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1=head1 NAME
2
3perlfunc - Perl builtin functions
4
5=head1 DESCRIPTION
6
7The functions in this section can serve as terms in an expression.
8They fall into two major categories: list operators and named unary
9operators. These differ in their precedence relationship with a
10following comma. (See the precedence table in L<perlop>.) List
11operators take more than one argument, while unary operators can never
12take more than one argument. Thus, a comma terminates the argument of
13a unary operator, but merely separates the arguments of a list
14operator. A unary operator generally provides a scalar context to its
2b5ab1e7 15argument, while a list operator may provide either scalar or list
a0d0e21e 16contexts for its arguments. If it does both, the scalar arguments will
5f05dabc 17be first, and the list argument will follow. (Note that there can ever
0f31cffe 18be only one such list argument.) For instance, splice() has three scalar
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19arguments followed by a list, whereas gethostbyname() has four scalar
20arguments.
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21
22In the syntax descriptions that follow, list operators that expect a
23list (and provide list context for the elements of the list) are shown
24with LIST as an argument. Such a list may consist of any combination
25of scalar arguments or list values; the list values will be included
26in the list as if each individual element were interpolated at that
27point in the list, forming a longer single-dimensional list value.
28Elements of the LIST should be separated by commas.
29
30Any function in the list below may be used either with or without
31parentheses around its arguments. (The syntax descriptions omit the
5f05dabc 32parentheses.) If you use the parentheses, the simple (but occasionally
19799a22 33surprising) rule is this: It I<looks> like a function, therefore it I<is> a
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34function, and precedence doesn't matter. Otherwise it's a list
35operator or unary operator, and precedence does matter. And whitespace
36between the function and left parenthesis doesn't count--so you need to
37be careful sometimes:
38
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39 print 1+2+4; # Prints 7.
40 print(1+2) + 4; # Prints 3.
41 print (1+2)+4; # Also prints 3!
42 print +(1+2)+4; # Prints 7.
43 print ((1+2)+4); # Prints 7.
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44
45If you run Perl with the B<-w> switch it can warn you about this. For
46example, the third line above produces:
47
48 print (...) interpreted as function at - line 1.
49 Useless use of integer addition in void context at - line 1.
50
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51A few functions take no arguments at all, and therefore work as neither
52unary nor list operators. These include such functions as C<time>
53and C<endpwent>. For example, C<time+86_400> always means
54C<time() + 86_400>.
55
a0d0e21e 56For functions that can be used in either a scalar or list context,
54310121 57nonabortive failure is generally indicated in a scalar context by
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58returning the undefined value, and in a list context by returning the
59null list.
60
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61Remember the following important rule: There is B<no rule> that relates
62the behavior of an expression in list context to its behavior in scalar
63context, or vice versa. It might do two totally different things.
a0d0e21e 64Each operator and function decides which sort of value it would be most
2b5ab1e7 65appropriate to return in scalar context. Some operators return the
5a964f20 66length of the list that would have been returned in list context. Some
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67operators return the first value in the list. Some operators return the
68last value in the list. Some operators return a count of successful
69operations. In general, they do what you want, unless you want
70consistency.
71
d1be9408 72A named array in scalar context is quite different from what would at
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73first glance appear to be a list in scalar context. You can't get a list
74like C<(1,2,3)> into being in scalar context, because the compiler knows
75the context at compile time. It would generate the scalar comma operator
76there, not the list construction version of the comma. That means it
77was never a list to start with.
78
79In general, functions in Perl that serve as wrappers for system calls
f86cebdf 80of the same name (like chown(2), fork(2), closedir(2), etc.) all return
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81true when they succeed and C<undef> otherwise, as is usually mentioned
82in the descriptions below. This is different from the C interfaces,
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83which return C<-1> on failure. Exceptions to this rule are C<wait>,
84C<waitpid>, and C<syscall>. System calls also set the special C<$!>
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85variable on failure. Other functions do not, except accidentally.
86
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87=head2 Perl Functions by Category
88
89Here are Perl's functions (including things that look like
5a964f20 90functions, like some keywords and named operators)
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91arranged by category. Some functions appear in more
92than one place.
93
13a2d996 94=over 4
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95
96=item Functions for SCALARs or strings
97
22fae026 98C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>,
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99C<length>, C<oct>, C<ord>, C<pack>, C<q/STRING/>, C<qq/STRING/>, C<reverse>,
100C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///>
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101
102=item Regular expressions and pattern matching
103
ab4f32c2 104C<m//>, C<pos>, C<quotemeta>, C<s///>, C<split>, C<study>, C<qr//>
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105
106=item Numeric functions
107
22fae026
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108C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>,
109C<sin>, C<sqrt>, C<srand>
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110
111=item Functions for real @ARRAYs
112
22fae026 113C<pop>, C<push>, C<shift>, C<splice>, C<unshift>
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114
115=item Functions for list data
116
ab4f32c2 117C<grep>, C<join>, C<map>, C<qw/STRING/>, C<reverse>, C<sort>, C<unpack>
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118
119=item Functions for real %HASHes
120
22fae026 121C<delete>, C<each>, C<exists>, C<keys>, C<values>
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122
123=item Input and output functions
124
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125C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
126C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
127C<readdir>, C<rewinddir>, C<seek>, C<seekdir>, C<select>, C<syscall>,
128C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>,
129C<warn>, C<write>
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130
131=item Functions for fixed length data or records
132
22fae026 133C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec>
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134
135=item Functions for filehandles, files, or directories
136
22fae026 137C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>,
5ff3f7a4 138C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>,
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139C<readlink>, C<rename>, C<rmdir>, C<stat>, C<symlink>, C<sysopen>,
140C<umask>, C<unlink>, C<utime>
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141
142=item Keywords related to the control flow of your perl program
143
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144C<caller>, C<continue>, C<die>, C<do>, C<dump>, C<eval>, C<exit>,
145C<goto>, C<last>, C<next>, C<redo>, C<return>, C<sub>, C<wantarray>
cb1a09d0 146
54310121 147=item Keywords related to scoping
cb1a09d0 148
4375e838 149C<caller>, C<import>, C<local>, C<my>, C<our>, C<package>, C<use>
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150
151=item Miscellaneous functions
152
4375e838 153C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<our>, C<reset>,
22fae026 154C<scalar>, C<undef>, C<wantarray>
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155
156=item Functions for processes and process groups
157
22fae026 158C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
ab4f32c2 159C<pipe>, C<qx/STRING/>, C<setpgrp>, C<setpriority>, C<sleep>, C<system>,
22fae026 160C<times>, C<wait>, C<waitpid>
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161
162=item Keywords related to perl modules
163
22fae026 164C<do>, C<import>, C<no>, C<package>, C<require>, C<use>
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165
166=item Keywords related to classes and object-orientedness
167
22fae026
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168C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
169C<untie>, C<use>
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170
171=item Low-level socket functions
172
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173C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
174C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
737dd4b4 175C<socket>, C<socketpair>
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176
177=item System V interprocess communication functions
178
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179C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
180C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>
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181
182=item Fetching user and group info
183
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184C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
185C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
186C<getpwuid>, C<setgrent>, C<setpwent>
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187
188=item Fetching network info
189
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190C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
191C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
192C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
193C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
194C<setnetent>, C<setprotoent>, C<setservent>
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195
196=item Time-related functions
197
22fae026 198C<gmtime>, C<localtime>, C<time>, C<times>
cb1a09d0 199
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200=item Functions new in perl5
201
22fae026 202C<abs>, C<bless>, C<chomp>, C<chr>, C<exists>, C<formline>, C<glob>,
b76cc8ba 203C<import>, C<lc>, C<lcfirst>, C<map>, C<my>, C<no>, C<our>, C<prototype>,
4375e838 204C<qx>, C<qw>, C<readline>, C<readpipe>, C<ref>, C<sub*>, C<sysopen>, C<tie>,
22fae026 205C<tied>, C<uc>, C<ucfirst>, C<untie>, C<use>
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206
207* - C<sub> was a keyword in perl4, but in perl5 it is an
5a964f20 208operator, which can be used in expressions.
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209
210=item Functions obsoleted in perl5
211
22fae026 212C<dbmclose>, C<dbmopen>
37798a01 213
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214=back
215
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216=head2 Portability
217
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218Perl was born in Unix and can therefore access all common Unix
219system calls. In non-Unix environments, the functionality of some
220Unix system calls may not be available, or details of the available
221functionality may differ slightly. The Perl functions affected
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222by this are:
223
224C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>,
225C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>,
226C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>,
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227C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostbyname>,
228C<gethostent>, C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
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229C<getppid>, C<getprgp>, C<getpriority>, C<getprotobynumber>,
230C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>,
231C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>,
232C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>,
2b5ab1e7 233C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<rename>, C<select>, C<semctl>,
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234C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>,
235C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>,
236C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>,
737dd4b4 237C<shmwrite>, C<socket>, C<socketpair>,
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238C<stat>, C<symlink>, C<syscall>, C<sysopen>, C<system>,
239C<times>, C<truncate>, C<umask>, C<unlink>,
2b5ab1e7 240C<utime>, C<wait>, C<waitpid>
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241
242For more information about the portability of these functions, see
243L<perlport> and other available platform-specific documentation.
244
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245=head2 Alphabetical Listing of Perl Functions
246
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247=over 8
248
5b3c99c0 249=item -X FILEHANDLE
a0d0e21e 250
5b3c99c0 251=item -X EXPR
a0d0e21e 252
5b3c99c0 253=item -X
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254
255A file test, where X is one of the letters listed below. This unary
256operator takes one argument, either a filename or a filehandle, and
257tests the associated file to see if something is true about it. If the
7660c0ab 258argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
19799a22 259Unless otherwise documented, it returns C<1> for true and C<''> for false, or
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260the undefined value if the file doesn't exist. Despite the funny
261names, precedence is the same as any other named unary operator, and
262the argument may be parenthesized like any other unary operator. The
263operator may be any of:
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264X<-r>X<-w>X<-x>X<-o>X<-R>X<-W>X<-X>X<-O>X<-e>X<-z>X<-s>X<-f>X<-d>X<-l>X<-p>
265X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
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266
267 -r File is readable by effective uid/gid.
268 -w File is writable by effective uid/gid.
269 -x File is executable by effective uid/gid.
270 -o File is owned by effective uid.
271
272 -R File is readable by real uid/gid.
273 -W File is writable by real uid/gid.
274 -X File is executable by real uid/gid.
275 -O File is owned by real uid.
276
277 -e File exists.
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278 -z File has zero size (is empty).
279 -s File has nonzero size (returns size in bytes).
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280
281 -f File is a plain file.
282 -d File is a directory.
283 -l File is a symbolic link.
9c4d0f16 284 -p File is a named pipe (FIFO), or Filehandle is a pipe.
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285 -S File is a socket.
286 -b File is a block special file.
287 -c File is a character special file.
288 -t Filehandle is opened to a tty.
289
290 -u File has setuid bit set.
291 -g File has setgid bit set.
292 -k File has sticky bit set.
293
121910a4 294 -T File is an ASCII text file (heuristic guess).
2cdbc966 295 -B File is a "binary" file (opposite of -T).
a0d0e21e 296
95a3fe12 297 -M Script start time minus file modification time, in days.
a0d0e21e 298 -A Same for access time.
95a3fe12 299 -C Same for inode change time (Unix, may differ for other platforms)
a0d0e21e 300
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301Example:
302
303 while (<>) {
5b3eff12 304 chomp;
a0d0e21e 305 next unless -f $_; # ignore specials
5a964f20 306 #...
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307 }
308
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309The interpretation of the file permission operators C<-r>, C<-R>,
310C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
311of the file and the uids and gids of the user. There may be other
312reasons you can't actually read, write, or execute the file. Such
313reasons may be for example network filesystem access controls, ACLs
314(access control lists), read-only filesystems, and unrecognized
315executable formats.
316
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317Also note that, for the superuser on the local filesystems, the C<-r>,
318C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
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319if any execute bit is set in the mode. Scripts run by the superuser
320may thus need to do a stat() to determine the actual mode of the file,
2b5ab1e7 321or temporarily set their effective uid to something else.
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322
323If you are using ACLs, there is a pragma called C<filetest> that may
324produce more accurate results than the bare stat() mode bits.
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325When under the C<use filetest 'access'> the above-mentioned filetests
326will test whether the permission can (not) be granted using the
468541a8 327access() family of system calls. Also note that the C<-x> and C<-X> may
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328under this pragma return true even if there are no execute permission
329bits set (nor any extra execute permission ACLs). This strangeness is
330due to the underlying system calls' definitions. Read the
331documentation for the C<filetest> pragma for more information.
332
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333Note that C<-s/a/b/> does not do a negated substitution. Saying
334C<-exp($foo)> still works as expected, however--only single letters
335following a minus are interpreted as file tests.
336
337The C<-T> and C<-B> switches work as follows. The first block or so of the
338file is examined for odd characters such as strange control codes or
61eff3bc 339characters with the high bit set. If too many strange characters (>30%)
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340are found, it's a C<-B> file, otherwise it's a C<-T> file. Also, any file
341containing null in the first block is considered a binary file. If C<-T>
9124316e 342or C<-B> is used on a filehandle, the current IO buffer is examined
19799a22 343rather than the first block. Both C<-T> and C<-B> return true on a null
54310121 344file, or a file at EOF when testing a filehandle. Because you have to
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345read a file to do the C<-T> test, on most occasions you want to use a C<-f>
346against the file first, as in C<next unless -f $file && -T $file>.
a0d0e21e 347
19799a22 348If any of the file tests (or either the C<stat> or C<lstat> operators) are given
28757baa 349the special filehandle consisting of a solitary underline, then the stat
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350structure of the previous file test (or stat operator) is used, saving
351a system call. (This doesn't work with C<-t>, and you need to remember
352that lstat() and C<-l> will leave values in the stat structure for the
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353symbolic link, not the real file.) (Also, if the stat buffer was filled by
354a C<lstat> call, C<-T> and C<-B> will reset it with the results of C<stat _>).
355Example:
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356
357 print "Can do.\n" if -r $a || -w _ || -x _;
358
359 stat($filename);
360 print "Readable\n" if -r _;
361 print "Writable\n" if -w _;
362 print "Executable\n" if -x _;
363 print "Setuid\n" if -u _;
364 print "Setgid\n" if -g _;
365 print "Sticky\n" if -k _;
366 print "Text\n" if -T _;
367 print "Binary\n" if -B _;
368
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369As of Perl 5.9.1, as a form of purely syntactic sugar, you can stack file
370test operators, in a way that C<-f -w -x $file> is equivalent to
371C<-x $file && -w _ && -f _>. (This is only syntax fancy : if you use
372the return value of C<-f $file> as an argument to another filetest
373operator, no special magic will happen.)
374
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375=item abs VALUE
376
54310121 377=item abs
bbce6d69 378
a0d0e21e 379Returns the absolute value of its argument.
7660c0ab 380If VALUE is omitted, uses C<$_>.
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381
382=item accept NEWSOCKET,GENERICSOCKET
383
f86cebdf 384Accepts an incoming socket connect, just as the accept(2) system call
19799a22 385does. Returns the packed address if it succeeded, false otherwise.
2b5ab1e7 386See the example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 387
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388On systems that support a close-on-exec flag on files, the flag will
389be set for the newly opened file descriptor, as determined by the
390value of $^F. See L<perlvar/$^F>.
391
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392=item alarm SECONDS
393
54310121 394=item alarm
bbce6d69 395
a0d0e21e 396Arranges to have a SIGALRM delivered to this process after the
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397specified number of wallclock seconds have elapsed. If SECONDS is not
398specified, the value stored in C<$_> is used. (On some machines,
399unfortunately, the elapsed time may be up to one second less or more
400than you specified because of how seconds are counted, and process
401scheduling may delay the delivery of the signal even further.)
402
403Only one timer may be counting at once. Each call disables the
404previous timer, and an argument of C<0> may be supplied to cancel the
405previous timer without starting a new one. The returned value is the
406amount of time remaining on the previous timer.
a0d0e21e 407
4633a7c4 408For delays of finer granularity than one second, you may use Perl's
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409four-argument version of select() leaving the first three arguments
410undefined, or you might be able to use the C<syscall> interface to
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411access setitimer(2) if your system supports it. The Time::HiRes
412module (from CPAN, and starting from Perl 5.8 part of the standard
413distribution) may also prove useful.
2b5ab1e7 414
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415It is usually a mistake to intermix C<alarm> and C<sleep> calls.
416(C<sleep> may be internally implemented in your system with C<alarm>)
a0d0e21e 417
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418If you want to use C<alarm> to time out a system call you need to use an
419C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
f86cebdf 420fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
19799a22 421restart system calls on some systems. Using C<eval>/C<die> always works,
5a964f20 422modulo the caveats given in L<perlipc/"Signals">.
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423
424 eval {
f86cebdf 425 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
36477c24 426 alarm $timeout;
ff68c719 427 $nread = sysread SOCKET, $buffer, $size;
36477c24 428 alarm 0;
ff68c719 429 };
ff68c719 430 if ($@) {
f86cebdf 431 die unless $@ eq "alarm\n"; # propagate unexpected errors
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432 # timed out
433 }
434 else {
435 # didn't
436 }
437
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438For more information see L<perlipc>.
439
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440=item atan2 Y,X
441
442Returns the arctangent of Y/X in the range -PI to PI.
443
ca6e1c26 444For the tangent operation, you may use the C<Math::Trig::tan>
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445function, or use the familiar relation:
446
447 sub tan { sin($_[0]) / cos($_[0]) }
448
a0d0e21e
LW
449=item bind SOCKET,NAME
450
451Binds a network address to a socket, just as the bind system call
19799a22 452does. Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
453packed address of the appropriate type for the socket. See the examples in
454L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 455
fae2c0fb 456=item binmode FILEHANDLE, LAYER
1c1fc3ea 457
a0d0e21e
LW
458=item binmode FILEHANDLE
459
1cbfc93d
NIS
460Arranges for FILEHANDLE to be read or written in "binary" or "text"
461mode on systems where the run-time libraries distinguish between
462binary and text files. If FILEHANDLE is an expression, the value is
463taken as the name of the filehandle. Returns true on success,
b5fe5ca2 464otherwise it returns C<undef> and sets C<$!> (errno).
1cbfc93d 465
d807c6f4
JH
466On some systems (in general, DOS and Windows-based systems) binmode()
467is necessary when you're not working with a text file. For the sake
468of portability it is a good idea to always use it when appropriate,
469and to never use it when it isn't appropriate. Also, people can
470set their I/O to be by default UTF-8 encoded Unicode, not bytes.
471
472In other words: regardless of platform, use binmode() on binary data,
473like for example images.
474
475If LAYER is present it is a single string, but may contain multiple
476directives. The directives alter the behaviour of the file handle.
477When LAYER is present using binmode on text file makes sense.
478
fae2c0fb 479If LAYER is omitted or specified as C<:raw> the filehandle is made
0226bbdb
NIS
480suitable for passing binary data. This includes turning off possible CRLF
481translation and marking it as bytes (as opposed to Unicode characters).
749683d2
YST
482Note that, despite what may be implied in I<"Programming Perl"> (the
483Camel) or elsewhere, C<:raw> is I<not> the simply inverse of C<:crlf>
fae2c0fb 484-- other layers which would affect binary nature of the stream are
0226bbdb
NIS
485I<also> disabled. See L<PerlIO>, L<perlrun> and the discussion about the
486PERLIO environment variable.
01e6739c 487
d807c6f4
JH
488The C<:bytes>, C<:crlf>, and C<:utf8>, and any other directives of the
489form C<:...>, are called I/O I<layers>. The C<open> pragma can be used to
490establish default I/O layers. See L<open>.
491
fae2c0fb
RGS
492I<The LAYER parameter of the binmode() function is described as "DISCIPLINE"
493in "Programming Perl, 3rd Edition". However, since the publishing of this
494book, by many known as "Camel III", the consensus of the naming of this
495functionality has moved from "discipline" to "layer". All documentation
496of this version of Perl therefore refers to "layers" rather than to
497"disciplines". Now back to the regularly scheduled documentation...>
498
01e6739c 499To mark FILEHANDLE as UTF-8, use C<:utf8>.
1cbfc93d 500
ed53a2bb 501In general, binmode() should be called after open() but before any I/O
01e6739c
NIS
502is done on the filehandle. Calling binmode() will normally flush any
503pending buffered output data (and perhaps pending input data) on the
fae2c0fb 504handle. An exception to this is the C<:encoding> layer that
01e6739c 505changes the default character encoding of the handle, see L<open>.
fae2c0fb 506The C<:encoding> layer sometimes needs to be called in
3874323d
JH
507mid-stream, and it doesn't flush the stream. The C<:encoding>
508also implicitly pushes on top of itself the C<:utf8> layer because
509internally Perl will operate on UTF-8 encoded Unicode characters.
16fe6d59 510
19799a22 511The operating system, device drivers, C libraries, and Perl run-time
30168b04
GS
512system all work together to let the programmer treat a single
513character (C<\n>) as the line terminator, irrespective of the external
514representation. On many operating systems, the native text file
515representation matches the internal representation, but on some
516platforms the external representation of C<\n> is made up of more than
517one character.
518
68bd7414
NIS
519Mac OS, all variants of Unix, and Stream_LF files on VMS use a single
520character to end each line in the external representation of text (even
5e12dbfa 521though that single character is CARRIAGE RETURN on Mac OS and LINE FEED
01e6739c
NIS
522on Unix and most VMS files). In other systems like OS/2, DOS and the
523various flavors of MS-Windows your program sees a C<\n> as a simple C<\cJ>,
524but what's stored in text files are the two characters C<\cM\cJ>. That
525means that, if you don't use binmode() on these systems, C<\cM\cJ>
526sequences on disk will be converted to C<\n> on input, and any C<\n> in
527your program will be converted back to C<\cM\cJ> on output. This is what
528you want for text files, but it can be disastrous for binary files.
30168b04
GS
529
530Another consequence of using binmode() (on some systems) is that
531special end-of-file markers will be seen as part of the data stream.
532For systems from the Microsoft family this means that if your binary
4375e838 533data contains C<\cZ>, the I/O subsystem will regard it as the end of
30168b04
GS
534the file, unless you use binmode().
535
536binmode() is not only important for readline() and print() operations,
537but also when using read(), seek(), sysread(), syswrite() and tell()
538(see L<perlport> for more details). See the C<$/> and C<$\> variables
539in L<perlvar> for how to manually set your input and output
540line-termination sequences.
a0d0e21e 541
4633a7c4 542=item bless REF,CLASSNAME
a0d0e21e
LW
543
544=item bless REF
545
2b5ab1e7
TC
546This function tells the thingy referenced by REF that it is now an object
547in the CLASSNAME package. If CLASSNAME is omitted, the current package
19799a22 548is used. Because a C<bless> is often the last thing in a constructor,
2b5ab1e7
TC
549it returns the reference for convenience. Always use the two-argument
550version if the function doing the blessing might be inherited by a
551derived class. See L<perltoot> and L<perlobj> for more about the blessing
552(and blessings) of objects.
a0d0e21e 553
57668c4d 554Consider always blessing objects in CLASSNAMEs that are mixed case.
2b5ab1e7
TC
555Namespaces with all lowercase names are considered reserved for
556Perl pragmata. Builtin types have all uppercase names, so to prevent
557confusion, you may wish to avoid such package names as well. Make sure
558that CLASSNAME is a true value.
60ad88b8
GS
559
560See L<perlmod/"Perl Modules">.
561
a0d0e21e
LW
562=item caller EXPR
563
564=item caller
565
5a964f20 566Returns the context of the current subroutine call. In scalar context,
28757baa 567returns the caller's package name if there is a caller, that is, if
19799a22 568we're in a subroutine or C<eval> or C<require>, and the undefined value
5a964f20 569otherwise. In list context, returns
a0d0e21e 570
748a9306 571 ($package, $filename, $line) = caller;
a0d0e21e
LW
572
573With EXPR, it returns some extra information that the debugger uses to
574print a stack trace. The value of EXPR indicates how many call frames
575to go back before the current one.
576
f3aa04c2 577 ($package, $filename, $line, $subroutine, $hasargs,
e476b1b5 578 $wantarray, $evaltext, $is_require, $hints, $bitmask) = caller($i);
e7ea3e70 579
951ba7fe 580Here $subroutine may be C<(eval)> if the frame is not a subroutine
19799a22 581call, but an C<eval>. In such a case additional elements $evaltext and
7660c0ab 582C<$is_require> are set: C<$is_require> is true if the frame is created by a
19799a22 583C<require> or C<use> statement, $evaltext contains the text of the
277ddfaf 584C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
951ba7fe 585$filename is C<(eval)>, but $evaltext is undefined. (Note also that
0fc9dec4
RGS
586each C<use> statement creates a C<require> frame inside an C<eval EXPR>
587frame.) $subroutine may also be C<(unknown)> if this particular
588subroutine happens to have been deleted from the symbol table.
589C<$hasargs> is true if a new instance of C<@_> was set up for the frame.
590C<$hints> and C<$bitmask> contain pragmatic hints that the caller was
591compiled with. The C<$hints> and C<$bitmask> values are subject to change
592between versions of Perl, and are not meant for external use.
748a9306
LW
593
594Furthermore, when called from within the DB package, caller returns more
7660c0ab 595detailed information: it sets the list variable C<@DB::args> to be the
54310121 596arguments with which the subroutine was invoked.
748a9306 597
7660c0ab 598Be aware that the optimizer might have optimized call frames away before
19799a22 599C<caller> had a chance to get the information. That means that C<caller(N)>
7660c0ab 600might not return information about the call frame you expect it do, for
b76cc8ba 601C<< N > 1 >>. In particular, C<@DB::args> might have information from the
19799a22 602previous time C<caller> was called.
7660c0ab 603
a0d0e21e
LW
604=item chdir EXPR
605
ffce7b87 606Changes the working directory to EXPR, if possible. If EXPR is omitted,
0bfc1ec4 607changes to the directory specified by C<$ENV{HOME}>, if set; if not,
ffce7b87 608changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
b4ad75f0
AMS
609variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
610neither is set, C<chdir> does nothing. It returns true upon success,
611false otherwise. See the example under C<die>.
a0d0e21e
LW
612
613=item chmod LIST
614
615Changes the permissions of a list of files. The first element of the
4633a7c4 616list must be the numerical mode, which should probably be an octal
4ad40acf 617number, and which definitely should I<not> be a string of octal digits:
2f9daede 618C<0644> is okay, C<'0644'> is not. Returns the number of files
dc848c6f 619successfully changed. See also L</oct>, if all you have is a string.
a0d0e21e
LW
620
621 $cnt = chmod 0755, 'foo', 'bar';
622 chmod 0755, @executables;
f86cebdf
GS
623 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
624 # --w----r-T
2f9daede
TPG
625 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
626 $mode = 0644; chmod $mode, 'foo'; # this is best
a0d0e21e 627
ca6e1c26
JH
628You can also import the symbolic C<S_I*> constants from the Fcntl
629module:
630
631 use Fcntl ':mode';
632
633 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
634 # This is identical to the chmod 0755 of the above example.
635
a0d0e21e
LW
636=item chomp VARIABLE
637
313c9f5c 638=item chomp( LIST )
a0d0e21e
LW
639
640=item chomp
641
2b5ab1e7
TC
642This safer version of L</chop> removes any trailing string
643that corresponds to the current value of C<$/> (also known as
28757baa
PP
644$INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
645number of characters removed from all its arguments. It's often used to
646remove the newline from the end of an input record when you're worried
2b5ab1e7
TC
647that the final record may be missing its newline. When in paragraph
648mode (C<$/ = "">), it removes all trailing newlines from the string.
4c5a6083
GS
649When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
650a reference to an integer or the like, see L<perlvar>) chomp() won't
b76cc8ba 651remove anything.
19799a22 652If VARIABLE is omitted, it chomps C<$_>. Example:
a0d0e21e
LW
653
654 while (<>) {
655 chomp; # avoid \n on last field
656 @array = split(/:/);
5a964f20 657 # ...
a0d0e21e
LW
658 }
659
4bf21a6d
RD
660If VARIABLE is a hash, it chomps the hash's values, but not its keys.
661
a0d0e21e
LW
662You can actually chomp anything that's an lvalue, including an assignment:
663
664 chomp($cwd = `pwd`);
665 chomp($answer = <STDIN>);
666
667If you chomp a list, each element is chomped, and the total number of
668characters removed is returned.
669
442a8c12
NC
670If the C<encoding> pragma is in scope then the lengths returned are
671calculated from the length of C<$/> in Unicode characters, which is not
672always the same as the length of C<$/> in the native encoding.
673
15e44fd8
RGS
674Note that parentheses are necessary when you're chomping anything
675that is not a simple variable. This is because C<chomp $cwd = `pwd`;>
676is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as
677C<chomp( $cwd = `pwd` )> which you might expect. Similarly,
678C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than
679as C<chomp($a, $b)>.
680
a0d0e21e
LW
681=item chop VARIABLE
682
313c9f5c 683=item chop( LIST )
a0d0e21e
LW
684
685=item chop
686
687Chops off the last character of a string and returns the character
5b3eff12 688chopped. It is much more efficient than C<s/.$//s> because it neither
7660c0ab 689scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
4bf21a6d
RD
690If VARIABLE is a hash, it chops the hash's values, but not its keys.
691
5b3eff12 692You can actually chop anything that's an lvalue, including an assignment.
a0d0e21e
LW
693
694If you chop a list, each element is chopped. Only the value of the
19799a22 695last C<chop> is returned.
a0d0e21e 696
19799a22 697Note that C<chop> returns the last character. To return all but the last
748a9306
LW
698character, use C<substr($string, 0, -1)>.
699
15e44fd8
RGS
700See also L</chomp>.
701
a0d0e21e
LW
702=item chown LIST
703
704Changes the owner (and group) of a list of files. The first two
19799a22
GS
705elements of the list must be the I<numeric> uid and gid, in that
706order. A value of -1 in either position is interpreted by most
707systems to leave that value unchanged. Returns the number of files
708successfully changed.
a0d0e21e
LW
709
710 $cnt = chown $uid, $gid, 'foo', 'bar';
711 chown $uid, $gid, @filenames;
712
54310121 713Here's an example that looks up nonnumeric uids in the passwd file:
a0d0e21e
LW
714
715 print "User: ";
19799a22 716 chomp($user = <STDIN>);
5a964f20 717 print "Files: ";
19799a22 718 chomp($pattern = <STDIN>);
a0d0e21e
LW
719
720 ($login,$pass,$uid,$gid) = getpwnam($user)
721 or die "$user not in passwd file";
722
5a964f20 723 @ary = glob($pattern); # expand filenames
a0d0e21e
LW
724 chown $uid, $gid, @ary;
725
54310121 726On most systems, you are not allowed to change the ownership of the
4633a7c4
LW
727file unless you're the superuser, although you should be able to change
728the group to any of your secondary groups. On insecure systems, these
729restrictions may be relaxed, but this is not a portable assumption.
19799a22
GS
730On POSIX systems, you can detect this condition this way:
731
732 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
733 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
4633a7c4 734
a0d0e21e
LW
735=item chr NUMBER
736
54310121 737=item chr
bbce6d69 738
a0d0e21e 739Returns the character represented by that NUMBER in the character set.
a0ed51b3 740For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
1e54db1a
JH
741chr(0x263a) is a Unicode smiley face. Note that characters from 128
742to 255 (inclusive) are by default not encoded in UTF-8 Unicode for
743backward compatibility reasons (but see L<encoding>).
aaa68c4a 744
974da8e5
JH
745If NUMBER is omitted, uses C<$_>.
746
b76cc8ba 747For the reverse, use L</ord>.
a0d0e21e 748
974da8e5
JH
749Note that under the C<bytes> pragma the NUMBER is masked to
750the low eight bits.
751
752See L<perlunicode> and L<encoding> for more about Unicode.
bbce6d69 753
a0d0e21e
LW
754=item chroot FILENAME
755
54310121 756=item chroot
bbce6d69 757
5a964f20 758This function works like the system call by the same name: it makes the
4633a7c4 759named directory the new root directory for all further pathnames that
951ba7fe 760begin with a C</> by your process and all its children. (It doesn't
28757baa 761change your current working directory, which is unaffected.) For security
4633a7c4 762reasons, this call is restricted to the superuser. If FILENAME is
19799a22 763omitted, does a C<chroot> to C<$_>.
a0d0e21e
LW
764
765=item close FILEHANDLE
766
6a518fbc
TP
767=item close
768
9124316e
JH
769Closes the file or pipe associated with the file handle, returning
770true only if IO buffers are successfully flushed and closes the system
771file descriptor. Closes the currently selected filehandle if the
772argument is omitted.
fb73857a
PP
773
774You don't have to close FILEHANDLE if you are immediately going to do
19799a22
GS
775another C<open> on it, because C<open> will close it for you. (See
776C<open>.) However, an explicit C<close> on an input file resets the line
777counter (C<$.>), while the implicit close done by C<open> does not.
fb73857a 778
dede8123
RGS
779If the file handle came from a piped open, C<close> will additionally
780return false if one of the other system calls involved fails, or if the
fb73857a 781program exits with non-zero status. (If the only problem was that the
dede8123 782program exited non-zero, C<$!> will be set to C<0>.) Closing a pipe
2b5ab1e7 783also waits for the process executing on the pipe to complete, in case you
b76cc8ba 784want to look at the output of the pipe afterwards, and
2b5ab1e7 785implicitly puts the exit status value of that command into C<$?>.
5a964f20 786
73689b13
GS
787Prematurely closing the read end of a pipe (i.e. before the process
788writing to it at the other end has closed it) will result in a
789SIGPIPE being delivered to the writer. If the other end can't
790handle that, be sure to read all the data before closing the pipe.
791
fb73857a 792Example:
a0d0e21e 793
fb73857a
PP
794 open(OUTPUT, '|sort >foo') # pipe to sort
795 or die "Can't start sort: $!";
5a964f20 796 #... # print stuff to output
fb73857a
PP
797 close OUTPUT # wait for sort to finish
798 or warn $! ? "Error closing sort pipe: $!"
799 : "Exit status $? from sort";
800 open(INPUT, 'foo') # get sort's results
801 or die "Can't open 'foo' for input: $!";
a0d0e21e 802
5a964f20
TC
803FILEHANDLE may be an expression whose value can be used as an indirect
804filehandle, usually the real filehandle name.
a0d0e21e
LW
805
806=item closedir DIRHANDLE
807
19799a22 808Closes a directory opened by C<opendir> and returns the success of that
5a964f20
TC
809system call.
810
a0d0e21e
LW
811=item connect SOCKET,NAME
812
813Attempts to connect to a remote socket, just as the connect system call
19799a22 814does. Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
815packed address of the appropriate type for the socket. See the examples in
816L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 817
cb1a09d0
AD
818=item continue BLOCK
819
820Actually a flow control statement rather than a function. If there is a
98293880
JH
821C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
822C<foreach>), it is always executed just before the conditional is about to
823be evaluated again, just like the third part of a C<for> loop in C. Thus
cb1a09d0
AD
824it can be used to increment a loop variable, even when the loop has been
825continued via the C<next> statement (which is similar to the C C<continue>
826statement).
827
98293880 828C<last>, C<next>, or C<redo> may appear within a C<continue>
19799a22
GS
829block. C<last> and C<redo> will behave as if they had been executed within
830the main block. So will C<next>, but since it will execute a C<continue>
1d2dff63
GS
831block, it may be more entertaining.
832
833 while (EXPR) {
834 ### redo always comes here
835 do_something;
836 } continue {
837 ### next always comes here
838 do_something_else;
839 # then back the top to re-check EXPR
840 }
841 ### last always comes here
842
843Omitting the C<continue> section is semantically equivalent to using an
19799a22 844empty one, logically enough. In that case, C<next> goes directly back
1d2dff63
GS
845to check the condition at the top of the loop.
846
a0d0e21e
LW
847=item cos EXPR
848
d6217f1e
GS
849=item cos
850
5a964f20 851Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
7660c0ab 852takes cosine of C<$_>.
a0d0e21e 853
ca6e1c26 854For the inverse cosine operation, you may use the C<Math::Trig::acos()>
28757baa
PP
855function, or use this relation:
856
857 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
858
a0d0e21e
LW
859=item crypt PLAINTEXT,SALT
860
f86cebdf 861Encrypts a string exactly like the crypt(3) function in the C library
4633a7c4
LW
862(assuming that you actually have a version there that has not been
863extirpated as a potential munition). This can prove useful for checking
864the password file for lousy passwords, amongst other things. Only the
865guys wearing white hats should do this.
a0d0e21e 866
a6d05634 867Note that L<crypt|/crypt> is intended to be a one-way function, much like
85c16d83
JH
868breaking eggs to make an omelette. There is no (known) corresponding
869decrypt function (in other words, the crypt() is a one-way hash
870function). As a result, this function isn't all that useful for
11155c91 871cryptography. (For that, see your nearby CPAN mirror.)
2f9daede 872
85c16d83
JH
873When verifying an existing encrypted string you should use the
874encrypted text as the salt (like C<crypt($plain, $crypted) eq
8e2ffcbe 875$crypted>). This allows your code to work with the standard L<crypt|/crypt>
85c16d83
JH
876and with more exotic implementations. In other words, do not assume
877anything about the returned string itself, or how many bytes in
878the encrypted string matter.
879
880Traditionally the result is a string of 13 bytes: two first bytes of
881the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
882the first eight bytes of the encrypted string mattered, but
883alternative hashing schemes (like MD5), higher level security schemes
884(like C2), and implementations on non-UNIX platforms may produce
885different strings.
886
887When choosing a new salt create a random two character string whose
888characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
d3989d75
CW
889'/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of
890characters is just a recommendation; the characters allowed in
891the salt depend solely on your system's crypt library, and Perl can't
892restrict what salts C<crypt()> accepts.
e71965be 893
a0d0e21e
LW
894Here's an example that makes sure that whoever runs this program knows
895their own password:
896
897 $pwd = (getpwuid($<))[1];
a0d0e21e
LW
898
899 system "stty -echo";
900 print "Password: ";
e71965be 901 chomp($word = <STDIN>);
a0d0e21e
LW
902 print "\n";
903 system "stty echo";
904
e71965be 905 if (crypt($word, $pwd) ne $pwd) {
a0d0e21e
LW
906 die "Sorry...\n";
907 } else {
908 print "ok\n";
54310121 909 }
a0d0e21e 910
9f8f0c9d 911Of course, typing in your own password to whoever asks you
748a9306 912for it is unwise.
a0d0e21e 913
8e2ffcbe 914The L<crypt|/crypt> function is unsuitable for encrypting large quantities
19799a22
GS
915of data, not least of all because you can't get the information
916back. Look at the F<by-module/Crypt> and F<by-module/PGP> directories
917on your favorite CPAN mirror for a slew of potentially useful
918modules.
919
f2791508
JH
920If using crypt() on a Unicode string (which I<potentially> has
921characters with codepoints above 255), Perl tries to make sense
922of the situation by trying to downgrade (a copy of the string)
923the string back to an eight-bit byte string before calling crypt()
924(on that copy). If that works, good. If not, crypt() dies with
925C<Wide character in crypt>.
85c16d83 926
aa689395 927=item dbmclose HASH
a0d0e21e 928
19799a22 929[This function has been largely superseded by the C<untie> function.]
a0d0e21e 930
aa689395 931Breaks the binding between a DBM file and a hash.
a0d0e21e 932
19799a22 933=item dbmopen HASH,DBNAME,MASK
a0d0e21e 934
19799a22 935[This function has been largely superseded by the C<tie> function.]
a0d0e21e 936
7b8d334a 937This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
19799a22
GS
938hash. HASH is the name of the hash. (Unlike normal C<open>, the first
939argument is I<not> a filehandle, even though it looks like one). DBNAME
aa689395
PP
940is the name of the database (without the F<.dir> or F<.pag> extension if
941any). If the database does not exist, it is created with protection
19799a22
GS
942specified by MASK (as modified by the C<umask>). If your system supports
943only the older DBM functions, you may perform only one C<dbmopen> in your
aa689395 944program. In older versions of Perl, if your system had neither DBM nor
19799a22 945ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
aa689395
PP
946sdbm(3).
947
948If you don't have write access to the DBM file, you can only read hash
949variables, not set them. If you want to test whether you can write,
19799a22 950either use file tests or try setting a dummy hash entry inside an C<eval>,
aa689395 951which will trap the error.
a0d0e21e 952
19799a22
GS
953Note that functions such as C<keys> and C<values> may return huge lists
954when used on large DBM files. You may prefer to use the C<each>
a0d0e21e
LW
955function to iterate over large DBM files. Example:
956
957 # print out history file offsets
958 dbmopen(%HIST,'/usr/lib/news/history',0666);
959 while (($key,$val) = each %HIST) {
960 print $key, ' = ', unpack('L',$val), "\n";
961 }
962 dbmclose(%HIST);
963
cb1a09d0 964See also L<AnyDBM_File> for a more general description of the pros and
184e9718 965cons of the various dbm approaches, as well as L<DB_File> for a particularly
cb1a09d0 966rich implementation.
4633a7c4 967
2b5ab1e7
TC
968You can control which DBM library you use by loading that library
969before you call dbmopen():
970
971 use DB_File;
972 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
973 or die "Can't open netscape history file: $!";
974
a0d0e21e
LW
975=item defined EXPR
976
54310121 977=item defined
bbce6d69 978
2f9daede
TPG
979Returns a Boolean value telling whether EXPR has a value other than
980the undefined value C<undef>. If EXPR is not present, C<$_> will be
981checked.
982
983Many operations return C<undef> to indicate failure, end of file,
984system error, uninitialized variable, and other exceptional
985conditions. This function allows you to distinguish C<undef> from
986other values. (A simple Boolean test will not distinguish among
7660c0ab 987C<undef>, zero, the empty string, and C<"0">, which are all equally
2f9daede 988false.) Note that since C<undef> is a valid scalar, its presence
19799a22 989doesn't I<necessarily> indicate an exceptional condition: C<pop>
2f9daede
TPG
990returns C<undef> when its argument is an empty array, I<or> when the
991element to return happens to be C<undef>.
992
f10b0346
GS
993You may also use C<defined(&func)> to check whether subroutine C<&func>
994has ever been defined. The return value is unaffected by any forward
04891299 995declarations of C<&func>. Note that a subroutine which is not defined
847c7ebe
DD
996may still be callable: its package may have an C<AUTOLOAD> method that
997makes it spring into existence the first time that it is called -- see
998L<perlsub>.
f10b0346
GS
999
1000Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
1001used to report whether memory for that aggregate has ever been
1002allocated. This behavior may disappear in future versions of Perl.
1003You should instead use a simple test for size:
1004
1005 if (@an_array) { print "has array elements\n" }
1006 if (%a_hash) { print "has hash members\n" }
2f9daede
TPG
1007
1008When used on a hash element, it tells you whether the value is defined,
dc848c6f 1009not whether the key exists in the hash. Use L</exists> for the latter
2f9daede 1010purpose.
a0d0e21e
LW
1011
1012Examples:
1013
1014 print if defined $switch{'D'};
1015 print "$val\n" while defined($val = pop(@ary));
1016 die "Can't readlink $sym: $!"
1017 unless defined($value = readlink $sym);
a0d0e21e 1018 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
2f9daede 1019 $debugging = 0 unless defined $debugging;
a0d0e21e 1020
19799a22 1021Note: Many folks tend to overuse C<defined>, and then are surprised to
7660c0ab 1022discover that the number C<0> and C<""> (the zero-length string) are, in fact,
2f9daede 1023defined values. For example, if you say
a5f75d66
AD
1024
1025 "ab" =~ /a(.*)b/;
1026
7660c0ab 1027The pattern match succeeds, and C<$1> is defined, despite the fact that it
a5f75d66 1028matched "nothing". But it didn't really match nothing--rather, it
2b5ab1e7 1029matched something that happened to be zero characters long. This is all
a5f75d66 1030very above-board and honest. When a function returns an undefined value,
2f9daede 1031it's an admission that it couldn't give you an honest answer. So you
19799a22 1032should use C<defined> only when you're questioning the integrity of what
7660c0ab 1033you're trying to do. At other times, a simple comparison to C<0> or C<""> is
2f9daede
TPG
1034what you want.
1035
dc848c6f 1036See also L</undef>, L</exists>, L</ref>.
2f9daede 1037
a0d0e21e
LW
1038=item delete EXPR
1039
01020589
GS
1040Given an expression that specifies a hash element, array element, hash slice,
1041or array slice, deletes the specified element(s) from the hash or array.
8216c1fd 1042In the case of an array, if the array elements happen to be at the end,
b76cc8ba 1043the size of the array will shrink to the highest element that tests
8216c1fd 1044true for exists() (or 0 if no such element exists).
a0d0e21e 1045
eba0920a
EM
1046Returns a list with the same number of elements as the number of elements
1047for which deletion was attempted. Each element of that list consists of
1048either the value of the element deleted, or the undefined value. In scalar
1049context, this means that you get the value of the last element deleted (or
1050the undefined value if that element did not exist).
1051
1052 %hash = (foo => 11, bar => 22, baz => 33);
1053 $scalar = delete $hash{foo}; # $scalar is 11
1054 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
1055 @array = delete @hash{qw(foo bar baz)}; # @array is (undef,undef,33)
1056
1057Deleting from C<%ENV> modifies the environment. Deleting from
01020589
GS
1058a hash tied to a DBM file deletes the entry from the DBM file. Deleting
1059from a C<tie>d hash or array may not necessarily return anything.
1060
8ea97a1e
GS
1061Deleting an array element effectively returns that position of the array
1062to its initial, uninitialized state. Subsequently testing for the same
8216c1fd
GS
1063element with exists() will return false. Note that deleting array
1064elements in the middle of an array will not shift the index of the ones
1065after them down--use splice() for that. See L</exists>.
8ea97a1e 1066
01020589 1067The following (inefficiently) deletes all the values of %HASH and @ARRAY:
a0d0e21e 1068
5f05dabc
PP
1069 foreach $key (keys %HASH) {
1070 delete $HASH{$key};
a0d0e21e
LW
1071 }
1072
01020589
GS
1073 foreach $index (0 .. $#ARRAY) {
1074 delete $ARRAY[$index];
1075 }
1076
1077And so do these:
5f05dabc 1078
01020589
GS
1079 delete @HASH{keys %HASH};
1080
9740c838 1081 delete @ARRAY[0 .. $#ARRAY];
5f05dabc 1082
2b5ab1e7 1083But both of these are slower than just assigning the empty list
01020589
GS
1084or undefining %HASH or @ARRAY:
1085
1086 %HASH = (); # completely empty %HASH
1087 undef %HASH; # forget %HASH ever existed
2b5ab1e7 1088
01020589
GS
1089 @ARRAY = (); # completely empty @ARRAY
1090 undef @ARRAY; # forget @ARRAY ever existed
2b5ab1e7
TC
1091
1092Note that the EXPR can be arbitrarily complicated as long as the final
01020589
GS
1093operation is a hash element, array element, hash slice, or array slice
1094lookup:
a0d0e21e
LW
1095
1096 delete $ref->[$x][$y]{$key};
5f05dabc 1097 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
a0d0e21e 1098
01020589
GS
1099 delete $ref->[$x][$y][$index];
1100 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1101
a0d0e21e
LW
1102=item die LIST
1103
19799a22
GS
1104Outside an C<eval>, prints the value of LIST to C<STDERR> and
1105exits with the current value of C<$!> (errno). If C<$!> is C<0>,
61eff3bc
JH
1106exits with the value of C<<< ($? >> 8) >>> (backtick `command`
1107status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside
19799a22
GS
1108an C<eval(),> the error message is stuffed into C<$@> and the
1109C<eval> is terminated with the undefined value. This makes
1110C<die> the way to raise an exception.
a0d0e21e
LW
1111
1112Equivalent examples:
1113
1114 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
54310121 1115 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
a0d0e21e 1116
ccac6780 1117If the last element of LIST does not end in a newline, the current
df37ec69
WW
1118script line number and input line number (if any) are also printed,
1119and a newline is supplied. Note that the "input line number" (also
1120known as "chunk") is subject to whatever notion of "line" happens to
1121be currently in effect, and is also available as the special variable
1122C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1123
1124Hint: sometimes appending C<", stopped"> to your message will cause it
1125to make better sense when the string C<"at foo line 123"> is appended.
1126Suppose you are running script "canasta".
a0d0e21e
LW
1127
1128 die "/etc/games is no good";
1129 die "/etc/games is no good, stopped";
1130
1131produce, respectively
1132
1133 /etc/games is no good at canasta line 123.
1134 /etc/games is no good, stopped at canasta line 123.
1135
2b5ab1e7 1136See also exit(), warn(), and the Carp module.
a0d0e21e 1137
7660c0ab
A
1138If LIST is empty and C<$@> already contains a value (typically from a
1139previous eval) that value is reused after appending C<"\t...propagated">.
fb73857a
PP
1140This is useful for propagating exceptions:
1141
1142 eval { ... };
1143 die unless $@ =~ /Expected exception/;
1144
ad216e65
JH
1145If LIST is empty and C<$@> contains an object reference that has a
1146C<PROPAGATE> method, that method will be called with additional file
1147and line number parameters. The return value replaces the value in
16869676 1148C<$@>. ie. as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >>
ad216e65
JH
1149were called.
1150
7660c0ab 1151If C<$@> is empty then the string C<"Died"> is used.
fb73857a 1152
52531d10
GS
1153die() can also be called with a reference argument. If this happens to be
1154trapped within an eval(), $@ contains the reference. This behavior permits
1155a more elaborate exception handling implementation using objects that
4375e838 1156maintain arbitrary state about the nature of the exception. Such a scheme
52531d10
GS
1157is sometimes preferable to matching particular string values of $@ using
1158regular expressions. Here's an example:
1159
1160 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1161 if ($@) {
1162 if (ref($@) && UNIVERSAL::isa($@,"Some::Module::Exception")) {
1163 # handle Some::Module::Exception
1164 }
1165 else {
1166 # handle all other possible exceptions
1167 }
1168 }
1169
19799a22 1170Because perl will stringify uncaught exception messages before displaying
52531d10
GS
1171them, you may want to overload stringification operations on such custom
1172exception objects. See L<overload> for details about that.
1173
19799a22
GS
1174You can arrange for a callback to be run just before the C<die>
1175does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1176handler will be called with the error text and can change the error
1177message, if it sees fit, by calling C<die> again. See
1178L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
1179L<"eval BLOCK"> for some examples. Although this feature was meant
1180to be run only right before your program was to exit, this is not
1181currently the case--the C<$SIG{__DIE__}> hook is currently called
1182even inside eval()ed blocks/strings! If one wants the hook to do
1183nothing in such situations, put
fb73857a
PP
1184
1185 die @_ if $^S;
1186
19799a22
GS
1187as the first line of the handler (see L<perlvar/$^S>). Because
1188this promotes strange action at a distance, this counterintuitive
b76cc8ba 1189behavior may be fixed in a future release.
774d564b 1190
a0d0e21e
LW
1191=item do BLOCK
1192
1193Not really a function. Returns the value of the last command in the
1194sequence of commands indicated by BLOCK. When modified by a loop
98293880
JH
1195modifier, executes the BLOCK once before testing the loop condition.
1196(On other statements the loop modifiers test the conditional first.)
a0d0e21e 1197
4968c1e4 1198C<do BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7
TC
1199C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1200See L<perlsyn> for alternative strategies.
4968c1e4 1201
a0d0e21e
LW
1202=item do SUBROUTINE(LIST)
1203
1204A deprecated form of subroutine call. See L<perlsub>.
1205
1206=item do EXPR
1207
1208Uses the value of EXPR as a filename and executes the contents of the
1209file as a Perl script. Its primary use is to include subroutines
1210from a Perl subroutine library.
1211
1212 do 'stat.pl';
1213
1214is just like
1215
986b19de 1216 eval `cat stat.pl`;
a0d0e21e 1217
2b5ab1e7
TC
1218except that it's more efficient and concise, keeps track of the current
1219filename for error messages, searches the @INC libraries, and updates
1220C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1221variables. It also differs in that code evaluated with C<do FILENAME>
1222cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1223same, however, in that it does reparse the file every time you call it,
1224so you probably don't want to do this inside a loop.
a0d0e21e 1225
8e30cc93 1226If C<do> cannot read the file, it returns undef and sets C<$!> to the
2b5ab1e7 1227error. If C<do> can read the file but cannot compile it, it
8e30cc93
G
1228returns undef and sets an error message in C<$@>. If the file is
1229successfully compiled, C<do> returns the value of the last expression
1230evaluated.
1231
a0d0e21e 1232Note that inclusion of library modules is better done with the
19799a22 1233C<use> and C<require> operators, which also do automatic error checking
4633a7c4 1234and raise an exception if there's a problem.
a0d0e21e 1235
5a964f20
TC
1236You might like to use C<do> to read in a program configuration
1237file. Manual error checking can be done this way:
1238
b76cc8ba 1239 # read in config files: system first, then user
f86cebdf 1240 for $file ("/share/prog/defaults.rc",
b76cc8ba 1241 "$ENV{HOME}/.someprogrc")
2b5ab1e7 1242 {
5a964f20 1243 unless ($return = do $file) {
f86cebdf
GS
1244 warn "couldn't parse $file: $@" if $@;
1245 warn "couldn't do $file: $!" unless defined $return;
1246 warn "couldn't run $file" unless $return;
5a964f20
TC
1247 }
1248 }
1249
a0d0e21e
LW
1250=item dump LABEL
1251
1614b0e3
JD
1252=item dump
1253
19799a22
GS
1254This function causes an immediate core dump. See also the B<-u>
1255command-line switch in L<perlrun>, which does the same thing.
1256Primarily this is so that you can use the B<undump> program (not
1257supplied) to turn your core dump into an executable binary after
1258having initialized all your variables at the beginning of the
1259program. When the new binary is executed it will begin by executing
1260a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1261Think of it as a goto with an intervening core dump and reincarnation.
1262If C<LABEL> is omitted, restarts the program from the top.
1263
1264B<WARNING>: Any files opened at the time of the dump will I<not>
1265be open any more when the program is reincarnated, with possible
b76cc8ba 1266resulting confusion on the part of Perl.
19799a22
GS
1267
1268This function is now largely obsolete, partly because it's very
1269hard to convert a core file into an executable, and because the
1270real compiler backends for generating portable bytecode and compilable
ac206dc8
RGS
1271C code have superseded it. That's why you should now invoke it as
1272C<CORE::dump()>, if you don't want to be warned against a possible
1273typo.
19799a22
GS
1274
1275If you're looking to use L<dump> to speed up your program, consider
1276generating bytecode or native C code as described in L<perlcc>. If
1277you're just trying to accelerate a CGI script, consider using the
210b36aa 1278C<mod_perl> extension to B<Apache>, or the CPAN module, CGI::Fast.
19799a22 1279You might also consider autoloading or selfloading, which at least
b76cc8ba 1280make your program I<appear> to run faster.
5a964f20 1281
aa689395
PP
1282=item each HASH
1283
5a964f20 1284When called in list context, returns a 2-element list consisting of the
aa689395 1285key and value for the next element of a hash, so that you can iterate over
74fc8b5f 1286it. When called in scalar context, returns only the key for the next
e902a979 1287element in the hash.
2f9daede 1288
ab192400 1289Entries are returned in an apparently random order. The actual random
504f80c1
JH
1290order is subject to change in future versions of perl, but it is
1291guaranteed to be in the same order as either the C<keys> or C<values>
4546b9e6
JH
1292function would produce on the same (unmodified) hash. Since Perl
12935.8.1 the ordering is different even between different runs of Perl
1294for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
ab192400
GS
1295
1296When the hash is entirely read, a null array is returned in list context
19799a22
GS
1297(which when assigned produces a false (C<0>) value), and C<undef> in
1298scalar context. The next call to C<each> after that will start iterating
1299again. There is a single iterator for each hash, shared by all C<each>,
1300C<keys>, and C<values> function calls in the program; it can be reset by
2f9daede
TPG
1301reading all the elements from the hash, or by evaluating C<keys HASH> or
1302C<values HASH>. If you add or delete elements of a hash while you're
74fc8b5f
MJD
1303iterating over it, you may get entries skipped or duplicated, so
1304don't. Exception: It is always safe to delete the item most recently
1305returned by C<each()>, which means that the following code will work:
1306
1307 while (($key, $value) = each %hash) {
1308 print $key, "\n";
1309 delete $hash{$key}; # This is safe
1310 }
aa689395 1311
f86cebdf 1312The following prints out your environment like the printenv(1) program,
aa689395 1313only in a different order:
a0d0e21e
LW
1314
1315 while (($key,$value) = each %ENV) {
1316 print "$key=$value\n";
1317 }
1318
19799a22 1319See also C<keys>, C<values> and C<sort>.
a0d0e21e
LW
1320
1321=item eof FILEHANDLE
1322
4633a7c4
LW
1323=item eof ()
1324
a0d0e21e
LW
1325=item eof
1326
1327Returns 1 if the next read on FILEHANDLE will return end of file, or if
1328FILEHANDLE is not open. FILEHANDLE may be an expression whose value
5a964f20 1329gives the real filehandle. (Note that this function actually
19799a22 1330reads a character and then C<ungetc>s it, so isn't very useful in an
748a9306 1331interactive context.) Do not read from a terminal file (or call
19799a22 1332C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
748a9306
LW
1333as terminals may lose the end-of-file condition if you do.
1334
820475bd
GS
1335An C<eof> without an argument uses the last file read. Using C<eof()>
1336with empty parentheses is very different. It refers to the pseudo file
1337formed from the files listed on the command line and accessed via the
61eff3bc
JH
1338C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1339as a normal filehandle is, an C<eof()> before C<< <> >> has been
820475bd 1340used will cause C<@ARGV> to be examined to determine if input is
67408cae 1341available. Similarly, an C<eof()> after C<< <> >> has returned
efdd0218
RB
1342end-of-file will assume you are processing another C<@ARGV> list,
1343and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1344see L<perlop/"I/O Operators">.
820475bd 1345
61eff3bc 1346In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
820475bd
GS
1347detect the end of each file, C<eof()> will only detect the end of the
1348last file. Examples:
a0d0e21e 1349
748a9306
LW
1350 # reset line numbering on each input file
1351 while (<>) {
b76cc8ba 1352 next if /^\s*#/; # skip comments
748a9306 1353 print "$.\t$_";
5a964f20
TC
1354 } continue {
1355 close ARGV if eof; # Not eof()!
748a9306
LW
1356 }
1357
a0d0e21e
LW
1358 # insert dashes just before last line of last file
1359 while (<>) {
6ac88b13 1360 if (eof()) { # check for end of last file
a0d0e21e
LW
1361 print "--------------\n";
1362 }
1363 print;
6ac88b13 1364 last if eof(); # needed if we're reading from a terminal
a0d0e21e
LW
1365 }
1366
a0d0e21e 1367Practical hint: you almost never need to use C<eof> in Perl, because the
3ce0d271
GS
1368input operators typically return C<undef> when they run out of data, or if
1369there was an error.
a0d0e21e
LW
1370
1371=item eval EXPR
1372
1373=item eval BLOCK
1374
c7cc6f1c
GS
1375In the first form, the return value of EXPR is parsed and executed as if it
1376were a little Perl program. The value of the expression (which is itself
5a964f20 1377determined within scalar context) is first parsed, and if there weren't any
be3174d2
GS
1378errors, executed in the lexical context of the current Perl program, so
1379that any variable settings or subroutine and format definitions remain
1380afterwards. Note that the value is parsed every time the eval executes.
1381If EXPR is omitted, evaluates C<$_>. This form is typically used to
1382delay parsing and subsequent execution of the text of EXPR until run time.
c7cc6f1c
GS
1383
1384In the second form, the code within the BLOCK is parsed only once--at the
1385same time the code surrounding the eval itself was parsed--and executed
1386within the context of the current Perl program. This form is typically
1387used to trap exceptions more efficiently than the first (see below), while
1388also providing the benefit of checking the code within BLOCK at compile
1389time.
1390
1391The final semicolon, if any, may be omitted from the value of EXPR or within
1392the BLOCK.
1393
1394In both forms, the value returned is the value of the last expression
5a964f20 1395evaluated inside the mini-program; a return statement may be also used, just
c7cc6f1c 1396as with subroutines. The expression providing the return value is evaluated
5a964f20 1397in void, scalar, or list context, depending on the context of the eval itself.
c7cc6f1c 1398See L</wantarray> for more on how the evaluation context can be determined.
a0d0e21e 1399
19799a22
GS
1400If there is a syntax error or runtime error, or a C<die> statement is
1401executed, an undefined value is returned by C<eval>, and C<$@> is set to the
a0d0e21e 1402error message. If there was no error, C<$@> is guaranteed to be a null
19799a22 1403string. Beware that using C<eval> neither silences perl from printing
c7cc6f1c 1404warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
d9984052
A
1405To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1406turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1407See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
a0d0e21e 1408
19799a22
GS
1409Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1410determining whether a particular feature (such as C<socket> or C<symlink>)
a0d0e21e
LW
1411is implemented. It is also Perl's exception trapping mechanism, where
1412the die operator is used to raise exceptions.
1413
1414If the code to be executed doesn't vary, you may use the eval-BLOCK
1415form to trap run-time errors without incurring the penalty of
1416recompiling each time. The error, if any, is still returned in C<$@>.
1417Examples:
1418
54310121 1419 # make divide-by-zero nonfatal
a0d0e21e
LW
1420 eval { $answer = $a / $b; }; warn $@ if $@;
1421
1422 # same thing, but less efficient
1423 eval '$answer = $a / $b'; warn $@ if $@;
1424
1425 # a compile-time error
5a964f20 1426 eval { $answer = }; # WRONG
a0d0e21e
LW
1427
1428 # a run-time error
1429 eval '$answer ='; # sets $@
1430
2b5ab1e7
TC
1431Due to the current arguably broken state of C<__DIE__> hooks, when using
1432the C<eval{}> form as an exception trap in libraries, you may wish not
1433to trigger any C<__DIE__> hooks that user code may have installed.
1434You can use the C<local $SIG{__DIE__}> construct for this purpose,
1435as shown in this example:
774d564b
PP
1436
1437 # a very private exception trap for divide-by-zero
f86cebdf
GS
1438 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1439 warn $@ if $@;
774d564b
PP
1440
1441This is especially significant, given that C<__DIE__> hooks can call
19799a22 1442C<die> again, which has the effect of changing their error messages:
774d564b
PP
1443
1444 # __DIE__ hooks may modify error messages
1445 {
f86cebdf
GS
1446 local $SIG{'__DIE__'} =
1447 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
c7cc6f1c
GS
1448 eval { die "foo lives here" };
1449 print $@ if $@; # prints "bar lives here"
774d564b
PP
1450 }
1451
19799a22 1452Because this promotes action at a distance, this counterintuitive behavior
2b5ab1e7
TC
1453may be fixed in a future release.
1454
19799a22 1455With an C<eval>, you should be especially careful to remember what's
a0d0e21e
LW
1456being looked at when:
1457
1458 eval $x; # CASE 1
1459 eval "$x"; # CASE 2
1460
1461 eval '$x'; # CASE 3
1462 eval { $x }; # CASE 4
1463
5a964f20 1464 eval "\$$x++"; # CASE 5
a0d0e21e
LW
1465 $$x++; # CASE 6
1466
2f9daede 1467Cases 1 and 2 above behave identically: they run the code contained in
19799a22 1468the variable $x. (Although case 2 has misleading double quotes making
2f9daede 1469the reader wonder what else might be happening (nothing is).) Cases 3
7660c0ab 1470and 4 likewise behave in the same way: they run the code C<'$x'>, which
19799a22 1471does nothing but return the value of $x. (Case 4 is preferred for
2f9daede
TPG
1472purely visual reasons, but it also has the advantage of compiling at
1473compile-time instead of at run-time.) Case 5 is a place where
19799a22 1474normally you I<would> like to use double quotes, except that in this
2f9daede
TPG
1475particular situation, you can just use symbolic references instead, as
1476in case 6.
a0d0e21e 1477
4968c1e4 1478C<eval BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7 1479C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
4968c1e4 1480
d819b83a
DM
1481Note that as a very special case, an C<eval ''> executed within the C<DB>
1482package doesn't see the usual surrounding lexical scope, but rather the
1483scope of the first non-DB piece of code that called it. You don't normally
1484need to worry about this unless you are writing a Perl debugger.
1485
a0d0e21e
LW
1486=item exec LIST
1487
8bf3b016
GS
1488=item exec PROGRAM LIST
1489
19799a22
GS
1490The C<exec> function executes a system command I<and never returns>--
1491use C<system> instead of C<exec> if you want it to return. It fails and
1492returns false only if the command does not exist I<and> it is executed
fb73857a 1493directly instead of via your system's command shell (see below).
a0d0e21e 1494
19799a22
GS
1495Since it's a common mistake to use C<exec> instead of C<system>, Perl
1496warns you if there is a following statement which isn't C<die>, C<warn>,
1497or C<exit> (if C<-w> is set - but you always do that). If you
1498I<really> want to follow an C<exec> with some other statement, you
55d729e4
GS
1499can use one of these styles to avoid the warning:
1500
5a964f20
TC
1501 exec ('foo') or print STDERR "couldn't exec foo: $!";
1502 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
55d729e4 1503
5a964f20 1504If there is more than one argument in LIST, or if LIST is an array
f86cebdf 1505with more than one value, calls execvp(3) with the arguments in LIST.
5a964f20
TC
1506If there is only one scalar argument or an array with one element in it,
1507the argument is checked for shell metacharacters, and if there are any,
1508the entire argument is passed to the system's command shell for parsing
1509(this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1510If there are no shell metacharacters in the argument, it is split into
b76cc8ba 1511words and passed directly to C<execvp>, which is more efficient.
19799a22 1512Examples:
a0d0e21e 1513
19799a22
GS
1514 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1515 exec "sort $outfile | uniq";
a0d0e21e
LW
1516
1517If you don't really want to execute the first argument, but want to lie
1518to the program you are executing about its own name, you can specify
1519the program you actually want to run as an "indirect object" (without a
1520comma) in front of the LIST. (This always forces interpretation of the
54310121 1521LIST as a multivalued list, even if there is only a single scalar in
a0d0e21e
LW
1522the list.) Example:
1523
1524 $shell = '/bin/csh';
1525 exec $shell '-sh'; # pretend it's a login shell
1526
1527or, more directly,
1528
1529 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1530
bb32b41a
GS
1531When the arguments get executed via the system shell, results will
1532be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1533for details.
1534
19799a22
GS
1535Using an indirect object with C<exec> or C<system> is also more
1536secure. This usage (which also works fine with system()) forces
1537interpretation of the arguments as a multivalued list, even if the
1538list had just one argument. That way you're safe from the shell
1539expanding wildcards or splitting up words with whitespace in them.
5a964f20
TC
1540
1541 @args = ( "echo surprise" );
1542
2b5ab1e7 1543 exec @args; # subject to shell escapes
f86cebdf 1544 # if @args == 1
2b5ab1e7 1545 exec { $args[0] } @args; # safe even with one-arg list
5a964f20
TC
1546
1547The first version, the one without the indirect object, ran the I<echo>
1548program, passing it C<"surprise"> an argument. The second version
1549didn't--it tried to run a program literally called I<"echo surprise">,
1550didn't find it, and set C<$?> to a non-zero value indicating failure.
1551
0f897271
GS
1552Beginning with v5.6.0, Perl will attempt to flush all files opened for
1553output before the exec, but this may not be supported on some platforms
1554(see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1555in English) or call the C<autoflush()> method of C<IO::Handle> on any
1556open handles in order to avoid lost output.
1557
19799a22 1558Note that C<exec> will not call your C<END> blocks, nor will it call
7660c0ab
A
1559any C<DESTROY> methods in your objects.
1560
a0d0e21e
LW
1561=item exists EXPR
1562
01020589 1563Given an expression that specifies a hash element or array element,
8ea97a1e
GS
1564returns true if the specified element in the hash or array has ever
1565been initialized, even if the corresponding value is undefined. The
1566element is not autovivified if it doesn't exist.
a0d0e21e 1567
01020589
GS
1568 print "Exists\n" if exists $hash{$key};
1569 print "Defined\n" if defined $hash{$key};
1570 print "True\n" if $hash{$key};
1571
1572 print "Exists\n" if exists $array[$index];
1573 print "Defined\n" if defined $array[$index];
1574 print "True\n" if $array[$index];
a0d0e21e 1575
8ea97a1e 1576A hash or array element can be true only if it's defined, and defined if
a0d0e21e
LW
1577it exists, but the reverse doesn't necessarily hold true.
1578
afebc493
GS
1579Given an expression that specifies the name of a subroutine,
1580returns true if the specified subroutine has ever been declared, even
1581if it is undefined. Mentioning a subroutine name for exists or defined
847c7ebe
DD
1582does not count as declaring it. Note that a subroutine which does not
1583exist may still be callable: its package may have an C<AUTOLOAD>
1584method that makes it spring into existence the first time that it is
1585called -- see L<perlsub>.
afebc493
GS
1586
1587 print "Exists\n" if exists &subroutine;
1588 print "Defined\n" if defined &subroutine;
1589
a0d0e21e 1590Note that the EXPR can be arbitrarily complicated as long as the final
afebc493 1591operation is a hash or array key lookup or subroutine name:
a0d0e21e 1592
2b5ab1e7
TC
1593 if (exists $ref->{A}->{B}->{$key}) { }
1594 if (exists $hash{A}{B}{$key}) { }
1595
01020589
GS
1596 if (exists $ref->{A}->{B}->[$ix]) { }
1597 if (exists $hash{A}{B}[$ix]) { }
1598
afebc493
GS
1599 if (exists &{$ref->{A}{B}{$key}}) { }
1600
01020589
GS
1601Although the deepest nested array or hash will not spring into existence
1602just because its existence was tested, any intervening ones will.
61eff3bc 1603Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
01020589
GS
1604into existence due to the existence test for the $key element above.
1605This happens anywhere the arrow operator is used, including even:
5a964f20 1606
2b5ab1e7
TC
1607 undef $ref;
1608 if (exists $ref->{"Some key"}) { }
1609 print $ref; # prints HASH(0x80d3d5c)
1610
1611This surprising autovivification in what does not at first--or even
1612second--glance appear to be an lvalue context may be fixed in a future
5a964f20 1613release.
a0d0e21e 1614
afebc493
GS
1615Use of a subroutine call, rather than a subroutine name, as an argument
1616to exists() is an error.
1617
1618 exists &sub; # OK
1619 exists &sub(); # Error
1620
a0d0e21e
LW
1621=item exit EXPR
1622
2b5ab1e7 1623Evaluates EXPR and exits immediately with that value. Example:
a0d0e21e
LW
1624
1625 $ans = <STDIN>;
1626 exit 0 if $ans =~ /^[Xx]/;
1627
19799a22 1628See also C<die>. If EXPR is omitted, exits with C<0> status. The only
2b5ab1e7
TC
1629universally recognized values for EXPR are C<0> for success and C<1>
1630for error; other values are subject to interpretation depending on the
1631environment in which the Perl program is running. For example, exiting
163269 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1633the mailer to return the item undelivered, but that's not true everywhere.
a0d0e21e 1634
19799a22
GS
1635Don't use C<exit> to abort a subroutine if there's any chance that
1636someone might want to trap whatever error happened. Use C<die> instead,
1637which can be trapped by an C<eval>.
28757baa 1638
19799a22 1639The exit() function does not always exit immediately. It calls any
2b5ab1e7 1640defined C<END> routines first, but these C<END> routines may not
19799a22 1641themselves abort the exit. Likewise any object destructors that need to
2b5ab1e7
TC
1642be called are called before the real exit. If this is a problem, you
1643can call C<POSIX:_exit($status)> to avoid END and destructor processing.
87275199 1644See L<perlmod> for details.
5a964f20 1645
a0d0e21e
LW
1646=item exp EXPR
1647
54310121 1648=item exp
bbce6d69 1649
b76cc8ba 1650Returns I<e> (the natural logarithm base) to the power of EXPR.
a0d0e21e
LW
1651If EXPR is omitted, gives C<exp($_)>.
1652
1653=item fcntl FILEHANDLE,FUNCTION,SCALAR
1654
f86cebdf 1655Implements the fcntl(2) function. You'll probably have to say
a0d0e21e
LW
1656
1657 use Fcntl;
1658
0ade1984 1659first to get the correct constant definitions. Argument processing and
b76cc8ba 1660value return works just like C<ioctl> below.
a0d0e21e
LW
1661For example:
1662
1663 use Fcntl;
5a964f20
TC
1664 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1665 or die "can't fcntl F_GETFL: $!";
1666
554ad1fc 1667You don't have to check for C<defined> on the return from C<fcntl>.
951ba7fe
GS
1668Like C<ioctl>, it maps a C<0> return from the system call into
1669C<"0 but true"> in Perl. This string is true in boolean context and C<0>
2b5ab1e7
TC
1670in numeric context. It is also exempt from the normal B<-w> warnings
1671on improper numeric conversions.
5a964f20 1672
19799a22 1673Note that C<fcntl> will produce a fatal error if used on a machine that
2b5ab1e7
TC
1674doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1675manpage to learn what functions are available on your system.
a0d0e21e 1676
be2f7487 1677Here's an example of setting a filehandle named C<REMOTE> to be
1678non-blocking at the system level. You'll have to negotiate C<$|>
1679on your own, though.
1680
1681 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
1682
1683 $flags = fcntl(REMOTE, F_GETFL, 0)
1684 or die "Can't get flags for the socket: $!\n";
1685
1686 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
1687 or die "Can't set flags for the socket: $!\n";
1688
a0d0e21e
LW
1689=item fileno FILEHANDLE
1690
2b5ab1e7
TC
1691Returns the file descriptor for a filehandle, or undefined if the
1692filehandle is not open. This is mainly useful for constructing
19799a22 1693bitmaps for C<select> and low-level POSIX tty-handling operations.
2b5ab1e7
TC
1694If FILEHANDLE is an expression, the value is taken as an indirect
1695filehandle, generally its name.
5a964f20 1696
b76cc8ba 1697You can use this to find out whether two handles refer to the
5a964f20
TC
1698same underlying descriptor:
1699
1700 if (fileno(THIS) == fileno(THAT)) {
1701 print "THIS and THAT are dups\n";
b76cc8ba
NIS
1702 }
1703
1704(Filehandles connected to memory objects via new features of C<open> may
1705return undefined even though they are open.)
1706
a0d0e21e
LW
1707
1708=item flock FILEHANDLE,OPERATION
1709
19799a22
GS
1710Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1711for success, false on failure. Produces a fatal error if used on a
2b5ab1e7 1712machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
19799a22 1713C<flock> is Perl's portable file locking interface, although it locks
2b5ab1e7
TC
1714only entire files, not records.
1715
1716Two potentially non-obvious but traditional C<flock> semantics are
1717that it waits indefinitely until the lock is granted, and that its locks
1718B<merely advisory>. Such discretionary locks are more flexible, but offer
19799a22
GS
1719fewer guarantees. This means that files locked with C<flock> may be
1720modified by programs that do not also use C<flock>. See L<perlport>,
2b5ab1e7
TC
1721your port's specific documentation, or your system-specific local manpages
1722for details. It's best to assume traditional behavior if you're writing
1723portable programs. (But if you're not, you should as always feel perfectly
1724free to write for your own system's idiosyncrasies (sometimes called
1725"features"). Slavish adherence to portability concerns shouldn't get
1726in the way of your getting your job done.)
a3cb178b 1727
8ebc5c01
PP
1728OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1729LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
ea3105be 1730you can use the symbolic names if you import them from the Fcntl module,
68dc0745
PP
1731either individually, or as a group using the ':flock' tag. LOCK_SH
1732requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
ea3105be
GS
1733releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1734LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
68dc0745
PP
1735waiting for the lock (check the return status to see if you got it).
1736
2b5ab1e7
TC
1737To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1738before locking or unlocking it.
8ebc5c01 1739
f86cebdf 1740Note that the emulation built with lockf(3) doesn't provide shared
8ebc5c01 1741locks, and it requires that FILEHANDLE be open with write intent. These
2b5ab1e7 1742are the semantics that lockf(3) implements. Most if not all systems
f86cebdf 1743implement lockf(3) in terms of fcntl(2) locking, though, so the
8ebc5c01
PP
1744differing semantics shouldn't bite too many people.
1745
becacb53
TM
1746Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
1747be open with read intent to use LOCK_SH and requires that it be open
1748with write intent to use LOCK_EX.
1749
19799a22
GS
1750Note also that some versions of C<flock> cannot lock things over the
1751network; you would need to use the more system-specific C<fcntl> for
f86cebdf
GS
1752that. If you like you can force Perl to ignore your system's flock(2)
1753function, and so provide its own fcntl(2)-based emulation, by passing
8ebc5c01
PP
1754the switch C<-Ud_flock> to the F<Configure> program when you configure
1755perl.
4633a7c4
LW
1756
1757Here's a mailbox appender for BSD systems.
a0d0e21e 1758
7e1af8bc 1759 use Fcntl ':flock'; # import LOCK_* constants
a0d0e21e
LW
1760
1761 sub lock {
7e1af8bc 1762 flock(MBOX,LOCK_EX);
a0d0e21e
LW
1763 # and, in case someone appended
1764 # while we were waiting...
1765 seek(MBOX, 0, 2);
1766 }
1767
1768 sub unlock {
7e1af8bc 1769 flock(MBOX,LOCK_UN);
a0d0e21e
LW
1770 }
1771
1772 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1773 or die "Can't open mailbox: $!";
1774
1775 lock();
1776 print MBOX $msg,"\n\n";
1777 unlock();
1778
2b5ab1e7
TC
1779On systems that support a real flock(), locks are inherited across fork()
1780calls, whereas those that must resort to the more capricious fcntl()
1781function lose the locks, making it harder to write servers.
1782
cb1a09d0 1783See also L<DB_File> for other flock() examples.
a0d0e21e
LW
1784
1785=item fork
1786
2b5ab1e7
TC
1787Does a fork(2) system call to create a new process running the
1788same program at the same point. It returns the child pid to the
1789parent process, C<0> to the child process, or C<undef> if the fork is
1790unsuccessful. File descriptors (and sometimes locks on those descriptors)
1791are shared, while everything else is copied. On most systems supporting
1792fork(), great care has gone into making it extremely efficient (for
1793example, using copy-on-write technology on data pages), making it the
1794dominant paradigm for multitasking over the last few decades.
5a964f20 1795
0f897271
GS
1796Beginning with v5.6.0, Perl will attempt to flush all files opened for
1797output before forking the child process, but this may not be supported
1798on some platforms (see L<perlport>). To be safe, you may need to set
1799C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1800C<IO::Handle> on any open handles in order to avoid duplicate output.
a0d0e21e 1801
19799a22 1802If you C<fork> without ever waiting on your children, you will
2b5ab1e7
TC
1803accumulate zombies. On some systems, you can avoid this by setting
1804C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1805forking and reaping moribund children.
cb1a09d0 1806
28757baa
PP
1807Note that if your forked child inherits system file descriptors like
1808STDIN and STDOUT that are actually connected by a pipe or socket, even
2b5ab1e7 1809if you exit, then the remote server (such as, say, a CGI script or a
19799a22 1810backgrounded job launched from a remote shell) won't think you're done.
2b5ab1e7 1811You should reopen those to F</dev/null> if it's any issue.
28757baa 1812
cb1a09d0
AD
1813=item format
1814
19799a22 1815Declare a picture format for use by the C<write> function. For
cb1a09d0
AD
1816example:
1817
54310121 1818 format Something =
cb1a09d0
AD
1819 Test: @<<<<<<<< @||||| @>>>>>
1820 $str, $%, '$' . int($num)
1821 .
1822
1823 $str = "widget";
184e9718 1824 $num = $cost/$quantity;
cb1a09d0
AD
1825 $~ = 'Something';
1826 write;
1827
1828See L<perlform> for many details and examples.
1829
8903cb82 1830=item formline PICTURE,LIST
a0d0e21e 1831
5a964f20 1832This is an internal function used by C<format>s, though you may call it,
a0d0e21e
LW
1833too. It formats (see L<perlform>) a list of values according to the
1834contents of PICTURE, placing the output into the format output
7660c0ab 1835accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
19799a22 1836Eventually, when a C<write> is done, the contents of
a0d0e21e 1837C<$^A> are written to some filehandle, but you could also read C<$^A>
7660c0ab 1838yourself and then set C<$^A> back to C<"">. Note that a format typically
19799a22 1839does one C<formline> per line of form, but the C<formline> function itself
748a9306 1840doesn't care how many newlines are embedded in the PICTURE. This means
4633a7c4 1841that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
748a9306
LW
1842You may therefore need to use multiple formlines to implement a single
1843record format, just like the format compiler.
1844
19799a22 1845Be careful if you put double quotes around the picture, because an C<@>
748a9306 1846character may be taken to mean the beginning of an array name.
19799a22 1847C<formline> always returns true. See L<perlform> for other examples.
a0d0e21e
LW
1848
1849=item getc FILEHANDLE
1850
1851=item getc
1852
1853Returns the next character from the input file attached to FILEHANDLE,
b5fe5ca2
SR
1854or the undefined value at end of file, or if there was an error (in
1855the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
1856STDIN. This is not particularly efficient. However, it cannot be
1857used by itself to fetch single characters without waiting for the user
1858to hit enter. For that, try something more like:
4633a7c4
LW
1859
1860 if ($BSD_STYLE) {
1861 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1862 }
1863 else {
54310121 1864 system "stty", '-icanon', 'eol', "\001";
4633a7c4
LW
1865 }
1866
1867 $key = getc(STDIN);
1868
1869 if ($BSD_STYLE) {
1870 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1871 }
1872 else {
5f05dabc 1873 system "stty", 'icanon', 'eol', '^@'; # ASCII null
4633a7c4
LW
1874 }
1875 print "\n";
1876
54310121
PP
1877Determination of whether $BSD_STYLE should be set
1878is left as an exercise to the reader.
cb1a09d0 1879
19799a22 1880The C<POSIX::getattr> function can do this more portably on
2b5ab1e7
TC
1881systems purporting POSIX compliance. See also the C<Term::ReadKey>
1882module from your nearest CPAN site; details on CPAN can be found on
1883L<perlmodlib/CPAN>.
a0d0e21e
LW
1884
1885=item getlogin
1886
5a964f20
TC
1887Implements the C library function of the same name, which on most
1888systems returns the current login from F</etc/utmp>, if any. If null,
19799a22 1889use C<getpwuid>.
a0d0e21e 1890
f86702cc 1891 $login = getlogin || getpwuid($<) || "Kilroy";
a0d0e21e 1892
19799a22
GS
1893Do not consider C<getlogin> for authentication: it is not as
1894secure as C<getpwuid>.
4633a7c4 1895
a0d0e21e
LW
1896=item getpeername SOCKET
1897
1898Returns the packed sockaddr address of other end of the SOCKET connection.
1899
4633a7c4
LW
1900 use Socket;
1901 $hersockaddr = getpeername(SOCK);
19799a22 1902 ($port, $iaddr) = sockaddr_in($hersockaddr);
4633a7c4
LW
1903 $herhostname = gethostbyaddr($iaddr, AF_INET);
1904 $herstraddr = inet_ntoa($iaddr);
a0d0e21e
LW
1905
1906=item getpgrp PID
1907
47e29363 1908Returns the current process group for the specified PID. Use
7660c0ab 1909a PID of C<0> to get the current process group for the
4633a7c4 1910current process. Will raise an exception if used on a machine that
f86cebdf 1911doesn't implement getpgrp(2). If PID is omitted, returns process
19799a22 1912group of current process. Note that the POSIX version of C<getpgrp>
7660c0ab 1913does not accept a PID argument, so only C<PID==0> is truly portable.
a0d0e21e
LW
1914
1915=item getppid
1916
1917Returns the process id of the parent process.
1918
4d76a344
RGS
1919Note for Linux users: on Linux, the C functions C<getpid()> and
1920C<getppid()> return different values from different threads. In order to
1921be portable, this behavior is not reflected by the perl-level function
1922C<getppid()>, that returns a consistent value across threads. If you want
e3256f86
RGS
1923to call the underlying C<getppid()>, you may use the CPAN module
1924C<Linux::Pid>.
4d76a344 1925
a0d0e21e
LW
1926=item getpriority WHICH,WHO
1927
4633a7c4
LW
1928Returns the current priority for a process, a process group, or a user.
1929(See L<getpriority(2)>.) Will raise a fatal exception if used on a
f86cebdf 1930machine that doesn't implement getpriority(2).
a0d0e21e
LW
1931
1932=item getpwnam NAME
1933
1934=item getgrnam NAME
1935
1936=item gethostbyname NAME
1937
1938=item getnetbyname NAME
1939
1940=item getprotobyname NAME
1941
1942=item getpwuid UID
1943
1944=item getgrgid GID
1945
1946=item getservbyname NAME,PROTO
1947
1948=item gethostbyaddr ADDR,ADDRTYPE
1949
1950=item getnetbyaddr ADDR,ADDRTYPE
1951
1952=item getprotobynumber NUMBER
1953
1954=item getservbyport PORT,PROTO
1955
1956=item getpwent
1957
1958=item getgrent
1959
1960=item gethostent
1961
1962=item getnetent
1963
1964=item getprotoent
1965
1966=item getservent
1967
1968=item setpwent
1969
1970=item setgrent
1971
1972=item sethostent STAYOPEN
1973
1974=item setnetent STAYOPEN
1975
1976=item setprotoent STAYOPEN
1977
1978=item setservent STAYOPEN
1979
1980=item endpwent
1981
1982=item endgrent
1983
1984=item endhostent
1985
1986=item endnetent
1987
1988=item endprotoent
1989
1990=item endservent
1991
1992These routines perform the same functions as their counterparts in the
5a964f20 1993system library. In list context, the return values from the
a0d0e21e
LW
1994various get routines are as follows:
1995
1996 ($name,$passwd,$uid,$gid,
6ee623d5 1997 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
a0d0e21e
LW
1998 ($name,$passwd,$gid,$members) = getgr*
1999 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
2000 ($name,$aliases,$addrtype,$net) = getnet*
2001 ($name,$aliases,$proto) = getproto*
2002 ($name,$aliases,$port,$proto) = getserv*
2003
2004(If the entry doesn't exist you get a null list.)
2005
4602f195
JH
2006The exact meaning of the $gcos field varies but it usually contains
2007the real name of the user (as opposed to the login name) and other
2008information pertaining to the user. Beware, however, that in many
2009system users are able to change this information and therefore it
106325ad 2010cannot be trusted and therefore the $gcos is tainted (see
2959b6e3
JH
2011L<perlsec>). The $passwd and $shell, user's encrypted password and
2012login shell, are also tainted, because of the same reason.
4602f195 2013
5a964f20 2014In scalar context, you get the name, unless the function was a
a0d0e21e
LW
2015lookup by name, in which case you get the other thing, whatever it is.
2016(If the entry doesn't exist you get the undefined value.) For example:
2017
5a964f20
TC
2018 $uid = getpwnam($name);
2019 $name = getpwuid($num);
2020 $name = getpwent();
2021 $gid = getgrnam($name);
08a33e13 2022 $name = getgrgid($num);
5a964f20
TC
2023 $name = getgrent();
2024 #etc.
a0d0e21e 2025
4602f195
JH
2026In I<getpw*()> the fields $quota, $comment, and $expire are special
2027cases in the sense that in many systems they are unsupported. If the
2028$quota is unsupported, it is an empty scalar. If it is supported, it
2029usually encodes the disk quota. If the $comment field is unsupported,
2030it is an empty scalar. If it is supported it usually encodes some
2031administrative comment about the user. In some systems the $quota
2032field may be $change or $age, fields that have to do with password
2033aging. In some systems the $comment field may be $class. The $expire
2034field, if present, encodes the expiration period of the account or the
2035password. For the availability and the exact meaning of these fields
2036in your system, please consult your getpwnam(3) documentation and your
2037F<pwd.h> file. You can also find out from within Perl what your
2038$quota and $comment fields mean and whether you have the $expire field
2039by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2040C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
2041files are only supported if your vendor has implemented them in the
2042intuitive fashion that calling the regular C library routines gets the
5d3a0a3b
GS
2043shadow versions if you're running under privilege or if there exists
2044the shadow(3) functions as found in System V ( this includes Solaris
2045and Linux.) Those systems which implement a proprietary shadow password
2046facility are unlikely to be supported.
6ee623d5 2047
19799a22 2048The $members value returned by I<getgr*()> is a space separated list of
a0d0e21e
LW
2049the login names of the members of the group.
2050
2051For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2052C, it will be returned to you via C<$?> if the function call fails. The
7660c0ab 2053C<@addrs> value returned by a successful call is a list of the raw
a0d0e21e
LW
2054addresses returned by the corresponding system library call. In the
2055Internet domain, each address is four bytes long and you can unpack it
2056by saying something like:
2057
2058 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
2059
2b5ab1e7
TC
2060The Socket library makes this slightly easier:
2061
2062 use Socket;
2063 $iaddr = inet_aton("127.1"); # or whatever address
2064 $name = gethostbyaddr($iaddr, AF_INET);
2065
2066 # or going the other way
19799a22 2067 $straddr = inet_ntoa($iaddr);
2b5ab1e7 2068
19799a22
GS
2069If you get tired of remembering which element of the return list
2070contains which return value, by-name interfaces are provided
2071in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2072C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2073and C<User::grent>. These override the normal built-ins, supplying
2074versions that return objects with the appropriate names
2075for each field. For example:
5a964f20
TC
2076
2077 use File::stat;
2078 use User::pwent;
2079 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2080
b76cc8ba
NIS
2081Even though it looks like they're the same method calls (uid),
2082they aren't, because a C<File::stat> object is different from
19799a22 2083a C<User::pwent> object.
5a964f20 2084
a0d0e21e
LW
2085=item getsockname SOCKET
2086
19799a22
GS
2087Returns the packed sockaddr address of this end of the SOCKET connection,
2088in case you don't know the address because you have several different
2089IPs that the connection might have come in on.
a0d0e21e 2090
4633a7c4
LW
2091 use Socket;
2092 $mysockaddr = getsockname(SOCK);
19799a22 2093 ($port, $myaddr) = sockaddr_in($mysockaddr);
b76cc8ba 2094 printf "Connect to %s [%s]\n",
19799a22
GS
2095 scalar gethostbyaddr($myaddr, AF_INET),
2096 inet_ntoa($myaddr);
a0d0e21e
LW
2097
2098=item getsockopt SOCKET,LEVEL,OPTNAME
2099
636e6b1f
TH
2100Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
2101Options may exist at multiple protocol levels depending on the socket
2102type, but at least the uppermost socket level SOL_SOCKET (defined in the
2103C<Socket> module) will exist. To query options at another level the
2104protocol number of the appropriate protocol controlling the option
2105should be supplied. For example, to indicate that an option is to be
2106interpreted by the TCP protocol, LEVEL should be set to the protocol
2107number of TCP, which you can get using getprotobyname.
2108
2109The call returns a packed string representing the requested socket option,
2110or C<undef> if there is an error (the error reason will be in $!). What
2111exactly is in the packed string depends in the LEVEL and OPTNAME, consult
2112your system documentation for details. A very common case however is that
2113the option is an integer, in which case the result will be an packed
2114integer which you can decode using unpack with the C<i> (or C<I>) format.
2115
2116An example testing if Nagle's algorithm is turned on on a socket:
2117
2118 use Socket;
2119
2120 defined(my $tcp = getprotobyname("tcp"))
2121 or die "Could not determine the protocol number for tcp";
2122 # my $tcp = Socket::IPPROTO_TCP; # Alternative
2123 my $packed = getsockopt($socket, $tcp, Socket::TCP_NODELAY)
2124 or die "Could not query TCP_NODELAY SOCKEt option: $!";
2125 my $nodelay = unpack("I", $packed);
2126 print "Nagle's algorithm is turned ", $nodelay ? "off\n" : "on\n";
2127
a0d0e21e
LW
2128
2129=item glob EXPR
2130
0a753a76
PP
2131=item glob
2132
d9a9d457
JL
2133In list context, returns a (possibly empty) list of filename expansions on
2134the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
2135scalar context, glob iterates through such filename expansions, returning
2136undef when the list is exhausted. This is the internal function
2137implementing the C<< <*.c> >> operator, but you can use it directly. If
2138EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2139more detail in L<perlop/"I/O Operators">.
a0d0e21e 2140
3a4b19e4
GS
2141Beginning with v5.6.0, this operator is implemented using the standard
2142C<File::Glob> extension. See L<File::Glob> for details.
2143
a0d0e21e
LW
2144=item gmtime EXPR
2145
d1be9408 2146Converts a time as returned by the time function to an 8-element list
54310121 2147with the time localized for the standard Greenwich time zone.
4633a7c4 2148Typically used as follows:
a0d0e21e 2149
b76cc8ba 2150 # 0 1 2 3 4 5 6 7
48a26b3a 2151 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday) =
a0d0e21e
LW
2152 gmtime(time);
2153
48a26b3a
GS
2154All list elements are numeric, and come straight out of the C `struct
2155tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2156specified time. $mday is the day of the month, and $mon is the month
2157itself, in the range C<0..11> with 0 indicating January and 11
2158indicating December. $year is the number of years since 1900. That
2159is, $year is C<123> in year 2023. $wday is the day of the week, with
21600 indicating Sunday and 3 indicating Wednesday. $yday is the day of
b76cc8ba 2161the year, in the range C<0..364> (or C<0..365> in leap years.)
48a26b3a
GS
2162
2163Note that the $year element is I<not> simply the last two digits of
2164the year. If you assume it is, then you create non-Y2K-compliant
2165programs--and you wouldn't want to do that, would you?
2f9daede 2166
abd75f24
GS
2167The proper way to get a complete 4-digit year is simply:
2168
2169 $year += 1900;
2170
2171And to get the last two digits of the year (e.g., '01' in 2001) do:
2172
2173 $year = sprintf("%02d", $year % 100);
2174
48a26b3a 2175If EXPR is omitted, C<gmtime()> uses the current time (C<gmtime(time)>).
a0d0e21e 2176
48a26b3a 2177In scalar context, C<gmtime()> returns the ctime(3) value:
0a753a76
PP
2178
2179 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
2180
fe86afc2
NC
2181If you need local time instead of GMT use the L</localtime> builtin.
2182See also the C<timegm> function provided by the C<Time::Local> module,
2183and the strftime(3) and mktime(3) functions available via the L<POSIX> module.
7660c0ab 2184
fe86afc2
NC
2185This scalar value is B<not> locale dependent (see L<perllocale>), but is
2186instead a Perl builtin. To get somewhat similar but locale dependent date
2187strings, see the example in L</localtime>.
0a753a76 2188
a0d0e21e
LW
2189=item goto LABEL
2190
748a9306
LW
2191=item goto EXPR
2192
a0d0e21e
LW
2193=item goto &NAME
2194
7660c0ab 2195The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
a0d0e21e 2196execution there. It may not be used to go into any construct that
7660c0ab 2197requires initialization, such as a subroutine or a C<foreach> loop. It
0a753a76 2198also can't be used to go into a construct that is optimized away,
19799a22 2199or to get out of a block or subroutine given to C<sort>.
0a753a76 2200It can be used to go almost anywhere else within the dynamic scope,
a0d0e21e 2201including out of subroutines, but it's usually better to use some other
19799a22 2202construct such as C<last> or C<die>. The author of Perl has never felt the
7660c0ab 2203need to use this form of C<goto> (in Perl, that is--C is another matter).
1b6921cb
BT
2204(The difference being that C does not offer named loops combined with
2205loop control. Perl does, and this replaces most structured uses of C<goto>
2206in other languages.)
a0d0e21e 2207
7660c0ab
A
2208The C<goto-EXPR> form expects a label name, whose scope will be resolved
2209dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
748a9306
LW
2210necessarily recommended if you're optimizing for maintainability:
2211
2212 goto ("FOO", "BAR", "GLARCH")[$i];
2213
1b6921cb
BT
2214The C<goto-&NAME> form is quite different from the other forms of
2215C<goto>. In fact, it isn't a goto in the normal sense at all, and
2216doesn't have the stigma associated with other gotos. Instead, it
2217exits the current subroutine (losing any changes set by local()) and
2218immediately calls in its place the named subroutine using the current
2219value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2220load another subroutine and then pretend that the other subroutine had
2221been called in the first place (except that any modifications to C<@_>
6cb9131c
GS
2222in the current subroutine are propagated to the other subroutine.)
2223After the C<goto>, not even C<caller> will be able to tell that this
2224routine was called first.
2225
2226NAME needn't be the name of a subroutine; it can be a scalar variable
2227containing a code reference, or a block which evaluates to a code
2228reference.
a0d0e21e
LW
2229
2230=item grep BLOCK LIST
2231
2232=item grep EXPR,LIST
2233
2b5ab1e7
TC
2234This is similar in spirit to, but not the same as, grep(1) and its
2235relatives. In particular, it is not limited to using regular expressions.
2f9daede 2236
a0d0e21e 2237Evaluates the BLOCK or EXPR for each element of LIST (locally setting
7660c0ab 2238C<$_> to each element) and returns the list value consisting of those
19799a22
GS
2239elements for which the expression evaluated to true. In scalar
2240context, returns the number of times the expression was true.
a0d0e21e
LW
2241
2242 @foo = grep(!/^#/, @bar); # weed out comments
2243
2244or equivalently,
2245
2246 @foo = grep {!/^#/} @bar; # weed out comments
2247
be3174d2
GS
2248Note that C<$_> is an alias to the list value, so it can be used to
2249modify the elements of the LIST. While this is useful and supported,
2250it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
2251Similarly, grep returns aliases into the original list, much as a for
2252loop's index variable aliases the list elements. That is, modifying an
19799a22
GS
2253element of a list returned by grep (for example, in a C<foreach>, C<map>
2254or another C<grep>) actually modifies the element in the original list.
2b5ab1e7 2255This is usually something to be avoided when writing clear code.
a0d0e21e 2256
a4fb8298
RGS
2257If C<$_> is lexical in the scope where the C<grep> appears (because it has
2258been declared with C<my $_>) then, in addition the be locally aliased to
2259the list elements, C<$_> keeps being lexical inside the block; i.e. it
2260can't be seen from the outside, avoiding any potential side-effects.
2261
19799a22 2262See also L</map> for a list composed of the results of the BLOCK or EXPR.
38325410 2263
a0d0e21e
LW
2264=item hex EXPR
2265
54310121 2266=item hex
bbce6d69 2267
2b5ab1e7
TC
2268Interprets EXPR as a hex string and returns the corresponding value.
2269(To convert strings that might start with either 0, 0x, or 0b, see
2270L</oct>.) If EXPR is omitted, uses C<$_>.
2f9daede
TPG
2271
2272 print hex '0xAf'; # prints '175'
2273 print hex 'aF'; # same
a0d0e21e 2274
19799a22 2275Hex strings may only represent integers. Strings that would cause
53305cf1
NC
2276integer overflow trigger a warning. Leading whitespace is not stripped,
2277unlike oct().
19799a22 2278
a0d0e21e
LW
2279=item import
2280
19799a22 2281There is no builtin C<import> function. It is just an ordinary
4633a7c4 2282method (subroutine) defined (or inherited) by modules that wish to export
19799a22 2283names to another module. The C<use> function calls the C<import> method
cea6626f 2284for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
a0d0e21e
LW
2285
2286=item index STR,SUBSTR,POSITION
2287
2288=item index STR,SUBSTR
2289
2b5ab1e7
TC
2290The index function searches for one string within another, but without
2291the wildcard-like behavior of a full regular-expression pattern match.
2292It returns the position of the first occurrence of SUBSTR in STR at
2293or after POSITION. If POSITION is omitted, starts searching from the
2294beginning of the string. The return value is based at C<0> (or whatever
2295you've set the C<$[> variable to--but don't do that). If the substring
2296is not found, returns one less than the base, ordinarily C<-1>.
a0d0e21e
LW
2297
2298=item int EXPR
2299
54310121 2300=item int
bbce6d69 2301
7660c0ab 2302Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2b5ab1e7
TC
2303You should not use this function for rounding: one because it truncates
2304towards C<0>, and two because machine representations of floating point
2305numbers can sometimes produce counterintuitive results. For example,
2306C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2307because it's really more like -268.99999999999994315658 instead. Usually,
19799a22 2308the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2b5ab1e7 2309functions will serve you better than will int().
a0d0e21e
LW
2310
2311=item ioctl FILEHANDLE,FUNCTION,SCALAR
2312
2b5ab1e7 2313Implements the ioctl(2) function. You'll probably first have to say
a0d0e21e 2314
4633a7c4 2315 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
a0d0e21e 2316
2b5ab1e7 2317to get the correct function definitions. If F<ioctl.ph> doesn't
a0d0e21e 2318exist or doesn't have the correct definitions you'll have to roll your
61eff3bc 2319own, based on your C header files such as F<< <sys/ioctl.h> >>.
5a964f20 2320(There is a Perl script called B<h2ph> that comes with the Perl kit that
54310121 2321may help you in this, but it's nontrivial.) SCALAR will be read and/or
4633a7c4 2322written depending on the FUNCTION--a pointer to the string value of SCALAR
19799a22 2323will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
4633a7c4
LW
2324has no string value but does have a numeric value, that value will be
2325passed rather than a pointer to the string value. To guarantee this to be
19799a22
GS
2326true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2327functions may be needed to manipulate the values of structures used by
b76cc8ba 2328C<ioctl>.
a0d0e21e 2329
19799a22 2330The return value of C<ioctl> (and C<fcntl>) is as follows:
a0d0e21e
LW
2331
2332 if OS returns: then Perl returns:
2333 -1 undefined value
2334 0 string "0 but true"
2335 anything else that number
2336
19799a22 2337Thus Perl returns true on success and false on failure, yet you can
a0d0e21e
LW
2338still easily determine the actual value returned by the operating
2339system:
2340
2b5ab1e7 2341 $retval = ioctl(...) || -1;
a0d0e21e
LW
2342 printf "System returned %d\n", $retval;
2343
be2f7487 2344The special string C<"0 but true"> is exempt from B<-w> complaints
5a964f20
TC
2345about improper numeric conversions.
2346
a0d0e21e
LW
2347=item join EXPR,LIST
2348
2b5ab1e7
TC
2349Joins the separate strings of LIST into a single string with fields
2350separated by the value of EXPR, and returns that new string. Example:
a0d0e21e 2351
2b5ab1e7 2352 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
a0d0e21e 2353
eb6e2d6f
GS
2354Beware that unlike C<split>, C<join> doesn't take a pattern as its
2355first argument. Compare L</split>.
a0d0e21e 2356
aa689395
PP
2357=item keys HASH
2358
504f80c1
JH
2359Returns a list consisting of all the keys of the named hash.
2360(In scalar context, returns the number of keys.)
2361
2362The keys are returned in an apparently random order. The actual
2363random order is subject to change in future versions of perl, but it
2364is guaranteed to be the same order as either the C<values> or C<each>
4546b9e6
JH
2365function produces (given that the hash has not been modified). Since
2366Perl 5.8.1 the ordering is different even between different runs of
2367Perl for security reasons (see L<perlsec/"Algorithmic Complexity
d6df3700 2368Attacks">).
504f80c1
JH
2369
2370As a side effect, calling keys() resets the HASH's internal iterator,
2f65b2f0
RGS
2371see L</each>. (In particular, calling keys() in void context resets
2372the iterator with no other overhead.)
a0d0e21e 2373
aa689395 2374Here is yet another way to print your environment:
a0d0e21e
LW
2375
2376 @keys = keys %ENV;
2377 @values = values %ENV;
b76cc8ba 2378 while (@keys) {
a0d0e21e
LW
2379 print pop(@keys), '=', pop(@values), "\n";
2380 }
2381
2382or how about sorted by key:
2383
2384 foreach $key (sort(keys %ENV)) {
2385 print $key, '=', $ENV{$key}, "\n";
2386 }
2387
8ea1e5d4
GS
2388The returned values are copies of the original keys in the hash, so
2389modifying them will not affect the original hash. Compare L</values>.
2390
19799a22 2391To sort a hash by value, you'll need to use a C<sort> function.
aa689395 2392Here's a descending numeric sort of a hash by its values:
4633a7c4 2393
5a964f20 2394 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
4633a7c4
LW
2395 printf "%4d %s\n", $hash{$key}, $key;
2396 }
2397
19799a22 2398As an lvalue C<keys> allows you to increase the number of hash buckets
aa689395
PP
2399allocated for the given hash. This can gain you a measure of efficiency if
2400you know the hash is going to get big. (This is similar to pre-extending
2401an array by assigning a larger number to $#array.) If you say
55497cff
PP
2402
2403 keys %hash = 200;
2404
ab192400
GS
2405then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2406in fact, since it rounds up to the next power of two. These
55497cff
PP
2407buckets will be retained even if you do C<%hash = ()>, use C<undef
2408%hash> if you want to free the storage while C<%hash> is still in scope.
2409You can't shrink the number of buckets allocated for the hash using
19799a22 2410C<keys> in this way (but you needn't worry about doing this by accident,
55497cff
PP
2411as trying has no effect).
2412
19799a22 2413See also C<each>, C<values> and C<sort>.
ab192400 2414
b350dd2f 2415=item kill SIGNAL, LIST
a0d0e21e 2416
b350dd2f 2417Sends a signal to a list of processes. Returns the number of
517db077
GS
2418processes successfully signaled (which is not necessarily the
2419same as the number actually killed).
a0d0e21e
LW
2420
2421 $cnt = kill 1, $child1, $child2;
2422 kill 9, @goners;
2423
b350dd2f 2424If SIGNAL is zero, no signal is sent to the process. This is a
1e9c1022 2425useful way to check that a child process is alive and hasn't changed
b350dd2f
GS
2426its UID. See L<perlport> for notes on the portability of this
2427construct.
2428
2429Unlike in the shell, if SIGNAL is negative, it kills
4633a7c4
LW
2430process groups instead of processes. (On System V, a negative I<PROCESS>
2431number will also kill process groups, but that's not portable.) That
2432means you usually want to use positive not negative signals. You may also
1e9c1022
JL
2433use a signal name in quotes.
2434
2435See L<perlipc/"Signals"> for more details.
a0d0e21e
LW
2436
2437=item last LABEL
2438
2439=item last
2440
2441The C<last> command is like the C<break> statement in C (as used in
2442loops); it immediately exits the loop in question. If the LABEL is
2443omitted, the command refers to the innermost enclosing loop. The
2444C<continue> block, if any, is not executed:
2445
4633a7c4
LW
2446 LINE: while (<STDIN>) {
2447 last LINE if /^$/; # exit when done with header
5a964f20 2448 #...
a0d0e21e
LW
2449 }
2450
4968c1e4 2451C<last> cannot be used to exit a block which returns a value such as
2b5ab1e7
TC
2452C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2453a grep() or map() operation.
4968c1e4 2454
6c1372ed
GS
2455Note that a block by itself is semantically identical to a loop
2456that executes once. Thus C<last> can be used to effect an early
2457exit out of such a block.
2458
98293880
JH
2459See also L</continue> for an illustration of how C<last>, C<next>, and
2460C<redo> work.
1d2dff63 2461
a0d0e21e
LW
2462=item lc EXPR
2463
54310121 2464=item lc
bbce6d69 2465
d1be9408 2466Returns a lowercased version of EXPR. This is the internal function
ad0029c4
JH
2467implementing the C<\L> escape in double-quoted strings. Respects
2468current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
983ffd37 2469and L<perlunicode> for more details about locale and Unicode support.
a0d0e21e 2470
7660c0ab 2471If EXPR is omitted, uses C<$_>.
bbce6d69 2472
a0d0e21e
LW
2473=item lcfirst EXPR
2474
54310121 2475=item lcfirst
bbce6d69 2476
ad0029c4
JH
2477Returns the value of EXPR with the first character lowercased. This
2478is the internal function implementing the C<\l> escape in
2479double-quoted strings. Respects current LC_CTYPE locale if C<use
983ffd37
JH
2480locale> in force. See L<perllocale> and L<perlunicode> for more
2481details about locale and Unicode support.
a0d0e21e 2482
7660c0ab 2483If EXPR is omitted, uses C<$_>.
bbce6d69 2484
a0d0e21e
LW
2485=item length EXPR
2486
54310121 2487=item length
bbce6d69 2488
974da8e5 2489Returns the length in I<characters> of the value of EXPR. If EXPR is
b76cc8ba 2490omitted, returns length of C<$_>. Note that this cannot be used on
2b5ab1e7
TC
2491an entire array or hash to find out how many elements these have.
2492For that, use C<scalar @array> and C<scalar keys %hash> respectively.
a0d0e21e 2493
974da8e5
JH
2494Note the I<characters>: if the EXPR is in Unicode, you will get the
2495number of characters, not the number of bytes. To get the length
2496in bytes, use C<do { use bytes; length(EXPR) }>, see L<bytes>.
2497
a0d0e21e
LW
2498=item link OLDFILE,NEWFILE
2499
19799a22 2500Creates a new filename linked to the old filename. Returns true for
b76cc8ba 2501success, false otherwise.
a0d0e21e
LW
2502
2503=item listen SOCKET,QUEUESIZE
2504
19799a22 2505Does the same thing that the listen system call does. Returns true if
b76cc8ba 2506it succeeded, false otherwise. See the example in
cea6626f 2507L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
2508
2509=item local EXPR
2510
19799a22 2511You really probably want to be using C<my> instead, because C<local> isn't
b76cc8ba 2512what most people think of as "local". See
13a2d996 2513L<perlsub/"Private Variables via my()"> for details.
2b5ab1e7 2514
5a964f20
TC
2515A local modifies the listed variables to be local to the enclosing
2516block, file, or eval. If more than one value is listed, the list must
2517be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2518for details, including issues with tied arrays and hashes.
a0d0e21e 2519
a0d0e21e
LW
2520=item localtime EXPR
2521
19799a22 2522Converts a time as returned by the time function to a 9-element list
5f05dabc 2523with the time analyzed for the local time zone. Typically used as
a0d0e21e
LW
2524follows:
2525
54310121 2526 # 0 1 2 3 4 5 6 7 8
a0d0e21e
LW
2527 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2528 localtime(time);
2529
48a26b3a
GS
2530All list elements are numeric, and come straight out of the C `struct
2531tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2532specified time. $mday is the day of the month, and $mon is the month
2533itself, in the range C<0..11> with 0 indicating January and 11
2534indicating December. $year is the number of years since 1900. That
2535is, $year is C<123> in year 2023. $wday is the day of the week, with
25360 indicating Sunday and 3 indicating Wednesday. $yday is the day of
874b1813 2537the year, in the range C<0..364> (or C<0..365> in leap years.) $isdst
48a26b3a
GS
2538is true if the specified time occurs during daylight savings time,
2539false otherwise.
2540
2541Note that the $year element is I<not> simply the last two digits of
2542the year. If you assume it is, then you create non-Y2K-compliant
2543programs--and you wouldn't want to do that, would you?
54310121 2544
abd75f24
GS
2545The proper way to get a complete 4-digit year is simply:
2546
2547 $year += 1900;
2548
2549And to get the last two digits of the year (e.g., '01' in 2001) do:
2550
2551 $year = sprintf("%02d", $year % 100);
2552
48a26b3a 2553If EXPR is omitted, C<localtime()> uses the current time (C<localtime(time)>).
a0d0e21e 2554
48a26b3a 2555In scalar context, C<localtime()> returns the ctime(3) value:
a0d0e21e 2556
5f05dabc 2557 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
a0d0e21e 2558
fe86afc2
NC
2559This scalar value is B<not> locale dependent but is a Perl builtin. For GMT
2560instead of local time use the L</gmtime> builtin. See also the
2561C<Time::Local> module (to convert the second, minutes, hours, ... back to
2562the integer value returned by time()), and the L<POSIX> module's strftime(3)
2563and mktime(3) functions.
2564
2565To get somewhat similar but locale dependent date strings, set up your
2566locale environment variables appropriately (please see L<perllocale>) and
2567try for example:
a3cb178b 2568
5a964f20 2569 use POSIX qw(strftime);
2b5ab1e7 2570 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
fe86afc2
NC
2571 # or for GMT formatted appropriately for your locale:
2572 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
a3cb178b
GS
2573
2574Note that the C<%a> and C<%b>, the short forms of the day of the week
2575and the month of the year, may not necessarily be three characters wide.
a0d0e21e 2576
07698885 2577=item lock THING
19799a22 2578
01e6739c 2579This function places an advisory lock on a shared variable, or referenced
03730085 2580object contained in I<THING> until the lock goes out of scope.
a6d5524e 2581
f3a23afb 2582lock() is a "weak keyword" : this means that if you've defined a function
67408cae 2583by this name (before any calls to it), that function will be called
03730085
AB
2584instead. (However, if you've said C<use threads>, lock() is always a
2585keyword.) See L<threads>.
19799a22 2586
a0d0e21e
LW
2587=item log EXPR
2588
54310121 2589=item log
bbce6d69 2590
2b5ab1e7
TC
2591Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2592returns log of C<$_>. To get the log of another base, use basic algebra:
19799a22 2593The base-N log of a number is equal to the natural log of that number
2b5ab1e7
TC
2594divided by the natural log of N. For example:
2595
2596 sub log10 {
2597 my $n = shift;
2598 return log($n)/log(10);
b76cc8ba 2599 }
2b5ab1e7
TC
2600
2601See also L</exp> for the inverse operation.
a0d0e21e 2602
a0d0e21e
LW
2603=item lstat EXPR
2604
54310121 2605=item lstat
bbce6d69 2606
19799a22 2607Does the same thing as the C<stat> function (including setting the
5a964f20
TC
2608special C<_> filehandle) but stats a symbolic link instead of the file
2609the symbolic link points to. If symbolic links are unimplemented on
c837d5b4
DP
2610your system, a normal C<stat> is done. For much more detailed
2611information, please see the documentation for C<stat>.
a0d0e21e 2612
7660c0ab 2613If EXPR is omitted, stats C<$_>.
bbce6d69 2614
a0d0e21e
LW
2615=item m//
2616
2617The match operator. See L<perlop>.
2618
2619=item map BLOCK LIST
2620
2621=item map EXPR,LIST
2622
19799a22
GS
2623Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2624C<$_> to each element) and returns the list value composed of the
2625results of each such evaluation. In scalar context, returns the
2626total number of elements so generated. Evaluates BLOCK or EXPR in
2627list context, so each element of LIST may produce zero, one, or
2628more elements in the returned value.
dd99ebda 2629
a0d0e21e
LW
2630 @chars = map(chr, @nums);
2631
2632translates a list of numbers to the corresponding characters. And
2633
4633a7c4 2634 %hash = map { getkey($_) => $_ } @array;
a0d0e21e
LW
2635
2636is just a funny way to write
2637
2638 %hash = ();
2639 foreach $_ (@array) {
4633a7c4 2640 $hash{getkey($_)} = $_;
a0d0e21e
LW
2641 }
2642
be3174d2
GS
2643Note that C<$_> is an alias to the list value, so it can be used to
2644modify the elements of the LIST. While this is useful and supported,
2645it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
2646Using a regular C<foreach> loop for this purpose would be clearer in
2647most cases. See also L</grep> for an array composed of those items of
2648the original list for which the BLOCK or EXPR evaluates to true.
fb73857a 2649
a4fb8298
RGS
2650If C<$_> is lexical in the scope where the C<map> appears (because it has
2651been declared with C<my $_>) then, in addition the be locally aliased to
2652the list elements, C<$_> keeps being lexical inside the block; i.e. it
2653can't be seen from the outside, avoiding any potential side-effects.
2654
205fdb4d
NC
2655C<{> starts both hash references and blocks, so C<map { ...> could be either
2656the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look
2657ahead for the closing C<}> it has to take a guess at which its dealing with
2658based what it finds just after the C<{>. Usually it gets it right, but if it
2659doesn't it won't realize something is wrong until it gets to the C<}> and
2660encounters the missing (or unexpected) comma. The syntax error will be
2661reported close to the C<}> but you'll need to change something near the C<{>
2662such as using a unary C<+> to give perl some help:
2663
2664 %hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong
2665 %hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right
2666 %hash = map { ("\L$_", 1) } @array # this also works
2667 %hash = map { lc($_), 1 } @array # as does this.
2668 %hash = map +( lc($_), 1 ), @array # this is EXPR and works!
cea6626f 2669
205fdb4d
NC
2670 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
2671
2672or to force an anon hash constructor use C<+{>
2673
2674 @hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end
2675
2676and you get list of anonymous hashes each with only 1 entry.
2677
19799a22 2678=item mkdir FILENAME,MASK
a0d0e21e 2679
5a211162
GS
2680=item mkdir FILENAME
2681
0591cd52 2682Creates the directory specified by FILENAME, with permissions
19799a22
GS
2683specified by MASK (as modified by C<umask>). If it succeeds it
2684returns true, otherwise it returns false and sets C<$!> (errno).
5a211162 2685If omitted, MASK defaults to 0777.
0591cd52 2686
19799a22 2687In general, it is better to create directories with permissive MASK,
0591cd52 2688and let the user modify that with their C<umask>, than it is to supply
19799a22 2689a restrictive MASK and give the user no way to be more permissive.
0591cd52
NT
2690The exceptions to this rule are when the file or directory should be
2691kept private (mail files, for instance). The perlfunc(1) entry on
19799a22 2692C<umask> discusses the choice of MASK in more detail.
a0d0e21e 2693
cc1852e8
JH
2694Note that according to the POSIX 1003.1-1996 the FILENAME may have any
2695number of trailing slashes. Some operating and filesystems do not get
2696this right, so Perl automatically removes all trailing slashes to keep
2697everyone happy.
2698
a0d0e21e
LW
2699=item msgctl ID,CMD,ARG
2700
f86cebdf 2701Calls the System V IPC function msgctl(2). You'll probably have to say
0ade1984
JH
2702
2703 use IPC::SysV;
2704
7660c0ab
A
2705first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2706then ARG must be a variable which will hold the returned C<msqid_ds>
951ba7fe
GS
2707structure. Returns like C<ioctl>: the undefined value for error,
2708C<"0 but true"> for zero, or the actual return value otherwise. See also
4755096e 2709L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation.
a0d0e21e
LW
2710
2711=item msgget KEY,FLAGS
2712
f86cebdf 2713Calls the System V IPC function msgget(2). Returns the message queue
4755096e
GS
2714id, or the undefined value if there is an error. See also
2715L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation.
a0d0e21e 2716
a0d0e21e
LW
2717=item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2718
2719Calls the System V IPC function msgrcv to receive a message from
2720message queue ID into variable VAR with a maximum message size of
41d6edb2
JH
2721SIZE. Note that when a message is received, the message type as a
2722native long integer will be the first thing in VAR, followed by the
2723actual message. This packing may be opened with C<unpack("l! a*")>.
2724Taints the variable. Returns true if successful, or false if there is
4755096e
GS
2725an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and
2726C<IPC::SysV::Msg> documentation.
41d6edb2
JH
2727
2728=item msgsnd ID,MSG,FLAGS
2729
2730Calls the System V IPC function msgsnd to send the message MSG to the
2731message queue ID. MSG must begin with the native long integer message
2732type, and be followed by the length of the actual message, and finally
2733the message itself. This kind of packing can be achieved with
2734C<pack("l! a*", $type, $message)>. Returns true if successful,
2735or false if there is an error. See also C<IPC::SysV>
2736and C<IPC::SysV::Msg> documentation.
a0d0e21e
LW
2737
2738=item my EXPR
2739
307ea6df
JH
2740=item my TYPE EXPR
2741
1d2de774 2742=item my EXPR : ATTRS
09bef843 2743
1d2de774 2744=item my TYPE EXPR : ATTRS
307ea6df 2745
19799a22 2746A C<my> declares the listed variables to be local (lexically) to the
1d2de774
JH
2747enclosing block, file, or C<eval>. If more than one value is listed,
2748the list must be placed in parentheses.
307ea6df 2749
1d2de774
JH
2750The exact semantics and interface of TYPE and ATTRS are still
2751evolving. TYPE is currently bound to the use of C<fields> pragma,
307ea6df
JH
2752and attributes are handled using the C<attributes> pragma, or starting
2753from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
2754L<perlsub/"Private Variables via my()"> for details, and L<fields>,
2755L<attributes>, and L<Attribute::Handlers>.
4633a7c4 2756
a0d0e21e
LW
2757=item next LABEL
2758
2759=item next
2760
2761The C<next> command is like the C<continue> statement in C; it starts
2762the next iteration of the loop:
2763
4633a7c4
LW
2764 LINE: while (<STDIN>) {
2765 next LINE if /^#/; # discard comments
5a964f20 2766 #...
a0d0e21e
LW
2767 }
2768
2769Note that if there were a C<continue> block on the above, it would get
2770executed even on discarded lines. If the LABEL is omitted, the command
2771refers to the innermost enclosing loop.
2772
4968c1e4 2773C<next> cannot be used to exit a block which returns a value such as
2b5ab1e7
TC
2774C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2775a grep() or map() operation.
4968c1e4 2776
6c1372ed
GS
2777Note that a block by itself is semantically identical to a loop
2778that executes once. Thus C<next> will exit such a block early.
2779
98293880
JH
2780See also L</continue> for an illustration of how C<last>, C<next>, and
2781C<redo> work.
1d2dff63 2782
4a66ea5a
RGS
2783=item no Module VERSION LIST
2784
2785=item no Module VERSION
2786
a0d0e21e
LW
2787=item no Module LIST
2788
4a66ea5a
RGS
2789=item no Module
2790
593b9c14 2791See the C<use> function, of which C<no> is the opposite.
a0d0e21e
LW
2792
2793=item oct EXPR
2794
54310121 2795=item oct
bbce6d69 2796
4633a7c4 2797Interprets EXPR as an octal string and returns the corresponding
4f19785b
WSI
2798value. (If EXPR happens to start off with C<0x>, interprets it as a
2799hex string. If EXPR starts off with C<0b>, it is interpreted as a
53305cf1
NC
2800binary string. Leading whitespace is ignored in all three cases.)
2801The following will handle decimal, binary, octal, and hex in the standard
2802Perl or C notation:
a0d0e21e
LW
2803
2804 $val = oct($val) if $val =~ /^0/;
2805
19799a22
GS
2806If EXPR is omitted, uses C<$_>. To go the other way (produce a number
2807in octal), use sprintf() or printf():
2808
2809 $perms = (stat("filename"))[2] & 07777;
2810 $oct_perms = sprintf "%lo", $perms;
2811
2812The oct() function is commonly used when a string such as C<644> needs
2813to be converted into a file mode, for example. (Although perl will
2814automatically convert strings into numbers as needed, this automatic
2815conversion assumes base 10.)
a0d0e21e
LW
2816
2817=item open FILEHANDLE,EXPR
2818
68bd7414
NIS
2819=item open FILEHANDLE,MODE,EXPR
2820
2821=item open FILEHANDLE,MODE,EXPR,LIST
2822
ba964c95
T
2823=item open FILEHANDLE,MODE,REFERENCE
2824
a0d0e21e
LW
2825=item open FILEHANDLE
2826
2827Opens the file whose filename is given by EXPR, and associates it with
ed53a2bb
JH
2828FILEHANDLE.
2829
2830(The following is a comprehensive reference to open(): for a gentler
2831introduction you may consider L<perlopentut>.)
2832
a28cd5c9
NT
2833If FILEHANDLE is an undefined scalar variable (or array or hash element)
2834the variable is assigned a reference to a new anonymous filehandle,
2835otherwise if FILEHANDLE is an expression, its value is used as the name of
2836the real filehandle wanted. (This is considered a symbolic reference, so
2837C<use strict 'refs'> should I<not> be in effect.)
ed53a2bb
JH
2838
2839If EXPR is omitted, the scalar variable of the same name as the
2840FILEHANDLE contains the filename. (Note that lexical variables--those
2841declared with C<my>--will not work for this purpose; so if you're
67408cae 2842using C<my>, specify EXPR in your call to open.)
ed53a2bb
JH
2843
2844If three or more arguments are specified then the mode of opening and
2845the file name are separate. If MODE is C<< '<' >> or nothing, the file
2846is opened for input. If MODE is C<< '>' >>, the file is truncated and
2847opened for output, being created if necessary. If MODE is C<<< '>>' >>>,
b76cc8ba 2848the file is opened for appending, again being created if necessary.
5a964f20 2849
ed53a2bb
JH
2850You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to
2851indicate that you want both read and write access to the file; thus
2852C<< '+<' >> is almost always preferred for read/write updates--the C<<
2853'+>' >> mode would clobber the file first. You can't usually use
2854either read-write mode for updating textfiles, since they have
2855variable length records. See the B<-i> switch in L<perlrun> for a
2856better approach. The file is created with permissions of C<0666>
2857modified by the process' C<umask> value.
2858
2859These various prefixes correspond to the fopen(3) modes of C<'r'>,
2860C<'r+'>, C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
5f05dabc 2861
6170680b
IZ
2862In the 2-arguments (and 1-argument) form of the call the mode and
2863filename should be concatenated (in this order), possibly separated by
68bd7414
NIS
2864spaces. It is possible to omit the mode in these forms if the mode is
2865C<< '<' >>.
6170680b 2866
7660c0ab 2867If the filename begins with C<'|'>, the filename is interpreted as a
5a964f20 2868command to which output is to be piped, and if the filename ends with a
f244e06d
GS
2869C<'|'>, the filename is interpreted as a command which pipes output to
2870us. See L<perlipc/"Using open() for IPC">
19799a22 2871for more examples of this. (You are not allowed to C<open> to a command
5a964f20 2872that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
4a4eefd0
GS
2873and L<perlipc/"Bidirectional Communication with Another Process">
2874for alternatives.)
cb1a09d0 2875
ed53a2bb
JH
2876For three or more arguments if MODE is C<'|-'>, the filename is
2877interpreted as a command to which output is to be piped, and if MODE
2878is C<'-|'>, the filename is interpreted as a command which pipes
2879output to us. In the 2-arguments (and 1-argument) form one should
2880replace dash (C<'-'>) with the command.
2881See L<perlipc/"Using open() for IPC"> for more examples of this.
2882(You are not allowed to C<open> to a command that pipes both in I<and>
2883out, but see L<IPC::Open2>, L<IPC::Open3>, and
2884L<perlipc/"Bidirectional Communication"> for alternatives.)
2885
2886In the three-or-more argument form of pipe opens, if LIST is specified
2887(extra arguments after the command name) then LIST becomes arguments
2888to the command invoked if the platform supports it. The meaning of
2889C<open> with more than three arguments for non-pipe modes is not yet
2890specified. Experimental "layers" may give extra LIST arguments
2891meaning.
6170680b
IZ
2892
2893In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
b76cc8ba 2894and opening C<< '>-' >> opens STDOUT.
6170680b 2895
fae2c0fb
RGS
2896You may use the three-argument form of open to specify IO "layers"
2897(sometimes also referred to as "disciplines") to be applied to the handle
2898that affect how the input and output are processed (see L<open> and
2899L<PerlIO> for more details). For example
7207e29d 2900
9124316e
JH
2901 open(FH, "<:utf8", "file")
2902
2903will open the UTF-8 encoded file containing Unicode characters,
fae2c0fb
RGS
2904see L<perluniintro>. (Note that if layers are specified in the
2905three-arg form then default layers set by the C<open> pragma are
01e6739c 2906ignored.)
ed53a2bb
JH
2907
2908Open returns nonzero upon success, the undefined value otherwise. If
2909the C<open> involved a pipe, the return value happens to be the pid of
2910the subprocess.
cb1a09d0 2911
ed53a2bb
JH
2912If you're running Perl on a system that distinguishes between text
2913files and binary files, then you should check out L</binmode> for tips
2914for dealing with this. The key distinction between systems that need
2915C<binmode> and those that don't is their text file formats. Systems
8939ba94 2916like Unix, Mac OS, and Plan 9, which delimit lines with a single
ed53a2bb
JH
2917character, and which encode that character in C as C<"\n">, do not
2918need C<binmode>. The rest need it.
cb1a09d0 2919
fb73857a 2920When opening a file, it's usually a bad idea to continue normal execution
19799a22
GS
2921if the request failed, so C<open> is frequently used in connection with
2922C<die>. Even if C<die> won't do what you want (say, in a CGI script,
fb73857a 2923where you want to make a nicely formatted error message (but there are
5a964f20 2924modules that can help with that problem)) you should always check
19799a22 2925the return value from opening a file. The infrequent exception is when
fb73857a
PP
2926working with an unopened filehandle is actually what you want to do.
2927
ed53a2bb
JH
2928As a special case the 3 arg form with a read/write mode and the third
2929argument being C<undef>:
b76cc8ba
NIS
2930
2931 open(TMP, "+>", undef) or die ...
2932
f253e835
JH
2933opens a filehandle to an anonymous temporary file. Also using "+<"
2934works for symmetry, but you really should consider writing something
2935to the temporary file first. You will need to seek() to do the
2936reading.
b76cc8ba 2937
2ce64696
JC
2938Since v5.8.0, perl has built using PerlIO by default. Unless you've
2939changed this (ie Configure -Uuseperlio), you can open file handles to
2940"in memory" files held in Perl scalars via:
ba964c95 2941
b996200f
SB
2942 open($fh, '>', \$variable) || ..
2943
2944Though if you try to re-open C<STDOUT> or C<STDERR> as an "in memory"
2945file, you have to close it first:
2946
2947 close STDOUT;
2948 open STDOUT, '>', \$variable or die "Can't open STDOUT: $!";
ba964c95 2949
cb1a09d0 2950Examples:
a0d0e21e
LW
2951
2952 $ARTICLE = 100;
2953 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2954 while (<ARTICLE>) {...
2955
6170680b 2956 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
fb73857a 2957 # if the open fails, output is discarded
a0d0e21e 2958
6170680b 2959 open(DBASE, '+<', 'dbase.mine') # open for update
fb73857a 2960 or die "Can't open 'dbase.mine' for update: $!";
cb1a09d0 2961
6170680b
IZ
2962 open(DBASE, '+<dbase.mine') # ditto
2963 or die "Can't open 'dbase.mine' for update: $!";
2964
2965 open(ARTICLE, '-|', "caesar <$article") # decrypt article
fb73857a 2966 or die "Can't start caesar: $!";
a0d0e21e 2967
6170680b
IZ
2968 open(ARTICLE, "caesar <$article |") # ditto
2969 or die "Can't start caesar: $!";
2970
2359510d 2971 open(EXTRACT, "|sort >Tmp$$") # $$ is our process id
fb73857a 2972 or die "Can't start sort: $!";
a0d0e21e 2973
ba964c95
T
2974 # in memory files
2975 open(MEMORY,'>', \$var)
2976 or die "Can't open memory file: $!";
2977 print MEMORY "foo!\n"; # output will end up in $var
2978
a0d0e21e
LW
2979 # process argument list of files along with any includes
2980
2981 foreach $file (@ARGV) {
2982 process($file, 'fh00');
2983 }
2984
2985 sub process {
5a964f20 2986 my($filename, $input) = @_;
a0d0e21e
LW
2987 $input++; # this is a string increment
2988 unless (open($input, $filename)) {
2989 print STDERR "Can't open $filename: $!\n";
2990 return;
2991 }
2992
5a964f20 2993 local $_;
a0d0e21e
LW
2994 while (<$input>) { # note use of indirection
2995 if (/^#include "(.*)"/) {
2996 process($1, $input);
2997 next;
2998 }
5a964f20 2999 #... # whatever
a0d0e21e
LW
3000 }
3001 }
3002
2ce64696
JC
3003See L<perliol/> for detailed info on PerlIO.
3004
a0d0e21e 3005You may also, in the Bourne shell tradition, specify an EXPR beginning
00cafafa
JH
3006with C<< '>&' >>, in which case the rest of the string is interpreted
3007as the name of a filehandle (or file descriptor, if numeric) to be
3008duped (as L<dup(2)>) and opened. You may use C<&> after C<< > >>,
3009C<<< >> >>>, C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>.
3010The mode you specify should match the mode of the original filehandle.
3011(Duping a filehandle does not take into account any existing contents
3012of IO buffers.) If you use the 3 arg form then you can pass either a
3013number, the name of a filehandle or the normal "reference to a glob".
6170680b 3014
eae1b76b
SB
3015Here is a script that saves, redirects, and restores C<STDOUT> and
3016C<STDERR> using various methods:
a0d0e21e
LW
3017
3018 #!/usr/bin/perl
eae1b76b
SB
3019 open my $oldout, ">&STDOUT" or die "Can't dup STDOUT: $!";
3020 open OLDERR, ">&", \*STDERR or die "Can't dup STDERR: $!";
818c4caa 3021
eae1b76b
SB
3022 open STDOUT, '>', "foo.out" or die "Can't redirect STDOUT: $!";
3023 open STDERR, ">&STDOUT" or die "Can't dup STDOUT: $!";
a0d0e21e 3024
eae1b76b
SB
3025 select STDERR; $| = 1; # make unbuffered
3026 select STDOUT; $| = 1; # make unbuffered
a0d0e21e
LW
3027
3028 print STDOUT "stdout 1\n"; # this works for
3029 print STDERR "stderr 1\n"; # subprocesses too
3030
eae1b76b
SB
3031 open STDOUT, ">&", $oldout or die "Can't dup \$oldout: $!";
3032 open STDERR, ">&OLDERR" or die "Can't dup OLDERR: $!";
a0d0e21e
LW
3033
3034 print STDOUT "stdout 2\n";
3035 print STDERR "stderr 2\n";
3036
ef8b303f
JH
3037If you specify C<< '<&=X' >>, where C<X> is a file descriptor number
3038or a filehandle, then Perl will do an equivalent of C's C<fdopen> of
3039that file descriptor (and not call L<dup(2)>); this is more
3040parsimonious of file descriptors. For example:
a0d0e21e 3041
00cafafa 3042 # open for input, reusing the fileno of $fd
a0d0e21e 3043 open(FILEHANDLE, "<&=$fd")
df632fdf 3044
b76cc8ba 3045or
df632fdf 3046
b76cc8ba 3047 open(FILEHANDLE, "<&=", $fd)
a0d0e21e 3048
00cafafa
JH
3049or
3050
3051 # open for append, using the fileno of OLDFH
3052 open(FH, ">>&=", OLDFH)
3053
3054or
3055
3056 open(FH, ">>&=OLDFH")
3057
ef8b303f
JH
3058Being parsimonious on filehandles is also useful (besides being
3059parsimonious) for example when something is dependent on file
3060descriptors, like for example locking using flock(). If you do just
3061C<< open(A, '>>&B') >>, the filehandle A will not have the same file
3062descriptor as B, and therefore flock(A) will not flock(B), and vice
3063versa. But with C<< open(A, '>>&=B') >> the filehandles will share
3064the same file descriptor.
3065
3066Note that if you are using Perls older than 5.8.0, Perl will be using
3067the standard C libraries' fdopen() to implement the "=" functionality.
3068On many UNIX systems fdopen() fails when file descriptors exceed a
3069certain value, typically 255. For Perls 5.8.0 and later, PerlIO is
3070most often the default.
4af147f6 3071
df632fdf
JH
3072You can see whether Perl has been compiled with PerlIO or not by
3073running C<perl -V> and looking for C<useperlio=> line. If C<useperlio>
3074is C<define>, you have PerlIO, otherwise you don't.
3075
6170680b
IZ
3076If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
3077with 2-arguments (or 1-argument) form of open(), then
a0d0e21e 3078there is an implicit fork done, and the return value of open is the pid
7660c0ab 3079of the child within the parent process, and C<0> within the child
184e9718 3080process. (Use C<defined($pid)> to determine whether the open was successful.)
a0d0e21e
LW
3081The filehandle behaves normally for the parent, but i/o to that
3082filehandle is piped from/to the STDOUT/STDIN of the child process.
3083In the child process the filehandle isn't opened--i/o happens from/to
3084the new STDOUT or STDIN. Typically this is used like the normal
3085piped open when you want to exercise more control over just how the
3086pipe command gets executed, such as when you are running setuid, and
54310121 3087don't want to have to scan shell commands for metacharacters.
6170680b 3088The following triples are more or less equivalent:
a0d0e21e
LW
3089
3090 open(FOO, "|tr '[a-z]' '[A-Z]'");
6170680b
IZ
3091 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
3092 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
b76cc8ba 3093 open(FOO, '|-', "tr", '[a-z]', '[A-Z]');
a0d0e21e
LW
3094
3095 open(FOO, "cat -n '$file'|");
6170680b
IZ
3096 open(FOO, '-|', "cat -n '$file'");
3097 open(FOO, '-|') || exec 'cat', '-n', $file;
b76cc8ba
NIS
3098 open(FOO, '-|', "cat", '-n', $file);
3099
3100The last example in each block shows the pipe as "list form", which is
64da03b2
JH
3101not yet supported on all platforms. A good rule of thumb is that if
3102your platform has true C<fork()> (in other words, if your platform is
3103UNIX) you can use the list form.
a0d0e21e 3104
4633a7c4
LW
3105See L<perlipc/"Safe Pipe Opens"> for more examples of this.
3106
0f897271
GS
3107Beginning with v5.6.0, Perl will attempt to flush all files opened for
3108output before any operation that may do a fork, but this may not be
3109supported on some platforms (see L<perlport>). To be safe, you may need
3110to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
3111of C<IO::Handle> on any open handles.
3112
ed53a2bb
JH
3113On systems that support a close-on-exec flag on files, the flag will
3114be set for the newly opened file descriptor as determined by the value
3115of $^F. See L<perlvar/$^F>.
a0d0e21e 3116
0dccf244
CS
3117Closing any piped filehandle causes the parent process to wait for the
3118child to finish, and returns the status value in C<$?>.
3119
ed53a2bb
JH
3120The filename passed to 2-argument (or 1-argument) form of open() will
3121have leading and trailing whitespace deleted, and the normal
3122redirection characters honored. This property, known as "magic open",
5a964f20 3123can often be used to good effect. A user could specify a filename of
7660c0ab 3124F<"rsh cat file |">, or you could change certain filenames as needed:
5a964f20
TC
3125
3126 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
3127 open(FH, $filename) or die "Can't open $filename: $!";
3128
6170680b
IZ
3129Use 3-argument form to open a file with arbitrary weird characters in it,
3130
3131 open(FOO, '<', $file);
3132
3133otherwise it's necessary to protect any leading and trailing whitespace:
5a964f20
TC
3134
3135 $file =~ s#^(\s)#./$1#;
3136 open(FOO, "< $file\0");
3137
a31a806a 3138(this may not work on some bizarre filesystems). One should
106325ad 3139conscientiously choose between the I<magic> and 3-arguments form
6170680b
IZ
3140of open():
3141
3142 open IN, $ARGV[0];
3143
3144will allow the user to specify an argument of the form C<"rsh cat file |">,
3145but will not work on a filename which happens to have a trailing space, while
3146
3147 open IN, '<', $ARGV[0];
3148
3149will have exactly the opposite restrictions.
3150
19799a22 3151If you want a "real" C C<open> (see L<open(2)> on your system), then you
6170680b
IZ
3152should use the C<sysopen> function, which involves no such magic (but
3153may use subtly different filemodes than Perl open(), which is mapped
3154to C fopen()). This is
5a964f20
TC
3155another way to protect your filenames from interpretation. For example:
3156
3157 use IO::Handle;
3158 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
3159 or die "sysopen $path: $!";
3160 $oldfh = select(HANDLE); $| = 1; select($oldfh);
38762f02 3161 print HANDLE "stuff $$\n";
5a964f20
TC
3162 seek(HANDLE, 0, 0);
3163 print "File contains: ", <HANDLE>;
3164
7660c0ab
A
3165Using the constructor from the C<IO::Handle> package (or one of its
3166subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
5a964f20
TC
3167filehandles that have the scope of whatever variables hold references to
3168them, and automatically close whenever and however you leave that scope:
c07a80fd 3169
5f05dabc 3170 use IO::File;
5a964f20 3171 #...
c07a80fd
PP
3172 sub read_myfile_munged {
3173 my $ALL = shift;
5f05dabc 3174 my $handle = new IO::File;
c07a80fd
PP
3175 open($handle, "myfile") or die "myfile: $!";
3176 $first = <$handle>
3177 or return (); # Automatically closed here.
3178 mung $first or die "mung failed"; # Or here.
3179 return $first, <$handle> if $ALL; # Or here.
3180 $first; # Or here.
3181 }
3182
b687b08b 3183See L</seek> for some details about mixing reading and writing.
a0d0e21e
LW
3184
3185=item opendir DIRHANDLE,EXPR
3186
19799a22
GS
3187Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
3188C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
a28cd5c9
NT
3189DIRHANDLE may be an expression whose value can be used as an indirect
3190dirhandle, usually the real dirhandle name. If DIRHANDLE is an undefined
3191scalar variable (or array or hash element), the variable is assigned a
3192reference to a new anonymous dirhandle.
a0d0e21e
LW
3193DIRHANDLEs have their own namespace separate from FILEHANDLEs.
3194
3195=item ord EXPR
3196
54310121 3197=item ord
bbce6d69 3198
121910a4
JH
3199Returns the numeric (the native 8-bit encoding, like ASCII or EBCDIC,
3200or Unicode) value of the first character of EXPR. If EXPR is omitted,
3201uses C<$_>.
3202
3203For the reverse, see L</chr>.
3204See L<perlunicode> and L<encoding> for more about Unicode.
a0d0e21e 3205
77ca0c92
LW
3206=item our EXPR
3207
307ea6df
JH
3208=item our EXPR TYPE
3209
1d2de774 3210=item our EXPR : ATTRS
9969eac4 3211
1d2de774 3212=item our TYPE EXPR : ATTRS
307ea6df 3213
77ca0c92
LW
3214An C<our> declares the listed variables to be valid globals within
3215the enclosing block, file, or C<eval>. That is, it has the same
3216scoping rules as a "my" declaration, but does not create a local
3217variable. If more than one value is listed, the list must be placed
3218in parentheses. The C<our> declaration has no semantic effect unless
3219"use strict vars" is in effect, in which case it lets you use the
3220declared global variable without qualifying it with a package name.
3221(But only within the lexical scope of the C<our> declaration. In this
3222it differs from "use vars", which is package scoped.)
3223
f472eb5c
GS
3224An C<our> declaration declares a global variable that will be visible
3225across its entire lexical scope, even across package boundaries. The
3226package in which the variable is entered is determined at the point
3227of the declaration, not at the point of use. This means the following
3228behavior holds:
3229
3230 package Foo;
3231 our $bar; # declares $Foo::bar for rest of lexical scope
3232 $bar = 20;
3233
3234 package Bar;
3235 print $bar; # prints 20
3236
3237Multiple C<our> declarations in the same lexical scope are allowed
3238if they are in different packages. If they happened to be in the same
3239package, Perl will emit warnings if you have asked for them.
3240
3241 use warnings;
3242 package Foo;
3243 our $bar; # declares $Foo::bar for rest of lexical scope
3244 $bar = 20;
3245
3246 package Bar;
3247 our $bar = 30; # declares $Bar::bar for rest of lexical scope
3248 print $bar; # prints 30
3249
3250 our $bar; # emits warning
3251
9969eac4 3252An C<our> declaration may also have a list of attributes associated
307ea6df
JH
3253with it.
3254
1d2de774
JH
3255The exact semantics and interface of TYPE and ATTRS are still
3256evolving. TYPE is currently bound to the use of C<fields> pragma,
307ea6df
JH
3257and attributes are handled using the C<attributes> pragma, or starting
3258from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
3259L<perlsub/"Private Variables via my()"> for details, and L<fields>,
3260L<attributes>, and L<Attribute::Handlers>.
3261
3262The only currently recognized C<our()> attribute is C<unique> which
3263indicates that a single copy of the global is to be used by all
3264interpreters should the program happen to be running in a
3265multi-interpreter environment. (The default behaviour would be for
3266each interpreter to have its own copy of the global.) Examples:
9969eac4 3267
51d2bbcc
JH
3268 our @EXPORT : unique = qw(foo);
3269 our %EXPORT_TAGS : unique = (bar => [qw(aa bb cc)]);
3270 our $VERSION : unique = "1.00";
9969eac4 3271
96fa8c42 3272Note that this attribute also has the effect of making the global
72e53bfb
JH
3273readonly when the first new interpreter is cloned (for example,