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Fix missing functions when splitting perlfunc
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1=head1 NAME
2
3perlfunc - Perl builtin functions
4
5=head1 DESCRIPTION
6
7The functions in this section can serve as terms in an expression.
8They fall into two major categories: list operators and named unary
9operators. These differ in their precedence relationship with a
10following comma. (See the precedence table in L<perlop>.) List
11operators take more than one argument, while unary operators can never
12take more than one argument. Thus, a comma terminates the argument of
13a unary operator, but merely separates the arguments of a list
14operator. A unary operator generally provides a scalar context to its
2b5ab1e7 15argument, while a list operator may provide either scalar or list
a0d0e21e 16contexts for its arguments. If it does both, the scalar arguments will
5f05dabc 17be first, and the list argument will follow. (Note that there can ever
0f31cffe 18be only one such list argument.) For instance, splice() has three scalar
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19arguments followed by a list, whereas gethostbyname() has four scalar
20arguments.
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21
22In the syntax descriptions that follow, list operators that expect a
23list (and provide list context for the elements of the list) are shown
24with LIST as an argument. Such a list may consist of any combination
25of scalar arguments or list values; the list values will be included
26in the list as if each individual element were interpolated at that
27point in the list, forming a longer single-dimensional list value.
28Elements of the LIST should be separated by commas.
29
30Any function in the list below may be used either with or without
31parentheses around its arguments. (The syntax descriptions omit the
5f05dabc 32parentheses.) If you use the parentheses, the simple (but occasionally
19799a22 33surprising) rule is this: It I<looks> like a function, therefore it I<is> a
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34function, and precedence doesn't matter. Otherwise it's a list
35operator or unary operator, and precedence does matter. And whitespace
36between the function and left parenthesis doesn't count--so you need to
37be careful sometimes:
38
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39 print 1+2+4; # Prints 7.
40 print(1+2) + 4; # Prints 3.
41 print (1+2)+4; # Also prints 3!
42 print +(1+2)+4; # Prints 7.
43 print ((1+2)+4); # Prints 7.
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44
45If you run Perl with the B<-w> switch it can warn you about this. For
46example, the third line above produces:
47
48 print (...) interpreted as function at - line 1.
49 Useless use of integer addition in void context at - line 1.
50
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51A few functions take no arguments at all, and therefore work as neither
52unary nor list operators. These include such functions as C<time>
53and C<endpwent>. For example, C<time+86_400> always means
54C<time() + 86_400>.
55
a0d0e21e 56For functions that can be used in either a scalar or list context,
54310121 57nonabortive failure is generally indicated in a scalar context by
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58returning the undefined value, and in a list context by returning the
59null list.
60
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61Remember the following important rule: There is B<no rule> that relates
62the behavior of an expression in list context to its behavior in scalar
63context, or vice versa. It might do two totally different things.
a0d0e21e 64Each operator and function decides which sort of value it would be most
2b5ab1e7 65appropriate to return in scalar context. Some operators return the
5a964f20 66length of the list that would have been returned in list context. Some
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67operators return the first value in the list. Some operators return the
68last value in the list. Some operators return a count of successful
69operations. In general, they do what you want, unless you want
70consistency.
71
d1be9408 72A named array in scalar context is quite different from what would at
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73first glance appear to be a list in scalar context. You can't get a list
74like C<(1,2,3)> into being in scalar context, because the compiler knows
75the context at compile time. It would generate the scalar comma operator
76there, not the list construction version of the comma. That means it
77was never a list to start with.
78
79In general, functions in Perl that serve as wrappers for system calls
f86cebdf 80of the same name (like chown(2), fork(2), closedir(2), etc.) all return
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81true when they succeed and C<undef> otherwise, as is usually mentioned
82in the descriptions below. This is different from the C interfaces,
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83which return C<-1> on failure. Exceptions to this rule are C<wait>,
84C<waitpid>, and C<syscall>. System calls also set the special C<$!>
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85variable on failure. Other functions do not, except accidentally.
86
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87=head2 Perl Functions by Category
88
89Here are Perl's functions (including things that look like
5a964f20 90functions, like some keywords and named operators)
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91arranged by category. Some functions appear in more
92than one place.
93
13a2d996 94=over 4
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95
96=item Functions for SCALARs or strings
97
22fae026 98C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>,
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99C<length>, C<oct>, C<ord>, C<pack>, C<q/STRING/>, C<qq/STRING/>, C<reverse>,
100C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///>
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101
102=item Regular expressions and pattern matching
103
ab4f32c2 104C<m//>, C<pos>, C<quotemeta>, C<s///>, C<split>, C<study>, C<qr//>
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105
106=item Numeric functions
107
22fae026
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108C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>,
109C<sin>, C<sqrt>, C<srand>
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110
111=item Functions for real @ARRAYs
112
22fae026 113C<pop>, C<push>, C<shift>, C<splice>, C<unshift>
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114
115=item Functions for list data
116
ab4f32c2 117C<grep>, C<join>, C<map>, C<qw/STRING/>, C<reverse>, C<sort>, C<unpack>
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118
119=item Functions for real %HASHes
120
22fae026 121C<delete>, C<each>, C<exists>, C<keys>, C<values>
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122
123=item Input and output functions
124
22fae026
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125C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
126C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
127C<readdir>, C<rewinddir>, C<seek>, C<seekdir>, C<select>, C<syscall>,
128C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>,
129C<warn>, C<write>
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130
131=item Functions for fixed length data or records
132
22fae026 133C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec>
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134
135=item Functions for filehandles, files, or directories
136
22fae026 137C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>,
5ff3f7a4 138C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>,
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139C<readlink>, C<rename>, C<rmdir>, C<stat>, C<symlink>, C<sysopen>,
140C<umask>, C<unlink>, C<utime>
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141
142=item Keywords related to the control flow of your perl program
143
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144C<caller>, C<continue>, C<die>, C<do>, C<dump>, C<eval>, C<exit>,
145C<goto>, C<last>, C<next>, C<redo>, C<return>, C<sub>, C<wantarray>
cb1a09d0 146
54310121 147=item Keywords related to scoping
cb1a09d0 148
4375e838 149C<caller>, C<import>, C<local>, C<my>, C<our>, C<package>, C<use>
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150
151=item Miscellaneous functions
152
4375e838 153C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<our>, C<reset>,
22fae026 154C<scalar>, C<undef>, C<wantarray>
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155
156=item Functions for processes and process groups
157
22fae026 158C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
ab4f32c2 159C<pipe>, C<qx/STRING/>, C<setpgrp>, C<setpriority>, C<sleep>, C<system>,
22fae026 160C<times>, C<wait>, C<waitpid>
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161
162=item Keywords related to perl modules
163
22fae026 164C<do>, C<import>, C<no>, C<package>, C<require>, C<use>
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165
166=item Keywords related to classes and object-orientedness
167
22fae026
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168C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
169C<untie>, C<use>
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170
171=item Low-level socket functions
172
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173C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
174C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
737dd4b4 175C<socket>, C<socketpair>
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176
177=item System V interprocess communication functions
178
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179C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
180C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>
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181
182=item Fetching user and group info
183
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184C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
185C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
186C<getpwuid>, C<setgrent>, C<setpwent>
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187
188=item Fetching network info
189
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190C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
191C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
192C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
193C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
194C<setnetent>, C<setprotoent>, C<setservent>
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195
196=item Time-related functions
197
22fae026 198C<gmtime>, C<localtime>, C<time>, C<times>
cb1a09d0 199
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200=item Functions new in perl5
201
22fae026 202C<abs>, C<bless>, C<chomp>, C<chr>, C<exists>, C<formline>, C<glob>,
b76cc8ba 203C<import>, C<lc>, C<lcfirst>, C<map>, C<my>, C<no>, C<our>, C<prototype>,
4375e838 204C<qx>, C<qw>, C<readline>, C<readpipe>, C<ref>, C<sub*>, C<sysopen>, C<tie>,
22fae026 205C<tied>, C<uc>, C<ucfirst>, C<untie>, C<use>
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206
207* - C<sub> was a keyword in perl4, but in perl5 it is an
5a964f20 208operator, which can be used in expressions.
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209
210=item Functions obsoleted in perl5
211
22fae026 212C<dbmclose>, C<dbmopen>
37798a01 213
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214=back
215
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216=head2 Portability
217
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218Perl was born in Unix and can therefore access all common Unix
219system calls. In non-Unix environments, the functionality of some
220Unix system calls may not be available, or details of the available
221functionality may differ slightly. The Perl functions affected
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222by this are:
223
224C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>,
225C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>,
226C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>,
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227C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostbyname>,
228C<gethostent>, C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
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229C<getppid>, C<getprgp>, C<getpriority>, C<getprotobynumber>,
230C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>,
231C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>,
232C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>,
2b5ab1e7 233C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<rename>, C<select>, C<semctl>,
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234C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>,
235C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>,
236C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>,
737dd4b4 237C<shmwrite>, C<socket>, C<socketpair>,
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238C<stat>, C<symlink>, C<syscall>, C<sysopen>, C<system>,
239C<times>, C<truncate>, C<umask>, C<unlink>,
2b5ab1e7 240C<utime>, C<wait>, C<waitpid>
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241
242For more information about the portability of these functions, see
243L<perlport> and other available platform-specific documentation.
244
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245=head2 Alphabetical Listing of Perl Functions
246
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247=over 8
248
5b3c99c0 249=item -X FILEHANDLE
a0d0e21e 250
5b3c99c0 251=item -X EXPR
a0d0e21e 252
5b3c99c0 253=item -X
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254
255A file test, where X is one of the letters listed below. This unary
256operator takes one argument, either a filename or a filehandle, and
257tests the associated file to see if something is true about it. If the
7660c0ab 258argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
19799a22 259Unless otherwise documented, it returns C<1> for true and C<''> for false, or
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260the undefined value if the file doesn't exist. Despite the funny
261names, precedence is the same as any other named unary operator, and
262the argument may be parenthesized like any other unary operator. The
263operator may be any of:
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264X<-r>X<-w>X<-x>X<-o>X<-R>X<-W>X<-X>X<-O>X<-e>X<-z>X<-s>X<-f>X<-d>X<-l>X<-p>
265X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
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266
267 -r File is readable by effective uid/gid.
268 -w File is writable by effective uid/gid.
269 -x File is executable by effective uid/gid.
270 -o File is owned by effective uid.
271
272 -R File is readable by real uid/gid.
273 -W File is writable by real uid/gid.
274 -X File is executable by real uid/gid.
275 -O File is owned by real uid.
276
277 -e File exists.
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278 -z File has zero size (is empty).
279 -s File has nonzero size (returns size in bytes).
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280
281 -f File is a plain file.
282 -d File is a directory.
283 -l File is a symbolic link.
9c4d0f16 284 -p File is a named pipe (FIFO), or Filehandle is a pipe.
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285 -S File is a socket.
286 -b File is a block special file.
287 -c File is a character special file.
288 -t Filehandle is opened to a tty.
289
290 -u File has setuid bit set.
291 -g File has setgid bit set.
292 -k File has sticky bit set.
293
121910a4 294 -T File is an ASCII text file (heuristic guess).
2cdbc966 295 -B File is a "binary" file (opposite of -T).
a0d0e21e 296
95a3fe12 297 -M Script start time minus file modification time, in days.
a0d0e21e 298 -A Same for access time.
95a3fe12 299 -C Same for inode change time (Unix, may differ for other platforms)
a0d0e21e 300
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301Example:
302
303 while (<>) {
5b3eff12 304 chomp;
a0d0e21e 305 next unless -f $_; # ignore specials
5a964f20 306 #...
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307 }
308
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309The interpretation of the file permission operators C<-r>, C<-R>,
310C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
311of the file and the uids and gids of the user. There may be other
312reasons you can't actually read, write, or execute the file. Such
313reasons may be for example network filesystem access controls, ACLs
314(access control lists), read-only filesystems, and unrecognized
315executable formats.
316
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317Also note that, for the superuser on the local filesystems, the C<-r>,
318C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
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319if any execute bit is set in the mode. Scripts run by the superuser
320may thus need to do a stat() to determine the actual mode of the file,
2b5ab1e7 321or temporarily set their effective uid to something else.
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322
323If you are using ACLs, there is a pragma called C<filetest> that may
324produce more accurate results than the bare stat() mode bits.
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325When under the C<use filetest 'access'> the above-mentioned filetests
326will test whether the permission can (not) be granted using the
468541a8 327access() family of system calls. Also note that the C<-x> and C<-X> may
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328under this pragma return true even if there are no execute permission
329bits set (nor any extra execute permission ACLs). This strangeness is
330due to the underlying system calls' definitions. Read the
331documentation for the C<filetest> pragma for more information.
332
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333Note that C<-s/a/b/> does not do a negated substitution. Saying
334C<-exp($foo)> still works as expected, however--only single letters
335following a minus are interpreted as file tests.
336
337The C<-T> and C<-B> switches work as follows. The first block or so of the
338file is examined for odd characters such as strange control codes or
61eff3bc 339characters with the high bit set. If too many strange characters (>30%)
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340are found, it's a C<-B> file, otherwise it's a C<-T> file. Also, any file
341containing null in the first block is considered a binary file. If C<-T>
9124316e 342or C<-B> is used on a filehandle, the current IO buffer is examined
19799a22 343rather than the first block. Both C<-T> and C<-B> return true on a null
54310121 344file, or a file at EOF when testing a filehandle. Because you have to
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345read a file to do the C<-T> test, on most occasions you want to use a C<-f>
346against the file first, as in C<next unless -f $file && -T $file>.
a0d0e21e 347
19799a22 348If any of the file tests (or either the C<stat> or C<lstat> operators) are given
28757baa 349the special filehandle consisting of a solitary underline, then the stat
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350structure of the previous file test (or stat operator) is used, saving
351a system call. (This doesn't work with C<-t>, and you need to remember
352that lstat() and C<-l> will leave values in the stat structure for the
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353symbolic link, not the real file.) (Also, if the stat buffer was filled by
354a C<lstat> call, C<-T> and C<-B> will reset it with the results of C<stat _>).
355Example:
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356
357 print "Can do.\n" if -r $a || -w _ || -x _;
358
359 stat($filename);
360 print "Readable\n" if -r _;
361 print "Writable\n" if -w _;
362 print "Executable\n" if -x _;
363 print "Setuid\n" if -u _;
364 print "Setgid\n" if -g _;
365 print "Sticky\n" if -k _;
366 print "Text\n" if -T _;
367 print "Binary\n" if -B _;
368
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 $@ eq "alarm\n"; # propagate unexpected errors
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426 # timed out
427 }
428 else {
429 # didn't
430 }
431
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432=item atan2 Y,X
433
434Returns the arctangent of Y/X in the range -PI to PI.
435
ca6e1c26 436For the tangent operation, you may use the C<Math::Trig::tan>
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437function, or use the familiar relation:
438
439 sub tan { sin($_[0]) / cos($_[0]) }
440
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441=item bind SOCKET,NAME
442
443Binds a network address to a socket, just as the bind system call
19799a22 444does. Returns true if it succeeded, false otherwise. NAME should be a
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445packed address of the appropriate type for the socket. See the examples in
446L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 447
fae2c0fb 448=item binmode FILEHANDLE, LAYER
1c1fc3ea 449
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450=item binmode FILEHANDLE
451
1cbfc93d
NIS
452Arranges for FILEHANDLE to be read or written in "binary" or "text"
453mode on systems where the run-time libraries distinguish between
454binary and text files. If FILEHANDLE is an expression, the value is
455taken as the name of the filehandle. Returns true on success,
b5fe5ca2 456otherwise it returns C<undef> and sets C<$!> (errno).
1cbfc93d 457
fae2c0fb 458If LAYER is omitted or specified as C<:raw> the filehandle is made
0226bbdb
NIS
459suitable for passing binary data. This includes turning off possible CRLF
460translation and marking it as bytes (as opposed to Unicode characters).
461Note that as desipite what may be implied in I<"Programming Perl">
462(the Camel) or elsewhere C<:raw> is I<not> the simply inverse of C<:crlf>
fae2c0fb 463-- other layers which would affect binary nature of the stream are
0226bbdb
NIS
464I<also> disabled. See L<PerlIO>, L<perlrun> and the discussion about the
465PERLIO environment variable.
01e6739c 466
fae2c0fb
RGS
467I<The LAYER parameter of the binmode() function is described as "DISCIPLINE"
468in "Programming Perl, 3rd Edition". However, since the publishing of this
469book, by many known as "Camel III", the consensus of the naming of this
470functionality has moved from "discipline" to "layer". All documentation
471of this version of Perl therefore refers to "layers" rather than to
472"disciplines". Now back to the regularly scheduled documentation...>
473
01e6739c
NIS
474On some systems (in general, DOS and Windows-based systems) binmode()
475is necessary when you're not working with a text file. For the sake
476of portability it is a good idea to always use it when appropriate,
477and to never use it when it isn't appropriate.
478
479In other words: regardless of platform, use binmode() on binary files
480(like for example images).
481
fae2c0fb 482If LAYER is present it is a single string, but may contain
01e6739c 483multiple directives. The directives alter the behaviour of the
fae2c0fb 484file handle. When LAYER is present using binmode on text
01e6739c
NIS
485file makes sense.
486
487To mark FILEHANDLE as UTF-8, use C<:utf8>.
1cbfc93d
NIS
488
489The C<:bytes>, C<:crlf>, and C<:utf8>, and any other directives of the
fae2c0fb
RGS
490form C<:...>, are called I/O I<layers>. The C<open> pragma can be used to
491establish default I/O layers. See L<open>.
1cbfc93d 492
ed53a2bb 493In general, binmode() should be called after open() but before any I/O
01e6739c
NIS
494is done on the filehandle. Calling binmode() will normally flush any
495pending buffered output data (and perhaps pending input data) on the
fae2c0fb 496handle. An exception to this is the C<:encoding> layer that
01e6739c 497changes the default character encoding of the handle, see L<open>.
fae2c0fb 498The C<:encoding> layer sometimes needs to be called in
ed53a2bb 499mid-stream, and it doesn't flush the stream.
16fe6d59 500
19799a22 501The operating system, device drivers, C libraries, and Perl run-time
30168b04
GS
502system all work together to let the programmer treat a single
503character (C<\n>) as the line terminator, irrespective of the external
504representation. On many operating systems, the native text file
505representation matches the internal representation, but on some
506platforms the external representation of C<\n> is made up of more than
507one character.
508
68bd7414
NIS
509Mac OS, all variants of Unix, and Stream_LF files on VMS use a single
510character to end each line in the external representation of text (even
5e12dbfa 511though that single character is CARRIAGE RETURN on Mac OS and LINE FEED
01e6739c
NIS
512on Unix and most VMS files). In other systems like OS/2, DOS and the
513various flavors of MS-Windows your program sees a C<\n> as a simple C<\cJ>,
514but what's stored in text files are the two characters C<\cM\cJ>. That
515means that, if you don't use binmode() on these systems, C<\cM\cJ>
516sequences on disk will be converted to C<\n> on input, and any C<\n> in
517your program will be converted back to C<\cM\cJ> on output. This is what
518you want for text files, but it can be disastrous for binary files.
30168b04
GS
519
520Another consequence of using binmode() (on some systems) is that
521special end-of-file markers will be seen as part of the data stream.
522For systems from the Microsoft family this means that if your binary
4375e838 523data contains C<\cZ>, the I/O subsystem will regard it as the end of
30168b04
GS
524the file, unless you use binmode().
525
526binmode() is not only important for readline() and print() operations,
527but also when using read(), seek(), sysread(), syswrite() and tell()
528(see L<perlport> for more details). See the C<$/> and C<$\> variables
529in L<perlvar> for how to manually set your input and output
530line-termination sequences.
a0d0e21e 531
4633a7c4 532=item bless REF,CLASSNAME
a0d0e21e
LW
533
534=item bless REF
535
2b5ab1e7
TC
536This function tells the thingy referenced by REF that it is now an object
537in the CLASSNAME package. If CLASSNAME is omitted, the current package
19799a22 538is used. Because a C<bless> is often the last thing in a constructor,
2b5ab1e7
TC
539it returns the reference for convenience. Always use the two-argument
540version if the function doing the blessing might be inherited by a
541derived class. See L<perltoot> and L<perlobj> for more about the blessing
542(and blessings) of objects.
a0d0e21e 543
57668c4d 544Consider always blessing objects in CLASSNAMEs that are mixed case.
2b5ab1e7
TC
545Namespaces with all lowercase names are considered reserved for
546Perl pragmata. Builtin types have all uppercase names, so to prevent
547confusion, you may wish to avoid such package names as well. Make sure
548that CLASSNAME is a true value.
60ad88b8
GS
549
550See L<perlmod/"Perl Modules">.
551
a0d0e21e
LW
552=item caller EXPR
553
554=item caller
555
5a964f20 556Returns the context of the current subroutine call. In scalar context,
28757baa 557returns the caller's package name if there is a caller, that is, if
19799a22 558we're in a subroutine or C<eval> or C<require>, and the undefined value
5a964f20 559otherwise. In list context, returns
a0d0e21e 560
748a9306 561 ($package, $filename, $line) = caller;
a0d0e21e
LW
562
563With EXPR, it returns some extra information that the debugger uses to
564print a stack trace. The value of EXPR indicates how many call frames
565to go back before the current one.
566
f3aa04c2 567 ($package, $filename, $line, $subroutine, $hasargs,
e476b1b5 568 $wantarray, $evaltext, $is_require, $hints, $bitmask) = caller($i);
e7ea3e70 569
951ba7fe 570Here $subroutine may be C<(eval)> if the frame is not a subroutine
19799a22 571call, but an C<eval>. In such a case additional elements $evaltext and
7660c0ab 572C<$is_require> are set: C<$is_require> is true if the frame is created by a
19799a22 573C<require> or C<use> statement, $evaltext contains the text of the
277ddfaf 574C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
951ba7fe 575$filename is C<(eval)>, but $evaltext is undefined. (Note also that
0fc9dec4
RGS
576each C<use> statement creates a C<require> frame inside an C<eval EXPR>
577frame.) $subroutine may also be C<(unknown)> if this particular
578subroutine happens to have been deleted from the symbol table.
579C<$hasargs> is true if a new instance of C<@_> was set up for the frame.
580C<$hints> and C<$bitmask> contain pragmatic hints that the caller was
581compiled with. The C<$hints> and C<$bitmask> values are subject to change
582between versions of Perl, and are not meant for external use.
748a9306
LW
583
584Furthermore, when called from within the DB package, caller returns more
7660c0ab 585detailed information: it sets the list variable C<@DB::args> to be the
54310121 586arguments with which the subroutine was invoked.
748a9306 587
7660c0ab 588Be aware that the optimizer might have optimized call frames away before
19799a22 589C<caller> had a chance to get the information. That means that C<caller(N)>
7660c0ab 590might not return information about the call frame you expect it do, for
b76cc8ba 591C<< N > 1 >>. In particular, C<@DB::args> might have information from the
19799a22 592previous time C<caller> was called.
7660c0ab 593
a0d0e21e
LW
594=item chdir EXPR
595
ffce7b87 596Changes the working directory to EXPR, if possible. If EXPR is omitted,
0bfc1ec4 597changes to the directory specified by C<$ENV{HOME}>, if set; if not,
ffce7b87 598changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
b4ad75f0
AMS
599variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
600neither is set, C<chdir> does nothing. It returns true upon success,
601false otherwise. See the example under C<die>.
a0d0e21e
LW
602
603=item chmod LIST
604
605Changes the permissions of a list of files. The first element of the
4633a7c4 606list must be the numerical mode, which should probably be an octal
2f9daede
TPG
607number, and which definitely should I<not> a string of octal digits:
608C<0644> is okay, C<'0644'> is not. Returns the number of files
dc848c6f 609successfully changed. See also L</oct>, if all you have is a string.
a0d0e21e
LW
610
611 $cnt = chmod 0755, 'foo', 'bar';
612 chmod 0755, @executables;
f86cebdf
GS
613 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
614 # --w----r-T
2f9daede
TPG
615 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
616 $mode = 0644; chmod $mode, 'foo'; # this is best
a0d0e21e 617
ca6e1c26
JH
618You can also import the symbolic C<S_I*> constants from the Fcntl
619module:
620
621 use Fcntl ':mode';
622
623 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
624 # This is identical to the chmod 0755 of the above example.
625
a0d0e21e
LW
626=item chomp VARIABLE
627
313c9f5c 628=item chomp( LIST )
a0d0e21e
LW
629
630=item chomp
631
2b5ab1e7
TC
632This safer version of L</chop> removes any trailing string
633that corresponds to the current value of C<$/> (also known as
28757baa
PP
634$INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
635number of characters removed from all its arguments. It's often used to
636remove the newline from the end of an input record when you're worried
2b5ab1e7
TC
637that the final record may be missing its newline. When in paragraph
638mode (C<$/ = "">), it removes all trailing newlines from the string.
4c5a6083
GS
639When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
640a reference to an integer or the like, see L<perlvar>) chomp() won't
b76cc8ba 641remove anything.
19799a22 642If VARIABLE is omitted, it chomps C<$_>. Example:
a0d0e21e
LW
643
644 while (<>) {
645 chomp; # avoid \n on last field
646 @array = split(/:/);
5a964f20 647 # ...
a0d0e21e
LW
648 }
649
4bf21a6d
RD
650If VARIABLE is a hash, it chomps the hash's values, but not its keys.
651
a0d0e21e
LW
652You can actually chomp anything that's an lvalue, including an assignment:
653
654 chomp($cwd = `pwd`);
655 chomp($answer = <STDIN>);
656
657If you chomp a list, each element is chomped, and the total number of
658characters removed is returned.
659
15e44fd8
RGS
660Note that parentheses are necessary when you're chomping anything
661that is not a simple variable. This is because C<chomp $cwd = `pwd`;>
662is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as
663C<chomp( $cwd = `pwd` )> which you might expect. Similarly,
664C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than
665as C<chomp($a, $b)>.
666
a0d0e21e
LW
667=item chop VARIABLE
668
313c9f5c 669=item chop( LIST )
a0d0e21e
LW
670
671=item chop
672
673Chops off the last character of a string and returns the character
5b3eff12 674chopped. It is much more efficient than C<s/.$//s> because it neither
7660c0ab 675scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
4bf21a6d
RD
676If VARIABLE is a hash, it chops the hash's values, but not its keys.
677
5b3eff12 678You can actually chop anything that's an lvalue, including an assignment.
a0d0e21e
LW
679
680If you chop a list, each element is chopped. Only the value of the
19799a22 681last C<chop> is returned.
a0d0e21e 682
19799a22 683Note that C<chop> returns the last character. To return all but the last
748a9306
LW
684character, use C<substr($string, 0, -1)>.
685
15e44fd8
RGS
686See also L</chomp>.
687
a0d0e21e
LW
688=item chown LIST
689
690Changes the owner (and group) of a list of files. The first two
19799a22
GS
691elements of the list must be the I<numeric> uid and gid, in that
692order. A value of -1 in either position is interpreted by most
693systems to leave that value unchanged. Returns the number of files
694successfully changed.
a0d0e21e
LW
695
696 $cnt = chown $uid, $gid, 'foo', 'bar';
697 chown $uid, $gid, @filenames;
698
54310121 699Here's an example that looks up nonnumeric uids in the passwd file:
a0d0e21e
LW
700
701 print "User: ";
19799a22 702 chomp($user = <STDIN>);
5a964f20 703 print "Files: ";
19799a22 704 chomp($pattern = <STDIN>);
a0d0e21e
LW
705
706 ($login,$pass,$uid,$gid) = getpwnam($user)
707 or die "$user not in passwd file";
708
5a964f20 709 @ary = glob($pattern); # expand filenames
a0d0e21e
LW
710 chown $uid, $gid, @ary;
711
54310121 712On most systems, you are not allowed to change the ownership of the
4633a7c4
LW
713file unless you're the superuser, although you should be able to change
714the group to any of your secondary groups. On insecure systems, these
715restrictions may be relaxed, but this is not a portable assumption.
19799a22
GS
716On POSIX systems, you can detect this condition this way:
717
718 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
719 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
4633a7c4 720
a0d0e21e
LW
721=item chr NUMBER
722
54310121 723=item chr
bbce6d69 724
a0d0e21e 725Returns the character represented by that NUMBER in the character set.
a0ed51b3 726For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
121910a4
JH
727chr(0x263a) is a Unicode smiley face. Note that characters from 127
728to 255 (inclusive) are by default not encoded in Unicode for backward
729compatibility reasons (but see L<encoding>).
aaa68c4a 730
b76cc8ba 731For the reverse, use L</ord>.
121910a4 732See L<perlunicode> and L<encoding> for more about Unicode.
a0d0e21e 733
7660c0ab 734If NUMBER is omitted, uses C<$_>.
bbce6d69 735
a0d0e21e
LW
736=item chroot FILENAME
737
54310121 738=item chroot
bbce6d69 739
5a964f20 740This function works like the system call by the same name: it makes the
4633a7c4 741named directory the new root directory for all further pathnames that
951ba7fe 742begin with a C</> by your process and all its children. (It doesn't
28757baa 743change your current working directory, which is unaffected.) For security
4633a7c4 744reasons, this call is restricted to the superuser. If FILENAME is
19799a22 745omitted, does a C<chroot> to C<$_>.
a0d0e21e
LW
746
747=item close FILEHANDLE
748
6a518fbc
TP
749=item close
750
9124316e
JH
751Closes the file or pipe associated with the file handle, returning
752true only if IO buffers are successfully flushed and closes the system
753file descriptor. Closes the currently selected filehandle if the
754argument is omitted.
fb73857a
PP
755
756You don't have to close FILEHANDLE if you are immediately going to do
19799a22
GS
757another C<open> on it, because C<open> will close it for you. (See
758C<open>.) However, an explicit C<close> on an input file resets the line
759counter (C<$.>), while the implicit close done by C<open> does not.
fb73857a 760
19799a22
GS
761If the file handle came from a piped open C<close> will additionally
762return false if one of the other system calls involved fails or if the
fb73857a 763program exits with non-zero status. (If the only problem was that the
b76cc8ba 764program exited non-zero C<$!> will be set to C<0>.) Closing a pipe
2b5ab1e7 765also waits for the process executing on the pipe to complete, in case you
b76cc8ba 766want to look at the output of the pipe afterwards, and
2b5ab1e7 767implicitly puts the exit status value of that command into C<$?>.
5a964f20 768
73689b13
GS
769Prematurely closing the read end of a pipe (i.e. before the process
770writing to it at the other end has closed it) will result in a
771SIGPIPE being delivered to the writer. If the other end can't
772handle that, be sure to read all the data before closing the pipe.
773
fb73857a 774Example:
a0d0e21e 775
fb73857a
PP
776 open(OUTPUT, '|sort >foo') # pipe to sort
777 or die "Can't start sort: $!";
5a964f20 778 #... # print stuff to output
fb73857a
PP
779 close OUTPUT # wait for sort to finish
780 or warn $! ? "Error closing sort pipe: $!"
781 : "Exit status $? from sort";
782 open(INPUT, 'foo') # get sort's results
783 or die "Can't open 'foo' for input: $!";
a0d0e21e 784
5a964f20
TC
785FILEHANDLE may be an expression whose value can be used as an indirect
786filehandle, usually the real filehandle name.
a0d0e21e
LW
787
788=item closedir DIRHANDLE
789
19799a22 790Closes a directory opened by C<opendir> and returns the success of that
5a964f20
TC
791system call.
792
793DIRHANDLE may be an expression whose value can be used as an indirect
794dirhandle, usually the real dirhandle name.
a0d0e21e
LW
795
796=item connect SOCKET,NAME
797
798Attempts to connect to a remote socket, just as the connect system call
19799a22 799does. Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
800packed address of the appropriate type for the socket. See the examples in
801L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 802
cb1a09d0
AD
803=item continue BLOCK
804
805Actually a flow control statement rather than a function. If there is a
98293880
JH
806C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
807C<foreach>), it is always executed just before the conditional is about to
808be evaluated again, just like the third part of a C<for> loop in C. Thus
cb1a09d0
AD
809it can be used to increment a loop variable, even when the loop has been
810continued via the C<next> statement (which is similar to the C C<continue>
811statement).
812
98293880 813C<last>, C<next>, or C<redo> may appear within a C<continue>
19799a22
GS
814block. C<last> and C<redo> will behave as if they had been executed within
815the main block. So will C<next>, but since it will execute a C<continue>
1d2dff63
GS
816block, it may be more entertaining.
817
818 while (EXPR) {
819 ### redo always comes here
820 do_something;
821 } continue {
822 ### next always comes here
823 do_something_else;
824 # then back the top to re-check EXPR
825 }
826 ### last always comes here
827
828Omitting the C<continue> section is semantically equivalent to using an
19799a22 829empty one, logically enough. In that case, C<next> goes directly back
1d2dff63
GS
830to check the condition at the top of the loop.
831
a0d0e21e
LW
832=item cos EXPR
833
d6217f1e
GS
834=item cos
835
5a964f20 836Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
7660c0ab 837takes cosine of C<$_>.
a0d0e21e 838
ca6e1c26 839For the inverse cosine operation, you may use the C<Math::Trig::acos()>
28757baa
PP
840function, or use this relation:
841
842 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
843
a0d0e21e
LW
844=item crypt PLAINTEXT,SALT
845
f86cebdf 846Encrypts a string exactly like the crypt(3) function in the C library
4633a7c4
LW
847(assuming that you actually have a version there that has not been
848extirpated as a potential munition). This can prove useful for checking
849the password file for lousy passwords, amongst other things. Only the
850guys wearing white hats should do this.
a0d0e21e 851
a6d05634 852Note that L<crypt|/crypt> is intended to be a one-way function, much like
85c16d83
JH
853breaking eggs to make an omelette. There is no (known) corresponding
854decrypt function (in other words, the crypt() is a one-way hash
855function). As a result, this function isn't all that useful for
11155c91 856cryptography. (For that, see your nearby CPAN mirror.)
2f9daede 857
85c16d83
JH
858When verifying an existing encrypted string you should use the
859encrypted text as the salt (like C<crypt($plain, $crypted) eq
8e2ffcbe 860$crypted>). This allows your code to work with the standard L<crypt|/crypt>
85c16d83
JH
861and with more exotic implementations. In other words, do not assume
862anything about the returned string itself, or how many bytes in
863the encrypted string matter.
864
865Traditionally the result is a string of 13 bytes: two first bytes of
866the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
867the first eight bytes of the encrypted string mattered, but
868alternative hashing schemes (like MD5), higher level security schemes
869(like C2), and implementations on non-UNIX platforms may produce
870different strings.
871
872When choosing a new salt create a random two character string whose
873characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
874'/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>).
e71965be 875
a0d0e21e
LW
876Here's an example that makes sure that whoever runs this program knows
877their own password:
878
879 $pwd = (getpwuid($<))[1];
a0d0e21e
LW
880
881 system "stty -echo";
882 print "Password: ";
e71965be 883 chomp($word = <STDIN>);
a0d0e21e
LW
884 print "\n";
885 system "stty echo";
886
e71965be 887 if (crypt($word, $pwd) ne $pwd) {
a0d0e21e
LW
888 die "Sorry...\n";
889 } else {
890 print "ok\n";
54310121 891 }
a0d0e21e 892
9f8f0c9d 893Of course, typing in your own password to whoever asks you
748a9306 894for it is unwise.
a0d0e21e 895
8e2ffcbe 896The L<crypt|/crypt> function is unsuitable for encrypting large quantities
19799a22
GS
897of data, not least of all because you can't get the information
898back. Look at the F<by-module/Crypt> and F<by-module/PGP> directories
899on your favorite CPAN mirror for a slew of potentially useful
900modules.
901
f2791508
JH
902If using crypt() on a Unicode string (which I<potentially> has
903characters with codepoints above 255), Perl tries to make sense
904of the situation by trying to downgrade (a copy of the string)
905the string back to an eight-bit byte string before calling crypt()
906(on that copy). If that works, good. If not, crypt() dies with
907C<Wide character in crypt>.
85c16d83 908
aa689395 909=item dbmclose HASH
a0d0e21e 910
19799a22 911[This function has been largely superseded by the C<untie> function.]
a0d0e21e 912
aa689395 913Breaks the binding between a DBM file and a hash.
a0d0e21e 914
19799a22 915=item dbmopen HASH,DBNAME,MASK
a0d0e21e 916
19799a22 917[This function has been largely superseded by the C<tie> function.]
a0d0e21e 918
7b8d334a 919This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
19799a22
GS
920hash. HASH is the name of the hash. (Unlike normal C<open>, the first
921argument is I<not> a filehandle, even though it looks like one). DBNAME
aa689395
PP
922is the name of the database (without the F<.dir> or F<.pag> extension if
923any). If the database does not exist, it is created with protection
19799a22
GS
924specified by MASK (as modified by the C<umask>). If your system supports
925only the older DBM functions, you may perform only one C<dbmopen> in your
aa689395 926program. In older versions of Perl, if your system had neither DBM nor
19799a22 927ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
aa689395
PP
928sdbm(3).
929
930If you don't have write access to the DBM file, you can only read hash
931variables, not set them. If you want to test whether you can write,
19799a22 932either use file tests or try setting a dummy hash entry inside an C<eval>,
aa689395 933which will trap the error.
a0d0e21e 934
19799a22
GS
935Note that functions such as C<keys> and C<values> may return huge lists
936when used on large DBM files. You may prefer to use the C<each>
a0d0e21e
LW
937function to iterate over large DBM files. Example:
938
939 # print out history file offsets
940 dbmopen(%HIST,'/usr/lib/news/history',0666);
941 while (($key,$val) = each %HIST) {
942 print $key, ' = ', unpack('L',$val), "\n";
943 }
944 dbmclose(%HIST);
945
cb1a09d0 946See also L<AnyDBM_File> for a more general description of the pros and
184e9718 947cons of the various dbm approaches, as well as L<DB_File> for a particularly
cb1a09d0 948rich implementation.
4633a7c4 949
2b5ab1e7
TC
950You can control which DBM library you use by loading that library
951before you call dbmopen():
952
953 use DB_File;
954 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
955 or die "Can't open netscape history file: $!";
956
a0d0e21e
LW
957=item defined EXPR
958
54310121 959=item defined
bbce6d69 960
2f9daede
TPG
961Returns a Boolean value telling whether EXPR has a value other than
962the undefined value C<undef>. If EXPR is not present, C<$_> will be
963checked.
964
965Many operations return C<undef> to indicate failure, end of file,
966system error, uninitialized variable, and other exceptional
967conditions. This function allows you to distinguish C<undef> from
968other values. (A simple Boolean test will not distinguish among
7660c0ab 969C<undef>, zero, the empty string, and C<"0">, which are all equally
2f9daede 970false.) Note that since C<undef> is a valid scalar, its presence
19799a22 971doesn't I<necessarily> indicate an exceptional condition: C<pop>
2f9daede
TPG
972returns C<undef> when its argument is an empty array, I<or> when the
973element to return happens to be C<undef>.
974
f10b0346
GS
975You may also use C<defined(&func)> to check whether subroutine C<&func>
976has ever been defined. The return value is unaffected by any forward
847c7ebe
DD
977declarations of C<&foo>. Note that a subroutine which is not defined
978may still be callable: its package may have an C<AUTOLOAD> method that
979makes it spring into existence the first time that it is called -- see
980L<perlsub>.
f10b0346
GS
981
982Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
983used to report whether memory for that aggregate has ever been
984allocated. This behavior may disappear in future versions of Perl.
985You should instead use a simple test for size:
986
987 if (@an_array) { print "has array elements\n" }
988 if (%a_hash) { print "has hash members\n" }
2f9daede
TPG
989
990When used on a hash element, it tells you whether the value is defined,
dc848c6f 991not whether the key exists in the hash. Use L</exists> for the latter
2f9daede 992purpose.
a0d0e21e
LW
993
994Examples:
995
996 print if defined $switch{'D'};
997 print "$val\n" while defined($val = pop(@ary));
998 die "Can't readlink $sym: $!"
999 unless defined($value = readlink $sym);
a0d0e21e 1000 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
2f9daede 1001 $debugging = 0 unless defined $debugging;
a0d0e21e 1002
19799a22 1003Note: Many folks tend to overuse C<defined>, and then are surprised to
7660c0ab 1004discover that the number C<0> and C<""> (the zero-length string) are, in fact,
2f9daede 1005defined values. For example, if you say
a5f75d66
AD
1006
1007 "ab" =~ /a(.*)b/;
1008
7660c0ab 1009The pattern match succeeds, and C<$1> is defined, despite the fact that it
a5f75d66 1010matched "nothing". But it didn't really match nothing--rather, it
2b5ab1e7 1011matched something that happened to be zero characters long. This is all
a5f75d66 1012very above-board and honest. When a function returns an undefined value,
2f9daede 1013it's an admission that it couldn't give you an honest answer. So you
19799a22 1014should use C<defined> only when you're questioning the integrity of what
7660c0ab 1015you're trying to do. At other times, a simple comparison to C<0> or C<""> is
2f9daede
TPG
1016what you want.
1017
dc848c6f 1018See also L</undef>, L</exists>, L</ref>.
2f9daede 1019
a0d0e21e
LW
1020=item delete EXPR
1021
01020589
GS
1022Given an expression that specifies a hash element, array element, hash slice,
1023or array slice, deletes the specified element(s) from the hash or array.
8216c1fd 1024In the case of an array, if the array elements happen to be at the end,
b76cc8ba 1025the size of the array will shrink to the highest element that tests
8216c1fd 1026true for exists() (or 0 if no such element exists).
a0d0e21e 1027
01020589
GS
1028Returns each element so deleted or the undefined value if there was no such
1029element. Deleting from C<$ENV{}> modifies the environment. Deleting from
1030a hash tied to a DBM file deletes the entry from the DBM file. Deleting
1031from a C<tie>d hash or array may not necessarily return anything.
1032
8ea97a1e
GS
1033Deleting an array element effectively returns that position of the array
1034to its initial, uninitialized state. Subsequently testing for the same
8216c1fd
GS
1035element with exists() will return false. Note that deleting array
1036elements in the middle of an array will not shift the index of the ones
1037after them down--use splice() for that. See L</exists>.
8ea97a1e 1038
01020589 1039The following (inefficiently) deletes all the values of %HASH and @ARRAY:
a0d0e21e 1040
5f05dabc
PP
1041 foreach $key (keys %HASH) {
1042 delete $HASH{$key};
a0d0e21e
LW
1043 }
1044
01020589
GS
1045 foreach $index (0 .. $#ARRAY) {
1046 delete $ARRAY[$index];
1047 }
1048
1049And so do these:
5f05dabc 1050
01020589
GS
1051 delete @HASH{keys %HASH};
1052
9740c838 1053 delete @ARRAY[0 .. $#ARRAY];
5f05dabc 1054
2b5ab1e7 1055But both of these are slower than just assigning the empty list
01020589
GS
1056or undefining %HASH or @ARRAY:
1057
1058 %HASH = (); # completely empty %HASH
1059 undef %HASH; # forget %HASH ever existed
2b5ab1e7 1060
01020589
GS
1061 @ARRAY = (); # completely empty @ARRAY
1062 undef @ARRAY; # forget @ARRAY ever existed
2b5ab1e7
TC
1063
1064Note that the EXPR can be arbitrarily complicated as long as the final
01020589
GS
1065operation is a hash element, array element, hash slice, or array slice
1066lookup:
a0d0e21e
LW
1067
1068 delete $ref->[$x][$y]{$key};
5f05dabc 1069 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
a0d0e21e 1070
01020589
GS
1071 delete $ref->[$x][$y][$index];
1072 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1073
a0d0e21e
LW
1074=item die LIST
1075
19799a22
GS
1076Outside an C<eval>, prints the value of LIST to C<STDERR> and
1077exits with the current value of C<$!> (errno). If C<$!> is C<0>,
61eff3bc
JH
1078exits with the value of C<<< ($? >> 8) >>> (backtick `command`
1079status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside
19799a22
GS
1080an C<eval(),> the error message is stuffed into C<$@> and the
1081C<eval> is terminated with the undefined value. This makes
1082C<die> the way to raise an exception.
a0d0e21e
LW
1083
1084Equivalent examples:
1085
1086 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
54310121 1087 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
a0d0e21e 1088
ccac6780 1089If the last element of LIST does not end in a newline, the current
df37ec69
WW
1090script line number and input line number (if any) are also printed,
1091and a newline is supplied. Note that the "input line number" (also
1092known as "chunk") is subject to whatever notion of "line" happens to
1093be currently in effect, and is also available as the special variable
1094C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1095
1096Hint: sometimes appending C<", stopped"> to your message will cause it
1097to make better sense when the string C<"at foo line 123"> is appended.
1098Suppose you are running script "canasta".
a0d0e21e
LW
1099
1100 die "/etc/games is no good";
1101 die "/etc/games is no good, stopped";
1102
1103produce, respectively
1104
1105 /etc/games is no good at canasta line 123.
1106 /etc/games is no good, stopped at canasta line 123.
1107
2b5ab1e7 1108See also exit(), warn(), and the Carp module.
a0d0e21e 1109
7660c0ab
A
1110If LIST is empty and C<$@> already contains a value (typically from a
1111previous eval) that value is reused after appending C<"\t...propagated">.
fb73857a
PP
1112This is useful for propagating exceptions:
1113
1114 eval { ... };
1115 die unless $@ =~ /Expected exception/;
1116
ad216e65
JH
1117If LIST is empty and C<$@> contains an object reference that has a
1118C<PROPAGATE> method, that method will be called with additional file
1119and line number parameters. The return value replaces the value in
67408cae 1120C<$@>. ie. as if C<<$@ = eval { $@->PROPAGATE(__FILE__, __LINE__) };>>
ad216e65
JH
1121were called.
1122
7660c0ab 1123If C<$@> is empty then the string C<"Died"> is used.
fb73857a 1124
52531d10
GS
1125die() can also be called with a reference argument. If this happens to be
1126trapped within an eval(), $@ contains the reference. This behavior permits
1127a more elaborate exception handling implementation using objects that
4375e838 1128maintain arbitrary state about the nature of the exception. Such a scheme
52531d10
GS
1129is sometimes preferable to matching particular string values of $@ using
1130regular expressions. Here's an example:
1131
1132 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1133 if ($@) {
1134 if (ref($@) && UNIVERSAL::isa($@,"Some::Module::Exception")) {
1135 # handle Some::Module::Exception
1136 }
1137 else {
1138 # handle all other possible exceptions
1139 }
1140 }
1141
19799a22 1142Because perl will stringify uncaught exception messages before displaying
52531d10
GS
1143them, you may want to overload stringification operations on such custom
1144exception objects. See L<overload> for details about that.
1145
19799a22
GS
1146You can arrange for a callback to be run just before the C<die>
1147does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1148handler will be called with the error text and can change the error
1149message, if it sees fit, by calling C<die> again. See
1150L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
1151L<"eval BLOCK"> for some examples. Although this feature was meant
1152to be run only right before your program was to exit, this is not
1153currently the case--the C<$SIG{__DIE__}> hook is currently called
1154even inside eval()ed blocks/strings! If one wants the hook to do
1155nothing in such situations, put
fb73857a
PP
1156
1157 die @_ if $^S;
1158
19799a22
GS
1159as the first line of the handler (see L<perlvar/$^S>). Because
1160this promotes strange action at a distance, this counterintuitive
b76cc8ba 1161behavior may be fixed in a future release.
774d564b 1162
a0d0e21e
LW
1163=item do BLOCK
1164
1165Not really a function. Returns the value of the last command in the
1166sequence of commands indicated by BLOCK. When modified by a loop
98293880
JH
1167modifier, executes the BLOCK once before testing the loop condition.
1168(On other statements the loop modifiers test the conditional first.)
a0d0e21e 1169
4968c1e4 1170C<do BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7
TC
1171C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1172See L<perlsyn> for alternative strategies.
4968c1e4 1173
a0d0e21e
LW
1174=item do SUBROUTINE(LIST)
1175
1176A deprecated form of subroutine call. See L<perlsub>.
1177
1178=item do EXPR
1179
1180Uses the value of EXPR as a filename and executes the contents of the
1181file as a Perl script. Its primary use is to include subroutines
1182from a Perl subroutine library.
1183
1184 do 'stat.pl';
1185
1186is just like
1187
986b19de 1188 eval `cat stat.pl`;
a0d0e21e 1189
2b5ab1e7
TC
1190except that it's more efficient and concise, keeps track of the current
1191filename for error messages, searches the @INC libraries, and updates
1192C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1193variables. It also differs in that code evaluated with C<do FILENAME>
1194cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1195same, however, in that it does reparse the file every time you call it,
1196so you probably don't want to do this inside a loop.
a0d0e21e 1197
8e30cc93 1198If C<do> cannot read the file, it returns undef and sets C<$!> to the
2b5ab1e7 1199error. If C<do> can read the file but cannot compile it, it
8e30cc93
G
1200returns undef and sets an error message in C<$@>. If the file is
1201successfully compiled, C<do> returns the value of the last expression
1202evaluated.
1203
a0d0e21e 1204Note that inclusion of library modules is better done with the
19799a22 1205C<use> and C<require> operators, which also do automatic error checking
4633a7c4 1206and raise an exception if there's a problem.
a0d0e21e 1207
5a964f20
TC
1208You might like to use C<do> to read in a program configuration
1209file. Manual error checking can be done this way:
1210
b76cc8ba 1211 # read in config files: system first, then user
f86cebdf 1212 for $file ("/share/prog/defaults.rc",
b76cc8ba 1213 "$ENV{HOME}/.someprogrc")
2b5ab1e7 1214 {
5a964f20 1215 unless ($return = do $file) {
f86cebdf
GS
1216 warn "couldn't parse $file: $@" if $@;
1217 warn "couldn't do $file: $!" unless defined $return;
1218 warn "couldn't run $file" unless $return;
5a964f20
TC
1219 }
1220 }
1221
a0d0e21e
LW
1222=item dump LABEL
1223
1614b0e3
JD
1224=item dump
1225
19799a22
GS
1226This function causes an immediate core dump. See also the B<-u>
1227command-line switch in L<perlrun>, which does the same thing.
1228Primarily this is so that you can use the B<undump> program (not
1229supplied) to turn your core dump into an executable binary after
1230having initialized all your variables at the beginning of the
1231program. When the new binary is executed it will begin by executing
1232a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1233Think of it as a goto with an intervening core dump and reincarnation.
1234If C<LABEL> is omitted, restarts the program from the top.
1235
1236B<WARNING>: Any files opened at the time of the dump will I<not>
1237be open any more when the program is reincarnated, with possible
b76cc8ba 1238resulting confusion on the part of Perl.
19799a22
GS
1239
1240This function is now largely obsolete, partly because it's very
1241hard to convert a core file into an executable, and because the
1242real compiler backends for generating portable bytecode and compilable
ac206dc8
RGS
1243C code have superseded it. That's why you should now invoke it as
1244C<CORE::dump()>, if you don't want to be warned against a possible
1245typo.
19799a22
GS
1246
1247If you're looking to use L<dump> to speed up your program, consider
1248generating bytecode or native C code as described in L<perlcc>. If
1249you're just trying to accelerate a CGI script, consider using the
210b36aa 1250C<mod_perl> extension to B<Apache>, or the CPAN module, CGI::Fast.
19799a22 1251You might also consider autoloading or selfloading, which at least
b76cc8ba 1252make your program I<appear> to run faster.
5a964f20 1253
aa689395
PP
1254=item each HASH
1255
5a964f20 1256When called in list context, returns a 2-element list consisting of the
aa689395 1257key and value for the next element of a hash, so that you can iterate over
74fc8b5f 1258it. When called in scalar context, returns only the key for the next
e902a979 1259element in the hash.
2f9daede 1260
ab192400
GS
1261Entries are returned in an apparently random order. The actual random
1262order is subject to change in future versions of perl, but it is guaranteed
19799a22 1263to be in the same order as either the C<keys> or C<values> function
ab192400
GS
1264would produce on the same (unmodified) hash.
1265
1266When the hash is entirely read, a null array is returned in list context
19799a22
GS
1267(which when assigned produces a false (C<0>) value), and C<undef> in
1268scalar context. The next call to C<each> after that will start iterating
1269again. There is a single iterator for each hash, shared by all C<each>,
1270C<keys>, and C<values> function calls in the program; it can be reset by
2f9daede
TPG
1271reading all the elements from the hash, or by evaluating C<keys HASH> or
1272C<values HASH>. If you add or delete elements of a hash while you're
74fc8b5f
MJD
1273iterating over it, you may get entries skipped or duplicated, so
1274don't. Exception: It is always safe to delete the item most recently
1275returned by C<each()>, which means that the following code will work:
1276
1277 while (($key, $value) = each %hash) {
1278 print $key, "\n";
1279 delete $hash{$key}; # This is safe
1280 }
aa689395 1281
f86cebdf 1282The following prints out your environment like the printenv(1) program,
aa689395 1283only in a different order:
a0d0e21e
LW
1284
1285 while (($key,$value) = each %ENV) {
1286 print "$key=$value\n";
1287 }
1288
19799a22 1289See also C<keys>, C<values> and C<sort>.
a0d0e21e
LW
1290
1291=item eof FILEHANDLE
1292
4633a7c4
LW
1293=item eof ()
1294
a0d0e21e
LW
1295=item eof
1296
1297Returns 1 if the next read on FILEHANDLE will return end of file, or if
1298FILEHANDLE is not open. FILEHANDLE may be an expression whose value
5a964f20 1299gives the real filehandle. (Note that this function actually
19799a22 1300reads a character and then C<ungetc>s it, so isn't very useful in an
748a9306 1301interactive context.) Do not read from a terminal file (or call
19799a22 1302C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
748a9306
LW
1303as terminals may lose the end-of-file condition if you do.
1304
820475bd
GS
1305An C<eof> without an argument uses the last file read. Using C<eof()>
1306with empty parentheses is very different. It refers to the pseudo file
1307formed from the files listed on the command line and accessed via the
61eff3bc
JH
1308C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1309as a normal filehandle is, an C<eof()> before C<< <> >> has been
820475bd 1310used will cause C<@ARGV> to be examined to determine if input is
67408cae 1311available. Similarly, an C<eof()> after C<< <> >> has returned
efdd0218
RB
1312end-of-file will assume you are processing another C<@ARGV> list,
1313and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1314see L<perlop/"I/O Operators">.
820475bd 1315
61eff3bc 1316In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
820475bd
GS
1317detect the end of each file, C<eof()> will only detect the end of the
1318last file. Examples:
a0d0e21e 1319
748a9306
LW
1320 # reset line numbering on each input file
1321 while (<>) {
b76cc8ba 1322 next if /^\s*#/; # skip comments
748a9306 1323 print "$.\t$_";
5a964f20
TC
1324 } continue {
1325 close ARGV if eof; # Not eof()!
748a9306
LW
1326 }
1327
a0d0e21e
LW
1328 # insert dashes just before last line of last file
1329 while (<>) {
5a964f20 1330 if (eof()) { # check for end of current file
a0d0e21e 1331 print "--------------\n";
2b5ab1e7 1332 close(ARGV); # close or last; is needed if we
748a9306 1333 # are reading from the terminal
a0d0e21e
LW
1334 }
1335 print;
1336 }
1337
a0d0e21e 1338Practical hint: you almost never need to use C<eof> in Perl, because the
3ce0d271
GS
1339input operators typically return C<undef> when they run out of data, or if
1340there was an error.
a0d0e21e
LW
1341
1342=item eval EXPR
1343
1344=item eval BLOCK
1345
c7cc6f1c
GS
1346In the first form, the return value of EXPR is parsed and executed as if it
1347were a little Perl program. The value of the expression (which is itself
5a964f20 1348determined within scalar context) is first parsed, and if there weren't any
be3174d2
GS
1349errors, executed in the lexical context of the current Perl program, so
1350that any variable settings or subroutine and format definitions remain
1351afterwards. Note that the value is parsed every time the eval executes.
1352If EXPR is omitted, evaluates C<$_>. This form is typically used to
1353delay parsing and subsequent execution of the text of EXPR until run time.
c7cc6f1c
GS
1354
1355In the second form, the code within the BLOCK is parsed only once--at the
1356same time the code surrounding the eval itself was parsed--and executed
1357within the context of the current Perl program. This form is typically
1358used to trap exceptions more efficiently than the first (see below), while
1359also providing the benefit of checking the code within BLOCK at compile
1360time.
1361
1362The final semicolon, if any, may be omitted from the value of EXPR or within
1363the BLOCK.
1364
1365In both forms, the value returned is the value of the last expression
5a964f20 1366evaluated inside the mini-program; a return statement may be also used, just
c7cc6f1c 1367as with subroutines. The expression providing the return value is evaluated
5a964f20 1368in void, scalar, or list context, depending on the context of the eval itself.
c7cc6f1c 1369See L</wantarray> for more on how the evaluation context can be determined.
a0d0e21e 1370
19799a22
GS
1371If there is a syntax error or runtime error, or a C<die> statement is
1372executed, an undefined value is returned by C<eval>, and C<$@> is set to the
a0d0e21e 1373error message. If there was no error, C<$@> is guaranteed to be a null
19799a22 1374string. Beware that using C<eval> neither silences perl from printing
c7cc6f1c 1375warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
d9984052
A
1376To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1377turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1378See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
a0d0e21e 1379
19799a22
GS
1380Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1381determining whether a particular feature (such as C<socket> or C<symlink>)
a0d0e21e
LW
1382is implemented. It is also Perl's exception trapping mechanism, where
1383the die operator is used to raise exceptions.
1384
1385If the code to be executed doesn't vary, you may use the eval-BLOCK
1386form to trap run-time errors without incurring the penalty of
1387recompiling each time. The error, if any, is still returned in C<$@>.
1388Examples:
1389
54310121 1390 # make divide-by-zero nonfatal
a0d0e21e
LW
1391 eval { $answer = $a / $b; }; warn $@ if $@;
1392
1393 # same thing, but less efficient
1394 eval '$answer = $a / $b'; warn $@ if $@;
1395
1396 # a compile-time error
5a964f20 1397 eval { $answer = }; # WRONG
a0d0e21e
LW
1398
1399 # a run-time error
1400 eval '$answer ='; # sets $@
1401
2b5ab1e7
TC
1402Due to the current arguably broken state of C<__DIE__> hooks, when using
1403the C<eval{}> form as an exception trap in libraries, you may wish not
1404to trigger any C<__DIE__> hooks that user code may have installed.
1405You can use the C<local $SIG{__DIE__}> construct for this purpose,
1406as shown in this example:
774d564b
PP
1407
1408 # a very private exception trap for divide-by-zero
f86cebdf
GS
1409 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1410 warn $@ if $@;
774d564b
PP
1411
1412This is especially significant, given that C<__DIE__> hooks can call
19799a22 1413C<die> again, which has the effect of changing their error messages:
774d564b
PP
1414
1415 # __DIE__ hooks may modify error messages
1416 {
f86cebdf
GS
1417 local $SIG{'__DIE__'} =
1418 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
c7cc6f1c
GS
1419 eval { die "foo lives here" };
1420 print $@ if $@; # prints "bar lives here"
774d564b
PP
1421 }
1422
19799a22 1423Because this promotes action at a distance, this counterintuitive behavior
2b5ab1e7
TC
1424may be fixed in a future release.
1425
19799a22 1426With an C<eval>, you should be especially careful to remember what's
a0d0e21e
LW
1427being looked at when:
1428
1429 eval $x; # CASE 1
1430 eval "$x"; # CASE 2
1431
1432 eval '$x'; # CASE 3
1433 eval { $x }; # CASE 4
1434
5a964f20 1435 eval "\$$x++"; # CASE 5
a0d0e21e
LW
1436 $$x++; # CASE 6
1437
2f9daede 1438Cases 1 and 2 above behave identically: they run the code contained in
19799a22 1439the variable $x. (Although case 2 has misleading double quotes making
2f9daede 1440the reader wonder what else might be happening (nothing is).) Cases 3
7660c0ab 1441and 4 likewise behave in the same way: they run the code C<'$x'>, which
19799a22 1442does nothing but return the value of $x. (Case 4 is preferred for
2f9daede
TPG
1443purely visual reasons, but it also has the advantage of compiling at
1444compile-time instead of at run-time.) Case 5 is a place where
19799a22 1445normally you I<would> like to use double quotes, except that in this
2f9daede
TPG
1446particular situation, you can just use symbolic references instead, as
1447in case 6.
a0d0e21e 1448
4968c1e4 1449C<eval BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7 1450C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
4968c1e4 1451
d819b83a
DM
1452Note that as a very special case, an C<eval ''> executed within the C<DB>
1453package doesn't see the usual surrounding lexical scope, but rather the
1454scope of the first non-DB piece of code that called it. You don't normally
1455need to worry about this unless you are writing a Perl debugger.
1456
a0d0e21e
LW
1457=item exec LIST
1458
8bf3b016
GS
1459=item exec PROGRAM LIST
1460
19799a22
GS
1461The C<exec> function executes a system command I<and never returns>--
1462use C<system> instead of C<exec> if you want it to return. It fails and
1463returns false only if the command does not exist I<and> it is executed
fb73857a 1464directly instead of via your system's command shell (see below).
a0d0e21e 1465
19799a22
GS
1466Since it's a common mistake to use C<exec> instead of C<system>, Perl
1467warns you if there is a following statement which isn't C<die>, C<warn>,
1468or C<exit> (if C<-w> is set - but you always do that). If you
1469I<really> want to follow an C<exec> with some other statement, you
55d729e4
GS
1470can use one of these styles to avoid the warning:
1471
5a964f20
TC
1472 exec ('foo') or print STDERR "couldn't exec foo: $!";
1473 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
55d729e4 1474
5a964f20 1475If there is more than one argument in LIST, or if LIST is an array
f86cebdf 1476with more than one value, calls execvp(3) with the arguments in LIST.
5a964f20
TC
1477If there is only one scalar argument or an array with one element in it,
1478the argument is checked for shell metacharacters, and if there are any,
1479the entire argument is passed to the system's command shell for parsing
1480(this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1481If there are no shell metacharacters in the argument, it is split into
b76cc8ba 1482words and passed directly to C<execvp>, which is more efficient.
19799a22 1483Examples:
a0d0e21e 1484
19799a22
GS
1485 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1486 exec "sort $outfile | uniq";
a0d0e21e
LW
1487
1488If you don't really want to execute the first argument, but want to lie
1489to the program you are executing about its own name, you can specify
1490the program you actually want to run as an "indirect object" (without a
1491comma) in front of the LIST. (This always forces interpretation of the
54310121 1492LIST as a multivalued list, even if there is only a single scalar in
a0d0e21e
LW
1493the list.) Example:
1494
1495 $shell = '/bin/csh';
1496 exec $shell '-sh'; # pretend it's a login shell
1497
1498or, more directly,
1499
1500 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1501
bb32b41a
GS
1502When the arguments get executed via the system shell, results will
1503be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1504for details.
1505
19799a22
GS
1506Using an indirect object with C<exec> or C<system> is also more
1507secure. This usage (which also works fine with system()) forces
1508interpretation of the arguments as a multivalued list, even if the
1509list had just one argument. That way you're safe from the shell
1510expanding wildcards or splitting up words with whitespace in them.
5a964f20
TC
1511
1512 @args = ( "echo surprise" );
1513
2b5ab1e7 1514 exec @args; # subject to shell escapes
f86cebdf 1515 # if @args == 1
2b5ab1e7 1516 exec { $args[0] } @args; # safe even with one-arg list
5a964f20
TC
1517
1518The first version, the one without the indirect object, ran the I<echo>
1519program, passing it C<"surprise"> an argument. The second version
1520didn't--it tried to run a program literally called I<"echo surprise">,
1521didn't find it, and set C<$?> to a non-zero value indicating failure.
1522
0f897271
GS
1523Beginning with v5.6.0, Perl will attempt to flush all files opened for
1524output before the exec, but this may not be supported on some platforms
1525(see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1526in English) or call the C<autoflush()> method of C<IO::Handle> on any
1527open handles in order to avoid lost output.
1528
19799a22 1529Note that C<exec> will not call your C<END> blocks, nor will it call
7660c0ab
A
1530any C<DESTROY> methods in your objects.
1531
a0d0e21e
LW
1532=item exists EXPR
1533
01020589 1534Given an expression that specifies a hash element or array element,
8ea97a1e
GS
1535returns true if the specified element in the hash or array has ever
1536been initialized, even if the corresponding value is undefined. The
1537element is not autovivified if it doesn't exist.
a0d0e21e 1538
01020589
GS
1539 print "Exists\n" if exists $hash{$key};
1540 print "Defined\n" if defined $hash{$key};
1541 print "True\n" if $hash{$key};
1542
1543 print "Exists\n" if exists $array[$index];
1544 print "Defined\n" if defined $array[$index];
1545 print "True\n" if $array[$index];
a0d0e21e 1546
8ea97a1e 1547A hash or array element can be true only if it's defined, and defined if
a0d0e21e
LW
1548it exists, but the reverse doesn't necessarily hold true.
1549
afebc493
GS
1550Given an expression that specifies the name of a subroutine,
1551returns true if the specified subroutine has ever been declared, even
1552if it is undefined. Mentioning a subroutine name for exists or defined
847c7ebe
DD
1553does not count as declaring it. Note that a subroutine which does not
1554exist may still be callable: its package may have an C<AUTOLOAD>
1555method that makes it spring into existence the first time that it is
1556called -- see L<perlsub>.
afebc493
GS
1557
1558 print "Exists\n" if exists &subroutine;
1559 print "Defined\n" if defined &subroutine;
1560
a0d0e21e 1561Note that the EXPR can be arbitrarily complicated as long as the final
afebc493 1562operation is a hash or array key lookup or subroutine name:
a0d0e21e 1563
2b5ab1e7
TC
1564 if (exists $ref->{A}->{B}->{$key}) { }
1565 if (exists $hash{A}{B}{$key}) { }
1566
01020589
GS
1567 if (exists $ref->{A}->{B}->[$ix]) { }
1568 if (exists $hash{A}{B}[$ix]) { }
1569
afebc493
GS
1570 if (exists &{$ref->{A}{B}{$key}}) { }
1571
01020589
GS
1572Although the deepest nested array or hash will not spring into existence
1573just because its existence was tested, any intervening ones will.
61eff3bc 1574Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
01020589
GS
1575into existence due to the existence test for the $key element above.
1576This happens anywhere the arrow operator is used, including even:
5a964f20 1577
2b5ab1e7
TC
1578 undef $ref;
1579 if (exists $ref->{"Some key"}) { }
1580 print $ref; # prints HASH(0x80d3d5c)
1581
1582This surprising autovivification in what does not at first--or even
1583second--glance appear to be an lvalue context may be fixed in a future
5a964f20 1584release.
a0d0e21e 1585
afebc493
GS
1586Use of a subroutine call, rather than a subroutine name, as an argument
1587to exists() is an error.
1588
1589 exists &sub; # OK
1590 exists &sub(); # Error
1591
a0d0e21e
LW
1592=item exit EXPR
1593
2b5ab1e7 1594Evaluates EXPR and exits immediately with that value. Example:
a0d0e21e
LW
1595
1596 $ans = <STDIN>;
1597 exit 0 if $ans =~ /^[Xx]/;
1598
19799a22 1599See also C<die>. If EXPR is omitted, exits with C<0> status. The only
2b5ab1e7
TC
1600universally recognized values for EXPR are C<0> for success and C<1>
1601for error; other values are subject to interpretation depending on the
1602environment in which the Perl program is running. For example, exiting
160369 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1604the mailer to return the item undelivered, but that's not true everywhere.
a0d0e21e 1605
19799a22
GS
1606Don't use C<exit> to abort a subroutine if there's any chance that
1607someone might want to trap whatever error happened. Use C<die> instead,
1608which can be trapped by an C<eval>.
28757baa 1609
19799a22 1610The exit() function does not always exit immediately. It calls any
2b5ab1e7 1611defined C<END> routines first, but these C<END> routines may not
19799a22 1612themselves abort the exit. Likewise any object destructors that need to
2b5ab1e7
TC
1613be called are called before the real exit. If this is a problem, you
1614can call C<POSIX:_exit($status)> to avoid END and destructor processing.
87275199 1615See L<perlmod> for details.
5a964f20 1616
a0d0e21e
LW
1617=item exp EXPR
1618
54310121 1619=item exp
bbce6d69 1620
b76cc8ba 1621Returns I<e> (the natural logarithm base) to the power of EXPR.
a0d0e21e
LW
1622If EXPR is omitted, gives C<exp($_)>.
1623
1624=item fcntl FILEHANDLE,FUNCTION,SCALAR
1625
f86cebdf 1626Implements the fcntl(2) function. You'll probably have to say
a0d0e21e
LW
1627
1628 use Fcntl;
1629
0ade1984 1630first to get the correct constant definitions. Argument processing and
b76cc8ba 1631value return works just like C<ioctl> below.
a0d0e21e
LW
1632For example:
1633
1634 use Fcntl;
5a964f20
TC
1635 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1636 or die "can't fcntl F_GETFL: $!";
1637
19799a22 1638You don't have to check for C<defined> on the return from C<fnctl>.
951ba7fe
GS
1639Like C<ioctl>, it maps a C<0> return from the system call into
1640C<"0 but true"> in Perl. This string is true in boolean context and C<0>
2b5ab1e7
TC
1641in numeric context. It is also exempt from the normal B<-w> warnings
1642on improper numeric conversions.
5a964f20 1643
19799a22 1644Note that C<fcntl> will produce a fatal error if used on a machine that
2b5ab1e7
TC
1645doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1646manpage to learn what functions are available on your system.
a0d0e21e
LW
1647
1648=item fileno FILEHANDLE
1649
2b5ab1e7
TC
1650Returns the file descriptor for a filehandle, or undefined if the
1651filehandle is not open. This is mainly useful for constructing
19799a22 1652bitmaps for C<select> and low-level POSIX tty-handling operations.
2b5ab1e7
TC
1653If FILEHANDLE is an expression, the value is taken as an indirect
1654filehandle, generally its name.
5a964f20 1655
b76cc8ba 1656You can use this to find out whether two handles refer to the
5a964f20
TC
1657same underlying descriptor:
1658
1659 if (fileno(THIS) == fileno(THAT)) {
1660 print "THIS and THAT are dups\n";
b76cc8ba
NIS
1661 }
1662
1663(Filehandles connected to memory objects via new features of C<open> may
1664return undefined even though they are open.)
1665
a0d0e21e
LW
1666
1667=item flock FILEHANDLE,OPERATION
1668
19799a22
GS
1669Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1670for success, false on failure. Produces a fatal error if used on a
2b5ab1e7 1671machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
19799a22 1672C<flock> is Perl's portable file locking interface, although it locks
2b5ab1e7
TC
1673only entire files, not records.
1674
1675Two potentially non-obvious but traditional C<flock> semantics are
1676that it waits indefinitely until the lock is granted, and that its locks
1677B<merely advisory>. Such discretionary locks are more flexible, but offer
19799a22
GS
1678fewer guarantees. This means that files locked with C<flock> may be
1679modified by programs that do not also use C<flock>. See L<perlport>,
2b5ab1e7
TC
1680your port's specific documentation, or your system-specific local manpages
1681for details. It's best to assume traditional behavior if you're writing
1682portable programs. (But if you're not, you should as always feel perfectly
1683free to write for your own system's idiosyncrasies (sometimes called
1684"features"). Slavish adherence to portability concerns shouldn't get
1685in the way of your getting your job done.)
a3cb178b 1686
8ebc5c01
PP
1687OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1688LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
ea3105be 1689you can use the symbolic names if you import them from the Fcntl module,
68dc0745
PP
1690either individually, or as a group using the ':flock' tag. LOCK_SH
1691requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
ea3105be
GS
1692releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1693LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
68dc0745
PP
1694waiting for the lock (check the return status to see if you got it).
1695
2b5ab1e7
TC
1696To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1697before locking or unlocking it.
8ebc5c01 1698
f86cebdf 1699Note that the emulation built with lockf(3) doesn't provide shared
8ebc5c01 1700locks, and it requires that FILEHANDLE be open with write intent. These
2b5ab1e7 1701are the semantics that lockf(3) implements. Most if not all systems
f86cebdf 1702implement lockf(3) in terms of fcntl(2) locking, though, so the
8ebc5c01
PP
1703differing semantics shouldn't bite too many people.
1704
becacb53
TM
1705Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
1706be open with read intent to use LOCK_SH and requires that it be open
1707with write intent to use LOCK_EX.
1708
19799a22
GS
1709Note also that some versions of C<flock> cannot lock things over the
1710network; you would need to use the more system-specific C<fcntl> for
f86cebdf
GS
1711that. If you like you can force Perl to ignore your system's flock(2)
1712function, and so provide its own fcntl(2)-based emulation, by passing
8ebc5c01
PP
1713the switch C<-Ud_flock> to the F<Configure> program when you configure
1714perl.
4633a7c4
LW
1715
1716Here's a mailbox appender for BSD systems.
a0d0e21e 1717
7e1af8bc 1718 use Fcntl ':flock'; # import LOCK_* constants
a0d0e21e
LW
1719
1720 sub lock {
7e1af8bc 1721 flock(MBOX,LOCK_EX);
a0d0e21e
LW
1722 # and, in case someone appended
1723 # while we were waiting...
1724 seek(MBOX, 0, 2);
1725 }
1726
1727 sub unlock {
7e1af8bc 1728 flock(MBOX,LOCK_UN);
a0d0e21e
LW
1729 }
1730
1731 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1732 or die "Can't open mailbox: $!";
1733
1734 lock();
1735 print MBOX $msg,"\n\n";
1736 unlock();
1737
2b5ab1e7
TC
1738On systems that support a real flock(), locks are inherited across fork()
1739calls, whereas those that must resort to the more capricious fcntl()
1740function lose the locks, making it harder to write servers.
1741
cb1a09d0 1742See also L<DB_File> for other flock() examples.
a0d0e21e
LW
1743
1744=item fork
1745
2b5ab1e7
TC
1746Does a fork(2) system call to create a new process running the
1747same program at the same point. It returns the child pid to the
1748parent process, C<0> to the child process, or C<undef> if the fork is
1749unsuccessful. File descriptors (and sometimes locks on those descriptors)
1750are shared, while everything else is copied. On most systems supporting
1751fork(), great care has gone into making it extremely efficient (for
1752example, using copy-on-write technology on data pages), making it the
1753dominant paradigm for multitasking over the last few decades.
5a964f20 1754
0f897271
GS
1755Beginning with v5.6.0, Perl will attempt to flush all files opened for
1756output before forking the child process, but this may not be supported
1757on some platforms (see L<perlport>). To be safe, you may need to set
1758C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1759C<IO::Handle> on any open handles in order to avoid duplicate output.
a0d0e21e 1760
19799a22 1761If you C<fork> without ever waiting on your children, you will
2b5ab1e7
TC
1762accumulate zombies. On some systems, you can avoid this by setting
1763C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1764forking and reaping moribund children.
cb1a09d0 1765
28757baa
PP
1766Note that if your forked child inherits system file descriptors like
1767STDIN and STDOUT that are actually connected by a pipe or socket, even
2b5ab1e7 1768if you exit, then the remote server (such as, say, a CGI script or a
19799a22 1769backgrounded job launched from a remote shell) won't think you're done.
2b5ab1e7 1770You should reopen those to F</dev/null> if it's any issue.
28757baa 1771
cb1a09d0
AD
1772=item format
1773
19799a22 1774Declare a picture format for use by the C<write> function. For
cb1a09d0
AD
1775example:
1776
54310121 1777 format Something =
cb1a09d0
AD
1778 Test: @<<<<<<<< @||||| @>>>>>
1779 $str, $%, '$' . int($num)
1780 .
1781
1782 $str = "widget";
184e9718 1783 $num = $cost/$quantity;
cb1a09d0
AD
1784 $~ = 'Something';
1785 write;
1786
1787See L<perlform> for many details and examples.
1788
8903cb82 1789=item formline PICTURE,LIST
a0d0e21e 1790
5a964f20 1791This is an internal function used by C<format>s, though you may call it,
a0d0e21e
LW
1792too. It formats (see L<perlform>) a list of values according to the
1793contents of PICTURE, placing the output into the format output
7660c0ab 1794accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
19799a22 1795Eventually, when a C<write> is done, the contents of
a0d0e21e 1796C<$^A> are written to some filehandle, but you could also read C<$^A>
7660c0ab 1797yourself and then set C<$^A> back to C<"">. Note that a format typically
19799a22 1798does one C<formline> per line of form, but the C<formline> function itself
748a9306 1799doesn't care how many newlines are embedded in the PICTURE. This means
4633a7c4 1800that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
748a9306
LW
1801You may therefore need to use multiple formlines to implement a single
1802record format, just like the format compiler.
1803
19799a22 1804Be careful if you put double quotes around the picture, because an C<@>
748a9306 1805character may be taken to mean the beginning of an array name.
19799a22 1806C<formline> always returns true. See L<perlform> for other examples.
a0d0e21e
LW
1807
1808=item getc FILEHANDLE
1809
1810=item getc
1811
1812Returns the next character from the input file attached to FILEHANDLE,
b5fe5ca2
SR
1813or the undefined value at end of file, or if there was an error (in
1814the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
1815STDIN. This is not particularly efficient. However, it cannot be
1816used by itself to fetch single characters without waiting for the user
1817to hit enter. For that, try something more like:
4633a7c4
LW
1818
1819 if ($BSD_STYLE) {
1820 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1821 }
1822 else {
54310121 1823 system "stty", '-icanon', 'eol', "\001";
4633a7c4
LW
1824 }
1825
1826 $key = getc(STDIN);
1827
1828 if ($BSD_STYLE) {
1829 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1830 }
1831 else {
5f05dabc 1832 system "stty", 'icanon', 'eol', '^@'; # ASCII null
4633a7c4
LW
1833 }
1834 print "\n";
1835
54310121
PP
1836Determination of whether $BSD_STYLE should be set
1837is left as an exercise to the reader.
cb1a09d0 1838
19799a22 1839The C<POSIX::getattr> function can do this more portably on
2b5ab1e7
TC
1840systems purporting POSIX compliance. See also the C<Term::ReadKey>
1841module from your nearest CPAN site; details on CPAN can be found on
1842L<perlmodlib/CPAN>.
a0d0e21e
LW
1843
1844=item getlogin
1845
5a964f20
TC
1846Implements the C library function of the same name, which on most
1847systems returns the current login from F</etc/utmp>, if any. If null,
19799a22 1848use C<getpwuid>.
a0d0e21e 1849
f86702cc 1850 $login = getlogin || getpwuid($<) || "Kilroy";
a0d0e21e 1851
19799a22
GS
1852Do not consider C<getlogin> for authentication: it is not as
1853secure as C<getpwuid>.
4633a7c4 1854
a0d0e21e
LW
1855=item getpeername SOCKET
1856
1857Returns the packed sockaddr address of other end of the SOCKET connection.
1858
4633a7c4
LW
1859 use Socket;
1860 $hersockaddr = getpeername(SOCK);
19799a22 1861 ($port, $iaddr) = sockaddr_in($hersockaddr);
4633a7c4
LW
1862 $herhostname = gethostbyaddr($iaddr, AF_INET);
1863 $herstraddr = inet_ntoa($iaddr);
a0d0e21e
LW
1864
1865=item getpgrp PID
1866
47e29363 1867Returns the current process group for the specified PID. Use
7660c0ab 1868a PID of C<0> to get the current process group for the
4633a7c4 1869current process. Will raise an exception if used on a machine that
f86cebdf 1870doesn't implement getpgrp(2). If PID is omitted, returns process
19799a22 1871group of current process. Note that the POSIX version of C<getpgrp>
7660c0ab 1872does not accept a PID argument, so only C<PID==0> is truly portable.
a0d0e21e
LW
1873
1874=item getppid
1875
1876Returns the process id of the parent process.
1877
4d76a344
RGS
1878Note for Linux users: on Linux, the C functions C<getpid()> and
1879C<getppid()> return different values from different threads. In order to
1880be portable, this behavior is not reflected by the perl-level function
1881C<getppid()>, that returns a consistent value across threads. If you want
e3256f86
RGS
1882to call the underlying C<getppid()>, you may use the CPAN module
1883C<Linux::Pid>.
4d76a344 1884
a0d0e21e
LW
1885=item getpriority WHICH,WHO
1886
4633a7c4
LW
1887Returns the current priority for a process, a process group, or a user.
1888(See L<getpriority(2)>.) Will raise a fatal exception if used on a
f86cebdf 1889machine that doesn't implement getpriority(2).
a0d0e21e
LW
1890
1891=item getpwnam NAME
1892
1893=item getgrnam NAME
1894
1895=item gethostbyname NAME
1896
1897=item getnetbyname NAME
1898
1899=item getprotobyname NAME
1900
1901=item getpwuid UID
1902
1903=item getgrgid GID
1904
1905=item getservbyname NAME,PROTO
1906
1907=item gethostbyaddr ADDR,ADDRTYPE
1908
1909=item getnetbyaddr ADDR,ADDRTYPE
1910
1911=item getprotobynumber NUMBER
1912
1913=item getservbyport PORT,PROTO
1914
1915=item getpwent
1916
1917=item getgrent
1918
1919=item gethostent
1920
1921=item getnetent
1922
1923=item getprotoent
1924
1925=item getservent
1926
1927=item setpwent
1928
1929=item setgrent
1930
1931=item sethostent STAYOPEN
1932
1933=item setnetent STAYOPEN
1934
1935=item setprotoent STAYOPEN
1936
1937=item setservent STAYOPEN
1938
1939=item endpwent
1940
1941=item endgrent
1942
1943=item endhostent
1944
1945=item endnetent
1946
1947=item endprotoent
1948
1949=item endservent
1950
1951These routines perform the same functions as their counterparts in the
5a964f20 1952system library. In list context, the return values from the
a0d0e21e
LW
1953various get routines are as follows:
1954
1955 ($name,$passwd,$uid,$gid,
6ee623d5 1956 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
a0d0e21e
LW
1957 ($name,$passwd,$gid,$members) = getgr*
1958 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1959 ($name,$aliases,$addrtype,$net) = getnet*
1960 ($name,$aliases,$proto) = getproto*
1961 ($name,$aliases,$port,$proto) = getserv*
1962
1963(If the entry doesn't exist you get a null list.)
1964
4602f195
JH
1965The exact meaning of the $gcos field varies but it usually contains
1966the real name of the user (as opposed to the login name) and other
1967information pertaining to the user. Beware, however, that in many
1968system users are able to change this information and therefore it
106325ad 1969cannot be trusted and therefore the $gcos is tainted (see
2959b6e3
JH
1970L<perlsec>). The $passwd and $shell, user's encrypted password and
1971login shell, are also tainted, because of the same reason.
4602f195 1972
5a964f20 1973In scalar context, you get the name, unless the function was a
a0d0e21e
LW
1974lookup by name, in which case you get the other thing, whatever it is.
1975(If the entry doesn't exist you get the undefined value.) For example:
1976
5a964f20
TC
1977 $uid = getpwnam($name);
1978 $name = getpwuid($num);
1979 $name = getpwent();
1980 $gid = getgrnam($name);
08a33e13 1981 $name = getgrgid($num);
5a964f20
TC
1982 $name = getgrent();
1983 #etc.
a0d0e21e 1984
4602f195
JH
1985In I<getpw*()> the fields $quota, $comment, and $expire are special
1986cases in the sense that in many systems they are unsupported. If the
1987$quota is unsupported, it is an empty scalar. If it is supported, it
1988usually encodes the disk quota. If the $comment field is unsupported,
1989it is an empty scalar. If it is supported it usually encodes some
1990administrative comment about the user. In some systems the $quota
1991field may be $change or $age, fields that have to do with password
1992aging. In some systems the $comment field may be $class. The $expire
1993field, if present, encodes the expiration period of the account or the
1994password. For the availability and the exact meaning of these fields
1995in your system, please consult your getpwnam(3) documentation and your
1996F<pwd.h> file. You can also find out from within Perl what your
1997$quota and $comment fields mean and whether you have the $expire field
1998by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
1999C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
2000files are only supported if your vendor has implemented them in the
2001intuitive fashion that calling the regular C library routines gets the
5d3a0a3b
GS
2002shadow versions if you're running under privilege or if there exists
2003the shadow(3) functions as found in System V ( this includes Solaris
2004and Linux.) Those systems which implement a proprietary shadow password
2005facility are unlikely to be supported.
6ee623d5 2006
19799a22 2007The $members value returned by I<getgr*()> is a space separated list of
a0d0e21e
LW
2008the login names of the members of the group.
2009
2010For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2011C, it will be returned to you via C<$?> if the function call fails. The
7660c0ab 2012C<@addrs> value returned by a successful call is a list of the raw
a0d0e21e
LW
2013addresses returned by the corresponding system library call. In the
2014Internet domain, each address is four bytes long and you can unpack it
2015by saying something like:
2016
2017 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
2018
2b5ab1e7
TC
2019The Socket library makes this slightly easier:
2020
2021 use Socket;
2022 $iaddr = inet_aton("127.1"); # or whatever address
2023 $name = gethostbyaddr($iaddr, AF_INET);
2024
2025 # or going the other way
19799a22 2026 $straddr = inet_ntoa($iaddr);
2b5ab1e7 2027
19799a22
GS
2028If you get tired of remembering which element of the return list
2029contains which return value, by-name interfaces are provided
2030in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2031C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2032and C<User::grent>. These override the normal built-ins, supplying
2033versions that return objects with the appropriate names
2034for each field. For example:
5a964f20
TC
2035
2036 use File::stat;
2037 use User::pwent;
2038 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2039
b76cc8ba
NIS
2040Even though it looks like they're the same method calls (uid),
2041they aren't, because a C<File::stat> object is different from
19799a22 2042a C<User::pwent> object.
5a964f20 2043
a0d0e21e
LW
2044=item getsockname SOCKET
2045
19799a22
GS
2046Returns the packed sockaddr address of this end of the SOCKET connection,
2047in case you don't know the address because you have several different
2048IPs that the connection might have come in on.
a0d0e21e 2049
4633a7c4
LW
2050 use Socket;
2051 $mysockaddr = getsockname(SOCK);
19799a22 2052 ($port, $myaddr) = sockaddr_in($mysockaddr);
b76cc8ba 2053 printf "Connect to %s [%s]\n",
19799a22
GS
2054 scalar gethostbyaddr($myaddr, AF_INET),
2055 inet_ntoa($myaddr);
a0d0e21e
LW
2056
2057=item getsockopt SOCKET,LEVEL,OPTNAME
2058
5a964f20 2059Returns the socket option requested, or undef if there is an error.
a0d0e21e
LW
2060
2061=item glob EXPR
2062
0a753a76
PP
2063=item glob
2064
d9a9d457
JL
2065In list context, returns a (possibly empty) list of filename expansions on
2066the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
2067scalar context, glob iterates through such filename expansions, returning
2068undef when the list is exhausted. This is the internal function
2069implementing the C<< <*.c> >> operator, but you can use it directly. If
2070EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2071more detail in L<perlop/"I/O Operators">.
a0d0e21e 2072
3a4b19e4
GS
2073Beginning with v5.6.0, this operator is implemented using the standard
2074C<File::Glob> extension. See L<File::Glob> for details.
2075
a0d0e21e
LW
2076=item gmtime EXPR
2077
d1be9408 2078Converts a time as returned by the time function to an 8-element list
54310121 2079with the time localized for the standard Greenwich time zone.
4633a7c4 2080Typically used as follows:
a0d0e21e 2081
b76cc8ba 2082 # 0 1 2 3 4 5 6 7
48a26b3a 2083 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday) =
a0d0e21e
LW
2084 gmtime(time);
2085
48a26b3a
GS
2086All list elements are numeric, and come straight out of the C `struct
2087tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2088specified time. $mday is the day of the month, and $mon is the month
2089itself, in the range C<0..11> with 0 indicating January and 11
2090indicating December. $year is the number of years since 1900. That
2091is, $year is C<123> in year 2023. $wday is the day of the week, with
20920 indicating Sunday and 3 indicating Wednesday. $yday is the day of
b76cc8ba 2093the year, in the range C<0..364> (or C<0..365> in leap years.)
48a26b3a
GS
2094
2095Note that the $year element is I<not> simply the last two digits of
2096the year. If you assume it is, then you create non-Y2K-compliant
2097programs--and you wouldn't want to do that, would you?
2f9daede 2098
abd75f24
GS
2099The proper way to get a complete 4-digit year is simply:
2100
2101 $year += 1900;
2102
2103And to get the last two digits of the year (e.g., '01' in 2001) do:
2104
2105 $year = sprintf("%02d", $year % 100);
2106
48a26b3a 2107If EXPR is omitted, C<gmtime()> uses the current time (C<gmtime(time)>).
a0d0e21e 2108
48a26b3a 2109In scalar context, C<gmtime()> returns the ctime(3) value:
0a753a76
PP
2110
2111 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
2112
19799a22 2113Also see the C<timegm> function provided by the C<Time::Local> module,
f86cebdf 2114and the strftime(3) function available via the POSIX module.
7660c0ab 2115
2b5ab1e7
TC
2116This scalar value is B<not> locale dependent (see L<perllocale>), but
2117is instead a Perl builtin. Also see the C<Time::Local> module, and the
2118strftime(3) and mktime(3) functions available via the POSIX module. To
7660c0ab
A
2119get somewhat similar but locale dependent date strings, set up your
2120locale environment variables appropriately (please see L<perllocale>)
2121and try for example:
2122
2123 use POSIX qw(strftime);
2b5ab1e7 2124 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
7660c0ab 2125
2b5ab1e7
TC
2126Note that the C<%a> and C<%b> escapes, which represent the short forms
2127of the day of the week and the month of the year, may not necessarily
2128be three characters wide in all locales.
0a753a76 2129
a0d0e21e
LW
2130=item goto LABEL
2131
748a9306
LW
2132=item goto EXPR
2133
a0d0e21e
LW
2134=item goto &NAME
2135
7660c0ab 2136The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
a0d0e21e 2137execution there. It may not be used to go into any construct that
7660c0ab 2138requires initialization, such as a subroutine or a C<foreach> loop. It
0a753a76 2139also can't be used to go into a construct that is optimized away,
19799a22 2140or to get out of a block or subroutine given to C<sort>.
0a753a76 2141It can be used to go almost anywhere else within the dynamic scope,
a0d0e21e 2142including out of subroutines, but it's usually better to use some other
19799a22 2143construct such as C<last> or C<die>. The author of Perl has never felt the
7660c0ab 2144need to use this form of C<goto> (in Perl, that is--C is another matter).
1b6921cb
BT
2145(The difference being that C does not offer named loops combined with
2146loop control. Perl does, and this replaces most structured uses of C<goto>
2147in other languages.)
a0d0e21e 2148
7660c0ab
A
2149The C<goto-EXPR> form expects a label name, whose scope will be resolved
2150dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
748a9306
LW
2151necessarily recommended if you're optimizing for maintainability:
2152
2153 goto ("FOO", "BAR", "GLARCH")[$i];
2154
1b6921cb
BT
2155The C<goto-&NAME> form is quite different from the other forms of
2156C<goto>. In fact, it isn't a goto in the normal sense at all, and
2157doesn't have the stigma associated with other gotos. Instead, it
2158exits the current subroutine (losing any changes set by local()) and
2159immediately calls in its place the named subroutine using the current
2160value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2161load another subroutine and then pretend that the other subroutine had
2162been called in the first place (except that any modifications to C<@_>
6cb9131c
GS
2163in the current subroutine are propagated to the other subroutine.)
2164After the C<goto>, not even C<caller> will be able to tell that this
2165routine was called first.
2166
2167NAME needn't be the name of a subroutine; it can be a scalar variable
2168containing a code reference, or a block which evaluates to a code
2169reference.
a0d0e21e
LW
2170
2171=item grep BLOCK LIST
2172
2173=item grep EXPR,LIST
2174
2b5ab1e7
TC
2175This is similar in spirit to, but not the same as, grep(1) and its
2176relatives. In particular, it is not limited to using regular expressions.
2f9daede 2177
a0d0e21e 2178Evaluates the BLOCK or EXPR for each element of LIST (locally setting
7660c0ab 2179C<$_> to each element) and returns the list value consisting of those
19799a22
GS
2180elements for which the expression evaluated to true. In scalar
2181context, returns the number of times the expression was true.
a0d0e21e
LW
2182
2183 @foo = grep(!/^#/, @bar); # weed out comments
2184
2185or equivalently,
2186
2187 @foo = grep {!/^#/} @bar; # weed out comments
2188
be3174d2
GS
2189Note that C<$_> is an alias to the list value, so it can be used to
2190modify the elements of the LIST. While this is useful and supported,
2191it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
2192Similarly, grep returns aliases into the original list, much as a for
2193loop's index variable aliases the list elements. That is, modifying an
19799a22
GS
2194element of a list returned by grep (for example, in a C<foreach>, C<map>
2195or another C<grep>) actually modifies the element in the original list.
2b5ab1e7 2196This is usually something to be avoided when writing clear code.
a0d0e21e 2197
19799a22 2198See also L</map> for a list composed of the results of the BLOCK or EXPR.
38325410 2199
a0d0e21e
LW
2200=item hex EXPR
2201
54310121 2202=item hex
bbce6d69 2203
2b5ab1e7
TC
2204Interprets EXPR as a hex string and returns the corresponding value.
2205(To convert strings that might start with either 0, 0x, or 0b, see
2206L</oct>.) If EXPR is omitted, uses C<$_>.
2f9daede
TPG
2207
2208 print hex '0xAf'; # prints '175'
2209 print hex 'aF'; # same
a0d0e21e 2210
19799a22 2211Hex strings may only represent integers. Strings that would cause
53305cf1
NC
2212integer overflow trigger a warning. Leading whitespace is not stripped,
2213unlike oct().
19799a22 2214
a0d0e21e
LW
2215=item import
2216
19799a22 2217There is no builtin C<import> function. It is just an ordinary
4633a7c4 2218method (subroutine) defined (or inherited) by modules that wish to export
19799a22 2219names to another module. The C<use> function calls the C<import> method
cea6626f 2220for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
a0d0e21e
LW
2221
2222=item index STR,SUBSTR,POSITION
2223
2224=item index STR,SUBSTR
2225
2b5ab1e7
TC
2226The index function searches for one string within another, but without
2227the wildcard-like behavior of a full regular-expression pattern match.
2228It returns the position of the first occurrence of SUBSTR in STR at
2229or after POSITION. If POSITION is omitted, starts searching from the
2230beginning of the string. The return value is based at C<0> (or whatever
2231you've set the C<$[> variable to--but don't do that). If the substring
2232is not found, returns one less than the base, ordinarily C<-1>.
a0d0e21e
LW
2233
2234=item int EXPR
2235
54310121 2236=item int
bbce6d69 2237
7660c0ab 2238Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2b5ab1e7
TC
2239You should not use this function for rounding: one because it truncates
2240towards C<0>, and two because machine representations of floating point
2241numbers can sometimes produce counterintuitive results. For example,
2242C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2243because it's really more like -268.99999999999994315658 instead. Usually,
19799a22 2244the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2b5ab1e7 2245functions will serve you better than will int().
a0d0e21e
LW
2246
2247=item ioctl FILEHANDLE,FUNCTION,SCALAR
2248
2b5ab1e7 2249Implements the ioctl(2) function. You'll probably first have to say
a0d0e21e 2250
4633a7c4 2251 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
a0d0e21e 2252
2b5ab1e7 2253to get the correct function definitions. If F<ioctl.ph> doesn't
a0d0e21e 2254exist or doesn't have the correct definitions you'll have to roll your
61eff3bc 2255own, based on your C header files such as F<< <sys/ioctl.h> >>.
5a964f20 2256(There is a Perl script called B<h2ph> that comes with the Perl kit that
54310121 2257may help you in this, but it's nontrivial.) SCALAR will be read and/or
4633a7c4 2258written depending on the FUNCTION--a pointer to the string value of SCALAR
19799a22 2259will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
4633a7c4
LW
2260has no string value but does have a numeric value, that value will be
2261passed rather than a pointer to the string value. To guarantee this to be
19799a22
GS
2262true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2263functions may be needed to manipulate the values of structures used by
b76cc8ba 2264C<ioctl>.
a0d0e21e 2265
19799a22 2266The return value of C<ioctl> (and C<fcntl>) is as follows:
a0d0e21e
LW
2267
2268 if OS returns: then Perl returns:
2269 -1 undefined value
2270 0 string "0 but true"
2271 anything else that number
2272
19799a22 2273Thus Perl returns true on success and false on failure, yet you can
a0d0e21e
LW
2274still easily determine the actual value returned by the operating
2275system:
2276
2b5ab1e7 2277 $retval = ioctl(...) || -1;
a0d0e21e
LW
2278 printf "System returned %d\n", $retval;
2279
c2611fb3 2280The special string "C<0> but true" is exempt from B<-w> complaints
5a964f20
TC
2281about improper numeric conversions.
2282
19799a22
GS
2283Here's an example of setting a filehandle named C<REMOTE> to be
2284non-blocking at the system level. You'll have to negotiate C<$|>
2285on your own, though.
2286
2287 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
2288
2289 $flags = fcntl(REMOTE, F_GETFL, 0)
2290 or die "Can't get flags for the socket: $!\n";
2291
2292 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
2293 or die "Can't set flags for the socket: $!\n";
2294
a0d0e21e
LW
2295=item join EXPR,LIST
2296
2b5ab1e7
TC
2297Joins the separate strings of LIST into a single string with fields
2298separated by the value of EXPR, and returns that new string. Example:
a0d0e21e 2299
2b5ab1e7 2300 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
a0d0e21e 2301
eb6e2d6f
GS
2302Beware that unlike C<split>, C<join> doesn't take a pattern as its
2303first argument. Compare L</split>.
a0d0e21e 2304
aa689395
PP
2305=item keys HASH
2306
19799a22 2307Returns a list consisting of all the keys of the named hash. (In
1d2dff63 2308scalar context, returns the number of keys.) The keys are returned in
ab192400
GS
2309an apparently random order. The actual random order is subject to
2310change in future versions of perl, but it is guaranteed to be the same
19799a22 2311order as either the C<values> or C<each> function produces (given
ab192400
GS
2312that the hash has not been modified). As a side effect, it resets
2313HASH's iterator.
a0d0e21e 2314
aa689395 2315Here is yet another way to print your environment:
a0d0e21e
LW
2316
2317 @keys = keys %ENV;
2318 @values = values %ENV;
b76cc8ba 2319 while (@keys) {
a0d0e21e
LW
2320 print pop(@keys), '=', pop(@values), "\n";
2321 }
2322
2323or how about sorted by key:
2324
2325 foreach $key (sort(keys %ENV)) {
2326 print $key, '=', $ENV{$key}, "\n";
2327 }
2328
8ea1e5d4
GS
2329The returned values are copies of the original keys in the hash, so
2330modifying them will not affect the original hash. Compare L</values>.
2331
19799a22 2332To sort a hash by value, you'll need to use a C<sort> function.
aa689395 2333Here's a descending numeric sort of a hash by its values:
4633a7c4 2334
5a964f20 2335 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
4633a7c4
LW
2336 printf "%4d %s\n", $hash{$key}, $key;
2337 }
2338
19799a22 2339As an lvalue C<keys> allows you to increase the number of hash buckets
aa689395
PP
2340allocated for the given hash. This can gain you a measure of efficiency if
2341you know the hash is going to get big. (This is similar to pre-extending
2342an array by assigning a larger number to $#array.) If you say
55497cff
PP
2343
2344 keys %hash = 200;
2345
ab192400
GS
2346then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2347in fact, since it rounds up to the next power of two. These
55497cff
PP
2348buckets will be retained even if you do C<%hash = ()>, use C<undef
2349%hash> if you want to free the storage while C<%hash> is still in scope.
2350You can't shrink the number of buckets allocated for the hash using
19799a22 2351C<keys> in this way (but you needn't worry about doing this by accident,
55497cff
PP
2352as trying has no effect).
2353
19799a22 2354See also C<each>, C<values> and C<sort>.
ab192400 2355
b350dd2f 2356=item kill SIGNAL, LIST
a0d0e21e 2357
b350dd2f 2358Sends a signal to a list of processes. Returns the number of
517db077
GS
2359processes successfully signaled (which is not necessarily the
2360same as the number actually killed).
a0d0e21e
LW
2361
2362 $cnt = kill 1, $child1, $child2;
2363 kill 9, @goners;
2364
b350dd2f
GS
2365If SIGNAL is zero, no signal is sent to the process. This is a
2366useful way to check that the process is alive and hasn't changed
2367its UID. See L<perlport> for notes on the portability of this
2368construct.
2369
2370Unlike in the shell, if SIGNAL is negative, it kills
4633a7c4
LW
2371process groups instead of processes. (On System V, a negative I<PROCESS>
2372number will also kill process groups, but that's not portable.) That
2373means you usually want to use positive not negative signals. You may also
da0045b7 2374use a signal name in quotes. See L<perlipc/"Signals"> for details.
a0d0e21e
LW
2375
2376=item last LABEL
2377
2378=item last
2379
2380The C<last> command is like the C<break> statement in C (as used in
2381loops); it immediately exits the loop in question. If the LABEL is
2382omitted, the command refers to the innermost enclosing loop. The
2383C<continue> block, if any, is not executed:
2384
4633a7c4
LW
2385 LINE: while (<STDIN>) {
2386 last LINE if /^$/; # exit when done with header
5a964f20 2387 #...
a0d0e21e
LW
2388 }
2389
4968c1e4 2390C<last> cannot be used to exit a block which returns a value such as
2b5ab1e7
TC
2391C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2392a grep() or map() operation.
4968c1e4 2393
6c1372ed
GS
2394Note that a block by itself is semantically identical to a loop
2395that executes once. Thus C<last> can be used to effect an early
2396exit out of such a block.
2397
98293880
JH
2398See also L</continue> for an illustration of how C<last>, C<next>, and
2399C<redo> work.
1d2dff63 2400
a0d0e21e
LW
2401=item lc EXPR
2402
54310121 2403=item lc
bbce6d69 2404
d1be9408 2405Returns a lowercased version of EXPR. This is the internal function
ad0029c4
JH
2406implementing the C<\L> escape in double-quoted strings. Respects
2407current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
983ffd37 2408and L<perlunicode> for more details about locale and Unicode support.
a0d0e21e 2409
7660c0ab 2410If EXPR is omitted, uses C<$_>.
bbce6d69 2411
a0d0e21e
LW
2412=item lcfirst EXPR
2413
54310121 2414=item lcfirst
bbce6d69 2415
ad0029c4
JH
2416Returns the value of EXPR with the first character lowercased. This
2417is the internal function implementing the C<\l> escape in
2418double-quoted strings. Respects current LC_CTYPE locale if C<use
983ffd37
JH
2419locale> in force. See L<perllocale> and L<perlunicode> for more
2420details about locale and Unicode support.
a0d0e21e 2421
7660c0ab 2422If EXPR is omitted, uses C<$_>.
bbce6d69 2423
a0d0e21e
LW
2424=item length EXPR
2425
54310121 2426=item length
bbce6d69 2427
a0ed51b3 2428Returns the length in characters of the value of EXPR. If EXPR is
b76cc8ba 2429omitted, returns length of C<$_>. Note that this cannot be used on
2b5ab1e7
TC
2430an entire array or hash to find out how many elements these have.
2431For that, use C<scalar @array> and C<scalar keys %hash> respectively.
a0d0e21e
LW
2432
2433=item link OLDFILE,NEWFILE
2434
19799a22 2435Creates a new filename linked to the old filename. Returns true for
b76cc8ba 2436success, false otherwise.
a0d0e21e
LW
2437
2438=item listen SOCKET,QUEUESIZE
2439
19799a22 2440Does the same thing that the listen system call does. Returns true if
b76cc8ba 2441it succeeded, false otherwise. See the example in
cea6626f 2442L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
2443
2444=item local EXPR
2445
19799a22 2446You really probably want to be using C<my> instead, because C<local> isn't
b76cc8ba 2447what most people think of as "local". See
13a2d996 2448L<perlsub/"Private Variables via my()"> for details.
2b5ab1e7 2449
5a964f20
TC
2450A local modifies the listed variables to be local to the enclosing
2451block, file, or eval. If more than one value is listed, the list must
2452be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2453for details, including issues with tied arrays and hashes.
a0d0e21e 2454
a0d0e21e
LW
2455=item localtime EXPR
2456
19799a22 2457Converts a time as returned by the time function to a 9-element list
5f05dabc 2458with the time analyzed for the local time zone. Typically used as
a0d0e21e
LW
2459follows:
2460
54310121 2461 # 0 1 2 3 4 5 6 7 8
a0d0e21e
LW
2462 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2463 localtime(time);
2464
48a26b3a
GS
2465All list elements are numeric, and come straight out of the C `struct
2466tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2467specified time. $mday is the day of the month, and $mon is the month
2468itself, in the range C<0..11> with 0 indicating January and 11
2469indicating December. $year is the number of years since 1900. That
2470is, $year is C<123> in year 2023. $wday is the day of the week, with
24710 indicating Sunday and 3 indicating Wednesday. $yday is the day of
874b1813 2472the year, in the range C<0..364> (or C<0..365> in leap years.) $isdst
48a26b3a
GS
2473is true if the specified time occurs during daylight savings time,
2474false otherwise.
2475
2476Note that the $year element is I<not> simply the last two digits of
2477the year. If you assume it is, then you create non-Y2K-compliant
2478programs--and you wouldn't want to do that, would you?
54310121 2479
abd75f24
GS
2480The proper way to get a complete 4-digit year is simply:
2481
2482 $year += 1900;
2483
2484And to get the last two digits of the year (e.g., '01' in 2001) do:
2485
2486 $year = sprintf("%02d", $year % 100);
2487
48a26b3a 2488If EXPR is omitted, C<localtime()> uses the current time (C<localtime(time)>).
a0d0e21e 2489
48a26b3a 2490In scalar context, C<localtime()> returns the ctime(3) value:
a0d0e21e 2491
5f05dabc 2492 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
a0d0e21e 2493
a3cb178b 2494This scalar value is B<not> locale dependent, see L<perllocale>, but
68f8bed4
JH
2495instead a Perl builtin. Also see the C<Time::Local> module
2496(to convert the second, minutes, hours, ... back to seconds since the
2497stroke of midnight the 1st of January 1970, the value returned by
ca6e1c26 2498time()), and the strftime(3) and mktime(3) functions available via the
68f8bed4
JH
2499POSIX module. To get somewhat similar but locale dependent date
2500strings, set up your locale environment variables appropriately
2501(please see L<perllocale>) and try for example:
a3cb178b 2502
5a964f20 2503 use POSIX qw(strftime);
2b5ab1e7 2504 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
a3cb178b
GS
2505
2506Note that the C<%a> and C<%b>, the short forms of the day of the week
2507and the month of the year, may not necessarily be three characters wide.
a0d0e21e 2508
07698885 2509=item lock THING
19799a22 2510
01e6739c 2511This function places an advisory lock on a shared variable, or referenced
03730085 2512object contained in I<THING> until the lock goes out of scope.
a6d5524e 2513
f3a23afb 2514lock() is a "weak keyword" : this means that if you've defined a function
67408cae 2515by this name (before any calls to it), that function will be called
03730085
AB
2516instead. (However, if you've said C<use threads>, lock() is always a
2517keyword.) See L<threads>.
19799a22 2518
a0d0e21e
LW
2519=item log EXPR
2520
54310121 2521=item log
bbce6d69 2522
2b5ab1e7
TC
2523Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2524returns log of C<$_>. To get the log of another base, use basic algebra:
19799a22 2525The base-N log of a number is equal to the natural log of that number
2b5ab1e7
TC
2526divided by the natural log of N. For example:
2527
2528 sub log10 {
2529 my $n = shift;
2530 return log($n)/log(10);
b76cc8ba 2531 }
2b5ab1e7
TC
2532
2533See also L</exp> for the inverse operation.
a0d0e21e 2534
a0d0e21e
LW
2535=item lstat EXPR
2536
54310121 2537=item lstat
bbce6d69 2538
19799a22 2539Does the same thing as the C<stat> function (including setting the
5a964f20
TC
2540special C<_> filehandle) but stats a symbolic link instead of the file
2541the symbolic link points to. If symbolic links are unimplemented on
c837d5b4
DP
2542your system, a normal C<stat> is done. For much more detailed
2543information, please see the documentation for C<stat>.
a0d0e21e 2544
7660c0ab 2545If EXPR is omitted, stats C<$_>.
bbce6d69 2546
a0d0e21e
LW
2547=item m//
2548
2549The match operator. See L<perlop>.
2550
2551=item map BLOCK LIST
2552
2553=item map EXPR,LIST
2554
19799a22
GS
2555Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2556C<$_> to each element) and returns the list value composed of the
2557results of each such evaluation. In scalar context, returns the
2558total number of elements so generated. Evaluates BLOCK or EXPR in
2559list context, so each element of LIST may produce zero, one, or
2560more elements in the returned value.
dd99ebda 2561
a0d0e21e
LW
2562 @chars = map(chr, @nums);
2563
2564translates a list of numbers to the corresponding characters. And
2565
4633a7c4 2566 %hash = map { getkey($_) => $_ } @array;
a0d0e21e
LW
2567
2568is just a funny way to write
2569
2570 %hash = ();
2571 foreach $_ (@array) {
4633a7c4 2572 $hash{getkey($_)} = $_;
a0d0e21e
LW
2573 }
2574
be3174d2
GS
2575Note that C<$_> is an alias to the list value, so it can be used to
2576modify the elements of the LIST. While this is useful and supported,
2577it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
2578Using a regular C<foreach> loop for this purpose would be clearer in
2579most cases. See also L</grep> for an array composed of those items of
2580the original list for which the BLOCK or EXPR evaluates to true.
fb73857a 2581
205fdb4d
NC
2582C<{> starts both hash references and blocks, so C<map { ...> could be either
2583the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look
2584ahead for the closing C<}> it has to take a guess at which its dealing with
2585based what it finds just after the C<{>. Usually it gets it right, but if it
2586doesn't it won't realize something is wrong until it gets to the C<}> and
2587encounters the missing (or unexpected) comma. The syntax error will be
2588reported close to the C<}> but you'll need to change something near the C<{>
2589such as using a unary C<+> to give perl some help:
2590
2591 %hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong
2592 %hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right
2593 %hash = map { ("\L$_", 1) } @array # this also works
2594 %hash = map { lc($_), 1 } @array # as does this.
2595 %hash = map +( lc($_), 1 ), @array # this is EXPR and works!
cea6626f 2596
205fdb4d
NC
2597 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
2598
2599or to force an anon hash constructor use C<+{>
2600
2601 @hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end
2602
2603and you get list of anonymous hashes each with only 1 entry.
2604
19799a22 2605=item mkdir FILENAME,MASK
a0d0e21e 2606
5a211162
GS
2607=item mkdir FILENAME
2608
0591cd52 2609Creates the directory specified by FILENAME, with permissions
19799a22
GS
2610specified by MASK (as modified by C<umask>). If it succeeds it
2611returns true, otherwise it returns false and sets C<$!> (errno).
5a211162 2612If omitted, MASK defaults to 0777.
0591cd52 2613
19799a22 2614In general, it is better to create directories with permissive MASK,
0591cd52 2615and let the user modify that with their C<umask>, than it is to supply
19799a22 2616a restrictive MASK and give the user no way to be more permissive.
0591cd52
NT
2617The exceptions to this rule are when the file or directory should be
2618kept private (mail files, for instance). The perlfunc(1) entry on
19799a22 2619C<umask> discusses the choice of MASK in more detail.
a0d0e21e 2620
cc1852e8
JH
2621Note that according to the POSIX 1003.1-1996 the FILENAME may have any
2622number of trailing slashes. Some operating and filesystems do not get
2623this right, so Perl automatically removes all trailing slashes to keep
2624everyone happy.
2625
a0d0e21e
LW
2626=item msgctl ID,CMD,ARG
2627
f86cebdf 2628Calls the System V IPC function msgctl(2). You'll probably have to say
0ade1984
JH
2629
2630 use IPC::SysV;
2631
7660c0ab
A
2632first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2633then ARG must be a variable which will hold the returned C<msqid_ds>
951ba7fe
GS
2634structure. Returns like C<ioctl>: the undefined value for error,
2635C<"0 but true"> for zero, or the actual return value otherwise. See also
4755096e 2636L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation.
a0d0e21e
LW
2637
2638=item msgget KEY,FLAGS
2639
f86cebdf 2640Calls the System V IPC function msgget(2). Returns the message queue
4755096e
GS
2641id, or the undefined value if there is an error. See also
2642L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation.
a0d0e21e 2643
a0d0e21e
LW
2644=item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2645
2646Calls the System V IPC function msgrcv to receive a message from
2647message queue ID into variable VAR with a maximum message size of
41d6edb2
JH
2648SIZE. Note that when a message is received, the message type as a
2649native long integer will be the first thing in VAR, followed by the
2650actual message. This packing may be opened with C<unpack("l! a*")>.
2651Taints the variable. Returns true if successful, or false if there is
4755096e
GS
2652an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and
2653C<IPC::SysV::Msg> documentation.
41d6edb2
JH
2654
2655=item msgsnd ID,MSG,FLAGS
2656
2657Calls the System V IPC function msgsnd to send the message MSG to the
2658message queue ID. MSG must begin with the native long integer message
2659type, and be followed by the length of the actual message, and finally
2660the message itself. This kind of packing can be achieved with
2661C<pack("l! a*", $type, $message)>. Returns true if successful,
2662or false if there is an error. See also C<IPC::SysV>
2663and C<IPC::SysV::Msg> documentation.
a0d0e21e
LW
2664
2665=item my EXPR
2666
307ea6df
JH
2667=item my TYPE EXPR
2668
1d2de774 2669=item my EXPR : ATTRS
09bef843 2670
1d2de774 2671=item my TYPE EXPR : ATTRS
307ea6df 2672
19799a22 2673A C<my> declares the listed variables to be local (lexically) to the
1d2de774
JH
2674enclosing block, file, or C<eval>. If more than one value is listed,
2675the list must be placed in parentheses.
307ea6df 2676
1d2de774
JH
2677The exact semantics and interface of TYPE and ATTRS are still
2678evolving. TYPE is currently bound to the use of C<fields> pragma,
307ea6df
JH
2679and attributes are handled using the C<attributes> pragma, or starting
2680from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
2681L<perlsub/"Private Variables via my()"> for details, and L<fields>,
2682L<attributes>, and L<Attribute::Handlers>.
4633a7c4 2683
a0d0e21e
LW
2684=item next LABEL
2685
2686=item next
2687
2688The C<next> command is like the C<continue> statement in C; it starts
2689the next iteration of the loop:
2690
4633a7c4
LW
2691 LINE: while (<STDIN>) {
2692 next LINE if /^#/; # discard comments
5a964f20 2693 #...
a0d0e21e
LW
2694 }
2695
2696Note that if there were a C<continue> block on the above, it would get
2697executed even on discarded lines. If the LABEL is omitted, the command
2698refers to the innermost enclosing loop.
2699
4968c1e4 2700C<next> cannot be used to exit a block which returns a value such as
2b5ab1e7
TC
2701C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2702a grep() or map() operation.
4968c1e4 2703
6c1372ed
GS
2704Note that a block by itself is semantically identical to a loop
2705that executes once. Thus C<next> will exit such a block early.
2706
98293880
JH
2707See also L</continue> for an illustration of how C<last>, C<next>, and
2708C<redo> work.
1d2dff63 2709
4a66ea5a
RGS
2710=item no Module VERSION LIST
2711
2712=item no Module VERSION
2713
a0d0e21e
LW
2714=item no Module LIST
2715
4a66ea5a
RGS
2716=item no Module
2717
a6d05634 2718See the C<use> function, which C<no> is the opposite of.
a0d0e21e
LW
2719
2720=item oct EXPR
2721
54310121 2722=item oct
bbce6d69 2723
4633a7c4 2724Interprets EXPR as an octal string and returns the corresponding
4f19785b
WSI
2725value. (If EXPR happens to start off with C<0x>, interprets it as a
2726hex string. If EXPR starts off with C<0b>, it is interpreted as a
53305cf1
NC
2727binary string. Leading whitespace is ignored in all three cases.)
2728The following will handle decimal, binary, octal, and hex in the standard
2729Perl or C notation:
a0d0e21e
LW
2730
2731 $val = oct($val) if $val =~ /^0/;
2732
19799a22
GS
2733If EXPR is omitted, uses C<$_>. To go the other way (produce a number
2734in octal), use sprintf() or printf():
2735
2736 $perms = (stat("filename"))[2] & 07777;
2737 $oct_perms = sprintf "%lo", $perms;
2738
2739The oct() function is commonly used when a string such as C<644> needs
2740to be converted into a file mode, for example. (Although perl will
2741automatically convert strings into numbers as needed, this automatic
2742conversion assumes base 10.)
a0d0e21e
LW
2743
2744=item open FILEHANDLE,EXPR
2745
68bd7414
NIS
2746=item open FILEHANDLE,MODE,EXPR
2747
2748=item open FILEHANDLE,MODE,EXPR,LIST
2749
ba964c95
T
2750=item open FILEHANDLE,MODE,REFERENCE
2751
a0d0e21e
LW
2752=item open FILEHANDLE
2753
2754Opens the file whose filename is given by EXPR, and associates it with
ed53a2bb
JH
2755FILEHANDLE.
2756
2757(The following is a comprehensive reference to open(): for a gentler
2758introduction you may consider L<perlopentut>.)
2759
2760If FILEHANDLE is an undefined lexical (C<my>) variable the variable is
2761assigned a reference to a new anonymous filehandle, otherwise if
2762FILEHANDLE is an expression, its value is used as the name of the real
2763filehandle wanted. (This is considered a symbolic reference, so C<use
2764strict 'refs'> should I<not> be in effect.)
2765
2766If EXPR is omitted, the scalar variable of the same name as the
2767FILEHANDLE contains the filename. (Note that lexical variables--those
2768declared with C<my>--will not work for this purpose; so if you're
67408cae 2769using C<my>, specify EXPR in your call to open.)
ed53a2bb
JH
2770
2771If three or more arguments are specified then the mode of opening and
2772the file name are separate. If MODE is C<< '<' >> or nothing, the file
2773is opened for input. If MODE is C<< '>' >>, the file is truncated and
2774opened for output, being created if necessary. If MODE is C<<< '>>' >>>,
b76cc8ba 2775the file is opened for appending, again being created if necessary.
5a964f20 2776
ed53a2bb
JH
2777You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to
2778indicate that you want both read and write access to the file; thus
2779C<< '+<' >> is almost always preferred for read/write updates--the C<<
2780'+>' >> mode would clobber the file first. You can't usually use
2781either read-write mode for updating textfiles, since they have
2782variable length records. See the B<-i> switch in L<perlrun> for a
2783better approach. The file is created with permissions of C<0666>
2784modified by the process' C<umask> value.
2785
2786These various prefixes correspond to the fopen(3) modes of C<'r'>,
2787C<'r+'>, C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
5f05dabc 2788
6170680b
IZ
2789In the 2-arguments (and 1-argument) form of the call the mode and
2790filename should be concatenated (in this order), possibly separated by
68bd7414
NIS
2791spaces. It is possible to omit the mode in these forms if the mode is
2792C<< '<' >>.
6170680b 2793
7660c0ab 2794If the filename begins with C<'|'>, the filename is interpreted as a
5a964f20 2795command to which output is to be piped, and if the filename ends with a
f244e06d
GS
2796C<'|'>, the filename is interpreted as a command which pipes output to
2797us. See L<perlipc/"Using open() for IPC">
19799a22 2798for more examples of this. (You are not allowed to C<open> to a command
5a964f20 2799that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
4a4eefd0
GS
2800and L<perlipc/"Bidirectional Communication with Another Process">
2801for alternatives.)
cb1a09d0 2802
ed53a2bb
JH
2803For three or more arguments if MODE is C<'|-'>, the filename is
2804interpreted as a command to which output is to be piped, and if MODE
2805is C<'-|'>, the filename is interpreted as a command which pipes
2806output to us. In the 2-arguments (and 1-argument) form one should
2807replace dash (C<'-'>) with the command.
2808See L<perlipc/"Using open() for IPC"> for more examples of this.
2809(You are not allowed to C<open> to a command that pipes both in I<and>
2810out, but see L<IPC::Open2>, L<IPC::Open3>, and
2811L<perlipc/"Bidirectional Communication"> for alternatives.)
2812
2813In the three-or-more argument form of pipe opens, if LIST is specified
2814(extra arguments after the command name) then LIST becomes arguments
2815to the command invoked if the platform supports it. The meaning of
2816C<open> with more than three arguments for non-pipe modes is not yet
2817specified. Experimental "layers" may give extra LIST arguments
2818meaning.
6170680b
IZ
2819
2820In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
b76cc8ba 2821and opening C<< '>-' >> opens STDOUT.
6170680b 2822
fae2c0fb
RGS
2823You may use the three-argument form of open to specify IO "layers"
2824(sometimes also referred to as "disciplines") to be applied to the handle
2825that affect how the input and output are processed (see L<open> and
2826L<PerlIO> for more details). For example
7207e29d 2827
9124316e
JH
2828 open(FH, "<:utf8", "file")
2829
2830will open the UTF-8 encoded file containing Unicode characters,
fae2c0fb
RGS
2831see L<perluniintro>. (Note that if layers are specified in the
2832three-arg form then default layers set by the C<open> pragma are
01e6739c 2833ignored.)
ed53a2bb
JH
2834
2835Open returns nonzero upon success, the undefined value otherwise. If
2836the C<open> involved a pipe, the return value happens to be the pid of
2837the subprocess.
cb1a09d0 2838
ed53a2bb
JH
2839If you're running Perl on a system that distinguishes between text
2840files and binary files, then you should check out L</binmode> for tips
2841for dealing with this. The key distinction between systems that need
2842C<binmode> and those that don't is their text file formats. Systems
8939ba94 2843like Unix, Mac OS, and Plan 9, which delimit lines with a single
ed53a2bb
JH
2844character, and which encode that character in C as C<"\n">, do not
2845need C<binmode>. The rest need it.
cb1a09d0 2846
fb73857a 2847When opening a file, it's usually a bad idea to continue normal execution
19799a22
GS
2848if the request failed, so C<open> is frequently used in connection with
2849C<die>. Even if C<die> won't do what you want (say, in a CGI script,
fb73857a 2850where you want to make a nicely formatted error message (but there are
5a964f20 2851modules that can help with that problem)) you should always check
19799a22 2852the return value from opening a file. The infrequent exception is when
fb73857a
PP
2853working with an unopened filehandle is actually what you want to do.
2854
ed53a2bb
JH
2855As a special case the 3 arg form with a read/write mode and the third
2856argument being C<undef>:
b76cc8ba
NIS
2857
2858 open(TMP, "+>", undef) or die ...
2859
2860opens a filehandle to an anonymous temporary file.
2861
ba964c95
T
2862File handles can be opened to "in memory" files held in Perl scalars via:
2863
b996200f
SB
2864 open($fh, '>', \$variable) || ..
2865
2866Though if you try to re-open C<STDOUT> or C<STDERR> as an "in memory"
2867file, you have to close it first:
2868
2869 close STDOUT;
2870 open STDOUT, '>', \$variable or die "Can't open STDOUT: $!";
ba964c95 2871
cb1a09d0 2872Examples:
a0d0e21e
LW
2873
2874 $ARTICLE = 100;
2875 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2876 while (<ARTICLE>) {...
2877
6170680b 2878 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
fb73857a 2879 # if the open fails, output is discarded
a0d0e21e 2880
6170680b 2881 open(DBASE, '+<', 'dbase.mine') # open for update
fb73857a 2882 or die "Can't open 'dbase.mine' for update: $!";
cb1a09d0 2883
6170680b
IZ
2884 open(DBASE, '+<dbase.mine') # ditto
2885 or die "Can't open 'dbase.mine' for update: $!";
2886
2887 open(ARTICLE, '-|', "caesar <$article") # decrypt article
fb73857a 2888 or die "Can't start caesar: $!";
a0d0e21e 2889
6170680b
IZ
2890 open(ARTICLE, "caesar <$article |") # ditto
2891 or die "Can't start caesar: $!";
2892
2893 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
fb73857a 2894 or die "Can't start sort: $!";
a0d0e21e 2895
ba964c95
T
2896 # in memory files
2897 open(MEMORY,'>', \$var)
2898 or die "Can't open memory file: $!";
2899 print MEMORY "foo!\n"; # output will end up in $var
2900
a0d0e21e
LW
2901 # process argument list of files along with any includes
2902
2903 foreach $file (@ARGV) {
2904 process($file, 'fh00');
2905 }
2906
2907 sub process {
5a964f20 2908 my($filename, $input) = @_;
a0d0e21e
LW
2909 $input++; # this is a string increment
2910 unless (open($input, $filename)) {
2911 print STDERR "Can't open $filename: $!\n";
2912 return;
2913 }
2914
5a964f20 2915 local $_;
a0d0e21e
LW
2916 while (<$input>) { # note use of indirection
2917 if (/^#include "(.*)"/) {
2918 process($1, $input);
2919 next;
2920 }
5a964f20 2921 #... # whatever
a0d0e21e
LW
2922 }
2923 }
2924
2925You may also, in the Bourne shell tradition, specify an EXPR beginning
61eff3bc 2926with C<< '>&' >>, in which case the rest of the string is interpreted as the
5a964f20 2927name of a filehandle (or file descriptor, if numeric) to be
61eff3bc
JH
2928duped and opened. You may use C<&> after C<< > >>, C<<< >> >>>,
2929C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>. The
a0d0e21e 2930mode you specify should match the mode of the original filehandle.
184e9718 2931(Duping a filehandle does not take into account any existing contents of
9124316e 2932IO buffers.) If you use the 3 arg form then you can pass either a number,
b76cc8ba 2933the name of a filehandle or the normal "reference to a glob".
6170680b 2934
eae1b76b
SB
2935Here is a script that saves, redirects, and restores C<STDOUT> and
2936C<STDERR> using various methods:
a0d0e21e
LW
2937
2938 #!/usr/bin/perl
eae1b76b
SB
2939 open my $oldout, ">&STDOUT" or die "Can't dup STDOUT: $!";
2940 open OLDERR, ">&", \*STDERR or die "Can't dup STDERR: $!";
818c4caa 2941
eae1b76b
SB
2942 open STDOUT, '>', "foo.out" or die "Can't redirect STDOUT: $!";
2943 open STDERR, ">&STDOUT" or die "Can't dup STDOUT: $!";
a0d0e21e 2944
eae1b76b
SB
2945 select STDERR; $| = 1; # make unbuffered
2946 select STDOUT; $| = 1; # make unbuffered
a0d0e21e
LW
2947
2948 print STDOUT "stdout 1\n"; # this works for
2949 print STDERR "stderr 1\n"; # subprocesses too
2950
eae1b76b
SB
2951 close STDOUT;
2952 close STDERR;
a0d0e21e 2953
eae1b76b
SB
2954 open STDOUT, ">&", $oldout or die "Can't dup \$oldout: $!";
2955 open STDERR, ">&OLDERR" or die "Can't dup OLDERR: $!";
a0d0e21e
LW
2956
2957 print STDOUT "stdout 2\n";
2958 print STDERR "stderr 2\n";
2959
df632fdf
JH
2960If you specify C<< '<&=N' >>, where C<N> is a number, then Perl will
2961do an equivalent of C's C<fdopen> of that file descriptor; this is
2962more parsimonious of file descriptors. For example:
a0d0e21e
LW
2963
2964 open(FILEHANDLE, "<&=$fd")
df632fdf 2965
b76cc8ba 2966or
df632fdf 2967
b76cc8ba 2968 open(FILEHANDLE, "<&=", $fd)
a0d0e21e 2969
df632fdf
JH
2970Note that if Perl is using the standard C libraries' fdopen() then on
2971many UNIX systems, fdopen() is known to fail when file descriptors
4af147f6 2972exceed a certain value, typically 255. If you need more file
b76cc8ba 2973descriptors than that, consider rebuilding Perl to use the C<PerlIO>.
4af147f6 2974
df632fdf
JH
2975You can see whether Perl has been compiled with PerlIO or not by
2976running C<perl -V> and looking for C<useperlio=> line. If C<useperlio>
2977is C<define>, you have PerlIO, otherwise you don't.
2978
6170680b
IZ
2979If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
2980with 2-arguments (or 1-argument) form of open(), then
a0d0e21e 2981there is an implicit fork done, and the return value of open is the pid
7660c0ab 2982of the child within the parent process, and C<0> within the child
184e9718 2983process. (Use C<defined($pid)> to determine whether the open was successful.)
a0d0e21e
LW
2984The filehandle behaves normally for the parent, but i/o to that
2985filehandle is piped from/to the STDOUT/STDIN of the child process.
2986In the child process the filehandle isn't opened--i/o happens from/to
2987the new STDOUT or STDIN. Typically this is used like the normal
2988piped open when you want to exercise more control over just how the
2989pipe command gets executed, such as when you are running setuid, and
54310121 2990don't want to have to scan shell commands for metacharacters.
6170680b 2991The following triples are more or less equivalent:
a0d0e21e
LW
2992
2993 open(FOO, "|tr '[a-z]' '[A-Z]'");
6170680b
IZ
2994 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
2995 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
b76cc8ba 2996 open(FOO, '|-', "tr", '[a-z]', '[A-Z]');
a0d0e21e
LW
2997
2998 open(FOO, "cat -n '$file'|");
6170680b
IZ
2999 open(FOO, '-|', "cat -n '$file'");
3000 open(FOO, '-|') || exec 'cat', '-n', $file;
b76cc8ba
NIS
3001 open(FOO, '-|', "cat", '-n', $file);
3002
3003The last example in each block shows the pipe as "list form", which is
64da03b2
JH
3004not yet supported on all platforms. A good rule of thumb is that if
3005your platform has true C<fork()> (in other words, if your platform is
3006UNIX) you can use the list form.
a0d0e21e 3007
4633a7c4
LW
3008See L<perlipc/"Safe Pipe Opens"> for more examples of this.
3009
0f897271
GS
3010Beginning with v5.6.0, Perl will attempt to flush all files opened for
3011output before any operation that may do a fork, but this may not be
3012supported on some platforms (see L<perlport>). To be safe, you may need
3013to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
3014of C<IO::Handle> on any open handles.
3015
ed53a2bb
JH
3016On systems that support a close-on-exec flag on files, the flag will
3017be set for the newly opened file descriptor as determined by the value
3018of $^F. See L<perlvar/$^F>.
a0d0e21e 3019
0dccf244
CS
3020Closing any piped filehandle causes the parent process to wait for the
3021child to finish, and returns the status value in C<$?>.
3022
ed53a2bb
JH
3023The filename passed to 2-argument (or 1-argument) form of open() will
3024have leading and trailing whitespace deleted, and the normal
3025redirection characters honored. This property, known as "magic open",
5a964f20 3026can often be used to good effect. A user could specify a filename of
7660c0ab 3027F<"rsh cat file |">, or you could change certain filenames as needed:
5a964f20
TC
3028
3029 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
3030 open(FH, $filename) or die "Can't open $filename: $!";
3031
6170680b
IZ
3032Use 3-argument form to open a file with arbitrary weird characters in it,
3033
3034 open(FOO, '<', $file);
3035
3036otherwise it's necessary to protect any leading and trailing whitespace:
5a964f20
TC
3037
3038 $file =~ s#^(\s)#./$1#;
3039 open(FOO, "< $file\0");
3040
a31a806a 3041(this may not work on some bizarre filesystems). One should
106325ad 3042conscientiously choose between the I<magic> and 3-arguments form
6170680b
IZ
3043of open():
3044
3045 open IN, $ARGV[0];
3046
3047will allow the user to specify an argument of the form C<"rsh cat file |">,
3048but will not work on a filename which happens to have a trailing space, while
3049
3050 open IN, '<', $ARGV[0];
3051
3052will have exactly the opposite restrictions.
3053
19799a22 3054If you want a "real" C C<open> (see L<open(2)> on your system), then you
6170680b
IZ
3055should use the C<sysopen> function, which involves no such magic (but
3056may use subtly different filemodes than Perl open(), which is mapped
3057to C fopen()). This is
5a964f20
TC
3058another way to protect your filenames from interpretation. For example:
3059
3060 use IO::Handle;
3061 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
3062 or die "sysopen $path: $!";
3063 $oldfh = select(HANDLE); $| = 1; select($oldfh);
38762f02 3064 print HANDLE "stuff $$\n";
5a964f20
TC
3065 seek(HANDLE, 0, 0);
3066 print "File contains: ", <HANDLE>;
3067
7660c0ab
A
3068Using the constructor from the C<IO::Handle> package (or one of its
3069subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
5a964f20
TC
3070filehandles that have the scope of whatever variables hold references to
3071them, and automatically close whenever and however you leave that scope:
c07a80fd 3072
5f05dabc 3073 use IO::File;
5a964f20 3074 #...
c07a80fd
PP
3075 sub read_myfile_munged {
3076 my $ALL = shift;
5f05dabc 3077 my $handle = new IO::File;
c07a80fd
PP
3078 open($handle, "myfile") or die "myfile: $!";
3079 $first = <$handle>
3080 or return (); # Automatically closed here.
3081 mung $first or die "mung failed"; # Or here.
3082 return $first, <$handle> if $ALL; # Or here.
3083 $first; # Or here.
3084 }
3085
b687b08b 3086See L</seek> for some details about mixing reading and writing.
a0d0e21e
LW
3087
3088=item opendir DIRHANDLE,EXPR
3089
19799a22
GS
3090Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
3091C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
a0d0e21e
LW
3092DIRHANDLEs have their own namespace separate from FILEHANDLEs.
3093
3094=item ord EXPR
3095
54310121 3096=item ord
bbce6d69 3097
121910a4
JH
3098Returns the numeric (the native 8-bit encoding, like ASCII or EBCDIC,
3099or Unicode) value of the first character of EXPR. If EXPR is omitted,
3100uses C<$_>.
3101
3102For the reverse, see L</chr>.
3103See L<perlunicode> and L<encoding> for more about Unicode.
a0d0e21e 3104
77ca0c92
LW
3105=item our EXPR
3106
307ea6df
JH
3107=item our EXPR TYPE
3108
1d2de774 3109=item our EXPR : ATTRS
9969eac4 3110
1d2de774 3111=item our TYPE EXPR : ATTRS
307ea6df 3112
77ca0c92
LW
3113An C<our> declares the listed variables to be valid globals within
3114the enclosing block, file, or C<eval>. That is, it has the same
3115scoping rules as a "my" declaration, but does not create a local
3116variable. If more than one value is listed, the list must be placed
3117in parentheses. The C<our> declaration has no semantic effect unless
3118"use strict vars" is in effect, in which case it lets you use the
3119declared global variable without qualifying it with a package name.
3120(But only within the lexical scope of the C<our> declaration. In this
3121it differs from "use vars", which is package scoped.)
3122
f472eb5c
GS
3123An C<our> declaration declares a global variable that will be visible
3124across its entire lexical scope, even across package boundaries. The
3125package in which the variable is entered is determined at the point
3126of the declaration, not at the point of use. This means the following
3127behavior holds:
3128
3129 package Foo;
3130 our $bar; # declares $Foo::bar for rest of lexical scope
3131 $bar = 20;
3132
3133 package Bar;
3134 print $bar; # prints 20
3135
3136Multiple C<our> declarations in the same lexical scope are allowed
3137if they are in different packages. If they happened to be in the same
3138package, Perl will emit warnings if you have asked for them.
3139
3140 use warnings;
3141 package Foo;
3142 our $bar; # declares $Foo::bar for rest of lexical scope
3143 $bar = 20;
3144
3145 package Bar;
3146 our $bar = 30; # declares $Bar::bar for rest of lexical scope
3147 print $bar; # prints 30
3148
3149 our $bar; # emits warning
3150
9969eac4 3151An C<our> declaration may also have a list of attributes associated
307ea6df
JH
3152with it.
3153
1d2de774
JH
3154The exact semantics and interface of TYPE and ATTRS are still
3155evolving. TYPE is currently bound to the use of C<fields> pragma,
307ea6df
JH
3156and attributes are handled using the C<attributes> pragma, or starting
3157from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
3158L<perlsub/"Private Variables via my()"> for details, and L<fields>,
3159L<attributes>, and L<Attribute::Handlers>.
3160
3161The only currently recognized C<our()> attribute is C<unique> which
3162indicates that a single copy of the global is to be used by all
3163interpreters should the program happen to be running in a
3164multi-interpreter environment. (The default behaviour would be for
3165each interpreter to have its own copy of the global.) Examples:
9969eac4 3166
51d2bbcc
JH
3167 our @EXPORT : unique = qw(foo);
3168 our %EXPORT_TAGS : unique = (bar => [qw(aa bb cc)]);
3169 our $VERSION : unique = "1.00";
9969eac4 3170
96fa8c42 3171Note that this attribute also has the effect of making the global
72e53bfb
JH
3172readonly when the first new interpreter is cloned (for example,
3173when the first new thread is created).
96fa8c42 3174
9969eac4
BS
3175Multi-interpreter environments can come to being either through the
3176fork() emulation on Windows platforms, or by embedding perl in a
51d2bbcc 3177multi-threaded application. The C<unique> attribute does nothing in
9969eac4
BS
3178all other environments.
3179
a0d0e21e
LW
3180=item pack TEMPLATE,LIST
3181
2b6c5635
GS
3182Takes a LIST of values and converts it into a string using the rules
3183given by the TEMPLATE. The resulting string is the concatenation of
3184the converted values. Typically, each converted value looks
3185like its machine-level representation. For example, on 32-bit machines
3186a converted integer may be represented by a sequence of 4 bytes.
3187
18529408
IZ
3188The TEMPLATE is a sequence of characters that give the order and type
3189of values, as follows:
a0d0e21e 3190
5a929a98 3191 a A string with arbitrary binary data, will be null padded.
121910a4
JH
3192 A A text (ASCII) string, will be space padded.
3193 Z A null terminated (ASCIZ) string, will be null padded.
5a929a98 3194
2b6c5635
GS
3195 b A bit string (ascending bit order inside each byte, like vec()).
3196 B A bit string (descending bit order inside each byte).
a0d0e21e
LW
3197 h A hex string (low nybble first).
3198 H A hex string (high nybble first).
3199
3200 c A signed char value.
a0ed51b3 3201 C An unsigned char value. Only does bytes. See U for Unicode.
96e4d5b1 3202
a0d0e21e
LW
3203 s A signed short value.
3204 S An unsigned short value.
96e4d5b1 3205 (This 'short' is _exactly_ 16 bits, which may differ from
851646ae
JH
3206 what a local C compiler calls 'short'. If you want
3207 native-length shorts, use the '!' suffix.)
96e4d5b1 3208
a0d0e21e
LW
3209 i A signed integer value.
3210 I An unsigned integer value.
19799a22 3211 (This 'integer' is _at_least_ 32 bits wide. Its exact
f86cebdf
GS
3212 size depends on what a local C compiler calls 'int',
3213 and may even be larger than the 'long' described in
3214 the next item.)
96e4d5b1 3215
a0d0e21e
LW
3216 l A signed long value.
3217 L An unsigned long value.
96e4d5b1 3218 (This 'long' is _exactly_ 32 bits, which may differ from
851646ae
JH
3219 what a local C compiler calls 'long'. If you want
3220 native-length longs, use the '!' suffix.)
a0d0e21e 3221
5d11dd56
G
3222 n An unsigned short in "network" (big-endian) order.
3223 N An unsigned long in "network" (big-endian) order.
3224 v An unsigned short in "VAX" (little-endian) order.
3225 V An unsigned long in "VAX" (little-endian) order.
96e4d5b1
PP
3226 (These 'shorts' and 'longs' are _exactly_ 16 bits and
3227 _exactly_ 32 bits, respectively.)
a0d0e21e 3228
dae0da7a
JH
3229 q A signed quad (64-bit) value.
3230 Q An unsigned quad value.
851646ae
JH
3231 (Quads are available only if your system supports 64-bit
3232 integer values _and_ if Perl has been compiled to support those.
dae0da7a
JH
3233 Causes a fatal error otherwise.)
3234
92d41999
JH
3235 j A signed integer value (a Perl internal integer, IV).
3236 J An unsigned integer value (a Perl internal unsigned integer, UV).
3237
a0d0e21e
LW
3238 f A single-precision float in the native format.
3239 d A double-precision float in the native format.
3240
92d41999
JH
3241 F A floating point value in the native native format
3242 (a Perl internal floating point value, NV).
3243 D A long double-precision float in the native format.
3244 (Long doubles are available only if your system supports long
3245 double values _and_ if Perl has been compiled to support those.
3246 Causes a fatal error otherwise.)
3247
a0d0e21e
LW
3248 p A pointer to a null-terminated string.
3249 P A pointer to a structure (fixed-length string).
3250
3251 u A uuencoded string.
ad0029c4
JH
3252 U A Unicode character number. Encodes to UTF-8 internally
3253 (or UTF-EBCDIC in EBCDIC platforms).
a0d0e21e 3254