls older than 5.8.0, Perl will be using
3097the standard C libraries' fdopen() to implement the "=" functionality.
3098On many UNIX systems fdopen() fails when file descriptors exceed a
3099certain value, typically 255. For Perls 5.8.0 and later, PerlIO is
3100most often the default.
4af147f6 3101
df632fdf
JH
3102You can see whether Perl has been compiled with PerlIO or not by
3103running C<perl -V> and looking for C<useperlio=> line. If C<useperlio>
3104is C<define>, you have PerlIO, otherwise you don't.
3105
6170680b
IZ
3106If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
3107with 2-arguments (or 1-argument) form of open(), then
a0d0e21e 3108there is an implicit fork done, and the return value of open is the pid
7660c0ab 3109of the child within the parent process, and C<0> within the child
184e9718 3110process. (Use C<defined($pid)> to determine whether the open was successful.)
a0d0e21e
LW
3111The filehandle behaves normally for the parent, but i/o to that
3112filehandle is piped from/to the STDOUT/STDIN of the child process.
3113In the child process the filehandle isn't opened--i/o happens from/to
3114the new STDOUT or STDIN. Typically this is used like the normal
3115piped open when you want to exercise more control over just how the
3116pipe command gets executed, such as when you are running setuid, and
54310121 3117don't want to have to scan shell commands for metacharacters.
6170680b 3118The following triples are more or less equivalent:
a0d0e21e
LW
3119
3120 open(FOO, "|tr '[a-z]' '[A-Z]'");
6170680b
IZ
3121 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
3122 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
b76cc8ba 3123 open(FOO, '|-', "tr", '[a-z]', '[A-Z]');
a0d0e21e
LW
3124
3125 open(FOO, "cat -n '$file'|");
6170680b
IZ
3126 open(FOO, '-|', "cat -n '$file'");
3127 open(FOO, '-|') || exec 'cat', '-n', $file;
b76cc8ba
NIS
3128 open(FOO, '-|', "cat", '-n', $file);
3129
3130The last example in each block shows the pipe as "list form", which is
64da03b2
JH
3131not yet supported on all platforms. A good rule of thumb is that if
3132your platform has true C<fork()> (in other words, if your platform is
3133UNIX) you can use the list form.
a0d0e21e 3134
4633a7c4
LW
3135See L<perlipc/"Safe Pipe Opens"> for more examples of this.
3136
0f897271
GS
3137Beginning with v5.6.0, Perl will attempt to flush all files opened for
3138output before any operation that may do a fork, but this may not be
3139supported on some platforms (see L<perlport>). To be safe, you may need
3140to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
3141of C<IO::Handle> on any open handles.
3142
ed53a2bb
JH
3143On systems that support a close-on-exec flag on files, the flag will
3144be set for the newly opened file descriptor as determined by the value
3145of $^F. See L<perlvar/$^F>.
a0d0e21e 3146
0dccf244 3147Closing any piped filehandle causes the parent process to wait for the
e5218da5
GA
3148child to finish, and returns the status value in C<$?> and
3149C<${^CHILD_ERROR_NATIVE}>.
0dccf244 3150
ed53a2bb
JH
3151The filename passed to 2-argument (or 1-argument) form of open() will
3152have leading and trailing whitespace deleted, and the normal
3153redirection characters honored. This property, known as "magic open",
5a964f20 3154can often be used to good effect. A user could specify a filename of
7660c0ab 3155F<"rsh cat file |">, or you could change certain filenames as needed:
5a964f20
TC
3156
3157 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
3158 open(FH, $filename) or die "Can't open $filename: $!";
3159
6170680b
IZ
3160Use 3-argument form to open a file with arbitrary weird characters in it,
3161
3162 open(FOO, '<', $file);
3163
3164otherwise it's necessary to protect any leading and trailing whitespace:
5a964f20
TC
3165
3166 $file =~ s#^(\s)#./$1#;
3167 open(FOO, "< $file\0");
3168
a31a806a 3169(this may not work on some bizarre filesystems). One should
106325ad 3170conscientiously choose between the I<magic> and 3-arguments form
6170680b
IZ
3171of open():
3172
3173 open IN, $ARGV[0];
3174
3175will allow the user to specify an argument of the form C<"rsh cat file |">,
3176but will not work on a filename which happens to have a trailing space, while
3177
3178 open IN, '<', $ARGV[0];
3179
3180will have exactly the opposite restrictions.
3181
19799a22 3182If you want a "real" C C<open> (see L<open(2)> on your system), then you
6170680b
IZ
3183should use the C<sysopen> function, which involves no such magic (but
3184may use subtly different filemodes than Perl open(), which is mapped
3185to C fopen()). This is
5a964f20
TC
3186another way to protect your filenames from interpretation. For example:
3187
3188 use IO::Handle;
3189 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
3190 or die "sysopen $path: $!";
3191 $oldfh = select(HANDLE); $| = 1; select($oldfh);
38762f02 3192 print HANDLE "stuff $$\n";
5a964f20
TC
3193 seek(HANDLE, 0, 0);
3194 print "File contains: ", <HANDLE>;
3195
7660c0ab
A
3196Using the constructor from the C<IO::Handle> package (or one of its
3197subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
5a964f20
TC
3198filehandles that have the scope of whatever variables hold references to
3199them, and automatically close whenever and however you leave that scope:
c07a80fd 3200
5f05dabc 3201 use IO::File;
5a964f20 3202 #...
c07a80fd
PP
3203 sub read_myfile_munged {
3204 my $ALL = shift;
5f05dabc 3205 my $handle = new IO::File;
c07a80fd
PP
3206 open($handle, "myfile") or die "myfile: $!";
3207 $first = <$handle>
3208 or return (); # Automatically closed here.
3209 mung $first or die "mung failed"; # Or here.
3210 return $first, <$handle> if $ALL; # Or here.
3211 $first; # Or here.
3212 }
3213
b687b08b 3214See L</seek> for some details about mixing reading and writing.
a0d0e21e
LW
3215
3216=item opendir DIRHANDLE,EXPR
3217
19799a22
GS
3218Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
3219C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
a28cd5c9
NT
3220DIRHANDLE may be an expression whose value can be used as an indirect
3221dirhandle, usually the real dirhandle name. If DIRHANDLE is an undefined
3222scalar variable (or array or hash element), the variable is assigned a
3223reference to a new anonymous dirhandle.
a0d0e21e
LW
3224DIRHANDLEs have their own namespace separate from FILEHANDLEs.
3225
3226=item ord EXPR
3227
54310121 3228=item ord
bbce6d69 3229
121910a4
JH
3230Returns the numeric (the native 8-bit encoding, like ASCII or EBCDIC,
3231or Unicode) value of the first character of EXPR. If EXPR is omitted,
3232uses C<$_>.
3233
3234For the reverse, see L</chr>.
3235See L<perlunicode> and L<encoding> for more about Unicode.
a0d0e21e 3236
77ca0c92
LW
3237=item our EXPR
3238
307ea6df
JH
3239=item our EXPR TYPE
3240
1d2de774 3241=item our EXPR : ATTRS
9969eac4 3242
1d2de774 3243=item our TYPE EXPR : ATTRS
307ea6df 3244
85d8b7d5
MS
3245C<our> associates a simple name with a package variable in the current
3246package for the remander of the lexical scope. The listed variables
3247are declared to be valid globals within the enclosing block, file, or
3248C<eval>. That is, it has the same scoping rules as a "my"
3249declaration, but does not create a local variable. When C<use strict
3250'vars'> is in effect, the C<our> declaration lets you use the declared
3251global variable without qualifying it with a package name. (But only
3252within the lexical scope of the C<our> declaration. In this it
3253differs from "use vars", which is package scoped.)