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1=head1 NAME
2
3perlfunc - Perl builtin functions
4
5=head1 DESCRIPTION
6
7The functions in this section can serve as terms in an expression.
8They fall into two major categories: list operators and named unary
9operators. These differ in their precedence relationship with a
10following comma. (See the precedence table in L<perlop>.) List
11operators take more than one argument, while unary operators can never
12take more than one argument. Thus, a comma terminates the argument of
13a unary operator, but merely separates the arguments of a list
14operator. A unary operator generally provides a scalar context to its
2b5ab1e7 15argument, while a list operator may provide either scalar or list
a0d0e21e 16contexts for its arguments. If it does both, the scalar arguments will
5f05dabc 17be first, and the list argument will follow. (Note that there can ever
0f31cffe 18be only one such list argument.) For instance, splice() has three scalar
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19arguments followed by a list, whereas gethostbyname() has four scalar
20arguments.
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21
22In the syntax descriptions that follow, list operators that expect a
23list (and provide list context for the elements of the list) are shown
24with LIST as an argument. Such a list may consist of any combination
25of scalar arguments or list values; the list values will be included
26in the list as if each individual element were interpolated at that
27point in the list, forming a longer single-dimensional list value.
28Elements of the LIST should be separated by commas.
29
30Any function in the list below may be used either with or without
31parentheses around its arguments. (The syntax descriptions omit the
5f05dabc 32parentheses.) If you use the parentheses, the simple (but occasionally
19799a22 33surprising) rule is this: It I<looks> like a function, therefore it I<is> a
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34function, and precedence doesn't matter. Otherwise it's a list
35operator or unary operator, and precedence does matter. And whitespace
36between the function and left parenthesis doesn't count--so you need to
37be careful sometimes:
38
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39 print 1+2+4; # Prints 7.
40 print(1+2) + 4; # Prints 3.
41 print (1+2)+4; # Also prints 3!
42 print +(1+2)+4; # Prints 7.
43 print ((1+2)+4); # Prints 7.
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44
45If you run Perl with the B<-w> switch it can warn you about this. For
46example, the third line above produces:
47
48 print (...) interpreted as function at - line 1.
49 Useless use of integer addition in void context at - line 1.
50
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51A few functions take no arguments at all, and therefore work as neither
52unary nor list operators. These include such functions as C<time>
53and C<endpwent>. For example, C<time+86_400> always means
54C<time() + 86_400>.
55
a0d0e21e 56For functions that can be used in either a scalar or list context,
54310121 57nonabortive failure is generally indicated in a scalar context by
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58returning the undefined value, and in a list context by returning the
59null list.
60
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61Remember the following important rule: There is B<no rule> that relates
62the behavior of an expression in list context to its behavior in scalar
63context, or vice versa. It might do two totally different things.
a0d0e21e 64Each operator and function decides which sort of value it would be most
2b5ab1e7 65appropriate to return in scalar context. Some operators return the
5a964f20 66length of the list that would have been returned in list context. Some
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67operators return the first value in the list. Some operators return the
68last value in the list. Some operators return a count of successful
69operations. In general, they do what you want, unless you want
70consistency.
71
d1be9408 72A named array in scalar context is quite different from what would at
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73first glance appear to be a list in scalar context. You can't get a list
74like C<(1,2,3)> into being in scalar context, because the compiler knows
75the context at compile time. It would generate the scalar comma operator
76there, not the list construction version of the comma. That means it
77was never a list to start with.
78
79In general, functions in Perl that serve as wrappers for system calls
f86cebdf 80of the same name (like chown(2), fork(2), closedir(2), etc.) all return
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81true when they succeed and C<undef> otherwise, as is usually mentioned
82in the descriptions below. This is different from the C interfaces,
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83which return C<-1> on failure. Exceptions to this rule are C<wait>,
84C<waitpid>, and C<syscall>. System calls also set the special C<$!>
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85variable on failure. Other functions do not, except accidentally.
86
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87=head2 Perl Functions by Category
88
89Here are Perl's functions (including things that look like
5a964f20 90functions, like some keywords and named operators)
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91arranged by category. Some functions appear in more
92than one place.
93
13a2d996 94=over 4
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95
96=item Functions for SCALARs or strings
97
22fae026 98C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>,
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99C<length>, C<oct>, C<ord>, C<pack>, C<q/STRING/>, C<qq/STRING/>, C<reverse>,
100C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///>
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101
102=item Regular expressions and pattern matching
103
ab4f32c2 104C<m//>, C<pos>, C<quotemeta>, C<s///>, C<split>, C<study>, C<qr//>
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105
106=item Numeric functions
107
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108C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>,
109C<sin>, C<sqrt>, C<srand>
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110
111=item Functions for real @ARRAYs
112
22fae026 113C<pop>, C<push>, C<shift>, C<splice>, C<unshift>
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114
115=item Functions for list data
116
ab4f32c2 117C<grep>, C<join>, C<map>, C<qw/STRING/>, C<reverse>, C<sort>, C<unpack>
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118
119=item Functions for real %HASHes
120
22fae026 121C<delete>, C<each>, C<exists>, C<keys>, C<values>
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122
123=item Input and output functions
124
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125C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
126C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
127C<readdir>, C<rewinddir>, C<seek>, C<seekdir>, C<select>, C<syscall>,
128C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>,
129C<warn>, C<write>
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130
131=item Functions for fixed length data or records
132
22fae026 133C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec>
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134
135=item Functions for filehandles, files, or directories
136
22fae026 137C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>,
5ff3f7a4 138C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>,
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139C<readlink>, C<rename>, C<rmdir>, C<stat>, C<symlink>, C<sysopen>,
140C<umask>, C<unlink>, C<utime>
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141
142=item Keywords related to the control flow of your perl program
143
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144C<caller>, C<continue>, C<die>, C<do>, C<dump>, C<eval>, C<exit>,
145C<goto>, C<last>, C<next>, C<redo>, C<return>, C<sub>, C<wantarray>
cb1a09d0 146
54310121 147=item Keywords related to scoping
cb1a09d0 148
4375e838 149C<caller>, C<import>, C<local>, C<my>, C<our>, C<package>, C<use>
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150
151=item Miscellaneous functions
152
4375e838 153C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<our>, C<reset>,
22fae026 154C<scalar>, C<undef>, C<wantarray>
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155
156=item Functions for processes and process groups
157
22fae026 158C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
ab4f32c2 159C<pipe>, C<qx/STRING/>, C<setpgrp>, C<setpriority>, C<sleep>, C<system>,
22fae026 160C<times>, C<wait>, C<waitpid>
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161
162=item Keywords related to perl modules
163
22fae026 164C<do>, C<import>, C<no>, C<package>, C<require>, C<use>
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165
166=item Keywords related to classes and object-orientedness
167
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168C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
169C<untie>, C<use>
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170
171=item Low-level socket functions
172
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173C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
174C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
737dd4b4 175C<socket>, C<socketpair>
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176
177=item System V interprocess communication functions
178
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179C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
180C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>
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181
182=item Fetching user and group info
183
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184C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
185C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
186C<getpwuid>, C<setgrent>, C<setpwent>
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187
188=item Fetching network info
189
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190C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
191C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
192C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
193C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
194C<setnetent>, C<setprotoent>, C<setservent>
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195
196=item Time-related functions
197
22fae026 198C<gmtime>, C<localtime>, C<time>, C<times>
cb1a09d0 199
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200=item Functions new in perl5
201
22fae026 202C<abs>, C<bless>, C<chomp>, C<chr>, C<exists>, C<formline>, C<glob>,
b76cc8ba 203C<import>, C<lc>, C<lcfirst>, C<map>, C<my>, C<no>, C<our>, C<prototype>,
4375e838 204C<qx>, C<qw>, C<readline>, C<readpipe>, C<ref>, C<sub*>, C<sysopen>, C<tie>,
22fae026 205C<tied>, C<uc>, C<ucfirst>, C<untie>, C<use>
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206
207* - C<sub> was a keyword in perl4, but in perl5 it is an
5a964f20 208operator, which can be used in expressions.
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209
210=item Functions obsoleted in perl5
211
22fae026 212C<dbmclose>, C<dbmopen>
37798a01 213
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214=back
215
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216=head2 Portability
217
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218Perl was born in Unix and can therefore access all common Unix
219system calls. In non-Unix environments, the functionality of some
220Unix system calls may not be available, or details of the available
221functionality may differ slightly. The Perl functions affected
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222by this are:
223
224C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>,
225C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>,
226C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>,
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227C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostbyname>,
228C<gethostent>, C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
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229C<getppid>, C<getprgp>, C<getpriority>, C<getprotobynumber>,
230C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>,
231C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>,
232C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>,
2b5ab1e7 233C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<rename>, C<select>, C<semctl>,
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234C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>,
235C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>,
236C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>,
737dd4b4 237C<shmwrite>, C<socket>, C<socketpair>,
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238C<stat>, C<symlink>, C<syscall>, C<sysopen>, C<system>,
239C<times>, C<truncate>, C<umask>, C<unlink>,
2b5ab1e7 240C<utime>, C<wait>, C<waitpid>
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241
242For more information about the portability of these functions, see
243L<perlport> and other available platform-specific documentation.
244
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245=head2 Alphabetical Listing of Perl Functions
246
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247=over 8
248
5b3c99c0 249=item -X FILEHANDLE
a0d0e21e 250
5b3c99c0 251=item -X EXPR
a0d0e21e 252
5b3c99c0 253=item -X
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254
255A file test, where X is one of the letters listed below. This unary
256operator takes one argument, either a filename or a filehandle, and
257tests the associated file to see if something is true about it. If the
7660c0ab 258argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
19799a22 259Unless otherwise documented, it returns C<1> for true and C<''> for false, or
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260the undefined value if the file doesn't exist. Despite the funny
261names, precedence is the same as any other named unary operator, and
262the argument may be parenthesized like any other unary operator. The
263operator may be any of:
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264X<-r>X<-w>X<-x>X<-o>X<-R>X<-W>X<-X>X<-O>X<-e>X<-z>X<-s>X<-f>X<-d>X<-l>X<-p>
265X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
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266
267 -r File is readable by effective uid/gid.
268 -w File is writable by effective uid/gid.
269 -x File is executable by effective uid/gid.
270 -o File is owned by effective uid.
271
272 -R File is readable by real uid/gid.
273 -W File is writable by real uid/gid.
274 -X File is executable by real uid/gid.
275 -O File is owned by real uid.
276
277 -e File exists.
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278 -z File has zero size (is empty).
279 -s File has nonzero size (returns size in bytes).
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280
281 -f File is a plain file.
282 -d File is a directory.
283 -l File is a symbolic link.
9c4d0f16 284 -p File is a named pipe (FIFO), or Filehandle is a pipe.
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285 -S File is a socket.
286 -b File is a block special file.
287 -c File is a character special file.
288 -t Filehandle is opened to a tty.
289
290 -u File has setuid bit set.
291 -g File has setgid bit set.
292 -k File has sticky bit set.
293
121910a4 294 -T File is an ASCII text file (heuristic guess).
2cdbc966 295 -B File is a "binary" file (opposite of -T).
a0d0e21e 296
95a3fe12 297 -M Script start time minus file modification time, in days.
a0d0e21e 298 -A Same for access time.
95a3fe12 299 -C Same for inode change time (Unix, may differ for other platforms)
a0d0e21e 300
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301Example:
302
303 while (<>) {
5b3eff12 304 chomp;
a0d0e21e 305 next unless -f $_; # ignore specials
5a964f20 306 #...
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307 }
308
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309The interpretation of the file permission operators C<-r>, C<-R>,
310C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
311of the file and the uids and gids of the user. There may be other
312reasons you can't actually read, write, or execute the file. Such
313reasons may be for example network filesystem access controls, ACLs
314(access control lists), read-only filesystems, and unrecognized
315executable formats.
316
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317Also note that, for the superuser on the local filesystems, the C<-r>,
318C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
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319if any execute bit is set in the mode. Scripts run by the superuser
320may thus need to do a stat() to determine the actual mode of the file,
2b5ab1e7 321or temporarily set their effective uid to something else.
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322
323If you are using ACLs, there is a pragma called C<filetest> that may
324produce more accurate results than the bare stat() mode bits.
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325When under the C<use filetest 'access'> the above-mentioned filetests
326will test whether the permission can (not) be granted using the
468541a8 327access() family of system calls. Also note that the C<-x> and C<-X> may
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328under this pragma return true even if there are no execute permission
329bits set (nor any extra execute permission ACLs). This strangeness is
330due to the underlying system calls' definitions. Read the
331documentation for the C<filetest> pragma for more information.
332
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333Note that C<-s/a/b/> does not do a negated substitution. Saying
334C<-exp($foo)> still works as expected, however--only single letters
335following a minus are interpreted as file tests.
336
337The C<-T> and C<-B> switches work as follows. The first block or so of the
338file is examined for odd characters such as strange control codes or
61eff3bc 339characters with the high bit set. If too many strange characters (>30%)
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340are found, it's a C<-B> file, otherwise it's a C<-T> file. Also, any file
341containing null in the first block is considered a binary file. If C<-T>
9124316e 342or C<-B> is used on a filehandle, the current IO buffer is examined
19799a22 343rather than the first block. Both C<-T> and C<-B> return true on a null
54310121 344file, or a file at EOF when testing a filehandle. Because you have to
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345read a file to do the C<-T> test, on most occasions you want to use a C<-f>
346against the file first, as in C<next unless -f $file && -T $file>.
a0d0e21e 347
19799a22 348If any of the file tests (or either the C<stat> or C<lstat> operators) are given
28757baa 349the special filehandle consisting of a solitary underline, then the stat
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350structure of the previous file test (or stat operator) is used, saving
351a system call. (This doesn't work with C<-t>, and you need to remember
352that lstat() and C<-l> will leave values in the stat structure for the
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353symbolic link, not the real file.) (Also, if the stat buffer was filled by
354a C<lstat> call, C<-T> and C<-B> will reset it with the results of C<stat _>).
355Example:
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356
357 print "Can do.\n" if -r $a || -w _ || -x _;
358
359 stat($filename);
360 print "Readable\n" if -r _;
361 print "Writable\n" if -w _;
362 print "Executable\n" if -x _;
363 print "Setuid\n" if -u _;
364 print "Setgid\n" if -g _;
365 print "Sticky\n" if -k _;
366 print "Text\n" if -T _;
367 print "Binary\n" if -B _;
368
369=item abs VALUE
370
54310121 371=item abs
bbce6d69 372
a0d0e21e 373Returns the absolute value of its argument.
7660c0ab 374If VALUE is omitted, uses C<$_>.
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375
376=item accept NEWSOCKET,GENERICSOCKET
377
f86cebdf 378Accepts an incoming socket connect, just as the accept(2) system call
19799a22 379does. Returns the packed address if it succeeded, false otherwise.
2b5ab1e7 380See the example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 381
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382On systems that support a close-on-exec flag on files, the flag will
383be set for the newly opened file descriptor, as determined by the
384value of $^F. See L<perlvar/$^F>.
385
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386=item alarm SECONDS
387
54310121 388=item alarm
bbce6d69 389
a0d0e21e 390Arranges to have a SIGALRM delivered to this process after the
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391specified number of wallclock seconds have elapsed. If SECONDS is not
392specified, the value stored in C<$_> is used. (On some machines,
393unfortunately, the elapsed time may be up to one second less or more
394than you specified because of how seconds are counted, and process
395scheduling may delay the delivery of the signal even further.)
396
397Only one timer may be counting at once. Each call disables the
398previous timer, and an argument of C<0> may be supplied to cancel the
399previous timer without starting a new one. The returned value is the
400amount of time remaining on the previous timer.
a0d0e21e 401
4633a7c4 402For delays of finer granularity than one second, you may use Perl's
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403four-argument version of select() leaving the first three arguments
404undefined, or you might be able to use the C<syscall> interface to
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405access setitimer(2) if your system supports it. The Time::HiRes
406module (from CPAN, and starting from Perl 5.8 part of the standard
407distribution) may also prove useful.
2b5ab1e7 408
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409It is usually a mistake to intermix C<alarm> and C<sleep> calls.
410(C<sleep> may be internally implemented in your system with C<alarm>)
a0d0e21e 411
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412If you want to use C<alarm> to time out a system call you need to use an
413C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
f86cebdf 414fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
19799a22 415restart system calls on some systems. Using C<eval>/C<die> always works,
5a964f20 416modulo the caveats given in L<perlipc/"Signals">.
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417
418 eval {
f86cebdf 419 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
36477c24 420 alarm $timeout;
ff68c719 421 $nread = sysread SOCKET, $buffer, $size;
36477c24 422 alarm 0;
ff68c719 423 };
ff68c719 424 if ($@) {
f86cebdf 425 die unless $@ eq "alarm\n"; # propagate unexpected errors
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426 # timed out
427 }
428 else {
429 # didn't
430 }
431
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432For more information see L<perlipc>.
433
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434=item atan2 Y,X
435
436Returns the arctangent of Y/X in the range -PI to PI.
437
ca6e1c26 438For the tangent operation, you may use the C<Math::Trig::tan>
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439function, or use the familiar relation:
440
441 sub tan { sin($_[0]) / cos($_[0]) }
442
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443=item bind SOCKET,NAME
444
445Binds a network address to a socket, just as the bind system call
19799a22 446does. Returns true if it succeeded, false otherwise. NAME should be a
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447packed address of the appropriate type for the socket. See the examples in
448L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 449
fae2c0fb 450=item binmode FILEHANDLE, LAYER
1c1fc3ea 451
a0d0e21e
LW
452=item binmode FILEHANDLE
453
1cbfc93d
NIS
454Arranges for FILEHANDLE to be read or written in "binary" or "text"
455mode on systems where the run-time libraries distinguish between
456binary and text files. If FILEHANDLE is an expression, the value is
457taken as the name of the filehandle. Returns true on success,
b5fe5ca2 458otherwise it returns C<undef> and sets C<$!> (errno).
1cbfc93d 459
d807c6f4
JH
460On some systems (in general, DOS and Windows-based systems) binmode()
461is necessary when you're not working with a text file. For the sake
462of portability it is a good idea to always use it when appropriate,
463and to never use it when it isn't appropriate. Also, people can
464set their I/O to be by default UTF-8 encoded Unicode, not bytes.
465
466In other words: regardless of platform, use binmode() on binary data,
467like for example images.
468
469If LAYER is present it is a single string, but may contain multiple
470directives. The directives alter the behaviour of the file handle.
471When LAYER is present using binmode on text file makes sense.
472
fae2c0fb 473If LAYER is omitted or specified as C<:raw> the filehandle is made
0226bbdb
NIS
474suitable for passing binary data. This includes turning off possible CRLF
475translation and marking it as bytes (as opposed to Unicode characters).
2a131c56 476Note that as despite what may be implied in I<"Programming Perl">
0226bbdb 477(the Camel) or elsewhere C<:raw> is I<not> the simply inverse of C<:crlf>
fae2c0fb 478-- other layers which would affect binary nature of the stream are
0226bbdb
NIS
479I<also> disabled. See L<PerlIO>, L<perlrun> and the discussion about the
480PERLIO environment variable.
01e6739c 481
d807c6f4
JH
482The C<:bytes>, C<:crlf>, and C<:utf8>, and any other directives of the
483form C<:...>, are called I/O I<layers>. The C<open> pragma can be used to
484establish default I/O layers. See L<open>.
485
fae2c0fb
RGS
486I<The LAYER parameter of the binmode() function is described as "DISCIPLINE"
487in "Programming Perl, 3rd Edition". However, since the publishing of this
488book, by many known as "Camel III", the consensus of the naming of this
489functionality has moved from "discipline" to "layer". All documentation
490of this version of Perl therefore refers to "layers" rather than to
491"disciplines". Now back to the regularly scheduled documentation...>
492
01e6739c 493To mark FILEHANDLE as UTF-8, use C<:utf8>.
1cbfc93d 494
ed53a2bb 495In general, binmode() should be called after open() but before any I/O
01e6739c
NIS
496is done on the filehandle. Calling binmode() will normally flush any
497pending buffered output data (and perhaps pending input data) on the
fae2c0fb 498handle. An exception to this is the C<:encoding> layer that
01e6739c 499changes the default character encoding of the handle, see L<open>.
fae2c0fb 500The C<:encoding> layer sometimes needs to be called in
3874323d
JH
501mid-stream, and it doesn't flush the stream. The C<:encoding>
502also implicitly pushes on top of itself the C<:utf8> layer because
503internally Perl will operate on UTF-8 encoded Unicode characters.
16fe6d59 504
19799a22 505The operating system, device drivers, C libraries, and Perl run-time
30168b04
GS
506system all work together to let the programmer treat a single
507character (C<\n>) as the line terminator, irrespective of the external
508representation. On many operating systems, the native text file
509representation matches the internal representation, but on some
510platforms the external representation of C<\n> is made up of more than
511one character.
512
68bd7414
NIS
513Mac OS, all variants of Unix, and Stream_LF files on VMS use a single
514character to end each line in the external representation of text (even
5e12dbfa 515though that single character is CARRIAGE RETURN on Mac OS and LINE FEED
01e6739c
NIS
516on Unix and most VMS files). In other systems like OS/2, DOS and the
517various flavors of MS-Windows your program sees a C<\n> as a simple C<\cJ>,
518but what's stored in text files are the two characters C<\cM\cJ>. That
519means that, if you don't use binmode() on these systems, C<\cM\cJ>
520sequences on disk will be converted to C<\n> on input, and any C<\n> in
521your program will be converted back to C<\cM\cJ> on output. This is what
522you want for text files, but it can be disastrous for binary files.
30168b04
GS
523
524Another consequence of using binmode() (on some systems) is that
525special end-of-file markers will be seen as part of the data stream.
526For systems from the Microsoft family this means that if your binary
4375e838 527data contains C<\cZ>, the I/O subsystem will regard it as the end of
30168b04
GS
528the file, unless you use binmode().
529
530binmode() is not only important for readline() and print() operations,
531but also when using read(), seek(), sysread(), syswrite() and tell()
532(see L<perlport> for more details). See the C<$/> and C<$\> variables
533in L<perlvar> for how to manually set your input and output
534line-termination sequences.
a0d0e21e 535
4633a7c4 536=item bless REF,CLASSNAME
a0d0e21e
LW
537
538=item bless REF
539
2b5ab1e7
TC
540This function tells the thingy referenced by REF that it is now an object
541in the CLASSNAME package. If CLASSNAME is omitted, the current package
19799a22 542is used. Because a C<bless> is often the last thing in a constructor,
2b5ab1e7
TC
543it returns the reference for convenience. Always use the two-argument
544version if the function doing the blessing might be inherited by a
545derived class. See L<perltoot> and L<perlobj> for more about the blessing
546(and blessings) of objects.
a0d0e21e 547
57668c4d 548Consider always blessing objects in CLASSNAMEs that are mixed case.
2b5ab1e7
TC
549Namespaces with all lowercase names are considered reserved for
550Perl pragmata. Builtin types have all uppercase names, so to prevent
551confusion, you may wish to avoid such package names as well. Make sure
552that CLASSNAME is a true value.
60ad88b8
GS
553
554See L<perlmod/"Perl Modules">.
555
a0d0e21e
LW
556=item caller EXPR
557
558=item caller
559
5a964f20 560Returns the context of the current subroutine call. In scalar context,
28757baa 561returns the caller's package name if there is a caller, that is, if
19799a22 562we're in a subroutine or C<eval> or C<require>, and the undefined value
5a964f20 563otherwise. In list context, returns
a0d0e21e 564
748a9306 565 ($package, $filename, $line) = caller;
a0d0e21e
LW
566
567With EXPR, it returns some extra information that the debugger uses to
568print a stack trace. The value of EXPR indicates how many call frames
569to go back before the current one.
570
f3aa04c2 571 ($package, $filename, $line, $subroutine, $hasargs,
e476b1b5 572 $wantarray, $evaltext, $is_require, $hints, $bitmask) = caller($i);
e7ea3e70 573
951ba7fe 574Here $subroutine may be C<(eval)> if the frame is not a subroutine
19799a22 575call, but an C<eval>. In such a case additional elements $evaltext and
7660c0ab 576C<$is_require> are set: C<$is_require> is true if the frame is created by a
19799a22 577C<require> or C<use> statement, $evaltext contains the text of the
277ddfaf 578C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
951ba7fe 579$filename is C<(eval)>, but $evaltext is undefined. (Note also that
0fc9dec4
RGS
580each C<use> statement creates a C<require> frame inside an C<eval EXPR>
581frame.) $subroutine may also be C<(unknown)> if this particular
582subroutine happens to have been deleted from the symbol table.
583C<$hasargs> is true if a new instance of C<@_> was set up for the frame.
584C<$hints> and C<$bitmask> contain pragmatic hints that the caller was
585compiled with. The C<$hints> and C<$bitmask> values are subject to change
586between versions of Perl, and are not meant for external use.
748a9306
LW
587
588Furthermore, when called from within the DB package, caller returns more
7660c0ab 589detailed information: it sets the list variable C<@DB::args> to be the
54310121 590arguments with which the subroutine was invoked.
748a9306 591
7660c0ab 592Be aware that the optimizer might have optimized call frames away before
19799a22 593C<caller> had a chance to get the information. That means that C<caller(N)>
7660c0ab 594might not return information about the call frame you expect it do, for
b76cc8ba 595C<< N > 1 >>. In particular, C<@DB::args> might have information from the
19799a22 596previous time C<caller> was called.
7660c0ab 597
a0d0e21e
LW
598=item chdir EXPR
599
ffce7b87 600Changes the working directory to EXPR, if possible. If EXPR is omitted,
0bfc1ec4 601changes to the directory specified by C<$ENV{HOME}>, if set; if not,
ffce7b87 602changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
b4ad75f0
AMS
603variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
604neither is set, C<chdir> does nothing. It returns true upon success,
605false otherwise. See the example under C<die>.
a0d0e21e
LW
606
607=item chmod LIST
608
609Changes the permissions of a list of files. The first element of the
4633a7c4 610list must be the numerical mode, which should probably be an octal
2f9daede
TPG
611number, and which definitely should I<not> a string of octal digits:
612C<0644> is okay, C<'0644'> is not. Returns the number of files
dc848c6f 613successfully changed. See also L</oct>, if all you have is a string.
a0d0e21e
LW
614
615 $cnt = chmod 0755, 'foo', 'bar';
616 chmod 0755, @executables;
f86cebdf
GS
617 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
618 # --w----r-T
2f9daede
TPG
619 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
620 $mode = 0644; chmod $mode, 'foo'; # this is best
a0d0e21e 621
ca6e1c26
JH
622You can also import the symbolic C<S_I*> constants from the Fcntl
623module:
624
625 use Fcntl ':mode';
626
627 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
628 # This is identical to the chmod 0755 of the above example.
629
a0d0e21e
LW
630=item chomp VARIABLE
631
313c9f5c 632=item chomp( LIST )
a0d0e21e
LW
633
634=item chomp
635
2b5ab1e7
TC
636This safer version of L</chop> removes any trailing string
637that corresponds to the current value of C<$/> (also known as
28757baa
PP
638$INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
639number of characters removed from all its arguments. It's often used to
640remove the newline from the end of an input record when you're worried
2b5ab1e7
TC
641that the final record may be missing its newline. When in paragraph
642mode (C<$/ = "">), it removes all trailing newlines from the string.
4c5a6083
GS
643When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
644a reference to an integer or the like, see L<perlvar>) chomp() won't
b76cc8ba 645remove anything.
19799a22 646If VARIABLE is omitted, it chomps C<$_>. Example:
a0d0e21e
LW
647
648 while (<>) {
649 chomp; # avoid \n on last field
650 @array = split(/:/);
5a964f20 651 # ...
a0d0e21e
LW
652 }
653
4bf21a6d
RD
654If VARIABLE is a hash, it chomps the hash's values, but not its keys.
655
a0d0e21e
LW
656You can actually chomp anything that's an lvalue, including an assignment:
657
658 chomp($cwd = `pwd`);
659 chomp($answer = <STDIN>);
660
661If you chomp a list, each element is chomped, and the total number of
662characters removed is returned.
663
15e44fd8
RGS
664Note that parentheses are necessary when you're chomping anything
665that is not a simple variable. This is because C<chomp $cwd = `pwd`;>
666is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as
667C<chomp( $cwd = `pwd` )> which you might expect. Similarly,
668C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than
669as C<chomp($a, $b)>.
670
a0d0e21e
LW
671=item chop VARIABLE
672
313c9f5c 673=item chop( LIST )
a0d0e21e
LW
674
675=item chop
676
677Chops off the last character of a string and returns the character
5b3eff12 678chopped. It is much more efficient than C<s/.$//s> because it neither
7660c0ab 679scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
4bf21a6d
RD
680If VARIABLE is a hash, it chops the hash's values, but not its keys.
681
5b3eff12 682You can actually chop anything that's an lvalue, including an assignment.
a0d0e21e
LW
683
684If you chop a list, each element is chopped. Only the value of the
19799a22 685last C<chop> is returned.
a0d0e21e 686
19799a22 687Note that C<chop> returns the last character. To return all but the last
748a9306
LW
688character, use C<substr($string, 0, -1)>.
689
15e44fd8
RGS
690See also L</chomp>.
691
a0d0e21e
LW
692=item chown LIST
693
694Changes the owner (and group) of a list of files. The first two
19799a22
GS
695elements of the list must be the I<numeric> uid and gid, in that
696order. A value of -1 in either position is interpreted by most
697systems to leave that value unchanged. Returns the number of files
698successfully changed.
a0d0e21e
LW
699
700 $cnt = chown $uid, $gid, 'foo', 'bar';
701 chown $uid, $gid, @filenames;
702
54310121 703Here's an example that looks up nonnumeric uids in the passwd file:
a0d0e21e
LW
704
705 print "User: ";
19799a22 706 chomp($user = <STDIN>);
5a964f20 707 print "Files: ";
19799a22 708 chomp($pattern = <STDIN>);
a0d0e21e
LW
709
710 ($login,$pass,$uid,$gid) = getpwnam($user)
711 or die "$user not in passwd file";
712
5a964f20 713 @ary = glob($pattern); # expand filenames
a0d0e21e
LW
714 chown $uid, $gid, @ary;
715
54310121 716On most systems, you are not allowed to change the ownership of the
4633a7c4
LW
717file unless you're the superuser, although you should be able to change
718the group to any of your secondary groups. On insecure systems, these
719restrictions may be relaxed, but this is not a portable assumption.
19799a22
GS
720On POSIX systems, you can detect this condition this way:
721
722 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
723 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
4633a7c4 724
a0d0e21e
LW
725=item chr NUMBER
726
54310121 727=item chr
bbce6d69 728
a0d0e21e 729Returns the character represented by that NUMBER in the character set.
a0ed51b3 730For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
121910a4
JH
731chr(0x263a) is a Unicode smiley face. Note that characters from 127
732to 255 (inclusive) are by default not encoded in Unicode for backward
733compatibility reasons (but see L<encoding>).
aaa68c4a 734
974da8e5
JH
735If NUMBER is omitted, uses C<$_>.
736
b76cc8ba 737For the reverse, use L</ord>.
a0d0e21e 738
974da8e5
JH
739Note that under the C<bytes> pragma the NUMBER is masked to
740the low eight bits.
741
742See L<perlunicode> and L<encoding> for more about Unicode.
bbce6d69 743
a0d0e21e
LW
744=item chroot FILENAME
745
54310121 746=item chroot
bbce6d69 747
5a964f20 748This function works like the system call by the same name: it makes the
4633a7c4 749named directory the new root directory for all further pathnames that
951ba7fe 750begin with a C</> by your process and all its children. (It doesn't
28757baa 751change your current working directory, which is unaffected.) For security
4633a7c4 752reasons, this call is restricted to the superuser. If FILENAME is
19799a22 753omitted, does a C<chroot> to C<$_>.
a0d0e21e
LW
754
755=item close FILEHANDLE
756
6a518fbc
TP
757=item close
758
9124316e
JH
759Closes the file or pipe associated with the file handle, returning
760true only if IO buffers are successfully flushed and closes the system
761file descriptor. Closes the currently selected filehandle if the
762argument is omitted.
fb73857a
PP
763
764You don't have to close FILEHANDLE if you are immediately going to do
19799a22
GS
765another C<open> on it, because C<open> will close it for you. (See
766C<open>.) However, an explicit C<close> on an input file resets the line
767counter (C<$.>), while the implicit close done by C<open> does not.
fb73857a 768
19799a22
GS
769If the file handle came from a piped open C<close> will additionally
770return false if one of the other system calls involved fails or if the
fb73857a 771program exits with non-zero status. (If the only problem was that the
b76cc8ba 772program exited non-zero C<$!> will be set to C<0>.) Closing a pipe
2b5ab1e7 773also waits for the process executing on the pipe to complete, in case you
b76cc8ba 774want to look at the output of the pipe afterwards, and
2b5ab1e7 775implicitly puts the exit status value of that command into C<$?>.
5a964f20 776
73689b13
GS
777Prematurely closing the read end of a pipe (i.e. before the process
778writing to it at the other end has closed it) will result in a
779SIGPIPE being delivered to the writer. If the other end can't
780handle that, be sure to read all the data before closing the pipe.
781
fb73857a 782Example:
a0d0e21e 783
fb73857a
PP
784 open(OUTPUT, '|sort >foo') # pipe to sort
785 or die "Can't start sort: $!";
5a964f20 786 #... # print stuff to output
fb73857a
PP
787 close OUTPUT # wait for sort to finish
788 or warn $! ? "Error closing sort pipe: $!"
789 : "Exit status $? from sort";
790 open(INPUT, 'foo') # get sort's results
791 or die "Can't open 'foo' for input: $!";
a0d0e21e 792
5a964f20
TC
793FILEHANDLE may be an expression whose value can be used as an indirect
794filehandle, usually the real filehandle name.
a0d0e21e
LW
795
796=item closedir DIRHANDLE
797
19799a22 798Closes a directory opened by C<opendir> and returns the success of that
5a964f20
TC
799system call.
800
a0d0e21e
LW
801=item connect SOCKET,NAME
802
803Attempts to connect to a remote socket, just as the connect system call
19799a22 804does. Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
805packed address of the appropriate type for the socket. See the examples in
806L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 807
cb1a09d0
AD
808=item continue BLOCK
809
810Actually a flow control statement rather than a function. If there is a
98293880
JH
811C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
812C<foreach>), it is always executed just before the conditional is about to
813be evaluated again, just like the third part of a C<for> loop in C. Thus
cb1a09d0
AD
814it can be used to increment a loop variable, even when the loop has been
815continued via the C<next> statement (which is similar to the C C<continue>
816statement).
817
98293880 818C<last>, C<next>, or C<redo> may appear within a C<continue>
19799a22
GS
819block. C<last> and C<redo> will behave as if they had been executed within
820the main block. So will C<next>, but since it will execute a C<continue>
1d2dff63
GS
821block, it may be more entertaining.
822
823 while (EXPR) {
824 ### redo always comes here
825 do_something;
826 } continue {
827 ### next always comes here
828 do_something_else;
829 # then back the top to re-check EXPR
830 }
831 ### last always comes here
832
833Omitting the C<continue> section is semantically equivalent to using an
19799a22 834empty one, logically enough. In that case, C<next> goes directly back
1d2dff63
GS
835to check the condition at the top of the loop.
836
a0d0e21e
LW
837=item cos EXPR
838
d6217f1e
GS
839=item cos
840
5a964f20 841Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
7660c0ab 842takes cosine of C<$_>.
a0d0e21e 843
ca6e1c26 844For the inverse cosine operation, you may use the C<Math::Trig::acos()>
28757baa
PP
845function, or use this relation:
846
847 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
848
a0d0e21e
LW
849=item crypt PLAINTEXT,SALT
850
f86cebdf 851Encrypts a string exactly like the crypt(3) function in the C library
4633a7c4
LW
852(assuming that you actually have a version there that has not been
853extirpated as a potential munition). This can prove useful for checking
854the password file for lousy passwords, amongst other things. Only the
855guys wearing white hats should do this.
a0d0e21e 856
a6d05634 857Note that L<crypt|/crypt> is intended to be a one-way function, much like
85c16d83
JH
858breaking eggs to make an omelette. There is no (known) corresponding
859decrypt function (in other words, the crypt() is a one-way hash
860function). As a result, this function isn't all that useful for
11155c91 861cryptography. (For that, see your nearby CPAN mirror.)
2f9daede 862
85c16d83
JH
863When verifying an existing encrypted string you should use the
864encrypted text as the salt (like C<crypt($plain, $crypted) eq
8e2ffcbe 865$crypted>). This allows your code to work with the standard L<crypt|/crypt>
85c16d83
JH
866and with more exotic implementations. In other words, do not assume
867anything about the returned string itself, or how many bytes in
868the encrypted string matter.
869
870Traditionally the result is a string of 13 bytes: two first bytes of
871the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
872the first eight bytes of the encrypted string mattered, but
873alternative hashing schemes (like MD5), higher level security schemes
874(like C2), and implementations on non-UNIX platforms may produce
875different strings.
876
877When choosing a new salt create a random two character string whose
878characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
d3989d75
CW
879'/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of
880characters is just a recommendation; the characters allowed in
881the salt depend solely on your system's crypt library, and Perl can't
882restrict what salts C<crypt()> accepts.
e71965be 883
a0d0e21e
LW
884Here's an example that makes sure that whoever runs this program knows
885their own password:
886
887 $pwd = (getpwuid($<))[1];
a0d0e21e
LW
888
889 system "stty -echo";
890 print "Password: ";
e71965be 891 chomp($word = <STDIN>);
a0d0e21e
LW
892 print "\n";
893 system "stty echo";
894
e71965be 895 if (crypt($word, $pwd) ne $pwd) {
a0d0e21e
LW
896 die "Sorry...\n";
897 } else {
898 print "ok\n";
54310121 899 }
a0d0e21e 900
9f8f0c9d 901Of course, typing in your own password to whoever asks you
748a9306 902for it is unwise.
a0d0e21e 903
8e2ffcbe 904The L<crypt|/crypt> function is unsuitable for encrypting large quantities
19799a22
GS
905of data, not least of all because you can't get the information
906back. Look at the F<by-module/Crypt> and F<by-module/PGP> directories
907on your favorite CPAN mirror for a slew of potentially useful
908modules.
909
f2791508
JH
910If using crypt() on a Unicode string (which I<potentially> has
911characters with codepoints above 255), Perl tries to make sense
912of the situation by trying to downgrade (a copy of the string)
913the string back to an eight-bit byte string before calling crypt()
914(on that copy). If that works, good. If not, crypt() dies with
915C<Wide character in crypt>.
85c16d83 916
aa689395 917=item dbmclose HASH
a0d0e21e 918
19799a22 919[This function has been largely superseded by the C<untie> function.]
a0d0e21e 920
aa689395 921Breaks the binding between a DBM file and a hash.
a0d0e21e 922
19799a22 923=item dbmopen HASH,DBNAME,MASK
a0d0e21e 924
19799a22 925[This function has been largely superseded by the C<tie> function.]
a0d0e21e 926
7b8d334a 927This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
19799a22
GS
928hash. HASH is the name of the hash. (Unlike normal C<open>, the first
929argument is I<not> a filehandle, even though it looks like one). DBNAME
aa689395
PP
930is the name of the database (without the F<.dir> or F<.pag> extension if
931any). If the database does not exist, it is created with protection
19799a22
GS
932specified by MASK (as modified by the C<umask>). If your system supports
933only the older DBM functions, you may perform only one C<dbmopen> in your
aa689395 934program. In older versions of Perl, if your system had neither DBM nor
19799a22 935ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
aa689395
PP
936sdbm(3).
937
938If you don't have write access to the DBM file, you can only read hash
939variables, not set them. If you want to test whether you can write,
19799a22 940either use file tests or try setting a dummy hash entry inside an C<eval>,
aa689395 941which will trap the error.
a0d0e21e 942
19799a22
GS
943Note that functions such as C<keys> and C<values> may return huge lists
944when used on large DBM files. You may prefer to use the C<each>
a0d0e21e
LW
945function to iterate over large DBM files. Example:
946
947 # print out history file offsets
948 dbmopen(%HIST,'/usr/lib/news/history',0666);
949 while (($key,$val) = each %HIST) {
950 print $key, ' = ', unpack('L',$val), "\n";
951 }
952 dbmclose(%HIST);
953
cb1a09d0 954See also L<AnyDBM_File> for a more general description of the pros and
184e9718 955cons of the various dbm approaches, as well as L<DB_File> for a particularly
cb1a09d0 956rich implementation.
4633a7c4 957
2b5ab1e7
TC
958You can control which DBM library you use by loading that library
959before you call dbmopen():
960
961 use DB_File;
962 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
963 or die "Can't open netscape history file: $!";
964
a0d0e21e
LW
965=item defined EXPR
966
54310121 967=item defined
bbce6d69 968
2f9daede
TPG
969Returns a Boolean value telling whether EXPR has a value other than
970the undefined value C<undef>. If EXPR is not present, C<$_> will be
971checked.
972
973Many operations return C<undef> to indicate failure, end of file,
974system error, uninitialized variable, and other exceptional
975conditions. This function allows you to distinguish C<undef> from
976other values. (A simple Boolean test will not distinguish among
7660c0ab 977C<undef>, zero, the empty string, and C<"0">, which are all equally
2f9daede 978false.) Note that since C<undef> is a valid scalar, its presence
19799a22 979doesn't I<necessarily> indicate an exceptional condition: C<pop>
2f9daede
TPG
980returns C<undef> when its argument is an empty array, I<or> when the
981element to return happens to be C<undef>.
982
f10b0346
GS
983You may also use C<defined(&func)> to check whether subroutine C<&func>
984has ever been defined. The return value is unaffected by any forward
04891299 985declarations of C<&func>. Note that a subroutine which is not defined
847c7ebe
DD
986may still be callable: its package may have an C<AUTOLOAD> method that
987makes it spring into existence the first time that it is called -- see
988L<perlsub>.
f10b0346
GS
989
990Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
991used to report whether memory for that aggregate has ever been
992allocated. This behavior may disappear in future versions of Perl.
993You should instead use a simple test for size:
994
995 if (@an_array) { print "has array elements\n" }
996 if (%a_hash) { print "has hash members\n" }
2f9daede
TPG
997
998When used on a hash element, it tells you whether the value is defined,
dc848c6f 999not whether the key exists in the hash. Use L</exists> for the latter
2f9daede 1000purpose.
a0d0e21e
LW
1001
1002Examples:
1003
1004 print if defined $switch{'D'};
1005 print "$val\n" while defined($val = pop(@ary));
1006 die "Can't readlink $sym: $!"
1007 unless defined($value = readlink $sym);
a0d0e21e 1008 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
2f9daede 1009 $debugging = 0 unless defined $debugging;
a0d0e21e 1010
19799a22 1011Note: Many folks tend to overuse C<defined>, and then are surprised to
7660c0ab 1012discover that the number C<0> and C<""> (the zero-length string) are, in fact,
2f9daede 1013defined values. For example, if you say
a5f75d66
AD
1014
1015 "ab" =~ /a(.*)b/;
1016
7660c0ab 1017The pattern match succeeds, and C<$1> is defined, despite the fact that it
a5f75d66 1018matched "nothing". But it didn't really match nothing--rather, it
2b5ab1e7 1019matched something that happened to be zero characters long. This is all
a5f75d66 1020very above-board and honest. When a function returns an undefined value,
2f9daede 1021it's an admission that it couldn't give you an honest answer. So you
19799a22 1022should use C<defined> only when you're questioning the integrity of what
7660c0ab 1023you're trying to do. At other times, a simple comparison to C<0> or C<""> is
2f9daede
TPG
1024what you want.
1025
dc848c6f 1026See also L</undef>, L</exists>, L</ref>.
2f9daede 1027
a0d0e21e
LW
1028=item delete EXPR
1029
01020589
GS
1030Given an expression that specifies a hash element, array element, hash slice,
1031or array slice, deletes the specified element(s) from the hash or array.
8216c1fd 1032In the case of an array, if the array elements happen to be at the end,
b76cc8ba 1033the size of the array will shrink to the highest element that tests
8216c1fd 1034true for exists() (or 0 if no such element exists).
a0d0e21e 1035
01020589
GS
1036Returns each element so deleted or the undefined value if there was no such
1037element. Deleting from C<$ENV{}> modifies the environment. Deleting from
1038a hash tied to a DBM file deletes the entry from the DBM file. Deleting
1039from a C<tie>d hash or array may not necessarily return anything.
1040
8ea97a1e
GS
1041Deleting an array element effectively returns that position of the array
1042to its initial, uninitialized state. Subsequently testing for the same
8216c1fd
GS
1043element with exists() will return false. Note that deleting array
1044elements in the middle of an array will not shift the index of the ones
1045after them down--use splice() for that. See L</exists>.
8ea97a1e 1046
01020589 1047The following (inefficiently) deletes all the values of %HASH and @ARRAY:
a0d0e21e 1048
5f05dabc
PP
1049 foreach $key (keys %HASH) {
1050 delete $HASH{$key};
a0d0e21e
LW
1051 }
1052
01020589
GS
1053 foreach $index (0 .. $#ARRAY) {
1054 delete $ARRAY[$index];
1055 }
1056
1057And so do these:
5f05dabc 1058
01020589
GS
1059 delete @HASH{keys %HASH};
1060
9740c838 1061 delete @ARRAY[0 .. $#ARRAY];
5f05dabc 1062
2b5ab1e7 1063But both of these are slower than just assigning the empty list
01020589
GS
1064or undefining %HASH or @ARRAY:
1065
1066 %HASH = (); # completely empty %HASH
1067 undef %HASH; # forget %HASH ever existed
2b5ab1e7 1068
01020589
GS
1069 @ARRAY = (); # completely empty @ARRAY
1070 undef @ARRAY; # forget @ARRAY ever existed
2b5ab1e7
TC
1071
1072Note that the EXPR can be arbitrarily complicated as long as the final
01020589
GS
1073operation is a hash element, array element, hash slice, or array slice
1074lookup:
a0d0e21e
LW
1075
1076 delete $ref->[$x][$y]{$key};
5f05dabc 1077 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
a0d0e21e 1078
01020589
GS
1079 delete $ref->[$x][$y][$index];
1080 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1081
a0d0e21e
LW
1082=item die LIST
1083
19799a22
GS
1084Outside an C<eval>, prints the value of LIST to C<STDERR> and
1085exits with the current value of C<$!> (errno). If C<$!> is C<0>,
61eff3bc
JH
1086exits with the value of C<<< ($? >> 8) >>> (backtick `command`
1087status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside
19799a22
GS
1088an C<eval(),> the error message is stuffed into C<$@> and the
1089C<eval> is terminated with the undefined value. This makes
1090C<die> the way to raise an exception.
a0d0e21e
LW
1091
1092Equivalent examples:
1093
1094 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
54310121 1095 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
a0d0e21e 1096
ccac6780 1097If the last element of LIST does not end in a newline, the current
df37ec69
WW
1098script line number and input line number (if any) are also printed,
1099and a newline is supplied. Note that the "input line number" (also
1100known as "chunk") is subject to whatever notion of "line" happens to
1101be currently in effect, and is also available as the special variable
1102C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1103
1104Hint: sometimes appending C<", stopped"> to your message will cause it
1105to make better sense when the string C<"at foo line 123"> is appended.
1106Suppose you are running script "canasta".
a0d0e21e
LW
1107
1108 die "/etc/games is no good";
1109 die "/etc/games is no good, stopped";
1110
1111produce, respectively
1112
1113 /etc/games is no good at canasta line 123.
1114 /etc/games is no good, stopped at canasta line 123.
1115
2b5ab1e7 1116See also exit(), warn(), and the Carp module.
a0d0e21e 1117
7660c0ab
A
1118If LIST is empty and C<$@> already contains a value (typically from a
1119previous eval) that value is reused after appending C<"\t...propagated">.
fb73857a
PP
1120This is useful for propagating exceptions:
1121
1122 eval { ... };
1123 die unless $@ =~ /Expected exception/;
1124
ad216e65
JH
1125If LIST is empty and C<$@> contains an object reference that has a
1126C<PROPAGATE> method, that method will be called with additional file
1127and line number parameters. The return value replaces the value in
16869676 1128C<$@>. ie. as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >>
ad216e65
JH
1129were called.
1130
7660c0ab 1131If C<$@> is empty then the string C<"Died"> is used.
fb73857a 1132
52531d10
GS
1133die() can also be called with a reference argument. If this happens to be
1134trapped within an eval(), $@ contains the reference. This behavior permits
1135a more elaborate exception handling implementation using objects that
4375e838 1136maintain arbitrary state about the nature of the exception. Such a scheme
52531d10
GS
1137is sometimes preferable to matching particular string values of $@ using
1138regular expressions. Here's an example:
1139
1140 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1141 if ($@) {
1142 if (ref($@) && UNIVERSAL::isa($@,"Some::Module::Exception")) {
1143 # handle Some::Module::Exception
1144 }
1145 else {
1146 # handle all other possible exceptions
1147 }
1148 }
1149
19799a22 1150Because perl will stringify uncaught exception messages before displaying
52531d10
GS
1151them, you may want to overload stringification operations on such custom
1152exception objects. See L<overload> for details about that.
1153
19799a22
GS
1154You can arrange for a callback to be run just before the C<die>
1155does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1156handler will be called with the error text and can change the error
1157message, if it sees fit, by calling C<die> again. See
1158L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
1159L<"eval BLOCK"> for some examples. Although this feature was meant
1160to be run only right before your program was to exit, this is not
1161currently the case--the C<$SIG{__DIE__}> hook is currently called
1162even inside eval()ed blocks/strings! If one wants the hook to do
1163nothing in such situations, put
fb73857a
PP
1164
1165 die @_ if $^S;
1166
19799a22
GS
1167as the first line of the handler (see L<perlvar/$^S>). Because
1168this promotes strange action at a distance, this counterintuitive
b76cc8ba 1169behavior may be fixed in a future release.
774d564b 1170
a0d0e21e
LW
1171=item do BLOCK
1172
1173Not really a function. Returns the value of the last command in the
1174sequence of commands indicated by BLOCK. When modified by a loop
98293880
JH
1175modifier, executes the BLOCK once before testing the loop condition.
1176(On other statements the loop modifiers test the conditional first.)
a0d0e21e 1177
4968c1e4 1178C<do BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7
TC
1179C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1180See L<perlsyn> for alternative strategies.
4968c1e4 1181
a0d0e21e
LW
1182=item do SUBROUTINE(LIST)
1183
1184A deprecated form of subroutine call. See L<perlsub>.
1185
1186=item do EXPR
1187
1188Uses the value of EXPR as a filename and executes the contents of the
1189file as a Perl script. Its primary use is to include subroutines
1190from a Perl subroutine library.
1191
1192 do 'stat.pl';
1193
1194is just like
1195
986b19de 1196 eval `cat stat.pl`;
a0d0e21e 1197
2b5ab1e7
TC
1198except that it's more efficient and concise, keeps track of the current
1199filename for error messages, searches the @INC libraries, and updates
1200C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1201variables. It also differs in that code evaluated with C<do FILENAME>
1202cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1203same, however, in that it does reparse the file every time you call it,
1204so you probably don't want to do this inside a loop.
a0d0e21e 1205
8e30cc93 1206If C<do> cannot read the file, it returns undef and sets C<$!> to the
2b5ab1e7 1207error. If C<do> can read the file but cannot compile it, it
8e30cc93
G
1208returns undef and sets an error message in C<$@>. If the file is
1209successfully compiled, C<do> returns the value of the last expression
1210evaluated.
1211
a0d0e21e 1212Note that inclusion of library modules is better done with the
19799a22 1213C<use> and C<require> operators, which also do automatic error checking
4633a7c4 1214and raise an exception if there's a problem.
a0d0e21e 1215
5a964f20
TC
1216You might like to use C<do> to read in a program configuration
1217file. Manual error checking can be done this way:
1218
b76cc8ba 1219 # read in config files: system first, then user
f86cebdf 1220 for $file ("/share/prog/defaults.rc",
b76cc8ba 1221 "$ENV{HOME}/.someprogrc")
2b5ab1e7 1222 {
5a964f20 1223 unless ($return = do $file) {
f86cebdf
GS
1224 warn "couldn't parse $file: $@" if $@;
1225 warn "couldn't do $file: $!" unless defined $return;
1226 warn "couldn't run $file" unless $return;
5a964f20
TC
1227 }
1228 }
1229
a0d0e21e
LW
1230=item dump LABEL
1231
1614b0e3
JD
1232=item dump
1233
19799a22
GS
1234This function causes an immediate core dump. See also the B<-u>
1235command-line switch in L<perlrun>, which does the same thing.
1236Primarily this is so that you can use the B<undump> program (not
1237supplied) to turn your core dump into an executable binary after
1238having initialized all your variables at the beginning of the
1239program. When the new binary is executed it will begin by executing
1240a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1241Think of it as a goto with an intervening core dump and reincarnation.
1242If C<LABEL> is omitted, restarts the program from the top.
1243
1244B<WARNING>: Any files opened at the time of the dump will I<not>
1245be open any more when the program is reincarnated, with possible
b76cc8ba 1246resulting confusion on the part of Perl.
19799a22
GS
1247
1248This function is now largely obsolete, partly because it's very
1249hard to convert a core file into an executable, and because the
1250real compiler backends for generating portable bytecode and compilable
ac206dc8
RGS
1251C code have superseded it. That's why you should now invoke it as
1252C<CORE::dump()>, if you don't want to be warned against a possible
1253typo.
19799a22
GS
1254
1255If you're looking to use L<dump> to speed up your program, consider
1256generating bytecode or native C code as described in L<perlcc>. If
1257you're just trying to accelerate a CGI script, consider using the
210b36aa 1258C<mod_perl> extension to B<Apache>, or the CPAN module, CGI::Fast.
19799a22 1259You might also consider autoloading or selfloading, which at least
b76cc8ba 1260make your program I<appear> to run faster.
5a964f20 1261
aa689395
PP
1262=item each HASH
1263
5a964f20 1264When called in list context, returns a 2-element list consisting of the
aa689395 1265key and value for the next element of a hash, so that you can iterate over
74fc8b5f 1266it. When called in scalar context, returns only the key for the next
e902a979 1267element in the hash.
2f9daede 1268
ab192400 1269Entries are returned in an apparently random order. The actual random
504f80c1
JH
1270order is subject to change in future versions of perl, but it is
1271guaranteed to be in the same order as either the C<keys> or C<values>
4546b9e6
JH
1272function would produce on the same (unmodified) hash. Since Perl
12735.8.1 the ordering is different even between different runs of Perl
1274for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
ab192400
GS
1275
1276When the hash is entirely read, a null array is returned in list context
19799a22
GS
1277(which when assigned produces a false (C<0>) value), and C<undef> in
1278scalar context. The next call to C<each> after that will start iterating
1279again. There is a single iterator for each hash, shared by all C<each>,
1280C<keys>, and C<values> function calls in the program; it can be reset by
2f9daede
TPG
1281reading all the elements from the hash, or by evaluating C<keys HASH> or
1282C<values HASH>. If you add or delete elements of a hash while you're
74fc8b5f
MJD
1283iterating over it, you may get entries skipped or duplicated, so
1284don't. Exception: It is always safe to delete the item most recently
1285returned by C<each()>, which means that the following code will work:
1286
1287 while (($key, $value) = each %hash) {
1288 print $key, "\n";
1289 delete $hash{$key}; # This is safe
1290 }
aa689395 1291
f86cebdf 1292The following prints out your environment like the printenv(1) program,
aa689395 1293only in a different order:
a0d0e21e
LW
1294
1295 while (($key,$value) = each %ENV) {
1296 print "$key=$value\n";
1297 }
1298
19799a22 1299See also C<keys>, C<values> and C<sort>.
a0d0e21e
LW
1300
1301=item eof FILEHANDLE
1302
4633a7c4
LW
1303=item eof ()
1304
a0d0e21e
LW
1305=item eof
1306
1307Returns 1 if the next read on FILEHANDLE will return end of file, or if
1308FILEHANDLE is not open. FILEHANDLE may be an expression whose value
5a964f20 1309gives the real filehandle. (Note that this function actually
19799a22 1310reads a character and then C<ungetc>s it, so isn't very useful in an
748a9306 1311interactive context.) Do not read from a terminal file (or call
19799a22 1312C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
748a9306
LW
1313as terminals may lose the end-of-file condition if you do.
1314
820475bd
GS
1315An C<eof> without an argument uses the last file read. Using C<eof()>
1316with empty parentheses is very different. It refers to the pseudo file
1317formed from the files listed on the command line and accessed via the
61eff3bc
JH
1318C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1319as a normal filehandle is, an C<eof()> before C<< <> >> has been
820475bd 1320used will cause C<@ARGV> to be examined to determine if input is
67408cae 1321available. Similarly, an C<eof()> after C<< <> >> has returned
efdd0218
RB
1322end-of-file will assume you are processing another C<@ARGV> list,
1323and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1324see L<perlop/"I/O Operators">.
820475bd 1325
61eff3bc 1326In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
820475bd
GS
1327detect the end of each file, C<eof()> will only detect the end of the
1328last file. Examples:
a0d0e21e 1329
748a9306
LW
1330 # reset line numbering on each input file
1331 while (<>) {
b76cc8ba 1332 next if /^\s*#/; # skip comments
748a9306 1333 print "$.\t$_";
5a964f20
TC
1334 } continue {
1335 close ARGV if eof; # Not eof()!
748a9306
LW
1336 }
1337
a0d0e21e
LW
1338 # insert dashes just before last line of last file
1339 while (<>) {
6ac88b13 1340 if (eof()) { # check for end of last file
a0d0e21e
LW
1341 print "--------------\n";
1342 }
1343 print;
6ac88b13 1344 last if eof(); # needed if we're reading from a terminal
a0d0e21e
LW
1345 }
1346
a0d0e21e 1347Practical hint: you almost never need to use C<eof> in Perl, because the
3ce0d271
GS
1348input operators typically return C<undef> when they run out of data, or if
1349there was an error.
a0d0e21e
LW
1350
1351=item eval EXPR
1352
1353=item eval BLOCK
1354
c7cc6f1c
GS
1355In the first form, the return value of EXPR is parsed and executed as if it
1356were a little Perl program. The value of the expression (which is itself
5a964f20 1357determined within scalar context) is first parsed, and if there weren't any
be3174d2
GS
1358errors, executed in the lexical context of the current Perl program, so
1359that any variable settings or subroutine and format definitions remain
1360afterwards. Note that the value is parsed every time the eval executes.
1361If EXPR is omitted, evaluates C<$_>. This form is typically used to
1362delay parsing and subsequent execution of the text of EXPR until run time.
c7cc6f1c
GS
1363
1364In the second form, the code within the BLOCK is parsed only once--at the
1365same time the code surrounding the eval itself was parsed--and executed
1366within the context of the current Perl program. This form is typically
1367used to trap exceptions more efficiently than the first (see below), while
1368also providing the benefit of checking the code within BLOCK at compile
1369time.
1370
1371The final semicolon, if any, may be omitted from the value of EXPR or within
1372the BLOCK.
1373
1374In both forms, the value returned is the value of the last expression
5a964f20 1375evaluated inside the mini-program; a return statement may be also used, just
c7cc6f1c 1376as with subroutines. The expression providing the return value is evaluated
5a964f20 1377in void, scalar, or list context, depending on the context of the eval itself.
c7cc6f1c 1378See L</wantarray> for more on how the evaluation context can be determined.
a0d0e21e 1379
19799a22
GS
1380If there is a syntax error or runtime error, or a C<die> statement is
1381executed, an undefined value is returned by C<eval>, and C<$@> is set to the
a0d0e21e 1382error message. If there was no error, C<$@> is guaranteed to be a null
19799a22 1383string. Beware that using C<eval> neither silences perl from printing
c7cc6f1c 1384warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
d9984052
A
1385To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1386turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1387See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
a0d0e21e 1388
19799a22
GS
1389Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1390determining whether a particular feature (such as C<socket> or C<symlink>)
a0d0e21e
LW
1391is implemented. It is also Perl's exception trapping mechanism, where
1392the die operator is used to raise exceptions.
1393
1394If the code to be executed doesn't vary, you may use the eval-BLOCK
1395form to trap run-time errors without incurring the penalty of
1396recompiling each time. The error, if any, is still returned in C<$@>.
1397Examples:
1398
54310121 1399 # make divide-by-zero nonfatal
a0d0e21e
LW
1400 eval { $answer = $a / $b; }; warn $@ if $@;
1401
1402 # same thing, but less efficient
1403 eval '$answer = $a / $b'; warn $@ if $@;
1404
1405 # a compile-time error
5a964f20 1406 eval { $answer = }; # WRONG
a0d0e21e
LW
1407
1408 # a run-time error
1409 eval '$answer ='; # sets $@
1410
2b5ab1e7
TC
1411Due to the current arguably broken state of C<__DIE__> hooks, when using
1412the C<eval{}> form as an exception trap in libraries, you may wish not
1413to trigger any C<__DIE__> hooks that user code may have installed.
1414You can use the C<local $SIG{__DIE__}> construct for this purpose,
1415as shown in this example:
774d564b
PP
1416
1417 # a very private exception trap for divide-by-zero
f86cebdf
GS
1418 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1419 warn $@ if $@;
774d564b
PP
1420
1421This is especially significant, given that C<__DIE__> hooks can call
19799a22 1422C<die> again, which has the effect of changing their error messages:
774d564b
PP
1423
1424 # __DIE__ hooks may modify error messages
1425 {
f86cebdf
GS
1426 local $SIG{'__DIE__'} =
1427 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
c7cc6f1c
GS
1428 eval { die "foo lives here" };
1429 print $@ if $@; # prints "bar lives here"
774d564b
PP
1430 }
1431
19799a22 1432Because this promotes action at a distance, this counterintuitive behavior
2b5ab1e7
TC
1433may be fixed in a future release.
1434
19799a22 1435With an C<eval>, you should be especially careful to remember what's
a0d0e21e
LW
1436being looked at when:
1437
1438 eval $x; # CASE 1
1439 eval "$x"; # CASE 2
1440
1441 eval '$x'; # CASE 3
1442 eval { $x }; # CASE 4
1443
5a964f20 1444 eval "\$$x++"; # CASE 5
a0d0e21e
LW
1445 $$x++; # CASE 6
1446
2f9daede 1447Cases 1 and 2 above behave identically: they run the code contained in
19799a22 1448the variable $x. (Although case 2 has misleading double quotes making
2f9daede 1449the reader wonder what else might be happening (nothing is).) Cases 3
7660c0ab 1450and 4 likewise behave in the same way: they run the code C<'$x'>, which
19799a22 1451does nothing but return the value of $x. (Case 4 is preferred for
2f9daede
TPG
1452purely visual reasons, but it also has the advantage of compiling at
1453compile-time instead of at run-time.) Case 5 is a place where
19799a22 1454normally you I<would> like to use double quotes, except that in this
2f9daede
TPG
1455particular situation, you can just use symbolic references instead, as
1456in case 6.
a0d0e21e 1457
4968c1e4 1458C<eval BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7 1459C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
4968c1e4 1460
d819b83a
DM
1461Note that as a very special case, an C<eval ''> executed within the C<DB>
1462package doesn't see the usual surrounding lexical scope, but rather the
1463scope of the first non-DB piece of code that called it. You don't normally
1464need to worry about this unless you are writing a Perl debugger.
1465
a0d0e21e
LW
1466=item exec LIST
1467
8bf3b016
GS
1468=item exec PROGRAM LIST
1469
19799a22
GS
1470The C<exec> function executes a system command I<and never returns>--
1471use C<system> instead of C<exec> if you want it to return. It fails and
1472returns false only if the command does not exist I<and> it is executed
fb73857a 1473directly instead of via your system's command shell (see below).
a0d0e21e 1474
19799a22
GS
1475Since it's a common mistake to use C<exec> instead of C<system>, Perl
1476warns you if there is a following statement which isn't C<die>, C<warn>,
1477or C<exit> (if C<-w> is set - but you always do that). If you
1478I<really> want to follow an C<exec> with some other statement, you
55d729e4
GS
1479can use one of these styles to avoid the warning:
1480
5a964f20
TC
1481 exec ('foo') or print STDERR "couldn't exec foo: $!";
1482 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
55d729e4 1483
5a964f20 1484If there is more than one argument in LIST, or if LIST is an array
f86cebdf 1485with more than one value, calls execvp(3) with the arguments in LIST.
5a964f20
TC
1486If there is only one scalar argument or an array with one element in it,
1487the argument is checked for shell metacharacters, and if there are any,
1488the entire argument is passed to the system's command shell for parsing
1489(this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1490If there are no shell metacharacters in the argument, it is split into
b76cc8ba 1491words and passed directly to C<execvp>, which is more efficient.
19799a22 1492Examples:
a0d0e21e 1493
19799a22
GS
1494 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1495 exec "sort $outfile | uniq";
a0d0e21e
LW
1496
1497If you don't really want to execute the first argument, but want to lie
1498to the program you are executing about its own name, you can specify
1499the program you actually want to run as an "indirect object" (without a
1500comma) in front of the LIST. (This always forces interpretation of the
54310121 1501LIST as a multivalued list, even if there is only a single scalar in
a0d0e21e
LW
1502the list.) Example:
1503
1504 $shell = '/bin/csh';
1505 exec $shell '-sh'; # pretend it's a login shell
1506
1507or, more directly,
1508
1509 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1510
bb32b41a
GS
1511When the arguments get executed via the system shell, results will
1512be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1513for details.
1514
19799a22
GS
1515Using an indirect object with C<exec> or C<system> is also more
1516secure. This usage (which also works fine with system()) forces
1517interpretation of the arguments as a multivalued list, even if the
1518list had just one argument. That way you're safe from the shell
1519expanding wildcards or splitting up words with whitespace in them.
5a964f20
TC
1520
1521 @args = ( "echo surprise" );
1522
2b5ab1e7 1523 exec @args; # subject to shell escapes
f86cebdf 1524 # if @args == 1
2b5ab1e7 1525 exec { $args[0] } @args; # safe even with one-arg list
5a964f20
TC
1526
1527The first version, the one without the indirect object, ran the I<echo>
1528program, passing it C<"surprise"> an argument. The second version
1529didn't--it tried to run a program literally called I<"echo surprise">,
1530didn't find it, and set C<$?> to a non-zero value indicating failure.
1531
0f897271
GS
1532Beginning with v5.6.0, Perl will attempt to flush all files opened for
1533output before the exec, but this may not be supported on some platforms
1534(see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1535in English) or call the C<autoflush()> method of C<IO::Handle> on any
1536open handles in order to avoid lost output.
1537
19799a22 1538Note that C<exec> will not call your C<END> blocks, nor will it call
7660c0ab
A
1539any C<DESTROY> methods in your objects.
1540
a0d0e21e
LW
1541=item exists EXPR
1542
01020589 1543Given an expression that specifies a hash element or array element,
8ea97a1e
GS
1544returns true if the specified element in the hash or array has ever
1545been initialized, even if the corresponding value is undefined. The
1546element is not autovivified if it doesn't exist.
a0d0e21e 1547
01020589
GS
1548 print "Exists\n" if exists $hash{$key};
1549 print "Defined\n" if defined $hash{$key};
1550 print "True\n" if $hash{$key};
1551
1552 print "Exists\n" if exists $array[$index];
1553 print "Defined\n" if defined $array[$index];
1554 print "True\n" if $array[$index];
a0d0e21e 1555
8ea97a1e 1556A hash or array element can be true only if it's defined, and defined if
a0d0e21e
LW
1557it exists, but the reverse doesn't necessarily hold true.
1558
afebc493
GS
1559Given an expression that specifies the name of a subroutine,
1560returns true if the specified subroutine has ever been declared, even
1561if it is undefined. Mentioning a subroutine name for exists or defined
847c7ebe
DD
1562does not count as declaring it. Note that a subroutine which does not
1563exist may still be callable: its package may have an C<AUTOLOAD>
1564method that makes it spring into existence the first time that it is
1565called -- see L<perlsub>.
afebc493
GS
1566
1567 print "Exists\n" if exists &subroutine;
1568 print "Defined\n" if defined &subroutine;
1569
a0d0e21e 1570Note that the EXPR can be arbitrarily complicated as long as the final
afebc493 1571operation is a hash or array key lookup or subroutine name:
a0d0e21e 1572
2b5ab1e7
TC
1573 if (exists $ref->{A}->{B}->{$key}) { }
1574 if (exists $hash{A}{B}{$key}) { }
1575
01020589
GS
1576 if (exists $ref->{A}->{B}->[$ix]) { }
1577 if (exists $hash{A}{B}[$ix]) { }
1578
afebc493
GS
1579 if (exists &{$ref->{A}{B}{$key}}) { }
1580
01020589
GS
1581Although the deepest nested array or hash will not spring into existence
1582just because its existence was tested, any intervening ones will.
61eff3bc 1583Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
01020589
GS
1584into existence due to the existence test for the $key element above.
1585This happens anywhere the arrow operator is used, including even:
5a964f20 1586
2b5ab1e7
TC
1587 undef $ref;
1588 if (exists $ref->{"Some key"}) { }
1589 print $ref; # prints HASH(0x80d3d5c)
1590
1591This surprising autovivification in what does not at first--or even
1592second--glance appear to be an lvalue context may be fixed in a future
5a964f20 1593release.
a0d0e21e 1594
afebc493
GS
1595Use of a subroutine call, rather than a subroutine name, as an argument
1596to exists() is an error.
1597
1598 exists &sub; # OK
1599 exists &sub(); # Error
1600
a0d0e21e
LW
1601=item exit EXPR
1602
2b5ab1e7 1603Evaluates EXPR and exits immediately with that value. Example:
a0d0e21e
LW
1604
1605 $ans = <STDIN>;
1606 exit 0 if $ans =~ /^[Xx]/;
1607
19799a22 1608See also C<die>. If EXPR is omitted, exits with C<0> status. The only
2b5ab1e7
TC
1609universally recognized values for EXPR are C<0> for success and C<1>
1610for error; other values are subject to interpretation depending on the
1611environment in which the Perl program is running. For example, exiting
161269 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1613the mailer to return the item undelivered, but that's not true everywhere.
a0d0e21e 1614
19799a22
GS
1615Don't use C<exit> to abort a subroutine if there's any chance that
1616someone might want to trap whatever error happened. Use C<die> instead,
1617which can be trapped by an C<eval>.
28757baa 1618
19799a22 1619The exit() function does not always exit immediately. It calls any
2b5ab1e7 1620defined C<END> routines first, but these C<END> routines may not
19799a22 1621themselves abort the exit. Likewise any object destructors that need to
2b5ab1e7
TC
1622be called are called before the real exit. If this is a problem, you
1623can call C<POSIX:_exit($status)> to avoid END and destructor processing.
87275199 1624See L<perlmod> for details.
5a964f20 1625
a0d0e21e
LW
1626=item exp EXPR
1627
54310121 1628=item exp
bbce6d69 1629
b76cc8ba 1630Returns I<e> (the natural logarithm base) to the power of EXPR.
a0d0e21e
LW
1631If EXPR is omitted, gives C<exp($_)>.
1632
1633=item fcntl FILEHANDLE,FUNCTION,SCALAR
1634
f86cebdf 1635Implements the fcntl(2) function. You'll probably have to say
a0d0e21e
LW
1636
1637 use Fcntl;
1638
0ade1984 1639first to get the correct constant definitions. Argument processing and
b76cc8ba 1640value return works just like C<ioctl> below.
a0d0e21e
LW
1641For example:
1642
1643 use Fcntl;
5a964f20
TC
1644 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1645 or die "can't fcntl F_GETFL: $!";
1646
554ad1fc 1647You don't have to check for C<defined> on the return from C<fcntl>.
951ba7fe
GS
1648Like C<ioctl>, it maps a C<0> return from the system call into
1649C<"0 but true"> in Perl. This string is true in boolean context and C<0>
2b5ab1e7
TC
1650in numeric context. It is also exempt from the normal B<-w> warnings
1651on improper numeric conversions.
5a964f20 1652
19799a22 1653Note that C<fcntl> will produce a fatal error if used on a machine that
2b5ab1e7
TC
1654doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1655manpage to learn what functions are available on your system.
a0d0e21e
LW
1656
1657=item fileno FILEHANDLE
1658
2b5ab1e7
TC
1659Returns the file descriptor for a filehandle, or undefined if the
1660filehandle is not open. This is mainly useful for constructing
19799a22 1661bitmaps for C<select> and low-level POSIX tty-handling operations.
2b5ab1e7
TC
1662If FILEHANDLE is an expression, the value is taken as an indirect
1663filehandle, generally its name.
5a964f20 1664
b76cc8ba 1665You can use this to find out whether two handles refer to the
5a964f20
TC
1666same underlying descriptor:
1667
1668 if (fileno(THIS) == fileno(THAT)) {
1669 print "THIS and THAT are dups\n";
b76cc8ba
NIS
1670 }
1671
1672(Filehandles connected to memory objects via new features of C<open> may
1673return undefined even though they are open.)
1674
a0d0e21e
LW
1675
1676=item flock FILEHANDLE,OPERATION
1677
19799a22
GS
1678Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1679for success, false on failure. Produces a fatal error if used on a
2b5ab1e7 1680machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
19799a22 1681C<flock> is Perl's portable file locking interface, although it locks
2b5ab1e7
TC
1682only entire files, not records.
1683
1684Two potentially non-obvious but traditional C<flock> semantics are
1685that it waits indefinitely until the lock is granted, and that its locks
1686B<merely advisory>. Such discretionary locks are more flexible, but offer
19799a22
GS
1687fewer guarantees. This means that files locked with C<flock> may be
1688modified by programs that do not also use C<flock>. See L<perlport>,
2b5ab1e7
TC
1689your port's specific documentation, or your system-specific local manpages
1690for details. It's best to assume traditional behavior if you're writing
1691portable programs. (But if you're not, you should as always feel perfectly
1692free to write for your own system's idiosyncrasies (sometimes called
1693"features"). Slavish adherence to portability concerns shouldn't get
1694in the way of your getting your job done.)
a3cb178b 1695
8ebc5c01
PP
1696OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1697LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
ea3105be 1698you can use the symbolic names if you import them from the Fcntl module,
68dc0745
PP
1699either individually, or as a group using the ':flock' tag. LOCK_SH
1700requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
ea3105be
GS
1701releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1702LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
68dc0745
PP
1703waiting for the lock (check the return status to see if you got it).
1704
2b5ab1e7
TC
1705To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1706before locking or unlocking it.
8ebc5c01 1707
f86cebdf 1708Note that the emulation built with lockf(3) doesn't provide shared
8ebc5c01 1709locks, and it requires that FILEHANDLE be open with write intent. These
2b5ab1e7 1710are the semantics that lockf(3) implements. Most if not all systems
f86cebdf 1711implement lockf(3) in terms of fcntl(2) locking, though, so the
8ebc5c01
PP
1712differing semantics shouldn't bite too many people.
1713
becacb53
TM
1714Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
1715be open with read intent to use LOCK_SH and requires that it be open
1716with write intent to use LOCK_EX.
1717
19799a22
GS
1718Note also that some versions of C<flock> cannot lock things over the
1719network; you would need to use the more system-specific C<fcntl> for
f86cebdf
GS
1720that. If you like you can force Perl to ignore your system's flock(2)
1721function, and so provide its own fcntl(2)-based emulation, by passing
8ebc5c01
PP
1722the switch C<-Ud_flock> to the F<Configure> program when you configure
1723perl.
4633a7c4
LW
1724
1725Here's a mailbox appender for BSD systems.
a0d0e21e 1726
7e1af8bc 1727 use Fcntl ':flock'; # import LOCK_* constants
a0d0e21e
LW
1728
1729 sub lock {
7e1af8bc 1730 flock(MBOX,LOCK_EX);
a0d0e21e
LW
1731 # and, in case someone appended
1732 # while we were waiting...
1733 seek(MBOX, 0, 2);
1734 }
1735
1736 sub unlock {
7e1af8bc 1737 flock(MBOX,LOCK_UN);
a0d0e21e
LW
1738 }
1739
1740 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1741 or die "Can't open mailbox: $!";
1742
1743 lock();
1744 print MBOX $msg,"\n\n";
1745 unlock();
1746
2b5ab1e7
TC
1747On systems that support a real flock(), locks are inherited across fork()
1748calls, whereas those that must resort to the more capricious fcntl()
1749function lose the locks, making it harder to write servers.
1750
cb1a09d0 1751See also L<DB_File> for other flock() examples.
a0d0e21e
LW
1752
1753=item fork
1754
2b5ab1e7
TC
1755Does a fork(2) system call to create a new process running the
1756same program at the same point. It returns the child pid to the
1757parent process, C<0> to the child process, or C<undef> if the fork is
1758unsuccessful. File descriptors (and sometimes locks on those descriptors)
1759are shared, while everything else is copied. On most systems supporting
1760fork(), great care has gone into making it extremely efficient (for
1761example, using copy-on-write technology on data pages), making it the
1762dominant paradigm for multitasking over the last few decades.
5a964f20 1763
0f897271
GS
1764Beginning with v5.6.0, Perl will attempt to flush all files opened for
1765output before forking the child process, but this may not be supported
1766on some platforms (see L<perlport>). To be safe, you may need to set
1767C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1768C<IO::Handle> on any open handles in order to avoid duplicate output.
a0d0e21e 1769
19799a22 1770If you C<fork> without ever waiting on your children, you will
2b5ab1e7
TC
1771accumulate zombies. On some systems, you can avoid this by setting
1772C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1773forking and reaping moribund children.
cb1a09d0 1774
28757baa
PP
1775Note that if your forked child inherits system file descriptors like
1776STDIN and STDOUT that are actually connected by a pipe or socket, even
2b5ab1e7 1777if you exit, then the remote server (such as, say, a CGI script or a
19799a22 1778backgrounded job launched from a remote shell) won't think you're done.
2b5ab1e7 1779You should reopen those to F</dev/null> if it's any issue.
28757baa 1780
cb1a09d0
AD
1781=item format
1782
19799a22 1783Declare a picture format for use by the C<write> function. For
cb1a09d0
AD
1784example:
1785
54310121 1786 format Something =
cb1a09d0
AD
1787 Test: @<<<<<<<< @||||| @>>>>>
1788 $str, $%, '$' . int($num)
1789 .
1790
1791 $str = "widget";
184e9718 1792 $num = $cost/$quantity;
cb1a09d0
AD
1793 $~ = 'Something';
1794 write;
1795
1796See L<perlform> for many details and examples.
1797
8903cb82 1798=item formline PICTURE,LIST
a0d0e21e 1799
5a964f20 1800This is an internal function used by C<format>s, though you may call it,
a0d0e21e
LW
1801too. It formats (see L<perlform>) a list of values according to the
1802contents of PICTURE, placing the output into the format output
7660c0ab 1803accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
19799a22 1804Eventually, when a C<write> is done, the contents of
a0d0e21e 1805C<$^A> are written to some filehandle, but you could also read C<$^A>
7660c0ab 1806yourself and then set C<$^A> back to C<"">. Note that a format typically
19799a22 1807does one C<formline> per line of form, but the C<formline> function itself
748a9306 1808doesn't care how many newlines are embedded in the PICTURE. This means
4633a7c4 1809that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
748a9306
LW
1810You may therefore need to use multiple formlines to implement a single
1811record format, just like the format compiler.
1812
19799a22 1813Be careful if you put double quotes around the picture, because an C<@>
748a9306 1814character may be taken to mean the beginning of an array name.
19799a22 1815C<formline> always returns true. See L<perlform> for other examples.
a0d0e21e
LW
1816
1817=item getc FILEHANDLE
1818
1819=item getc
1820
1821Returns the next character from the input file attached to FILEHANDLE,
b5fe5ca2
SR
1822or the undefined value at end of file, or if there was an error (in
1823the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
1824STDIN. This is not particularly efficient. However, it cannot be
1825used by itself to fetch single characters without waiting for the user
1826to hit enter. For that, try something more like:
4633a7c4
LW
1827
1828 if ($BSD_STYLE) {
1829 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1830 }
1831 else {
54310121 1832 system "stty", '-icanon', 'eol', "\001";
4633a7c4
LW
1833 }
1834
1835 $key = getc(STDIN);
1836
1837 if ($BSD_STYLE) {
1838 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1839 }
1840 else {
5f05dabc 1841 system "stty", 'icanon', 'eol', '^@'; # ASCII null
4633a7c4
LW
1842 }
1843 print "\n";
1844
54310121
PP
1845Determination of whether $BSD_STYLE should be set
1846is left as an exercise to the reader.
cb1a09d0 1847
19799a22 1848The C<POSIX::getattr> function can do this more portably on
2b5ab1e7
TC
1849systems purporting POSIX compliance. See also the C<Term::ReadKey>
1850module from your nearest CPAN site; details on CPAN can be found on
1851L<perlmodlib/CPAN>.
a0d0e21e
LW
1852
1853=item getlogin
1854
5a964f20
TC
1855Implements the C library function of the same name, which on most
1856systems returns the current login from F</etc/utmp>, if any. If null,
19799a22 1857use C<getpwuid>.
a0d0e21e 1858
f86702cc 1859 $login = getlogin || getpwuid($<) || "Kilroy";
a0d0e21e 1860
19799a22
GS
1861Do not consider C<getlogin> for authentication: it is not as
1862secure as C<getpwuid>.
4633a7c4 1863
a0d0e21e
LW
1864=item getpeername SOCKET
1865
1866Returns the packed sockaddr address of other end of the SOCKET connection.
1867
4633a7c4
LW
1868 use Socket;
1869 $hersockaddr = getpeername(SOCK);
19799a22 1870 ($port, $iaddr) = sockaddr_in($hersockaddr);
4633a7c4
LW
1871 $herhostname = gethostbyaddr($iaddr, AF_INET);
1872 $herstraddr = inet_ntoa($iaddr);
a0d0e21e
LW
1873
1874=item getpgrp PID
1875
47e29363 1876Returns the current process group for the specified PID. Use
7660c0ab 1877a PID of C<0> to get the current process group for the
4633a7c4 1878current process. Will raise an exception if used on a machine that
f86cebdf 1879doesn't implement getpgrp(2). If PID is omitted, returns process
19799a22 1880group of current process. Note that the POSIX version of C<getpgrp>
7660c0ab 1881does not accept a PID argument, so only C<PID==0> is truly portable.
a0d0e21e
LW
1882
1883=item getppid
1884
1885Returns the process id of the parent process.
1886
4d76a344
RGS
1887Note for Linux users: on Linux, the C functions C<getpid()> and
1888C<getppid()> return different values from different threads. In order to
1889be portable, this behavior is not reflected by the perl-level function
1890C<getppid()>, that returns a consistent value across threads. If you want
e3256f86
RGS
1891to call the underlying C<getppid()>, you may use the CPAN module
1892C<Linux::Pid>.
4d76a344 1893
a0d0e21e
LW
1894=item getpriority WHICH,WHO
1895
4633a7c4
LW
1896Returns the current priority for a process, a process group, or a user.
1897(See L<getpriority(2)>.) Will raise a fatal exception if used on a
f86cebdf 1898machine that doesn't implement getpriority(2).
a0d0e21e
LW
1899
1900=item getpwnam NAME
1901
1902=item getgrnam NAME
1903
1904=item gethostbyname NAME
1905
1906=item getnetbyname NAME
1907
1908=item getprotobyname NAME
1909
1910=item getpwuid UID
1911
1912=item getgrgid GID
1913
1914=item getservbyname NAME,PROTO
1915
1916=item gethostbyaddr ADDR,ADDRTYPE
1917
1918=item getnetbyaddr ADDR,ADDRTYPE
1919
1920=item getprotobynumber NUMBER
1921
1922=item getservbyport PORT,PROTO
1923
1924=item getpwent
1925
1926=item getgrent
1927
1928=item gethostent
1929
1930=item getnetent
1931
1932=item getprotoent
1933
1934=item getservent
1935
1936=item setpwent
1937
1938=item setgrent
1939
1940=item sethostent STAYOPEN
1941
1942=item setnetent STAYOPEN
1943
1944=item setprotoent STAYOPEN
1945
1946=item setservent STAYOPEN
1947
1948=item endpwent
1949
1950=item endgrent
1951
1952=item endhostent
1953
1954=item endnetent
1955
1956=item endprotoent
1957
1958=item endservent
1959
1960These routines perform the same functions as their counterparts in the
5a964f20 1961system library. In list context, the return values from the
a0d0e21e
LW
1962various get routines are as follows:
1963
1964 ($name,$passwd,$uid,$gid,
6ee623d5 1965 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
a0d0e21e
LW
1966 ($name,$passwd,$gid,$members) = getgr*
1967 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1968 ($name,$aliases,$addrtype,$net) = getnet*
1969 ($name,$aliases,$proto) = getproto*
1970 ($name,$aliases,$port,$proto) = getserv*
1971
1972(If the entry doesn't exist you get a null list.)
1973
4602f195
JH
1974The exact meaning of the $gcos field varies but it usually contains
1975the real name of the user (as opposed to the login name) and other
1976information pertaining to the user. Beware, however, that in many
1977system users are able to change this information and therefore it
106325ad 1978cannot be trusted and therefore the $gcos is tainted (see
2959b6e3
JH
1979L<perlsec>). The $passwd and $shell, user's encrypted password and
1980login shell, are also tainted, because of the same reason.
4602f195 1981
5a964f20 1982In scalar context, you get the name, unless the function was a
a0d0e21e
LW
1983lookup by name, in which case you get the other thing, whatever it is.
1984(If the entry doesn't exist you get the undefined value.) For example:
1985
5a964f20
TC
1986 $uid = getpwnam($name);
1987 $name = getpwuid($num);
1988 $name = getpwent();
1989 $gid = getgrnam($name);
08a33e13 1990 $name = getgrgid($num);
5a964f20
TC
1991 $name = getgrent();
1992 #etc.
a0d0e21e 1993
4602f195
JH
1994In I<getpw*()> the fields $quota, $comment, and $expire are special
1995cases in the sense that in many systems they are unsupported. If the
1996$quota is unsupported, it is an empty scalar. If it is supported, it
1997usually encodes the disk quota. If the $comment field is unsupported,
1998it is an empty scalar. If it is supported it usually encodes some
1999administrative comment about the user. In some systems the $quota
2000field may be $change or $age, fields that have to do with password
2001aging. In some systems the $comment field may be $class. The $expire
2002field, if present, encodes the expiration period of the account or the
2003password. For the availability and the exact meaning of these fields
2004in your system, please consult your getpwnam(3) documentation and your
2005F<pwd.h> file. You can also find out from within Perl what your
2006$quota and $comment fields mean and whether you have the $expire field
2007by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2008C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
2009files are only supported if your vendor has implemented them in the
2010intuitive fashion that calling the regular C library routines gets the
5d3a0a3b
GS
2011shadow versions if you're running under privilege or if there exists
2012the shadow(3) functions as found in System V ( this includes Solaris
2013and Linux.) Those systems which implement a proprietary shadow password
2014facility are unlikely to be supported.
6ee623d5 2015
19799a22 2016The $members value returned by I<getgr*()> is a space separated list of
a0d0e21e
LW
2017the login names of the members of the group.
2018
2019For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2020C, it will be returned to you via C<$?> if the function call fails. The
7660c0ab 2021C<@addrs> value returned by a successful call is a list of the raw
a0d0e21e
LW
2022addresses returned by the corresponding system library call. In the
2023Internet domain, each address is four bytes long and you can unpack it
2024by saying something like:
2025
2026 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
2027
2b5ab1e7
TC
2028The Socket library makes this slightly easier:
2029
2030 use Socket;
2031 $iaddr = inet_aton("127.1"); # or whatever address
2032 $name = gethostbyaddr($iaddr, AF_INET);
2033
2034 # or going the other way
19799a22 2035 $straddr = inet_ntoa($iaddr);
2b5ab1e7 2036
19799a22
GS
2037If you get tired of remembering which element of the return list
2038contains which return value, by-name interfaces are provided
2039in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2040C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2041and C<User::grent>. These override the normal built-ins, supplying
2042versions that return objects with the appropriate names
2043for each field. For example:
5a964f20
TC
2044
2045 use File::stat;
2046 use User::pwent;
2047 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2048
b76cc8ba
NIS
2049Even though it looks like they're the same method calls (uid),
2050they aren't, because a C<File::stat> object is different from
19799a22 2051a C<User::pwent> object.
5a964f20 2052
a0d0e21e
LW
2053=item getsockname SOCKET
2054
19799a22
GS
2055Returns the packed sockaddr address of this end of the SOCKET connection,
2056in case you don't know the address because you have several different
2057IPs that the connection might have come in on.
a0d0e21e 2058
4633a7c4
LW
2059 use Socket;
2060 $mysockaddr = getsockname(SOCK);
19799a22 2061 ($port, $myaddr) = sockaddr_in($mysockaddr);
b76cc8ba 2062 printf "Connect to %s [%s]\n",
19799a22
GS
2063 scalar gethostbyaddr($myaddr, AF_INET),
2064 inet_ntoa($myaddr);
a0d0e21e
LW
2065
2066=item getsockopt SOCKET,LEVEL,OPTNAME
2067
5a964f20 2068Returns the socket option requested, or undef if there is an error.
a0d0e21e
LW
2069
2070=item glob EXPR
2071
0a753a76
PP
2072=item glob
2073
d9a9d457
JL
2074In list context, returns a (possibly empty) list of filename expansions on
2075the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
2076scalar context, glob iterates through such filename expansions, returning
2077undef when the list is exhausted. This is the internal function
2078implementing the C<< <*.c> >> operator, but you can use it directly. If
2079EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2080more detail in L<perlop/"I/O Operators">.
a0d0e21e 2081
3a4b19e4
GS
2082Beginning with v5.6.0, this operator is implemented using the standard
2083C<File::Glob> extension. See L<File::Glob> for details.
2084
a0d0e21e
LW
2085=item gmtime EXPR
2086
d1be9408 2087Converts a time as returned by the time function to an 8-element list
54310121 2088with the time localized for the standard Greenwich time zone.
4633a7c4 2089Typically used as follows:
a0d0e21e 2090
b76cc8ba 2091 # 0 1 2 3 4 5 6 7
48a26b3a 2092 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday) =
a0d0e21e
LW
2093 gmtime(time);
2094
48a26b3a
GS
2095All list elements are numeric, and come straight out of the C `struct
2096tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2097specified time. $mday is the day of the month, and $mon is the month
2098itself, in the range C<0..11> with 0 indicating January and 11
2099indicating December. $year is the number of years since 1900. That
2100is, $year is C<123> in year 2023. $wday is the day of the week, with
21010 indicating Sunday and 3 indicating Wednesday. $yday is the day of
b76cc8ba 2102the year, in the range C<0..364> (or C<0..365> in leap years.)
48a26b3a
GS
2103
2104Note that the $year element is I<not> simply the last two digits of
2105the year. If you assume it is, then you create non-Y2K-compliant
2106programs--and you wouldn't want to do that, would you?
2f9daede 2107
abd75f24
GS
2108The proper way to get a complete 4-digit year is simply:
2109
2110 $year += 1900;
2111
2112And to get the last two digits of the year (e.g., '01' in 2001) do:
2113
2114 $year = sprintf("%02d", $year % 100);
2115
48a26b3a 2116If EXPR is omitted, C<gmtime()> uses the current time (C<gmtime(time)>).
a0d0e21e 2117
48a26b3a 2118In scalar context, C<gmtime()> returns the ctime(3) value:
0a753a76
PP
2119
2120 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
2121
19799a22 2122Also see the C<timegm> function provided by the C<Time::Local> module,
f86cebdf 2123and the strftime(3) function available via the POSIX module.
7660c0ab 2124
2b5ab1e7
TC
2125This scalar value is B<not> locale dependent (see L<perllocale>), but
2126is instead a Perl builtin. Also see the C<Time::Local> module, and the
2127strftime(3) and mktime(3) functions available via the POSIX module. To
7660c0ab
A
2128get somewhat similar but locale dependent date strings, set up your
2129locale environment variables appropriately (please see L<perllocale>)
2130and try for example:
2131
2132 use POSIX qw(strftime);
2b5ab1e7 2133 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
7660c0ab 2134
2b5ab1e7
TC
2135Note that the C<%a> and C<%b> escapes, which represent the short forms
2136of the day of the week and the month of the year, may not necessarily
2137be three characters wide in all locales.
0a753a76 2138
a0d0e21e
LW
2139=item goto LABEL
2140
748a9306
LW
2141=item goto EXPR
2142
a0d0e21e
LW
2143=item goto &NAME
2144
7660c0ab 2145The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
a0d0e21e 2146execution there. It may not be used to go into any construct that
7660c0ab 2147requires initialization, such as a subroutine or a C<foreach> loop. It
0a753a76 2148also can't be used to go into a construct that is optimized away,
19799a22 2149or to get out of a block or subroutine given to C<sort>.
0a753a76 2150It can be used to go almost anywhere else within the dynamic scope,
a0d0e21e 2151including out of subroutines, but it's usually better to use some other
19799a22 2152construct such as C<last> or C<die>. The author of Perl has never felt the
7660c0ab 2153need to use this form of C<goto> (in Perl, that is--C is another matter).
1b6921cb
BT
2154(The difference being that C does not offer named loops combined with
2155loop control. Perl does, and this replaces most structured uses of C<goto>
2156in other languages.)
a0d0e21e 2157
7660c0ab
A
2158The C<goto-EXPR> form expects a label name, whose scope will be resolved
2159dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
748a9306
LW
2160necessarily recommended if you're optimizing for maintainability:
2161
2162 goto ("FOO", "BAR", "GLARCH")[$i];
2163
1b6921cb
BT
2164The C<goto-&NAME> form is quite different from the other forms of
2165C<goto>. In fact, it isn't a goto in the normal sense at all, and
2166doesn't have the stigma associated with other gotos. Instead, it
2167exits the current subroutine (losing any changes set by local()) and
2168immediately calls in its place the named subroutine using the current
2169value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2170load another subroutine and then pretend that the other subroutine had
2171been called in the first place (except that any modifications to C<@_>
6cb9131c
GS
2172in the current subroutine are propagated to the other subroutine.)
2173After the C<goto>, not even C<caller> will be able to tell that this
2174routine was called first.
2175
2176NAME needn't be the name of a subroutine; it can be a scalar variable
2177containing a code reference, or a block which evaluates to a code
2178reference.
a0d0e21e
LW
2179
2180=item grep BLOCK LIST
2181
2182=item grep EXPR,LIST
2183
2b5ab1e7
TC
2184This is similar in spirit to, but not the same as, grep(1) and its
2185relatives. In particular, it is not limited to using regular expressions.
2f9daede 2186
a0d0e21e 2187Evaluates the BLOCK or EXPR for each element of LIST (locally setting
7660c0ab 2188C<$_> to each element) and returns the list value consisting of those
19799a22
GS
2189elements for which the expression evaluated to true. In scalar
2190context, returns the number of times the expression was true.
a0d0e21e
LW
2191
2192 @foo = grep(!/^#/, @bar); # weed out comments
2193
2194or equivalently,
2195
2196 @foo = grep {!/^#/} @bar; # weed out comments
2197
be3174d2
GS
2198Note that C<$_> is an alias to the list value, so it can be used to
2199modify the elements of the LIST. While this is useful and supported,
2200it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
2201Similarly, grep returns aliases into the original list, much as a for
2202loop's index variable aliases the list elements. That is, modifying an
19799a22
GS
2203element of a list returned by grep (for example, in a C<foreach>, C<map>
2204or another C<grep>) actually modifies the element in the original list.
2b5ab1e7 2205This is usually something to be avoided when writing clear code.
a0d0e21e 2206
19799a22 2207See also L</map> for a list composed of the results of the BLOCK or EXPR.
38325410 2208
a0d0e21e
LW
2209=item hex EXPR
2210
54310121 2211=item hex
bbce6d69 2212
2b5ab1e7
TC
2213Interprets EXPR as a hex string and returns the corresponding value.
2214(To convert strings that might start with either 0, 0x, or 0b, see
2215L</oct>.) If EXPR is omitted, uses C<$_>.
2f9daede
TPG
2216
2217 print hex '0xAf'; # prints '175'
2218 print hex 'aF'; # same
a0d0e21e 2219
19799a22 2220Hex strings may only represent integers. Strings that would cause
53305cf1
NC
2221integer overflow trigger a warning. Leading whitespace is not stripped,
2222unlike oct().
19799a22 2223
a0d0e21e
LW
2224=item import
2225
19799a22 2226There is no builtin C<import> function. It is just an ordinary
4633a7c4 2227method (subroutine) defined (or inherited) by modules that wish to export
19799a22 2228names to another module. The C<use> function calls the C<import> method
cea6626f 2229for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
a0d0e21e
LW
2230
2231=item index STR,SUBSTR,POSITION
2232
2233=item index STR,SUBSTR
2234
2b5ab1e7
TC
2235The index function searches for one string within another, but without
2236the wildcard-like behavior of a full regular-expression pattern match.
2237It returns the position of the first occurrence of SUBSTR in STR at
2238or after POSITION. If POSITION is omitted, starts searching from the
2239beginning of the string. The return value is based at C<0> (or whatever
2240you've set the C<$[> variable to--but don't do that). If the substring
2241is not found, returns one less than the base, ordinarily C<-1>.
a0d0e21e
LW
2242
2243=item int EXPR
2244
54310121 2245=item int
bbce6d69 2246
7660c0ab 2247Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2b5ab1e7
TC
2248You should not use this function for rounding: one because it truncates
2249towards C<0>, and two because machine representations of floating point
2250numbers can sometimes produce counterintuitive results. For example,
2251C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2252because it's really more like -268.99999999999994315658 instead. Usually,
19799a22 2253the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2b5ab1e7 2254functions will serve you better than will int().
a0d0e21e
LW
2255
2256=item ioctl FILEHANDLE,FUNCTION,SCALAR
2257
2b5ab1e7 2258Implements the ioctl(2) function. You'll probably first have to say
a0d0e21e 2259
4633a7c4 2260 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
a0d0e21e 2261
2b5ab1e7 2262to get the correct function definitions. If F<ioctl.ph> doesn't
a0d0e21e 2263exist or doesn't have the correct definitions you'll have to roll your
61eff3bc 2264own, based on your C header files such as F<< <sys/ioctl.h> >>.
5a964f20 2265(There is a Perl script called B<h2ph> that comes with the Perl kit that
54310121 2266may help you in this, but it's nontrivial.) SCALAR will be read and/or
4633a7c4 2267written depending on the FUNCTION--a pointer to the string value of SCALAR
19799a22 2268will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
4633a7c4
LW
2269has no string value but does have a numeric value, that value will be
2270passed rather than a pointer to the string value. To guarantee this to be
19799a22
GS
2271true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2272functions may be needed to manipulate the values of structures used by
b76cc8ba 2273C<ioctl>.
a0d0e21e 2274
19799a22 2275The return value of C<ioctl> (and C<fcntl>) is as follows:
a0d0e21e
LW
2276
2277 if OS returns: then Perl returns:
2278 -1 undefined value
2279 0 string "0 but true"
2280 anything else that number
2281
19799a22 2282Thus Perl returns true on success and false on failure, yet you can
a0d0e21e
LW
2283still easily determine the actual value returned by the operating
2284system:
2285
2b5ab1e7 2286 $retval = ioctl(...) || -1;
a0d0e21e
LW
2287 printf "System returned %d\n", $retval;
2288
c2611fb3 2289The special string "C<0> but true" is exempt from B<-w> complaints
5a964f20
TC
2290about improper numeric conversions.
2291
19799a22
GS
2292Here's an example of setting a filehandle named C<REMOTE> to be
2293non-blocking at the system level. You'll have to negotiate C<$|>
2294on your own, though.
2295
2296 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
2297
2298 $flags = fcntl(REMOTE, F_GETFL, 0)
2299 or die "Can't get flags for the socket: $!\n";
2300
2301 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
2302 or die "Can't set flags for the socket: $!\n";
2303
a0d0e21e
LW
2304=item join EXPR,LIST
2305
2b5ab1e7
TC
2306Joins the separate strings of LIST into a single string with fields
2307separated by the value of EXPR, and returns that new string. Example:
a0d0e21e 2308
2b5ab1e7 2309 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
a0d0e21e 2310
eb6e2d6f
GS
2311Beware that unlike C<split>, C<join> doesn't take a pattern as its
2312first argument. Compare L</split>.
a0d0e21e 2313
aa689395
PP
2314=item keys HASH
2315
504f80c1
JH
2316Returns a list consisting of all the keys of the named hash.
2317(In scalar context, returns the number of keys.)
2318
2319The keys are returned in an apparently random order. The actual
2320random order is subject to change in future versions of perl, but it
2321is guaranteed to be the same order as either the C<values> or C<each>
4546b9e6
JH
2322function produces (given that the hash has not been modified). Since
2323Perl 5.8.1 the ordering is different even between different runs of
2324Perl for security reasons (see L<perlsec/"Algorithmic Complexity
d6df3700 2325Attacks">).
504f80c1
JH
2326
2327As a side effect, calling keys() resets the HASH's internal iterator,
2328see L</each>.
a0d0e21e 2329
aa689395 2330Here is yet another way to print your environment:
a0d0e21e
LW
2331
2332 @keys = keys %ENV;
2333 @values = values %ENV;
b76cc8ba 2334 while (@keys) {
a0d0e21e
LW
2335 print pop(@keys), '=', pop(@values), "\n";
2336 }
2337
2338or how about sorted by key:
2339
2340 foreach $key (sort(keys %ENV)) {
2341 print $key, '=', $ENV{$key}, "\n";
2342 }
2343
8ea1e5d4
GS
2344The returned values are copies of the original keys in the hash, so
2345modifying them will not affect the original hash. Compare L</values>.
2346
19799a22 2347To sort a hash by value, you'll need to use a C<sort> function.
aa689395 2348Here's a descending numeric sort of a hash by its values:
4633a7c4 2349
5a964f20 2350 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
4633a7c4
LW
2351 printf "%4d %s\n", $hash{$key}, $key;
2352 }
2353
19799a22 2354As an lvalue C<keys> allows you to increase the number of hash buckets
aa689395
PP
2355allocated for the given hash. This can gain you a measure of efficiency if
2356you know the hash is going to get big. (This is similar to pre-extending
2357an array by assigning a larger number to $#array.) If you say
55497cff
PP
2358
2359 keys %hash = 200;
2360
ab192400
GS
2361then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2362in fact, since it rounds up to the next power of two. These
55497cff
PP
2363buckets will be retained even if you do C<%hash = ()>, use C<undef
2364%hash> if you want to free the storage while C<%hash> is still in scope.
2365You can't shrink the number of buckets allocated for the hash using
19799a22 2366C<keys> in this way (but you needn't worry about doing this by accident,
55497cff
PP
2367as trying has no effect).
2368
19799a22 2369See also C<each>, C<values> and C<sort>.
ab192400 2370
b350dd2f 2371=item kill SIGNAL, LIST
a0d0e21e 2372
b350dd2f 2373Sends a signal to a list of processes. Returns the number of
517db077
GS
2374processes successfully signaled (which is not necessarily the
2375same as the number actually killed).
a0d0e21e
LW
2376
2377 $cnt = kill 1, $child1, $child2;
2378 kill 9, @goners;
2379
b350dd2f 2380If SIGNAL is zero, no signal is sent to the process. This is a
1e9c1022 2381useful way to check that a child process is alive and hasn't changed
b350dd2f
GS
2382its UID. See L<perlport> for notes on the portability of this
2383construct.
2384
2385Unlike in the shell, if SIGNAL is negative, it kills
4633a7c4
LW
2386process groups instead of processes. (On System V, a negative I<PROCESS>
2387number will also kill process groups, but that's not portable.) That
2388means you usually want to use positive not negative signals. You may also
1e9c1022
JL
2389use a signal name in quotes.
2390
2391See L<perlipc/"Signals"> for more details.
a0d0e21e
LW
2392
2393=item last LABEL
2394
2395=item last
2396
2397The C<last> command is like the C<break> statement in C (as used in
2398loops); it immediately exits the loop in question. If the LABEL is
2399omitted, the command refers to the innermost enclosing loop. The
2400C<continue> block, if any, is not executed:
2401
4633a7c4
LW
2402 LINE: while (<STDIN>) {
2403 last LINE if /^$/; # exit when done with header
5a964f20 2404 #...
a0d0e21e
LW
2405 }
2406
4968c1e4 2407C<last> cannot be used to exit a block which returns a value such as
2b5ab1e7
TC
2408C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2409a grep() or map() operation.
4968c1e4 2410
6c1372ed
GS
2411Note that a block by itself is semantically identical to a loop
2412that executes once. Thus C<last> can be used to effect an early
2413exit out of such a block.
2414
98293880
JH
2415See also L</continue> for an illustration of how C<last>, C<next>, and
2416C<redo> work.
1d2dff63 2417
a0d0e21e
LW
2418=item lc EXPR
2419
54310121 2420=item lc
bbce6d69 2421
d1be9408 2422Returns a lowercased version of EXPR. This is the internal function
ad0029c4
JH
2423implementing the C<\L> escape in double-quoted strings. Respects
2424current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
983ffd37 2425and L<perlunicode> for more details about locale and Unicode support.
a0d0e21e 2426
7660c0ab 2427If EXPR is omitted, uses C<$_>.
bbce6d69 2428
a0d0e21e
LW
2429=item lcfirst EXPR
2430
54310121 2431=item lcfirst
bbce6d69 2432
ad0029c4
JH
2433Returns the value of EXPR with the first character lowercased. This
2434is the internal function implementing the C<\l> escape in
2435double-quoted strings. Respects current LC_CTYPE locale if C<use
983ffd37
JH
2436locale> in force. See L<perllocale> and L<perlunicode> for more
2437details about locale and Unicode support.
a0d0e21e 2438
7660c0ab 2439If EXPR is omitted, uses C<$_>.
bbce6d69 2440
a0d0e21e
LW
2441=item length EXPR
2442
54310121 2443=item length
bbce6d69 2444
974da8e5 2445Returns the length in I<characters> of the value of EXPR. If EXPR is
b76cc8ba 2446omitted, returns length of C<$_>. Note that this cannot be used on
2b5ab1e7
TC
2447an entire array or hash to find out how many elements these have.
2448For that, use C<scalar @array> and C<scalar keys %hash> respectively.
a0d0e21e 2449
974da8e5
JH
2450Note the I<characters>: if the EXPR is in Unicode, you will get the
2451number of characters, not the number of bytes. To get the length
2452in bytes, use C<do { use bytes; length(EXPR) }>, see L<bytes>.
2453
a0d0e21e
LW
2454=item link OLDFILE,NEWFILE
2455
19799a22 2456Creates a new filename linked to the old filename. Returns true for
b76cc8ba 2457success, false otherwise.
a0d0e21e
LW
2458
2459=item listen SOCKET,QUEUESIZE
2460
19799a22 2461Does the same thing that the listen system call does. Returns true if
b76cc8ba 2462it succeeded, false otherwise. See the example in
cea6626f 2463L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
2464
2465=item local EXPR
2466
19799a22 2467You really probably want to be using C<my> instead, because C<local> isn't
b76cc8ba 2468what most people think of as "local". See
13a2d996 2469L<perlsub/"Private Variables via my()"> for details.
2b5ab1e7 2470
5a964f20
TC
2471A local modifies the listed variables to be local to the enclosing
2472block, file, or eval. If more than one value is listed, the list must
2473be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2474for details, including issues with tied arrays and hashes.
a0d0e21e 2475
a0d0e21e
LW
2476=item localtime EXPR
2477
19799a22 2478Converts a time as returned by the time function to a 9-element list
5f05dabc 2479with the time analyzed for the local time zone. Typically used as
a0d0e21e
LW
2480follows:
2481
54310121 2482 # 0 1 2 3 4 5 6 7 8
a0d0e21e
LW
2483 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2484 localtime(time);
2485
48a26b3a
GS
2486All list elements are numeric, and come straight out of the C `struct
2487tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2488specified time. $mday is the day of the month, and $mon is the month
2489itself, in the range C<0..11> with 0 indicating January and 11
2490indicating December. $year is the number of years since 1900. That
2491is, $year is C<123> in year 2023. $wday is the day of the week, with
24920 indicating Sunday and 3 indicating Wednesday. $yday is the day of
874b1813 2493the year, in the range C<0..364> (or C<0..365> in leap years.) $isdst
48a26b3a
GS
2494is true if the specified time occurs during daylight savings time,
2495false otherwise.
2496
2497Note that the $year element is I<not> simply the last two digits of
2498the year. If you assume it is, then you create non-Y2K-compliant
2499programs--and you wouldn't want to do that, would you?
54310121 2500
abd75f24
GS
2501The proper way to get a complete 4-digit year is simply:
2502
2503 $year += 1900;
2504
2505And to get the last two digits of the year (e.g., '01' in 2001) do:
2506
2507 $year = sprintf("%02d", $year % 100);
2508
48a26b3a 2509If EXPR is omitted, C<localtime()> uses the current time (C<localtime(time)>).
a0d0e21e 2510
48a26b3a 2511In scalar context, C<localtime()> returns the ctime(3) value:
a0d0e21e 2512
5f05dabc 2513 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
a0d0e21e 2514
a3cb178b 2515This scalar value is B<not> locale dependent, see L<perllocale>, but
68f8bed4
JH
2516instead a Perl builtin. Also see the C<Time::Local> module
2517(to convert the second, minutes, hours, ... back to seconds since the
2518stroke of midnight the 1st of January 1970, the value returned by
ca6e1c26 2519time()), and the strftime(3) and mktime(3) functions available via the
68f8bed4
JH
2520POSIX module. To get somewhat similar but locale dependent date
2521strings, set up your locale environment variables appropriately
2522(please see L<perllocale>) and try for example:
a3cb178b 2523
5a964f20 2524 use POSIX qw(strftime);
2b5ab1e7 2525 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
a3cb178b
GS
2526
2527Note that the C<%a> and C<%b>, the short forms of the day of the week
2528and the month of the year, may not necessarily be three characters wide.
a0d0e21e 2529
07698885 2530=item lock THING
19799a22 2531
01e6739c 2532This function places an advisory lock on a shared variable, or referenced
03730085 2533object contained in I<THING> until the lock goes out of scope.
a6d5524e 2534
f3a23afb 2535lock() is a "weak keyword" : this means that if you've defined a function
67408cae 2536by this name (before any calls to it), that function will be called
03730085
AB
2537instead. (However, if you've said C<use threads>, lock() is always a
2538keyword.) See L<threads>.
19799a22 2539
a0d0e21e
LW
2540=item log EXPR
2541
54310121 2542=item log
bbce6d69 2543
2b5ab1e7
TC
2544Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2545returns log of C<$_>. To get the log of another base, use basic algebra:
19799a22 2546The base-N log of a number is equal to the natural log of that number
2b5ab1e7
TC
2547divided by the natural log of N. For example:
2548
2549 sub log10 {
2550 my $n = shift;
2551 return log($n)/log(10);
b76cc8ba 2552 }
2b5ab1e7
TC
2553
2554See also L</exp> for the inverse operation.
a0d0e21e 2555
a0d0e21e
LW
2556=item lstat EXPR
2557
54310121 2558=item lstat
bbce6d69 2559
19799a22 2560Does the same thing as the C<stat> function (including setting the
5a964f20
TC
2561special C<_> filehandle) but stats a symbolic link instead of the file
2562the symbolic link points to. If symbolic links are unimplemented on
c837d5b4
DP
2563your system, a normal C<stat> is done. For much more detailed
2564information, please see the documentation for C<stat>.
a0d0e21e 2565
7660c0ab 2566If EXPR is omitted, stats C<$_>.
bbce6d69 2567
a0d0e21e
LW
2568=item m//
2569
2570The match operator. See L<perlop>.
2571
2572=item map BLOCK LIST
2573
2574=item map EXPR,LIST
2575
19799a22
GS
2576Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2577C<$_> to each element) and returns the list value composed of the
2578results of each such evaluation. In scalar context, returns the
2579total number of elements so generated. Evaluates BLOCK or EXPR in
2580list context, so each element of LIST may produce zero, one, or
2581more elements in the returned value.
dd99ebda 2582
a0d0e21e
LW
2583 @chars = map(chr, @nums);
2584
2585translates a list of numbers to the corresponding characters. And
2586
4633a7c4 2587 %hash = map { getkey($_) => $_ } @array;
a0d0e21e
LW
2588
2589is just a funny way to write
2590
2591 %hash = ();
2592 foreach $_ (@array) {
4633a7c4 2593 $hash{getkey($_)} = $_;
a0d0e21e
LW
2594 }
2595
be3174d2
GS
2596Note that C<$_> is an alias to the list value, so it can be used to
2597modify the elements of the LIST. While this is useful and supported,
2598it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
2599Using a regular C<foreach> loop for this purpose would be clearer in
2600most cases. See also L</grep> for an array composed of those items of
2601the original list for which the BLOCK or EXPR evaluates to true.
fb73857a 2602
205fdb4d
NC
2603C<{> starts both hash references and blocks, so C<map { ...> could be either
2604the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look
2605ahead for the closing C<}> it has to take a guess at which its dealing with
2606based what it finds just after the C<{>. Usually it gets it right, but if it
2607doesn't it won't realize something is wrong until it gets to the C<}> and
2608encounters the missing (or unexpected) comma. The syntax error will be
2609reported close to the C<}> but you'll need to change something near the C<{>
2610such as using a unary C<+> to give perl some help:
2611
2612 %hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong
2613 %hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right
2614 %hash = map { ("\L$_", 1) } @array # this also works
2615 %hash = map { lc($_), 1 } @array # as does this.
2616 %hash = map +( lc($_), 1 ), @array # this is EXPR and works!
cea6626f 2617
205fdb4d
NC
2618 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
2619
2620or to force an anon hash constructor use C<+{>
2621
2622 @hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end
2623
2624and you get list of anonymous hashes each with only 1 entry.
2625
19799a22 2626=item mkdir FILENAME,MASK
a0d0e21e 2627
5a211162
GS
2628=item mkdir FILENAME
2629
0591cd52 2630Creates the directory specified by FILENAME, with permissions
19799a22
GS
2631specified by MASK (as modified by C<umask>). If it succeeds it
2632returns true, otherwise it returns false and sets C<$!> (errno).
5a211162 2633If omitted, MASK defaults to 0777.
0591cd52 2634
19799a22 2635In general, it is better to create directories with permissive MASK,
0591cd52 2636and let the user modify that with their C<umask>, than it is to supply
19799a22 2637a restrictive MASK and give the user no way to be more permissive.
0591cd52
NT
2638The exceptions to this rule are when the file or directory should be
2639kept private (mail files, for instance). The perlfunc(1) entry on
19799a22 2640C<umask> discusses the choice of MASK in more detail.
a0d0e21e 2641
cc1852e8
JH
2642Note that according to the POSIX 1003.1-1996 the FILENAME may have any
2643number of trailing slashes. Some operating and filesystems do not get
2644this right, so Perl automatically removes all trailing slashes to keep
2645everyone happy.
2646
a0d0e21e
LW
2647=item msgctl ID,CMD,ARG
2648
f86cebdf 2649Calls the System V IPC function msgctl(2). You'll probably have to say
0ade1984
JH
2650
2651 use IPC::SysV;
2652
7660c0ab
A
2653first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2654then ARG must be a variable which will hold the returned C<msqid_ds>
951ba7fe
GS
2655structure. Returns like C<ioctl>: the undefined value for error,
2656C<"0 but true"> for zero, or the actual return value otherwise. See also
4755096e 2657L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation.
a0d0e21e
LW
2658
2659=item msgget KEY,FLAGS
2660
f86cebdf 2661Calls the System V IPC function msgget(2). Returns the message queue
4755096e
GS
2662id, or the undefined value if there is an error. See also
2663L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation.
a0d0e21e 2664
a0d0e21e
LW
2665=item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2666
2667Calls the System V IPC function msgrcv to receive a message from
2668message queue ID into variable VAR with a maximum message size of
41d6edb2
JH
2669SIZE. Note that when a message is received, the message type as a
2670native long integer will be the first thing in VAR, followed by the
2671actual message. This packing may be opened with C<unpack("l! a*")>.
2672Taints the variable. Returns true if successful, or false if there is
4755096e
GS
2673an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and
2674C<IPC::SysV::Msg> documentation.
41d6edb2
JH
2675
2676=item msgsnd ID,MSG,FLAGS
2677
2678Calls the System V IPC function msgsnd to send the message MSG to the
2679message queue ID. MSG must begin with the native long integer message
2680type, and be followed by the length of the actual message, and finally
2681the message itself. This kind of packing can be achieved with
2682C<pack("l! a*", $type, $message)>. Returns true if successful,
2683or false if there is an error. See also C<IPC::SysV>
2684and C<IPC::SysV::Msg> documentation.
a0d0e21e
LW
2685
2686=item my EXPR
2687
307ea6df
JH
2688=item my TYPE EXPR
2689
1d2de774 2690=item my EXPR : ATTRS
09bef843 2691
1d2de774 2692=item my TYPE EXPR : ATTRS
307ea6df 2693
19799a22 2694A C<my> declares the listed variables to be local (lexically) to the
1d2de774
JH
2695enclosing block, file, or C<eval>. If more than one value is listed,
2696the list must be placed in parentheses.
307ea6df 2697
1d2de774
JH
2698The exact semantics and interface of TYPE and ATTRS are still
2699evolving. TYPE is currently bound to the use of C<fields> pragma,
307ea6df
JH
2700and attributes are handled using the C<attributes> pragma, or starting
2701from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
2702L<perlsub/"Private Variables via my()"> for details, and L<fields>,
2703L<attributes>, and L<Attribute::Handlers>.
4633a7c4 2704
a0d0e21e
LW
2705=item next LABEL
2706
2707=item next
2708
2709The C<next> command is like the C<continue> statement in C; it starts
2710the next iteration of the loop:
2711
4633a7c4
LW
2712 LINE: while (<STDIN>) {
2713 next LINE if /^#/; # discard comments
5a964f20 2714 #...
a0d0e21e
LW
2715 }
2716
2717Note that if there were a C<continue> block on the above, it would get
2718executed even on discarded lines. If the LABEL is omitted, the command
2719refers to the innermost enclosing loop.
2720
4968c1e4 2721C<next> cannot be used to exit a block which returns a value such as
2b5ab1e7
TC
2722C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2723a grep() or map() operation.
4968c1e4 2724
6c1372ed
GS
2725Note that a block by itself is semantically identical to a loop
2726that executes once. Thus C<next> will exit such a block early.
2727
98293880
JH
2728See also L</continue> for an illustration of how C<last>, C<next>, and
2729C<redo> work.
1d2dff63 2730
4a66ea5a
RGS
2731=item no Module VERSION LIST
2732
2733=item no Module VERSION
2734
a0d0e21e
LW
2735=item no Module LIST
2736
4a66ea5a
RGS
2737=item no Module
2738
593b9c14 2739See the C<use> function, of which C<no> is the opposite.
a0d0e21e
LW
2740
2741=item oct EXPR
2742
54310121 2743=item oct
bbce6d69 2744
4633a7c4 2745Interprets EXPR as an octal string and returns the corresponding
4f19785b
WSI
2746value. (If EXPR happens to start off with C<0x>, interprets it as a
2747hex string. If EXPR starts off with C<0b>, it is interpreted as a
53305cf1
NC
2748binary string. Leading whitespace is ignored in all three cases.)
2749The following will handle decimal, binary, octal, and hex in the standard
2750Perl or C notation:
a0d0e21e
LW
2751
2752 $val = oct($val) if $val =~ /^0/;
2753
19799a22
GS
2754If EXPR is omitted, uses C<$_>. To go the other way (produce a number
2755in octal), use sprintf() or printf():
2756
2757 $perms = (stat("filename"))[2] & 07777;
2758 $oct_perms = sprintf "%lo", $perms;
2759
2760The oct() function is commonly used when a string such as C<644> needs
2761to be converted into a file mode, for example. (Although perl will
2762automatically convert strings into numbers as needed, this automatic
2763conversion assumes base 10.)
a0d0e21e
LW
2764
2765=item open FILEHANDLE,EXPR
2766
68bd7414
NIS
2767=item open FILEHANDLE,MODE,EXPR
2768
2769=item open FILEHANDLE,MODE,EXPR,LIST
2770
ba964c95
T
2771=item open FILEHANDLE,MODE,REFERENCE
2772
a0d0e21e
LW
2773=item open FILEHANDLE
2774
2775Opens the file whose filename is given by EXPR, and associates it with
ed53a2bb
JH
2776FILEHANDLE.
2777
2778(The following is a comprehensive reference to open(): for a gentler
2779introduction you may consider L<perlopentut>.)
2780
a28cd5c9
NT
2781If FILEHANDLE is an undefined scalar variable (or array or hash element)
2782the variable is assigned a reference to a new anonymous filehandle,
2783otherwise if FILEHANDLE is an expression, its value is used as the name of
2784the real filehandle wanted. (This is considered a symbolic reference, so
2785C<use strict 'refs'> should I<not> be in effect.)
ed53a2bb
JH
2786
2787If EXPR is omitted, the scalar variable of the same name as the
2788FILEHANDLE contains the filename. (Note that lexical variables--those
2789declared with C<my>--will not work for this purpose; so if you're
67408cae 2790using C<my>, specify EXPR in your call to open.)
ed53a2bb
JH
2791
2792If three or more arguments are specified then the mode of opening and
2793the file name are separate. If MODE is C<< '<' >> or nothing, the file
2794is opened for input. If MODE is C<< '>' >>, the file is truncated and
2795opened for output, being created if necessary. If MODE is C<<< '>>' >>>,
b76cc8ba 2796the file is opened for appending, again being created if necessary.
5a964f20 2797
ed53a2bb
JH
2798You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to
2799indicate that you want both read and write access to the file; thus
2800C<< '+<' >> is almost always preferred for read/write updates--the C<<
2801'+>' >> mode would clobber the file first. You can't usually use
2802either read-write mode for updating textfiles, since they have
2803variable length records. See the B<-i> switch in L<perlrun> for a
2804better approach. The file is created with permissions of C<0666>
2805modified by the process' C<umask> value.
2806
2807These various prefixes correspond to the fopen(3) modes of C<'r'>,
2808C<'r+'>, C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
5f05dabc 2809
6170680b
IZ
2810In the 2-arguments (and 1-argument) form of the call the mode and
2811filename should be concatenated (in this order), possibly separated by
68bd7414
NIS
2812spaces. It is possible to omit the mode in these forms if the mode is
2813C<< '<' >>.
6170680b 2814
7660c0ab 2815If the filename begins with C<'|'>, the filename is interpreted as a
5a964f20 2816command to which output is to be piped, and if the filename ends with a
f244e06d
GS
2817C<'|'>, the filename is interpreted as a command which pipes output to
2818us. See L<perlipc/"Using open() for IPC">
19799a22 2819for more examples of this. (You are not allowed to C<open> to a command
5a964f20 2820that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
4a4eefd0
GS
2821and L<perlipc/"Bidirectional Communication with Another Process">
2822for alternatives.)
cb1a09d0 2823
ed53a2bb
JH
2824For three or more arguments if MODE is C<'|-'>, the filename is
2825interpreted as a command to which output is to be piped, and if MODE
2826is C<'-|'>, the filename is interpreted as a command which pipes
2827output to us. In the 2-arguments (and 1-argument) form one should
2828replace dash (C<'-'>) with the command.
2829See L<perlipc/"Using open() for IPC"> for more examples of this.
2830(You are not allowed to C<open> to a command that pipes both in I<and>
2831out, but see L<IPC::Open2>, L<IPC::Open3>, and
2832L<perlipc/"Bidirectional Communication"> for alternatives.)
2833
2834In the three-or-more argument form of pipe opens, if LIST is specified
2835(extra arguments after the command name) then LIST becomes arguments
2836to the command invoked if the platform supports it. The meaning of
2837C<open> with more than three arguments for non-pipe modes is not yet
2838specified. Experimental "layers" may give extra LIST arguments
2839meaning.
6170680b
IZ
2840
2841In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
b76cc8ba 2842and opening C<< '>-' >> opens STDOUT.
6170680b 2843
fae2c0fb
RGS
2844You may use the three-argument form of open to specify IO "layers"
2845(sometimes also referred to as "disciplines") to be applied to the handle
2846that affect how the input and output are processed (see L<open> and
2847L<PerlIO> for more details). For example
7207e29d 2848
9124316e
JH
2849 open(FH, "<:utf8", "file")
2850
2851will open the UTF-8 encoded file containing Unicode characters,
fae2c0fb
RGS
2852see L<perluniintro>. (Note that if layers are specified in the
2853three-arg form then default layers set by the C<open> pragma are
01e6739c 2854ignored.)
ed53a2bb
JH
2855
2856Open returns nonzero upon success, the undefined value otherwise. If
2857the C<open> involved a pipe, the return value happens to be the pid of
2858the subprocess.
cb1a09d0 2859
ed53a2bb
JH
2860If you're running Perl on a system that distinguishes between text
2861files and binary files, then you should check out L</binmode> for tips
2862for dealing with this. The key distinction between systems that need
2863C<binmode> and those that don't is their text file formats. Systems
8939ba94 2864like Unix, Mac OS, and Plan 9, which delimit lines with a single
ed53a2bb
JH
2865character, and which encode that character in C as C<"\n">, do not
2866need C<binmode>. The rest need it.
cb1a09d0 2867
fb73857a 2868When opening a file, it's usually a bad idea to continue normal execution
19799a22
GS
2869if the request failed, so C<open> is frequently used in connection with
2870C<die>. Even if C<die> won't do what you want (say, in a CGI script,
fb73857a 2871where you want to make a nicely formatted error message (but there are
5a964f20 2872modules that can help with that problem)) you should always check
19799a22 2873the return value from opening a file. The infrequent exception is when
fb73857a
PP
2874working with an unopened filehandle is actually what you want to do.
2875
ed53a2bb
JH
2876As a special case the 3 arg form with a read/write mode and the third
2877argument being C<undef>:
b76cc8ba
NIS
2878
2879 open(TMP, "+>", undef) or die ...
2880
f253e835
JH
2881opens a filehandle to an anonymous temporary file. Also using "+<"
2882works for symmetry, but you really should consider writing something
2883to the temporary file first. You will need to seek() to do the
2884reading.
b76cc8ba 2885
ba964c95
T
2886File handles can be opened to "in memory" files held in Perl scalars via:
2887
b996200f
SB
2888 open($fh, '>', \$variable) || ..
2889
2890Though if you try to re-open C<STDOUT> or C<STDERR> as an "in memory"
2891file, you have to close it first:
2892
2893 close STDOUT;
2894 open STDOUT, '>', \$variable or die "Can't open STDOUT: $!";
ba964c95 2895
cb1a09d0 2896Examples:
a0d0e21e
LW
2897
2898 $ARTICLE = 100;
2899 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2900 while (<ARTICLE>) {...
2901
6170680b 2902 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
fb73857a 2903 # if the open fails, output is discarded
a0d0e21e 2904
6170680b 2905 open(DBASE, '+<', 'dbase.mine') # open for update
fb73857a 2906 or die "Can't open 'dbase.mine' for update: $!";
cb1a09d0 2907
6170680b
IZ
2908 open(DBASE, '+<dbase.mine') # ditto
2909 or die "Can't open 'dbase.mine' for update: $!";
2910
2911 open(ARTICLE, '-|', "caesar <$article") # decrypt article
fb73857a 2912 or die "Can't start caesar: $!";
a0d0e21e 2913
6170680b
IZ
2914 open(ARTICLE, "caesar <$article |") # ditto
2915 or die "Can't start caesar: $!";
2916
2917 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
fb73857a 2918 or die "Can't start sort: $!";
a0d0e21e 2919
ba964c95
T
2920 # in memory files
2921 open(MEMORY,'>', \$var)
2922 or die "Can't open memory file: $!";
2923 print MEMORY "foo!\n"; # output will end up in $var
2924
a0d0e21e
LW
2925 # process argument list of files along with any includes
2926
2927 foreach $file (@ARGV) {
2928 process($file, 'fh00');
2929 }
2930
2931 sub process {
5a964f20 2932 my($filename, $input) = @_;
a0d0e21e
LW
2933 $input++; # this is a string increment
2934 unless (open($input, $filename)) {
2935 print STDERR "Can't open $filename: $!\n";
2936 return;
2937 }
2938
5a964f20 2939 local $_;
a0d0e21e
LW
2940 while (<$input>) { # note use of indirection
2941 if (/^#include "(.*)"/) {
2942 process($1, $input);
2943 next;
2944 }
5a964f20 2945 #... # whatever
a0d0e21e
LW
2946 }
2947 }
2948
2949You may also, in the Bourne shell tradition, specify an EXPR beginning
61eff3bc 2950with C<< '>&' >>, in which case the rest of the string is interpreted as the
5a964f20 2951name of a filehandle (or file descriptor, if numeric) to be
61eff3bc
JH
2952duped and opened. You may use C<&> after C<< > >>, C<<< >> >>>,
2953C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>. The
a0d0e21e 2954mode you specify should match the mode of the original filehandle.
184e9718 2955(Duping a filehandle does not take into account any existing contents of
9124316e 2956IO buffers.) If you use the 3 arg form then you can pass either a number,
b76cc8ba 2957the name of a filehandle or the normal "reference to a glob".
6170680b 2958
eae1b76b
SB
2959Here is a script that saves, redirects, and restores C<STDOUT> and
2960C<STDERR> using various methods:
a0d0e21e
LW
2961
2962 #!/usr/bin/perl
eae1b76b
SB
2963 open my $oldout, ">&STDOUT" or die "Can't dup STDOUT: $!";
2964 open OLDERR, ">&", \*STDERR or die "Can't dup STDERR: $!";
818c4caa 2965
eae1b76b
SB
2966 open STDOUT, '>', "foo.out" or die "Can't redirect STDOUT: $!";
2967 open STDERR, ">&STDOUT" or die "Can't dup STDOUT: $!";
a0d0e21e 2968
eae1b76b
SB
2969 select STDERR; $| = 1; # make unbuffered
2970 select STDOUT; $| = 1; # make unbuffered
a0d0e21e
LW
2971
2972 print STDOUT "stdout 1\n"; # this works for
2973 print STDERR "stderr 1\n"; # subprocesses too
2974
eae1b76b
SB
2975 close STDOUT;
2976 close STDERR;
a0d0e21e 2977
eae1b76b
SB
2978 open STDOUT, ">&", $oldout or die "Can't dup \$oldout: $!";
2979 open STDERR, ">&OLDERR" or die "Can't dup OLDERR: $!";
a0d0e21e
LW
2980
2981 print STDOUT "stdout 2\n";
2982 print STDERR "stderr 2\n";
2983
df632fdf
JH
2984If you specify C<< '<&=N' >>, where C<N> is a number, then Perl will
2985do an equivalent of C's C<fdopen> of that file descriptor; this is
2986more parsimonious of file descriptors. For example:
a0d0e21e
LW
2987
2988 open(FILEHANDLE, "<&=$fd")
df632fdf 2989
b76cc8ba 2990or
df632fdf 2991
b76cc8ba 2992 open(FILEHANDLE, "<&=", $fd)
a0d0e21e 2993
df632fdf
JH
2994Note that if Perl is using the standard C libraries' fdopen() then on
2995many UNIX systems, fdopen() is known to fail when file descriptors
4af147f6 2996exceed a certain value, typically 255. If you need more file
b76cc8ba 2997descriptors than that, consider rebuilding Perl to use the C<PerlIO>.
4af147f6 2998
df632fdf
JH
2999You can see whether Perl has been compiled with PerlIO or not by
3000running C<perl -V> and looking for C<useperlio=> line. If C<useperlio>
3001is C<define>, you have PerlIO, otherwise you don't.
3002
6170680b
IZ
3003If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
3004with 2-arguments (or 1-argument) form of open(), then
a0d0e21e 3005there is an implicit fork done, and the return value of open is the pid
7660c0ab 3006of the child within the parent process, and C<0> within the child
184e9718 3007process. (Use C<defined($pid)> to determine whether the open was successful.)
a0d0e21e
LW
3008The filehandle behaves normally for the parent, but i/o to that
3009filehandle is piped from/to the STDOUT/STDIN of the child process.
3010In the child process the filehandle isn't opened--i/o happens from/to
3011the new STDOUT or STDIN. Typically this is used like the normal
3012piped open when you want to exercise more control over just how the
3013pipe command gets executed, such as when you are running setuid, and
54310121 3014don't want to have to scan shell commands for metacharacters.
6170680b 3015The following triples are more or less equivalent:
a0d0e21e
LW
3016
3017 open(FOO, "|tr '[a-z]' '[A-Z]'");
6170680b
IZ
3018 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
3019 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
b76cc8ba 3020 open(FOO, '|-', "tr", '[a-z]', '[A-Z]');
a0d0e21e
LW
3021
3022 open(FOO, "cat -n '$file'|");
6170680b
IZ
3023 open(FOO, '-|', "cat -n '$file'");
3024 open(FOO, '-|') || exec 'cat', '-n', $file;
b76cc8ba
NIS
3025 open(FOO, '-|', "cat", '-n', $file);
3026
3027The last example in each block shows the pipe as "list form", which is
64da03b2
JH
3028not yet supported on all platforms. A good rule of thumb is that if
3029your platform has true C<fork()> (in other words, if your platform is
3030UNIX) you can use the list form.
a0d0e21e 3031
4633a7c4
LW
3032See L<perlipc/"Safe Pipe Opens"> for more examples of this.
3033
0f897271
GS
3034Beginning with v5.6.0, Perl will attempt to flush all files opened for
3035output before any operation that may do a fork, but this may not be
3036supported on some platforms (see L<perlport>). To be safe, you may need
3037to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
3038of C<IO::Handle> on any open handles.
3039
ed53a2bb
JH
3040On systems that support a close-on-exec flag on files, the flag will
3041be set for the newly opened file descriptor as determined by the value
3042of $^F. See L<perlvar/$^F>.
a0d0e21e 3043
0dccf244
CS
3044Closing any piped filehandle causes the parent process to wait for the
3045child to finish, and returns the status value in C<$?>.
3046
ed53a2bb
JH
3047The filename passed to 2-argument (or 1-argument) form of open() will
3048have leading and trailing whitespace deleted, and the normal
3049redirection characters honored. This property, known as "magic open",
5a964f20 3050can often be used to good effect. A user could specify a filename of
7660c0ab 3051F<"rsh cat file |">, or you could change certain filenames as needed:
5a964f20
TC
3052
3053 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
3054 open(FH, $filename) or die "Can't open $filename: $!";
3055
6170680b
IZ
3056Use 3-argument form to open a file with arbitrary weird characters in it,
3057
3058 open(FOO, '<', $file);
3059
3060otherwise it's necessary to protect any leading and trailing whitespace:
5a964f20
TC
3061
3062 $file =~ s#^(\s)#./$1#;
3063 open(FOO, "< $file\0");
3064
a31a806a 3065(this may not work on some bizarre filesystems). One should
106325ad 3066conscientiously choose between the I<magic> and 3-arguments form
6170680b
IZ
3067of open():
3068
3069 open IN, $ARGV[0];
3070
3071will allow the user to specify an argument of the form C<"rsh cat file |">,
3072but will not work on a filename which happens to have a trailing space, while
3073
3074 open IN, '<', $ARGV[0];
3075
3076will have exactly the opposite restrictions.
3077
19799a22 3078If you want a "real" C C<open> (see L<open(2)> on your system), then you
6170680b
IZ
3079should use the C<sysopen> function, which involves no such magic (but
3080may use subtly different filemodes than Perl open(), which is mapped
3081to C fopen()). This is
5a964f20
TC
3082another way to protect your filenames from interpretation. For example:
3083
3084 use IO::Handle;
3085 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
3086 or die "sysopen $path: $!";
3087 $oldfh = select(HANDLE); $| = 1; select($oldfh);
38762f02 3088 print HANDLE "stuff $$\n";
5a964f20
TC
3089 seek(HANDLE, 0, 0);
3090 print "File contains: ", <HANDLE>;
3091
7660c0ab
A
3092Using the constructor from the C<IO::Handle> package (or one of its
3093subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
5a964f20
TC
3094filehandles that have the scope of whatever variables hold references to
3095them, and automatically close whenever and however you leave that scope:
c07a80fd 3096
5f05dabc 3097 use IO::File;
5a964f20 3098 #...
c07a80fd
PP
3099 sub read_myfile_munged {
3100 my $ALL = shift;
5f05dabc 3101 my $handle = new IO::File;
c07a80fd
PP
3102 open($handle, "myfile") or die "myfile: $!";
3103 $first = <$handle>
3104 or return (); # Automatically closed here.
3105 mung $first or die "mung failed"; # Or here.
3106 return $first, <$handle> if $ALL; # Or here.
3107 $first; # Or here.
3108 }
3109
b687b08b 3110See L</seek> for some details about mixing reading and writing.
a0d0e21e
LW
3111
3112=item opendir DIRHANDLE,EXPR
3113
19799a22
GS
3114Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
3115C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
a28cd5c9
NT
3116DIRHANDLE may be an expression whose value can be used as an indirect
3117dirhandle, usually the real dirhandle name. If DIRHANDLE is an undefined
3118scalar variable (or array or hash element), the variable is assigned a
3119reference to a new anonymous dirhandle.
a0d0e21e
LW
3120DIRHANDLEs have their own namespace separate from FILEHANDLEs.
3121
3122=item ord EXPR
3123
54310121 3124=item ord
bbce6d69 3125
121910a4
JH
3126Returns the numeric (the native 8-bit encoding, like ASCII or EBCDIC,
3127or Unicode) value of the first character of EXPR. If EXPR is omitted,
3128uses C<$_>.
3129
3130For the reverse, see L</chr>.
3131See L<perlunicode> and L<encoding> for more about Unicode.
a0d0e21e 3132
77ca0c92
LW
3133=item our EXPR
3134
307ea6df
JH
3135=item our EXPR TYPE
3136
1d2de774 3137=item our EXPR : ATTRS
9969eac4 3138
1d2de774 3139=item our TYPE EXPR : ATTRS
307ea6df 3140
77ca0c92
LW
3141An C<our> declares the listed variables to be valid globals within
3142the enclosing block, file, or C<eval>. That is, it has the same
3143scoping rules as a "my" declaration, but does not create a local
3144variable. If more than one value is listed, the list must be placed
3145in parentheses. The C<our> declaration has no semantic effect unless
3146"use strict vars" is in effect, in which case it lets you use the
3147declared global variable without qualifying it with a package name.
3148(But only within the lexical scope of the C<our> declaration. In this
3149it differs from "use vars", which is package scoped.)
3150
f472eb5c
GS
3151An C<our> declaration declares a global variable that will be visible
3152across its entire lexical scope, even across package boundaries. The
3153package in which the variable is entered is determined at the point
3154of the declaration, not at the point of use. This means the following
3155behavior holds:
3156
3157 package Foo;
3158 our $bar; # declares $Foo::bar for rest of lexical scope
3159 $bar = 20;
3160
3161 package Bar;
3162 print $bar; # prints 20
3163
3164Multiple C<our> declarations in the same lexical scope are allowed
3165if they are in different packages. If they happened to be in the same
3166package, Perl will emit warnings if you have asked for them.
3167
3168 use warnings;
3169 package Foo;
3170 our $bar; # declares $Foo::bar for rest of lexical scope
3171 $bar = 20;
3172
3173 package Bar;
3174 our $bar = 30; # declares $Bar::bar for rest of lexical scope
3175 print $bar; # prints 30
3176
3177 our $bar; # emits warning
3178
9969eac4 3179An C<our> declaration may also have a list of attributes associated
307ea6df
JH
3180with it.
3181
1d2de774
JH
3182The exact semantics and interface of TYPE and ATTRS are still
3183evolving. TYPE is currently bound to the use of C<fields> pragma,
307ea6df
JH
3184and attributes are handled using the C<attributes> pragma, or starting
3185from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
3186L<perlsub/"Private Variables via my()"> for details, and L<fields>,
3187L<attributes>, and L<Attribute::Handlers>.
3188
3189The only currently recognized C<our()> attribute is C<unique> which
3190indicates that a single copy of the global is to be used by all
3191interpreters should the program happen to be running in a
3192multi-interpreter environment. (The default behaviour would be for
3193each interpreter to have its own copy of the global.) Examples:
9969eac4 3194
51d2bbcc
JH
3195 our @EXPORT : unique = qw(foo);
3196 our %EXPORT_TAGS : unique = (bar => [qw(aa bb cc)]);
3197 our $VERSION : unique = "1.00";
9969eac4 3198
96fa8c42 3199Note that this attribute also has the effect of making the global
72e53bfb
JH
3200readonly when the first new interpreter is cloned (for example,
3201when the first new thread is created).
96fa8c42 3202
9969eac4
BS
3203Multi-interpreter environments can come to being either through the
3204fork() emulation on Windows platforms, or by embedding perl in a
51d2bbcc 3205multi-threaded application. The C<unique> attribute does nothing in
9969eac4
BS
3206all other environments.
3207
a0d0e21e
LW
3208=item pack TEMPLATE,LIST
3209
2b6c5635
GS
3210Takes a LIST of values and converts it into a string using the rules
3211given by the TEMPLATE. The resulting string is the concatenation of
3212the converted values. Typically, each converted value looks
3213like its machine-level representation. For example, on 32-bit machines
3214a converted integer may be represented by a sequence of 4 bytes.
3215
18529408
IZ
3216The TEMPLATE is a sequence of characters that give the order and type
3217of values, as follows:
a0d0e21e 3218
5a929a98 3219 a A string with arbitrary binary data, will be null padded.
121910a4
JH
3220 A A text (ASCII) string, will be space padded.
3221 Z A null terminated (ASCIZ) string, will be null padded.
5a929a98 3222
2b6c5635
GS
3223 b A bit string (ascending bit order inside each byte, like vec()).
3224 B A bit string (descending bit order inside each byte).
a0d0e21e
LW
3225 h A hex string (low nybble first).
3226 H A hex string (high nybble first).
3227
3228 c A signed char value.
a0ed51b3 3229 C An unsigned char value. Only does bytes. See U for Unicode.
96e4d5b1 3230
a0d0e21e
LW
3231 s A signed short value.
3232 S An unsigned short value.
96e4d5b1 3233 (This 'short' is _exactly_ 16 bits, which may differ from
851646ae
JH
3234 what a local C compiler calls 'short'. If you want
3235 native-length shorts, use the '!' suffix.)
96e4d5b1 3236
a0d0e21e
LW
3237 i A signed integer value.
3238 I An unsigned integer value.
19799a22 3239 (This 'integer' is _at_least_ 32 bits wide. Its exact
f86cebdf
GS
3240 size depends on what a local C compiler calls 'int',
3241 and may even be larger than the 'long' described in
3242 the next item.)
96e4d5b1 3243
a0d0e21e
LW
3244 l A signed long value.
3245 L An unsigned long value.
96e4d5b1 3246 (This 'long' is _exactly_ 32 bits, which may differ from
851646ae
JH
3247 what a local C compiler calls 'long'. If you want
3248 native-length longs, use the '!' suffix.)
a0d0e21e 3249
5d11dd56
G
3250 n An unsigned short in "network" (big-endian) order.
3251 N An unsigned long in "network" (big-endian) order.
3252 v An unsigned short in "VAX" (little-endian) order.
3253 V An unsigned long in "VAX" (little-endian) order.
96e4d5b1
PP