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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
ea63ef19 1209file as a Perl script.
a0d0e21e
LW
1210
1211 do 'stat.pl';
1212
1213is just like
1214
986b19de 1215 eval `cat stat.pl`;
a0d0e21e 1216
2b5ab1e7 1217except that it's more efficient and concise, keeps track of the current
ea63ef19 1218filename for error messages, searches the @INC directories, and updates
2b5ab1e7
TC
1219C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1220variables. It also differs in that code evaluated with C<do FILENAME>
1221cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1222same, however, in that it does reparse the file every time you call it,
1223so you probably don't want to do this inside a loop.
a0d0e21e 1224
8e30cc93 1225If C<do> cannot read the file, it returns undef and sets C<$!> to the
2b5ab1e7 1226error. If C<do> can read the file but cannot compile it, it
8e30cc93
G
1227returns undef and sets an error message in C<$@>. If the file is
1228successfully compiled, C<do> returns the value of the last expression
1229evaluated.
1230
a0d0e21e 1231Note that inclusion of library modules is better done with the
19799a22 1232C<use> and C<require> operators, which also do automatic error checking
4633a7c4 1233and raise an exception if there's a problem.
a0d0e21e 1234
5a964f20
TC
1235You might like to use C<do> to read in a program configuration
1236file. Manual error checking can be done this way:
1237
b76cc8ba 1238 # read in config files: system first, then user
f86cebdf 1239 for $file ("/share/prog/defaults.rc",
b76cc8ba 1240 "$ENV{HOME}/.someprogrc")
2b5ab1e7 1241 {
5a964f20 1242 unless ($return = do $file) {
f86cebdf
GS
1243 warn "couldn't parse $file: $@" if $@;
1244 warn "couldn't do $file: $!" unless defined $return;
1245 warn "couldn't run $file" unless $return;
5a964f20
TC
1246 }
1247 }
1248
a0d0e21e
LW
1249=item dump LABEL
1250
1614b0e3
JD
1251=item dump
1252
19799a22
GS
1253This function causes an immediate core dump. See also the B<-u>
1254command-line switch in L<perlrun>, which does the same thing.
1255Primarily this is so that you can use the B<undump> program (not
1256supplied) to turn your core dump into an executable binary after
1257having initialized all your variables at the beginning of the
1258program. When the new binary is executed it will begin by executing
1259a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1260Think of it as a goto with an intervening core dump and reincarnation.
1261If C<LABEL> is omitted, restarts the program from the top.
1262
1263B<WARNING>: Any files opened at the time of the dump will I<not>
1264be open any more when the program is reincarnated, with possible
b76cc8ba 1265resulting confusion on the part of Perl.
19799a22
GS
1266
1267This function is now largely obsolete, partly because it's very
1268hard to convert a core file into an executable, and because the
1269real compiler backends for generating portable bytecode and compilable
ac206dc8
RGS
1270C code have superseded it. That's why you should now invoke it as
1271C<CORE::dump()>, if you don't want to be warned against a possible
1272typo.
19799a22
GS
1273
1274If you're looking to use L<dump> to speed up your program, consider
1275generating bytecode or native C code as described in L<perlcc>. If
1276you're just trying to accelerate a CGI script, consider using the
210b36aa 1277C<mod_perl> extension to B<Apache>, or the CPAN module, CGI::Fast.
19799a22 1278You might also consider autoloading or selfloading, which at least
b76cc8ba 1279make your program I<appear> to run faster.
5a964f20 1280
aa689395
PP
1281=item each HASH
1282
5a964f20 1283When called in list context, returns a 2-element list consisting of the
aa689395 1284key and value for the next element of a hash, so that you can iterate over
74fc8b5f 1285it. When called in scalar context, returns only the key for the next
e902a979 1286element in the hash.
2f9daede 1287
ab192400 1288Entries are returned in an apparently random order. The actual random
504f80c1
JH
1289order is subject to change in future versions of perl, but it is
1290guaranteed to be in the same order as either the C<keys> or C<values>
4546b9e6
JH
1291function would produce on the same (unmodified) hash. Since Perl
12925.8.1 the ordering is different even between different runs of Perl
1293for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
ab192400
GS
1294
1295When the hash is entirely read, a null array is returned in list context
19799a22
GS
1296(which when assigned produces a false (C<0>) value), and C<undef> in
1297scalar context. The next call to C<each> after that will start iterating
1298again. There is a single iterator for each hash, shared by all C<each>,
1299C<keys>, and C<values> function calls in the program; it can be reset by
2f9daede
TPG
1300reading all the elements from the hash, or by evaluating C<keys HASH> or
1301C<values HASH>. If you add or delete elements of a hash while you're
74fc8b5f
MJD
1302iterating over it, you may get entries skipped or duplicated, so
1303don't. Exception: It is always safe to delete the item most recently
1304returned by C<each()>, which means that the following code will work:
1305
1306 while (($key, $value) = each %hash) {
1307 print $key, "\n";
1308 delete $hash{$key}; # This is safe
1309 }
aa689395 1310
f86cebdf 1311The following prints out your environment like the printenv(1) program,
aa689395 1312only in a different order:
a0d0e21e
LW
1313
1314 while (($key,$value) = each %ENV) {
1315 print "$key=$value\n";
1316 }
1317
19799a22 1318See also C<keys>, C<values> and C<sort>.
a0d0e21e
LW
1319
1320=item eof FILEHANDLE
1321
4633a7c4
LW
1322=item eof ()
1323
a0d0e21e
LW
1324=item eof
1325
1326Returns 1 if the next read on FILEHANDLE will return end of file, or if
1327FILEHANDLE is not open. FILEHANDLE may be an expression whose value
5a964f20 1328gives the real filehandle. (Note that this function actually
19799a22 1329reads a character and then C<ungetc>s it, so isn't very useful in an
748a9306 1330interactive context.) Do not read from a terminal file (or call
19799a22 1331C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
748a9306
LW
1332as terminals may lose the end-of-file condition if you do.
1333
820475bd
GS
1334An C<eof> without an argument uses the last file read. Using C<eof()>
1335with empty parentheses is very different. It refers to the pseudo file
1336formed from the files listed on the command line and accessed via the
61eff3bc
JH
1337C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1338as a normal filehandle is, an C<eof()> before C<< <> >> has been
820475bd 1339used will cause C<@ARGV> to be examined to determine if input is
67408cae 1340available. Similarly, an C<eof()> after C<< <> >> has returned
efdd0218
RB
1341end-of-file will assume you are processing another C<@ARGV> list,
1342and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1343see L<perlop/"I/O Operators">.
820475bd 1344
61eff3bc 1345In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
820475bd
GS
1346detect the end of each file, C<eof()> will only detect the end of the
1347last file. Examples:
a0d0e21e 1348
748a9306
LW
1349 # reset line numbering on each input file
1350 while (<>) {
b76cc8ba 1351 next if /^\s*#/; # skip comments
748a9306 1352 print "$.\t$_";
5a964f20
TC
1353 } continue {
1354 close ARGV if eof; # Not eof()!
748a9306
LW
1355 }
1356
a0d0e21e
LW
1357 # insert dashes just before last line of last file
1358 while (<>) {
6ac88b13 1359 if (eof()) { # check for end of last file
a0d0e21e
LW
1360 print "--------------\n";
1361 }
1362 print;
6ac88b13 1363 last if eof(); # needed if we're reading from a terminal
a0d0e21e
LW
1364 }
1365
a0d0e21e 1366Practical hint: you almost never need to use C<eof> in Perl, because the
3ce0d271
GS
1367input operators typically return C<undef> when they run out of data, or if
1368there was an error.
a0d0e21e
LW
1369
1370=item eval EXPR
1371
1372=item eval BLOCK
1373
c7cc6f1c
GS
1374In the first form, the return value of EXPR is parsed and executed as if it
1375were a little Perl program. The value of the expression (which is itself
5a964f20 1376determined within scalar context) is first parsed, and if there weren't any
be3174d2
GS
1377errors, executed in the lexical context of the current Perl program, so
1378that any variable settings or subroutine and format definitions remain
1379afterwards. Note that the value is parsed every time the eval executes.
1380If EXPR is omitted, evaluates C<$_>. This form is typically used to
1381delay parsing and subsequent execution of the text of EXPR until run time.
c7cc6f1c
GS
1382
1383In the second form, the code within the BLOCK is parsed only once--at the
1384same time the code surrounding the eval itself was parsed--and executed
1385within the context of the current Perl program. This form is typically
1386used to trap exceptions more efficiently than the first (see below), while
1387also providing the benefit of checking the code within BLOCK at compile
1388time.
1389
1390The final semicolon, if any, may be omitted from the value of EXPR or within
1391the BLOCK.
1392
1393In both forms, the value returned is the value of the last expression
5a964f20 1394evaluated inside the mini-program; a return statement may be also used, just
c7cc6f1c 1395as with subroutines. The expression providing the return value is evaluated
5a964f20 1396in void, scalar, or list context, depending on the context of the eval itself.
c7cc6f1c 1397See L</wantarray> for more on how the evaluation context can be determined.
a0d0e21e 1398
19799a22
GS
1399If there is a syntax error or runtime error, or a C<die> statement is
1400executed, an undefined value is returned by C<eval>, and C<$@> is set to the
a0d0e21e 1401error message. If there was no error, C<$@> is guaranteed to be a null
19799a22 1402string. Beware that using C<eval> neither silences perl from printing
c7cc6f1c 1403warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
d9984052
A
1404To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1405turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1406See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
a0d0e21e 1407
19799a22
GS
1408Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1409determining whether a particular feature (such as C<socket> or C<symlink>)
a0d0e21e
LW
1410is implemented. It is also Perl's exception trapping mechanism, where
1411the die operator is used to raise exceptions.
1412
1413If the code to be executed doesn't vary, you may use the eval-BLOCK
1414form to trap run-time errors without incurring the penalty of
1415recompiling each time. The error, if any, is still returned in C<$@>.
1416Examples:
1417
54310121 1418 # make divide-by-zero nonfatal
a0d0e21e
LW
1419 eval { $answer = $a / $b; }; warn $@ if $@;
1420
1421 # same thing, but less efficient
1422 eval '$answer = $a / $b'; warn $@ if $@;
1423
1424 # a compile-time error
5a964f20 1425 eval { $answer = }; # WRONG
a0d0e21e
LW
1426
1427 # a run-time error
1428 eval '$answer ='; # sets $@
1429
2b5ab1e7
TC
1430Due to the current arguably broken state of C<__DIE__> hooks, when using
1431the C<eval{}> form as an exception trap in libraries, you may wish not
1432to trigger any C<__DIE__> hooks that user code may have installed.
1433You can use the C<local $SIG{__DIE__}> construct for this purpose,
1434as shown in this example:
774d564b
PP
1435
1436 # a very private exception trap for divide-by-zero
f86cebdf
GS
1437 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1438 warn $@ if $@;
774d564b
PP
1439
1440This is especially significant, given that C<__DIE__> hooks can call
19799a22 1441C<die> again, which has the effect of changing their error messages:
774d564b
PP
1442
1443 # __DIE__ hooks may modify error messages
1444 {
f86cebdf
GS
1445 local $SIG{'__DIE__'} =
1446 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
c7cc6f1c
GS
1447 eval { die "foo lives here" };
1448 print $@ if $@; # prints "bar lives here"
774d564b
PP
1449 }
1450
19799a22 1451Because this promotes action at a distance, this counterintuitive behavior
2b5ab1e7
TC
1452may be fixed in a future release.
1453
19799a22 1454With an C<eval>, you should be especially careful to remember what's
a0d0e21e
LW
1455being looked at when:
1456
1457 eval $x; # CASE 1
1458 eval "$x"; # CASE 2
1459
1460 eval '$x'; # CASE 3
1461 eval { $x }; # CASE 4
1462
5a964f20 1463 eval "\$$x++"; # CASE 5
a0d0e21e
LW
1464 $$x++; # CASE 6
1465
2f9daede 1466Cases 1 and 2 above behave identically: they run the code contained in
19799a22 1467the variable $x. (Although case 2 has misleading double quotes making
2f9daede 1468the reader wonder what else might be happening (nothing is).) Cases 3
7660c0ab 1469and 4 likewise behave in the same way: they run the code C<'$x'>, which
19799a22 1470does nothing but return the value of $x. (Case 4 is preferred for
2f9daede
TPG
1471purely visual reasons, but it also has the advantage of compiling at
1472compile-time instead of at run-time.) Case 5 is a place where
19799a22 1473normally you I<would> like to use double quotes, except that in this
2f9daede
TPG
1474particular situation, you can just use symbolic references instead, as
1475in case 6.
a0d0e21e 1476
4968c1e4 1477C<eval BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7 1478C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
4968c1e4 1479
d819b83a
DM
1480Note that as a very special case, an C<eval ''> executed within the C<DB>
1481package doesn't see the usual surrounding lexical scope, but rather the
1482scope of the first non-DB piece of code that called it. You don't normally
1483need to worry about this unless you are writing a Perl debugger.
1484
a0d0e21e
LW
1485=item exec LIST
1486
8bf3b016
GS
1487=item exec PROGRAM LIST
1488
19799a22
GS
1489The C<exec> function executes a system command I<and never returns>--
1490use C<system> instead of C<exec> if you want it to return. It fails and
1491returns false only if the command does not exist I<and> it is executed
fb73857a 1492directly instead of via your system's command shell (see below).
a0d0e21e 1493
19799a22
GS
1494Since it's a common mistake to use C<exec> instead of C<system>, Perl
1495warns you if there is a following statement which isn't C<die>, C<warn>,
1496or C<exit> (if C<-w> is set - but you always do that). If you
1497I<really> want to follow an C<exec> with some other statement, you
55d729e4
GS
1498can use one of these styles to avoid the warning:
1499
5a964f20
TC
1500 exec ('foo') or print STDERR "couldn't exec foo: $!";
1501 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
55d729e4 1502
5a964f20 1503If there is more than one argument in LIST, or if LIST is an array
f86cebdf 1504with more than one value, calls execvp(3) with the arguments in LIST.
5a964f20
TC
1505If there is only one scalar argument or an array with one element in it,
1506the argument is checked for shell metacharacters, and if there are any,
1507the entire argument is passed to the system's command shell for parsing
1508(this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1509If there are no shell metacharacters in the argument, it is split into
b76cc8ba 1510words and passed directly to C<execvp>, which is more efficient.
19799a22 1511Examples:
a0d0e21e 1512
19799a22
GS
1513 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1514 exec "sort $outfile | uniq";
a0d0e21e
LW
1515
1516If you don't really want to execute the first argument, but want to lie
1517to the program you are executing about its own name, you can specify
1518the program you actually want to run as an "indirect object" (without a
1519comma) in front of the LIST. (This always forces interpretation of the
54310121 1520LIST as a multivalued list, even if there is only a single scalar in
a0d0e21e
LW
1521the list.) Example:
1522
1523 $shell = '/bin/csh';
1524 exec $shell '-sh'; # pretend it's a login shell
1525
1526or, more directly,
1527
1528 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1529
bb32b41a
GS
1530When the arguments get executed via the system shell, results will
1531be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1532for details.
1533
19799a22
GS
1534Using an indirect object with C<exec> or C<system> is also more
1535secure. This usage (which also works fine with system()) forces
1536interpretation of the arguments as a multivalued list, even if the
1537list had just one argument. That way you're safe from the shell
1538expanding wildcards or splitting up words with whitespace in them.
5a964f20
TC
1539
1540 @args = ( "echo surprise" );
1541
2b5ab1e7 1542 exec @args; # subject to shell escapes
f86cebdf 1543 # if @args == 1
2b5ab1e7 1544 exec { $args[0] } @args; # safe even with one-arg list
5a964f20
TC
1545
1546The first version, the one without the indirect object, ran the I<echo>
1547program, passing it C<"surprise"> an argument. The second version
1548didn't--it tried to run a program literally called I<"echo surprise">,
1549didn't find it, and set C<$?> to a non-zero value indicating failure.
1550
0f897271
GS
1551Beginning with v5.6.0, Perl will attempt to flush all files opened for
1552output before the exec, but this may not be supported on some platforms
1553(see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1554in English) or call the C<autoflush()> method of C<IO::Handle> on any
1555open handles in order to avoid lost output.
1556
19799a22 1557Note that C<exec> will not call your C<END> blocks, nor will it call
7660c0ab
A
1558any C<DESTROY> methods in your objects.
1559
a0d0e21e
LW
1560=item exists EXPR
1561
01020589 1562Given an expression that specifies a hash element or array element,
8ea97a1e
GS
1563returns true if the specified element in the hash or array has ever
1564been initialized, even if the corresponding value is undefined. The
1565element is not autovivified if it doesn't exist.
a0d0e21e 1566
01020589
GS
1567 print "Exists\n" if exists $hash{$key};
1568 print "Defined\n" if defined $hash{$key};
1569 print "True\n" if $hash{$key};
1570
1571 print "Exists\n" if exists $array[$index];
1572 print "Defined\n" if defined $array[$index];
1573 print "True\n" if $array[$index];
a0d0e21e 1574
8ea97a1e 1575A hash or array element can be true only if it's defined, and defined if
a0d0e21e
LW
1576it exists, but the reverse doesn't necessarily hold true.
1577
afebc493
GS
1578Given an expression that specifies the name of a subroutine,
1579returns true if the specified subroutine has ever been declared, even
1580if it is undefined. Mentioning a subroutine name for exists or defined
847c7ebe
DD
1581does not count as declaring it. Note that a subroutine which does not
1582exist may still be callable: its package may have an C<AUTOLOAD>
1583method that makes it spring into existence the first time that it is
1584called -- see L<perlsub>.
afebc493
GS
1585
1586 print "Exists\n" if exists &subroutine;
1587 print "Defined\n" if defined &subroutine;
1588
a0d0e21e 1589Note that the EXPR can be arbitrarily complicated as long as the final
afebc493 1590operation is a hash or array key lookup or subroutine name:
a0d0e21e 1591
2b5ab1e7
TC
1592 if (exists $ref->{A}->{B}->{$key}) { }
1593 if (exists $hash{A}{B}{$key}) { }
1594
01020589
GS
1595 if (exists $ref->{A}->{B}->[$ix]) { }
1596 if (exists $hash{A}{B}[$ix]) { }
1597
afebc493
GS
1598 if (exists &{$ref->{A}{B}{$key}}) { }
1599
01020589
GS
1600Although the deepest nested array or hash will not spring into existence
1601just because its existence was tested, any intervening ones will.
61eff3bc 1602Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
01020589
GS
1603into existence due to the existence test for the $key element above.
1604This happens anywhere the arrow operator is used, including even:
5a964f20 1605
2b5ab1e7
TC
1606 undef $ref;
1607 if (exists $ref->{"Some key"}) { }
1608 print $ref; # prints HASH(0x80d3d5c)
1609
1610This surprising autovivification in what does not at first--or even
1611second--glance appear to be an lvalue context may be fixed in a future
5a964f20 1612release.
a0d0e21e 1613
afebc493
GS
1614Use of a subroutine call, rather than a subroutine name, as an argument
1615to exists() is an error.
1616
1617 exists &sub; # OK
1618 exists &sub(); # Error
1619
a0d0e21e
LW
1620=item exit EXPR
1621
2b5ab1e7 1622Evaluates EXPR and exits immediately with that value. Example:
a0d0e21e
LW
1623
1624 $ans = <STDIN>;
1625 exit 0 if $ans =~ /^[Xx]/;
1626
19799a22 1627See also C<die>. If EXPR is omitted, exits with C<0> status. The only
2b5ab1e7
TC
1628universally recognized values for EXPR are C<0> for success and C<1>
1629for error; other values are subject to interpretation depending on the
1630environment in which the Perl program is running. For example, exiting
163169 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1632the mailer to return the item undelivered, but that's not true everywhere.
a0d0e21e 1633
19799a22
GS
1634Don't use C<exit> to abort a subroutine if there's any chance that
1635someone might want to trap whatever error happened. Use C<die> instead,
1636which can be trapped by an C<eval>.
28757baa 1637
19799a22 1638The exit() function does not always exit immediately. It calls any
2b5ab1e7 1639defined C<END> routines first, but these C<END> routines may not
19799a22 1640themselves abort the exit. Likewise any object destructors that need to
2b5ab1e7
TC
1641be called are called before the real exit. If this is a problem, you
1642can call C<POSIX:_exit($status)> to avoid END and destructor processing.
87275199 1643See L<perlmod> for details.
5a964f20 1644
a0d0e21e
LW
1645=item exp EXPR
1646
54310121 1647=item exp
bbce6d69 1648
b76cc8ba 1649Returns I<e> (the natural logarithm base) to the power of EXPR.
a0d0e21e
LW
1650If EXPR is omitted, gives C<exp($_)>.
1651
1652=item fcntl FILEHANDLE,FUNCTION,SCALAR
1653
f86cebdf 1654Implements the fcntl(2) function. You'll probably have to say
a0d0e21e
LW
1655
1656 use Fcntl;
1657
0ade1984 1658first to get the correct constant definitions. Argument processing and
b76cc8ba 1659value return works just like C<ioctl> below.
a0d0e21e
LW
1660For example:
1661
1662 use Fcntl;
5a964f20
TC
1663 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1664 or die "can't fcntl F_GETFL: $!";
1665
554ad1fc 1666You don't have to check for C<defined> on the return from C<fcntl>.
951ba7fe
GS
1667Like C<ioctl>, it maps a C<0> return from the system call into
1668C<"0 but true"> in Perl. This string is true in boolean context and C<0>
2b5ab1e7
TC
1669in numeric context. It is also exempt from the normal B<-w> warnings
1670on improper numeric conversions.
5a964f20 1671
19799a22 1672Note that C<fcntl> will produce a fatal error if used on a machine that
2b5ab1e7
TC
1673doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1674manpage to learn what functions are available on your system.
a0d0e21e 1675
be2f7487 1676Here's an example of setting a filehandle named C<REMOTE> to be
1677non-blocking at the system level. You'll have to negotiate C<$|>
1678on your own, though.
1679
1680 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
1681
1682 $flags = fcntl(REMOTE, F_GETFL, 0)
1683 or die "Can't get flags for the socket: $!\n";
1684
1685 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
1686 or die "Can't set flags for the socket: $!\n";
1687
a0d0e21e
LW
1688=item fileno FILEHANDLE
1689
2b5ab1e7
TC
1690Returns the file descriptor for a filehandle, or undefined if the
1691filehandle is not open. This is mainly useful for constructing
19799a22 1692bitmaps for C<select> and low-level POSIX tty-handling operations.
2b5ab1e7
TC
1693If FILEHANDLE is an expression, the value is taken as an indirect
1694filehandle, generally its name.
5a964f20 1695
b76cc8ba 1696You can use this to find out whether two handles refer to the
5a964f20
TC
1697same underlying descriptor:
1698
1699 if (fileno(THIS) == fileno(THAT)) {
1700 print "THIS and THAT are dups\n";
b76cc8ba
NIS
1701 }
1702
1703(Filehandles connected to memory objects via new features of C<open> may
1704return undefined even though they are open.)
1705
a0d0e21e
LW
1706
1707=item flock FILEHANDLE,OPERATION
1708
19799a22
GS
1709Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1710for success, false on failure. Produces a fatal error if used on a
2b5ab1e7 1711machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
19799a22 1712C<flock> is Perl's portable file locking interface, although it locks
2b5ab1e7
TC
1713only entire files, not records.
1714
1715Two potentially non-obvious but traditional C<flock> semantics are
1716that it waits indefinitely until the lock is granted, and that its locks
1717B<merely advisory>. Such discretionary locks are more flexible, but offer
19799a22
GS
1718fewer guarantees. This means that files locked with C<flock> may be
1719modified by programs that do not also use C<flock>. See L<perlport>,
2b5ab1e7
TC
1720your port's specific documentation, or your system-specific local manpages
1721for details. It's best to assume traditional behavior if you're writing
1722portable programs. (But if you're not, you should as always feel perfectly
1723free to write for your own system's idiosyncrasies (sometimes called
1724"features"). Slavish adherence to portability concerns shouldn't get
1725in the way of your getting your job done.)
a3cb178b 1726
8ebc5c01
PP
1727OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1728LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
ea3105be 1729you can use the symbolic names if you import them from the Fcntl module,
68dc0745
PP
1730either individually, or as a group using the ':flock' tag. LOCK_SH
1731requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
ea3105be
GS
1732releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1733LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
68dc0745
PP
1734waiting for the lock (check the return status to see if you got it).
1735
2b5ab1e7
TC
1736To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1737before locking or unlocking it.
8ebc5c01 1738
f86cebdf 1739Note that the emulation built with lockf(3) doesn't provide shared
8ebc5c01 1740locks, and it requires that FILEHANDLE be open with write intent. These
2b5ab1e7 1741are the semantics that lockf(3) implements. Most if not all systems
f86cebdf 1742implement lockf(3) in terms of fcntl(2) locking, though, so the
8ebc5c01
PP
1743differing semantics shouldn't bite too many people.
1744
becacb53
TM
1745Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
1746be open with read intent to use LOCK_SH and requires that it be open
1747with write intent to use LOCK_EX.
1748
19799a22
GS
1749Note also that some versions of C<flock> cannot lock things over the
1750network; you would need to use the more system-specific C<fcntl> for
f86cebdf
GS
1751that. If you like you can force Perl to ignore your system's flock(2)
1752function, and so provide its own fcntl(2)-based emulation, by passing
8ebc5c01
PP
1753the switch C<-Ud_flock> to the F<Configure> program when you configure
1754perl.
4633a7c4
LW
1755
1756Here's a mailbox appender for BSD systems.
a0d0e21e 1757
7e1af8bc 1758 use Fcntl ':flock'; # import LOCK_* constants
a0d0e21e
LW
1759
1760 sub lock {
7e1af8bc 1761 flock(MBOX,LOCK_EX);
a0d0e21e
LW
1762 # and, in case someone appended
1763 # while we were waiting...
1764 seek(MBOX, 0, 2);
1765 }
1766
1767 sub unlock {
7e1af8bc 1768 flock(MBOX,LOCK_UN);
a0d0e21e
LW
1769 }
1770
1771 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1772 or die "Can't open mailbox: $!";
1773
1774 lock();
1775 print MBOX $msg,"\n\n";
1776 unlock();
1777
2b5ab1e7
TC
1778On systems that support a real flock(), locks are inherited across fork()
1779calls, whereas those that must resort to the more capricious fcntl()
1780function lose the locks, making it harder to write servers.
1781
cb1a09d0 1782See also L<DB_File> for other flock() examples.
a0d0e21e
LW
1783
1784=item fork
1785
2b5ab1e7
TC
1786Does a fork(2) system call to create a new process running the
1787same program at the same point. It returns the child pid to the
1788parent process, C<0> to the child process, or C<undef> if the fork is
1789unsuccessful. File descriptors (and sometimes locks on those descriptors)
1790are shared, while everything else is copied. On most systems supporting
1791fork(), great care has gone into making it extremely efficient (for
1792example, using copy-on-write technology on data pages), making it the
1793dominant paradigm for multitasking over the last few decades.
5a964f20 1794
0f897271
GS
1795Beginning with v5.6.0, Perl will attempt to flush all files opened for
1796output before forking the child process, but this may not be supported
1797on some platforms (see L<perlport>). To be safe, you may need to set
1798C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1799C<IO::Handle> on any open handles in order to avoid duplicate output.
a0d0e21e 1800
19799a22 1801If you C<fork> without ever waiting on your children, you will
2b5ab1e7
TC
1802accumulate zombies. On some systems, you can avoid this by setting
1803C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1804forking and reaping moribund children.
cb1a09d0 1805
28757baa
PP
1806Note that if your forked child inherits system file descriptors like
1807STDIN and STDOUT that are actually connected by a pipe or socket, even
2b5ab1e7 1808if you exit, then the remote server (such as, say, a CGI script or a
19799a22 1809backgrounded job launched from a remote shell) won't think you're done.
2b5ab1e7 1810You should reopen those to F</dev/null> if it's any issue.
28757baa 1811
cb1a09d0
AD
1812=item format
1813
19799a22 1814Declare a picture format for use by the C<write> function. For
cb1a09d0
AD
1815example:
1816
54310121 1817 format Something =
cb1a09d0
AD
1818 Test: @<<<<<<<< @||||| @>>>>>
1819 $str, $%, '$' . int($num)
1820 .
1821
1822 $str = "widget";
184e9718 1823 $num = $cost/$quantity;
cb1a09d0
AD
1824 $~ = 'Something';
1825 write;
1826
1827See L<perlform> for many details and examples.
1828
8903cb82 1829=item formline PICTURE,LIST
a0d0e21e 1830
5a964f20 1831This is an internal function used by C<format>s, though you may call it,
a0d0e21e
LW
1832too. It formats (see L<perlform>) a list of values according to the
1833contents of PICTURE, placing the output into the format output
7660c0ab 1834accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
19799a22 1835Eventually, when a C<write> is done, the contents of
a0d0e21e 1836C<$^A> are written to some filehandle, but you could also read C<$^A>
7660c0ab 1837yourself and then set C<$^A> back to C<"">. Note that a format typically
19799a22 1838does one C<formline> per line of form, but the C<formline> function itself
748a9306 1839doesn't care how many newlines are embedded in the PICTURE. This means
4633a7c4 1840that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
748a9306
LW
1841You may therefore need to use multiple formlines to implement a single
1842record format, just like the format compiler.
1843
19799a22 1844Be careful if you put double quotes around the picture, because an C<@>
748a9306 1845character may be taken to mean the beginning of an array name.
19799a22 1846C<formline> always returns true. See L<perlform> for other examples.
a0d0e21e
LW
1847
1848=item getc FILEHANDLE
1849
1850=item getc
1851
1852Returns the next character from the input file attached to FILEHANDLE,
b5fe5ca2
SR
1853or the undefined value at end of file, or if there was an error (in
1854the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
1855STDIN. This is not particularly efficient. However, it cannot be
1856used by itself to fetch single characters without waiting for the user
1857to hit enter. For that, try something more like:
4633a7c4
LW
1858
1859 if ($BSD_STYLE) {
1860 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1861 }
1862 else {
54310121 1863 system "stty", '-icanon', 'eol', "\001";
4633a7c4
LW
1864 }
1865
1866 $key = getc(STDIN);
1867
1868 if ($BSD_STYLE) {
1869 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1870 }
1871 else {
5f05dabc 1872 system "stty", 'icanon', 'eol', '^@'; # ASCII null
4633a7c4
LW
1873 }
1874 print "\n";
1875
54310121
PP
1876Determination of whether $BSD_STYLE should be set
1877is left as an exercise to the reader.
cb1a09d0 1878
19799a22 1879The C<POSIX::getattr> function can do this more portably on
2b5ab1e7
TC
1880systems purporting POSIX compliance. See also the C<Term::ReadKey>
1881module from your nearest CPAN site; details on CPAN can be found on
1882L<perlmodlib/CPAN>.
a0d0e21e
LW
1883
1884=item getlogin
1885
5a964f20
TC
1886Implements the C library function of the same name, which on most
1887systems returns the current login from F</etc/utmp>, if any. If null,
19799a22 1888use C<getpwuid>.
a0d0e21e 1889
f86702cc 1890 $login = getlogin || getpwuid($<) || "Kilroy";
a0d0e21e 1891
19799a22
GS
1892Do not consider C<getlogin> for authentication: it is not as
1893secure as C<getpwuid>.
4633a7c4 1894
a0d0e21e
LW
1895=item getpeername SOCKET
1896
1897Returns the packed sockaddr address of other end of the SOCKET connection.
1898
4633a7c4
LW
1899 use Socket;
1900 $hersockaddr = getpeername(SOCK);
19799a22 1901 ($port, $iaddr) = sockaddr_in($hersockaddr);
4633a7c4
LW
1902 $herhostname = gethostbyaddr($iaddr, AF_INET);
1903 $herstraddr = inet_ntoa($iaddr);
a0d0e21e
LW
1904
1905=item getpgrp PID
1906
47e29363 1907Returns the current process group for the specified PID. Use
7660c0ab 1908a PID of C<0> to get the current process group for the
4633a7c4 1909current process. Will raise an exception if used on a machine that
f86cebdf 1910doesn't implement getpgrp(2). If PID is omitted, returns process
19799a22 1911group of current process. Note that the POSIX version of C<getpgrp>
7660c0ab 1912does not accept a PID argument, so only C<PID==0> is truly portable.
a0d0e21e
LW
1913
1914=item getppid
1915
1916Returns the process id of the parent process.
1917
4d76a344
RGS
1918Note for Linux users: on Linux, the C functions C<getpid()> and
1919C<getppid()> return different values from different threads. In order to
1920be portable, this behavior is not reflected by the perl-level function
1921C<getppid()>, that returns a consistent value across threads. If you want
e3256f86
RGS
1922to call the underlying C<getppid()>, you may use the CPAN module
1923C<Linux::Pid>.
4d76a344 1924
a0d0e21e
LW
1925=item getpriority WHICH,WHO
1926
4633a7c4
LW
1927Returns the current priority for a process, a process group, or a user.
1928(See L<getpriority(2)>.) Will raise a fatal exception if used on a
f86cebdf 1929machine that doesn't implement getpriority(2).
a0d0e21e
LW
1930
1931=item getpwnam NAME
1932
1933=item getgrnam NAME
1934
1935=item gethostbyname NAME
1936
1937=item getnetbyname NAME
1938
1939=item getprotobyname NAME
1940
1941=item getpwuid UID
1942
1943=item getgrgid GID
1944
1945=item getservbyname NAME,PROTO
1946
1947=item gethostbyaddr ADDR,ADDRTYPE
1948
1949=item getnetbyaddr ADDR,ADDRTYPE
1950
1951=item getprotobynumber NUMBER
1952
1953=item getservbyport PORT,PROTO
1954
1955=item getpwent
1956
1957=item getgrent
1958
1959=item gethostent
1960
1961=item getnetent
1962
1963=item getprotoent
1964
1965=item getservent
1966
1967=item setpwent
1968
1969=item setgrent
1970
1971=item sethostent STAYOPEN
1972
1973=item setnetent STAYOPEN
1974
1975=item setprotoent STAYOPEN
1976
1977=item setservent STAYOPEN
1978
1979=item endpwent
1980
1981=item endgrent
1982
1983=item endhostent
1984
1985=item endnetent
1986
1987=item endprotoent
1988
1989=item endservent
1990
1991These routines perform the same functions as their counterparts in the
5a964f20 1992system library. In list context, the return values from the
a0d0e21e
LW
1993various get routines are as follows:
1994
1995 ($name,$passwd,$uid,$gid,
6ee623d5 1996 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
a0d0e21e
LW
1997 ($name,$passwd,$gid,$members) = getgr*
1998 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1999 ($name,$aliases,$addrtype,$net) = getnet*
2000 ($name,$aliases,$proto) = getproto*
2001 ($name,$aliases,$port,$proto) = getserv*
2002
2003(If the entry doesn't exist you get a null list.)
2004
4602f195
JH
2005The exact meaning of the $gcos field varies but it usually contains
2006the real name of the user (as opposed to the login name) and other
2007information pertaining to the user. Beware, however, that in many
2008system users are able to change this information and therefore it
106325ad 2009cannot be trusted and therefore the $gcos is tainted (see
2959b6e3
JH
2010L<perlsec>). The $passwd and $shell, user's encrypted password and
2011login shell, are also tainted, because of the same reason.
4602f195 2012
5a964f20 2013In scalar context, you get the name, unless the function was a
a0d0e21e
LW
2014lookup by name, in which case you get the other thing, whatever it is.
2015(If the entry doesn't exist you get the undefined value.) For example:
2016
5a964f20
TC
2017 $uid = getpwnam($name);
2018 $name = getpwuid($num);
2019 $name = getpwent();
2020 $gid = getgrnam($name);
08a33e13 2021 $name = getgrgid($num);
5a964f20
TC
2022 $name = getgrent();
2023 #etc.
a0d0e21e 2024
4602f195
JH
2025In I<getpw*()> the fields $quota, $comment, and $expire are special
2026cases in the sense that in many systems they are unsupported. If the
2027$quota is unsupported, it is an empty scalar. If it is supported, it
2028usually encodes the disk quota. If the $comment field is unsupported,
2029it is an empty scalar. If it is supported it usually encodes some
2030administrative comment about the user. In some systems the $quota
2031field may be $change or $age, fields that have to do with password
2032aging. In some systems the $comment field may be $class. The $expire
2033field, if present, encodes the expiration period of the account or the
2034password. For the availability and the exact meaning of these fields
2035in your system, please consult your getpwnam(3) documentation and your
2036F<pwd.h> file. You can also find out from within Perl what your
2037$quota and $comment fields mean and whether you have the $expire field
2038by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2039C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
2040files are only supported if your vendor has implemented them in the
2041intuitive fashion that calling the regular C library routines gets the
5d3a0a3b
GS
2042shadow versions if you're running under privilege or if there exists
2043the shadow(3) functions as found in System V ( this includes Solaris
2044and Linux.) Those systems which implement a proprietary shadow password
2045facility are unlikely to be supported.
6ee623d5 2046
19799a22 2047The $members value returned by I<getgr*()> is a space separated list of
a0d0e21e
LW
2048the login names of the members of the group.
2049
2050For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2051C, it will be returned to you via C<$?> if the function call fails. The
7660c0ab 2052C<@addrs> value returned by a successful call is a list of the raw
a0d0e21e
LW
2053addresses returned by the corresponding system library call. In the
2054Internet domain, each address is four bytes long and you can unpack it
2055by saying something like:
2056
f337b084 2057 ($a,$b,$c,$d) = unpack('W4',$addr[0]);
a0d0e21e 2058
2b5ab1e7
TC
2059The Socket library makes this slightly easier:
2060
2061 use Socket;
2062 $iaddr = inet_aton("127.1"); # or whatever address
2063 $name = gethostbyaddr($iaddr, AF_INET);
2064
2065 # or going the other way
19799a22 2066 $straddr = inet_ntoa($iaddr);
2b5ab1e7 2067
19799a22
GS
2068If you get tired of remembering which element of the return list
2069contains which return value, by-name interfaces are provided
2070in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2071C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2072and C<User::grent>. These override the normal built-ins, supplying
2073versions that return objects with the appropriate names
2074for each field. For example:
5a964f20
TC
2075
2076 use File::stat;
2077 use User::pwent;
2078 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2079
b76cc8ba
NIS
2080Even though it looks like they're the same method calls (uid),
2081they aren't, because a C<File::stat> object is different from
19799a22 2082a C<User::pwent> object.
5a964f20 2083
a0d0e21e
LW
2084=item getsockname SOCKET
2085
19799a22
GS
2086Returns the packed sockaddr address of this end of the SOCKET connection,
2087in case you don't know the address because you have several different
2088IPs that the connection might have come in on.
a0d0e21e 2089
4633a7c4
LW
2090 use Socket;
2091 $mysockaddr = getsockname(SOCK);
19799a22 2092 ($port, $myaddr) = sockaddr_in($mysockaddr);
b76cc8ba 2093 printf "Connect to %s [%s]\n",
19799a22
GS
2094 scalar gethostbyaddr($myaddr, AF_INET),
2095 inet_ntoa($myaddr);
a0d0e21e
LW
2096
2097=item getsockopt SOCKET,LEVEL,OPTNAME
2098
636e6b1f
TH
2099Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
2100Options may exist at multiple protocol levels depending on the socket
2101type, but at least the uppermost socket level SOL_SOCKET (defined in the
2102C<Socket> module) will exist. To query options at another level the
2103protocol number of the appropriate protocol controlling the option
2104should be supplied. For example, to indicate that an option is to be
2105interpreted by the TCP protocol, LEVEL should be set to the protocol
2106number of TCP, which you can get using getprotobyname.
2107
2108The call returns a packed string representing the requested socket option,
2109or C<undef> if there is an error (the error reason will be in $!). What
2110exactly is in the packed string depends in the LEVEL and OPTNAME, consult
2111your system documentation for details. A very common case however is that
2112the option is an integer, in which case the result will be an packed
2113integer which you can decode using unpack with the C<i> (or C<I>) format.
2114
2115An example testing if Nagle's algorithm is turned on on a socket:
2116
4852725b 2117 use Socket qw(:all);
636e6b1f
TH
2118
2119 defined(my $tcp = getprotobyname("tcp"))
2120 or die "Could not determine the protocol number for tcp";
4852725b
DD
2121 # my $tcp = IPPROTO_TCP; # Alternative
2122 my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
2123 or die "Could not query TCP_NODELAY socket option: $!";
636e6b1f
TH
2124 my $nodelay = unpack("I", $packed);
2125 print "Nagle's algorithm is turned ", $nodelay ? "off\n" : "on\n";
2126
a0d0e21e
LW
2127
2128=item glob EXPR
2129
0a753a76
PP
2130=item glob
2131
d9a9d457
JL
2132In list context, returns a (possibly empty) list of filename expansions on
2133the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
2134scalar context, glob iterates through such filename expansions, returning
2135undef when the list is exhausted. This is the internal function
2136implementing the C<< <*.c> >> operator, but you can use it directly. If
2137EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2138more detail in L<perlop/"I/O Operators">.
a0d0e21e 2139
3a4b19e4
GS
2140Beginning with v5.6.0, this operator is implemented using the standard
2141C<File::Glob> extension. See L<File::Glob> for details.
2142
a0d0e21e
LW
2143=item gmtime EXPR
2144
d1be9408 2145Converts a time as returned by the time function to an 8-element list
54310121 2146with the time localized for the standard Greenwich time zone.
4633a7c4 2147Typically used as follows:
a0d0e21e 2148
b76cc8ba 2149 # 0 1 2 3 4 5 6 7
48a26b3a 2150 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday) =
a0d0e21e
LW
2151 gmtime(time);
2152
48a26b3a
GS
2153All list elements are numeric, and come straight out of the C `struct
2154tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2155specified time. $mday is the day of the month, and $mon is the month
2156itself, in the range C<0..11> with 0 indicating January and 11
2157indicating December. $year is the number of years since 1900. That
2158is, $year is C<123> in year 2023. $wday is the day of the week, with
21590 indicating Sunday and 3 indicating Wednesday. $yday is the day of
b76cc8ba 2160the year, in the range C<0..364> (or C<0..365> in leap years.)
48a26b3a
GS
2161
2162Note that the $year element is I<not> simply the last two digits of
2163the year. If you assume it is, then you create non-Y2K-compliant
2164programs--and you wouldn't want to do that, would you?
2f9daede 2165
abd75f24
GS
2166The proper way to get a complete 4-digit year is simply:
2167
2168 $year += 1900;
2169
2170And to get the last two digits of the year (e.g., '01' in 2001) do:
2171
2172 $year = sprintf("%02d", $year % 100);
2173
48a26b3a 2174If EXPR is omitted, C<gmtime()> uses the current time (C<gmtime(time)>).
a0d0e21e 2175
48a26b3a 2176In scalar context, C<gmtime()> returns the ctime(3) value:
0a753a76
PP
2177
2178 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
2179
fe86afc2
NC
2180If you need local time instead of GMT use the L</localtime> builtin.
2181See also the C<timegm> function provided by the C<Time::Local> module,
2182and the strftime(3) and mktime(3) functions available via the L<POSIX> module.
7660c0ab 2183
fe86afc2
NC
2184This scalar value is B<not> locale dependent (see L<perllocale>), but is
2185instead a Perl builtin. To get somewhat similar but locale dependent date
2186strings, see the example in L</localtime>.
0a753a76 2187
a0d0e21e
LW
2188=item goto LABEL
2189
748a9306
LW
2190=item goto EXPR
2191
a0d0e21e
LW
2192=item goto &NAME
2193
7660c0ab 2194The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
a0d0e21e 2195execution there. It may not be used to go into any construct that
7660c0ab 2196requires initialization, such as a subroutine or a C<foreach> loop. It
0a753a76 2197also can't be used to go into a construct that is optimized away,
19799a22 2198or to get out of a block or subroutine given to C<sort>.
0a753a76 2199It can be used to go almost anywhere else within the dynamic scope,
a0d0e21e 2200including out of subroutines, but it's usually better to use some other
19799a22 2201construct such as C<last> or C<die>. The author of Perl has never felt the
7660c0ab 2202need to use this form of C<goto> (in Perl, that is--C is another matter).
1b6921cb
BT
2203(The difference being that C does not offer named loops combined with
2204loop control. Perl does, and this replaces most structured uses of C<goto>
2205in other languages.)
a0d0e21e 2206
7660c0ab
A
2207The C<goto-EXPR> form expects a label name, whose scope will be resolved
2208dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
748a9306
LW
2209necessarily recommended if you're optimizing for maintainability:
2210
2211 goto ("FOO", "BAR", "GLARCH")[$i];
2212
1b6921cb
BT
2213The C<goto-&NAME> form is quite different from the other forms of
2214C<goto>. In fact, it isn't a goto in the normal sense at all, and
2215doesn't have the stigma associated with other gotos. Instead, it
2216exits the current subroutine (losing any changes set by local()) and
2217immediately calls in its place the named subroutine using the current
2218value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2219load another subroutine and then pretend that the other subroutine had
2220been called in the first place (except that any modifications to C<@_>
6cb9131c
GS
2221in the current subroutine are propagated to the other subroutine.)
2222After the C<goto>, not even C<caller> will be able to tell that this
2223routine was called first.
2224
2225NAME needn't be the name of a subroutine; it can be a scalar variable
2226containing a code reference, or a block which evaluates to a code
2227reference.
a0d0e21e
LW
2228
2229=item grep BLOCK LIST
2230
2231=item grep EXPR,LIST
2232
2b5ab1e7
TC
2233This is similar in spirit to, but not the same as, grep(1) and its
2234relatives. In particular, it is not limited to using regular expressions.
2f9daede 2235
a0d0e21e 2236Evaluates the BLOCK or EXPR for each element of LIST (locally setting
7660c0ab 2237C<$_> to each element) and returns the list value consisting of those
19799a22
GS
2238elements for which the expression evaluated to true. In scalar
2239context, returns the number of times the expression was true.
a0d0e21e
LW
2240
2241 @foo = grep(!/^#/, @bar); # weed out comments
2242
2243or equivalently,
2244
2245 @foo = grep {!/^#/} @bar; # weed out comments
2246
be3174d2
GS
2247Note that C<$_> is an alias to the list value, so it can be used to
2248modify the elements of the LIST. While this is useful and supported,
2249it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
2250Similarly, grep returns aliases into the original list, much as a for
2251loop's index variable aliases the list elements. That is, modifying an
19799a22
GS
2252element of a list returned by grep (for example, in a C<foreach>, C<map>
2253or another C<grep>) actually modifies the element in the original list.
2b5ab1e7 2254This is usually something to be avoided when writing clear code.
a0d0e21e 2255
a4fb8298
RGS
2256If C<$_> is lexical in the scope where the C<grep> appears (because it has
2257been declared with C<my $_>) then, in addition the be locally aliased to
2258the list elements, C<$_> keeps being lexical inside the block; i.e. it
2259can't be seen from the outside, avoiding any potential side-effects.
2260
19799a22 2261See also L</map> for a list composed of the results of the BLOCK or EXPR.
38325410 2262
a0d0e21e
LW
2263=item hex EXPR
2264
54310121 2265=item hex
bbce6d69 2266
2b5ab1e7 2267Interprets EXPR as a hex string and returns the corresponding value.
38366c11 2268(To convert strings that might start with either C<0>, C<0x>, or C<0b>, see
2b5ab1e7 2269L</oct>.) If EXPR is omitted, uses C<$_>.
2f9daede
TPG
2270
2271 print hex '0xAf'; # prints '175'
2272 print hex 'aF'; # same
a0d0e21e 2273
19799a22 2274Hex strings may only represent integers. Strings that would cause
53305cf1 2275integer overflow trigger a warning. Leading whitespace is not stripped,
38366c11
DN
2276unlike oct(). To present something as hex, look into L</printf>,
2277L</sprintf>, or L</unpack>.
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
ba053783
AL
2522=item localtime
2523
19799a22 2524Converts a time as returned by the time function to a 9-element list
5f05dabc 2525with the time analyzed for the local time zone. Typically used as
a0d0e21e
LW
2526follows:
2527
54310121 2528 # 0 1 2 3 4 5 6 7 8
a0d0e21e 2529 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
ba053783 2530 localtime(time);
a0d0e21e 2531
48a26b3a 2532All list elements are numeric, and come straight out of the C `struct
ba053783
AL
2533tm'. C<$sec>, C<$min>, and C<$hour> are the seconds, minutes, and hours
2534of the specified time.
48a26b3a 2535
ba053783
AL
2536C<$mday> is the day of the month, and C<$mon> is the month itself, in
2537the range C<0..11> with 0 indicating January and 11 indicating December.
2538This makes it easy to get a month name from a list:
54310121 2539
ba053783
AL
2540 my @abbr = qw( Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec );
2541 print "$abbr[$mon] $mday";
2542 # $mon=9, $mday=18 gives "Oct 18"
abd75f24 2543
ba053783
AL
2544C<$year> is the number of years since 1900, not just the last two digits
2545of the year. That is, C<$year> is C<123> in year 2023. The proper way
2546to get a complete 4-digit year is simply:
abd75f24 2547
ba053783 2548 $year += 1900;
abd75f24 2549
ba053783
AL
2550To get the last two digits of the year (e.g., '01' in 2001) do:
2551
2552 $year = sprintf("%02d", $year % 100);
2553
2554C<$wday> is the day of the week, with 0 indicating Sunday and 3 indicating
2555Wednesday. C<$yday> is the day of the year, in the range C<0..364>
2556(or C<0..365> in leap years.)
2557
2558C<$isdst> is true if the specified time occurs during Daylight Saving
2559Time, false otherwise.
abd75f24 2560
48a26b3a 2561If EXPR is omitted, C<localtime()> uses the current time (C<localtime(time)>).
a0d0e21e 2562
48a26b3a 2563In scalar context, C<localtime()> returns the ctime(3) value:
a0d0e21e 2564
5f05dabc 2565 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
a0d0e21e 2566
fe86afc2
NC
2567This scalar value is B<not> locale dependent but is a Perl builtin. For GMT
2568instead of local time use the L</gmtime> builtin. See also the
2569C<Time::Local> module (to convert the second, minutes, hours, ... back to
2570the integer value returned by time()), and the L<POSIX> module's strftime(3)
2571and mktime(3) functions.
2572
2573To get somewhat similar but locale dependent date strings, set up your
2574locale environment variables appropriately (please see L<perllocale>) and
2575try for example:
a3cb178b 2576
5a964f20 2577 use POSIX qw(strftime);
2b5ab1e7 2578 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
fe86afc2
NC
2579 # or for GMT formatted appropriately for your locale:
2580 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
a3cb178b
GS
2581
2582Note that the C<%a> and C<%b>, the short forms of the day of the week
2583and the month of the year, may not necessarily be three characters wide.
a0d0e21e 2584
07698885 2585=item lock THING
19799a22 2586
01e6739c 2587This function places an advisory lock on a shared variable, or referenced
03730085 2588object contained in I<THING> until the lock goes out of scope.
a6d5524e 2589
f3a23afb 2590lock() is a "weak keyword" : this means that if you've defined a function
67408cae 2591by this name (before any calls to it), that function will be called
03730085
AB
2592instead. (However, if you've said C<use threads>, lock() is always a
2593keyword.) See L<threads>.
19799a22 2594
a0d0e21e
LW
2595=item log EXPR
2596
54310121 2597=item log
bbce6d69 2598
2b5ab1e7
TC
2599Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2600returns log of C<$_>. To get the log of another base, use basic algebra:
19799a22 2601The base-N log of a number is equal to the natural log of that number
2b5ab1e7
TC
2602divided by the natural log of N. For example:
2603
2604 sub log10 {
2605 my $n = shift;
2606 return log($n)/log(10);
b76cc8ba 2607 }
2b5ab1e7
TC
2608
2609See also L</exp> for the inverse operation.
a0d0e21e 2610
a0d0e21e
LW
2611=item lstat EXPR
2612
54310121 2613=item lstat
bbce6d69 2614
19799a22 2615Does the same thing as the C<stat> function (including setting the
5a964f20
TC
2616special C<_> filehandle) but stats a symbolic link instead of the file
2617the symbolic link points to. If symbolic links are unimplemented on
c837d5b4
DP
2618your system, a normal C<stat> is done. For much more detailed
2619information, please see the documentation for C<stat>.
a0d0e21e 2620
7660c0ab 2621If EXPR is omitted, stats C<$_>.
bbce6d69 2622
a0d0e21e
LW
2623=item m//
2624
2625The match operator. See L<perlop>.
2626
2627=item map BLOCK LIST
2628
2629=item map EXPR,LIST
2630
19799a22
GS
2631Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2632C<$_> to each element) and returns the list value composed of the
2633results of each such evaluation. In scalar context, returns the
2634total number of elements so generated. Evaluates BLOCK or EXPR in
2635list context, so each element of LIST may produce zero, one, or
2636more elements in the returned value.
dd99ebda 2637
a0d0e21e
LW
2638 @chars = map(chr, @nums);
2639
2640translates a list of numbers to the corresponding characters. And
2641
4633a7c4 2642 %hash = map { getkey($_) => $_ } @array;
a0d0e21e
LW
2643
2644is just a funny way to write
2645
2646 %hash = ();
2647 foreach $_ (@array) {
4633a7c4 2648 $hash{getkey($_)} = $_;
a0d0e21e
LW
2649 }
2650
be3174d2
GS
2651Note that C<$_> is an alias to the list value, so it can be used to
2652modify the elements of the LIST. While this is useful and supported,
2653it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
2654Using a regular C<foreach> loop for this purpose would be clearer in
2655most cases. See also L</grep> for an array composed of those items of
2656the original list for which the BLOCK or EXPR evaluates to true.
fb73857a 2657
a4fb8298
RGS
2658If C<$_> is lexical in the scope where the C<map> appears (because it has
2659been declared with C<my $_>) then, in addition the be locally aliased to
2660the list elements, C<$_> keeps being lexical inside the block; i.e. it
2661can't be seen from the outside, avoiding any potential side-effects.
2662
205fdb4d
NC
2663C<{> starts both hash references and blocks, so C<map { ...> could be either
2664the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look
2665ahead for the closing C<}> it has to take a guess at which its dealing with
2666based what it finds just after the C<{>. Usually it gets it right, but if it
2667doesn't it won't realize something is wrong until it gets to the C<}> and
2668encounters the missing (or unexpected) comma. The syntax error will be
2669reported close to the C<}> but you'll need to change something near the C<{>
2670such as using a unary C<+> to give perl some help:
2671
2672 %hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong
2673 %hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right
2674 %hash = map { ("\L$_", 1) } @array # this also works
2675 %hash = map { lc($_), 1 } @array # as does this.
2676 %hash = map +( lc($_), 1 ), @array # this is EXPR and works!
cea6626f 2677
205fdb4d
NC
2678 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
2679
2680or to force an anon hash constructor use C<+{>
2681
2682 @hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end
2683
2684and you get list of anonymous hashes each with only 1 entry.
2685
19799a22 2686=item mkdir FILENAME,MASK
a0d0e21e 2687
5a211162
GS
2688=item mkdir FILENAME
2689
0591cd52 2690Creates the directory specified by FILENAME, with permissions
19799a22
GS
2691specified by MASK (as modified by C<umask>). If it succeeds it
2692returns true, otherwise it returns false and sets C<$!> (errno).
5a211162 2693If omitted, MASK defaults to 0777.
0591cd52 2694
19799a22 2695In general, it is better to create directories with permissive MASK,
0591cd52 2696and let the user modify that with their C<umask>, than it is to supply
19799a22 2697a restrictive MASK and give the user no way to be more permissive.
0591cd52
NT
2698The exceptions to this rule are when the file or directory should be
2699kept private (mail files, for instance). The perlfunc(1) entry on
19799a22 2700C<umask> discusses the choice of MASK in more detail.
a0d0e21e 2701
cc1852e8
JH
2702Note that according to the POSIX 1003.1-1996 the FILENAME may have any
2703number of trailing slashes. Some operating and filesystems do not get
2704this right, so Perl automatically removes all trailing slashes to keep
2705everyone happy.
2706
a0d0e21e
LW
2707=item msgctl ID,CMD,ARG
2708
f86cebdf 2709Calls the System V IPC function msgctl(2). You'll probably have to say
0ade1984
JH
2710
2711 use IPC::SysV;
2712
7660c0ab
A
2713first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2714then ARG must be a variable which will hold the returned C<msqid_ds>
951ba7fe
GS
2715structure. Returns like C<ioctl>: the undefined value for error,
2716C<"0 but true"> for zero, or the actual return value otherwise. See also
4755096e 2717L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation.
a0d0e21e
LW
2718
2719=item msgget KEY,FLAGS
2720
f86cebdf 2721Calls the System V IPC function msgget(2). Returns the message queue
4755096e
GS
2722id, or the undefined value if there is an error. See also
2723L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation.
a0d0e21e 2724
a0d0e21e
LW
2725=item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2726
2727Calls the System V IPC function msgrcv to receive a message from
2728message queue ID into variable VAR with a maximum message size of
41d6edb2
JH
2729SIZE. Note that when a message is received, the message type as a
2730native long integer will be the first thing in VAR, followed by the
2731actual message. This packing may be opened with C<unpack("l! a*")>.
2732Taints the variable. Returns true if successful, or false if there is
4755096e
GS
2733an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and
2734C<IPC::SysV::Msg> documentation.
41d6edb2
JH
2735
2736=item msgsnd ID,MSG,FLAGS
2737
2738Calls the System V IPC function msgsnd to send the message MSG to the
2739message queue ID. MSG must begin with the native long integer message
2740type, and be followed by the length of the actual message, and finally
2741the message itself. This kind of packing can be achieved with
2742C<pack("l! a*", $type, $message)>. Returns true if successful,
2743or false if there is an error. See also C<IPC::SysV>
2744and C<IPC::SysV::Msg> documentation.
a0d0e21e
LW
2745
2746=item my EXPR
2747
307ea6df
JH
2748=item my TYPE EXPR
2749
1d2de774 2750=item my EXPR : ATTRS
09bef843 2751
1d2de774 2752=item my TYPE EXPR : ATTRS
307ea6df 2753
19799a22 2754A C<my> declares the listed variables to be local (lexically) to the
1d2de774
JH
2755enclosing block, file, or C<eval>. If more than one value is listed,
2756the list must be placed in parentheses.
307ea6df 2757
1d2de774
JH
2758The exact semantics and interface of TYPE and ATTRS are still
2759evolving. TYPE is currently bound to the use of C<fields> pragma,
307ea6df
JH
2760and attributes are handled using the C<attributes> pragma, or starting
2761from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
2762L<perlsub/"Private Variables via my()"> for details, and L<fields>,
2763L<attributes>, and L<Attribute::Handlers>.
4633a7c4 2764
a0d0e21e
LW
2765=item next LABEL
2766
2767=item next
2768
2769The C<next> command is like the C<continue> statement in C; it starts
2770the next iteration of the loop:
2771
4633a7c4
LW
2772 LINE: while (<STDIN>) {
2773 next LINE if /^#/; # discard comments
5a964f20 2774 #...
a0d0e21e
LW
2775 }
2776
2777Note that if there were a C<continue> block on the above, it would get
2778executed even on discarded lines. If the LABEL is omitted, the command
2779refers to the innermost enclosing loop.
2780
4968c1e4 2781C<next> cannot be used to exit a block which returns a value such as
2b5ab1e7
TC
2782C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2783a grep() or map() operation.
4968c1e4 2784
6c1372ed
GS
2785Note that a block by itself is semantically identical to a loop
2786that executes once. Thus C<next> will exit such a block early.
2787
98293880
JH
2788See also L</continue> for an illustration of how C<last>, C<next>, and
2789C<redo> work.
1d2dff63 2790
4a66ea5a
RGS
2791=item no Module VERSION LIST
2792
2793=item no Module VERSION
2794
a0d0e21e
LW
2795=item no Module LIST
2796
4a66ea5a
RGS
2797=item no Module
2798
593b9c14 2799See the C<use> function, of which C<no> is the opposite.
a0d0e21e
LW
2800
2801=item oct EXPR
2802
54310121 2803=item oct
bbce6d69 2804
4633a7c4 2805Interprets EXPR as an octal string and returns the corresponding
4f19785b
WSI
2806value. (If EXPR happens to start off with C<0x>, interprets it as a
2807hex string. If EXPR starts off with C<0b>, it is interpreted as a
53305cf1
NC
2808binary string. Leading whitespace is ignored in all three cases.)
2809The following will handle decimal, binary, octal, and hex in the standard
2810Perl or C notation:
a0d0e21e
LW
2811
2812 $val = oct($val) if $val =~ /^0/;
2813
19799a22
GS
2814If EXPR is omitted, uses C<$_>. To go the other way (produce a number
2815in octal), use sprintf() or printf():
2816
2817 $perms = (stat("filename"))[2] & 07777;
2818 $oct_perms = sprintf "%lo", $perms;
2819
2820The oct() function is commonly used when a string such as C<644> needs
2821to be converted into a file mode, for example. (Although perl will
2822automatically convert strings into numbers as needed, this automatic
2823conversion assumes base 10.)
a0d0e21e
LW
2824
2825=item open FILEHANDLE,EXPR
2826
68bd7414
NIS
2827=item open FILEHANDLE,MODE,EXPR
2828
2829=item open FILEHANDLE,MODE,EXPR,LIST
2830
ba964c95
T
2831=item open FILEHANDLE,MODE,REFERENCE
2832
a0d0e21e
LW
2833=item open FILEHANDLE
2834
2835Opens the file whose filename is given by EXPR, and associates it with
ed53a2bb
JH
2836FILEHANDLE.
2837
2838(The following is a comprehensive reference to open(): for a gentler
2839introduction you may consider L<perlopentut>.)
2840
a28cd5c9
NT
2841If FILEHANDLE is an undefined scalar variable (or array or hash element)
2842the variable is assigned a reference to a new anonymous filehandle,
2843otherwise if FILEHANDLE is an expression, its value is used as the name of
2844the real filehandle wanted. (This is considered a symbolic reference, so
2845C<use strict 'refs'> should I<not> be in effect.)
ed53a2bb
JH
2846
2847If EXPR is omitted, the scalar variable of the same name as the
2848FILEHANDLE contains the filename. (Note that lexical variables--those
2849declared with C<my>--will not work for this purpose; so if you're
67408cae 2850using C<my>, specify EXPR in your call to open.)
ed53a2bb
JH
2851
2852If three or more arguments are specified then the mode of opening and
2853the file name are separate. If MODE is C<< '<' >> or nothing, the file
2854is opened for input. If MODE is C<< '>' >>, the file is truncated and
2855opened for output, being created if necessary. If MODE is C<<< '>>' >>>,
b76cc8ba 2856the file is opened for appending, again being created if necessary.
5a964f20 2857
ed53a2bb
JH
2858You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to
2859indicate that you want both read and write access to the file; thus
2860C<< '+<' >> is almost always preferred for read/write updates--the C<<
2861'+>' >> mode would clobber the file first. You can't usually use
2862either read-write mode for updating textfiles, since they have
2863variable length records. See the B<-i> switch in L<perlrun> for a
2864better approach. The file is created with permissions of C<0666>
2865modified by the process' C<umask> value.
2866
2867These various prefixes correspond to the fopen(3) modes of C<'r'>,
2868C<'r+'>, C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
5f05dabc 2869
6170680b
IZ
2870In the 2-arguments (and 1-argument) form of the call the mode and
2871filename should be concatenated (in this order), possibly separated by
68bd7414
NIS
2872spaces. It is possible to omit the mode in these forms if the mode is
2873C<< '<' >>.
6170680b 2874
7660c0ab 2875If the filename begins with C<'|'>, the filename is interpreted as a
5a964f20 2876command to which output is to be piped, and if the filename ends with a
f244e06d
GS
2877C<'|'>, the filename is interpreted as a command which pipes output to
2878us. See L<perlipc/"Using open() for IPC">
19799a22 2879for more examples of this. (You are not allowed to C<open> to a command
5a964f20 2880that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
4a4eefd0
GS
2881and L<perlipc/"Bidirectional Communication with Another Process">
2882for alternatives.)
cb1a09d0 2883
ed53a2bb
JH
2884For three or more arguments if MODE is C<'|-'>, the filename is
2885interpreted as a command to which output is to be piped, and if MODE
2886is C<'-|'>, the filename is interpreted as a command which pipes
2887output to us. In the 2-arguments (and 1-argument) form one should
2888replace dash (C<'-'>) with the command.
2889See L<perlipc/"Using open() for IPC"> for more examples of this.
2890(You are not allowed to C<open> to a command that pipes both in I<and>
2891out, but see L<IPC::Open2>, L<IPC::Open3>, and
2892L<perlipc/"Bidirectional Communication"> for alternatives.)
2893
2894In the three-or-more argument form of pipe opens, if LIST is specified
2895(extra arguments after the command name) then LIST becomes arguments
2896to the command invoked if the platform supports it. The meaning of
2897C<open> with more than three arguments for non-pipe modes is not yet
2898specified. Experimental "layers" may give extra LIST arguments
2899meaning.
6170680b
IZ
2900
2901In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
b76cc8ba 2902and opening C<< '>-' >> opens STDOUT.
6170680b 2903
fae2c0fb
RGS
2904You may use the three-argument form of open to specify IO "layers"
2905(sometimes also referred to as "disciplines") to be applied to the handle
2906that affect how the input and output are processed (see L<open> and
2907L<PerlIO> for more details). For example
7207e29d 2908
9124316e
JH
2909 open(FH, "<:utf8", "file")
2910
2911will open the UTF-8 encoded file containing Unicode characters,
fae2c0fb
RGS
2912see L<perluniintro>. (Note that if layers are specified in the
2913three-arg form then default layers set by the C<open> pragma are
01e6739c 2914ignored.)
ed53a2bb
JH
2915
2916Open returns nonzero upon success, the undefined value otherwise. If
2917the C<open> involved a pipe, the return value happens to be the pid of
2918the subprocess.
cb1a09d0 2919
ed53a2bb
JH
2920If you're running Perl on a system that distinguishes between text
2921files and binary files, then you should check out L</binmode> for tips
2922for dealing with this. The key distinction between systems that need
2923C<binmode> and those that don't is their text file formats. Systems
8939ba94 2924like Unix, Mac OS, and Plan 9, which delimit lines with a single
ed53a2bb
JH
2925character, and which encode that character in C as C<"\n">, do not
2926need C<binmode>. The rest need it.
cb1a09d0 2927
fb73857a 2928When opening a file, it's usually a bad idea to continue normal execution
19799a22
GS
2929if the request failed, so C<open> is frequently used in connection with
2930C<die>. Even if C<die> won't do what you want (say, in a CGI script,
fb73857a 2931where you want to make a nicely formatted error message (but there are
5a964f20 2932modules that can help with that problem)) you should always check
19799a22 2933the return value from opening a file. The infrequent exception is when
fb73857a
PP
2934working with an unopened filehandle is actually what you want to do.
2935
ed53a2bb
JH
2936As a special case the 3 arg form with a read/write mode and the third
2937argument being C<undef>:
b76cc8ba
NIS
2938
2939 open(TMP, "+>", undef) or die ...
2940
f253e835
JH
2941opens a filehandle to an anonymous temporary file. Also using "+<"
2942works for symmetry, but you really should consider writing something
2943to the temporary file first. You will need to seek() to do the
2944reading.
b76cc8ba 2945
2ce64696
JC
2946Since v5.8.0, perl has built using PerlIO by default. Unless you've
2947changed this (ie Configure -Uuseperlio), you can open file handles to
2948"in memory" files held in Perl scalars via:
ba964c95 2949
b996200f
SB
2950 open($fh, '>', \$variable) || ..
2951
2952Though if you try to re-open C<STDOUT> or C<STDERR> as an "in memory"
2953file, you have to close it first:
2954
2955 close STDOUT;
2956 open STDOUT, '>', \$variable or die "Can't open STDOUT: $!";
ba964c95 2957
cb1a09d0 2958Examples:
a0d0e21e
LW
2959
2960 $ARTICLE = 100;
2961 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2962 while (<ARTICLE>) {...
2963
6170680b 2964 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
fb73857a 2965 # if the open fails, output is discarded
a0d0e21e 2966
6170680b 2967 open(DBASE, '+<', 'dbase.mine') # open for update
fb73857a 2968 or die "Can't open 'dbase.mine' for update: $!";
cb1a09d0 2969
6170680b
IZ
2970 open(DBASE, '+<dbase.mine') # ditto
2971 or die "Can't open 'dbase.mine' for update: $!";
2972
2973 open(ARTICLE, '-|', "caesar <$article") # decrypt article
fb73857a 2974 or die "Can't start caesar: $!";
a0d0e21e 2975
6170680b
IZ
2976 open(ARTICLE, "caesar <$article |") # ditto
2977 or die "Can't start caesar: $!";
2978
2359510d 2979 open(EXTRACT, "|sort >Tmp$$") # $$ is our process id
fb73857a 2980 or die "Can't start sort: $!";
a0d0e21e 2981
ba964c95
T
2982 # in memory files
2983 open(MEMORY,'>', \$var)
2984 or die "Can't open memory file: $!";
2985 print MEMORY "foo!\n"; # output will end up in $var
2986
a0d0e21e
LW
2987 # process argument list of files along with any includes
2988
2989 foreach $file (@ARGV) {
2990 process($file, 'fh00');
2991 }
2992
2993 sub process {
5a964f20 2994 my($filename, $input) = @_;
a0d0e21e
LW
2995 $input++; # this is a string increment
2996 unless (open($input, $filename)) {
2997 print STDERR "Can't open $filename: $!\n";
2998 return;
2999 }
3000
5a964f20 3001 local $_;
a0d0e21e
LW
3002 while (<$input>) { # note use of indirection
3003 if (/^#include "(.*)"/) {
3004 process($1, $input);
3005 next;
3006 }
5a964f20 3007 #... # whatever
a0d0e21e
LW
3008 }
3009 }
3010
ae4c5402 3011See L<perliol> for detailed info on PerlIO.
2ce64696 3012
a0d0e21e 3013You may also, in the Bourne shell tradition, specify an EXPR beginning
00cafafa
JH
3014with C<< '>&' >>, in which case the rest of the string is interpreted
3015as the name of a filehandle (or file descriptor, if numeric) to be
3016duped (as L<dup(2)>) and opened. You may use C<&> after C<< > >>,
3017C<<< >> >>>, C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>.
3018The mode you specify should match the mode of the original filehandle.
3019(Duping a filehandle does not take into account any existing contents
3020of IO buffers.) If you use the 3 arg form then you can pass either a
3021number, the name of a filehandle or the normal "reference to a glob".
6170680b 3022
eae1b76b
SB
3023Here is a script that saves, redirects, and restores C<STDOUT> and
3024C<STDERR> using various methods:
a0d0e21e
LW
3025
3026 #!/usr/bin/perl
eae1b76b
SB
3027 open my $oldout, ">&STDOUT" or die "Can't dup STDOUT: $!";
3028 open OLDERR, ">&", \*STDERR or die "Can't dup STDERR: $!";
818c4caa 3029
eae1b76b
SB
3030 open STDOUT, '>', "foo.out" or die "Can't redirect STDOUT: $!";
3031 open STDERR, ">&STDOUT" or die "Can't dup STDOUT: $!";
a0d0e21e 3032
eae1b76b
SB
3033 select STDERR; $| = 1; # make unbuffered
3034 select STDOUT; $| = 1; # make unbuffered
a0d0e21e
LW
3035
3036 print STDOUT "stdout 1\n"; # this works for
3037 print STDERR "stderr 1\n"; # subprocesses too
3038
eae1b76b
SB
3039 open STDOUT, ">&", $oldout or die "Can't dup \$oldout: $!";
3040 open STDERR, ">&OLDERR" or die "Can't dup OLDERR: $!";
a0d0e21e
LW
3041
3042 print STDOUT "stdout 2\n";
3043 print STDERR "stderr 2\n";
3044
ef8b303f
JH
3045If you specify C<< '<&=X' >>, where C<X> is a file descriptor number
3046or a filehandle, then Perl will do an equivalent of C's C<fdopen> of
3047that file descriptor (and not call L<dup(2)>); this is more
3048parsimonious of file descriptors. For example:
a0d0e21e 3049
00cafafa 3050 # open for input, reusing the fileno of $fd
a0d0e21e 3051 open(FILEHANDLE, "<&=$fd")
df632fdf 3052
b76cc8ba 3053or
df632fdf 3054
b76cc8ba 3055 open(FILEHANDLE, "<&=", $fd)
a0d0e21e 3056
00cafafa
JH
3057or
3058
3059 # open for append, using the fileno of OLDFH
3060 open(FH, ">>&=", OLDFH)
3061
3062or
3063
3064 open(FH, ">>&=OLDFH")
3065
ef8b303f
JH
3066Being parsimonious on filehandles is also useful (besides being
3067parsimonious) for example when something is dependent on file
3068descriptors, like for example locking using flock(). If you do just
3069C<< open(A, '>>&B') >>, the filehandle A will not have the same file
3070descriptor as B, and therefore flock(A) will not flock(B), and vice
3071versa. But with C<< open(A, '>>&=B') >> the filehandles will share
3072the same file descriptor.
3073
3074Note that if you are using Perls older than 5.8.0, Perl will be using
3075the standard C libraries' fdopen() to implement the "=" functionality.
3076On many UNIX systems fdopen() fails when file descriptors exceed a
3077certain value, typically 255. For Perls 5.8.0 and later, PerlIO is
3078most often the default.
4af147f6 3079
df632fdf
JH
3080You can see whether Perl has been compiled with PerlIO or not by
3081running C<perl -V> and looking for C<useperlio=> line. If C<useperlio>
3082is C<define>, you have PerlIO, otherwise you don't.
3083
6170680b
IZ
3084If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
3085with 2-arguments (or 1-argument) form of open(), then
a0d0e21e 3086there is an implicit fork done, and the return value of open is the pid
7660c0ab 3087of the child within the parent process, and C<0> within the child
184e9718 3088process. (Use C<defined($pid)> to determine whether the open was successful.)
a0d0e21e
LW
3089The filehandle behaves normally for the parent, but i/o to that
3090filehandle is piped from/to the STDOUT/STDIN of the child process.
3091In the child process the filehandle isn't opened--i/o happens from/to
3092the new STDOUT or STDIN. Typically this is used like the normal
3093piped open when you want to exercise more control over just how the
3094pipe command gets executed, such as when you are running setuid, and
54310121 3095don't want to have to scan shell commands for metacharacters.
6170680b 3096The following triples are more or less equivalent:
a0d0e21e
LW
3097
3098 open(FOO, "|tr '[a-z]' '[A-Z]'");
6170680b
IZ
3099 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
3100 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
b76cc8ba 3101 open(FOO, '|-', "tr", '[a-z]', '[A-Z]');
a0d0e21e
LW
3102
3103 open(FOO, "cat -n '$file'|");
6170680b
IZ
3104 open(FOO, '-|', "cat -n '$file'");
3105 open(FOO, '-|') || exec 'cat', '-n', $file;
b76cc8ba
NIS
3106 open(FOO, '-|', "cat", '-n', $file);
3107
3108The last example in each block shows the pipe as "list form", which is
64da03b2
JH
3109not yet supported on all platforms. A good rule of thumb is that if
3110your platform has true C<fork()> (in other words, if your platform is
3111UNIX) you can use the list form.
a0d0e21e 3112
4633a7c4
LW
3113See L<perlipc/"Safe Pipe Opens"> for more examples of this.
3114
0f897271
GS
3115Beginning with v5.6.0, Perl will attempt to flush all files opened for
3116output before any operation that may do a fork, but this may not be
3117supported on some platforms (see L<perlport>). To be safe, you may need
3118to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
3119of C<IO::Handle> on any open handles.
3120
ed53a2bb
JH
3121On systems that support a close-on-exec flag on files, the flag will
3122be set for the newly opened file descriptor as determined by the value
3123of $^F. See L<perlvar/$^F>.
a0d0e21e 3124
0dccf244
CS
3125Closing any piped filehandle causes the parent process to wait for the
3126child to finish, and returns the status value in C<$?>.
3127
ed53a2bb
JH
3128The filename passed to 2-argument (or 1-argument) form of open() will
3129have leading and trailing whitespace deleted, and the normal
3130redirection characters honored. This property, known as "magic open",
5a964f20 3131can often be used to good effect. A user could specify a filename of
7660c0ab 3132F<"rsh cat file |">, or you could change certain filenames as needed:
5a964f20
TC
3133
3134 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
3135 open(FH, $filename) or die "Can't open $filename: $!";
3136
6170680b
IZ
3137Use 3-argument form to open a file with arbitrary weird characters in it,
3138
3139 open(FOO, '<', $file);
3140
3141otherwise it's necessary to protect any leading and trailing whitespace:
5a964f20
TC
3142
3143 $file =~ s#^(\s)#./$1#;
3144 open(FOO, "< $file\0");
3145
a31a806a 3146(this may not work on some bizarre filesystems). One should
106325ad 3147conscientiously choose between the I<magic> and 3-arguments form
6170680b
IZ
3148of open():
3149
3150 open IN, $ARGV[0];
3151
3152will allow the user to specify an argument of the form C<"rsh cat file |">,
3153but will not work on a filename which happens to have a trailing space, while
3154
3155 open IN, '<', $ARGV[0];
3156
3157will have exactly the opposite restrictions.
3158
19799a22 3159If you want a "real" C C<open> (see L<open(2)> on your system), then you
6170680b
IZ
3160should use the C<sysopen> function, which involves no such magic (but
3161may use subtly different filemodes than Perl open(), which is mapped
3162to C fopen()). This is
5a964f20
TC
3163another way to protect your filenames from interpretation. For example:
3164
3165 use IO::Handle;
3166 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
3167 or die "sysopen $path: $!";
3168 $oldfh = select(HANDLE); $| = 1; select($oldfh);
38762f02 3169 print HANDLE "stuff $$\n";
5a964f20
TC
3170 seek(HANDLE, 0, 0);
3171 print "File contains: ", <HANDLE>;
3172
7660c0ab
A
3173Using the constructor from the C<IO::Handle> package (or one of its
3174subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
5a964f20
TC
3175filehandles that have the scope of whatever variables hold references to
3176them, and automatically close whenever and however you leave that scope:
c07a80fd 3177
5f05dabc 3178 use IO::File;
5a964f20 3179 #...
c07a80fd
PP
3180 sub read_myfile_munged {
3181 my $ALL = shift;
5f05dabc 3182 my $handle = new IO::File;
c07a80fd
PP
3183 open($handle, "myfile") or die "myfile: $!";
3184 $first = <$handle>
3185 or return (); # Automatically closed here.
3186 mung $first or die "mung failed"; # Or here.
3187 return $first, <$handle> if $ALL; # Or here.
3188 $first; # Or here.
3189 }
3190
b687b08b 3191See L</seek> for some details about mixing reading and writing.
a0d0e21e
LW
3192
3193=item opendir DIRHANDLE,EXPR
3194
19799a22
GS
3195Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
3196C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
a28cd5c9
NT
3197DIRHANDLE may be an expression whose value can be used as an indirect
3198dirhandle, usually the real dirhandle name. If DIRHANDLE is an undefined
3199scalar variable (or array or hash element), the variable is assigned a
3200reference to a new anonymous dirhandle.
a0d0e21e
LW
3201DIRHANDLEs have their own namespace separate from FILEHANDLEs.
3202
3203=item ord EXPR
3204
54310121 3205=item ord
bbce6d69 3206
121910a4
JH
3207Returns the numeric (the native 8-bit encoding, like ASCII or EBCDIC,
3208or Unicode) value of the first character of EXPR. If EXPR is omitted,
3209uses C<$_>.
3210
3211For the reverse, see L</chr>.
3212See L<perlunicode> and L<encoding> for more about Unicode.
a0d0e21e 3213
77ca0c92
LW
3214=item our EXPR
3215
307ea6df
JH
3216=item our EXPR TYPE
3217
1d2de774 3218=item our EXPR : ATTRS
9969eac4 3219
1d2de774 3220=item our TYPE EXPR : ATTRS
307ea6df 3221
77ca0c92
LW
3222An C<our> declares the listed variables to be valid globals within
3223the enclosing block, file, or C<eval>. That is, it has the same
3224scoping rules as a "my" declaration, but does not create a local
3225variable. If more than one value is listed, the list must be placed
3226in parentheses. The C<our> declaration has no semantic effect unless
3227"use strict vars" is in effect, in which case it lets you use the
3228declared global variable without qualifying it with a package name.
3229(But only within the lexical scope of the C<our> declaration. In this
3230it differs from "use vars", which is package scoped.)
3231
f472eb5c
GS
3232An C<our> declaration declares a global variable that will be visible
3233across its entire lexical scope, even across package boundaries. The
3234package in which the variable is entered is determined at the point
3235of the declaration, not at the point of use. This means the following
3236behavior holds:
3237
3238 package Foo;
3239 our $bar; # declares $Foo::bar for rest of lexical scope
3240 $bar = 20;
3241
3242 package Bar;
3243 print $bar; # prints 20
3244
3245Multiple C<our> declarations in the same lexical scope are allowed
3246if they are in different packages. If they happened to be in the same
3247package, Perl will emit warnings if you have asked for them.
3248
3249 use warnings;
3250 package Foo;
3251 our $bar; # declares $Foo::bar for rest of lexical scope
3252 $bar = 20;
3253
3254 package Bar;
3255 our $bar = 30; # declares $Bar::bar for rest of lexical scope
3256 print $bar; # prints 30
3257
3258 our $bar; # emits warning
3259
9969eac4 3260An C<our> declaration may also have a list of attributes associated
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