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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
68dc0745 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
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72An named array in scalar context is quite different from what would at
73first glance appear to be a list in scalar context. You can't get a list
74like C<(1,2,3)> into being in scalar context, because the compiler knows
75the context at compile time. It would generate the scalar comma operator
76there, not the list construction version of the comma. That means it
77was never a list to start with.
78
79In general, functions in Perl that serve as wrappers for system calls
f86cebdf 80of the same name (like chown(2), fork(2), closedir(2), etc.) all return
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81true when they succeed and C<undef> otherwise, as is usually mentioned
82in the descriptions below. This is different from the C interfaces,
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83which return C<-1> on failure. Exceptions to this rule are C<wait>,
84C<waitpid>, and C<syscall>. System calls also set the special C<$!>
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85variable on failure. Other functions do not, except accidentally.
86
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87=head2 Perl Functions by Category
88
89Here are Perl's functions (including things that look like
5a964f20 90functions, like some keywords and named operators)
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91arranged by category. Some functions appear in more
92than one place.
93
13a2d996 94=over 4
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95
96=item Functions for SCALARs or strings
97
22fae026 98C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>,
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99C<length>, C<oct>, C<ord>, C<pack>, C<q/STRING/>, C<qq/STRING/>, C<reverse>,
100C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///>
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101
102=item Regular expressions and pattern matching
103
ab4f32c2 104C<m//>, C<pos>, C<quotemeta>, C<s///>, C<split>, C<study>, C<qr//>
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105
106=item Numeric functions
107
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108C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>,
109C<sin>, C<sqrt>, C<srand>
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110
111=item Functions for real @ARRAYs
112
22fae026 113C<pop>, C<push>, C<shift>, C<splice>, C<unshift>
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114
115=item Functions for list data
116
ab4f32c2 117C<grep>, C<join>, C<map>, C<qw/STRING/>, C<reverse>, C<sort>, C<unpack>
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118
119=item Functions for real %HASHes
120
22fae026 121C<delete>, C<each>, C<exists>, C<keys>, C<values>
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122
123=item Input and output functions
124
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125C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
126C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
127C<readdir>, C<rewinddir>, C<seek>, C<seekdir>, C<select>, C<syscall>,
128C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>,
129C<warn>, C<write>
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130
131=item Functions for fixed length data or records
132
22fae026 133C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec>
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134
135=item Functions for filehandles, files, or directories
136
22fae026 137C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>,
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138C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>,
139C<readlink>, C<rename>, C<rmdir>, C<stat>, C<symlink>, C<umask>,
140C<unlink>, C<utime>
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141
142=item Keywords related to the control flow of your perl program
143
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144C<caller>, C<continue>, C<die>, C<do>, C<dump>, C<eval>, C<exit>,
145C<goto>, C<last>, C<next>, C<redo>, C<return>, C<sub>, C<wantarray>
cb1a09d0 146
54310121 147=item Keywords related to scoping
cb1a09d0 148
4375e838 149C<caller>, C<import>, C<local>, C<my>, C<our>, C<package>, C<use>
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150
151=item Miscellaneous functions
152
4375e838 153C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<our>, C<reset>,
22fae026 154C<scalar>, C<undef>, C<wantarray>
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155
156=item Functions for processes and process groups
157
22fae026 158C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
ab4f32c2 159C<pipe>, C<qx/STRING/>, C<setpgrp>, C<setpriority>, C<sleep>, C<system>,
22fae026 160C<times>, C<wait>, C<waitpid>
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161
162=item Keywords related to perl modules
163
22fae026 164C<do>, C<import>, C<no>, C<package>, C<require>, C<use>
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165
166=item Keywords related to classes and object-orientedness
167
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168C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
169C<untie>, C<use>
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170
171=item Low-level socket functions
172
22fae026
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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>,
80cbd5ad 175C<sockatmark>, C<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
37798a01 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>
37798a01 206
207* - C<sub> was a keyword in perl4, but in perl5 it is an
5a964f20 208operator, which can be used in expressions.
37798a01 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>,
227C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostent>,
228C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
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>,
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237C<shmwrite>, C<sockatmark>, C<socket>, C<socketpair>,
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
22fae026 249=item I<-X> FILEHANDLE
a0d0e21e 250
22fae026 251=item I<-X> EXPR
a0d0e21e 252
22fae026 253=item I<-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
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294 -T File is an ASCII text file.
295 -B File is a "binary" file (opposite of -T).
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296
297 -M Age of file in days when script started.
298 -A Same for access time.
299 -C Same for inode change time.
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>
342or C<-B> is used on a filehandle, the current stdio 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
353symbolic link, not the real file.) Example:
354
355 print "Can do.\n" if -r $a || -w _ || -x _;
356
357 stat($filename);
358 print "Readable\n" if -r _;
359 print "Writable\n" if -w _;
360 print "Executable\n" if -x _;
361 print "Setuid\n" if -u _;
362 print "Setgid\n" if -g _;
363 print "Sticky\n" if -k _;
364 print "Text\n" if -T _;
365 print "Binary\n" if -B _;
366
367=item abs VALUE
368
54310121 369=item abs
bbce6d69 370
a0d0e21e 371Returns the absolute value of its argument.
7660c0ab 372If VALUE is omitted, uses C<$_>.
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373
374=item accept NEWSOCKET,GENERICSOCKET
375
f86cebdf 376Accepts an incoming socket connect, just as the accept(2) system call
19799a22 377does. Returns the packed address if it succeeded, false otherwise.
2b5ab1e7 378See the example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 379
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380On systems that support a close-on-exec flag on files, the flag will
381be set for the newly opened file descriptor, as determined by the
382value of $^F. See L<perlvar/$^F>.
383
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384=item alarm SECONDS
385
54310121 386=item alarm
bbce6d69 387
a0d0e21e 388Arranges to have a SIGALRM delivered to this process after the
bbce6d69 389specified number of seconds have elapsed. If SECONDS is not specified,
7660c0ab 390the value stored in C<$_> is used. (On some machines,
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391unfortunately, the elapsed time may be up to one second less than you
392specified because of how seconds are counted.) Only one timer may be
393counting at once. Each call disables the previous timer, and an
7660c0ab 394argument of C<0> may be supplied to cancel the previous timer without
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395starting a new one. The returned value is the amount of time remaining
396on the previous timer.
397
4633a7c4 398For delays of finer granularity than one second, you may use Perl's
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399four-argument version of select() leaving the first three arguments
400undefined, or you might be able to use the C<syscall> interface to
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401access setitimer(2) if your system supports it. The Time::HiRes
402module (from CPAN, and starting from Perl 5.8 part of the standard
403distribution) may also prove useful.
2b5ab1e7 404
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405It is usually a mistake to intermix C<alarm> and C<sleep> calls.
406(C<sleep> may be internally implemented in your system with C<alarm>)
a0d0e21e 407
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408If you want to use C<alarm> to time out a system call you need to use an
409C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
f86cebdf 410fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
19799a22 411restart system calls on some systems. Using C<eval>/C<die> always works,
5a964f20 412modulo the caveats given in L<perlipc/"Signals">.
ff68c719 413
414 eval {
f86cebdf 415 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
36477c24 416 alarm $timeout;
ff68c719 417 $nread = sysread SOCKET, $buffer, $size;
36477c24 418 alarm 0;
ff68c719 419 };
ff68c719 420 if ($@) {
f86cebdf 421 die unless $@ eq "alarm\n"; # propagate unexpected errors
ff68c719 422 # timed out
423 }
424 else {
425 # didn't
426 }
427
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428=item atan2 Y,X
429
430Returns the arctangent of Y/X in the range -PI to PI.
431
ca6e1c26 432For the tangent operation, you may use the C<Math::Trig::tan>
28757baa 433function, or use the familiar relation:
434
435 sub tan { sin($_[0]) / cos($_[0]) }
436
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437=item bind SOCKET,NAME
438
439Binds a network address to a socket, just as the bind system call
19799a22 440does. Returns true if it succeeded, false otherwise. NAME should be a
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441packed address of the appropriate type for the socket. See the examples in
442L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 443
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444=item binmode FILEHANDLE, DISCIPLINE
445
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446=item binmode FILEHANDLE
447
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448Arranges for FILEHANDLE to be read or written in "binary" or "text" mode
449on systems where the run-time libraries distinguish between binary and
30168b04 450text files. If FILEHANDLE is an expression, the value is taken as the
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451name of the filehandle. DISCIPLINE can be either of C<":raw"> for
452binary mode or C<":crlf"> for "text" mode. If the DISCIPLINE is
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453omitted, it defaults to C<":raw">. Returns true on success, C<undef> on
454failure.
30168b04 455
16fe6d59
GS
456binmode() should be called after open() but before any I/O is done on
457the filehandle.
458
16fe6d59
GS
459On some systems binmode() is necessary when you're not working with a
460text file. For the sake of portability it is a good idea to always use
461it when appropriate, and to never use it when it isn't appropriate.
30168b04
GS
462
463In other words: Regardless of platform, use binmode() on binary
464files, and do not use binmode() on text files.
19799a22 465
16fe6d59
GS
466The C<open> pragma can be used to establish default disciplines.
467See L<open>.
468
19799a22 469The operating system, device drivers, C libraries, and Perl run-time
30168b04
GS
470system all work together to let the programmer treat a single
471character (C<\n>) as the line terminator, irrespective of the external
472representation. On many operating systems, the native text file
473representation matches the internal representation, but on some
474platforms the external representation of C<\n> is made up of more than
475one character.
476
68bd7414
NIS
477Mac OS, all variants of Unix, and Stream_LF files on VMS use a single
478character to end each line in the external representation of text (even
5e12dbfa 479though that single character is CARRIAGE RETURN on Mac OS and LINE FEED
68bd7414
NIS
480on Unix and most VMS files). Consequently binmode() has no effect on
481these operating systems. In other systems like OS/2, DOS and the various
482flavors of MS-Windows your program sees a C<\n> as a simple C<\cJ>, but
483what's stored in text files are the two characters C<\cM\cJ>. That means
484that, if you don't use binmode() on these systems, C<\cM\cJ> sequences on
485disk will be converted to C<\n> on input, and any C<\n> in your program
5e12dbfa
PP
486will be converted back to C<\cM\cJ> on output. This is what you want for
487text files, but it can be disastrous for binary files.
30168b04
GS
488
489Another consequence of using binmode() (on some systems) is that
490special end-of-file markers will be seen as part of the data stream.
491For systems from the Microsoft family this means that if your binary
4375e838 492data contains C<\cZ>, the I/O subsystem will regard it as the end of
30168b04
GS
493the file, unless you use binmode().
494
495binmode() is not only important for readline() and print() operations,
496but also when using read(), seek(), sysread(), syswrite() and tell()
497(see L<perlport> for more details). See the C<$/> and C<$\> variables
498in L<perlvar> for how to manually set your input and output
499line-termination sequences.
a0d0e21e 500
4633a7c4 501=item bless REF,CLASSNAME
a0d0e21e
LW
502
503=item bless REF
504
2b5ab1e7
TC
505This function tells the thingy referenced by REF that it is now an object
506in the CLASSNAME package. If CLASSNAME is omitted, the current package
19799a22 507is used. Because a C<bless> is often the last thing in a constructor,
2b5ab1e7
TC
508it returns the reference for convenience. Always use the two-argument
509version if the function doing the blessing might be inherited by a
510derived class. See L<perltoot> and L<perlobj> for more about the blessing
511(and blessings) of objects.
a0d0e21e 512
57668c4d 513Consider always blessing objects in CLASSNAMEs that are mixed case.
2b5ab1e7
TC
514Namespaces with all lowercase names are considered reserved for
515Perl pragmata. Builtin types have all uppercase names, so to prevent
516confusion, you may wish to avoid such package names as well. Make sure
517that CLASSNAME is a true value.
60ad88b8
GS
518
519See L<perlmod/"Perl Modules">.
520
a0d0e21e
LW
521=item caller EXPR
522
523=item caller
524
5a964f20 525Returns the context of the current subroutine call. In scalar context,
28757baa 526returns the caller's package name if there is a caller, that is, if
19799a22 527we're in a subroutine or C<eval> or C<require>, and the undefined value
5a964f20 528otherwise. In list context, returns
a0d0e21e 529
748a9306 530 ($package, $filename, $line) = caller;
a0d0e21e
LW
531
532With EXPR, it returns some extra information that the debugger uses to
533print a stack trace. The value of EXPR indicates how many call frames
534to go back before the current one.
535
f3aa04c2 536 ($package, $filename, $line, $subroutine, $hasargs,
e476b1b5 537 $wantarray, $evaltext, $is_require, $hints, $bitmask) = caller($i);
e7ea3e70 538
951ba7fe 539Here $subroutine may be C<(eval)> if the frame is not a subroutine
19799a22 540call, but an C<eval>. In such a case additional elements $evaltext and
7660c0ab 541C<$is_require> are set: C<$is_require> is true if the frame is created by a
19799a22 542C<require> or C<use> statement, $evaltext contains the text of the
277ddfaf 543C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
951ba7fe 544$filename is C<(eval)>, but $evaltext is undefined. (Note also that
dc848c6f 545each C<use> statement creates a C<require> frame inside an C<eval EXPR>)
277ddfaf 546frame. C<$hasargs> is true if a new instance of C<@_> was set up for the
e476b1b5
GS
547frame. C<$hints> and C<$bitmask> contain pragmatic hints that the caller
548was compiled with. The C<$hints> and C<$bitmask> values are subject to
549change between versions of Perl, and are not meant for external use.
748a9306
LW
550
551Furthermore, when called from within the DB package, caller returns more
7660c0ab 552detailed information: it sets the list variable C<@DB::args> to be the
54310121 553arguments with which the subroutine was invoked.
748a9306 554
7660c0ab 555Be aware that the optimizer might have optimized call frames away before
19799a22 556C<caller> had a chance to get the information. That means that C<caller(N)>
7660c0ab 557might not return information about the call frame you expect it do, for
b76cc8ba 558C<< N > 1 >>. In particular, C<@DB::args> might have information from the
19799a22 559previous time C<caller> was called.
7660c0ab 560
a0d0e21e
LW
561=item chdir EXPR
562
2b5ab1e7 563Changes the working directory to EXPR, if possible. If EXPR is omitted,
0bfc1ec4
GS
564changes to the directory specified by C<$ENV{HOME}>, if set; if not,
565changes to the directory specified by C<$ENV{LOGDIR}>. If neither is
566set, C<chdir> does nothing. It returns true upon success, false
567otherwise. See the example under C<die>.
a0d0e21e
LW
568
569=item chmod LIST
570
571Changes the permissions of a list of files. The first element of the
4633a7c4 572list must be the numerical mode, which should probably be an octal
2f9daede
TP
573number, and which definitely should I<not> a string of octal digits:
574C<0644> is okay, C<'0644'> is not. Returns the number of files
dc848c6f 575successfully changed. See also L</oct>, if all you have is a string.
a0d0e21e
LW
576
577 $cnt = chmod 0755, 'foo', 'bar';
578 chmod 0755, @executables;
f86cebdf
GS
579 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
580 # --w----r-T
2f9daede
TP
581 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
582 $mode = 0644; chmod $mode, 'foo'; # this is best
a0d0e21e 583
ca6e1c26
JH
584You can also import the symbolic C<S_I*> constants from the Fcntl
585module:
586
587 use Fcntl ':mode';
588
589 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
590 # This is identical to the chmod 0755 of the above example.
591
a0d0e21e
LW
592=item chomp VARIABLE
593
594=item chomp LIST
595
596=item chomp
597
2b5ab1e7
TC
598This safer version of L</chop> removes any trailing string
599that corresponds to the current value of C<$/> (also known as
28757baa 600$INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
601number of characters removed from all its arguments. It's often used to
602remove the newline from the end of an input record when you're worried
2b5ab1e7
TC
603that the final record may be missing its newline. When in paragraph
604mode (C<$/ = "">), it removes all trailing newlines from the string.
4c5a6083
GS
605When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
606a reference to an integer or the like, see L<perlvar>) chomp() won't
b76cc8ba 607remove anything.
19799a22 608If VARIABLE is omitted, it chomps C<$_>. Example:
a0d0e21e
LW
609
610 while (<>) {
611 chomp; # avoid \n on last field
612 @array = split(/:/);
5a964f20 613 # ...
a0d0e21e
LW
614 }
615
4bf21a6d
RD
616If VARIABLE is a hash, it chomps the hash's values, but not its keys.
617
a0d0e21e
LW
618You can actually chomp anything that's an lvalue, including an assignment:
619
620 chomp($cwd = `pwd`);
621 chomp($answer = <STDIN>);
622
623If you chomp a list, each element is chomped, and the total number of
624characters removed is returned.
625
626=item chop VARIABLE
627
628=item chop LIST
629
630=item chop
631
632Chops off the last character of a string and returns the character
5b3eff12 633chopped. It is much more efficient than C<s/.$//s> because it neither
7660c0ab 634scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
4bf21a6d
RD
635If VARIABLE is a hash, it chops the hash's values, but not its keys.
636
5b3eff12 637You can actually chop anything that's an lvalue, including an assignment.
a0d0e21e
LW
638
639If you chop a list, each element is chopped. Only the value of the
19799a22 640last C<chop> is returned.
a0d0e21e 641
19799a22 642Note that C<chop> returns the last character. To return all but the last
748a9306
LW
643character, use C<substr($string, 0, -1)>.
644
a0d0e21e
LW
645=item chown LIST
646
647Changes the owner (and group) of a list of files. The first two
19799a22
GS
648elements of the list must be the I<numeric> uid and gid, in that
649order. A value of -1 in either position is interpreted by most
650systems to leave that value unchanged. Returns the number of files
651successfully changed.
a0d0e21e
LW
652
653 $cnt = chown $uid, $gid, 'foo', 'bar';
654 chown $uid, $gid, @filenames;
655
54310121 656Here's an example that looks up nonnumeric uids in the passwd file:
a0d0e21e
LW
657
658 print "User: ";
19799a22 659 chomp($user = <STDIN>);
5a964f20 660 print "Files: ";
19799a22 661 chomp($pattern = <STDIN>);
a0d0e21e
LW
662
663 ($login,$pass,$uid,$gid) = getpwnam($user)
664 or die "$user not in passwd file";
665
5a964f20 666 @ary = glob($pattern); # expand filenames
a0d0e21e
LW
667 chown $uid, $gid, @ary;
668
54310121 669On most systems, you are not allowed to change the ownership of the
4633a7c4
LW
670file unless you're the superuser, although you should be able to change
671the group to any of your secondary groups. On insecure systems, these
672restrictions may be relaxed, but this is not a portable assumption.
19799a22
GS
673On POSIX systems, you can detect this condition this way:
674
675 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
676 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
4633a7c4 677
a0d0e21e
LW
678=item chr NUMBER
679
54310121 680=item chr
bbce6d69 681
a0d0e21e 682Returns the character represented by that NUMBER in the character set.
a0ed51b3 683For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
ad0029c4
JH
684chr(0x263a) is a Unicode smiley face. Note that characters from
685127 to 255 (inclusive) are not encoded in Unicode for backward
686compatibility reasons.
aaa68c4a 687
b76cc8ba 688For the reverse, use L</ord>.
2b5ab1e7 689See L<utf8> for more about Unicode.
a0d0e21e 690
7660c0ab 691If NUMBER is omitted, uses C<$_>.
bbce6d69 692
a0d0e21e
LW
693=item chroot FILENAME
694
54310121 695=item chroot
bbce6d69 696
5a964f20 697This function works like the system call by the same name: it makes the
4633a7c4 698named directory the new root directory for all further pathnames that
951ba7fe 699begin with a C</> by your process and all its children. (It doesn't
28757baa 700change your current working directory, which is unaffected.) For security
4633a7c4 701reasons, this call is restricted to the superuser. If FILENAME is
19799a22 702omitted, does a C<chroot> to C<$_>.
a0d0e21e
LW
703
704=item close FILEHANDLE
705
6a518fbc
TP
706=item close
707
19799a22 708Closes the file or pipe associated with the file handle, returning true
a0d0e21e 709only if stdio successfully flushes buffers and closes the system file
19799a22 710descriptor. Closes the currently selected filehandle if the argument
6a518fbc 711is omitted.
fb73857a 712
713You don't have to close FILEHANDLE if you are immediately going to do
19799a22
GS
714another C<open> on it, because C<open> will close it for you. (See
715C<open>.) However, an explicit C<close> on an input file resets the line
716counter (C<$.>), while the implicit close done by C<open> does not.
fb73857a 717
19799a22
GS
718If the file handle came from a piped open C<close> will additionally
719return false if one of the other system calls involved fails or if the
fb73857a 720program exits with non-zero status. (If the only problem was that the
b76cc8ba 721program exited non-zero C<$!> will be set to C<0>.) Closing a pipe
2b5ab1e7 722also waits for the process executing on the pipe to complete, in case you
b76cc8ba 723want to look at the output of the pipe afterwards, and
2b5ab1e7 724implicitly puts the exit status value of that command into C<$?>.
5a964f20 725
73689b13
GS
726Prematurely closing the read end of a pipe (i.e. before the process
727writing to it at the other end has closed it) will result in a
728SIGPIPE being delivered to the writer. If the other end can't
729handle that, be sure to read all the data before closing the pipe.
730
fb73857a 731Example:
a0d0e21e 732
fb73857a 733 open(OUTPUT, '|sort >foo') # pipe to sort
734 or die "Can't start sort: $!";
5a964f20 735 #... # print stuff to output
fb73857a 736 close OUTPUT # wait for sort to finish
737 or warn $! ? "Error closing sort pipe: $!"
738 : "Exit status $? from sort";
739 open(INPUT, 'foo') # get sort's results
740 or die "Can't open 'foo' for input: $!";
a0d0e21e 741
5a964f20
TC
742FILEHANDLE may be an expression whose value can be used as an indirect
743filehandle, usually the real filehandle name.
a0d0e21e
LW
744
745=item closedir DIRHANDLE
746
19799a22 747Closes a directory opened by C<opendir> and returns the success of that
5a964f20
TC
748system call.
749
750DIRHANDLE may be an expression whose value can be used as an indirect
751dirhandle, usually the real dirhandle name.
a0d0e21e
LW
752
753=item connect SOCKET,NAME
754
755Attempts to connect to a remote socket, just as the connect system call
19799a22 756does. Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
757packed address of the appropriate type for the socket. See the examples in
758L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 759
cb1a09d0
AD
760=item continue BLOCK
761
762Actually a flow control statement rather than a function. If there is a
98293880
JH
763C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
764C<foreach>), it is always executed just before the conditional is about to
765be evaluated again, just like the third part of a C<for> loop in C. Thus
cb1a09d0
AD
766it can be used to increment a loop variable, even when the loop has been
767continued via the C<next> statement (which is similar to the C C<continue>
768statement).
769
98293880 770C<last>, C<next>, or C<redo> may appear within a C<continue>
19799a22
GS
771block. C<last> and C<redo> will behave as if they had been executed within
772the main block. So will C<next>, but since it will execute a C<continue>
1d2dff63
GS
773block, it may be more entertaining.
774
775 while (EXPR) {
776 ### redo always comes here
777 do_something;
778 } continue {
779 ### next always comes here
780 do_something_else;
781 # then back the top to re-check EXPR
782 }
783 ### last always comes here
784
785Omitting the C<continue> section is semantically equivalent to using an
19799a22 786empty one, logically enough. In that case, C<next> goes directly back
1d2dff63
GS
787to check the condition at the top of the loop.
788
a0d0e21e
LW
789=item cos EXPR
790
d6217f1e
GS
791=item cos
792
5a964f20 793Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
7660c0ab 794takes cosine of C<$_>.
a0d0e21e 795
ca6e1c26 796For the inverse cosine operation, you may use the C<Math::Trig::acos()>
28757baa 797function, or use this relation:
798
799 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
800
a0d0e21e
LW
801=item crypt PLAINTEXT,SALT
802
f86cebdf 803Encrypts a string exactly like the crypt(3) function in the C library
4633a7c4
LW
804(assuming that you actually have a version there that has not been
805extirpated as a potential munition). This can prove useful for checking
806the password file for lousy passwords, amongst other things. Only the
807guys wearing white hats should do this.
a0d0e21e 808
19799a22 809Note that C<crypt> is intended to be a one-way function, much like breaking
11155c91
CS
810eggs to make an omelette. There is no (known) corresponding decrypt
811function. As a result, this function isn't all that useful for
812cryptography. (For that, see your nearby CPAN mirror.)
2f9daede 813
e71965be
RS
814When verifying an existing encrypted string you should use the encrypted
815text as the salt (like C<crypt($plain, $crypted) eq $crypted>). This
19799a22 816allows your code to work with the standard C<crypt> and with more
e71965be
RS
817exotic implementations. When choosing a new salt create a random two
818character string whose characters come from the set C<[./0-9A-Za-z]>
819(like C<join '', ('.', '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>).
820
a0d0e21e
LW
821Here's an example that makes sure that whoever runs this program knows
822their own password:
823
824 $pwd = (getpwuid($<))[1];
a0d0e21e
LW
825
826 system "stty -echo";
827 print "Password: ";
e71965be 828 chomp($word = <STDIN>);
a0d0e21e
LW
829 print "\n";
830 system "stty echo";
831
e71965be 832 if (crypt($word, $pwd) ne $pwd) {
a0d0e21e
LW
833 die "Sorry...\n";
834 } else {
835 print "ok\n";
54310121 836 }
a0d0e21e 837
9f8f0c9d 838Of course, typing in your own password to whoever asks you
748a9306 839for it is unwise.
a0d0e21e 840
19799a22
GS
841The L<crypt> function is unsuitable for encrypting large quantities
842of data, not least of all because you can't get the information
843back. Look at the F<by-module/Crypt> and F<by-module/PGP> directories
844on your favorite CPAN mirror for a slew of potentially useful
845modules.
846
aa689395 847=item dbmclose HASH
a0d0e21e 848
19799a22 849[This function has been largely superseded by the C<untie> function.]
a0d0e21e 850
aa689395 851Breaks the binding between a DBM file and a hash.
a0d0e21e 852
19799a22 853=item dbmopen HASH,DBNAME,MASK
a0d0e21e 854
19799a22 855[This function has been largely superseded by the C<tie> function.]
a0d0e21e 856
7b8d334a 857This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
19799a22
GS
858hash. HASH is the name of the hash. (Unlike normal C<open>, the first
859argument is I<not> a filehandle, even though it looks like one). DBNAME
aa689395 860is the name of the database (without the F<.dir> or F<.pag> extension if
861any). If the database does not exist, it is created with protection
19799a22
GS
862specified by MASK (as modified by the C<umask>). If your system supports
863only the older DBM functions, you may perform only one C<dbmopen> in your
aa689395 864program. In older versions of Perl, if your system had neither DBM nor
19799a22 865ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
aa689395 866sdbm(3).
867
868If you don't have write access to the DBM file, you can only read hash
869variables, not set them. If you want to test whether you can write,
19799a22 870either use file tests or try setting a dummy hash entry inside an C<eval>,
aa689395 871which will trap the error.
a0d0e21e 872
19799a22
GS
873Note that functions such as C<keys> and C<values> may return huge lists
874when used on large DBM files. You may prefer to use the C<each>
a0d0e21e
LW
875function to iterate over large DBM files. Example:
876
877 # print out history file offsets
878 dbmopen(%HIST,'/usr/lib/news/history',0666);
879 while (($key,$val) = each %HIST) {
880 print $key, ' = ', unpack('L',$val), "\n";
881 }
882 dbmclose(%HIST);
883
cb1a09d0 884See also L<AnyDBM_File> for a more general description of the pros and
184e9718 885cons of the various dbm approaches, as well as L<DB_File> for a particularly
cb1a09d0 886rich implementation.
4633a7c4 887
2b5ab1e7
TC
888You can control which DBM library you use by loading that library
889before you call dbmopen():
890
891 use DB_File;
892 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
893 or die "Can't open netscape history file: $!";
894
a0d0e21e
LW
895=item defined EXPR
896
54310121 897=item defined
bbce6d69 898
2f9daede
TP
899Returns a Boolean value telling whether EXPR has a value other than
900the undefined value C<undef>. If EXPR is not present, C<$_> will be
901checked.
902
903Many operations return C<undef> to indicate failure, end of file,
904system error, uninitialized variable, and other exceptional
905conditions. This function allows you to distinguish C<undef> from
906other values. (A simple Boolean test will not distinguish among
7660c0ab 907C<undef>, zero, the empty string, and C<"0">, which are all equally
2f9daede 908false.) Note that since C<undef> is a valid scalar, its presence
19799a22 909doesn't I<necessarily> indicate an exceptional condition: C<pop>
2f9daede
TP
910returns C<undef> when its argument is an empty array, I<or> when the
911element to return happens to be C<undef>.
912
f10b0346
GS
913You may also use C<defined(&func)> to check whether subroutine C<&func>
914has ever been defined. The return value is unaffected by any forward
847c7ebe
DD
915declarations of C<&foo>. Note that a subroutine which is not defined
916may still be callable: its package may have an C<AUTOLOAD> method that
917makes it spring into existence the first time that it is called -- see
918L<perlsub>.
f10b0346
GS
919
920Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
921used to report whether memory for that aggregate has ever been
922allocated. This behavior may disappear in future versions of Perl.
923You should instead use a simple test for size:
924
925 if (@an_array) { print "has array elements\n" }
926 if (%a_hash) { print "has hash members\n" }
2f9daede
TP
927
928When used on a hash element, it tells you whether the value is defined,
dc848c6f 929not whether the key exists in the hash. Use L</exists> for the latter
2f9daede 930purpose.
a0d0e21e
LW
931
932Examples:
933
934 print if defined $switch{'D'};
935 print "$val\n" while defined($val = pop(@ary));
936 die "Can't readlink $sym: $!"
937 unless defined($value = readlink $sym);
a0d0e21e 938 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
2f9daede 939 $debugging = 0 unless defined $debugging;
a0d0e21e 940
19799a22 941Note: Many folks tend to overuse C<defined>, and then are surprised to
7660c0ab 942discover that the number C<0> and C<""> (the zero-length string) are, in fact,
2f9daede 943defined values. For example, if you say
a5f75d66
AD
944
945 "ab" =~ /a(.*)b/;
946
7660c0ab 947The pattern match succeeds, and C<$1> is defined, despite the fact that it
a5f75d66 948matched "nothing". But it didn't really match nothing--rather, it
2b5ab1e7 949matched something that happened to be zero characters long. This is all
a5f75d66 950very above-board and honest. When a function returns an undefined value,
2f9daede 951it's an admission that it couldn't give you an honest answer. So you
19799a22 952should use C<defined> only when you're questioning the integrity of what
7660c0ab 953you're trying to do. At other times, a simple comparison to C<0> or C<""> is
2f9daede
TP
954what you want.
955
dc848c6f 956See also L</undef>, L</exists>, L</ref>.
2f9daede 957
a0d0e21e
LW
958=item delete EXPR
959
01020589
GS
960Given an expression that specifies a hash element, array element, hash slice,
961or array slice, deletes the specified element(s) from the hash or array.
8216c1fd 962In the case of an array, if the array elements happen to be at the end,
b76cc8ba 963the size of the array will shrink to the highest element that tests
8216c1fd 964true for exists() (or 0 if no such element exists).
a0d0e21e 965
01020589
GS
966Returns each element so deleted or the undefined value if there was no such
967element. Deleting from C<$ENV{}> modifies the environment. Deleting from
968a hash tied to a DBM file deletes the entry from the DBM file. Deleting
969from a C<tie>d hash or array may not necessarily return anything.
970
8ea97a1e
GS
971Deleting an array element effectively returns that position of the array
972to its initial, uninitialized state. Subsequently testing for the same
8216c1fd
GS
973element with exists() will return false. Note that deleting array
974elements in the middle of an array will not shift the index of the ones
975after them down--use splice() for that. See L</exists>.
8ea97a1e 976
01020589 977The following (inefficiently) deletes all the values of %HASH and @ARRAY:
a0d0e21e 978
5f05dabc 979 foreach $key (keys %HASH) {
980 delete $HASH{$key};
a0d0e21e
LW
981 }
982
01020589
GS
983 foreach $index (0 .. $#ARRAY) {
984 delete $ARRAY[$index];
985 }
986
987And so do these:
5f05dabc 988
01020589
GS
989 delete @HASH{keys %HASH};
990
9740c838 991 delete @ARRAY[0 .. $#ARRAY];
5f05dabc 992
2b5ab1e7 993But both of these are slower than just assigning the empty list
01020589
GS
994or undefining %HASH or @ARRAY:
995
996 %HASH = (); # completely empty %HASH
997 undef %HASH; # forget %HASH ever existed
2b5ab1e7 998
01020589
GS
999 @ARRAY = (); # completely empty @ARRAY
1000 undef @ARRAY; # forget @ARRAY ever existed
2b5ab1e7
TC
1001
1002Note that the EXPR can be arbitrarily complicated as long as the final
01020589
GS
1003operation is a hash element, array element, hash slice, or array slice
1004lookup:
a0d0e21e
LW
1005
1006 delete $ref->[$x][$y]{$key};
5f05dabc 1007 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
a0d0e21e 1008
01020589
GS
1009 delete $ref->[$x][$y][$index];
1010 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1011
a0d0e21e
LW
1012=item die LIST
1013
19799a22
GS
1014Outside an C<eval>, prints the value of LIST to C<STDERR> and
1015exits with the current value of C<$!> (errno). If C<$!> is C<0>,
61eff3bc
JH
1016exits with the value of C<<< ($? >> 8) >>> (backtick `command`
1017status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside
19799a22
GS
1018an C<eval(),> the error message is stuffed into C<$@> and the
1019C<eval> is terminated with the undefined value. This makes
1020C<die> the way to raise an exception.
a0d0e21e
LW
1021
1022Equivalent examples:
1023
1024 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
54310121 1025 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
a0d0e21e
LW
1026
1027If the value of EXPR does not end in a newline, the current script line
1028number and input line number (if any) are also printed, and a newline
883faa13
GS
1029is supplied. Note that the "input line number" (also known as "chunk")
1030is subject to whatever notion of "line" happens to be currently in
1031effect, and is also available as the special variable C<$.>.
1032See L<perlvar/"$/"> and L<perlvar/"$.">.
1033
1034Hint: sometimes appending C<", stopped"> to your message
7660c0ab 1035will cause it to make better sense when the string C<"at foo line 123"> is
a0d0e21e
LW
1036appended. Suppose you are running script "canasta".
1037
1038 die "/etc/games is no good";
1039 die "/etc/games is no good, stopped";
1040
1041produce, respectively
1042
1043 /etc/games is no good at canasta line 123.
1044 /etc/games is no good, stopped at canasta line 123.
1045
2b5ab1e7 1046See also exit(), warn(), and the Carp module.
a0d0e21e 1047
7660c0ab
A
1048If LIST is empty and C<$@> already contains a value (typically from a
1049previous eval) that value is reused after appending C<"\t...propagated">.
fb73857a 1050This is useful for propagating exceptions:
1051
1052 eval { ... };
1053 die unless $@ =~ /Expected exception/;
1054
7660c0ab 1055If C<$@> is empty then the string C<"Died"> is used.
fb73857a 1056
52531d10
GS
1057die() can also be called with a reference argument. If this happens to be
1058trapped within an eval(), $@ contains the reference. This behavior permits
1059a more elaborate exception handling implementation using objects that
4375e838 1060maintain arbitrary state about the nature of the exception. Such a scheme
52531d10
GS
1061is sometimes preferable to matching particular string values of $@ using
1062regular expressions. Here's an example:
1063
1064 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1065 if ($@) {
1066 if (ref($@) && UNIVERSAL::isa($@,"Some::Module::Exception")) {
1067 # handle Some::Module::Exception
1068 }
1069 else {
1070 # handle all other possible exceptions
1071 }
1072 }
1073
19799a22 1074Because perl will stringify uncaught exception messages before displaying
52531d10
GS
1075them, you may want to overload stringification operations on such custom
1076exception objects. See L<overload> for details about that.
1077
19799a22
GS
1078You can arrange for a callback to be run just before the C<die>
1079does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1080handler will be called with the error text and can change the error
1081message, if it sees fit, by calling C<die> again. See
1082L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
1083L<"eval BLOCK"> for some examples. Although this feature was meant
1084to be run only right before your program was to exit, this is not
1085currently the case--the C<$SIG{__DIE__}> hook is currently called
1086even inside eval()ed blocks/strings! If one wants the hook to do
1087nothing in such situations, put
fb73857a 1088
1089 die @_ if $^S;
1090
19799a22
GS
1091as the first line of the handler (see L<perlvar/$^S>). Because
1092this promotes strange action at a distance, this counterintuitive
b76cc8ba 1093behavior may be fixed in a future release.
774d564b 1094
a0d0e21e
LW
1095=item do BLOCK
1096
1097Not really a function. Returns the value of the last command in the
1098sequence of commands indicated by BLOCK. When modified by a loop
98293880
JH
1099modifier, executes the BLOCK once before testing the loop condition.
1100(On other statements the loop modifiers test the conditional first.)
a0d0e21e 1101
4968c1e4 1102C<do BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7
TC
1103C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1104See L<perlsyn> for alternative strategies.
4968c1e4 1105
a0d0e21e
LW
1106=item do SUBROUTINE(LIST)
1107
1108A deprecated form of subroutine call. See L<perlsub>.
1109
1110=item do EXPR
1111
1112Uses the value of EXPR as a filename and executes the contents of the
1113file as a Perl script. Its primary use is to include subroutines
1114from a Perl subroutine library.
1115
1116 do 'stat.pl';
1117
1118is just like
1119
986b19de 1120 eval `cat stat.pl`;
a0d0e21e 1121
2b5ab1e7
TC
1122except that it's more efficient and concise, keeps track of the current
1123filename for error messages, searches the @INC libraries, and updates
1124C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1125variables. It also differs in that code evaluated with C<do FILENAME>
1126cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1127same, however, in that it does reparse the file every time you call it,
1128so you probably don't want to do this inside a loop.
a0d0e21e 1129
8e30cc93 1130If C<do> cannot read the file, it returns undef and sets C<$!> to the
2b5ab1e7 1131error. If C<do> can read the file but cannot compile it, it
8e30cc93
MG
1132returns undef and sets an error message in C<$@>. If the file is
1133successfully compiled, C<do> returns the value of the last expression
1134evaluated.
1135
a0d0e21e 1136Note that inclusion of library modules is better done with the
19799a22 1137C<use> and C<require> operators, which also do automatic error checking
4633a7c4 1138and raise an exception if there's a problem.
a0d0e21e 1139
5a964f20
TC
1140You might like to use C<do> to read in a program configuration
1141file. Manual error checking can be done this way:
1142
b76cc8ba 1143 # read in config files: system first, then user
f86cebdf 1144 for $file ("/share/prog/defaults.rc",
b76cc8ba 1145 "$ENV{HOME}/.someprogrc")
2b5ab1e7 1146 {
5a964f20 1147 unless ($return = do $file) {
f86cebdf
GS
1148 warn "couldn't parse $file: $@" if $@;
1149 warn "couldn't do $file: $!" unless defined $return;
1150 warn "couldn't run $file" unless $return;
5a964f20
TC
1151 }
1152 }
1153
a0d0e21e
LW
1154=item dump LABEL
1155
1614b0e3
JD
1156=item dump
1157
19799a22
GS
1158This function causes an immediate core dump. See also the B<-u>
1159command-line switch in L<perlrun>, which does the same thing.
1160Primarily this is so that you can use the B<undump> program (not
1161supplied) to turn your core dump into an executable binary after
1162having initialized all your variables at the beginning of the
1163program. When the new binary is executed it will begin by executing
1164a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1165Think of it as a goto with an intervening core dump and reincarnation.
1166If C<LABEL> is omitted, restarts the program from the top.
1167
1168B<WARNING>: Any files opened at the time of the dump will I<not>
1169be open any more when the program is reincarnated, with possible
b76cc8ba 1170resulting confusion on the part of Perl.
19799a22
GS
1171
1172This function is now largely obsolete, partly because it's very
1173hard to convert a core file into an executable, and because the
1174real compiler backends for generating portable bytecode and compilable
1175C code have superseded it.
1176
1177If you're looking to use L<dump> to speed up your program, consider
1178generating bytecode or native C code as described in L<perlcc>. If
1179you're just trying to accelerate a CGI script, consider using the
1180C<mod_perl> extension to B<Apache>, or the CPAN module, Fast::CGI.
1181You might also consider autoloading or selfloading, which at least
b76cc8ba 1182make your program I<appear> to run faster.
5a964f20 1183
aa689395 1184=item each HASH
1185
5a964f20 1186When called in list context, returns a 2-element list consisting of the
aa689395 1187key and value for the next element of a hash, so that you can iterate over
74fc8b5f 1188it. When called in scalar context, returns only the key for the next
e902a979 1189element in the hash.
2f9daede 1190
ab192400
GS
1191Entries are returned in an apparently random order. The actual random
1192order is subject to change in future versions of perl, but it is guaranteed
19799a22 1193to be in the same order as either the C<keys> or C<values> function
ab192400
GS
1194would produce on the same (unmodified) hash.
1195
1196When the hash is entirely read, a null array is returned in list context
19799a22
GS
1197(which when assigned produces a false (C<0>) value), and C<undef> in
1198scalar context. The next call to C<each> after that will start iterating
1199again. There is a single iterator for each hash, shared by all C<each>,
1200C<keys>, and C<values> function calls in the program; it can be reset by
2f9daede
TP
1201reading all the elements from the hash, or by evaluating C<keys HASH> or
1202C<values HASH>. If you add or delete elements of a hash while you're
74fc8b5f
MJD
1203iterating over it, you may get entries skipped or duplicated, so
1204don't. Exception: It is always safe to delete the item most recently
1205returned by C<each()>, which means that the following code will work:
1206
1207 while (($key, $value) = each %hash) {
1208 print $key, "\n";
1209 delete $hash{$key}; # This is safe
1210 }
aa689395 1211
f86cebdf 1212The following prints out your environment like the printenv(1) program,
aa689395 1213only in a different order:
a0d0e21e
LW
1214
1215 while (($key,$value) = each %ENV) {
1216 print "$key=$value\n";
1217 }
1218
19799a22 1219See also C<keys>, C<values> and C<sort>.
a0d0e21e
LW
1220
1221=item eof FILEHANDLE
1222
4633a7c4
LW
1223=item eof ()
1224
a0d0e21e
LW
1225=item eof
1226
1227Returns 1 if the next read on FILEHANDLE will return end of file, or if
1228FILEHANDLE is not open. FILEHANDLE may be an expression whose value
5a964f20 1229gives the real filehandle. (Note that this function actually
19799a22 1230reads a character and then C<ungetc>s it, so isn't very useful in an
748a9306 1231interactive context.) Do not read from a terminal file (or call
19799a22 1232C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
748a9306
LW
1233as terminals may lose the end-of-file condition if you do.
1234
820475bd
GS
1235An C<eof> without an argument uses the last file read. Using C<eof()>
1236with empty parentheses is very different. It refers to the pseudo file
1237formed from the files listed on the command line and accessed via the
61eff3bc
JH
1238C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1239as a normal filehandle is, an C<eof()> before C<< <> >> has been
820475bd
GS
1240used will cause C<@ARGV> to be examined to determine if input is
1241available.
1242
61eff3bc 1243In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
820475bd
GS
1244detect the end of each file, C<eof()> will only detect the end of the
1245last file. Examples:
a0d0e21e 1246
748a9306
LW
1247 # reset line numbering on each input file
1248 while (<>) {
b76cc8ba 1249 next if /^\s*#/; # skip comments
748a9306 1250 print "$.\t$_";
5a964f20
TC
1251 } continue {
1252 close ARGV if eof; # Not eof()!
748a9306
LW
1253 }
1254
a0d0e21e
LW
1255 # insert dashes just before last line of last file
1256 while (<>) {
5a964f20 1257 if (eof()) { # check for end of current file
a0d0e21e 1258 print "--------------\n";
2b5ab1e7 1259 close(ARGV); # close or last; is needed if we
748a9306 1260 # are reading from the terminal
a0d0e21e
LW
1261 }
1262 print;
1263 }
1264
a0d0e21e 1265Practical hint: you almost never need to use C<eof> in Perl, because the
3ce0d271
GS
1266input operators typically return C<undef> when they run out of data, or if
1267there was an error.
a0d0e21e
LW
1268
1269=item eval EXPR
1270
1271=item eval BLOCK
1272
c7cc6f1c
GS
1273In the first form, the return value of EXPR is parsed and executed as if it
1274were a little Perl program. The value of the expression (which is itself
5a964f20 1275determined within scalar context) is first parsed, and if there weren't any
be3174d2
GS
1276errors, executed in the lexical context of the current Perl program, so
1277that any variable settings or subroutine and format definitions remain
1278afterwards. Note that the value is parsed every time the eval executes.
1279If EXPR is omitted, evaluates C<$_>. This form is typically used to
1280delay parsing and subsequent execution of the text of EXPR until run time.
c7cc6f1c
GS
1281
1282In the second form, the code within the BLOCK is parsed only once--at the
1283same time the code surrounding the eval itself was parsed--and executed
1284within the context of the current Perl program. This form is typically
1285used to trap exceptions more efficiently than the first (see below), while
1286also providing the benefit of checking the code within BLOCK at compile
1287time.
1288
1289The final semicolon, if any, may be omitted from the value of EXPR or within
1290the BLOCK.
1291
1292In both forms, the value returned is the value of the last expression
5a964f20 1293evaluated inside the mini-program; a return statement may be also used, just
c7cc6f1c 1294as with subroutines. The expression providing the return value is evaluated
5a964f20 1295in void, scalar, or list context, depending on the context of the eval itself.
c7cc6f1c 1296See L</wantarray> for more on how the evaluation context can be determined.
a0d0e21e 1297
19799a22
GS
1298If there is a syntax error or runtime error, or a C<die> statement is
1299executed, an undefined value is returned by C<eval>, and C<$@> is set to the
a0d0e21e 1300error message. If there was no error, C<$@> is guaranteed to be a null
19799a22 1301string. Beware that using C<eval> neither silences perl from printing
c7cc6f1c
GS
1302warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1303To do either of those, you have to use the C<$SIG{__WARN__}> facility. See
1304L</warn> and L<perlvar>.
a0d0e21e 1305
19799a22
GS
1306Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1307determining whether a particular feature (such as C<socket> or C<symlink>)
a0d0e21e
LW
1308is implemented. It is also Perl's exception trapping mechanism, where
1309the die operator is used to raise exceptions.
1310
1311If the code to be executed doesn't vary, you may use the eval-BLOCK
1312form to trap run-time errors without incurring the penalty of
1313recompiling each time. The error, if any, is still returned in C<$@>.
1314Examples:
1315
54310121 1316 # make divide-by-zero nonfatal
a0d0e21e
LW
1317 eval { $answer = $a / $b; }; warn $@ if $@;
1318
1319 # same thing, but less efficient
1320 eval '$answer = $a / $b'; warn $@ if $@;
1321
1322 # a compile-time error
5a964f20 1323 eval { $answer = }; # WRONG
a0d0e21e
LW
1324
1325 # a run-time error
1326 eval '$answer ='; # sets $@
1327
2b5ab1e7
TC
1328Due to the current arguably broken state of C<__DIE__> hooks, when using
1329the C<eval{}> form as an exception trap in libraries, you may wish not
1330to trigger any C<__DIE__> hooks that user code may have installed.
1331You can use the C<local $SIG{__DIE__}> construct for this purpose,
1332as shown in this example:
774d564b 1333
1334 # a very private exception trap for divide-by-zero
f86cebdf
GS
1335 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1336 warn $@ if $@;
774d564b 1337
1338This is especially significant, given that C<__DIE__> hooks can call
19799a22 1339C<die> again, which has the effect of changing their error messages:
774d564b 1340
1341 # __DIE__ hooks may modify error messages
1342 {
f86cebdf
GS
1343 local $SIG{'__DIE__'} =
1344 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
c7cc6f1c
GS
1345 eval { die "foo lives here" };
1346 print $@ if $@; # prints "bar lives here"
774d564b 1347 }
1348
19799a22 1349Because this promotes action at a distance, this counterintuitive behavior
2b5ab1e7
TC
1350may be fixed in a future release.
1351
19799a22 1352With an C<eval>, you should be especially careful to remember what's
a0d0e21e
LW
1353being looked at when:
1354
1355 eval $x; # CASE 1
1356 eval "$x"; # CASE 2
1357
1358 eval '$x'; # CASE 3
1359 eval { $x }; # CASE 4
1360
5a964f20 1361 eval "\$$x++"; # CASE 5
a0d0e21e
LW
1362 $$x++; # CASE 6
1363
2f9daede 1364Cases 1 and 2 above behave identically: they run the code contained in
19799a22 1365the variable $x. (Although case 2 has misleading double quotes making
2f9daede 1366the reader wonder what else might be happening (nothing is).) Cases 3
7660c0ab 1367and 4 likewise behave in the same way: they run the code C<'$x'>, which
19799a22 1368does nothing but return the value of $x. (Case 4 is preferred for
2f9daede
TP
1369purely visual reasons, but it also has the advantage of compiling at
1370compile-time instead of at run-time.) Case 5 is a place where
19799a22 1371normally you I<would> like to use double quotes, except that in this
2f9daede
TP
1372particular situation, you can just use symbolic references instead, as
1373in case 6.
a0d0e21e 1374
4968c1e4 1375C<eval BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7 1376C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
4968c1e4 1377
a0d0e21e
LW
1378=item exec LIST
1379
8bf3b016
GS
1380=item exec PROGRAM LIST
1381
19799a22
GS
1382The C<exec> function executes a system command I<and never returns>--
1383use C<system> instead of C<exec> if you want it to return. It fails and
1384returns false only if the command does not exist I<and> it is executed
fb73857a 1385directly instead of via your system's command shell (see below).
a0d0e21e 1386
19799a22
GS
1387Since it's a common mistake to use C<exec> instead of C<system>, Perl
1388warns you if there is a following statement which isn't C<die>, C<warn>,
1389or C<exit> (if C<-w> is set - but you always do that). If you
1390I<really> want to follow an C<exec> with some other statement, you
55d729e4
GS
1391can use one of these styles to avoid the warning:
1392
5a964f20
TC
1393 exec ('foo') or print STDERR "couldn't exec foo: $!";
1394 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
55d729e4 1395
5a964f20 1396If there is more than one argument in LIST, or if LIST is an array
f86cebdf 1397with more than one value, calls execvp(3) with the arguments in LIST.
5a964f20
TC
1398If there is only one scalar argument or an array with one element in it,
1399the argument is checked for shell metacharacters, and if there are any,
1400the entire argument is passed to the system's command shell for parsing
1401(this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1402If there are no shell metacharacters in the argument, it is split into
b76cc8ba 1403words and passed directly to C<execvp>, which is more efficient.
19799a22 1404Examples:
a0d0e21e 1405
19799a22
GS
1406 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1407 exec "sort $outfile | uniq";
a0d0e21e
LW
1408
1409If you don't really want to execute the first argument, but want to lie
1410to the program you are executing about its own name, you can specify
1411the program you actually want to run as an "indirect object" (without a
1412comma) in front of the LIST. (This always forces interpretation of the
54310121 1413LIST as a multivalued list, even if there is only a single scalar in
a0d0e21e
LW
1414the list.) Example:
1415
1416 $shell = '/bin/csh';
1417 exec $shell '-sh'; # pretend it's a login shell
1418
1419or, more directly,
1420
1421 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1422
bb32b41a
GS
1423When the arguments get executed via the system shell, results will
1424be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1425for details.
1426
19799a22
GS
1427Using an indirect object with C<exec> or C<system> is also more
1428secure. This usage (which also works fine with system()) forces
1429interpretation of the arguments as a multivalued list, even if the
1430list had just one argument. That way you're safe from the shell
1431expanding wildcards or splitting up words with whitespace in them.
5a964f20
TC
1432
1433 @args = ( "echo surprise" );
1434
2b5ab1e7 1435 exec @args; # subject to shell escapes
f86cebdf 1436 # if @args == 1
2b5ab1e7 1437 exec { $args[0] } @args; # safe even with one-arg list
5a964f20
TC
1438
1439The first version, the one without the indirect object, ran the I<echo>
1440program, passing it C<"surprise"> an argument. The second version
1441didn't--it tried to run a program literally called I<"echo surprise">,
1442didn't find it, and set C<$?> to a non-zero value indicating failure.
1443
0f897271
GS
1444Beginning with v5.6.0, Perl will attempt to flush all files opened for
1445output before the exec, but this may not be supported on some platforms
1446(see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1447in English) or call the C<autoflush()> method of C<IO::Handle> on any
1448open handles in order to avoid lost output.
1449
19799a22 1450Note that C<exec> will not call your C<END> blocks, nor will it call
7660c0ab
A
1451any C<DESTROY> methods in your objects.
1452
a0d0e21e
LW
1453=item exists EXPR
1454
01020589 1455Given an expression that specifies a hash element or array element,
8ea97a1e
GS
1456returns true if the specified element in the hash or array has ever
1457been initialized, even if the corresponding value is undefined. The
1458element is not autovivified if it doesn't exist.
a0d0e21e 1459
01020589
GS
1460 print "Exists\n" if exists $hash{$key};
1461 print "Defined\n" if defined $hash{$key};
1462 print "True\n" if $hash{$key};
1463
1464 print "Exists\n" if exists $array[$index];
1465 print "Defined\n" if defined $array[$index];
1466 print "True\n" if $array[$index];
a0d0e21e 1467
8ea97a1e 1468A hash or array element can be true only if it's defined, and defined if
a0d0e21e
LW
1469it exists, but the reverse doesn't necessarily hold true.
1470
afebc493
GS
1471Given an expression that specifies the name of a subroutine,
1472returns true if the specified subroutine has ever been declared, even
1473if it is undefined. Mentioning a subroutine name for exists or defined
847c7ebe
DD
1474does not count as declaring it. Note that a subroutine which does not
1475exist may still be callable: its package may have an C<AUTOLOAD>
1476method that makes it spring into existence the first time that it is
1477called -- see L<perlsub>.
afebc493
GS
1478
1479 print "Exists\n" if exists &subroutine;
1480 print "Defined\n" if defined &subroutine;
1481
a0d0e21e 1482Note that the EXPR can be arbitrarily complicated as long as the final
afebc493 1483operation is a hash or array key lookup or subroutine name:
a0d0e21e 1484
2b5ab1e7
TC
1485 if (exists $ref->{A}->{B}->{$key}) { }
1486 if (exists $hash{A}{B}{$key}) { }
1487
01020589
GS
1488 if (exists $ref->{A}->{B}->[$ix]) { }
1489 if (exists $hash{A}{B}[$ix]) { }
1490
afebc493
GS
1491 if (exists &{$ref->{A}{B}{$key}}) { }
1492
01020589
GS
1493Although the deepest nested array or hash will not spring into existence
1494just because its existence was tested, any intervening ones will.
61eff3bc 1495Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
01020589
GS
1496into existence due to the existence test for the $key element above.
1497This happens anywhere the arrow operator is used, including even:
5a964f20 1498
2b5ab1e7
TC
1499 undef $ref;
1500 if (exists $ref->{"Some key"}) { }
1501 print $ref; # prints HASH(0x80d3d5c)
1502
1503This surprising autovivification in what does not at first--or even
1504second--glance appear to be an lvalue context may be fixed in a future
5a964f20 1505release.
a0d0e21e 1506
479ba383
GS
1507See L<perlref/"Pseudo-hashes: Using an array as a hash"> for specifics
1508on how exists() acts when used on a pseudo-hash.
e0478e5a 1509
afebc493
GS
1510Use of a subroutine call, rather than a subroutine name, as an argument
1511to exists() is an error.
1512
1513 exists &sub; # OK
1514 exists &sub(); # Error
1515
a0d0e21e
LW
1516=item exit EXPR
1517
2b5ab1e7 1518Evaluates EXPR and exits immediately with that value. Example:
a0d0e21e
LW
1519
1520 $ans = <STDIN>;
1521 exit 0 if $ans =~ /^[Xx]/;
1522
19799a22 1523See also C<die>. If EXPR is omitted, exits with C<0> status. The only
2b5ab1e7
TC
1524universally recognized values for EXPR are C<0> for success and C<1>
1525for error; other values are subject to interpretation depending on the
1526environment in which the Perl program is running. For example, exiting
152769 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1528the mailer to return the item undelivered, but that's not true everywhere.
a0d0e21e 1529
19799a22
GS
1530Don't use C<exit> to abort a subroutine if there's any chance that
1531someone might want to trap whatever error happened. Use C<die> instead,
1532which can be trapped by an C<eval>.
28757baa 1533
19799a22 1534The exit() function does not always exit immediately. It calls any
2b5ab1e7 1535defined C<END> routines first, but these C<END> routines may not
19799a22 1536themselves abort the exit. Likewise any object destructors that need to
2b5ab1e7
TC
1537be called are called before the real exit. If this is a problem, you
1538can call C<POSIX:_exit($status)> to avoid END and destructor processing.
87275199 1539See L<perlmod> for details.
5a964f20 1540
a0d0e21e
LW
1541=item exp EXPR
1542
54310121 1543=item exp
bbce6d69 1544
b76cc8ba 1545Returns I<e> (the natural logarithm base) to the power of EXPR.
a0d0e21e
LW
1546If EXPR is omitted, gives C<exp($_)>.
1547
1548=item fcntl FILEHANDLE,FUNCTION,SCALAR
1549
f86cebdf 1550Implements the fcntl(2) function. You'll probably have to say
a0d0e21e
LW
1551
1552 use Fcntl;
1553
0ade1984 1554first to get the correct constant definitions. Argument processing and
b76cc8ba 1555value return works just like C<ioctl> below.
a0d0e21e
LW
1556For example:
1557
1558 use Fcntl;
5a964f20
TC
1559 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1560 or die "can't fcntl F_GETFL: $!";
1561
19799a22 1562You don't have to check for C<defined> on the return from C<fnctl>.
951ba7fe
GS
1563Like C<ioctl>, it maps a C<0> return from the system call into
1564C<"0 but true"> in Perl. This string is true in boolean context and C<0>
2b5ab1e7
TC
1565in numeric context. It is also exempt from the normal B<-w> warnings
1566on improper numeric conversions.
5a964f20 1567
19799a22 1568Note that C<fcntl> will produce a fatal error if used on a machine that
2b5ab1e7
TC
1569doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1570manpage to learn what functions are available on your system.
a0d0e21e
LW
1571
1572=item fileno FILEHANDLE
1573
2b5ab1e7
TC
1574Returns the file descriptor for a filehandle, or undefined if the
1575filehandle is not open. This is mainly useful for constructing
19799a22 1576bitmaps for C<select> and low-level POSIX tty-handling operations.
2b5ab1e7
TC
1577If FILEHANDLE is an expression, the value is taken as an indirect
1578filehandle, generally its name.
5a964f20 1579
b76cc8ba 1580You can use this to find out whether two handles refer to the
5a964f20
TC
1581same underlying descriptor:
1582
1583 if (fileno(THIS) == fileno(THAT)) {
1584 print "THIS and THAT are dups\n";
b76cc8ba
NIS
1585 }
1586
1587(Filehandles connected to memory objects via new features of C<open> may
1588return undefined even though they are open.)
1589
a0d0e21e
LW
1590
1591=item flock FILEHANDLE,OPERATION
1592
19799a22
GS
1593Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1594for success, false on failure. Produces a fatal error if used on a
2b5ab1e7 1595machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
19799a22 1596C<flock> is Perl's portable file locking interface, although it locks
2b5ab1e7
TC
1597only entire files, not records.
1598
1599Two potentially non-obvious but traditional C<flock> semantics are
1600that it waits indefinitely until the lock is granted, and that its locks
1601B<merely advisory>. Such discretionary locks are more flexible, but offer
19799a22
GS
1602fewer guarantees. This means that files locked with C<flock> may be
1603modified by programs that do not also use C<flock>. See L<perlport>,
2b5ab1e7
TC
1604your port's specific documentation, or your system-specific local manpages
1605for details. It's best to assume traditional behavior if you're writing
1606portable programs. (But if you're not, you should as always feel perfectly
1607free to write for your own system's idiosyncrasies (sometimes called
1608"features"). Slavish adherence to portability concerns shouldn't get
1609in the way of your getting your job done.)
a3cb178b 1610
8ebc5c01 1611OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1612LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
ea3105be 1613you can use the symbolic names if you import them from the Fcntl module,
68dc0745 1614either individually, or as a group using the ':flock' tag. LOCK_SH
1615requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
ea3105be
GS
1616releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1617LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
68dc0745 1618waiting for the lock (check the return status to see if you got it).
1619
2b5ab1e7
TC
1620To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1621before locking or unlocking it.
8ebc5c01 1622
f86cebdf 1623Note that the emulation built with lockf(3) doesn't provide shared
8ebc5c01 1624locks, and it requires that FILEHANDLE be open with write intent. These
2b5ab1e7 1625are the semantics that lockf(3) implements. Most if not all systems
f86cebdf 1626implement lockf(3) in terms of fcntl(2) locking, though, so the
8ebc5c01 1627differing semantics shouldn't bite too many people.
1628
19799a22
GS
1629Note also that some versions of C<flock> cannot lock things over the
1630network; you would need to use the more system-specific C<fcntl> for
f86cebdf
GS
1631that. If you like you can force Perl to ignore your system's flock(2)
1632function, and so provide its own fcntl(2)-based emulation, by passing
8ebc5c01 1633the switch C<-Ud_flock> to the F<Configure> program when you configure
1634perl.
4633a7c4
LW
1635
1636Here's a mailbox appender for BSD systems.
a0d0e21e 1637
7e1af8bc 1638 use Fcntl ':flock'; # import LOCK_* constants
a0d0e21e
LW
1639
1640 sub lock {
7e1af8bc 1641 flock(MBOX,LOCK_EX);
a0d0e21e
LW
1642 # and, in case someone appended
1643 # while we were waiting...
1644 seek(MBOX, 0, 2);
1645 }
1646
1647 sub unlock {
7e1af8bc 1648 flock(MBOX,LOCK_UN);
a0d0e21e
LW
1649 }
1650
1651 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1652 or die "Can't open mailbox: $!";
1653
1654 lock();
1655 print MBOX $msg,"\n\n";
1656 unlock();
1657
2b5ab1e7
TC
1658On systems that support a real flock(), locks are inherited across fork()
1659calls, whereas those that must resort to the more capricious fcntl()
1660function lose the locks, making it harder to write servers.
1661
cb1a09d0 1662See also L<DB_File> for other flock() examples.
a0d0e21e
LW
1663
1664=item fork
1665
2b5ab1e7
TC
1666Does a fork(2) system call to create a new process running the
1667same program at the same point. It returns the child pid to the
1668parent process, C<0> to the child process, or C<undef> if the fork is
1669unsuccessful. File descriptors (and sometimes locks on those descriptors)
1670are shared, while everything else is copied. On most systems supporting
1671fork(), great care has gone into making it extremely efficient (for
1672example, using copy-on-write technology on data pages), making it the
1673dominant paradigm for multitasking over the last few decades.
5a964f20 1674
0f897271
GS
1675Beginning with v5.6.0, Perl will attempt to flush all files opened for
1676output before forking the child process, but this may not be supported
1677on some platforms (see L<perlport>). To be safe, you may need to set
1678C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1679C<IO::Handle> on any open handles in order to avoid duplicate output.
a0d0e21e 1680
19799a22 1681If you C<fork> without ever waiting on your children, you will
2b5ab1e7
TC
1682accumulate zombies. On some systems, you can avoid this by setting
1683C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1684forking and reaping moribund children.
cb1a09d0 1685
28757baa 1686Note that if your forked child inherits system file descriptors like
1687STDIN and STDOUT that are actually connected by a pipe or socket, even
2b5ab1e7 1688if you exit, then the remote server (such as, say, a CGI script or a
19799a22 1689backgrounded job launched from a remote shell) won't think you're done.
2b5ab1e7 1690You should reopen those to F</dev/null> if it's any issue.
28757baa 1691
cb1a09d0
AD
1692=item format
1693
19799a22 1694Declare a picture format for use by the C<write> function. For
cb1a09d0
AD
1695example:
1696
54310121 1697 format Something =
cb1a09d0
AD
1698 Test: @<<<<<<<< @||||| @>>>>>
1699 $str, $%, '$' . int($num)
1700 .
1701
1702 $str = "widget";
184e9718 1703 $num = $cost/$quantity;
cb1a09d0
AD
1704 $~ = 'Something';
1705 write;
1706
1707See L<perlform> for many details and examples.
1708
8903cb82 1709=item formline PICTURE,LIST
a0d0e21e 1710
5a964f20 1711This is an internal function used by C<format>s, though you may call it,
a0d0e21e
LW
1712too. It formats (see L<perlform>) a list of values according to the
1713contents of PICTURE, placing the output into the format output
7660c0ab 1714accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
19799a22 1715Eventually, when a C<write> is done, the contents of
a0d0e21e 1716C<$^A> are written to some filehandle, but you could also read C<$^A>
7660c0ab 1717yourself and then set C<$^A> back to C<"">. Note that a format typically
19799a22 1718does one C<formline> per line of form, but the C<formline> function itself
748a9306 1719doesn't care how many newlines are embedded in the PICTURE. This means
4633a7c4 1720that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
748a9306
LW
1721You may therefore need to use multiple formlines to implement a single
1722record format, just like the format compiler.
1723
19799a22 1724Be careful if you put double quotes around the picture, because an C<@>
748a9306 1725character may be taken to mean the beginning of an array name.
19799a22 1726C<formline> always returns true. See L<perlform> for other examples.
a0d0e21e
LW
1727
1728=item getc FILEHANDLE
1729
1730=item getc
1731
1732Returns the next character from the input file attached to FILEHANDLE,
2b5ab1e7
TC
1733or the undefined value at end of file, or if there was an error.
1734If FILEHANDLE is omitted, reads from STDIN. This is not particularly
1735efficient. However, it cannot be used by itself to fetch single
1736characters without waiting for the user to hit enter. For that, try
1737something more like:
4633a7c4
LW
1738
1739 if ($BSD_STYLE) {
1740 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1741 }
1742 else {
54310121 1743 system "stty", '-icanon', 'eol', "\001";
4633a7c4
LW
1744 }
1745
1746 $key = getc(STDIN);
1747
1748 if ($BSD_STYLE) {
1749 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1750 }
1751 else {
5f05dabc 1752 system "stty", 'icanon', 'eol', '^@'; # ASCII null
4633a7c4
LW
1753 }
1754 print "\n";
1755
54310121 1756Determination of whether $BSD_STYLE should be set
1757is left as an exercise to the reader.
cb1a09d0 1758
19799a22 1759The C<POSIX::getattr> function can do this more portably on
2b5ab1e7
TC
1760systems purporting POSIX compliance. See also the C<Term::ReadKey>
1761module from your nearest CPAN site; details on CPAN can be found on
1762L<perlmodlib/CPAN>.
a0d0e21e
LW
1763
1764=item getlogin
1765
5a964f20
TC
1766Implements the C library function of the same name, which on most
1767systems returns the current login from F</etc/utmp>, if any. If null,
19799a22 1768use C<getpwuid>.
a0d0e21e 1769
f86702cc 1770 $login = getlogin || getpwuid($<) || "Kilroy";
a0d0e21e 1771
19799a22
GS
1772Do not consider C<getlogin> for authentication: it is not as
1773secure as C<getpwuid>.
4633a7c4 1774
a0d0e21e
LW
1775=item getpeername SOCKET
1776
1777Returns the packed sockaddr address of other end of the SOCKET connection.
1778
4633a7c4
LW
1779 use Socket;
1780 $hersockaddr = getpeername(SOCK);
19799a22 1781 ($port, $iaddr) = sockaddr_in($hersockaddr);
4633a7c4
LW
1782 $herhostname = gethostbyaddr($iaddr, AF_INET);
1783 $herstraddr = inet_ntoa($iaddr);
a0d0e21e
LW
1784
1785=item getpgrp PID
1786
47e29363 1787Returns the current process group for the specified PID. Use
7660c0ab 1788a PID of C<0> to get the current process group for the
4633a7c4 1789current process. Will raise an exception if used on a machine that
f86cebdf 1790doesn't implement getpgrp(2). If PID is omitted, returns process
19799a22 1791group of current process. Note that the POSIX version of C<getpgrp>
7660c0ab 1792does not accept a PID argument, so only C<PID==0> is truly portable.
a0d0e21e
LW
1793
1794=item getppid
1795
1796Returns the process id of the parent process.
1797
1798=item getpriority WHICH,WHO
1799
4633a7c4
LW
1800Returns the current priority for a process, a process group, or a user.
1801(See L<getpriority(2)>.) Will raise a fatal exception if used on a
f86cebdf 1802machine that doesn't implement getpriority(2).
a0d0e21e
LW
1803
1804=item getpwnam NAME
1805
1806=item getgrnam NAME
1807
1808=item gethostbyname NAME
1809
1810=item getnetbyname NAME
1811
1812=item getprotobyname NAME
1813
1814=item getpwuid UID
1815
1816=item getgrgid GID
1817
1818=item getservbyname NAME,PROTO
1819
1820=item gethostbyaddr ADDR,ADDRTYPE
1821
1822=item getnetbyaddr ADDR,ADDRTYPE
1823
1824=item getprotobynumber NUMBER
1825
1826=item getservbyport PORT,PROTO
1827
1828=item getpwent
1829
1830=item getgrent
1831
1832=item gethostent
1833
1834=item getnetent
1835
1836=item getprotoent
1837
1838=item getservent
1839
1840=item setpwent
1841
1842=item setgrent
1843
1844=item sethostent STAYOPEN
1845
1846=item setnetent STAYOPEN
1847
1848=item setprotoent STAYOPEN
1849
1850=item setservent STAYOPEN
1851
1852=item endpwent
1853
1854=item endgrent
1855
1856=item endhostent
1857
1858=item endnetent
1859
1860=item endprotoent
1861
1862=item endservent
1863
1864These routines perform the same functions as their counterparts in the
5a964f20 1865system library. In list context, the return values from the
a0d0e21e
LW
1866various get routines are as follows:
1867
1868 ($name,$passwd,$uid,$gid,
6ee623d5 1869 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
a0d0e21e
LW
1870 ($name,$passwd,$gid,$members) = getgr*
1871 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1872 ($name,$aliases,$addrtype,$net) = getnet*
1873 ($name,$aliases,$proto) = getproto*
1874 ($name,$aliases,$port,$proto) = getserv*
1875
1876(If the entry doesn't exist you get a null list.)
1877
4602f195
JH
1878The exact meaning of the $gcos field varies but it usually contains
1879the real name of the user (as opposed to the login name) and other
1880information pertaining to the user. Beware, however, that in many
1881system users are able to change this information and therefore it
106325ad 1882cannot be trusted and therefore the $gcos is tainted (see
2959b6e3
JH
1883L<perlsec>). The $passwd and $shell, user's encrypted password and
1884login shell, are also tainted, because of the same reason.
4602f195 1885
5a964f20 1886In scalar context, you get the name, unless the function was a
a0d0e21e
LW
1887lookup by name, in which case you get the other thing, whatever it is.
1888(If the entry doesn't exist you get the undefined value.) For example:
1889
5a964f20
TC
1890 $uid = getpwnam($name);
1891 $name = getpwuid($num);
1892 $name = getpwent();
1893 $gid = getgrnam($name);
1894 $name = getgrgid($num;
1895 $name = getgrent();
1896 #etc.
a0d0e21e 1897
4602f195
JH
1898In I<getpw*()> the fields $quota, $comment, and $expire are special
1899cases in the sense that in many systems they are unsupported. If the
1900$quota is unsupported, it is an empty scalar. If it is supported, it
1901usually encodes the disk quota. If the $comment field is unsupported,
1902it is an empty scalar. If it is supported it usually encodes some
1903administrative comment about the user. In some systems the $quota
1904field may be $change or $age, fields that have to do with password
1905aging. In some systems the $comment field may be $class. The $expire
1906field, if present, encodes the expiration period of the account or the
1907password. For the availability and the exact meaning of these fields
1908in your system, please consult your getpwnam(3) documentation and your
1909F<pwd.h> file. You can also find out from within Perl what your
1910$quota and $comment fields mean and whether you have the $expire field
1911by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
1912C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
1913files are only supported if your vendor has implemented them in the
1914intuitive fashion that calling the regular C library routines gets the
5d3a0a3b
GS
1915shadow versions if you're running under privilege or if there exists
1916the shadow(3) functions as found in System V ( this includes Solaris
1917and Linux.) Those systems which implement a proprietary shadow password
1918facility are unlikely to be supported.
6ee623d5 1919
19799a22 1920The $members value returned by I<getgr*()> is a space separated list of
a0d0e21e
LW
1921the login names of the members of the group.
1922
1923For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1924C, it will be returned to you via C<$?> if the function call fails. The
7660c0ab 1925C<@addrs> value returned by a successful call is a list of the raw
a0d0e21e
LW
1926addresses returned by the corresponding system library call. In the
1927Internet domain, each address is four bytes long and you can unpack it
1928by saying something like:
1929
1930 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1931
2b5ab1e7
TC
1932The Socket library makes this slightly easier:
1933
1934 use Socket;
1935 $iaddr = inet_aton("127.1"); # or whatever address
1936 $name = gethostbyaddr($iaddr, AF_INET);
1937
1938 # or going the other way
19799a22 1939 $straddr = inet_ntoa($iaddr);
2b5ab1e7 1940
19799a22
GS
1941If you get tired of remembering which element of the return list
1942contains which return value, by-name interfaces are provided
1943in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
1944C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
1945and C<User::grent>. These override the normal built-ins, supplying
1946versions that return objects with the appropriate names
1947for each field. For example:
5a964f20
TC
1948
1949 use File::stat;
1950 use User::pwent;
1951 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
1952
b76cc8ba
NIS
1953Even though it looks like they're the same method calls (uid),
1954they aren't, because a C<File::stat> object is different from
19799a22 1955a C<User::pwent> object.
5a964f20 1956
a0d0e21e
LW
1957=item getsockname SOCKET
1958
19799a22
GS
1959Returns the packed sockaddr address of this end of the SOCKET connection,
1960in case you don't know the address because you have several different
1961IPs that the connection might have come in on.
a0d0e21e 1962
4633a7c4
LW
1963 use Socket;
1964 $mysockaddr = getsockname(SOCK);
19799a22 1965 ($port, $myaddr) = sockaddr_in($mysockaddr);
b76cc8ba 1966 printf "Connect to %s [%s]\n",
19799a22
GS
1967 scalar gethostbyaddr($myaddr, AF_INET),
1968 inet_ntoa($myaddr);
a0d0e21e
LW
1969
1970=item getsockopt SOCKET,LEVEL,OPTNAME
1971
5a964f20 1972Returns the socket option requested, or undef if there is an error.
a0d0e21e
LW
1973
1974=item glob EXPR
1975
0a753a76 1976=item glob
1977
2b5ab1e7
TC
1978Returns the value of EXPR with filename expansions such as the
1979standard Unix shell F</bin/csh> would do. This is the internal function
61eff3bc
JH
1980implementing the C<< <*.c> >> operator, but you can use it directly.
1981If EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is
2b5ab1e7 1982discussed in more detail in L<perlop/"I/O Operators">.
a0d0e21e 1983
3a4b19e4
GS
1984Beginning with v5.6.0, this operator is implemented using the standard
1985C<File::Glob> extension. See L<File::Glob> for details.
1986
a0d0e21e
LW
1987=item gmtime EXPR
1988
48a26b3a 1989Converts a time as returned by the time function to a 8-element list
54310121 1990with the time localized for the standard Greenwich time zone.
4633a7c4 1991Typically used as follows:
a0d0e21e 1992
b76cc8ba 1993 # 0 1 2 3 4 5 6 7
48a26b3a 1994 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday) =
a0d0e21e
LW
1995 gmtime(time);
1996
48a26b3a
GS
1997All list elements are numeric, and come straight out of the C `struct
1998tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
1999specified time. $mday is the day of the month, and $mon is the month
2000itself, in the range C<0..11> with 0 indicating January and 11
2001indicating December. $year is the number of years since 1900. That
2002is, $year is C<123> in year 2023. $wday is the day of the week, with
20030 indicating Sunday and 3 indicating Wednesday. $yday is the day of
b76cc8ba 2004the year, in the range C<0..364> (or C<0..365> in leap years.)
48a26b3a
GS
2005
2006Note that the $year element is I<not> simply the last two digits of
2007the year. If you assume it is, then you create non-Y2K-compliant
2008programs--and you wouldn't want to do that, would you?
2f9daede 2009
abd75f24
GS
2010The proper way to get a complete 4-digit year is simply:
2011
2012 $year += 1900;
2013
2014And to get the last two digits of the year (e.g., '01' in 2001) do:
2015
2016 $year = sprintf("%02d", $year % 100);
2017
48a26b3a 2018If EXPR is omitted, C<gmtime()> uses the current time (C<gmtime(time)>).
a0d0e21e 2019
48a26b3a 2020In scalar context, C<gmtime()> returns the ctime(3) value:
0a753a76 2021
2022 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
2023
19799a22 2024Also see the C<timegm> function provided by the C<Time::Local> module,
f86cebdf 2025and the strftime(3) function available via the POSIX module.
7660c0ab 2026
2b5ab1e7
TC
2027This scalar value is B<not> locale dependent (see L<perllocale>), but
2028is instead a Perl builtin. Also see the C<Time::Local> module, and the
2029strftime(3) and mktime(3) functions available via the POSIX module. To
7660c0ab
A
2030get somewhat similar but locale dependent date strings, set up your
2031locale environment variables appropriately (please see L<perllocale>)
2032and try for example:
2033
2034 use POSIX qw(strftime);
2b5ab1e7 2035 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
7660c0ab 2036
2b5ab1e7
TC
2037Note that the C<%a> and C<%b> escapes, which represent the short forms
2038of the day of the week and the month of the year, may not necessarily
2039be three characters wide in all locales.
0a753a76 2040
a0d0e21e
LW
2041=item goto LABEL
2042
748a9306
LW
2043=item goto EXPR
2044
a0d0e21e
LW
2045=item goto &NAME
2046
7660c0ab 2047The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
a0d0e21e 2048execution there. It may not be used to go into any construct that
7660c0ab 2049requires initialization, such as a subroutine or a C<foreach> loop. It
0a753a76 2050also can't be used to go into a construct that is optimized away,
19799a22 2051or to get out of a block or subroutine given to C<sort>.
0a753a76 2052It can be used to go almost anywhere else within the dynamic scope,
a0d0e21e 2053including out of subroutines, but it's usually better to use some other
19799a22 2054construct such as C<last> or C<die>. The author of Perl has never felt the
7660c0ab 2055need to use this form of C<goto> (in Perl, that is--C is another matter).
a0d0e21e 2056
7660c0ab
A
2057The C<goto-EXPR> form expects a label name, whose scope will be resolved
2058dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
748a9306
LW
2059necessarily recommended if you're optimizing for maintainability:
2060
2061 goto ("FOO", "BAR", "GLARCH")[$i];
2062
6cb9131c
GS
2063The C<goto-&NAME> form is quite different from the other forms of C<goto>.
2064In fact, it isn't a goto in the normal sense at all, and doesn't have
2065the stigma associated with other gotos. Instead, it
2066substitutes a call to the named subroutine for the currently running
2067subroutine. This is used by C<AUTOLOAD> subroutines that wish to load
2068another subroutine and then pretend that the other subroutine had been
2069called in the first place (except that any modifications to C<@_>
2070in the current subroutine are propagated to the other subroutine.)
2071After the C<goto>, not even C<caller> will be able to tell that this
2072routine was called first.
2073
2074NAME needn't be the name of a subroutine; it can be a scalar variable
2075containing a code reference, or a block which evaluates to a code
2076reference.
a0d0e21e
LW
2077
2078=item grep BLOCK LIST
2079
2080=item grep EXPR,LIST
2081
2b5ab1e7
TC
2082This is similar in spirit to, but not the same as, grep(1) and its
2083relatives. In particular, it is not limited to using regular expressions.
2f9daede 2084
a0d0e21e 2085Evaluates the BLOCK or EXPR for each element of LIST (locally setting
7660c0ab 2086C<$_> to each element) and returns the list value consisting of those
19799a22
GS
2087elements for which the expression evaluated to true. In scalar
2088context, returns the number of times the expression was true.
a0d0e21e
LW
2089
2090 @foo = grep(!/^#/, @bar); # weed out comments
2091
2092or equivalently,
2093
2094 @foo = grep {!/^#/} @bar; # weed out comments
2095
be3174d2
GS
2096Note that C<$_> is an alias to the list value, so it can be used to
2097modify the elements of the LIST. While this is useful and supported,
2098it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
2099Similarly, grep returns aliases into the original list, much as a for
2100loop's index variable aliases the list elements. That is, modifying an
19799a22
GS
2101element of a list returned by grep (for example, in a C<foreach>, C<map>
2102or another C<grep>) actually modifies the element in the original list.
2b5ab1e7 2103This is usually something to be avoided when writing clear code.
a0d0e21e 2104
19799a22 2105See also L</map> for a list composed of the results of the BLOCK or EXPR.
38325410 2106
a0d0e21e
LW
2107=item hex EXPR
2108
54310121 2109=item hex
bbce6d69 2110
2b5ab1e7
TC
2111Interprets EXPR as a hex string and returns the corresponding value.
2112(To convert strings that might start with either 0, 0x, or 0b, see
2113L</oct>.) If EXPR is omitted, uses C<$_>.
2f9daede
TP
2114
2115 print hex '0xAf'; # prints '175'
2116 print hex 'aF'; # same
a0d0e21e 2117
19799a22 2118Hex strings may only represent integers. Strings that would cause
c6edd1b7 2119integer overflow trigger a warning.
19799a22 2120
a0d0e21e
LW
2121=item import
2122
19799a22 2123There is no builtin C<import> function. It is just an ordinary
4633a7c4 2124method (subroutine) defined (or inherited) by modules that wish to export
19799a22 2125names to another module. The C<use> function calls the C<import> method
cea6626f 2126for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
a0d0e21e
LW
2127
2128=item index STR,SUBSTR,POSITION
2129
2130=item index STR,SUBSTR
2131
2b5ab1e7
TC
2132The index function searches for one string within another, but without
2133the wildcard-like behavior of a full regular-expression pattern match.
2134It returns the position of the first occurrence of SUBSTR in STR at
2135or after POSITION. If POSITION is omitted, starts searching from the
2136beginning of the string. The return value is based at C<0> (or whatever
2137you've set the C<$[> variable to--but don't do that). If the substring
2138is not found, returns one less than the base, ordinarily C<-1>.
a0d0e21e
LW
2139
2140=item int EXPR
2141
54310121 2142=item int
bbce6d69 2143
7660c0ab 2144Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2b5ab1e7
TC
2145You should not use this function for rounding: one because it truncates
2146towards C<0>, and two because machine representations of floating point
2147numbers can sometimes produce counterintuitive results. For example,
2148C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2149because it's really more like -268.99999999999994315658 instead. Usually,
19799a22 2150the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2b5ab1e7 2151functions will serve you better than will int().
a0d0e21e
LW
2152
2153=item ioctl FILEHANDLE,FUNCTION,SCALAR
2154
2b5ab1e7 2155Implements the ioctl(2) function. You'll probably first have to say
a0d0e21e 2156
4633a7c4 2157 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
a0d0e21e 2158
2b5ab1e7 2159to get the correct function definitions. If F<ioctl.ph> doesn't
a0d0e21e 2160exist or doesn't have the correct definitions you'll have to roll your
61eff3bc 2161own, based on your C header files such as F<< <sys/ioctl.h> >>.
5a964f20 2162(There is a Perl script called B<h2ph> that comes with the Perl kit that
54310121 2163may help you in this, but it's nontrivial.) SCALAR will be read and/or
4633a7c4 2164written depending on the FUNCTION--a pointer to the string value of SCALAR
19799a22 2165will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
4633a7c4
LW
2166has no string value but does have a numeric value, that value will be
2167passed rather than a pointer to the string value. To guarantee this to be
19799a22
GS
2168true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2169functions may be needed to manipulate the values of structures used by
b76cc8ba 2170C<ioctl>.
a0d0e21e 2171
19799a22 2172The return value of C<ioctl> (and C<fcntl>) is as follows:
a0d0e21e
LW
2173
2174 if OS returns: then Perl returns:
2175 -1 undefined value
2176 0 string "0 but true"
2177 anything else that number
2178
19799a22 2179Thus Perl returns true on success and false on failure, yet you can
a0d0e21e
LW
2180still easily determine the actual value returned by the operating
2181system:
2182
2b5ab1e7 2183 $retval = ioctl(...) || -1;
a0d0e21e
LW
2184 printf "System returned %d\n", $retval;
2185
c2611fb3 2186The special string "C<0> but true" is exempt from B<-w> complaints
5a964f20
TC
2187about improper numeric conversions.
2188
19799a22
GS
2189Here's an example of setting a filehandle named C<REMOTE> to be
2190non-blocking at the system level. You'll have to negotiate C<$|>
2191on your own, though.
2192
2193 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
2194
2195 $flags = fcntl(REMOTE, F_GETFL, 0)
2196 or die "Can't get flags for the socket: $!\n";
2197
2198 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
2199 or die "Can't set flags for the socket: $!\n";
2200
a0d0e21e
LW
2201=item join EXPR,LIST
2202
2b5ab1e7
TC
2203Joins the separate strings of LIST into a single string with fields
2204separated by the value of EXPR, and returns that new string. Example:
a0d0e21e 2205
2b5ab1e7 2206 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
a0d0e21e 2207
eb6e2d6f
GS
2208Beware that unlike C<split>, C<join> doesn't take a pattern as its
2209first argument. Compare L</split>.
a0d0e21e 2210
aa689395 2211=item keys HASH
2212
19799a22 2213Returns a list consisting of all the keys of the named hash. (In
1d2dff63 2214scalar context, returns the number of keys.) The keys are returned in
ab192400
GS
2215an apparently random order. The actual random order is subject to
2216change in future versions of perl, but it is guaranteed to be the same
19799a22 2217order as either the C<values> or C<each> function produces (given
ab192400
GS
2218that the hash has not been modified). As a side effect, it resets
2219HASH's iterator.
a0d0e21e 2220
aa689395 2221Here is yet another way to print your environment:
a0d0e21e
LW
2222
2223 @keys = keys %ENV;
2224 @values = values %ENV;
b76cc8ba 2225 while (@keys) {
a0d0e21e
LW
2226 print pop(@keys), '=', pop(@values), "\n";
2227 }
2228
2229or how about sorted by key:
2230
2231 foreach $key (sort(keys %ENV)) {
2232 print $key, '=', $ENV{$key}, "\n";
2233 }
2234
8ea1e5d4
GS
2235The returned values are copies of the original keys in the hash, so
2236modifying them will not affect the original hash. Compare L</values>.
2237
19799a22 2238To sort a hash by value, you'll need to use a C<sort> function.
aa689395 2239Here's a descending numeric sort of a hash by its values:
4633a7c4 2240
5a964f20 2241 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
4633a7c4
LW
2242 printf "%4d %s\n", $hash{$key}, $key;
2243 }
2244
19799a22 2245As an lvalue C<keys> allows you to increase the number of hash buckets
aa689395 2246allocated for the given hash. This can gain you a measure of efficiency if
2247you know the hash is going to get big. (This is similar to pre-extending
2248an array by assigning a larger number to $#array.) If you say
55497cff 2249
2250 keys %hash = 200;
2251
ab192400
GS
2252then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2253in fact, since it rounds up to the next power of two. These
55497cff 2254buckets will be retained even if you do C<%hash = ()>, use C<undef
2255%hash> if you want to free the storage while C<%hash> is still in scope.
2256You can't shrink the number of buckets allocated for the hash using
19799a22 2257C<keys> in this way (but you needn't worry about doing this by accident,
55497cff 2258as trying has no effect).
2259
19799a22 2260See also C<each>, C<values> and C<sort>.
ab192400 2261
b350dd2f 2262=item kill SIGNAL, LIST
a0d0e21e 2263
b350dd2f 2264Sends a signal to a list of processes. Returns the number of
517db077
GS
2265processes successfully signaled (which is not necessarily the
2266same as the number actually killed).
a0d0e21e
LW
2267
2268 $cnt = kill 1, $child1, $child2;
2269 kill 9, @goners;
2270
b350dd2f
GS
2271If SIGNAL is zero, no signal is sent to the process. This is a
2272useful way to check that the process is alive and hasn't changed
2273its UID. See L<perlport> for notes on the portability of this
2274construct.
2275
2276Unlike in the shell, if SIGNAL is negative, it kills
4633a7c4
LW
2277process groups instead of processes. (On System V, a negative I<PROCESS>
2278number will also kill process groups, but that's not portable.) That
2279means you usually want to use positive not negative signals. You may also
da0045b7 2280use a signal name in quotes. See L<perlipc/"Signals"> for details.
a0d0e21e
LW
2281
2282=item last LABEL
2283
2284=item last
2285
2286The C<last> command is like the C<break> statement in C (as used in
2287loops); it immediately exits the loop in question. If the LABEL is
2288omitted, the command refers to the innermost enclosing loop. The
2289C<continue> block, if any, is not executed:
2290
4633a7c4
LW
2291 LINE: while (<STDIN>) {
2292 last LINE if /^$/; # exit when done with header
5a964f20 2293 #...
a0d0e21e
LW
2294 }
2295
4968c1e4 2296C<last> cannot be used to exit a block which returns a value such as
2b5ab1e7
TC
2297C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2298a grep() or map() operation.
4968c1e4 2299
6c1372ed
GS
2300Note that a block by itself is semantically identical to a loop
2301that executes once. Thus C<last> can be used to effect an early
2302exit out of such a block.
2303
98293880
JH
2304See also L</continue> for an illustration of how C<last>, C<next>, and
2305C<redo> work.
1d2dff63 2306
a0d0e21e
LW
2307=item lc EXPR
2308
54310121 2309=item lc
bbce6d69 2310
a0d0e21e 2311Returns an lowercased version of EXPR. This is the internal function
ad0029c4
JH
2312implementing the C<\L> escape in double-quoted strings. Respects
2313current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
2314and L<perlunicode>.
a0d0e21e 2315
7660c0ab 2316If EXPR is omitted, uses C<$_>.
bbce6d69 2317
a0d0e21e
LW
2318=item lcfirst EXPR
2319
54310121 2320=item lcfirst
bbce6d69 2321
ad0029c4
JH
2322Returns the value of EXPR with the first character lowercased. This
2323is the internal function implementing the C<\l> escape in
2324double-quoted strings. Respects current LC_CTYPE locale if C<use
2325locale> in force. See L<perllocale> and L<perlunicode>.
a0d0e21e 2326
7660c0ab 2327If EXPR is omitted, uses C<$_>.
bbce6d69 2328
a0d0e21e
LW
2329=item length EXPR
2330
54310121 2331=item length
bbce6d69 2332
a0ed51b3 2333Returns the length in characters of the value of EXPR. If EXPR is
b76cc8ba 2334omitted, returns length of C<$_>. Note that this cannot be used on
2b5ab1e7
TC
2335an entire array or hash to find out how many elements these have.
2336For that, use C<scalar @array> and C<scalar keys %hash> respectively.
a0d0e21e
LW
2337
2338=item link OLDFILE,NEWFILE
2339
19799a22 2340Creates a new filename linked to the old filename. Returns true for
b76cc8ba 2341success, false otherwise.
a0d0e21e
LW
2342
2343=item listen SOCKET,QUEUESIZE
2344
19799a22 2345Does the same thing that the listen system call does. Returns true if
b76cc8ba 2346it succeeded, false otherwise. See the example in
cea6626f 2347L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
2348
2349=item local EXPR
2350
19799a22 2351You really probably want to be using C<my> instead, because C<local> isn't
b76cc8ba 2352what most people think of as "local". See
13a2d996 2353L<perlsub/"Private Variables via my()"> for details.
2b5ab1e7 2354
5a964f20
TC
2355A local modifies the listed variables to be local to the enclosing
2356block, file, or eval. If more than one value is listed, the list must
2357be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2358for details, including issues with tied arrays and hashes.
a0d0e21e 2359
a0d0e21e
LW
2360=item localtime EXPR
2361
19799a22 2362Converts a time as returned by the time function to a 9-element list
5f05dabc 2363with the time analyzed for the local time zone. Typically used as
a0d0e21e
LW
2364follows:
2365
54310121 2366 # 0 1 2 3 4 5 6 7 8
a0d0e21e
LW
2367 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2368 localtime(time);
2369
48a26b3a
GS
2370All list elements are numeric, and come straight out of the C `struct
2371tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2372specified time. $mday is the day of the month, and $mon is the month
2373itself, in the range C<0..11> with 0 indicating January and 11
2374indicating December. $year is the number of years since 1900. That
2375is, $year is C<123> in year 2023. $wday is the day of the week, with
23760 indicating Sunday and 3 indicating Wednesday. $yday is the day of
874b1813 2377the year, in the range C<0..364> (or C<0..365> in leap years.) $isdst
48a26b3a
GS
2378is true if the specified time occurs during daylight savings time,
2379false otherwise.
2380
2381Note that the $year element is I<not> simply the last two digits of
2382the year. If you assume it is, then you create non-Y2K-compliant
2383programs--and you wouldn't want to do that, would you?
54310121 2384
abd75f24
GS
2385The proper way to get a complete 4-digit year is simply:
2386
2387 $year += 1900;
2388
2389And to get the last two digits of the year (e.g., '01' in 2001) do:
2390
2391 $year = sprintf("%02d", $year % 100);
2392
48a26b3a 2393If EXPR is omitted, C<localtime()> uses the current time (C<localtime(time)>).
a0d0e21e 2394
48a26b3a 2395In scalar context, C<localtime()> returns the ctime(3) value:
a0d0e21e 2396
5f05dabc 2397 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
a0d0e21e 2398
a3cb178b 2399This scalar value is B<not> locale dependent, see L<perllocale>, but
68f8bed4
JH
2400instead a Perl builtin. Also see the C<Time::Local> module
2401(to convert the second, minutes, hours, ... back to seconds since the
2402stroke of midnight the 1st of January 1970, the value returned by
ca6e1c26 2403time()), and the strftime(3) and mktime(3) functions available via the
68f8bed4
JH
2404POSIX module. To get somewhat similar but locale dependent date
2405strings, set up your locale environment variables appropriately
2406(please see L<perllocale>) and try for example:
a3cb178b 2407
5a964f20 2408 use POSIX qw(strftime);
2b5ab1e7 2409 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
a3cb178b
GS
2410
2411Note that the C<%a> and C<%b>, the short forms of the day of the week
2412and the month of the year, may not necessarily be three characters wide.
a0d0e21e 2413
19799a22
GS
2414=item lock
2415
2416 lock I<THING>
2417
2418This function places an advisory lock on a variable, subroutine,
2419or referenced object contained in I<THING> until the lock goes out
2420of scope. This is a built-in function only if your version of Perl
2421was built with threading enabled, and if you've said C<use Threads>.
2422Otherwise a user-defined function by this name will be called. See
2423L<Thread>.
2424
a0d0e21e
LW
2425=item log EXPR
2426
54310121 2427=item log
bbce6d69 2428
2b5ab1e7
TC
2429Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2430returns log of C<$_>. To get the log of another base, use basic algebra:
19799a22 2431The base-N log of a number is equal to the natural log of that number
2b5ab1e7
TC
2432divided by the natural log of N. For example:
2433
2434 sub log10 {
2435 my $n = shift;
2436 return log($n)/log(10);
b76cc8ba 2437 }
2b5ab1e7
TC
2438
2439See also L</exp> for the inverse operation.
a0d0e21e 2440
a0d0e21e
LW
2441=item lstat EXPR
2442
54310121 2443=item lstat
bbce6d69 2444
19799a22 2445Does the same thing as the C<stat> function (including setting the
5a964f20
TC
2446special C<_> filehandle) but stats a symbolic link instead of the file
2447the symbolic link points to. If symbolic links are unimplemented on
19799a22 2448your system, a normal C<stat> is done.
a0d0e21e 2449
7660c0ab 2450If EXPR is omitted, stats C<$_>.
bbce6d69 2451
a0d0e21e
LW
2452=item m//
2453
2454The match operator. See L<perlop>.
2455
2456=item map BLOCK LIST
2457
2458=item map EXPR,LIST
2459
19799a22
GS
2460Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2461C<$_> to each element) and returns the list value composed of the
2462results of each such evaluation. In scalar context, returns the
2463total number of elements so generated. Evaluates BLOCK or EXPR in
2464list context, so each element of LIST may produce zero, one, or
2465more elements in the returned value.
dd99ebda 2466
a0d0e21e
LW
2467 @chars = map(chr, @nums);
2468
2469translates a list of numbers to the corresponding characters. And
2470
4633a7c4 2471 %hash = map { getkey($_) => $_ } @array;
a0d0e21e
LW
2472
2473is just a funny way to write
2474
2475 %hash = ();
2476 foreach $_ (@array) {
4633a7c4 2477 $hash{getkey($_)} = $_;
a0d0e21e
LW
2478 }
2479
be3174d2
GS
2480Note that C<$_> is an alias to the list value, so it can be used to
2481modify the elements of the LIST. While this is useful and supported,
2482it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
2483Using a regular C<foreach> loop for this purpose would be clearer in
2484most cases. See also L</grep> for an array composed of those items of
2485the original list for which the BLOCK or EXPR evaluates to true.
fb73857a 2486
205fdb4d
NC
2487C<{> starts both hash references and blocks, so C<map { ...> could be either
2488the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look
2489ahead for the closing C<}> it has to take a guess at which its dealing with
2490based what it finds just after the C<{>. Usually it gets it right, but if it
2491doesn't it won't realize something is wrong until it gets to the C<}> and
2492encounters the missing (or unexpected) comma. The syntax error will be
2493reported close to the C<}> but you'll need to change something near the C<{>
2494such as using a unary C<+> to give perl some help:
2495
2496 %hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong
2497 %hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right
2498 %hash = map { ("\L$_", 1) } @array # this also works
2499 %hash = map { lc($_), 1 } @array # as does this.
2500 %hash = map +( lc($_), 1 ), @array # this is EXPR and works!
cea6626f 2501
205fdb4d
NC
2502 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
2503
2504or to force an anon hash constructor use C<+{>
2505
2506 @hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end
2507
2508and you get list of anonymous hashes each with only 1 entry.
2509
19799a22 2510=item mkdir FILENAME,MASK
a0d0e21e 2511
5a211162
GS
2512=item mkdir FILENAME
2513
0591cd52 2514Creates the directory specified by FILENAME, with permissions
19799a22
GS
2515specified by MASK (as modified by C<umask>). If it succeeds it
2516returns true, otherwise it returns false and sets C<$!> (errno).
5a211162 2517If omitted, MASK defaults to 0777.
0591cd52 2518
19799a22 2519In general, it is better to create directories with permissive MASK,
0591cd52 2520and let the user modify that with their C<umask>, than it is to supply
19799a22 2521a restrictive MASK and give the user no way to be more permissive.
0591cd52
NT
2522The exceptions to this rule are when the file or directory should be
2523kept private (mail files, for instance). The perlfunc(1) entry on
19799a22 2524C<umask> discusses the choice of MASK in more detail.
a0d0e21e 2525
cc1852e8
JH
2526Note that according to the POSIX 1003.1-1996 the FILENAME may have any
2527number of trailing slashes. Some operating and filesystems do not get
2528this right, so Perl automatically removes all trailing slashes to keep
2529everyone happy.
2530
a0d0e21e
LW
2531=item msgctl ID,CMD,ARG
2532
f86cebdf 2533Calls the System V IPC function msgctl(2). You'll probably have to say
0ade1984
JH
2534
2535 use IPC::SysV;
2536
7660c0ab
A
2537first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2538then ARG must be a variable which will hold the returned C<msqid_ds>
951ba7fe
GS
2539structure. Returns like C<ioctl>: the undefined value for error,
2540C<"0 but true"> for zero, or the actual return value otherwise. See also
4755096e 2541L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation.
a0d0e21e
LW
2542
2543=item msgget KEY,FLAGS
2544
f86cebdf 2545Calls the System V IPC function msgget(2). Returns the message queue
4755096e
GS
2546id, or the undefined value if there is an error. See also
2547L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation.
a0d0e21e 2548
a0d0e21e
LW
2549=item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2550
2551Calls the System V IPC function msgrcv to receive a message from
2552message queue ID into variable VAR with a maximum message size of
41d6edb2
JH
2553SIZE. Note that when a message is received, the message type as a
2554native long integer will be the first thing in VAR, followed by the
2555actual message. This packing may be opened with C<unpack("l! a*")>.
2556Taints the variable. Returns true if successful, or false if there is
4755096e
GS
2557an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and
2558C<IPC::SysV::Msg> documentation.
41d6edb2
JH
2559
2560=item msgsnd ID,MSG,FLAGS
2561
2562Calls the System V IPC function msgsnd to send the message MSG to the
2563message queue ID. MSG must begin with the native long integer message
2564type, and be followed by the length of the actual message, and finally
2565the message itself. This kind of packing can be achieved with
2566C<pack("l! a*", $type, $message)>. Returns true if successful,
2567or false if there is an error. See also C<IPC::SysV>
2568and C<IPC::SysV::Msg> documentation.
a0d0e21e
LW
2569
2570=item my EXPR
2571
09bef843
SB
2572=item my EXPR : ATTRIBUTES
2573
19799a22
GS
2574A C<my> declares the listed variables to be local (lexically) to the
2575enclosing block, file, or C<eval>. If
5f05dabc 2576more than one value is listed, the list must be placed in parentheses. See
cb1a09d0 2577L<perlsub/"Private Variables via my()"> for details.
4633a7c4 2578
a0d0e21e
LW
2579=item next LABEL
2580
2581=item next
2582
2583The C<next> command is like the C<continue> statement in C; it starts
2584the next iteration of the loop:
2585
4633a7c4
LW
2586 LINE: while (<STDIN>) {
2587 next LINE if /^#/; # discard comments
5a964f20 2588 #...
a0d0e21e
LW
2589 }
2590
2591Note that if there were a C<continue> block on the above, it would get
2592executed even on discarded lines. If the LABEL is omitted, the command
2593refers to the innermost enclosing loop.
2594
4968c1e4 2595C<next> cannot be used to exit a block which returns a value such as
2b5ab1e7
TC
2596C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2597a grep() or map() operation.
4968c1e4 2598
6c1372ed
GS
2599Note that a block by itself is semantically identical to a loop
2600that executes once. Thus C<next> will exit such a block early.
2601
98293880
JH
2602See also L</continue> for an illustration of how C<last>, C<next>, and
2603C<redo> work.
1d2dff63 2604
a0d0e21e
LW
2605=item no Module LIST
2606
7660c0ab 2607See the L</use> function, which C<no> is the opposite of.
a0d0e21e
LW
2608
2609=item oct EXPR
2610
54310121 2611=item oct
bbce6d69 2612
4633a7c4 2613Interprets EXPR as an octal string and returns the corresponding
4f19785b
WSI
2614value. (If EXPR happens to start off with C<0x>, interprets it as a
2615hex string. If EXPR starts off with C<0b>, it is interpreted as a
2616binary string.) The following will handle decimal, binary, octal, and
4633a7c4 2617hex in the standard Perl or C notation:
a0d0e21e
LW
2618
2619 $val = oct($val) if $val =~ /^0/;
2620
19799a22
GS
2621If EXPR is omitted, uses C<$_>. To go the other way (produce a number
2622in octal), use sprintf() or printf():
2623
2624 $perms = (stat("filename"))[2] & 07777;
2625 $oct_perms = sprintf "%lo", $perms;
2626
2627The oct() function is commonly used when a string such as C<644> needs
2628to be converted into a file mode, for example. (Although perl will
2629automatically convert strings into numbers as needed, this automatic
2630conversion assumes base 10.)
a0d0e21e
LW
2631
2632=item open FILEHANDLE,EXPR
2633
68bd7414
NIS
2634=item open FILEHANDLE,MODE,EXPR
2635
2636=item open FILEHANDLE,MODE,EXPR,LIST
2637
a0d0e21e
LW
2638=item open FILEHANDLE
2639
2640Opens the file whose filename is given by EXPR, and associates it with
68bd7414
NIS
2641FILEHANDLE. If FILEHANDLE is an undefined lexical (C<my>) variable the variable is
2642assigned a reference to a new anonymous filehandle, otherwise if FILEHANDLE is an expression,
2643its value is used as the name of the real filehandle wanted. (This is considered a symbolic
d6fd2b02
GS
2644reference, so C<use strict 'refs'> should I<not> be in effect.)
2645
2646If EXPR is omitted, the scalar
5f05dabc 2647variable of the same name as the FILEHANDLE contains the filename.
19799a22
GS
2648(Note that lexical variables--those declared with C<my>--will not work
2649for this purpose; so if you're using C<my>, specify EXPR in your call
2b5ab1e7
TC
2650to open.) See L<perlopentut> for a kinder, gentler explanation of opening
2651files.
5f05dabc 2652
68bd7414
NIS
2653If three or more arguments are specified then the mode of opening and the file name
2654are separate. If MODE is C<< '<' >> or nothing, the file is opened for input.
61eff3bc
JH
2655If MODE is C<< '>' >>, the file is truncated and opened for
2656output, being created if necessary. If MODE is C<<< '>>' >>>,
b76cc8ba 2657the file is opened for appending, again being created if necessary.
61eff3bc
JH
2658You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to indicate that
2659you want both read and write access to the file; thus C<< '+<' >> is almost
2660always preferred for read/write updates--the C<< '+>' >> mode would clobber the
5a964f20
TC
2661file first. You can't usually use either read-write mode for updating
2662textfiles, since they have variable length records. See the B<-i>
0591cd52
NT
2663switch in L<perlrun> for a better approach. The file is created with
2664permissions of C<0666> modified by the process' C<umask> value.
5a964f20 2665
61eff3bc
JH
2666These various prefixes correspond to the fopen(3) modes of C<'r'>, C<'r+'>,
2667C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
5f05dabc 2668
6170680b
IZ
2669In the 2-arguments (and 1-argument) form of the call the mode and
2670filename should be concatenated (in this order), possibly separated by
68bd7414
NIS
2671spaces. It is possible to omit the mode in these forms if the mode is
2672C<< '<' >>.
6170680b 2673
7660c0ab 2674If the filename begins with C<'|'>, the filename is interpreted as a
5a964f20 2675command to which output is to be piped, and if the filename ends with a
f244e06d
GS
2676C<'|'>, the filename is interpreted as a command which pipes output to
2677us. See L<perlipc/"Using open() for IPC">
19799a22 2678for more examples of this. (You are not allowed to C<open> to a command
5a964f20 2679that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
4a4eefd0
GS
2680and L<perlipc/"Bidirectional Communication with Another Process">
2681for alternatives.)
cb1a09d0 2682
68bd7414 2683For three or more arguments if MODE is C<'|-'>, the filename is interpreted as a
6170680b
IZ
2684command to which output is to be piped, and if MODE is
2685C<'-|'>, the filename is interpreted as a command which pipes output to
2686us. In the 2-arguments (and 1-argument) form one should replace dash
2687(C<'-'>) with the command. See L<perlipc/"Using open() for IPC">
2688for more examples of this. (You are not allowed to C<open> to a command
2689that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
68bd7414
NIS
2690and L<perlipc/"Bidirectional Communication"> for alternatives.) In 3+ arg form of
2691pipe opens then if LIST is specified (extra arguments after the command name) then
2692LIST becomes arguments to the command invoked if the platform supports it.
2693The meaning of C<open> with more than three arguments for non-pipe modes
2694is not yet specified. Experimental "layers" may give extra LIST arguments meaning.
6170680b
IZ
2695
2696In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
b76cc8ba 2697and opening C<< '>-' >> opens STDOUT.
6170680b
IZ
2698
2699Open returns
19799a22 2700nonzero upon success, the undefined value otherwise. If the C<open>
4633a7c4 2701involved a pipe, the return value happens to be the pid of the
54310121 2702subprocess.
cb1a09d0
AD
2703
2704If you're unfortunate enough to be running Perl on a system that
2705distinguishes between text files and binary files (modern operating
2706systems don't care), then you should check out L</binmode> for tips for
19799a22 2707dealing with this. The key distinction between systems that need C<binmode>
5a964f20
TC
2708and those that don't is their text file formats. Systems like Unix, MacOS, and
2709Plan9, which delimit lines with a single character, and which encode that
19799a22 2710character in C as C<"\n">, do not need C<binmode>. The rest need it.
cb1a09d0 2711
68bd7414
NIS
2712In the three argument form MODE may also contain a list of IO "layers" (see L<open> and
2713L<PerlIO> for more details) to be applied to the handle. This can be used to achieve the
2714effect of C<binmode> as well as more complex behaviours.
2715
fb73857a 2716When opening a file, it's usually a bad idea to continue normal execution
19799a22
GS
2717if the request failed, so C<open> is frequently used in connection with
2718C<die>. Even if C<die> won't do what you want (say, in a CGI script,
fb73857a 2719where you want to make a nicely formatted error message (but there are
5a964f20 2720modules that can help with that problem)) you should always check
19799a22 2721the return value from opening a file. The infrequent exception is when
fb73857a 2722working with an unopened filehandle is actually what you want to do.
2723
b76cc8ba
NIS
2724As a special case the 3 arg form with a read/write mode and the third argument
2725being C<undef>:
2726
2727 open(TMP, "+>", undef) or die ...
2728
2729opens a filehandle to an anonymous temporary file.
2730
68bd7414 2731
cb1a09d0 2732Examples:
a0d0e21e
LW
2733
2734 $ARTICLE = 100;
2735 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2736 while (<ARTICLE>) {...
2737
6170680b 2738 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
fb73857a 2739 # if the open fails, output is discarded
a0d0e21e 2740
6170680b 2741 open(DBASE, '+<', 'dbase.mine') # open for update
fb73857a 2742 or die "Can't open 'dbase.mine' for update: $!";
cb1a09d0 2743
6170680b
IZ
2744 open(DBASE, '+<dbase.mine') # ditto
2745 or die "Can't open 'dbase.mine' for update: $!";
2746
2747 open(ARTICLE, '-|', "caesar <$article") # decrypt article
fb73857a 2748 or die "Can't start caesar: $!";
a0d0e21e 2749
6170680b
IZ
2750 open(ARTICLE, "caesar <$article |") # ditto
2751 or die "Can't start caesar: $!";
2752
2753 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
fb73857a 2754 or die "Can't start sort: $!";
a0d0e21e
LW
2755
2756 # process argument list of files along with any includes
2757
2758 foreach $file (@ARGV) {
2759 process($file, 'fh00');
2760 }
2761
2762 sub process {
5a964f20 2763 my($filename, $input) = @_;
a0d0e21e
LW
2764 $input++; # this is a string increment
2765 unless (open($input, $filename)) {
2766 print STDERR "Can't open $filename: $!\n";
2767 return;
2768 }
2769
5a964f20 2770 local $_;
a0d0e21e
LW
2771 while (<$input>) { # note use of indirection
2772 if (/^#include "(.*)"/) {
2773 process($1, $input);
2774 next;
2775 }
5a964f20 2776 #... # whatever
a0d0e21e
LW
2777 }
2778 }
2779
2780You may also, in the Bourne shell tradition, specify an EXPR beginning
61eff3bc 2781with C<< '>&' >>, in which case the rest of the string is interpreted as the
5a964f20 2782name of a filehandle (or file descriptor, if numeric) to be
61eff3bc
JH
2783duped and opened. You may use C<&> after C<< > >>, C<<< >> >>>,
2784C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>. The
a0d0e21e 2785mode you specify should match the mode of the original filehandle.
184e9718 2786(Duping a filehandle does not take into account any existing contents of
b76cc8ba
NIS
2787stdio buffers.) If you use the 3 arg form then you can pass either a number,
2788the name of a filehandle or the normal "reference to a glob".
6170680b 2789
a0d0e21e
LW
2790Here is a script that saves, redirects, and restores STDOUT and
2791STDERR:
2792
2793 #!/usr/bin/perl
b76cc8ba 2794 open(my $oldout, ">&", \*STDOUT);
5a964f20 2795 open(OLDERR, ">&STDERR");
a0d0e21e 2796
6170680b
IZ
2797 open(STDOUT, '>', "foo.out") || die "Can't redirect stdout";
2798 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
a0d0e21e
LW
2799
2800 select(STDERR); $| = 1; # make unbuffered
2801 select(STDOUT); $| = 1; # make unbuffered
2802
2803 print STDOUT "stdout 1\n"; # this works for
2804 print STDERR "stderr 1\n"; # subprocesses too
2805
2806 close(STDOUT);
2807 close(STDERR);
2808
5a964f20
TC
2809 open(STDOUT, ">&OLDOUT");
2810 open(STDERR, ">&OLDERR");
a0d0e21e
LW
2811
2812 print STDOUT "stdout 2\n";
2813 print STDERR "stderr 2\n";
2814
df632fdf
JH
2815If you specify C<< '<&=N' >>, where C<N> is a number, then Perl will
2816do an equivalent of C's C<fdopen> of that file descriptor; this is
2817more parsimonious of file descriptors. For example:
a0d0e21e
LW
2818
2819 open(FILEHANDLE, "<&=$fd")
df632fdf 2820
b76cc8ba 2821or
df632fdf 2822
b76cc8ba 2823 open(FILEHANDLE, "<&=", $fd)
a0d0e21e 2824
df632fdf
JH
2825Note that if Perl is using the standard C libraries' fdopen() then on
2826many UNIX systems, fdopen() is known to fail when file descriptors
4af147f6 2827exceed a certain value, typically 255. If you need more file
b76cc8ba 2828descriptors than that, consider rebuilding Perl to use the C<PerlIO>.
4af147f6 2829
df632fdf
JH
2830You can see whether Perl has been compiled with PerlIO or not by
2831running C<perl -V> and looking for C<useperlio=> line. If C<useperlio>
2832is C<define>, you have PerlIO, otherwise you don't.
2833
6170680b
IZ
2834If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
2835with 2-arguments (or 1-argument) form of open(), then
a0d0e21e 2836there is an implicit fork done, and the return value of open is the pid
7660c0ab 2837of the child within the parent process, and C<0> within the child
184e9718 2838process. (Use C<defined($pid)> to determine whether the open was successful.)
a0d0e21e
LW
2839The filehandle behaves normally for the parent, but i/o to that
2840filehandle is piped from/to the STDOUT/STDIN of the child process.
2841In the child process the filehandle isn't opened--i/o happens from/to
2842the new STDOUT or STDIN. Typically this is used like the normal
2843piped open when you want to exercise more control over just how the
2844pipe command gets executed, such as when you are running setuid, and
54310121 2845don't want to have to scan shell commands for metacharacters.
6170680b 2846The following triples are more or less equivalent:
a0d0e21e
LW
2847
2848 open(FOO, "|tr '[a-z]' '[A-Z]'");
6170680b
IZ
2849 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
2850 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
b76cc8ba 2851 open(FOO, '|-', "tr", '[a-z]', '[A-Z]');
a0d0e21e
LW
2852
2853 open(FOO, "cat -n '$file'|");
6170680b
IZ
2854 open(FOO, '-|', "cat -n '$file'");
2855 open(FOO, '-|') || exec 'cat', '-n', $file;
b76cc8ba
NIS
2856 open(FOO, '-|', "cat", '-n', $file);
2857
2858The last example in each block shows the pipe as "list form", which is
2859not yet supported on all platforms.
a0d0e21e 2860
4633a7c4
LW
2861See L<perlipc/"Safe Pipe Opens"> for more examples of this.
2862
0f897271
GS
2863Beginning with v5.6.0, Perl will attempt to flush all files opened for
2864output before any operation that may do a fork, but this may not be
2865supported on some platforms (see L<perlport>). To be safe, you may need
2866to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
2867of C<IO::Handle> on any open handles.
2868
2869On systems that support a
45bc9206
GS
2870close-on-exec flag on files, the flag will be set for the newly opened
2871file descriptor as determined by the value of $^F. See L<perlvar/$^F>.
a0d0e21e 2872
0dccf244
CS
2873Closing any piped filehandle causes the parent process to wait for the
2874child to finish, and returns the status value in C<$?>.
2875
6170680b
IZ
2876The filename passed to 2-argument (or 1-argument) form of open()
2877will have leading and trailing
f86cebdf 2878whitespace deleted, and the normal redirection characters
b76cc8ba 2879honored. This property, known as "magic open",
5a964f20 2880can often be used to good effect. A user could specify a filename of
7660c0ab 2881F<"rsh cat file |">, or you could change certain filenames as needed:
5a964f20
TC
2882
2883 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
2884 open(FH, $filename) or die "Can't open $filename: $!";
2885
6170680b
IZ
2886Use 3-argument form to open a file with arbitrary weird characters in it,
2887
2888 open(FOO, '<', $file);
2889
2890otherwise it's necessary to protect any leading and trailing whitespace:
5a964f20
TC
2891
2892 $file =~ s#^(\s)#./$1#;
2893 open(FOO, "< $file\0");
2894
a31a806a 2895(this may not work on some bizarre filesystems). One should
106325ad 2896conscientiously choose between the I<magic> and 3-arguments form
6170680b
IZ
2897of open():
2898
2899 open IN, $ARGV[0];
2900
2901will allow the user to specify an argument of the form C<"rsh cat file |">,
2902but will not work on a filename which happens to have a trailing space, while
2903
2904 open IN, '<', $ARGV[0];
2905
2906will have exactly the opposite restrictions.
2907
19799a22 2908If you want a "real" C C<open> (see L<open(2)> on your system), then you
6170680b
IZ
2909should use the C<sysopen> function, which involves no such magic (but
2910may use subtly different filemodes than Perl open(), which is mapped
2911to C fopen()). This is
5a964f20
TC
2912another way to protect your filenames from interpretation. For example:
2913
2914 use IO::Handle;
2915 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
2916 or die "sysopen $path: $!";
2917 $oldfh = select(HANDLE); $| = 1; select($oldfh);
38762f02 2918 print HANDLE "stuff $$\n";
5a964f20
TC
2919 seek(HANDLE, 0, 0);
2920 print "File contains: ", <HANDLE>;
2921
7660c0ab
A
2922Using the constructor from the C<IO::Handle> package (or one of its
2923subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
5a964f20
TC
2924filehandles that have the scope of whatever variables hold references to
2925them, and automatically close whenever and however you leave that scope:
c07a80fd 2926
5f05dabc 2927 use IO::File;
5a964f20 2928 #...
c07a80fd 2929 sub read_myfile_munged {
2930 my $ALL = shift;
5f05dabc 2931 my $handle = new IO::File;
c07a80fd 2932 open($handle, "myfile") or die "myfile: $!";
2933 $first = <$handle>
2934 or return (); # Automatically closed here.
2935 mung $first or die "mung failed"; # Or here.
2936 return $first, <$handle> if $ALL; # Or here.
2937 $first; # Or here.
2938 }
2939
b687b08b 2940See L</seek> for some details about mixing reading and writing.
a0d0e21e
LW
2941
2942=item opendir DIRHANDLE,EXPR
2943
19799a22
GS
2944Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
2945C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
a0d0e21e
LW
2946DIRHANDLEs have their own namespace separate from FILEHANDLEs.
2947
2948=item ord EXPR
2949
54310121 2950=item ord
bbce6d69 2951
a0ed51b3 2952Returns the numeric (ASCII or Unicode) value of the first character of EXPR. If
7660c0ab 2953EXPR is omitted, uses C<$_>. For the reverse, see L</chr>.
2b5ab1e7 2954See L<utf8> for more about Unicode.
a0d0e21e 2955
77ca0c92
LW
2956=item our EXPR
2957
9969eac4
BS
2958=item our EXPR : ATTRIBUTES
2959
77ca0c92
LW
2960An C<our> declares the listed variables to be valid globals within
2961the enclosing block, file, or C<eval>. That is, it has the same
2962scoping rules as a "my" declaration, but does not create a local
2963variable. If more than one value is listed, the list must be placed
2964in parentheses. The C<our> declaration has no semantic effect unless
2965"use strict vars" is in effect, in which case it lets you use the
2966declared global variable without qualifying it with a package name.
2967(But only within the lexical scope of the C<our> declaration. In this
2968it differs from "use vars", which is package scoped.)
2969
f472eb5c
GS
2970An C<our> declaration declares a global variable that will be visible
2971across its entire lexical scope, even across package boundaries. The
2972package in which the variable is entered is determined at the point
2973of the declaration, not at the point of use. This means the following
2974behavior holds:
2975
2976 package Foo;
2977 our $bar; # declares $Foo::bar for rest of lexical scope
2978 $bar = 20;
2979
2980 package Bar;
2981 print $bar; # prints 20
2982
2983Multiple C<our> declarations in the same lexical scope are allowed
2984if they are in different packages. If they happened to be in the same
2985package, Perl will emit warnings if you have asked for them.
2986
2987 use warnings;
2988 package Foo;
2989 our $bar; # declares $Foo::bar for rest of lexical scope
2990 $bar = 20;
2991
2992 package Bar;
2993 our $bar = 30; # declares $Bar::bar for rest of lexical scope
2994 print $bar; # prints 30
2995
2996 our $bar; # emits warning
2997
9969eac4
BS
2998An C<our> declaration may also have a list of attributes associated
2999with it. B<WARNING>: This is an experimental feature that may be
3000changed or removed in future releases of Perl. It should not be
3001relied upon.
3002
51d2bbcc 3003The only currently recognized attribute is C<unique> which indicates
9969eac4
BS
3004that a single copy of the global is to be used by all interpreters
3005should the program happen to be running in a multi-interpreter
3006environment. (The default behaviour would be for each interpreter to
3007have its own copy of the global.) In such an environment, this
3008attribute also has the effect of making the global readonly.
3009Examples:
3010
51d2bbcc
JH
3011 our @EXPORT : unique = qw(foo);
3012 our %EXPORT_TAGS : unique = (bar => [qw(aa bb cc)]);
3013 our $VERSION : unique = "1.00";
9969eac4
BS
3014
3015Multi-interpreter environments can come to being either through the
3016fork() emulation on Windows platforms, or by embedding perl in a
51d2bbcc 3017multi-threaded application. The C<unique> attribute does nothing in
9969eac4
BS
3018all other environments.
3019
a0d0e21e
LW
3020=item pack TEMPLATE,LIST
3021
2b6c5635
GS
3022Takes a LIST of values and converts it into a string using the rules
3023given by the TEMPLATE. The resulting string is the concatenation of
3024the converted values. Typically, each converted value looks
3025like its machine-level representation. For example, on 32-bit machines
3026a converted integer may be represented by a sequence of 4 bytes.
3027
3028The TEMPLATE is a
a0d0e21e
LW
3029sequence of characters that give the order and type of values, as
3030follows:
3031
5a929a98 3032 a A string with arbitrary binary data, will be null padded.
4375e838 3033 A An ASCII string, will be space padded.
5a929a98
VU
3034 Z A null terminated (asciz) string, will be null padded.
3035
2b6c5635
GS
3036 b A bit string (ascending bit order inside each byte, like vec()).
3037 B A bit string (descending bit order inside each byte).
a0d0e21e
LW
3038 h A hex string (low nybble first).
3039 H A hex string (high nybble first).
3040
3041 c A signed char value.
a0ed51b3 3042 C An unsigned char value. Only does bytes. See U for Unicode.
96e4d5b1 3043
a0d0e21e
LW
3044 s A signed short value.
3045 S An unsigned short value.
96e4d5b1 3046 (This 'short' is _exactly_ 16 bits, which may differ from
851646ae
JH
3047 what a local C compiler calls 'short'. If you want
3048 native-length shorts, use the '!' suffix.)
96e4d5b1 3049
a0d0e21e
LW
3050 i A signed integer value.
3051 I An unsigned integer value.
19799a22 3052 (This 'integer' is _at_least_ 32 bits wide. Its exact
f86cebdf
GS
3053 size depends on what a local C compiler calls 'int',
3054 and may even be larger than the 'long' described in
3055 the next item.)
96e4d5b1 3056
a0d0e21e
LW
3057 l A signed long value.
3058 L An unsigned long value.
96e4d5b1 3059 (This 'long' is _exactly_ 32 bits, which may differ from
851646ae
JH
3060 what a local C compiler calls 'long'. If you want
3061 native-length longs, use the '!' suffix.)
a0d0e21e 3062
5d11dd56
MG
3063 n An unsigned short in "network" (big-endian) order.
3064 N An unsigned long in "network" (big-endian) order.
3065 v An unsigned short in "VAX" (little-endian) order.
3066 V An unsigned long in "VAX" (little-endian) order.
96e4d5b1 3067 (These 'shorts' and 'longs' are _exactly_ 16 bits and
3068 _exactly_ 32 bits, respectively.)
a0d0e21e 3069
dae0da7a
JH
3070 q A signed quad (64-bit) value.
3071 Q An unsigned quad value.
851646ae
JH
3072 (Quads are available only if your system supports 64-bit
3073 integer values _and_ if Perl has been compiled to support those.
dae0da7a
JH
3074 Causes a fatal error otherwise.)
3075
a0d0e21e
LW
3076 f A single-precision float in the native format.
3077 d A double-precision float in the native format.
3078
3079 p A pointer to a null-terminated string.
3080 P A pointer to a structure (fixed-length string).
3081
3082 u A uuencoded string.
ad0029c4
JH
3083 U A Unicode character number. Encodes to UTF-8 internally
3084 (or UTF-EBCDIC in EBCDIC platforms).
a0d0e21e 3085
96e4d5b1 3086 w A BER compressed integer. Its bytes represent an unsigned
f86cebdf
GS
3087 integer in base 128, most significant digit first, with as
3088 few digits as possible. Bit eight (the high bit) is set
3089 on each byte except the last.
def98dd4 3090
a0d0e21e
LW
3091 x A null byte.
3092 X Back up a byte.
3093 @ Null fill to absolute position.
3094
5a929a98
VU
3095The following rules apply:
3096
3097=over 8
3098
3099=item *
3100
5a964f20 3101Each letter may optionally be followed by a number giving a repeat
951ba7fe
GS
3102count. With all types except C<a>, C<A>, C<Z>, C<b>, C<B>, C<h>,
3103C<H>, and C<P> the pack function will gobble up that many values from
5a929a98 3104the LIST. A C<*> for the repeat count means to use however many items are
951ba7fe
GS
3105left, except for C<@>, C<x>, C<X>, where it is equivalent
3106to C<0>, and C<u>, where it is equivalent to 1 (or 45, what is the
2b6c5635
GS
3107same).
3108
951ba7fe 3109When used with C<Z>, C<*> results in the addition of a trailing null
2b6c5635
GS
3110byte (so the packed result will be one longer than the byte C<length>
3111of the item).
3112
951ba7fe 3113The repeat count for C<u> is interpreted as the maximal number of bytes
2b6c5635 3114to encode per line of output, with 0 and 1 replaced by 45.
5a929a98
VU
3115
3116=item *
3117
951ba7fe 3118The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
5a929a98 3119string of length count, padding with nulls or spaces as necessary. When
951ba7fe
GS
3120unpacking, C<A> strips trailing spaces and nulls, C<Z> strips everything
3121after the first null, and C<a> returns data verbatim. When packing,
3122C<a>, and C<Z> are equivalent.
2b6c5635
GS
3123
3124If the value-to-pack is too long, it is truncated. If too long and an
951ba7fe
GS
3125explicit count is provided, C<Z> packs only C<$count-1> bytes, followed
3126by a null byte. Thus C<Z> always packs a trailing null byte under
2b6c5635 3127all circumstances.
5a929a98
VU
3128
3129=item *
3130
951ba7fe 3131Likewise, the C<b> and C<B> fields pack a string that many bits long.
c73032f5
IZ
3132Each byte of the input field of pack() generates 1 bit of the result.
3133Each result bit is based on the least-significant bit of the corresponding
3134input byte, i.e., on C<ord($byte)%2>. In particular, bytes C<"0"> and
3135C<"1"> generate bits 0 and 1, as do bytes C<"\0"> and C<"\1">.
3136
3137Starting from the beginning of the input string of pack(), each 8-tuple
951ba7fe 3138of bytes is converted to 1 byte of output. With format C<b>
c73032f5 3139the first byte of the 8-tuple determines the least-significant bit of a
951ba7fe 3140byte, and with format C<B> it determines the most-significant bit of
c73032f5
IZ
3141a byte.
3142
3143If the length of the input string is not exactly divisible by 8, the
3144remainder is packed as if the input string were padded by null bytes
3145at the end. Similarly, during unpack()ing the "extra" bits are ignored.
3146
3147If the input string of pack() is longer than needed, extra bytes are ignored.
2b6c5635
GS
3148A C<*> for the repeat count of pack() means to use all the bytes of
3149the input field. On unpack()ing the bits are converted to a string
3150of C<"0">s and C<"1">s.
5a929a98
VU
3151
3152=item *
3153
951ba7fe 3154The C<h> and C<H> fields pack a string that many nybbles (4-bit groups,
851646ae 3155representable as hexadecimal digits, 0-9a-f) long.
5a929a98 3156
c73032f5
IZ
3157Each byte of the input field of pack() generates 4 bits of the result.
3158For non-alphabetical bytes the result is based on the 4 least-significant
3159bits of the input byte, i.e., on C<ord($byte)%16>. In particular,
3160bytes C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
3161C<"\0"> and C<"\1">. For bytes C<"a".."f"> and C<"A".."F"> the result
3162is compatible with the usual hexadecimal digits, so that C<"a"> and
3163C<"A"> both generate the nybble C<0xa==10>. The result for bytes
3164C<"g".."z"> and C<"G".."Z"> is not well-defined.
3165
3166Starting from the beginning of the input string of pack(), each pair
951ba7fe 3167of bytes is converted to 1 byte of output. With format C<h> the
c73032f5 3168first byte of the pair determines the least-significant nybble of the
951ba7fe 3169output byte, and with format C<H> it determines the most-significant
c73032f5
IZ
3170nybble.
3171
3172If the length of the input string is not even, it behaves as if padded
3173by a null byte at the end. Similarly, during unpack()ing the "extra"
3174nybbles are ignored.
3175
3176If the input string of pack() is longer than needed, extra bytes are ignored.
3177A C<*> for the repeat count of pack() means to use all the bytes of
3178the input field. On unpack()ing the bits are converted to a string
3179of hexadecimal digits.
3180
5a929a98
VU
3181=item *
3182
951ba7fe 3183The C<p> type packs a pointer to a null-terminated string. You are
5a929a98
VU
3184responsible for ensuring the string is not a temporary value (which can
3185potentially get deallocated before you get around to using the packed result).
951ba7fe
GS
3186The C<P> type packs a pointer to a structure of the size indicated by the
3187length. A NULL pointer is created if the corresponding value for C<p> or
3188C<P> is C<undef>, similarly for unpack().
5a929a98
VU
3189
3190=item *
3191
951ba7fe
GS
3192The C</> template character allows packing and unpacking of strings where
3193the packed structure contains a byte count followed by the string itself.
17f4a12d 3194You write I<length-item>C</>I<string-item>.
43192e07
IP
3195
3196The I<length-item> can be any C<pack> template letter,
3197and describes how the length value is packed.
3198The ones likely to be of most use are integer-packing ones like
951ba7fe
GS
3199C<n> (for Java strings), C<w> (for ASN.1 or SNMP)
3200and C<N> (for Sun XDR).
43192e07
IP
3201
3202The I<string-item> must, at present, be C<"A*">, C<"a*"> or C<"Z*">.
3203For C<unpack> the length of the string is obtained from the I<length-item>,
3204but if you put in the '*' it will be ignored.
3205
17f4a12d
IZ
3206 unpack 'C/a', "\04Gurusamy"; gives 'Guru'
3207 unpack 'a3/A* A*', '007 Bond J '; gives (' Bond','J')
3208 pack 'n/a* w/a*','hello,','world'; gives "\000\006hello,\005world"
43192e07
IP
3209
3210The I<length-item> is not returned explicitly from C<unpack>.
3211
951ba7fe
GS
3212Adding a count to the I<length-item> letter is unlikely to do anything
3213useful, unless that letter is C<A>, C<a> or C<Z>. Packing with a
3214I<length-item> of C<a> or C<Z> may introduce C<"\000"> characters,
43192e07
IP
3215which Perl does not regard as legal in numeric strings.
3216
3217=item *
3218
951ba7fe
GS
3219The integer types C<s>, C<S>, C<l>, and C<L> may be
3220immediately followed by a C<!> suffix to signify native shorts or
3221longs--as you can see from above for example a bare C<l> does mean
851646ae
JH
3222exactly 32 bits, the native C<long> (as seen by the local C compiler)
3223may be larger. This is an issue mainly in 64-bit platforms. You can
951ba7fe 3224see whether using C<!> makes any difference by
726ea183 3225
4d0c1c44
GS
3226 print length(pack("s")), " ", length(pack("s!")), "\n";
3227 print length(pack("l")), " ", length(pack("l!")), "\n";
ef54e1a4 3228
951ba7fe
GS
3229C<i!> and C<I!> also work but only because of completeness;
3230they are identical to C<i> and C<I>.
ef54e1a4 3231
19799a22
GS
3232The actual sizes (in bytes) of native shorts, ints, longs, and long
3233longs on the platform where Perl was built are also available via
3234L<Config>:
3235
3236 use Config;
3237 print $Config{shortsize}, "\n";
3238 print $Config{intsize}, "\n";
3239 print $Config{longsize}, "\n";
3240 print $Config{longlongsize}, "\n";
ef54e1a4 3241
5074e145 3242(The C<$Config{longlongsize}> will be undefine if your system does
b76cc8ba 3243not support long longs.)
851646ae 3244
ef54e1a4
JH
3245=item *
3246
951ba7fe 3247The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, and C<L>
ef54e1a4
JH
3248are inherently non-portable between processors and operating systems
3249because they obey the native byteorder and endianness. For example a
82e239e7 32504-byte integer 0x12345678 (305419896 decimal) would be ordered natively
ef54e1a4 3251(arranged in and handled by the CPU registers) into bytes as
61eff3bc 3252
b35e152f
JJ
3253 0x12 0x34 0x56 0x78 # big-endian
3254 0x78 0x56 0x34 0x12 # little-endian
61eff3bc 3255
b84d4f81
JH
3256Basically, the Intel and VAX CPUs are little-endian, while everybody
3257else, for example Motorola m68k/88k, PPC, Sparc, HP PA, Power, and
3258Cray are big-endian. Alpha and MIPS can be either: Digital/Compaq
82e239e7
JH
3259used/uses them in little-endian mode; SGI/Cray uses them in big-endian
3260mode.
719a3cf5 3261
19799a22 3262The names `big-endian' and `little-endian' are comic references to
ef54e1a4
JH
3263the classic "Gulliver's Travels" (via the paper "On Holy Wars and a
3264Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and
19799a22